Ionomer Charge Density Modulates Interfacial Water and Reaction Intermediates for CO ₂ Electrolysis to C ₂ Products

High Resolution Image Download MS PowerPoint Slide This study reports that an optimal charge density in ionomers enhances the formation of C 2 products in Cu-catalyzed electrochemical CO 2 reduction reaction (CO 2 RR), challenging the established view in water electrolysis that higher charge density improves performance. Systematic variation of ion-exchange capacity (IEC) shows that it governs interfacial microenvironments by balancing the density of charged groups, ionic conductivity, water uptake, and hydrophobicity. An intermediate IEC creates a microenvironment that suppresses the hydrogen evolution reaction (HER) and favors the C 2 formation, achieving a Faradaic efficiency (FE C 2 ) of 38.2% with moderately active Cu catalysts in a two-compartment cell. In situ vibrational spectroscopy and molecular dynamics simulations reveal the orientations and dynamics of interfacial water, CO 2, and *CO intermediates, as well as the hydrogen bond (H-bond) network of water under applied potentials. The optimized ionomer induces interfacial water molecules so that their hydrogen atoms point away from the electrode, thereby strengthening H-bonds and suppressing HER. It simultaneously tilts CO 2 relative to the electrode and thus increases the population of linearly bound *CO intermediates that facilitate C–C coupling. Incorporating the optimized ionomer into a flow cell further delivers partial current densities for C 2+ products above −500 mA cm –2 with FE C 2+ over 70%. These findings reveal an unexplored role of IEC in tuning interfacial microenvironments and provide design principles for ionomers that selectively promote C 2+ formation in CO 2 RR.