Next-generation PBI-based ISMs: game changer of low-alkali water electrolysis
Water electrolysis is a promising method for producing green hydrogen from renewable energy sources. The mainstream membrane-based water electrolysis technologies include proton exchange membrane water electrolysis (PEMWE), alkaline water electrolysis (AWE), and anion exchange membrane water electrolysis (AEMWE). Each technology has inherent limitations: PEMWE requires expensive platinum group metal catalysts and fluorinated membranes; AWE uses porous diaphragms, limiting operational window and differential pressure capability; AEMWE suffers from quaternary ammonium group degradation under alkaline conditions. Ion-solvating membranes (ISMs), dense polymeric membranes with solvation sites that absorb aqueous alkaline electrolytes, offer an alternative. Most ISMs are based on polybenzimidazole (PBI), but achieving high efficiency at low alkali concentrations remains a bottleneck. Introducing sulfonic acid moieties into PBI enhances water and KOH uptake, improving conductivity, but alkali stability often requires crosslinking, which hampers scale-up. Henkensmeier et al. adapted sulfonated polybenzimidazole (SPBI) membranes originally for PEM fuel cells to ISMWE. By copolymerizing sulfonated and non-sulfonated monomers, they created a series of SPBI membranes with varying sulfonation degrees. The sulfonated segments boost electrolyte uptake and conductivity, while non-sulfonated units ensure alkali resistance and mechanical stability. Deprotonated benzimidazole rings increase electron density on ether carbons, enhancing stability in basic environments. The 50SOPBI-act membrane exhibited promising performance, with record ionic conductivity of 135 mS cm−1 at room temperature and 358 mS cm−1 at 80 °C in 1 M KOH, outperforming existing AEMWE and ISMWE systems.