Lowering the Coordination of Octahedra in Spinel Oxides by the Robust Fe–N Bonds for Enhancing Oxygen Evolution Reaction

The coordination environment of octahedra in spinel oxides plays a vital role in regulating the OER performance. But selectively engineering the octahedral units is very challenging for the design of low-cost and high-efficient electrocatalysts. In this work, the low Fe coordinated CoFe2O4 (CoFe2O4–Cd) is successfully obtained by taking advantage of the different bond strength between metal centers and N atoms in 7,7,8,8-tetracyanoquinodimethane-decorated CoFe-MOF-74 (CoFe-MOF-74/TCNQ) and shows promising physical and chemical properties. DFT calculations reveal that the low-coordinated Fe improves the electrochemical symmetry of catalysts and reduces the free energy barrier of the potential-determining step, resulting in the high activity and stability of CoFe2O4–Cd. Systematic experimental and theoretical analyses demonstrate that TCNQ motivates the more electron-occupied states at the Fermi level of CoFe-MOF-74 and enhances the hybridization of Co 3d, Fe 3d, and N 2p, leading to the formation of the Co/Fe–N bond. Owing to the stronger Fe–N bonds, CoFe-MOF-74/TCNQ realizes the step-by-step phase transformation upon electrochemical activation, achieving the CoFe2O4–Cd. This work advances the current comprehension of the controllable regulation of coordination environment of octahedra in spinel oxides, which in turn contributes to the design of high-performance electrocatalysts for OER.