Water dissociation reaction generated in an ion exchange membrane

Abstract

Abstract Based on pH measurement in water dissociation experiments, the mechanism of a water dissociation (water splitting) reaction is discussed assuming that the reaction is generated in a reaction layer formed in an ion exchange membrane and the reaction layer extends throughout the space in the membrane. The process is established to estimate the current efficiency for H+ and OH− ions in the water dissociation reaction, area and specific electric resistance of the reaction layer, potential gradient and potential difference in the reaction layer and forward reaction rate constant of the water dissociation reaction. The forward reaction rate constant is expressed by the function of potential gradient and electric potential in the reaction layer and the function of the reaction rate characteristic parameter. The water dissociation reaction is influenced by the potential gradient applied to the reaction layer and the electric potential induced by ion exchange groups in the reaction layer. Namely, the intensity of the water dissociation reaction (generation and transport of co-ions in the reaction layer) is decreased by repulsive force between ion exchange groups in the reaction layer and co-ions generated by the water splitting reactions. H+ ion current efficiency in a cation exchange membrane is less than OH− ion current efficiency in an anion exchange membrane, because the repulsive force against co-ions in the cation exchange membrane is larger than that in the anion exchange membrane. In spite of the natures of the water dissociation reaction mentioned above, the contribution of H+ and OH− ions to an electric current is insufficient and the rest of the electric current is carried by electrolyte ions supplied by accelerated solution convection in a boundary layer. In order to understand the mechanism of the water dissociation reaction, it is not necessary to be based on the concept of the protonation and deprotonation reaction. The influence of the second Wien effect on the water dissociation reaction is negligible because the potential gradient in the reaction layer is low.