ZnS-assisted evolution of N,S-doped hierarchical porous carbon nanofiber membrane with highly exposed Fe-N4/C sites for rechargeable Zn-air battery

Binder-free bifunctional electrocatalysts are attractive for rechargeable Zn-air batteries (ZABs) in grid-scale energy storage and flexible electronics, but suffering from the sluggish mass transport and inadequate catalytic capability. Herein, we propose a scalable approach of in-situ engineering highly exposed Fe-N4/Cx sites on the N,S-doped porous carbon nanofiber membrane as a binder-free air electrode catalyst for ZABs. ZnS nanospheres are firstly used as integrated structure-directing agents to facilitate the electronic modulation of Fe-N4/Cx sites by S doping and construct the hierarchical macro/meso/micropores at high temperature. Neither additional step for removal of ZnS nanospheres nor doping process is required, significantly simplifying the pore formation process and improving the S doping efficiency. Benefiting from the enhanced intrinsic activity of high-density Fe-N4/Cx sites (23.53 μmol g−1) and the optimized mass transport of carbon nanofibers, as-synthesized electrocatalyst shows a positive half-wave potential of 0.89 V for oxygen reduction reaction and a small overpotential of 0.47 V at 10 mA cm−2 for oxygen evolution reaction. When used as the air cathode catalyst for ZABs, it delivers a high specific capacity of 699 mAh g−1 at 5 mA cm−2, a large peak power density of 228 mW cm−2 and a prolonged cycling over 1000 h. At 10 mA cm−2, a robust structure with atomically dispersed Fe is also remained after cycling for 420 h. Due to the flexible properties of the electrocatalyst, as-assembled quasi-solid-state ZAB shows stable cycling over 90 h at alternately flat/bent states, demonstrating great prospects in flexible electronic device applications.