Integrating V-doped CoP on Ti3C2Tx MXene-incorporated hollow carbon nanofibers as a freestanding positrode and MOF-derived carbon nanotube negatrode for flexible supercapacitors

Novel architectural electrode materials that are freestanding and exhibit improved electrochemical performances in flexible asymmetric supercapacitors are urgently needed; however, designing these materials is challenging. Herein, a Ti3C2TX MXene-aligned hollow carbon fiber (MX/HCF) is engineered and vanadium-doped cobalt phosphide nanorod arrays are grown on it (V-CoP@MX/HCF) and applied for supercapacitor positrode. V doping modulates the surface structure and electronic environment of CoP nanorods grown over conductive and flexible MX/HCFs. As expected, the optimized V-CoP@MX/HCF exhibits a better electrochemical performance (1896.8 F g−1) than that of CoP@MX/HCF and CoP@HCF. For the negative electrode, zeolitic imidazolate framework-67 (ZIF-67) grown on electrospun polyacrylonitrile (PAN) fibers is converted to cobalt nanoparticle-encapsulated nitrogen-doped carbon nanotubes at carbon nanofibers (Co-CNT@CNF) by a simple heat treatment without the burden of external catalysts and reducing gases. The as-designed freestanding Co-CNT@CNF delivers a specific capacitance of 405.5 F g−1 with superior cycling stability. A flexible asymmetric supercapacitor (V-CoP@MX/HCF//Co-CNT@CNF) is designed that unveil remarkable electrochemical properties, such as a high energy density of 72.4 Wh kg−1 at 800.12 W kg−1 and good capacitance retention (91.4 %) after 10,000 charge/discharge cycles. Furthermore, the device can maintain a consistent performance regardless of the bending degree. More importantly, this work provides new opportunities to rationally design novel freestanding cathodes and anodes for high-performance flexible asymmetric supercapacitors.