In situ encapsulation of metal sulfide into hierarchical nanostructured electrospun nanofibers as self-supported electrodes for flexible quasi-solid-state supercapacitors

Flexible freestanding electrodes, with high electrochemical performance and long cycle stability, are the key units essential to realizing flexible solid-state supercapacitors and fulfilling the demand for portable electronic equipment. Herein, we report hierarchical "tube-on-fiber" nanostructures synthesized via melamine-assisted calcination and vulcanization processes on electrospun fibers, composed of zeolitic imidazolate framework-67 and polyacrylonitrile. The hierarchical nanostructure is devised to prevent the agglomeration of Co-mixed cobalt sulfide nanoparticles, and the in situ growth of Co-catalyzed carbon nanotubes (CNTs) on carbon nanofibers is extremely beneficial to improving electronic conductivity. Based on these advantages, the hierarchical nanostructured composite of carbon-encapsulated metal sulfide (Co-S@CNF-CNT-3) delivers a large mass-specific capacitance of 416.5 F g(-1) at 0.2 A g(-1). In addition, a quasi-solid-state asymmetric supercapacitor with Co-S@CNF-CNT-3 as the cathode and C@CNF-CNT-3 as the anode (defined as Co-S@CNF-CNT-3//C@CNF-CNT-3) is assembled. It shows a high energy density of 10.3 Wh kg(-1) with a power density of 320 W kg(-1) at 0.4 A g(-1) or a superior power density of 8000 W kg(-1) with an energy density of 7.56 Wh kg(-1) at 10 A g(-1). Furthermore, the assembled hybrid supercapacitor shows an outstanding capacitance retention of 96.9% after 10 000 cycles at 5 A g(-1) and good mechanical flexibility, illustrating its promising potential for practical flexible supercapacitors.