Preparation and electrochemical characterization of polyaniline/multi-walled carbon nanotubes composites for supercapacitor
Introduction
Electrochemical capacitors (ECs), known as supercapacitors, have attracted great interest as promising energy storage devices due to their high power energy density and long cycle performance than conventional dielectric capacitors [1], [2], [3], [4], [5]. ECs are being considered for all kinds of applications such as power sources for camera flash equipment, lasers, pulsed light generators, and as backup power source for computer memory [6], [7]. They also have become of interest in hybrid electric vehicles as an auxiliary power source in combination with battery. ECs can provide the peak power in such hybrid systems when accelerating and thus the batteries can be optimized primarily for higher energy density and better cycle-life [8].
A great attention has been focused on conducting polymer-based electrochemical capacitors owing to their remarkable merits such as high capacitive energy density and low material cost. Contrarily to activated carbons where only the surface is used for charge accumulation, in the case of conducting polymer, the total mass and volume is involved in charge storage. Among various conducting polymer, polyaniline has emerged as the one of the most promising class of active materials for electrochemical capacitor, due to its high capacitive characteristics, easy processing and environmental friendliness [9], [10], [11], [12]. However, the conducting polymers also have disadvantages that include lower cycle-life and slow kinetics of ion transport because the redox sites in polymer backbone are not sufficiently stable during the many cycled redox processes [13].
Therefore, suitable materials with high stability, conductivity, nanosize, high surface area and good capacitive characteristics have been considered as filler in order to overcome the stability problem of conducting polymer with cycling. Owing to much unique structural, mechanical, electronic properties and high stability [14], carbon nanotubes (CNTs) show tremendous potential for a vast range of applications, including mechanically reinforced composites [15], [16], [17], [18], tips for scanning probe microscopy [19] and sensor [20]. Moreover, CNTs also possess the suitable properties such as high surface area, excellent electrical conductivity and interconnectivity. So CNTs have been predicted as excellent filler for polymer on improving the electric conductivity as well as the mechanical properties. Recently, obvious progress has been made in designing and fabricating PANI/CNTs composites [21], [22], [23], [24], [25], [26], [27]. Among these studies, Gupta and Miura reported the electrochemical polymerization of PANI/SWNTs composites and studied the capacitor behavior of the composites in acid system [28], [29]. Khomenko et al. reported the synthesis of PANI/MWNTs nanocomposites by polycondensation of aniline by K2Cr2O7 as oxidant and studied the capacitor behavior of the composites in acid system [30], [31]. However, the capacitor behavior of PANI/MWNTs composites has seldom been studied in detail in the neutral solution.
In our work, the fabrication of PANI/MWNTs composites is done by an in situ chemical oxidative polymerization method and characterized by TEM, XRD. The electrochemical capacitance performances of PANI/MWNTs in neutral system (NaNO3) are investigated. The improvement mechanisms of the capacitance of the composites are also discussed in detail.
Section snippets
Preparation of PANI/MWNTs composites
MWNTs synthesized by the catalytic chemical vapor deposition (CVD) method had been purified according to the method described in previous works [32]. The resulted MWNTs with a purity exceeding 98% had many carboxylic acid groups at the defect sites that can improve the solubility of MWNTs in HCl solution.
The composites of protonic acid doped PANI with MWNTs were synthesized via an in situ chemical oxidative polymerization method. Thirty millilitres solution of 1.0 M HCl (37 wt%, Aldrich)
TEM analysis
The morphology of purified MWNTs and PANI/MWNTs composites was characterized using TEM. Typical images of purified MWNTs and PANI/MWNTs composites are shown as Fig. 1a and b, respectively. As shown in Fig. 1a, purified MWNTs show very clean wall and central hollow tubular structure. The outer diameters of most MWNTs range from 20 to 40 nm, and their lengths are several micrometers. From Fig. 1b, it is clear that a layer of uniformly PANI film is presented on MWNTs’ surface, and the diameters of
Conclusions
In this work, PANI/MWNTs composites as the new electrode materials used for supercapacitors were prepared by a way of in situ chemical oxidative polymerization. The capacitor behaviors of the composites in neutral system (NaNO3) were tested in detail and the improvement mechanisms of capacitor value of the composites were discussed by impedance measurements. High value of capacitance up to 328 F g−1 of the composites have been obtained. The results indicated that MWNTs have an obvious improvement
Acknowledgements
The authors are thankful for the support provided by National Nature Science Foundation of China (No. 60471014) and 2006 Hatch Project Funds of National University Science Park of Lanzhou University.
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