Effect of Annealing Treatment on Electrocatalytic Properties of Copper Electrodes toward Enhanced CO2 Reduction

Abstract

AbstractCO2 reduction (CO2RR) into added‐value chemicals is a key contribution to solve energetic and environmental issues. The performance of Cu‐based electrodes towards CO2RR is strongly affected by pretreatments and presence of impurities, such as copper oxides. We evaluated the effect of different treatments, electropolishing (Cu‐p), and annealing treatments under oxidizing (Cu‐Air) and reducing (Cu‐H2) conditions, on the performance of Cu electrodes designed for CO2RR. Characterizations revealed the presence of Cu2O and CuO layers on the surface of the Cu‐Air electrode. All electrodes formed only HCOO− ions as liquid‐phase products, with yield of 480 mg L−1 of HCOO− and Faradaic efficiency (FE) of 30% for the Cu‐Air electrode and 25 mg L−1 with FE of 16%, and 26 mg L−1 with FE of 10% for the Cu‐p and Cu‐H2 electrodes, respectively. Overall, the HCOO− concentration increased with an increasing KHCO3 concentration, while the FE increased up to 46%, due to insertion of CO2 into the electrical double layer, generating an intermediate specie prior to the electrochemical CO2RR. CO2RR tests performed on the Cu‐Air and Cu‐p electrodes under galvanostatic conditions showed HCOO− FE of 32% and 16%, respectively. Therefore, the increasing local pH mechanism does not play a key role in the liquid‐phase products of CO2RR. The activity of the Cu‐Air electrode for CO2RR remained stable over 64 hours. It was proposed that Cu1+ was the active site responsible for CO2RR. CO, C2H4, C2H6 and H2 were formed by gas‐phase electrochemical CO2RR using the Cu‐Air electrode, with total FE higher than 65%. We have uncovered the role played by the type of pre‐treatment in the performance of Cu‐based electrodes on CO2RR and the reason for the increased electrocatalytic activity of oxidized Cu electrodes.