Understanding CO2 reduction selectivity in silver and gold electrocatalysts using atomically precise nanoclusters

Abstract

Au and Ag electrodes have been widely utilized as catalysts for the electrochemical CO₂ reduction reaction (CO₂RR) in aqueous solutions. Despite significant differences in their CO₂RR and competing hydrogen evolution reaction (HER) activities, the fundamental reasons for these distinctions remain unclear. Herein, we present a comparative analysis employing atomically precise Ag and Au nanocluster (NC) catalysts, Ag₂₅(SR)₁₈ and Au₂₅(SR)₁₈ (where SR represents a thiolate ligand), to elucidate the molecular-level mechanisms governing the CO₂RR and the HER on Ag and Au surfaces. While Au₂₅(SR)₁₈ NCs demonstrate a lower onset potential for the CO₂RR compared to Ag₂₅(SR)₁₈ NCs, the CO current density of the Au₂₅(SR)₁₈ NCs reaches a plateau and even decreases at high overpotentials due to the increased HER. Electrokinetic and in situ infrared spectroscopy investigations using well-defined model NC catalysts revealed that both Ag₂₅(SR)₁₈ and Au₂₅(SR)₁₈ NCs facilitate a CO₂RR pathway through enhanced water dissociation (WD) kinetics. However, these NCs display significantly different HER behaviors under CO₂: Au₂₅(SR)₁₈ exhibits enhanced activity, while Ag₂₅(SR)₁₈ is significantly suppressed. Theoretical studies indicate that the enhanced WD kinetics stem from direct proton abstraction by the CO₂RR intermediate. Adsorption geometry analyses further demonstrate that their differing selectivities arise from distinct HER active sites, thereby explaining the variations in CO₂RR and HER activities between Ag₂₅(SR)₁₈ and Au₂₅(SR)₁₈ catalysts.