Direct methanol fuel cells (DMFC) are appealing as a power source for portable
devices as they do not require recharging with an electrical outlet. However, the DMFC
technology is confronted with the high crossover of methanol fuel from the anode to the
cathode through the currently used Nafion membrane, which not only wastes the fuel but
also poisons the cathode platinum catalyst. With an aim to overcome the problems
encountered with the Nafion membrane, this dissertation focuses on the design and
development of new polymeric membrane materials for DMFC and a fundamental
understanding of their structure-property-performance relationships.
Several polymeric blend membranes based on acid-base interactions between an
aromatic acidic polymer such as sulfonated ploy(ether ether ketone) (SPEEK) and an
aromatic basic polymer such as heterocycle tethered poly(sulfone) (PSf) have been
explored. Various heterochylces like nitro-benzimidazole (NBIm), 1H-Perimidine
(PImd), and 5-amino-benzotriazole (BTraz) have been tethered to PSf to understand the influence of pKa values and the size of the hetrocycles. The blend membranes show
lower methanol crossover and better performance in DMFC than plain SPEEK due to an
enhancement in proton conductivity through acid-base interactions and an insertion of the
heterocycle side groups into the ionic clusters of SPEEK as indicated by small angle Xray
scattering and TEM data. The SPEEK/PSf-PImd blend membrane shows the lowest
methanol crossover due to the larger size of the side groups, while the SPEEK/PSf-BTraz
blend membrane shows the highest proton conductivity and maximum power density.
To further investigate the methanol-blocking effect of the heterocycles, N,N’-Bis-
(1H-benzimidazol-2-yl)-isophthalamide (BBImIP) having two amino-benzimidazole
groups bonded to a phenyl ring has been incorporated into sulfonated polysulfone (SPSf)
and SPEEK membranes. With two 2-amino-benzimidazole groups, which could greatly
increase the proton transfer sites, and three phenyl rings, which are compatible with the
aromatic polymers, the BBImIP/SPSf and BBImIP/SPEEK blend membranes show
suppressed methanol crossover and increased fuel cell performance in DMFC.
Novel sulfonated copolymers based on poly(aryl ether sulfone) (SPS-DP) that
exhibit low methanol crossover have been synthesized and explored as a methanol-barrier
center layer in a multilayer membrane configuration having SPEEK as the outer layers.
These multilayer membranes exhibit better performance in DMFC than plain SPEEK and
Nafion 115 membranes due to suppressed methanol crossover.
To address the issue of incompatibility between the new hydrocarbon-based
membranes synthesized and the Nafion ionomer used in the catalyst layer in fabricating
membrane-electrode assemblies (MEAs), the MEAs have been fabricated with the
SPEEK membranes and 10 to 30 % SPEEK ionomer in the catalyst layer. These MEAs
exhibit better performance in DMFC compared to the MEAs fabricated with the SPEEK
membranes and Nafion ionomer in the catalyst layer due to lower interfacial resistance. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/6580 |
| Date | 20 October 2009 |
| Creators | Li, Wen |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Format | electronic |
| Rights | Copyright is held by the author. Presentation of this material on the Libraries' web site by University Libraries, The University of Texas at Austin was made possible under a limited license grant from the author who has retained all copyrights in the works. |
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