Nanocluster Surface Microenvironment Modulates Electrocatalytic CO₂ Reduction

Abstract The catalytic activity and product selectivity of the electrochemical CO₂ reduction reaction (eCO₂RR) depend strongly on the local microenvironment of mass diffusion at the nanostructured catalyst and electrolyte interface. Achieving a molecular‐level understanding of the electrocatalytic reaction requires the development of tunable metal‐ligand interfacial structures with atomic precision, which is highly challenging. Here, the synthesis and molecular structure of a 25‐atom silver nanocluster interfaced with an organic shell comprising 18 thiolate ligands are presented. The locally induced hydrophobicity by bulky alkyl functionality near the surface of the Ag₂₅ cluster dramatically enhances the eCO₂RR activity (CO Faradaic efficiency, FE_CO: 90.3%) with higher CO partial current density (j_CO) in an H‐cell compared to Ag₂₅ cluster (FE_CO: 66.6%) with confined hydrophilicity, which modulates surface interactions with water and CO₂. Remarkably, the hydrophobic Ag₂₅ cluster exhibits j_CO as high as −240 mA cm⁻² with FE_CO >90% at −3.4 V cell potential in a gas‐fed membrane electrode assembly device. Furthermore, this cluster demonstrates stable eCO₂RR over 120 h. Operando surface‐enhanced infrared absorption spectroscopy and theoretical simulations reveal how the ligands alter the neighboring water structure and *CO intermediates, impacting the intrinsic eCO₂RR activity, which provides atomistic mechanistic insights into the crucial role of confined hydrophobicity.