Preparation Of Coal-based Graded Porous Carbon Fibers And Research Of Electrochemical Performance

The supercapacitor has the advantages of high power density,fast charge and discharge,strong charge storage capacity,long service life,wide operating temperature range,low cost and no pollution to the environment,which makes it an important energy storage device.Electrode material is a key factor that determines the electrochemical performance of supercapacitors.As an important electrode material,the specific surface area,pore volume,and pore size distribution of carbon fiber are important parameters that affect its electrochemical performance.However,it still faces many challenges that the design and preparation of porous carbon fibers with large specific surface area,rich pore structure and narrow pore size distribution.This article combines electrospinning technology,pore-forming technology and carbonization technology,and takes rich coal resources in Xinjiang as carbon source,and obtains oxidized coal through liquid-phase oxidation modification treatment of raw coal.The porous carbon fiber precursor is prepared by the electrostatic spinning supplemented with immersion water washing,which takes advantage of the characteristic that oxidized coal can be partially dissolved in water.On this basis,it is prepared by adding a pore-forming agent to the spinning solution that three-dimensional porous carbon fibers and hollow porous carbon fiber precursor.After the pre-oxidation precursor and carbonization of the obtained precursor,porous carbon fibers with unique pore structure and large specific surface area were obtained.The article studies the effects of pore-forming technology and the amount of pore-forming agent on the structure,morphology and electrochemical performance of carbon fiber,and finally prepares a porous carbon fiber material with excellent electrochemical performance.The main research contents of this paper are as follows:1.Oxidized coal is obtained by modifying the coal production in Heishan of Xinjiang by mixed acid liquid phase oxidation.The oxidized coal is used as the carbon source,and polyacrylonitrile(PAN)is used as the spinning forming agent,then the electrospun fiber is prepared by the electrostatic spinning method.The membrane uses the characteristic that oxidized coal can partially dissolve in water,and the electrospun fiber membrane is washed with water to make pores.With the extension of the immersion time and the increase of the number of washing treatment,some small polar molecules in the oxidized coal are removed.After water washing pretreatment,pre-oxidation and carbonization,the specific surface area and total pore volume of the samples PCNFs_6-6 reached 481.1 m²·g^-1 and 0.344 cm³·g^-1,respectively.PCNFs_6-6shows good electric double layer characteristics,and the specific capacitance can reach to 205 F·g^-1 and 163 F·g^-1 at current densities of 1 A·g^-1 and 20 A·g^-1 in the three-electrode system,respectively.2.In order to further enrich the pore structure of carbon fibers,polyethylene glycol(PEG),which is soluble in water and N,N-dimethylformamide(DMF),is selected as the soft template,and the synergistic pre-treatment of PEG and oxidized coal is used.Three-dimensional porous carbon fiber with interpenetrating pore structures is obtained by pore-making.This paper also studies the effect of PEG addition on the pore structure,graphitization and electrochemical performance of carbon fibers.Under the best preparation conditions,the specific surface area and total pore volume of the samples PCNFs_0.6 can reach to 529.4 m²·g^-1 and 0.288 cm³·g^-1.The specific capacitances of the electrode materials can reach to 252 F·g^-1 and 197 F·g^-1 in the three-electrode system at1 A·g^-1 and 20 A·g^-1 current densities,respectively.At the same current density,the specific capacitances of the symmetric supercapacitors assembled from them reach 188F·g^-1 and 159 F·g^-1.The capacity retention rate is 99.99%after 10000 times charge-discharge test at a current density of 5 A·g^-1,and its excellent electrochemical performance is mainly due to the large surface area and the three-dimensional porous structure that penetrates inside and outside.3.Dissolve PEG,PAN in DMF as the core layer spinning solution,PEG,PAN and oxidized coal in DMF as the shell layer spinning solution.The preparation of electrospun fiber membrane is completed by electrospinning method.When the fiber membrane is washed with water,the concentration difference of PEG in the core layer,the shell layer and the aqueous phase is utilized.Under the induction of the concentration gradient,the PEG in the shell layer and the core layer gradually dissolves in the water.After migrating to the shell and then carbonizing,HPCNFs with hollow structure were finally obtained.The effect of the ratio of PEG and PAN in the core layer solution on the formation of carbon fiber hollow structure and its electrochemical performance was studied.At current densities of 1 A·g^-1 and 20 A·g^-1 in the three-electrode system,the specific capacitances of the sample HPCNFs reached 218F·g^-1 and 115 F·g^-1,respectively.