Low-Potential Interfacial Oxidation of Piperidinium Ionomers to Formate Contributes to Hydrogen Evolution in AEM Water Electrolyzers

Abstract

Piperidinium-functionalized ionomers are widely used alkaline-stable binders in anion exchange membrane water electrolyzers (AEMWEs). Although oxidative degradation has been demonstrated under strongly polarized, reactive oxygen species (ROS)-generating conditions, whether it can initiate through a distinct low-potential interfacial pathway before appreciable oxygen evolution remained unresolved. Here, we show that piperidinium groups undergo potential-induced interfacial oxidation on IrO2 near 1.3 V vs the reversible hydrogen electrode under deoxygenated conditions, demonstrating that dissolved O2 and diffusible O2-derived oxidants are not required. Quantitative 13C nuclear magnetic resonance spectroscopy identifies formate as the dominant soluble product, accompanied by ion-exchange-capacity loss and damage to a backbone-integrated structural unit. Faradaic-efficiency measurements in a pure-water-fed AEMWE show a 16.9% H2-normalized O2 deficit at 1.6 V, consistent with a substantial non-oxygen evolution reaction (OER) contribution from piperidinium oxidation. These findings distinguish low-potential interfacial oxidation from diffusible-ROS-mediated degradation and establish anodic cation stability as critical to durable pure-water-fed AEMWEs and reliable OER catalyst evaluation.