06) 1466–1469www.elsevier.com/locate/matlet

Materials Letters 60 (20

High performance electrochemical supercapacitor from electrochemicallysynthesized nanostructured polyaniline

Vinay Gupta a,b,⁎, Norio Miura a

a Art, Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japanb Japan Science and Technology Agency, Saitama, 332-0012, Japan

Received 12 September 2005; accepted 15 November 2005Available online 7 December 2005

Abstract

Polyaniline nanowires were electrochemically deposited on stainless steel electrode at the potential of 0.75 V vs. SCE and characterized bycyclic voltammetry in 1 M H2SO4 electrolyte for supercapacitive properties. A high specific capacitance of 775 F g−1 was obtained at the sweeprate of 10 mV s−1. A long-term cyclic stability of the polyaniline nanowires demonstrated its implications for the high performancesupercapacitors.© 2005 Elsevier B.V. All rights reserved.

Keywords: Polyaniline; Deposition; Nanowire; Cyclic voltammetry; Supercapacitor

Electrochemical supercapacitors are the charge-storagedevices having high power density and long cyclic life [1–5].The increasing pollution due to electrical vehicles and explosivegrowth of portable electronic devices have pushed thedevelopment of high performance supercapacitors as the urgentrequirement. Supercapacitors store energy in the form of chargeat the electrode/electrolyte interface and can be divided into twocategories: (i) redox supercapacitors, in which the pseudocapa-citance arises from faradic reactions occurring at the electrodeinterface and (ii) electric double layer capacitors (EDLCs), inwhich the capacitance arises from the charge separation at theelectrode/electrolyte interface.

The main materials that have been studied for the super-capacitor electrode are (i) carbons, (ii) metal oxides and (iii)polymers. The polymers are considered the most promisingmaterial in the supercapacitors. Among the polymers, such aspolymethyl methacrylate (PMMA) [6], p-phenylenevinylene(PPV) [7], polypyrrole (PPy) [8–10] and polyaniline (PANI)[11–13], polyaniline is considered the most promising material

⁎ Corresponding author. KASTEC, Kyushu University, Kasuga-shi, Fukuoka816-8580, Japan. Tel./fax: +81 92 583 7886.

E-mail address: drvinaygupta@netscape.net (V. Gupta).

0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2005.11.047

in the supercapacitors due to its high capacitive characteristics,low cost and ease of synthesis.

The materials in the nano-size form with high surface areaand high porosity give the best performances as the electrodematerials for supercapacitors because of their distinctivecharacteristics of conducting pathways, surface interactions,and nanoscale dimensions. Therefore, the synthesis and thecapacitive characterization of the high surface area nanomater-ials such as nanotubes, nanowires, [14–16] etc. have beencarried out extensively in the past few years. Consequently,different indirect methods were used to synthesized nanosizedpolyaniline, such as template synthesis [17], self-assembly [18],emulsions [19] and interfacial polymerization [20]. However,such methods require relatively large amounts of surfactants,which are rather tedious to recycle after polymerization, and it isdifficult to attach nanosized polyaniline onto a substrate withoutinvolving large contact resistance. Therefore nanosized polyani-line synthesized by such methods is not suitable for super-capacitors applications. The best way is direct deposition of thenanostructured polyaniline onto a substrate electrode. In thepresent work, for the first time, we have performed cyclicvotammetric measurements of the polyaniline nanowireselectrochemically deposited directly on the stainless steelelectrode for supercapacitor application.

Fig. 1. (a) and (b) SEM images of the polyaniline nanowires at differentmagnification.

Fig. 3. X-ray diffraction pattern of the original polyaniline nanowires.

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The polyaniline was potentiostatically deposited on to 1×1cm stainless-steel plates (SS) (grade 304, 0.2 mm thick) at 0.75V vs. SCE. Research grade SS was obtained from the Nilaco®Corporation. Before the deposition, the SS was polished withemery paper to a rough finish, washed free of emery particlesand then air-dried. The electrochemical deposition wasperformed using auto-lab® PGSTAT 30 instrument (Ecochemie, Netherlands) connected to a three-electrode cell. Thethree-electrode configuration contains platinum (Pt) as thecounter electrode, saturated calomel electrode (SCE) as thereference electrode and SS as the working electrode. Anelectrolyte solution of 1 M H2SO4+0.05 M polyaniline wasused for the electrochemical deposition of polyaniline nano-wires on the SS electrodes. Subsequent to the deposition, theelectrode was washed in distilled water and stirred by using amagnetic paddle. Thereafter, the electrodes were dried in ovenat 40 °C for a day. The weight of the electrochemicallydeposited polyaniline was measured by means of Sartorius

Fig. 2. (a) and (b) Schematics of the possible growth process of the polyanilinenanowires.

microbalance (Model BP211D). The microstructure and thethickness of the deposit were evaluated by means of JEOLscanning electron microscope (FE-SEM, JEOL, JSM-6340F).The X-ray diffraction pattern was obtained by means of RigakuX-ray diffractometer (model R1NT2100) using CuKα radia-tions. The photospectra was obtained by means of Rigaku UV–vis-NIR spectrophotometer (model ultraspec 3300 pro). Thecyclic voltammetric measurements were performed in 1 MH2SO4 solution.

Fig. 1 shows the SEM images of the polyaniline nanowiresformed after 10 min deposition time. The diameter of thenanowires is in the range of 30∼60 nm. The thickness andthe size of the nanowire film is ∼20 μm and 1×1 cm,respectively. The nanowire's network is highly porous withinterconnectivity. A possible growth process of such nano-wires is proposed as shown in Fig. 2(a) and (b). Thepolyaniline nanowires are expected to growth via seedlinggrowth process, in which further polyaniline is deposited onthe initially deposited nanosized granules (Fig. 2(a)). As thedeposition progress, an aligned nanowire network is formed(Fig. 2(b)). A further deposition results in extended lengthand in the misalignment of nanowires and cross-links areformed [16].

Fig. 3 shows the X-ray diffraction spectra of the polyanilinenanowires. A very broad peak was observed centered at 2θ

Fig. 4. UV–vis-NIR spectra of polyaniline nanowires in dedoped state in NMPsolution.

Fig. 5. Cyclic voltammograms of polyanile nanowires at different sweep rates in1 M H2SO4 electrolyte. Fig. 7. Cyclic-life data of the polyaniline nanowires capacitor. The specific

capacitance was calculated for the cyclic voltammetry at the sweep rate of 100mV s−1 in 1 M H2SO4 electrolyte.

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∼19° with a shoulder at 2θ∼26°. This suggests that the natureof the nanowires is amorphous. Fig. 4 shows the UV/visiblephoto spectra of the polyaniline nanowires in the dedoped statein N-methyl-2-pyrrolidone (NMP) solution. Before obtainingthe photospectra, the polyaniline nanowires were washed in 0.1M NH4OH solution. In this process, original green polyanilinebecomes blue-purple, indicating that the doped emeraldine formhas been deprotonated to emeraldine base. A few drops of thisbase formed a clear blue solution in the NMP, giving UV–vis-NIR absorbance photospectra with maxima at 638 and 332 nm,

Fig. 6. (a) Cyclic voltammetric current density of polyaniline nanowires versusapplied scan rate and (b) specific capacitance of polyaniline nanowirescalculated from CV vs. the scan rate.

in excellent agreement with the literature results for indirectlyprepared polyaniline nanostructures [21].

Fig. 5 shows the cyclic voltammogram of polyanilinenanowires/SS electrode in 1 M H2SO4 electrolyte in thepotential range of 0 and 0.7 V vs. SCE. The near rectangular-shaped cyclic voltammograms (CV) at low scan rate and highoverall current suggests the highly capacitive behavior of thepolyaniline nanowires. The CVs were also recorded in thesame electrolyte with bare SS electrode, but no capacitivebehavior was observed. Hence the capacitive behavior ofpolyaniline nanowires/SS electrode is entirely due to polyani-line nanowires. The CV current densities and the calculatedspecific capacitance values are shown in Fig. 6(a) and (b),respectively. The specific capacitance (SC) value of 775 Fg−1 was obtained at the scan rate of 10 mV s−1 whereas theSC value of 562 F g−1 was obtained at the high scan rate of200 mV s−1. This decrease of 25% in the specific capacitanceat high scan rates is much lower than in the case of metaloxides where decrease of 50∼80% was reported between 10and 200 mV s−1 [22]. Moreover the CV current increaseslinearly with the increase in the scan rate as shown in Fig. 6(a). This implies highly stable supercapacitive characteristicsof the polyaniline nanowires.

Fig. 7 shows the cyclic stability of the polyanilinenanowires at the sweep rate of 100 mV s−1 for 1500 cycles.There is a small decrease in the specific capacitance value inthe first 100 cycles and thereafter the specific capacitanceremains almost constant. The decrease in the specificcapacitance was 8% in the first 500 cycles and 1% in thesubsequent 1000 cycles, indicating high stability of theelectrode for long cyclic life.

Acknowledgements

This present work was supported by Japan Science andTechnology (JST) agency through “Core research for EvolutionScience and Technology (CREST)” under the project “Devel-opment of advanced nanostructured materials for energyconversion and storage”.

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