Journal of Nanoparticle Research

, Volume 11, Issue 3, pp 725–729

A non-aqueous electrolyte-based asymmetric supercapacitor with polymer and metal oxide/multiwalled carbon nanotube electrodes

Authors

  • F. Estaline Amitha
    • Alternative Energy Technology Laboratory, Department of PhysicsIndian Institute of Technology Madras
  • A. Leela Mohana Reddy
    • Alternative Energy Technology Laboratory, Department of PhysicsIndian Institute of Technology Madras
    • Alternative Energy Technology Laboratory, Department of PhysicsIndian Institute of Technology Madras
Brief Communication

DOI: 10.1007/s11051-008-9497-6

Cite this article as:
Estaline Amitha, F., Leela Mohana Reddy, A. & Ramaprabhu, S. J Nanopart Res (2009) 11: 725. doi:10.1007/s11051-008-9497-6

Abstract

A supercapacitor using non-aqueous electrolyte and multiwalled carbon nanotube (MWNTs) nanocomposite electrodes has been designed with polymer and metal oxide loaded carbon nanotubes as electrodes. These nanocomposites were coated on the carbon paper with Nafion solution to obtain the flexible electrodes. Carbon paper with the nanocomposite coating was pressed on either sides of the Nafion membrane, which acts both as a separator and as an electrolyte. The performance of asymmetric assembly of electrochemical double layer capacitor with polymer- and metal oxide-dispersed MWNTs composite materials with non-aqueous Nafion electrolyte is compared with symmetric assemblies, and the results are discussed.

Keywords

Solid Nafion electrolyteMWNTsPANI/MWNTs and TiO2/MWNTsAsymmetric supercapacitorNanocomposites

Introduction

The present trend in the research of supercapacitor is to develop economical electrode material with high performance. Recently, carbon nanotubes (CNTs) have been recognized as potential electrode materials for electrochemical supercapacitors, owing to their properties such as high chemical stability, low mass density, low resistivity, narrow distribution of mesopore sizes, and large surface area (Portet et al. 2005; Beguin et al. 2005; Frackowiak et al. 2002; Yoon et al. 2004; Chatterjee et al. 2003; Chen et al. 2002). The capacitance of CNT-based electrodes for the supercapacitor can be further increased by modifying their surface with conducting polymers (An et al. 2002) or with transition metal oxides (Wang et al. 2005; Arabale et al. 2003). Asymmetric supercapacitor gives the opportunity to use two different compounds as electrodes with complementary properties which takes advantage of the best properties of each component, trying to decrease or eliminate their drawbacks and to get synergic effect. In addition, all solid-state supercapacitor can be developed by the use of non-aqueous polymer electrolyte such as Nafion which can be integrated to a fuel cell.

We present here the design and development of an asymmetric supercapacitor made up of polymer-dispersed multiwalled nanotubes (MWNTs) and metal oxide-dispersed MWNTs composites. Nafion membrane was used as the electrolyte which also serves as the separator. Morphological characterizations of polymer and metal oxide-dispersed MWNTs have been carried out using scanning electron microscopy (SEM) and transmission electron microscopy (TEM and HRTEM). Electrochemical performances of these electrodes have been investigated using cyclic voltammetry (CV), galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS).

Experimental

Preparation of TiO2/MWNTs, PPY/MWNTs, and PANI/MWNTs

Synthesis and functionalization of MWNTs were done using procedure described elsewhere (Shaijumon et al. 2005; Leela Mohana Reddy and Ramaprabhu 2007). TiO2 was coated over MWNTs by sol–gel method as briefly mentioned: Functionalised MWNTs were dispersed in dilute nitric acid (pH 0.5) by ultrasonic agitation, and titanium tetraisopropoxide was added drop by drop maintaining the volume ratio of titanium tetraisopropoxide to water 1:4. The sol obtained was stirred for 2 days in air at room temperature. The obtained turbid suspension was centrifuged at 6,000 rpm, and the resultant residue was washed twice with distilled water. As-synthesized TiO2/MWNT composites were heat treated at 350 °C for 2 h in air.

The PPY/MWNTs composites were first prepared by chemical oxidative polymerization of a monomer (pyrrole) on the surface of MWNT in order to overcome the drawbacks of electrochemical method, since it is not suitable for preparing composites with a large proportion of polymer and also has a limitation on mass production (Khomenko et al. 2005). Composites were prepared by immersion of the MWNTs (0.1 g) into an aqueous solution of monomer (pyrrole) and addition of an oxidant to this solution. PPY was deposited on MWNTs by chemical polymerization of 0.5 mL pyrrole with 1.2 g of FeCl3 in 50 mL of 0.1 mol/L HCl. PANI/MWNTs nanocomposite was synthesized by polycondensation of 0.4 mL aniline by 0.4 g K2Cr2O7 in 50 mL of 1 mol/L HCl. The nanocomposite materials were then filtered and washed with large amount of water and subsequently with ethanol to remove the residual oxidant. Finally, all composites were washed with acetone and dried at 60 °C.

Fabrication of supercapacitor electrodes and membrane electrode assembly

The required quantity of electrode material was suspended in deionized water and ultrasonicated by adding 5 wt% Nafion solution. This suspension was spread uniformly over the gas diffusion-layered carbon paper (Toray) by spin-coating. The electrode surface area used was of 6.25 cm2 with nanocomposite loadings of 1.6 mg/cm2 on the electrodes. A Nafion membrane (Nafion 1110®), a solid electrolyte, cut in the dimension of 3 × 3 cm2 was pretreated by boiling in a solution of 5% H2O2 and 1 M H2SO4 at 80 °C for 30 min and washed several times with deionized water. Both the electrodes were then pressed on either sides of the pretreated Nafion® membrane at a pressure of 50 kg/cm2 and 130 °C for 2 min. Five different membrane electrode assemblies were prepared for supercapacitors using MWNTs, TiO2/MWNTs, PANI/MWNTs, PPY/MWNTs in symmetric design for comparison with the asymmetry design in which one electrode was coated with TiO2/MWNTs and the other with PANI/MWNTs. This design of the super capacitor is very compact, and the performance of the supercapacitor was carefully studied using an electrochemical workstation PSGSTAT-30 (AUTOLAB). Scheme 1 illustrates the steps involved in the development of CNT-based asymmetric supercapacitor.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9497-6/MediaObjects/11051_2008_9497_Sch1_HTML.gif
Scheme 1

Schematic representation of the steps involved in the development of carbon nanotube-based asymmetric supercapacitor

Results and discussions

Characterization

The SEM and TEM images of polymer-coated MWNTs and metal oxide-dispersed MWNTs indicate the uniform coating/dispersion of the polymer/metal oxide. Figure 1a–c shows the TEM images of MWNTs, PANI/MWNTs, and TiO2/MWNTs. TEM images of TiO2/MWNTs indicate the uniform distribution of nanocrystalline metal oxide particles of size of about 3–5 nm on the MWNTs.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9497-6/MediaObjects/11051_2008_9497_Fig1_HTML.jpg
Fig. 1

(a) HRTEM image of purified MWNTs synthesized by CVD technique, (b) TEM image of PANI/MWNTs, and (c) TEM image uniform-dispersed TiO2/MWNTs

Electrochemical analysis

The CV response of the different electrode materials at a scan rate of 2 mV/s was carried out at potentials between −1.0 and 1.0 V. The MWNT presents the typical box-like curve, expected for an ideal capacitor. However, there are oxidation peaks observed in the CV for polymer- and metal oxide-dispersed MWNT electrodes, which are attributed to redox reactions due to the functional groups on nanotubes (Frackowiak et al. 2000). The reasonable symmetry of the curves for MWNT electrodes is due to the capacitance arising solely due to the double layer, whereas the lack of symmetry of the curves in polymer- and metal oxide-dispersed MWNTs electrodes is probably due to combination of double layer and pseudocapacitances contributing to the total capacitance (Fig. 2). The specific capacitance has been obtained from the CV curve according to following equation:
$$ C_{\text{sp}} = \frac{i}{s\,m} $$
(1)
where i is the average cathodic current, s is the potential sweep rate, and m is the mass of each electrode.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9497-6/MediaObjects/11051_2008_9497_Fig2_HTML.gif
Fig. 2

Cyclic voltammograms of symmetric assembly of supercapacitor having pure MWNTs, PANI/MWNTs as electrode materials, and cyclic voltammograms of asymmetric assembly of supercapacitor having PANI/MWNTs and TiO2/MWNTs as electrode materials, at a scan rate of 2 mV/s

The galvanostatic charge–discharge behavior of the different electrodes have been studied with an applied constant current of 10 mA in the potential range between 0 and +1 V. The symmetry of the charge and discharge characteristics shows good capacitive behavior (Fig. 3). The specific capacitance has been evaluated from the charge–discharge curves, according to the following equation:
$$ C_{\text{sp}} = \frac{I}{{m({\text{d}}V/{\text{d}}t)}} $$
(2)
where I is the applied current and m is the mass of each electrode.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9497-6/MediaObjects/11051_2008_9497_Fig3_HTML.gif
Fig. 3

Galvanostatic charge–discharge of symmetric assembly of supercapacitor having pure MWNTs, TiO2/MWNTs as electrode materials, and galvanostatic charge–discharge of asymmetric assembly of supercapacitor having PANI/MWNTs and TiO2/MWNTs as electrode materials, at an applied constant current of 10 mA

Electrochemical impedance spectroscopy measurements were carried out at a dc bias of 0 V with sinusoidal signal of 10 mV over the frequency range from 40 kHz and 10 MHz. Figure 4 presents complex-plane impedance plots for the present composites. At lower frequency, the imaginary part of impedance sharply increases due to the capacitive behavior of electrode. A slight variation from the ideal capacitive behavior could be attributed to the pore size distribution of MWNTs (Song et al. 2000). The ac impedance method has also been used to measure the specific capacitance of the electrodes, which is influenced by the frequency, especially for porous electrodes, where almost no current flows down the pore at higher frequencies (Arabale et al. 2003).
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9497-6/MediaObjects/11051_2008_9497_Fig4_HTML.gif
Fig. 4

Complex-plane impedance spectra of symmetric assembly of supercapacitor having pure MWNTs, PANI/MWNTs, TiO2/MWNTs as electrode materials, and complex-plane impedance spectra of asymmetric assembly of supercapacitor having PANI/MWNT and TiO2/MWNTs as electrode materials

The average specific capacitances measured using the three electrochemical techniques for the symmetric arrangement of the pure MWNTs, PPY/MWNTs, PANI/MWNTs, and TiO2/MWNTs nanocomposite electrodes were 80, 116, 175, and 210 F/g respectively. The capacitance values thus obtained here are comparatively more than literature values (30 and 80 F/g for MWNTs and RuO2/MWNTs, respectively) (Kötz and Carlen 2000). Though PANI/MWNTs electrodes show a less capacitance than that of previously reported 328 F/g (Dong et al. 2007), the advantage in the present method is complete elimination of liquid electrolyte. In the present work, the asymmetric assembly by using PANI/MWNTs and TiO2/MWNTs as electrodes shows a capacitance of 345 F/g with Nafion solid electrolyte. The increase in the capacitance of MWNTs is mainly due to a homogeneous inter-distribution of carbon/Nafion in electrodes, an excellent adhesion realized between electrodes and Nafion membrane, a good contact between electrodes and end plates (current collectors) and fast proton transport in smaller carbon pores. The further increase in the capacitance for polymer- and metal oxide-dispersed MWNTs is due to uniform dispersion of polymer and metal oxide particles over functionalized MWNTs, which in turn modify the microstructure and morphology of MWNT, allowing the polymer and metal oxides to be available for the electrochemical reactions and improves the efficiency of the composites. The progressive redox reactions occurring at the surface and bulk of polymer and metal oxides through faradiac charge transfer between electrolyte and electrode result in the enhancement of the specific capacitance of polymer and metal oxide-dispersed MWNTs from pure MWNTs.

Conclusion

A uniform distribution of nanocrystalline TiO2 particles of size of about 3–5 nm on the MWNTs is achieved by chemical method. In situ polymerization technique leads to the uniform distribution of polymer over the surface of MWNTs. From systematic study of different electrode materials using both symmetric and asymmetric combinations, a maximum specific capacitance of 345 F/g has been obtained with asymmetric assembly of PANI/MWNTs and TiO2/MWNTs. Due to the use of non-aqueous electrolyte and versatility of the asymmetry process, the current approach will provide a promising technique for energy-storage applications.

Acknowledgments

We thank NRB and DRDO, Government of India, for the financial support.

Copyright information

© Springer Science+Business Media B.V. 2008