Oxidation State Engineering in Octahedral Ni by Anchored Sulfate to Boost Intrinsic Oxygen Evolution Activity

Promoting the electron occupancy of active sites to unity is an effective method to enhance the oxygen evolution reaction (OER) performance of spinel oxides, but it remains a great challenge. Here, an in situ approach is developed to modify the valence state of octahedral Ni cations in NiFe₂O₄ inverse spinel via surface sulfates (SO₄^2-). Different from previous studies, SO₄^2- is directly anchored on the spinel surface instead of forming from uncontrolled conversion or surface reconstruction. Experiment and theoretical calculations reveal the precise adsorption sites and spatial arrangement for SO₄^2- species. As a main promoting factor, surface SO₄^2- effectively converts the crystal field stable Ni state (t_2g⁶e_g²) to the near-unity e_g electron state (t_2g⁶e_g¹). Moreover, the inevitable oxygen vacancies (V_o) further optimize the energy barrier of the potential-determining step (from OH* to O*). This co-modification strategy enhances turnover frequency-based electrocatalytic activity about two orders higher than the control sample without surface sulfates. This work may provide insight into the OER activity enhancement mechanism by the oxyanion groups.