Porous Carbon Prepared by Chemical Activation for High-Energy Supercapacitors in Ionic Liquid Electrolyte
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Supercapacitor (SC), a high-power, safe and long-lifetime energy storage device, has been considered as a potential candidate to replace the use of rechargeable batteries for mobile high-power application. Due to the relatively low energy density of the supercapacitor compared to rechargeable batteries, the wide-scale application of the supercapacitor is limited. To improve the specific energy of SCs, it is efficient to improve the double layer capacitance of the carbon electrode in ionic liquid electrolyte as the ionic liquid electrolyte provides a wide voltage window. In the work presented in this thesis, we have made an overview of the current understanding about the influence of the active electrode material on the double layer capacitance. Based on the existing comprehension of the charged electrode/electrolyte interface, we built up an ion packing model, which can estimate the gravimetric capacitance according to the pore size distribution. We validate the model by comparing the experimental capacitance, from both our work and literature, with the model estimated capacitance. The effect of the cylindrical pore properties on the gravimetric capacitance has been comprehensively studied and a strategy to improve the gravimetric capacitance of the porous material has been suggested. Porous carbon material with capacitance-favorable ion packing method (optimized pore size distribution, large nanopores volume and large specific surface area) is synthesised from polyaniline (PANI) by KOH chemical activation and is shown to deliver a high gravimetric capacitance. To further improve the performance of the PANI-derived porous carbon material, efforts are also applied to improve the rate capability and the volumetric capacitance while maintain the high gravimetric capacitance. We offer a systematic strategy to improve the SC performance comprehensively with high energy (both gravimetrically and volumetrically) and high output power capability. Moreover, pine tree powder, much cheaper precursor compared to the PANI, was used as the activation precursor to synthesis porous carbon. Instead of the KOH activation, a new method of H3PO4 impregnated NH3 activation was applied to the biomass. Compared to the porous carbon treated by the KOH activation, the porous carbon synthesised by the new activation method showed improved surface area and performance in the SC using non-aqueous electrolytes.