In the sulfur-iodine (S-I) thermochemical cycle for hydrogen production, the chemistry is dictated by the oxidation and reduction behaviors of sulfur and iodine. Hydrogen is produced from the decomposition of hydriodic acid (HI). To form HI, the Bunsen reaction is conducted that produces an HI and sulfuric acid mixture, which is then separated. Inputs into the Bunsen reaction are water, SO2 and I2. Thus, sulfur is cycled between its +4 and +6 oxidation states, SO2 and H2SO4, respectively.

Necessary SO2 for the Bunsen reaction is provided by the decomposition of sulfuric acid. Typically, the decomposition of sulfuric acid is performed over a catalyst, such as platinum. However, even at the highest temperatures (~850 ºC) and using the most active catalyst materials, the reaction remains equilibrium limited, which limits the overall conversion and creates significant amounts of unreacted sulfuric acid that requires recycle. From the sulfuric acid that is decomposed during the reaction, water is liberated, which occurs when the acid initially dissociates to SO3 and water on heating. Thus, the effluent from the decomposer is the desired SO2, and diluted sulfuric acid. To recycle the acid, water must be removed. Distillation is a natural option and can perform the concentration if enough heat is applied. An issue with distillation is thermally induced corrosion of equipment, which surprisingly is a more significant issue for dilute sulfuric acid than for concentrated acid, especially at the vapor-liquid interface. Thus, an alternate technology was sought that could perform the separation at lower temperature.

Previous work applied to S-I cycle was focused on HI concentration. Nafion-® membranes were found to be effective in the removal of water from concentrated HI and HI/I2 solutions using pervaporation at temperatures as high as 140 ºC. Performance of the membranes was considered excellent with both high flux rates and separation factors with no significant degradation of the membranes. This technology was applied to sulfuric acid with similar results. Flux of water through the membranes was found to be commercially competitive with high selectivity. In this paper, performance of two differing thicknesses of Nafion® will be discussed including a stability study of the membranes using Dynamic Mechanical Analysis. Our results indicate that the Nafion® materials are stable and perform predictably; however there is some slight embrittlement of the membranes upon exposure to acid. The embrittlement has been characterized instrumentally, but does not appear to have an effect on water transport through the membranes as reflected by permeability and selectivity data. Thus, this technology may be applicable to the S-I cycle and to any other cycle employing sulfur. Moreover, there also may be industrial applications considering the ubiquitous nature of sulfuric acid as a commodity.