Preparation And Electrochemical Performance Of Porous Carbon And Its Compounds

Today, the excessive consumption of fossil energy had prompted people to turn their attention to the development of clean energy in order to alleviate the fossil energy crisis. The key to efficient use of clean energy was energy storage and convert devices. Carbon materials had been playing a significant role in alternative clean and sustainable energy technologies. Porous carbon as an important member of the carbon materials, with advantages of high specific surface area, excellent electrical conductivity, well controlled pore structure, wide source, low cost and stable physical and chemical properties, has received wide attention for researchers from domestic and oversea. In this paper, porous carbon and its composites with carbon nanotubes(CNTs) as electrochemical capacitor and lithium ion battery electrodes were proposed. The composition, structure and morphology of the products were characterized by scanning electron microscope(SEM), transmission electron microscopy(TEM), Brunauer-Emmett-Teller(BET), X-ray diffraction(XRD), X-ray photoelectron spectroscopy(XPS) and Infrared spectra(IR). The electrochemical performance of the products was estimated by cyclic voltammetry(CV), galvanostatic charge/discharge(CD) and electrochemical impedance spectroscopy(EIS) techniques. The morphology, nanostructure, preparation and electrochemical performance of porous carbon and its composites with CNTs were studied systematically, and had been achieved the anticipated research goal.The introduction outlines the classification and advantages of porous carbons, and then emphatically introduces the preparation methods, the modification measures and the main application in electrochemistry of porous carbons.First, porous carbon materials were prepared from freeze-dried porous carrots as carbon sources through carbonization at 600 oC in nitrogen atmosphere, followed by KOH activation. Electrochemical results indicated that, after being activated, porous carbon showed a significant improvement in specific surface area(from 7.0 m2 g-1 to 147.2 m2 g-1). Moreover, activated carbon displayed a maximum specific capacitance of 414 F g-1. Even when the current increased to 4 A g-1, its capacitance retention still reached 74.5%. While non-activated carbon had only 253 F g-1 and 45.1% retention. Additionally, activated carbon exhibited high electrochemical stability with 94% retention after 8000 cycles. The remarkable improvement in capacitive performance was strongly related to the significant improvement of specific surface area and the increase of mesopore.Second, N-doped porous carbons possessing cross-linking nanoflake networks were prepared by an easy yet efficient copyrolysis of inexpensive melamine and polyethylene glycol(PEG) precursors and the subsequent etching removal of magnesium oxide(MgO). A systematic study was focused specifically on influences of PEG-MgO-melamine mass ratio and pyrolysis temperature on the morphology, structure and composition. Interestingly, the supercapacitance performance of the porous carbon could be easily modulated by adjusting the mass ratio of the precursors and templates, as well as the carbonization temperature. The newly porous carbons obtained under the desired control(10:7:3 mass ratio and 700 oC treatment temperature) had cross-linking network structures with high specific surface area and total pore volume of 370.8 m2 g-1 and 1.65 cm3 g-1, high nitrogen content of 8.54 at.% and well-preserved pore structures. These virtues significantly contributed to its high specific capacitance of 485 F g-1 at 1.0 A g-1, and long-term cyclability with no capacitance falloff at high current loading of 5.0 A g-1 on 8000 successive cycling. By virtue of the simplicity and wide availability of fabrication method and also superior performance, such optimal carbons may promise great potential for practical applications.Finally, the approach for porous carbon(PC) and carbon nanotube(CNT) was explored. On the basis of the above chapter, different proportion of carbon nanotubes was added into the precursor, a series of PC/CNT composites were successfully prepared by carbonization and then etching of MgO. Physical tests showed that the porous carbon and carbon nanotubes formed a relatively ideal composite structure. The composites having a higher specific surface area, the PC@3%CNT product could reach 1015.2 m2 g-1. Electrochemical tests showed that the suitable addition of carbon nanotubes could effectively reduce the charge transfer resistance(Rct) of porous carbons. The porous carbon with 3% CNT(PC@3%CNT) exhibited the lowest Rct(70.5 Ω) and highest discharge capacity about 714.5 mAh g-1 after 100 cycles at a current density of 200 mA g-1. The results of this work suggested that the obtained porous carbons could be employed as a promising material for lithium ion batteries and also provided new ideas for the preparation of other porous carbon/carbon nanotube composites.