Affinity of ion-exchange membranes for HI–I2–H2O mixture

• Table 1 was added in Section 3.2 to explain about the membrane properties.
• The calculation error bars of Gibbs free energy were added in Fig. 7, Fig. 8 and Fig. 9 in order to clarify the accuracy of the results.
• We revised the expressions of the parts pointed out by the reviews comments for the better understandings of the reader.

Electro-electrodialysis (EED) has been applied to thermochemical water-splitting hydrogen production iodine–sulfur process in order to enhance HI in a HI–I2–H2O mixture (HIx solution). In this paper, in order to understand the behavior of the components (I2, H+, I−, and H2O) of the HIx solution in the membranes on absorption equilibrium, each component absorbed in a styrene-grafted poly-(ethylene-co-tetrafluoroethylene) (ETFE-St) membrane and Nafion 212 were evaluated. The behavior of each component in the membrane was as follows. The I2 content in the ETFE-St membrane increased with an increase in the I2 concentration of the immersed HIx solution, whereas I2 absorption by Nafion 212 was minimal. The strong affinity of the ETFE-St membrane for I2 was attributed to the formation of a charge-transfer complex with electron-donating aromatic groups. The ETFE-St membrane absorbed I− (also H+ under the neutral condition) in a HI-molality-dependent manner, whereas I− (H+) absorption by Nafion 212 was negligible. This difference was attributed to the I3−-complex formation in the ETFE-St membrane, which was precluded in Nafion 212. The absorption of H2O was greater for the ETFE-St membrane than for Nafion 212. Incorporation into Nafion 212 would not greatly stabilize H2O while stabilizing H+ by a strong electrical attraction. These results accordingly clarified the affinity of the membranes for the HIx solution based on the excess Gibbs free energy calculated from the measured composition data and the estimated activity coefficients. The excess Gibbs free energy for I2 was larger in the membranes than in the immersed solution, indicating a destabilization of I2 in the membranes. Importantly, this difference agreed with the energy change estimated under the assumption that I2 could form I3− or the charge-transfer complex. The affinity for I2 as well as between the sulfonic acid groups and H+ or H2O is a factor determining the absorbability of the HIx solution.