REDOX-ACTIVE POLYMERS AS CATHODE MATERIALS FOR ENERGY STORAGE APPLICATIONS
Organic and polymeric electrodes have great properties and tenability enabling them to become high-energy and high-power cathode materials for electrical energy storage(EES) applications. Two main classes of polymeric materials were explored, investigated, and evaluated as promising electrodes for EES applications: thin-film and bulk polymers. In the first half of this dissertation, I studied, in detail, the electrosynthesis of a library of polypyrrole anchored redox-active pendant decorated thin-film polymers. The polymer architecture was designed to boost electronic conductivity of the polymer by installing a conducting polymer as backbone, and energy density was maximized by employing highly reversible redox-active moieties including 1,4-dimethoxybenzene (DMB), N,N,N’,N’-tetramethylphenylenediaine (TMPD). By having two stable redox couples at high potentials, PPy-5C-TMPD was shown to have the most promising electrochemical properties, amongst the molecules studied. However, this thin-film polymer exhibited severe charge trapping behavior, in which half of the capacity was missing during reduction of the polymer, as if anions (or charges) were "trapped" inside the polymer film. This so-called "charge trapping" phenomenon was studied in detail to unravel its origin, and results indicated that the phenomenon appears to be specific to TMPD as a redox-active pendant. Substitution of the redox-active pendant to equally high-energy 5,5’-bis(methylthio)-2,2’- bithiophene (BMTbT) resulted in no signs of charge trapping. I then explored bulk polymer materials, by coupling redox-active moieties in alternating fashion by Buchwald-Hartwig coupling to form main-chain redox-active polymers. By coupling phenothiazine with N,N’-dimethylphenylenediamine (PT- DMPD), the resulting polymers showed three main redox couples. EQCM and other mechanistic studies suggested that the redox activities of PT-DMPD involved the exchange of three electrons. As battery material, two of the redox processes could be accessed. Initial cycling performance of PT-DMPD showed capacity fade likely due to polymer dissolution. Cross-linking of PT units by a butyl alkyl linker enabled different degrees of cross-linked polymers to be prepared. Upon optimization, 10%CL PT- DMPD showed the best performance, reaching close to 100 % of its theoretical capacity. An impressive rate performance was also achieved with this material, reaching 120 C with 82 % capacity retention, an unprecedented rate performance among organic materials. These studies on organic electrode materials have demonstrated promising electrochemical properties of these materials for EES applications, and device level testing suggested potentials for practical application. Using these results as a part of motivation, future researchers should focus on better design while maintaining chemical stability of the polymers, and perhaps aided by our studies, find better performing organic materials for EES application.
Analytical chemistry; Lithium-ion battery; Organic cathodes; Redox-active polymers; Chemistry; Electrochemistry; Polymer chemistry
Abruna, Hector D.
Coates, Geoffrey; Fors, Brett P.
Chemistry and Chemical Biology
Ph. D., Chemistry and Chemical Biology
Doctor of Philosophy
dissertation or thesis