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 9.9A/dm² and the cell temperature of 110°C. The molality of HI in catholyte increased from the starting value of 10.6mol/kg to a final one of over 15mol/kg, whereas that of anolyte decreased from 10.2 to 5.2mol/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 1mol/l HCl and static transport number in 0.1 and 0.01mol/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 69.8kJ/mol-H₂ to produce the hydrogen of 1mol. 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.