Origin of Unexpected Products from CO2 Electroreduction on Silver: The Role of ppb-Level Copper Impurities in the Electrolyte

Silver (Ag) electrodes are among the best catalysts for producing CO in electrochemical CO2 reduction (CO2R). Beyond the catalyst material, it is increasingly recognized that electrolyte effects and dynamic processes that shape the interfacial microenvironment play critical roles in determining product selectivity. Herein, we reported unique observations of a variety of other CO2R products from Ag foil electrodes, highly dependent on electrolyte concentration and impurities. Notably, we found that trace levels of Cu (<5 ppb) could induce significant selectivity toward methane (CH4). This suggested that (sub)monolayer amounts of impurity metal deposited on Ag electrodes can drastically alter product selectivity. Density functional theory attributed this surprising CH4 selectivity to a d-band center upshift induced by the lattice mismatch with the Ag substrate. We furthermore observed that product selectivity shifts significantly over time during extended testing, attributable to dynamic electrolyte changes enabled by cation-exchange membranes. This study offers new insights into the behavior of well-studied Ag catalysts, highlighting the importance of trace electrolyte constituents and suggesting a possible new strategy for selectivity tuning