Effect of HIx solution concentration on ion-exchange membrane performance in electro-electrodialysis

Tanaka, Nobuyuki ; Yamaki, Tetsuya*; Asano, Masaharu*; Terai, Takayuki*

Electro-electrodialysis (EED) has been applied to thermochemical water-splitting iodine-sulfur process for hydrogen production in order to enhance the HI component in the HI-I-HO mixture (HIx solution). In this paper, HIx solution concentration dependence of the conductivity () and transport number () as the membrane-performance indexes of Nafion 212 and a styrene-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE) membrane was experimentally investigated. From the theoretical analysis for the measurements based on the new EED model, the effects of the H and I content and the diffusion coefficients of H and I in the evaluated membranes on and were as follows: HIx solution concentration dependence of ion contents exhibited a significantly small or different variation in comparison with that of and t, which indicates that the dominant factors for and would not be ion contents; thereby, the effect of ion diffusion coefficients should not be ruled out. For of Nafion 212, H diffusion is the dominant factor because the existence of I was negligible, whereas for the ETFE-St membrane, the effect of I diffusion should be considered. I diffusion could more strongly affect for the evaluated membranes rather than H diffusion, which indicates that H selectivity of the membranes could be relatively determined by the variation of the I behavior in the membranes.