Synthesis and Characterization of Pyridinium Derivatives for Application in Non-Aqueous Redox Flow Batteries
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As renewable energy sources are further explored for integration into the electric grid on which we all rely, the question of how to meet energy storage demand in an equally “green” way arises. One potential solution to this demand is redox flow batteries. Redox flow batteries stand to become scalable grid-level energy storage systems due, in large part, to the separation of power and energy in the system inherent in structure. Many types of redox flow batteries have been thoroughly studied, with the bulk of them being aqueous systems relying on inorganic compounds. A major shift in the design of these batteries is the transition from inorganic and aqueous based systems to all-organic non-aqueous batteries to take advantage of the extended electrochemical window of solvents such as acetonitrile.
The objective of this thesis is to explore the electrochemical properties of di-pyridinium carbonyl species to determine their potential competency as redox flow battery anolytes. A suite of dipyridinium carbonyl compounds were synthesized and characterized electrochemically. Based on the literature surrounding pyridinium based anolytes, it was hypothesized that bis-4-(N-methyl pyridinium) carbonyl would prove to be the compound with the greatest competency regarding stability of the radical cation species and reversibility of the redox events in solution. This was found not to be the case; bis-2-(N-methyl pyridinium) carbonyl proves to be significantly more competent in terms of stability and reversibility. To further investigate this, a series of ortho- and para-benzoyl functionalized N-methyl pyridinium compounds were studied computationally using the hybrid (U)B3LYP functional and the 6-311G++(d) basis set. Hammett analysis of these various compounds showed that the stability of radical species does improve with increasing electron withdrawing character of the substituent but is improved in the ortho- functionalized over the para- functionalized. These computational findings in conjunction with the experimental observations of stability and reversibility of di-pyridinium carbonyl electrochemical events indicates that the presence of a highly electron withdrawing moiety stabilizes the carbon ortho to the heteroatom as a point of radical localization. These observations may be used in future work to design ortho-functionalized derivatives with improved electrochemical properties in regard to redox flow battery applications.