B, N, F tri-doped lignin-derived carbon nanofibers as an efficient metal-free bifunctional electrocatalyst for ORR and OER in rechargeable liquid/solid-state Zn-air batteries

• The B, N, F tri-doped lignin-derived carbon nanofibers (BNF-LCFs) have been prepared by electrospinning and pyrolysis. • The specific synergistic effects between B, N and F dopants are essential for the super performance of the BNF-LCF catalyst. • The BNF-LCF catalyst exhibits superior ORR and OER bifunctional catalytic performance. • The BNF-LCF catalyst shows great potential in liquid/solid-state Zn-air batteries. The exploitation of cost-effective and high-efficiency bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is vital for the development of rechargeable metal-air batteries. Herein, B, N and F tri-doped lignin-based carbon porous nanofibers (BNF-LCFs) were prepared by electrospinning and pyrolysis without extra post-treatment using biomass lignin as carbon precursor, PVP as spinning additive, zinc borate as boron source, ammonium fluoride as fluorine source and partial nitrogen source. This method is simple, efficient, and environmentally friendly. Benefiting from the synergistic effect of B, N and F heteroatoms, large specific surface area and abundant defect sites, the BNF-LCF catalyst exhibits impressive bifunctional electrocatalytic performance towards ORR and OER with a small potential gap (ΔE) of 0.728 V. It outperforms the commercial Pt/C + RuO 2 and most recently-reported non-metal carbon-based electrocatalysts. The liquid Zn-air batteries (ZABs) assembled with BNF-LCFs present a high open circuit potential of 1.536 V, a large specific capacity of 791.5 mAh g −1 and satisfactory cycling stability, superior to Pt + RuO 2 -based ZABs. Furthermore, the solid-state ZABs assembled with BNF-LCFs not only deliver excellent electrochemical performance, but also exhibit admirable mechanical flexibility and cycling stability, indicating favorable application prospects in flexible and wearable electronic devices.