NiCo₂O₄ Nanosheets on Hollow Carbon Nanofibers for Flexible Solid-State Supercapacitors

The carbon nanofiber-based all-solid-state supercapacitors hold great promise in powering portable electronics, while the low specific capacitance and intrinsic fragility have hindered their practical application. Here, flexible N-doped hollow carbon nanofibers (HCNFs) were prepared using poly(acrylonitrile-co-acrylamide) as a precursor by the coaxial electrospinning method. The leaf-like NiCo2O4 nanosheets are located on the inner and outer walls of the carbon layer via a simple solvothermal method to construct 3D freestanding NiCo2O4/HCNFs electrodes for a flexible all-solid-state supercapacitor. The hollow structure of HCNFs provides both inner and outer surfaces for NiCo2O4 nanosheets and shortens the distance of ion transportation. NiCo2O4/HCNFs possesses an enhanced specific capacitance of 1864.0 F g–1 at 1 A g–1 and a good capacitance retention capability of 91.7% after 5000 times cycling, which is one of the best reported cycling stabilities to date. The symmetrical all-solid-state supercapacitors (ASSCs) were directly prepared with NiCo2O4/HCNFs as an electrode without adding any conductive agents and binders, which have good flexibility and a maximum energy density of 44.3 W h kg–1 at a specific power of 814.8 W kg–1. Five series-connected ASSCs assembled by NiCo2O4/HCNFs can lighten a “DHU” logo composed of 36 light-emitting diodes, indicating their potential application prospect in wearable and portable devices.