High electroactive material loading on a carbon nanotube/carbon nanofiber as an advanced free-standing electrode for asymmetric supercapacitors.

3D hierarchical nanocomposites always lead to excellent electrochemical properties. In this research, we have successfully designed and synthesized a hierarchical three-dimensional cobalt sulfide encapsulated carbon nanotube/carbon nanofiber (Co9S8@CNT/CNF) composite, resulting in a high-performance supercapacitor with an unrivalled capacitance of ≈1580 F g-1 at a current density of 1 A g-1. An asymmetric supercapacitor (ASC) based on these 3D hierarchical Co9S8@CNT/CNF composites exhibits a maximum energy density of 58 W h kg-1 at a power density of 1000 W kg-1 and maintains it at 38 W h kg-1 at a higher power density of 16.7 kW kg-1. Even after 10 000 charge/discharge cycles, the ASC still retains 93% of the initial capacitance at a current density of 5 A g-1. A large number of aligned CNF nanochannels combined with the synergistic effects of cobalt sulfide nanowires encapsulated in CNTs obviously accelerates the ion transfer speed and shortens the ion transfer distance, thereby boosting both power density and energy density.