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 m²/g for MoO₃@CNT and 343 m²/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 free-standing 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.