| Electrochemical capacitors, or supercapacitors, are energy storage devices that can charge and discharge high amounts of energy at high rates. Their high energy/high power characteristic enables them to have potential applications in power grid stabilization, hybrid electric vehicles (HEVs) and consumer electronics. Supercapacitors have rapid charge and discharge times, high charge efficiency, long cycle life and relatively lower cost. Despite these attractive properties, commercial supercapacitors still fall behind batteries in terms of energy density. Current research on supercapacitors focuses mainly on electrode fabrication, choice of electrolyte and electrode/electrolyte compatibility. Porous carbons are utilized as electrodes due to their electrochemical stability and their good electrical conductivity. In this work, carbon nanofibers (CNFs) from different polymer precursors are explored. Electrospinning was employed to obtain freestanding fiber mat that can be pyrolyzed to form binder-free CNF electrodes. The careful choice of the polymer precursor and thermal treatment protocols direct in achieving high energy storage capability. An ionic liquid, 1,3-ethylmethylimidazolium bis(trifluoromethylsulfonyl) imide, was used to achieve a wider working voltage than in organic (2.5 V to 2.7 V) or aqueous (1 V) electrolytes. Chapter 1 gives a general introduction on supercapacitors, which focuses on carbon-based electrodes. Carbon nanofiber preparation, from polymer precursor preparation, electrospinning and thermal treatments, is described. Chapter 2 describes the preparation of carbon nanofibers with improved mesoporosity derived from ZIF-8-incorporated polyacrylonitrile (PAN). Chapter 3 shows the use a porous polymer with a high fractional free volume, PIM-1, as a single precursor for high surface area carbon nanofibers. Chapter 4 describes the use of polymer blends containing PAN and 6FDA-NDA to improve the surface area of polyacrylonitrile. Chapter 5 describes the use of ammonium bicarbonate as a low-cost pore-forming agent to increase the surface area of PAN. An improvement in electrochemical performance by the use of activation by CO2 instead of steam is also discussed. |
