Rational Ionomer Design Directs Hydrogenation Pathways and Reactant Accessibility in Electrochemical Nitrate Reduction

Abstract

Ionomer engineering offers a powerful yet underexplored strategy for electrochemical nitrate reduction (NO3RR), since the polymer tailors the microenvironment at the catalyst surface. Here, we rationally design high-performance ionomers by applying fixed-charge sign and hydration as design criteria. Synthetic anion-exchange ionomers (AEIs) enhance NO3– accessibility, and the highest AEI hydration is accompanied by a shift in the dominant hydrogenation pathway toward a favorable *H-mediated regime. Time-dependent attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) coupled with numerical simulation yields a 40.7-fold higher NO3– diffusion coefficient for the AEI than for Nafion. Potential-dependent in situ ATR-SEIRAS combined with kinetic isotope effect measurements and *H scavenging supports a pathway shift arising from a hydrated microenvironment enriched in free and weakly hydrogen-bonded water. The AEI-modified commercial Cu nanoparticle electrode reaches an NH3 yield rate of 13.8 mg h–1cm–2 and 98.0% Faradaic efficiency, positioning the fixed-charge sign and hydration as accessible design descriptors potentially transferable to other electrocatalytic reactions.