Examination Committee

Prof Qiong LUO, CSE/HKUST (Chairperson)
Prof Zhiyong FAN, ECE/HKUST (Thesis Supervisor)
Prof Zijian ZHENG, Institute of Textiles and Clothing, The Hong Kong Polytechnic University (External Examiner)
Prof Kevin CHEN, ECE/HKUST
Prof James SHE, ECE/HKUST
Prof Qing CHEN, MAE/HKUST

Abstract

Electrochemical capacitors (ECs) possess large specific power, fast charge/discharge rate, and long cycle life, etc., and these appealing features offer ECs possibilities in a broad range of energy-related applications such as regenerative breaks in electric vehicles, memory back-up systems and pacemakers in medical treatment, etc. However, these devices suffer from low specific energy/volumetric capacitance because the charge stored mainly comes from surface electrochemical activity, either physically ion adsorption or chemcially surface redox reaction, and the low specific energy/volumetric capacitance leads to the failure of devices in long time power delivery.  
 
Recently, engineering of three-dimensionally (3-D) ECs electrode is regarded as the promising way to enhance the specific energy/volumetric capacitance of the ECs, mainly because 1) 3-D architecture has large surface area, which is capable to load larger amount of active materials in comparison with the planar architecture. 2) 3-D ECs electrode possesses shorter ion diffusion and electron transfer pathway. 3) The anchor effect of 3-D electrode could fasten the active materials located at its surface, thus releasing the mechanical stress during the charge/discharge process.
 
In this thesis, we firstly engineered a unique ECs electrode based on free-standing 3-D gold (Au) nanospikes (NSPs) film. The large surface area of NSPs is utilized to achieve high areal capacitance together with thin layer of MnO2 coating on both of front and back sides of the NSPs. The all-solid-state symmetric supercapacitors based on MnO2/Au/MnO2 NSPs (MAMNSPs) electrodes have been fabricated and systematic performance characterizations showed that the devices have highly attractive volumetric capacitance and specific energy.  Secondly, to achieve mechanically stable 3-D architecture, a unique 3-D Nanopores (NPs) with larger surface area and higher aspect ratio was constructed via a cost-effective ultrasonic spray pyrolysis (USP) method. The unique hierarchical MnO2/FTO/AAO NPs (MFANPs) pseudocapacitor electrode based on FTO NPs arrays achieves both high areal and volumetric capacitance, together with a remarkable capacitance enhancement in comparison to planar electrode. Thirdly, to further decrease the “dead volume“ of ECs electrode, a 3-D interconnected nanoporous (INPOS) architecture was achieved via soft anodization of aluminum alloy. Such structure inherits all the structural merits of 3-D NPs, including large surface area, efficient electron transport, as well as good structural stability, while superior to 3-D NPs in higher porosity and better ion accessibility. Benefiting from the above superiority, the pseudocapacitor electrode built based on 3-D INPOS architecture achieves both high areal and volumetric capacitance, together with largely enhanced rate capability.