Hydroxyl radicals dominate reoxidation of oxide-derived Cu in electrochemical CO₂ reduction

Cu^δ+ sites on the surface of oxide-derived copper (OD-Cu) are of vital importance in electrochemical CO₂ reduction reaction (CO₂RR). However, the underlying reason for the dynamically existing Cu^δ+ species, although thermodynamically unstable under reductive CO₂RR conditions, remains uncovered. Here, by using electron paramagnetic resonance, we identify the highly oxidative hydroxyl radicals (OH^•) formed at room temperature in HCO₃^- 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 HCO₃^- and H₂O 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_x 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^δ+ species in CO₂RR. This insight is supposed to thrust an understanding of the crucial role of electrolytes in CO₂RR.