Alkaline Anion Exchange Membranes for Fuel Cells Build the fuel cells of the future with this cutting-edge material Alkaline anion exchange membranes (AAEMs) are cutting-edge polyelectrolyte materials with growing renewable energy applications including fuel cells, batteries, hydrogen electrolyzers and electrodialysis technologies. Their use in relatively new alkaline exchange membrane fuel cells (AEMFCs) is designed to produce cost-effective clean energy (electricity) produced by a chemical reaction. Rigorous studies are being conducted to meet the requirements of AAEMs precisely tailored for high anion conductivity and durability for future high energy efficient devices. Hence, over the past few years the academic and industrial scientific communities have explored various polymeric, composite and inorganic materials and studied their properties as a potential AAEM. The accumulated literature in this area of investigation is vast and in order to provide the community with the tools needed to strive forward, there is a clear need to condense this information in a single volume. Alkaline Anion Exchange Membranes for Fuel Cells meets this need with a comprehensive overview of the properties of these membranes and their applications. The book considers recent developments, common challenges, and the long-term prospects for this field of research and engineering. It constitutes a one-stop resource for the development and production of AAEM fuel cells and related electrochemical applications. Alkaline Anion Exchange Membranes for Fuel Cells readers will find: Discussion of electrochemical applications like redox flow batteries, water electrolysis, and many more Detailed treatment of specially tailored cationic groups such as quaternary ammonium and guanidinium Expert advice on efficient fabrication and electrode assembly Alkaline Anion Exchange Membranes for Fuel Cells is ideal for electrochemists, materials scientists, polymer chemists, electrical engineers, and anyone working in power technology or related fields.

Advisor: Yossef A. Elabd.

The advantages of alkaline anion exchange membrane fuel cells (AAEMFCs) over proton exchange membrane fuel cells is the motivation for this dissertation. The objectives of this dissertation were to develop durable membranes with high anion conductivity and an understanding of the ion conductivity relationship with morphology. The research results presented in this dissertation focuses on developing different architectures of ionic copolymers including diblock copolymers and random copolymers for AAEMFCs. A novel, and stable cobaltocenium cation, was incorporated into polymer for stable AAEM. Because of its 18 electron closed valence-shell configuration, the cobaltocenium cation is promising for use in AAEMFC. Two block copolymers, polystyrene-b-poly(vinyl benzyl trimethyl ammonium hydroxide) (PS-b-[PVBTMA][OH]) and poly(vinyl benzyl trimethyl ammonium bromide)-bpoly(methylbutylene) ([PVBTMA][Br]-b-PMB), were studied in chapters 2 and 3 respectively. The major difference between these two chapters was the type of hydrophobic block employed. The membranes fabricated from PS-b-[PVBTMA][OH] were too brittle to be mechanically durable nor flexible enough for use as membranes due to the high Tg of polystyrene. The flexible, and robust, [PVBTMA][Br]-b-PMB membranes were successfully fabricated because of the low Tg and fully saturated backbone of poly(methylbutylene). The morphological structures were characterized by environmental controlled scattering experiments. The morphology relationship with ion conductivity was investigated in terms of the type of structure, various degree of ion content and degree of orientation of structure. In chapter 2, block copolymers of PS-b-[PVBTMA][OH] were synthesized by sequential monomer addition by ATRP and then post polymerization anion exchange from tetrafluoroborate to the hydroxide counter anion. The morphology of the membranes of PS-b-[PVBTMA][BF4] and PS-b-[PVBTMA][OH] block copolymers were determined by small angle X-ray scattering (SAXS) at different humidity and temperature conditions. The effects of the morphologies on the ionic conductivity, measured by impedance spectroscopy, were investigated in terms of type of structure, size of d-spacing and presence of grain boundaries. The block copolymers of [PVBTMA][Br]-b-PMB membranes were successfully fabricated in chapter 3. The membranes cast from different solvents exhibited different degree of structural ordering and values of ionic conductivity. The conductivity dependence on humidity, temperature and casting solvents were fully studied to understand the relationship between conductivities and morphologies. The membranes cast from THF showed highest bromide conductivity (0.02 S/cm) at 90 oC and 95% RH. High bromide conductivity (~0.04 S/cm) and a low percolation point were achieved because of the formation of well-connected ion conducting channel. Effects of ion clusters on conductivities were studied by SANS and SAXS. Increasing the degree of functionality in the ionic domain is another avenue to eliminate ion cluster and achieve high ion conductivity in block copolymers. Investigations of cross-linked polyisoprene-ran-poly(vinyl benzyl trimethyl ammonium chloride) (PI-ran-[PVBTMA][Cl]) in chapter 4 was explored to fabricate robust AAEMs and to offer comparison of block copolymers to random copolymers in terms of ion conductivity, water uptake and morphology. The random copolymers were solvent processable, and were cross-linked by thermal treatment. High chloride ion conductivity (0.061 S/cm at 90oC and 95% RH) could be achieved. The ion conductivities were influenced by water uptake and ion exchange capacity of the membranes. The ion cluster effects on the conductivities were studied by SAXS as well. Finally, the comparison of ionic block copolymers and random copolymers membranes indicated that the ionic block copolymers membranes showed lower percolation point, lower water uptake and higher ion conductivity with the similar ion content relative to random copolymers membranes. Therefore, using ionic block copolymers as AAEM is promising for achieving higher performance. In chapter 5, a novel monomer, styrene cobaltocenium hexafluorophosphate (StCo+PF6 - ), was synthesized by a one-pot reaction without the need for purification by column chromatography. It showed excellent alkaline stability (negligible degradation after 7 days at 80oC in 2 M KOH solution) because of its 18 electron closed valence-shell configuration and the steric hindrance of the phenyl group. The excellent alkaline stabilities of phenyl cobaltocenium confirmed that membranes containing cobaltocenium are promising for use in AAEMFC. The dissertation concludes with a summary chapter 6 where the major results from the previous chapters are discussed. Suggestions are also offered for future investigations.

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 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.

Fuel cells are devices that convert the chemical energy stored in a fuel directly into electricity and have the potential to serve as a highly efficient and environmentally sustainable power generation technology for stationary and mobile applications. Within a fuel cell, the polymer electrolyte membrane serves as the ion conducting medium between the anode and cathode, making it a central, and often performance-limiting component of the fuel cell. The most common polymer electrolyte membrane fuel cells operate under acidic conditions and are therefore proton conducting. Although proton exchange membrane (PEM) fuel cells are well developed and can offer excellent performance, they rely almost exclusively on platinum, a very expensive and scarce noble metal. This dependence on platinum has severely hindered wide scale commercialization of PEM fuel cell technologies. By comparison, alkaline fuel cells that employ hydroxide conducting alkaline anion exchange membranes (AAEMs) are relatively unexplored. A major advantage of alkaline fuel cells, when compared to acidic fuel cells, is their enhanced reaction kinetics for oxygen reduction, permitting the use of less costly, non-noble metal catalysts (e.g. Ni). Therefore, high performance AAEMs could significantly advance fuel cell technologies. We have been working to develop new polymeric materials that can serve as effective AAEMs. Prior work in this area has mainly focused on re-engineering existing materials to access AAEMs. In contrast, we approached this problem from a synthetic perspective by designing and synthesizing materials from the ground up. Herein, the synthesis of two separate AAEM systems that are synthesized via ring-opening metathesis polymerization are described. The first route involves the copolymerization of a tetraalkylammonium-functionalized norbornene with dicyclopentadiene. The crosslinked thin films generated are mechanically strong and exhibit exceptional methanol tolerance. The second route involves the synthesis of a solvent processable, tetraalkylammonium-functionalized polyethylene for use as an AAEM. The membranes are insoluble in both pure water and aqueous methanol but exhibit excellent solubility in a variety of other aqueous alcohols. These solubility characteristics extend the utility of this system for use as both an AAEM and ionomer electrode material from a single polymer composition. The AAEMs generated are mechanically strong and exhibit high hydroxide conductivities. Lastly, we have developed a standardized procedure for measuring the alkaline stability of a benzyltrimethylammonium (BTMA) model compound and a BTMA functionalized polyethylene. The procedure is broadly applicable and should serve as a testing method to better understand other systems, specifically those based on novel cations. Applying this procedure should facilitate the discovery of AAEMs with increased base stability, thus enabling high temperature AAEM fuel cell operation.

Examines the important topic of fuel cell science by way of combining membrane design, chemical degradation mechanisms, and stabilization strategies This book describes the mechanism of membrane degradation and stabilization, as well as the search for stable membranes that can be used in alkaline fuel cells. Arranged in ten chapters, the book presents detailed studies that can help readers understand the attack and degradation mechanisms of polymer membranes and mitigation strategies. Coverage starts from fundamentals and moves to different fuel cell membrane types and methods to profile and analyze them. The Chemistry of Membranes Used in Fuel Cells: Degradation and Stabilization features chapters on: Fuel Cell Fundamentals: The Evolution of Fuel Cells and their Components; Degradation Mechanism of Perfluorinated Membranes; Ranking the Stability of Perfluorinated Membranes Used in Fuel Cells to Attack by Hydroxyl Radicals; Stabilization Mechanism of Perfluorinated Membranes by Ce(III) and Mn(II); Hydrocarbon Proton Exchange Membranes; Stabilization of Perfluorinated Membranes Using Nanoparticle Additives; Degradation Mechanism in Aquivion Perfluorinated Membranes and Stabilization Strategies; Anion Exchange Membrane Fuel Cells: Synthesis and Stability; In-depth Profiling of Degradation Processes in Nafion Due to Pt Dissolution and Migration into the Membrane; and Quantum Mechanical Calculations of the Degradation Mechanism in Perfluorinated Membranes. Brings together aspects of membrane design, chemical degradation mechanisms and stabilization strategies Emphasizes chemistry of fuel cells, which is underemphasized in other books Includes discussion of fuel cell performance and behavior, analytical profiling methods, and quantum mechanical calculations The Chemistry of Membranes Used in Fuel Cells is an ideal book for polymer scientists, chemists, chemical engineers, electrochemists, material scientists, energy and electrical engineers, and physicists. It is also important for grad students studying advanced polymers and applications.