Nanocluster Surface Microenvironment Modulates Electrocatalytic CO<sub>2</sub> Reduction

Abstract

The catalytic activity and product selectivity of the electrochemical CO2 reduction reaction (eCO(2)RR) 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 Ag-25 cluster dramatically enhances the eCO(2)RR activity (CO Faradaic efficiency, FECO: 90.3%) with higher CO partial current density (j(CO)) in an H-cell compared to Ag-25 cluster (FECO: 66.6%) with confined hydrophilicity, which modulates surface interactions with water and CO2. Remarkably, the hydrophobic Ag-25 cluster exhibits j(CO) as high as -240 mA cm(-2) with FECO >90% at -3.4 V cell potential in a gas-fed membrane electrode assembly device. Furthermore, this cluster demonstrates stable eCO(2)RR 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 eCO(2)RR activity, which provides atomistic mechanistic insights into the crucial role of confined hydrophobicity.