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-OV-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.