Conducting polymer composites and their electrochemical and charge transport properties
Creators: name: Mayevsky, David
Publisher: name: Monash University
Year: 2017
Subjects: subject: 91299 Materials Engineering not elsewhere classified
subject: FOS: Materials engineering
subject: FOS: Materials engineering

description: At the commencement of this project, the objective was to manufacture new inexpensive electrocatalytic systems based on conducting polymers in order to be able to generate hydrogen gas from water. At the time, one system, polythiophene, had been shown to work as a photo-electrocatalyst, and the objective was to manufacture new systems, that could function simply as catalysts. <br> It was discovered that to approach the task of manufacturing an electrocatalyst based on conducting polymers is a non-trivial goal. This thesis therefore details numerous developments made towards understanding of the intricacies involved in catalyst validation and the understanding of charge transport and charge transfer in conducting polymers and their composites. <br> In Chapter 1 the motivation behind the project is outlined and literature from various backgrounds is explored in the context of the work performed during the project. The primary areas discussed are the polymer manufacturing, charge transport and catalysis fields with a particular focus on conducting polymers. <br> Chapter 2 outlines the experimental methodologies used. <br> In Chapter 3 a new manufacturing technique is outlined that enables evaporation of a monomer below its melting temperature. This is performed by dispersing the intended evaporant monomer in a high molecular weight polymer or an ionic liquid. This is an important development, as previously vapour phase polymerisation could only be performed at temperatures where the monomer had melted. This work is outlined in the publication ‘<i>A simple technique for performing</i> <i>evaporation of quaterthiophene below the melting temperature vapour deposition</i>,’ published in RSC Advances. <br> Chapter 4 describes the technique of polymer blending and how blending can be used to decouple the charge transport and catalytic properties of a material. Using this approach, new conducting polymer/quinoid composites are reported. The pEDOT:vitamin K composite was manufactured and it was demonstrated that 100% of the vitamin K groups in the entire composite were electroactive, and bonding protons, and that the composite was stable up to 500 cycles. The manuscript titled ‘<i>pEDOT:vitamin K composites: electrochemistry on stable proton bonding thin film electrodes</i>’ outlining this work is currently in review at Electrochimica Acta. <br> Chapter 5 explores an intriguing behaviour of this composite in the presence of buffered solutions. It was demonstrated that the material was sensitive to the concentration of buffering ions in solution. Where the concentration of buffering ions was low, it was found that a higher energy was required to perform the reduction reaction on the quinoid in the composite, without a change in pH. It was demonstrated that the shift of energy for the reaction followed the shift of the energy predicted by the Nernst equation. As a consequence, the concept of ‘availability’ was outlined, where the availability of protons is the sum of the concentration of the buffer (the concentration of undissociated protons) + the concentration of H<sub>3</sub>O<sup>+</sup>. It was demonstrated that the electrolytic performance of a gold electrode followed the same trend, where more energy is required to perform the hydrogen evolution reaction in solutions with a low buffer concentration, without a change in pH. A patent was filed for an electrode that is capable of measuring this trend. <br> A new composite of pEDOT:quinoxaline was manufactured that stores 2 electrons at an energy appropriate for the water reduction reaction. It was hoped that the quinoxaline would function as a catalyst for the water reduction reaction, however, it did not. The reasons for this merit further exploration, however we suggest that it may be a consequence of a combination of factors. These include; an insufficient overpotential for the reaction, the protons being too far away, or the structure having a high energy ‘intermediate’ state before the evolution of the protons. This work is outlined in Chapter 5 of this thesis. <br> The structural analysis of conducting polymer blends revealed that the limiting factor for conductivity in doped conducting polymer was not limited by π stacking of the system. In Chapter 6 it is demonstrated for the same film that in transitioning from a disordered to an ordered state the resistance of PEDOT-based film remains the same. The consequence of this observation is that charge transport can be measured during the vapour phase polymerisation process. This work is outlined in the publication ‘<i>Decoupling order and conductivity in doped conducting polymers</i>,’ and has been published in Physical Chemistry Chemical Physics. <br> Chapter 7 summarises the main conclusions and outcomes of this thesis and discusses avenues for future research. <br> While no electrocatalysts were manufactured, developments were made in the understanding of the reactants involved in the electrolysis of water. New composites were manufactured that interact with water and bond proton, and a new understanding of relationship between structure and charge transfer/transport in conducting polymer composites was developed. <br> <br>