High energy density asymmetric pseudocapacitors fabricated by graphene/carbon nanotube/MnO₂ plus carbon nanotubes nanocomposites electrode

Novel graphene/carbon nanotubes (CNTs)/manganese oxide (MnO₂) nanocomposites plus CNTs (GMC + C) and graphene/CNTs hybrid (GC) thin-film electrodes are prepared by electrophoretic deposition (EPD). These nanocomposite electrodes exhibit high surface area and interconnected pore networks. The GMC + C nanocomposite electrode shows excellent specific capacitance of 964 F g ^-1 at 1 A g^-1, rate capability with the residual capacitance of 529 F g^-1 at 500 mV s^-1, and fast Na ⁺ diffusion with intercalation value of 6.34 × 10^-7 cm² s^-1, and deintercalation value of 8.86 × 10 ^-7 cm² s^-1. Such excellent pseudocapacitive performances are attributed to low ion/electron transport resistances and short ion/electron diffusion lengths. Furthermore, novel aqueous electrolyte-based asymmetric pseudocapacitor having 1.8 V cell voltage is successfully fabricated using GMC + C nanocomposite as a cathode and GC nanocomposite as an anode. The optimized asymmetric pseudocapacitor possesses superior performance with a maximum energy density of record high 304 Wh kg^-1 and retaining 56.2% of its initial specific energy density at the power density up to 242 kW kg^-1. In addition, the asymmetric cell configuration also shows excellent cycling stability with 89% specific capacitance maintained after 10,000 cycles. These results suggest that our designed asymmetric pseudocapacitors have a high potential for practical applications.