Preparation And Capacitive Properties Of Pitch-based Carbon Fiber

With the problem of energy shortage and environmental pollution becoming more and more serious,the search and development of efficient energy storage and transformation technology has become one of the focuses of research.Xinjiang is an important petroleum production and processing base in China,petroleum pitch,a by-product of petroleum refining with abundant production,low price and high carbon content as a by-product of petroleum refining,therefore,it is of great theoretical significance and practical application value to prepare carbon materials with high added value using petroleum pitch as raw material.Based on xinjiang karamay petroleum pitch as raw material,mesophase pitch was obtained by liquid-phase oxidation method,and the carbon fiber was prepared by electrospinning,pre-oxidation and carbonization,the composition,structure and morphology of the carbon fiber were controlled by introducing cross-linking agent,pore-forming agent and N/S doping,the electrochemical properties in three electrodes,two electrodes and different electrolytes were studied.The main research contents and results are as follows:1.The mesophase pitch obtained by liquid phase oxidation treatment was co-spun with triblock copolymer F127 and polyacrylonitrile(PAN),after pre-oxidation and carbonization of electrospun fibers,pitch-based carbon fibers were successfully prepared.F127 acts as both a crosslinking agent and a pore-forming agent,which makes the prepared carbon fiber have a crosslinking net-like porous structure,the crosslinking structure can greatly improve the electrical conductivity of the carbon fiber,while the porous structure provides more active sites for charge storage.The specific capacitance of the sample FPCNF-0.1 in the three-electrode system(6 mol L^-1 KOH)reached 234.3F g^-1(1 A g^-1),and the capacity remained 66.7%when the current density increased to50 A g^-1.In the symmetrical two-electrode system,the specific capacitance of FPCNF-0.1 can still reach 173.5 F g^-1 at 1 A g^-1,and the capacity retention rate(63.5%)at 50 A g^-1 is similar to that in the three-electrode system;Under load of 12 mg cm^-2,the mass specific capacitance and area specific capacitance of FPCNF-0.1 at 1 A g^-1were 90.2 F g^-1 and 1082 m F cm^-2,respectively.2.The pitch-based carbon fiber with nitrogen doped crosslinked porous structure was successfully prepared by co-spinning mesophase asphalt with dicyandiamide and PAN.In the three-electrode system(6 mol L^-1 KOH),the specific capacitance of the sample NPCNF-0.4 is 268.2 F g^-1(1 A g^-1),and the proportion of pseudocapacitance provided by heteroatom is 37.7%;In the two-electrode system,the specific capacitance of NPCNF-0.4 at 1 A g^-1 is 202.3 F g^-1,and the capacity retention rate is 99%after150000 cycles of charge and discharge at 5 A g^-1,showing good cycling stability.In addition,the energy density of NPCNF-0.4 can reach 10.1 Wh kg^-1when the power density is 450 W kg^-1 in the symmetric two-electrode system with 1 mol L^-1 Na₂SO₄ as the electrolyte.The excellent electrochemical performance can be attributed to the following two aspects:on the one hand,dicyandiamide not only acts as a crosslinking agent and pore-forming agent,but also increases the N content in the carbon fiber,which not only improves the wettability of the carbon fiber,but also provides pseudo capacitance;On the other hand,the unique cross-linked porous structure is conducive to adsorption-desorption and rapid transport of ions.3.N,S co-doped hierarchical porous pitch-based carbon fibers were prepared by co-spinning mesophase pitch with poly(methyl methacrylate),thiourea and PAN.In the three-electrode system(6 mol L^-1 KOH),the specific capacitance of the sample SPPCNF-0.3 at 1 A g^-1 is 250.6 F g^-1 under the optimal preparation conditions,and the capacity retention rate at 50 A g^-1 is 73%.In the symmetric two-electrode system,when the load is 16 mg cm^-2,the mass specific capacitance of SPPCNF-0.3 at 1 A g^-1 is 158.8F g^-1,and the area specific capacitance is as high as 2540 m F cm^-2.In the symmetric two-electrode system with 1 mol L^-1Na₂SO₄ as the electrolyte,the energy density is as high as 11.6 Wh kg^-1 at a power density of 400 W kg^-1,and 7.2 Wh kg^-1 even at a high power density of 20,000 W kg^-1.The excellent electrochemical performance can be attributed to the synergistic effect of reasonable pore structure and heteroatom doping of carbon fiber.