Hydroxyl radicals dominate reoxidation of oxide-derived Cu in electrochemical CO reduction.

Cu sites on the surface of oxide-derived copper (OD-Cu) are of vital importance in electrochemical CO reduction reaction (CORR). However, the underlying reason for the dynamically existing Cu species, although thermodynamically unstable under reductive CORR conditions, remains uncovered. Here, by using electron paramagnetic resonance, we identify the highly oxidative hydroxyl radicals (OH) formed at room temperature in HCO solutions. In combination with in situ Raman spectroscopy, secondary ion mass spectrometry, and isotope-labelling, we demonstrate a dynamic reduction/reoxidation behavior at the surface of OD-Cu and reveal that the fast oxygen exchange between HCO and HO provides oxygen sources for the formation of OH radicals. In addition, their continuous generations can cause spontaneous oxidation of Cu electrodes and produce surface CuO species. Significantly, this work suggests that there is a "seesaw-effect" between the cathodic reduction and the OH-induced reoxidation, determining the chemical state and content of Cu species in CORR. This insight is supposed to thrust an understanding of the crucial role of electrolytes in CORR.