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.