Preparation and capacitance of graphene/multiwall carbon nanotubes/MnO2 hybrid material for high-performance asymmetrical electrochemical capacitor

Graphene/multiwall carbon nanotubes/MnO2 (GR/MCNTs/MnO2) hybrid material with a specific capacitance of 126 Fg(-1) within a potential window of 0-1.1V vs. saturated calomel electrode has been synthesized by a simple redox reaction between graphene/multiwall carbon nanotubes (GR/MCNTs) and KMnO4 at room temperature. The morphology and structure of the obtained material are examined by XRD, SEM and TEM. The electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The mass percentage of MnO2 with layered structure is 37% in the hybrid material. An asymmetrical electrochemical capacitor (EC) is assembled using GR/MCNT/MnO2 hybrid material as positive electrode and GR/MCNT material as negative electrode, respectively. The electrochemical properties of the two electrodes and the asymmetrical EC are investigated in I mol L-1 Na2SO4 aqueous electrolyte. The asymmetrical EC can cycle reversibly in a cell potential of 0-2.0 V and gives a high energy density of 28.33 Wh kg(-1), which is much higher than those of symmetrical ECs based on GR/MCNT/MnO2 (6.20 Wh kg(-1)) and GR/MCNT (3.92 Wh kg(-1)). Moreover, the asymmetrical EC presents a high power density (5 kW kg(-1) at 13.33 Wh kg(-1)) and excellent cycling performance of 83% retention after 2500 cycles. (C) 2012 Elsevier Ltd. All rights reserved.