Cu⁺-Driven Formate Selectivity in Oxidation-State-Dependent CO₂ Reduction on Cu₂O|Graphene

Electrochemical CO₂ reduction to formate on copper oxide catalysts is often limited by poor selectivity due to competing multi-electron pathways and by rapid reduction of active sites accompanied by severe structural reconstruction under cathodic bias. Here, we report a well-defined Cu₂O|graphene heterostructure that enables highly selective CO₂-to-formate conversion while maintaining a Cu⁺-state active phase during electrocatalysis. The electrochemical reactivity of distinct Cu oxidation states (Cu⁰, Cu⁺, and Cu²⁺) supported on graphene was systematically compared, revealing pronounced oxidation-state-dependent CO₂ reduction behavior. Strong electronic coupling at the Cu₂O|graphene interface regulated interfacial electron transfer, suppressing over-reduction to metallic Cu and preserving the catalytic structure during electrolysis. Time-resolved ex situ X-ray diffraction, Cu LMM Auger spectroscopy, Cu K-edge XANES, in situ Raman spectroscopy and high-resolution transmission electron microscopy collectively suggested the retention of Cu⁺-rich environment on graphene during CO₂RR, in contrast to Cu|graphene and CuO|graphene. Consequently, stabilized Cu⁺ sites at the Cu₂O|graphene interface favored a proton-coupled electron transfer pathway via the *OCHO intermediate, delivering a high formate Faradaic efficiency of 78% at -0.86 V vs. RHE. This work establishes interfacial electronic coupling as an effective strategy to stabilize metastable Cu⁺ active sites for selective CO₂-to-formate conversion.