RESUMEN
We investigated the relationship between grain boundary (GB) oxidation of Cu-Ag thin-film catalysts and selectivity of the (photo)electrochemical CO2 reduction reaction (CO2 RR). The change in the thickness of the Cu thin film accompanies the variation of GB density, and the Ag layer (3 nm) has an island-like morphology on the Cu thin film. Therefore, oxygen from ambient air penetrates into the Cu thin film through the GB of Cu and binds with it because the uncoordinated Cu atoms at the GBs are unstable. It was found that the Cu thin film with a small grain size was susceptible to spontaneous oxidation and degraded the faradaic efficiency (FE) of CO and CH4. However, a relatively thick (≥80 nm) Cu layer was effective in preventing the GB oxidation and realized catalytic properties similar to those of bulk Cu-Ag catalysts. The optimized Cu (100 nm)-Ag (3 nm) thin film exhibited a unique bifunctional characteristic, which enables selective production of both CO (FECO = 79.8%) and CH4 (FECH4 = 59.3%) at a reductive potential of -1.0 and -1.4 VRHE, respectively. Moreover, the Cu-Ag thin film was used as a cocatalyst for photo-electrochemical CO2 reduction by patterning the Cu-Ag thin film and a SiO2 passivation layer on a p-type Si photocathode. This novel architecture improved the selectivity of CO and CH4 under light illumination (100 mW/cm2).
RESUMEN
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.