Synergistic regulation of oxygen vacancies and Cu⁺/Cu⁰ ratio enables robust urea electrosynthesis

Cu-based catalysts are able to activate both CO₂ and NO₃^- for CN coupling, enabling them with the capability to replace energy-intensive industrial urea production. However, the in situ electrochemical reconfiguration occurring at the reduction potential causes Cu⁰ accumulation and the imbalance of the Cu⁺/Cu⁰ ratio, resulting in a substantial decrease of their catalytic activity. In this work, CeO₂-CuO_x composite catalysts with a tailored Cu⁺/Cu⁰ ratio and a high concentration of oxygen vacancies (OVs) are synthesized via a one-step method. This approach leverages the synergistic effect between Ce³⁺/Ce⁴⁺ and OVs to mitigate the excessive accumulation of Cu⁰ and optimize the Cu⁺/Cu⁰ ratio. Electrochemical experiments coupled with in situ characterization demonstrate that CeO₂ effectively stabilizes Cu⁺ species, endowing CeO₂-CuO_x-10% (with an optimal Cu⁺/Cu⁰ ratio of ≈2:1) with the ability to synergistically facilitate the formation of the key intermediate *NH, stabilize *CO in a highly reactive top conformation, and inhibit the competing Hydrogen Evolution Reaction (HER), thereby achieving efficient CN coupling. Hence, CeO₂-CuO_x-10% achieves remarkable urea yield of 50.14 mmol h^-1 g^-1 at -1.0 V (vs. Reversible Hydrogen Electrode (RHE)), and a high Faraday efficiency (FE) of 42% at -0.6 V (vs. RHE), demonstrating an excellent electrochemical performance. This work illustrates the critical role of valence scaling and defect engineering in improving the performance of electrocatalysts, which may provide new ideas for designing highly efficient CN coupling electrocatalysts.