Quenching-Induced Octahedral Fe Vacancies in CoFe Spinel Nanosheets for Boosted Oxygen Evolution

Introducing cation vacancies into transition-metal oxides has emerged as an effective strategy to enhance oxygen evolution reaction (OER) activity, yet most approaches rely on complex synthesis or chemical additives, and establishing clear vacancy-structure-activity relationships remains challenging. Here, we report a facile liquid-nitrogen quenching strategy to introduce Fe vacancies into CoFe spinel nanosheets (V_Fe-CFO). X-ray absorption spectroscopy (XAS) and pair distribution function (PDF) analyses reveal the preferential formation of Fe vacancies at octahedral sites and their influence on local structural rearrangements among neighboring metal centers. The optimized V_Fe-CFO exhibits markedly enhanced OER performance, delivering 10 mA cm^-2 at an overpotential of 244 mV and maintaining excellent durability over 330 h under dynamic current densities ranging from 10 to 400 mA cm^-2. In situ Raman spectroscopy combined with density functional theory (DFT) calculations provides insight into how the operando-formed CoFe oxyhydroxide layer retains Fe-vacancy-related structural features, which are associated with a reduced energy barrier for the *O → *OOH step along the adsorbate evolution mechanism (AEM). This work highlights the importance of cation-vacancy engineering in spinel oxides and provides a practical strategy for improving their OER performance.