7 - Water Dissociation

Abstract

The water dissociation is generated under an applied potential field within the reaction layer formed in the membrane. With an applied potential field, the forward reaction constant ka is increased due to the potential difference applied to the reaction layer, while the reverse reaction rate constant kb remains being unchanged. Based on the experimental observation and theoretical discussion, the thickness and the electric resistance of the reaction layer and H+ and OH− ion concentration distribution in the reaction layer are calculated. The intensity of the reaction is catalytically accelerated by ion exchange groups in the membrane and is increased with the potential gradient applied to the reaction layer. The water dissociation reaction generates H+ ions and OH− ions, which become, respectively, counter-ions and co-ions in the cation exchange membrane, while they become, respectively, co-ions and counter-ions in the anion exchange membrane. When electric potential in the membrane is increased, the co-ion generation is suppressed due to generation of repulsive force between ion exchange groups and co-ions generated by the water splitting reaction. The intensity of the water dissociation reaction relates to the dissociation strength of ion exchange groups, which is understandable from the pK values of the ion exchange groups. Metallic hydroxides such as Fe(OH)3, Mg(OH)2, Ni(OH)2, Co(OH)2, Mn(OH)2, etc. induce an extremely strong catalytic water dissociation reaction under an applied potential gradient.