| Due to the deep concerns of environmental issues and the consumption of fossil fuels, such as petroleum, natural gas and coal, as well as the accelerated greenhouse effect, the renewable energy sources e.g., wind, solar and hydroelectricity have attracted enormous interests. The large fluctuations of these renewable energy sources in power output have brought the vigorous development of the area of energy storage system. Supercapacitors, which bridge the gap of power and energy between batteries and dielectric capacitors, have developed fast in the last decades among these energy storage devices. Although batteries can store a large quantity of energy, they release energy in a slow rate, resulting in a very low power density with a limited lifetime. On the other hand, dielectric capacitors can be charged and discharged at high rate and hence possess very high power density, but their energy density is low. With relatively high power, mid-high energy density and long cycle lifetime, supercapacitors are attractive for many applications, such as in grid scale renewable energy storage and in hybrid electric vehicles where high energy, high power and reasonable lifetime are all required. However, the current commercial supercapacitor product still suffers from the low energy density (less than 10 Wh/kg) and low power density (1 kW/kg). Hence, it is highly desired to further improve electrochemcial performance of supercapacitors cells for the advanced and wide applications. In this dissertation, different supercapacitor cells are introduced to improve the performance by several strategies such as controlling the nanomorphology of electrodes and optimizing the cell configuration.;The tortuous ion transport pathways formed in activated carbon, which is widely used as electrodes in the current commerical products, have influenced the power denisty of the cell. To overcome this drawback, the aligned carbon nanotubes (A-CNTs) were investigated in this dissetation due to the superior electrical conductivity and parallel ion pathways of electrodes. Meanwhile, to achieve high volumetric energy and power densities of the cells, a unique mechanical densification method was developed to allow the density of A-CNTs to be tuned precisely over a broad range from 1% volume fraction (Vf) to 40% Vf while preserving the straight ion pathway between A-CNTs. As a result, the supercapacitors fabricated from 40% volumetric fraction (Vf) of A-CNTs as the electrodes with the thickness of 0.8 mm exhibit a power density of 25 kW L-1 (50 kW kg-1), which is much higher than that of the A-CNTs electrodes with similar thickness fabricated by other methods and that of activated carbon electrodes.;Pseudocapacitive materials, such as conducitng polymers and transition metal oxides, can be incorporated into the electrode to increase the specific capacitance because the whole bulk (not only the surface for pure carbon electrode) of pseudocapacitive material has involved the electrochemical energy storage. Poly(ethylenedioxythiophene) (PEDOT) was studied as the pseudocapacitive materials in this dissertation. The conformal coating of PEDOT on A-CNTs can exhibit long cycle life compared with pure PEDOT or PEDOT coated on random CNTs since the A-CNTs can provide a mechanical structure to absorb the large volume change of PEDOT during the charge and discharge processes. The unique mechanical densification method was also used to densify the composite to 5% to improve the volumetric performance of the cell. Symmetric supercapacitors using the 5% compacted A-CNTs coated with 10nm thickness PEDOT as electrodes, as well as using BMI-BF4/PC as electrolyte can achieve the highest volumetric specific capacitance of 92.79 F cm-3. Meanwhile, the QV curve based on CV curve or galvanostatic curve was firstly introduced. This new investigation method can be used to evaluate the cell energy loss and coulombic efficiency better compared with the conventional one. (Abstract shortened by ProQuest.). |
