Electrochemical Reduction of CO at a Copper Electrode

Abstract

CO 2 is electrochemically reduced to CH 4, C 2 H 4, and alcohols in aqueous electrolytes at Cu electrode with high current density. CO 2 is initially reduced to adsorbed CO and further to hydrocarbons and alcohols. This paper describes macroscopic electrolytic reduction of CO at a Cu electrode in various electrolyte solutions in order to reveal the unique properties of Cu electrode in comparison with Fe and Ni electrodes. The reaction products from the Cu electrode are CH 4, C 2 H 4, C 2 H 5 OH, n -C 3 H 7 OH, CH 3 CHO, and C 2 H 5 CHO. Neither C 2 H 6 nor CH 3 OH is produced. CH 4 is favorably produced in aqueous KHCO 3 of high concentrations ( e.g . 0.3 mol dm -3 ), whereas C 2 H 4 and C 2 H 5 OH are produced in dilute KHCO 3 solutions (0.03 mol dm -3 ). Such product selectivity is derived from the electrogenerated OH - in the cathodic reaction, as is the case in the CO 2 reduction. The partial current densities of CH 4, C 2 H 4, and C 2 H 5 OH are correlated with the electrode potential. Tafel relationships hold for C 2 H 4 and C 2 H 5 OH irrespective of pH. The partial current of CH 4 formation is proportional to proton activity and also follows the Tafel relationship. The transfer coefficients are 0.35 for C 2 H 4 formation and 1.33 for CH 4 formation. These facts indicate that the reaction paths of CH 4 and C 2 H 4 formations are separated at an early stage of CO reduction. Ethanol and n -propanol may be reduced from acetaldehyde and propionaldehyde intermediately formed in the reaction. The molecular reaction path is discussed with regard to these experimental results.