Hydroxyl radicals dominate reoxidation of oxide-derived Cu in electrochemical CO 2 reduction.

Cu δ+ sites on the surface of oxide-derived copper (OD-Cu) are of vital importance in electrochemical CO 2 reduction reaction (CO 2 RR). However, the underlying reason for the dynamically existing Cu δ+ species, although thermodynamically unstable under reductive CO 2 RR conditions, remains uncovered. Here, by using electron paramagnetic resonance, we identify the highly oxidative hydroxyl radicals (OH • ) formed at room temperature in HCO 3 - 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 3 - and H 2 O 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 CO 2 RR. This insight is supposed to thrust an understanding of the crucial role of electrolytes in CO 2 RR.