Date of Award

1-2018

Document Type

Dissertation

Publisher

Santa Clara : Santa Clara University, 2018.

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical Engineering

First Advisor

Carl Y. Yang

Abstract

A model of carbon nanotube (CNT) ultracapacitor (CNU) as a high-performance energy storage device is developed based on simulations of electrolyte ion motions between cathode and anode. Using a molecular dynamics (MD) approach, the equilibrium positions of electrode charges interacting through Coulomb potential are determined, which in turn yield the equipotential surface and electric field associated with the capacitor. With an applied AC voltage, the current is computed from the nanotube and electrolyte particle distribution and interaction, resulting in a frequency-dependent CNU impedance. From the current and impedance profiles, the Nyquist and Cyclic Voltammetry plots are then extracted. Results of these calculations compare well with existing experimental data. A lumped-element equivalent circuit for the CNU is proposed and the impedance computed from this circuit correlates well with the simulated and measured impedances.

Further, a methodology is developed to optimize vertically grown carbon nanotube CNU geometrical features such as CNT length, electrode-to-electrode separation, and CNT packing density. The electric field and electrolyte ionic motion within the CNU are critical in determining device performance. Using a particle-based model (PBM) based on MD techniques, developed for this purpose, the electric field in the device is computed, the electrolyte ionic motion in the device volume is tracked, and the CNU electrical performance as a function of the aforementioned geometrical features is determined. Interestingly, the PBM predicts an optimal CNT packing density for the UC electrodes. Electrolyte ionic trapping occurs in the high CNT density regime, which limits the electrolyte ions from forming a double layer capacitance. In this regime, the CNU capacitance does not increase with the CNT packing density as expected, but decreases significantly. The results compare well with existing experimental data and the PBM methodology can be applied to an ultracapacitor built from any metallic electrode materials, as well as vertically aligned CNTs studied here.

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