Improvement of the thermochemical water-splitting IS (iodine–sulfur) process by electro-electrodialysis

The electro-electrodialysis (EED) was carried out to improve HI concentration process in the thermochemical water-splitting IS (iodine–sulfur) process. The HI concentration from HIx solution (molar ratio of HI:H2O:I2=1:5:4) by EED was examined using a commercial cation exchange membrane (Nafion 117) at the current density of 9.9 A/dm2 and the cell temperature of 110 °C. The molality of HI in catholyte increased from the starting value of 10.6 mol/kg to a final one of over 15 mol/kg, whereas that of anolyte decreased from 10.2 to 5.2 mol/kg. Durability test of the membrane was carried out by soaking in HIx solution (molar ratio of HI:H2O:I2=1:5:0.5) at 120 °C. The membrane properties (membrane area resistance in 1 mol/l HCl and static transport number in 0.1 and 0.01 mol/l HCl) were measured as the indicator of the stability. After the soaking time of 3 months, the area resistance of membrane increased a little, and the static transport number decreased a little. The electrical energy demand of EED obtained in this study was 69.8 kJ/mol-H2 to produce the hydrogen of 1 mol. It is expected that the application of EED in IS process would be very useful with proposing the possibility of the larger improvement in thermal efficiency.