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

Abstract CO 2 reduction (CO 2 RR) into added‐value chemicals is a key contribution to solve energetic and environmental issues. The performance of Cu‐based electrodes towards CO 2 RR 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‐H 2 ) conditions, on the performance of Cu electrodes designed for CO 2 RR. Characterizations revealed the presence of Cu 2 O 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‐H 2 electrodes, respectively. Overall, the HCOO − concentration increased with an increasing KHCO 3 concentration, while the FE increased up to 46%, due to insertion of CO 2 into the electrical double layer, generating an intermediate specie prior to the electrochemical CO 2 RR. CO 2 RR 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 CO 2 RR. The activity of the Cu‐Air electrode for CO 2 RR remained stable over 64 hours. It was proposed that Cu 1+ was the active site responsible for CO 2 RR. CO, C 2 H 4 , C 2 H 6 and H 2 were formed by gas‐phase electrochemical CO 2 RR 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 CO 2 RR and the reason for the increased electrocatalytic activity of oxidized Cu electrodes.