Mitigating hydrogen evolution via ionomer structure for efficient CO<sub>2</sub> reduction in acidic membrane electrode assembly electrolyzers

Electrocatalytic CO2 reduction in acidic media suppresses (bi)carbonate formation and improves CO2 single-pass conversion, but the catalyst layer in contact with the hydrated proton environment usually leads to severe competing hydrogen evolution reaction (HER). Regulating the interfacial microenvironment is an effective strategy to inhibit the migration of H+ during the electrolysis process and block the contact between H2O and the catalyst. In this work, we systematically investigate the influence of ionomer side chain on the interfacial water structure in an acidic membrane electrode assembly electrolyzer. The short side chain ionomer D79, characterized by a high density of sulfonic acid groups, generates abundant nanoscale micropores and promotes a more ordered dipole alignment of water molecules. In situ attenuated total reflection surface-enhanced infrared absorption spectroscopy reveals that over 60% of the interfacial water exists as strongly hydrogen-bonded water on the catalyst surface under an applied potential, which is conducive to suppressing HER from protons and free water. Optimizing the ionomer ratio to 20 wt% balances CO2 transport and hydrated proton migration. Consequently, the D79-modified electrode achieves a CO Faradaic efficiency (FECO) of 91.1% at 500 mA cm-2, significantly outperforming most reported studies. This work demonstrates that ionomer side chains can effectively regulate the hydrogen-bond network and suppress competitive HER, offering critical insights into interfacial microenvironment engineering for acidic CO2 electrolysis.