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 (εd ) 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.
Keywords: FeCo 2O 4 nanofibers; Zn-air batteries; d-band center; magnetic field application.
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