Spontaneously Formed CuS Catalysts for Selective and Stable Electrochemical Reduction of Industrial CO Gas to Formate.

The electrochemical CO 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 CO gas, the conversion of industrial CO gas has been limited. Here, we fabricated the copper sulfide catalysts (CuS), which were spontaneously formed by dipping a Cu foil into a laboratory-prepared industrial CO-purged 0.1 M KHCO electrolyte. Because industrial CO contains HS 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 CuS 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 CuS NPs, the Faradaic efficiency (FE) of formate was improved from 22.7 to 72.0% at -0.6 V. Additionally, CuS catalysts showed excellent stability in reducing industrial CO to formate. The change in FE was hardly observed even after long-term (72 h) operation. This study experimentally demonstrated that spontaneously formed CuS catalysts are efficient and stable for reducing the industrial CO gas to formate.