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Permanent link (DOI): https://doi.org/10.7939/R3DR2PH5B

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Electrodeposition of Mn Oxide Supercapacitors Open Access

Descriptions

Other title
Subject/Keyword
Supercapacitor
Nucleation and Growth
Mn oxide
PEDOT
Type of item
Thesis
Degree grantor
University of Alberta
Author or creator
Clark, Michael P
Supervisor and department
Ivey, Douglas (Chemcial and Materials Engineering)
Examining committee member and department
Luo, Jingli (Chemical and Materials Engineering)
Sharp, David (Chemical and Materials Engineering)
Etsell, Thomas (Chemical and Materials Engineering)
Ivey, Douglas (Chemical and Materials Engineering)
Department
Department of Chemical and Materials Engineering
Specialization
Materials Engineering
Date accepted
2015-05-15T08:48:06Z
Graduation date
2015-11
Degree
Master of Science
Degree level
Master's
Abstract
In today’s society, the need for renewable energy sources is driving the development of new energy storage devices. Supercapacitors are one such device and are characterized by their large power density, long cycle life, and environmental friendliness. Many novel materials and architectures have been developed to produce supercapacitors with high specific capacitances (F/g). However, many of these materials are not practical for commercial applications because of their high cost and labor intensive production. The goal of this thesis is to produce high performance supercapacitor electrodes using inexpensive materials and an inexpensive and simple electrodeposition technique developed by our research group. By modifying the electrodeposition process previously developed by our research group, fibrous Mn oxide rods were deposited onto Ni foam substrates. One major factor affecting supercapacitor performance is the electrode surface area; the fibrous micron scale rods greatly improve the electrode surface area and consequently capacitive performance. Deposits were characterized using a variety of materials and electrochemical techniques including scanning electron microscopy (SEM), transmission electron microscopy (TEM) and cyclic voltammetry (CV). The electrodeposited rods are 15-20 µm long, around 2 µm wide, and are petal shaped, with the bases of rods being narrower than the tops. The rods are composed of many sheets, each only a few nanometers wide. TEM analysis revealed that the sheets are nanocrystalline with a grain size of about 5 nm. Electron diffraction patterns can be indexed to either cubic spinel Mn3O4 or hexagonal birnessite MnO2. X-ray photoelectron spectroscopy (XPS) indicated that Mn is present in the deposit as a mixture of oxidation states. Mn oxide deposits on Ni foam exhibited a capacitance of 144 F/g (450 mF/cm2), at a galvanostatic charge/discharge rate of 0.5 mA/cm2. During extended cycling, dissolution and redeposition of Mn oxide caused cracking and peeling of the deposit. After 500 cycles, a change in oxidation state is observed, with the mixed Mn2+/Mn3+ state being oxidized to Mn4+. Despite the damage caused by cycling, redeposition and the oxidation state change lead to a capacitance increase of 13% over 500 cycles. With the aim of improving capacitive performance, a conductive polymer coating of polyethylenedioxythiophene (PEDOT) was applied to Mn oxide deposits using an electropolymerization technique. Helium ion imaging revealed that the PEDOT conformally coated the fibrous Mn oxide rods. The PEDOT coating improved the initial capacitance from 144 to 217 F/g (450 to 690 F/cm2). The PEDOT coating successfully prevented dissolution during cycling. Although capacitance dropped by 9% over 500 cycles, the capacitance at cycle 500 for deposits with PEDOT was still larger than deposits without PEDOT. Deposits were annealed in air and forming gas (95% N2 and 5% H2) at 350˚C for 1 h. Both annealed deposits experienced an increase in crystallinity with annealing. Deposits annealed in air retained their cubic spinel Mn3O4 structure, while deposits annealed in forming gas transformed to a tetragonal spinel Mn3O4 structure. The capacitances of deposits annealed in air and forming gas were 93 and 56 F/g (230 and 170 mF/cm2), respectively, measured by CV at a scan rate of 10 mV/s. The nucleation and growth of electrodeposited Mn oxide rods was investigated by preparing deposits on Au coated Si at varying deposition times between 0.5 s and 5 min. The deposits were investigated using high resolution SEM and TEM. A model for the nucleation and growth of Mn oxide rods has been proposed. TEM analysis of 3 s and 6 s deposits shows that the sheets are initially amorphous and then begin to crystallize into a cubic spinel Mn3O4 crystal structure. High resolution imaging of the 6 s sample shows small crystalline regions (~5 nm in size) within an amorphous matrix.
Language
English
DOI
doi:10.7939/R3DR2PH5B
Rights
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Clark M, Ivey D, "Electrodeposition of Nanoscale Manganese Oxide onto Nickel Foam for Energy Storage Applications" ECS Transactions, 64 (24) 57-67 (2015)

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