(360ar) Identifying the Stoichiometry of the Metastable Cu3+ State in Alkaline Electrochemical Systems Using DFT-Based Theoretical Raman Standards

While normally used for CO₂ reduction, copper electrocatalysts have recently shown promise for biomass upgrading¹ and the oxygen evolution reaction (OER), with overpotentials comparable to the state-of-the-art OER catalysts² 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.³ 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 this CuOOH species as d⁹L Cu²⁺ (where dⁿ denotes the n number of electrons in the d orbital of copper and L denotes an electron hole in the ligand, oxygen) by Bader charge and density of state analysis. This is similar to other formal Cu³⁺ species, where electron density is removed from the bonding p-orbital due to inverted ligand behavior.⁴ We also performed calculations of hydroxide adsorption on a CuO(111) facet and provided an explanation for the background oxidation current between the Cu²⁺ and formal Cu³⁺ states. There are two unique copper atoms in the experimentally-verified CuO crystal structure with one of these unique atoms being more accessible to solution species. The hydroxide species prefer adsorption on the bridge site between the surface-exposed copper atoms (E_ads ~0.62 eV). However, there is a surface-coverage dependency that will increase the E_ads up to 1.92 eV for total site coverage. We suggest the background oxidation current between the two formal oxidation states is due to continuous partial oxidation of Cu atoms by hydroxide adsorption, which requires increased applied potentials as surface coverage increases. We hope the identification of the metastable Cu³⁺ species and other atomic insight leads to further rational design of copper electrocatalysts.

(1) Ostervold, L.; Perez Bakovic, S. I.; Hestekin, J.; Greenlee, L. F. Electrochemical biomass upgrading: degradation of glucose to lactic acid on a copper(ii) electrode. RSC Advances 2021, 11 (50), 31208-31218, 10.1039/D1RA06737K. DOI: 10.1039/D1RA06737K.

(2) Roger, I.; Shipman, M. A.; Symes, M. D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nature Reviews Chemistry 2017, 1 (1). DOI: 10.1038/s41570-016-0003.

(3) Deng, Y.; Handoko, A. D.; Du, Y.; Xi, S.; Yeo, B. S. In situ Raman spectroscopy of copper and copper oxide surfaces during electrochemical oxygen evolution reaction: identification of CuIII oxides as catalytically active species. Acs Catalysis 2016, 6 (4), 2473-2481.

(4) Steen, J. S.; Knizia, G.; Klein, J. sigma-Noninnocence: Masked Phenyl-Cation Transfer at Formal Ni-IV. Angewandte Chemie-International Edition 2019, 58 (37), 13133-13139. DOI: 10.1002/anie.201906658. DiMucci, I. M.; Lukens, J. T.; Chatterjee, S.; Carsch, K. M.; Titus, C. J.; Lee, S. J.; Nordlund, D.; Betley, T. A.; MacMillan, S. N.; Lancaster, K. M. The myth of d8 copper (III). Journal of the American Chemical Society 2019, 141 (46), 18508-18520.