Author ORCID Identifier

https://orcid.org/0000-0003-0953-8686

Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. Fuqian Yang

Abstract

The vigorous development of human civilization has significantly increased the energy consumption in recent years. There is a great need to use renewable energy sources to substitute the depleting traditional fossil fuels, such as crude oil, natural gas and coal. The development of low-cost and high-performance energy storage devices (ESDs) and systems have drawn great attention due to their feasibility as backup power supply and their applications in portable electronics and electric vehicles. Supercapacitors are among the most important ESDs because of their long charging-discharging cycle life, high power capability and a large operating temperature range. In this thesis, high-performance activated carbons (ACs)-based SCs have been synthesized from two biomass materials in both “bottom-up” and “top-down” patterns, including high fructose corn syrup and soybean residues, which are economic and environmental friendly.

Firstly, a hydrothermal carbonization (HTC) - physical activation method is presented to synthesize activated carbons from high fructose corn syrup (HFCS). The effect of the activation time on the geometrical and porous characteristics of the ACs is investigated. The electrochemical performance of the supercapacitor cells made from AC treated at 850ºC for 4 hours are found as the best with a specific capacitance of 168 F/g at 0.2 A/g in 6 M KOH aqueous system. Secondly, a two-step HTC process followed by a physical activation to prepare activated carbons from soybean residue is presented. The effect of activation temperature on geometrical and porous characteristics of the ACs is studied. The ACs activated at 850ºC are found partly crystallized and exhibit a specific capacitance of 227 F/g at 2 mV/s.

To understand the effect of mechanical deformation of the electrode materials on the electrochemical performance of electrical double-layer supercapacitors, a series of compression tests of HFCS-based ACs are further conducted in both dry and wet conditions. The nominal stiffness of the compressed ACs is calculated from the unloading curves. For both dry and wet disks the stiffness get increased with increased compression load, where the wet ones get higher stiffness than that of the dry ones. A simple model of porous materials is used to explain the increase in the stiffness of a compressed disk with the increase of pressure.

Lastly, the effect of mechanical deformation on the electrochemical impedance of HFCS-based ACs is studied. When increasing the mechanical pressure from 4 to 81.5 KPa, the system resistance shows a relatively stable trend around 1 ohm, while the charge transfer resistance shows a dramatic dependence on mechanical pressure decreasing from 420 ohms to 1.5 ohms.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.045

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