Fe₃O₄nanoparticles 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 (ASC). An appropriate amount of water in ethylene glycol can accelerate the formation of Fe₃O₄and reduce the average size of Fe₃O₄to around 20 nm. However, spherical Fe₃O₄particles larger than 100 nm will form in pure ethylene glycol for long reaction time. The Fe₃O₄/CNT composite with small Fe₃O₄nanoparticles 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^-1at 1 A g^-1, and remains 443 C g^-1at 10 A g^-1. The inferior long-term cycling stability is due to the phase transition of Fe₃O₄and a reductive effect to form metallic Fe. An ASC using Fe₃O₄/CNT and NiCoO₂/C composites as anode and cathode, respectively, delivers a high energy density of 58.1 Wh kg^-1at a power density of 1007 W kg^-1in a voltage window of 1.67 V and has a capacity retention of 63% after 5000 cycles. The self-discharge behavior of the ASC is also investigated.