Moderate Preparation Of Coal-based Porous Carbon For Supercapacitor

Xinjiang is rich in coal resources, mainly non-viscious, but the lack of water and serious environmental issues in Xinjiang limits the development of its coal-to-oil industry. Meanwhile, in recent years, environmental pollution and energy demand make all the countries raise a higher requirement for developing renewable energy sources. Supercapacitor is a new type of energy storage device with high power density, long cycle life, low-cost, maintenance-free. And it will become an important part of the future of electric vehicles and natural energy storage. Due to the high specific surface area, high conductivity, good chemical stability, low cost and facile preparation, etc., the porous carbon materials have been the preferred electrode materials for supercapacitor. With high carbon content, low ash content, Xinjiang non-viscous coal,is a suitable raw material for preparation porous carbon electrode material. Simple and green process conditions to achieve coal-based porous carbon electrode materials with low energy consumption and little pollution will undoubtedly contribute to clean utilization of coal and the development of supercapacitor electrode material. Meanwhile, this is in accordance with with the ease of bad climatic environment, the progress of social progress and economic development.In this thesis, Xinjiang non-viscous coal was used to prepare porous carbon through steam physical activation method. Activation time, raw material particle size, and removing ash process were investigated. In addition, the raw coal was firstly carbonizedand then ash was removed. After this, melamine (urea), boric acid and both of them was introduced into the carbonisate, respectively and then the mixture was subject to steam activation. Thus, a series of nitrogen-doped, boron-doped and nitrogen and boron co-doped coal-based porous carbons were obtained. The effects of doping on activation and electrochemical performance are discussed. The results include the following four aspects:(1) Preparation of coal-based porous carbon materials through steam physical activation. The results show that the yield decreases with prolonged activation time, accompanying by the increase of the specific surface area. However, if the activation time is too long, the micropores will decrease, so the appropriate activation time is2h in this given condition; Raw material particle size in the range of75-250μm has little effect on the activation. Only when the particle size continues to increase or decrease, activation will be weakened or intensified; It is appropriate to remove ash after activation. Thus, activation yield is higher, ash content of the obtained sample is lower. In the meantime, handling capacity will be highly reduced. For the sample with removing ash after activation, the capacitance value of up to149F g-1is achieved in a three-electrode system in6M KOH electrolyte. The assembled two-electrode cell presents superb long cycling stability. After a consective30000cycles, almost no the capacitance decay is observed.(2) Melamine and urea are used to be involved in steam activation, respectively, and N-doped coal-based porous carbons are obtained. Experimental results show that N-doping promotes the steam activation and doping with melamine can obtain a higher nitrogen content. The porosity of the sample activated in800℃with doping can be comparable with that avtivated in850℃without doping. More importantly, the activation with the introduction of N can increase the yield and nitrogen content is up to2.048%. The capacitance value increases by30%, and a good long cycle performance is demonstrated through by almost no capacitance decay after30000cycles.(3) Boric acid is used to involve in steam activation. Similarly, boron acid contributes to the activation process and the highest boron content is0.69wt.%. The capacitance value increases by30%, and the capacitance retention can reach86%at10A g-1. The supercapacitor cell presents good long cycle performance and the retention is95%after a consective20000cycles.(4) As is expeted, the use of boric acid and melamine together accelerates the activation. Meanwhile, N and B co-doped coal-based porous carbon is obtained. In all, capacitance value and rate performance have improved remarkably. In three-electrode system, the capacitance value of189F g-1is obtained at0.5A g-1, increasing by75%compared with the sample without doping. The two-electrode supercapacitor cell exhibits an excellent long cycle life performance. After20000cycles, there is almost no capacitance decay.