Electrochemical Studies Of PEDOT : Microscopy, Electrooxidation Of Small Organic Molecules And Phenol, And Supercapacitor Studies

Abstract

Following the discovery of electronic conductivity in doped polyacetylene, various studies on conducting polymers have been investigated. These polymers are essentially characterized by the presence of conjugated bonding on polymer backbone, which facilitates formation of polarons and bipolarons as charge carriers. Poly(3,4-ethylenedioxythiophene) (PEDOT) is an interesting polymer because of high electronic conductivity, ease of synthesis and high chemical stability. Electrochemically prepared PEDOT is more interesting than the polymer prepared by chemical routes because it adheres to the electrodes surface and the PEDOT coated electrodes can directly be used for various applications such as batteries, supercapacitor, sensors, etc. A majority of the studies described in the thesis are based on PEDOT. Studies on polyanthanilic acid and reduction of hydrogen peroxide on stainless steel substrate are also included. Chapter 1 provides an introduction to conducting polymers with a focus on synthesis, electrochemical characterization and applications of PEDOT. In Chapter 2, microscopic and impedance spectroscopic characterization of PEDOT coated on stainless steel (SS) and indium tin oxide (ITO) coated glass substrates are described. Electrosynthesis of PEDOT is carried out on SS electrodes by three different techniques, namely, potentiostatic, galvanostatic and potentiodynamic techniques. The SEM images of PEDOT prepared by the galvanostatatic and potentiostatic routes indicate globular morphology. However, it is seen that porosity increases by increasing the current or the potential. In the cases of both galvanostatic and potentiostatic routes, the oxidation of EDOT to form PEDOT takes place continuously during preparation. However, in the case of potentiodynamic experiment between 0 and 0.9 V vs. SCE (saturated calomel electrode), the formation of PEDOT occurs only when the potential is greater than 0.70 V. During multicycle preparation to grow thicker films of PEDOT, formation of PEDOT takes place layer by layer, a layer of PEDOT being formed in each potential cycle. PEDOT prepared in the potential ranges 0-0.90 V and 0-1.0 V show globular morphology similar to the morphology of the galvanostatically and potentiostatically prepared polymer. If prepared in the potential ranges 0-1.1 V and 0-1.2 V, the PEDOT films have rod-like and fibrous morphology. This is attributed to larger amount of PEDOT formed in each cycle in comparison with lower potential ranges and also to partial oxidation of PEDOT at potentials ≥ 1.10 V. PEDOT is also electrochemically prepared on ITO coated glass substrate. Preparation is carried out under potentiostatic conditions in the potential range between 0.9 and 1.2 V. Atomic force microscopy (AFM) studies indicate a globular topography for PEDOT films prepared on ITO coated glass plates. The height and width of globules increase with an increase in deposition potential. The PEDOT coated SS electrodes are subjected to electrochemical impedance spectroscopy studies in 0.1 M H2SO4. The Nyquist plot of impedance consists of a depressed semicircle, which arises due to a parallel combination of the polymer resistance and double-layer capacitance (Cdl). Impedance data are analyzed. Studies on electrooxidation of methanol, formic acid, formaldehyde and ethanol on nanocluster of Pt and Pt-Ru deposited on PEDOT/C electrode are reported in Chapter 3. Studies on electrooxidation of small molecules are important in view of their promising applications in fuel cells. Films of PEDOT are electrochemically deposited on carbon paper. Nanoclusters of Pt and bimetallic Pt-Ru catalysts are potentiostatically deposited on PEDOT/C electrodes. Catalysts are also prepared on bare carbon paper for studying the effect of PEDOT. The presence of PEDOT film on carbon paper allows the formation of uniform, well dispersed nanoclusters of Pt as well as Pt-Ru catalysts. TEM studies suggest that the nanoclusters of about 50 nm consist of nanoparticles of about 5 nm in diameter. Electrooxidation of methanol, formic acid, formaldehyde and ethanol are studied on Pt-PEDOT/C and PtRu-PEDOT/C electrodes by cyclic voltammetry and chronoamperometry. The data for oxidation of these small organic molecules reveal that PEDOT imparts a greater catalytic activity for the Pt and Pt-Ru catalysts. Results of these studies are described in Chapter 3. In Chapter 4, PEDOT is coated on SS substrate to investigate phenol oxidation. Studies on electrochemical oxidation of phenol are interesting because it is important to remove phenol from contaminated water or industrial effluents. Deactivation of the anode due to the formation and adsorption of polyoxyphenylene on its surface is a common problem for a variety of electrode materials, during phenol oxidation. Investigations on suitable anode materials, which can undergo no or moderate poisoning by polyoxyphenylene, are interesting. In the present study, it is shown that the electrooxidation rate of phenol is greater on PEDOT/SS electrodes than on Pt. Deactivation of PEDOT/SS electrode is slower in relation to Pt. The oxidation of phenol on PEDOT/SS electrode occurs to form both polyoxyphenylene and benzoquinone in parallel. Cyclic voltammetry of phenol oxidation is studied by varying the concentration of phenol, sweep rate and thickness of PEDOT. Ac impedance studies indicate a gradual increase in polymer resistance due to adsorption of polyoxyphenylene during multi sweep cyclic voltammetry. This investigation reveals that PEDOT coated on a common metal or alloy such as SS is useful for studying electrooxidation of phenol, which is generally studied on a noble metal based electrodes. Electrochemically prepared PEDOT is used for supercapacitor studies and the results are presented in Chapter 5. Generally, electronically conducting polymers possess high capacitive properties due to pseudo-faradaic reactions. PEDOT/SS electrodes prepared in 0.1 M H2SO4 are found to yield higher specific capacitance (SC) than the electrodes prepared from neutral aqueous electrolyte. The effects of concentration of H2SO4, concentration of SDS, potential of deposition and nature of supporting electrolytes used for capacitor studies on SC of the PEDOT/SS electrodes are studied. Specific capacitance values as high as 250 F g-1 in 1 M oxalic acid are obtained during the initial stages of cycling. However, there is a decrease in SC on repeated charge-discharge cycling. Spectroscopic data reflect structural changes in PEDOT on extended cycling. Self-doped PANI is expected to possess superior electrochemical characteristics in relation to PANI. The self-doping is due to the presence of an acidic group on the polymer chain. However, self-doped PANI is soluble in acidic solutions against insolubility of PANI. In the present study, poly(anthranilic acid), PANA, is encapsulated in porous Nafion membrane by chemical and a novel electrochemical methods. PANA present in solid form in Nafion membrane does not undergo dissolution in acidic solutions. The methods of preparation and various electrochemical, optical and spectroscopic characterizations studies of PANA-Nafion are described in Chapter 6. Electroreduction of H2O2 is studied on sand-blasted stainless steel (SSS) electrode in an aqueous solution of NaClO4 and the details are reported in Chapter 7. The cyclic voltammetric reduction of H2O2 at low concentrations is characterized by a cathodic peak at -0.40 V versus standard calomel electrode (SCE). Cyclic voltammetry is studied by varying the concentration of H2O2 in the range from 0.2 mM to 20 mM and the sweep rate in the range from 2 to 100 mV s-1. Cyclic voltammograms at concentrations of H2O2 higher than 2 mM or at high sweep rates consist of an additional current peak, which may be due to the reduction of adsorbed species formed during the reduction of H2O2. Amperometric determination of H2O2 at -0.50 V vs. SCE provides the detection limit of 5 μM H2O2. A plot of current density versus concentration has two linear segments suggesting a change in the mechanism of H2O2 reduction at concentrations of H2O2 ≥ 2 mM. From the rotating disc electrode study, diffusion co-efficient of H2O2 and rate constant for reduction of H2O2 are evaluated. Thus, stainless steel, which is inexpensive and a common alloy, is useful for studying electrochemical reduction of H2O2 and also for analytical application. This work is initiated to study the reduction of H2O2 on PEDOT/SS electrodes. As a result of preliminary experiments, it is found that PEDOT does not exhibit any influence on the kinetics of H2O2 reduction. Therefore studies conducted using bare stainless steel are included in this chapter. Results of the above studies are described in the thesis.