Date of Award

2014

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

Chemistry

First Advisor

Brian C Benicewicz

Abstract

The overall goal of the work presented within this thesis is to investigate, understand, and develop structure-property relationships in regards to polymer structure, membrane stability, and device performance. Previous work has shown that material properties such as acid loading, proton conductivity, mechanical integrity, and device performance can be attributed to polymer structure, chemical functionality, and polymer architecture (homo, random, co-polymer etc.). Developing and understanding these relationships is critical to the advancement of fuel cell technology. Within this thesis, several of these relationships are investigated and reported.

Novel polymers and copolymer systems have been developed in this research and have significantly progressed the understanding of polybenzimidazole chemistries. The research reported herein contributes to the ever-developing pool of knowledge regarding polybenzimidazole chemistry and material applications.

Extensive work has been done concerning polymer structural design and investigation with regards to sequence isomerism through the design, synthesis, and utilization of novel monomer compounds. A novel diacid monomer, 2,2'-bisbenzimidazole-5,5'-dicarboxylic acid (BBDCA), was synthesized and polymerized with 3,3',4,4'-tetraaminobiphenyl (TAB) to prepare a polymer, termed i-AB-PBI, that was composed of repeating 2,5-benzimidazole units. The i-AB-PBI incorporates head-to-head, tail-to-tail, and head-to-tail benzimidazole orientations. This is in contrast to the previously known AB-PBI which incorporates only head-to-tail benzimidazole sequences. Until this work, no other AB-type polybenzimidazole existed. AB-type polybenzimidazoles are fundamental in the family of polybenzimidazoles because they only incorporate the benzimidazole motif, i.e. no other spacer or functionality is incorporated in the polymer structure. These polymers were synthesized and characterized by molecular weight, membrane composition, ionic conductivity, thermal, mechanical, and fuel cell performance properties. A detailed report of this study is presented in Chapter 3.

This work was further progressed by the study of the AB-PBI and i-AB-PBI random copolymer system. The random copolymer system of these polymers, termed r-AB-PBI, is unique in that the chemical structure and functionality does not change across the copolymer compositional spectrum. The r-AB-PBI copolymer system introduces a sequence distribution effect with regards to benzimidazole orientation, as well as randomization of benzimidazole sequence between two well defined sequenced AB-type polymers. These polymers were synthesized and characterized by molecular weight, membrane composition, ionic conductivity, thermal, mechanical, and fuel cell performance properties. A detailed report of the unique r-AB-PBI system is described in Chapter 4.

Another set of random copolymers was developed that incorporates the AB-type polybenzimidazoles and a para-phenyl-polybenzimidazole (p-PBI). AB-PBI and p-PBI random copolymers were developed and compared to random copolymers of i-AB-PBI and p-PBI. The unique combination and contrast of properties in the AB/p-PBI and i-AB/p-PBI systems provided insight into effects resulting from structural modification, sequence orientation, stability, and randomization. The p-PBI and AB-PBI copolymers address aspects of acid loading, stability, and structural modification, whereas the p-PBI and i-AB-PBI copolymers address comparisons of sequence isomerism and randomization between two high performance PEM materials. All copolymer materials were characterized by molecular weight, membrane composition, mechanical properties, ionic conductivities, and fuel cell performances. A detailed report of these copolymer systems and insights gained thereof are described in Chapter 5.

A sulfonated polybenzimidazole membrane (s-PBI) doped with sulfuric acid was utilized for the first time in an all-Vanadium redox flow battery (VFRB). This is the first reported utilization of polybenzimidazole in an energy storage device. Performance investigations of s-PBI in a VRFB were conducted and compared to Nafion 117, and Nafion 212. The s-PBI membranes developed were characterized in terms of molecular weight, ionic conductivity, mechanical properties, membrane composition, and overall VFRB performances under a variety of conditions. A detailed report of this work is provided in Chapter 6.

A sulfonated polybenzimidazole (s-PBI) membrane doped with sulfuric acid was utilized in the Hybrid Sulfur Electrolyzer. This work is the first report of a polybenzimidazole material being utilized in a Hybrid Sulfur Electrolyzer. Extensive research was conducted regarding device performance under a variety of conditions. All s-PBI membranes were conditioned through an acid exchange process, verified by titration, and characterized in terms of molecular weight, membrane composition, mechanical properties, ionic conductivity, and overall device performance. The s-PBI utilized in the Hybrid Sulfur Electrolyzer was compared to Nafion 117 and Nafion 112. Prior to this work, high-temperature device operation of a Hybrid Sulfur Electrolyzer had not been evaluated. A detailed overview of this work is reported in Chapter 7.

In addition to developments of novel PBI chemistries and device applications, extensive work was conducted in regards to the development of a new solution polymerization method for PBI materials. Prior to this work, polymerization of high-molecular weight polybenzimidazole in an organic solvent had not been reported. The work conducted resulted in a viable solution polymerization method of PBI in dimethylacetamide (DMAc). Through investigation of a series of monomer functionalities, a bisulfite adduct derivative of isophthalaldehyde was developed which allowed for the synthesis of high-molecular weight PBI in DMAc at high concentrations. This work has been patented, and provides a practical synthetic avenue for the synthesis of a multitude of polybenzimidazole derivatives. PBI developed from this process was characterized in terms of NMR, IR, DSC, TGA, and molecular weight and compared to commercially produced PBI. A detailed overview of this work is reported in Chapter 8.

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