Electrolyte Competition Controls Surface Binding of CO Intermediates to CO2 Reduction Catalysts

Adsorbed CO is a critical intermediate in the electrocatalytic reduction of CO2 to fuels. The directed design of CO2RR electrocatalysts has centered on strategies to understand and optimize the differences in CO adsorption enthalpy across surfaces. This approach has largely ignored the role of competitive electrolyte adsorption in defining the CO surface population relevant for catalysis. Using in situ infrared spectroelectrochemistry and voltammetry, we uncover the contrasting influence of electrolyte competition on reversible CO binding to Au and Cu catalysts. Although reversible CO binding to Au surfaces is primarily driven by the adsorption processes associated with interfacial water, CO binding to Cu surfaces requires the reductive displacement of adsorbed carbonate anions. The divergent role of electrolyte competition for CO adsorption on Au versus Cu leads to a ∼600 mV difference in the potential region where CO accumulates on the two surfaces. The contrasting CO adsorption stoichiometry on Au and Cu also explains their disparate reactivity: interfacial water adsorption contributes to CO liberation from Au surfaces, impeding further reduction, whereas carbonate desorption contributes to CO accumulation on Cu surfaces, allowing for further reduction to hydrocarbons. These studies provide direct insights into how electrolyte constituents fine-tune CO surface populations and, thereby, CO2-to-fuel reactivity.