**1. Introduction**

Due to diminishing fossil fuel resources and climate change, sustainable renewable energy sources are sought. Among the proposed clean energy sources, fuel cells are the latest developed energy conversion devices that convert the chemical energy of a fuel directly into electrical

© 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.

energy. Fuel cells technology has recently evolved to address the challenges of global energy. Among them, proton exchange membrane fuel cells (PEMFC) have attracted a considerable attention as a potential power source for automotive and stationary applications due to its low temperature operation conditions, high power density and high energy conversion efficiency. Hydrogen is regarded as the most attractive fuel for PEM FC, demonstrating excellent electrochemical reactivity, highest power density, zero emissions characteristics.

the dissolution and aggregation of metal nanoparticles, severe corrosion and oxidation of carbon materials in the actual fuel cell environment could lead to rapid loss of catalyst activity [19–21]. In this context, the unique properties of the graphene meet the basic requirements of an ideal catalyst support. Therefore, a notable effort was devoted to the design of new nano-

Iodine Doped Graphene for Enhanced Electrocatalytic Oxygen Reduction Reaction in PEM Fuel…

In the carbonaceous family, graphene is a graphite monolayer with a thickness of only

atom is covalently bonded to the other three. Graphene is the two-dimensional graphite variant; is composed of a planar (two-dimensional) arrangement of carbon atoms ensconced in a hexagonal structure. Graphene possesses unique properties such as high mobility of load carriers (up to 105 cm2 V−1 s−1) superconductivity, Hall effect at room temperature, high mechanical strength (130 GPa) and high specific surface area. These properties make the material under discussion an excellent support for catalysts used in electrochemical energy systems. Catalysts based on graphene doped with heteroatoms have proved a stable catalytic activity compared to other ORR composite electrocatalysts. As a catalytic support, compared to other carbonaceous support materials, the graphene combines the advantages of the traditional 2D graphite (high conductivity and high specific surface area) with porous structure and nonagglomerated morphology that can facilitate the deposition and dispersion of the catalyst. Moreover, the interconnected graphene network promotes the rapid transport of electrons between active sites and the electrode and increases the electroactive catalyst surface, all of which increase the catalytic activity and durability of these electrocatalysts for ORR [22–25]. The doped graphene with various heteroatoms removes the disadvantages and limitations described above by improving the catalytic performance of the electrocatalysts due to the high specific surface area and excellent conductivity. Graphene nanosheets (GNS) have exhibited large promising applications as a support for 2-D catalyst because of the following properties:

first, the graphene has a large theoretical surface area of more than 2000 m<sup>2</sup>

as much as that carbon nanotubes (CNTs); secondly, the graphene has a completely conju-

ical properties, and high thermal conductivity. The electronic property of the graphene is very important for its application in electrochemistry. Previous studies have shown that the mobility of electrons in the graphite suspended monolayer can reach around 2 × 105 cm2

at room temperature, which is higher than that of all other materials, including metals and carbon nanotubes. Doped graphene was noted to show high electrocatalytic activity for the

Previous studies have found that in the case of non-Pt graphene-based nanocatalysts, there is also a charge transfer process through the graphene-metal interface, which depends on the distance and the Fermi level difference between the graphene and the supported cata-

However, metal-based electrocatalysts often suffer from some drawbacks, particularly low acidity in the acid medium specific for PEMFC environment. Doping with non-metallic heteroatom is one of the most studied nanocatalysts for the ORR reaction in PEMFC because

hybrid structure, giving rise to very high electrical conductivity, excellent mechan-

hybridization state, arranged so that each carbon

http://dx.doi.org/10.5772/intechopen.76495

81

/g), which is twice

/(V s)

structured catalysts dispersed on graphene support.

0.34 nm. It consists of carbon atoms in a sp2

gated sp2

ORR reaction.

lysts [22].

The standard structure of a fuel cell consists in a solid electrolyte in contact to a porous anode and cathode on either side. A fuel cell consists of a fuel-fed anode and an oxygen-filled cathode separated by a solid polymer membrane. Diffusion layers for the reactants and product, bipolar plates to transport the reactants/product to/from the catalytic layers; end plates with current collectors and sealing gaskets are other basic components of a fuel cell. This configuration allows the transfer of ions between the two electrodes (components constituting the membrane-electrode assembly MEA). In a typical fuel cell, gaseous hydrogen reactant H2 is continuously fed to anode side and a gaseous oxidant (air or oxygen) is fed to the cathode compartment. The electrochemical reactions occur at electrodes and the electrons will be released to the external circuit.

Appreciable progress has been made over past 20 years regarding the development of PEMFC technology. However there are still several technical challenges that need to be addressed in order to promote their commercialization.

Among the metal catalysts for the anode and cathode reactions, platinum (Pt) exhibits the largest electrocatalytic activities for the electro-oxidation of small organic compounds in the fuel from the anode and the reduction of oxygen at the cathode. In order to obtain ideal electrocatalysts for FCs with high catalytic performance and low cost, efforts have been made to develop new structured catalysts.

Recently, several technical issues for the commercialization of PEMFC including water management at the cathode, resistance reduction of the electrolyte membrane, technical realization of electrode assembly (MEA) have been addressed. Thus, in order to improve transport of water generated from electrochemical reaction at the cathode, research has been conducted on developing the mesoporous structures into the electrode [1–9]. Pt nanoparticles based on graphene support have been extensively studied due to their catalytic properties and excellent corrosion and oxidation resistance [10–12]. However, the high cost and limited resources hinder the widespread use of Pt-based catalysts and the widespread commercialization of fuel cells. To reduce the cost of FCs, numerous studies on electrocatalysts as FC electrodes were focused on manufacturing and developing alternatives to non-precious metals [13–18].

One of the major fuel cell limitations is the low rate of oxygen reduction (ORR) at the cathode, which requires a large amount of expensive Pt/C platinum catalyst. Thus, ORR plays a critical role in determining the performance of a fuel cell. ORR is a multi-electron transfer reaction with two possible main pathways: a) a direct path in one step, involving the transfer of four electrons to directly produce H2 O; b) an indirect two-step pathway, involving the transfer of two electrons for the first stage and two second electrons for the second stage to obtain water. Previous studies have shown that, with the exception of metal catalyst degradation due to the dissolution and aggregation of metal nanoparticles, severe corrosion and oxidation of carbon materials in the actual fuel cell environment could lead to rapid loss of catalyst activity [19–21]. In this context, the unique properties of the graphene meet the basic requirements of an ideal catalyst support. Therefore, a notable effort was devoted to the design of new nanostructured catalysts dispersed on graphene support.

energy. Fuel cells technology has recently evolved to address the challenges of global energy. Among them, proton exchange membrane fuel cells (PEMFC) have attracted a considerable attention as a potential power source for automotive and stationary applications due to its low temperature operation conditions, high power density and high energy conversion efficiency. Hydrogen is regarded as the most attractive fuel for PEM FC, demonstrating excellent

The standard structure of a fuel cell consists in a solid electrolyte in contact to a porous anode and cathode on either side. A fuel cell consists of a fuel-fed anode and an oxygen-filled cathode separated by a solid polymer membrane. Diffusion layers for the reactants and product, bipolar plates to transport the reactants/product to/from the catalytic layers; end plates with current collectors and sealing gaskets are other basic components of a fuel cell. This configuration allows the transfer of ions between the two electrodes (components constituting the membrane-electrode assembly MEA). In a typical fuel cell, gaseous hydrogen reactant

 is continuously fed to anode side and a gaseous oxidant (air or oxygen) is fed to the cathode compartment. The electrochemical reactions occur at electrodes and the electrons will be

Appreciable progress has been made over past 20 years regarding the development of PEMFC technology. However there are still several technical challenges that need to be addressed in

Among the metal catalysts for the anode and cathode reactions, platinum (Pt) exhibits the largest electrocatalytic activities for the electro-oxidation of small organic compounds in the fuel from the anode and the reduction of oxygen at the cathode. In order to obtain ideal electrocatalysts for FCs with high catalytic performance and low cost, efforts have been made to

Recently, several technical issues for the commercialization of PEMFC including water management at the cathode, resistance reduction of the electrolyte membrane, technical realization of electrode assembly (MEA) have been addressed. Thus, in order to improve transport of water generated from electrochemical reaction at the cathode, research has been conducted on developing the mesoporous structures into the electrode [1–9]. Pt nanoparticles based on graphene support have been extensively studied due to their catalytic properties and excellent corrosion and oxidation resistance [10–12]. However, the high cost and limited resources hinder the widespread use of Pt-based catalysts and the widespread commercialization of fuel cells. To reduce the cost of FCs, numerous studies on electrocatalysts as FC electrodes were focused on manufacturing and developing alternatives to non-precious metals [13–18]. One of the major fuel cell limitations is the low rate of oxygen reduction (ORR) at the cathode, which requires a large amount of expensive Pt/C platinum catalyst. Thus, ORR plays a critical role in determining the performance of a fuel cell. ORR is a multi-electron transfer reaction with two possible main pathways: a) a direct path in one step, involving the transfer of four

two electrons for the first stage and two second electrons for the second stage to obtain water. Previous studies have shown that, with the exception of metal catalyst degradation due to

O; b) an indirect two-step pathway, involving the transfer of

electrochemical reactivity, highest power density, zero emissions characteristics.

H2

released to the external circuit.

80 Advances In Hydrogen Generation Technologies

develop new structured catalysts.

electrons to directly produce H2

order to promote their commercialization.

In the carbonaceous family, graphene is a graphite monolayer with a thickness of only 0.34 nm. It consists of carbon atoms in a sp2 hybridization state, arranged so that each carbon atom is covalently bonded to the other three. Graphene is the two-dimensional graphite variant; is composed of a planar (two-dimensional) arrangement of carbon atoms ensconced in a hexagonal structure. Graphene possesses unique properties such as high mobility of load carriers (up to 105 cm2 V−1 s−1) superconductivity, Hall effect at room temperature, high mechanical strength (130 GPa) and high specific surface area. These properties make the material under discussion an excellent support for catalysts used in electrochemical energy systems.

Catalysts based on graphene doped with heteroatoms have proved a stable catalytic activity compared to other ORR composite electrocatalysts. As a catalytic support, compared to other carbonaceous support materials, the graphene combines the advantages of the traditional 2D graphite (high conductivity and high specific surface area) with porous structure and nonagglomerated morphology that can facilitate the deposition and dispersion of the catalyst. Moreover, the interconnected graphene network promotes the rapid transport of electrons between active sites and the electrode and increases the electroactive catalyst surface, all of which increase the catalytic activity and durability of these electrocatalysts for ORR [22–25].

The doped graphene with various heteroatoms removes the disadvantages and limitations described above by improving the catalytic performance of the electrocatalysts due to the high specific surface area and excellent conductivity. Graphene nanosheets (GNS) have exhibited large promising applications as a support for 2-D catalyst because of the following properties: first, the graphene has a large theoretical surface area of more than 2000 m<sup>2</sup> /g), which is twice as much as that carbon nanotubes (CNTs); secondly, the graphene has a completely conjugated sp2 hybrid structure, giving rise to very high electrical conductivity, excellent mechanical properties, and high thermal conductivity. The electronic property of the graphene is very important for its application in electrochemistry. Previous studies have shown that the mobility of electrons in the graphite suspended monolayer can reach around 2 × 105 cm2 /(V s) at room temperature, which is higher than that of all other materials, including metals and carbon nanotubes. Doped graphene was noted to show high electrocatalytic activity for the ORR reaction.

Previous studies have found that in the case of non-Pt graphene-based nanocatalysts, there is also a charge transfer process through the graphene-metal interface, which depends on the distance and the Fermi level difference between the graphene and the supported catalysts [22].

However, metal-based electrocatalysts often suffer from some drawbacks, particularly low acidity in the acid medium specific for PEMFC environment. Doping with non-metallic heteroatom is one of the most studied nanocatalysts for the ORR reaction in PEMFC because it has been shown that heteroatom doping can induce redistribution of graft load. In this regard, heteroatom doped graphene such as N, S, P, B have recently demonstrated that they can effectively improve the ORR catalytic activity [26–30]. It has been suggested that the dopant (whether its electronegativity toward carbon is greater (such as N, S) or lower (such as B) could create electronically loaded sites favorable for O<sup>2</sup> adsorption. To reduce Pt loading and the cost of electrocatalysts in fuel cells, non-Pt catalysts supported on graphene have also been developed in recent years. For N-doped graphene, the ORR catalytic activity is strongly dependent on the nitrogen types and the doping concentration. Nitrogen-doped graphene nanocomposites, have demonstrated an improved electrocatalytic activity of ORR, due to an interpenetration network formed between N and graphene, which can efficiently accelerate the reaction, the transport of ions and electrons and therefore synergistically improves the catalytic activity of ORR. The interaction between graphene support and composites may also affect the stability of electrocatalysts. For example, the strong link between N-doped sites of graphene may result in increased resistance of hybrid catalysts [25].

Theoretically, adsorption of diatomic halogen molecules on graphene using functional Van der Waals has been studied, which includes nonlocal correlation effects, perfect for geometric optimization. The adsorption of halogen atoms on graphenes was studied using only a semi local function. It is accepted that an iodine atom can accept 0.5 electrons from the carbon substrate. The doping of the graphene through physical adsorption is particularly promising because it can increase the concentration of the carriers without affecting carrier mobility as in the case of chemical adsorbed dopants, where the covalent function can produce crystalline defects and irreversibly alter the electron structure. One of the most promising dopants that could be physically adsorbed on the graphite is iodine. Iodine is also considered to be a stable and practical dopant as compared to other halogen based dopants (Cl, Br and F) and compared to many other physically adsorbed dopants such as alkali metal dopants (K, Li, Na, etc.), acids (hydrochloric

Iodine Doped Graphene for Enhanced Electrocatalytic Oxygen Reduction Reaction in PEM Fuel…

racyanoquinodimethane), poly (4-vinylpyridine) and polyethyleneimine). However, the intercalation of iodine in the engraved Bernal multilayer graphite, which is required for potential electrode applications, was considered unlikely due to the strong interaction between the gra-

iodine-doped graphite, such as its chemical state, thermal stability, and working function have not been extensively investigated, even if they are critical parameters for successfully achieving graphite-based electrodes in industrial applications. It has been shown that the overlaid monolayer graphite and the double layer of graphite foil can be efficiently doped with iodine. The request of a chemically stable material and efficient ORR electrocatalyst, directed us to develop a new concept of electrode as alternative cathode catalyst/microporous layer. The recent new application of iodine-doped (I-doped) graphene as electrode in PEMFC has been recently recognized as an improved strategy for effective modification of cathode side efficiency [32–35]. The performed experimental studies revealed that the microporous layer (MPL) placed between catalyst layer (CL) and GDL have many advantages, like: keeping the hydration of the membrane and of the ionomer phase, preventing gas diffusion layer (GDL) flooding, especially at high current densities, forming a more intimate contact between CL and GDL.

The main objective of this work is to improve the ORR performance by including of the nanostructured I-doped graphene and to prove the efficiency of the developed cathode in PEMFC operation conditions. It is important to mention, that only few papers have been reported in respect to the cathode electrocatalyst for PEMFC containing I-doped graphene. In view of these facts, the purpose of this work is to provide valuable information about the recommendation of I-doped graphene as innovative ORR electrode in the PEMFC cathode, based on

performances obtained in electrochemical test in FC operation conditions.

, conc. H2

SO<sup>4</sup>

, KMnO4

and HCl were obtained from Oltchim SA Romania. Carbon paper gas diffusion layer

) and organic compounds (tetracyano quinodimethane, tetrafluorotet-

. In addition, the physicochemical properties of

http://dx.doi.org/10.5772/intechopen.76495

83

, HI were purchased from Sigma-Aldrich.,

acid, HNO<sup>3</sup>

**2. Experimental**

Graphite powder, K2

S2 O8 , P2 O5

**2.1. Materials**

H2 O2 and H2

SO<sup>4</sup>

phene layers and the high molecular size of I2

The main drawbacks of the mentioned catalysts are the preparation methods with multiple operating activities as well as the sophisticated equipment, making the processes less attractive to be transposed on a larger production scale. Other disadvantages are reaction conditions involving high temperatures for thermal decomposition, high vacuum or supercritical conditions.

Calculations using functional density theory (DFT) showed that the electrocatalytic activity of the heteroatom doped graphene is strongly dependent on the electronic spin density and the distribution of the electric charge density on the atoms. Catalytic active sites of doped graphene are typically high density spinning carbon atoms. N, P or B doped graphene introduces unpaired electrons and determines a local high density resulting in a high electrocatalytic ORR performance. Halogens are other important elements that offer new properties for alternative energy devices and technologies due to the effect of the difference in electronegativity between halogen atoms (x = 2.66–3.98) and C atoms (x = 2.55). They have a different electron loss capacity compared to O2 − (x = 3.44). It is important to note that iodine halide may form partially ionized bonds to promote the transfer of the burden due to its large atomic size (the largest in the halogen group) [31–35]. In addition, it is known that the sides of the halogenated graphite have sufficient possibilities to attract O<sup>2</sup> and to weaken the O─O bond from the adsorbed oxygen, thus facilitating efficient conversion into water after reduction and protonation.

Previous studies have shown that the single layered basal structure of graphene can guarantee its electrochemical durability. In fact, the carbon corrosion starts from graphite defects, and carbonaceous materials with several graphite layers usually exhibit fewer structural defects. Therefore, the intrinsic grafting capacity of the graphene could improve the durability of graphene composite materials. More efforts need to be devoted to scalable and reproducible synthesis, with a compositional and morphological control, as well as investigations into properties and catalytic mechanism. In this area, major efforts have been made to refine these properties, including the adsorption of halogen molecules on the surface of the monolayer graphene as a promising approach due to the diversity of halogen properties and the variety of formed structures. The adsorption of atoms and halogen molecules on the graphene layer has been studied both theoretically and experimentally to adjust the electronic structure of the graphene layer. Theoretically, adsorption of diatomic halogen molecules on graphene using functional Van der Waals has been studied, which includes nonlocal correlation effects, perfect for geometric optimization. The adsorption of halogen atoms on graphenes was studied using only a semi local function. It is accepted that an iodine atom can accept 0.5 electrons from the carbon substrate. The doping of the graphene through physical adsorption is particularly promising because it can increase the concentration of the carriers without affecting carrier mobility as in the case of chemical adsorbed dopants, where the covalent function can produce crystalline defects and irreversibly alter the electron structure. One of the most promising dopants that could be physically adsorbed on the graphite is iodine. Iodine is also considered to be a stable and practical dopant as compared to other halogen based dopants (Cl, Br and F) and compared to many other physically adsorbed dopants such as alkali metal dopants (K, Li, Na, etc.), acids (hydrochloric acid, HNO<sup>3</sup> and H2 SO<sup>4</sup> ) and organic compounds (tetracyano quinodimethane, tetrafluorotetracyanoquinodimethane), poly (4-vinylpyridine) and polyethyleneimine). However, the intercalation of iodine in the engraved Bernal multilayer graphite, which is required for potential electrode applications, was considered unlikely due to the strong interaction between the graphene layers and the high molecular size of I2 . In addition, the physicochemical properties of iodine-doped graphite, such as its chemical state, thermal stability, and working function have not been extensively investigated, even if they are critical parameters for successfully achieving graphite-based electrodes in industrial applications. It has been shown that the overlaid monolayer graphite and the double layer of graphite foil can be efficiently doped with iodine.

The request of a chemically stable material and efficient ORR electrocatalyst, directed us to develop a new concept of electrode as alternative cathode catalyst/microporous layer. The recent new application of iodine-doped (I-doped) graphene as electrode in PEMFC has been recently recognized as an improved strategy for effective modification of cathode side efficiency [32–35]. The performed experimental studies revealed that the microporous layer (MPL) placed between catalyst layer (CL) and GDL have many advantages, like: keeping the hydration of the membrane and of the ionomer phase, preventing gas diffusion layer (GDL) flooding, especially at high current densities, forming a more intimate contact between CL and GDL.

The main objective of this work is to improve the ORR performance by including of the nanostructured I-doped graphene and to prove the efficiency of the developed cathode in PEMFC operation conditions. It is important to mention, that only few papers have been reported in respect to the cathode electrocatalyst for PEMFC containing I-doped graphene. In view of these facts, the purpose of this work is to provide valuable information about the recommendation of I-doped graphene as innovative ORR electrode in the PEMFC cathode, based on performances obtained in electrochemical test in FC operation conditions.
