We report a coordinated research effort focused on the design, synthesis, and application of anion-exchange ionomers as tunable interfacial components for selective electrochemical CO2 reduction reaction (CO2RR). Systematic variations in both the side chain structure and the cationic group were found to influence microenvironments at the catalyst surface, affecting catalytic activity and product selectivity. Engineering the alkyl side chain length–ranging from short (methyl) to long (n-hexadecyl)–modulates interfacial hydrophobicity and the thermodynamic stability of key intermediates. Ionomers with n-hexadecyl chains most effectively suppress H2 and CH4 formation, whereas n-decyl analogs yield the highest C2H4 selectivity. Cu-based systems incorporating n-decyl ionomers exhibit partial current densities up to –210 mA cm–2 and achieve Faradaic efficiencies of 52% for C2H4 at 3.9 V in membrane electrode assemblies. In Ag-catalyzed systems, n-hexadecyl ionomers show CO Faradaic efficiencies of up to 90% and reduce hydrogen evolution to 4% by stabilizing an alkaline microenvironment at the catalyst interface. Ionomers with shorter side chains exhibit lower selectivity and limited ability to regulate interfacial conditions in systems requiring strong microenvironmental control. The nature of the cationic group incorporated in the polymer backbone further impacts interfacial charge distribution, ion pairing, and local pH regulation, providing an additional design variable to control microenvironments. Ionomers with distinct cationic functionalities demonstrate varying capabilities to stabilize reactive intermediates and suppress competing pathways. These studies collectively establish structure-performance relationships that link molecular ionomer design to interfacial control, providing a versatile platform for optimizing CO2RR selectivity and efficiency across catalytic systems.
