Steam Electrolysis

The process of the high-temperature electrolysis (HTE) of steam is a reverse reaction of the solid-oxide fuel cell (SOFC): an oxygen ionic conductor is usually used as a solid-oxide electrolyte as shown in Figure 4.1. Steam (H2O) is dissociated at the cathode with hydrogen (H2) molecules forming on the cathode surface: H2O(g) + 2e− → H2(g) + O2− while oxygen ions migrate simultaneously through oxygen vacancies in the lattice of the electrolyte material. Oxygen (O2) molecules form on the anode surface with the release of electrons: O2− → ½O2(g) + 2e−. The products, H2 and O2, are separated by the gas-tight electrolyte. Reactions on the two electrodes are summed up as,

H O(g) H (g) + O (g)2 2 2→

1 2

(4.1)

In the reaction, theoretical energy demand (ΔH) for water and steam decomposition is the sum of the Gibbs energy (ΔG) and the heat energy (TΔS). Figure 4.2 shows an energy demand for water and steam electrolysis. The electrical energy demand, ΔG, decreases with increasing temperature as shown in the Œgure; the ratio of ΔG to ΔH is about 93% at 100°C and about 70% at 1000°C. Thus, the HTE demands less electricity to produce hydrogen than does the conventional water electrolysis. This reaction can be expressed as follows:

∆ ∆G G RT a a a= +o H O 1 2

H O ln 2 2 2( / ) /

(4.2)

CONTENTS

4.1 Reaction Scheme ..................................................................................................................99 4.2 Research and Development Overview ........................................................................... 102 4.3 Experimental Studies in JAEA ......................................................................................... 103

4.3.1 Experimental Results with Tubular Cell ............................................................ 103 4.3.2 Experimental Results with Planar Cell .............................................................. 106

4.4 Experimental and Analytical Studies in Toshiba ......................................................... 108 4.4.1 Experimental Single Electrolysis Cell ................................................................. 108 4.4.2 Experimental Multielectrolysis-Cell Assembly ................................................. 109 4.4.3 Simulation Method for Thermo-Electrochemical Coupled Phenomena ....... 111 4.4.4 Design Analysis of the HTE Plant System ......................................................... 113

References ..................................................................................................................................... 115

where ΔGo is the standard Gibbs free energy change (per mole) for the reaction 4.1 at a temperature of T, R the gas constant, and a aH O2 2, , and aH O 2 the activities of H2, O2, and H2O in the cell. Equation 4.2 can be rewritten using relations of E = ΔG/2F and Eo = ΔGo/2F as

E E RT a a a= +o H O 1 2

H OF ln 2 2 2( / ) ( / ) /2 (4.3)

where F is the Faraday constant, and Eo (= ΔGo/2F) the standard electromotive force (EMF) of the following reaction in the quasi-static state: H2(g) + ½O2(g) → H2O(g).