Surface-optimized carbon nanocages with tailorable atomic Fe-N₄ sites to boost oxygen reduction in long stable zinc-air battery

High-efficiency and durable precious-metals-free electrocatalysts for oxygen reduction (ORR) are crucial to Zn-air battery (ZAB). Herein, a molten salt-assisted pyrolysis is reported to construct fully open carbon nanocages with highly atomically dispersed Fe-N₄ sites (Fe-SA/NC) as air cathode. The chemical etching in pyrolysis can induce such highly active surface structure with abundant surface Fe anchoring sites and regulate the uncoordinated pyridinic nitrogen. The theoretical investigations reveal that engineering the adjacent uncoordinated pyridinic nitrogen can efficiently modulate the charge distribution and lower D-band center position of FeN₄, thus reducing the dissociation energy of intermediate OH*. The Fe-SA/NC exhibit outstanding catalytic activity with 0.905 V (vs. RHE) half-wave potential and excellent cycling stability, surpassing commercial Pt/C and most reported M-N-C catalysts. The ZAB with the Fe-SA/NC as air cathode delivers a high power density of 240 mW cm⁻², large specific capacity (815 mAh g⁻¹) and long-term durability over 1050 h. This work highlights the rational design and fabrication of efficient M-N-C single-atom catalysts from the point of view of both morphological engineering and local coordination chemistry.