Activating the Lattice Oxygen Mechanism in NiCo2O4 through Fe Single-Atom Engineering for the Oxygen Evolution Reaction

NiCo₂O₄ is a promising oxygen evolution reaction (OER) electrocatalyst in alkaline media, owing to its mixed-valence Ni and Co cations that confer high activity and durability, exemplifying the potential of bimetallic spinel oxides for sustainable hydrogen production via water electrolysis. However, conventional spinel oxides typically follow the adsorbate evolution mechanism (AEM), in which three intermediates (*OH, *O, and *OOH) form sequentially. The adsorption energies of *OH and *OOH are constrained by a linear scaling relation with a fixed offset of approximately 3.2 eV, imposing a theoretical minimum overpotential of ~370 mV. To overcome this limitation, this study aims to shift the reaction pathway toward the lattice oxygen mechanism (LOM), which bypasses the scaling relation by involving lattice oxygen in O–O bond formation. To this end, Fe single-atom catalysts (SACs) were incorporated into NiCo₂O₄. The atomically dispersed Fe sites modulate the local electronic structure, enhancing metal 3d–oxygen 2p orbital hybridization and thus M–O bond covalency, which facilitates lattice oxygen activation. Their unsaturated coordination further promotes oxygen vacancy formation and lattice oxygen mobility, synergistically lowering the reaction energy barrier and accelerating OER kinetics. LOM activation was confirmed by operando isotope-labeling and electrochemical measurements in a TMAOH electrolyte, which revealed lattice oxygen participation, while complementary structural and spectroscopic analyses corroborated changes in coordination geometry and oxidation states upon SAC incorporation. Consequently, the SAC-modified spinel exhibits enhanced OER activity and robust electrochemical stability, demonstrating atomic-level catalyst tuning as a platform strategy for next-generation OER electrocatalysts with improved kinetics and long-term durability.