Fe3O4/Fe2O3/Fe nanoparticles anchored on N-doped hierarchically porous carbon nanospheres as a high-efficiency ORR electrocatalyst for rechargeable Zn–air batteries

N-doped hierarchical porous carbon nanospheres loaded with Fe₃O₄/Fe₂O₃/Fe nanoparticles are prepared and show excellent ORR activity and durability, based on which the assembled Zn-air battery has outstanding peak power density and specific capacity.