Active non-bonding oxygen mediate lattice oxygen oxidation on NiFe2O4 achieving efficient and stable water oxidation

The oxygen evolution reaction (OER) serves as a fundamental half–reaction in the electrolysis of water for hydrogen production, which is restricted by the sluggish OER reaction kinetics and unable to be practically applied. The traditional lattice oxygen oxidation mechanism (LOM) offers an advantageous route by circumventing the formation of M-OOH* in the adsorption evolution mechanism (AEM), thus enhancing the reaction kinetics of the OER but resulting in possible structural destabilization due to the decreased M–O bond order. Fortunately, the asymmetry of tetrahedral and octahedral sites in transition metal spinel oxides permits the existence of non-bonding oxygen, which could be activated by rational band structure design for direct O-O coupling, where the M–O bond maintains its initial bond order. Here, non-bonding oxygen was introduced into NiFe₂O₄ via annealing in an oxygen-deficient atmosphere. Then, in-situ grown sulfate species on octahedral nickel sites significantly improved the reactivity of the non-bonding oxygen electrons, thereby facilitating the transformation of the redox center from metal to oxygen. LOM based on non-bonding oxygen (LOM_NB) was successfully activated within NiFe₂O₄, exhibiting a low overpotential of 206 mV to achieve a current density of 10 mA cm^–2 and excellent durability of stable operation for over 150 h. Additionally, catalysts featuring varying band structures were synthesized for comparative analysis, and it was found that the reversible redox processes of non-bonding oxygen and the accumulation of non-bonding oxygen species containing 2p holes are critical prerequisites for triggering and sustaining the LOM_NB pathway in transition metal spinel oxides. These findings may provide valuable insights for the future development of spinel-oxide-based LOM catalysts.