(427h) Identifying the Stoichiometry of the Metastable Cu3+ State in Alkaline Electrochemical Systems

While normally used for CO₂ reduction, copper electrocatalysts have recently shown promise for biomass upgrading and the oxygen evolution reaction (OER), with overpotentials comparable to the state-of-the-art OER catalysts at 10 mA-cm^-2. However, likely due to the comparative lack in popularity for copper OER catalysts, the active site for the OER on copper is unknown, hindering further fundamental studies. The demonstrated low overpotentials for CuO-based materials combined with the low cost and high availability of copper justify further investigation into copper OER catalysts. A study by Deng et al. linked a distinct Raman signal to the active site for copper-based OER but did not provide further insight into the chemical nature of the active site. The metastability of this species makes investigations difficult, but here we solve the problem by using theoretical Raman standards based on density functional theory (DFT). First, we performed operando Raman during linear sweep voltammetry (LSV) to note a unique peak corresponding to a formal Cu³⁺ species ca. 587 cm^-1. We then screened for possible components by simulating the Raman spectra of ~30 structures with formal copper charges ≥2+. We further narrowed the possibilities by performing additional operando Raman experiments along with a series of cyclic voltammetry studies - providing a compelling case for a CuOOH species. After identifying the CuOOH species, we performed DFT calculations to determine the electronic structure of the copper (d⁹L Cu²⁺). We also provide an explanation for the background oxidation current between the Cu²⁺ and formal Cu³⁺ states as continued hydroxide adsorption due to an increasing adsorption barrier as surface-coverage increases. We hope the identification of the metastable Cu³⁺ species leads to further rational design of copper electrocatalysts.