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Synthesis and Characterization of Polymeric Anion Exchange Membranes

Abstract
As alkaline anion exchange membrane fuel cells (AAEMFC) are regarded as promising and important energy devices, the development of high performance anion exchange membranes are in urgent need, as well as fundamental investigation on the structure-property relationship, which are the motivation of this dissertation. Three different polymer systems are presented and focused on polymer synthesis, material morphology, and ion transport phenomena. Crosslinked membranes are promising as practical materials, however, the understanding and further improvement of its performance is hindered by the lack of an ordered morphology or well-defined chemical structure. In Chapter 2, a series of crosslinked membranes were design to bear cationic groups organized via covalent linkages, which were synthesized by sequential reversible addition-fragmentation chain transfer radical polymerization (RAFT), “click” chemistry, cast/crosslinking process, and solid state quaternization. Significant enhancement in conductivities was observed and presumably attributed to the formation of ion transport channels directed by polycation chains. Excellent membrane performance were observed, including conductivities, water diffusivities, and fuel cell power densities. In Chapter 3, phosphonium containing block copolymers were synthesized and subjected to morphology characterization. Using Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM), it was observed that these materials form well-ordered morphologies upon solvent casting, and the ionic block preferred to form a continuous phase. By comparing the anion conductivities, the matrix in a hexagonal phase was proved to be more efficient in ion transport than lamellae. Polycyclooctene (PCOE) based triblock copolymers were synthesized in Chapter 4, by using a special chain transfer agent (CTA) to mediate Ring-Opening Metathesis Polymerization (ROMP) and reversible addition-fragmentation chain transfer radical polymerization (RAFT). The well-defined melting transition (~50 oC) of PCOE enabled the investigation of the thermal transition in hydrophobic block affecting ionic domain behavior. Then metal ion doped star block copolymers were investigated in bulk and thin film forms to demonstrate that the star block copolymer architecture can facilitate microphase separation and thus the preparation of smaller features. Using an ortho-nitrobenzyl ester junction, triblock copolymers based on PEO and PSt were synthesized and applied to hierarchical pattern fabrication in self-assembled thin films. During these studies, the single monomer insertion methodology was developed for high efficiency synthesis of (multi)functional RAFT CTAs. The molecular characterization and controlled polymerization results were documented in Chapter 7. The last chapter contains outlooks based on the research in this dissertation. Methods to improve the previously presented materials were listed. Also, fundamental questions were raised on ion transport membranes, and possible ways to answer them were provided. In addition, potential research directions are proposed.
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