Oxygen Evolution Reaction of Amorphous/Crystalline Composites of NiFe(OH)x/NiFe2O4

Orbital structures are strongly correlated with catalytic performance, whereas their regulation strategy is still in pursuit. Herein, the Fe 3d and O 2p orbital hybridization was optimized by controlling the content of amorphous NiFe(OH)_x (a-NiFe(OH)_x), which was grown in situ on crystalline NiFe₂O₄ (c-NiFe₂O₄) using an ultrasonic reduction method. The results of electron energy loss spectroscopy (EELS) and X-ray absorption spectra (XAS) revealed that the Fe–O_a orbital hybridization in a-NiFe(OH)_x is effectively strengthened by jointing with the adjacent oxygen (O_c) in c-NiFe₂O₄, which is further confirmed by the higher antibonding orbital energies based on density functional theory (DFT) calculations. The resultant O_a–Fe–O_c at the composite interface leads to balanced adsorption and desorption energies. Accordingly, the optimal composite with strong Fe 3d-O 2p hybridization results in enhanced OER performance, and the overpotential is 150 mV, lower than that of the pristine sample. This work represents a promising approach to orbital hybridization via the introduction of an amorphous phase to construct highly efficient catalysts.