Adsorbed CO is a critical intermediate in the electrocatalytic reduction of CO<sub>2</sub> to fuels. Contemporary methods for probing the thermodynamics of CO adsorption ignore the role of the electrolyte. 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 requires the reductive displacement of adsorbed carbonate anions. Through variable temperature studies, we find that CO binding to Cu is enthalpically favored by ~36 kJ mol<sup>–1</sup> relative to CO adsorption on Au. The divergent CO adsorption stoichiometry on Au and Cu explains their disparate reactivity: water adsorption drives CO liberation from Au surfaces, impeding further reduction, whereas carbonate desorption drives CO accumulation on Cu, 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. <br>
Facet-Dependent Selectivity of Cu Catalysts in Electrochemical CO2 Reduction at Commercially Viable Current Densities
