Date of Award

8-2017

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemistry

Committee Member

Dr. Stephen E. Creager, Committee Chair

Committee Member

Dr. Joseph S. Thrasher

Committee Member

Dr. William T. Pennington

Abstract

Much interest exists in alkaline exchange membrane (AEM) fuel cells, not just because of the higher efficiency for the oxygen reduction reaction (ORR) under alkaline conditions, but also because of their inexpensiveness due to the possibility of using low-cost materials, such as non-platinum-group catalysts. Due to their high chemical stability and conductivity, tetrafluoroethylene (TFE)/ trifluorovinyl ether (TFVE) anion-exchange copolymers having pendant cationic groups attract great attention. In many cases the cationic group, often a quaternary ammonium group, is attached to a fluoropolymer major chain via a sulfonamide linkage. Alkaline stability of these ionomers is critically important for AEM fuel-cell applications. This research is a stability study of small-molecule cationic perfluoroalkyl sulfonamide model compounds in alkaline conditions. Model compounds are synthesized from n-perfluoroalkylsulfonyl fluoride and different amine precursors, followed by reaction with methyl iodide to make quaternary ammonium salts. Alkaline stability tests of the model compounds are then carried out in aqueous KOH solution at evaluated temperatures from 60-90 °C. Samples are heated starting from 60 °C for the first two days, and then the temperature is increased 10 °C every two days until reaching 90 °C. Decomposition products are identified by using high-temperature NMR spectroscopy at 65 °C, because the compounds have low solubility in water at room temperature, but in many cases will be completely dissolved at 65 °C. NMR spectroscopy is useful to detect the formation or degradation of functional groups. From the 1H and 19F NMR spectra, one can investigate the decomposition mechanism of sulfonamides and the quaternary ammonium group. Two possible decomposition mechanisms exist for the quaternary ammonium group, 1) Hofmann elimination and 2) nucleophilic attack. The NMR spectra suggest that the quaternary ammonium group is the first site of decomposition in strong alkaline conditions. Moreover, the concentration of base shows a larger effect on the degradation of the quaternary ammonium salts than temperature in the tests.

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