Significant Change of Metal Cations in Geometric Sites by Magnetic‐Field Annealing FeCo2O4 for Enhanced Oxygen Catalytic Activity

The application of magnetic fields in the oxygen reduction/evolution reaction (ORR/OER) testing for electrocatalysts has attracted increasing interest, but it is difficult to characterize on-site surface reconstruction. Here, a strategy is developed for annealing-treated FeCo₂ O₄ nanofibers at a magnetic field of 2500 Oe, named FeCo₂ O₄ -M, showing a right-shifted half-wave potential of 20 mV for the ORR and a left-shifted overpotential of 60 mV at 10 mV cm^-2 for the OER as compared with its counterpart. Magnetic characterizations indicate that FeCo₂ O₄ -M shows the spin-state transition of cations from a low-spin state to an intermediate-spin state compared with FeCo₂ O₄ . Mössbauer spectra show that the Fe³⁺ ion in the octahedral site (0.76) of FeCo₂ O₄ -M is more than that of FeCo₂ O₄ (0.71), indicating the effective stimulus of metal cations in geometric sites by magnetic-field annealing. Furthermore, theoretical calculations demonstrate that the d-band centers (ε_d ) of Co 3d and Fe 3d in the tetrahedral and octahedral sites of the FeCo₂ O₄ -M nanofibers shift close to the Fermi level, revealing the enhanced mechanism of the ORR/OER activity.