Highly alkaline electrolytes have been shown to improve the formation rate of C2+ products in the electrochemical reduction of carbon dioxide (CO2) and carbon monoxide (CO) on copper surfaces, with the assumption that higher OH− concentrations promote the C−C coupling chemistry. Herein, by systematically varying the concentration of Na+ and OH− at the same absolute electrode potential, we demonstrate that higher concentrations of cations (Na+), rather than OH−, exert the main promotional effect on the production of C2+ products. The impact of the nature and the concentration of cations on the electrochemical reduction of CO is supported by experiments in which a fraction or all of Na+ is chelated by a crown ether. Chelation of Na+ leads to drastic decrease in the formation rate of C2+ products. The promotional effect of OH− determined at the same potential on the reversible hydrogen electrode scale is likely caused by larger overpotentials at higher electrolyte pH.

Electrochemical reduction of carbon monoxide to high-value multi-carbon (C2+) products offers an appealing route to store sustainable energy and make use of the chief greenhouse gas leading to climate change, i.e., CO2. Among potential products, C2+ liquid products such as ethanol are of particular interest owing to their high energy density and industrial relevance. In this work, we demonstrate that Ag-modified oxide-derive Cu catalysts prepared via high-energy ball milling exhibit near 80% Faradaic efficiencies for C2+ liquid products at commercially relevant current densities (>100 mA cm−2) in the CO electroreduction in a microfluidic flow cell. Such performance is retained in an over 100-hour electrolysis in a 100 cm2 membrane electrode assembly (MEA) electrolyzer. A method based on surface-enhanced infrared absorption spectroscopy is developed to characterize the CO binding strength on the catalyst surface. The lower C and O affinities of the Cu–Ag interfacial sites in the prepared catalysts are proposed to be responsible for the enhanced selectivity for C2+ oxygenates, which is the experimental verification of recent computational predictions.

Highly alkaline electrolytes have been shown to improve the formation rate of C2+ products in the electrochemical reduction of carbon dioxide (CO2) and carbon monoxide (CO) on copper surfaces, with the assumption that higher OH− concentrations promote the C−C coupling chemistry. Herein, by systematically varying the concentration of Na+ and OH− at the same absolute electrode potential, we demonstrate that higher concentrations of cations (Na+), rather than OH−, exert the main promotional effect on the production of C2+ products. The impact of the nature and the concentration of cations on the electrochemical reduction of CO is supported by experiments in which a fraction or all of Na+ is chelated by a crown ether. Chelation of Na+ leads to drastic decrease in the formation rate of C2+ products. The promotional effect of OH− determined at the same potential on the reversible hydrogen electrode scale is likely caused by larger overpotentials at higher electrolyte pH.