Nano-Flower MnO 2 Coated Graphene Composite Electrodes for Energy Storage Devices
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Nano-Flower MnO2 Coated Graphene Composite Electrodes for Energy Storage Devices
Qian Cheng,1,2 Jie Tang,1,2 Jun Ma,1 Han Zhang,1 Norio Shinya,1 and Lu-Chang Qin3 1
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan
2
Doctoral Program in Materials Science and Engineering, University of Tsukuba, 1-1-1 Tennodai,
Tsukuba, 305-8577, Japan 3
Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel
Hill, NC 27599-3255, USA
ABSTRACT Graphene, two-dimensional layers of sp2-bonded carbon, has many unique properties. In this paper, graphene is decorated with flower-like MnO2 nanostructures for the application in energy storage devices. The as-prepared graphene and MnO2 nano-flowers, which were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were assembled into an asymmetric supercapacitor. The specific capacitance of the graphene electrode reached 245 F/g at a charging current of 1 mA. The MnO2 nano-flowers which consisted of tiny rods with a diameter of less than 10 nm were coated onto the graphene electrodes by electrodeposition. The specific capacitance after the MnO2 deposition is 328 F/g at the charging current of 1 mA with an energy density of 11.4Wh/kg and power density of 25.8 kW/kg. This work suggests that our graphene-based electrodes can be a promising candidate for highperformance energy storage devices. INTRODUCTION Research on supercapacitors has generated growing interests from both academia and industry in recent years. Supercapacitors are considered as a promising power storage device for backup power storage, peak power sources, and hybrid vehicles, due to their high power density, high charge and discharge rate, and long cycle life [1]. Graphene, parent of all graphitic structures ranging from graphite to carbon nanotubes and fullerenes, has become one of the most exciting topics of research in the last few years [2].This two-dimensional material constitutes a new type of nanostructured carbon comprising a single layer of carbon atoms arranged in the graphitic sp2 bonding configuration. It is distinctly different from carbon nanotubes and fullerenes and exhibits many unique properties. Graphene and chemically modified graphene sheets have shown a high electrical conductivity [3], high surface area, and good mechanical properties comparable with or even better than carbon nanotubes [4]. In addition, graphene-based materials can be easily obtained by simple chemical processing of graphite [5]. Furthermore, the graphene-based system of individual sheets does not depend on the distribution of pores in a solid support to offer its large surface area. Instead, every chemically modified graphene sheet can “move” physically to adjust itself to the different types of electrolytes. Therefore the access to the very high surface area of graphene-based materials by
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the electrolyte can be maintained while preserving the overall high electrical conductivity for the network [6-8]. To exploit the poten
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