Date of Award

12-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Roberts, Mark E

Committee Member

Getman, Rachel B

Committee Member

Husson, Scott M

Committee Member

Creager, Stephen E

Abstract

Supercapacitors (SCs) are promising energy storage devices because they deliver energy faster than Li-ion batteries and store larger amounts of charge compared to dielectric capacitors. SCs are classified in electrical double layer capacitors (EDLCs) and pseudocapacitors, based on their charge storage mechanism. EDLCs store charge electrostatically, i.e. by physical charge separation. This mechanism limits the storable amount of energy to the available surface area of the electrode, typically made of carbon materials, but grants good cycling stability of the SC device. Pseudocapacitor electrodes, commonly made of conducting polymers or metal oxides, store charge faradaically, i.e. through redox reactions throughout the bulk material, which allows them to store significantly larger amounts of energy than EDLCs, but their stability is compromised due to the partial irreversibility of the faradaic processes. To accomplish the commercialization of SCs, devices must show a combination of high charge storage capacities and long-term stability, besides being cost-effective. To tackle the current issues of SCs, this field of study has taken mainly two directions: 1) the development of new architectures and nanostructures of the active materials, which has shown to increase the surface area, enhance stability, and facilitate ion diffusion; and 2) fabrication of composites between non-faradaic (carbon), faradaic materials, and/or redox-active components to achieve a balance between the amount of energy stored and the stability. Following the first approach, a continuous process to grow vertically aligned carbon nanotubes (VACNTs) on cost-effective aluminum foil was developed. The resulting electrodes were analyzed as SC electrodes and in symmetric cells, and the influence of the arrangement of the nanotubes and the synthesis conditions was studied. The performance of the VACNTs produced continuously showed similar performance to the VACNTs produced stationarily and the ordered structure of the VACNTs showed superior performance compared to randomly oriented CNTs. To increase the energy density, the second approach was taken, by combining pre-synthesized conducting polymers (CPs) and carbon nanotubes (CNTs) using a facile scalable dispersion filtration method to produce free-standing electrodes. Composites with the three main CPs were prepared, analyzed in various electrolytes, and their performance was comparable with polymer/ CNT films prepared with more complex techniques such as in-situ polymerization and pellet pressing. Then, based on the idea that the quinone molecules present in lignin store charge by undergoing a 2 proton, 2 electron redox reaction, a composite between polypyrrole, a stable conducting polymer, and the prototypical molecule p-benzoquinone was fabricated by electropolymerization of pyrrole in the presence of the redox molecule. A significant increase in capacitance and capacity was obtained with respect to polypyrrole films. Furthermore, an important obstacle in the application of CPs in SCs is the lack of easily reduced (n-dopable) polymers. Poly(aminoanthraquinone) (PAQ) is a conjugated polymer that shows electroactivity in the negative potential range of 0 to -2 V, due to the redox moieties of the polymer. PAQ was electropolymerized on free-standing CNT films and its performance as anode for SCs was studied. The materials and processing techniques described in this dissertation are useful to further develop high power/high energy electrodes for SCs.

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