Spontaneously Formed CuS_x Catalysts for Selective and Stable Electrochemical Reduction of Industrial CO₂ Gas to Formate

The electrochemical CO2 reduction in aqueous media is a promising method for both the mitigation of climate changes and the generation of value-added fuels. Although many researchers have demonstrated selective and stable catalysts for electrochemical reduction of pure CO2 gas, the conversion of industrial CO2 gas has been limited. Here, we fabricated the copper sulfide catalysts (CuSx), which were spontaneously formed by dipping a Cu foil into a laboratory-prepared industrial CO2-purged 0.1 M KHCO3 electrolyte. Because industrial CO2 contains H2S gas, sulfur species dissolved in the electrolyte can easily react with the Cu foil. As the concentration of dissolved sulfur species increased, the reaction between the Cu foil and sulfur enhanced. As a result, the average size and surface density of CuSx nanoparticles (NPs) increased to 133.2 ± 33.1 nm and 86.2 ± 3.3%, respectively. Because of the larger amount of sulfur content and the enlarged electrochemical surface area of CuSx NPs, the Faradaic efficiency (FE) of formate was improved from 22.7 to 72.0% at −0.6 VRHE. Additionally, CuSx catalysts showed excellent stability in reducing industrial CO2 to formate. The change in FE was hardly observed even after long-term (72 h) operation. This study experimentally demonstrated that spontaneously formed CuSx catalysts are efficient and stable for reducing the industrial CO2 gas to formate.