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 (?G+) 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 (?G+ = 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 (?G+ = 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 NiFe2O4 in the pristine case, especially for the Co-site-assisted OER pathway, and may offer a basis for further progress and optimization.