Ionomer Charge Density Modulates Interfacial Water and Reaction Intermediates for CO2 Electrolysis to C2 Products

Abstract

This study reports that an optimal charge density in ionomers enhances the formation of C2 products in Cu-catalyzed electrochemical CO2 reduction reaction (CO2RR), 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 C2 formation, achieving a Faradaic efficiency (FEC2) 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, CO2, 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 CO2 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 C2+ products above −500 mA cm–2 with FEC2+ over 70%. These findings reveal an unexplored role of IEC in tuning interfacial microenvironments and provide design principles for ionomers that selectively promote C2+ formation in CO2RR.