Year

2018

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

High demand for energy from clean and renewable energy sources such as wind and solar have made the necessity for efficient energy storage systems. This has catapulted energy storage research on batteries and supercapacitors. These storage technologies, i.e. batteries and supercapacitors (carbon-based electric double layer capacitors), fall short, however, in terms of power performance and in their ability to store energy density, respectively. Pseudocapacitors combine both the features of batteries and supercapacitors. They possess high rates capability, and can store much more energy than a supercapacitor, although the ultimate performance of these devices strongly depends on the intrinsic properties of their constituent elements and their eventual architectural design. Thus, we have explored more economical and feasible way to develop multi-cationic oxide materials, namely, MnCo2O4, NiCo2O4, NiFe2O4 and Zn-Ni-Co oxide as pseudocapacitor electrodes.

The first part of this thesis looks into MnCo2O4 nanoflakes that were synthesized successfully via a hydrothermal technique in different morphologies depending on the amount of NH4F. The MnCo2O4 nanoflakes in combination with graphene nanoplatelets was deposited on Ni foam using an electrophoretic deposition technique. The as prepared composite electrode showed superior performance in terms of specific capacitance and cycling stability, as compared to the pristine MnCo2O4 system, due to the enhanced electronic conductivity resulting from bond formation between carbon and MnCo2O4. A high specific capacitance of ~ 1323 F g-1 was observed at 1 mV·s-1 scan rate. Noteworthy cycling stability was observed, even at the end of 10000 cycles of consecutive charging and discharging at a current density of 7.81 Ag-1.

The second part of the thesis focuses on NiCo2O4, which was considered to be one of the most promising materials for pseudocapacitor applications, although its unsatisfactory rate performance and cycling stability, due to its inherent low electrical conductivity, has limited its further development as a pseudocapacitor electrode. Our study tries to profitably exploit reduced graphene oxide (rGO) nanosheets as a conducting unit across the NiCo2O4 matrix to improve its overall electrochemical performance. This is done through a simple hydrothermal technique. The as-prepared NiCo2O4-rGO nancomposite consists of NiCo2O4 hexagons wrapped in conducting rGO sheets, which ensure a short ion diffusion distance, percolating electron conducting pathways, and stable structural integrity. Such a feasible design provides good synergism between the rGO and the NiCo2O4, resulting in better electrochemical performance. As a result, this nanocomposite displays an impressive overall electrochemical performance, such as a promising capacitance (1185 F g-1 at a current density of 2 A g-1) and remarkable cycling stability (98% capacitance retention after 10000 charge-discharge cycles at 2 A g-1). This facile method could be beneficial for preparing similar materials that require high electronic conductivity. The third section is an investigation of NiFe2O4 (NFO) nanoparticles embedded on graphene capsules (GCs), which were synthesized by a simple hydrothermal technique. This NFO–GCs electrode material was subjected to different types of electrochemical performance evaluation to investigate its feasibility as a pseudocapacitor electrode. The as-prepared NFO–GCs nanocomposite electrode exhibits a high specific capacitance of 1028 F g-1 at a current density of 2 A g-1 and 94% capacitance retention at the end of 10000 cycles of charge–discharge, whereas pristine NFO electrode shows 720 Fg-1 specific capacitance with 88% capacitance retention. The high specific capacitance, good rate capability, and excellent cycling stability of the NFO–GCs composite can be attributed to the effective synergism between the GCs and the NFO. The superior electrochemical performance of the NFO–GCs nanocomposite demonstrates the possible application of this material as a working electrode for fully functional pseudocapacitor devices.

The final part of the thesis focuses on porous Zn-Ni–Co oxide/CNTs (ZNCO) hexagonal nanoplate. This material is particularly interesting because of their large surface areas, easy electrolyte access to electrode, efficient electron transfer, fast ion transport, and good strain accommodation. Moreover, the electrochemical tests indicated that the as-prepared composite showed a positive synergism between the CNTs and the porous ZNCO, resulting in good pseudocapacitive behaviour in terms of high specific capacitance, excellent rate capability, and good cycling stability. These advantages, along with the ease of processing, make this (ZNCO-CNTs) composite a potential candidate for supercapacitor applications. In particular, these CNT wrapped multi-component metal oxides with a homogenous structure exhibit high specific capacitance of 2360 F g−1 at a current density of 2 A g-1 and a remarkable cycling stability of 96% capacitance retention over 10000 charge-discharge cycles. The ZNCO nanocomposite shows the maximum power density and maximum energy density of 355 W/kg and 50 Wh/kg, respectively. The as-prepared Zn- Ni-Co oxide CNTs nancomposite consists of Zn- Ni-Co oxide hexagons wrapped in conducting CNTs which ensure a short ion diffusion distance, percolating electron conducting pathways, and stable structural integrity. Such a feasible design could also provide good synergism between the CNTs and the Zn- Ni-Co oxide, resulting in better electrochemical performance. As a result, this nanocomposite displays impressive overall electrochemical performance. This facile and feasible method could be beneficial for preparing similar materials that require high electronic conductivity for supercapacitor applications. Thus the main objective of this thesis has been to explore new electrode materials for pseudocapacitors as a solution to the ever-rising demands for better performing devices exhibiting longer cycle life and improved safety.

FoR codes (2008)

0912 MATERIALS ENGINEERING, 0910 MANUFACTURING ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.