ELECTROCHEMICAL REDUCTION OF CO2 AT INTENTIONALLY OXIDIZED COPPER ELECTRODES

We observed CO2 reduction to CH3OH at various oxidized copper electrodes at 22-degrees-C. The electrode types included anodized Cu foil, Cu foil thermally oxidized in air, and air-oxidized Cu electrodeposited on anodized or air-oxidized Ti foil. The highest rates to date, 1 x 10(-4) mol cm-2 h-1 (geometrical area), were found using anodized Cu in 0.5M KHCO3, pH = 7.6, and -1.9 V (SCE). Faradaic yields for CH3OH depended on the current and reached about 240%. The onset potential for CH3OH formation was near -0.4 V (SCE). Hydrogen gas and small amounts of CO were also formed. A mechanism is proposed involving chemical reduction steps up to HCO(ad), followed by three hydrogenation steps to CH3OH. Absorption of oxygen by Cu(m) is implied. The stability of p-Cu2O semiconductor films is discussed in terms of breakdown currents due to band-to-band tunneling. The major impurities deposited on the Cu electrode during electrolysis are Zn and Fe at several atom percent each. An 8% level of Cl was found, originating from the HCl etch used to remove the native oxide before thermal oxidation.