Controllable synthesis of MnO2 nanostructures anchored on graphite foam with different morphologies for a high-performance asymmetric supercapacitor

MnO2 nanostructures with different morphologies (nanowires, nanowire bundles and flower-like nanosheet bundles) were synthesized by adjusting the concentration of KMnO4 solution using a simple and surfactant-free hydrothermal method. Both MnO2 nanowires and nanosheets have ultrathin thickness of about 10-30 nanometers. Such open' nanostructures can facilitate the electrolyte infiltration and increase the active material utilization efficiency. Owing to the high electron conductivity of the graphene/Ni foam (GNF) substrate as well as the large electrolyte/electrode contact interface, a specific capacitance of 201 F g(-1) can be obtained in 1 M Na2SO4 electrolyte. An asymmetric supercapacitor assembled with flower-like MnO2 nanosheet bundles on GNF and activated microwave exfoliated graphite oxide (a-MEGO) yields an energy density of 44.5 W h kg(-1) and a power density of 50.0 kW kg(-1), respectively. These unique and controllable MnO2 nanostructures may have enormous potential applications in the fields of energy storage and sensors.