UC Riverside

Fossil fuels have been used as the primary source of energy to meet the growing demand for electric energy consumption worldwide. The use of fossil fuels results in carbon dioxide and other greenhouse gas emissions, contributing to the acceleration of global warming and climate change. Therefore, more energy should be produced from renewable energy resources to lessen energy reliance on fossil fuels. In particular, solar energy is regarded as the most promising renewable energy resource alternative to fossil fuels because it is not only unlimited but also pollution-free. However, solar energy is highly dependent on weather and environmental conditions. The intermittency of solar photovoltaic (PV) systems can be alleviated by the use of batteries. In addition, the capacity limitations of batteries can also be overcome with the integration of PV systems. The traditional PV-plus-battery system has significant energy losses due to the connections between individual components, affecting the efficiency of overall system. The direct integration of PV-battery system has the potential to minimize the energy losses and increase overall system efficiency.The main objectives of this dissertation are focused on 1) developing the understanding of the theoretical aspects of a photo-rechargeable battery system combining solar cells with batteries into a single device architecture, 2) investigating the properties of graphene quantum dots (GQDs) for their application in photo-rechargeable batteries, and 3) studying the roles of GQDs and ruthenium-based dye (N719) as photosensitizers in a photo-rechargeable battery. Chapter 2 focuses on understanding the structure and operation mechanism of the photo-rechargeable battery, consisting of a TiO2 photoanode, a LiFePO4 cathode, and LiTFSI in PC electrolyte. The performance and stability tests of photo-rechargeable battery is examined by electrochemical measurements. Chapter 3 describes the synthesis of GQDs that exhibit a variety of excellent properties including high carrier transport mobility, large surface area, and tunable bandgaps. Oxidized GQDs are selectively synthesized by the electrochemical exfoliation method and a toluene post-treatment. The chemical, structural, and optical properties of synthesized GQDs are studied for their use in the photo-rechargeable battery. Chapter 4 discusses the effect of GQDs in the performance of the photo-rechargeable battery. The performance and characteristic tests of four different photoanodes: TiO2, GQDs/TiO2, N719/TiO2, N719/GQDs/TiO2, are investigated. In conclusion, the photo-rechargeable battery with N719/GQDs/TiO2, shows significantly improved conversion efficiency and cyclic performance under illumination compared to the photo-rechargeable battery with pristine TiO2. GQDs have a crucial role in reducing the internal resistance, transferring electrons between TiO2 and N719, and absorbing more dye. The conclusions and remarks of the dissertation are summarized in Chapter 5.