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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2014

First Advisor

E. Bryan Coughlin

Second Advisor

Ryan C. Hayward

Third Advisor

Mark T. Tuominen

Subject Categories

Energy Systems | Materials Science and Engineering | Polymer Chemistry

Abstract

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.

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