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-C-d) 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-C-d. 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-C-d. 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.