Atomic Fe-N4 /C in flexible carbon fiber membrane as binder-free air cathode for Zn-air batteries with stable cycling over 1000 hours.

Noble-metal-free, durable and high-efficiency electrocatalysts for oxygen reduction and evolution reaction (ORR/OER) are vital for rechargeable Zn-air batteries (ZABs). Herein, a flexible and free-standing carbon fiber membrane immobilized with atomically dispersed Fe-N4 /C catalysts (Fe/SNCFs-NH3 ) is synthesized and used as air cathode for ZABs. The intertwined carbon fibers with hierarchical nanopores facilitate the gas transportation, electrolyte infiltration and electron transfer. The larger specific surface area after NH3 activation exposes a high concentration of Fe-N4 /C sites that embedded in the carbon matrix. Modulation of local atomic configurations by sulfur doping in Fe/SNCFs-NH3 catalyst leads to excellent ORR and enhanced OER activities. As-synthesized Fe/SNCFs-NH3 catalyst demonstrates a positive half-wave potential of 0.89 V and a small Tafel slope of 70.82 mV dec-1 , outperforming the commercial Pt/C (0.86 V/94.74 mV dec-1 ) and most reported M-Nx /C (M = Fe, Co, Ni) catalysts. Experimental characterizations and theoretical calculations uncover the crucial role of S doping in regulating ORR and OER activity. The liquid-state ZABs with Fe/SNCFs-NH3 catalyst as air cathode deliver a large peak power density of 255.84 mW cm-2 and a long-term cycle durability over 1000 h. Solid-state ZAB shows stable cycling at various flat/bent/flat states, demonstrating great prospects in portable and wearable device applications. This article is protected by copyright. All rights reserved.