Polybenzimidazoles for Hydrogen Generation in the Hybrid Sulfur Electrolyzer
Authors: Garrick Taylor Reed , Wilkins Cody Herbert , Pingitore Andrew T , Benicewicz Brian C , Weidner John W. 
Published: February 2020

The hybrid sulfur thermochemical cycle has seen much attentional recently due to its potential to enable the production of clean hydrogen on a large scale with a higher efficiency than water electrolysis.[1-9] 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, including the inability to operate at elevated temperatures and the decreased performance seen when exposed to high acid concentrations.[7, 8] 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.[8] Previous operation has focused on low temperatures in for comparison to Nafion, however, high temperature operation offers a better utilization of the advantages of PBI. Here, we present the results of high temperature operation of the HyS electrolyzer using sulfonated PBI. The acid concentration and hydrogen production rates are also quantified. Current interrupt was used to determine the membrane resistance under different operating parameters. A discussion of economic operating targets will also be provided.
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