In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of Cu^III Oxides as Catalytically Active Species

Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, in situ Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu₂O, Cu­(OH)₂, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu­(OH)₂ during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm^–1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a Cu^III oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCu^IIIO₂ standard, we suggest that the chemical composition of the Cu^III oxide is CuO₂^–-like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurements at 1.7 V vs RHE. Cu^III oxide was observed only on the CuO and Cu­(OH)₂ electrodes. Interestingly, these two electrodes catalyzed the OER ∼10 times more efficiently than the Cu and Cu₂O catalysts. By correlating the intensity of the Raman band of Cu^III oxide and the extent of the OER activity, we propose that Cu^III species provides catalytically active sites for the electrochemical water oxidation. The formation of Cu^III oxides on CuO films during OER was also corroborated by in situ XANES measurements of the Cu K-edge. The catalytic role of Cu^III oxide in the O₂ evolution reaction is proposed and discussed.