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. Directed design of CO₂RR electrocatalysts have centered on strategies to understand and optimize the differences in CO adsorption enthalpy across surfaces. Yet, 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, we disclose the contrasting influence of electrolyte competition on reversible CO binding to Au and Cu catalysts. Whereas reversible CO binding to Au surfaces is driven by substitution and reorientation of adsorbed 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 vs. 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: water adsorption drives CO liberation from Au surfaces, impeding further reduction, whereas carbonate desorption drives CO accumulation on Cu surfaces, allowing for further reduction to hydrocarbons. These studies provide direct insight into how electrolyte constituents can serve as powerful design parameters for fine-tuning of CO surface populations and, thereby, CO₂-to-fuels reactivity.