Thermally Cured Sulfonated <i>para</i>‐PBI as Ion Solvating Membrane for Use in Water Electrolysis

Abstract A sulfonated para ‐polybenzimidazole (MS‐PBI) membrane is thermally cured at 350 °C for 120 minutes. The thus obtained cMS‐PBI membrane is insoluble in hot phosphoric acid solution (&lt;5% soluble fraction), and is free of the known breaking points under alkaline conditions, i.e., no aromatic ether bonds, no quaternary ammonium groups, no N ‐alkylated neutral or positively charged imidazole groups. Indeed, a 6 months long alkaline stability test in 2 M KOH at 80 °C shows that cMS‐PBI retains its dimensions and its weight without any sign of degradation. The room temperature conductivity increases within the first 24 days, and then remains constant at 192 ± 10 mS cm −1 . At 80 °C, the conductivity in 3 M KOH reaches 682 mS cm −1 . At 80 °C and with 2 M KOH feed solution, an electrolyzer using robust nickel foam electrodes has a performance of 0.975 A cm −2 at 2 V. With NiFe/Raney nickel, 3.52 A cm −2 (extrapolated) at 2 V are reached. Voltage remained stable in a 200 hours test. In conclusion, it is expected that ion solvating membranes like cMS‐PBI can substitute anion exchange membranes in water electrolyzers, and by this improve their lifetime and reduce the cost of green hydrogen.