Preparation And Electrochemical Performance Of Pitch-based Porous Carbon From Coal Liquefaction Residue

Coal liquefaction residue,accounting for about 30 wt.% of raw coal,would be produced in coal direct liquefaction process,and its efficient and clean utilization is beneficial to promote the economic efficiency and environmental benefit of the direct coal liquefaction process.In addition to insoluble organic matter,inorganic minerals and additional catalyst,about 50% coal liquefaction pitch rich in polycyclic aromatic hydrocarbons exists in coal liquefaction residue.The utilization methods of coal liquefaction residue mainly includes combustion,gasification,pyrolysis and the road asphalt modifiers,et al.However,these methods do not fully take advantages of coal liquefaction residue.In contrast,the pitch separated from coal liquefaction residue by solvent extraction is the high-quality raw material for preparing porous carbon.Porous carbon is the preferred material for supercapacitors because of its high specific surface area and adjustable pore structure,which would present an important meaning for the maximizing value of coal liquefaction residue.But the key is to explore the composition and properties of pitch in relation to the structure and electrochemical performance of porous carbon.Based on this,the efficient extraction of coal liquefaction residue was studied,and porous carbon used as electrode material for supercapacitor was prepared from the extracted product(coal liquefaction pitch).The effect of pore structure and surface properties on the capacitance of porous carbon was studied by changing the preparation conditions.The role of pitch light components in the preparation of porous carbon was investigated by solvent extraction.The effect of polymerization on the structure and electrochemical properties of porous carbon was investigated by modulating the properties of liquefaction pitch through polymerization treatment.The details are shown as follows.(1)The different extractants,such as aliphatic hydrocarbon(straight-chain alkane,and cyclane),aromatic hydrocarbon,oxygen-containing compounds(ethers,ketones,esters,and alcohols),and simulated coking benzene were selected for the extraction of coal liquefaction residue.The effects of solubility and polarity parameters of extractants on extraction rate and components of extracts were analyzed.The results showed that the contents of n-hexane soluble,n-hexane insoluble and toluene soluble,and toluene insoluble and tetrahydrofuran soluble components in coal liquefaction residue were 9.90%,31.53% and 15.10%,respectively.With the increase in solubility and polarity parameters,the extraction rate for coal liquefaction residue increased and then decreased.The influence of the solubility parameter on the extraction performance was mainly reflected in their matching with the solubility parameter of the extractable components.The effect of the polarity parameter was mainly the breaking of the association structure of coal liquefaction residue.When the solubility parameters of the extracts and extractants were similar,the extraction performance was better,such as aromatic hydrocarbon and tetrahydrofuran.The main components of these extracts were polycyclic aromatic hydrocarbons.The extractants with larger polar parameters were more likely to destroy the connective structure of the coal liquefaction residue,which was the reason that the relative contents of heteroatom-containing compounds in acetone,ethanol and methanol extracts were high.Simulated coking benzene was a good solvent for extracting coal liquefaction residue,which was easy to be recovered from solution,and the extraction rate of coal liquefaction residue could reach more than 50%.The extracts could be used as important feedstocks for producing high quality carbon material.(2)Porous carbon used as electrode material for supercapacitors was prepared from the extract of coal liquefaction residue(coal liquefaction pitch)through activation with KOH.The influences of preparation conditions on the structure and electrochemical properties of the porous carbon were investigated.By adjusting the ratio of KOH to liquefaction pitch,the activation temperature and the addition of magnesium acetate,the evolution of the pore structure of the porous carbon was analyzed,and the relationship between the pore structure and the electrochemical properties were studied.The results showed that the specific surface area and pore volume of porous carbon increased with the increase of mass ratio of KOH to liquefaction pitch or activation temperature,but excessive KOH and high activation temperature led to the collapse of the pore wall,which in turn affected its electrochemical properties.The porous carbon with pore volume less than 1 nm had a linear relationship with its double electric layer capacitance,and the surface groups would affect the pseudocapacitance.When the mass ratio of liquefaction pitch to KOH was 1:2 and the activation temperature was700 °C,the prepared sample exhibited an excellent capacitance performance with high specific capacitance of 329 F/g at the specific current of 0.5 A/g,and maintained 254 F/g at the high specific current of 20 A/g.And it maintained 96.37% of the initial performance after 10,000 cycles at the specific current of 1 A/g in 6 mol/L KOH electrolyte.The addition of magnesium acetate template could achieve hierarchical pore structure,and consequently to improve the cycle stability of porous carbon.The capacitance retention of porous carbon after 10000 cycles was increased from 96.37% to 99.30%.(3)The properties of the different components in liquefaction pitch were investigated by solvent extraction,and the effects of the different components on the structure and electrochemical properties of the porous carbon were analyzed.The results showed that with the light component content in raw pitch decreased,the specific surface area and total pore volume decreased,while the graphitization degree of porous carbon first increased and then decreased.Light components in pitch were conducive to form mesopores.The volatilization of light components not only produced a pore structure that was favorable for KOH diffusion,but also provided a large number of active sites for KOH activation.Porous carbon prepared from liquefaction pitch,medium-temperature pitch and high-temperature pitch with a certain amount of light component showed the better electrochemical performances.Their specific capacitances were 336,307 and 288 F/g at 0.5 A/g,respectively.(4)Based on the above influence mechanism,the characteristics of liquefaction pitch were modified in an autoclave at different temperatures to prepare porous carbon.The influence of pitch polymerization treatment on the structure and electrochemical properties of porous carbon were explored.The results showed that the composition and structure of liquefaction pitch could be modified by polymerization.The pitch underwent the reactions including alkyl side chain breaking,dehydrogenation and condensation and the structure changed significantly during the heat treatment process.With the increase of polymerization temperature,the H/C,N/C,S/C atomic ratio and alkyl side chain of the polymerization products decreased,while the O/C atomic ratio,aromaticity and thermal stability increased.The graphitization degree of porous carbons prepared by polymerization products at different temperatures increased,while specific surface area and total pore volume decreased with the increase of polymerization temperature.Polymerization treatment of the pitch was beneficial to improve the graphitization degree of porous carbons,but affected the activation effect of KOH.Porous carbon prepared by the polymerization product at 370 ℃ had an excellent electrochemical performance,which is mainly related to its large specific surface area and pore volume,suitable degree of graphitization and moderate oxygen doping on the surface.