Sn-Induced Stable Cu⁺ in Cu₂O/Cu₆Sn₅ Interface System for Stabilizing *OCHO in CO₂-to-Formate Electroreduction

The rational construction of Cu-based interface with stable Cu⁺ species is crucial in CO₂ electroreduction reactions. Herein, a Sn-induced Cu₂O/Cu₆Sn₅ interface system (denoted as Cu₂O/CuSn) with stable Cu⁺ is ingeniously constructed for the CO₂-to-formate electroreduction reaction. At -1.1 V vs RHE, Cu₂O/CuSn achieves a high-formate Faradaic efficiency of 91.3% and a partial current density of -53.87 mA cm^-2. Cu₂O/CuSn maintains formate selectivity above 80% over a wide potential range (-0.8 to -1.2 V vs RHE). Moreover, the Cu⁺ species exhibits high stability under reaction conditions. Cu-O bonds confirm the presence of oxidized Cu (mainly Cu₂O), while Cu-Cu and Cu-Sn coordination suggests the formation of CuSn alloy. The presence of *OCHO as a key intermediate elucidates the formate formation pathway (*CO₂^-→ *OCHO → *HCOOH → HCOO^-). Density functional theory (DFT) calculations reveal significant electron redistribution at the interface between Cu₂O and CuSn alloy, modulating the electronic structure and stabilizing *OCHO adsorption. The synergy effect between Cu₂O and CuSn alloy enhances CO₂ adsorption, reduces the energy barrier for *OCHO formation, and effectively suppresses hydrogen evolution reaction (HER), enabling efficient CO₂-to-formate conversion.