Abstract
This study proposes an effective method to inhibit the aggregation of graphene sheets (GS) by introducing the carbon nanotubes (CNTs) as nanospacers to form the 3D hierarchical graphene sheets-carbon nanotubes (GS-CNTs) structures, and then various electrochemical methods were used to construct the manganese oxide/graphene composite electrodes for supercapacitors. Graphene oxide (GO) was prepared from natural graphite through modified Hummer’s method. GO and CNTs were reduced to GS-CNTs by in-situ chemical reduction in the next step. Field emission scanning electron microscopy (FE-SEM) was utilized to investigate the nanostructures of GS-CNTs, which can observe that GS-CNTs show more porous morphology than that of GS. The a-MnO2 was uniformly deposited on GS-CNTs by potentiodynamic deposition, and FE-SEM was utilized to characterize the structures of a-MnO2/GS-CNTs composites. From the observations of FE-SEM images, a-MnO2 uniformly grows onto the whole framework of the 3D porous GS-CNTs composites in the form of flowery nanostructure. According to electrochemical analysis, the a-MnO2/GS-CNTs composite electrodes exhibit the highest specific capacitance (535 F/g at 5mVs-1) at all scan rates than that of the pure a-MnO2 and a-MnO2/GS electrodes. And the capacitance retention of a-MnO2/GS-CNTs electrodes can still maintain at 97% after 1500 charge/discharge cycles. The results of electrochemical impedance spectroscopy (EIS) reveal that the a-MnO2/GS-CNTs electrodes not only exhibit low internal resistance but also high speed of electrons transfer. From the Ragone plot, it can be seen that the power density and energy density of the a-MnO2/GS-CNTs electrode can achieve as high as 2.6kW/kg and 71.3 Wh/kg respectively under the charge/discharge time of 100 seconds. In summary, such the 3D hierarchical a-MnO2/GS-CNT with outstanding performances has been realized by an environment-friendly approach, which is a promising electrode material for the next generation of ECs. The a-MnO2-GS-CNTs electrode was simultaneously fabricated through one-step anodic composite deposition. The FE-SEM was utilized to characterize their structures. From FE-SEM, the morphologies of a-MnO2-GS-CNTs electrodes fabricated at low potentials show the two phases of individual GS-CNTs and a-MnO2, but the a-MnO2-GS-CNTs electrode fabricated at high potentials exhibit the similar morphology with the pure a-MnO2 electrode. From linear sweep voltammetry (LSV), the on-set potential of Mn(OAc)2/GS-CNTs solution is much slower than that of pure Mn(OAc)2 solution. From CV analysis, when the potential is 0.75 V, the a-MnO2-GS-CNTs-0.75 electrode possesses the highest specific capacitance (437.1 F/g at 5mVs-1) at each scan rate in comparison with other electrodes (a-MnO2-GS and a-MnO2). In summary, this work proposed a simple, one-step and efficient approach to simultaneously fabricate 3D hierarchical a-MnO2-GS-CNT with outstanding performances, which also provides a new concept to tailor/design a hierarchical metal oxide/carbon architecture with many different materials for the development of energy storage and conversion systems.