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

High Resolution Image Download MS PowerPoint Slide CO 2 electroreduction (CO 2 R) 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 (C 2+ ) 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 (−NH x ) function from branched polyethylenimine (PEI) with sulfonate (−SO 3 – ) 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 [CO 2 ]/[H 2 O] 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 K 2 /H 2 SO 4, pH = 2), the optimized polyionomer coating enables a C 2+ Faradaic efficiency of 61% at a single-pass CO 2 utilization of 84%, including a conversion efficiency of 64% toward C 2+, at a current density of at 0.3 A cm –2 ─an improvement of almost 30% in C 2+ selectivity and 35% in carbon utilization compared to monofunctional coatings. These findings expand the toolbox of strategies to modulate CO 2 R microenvironments toward improved performance.