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Proton exchange membrane for hydrogen fuel cells : self-humidification and crystallization

Authors Zhang, Wenjing
Issue Date 2008
Summary The objective of this thesis is to develop a self-humidification Nafion nanocomposite for hydrogen fuel cells. Nafion membrane developed by DuPont has been shown to have the highest proton conductivity and chemical stability. However, its lack of proton conductivity in the absence of hydration has been the limiting factor for its use in fuel cell batteries. We have systematically studied the use of stable Pt-clay fillers and a molecular lubricant for stronger and self-humidifying membranes in this research. Firstly, monolayer of Pt nanoparticles of diameters 2-3 nm with high crystallinity were successfully anchored onto exfoliated nano clay surfaces using a novel chemical vapor deposition (CVD) process. Exfoliated clay offered the maximum exposure area of the catalyst to the reactants. The strong interaction between the metal catalysts and the support exhibits superior stability against vigorous sonication for prolonged period and high resistance to thermal deterioration. Secondly, exfoliated Pt-clay nanoparticle was homogeneously dispersed in Nafion matrix. Systematic analysis showed that the recasting process produced a new self-humidifying exfoliated Pt-clay/Nafion nanocomposite membrane with high crystallinity and proton conductivity. In situ water production catalyzed by Pt nanocatalysts efficiently hydrated the Nafion membrane without any external humidification. The power density at 0.5V of a single cell made of Pt-clay/Nafion nanocomposite membrane is 170% higher than that made of a commercial Nafion 112 membrane. Efforts were also made to enhance the membrane crystallinity using a molecular lubricant. Both molecular dynamic simulation and experimental characterization show that the use of a molecular lubricant leads to a significant enhancement in crystallinity and membrane performance. Higher ordering of backbones makes the membrane mechanically robust. Sulfonic acid groups, largely extended outwards and exposed to water, strongly polarize the water molecules and result in the larger portion of nonfreezable water, which are crucial in proton conduction at low hydration levels. Given the evidences of self-humidification and molecular lubricant effects, we introduced both exfoliated Pt-clay nanocatalysts and molecular lubricant into the Nafion membrane and investigated the synergic effects. The result is a new nanocomposites membrane, having both self-humidification ability and high crystallinity.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2008
Language English
Format Thesis
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