Date of Award

Spring 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering - (Ph.D.)

Department

Electrical and Computer Engineering

First Advisor

Grebel, Haim

Second Advisor

Tsybeskov, Leonid

Third Advisor

Barat, Robert Benedict

Fourth Advisor

Ko, Dong Kyun

Fifth Advisor

Liu, Xuan

Abstract

A new approach to improve the capacitance of supercapacitors (SC) is proposed in this study. A typical SC is composed of an anode and a cathode; a separator in between them assures an unintentional discharge of the capacitor. The study focuses on a family of structured separators, either electronically active or passive which are called gates. An active structured separator layer has been fabricated and analyzed. The structured separator has characteristics of electrical diode and is fabricated out of functionalized carbon nanotubes (CNT). Improvement of the overall capacitance of SC, equipped with either active or passive structured separators demonstrated a large capacitance increase compared to SC which are interfaced with traditional separators.

Cyclic voltammetry (CV), Chrono-potentiometry (charge-discharge, C-D) and Electrochemical Impedance Spectroscopy (EIS) are used to assess the electrochemical characteristics of these novel devices. Raman spectroscopy is used to assess the quality of the structured CNT. Scanning electron microscope determines the surface morphology and porosity of the films. Current-voltage measurement takes place to ensure non-linear characteristics of the fabricated active separator layer. CV demonstrates that aqueous based electrolyte supercapacitors (SC) does not exhibit reaction peaks. Chrono-potentiometry demonstrates an overall 5-10% larger capacitance then traditional counterparts. EIS exhibits an unusual reduction of the cell's equivalent series resistance (ESR). Additional capacitance increase can be achieved when the active structured separator is biased between -0.1 V to +0.1 V. The electrical energy, invested in a biased gate is fully captured as a stored energy.

Similar study is performed by fabricating active separator interfacing with ionic liquid electrolyte. Active gates exhibit similar capacitance improvement even after many cycles of charge and discharge when they are interfaced with ionic liquid.

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