Modeling the Oxygen Evolution Reaction on Metal Oxides: The Infuence of Unrestricted DFT Calculations

Due to the contraints imposed on the optimization of oxygen evolution reaction (OER) catalysts, the correlations between adsorption energies of OER intermediates have received considerable interest. Computation has made important contributions uncovering and elucidating these correlations. The calculations have predominantly been of the spin-restricted type. Using both the restricted and unrestricted formalism, we have performed a DFT-PBE study of the key OER intermediates on 10 nonmagnetic metal oxide surfaces. We show that, depending on the binding energy, unrestricted DFT calculations may yield considerably higher binding energies than restricted DFT calculations. At low binding energies, spin quenching of the adsorbates is inefficient, leading to a strong open shell character that is not well described by restricted DFT. At high binding energies, spin quenching is complete, so that the unrestricted DFT binding energy equals the restricted DFT binding energy. We conclude therefore that although strong binding systems can be properly described by restricted DFT, unrestricted DFT is a necessity for weak binding systems, to which photocatalysts typically belong. The use of unrestricted DFT has little effect on the shape of volcano plots when Delta G(O) - Delta G(OH) is chosen as a descriptor. The location of the data points on the plot does show a considerable shift for weak binding substrates.