Evaluation on the electro-electrodialysis stacks for hydrogen iodide concentrating in iodine–sulphur cycle

• Improve EED's capacity by applying multi-unit stacks.
• Effects of temperature, current density and operating time were studied.
• The calculated apparent proton transport numbers were very close to 1.0.
• Counter current operation secures the uniform performance of each repeated unit of a stack.

In thermochemical water-splitting iodine–sulfur cycle, concentrating hydrogen iodine in the HI–H2O–I2 solution is crucial for the efficient hydrogen production. Electro-electrodialysis (EED) is a very promising HI concentrating method. In this paper, EED experiments were carried out using stacked cells, aiming at the scale up of EED equipment. Compared with the single-unit EED cell, the multi-cell EED stacks could concentrate HI in catholyte much more rapidly. During the EED process, the cell voltages increased gradually with the expansion of the concentration difference between catholyte and anolyte. For the stacks with more EED cells, the voltage increased much more steeply. High operating temperature ensured EED process carried out under low cell voltages and avoided voltage swelling. The apparent transport number (t+) of all the experiments were very close to 1, while the ratio of permeated quantities of water to H+ (β) changed in a range of 1.79–3.05, influenced by temperature, I2 content and current density.