Quantitatively Unraveling the Redox Shuttle of Spontaneous Oxidation/Electroreduction of CuO_x on Silver Nanowires Using in Situ X-ray Absorption Spectroscopy

Oxide-derived copper catalysts have been shown to enhance CO₂ reduction reaction (CO₂RR) activity with high selectivity toward hydrocarbon products. However, the chemical state of oxide-derived copper during the CO₂RR has remained elusive and is lacking in situ observations. Herein, a two-step process was developed to synthesize Ag nanowires coated with various thicknesses of a CuO_x layer for the CO₂RR. By employing in situ X-ray absorption spectroscopy, a strong correlation between the chemical state under reaction conditions and the CO₂RR product profile can be revealed to validate another competing reaction (i.e., the spontaneous oxidation of Cu(0) in aqueous electrolyte) that significantly governs the chemical state of active centers of Cu. In situ Raman spectroscopy reveals the existence of reoxidation behavior under cathodic potential, and the quantification analysis of reoxidized behavior is revealed to indicate that the reoxidation rate is independent of surface morphology and strongly proportional to the electrochemically surface area. The steady oxidation state of Cu in an in situ condition is the paramount key and dominates the products' profile of the CO₂RR rather than other factors (e.g., crystal facets, atomic arrangements, morphology, elements) that have been investigated in numerous reports.