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

Abstract

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.