A novel electrochemical cell design based on yarn or fiber-like carbon nanotube electrodes was developed as an engineered solution for flexible electrochemical devices, namely
electrical energy storage devices. Proof-of-concept cells consisting of porous CNT networks as the sole structural support, electronic conductor and active charge storage material were
fabricated and tested. The coaxial yarn cell provided a robust structure able to undergo flexural deformation with minimal impact on the energy storage performance. Greater than 95% of the
energy density and 99% of the power density was retained when wound around an 11 cm diameter cylinder. The electrochemical properties were characterized at stages throughout the fabrication
process to provide insights and potential directions for further development of these novel cell designs. To demonstrate the ability to improve the performance and extend the applicability of
the CNT network based cell design to various cell chemistries and material combinations, the versatile redox activity material vanadium oxide (VOx) was deposited onto
CNT yarns. A supercritical fluid deposition and in-situ oxidation process was utilized to create thin conformal coatings of vanadium oxide on carbon nanotube (CNT) surfaces throughout the
porous structure of CNT yarns. Half-cell electrochemical characterizations were conducted on carbon nanotube-vanadium oxide (CNT-VOx) yarn electrodes in an 8 M LiCl
aqueous electrolyte. The high surface area, interconnected pore structure and high electrical conductivity of the CNT yarn enabled extraordinary rate capabilities from the high capacity
Li/VOx system. Cyclic voltammetry tests with scan rates of several volts per second, requiring current densities of hundreds of amperes per gram of electrode mass
produced voltammograms with distinguishable redox peaks from Li-ion intercalation/deintercalation. Capacitances of over 150 F g-1 were achieved at a scan rate of 5
V s-1 over a 1.2 V potential window resulting in an energy density of > 32 Wh kg-1 (> 30 Wh L-1) for the
yarn electrode. The charge storage also showed good reversibility when cycled over this large potential window, maintaining 90% of the capacitance after 100 cycles at a scan rate of 2 V
s-1. Investigation into the structure-property relationship of the CNT yarn and the effects on electrochemical energy storage performance was conducted through
half-cell testing of single-filament CNT yarn electrodes. Electrochemical impedance spectroscopy (EIS) enabled the development of an equivalent circuit model based on representation of the
CNT yarn through a parallel configuration of a constant phase element (CPE) and Warburg impedance element. Various physical properties of the electrode and electrode-electrolyte system can be
obtained from model fitting of the experimental data, providing the ability to verify model accuracy through the comparison of known physical properties (e.g. electrode resistivity), as well
as the ability to gain insight into complex electrochemical phenomena from previously unmeasured properties. Analysis of the modeling results for the DC potential dependent EIS tests on
electrodes of various lengths suggest a relationship between the effective ion diffusion lengths, the extent of mobile charge carries in the CNT electronic structure and the fractal dimension
of the hierarchical self-similar CNT yarn electrodes. The unique concentric cylinder architecture of the flexible coaxial electrode cells displayed an unintended sensitivity of the cell
open-circuit voltage (OCV) to the atmosphere surrounding the cell; specifically to changes in the relative humidity. This potentiostatic responsiveness from the coaxial cell was characterized
experimentally and a hypothesis was developed relating the OCV response to humidity to the chemical potential energy that exists upon the establishments of concentration (activity) gradients.
The Nersnt relation was used to provide quantitative support of the hypothesis through the development of a time-dependent model of the OCV response to the relative humidity. Finite element
methods were used to approximate the 1D solution to Fick's diffusion laws to model the H₂O transport through the polymer electrolyte membrane and calculate the time-varying water
concentration profile. The model results provide strong support of the hypothesis that the open-circuit voltage response is based on the time-dependent concentration gradient of water between
the inner and outer electrodes of the coaxial cell. These results have implications for applications such as humidity sensors, wearable energy harvesting, and self-powered detection
devices. / A Dissertation submitted to the Materials Science and Engineering Program in partial fulfillment of the requirements for the degree of Doctor of
Philosophy. / Fall Semester, 2014. / November 7, 2014. / Includes bibliographical references. / Zhiyong Liang, Professor Directing Dissertation; Tao Liu, Committee Member; Jim Zheng, Committee Member; Steven Lenhert, Committee Member.
| Identifer | oai:union.ndltd.org:fsu.edu/oai:fsu.digital.flvc.org:fsu_253411 |
| Contributors | Smithyman, Jesse (authoraut), Liang, Zhiyong Richard (professor directing dissertation), Steigman, Albert E. (university representative), Liu, Tao, 1969- (committee member), Zheng, Jianping P. (committee member), Lenhert, Steven (committee member), Florida State University (degree granting institution), The Graduate School (degree granting college), Program in Materials Science (degree granting department) |
| Publisher | Florida State University, Florida State University |
| Source Sets | Florida State University |
| Language | English, English |
| Detected Language | English |
| Type | Text, text |
| Format | 1 online resource (163 pages), computer, application/pdf |
| Rights | This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s). The copyright in theses and dissertations completed at Florida State University is held by the students who author them. |
Page generated in 0.0131 seconds