Interfacial Water Structure Governs CO2 Electroreduction Selectivity on Copper via Surface Ligand Functionalization

Controlling product selectivity in Cu-catalyzed electrochemical CO2 reduction remains challenging, as competing proton-coupled electron transfer pathways are governed by the balance between *CO coupling and protonation kinetics. Here, we demonstrate that this balance is precisely tuned by engineering interfacial water through surface ligand functionalization, without altering the Cu active site's electronic properties. Alkanethiols with distinct terminal groups (-CH3, -COOH, -OH) were anchored on Cu (Cu-UDT, Cu-MUA, Cu-MUO), imparting varying surface hydrophobicity. In situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) reveals that Cu-MUA enforces a strongly hydrogen-bonded water network, whereas Cu-MUO promotes a predominantly free water environment, with minor perturbation of *CO in both cases. Time-resolved SEIRAS demonstrates that the divergent water structures dictate *CO decay kinetics in which Cu-MUA facilitates rapid *CO consumption via C-C coupling, yielding high ethylene selectivity, while Cu-MUO enables preferential *CO protonation to *CHO through enhanced proton supply, steering selectivity toward methane.