Electrochemical Reduction of  CO 2 at Intentionally Oxidized Copper Electrodes

We observed reduction to at various oxidized copper electrodes at 22°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, (geometrical area), were found using anodized Cu in ,, and −1.9 V (SCE). Faradaic yields for depended on the current and reached about 240%. The onset potential for formation was near −0.4 V (SCE). Hydrogen gas and small amounts of were also formed. A mechanism is proposed involving chemical reduction steps up to , followed by three hydrogenation steps to . Absorption of oxygen by Cum is implied. The stability of 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 etch used to remove the native oxide before thermal oxidation.