Co-Activated Interfacial Water Facilitates Proton Supply in CO2-to-Methane Electroreduction on a CuCo Single-Atom Alloy

Abstract Cu-based single-atom alloys (SAAs), particularly those containing Fe or Co, have been reported to enhance methane formation during electrochemical CO2 reduction, but whether isolated metal dopants act as direct reaction sites remains unresolved. Using Cu(111)-based SAA terrace models, we show that the isolated dopant sites examined remain accessible under reductive conditions. However, neither these dopant sites nor nearby Cu sites improve the adsorbate-mediated CO2-to-CH4 pathway relative to Cu(111), challenging the conventional intermediate-binding picture. For CuCo SAA, explicit-water AIMD simulations reveal that Co instead polarizes adjacent water into an O–H-elongated prereactive configuration. The water-dissociation barrier consequently decreases from 1.12 eV on Cu(111) to 0.67 eV, and the promotional effect persists when *CO binds to Co. These results identify Co-mediated interfacial proton supply as a mechanism beyond carbon-intermediate binding and suggest that interfacial water activation should be considered alongside adsorbate binding when evaluating Cu-based SAA catalysts.