Enhanced electrochemical reduction of CO2 to CO on Ag electrocatalysts with increased unoccupied density of states

CO2 conversion through catalytic processes in a selective and efficient manner is an essential technology for a sustainable carbon economy at present and in the future. Here, we fabricated five nanostructured Ag electrocatalysts and studied their CO2 electro-reduction properties compared to commercial Ag. Ag L-edge X-ray absorption near-edge spectroscopy was employed to characterize the electronic structure variation among the catalysts. It is found that an increased unoccupied density of states (DOS) of d-character of Ag endows the electrocatalyst with a higher selectivity and efficiency for CO2 reduction. The introduction of Ni into the Ag matrix reduces the unoccupied DOS of d-character as there is charge redistribution between Ag and Ni, worsening the active efficiency of CO production. Density functional theory calculations show that an increased unoccupied DOS optimizes the affinity of the catalyst surface to the intermediate COOH and CO, closer to the activity volcano, which facilitates CO formation. The nanoporous Ag electrocatalyst, made through anodization-reduction, possesses the highest unoccupied DOS of d-character among the samples, showing the best catalytic performance in the reduction of CO2 to CO. Meanwhile, its onset overpotential is 0.19 V and the highest faradaic efficiency reaches 90%. The electrocatalyst stays at this level for 10 h without any noticeable activity change, possessing great potential for applications in industry. This research provides a useful insight on the structure-property relationships of CO2 reduction catalysts, and a guideline for designing high-performance electrocatalysts.