Preparation Of Carbon Nanofibers By Degradative Solvent Extraction Product From Low-rank Coal And The Utilization For Supercapacitor

Supercapacitor is a new type of energy storage equipment with performance between traditional capacitor and battery.Supercapacitor shows many excellent properties such as rapid charging and discharging,long lifecycle,high efficiency and environmental friendly.Therefore,it is widely used in the area of vehicles,new energy power generation,military industry,information technology and so on.At present,among various carbon materials electrodes,electrospun carbon nanofibers show excellent conductivity and electrochemical performance.Moreover,electrospun carbon nanofiberscan can be directly use as the electrodes due to its flexibility which avoids the addition of binder and conductive additives.However,the complex preparation process and high cost of the precursor restrict its development.Therefore,it is very important to find a precursor with low-cost and relatively simple preparation process,which can also improve the properties of the carbon fiber.The author's team has proposed a novel degradative solvent extraction method to upgrade the low-rank under mild condition and the solvent can be easily recycled.Compared with raw coal,the extract obtained by degradative solvent extraction method showed higher carbon content and lower oxygen content.The extract was rich in aromatic compounds,had almost no ash and moisture with good thermoplasticity at high temperature which can be used as the precursor of carbon fibers.Thus,in this study,DP was fabricated into carbon nanofibers for the first time by electrospinning method and the possibility of using the DP-based carbon nanofibers as an electrode for the supercapacitor was also studied preliminarily.The details are as follows:Firstly,the precursor of carbon nanofibers was prepared by the degradative solvent extracts obtained from low-rank coal.Six kinds of low-rank coal were selected as raw materials to explore the influence of coal types on the degradative solvent extraction methods,and the most suitable extract was selected as the precursor of carbon nanofibers.The yields of high molecular weight extract(DP)and low molecular weight extract(SL)of six low-rank coals are about 7.6%?15.2%and 1.9%?8.9%,respectively.The carbon content and the aromatic content of DP and SL increased while the ash content decreased significantly.The softening points of two extract were 240-270? and 120-150?,respectively.The properties of the extract obtained from different kinds of raw coal are very similar,so the extract obtained from Hefeng coal with thehighest yield was selected as the precursors for further study.Then,polyacrylonitrile(PAN)was mixed with DP and SL in the ratio of 2:1,1:1,1:0 to improve the spinnability.The DP based and SL based carbon nanofibers was successfully prepared by electrospinning.However,due to the low molecular weight and softening point,the SL based carbon nanofiber was fragmentary,and has serious fracture and melting problem,which had a negative effect on the subsequent preparation of supercapacitor electorde,so the DP based carbon nanofiber was selected for further study.The DP based carbon nanofibers were flexible and formed good fiber structure.When the mixing ratio is2:1,the carbon nanofiber displayed the highest specific surface area,reached 471 m~2/g.Moreover,the degree of graphitization increased and the graphite microcrystalline structure was more ordered.Finally,activated carbon nanofibers were prepared by carbon dioxide activation method using DP as the precursor.When the activation temperature was 800?,the activation of the fibers was incomplete and the specific surface area was relatively low;when the activation temperature came to 1000?,almost all the fibers are burnt out due to the excessive activation.Therefore,900? was selected as the activation temperature for the preparation of activated carbon nanofibers.When the mixing ratio was 2:1,the specific surface area of the activated carbon nanofibers reached 1005 m~2/g,and the pore volume was 0.48 cm~3/g.With the addition of the extract DP,the graphitization degree increased and the i graphite microcrystalline was more ordered.The prepared activated carbon nanofibers were directly used as the electrode of supercapacitor.The DP based activated carbon nanofibers had higher specific capacitance,lower impedance,better high-rate handing capacity and cycle stability in 6M KOH electrolyte.When the mixing ratio was 2:1,the specific capacitance reached 192.6 F/g at the current density of 1 A/g;when the current raised to 10 A/g,the capacitance retention rate was 72.1%;after 1000 cycles at 5 A/g,the capacitance retention rate is 89.8%.In conclusion,this study proved that the extract obtained by degradative solvent extraction of low-rank coal is a low-cost precursor with simple preparation process for the electrospinning carbon nanofibers.Moreover,using extract as one of the precursor can effectively improve the quality of electrospun carbon nanofibers,which showed great potential as an excellent electrode material for supercapacitor utilization.