Beyond Conductive Supports: Carbon-Regulated Interfacial Water Microenvironment on Cu for Selective CO2 Electroreduction

Abstract Cu-based catalysts in contact with carbon supports are promising for electrochemical CO2 reduction, with their product selectivity strongly governed by the local interfacial microenvironment. Carbon supports are typically regarded as passive conductive scaffolds, yet their role in programming the Cu interfacial microenvironment remains underexplored. Here, we identify the carbon defect state as a structural descriptor for reorganizing interfacial water and regulating CO2 reduction pathways, establishing a defect water-*CO selectivity relationship across carbon powder (CP)-, fullerene (C60)-, and graphene (GNS)- supported Cu catalysts. CP/Cu strongly biases the reaction pathway toward CO and C2H4 formation (FECO = 48.7%, FEC2H4 = 50.6%), representing about a 3.7-fold enhancement in C2H4 selectivity compared with GNS/Cu. In sharp contrast, GNS/Cu redirects the reaction toward the hydrogenation-dominated CH4 pathway (FECH4 = 43.1%, approximately 15 times higher than that of CP/Cu), while C60/Cu yields a transitional selectivity regime between the two extremes. In situ infrared spectroscopy reveals carbon-dependent *CO adsorption and distinct interfacial hydrogen-bonding environments near Cu sites. These carbon-dependent water-related spectral signatures correlate with the divergent evolution of *CO-derived intermediates and the resulting product selectivity trends. This work identifies carbon supports as active microenvironment modulators and provides a defect engineering strategy for tuning Cu-based CO2 electroreduction.