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

Water electrolysis in the form of Anion Exchange Membrane Water Electrolysis is a well-established method to produce hydrogen from the renewable energy sources, whose implementation at the global level is hindered by the sluggish anodic oxygen evolution reaction (OER). In the search of efficient and non-critical-raw-material OER catalysts, spinel oxides are promising, but experiments on catalytic performance and stability are scattered over a wide range depending on protocols of materials preparation and treatment. To overcome this impasse and provide fundamental knowledge in the field, we conduct an extensive DFT investigation of OER energetics and reaction paths over the (001) facets of two spinel oxides: Nickel ferrite, NiFe₂O₄, and Cobalt ferrite, CoFe₂O₄. By cataloguing a wide set of intermediates and mechanistic pathways, including lattice oxygen mechanism (LOM) and adsorbate evolution mechanism (AEM), along with the rate-determining barriers  and transition-state structures, we computationally predict turnover frequencies (TOFs) in fair agreement with experimentally reported data, and suggest CoFe₂O₄ as a more promising OER catalyst than NiFe₂O₄ in the pristine case, a finding that may offer a basis for further progress and optimization. Then, by including other low-index facets [(110) and (111)], we also predict nanoparticle reshaping under diverse environment and operating conditions, as well as discuss the possibility and challenges of operando monitoring of these promising OER catalysts.