Highly Efficient and Durable Water Electrolysis at High KOH Concentration Enabled by Cationic Group-Free Ion Solvating Membranes in Free-standing Gel Form

Abstract

The degradation of fixed cationic groups in most anion exchange membranes (AEMs) under alkaline environments limits their durability for alkaline water electrolysis (AWE). Ion-solvating membranes (ISMs) have emerged as a promising alternative to address this issue. Herein, a cationic group-free ion solvating membrane in a free-standing gel form (ISM-PBI-FG) is presented, created through a sol-to-gel transformation process followed by KOH imbibing. This approach yields a 3D porous microstructure composed of entangled polybenzimidazole (PBI) nanofibrils, achieving 89% porosity, which enables ultrahigh alkali uptake (346% in 6 M KOH) and exceptional ionic conductivity (763 mS cm⁻¹ at 80 °C). The absence of cationic groups avoids the attack by OH⁻, thus ensures good alkaline stability with a conductivity retention rate of 91.6% over a 3120 h ex situ test in 6 M KOH. The resulting membrane delivered outstanding AWE performance with current densities of 5.5 A cm⁻² using platinum-group-metal (PGM) catalysts and 2.2 A cm⁻² using PGM-free catalysts at 2.0 V. Notably, the in situ electrolyzer device based on ISM-PBI-FG offers extensive operational flexibility ranging from 40–100 °C and demonstrates record durability in concentrated alkaline conditions, with a voltage decay rate of only 23.5µV h⁻¹, outperforming most reported AEMs and ISMs.