High energy density and enhanced stability of asymmetric supercapacitors with mesoporous MnO2@CNT and nanodot MoO3@CNT free-standing films

Asymmetric supercapacitors employ two different electrode materials with different working potentials and charge-storage mechanisms. One is for redox reactions or pseudocapacitance, similar to batteries, and the other for electric double-layer capacitance, similar to supercapacitors. This helps improve both energy density and power density. The choice of materials and control of nanostructures are the keys to enhancing electrochemical performance. Use of an aqueous electrolyte is desired for safety issues but the operating voltage window remains a challenge. We chose MoO3 and MnO2 for the two electrodes, where both exhibited pseudocapacitance with a high voltage window of 2 V. Each material was further nanostructured with carbon nanotubes to form MoO3 nanodots on CNT surfaces (MoO3@CNT) and mesoporous MnO2 embedded in CNT networks (MnO2@CNT). Therefore, the specific surface area improved to 68 m(2)/g for MoO3@CNT and 343 m(2)/g for MnO2@CNT, while the conductivity increased to 2.27 and 10.82 S/cm, respectively. For full-cell asymmetric supercapacitors with Na2SO4 as the electrolyte, a high energy density of 27.8 Wh/kg at a power density 524 W/kg or 9.8 Wh/kg at a high power density 10,000 W/kg was observed, where the power density was increased by a factor of 4 relative to the value reported with graphene oxide composites. Our ASCs exhibited excellent cycle stability with a capacitance retention of 96.8% after 10,000 cycles at 5 A/g. The simple self-assembly approach and freestanding nature of these metal oxide@CNT hybrid films offer high potential for the development of safe, low-cost, and wearable energy storage devices in the near future.