Electrochemical Performance of Polymer Derived Carbon Nanofibers and Tungsten Compound/Carbon Composites

Current energy demands and advancements in energy harvesting have driven research towards developing storage devices with high energy and power densities to better store and deliver charge. Battery devices fulfill many of these demands, but supercapacitor devices have garnered more favor due to their rapid charge rate capabilities and their vast cyclability. The different classifications of supercapcitors (electric double layer capacitors, pseudocapacitors, and hybrid supercapcitors) rely on either physical charge storage mechanisms which provide rapid delivery of charge, faradaic charge storage mechanisms which can be used to achieve how quantities of charge, or a combination of the two. While advancements in the field have been great, they are largely driven by trial-and-error approaches. To fully achieve the potential of supercapacitor devices, a better fundamental understanding of the aspects of the electrode materials and their influence over charge capabilities is needed. Chapter 1 introduces supercapacitor devices, and some of the materials that can be used to make them. It provides details on the production of activated carbon nanofibers (CNFs) derived from electrospun polyacrylonitrile (PAN) and tungsten compounds that were used in this work. Chapter 2 demonstrates the influence of miscibility in electrospun polymer blends on electrochemical performance on devices with ionic liquid electrolyte. In comparison to blends of PAN:polystyrene (PS) which have a miscibility parameter (MP) of 118, the studied blend of PAN:poly(styrene-co-acrylonitrile) has a miscibility parameter of 79. This resulted in a distinct morphology for phase separation of the blends, as well as a channel like morphology with interspersed domains. Devices from the PAN:SAN blend achieved a maximum power densities of approximately 17,500 W/kg when tested at 10 A/g in galvanostatic charge/discharge (GCD) and achieved a maximum energy density of 81 Wh/kg at 1750 W/kg when tested at 1 A/g. This device also boasted a capacitance retention of 82% after 3,500 cycles. Chapter 3 describes the preparation of hybrid electrode materials derived from polymer fibers with tungsten oxide nanoparticles and the influence of their degree of interaction on performance. A novel synthesis was described for the preparation of WO2.72, which normally is synthesized through a hydrothermal procedure in an autoclave, was prepared by in-situ synthesis of hybrid tungsten compound at CNFs with CO2 activation. This hybrid material (WO@CNF) was compared to pure WO2.72 and CNFs to determine the degree of interaction between the components in the hybrid and determine that interactions influence on the performance of the device. The WO@CNF material was found to have a low degree of interaction, but this still provided an improvement on the energy storage capabilities of the material over the physical mixture composite. Chapter 4 describes the modification of the synthesis used to make WO2.72 nanoparticles in carbon fibers to produce WN nanoparticles and the comparison of their electrochemical performance. Upon carbonization without activation, WN was found in the WO@CNF hybrid material. Metal nitrides possess favorable qualities for pseudocapacitors, and hybrid supercapacitors compared to metal oxides, and WN is not widely researched for these devices. Utilizing ammonia activation drove the synthesis towards WN to form WN@CNF hybrid materials. The WN@CNF produced carbons with a higher graphitic quality (Id:Ig ratio of 0.77 compared to 0.91 for WO@CNF), and achieved comparable power densities, but possessed lower energy densities.