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[en] Some rcent advances of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) have enabled the chemical composition and the electrical potential profile at a liquid/electrode interface under electrochemical reaction conditions to be directly probed. In this work, we apply this operando technique to study the surface chemical composition evolution on a Co metal electrode in 0.1 M KOH aqueous solution under various electrical biases. It is found that an ~12.2 nm-thick layer of Co(OH)₂ forms at a potential of about -0.4 V_Ag/AgCl, and upon increasing the anodic potential to about +0.4 V_Ag/AgCl, this layer is partially oxidized into cobalt oxyhydroxide (CoOOH). A CoOOH/Co(OH)₂ mixture layer is formed on the top of the electrode surface. Finally, the oxidized surface layer can be reduced to Co0 at a cathodic potential of -1.35 VAg/Cl. Our observations indicate that the ultrathin layer containing cobalt oxyhydroxide is the active phase for oxygen evolution reaction (OER) on a Co electrode in an alkaline electrolyte, consistent with previous studies.

[en] The oxygen K-edge absorption spectra (XAS) of aqueous chloride solutions are measured for Li⁺, Na⁺, K⁺, NH⁺, C(NH2)₃⁺, Mg²⁺ and Ca²⁺ and 4 M cation concentrations. Density functional theory calculation have indicated that the ion-specific spectral variations arise from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, as a result of differences in charge transfer from the water molecules onto the divalent cations

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