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 (-NH_x) 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.