<i>In Situ</i> Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of Cu<sup>III</sup> Oxides as Catalytically Active Species

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