Vertically Aligned Carbon Nanotubes on Carbon Nanofibers: A Hierarchical Three-Dimensional Carbon Nanostructure for High-Energy Flexible Supercapacitors

Hierarchical structures enable high-performance power sources. We report here the preparation of vertically aligned carbon nanotubes directly grown on carbon nanofibers (VACNTs/CNFs) by combining electrospinning with pyrolysis technologies. The structure and morphology of VACNTs/CNFs could be precisely tuned and controlled by adjusting the percentage of reactants. The desired VACNTs/CNFs could not only possess high electric conductivity for efficient charge transport but could also increase surface area for accessing more electrolyte ions. When using an ionic liquid electrolyte, VACNTs/CNFs-based electric double layer (EDL) flexible supercapacitors can deliver a high specific energy of 70.7 Wh/kg at a current density of 0.5 A/g and at 30 °C, and an ultrahigh-energy density of 98.8 Wh/kg at a current density of 1.0 A/g and at 60 °C. Even after 20 000 charging/discharging cycles, the EDL capacitor still retains 97.0% of the initial capacitance. The excellent performance highlights the important role of the branched VACNTs in storing and accumulating charge and the CNF backbone in transporting charge, thereby boosting both power density and energy density.