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Document Type

Campus-Only Access for Five (5) Years

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2016

Month Degree Awarded

September

First Advisor

Coughlin, E. Bryan

Subject Categories

Condensed Matter Physics | Energy Systems | Membrane Science | Polymer and Organic Materials | Polymer Chemistry | Polymer Science

Abstract

Fuel cells are one of the oldest sustainable energy generation devices, converting chemical energy into electrical energy via reverse-electrolysis reactions. With the rapid development of polymer science, solid polymer electrolyte (SPE) membranes replaced the conventional liquid ion transport media, rendering low-temperature fuel cells more accessible for applications in portable electronics and transportation. However, SPE fuel cells are still far from commercialization due to high operation cost, and insufficient lifetime and performance limitations. Anion exchange membrane fuel cells (AEMFCs) are inexpensive alternatives to current proton exchange membrane fuel cell (PEMFC) technology, which relies on utilizing expensive noble-metal catalysts and perfluorinated SPE materials. Unlike PEMFCs, there is not an ideal AEM material that provides efficient ion transport, while being mechanically robust and chemically stable under strong alkaline conditions. The objectives of this dissertation are to investigate macromolecular design parameters to obtain robust membranes with efficient ion conductivities, and molecular design parameters to obtain alkaline stable ammonium cations as an alternative to the benchmark benzyltrimethylammonium (BTMA) cation.

Macromolecular design parameters were explored by systematic variations of polymer architecture from random, to graft, to symmetric pentablock copolymer structures. Solvent processable random copolymers of polyisoprene-ran-poly(vinyl- benzyltrimethylammonium chloride) were synthesized via polymerization of commercially available monomers. Robust membranes were obtained by thermal or photocross-linking of unsaturated isoprene units. Depending on the copolymer composition, choice of cross-linking method, and the hydrophobicity of the cross-linker, microphase-separated morphologies were obtained forming a connected network of ion clusters. Connectivity improved ion conductivity by two to three orders of magnitude even at low hydration numbers.

Connected ionic networks with larger domain sizes were obtained when polymer chains with fixed cations were grafted onto a hydrophobic backbone. Systematic change of graft length and graft density showed a strong correlation with domain connectivity. At a fixed graft density, increasing graft length improved domain connectivity and ion conductivity at the expense of excessive water uptake and dimensional instability. At a fixed graft length, increased graft density improved domain connectivity due to decreased domain size and distance, without compromising membrane dimensional stability. Compared to analogous random copolymers two to three times higher ion conductivities were obtained at relatively low hydration, reaching chloride ion conductivities as high as 50 mS/cm at 60 oC and 95 % relative humidity.

A symmetric ABCBA pentablock was functionalized to obtain a midblock quaternary ammonium functionalized polymer that are analogous to midblock sulfonated Nexar® pentablock copolymers which have been commercialized by Kraton Polymers. X-ray scattering and transmission electron microscopy revealed formation of a microphase-separated inverse morphology where the minor ionic component formed the connected phase. Membranes had elastomeric properties and superior water management to graft copolymers while providing two to three times higher ion conductivity at an equivalent ion concentration. This work represents the first example of a midblock quaternized pentablock copolymer and the investigation of the structure-morphology-property relationships.

Lastly, improved alkaline stability of hexyltrimethylammonium (HTMA) cations were investigated on a molecular level, by systematic structural design. Phenyl, phenyl ether, and benzyl ether attached HTMA small molecule cations were synthesized. These three spacer-modified cations were found to be six to ten times more stable than the conventional BTMA cation. The linker chemistry did not influence the overall alkaline stability, enabling easy access to stable ammonium cations. Analogous styrenic monomers, and their homopolymers were synthesized. High stability of the homopolymer cations was confirmed in comparison to poly(BTMA). This study provided a deeper understanding of ammonium degradation mechanisms under strong alkaline conditions, and proposed monomer designs for easy incorporation of stable ammonium cations onto polymers.

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