Preface

The ever-increasing global population makes an increasing demand on energy. Fossil fuels are at the forefront of energy sources that we still need. However, due to the limited resour‐ ces of these fuels and global climate change, the availability of alternative sources and ener‐ gy carriers has gained importance. For this reason, studies have been accelerated to obtain blue energy (water) and green energy (wind, solar, etc.) from new technologies to meet the increasing energy demand of human society. Among energy sources, hydrogen gas is clean and renewable and has the potential to solve the growing energy crisis in today's society because of its high-energy density and noncarbon fuel properties. It is also used for many potential applications in nonpolluting vehicles, fuel cells, home heating systems, and air‐ craft. In addition, using hydrogen as an energy carrier is a long-term option to reduce car‐ bon dioxide emissions worldwide by obtaining high-value hydrocarbons through the hydrogenation of carbon dioxide. Water is considered as an ideal hydrogen source because it is clean, abundant, and renewable. Hydrogen production from water is achieved by water splitting using several methods such as thermal decomposition, electrolysis, photolysis, and photoelectrochemical methods.

This book presents the recent progresses and developments in water-splitting processes as well as other hydrogen generation technologies with challenges and future perspectives from the point of energy sustainability. In the first two chapters, the electrolysis of water, which attracts great attention due to its sustainability and renewability, is presented with the theoretical foundations of the operating principles of different types of electrolyzers. The most important technologies, such as alkaline electrolysis, proton exchange membrane elec‐ trolysis, and solid oxide high-temperature electrolysis, are addressed. In Chapter 3, the re‐ cent technological progress in light hydrocarbons regarding the sustainable hydrogen production is highligted, while the following two chapters evaluate the hydrogen produc‐ tion from ethanol by steam and use of hydrogen in a proton exchange membrane fuel cell. In the last chapter, a detailed performance model and optimization strategy is proposed for standalone operation of a concentrated photovoltaic system, with hydrogen production as an energy storage.

> **Asst. Prof. Murat Eyvaz** Co-Head of Department Department of Environmental Engineering Deputy Director Science and Technology Application and Research Center Gebze Technical University

Turkey

**Chapter 1**

Provisional chapter

**Hydrogen Generation by Water Electrolysis**

Hydrogen is a promising energy vector for the future. Among the different methods of its production, the electrolysis of water has attracted great attention because it is a sustainable and renewable chemical technology. Thus, hydrogen represents a suitable energy vector for the storage of intermittent energies. This chapter is devoted to the hydrogen generation by water electrolysis as an important part of both existing and emerging industrial electrochemical processes. It aims to give an insight into the theoretical foundations of the operating principles of different types of electrolyzers. Also, it is developed in this chapter, the thermodynamic and kinetic aspects of the reactions taking place at the electrodes of water electrolysis. The evolution reaction of hydrogen has a rapid kinetics, and thus, the polarization of the cathode is not critical. On the other hand, the evolution reaction of oxygen is characterized by a very slow kinetics and is thus responsible for most of the overvoltage in the electrolysis of water. The most important technologies of water electrolysis are addressed: alkaline electrolysis, proton exchange membrane electrolysis,

DOI: 10.5772/intechopen.76814

Keywords: hydrogen, oxygen, water electrolysis, catalyst, electrolyte, alkaline, polymer,

The United Nations (UN) published The Sustainable Development Program in 2015, which is an action plan for humanity, the planet, and prosperity. A total of 17 sustainable development goals and 169 targets are announced, which will stimulate action over the next 15 years in areas of critical importance for humanity and the planet (People, Planet, Prosperity, Peace, Partnership). The seventh goal is to ensure access to affordable, reliable, sustainable, and modern energy for all [1]. The hydrogen economy is seen as an instrument for the transformation of the energy system [2]. Hydrogen is the fuel most often used in fuel cells. It can have

> © 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.

Hydrogen Generation by Water Electrolysis

Youssef Naimi and Amal Antar

Youssef Naimi and Amal Antar

http://dx.doi.org/10.5772/intechopen.76814

Abstract

solid oxide

1. Introduction

Additional information is available at the end of the chapter

and solid oxide high-temperature electrolysis.

Additional information is available at the end of the chapter

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

#### **Hydrogen Generation by Water Electrolysis** Hydrogen Generation by Water Electrolysis

DOI: 10.5772/intechopen.76814

#### Youssef Naimi and Amal Antar Youssef Naimi and Amal Antar

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.76814

#### Abstract

Hydrogen is a promising energy vector for the future. Among the different methods of its production, the electrolysis of water has attracted great attention because it is a sustainable and renewable chemical technology. Thus, hydrogen represents a suitable energy vector for the storage of intermittent energies. This chapter is devoted to the hydrogen generation by water electrolysis as an important part of both existing and emerging industrial electrochemical processes. It aims to give an insight into the theoretical foundations of the operating principles of different types of electrolyzers. Also, it is developed in this chapter, the thermodynamic and kinetic aspects of the reactions taking place at the electrodes of water electrolysis. The evolution reaction of hydrogen has a rapid kinetics, and thus, the polarization of the cathode is not critical. On the other hand, the evolution reaction of oxygen is characterized by a very slow kinetics and is thus responsible for most of the overvoltage in the electrolysis of water. The most important technologies of water electrolysis are addressed: alkaline electrolysis, proton exchange membrane electrolysis, and solid oxide high-temperature electrolysis.

Keywords: hydrogen, oxygen, water electrolysis, catalyst, electrolyte, alkaline, polymer, solid oxide

### 1. Introduction

The United Nations (UN) published The Sustainable Development Program in 2015, which is an action plan for humanity, the planet, and prosperity. A total of 17 sustainable development goals and 169 targets are announced, which will stimulate action over the next 15 years in areas of critical importance for humanity and the planet (People, Planet, Prosperity, Peace, Partnership). The seventh goal is to ensure access to affordable, reliable, sustainable, and modern energy for all [1]. The hydrogen economy is seen as an instrument for the transformation of the energy system [2]. Hydrogen is the fuel most often used in fuel cells. It can have

© 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.

several provenances (electrolysis of water, cracking or reforming of petroleum products), with obvious implications on its degree of purity and consequently on the choice of catalyst, electrolyte, and operating conditions. The production of hydrogen by the electrolysis technique is very interesting because it can use a non-greenhouse gas energy source (renewable or nuclear energy). In addition, it remains the basic technique for providing applications that require small volumes of high purity hydrogen, including the semiconductor and food industry.

2. Principle of water electrolysis

positive terminal of the DC source.

<sup>H</sup>2<sup>O</sup> ! <sup>1</sup> 2

<sup>2</sup>OH� ! <sup>1</sup>

The global reaction for the two cases is:

H2O ! H<sup>2</sup> þ

reaction can be written as:

3. Thermodynamic

2

1 2

ing in a chemical reaction (aA þ bB þ … ! mM þ nN þ …):

Water electrolysis is the process whereby water is split into hydrogen and oxygen through the application of electrical energy, as in Eq. (6). Typically, a water electrolysis unit consists of an anode, a cathode separated with an electrolyte, and a power supply. The electrolyte can be made of an aqueous solution containing ions, a proton exchange membrane (PEM) or an oxygen ion exchange ceramic membrane. A direct current (DC) is applied from the negative terminal of the DC source to the cathode (seat of the reduction reaction), where the hydrogen is produced. At the anode, the electrons produced by the electrochemical reaction return to the

For the case of water electrolysis in an acid aqueous electrolyte, the processes that occur at the

� E<sup>0</sup>

� E<sup>0</sup>

Electrolysis of water is not a spontaneous phenomenon because the standard global reaction potential is negative. Therefore, it needs an external intervention (power source) and the global

H2O þ electricity ! H<sup>2</sup> þ

The equation of the German chemist Walther Nernst can be obtained from thermodynamics. The variation of Gibbs free energy is a function of the concentrations of the species participat-

<sup>Δ</sup><sup>G</sup> <sup>¼</sup> <sup>Δ</sup>G<sup>0</sup> <sup>þ</sup> RT∙ln am

<sup>H</sup>2O=O<sup>2</sup>

<sup>H</sup><sup>þ</sup> =<sup>H</sup><sup>2</sup>

<sup>H</sup>2<sup>O</sup>=<sup>H</sup><sup>2</sup>

OH� =<sup>O</sup><sup>2</sup>

> � <sup>E</sup><sup>0</sup> OH� =<sup>O</sup><sup>2</sup>

1 2

M∙an <sup>N</sup>∙…

!

aa A∙ab <sup>B</sup>∙…

<sup>H</sup>2<sup>O</sup>=<sup>H</sup><sup>2</sup>

¼ 1:23V=ENH (1)

Hydrogen Generation by Water Electrolysis http://dx.doi.org/10.5772/intechopen.76814 3

¼ 0:00V=ENH (2)

¼ �0:83V=ENH (3)

¼ 0:4 V=ENH (4)

¼ �1:23 V (5)

(7)

O<sup>2</sup> (6)

anode and the cathode are described, respectively, by Eqs. 1 and 2:

O<sup>2</sup> þ 2H<sup>þ</sup> þ 2e

<sup>2</sup>H<sup>þ</sup> <sup>þ</sup> <sup>2</sup>e� ! <sup>H</sup><sup>2</sup> <sup>E</sup><sup>0</sup>

<sup>2</sup>H2<sup>O</sup> <sup>þ</sup> <sup>2</sup>e� ! <sup>H</sup><sup>2</sup> <sup>þ</sup> <sup>2</sup>OH� <sup>E</sup><sup>0</sup>

O<sup>2</sup> þ H2O þ 2e

The half reactions occurring on the cathode and anode, respectively, can be written as:

<sup>O</sup><sup>2</sup> <sup>Δ</sup>E<sup>0</sup> <sup>¼</sup> <sup>E</sup><sup>0</sup>

Hydrogen is the lightest chemical element. Its molecules contain two hydrogen atoms. If this gas does not exist in its natural state, it is found in many molecules: water, sugar, proteins, hydrocarbons, and so on.

Hydrogen is a very light gas, colorless, odorless, and extremely flammable and reacts very easily in the presence of other chemicals. The properties of this gas are summarized in Table 1 [3].

The advantages of using hydrogen as a fuel in fuel cells are:


Its low density under normal conditions, the difficulty of storage, and the risk of explosion can summarize the major drawbacks of the use of pure hydrogen in fuel cells.

Notwithstanding the increasing interest in hydrogen as an energy carrier, its main uses continue to be in petroleum refining, ammonia production, metal refining, and electronics fabrication, with an average worldwide consumption of about 40 million tons [4–8]. This large-scale hydrogen consumption consequently requires large-scale hydrogen production. Presently, the technologies that dominate hydrogen production include reforming of natural gas [9], gasification of coal and petroleum coke [10–12], as well as gasification and reforming of heavy oil [13, 14]. Although water electrolysis has been known for around 200 years [15, 16], it still contributes only a minor fraction of the total hydrogen production (4% of the worldwide hydrogen production) [17, 18]. When compared to other available methods, water electrolysis has the advantage of producing extremely pure hydrogen (>99.9%), ideal for some high valueadded processes such as the manufacture of electronic components [4].


Table 1. Characteristics of hydrogen [3].

### 2. Principle of water electrolysis

several provenances (electrolysis of water, cracking or reforming of petroleum products), with obvious implications on its degree of purity and consequently on the choice of catalyst, electrolyte, and operating conditions. The production of hydrogen by the electrolysis technique is very interesting because it can use a non-greenhouse gas energy source (renewable or nuclear energy). In addition, it remains the basic technique for providing applications that require small volumes

Hydrogen is the lightest chemical element. Its molecules contain two hydrogen atoms. If this gas does not exist in its natural state, it is found in many molecules: water, sugar, proteins,

Hydrogen is a very light gas, colorless, odorless, and extremely flammable and reacts very easily in the presence of other chemicals. The properties of this gas are summarized in Table 1 [3].

Its low density under normal conditions, the difficulty of storage, and the risk of explosion can

Notwithstanding the increasing interest in hydrogen as an energy carrier, its main uses continue to be in petroleum refining, ammonia production, metal refining, and electronics fabrication, with an average worldwide consumption of about 40 million tons [4–8]. This large-scale hydrogen consumption consequently requires large-scale hydrogen production. Presently, the technologies that dominate hydrogen production include reforming of natural gas [9], gasification of coal and petroleum coke [10–12], as well as gasification and reforming of heavy oil [13, 14]. Although water electrolysis has been known for around 200 years [15, 16], it still contributes only a minor fraction of the total hydrogen production (4% of the worldwide hydrogen production) [17, 18]. When compared to other available methods, water electrolysis has the advantage of producing extremely pure hydrogen (>99.9%), ideal for some high value-

Molecular weight 2016 g/mol Melting point 259C Boiling point (1.013 bar) 252.8C Evaporation heat (1.013 bar at boiling point) 454.3 kJ/mol Density in the gas phase (1.013 and at 21C) 0.0696 kg/m<sup>3</sup> Solubility in water (1.013 bar and 0C) 0.0214 vol/vol

of high purity hydrogen, including the semiconductor and food industry.

The advantages of using hydrogen as a fuel in fuel cells are:

• unlimited availability (as long as you can break down the water),

summarize the major drawbacks of the use of pure hydrogen in fuel cells.

added processes such as the manufacture of electronic components [4].

• its harmless combustion product (H2O) for the environment.

hydrocarbons, and so on.

2 Advances In Hydrogen Generation Technologies

• its high electrochemical reactivity, • its high theoretical energy density,

Table 1. Characteristics of hydrogen [3].

Water electrolysis is the process whereby water is split into hydrogen and oxygen through the application of electrical energy, as in Eq. (6). Typically, a water electrolysis unit consists of an anode, a cathode separated with an electrolyte, and a power supply. The electrolyte can be made of an aqueous solution containing ions, a proton exchange membrane (PEM) or an oxygen ion exchange ceramic membrane. A direct current (DC) is applied from the negative terminal of the DC source to the cathode (seat of the reduction reaction), where the hydrogen is produced. At the anode, the electrons produced by the electrochemical reaction return to the positive terminal of the DC source.

For the case of water electrolysis in an acid aqueous electrolyte, the processes that occur at the anode and the cathode are described, respectively, by Eqs. 1 and 2:

$$\text{H}\_2\text{O} \rightarrow \frac{1}{2}\text{O}\_2 + 2\text{H}^+ + 2\text{e}^- \qquad\qquad \text{E}^0\_{\text{H}\_2\text{O}/\_{\text{O}\_2}} = 1.23\text{V/ENH} \tag{1}$$

$$2H^{+} + 2e^{-} \rightarrow H\_{2} \qquad\qquad\qquad \qquad \begin{aligned} &E\_{H^{+}}^{0}{}\_{/H\_{2}} = 0.00V/\text{ENH} \end{aligned} \tag{2}$$

The half reactions occurring on the cathode and anode, respectively, can be written as:

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{OH}^-\qquad\qquad\qquad \qquad \qquad E\_{\text{H}\_2\text{O}}^0 \!\!/\_{\text{H}\_2} = -0.83\text{V}/\text{ENH}\tag{3}$$

$$2\text{OH}^- \rightarrow \frac{1}{2}\text{O}\_2 + \text{H}\_2\text{O} + 2\text{e}^- \qquad\qquad\qquad \qquad \qquad \qquad \qquad \qquad \text{E}\_{\text{OH}^-}^0 \text{ }\_{\text{O}\_2} = 0.4\text{ V/ENH} \tag{4}$$

The global reaction for the two cases is:

$$\mathrm{H\_2O} \rightarrow \mathrm{H\_2} + \frac{1}{2} \mathrm{O\_2} \qquad \qquad \Delta \mathrm{E^0} = \mathrm{E\_{H\_2O}^0}\_{\mathrm{H\_2O}} \, - \, \mathrm{E\_{OH^-}^0}\_{\mathrm{O\_2}} \, = -1.23 \,\mathrm{V} \tag{5}$$

Electrolysis of water is not a spontaneous phenomenon because the standard global reaction potential is negative. Therefore, it needs an external intervention (power source) and the global reaction can be written as:

$$\text{H}\_2\text{O} + \text{electricity} \rightarrow \text{H}\_2 + \frac{1}{2}\text{O}\_2\tag{6}$$

#### 3. Thermodynamic

The equation of the German chemist Walther Nernst can be obtained from thermodynamics. The variation of Gibbs free energy is a function of the concentrations of the species participating in a chemical reaction (aA þ bB þ … ! mM þ nN þ …):

$$
\Delta G = \Delta G^0 + RT \cdot \ln \left( \frac{a\_M^m \cdot a\_N^n \cdot \dots}{a\_A^a \cdot a\_B^b \cdot \dots} \right) \tag{7}
$$

where aa A, ab <sup>B</sup>, …, am M, an <sup>N</sup>, …, are the activities of the species.

Some species involved in the electrode reaction are solids or pure liquids. For these pure substances, the activity is constant and its value is considered unitary. The activity of the gases is usually taken to be the partial pressure of the gases expressed in the atmosphere, and the activity of the ions in dilute solution is generally considered to be their molar concentration. By substituting in Eq. (7) the reactions, and dividing each member of the equation by -nF, we obtain the Nernst equation. Nernst's equation expresses the relationship between the potential of an electrochemical cell and the concentrations of its constituents at equilibrium. In the specific case of an electrochemical cell, it is written:

$$
\Delta E\_{\text{myle}} = \left( E\_{\text{Caathode}}^0 - E\_{\text{Anode}}^0 \right) - \frac{RT}{nF} \ln \left( \frac{a\_M^m \cdot a\_N^u \dots}{a\_A^a \cdot a\_B^b \dots} \right) \tag{8}
$$

#### 3.1. Faraday's law

In 1832, Michael Faraday stated his two laws of electrolysis:


The quantity of material (m) produced is:

$$\mathbf{m} = \mathbf{k} \cdot \int\_0^t \mathbf{I} \cdot \mathbf{d}t \tag{9}$$

ΔE ¼ ΔEthe þ η<sup>a</sup> þ η<sup>c</sup> þ R∙I (11)

with: η<sup>a</sup> ð Þ V is the anodic overvoltage; η<sup>c</sup> ð Þ V is the cathodic overvoltage; R (Ω) is the global

Figure 1 shows the relationship between the electrolyzer cell potential and operating temperature [19–22]. The cell potential-temperature plane is divided into three zones by the so-called equilibrium voltage line and thermo-neutral voltage line. The equilibrium voltage is the theoretical minimum potential required to dissociate water by electrolysis, below which the electrolysis of water cannot proceed. The equilibrium voltage decreases with increasing temperature. The thermo-neutral voltage is the actual minimum voltage that has to be applied to the electrolysis cell, below which the electrolysis is endothermic and above which, exothermic. The thermoneutral voltage naturally includes the overpotentials of the electrodes, which are only weakly dependent on temperature. Thus, the thermo-neutral voltage only exhibits a slight increase with temperature. If water electrolysis takes place in the shaded area in Figure 4, the reaction will be

As these electrochemical reactions are heterogeneous surface processes, it is convenient to

<sup>A</sup> ∙ dm dt .

<sup>A</sup>∙dt <sup>¼</sup> <sup>I</sup>

<sup>A</sup>∙n∙F. Therefore, the expression for

Hydrogen Generation by Water Electrolysis http://dx.doi.org/10.5772/intechopen.76814 5

O þ ne� ⇄ R (12)

resistance and I (A) is the current.

endothermic.

3.3. Electrodes polarizations

For a general electrochemical reaction:

current density, <sup>j</sup> <sup>¼</sup> <sup>I</sup>

relate the reaction rate to the electrode area, A, as dm

<sup>A</sup>, may be rewritten as<sup>j</sup> <sup>¼</sup> nF

Figure 1. Cell potential for hydrogen production by water electrolysis as a function of temperature [19].

where k is a proportionality constant and I is the instantaneous current flowing through the cell. In a cell in which a continuous current circulates, the majority of this current is connected to chemical reactions (faradic current) and a small part, often negligible can be used for other purposes (non-Faradic current). Thus, the amount of material that forms or disappears at the electrodes is proportional to the intensity of the current and the duration of the electrolysis t. Knowing the number of moles is ð Þ m=M , which corresponds to a quantity of electricity <sup>Q</sup> <sup>¼</sup> <sup>m</sup> <sup>M</sup> <sup>∙</sup>N∙<sup>F</sup> <sup>¼</sup> <sup>I</sup>∙<sup>t</sup> � �. Hence, Faraday's law:

$$m = \frac{I \cdot t \cdot M}{nF} \tag{10}$$

With: m is the mass of substances formed (g); M is the molar mass of substances formed (g/mol.); n is number of exchanged electrons, I is the current in amperes (A), Q ¼ I∙t is the quantity of electricity in coulomb (C), t is the time (s).

#### 3.2. Cell voltage (difference of potential)

The potential difference for a cell of an electrolyzer, which is always ΔE ¼ 1:8˜2:0 V at the current density of <sup>j</sup> <sup>¼</sup> <sup>1000</sup> � <sup>300</sup> Am<sup>2</sup> in industry water electrolysis [17], is an addition of four terms:

$$
\Delta E = \Delta \mathbf{E}\_{\text{the}} + \mathfrak{n}\_{\text{a}} + \mathfrak{n}\_{\text{c}} + \mathbf{R} \cdot \mathbf{I} \tag{11}
$$

with: η<sup>a</sup> ð Þ V is the anodic overvoltage; η<sup>c</sup> ð Þ V is the cathodic overvoltage; R (Ω) is the global resistance and I (A) is the current.

Figure 1 shows the relationship between the electrolyzer cell potential and operating temperature [19–22]. The cell potential-temperature plane is divided into three zones by the so-called equilibrium voltage line and thermo-neutral voltage line. The equilibrium voltage is the theoretical minimum potential required to dissociate water by electrolysis, below which the electrolysis of water cannot proceed. The equilibrium voltage decreases with increasing temperature. The thermo-neutral voltage is the actual minimum voltage that has to be applied to the electrolysis cell, below which the electrolysis is endothermic and above which, exothermic. The thermoneutral voltage naturally includes the overpotentials of the electrodes, which are only weakly dependent on temperature. Thus, the thermo-neutral voltage only exhibits a slight increase with temperature. If water electrolysis takes place in the shaded area in Figure 4, the reaction will be endothermic.

#### 3.3. Electrodes polarizations

where aa

A, ab

3.1. Faraday's law

<sup>Q</sup> <sup>¼</sup> <sup>m</sup>

<sup>B</sup>, …, am M, an

4 Advances In Hydrogen Generation Technologies

specific case of an electrochemical cell, it is written:

The quantity of material (m) produced is:

<sup>M</sup> <sup>∙</sup>N∙<sup>F</sup> <sup>¼</sup> <sup>I</sup>∙<sup>t</sup> � �. Hence, Faraday's law:

3.2. Cell voltage (difference of potential)

quantity of electricity in coulomb (C), t is the time (s).

<sup>Δ</sup>Enpile <sup>¼</sup> <sup>E</sup><sup>0</sup>

In 1832, Michael Faraday stated his two laws of electrolysis:

<sup>N</sup>, …, are the activities of the species.

Cathode � <sup>E</sup><sup>0</sup>

tional to the quantity of electricity that passes through the electrolyte.

ity are proportional to the equivalent weight of each substance.

Some species involved in the electrode reaction are solids or pure liquids. For these pure substances, the activity is constant and its value is considered unitary. The activity of the gases is usually taken to be the partial pressure of the gases expressed in the atmosphere, and the activity of the ions in dilute solution is generally considered to be their molar concentration. By substituting in Eq. (7) the reactions, and dividing each member of the equation by -nF, we obtain the Nernst equation. Nernst's equation expresses the relationship between the potential of an electrochemical cell and the concentrations of its constituents at equilibrium. In the

> Anode � � � RT

1. The weights of substances formed at an electrode during electrolysis are directly propor-

2. The weights of different substances formed by the passage of the same quantity of electric-

ðt 0

where k is a proportionality constant and I is the instantaneous current flowing through the cell. In a cell in which a continuous current circulates, the majority of this current is connected to chemical reactions (faradic current) and a small part, often negligible can be used for other purposes (non-Faradic current). Thus, the amount of material that forms or disappears at the electrodes is proportional to the intensity of the current and the duration of the electrolysis t. Knowing the number of moles is ð Þ m=M , which corresponds to a quantity of electricity

<sup>m</sup> <sup>¼</sup> <sup>I</sup>∙t∙<sup>M</sup>

With: m is the mass of substances formed (g); M is the molar mass of substances formed (g/mol.); n is number of exchanged electrons, I is the current in amperes (A), Q ¼ I∙t is the

The potential difference for a cell of an electrolyzer, which is always ΔE ¼ 1:8˜2:0 V at the current density of <sup>j</sup> <sup>¼</sup> <sup>1000</sup> � <sup>300</sup> Am<sup>2</sup> in industry water electrolysis [17], is an addition of four terms:

m ¼ k∙

nF ln <sup>a</sup><sup>m</sup>

M:an <sup>N</sup>…

!

I∙dt (9)

nF (10)

(8)

aa A:ab <sup>B</sup>…

> As these electrochemical reactions are heterogeneous surface processes, it is convenient to relate the reaction rate to the electrode area, A, as dm <sup>A</sup>∙dt <sup>¼</sup> <sup>I</sup> <sup>A</sup>∙n∙F. Therefore, the expression for current density, <sup>j</sup> <sup>¼</sup> <sup>I</sup> <sup>A</sup>, may be rewritten as<sup>j</sup> <sup>¼</sup> nF <sup>A</sup> ∙ dm dt .

For a general electrochemical reaction:

$$O + ne^- \rightleftharpoons R\tag{12}$$

Figure 1. Cell potential for hydrogen production by water electrolysis as a function of temperature [19].

#### 6 Advances In Hydrogen Generation Technologies

Under nonequilibrium potential conditions, the equation that best describes the current density versus potential is the Butler-Volmer expression:

$$\overrightarrow{j}\_{a} = \overrightarrow{j}\_{a} + \overrightarrow{j}\_{c} = nF\overrightarrow{k}\_{0}\mathsf{C}\_{R}(0, t)e^{\left(\frac{a\_{a}nI\left(\frac{\varepsilon}{kT}\right)}{kT}\right)} - nF\overrightarrow{k}\_{0}\mathsf{C}\_{O}(0, t)e^{\left(\frac{a\_{a}nI\left(\frac{\varepsilon}{kT}\right)}{kT}\right)}\tag{13}$$

where j a ! and j c are, respectively, the anodic and cathodic current density; k<sup>0</sup> and k<sup>0</sup> ! are, respectively, the rates constants of the anodic and cathodic reaction; α<sup>a</sup> and α<sup>c</sup> are, respectively, the anodic and cathodic exchanges coefficients; E<sup>0</sup> is the standard potential.

Under the control of electron transfer rate, (the concentration of the electrodes' surface is equal to the concentration in the bulk), this equation can be expressed as current density versus overpotential (η ¼ E � Éeq):

$$j = j\_0 j \left( e^{\left(\frac{a\_d n \mathbb{F} \eta}{kT}\right)} - e^{\left(\frac{a\_c n \mathbb{F} \eta}{kT}\right)} \right) \tag{14}$$

(ΔHads,Pd,298ð Þ¼ H<sup>2</sup> 83:kJ mol:

the rate-determining step.

Cappadonia et al. [29]:

and Pt at the apex of the volcano curve [27].

mol:

�1); for Ni, the heat of adsorption is <sup>Δ</sup>Hads,Ni, <sup>298</sup>ð Þ¼ <sup>H</sup><sup>2</sup> <sup>105</sup> kJ

Hydrogen Generation by Water Electrolysis http://dx.doi.org/10.5772/intechopen.76814 7

�<sup>1</sup> [25]. Electrode properties, type and concentration of the electrolyte, and temperature are parameters that also influence hydrogen formation. If hydrogen adsorption is the ratedetermining step, electrode materials with more edges and cavities in their surface structure will favor electron transfer and create more centers for hydrogen adsorption. If hydrogen desorption is the rate-determining step, physical properties such as surface roughness or perforation will prevent bubbles from growing and increase electron transfer by adding reaction area, consequently increasing the rate of electrolysis [26]. When the overpotential is low, electron transfer is not as fast as desorption and hydrogen adsorption will be the ratedetermining step. In contrast, when the potential is high enough, hydrogen desorption will be

The hydrogen adsorption energy is a good parameter to identify the most promising materials for the HER. If the activities for the HER of the coinage metals (IB metals: Au, Cu, Ag), the platinum group (Pt, Ir, Ru) metals and the valve metal (Ti) in 0.1 M HClO4, are plotted as a function of the M–Hads binding energy, a volcano relationship is established (Figure 2) with Ir

Further analysis of Figure 2 reveals that the IB group elements are positioned on the ascending slope of the volcano with the order activity increasing from Au < Cu < Ag. Figure 1 also shows that the elements that interact strongly with Hads (such as Ru and Ti) are positioned on the descending slope of the volcano, supporting previous suggestions that the M–Hads binding energy can be used as a descriptor for the HER. Not in passing, given that recent analysis has demonstrated that neither Ru nor Ti are bare metals in the HER region, it is suggested that, in fact, experimentally it is very difficult (impossible) to determine unambiguously solely based on the M–Hads energetics what would be the correct position of these two elements in the observed volcano relationship. This is most likely also true for the HER in alkaline solutions,

The HER exchange current of Pt in acid media is at least two orders of magnitude higher than that in alkaline electrolytes, including KOH. This is due to the shorter Pt � Hads distance in alkaline media, as suggested by theoretical estimates. It has been claimed that Ni(OH)2 nanoclusters on Pt surface enhance HER rates in 0.1 M KOH by one order of magnitude [26], although no theoretical explanation for this synergistic effect has been attempted. The long-term stability of Ni(OH)2 in

The mechanism of the oxygen evolution reaction (OER) is more complex than that suggested for HER. The most generally accepted mechanism for the OER is that described by

OH� ⇄ OHads þ e

OH� þ OHads ⇄ Oads þ H2O þ e

� (20)

Oads þ Oads ⇄ O<sup>2</sup> (22)

� (21)

when the rates of the reaction are much slower than in acidic environments [22].

the strongly reducing environment occurring at the cathode is also not discussed.

The anodic and cathodic exchanges coefficients (αa, and αc) are related (α<sup>a</sup> þ α<sup>c</sup> ¼ 1), and generally, α ≈ α<sup>a</sup> ≈ α<sup>c</sup> ≈ <sup>1</sup> <sup>2</sup>. For a given single-step reaction at a constant temperature, the j versus η characteristics will depend on j <sup>0</sup>, αa, and αc.

For large η values, the Butler-Volmer equations can be simplified to give the Tafel equation ð Þ j j η ¼ a∙ log j jj þ b :

$$\text{For } \eta \ll 0: \log\left(j\right) = -\log\left(j\_0\right) - \frac{\alpha\_\epsilon nF}{2.3RT} \cdot \eta \tag{15}$$

$$\text{For } \eta \gg 0: \log \left( j \right) = -\log \left( j\_0 \right) + \frac{\alpha\_d nF}{2.3RT} \cdot \eta \tag{16}$$

For the hydrogen evolution reaction (HER), the Volmer-Tafel and Volmer-Heyrovský mechanisms are often proposed and well known [16, 23, 24]. The first step (Eq. 17) involves the formation of adsorbed hydrogen, which is then followed by either chemical desorption (Eq. 18) or electrochemical desorption (Eq. 19), where Hads is an adsorbed hydrogen atom.

$$H\_2O + e^- \rightleftharpoons H\_{ads} + OH^- \qquad\qquad\text{Volmer step}\tag{17}$$

$$\text{2H}\_{\text{ads}} \rightleftharpoons \text{H}\_{\text{2}} \tag{18}$$

$$\rm H\_{ads} + H\_2O + e^- \rightleftharpoons H\_2 + OH^- \qquad \quad \text{Heyrovsky} \\ \rm step \tag{19}$$

For the hydrogen evolution reaction (HER), the overpotential, η<sup>H</sup><sup>2</sup> , is generally calculated by the Tafel equation. Hydrogen formation is intrinsically determined by the strength of the bond between hydrogen and the electrode surface. Pd has the lowest heat of adsorption of hydrogen (ΔHads,Pd,298ð Þ¼ H<sup>2</sup> 83:kJ mol: �1); for Ni, the heat of adsorption is <sup>Δ</sup>Hads,Ni, <sup>298</sup>ð Þ¼ <sup>H</sup><sup>2</sup> <sup>105</sup> kJ mol: �<sup>1</sup> [25]. Electrode properties, type and concentration of the electrolyte, and temperature are parameters that also influence hydrogen formation. If hydrogen adsorption is the ratedetermining step, electrode materials with more edges and cavities in their surface structure will favor electron transfer and create more centers for hydrogen adsorption. If hydrogen desorption is the rate-determining step, physical properties such as surface roughness or perforation will prevent bubbles from growing and increase electron transfer by adding reaction area, consequently increasing the rate of electrolysis [26]. When the overpotential is low, electron transfer is not as fast as desorption and hydrogen adsorption will be the ratedetermining step. In contrast, when the potential is high enough, hydrogen desorption will be the rate-determining step.

Under nonequilibrium potential conditions, the equation that best describes the current den-

<sup>α</sup>anF E�E<sup>0</sup> ð Þ RT 

respectively, the rates constants of the anodic and cathodic reaction; α<sup>a</sup> and α<sup>c</sup> are, respectively,

Under the control of electron transfer rate, (the concentration of the electrodes' surface is equal to the concentration in the bulk), this equation can be expressed as current density versus

<sup>0</sup>j e <sup>α</sup>anF<sup>η</sup> ð Þ RT � <sup>e</sup>

The anodic and cathodic exchanges coefficients (αa, and αc) are related (α<sup>a</sup> þ α<sup>c</sup> ¼ 1), and

For large η values, the Butler-Volmer equations can be simplified to give the Tafel equation

For the hydrogen evolution reaction (HER), the Volmer-Tafel and Volmer-Heyrovský mechanisms are often proposed and well known [16, 23, 24]. The first step (Eq. 17) involves the formation of adsorbed hydrogen, which is then followed by either chemical desorption (Eq. 18)

the Tafel equation. Hydrogen formation is intrinsically determined by the strength of the bond between hydrogen and the electrode surface. Pd has the lowest heat of adsorption of hydrogen

are, respectively, the anodic and cathodic current density; k<sup>0</sup>

� nFk<sup>0</sup> !

<sup>α</sup>cnF<sup>η</sup> ð Þ RT 

> 0 � <sup>α</sup>cnF

0 <sup>þ</sup> αanF

� ⇄ Hads þ OH� Volmer step (17)

� ⇄ H<sup>2</sup> þ OH� Heyrovsḱy step (19)

2Hads ⇄ H<sup>2</sup> Tafel step (18)

<sup>2</sup>. For a given single-step reaction at a constant temperature, the j versus

COð Þ 0; t e

<sup>α</sup>cnF E�E<sup>0</sup> ð Þ RT 

<sup>2</sup>:3RT <sup>∙</sup><sup>η</sup> (15)

<sup>2</sup>:3RT <sup>∙</sup><sup>η</sup> (16)

, is generally calculated by

and k<sup>0</sup> ! are,

(13)

(14)

sity versus potential is the Butler-Volmer expression:

¼ nFk<sup>0</sup> 

CRð Þ 0; t e

the anodic and cathodic exchanges coefficients; E<sup>0</sup> is the standard potential.

j ¼ j

<sup>0</sup>, αa, and αc.

For η ≪ 0 : log ðÞ¼� j log j

For η ≫ 0 : log ðÞ¼� j log j

or electrochemical desorption (Eq. 19), where Hads is an adsorbed hydrogen atom.

H2O þ e

Hads þ H2O þ e

For the hydrogen evolution reaction (HER), the overpotential, η<sup>H</sup><sup>2</sup>

j ¼ j a ! þ j c 

6 Advances In Hydrogen Generation Technologies

overpotential (η ¼ E � Éeq):

generally, α ≈ α<sup>a</sup> ≈ α<sup>c</sup> ≈ <sup>1</sup>

ð Þ j j η ¼ a∙ log j jj þ b :

η characteristics will depend on j

where j a ! and j c 

> The hydrogen adsorption energy is a good parameter to identify the most promising materials for the HER. If the activities for the HER of the coinage metals (IB metals: Au, Cu, Ag), the platinum group (Pt, Ir, Ru) metals and the valve metal (Ti) in 0.1 M HClO4, are plotted as a function of the M–Hads binding energy, a volcano relationship is established (Figure 2) with Ir and Pt at the apex of the volcano curve [27].

> Further analysis of Figure 2 reveals that the IB group elements are positioned on the ascending slope of the volcano with the order activity increasing from Au < Cu < Ag. Figure 1 also shows that the elements that interact strongly with Hads (such as Ru and Ti) are positioned on the descending slope of the volcano, supporting previous suggestions that the M–Hads binding energy can be used as a descriptor for the HER. Not in passing, given that recent analysis has demonstrated that neither Ru nor Ti are bare metals in the HER region, it is suggested that, in fact, experimentally it is very difficult (impossible) to determine unambiguously solely based on the M–Hads energetics what would be the correct position of these two elements in the observed volcano relationship. This is most likely also true for the HER in alkaline solutions, when the rates of the reaction are much slower than in acidic environments [22].

> The HER exchange current of Pt in acid media is at least two orders of magnitude higher than that in alkaline electrolytes, including KOH. This is due to the shorter Pt � Hads distance in alkaline media, as suggested by theoretical estimates. It has been claimed that Ni(OH)2 nanoclusters on Pt surface enhance HER rates in 0.1 M KOH by one order of magnitude [26], although no theoretical explanation for this synergistic effect has been attempted. The long-term stability of Ni(OH)2 in the strongly reducing environment occurring at the cathode is also not discussed.

> The mechanism of the oxygen evolution reaction (OER) is more complex than that suggested for HER. The most generally accepted mechanism for the OER is that described by Cappadonia et al. [29]:

$$\text{OH}^- \rightleftharpoons \text{OH}\_{\text{ads}} + e^- \tag{20}$$

$$2\text{ }OH^- + \text{ }OH\_{\text{ads}} \rightleftharpoons \text{O}\_{\text{ads}} + \text{H}\_2\text{O} + e^- \tag{21}$$

$$O\_{ads} + O\_{ads} \rightleftharpoons O\_2 \tag{22}$$

<sup>R</sup> <sup>¼</sup> <sup>X</sup> i

wires, connectors, and electrodes. This part of the resistance can be reduced by reducing the length of the wire, increasing the cross-section area and adopting more conductive wire

The ionic solution conductivity χ is a function of concentration and temperature. For an ionic solution containing ions (i), charged <sup>þ</sup>zi or �zi and at the concentration Ci in mol:∙m�3, the

The presence of bubbles in the electrolyte solution and on the electrode surfaces causes additional resistances to the ionic transfer and surface electrochemical reactions. One of the accepted

where κ is the specific conductivity of the gas-free electrolyte solution; f is the volume fraction

Convective mass transfer plays an important role in the ionic transfer, heat dissipation and distribution, and gas bubble behavior in the electrolyte. The viscosity and flow field of the electrolyte determines the mass (ionic) transfer, temperature distribution and bubble sizes, bubble detachment and rising velocity, and in turn influence the current and potential distributions in the electrolysis cell. As the water electrolysis progresses the concentration of the electrolyte increases, resulting in an increase in the viscosity. Water is usually continuously added to the system to maintain a constant electrolyte concentration and thus the viscosity.

The conductivity of the solution is enhanced by the use of strong electrolytes that deliver ions with high mobility [43], such as sodium, potassium for positive ions, and hydroxide or chlorides as negative ions. During electrolysis, the water molecules move to the cathode by diffusion as they are consumed, and the hydroxide ions move to the anode by migration because they have an opposite charge and diffusion because they are consumed. A diaphragm

m�<sup>1</sup> � �, is:

<sup>χ</sup> <sup>¼</sup> <sup>X</sup> i

theoretical equations to study the bubble effect in the electrolyte is given as follows [41]:

with: λ<sup>i</sup> is the equivalent conductivity of the ion (i) in, S∙m<sup>2</sup>∙mol:

where χ<sup>i</sup> Ω�<sup>1</sup>

conductivity of the solution, noted χ Ω�<sup>1</sup>

of gas in the solution [42].

3.5. Transport resistances

4. Alkaline electrolysis

material.

li Ai∙χ<sup>i</sup>

m�<sup>1</sup> � � is the electrical conductivity for each component of the circuit, including

λi∙zi∙Ci (24)

Hydrogen Generation by Water Electrolysis http://dx.doi.org/10.5772/intechopen.76814

�<sup>1</sup> .

κ<sup>g</sup> ¼ κð Þ 1 � 1:5∙f (25)

(23)

9

Figure 2. HER activity, overpotential (η) at 5 mA cm2, measured in 0.1 M HClO4 (pH = 1) as a function of calculated M–H binding energy for several metals (volcano plot) [20].

The mechanism is controlled by the charge transfer (step 20 or 21) at low temperatures. On the other hand, at high temperatures, the recombination step (Eq. 22) controls the reaction on Ni electrode [22–30].

Generally, acid solutions or PEMs are used as electrolytes in water electrolyzers because acidic media show high ionic conductivity and are free from carbonate formation, as compared with alkaline electrolytes. Consequently, noble metals are used as electrocatalysts for OER in acidic media. Ruthenium and iridium have shown strong activity for OER, but they were passivated at very high anode potentials [31–36]. Bifunctional electrocatalysts, which can work for both oxygen evolution and oxygen reduction, have also been proposed for water electrolysis. A typical bifunctional electrocatalyst is composed of a noble metal oxide such as IrO2. For unsupported bifunctional electrocatalysts, Pt-MOx (M = Ru, Ir, Na), bimetallic (e.g., Pt-Ir), and trimetallic (e.g., Pt4.5Ru4Ir0.5) materials have been developed [37–40].

At high current densities, are added to the polarization of the electrodes other resistances: ohmic losses in the electrolyte, resistances from bubbles, diaphragm, and ion transfer.

#### 3.4. Electrical resistance

The electrical resistance in a water electrolysis system has three main components: (1) the resistance in the system circuits; (2) the mass transport phenomena including ions transfer in the electrolyte; (3) the gas bubbles covering the electrode surfaces and the diaphragm [15].

The nature and the dimensions of the materials used in the electrodes and the connections and the electric circuit, the methods of their preparations are responsible for the electrical resistance of the system. It can be expressed as follows:

Hydrogen Generation by Water Electrolysis http://dx.doi.org/10.5772/intechopen.76814 9

$$R = \sum\_{i} \frac{l\_i}{A\_{i^\*} \chi\_i} \tag{23}$$

where χ<sup>i</sup> Ω�<sup>1</sup> m�<sup>1</sup> � � is the electrical conductivity for each component of the circuit, including wires, connectors, and electrodes. This part of the resistance can be reduced by reducing the length of the wire, increasing the cross-section area and adopting more conductive wire material.

The ionic solution conductivity χ is a function of concentration and temperature. For an ionic solution containing ions (i), charged <sup>þ</sup>zi or �zi and at the concentration Ci in mol:∙m�3, the conductivity of the solution, noted χ Ω�<sup>1</sup> m�<sup>1</sup> � �, is:

$$\chi = \sum\_{i} \lambda\_{i} \mathbf{z}\_{i} \cdot \mathbf{C}\_{i} \tag{24}$$

with: λ<sup>i</sup> is the equivalent conductivity of the ion (i) in, S∙m<sup>2</sup>∙mol: �<sup>1</sup> .

The presence of bubbles in the electrolyte solution and on the electrode surfaces causes additional resistances to the ionic transfer and surface electrochemical reactions. One of the accepted theoretical equations to study the bubble effect in the electrolyte is given as follows [41]:

$$
\kappa\_{\mathcal{S}} = \kappa (1 - 1.5\mathcal{f}) \tag{25}
$$

where κ is the specific conductivity of the gas-free electrolyte solution; f is the volume fraction of gas in the solution [42].

#### 3.5. Transport resistances

The mechanism is controlled by the charge transfer (step 20 or 21) at low temperatures. On the other hand, at high temperatures, the recombination step (Eq. 22) controls the reaction on Ni

Figure 2. HER activity, overpotential (η) at 5 mA cm2, measured in 0.1 M HClO4 (pH = 1) as a function of calculated

Generally, acid solutions or PEMs are used as electrolytes in water electrolyzers because acidic media show high ionic conductivity and are free from carbonate formation, as compared with alkaline electrolytes. Consequently, noble metals are used as electrocatalysts for OER in acidic media. Ruthenium and iridium have shown strong activity for OER, but they were passivated at very high anode potentials [31–36]. Bifunctional electrocatalysts, which can work for both oxygen evolution and oxygen reduction, have also been proposed for water electrolysis. A typical bifunctional electrocatalyst is composed of a noble metal oxide such as IrO2. For unsupported bifunctional electrocatalysts, Pt-MOx (M = Ru, Ir, Na), bimetallic (e.g., Pt-Ir),

At high current densities, are added to the polarization of the electrodes other resistances:

The electrical resistance in a water electrolysis system has three main components: (1) the resistance in the system circuits; (2) the mass transport phenomena including ions transfer in the electrolyte; (3) the gas bubbles covering the electrode surfaces and the diaphragm [15].

The nature and the dimensions of the materials used in the electrodes and the connections and the electric circuit, the methods of their preparations are responsible for the electrical resistance

ohmic losses in the electrolyte, resistances from bubbles, diaphragm, and ion transfer.

and trimetallic (e.g., Pt4.5Ru4Ir0.5) materials have been developed [37–40].

electrode [22–30].

3.4. Electrical resistance

of the system. It can be expressed as follows:

M–H binding energy for several metals (volcano plot) [20].

8 Advances In Hydrogen Generation Technologies

Convective mass transfer plays an important role in the ionic transfer, heat dissipation and distribution, and gas bubble behavior in the electrolyte. The viscosity and flow field of the electrolyte determines the mass (ionic) transfer, temperature distribution and bubble sizes, bubble detachment and rising velocity, and in turn influence the current and potential distributions in the electrolysis cell. As the water electrolysis progresses the concentration of the electrolyte increases, resulting in an increase in the viscosity. Water is usually continuously added to the system to maintain a constant electrolyte concentration and thus the viscosity.

#### 4. Alkaline electrolysis

The conductivity of the solution is enhanced by the use of strong electrolytes that deliver ions with high mobility [43], such as sodium, potassium for positive ions, and hydroxide or chlorides as negative ions. During electrolysis, the water molecules move to the cathode by diffusion as they are consumed, and the hydroxide ions move to the anode by migration because they have an opposite charge and diffusion because they are consumed. A diaphragm separates the two anode and cathode compartments and the gases formed are thus collected: hydrogen at the cathode and oxygen at the anode as shown in Figure 3.

Concentrated solutions of potassium hydroxide are generally used as the electrolytic solution because they have very high conductivities and fewer corrosion problems compared with other alkaline electrolytes. The electrode materials often used are based on nickel because of its low cost, high activity [44].

Electrolysis cells can be of two types of configurations: monopolar and bipolar [14]. Figure 4(a) gives a schematic of the monopolar configuration. The electrodes are altered in the electrolyzer and are all directly connected to the terminals of the DC power supply: the anodes at the positive terminal and the cathodes at the negative terminal. The total voltage applied to the entire electrolysis cell is essentially the same as that applied to the individual pairs of the electrodes in the cell (Utot ¼ Ui), but the current is subdivided between the different unit cells (Itot ¼ n � IiÞ. Figure 4(b) depicts conflation in bipolar mode. Only the two end electrodes are connected directly to the DC power source. The other inner electrodes have a dual role: one side acts as the cathode for a unit cell and the other side acts as the anode for the adjacent unit cell. These cells are electrically linked thanks to their electrodes which are bipolar and ionically via the electrolytic solution. The total voltage of the cell is the sum of the individual voltages of

the unit cells (Utot ¼ n � Ui), but the current for each unit cell is equal to the output current of the generator (Itot ¼ IiÞ. The electrical energy consumed is the same in the two configurations. The wide range of flammability limits of the mixture of hydrogen and oxygen requires a careful design of the electrolyzer system. The separator diaphragm (or membrane) must avoid the mixing of the two gases inside the cell. Furthermore, the corrosive nature of the electrolyte does not allow leaks that are often likely to take place at the connections and seals of the electrolyzer. The bipolar configuration is more risky in mixing oxygen and hydrogen because of their simultaneous productions on the same bipolar electrode (on each side) and also

Hydrogen Generation by Water Electrolysis http://dx.doi.org/10.5772/intechopen.76814 11

Figure 4. Schematics of cell configurations of monopolar (a) and bipolar (b) electrolyzers [14].

Obviously, the life of the system is an important criterion. It is extremely linked to the quality of the materials used. Indeed, these materials must be resistant to high concentrations of the alkaline electrolyte and operating conditions of the electrolyzer (pressure and temperature). In particular, connections and seals are subject to corrosion, which is why it is recommended to

electrolyte leakage as the monopolar design.

use sealing materials that are also stable in this environment [14].

Figure 3. Principle of an alkaline water electrolysis.

Figure 4. Schematics of cell configurations of monopolar (a) and bipolar (b) electrolyzers [14].

separates the two anode and cathode compartments and the gases formed are thus collected:

Concentrated solutions of potassium hydroxide are generally used as the electrolytic solution because they have very high conductivities and fewer corrosion problems compared with other alkaline electrolytes. The electrode materials often used are based on nickel because of

Electrolysis cells can be of two types of configurations: monopolar and bipolar [14]. Figure 4(a) gives a schematic of the monopolar configuration. The electrodes are altered in the electrolyzer and are all directly connected to the terminals of the DC power supply: the anodes at the positive terminal and the cathodes at the negative terminal. The total voltage applied to the entire electrolysis cell is essentially the same as that applied to the individual pairs of the electrodes in the cell (Utot ¼ Ui), but the current is subdivided between the different unit cells (Itot ¼ n � IiÞ. Figure 4(b) depicts conflation in bipolar mode. Only the two end electrodes are connected directly to the DC power source. The other inner electrodes have a dual role: one side acts as the cathode for a unit cell and the other side acts as the anode for the adjacent unit cell. These cells are electrically linked thanks to their electrodes which are bipolar and ionically via the electrolytic solution. The total voltage of the cell is the sum of the individual voltages of

hydrogen at the cathode and oxygen at the anode as shown in Figure 3.

its low cost, high activity [44].

10 Advances In Hydrogen Generation Technologies

Figure 3. Principle of an alkaline water electrolysis.

the unit cells (Utot ¼ n � Ui), but the current for each unit cell is equal to the output current of the generator (Itot ¼ IiÞ. The electrical energy consumed is the same in the two configurations.

The wide range of flammability limits of the mixture of hydrogen and oxygen requires a careful design of the electrolyzer system. The separator diaphragm (or membrane) must avoid the mixing of the two gases inside the cell. Furthermore, the corrosive nature of the electrolyte does not allow leaks that are often likely to take place at the connections and seals of the electrolyzer. The bipolar configuration is more risky in mixing oxygen and hydrogen because of their simultaneous productions on the same bipolar electrode (on each side) and also electrolyte leakage as the monopolar design.

Obviously, the life of the system is an important criterion. It is extremely linked to the quality of the materials used. Indeed, these materials must be resistant to high concentrations of the alkaline electrolyte and operating conditions of the electrolyzer (pressure and temperature). In particular, connections and seals are subject to corrosion, which is why it is recommended to use sealing materials that are also stable in this environment [14].
