N-doped Cu₂O with the tunable Cu⁰ and Cu⁺ sites for selective CO₂ electrochemical reduction to ethylene

The electrochemical carbon dioxide reduction reaction (CO₂RR) to high value-added fuels or chemicals driven by the renewable energy is promising to alleviate global warming. However, the selective CO₂ reduction to C₂ products remains challenge. Cu-based catalyst with the specific Cu⁰ and Cu⁺ sites is important to generate C₂ products. This work used nitrogen (N) to tune amounts of Cu⁰ and Cu⁺ sites in Cu₂O catalysts and improve C₂-product conversion. The controllable Cu⁰/Cu⁺ ratio of Cu₂O catalyst from 0.16 to 15.19 was achieved by adjusting the N doping amount using NH₃/Ar plasma treatment. The major theme of this work was clarifying a volcano curve of the ethylene Faraday efficiency as a function of the Cu⁰/Cu⁺ ratio. The optimal Cu⁰/Cu⁺ ratio was determined as 0.43 for selective electroreduction CO₂ to ethylene. X-ray spectroscopy and density functional theory (DFT) calculations were employed to elucidate that the strong interaction between N and Cu increased the binding energy of NCu bond and stabilize Cu⁺, resulting in a 92.3% reduction in the potential energy change for *CO-*CO dimerization. This study is inspiring in designing high performance electrocatalysts for CO₂ conversion.