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.

Automated summary

Flexible freestanding electrodes with hierarchical tube-on-fiber nanostructures were successfully synthesized for flexible solid-state supercapacitors, demonstrating high electrochemical performance and long cycle stability. The assembled hybrid supercapacitor showed promising potential for practical applications with high energy density, superior power density, outstanding capacitance retention after 10,000 cycles, and good mechanical flexibility.