| Recently, many efforts have been made to search supercapacitor electrode materials that can be used practically as high power sources to supply large pulsed current. Among various available candidates, microporous activated carbons are mostly investigated due to their large surface areas, good electric conductivity, excellent chemical stability and relatively low cost. The key factors that dictate the selection of carbon materials for supercapacitor electrodes are the following: high specific surface areas for charge storage, suitable surface functional groups to enhance the capacitance by additional faradaic redox reaction and improve the wettability of carbon surface, and large pore size to facilitate the ions diffusion with a high speed.With the development of microelectromechanical systems (MEMS) and very large-scale integration (VLSI), there is an increasing requirement in the miniaturization and integration of power sources. The reduction in size and power requirement of electronic devices is the major driving force behind the development of thin-film batteries. Applications focus on the improvement of existing consumer and medical products, such as smart cards, sensors, portable electronic devices, as well as on the integration with electronic chips and microelectromechanical systems. With better integration compatibility and electrochemical performance, thin-film battery becomes the optimal choice for miniaturization and integration of MEMS and VLSI power.This thesis includes two major parts. Firstly, nitrogen-contained mesoporous carbon spheres were fabricated and used for supercapacitors. Their structure, morphology and electrochemical behaviors were investigated in great detail. Secondly, Ni(0H)2 thin-film electrodes were successfully fabricated by a novel and facile route of ink-jet printing technique. Their electrochemical performances were also investigated.The main results are as follows.(1) we demonstrate a facile polymerization-induced colloid aggregation method to synthesize a kind of mesoporous carbon spheres containing in-frame incorporated nitrogen using melamine-formaldehyde resin as a carbon precursor. The obtained MCS materials simultaneously possess the following characteristics: high specific surface areas contributed mainly by mesopores with uniform pore size, and suitable quantity of nitrogen on the surface of the materials. The precursors used in this simple process are commercially available and very cheap, which will be favorable in the preparation of MCS on a large scale. Their surface areas can be varied from 765 to 1480 m2/g with adjusting the ratio among melamine resin, formaldehyde and silica. As the electrode material for supercapacitor in 5 mol/L H2SO4, the MCS products present excellent specific capacitance as 226 F/g. Its specific capacitance can still remain 214 F/g at 5 A/g. The superior electrochemical performance of MCS is associated with the following characteristics: high specific surface area (～1480 m2/g) contributed mainly by the mesopores, uniform pore size as large as 29 nm and moderate content of nitrogen.(2) One of the most important characters of supercapacitor is high power density, which demand carbon materials used for supercapacitor possess good conductivity. Here we enhanced the carbonization temperature to 1000℃to obtain mesoporous carbon spheres with good graphitized nanostructures. The obtained carbons have good specific capacitance and rate capability as an EDLC electrode when constantly charged/discharged over a wide loading current range (1-50 A/g).Generally, the electrolyte can be classified as aqueous and organic medium. In the aqueous solutions, the operating voltage region is restricted to be ca. 1.23 V due to the thermodynamic electrochemical window of water. The electrical energy accumulated in supercapacitor can be significantly enhanced by the selection of organic medium where the decomposition potential window of the electrolyte can reach to 2 - 4.2 V. The carbon material product presents a high specific capacitance as 159 F/g at 0.5 A/g in organic electrolyte in the potential range of 1.8V to 3.8V (vs Li). The high specific capacitance of the carbon material is believed to be associated with its suitable nitrogen content that can afford pseudocapacitanc as well as the high specific surface area. From Nyquist we conclude that the double layer capacitance is 122 F/g before cycling and after 1000 cycles it still kept on 127 F/g. This phenomenon indicates that the MCS particles can be used for 1000 cycles without aggregation, and the nitrogen functional groups may be very stable.(3) Nitrogen groups can influence the electrochemical performance of carbon materials greatly. Variation in carbonization temperature can result in the MCS materials with different nitrogen content and graphitized nanostructures. Increasing the temperature from 700 to 1000℃, the nitrogen content decrease from 14.9 to 8.9 wt.%. The lower carbonization temperature, the higher the specific capacitance and the poorer the power performance. We conclude that: a. even at a high loading current density, the nitrogen can afford pseudocapacitanc at the same. And b. high-carbonized temperature will lead to the decrease of the equivalent series resistance.The amounts of nitrogen increase by the protection of Fe powder in the carbonization process. The results indicate that the N-6 has been chemically transformed into nitrogen species with higher binding energies through the condensation reaction during the carbonization process. The higher the nitrogen content, the higher the specific capacitance when the specific surface areas of carbon materials are similar.In KOH electrolyte, the significant presence of pseudocapacitive interactions is clear as well as in H2SO4. And we conclude form the experiments that the faradaic interactions between H3O+ and the nitrogen functionalities are stronger than those of K+ and that they determine the overall capacitive performance.(4) The key procedure for the ink-jet printing process is to obtain the stability of nano-sized materials in the dispersion system. The stable Ni(OH)2 "inks" containing binder were successfully prepared by employing both wet ball-milling technology and steric polymeric dispersant. The morphology, structure, and electrochemical performance of Ni(OH)2 thin film electrodes were investigated by scanning electron microscopy (SEM), cyclic voltammograms (CV), galvanostatic charge-discharge and electrochemical impedance measurements. SEM images show uniform distribution of as-printed Ni(0H)2 thin film electrodes. The thickness of thin film electrodes were about 0.6μm by the cross-sectional profile of SEM observation. Galvanostatic charge-discharge shows that the capacity of Ni(OH)2 film is about 260 mAh/g and stably retained after 100 cycles at a high current density of 4.16 A/g. The high charge/discharge rate capability can be attributed to the following reasons: easy proton diffusion in the nano-sized particles of Ni(OH)2 especially followed by the ball-milling process, very thin film and high surface area of nano-Ni(OH)2 particles. The ink-jet printing method shows the convenient, feasible, and inexpensive property for fabricating the Ni(OH)2 thin films. |
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