ZnS-assisted evolution of N,S-doped hierarchical porous carbon nanofiber membrane with highly exposed Fe-N_(4)/C_(x) sites for rechargeable Zn-air battery

Binder-free bifunctional electrocatalysts are attractive for rechargeable Zn-air batteries(ZABs)in gridscale 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-N_(4)/Cxsites 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-N_(4)/Cxsites 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-N_(4)/Cxsites(23.53μmol 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 m A cm^(-2)for oxygen evolution reaction.When used as the air cathode catalyst for ZABs,it delivers a high specific capacity of 699 m Ah g^(-1)at 5 m A cm^(-2),a large peak power density of 228 m W cm^(-2)and a prolonged cycling over 1000 h.At 10 m A 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.