Synthesis and electrochemical characterizations of amorphous manganese oxide and single walled carbon nanotube composites as supercapacitor electrode materials
Introduction
Development of metal oxide-carbon nanotube (CNT) composites have gained interest in recent years owing to their potential applications requiring both high energy and high power densities, which are much sought in present day portable electronics.
Various metal oxides, such as RuO2, Co3O4, NiO, Fe2O3, Ir2O3, SnO2, MnO2 etc., are being studied for the supercapacitor applications [1], [2], [3], [4], [5], [6], [7]. Manganese oxide is one of the most promising pseudocapacitor electrode materials with respect to both its specific capacitance and cost effectiveness. Similarly, various carbonaceous materials, such as CNTs, mesoporous carbon, carbon blacks, and activated carbons have been studied as electrodes for electrical double layer capacitors (EDLCs) [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].
CNTs are interesting class of materials that find applications in variety of energy storage/conversion applications, such as lithium ion battery anodes, catalyst supports in direct methanol fuel cells and electrodes for the EDLC based supercapacitors [10], [11], [12]. The main criteria for the EDLC based supercapacitor electrodes is the large surface area which contributes to the formation of a double layer at the electrode–electrolyte interface. While various carbonaceous materials, such as activated carbon, disordered carbon, mesoporous carbon and CNTs are under close scrutiny for use as EDLC electrode materials [10], [11], [12], [13], [14], [15], composites involving, carbon or CNT and RuO2 or MnO2 oxides, have also been reported [20], [21], [22], [23]. Among them, all the above preliminary studies involving MnO2 and CNT composites focused on optimal composition of CNTs and their rate capability for few cycles, and showed promising. However, long cycle performance at a considerably higher charge–discharge current has not been reported so far. Obviously, long cycle and high rate stability of an energy storage system is a very important parameter for its applications and needs to be evaluated. In this paper, we employed a simple precipitation technique to prepare the homogeneous composites of MnO2:SWNTs at room temperature and investigated their electrochemical properties as a supercapacitor electrode material cycled at a high specific current. The long cycle performance of the MnO2:SWNT composites at a high current has therefore been reported for the first time.
Section snippets
Experimental
The MnO2:SWNT composites with different weight ratios of 5–40 wt% SWNTs were prepared by a simple precipitation technique developed in our lab [24]. Briefly, the starting materials for the preparation of MnO2 are KMnO4 and ethanol. Firstly, the KMnO4 was made a saturated solution in deionized water. The SWNTs were procured in purified form from Helix materials Inc. and used as received. According to their technical data sheet, the diameter distribution is <2 nm and length 0.5–40 μm and exist as
Results and discussion
Structural characterizations for the room temperature synthesized a-MnO2 have been done using TEM, X-ray diffraction (XRD) technique, X-ray photoelectron spectroscopy (XPS), BET surface area measurements and the details are presented elsewhere [24]. The X-ray diffraction studies revealed an amorphous structure for the as synthesized material. Since the oxidation state of Mn is very critical for the electrochemical performance of the system as an electrode, XPS studies was performed and the Mn
Conclusions
A simple method for forming a homogenous composite of a-MnO2 and SWNTs has been demonstrated and for the first time a long cycle performance study at a high current for the composites has been studied. TEM observations showed entangled SWNT homogeneously mixed with the MnO2 nanoclusters. The EIS measurements showed a decrease in resistance with respect to the increase of SWNT content in the composites. The cyclic voltammetric studies showed typical capacitive behavior for the pure MnO2, the
Acknowledgements
The authors gratefully acknowledge financial support from National Science Foundation under the NSF award number DMI-0457555 and Louisiana Board of Regents under the award number LEQSF(2005-08)-RD-B-05.
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