Design, synthesis, and application of base-stable cations in anion exchange membranes for alkaline fuel cells

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Fuel cells are devices that convert the energy stored in chemical bonds into electrical energy that can be used to do useful work. They are interesting as a source of energy because they possess high energy density, emit few by-products, have high efficiency and are capable of uninterrupted power generation. In particular, alkaline fuel cells (AFCs) are attractive because they enable the use of non-noble metal catalysts, which can significantly reduce production costs. The development of alkaline anion exchange membranes (AAEMs) is necessary for further development of AFCs and polymers that conduct hydroxide efficiently, while maintaining good mechanical strength are needed. Despite significant efforts to achieve viable polymer electrolytes, a standard AAEM has not been realized or successfully commercialized. Many of the polymer backbones and cations that constitute AAEMs have limited stability under the alkaline conditions required for operation. Developing cations that are resistant to degradation with bases and nucleophiles and incorporating them into inert polymer architectures is required for effective AAEMs. Moreover, designing methods that accurately characterize the stability properties and maximize the information acquired from studies will reduce the resources needed to achieve these goals. This dissertation describes the design, synthesis, and characterization of base-stable organic cations and incorporation into polymer electrolytes for AFCs. We synthesized a tetrakis(dialkylamino)phosphonium functionalized cis-cyclooctene (COE) monomer and copolymerized it with COE using Grubbs’ second generation catalyst for ring opening metathesis polymerization (ROMP). After hydrogenation with Crabtree’s catalyst, the polymer was essentially composed of polyethylene with phosphonium cations covalently linked to the backbone. The polymer exhibited a room temperature (22 °C) hydroxide conductivity of 22 mS/cm. After storing strips of the polymer membrane in caustic alkaline solutions (1 M KOH @ 80 °C or 15 M KOH @ 22 °C), the conductivity was reanalyzed and remained unchanged for 22 and 138 days, respectively. A 1H NMR spectroscopy protocol was developed to quantitatively assess the stability of a wide variety of organic cations under harsh alkaline conditions. We selected methanol-d3 as the reaction solvent to fully dissolve organic cations and their degradation products. Moreover, the use of methanol that is not fully deuterated prevents a hydrogen/deuterium exchange process that limits the amount of useful information that can be obtained during an experiment. The solutions were stored in flame-sealed NMR tubes to prevent the loss of volatile compounds and heated to 80 °C, a relevant fuel cell temperature. A minimum ratio of 1:10 was used between the cation and hydroxide molarities. A TMS derivative was used as an internal standard to determine the amount of cation remaining in solution over time. Several cations that are of interest to the AAEM community were analyzed over 30 days and compared to the stability of benzyl trimethylammonium (BTMA). When possible, degradation products were identified with 1H NMR and high resolution mass spectrometry (HRMS). Finally, we synthesized a series of imidazolium cations with varying substituents on the ring positions and tested their alkaline stability using our protocol. We found that substituents at the C2 position were important and substituted aryl groups were the most effective at preventing degradation. Imidazolium stability was further improved by placing methyl groups at the C4 and C5 positions. Long chain alkyl groups, such as n-butyl groups, at the N1 and N3 positions were the most effective at hindering reactions with hydroxide and methoxide. Ultimately, we achieved imidazolium cations that were completely resistant to degradation for 30 days at 1 M, 2 M, and 5 M KOH concentrations, in methanol-d3 at 80 °C.
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Coates, Geoffrey
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Dichtel, William R.; Abruña, Héctor D.
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Chemistry and Chemical Biology
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Ph. D., Chemistry and Chemical Biology
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Doctor of Philosophy
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bibid: 10427035
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