CHARGE TRANSFER PROCESSES IN ELECTROACTIVE POLYMER FILMS Steven Holdcroft B.Sc. (Honours), University of Salford, U.K., 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR 'IXE DEGREE OF DOCTOR OF PHILOSOPHY in the Department 0f Chemistry @ Steven Holdcroft Simon Fraser University June 1987 All rights reserved. This thesis may not be reproduced in whole or in part, by photocopy or by other means, without the permission of the author. APPROVAL Name : Steven Holdcroft Degree : Doctor of Philosophy Title of Thesis: Charge Transfer Processes in Electroactive Polymer Films Examining Committee: Chairman: Dr. F. W. B. Einstein Dr. B. L. Funt Senior Supervisor Dr. S. R. xorrison Dr. T. N. Bell Dr. R. J . ~ushlev Dr. R. N. O'Brien External Examiner Professor, Department of Chemistry, University of Victoria Date of approval: / .pJ&Gc p-gj_ PARTIAL COPYRIGHT LICENSE I hereby grant to Slmon Fraser University the right to lend my thesis, proJect or extended essay (the tltle of which is shown below) to users of the Simon Fraser University Llbrary, and to make partial or single copies only for such users or in response to a request from the library of any other university, or other educational Institution, on its own behalf or for one of its users, I further agree that permission for multiple copying of this work for scholarly purposes may be granted by me or the Dean of Graduate Studies. It is understood that copying or publication of this work for financial gain shall not be allowed without my written permission. T it l e of Thes i s/Project/Extended Essay Author: (signature) - iii- ABSTRACT Polymer modified electrodes have potential applications in energy conversion and storage systems, sensors, displays, chemical synthesis and microelectronic devices. This work investigates the charge transfer processes accompanying electrochemical reactions at polymer modified electrodes. Thin films of poly-[(9,10 anthraquinone-2-carbony1)styrene-co-styrene (PAQ) were deposited on Pt electrodes and their electrochemical properties investigated. PAQ films were electrocatalytic towards O2 reduction in non- aqueous media. The kinetic parameters were investigated and the current densities associated with permeation of 02,charge propagation and rate of cross reaction were determined. Theoretical plots of catalytic efficiency vs. film thickness were developed. The experimental data are consistent with such plots and indicate that the catalytic efficiency is insensitive to film thickness. The protection of silicon semiconductors by PAQ films and the transfer, of conduction band carriers through PAQ to oxidising agents in solution were investigated. PAQ coated Si electrodes exhibited superior stability for the reduction of O2 over bare Si in the presence and absence of AQ in solution. Films of polypyrrole (PP) were electrodeposited on electrodes. PP coated electrodes, in conjunction with PAQ coated electrodes, were employed as cathodes in simple electrochemical rechargeable cells. The charge storage capabilities of these cells were determined. Photo-assisted rechargeable cells were devised using PAQ coated semiconductor anodes and PP coated cathodes. The energetics of electron transfer at the Si/PP interface were studied using a.c. impedance techniques, voltammetry and open circuit voltage - iv- measurements. The data are interpreted on the basis of facile electron transfer between silicon semiconductor surface states and the polypyrrole film. The conductive nature of PP prompted the fabrication and investigation of PP films containing Pt particles for electrocatalytic purposes. The distribution of Pt, as clarified by Auger electron spectroscopy, can be made homogeneous through the film, or localised at specified regions by a number of film forming techniques. PP/Pt films electrocatalyse O2 reduction within a narrow potential window. Rotating disk electrode studies, using films of various thickness and Pt loading, indicate that the catalytic current obtained for films containing homogeneously dispersed Pt is strongly dependent on the rate of Ot permeation. -vi- ACKNOWLEDGEMENTS My thanks go to: Dr. 8. L. Funt for his steady support and supervision. Dr. S. R. Morrison and the Energy Research Institute for helpful comments and discussions. Dr. P. P. M. Hoang and Messrs. S. V. Lowen, F. Orfino, G. H. Fritzke and E. M. Peters for assistance in experimental tasks. Members and fellow graduate workers of the Department of Chemistry for friendship and useful discussions. I would like to express my gratitude to Drs. B. L. Fune and S. R. Morrison, the Department of Chemistry, and Simon Fraser University for financial assistance. Last, but not least, I thank my wife, Amanda, for her patience and encouragement while preparing this thesis. -vii- LIST OF ABBREVIATIONS A Electrode area A Substrate A/B Substrate couple AES Auger electron spectroscopy AQ Anthraquinone ~PY 2,2'-bipyridine BQ Benzoquinone C Capacitance C-V Capacitance-voltage (CH), Polyacetylene CH3 CN Acetonitrile (C4H3N)x Polypyrrole Co & CR Bulk concentration of oxidised and reduced species toe & cRe Concentration of oxidised and reduced species at the electrode surface Cs c Space charge capacitance d Film thickness DfiL,& D Diffusion coefficient of substrate in the film and solution DE Electron diffusion coefficient Do & DR Diffusion coefficient of species 0 & R DMSO Dimethyl sulphoxide E Kinetic control by diffusion of electrons in the film Conduction band energy Ec B E~ Barrier height ED Energy density ESD Electricity storage density Potential of anodic peak E~a Potential of cathodic peak Electrochemical potential of polypyrrole Valence band energy Formal potential of redox couple Half wave potential Rest potential of polypyrrole film Faraday Glassy carbon Levich current Current density associated with charge propagation through the film Current density associated with rate of cross reaction Limiting current Peak current Current density associated with permeation of substrate in the polymer film Current density associated with the interplay of iE, is, iK and iA Equilibrium constant Rate constant for cross reaction between the substrate and mediator Standard heterogeneous electron transfer rate constant Number of electrons involved in reaction n~ Concentration (AES) P Oxidised form of catalyst PAQ Poly[p-(9,lO anthraquinone-2-carbony1)styrenel-co-styrene PEC Photoelectrochemical cell PP Polypyrrole P/Q Mediator couple Reduced form of catalyst - ix- Charge associated with PP formation Qf o r m Charge required to reduce a polypyrrole film Qr ed Charge associated with electrochemical doping of PP Qox Total charge passed during polymerisation Qtot R Gas constant R Kinetic control by the cross exchange reaction RDE Rotating disk electrode RRDE Rotating ring-disk electrode S Kinetic control by the diffusion of the substrate in the film SC Semiconductor b SCE Saturated calomel electrode Temperature TEAP Tetraethylammonium perchlorate Flat band ptential Open circuit voltage Open circuit photovoltage Diffusion layer Peak width at half height Surface coverage of attached electroactive material Quantity of material applied to the electrode Observed surface coverage of electroactive material Surface coverage of catalyst P Total coverage of attached electroactive material Partition coefficient Density Inelastic mean free path of Auger electrons Kinematic viscosity of solution Angular velocity (rad/s) TABLE OF CONTENTS APPROVAL ii ABSTRACT iii DEDICATION ACKNOWLEDGEMENTS LIST OF ABBREVIATIONS TABLE OF CONTENTS LIST OF TABLES xiv LIST OF FIGURES GENERAL INTRODUCTION I. THEORETICAL ASPECTS 1.1. ANALYTICAL TECHNIQUES 1.1.1. ELECTROANALYTICAL TECHNIQUES 1. CYCLIC VOLTAMMETRY 2. CHRONOAMPEROMETRY 3. ROTATING DISK VOLTAMMETRY 1.1.2. ELECTROANALTYTICAL TECHNIQUES APPLIED TO POLYMER FILMS 1. THEORY OF VOLTAMMETRY AND CHARGE TRANSPORT 2. THEORY OF ELECTROCATALYSIS BY REDOX POLYMER FILMS 1.1.3. SPECTRAL TECHNIQUES 1. ABSORPTION SPECTROPHOTOMETRY 2. AUGER ELECTRON SPECTROSCOPY (AES) SEMICONDUCTOR ELECTROCHEMISTRY 1.2.1. THE ELECTRICAL DOUBLE LAYER MODEL 1.2.2. THE SEMICONDUCTOR SPACE CHARGE LAYER 1.2.3. BAND EDGE UNPINNING 1.2.4. THE CONDUCTION AND VALENCE BAND ENERGY 11. ELECTROCHEMICAL STUDIES OF ANTHRAQUINONE REDOX POLYMER FILMS ON CONDUCTIVE AND SEMICONDUCTIVE SUBSTRATES 11.1. INTRODUCTION 11.1.1. TYPES OF REDOX POLYMER FILM 11.1.2. VOLTAMMETRY 11.1.3. CHARGE TRANSPORT 11.1.4. ELECTROCATALYSIS 11.1.5. SEMICONDUCTOR ELECTRODES MODIFIED BY REDOX POLYMER FILMS 11.2. EXPERIMENTAL 11.2.1. CHEMICALS 11.2.2. FILM PREPARATION 11.2.3. ELECTROCHEMISTRY 11.3. RESULTS AND DISCUSSION 11.3.1. VOLTAMMETRY AND CHARGE TRANSPORT 11.3.2. ELECTROCATALYSIS OF O2 11.3.3. THE ELECTROCHEMICAL BEHAVIOUR OF Si ELECTRODES MODIFIED BY FILMS OF PAQ 11.4. CONCLUSION 111. CHARGE STORAGE PROPERTIES OF PAQ AND POLYPYRROLE FILMS 111.1. INTRODUCTION 111.1.1. ELECTROCHEMICAL PROPERTIES OF POLYPYRJXOLE 111.1.2. PRINCIPLES OF CHARGE STORAGE TECHNOLOGY 111.1.3. RECHARGEABLE STORAGE CELLS BASED ON ORGANIC CONDUCTING POLYMERS 111.2. EXPERIMENTAL 111.2.1. CHEMICALS 111.2.2. FILM PREPARATION 111.2.3. ELECTROCHEMISTRY -xii- 1. CHRONOCOULOMETRY 2. CYCLIC VOLTAMMETRY 3. DISCHARGE CHARACTERISTICS 111.3. RESULTS AND DISCUSSION 111.3.1. ELECTROCHEMISTRY OF PAQ ANDPP COATED ELECTRODES 111.3.2. Pt/PAQ//PP/Pt RECHARGEABLE CELLS 111.3.3. p-SEMICONDUCTOR/PAQ//PP/Pt RECHARGEABLE CELLS 111.4. CONCLUSION IV. THE ENERGETICS OF ELECTRON TRANSFER AT THE POLYPYRROLE/SILICON INTERFACE IV.l. INTRODUCTION IV.l.l. ELECTROCHEMISTRY WITH SILICON ELECTRODES IV.1.2. POLYPYRROLE