Date of Award

Fall 2021

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science

Committee Chair

Jack Skinner

First Advisor

David Bahr

Second Advisor

Jerome Downey

Third Advisor

John Kirtley

Fourth Advisor

Atish Mitra

Abstract

Supercapacitors are an energy storage technologycombining properties of both capacitors and batteries to deliver high power and energydensities. Supercapacitors store charge through electrostatic and faradaic interactions between the electrode and ionic electrolyte. By improving the physical structure of electrodes, the interfacial area where energy storage occurs can be increased and electrochemical performance improved. New, fibrous, manganese oxide web-based structures tested for use as a supercapacitor electrode were fabricated by electrospinning and direct calcinationof metal salt-containing polymer fibers, and the effects of fabrication parameters on electrochemical performance were investigated. Data show that high polymer concentrations and low oxide precursor concentrations during electrospinning form porous fibers with increased surface area, resulting in capacitance values up to 76 % greater than electrodes prepared with low polymer and high precursor concentrations. Post-electrospinning vapor melting treatments improved mechanical stability of the fiber mats to prevent delamination during calcination, increasing active mass of the prepared electrodes and improving performance by over 50 %. Calcining the structures for at least 4 h improves structural and electrical properties, increasing capacitance by up to 140 % compared to 2 h calcination, but extended calcination times past 4 h have no further beneficial effects.Electrochemical impedance spectroscopy and linear sweep voltammetry on electrospun web electrodes areused to extract system parameters including double layer capacitance and charge transfer resistance. The measured parameters are combined with mathematical models to develop a theoretical description of discharge behavior in electrospun electrodes with varying fiber sizes, porosities, and materials. Modeled discharge curves are used to calculate power and energy densities over current densities ranging from 5 to 5000 A/g and predict that the electrospun electrodes should exhibit remarkably stable power density over a large range of energy densities. The geometry of a fabricated electrode is used to predict relationships between fiber diameter,

ivporosity, and surface area. The predictions are used to examine the effect of fiber diameter on the performance of an electrospun system. At low porosity, electrode energy density is maximized by minimizing void space in the electrode. Parametric manipulation of the model shows thatimprovements to electrode conductivity and the material’s specific capacitance are promising, high-impact areas for optimization, while electrolyte conductivity and exchange current density have minimal effects. The model is also expanded to MnO2, Fe2O3, Co3O4, V2O5, and WO3in order to predict suitability for use in electrospun web electrodes. The unexpectedly good performance of low-capacitance materials with high conductivities reveals the complex relationship between material parameters and electrospun electrode performance. The model is useful for predicting the effects of changing electrospun electrode parameters while decreasing the amount of necessary experimentation.The work presented in this dissertation has demonstrated the suitability of electrospun structures for use assupercapacitor electrodesand provides insight into the fabrication conditions that improve capacitance. The model produced is a powerful tool for predicting materials and fiber sizes that are well-suited to the application, providing the potential for electrospun electrode fabrication methods to be expanded into higher-performing materials for improved low cost energy storage.

Comments

A dissertation submitted in partial fulfillment of requirements for the degree of Doctor of Philosophy: Materials Science.

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