Graduation Year

2015

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Electrical Engineering

Major Professor

Elias Stefanakos, Ph.D.

Co-Major Professor

G. Y. Goswami, Ph.D.

Committee Member

Manoj K. Ram, Ph.D.

Committee Member

Arash Takshi, Ph.D.

Committee Member

Sarath Witanachchi, Ph.D.

Keywords

Electrochemical Characterization, Electrode Deposition, Electrolytes, Energy Density, Specific Capacitance

Abstract

The rising energy consumption worldwide is leading to significant increases in energy production with fossil fuels being the major energy source. The negative environmental impact of fossil fuel use and its finite nature requires the use of alternative sources of energy. Solar energy is a clean alternative energy source; however, its intermittent nature is a major impediment that needs to be reduced or eliminated by the development of cost effective energy storage. Thermal storage in tanks filled typically with molten salt at elevated temperatures is widely used in concentrating solar power plants to generate electricity during periods of low daytime solar radiation or night time. Similarly, electrical storage in batteries, etc. is used in conjunction with photovoltaic solar power plants.

Electrochemical supercapacitors can be effectively used for electrical storage, either alone or in a hybrid configuration with batteries, for large scale energy storage as well as in electric vehicles and portable electronics. Unlike batteries’, supercapacitor electrodes can be made of materials that are either less toxic or biodegradable and can provide almost instantaneous power due to their unique charge storage mechanism similar to conventional capacitors found in most electronics. Unfortunately, the same storage mechanism prevents supercapacitors from having high energy density. The purpose of this dissertation is to investigate organic and inorganic electrode materials that can increase the specific capacitance and energy density of supercapacitors. Additionally, certain types of supercapacitor electrode materials store the charges at the electrode/electrolyte interface preventing any deformation of the material and thus increasing its cycle life by two to three orders of magnitude.

Transition metal oxides, layered transition metal chalcogenides, and their composites with graphene and conducting polymers have been synthesized, characterized, and their electrochemical performances evaluated for suitability as electrode materials for supercapacitor applications. Morphology and crystalline structure characterization methods used, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR), were used throughout this work. Electrochemical characterization involved cyclic voltammetry (CV), constant current charge and discharge (CCCD), and electrochemical impedance spectroscopy (EIS) in two and three electrode configuration using aqueous and organic electrolytes.

Ruthenium oxide-graphene (RuO2-G) electrodes were tested in the two-electrode cell configuration and exhibit an areal capacitance of 187.5 mF cm-2 in 2M H2SO4 at a RuO2:G ratio of 10:1. Due to RuO2 high toxicity, scarcity, and high cost, manganese oxide-graphene (MnO-G) was used as an alternative but its low specific capacitance remains a major stumbling block.

The electrodes’ mass loading was studied in detail to understand the effects of thickness on the measured specific capacitance. Layered transition metal chalcogenides are structurally similar to graphene but possess different characteristics. Molybdenum sulfide (MoS2) is a two-dimensional material that has lower conductivity than graphene but larger sheet spacing making it easy for other materials to intercalate and form composites such as molybdenum sulfide-polyaniline (MoS2-PANI). MoS2-PANI electrodes, with different thicknesses, were measure in a three-electrode cell configuration resulting in gravimetric capacitance of 203 F g-1 for the thinnest electrode and areal capacitance of 358 mF cm-2 for the thickest electrode; all measurements performed using 1M H2SO4 aqueous electrolyte.

Attempts were also made to reduce the supercapacitor self-discharge by depositing on the electrode a blocking thin layer of barium strontium titanate (BST). The results were rather inconclusive because of the large thickness of the deposited BST layer. However, they strongly suggest that a very thin BST layer could improve the overall capacitance because of the very large dielectric constant of the BST material. Additional work is required to determine its effects on self-discharge.

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