Cobalt oxide (Co3O4) nanoparticles uniformly anchored on vertically aligned carbon nanotube arrays (VACNTs) are synthesized by means of an effective supercritical carbon dioxide (scCO2) assisted impregnation followed by a thermal annealing process. Benefiting from their desirable 3D nanostructure, the Co3O4/VACNTs (CVN) composites manifest a high specific capacitance of 833 F/g at a current density of 1A/g and a good rate capacity with capacitance retention of 89% even after 20‐fold increase in current density. Coupled with a Fe3O4/VACNTs (FVN) anode (350 F/g at 1 A/g) prepared by the same method, the assembled CVN//FVN asymmetric supercapacitor (ASC) delivers a high energy density of 36 Wh/kg (1.65 mWh/cm3) at a power density of 0.93 kW/kg (0.05 W/cm3), and a maximum power density of 13.6 kW/kg (0.62 W/cm3) at an energy density of 10.6 Wh/kg (0.51 mWh/cm3). Moreover, even after 5000 charge‐discharge cycles at a current density of 5 A/g, the ASC still retains 87% of its initial specific capacitance. The extraordinary electrochemical performance of the ASC device demonstrates the promise of our VACNTs‐based pseudocapacitive electrode for future energy storage systems.

https://doi.org/10.1002/celc.201800138