Molecular-Scale Insights into Electrochemical Reduction of CO₂ on Hydrophobically Modified Cu Surfaces

Depressing the competitive hydrogen evolution reaction (HER) to promote current efficiency toward carbon-based chemicals in the electrocatalytic CO₂ reduction reaction (CO₂RR) is desirable. A strategy is to apply the hydrophobically molecular-modified electrodes. However, the molecular-scale catalytic process remains poorly understood. Using alkanethiol-modified hydrophobic Cu as an electrode and CO₂-saturated KHCO₃ as an electrolyte, we reveal that H₂O, rather than HCO₃^-, is the major H⁺ source for the HER, determined by differential electrochemical mass spectrometry with isotopic labeling. As a result, using in situ Raman, we find that the hydrophobic molecules screen the cathodic electric field effect on the reorientation of interfacial H₂O to a "H-down" configuration toward Cu surfaces that corresponds to the decreased content of H-bonding-free water, leading to unfavorable H₂O dissociation and thus decreased H⁺ source for the HER. Further, density functional theory calculations suggest that the absorbed alkanethiol molecules alter the electronic structure of Cu sites, thus decreasing the formation energy barrier of CO₂RR intermediates, which consequently increases the CO₂RR selectivity. This work provides a molecular-level understanding of improved CO₂RR on hydrophobically molecule-modified catalysts and presents general references for catalytic systems having H₂O-involved competitive HER.