Efficient bifunctional oxygen electrocatalysts for ORR and OER are fundamental to the development of high performance metal-air batteries. Herein, a facile cost-efficient two-step pyrolysis strategy for the fabrication of a bifunctional oxygen electrocatalyst has been proposed. The efficient non-precious-metal-based electrocatalyst, Fe/Fe3C@Fe-Nx-C consists of highly curved onion-like carbon shells that encapsulate Fe/Fe3C nanoparticles, distributed on an extensively porous graphitic carbon aerogel. The obtained Fe/Fe3C@Fe-Nx-C aerogel exhibited superb electrochemical activity, excellent durability, and high methanol tolerance. The experimental results indicated that the assembly of onion-like carbon shells with encapsulated Fe/Fe3C yielded highly curved carbon surfaces with abundant Fe-Nx active sites, a porous structure, and enhanced electrocatalytic activity towards ORR and OER, hence displaying promising potential for application as an air cathode in rechargeable Zn-air batteries. The constructed Zn-air battery possessed an exceptional peak power density of ~147 mW cm−2, outstanding cycling stability (200 cycles, 1 h per cycle), and a small voltage gap of 0.87 V. This study offers valuable insights regarding the construction of low-cost and highly active bifunctional oxygen electrocatalysts for efficient air batteries.