Hao Yang
Published: 2013
Total Pages: 73
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Abstract: Although great efforts have been made on development of high performance Li-ion batteries and fuel cells in the past, the slow power capability and high maintenance cost have kept them away from many applications. Recently, supercapacitors have drawn great attention because of their high charge/discharge rate, long life cycle, outstanding power density and no short circuit concern. However, supercapacitors generally exhibit low energy density. The objective of this thesis research is to develop graphene-based supercapacitors with simultaneously high power density and energy density at low production cost. Supercapacitors, also known as ultracapacitors or electrochemical capacitors, store energy as electrical charge on highly porous materials. Currently one major challenge that keeps supercapacitors from their promising applications is their low energy density. One promising electrode material candidate for electric double-layer (EDL) supercapacitors is graphene. Graphene, due to its unique lattice structure, exhibits appealing electrical properties, chemical stability and high surface area. Ideally a monolayer of sp2 bonded carbon atoms can reach a specific capacitance up to ~550 F/g as well as a high surface area of 2675 m2/g. So far, a variety of methods have been developed to synthesis graphene starting from graphite, but the cost, graphene quality and productivity remain main obstacles for their industrial application. The porous graphene material reported in this thesis was synthesized by a scalable oxidation-reduction method involving a rapid annealing process. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images revealed the morphology and successful exfoliation of reduced graphene oxide (rGO). The interlayer distance characterized by X-ray diffraction (XRD) is 3.64 Å (24.44°) suggesting the removal of oxygen-containing functional groups, such as carbonyl, hydroxyl and carboxyl groups. In the X-ray photoelectron spectroscopy (XPS), the C/O ratio increases from ~2 to ~5 with O1s peak reduced significantly from graphite oxide (GO) to reduced graphene oxide. Furthermore, the successful reduction was verified by the low intensities of oxygen-related peaks in Fourier transform infrared spectroscopy (FTIR). In addition, the high Brunauer-Emmett-Teller (BET) specific surface area of 410 m2/g and mesoporous structure of the synthesized material would be beneficial to the improvement of charge-storage capability and thus energy density in supercapacitors. To evaluate the electrochemical performance of graphene electrodes, supercapacitors were assembled in symmetrical cell geometry. The near rectangular cyclic voltammetry (CV) curves with EMIMBF4 and LiPF6 at scan rate of 100mV/s suggest very efficient charge transfer within the porous graphene electrodes. The triangle charge-discharge responses with a small voltage drop and vertical spike in the low frequency region of a Nyquist plot indicates an ideal capacitor performance. The specific capacitance of 306.03 F/g and energy density of 148.75 Wh/kg at 1A/g were realized with highly porous graphene electrodes. Meanwhile, the power density extracted at 8A/g reaches ~10 kW/kg, thus, making it suitable for high power applications. Compared with previously investigated carbon-based EDL capacitors, the supercapacitor based on the annealed graphene electrode is a milestone in terms of capacitance and energy density. Moreover, the supercapacitors assembled with graphene electrodes shows excellent stability for 10,000 charge-discharge cycles.