(510g) Role of Cu+3 in Oxygen Evolution Activity of Copper-Oxides

Significant progress is currently being made in understanding the behavior of copper oxide (CuO) at positive potentials, ~1.7 V, under electrochemical conditions. While it is assumed to be Cu₂O₃ at this potential with ∆G = 208.5 kJ/mol, there is no experimental evidence. However, our calculations show that bulk CuOOH has ∆G ~210 kJ/mol, indicating Cu⁺³ possibly in the form of bulk CuOOH. To verify this, we employ a combination of density functional theory (DFT) calculations and simulated Raman spectra. This approach allows us to examine the electrocatalytic activity of CuOOH polymorphs, including Cmc21 (β-CuOOH), Pmn21 (γ-CuOOH), and C2/m (δ-CuOOH), for the OER at this potential. Our calculations indicate a presence of stable square planar, non-magnetic γ/δ-CuOOH phase, and the volume Pourbaix diagram highlights γ/δ-CuOOH as the newly discovered active phase governing the OER in alkaline media. This finding is further supported by simulated Raman spectroscopy, revealing distinct features corresponding to Cu³⁺ species at 552 and 573 cm^-1, matching experimental data peak at 587 cm^-1 only present under highly oxidizing conditions. Next, we compare the theoretical overpotentials of all CuOOH surfaces with O and OH-vacancy at the bridge site as the active site, and the surface-adsorbed OH* on a CuO surface. We find that the (021) surface of CuOOH and the (002) surface of CuOOH are the most active, characterized by an overpotential ranging from 0.36V to 0.50 V. Lastly, we demonstrate that the overpotential of the CuOOH (021) surface can be improved further by surface substitution of Cu by Fe, and explain the experimentally observed enhancement in the OER activity of Fe_yCu_1−yOOH thin films. This work expands our knowledge in understanding the active phases governing OER in alkaline media but also paves the way for the development of more efficient electrocatalysts for renewable energy applications.