Hydrothermal synthesis of a MnOOH/three-dimensional reduced graphene oxide composite and its electrochemical properties for supercapacitors

A manganese oxyhydroxide/three-dimensional reduced graphene oxide (MnOOH/3D-rGO) composite was prepared by a two-step hydrothermal procedure. Powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy analyses show that MnOOH nanoneedles grow on the porous 3D-rGO skeleton. The MnOOH/3D-rGO composite displays a high specific capacitance (327 F g(-1) at 0.2 A g(-1) in 1.0 M Na2SO4 electrolyte) and excellent cycling stability with 96.7% capacitance retention after 1000 cycles. The asymmetric supercapacitors made with MnOOH/3D-rGO and activated carbon exhibit excellent performances in energy storage. At a power density of 378 W kg(-1), the devices can deliver a maximum energy density of 52.7 W h kg(-1). The excellent electrochemical performance, including high energy density, high specific capacitance and long cycle lifetime, of the MnOOH/3D-rGO composite may be due to the synergistic effect of the 3D interpenetrating microstructure of the conductive rGO and the nanoneedle structure of MnOOH. The 3D-rGO can provide facile routes for the transfer of the ions and electrons and the nanoneedle structure is beneficial for the intercalation-deintercalation processes of Na+ ions in the MnOOH.