Electrochemical Studies Of Polyaniline And Some Of Its Applications

Abstract

The studies reported in the thesis deal with surface modification of non-platinum metals by coating with electronically conducting polymers, namely, polyanilne (PANI) and polypyrrole (PPY). The oxidation of Г/I2, hydroquinone/quinine and [Fe(CN)6]3-/ [ Fe(CN)6]4-are studied by cyclic voltammetry and chronoamperometry experiments. It has been shown tht the redox reactions, which do not occur on bare stainless steel electrode, occur through electron-transfer mediated by conducting polymers. The effect of heating of polyaniline (PANI) at 80 0C on its electrochemical activity is studies. Although the thermogravimetric analysis indicates that PANI is stable at temperatures up to about 250 0C and it undergoes decomposition at higher temperatures, its intrinsic redox electrochemical activity decreases with duration of heating at a temperature as low as 80 0C . The polymer completely loses its electrochemical activity. The decrease in lectrochemical activity of PANI is attributed to an irreversible loss of water molecules. The reaction order for degradation of PANI is found to be close to unity, and a value of 1.63 X 10-4 s-1 is obtained for the rate constant. The deactivated PANI does not recover its electrochemical activity even after a prolonged treatment in acidic electrolytes. The electrodeposition of PANI is carried out by galvanostatic, potentiostatic and potentiodynamic methods. The impedance data reflect a marked difference between the PANI deposited by static and dynamic methods. Furthermore, the impedance parameters vary with the sweep rate used in potentiodynamic method. Electrochemical impedance spectra of the electrodes are analyzed using a transmission line model consisting of two rails of finite resistances. Electrochemical deposition of polyaniline (PANI) is carried out on a porous carbon substrate for supercapacitor studies. PANI deposited at 100 mV s-1 sweep rate by potentiodynamic technique on porous carbon substrate is found to possess superior capacitance properties. Capacitance values as high as 1600 F g-1 are obtained and PANI coated carbon electrodes facilitate charge-discharge current densities as hgh as 45 mA cm-2 (19.8 A g-1 ). Electrodes are found to be fairly stable over a long cycle-life, although there is some capacitance loss during the initial stages of cycling. Electrooxidation of ascorbic acid on polyaniline is studied in a fuel-cell. Ascorbic acid (H2A) is employed as fuel and polyaniline (PANI) as the catalyst. H2A is an environmentally and biologically friendly molecule. The catalyst, namely PANI does not consist of any platinum group metal, and at 70 0C , a maximum power density of 4.3 mW cm-2 is obtained at a current density of 15 mA cm-2 . Also, studies on anodically deposited RuO2 for capacitor applications are reported. Cathodic deposition of RuO2 generally produces a mixture of Ru and RuO2 . On the other hand, the anodic depsotion on SS substrates is found to produce RUO2 which is characterized with high power supercapacitance properties. A capacitance value of 274 F g-1 is obtained at a current density of 20mA cm-2 .