Nuclear Hydrogen Production Handbook Water Electrolysis

This article was downloaded by: 10.3.98.104 On: 28 Sep 2021 Access details: subscription number Publisher: CRC Press Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: 5 Howick Place, London SW1P 1WG, UK Nuclear Hydrogen Production Handbook Xing L. Yan, Ryutaro Hino Water Electrolysis Publication details https://www.routledgehandbooks.com/doi/10.1201/b10789-6 Seiji Kasahara Published online on: 28 Mar 2011 How to cite :- Seiji Kasahara. 28 Mar 2011, Water Electrolysis from: Nuclear Hydrogen Production, Handbook CRC Press Accessed on: 28 Sep 2021 https://www.routledgehandbooks.com/doi/10.1201/b10789-6 PLEASE SCROLL DOWN FOR DOCUMENT Full terms and conditions of use: https://www.routledgehandbooks.com/legal-notices/terms This Document PDF may be used for research, teaching and private study purposes. 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The publisher shall not be liable for an loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. 3 Water Electrolysis Seiji Kasahara Contents 3.1 Introduction ..........................................................................................................................83 3.2 Principle ................................................................................................................................84 3.3 Alkaline Water Electrolysis ................................................................................................86 3.3.1 Outline .......................................................................................................................86 3.3.2 Research and Development ....................................................................................87 3.3.3 Industrialization ......................................................................................................89 3.4 Polymer Electrolyte Water Electrolysis .............................................................................92 3.4.1 Outline .......................................................................................................................92 3.4.2 Research and Development ....................................................................................93 3.4.3 Industrialization ......................................................................................................95 Nomenclature ................................................................................................................................95 References .......................................................................................................................................96 3.1 Introduction Electrolysis of water is a method of producing hydrogen, and by-product oxygen, by the direct decomposition of water molecules using electric energy. Water electrolysis was already commercially practiced in 1890s. Some installations were operating around the beginning of the twentieth century. In the 1920s and 1930s, several plants in over 10 MWe size were constructed [1]. Industrial research and development to improve the economical performance of the method have continued to this very date. The newly developed electrolysis cells include the high-pressure designs and state-of-the-art membrane electrolyte designs. Electrolysis of water supplies only a few percentage of world hydrogen used today. The method is preferred for the production of high purity hydrogen and oxygen. It is used more often in places where hydropower is abundantly produced, for example, Iceland and Norway. Its wider use in industrial applications has been limited mainly because the cost of electricity remains high. In contrast, hydrogen produced by reforming fossil-fuel resources, chiefly methane or natural gas, has been developed and made economical and supplies the remainder of the world hydrogen demand now [2]. This chapter discusses electrolysis of liquid water, that is, alkaline water electrolysis and polymer electrolyte water electrolysis. The discussion is based on review of the literature [3–6]. 83 Downloaded By: 10.3.98.104 At: 11:18 28 Sep 2021; For: 9781439810842, chapter3, 10.1201/b10789-6 © 2011 by Taylor and Francis Group, LLC 84 Nuclear Hydrogen Production Handbook 3.2 Principle Heat and work are added to and taken from a reaction to maintain enthalpy and entropy balance of the reaction in accordance with the equation below. ΔH = ΔG + TΔS (3.1) When reaction temperature (and pressure, to be accurate) are decided, heat and work requirement are fixed. In the case of water decomposition, the relation of heat and work is schematically illustrated in Figure 3.1. A certain work input is needed to obtain products when the temperature is below Td. Note that the product gases of H2 and O2 are the same as the initial pressure. H2 and O2 of lower pressure are made by thermal equilibrium of water decomposition at lower temperature. To increase the pressure of these gases to the initial pressure requires a certain kind of work. Electrolysis of water can be regarded as a reaction of which the work requirement is provided as electricity. Electricity demand depends on temperature. The requirement is smaller at higher temperature. Electrolysis at several hundred to 1000°C is called high-temperature steam electrolysis. This type is explained in Chapter 4 because the technology is different from electrolysis at ambient temperature. Here, electrolysis methods applied to liquid water are discussed. Electrolysis of water is a combination of two half-reactions as shown below. The equations are different by electrolyte type. Acid electrolyte: + − Anode: H2O → 2H + 0.5O2 + 2e (3.2) + − Cathode: 2H + 2e → H2 (3.3) Alkaline electrolyte: − − Anode: 2OH → H2O + 0.5O2 + 2e (3.4) − − Cathode: 2H2O + 2e → H2 + 2OH (3.5) Theoretical voltage of electrolysis is described as ΔGe = nFE (3.6) ΔH, ΔG ΔH = TΔS(heat) Δ Δ ΔH G + T S ΔG(work) T 0 T Td (Phase shift is not described for simplification) FIGURE 3.1 G–T diagram of decomposition of water. Downloaded By: 10.3.98.104 At: 11:18 28 Sep 2021; For: 9781439810842, chapter3, 10.1201/b10789-6 © 2011 by Taylor and Francis Group, LLC Water Electrolysis 85 ΔGe depends on temperature, composition of electrolyte, and pressure of gas as in Equation 3.7. From Equations 3.6 and 3.7, theoretical voltage is described as in Equation 3.8. 05. f f HO22 0 00 ⋅ ∆∆GGee=+RT ⋅ ln p p (3.7) a HO2 05. fHO22 f 0 RT ⋅ EE=+ ⋅ ln 00 (3.8) nF p p a HO2 Actual cell voltage is greater than theoretical voltage because of over potential of electrodes and ohmic resistance of cell components as in Equation 3.9. Ecell = E + Eov.pot.A + Eov.pot.C + Eohm (3.9) Breakdown of actual cell voltage is illustrated in Figure 3.2. Over potential of electrodes means excess voltage to theoretical cell voltage in order to progress cell reactions at practi- cal rate. Over potential is made from the composition difference in between bulk electrolyte and around electrodes. The approximate value of over potentials is described by Tafel Equation 3.10. Eov.pot. = C1 + C2 ln i (3.10) Cathode over ) potential, Eov.pot.C (V Anode over cell E potential, Eov.pot.A Cell voltage, Ohmic resistance, Eohm E eoretical voltage, E 0 Current density, i (A/cm2) FIGURE 3.2 Breakdown of cell voltage. Downloaded By: 10.3.98.104 At: 11:18 28 Sep 2021; For: 9781439810842, chapter3, 10.1201/b10789-6 © 2011 by Taylor and Francis Group, LLC 86 Nuclear Hydrogen Production Handbook Ohmic resistance is made from electric resistance of components: electrolyte, separator, gas bubble between electrodes, outer electric circuit and electrodes. Ohmic resistance is approximately linear to current density. The total cell voltage is high in large current density. When current density is large, operation cost is higher because greater electric power is required. However, cell size can be made small and cell cost can be low. Designing the total cell system and optimization of cell operation are required for lower total cost. Efficiency of a water electrolysis cell is defined in Equation 3.11 as the ratio of reaction enthalpy to the electric energy supplied to the cell. Not only work of ΔG but heat of TΔS should be supplied to operate the cell. This is the reason to use ΔH, not ΔG as numerator. This efficiency can be defined as ratio of thermoneutral voltage, EH defined by Equation 3.12 to actual cell voltage in Equation 3.13. Theoretically, heat production by Joule loss is the same as heat requirement and no heat supply is required at thermoneutral voltage. ∆H EH ηel. == (3.11) W Ecell ∆H EH = (3.12) nF W Ecell = (3.13) nF It is noted that the efficiency of water electrolysis is a different concept from other efficiency, such as that of thermochemical water splitting (see Chapter, Section 5.1). While the calculation in Equation 3.11 uses electric work, the efficiency of thermochemical water splitting uses heat. When efficiency of electrolysis is compared with other methods, the same definition in those methods has to be used. 3.3 Alkaline Water Electrolysis 3.3.1 Outline Figure 3.3 is a schematic of an alkaline-water electrolysis (AWE) cell.

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