Sulfonated polybenzimidazole for low-alkalinity ion solvating membrane water electrolysis

Ion solvating membranes based on polybenzimidazole (PBI) are alternatives to diaphragms in alkaline water electrolysers but can typically only operate with electrolyte concentrations of 15–30 wt% KOH. Sulfonation of the membrane broadens the operational range to 0.1 wt%–30 wt%; however, the swelling of sulfonated para -PBI means that crosslinkers are needed, complicating membrane fabrication and decreasing alkaline stability. Here we report a non-crosslinked PBI membrane with a 50% degree of sulfonation that shows a high room temperature conductivity in 1 M KOH of 135 mS cm −1 . We did not observe degradation in a 6-month alkaline stability test at 80 °C. Using this membrane in an anion-exchange membrane water electrolyser, we report a current density of 4.8 A cm −2 at 2 V (3 M KOH at 80 °C; Pt and NiFe electrode catalysts); the H 2 crossover to the O 2 side remained <2%. Using non-platinum group metal electrodes and a polyphenylene sulfide-reinforced membrane, a cell operated for >1,000 h without failure.