The hybrid sulfur thermochemical cycle has seen much attention recently due to its potential to enable the production of clean hydrogen on a large scale with a higher energy efficiency than water electrolysis. The two step hybrid sulfur (HyS) process involves the high temperature decomposition of sulfuric acid to produce sulfur dioxide, oxygen, and water, as well as a low temperature electrochemical oxidation of sulfur dioxide in the presence of water to produce sulfuric acid and gaseous hydrogen. Due to the internal recycling of sulfur compounds in the HyS process, the overall balance is the decomposition of water to form gaseous hydrogen and oxygen. This process is interesting because the high temperature decomposition step could be coupled to next generation solar power plants or high temperature nuclear reactors in order to produce hydrogen for other applications.

Using a proton exchange membrane such as Nafion in the HyS electrolyzer has been thoroughly examined via the prediction of mass transport through the membrane as a function of operating potential and other design variables. However, Nafion presents several drawbacks, such as the inability to operate at elevated temperatures and the decreased performance seen when exposed to high acid concentrations. Previously, we showed that acid doped polybenzimidazole (PBI) membranes are an alternative to Nafion because they do not rely on water for proton conductivity, and therefore offer the possibility of operating at higher acid concentrations in order to minimize energy requirements necessary for water separation, as well as operation at higher temperatures in order to minimize voltage losses. Through the successful operation of the HyS electrolyzer using sulfuric acid doped PBI membranes, we have determined that despite the relative thickness of s-PBI, the area-specific resistance of s-PBI compares favorably with Nafion and is not adversely affected by the sulfuric acid concentration in the electrolyzer. Through further characterization of the membrane and the electrolyzer, we have been able to refine a model for high temperature and high pressure operation of the electrolyzer allowing for further analysis of the system in order to determine operating conditions that allow for economically viable operation of the HyS electrolyzer.

The dependence of the individual potential contributions on temperature and pressure will be discussed, as well as trends seen in the membrane conductivity as a function of operating conditions and produced acid concentrations. A discussion of model results illustrating future membrane design considerations will also be presented.