Formation Mechanism Of Hollow Porous PAN Fibers And Structure Regulation Of Hollow Carbon Fibers

Hollow porous carbon fiber(HPCF)has high specific surface area,available cavity,adjustable pore structure,controllable morphology and composition,and has a wide range of applications in thermal insulation materials,wastewater treatment,adsorption separation and other fields.In view of the problems of complex technology and multiple processing steps in the current preparation process of hollow porous carbon fiber,this paper rapidly prepared hollow porous PAN fiber with the cavity and bubble structure of natural wheat straw by wet spinning co-foaming technique,and prepared hollow porous carbon fiber based on this precursor system.In this method,the foaming agent diphenylmethyl diisocyanate(MDI)was mixed into polyacrylonitrile(PAN)/N,N dimethyldiformamide(DMF)solution,and water was used as coagulation bath for wet spinning.The chemical reaction of MDI and water was used to generate CO2 to make holes in the PAN fiber.On this basis,KOH activation method and template method of adding different nanoparticles were used to regulate the pore structure and properties of hollow porous carbon fiber.PAN fibers and hollow porous carbon fibers were characterized by SEM,DSC,FTIR,XRD,BET,etc.The forming mechanism of hollow porous PAN fibers was discussed,and the effects of KOH activation and doping of different nanoparticles on the structure and properties of hollow porous carbon fibers were studied.Findings are as follows:(1)When the MDI ratio reached 10%or above,the hollow structure appeared in the cross section of the porous PAN fiber,and the hollowness increased with the increase of the MDI ratio,up to 65%.The formation of the hollow porous structure is attributed to the three phase separation of PAN/DMF/H₂O in wet spinning and the chemical foaming reaction between MDI and H₂O.During wet spinning,the phase separation is caused by the bidirectional diffusion of DMF and water and the solidification of PAN.The foaming reaction not only promotes the phase separation,but also the product polyurea acts as a heterogeneous nucleating agent,which limits the growth of bubbles,and makes the bubbles dispersed more evenly,so that the prepared PAN fiber section has a uniform and symmetrical pore structure.(2)The hollow porous carbon fibers with hierarchical structure were prepared by KOH activation.The results show that the micropore volume and specific surface area of carbon fiber increase due to the etching effect of KOH.In addition,organic polytetrafluoroethylene(PTFE)nanoparticles and inorganic calcium carbonate(Nano-Ca CO₃)nanoparticles were added in different proportions to regulate the pore structure of porous carbon fiber with hollow structure.The results show that the volume of micropores in carbon fiber system increases with the addition of nano-particles.With the introduction of nano-sized calcium carbonate,the specific capacitance of the prepared carbon fiber is195 F g^-1 at the current density of 0.5 A g^-1,it still maintains a capacity retention rate of99%after 5000 cycles of charge and discharge under a current density of 10 A g^-1and has excellent long-cycle stability.