Acidic CO2 electrolysis offers great potential for high-efficiency single-pass CO2 conversion to valuable fuels and chemicals, yet is challenged by the kinetically favorable hydrogen evolution reaction. Electrode coatings can engineer a favorable electrode–electrolyte interface by regulating interfacial ion flux to steer the reaction pathway toward CO2 electroreduction; however, rational design rules for such coatings remain elusive. Here, we decouple the roles of binders and inorganic fillers in the coatings and demonstrate that coupling a cation-exchange ionomer (CEI) with a proton-blocking oxide yields a durable coating that enables efficient acidic CO2 electrolysis. Combined experimental and simulation results reveal that this optimal formulation promotes inward K+ transport while restricting H+ influx and outward OH− transport, thereby establishing a K+-enriched and alkaline interfacial microenvironment that enhances CO2/CO adsorption and lowers the *CO dimerization barrier. As a representative demonstration, an Al2O3/CEI-coated Cu electrode achieves a maximum Faradaic efficiency (FE) of 81.3% for multi-carbon (C2+) products at 600 mA cm−2 and remarkably durable operation for 200 h at 200 mA cm−2. Furthermore, this proposed electrode coating strategy demonstrates the broad generality in engineering interfacial microenvironments across distinct catalyst platforms.