Interfacial Dynamic Oxygen Exchange Preserves C─O Bonds for Selective CO₂-to-Ethanol Electrosynthesis

Intermediate-valence copper (Cu⁺) is essential for preserving C─O bonds during the electrochemical reduction of CO₂ to ethanol, yet its progressive over-reduction to Cu⁰ under operating potentials inevitably dictates C─O bond cleavage and shifts selectivity toward ethylene. Herein, we propose an interfacial dynamic oxygen exchange strategy to stabilize Cu⁺ sites and steer the ethanol reaction pathway. We realize this mechanism by engineering a few-layer ceria-coated cuprous oxide (Cu₂O@CeO₂) catalyst featuring an oxygen vacancy-rich heterointerface (Ce-O_V-Cu). Operando spectroscopic measurements and density functional theory calculations reveal that these interfacial oxygen vacancies act as core mediators; by continuously capturing and migrating oxygen species derived from CO₂, they effectively arrest the reduction of adjacent Cu⁺ siteversuss. Crucially, this dynamic interface dictates the asymmetric C─C coupling of *CH₂ and *CHO, successfully preserving the C-O bond during the subsequent protonation of *CH₂CHO to *CH₃CHO. Consequently, the optimized catalyst delivers an outstanding ethanol Faradaic efficiency of 68.5% at -1.1 V versus RHE and exhibits robust operational stability exceeding 150 h, substantially outperforming pristine Cu₂O. This study establishes vacancy-mediated dynamic oxygen exchange as a robust strategy for preserving key oxygen-containing functional groups in highly selective CO₂-to-ethanol electrosynthesis.