N, F and S doped carbon nanofibers generated from electrospun polymerized ionic liquids for metal-free bifunctional oxygen electrocatalysis

The rational development of promising oxygen electrocatalysts with remarkable electrocatalytic activity, high stability and low cost is highly important for wide-scale application of sustainable energy technologies. Here, a novel free-standing metal-free carbon-based bifunctional electrocatalyst (NFS-CNF) with uniform multiple heteroatom (N, F and S) doping and large specific surface area (1450.7 m² g⁻¹), was firstly synthesized by direct pyrolysis of electrospun polyacrylonitrile (PAN)/polymerized ionic liquid (PIL) nanofibers. The as-prepared NFS-CNF presents superior oxygen catalytic performance as well as excellent durability, exhibiting a positive half-wave potential (0.91 V) for oxygen reduction reaction (ORR) and a low overpotential (380 mV at 10 mA cm⁻²) for oxygen evolution reaction (OER) in alkaline medium. Notably, the outstanding bifunctional ORR/OER activity (ΔE=0.70 V) endows remarkable Zn-air battery performance with a peak power density of 127.5 mW cm⁻², a high specific capacity of 826.4 mAh g_Zn⁻¹ (corresponding to an energy density of ~991 Wh kg_Zn⁻¹), a stable cyclability after 600 cycles at 10 mA cm⁻² with a voltage gap increase as small as 0.1 V. Density functional theory (DFT) calculations confirm that the N, F, S tri-doping contributes to the improvement in the electrochemical properties of our material. Our work firstly experimentally demonstrates a simple pathway of combining PIL precursor and electrospinning technology to achieve multiple heteroatom doping in metal-free nanofibrous bifunctional oxygen electrocatalysts with greatly enhanced electrochemical activity and durability. Additionally, the realization of efficient rechargeable Zn-air batteries suggests the promising future of our NFS-CNF pieces as robust free-standing air cathodes in various sustainable energy applications.