Thermally Cured Sulfonated Para-PBI As Ion Solvating Membrane for Use in Water Electrolysis

Abstract

A growing demand for green hydrogen necessitates durable, high‐performance membranes in alkaline water electrolyzers. In this work, a sulfonated para‐polybenzimidazole precursor (MS‐PBI) was thermally cured at 350 °C for 120 min to produce a crosslinked cMS‐PBI membrane. The cured material exhibits < 5 wt % solubility in hot H₃PO₄, confirming extensive network formation via sulfonic acid–imidazole crosslinking and the absence of labile ether or N‐alkylated imidazole linkages, which are known alkaline‐degradation points. Detailed spectroscopic and mechanical analyses show no aromatic ether bonds, quaternary ammonium sites, or positively charged imidazole moieties, ensuring chemical stability under harsh conditions. In a 6-month stability test (2 M KOH, 80 °C), cMS-PBI maintained its original dimensions, mass, and tensile properties without observable degradation or dimensional swelling. Ionic conductivity at room temperature rose during the first 24 days and plateaued at 192 ± 10 mS cm⁻¹; at 80 °C in 3 M KOH, it reached 682 mS cm⁻¹, underscoring sustained ion mobility. Single‐cell electrolyzer evaluation with robust Ni-foam electrodes and 2 M KOH feed at 80 °C delivered 0.975 A cm⁻² at 2 V, while using NiFe/Raney Ni catalysts achieved an extrapolated 3.52 A cm⁻² at 2 V, with voltage stability maintained over a 200 h accelerated stress test. To assess real‐world applicability, an accelerated stress protocol simulating intermittent power input from renewable sources (daily on/off cycling and current‐density modulation) was applied. cMS-PBI retained > 95 % of its initial conductivity and mechanical integrity after 500 on/off cycles, demonstrating compatibility with variable‐output energy systems. Furthermore, preliminary scale‐up trials using 50 cm² membrane sheets in a stack module confirmed uniform curing, low gas crossover (< 0.1 mL min⁻¹ cm⁻²), and consistent performance over 100 h at industrial‐relevant current densities. These results validate that ion-solvating, crosslinked PBI membranes such as cMS-PBI not only match or exceed conventional anion exchange membranes in performance and durability but also offer simplified fabrication and cost advantages. Their robust alkaline stability and renewable‐compatible cycling behavior position them as strong candidates for next-generation green hydrogen electrolyzers.