Cu ⁺ ‐Driven Formate Selectivity in Oxidation‐State‐Dependent CO ₂ Reduction on Cu ₂ O|Graphene

ABSTRACT

Electrochemical CO ₂ 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 Cu ₂ O|graphene heterostructure that enables highly selective CO ₂ ‐to‐formate conversion while maintaining a Cu ⁺ ‐state active phase during electrocatalysis. The electrochemical reactivity of distinct Cu oxidation states (Cu ⁰ , Cu ⁺ , and Cu ²⁺ ) supported on graphene was systematically compared, revealing pronounced oxidation‐state‐dependent CO ₂ reduction behavior. Strong electronic coupling at the Cu ₂ O|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 CO ₂ RR, in contrast to Cu|graphene and CuO|graphene. Consequently, stabilized Cu ⁺ sites at the Cu ₂ O|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 CO ₂ ‐to‐formate conversion.