Synthesis and capacitive properties of carbonaceous sphere@MnO 2 rattle-type hollow structures
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Carbonaceous sphere@MnO2 rattle-type hollow spheres were synthesized under mild experimental conditions. The as-prepared hollow structures were characterized using scanning electron microscope, transmission electron microscope, x-ray diffraction, x-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, thermogravimetric analysis, and nitrogen adsorption techniques. The characterization data showed the formation of rattle-type hollow structures with a mesoporous MnO2 shell and a carbonaceous sphere core. The composition and shell thickness of the hollow spheres can be controlled experimentally. The capacitive performance of the hollow structures was evaluated by using both cycle voltammetry and charge–discharge methods. The results demonstrated a specific capacitance as high as 184 F/g at a current density of 125 mA/g. The good electrocapacitive performance resulted from the mesoporous structure and high surface area of the MnO2-based hollow spheres.
I. INTRODUCTION
Supercapcitors, also known as ultracapacitors or electrochemical double-layer capacitors (EDLCs), are energy storage devices that possess a higher specific power density than most batteries and a higher specific energy density than conventional capacitors.1,2 Such energy storage devices can be used as a stand-alone energy supply or in combination with batteries as a hybrid energy system.3 Supercapacitors store energy using either ion adsorption (namely EDLCs) or fast and reversible faradic reactions (i.e., pseudocapacitors).1,4 These two mechanisms can sometimes function simultaneously depending on the nature of the electrode material. Carbon materials are generally used as the electrode materials for EDLCs because of their unique properties, such as electrical conductivity, high surface area, low cost, and electrochemical stability.1,3–5 Transition metal oxides such as ruthenium dioxide (RuO2) and manganese dioxide (MnO2) and conducting polymers such as polyaniline and poly(3,4-ethylenedioxythiophene) have also been shown to be potential pseudocapacitor electrode materials.3,4,6 While hydrous RuO2 possesses a high pseudocapacitances (as high as 720 F/g at a scan rate of 2 mV/s),7 the high cost of this precious metal is a great hurdle in practical applications. Manganese oxides with a theoretical specific capacitance of about 1370 F/g for a redox process involving one electron per manganese atom8 have attracted a a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0189
1476
http://journals.cambridge.org
J. Mater. Res., Vol. 25, No. 8, Aug 2010 Downloaded: 14 Mar 2015
great deal of attention due to their distinctive properties, such as good capacitive performance, low cost, and environmental benigness.6,9 Lee et al.10 first described the pseudocapacitive behaviors of amorphous manganese oxides. Brousse et al.11 investigated the capacitive performance of amorphous and crystallized manganese dioxide (MnO2). The amorphous MnO2 exhibited a maximum capacitance of about 150 F
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