Hydrophobic modification regulates the water microenvironment at the interface of Cu-based materials to promote E-CO 2 RR

Electrocatalytic CO2 reduction (E-CO2RR) powered by renewable energy is vital for carbon cycling and carbon neutrality. Cu-based catalysts uniquely produce multi-carbon (C2+) products, yet the interfacial water microenvironment and reaction kinetics remain poorly understood. This work fabricates hydrophobic HY-Cu2O (100) catalysts by self-assembling octadecanethiol on cubic (100)-faced Cu2O nanoparticles to tune interfacial water. Integrated characterizations, electrochemistry, and in situ Attenuated Total Reflection Surface-Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) reveal how hydrophobicity regulates E-CO2RR performance. Hydrophilic/hydrophobic contact-angle measurements reveal that the surface wetting angle changes from 46.1° (hydrophilic) to 133.5° (hydrophobic). X-ray diffraction (XRD) confirms that the crystal structure remains unchanged. X-ray photoelectron spectroscopy (XPS) results demonstrate that the Cu-S coordination environment of the material is modified. Further inference suggests that the formation of Cu-S bonds verifies the successful functional grafting of octadecyl chains onto the Cu2O (100) surface, which alters the surface hydrophilicity/hydrophobicity without inducing any variation in the crystal structure. In 1 M KOH flow cells, HY-Cu2O delivers 45.2% Faradaic efficiency for ethylene at -250 mA∙cm-2 and suppresses parasitic Hydrogen Evolution Reaction (HER), far exceeding bare Cu2O. In situ spectra and theory verify that hydrophobic interfaces accumulate K+-bound free water, disrupt interfacial hydrogen bonds, accelerate *CO protonation, and boost C-C coupling for C2+ intermediates. Higher surface CO coverage further kinetically favors C2+ formation. This work establishes an effective hydrophobic modification approach, clarifies the central role of interfacial water in ethylene selectivity, and offers guidance for the design of advanced CO2RR catalysts.