Date of Award

2010

Document Type

Campus Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

Chemistry

First Advisor

Brian C. Benicewicz

Abstract

Proton exchange membrane fuel cells (PEMFCs) have recently been acknowledged as one of the most promising alternative power sources due to their clean and efficient operation, as well as their numerous applications in transportation, stationary, cogeneration, and portable systems. High temperature PEMFCs operating at temperatures over 120°C have been extensively studied due to their beneficial aspects, such as faster electrode kinetics, simplified water management, increased reactant gas impurity tolerance, and better utilization of system-generated heat. Polybenzimidazoles (PBIs) have been identified as one of the most prominent candidates for high temperature PEMFCs due to their excellent thermal and oxidative stability, as well as their proton conducting ability.

Recently, a novel process was developed to produce gel membranes suitable for high temperature PEMFCs where polyphosphoric acid (PPA) is used as a condensation agent and as a casting solvent in a one-pot synthesis of polymer suitable for proton exchange membranes. This process, termed the "PPA Process," produces high molecular weight polymer with a membrane structure capable of attaining high phosphoric acid doping levels while maintaining good mechanical properties and excellent long-term stability.

Polyphenylquinoxalines (PPQs), a novel chemistry for the PPA Process, were developed for use as proton exchange membranes. PPQs were synthesized via the polycondensation of 3,3',4,4'-tetraaminobiphenyl and 1,4-bisbenzil in polyphosphoric acid. It was found that PPQ homopolymers were unable to form stable gel membranes via the PPA Process. Thus, PPQ copolymers with PBI were investigated. A series of PPQ/PBI copolymers was produced and found to have greatly improved Young's Modulus as compared to p-PBI membranes produced by the PPA Process. A PPQ/PBI copolymer containing 58 mol% PPQ (PPQ-58) was found to possess the best mechanical properties, proton conductivity, and fuel cell performance, among the PPQ/PBI copolymers investigated.

To better understand the structure-property relationships of PBI-based membranes produced by the PPA Process, several novel PBI-based proton exchange membranes were investigated as prepared via the polycondensation of 3,3',4,4'-tetraaminobiphenyl with diacids functionalized with bulky phenyl groups in polyphosphoric acid (PPA) according to the PPA Process. The chemistries investigated include phenyl-functionalized meta-PBI homopolymer, phenyl-functionalized meta-PBI copolymers with meta-PBI and with p-PBI, and 2-bromo-para-PBI. Phenyl-m-PBI homopolymer was found to be poorly soluble in PPA due to an enhanced crystallinity induced by the phenyl side chain, and were therefore unsuitable for membrane formation. Random copolymers based on m-PBI and p-PBI containing phenyl¬-m-PBI, and 2-bromo-p-PBI were successfully synthesized and studied as proton exchange membranes in high temperatures fuel cells.

In comparison to m-PBI homopolymer produced via the PPA Process, phenyl-m-PBI/m-PBI copolymers showed enhanced proton conductivities and phosphoric acid uptake, but lower fuel cell performance overall. Functionalized para-PBI and copolymers developed herein were shown to have lower fuel cell performance than p-PBI homopolymer membranes developed by the PPA Process.

Share

COinS