Hydroxide Is Not a Promoter of C₂₊ Product Formation in the Electrochemical Reduction of CO on Copper

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.