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

Electro-electrodialysis (EED) has been applied to the thermochemical water-splitting iodine-sulfur process for hydrogen production to enhance the hydrogen iodide (HI) component in the HI-I-2-H2O mixture (HIx solution). In this work, the HIx solution concentration dependence of conductivity and transport number as the membraneperformance indexes of Nafion 212 and a styrene-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE) membrane was investigated experimentally. From a theoretical analysis of measurements based on the new EED model, the effects of the H+ and I- contents and the diffusion coefficients of H+ and I- in the evaluated membranes on conductivity and transport number were as follows: HIx solution concentration dependence of the H+ and I- contents exhibited a significantly small or different variation in comparison with that of conductivity and transport number, indicating that the dominant factors for conductivity and transport number would not be the H+ and I- contents; thereby, the effect of the diffusion coefficients of H+ and I- should not be ruled out. In terms of the conductivity of Nafion 212, the diffusion coefficient of H+ is the dominant factor because the existence of I- was negligible, whereas in the case of the ETFE-St membrane, the effect of the diffusion coefficient of I- should be considered. The diffusion coefficient of I- could more strongly affect transport number for the evaluated membranes rather than the diffusion coefficient of H+, indicating that the H+ selectivity of the membranes could be determined relatively based on variations of I- behavior in the membranes.