Chemostructurally Stable Polyionomer Coatings Regulate Proton-Intermediate Landscape in Acidic CO2 Electrolysis

CO2 electroreduction (CO2R) in acidic media offers a path to high carbon utilization via local carbonate regeneration. However, this proton-rich environment challenges achieving a combined selectivity and rate toward multicarbon (C2+) products due to proton and intermediate competition. Here, we demonstrate a strategy to modulate local protons and intermediates, at these settings, using a polyionomer coating over benchmark copper gas diffusion electrodes. The polyionomer integrates amine (−NHx) function from branched polyethylenimine (PEI) with sulfonate (−SO3–) and amphiphilic functions from PFSA. We show that their chemical structure enables H-bonding interaction, leading to a stereochemical assembly that retains a structure–property relationship through a wide pH range (2–14). PFSA domains modulate *CO intermediates and local [CO2]/[H2O] and K+ environment, while partially protonated amines provide further control over proton availability and intermediate stabilization, which in combination enhance C–C coupling. When implemented in a flow cell (0.5 M K2/H2SO4, pH = 2), the optimized polyionomer coating enables a C2+ Faradaic efficiency of 61% at a single-pass CO2 utilization of 84%, including a conversion efficiency of 64% toward C2+, at a current density of at 0.3 A cm–2─an improvement of almost 30% in C2+ selectivity and 35% in carbon utilization compared to monofunctional coatings. These findings expand the toolbox of strategies to modulate CO2R microenvironments toward improved performance.