Electrodeposition of Mn Oxide Supercapacitors

• Author / Creator

  Clark, Michael P

• 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.

• Subjects / Keywords

  • Mn oxide
  • PEDOT
  • Supercapacitor
  • Nucleation and Growth

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3DR2PH5B

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.