Identifying the Local Atomic Environment of the “Cu3+” State in Alkaline Electrochemical Systems

CuO-based catalysts are active for the oxygen evolution reaction (OER), although the active form of copper for the OER is still unknown. We combine operando Raman experiments and density functional theory (DFT) electronic structure calculations to determine the form of Cu(O)_xOH_y present under OER conditions. Raman spectra show a distinct feature related to the active "Cu³⁺" species, which is only present under highly oxidizing conditions. DFT is used to produce theoretical Raman standards and match the unique Raman feature of copper under OER potentials. This method identifies a range of Cu³⁺-containing compounds which match the distinct Raman feature. We then integrate experimental electrochemistry to progressively eliminate possible structures and determine the stoichiometry of the active form as CuOOH, which likely takes the form of a surface-adsorbed hydroxide on a CuO surface. Bader charge analysis, site-projected wavefunctions, and density of states analysis show that electron density is removed from O 2p orbitals upon hydroxide adsorption, suggesting that the electronic structure exhibits d⁹L Cu²⁺ behavior rather than the local electronic structure of a formal Cu³⁺.