Freestanding nanofiber electrodes for supercapacitors

Daniel Wayne Lawrence

doi:10.17918/etd-6917

Supercapacitors are new and promising electrochemical energy storage devices that possess a much higher energy per unit mass than conventional capacitors while maintaining very high power handling capabilities. However, the current energy storage capability of conventional supercapacitors is still much less than that of batteries. The energy storage potential of a supercapacitor is dependent on the total charge it can store and the potential at which storage occurs. Improvements are constantly being made through alteration to device capacitance through implementing more capacitive materials and increasing the surface area where charge storage can take place. Perhaps more significantly, serious effort has been taken to increase the operational potential of supercapacitor devices to greatly increasing energy storage. Recent efforts towards increasing this potential have been made through implementing electrolytes stable at higher potentials, most notably room temperature ionic liquids, as well as designing around the positive and negative potential limits based on electrode-electrolyte interaction, maximizing the available window through an asymmetric device design. Electrospun porous carbon nanofibers are used as electrode materials in supercapacitor devices due to their high conductivity, wettability, and freestanding structure. Further treatments of the fibers result in a high specific surface area with a hierarchical porous structure that facilitates electrolyte diffusion and ion adsorption for rapid, low-resistance charge storage. In this work two supercapacitor designs are explored using these carbon nanofiber electrodes that aim to maximize the operational potential voltage of the supercapacitor device. The first is a solid-state supercapacitor with silica-based ionic liquid gel electrolyte is developed that achieves a specific capacitance of 144 F g-1, and energy density of 61 Wh kg-1 based on the mass of the electrode active material. An emphasis is placed on understanding the novel electrolyte design and its strengths and weakness compared to an unmodified ionic liquid electrolyte. The second is a hybrid electrode consisting of a porous carbon nanofiber substrate with a thin film surface deposition of the conductive polymer polyaniline, which is employed in an asymmetric supercapacitor design in order to maximize its available potential window in an aqueous electrolyte. The fundamentals of the more complicated asymmetric design are explored in detail to ensure the optimization of device performance.

Freestanding nanofiber electrodes for supercapacitors

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Freestanding nanofiber electrodes for supercapacitors

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