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Cross Linked Sulphonated Poly(Ether Ether Ketone) for the Development of Polymer Electrolyte Membrane Fuel Cell

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Cross Linked Sulphonated Poly(Ether Ether Ketone) for the Development of Polymer Electrolyte Membrane Fuel Cell Cross Linked Sulphonated Poly(ether ether ketone) for the Development of Polymer Electrolyte Membrane Fuel Cell Abdul Ghaffar Al Lafi, MSc. A Thesis Submitted for the Degree of Doctor of Philosophy The School of Metallurgy and Materials, The College of Engineering and Physical Sciences, The University of Birmingham, Birmingham, UK. September 2009 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Synopsis Ion irradiation has been investigated as a route for the preparation of mechanically stable and highly durable cross linked sulphonated PEEK for fuel cell application. Ion irradiation was carried out using the University of Birmingham’s Scanditronix MC-40 Cyclotron operating at 11.7 MeV for H+ and 30 MeV for He2+ and the irradiated materials were characterized focusing on structural, thermal, morphological as well as dielectrical properties. Alterations produced in the molecular structure of amorphous PEEK by ion irradiation have been interpreted as due to chain scission and formation of cross links, as confirmed by sol-gel analysis in MSA using the well known Charlesby–Pinner equation. The thermal decomposition of irradiated PEEK was similar to that of untreated PEEK in that it occurred by a random chain scission process. The thermal decomposition temperature and kinetic data for irradiated PEEK films quantitatively suggest that these films still have sufficient thermal stability for long term applications as fuel cell membranes. The observed Tg increased linearly with cross link density in accordance with the DiBenedetto equation. The DSC results also indicated that the cross links which accompanied irradiation retard the crystallization, but no changes were observed in the mechanism of crystallization. The apparent activation enthalpy of the glass forming process, determined from DRS, however, decreased and analysis of the dielectric response by Cole-Cole, Havrilak-Negami and Kohlrausch-Williams-Watts equations indicated that the dipole relaxation was broadened and becoming more asymmetric with cross link density. This was interpreted as the cross linked network progressively restricting the length of the chain segments involved in dipole relation process yet adding to the complexity of the modes of segmental motions. The sulphonation of the cross linked PEEK was investigated in concentrated sulphuric acid following the kinetics of the reaction at room temperature. The rate of reaction decreased with the degree of cross linking and the progress with time was consistent with diffusion control of ii the sulphuric acid into the cross linked matrix. The results were consistent with the efficiency of the ions in cross linking PEEK and in particular with the differences in their linear energy transfer (LET). The materials properties of different cross linked SPEEK membranes have been investigated focusing on water uptake kinetics, stability tests and the performance in a single fuel cell. Increasing cross link density resulted in more bound water present in the equilibrated membranes and increasing the ion exchange capacity (IEC) gave rise to more free water. The results indicated that the cross linked membranes have lower methanol permeability and electro-osmotic drag as well as improved mechanical stability. The presence of a nano- structure in the cross linked membranes was confirmed and the sizes of pore present were comparable to those of Nafion. The effects of cross linking and IEC on the membranes stability in chemical, oxidative and thermal environments have also been considered. Cross linking had little effect in improving both thermal and oxidative stabilities but its effect in improving the chemical stability in particular in methanol solution was pronounced. The polarisation curves indicated that film thickness, IEC, cross link density as well as temperature affect the performance of the PEM fuel cell, but that the IEC values had a greater effect than cross link density. The performance of the cross linked membranes was very similar to that of non cross linked PEM suggesting the same mechanism of proton transport was present in both systems. The cross linked membranes showed better performance (higher power output and current density) compared to those of the non cross linked solvent cast SPEEK membranes of similar IEC due to the solvent effect. The measurement of power output and energy efficiency suggested that the cross linked PEMs produced in this work are promising candidates to replace Nafion membranes but more information is required, in particular on their long term stability under fuel cell operating conditions, and also in understanding the relationship between material properties and cross linking density. iii Acknowledgments I would like to acknowledge the Atomic Energy Commission of Syria for financial support throughout the time of the PhD course. I would like also to acknowledge the School of Metallurgy and Materials and the School of Physics and astronomy of the University of Birmingham for covering all materials and experimental costs. I would like to express my gratitude to the many people who made this work possible. First I am deeply indebted to my supervisor Prof. James N. Hay who provided guidance and profound expertise and wisdom throughout the course of this research. He was always willing to answer my every question, no matter how trivial. I feel extremely honoured to work under his great guidance. Without his help and support, this work would never have been accomplished. I am also deeply grateful to Dr. Mike J. Jenkins for his co-supervision, constant guidance, valuable advice and support in difficult times throughout the study, and I would like to give a very special thanks to Mr. Frank Biddlestone for his technical support. His broad experience, logical way of thinking and constructive comments has been of great value for me in doing the experimental part of the research. I would like to thank Prof. David J. Parker for allowing me to use the irradiation facilities, MC-40, and for useful discussion. I would like to thank Mr. John Lane for his kind helps with the fabrication of materials involved in the fuel cell design, and all my friends in the polymer lab for their great help and discussion. I would above all like to thank my parents for always believing in me and teaching me that any goal I set forth is attainable. Their support, love and prayers helped me through the time of this degree and I can never thank them enough. Finally, and most importantly, I would like to give my special thanks to my wife Safaa Eissa, for always being there for me. Thank you for your constant and endless love, support, understanding, patience and encouragement. I could not have succeeded without her love and understanding. iv In the Name of Almighty ALLAH (GOD), the Most Merciful, the Most Compassionate v To my parents… To my wife and my kids… vi Table of Contents Synopsis………………………………………………………………………………….…….ii Acknowledgements……………………………………………………………………….…...iv Dedications ……………………………………………………………………………….......vi Table of Contents………………………………………………………………………..…....vii Abbreviations..…………………………………………………………………………….….xv Symbols…………………………………………………………………………….……….xvii List of Figures………………………………………………………………………………...xx List of Schemes………………………………………………………………………….….xxix List of Tables…………………………………………………………………………….….xxx Chapter 1 Polymer Electrolyte Membrane Fuel Cells: Systems and Applications 1.1 Introduction ..........................................................................................................................1 1.2 Proton Exchange Membrane Fuel Cells ...............................................................................5 1.3 Review on the Nafion Based PEMFC Technology: Achievements and Challenges ...........7 1.3.1 Water Management........................................................................................................8 1.3.2 The CO Poisoning Effect...............................................................................................9 1.3.3 Direct Hydrogen ............................................................................................................9 1.3.4 Methanol Reformate....................................................................................................10 1.3.5 Direct Methanol...........................................................................................................11 1.3.6 Heat Recovery .............................................................................................................12 1.4 The Need for Developing High Temperature PEM............................................................12 vii 1.5 High Temperature Polymer Electrolyte Membranes..........................................................13 1.5.1 Modification of PFSA Membranes .............................................................................14 1.5.2 Alternative Sulphonated Polymer Membranes and Their Composites .......................16 1.5.2.1 Fluoro-Polymers ...................................................................................................16
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