Heterojunction engineering of Fe/Fe₃C@NC: enabling kilohour-scale operation in high-energy-density Zn-air batteries

Efficient and durable bifunctional oxygen electrocatalysts represent a cornerstone for high-energy-density rechargeable metal-air batteries. Herein, we report a facile one-step pyrolysis synthesis of nitrogen-doped graphite carbon-encapsulated Fe/Fe₃C heterojunction nanoparticles (Fe/Fe₃C@NC) with a well-defined core-shell architecture. The Fe/Fe₃C heterojunction exhibits prominent interfacial electron transfer from Fe to Fe₃C. This electron-rich Fe₃C phase enables superior adsorption modulation toward oxygen intermediates, thereby endowing the catalyst with exceptional bifunctional oxygen electrocatalytic activity. In alkaline media, Fe/Fe₃C@NC outperforms platinum-group metal (PGM) benchmarks in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), achieving a smaller OER/ORR overpotential difference than Pt/C + RuO₂. When integrated into aqueous zinc-air batteries, Fe/Fe₃C@NC maintains an impressive 92.6% of its initial round-trip efficiency (RTE) over 1900 h (5700+ cycles), far exceeding the PGM-based control (23.3% RTE decay within 50 h, 150 cycles). Structural characterizations confirm that Fe/Fe₃C@NC retains its morphology and crystallinity after prolonged cycling, highlighting the critical role of the core-shell architecture in preserving structural/compositional integrity. This work not only elucidates the pivotal role of the Fe/Fe₃C heterojunctions in enhancing bifunctional oxygen electrocatalysis but also provides a scalable carbon-encapsulation strategy for designing low-cost, high-performance, and long-lifespan non-noble metal bifunctional catalysts for next-generation metal-air batteries.