Polybenzimidazole Membranes for Hydrogen Production in the Hybrid Sulfur Electrolyzer

The hybrid sulfur thermochemical cycle has seen much attention lately because of its potential to provide clean hydrogen on a large scale at a higher efficiency than water electrolysis. The two step HyS process relies on high temperature decomposition of H₂SO4 to SO₂, O₂, and H₂O, and the low temperature electrochemical oxidation of SO₂ in the presence of water to produce H₂SO₄ and H₂. Because of internal recycling of the sulfur compounds, the overall process is the decomposition of water to form H₂ and O₂. This is an interesting process because the high temperature decomposition step could be coupled to next generation power plants or high-temperature solar arrays to enable the production of H₂ for other applications.

                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. 

Figure 1. Sulfuric acid concentration for s-PBI membranes as a function of temperature and current density. Lines are used to connect the data points and are meant as a guide for the eye.

Figure 2: Current-voltage curves from a HyS electrolyzer using s-PBI membranes. These curves correspond to the curves seen in Figure 1.

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