Polybenzimidazole Membranes for Hydrogen Production in the Hybrid Sulfur Electrolyzer
For a gas-fed anode using a proton exchange membrane such as Nafion in the electrolyzer, we have predicted water transport and used that to calculate cell voltages and sulfuric acid concentrations as a function of operating and design variables. Acid-doped polybenximidazole (PBI) membranes are an alternative to Nafion because they do not rely on water for their proton conductivity, and therefore they offer the possibility of operating at high acid concentrations and higher temperatures to minimize voltage losses. Early studies relied on doping the PBI membranes with concentrated solutions of phosphoric acid to increase membrane conductivity. However, leaching of the phosphoric acid resulted in a gradual loss of conductivity.
More recently, an alternative casting and doping procedure was developed for PBI membrane fabrication. Sulfonated PBI (s-PBI) membranes can be prepared using the same process starting with sulfonated monomers to impart an additional acid moiety in the polymer structure to enhance conductivity. In our research, we were able to use s-PBI membranes in the HyS electrolyzer and compared it to data collected from a Nafion-based cell.
We have successfully operated the HyS electrolyzer using sulfuric acid-doped s-PBI membranes. We have shown 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 at the anode. Also, s-PBI membranes provide the option of operating the cell at significantly elevated temperatures to reduce kinetic resistance. Data has also been compared to a model that has been developed to predict HyS electrolyzer performance under a variety of different operating conditions.
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