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Understanding Ion and Solvent Transport in Anion Exchange Membranes Under Humidified Conditions

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Understanding Ion and Solvent Transport in Anion Exchange Membranes Under Humidified Conditions UNDERSTANDING ION AND SOLVENT TRANSPORT IN ANION EXCHANGE MEMBRANES UNDER HUMIDIFIED CONDITIONS by Himanshu Sarode © Copyright by Himanshu Sarode, 2015 All rights reserved A thesis submitted to the Faculty and Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Chemical Engineering). Golden, Colorado Date _____________________ Signed: ___________________________ Himanshu N. Sarode Signed: ___________________________ Dr. Andrew M. Herring Thesis Advisor Golden, Colorado Date _____________________ Signed: ___________________________ Dr. David W. M. Marr Professor and Head Department of Chemical and Biological Engineering ii ABSTRACT Anion exchange membranes (AEM) have been studied for more than a decade for potential applications in low temperature fuel cells and other electrochemical devices. They offer the advantage of faster reaction kinetics under alkaline conditions and ability to perform without costly platinum catalyst. Inherently slow diffusion of hydroxide ions compared to protons is a primary reason for synthesizing and studying the ion transport properties in AEMs. The aim of this thesis is to understand ion transport in novel AEMs using Pulse Gradient stimulated Spin Echo Nuclear Magnetic Resonance technique (PGSE NMR), water uptake, ionic conductivity, Small Angle X- ray Scattering (SAXS) etc. All experiments were performed under humidified conditions (80-95% relative humidity) and fuel cell operating temperatures of 30-90°C. In this work, the NMR tube design was modified for humidifying the entire NMR tube evenly from our previous design. We have developed a new protocol for replacing caustic hydroxide with harmless fluoride or bicarbonate ions for 19F and 13C NMR diffusion experiments. After performing these NMR experiments, we have obtained in-depth understanding of the morphology linked ion transport in AEMs. We have obtained the highest fluoride self-diffusion coefficient of > 1 x 10-5 cm2/sec (@ 55°C) for ETFE-g-PVBTMA membrane which is a result of low tortuosity of 1 obtained for the membrane. This faster fluoride transport combined with low tortuosity of the membrane resulted in > 100mS/cm hydroxide conductivity for the membrane. Polycyclooctene (PCOE) based triblock copolymers are also studied for in-depth understanding of molecular weight, IEC, mechanical and transport properties. Effect of melting temperature of PCOE has favorable effect on increasing ion conductivity and lowering activation energy. Mechanical properties of these types of membranes were studied showing detrimental effect of water plasticization which results in unsuitable mechanical properties. iii Hydroxide conductivity was studied to measure the effectiveness of AEMs for practical applications. PPO-b-PVBTMA membrane showed more than 100mS/cm conductivity and PCOE based membranes showed ~ 70mS/cm conductivity which is a combined effect of Grotthuss hopping and vehicular mode of ion transport, which lowers the activation energy to < 14 kJ/mol. Overall this thesis sheds light on one of the most important aspect of AEMs: ion/solvent transport, we have studied effect of membrane chemistry, IEC, morphology, polymer molecular weight on self-diffusion, ionic conductivity to have a better understanding for development of a good AEM for practical applications. iv TABLE OF CONTENTS ABSTRACT ................................................................................................................................. iii LIST OF FIGURES ....................................................................................................................... ix LIST OF TABLES ....................................................................................................................... xiii ACKNOWLEDGEMENTS ......................................................................................................... xiv CHAPTER 1 INTRODUCTION .....................................................................................................1 1.1 Anion Exchange Membranes ........................................................................................ 3 CHAPTER 2 EXPERIMENTAL METHODS ..............................................................................14 2.1 Ionic Conductivity Measurements in Aqueous Solution ............................................ 14 2.2 Ion Exchange Capacity (IEC) ..................................................................................... 15 2.3 Ionic conductivity measurements for AEMs .............................................................. 15 2.4 Pulse Gradient stimulated Echo Nuclear Magnetic Resonance (PGSE-NMR) .......... 16 2.5 Water uptake measurements ....................................................................................... 19 2.6 Mechanical Characterization of AEMs ....................................................................... 20 2.7 Small Angle X-ray Scattering under temperature and humidity control .................... 21 CHAPTER 3 INSIGHTS INTO THE TRANSPORT OF AQUEOUS QUATERNARY AMMONIUM CATIONS: A COMBINED EXPERIMENTAL AND COMPUTATIONAL STUDY ................................................................................23 3.1 Abstract ....................................................................................................................... 23 3.2 Introduction ................................................................................................................. 24 3.3 Methods....................................................................................................................... 28 3.3.1 Experimental details..................................................................................... 28 3.3.2 Simulation Details ........................................................................................ 32 3.4 Results ......................................................................................................................... 34 3.4.1 Conductivity of quaternary ammonium hydroxides .................................... 34 v 3.4.2 Effect of CO2 on conductivity...................................................................... 36 3.4.3 Self-diffusion coefficients ............................................................................ 37 3.4.4 Residence time ............................................................................................. 43 3.4.5 Structure and solvation of ammonium cations............................................. 44 3.5 Conclusions ................................................................................................................. 46 3.6 Acknowledgements ..................................................................................................... 47 CHAPTER 4 UNDERSTANDING ANION EXCHANGE MEMBRANE ION TRANSPORT USING FLUORIDE SELF DIFFUSION ................................................................48 4.1 Introduction ................................................................................................................. 48 4.2 Experimental ............................................................................................................... 49 4.3 Results and Discussion ............................................................................................... 52 4.4 Conclusion .................................................................................................................. 57 CHAPTER 5 ION TRANSPORT IN A CHEMICALLY STABLE ANION EXCHANGE MEMBRANE, POLY (2,6-DIMETHYL-1,4-PHENYLENE OXIDE)-B- POLY(VINYLBENZYLTRIMETHYLAMMONIUM) .........................................58 5.1 Introduction ................................................................................................................. 58 5.2 Experimental ............................................................................................................... 59 5.3 Calculation of fluoride conductivity based on Nernst Einstein equation ................... 61 5.4 Results and Discussion ............................................................................................... 61 5.5 Conclusions ................................................................................................................. 69 CHAPTER 6 UNDERSTANDING ION AND SOLVENT TRANSPORT IN POLYETHYLENE BASED ANION EXCHANGE MEMBRANE FOR ELECTROCHEMICAL APPLICATIONS .....................................................................................................71 6.1 Introduction ................................................................................................................. 71 6.2 Experimental Procedure: ............................................................................................. 72 6.2.1 Ion Exchange Procedure .............................................................................. 73 vi 6.2.2 Ion Exchange Capacity Measurement ......................................................... 74 6.2.3 Ionic Conductivity Measurements ............................................................... 74 6.2.4 PGSE NMR Measurements: ........................................................................ 75 6.3 Results and Discussion ............................................................................................... 76 6.4 Conclusions ................................................................................................................. 84 6.5 Acknowledgements ....................................................................................................
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