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.