Preparation And Electrochemical Performance Of Coal-based Porous Carbon And Molybdenum-based Composites

Due to their good thermal and high mechanical stability,excellent electrical conductivity,developed pore structure and large specific surface area,porous carbon materials show great potential applications in the fields of adsorption,separation,catalysis,gas storage,energy storage and conversion.Especially in the field of energy storage,porous carbon materials can be used as activated materials,conductive agents,coatings,flexible substrates,carriers,which play an important role in electrochemical energy storage systems.Coal is an abundant and cheap natural resource in Xinjiang,China,which is of good quality and various types.To meet the regional demand of efficient use of coal,it is very important to prepare coal-based carbon materials and composites with high performance and quality.And studies on their application in the field of energy storage is of great significance to the extension of the coal chemical industry chain and the promotion of the resource advantages of coal into technological and economic advantages.In order to realize the high value utilization of coal resource,this paper develops coal-based porous carbon materials and their composites by two methods,the salt template method and the molten-salt-assisted method.The above methods are simple,environmentally friendly and can be extended to the preparation of other type porous materials.At the same time,the electrochemical properties of the prepared materials as lithium-ion battery andoes and supercapacitor electrode materials are studied.These results provide a theoretical basis for the industrial production of high-performance coal-based carbon materials and their composite materials.The main contents and results are as follows:1.Coal-based multiscale porous carbon materials?CPCs?have been successfully fabricated through a friendly method with NaCl,Na2CO3,and Na2SiO3 as structural templates,instead of alkali activation at a relatively low temperature.The amount and type of salts on the specific surface area and porous structure of carbon materials were investigated.The optimal product?CPCs-20?possesses a high surface area of 1100 m²g^-1.As electrode materials of supercapacitors,CPCs-20 presents a specific capacitance of 304 F g^-1 at 1 A g^-1 and superior stability over 10,000 cycles at 4 A g^-1 owing to its hierarchical porosity with higher surface area,promoted diffusion of electrolyte and increased conductivity.It is also worth mentioning that a symmetric device based on CPCs-20 can light a light-emitting diode?LED?for 30 min.Furthermore,CPCs-20 as the anode for Li-ion batteries?LIBs?exhibits a high reversible capacity of 450 mAh g^-1 at 0.2 A g^-1,excellent rate capability,and outstanding cycling performance.2.As a special“wet chemical”synthesis route,the molten-salt-assisted method has many advangtages on preparation of nanomaterials.In this part,we prepared porous carbon materials by the simple method with ZnCl2/KCl or LiCl/KCl as the molten salt system and coal as the carbon source.The molten salts have strong pole during the calcination process,which can act as solvent to"dissolve"the coal and as"molecular template"to induce the formation of porous carbon with different morphologies.When the ZnCl2/KCl molten salt system is used,the obtained porous carbon was formed by the accumulation of nano-sized spherical carbon particles.With the temperature increased,the spacing between the carbon particles became larger and the structure was more loose.The optimal product?C-700?obtained at 700°C had a specific surface area of 1406 m² g^-1,and exhibited excellent electrochemical performance when used as LIBs anodes.Similarly,when C-700 was used for supercapacitor electrode materials,the specific capacitance is 231 F g^-1 at 1 A g^-1.Even at a high current density of 20 A g^-1,the specific capacitance can still be as high as 164.9 F g^-1,and the capacitance retention rate is 71.3%,indicating that it has excellent rate performance.In addition,after 20,000 cycles at a current density of 4 A g^-1,the discharge capacity increased by 5.3%from the initial value.When LiCl/KCl is used as the"solvent",the"graphene-like"ultra-thin nanosheets can be obtained,and the nanosheets connected to each other to form porous structure.The N-doped nanosheets become thicker,but the pores formed by the interconnection are more abundant,and the N-doped improves the conductivity of the porous carbon nanosheets,thereby increasing their electrochemical performance.The N-doped porous carbon exhibits excellent rate performance and cycle stability.At a relatively high current density(6 A g^-1),the reversible capacity is also as high as352.1 mAh g^-1.The reversible capacity is still as high as 344.6 mAh g^-1 after 500cycles at 5 A g^-1.3.Considering the advantages of molybdenum-based metal oxides as anode materials for LIBs,MoO2/C composites are obtained by one-step calcination using the salt template method.The MoO2/C composites obtained by the salt template method have a multi-stage pore structure.We studied the effect of calcination temperature on the structure and electrochemical properties of the obtained material.The optimal product?MoO2/C-650?exhibited excellent electrochemical performance when used as LIBs anodes.At 0.2 A g^-1,the reversible capacitie of MoO2/C-650 after 350 cycles was 988.1 mAh g^-1.4.Using ZnCl2/KCl as the molten salt system,the MoO2/C composite was successfully prepared by one-step calcination.In the composites,the MoO2nanocrystals are uniformly dispersed on the carbon layer.Carbon layer not only improves the conductivity of the material,but also buffers the volume change and aggregation of the MoO2 nanocrystal during the charge-discharge cycles,showing excellent performance.At 0.2 and 1 A g^-1,the reversible capacities after 100 cycles were 960 and 700.3 mAh g^-1,respectively.It has been found that the composites have a capacitive behavior of surface diffusion during the electrochemical reaction.Using LiCl/KCl as the molten salt system,the MoO2/C hybrid material was also successfully prepared.The MoO2 nanoparticles in the hybrid structure are uniformly dispersed and firmly fixed,and the MoO2 nanoparticles form a bicontinuous phase with the carbon layer,which can highlight the advantages of complementary phases during the electrochemical reaction process,and maintain the mechanical properties and structural stability of the electrode materials,thus preventing the MoO2nanoparticles aggregation/pulverization after repeat charge-discharge processes.Therefore,the hybrid material exhibits more excellent cycle life and rate performance.At a current density of 4 A g^-1,the capacitance can be maintained at 507 mAh g^-1.5.In order to obtain the LIBs anode material with much better electrochemical performance,the highly crystalized Co2Mo3O8 hexagonal nanoplates interconnected by coal-derived carbon have been successfully fabricated by the molten-salt-assisted method.The two-dimensional nanosheets of Co2Mo3O8 in the hybrid material are connected by coal-based porous carbon,and the surface of Co2Mo3O8 is also coated with a thin carbon film.This special hybrid structure promotes the compatibility between the two phases and the synergy of the two phases can be fully utilized,so that the material has the high reversible capacity,excellent rate performance and cycle stability.At current densities of 0.2 and 0.5 A g^-1,the reversible capacity is as high as1075 and 874 mAh g^-1 after 100 cycles.When the current density is increased to 4 A g^-1,the discharge capacity can still reach 533 mAh g^-1,showing excellent rate performance.