Fe3O4 nanoparticles anchored on carbon nanotubes as high-performance anodes for asymmetric supercapacitors.

Fe3O4/CNT composites are synthesized with ethylene glycol as solvent by a one-step solvothermal method and used as anode materials for asymmetric supercapacitors. An appropriate amount of water in ethylene glycol can accelerate the formation of Fe3O4 and reduce the average size of Fe3O4 to around 20 nm. However, spherical Fe3O4 particles larger than 100 nm will form in pure ethylene glycol for long reaction time. The Fe3O4/CNT composite with small Fe3O4 nanoparticles exhibits a high specific surface area, promoted electron transfer ability, as well as a high utilization rate of active materials. The optimized electrode shows a high specific capacity of 689 C g-1 at 1 A g-1, and remains 443 C g-1 at 10 A g-1. The inferior long-term cycling stability is due to the phase transition of Fe3O4 and a reductive effect to form metallic Fe. An asymmetric supercapacitor using Fe3O4/CNT and NiCoO2/C composites as anode and cathode, respectively, delivers a high energy density of 58.1 Wh kg-1 at a power density of 1007 W kg-1 in a voltage window of 1.67 V and has a capacity retention of 63% after 5000 cycles. The self-discharge behavior of the asymmetric supercapacitor is also investigated.