Lawn-like FeCo2S4 hollow nanoneedle arrays on flexible carbon nanofiber film as binder-free electrodes for high-performance asymmetric pseudocapacitors

Herein, we report the rational design and synthesis of a novel flexible supercapacitor electrode consisting of lawn-like FeCo₂S₄ hollow nanoneedle arrays (FeCo₂S₄ HNNA) on electrospun carbon nanofiber (CNF) film via a two-step sulfidation method. In this work, the macroporous CNF film served as flexible/conducting substrate not only promotes the homogeneous distribution of FeCo₂S₄ hollow nanoneedles, but also enhance the electrical conductivity of the composite film. Especially, the tip-welded FeCo₂S₄ nanoneedles with unique interior channels and perpendicular orientation possess numerous accessible electroactive sites and short electrolyte transfer paths for efficient energy storage, and also buffer the drastic volume change during the repetitive charge-discharge process. As a result, prepared FeCo₂S₄ HNNA/CNF composite electrode exhibits a high specific capacitance of 2476 F g⁻¹ at 1 A g⁻¹, and excellent capacitance retention of 93% after 5000 cycles, which is competitive among those previously reported ternary metal sulfide-based electrodes. Asymmetric supercapacitor device fabricated using FeCo₂S₄ HNNA/CNF as positive electrode also delivers a high energy density of 88.5 Wh kg⁻¹ at a power density of 800 W kg⁻¹, as well as a superior cycling stability of 81.2% capacity retention after 5000 cycles.