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Supercapacitors are widely used in our everyday life. They can supply power for portable electronic devices and electric vehicles as well as be utilized for energy recovery systems to improve energy efficiency. It is reported that the global supercapacitor market will be $2.12 billion by 2020 with an annual growth rate of 20.7%. The popularity of supercapacitors is due to their high power density, long cycle life and ability to work at low temperatures. However, state-of-art supercapacitors suffer from relatively low energy density, which has prevented them from being used in more applications. Besides, commercial cells are assembled with organic electrolytes to enlarge their potential window, which are expensive and flammable, far away from meeting the standards of ideal energy storage devices. In order to enhance the performance of current devices, advanced electrode materials, electrolytes as well as novel separators are needed. In this thesis, a hybrid material composed of carbon and nanostructured metal oxides as electrode materials is investigated. With the synergetic effects between the two compositions, the charge storage capability of the hybrid material is enhanced. To further boost the energy density of devices, the performance of commercial activated carbon supercapacitors was enhanced by a combination of a laser scribing process and a redox electrolyte. In this way, the energy density obtained was nine times higher than the commercial device, while using a safer and less expensive aqueous electrolyte. Additionally, supercapacitors were integrated with energy harvesting devices to build self-powered systems. A self-charging power pack was fabricated based on a combination of a solar cell and a supercapacitor which is able to harvest 2.92% of the solar energy. Moreover, a self-charging supercapacitor was fabricated by using a piezoelectric separator. The device can harvest energy from the ambient environment and be charged to 0.2 V simply by pressing with one’s palm.