Significant Change of Metal Cations in Geometric Sites by Magnetic‐Field Annealing FeCo<sub>2</sub>O<sub>4</sub> for Enhanced Oxygen Catalytic Activity
Zhengmei Zhang, Junfu Li, Jinmei Qian, Zhiwei Li, Lei Jia, Daqiang Gao, Desheng Xue
Abstract
Abstract 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 FeCo 2 O 4 nanofibers at a magnetic field of 2500 Oe, named FeCo 2 O 4 ‐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 FeCo 2 O 4 ‐M shows the spin‐state transition of cations from a low‐spin state to an intermediate‐spin state compared with FeCo 2 O 4 . Mössbauer spectra show that the Fe 3+ ion in the octahedral site (0.76) of FeCo 2 O 4 ‐M is more than that of FeCo 2 O 4 (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 FeCo 2 O 4 ‐M nanofibers shift close to the Fermi level, revealing the enhanced mechanism of the ORR/OER activity.