Supercapacitor can attain greater energy density than those of conventional capacitors and greater power density than batteries; this has kindled the interests of the researchers in this field of energy storage. As a result, supercapacitors may become an attractive power solution for an increasing number of applications. Electrical double layer capacitors (EDLCs) has the following advantages in energy storage system: long cycle life, high current capability, easily monitored state of charge, very high efficiency, wide voltage range, wide temperature range, long operational life, and ease of maintenance. Several attempts have been made to obtain fundamentally new materials and technologies for the design and manufacture of electrochemical supercapacitors. The present trend in research is focused on developing new supercapacitor electrode materials. Carbon nanotubes (CNTs) are materials with interesting properties such as increased accessible surface area, low mass density, and high electronic conductivity. These materials have then been studied by several authors in supercapacitor applications, mainly in aqueous electrolytes 12345.

Transition metal oxides have been explored as potential electrode materials for use in supercapacitors; their charge-storage mechanisms are based predominantly on pseudocapacitance 67 RuO₂ has been found to have high capacitance due to redox transitions that even penetrates into the bulk of the material, however, the cost of Ru has retarded its commercial acceptance 89. Cheap metal oxides with comparable characteristics are being investigated, e.g., oxides of Ni, Co, In, Sn, Mn, etc and conducting polymers are another class of material under investigation due to their excellent electrochemical properties and low cost. Conducting polymer for redox supercapacitor has characteristic advantages, such as fast kinetics of charge/discharge process and high charge storage capacitance 10. The selection of Polyaniline (PANI) as electrode material for the supercapacitor is for the reason that it has good stability and reasonably priced 11121314.

An asymmetric configuration is obtained by using two different active materials as two electrodes. The asymmetric supercapacitor also improves power density by using high surface area nanostructured/nanofibrous electrode materials. It is demonstrated that a successful application of conducting polymers and metal oxides in supercapacitor technologies is possible only in an asymmetric configuration, i.e., with electrodes of different nature for positive and negative polarizations. Stacking of supercapacitors is desirable to produce devices that can operate at application voltages. By stacking two conventional double layer supercapacitor into a single component it is possible to get higher working voltage, thus enabling the use of supercapacitor for practical applications.

The objective of this work is to combine the advantages of the carbon nanotubes along with the transition metal oxide and polymer to fabricate the supercapacitor electrodes in asymmetric assembly. Further we have made use of the solid electrolyte to obtain all solid component supercapacitor with compact and flexible design. Two conventional supercapacitors are combined together to get a supercapacitor stack with a higher performance.

MWNTs were synthesized by CVD using Mm based AB₃ alloy hydride as catalyst prepared by hydrogenation and dehydrogenation cycles. Figure 1a shows the XRD pattern of the as grown MWNTs with peaks corresponding to hexagonal graphite and catalytic impurities. Figure 1b shows the XRD pattern of purified sample which illustrates that the catalytic impurities have been efficiently removed during acid treatment 17. The XRD pattern of TiO₂/MWNT nanocomposite material (Fig. 1c) shows the reflections of TiO₂ along with that for graphitic carbon. The broad peaks of TiO₂ reveal the presence of nanostructured metal oxide crystals.

Thermogravimetric measurements were performed using a STA 409PC TGA-DTA analyzer. The weight of the sample has been recorded as a function of temperature in the ranges of 25–950 °C. In the as grown sample the weight loss between 500 and 680 °C in the TG curves is attributed to the oxidation of MWNTs. Final residual weights of ∼27% were obtained for as grown MWNTs due to the presence of catalytic impurities. The weight loss between 500 and 800 °C in the purified sample is due to the oxidation of MWNTs and the residual weight is ∼0% revealing a purity of about ∼98% for the purified sample (Fig. 2a, b) 17. The TGA of TiO₂/MWNTs shows weight loss between 500 and 750 °C which corresponds to oxidation of MWNTs and shows a residual weight of ∼70% which is due to the presence of TiO₂particles. The PANI/MWNTs shows weight loss from 200 °C which corresponds to the decomposition of polymer.

Figure 3 (a) SEM, (b) TEM and (c) HRTEM images confirm the good quality of the MWNTs obtained by CVD technique. The HRTEM image reveals the multi-walled nature of carbon nanotubes with each graphene layer being clearly distinguishable since the graphene sheets with a spacing of ∼ 0.34 nm are stacked parallel to the growth axis of carbon nanotubes. Figure 4(a, b) shows the SEM and TEM images of PANI/MWNTs composites which discloses the uniform dispersion of polymer over MWNTs surface. SEM and TEM images of TiO₂/MWNTs (Fig. 4(c, d)) indicate the uniform distribution of nano crystalline metal oxide particles of size of about 3–5 nm on the MWNTs.

The electrochemical properties of the flexible supercapacitor stack with solid electrolyte were studied. The cyclic voltammetry (CV) response of the super stack at scan rate 0.1 Vs⁻¹is shown in Fig. 5. There is a reasonable symmetry to the curves and the specific capacitance has been obtained from the CV curve according to following equation,

where i is the average cathodic current, dV/dt is the potential sweep rate and m is the mass of each electrode.

Figure 6 shows the galvanostatic charge-discharge behavior of the supercapacitor stack with asymmetric assembly using PANI/MWNTs and TiO₂/MWNTs with an applied constant current of 10 mA in the potential range between 0 and +3 V. The symmetry of the charge and discharge characteristics shows good capacitive behavior. The specific capacitance has been evaluated from the charge-discharge curves, according to the following equation:

where I is the applied current and m is the mass of each electrode.

Electrochemical Impedance Spectroscopy (EIS) measurements were carried out at a dc bias of 0 V with sinusoidal signal of 10 mV over the frequency range from 40 kHz to 10 MHz. Figure 7 presents complex-plane impedance plots for supercapacitor stack with PANI/MWNTs and TiO₂/MWNTs as the electrode materials with asymmetric assembly. At lower frequency, the imaginary part of impedance sharply increases: this is the capacitive behavior of electrode. The impedance plot should theoretically be a vertical line, parallel to the imaginary axis. In fact, the difference between this theoretical prediction and experiment can be observed. The impedance behavior of supercapacitor stack with PANI/MWNTs and TiO₂/MWNTs composites as electrodes show a deviation from ideal capacitor. 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 18.

The average specific capacitances measured using the three electrochemical techniques of the supercapacitor stack with asymmetric assembly using polymer/MWNTs and metal oxide/MWNTs as electrodes and non-aqeuous nafion membrane as electrolyte is 240 F/g. The capacitance values thus obtained here are comparatively more than literature values (30 and 80 F/g for MWNTs, RuO₂/MWNTs, respectively) 19. The capacitance of the MWNTs is mainly due to several factors including (1) a homogeneous inter-distribution of carbon/Nafion in electrodes, (2) an excellent adhesion realized between electrodes and Nafion membrane, (3) a good contact between electrodes and end plates, (4) fast proton transport in smaller carbon pores. Further uniform dispersion of polymer and metal oxide particles over functionalized MWNTs also contributes to the increase in the capacitance. Since the specific double layer capacitance arises from the ionic double layer at the electrode/electrolyte interface, the accessibility of the active layer depends on the diffusion of solvated ions and more precisely on the pore size distribution. The central hollow core of the CNTs is also accessible for the double layer charging and thus the purified MWNTs with opened tips along with the unique network of mesopores formed by the entanglement of CNTs would have acted as sites for the accumulation of charges. The enhancement of the specific capacitances can be attributed to the presence of polymer and metal oxide attached to the surface of MWNTs, which in turn modify the microstructure and morphology of MWNTs, 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 results in the enhancement of the specific capacitance of polymer and metal oxide dispersed MWNTs from pure MWNTs.

A non-aqueous electrolyte based supercapacitor stack has been fabricated successfully using asymmetric combination of PANI/MWNTs and TiO₂/MWNTs coated flexible electrode materials. A maximum specific capacitance of 240 F/g has been derived from the supercapacitor stack.