Electrochemical Reduction of CO at a Copper Electrode

CO2 is electrochemically reduced to CH4, C2H4, and alcohols in aqueous electrolytes at Cu electrode with high current density. CO2 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 CH4, C2H4, C2H5OH, n-C3H7OH, CH3CHO, and C2H5CHO. Neither C2H6 nor CH3OH is produced. CH4 is favorably produced in aqueous KHCO3 of high concentrations (e.g. 0.3 mol dm-3), whereas C2H4 and C2H5OH are produced in dilute KHCO3 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 CO2 reduction. The partial current densities of CH4, C2H4, and C2H5OH are correlated with the electrode potential. Tafel relationships hold for C2H4 and C2H5OH irrespective of pH. The partial current of CH4 formation is proportional to proton activity and also follows the Tafel relationship. The transfer coefficients are 0.35 for C2H4 formation and 1.33 for CH4 formation. These facts indicate that the reaction paths of CH4 and C2H4 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.