Study On Controllable Preparation And Electrochemical Performance Of Electrospinning PVA-based Porous Carbon Nanofiber Membrane With High Porosity

Porous carbon nanofibers（PCNFs）have vital applications in fields as diverse as environmen-tal,energy and catalysis due to their high aspect ratio,large specific surface area,adjustable pore structure,and excellent stability.Among them,the hierarchical porous structure carbon material exhibits excellent electrochemical performance for energy storage devices because it can form interconnected macropores and uniformly distributed micropores and mesopores on nano-scale fibers.Such high-porosity PCNFs can provide sufficient reactive sites for active materials and convenient channels for rapid ion transport,when they are used as electrode materials.However,the high pore volume ratio is generally inversely proportional to outstanding conductivity.Poor electronic conductivity results in electrode polarization,which hinders the development of energy storage equipment in terms of rate and cycle stability.In addition,as smart wearable products gradually eagerly look forward to the public life,the research on flexible electrode materials has drawn attention,and the freestanding PCNFs films are expected to become the most competitive candidates due to their unique mechanical strength.Therefore,this paper a chemical crosslinking electrospinning and a macro-micro dual phase separation method to to synthesize PCNFs free-standing membranes,which has a whole hierarchical porous structure with high porosity and conductivity.At the same time,its electrochemical energy storage characteristics were character-ized.Moreover,based on PAN fiber membrane substrate,flexible PAN-PCNFs composite carbon fiber membrane was prepared and its performance was characterized.The specific research content of the paper is as follows:1.We selected poly（tetrafluoroethylene）（PTFE）NPs,poly（vinyl alcohol）（PVA）,and boric acid（BA）as pore inducer,carbon precursor,and crosslinking agent,respectively.A sol-gel electrospinning combined with a one-step carbonization method was used to form a porous carbon fiber membrane in situ.By changing the content of boric acid in the spinning solution,the atmo-sphere and temperature of high temperature carbonization,the composition,pore structure and morphology of PCNFs were adjusted.The results showed that the primary fiber with boric acid added after high temperature heat treatment at 1000℃under nitrogen atmosphere was trans-formed into B-F-N triple-doped PCNFs.This PCNFs have a uniformly distributed inter-connected porous microstructure and a specific surface area up to 750 m² g^-1,showing excellent performance in electron conduction and liquid adsorption.2.Based on the above studies,PCNFs were synthesized under the condition that the calcin-ing protective gas was N₂,and boric acid and PVA were mixed at a mass ratio of 3μL/g.The effects of different polymer mass ratios and calcination temperatures on the morphology,porosity and conductivity of PCNFs were studied to achieve controllable preparation of porous carbon nanofiber freestanding films.The resultsnt material has a porosity>80%,a specific surface area of more than 750 m² g^-1,and a high conductivity of about 980 S cm^-1.As the electrode of all carbon supercapacitor delivered a high power density of 1.75 kW kg^-11 and a large energy density of 42.77 Wh kg^-11 at 1 A g^-1,and the Li-sulfur batteries exhibited a high discharge capacity of>1000 mA h g^-11 at 1 C over 300 cycles.3.An electrospun PAN fiber membrane was used as a substrate,the precursor spinning solution of PCNFs was mixed with it by spinning to prepare a flexible composite carbon fiber membrane through high temperature calcination.We explored the relationship between the change in the mass ratio of PAN and PCNFs and the flexibility of the composite film.When PAN and PCNFs were mixed at a ratio of 1:3,a 360°freely folded and curled fiber film was obtained,and showed an excellent electrical performance in both supercapacitors and lithium-sulfur batteries.This paper provides an economical and environmentally friendly method for the scalable fabrication of PCNFs with ultrahigh porosity and outstanding conductivity.Both the macrophase separation of PTFE NPs and the microphase separation of PVA and BA contributed to the formation of highly controlled trimodal porous structures that contained macro,meso and micro pores.Such freestanding PCNF films exhibited integrated characteristics of porous nanostructure,and shows excellent flexibility when compounded with the PAN membrane substrate,providing a new direction for its development in the energy field.