Analysis Of Polypyrrole

J. Chem. Sci., Vol. 120, No. 1, January 2008, pp. 25–31. © Indian Academy of Sciences.

Analysis of polypyrrole-coated stainless steel electrodes – Estimation of specific capacitances and construction of equivalent circuits
R RAMYA and M V SANGARANARAYANAN*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036 e-mail: sangara@iitm.ac.in Abstract. The galvanostaticpolymerization of pyrrole is carried out on stainless steel electrodes using p-toluene sulphonic acid. The morphology of the film is studied from Scanning Electron Microscopy (SEM) measurements while the nature of the substrate is analysed using Energy Dispersive X-ray Spectroscopy (EDAX) technique. The electrochemical behaviour is studied using cyclic voltammetry, charge– discharge analysis andimpedance spectroscopy. The feasibility of the electrode for supercapacitor applications is investigated. The specific capacitance is estimated as ~102 Farads per gram with 103 charge– discharge cycles. A plausible equivalent circuit for the system is proposed and the circuit parameters are obtained by non-linear regression analysis. Keywords. Polypyrrole; equivalent circuit; impedance spectroscopy;specific capacitance; charge– discharge; supercapacitors.

1.

Introduction

The development of electrochemical supercapacitors occupies a pivotal role in the context of electrochemical energy storage and conversion devices.1 The major advantages as envisaged for the electrochemical supercapacitors are as follows:2 (i) rapid rates of charge–discharge rate, (ii) high power densities, (iii)large cycle life and (iv) ease of fabrication. There exist three main classes of electrochemical supercapacitors viz. oxide based,3–5 carbon nanotubes6–8 and conducting polymers9 based. The feasibility of Under Potential Deposition (UPD) systems as suitable candidates is also being pursued.10 Each of these systems has distinct advantages and limitations. The conducting polymers based systems providelong-term stability and yield large number of chargedischarge cycles. (~ 106) in energy storage devices. Among a variety of polymers studied extensively in energy storage and conversion devices, mention may be made of polyacetylene,11 polyaniline,12 polythiophene,13 polyindole,14 polypyrrole,15 polyphenelyne16, etc. The polypyrrole-based systems are stable under different environmental conditionsand are employed in sensors17 and actuators.18 The objectives of this Communication are (i) to prepare polypyrrole coated stainless steel electrodes using p-toluene sul*For correspondence

phonic acid as dopant; (ii) to analyse the performance of the electrodes as supercapacitors using cyclic voltammetry, galvanostatic charge–discharge and impedance spectroscopy; (iii) to characterize theelectrodes using SEM and EDAX measurements and (iv) to propose a plausible equivalent circuit model and reproduce theoretically Nyquist and Bode plots using non-linear regression analysis. 2. 2.1 Experimental Electrodes

Stainless steel (SS) foils commercially available were cut into electrodes of 1 cm2 area and 1 mm thickness. SS foil was polished with emery paper to a smooth finish, andultrasonicated before use. These were employed as working electrodes. Platinum wire (Bioanalytical systems, USA) was used as the counter electrode while Ag/AgCl (Bioanalytical systems, USA) served as the reference electrode. The temperature was maintained at 24 ± 1°C. 2.2 Chemicals Pyrrole (Sigma Aldrich) was distilled under reduced pressure and stored at 4°C in an inert atmosphere; ptoluenesulphonic acid(Thomas Baker, India) was used as received.
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R Ramya and M V Sangaranarayanan

2.3

Electrochemical measurements

Cyclic voltammetry, Galvanostatic charge/discharge experiments and impedance analysis were done using the electrochemical workstation CHI 660B (CH Instruments, USA). Impedance measurements were carried out using 5 mV as the excitation signal with a frequency range of...