Hetero-nanoparticles encapsulated in nitrogen-doped carbon show promising potential application in oxygen reduction reaction (ORR). Herein, a covalent-organic frameworks (COFs) based composite with FeN species and Fe/Fe₃C nanoparticles embedded in N-doped carbon nanosheets (Fe/Fe₃C@FeN-Cs) has been successfully synthesized by an in-situ reduction of pre-designed Fe-N₂-O₂ units. Benefit from the compositional and structural synergistic effect, the optimized Fe/Fe₃C@FeN-C-900 displays excellent ORR activity in both acidic (E_1/2 = 0.77 V) and alkaline (E_1/2 = 0.88 V) electrolytes, which are comparable and even superior to the benchmark Pt/C. Impressively, the maximum power density of fuel cell using Fe/Fe₃C@FeN-C-900 as cathode catalyst reaches 408 mW cm^-2. Moreover, the Fe/Fe₃C@FeN-C-900-drived rechargeable zinc-air batteries (ZABs) deliver a higher power density (156.8 mW cm^-2), larger specific capacity (775 mA h g^-1) and better cycling stability (270 cycles) as compared with the state-of-the-art Pt/C counterpart. The interfacial electron transfer between Fe/Fe₃C nanoparticles and neighboring FeN species, as well as the porous carbon architecture contribute to the excellent ORR performance of Fe/Fe₃C@FeN-C-900, resulting in the easily adsorption/activation of O₂ and accelerated ORR kinetics because of the fast mass transfer. The COF-derived synthetic strategy inspires new perspectives to develop hetero-nanoparticle composites as high-efficiency ORR electrocatalysts for the economical-practical energy conversion devices.