Polybenzimidazole based segmented block copolymers for high temperature fuel cell applications

• Main Author: Scanlon, Eugene
• Format: Dissertation
• Language: English
• Published: Ann Arbor ProQuest Dissertations & Theses 2005
• Subjects: Polymer chemistry; Polymers
• ISBN: 0542501422; 9780542501425

Polymer electrolyte membrane fuel cells (PEMFC) are the ongoing focus of current research for potential applications ranging from mobile devices utilizing micro fuel cells to automotive applications and stationary devices generating power on the kW scale.
PEMFC's employing polybenzimidazole (PBI) membranes with a phosphoric acid electrolyte have been recognized for their outstanding high temperature performance (>160°C). High temperature operation provides favorable electrode kinetics and an increased tolerance to fuel impurities. Conventional perfluorinated ionomers such as NafionRTM are limited to temperatures of 100°C or less due their reliance on water to conduct protons. This reliance on water imposes additional hardware requirements to maintain high membrane hydration levels. In general, hydrated perfluorinated ionomers exhibit low tolerance to CO due to their low operating temperatures.
Acid doped polymer membranes were prepared using a novel process coined the "PPA Process" where the hot polymerization solution of polymer dissolved in PPA was cast directly upon completion of the polymerization. Stable gel membranes were obtained after the films were allowed to absorb atmospheric moisture causing the PPA to hydrolyze to phosphoric acid.
The hydrolysis process was studied at both high and low relative humidity conditions. All of the gel membranes possessed high levels of phosphoric acid as measured by titration and excellent ionic conductivities were exhibited at elevated temperatures for all samples which remained stable during the experiment.
A series of segmented block copolymers, random copolymers and a blend of homopolymers based on the polybenzimidazole repeat unit were synthesized using polyphosphoric acid (PPA) as both reaction solvent and condensation reagent. Comparisons were made using the repeat units meta sulfone PBI ( m-SPBI) from isophthalic acid (IA) and 3,3-4,4'-tetraaminodiphenyl sulfone (TAS) and p-PBI from terephthalic acid (TA) and 3,3'-4,4'-tetraaminobiphenyl (TAB). High polymer was obtained in all cases. The properties of the segmented block copolymer membranes were further studied as a function of the mole ratio of the two segments.
MEAs assembled from segmented block copolymer membranes exhibited good long term stability and were capable of operating on a variety of gas streams at high temperatures and pressures including reformed synthetic reformate with a CO level of 2000ppm.