Oxidative stability as a guiding principle for durable ionomer cations in pure-water electrolysis

Cation degradation in ionomers critically limits the durability of pure-water-fed anion exchange membrane water electrolysis, particularly at the anode where high overpotentials and near-neutral pH induce harsh oxidative stress. However, prevailing cation design remains rooted in alkali-stability frameworks, which fall short under these oxidative conditions. Here, a multiscale approach combining molecular degradation and operando diagnostics identifies electrochemical oxidation as the dominant anodic cation degradation pathway in anion exchange membrane water electrolysis, prompting a shift toward oxidation-resistant cation design. Electrolyzers incorporating oxidation-tolerant trimethylammonium-functionalized ionomers achieve 360-hour stability at 1 A cm⁻², doubling the durability of alkali-stable N,N-dimethylpiperidinium-based ionomers. Mechanistic studies under simulated anode conditions reveal electrochemical oxidative degradation pathways of cations, establishing a molecular-level evaluation framework. Operando diagnostics with reference-integrated electrolyzers reveal that cation oxidation elevates ohmic and ionic transport resistances. This multiscale study directly traces device failure to cation oxidation, redefining cation design around oxidative stability to enhance durability in pure-water-fed anion exchange membrane water electrolysis.