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Designing Polymer Electrolytes for Alkaline Anion

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Designing Polymer Electrolytes for Alkaline Anion DESIGNING POLYMER ELECTROLYTES FOR ALKALINE ANION EXCHANGE MEMBRANE FUEL CELLS A DISSERTATION SUBMITTED TO THE DEPARTMENT OF CHEMICAL ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Steve Sidi He May 2015 © 2015 by Steve Sidi He. All Rights Reserved. Re-distributed by Stanford University under license with the author. This dissertation is online at: http://purl.stanford.edu/wh023dj3361 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Curtis Frank, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Thomas Jaramillo I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Andrew Spakowitz Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost for Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Abstract Increasing global demand and dependence on fossil fuels, coupled with environmental concerns arising from their use, have sparked interest in alternative energy sources. Hydrogen-powered fuel cells are a promising solution, offering a clean, scalable method for energy production. The most prominent low-temperature fuel cell devices today operate under an acidic environment, using a semi-permeable proton exchange membrane (PEM) to separate the two electrodes. However, their caustic operating conditions present unique stability and activity issues for the metal catalysts and ultimately necessitates the use of platinum-group materials, severely limiting commercial viability. A potential solution is to operate the fuel cell device under an alkaline environment using an anion exchange membrane (AEM), transporting hydroxide ions in lieu of protons. The basic environment opens the door for cheaper catalysts based on nickel and molybdenum, eliminating the cost barrier associated with PEM fuel cells. Unfortunately, typical AEMs exhibit poorer ionic conductivity and stability compared to traditional acidic membranes (e.g. Nafion), offsetting any potential cost advantage they may afford. This dissertation discusses design rationales towards enhancing the macroscopic properties of AEMs. Specifically, I present two experimental design motifs for improving the device viability of AEMs. In the first case, I present a semi- interpenetrating network design where a linear AEM ionomer is stabilized by a crosslinked poly(styrene-co-divinylbenzene) matrix. The crosslinked network acts as a reinforcing scaffold, dramatically increasing dimensional stability while maintaining iv excellent anion conductivity. Prototypical single-stack fuel cells with enhanced performance and stability have been fabricated from these materials, validating the design choices. In the second approach, I demonstrate the ability to increase hydroxide conductivity by tuning the nanostructure of the polymer electrolyte. Specifically, I show that tethering hydrophilic poly(ethylene glycol) grafts onto a benzyltrimethylammonium polysulfone benchmark AEM results in phase-separated, water-rich domains on the order of 5 to 10 nm. These domains serve as an ion transport pathway, facilitating the diffusion of hydroxide anions and consequently enhancing the efficiency of hydroxide conduction. Finally, in order to better understand the phase behavior and structure-property relationships of typical AEM materials, we have developed coarse-grained simulations and fundamental polymer theory to elucidate the thermodynamic behavior of random copolymers. We find that both the stochastic distribution of monomers along the polymer backbone as well as the overall stiffness of the polymer chain heavily influences its phase behavior (i.e., morphology and critical point). The ultimate objective is to provide not only a theoretical basis for understanding and explaining structure-property relationships in existing AEM materials, but to provide a set of general design guidelines moving forward. v Acknowledgments I would like to begin by thanking the all the people at Stanford who have supported me throughout my Ph.D. career, without whom I would not be in a position to write this very sentence. I am truly lucky to have found a research advisor, Prof. Curtis W. Frank, whose encouragement and support has have had a highly positive influence on me during my time at Stanford. Curt’s flexibility and understanding has allowed me to pursue my own research ideas, allowing me to grow professionally in a way that I otherwise could not have. I would also like to thank Prof. Andrew Spakowitz for providing me an opportunity to work on polymer theory and simulations, the nature of which is a far cry from the experimental studies I had become familiar with. This experience has opened me to new perspectives and understanding of polymer science, tremendously accelerating my scientific growth. Andy has been highly encouraging and has always been open and receptive to my questions, no matter how trivial they might seem. Prof. Thomas Jaramillo has been a great resource for not only helping me understand electrochemistry and energy conversion technologies, but how they fit into the overall economic picture; bridging fundamental science with practical implementation has helped shaped me into a better engineer. In addition, I would like to thank Prof. Do Y. Yoon for his countless advice, comments and critiques throughout the years. Do has been a great source of knowledge on general polymer science, and his extensive experience has provided unique viewpoints and interpretation of my data. vi My friends and fellow graduate students have played a critical role not only in my graduate experience, but my research as well. I especially thank Elyse Coletta, who had worked on a similar project concerning proton exchange membranes, for all her advice and our countless discussions. Desmond Ng was a great resource for teaching me how to fabricate membrane electrode assemblies from my materials. Shifan Mao has been an instrumental collaborator in the theory and simulation work and was always available to answer my questions and concerns. Moreover, I would like to thank the various undergraduate and graduate rotation students I had the pleasure of working with: Sumit Mitra, Nathaniel Morrison, Joseph Troderman, Leo Shaw, Jeff Lopez and Alaina Strickler. I especially thank Alaina Strickler, who played a key role in the protocol development for the semi-interpenetrating network work. I thank the administrative staff in the Department of Chemical Engineering at Stanford for all their help. In particular, Jeanne Cosby, Jeannie Lewindowski, Pamela Dixon and Pam Juanes guaranteed everything went smoothly, from setting up purchasing accounts to making room and equipment reservations. Finally, this work would not have been possible without funding and support from the TomKat Center for Sustainable Energy and the Precourt Institute for Energy. vii Dedication I would like to dedicate this thesis to my family. My parents immigrated to the United States when I was still a toddler to seek out a better life; growing up here has opened up all sorts of opportunities for me that would not have been possible had we stayed in China. I thank them for not only their sacrifice, but also for their support, care and trust to let me carve out my own path in life. In addition, I want to thank my wife, Marina Kim. She believed in me even when I doubted myself, and has been a constant source of comfort, encouragement and support. In retrospect, none of this would have been possible without Dr. Dena Leggett, my high school AP Chemistry teacher. Prior to her class, I had no idea what chemistry was or what a scientific career entailed. Indeed, the whole endeavor at the time seemed daunting after hearing horror stories about “stoichiometry” from upperclassman. Yet, my experience was totally the opposite – Dr. Leggett made chemistry intuitive, interesting and, above all, fun. I am eternally grateful to her for opening my eyes to the world of science and engineering; if not for her, I would not be where I am today. viii Table of Contents Chapter 1: Introduction .................................................................................................... 1 1.1 Background ............................................................................................................................. 1 1.2 Motivation and Outline ....................................................................................................... 4 Chapter 2: Background ..................................................................................................... 7 2.1 Hydroxide Transport in Anion Exchange Membranes ............................................. 7 2.2 Major Challenges ................................................................................................................ 11 2.2.1 Low Ionic Conductivity ............................................................................................................. 11 2.2.2 Poor Chemical Stability ...........................................................................................................
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