Abstract
Fe-based materials are famous electrocatalysts in oxygen reduction reaction (ORR), but their selectivity towards four- or two-electron ORR still requires further research in detail. Herein, two Fe-based ORR electrocatalysts are synthesized from MIL-101, a metal–organic framework (MOF). After calcined with dicyandiamide (DCDA) under N2 protection, Fe3O4@NCNTs (NCNTs = N-doped carbon nanotubes) is synthesized, in which small Fe3O4 particles reside in NCNTs homogeneously. It is a typical four-electron electrocatalyst in both alkaline and neutral electrolyte. Without DCDA, Fe2O3 forms and it acts as two-electron electrocatalyst in ORR. Theoretical calculation suggests the discrepancy in ORR selectivity between Fe3O4@NCNTs and Fe2O3 roots from their different interactions with OOH*. If Fe3O4@NCNTs serves as cathode material, peak power density of Zn-air battery reaches 125.1 mW·cm−2 in alkaline electrolyte. When discharge at 10 mA·cm−2, the specific capacitance and energy density reach 799.7 mAh·g−1 and 839.7 Wh·kg−1. The performance of Fe3O4@NCNTs is also well kept in neutral Zn-air battery. We expect this work elucidates the effect of structure on ORR selectivity for Fe-based electrocatalysts.
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