**Meet the editors**

Dr. Abhijit Ray is an Associate Professor at the Pandit Deendayal Petroleum University (PDPU), India and currently heading the Department of Solar Energy. His current research works are focused on the development of eco-friendly functional materials and devices for photovoltaic, photo-/electro-chemical and capacitive energy conversion and storage applications.

Prof. Indrajit Mukhopadhyay is a Professor of PDPU, heading the Solar Research and Development Center at PDPU. He has been involved in establishing a modern laboratory with state-of-the-art facilities for carrying out frontier research in the field of solar photovoltaics.

Dr. Ranjan K. Pati is an Assistant Professor at the School of Petroleum Technology, PDPU. His research focuses are nanomaterials, oxide ceramics, and their application on fuel cell technology.

Contents

**Preface VII**

**Section 1 Electrocatalysts for Fuel Cells 1**

Chapter 2 **Oxygen Reduction Reaction 25** Lindiwe Khotseng

Chapter 1 **Theoretical Basis of Electrocatalysis 3** Chi Ho Lee and Sang Uck Lee

Winston Duo Wu and Dan Xu

Varlam and Marius Enachescu

**Section 2 Electrocatalysts for Water Splitting 93**

**Application in PEM Fuel Cells 69**

Chapter 3 **Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction 51**

Chapter 4 **Noble Metal Dispersed on Reduced Graphene Oxide and Its**

Chapter 5 **Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction 95** Valeriy Kublanovsky and Yuliya Yapontseva

Adriana Marinoiu, Mircea Raceanu, Elena Carcadea, Aida Pantazi, Raluca Mesterca, Oana Tutunaru, Simona Nica, Daniela Bala, Mihai

## Contents

**Preface XI**


Preface

Electrocatalysts play a major role in two forms of green energy conversions: the controlled reaction of hydrogen and oxygen producing electricity in fuel cells, and the water-splitting reaction where water is reduced into hydrogen and oxygen in an up-hill thermodynamic process. They are equally important in new energy storage technologies, such as in re‐ chargeable metal-air batteries, where reduction of oxygen must be assisted by a catalyst. The science of electrocatalysis, which originated as a special field in electrochemistry, has seen tremendous progress after the late eighties due to several applications not only in chemistry, but also in chemical and industrial engineering. The field of electrocatalysis is still one of the most studied fields in research and development with emerging and advanced materials. Some of these materials are nano-carbon forms, including graphene, other materials are nanostructured two-dimensional materials, such as transition metal dichalcogenides (TMDCs) (MoS2, WS2 etc), metal-carbides, -nitrides, and –phosphide etc. This book is intend‐ ed to bring together various theoretical and experimental aspects of designing efficient elec‐ trocatalysts for two major applications of oxygen reduction reactions in the fuel cells and

This book starts with a theoretical understanding of electrocatalysis in the framework of den‐ sity functional theory followed by a vivid review of oxygen reduction reactions. A special emphasis has been placed on electrocatalysts for a proton-exchange membrane-based fuel cell where graphene with noble metal dispersion plays a significant role in electron transfer at thermodynamically favourable conditions. The latter part of the book deals with two major TMDCs with high economic viability, MoS2 and WS2 for their prospects in HER activities.

We trust that readers with a specific interest in the subject and above research, development

**Dr. Abhijit Ray**

**Dr. Ranjan K. Pati**

**Prof. Indrajit Mukhopadhyay**

Pandit Deendayal Petroleum University, India

Pandit Deendayal Petroleum University, India

Pandit Deendayal Petroleum University, India

hydrogen evolution reaction (HER) in water-splitting cells.

and applications will find the book convenient and informative.

## Preface

Electrocatalysts play a major role in two forms of green energy conversions: the controlled reaction of hydrogen and oxygen producing electricity in fuel cells, and the water-splitting reaction where water is reduced into hydrogen and oxygen in an up-hill thermodynamic process. They are equally important in new energy storage technologies, such as in re‐ chargeable metal-air batteries, where reduction of oxygen must be assisted by a catalyst. The science of electrocatalysis, which originated as a special field in electrochemistry, has seen tremendous progress after the late eighties due to several applications not only in chemistry, but also in chemical and industrial engineering. The field of electrocatalysis is still one of the most studied fields in research and development with emerging and advanced materials. Some of these materials are nano-carbon forms, including graphene, other materials are nanostructured two-dimensional materials, such as transition metal dichalcogenides (TMDCs) (MoS2, WS2 etc), metal-carbides, -nitrides, and –phosphide etc. This book is intend‐ ed to bring together various theoretical and experimental aspects of designing efficient elec‐ trocatalysts for two major applications of oxygen reduction reactions in the fuel cells and hydrogen evolution reaction (HER) in water-splitting cells.

This book starts with a theoretical understanding of electrocatalysis in the framework of den‐ sity functional theory followed by a vivid review of oxygen reduction reactions. A special emphasis has been placed on electrocatalysts for a proton-exchange membrane-based fuel cell where graphene with noble metal dispersion plays a significant role in electron transfer at thermodynamically favourable conditions. The latter part of the book deals with two major TMDCs with high economic viability, MoS2 and WS2 for their prospects in HER activities.

We trust that readers with a specific interest in the subject and above research, development and applications will find the book convenient and informative.

> **Dr. Abhijit Ray** Pandit Deendayal Petroleum University, India

> **Prof. Indrajit Mukhopadhyay** Pandit Deendayal Petroleum University, India

> **Dr. Ranjan K. Pati** Pandit Deendayal Petroleum University, India

**Section 1**

**Electrocatalysts for Fuel Cells**

**Section 1**

**Electrocatalysts for Fuel Cells**

**Chapter 1**

Provisional chapter

**Theoretical Basis of Electrocatalysis**

Theoretical Basis of Electrocatalysis

Additional information is available at the end of the chapter

In this chapter, we introduce the density functional theory (DFT)-based computational approaches to the study of various electrochemical reactions (hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR)) occurring on heterogeneous catalysis surfaces. A detailed computational approach to the theoretical interpretation of electrochemical reactions and structure-catalytic activity relationships for graphene-based catalysts will be discussed. The electrocatalytic activity of catalysis can be theoretically evaluated by overpotential value determined from free energy diagram (FED) of electrochemical reactions. By comparing electrocatalytic activity of systematically designed graphene-based catalysts, we will discuss the structure-catalytic activity relation-

DOI: 10.5772/intechopen.77109

ships, especially the electronic and geometrical effects of heteroatom dopants.

Keywords: DFT, electrocatalysis, HER, OER, ORR, FED, overpotential, dopant, carbon

With the climate change, fast consumption of fossil fuels, and environment situations due to carbon release, the research and development of clean energy is of vital importance in the coming decades. Promising applications of electrocatalysis for clean energy conversion, for example fuel cells, water electrolysis, metal-air batteries, and CO2 to fuel conversion, are the subjects of both extensive fundamental and utilitarian studies. These technologies play a crucial role in the future of sustainable energy utilization infrastructure, and thus huge research efforts have been dedicated to improving the electrocatalytic activity of these reactions, which include electrocatalytic oxygen reduction reaction (ORR), and hydrogen oxidation reaction (HOR) that occur on the cathode and anode of a hydrogen-oxygen fuel cell, respectively, and hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

Additional information is available at the end of the chapter

Chi Ho Lee and Sang Uck Lee

Chi Ho Lee and Sang Uck Lee

Abstract

1. Introduction

http://dx.doi.org/10.5772/intechopen.77109

#### **Chapter 1** Provisional chapter

#### **Theoretical Basis of Electrocatalysis** Theoretical Basis of Electrocatalysis

Chi Ho Lee and Sang Uck Lee Chi Ho Lee and Sang Uck Lee

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77109

#### Abstract

In this chapter, we introduce the density functional theory (DFT)-based computational approaches to the study of various electrochemical reactions (hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR)) occurring on heterogeneous catalysis surfaces. A detailed computational approach to the theoretical interpretation of electrochemical reactions and structure-catalytic activity relationships for graphene-based catalysts will be discussed. The electrocatalytic activity of catalysis can be theoretically evaluated by overpotential value determined from free energy diagram (FED) of electrochemical reactions. By comparing electrocatalytic activity of systematically designed graphene-based catalysts, we will discuss the structure-catalytic activity relationships, especially the electronic and geometrical effects of heteroatom dopants.

DOI: 10.5772/intechopen.77109

Keywords: DFT, electrocatalysis, HER, OER, ORR, FED, overpotential, dopant, carbon

### 1. Introduction

With the climate change, fast consumption of fossil fuels, and environment situations due to carbon release, the research and development of clean energy is of vital importance in the coming decades. Promising applications of electrocatalysis for clean energy conversion, for example fuel cells, water electrolysis, metal-air batteries, and CO2 to fuel conversion, are the subjects of both extensive fundamental and utilitarian studies. These technologies play a crucial role in the future of sustainable energy utilization infrastructure, and thus huge research efforts have been dedicated to improving the electrocatalytic activity of these reactions, which include electrocatalytic oxygen reduction reaction (ORR), and hydrogen oxidation reaction (HOR) that occur on the cathode and anode of a hydrogen-oxygen fuel cell, respectively, and hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

cathode and the anode of an electrolytic cell producing gaseous molecular hydrogen and oxygen, respectively. These reactions play an important role in regenerative fuel cells and dominate their overall performance [1–5]. Understanding the HER/OER/ORR mechanisms of various catalysts could provide design guidelines for material and process development, as well as facilitating the discovery of new catalysts. Above all, the detailed OER/ORR mechanisms in acid/alkaline environment are still being studied. Generally, OER/ORR can proceed in Langmuir-Hinshelwood (LH) or Eley-Rideal (ER) mechanisms [6]. The LH mechanism comprises all reactive intermediates on the surface while the ER mechanism includes species from the electrolyte that reacts with the surface intermediate. Despite the controversy over the mechanism, ER mechanism is generally accepted with lower reaction energy barrier than that of LH mechanism [7], and many researchers have conducted theoretical studies on OER/ORR based on the ER mechanism. However, there are two feasible reaction pathways in ER mechanism, two-step pathway and four-step pathway depending on the relative stability of O\* and OOH\* intermediates generated after the adsorption of O2 on the catalyst [8]. Thus, we sought to describe the detailed reaction pathway of the OER/ORR as well as proposing solutions for the determination of the preferred reaction pathway on the ER mechanism.

In this chapter, we will introduce metal-free bifunctional electrocatalysts of the heteroatomdoped graphenes (GXs, where G and X represent graphene and the heteroatom dopant) for HER [17] and the heteroatom-doped C3N4 (XY-C3N4s, where X and Y indicate the dopant and doping site on C3N4, respectively) for OER/ORR [18–20]. From the doping effect which shows better performance for HER/OER/ORR, we first present evidence that structural deformation and periodic lattice defects play the fundamental role in the HER activity of GXs by adjusting the electronic properties of graphene. We found that graphene doped with third row elements has higher HER activity with out-of-plane structural deformation compared to graphene doped with second row elements, in which graphene tends to maintain its planar structure. We systematically describe a structure-activity relationship in GXs for HER based on a thorough understanding of the effects of dopants, respectively. In addition, the third row elementsdoped graphenes (GSi, GP and GS) show an interesting regularity described by a simple 3 N rule: GXs give outstanding HER activity with sustained metallic property when its primitive cell size has 3 � 3 N (N is integral) supercell size of pure graphene. Secondly, we describe not only the detailed OER/ORR mechanisms but also improved OER/ORR activity of C3N4 by introducing dopants such as P or S into the C3N4 matrix. Especially, we explore the causes of variation in HER/OER/ORR performance with respect to the type of dopant by comparing

From these geometric and electronic structures, we demonstrated that GXs [17] and XY-C3N4 [18–20] show outstanding HER/OER/ORR activity with synergistic geometric and electronic

improve electrical conductance by modulating the electronic structure with extra electrons from dopants. Our theoretical investigations suggest that the synergistic effect between geometric and electronic factors plays an important role in HER/OER/ORR catalytic activities. It can be emphasized that there is a close correlation between the geometric/electronic structure and HER/OER/ORR catalytic activities. This understanding of the structure-activity relationship will give an insight into the development of new highly efficient electrocatalytic materials.

HER is a multistep process that takes place on the surface of catalyst, and there are two proposed mechanisms: Volmer-Heyrovsky and Volmer-Tafel. Both Volmer-Heyrovsky and Volmer-Tafel mechanisms describe the hydrogen atom adsorption and hydrogen molecule desorption reactions among (1) an initial state 2H<sup>þ</sup> þ 2e�, (2) an intermediate adsorbed state

active site and adsorbed hydrogen atom on the surface of the catalyst, respectively. Because the initial and final states are equivalent at equilibrium reduction potential, U = 0, the Gibbs free energy of the intermediate state, ΔGH j j <sup>∗</sup> , has been considered as a major descriptor of the HER activity for a wide variety of catalysts. Therefore, the optimum value of ΔGH j j <sup>∗</sup> should be zero for a spontaneous reaction without activation energy barrier. The Pt catalyst facilitates HER

, and (3) a final product state 2H<sup>þ</sup> <sup>þ</sup> 2e�, where the <sup>∗</sup> and <sup>H</sup><sup>∗</sup> denote the


Theoretical Basis of Electrocatalysis

5

http://dx.doi.org/10.5772/intechopen.77109

geometric and electronic structures of GXs and XY-C3N4s.

effects, which coordinatively increase unsaturated sp<sup>3</sup>

2. Theoretical background

2.1. Hydrogen evolution reaction

<sup>H</sup><sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� or 2H<sup>∗</sup>

Precious metals such as platinum (Pt), iridium (Ir), and ruthenium (Ru)-based catalysts [9–11] are generally needed to promote the HER for the generation of hydrogen fuel from the electrochemical splitting of water, the ORR in fuel cells for energy conversion, and the OER in metal-air batteries for energy storage. Besides the requirement for high catalytic activity, other issues related to these catalysts are their limited reserves and comparatively high cost, which have precluded these renewable energy technologies from large-scale commercial applications. In this regard, huge amount of efforts has been devoted to develop novel electrocatalysts to completely or partially replace precious metal catalysts in energy technologies. Along with the intensive research efforts in developing nonprecious electrocatalysts to reduce or to replace precious metal catalysts, various carbon-based, metal-free catalysts have been extensively studied because they have unique advantages for designated catalysis due to their tunable molecular structures, abundance and strong tolerance to acid/alkaline environments when used as alternative HER/OER/ORR catalysts. A rapidly growing field of metal-free catalysis based on carbon-based materials has developed, and a substantial amount of literature in both on the theoretical and experimental fields has been generated. Recent studies have revealed that graphene, [12] graphite, [13] vertically aligned nitrogen-doped carbon nanotubes (VA-NCNTs), [14] heteroatom-doped CNTs, [15] and nitrogen-doped graphene sheets [16] have excellent catalytic performance. The presence of N in N-doped graphene leads to more chemically active sites, a high density of defects and high electrochemical activity. Due to these enhanced electronic properties, N-doped catalysts in the C network are attractive for a wide range of applications, including as metal-free catalysts for HER/OER/ORR in fuel cell systems. Recently, carbon nitride-based catalysts (C3N4 and C2N) with N-rich including both graphitic and pyridinic N moieties is a promising catalyst due to its competitiveness over a wide range of electrocatalyst processes, despite pure C3N4 and C2N itself being inert with regard to HER/OER/ORR activity. Here, we attempted to enhance the catalytic activity of graphene, C3N4 and C2N by introducing heteroatoms, which is an effective way to manipulate its electronic structure and electrochemical properties.

In this chapter, we will introduce metal-free bifunctional electrocatalysts of the heteroatomdoped graphenes (GXs, where G and X represent graphene and the heteroatom dopant) for HER [17] and the heteroatom-doped C3N4 (XY-C3N4s, where X and Y indicate the dopant and doping site on C3N4, respectively) for OER/ORR [18–20]. From the doping effect which shows better performance for HER/OER/ORR, we first present evidence that structural deformation and periodic lattice defects play the fundamental role in the HER activity of GXs by adjusting the electronic properties of graphene. We found that graphene doped with third row elements has higher HER activity with out-of-plane structural deformation compared to graphene doped with second row elements, in which graphene tends to maintain its planar structure. We systematically describe a structure-activity relationship in GXs for HER based on a thorough understanding of the effects of dopants, respectively. In addition, the third row elementsdoped graphenes (GSi, GP and GS) show an interesting regularity described by a simple 3 N rule: GXs give outstanding HER activity with sustained metallic property when its primitive cell size has 3 � 3 N (N is integral) supercell size of pure graphene. Secondly, we describe not only the detailed OER/ORR mechanisms but also improved OER/ORR activity of C3N4 by introducing dopants such as P or S into the C3N4 matrix. Especially, we explore the causes of variation in HER/OER/ORR performance with respect to the type of dopant by comparing geometric and electronic structures of GXs and XY-C3N4s.

From these geometric and electronic structures, we demonstrated that GXs [17] and XY-C3N4 [18–20] show outstanding HER/OER/ORR activity with synergistic geometric and electronic effects, which coordinatively increase unsaturated sp<sup>3</sup> -C via structural deformation and improve electrical conductance by modulating the electronic structure with extra electrons from dopants. Our theoretical investigations suggest that the synergistic effect between geometric and electronic factors plays an important role in HER/OER/ORR catalytic activities. It can be emphasized that there is a close correlation between the geometric/electronic structure and HER/OER/ORR catalytic activities. This understanding of the structure-activity relationship will give an insight into the development of new highly efficient electrocatalytic materials.

### 2. Theoretical background

cathode and the anode of an electrolytic cell producing gaseous molecular hydrogen and oxygen, respectively. These reactions play an important role in regenerative fuel cells and dominate their overall performance [1–5]. Understanding the HER/OER/ORR mechanisms of various catalysts could provide design guidelines for material and process development, as well as facilitating the discovery of new catalysts. Above all, the detailed OER/ORR mechanisms in acid/alkaline environment are still being studied. Generally, OER/ORR can proceed in Langmuir-Hinshelwood (LH) or Eley-Rideal (ER) mechanisms [6]. The LH mechanism comprises all reactive intermediates on the surface while the ER mechanism includes species from the electrolyte that reacts with the surface intermediate. Despite the controversy over the mechanism, ER mechanism is generally accepted with lower reaction energy barrier than that of LH mechanism [7], and many researchers have conducted theoretical studies on OER/ORR based on the ER mechanism. However, there are two feasible reaction pathways in ER mechanism, two-step pathway and four-step pathway depending on the relative stability of O\* and OOH\* intermediates generated after the adsorption of O2 on the catalyst [8]. Thus, we sought to describe the detailed reaction pathway of the OER/ORR as well as proposing solutions for

Precious metals such as platinum (Pt), iridium (Ir), and ruthenium (Ru)-based catalysts [9–11] are generally needed to promote the HER for the generation of hydrogen fuel from the electrochemical splitting of water, the ORR in fuel cells for energy conversion, and the OER in metal-air batteries for energy storage. Besides the requirement for high catalytic activity, other issues related to these catalysts are their limited reserves and comparatively high cost, which have precluded these renewable energy technologies from large-scale commercial applications. In this regard, huge amount of efforts has been devoted to develop novel electrocatalysts to completely or partially replace precious metal catalysts in energy technologies. Along with the intensive research efforts in developing nonprecious electrocatalysts to reduce or to replace precious metal catalysts, various carbon-based, metal-free catalysts have been extensively studied because they have unique advantages for designated catalysis due to their tunable molecular structures, abundance and strong tolerance to acid/alkaline environments when used as alternative HER/OER/ORR catalysts. A rapidly growing field of metal-free catalysis based on carbon-based materials has developed, and a substantial amount of literature in both on the theoretical and experimental fields has been generated. Recent studies have revealed that graphene, [12] graphite, [13] vertically aligned nitrogen-doped carbon nanotubes (VA-NCNTs), [14] heteroatom-doped CNTs, [15] and nitrogen-doped graphene sheets [16] have excellent catalytic performance. The presence of N in N-doped graphene leads to more chemically active sites, a high density of defects and high electrochemical activity. Due to these enhanced electronic properties, N-doped catalysts in the C network are attractive for a wide range of applications, including as metal-free catalysts for HER/OER/ORR in fuel cell systems. Recently, carbon nitride-based catalysts (C3N4 and C2N) with N-rich including both graphitic and pyridinic N moieties is a promising catalyst due to its competitiveness over a wide range of electrocatalyst processes, despite pure C3N4 and C2N itself being inert with regard to HER/OER/ORR activity. Here, we attempted to enhance the catalytic activity of graphene, C3N4 and C2N by introducing heteroatoms, which is an effective way to manipulate its electronic structure and electrochemical

the determination of the preferred reaction pathway on the ER mechanism.

4 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

properties.

### 2.1. Hydrogen evolution reaction

HER is a multistep process that takes place on the surface of catalyst, and there are two proposed mechanisms: Volmer-Heyrovsky and Volmer-Tafel. Both Volmer-Heyrovsky and Volmer-Tafel mechanisms describe the hydrogen atom adsorption and hydrogen molecule desorption reactions among (1) an initial state 2H<sup>þ</sup> þ 2e�, (2) an intermediate adsorbed state <sup>H</sup><sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>� or 2H<sup>∗</sup> , and (3) a final product state 2H<sup>þ</sup> <sup>þ</sup> 2e�, where the <sup>∗</sup> and <sup>H</sup><sup>∗</sup> denote the active site and adsorbed hydrogen atom on the surface of the catalyst, respectively. Because the initial and final states are equivalent at equilibrium reduction potential, U = 0, the Gibbs free energy of the intermediate state, ΔGH j j <sup>∗</sup> , has been considered as a major descriptor of the HER activity for a wide variety of catalysts. Therefore, the optimum value of ΔGH j j <sup>∗</sup> should be zero for a spontaneous reaction without activation energy barrier. The Pt catalyst facilitates HER with a low activation energy, ΔGH j j�<sup>∗</sup> 0.09 eV [21]. In this work, we have considered the Volmer-Tafel mechanism to calculate the Gibbs free energy of the intermediate state, ΔGVolmer <sup>H</sup><sup>∗</sup> <sup>θ</sup>H1 ð Þ<sup>∗</sup> , <sup>Δ</sup>GVolmer <sup>H</sup><sup>∗</sup> <sup>θ</sup>H2 ð Þ<sup>∗</sup> , <sup>Δ</sup>GHeyrovsky <sup>H</sup><sup>∗</sup> <sup>θ</sup>H1 ð Þ<sup>∗</sup> , and <sup>Δ</sup>GTafel <sup>H</sup><sup>∗</sup> θH2 ð Þ<sup>∗</sup> with different hydrogen coverage at the active sites, θH1<sup>∗</sup> , and θH2<sup>∗</sup> , as shown in Figure 1. The Gibbs free energy of the adsorbed hydrogen is calculated as:

$$
\Delta G\_{H^\*} = \Delta E\_{H^\*} + \Delta E\_{ZPE} - T\Delta S\_H \tag{1}
$$

where n, H<sup>∗</sup> and θH<sup>∗</sup> refer to the number of hydrogen atoms, adsorbed hydrogen on the

Table 1. Zero point energy (ZPE) and entropic (TS) correction for heteroatom doped-graphenes (G, GB, GN, GP, and

eV ZPE TS △ZPE <sup>T</sup>△<sup>S</sup> △ZPE- T△<sup>S</sup>

G-H\* 0.25 — 0.11 �0.21 0.32 GB-H\* 0.25 — 0.12 �0.21 0.32 GN-H\* 0.29 — 0.16 �0.21 0.36 GP-H\* 0.30 — 0.17 �0.21 0.37 GS-H\* 0.31 — 0.18 �0.21 0.38 GSi-H\* 0.30 — 0.17 �0.21 0.37 H2 0.27 0.41 — ——

In contrast to the single hydrogen reaction of the Volmer step, two hydrogen atoms mediate the Tafel step. Therefore, we obtain the Gibbs free energy of the intermediate state during the Volmer and Tafel steps with the following equations to determine the different hydrogen

The generally acceptable OER mechanism is the four-electron associative mechanism in alka-

OH� <sup>þ</sup> <sup>∗</sup> ! OH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>

OH<sup>∗</sup> <sup>þ</sup> OH� ! <sup>O</sup><sup>∗</sup> <sup>þ</sup> H2O lðÞþ <sup>e</sup>

<sup>O</sup><sup>∗</sup> <sup>þ</sup> OH� ! OOH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>

OOH<sup>∗</sup> <sup>þ</sup> OH� ! <sup>∗</sup> <sup>þ</sup> O2ð Þþ <sup>g</sup> H2O lðÞþ <sup>e</sup>

where \* represents the active site on the surface, (l) and (g) refer to liquid and gas phases,

, OH\* and OOH\* are adsorbed intermediates.

line media. The four elementary steps of OER mechanism are described as follows:

<sup>H</sup><sup>∗</sup> þ ΔEZPE � TΔS (4)

Theoretical Basis of Electrocatalysis

7

http://dx.doi.org/10.5772/intechopen.77109

<sup>H</sup><sup>∗</sup> þ ΔEZPE � TΔS (6)

� (8)

� (10)

� (9)

� (11)

ð5Þ

ð7Þ

surface, and hydrogen coverage, respectively.

ΔGVolmer

ΔGTafel

2.2. Oxygen evolution reaction and oxygen reduction reaction

2.2.1. Reaction pathways in alkaline media

respectively, and O\*

<sup>H</sup><sup>∗</sup> <sup>¼</sup> <sup>Δ</sup>EVolmer

<sup>H</sup><sup>∗</sup> <sup>¼</sup> <sup>Δ</sup>ETafel

coverages of active sites.

GSi) at 298 K.

where is hydrogen adsorption energy and is the difference in zero point energy (ZPE) between the adsorbed state and the gas phase. refers to the entropy of adsorption of which is the where S0 H2 is the entropy of H2 in the gas phase at STP. ZPE and entropic corrections, of the heteroatom-doped graphenes (GXs, where G and X represent graphene and the heteroatom dopant), are listed in Table 1. The hydrogen adsorption energy can be defined in two ways: the integral and differential H adsorption energy as a function of the H coverage in Eqs. (2) and (3), respectively.

The gas phase values were from reference 17, while the values for the adsorbed species were taken from DFT calculations. The same values for the adsorbed species for all the NXN models were used, as vibrational frequencies have been found to depend much less on the surface than the bond strength.

$$E\_{H^{\*}}^{int}(n) = 1/2[E(surf + nH^{\*}) - E(surf)] - 1/2E(H\_{2})\tag{2}$$

$$E\_{H^{\bullet}}^{\mathrm{diff}}(\theta\_{H^{\bullet}}) = \delta E\_{H^{\bullet}}^{\mathrm{int}}(\mathfrak{n})/\delta \mathfrak{n} = \left[ E\_{H^{\bullet}}^{\mathrm{int}}(\mathfrak{n}) - E\_{H^{\bullet}}^{\mathrm{int}}(\mathfrak{n} - 1) \right]/\Delta \mathfrak{n} \tag{3}$$

Figure 1. Schematic of Volmer-Heyrovsky and Volmer-Tafel mechanisms. ΔGVolmer <sup>H</sup><sup>∗</sup> <sup>θ</sup>H1 ð Þ<sup>∗</sup> , <sup>Δ</sup>GVolmer <sup>H</sup><sup>∗</sup> <sup>θ</sup>H2 ð Þ<sup>∗</sup> , <sup>Δ</sup>GHeyrovsky H∗ <sup>θ</sup>H1 ð Þ<sup>∗</sup> , and <sup>Δ</sup>GTafel <sup>H</sup><sup>∗</sup> θH2 ð Þ<sup>∗</sup> are free energies of the first-, and second-Volmer steps, Hyrovsky and Tafel step, where θH1<sup>∗</sup> , and θH2<sup>∗</sup> indicate different hydrogen coverage of active sites.


Table 1. Zero point energy (ZPE) and entropic (TS) correction for heteroatom doped-graphenes (G, GB, GN, GP, and GSi) at 298 K.

where n, H<sup>∗</sup> and θH<sup>∗</sup> refer to the number of hydrogen atoms, adsorbed hydrogen on the surface, and hydrogen coverage, respectively.

In contrast to the single hydrogen reaction of the Volmer step, two hydrogen atoms mediate the Tafel step. Therefore, we obtain the Gibbs free energy of the intermediate state during the Volmer and Tafel steps with the following equations to determine the different hydrogen coverages of active sites.

$$
\Delta G\_{H^{\ast}}^{\text{Volmer}} = \Delta E\_{H^{\ast}}^{\text{Volmer}} + \Delta E\_{\text{ZPE}} - T\Delta S \tag{4}
$$

$$
\Delta \mathsf{E}\_{\mathsf{II}\_{\star}}^{\mathsf{IV}\_{\mathsf{V}\mathsf{ol}\mathsf{inv}\mathsf{r}}} = \mathsf{E}\_{\mathsf{II}\_{\star}}^{\mathsf{diff}}(\mathsf{O}\_{\mathsf{H}^{\*}}) = \mathsf{I} \mathsf{E}\_{\mathsf{I}^{\mathsf{int}}}^{\mathsf{III}}(n) - \mathsf{E}\_{\mathsf{II}\_{\star}}^{\mathsf{III}}(n-1)\mathsf{I}\langle\Delta\rangle\mathsf{I} \qquad \Delta n = 1 \tag{5}
$$

$$
\Delta G\_{H^{\text{Tafel}}}^{\text{Tafel}} = \Delta E\_{H^{\text{Tafel}}}^{\text{Tafel}} + \Delta E\_{\text{ZPE}} - T\Delta \text{S} \tag{6}
$$

$$
\Delta \mathsf{E}\_{\mathsf{II}^{\star}}^{\mathsf{i\&}\mathsf{i\&}\mathsf{f}\mathsf{i}} = \mathsf{E}\_{\mathsf{II}^{\star}}^{\mathsf{i\&}\mathsf{i\&}\mathsf{f}}(\mathsf{O}\_{\mathsf{H}^{\perp}}) = \mathsf{I}\mathsf{E}\_{\mathsf{II}^{\star}}^{\mathsf{i\&}\mathsf{i\&}}(\mathsf{n}-\mathsf{2}) - \mathsf{E}\_{\mathsf{II}^{\star}}^{\mathsf{i\&}\mathsf{i\&}}(\mathsf{n})\mathsf{I}\mathsf{V}\Delta\mathsf{n}\mathsf{j} \qquad \Delta\mathsf{n} = \mathsf{2} \tag{7}
$$

#### 2.2. Oxygen evolution reaction and oxygen reduction reaction

#### 2.2.1. Reaction pathways in alkaline media

with a low activation energy, ΔGH j j�<sup>∗</sup> 0.09 eV [21]. In this work, we have considered the Volmer-Tafel mechanism to calculate the Gibbs free energy of the intermediate state,

<sup>H</sup><sup>∗</sup> <sup>θ</sup>H1 ð Þ<sup>∗</sup> , and <sup>Δ</sup>GTafel

coverage at the active sites, θH1<sup>∗</sup> , and θH2<sup>∗</sup> , as shown in Figure 1. The Gibbs free energy of the

where is hydrogen adsorption energy and is the difference in zero point energy (ZPE) between the adsorbed state and the gas phase. refers to the entropy of adsorption of

ZPE and entropic corrections, of the heteroatom-doped graphenes (GXs, where G and X represent graphene and the heteroatom dopant), are listed in Table 1. The hydrogen adsorption energy can be defined in two ways: the integral and differential H adsorption

The gas phase values were from reference 17, while the values for the adsorbed species were taken from DFT calculations. The same values for the adsorbed species for all the NXN models were used, as vibrational frequencies have been found to depend much less on the

<sup>H</sup><sup>∗</sup> ð Þ <sup>n</sup> <sup>=</sup>δ<sup>n</sup> <sup>¼</sup> <sup>E</sup>int

<sup>H</sup><sup>∗</sup> θH2 ð Þ<sup>∗</sup> with different hydrogen

ΔGH<sup>∗</sup> ¼ ΔEH<sup>∗</sup> þ ΔEZPE � TΔSH (1)

<sup>H</sup><sup>∗</sup> ð Þ¼ <sup>n</sup> <sup>1</sup>=2 E surf <sup>þ</sup> nH<sup>∗</sup> <sup>½</sup> ð Þ� E surf ð Þ� � <sup>1</sup>=2E Hð Þ<sup>2</sup> (2)

<sup>H</sup><sup>∗</sup> ð Þ� <sup>n</sup> Eint

<sup>H</sup><sup>∗</sup> θH2 ð Þ<sup>∗</sup> are free energies of the first-, and second-Volmer steps, Hyrovsky and Tafel step, where θH1<sup>∗</sup> ,

H2 is the entropy of H2 in the gas phase at STP.

<sup>H</sup><sup>∗</sup> ð Þ <sup>n</sup> � <sup>1</sup> <sup>=</sup>Δ<sup>n</sup> (3)

<sup>H</sup><sup>∗</sup> <sup>θ</sup>H1 ð Þ<sup>∗</sup> , <sup>Δ</sup>GVolmer

<sup>H</sup><sup>∗</sup> <sup>θ</sup>H2 ð Þ<sup>∗</sup> , <sup>Δ</sup>GHeyrovsky H∗

ΔGVolmer

<sup>H</sup><sup>∗</sup> <sup>θ</sup>H1 ð Þ<sup>∗</sup> , <sup>Δ</sup>GVolmer

adsorbed hydrogen is calculated as:

surface than the bond strength.

<sup>θ</sup>H1 ð Þ<sup>∗</sup> , and <sup>Δ</sup>GTafel

Eint

Ediff

<sup>H</sup><sup>∗</sup> <sup>θ</sup><sup>H</sup> ð Þ¼ <sup>∗</sup> <sup>δ</sup>Eint

Figure 1. Schematic of Volmer-Heyrovsky and Volmer-Tafel mechanisms. ΔGVolmer

and θH2<sup>∗</sup> indicate different hydrogen coverage of active sites.

<sup>H</sup><sup>∗</sup> <sup>θ</sup>H2 ð Þ<sup>∗</sup> , <sup>Δ</sup>GHeyrovsky

6 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

which is the where S0

energy as a function of the H coverage in Eqs. (2) and (3), respectively.

The generally acceptable OER mechanism is the four-electron associative mechanism in alkaline media. The four elementary steps of OER mechanism are described as follows:

$$
\cdot OH^- + \* \rightarrow OH^\* + e^- \tag{8}
$$

$$\text{OH}^\* + \text{OH}^- \rightarrow \text{O}^\* + \text{H}\_2\text{O}(l) + e^- \tag{9}$$

$$\text{O}^\* + \text{OH}^- \rightarrow \text{OOH}^\* + e^- \tag{10}$$

$$\rm OOH^\* + \rm OH^- \longrightarrow \* + \rm O\_2(g) + H\_2O(l) + e^- \tag{11}$$

where \* represents the active site on the surface, (l) and (g) refer to liquid and gas phases, respectively, and O\* , OH\* and OOH\* are adsorbed intermediates.

In contrast to OER, ORR can proceed either by a two-step or four-step pathways depending on the relative stability of O\* and OOH\* intermediates generated after the adsorption of O2 on the catalyst. The two-step pathway of ORR in alkaline environment is summarized using the following elementary steps,

$$2\text{O}\_2 + \text{H}\_2\text{O}(l) + 2e^- \rightarrow \text{O}^\* + 2\text{OH}^- \tag{12}$$

2.2.3. The chemical potentials of OH�, e�, H2O, and O2

potentials, the equations are expressed as follows:

further as follows:

In alkaline environment, the standard oxygen reduction reaction is described as follows:

and the standard reduction potential ( ) is 0.402 V at T = 298.15 K. In terms of chemical

The left side in (Eq. (24)), the chemical potentials of electron and hydroxide, could be derived

<sup>μ</sup><sup>e</sup>� <sup>¼</sup> <sup>μ</sup><sup>0</sup>

where represents the shift in electron energy when a bias is applied and ,

represent the chemical potentials of electron and hydroxide at standard conditions

<sup>e</sup>- � eU - <sup>μ</sup><sup>0</sup>

at standard and equilibrium conditions aOH- <sup>¼</sup> <sup>1</sup>; <sup>T</sup> <sup>¼</sup> <sup>298</sup>:15K; and eU <sup>¼</sup> <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:402V . In the case of the right side in Eq. (24), the chemical potentials of liquid water and oxygen molecule ( and ) can be calculated from the approximations proposed by Norskov et al., where the chemical potential of liquid water ( ) is equal to the chemical potential of

Moreover, the chemical potential of oxygen molecule ( ) is derived from the standard free

Because the experimental standard free energy change is �2.46 eV, the equation can be written as:

O2ð Þ<sup>g</sup> � <sup>G</sup><sup>0</sup>

. Therefore,

DFT <sup>þ</sup> ZPEH2O gð Þ � TS<sup>0</sup>

1=2O2ð Þþ g H2ð Þ\$g H2O lð Þ (30)

H2ð Þ<sup>g</sup> ¼ �2:46eV (31)

OH- þ κBTlnaOH�

<sup>μ</sup>OH� <sup>¼</sup> <sup>μ</sup><sup>0</sup>

<sup>e</sup>- � <sup>μ</sup><sup>0</sup>

<sup>μ</sup><sup>e</sup>� � <sup>μ</sup>OH� <sup>¼</sup> <sup>μ</sup><sup>0</sup>

water in the gas phase ( ), at T = 298.15 K and 0.035 bars.

G0

energy change of the reaction:

<sup>μ</sup>H2O lð Þ <sup>¼</sup> <sup>μ</sup>H2O gð Þ <sup>¼</sup> <sup>E</sup>H2O gð Þ

H2O lð Þ � <sup>1</sup>=2G<sup>0</sup>

<sup>μ</sup><sup>e</sup>� � <sup>μ</sup>OH� <sup>¼</sup> <sup>μ</sup><sup>0</sup>

O2ð Þþ <sup>g</sup> 2H2O lðÞþ 4e� ! 4OH� <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:402V (22)

<sup>μ</sup>o2ð Þ<sup>g</sup> <sup>þ</sup> <sup>2</sup>μH2O lð Þ <sup>þ</sup> <sup>4</sup>μ<sup>e</sup>� <sup>¼</sup> <sup>4</sup>μOH� (23)

<sup>e</sup>- � eU (25)

Theoretical Basis of Electrocatalysis

9

http://dx.doi.org/10.5772/intechopen.77109

(27)

H2O gð Þ (29)

OH--eU <sup>¼</sup> -1=<sup>4</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> <sup>þ</sup> <sup>2</sup>μH2O lð Þ (28)

OH- þ κBTlnaOH� (26)

<sup>μ</sup><sup>e</sup>� � <sup>μ</sup>OH� ¼ �1=<sup>4</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> <sup>þ</sup> <sup>2</sup>μH2O lð Þ (24)

$$2\text{O}^\* + 2\text{OH}^- + H\_2\text{O}(l) + 2e^- \rightarrow 4\text{OH}^- \tag{13}$$

whereas the four-step pathway has following elementary steps:

$$O\_2 + H\_2O(l) + e^- \rightarrow OOH^\* + OH^- \tag{14}$$

$$\text{OOH}^\* + \text{e}^- \rightarrow \text{O}^\* + \text{OH}^- \tag{15}$$

$$\text{O}^\* + \text{H}\_2\text{O}(l) + e^- \rightarrow \text{OH}^\* + \text{OH}^- \tag{16}$$

$$OH^\* + e^- \to \* + OH^- \tag{17}$$

Looking at the elementary reaction steps of ORR, both reaction pathways lead to the same final products as 4OH� and the different intermediate states after the adsorption of O2 on the catalyst, O\* in (Eq. (12)) and OOH\* in (Eq. (14)). Therefore, it is worth mentioning that the transformation step of to in (Eq. (12)) or to in (Eq. (14)) can be important index to determine the ORR pathway.

#### 2.2.2. Derivation of the free energy relations

The free energy change of each elementary reaction of OER in alkaline media can be expressed as follows:

$$
\Delta \mathcal{G}\_{\rm I} = \mathcal{G}\_{\rm OH^{\*}} + \mu\_{e^{-}} - \left(\mu\_{\rm OH^{-}} + \mathcal{G}\_{\rm \*}\right) \tag{18}
$$

$$
\Delta \mathcal{G}\_2 = \mathcal{G}\_{O^\*} + \mu\_{H\_2O(l)} + \mu\_{e^-} - \left(\mathcal{G}\_{OH^\*} + \mu\_{OH^-}\right) \tag{19}
$$

$$
\Delta \mathcal{G}\_3 = \mathcal{G}\_{\text{OOH}^\*} + \mu\_{e^-} - \left(\mathcal{G}\_{O^\*} + \mu\_{\text{OH}^-}\right) \tag{20}
$$

$$
\Delta \mathcal{G}\_{\mathsf{4}} = \mathsf{G}\_{\mathsf{\*}} + \mu\_{O\_{2}} + \mu\_{H\_{2}O(l)} + \mu\_{\mathfrak{e}^{-}} - \left(\mathsf{G}\_{\mathsf{OOH^{\*}}} + \mu\_{\mathsf{OH^{-}}}\right) \tag{21}
$$

Therefore, the free energy change of each elementary reaction can be calculated using (1) the chemical potentials of hydroxide, electron, liquid water and oxygen molecule (μOH� , <sup>μ</sup><sup>e</sup>� , <sup>μ</sup>H2O lð Þ and μO2 ) and (2) the free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of catalyst (\*). In the following paragraph, we describe the theoretical method to calculate the free energy change of each elementary reaction of the proposed reaction path based on the relation between the standard electrode potential and the chemical potential of hydroxides and electrons in alkaline environment.

### 2.2.3. The chemical potentials of OH�, e�, H2O, and O2

In contrast to OER, ORR can proceed either by a two-step or four-step pathways depending on the relative stability of O\* and OOH\* intermediates generated after the adsorption of O2 on the catalyst. The two-step pathway of ORR in alkaline environment is summarized using the

whereas the four-step pathway has following elementary steps:

8 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

O2 þ H2O lðÞþ e

OOH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>

<sup>O</sup><sup>∗</sup> <sup>þ</sup> H2O lðÞþ <sup>e</sup>

OH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>

Looking at the elementary reaction steps of ORR, both reaction pathways lead to the same final products as 4OH� and the different intermediate states after the adsorption of O2 on the catalyst, O\* in (Eq. (12)) and OOH\* in (Eq. (14)). Therefore, it is worth mentioning that the transformation step of to in (Eq. (12)) or to in (Eq. (14)) can be important index to determine the

The free energy change of each elementary reaction of OER in alkaline media can be expressed

ΔG1 ¼ GOH<sup>∗</sup> þ μ<sup>e</sup>� � μOH� þ G<sup>∗</sup>

<sup>Δ</sup>G2 <sup>¼</sup> GO<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>H2O lð Þ <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � GOH<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>OH�

ΔG3 ¼ GOOH<sup>∗</sup> þ μ<sup>e</sup>� � GO<sup>∗</sup> þ μOH�

<sup>Δ</sup>G4 <sup>¼</sup> <sup>G</sup><sup>∗</sup> <sup>þ</sup> <sup>μ</sup>O2 <sup>þ</sup> <sup>μ</sup>H2O lð Þ <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � GOOH<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>OH�

Therefore, the free energy change of each elementary reaction can be calculated using (1) the chemical potentials of hydroxide, electron, liquid water and oxygen molecule (μOH� , <sup>μ</sup><sup>e</sup>� , <sup>μ</sup>H2O lð Þ

catalyst (\*). In the following paragraph, we describe the theoretical method to calculate the free energy change of each elementary reaction of the proposed reaction path based on the relation between the standard electrode potential and the chemical potential of hydroxides and elec-

) and (2) the free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of

O2 <sup>þ</sup> H2O lðÞþ 2e� ! <sup>O</sup><sup>∗</sup> <sup>þ</sup> 2OH� (12)

<sup>O</sup><sup>∗</sup> <sup>þ</sup> 2OH� <sup>þ</sup> H2O lðÞþ 2e� ! 4OH� (13)

� ! OOH<sup>∗</sup> <sup>þ</sup> OH� (14)

� ! <sup>O</sup><sup>∗</sup> <sup>þ</sup> OH� (15)

� ! ∗ þ OH� (17)

� ! OH<sup>∗</sup> <sup>þ</sup> OH� (16)

(18)

(20)

(19)

(21)

following elementary steps,

ORR pathway.

as follows:

and μO2

trons in alkaline environment.

2.2.2. Derivation of the free energy relations

In alkaline environment, the standard oxygen reduction reaction is described as follows:

$$2H\_2(g) + 2H\_2O(l) + 4e^- \rightarrow 4OH^- \qquad E^0 = 0.402V \tag{22}$$

and the standard reduction potential ( ) is 0.402 V at T = 298.15 K. In terms of chemical potentials, the equations are expressed as follows:

$$
\mu\_{o\_2(\mathcal{g})} + 2\mu\_{H\_2O(l)} + 4\mu\_{e^-} = 4\mu\_{OH^-} \tag{23}
$$

$$
\mu\_{e^-} - \mu\_{OH^-} = -1/4 \left( \mu\_{O\_2(g)} + 2\mu\_{H\_2O(l)} \right) \tag{24}
$$

The left side in (Eq. (24)), the chemical potentials of electron and hydroxide, could be derived further as follows:

$$
\mu\_{e^{-}} = \mu\_{e^{\*}}^{0} - e \mathcal{U} \tag{25}
$$

$$
\mu\_{\rm OH^{-}} = \mu\_{\rm OH^{-}}^{0} + \kappa\_{\rm B} \, Tlma\_{\rm OH^{-}} \tag{26}
$$

where represents the shift in electron energy when a bias is applied and , represent the chemical potentials of electron and hydroxide at standard conditions . Therefore,

$$
\mu\_{\varepsilon^-} - \mu\_{OH^-} = \left(\mu\_{\varepsilon^-}^0 - \varepsilon \text{II}\right) \cdot \left(\mu\_{OH^-}^0 + \kappa\_B \text{Tlma}\_{OH^-}\right) \tag{27}
$$

$$
\mu\_{e^-} - \mu\_{OH^-} = \mu\_{e^\*}^0 - \mu\_{OH^-}^0 \text{-e} \text{II} = \text{-1/4} \left( \mu\_{O\_2(g)} + 2\mu\_{H\_2O(l)} \right) \tag{28}
$$

at standard and equilibrium conditions aOH- <sup>¼</sup> <sup>1</sup>; <sup>T</sup> <sup>¼</sup> <sup>298</sup>:15K; and eU <sup>¼</sup> <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:402V . In the case of the right side in Eq. (24), the chemical potentials of liquid water and oxygen molecule ( and ) can be calculated from the approximations proposed by Norskov et al., where the chemical potential of liquid water ( ) is equal to the chemical potential of water in the gas phase ( ), at T = 298.15 K and 0.035 bars.

$$
\mu\_{H\_2O(l)} = \mu\_{H\_2O(g)} = E\_{DFT}^{H\_2O(g)} + ZPE\_{H\_2O(g)} - TS\_{H\_2O(g)}^0 \tag{29}
$$

Moreover, the chemical potential of oxygen molecule ( ) is derived from the standard free energy change of the reaction:

$$11/2O\_2(g) + H\_2(g) \leftrightarrow H\_2O(l) \tag{30}$$

Because the experimental standard free energy change is �2.46 eV, the equation can be written as:

$$\rm{G}\_{H\_2O(l)}^0 - 1/2 \rm{G}\_{O\_2(g)}^0 - \rm{G}\_{H\_2(g)}^0 = -2.46 eV \tag{31}$$

Therefore, the chemical potential of oxygen molecule ( ) can be approximately calculated as follows:

$$\begin{split} \mu\_{O\_2(\mathfrak{g})} &= 4.92 + 2 \left( E\_{DFT}^{H\_2O(\mathfrak{g})} + ZPE\_{H\_2O(\mathfrak{g})} - TS\_{H\_2O(\mathfrak{g})}^0 \right) \\ &- 2 \left( E\_{DFT}^{H\_2(\mathfrak{g})} + ZPE\_{H\_2(\mathfrak{g})} - TS\_{H\_2(\mathfrak{g})}^0 \right) \end{split} \tag{32}$$

GOH<sup>∗</sup> <sup>¼</sup> <sup>E</sup>OH<sup>∗</sup>

DFT <sup>þ</sup> ZPE � TS0 � � � <sup>E</sup><sup>∗</sup> DFT h i <sup>þ</sup> <sup>μ</sup><sup>0</sup>

DFT <sup>þ</sup> ZPE � TS<sup>0</sup> � � � <sup>E</sup><sup>∗</sup>

DFT h i <sup>þ</sup> <sup>9</sup>:952eV

The second step is oxidation of the OH\* species to O\* with release of water and an electron:

The relation for in terms of DFT energies is similar to the relation for (Eq. (35)). Replacing

The third step is represented by formation of the OOH\* on top of oxygen with a release of an

The relation for GOOH<sup>∗</sup> and GO<sup>∗</sup> in terms of DFT energies is similar to the relation for (Eq. (35)).

ΔG3 ¼ GOOH<sup>∗</sup> þ μ<sup>e</sup>� � GO<sup>∗</sup> þ μOH�

<sup>¼</sup> GOOH<sup>∗</sup> � GO <sup>ð</sup> <sup>∗</sup> Þ þ <sup>μ</sup><sup>0</sup>

<sup>Δ</sup>G4 <sup>¼</sup> <sup>G</sup><sup>∗</sup> <sup>þ</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> <sup>þ</sup> <sup>μ</sup>H2O lð Þ <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � GOOH<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>OH�

� � <sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>¼</sup> <sup>G</sup><sup>∗</sup> <sup>þ</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> <sup>þ</sup> <sup>μ</sup>H2O lð Þ � GOOH<sup>∗</sup>

<sup>Δ</sup>G2 <sup>¼</sup> GO<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>H2O lð Þ <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � GOH<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>OH�

� � <sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>¼</sup> GO<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>H2O lð Þ � GOH<sup>∗</sup>

again the same equations as in the case for the first step in (Eq. (38)) we get:

DFT <sup>þ</sup> EH2O gð Þ DFT � � � <sup>E</sup>OH<sup>∗</sup>

The same equations are replaced gradually in the (Eq. (40)) as follows:

DFT � <sup>E</sup><sup>O</sup><sup>∗</sup>

<sup>Δ</sup>G3 <sup>¼</sup> <sup>E</sup>OOH<sup>∗</sup>

Step 4: OOH<sup>∗</sup> <sup>þ</sup> OH� ! <sup>∗</sup> <sup>þ</sup> O2ð Þþ <sup>g</sup> H2O lð Þþ <sup>e</sup>� The last step is the evolution of oxygen molecule:

Therefore, we leave from the following equation:

Replacing (Eq. (33)), (Eq. (35)) and (Eq. (36)) in (Eq. (34)) we get:

<sup>Δ</sup>G1 <sup>¼</sup> <sup>E</sup>OH<sup>∗</sup>

Step 2: OH<sup>∗</sup> <sup>þ</sup> OH� ! <sup>O</sup><sup>∗</sup> <sup>þ</sup> H2O lð Þþ <sup>e</sup>�

<sup>Δ</sup>G2 <sup>¼</sup> <sup>E</sup><sup>O</sup><sup>∗</sup>

Step 3: <sup>O</sup><sup>∗</sup> <sup>þ</sup> OH� ! OOH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>�

electron:

<sup>¼</sup> EOH<sup>∗</sup>

DFT <sup>þ</sup> ZPEOH<sup>∗</sup> � TS<sup>0</sup>

OH<sup>∗</sup> (35)

http://dx.doi.org/10.5772/intechopen.77109

Theoretical Basis of Electrocatalysis

OH- � eU � � (38)

OH� � eU � � (40)

OH� � eU � � (42)

(37)

11

G<sup>∗</sup> ¼ GOH<sup>∗</sup> (36)

<sup>e</sup>- � <sup>μ</sup><sup>0</sup> OH- � eU � �

� �

<sup>e</sup>- � <sup>μ</sup><sup>0</sup>

DFT <sup>þ</sup> <sup>Δ</sup>ZPE-TΔS0 � � h i <sup>þ</sup> <sup>9</sup>:952eV (39)

� �

<sup>e</sup>� � <sup>μ</sup><sup>0</sup>

DFT <sup>þ</sup> <sup>Δ</sup>ZPE-TΔS0 � � h i <sup>þ</sup> <sup>9</sup>:952eV (41)

� �

<sup>e</sup>� � <sup>μ</sup><sup>0</sup>

Therefore, Eq. (24) can be written as:

$$
\mu\_{e^-} - \mu\_{OH^-} - \text{e}\mathcal{U} = -1/4 \left(\mu\_{O\_2(\mathfrak{g})} + 2\mu\_{H\_2O(\mathfrak{g})}\right) \tag{33}
$$

Finally, we can obtain value as 9.952 eV using the (Eq. (29)), (Eq. (32)) and (Eq. (33)) [20]. The enthalpy change (EDFT), the zero-point energy correction (ΔZPE) calculated by the DFT calculations of vibrational frequencies, and entropy corrections (TS) are listed in Table 2.

Gas phase H2O at 0.035 bar was used as the reference state because at this pressure gas phase H2O is in equilibrium with liquid water at 300 K. The same values for the adsorbed species for all the models were used, as vibrational frequencies have been found to depend much less on the surface than the bond strength. We took the standard entropies from thermodynamic tables for gas phase molecules.

### 2.2.4. The free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of catalyst (\*)

Step 1. OH� <sup>þ</sup> <sup>∗</sup> ! OH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>�

The first step is the adsorption step of active site with a release of an electron:

$$\begin{split} \Delta \mathbf{G}\_{\rm I} &= \mathbf{G}\_{\rm OH^{\*}} + \boldsymbol{\mu}\_{\rm e^{-}} - \left( \boldsymbol{\mu}\_{\rm OH^{-}} + \mathbf{G}\_{\rm \*} \right) \\ &= \left( \mathbf{G}\_{\rm OH^{\*}} - \mathbf{G}\_{\rm \*} \right) + \left( \boldsymbol{\mu}\_{\rm e^{-}}^{0} - \boldsymbol{\mu}\_{\rm OH^{-}}^{0} - \boldsymbol{\nu} \boldsymbol{U} \right) \end{split} \tag{34}$$

where respectively and could be expressed by DFT energies:


Table 2. Total energies (E) of H2O and H2 and zero point energy (ZPE) corrections and entropic contributions (TS) to the free energies.

Theoretical Basis of Electrocatalysis http://dx.doi.org/10.5772/intechopen.77109 11

$$\mathbf{G}\_{\rm OH^{\*}} = \mathbf{E}\_{\rm DFT}^{\rm OH^{\*}} + \mathbf{Z}\mathbf{P}\mathbf{E}\_{\rm OH^{\*}} - \mathbf{T}\mathbf{S}\_{\rm OH^{\*}}^{0} \tag{35}$$

$$\mathcal{G}\_{\*} = \mathcal{G}\_{\text{OH}^{\*}} \tag{36}$$

Replacing (Eq. (33)), (Eq. (35)) and (Eq. (36)) in (Eq. (34)) we get:

$$\begin{split} \Delta \mathbf{G}\_{\rm I} &= \left[ \left( E\_{\rm DFT}^{\rm OH^{\*}} + ZPE - TS^{0} \right) - E\_{\rm DFT}^{\*} \right] + \left( \mu\_{\rm e^{\*}}^{0} - \mu\_{\rm OH^{\*}}^{0} - e \boldsymbol{\mathcal{U}} \right) \\ &= \left[ \left( E\_{\rm DFT}^{\rm OH^{\*}} + ZPE - TS^{0} \right) - E\_{\rm DFT}^{\*} \right] + 9.952 \boldsymbol{\mathcal{e}} \boldsymbol{V} \end{split} \tag{37}$$

Step 2: OH<sup>∗</sup> <sup>þ</sup> OH� ! <sup>O</sup><sup>∗</sup> <sup>þ</sup> H2O lð Þþ <sup>e</sup>�

Therefore, the chemical potential of oxygen molecule ( ) can be approximately calculated

<sup>μ</sup><sup>e</sup>� � <sup>μ</sup>OH� � eU ¼ �1=<sup>4</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> <sup>þ</sup> <sup>2</sup>μH2O gð Þ

Finally, we can obtain value as 9.952 eV using the (Eq. (29)), (Eq. (32)) and (Eq. (33)) [20]. The enthalpy change (EDFT), the zero-point energy correction (ΔZPE) calculated by the DFT calculations of vibrational frequencies, and entropy corrections (TS) are listed in

Gas phase H2O at 0.035 bar was used as the reference state because at this pressure gas phase H2O is in equilibrium with liquid water at 300 K. The same values for the adsorbed species for all the models were used, as vibrational frequencies have been found to depend much less on the surface than the bond strength. We took the standard entropies from thermodynamic

2.2.4. The free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of catalyst (\*)

ΔG1 ¼ GOH<sup>∗</sup> þ μ<sup>e</sup>� � μOH� þ G<sup>∗</sup>

<sup>¼</sup> GOH ð Þþ <sup>∗</sup> � <sup>G</sup><sup>∗</sup> <sup>μ</sup><sup>0</sup>

Species E (eV) ZPE (eV) TS (eV) H2O (0.035 bar) �14.22 0.56 0.67 H2 �6.76 0.27 0.41 O\* — 0.09 0.05 OH\* — 0.41 0.07 OOH\* — 0.46 0.16

Table 2. Total energies (E) of H2O and H2 and zero point energy (ZPE) corrections and entropic contributions (TS) to the

<sup>e</sup>� � <sup>μ</sup><sup>0</sup>

OH� � eU (34)

The first step is the adsorption step of active site with a release of an electron:

where respectively and could be expressed by DFT energies:

DFT <sup>þ</sup> ZPEH2O gð Þ � TS0

DFT <sup>þ</sup> ZPEH2ð Þ<sup>g</sup> � TS<sup>0</sup>

H2O gð Þ

(33)

H2ð Þg (32)

<sup>μ</sup>O2ð Þ<sup>g</sup> <sup>¼</sup> <sup>4</sup>:<sup>92</sup> <sup>þ</sup> 2 EH2O gð Þ

10 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

� 2 EH2ð Þ<sup>g</sup>

as follows:

Table 2.

free energies.

Therefore, Eq. (24) can be written as:

tables for gas phase molecules.

Step 1. OH� <sup>þ</sup> <sup>∗</sup> ! OH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>�

The second step is oxidation of the OH\* species to O\* with release of water and an electron:

$$\begin{split} \Delta \mathcal{G}\_{2} &= \mathcal{G}\_{O^{\prime}} + \mu\_{H\_{2}O(l)} + \mu\_{\varepsilon^{-}} - \left( \mathcal{G}\_{OH^{\prime}} + \mu\_{OH^{-}} \right) \\ &= \left( \mathcal{G}\_{O^{\prime}} + \mu\_{H\_{2}O(l)} - \mathcal{G}\_{OH^{\prime}} \right) + \left( \mu\_{\varepsilon^{-}}^{0} - \mu\_{OH^{\prime}}^{0} - eII \right) \end{split} \tag{38}$$

The relation for in terms of DFT energies is similar to the relation for (Eq. (35)). Replacing again the same equations as in the case for the first step in (Eq. (38)) we get:

$$
\Delta G\_2 = \left[ \left( E\_{\rm DFT}^{O^\*} + E\_{\rm DFT}^{H\_2O(g)} \right) - E\_{\rm DFT}^{OH^\*} + \left( \Delta ZPE \text{-} T\Delta S^0 \right) \right] + 9.952 eV \tag{39}
$$

Step 3: <sup>O</sup><sup>∗</sup> <sup>þ</sup> OH� ! OOH<sup>∗</sup> <sup>þ</sup> <sup>e</sup>�

The third step is represented by formation of the OOH\* on top of oxygen with a release of an electron:

$$\begin{split} \Delta \mathbf{G}\_3 &= \mathbf{G}\_{\text{OOH}^\*} + \boldsymbol{\mu}\_{\text{e}^-} - \left( \mathbf{G}\_{\text{O}^\*} + \boldsymbol{\mu}\_{\text{OH}^-} \right) \\ &= \left( \mathbf{G}\_{\text{OOH}^\*} - \mathbf{G}\_{\text{O}^\*} \right) + \left( \boldsymbol{\mu}\_{\text{e}^-}^0 - \boldsymbol{\mu}\_{\text{OH}^-}^0 - e \boldsymbol{I} \boldsymbol{I} \right) \end{split} \tag{40}$$

The relation for GOOH<sup>∗</sup> and GO<sup>∗</sup> in terms of DFT energies is similar to the relation for (Eq. (35)). The same equations are replaced gradually in the (Eq. (40)) as follows:

$$
\Delta G\_3 = \left[ E\_{DFT}^{OOH^\*} - E\_{DFT}^{O^\*} + \left( \Delta ZPE \text{-} T\Delta S^0 \right) \right] + 9.952 eV \tag{41}
$$

Step 4: OOH<sup>∗</sup> <sup>þ</sup> OH� ! <sup>∗</sup> <sup>þ</sup> O2ð Þþ <sup>g</sup> H2O lð Þþ <sup>e</sup>�

The last step is the evolution of oxygen molecule:

$$\begin{split} \Delta \mathbf{G}\_{\mathsf{4}} &= \mathsf{G}\_{\mathsf{\*}} + \mu\_{\mathrm{O}\_{2}(\mathsf{g})} + \mu\_{\mathrm{H}\_{2}\mathrm{O}(\mathsf{l})} + \mu\_{\mathrm{e}^{-}} - \left( \mathsf{G}\_{\mathrm{OOH^{\*}}} + \mu\_{\mathrm{OH^{-}}} \right) \\ &= \left( \mathsf{G}\_{\mathsf{\*}} + \mu\_{\mathrm{O}\_{2}(\mathsf{g})} + \mu\_{\mathrm{H}\_{2}\mathrm{O}(\mathsf{l})} - \mathsf{G}\_{\mathrm{OOH^{\*}}} \right) + \left( \mu\_{\mathrm{e}^{-}}^{0} - \mu\_{\mathrm{OH^{-}}}^{0} - \mathrm{e} \mathrm{U} \right) \end{split} \tag{42}$$

Therefore, we leave from the following equation:

$$\Delta\text{G}\_{4} = \left[E\_{\text{DFT}}^{\ast} + \left(4.92 + 2\left(E\_{\text{DFT}}^{H\_{2}\text{O}(\text{g})} - E\_{\text{DFT}}^{H\_{2}\text{(g)}}\right)\right) + E\_{\text{DFT}}^{H\_{2}\text{O}(\text{g})}\right] - E\_{\text{DFT}}^{\text{OOH}^{\ast}} + \left(\Delta\text{ZPE-T}\Delta S^{0}\right)] + 9.952 eV\tag{43}$$

Finally, the summation of ΔG1, ΔG2, ΔG3, and ΔG4 should be 1.608 eV.

The reaction free energies of O\*, OH\* and OOH\* species on the surface of catalyst are corrected by ZPE and TS. Moreover, the free energy relations of the ORR in alkaline media can also be explained in the same vein as the OER above mentioned [20].

#### 2.2.5. Reaction pathways in acidic media

The generally acceptable OER mechanism is the four-electron associative mechanism in acidic media. The four elementary steps of OER mechanism are described as follows:

$$H\_2O(l) + \* \to OH^\* + H^+ + e^- \tag{44}$$

$$\text{OH}^\* \rightarrow \text{O}^\* + \text{H}^+ + \text{e}^- \tag{45}$$

ΔG2 ¼ GO<sup>∗</sup> þ μ<sup>H</sup><sup>þ</sup> þ μ<sup>e</sup>� � GOH ð Þ<sup>∗</sup> (53)

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<sup>Δ</sup>G3 <sup>¼</sup> GOOH<sup>∗</sup> <sup>þ</sup> <sup>μ</sup><sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � GO<sup>∗</sup> <sup>þ</sup> <sup>μ</sup>H2O lð Þ (54)

<sup>Δ</sup>G3 <sup>¼</sup> <sup>G</sup><sup>∗</sup> <sup>þ</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> <sup>þ</sup> <sup>μ</sup><sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � GOOH ð Þ<sup>∗</sup> (55)

� ! <sup>1</sup>=2H2ð Þ<sup>g</sup> <sup>E</sup><sup>0</sup> <sup>¼</sup> <sup>0</sup>:00 V (56)

<sup>μ</sup><sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>e</sup>� <sup>¼</sup> <sup>1</sup>=2μH2ð Þ<sup>g</sup> (57)

) is 0.00 V at T = 298.15 K. In terms of chemical

<sup>H</sup><sup>þ</sup> � κBTlnaH<sup>þ</sup> (58)

<sup>e</sup>� � eU (59)

H2 � κBTlnpH2 (60)

H2ð Þ<sup>g</sup> (61)

Therefore, the free energy change of each elementary reaction can be calculated using (1) the chemical potentials of proton, electron, liquid water and oxygen molecule ( ), and (2) the free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of catalyst (\*). In the following paragraph, we describe the theoretical method to calculate the free energy change of each elementary reaction of the proposed reaction path based on the relation between the standard electrode potential and the chemical

potential of protons and electrons in acidic environment.

Hþð Þþ aq e

, and e�

In acidic environment, the standard hydrogen electrode is based on the redox half-cell,

In (Eq. (57)), the chemical potentials of proton, electron and hydrogen could be derived further

<sup>μ</sup><sup>H</sup><sup>þ</sup> <sup>¼</sup> <sup>μ</sup><sup>0</sup>

<sup>μ</sup>H2ð Þ<sup>g</sup> <sup>¼</sup> <sup>μ</sup><sup>0</sup>

. Therefore,

μ0 <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>0</sup>

hydrogen on the DFT scale from computational point of view:

<sup>μ</sup><sup>e</sup>� <sup>¼</sup> <sup>μ</sup><sup>0</sup>

where represents the shift in electron energy when a bias is applied and , represent the chemical potentials of electron and proton at standard conditions

<sup>e</sup>- <sup>¼</sup> <sup>1</sup>=2μ<sup>0</sup>

at standard and equilibrium conditions ( and ). In the case of the right side in (Eq. (61)), we can define the standard chemical potential of

2.2.7. The chemical potentials of H<sup>+</sup>

as follows:

and the standard reduction potential (E<sup>0</sup>

potentials, the equation is expressed as follows:

$$O^\* + H\_2O(l) \to OOH^\* + H^+ + e^- \tag{46}$$

$$\text{COOH}^\* \rightarrow \text{\*} + \text{O}\_2(\text{g}) + \text{H}^+ + \text{e}^- \tag{47}$$

where \* represents the active site on the surface, (l) and ð Þg refer to liquid and gas phases, respectively, and O\* , OH\* and OOH\* are adsorbed intermediates. The ORR can proceed completely via a direct four-electron process in which O2(g) is reduced directly to water H2O (g), without involvement of hydrogen peroxide. The ORR mechanism is summarized using the following elementary steps,

$$O\_2(g) + H^+ + e^- \rightarrow OOH^\* \tag{48}$$

$$\text{COOH}^\* + \text{H}^+ + \text{e}^- \rightarrow \text{O}^\* + \text{H}\_2\text{O}(l) \tag{49}$$

$$O^\* + H^+ + e^- \to OH^\* \tag{50}$$

$$\text{OH}^\* + \text{H}^+ + \text{e}^- \rightarrow \text{\*} + \text{H}\_2\text{O}(l) \tag{51}$$

Here, we took the OER reactions ((44)–(47)) to derive the thermochemistry of both OER/ORR, because the ORR reactions (Eqs. ((48)–(51)) are inversed from the OER reactions (Eqs. ((44)–(47)). The catalytic activity of the OER/ORR processes can be determined by examining the reaction free energies of the different elementary steps.

#### 2.2.6. Derivation of the free energy relations

The free energy change of each elementary reaction of OER can be expressed as follows:

$$
\Delta \mathcal{G}\_I = \mathcal{G}\_{\partial H^\*} + \mu\_{H^+} + \mu\_{e^-} - \left(\mu\_{H\_2O(l)} + \mathcal{G}\_\*\right) \tag{52}
$$

$$
\Delta \mathcal{G}\_2 = \mathcal{G}\_{O^\*} + \mu\_{H^+} + \mu\_{e^-} - (\mathcal{G}\_{OH^\*}) \tag{53}
$$

$$
\Delta \mathcal{G}\_3 = \mathcal{G}\_{OOH^\*} + \mu\_{H^+} + \mu\_{\varepsilon^-} - \left(\mathcal{G}\_{O^\*} + \mu\_{H\_2O(l)}\right) \tag{54}
$$

$$
\Delta \mathcal{G}\_3 = \mathcal{G}\_\* + \mu\_{O\_2(\mathfrak{g})} + \mu\_{H^+} + \mu\_{e^-} - (\mathcal{G}\_{\text{OOH''}}) \tag{55}
$$

Therefore, the free energy change of each elementary reaction can be calculated using (1) the chemical potentials of proton, electron, liquid water and oxygen molecule ( ), and (2) the free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of catalyst (\*). In the following paragraph, we describe the theoretical method to calculate the free energy change of each elementary reaction of the proposed reaction path based on the relation between the standard electrode potential and the chemical potential of protons and electrons in acidic environment.

#### 2.2.7. The chemical potentials of H<sup>+</sup> , and e�

<sup>Δ</sup>G4 <sup>¼</sup> <sup>E</sup><sup>∗</sup>

DFT <sup>þ</sup> <sup>4</sup>:<sup>92</sup> <sup>þ</sup> 2 EH2O gð Þ

12 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

2.2.5. Reaction pathways in acidic media

respectively, and O\*

following elementary steps,

free energies of the different elementary steps.

2.2.6. Derivation of the free energy relations

DFT � EH2ð Þ<sup>g</sup> DFT

Finally, the summation of ΔG1, ΔG2, ΔG3, and ΔG4 should be 1.608 eV.

<sup>þ</sup> EH2O gð Þ DFT

The reaction free energies of O\*, OH\* and OOH\* species on the surface of catalyst are corrected by ZPE and TS. Moreover, the free energy relations of the ORR in alkaline media can also be

The generally acceptable OER mechanism is the four-electron associative mechanism in acidic

H2O lðÞþ <sup>∗</sup> ! OH<sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

OH<sup>∗</sup> ! <sup>O</sup><sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

<sup>O</sup><sup>∗</sup> <sup>þ</sup> H2O lðÞ! OOH<sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

OOH<sup>∗</sup> ! <sup>∗</sup> <sup>þ</sup> O2ð Þþ <sup>g</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

where \* represents the active site on the surface, (l) and ð Þg refer to liquid and gas phases,

completely via a direct four-electron process in which O2(g) is reduced directly to water H2O (g), without involvement of hydrogen peroxide. The ORR mechanism is summarized using the

Here, we took the OER reactions ((44)–(47)) to derive the thermochemistry of both OER/ORR, because the ORR reactions (Eqs. ((48)–(51)) are inversed from the OER reactions (Eqs. ((44)–(47)). The catalytic activity of the OER/ORR processes can be determined by examining the reaction

The free energy change of each elementary reaction of OER can be expressed as follows:

<sup>Δ</sup>G1 <sup>¼</sup> GOH<sup>∗</sup> <sup>þ</sup> <sup>μ</sup><sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � <sup>μ</sup>H2O lð Þ <sup>þ</sup> <sup>G</sup><sup>∗</sup>

� �

O2ð Þþ g H<sup>þ</sup> þ e

<sup>O</sup><sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

OOH<sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

OH<sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>

, OH\* and OOH\* are adsorbed intermediates. The ORR can proceed

media. The four elementary steps of OER mechanism are described as follows:

� EOOH<sup>∗</sup>

DFT <sup>þ</sup> <sup>Δ</sup>ZPE-TΔS<sup>0</sup> � �� þ <sup>9</sup>:952eV

� (44)

� (46)

� (47)

� (45)

� ! OOH<sup>∗</sup> (48)

� ! <sup>O</sup><sup>∗</sup> <sup>þ</sup> H2O lð Þ (49)

� ! ∗ þ H2O lð Þ (51)

� ! OH<sup>∗</sup> (50)

(43)

(52)

� � � �

explained in the same vein as the OER above mentioned [20].

h i

In acidic environment, the standard hydrogen electrode is based on the redox half-cell,

$$\text{H}^+(aq) + e^- \rightarrow \text{1/2}H\_2(\text{g}) \qquad E^0 = 0.00 \text{ V} \tag{56}$$

and the standard reduction potential (E<sup>0</sup> ) is 0.00 V at T = 298.15 K. In terms of chemical potentials, the equation is expressed as follows:

$$
\mu\_{H^{+}} + \mu\_{e^{-}} = 1/2\mu\_{H\_{2}(\mathfrak{g})} \tag{57}
$$

In (Eq. (57)), the chemical potentials of proton, electron and hydrogen could be derived further as follows:

$$
\mu\_{H^{+}} = \mu\_{H^{+}}^{0} - \kappa\_{B} T \text{Im} \mathfrak{a}\_{H^{+}} \tag{58}
$$

$$
\mu\_{e^{-}} = \mu\_{e^{-}}^{0} - e\mathcal{U} \tag{59}
$$

$$
\mu\_{H\_2(\mathcal{g})} = \mu\_{H\_2}^0 - \kappa\_\mathcal{B} \operatorname{Thup}\_{H\_2} \tag{60}
$$

where represents the shift in electron energy when a bias is applied and , represent the chemical potentials of electron and proton at standard conditions . Therefore,

$$
\mu\_{H^{+}}^{0} + \mu\_{\mathfrak{e}^{\bullet}}^{0} = 1/2\mu\_{H\_{2}(\mathfrak{g})}^{0} \tag{61}
$$

at standard and equilibrium conditions ( and ). In the case of the right side in (Eq. (61)), we can define the standard chemical potential of hydrogen on the DFT scale from computational point of view:

$$
\mu\_{H\_2}^0 = E\_{\rm DFT}^{H\_2(\rm g)} + ZPE\_{H\_2(\rm g)} - TS\_{H\_2(\rm g)}^0 \tag{62}
$$

The third step is represented by formation of the O\* on top of oxygen with a release of a proton

The relation for and GO<sup>∗</sup> in terms of DFT energies is similar to the relation for (Eq. (64)).

DFT � � <sup>þ</sup> <sup>Δ</sup>ZPE-TΔS0 � � h i <sup>þ</sup> <sup>1</sup>=2EH2ð Þ<sup>g</sup>

<sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>0</sup>

, O\* and OOH\* species on the surface of catalyst considered by

DFT � <sup>E</sup>H2ð Þ<sup>g</sup> DFT � � � � <sup>þ</sup> <sup>E</sup>OOH<sup>∗</sup> DFT h i

<sup>e</sup>� � eU � � (69)

DFT � eU � � (70)

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<sup>e</sup>� � eU � � (71)

DFT � eU � � (72)

� � (73)

� � (75)

� � (74)

ΔG3 ¼ GOOH<sup>∗</sup> þ μ<sup>H</sup><sup>þ</sup> þ μ<sup>e</sup>� � GO<sup>∗</sup> <sup>¼</sup> GOOH<sup>∗</sup> � GO <sup>ð</sup> <sup>∗</sup> Þ þ <sup>μ</sup><sup>0</sup>

<sup>Δ</sup>G4 <sup>¼</sup> <sup>G</sup><sup>∗</sup> <sup>þ</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> <sup>þ</sup> <sup>μ</sup><sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � GOOH<sup>∗</sup>

DFT <sup>þ</sup> <sup>4</sup>:<sup>92</sup> <sup>þ</sup> 2 EH2O gð Þ

<sup>þ</sup> <sup>Δ</sup>ZPE � <sup>T</sup>ΔS<sup>0</sup> � � <sup>þ</sup> <sup>1</sup>=2EH2ð Þ<sup>g</sup>

ZPE and TS. Moreover, the free energy relations of the ORR in acidic media can also be

We can deduce an important parameter of electrocatalytic activity from the calculated ΔG, the magnitude of the potential-determining step (GHER and GOER/ORR) in consecutive reaction steps. This is the specific reaction step with the largest ΔG in the HER and OER/ORR elementary reaction steps, that is, the concluding step to achieve a downhill reaction in the free energy diagram (FED) with increasing potential N, as shown in Figure 2 for HER and Figure 3 for

<sup>G</sup>HER <sup>¼</sup> max <sup>Δ</sup>G0

GORR <sup>¼</sup> max <sup>Δ</sup>G<sup>0</sup>

<sup>G</sup>OER=ORR <sup>¼</sup> max <sup>Δ</sup>G<sup>0</sup>

<sup>1</sup>; ΔG<sup>0</sup> 2

<sup>1</sup>; ΔG<sup>0</sup> 2

<sup>2</sup>; ΔG<sup>0</sup>

<sup>3</sup>; ΔG<sup>0</sup> 4

<sup>1</sup>; ΔG<sup>0</sup>

� � <sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>¼</sup> <sup>G</sup><sup>∗</sup> <sup>þ</sup> <sup>μ</sup>O2ð Þ<sup>g</sup> � GOOH<sup>∗</sup>

Finally, the summation of ΔG1, ΔG2, ΔG3, and ΔG4 should be 4.92 eV.

The same equations are replaced gradually in the (Eq. (69)) as follows:

DFT � <sup>E</sup><sup>O</sup><sup>∗</sup>

<sup>Δ</sup>G3 <sup>¼</sup> <sup>E</sup>OOH<sup>∗</sup>

The last step is the evolution of oxygen molecule:

Therefore, we leave from the following equation:

<sup>Δ</sup>G4 <sup>¼</sup> <sup>E</sup><sup>∗</sup>

explained in the same vein as the OER above mentioned.

2.3. Free energy diagram (FED) and Overpotential(η)

The reaction free energies of OH\*

and an electron:

Step 4:

OER/ORR:

Another approximation is that for liquid water and oxygen molecule, and can also be explained in the same vein as the OER in the alkaline media above mentioned.

2.2.8. The free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of catalyst (\*)

Step 1:

The first step is the adsorption step of OH\* on active site with a release of a proton and an electron:

$$\begin{split} \Delta \mathbf{G}\_{l} &= \mathbf{G}\_{\text{OH}^{\circ}} + \boldsymbol{\mu}\_{H^{+}} + \boldsymbol{\mu}\_{\text{e}^{-}} - \left( \boldsymbol{\mu}\_{H\_{2}\text{O}(l)} + \mathbf{G}\_{\ast} \right) \\ &= \left( \mathbf{G}\_{\text{OH}^{\circ}} - \boldsymbol{\mu}\_{H\_{2}\text{O}(l)} + \mathbf{G}\_{\ast} \right) + \left( \boldsymbol{\mu}\_{H^{+}}^{0} + \boldsymbol{\mu}\_{\text{e}^{-}}^{0} - \boldsymbol{e} \boldsymbol{I} \right) \end{split} \tag{63}$$

where GOH<sup>∗</sup> and G<sup>∗</sup> are expressed by DFT energies:

$$\mathbf{G}\_{\rm OH^{\ast}} = \mathbf{E}\_{\rm DFT}^{\rm OH^{\ast}} + \mathbf{ZPE}\_{\rm OH^{\ast}} - \mathbf{TS}\_{\rm OH}^{0} \tag{64}$$

$$G\_\* = E\_{DFT}^\* \tag{65}$$

Replacing (Eq. (29)), (Eq. (62)), (Eq. (64)) and (Eq. (65)) in (Eq. (63)) we get:

$$\begin{aligned} \Delta \mathcal{G}\_{I} &= \left[ \left( E\_{DFT}^{OH^{\*}} - E\_{DFT}^{H\_{2}O(g)} - E\_{DFT}^{\*} \right) + \left( \Delta ZPE - T \Delta S^{0} \right) \right] \\ &+ \left( \mu\_{H^{+}}^{0} + \mu\_{e^{-}}^{0} - eII \right) \\ &= \left[ \left( E\_{DFT}^{OH^{\*}} - E\_{DFT}^{H\_{2}O(g)} - E\_{DFT}^{\*} \right) + \left( \Delta ZPE - T \Delta S^{0} \right) \right] \\ &+ \left( 1/2 E\_{DFT}^{H\_{2}(g)} - eII \right) \end{aligned} \tag{66}$$

Step 2:

The second step is oxidation of the OH\* species to O\* with release of a proton and an electron:

$$\begin{split} \Delta \mathbf{G}\_2 &= \mathbf{G}\_{O^\*} + \boldsymbol{\mu}\_{H^+} + \boldsymbol{\mu}\_{\boldsymbol{e^-}} - \mathbf{G}\_{OH^\*} \\ &= (\mathbf{G}\_{O^\*} - \mathbf{G}\_{OH^\*}) + \left( \boldsymbol{\mu}\_{H^+}^0 + \boldsymbol{\mu}\_{\boldsymbol{e^-}}^0 - \boldsymbol{e} \boldsymbol{U} \right) \end{split} \tag{67}$$

The relation for in terms of DFT energies is similar to the relation for (Eq. (64)). Replacing again the same equations as in the case for the first step in (Eq. (66)), we get:

$$
\Delta \mathbf{G}\_2 = \left[ \left( E\_{\mathrm{DFT}}^{O^\*} - E\_{\mathrm{DFT}}^{\mathrm{OH^\*}} \right) + \left( \Delta \mathbf{ZPE} - T \Delta \mathbf{S}^0 \right) \right] + \left( \mathbf{1} / 2 E\_{\mathrm{DFT}}^{H\_2(\xi)} - \varepsilon \mathrm{II} \right) \tag{68}
$$

Step 3: <sup>O</sup><sup>∗</sup> ! OOH<sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>�

The third step is represented by formation of the O\* on top of oxygen with a release of a proton and an electron:

$$\begin{split} \Delta \mathbf{G}\_3 &= \mathbf{G}\_{\mathrm{OOH}^\*} + \boldsymbol{\mu}\_{\mathrm{H}^+} + \boldsymbol{\mu}\_{\mathrm{e}^-} - \mathbf{G}\_{\mathrm{O}^\*} \\ &= (\mathbf{G}\_{\mathrm{OOH}^\*} - \mathbf{G}\_{\mathrm{O}^\*}) + \left(\boldsymbol{\mu}\_{\mathrm{H}^+}^0 + \boldsymbol{\mu}\_{\mathrm{e}^-}^0 - \mathrm{e} \mathrm{I} \right) \end{split} \tag{69}$$

The relation for and GO<sup>∗</sup> in terms of DFT energies is similar to the relation for (Eq. (64)). The same equations are replaced gradually in the (Eq. (69)) as follows:

$$
\Delta \mathbf{G}\_3 = \left[ \left( E\_{\mathrm{DFT}}^{\mathrm{OOH}^\*} - E\_{\mathrm{DFT}}^{\mathrm{O}^\*} \right) + \left( \Delta \mathbf{ZPE-T\Delta S}^0 \right) \right] + \left( 1/2 E\_{\mathrm{DFT}}^{H\_2(\mathrm{g})} - e \mathrm{I} \mathrm{I} \right) \tag{70}
$$

Step 4:

μ0

14 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

Step 1:

electron:

Step 2:

H2 <sup>¼</sup> <sup>E</sup>H2ð Þ<sup>g</sup>

DFT <sup>þ</sup> ZPEH2ð Þ<sup>g</sup> � TS0

Another approximation is that for liquid water and oxygen molecule, and can

The first step is the adsorption step of OH\* on active site with a release of a proton and an

� �

<sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>e</sup>� � eU

� � (63)

OH (64)

(66)

(68)

DFT (65)

<sup>þ</sup> <sup>μ</sup><sup>0</sup>

DFT <sup>þ</sup> ZPEOH<sup>∗</sup> � TS<sup>0</sup>

DFT

DFT

<sup>þ</sup> <sup>Δ</sup>ZPE � <sup>T</sup>ΔS0 � � h i

<sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>e</sup>� � eU

<sup>þ</sup> <sup>1</sup>=2EH2ð Þ<sup>g</sup>

� � (67)

DFT � eU � �

<sup>þ</sup> <sup>Δ</sup>ZPE � <sup>T</sup>ΔS0 � � h i

<sup>G</sup><sup>∗</sup> <sup>¼</sup> <sup>E</sup><sup>∗</sup>

DFT � <sup>E</sup><sup>∗</sup>

DFT � <sup>E</sup><sup>∗</sup>

The second step is oxidation of the OH\* species to O\* with release of a proton and an electron:

The relation for in terms of DFT energies is similar to the relation for (Eq. (64)). Replacing

also be explained in the same vein as the OER in the alkaline media above mentioned.

<sup>Δ</sup>G1 <sup>¼</sup> GOH<sup>∗</sup> <sup>þ</sup> <sup>μ</sup><sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>e</sup>� � <sup>μ</sup>H2O lð Þ <sup>þ</sup> <sup>G</sup><sup>∗</sup>

<sup>¼</sup> GOH<sup>∗</sup> � <sup>μ</sup>H2O lð Þ <sup>þ</sup> <sup>G</sup><sup>∗</sup> � �

GOH<sup>∗</sup> <sup>¼</sup> <sup>E</sup>OH<sup>∗</sup>

Replacing (Eq. (29)), (Eq. (62)), (Eq. (64)) and (Eq. (65)) in (Eq. (63)) we get:

DFT � EH2O gð Þ

DFT � EH2O gð Þ

<sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>þ</sup> <sup>1</sup>=2EH2ð Þ<sup>g</sup>

� �

� �

DFT � eU � �

ΔG2 ¼ GO<sup>∗</sup> þ μ<sup>H</sup><sup>þ</sup> þ μ<sup>e</sup>� � GOH<sup>∗</sup> <sup>¼</sup> GO<sup>∗</sup> � GOH ð Þþ <sup>∗</sup> <sup>μ</sup><sup>0</sup>

again the same equations as in the case for the first step in (Eq. (66)), we get:

<sup>þ</sup> <sup>Δ</sup>ZPE � <sup>T</sup>ΔS0 � � h i

DFT � <sup>E</sup>OH<sup>∗</sup> DFT � �

<sup>e</sup>� � eU � �

where GOH<sup>∗</sup> and G<sup>∗</sup> are expressed by DFT energies:

<sup>Δ</sup>G1 <sup>¼</sup> <sup>E</sup>OH<sup>∗</sup>

<sup>Δ</sup>G2 <sup>¼</sup> EO<sup>∗</sup>

Step 3: <sup>O</sup><sup>∗</sup> ! OOH<sup>∗</sup> <sup>þ</sup> <sup>H</sup><sup>þ</sup> <sup>þ</sup> <sup>e</sup>�

<sup>þ</sup> <sup>μ</sup><sup>0</sup>

<sup>¼</sup> <sup>E</sup>OH<sup>∗</sup>

2.2.8. The free energies of each intermediate (GOH<sup>∗</sup> , GO<sup>∗</sup> , and GOOH<sup>∗</sup> ) on the surface of catalyst (\*)

H2ð Þ<sup>g</sup> (62)

The last step is the evolution of oxygen molecule:

$$\begin{split} \Delta \mathbf{G}\_{4} &= \mathbf{G}\_{\*} + \mu\_{O\_{2}(\mathfrak{g})} + \mu\_{H^{+}} + \mu\_{\varepsilon^{-}} - \mathbf{G}\_{\mathrm{OOH^{\*}}} \\ &= \left( \mathbf{G}\_{\*} + \mu\_{O\_{2}(\mathfrak{g})} - \mathbf{G}\_{\mathrm{OOH^{\*}}} \right) + \left( \mu\_{H^{+}}^{0} + \mu\_{\varepsilon^{-}}^{0} - \varepsilon \mathrm{U} \right) \end{split} \tag{71}$$

Therefore, we leave from the following equation:

$$\begin{split} \Delta \mathbf{G}\_{4} &= \left[ E\_{DFT}^{\*} + \left( 4.92 + 2 \left( E\_{DFT}^{H\_{2}O(\emptyset)} - E\_{DFT}^{H\_{2}(\emptyset)} \right) \right) + E\_{DFT}^{OOH^{\*}} \right] \\ &+ \left( \Delta ZPE - T\Delta S^{0} \right) + \left( 1/2E\_{DFT}^{H\_{2}(\emptyset)} - eI \right) \end{split} \tag{72}$$

Finally, the summation of ΔG1, ΔG2, ΔG3, and ΔG4 should be 4.92 eV.

The reaction free energies of OH\* , O\* and OOH\* species on the surface of catalyst considered by ZPE and TS. Moreover, the free energy relations of the ORR in acidic media can also be explained in the same vein as the OER above mentioned.

#### 2.3. Free energy diagram (FED) and Overpotential(η)

We can deduce an important parameter of electrocatalytic activity from the calculated ΔG, the magnitude of the potential-determining step (GHER and GOER/ORR) in consecutive reaction steps. This is the specific reaction step with the largest ΔG in the HER and OER/ORR elementary reaction steps, that is, the concluding step to achieve a downhill reaction in the free energy diagram (FED) with increasing potential N, as shown in Figure 2 for HER and Figure 3 for OER/ORR:

$$G^{HER} = \max\left[\Delta G\_1^0, \Delta G\_2^0\right] \tag{73}$$

$$G^{OER/ORR} = \max\left[\Delta\mathbf{G}\_1^0, \Delta\mathbf{G}\_2^0, \Delta\mathbf{G}\_3^0, \Delta\mathbf{G}\_4^0\right] \tag{74}$$

$$\mathbf{G}^{\rm ORR} = \max \left[ \Delta \mathbf{G}\_1^0, \Delta \mathbf{G}\_2^0 \right] \tag{75}$$

The theoretical overpotential at standard conditions is then given by (Eq. (76)) in acidic and

Figure 3. (a) Primitive cells of single heteroatom doped-graphene (GX, where G means graphene and X is B, N, Si, P, and S dopant) and (b) HER activities ΔGVolmer of GX as a function of primitive cell size. Red mark indicates dopant. There are

� x V,

x ¼ 1:230 V acidic ð Þ, and 0:402 V alkaline ð Þ

In the case of HER pathways, the Volmer-Heyrovsky reaction is an electrochemical reaction involving electrons at all steps, as shown in Figure 2(a). Therefore, when the external potential (U ¼ 0:50V) is applied, the chemical potential of steps involving electrons shifts by the external potential (U ¼ 0:50V), and the Volmer-Heyrovsky reaction changes from uphill to downhill reaction. In contrast to the Volmer-Heyrovsky reaction, the Volmer-Tafel reaction is an electrochemical reaction involving electrons only in the Volmer step, as shown in Figure 2(b). Therefore, when the adsorption of hydrogen is unstable, the downhill reaction is possible by applying the external potential (U ¼ 0:25V), but when the adsorption of hydrogen is stable, the reduction of H+ is spontaneous reaction, and the desorption of H2 in the Tafel step is the rate determining step. However, the desorption of H2 is a thermodynamic reaction, which is not involved in electrons, not an electrochemical reaction. In the case of OER/ORR pathways, the theoretical overpotential (ηΟΕΡ/ΟΡΡ) represents the relative stability of the intermediates between (4OH� + \*)alkaline, (2H2O(l) + \*)acidic and (O2(g) + \*), and vice versa, as shown in Figure 2 (c)-(e). In addition, it can be calculated by applying standard density functional theory (DFT)

<sup>η</sup>OER=ORR <sup>¼</sup> <sup>G</sup>OER=ORR=<sup>e</sup>

two different types of structures depending on dopant, (c) in-plane and (d) out-of-plane.

<sup>η</sup>HER <sup>¼</sup> <sup>G</sup>HER=<sup>e</sup> � x V, x <sup>¼</sup> <sup>0</sup>:000 V (76)

(77)

Theoretical Basis of Electrocatalysis

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alkaline conditions, respectively:

Figure 2. Free energy diagrams (FEDs) of (a) Volmer-Heyrovsky pathway and (b) Volmer-Tafel pathway for HER, and (c) four-step associative OER pathway, and (d) four-step ORR pathway in alkaline/acidic media, and (e) two-step ORR pathway in alkaline media at zero potential ð Þ U ¼ 0 V , at equilibrium reduction potential ð Þ U ¼ x V and at overpotential . Thexvalues indicate 0.402 V (alkaline) or 1.230 V (acidic).

Theoretical Basis of Electrocatalysis http://dx.doi.org/10.5772/intechopen.77109 17

Figure 3. (a) Primitive cells of single heteroatom doped-graphene (GX, where G means graphene and X is B, N, Si, P, and S dopant) and (b) HER activities ΔGVolmer of GX as a function of primitive cell size. Red mark indicates dopant. There are two different types of structures depending on dopant, (c) in-plane and (d) out-of-plane.

The theoretical overpotential at standard conditions is then given by (Eq. (76)) in acidic and alkaline conditions, respectively:

$$
\boldsymbol{\eta}^{HER} = \left(\mathbf{G}^{HER}/\mathbf{e}\right) - \mathbf{x}\,\,\mathbf{V}, \quad \mathbf{x} = 0.000\,\,\mathbf{V} \tag{76}
$$

$$\begin{aligned} \eta^{\text{OER/ORR}} &= \left( G^{\text{OER/ORR}}/\mathfrak{e} \right) - \text{x } V, \\ \mathbf{x} &= 1.230 \text{ V (acidic)}, \text{ and } 0.402 \text{ V (alkaline)} \end{aligned} \tag{77}$$

In the case of HER pathways, the Volmer-Heyrovsky reaction is an electrochemical reaction involving electrons at all steps, as shown in Figure 2(a). Therefore, when the external potential (U ¼ 0:50V) is applied, the chemical potential of steps involving electrons shifts by the external potential (U ¼ 0:50V), and the Volmer-Heyrovsky reaction changes from uphill to downhill reaction. In contrast to the Volmer-Heyrovsky reaction, the Volmer-Tafel reaction is an electrochemical reaction involving electrons only in the Volmer step, as shown in Figure 2(b). Therefore, when the adsorption of hydrogen is unstable, the downhill reaction is possible by applying the external potential (U ¼ 0:25V), but when the adsorption of hydrogen is stable, the reduction of H+ is spontaneous reaction, and the desorption of H2 in the Tafel step is the rate determining step. However, the desorption of H2 is a thermodynamic reaction, which is not involved in electrons, not an electrochemical reaction. In the case of OER/ORR pathways, the theoretical overpotential (ηΟΕΡ/ΟΡΡ) represents the relative stability of the intermediates between (4OH� + \*)alkaline, (2H2O(l) + \*)acidic and (O2(g) + \*), and vice versa, as shown in Figure 2 (c)-(e). In addition, it can be calculated by applying standard density functional theory (DFT)

Figure 2. Free energy diagrams (FEDs) of (a) Volmer-Heyrovsky pathway and (b) Volmer-Tafel pathway for HER, and (c) four-step associative OER pathway, and (d) four-step ORR pathway in alkaline/acidic media, and (e) two-step ORR pathway in alkaline media at zero potential ð Þ U ¼ 0 V , at equilibrium reduction potential ð Þ U ¼ x V and at overpotential

. Thexvalues indicate 0.402 V (alkaline) or 1.230 V (acidic).

16 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

in combination with the computational standard hydrogen electrode (SHE) model. Because the equilibrium reduction potential is U ¼ 1:230 V acidic ð Þ and 0:402 V alkaline ð Þ, the chemical potential difference between (4OH�) alkaline, (2H2O(l))acidic and O2(g) should be 1.608 and 4.920 V, respectively. However, the actual catalytic behaviors deviate from the ideal case due to correlation with binding energies of the intermediates. Therefore, most catalysts require overpotential (ηHER and ηOER/ORR) in order to achieve an overall downhill reaction. Consequently, the ηHER and ηOER/ORR the important indicators of catalytic activities of a catalyst, and the lower ηHER and ηOER/ORR indicate a thermodynamically superior catalyst.

systematically changed the structures of GXs from in-plane to out-of-plane structures and vice versa. These calculations clearly show that dopant can produce n- and p-type doping states as well as a localized state depending on the structure of GXs induced by the type of dopants. When the out-of-plane (in-plane) deformation is applied in the in-plane (out-of-plane) GXs, band structures change from the p-type doping state (localized state) to the localized state (ptype doping state). Therefore, it is worth mentioning that the localized electronic states can be

In this work, we have systematically investigated a metal-free bifunctional electrocatalyst of heteroatom-doped carbon nitride (XY-C3N4, where X and Y indicate the dopant and doping site on C3N4, respectively) for oxygen evolution and oxygen reduction reactions (OER and ORR) in alkaline media, considering the possible reaction pathways based on the Eley-Rideal (ER) mechanism as well as the doping effects on electrocatalytic activity. Moreover, we determined that the relative stability of O\* and OOH\* intermediates was a key factor determining the ORR pathway; accordingly, ORR follows a two-step reaction pathway governed by O\* rather

most stable structures of heteroatom-doped C3N4 structures, XY-C3N4, where X = P, S, or PS and Y = C or N [18, 20]. By considering the Ef (formation energy) results as well as the various doping sites, we determined the most stable XY-C3N4 structures, PCA-C3N4, PNB-C3N4, SCB-C3N4, SNB-C3N4, PCASCB-C3N4, PNASNB-C3N4, and PCASNB-C3N4, for the investigation of OER/ ORR bifunctional electrocatalytic activities and their reaction mechanisms in alkaline media.

Figure 4 (a) and (c) show the volcano plot of OER and ORR in alkaline media at all possible active sites on XY-C3N4, which represents the apparent catalytic activity, respectively. This theoretical analysis reveals that the PCSC-C3N4 structure has minimum ηOER (0.42 V) and ηORR (0.27 V). The ηOER/ORR value of PCSC-C3N4 is comparable to those of the best conventional catalysts (0.42 V for OER on RuO2 and 0.45 V for ORR on Pt). Most elementary steps in OER have an uphill reaction at 0.00 V and at equilibrium potential of 0.402 V. Analyzing the FED at the equilibrium potential of 0.402 V, we can define the rate determination step as the formation of OOH\* from O\* on PCSC-C3N4, as shown in Figure 4 (b). Therefore, the ΔGat the rate determination step, 0.42 V, become the ηOER for facilitating the OER as a spontaneously downhill reaction. By applying the increased electrode potential of 0.82 V, all elementary reactions can be downhill reactions for spontaneous OER. In the case of the ORR, we considered the O2 adsorption free energy (ΔGO2\*) and ηORR are highly correlated to determine ORR activity. If the O2 adsorption reaction is very difficult to achieve as an initiation step in ORR, that material cannot have ORR activity, even though it can have a possibility to have a very small ηORR. Therefore, we defined a new indicator of ORR activity as figure of merit (FOM), which is defined by FOM = (ηORR <sup>+</sup> <sup>Δ</sup>GO2\*). Using FOM, we constructed a volcano plot of ORR at all possible active sites on XY-C3N4 in order to compare the ORR catalytic activity, as

shown in Figure 4 (c), where the PCSC-C3N4 structure have minimum ηORR of 0.27 V.

. Firstly, we calculated relative Ef and the

Theoretical Basis of Electrocatalysis

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associated with physical regularity of HER activities on the out-of-plane GXs.

3.2. Heteroatom doped C3N4 (XY-C3N4) for OER/ORR bi-functional catalyst

than a four-step reaction pathway governed by OOH\*

3.2.1. OER and ORR catalytic activity on XY-C3N4

### 3. Structure-catalytic activity relationships

#### 3.1. Heteroatom doped graphene (GX) for HER catalyst

Figure 3 shows a schematic of the heteroatom doped-graphene (GX, where G means graphene and X is B, N, Si, P, and S dopant) structures. We have used the second row elements (B and N) and the third row elements (Si, P, and S) in the periodic table in order to investigate the structural and electronic doping effects on HER activity because the third row elements are relatively larger than the second row elements and p- and n-type doping effects can be expected from the electron deficient B, and electron rich N, P, and S elements give in-plane and out-of-plane structures, respectively, due to the size of the dopants.

### 3.1.1. Structural and electronic doping effects on HER activity

On the atomic orbital hybridization characters of adjacent carbon atoms of dopant, the natural bond orbital (NBO) analysis show an increased p orbital contribution from sp<sup>2</sup> to sp<sup>3</sup> hybridization of carbons adjacent to the dopant due to structural deformation from in-plane to out-ofplane. Compared to sp<sup>2</sup> -hybridized carbon, sp3 -hybridized carbons more readily form an extra a hydrogen atom without additional structural change. Therefore, in out-of-plane structures, the subsequent two hydrogen atoms prefer to bind to only sp<sup>3</sup> hybridized carbons adjacent to the dopant. However, in the case of in-plane structures having only sp<sup>2</sup> -hybridized carbons, the first hydrogen atom should result in structural deformation to form sp<sup>3</sup> -hybridized carbons. Therefore, the first hydrogen adsorption on the in-plane structure is less favorable than the reaction on out-of-plane structures. The second hydrogen atom can favorably bind to sp<sup>3</sup> hybridized second neighboring carbons of dopant. Consequently, structural deformation with dopants that are third row elements is associated with improved HER activity due to atomic orbital hybridization, as shown in Figure 3(b). Looking at the electronic structures of GXs, pand n-type doping effects are also expected from electron deficient and rich elements. In the case of in-plane GXs, electron deficient boron shifts its band structure up by withdrawing an electron from graphene, and electron rich nitrogen shifts its band structure down by donating an electron to graphene. Interestingly, in the case of out-of-plane GXs, electron rich phosphorous and sulfur dopant have no associated band shift. The origin of the flat band can be understood based on the localization of an extra electron onto the dopant site. In order to verify the relationship between geometric and electronic structures of GXs, we have systematically changed the structures of GXs from in-plane to out-of-plane structures and vice versa. These calculations clearly show that dopant can produce n- and p-type doping states as well as a localized state depending on the structure of GXs induced by the type of dopants. When the out-of-plane (in-plane) deformation is applied in the in-plane (out-of-plane) GXs, band structures change from the p-type doping state (localized state) to the localized state (ptype doping state). Therefore, it is worth mentioning that the localized electronic states can be associated with physical regularity of HER activities on the out-of-plane GXs.

### 3.2. Heteroatom doped C3N4 (XY-C3N4) for OER/ORR bi-functional catalyst

In this work, we have systematically investigated a metal-free bifunctional electrocatalyst of heteroatom-doped carbon nitride (XY-C3N4, where X and Y indicate the dopant and doping site on C3N4, respectively) for oxygen evolution and oxygen reduction reactions (OER and ORR) in alkaline media, considering the possible reaction pathways based on the Eley-Rideal (ER) mechanism as well as the doping effects on electrocatalytic activity. Moreover, we determined that the relative stability of O\* and OOH\* intermediates was a key factor determining the ORR pathway; accordingly, ORR follows a two-step reaction pathway governed by O\* rather than a four-step reaction pathway governed by OOH\* . Firstly, we calculated relative Ef and the most stable structures of heteroatom-doped C3N4 structures, XY-C3N4, where X = P, S, or PS and Y = C or N [18, 20]. By considering the Ef (formation energy) results as well as the various doping sites, we determined the most stable XY-C3N4 structures, PCA-C3N4, PNB-C3N4, SCB-C3N4, SNB-C3N4, PCASCB-C3N4, PNASNB-C3N4, and PCASNB-C3N4, for the investigation of OER/ ORR bifunctional electrocatalytic activities and their reaction mechanisms in alkaline media.

### 3.2.1. OER and ORR catalytic activity on XY-C3N4

in combination with the computational standard hydrogen electrode (SHE) model. Because the equilibrium reduction potential is U ¼ 1:230 V acidic ð Þ and 0:402 V alkaline ð Þ, the chemical

4.920 V, respectively. However, the actual catalytic behaviors deviate from the ideal case due to correlation with binding energies of the intermediates. Therefore, most catalysts require overpotential (ηHER and ηOER/ORR) in order to achieve an overall downhill reaction. Consequently, the ηHER and ηOER/ORR the important indicators of catalytic activities of a catalyst, and

Figure 3 shows a schematic of the heteroatom doped-graphene (GX, where G means graphene and X is B, N, Si, P, and S dopant) structures. We have used the second row elements (B and N) and the third row elements (Si, P, and S) in the periodic table in order to investigate the structural and electronic doping effects on HER activity because the third row elements are relatively larger than the second row elements and p- and n-type doping effects can be expected from the electron deficient B, and electron rich N, P, and S elements give in-plane

On the atomic orbital hybridization characters of adjacent carbon atoms of dopant, the natural bond orbital (NBO) analysis show an increased p orbital contribution from sp<sup>2</sup> to sp<sup>3</sup> hybridization of carbons adjacent to the dopant due to structural deformation from in-plane to out-of-

a hydrogen atom without additional structural change. Therefore, in out-of-plane structures, the subsequent two hydrogen atoms prefer to bind to only sp<sup>3</sup> hybridized carbons adjacent to

bons. Therefore, the first hydrogen adsorption on the in-plane structure is less favorable than the reaction on out-of-plane structures. The second hydrogen atom can favorably bind to sp<sup>3</sup>

hybridized second neighboring carbons of dopant. Consequently, structural deformation with dopants that are third row elements is associated with improved HER activity due to atomic orbital hybridization, as shown in Figure 3(b). Looking at the electronic structures of GXs, pand n-type doping effects are also expected from electron deficient and rich elements. In the case of in-plane GXs, electron deficient boron shifts its band structure up by withdrawing an electron from graphene, and electron rich nitrogen shifts its band structure down by donating an electron to graphene. Interestingly, in the case of out-of-plane GXs, electron rich phosphorous and sulfur dopant have no associated band shift. The origin of the flat band can be understood based on the localization of an extra electron onto the dopant site. In order to verify the relationship between geometric and electronic structures of GXs, we have

the lower ηHER and ηOER/ORR indicate a thermodynamically superior catalyst.

and out-of-plane structures, respectively, due to the size of the dopants.


the dopant. However, in the case of in-plane structures having only sp<sup>2</sup>

the first hydrogen atom should result in structural deformation to form sp<sup>3</sup>

alkaline, (2H2O(l))acidic and O2(g) should be 1.608 and





potential difference between (4OH�)

3. Structure-catalytic activity relationships

18 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

3.1. Heteroatom doped graphene (GX) for HER catalyst

3.1.1. Structural and electronic doping effects on HER activity

plane. Compared to sp<sup>2</sup>

Figure 4 (a) and (c) show the volcano plot of OER and ORR in alkaline media at all possible active sites on XY-C3N4, which represents the apparent catalytic activity, respectively. This theoretical analysis reveals that the PCSC-C3N4 structure has minimum ηOER (0.42 V) and ηORR (0.27 V). The ηOER/ORR value of PCSC-C3N4 is comparable to those of the best conventional catalysts (0.42 V for OER on RuO2 and 0.45 V for ORR on Pt). Most elementary steps in OER have an uphill reaction at 0.00 V and at equilibrium potential of 0.402 V. Analyzing the FED at the equilibrium potential of 0.402 V, we can define the rate determination step as the formation of OOH\* from O\* on PCSC-C3N4, as shown in Figure 4 (b). Therefore, the ΔGat the rate determination step, 0.42 V, become the ηOER for facilitating the OER as a spontaneously downhill reaction. By applying the increased electrode potential of 0.82 V, all elementary reactions can be downhill reactions for spontaneous OER. In the case of the ORR, we considered the O2 adsorption free energy (ΔGO2\*) and ηORR are highly correlated to determine ORR activity. If the O2 adsorption reaction is very difficult to achieve as an initiation step in ORR, that material cannot have ORR activity, even though it can have a possibility to have a very small ηORR. Therefore, we defined a new indicator of ORR activity as figure of merit (FOM), which is defined by FOM = (ηORR <sup>+</sup> <sup>Δ</sup>GO2\*). Using FOM, we constructed a volcano plot of ORR at all possible active sites on XY-C3N4 in order to compare the ORR catalytic activity, as shown in Figure 4 (c), where the PCSC-C3N4 structure have minimum ηORR of 0.27 V.

electron-rich heteroatom doping will induce electronic structure changes from a non-metallic doping effect to a metallic doping effect due to a downward band shift. As a result, PCSC-C3N4 shows the best OER/ORR activity by maintaining a metallic property despite the presence of out-of-plane deformation. Consequently, it can be emphasized that there is a close correlation between the electronic/geometric structure and OER/ORR catalytic activities, and the best bifunctional OER/ORR catalytic activity of P,S co-doped PCSC-C3N4 is attributed to a synergis-

We have systematically investigated the detailed mechanisms of HER/OER/ORR and the synergistic effect between geometric and electronic factors plays an important role in HER/ OER/ORR catalytic activities of GXs and XY-C3N4. In this work, we demonstrated that the HER/OER/ORR activity of GXs and XY-C3N4 can be modulated by structural and electronic factors, including structural deformation with dopants. These structural and electronic factors enhance adsorbent binding strength during the reactions in HER/OER/ORR by generating sp<sup>3</sup>

hybridized atoms and facilitating charge transfer between adsorbents and GXs/XY-C3N4 as metallic properties. Additionally, we re-evaluated the generally accepted ER mechanism of OER/ORR by comparing the stability of intermediates governing the reactions, where we found that the OER/ORR respectively follows four-step and two-step reaction pathway. We also elucidated the importance of the O2 adsorption free energy (ΔGO2\*) in ORR activity. Considering the <sup>Δ</sup>GO2\* with a FOM, FOM = (ηORR <sup>+</sup> <sup>Δ</sup>GO2\*), we successfully represented the ORR activity of XY-C3N4. We believe that the understanding of the detailed mechanism as well as the relationship of structure-electrocatalytic activity of the HER/OER/ORR can facilitate

This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2018R1A2B6006320). This work was also supported by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing


21

Theoretical Basis of Electrocatalysis

http://dx.doi.org/10.5772/intechopen.77109

tic effect between the electronic and geometric effects.

development of new electrocatalytic materials.

resources including technical support (KSC-2017-C3-0032).

\*Address all correspondence to: sulee@hanyang.ac.kr

1 Bionano Technology, Hanyang University, Ansan, Korea

2 Chemical and Molecular Engineering, Hanyang University, Ansan, Korea

Acknowledgements

Author details

Chi Ho Lee<sup>1</sup> and Sang Uck Lee1,2\*

4. Conclusion

Figure 4. The volcano plots of (a) OER and (c) ORR at all possible active sites on XY-C3N4 and the free energy diagrams (FEDs) of PCSC-C3N4 structure having the best catalytic activity.

#### 3.2.2. Structural and electronic doping effects on OER/ORR activity

The synergistic effect of P, S co-doping can be explained based on the geometric and electronic effects of heteroatoms [17–20]. Considering the atomic size of heteroatoms, the relatively larger S and P dopants can cause structural deformation of XY-C3N4, which improves the OER and ORR activity by enhancing the stability of intermediates with increasing p orbital character of active sites adjacent to the dopant from sp<sup>2</sup> to sp<sup>3</sup> hybridization. Compared to sp<sup>2</sup> hybridized orbitals on pure C3N4, sp<sup>3</sup> character of active sites on XY-C3N4 is more suitable for forming chemical bonds with intermediate species. Therefore, in out-of-plane structures, the intermediates prefer to bind to sp3 -hybridized active sites. Moreover, to verify the relationship between geometric and electronic structures of XY-C3N4, we intentionally changed the structures of XY-C3N4 from in-plane to out-of-plane to increase the activity for binding intermediate species by increasing the sp<sup>3</sup> character of active sites on XY-C3N4. We investigated density of state (DOS) of pure C3N4 and XY-C3N4 (X = P, S, or PS and Y = N) to determine at what condition the OER/ORR exhibit outstanding performance [18, 20]. It can be expected that electron-rich heteroatom doping will induce electronic structure changes from a non-metallic doping effect to a metallic doping effect due to a downward band shift. As a result, PCSC-C3N4 shows the best OER/ORR activity by maintaining a metallic property despite the presence of out-of-plane deformation. Consequently, it can be emphasized that there is a close correlation between the electronic/geometric structure and OER/ORR catalytic activities, and the best bifunctional OER/ORR catalytic activity of P,S co-doped PCSC-C3N4 is attributed to a synergistic effect between the electronic and geometric effects.

### 4. Conclusion

We have systematically investigated the detailed mechanisms of HER/OER/ORR and the synergistic effect between geometric and electronic factors plays an important role in HER/ OER/ORR catalytic activities of GXs and XY-C3N4. In this work, we demonstrated that the HER/OER/ORR activity of GXs and XY-C3N4 can be modulated by structural and electronic factors, including structural deformation with dopants. These structural and electronic factors enhance adsorbent binding strength during the reactions in HER/OER/ORR by generating sp<sup>3</sup> hybridized atoms and facilitating charge transfer between adsorbents and GXs/XY-C3N4 as metallic properties. Additionally, we re-evaluated the generally accepted ER mechanism of OER/ORR by comparing the stability of intermediates governing the reactions, where we found that the OER/ORR respectively follows four-step and two-step reaction pathway. We also elucidated the importance of the O2 adsorption free energy (ΔGO2\*) in ORR activity. Considering the <sup>Δ</sup>GO2\* with a FOM, FOM = (ηORR <sup>+</sup> <sup>Δ</sup>GO2\*), we successfully represented the ORR activity of XY-C3N4. We believe that the understanding of the detailed mechanism as well as the relationship of structure-electrocatalytic activity of the HER/OER/ORR can facilitate development of new electrocatalytic materials.

### Acknowledgements

This research was supported by grants from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2018R1A2B6006320). This work was also supported by the Supercomputing Center/Korea Institute of Science and Technology Information with supercomputing resources including technical support (KSC-2017-C3-0032).

### Author details

3.2.2. Structural and electronic doping effects on OER/ORR activity

(FEDs) of PCSC-C3N4 structure having the best catalytic activity.

20 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

diates prefer to bind to sp3

The synergistic effect of P, S co-doping can be explained based on the geometric and electronic effects of heteroatoms [17–20]. Considering the atomic size of heteroatoms, the relatively larger S and P dopants can cause structural deformation of XY-C3N4, which improves the OER and ORR activity by enhancing the stability of intermediates with increasing p orbital character of active sites adjacent to the dopant from sp<sup>2</sup> to sp<sup>3</sup> hybridization. Compared to sp<sup>2</sup> hybridized orbitals on pure C3N4, sp<sup>3</sup> character of active sites on XY-C3N4 is more suitable for forming chemical bonds with intermediate species. Therefore, in out-of-plane structures, the interme-

Figure 4. The volcano plots of (a) OER and (c) ORR at all possible active sites on XY-C3N4 and the free energy diagrams

between geometric and electronic structures of XY-C3N4, we intentionally changed the structures of XY-C3N4 from in-plane to out-of-plane to increase the activity for binding intermediate species by increasing the sp<sup>3</sup> character of active sites on XY-C3N4. We investigated density of state (DOS) of pure C3N4 and XY-C3N4 (X = P, S, or PS and Y = N) to determine at what condition the OER/ORR exhibit outstanding performance [18, 20]. It can be expected that


Chi Ho Lee<sup>1</sup> and Sang Uck Lee1,2\*

\*Address all correspondence to: sulee@hanyang.ac.kr


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**Chapter 2**

**Provisional chapter**

**Oxygen Reduction Reaction**

**Oxygen Reduction Reaction**

http://dx.doi.org/10.5772/intechopen.79098

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

DOI: 10.5772/intechopen.79098

In this chapter, the oxygen reduction reaction (ORR), which is one of the most important reactions in energy conversion systems such as fuel cells, including its reaction kinetics, is presented. Recent developments in electrocatalysts for ORR in fuel cells, including low and non-Pt electrocatalysts, metal oxides, transition metal macrocycles and chalgogenides, are discussed. Understanding of the interdependence of size, shape and activity of the electrocatalysts is evaluated. The recent development of ORR electrocatalysts with novel nanostructures is also reported. The mechanism catalysed by these electrocatalysts

is presented. Finally, the perspectives of future trends for ORR are discussed.

novel nanostructures, polymer membrane fuel cells

alternate electrocatalysts that can be used.

a four-electron reduction pathway from O2

**Keywords:** oxygen reduction reaction, electrocatalysts, reaction kinetics, mechanism,

Oxygen reduction reaction (ORR) has been the subject of extensive investigation over the last century [1]. This is largely because ORR is of major importance to energy conversion, in particular in the field of fuel cells and metal-air batteries [1–3]. ORR is the most important cathodic process in polymer electrolyte membrane fuel cells (PEMFCs) [4]. Among all catalysts evaluated, Pt is still the best catalyst for ORR. The major obstacle with Pt is that it belongs to the platinum group of metals, which are rare metals, hence too expensive for feasible commercialisation of fuel cells. This has led to more research being conducted in an effort to find

Oxygen reduction in aqueous solutions occurs mainly through two different pathways: either

to H2

. The most accepted mechanism of ORR was first proposed by Damjanovic et al. [5, 6]

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

O or a two-electron pathway from O2

to

Lindiwe Khotseng

Lindiwe Khotseng

**Abstract**

**1. Introduction**

H2 O2

#### **Chapter 2 Provisional chapter**

#### **Oxygen Reduction Reaction Oxygen Reduction Reaction**

#### Lindiwe Khotseng Lindiwe Khotseng

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79098

#### **Abstract**

In this chapter, the oxygen reduction reaction (ORR), which is one of the most important reactions in energy conversion systems such as fuel cells, including its reaction kinetics, is presented. Recent developments in electrocatalysts for ORR in fuel cells, including low and non-Pt electrocatalysts, metal oxides, transition metal macrocycles and chalgogenides, are discussed. Understanding of the interdependence of size, shape and activity of the electrocatalysts is evaluated. The recent development of ORR electrocatalysts with novel nanostructures is also reported. The mechanism catalysed by these electrocatalysts is presented. Finally, the perspectives of future trends for ORR are discussed.

DOI: 10.5772/intechopen.79098

**Keywords:** oxygen reduction reaction, electrocatalysts, reaction kinetics, mechanism, novel nanostructures, polymer membrane fuel cells

### **1. Introduction**

Oxygen reduction reaction (ORR) has been the subject of extensive investigation over the last century [1]. This is largely because ORR is of major importance to energy conversion, in particular in the field of fuel cells and metal-air batteries [1–3]. ORR is the most important cathodic process in polymer electrolyte membrane fuel cells (PEMFCs) [4]. Among all catalysts evaluated, Pt is still the best catalyst for ORR. The major obstacle with Pt is that it belongs to the platinum group of metals, which are rare metals, hence too expensive for feasible commercialisation of fuel cells. This has led to more research being conducted in an effort to find alternate electrocatalysts that can be used.

Oxygen reduction in aqueous solutions occurs mainly through two different pathways: either a four-electron reduction pathway from O2 to H2 O or a two-electron pathway from O2 to H2 O2 . The most accepted mechanism of ORR was first proposed by Damjanovic et al. [5, 6]

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and later modified by Wroblowa et al. [7], making it easier to understand the complicated reaction pathway of oxygen on the metal surface. They suggest that ORR proceeds along two parallel reaction pathways with rates that are comparable. In PEMFCs, a four-electron transfer is preferred.

The ORR is alkaline media versus reversible hydrogen electrode (RHE) at 25°, and its thermodynamic potentials at standard conditions are presented as follows [3, 8, 9]:

$$\rm O\_2 \star 2\, H\_2O \star 4\, e^- \rightarrow 4\, OH^- \; E^\circ = 0.401\quad V \tag{1}$$

The difference between *E* and *Eeq* is called polarisation.

environment while still electrically coupled.

Volmer equation and is given as follows [13]:

*i* = *i*

where *i* is the ORR current density, *i*

a fuel cell.

There are three distinct types of polarisation expressed in Eq. (8):

ƞ = ƞ*act* + ƞ*conc* + *iR* (8)

ƞ*act* is the activation over-potential, a function describing the charge transfer kinetics of an electrochemical reaction. ƞ*act* is always present and mostly dominant at small polarisation currents. ƞ*conc* is the concentration over-potential, a function describing the mass transport limitations associated with electrochemical processes. ƞ*conc* is predominant at larger polarisation currents. *iR* is the ohmic drop. This function takes into account the electrolytic resistivity of an environment when the anodic and cathodic elements of a corrosion reaction are separated by this

The graph in **Figure 1** depicts a polarisation curve showing the electrochemical efficiency of

The expression relating the over-potential, ƞ, and the net current is known as the Butler-

electrons transferred in the rate-determining step, *β* is the transfer coefficient, β is the

**Figure 1.** The polarisation curve shows the electrochemical efficiency of the PEMFC at any operating current [12].

*RT* <sup>ƞ</sup>) <sup>−</sup> exp(−[1 <sup>−</sup> *<sup>β</sup>*] \_\_\_\_\_\_\_\_ *nF*

*RT* ƞ) } (9)

Oxygen Reduction Reaction

27

http://dx.doi.org/10.5772/intechopen.79098

is the exchange current density, *n* is the number of

*<sup>o</sup>* {exp (<sup>β</sup> \_\_\_\_\_\_\_\_ *nF*

*o*

The pathway involving the hydrogen peroxide is expressed as follows [10]:

$$\rm O\_2 + H\_2O + 2e^- \rightarrow HO\_2^- + OH^- \rightarrow ^\cdot = 0.080\quad V \tag{2}$$

$$\text{HO}\_2^- + \text{H}\_2\text{O} + 2\text{ e}^- \rightarrow 3\text{OH}^-\text{ E}^\cdot = 0.880\text{ V} \tag{3}$$

The ORR is acidic media versus RHE at 25°, and its thermodynamic potentials at standard conditions are presented as follows:

Direct four-electron reduction:

$$\text{O}\_2 \text{\textdegree 4H}^\* \text{\textdegree 4e}^- \text{\textdegree 2H}\_2\text{O} \text{\textdegree E}^\prime \text{\textdegree 1.230 V} \tag{4}$$

Indirect reduction:

$$\text{O}\_2 + 2\,\text{H}^+ + 2\,\text{e}^- \to \text{H}\_2\text{O}\_2 \quad \text{E}^\circ = 0.680 \text{ V} \tag{5}$$

$$\mathrm{H\_2O\_2 + 2H^+ + 2e^- \to 2H\_2O} \to \mathrm{'} = 1.776 \text{ V} \tag{6a}$$

Or chemical decomposition:

$$2\,\mathrm{H}\_{2}\mathrm{O}\_{2} \rightarrow 2\,\mathrm{H}\_{2}\mathrm{O} + \mathrm{O}\_{2} \quad \mathrm{E}^{\circ} = 1.776\ \mathrm{V} \tag{6b}$$

### **2. Kinetics of ORR**

It is desirable for the ORR to occur at potentials close to thermodynamic potentials as much as possible. For the thermodynamic potentials to be obtained, the charge transfer kinetics of the ORR must be quick. It has been reported that the kinetics of fuel cells at cathode are slow, hence show over-potential ƞ as in Eq. (7) [3, 11]:

$$
\eta = E - E\_{eq} \tag{7}
$$

*E* is the resultant potential and *Eeq* is the equilibrium potential.

The difference between *E* and *Eeq* is called polarisation.

and later modified by Wroblowa et al. [7], making it easier to understand the complicated reaction pathway of oxygen on the metal surface. They suggest that ORR proceeds along two parallel reaction pathways with rates that are comparable. In PEMFCs, a four-electron

The ORR is alkaline media versus reversible hydrogen electrode (RHE) at 25°, and its thermo-

O2 + 2 H2 O + 4 e<sup>−</sup> → 4 OH<sup>−</sup> E° = 0.401 V (1)

The ORR is acidic media versus RHE at 25°, and its thermodynamic potentials at standard

O2 + 4 H+ + 4 e<sup>−</sup> → 2 H2 O E° = 1.230 V (4)

O2 + 2 H+ + 2 e<sup>−</sup> → H2 O2 E° = 0.680 V (5)

H2 O2 + 2 H+ + 2 e<sup>−</sup> → 2 H2 O E° = 1.776 V (6a)

2 H2 O2 → 2 H2 O + O2 E° = 1.776 V (6b)

It is desirable for the ORR to occur at potentials close to thermodynamic potentials as much as possible. For the thermodynamic potentials to be obtained, the charge transfer kinetics of the ORR must be quick. It has been reported that the kinetics of fuel cells at cathode are slow,

ƞ = *E* − *Eeq* (7)

<sup>−</sup> + OH<sup>−</sup> E° = 0.080 V (2)

<sup>−</sup> + H2 O + 2 e<sup>−</sup> → 3 OH<sup>−</sup> E° = 0.880 V (3)

dynamic potentials at standard conditions are presented as follows [3, 8, 9]:

The pathway involving the hydrogen peroxide is expressed as follows [10]:

O2 + H2 O + 2 e<sup>−</sup> → HO2

26 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

HO2

Direct four-electron reduction:

Or chemical decomposition:

**2. Kinetics of ORR**

hence show over-potential ƞ as in Eq. (7) [3, 11]:

*E* is the resultant potential and *Eeq* is the equilibrium potential.

Indirect reduction:

conditions are presented as follows:

transfer is preferred.

There are three distinct types of polarisation expressed in Eq. (8):

$$
\eta = \eta\_{at} + \eta\_{conc} + iR \tag{8}
$$

ƞ*act* is the activation over-potential, a function describing the charge transfer kinetics of an electrochemical reaction. ƞ*act* is always present and mostly dominant at small polarisation currents.

ƞ*conc* is the concentration over-potential, a function describing the mass transport limitations associated with electrochemical processes. ƞ*conc* is predominant at larger polarisation currents.

*iR* is the ohmic drop. This function takes into account the electrolytic resistivity of an environment when the anodic and cathodic elements of a corrosion reaction are separated by this environment while still electrically coupled.

The graph in **Figure 1** depicts a polarisation curve showing the electrochemical efficiency of a fuel cell.

The expression relating the over-potential, ƞ, and the net current is known as the Butler-Volmer equation and is given as follows [13]:

$$i = i\_o \left\{ \exp\left(\beta \frac{nF}{RT} \eta\right) - \exp\left(-\left[1 - \beta\right] \frac{nF}{RT} \eta\right) \right\} \tag{9}$$

where *i* is the ORR current density, *i o* is the exchange current density, *n* is the number of electrons transferred in the rate-determining step, *β* is the transfer coefficient, β is the

**Figure 1.** The polarisation curve shows the electrochemical efficiency of the PEMFC at any operating current [12].

**Figure 2.** Current-potential relationship of a metal dissolution (M → Mn+)/deposition (Mn+ → M) process [13].

over-potential of ORR, *F* is the Faraday constant, *R* is the gas constant and *T* is the temperature in K. The first term in the Butler-Volmer equation represents the anodic reaction/metal dissolution, while the second term represents the cathodic reaction/metal deposition. A plot of the Butler-Volmer equation gives the polarisation curve as shown in **Figure 2**.

There are two limitations in the Butler-Volmer equation.

**1.** A low over-potential region, also known as polarisation resistance/charge transfer resistance, where the Butler-Volmer equation simplifies to [13]

 *i* = *i <sup>c</sup>* = *i <sup>o</sup>* exp {(1 <sup>−</sup> *<sup>β</sup>*) \_\_\_\_\_\_\_\_ *nF RT* ƞ} (10)

$$
\eta\_{\iota} = b\_{\iota} \log \left( \frac{\mathrm{i}\mathfrak{c}}{\mathrm{i}\mathfrak{o}} \right) \tag{11}
$$

A plot of ƞ versus log *i*

and the electrode material used.

**3. Electrocatalysts for ORR**

four-electron reduction of O2

**3.1. Single-metal catalyst**

2.303 \_\_\_\_ *RT*

*c*

*nF*, known as the Tafel slope and the intercept yielding the *i*

information about the mechanism of the ORR. The higher the Tafel slope, the faster the overpotential increases with current density. With a low Tafel slope, the electrochemical reaction is able to obtain a high current density, at low over-potential. For ORR, two Tafel slopes, 60 mV dec−1 and 120 mV dec−1, respectively, are obtained, depending on the potential range

The kinetics of the ORR at the cathode are very important, as they are the factors for the performance of PEMFCs [14, 15]. There are several issues that need to be addressed, including slow reaction kinetics at the cathode, which are due to highly irreversible ORR, and fuel crossover in the cathode, which causes a mixed potential, leading to potential loss and 25% reduction in efficiency, hence reducing the ORR performance [16–19]. An electrocatalyst is used to induce a

the electrocatalyst that is currently used for ORR reactions, as it is the only commercially available catalyst with sensible activity and stability for PEMFCs, although it offers limited commercialisation of fuel cells due to its limited availability and high cost [4, 20]. It is also reported that Pt still shows over-potentials of over 400 mV from the equilibrium reversible potentials (1.19 V at 80°C) [21]. These high potentials result in the formation of adsorbed species on a platinum surface that restrain the ORR and hence result in performance loss [2]. Considerable research has been conducted to try to (1) reduce the costs of fuel cells, which is one of the stumbling blocks in fuel cell commercialisation using low-cost non-Pt catalysts such as supported platinum group metals Pd, Ir and Ru; (2) improve the electrocatalytic activity of the cathode catalyst, which includes using bimetallic alloy catalysts, transition metal macrocyanides, transition metal chalcogenides and metal oxides in order to improve the ORR kinetics on the new catalyst; and (3) fabricate Pt with novel nanostructures such as nanotubes, graphene and carbon nanofibres (CNFs), as it is known that supports may significantly affect the performance of the catalyst. However, these efforts are still in the research stage, as their activity and stability are still lower than that of the Pt catalyst.

Other noble metals, such as Pd, Ag, Rh, Ir and Ru, have also been used as cathode materials for ORR [22]. Among these, Pd, which has the same electron configuration and lattice constant as Pt, as they belong to the same row in the periodic table, showed the most improved ORR towards alcohol in an alkaline medium, while it is reported to be inactive in an acid medium [4, 23]. It has been showed that Pd/C is less sensitive to alcohol contamination compared to Pt/C, hence more tolerant to alcohol crossover [16, 24, 25]. However, when comparing the ORR activity of Pd/C to Pt/C, the former has less activity and high potentials of amount 0.8 V versus NHE, hence less stability, which prevents its replacement as the preferred ORR catalyst over Pt/C [26]. The noble metals in terms of ORR activity follow the trend: Pt > Pd > Ir > Rh [27]. Wang [18] reports that Ru can undergo a four-electron reduction reaction. Ag is reported

also known as the Tafel plot and gives a linear relationship, with slope

to water by utilising the protons that permeate from the anode. Pt is

*o*

. The Tafel slope gives the

http://dx.doi.org/10.5772/intechopen.79098

Oxygen Reduction Reaction

29

$$\mathbf{b}\_c = -2.303 \begin{array}{c} \text{RT} \\ \text{(1 - \beta)} \eta F \end{array} \tag{12}$$

**2.** At high over-potentials region, the Butler-Volmer equation simplifies to Tafel equation

$$i\_a = \ i\_o \exp\left(\beta \frac{nF}{RT} \eta\right) \tag{13}$$

$$
\eta\_a = b\_a \log \left(\frac{\dot{a}}{\dot{a}}\right) \tag{14}
$$

$$\mathbf{b}\_{\text{a}} = \text{2.303 } \frac{\text{RT}}{\rho \text{nF}} \tag{15}$$

A plot of ƞ versus log *i c* also known as the Tafel plot and gives a linear relationship, with slope 2.303 \_\_\_\_ *RT nF*, known as the Tafel slope and the intercept yielding the *i o* . The Tafel slope gives the information about the mechanism of the ORR. The higher the Tafel slope, the faster the overpotential increases with current density. With a low Tafel slope, the electrochemical reaction is able to obtain a high current density, at low over-potential. For ORR, two Tafel slopes, 60 mV dec−1 and 120 mV dec−1, respectively, are obtained, depending on the potential range and the electrode material used.

### **3. Electrocatalysts for ORR**

The kinetics of the ORR at the cathode are very important, as they are the factors for the performance of PEMFCs [14, 15]. There are several issues that need to be addressed, including slow reaction kinetics at the cathode, which are due to highly irreversible ORR, and fuel crossover in the cathode, which causes a mixed potential, leading to potential loss and 25% reduction in efficiency, hence reducing the ORR performance [16–19]. An electrocatalyst is used to induce a four-electron reduction of O2 to water by utilising the protons that permeate from the anode. Pt is the electrocatalyst that is currently used for ORR reactions, as it is the only commercially available catalyst with sensible activity and stability for PEMFCs, although it offers limited commercialisation of fuel cells due to its limited availability and high cost [4, 20]. It is also reported that Pt still shows over-potentials of over 400 mV from the equilibrium reversible potentials (1.19 V at 80°C) [21]. These high potentials result in the formation of adsorbed species on a platinum surface that restrain the ORR and hence result in performance loss [2]. Considerable research has been conducted to try to (1) reduce the costs of fuel cells, which is one of the stumbling blocks in fuel cell commercialisation using low-cost non-Pt catalysts such as supported platinum group metals Pd, Ir and Ru; (2) improve the electrocatalytic activity of the cathode catalyst, which includes using bimetallic alloy catalysts, transition metal macrocyanides, transition metal chalcogenides and metal oxides in order to improve the ORR kinetics on the new catalyst; and (3) fabricate Pt with novel nanostructures such as nanotubes, graphene and carbon nanofibres (CNFs), as it is known that supports may significantly affect the performance of the catalyst. However, these efforts are still in the research stage, as their activity and stability are still lower than that of the Pt catalyst.

### **3.1. Single-metal catalyst**

over-potential of ORR, *F* is the Faraday constant, *R* is the gas constant and *T* is the temperature in K. The first term in the Butler-Volmer equation represents the anodic reaction/metal dissolution, while the second term represents the cathodic reaction/metal deposition. A plot

**1.** A low over-potential region, also known as polarisation resistance/charge transfer resist-

**2.** At high over-potentials region, the Butler-Volmer equation simplifies to Tafel equation

*<sup>o</sup>* exp (<sup>β</sup> \_\_\_\_\_\_\_\_ *nF*

*<sup>o</sup>* exp {(1 <sup>−</sup> *<sup>β</sup>*) \_\_\_\_\_\_\_\_ *nF*

\_\_*ic*

\_\_*ia*

*RT* ƞ} (10)

*io*) (11)

(1 <sup>−</sup> *<sup>β</sup>*)*nF* (12)

*RT* ƞ) (13)

*io*) (14)

*nF* (15)

of the Butler-Volmer equation gives the polarisation curve as shown in **Figure 2**.

**Figure 2.** Current-potential relationship of a metal dissolution (M → Mn+)/deposition (Mn+ → M) process [13].

*<sup>c</sup>* = *i*

*<sup>a</sup>* = *i*

There are two limitations in the Butler-Volmer equation.

28 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

ƞ*<sup>c</sup>* = *bc log* (

ƞ*<sup>a</sup>* = *ba log* (

ba <sup>=</sup> 2.303 \_\_\_\_ *RT*

bc <sup>=</sup> <sup>−</sup>2.303 \_\_\_\_\_\_\_ *RT*

*i* = *i*

*i*

ance, where the Butler-Volmer equation simplifies to [13]

Other noble metals, such as Pd, Ag, Rh, Ir and Ru, have also been used as cathode materials for ORR [22]. Among these, Pd, which has the same electron configuration and lattice constant as Pt, as they belong to the same row in the periodic table, showed the most improved ORR towards alcohol in an alkaline medium, while it is reported to be inactive in an acid medium [4, 23]. It has been showed that Pd/C is less sensitive to alcohol contamination compared to Pt/C, hence more tolerant to alcohol crossover [16, 24, 25]. However, when comparing the ORR activity of Pd/C to Pt/C, the former has less activity and high potentials of amount 0.8 V versus NHE, hence less stability, which prevents its replacement as the preferred ORR catalyst over Pt/C [26]. The noble metals in terms of ORR activity follow the trend: Pt > Pd > Ir > Rh [27]. Wang [18] reports that Ru can undergo a four-electron reduction reaction. Ag is reported

The effect of Pd on Pt catalysts has been reported by various researchers [33–36], who all reported an improved electrocatalytic activity towards ORR. The improved electrocatalytic activity of Pt alloys has been ascribed to geometric effect, namely a decreased Pt-Pt bond distance, the dissolution of more oxidisable alloying components, a change in surface structure and electronic effect, that is, an increase in Pt d-band vacancy [31, 37]. Other noble metals (Au, Ir, Rh and Ru) were also studied as Pt alloys. PtAu/CNT (carbon nanotubes) was reported to show an improved electrocatalytic activity in an alkaline membrane compared to Pt/CNT [38–40]. Pd-Pt alloy had the highest electrocatalytic activity towards ORR compared to other Pt-alloyed noble metals. Fu et al. [41] report on an improved ORR electrocatalytic activity and a larger diffusion-limiting current density compared to commercial Pd black. The improvement of Pt with noble metals (Pd, Ag and Au) is attributed to these metals having a fully occupied d-orbital. The d-orbital coupling effect between metals decreases the Gibbs free energy

Oxygen Reduction Reaction

31

http://dx.doi.org/10.5772/intechopen.79098

Binary Pd catalysts (Cu, Ni, Fe, Co, W and Mo) have been identified as promising cathode catalysts for ORR with an improved electrocatalytic activity and stability compared to Pd alone due to changes in Pd-Pd bond length, modification of the electron configuration and change of surface species and compositions [4, 25]. Kim et al. [42] used PdSn/C as a cathode for direct methanol fuel cells. Their results exhibited a high performance in high methanol concentrations compared to commercial Pt/C. Wang [18] reports on PdCo catalysts tested for ORR. The results showed improved activities towards ORR, with PdCo improvement only observed when less than 20% Co was used. Meng et al. [4] report on PdFe, PdCu, PdAg and PdCo, which all showed an improved electrocatalytic activity compared to Pd alone. In the presence of alcohol, the binary Pd catalysts showed an improved electrocatalytic performance

Although bimetallic catalysts (Pt with Fe, Ni, Co, etc.) have shown improvement in the performance of the ORR compared to Pt [27], the lack of preparation methods to control large synthesis has limited its use in commercial devices [33]. In addition, the dissolution of transition metals alloyed in the Pt-M catalyst is a major drawback, because these transition metals are electrochemically soluble at a potential range between 0.3 and 1.0 V versus NHE in acidic

One possible way to overcome the dissolution of transition metals and improve stability of catalysts is by preparing multi-component catalysts (three and more electrocatalyst alloys). The multi-component catalysts are able to shift the d-band centre by a strain effect (caused by the lattice mismatch in the multi-component system) and lower the adsorption energy of

Tiwari et al. [37] report on various multi-component catalysts that have been synthesised over the years with either Pt or Pd, including PtTiM (M = Co, Cr, Cu, Fe, Mn, Mo, Ni, Pd, Ta, V, W

component catalysts, they reported an improved electrocatalytic activity towards ORR compared to pure Pt and the possibility of reducing the costs of the electrocatalysts for PEMFCs.

, PtIrCo, PdFePt, PtCuCoNi, PdSnPt and PdCoPt. From the multi-

surface oxygenated intermediates, thereby enhancing the surface catalytic activity.

for the electron-free steps in ORR, resulting in improved ORR kinetics [37].

for ORR compared to Pt.

and Za), PtCuCo, PtCoSe2

media [43].

**Figure 3.** Oxygen reduction activities as a function of oxygen-binding energy [27].

to show less electrocatalytic activity towards ORR compared to Pt, but is more stable than Pt cathodes during long-term operations [4].

There are several metals other than noble metals that were also evaluated as cathode catalysts for ORR. **Figure 3** shows a comparison of the activities of various catalysts as a function of binding energy. These catalysts showed less catalytic activity towards ORR compared to Pt, with less electrochemical stability [28].

### **3.2. Bimetallic-alloyed catalysts**

Transition metals such as Fe, Ni, Co and Cr have been extensively studied due to their improved ORR electrocatalytic activity as alloys for Pt in the presence of a support [17, 28]. Yuan et al. [17] prepared PtFe/C using the impregnation method. Current densities of PtFe/C for ORR in methanol were higher (78.6 mA cm−2) than in Pt/C (65.0 mA cm−2), but was lower in methanol-free solutions, indicating that PtFe/C is a better methanol-tolerant catalyst compared to Pt/C. In terms of power density, the PtFe/C showed an improvement of 20–30% compared to Pt/C [17]. Other researchers [29, 30] also reported better performance of PtFe/C for ORR. Yang et al. [31] report on PtNi/C prepared via the carbonyl complex route an improved mass and specific activity of PtNi/C compared to Pt/C with an improved electrocatalytic activity towards ORR. Pt-Bi/C showed improved methanol tolerance for ORR compared to Pt/C [3]. Remona and Phani [32] synthesised PtBi/C by micro-emulsion. The PtBi/C displayed a higher methanol tolerance compared to mass activity (1.5 times higher) for ORR compared to Pt/C. The high ORR activity of PtBi/C was due to distortion (internal stress) of three Pt sites by Bi.

The effect of Pd on Pt catalysts has been reported by various researchers [33–36], who all reported an improved electrocatalytic activity towards ORR. The improved electrocatalytic activity of Pt alloys has been ascribed to geometric effect, namely a decreased Pt-Pt bond distance, the dissolution of more oxidisable alloying components, a change in surface structure and electronic effect, that is, an increase in Pt d-band vacancy [31, 37]. Other noble metals (Au, Ir, Rh and Ru) were also studied as Pt alloys. PtAu/CNT (carbon nanotubes) was reported to show an improved electrocatalytic activity in an alkaline membrane compared to Pt/CNT [38–40]. Pd-Pt alloy had the highest electrocatalytic activity towards ORR compared to other Pt-alloyed noble metals. Fu et al. [41] report on an improved ORR electrocatalytic activity and a larger diffusion-limiting current density compared to commercial Pd black. The improvement of Pt with noble metals (Pd, Ag and Au) is attributed to these metals having a fully occupied d-orbital. The d-orbital coupling effect between metals decreases the Gibbs free energy for the electron-free steps in ORR, resulting in improved ORR kinetics [37].

Binary Pd catalysts (Cu, Ni, Fe, Co, W and Mo) have been identified as promising cathode catalysts for ORR with an improved electrocatalytic activity and stability compared to Pd alone due to changes in Pd-Pd bond length, modification of the electron configuration and change of surface species and compositions [4, 25]. Kim et al. [42] used PdSn/C as a cathode for direct methanol fuel cells. Their results exhibited a high performance in high methanol concentrations compared to commercial Pt/C. Wang [18] reports on PdCo catalysts tested for ORR. The results showed improved activities towards ORR, with PdCo improvement only observed when less than 20% Co was used. Meng et al. [4] report on PdFe, PdCu, PdAg and PdCo, which all showed an improved electrocatalytic activity compared to Pd alone. In the presence of alcohol, the binary Pd catalysts showed an improved electrocatalytic performance for ORR compared to Pt.

**Figure 3.** Oxygen reduction activities as a function of oxygen-binding energy [27].

30 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

cathodes during long-term operations [4].

with less electrochemical stability [28].

**3.2. Bimetallic-alloyed catalysts**

to show less electrocatalytic activity towards ORR compared to Pt, but is more stable than Pt

There are several metals other than noble metals that were also evaluated as cathode catalysts for ORR. **Figure 3** shows a comparison of the activities of various catalysts as a function of binding energy. These catalysts showed less catalytic activity towards ORR compared to Pt,

Transition metals such as Fe, Ni, Co and Cr have been extensively studied due to their improved ORR electrocatalytic activity as alloys for Pt in the presence of a support [17, 28]. Yuan et al. [17] prepared PtFe/C using the impregnation method. Current densities of PtFe/C for ORR in methanol were higher (78.6 mA cm−2) than in Pt/C (65.0 mA cm−2), but was lower in methanol-free solutions, indicating that PtFe/C is a better methanol-tolerant catalyst compared to Pt/C. In terms of power density, the PtFe/C showed an improvement of 20–30% compared to Pt/C [17]. Other researchers [29, 30] also reported better performance of PtFe/C for ORR. Yang et al. [31] report on PtNi/C prepared via the carbonyl complex route an improved mass and specific activity of PtNi/C compared to Pt/C with an improved electrocatalytic activity towards ORR. Pt-Bi/C showed improved methanol tolerance for ORR compared to Pt/C [3]. Remona and Phani [32] synthesised PtBi/C by micro-emulsion. The PtBi/C displayed a higher methanol tolerance compared to mass activity (1.5 times higher) for ORR compared to Pt/C. The high

ORR activity of PtBi/C was due to distortion (internal stress) of three Pt sites by Bi.

Although bimetallic catalysts (Pt with Fe, Ni, Co, etc.) have shown improvement in the performance of the ORR compared to Pt [27], the lack of preparation methods to control large synthesis has limited its use in commercial devices [33]. In addition, the dissolution of transition metals alloyed in the Pt-M catalyst is a major drawback, because these transition metals are electrochemically soluble at a potential range between 0.3 and 1.0 V versus NHE in acidic media [43].

One possible way to overcome the dissolution of transition metals and improve stability of catalysts is by preparing multi-component catalysts (three and more electrocatalyst alloys). The multi-component catalysts are able to shift the d-band centre by a strain effect (caused by the lattice mismatch in the multi-component system) and lower the adsorption energy of surface oxygenated intermediates, thereby enhancing the surface catalytic activity.

Tiwari et al. [37] report on various multi-component catalysts that have been synthesised over the years with either Pt or Pd, including PtTiM (M = Co, Cr, Cu, Fe, Mn, Mo, Ni, Pd, Ta, V, W and Za), PtCuCo, PtCoSe2 , PtIrCo, PdFePt, PtCuCoNi, PdSnPt and PdCoPt. From the multicomponent catalysts, they reported an improved electrocatalytic activity towards ORR compared to pure Pt and the possibility of reducing the costs of the electrocatalysts for PEMFCs.

More research is also done on trying to improve the kinetics of Pt catalysts through the development of new and the optimisation of existing synthesis methods that can control the shape and surface structure of the electrocatalyst in order to improve its performance [8, 33, 44, 45]. There has been contracting reports on particle size and specific activity. Some researchers report a decrease in specific activity with a decrease in particle size, while other researchers report an increase in specific activity with a decrease in particle size. The discrepancy may be the result of different electrocatalysts with different shapes and degree of agglomeration [46]. It has been reported that metallic nanostructures with different shapes display unique chemical and physical properties [47]. Significant progress has been made for size-controlled spherical nanostructures, but only limited progress has been made with non-spherical nanostructures, which are reported to show more improved ORR electrocatalytic activity for fuel cells compared to spherical-shaped electrocatalysts, the reason being that the highly symmetric face-centred cubic crystal structure of Pt-based catalysts makes it not easy to obtain a non-spherical shape, which involves a competition over the desire to minimise the surface energy through the formation of thermodynamically stable spherical shapes. Hao et al. [48] report that cubic Pd nanocrystals exhibit better performance in methanol compared to the spherical Pd catalyst of a similar size. The improvement was attributed to cubic nanocrystals that contain a more electroactive surface area compared to its spherical counterpart. It has also been reported that Pt electrocatalysts with a tetrahedral shape showed much improved reaction kinetics compared to the spherical Pt nanoparticles. It has been reported that lowindex crystal planes give poor electrocatalytic properties, while high-index planes give a high electrocatalytic activity and stability [47, 49]. Pt nanocubes have been reported to have much improved specific activity for ORR compared to commercial Pt catalysts (spherical) [47, 50]. Kuai et al. [50] report on uniform, high-yield icosahedral Ag and Au nanoparticles prepared using a hydrothermal system in the presence of polyvinylpyrrolidone and ammonia. The prepared Au and Ag nanoparticles showed an improved ORR electrocatalytic activity and excellent stability compared to spherical Pt/C nanoparticles.

The form or shape of the electrocatalysts depends on the synthesis method and the various parameters used. **Figure 4** shows the formation of various forms of Pt nanocubes using different strategies based on the polyol method [47]. The formation of Pt tubes can be obtained through two different routes, either galvanic displacement or using selected templates [47]. Researchers have synthesised metal nanoparticles with various shapes [32].

Researchers are reporting an improved electrocatalytic activity of ORR using various synthesis methods focusing on surface structure. Lim et al. [14] report on the synthesis of Pt/C using the modified polyol method using ethylene glycol as the reducing agent. The prepared catalyst had the highest mass activity, which was 1.7 times higher than the commercial Pt/C [14]. The improvement was attributed to small and uniform particle size and better dispersion. Adonisi et al. [51] report on various Pt-based catalysts synthesised using the Bonnemann method. Some of those catalysts showed an improved ORR electrocatalytic activity compared to commercial Pt/C. **Figure 5** shows a graph depicting 20% Pt/C commercial compared to 20% Pt/C synthesised catalysts using the Bonnemann method. From the graph, it can be observed that the 20% Pt/C commercial catalyst was found at lower current densities than the prepared 20% Pt/C catalyst. The mass activities of the 20% Pt/C commercial and 20% Pt/C prepared catalysts were found to be 12.6 and 15.8 A/g, respectively, and the specific activities were

found to be 0.060 and 0.063 A cm−2, respectively. This improvement was attributed to the particle size of the prepared catalysts, which was smaller than the particle size for the com-


Oxygen Reduction Reaction

33

http://dx.doi.org/10.5772/intechopen.79098

**Figure 5.** Cyclic voltammograms for ORR of 20% Pt/C commercial and prepared electrocatalysts in O<sup>2</sup>

**Figure 4.** Different routes for the synthesis of Pt nanocubes along with their electron micrographs [47].

mercial Pt/C catalyst.

H2SO4 at a scan rate of 20 mV [51].

More research is also done on trying to improve the kinetics of Pt catalysts through the development of new and the optimisation of existing synthesis methods that can control the shape and surface structure of the electrocatalyst in order to improve its performance [8, 33, 44, 45]. There has been contracting reports on particle size and specific activity. Some researchers report a decrease in specific activity with a decrease in particle size, while other researchers report an increase in specific activity with a decrease in particle size. The discrepancy may be the result of different electrocatalysts with different shapes and degree of agglomeration [46]. It has been reported that metallic nanostructures with different shapes display unique chemical and physical properties [47]. Significant progress has been made for size-controlled spherical nanostructures, but only limited progress has been made with non-spherical nanostructures, which are reported to show more improved ORR electrocatalytic activity for fuel cells compared to spherical-shaped electrocatalysts, the reason being that the highly symmetric face-centred cubic crystal structure of Pt-based catalysts makes it not easy to obtain a non-spherical shape, which involves a competition over the desire to minimise the surface energy through the formation of thermodynamically stable spherical shapes. Hao et al. [48] report that cubic Pd nanocrystals exhibit better performance in methanol compared to the spherical Pd catalyst of a similar size. The improvement was attributed to cubic nanocrystals that contain a more electroactive surface area compared to its spherical counterpart. It has also been reported that Pt electrocatalysts with a tetrahedral shape showed much improved reaction kinetics compared to the spherical Pt nanoparticles. It has been reported that lowindex crystal planes give poor electrocatalytic properties, while high-index planes give a high electrocatalytic activity and stability [47, 49]. Pt nanocubes have been reported to have much improved specific activity for ORR compared to commercial Pt catalysts (spherical) [47, 50]. Kuai et al. [50] report on uniform, high-yield icosahedral Ag and Au nanoparticles prepared using a hydrothermal system in the presence of polyvinylpyrrolidone and ammonia. The prepared Au and Ag nanoparticles showed an improved ORR electrocatalytic activity and

The form or shape of the electrocatalysts depends on the synthesis method and the various parameters used. **Figure 4** shows the formation of various forms of Pt nanocubes using different strategies based on the polyol method [47]. The formation of Pt tubes can be obtained through two different routes, either galvanic displacement or using selected templates [47].

Researchers are reporting an improved electrocatalytic activity of ORR using various synthesis methods focusing on surface structure. Lim et al. [14] report on the synthesis of Pt/C using the modified polyol method using ethylene glycol as the reducing agent. The prepared catalyst had the highest mass activity, which was 1.7 times higher than the commercial Pt/C [14]. The improvement was attributed to small and uniform particle size and better dispersion. Adonisi et al. [51] report on various Pt-based catalysts synthesised using the Bonnemann method. Some of those catalysts showed an improved ORR electrocatalytic activity compared to commercial Pt/C. **Figure 5** shows a graph depicting 20% Pt/C commercial compared to 20% Pt/C synthesised catalysts using the Bonnemann method. From the graph, it can be observed that the 20% Pt/C commercial catalyst was found at lower current densities than the prepared 20% Pt/C catalyst. The mass activities of the 20% Pt/C commercial and 20% Pt/C prepared catalysts were found to be 12.6 and 15.8 A/g, respectively, and the specific activities were

excellent stability compared to spherical Pt/C nanoparticles.

32 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

Researchers have synthesised metal nanoparticles with various shapes [32].

**Figure 4.** Different routes for the synthesis of Pt nanocubes along with their electron micrographs [47].

**Figure 5.** Cyclic voltammograms for ORR of 20% Pt/C commercial and prepared electrocatalysts in O<sup>2</sup> -saturated 0.5 M H2SO4 at a scan rate of 20 mV [51].

found to be 0.060 and 0.063 A cm−2, respectively. This improvement was attributed to the particle size of the prepared catalysts, which was smaller than the particle size for the commercial Pt/C catalyst.

Post-treatment after synthesis has been reported to change the physicochemical properties of the electrocatalysts. Heat treatment is considered as one of the important and sometimes necessary steps to improve the activity of the catalysts [52]. Heat treatment involves heating the catalyst under inert (N<sup>2</sup> , Ar or He) or reducing H2 atmosphere in the temperature range of 80–900°C for 1–4 h [52]. The benefit of heat treatment is the removal of impurities resulting from the preparation stages, allowing uniform dispersion and stable distribution of the catalyst on the support, thereby improving the electrocatalytic activity of the prepared catalyst. It has been determined that the electrocatalytic reduction of oxygen on the catalyst can be influenced by the particle size and surface structure, and hence treatment can have an effect on ORR activity and stability by altering the surface structure of the catalyst [32]. Various researchers have worked on heat treatment of mono and bimetallic catalysts for ORR [50]. They all concluded that heat treatment improves alloying of the catalysts, which decreases the Pt-Pt distance and hence d-band vacancy of the Pt and thus improves the electroactivity of the catalyst [53]. Jeyabharathi et al. [54] report on improved methanol tolerance of PtSn/C after heat treatment, while the ORR activity remained intact. Sarkar et al. report on PdW synthesised using thermal composition followed by annealing at 800°C with an improved electrocatalytic activity for ORR and catalyst durability with an improved methanol tolerance compared to Pt [32].

**3.4. Metal oxides**

ports. Metal oxides such as IrO2

cally active compared to PtTiO2

CNT compared to TiO<sup>2</sup>

report on CoFe2

CNT that was provided by the TiO<sup>2</sup>

compared the activity for PtRu/TiO<sup>2</sup>

O4

**3.5. Novel nanostructures with electrocatalysts**

Another route to stabilising nanoparticles is the development of metal oxide composite sup-

, ZrO2

ORR catalysts in basic acidic media [62–66]. Nanoparticles on metal oxides are not able to improve the electrocatalytic activity due to their limited electron conductivity, but are reported to have excellent corrosion resistance in various electrolyte media [66, 67]. Researchers use metal oxides in combination with carbon supports that have desirable properties such as a high surface area and a high electric conductivity. The metal oxides combined with carbon supports are reported to improve the stability and the electrocatalytic activity of the electrode material. Carbon surfaces are functionalised before they are used as supports for catalysts in order to improve their surface properties, but the disadvantage of functionalisation is that it accelerates the degradation process of the support material. The presence of the metal oxide delays the

, which has limited electron conductivity. Pt/TiO<sup>2</sup>

ment in activity and thermal stability for ORR compared to Pt/C [69]. Khotseng et al. [70]

electroactive surface area and activity of commercial Pt/C and PtRu/C compared to PtRu/TiO<sup>2</sup> towards ORR. When durability studies were performed for the same catalysts, the PtRu/TiO<sup>2</sup> recorded a loss of 29% compared to Pt/C and PtRu/C, which recorded a loss of 64 and 32%, respectively. Li et al. [71] reported an improved oxygen reduction activity, a better durability

Most metal oxides were found to be unstable in acidic media. To overcome this instability, conducting polymer polypyrrole (Ppy) was used against the dissolution of metal oxides. During synthesis, the metal oxides were sandwiched between the Ppy layers. Through this research, an improved electrochemical stability of the metal oxides was achieved [18]. Singh et al. [72]

towards ORR at high cathodic potentials was obtained with stability in acidic media [72].

Although Pt-based catalysts have been widely studied due to their high current density and low over-potential, when used as cathode catalysts, their activity is lowered due to slow reaction kinetics. More research is required to try to improve the catalyst activity. One of the focus areas is looking into loading Pt nanostructures with a high activity on the surface of supporting materials with (1) low cost, (2) good electrical conductivity, (3) strong catalyst-support interaction that is influenced by its surface functionalities to limit the possible deactivation of the electrocatalyst and allow for efficient charge transport, (4) large surface area and (5) good resistance to corrosion to allow high stability [8, 22, 24, 73–75]. Carbon black (CB) is the mostused support for Pt and Pt alloy catalysts. CB is thermochemically unstable and hence suffers from corrosion, leading to performance degradation and durability issues and high potential

and a higher methanol tolerance capability in alkaline solution compared to Pt/C.

, TiO2

, while good stability was observed for both PtTiO2

oxides sandwiched between Ppy layers. A high electrocatalytic activity

and SnO2

and PtTiO2

support. This was attributed to the high conductivity of

to commercial PtRu/C and Pt/C. They reported a high

have also been studied as

Oxygen Reduction Reaction

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http://dx.doi.org/10.5772/intechopen.79098

/CNT synthesised

/C showed improve-

and PtTiO2

/

/CNT was more electrocatalyti-

, NiO, CeO<sup>2</sup>

corrosion process. Montero-Ocampo et al. [68] report on PtTiO2

using metal organic chemical vapour deposition. The PtTiO2

#### **3.3. Transition metal macrocycles and chalgogenides**

Transition metals such as macrocycles and chalgogenides have been used as ORR catalysts since the 1960s due to their inactivity towards the oxidation of methanol [55]. Other than noble metals, they are the most-studied electrocatalysts for oxygen reduction. The study of ruthenium chalgogenides Rux Sey and Pt for ORR has shown that the performance of Rux Sey is slightly weaker than Pt and that the difference was their behaviour in the presence or absence of methanol. Under these conditions, the electroactivity of Rux Sey is not changed, while for Pt, the potential shifts to the negative direction (120–150 mV). A similar behaviour was observed when Rux Sey was embedded in a polymetric matrix, such as polyaniline. RuSM (M = Rh, Re, Mo, etc.) when used as catalysts, methanol oxidation on the cathode was suppressed or avoided leading to a reduced mixed potential. The results confirm that the chalgogenide of Ru is insensitive to methanol, in contrast to the Pt catalyst [56]. The main concern with this approach was the low power output due to low activity of these catalysts for ORR, compared to the Pt catalyst [17]. Cobalt and iron phthalocyanide are the most-studied transition metals as centres for macrocycling rings as catalysts for ORR in fuel cells [56, 57]. These ORR catalysts have shown that a number of metal chelates will chemisorb oxygen [58]. A fuel cell with an iron phthalocyanide cathode can only be stable for up to 10 h [17], but has shown improved activity towards ORR in alkaline media [44]. Zagal et al. [59] report that when Fe chelates with N4 , a four-electron ORR occurs. Co phthalocyanide has demonstrated similar ORR kinetics as commercial Pt/C, as it also leads to a four-electron process per oxygen molecule, that is, to water, but at lower potentials (0.25 and −0.25 versus RHE) [44, 56]. However, these compounds are not completely stable under strong acid conditions [17, 56]. They decompose via hydrolysis in the electrolyte and attack the macrocyanide via peroxide, causing poor performance and stability [58, 60, 61]. Transition metal chalgogenides are more stable and show an improved electrocatalytic activity at temperatures above 800°C.

### **3.4. Metal oxides**

Post-treatment after synthesis has been reported to change the physicochemical properties of the electrocatalysts. Heat treatment is considered as one of the important and sometimes necessary steps to improve the activity of the catalysts [52]. Heat treatment involves heating

of 80–900°C for 1–4 h [52]. The benefit of heat treatment is the removal of impurities resulting from the preparation stages, allowing uniform dispersion and stable distribution of the catalyst on the support, thereby improving the electrocatalytic activity of the prepared catalyst. It has been determined that the electrocatalytic reduction of oxygen on the catalyst can be influenced by the particle size and surface structure, and hence treatment can have an effect on ORR activity and stability by altering the surface structure of the catalyst [32]. Various researchers have worked on heat treatment of mono and bimetallic catalysts for ORR [50]. They all concluded that heat treatment improves alloying of the catalysts, which decreases the Pt-Pt distance and hence d-band vacancy of the Pt and thus improves the electroactivity of the catalyst [53]. Jeyabharathi et al. [54] report on improved methanol tolerance of PtSn/C after heat treatment, while the ORR activity remained intact. Sarkar et al. report on PdW synthesised using thermal composition followed by annealing at 800°C with an improved electrocatalytic activity for

atmosphere in the temperature range

Sey is

is not changed, while for Pt,

, Ar or He) or reducing H2

ORR and catalyst durability with an improved methanol tolerance compared to Pt [32].

Transition metals such as macrocycles and chalgogenides have been used as ORR catalysts since the 1960s due to their inactivity towards the oxidation of methanol [55]. Other than noble metals, they are the most-studied electrocatalysts for oxygen reduction. The study of

slightly weaker than Pt and that the difference was their behaviour in the presence or absence

the potential shifts to the negative direction (120–150 mV). A similar behaviour was observed

Re, Mo, etc.) when used as catalysts, methanol oxidation on the cathode was suppressed or avoided leading to a reduced mixed potential. The results confirm that the chalgogenide of Ru is insensitive to methanol, in contrast to the Pt catalyst [56]. The main concern with this approach was the low power output due to low activity of these catalysts for ORR, compared to the Pt catalyst [17]. Cobalt and iron phthalocyanide are the most-studied transition metals as centres for macrocycling rings as catalysts for ORR in fuel cells [56, 57]. These ORR catalysts have shown that a number of metal chelates will chemisorb oxygen [58]. A fuel cell with an iron phthalocyanide cathode can only be stable for up to 10 h [17], but has shown improved activity towards ORR in alkaline media [44]. Zagal et al. [59] report that when Fe chelates with

, a four-electron ORR occurs. Co phthalocyanide has demonstrated similar ORR kinetics as commercial Pt/C, as it also leads to a four-electron process per oxygen molecule, that is, to water, but at lower potentials (0.25 and −0.25 versus RHE) [44, 56]. However, these compounds are not completely stable under strong acid conditions [17, 56]. They decompose via hydrolysis in the electrolyte and attack the macrocyanide via peroxide, causing poor performance and stability [58, 60, 61]. Transition metal chalgogenides are more stable and show an

was embedded in a polymetric matrix, such as polyaniline. RuSM (M = Rh,

and Pt for ORR has shown that the performance of Rux

Sey

**3.3. Transition metal macrocycles and chalgogenides**

34 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

Sey

of methanol. Under these conditions, the electroactivity of Rux

improved electrocatalytic activity at temperatures above 800°C.

the catalyst under inert (N<sup>2</sup>

ruthenium chalgogenides Rux

Sey

when Rux

N4

Another route to stabilising nanoparticles is the development of metal oxide composite supports. Metal oxides such as IrO2 , NiO, CeO<sup>2</sup> , ZrO2 , TiO2 and SnO2 have also been studied as ORR catalysts in basic acidic media [62–66]. Nanoparticles on metal oxides are not able to improve the electrocatalytic activity due to their limited electron conductivity, but are reported to have excellent corrosion resistance in various electrolyte media [66, 67]. Researchers use metal oxides in combination with carbon supports that have desirable properties such as a high surface area and a high electric conductivity. The metal oxides combined with carbon supports are reported to improve the stability and the electrocatalytic activity of the electrode material. Carbon surfaces are functionalised before they are used as supports for catalysts in order to improve their surface properties, but the disadvantage of functionalisation is that it accelerates the degradation process of the support material. The presence of the metal oxide delays the corrosion process. Montero-Ocampo et al. [68] report on PtTiO2 and PtTiO2 /CNT synthesised using metal organic chemical vapour deposition. The PtTiO2 /CNT was more electrocatalytically active compared to PtTiO2 , while good stability was observed for both PtTiO2 and PtTiO2 / CNT that was provided by the TiO<sup>2</sup> support. This was attributed to the high conductivity of CNT compared to TiO<sup>2</sup> , which has limited electron conductivity. Pt/TiO<sup>2</sup> /C showed improvement in activity and thermal stability for ORR compared to Pt/C [69]. Khotseng et al. [70] compared the activity for PtRu/TiO<sup>2</sup> to commercial PtRu/C and Pt/C. They reported a high electroactive surface area and activity of commercial Pt/C and PtRu/C compared to PtRu/TiO<sup>2</sup> towards ORR. When durability studies were performed for the same catalysts, the PtRu/TiO<sup>2</sup> recorded a loss of 29% compared to Pt/C and PtRu/C, which recorded a loss of 64 and 32%, respectively. Li et al. [71] reported an improved oxygen reduction activity, a better durability and a higher methanol tolerance capability in alkaline solution compared to Pt/C.

Most metal oxides were found to be unstable in acidic media. To overcome this instability, conducting polymer polypyrrole (Ppy) was used against the dissolution of metal oxides. During synthesis, the metal oxides were sandwiched between the Ppy layers. Through this research, an improved electrochemical stability of the metal oxides was achieved [18]. Singh et al. [72] report on CoFe2 O4 oxides sandwiched between Ppy layers. A high electrocatalytic activity towards ORR at high cathodic potentials was obtained with stability in acidic media [72].

### **3.5. Novel nanostructures with electrocatalysts**

Although Pt-based catalysts have been widely studied due to their high current density and low over-potential, when used as cathode catalysts, their activity is lowered due to slow reaction kinetics. More research is required to try to improve the catalyst activity. One of the focus areas is looking into loading Pt nanostructures with a high activity on the surface of supporting materials with (1) low cost, (2) good electrical conductivity, (3) strong catalyst-support interaction that is influenced by its surface functionalities to limit the possible deactivation of the electrocatalyst and allow for efficient charge transport, (4) large surface area and (5) good resistance to corrosion to allow high stability [8, 22, 24, 73–75]. Carbon black (CB) is the mostused support for Pt and Pt alloy catalysts. CB is thermochemically unstable and hence suffers from corrosion, leading to performance degradation and durability issues and high potential [66, 76]. Nanostructured carbon materials, for example, mesoporous carbon, CNFs, CNTs and graphene, have been studied extensively as support materials for electrocatalysts, as they have been identified as some of the most promising materials for PEMFCs due to their high chemical stabilities, high electric conductivities and improved mass transport capabilities [32, 77].

CNTs are attractive support materials in fuel cell applications and are by far the most-explored carbon nanostructures as catalyst supports in fuel cells due to their excellent mechanical strength, a high surface area and a high electric conductivity and because they have reported to show an improved catalytic activity [22, 32, 78] compared to CB. The carbon surface is functionalised to provide oxygen-binding groups for the growth of metal catalyst ions [79]. CNTs can be single-walled (SWCNT) or multi-walled (MWCNT), depending on the structure. Both SWCNT and MWCNT have been used as support materials to disperse the electrocatalyst and have been reported to show an enhanced electrocatalytic activity towards ORR. SWCNTs have unique electrical and electronic properties, a wide electrochemical stability and high surface areas [9]. When compared to commercial Pt/C (from ETEK) in acidic media, the SWCNT showed an improved electrocatalytic activity performance, with the negative shift of onset potential by 10 mV compared to Pt/C, whose onset potential moved to a higher potential by 15 mV [15, 73]. Jukk et al. [80] report on Pd/MWCNT having an enhanced electrocatalytic activity compared to Pd/C for ORR. Wang et al. [81] reported on Pt/MWCNT, which showed an improved electrocatalytic activity for ORR compared to Pt/C. Khotseng et al. [70] compared PtRu/MWCNT with commercial Pt/C, PtRu/C and prepared PtRu/TiO<sup>2</sup> and PtRu/MoO<sup>2</sup> . From **Figure 6** and **Table 1**, it is observed that PtRu/MWCNT has the highest mass and specific activity at 0.9 V compared to commercial Pt/C, PtRu/C and prepared PtRu/TiO<sup>2</sup> PtRu/MoO<sup>2</sup> with the highest current density towards ORR.

The two main functionalities are oxygen, namely carboxyl (▬COOH), hydroxyl (▬OH) and carbonyl (▬C=O), and nitrogen groups. Modified CNTs with nitrogen functional groups have been reported to show a much improved electrocatalytic activity towards ORR through forming thermally stable structures during heat treatment [82, 83]. Nitrogen is known to efficiently create defects on carbon materials, which might increase the edge plane exposure and thus improve the ORR activity [29]. Ghosh and Raj [84] report on an improved electrocatalytic activity towards ORR for N-doped CNTs. Wang et al. [85] report on a sponge-like nitrogen containing carbon with a high electrocatalytic ORR activity compared to commercial Pt/C with a considerably higher methanol tolerance. One distinct advantage offered by CNTs is their high resistance towards corrosion compared to CB, and hence they have an enhanced

**Table 1.** Studying the activity of electrocatalysts towards ORR in comparison with commercial Pt/C, PtRu/C and PtRu/TiO<sup>2</sup>

.

37

Oxygen Reduction Reaction

http://dx.doi.org/10.5772/intechopen.79098

**ORR catalytic activity at 0.9 V**

**Catalyst MA (mA mg−1) SA (mA cm−2)**

Pt/C 85.85 0.188 PtRu/C 463 1.66 PtRu/MWCNT 35.6 × 103 111 PtRu/TiO<sup>2</sup> 18.94 1.04 PtRu/MoO<sup>2</sup> 997 17.82

CNFs have been reported to show an improved electrocatalytic activity towards ORR compared to CB [66]. Yang et al. [86] report on an improved electrocatalytic activity for ORR for Pd/CNF. The biggest difference between CNTs and CNFs is their exposure of active edge planes. For CNTs, the basal planes are exposed, while for CNFs, edge planes are

Graphene and graphene oxide (GRO) have also been investigated as another support material for electrocatalysts in fuel cells due to their high electron transfer rate, a large surface area and a high conductivity [64]. When compared to CNTs, they have a higher surface area and a similar electric conductivity for electrochemical applications and can also be produced at a lower cost compared to CNTs [21]. In graphene, both basal and edge planes interact with the electrocatalysts, while for CNTs, only basal planes are exposed [66]. A surface built up only of basal planes is said to have a homogeneous surface, while a surface built up of both basal and edge planes is said to have a heterogeneous surface. Heterogeneous surfaces are reported to better stabilise the metal in a highly dispersed state. It has been reported that Pd/GRO shows a better ORR activity and forms a four-electron oxygen-reduction process compared to Pt [87–89]. N-doped graphene has been reported to show an improved electrocatalytic activity towards ORR compared to graphene in acidic and alkaline media [90]. Lu et al. [91] report a superior electrocatalytic activity of N-GRO compared to GRO. The fast electron transfer rate of graphene can particularly facilitate ORR

electrochemical stability compared to CB [66, 84].

exposed [66].

much quicker in fuel cells [66].

**Figure 6.** ORR polarisation curves of PtRu/MWCNT PtRu/MWCNT compared to Pt/C, PtRu/C PtRu/TiO<sup>2</sup> and PtRu/ MoO2 commercial catalysts in O2 -saturated 0.1 M HClO4 at 20 m V/s and 1600 rpm [70].


**Table 1.** Studying the activity of electrocatalysts towards ORR in comparison with commercial Pt/C, PtRu/C and PtRu/TiO<sup>2</sup> .

The two main functionalities are oxygen, namely carboxyl (▬COOH), hydroxyl (▬OH) and carbonyl (▬C=O), and nitrogen groups. Modified CNTs with nitrogen functional groups have been reported to show a much improved electrocatalytic activity towards ORR through forming thermally stable structures during heat treatment [82, 83]. Nitrogen is known to efficiently create defects on carbon materials, which might increase the edge plane exposure and thus improve the ORR activity [29]. Ghosh and Raj [84] report on an improved electrocatalytic activity towards ORR for N-doped CNTs. Wang et al. [85] report on a sponge-like nitrogen containing carbon with a high electrocatalytic ORR activity compared to commercial Pt/C with a considerably higher methanol tolerance. One distinct advantage offered by CNTs is their high resistance towards corrosion compared to CB, and hence they have an enhanced electrochemical stability compared to CB [66, 84].

CNFs have been reported to show an improved electrocatalytic activity towards ORR compared to CB [66]. Yang et al. [86] report on an improved electrocatalytic activity for ORR for Pd/CNF. The biggest difference between CNTs and CNFs is their exposure of active edge planes. For CNTs, the basal planes are exposed, while for CNFs, edge planes are exposed [66].

Graphene and graphene oxide (GRO) have also been investigated as another support material for electrocatalysts in fuel cells due to their high electron transfer rate, a large surface area and a high conductivity [64]. When compared to CNTs, they have a higher surface area and a similar electric conductivity for electrochemical applications and can also be produced at a lower cost compared to CNTs [21]. In graphene, both basal and edge planes interact with the electrocatalysts, while for CNTs, only basal planes are exposed [66]. A surface built up only of basal planes is said to have a homogeneous surface, while a surface built up of both basal and edge planes is said to have a heterogeneous surface. Heterogeneous surfaces are reported to better stabilise the metal in a highly dispersed state. It has been reported that Pd/GRO shows a better ORR activity and forms a four-electron oxygen-reduction process compared to Pt [87–89]. N-doped graphene has been reported to show an improved electrocatalytic activity towards ORR compared to graphene in acidic and alkaline media [90]. Lu et al. [91] report a superior electrocatalytic activity of N-GRO compared to GRO. The fast electron transfer rate of graphene can particularly facilitate ORR much quicker in fuel cells [66].

**Figure 6.** ORR polarisation curves of PtRu/MWCNT PtRu/MWCNT compared to Pt/C, PtRu/C PtRu/TiO<sup>2</sup>

[66, 76]. Nanostructured carbon materials, for example, mesoporous carbon, CNFs, CNTs and graphene, have been studied extensively as support materials for electrocatalysts, as they have been identified as some of the most promising materials for PEMFCs due to their high chemical stabilities, high electric conductivities and improved mass transport capabilities [32, 77]. CNTs are attractive support materials in fuel cell applications and are by far the most-explored carbon nanostructures as catalyst supports in fuel cells due to their excellent mechanical strength, a high surface area and a high electric conductivity and because they have reported to show an improved catalytic activity [22, 32, 78] compared to CB. The carbon surface is functionalised to provide oxygen-binding groups for the growth of metal catalyst ions [79]. CNTs can be single-walled (SWCNT) or multi-walled (MWCNT), depending on the structure. Both SWCNT and MWCNT have been used as support materials to disperse the electrocatalyst and have been reported to show an enhanced electrocatalytic activity towards ORR. SWCNTs have unique electrical and electronic properties, a wide electrochemical stability and high surface areas [9]. When compared to commercial Pt/C (from ETEK) in acidic media, the SWCNT showed an improved electrocatalytic activity performance, with the negative shift of onset potential by 10 mV compared to Pt/C, whose onset potential moved to a higher potential by 15 mV [15, 73]. Jukk et al. [80] report on Pd/MWCNT having an enhanced electrocatalytic activity compared to Pd/C for ORR. Wang et al. [81] reported on Pt/MWCNT, which showed an improved electrocatalytic activity for ORR compared to Pt/C. Khotseng et al. [70] compared

at 20 m V/s and 1600 rpm [70].


PtRu/MWCNT with commercial Pt/C, PtRu/C and prepared PtRu/TiO<sup>2</sup>

with the highest current density towards ORR.

36 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

activity at 0.9 V compared to commercial Pt/C, PtRu/C and prepared PtRu/TiO<sup>2</sup>

**Figure 6** and **Table 1**, it is observed that PtRu/MWCNT has the highest mass and specific

MoO2

commercial catalysts in O2

and PtRu/

and PtRu/MoO<sup>2</sup>

. From

PtRu/MoO<sup>2</sup>

Other nanostructured carbon supports such as mesoporous carbon, carbon nanocoils and carbon aerogel have also been used as support material for cathode catalysts and have been reported to show an improved ORR electrocatalytic activity [76].

constant k3

electrolyte solution (k5

high current density range:

Dissociative mechanism:

In this mechanism, no H2

tive steps, forming H2

Associative mechanism:

In this mechanism, no H2

O2

O2

The rate expression under Temkin conditions of adsorption is

where *k* is the rate constant and ƞ is the over-potential.

which can be reduced further to form water.

, or be chemically decomposed on the electrode surface (k4

Oxygen reduction on a Pt catalyst in acid media occurs via dissociative adsorption of O2

by the protonation of the adsorbed species, with the former being the rate-determining step [55]. The main steps in the mechanism of ORR are given subsequently. One is known as dissociative mechanism for a low current density range and the other associative mechanism for a

1/2O2 + Pt → PtO (16)

PtO + H+ + e<sup>−</sup> → Pt − OH (17)

Pt − OH + H+ + e<sup>−</sup> → H2 O + Pt (18)

is formed. On the Pt surface, the O2

formed. This mechanism can be considered as the direct four-electron reduction reaction.

O. Because there is no adsorption of O2

bond and forms adsorbed atomic O with further gain of two electrons, in the two consecu-

O2 + 2Pt → Pt2 O2 (19)

Pt2 O2 + H+ + e<sup>−</sup> → Pt2 − O2 H (20)

Pt2 − O2 H → Pt − OH + Pt − O (21)

Pt − OH + Pt − O + H+ + e<sup>−</sup> → Pt − OH + Pt − OH (22)

Pt − OH + Pt − OH + 2H+ + 2e<sup>−</sup> → 2Pt + 2H2 O (23)

O▬O may not be broken down in the following steps, resulting in the formation of H2

therm, that is, the adsorption of reaction intermediates Oads, OHads and HO2ads [98, 99].

Pt shows two Tafel slope regions. At a high potential, low current density (>0.8 V), the electrode surface is a mixture of Pt and PtO with the Tafel slope of 60 mV dec−1 and the reaction order 0.5 with respect to pH in alkaline media. The fractional reaction order was represented in terms of the first electrochemical step as a rate-determining step under the Temkin iso-

*j* = *k* [*O*2] [*H*+] ½ exp [−ƞ*F*/*RT*] = 374 *k* [*O*2] [*OH*<sup>−</sup>] ½ exp [−ƞ*F*/*RT*] (24)

is involved as well. Because there is adsorbed O2

). For ORR in fuel cells, the direct four-electron process is required.

), or be desorbed in the

Oxygen Reduction Reaction

http://dx.doi.org/10.5772/intechopen.79098

adsorption breaks the O▬O

O2

on the surface,

O2 ,

can be

on the surface, no H2

followed

39

Although carbon supports have been reported to show an improved ORR electrocatalytic activity, carbon oxidation or corrosion due to the presence of O2 and/or high electrode potential has been identified as one of the major causes of failure for PEMFC degradation [67]. Noncarbon supports such as electrically conducting polymers, for example, polyaniline, Ppy and mesoporous silica, have also been used as supports to improve the stability of the electrode materials. Shurma and Pollet [66] and Wang et al. [67] report on various non-carbon supports for electrocatalysts for fuel cells. However, it is reported that with non-carbon supports, no major breakthrough has been achieved as yet [66].

#### **3.6. Anion exchange membranes**

ORR is also studied in alkaline media using anion exchange membranes (AEMs) [92]. The significant reason for the change in electrolyte membrane from acid to alkaline is the improved electrokinetics of ORR in alkaline [93]. Pd is emerging as an alternative catalyst compared to Pt in alkaline. It is reported that more ORR catalysts are available for alkaline solutions compared to acidic solutions, due to excessive corrosion in acidic media. Pt/C in basic media is said to enhance ORR towards alcohol [94, 95], while non-Pt catalysts also showed an improved ORR when employed. In addition, in alkaline media, Pt/C is more tolerant to alcohol crossover due to its inactivity in alcohol oxidation reaction. Pd alloys are reported to be comparable or slightly better than Pt/C [4]. Kim et al. reported on PdSn using anion exchange membrane (AEM) which showed an improved ORR electrocatalytic activity with a high methanol tolerance compared to commercial Pt/C tested in proton exchange membrane [42]. He and Cairns [96] report on various electrocatalysts for ORR in AEM.

### **4. Mechanisms for ORR in the presence of an electrocatalyst**

Oxygen reduction on Pt is one of the most extensively studied mechanisms [3]. It involves a multi-electron process with a number of elementary steps, involving different reaction intermediates. The mechanism can be shown schematically as follows [97] (**Figure 7**).

From the mechanism, only two products are observed with ORR on Pt, either H2 O, which can directly form through a four-electron reduction with the rate constant k1 , or adsorbed hydrogen peroxide (H2 O2 , ads), which is through a two-electron process with the rate constant k2 , which can be reduced further by another two-electron process to form water with rate

**Figure 7.** A simplified schematic pathway of oxygen reduction reaction for both acidic and alkaline media [7].

constant k3 , or be chemically decomposed on the electrode surface (k4 ), or be desorbed in the electrolyte solution (k5 ). For ORR in fuel cells, the direct four-electron process is required.

Oxygen reduction on a Pt catalyst in acid media occurs via dissociative adsorption of O2 followed by the protonation of the adsorbed species, with the former being the rate-determining step [55].

The main steps in the mechanism of ORR are given subsequently. One is known as dissociative mechanism for a low current density range and the other associative mechanism for a high current density range:

Dissociative mechanism:

Other nanostructured carbon supports such as mesoporous carbon, carbon nanocoils and carbon aerogel have also been used as support material for cathode catalysts and have been

Although carbon supports have been reported to show an improved ORR electrocatalytic

tial has been identified as one of the major causes of failure for PEMFC degradation [67]. Noncarbon supports such as electrically conducting polymers, for example, polyaniline, Ppy and mesoporous silica, have also been used as supports to improve the stability of the electrode materials. Shurma and Pollet [66] and Wang et al. [67] report on various non-carbon supports for electrocatalysts for fuel cells. However, it is reported that with non-carbon supports, no

ORR is also studied in alkaline media using anion exchange membranes (AEMs) [92]. The significant reason for the change in electrolyte membrane from acid to alkaline is the improved electrokinetics of ORR in alkaline [93]. Pd is emerging as an alternative catalyst compared to Pt in alkaline. It is reported that more ORR catalysts are available for alkaline solutions compared to acidic solutions, due to excessive corrosion in acidic media. Pt/C in basic media is said to enhance ORR towards alcohol [94, 95], while non-Pt catalysts also showed an improved ORR when employed. In addition, in alkaline media, Pt/C is more tolerant to alcohol crossover due to its inactivity in alcohol oxidation reaction. Pd alloys are reported to be comparable or slightly better than Pt/C [4]. Kim et al. reported on PdSn using anion exchange membrane (AEM) which showed an improved ORR electrocatalytic activity with a high methanol tolerance compared to commercial Pt/C tested in proton exchange membrane

Oxygen reduction on Pt is one of the most extensively studied mechanisms [3]. It involves a multi-electron process with a number of elementary steps, involving different reaction inter-

, which can be reduced further by another two-electron process to form water with rate

, ads), which is through a two-electron process with the rate constant

[42]. He and Cairns [96] report on various electrocatalysts for ORR in AEM.

**4. Mechanisms for ORR in the presence of an electrocatalyst**

mediates. The mechanism can be shown schematically as follows [97] (**Figure 7**).

can directly form through a four-electron reduction with the rate constant k1

From the mechanism, only two products are observed with ORR on Pt, either H2

**Figure 7.** A simplified schematic pathway of oxygen reduction reaction for both acidic and alkaline media [7].

and/or high electrode poten-

O, which

, or adsorbed

reported to show an improved ORR electrocatalytic activity [76].

38 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

activity, carbon oxidation or corrosion due to the presence of O2

major breakthrough has been achieved as yet [66].

**3.6. Anion exchange membranes**

hydrogen peroxide (H2

k2

O2

$$\text{1/2O}\_2 + \text{Pt} \rightarrow \text{PtO} \tag{16}$$

$$\text{PtO} + \text{H}^+ + \text{e}^- \rightarrow \text{Pt}-\text{OH} \tag{17}$$

$$\text{Pt}-\text{OH} + \text{H}^\* + \text{e}^- \rightarrow \text{H}\_2\text{O} + \text{Pt} \tag{18}$$

In this mechanism, no H2 O2 is formed. On the Pt surface, the O2 adsorption breaks the O▬O bond and forms adsorbed atomic O with further gain of two electrons, in the two consecutive steps, forming H2 O. Because there is no adsorption of O2 on the surface, no H2 O2 can be formed. This mechanism can be considered as the direct four-electron reduction reaction.

Associative mechanism:

$$\text{O}\_2 + 2\text{Pt} \rightarrow \text{Pt}\_2\text{O}\_2 \tag{19}$$

$$\text{Pt}\_2\text{O}\_2 + \text{H}^\* + \text{e}^- \rightarrow \text{Pt}\_2 - \text{O}\_2\text{H} \tag{20}$$

$$\text{Pt}\_2\text{--O}\_2\text{H} \rightarrow \text{Pt}-\text{OH} + \text{Pt}-\text{O} \tag{21}$$

$$\text{Pt}-\text{OH} + \text{Pt}-\text{O} + \text{H}^+ + \text{e}^- \rightarrow \text{Pt}-\text{OH} + \text{Pt}-\text{OH} \tag{22}$$

$$\text{Pt}-\text{OH} + \text{Pt}-\text{OH} + 2\text{H}^+ + 2\text{e}^- \rightarrow 2\text{Pt} + 2\text{H}\_2\text{O} \tag{23}$$

In this mechanism, no H2 O2 is involved as well. Because there is adsorbed O2 on the surface, O▬O may not be broken down in the following steps, resulting in the formation of H2 O2 , which can be reduced further to form water.

Pt shows two Tafel slope regions. At a high potential, low current density (>0.8 V), the electrode surface is a mixture of Pt and PtO with the Tafel slope of 60 mV dec−1 and the reaction order 0.5 with respect to pH in alkaline media. The fractional reaction order was represented in terms of the first electrochemical step as a rate-determining step under the Temkin isotherm, that is, the adsorption of reaction intermediates Oads, OHads and HO2ads [98, 99].

The rate expression under Temkin conditions of adsorption is

$$j = k \left[ O\_2 \right] \left[ H^\* \right] \uplus \exp \left[ -\eta F/RT \right] = 374 \ k \left[ O\_2 \right] \left[ OH^\* \right] \uplus \exp \left[ -\eta F/RT \right] \tag{24}$$

where *k* is the rate constant and ƞ is the over-potential.

At a low potential, high current density (<0.8 V), the electrode surface is a pure Pt with the Tafel slope of 120 mV dec−1 and the reaction order 0 with respect to pH in alkaline media, with H2 O as the reacting species. The adsorption of intermediate species to a Langmuir isotherm under Temkin conditions no longer holds.

The rate expression under Langmuir conditions is

$$j = k \left[ O\_2 \right] \exp \left[ -\beta \,\text{qF/RT} \right] \tag{25}$$

where *β* is the symmetry factor.

The reaction is of the first order with respect to O<sup>2</sup> in solutions. It was found that the H2 O2 formed was greater in an alkaline solution than in an acidic one [4, 94]. In alkaline solutions, about 80% of the reduction current is through the direct reduction and the other current forms H2 O2 , which leads to a complicated mechanism.

Various models representing the adsorbed states of oxygen are represented in **Figure 8**.

**Figure 8(a)** is known as the bridge model. It is a 2:2 complex of metal oxygen where the bonding arises from the interaction between the d-orbital on the metal with a Π\* and Π orbital combination on O2 [101]. This gives rise to a singlet or a triplet nature of di-oxygen orbitals

and determines the bridge or a transmode of interaction of di-oxygen with the metal [101]. **Figure 8(b)** is known as the Griffiths model. It is a 2:1 metal-di-oxygen complex structure, which involves a side on the interaction of oxygen with metal. This type of bonding can be viewed as rising from two contributions: (1) σ-type bonding is formed by overlapping

[102]. **Figure 8(c)** is known as the Pauling model. It is a 1:1 metal-oxygen complex structure,

the metal. The metal's two d-orbitals, namely dxz and dyz, then interact with the Π\* orbitals

Griffiths and Pauling models are the preferred models due to the donating abilities of the

ORR has a huge role to play in fuel cell development. Comparing the ORR electrocatalytic activity of Pt with other single metals, Pt shows the most improved electrocatalytic activity, but its large-scale applications are limited by its high cost and scarcity. The addition of a second metal to a metal electrocatalyst decreases its particle size (large surface area), which leads to an increased lattice strain and hence an increased electrocatalytic activity [28]. It has been concluded that multi-component catalysts improve ORR activity, although it is not conclusive on which multi-catalyst shows the most improved ORR activity, as various researchers report on different multi-component catalysts as the most improved. Although improvement has been obtained, the Pt loading required is still too high to produce PEMFCs at commercially viable prices [22]. The transition metal phthalocyanine is offering reasonable performance as an ORR catalyst, although it suffers from lack of long-

The challenge remaining is optimising the synthesis method in order to control the shape and the surface structure of especially non-spherical electrocatalysts, which are reported to show the most improved ORR electrocatalytic activity with most stable electrocatalysts, while they con-

Catalyst support is one of the critical components in improving the electrocatalytic activity of PEMFCs, as they are responsible for parameters that govern the performance of the fuel cell, that is, particle size, catalyst dispersion and stability [8]. Comparison of the carbon supports, CNTs and graphene supports provides considerable advantages concerning mass and charge transport. The disadvantage of using these supports is the costs [25]. In addition, the deposition, distribution and crystallite size of metal nanoparticles are affected by the synthesis

Using an alkaline medium, Pt-free nanoparticles can be used as electrocatalysts for fuel cells, with a reduced alcohol crossover, an improved ORR kinetics and limited risks of corrosion.

tribute to the lowering of Pt usage and hence cost reduction of the PEMFC.

method and oxidation treatment of carbon supports.

orbitals on the metal; (2) Π back-bond interaction

with metal. In this model, the σ bond is formed by the

arises

41

orbital on

molecule. The

2

Oxygen Reduction Reaction

http://dx.doi.org/10.5772/intechopen.79098

2

between the metal d Π orbitals and partially occupied Π\* antibonding orbital on O<sup>2</sup>

donation of electron density from the σ-rich orbital of di-oxygen to the acceptor d<sup>z</sup>

of di-oxygen, with the corresponding charge transfer from the metal to the O2

filled Π and σ orbitals of the di-oxygen molecule, respectively [103].

between the Π orbitals of oxygen and the d<sup>z</sup>

which is an end-on interaction of O2

**5. Conclusion**

term stability.

**Figure 8.** Oxygen reduction on Pt from the (a) bridge model, (b) Griffiths model and (c) Pauling model [100].

and determines the bridge or a transmode of interaction of di-oxygen with the metal [101]. **Figure 8(b)** is known as the Griffiths model. It is a 2:1 metal-di-oxygen complex structure, which involves a side on the interaction of oxygen with metal. This type of bonding can be viewed as rising from two contributions: (1) σ-type bonding is formed by overlapping between the Π orbitals of oxygen and the d<sup>z</sup> 2 orbitals on the metal; (2) Π back-bond interaction between the metal d Π orbitals and partially occupied Π\* antibonding orbital on O<sup>2</sup> arises [102]. **Figure 8(c)** is known as the Pauling model. It is a 1:1 metal-oxygen complex structure, which is an end-on interaction of O2 with metal. In this model, the σ bond is formed by the donation of electron density from the σ-rich orbital of di-oxygen to the acceptor d<sup>z</sup> 2 orbital on the metal. The metal's two d-orbitals, namely dxz and dyz, then interact with the Π\* orbitals of di-oxygen, with the corresponding charge transfer from the metal to the O2 molecule. The Griffiths and Pauling models are the preferred models due to the donating abilities of the filled Π and σ orbitals of the di-oxygen molecule, respectively [103].

### **5. Conclusion**

At a low potential, high current density (<0.8 V), the electrode surface is a pure Pt with the Tafel slope of 120 mV dec−1 and the reaction order 0 with respect to pH in alkaline media, with

*j* = *k* [*O*2] exp [−*β* ƞ*F*/*RT*] (25)

formed was greater in an alkaline solution than in an acidic one [4, 94]. In alkaline solutions, about 80% of the reduction current is through the direct reduction and the other current forms

**Figure 8(a)** is known as the bridge model. It is a 2:2 complex of metal oxygen where the bonding arises from the interaction between the d-orbital on the metal with a Π\* and Π orbital

[101]. This gives rise to a singlet or a triplet nature of di-oxygen orbitals

Various models representing the adsorbed states of oxygen are represented in **Figure 8**.

**Figure 8.** Oxygen reduction on Pt from the (a) bridge model, (b) Griffiths model and (c) Pauling model [100].

in solutions. It was found that the H2

O2

O as the reacting species. The adsorption of intermediate species to a Langmuir isotherm

H2

H2 O2

combination on O2

under Temkin conditions no longer holds.

where *β* is the symmetry factor.

The rate expression under Langmuir conditions is

40 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

The reaction is of the first order with respect to O<sup>2</sup>

, which leads to a complicated mechanism.

ORR has a huge role to play in fuel cell development. Comparing the ORR electrocatalytic activity of Pt with other single metals, Pt shows the most improved electrocatalytic activity, but its large-scale applications are limited by its high cost and scarcity. The addition of a second metal to a metal electrocatalyst decreases its particle size (large surface area), which leads to an increased lattice strain and hence an increased electrocatalytic activity [28]. It has been concluded that multi-component catalysts improve ORR activity, although it is not conclusive on which multi-catalyst shows the most improved ORR activity, as various researchers report on different multi-component catalysts as the most improved. Although improvement has been obtained, the Pt loading required is still too high to produce PEMFCs at commercially viable prices [22]. The transition metal phthalocyanine is offering reasonable performance as an ORR catalyst, although it suffers from lack of longterm stability.

The challenge remaining is optimising the synthesis method in order to control the shape and the surface structure of especially non-spherical electrocatalysts, which are reported to show the most improved ORR electrocatalytic activity with most stable electrocatalysts, while they contribute to the lowering of Pt usage and hence cost reduction of the PEMFC.

Catalyst support is one of the critical components in improving the electrocatalytic activity of PEMFCs, as they are responsible for parameters that govern the performance of the fuel cell, that is, particle size, catalyst dispersion and stability [8]. Comparison of the carbon supports, CNTs and graphene supports provides considerable advantages concerning mass and charge transport. The disadvantage of using these supports is the costs [25]. In addition, the deposition, distribution and crystallite size of metal nanoparticles are affected by the synthesis method and oxidation treatment of carbon supports.

Using an alkaline medium, Pt-free nanoparticles can be used as electrocatalysts for fuel cells, with a reduced alcohol crossover, an improved ORR kinetics and limited risks of corrosion. Recently, research has been focused on using AEMs. The main advantage of using AEM fuel cells over PEMFCs is that it allows for the use of less expensive, Pt-free electrocatalysts. AEM fuel cells promise to solve the cost barriers of PEMFCs.

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### **Acknowledgements**

I thank ESKOM (TESP), NRF (THUTHUKA), Ithemba Labs, Physics Department (UWC), Chemical Engineering Department (UCT) and Chemistry Department (UWC).

### **Conflict of interest**

I have no conflict of interest to declare.

### **Nomenclature**


### **Author details**

Lindiwe Khotseng Address all correspondence to: lkhotseng@uwc.ac.za Department of Chemistry, University of the Western Cape, South Africa

### **References**

Recently, research has been focused on using AEMs. The main advantage of using AEM fuel cells over PEMFCs is that it allows for the use of less expensive, Pt-free electrocatalysts. AEM

I thank ESKOM (TESP), NRF (THUTHUKA), Ithemba Labs, Physics Department (UWC),

Chemical Engineering Department (UCT) and Chemistry Department (UWC).

fuel cells promise to solve the cost barriers of PEMFCs.

42 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

**Acknowledgements**

**Conflict of interest**

**Nomenclature**

I have no conflict of interest to declare.

AEM anion exchange membrane

MWCNT multi-walled carbon nanotube

RHE reversible hydrogen electrode SWCNT single-walled carbon nanotube

Address all correspondence to: lkhotseng@uwc.ac.za

Department of Chemistry, University of the Western Cape, South Africa

PEMFC polymer electrolyte membrane fuel cell

NHE normal hydrogen electrode ORR oxygen reduction reaction

CB carbon black

Ppy polypyrrole

**Author details**

Lindiwe Khotseng

CNF carbon nanofibre CNT carbon nanotube GRO graphene oxide


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**Chapter 3**

**Provisional chapter**

**Active Sites Derived from Heteroatom Doping in**

**Active Sites Derived from Heteroatom Doping in** 

DOI: 10.5772/intechopen.77048

The oxygen reduction reaction (ORR) is a key cathode reaction in fuel cells. Due to the sluggish kinetics of the ORR, various kinds of catalysts have been developed to compensate for the shortcomings of the cathode reaction. Carbon materials are considered ideal cathode catalysts. In particular, heteroatom doping is essential to achieve an excellent ORR activity. Interestingly, doping trace amounts of metals in carbon materials plays an important role in enhancing the electrocatalytic activities. This chapter describes the recent advancements with regard to heteroatom-doped carbons and discusses the active sites decorated in the carbon matrix in terms of their configurations and contents, as well as their effectiveness in boosting the ORR performance. Furthermore, trace metal

Owing to the limited supply of fossil fuels and increasing number of environmental crises, sustainable energy conversion and storage devices such as fuel cells and metal-air batteries have attracted significant attention [1]. The oxygen reduction reaction (ORR) is a key cathode reaction in such systems [2–4]. The sluggish kinetics of the ORR at the cathode owing to a higher overpotential than in the anode limits the wide commercialization of these devices. Currently, only the state-of-the-art platinum/carbon black catalyst (Pt/C) has been widely used in practical applications of proton exchange membrane (PEM) fuel cells [5]. Unfortunately, the scarcity and high cost of Pt and the poor durability limits the wide commercialization of

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Carbon Materials for Oxygen Reduction Reaction**

**Carbon Materials for Oxygen Reduction Reaction**

Winston Duo Wu and Dan Xu

Winston Duo Wu and Dan Xu

http://dx.doi.org/10.5772/intechopen.77048

**Abstract**

heteroatom doping

**1. Introduction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

residues and metal-free catalysts for the ORR are clarified.

**Keywords:** oxygen reduction reaction, trace metal residues, active sites,


#### **Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction**

DOI: 10.5772/intechopen.77048

Winston Duo Wu and Dan Xu Winston Duo Wu and Dan Xu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77048

#### **Abstract**

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50 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

The oxygen reduction reaction (ORR) is a key cathode reaction in fuel cells. Due to the sluggish kinetics of the ORR, various kinds of catalysts have been developed to compensate for the shortcomings of the cathode reaction. Carbon materials are considered ideal cathode catalysts. In particular, heteroatom doping is essential to achieve an excellent ORR activity. Interestingly, doping trace amounts of metals in carbon materials plays an important role in enhancing the electrocatalytic activities. This chapter describes the recent advancements with regard to heteroatom-doped carbons and discusses the active sites decorated in the carbon matrix in terms of their configurations and contents, as well as their effectiveness in boosting the ORR performance. Furthermore, trace metal residues and metal-free catalysts for the ORR are clarified.

**Keywords:** oxygen reduction reaction, trace metal residues, active sites, heteroatom doping

### **1. Introduction**

Owing to the limited supply of fossil fuels and increasing number of environmental crises, sustainable energy conversion and storage devices such as fuel cells and metal-air batteries have attracted significant attention [1]. The oxygen reduction reaction (ORR) is a key cathode reaction in such systems [2–4]. The sluggish kinetics of the ORR at the cathode owing to a higher overpotential than in the anode limits the wide commercialization of these devices. Currently, only the state-of-the-art platinum/carbon black catalyst (Pt/C) has been widely used in practical applications of proton exchange membrane (PEM) fuel cells [5]. Unfortunately, the scarcity and high cost of Pt and the poor durability limits the wide commercialization of

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

these devices. Therefore, the development of low-cost and highly efficient catalysts for the ORR has become a "hot topic".

oxygen intermediates and modify the charge/spin distribution of the catalyst. Although, there is strong controversy about the role of active sites and mechanisms, the heteroatoms, even the

Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction

Decreasing the loading amount of Pt and Pt-based materials to enhance the performances of fuel cells is a wise choice. A series of Pt-based catalysts were prepared. For example, Xia et al. reported the synthesis of Pt─Ag alloy nanocages (**Figure 1**) [6]. Due to the ligand effects from

specific activity toward oxygen reduction compared with that of the state-of-the-art commercial Pt/C catalyst. Furthermore, hollow Pt─M (M = Ni, Co) nanoparticle-decorated graphene was designed as an electrocatalyst for the ORR. Due to the hollow interior, the amount of buried nonfunctional precious metal atoms decreased and hence enhanced the electrocatalytic activity and durability toward the ORR [7]. Based on the same mechanism, a hollow structure of a Pt catalyst was also reported by Li and co-workers. The obtained icosahedral Pt-enriched nanocages demonstrated a superior ORR activity [24]. In addition, Pt nanoparticles stabilized by a graphitic step-edge and combined with the effect of nanoscale confinement showed high electrochemical stability outperforming that of a commercial Pt/C [25]. Interestingly, Adzic and co-workers used titanium nickel binary nitride as a support and then several layers of Pt atoms were deposited on the robust support. The obtained catalyst exhibited high mass activity and specific activity compared with the commercial Pt/C catalyst; this result was mainly due to the synergistic effect of Ni doping and the strong interaction between the Pt

ORR catalyst and demonstrated that the Pt─O interactions improved the ORR activity [26]. Ultrathin Rh-doped Pt nanowires synthesized by Zeng and co-workers achieved remarkable activity and durability toward the ORR due to the doping of the Rh atoms and high utiliza-

All in all, despite numerous progresses made in exploring novel type of ORR catalyst, the state-of-the-art Pt/C catalysts still dominate. Pt-based multimetallic catalysts with various

**Figure 1.** Schematic illustration depicting the major steps involved in the formation of Pt─Ag nanocages with high ORR activity: (a, b) alloying of Ag with Pt to generate Pt─Ag nanocages during the galvanic replacement reaction between Ag

and a Pt(II) precursor and (c) dealloying of Ag from Pt during the accelerated durability test [6].

layer and the support [10]. Mukerjee and co-workers designed a Pt/NbOx

tion efficiency of the Pt atoms [27]. Furthermore, Pt<sup>3</sup>

activity and stability toward the ORR [8].

transition state can be stabilized, demonstrating a high

http://dx.doi.org/10.5772/intechopen.77048

53

/C system as an

Ni nanowires also showed extraordinary

edges and defects that can function as active sites for the ORR.

**2.1. Pt-based active sites**

the electronegative Ag atoms, the O2

Earlier studies have focused on tuning the surface structure [6–8] and electronic structure [9] of Pt as well as the electrocatalyst supports [10, 11] to achieve optimum ORR activity by using the least amount of Pt. In addition, various kinds of catalysts, including transition metals and their alloys, transition metal oxides/nitrides/sulfides, as well as mixed-valence metal oxides [12, 13], carbon-based metal-free catalysts [14], and others have been developed to promote the sluggish kinetics of the ORR at the cathode. Among the catalysts studied, heteroatomdoped carbon materials are considered ideal cathode catalysts due to the high surface area, good electrical conductivity, and densely distributed active sites. Presently, carbon black (CB) [15], graphene [16, 17], carbon nanotubes (CNTs) [17, 18], and porous carbon [19] are used as support materials in carbon-based electrocatalysts. Although topological defects contribute to the intrinsic activity of nanocarbon catalysts, various kinds of active sites have been created in nanocarbon catalysts to further enhance their activity [20, 21]. For example, transition metals such as Fe, Co, Ni, Cu, Zn, and Mn display fairly strong adsorption toward oxygen in the ORR, which can enhance the ORR efficiency and performance [22, 23]. Furthermore, metalfree catalysts have also become an intriguing research area. Non-metal heteroatoms, such as N, S, B, and P, in carbon materials can serve as active sites for the ORR. The heteroatoms can alter the electronic distribution of the carbon framework and effectively increase the defects in the carbon structure, contributing to O2 adsorption and O─O bond breaking during the ORR catalytic reaction and concurrently promoting the catalytic performance of the carbon materials. Although the individual roles of different atoms and their synergistic effects in facilitating the ORR activity are still under debate, numerous achievements have been made toward the rational design and synthesis of carbon materials with a high surface area, high electrical conductivity, and multiple heteroatom doping to achieve extraordinary ORR activities.

In fact, the ORR activity and durability of heteroatom-doped carbon even outperform those of commercially available Pt-based catalysts. Here, we aim to assemble a review of the significant scientific progress in the design and synthesis of carbon-based electrocatalysts. We discuss the activity of different doping sites to provide an understanding of the mode of heteroatom doping and the role of heteroatoms in ORR, especially their content effects in ORR.

### **2. Active sites in the ORR**

The ORR proceeds through two pathways—partial reduction and full reduction—and involves a two-electron pathway and a four-electron pathway, respectively. The four-electron route is highly preferred due to its high efficiency in fuel cell technology. The transformation of O2 to OOH\* is the first step in this route and is also a rate-determining step. The highly active catalytic centers must be favorable to enhance the binding energy between the oxygen intermediates and the catalyst surface but weak enough that the oxygen intermediates dissociate from the catalyst surface to prevent poisoning the catalyst. Therefore, goals for designing an ORR catalyst with high efficiency are to tune the adsorption energies of the oxygen intermediates and modify the charge/spin distribution of the catalyst. Although, there is strong controversy about the role of active sites and mechanisms, the heteroatoms, even the edges and defects that can function as active sites for the ORR.

### **2.1. Pt-based active sites**

these devices. Therefore, the development of low-cost and highly efficient catalysts for the

Earlier studies have focused on tuning the surface structure [6–8] and electronic structure [9] of Pt as well as the electrocatalyst supports [10, 11] to achieve optimum ORR activity by using the least amount of Pt. In addition, various kinds of catalysts, including transition metals and their alloys, transition metal oxides/nitrides/sulfides, as well as mixed-valence metal oxides [12, 13], carbon-based metal-free catalysts [14], and others have been developed to promote the sluggish kinetics of the ORR at the cathode. Among the catalysts studied, heteroatomdoped carbon materials are considered ideal cathode catalysts due to the high surface area, good electrical conductivity, and densely distributed active sites. Presently, carbon black (CB) [15], graphene [16, 17], carbon nanotubes (CNTs) [17, 18], and porous carbon [19] are used as support materials in carbon-based electrocatalysts. Although topological defects contribute to the intrinsic activity of nanocarbon catalysts, various kinds of active sites have been created in nanocarbon catalysts to further enhance their activity [20, 21]. For example, transition metals such as Fe, Co, Ni, Cu, Zn, and Mn display fairly strong adsorption toward oxygen in the ORR, which can enhance the ORR efficiency and performance [22, 23]. Furthermore, metalfree catalysts have also become an intriguing research area. Non-metal heteroatoms, such as N, S, B, and P, in carbon materials can serve as active sites for the ORR. The heteroatoms can alter the electronic distribution of the carbon framework and effectively increase the defects in

catalytic reaction and concurrently promoting the catalytic performance of the carbon materials. Although the individual roles of different atoms and their synergistic effects in facilitating the ORR activity are still under debate, numerous achievements have been made toward the rational design and synthesis of carbon materials with a high surface area, high electrical conductivity, and multiple heteroatom doping to achieve extraordinary ORR activities.

In fact, the ORR activity and durability of heteroatom-doped carbon even outperform those of commercially available Pt-based catalysts. Here, we aim to assemble a review of the significant scientific progress in the design and synthesis of carbon-based electrocatalysts. We discuss the activity of different doping sites to provide an understanding of the mode of heteroatom doping and the role of heteroatoms in ORR, especially their content effects in ORR.

The ORR proceeds through two pathways—partial reduction and full reduction—and involves a two-electron pathway and a four-electron pathway, respectively. The four-electron route is highly preferred due to its high efficiency in fuel cell technology. The transformation

 to OOH\* is the first step in this route and is also a rate-determining step. The highly active catalytic centers must be favorable to enhance the binding energy between the oxygen intermediates and the catalyst surface but weak enough that the oxygen intermediates dissociate from the catalyst surface to prevent poisoning the catalyst. Therefore, goals for designing an ORR catalyst with high efficiency are to tune the adsorption energies of the

adsorption and O─O bond breaking during the ORR

ORR has become a "hot topic".

52 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

the carbon structure, contributing to O2

**2. Active sites in the ORR**

of O2

Decreasing the loading amount of Pt and Pt-based materials to enhance the performances of fuel cells is a wise choice. A series of Pt-based catalysts were prepared. For example, Xia et al. reported the synthesis of Pt─Ag alloy nanocages (**Figure 1**) [6]. Due to the ligand effects from the electronegative Ag atoms, the O2 transition state can be stabilized, demonstrating a high specific activity toward oxygen reduction compared with that of the state-of-the-art commercial Pt/C catalyst. Furthermore, hollow Pt─M (M = Ni, Co) nanoparticle-decorated graphene was designed as an electrocatalyst for the ORR. Due to the hollow interior, the amount of buried nonfunctional precious metal atoms decreased and hence enhanced the electrocatalytic activity and durability toward the ORR [7]. Based on the same mechanism, a hollow structure of a Pt catalyst was also reported by Li and co-workers. The obtained icosahedral Pt-enriched nanocages demonstrated a superior ORR activity [24]. In addition, Pt nanoparticles stabilized by a graphitic step-edge and combined with the effect of nanoscale confinement showed high electrochemical stability outperforming that of a commercial Pt/C [25]. Interestingly, Adzic and co-workers used titanium nickel binary nitride as a support and then several layers of Pt atoms were deposited on the robust support. The obtained catalyst exhibited high mass activity and specific activity compared with the commercial Pt/C catalyst; this result was mainly due to the synergistic effect of Ni doping and the strong interaction between the Pt layer and the support [10]. Mukerjee and co-workers designed a Pt/NbOx /C system as an ORR catalyst and demonstrated that the Pt─O interactions improved the ORR activity [26]. Ultrathin Rh-doped Pt nanowires synthesized by Zeng and co-workers achieved remarkable activity and durability toward the ORR due to the doping of the Rh atoms and high utilization efficiency of the Pt atoms [27]. Furthermore, Pt<sup>3</sup> Ni nanowires also showed extraordinary activity and stability toward the ORR [8].

All in all, despite numerous progresses made in exploring novel type of ORR catalyst, the state-of-the-art Pt/C catalysts still dominate. Pt-based multimetallic catalysts with various

**Figure 1.** Schematic illustration depicting the major steps involved in the formation of Pt─Ag nanocages with high ORR activity: (a, b) alloying of Ag with Pt to generate Pt─Ag nanocages during the galvanic replacement reaction between Ag and a Pt(II) precursor and (c) dealloying of Ag from Pt during the accelerated durability test [6].

finely tuned morphologies will represent a promising research area of ORR catalyst, due to the scarcity of Pt and the unsatisfactory long-term stability.

Other earth-abundant non-noble metal-based electrocatalysts are also emerging as a new generation of low-cost and high-performance alternatives [37–44]. For example, the ORR electrocatalyst properties of Cu, N-co-doped hierarchical porous carbon with a copper content of

Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction

an N-decorated graphene hybrid material, was prepared by Wu and co-workers. Abundant Co─N─C active sites and high electron transfer capacity made this hybrid active in the ORR in

graphene derived from waste paper (**Figure 3**) [40]. In this work, the cobalt(II)acetate-1,10-phenanthroline complex was selected as a precursor of both the catalyst and etcher; the control of the feed ratio of the complex, as well as the pyrolysis temperature and chemical compositions

O4

porous structure of the matrix. Lee and co-workers designed a nickel-containing nanoreactor (Ni@N-CNCs) [42]. Ni particles were encapsulated in N-decorated carbon nanocapsules. X-ray photoelectron spectroscopy (XPS) analysis showed the formation of N─C and Ni─N bonds in the nanoreactor and these two types of active sites significantly improved the catalytic activity. In addition, a nanoconfined effect improved the reaction rate. The electron transfer number of Ni@N-CNCs reached nearly 4.0 comparable to that of Pt/C. Successively, a manganese corrole

Additionally, bimetallic catalysts are promising candidates for the ORR. You and co-workers

C/tungsten carbide/graphitic carbon in which the Fe<sup>3</sup>

**Figure 3.** Schematic procedure for the preparation of functional porous graphene (Co3

(MCM stands for microporous carbonaceous material and PG stands for porous graphene) [40].

/Co) of the active sites could be finely tuned. This cost-efficient and green catalyst exhibited an efficient ORR activity with a performance comparable to that of a Pt/C catalyst due to

/C catalysts [44] were explored as efficient ORR catalysts.

nanoparticles@B,

55

/Co-decorated porous

C acted as the active

O4

http://dx.doi.org/10.5772/intechopen.77048

and graphene combined with the hierarchical

O4

/Co@PG) from waste paper

2.67 wt% were almost equal to those of a commercial Pt/C catalyst [37]. CoOx

an alkaline medium [39]. Recently, Deng and co-workers prepared Co3

the synergistic catalytic effects between Co<sup>3</sup>

complex [43] and ZnN*<sup>x</sup>*

synthesized Fe<sup>3</sup>

(Co3 O4

### **2.2. Non-noble metal-based active sites**

The search for Pt-free alternative catalysts with excellent ORR performance has attracted much attention. Accordingly, extensive efforts have been directed toward the design and synthesis of nonprecious metal-based catalysts for potential applications in fuel cells. Particularly, the co-doping of transition metals and non-metal heteroatoms have also been extensively studied due to their combined advantages. Fe─N─C catalysts have been broadly studied. Atomically dispersed Fe [28], Fe─Nx [29, 30], Fe<sup>3</sup> C [31, 32], and Fe<sup>3</sup> O4 [33] are all considered ORR active sites. Dong and co-workers demonstrated that atomically dispersed Fe (1–2 wt%) played a pivotal role in promoting the ORR performance [28]. Joo et al. prepared a catalyst consisting of Fe─Nx and Fe─Fe<sup>3</sup> C@C species (Fe content of 6.3 wt%) [31]. The experimental results indicated that the Fe─Nx sites played a dominant role in promoting the ORR via a 4-electron pathway, whereas the Fe─Fe<sup>3</sup> C@C sites played an auxiliary role. The authors also prepared a CNT/porphyrinic carbon (PC) catalyst with densely distributed active Fe─Nx sites (**Figure 2**). The Fe contents of the CNT/PC were 2.9 wt%. This catalyst showed very high ORR activity in both alkaline and acidic media [32]. Xia and co-workers prepared Fe<sup>3</sup> O4 nanoparticles encapsulated in hollow core-shell structured N-doped carbon spheres. The obtained catalyst exhibited an excellent catalytic performance toward the ORR [33].

In addition to Fe─N─C catalysts, S-doped Fe─N─C catalysts have also been systematically studied. For example, Wang and co-workers found that iron sulfides/nitrogen and sulfur dual-doped mesoporous graphitic carbon spheres demonstrated excellent electrocatalytic activities toward the ORR in alkaline and acidic media. The remarkable catalytic performance was ascribed to the iron sulfide nanocrystals with an iron content of 5.9 wt% [34]. Similarly, multi-source-derived S─Fe/N/C with the atom contents of Fe 1.38% has been reported by Wang and co-workers. Five types of Fe were detected: Fe, FeS, FeN, FeC, and Fe<sup>3</sup> O4 . Although the nature of the active sites was uncertain for the Fe/N/C, the S─Fe/N/C catalyst showed a highly efficient ORR activity [35, 36].

**Figure 2.** Synthetic scheme for the preparation of CNT/PC catalysts [32].

Other earth-abundant non-noble metal-based electrocatalysts are also emerging as a new generation of low-cost and high-performance alternatives [37–44]. For example, the ORR electrocatalyst properties of Cu, N-co-doped hierarchical porous carbon with a copper content of 2.67 wt% were almost equal to those of a commercial Pt/C catalyst [37]. CoOx nanoparticles@B, an N-decorated graphene hybrid material, was prepared by Wu and co-workers. Abundant Co─N─C active sites and high electron transfer capacity made this hybrid active in the ORR in an alkaline medium [39]. Recently, Deng and co-workers prepared Co3 O4 /Co-decorated porous graphene derived from waste paper (**Figure 3**) [40]. In this work, the cobalt(II)acetate-1,10-phenanthroline complex was selected as a precursor of both the catalyst and etcher; the control of the feed ratio of the complex, as well as the pyrolysis temperature and chemical compositions (Co3 O4 /Co) of the active sites could be finely tuned. This cost-efficient and green catalyst exhibited an efficient ORR activity with a performance comparable to that of a Pt/C catalyst due to the synergistic catalytic effects between Co<sup>3</sup> O4 and graphene combined with the hierarchical porous structure of the matrix. Lee and co-workers designed a nickel-containing nanoreactor (Ni@N-CNCs) [42]. Ni particles were encapsulated in N-decorated carbon nanocapsules. X-ray photoelectron spectroscopy (XPS) analysis showed the formation of N─C and Ni─N bonds in the nanoreactor and these two types of active sites significantly improved the catalytic activity. In addition, a nanoconfined effect improved the reaction rate. The electron transfer number of Ni@N-CNCs reached nearly 4.0 comparable to that of Pt/C. Successively, a manganese corrole complex [43] and ZnN*<sup>x</sup>* /C catalysts [44] were explored as efficient ORR catalysts.

finely tuned morphologies will represent a promising research area of ORR catalyst, due to

The search for Pt-free alternative catalysts with excellent ORR performance has attracted much attention. Accordingly, extensive efforts have been directed toward the design and synthesis of nonprecious metal-based catalysts for potential applications in fuel cells. Particularly, the co-doping of transition metals and non-metal heteroatoms have also been extensively studied due to their combined advantages. Fe─N─C catalysts have been broadly studied. Atomically

C [31, 32], and Fe<sup>3</sup>

sites. Dong and co-workers demonstrated that atomically dispersed Fe (1–2 wt%) played a pivotal role in promoting the ORR performance [28]. Joo et al. prepared a catalyst consist-

The Fe contents of the CNT/PC were 2.9 wt%. This catalyst showed very high ORR activ-

encapsulated in hollow core-shell structured N-doped carbon spheres. The obtained catalyst

In addition to Fe─N─C catalysts, S-doped Fe─N─C catalysts have also been systematically studied. For example, Wang and co-workers found that iron sulfides/nitrogen and sulfur dual-doped mesoporous graphitic carbon spheres demonstrated excellent electrocatalytic activities toward the ORR in alkaline and acidic media. The remarkable catalytic performance was ascribed to the iron sulfide nanocrystals with an iron content of 5.9 wt% [34]. Similarly, multi-source-derived S─Fe/N/C with the atom contents of Fe 1.38% has been reported by

the nature of the active sites was uncertain for the Fe/N/C, the S─Fe/N/C catalyst showed a

O4

C@C species (Fe content of 6.3 wt%) [31]. The experimental results

sites played a dominant role in promoting the ORR via a 4-electron

C@C sites played an auxiliary role. The authors also prepared a

[33] are all considered ORR active

O4

O4

. Although

sites (**Figure 2**).

nanoparticles

the scarcity of Pt and the unsatisfactory long-term stability.

54 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

[29, 30], Fe<sup>3</sup>

CNT/porphyrinic carbon (PC) catalyst with densely distributed active Fe─Nx

ity in both alkaline and acidic media [32]. Xia and co-workers prepared Fe<sup>3</sup>

Wang and co-workers. Five types of Fe were detected: Fe, FeS, FeN, FeC, and Fe<sup>3</sup>

exhibited an excellent catalytic performance toward the ORR [33].

**2.2. Non-noble metal-based active sites**

and Fe─Fe<sup>3</sup>

highly efficient ORR activity [35, 36].

**Figure 2.** Synthetic scheme for the preparation of CNT/PC catalysts [32].

dispersed Fe [28], Fe─Nx

indicated that the Fe─Nx

pathway, whereas the Fe─Fe<sup>3</sup>

ing of Fe─Nx

Additionally, bimetallic catalysts are promising candidates for the ORR. You and co-workers synthesized Fe<sup>3</sup> C/tungsten carbide/graphitic carbon in which the Fe<sup>3</sup> C acted as the active

**Figure 3.** Schematic procedure for the preparation of functional porous graphene (Co3 O4 /Co@PG) from waste paper (MCM stands for microporous carbonaceous material and PG stands for porous graphene) [40].

sites. Meanwhile, graphitic layers and tungsten carbide nanoparticles can improve the surface chemical stability of the Fe<sup>3</sup> C phase. The nanocomposite demonstrated high ORR efficiency via a four-electron pathway in a pH-neutral electrolyte [45]. Xu and co-workers used transition-metal spinels as a descriptor to identify the active sites in MnCo2 O4 for catalyzing the ORR. Mn cations played an important role in the ORR because of the Mn valence in the octahedral sites [46]. Moreover, PdCo bimetallic nanoparticle (∼8 wt%)-decorated N-doped porous carbon was fabricated by Yamauchi and co-workers. Due to the existence of the PdCo nanoparticles, the O─O bonds can be easily broken. More importantly, the three-dimensional ordered porous structure disperses the PdCo nanoparticles uniformly in the matrix. Therefore, this composite exhibited similar electrocatalytic activity to that of commercial Pt/C in alkaline media [47]. One step beyond is to further dope metal into the bimetallic catalysts. A cathode catalyst composed of a conformal film of the PrNi0.5Mn0.5O3 and exsoluted PrOx nanoparticles was designed by Liu and co-workers. Oxygen-vacancy-rich surfaces of the catalyst facilitated the electron transfer and hence dramatically enhanced the ORR kinetics and durability [48].

exposed active sites [55]. Task-specific tubular nanoporous polycarbazole-derived N-doped carbon nanotubes [56] and highly pyridinic nitrogen (up to 14.9 wt%)-doped ultra-hollow carbon frameworks [57] have also been reported. The design of carbon structures with multicomponent active centers can in principle enhance the ORR activity. Two-dimensional N- and S-co-doped graphitic sheets with abundant interfacial active sites showed catalytic activity comparable to that of the commercial Pt/C [58]. In addition, N- and P-co-doped porous carbon networks showed high activities and excellent durability for ORR [59]. More interestingly, an N- and S-doped carbon nanofiber network coated with N- and P-doped carbon nanoparticles showed superior ORR performance due to the integration of N, S, and P in the carbon

Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction

hybrid [60]. Zhi and co-workers studied the ORR activity of B-, P-, and S-doped g-C<sup>3</sup>

sites in the ORR with the catalytic activity trend of S-doped > P-doped > g-C3

pH-universal electrolyte is still a challenge.

trace amounts of gold (Au:Pd = 1:100) in Pd6

It has been reported that metallic impurities within sp2

**2.4. Trace metal-based active sites**

g-C3 N4 [61].

rated Pd3

XPS results suggested the formation of B─N, P─N, and S─C bonds. The experimental results indicated that S- and P-doping enhanced the electrocatalytic performance of ORR, whereas B-doping deactivated the ORR process. The authors proposed that B, S, and P atoms are active

Now, metal-free catalysts, commonly decorated with N, B, P, or S, have emerged as promising catalysts in ORR, due to the high methanol tolerance and excellent long-term stability. In fact, design and synthesis of metal-free catalysts with satisfactory ORR performance in

the electrochemistry of the materials [62, 63]. Jiang and co-workers prepared sulfur, trace nitrogen, and iron co-doped porous carbon foams with an Fe content of 0.27 at%. They proposed that C─S moieties and the synergistic effect of sulfur, the trace amount of nitrogen, and iron contributed to the efficient electrocatalytic properties [64]. Wang and co-workers deco-

the exposure of more active sites of the Pt, the obtained catalyst showed high catalytic activity and stability in the ORR with a 30-mV positive half-wave potential (E1/2) comparable to that of commercial Pt/C [65]. Surprisingly, the presence of trace Au (Au/Pt atomic ratio = 0.0005) can significantly enhance the ORR durability of a PtCu alloy [66]. Similarly, incorporating

durability of the ORR (**Figure 4**). Synchrotron X-ray absorption spectroscopy showed that gold replaced cobalt and copper on the surface and in the interior of the nanoparticles. This

Impressively, Schuhmann and co-workers systematically investigated the influence of trace metal residues on the ORR activity. They prepared a metal-free catalyst and then deliberately incorporated metal precursors into the catalyst. The experimental results indicated that the addition of Fe with 0.05 wt% to the metal-free catalyst significantly improved its ORR activity [68]. Pumera and co-workers found that a Mn content of 0.0018 wt% in graphene was sufficient to enhance the electrocatalytic properties toward the ORR. In addition, they claimed that the elemental analysis of the metal contents should be provided to prove that the catalyst is"metal-free" [69]. Recently, we prepared a catalyst for the ORR derived from *Gentiana scabra* 

study developed a new strategy for enhancing the stability of fuel-cell catalysts [67].

V bimetallic alloy nanoparticles with Pt (Pt to Pd nominal atomic ratio at 1:30). With

N4 . The 57

N4 > B-doped

carbon nanomaterials may dominate

http://dx.doi.org/10.5772/intechopen.77048

CoCu nanocatalysts can markedly improve the

Now, the non-noble metal-based ORR catalysts have received more concern, due to their low cost and earth abundance. Although the deep insights of the active sites are not clear, nonnoble metal-based ORR catalyst can outperform the catalytic performance of Pt/C. However, most synthetic strategies involved the random mixing of carbon precursors and dopants, followed by pyrolysis. Therefore, it is hard to tune the porous structures and the distribution of active sites. Future research should focus on the control of the morphology, composition, and active sites of the non-noble metal-based catalysts in high precision.

### **2.3. N-, S-, B-, and P-based active sites**

Along with great progress made in metal-based catalysts, metal-free catalysts have attracted significant attention. Due to their high catalytic performance, long-term stability, and durability, electronegative N-doped carbon materials have been broadly studied. Pyridinic-N and graphitic-N were found to be the most highly active sites for the ORR. It is still under debate which configuration contributes more to the ORR. In fact, it is difficult to determine the role of a single N-type because high-temperature pyrolysis tends to introduce a mixture of N types. Surprisingly, synthetic strategies favorable for obtaining specific types of C─N bonding have been developed [49, 50]. Specifically, Zhang and co-workers prepared pyridinic-N dominated graphene aerogels by the etching effect of NH<sup>3</sup> . The authors found that a long annealing time at 900°C was favorable for obtaining pyridinic-N. An N content of up to 12.2 at% with 90.4% pyridinic nitrogen has been achieved [51]. Furthermore, g-C<sup>3</sup> N4 is an ideal nitrogen dopant with an N content of 57.1 at%. A series of studies have been reported using g-C<sup>3</sup> N4 as both the template and nitrogen source [52–54]. Yan and co-workers reported a facile strategy for the synthesis of nitrogen-doped porous carbon with an N content of 10.49 at% [53]. Polypyrrole was used as the carbon and heteroatom sources and g-C3 N4 acted as a sacrificial template and nitrogen sources. The XPS spectra of N 1 s indicated the existence of pyridinic-N, pyrrolic-N, graphitic-N, and oxidized-N. Due to the high specific surface area and N content as well as massive edges and defects, the obtained porous carbon exhibited excellent ORR activity. In addition, nitrogen-doped carbon nanoparticle−carbon nanofiber composites prepared by Ishizaki and co-workers demonstrated long-term durability and high tolerance to methanol. This can be attributed to the high conductivity of the carbon nanofibers and the largely exposed active sites [55]. Task-specific tubular nanoporous polycarbazole-derived N-doped carbon nanotubes [56] and highly pyridinic nitrogen (up to 14.9 wt%)-doped ultra-hollow carbon frameworks [57] have also been reported. The design of carbon structures with multicomponent active centers can in principle enhance the ORR activity. Two-dimensional N- and S-co-doped graphitic sheets with abundant interfacial active sites showed catalytic activity comparable to that of the commercial Pt/C [58]. In addition, N- and P-co-doped porous carbon networks showed high activities and excellent durability for ORR [59]. More interestingly, an N- and S-doped carbon nanofiber network coated with N- and P-doped carbon nanoparticles showed superior ORR performance due to the integration of N, S, and P in the carbon hybrid [60]. Zhi and co-workers studied the ORR activity of B-, P-, and S-doped g-C<sup>3</sup> N4 . The XPS results suggested the formation of B─N, P─N, and S─C bonds. The experimental results indicated that S- and P-doping enhanced the electrocatalytic performance of ORR, whereas B-doping deactivated the ORR process. The authors proposed that B, S, and P atoms are active sites in the ORR with the catalytic activity trend of S-doped > P-doped > g-C3 N4 > B-doped g-C3 N4 [61].

Now, metal-free catalysts, commonly decorated with N, B, P, or S, have emerged as promising catalysts in ORR, due to the high methanol tolerance and excellent long-term stability. In fact, design and synthesis of metal-free catalysts with satisfactory ORR performance in pH-universal electrolyte is still a challenge.

### **2.4. Trace metal-based active sites**

sites. Meanwhile, graphitic layers and tungsten carbide nanoparticles can improve the sur-

ciency via a four-electron pathway in a pH-neutral electrolyte [45]. Xu and co-workers used

the ORR. Mn cations played an important role in the ORR because of the Mn valence in the octahedral sites [46]. Moreover, PdCo bimetallic nanoparticle (∼8 wt%)-decorated N-doped porous carbon was fabricated by Yamauchi and co-workers. Due to the existence of the PdCo nanoparticles, the O─O bonds can be easily broken. More importantly, the three-dimensional ordered porous structure disperses the PdCo nanoparticles uniformly in the matrix. Therefore, this composite exhibited similar electrocatalytic activity to that of commercial Pt/C in alkaline media [47]. One step beyond is to further dope metal into the bimetallic catalysts. A cathode

was designed by Liu and co-workers. Oxygen-vacancy-rich surfaces of the catalyst facilitated the electron transfer and hence dramatically enhanced the ORR kinetics and durability [48]. Now, the non-noble metal-based ORR catalysts have received more concern, due to their low cost and earth abundance. Although the deep insights of the active sites are not clear, nonnoble metal-based ORR catalyst can outperform the catalytic performance of Pt/C. However, most synthetic strategies involved the random mixing of carbon precursors and dopants, followed by pyrolysis. Therefore, it is hard to tune the porous structures and the distribution of active sites. Future research should focus on the control of the morphology, composition, and

Along with great progress made in metal-based catalysts, metal-free catalysts have attracted significant attention. Due to their high catalytic performance, long-term stability, and durability, electronegative N-doped carbon materials have been broadly studied. Pyridinic-N and graphitic-N were found to be the most highly active sites for the ORR. It is still under debate which configuration contributes more to the ORR. In fact, it is difficult to determine the role of a single N-type because high-temperature pyrolysis tends to introduce a mixture of N types. Surprisingly, synthetic strategies favorable for obtaining specific types of C─N bonding have been developed [49, 50]. Specifically, Zhang and co-workers prepared pyridinic-N dominated

at 900°C was favorable for obtaining pyridinic-N. An N content of up to 12.2 at% with 90.4%

template and nitrogen source [52–54]. Yan and co-workers reported a facile strategy for the synthesis of nitrogen-doped porous carbon with an N content of 10.49 at% [53]. Polypyrrole

nitrogen sources. The XPS spectra of N 1 s indicated the existence of pyridinic-N, pyrrolic-N, graphitic-N, and oxidized-N. Due to the high specific surface area and N content as well as massive edges and defects, the obtained porous carbon exhibited excellent ORR activity. In addition, nitrogen-doped carbon nanoparticle−carbon nanofiber composites prepared by Ishizaki and co-workers demonstrated long-term durability and high tolerance to methanol. This can be attributed to the high conductivity of the carbon nanofibers and the largely

with an N content of 57.1 at%. A series of studies have been reported using g-C<sup>3</sup>

transition-metal spinels as a descriptor to identify the active sites in MnCo2

catalyst composed of a conformal film of the PrNi0.5Mn0.5O3

56 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

active sites of the non-noble metal-based catalysts in high precision.

**2.3. N-, S-, B-, and P-based active sites**

graphene aerogels by the etching effect of NH<sup>3</sup>

pyridinic nitrogen has been achieved [51]. Furthermore, g-C<sup>3</sup>

was used as the carbon and heteroatom sources and g-C3

C phase. The nanocomposite demonstrated high ORR effi-

and exsoluted PrOx

. The authors found that a long annealing time

is an ideal nitrogen dopant

acted as a sacrificial template and

N4

as both the

N4

N4

O4

for catalyzing

nanoparticles

face chemical stability of the Fe<sup>3</sup>

It has been reported that metallic impurities within sp2 carbon nanomaterials may dominate the electrochemistry of the materials [62, 63]. Jiang and co-workers prepared sulfur, trace nitrogen, and iron co-doped porous carbon foams with an Fe content of 0.27 at%. They proposed that C─S moieties and the synergistic effect of sulfur, the trace amount of nitrogen, and iron contributed to the efficient electrocatalytic properties [64]. Wang and co-workers decorated Pd3 V bimetallic alloy nanoparticles with Pt (Pt to Pd nominal atomic ratio at 1:30). With the exposure of more active sites of the Pt, the obtained catalyst showed high catalytic activity and stability in the ORR with a 30-mV positive half-wave potential (E1/2) comparable to that of commercial Pt/C [65]. Surprisingly, the presence of trace Au (Au/Pt atomic ratio = 0.0005) can significantly enhance the ORR durability of a PtCu alloy [66]. Similarly, incorporating trace amounts of gold (Au:Pd = 1:100) in Pd6 CoCu nanocatalysts can markedly improve the durability of the ORR (**Figure 4**). Synchrotron X-ray absorption spectroscopy showed that gold replaced cobalt and copper on the surface and in the interior of the nanoparticles. This study developed a new strategy for enhancing the stability of fuel-cell catalysts [67].

Impressively, Schuhmann and co-workers systematically investigated the influence of trace metal residues on the ORR activity. They prepared a metal-free catalyst and then deliberately incorporated metal precursors into the catalyst. The experimental results indicated that the addition of Fe with 0.05 wt% to the metal-free catalyst significantly improved its ORR activity [68]. Pumera and co-workers found that a Mn content of 0.0018 wt% in graphene was sufficient to enhance the electrocatalytic properties toward the ORR. In addition, they claimed that the elemental analysis of the metal contents should be provided to prove that the catalyst is"metal-free" [69]. Recently, we prepared a catalyst for the ORR derived from *Gentiana scabra* 

**Figure 4.** Schematic illustration of the formation of Pd6 CoCu/C (a) and Au-Pd6 CoCu/C (b) [67].

*Bunge* (natural biomass) (**Figure 5**). Notably, the obtained catalyst showed an enhanced ORR activity compared with other biomass-derived carbon materials [70]. This could be partly due to the presence of intrinsic Fe species (about 744 mg kg−1). Fe not only facilitates the formation of catalytically active N─C sites but also increases the graphitization degree of carbon. Another possible reason is that trace levels of Fe residues (0.07 wt%) in carbon can dramatically enhance the ORR properties.

Trace metal as active sites in ORR depending more on the nanostructure of carbon matrix. It is time to take the step forward toward exploration of ORR electrocatalyst with uniform open

**Figure 5.** Schematic procedure for the synthesis of heteroatom doped porous carbon materials derived from Gentiana

Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction

http://dx.doi.org/10.5772/intechopen.77048

59

Cathode catalysts meet the criteria of promising ORR performance, high stability, high durability, and low cost and are highly desirable for automotive applications. The development of novel strategies for the rational design and synthesis of catalysts for meeting these performance goals is very important. Furthermore, it is significantly important to develop a theoretical and experiment-based in-depth understanding of the nature of active sites together with the underlying mechanism of the ORR. Close attention should be paid to catalysts with

Metal atoms, metal alloys, metal oxide/nitride/sulfide nanoparticles, non-metal heteroatoms (e.g., N, S, B, and P), and even topological defects and edges of the carbon support can serve as potential active sites. Assisted by theoretical calculations, the catalytic mechanism of the ORR can be easily predicted; thus, the rational design and synthesis of catalysts become more efficient. In addition, the composition, size, dispersion, morphology, and structure of the nanoparticles, the contents of the heteroatoms, as well as the nanostructure of the carbon support are probably influential in the ORR. Therefore, the precise control of the structure, electronic state, and density of the active sites at the atomistic level are favorable for achieving an optimum ORR performance. Probing the role of trace metals in the ORR further guides future research activities, thereby reducing the cost and dramatically improving the ORR

cavities and ordering distribution of the trace metal.

high ORR activities under alkaline, acidic, and neutral conditions.

**3. Conclusion(s)**

scabra Bunge [70].

performance.

In fact, controversy still exists over whether metal ions can function as active sites or just facilitate the formation of active sites. An understanding of active sites has been gained from experimental data and theoretical calculations [71]. An in situ poisoning experiment confirmed the existence of a metal-centered active site. For example, in the presence of CN− , the redox couple of Co(III)/Co(II) dramatically changed because CN<sup>−</sup> coordinated with the transition metal in the axial position. The blocking effect prevented the interaction of O<sup>2</sup> with the transition metal and hence reduced the electrocatalytic activity [72]. Similarly, H2 S has also been used in an in situ poisoning experiment. A significant deactivation of Fe─N─C catalysts for ORR was observed after the H2 S treatment due to the formation of Fe─S bonds. In addition, the XPS characterization indicated the existence of Fe─S type bonds. These results showed that Fe was indeed active sites in catalyzing ORR [73].

The experimental results indeed proved that trace metal residues enhanced the ORR activity. Here comes the question: "How metal-free are metal-free catalysts?" [68]. Pumera and coworkers investigate the electrochemical response of carbon nanotubes with different metallic impurities. They found that 100 ppm of Fe impurities (close to the detection limit) still dominated the electrochemical behavior of a CNT sample [74]. Therefore, definitive proof of metalfree catalysts is necessary. In addition, we highly suggest that there is no need to argue whether trace metal residue promotes the ORR activity. The abovementioned results clarify this.

Active Sites Derived from Heteroatom Doping in Carbon Materials for Oxygen Reduction Reaction http://dx.doi.org/10.5772/intechopen.77048 59

**Figure 5.** Schematic procedure for the synthesis of heteroatom doped porous carbon materials derived from Gentiana scabra Bunge [70].

Trace metal as active sites in ORR depending more on the nanostructure of carbon matrix. It is time to take the step forward toward exploration of ORR electrocatalyst with uniform open cavities and ordering distribution of the trace metal.

### **3. Conclusion(s)**

,

with

S has

coordinated with the

S treatment due to the formation of Fe─S bonds. In

CoCu/C (b) [67].

*Bunge* (natural biomass) (**Figure 5**). Notably, the obtained catalyst showed an enhanced ORR activity compared with other biomass-derived carbon materials [70]. This could be partly due to the presence of intrinsic Fe species (about 744 mg kg−1). Fe not only facilitates the formation of catalytically active N─C sites but also increases the graphitization degree of carbon. Another possible reason is that trace levels of Fe residues (0.07 wt%) in carbon can dramati-

CoCu/C (a) and Au-Pd6

In fact, controversy still exists over whether metal ions can function as active sites or just facilitate the formation of active sites. An understanding of active sites has been gained from experimental data and theoretical calculations [71]. An in situ poisoning experiment confirmed the existence of a metal-centered active site. For example, in the presence of CN−

transition metal in the axial position. The blocking effect prevented the interaction of O<sup>2</sup>

the transition metal and hence reduced the electrocatalytic activity [72]. Similarly, H2

also been used in an in situ poisoning experiment. A significant deactivation of Fe─N─C

addition, the XPS characterization indicated the existence of Fe─S type bonds. These results

The experimental results indeed proved that trace metal residues enhanced the ORR activity. Here comes the question: "How metal-free are metal-free catalysts?" [68]. Pumera and coworkers investigate the electrochemical response of carbon nanotubes with different metallic impurities. They found that 100 ppm of Fe impurities (close to the detection limit) still dominated the electrochemical behavior of a CNT sample [74]. Therefore, definitive proof of metalfree catalysts is necessary. In addition, we highly suggest that there is no need to argue whether trace metal residue promotes the ORR activity. The abovementioned results clarify this.

the redox couple of Co(III)/Co(II) dramatically changed because CN<sup>−</sup>

showed that Fe was indeed active sites in catalyzing ORR [73].

cally enhance the ORR properties.

**Figure 4.** Schematic illustration of the formation of Pd6

58 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

catalysts for ORR was observed after the H2

Cathode catalysts meet the criteria of promising ORR performance, high stability, high durability, and low cost and are highly desirable for automotive applications. The development of novel strategies for the rational design and synthesis of catalysts for meeting these performance goals is very important. Furthermore, it is significantly important to develop a theoretical and experiment-based in-depth understanding of the nature of active sites together with the underlying mechanism of the ORR. Close attention should be paid to catalysts with high ORR activities under alkaline, acidic, and neutral conditions.

Metal atoms, metal alloys, metal oxide/nitride/sulfide nanoparticles, non-metal heteroatoms (e.g., N, S, B, and P), and even topological defects and edges of the carbon support can serve as potential active sites. Assisted by theoretical calculations, the catalytic mechanism of the ORR can be easily predicted; thus, the rational design and synthesis of catalysts become more efficient. In addition, the composition, size, dispersion, morphology, and structure of the nanoparticles, the contents of the heteroatoms, as well as the nanostructure of the carbon support are probably influential in the ORR. Therefore, the precise control of the structure, electronic state, and density of the active sites at the atomistic level are favorable for achieving an optimum ORR performance. Probing the role of trace metals in the ORR further guides future research activities, thereby reducing the cost and dramatically improving the ORR performance.

For practical applications, the reduction in the cost of electrocatalyst is an important factor. Future studies should focus on the design and synthesis of platinum group metal (PGM)-free cathode catalysts achieving high efficiency and durability along with low costs. The 2020 target for PGM-free catalyst activity proposed by the Department of Energy (DOE) is to achieve 0.044 A/cm2 @ 0.9 VIR-free and a durability of 5000 h [75]. Overall, a bright future awaits for cathode catalysts.

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acsami.6b02630

### **Acknowledgements**

This work was supported by the National Nature Science Foundation of China (no. 21603156), Jiangsu Province Science Foundation for Youths (no. BK20170331).

### **Acronyms and abbreviations**


### **Author details**

Winston Duo Wu and Dan Xu\*

\*Address all correspondence to: dxu@suda.edu.cn

Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, China

### **References**

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[2] Qiao M, Tang C, Tanase LC, Teodorescu CM, Chen CM, Zhang Q, Titirici MM. Oxygenophilic ionic liquids promote the oxygen reduction reaction in Pt-free carbon electrocatalysts. Materials Horizons. 2017;**4**:895-899. DOI: 10.1039/c7mh00298j

For practical applications, the reduction in the cost of electrocatalyst is an important factor. Future studies should focus on the design and synthesis of platinum group metal (PGM)-free cathode catalysts achieving high efficiency and durability along with low costs. The 2020 target for PGM-free catalyst activity proposed by the Department of Energy (DOE) is to achieve

This work was supported by the National Nature Science Foundation of China (no. 21603156),

Jiangsu Province Science Foundation for Youths (no. BK20170331).

@ 0.9 VIR-free and a durability of 5000 h [75]. Overall, a bright future awaits for

0.044 A/cm2

cathode catalysts.

**Acknowledgements**

**Acronyms and abbreviations**

CB Carbon black

Winston Duo Wu and Dan Xu\*

chemrev.5b00462

**Author details**

**References**

CNT Carbon nanotube

ORR Oxygen reduction reaction PEM Proton exchange membrane

Pt/C Platinum/carbon black catalyst

60 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

XPS X-ray photoelectron spectroscopy

\*Address all correspondence to: dxu@suda.edu.cn

Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical

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Engineering and Materials Science, Soochow University, Suzhou, China


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**Chapter 4**

**Provisional chapter**

**Noble Metal Dispersed on Reduced Graphene Oxide**

**Noble Metal Dispersed on Reduced Graphene Oxide** 

Metal-dispersed nanoparticles on reduced graphene oxide as catalyst for oxygen reduction reaction (ORR) demonstrate promising applications in the energy sector. The catalyst activity enhancement and stability improvement investigated in this study are mandatory steps in obtaining feasible electrodes for PEMFC. The chapter deals with the synthesis of noble metal catalysts including platinum and gold nanoparticles dispersed on reduced graphene oxide (PtNPs/rGO and AuNPs/rGrO). The understanding of the correlations between the electrochemical activity on one side and the structure, composition and synthesis method on the other side are provided. Facile routes in order to prepare the well dispersed PtNPs/rGO and AuNPs/rGrO are included. The structure and morphology were characterized by different techniques, namely X-ray diffraction (XRD), Scanning Transmission Electron Microscopy (STEM), specific surface area measurements. In this context we report a hybrid derived electrocatalyst with increased electrochemical active area and enhanced mass-transport properties. The electrochemical performances of PtNPs/rGO and AuNPs/rGrO were tested and compared with a standard PEMFC configuration. The performed electrochemical characterization recommends the prepared materials as ORR electrocatalysts for the further fabrication of cathodes for PEM fuel cells. The research directions as well as perspectives on the subsequent development of more active and less expensive electro-

**Keywords:** fuel cell, cathode, doped graphenes, oxygen reduction reaction

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.80941

**and Its Application in PEM Fuel Cells**

**and Its Application in PEM Fuel Cells**

Adriana Marinoiu, Mircea Raceanu, Elena Carcadea,

Aida Pantazi, Raluca Mesterca, Oana Tutunaru, Simona Nica, Daniela Bala, Mihai Varlam and

Elena Carcadea, Aida Pantazi, Raluca Mesterca, Oana Tutunaru, Simona Nica, Daniela Bala,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80941

catalysts are established.

Adriana Marinoiu, Mircea Raceanu,

Mihai Varlam and Marius Enachescu

Marius Enachescu

**Abstract**

#### **Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells**

DOI: 10.5772/intechopen.80941

Adriana Marinoiu, Mircea Raceanu, Elena Carcadea, Aida Pantazi, Raluca Mesterca, Oana Tutunaru, Simona Nica, Daniela Bala, Mihai Varlam and Marius Enachescu Adriana Marinoiu, Mircea Raceanu, Elena Carcadea, Aida Pantazi, Raluca Mesterca, Oana Tutunaru, Simona Nica, Daniela Bala, Mihai Varlam and Marius Enachescu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80941

#### **Abstract**

Metal-dispersed nanoparticles on reduced graphene oxide as catalyst for oxygen reduction reaction (ORR) demonstrate promising applications in the energy sector. The catalyst activity enhancement and stability improvement investigated in this study are mandatory steps in obtaining feasible electrodes for PEMFC. The chapter deals with the synthesis of noble metal catalysts including platinum and gold nanoparticles dispersed on reduced graphene oxide (PtNPs/rGO and AuNPs/rGrO). The understanding of the correlations between the electrochemical activity on one side and the structure, composition and synthesis method on the other side are provided. Facile routes in order to prepare the well dispersed PtNPs/rGO and AuNPs/rGrO are included. The structure and morphology were characterized by different techniques, namely X-ray diffraction (XRD), Scanning Transmission Electron Microscopy (STEM), specific surface area measurements. In this context we report a hybrid derived electrocatalyst with increased electrochemical active area and enhanced mass-transport properties. The electrochemical performances of PtNPs/rGO and AuNPs/rGrO were tested and compared with a standard PEMFC configuration. The performed electrochemical characterization recommends the prepared materials as ORR electrocatalysts for the further fabrication of cathodes for PEM fuel cells. The research directions as well as perspectives on the subsequent development of more active and less expensive electrocatalysts are established.

**Keywords:** fuel cell, cathode, doped graphenes, oxygen reduction reaction

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction**

Almost 60 years have passed from Feynman's famous and incredible visionary talk "There is plenty of room at the bottom" and many of his statements are the reality today. The new field he was just introducing was defined as having an enormous number of technical applications. And so right he was! In this chapter, we are going to discuss one of those applications. Fuel cells are among the major scientific discoveries that had a strange "life". Despite their modern tech aura, the wonderful discovery of Mr. Grove, 150 years ago, had in the last 20 years an extremely variable popularity, reaching their peak at the end of the last century, when scientists and stock promoters envisioned a world run on clean and inexhaustible resource hydrogen. At that time, it was predicted that soon, cars will run on fuel cells and households will generate electricity from back-yard fuel cells. Improvements in stack design during that time led to increased power densities and lower costs. However, high manufacturing costs, marginal performance and short service life stood in the way of turning the hydrogen dream into reality. Nowadays, a new "wave" has emerged, partly due to exactly the new field introduced by Mr. Feynman—nanostructure-based catalysts.

PEMFC cells are currently the leading technology for light duty vehicles and in a smaller proportion for stationary and other applications. Commercialization of light duty FC vehicles started recently. The market for FC vehicles is still limited to the present day, mainly due to an insufficient hydrogen fueling infrastructure, but also to a considerable cost of FC systems able to achieve the target lifetime [1, 2]. For example, it is recognized that a decrease in platinum loading has a negative impact in respect to durability. In this particular domain of catalyst durability for FC systems the smetal-dispersed nanoparticles can concurrently lower the catalytic system cost playing a significant role in technology developments in the near future [3]. Moreover, most of the researches have been devoted to reduce the resistance of the electrolyte membrane by lowering the thickness, which caused inferior mechanical properties of the thinned membrane. The total cost of a fuel cells stack can be greatly reduced by reducing the catalyst load and using low- or non-Pt-based electrocatalysts. Therefore, the ways to improve the catalytic activity and reduce the cost of anodic and cathodic catalysis are critical issues to achieve the real fueling of the fuel cells. In PEM fuel cells, the efficiency of the cathode electrocatalyst plays an important role in increasing energy conversion efficiency and achieving higher power densities. In recent years, new 3D graphene-based materials have been developed, that have proved to be effective electrocatalysts for ORR [4]. The main directions for increasing the catalytic activity and reducing the production costs for the ORR catalysts were as follows: the development of nano-graphene nanostructures based on noble metals, either by doping with non-metals or by the deposition of non-metallic nanoparticles or metallic oxides with a

Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

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71

catalytic role; decreasing the noble metal content of the electrocatalyst.

In recent years, various catalyst support materials have been proposed to meet the challenges. In addition to traditional carbon materials, non-carbon materials such as metal oxides, electroconducting polymers, carbides, nitrides, etc. have been proposed and sustained as catalysts. The nanostructured materials meet the requirements of a high-surface catalyst with high active surface sites and a low metal load. Consequently, a number of non-metallic catalysts have been developed, but the catalytic performance has been modest [5, 6]. The ORR reaction at the cathode plays a critical role in the performance of a fuel cell. Although substantial advances have been made in cathodic catalysis over the last decades, the slow kinetics of ORR electron transfer

and overpotential are the disadvantages that limit the large-scale marketing of PEMFC.

graphene support nanomaterials are a promising class of fuel cell electrocatalysts.

The porous graphene has remarkable properties for electrochemical power systems [7]. In general, nanoelectrocatalysts based on graphene used in PEMFC can be classified into the following types, depending on the composition of the catalysts: (1) Pt-based nanocatalysts supported on graphene; (2) noble metals supported on graphene as electrocatalysts, including transition metals and their oxides, sulphide nanomaterials and metal complexes (3) Pt-free graphene-supported electrocatalysts, including alloys Pd, Au, Ag and nanomaterials; and (4) non-metal electrocatalysts, including surface functionalized graphene and heteroatom doped graphene. It should be noted that, despite the substantial advances made in the construction of advanced electrocatalysts based on graphene and the understanding of their electrocatalytic mechanism, a number of scientific and technical challenges still remain in order to achieve the commercialization of graphite-supported electrocatalysts in fuel cells. It is anticipated that

The development of human society requires more resources to meet the enhanced energy demand of society. With the rapid increase in energy demand in people's everyday lives, research into new environment-friendly energy sources and their practical applications have attracted increasing attention. Fuel cells demonstrated important advantages with distinguishing features compared to conventional power sources, such as internal combustion engines or batteries, namely: higher efficiency and silent operation in comparison to internal combustion engines; no pollution considering the only by-product at the point of use is water; the maintenance of fuel cells is simple; low temperature proton exchange fuel cells (PEMFC) have low heat transmission; operating time is much longer than in the case of batteries, since doubling the operating time needs only doubling the content of fuel and not the doubling of the capacity of the unit itself; as opposed to batteries, fuel cells have no "memory effect" when are replenished.

The main advantages of new nanostructured materials are their very high surface area to volume ratio and high absorption rates. There are nanoscale materials of various types, including nanoparticles, nano-powders, nano-rods, nanotubes, and nanowires. It is well known that nanomaterials have multiple dimensionalities, including zero dimensional, 1-D, 2-D and 3-D. One dimensional materials are widely used in various applications because of their high surface area and porosity. Carbon-type materials are known to have good features, such as abundance, stability, environmental safety and high durability. They exhibit high chemical stability over a wide temperature range in both acidic and alkaline conditions, making them the most suitable candidates for electrodes in electrochemical energy devices. There are many available carbon allotropes, such as buckminster fullerenes, carbon nanotubes, graphene and nano-diamonds. In the last years, graphene is considered as a major material in energy conversion and storage applications in general and in PEM fuel cells topic particularly. Therefore, one of the cornerstones of the push towards future improvements in present-day PEM fuel cell, and the research and development generated by this push, is the introduction of nanostructured catalysts in order to mitigate the main issues of the technology—high prices and low service life.

PEMFC cells are currently the leading technology for light duty vehicles and in a smaller proportion for stationary and other applications. Commercialization of light duty FC vehicles started recently. The market for FC vehicles is still limited to the present day, mainly due to an insufficient hydrogen fueling infrastructure, but also to a considerable cost of FC systems able to achieve the target lifetime [1, 2]. For example, it is recognized that a decrease in platinum loading has a negative impact in respect to durability. In this particular domain of catalyst durability for FC systems the smetal-dispersed nanoparticles can concurrently lower the catalytic system cost playing a significant role in technology developments in the near future [3]. Moreover, most of the researches have been devoted to reduce the resistance of the electrolyte membrane by lowering the thickness, which caused inferior mechanical properties of the thinned membrane.

**1. Introduction**

Almost 60 years have passed from Feynman's famous and incredible visionary talk "There is plenty of room at the bottom" and many of his statements are the reality today. The new field he was just introducing was defined as having an enormous number of technical applications. And so right he was! In this chapter, we are going to discuss one of those applications. Fuel cells are among the major scientific discoveries that had a strange "life". Despite their modern tech aura, the wonderful discovery of Mr. Grove, 150 years ago, had in the last 20 years an extremely variable popularity, reaching their peak at the end of the last century, when scientists and stock promoters envisioned a world run on clean and inexhaustible resource hydrogen. At that time, it was predicted that soon, cars will run on fuel cells and households will generate electricity from back-yard fuel cells. Improvements in stack design during that time led to increased power densities and lower costs. However, high manufacturing costs, marginal performance and short service life stood in the way of turning the hydrogen dream into reality. Nowadays, a new "wave" has emerged, partly due to exactly the new field intro-

The development of human society requires more resources to meet the enhanced energy demand of society. With the rapid increase in energy demand in people's everyday lives, research into new environment-friendly energy sources and their practical applications have attracted increasing attention. Fuel cells demonstrated important advantages with distinguishing features compared to conventional power sources, such as internal combustion engines or batteries, namely: higher efficiency and silent operation in comparison to internal combustion engines; no pollution considering the only by-product at the point of use is water; the maintenance of fuel cells is simple; low temperature proton exchange fuel cells (PEMFC) have low heat transmission; operating time is much longer than in the case of batteries, since doubling the operating time needs only doubling the content of fuel and not the doubling of the capacity of the unit itself; as opposed to batteries, fuel cells have no "memory effect" when are replenished. The main advantages of new nanostructured materials are their very high surface area to volume ratio and high absorption rates. There are nanoscale materials of various types, including nanoparticles, nano-powders, nano-rods, nanotubes, and nanowires. It is well known that nanomaterials have multiple dimensionalities, including zero dimensional, 1-D, 2-D and 3-D. One dimensional materials are widely used in various applications because of their high surface area and porosity. Carbon-type materials are known to have good features, such as abundance, stability, environmental safety and high durability. They exhibit high chemical stability over a wide temperature range in both acidic and alkaline conditions, making them the most suitable candidates for electrodes in electrochemical energy devices. There are many available carbon allotropes, such as buckminster fullerenes, carbon nanotubes, graphene and nano-diamonds. In the last years, graphene is considered as a major material in energy conversion and storage applications in general and in PEM fuel cells topic particularly. Therefore, one of the cornerstones of the push towards future improvements in present-day PEM fuel cell, and the research and development generated by this push, is the introduction of nanostructured catalysts in

order to mitigate the main issues of the technology—high prices and low service life.

duced by Mr. Feynman—nanostructure-based catalysts.

70 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

The total cost of a fuel cells stack can be greatly reduced by reducing the catalyst load and using low- or non-Pt-based electrocatalysts. Therefore, the ways to improve the catalytic activity and reduce the cost of anodic and cathodic catalysis are critical issues to achieve the real fueling of the fuel cells. In PEM fuel cells, the efficiency of the cathode electrocatalyst plays an important role in increasing energy conversion efficiency and achieving higher power densities. In recent years, new 3D graphene-based materials have been developed, that have proved to be effective electrocatalysts for ORR [4]. The main directions for increasing the catalytic activity and reducing the production costs for the ORR catalysts were as follows: the development of nano-graphene nanostructures based on noble metals, either by doping with non-metals or by the deposition of non-metallic nanoparticles or metallic oxides with a catalytic role; decreasing the noble metal content of the electrocatalyst.

In recent years, various catalyst support materials have been proposed to meet the challenges. In addition to traditional carbon materials, non-carbon materials such as metal oxides, electroconducting polymers, carbides, nitrides, etc. have been proposed and sustained as catalysts. The nanostructured materials meet the requirements of a high-surface catalyst with high active surface sites and a low metal load. Consequently, a number of non-metallic catalysts have been developed, but the catalytic performance has been modest [5, 6]. The ORR reaction at the cathode plays a critical role in the performance of a fuel cell. Although substantial advances have been made in cathodic catalysis over the last decades, the slow kinetics of ORR electron transfer and overpotential are the disadvantages that limit the large-scale marketing of PEMFC.

The porous graphene has remarkable properties for electrochemical power systems [7]. In general, nanoelectrocatalysts based on graphene used in PEMFC can be classified into the following types, depending on the composition of the catalysts: (1) Pt-based nanocatalysts supported on graphene; (2) noble metals supported on graphene as electrocatalysts, including transition metals and their oxides, sulphide nanomaterials and metal complexes (3) Pt-free graphene-supported electrocatalysts, including alloys Pd, Au, Ag and nanomaterials; and (4) non-metal electrocatalysts, including surface functionalized graphene and heteroatom doped graphene. It should be noted that, despite the substantial advances made in the construction of advanced electrocatalysts based on graphene and the understanding of their electrocatalytic mechanism, a number of scientific and technical challenges still remain in order to achieve the commercialization of graphite-supported electrocatalysts in fuel cells. It is anticipated that graphene support nanomaterials are a promising class of fuel cell electrocatalysts.

The catalytic activity of the Pt graphene composites can be improved by increasing the charge transfer from the catalysts to the graphene substrate. The chemically functionalized graphene with interlayer structure contains network defects (bare spots, holes) and surface functional groups (carbonyls, epoxies, hydroxyls, etc.) that can bind metal nanoparticles and can retain them on the graphene surface. A strong metal–substrate interaction was demonstrated to enable the stability of the nanocatalyst, which can be improved by dispersion. Using Density Functional Theory (DFT) it was found that surface defects in the graphene support can also act as strong traps for Pt nanoclusters, leading to long-term stability of Pt graphene composites.

via a facile process (**Figure 1**) described in detail elsewhere [11, 12]. Graphite oxide was pre-

Hummers' method, namely the graphitic powder was mixed with an oxidizing agent, filtered, washed and dried. Graphite oxide was dispersed in water by sonication for 2 h, forming the stable graphene oxide suspension (GO). Then, solutions of NaCl and the precursor of

PtCl6

stirring. The reaction mixture was stirred for another 24 h for the complete reduction, then washed with large amount of water several times, separated by filtration and centrifugation in order to remove residual ions. The obtained mass was then dried. pH plays an important role in the obtaining of the metal dispersed rGO, therefore the values of this parameter were mentioned in the experimental part (**Figure 1**) during each proposed fabrication steps. Aqueous NaOH and HCl solutions were used to adjust the pH value. In this work, we took into consideration the preparation of highly dispersed platinum and gold nanoparticles on reduced Graphene Oxide nanosheets with different metal compositions: Pt/rGO\_x and Au/ rGO\_y. It is known the fact that, during reduction step, graphene oxide gradually loses its oxygen-containing groups and could become hydrophobic, thus a direct reduction without a compatible polymer with ability of adsorption on the graphene surface, generally causes irreversible agglomerations. In our developed protocol synthesis PDDA was used as cationic polyelectrolyte owing to its expected reason of stabilizer effect, favoring the electrostatic interactions. Therefore, the bonding of reduced graphene oxide (rGO) with PDDA offers the

O5 , H2 SO<sup>4</sup>

Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

or HAuCl4

, and KMnO4

was added slowly to the reaction mixture under vigorous

) were added individually to GO

http://dx.doi.org/10.5772/intechopen.80941

according to the

73

pared by the oxidation of graphite powder using P2

suspension under continuous mechanical stirring (500 rpm for 1 h).

possibility of controlling the behaviors of RGO in suspensions.

**Figure 1.** Reaction routes for the synthesis of metal nanoparticles on graphene oxide.

corresponding metal (water solutions of H2

Freshly prepared solution of NaBH4

Metallic graphene hybrids have exhibited significant electrocatalytic activities when used as anodic and cathodic catalysts in PEM fuel cells. Both n- and p-type conductivities can be obtained by selecting metal dopants, and the carrier concentration can be controlled by the quantity of metal deposited [8]. For example, several transition metals with different work functions, such as Ti, Fe, Cu, Ag, Au, and Pt, have been used for graphene band modulation. In particular, band arrangement is not only determined by work function, but also affected by the interfacial interactions, as a consequence the metal structures would influence the charge transfer between metal and graphene. Moreover, first-principles calculations indicated that the carbon-vacancies attract and trap metal atoms [8]. The binding of a single metal atom nearby a single-vacancy (SV) on the graphene was investigated recently for Pt, Pd, Au, and Sn, using the density-functional theory (DFT). Regarding the pristine graphene, Pt, Pd, and Sn prefer to be adsorbed at the bridge site, while Au prefers the top site. On the single-vacancy graphene, all the metal atoms prefer to be trapped at the vacancy site and appear as dopants. The trapping abilities of the SV-graphene for different metal atoms, are different, that is, the Pt and Pd have the larger trapping zones. The diffusion barrier of a metal atom on the SV-graphene is much higher than that on the pristine graphene, and the Pt atom has the highest diffusion barrier from the SV site to the neighboring bridge sites. More electrons are transferred from the adatoms (or dopants) to the carbon atoms at the defect site, which induces changes in the electronic structures and properties of the systems. However, the Pt atom is easier to be incorporated into the SV-graphene [8].

In our previous efforts on the development of innovative fuel cell catalysts, an experimental approach of finding low cost alternative support material in PEMFC was developed [9]. The potential of using graphene oxide as support/catalyst for PEMFC was exploited. Moving forward, Pt/graphene nanocomposites were synthesized using a simple route involving chemical oxidation, exfoliation of graphene oxide, as well as functionalization with a compatible polymer. Poly-diallyldimethylammonium chloride (PDDA) was used as cationic polyelectrolyte, with ability of adsorption on the graphene surface through π−π and electrostatic interactions, acting as a stabilizer for controlled synthesis of Pt/graphene [9, 10].

### **2. Experimental**

### **2.1. Synthesis of graphene: noble metals nanocomposites**

Graphite powder, K2 S2 O8 , P2 O5 , conc. H2 SO<sup>4</sup> , KMnO4 , HI were purchased from Sigma-Aldrich, H2 O2 and HCl were obtained from Oltchim SA Romania. The electrocatalysts were synthesized via a facile process (**Figure 1**) described in detail elsewhere [11, 12]. Graphite oxide was prepared by the oxidation of graphite powder using P2 O5 , H2 SO<sup>4</sup> , and KMnO4 according to the Hummers' method, namely the graphitic powder was mixed with an oxidizing agent, filtered, washed and dried. Graphite oxide was dispersed in water by sonication for 2 h, forming the stable graphene oxide suspension (GO). Then, solutions of NaCl and the precursor of corresponding metal (water solutions of H2 PtCl6 or HAuCl4 ) were added individually to GO suspension under continuous mechanical stirring (500 rpm for 1 h).

The catalytic activity of the Pt graphene composites can be improved by increasing the charge transfer from the catalysts to the graphene substrate. The chemically functionalized graphene with interlayer structure contains network defects (bare spots, holes) and surface functional groups (carbonyls, epoxies, hydroxyls, etc.) that can bind metal nanoparticles and can retain them on the graphene surface. A strong metal–substrate interaction was demonstrated to enable the stability of the nanocatalyst, which can be improved by dispersion. Using Density Functional Theory (DFT) it was found that surface defects in the graphene support can also act as strong traps for Pt nanoclusters, leading to long-term stability of Pt graphene composites. Metallic graphene hybrids have exhibited significant electrocatalytic activities when used as anodic and cathodic catalysts in PEM fuel cells. Both n- and p-type conductivities can be obtained by selecting metal dopants, and the carrier concentration can be controlled by the quantity of metal deposited [8]. For example, several transition metals with different work functions, such as Ti, Fe, Cu, Ag, Au, and Pt, have been used for graphene band modulation. In particular, band arrangement is not only determined by work function, but also affected by the interfacial interactions, as a consequence the metal structures would influence the charge transfer between metal and graphene. Moreover, first-principles calculations indicated that the carbon-vacancies attract and trap metal atoms [8]. The binding of a single metal atom nearby a single-vacancy (SV) on the graphene was investigated recently for Pt, Pd, Au, and Sn, using the density-functional theory (DFT). Regarding the pristine graphene, Pt, Pd, and Sn prefer to be adsorbed at the bridge site, while Au prefers the top site. On the single-vacancy graphene, all the metal atoms prefer to be trapped at the vacancy site and appear as dopants. The trapping abilities of the SV-graphene for different metal atoms, are different, that is, the Pt and Pd have the larger trapping zones. The diffusion barrier of a metal atom on the SV-graphene is much higher than that on the pristine graphene, and the Pt atom has the highest diffusion barrier from the SV site to the neighboring bridge sites. More electrons are transferred from the adatoms (or dopants) to the carbon atoms at the defect site, which induces changes in the electronic structures and properties of the systems. However, the Pt

In our previous efforts on the development of innovative fuel cell catalysts, an experimental approach of finding low cost alternative support material in PEMFC was developed [9]. The potential of using graphene oxide as support/catalyst for PEMFC was exploited. Moving forward, Pt/graphene nanocomposites were synthesized using a simple route involving chemical oxidation, exfoliation of graphene oxide, as well as functionalization with a compatible polymer. Poly-diallyldimethylammonium chloride (PDDA) was used as cationic polyelectrolyte, with ability of adsorption on the graphene surface through π−π and electrostatic interactions,

atom is easier to be incorporated into the SV-graphene [8].

72 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

acting as a stabilizer for controlled synthesis of Pt/graphene [9, 10].

, conc. H2

SO<sup>4</sup>

, KMnO4

and HCl were obtained from Oltchim SA Romania. The electrocatalysts were synthesized

, HI were purchased from Sigma-Aldrich,

**2.1. Synthesis of graphene: noble metals nanocomposites**

S2 O8 , P2 O5

**2. Experimental**

Graphite powder, K2

H2 O2 Freshly prepared solution of NaBH4 was added slowly to the reaction mixture under vigorous stirring. The reaction mixture was stirred for another 24 h for the complete reduction, then washed with large amount of water several times, separated by filtration and centrifugation in order to remove residual ions. The obtained mass was then dried. pH plays an important role in the obtaining of the metal dispersed rGO, therefore the values of this parameter were mentioned in the experimental part (**Figure 1**) during each proposed fabrication steps. Aqueous NaOH and HCl solutions were used to adjust the pH value. In this work, we took into consideration the preparation of highly dispersed platinum and gold nanoparticles on reduced Graphene Oxide nanosheets with different metal compositions: Pt/rGO\_x and Au/ rGO\_y. It is known the fact that, during reduction step, graphene oxide gradually loses its oxygen-containing groups and could become hydrophobic, thus a direct reduction without a compatible polymer with ability of adsorption on the graphene surface, generally causes irreversible agglomerations. In our developed protocol synthesis PDDA was used as cationic polyelectrolyte owing to its expected reason of stabilizer effect, favoring the electrostatic interactions. Therefore, the bonding of reduced graphene oxide (rGO) with PDDA offers the possibility of controlling the behaviors of RGO in suspensions.

**Figure 1.** Reaction routes for the synthesis of metal nanoparticles on graphene oxide.

### **2.2. Catalysts characterization**

The microstructure and morphology of prepared samples were evaluated by using the following techniques: Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), specific surface area. Scanning Electron Microscopy (SEM) measurements were carried out using a Hitachi SU 8230 Scanning Electron Microscope equipped with EDS detector-analyzer. Hitachi HD 2700 Scanning Transmission Electron Microscope equipped with EDX Oxford detector-analyzer was used to perform graphene samples analysis. For the analysis, the samples in the form of powder were dispersed in bi-distilled water using a probe-type ultrasonic homogenizer and deposited on a standard Cu TEM grid. Also, EDX analysis was performed to determine the chemical composition of the samples. Autosorb IQ (Quantachrome, USA) instrumentation was used to perform the adsorption and desorption experiments at 77 K after initial pre-treatment of the samples by degassing at 115°C for 4 hours. The powder X-ray diffraction (XRD) analyses were performed at room temperature on a Rigaku SmartLab X Ray Diffractometer with Cu target Kβ<sup>1</sup> = 1.39217 Å, Kα<sup>1</sup> = 1.540598 Å and Kα<sup>2</sup> = 1.544426 Å. The diffraction data was recorded for 2θ angles using the following parameters: tube voltage = 45 kV, tube current = 200 mA, scan range: 5–90°. The identification of the phase was made by referring to the International Center for Diffraction Data—ICDD (PDF-2) database.

**3. Results and discussions**

structure of the graphene oxide support.

25 nm for Au/rGO and 7–15 nm for Pt/rGO.

and **3**).

rGO—down.

Firstly, the characterization of the prepared materials was performed to validate the microstructural quality and to confirm the metal presence and the structured morphology (**Figures 2**

Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

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75

The crystalline structure of Pt/rGO\_x and Au/rGO\_y composites was characterized by X-ray diffraction (XRD). XRD patterns of prepared doped-graphene with Pt or Au nanoparticles are presented in **Figure 4**. The prepared materials exhibited characteristic diffraction peaks at 39.76° (38.18°), 46.24° (44.39°), 67.46° (64.58°), 81.29° (77.55°)corresponding to the (111), (200), (220), (311) planes of face-centered cubic structure of Pt (PDF Card 00-004-0802) and Au (PDF Card 00-004-0784), respectively. The diffraction peaks in all samples are also assigned to the

Based on the intensity of the main peaks from the XRD patterns and using on the Scherrer's equation, the mean crystallites sizes were computed: Dhkl = Kλ/Bhklcos θ, where Dhkl is the crystallite size in the direction perpendicular to the lattice planes (hkl), K is a constant related to crystallite shape, λ is the X-ray wavelength in nm, and Bhkl is the peak width at half-maximum peak height. The crystalline size estimated from the Scherrer equation is between 14 and

**Figure 2.** XRD patterns of metal-dispersed nanoparticles on reduced graphene oxide composites: Pt/rGO—up and Au/

### **2.3. Electrode preparation and electrochemical measurements**

The modified carbon paste electrodes were prepared by mixing graphite powder with paraffin oil, then mixing in a mortar until a consistent uniformly wetted paste was obtained. The ratio of the two components was approximately 3:2 (w/w). The obtained paste was placed into a plastic syringe with an inner volume of 1.0 mL. The electrical contact was assured by a copper wire that was inserted into the back of the graphite paste. The modified electrodes were prepared by mixing certain amounts of carbon paste with platinum-doped reduced graphene oxide and gold-doped reduced graphene oxide in ratio of 109.7:0.3 and 109:1 (w/w), respectively. The obtained materials were pressed at the end of carbon paste from syringes. Thus, four electrodes were prepared: bare carbon paste electrode (CPE), graphene oxide modified carbon paste electrode (denoted by GO), platinum-doped reduced graphene oxide (denoted by Pt/rGO) and gold-doped reduced graphene oxide (denoted by Au/rGO). The surface of all electrodes was smoothed by polishing on a piece of weighing paper. Electrochemical measurements were carried out on a potentiostat/galvanostat system AutoLabPGStat 12, controlled by GPES (general purpose electrochemical system), electrochemical interface for Windows (version 4.9.007). Three electrodes in one compartment cell (10 mL) were used in all experiments. A glassy carbon electrode (Metrohm, 3 mm in diameter) and each modified carbon paste electrodes served as working electrodes. The counter electrode was a large area Pt wire. All experimental potentials were referred to Ag/AgCl, KClsat as reference electrode.

The electrochemical characterization of the modified carbon paste electrodes was carried out by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CV experiments were recorded in 0.5 M KCl solution containing 1.0 mmol L−<sup>1</sup> K3 Fe(CN)6 in the potential range of (−0.35) to (+0.8) V at scan rates of 20–120 mV s−<sup>1</sup> . DPV curves were recorded in the same potential domains with 5 mV potential and 25 mV as modulation amplitude.

### **3. Results and discussions**

**2.2. Catalysts characterization**

74 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

(PDF-2) database.

**2.3. Electrode preparation and electrochemical measurements**

were recorded in 0.5 M KCl solution containing 1.0 mmol L−<sup>1</sup>

potential domains with 5 mV potential and 25 mV as modulation amplitude.

of (−0.35) to (+0.8) V at scan rates of 20–120 mV s−<sup>1</sup>

The microstructure and morphology of prepared samples were evaluated by using the following techniques: Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), specific surface area. Scanning Electron Microscopy (SEM) measurements were carried out using a Hitachi SU 8230 Scanning Electron Microscope equipped with EDS detector-analyzer. Hitachi HD 2700 Scanning Transmission Electron Microscope equipped with EDX Oxford detector-analyzer was used to perform graphene samples analysis. For the analysis, the samples in the form of powder were dispersed in bi-distilled water using a probe-type ultrasonic homogenizer and deposited on a standard Cu TEM grid. Also, EDX analysis was performed to determine the chemical composition of the samples. Autosorb IQ (Quantachrome, USA) instrumentation was used to perform the adsorption and desorption experiments at 77 K after initial pre-treatment of the samples by degassing at 115°C for 4 hours. The powder X-ray diffraction (XRD) analyses were performed at room temperature on a Rigaku SmartLab X Ray Diffractometer with Cu target Kβ<sup>1</sup> = 1.39217 Å, Kα<sup>1</sup> = 1.540598 Å and Kα<sup>2</sup> = 1.544426 Å. The diffraction data was recorded for 2θ angles using the following parameters: tube voltage = 45 kV, tube current = 200 mA, scan range: 5–90°. The identification of the phase was made by referring to the International Center for Diffraction Data—ICDD

The modified carbon paste electrodes were prepared by mixing graphite powder with paraffin oil, then mixing in a mortar until a consistent uniformly wetted paste was obtained. The ratio of the two components was approximately 3:2 (w/w). The obtained paste was placed into a plastic syringe with an inner volume of 1.0 mL. The electrical contact was assured by a copper wire that was inserted into the back of the graphite paste. The modified electrodes were prepared by mixing certain amounts of carbon paste with platinum-doped reduced graphene oxide and gold-doped reduced graphene oxide in ratio of 109.7:0.3 and 109:1 (w/w), respectively. The obtained materials were pressed at the end of carbon paste from syringes. Thus, four electrodes were prepared: bare carbon paste electrode (CPE), graphene oxide modified carbon paste electrode (denoted by GO), platinum-doped reduced graphene oxide (denoted by Pt/rGO) and gold-doped reduced graphene oxide (denoted by Au/rGO). The surface of all electrodes was smoothed by polishing on a piece of weighing paper. Electrochemical measurements were carried out on a potentiostat/galvanostat system AutoLabPGStat 12, controlled by GPES (general purpose electrochemical system), electrochemical interface for Windows (version 4.9.007). Three electrodes in one compartment cell (10 mL) were used in all experiments. A glassy carbon electrode (Metrohm, 3 mm in diameter) and each modified carbon paste electrodes served as working electrodes. The counter electrode was a large area Pt wire. All experimental potentials were referred to Ag/AgCl, KClsat as reference electrode. The electrochemical characterization of the modified carbon paste electrodes was carried out by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CV experiments

K3

Fe(CN)6

. DPV curves were recorded in the same

in the potential range

Firstly, the characterization of the prepared materials was performed to validate the microstructural quality and to confirm the metal presence and the structured morphology (**Figures 2** and **3**).

The crystalline structure of Pt/rGO\_x and Au/rGO\_y composites was characterized by X-ray diffraction (XRD). XRD patterns of prepared doped-graphene with Pt or Au nanoparticles are presented in **Figure 4**. The prepared materials exhibited characteristic diffraction peaks at 39.76° (38.18°), 46.24° (44.39°), 67.46° (64.58°), 81.29° (77.55°)corresponding to the (111), (200), (220), (311) planes of face-centered cubic structure of Pt (PDF Card 00-004-0802) and Au (PDF Card 00-004-0784), respectively. The diffraction peaks in all samples are also assigned to the structure of the graphene oxide support.

Based on the intensity of the main peaks from the XRD patterns and using on the Scherrer's equation, the mean crystallites sizes were computed: Dhkl = Kλ/Bhklcos θ, where Dhkl is the crystallite size in the direction perpendicular to the lattice planes (hkl), K is a constant related to crystallite shape, λ is the X-ray wavelength in nm, and Bhkl is the peak width at half-maximum peak height. The crystalline size estimated from the Scherrer equation is between 14 and 25 nm for Au/rGO and 7–15 nm for Pt/rGO.

**Figure 2.** XRD patterns of metal-dispersed nanoparticles on reduced graphene oxide composites: Pt/rGO—up and Au/ rGO—down.

The morphology of reduced graphene oxide doped with gold or platinum samples (Pt/ rGO\_x and Au/rGO\_y) used in this work was examined using scanning transmission electron microscopy technique and the results are shown in **Figure 5**. The images were obtained with different detectors: SE—secondary electrons (left), ZC—phase contrast (middle) and TE—transmission electrons (right) at the same location on the sample. Structural studies were performed on all samples and obtained images were almost similar for each class of materials,

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The (200) plane in Platinum Nanoparticle with face-centered cubic structure was identified with a 1.922 Å d-spacing (ideal 1.962 Å)—up. Single layer graphene flake was identified with

The (111) plane in Gold Nanoparticle with face-centered cubic structure was identified with a 2.328 Å d-spacing (ideal 2.35 Å)—up. Single layer graphene flake was identified with a

**Figures 5** and **6** show the high resolution STEM micrographs, in which the interplanar distances can be clearly seen. The as marked interplanar distances are inserted in profiles from the orange marked areas, corresponding to the (111) and (200) face-centered cubic structure

STEM images showed the Pt and Au nanoparticles supported on the rGO as well-dispersed and well-separated metal nanoparticles, indicating a good spatial distribution of metal

The Energy-Dispersive X-ray spectra (EDX) were measured to characterize the elemental heterogeneity of PtNP/rGO\_x and AuNP/rGO\_y composites. The quantitative analysis was performed at 400x magnification. The EDX spectra for PtNP/rGO\_x show signals for carbon, oxygen and platinum with composition presented in **Table 1**. The EDX spectra for AuNP/ rGO\_y, present signals for carbon, oxygen and gold with composition presented in **Table 2**. With the desirable structural information presented so far, the materials with higher metal

The doping of Pt/Au into graphene oxide is expected to produce some changes in surface area and consequently to the pore size. Thus, due to the porous appearance of the materials confirmed by the performed analysis and taking into account the need for a high surface

**Figure 5.** Co-localized SE—secondary electrons (left), ZC—phase contrast (middle) and TE—transmission electrons

namely Pt/rGO\_x and Au/rGO\_y, respectively.

a 2.421 Å d-spacing (ideal 2.45 Å)—down.

2.461 Å d-spacing (ideal 2.45 Å)—down.

nanoparticles on the layered graphene sheets.

content, were further characterized and tested.

(right) images of the Au/rGO\_y sample.

of Pt and Au, respectively, which is in agreement to XRD results.

**Figure 3.** Co-localized SE—secondary electrons (left), ZC—phase contrast (middle) and TE—transmission electrons (right) images of the Pt/rGO\_x sample.

**Figure 4.** High resolution STEM images of the Pt/rGO\_x sample were recorded in the marked area in the co-localized images from above (left). The lattice constant from the area marked in orange was measured and the resulting profiles revealed Pt and graphene specific d-spacings (right).

These obvious characteristic peaks indicate a good crystallinity of the supported Pt/Au nanoparticles in the prepared composites.

The distance between two layers is an important parameter to evaluate the structural information of graphene. Due to the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the atomic-scale roughness arising from structural defects (sp3 bonding) generated on the originally atomically flat graphene sheet the d-spacing of the GO. The rGO (002) plane was observed at 2θ of about 23° indicating interatomic spacing of about 0.384 nm.

Both metal-dispersed nanoparticles on reduced graphene oxide presented similar XRD peaks corresponding to the face-centered cubic crystalline Pt/Au and no trace of other phases was detected, suggesting that the modification of the drying conditions does not provide a major influence to the crystalline structure.

The morphology of reduced graphene oxide doped with gold or platinum samples (Pt/ rGO\_x and Au/rGO\_y) used in this work was examined using scanning transmission electron microscopy technique and the results are shown in **Figure 5**. The images were obtained with different detectors: SE—secondary electrons (left), ZC—phase contrast (middle) and TE—transmission electrons (right) at the same location on the sample. Structural studies were performed on all samples and obtained images were almost similar for each class of materials, namely Pt/rGO\_x and Au/rGO\_y, respectively.

The (200) plane in Platinum Nanoparticle with face-centered cubic structure was identified with a 1.922 Å d-spacing (ideal 1.962 Å)—up. Single layer graphene flake was identified with a 2.421 Å d-spacing (ideal 2.45 Å)—down.

The (111) plane in Gold Nanoparticle with face-centered cubic structure was identified with a 2.328 Å d-spacing (ideal 2.35 Å)—up. Single layer graphene flake was identified with a 2.461 Å d-spacing (ideal 2.45 Å)—down.

**Figures 5** and **6** show the high resolution STEM micrographs, in which the interplanar distances can be clearly seen. The as marked interplanar distances are inserted in profiles from the orange marked areas, corresponding to the (111) and (200) face-centered cubic structure of Pt and Au, respectively, which is in agreement to XRD results.

STEM images showed the Pt and Au nanoparticles supported on the rGO as well-dispersed and well-separated metal nanoparticles, indicating a good spatial distribution of metal nanoparticles on the layered graphene sheets.

The Energy-Dispersive X-ray spectra (EDX) were measured to characterize the elemental heterogeneity of PtNP/rGO\_x and AuNP/rGO\_y composites. The quantitative analysis was performed at 400x magnification. The EDX spectra for PtNP/rGO\_x show signals for carbon, oxygen and platinum with composition presented in **Table 1**. The EDX spectra for AuNP/ rGO\_y, present signals for carbon, oxygen and gold with composition presented in **Table 2**.

With the desirable structural information presented so far, the materials with higher metal content, were further characterized and tested.

The doping of Pt/Au into graphene oxide is expected to produce some changes in surface area and consequently to the pore size. Thus, due to the porous appearance of the materials confirmed by the performed analysis and taking into account the need for a high surface

These obvious characteristic peaks indicate a good crystallinity of the supported Pt/Au

**Figure 4.** High resolution STEM images of the Pt/rGO\_x sample were recorded in the marked area in the co-localized images from above (left). The lattice constant from the area marked in orange was measured and the resulting profiles

**Figure 3.** Co-localized SE—secondary electrons (left), ZC—phase contrast (middle) and TE—transmission electrons

The distance between two layers is an important parameter to evaluate the structural information of graphene. Due to the presence of oxygen-containing functional groups attached on both sides of the graphene sheet and the atomic-scale roughness arising from structural defects (sp3 bonding) generated on the originally atomically flat graphene sheet the d-spacing of the GO. The rGO (002) plane was observed at 2θ of about 23° indicating interatomic spacing

Both metal-dispersed nanoparticles on reduced graphene oxide presented similar XRD peaks corresponding to the face-centered cubic crystalline Pt/Au and no trace of other phases was detected, suggesting that the modification of the drying conditions does not provide a major

nanoparticles in the prepared composites.

revealed Pt and graphene specific d-spacings (right).

(right) images of the Pt/rGO\_x sample.

76 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

influence to the crystalline structure.

of about 0.384 nm.

**Figure 5.** Co-localized SE—secondary electrons (left), ZC—phase contrast (middle) and TE—transmission electrons (right) images of the Au/rGO\_y sample.

area to provide an efficient ORR performance, we subsequently analyzed the surface area. The nitrogen adsorption–desorption isotherms were studied using Brunauer–Emmett–Teller (BET) and are provided in **Figure 7**. The hysteresis study also revealed that hysteresis loops showed parallel adsorption and desorption branches, regarded as Type H4 behavior among the IUPAC classification. This observation allows for a better understanding of the porous character of the prepared samples, demonstrating the presence of pores that are open at the end, but unconnected to each other. As shown, the isotherm curves of adsorption/desorption performance of samples were compatible with isotherm Type IV, with an abrupt increase at

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graphene oxide with highest metal content. Our measured surface area of the prepared doped graphene samples could be connected with the stacking structure and agglomerated morphology of the reduced graphene sheets and could be attributed to an increasing in the number of closed pores during platinum/gold doping. The estimated radii presented in **Table 3** suggest a hierarchical interconnected porous framework in the prepared doped graphene. Different pore types displayed different roles in the PEMFC electrochemical performance, thus the existence of various porous characteristics ensures sufficient space that enables the access of the

reactants to the catalytic sites and could accelerate the kinetic process of ion diffusion.

Based on the structural information presented above, the prepared Pt- and Au-doped graphene oxide was evaluated as ORR cathode under practical FC operation conditions. In comparison to commercial Pt/C, a better cathode performance, including the prepared catalytic

**)**

adsorbed at P/P0 = 0.99, using BJH method.

**(b) BJH pore radius (nm)**

for Pt- and Au-doped

high relative pressure, with respect to IUPAC classification.

**Figure 7.** BET isotherms corresponding to Pt/rGO\_x and Au/rGO\_y.

**Samples SBET (m2 g−<sup>1</sup>**

(b)BJH pore volume, taken from the volume of N2

**Table 3.** Textural properties of Pt/rGO and Au/rGO.

The BET specific surface areas are adequate, namely 158 and 107 m<sup>2</sup> g−<sup>1</sup>

**)(a) BJH pore volume (cm3 g−<sup>1</sup>**

Pt/rGO 158 0.101 1.857 Au/rGO 107 0.059 1.964

(a)BET surface area calculated from the linear part of the BET plot (P/P0 = 0.1–0.3).

**Figure 6.** High resolution STEM images of the Au/rGO\_y sample were recorded in the marked area in the co-localized images from above (left). The lattice constant from the area marked in orange was measured and the resulting profiles revealed Au and graphene specific d-spacings (right).


**Table 1.** EDAX quantitative analysis—composition profile for PtNP/rGO\_y.


**Table 2.** EDAX quantitative analysis—composition profile for AuNP/rGO\_y.

Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells http://dx.doi.org/10.5772/intechopen.80941 79

**Figure 7.** BET isotherms corresponding to Pt/rGO\_x and Au/rGO\_y.

area to provide an efficient ORR performance, we subsequently analyzed the surface area. The nitrogen adsorption–desorption isotherms were studied using Brunauer–Emmett–Teller (BET) and are provided in **Figure 7**. The hysteresis study also revealed that hysteresis loops showed parallel adsorption and desorption branches, regarded as Type H4 behavior among the IUPAC classification. This observation allows for a better understanding of the porous character of the prepared samples, demonstrating the presence of pores that are open at the end, but unconnected to each other. As shown, the isotherm curves of adsorption/desorption performance of samples were compatible with isotherm Type IV, with an abrupt increase at high relative pressure, with respect to IUPAC classification.

The BET specific surface areas are adequate, namely 158 and 107 m<sup>2</sup> g−<sup>1</sup> for Pt- and Au-doped graphene oxide with highest metal content. Our measured surface area of the prepared doped graphene samples could be connected with the stacking structure and agglomerated morphology of the reduced graphene sheets and could be attributed to an increasing in the number of closed pores during platinum/gold doping. The estimated radii presented in **Table 3** suggest a hierarchical interconnected porous framework in the prepared doped graphene. Different pore types displayed different roles in the PEMFC electrochemical performance, thus the existence of various porous characteristics ensures sufficient space that enables the access of the reactants to the catalytic sites and could accelerate the kinetic process of ion diffusion.

Based on the structural information presented above, the prepared Pt- and Au-doped graphene oxide was evaluated as ORR cathode under practical FC operation conditions. In comparison to commercial Pt/C, a better cathode performance, including the prepared catalytic


(b)BJH pore volume, taken from the volume of N2 adsorbed at P/P0 = 0.99, using BJH method.

**Table 3.** Textural properties of Pt/rGO and Au/rGO.

**Element AuNP/rGO\_1 AuNP/rGO\_2**

**Table 1.** EDAX quantitative analysis—composition profile for PtNP/rGO\_y.

**Element PtNP/rGO\_1 PtNP/rGO\_2**

revealed Au and graphene specific d-spacings (right).

78 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

**Table 2.** EDAX quantitative analysis—composition profile for AuNP/rGO\_y.

C 74.91 87.16 72.75 86.46 O 13.79 12.04 14.11 12.59 Au 11.30 0.80 13.14 0.95

C 58.96 86.01 62.47 85.87 O 10.25 11.22 11.57 11.98 Pt 30.78 2.77 25.96 2.20

**Weight % Atomic % Weight % Atomic %**

**Figure 6.** High resolution STEM images of the Au/rGO\_y sample were recorded in the marked area in the co-localized images from above (left). The lattice constant from the area marked in orange was measured and the resulting profiles

**Weight % Atomic % Weight % Atomic %**

system based on doped graphene can be easily anticipated, taking into account our recent results [11]. This improvement in performance was explained on the basis of higher electrical conductivity and durability parameters, essential for the PEMFC commercialization.

Electrochemical processes occur at the electrode/solution interface, in contrast to many other chemical measurements that involve bulk solutions. Due to the fact that the reaction is controlled by the electrode potential, electrochemistry is very sensitive and selective for the detection of electroactive species in extremely low limits of detection (nanomolar) and very small sample volumes (μL). The material used defines the performance, hence the continuous interest in developing new generation of electrodes. Carbon materials are widely used in industrial electrochemistry and modified electrodes are still developing with the aim of enhancing both the electrochemical characteristics and performance. There is a large range of carbon based forms available for use as an electrode material, with various allotropic forms exhibiting distinct properties. Among them, graphene materials present an enormous interest for electrochemists owing to their extraordinary physical, chemical and electrical properties. Graphenes are reported as performing electrode constituents for a wide variety of electrochemical applications, including the fabrication of energy storage devices, membrane material, and simultaneous characterization of ascorbic acid, dopamine and uric acid levels [13]. Graphene is the thinnest electrode material, but there are experimental parameters to be overcome: the first problem is the electrical connection of the graphene; the second issue is to avoid the aggregation of graphene sheets to form graphite through strong π–π interactions between the constituting sheets; the last limitation regards the quality of the obtained graphenes by various routes with different electrochemical properties.

The aim of the present research is the fundamental electrochemical characterization of modified graphene-based electrode materials. In electrochemical measurements, supporting electrolytes are widely used. They contain chemical species which are not electroactive in the range of used potentials and have higher conductivity and ionic strength in comparison with electroactive species. Therefore they increase the conductivity of the solution, maintain constant ionic strength and pH, eliminate the transport of electroactive species by ion migration. KCl solutions are widely used as supporting electrolyte, due to relatively high ion conductivity; also potassium ion has a smaller hydration sphere than other alkaline ions We focused our attention on developing novel electrodes using carbon paste as support to investigations of the classical ferri/ferro redox process with attention on the electrochemistry of Pt-doped reduced graphene oxide electrodes. All the modified electrodes were tested for the redox process of 1 mM potassium ferrocyanide(II) using 0.5 M KCl as electrolyte.

$$2\,\mathrm{K}\_{\mathrm{j}}[\mathrm{Fe}\,\mathrm{(CN)}\_{\mathrm{6}}] + 2\,\mathrm{KOH} \rightarrow 2\,\mathrm{K}\_{\mathrm{4}}[\mathrm{Fe}\,\mathrm{(CN)}\_{\mathrm{6}}] + \mathrm{H}\_{2}\mathrm{O} + \mathrm{O} \tag{1}$$

$$\text{Fe}^{3+} + \text{e}^{\cdot} \leftrightarrow \text{Fe}^{2+} \tag{2}$$

peak height as a function of the square-root of scan rate shows a highly linear response, indicating a diffusion electrochemical process in accordance with reported literature data [14].

Fe(CN)6

The focus herein is to electrochemically characterize reduced GO modified electrodes for efficiency towards ferri/ferro redox probe. For achieving this goal, carbon paste electrodes were prepared by mixing graphite powder with paraffin oil in a ratio of approximately 3:2 (w/w). The two components are hand mixed in a mortar until a consistent wet paste is formed. The obtained paste is placed into a plastic syringe with an inner volume of 1.0 mL. For assuring the electrical contact, a copper wire was inserted into the back of the carbon paste. A control experiment was first performed utilizing a bare carbon paste electrode. The electrochemical characteristic signatures are similar with those of the glassy

with smaller intensity of the peaks indicates that the bare carbon paste electrode is not sensitive to the chosen redox probe. However, the electrochemical process is also diffusion controlled, the voltammetric peak height as a function of the square-root of scan rate also

Next, the modified electrodes were prepared by mixing certain amounts of carbon paste with graphene oxide and then, carbon paste with metal-doped graphene oxide. We have previously observed that modified gold-doped reduced graphene oxide electrodes are sensitive for ferri/ferro redox systems. In all cases, the surface of the electrodes was smoothed by polishing with filter paper. All electrodes were kept in distilled water before and after measurements. The electrochemical measurements were carried out on a potentiostat galvanostat system using one compartment cell of 10 mL containing three electrodes. Each modified carbon paste electrode served as working electrodes. The counter electrode was a large area Pt wire and Ag/AgCl, KClsat constituted the reference electrode. The experiments were run in the potential

To ensure a better evaluation of the Pt-doped rGO modified electrode, similar experiments with GO-modified electrode were performed. By comparison with the bare carbon paste electrode, the experimental results showed better defined anodic and cathodic peaks related

being 325 mV. This, together

in 0.5 M KCl solution on glassy carbon electrode, v = 20–120 mV s−<sup>1</sup>

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Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

carbon, the peak-to-peak separations (ΔEp) at 100 mV s−<sup>1</sup>

showing a linear response (**Figure 9**).

**Figure 8.** Cyclic voltammograms for 1.0 mM K<sup>3</sup>

(left) and plot of I *vs*. v½ (right).

range of −0.35 to 0.8 V at various scan rate.

The electrochemical response of a standard glassy carbon was first tested. **Figure 8** depicts the cyclic voltammetry of the ferri/ferro redox system, scanned from 20 to 120 mV/s in the −0.35 to 0.80 V potential range. The peak-to-peak separation (ΔEp) at 100 mV s−<sup>1</sup> is 181 mV. Analysis of the peak-to-peak separation as a function of voltage scan rate indicates that the electrochemical process is quasi-reversible within the employed scan rates. Monitoring of the voltammetric

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system based on doped graphene can be easily anticipated, taking into account our recent results [11]. This improvement in performance was explained on the basis of higher electrical

Electrochemical processes occur at the electrode/solution interface, in contrast to many other chemical measurements that involve bulk solutions. Due to the fact that the reaction is controlled by the electrode potential, electrochemistry is very sensitive and selective for the detection of electroactive species in extremely low limits of detection (nanomolar) and very small sample volumes (μL). The material used defines the performance, hence the continuous interest in developing new generation of electrodes. Carbon materials are widely used in industrial electrochemistry and modified electrodes are still developing with the aim of enhancing both the electrochemical characteristics and performance. There is a large range of carbon based forms available for use as an electrode material, with various allotropic forms exhibiting distinct properties. Among them, graphene materials present an enormous interest for electrochemists owing to their extraordinary physical, chemical and electrical properties. Graphenes are reported as performing electrode constituents for a wide variety of electrochemical applications, including the fabrication of energy storage devices, membrane material, and simultaneous characterization of ascorbic acid, dopamine and uric acid levels [13]. Graphene is the thinnest electrode material, but there are experimental parameters to be overcome: the first problem is the electrical connection of the graphene; the second issue is to avoid the aggregation of graphene sheets to form graphite through strong π–π interactions between the constituting sheets; the last limitation regards the quality of the obtained

conductivity and durability parameters, essential for the PEMFC commercialization.

80 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

graphenes by various routes with different electrochemical properties.

process of 1 mM potassium ferrocyanide(II) using 0.5 M KCl as electrolyte.

0.80 V potential range. The peak-to-peak separation (ΔEp) at 100 mV s−<sup>1</sup>

2 K3[Fe (CN)

The aim of the present research is the fundamental electrochemical characterization of modified graphene-based electrode materials. In electrochemical measurements, supporting electrolytes are widely used. They contain chemical species which are not electroactive in the range of used potentials and have higher conductivity and ionic strength in comparison with electroactive species. Therefore they increase the conductivity of the solution, maintain constant ionic strength and pH, eliminate the transport of electroactive species by ion migration. KCl solutions are widely used as supporting electrolyte, due to relatively high ion conductivity; also potassium ion has a smaller hydration sphere than other alkaline ions We focused our attention on developing novel electrodes using carbon paste as support to investigations of the classical ferri/ferro redox process with attention on the electrochemistry of Pt-doped reduced graphene oxide electrodes. All the modified electrodes were tested for the redox

<sup>6</sup>] + 2KOH → 2 K4[Fe (CN)

Fe3+ + e‐ ↔ Fe2<sup>+</sup> (2)

The electrochemical response of a standard glassy carbon was first tested. **Figure 8** depicts the cyclic voltammetry of the ferri/ferro redox system, scanned from 20 to 120 mV/s in the −0.35 to

the peak-to-peak separation as a function of voltage scan rate indicates that the electrochemical process is quasi-reversible within the employed scan rates. Monitoring of the voltammetric

<sup>6</sup>] + H2 O + O (1)

is 181 mV. Analysis of

**Figure 8.** Cyclic voltammograms for 1.0 mM K<sup>3</sup> Fe(CN)6 in 0.5 M KCl solution on glassy carbon electrode, v = 20–120 mV s−<sup>1</sup> (left) and plot of I *vs*. v½ (right).

peak height as a function of the square-root of scan rate shows a highly linear response, indicating a diffusion electrochemical process in accordance with reported literature data [14].

The focus herein is to electrochemically characterize reduced GO modified electrodes for efficiency towards ferri/ferro redox probe. For achieving this goal, carbon paste electrodes were prepared by mixing graphite powder with paraffin oil in a ratio of approximately 3:2 (w/w). The two components are hand mixed in a mortar until a consistent wet paste is formed. The obtained paste is placed into a plastic syringe with an inner volume of 1.0 mL. For assuring the electrical contact, a copper wire was inserted into the back of the carbon paste. A control experiment was first performed utilizing a bare carbon paste electrode. The electrochemical characteristic signatures are similar with those of the glassy carbon, the peak-to-peak separations (ΔEp) at 100 mV s−<sup>1</sup> being 325 mV. This, together with smaller intensity of the peaks indicates that the bare carbon paste electrode is not sensitive to the chosen redox probe. However, the electrochemical process is also diffusion controlled, the voltammetric peak height as a function of the square-root of scan rate also showing a linear response (**Figure 9**).

Next, the modified electrodes were prepared by mixing certain amounts of carbon paste with graphene oxide and then, carbon paste with metal-doped graphene oxide. We have previously observed that modified gold-doped reduced graphene oxide electrodes are sensitive for ferri/ferro redox systems. In all cases, the surface of the electrodes was smoothed by polishing with filter paper. All electrodes were kept in distilled water before and after measurements. The electrochemical measurements were carried out on a potentiostat galvanostat system using one compartment cell of 10 mL containing three electrodes. Each modified carbon paste electrode served as working electrodes. The counter electrode was a large area Pt wire and Ag/AgCl, KClsat constituted the reference electrode. The experiments were run in the potential range of −0.35 to 0.8 V at various scan rate.

To ensure a better evaluation of the Pt-doped rGO modified electrode, similar experiments with GO-modified electrode were performed. By comparison with the bare carbon paste electrode, the experimental results showed better defined anodic and cathodic peaks related

**Figure 9.** Cyclic voltammograms for 1.0 mM K<sup>3</sup> Fe(CN)6 in 0.5 M KCl solution on bare carbon paste electrode, v = 20–120 mV s−<sup>1</sup> (left) and I *vs*. v½ plot (right).

Owing to the superior electrochemical response of the GO-modified electrode, we further explored whether the introduction of metal-doped rGO onto the carbon paste modified electrodes improves the electrochemical response of the studied redox system. Thus, we studied the Pt-doped rGO modified electrode. **Figure 12** shows an overlay of the cyclic voltammograms of Pt/rGO and GO modified electrodes. Analysis of the cyclic voltammograms reveals that the platinum-doped rGO modified electrode exhibits an enhancement of the current response when compared to GO which translates in better electrocatalytic activity for both oxidation and reduction processes, due to the metal presence. A better electrochemical performance than the non-metal modified GO electrode was recorded with a current 3.94 times higher than bare carbon paste electrode (**Table 4**). Both anodic and cathodic peak potentials

**Figure 12.** Overlay of the cyclic voltammograms for GO and Pt-doped rGO modified electrodes for the redox process of

).

in 0.5 M KCl solution on GO modified electrode, v = 20–120 mV s−<sup>1</sup>

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Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

Fe(CN)6

**Figure 11.** Cyclic voltammograms for 1.0 mM K<sup>3</sup>

(left) and I *vs*. v½ plot (right).

1.0 mM K<sup>3</sup>

Fe(CN)6

in 0.5 M KCl solution (v = 20 mV s−<sup>1</sup>

to Fe(CN)6 <sup>3</sup>−/Fe(CN)6 <sup>4</sup>− redox couple for slow (20 mV/s) and high scan rates (100 mV/s) (**Figure 10**). The peak separation potential ΔEp was observed as 182 mV, similar with the results observed for the standard glassy carbon electrode. A comparison of cyclic voltammetry measurements is presented in **Figure 11**. In the case of the GO-modified electrode, for both slow and fast sweep rates, both the anodic and the cathodic peaks are sharper and well defined when compared to bare carbon paste electrode. This highlights the positive effect of the graphene oxide on the carbon paste support used for the preparation of the modified electrodes, an improvement of the analytical signal (peak height) being clearly observed.

The voltammetric profile of GO modified electrode was next explored by sweep rate variation from 20, 40, 80 to 120 mV/s. The results show that the anodic peaks increases with increasing the sweep rate and moves to positive potentials. Also, by increasing the sweep rate, the peak shape does not modify, which leads to the conclusion that this modified electrode is sensitive for the electrochemical investigation of the ferri/ferro process. Analysis of the voltammetric peak height as a function of the square-root of the scan rate reveals a highly linear response as observed for the above two investigated electrodes.

**Figure 10.** Comparison of the cyclic voltammograms of the modified electrodes according to the legends performed at 20 mV/s (left) and 100 mV/s (right) scan rate.

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**Figure 11.** Cyclic voltammograms for 1.0 mM K<sup>3</sup> Fe(CN)6 in 0.5 M KCl solution on GO modified electrode, v = 20–120 mV s−<sup>1</sup> (left) and I *vs*. v½ plot (right).

to Fe(CN)6

v = 20–120 mV s−<sup>1</sup>

<sup>3</sup>−/Fe(CN)6

**Figure 9.** Cyclic voltammograms for 1.0 mM K<sup>3</sup>

(left) and I *vs*. v½ plot (right).

82 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

observed for the above two investigated electrodes.

20 mV/s (left) and 100 mV/s (right) scan rate.

<sup>4</sup>− redox couple for slow (20 mV/s) and high scan rates (100 mV/s)

in 0.5 M KCl solution on bare carbon paste electrode,

(**Figure 10**). The peak separation potential ΔEp was observed as 182 mV, similar with the results observed for the standard glassy carbon electrode. A comparison of cyclic voltammetry measurements is presented in **Figure 11**. In the case of the GO-modified electrode, for both slow and fast sweep rates, both the anodic and the cathodic peaks are sharper and well defined when compared to bare carbon paste electrode. This highlights the positive effect of the graphene oxide on the carbon paste support used for the preparation of the modified electrodes, an improvement of the analytical signal (peak height) being clearly observed.

Fe(CN)6

The voltammetric profile of GO modified electrode was next explored by sweep rate variation from 20, 40, 80 to 120 mV/s. The results show that the anodic peaks increases with increasing the sweep rate and moves to positive potentials. Also, by increasing the sweep rate, the peak shape does not modify, which leads to the conclusion that this modified electrode is sensitive for the electrochemical investigation of the ferri/ferro process. Analysis of the voltammetric peak height as a function of the square-root of the scan rate reveals a highly linear response as

**Figure 10.** Comparison of the cyclic voltammograms of the modified electrodes according to the legends performed at

Owing to the superior electrochemical response of the GO-modified electrode, we further explored whether the introduction of metal-doped rGO onto the carbon paste modified electrodes improves the electrochemical response of the studied redox system. Thus, we studied the Pt-doped rGO modified electrode. **Figure 12** shows an overlay of the cyclic voltammograms of Pt/rGO and GO modified electrodes. Analysis of the cyclic voltammograms reveals that the platinum-doped rGO modified electrode exhibits an enhancement of the current response when compared to GO which translates in better electrocatalytic activity for both oxidation and reduction processes, due to the metal presence. A better electrochemical performance than the non-metal modified GO electrode was recorded with a current 3.94 times higher than bare carbon paste electrode (**Table 4**). Both anodic and cathodic peak potentials

**Figure 12.** Overlay of the cyclic voltammograms for GO and Pt-doped rGO modified electrodes for the redox process of 1.0 mM K<sup>3</sup> Fe(CN)6 in 0.5 M KCl solution (v = 20 mV s−<sup>1</sup> ).


**Table 4.** Electrochemical data from CV measurements at 20 mV/s; I<sup>a</sup> and Ic represent the anodic and cathodic peak currents, ΔE is the separation between the peak potentials and A is the area of an electrode, with the corresponding measurements units.

are shifted by approximately −80 mV for Pt/rGO, when compared with the potential of bare carbon paste electrode. An increase in the peak currents and a decrease in the separation between the peak potentials (ΔEp) at 20 mV s−<sup>1</sup> were observed for all modified electrodes in comparison to the bare CPE (ΔEp = 385 mV), indicating that the electron transfer reaction was kinetically and thermodynamically favored at the graphene oxide modified electrode surface.

However, the parameter of most significant importance is represented by the position of the voltammetric peak, rather than the magnitude of the wave. In the case of the metal-doped graphene modified electrodes, the larger peak current is likely due to a slightly larger surface

**Figure 14.** Overlay of the cyclic voltammograms for GO, Pt-doped rGO and Au-doped rGO modified electrodes for the

).

in 0.5 M KCl solution (v = 20 mV s−<sup>1</sup>

in 0.5 M KCl solution on Pt/rGO modified electrode,

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85

Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

Fe(CN)6

area at the electrode (see **Table 4**).

redox process of 1.0 mM K<sup>3</sup>

Fe(CN)6

v = 20–120 mV s−<sup>1</sup>

**Figure 13.** Cyclic voltammograms for 1.0 mM K<sup>3</sup>

(left) and plot of I *vs*. v½ (right).

By increasing the scan rate, the intensity of the peak increases not only in the anodic direction, but also in the cathodic side. The peaks are not well defined, most likely because of the low concentration of the platinum-doped reduced graphene oxide in the modified electrode. The effect of varying the scan rate results in the slight shifting of the peak potentials to higher values, indicating a quasi-reversible electron transfer. The plot of the peak height *vs.* square-root of the scan rate is illustrated in **Figure 13**, on the right. Both, the anodic and the cathodic peak currents reveal linear response at all scan rates with very good correlation factors above 0.9. These results indicate that the electrochemical process is, as previously observed, controlled by the diffusion of the electroactive species. Modified electrodes containing platinum-doped graphene materials cause an increase of the current, however due to the low concentration, the peaks are not well defined. Reported literature data showed that the electrochemical response of graphene modified electrodes can be improved by increasing the amount of graphene into the electrode [15]. The electrochemical data and areas of the electrodes presented in **Table 4** reveal that the Pt-doped rGO modified electrode has a higher electroactive area than the graphene oxide electrode.

We have shown earlier that Au/rGO modified electrode exhibits an enhancement of the current response when compared to classical glassy carbon, which translates in better electrocatalytic activity for both oxidation and reduction processes. The Au/rGO modified electrode showed a current almost four times higher than bare carbon paste electrode.

A comparison of the cyclic voltammograms of GO, Pt- and Au-doped graphene material electrodes is shown in **Figure 14**. The cyclic voltammetric response of the gold-containing modified electrode shows well defined redox peaks with a peak-to-peak separation, ΔE, of 200 mV. Thus, it was clearly revealed that the insertion of gold onto the modified graphene oxide electrodes gives improved electrochemical response for the ferri/ferro redox signal. Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells http://dx.doi.org/10.5772/intechopen.80941 85

**Figure 13.** Cyclic voltammograms for 1.0 mM K<sup>3</sup> Fe(CN)6 in 0.5 M KCl solution on Pt/rGO modified electrode, v = 20–120 mV s−<sup>1</sup> (left) and plot of I *vs*. v½ (right).

are shifted by approximately −80 mV for Pt/rGO, when compared with the potential of bare carbon paste electrode. An increase in the peak currents and a decrease in the separation

currents, ΔE is the separation between the peak potentials and A is the area of an electrode, with the corresponding

 **(A) Ic**

1. CPE 2.40·10−<sup>6</sup> −3.23·10−<sup>6</sup> 325 0.0165 2. GO 6.14·10−<sup>6</sup> −5.99·10−<sup>6</sup> 182 0.0309 3. Au/rGO 9.45·10−<sup>6</sup> −1.47·10−<sup>5</sup> 200 0.2020 4. Pt/rGO 5.01·10−<sup>6</sup> −8.04·10−<sup>6</sup> 153 0.0660

comparison to the bare CPE (ΔEp = 385 mV), indicating that the electron transfer reaction was kinetically and thermodynamically favored at the graphene oxide modified electrode

By increasing the scan rate, the intensity of the peak increases not only in the anodic direction, but also in the cathodic side. The peaks are not well defined, most likely because of the low concentration of the platinum-doped reduced graphene oxide in the modified electrode. The effect of varying the scan rate results in the slight shifting of the peak potentials to higher values, indicating a quasi-reversible electron transfer. The plot of the peak height *vs.* square-root of the scan rate is illustrated in **Figure 13**, on the right. Both, the anodic and the cathodic peak currents reveal linear response at all scan rates with very good correlation factors above 0.9. These results indicate that the electrochemical process is, as previously observed, controlled by the diffusion of the electroactive species. Modified electrodes containing platinum-doped graphene materials cause an increase of the current, however due to the low concentration, the peaks are not well defined. Reported literature data showed that the electrochemical response of graphene modified electrodes can be improved by increasing the amount of graphene into the electrode [15]. The electrochemical data and areas of the electrodes presented in **Table 4** reveal that the Pt-doped rGO modified electrode has a higher electroactive area than the gra-

We have shown earlier that Au/rGO modified electrode exhibits an enhancement of the current response when compared to classical glassy carbon, which translates in better electrocatalytic activity for both oxidation and reduction processes. The Au/rGO modified electrode

A comparison of the cyclic voltammograms of GO, Pt- and Au-doped graphene material electrodes is shown in **Figure 14**. The cyclic voltammetric response of the gold-containing modified electrode shows well defined redox peaks with a peak-to-peak separation, ΔE, of 200 mV. Thus, it was clearly revealed that the insertion of gold onto the modified graphene oxide electrodes gives improved electrochemical response for the ferri/ferro redox signal.

showed a current almost four times higher than bare carbon paste electrode.

were observed for all modified electrodes in

represent the anodic and cathodic peak

 **(A) ΔE (mV) A (cm2**

and Ic

**)**

between the peak potentials (ΔEp) at 20 mV s−<sup>1</sup>

**Table 4.** Electrochemical data from CV measurements at 20 mV/s; I<sup>a</sup>

84 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

**Entry Electrode type Ia**

surface.

measurements units.

phene oxide electrode.

However, the parameter of most significant importance is represented by the position of the voltammetric peak, rather than the magnitude of the wave. In the case of the metal-doped graphene modified electrodes, the larger peak current is likely due to a slightly larger surface area at the electrode (see **Table 4**).

**Figure 14.** Overlay of the cyclic voltammograms for GO, Pt-doped rGO and Au-doped rGO modified electrodes for the redox process of 1.0 mM K<sup>3</sup> Fe(CN)6 in 0.5 M KCl solution (v = 20 mV s−<sup>1</sup> ).

In order to better investigate the electroanalytical outcome of these graphene modified electrodes, we performed differential pulse voltammetry (DPV) measurements. The DPV experiments were meant to support the cyclic voltammetry investigations, as it is clearly known that DPV can assist in resolving the signals due to species with close half-wave potentials, as it measures the difference between two currents, before the end of the pulse and before its application. The strength of this technique is evident when poor electrochemical signals are obtained in cyclic voltammetry, enabling the registration of well-defined signals through the elimination of the non-Faradaic processes. Moreover, DPV provides useful information when the resolving of the voltammetric signals given by two species with close half-wave potentials is needed, producing easily quantifiable peak shaped responses.

**Figure 15** compares the differential pulse voltammograms of the graphene modified carbon paste electrodes, as well as the classical glassy carbon electrode. In all cases, sharp and wellresolved peaks are observed. The anodic (right side) and cathodic (left side) peaks are higher for the metal-doped reduced graphene oxide modified electrodes with the highest peak current being observed for the gold doped reduced graphene oxide modified electrode. An important parameter of an electrode material is its electronic properties, namely, the density of electronic states (DOS). The DOS of graphene materials has been reported as being high and it can be increased by varying the amounts of loaded graphene. Gold has a DOS of 0.28 states atom−<sup>1</sup> eV−<sup>1</sup> with high conductivity due to high proportion of atomic orbitals. Thus, the Au-doped rGO modified electrodes are highly sensitive for electrochemical investigation of the ferri/ferro redox system.

The active surface areas of the graphene modified electrodes were estimated according to the slope of the Ia *vs.* square root of the scan rates plot for a 1.0 mM K3 [Fe(CN)6 ], solution using 0.50 M KCl electrolyte, according to Randles–Sevcik equation [16]:

$$I\_i = 2.69 \cdot 10^5 \cdot n^{32} \cdot D^{12} \cdot A \cdot \mathbf{c} \cdot \mathbf{v}^{12} \tag{3}$$

where Ia

the scan rate.

For n = 1 and D = 6.68 × 10−<sup>6</sup> cm2 s−<sup>1</sup>

system PEMFC with active area of 25 cm2

) gases (H2

and N2

NI c-RIO hardware was used to control the PEMFC system.

4000 with 0.4 mgPt cm−<sup>2</sup>

0.2 mgPt cm−<sup>2</sup>

150 mL min−<sup>1</sup>

times higher than the graphene oxide modified electrode.

as simultaneous CV characterization and degradation testing tool.

loading (case 2); commercial Pt/C 0.2 mgPt cm−<sup>2</sup>

refers to the anodic peak current, n to the electron transfer number, A to the surface

Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

areas were calculated for all the used modified electrodes. The results show that metal-doped graphene oxide materials cause an increase in the active surface of the electrode. The comparison between modified GO electrode and Au-doped rGO reveals a higher active surface of the latter electrode which recommends this material as an electrocatalyst for the ORR reaction in fuel cells. Also, the Pt-doped rGO modified electrode exhibits an electrode active surface two

Under PEMFC conditions at cell voltage of around 1.0 V during no-load, the Pt metal dissolution is expected. Moreover, Pt oxides are produced at potentials higher than 0.6 VSHE. In particularly at potentials above 0.95 VSHE, oxygen atoms can replace Pt atoms, thus the potential cycling cause remarkable changes of the catalyst structure. The catalyst exposure to such an accelerated test namely the potential cycling between 0.0–1.2 VSHE lead to a fast catalyst aging, opposed to the constant potential holding test [17]. Thus, the potential cycling could be used

For this purpose, the in-situ electrochemical evaluation was performed in a single fuel cell

electrodes and membrane electrode assembly fabrication procedure was reported in our previous studies [11–13]. For actual study, the Pt loading was established at 0.2 mgPt cm−<sup>2</sup>

anode of all developed fuel cells. The cathode catalyst layer was modified by taking into account three FC configurations, as follows: deposition of commercial Pt/C catalyst Hispec

loading (case 3).The fuel cell test station included specific devices such as configured workstation, fuel cell, DS electronic load, bubble-type humidifiers. The PEMFC was operated at 0.7 V for 1 h for membrane electrode assembly conditioning. After steady state operating conditions were maintained, the cyclic voltammetry measurements were performed in a H2

mode cell, from 0.025 to 1.2 V at a scan rate of 0.05 V/s. The flow rates of reactants (50 and

and pressure were fixed at 70°C and 1 bar pressure. The developed control system based on

Carbon corrosion causes the detachment of metal nanoparticles and thus electrical isolation and electrocatalytic inactivity. Taking into account the instabilities due to catalyst support which lead to diminishing of FC performance, we investigated the support aging. **Figures 16**–**18** present the cyclic voltammograms before and catalyst exposure to 2000 cycles between 0.025 and 1.2 V potential cycling. Our preliminary results indicated that the oxidation and reduction signals approx at 0.025–0.3 V decreased differently after accelerated test, indicating the loss in electrochemical surface area for all cases, more pronounced being recorded for commercial catalyst. In case 1 it is easily to observe the change in hydrogen adsorption/desorption on Pt both for hydrogen oxidation and hydrogen reduction. In case 2 and 3, insignificant changes

loading (case 1); commercial Pt/C 0.2 mgPt cm−<sup>2</sup>

[Fe(CN)6

http://dx.doi.org/10.5772/intechopen.80941

using the slope of the Ip–ν1/2 relation, the microscopic

(ElectroChem, USA). A detailed description of the

) were adjusted using flow controllers. The cell temperature

] and ν is

87

for

/N2

and Pt/rGO with

and Au/rGO with 0.2 mgAu cm−<sup>2</sup>

area of the electrode, D is the diffusion coefficient, c the concentration of K3

**Figure 15.** DPV voltammograms for graphene modified electrodes and commercially available glassy carbon electrode (GC): oxidation (left) and reduction (right) with optimized DPV parameters: modulation amplitude of 25 mV and step potential 5 mV.

where Ia refers to the anodic peak current, n to the electron transfer number, A to the surface area of the electrode, D is the diffusion coefficient, c the concentration of K3 [Fe(CN)6 ] and ν is the scan rate.

For n = 1 and D = 6.68 × 10−<sup>6</sup> cm2 s−<sup>1</sup> using the slope of the Ip–ν1/2 relation, the microscopic areas were calculated for all the used modified electrodes. The results show that metal-doped graphene oxide materials cause an increase in the active surface of the electrode. The comparison between modified GO electrode and Au-doped rGO reveals a higher active surface of the latter electrode which recommends this material as an electrocatalyst for the ORR reaction in fuel cells. Also, the Pt-doped rGO modified electrode exhibits an electrode active surface two times higher than the graphene oxide modified electrode.

Under PEMFC conditions at cell voltage of around 1.0 V during no-load, the Pt metal dissolution is expected. Moreover, Pt oxides are produced at potentials higher than 0.6 VSHE. In particularly at potentials above 0.95 VSHE, oxygen atoms can replace Pt atoms, thus the potential cycling cause remarkable changes of the catalyst structure. The catalyst exposure to such an accelerated test namely the potential cycling between 0.0–1.2 VSHE lead to a fast catalyst aging, opposed to the constant potential holding test [17]. Thus, the potential cycling could be used as simultaneous CV characterization and degradation testing tool.

For this purpose, the in-situ electrochemical evaluation was performed in a single fuel cell system PEMFC with active area of 25 cm2 (ElectroChem, USA). A detailed description of the electrodes and membrane electrode assembly fabrication procedure was reported in our previous studies [11–13]. For actual study, the Pt loading was established at 0.2 mgPt cm−<sup>2</sup> for anode of all developed fuel cells. The cathode catalyst layer was modified by taking into account three FC configurations, as follows: deposition of commercial Pt/C catalyst Hispec 4000 with 0.4 mgPt cm−<sup>2</sup> loading (case 1); commercial Pt/C 0.2 mgPt cm−<sup>2</sup> and Pt/rGO with 0.2 mgPt cm−<sup>2</sup> loading (case 2); commercial Pt/C 0.2 mgPt cm−<sup>2</sup> and Au/rGO with 0.2 mgAu cm−<sup>2</sup> loading (case 3).The fuel cell test station included specific devices such as configured workstation, fuel cell, DS electronic load, bubble-type humidifiers. The PEMFC was operated at 0.7 V for 1 h for membrane electrode assembly conditioning. After steady state operating conditions were maintained, the cyclic voltammetry measurements were performed in a H2 /N2 mode cell, from 0.025 to 1.2 V at a scan rate of 0.05 V/s. The flow rates of reactants (50 and 150 mL min−<sup>1</sup> ) gases (H2 and N2 ) were adjusted using flow controllers. The cell temperature and pressure were fixed at 70°C and 1 bar pressure. The developed control system based on NI c-RIO hardware was used to control the PEMFC system.

Carbon corrosion causes the detachment of metal nanoparticles and thus electrical isolation and electrocatalytic inactivity. Taking into account the instabilities due to catalyst support which lead to diminishing of FC performance, we investigated the support aging. **Figures 16**–**18** present the cyclic voltammograms before and catalyst exposure to 2000 cycles between 0.025 and 1.2 V potential cycling. Our preliminary results indicated that the oxidation and reduction signals approx at 0.025–0.3 V decreased differently after accelerated test, indicating the loss in electrochemical surface area for all cases, more pronounced being recorded for commercial catalyst. In case 1 it is easily to observe the change in hydrogen adsorption/desorption on Pt both for hydrogen oxidation and hydrogen reduction. In case 2 and 3, insignificant changes

**Figure 15.** DPV voltammograms for graphene modified electrodes and commercially available glassy carbon electrode (GC): oxidation (left) and reduction (right) with optimized DPV parameters: modulation amplitude of 25 mV and step

In order to better investigate the electroanalytical outcome of these graphene modified electrodes, we performed differential pulse voltammetry (DPV) measurements. The DPV experiments were meant to support the cyclic voltammetry investigations, as it is clearly known that DPV can assist in resolving the signals due to species with close half-wave potentials, as it measures the difference between two currents, before the end of the pulse and before its application. The strength of this technique is evident when poor electrochemical signals are obtained in cyclic voltammetry, enabling the registration of well-defined signals through the elimination of the non-Faradaic processes. Moreover, DPV provides useful information when the resolving of the voltammetric signals given by two species with close half-wave potentials

**Figure 15** compares the differential pulse voltammograms of the graphene modified carbon paste electrodes, as well as the classical glassy carbon electrode. In all cases, sharp and wellresolved peaks are observed. The anodic (right side) and cathodic (left side) peaks are higher for the metal-doped reduced graphene oxide modified electrodes with the highest peak current being observed for the gold doped reduced graphene oxide modified electrode. An important parameter of an electrode material is its electronic properties, namely, the density of electronic states (DOS). The DOS of graphene materials has been reported as being high and it can be increased by varying the amounts of loaded graphene. Gold has a DOS of 0.28

Au-doped rGO modified electrodes are highly sensitive for electrochemical investigation of

The active surface areas of the graphene modified electrodes were estimated according to the

with high conductivity due to high proportion of atomic orbitals. Thus, the

*<sup>a</sup>* = 2.69 · 105 · *n*<sup>3</sup>/2 · *D*1/2 · *A* · *c* · *v*1/2 (3)

[Fe(CN)6

], solution using

is needed, producing easily quantifiable peak shaped responses.

86 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

slope of the Ia *vs.* square root of the scan rates plot for a 1.0 mM K3

0.50 M KCl electrolyte, according to Randles–Sevcik equation [16]:

potential 5 mV.

states atom−<sup>1</sup> eV−<sup>1</sup>

the ferri/ferro redox system.

*I*

**Figure 16.** Cyclic voltammetry of commercial Pt/C for 2000 cycles in following operation conditions: temperature 70°C; scan rate 50 mV/s.

The graphene based material modified electrodes presented herein can significantly contribute to the fundamental understanding of graphene electrochemistry and its application as an electrode material. Future work can be directed to exploring these modified electrodes towards the application of graphene materials in ORR reactions for the development of per-

**Figure 18.** Cyclic voltammetry of Au/rGO for 2000 cycles in following operation conditions: temperature 70°C; scan rate

Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells

http://dx.doi.org/10.5772/intechopen.80941

89

In conclusion, we have successfully synthesized PtNP/rGO and Au NP/rGO nanomaterials. The successful incorporation of Au and Pt nanoparticles into the graphene oxide structure was confirmed. SEM and STEM images show the good spatial distribution of metal nanoparticles onto the layered graphene sheets. Electrocatalytic performance of the prepared materials towards the ORR reaction using cyclic voltammetry and differential pulse voltammetry show that metal-doped graphene oxide materials caused an increase in the active surface of the electrodes. The results obtained by electrochemical characterizations suggest that metaldispersed nanoparticles on reduced graphene oxide holds a great application potential as a promising electrocatalyst for oxygen reduction reaction due to the advantages of facile preparation and increased catalytic performance. It can be anticipated that the PtNP/rGO and AuNP/rGO composite materials hold great potential for developing novel ORR electrodes for PEM fuel cells. This imparts a high level of confidence that the materials developed as ORR electrocatalysts will be used in our future studies for the fabrication of cathodes for PEM fuel cell, the goal being the improvement of the fuel cell in terms of performance, life time and durability. Therefore, the reason for designed ORR electrocatalyst involves the advantage of improved catalytic activity due to the incorporation of a noble metal (platinum/gold) and

ameliorated durability from the 1-dimensional structure.

forming fuel cells.

50 mV/s.

**4. Conclusions**

**Figure 17.** Cyclic voltammetry of Pt/rGO for 2000 cycles in following operation conditions: temperature 70°C; scan rate 50 mV/s.

were observed for reduction signal, which recommend Pt- and Au-doped rGO as potential ORR catalyst for a more comprehensive investigation.

Clearly, there are many types of GO modified electrodes and each is performing for different redox system, but the electrochemistry of graphene materials is far from being fully explored. Noble Metal Dispersed on Reduced Graphene Oxide and Its Application in PEM Fuel Cells http://dx.doi.org/10.5772/intechopen.80941 89

**Figure 18.** Cyclic voltammetry of Au/rGO for 2000 cycles in following operation conditions: temperature 70°C; scan rate 50 mV/s.

The graphene based material modified electrodes presented herein can significantly contribute to the fundamental understanding of graphene electrochemistry and its application as an electrode material. Future work can be directed to exploring these modified electrodes towards the application of graphene materials in ORR reactions for the development of performing fuel cells.

### **4. Conclusions**

were observed for reduction signal, which recommend Pt- and Au-doped rGO as potential

**Figure 17.** Cyclic voltammetry of Pt/rGO for 2000 cycles in following operation conditions: temperature 70°C; scan rate

**Figure 16.** Cyclic voltammetry of commercial Pt/C for 2000 cycles in following operation conditions: temperature 70°C;

Clearly, there are many types of GO modified electrodes and each is performing for different redox system, but the electrochemistry of graphene materials is far from being fully explored.

ORR catalyst for a more comprehensive investigation.

88 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

scan rate 50 mV/s.

50 mV/s.

In conclusion, we have successfully synthesized PtNP/rGO and Au NP/rGO nanomaterials. The successful incorporation of Au and Pt nanoparticles into the graphene oxide structure was confirmed. SEM and STEM images show the good spatial distribution of metal nanoparticles onto the layered graphene sheets. Electrocatalytic performance of the prepared materials towards the ORR reaction using cyclic voltammetry and differential pulse voltammetry show that metal-doped graphene oxide materials caused an increase in the active surface of the electrodes. The results obtained by electrochemical characterizations suggest that metaldispersed nanoparticles on reduced graphene oxide holds a great application potential as a promising electrocatalyst for oxygen reduction reaction due to the advantages of facile preparation and increased catalytic performance. It can be anticipated that the PtNP/rGO and AuNP/rGO composite materials hold great potential for developing novel ORR electrodes for PEM fuel cells. This imparts a high level of confidence that the materials developed as ORR electrocatalysts will be used in our future studies for the fabrication of cathodes for PEM fuel cell, the goal being the improvement of the fuel cell in terms of performance, life time and durability. Therefore, the reason for designed ORR electrocatalyst involves the advantage of improved catalytic activity due to the incorporation of a noble metal (platinum/gold) and ameliorated durability from the 1-dimensional structure.

### **Acknowledgements**

This work is supported by the Ministry of Research and Innovation from Romania by the National Plan of R&D, Project No. PN 18 12 01 02, PN 18 12 01 04 and by Executive Agency for Higher Education, Research, Development and Innovation, under Project M-ERA.net 37/2016. The authors wish to thank Dr. Izabela Jinga from the Centre of Organic Chemistry 'Costin D. Nenitescu' Romanian Academy for her work in the electrochemical experimental study.

[5] Li M, Zhang L, Xu Q, Niu J, Xia Z. N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations. Journal of Catalysis.

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[9] Marinoiu A, Cobzaru C, Carcadea E, Raceanu M, Atkinson I, Varlam M, et al. An experimental approach for finding low cost alternative support material in PEM fuel cells.

[10] Marinoiu A, Teodorescu C, Carcadea E, Raceanu M, Varlam M, Cobzaru C, et al. Convenient graphene based materials as potential candidates for low cost fuel cell catalysts. Reaction Kinetics Mechanisms and Catalysis. 2016;**118**:281-296. DOI: 10.1007/

[11] Marinoiu A, Raceanu M, Carcadea E, Varlam M, Stefanescu I. Doped Graphene as non-metallic catalyst for fuel cells. Materials Science. 2017;**23**:108-113. DOI: 10.5755/j01.

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[13] Marinoiu A, Raceanu M, Carcadea E, Varlam M, Stefanescu I. Low cost iodine intercalated graphene for fuel cells electrodes. Applied Surface Science. 2017;**424**:93-100. DOI:

[14] Sun CL, Lee HH, Yang JM, Wu CC. The simultaneous electrochemical detection of ascorbic acid, dopamine, and uric acid using graphene/size-selected Pt nanocompos-

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ms.23.2.16216

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### **Conflict of interest**

The authors declare that there is no conflict of interest.

### **Author details**

Adriana Marinoiu1 \*, Mircea Raceanu1 , Elena Carcadea1 , Aida Pantazi2 , Raluca Mesterca2 , Oana Tutunaru2 , Simona Nica3 , Daniela Bala<sup>4</sup> , Mihai Varlam<sup>1</sup> and Marius Enachescu2

\*Address all correspondence to: adriana.marinoiu@icsi.ro

1 RD Institute for Cryogenics and Isotopic Technologies—ICSI, Rm Valcea, Romania

2 Center for Surface Science and Nanotechnology, Politehnica University of Bucharest, Romania

3 Centre of Organic Chemistry 'Costin D. Nenitescu' Romanian Academy, Romania

4 Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, Romania

### **References**


[5] Li M, Zhang L, Xu Q, Niu J, Xia Z. N-doped graphene as catalysts for oxygen reduction and oxygen evolution reactions: Theoretical considerations. Journal of Catalysis. 2014;**314**:66-72. DOI: 10.1016/j.jcat.2014.03.011

**Acknowledgements**

**Conflict of interest**

**Author details**

Adriana Marinoiu1

Oana Tutunaru2

Romania

Romania

**References**

C2NR33839D

10.1039/C4CC05055J

The authors declare that there is no conflict of interest.

90 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

\*, Mircea Raceanu1

\*Address all correspondence to: adriana.marinoiu@icsi.ro

, Simona Nica3

48-49. DOI: 10.1126/science.1172083

This work is supported by the Ministry of Research and Innovation from Romania by the National Plan of R&D, Project No. PN 18 12 01 02, PN 18 12 01 04 and by Executive Agency for Higher Education, Research, Development and Innovation, under Project M-ERA.net 37/2016. The authors wish to thank Dr. Izabela Jinga from the Centre of Organic Chemistry 'Costin D. Nenitescu' Romanian Academy for her work in the electrochemical experimental study.

, Elena Carcadea1

, Mihai Varlam<sup>1</sup>

, Daniela Bala<sup>4</sup>

1 RD Institute for Cryogenics and Isotopic Technologies—ICSI, Rm Valcea, Romania

3 Centre of Organic Chemistry 'Costin D. Nenitescu' Romanian Academy, Romania 4 Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest,

2 Center for Surface Science and Nanotechnology, Politehnica University of Bucharest,

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[3] Lale IS et al. Engineered catalyst layer design with graphene-carbon black hybrid supports for enhanced platinum utilization in PEM fuel cell. International Journal of

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, Aida Pantazi2

, Raluca Mesterca2

and Marius Enachescu2

,


**Section 2**

**Electrocatalysts for Water Splitting**

**Electrocatalysts for Water Splitting**

**Chapter 5**

**Provisional chapter**

**Electrocatalytic Properties of Molybdenum and**

**Electrocatalytic Properties of Molybdenum and** 

Valeriy Kublanovsky and Yuliya Yapontseva

Valeriy Kublanovsky and Yuliya Yapontseva

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

molybdenum, tungsten, spillover, corrosion

http://dx.doi.org/10.5772/intechopen.79058

**Abstract**

**1. Introduction**

**Tungsten Alloys in the Hydrogen Evolution Reaction**

The search for new electrode materials which are able to intensify the currently used process for the electrolytic production of pure hydrogen is an important scientific and technical problem. Promising materials for making electrodes or for the modification of currently used nickel cathodes are alloys of iron subgroup metals with molybdenum or tungsten, which show a higher catalytic activity in the hydrogen reduction reaction compared with pure nickel or cobalt and have a high corrosion resistance in aggressive media. The present chapter demonstrates that the catalytic, magnetic, and corrosive properties of coatings depend not only on the chemical composition of the alloy but also on its phase composition and morphology so that even coatings having the same chemical composition, but obtained under different conditions, possess different properties. Thus, by changing the electrolyte composition and the electrolysis parameters, one can obtain coatings with optimal set of functional properties and the distribution of metals on the surface which ensure hydrogen spillover and, hence, a high rate of the desired reaction.

**Keywords:** electrocatalytic activity, hydrogen evolution, electrodeposition,

The amount of energy consumed in the world has been steadily increasing in the recent decades, and the nonrenewable fossil energy sources account for over 80% of it [1]. In view of this, the search for efficient, environmentally appropriate methods for the generation of renewable energy is a vital present-day scientific problem [2]. One such promising trends is fuel cells [3, 4], the operation of which is directly bound up with the availability of high-purity hydrogen.

**Tungsten Alloys in the Hydrogen Evolution Reaction**

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.79058

#### **Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction**

DOI: 10.5772/intechopen.79058

Valeriy Kublanovsky and Yuliya Yapontseva Valeriy Kublanovsky and Yuliya Yapontseva

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79058

#### **Abstract**

The search for new electrode materials which are able to intensify the currently used process for the electrolytic production of pure hydrogen is an important scientific and technical problem. Promising materials for making electrodes or for the modification of currently used nickel cathodes are alloys of iron subgroup metals with molybdenum or tungsten, which show a higher catalytic activity in the hydrogen reduction reaction compared with pure nickel or cobalt and have a high corrosion resistance in aggressive media. The present chapter demonstrates that the catalytic, magnetic, and corrosive properties of coatings depend not only on the chemical composition of the alloy but also on its phase composition and morphology so that even coatings having the same chemical composition, but obtained under different conditions, possess different properties. Thus, by changing the electrolyte composition and the electrolysis parameters, one can obtain coatings with optimal set of functional properties and the distribution of metals on the surface which ensure hydrogen spillover and, hence, a high rate of the desired reaction.

**Keywords:** electrocatalytic activity, hydrogen evolution, electrodeposition, molybdenum, tungsten, spillover, corrosion

### **1. Introduction**

The amount of energy consumed in the world has been steadily increasing in the recent decades, and the nonrenewable fossil energy sources account for over 80% of it [1]. In view of this, the search for efficient, environmentally appropriate methods for the generation of renewable energy is a vital present-day scientific problem [2]. One such promising trends is fuel cells [3, 4], the operation of which is directly bound up with the availability of high-purity hydrogen.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

At the present time, the wide use of pure hydrogen is economically inexpedient in comparison, e.g., with the use of natural gas, because of the high cost of its production. The industrial method for hydrogen production by the electrolysis of alkaline aqueous solutions [5] has a number of disadvantages, such as the large expenditure of energy, low efficiency of the process, and the necessity of using noble metals as catalysts. Low carbon steel, which is corrosion-nonresistant in alkaline medium, and nickel also possess a catalytic effect in the hydrogen evolution reaction [6]; nevertheless, the problem of search for electrode materials with lower hydrogen evolution overpotential and high corrosion resistance remains vital. For instance, platinum group catalysts can be replaced by cheaper materials, such as nickel, molybdenum, and iron-chromium and iron-manganese alloys, in electrolysis in ionic liquids [7]. However, if the high cost of ionic liquids themselves is taken into account, there is no considerable reduction in the cost of materials for pure hydrogen production.

#### **1.1. Hydrogen electroreduction mechanism and spillover effect**

In the general form, the hydrogen electroreduction process in an alkaline medium is described by the Volmer (1)—Heyrovsky (2)—Tafel (3) mechanism with the following stages:

$$\text{Me} + \text{H}\_{2}\text{O} + \text{e} \rightarrow \text{MeH}\_{\text{ads}} \text{\textasciicircum} \tag{1}$$

$$\rm MeH\_{ads} + H\_2O + e \rightarrow H\_2 + Me + OH^- \tag{2}$$

$$\text{2MeH}\_{\text{ads}} \rightarrow \text{H}\_2 + \text{2Me} \tag{3}$$

hydrogen reduction reaction both in acidic [10–13] and in alkaline aqueous solutions [14–17], meet these requirements, and that the use of the electrochemical method for the deposition of thin alloy coatings makes it possible to reduce the cost of electrode materials and hence the

**Figure 1.** Dependence of the exchange current density of hydrogen evolution reaction in acid solutions against hydrogen-

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction

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97

The synergetic action of alloys in comparison with individual metals is explained by researchers not only by the type of formed alloy and its chemical composition but also by the difference in the mechanism of hydrogen electroreduction on the metals, as well as by the phenomenon of interfacial diffusion of adsorbed hydrogen, hydrogen spillover. The authors of [18] showed the frequent occurrence of this phenomenon in various catalytic processes and catalyst types. Spillover is called the transport of active species, sorbed or formed on one phase, to another phase, which does not sorb and form these species. A study of the synergetic effect of alloys of cobalt and nickel with molybdenum and tungsten with allowance for hydrogen spillover was carried out in [19] for hydrodesulphurization reaction as an example, and it was suggested that different process stages occur on different catalyst phases, between which hydrogen spillover takes place. The spillover phenomenon is of great practical importance, the study of which will help to design new multi-

As applied to the alloys of molybdenum and tungsten with iron group metals, the spillover phenomenon can account for the synergetic effect of these alloys, in the case of which the discharge step on Mo(W) takes place quickly, and on Co(Ni, Fe), the electrochemical desorption step is fast. Thus, in order that this mechanism may be effected, the active centers of different nature must be at the distance from one another that makes possible the surface diffusion of

phase catalysts, where the catalytic reaction takes place by interfacial diffusion.

hydrogen adatoms, and hence the alloy must be a solid solution or an intermetallic.

cost of produced hydrogen.

metal bond energy. Reprinted with permission from Ref. [8].

The rate of electrocatalytic reaction depends on the energy of adsorbed species, i.e., on the energy of hydrogen atoms. The plot of the exchange current density of hydrogen evolution reaction against hydrogen-metal bond energy passes through a maximum and has a volcanolike shape (**Figure 1**).

At low Ме▬Н bond energy, the process is controlled by the discharge step, reaction (1). At very high Ме▬Н bond energy, the process is controlled by the electrochemical desorption step, reaction (2). On nickel, cobalt, and iron, the rate-determining step changes from reaction (1) to (2) with increasing polarization. Thus, the most important parameter that determines electrocatalytic activity is the energy of bonding of adsorbed species to the catalyst [9]. When catalysts consisting of two metals are used, the formation of several alloy types, such as mechanical mixtures, solid solutions, and intermetallics is possible. From an analysis of the mechanism of electrocatalytic processes, metal-metal bond energy, and the properties of different alloy types, the authors of [9] conclude that the alloys formed by the metals that are on the different branches of the volcano plot and especially the alloys formed by d-metals with unoccupied d-orbitals (of groups IV–VI) and d-metals of group VIII with a large number of filled d-orbitals possess electrocatalytic activity.

It is known from literature that transition-metal alloys, viz. alloys of iron subgroup metals with molybdenum and tungsten, which show electrocatalytic properties with respect to Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction http://dx.doi.org/10.5772/intechopen.79058 97

At the present time, the wide use of pure hydrogen is economically inexpedient in comparison, e.g., with the use of natural gas, because of the high cost of its production. The industrial method for hydrogen production by the electrolysis of alkaline aqueous solutions [5] has a number of disadvantages, such as the large expenditure of energy, low efficiency of the process, and the necessity of using noble metals as catalysts. Low carbon steel, which is corrosion-nonresistant in alkaline medium, and nickel also possess a catalytic effect in the hydrogen evolution reaction [6]; nevertheless, the problem of search for electrode materials with lower hydrogen evolution overpotential and high corrosion resistance remains vital. For instance, platinum group catalysts can be replaced by cheaper materials, such as nickel, molybdenum, and iron-chromium and iron-manganese alloys, in electrolysis in ionic liquids [7]. However, if the high cost of ionic liquids themselves is taken into account, there is no

In the general form, the hydrogen electroreduction process in an alkaline medium is described

Me + H2 O + e → MeHads + OH<sup>−</sup> (1)

MeHads + H2 O + e → H2 + Me + OH<sup>−</sup> (2)

2MeHads → H2 + 2Me (3)

The rate of electrocatalytic reaction depends on the energy of adsorbed species, i.e., on the energy of hydrogen atoms. The plot of the exchange current density of hydrogen evolution reaction against hydrogen-metal bond energy passes through a maximum and has a volcano-

At low Ме▬Н bond energy, the process is controlled by the discharge step, reaction (1). At very high Ме▬Н bond energy, the process is controlled by the electrochemical desorption step, reaction (2). On nickel, cobalt, and iron, the rate-determining step changes from reaction (1) to (2) with increasing polarization. Thus, the most important parameter that determines electrocatalytic activity is the energy of bonding of adsorbed species to the catalyst [9]. When catalysts consisting of two metals are used, the formation of several alloy types, such as mechanical mixtures, solid solutions, and intermetallics is possible. From an analysis of the mechanism of electrocatalytic processes, metal-metal bond energy, and the properties of different alloy types, the authors of [9] conclude that the alloys formed by the metals that are on the different branches of the volcano plot and especially the alloys formed by d-metals with unoccupied d-orbitals (of groups IV–VI) and d-metals of group VIII with a large number of

It is known from literature that transition-metal alloys, viz. alloys of iron subgroup metals with molybdenum and tungsten, which show electrocatalytic properties with respect to

considerable reduction in the cost of materials for pure hydrogen production.

by the Volmer (1)—Heyrovsky (2)—Tafel (3) mechanism with the following stages:

**1.1. Hydrogen electroreduction mechanism and spillover effect**

96 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

like shape (**Figure 1**).

filled d-orbitals possess electrocatalytic activity.

**Figure 1.** Dependence of the exchange current density of hydrogen evolution reaction in acid solutions against hydrogenmetal bond energy. Reprinted with permission from Ref. [8].

hydrogen reduction reaction both in acidic [10–13] and in alkaline aqueous solutions [14–17], meet these requirements, and that the use of the electrochemical method for the deposition of thin alloy coatings makes it possible to reduce the cost of electrode materials and hence the cost of produced hydrogen.

The synergetic action of alloys in comparison with individual metals is explained by researchers not only by the type of formed alloy and its chemical composition but also by the difference in the mechanism of hydrogen electroreduction on the metals, as well as by the phenomenon of interfacial diffusion of adsorbed hydrogen, hydrogen spillover. The authors of [18] showed the frequent occurrence of this phenomenon in various catalytic processes and catalyst types. Spillover is called the transport of active species, sorbed or formed on one phase, to another phase, which does not sorb and form these species. A study of the synergetic effect of alloys of cobalt and nickel with molybdenum and tungsten with allowance for hydrogen spillover was carried out in [19] for hydrodesulphurization reaction as an example, and it was suggested that different process stages occur on different catalyst phases, between which hydrogen spillover takes place. The spillover phenomenon is of great practical importance, the study of which will help to design new multiphase catalysts, where the catalytic reaction takes place by interfacial diffusion.

As applied to the alloys of molybdenum and tungsten with iron group metals, the spillover phenomenon can account for the synergetic effect of these alloys, in the case of which the discharge step on Mo(W) takes place quickly, and on Co(Ni, Fe), the electrochemical desorption step is fast. Thus, in order that this mechanism may be effected, the active centers of different nature must be at the distance from one another that makes possible the surface diffusion of hydrogen adatoms, and hence the alloy must be a solid solution or an intermetallic.

## **2. Electrocatalytic properties of nickel alloys**

Nickel is the most commonly used cathode material in electrolyzers for hydrogen production [20] due to its catalytic properties, corrosion resistance in alkaline medium, and low cost. A possible way of intensifying the process and improving the required properties is electrode surface modification by molybdenum and tungsten alloys; therefore, many papers are devoted to the study of the electrodeposition and catalytic activity of NiMo and NiW alloys. For instance, the authors of [21] studied the electrodeposition of a NiMo alloy on a copper and a nickel substrate from a citrate electrolyte with рН 9.5 and a concentration ratio of the metals of 1/12. The best catalytic activity was found for a coating containing 41 wt.% molybdenum, and it was shown that the hydrogen evolution overpotential at the obtained alloy is lower compared with pure nickel in the model 1 М NaOH solution. In the study [22], a citrate electrolyte with рН 6.0 was also used, and it was shown that it is possible to deposit an alloy containing 28.5 аt.% Мо, which reduces the hydrogen evolution overpotential in 8 M NaOH at 90°С from 122 to 21 mV·dec-1 relative to nickel. The authors of [23] found that when a SAS is added to a citrate-ammonia electrolyte for deposition, nanocrystalline coatings having catalytic activity at a molybdenum content of 19.59 аt.% can be obtained. Chialvo and co-authors [24] studied the catalytic activity of thermal NiMo alloys as a function of the amount of molybdenum (0–25 аt.%) and showed that the higher the Мо content, the lower the hydrogen overpotential. From an analysis of literature data, it can be concluded that there is no unambiguous dependence of electrocatalytic activity on the chemical composition of the coating; for each particular deposition electrolyte, activity is proportional to the molybdenum content, but from the comparison of a large number of papers, it becomes clear that the determining factor is electrolysis conditions, which determine the morphology, the true surface area, the presence of cracks, or an oxide phase on the alloy surface. Therefore, the catalytic properties of electrocatalytic alloys are determined experimentally in each particular case.

coatings were deposited in a solution containing 0.1 mol L−1 of CoSO4

noticeable effect only at a concentration of 4.5 g L−1.

Cit (where Cit—citrate ions), 0.2 mol L−1 of K4

and in solutions containing SASs: 2 mL L−1 of water-soluble resin neonol, whose efficiency was shown when electrodepositing Co-W alloys from a citrate-ammonia electrolyte [27], and 1.5 g L−1 and 4.5 g L−1 of a nonionic SAS, OP-10 emulsifier. All experiments were performed under forced convection conditions in a current density range of 5.0–30.0 mА сm−2 at 50°С and

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction

It can be seen from **Figure 2** that the composition of the coatings deposited from a citratepyrophosphate electrolyte is constant regardless of deposition current density and addition of SASs. All coatings containing on an average 22 аt.% W, nevertheless, differ in the current efficiency of their deposition and in the morphology of the produced surface. A small increase in current efficiency for alloys is observed in the case of adding a SAS (neonol or OP-10) to the electrolyte. For instance, at 5.0 mА сm−2 in the presence of neonol, the current efficiency reaches 68%, and a compact, bright, adherent coating is formed. The addition of OP-10 has a

The addition of SASs to the electrolyte and deposition current density affects greatly the quality and morphology (**Figure 3**). For instance, in the electrolyte without additives at a current density of 5.0 mА сm−2, compact bright coatings are deposited. When the current density is increased to 10.0 mА сm−2, the coating becomes more stressed, and microcracks appear, and at 30.0 mА сm−2, the formation of spherulites is observed. The addition of neonol makes it possible to obtain high-quality fine-crystalline deposits in a wider current density range, and the addition OP-10, on the contrary, facilitates the formation of spherulites and favors surface

The electrocatalytic properties of СоW coatings in the hydrogen reduction reaction were investigated by stationary voltammetry in 1.0 and 6.0 mol L−1 KОН solutions because KOH is used in industrial water electrolysis, and under experimental conditions, i.e., at room tem-

**Figure 2.** Dependence of the composition (a) and current efficiency (b) of Co-W alloys obtained from electrolyte: 1—

without additives; 2—neonol; and 3 and 4—OP-10 (1.5 and 4.5 g L−1) on the deposition current density.

perature, the solution has a maximum electrical conductivity at 28 wt.% KОН.

0.2 mol L−1 of Na3

electrolyte рН 8.5.

development.

, 0.2 mol L−1 of Na2

http://dx.doi.org/10.5772/intechopen.79058

, 0.5 mol L−1 of Na2

P2 O7 WO4 ,

SO4

99

On the basis of investigations carried out by the authors of [25], it was concluded that molybdenum-bearing alloys are more active in the hydrogen evolution reaction in alkaline medium than tungsten-bearing alloys. When investigating the properties of Ni-Mo, Ni-W, Co-Mo, and Co-W alloys, it was also found that cobalt alloys show a higher catalytic activity than nickel alloys.

### **3. Electrocatalytic properties of cobalt alloys**

The main characteristics of electrocatalytic activity in HER are reaction exchange current and hydrogen reduction overpotential. Therefore, the main method for studying this process is voltammetry.

### **3.1. Tungsten alloys**

In the study [26], an investigation on the electrocatalytic properties of Co-W alloys electrodeposited from a polyligand citrate-pyrophosphate electrolyte had been carried out. The coatings were deposited in a solution containing 0.1 mol L−1 of CoSO4 , 0.2 mol L−1 of Na2 WO4 , 0.2 mol L−1 of Na3 Cit (where Cit—citrate ions), 0.2 mol L−1 of K4 P2 O7 , 0.5 mol L−1 of Na2 SO4 and in solutions containing SASs: 2 mL L−1 of water-soluble resin neonol, whose efficiency was shown when electrodepositing Co-W alloys from a citrate-ammonia electrolyte [27], and 1.5 g L−1 and 4.5 g L−1 of a nonionic SAS, OP-10 emulsifier. All experiments were performed under forced convection conditions in a current density range of 5.0–30.0 mА сm−2 at 50°С and electrolyte рН 8.5.

**2. Electrocatalytic properties of nickel alloys**

98 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

are determined experimentally in each particular case.

**3. Electrocatalytic properties of cobalt alloys**

alloys.

voltammetry.

**3.1. Tungsten alloys**

Nickel is the most commonly used cathode material in electrolyzers for hydrogen production [20] due to its catalytic properties, corrosion resistance in alkaline medium, and low cost. A possible way of intensifying the process and improving the required properties is electrode surface modification by molybdenum and tungsten alloys; therefore, many papers are devoted to the study of the electrodeposition and catalytic activity of NiMo and NiW alloys. For instance, the authors of [21] studied the electrodeposition of a NiMo alloy on a copper and a nickel substrate from a citrate electrolyte with рН 9.5 and a concentration ratio of the metals of 1/12. The best catalytic activity was found for a coating containing 41 wt.% molybdenum, and it was shown that the hydrogen evolution overpotential at the obtained alloy is lower compared with pure nickel in the model 1 М NaOH solution. In the study [22], a citrate electrolyte with рН 6.0 was also used, and it was shown that it is possible to deposit an alloy containing 28.5 аt.% Мо, which reduces the hydrogen evolution overpotential in 8 M NaOH at 90°С from 122 to 21 mV·dec-1 relative to nickel. The authors of [23] found that when a SAS is added to a citrate-ammonia electrolyte for deposition, nanocrystalline coatings having catalytic activity at a molybdenum content of 19.59 аt.% can be obtained. Chialvo and co-authors [24] studied the catalytic activity of thermal NiMo alloys as a function of the amount of molybdenum (0–25 аt.%) and showed that the higher the Мо content, the lower the hydrogen overpotential. From an analysis of literature data, it can be concluded that there is no unambiguous dependence of electrocatalytic activity on the chemical composition of the coating; for each particular deposition electrolyte, activity is proportional to the molybdenum content, but from the comparison of a large number of papers, it becomes clear that the determining factor is electrolysis conditions, which determine the morphology, the true surface area, the presence of cracks, or an oxide phase on the alloy surface. Therefore, the catalytic properties of electrocatalytic alloys

On the basis of investigations carried out by the authors of [25], it was concluded that molybdenum-bearing alloys are more active in the hydrogen evolution reaction in alkaline medium than tungsten-bearing alloys. When investigating the properties of Ni-Mo, Ni-W, Co-Mo, and Co-W alloys, it was also found that cobalt alloys show a higher catalytic activity than nickel

The main characteristics of electrocatalytic activity in HER are reaction exchange current and hydrogen reduction overpotential. Therefore, the main method for studying this process is

In the study [26], an investigation on the electrocatalytic properties of Co-W alloys electrodeposited from a polyligand citrate-pyrophosphate electrolyte had been carried out. The It can be seen from **Figure 2** that the composition of the coatings deposited from a citratepyrophosphate electrolyte is constant regardless of deposition current density and addition of SASs. All coatings containing on an average 22 аt.% W, nevertheless, differ in the current efficiency of their deposition and in the morphology of the produced surface. A small increase in current efficiency for alloys is observed in the case of adding a SAS (neonol or OP-10) to the electrolyte. For instance, at 5.0 mА сm−2 in the presence of neonol, the current efficiency reaches 68%, and a compact, bright, adherent coating is formed. The addition of OP-10 has a noticeable effect only at a concentration of 4.5 g L−1.

The addition of SASs to the electrolyte and deposition current density affects greatly the quality and morphology (**Figure 3**). For instance, in the electrolyte without additives at a current density of 5.0 mА сm−2, compact bright coatings are deposited. When the current density is increased to 10.0 mА сm−2, the coating becomes more stressed, and microcracks appear, and at 30.0 mА сm−2, the formation of spherulites is observed. The addition of neonol makes it possible to obtain high-quality fine-crystalline deposits in a wider current density range, and the addition OP-10, on the contrary, facilitates the formation of spherulites and favors surface development.

The electrocatalytic properties of СоW coatings in the hydrogen reduction reaction were investigated by stationary voltammetry in 1.0 and 6.0 mol L−1 KОН solutions because KOH is used in industrial water electrolysis, and under experimental conditions, i.e., at room temperature, the solution has a maximum electrical conductivity at 28 wt.% KОН.

**Figure 2.** Dependence of the composition (a) and current efficiency (b) of Co-W alloys obtained from electrolyte: 1 without additives; 2—neonol; and 3 and 4—OP-10 (1.5 and 4.5 g L−1) on the deposition current density.

solution: [Co]:[Mo] = 20:1; 10:1: 5:1; 1:1 in a current density range of 10–100 mА сm−2 at 50°С. The studies of hydrogen electroreduction on Со-Мо alloys were carried out in solutions with

KOH. The voltammetric measurements for the determination of the kinetic parameters of hydrogen reduction were made at 25°С. The current-potential curves were recorded under potentiostatic conditions with a step of 20 mV. Before each experiment, argon was passed

In order to show clearly the catalytic effect of the alloy in comparison with an individual metal, current-potential curves of hydrogen reduction on pure cobalt deposited from a citrate-pyrophosphate electrolyte at a current density of 30 mА сm−2 and a temperature of

**Figure 5** shows current-potential curves of hydrogen electroreduction in an acidic, an alkaline, and a neutral medium on electrolytic cobalt cathodes and cathodes made of Со-Мо

Because of differences in electroreduction mechanism at different pH values, the lowest hydrogen evolution overpotential on cobalt is observed in an acidic medium and the highest in a neutral medium, as is the case with the dependence obtained for a mercury electrode and described in [30]. In the case of cobalt cathode, the coefficient *b* of the Tafel portion of the polarization curve for the acidic, alkaline, and neutral media was 0.122, 0.142, and 0.125 V, and the exchange current density was 1.93, 4.59 × 10−2 and 6.31 × 10−3 mА сm−2, respectively.

**Figure 5** shows a considerable decrease in hydrogen evolution overpotential on the alloy. For instance, at the current density of 10 mА сm−2, the overpotential value in the alkaline medium

The plots shown in **Figure 6** have been obtained on alloys electrodeposited at different current densities of 10–100 mА сm−2. The abscissa of the figure gives the ratio of the amounts of the metals in the alloy, and not the exact percentage because of the peculiarities of EDX

**Figure 5.** Stationary current-potential curves of the electroreduction of hydrogen evolution on electrolytic cobalt (a) and Co-Mo alloys (b) with the ratio [Mo]/[Mo] + [Co] = 0.33 electrodeposited at the current density 30 mA cm−2, in media with

alloys electrodeposited at the same current density and temperature.

SO4

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction

; 0.5 mol L−1 Na2

SO4

http://dx.doi.org/10.5772/intechopen.79058

; and 1.0 mol L−1

101

different mineralization and рН, viz. 0.01 mol L−1 H2

through the solution for 30 min.

50°С have been obtained.

is lower by over 200 mV.

different pH: 1—H<sup>2</sup>

SO4

, 2—KOH, and 3—Na2

SO4 .

**Figure 3.** Microphotographs of the surface of alloys obtained from electrolytes (a) without additives and (b) neonol at a current density of 10.0 mА сm−2 and (c) OP-10 at 30.0 mА сm−2.

**Figure 4.** (a) Volt-ampere curves for the electroreduction of hydrogen in a solution of 1.0 mol L−1 KOH on cobalt (1) and CoW alloys (2 and 3) electrodeposited from the electrolyte without additives at 10.0 and 20.0 mA cm−2, respectively. (b) Volt-ampere curves for the electroreduction of hydrogen in 6.0 mol L−1 KOH solution on CoW alloys, obtained from electrolytes: 1—without additives; 2—neonol; and 3 and 4—OP-10 (1.5 and 4.5 g L−1) at a current density of 10.0 mA cm−2.

**Figure 4a** shows a considerable decrease in hydrogen overpotential on CoW alloys relative to electrolytic cobalt. For instance, at the current density of 30.0 mА сm−2, the overpotential decreases by 360 mV. As one would expect, a higher electrocatalytic activity is observed for the coating with more developed surface with spherulites morphology.

**Figure 4b** shows the effect of SASs added to an electrolyte for the deposition of CoW alloys on their electrocatalytic properties in a solution analogous to industrial electrolyte for hydrogen production. It can be seen that in this particular case, the addition of SAS rather has adverse effects, i.e., smoothing the surface during electrodeposition improves the mechanical and corrosive properties of the coating, but reduces its true surface area.

#### **3.2. Molybdenum alloys**

In the study [28], cobalt and Со-Мо alloys were deposited from a citrate-pyrophosphate electrolyte, proposed earlier [29], with the different concentration ratio of the metals in the solution: [Co]:[Mo] = 20:1; 10:1: 5:1; 1:1 in a current density range of 10–100 mА сm−2 at 50°С. The studies of hydrogen electroreduction on Со-Мо alloys were carried out in solutions with different mineralization and рН, viz. 0.01 mol L−1 H2 SO4 ; 0.5 mol L−1 Na2 SO4 ; and 1.0 mol L−1 KOH. The voltammetric measurements for the determination of the kinetic parameters of hydrogen reduction were made at 25°С. The current-potential curves were recorded under potentiostatic conditions with a step of 20 mV. Before each experiment, argon was passed through the solution for 30 min.

In order to show clearly the catalytic effect of the alloy in comparison with an individual metal, current-potential curves of hydrogen reduction on pure cobalt deposited from a citrate-pyrophosphate electrolyte at a current density of 30 mА сm−2 and a temperature of 50°С have been obtained.

**Figure 5** shows current-potential curves of hydrogen electroreduction in an acidic, an alkaline, and a neutral medium on electrolytic cobalt cathodes and cathodes made of Со-Мо alloys electrodeposited at the same current density and temperature.

Because of differences in electroreduction mechanism at different pH values, the lowest hydrogen evolution overpotential on cobalt is observed in an acidic medium and the highest in a neutral medium, as is the case with the dependence obtained for a mercury electrode and described in [30]. In the case of cobalt cathode, the coefficient *b* of the Tafel portion of the polarization curve for the acidic, alkaline, and neutral media was 0.122, 0.142, and 0.125 V, and the exchange current density was 1.93, 4.59 × 10−2 and 6.31 × 10−3 mА сm−2, respectively.

**Figure 5** shows a considerable decrease in hydrogen evolution overpotential on the alloy. For instance, at the current density of 10 mА сm−2, the overpotential value in the alkaline medium is lower by over 200 mV.

The plots shown in **Figure 6** have been obtained on alloys electrodeposited at different current densities of 10–100 mА сm−2. The abscissa of the figure gives the ratio of the amounts of the metals in the alloy, and not the exact percentage because of the peculiarities of EDX

**Figure 4a** shows a considerable decrease in hydrogen overpotential on CoW alloys relative to electrolytic cobalt. For instance, at the current density of 30.0 mА сm−2, the overpotential decreases by 360 mV. As one would expect, a higher electrocatalytic activity is observed for

**Figure 4.** (a) Volt-ampere curves for the electroreduction of hydrogen in a solution of 1.0 mol L−1 KOH on cobalt (1) and CoW alloys (2 and 3) electrodeposited from the electrolyte without additives at 10.0 and 20.0 mA cm−2, respectively. (b) Volt-ampere curves for the electroreduction of hydrogen in 6.0 mol L−1 KOH solution on CoW alloys, obtained from electrolytes: 1—without additives; 2—neonol; and 3 and 4—OP-10 (1.5 and 4.5 g L−1) at a current density of 10.0 mA cm−2.

**Figure 3.** Microphotographs of the surface of alloys obtained from electrolytes (a) without additives and (b) neonol at a

current density of 10.0 mА сm−2 and (c) OP-10 at 30.0 mА сm−2.

100 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

**Figure 4b** shows the effect of SASs added to an electrolyte for the deposition of CoW alloys on their electrocatalytic properties in a solution analogous to industrial electrolyte for hydrogen production. It can be seen that in this particular case, the addition of SAS rather has adverse effects, i.e., smoothing the surface during electrodeposition improves the mechanical and cor-

In the study [28], cobalt and Со-Мо alloys were deposited from a citrate-pyrophosphate electrolyte, proposed earlier [29], with the different concentration ratio of the metals in the

the coating with more developed surface with spherulites morphology.

rosive properties of the coating, but reduces its true surface area.

**3.2. Molybdenum alloys**

**Figure 5.** Stationary current-potential curves of the electroreduction of hydrogen evolution on electrolytic cobalt (a) and Co-Mo alloys (b) with the ratio [Mo]/[Mo] + [Co] = 0.33 electrodeposited at the current density 30 mA cm−2, in media with different pH: 1—H<sup>2</sup> SO4 , 2—KOH, and 3—Na2 SO4 .

bright coatings are deposited in spite of the presence of microcracks. Increasing the deposition current density to 100 mА сm−2 results in the formation of spherulites and considerable surface

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The conclusion that the electrocatalytic activity decreases with increasing refractory metal content may contradict papers, published earlier, on the properties of Co-Mo coatings [17, 14]. However, attention should be called on how the coating composition was changed. There are several ways of influencing the chemical composition of alloys, viz. changing the electrolyte composition and pH, temperature, current density, hydrodynamic regime, etc. For instance, in the study [14], a change in alloy composition is achieved by changing both deposition current density and electrolyte composition and in the study [17], by changing pH and, hence, the concentration ratio of different complex species that are able to be discharged to form an alloy. Thus, it is impossible to obtain a rigorous dependence of catalytic activity on the composition of coating deposited under different conditions, because the electrolysis conditions influence not only chemical but also phase composition, which in turn influences the

The effect of change in the composition of the solution for alloy electrodeposition on the electrocatalytic properties is clearly seen in **Figure 8**, which shows current-potential curves of hydrogen electroreduction on cobalt and alloys deposited at a current density of 30 mА сm−2 from electrolytes containing different sodium molybdate concentrations. It is seen from the figure that the highest hydrogen evolution overpotential is observed on cobalt deposits and the lowest on the alloy deposited from the electrolyte with [Co]:[Mo] = 10:1. The kinetic parameters of hydrogen electroreduction in an alkaline medium are presented in **Table 1**.

The difference in the value of hydrogen exchange current on alloys with the same chemical

**Figure 8.** Polarization curves obtained in KOH solution on cobalt (1) and Co-Mo alloys with the ratio of metals [Mo]/ [Mo]+[Co] equal to (2)—0.11, (3)—0.19, (4)—0.23, and (5)—0.33, respectively, electrodeposited at 30 mA·cm−2 from

development. This regularity is observed for all investigated solutions.

properties of the alloy as a whole.

composition is seen in **Figure 9**.

electrolytes [Co]:[Mo] = 20:1, 10:1, 5:1, 1:1, respectively.

**Figure 6.** Dependence of the exchange current densities of the hydrogen electroreduction reaction on the ratio of molybdenum to cobalt in alloys electrodeposited at current densities of 10, 30, and 100 mA cm−2 from the solution with a [Co]:[Mo] ratio of 1:1 in various media: 1—H2 SO4 , 2—KOH, and 3—Na2 SO4 .

analysis, in which the coating surface layer, on which a large amount of oxygen and carbon are adsorbed. The adsorbed light elements can be removed by polishing the surface or treating it with argon ions. Both in this and in the initial case, the ratio of the metals in the alloy remains constant, as was shown in [17], with a coating depth of up to 100 nm. Thus, the ratio of molybdenum and cobalt in the surface layer, determined by EDX analysis, corresponds to the volumetric chemical composition of the alloy.

In the alloys deposited from an electrolyte with the concentration ratio [Co]:[Mo] = 1:1, the amount of molybdenum decreases and the exchange current density of the hydrogen reduction reaction increases with increasing deposition current density, which can be accounted for in terms of the value which the surface morphology and the true electrode area have, as was shown in [17]. **Figure 7** shows surface micrographs of alloys electrodeposited at two current densities of 10 and 100 mА сm−2. It can be seen that at a low current density, smooth, fine-crystalline, and

**Figure 7.** The morphology of Co-Mo alloys precipitated from electrolytes [Co]:[Mo] = 1:1 (a and b) and [Co]:[Mo] = 5:1 (c and d) at current densities of: a and c—10 mA cm−2; b and d—100 mA cm−2 and the ratio of metals [Mo]/[Mo] + [Co] equal to 0.39, 0.27, 0.28, and 0.21, respectively.

bright coatings are deposited in spite of the presence of microcracks. Increasing the deposition current density to 100 mА сm−2 results in the formation of spherulites and considerable surface development. This regularity is observed for all investigated solutions.

The conclusion that the electrocatalytic activity decreases with increasing refractory metal content may contradict papers, published earlier, on the properties of Co-Mo coatings [17, 14]. However, attention should be called on how the coating composition was changed. There are several ways of influencing the chemical composition of alloys, viz. changing the electrolyte composition and pH, temperature, current density, hydrodynamic regime, etc. For instance, in the study [14], a change in alloy composition is achieved by changing both deposition current density and electrolyte composition and in the study [17], by changing pH and, hence, the concentration ratio of different complex species that are able to be discharged to form an alloy. Thus, it is impossible to obtain a rigorous dependence of catalytic activity on the composition of coating deposited under different conditions, because the electrolysis conditions influence not only chemical but also phase composition, which in turn influences the properties of the alloy as a whole.

The effect of change in the composition of the solution for alloy electrodeposition on the electrocatalytic properties is clearly seen in **Figure 8**, which shows current-potential curves of hydrogen electroreduction on cobalt and alloys deposited at a current density of 30 mА сm−2 from electrolytes containing different sodium molybdate concentrations. It is seen from the figure that the highest hydrogen evolution overpotential is observed on cobalt deposits and the lowest on the alloy deposited from the electrolyte with [Co]:[Mo] = 10:1. The kinetic parameters of hydrogen electroreduction in an alkaline medium are presented in **Table 1**.

analysis, in which the coating surface layer, on which a large amount of oxygen and carbon are adsorbed. The adsorbed light elements can be removed by polishing the surface or treating it with argon ions. Both in this and in the initial case, the ratio of the metals in the alloy remains constant, as was shown in [17], with a coating depth of up to 100 nm. Thus, the ratio of molybdenum and cobalt in the surface layer, determined by EDX analysis, corresponds to

**Figure 6.** Dependence of the exchange current densities of the hydrogen electroreduction reaction on the ratio of molybdenum to cobalt in alloys electrodeposited at current densities of 10, 30, and 100 mA cm−2 from the solution with

, 2—KOH, and 3—Na2

SO4 .

SO4

In the alloys deposited from an electrolyte with the concentration ratio [Co]:[Mo] = 1:1, the amount of molybdenum decreases and the exchange current density of the hydrogen reduction reaction increases with increasing deposition current density, which can be accounted for in terms of the value which the surface morphology and the true electrode area have, as was shown in [17]. **Figure 7** shows surface micrographs of alloys electrodeposited at two current densities of 10 and 100 mА сm−2. It can be seen that at a low current density, smooth, fine-crystalline, and

**Figure 7.** The morphology of Co-Mo alloys precipitated from electrolytes [Co]:[Mo] = 1:1 (a and b) and [Co]:[Mo] = 5:1 (c and d) at current densities of: a and c—10 mA cm−2; b and d—100 mA cm−2 and the ratio of metals [Mo]/[Mo] + [Co]

the volumetric chemical composition of the alloy.

102 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

a [Co]:[Mo] ratio of 1:1 in various media: 1—H2

equal to 0.39, 0.27, 0.28, and 0.21, respectively.

The difference in the value of hydrogen exchange current on alloys with the same chemical composition is seen in **Figure 9**.

**Figure 8.** Polarization curves obtained in KOH solution on cobalt (1) and Co-Mo alloys with the ratio of metals [Mo]/ [Mo]+[Co] equal to (2)—0.11, (3)—0.19, (4)—0.23, and (5)—0.33, respectively, electrodeposited at 30 mA·cm−2 from electrolytes [Co]:[Mo] = 20:1, 10:1, 5:1, 1:1, respectively.


In **Figure 9**, attention should be called on how the current efficiency of the deposition of the alloy and its electrocatalytic activity correlate with each other. In spite of the fact that the current efficiency was measured for a citrate-pyrophosphate electrolyte for deposition, this electrolyte is a weakly alkaline salt solutions, in which the alloys that are formed also exhibit electrocatalytic activity against the parallel cathodes process, with hydrogen electroreduction; therefore, the plots shown in **Figure 9** are of antibate character. Therefore, alloys, electrodeposited at high current densities up to 100 mA cm−2 and having the highest catalytic activity (for all deposition electrolytes) are deposited with the lowest current

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From the obtained data, it can be concluded that in the case of Со-Мо alloys electrodeposited from citrate-pyrophosphate electrolytes, the hydrogen exchange current decreases for each particular electrolyte with increasing amount of molybdenum in the alloy. However, in a series of electrolytes with the different concentration ratio of the discharging metals, the alloys that differ greatly in chemical composition can have the same electrocatalytic activity; this can be seen in **Figure 10**, which shows values of hydrogen exchange currents and stationary potentials for alloys deposited at the same current density, but from different

An extreme form is typical both of the plots of hydrogen exchange current density vs. the ratio of the alloy components and of the stationary potential values of the alloy in the КОН

Thus, the optimal electrolyte for the making of electrocatalytic Со-Мо alloys is the electrolyte with a cobalt and molybdenum concentration ratio of 10:1, which occupies an intermediate position in the series of investigated solutions; so, increasing or decreasing the concentration of sodium molybdate and hence of molybdenum in the alloy results in the deterioration of the

**Figure 10.** The dependence of the exchange current (1 and 2) and the stationary potential (3 and 4) in the KOH solution

on the molybdenum and cobalt ratio in alloys deposited at current densities, mА cm−2. 1: 3—30; and 2: 4—10.

efficiency.

electrolytes.

solution.

catalytic properties of the coating.

Here, jdep. is the current density of alloy deposition; Е<sup>0</sup> is the stationary potential of the alloy in a КОН solution; *а* and *b* are coefficients in the Tafel equation; and j<sup>0</sup> is hydrogen exchange current density. \* The ratio of the metals was calculated based on their atomic fraction in alloys.

**Table 1.** Kinetic parameters of hydrogen reduction reaction in a 1 mol L−1 KОН.

**Figure 9.** The dependence of the exchange current of the hydrogen reduction in a 1.0 mol L−1 KOH solution (1 and 2) and the current efficiency (3 and 4) of the Co-Mo alloys electrodeposited in the electrolytes [Co]:[Mo] = 5:1 and [Co]:[Mo] = 1:1 on the ratio of metals in the alloy.

In **Figure 9**, attention should be called on how the current efficiency of the deposition of the alloy and its electrocatalytic activity correlate with each other. In spite of the fact that the current efficiency was measured for a citrate-pyrophosphate electrolyte for deposition, this electrolyte is a weakly alkaline salt solutions, in which the alloys that are formed also exhibit electrocatalytic activity against the parallel cathodes process, with hydrogen electroreduction; therefore, the plots shown in **Figure 9** are of antibate character. Therefore, alloys, electrodeposited at high current densities up to 100 mA cm−2 and having the highest catalytic activity (for all deposition electrolytes) are deposited with the lowest current efficiency.

From the obtained data, it can be concluded that in the case of Со-Мо alloys electrodeposited from citrate-pyrophosphate electrolytes, the hydrogen exchange current decreases for each particular electrolyte with increasing amount of molybdenum in the alloy. However, in a series of electrolytes with the different concentration ratio of the discharging metals, the alloys that differ greatly in chemical composition can have the same electrocatalytic activity; this can be seen in **Figure 10**, which shows values of hydrogen exchange currents and stationary potentials for alloys deposited at the same current density, but from different electrolytes.

An extreme form is typical both of the plots of hydrogen exchange current density vs. the ratio of the alloy components and of the stationary potential values of the alloy in the КОН solution.

Thus, the optimal electrolyte for the making of electrocatalytic Со-Мо alloys is the electrolyte with a cobalt and molybdenum concentration ratio of 10:1, which occupies an intermediate position in the series of investigated solutions; so, increasing or decreasing the concentration of sodium molybdate and hence of molybdenum in the alloy results in the deterioration of the catalytic properties of the coating.

**Figure 10.** The dependence of the exchange current (1 and 2) and the stationary potential (3 and 4) in the KOH solution on the molybdenum and cobalt ratio in alloys deposited at current densities, mА cm−2. 1: 3—30; and 2: 4—10.

**Figure 9.** The dependence of the exchange current of the hydrogen reduction in a 1.0 mol L−1 KOH solution (1 and 2) and the current efficiency (3 and 4) of the Co-Mo alloys electrodeposited in the electrolytes [Co]:[Mo] = 5:1 and [Co]:[Mo] = 1:1

on the ratio of metals in the alloy.

**Electrolyte j**

**dep. (mА сm−2) Е<sup>0</sup>**

104 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

Here, jdep. is the current density of alloy deposition; Е<sup>0</sup>

The ratio of the metals was calculated based on their atomic fraction in alloys.

**Table 1.** Kinetic parameters of hydrogen reduction reaction in a 1 mol L−1 KОН.

are coefficients in the Tafel equation; and j<sup>0</sup>

\*

Co 30 −0.821 0.190 0.142 4.59 × 10−2 — [Co]:[Mo] = 20:1 10 −0.910 0.181 0.134 4.46 × 10−2 0.22

[Co]:[Mo] = 10:1 10 −0.940 0.125 0.091 9.23 × 10−2 0.25

[Co]:[Mo] = 5:1 10 −0.920 0.130 0.155 1.25 × 10−1 0.28

[Co]:[Mo] = 1:1 10 −0.900 0.189 0.146 5.08 × 10−2 0.39

 **(V)** *а b* **j**

30 −0.985 0.038 0.077 3.21 × 10−1 0.11

20 −1.005 0.045 0.058 1.67 × 10−1 0.23 30 −1.025 0.025 0.056 3.58 × 10−1 0.19

30 −0.980 0.050 0.062 1.56 × 10−1 0.23 100 −1.000 0.035 0.053 2.19 × 10−1 0.21

 −0.935 0.110 0.089 5.81 × 10−2 0.37 −0.980 0.083 0.080 9.17 × 10−2 0.33 −0.965 0.069 0.077 1.27 × 10−1 0.30 −1.010 0.043 0.068 2.33 × 10−1 0.27

is hydrogen exchange current density.

**0**

 **(mА сm−2) [Мо]/([Мо] + [Со])\***

is the stationary potential of the alloy in a КОН solution; *а* and *b*

It is known [10] that the electrocatalytic activity of electrode material can be increased by several ways, e.g., by increasing the true working surface area of the catalyst, viz. by making the catalyst not in the form of a continuous film, but in the form of nanofibers [31]. The true surface area must be estimated when fabricating continuous alloy coatings, as this was done in [17]; it should be noted, however, that the factor of surface development is not determining in the ascertainment of the causes of the electrocatalytic activity of the materials under investigation. Another important factor is the nature of the metals comprising the alloy, which was shown in [25], but the physicochemical properties of one metal do not account for the synergetic effect in the use of alloys. The phenomenon that describes most reliably the synergetic action of two or more metals is spillover effect, which was described in [10–12]. Since hydrogen evolution occurs on Со and Мо with different rate-determining steps, as was said above, the occurrence of hydrogen spillover makes it possible to bring about a fact reaction (1) on molybdenum atoms and a rapid electrochemical desorption (2) on cobalt atoms.

Because of this, the synergetic effect of the alloy will manifest itself when the atoms are arranged in an optimal manner to perform sequential reaction steps. This is possible when the alloy is an intermetallic, an amorphous phase, or solid solutions with nanosized crystals.

and compact deposits are formed. Since the chemical composition of the coatings deposited from a citrate-pyrophosphate electrolyte is almost constant, the dependence of polarization resistance on deposition current density can be attributed to a difference in surface morphology and the appearance of microcracks on increasing the deposition current. On the whole, the corrosion rate of the obtained alloys is comparable with that in an analogous solution of electrolytic chromium deposited from Cr(III) and Cr(VI) baths in the study [33] (**Figure 12**). The parameters of the corrosion process, determined by the electrochemical impedance method, have been calculated on the basis of an equivalent circuit comprising the ohmic resistance of the solution, corrosion resistance, a constant phase element, and Warburg impedance (**Table 3**). The corrosion studies, carried out by us, of CoW alloys in a 6.0 mol L−1 КОН solution showed that the corrosion resistance in a concentrated КОН solution (**Figure 12**) is two order of magnitude lower than in a model NaCl solution; nevertheless, the coatings can be considered corrosion proof. It was noted that the coatings obtained for the experiment are of the worst quality in comparison with others, since they are intermediate at the transition from fine crystalline to spherulite deposits. The deposits of these two types are dense and smooth, and only at 10.0 mА сm−2, the largest number of cracks is observed, which accordingly affects the properties of coatings; nevertheless, the coatings have a low corrosion rate and are passivated in alkaline solution.

**j (mА сm−2) W (аt.%) R (kОhm сm−2) Rp (kОhm сm−2) Еcorr (V) Jcorr × 105**

Here, j is alloy deposition current density; W is tungsten content, аt.%; R is corrosion resistance determined by the impedance method; Rp is corrosion resistance determined by voltammetry; Еcorr is corrosion potential; and jcorr is

1 5.0 23.3 18.0 41.2 −0.782 1.35 2 10.0 24.0 18.8 6.9 −0.834 8.53 3 20.0 20.8 12.8 16.6 −0.786 5.24  **(А сm−2)**

**Figure 11.** Electrochemical impedance spectra (a) and current-potential curves (b) obtained in the case of corrosion of coatings electrodeposited from a citrate-pyrophosphate electrolyte at the current densities mА cm−2: 1—5.0; 2—10.0; and

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3—20.0.

corrosion current density.

**Table 2.** Corrosion parameters of alloys in a 3.5% NaCl solution.

When we attempted to investigate the coatings obtained by us by X-ray phase analysis, we have not obtained somewhat well-defined peaks of phases, i.e., the coatings were either very fine-crystalline, below the device sensitivity level, or amorphous. However, different properties (e.g., corrosion or magnetic properties) of Co-Mo alloys of the same composition have also been obtained by us earlier. Their phase composition has been studied by us by stripping analysis and shown in [32].

### **4. Corrosion properties**

Some of the main characteristics of modern catalytic materials are not only their activity in the desired reaction but also mechanical strength and corrosion resistance in aggressive media; therefore, the comprehensive investigation of the physicochemical properties of the electrode material is an important scientific and technical problem.

### **4.1. Tungsten alloys**

In the study [26], the corrosion characteristics of electrolytic Co-W alloys deposited from a citrate-pyrophosphate electrolyte in 3.5% NaCl and 6 mol L−1 KOH solutions at 25°С have been determined by EIS and stationary voltammetry (**Figure 11**).

The impedance hodographs obtained at a stationary potential in a corrosive medium can be described by a circuit comprising the ohmic resistance of the solution, charge transfer (corrosion) resistance, and a constant phase element. The corrosion parameters calculated in accordance with this equivalent circuit are listed **Table 2**.

The coating deposited at a current density of 5.0 mА сm−2 has the highest corrosion stability because in the region of kinetic control of electrochemical reaction, most fine-crystalline

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction http://dx.doi.org/10.5772/intechopen.79058 107

It is known [10] that the electrocatalytic activity of electrode material can be increased by several ways, e.g., by increasing the true working surface area of the catalyst, viz. by making the catalyst not in the form of a continuous film, but in the form of nanofibers [31]. The true surface area must be estimated when fabricating continuous alloy coatings, as this was done in [17]; it should be noted, however, that the factor of surface development is not determining in the ascertainment of the causes of the electrocatalytic activity of the materials under investigation. Another important factor is the nature of the metals comprising the alloy, which was shown in [25], but the physicochemical properties of one metal do not account for the synergetic effect in the use of alloys. The phenomenon that describes most reliably the synergetic action of two or more metals is spillover effect, which was described in [10–12]. Since hydrogen evolution occurs on Со and Мо with different rate-determining steps, as was said above, the occurrence of hydrogen spillover makes it possible to bring about a fact reaction (1)

106 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

on molybdenum atoms and a rapid electrochemical desorption (2) on cobalt atoms.

analysis and shown in [32].

**4. Corrosion properties**

**4.1. Tungsten alloys**

material is an important scientific and technical problem.

dance with this equivalent circuit are listed **Table 2**.

been determined by EIS and stationary voltammetry (**Figure 11**).

Because of this, the synergetic effect of the alloy will manifest itself when the atoms are arranged in an optimal manner to perform sequential reaction steps. This is possible when the alloy is an intermetallic, an amorphous phase, or solid solutions with nanosized crystals. When we attempted to investigate the coatings obtained by us by X-ray phase analysis, we have not obtained somewhat well-defined peaks of phases, i.e., the coatings were either very fine-crystalline, below the device sensitivity level, or amorphous. However, different properties (e.g., corrosion or magnetic properties) of Co-Mo alloys of the same composition have also been obtained by us earlier. Their phase composition has been studied by us by stripping

Some of the main characteristics of modern catalytic materials are not only their activity in the desired reaction but also mechanical strength and corrosion resistance in aggressive media; therefore, the comprehensive investigation of the physicochemical properties of the electrode

In the study [26], the corrosion characteristics of electrolytic Co-W alloys deposited from a citrate-pyrophosphate electrolyte in 3.5% NaCl and 6 mol L−1 KOH solutions at 25°С have

The impedance hodographs obtained at a stationary potential in a corrosive medium can be described by a circuit comprising the ohmic resistance of the solution, charge transfer (corrosion) resistance, and a constant phase element. The corrosion parameters calculated in accor-

The coating deposited at a current density of 5.0 mА сm−2 has the highest corrosion stability because in the region of kinetic control of electrochemical reaction, most fine-crystalline

**Figure 11.** Electrochemical impedance spectra (a) and current-potential curves (b) obtained in the case of corrosion of coatings electrodeposited from a citrate-pyrophosphate electrolyte at the current densities mА cm−2: 1—5.0; 2—10.0; and 3—20.0.

and compact deposits are formed. Since the chemical composition of the coatings deposited from a citrate-pyrophosphate electrolyte is almost constant, the dependence of polarization resistance on deposition current density can be attributed to a difference in surface morphology and the appearance of microcracks on increasing the deposition current. On the whole, the corrosion rate of the obtained alloys is comparable with that in an analogous solution of electrolytic chromium deposited from Cr(III) and Cr(VI) baths in the study [33] (**Figure 12**).

The parameters of the corrosion process, determined by the electrochemical impedance method, have been calculated on the basis of an equivalent circuit comprising the ohmic resistance of the solution, corrosion resistance, a constant phase element, and Warburg impedance (**Table 3**).

The corrosion studies, carried out by us, of CoW alloys in a 6.0 mol L−1 КОН solution showed that the corrosion resistance in a concentrated КОН solution (**Figure 12**) is two order of magnitude lower than in a model NaCl solution; nevertheless, the coatings can be considered corrosion proof. It was noted that the coatings obtained for the experiment are of the worst quality in comparison with others, since they are intermediate at the transition from fine crystalline to spherulite deposits. The deposits of these two types are dense and smooth, and only at 10.0 mА сm−2, the largest number of cracks is observed, which accordingly affects the properties of coatings; nevertheless, the coatings have a low corrosion rate and are passivated in alkaline solution.


Here, j is alloy deposition current density; W is tungsten content, аt.%; R is corrosion resistance determined by the impedance method; Rp is corrosion resistance determined by voltammetry; Еcorr is corrosion potential; and jcorr is corrosion current density.

**Table 2.** Corrosion parameters of alloys in a 3.5% NaCl solution.

**Figure 12.** Spectra of electrochemical impedance (a) and current-potential curves (b), obtained in the case of corrosion in a 6.0 mol L−1 KOH solution, for coatings electrodeposited at a current density of 10.0 mA cm−2 from electrolytes 1—no additives; 2—neonol; 3 and 4—OP-10 (1.5 and 4.5 g L−1).

The corrosion test of the obtained coatings was carried out in a sulfate-chloride solution. The calculation of corrosion resistance from electrochemical impedance data has been performed on the basis of an equivalent circuit comprising the ohmic resistance of the solution, charge

**Figure 13.** Dependence of the ratio of the metals content (a) and current efficiency (b) for alloys electrodeposited from

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Alloy samples electrodeposited from an electrolyte with [Co]/[Mo] = 5/1 with increasing current density, i.e., with decreasing molybdenum content (**Figure 13**, curve 2), and an alloy electrodeposited from an analogous citrate electrolyte containing no pyrophosphate at 30 mА сm−2, have been investigated. **Figure 14** and **Table 4** show no clear correlation between the molybdenum content of the alloy and corrosion resistance. Besides, the coatings deposited from solutions with the same ratio of the metals in the presence and absence of pyrophosphate are similar in

The anticorrosion resistance of Co-Mo coatings is usually associated with the percentage content of molybdenum in them as a more corrosion-resistant metal, and the magnetic properties

**Figure 14.** Stationary current-potential curves (a) and electrochemical impedance spectra (b) of the corrosion of alloys deposited from polyligand electrolyte (1–3) and a citrate electrolyte (4) at the deposition current densities, mА cm−2:

anticorrosion properties despite the difference in their chemical composition.

electrolytes with a concentration ratios [Co]/[Mo] = 1/1 and [Co]/[Mo] = 5/1 on current density.

transfer resistance, and a constant phase element.

1—10; 2 and 4—30; and 3—50.


**Table 3.** Corrosion parameters of alloys in a 6 mol L−1 КОН solution.

#### **4.2. Molybdenum alloys**

To determine the corrosion properties of electrolytic Со-Мо alloys in the study [34], the coatings were deposited from a citrate-pyrophosphate electrolyte with different concentration ratios of the metals: [Co]/[Mo] = 1/1 and [Co]/[Mo] = 5/1. The aim of the authors was a large percentage of a component having ferromagnetic properties, i.e., cobalt. To fabricate cobaltrich alloys, electrolytes with low sodium molybdate content and, hence, with small amount of molybdenum in the coating are used.

As is seen from **Figure 13**, the amount of molybdenum in the alloy electrodeposited from an electrolyte at [Co]/[Mo] = 5/1 decreased relative to the alloy deposited from an electrolyte with [Co]/[Mo] = 1/1; in this case, however, no direct proportionality between the molybdenum content of the alloy and solution is observed. For instance, the concentration ratio of molybdate and cobalt in the electrolyte was 0.2 and changed from 0.4 to 0.27 in the coating. The amount of cobalt in the deposit changes only slightly on increasing the current density, but the percentage of nonmetallic impurities, mainly oxygen, increases, which affect the internal stresses in the deposit. Nevertheless, this change in the concentration ratio of the metals in the solution leads to a great increase in current efficiency for the alloy at low current densities (**Figure 13b**). Thus, it can be stated that in terms of the cobalt content of the alloy, the most optimal conditions for the deposition of magnetic coatings are current densities of 10–50 mА сm−2.

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction http://dx.doi.org/10.5772/intechopen.79058 109

**Figure 13.** Dependence of the ratio of the metals content (a) and current efficiency (b) for alloys electrodeposited from electrolytes with a concentration ratios [Co]/[Mo] = 1/1 and [Co]/[Mo] = 5/1 on current density.

The corrosion test of the obtained coatings was carried out in a sulfate-chloride solution. The calculation of corrosion resistance from electrochemical impedance data has been performed on the basis of an equivalent circuit comprising the ohmic resistance of the solution, charge transfer resistance, and a constant phase element.

Alloy samples electrodeposited from an electrolyte with [Co]/[Mo] = 5/1 with increasing current density, i.e., with decreasing molybdenum content (**Figure 13**, curve 2), and an alloy electrodeposited from an analogous citrate electrolyte containing no pyrophosphate at 30 mА сm−2, have been investigated. **Figure 14** and **Table 4** show no clear correlation between the molybdenum content of the alloy and corrosion resistance. Besides, the coatings deposited from solutions with the same ratio of the metals in the presence and absence of pyrophosphate are similar in anticorrosion properties despite the difference in their chemical composition.

The anticorrosion resistance of Co-Mo coatings is usually associated with the percentage content of molybdenum in them as a more corrosion-resistant metal, and the magnetic properties

**4.2. Molybdenum alloys**

of molybdenum in the coating are used.

additives; 2—neonol; 3 and 4—OP-10 (1.5 and 4.5 g L−1).

108 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

**Table 3.** Corrosion parameters of alloys in a 6 mol L−1 КОН solution.

To determine the corrosion properties of electrolytic Со-Мо alloys in the study [34], the coatings were deposited from a citrate-pyrophosphate electrolyte with different concentration ratios of the metals: [Co]/[Mo] = 1/1 and [Co]/[Mo] = 5/1. The aim of the authors was a large percentage of a component having ferromagnetic properties, i.e., cobalt. To fabricate cobaltrich alloys, electrolytes with low sodium molybdate content and, hence, with small amount

**Figure 12.** Spectra of electrochemical impedance (a) and current-potential curves (b), obtained in the case of corrosion in a 6.0 mol L−1 KOH solution, for coatings electrodeposited at a current density of 10.0 mA cm−2 from electrolytes 1—no

 **(А сm−2)**

**Electrolyte W (аt.%) R (kOhm сm−2) Rp (kOhm сm−2) Еcorr (V) Jcorr × 103**

 24.0 0.92 0.58 −1.03 0.84 23.7 0.46 0.37 −1.05 0.11 24.0 0.34 0.40 −1.04 0.17 22.6 0.40 0.48 −1.03 0.12

As is seen from **Figure 13**, the amount of molybdenum in the alloy electrodeposited from an electrolyte at [Co]/[Mo] = 5/1 decreased relative to the alloy deposited from an electrolyte with [Co]/[Mo] = 1/1; in this case, however, no direct proportionality between the molybdenum content of the alloy and solution is observed. For instance, the concentration ratio of molybdate and cobalt in the electrolyte was 0.2 and changed from 0.4 to 0.27 in the coating. The amount of cobalt in the deposit changes only slightly on increasing the current density, but the percentage of nonmetallic impurities, mainly oxygen, increases, which affect the internal stresses in the deposit. Nevertheless, this change in the concentration ratio of the metals in the solution leads to a great increase in current efficiency for the alloy at low current densities (**Figure 13b**). Thus, it can be stated that in terms of the cobalt content of the alloy, the most optimal condi-

tions for the deposition of magnetic coatings are current densities of 10–50 mА сm−2.

**Figure 14.** Stationary current-potential curves (a) and electrochemical impedance spectra (b) of the corrosion of alloys deposited from polyligand electrolyte (1–3) and a citrate electrolyte (4) at the deposition current densities, mА cm−2: 1—10; 2 and 4—30; and 3—50.


**Table 4.** Dependence of the corrosion parameters of Со-Мо alloys on alloy composition.

of coatings—with a cobalt content as a metal with ferromagnetic properties. However, the functional properties of coatings can depend not only on the chemical composition of the alloy but also on its phase composition, morphology, thickness, porosity, and other factors.

To study the phase composition, the electrochemical method of linear stripping voltammetric analysis (LSVA) was used [35]. An advantage of this method, besides simplicity, is the possibility to follow the dissolution dynamics of the alloy. Before the stripping analysis, a 0.5 μM thick layer of Co or Co-Mo alloys were deposited onto the working electrode from a citratepyrophosphate electrolyte with the concentration ratios of the metals [Co]/[Mo] = 1/1 and [Co]/

is saturation magnetization.

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111

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction

**Deposition electrolyte j (mА сm−2) t (°С) Н (Oe) MR (Gs) МS (Gs)** [Co]/[Mo] = 1/1 30 25 155 5.0 113 [Co]/[Mo] = 1/1 40 50 117 20.0 290 [Co]/[Mo] = 5/1 10 50 102 252 927

Here, Н is coercive force, MR is remanent magnetization, and М<sup>S</sup>

The electrolyte for the anodic dissolution of the obtained alloy deposits must meet the following requirements: it must have a high electrical conductivity, be nonaggressive toward the coating, and not cause spontaneous chemical dissolution. In this electrolyte, an electrochemical side reaction paralleling the anodic dissolution of the deposit under investigation must be ruled out. The use of various acids as working solutions was considered inexpedient since the electrochemical process in such electrolytes is paralleled by the chemical dissolution of the coating under analysis. Alkaline solutions are unsuitable for use as working electrolytes because of passivation processes on the surface of samples under investigation. The optimal solution for the anodic dissolution of coatings under investigation is NaCl solution, which makes it possible to obtain on the current-potential curves clear peaks corresponding to the dissolution potentials of the various phases of the alloy. The stripping analysis of coatings was performed in a 0.5 М NaCl solution at 25°С on a rotating disk electrode at a rotational

**Figure 15** shows a current-potential curve of the dissolution of freshly deposited cobalt (curve 1) in a 0.5 М NaCl solution. The observed dissolution peaks are traditionally attributed to the complex phase structure of metals and alloys, but their number depends on the electrolyte in which dissolution is performed [35]. The plot of cobalt dissolution current vs. potential exhibits three peaks, which we further use in order to distinguish the peaks of dissolution of the alloy from the peaks of cobalt dissolution. The figure shows plots except the oxygen and chlorine evolution curve. It can be seen that in the case of dissolution of alloys, the magnitude of the peaks and their number are different. For instance, for the alloys deposited from an electrolyte with the same concentration ratio of the metals, the magnitude of peak 1 is the same, but besides it, only one large peak is observed for the alloy obtained at 50°С and 40 mА сm−2, whereas two peaks are observed for the alloy obtained at 25°С and 30 mА сm−2. However, for the alloy deposited from an electrolyte with lower molybdate content, the magnitude of peak 1 is much larger than for other alloys. Based on the potentials of the peaks, the known cathodic quantity of electricity, the phase diagram of the double layer, and stoichiometry, it can be suggested that peaks 1 and 3 relate to cobalt dissolution and peak 2 to

the dissolution of a solid solution of the chemical compound СоМо in cobalt.

[Mo] = 5/1 at a current density of 10–100 mА сm−2 and a temperature of 25 and 50°С.

**Table 6.** Magnetic properties of alloys having the same chemical composition (21.2 аt.% Мо).

speed of 774 rpm.

When studying the corrosion and magnetic properties of Со-Мо coatings [32] deposited from a citrate-pyrophosphate electrolyte, it was found that for the coatings obtained under different electrolysis conditions and having the same chemical composition, the corrosion stability and magnetic parameters are different; it was suggested that the properties of the alloy largely depend on its phase composition.

In the study, the corrosion and magnetic properties of coatings with same Mo content of the alloy (21.2 аt.%), deposited from a citrate-pyrophosphate electrolyte under different electrolysis conditions, have been investigated. The corrosion stability of the coatings was studied in a solution containing 7 g L−1 Na2 SO4 and 7 g L−1 NaCl at рН 6.0 and 24 ± 1°С (**Table 5**). The magnetic properties (**Table 6**) of the obtained deposits were determined by means of a vibrating sample magnetometer in fields of up to 20 kOe at room temperature (**Table 5**).

The magnetic characteristics of such Alloys are also given. Their calculated values are listed in **Table 6**. The main criteria for evaluating the properties of soft-magnetic materials are low coercive force, low remanent magnetization and high saturation magnetization, as well as the ability to reach saturation magnetization in low intensity fields. It is evident from **Table 6** that the largest saturation magnetization values—927 Gs were obtained at a magnetic field intensity of 3 kOe for the alloys deposited from an electrolyte with a ratio of 5:1. For the coatings deposited from an electrolyte with a ratio of 1:1, no saturation magnetization is observed even at 20 kOe; the maximum value obtained under experimental conditions for alloys deposited at 25° and 50°С was 113 and 290 Gs, respectively. Thus, it can be concluded that the alloy deposited from an electrolyte with the concentration ratio of the metals [Co]/[Mo] = 5/1. We assumed that such a difference in functional properties for the coatings having the same chemical composition may be due to a difference in their phase composition.


**Table 5.** Corrosion properties of alloys having the same chemical composition (21.2 аt.% Мо).


**Table 6.** Magnetic properties of alloys having the same chemical composition (21.2 аt.% Мо).

of coatings—with a cobalt content as a metal with ferromagnetic properties. However, the functional properties of coatings can depend not only on the chemical composition of the alloy but also on its phase composition, morphology, thickness, porosity, and other factors. When studying the corrosion and magnetic properties of Со-Мо coatings [32] deposited from a citrate-pyrophosphate electrolyte, it was found that for the coatings obtained under different electrolysis conditions and having the same chemical composition, the corrosion stability and magnetic parameters are different; it was suggested that the properties of the alloy

**Electrolyte j (mА сm−2) Мо (аt.%) R (Оhm) Rp (Оhm) Еcorr (V) Jcorr (А сm−2)** Citrate-pyrophosphate 10 21.5 5.05 × 102 2.97 × 103 −0.627 3.58 × 10−4

Citrate 30 23.9 4.57 × 103 8.06 × 103 −0.804 2.83 × 10−5

30 17.4 5.71 × 103 1.77 × 104 −0.804 829 × 10−6 50 16.0 7.02 × 102 3.55 × 102 −0.677 6.40 × 10−4

In the study, the corrosion and magnetic properties of coatings with same Mo content of the alloy (21.2 аt.%), deposited from a citrate-pyrophosphate electrolyte under different electrolysis conditions, have been investigated. The corrosion stability of the coatings was studied in

magnetic properties (**Table 6**) of the obtained deposits were determined by means of a vibrat-

The magnetic characteristics of such Alloys are also given. Their calculated values are listed in **Table 6**. The main criteria for evaluating the properties of soft-magnetic materials are low coercive force, low remanent magnetization and high saturation magnetization, as well as the ability to reach saturation magnetization in low intensity fields. It is evident from **Table 6** that the largest saturation magnetization values—927 Gs were obtained at a magnetic field intensity of 3 kOe for the alloys deposited from an electrolyte with a ratio of 5:1. For the coatings deposited from an electrolyte with a ratio of 1:1, no saturation magnetization is observed even at 20 kOe; the maximum value obtained under experimental conditions for alloys deposited at 25° and 50°С was 113 and 290 Gs, respectively. Thus, it can be concluded that the alloy deposited from an electrolyte with the concentration ratio of the metals [Co]/[Mo] = 5/1. We assumed that such a difference in functional properties for the coatings having the same

ing sample magnetometer in fields of up to 20 kOe at room temperature (**Table 5**).

chemical composition may be due to a difference in their phase composition.

**Table 5.** Corrosion properties of alloys having the same chemical composition (21.2 аt.% Мо).

**Deposition electrolyte j (mА сm−2) t (°С) Rр (Оhm) Еcorr (V) Jcorr (А сm−2)** [Co]/[Mo] = 1/1 30 25 2.71 × 103 −0.804 8.44 × 10−5 [Co]/[Mo] = 1/1 40 50 9.82 × 102 −0.808 2.54 × 10−4 [Co]/[Mo] = 5/1 10 50 2.97 × 103 −0.627 3.58 × 10−4

and 7 g L−1 NaCl at рН 6.0 and 24 ± 1°С (**Table 5**). The

SO4

**Table 4.** Dependence of the corrosion parameters of Со-Мо alloys on alloy composition.

110 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

largely depend on its phase composition.

a solution containing 7 g L−1 Na2

Here, j is deposition current density.

To study the phase composition, the electrochemical method of linear stripping voltammetric analysis (LSVA) was used [35]. An advantage of this method, besides simplicity, is the possibility to follow the dissolution dynamics of the alloy. Before the stripping analysis, a 0.5 μM thick layer of Co or Co-Mo alloys were deposited onto the working electrode from a citratepyrophosphate electrolyte with the concentration ratios of the metals [Co]/[Mo] = 1/1 and [Co]/ [Mo] = 5/1 at a current density of 10–100 mА сm−2 and a temperature of 25 and 50°С.

The electrolyte for the anodic dissolution of the obtained alloy deposits must meet the following requirements: it must have a high electrical conductivity, be nonaggressive toward the coating, and not cause spontaneous chemical dissolution. In this electrolyte, an electrochemical side reaction paralleling the anodic dissolution of the deposit under investigation must be ruled out. The use of various acids as working solutions was considered inexpedient since the electrochemical process in such electrolytes is paralleled by the chemical dissolution of the coating under analysis. Alkaline solutions are unsuitable for use as working electrolytes because of passivation processes on the surface of samples under investigation. The optimal solution for the anodic dissolution of coatings under investigation is NaCl solution, which makes it possible to obtain on the current-potential curves clear peaks corresponding to the dissolution potentials of the various phases of the alloy. The stripping analysis of coatings was performed in a 0.5 М NaCl solution at 25°С on a rotating disk electrode at a rotational speed of 774 rpm.

**Figure 15** shows a current-potential curve of the dissolution of freshly deposited cobalt (curve 1) in a 0.5 М NaCl solution. The observed dissolution peaks are traditionally attributed to the complex phase structure of metals and alloys, but their number depends on the electrolyte in which dissolution is performed [35]. The plot of cobalt dissolution current vs. potential exhibits three peaks, which we further use in order to distinguish the peaks of dissolution of the alloy from the peaks of cobalt dissolution. The figure shows plots except the oxygen and chlorine evolution curve. It can be seen that in the case of dissolution of alloys, the magnitude of the peaks and their number are different. For instance, for the alloys deposited from an electrolyte with the same concentration ratio of the metals, the magnitude of peak 1 is the same, but besides it, only one large peak is observed for the alloy obtained at 50°С and 40 mА сm−2, whereas two peaks are observed for the alloy obtained at 25°С and 30 mА сm−2. However, for the alloy deposited from an electrolyte with lower molybdate content, the magnitude of peak 1 is much larger than for other alloys. Based on the potentials of the peaks, the known cathodic quantity of electricity, the phase diagram of the double layer, and stoichiometry, it can be suggested that peaks 1 and 3 relate to cobalt dissolution and peak 2 to the dissolution of a solid solution of the chemical compound СоМо in cobalt.

**Figure 15.** Anodic dissolution curves for pure cobalt (curve 1) and Co-Mo alloys (21.1 at.% Mo) deposited under the following conditions: 2—Co:Mo = 5:1, 50°C, 10 mA cm−2; 3—Co:Mo = 1:1, 25°C, 30 mA cm−2; 4—Co:Mo = 1:1, 50°С, 40 mА сm−2.

Thus, the alloy deposited from an electrolyte containing a small amount of molybdate contains a larger amount of cobalt bound into neither a chemical compound nor a solid solution; this can explain the 200 mV shift of the corrosion potential of this alloy to positive values, i.e., cobalt is a more electropositive metal than molybdenum, as well as the improvement of the magnetic properties of the alloy. The increase in peak 2 indicates an increase in the percentage of the molybdenum-rich phase, which results in the improvement of the corrosion properties of alloys.

Stripping analysis of alloys deposited from an electrolyte with [Co]/[Mo]=1/1 at different current densities is shown in **Figure 16**. It is seen from the figure that for the coatings obtained at lower deposition current densities 10–20 mА сm−2, three dissolution peaks are observed. For the alloys deposited at 30 and 40 mА сm−2, only two dissolution peaks are observed. The magnitude of peak 1 remains unchanged, and it can be assumed to correspond to the amount of cobalt that is directly on the electrode surface, i.e., dissolution of the less corrosion-resistant component than the alloy takes place, and since the electrode geometry does not change, the quantity of electricity for dissolution remains constant too. In the remaining alloy phase at different current densities, different redistribution of cobalt between the solid solution and the free α-phase takes place.

**5. Conclusions**

dissolution peaks.

the properties of exiting nickel cathodes.

the anodic dissolution peaks: 1—the first peak, and 2—the second peak.

Electrolytic alloys of molybdenum and tungsten with iron subgroup metals possess catalytic properties in the hydrogen evolution reaction and can be used as a coating which improves

**Figure 17.** (a) Anodic current-potential curves of the dissolution of Co-Mo alloys deposited from an electrolyte with [Co]/[Mo]=1/1 at 30 mA∙cm−2 and 50°C at different deposition time, s: 1—30, 2—60, 3—120, 4—210. (b) The magnitude of

**Figure 16.** (a) Anodic current-potential curves of the dissolution of Co-Mo alloys deposited from an electrolyte at [Co]/ [Mo] = 1/1 at 50°C at different current densities, mA cm−2: 1—10, 2—20, 3—30, 4—40 and (b) The magnitude of anodic

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction

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113

For each particular electrolyte and particular deposition conditions, the activity of the alloy and its corrosion stability increase with the amount of molybdenum, but the comparison of

The properties of alloys depend not only on their chemical composition but also on their phase composition. The alloy containing the same amount of molybdenum, but fabricated

data obtained by different authors does not give a clear correlation.

under different conditions, has different corrosion and magnetic properties.

To study the alloy formation process, anodic polarization curves have been obtained for Со-Мо alloys deposited from an electrolyte with [Co]/[Mo] = 1/1 under the conditions corresponding to the maximum current efficiency: a current density of 30 mА сm−2 and at temperature of 50°С [36]. The coatings were deposited during 30, 60, 120, and 210 s (**Figure 17**). All current-potential curves of the dissolution of Со-Мо alloys exhibit two peaks. As one would expect, the quantity of electricity for the dissolution of the solid solution increases linearly with deposition time. We failed, however, to determine the order of deposition of the components; using this method, it is impossible to either confirm or refute the hypothesis that cobalt deposits first, which catalyzes later the reduction of molybdenum. Under our experimental conditions, an alloy phase which grows uniformly with increasing deposition time is present in the alloy even within a deposition time of 30 s.

Electrocatalytic Properties of Molybdenum and Tungsten Alloys in the Hydrogen Evolution Reaction http://dx.doi.org/10.5772/intechopen.79058 113

**Figure 16.** (a) Anodic current-potential curves of the dissolution of Co-Mo alloys deposited from an electrolyte at [Co]/ [Mo] = 1/1 at 50°C at different current densities, mA cm−2: 1—10, 2—20, 3—30, 4—40 and (b) The magnitude of anodic dissolution peaks.

**Figure 17.** (a) Anodic current-potential curves of the dissolution of Co-Mo alloys deposited from an electrolyte with [Co]/[Mo]=1/1 at 30 mA∙cm−2 and 50°C at different deposition time, s: 1—30, 2—60, 3—120, 4—210. (b) The magnitude of the anodic dissolution peaks: 1—the first peak, and 2—the second peak.

### **5. Conclusions**

Thus, the alloy deposited from an electrolyte containing a small amount of molybdate contains a larger amount of cobalt bound into neither a chemical compound nor a solid solution; this can explain the 200 mV shift of the corrosion potential of this alloy to positive values, i.e., cobalt is a more electropositive metal than molybdenum, as well as the improvement of the magnetic properties of the alloy. The increase in peak 2 indicates an increase in the percentage of the molybdenum-rich phase, which results in the improvement of the corrosion properties of alloys. Stripping analysis of alloys deposited from an electrolyte with [Co]/[Mo]=1/1 at different current densities is shown in **Figure 16**. It is seen from the figure that for the coatings obtained at lower deposition current densities 10–20 mА сm−2, three dissolution peaks are observed. For the alloys deposited at 30 and 40 mА сm−2, only two dissolution peaks are observed. The magnitude of peak 1 remains unchanged, and it can be assumed to correspond to the amount of cobalt that is directly on the electrode surface, i.e., dissolution of the less corrosion-resistant component than the alloy takes place, and since the electrode geometry does not change, the quantity of electricity for dissolution remains constant too. In the remaining alloy phase at different current densities, different redistribution of cobalt between the solid solution and the free α-phase takes place. To study the alloy formation process, anodic polarization curves have been obtained for Со-Мо alloys deposited from an electrolyte with [Co]/[Mo] = 1/1 under the conditions corresponding to the maximum current efficiency: a current density of 30 mА сm−2 and at temperature of 50°С [36]. The coatings were deposited during 30, 60, 120, and 210 s (**Figure 17**). All current-potential curves of the dissolution of Со-Мо alloys exhibit two peaks. As one would expect, the quantity of electricity for the dissolution of the solid solution increases linearly with deposition time. We failed, however, to determine the order of deposition of the components; using this method, it is impossible to either confirm or refute the hypothesis that cobalt deposits first, which catalyzes later the reduction of molybdenum. Under our experimental conditions, an alloy phase which grows uniformly with increasing deposition time is present

**Figure 15.** Anodic dissolution curves for pure cobalt (curve 1) and Co-Mo alloys (21.1 at.% Mo) deposited under the following conditions: 2—Co:Mo = 5:1, 50°C, 10 mA cm−2; 3—Co:Mo = 1:1, 25°C, 30 mA cm−2; 4—Co:Mo = 1:1, 50°С, 40

in the alloy even within a deposition time of 30 s.

112 Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

mА сm−2.

Electrolytic alloys of molybdenum and tungsten with iron subgroup metals possess catalytic properties in the hydrogen evolution reaction and can be used as a coating which improves the properties of exiting nickel cathodes.

For each particular electrolyte and particular deposition conditions, the activity of the alloy and its corrosion stability increase with the amount of molybdenum, but the comparison of data obtained by different authors does not give a clear correlation.

The properties of alloys depend not only on their chemical composition but also on their phase composition. The alloy containing the same amount of molybdenum, but fabricated under different conditions, has different corrosion and magnetic properties.

The amorphous and nanocrystalline structure of electrolytic alloys makes hydrogen spillover possible, which greatly accelerates the hydrogen reduction process in alkaline electrolytes.

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Electrolytic coatings of refractory metals alloys have a wide range of physicochemical properties. Controlling the electrolysis modes allows precipitating and accurately controlling the chemical and phase composition of the coatings, and hence producing corrosion-resistant materials for soft magnetic materials, electrocatalysts, and wear-resistant coatings.

### **Author details**

Valeriy Kublanovsky\* and Yuliya Yapontseva

\*Address all correspondence to: kublanovsky@ionc.kar.net

V. I. Vernadskii Institute of General and Inorganic Chemistry of the Ukrainian NAS, Kyiv, Ukraine

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The amorphous and nanocrystalline structure of electrolytic alloys makes hydrogen spillover possible, which greatly accelerates the hydrogen reduction process in alkaline electrolytes.

Electrolytic coatings of refractory metals alloys have a wide range of physicochemical properties. Controlling the electrolysis modes allows precipitating and accurately controlling the chemical and phase composition of the coatings, and hence producing corrosion-resistant

V. I. Vernadskii Institute of General and Inorganic Chemistry of the Ukrainian NAS, Kyiv,

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Valeriy Kublanovsky\* and Yuliya Yapontseva

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\*Address all correspondence to: kublanovsky@ionc.kar.net


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## *Edited by Abhijit Ray, Indrajit Mukhopadhyay and Ranjan K. Pati*

The book starts with a theoretical understanding of electrocatalysis in the framework of density functional theory followed by a vivid review of oxygen reduction reactions. A special emphasis has been placed on electrocatalysts for a proton-exchange membrane-based fuel cell where graphene with noble metal dispersion plays a significant role in electron transfer at thermodynamically favourable conditions. The latter part of the book deals with two 2D materials with high economic viability and process ability and MoS2 and WS2 for their prospects in water-splitting from renewable energy.

Published in London, UK © 2018 IntechOpen © oneblink-cj / iStock

Electrocatalysts for Fuel Cells and Hydrogen Evolution - Theory to Design

Electrocatalysts for Fuel Cells

and Hydrogen Evolution

Theory to Design

*Edited by Abhijit Ray,* 

*Indrajit Mukhopadhyay and Ranjan K. Pati*