Preparation of perfluorinated ionomers for fuel cell applications

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University of Alabama Libraries

One of the major issues with the current membrane technology for polymer electrolyte membrane fuel cells is the low conductivity seen at low relative humidity. This dissertation discloses the preparation of perfluorinated polymers with higher densities of acid sites and higher conductivities to overcome this issue. These materials are prepared using a system designed to safely synthesize and polymerize tetrafluoroethylene (TFE) on a hundred gram scale. The copolymerization of TFE and perfluoro-2-(2-fluorosulfonylethoxy) propyl vinyl ether (PSEPVE) to prepare materials with varying ratios of the two monomers was carried out by solution, bulk, and emulsion polymerization techniques. Additionally, the homopolymer of PSEPVE has been prepared and characterized by MALDI-TOF mass spectrometry, which shows the low molecular weight distribution seen in other similar materials in the literature is due to a high rate of β-scission termination. Spectroscopic measurements and thermal analysis were carried out on these samples to obtain better characterization than was currently available. Producing polymers with a higher amount of PSEPVE, and thus higher density of acid sites, leads to the materials becoming water soluble after hydrolysis. However, addition of a curable ter-monomer allows the polymer chains to be crosslinked to regain water insolubility. Using this approach, water insoluble membranes with high densities of acid sites and conductivities up to 5.5 times higher than Nafion® 115, the standard benchmark for fuel cell membranes, have been produced. Preparation of high molecular weight, low EW copolymers of TFE and PSEPVE is difficult due to the reactivity ratios of the two monomers. Literature reactivity ratios for VDF and PSEPVE are more favorable for preparation of high molecular weight, low EW copolymers. Here, alternating copolymers of VDF and PSEPVE are prepared for the first time; where high molecular weight samples have been shown to possess low swelling characteristics in water. It has also been found the lower molecular weight samples that are soluble in perfluorohexane can be converted to perfluorinated polymers by direct fluorination with 20% elemental fluorine in nitrogen with 254 nm UV irradiation.

Electronic Thesis or Dissertation
Polymer chemistry, Organic chemistry, Alternative energy