Mechanisms of the Oxygen Evolution Reaction on NiFe₂O₄ and CoFe₂O₄ Inverse-Spinel Oxides

Spinel ferrites, especially Nickel ferrite, NiFe₂O₄, and Cobalt ferrite, CoFe₂O₄, are efficient and promising anode catalyst materials in the field of electrochemical water splitting. Using density functional theory, we extensively investigate and quantitatively model the mechanism and energetics of the oxygen evolution reaction (OER) on the (001) facets of their inverse-spinel structure, thought as the most abundant orientations under reaction conditions. We catalogue a wide set of intermediates and mechanistic pathways, including the lattice oxygen mechanism (LOM) and adsorbate evolution mechanism (AEM), along with critical (rate-determining) O-O bond formation barriers and transition-state structures. In the case of NiFe₂O₄, we predict a Fe-site-assisted LOM pathway as the preferred OER mechanism, with a barrier (ΔG ^⧧) of 0.84 eV at U = 1.63 V versus SHE and a turnover frequency (TOF) of 0.26 s^-1 at 0.40 V overpotential. In the case of CoFe₂O₄, we find that a Fe-site-assisted LOM pathway (ΔG ^⧧ = 0.79 eV at U = 1.63 V vs SHE, TOF = 1.81 s^-1 at 0.40 V overpotential) and a Co-site-assisted AEM pathway (ΔG ^⧧ = 0.79 eV at bias > U = 1.34 V vs SHE, TOF = 1.81 s^-1 at bias >1.34 V) could both play a role, suggesting a coexistence of active sites, in keeping with experimental observations. The computationally predicted turnover frequencies exhibit a fair agreement with experimentally reported data and suggest CoFe₂O₄ as a more promising OER catalyst than NiFe₂O ₄ in the pristine case, especially for the Co-site-assisted OER pathway, and may offer a basis for further progress and optimization.