CN115948821B - A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber and its preparation method - Google Patents

Abstract

The invention discloses a hollow, porous and multi-layer polyacrylonitrile-based carbon fiber, which comprises the following specific steps: (1) The sheath/core spinning solution is subjected to coaxial/sheath-core wet spinning to form a sheath-core structure yarn; (2) Placing the skin-core structure yarn in a baking oven at 30-80 ℃ for drying treatment for 180-400 min to obtain porous coaxial nascent fibers; (3) Pre-oxidizing the porous coaxial nascent fiber, heating to 200-250 ℃ at the speed of 2 ℃/min, and keeping for 60-180 min; (4) Heating the pre-oxidized fiber to 700-900 ℃ at a speed of 4 ℃/min under the protection of inert gas, and keeping for 60-180 min to obtain the hollow, porous and multi-layer polyacrylonitrile-based carbon fiber. The invention utilizes the non-solvent induced phase separation principle and utilizes the simultaneous phase separation and solidification on the two sides of the sheath-core to realize the characteristic that the cross section of the fiber skeleton presents heterogeneous multi-level.

Description

A kind of hollow, porous, multi-layered polyacrylonitrile-based carbon fiber and preparation method thereof

Technical field

The present invention relates to the technical field of porous carbon fiber preparation, and in particular to a hollow, porous, multi-layered polyacrylonitrile-based carbon fiber and a preparation method thereof.

Background technique

Porous fibers have many advantages such as large specific surface area, strong adsorption, and high porosity. They are widely used in adsorption, catalysis, filtration, electricity, thermal energy storage materials and other fields.

Currently, the more common methods for preparing porous fibers include activation method, template method and phase separation method. The activation method refers to the method of etching the formed carbon fiber material through oxidizing gas or chemical reagents to introduce the pore structure. It specifically includes two methods: physical activation and chemical activation. Among them, physical activation needs to be carried out under high temperature conditions, which consumes high energy, while chemical etching rules have strict requirements on reaction temperature and the amount of oxidant reactants. Improper control of conditions can easily cause significant damage to fiber strength, and the common disadvantages of both methods are The obtained pore structures are mostly micro/mesoporous structures with narrow mesopore size distribution, which is not conducive to rapid material transport. The template method refers to a method of obtaining a porous structure by adding decomposable micro-nano particles to fiber-forming polymers and coordinating subsequent processing steps such as acid hydrolysis or high-temperature treatment. The porogenic effect of the template method is closely related to the dispersion of micro-nano-scale porogens in the fiber matrix. There are also problems such as poor controllability of the pore structure and difficulty in achieving hierarchical pore structures. The phase separation law refers to the process of solvent-non-solvent mass transfer and solidification in the coagulation bath of the thin stream of wet spinning dope. Through the principle of phase separation caused by non-solvent, a polymer-rich phase is formed in the thin stream of spinning dope. and polymer-poor phase. After curing and drying, the polymer-rich phase forms the fiber skeleton, while the polymer-poor phase forms the pore structure in the fiber. By changing the polymer solid content, the type of non-solvent in the coagulation bath, and the solvent-nonsolvent Factors such as mass transfer speed can control the phase separation speed, thereby regulating the fiber pore structure and its size distribution. Therefore, the characteristic of this method is that the fiber porous structure is formed during the solidification and forming process of the virgin silk, and the preparation process is relatively simple and easy to implement, and the cost is relatively low. Theoretically, the pore structure of porous fibers prepared by this method has better designability and controllability. However, the structure of porous fibers currently prepared by this method is relatively simple. By adjusting process parameters, further enriching the pore hierarchical structure of porous carbon fibers will help further increase the specific surface area of porous carbon fibers, thereby improving the application performance of porous carbon fiber materials in adsorption filtration, thermal insulation, electrochemical energy storage and other fields.

Contents of the invention

In view of the above problems existing in the prior art, the present invention provides a hollow, porous, multi-layered polyacrylonitrile-based carbon fiber and a preparation method thereof. The present invention adopts a wet coaxial spinning forming method and utilizes the principle of phase separation induced by non-solvent to prepare hollow and porous fibers. At the same time, phase separation and solidification occur simultaneously on both sides of the sheath and core to form a fiber skeleton with a heterogeneous cross-section. Multi-level features.

The technical solution of the present invention is as follows:

The first object of the present invention is to provide a hollow, porous, multi-layered polyacrylonitrile-based carbon fiber, and its preparation method includes the following specific steps:

(1) The skin/core layer spinning liquid forms a skin-core structure filament through coaxial/skin-core wet spinning;

(2) Dry the sheath-core structural filament obtained in step (1) in an oven at 30-80°C for 180-400 minutes to obtain porous coaxial primary fibers;

(3) Pre-oxidize the porous coaxial nascent fiber prepared in step (2), raise the temperature to 200-250°C at a rate of 2°C/min, and maintain it for 60-180 minutes;

(4) Heat the pre-oxidized fiber in step (4) to 700-900°C at a rate of 4°C/min under the protection of inert gas (nitrogen or argon), and keep it for 60-180 minutes to obtain hollow, porous, multi-layered polypropylene Nitrile based carbon fiber.

In one embodiment of the present invention, in step (1), the raw material composition of the skin spinning solution includes polyacrylonitrile powder and dimethyl sulfoxide;

Preferably, the raw material composition of the skin spinning solution includes a functional conductive active material, and the functional conductive active material is one or more of graphene oxide, carbon nanotubes, and Mxene.

Mxene is a two-dimensional layered nano-transition metal/nitrogen/carbide. Currently, the most mature research and application is the TiC series of Mxene, which uses the MAX phase of 400 mesh Ti ₃ AlC ₂ as the raw material.

After hydrochloric acid + lithium fluoride etching, a single layer of Ti ₃ C ₂ can be obtained.

In one embodiment of the invention, the molecular weight of the polyacrylonitrile powder is 5,000 to 150,000, and the total solid content of polyacrylonitrile and functional conductive active materials in the cortex spinning liquid is 120 to 180 mg/mL; the cortex spinning liquid The medium-functional conductive active material is 0.5 to 50% of the mass of polyacrylonitrile.

In one embodiment of the present invention, the preparation method of the cortex spinning liquid is as follows: adding polyacrylonitrile powder to dimethyl sulfoxide, stirring in a water bath at 65°C for 2 to 5 hours, until the mixture is evenly dispersed to form a uniform Cortex spinning liquid; or add functional conductive active materials and polyacrylonitrile powder to dimethyl sulfoxide and stir in a 65°C water bath for 2 to 5 hours until the mixture is evenly dispersed to form a homogeneous cortex spinning liquid.

In one embodiment of the present invention, in step (1), the core layer spinning solution is a polyvinyl alcohol aqueous solution of 50 to 150 mg/mL.

In one embodiment of the present invention, the alcoholysis degree of the polyvinyl alcohol is 87% to 98.8%.

In one embodiment of the present invention, in step (1), the skin/core layer spinning liquid undergoes deaeration treatment before use. The deaeration method is: ultrasonic treatment at room temperature for 10 to 30 minutes.

In one embodiment of the present invention, the power of the ultrasound is 80-120W.

In one embodiment of the present invention, in step (1), during the coaxial/skin-core wet spinning process, the propelling speed of the core layer spinning solution is 0.2-0.4ml/min, and the advancing speed of the skin spinning solution is 0.7 ~1.2ml/min.

Preferably, the inner/outer diameters of the inner tube of the coaxial nozzle are 0.19~3mm/0.4~3.2mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 0.6~3.5mm/0.9~3.8mm respectively.

Further preferably, the inner and outer diameters of the inner tube of the coaxial nozzle are 0.41mm/0.72mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 1.01mm/1.49mm respectively.

In one embodiment of the present invention, in step (1), during the coaxial/sheath-core wet spinning process, the thin stream of solution ejected from the coaxial nozzle enters a coagulation bath of 20 to 70°C and is solidified and formed. The solidification time is 10～40min;

The coagulation bath is any of the following:

① Dimethyl sulfoxide aqueous solution with a volume content of 30 to 70%;

② Calcium chloride/ethanol/water solution with a calcium chloride solid content of 5 to 25 wt% and a volume ratio of ethanol to deionized water of 1:3 to 3:1;

③Deionized water.

In one embodiment of the present invention, the average outer diameter of the hollow, porous, multi-layered polyacrylonitrile-based carbon fiber is 0.2-1.26 mm; the average inner diameter, that is, the diameter of the hollow part, is 0.1-0.8 mm.

The beneficial technical effects of the present invention are:

The present invention adopts a wet coaxial spinning forming method, utilizes the principle of non-solvent-induced phase separation, and designs the components of the core layer spinning solution to make the PAN polymer or PAN polymer/nano composite spinning stock solution that constitutes the fiber skeleton. The thin flow simultaneously triggers solvent-non-solvent mass transfer on both sides of the outer and inner surfaces. With the extraction and precipitation of non-solvent in the spinning solution and the phase separation and solidification process, abundant pores are formed in the fiber skeleton, and appear as a dense cortex and loose macroscopic structures. The multi-level distribution characteristics of the pore layer and the dense support layer in the middle are spaced apart. After subsequent pre-oxidation and carbonization treatments, the core layer material is vaporized and precipitated, the carbon of the polypropylene fiber skeleton is converted into a conductive carbon skeleton, and the fiber structure is hollow.

The polyacrylonitrile-based carbon fiber of the present invention has the characteristics of hollow, porous and multi-layer structure; in terms of performance, it has the advantages of rich specific surface area, light weight, conductivity, etc., and is convenient for adsorbing and containing abundant gaseous or liquid fluids, and is suitable for adsorption, Filtration, thermal insulation, energy storage electrodes and other fields.

The preparation method of the present invention does not require the addition of porogens or complicated processing steps, and has the advantage of simple and easy process. In addition, according to the principle of non-solvent-induced phase separation, the fiber pore structure can be controlled by controlling factors such as the molecular weight of polyacrylonitrile, the solid content of the spinning solution, the moisture content of the coagulation bath and the PVA hydrogel in the core layer, and the coagulation conditions. , with better structural designability.

Description of the drawings

Figure 1 is a schematic diagram of the polyacrylonitrile-based carbon fiber structure of the present invention;

Figure 2 is a scanning electron microscope image of the fiber product obtained in Example 1 of the present invention;

Figure 3 is a scanning electron microscope image of the fiber product obtained in Example 1 of the present invention;

Figure 4 is a scanning electron microscope image of the fiber product obtained in Example 6 of the present invention;

Figure 5 is a comparison chart of the electrochemical performance of Examples 1-3 and Example 6 of the present invention used as fiber electrodes;

Figure 6 is a comparison chart of the conductive properties of Examples 1-3 and Example 6 of the present invention used as fiber electrodes;

Figure 7 is a comparison chart of the mechanical properties of Examples 1-3 and Example 6 of the present invention used as fiber electrodes.

Detailed ways

The present invention will be described in detail below with reference to the drawings and examples.

As shown in Figure 1. Among them, the outer and inner layers located in the cortex and core layer, during the curing process, the solvent in the surface spinning solution is preferentially extracted by the non-solvent in the coagulation bath, and is cured preferentially to form the inner and outer fiber skeleton "skin", including the hole size. It is small and has a relatively dense structure. It is the main support layer that provides fiber structure and strength. The middle part of the fiber is mainly composed of two porous support layers with similar structures. The porous structure appears as finger-shaped pores emitted along the radial direction of the fiber. The size is relatively small. Large, it not only helps reduce the density of the fiber, but also provides abundant internal space to absorb and store large amounts of gaseous or liquid fluids. In addition, there is a thin dense middle support layer between the two porous layers, which separates the two radial layers of pore structure, which not only enriches the hierarchical structure of the fibers, but also provides strong structural support for the central macroporous loose layer.

Example 1

A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber with PAN/MXene as the skin layer, and its preparation method includes the following specific steps:

(1) Take 0.5g of 400 mesh Ti ₃ AlC ₂ powder and add it to 10 ml of hydrochloric acid + lithium fluoride mixed solution (hydrochloric acid concentration is 9 mol/L, LiF: 0.8g), stir for 24 hours in a 35°C water bath, and remove aluminum by etching layer, then wash the acid through multiple centrifugations until the supernatant is neutral, then take out the precipitate at the bottom of the centrifuge tube, evenly disperse it in deionized water, centrifuge again and take the supernatant and freeze-dry it to obtain a dry, etched and peeled layer Processed MXene (monolithic or few lamellar Ti ₃ C ₂ ).

(2) Add 160 mg of the etched and stripped MXene to 4 ml of dimethyl sulfoxide solution, and conduct ultrasonic treatment for 30 minutes in an ice-water bath to form an MXene/DMSO dispersion;

(3) Add polyacrylonitrile powder with a molecular weight of 85,000 to the MXene/DMSO dispersion, control the total solid content of PAN and Mxene to 160 mg/mL, and then stir for 3 hours under 65°C water bath conditions to form PAN/MXene spinning liquid;

(4) Add polyvinyl alcohol masterbatch (degree of alcoholysis: 88%) into deionized water and stir at 95°C for 3 hours to prepare a 100 mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);

(5) Treat the two spinning solutions prepared in step (3) and step (4) with ultrasonic treatment (power: 100W) at room temperature for 15 minutes for defoaming. After the degassing is completed, the porous fiber spinning solution is obtained. silk liquid;

(6) Use PVA aqueous solution as the core layer and PAN/MXene spinning solution as the core layer for coaxial/skin-core wet spinning. The spinning solution is wet-spun into a coagulation bath at 25°C for coagulation and shaping. The coagulation time is 30 minutes. , forming a skin-core structure filament;

The core layer spinning solution advancement speed is 0.3ml/min, and the skin layer spinning solution advancement speed is 0.9ml/min;

The inner/outer diameters of the inner tube of the coaxial nozzle are 0.41mm/0.72mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 1.01mm/1.49mm respectively.

The coagulation bath is a calcium chloride/ethanol/water solution with a calcium chloride solid content of 5 wt% and an ethanol-deionized water volume ratio of 1:3.

(7) Rinse the sheath-core structure filaments with deionized water and dry them in an oven at 60°C for 300 minutes to obtain PAN/MXene porous coaxial primary fibers.

(8) Pre-oxidize the prepared porous coaxial nascent fibers, raise the temperature to 250°C at a rate of 2°C/min, and keep it for 120 minutes;

(9) The obtained pre-oxidized fiber was heated to 800°C at a rate of 4°C/min under nitrogen protection and kept for 120 minutes to obtain hollow, porous, multi-layered polyacrylonitrile-based carbon fiber (PAN/MXene@PVA). Its scanning electron microscope picture is shown in Figure 2.

As can be seen from Figure 2, the core layer PVA is removed by high-temperature carbonization, leaving only the skin PAN/MXene-based carbon layer. The radial erosive finger-like pores formed on the main cross-section of the hollow fiber can be observed, and there are also obvious layered boundaries. In this structure, during the coagulation process of wet spinning, the non-solvent (water) in the coagulation bath and the moisture in the PVA spinning liquid in the core layer carry out mass transfer and spinning simultaneously from both directions of the fiber's cortex and core layer. The DMSO in the thin stream is exchanged, and the macromolecules inside the fiber preferentially gather in the area where the non-solvent (water) has fully penetrated, so that the hole size of the inner and outer walls of the hollow fiber is fine and relatively dense, while the holes near the core are relatively large. , the migration speed of PAN macromolecules is the slowest at the junction of two-way phase separation, and finally condenses to form a relatively dense "middle support layer", thus making the fiber hollow, porous, and multi-layered. Possibly due to the presence of Mxene nanomaterials, the fiber cross-section channel size is relatively uniform, and the outer layer of the fiber is no longer smooth, but has a wrinkled and rough morphology, which is conducive to providing a high specific surface area and greater adsorption capacity.

Example 2

A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber with PAN/GO as the skin layer. The preparation method includes the following specific steps:

(1) Add 192 mg GO to 4 ml dimethyl sulfoxide solution, and conduct ultrasonic treatment for 30 minutes in an ice-water bath to form a GO/DMSO dispersion;

(2) Add polyacrylonitrile powder with a molecular weight of 85,000 to the GO/DMSO dispersion, control the total solid content of PAN and GO to 180 mg/mL, and then stir in a water bath at 65°C for 3 hours to form PAN/GO spinning liquid;

(3) Add polyvinyl alcohol masterbatch (degree of alcoholysis: 88%) into deionized water and stir at 95°C for 3 hours to prepare a 100 mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);

(4) Treat the two spinning liquids prepared in step (2) and step (3) with ultrasonic treatment (power: 100W) at room temperature for 15 minutes for defoaming. After the degassing is completed, the porous fiber spinning solution is obtained. silk liquid;

(5) Use PVA aqueous solution as the core layer and PAN/GO spinning solution as the core layer for coaxial/skin-core wet spinning. The spinning solution is wet-spun into a coagulation bath at 25°C for coagulation and shaping. The coagulation time is 30 minutes. , forming a skin-core structure filament;

The core layer spinning solution advancement speed is 0.3ml/min, and the skin layer spinning solution advancement speed is 0.9ml/min;

The inner/outer diameters of the inner tube of the coaxial nozzle are 0.41mm/0.72mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 1.01mm/1.49mm respectively.

The coagulation bath is a calcium chloride/ethanol/water solution with a calcium chloride solid content of 5 wt% and an ethanol-deionized water volume ratio of 1:3.

(6) Rinse the sheath-core structure filaments with deionized water and then dry them in a 60°C oven for 300 minutes to obtain PAN/GO porous coaxial primary fibers;

(7) Pre-oxidize the prepared porous coaxial nascent fibers, raise the temperature to 250°C at a rate of 2°C/min, and keep it for 120 minutes;

(8) The obtained pre-oxidized fiber was then heated to 800°C at a rate of 4°C/min under inert gas protection and kept for 120min to obtain hollow, porous, multi-layered polyacrylonitrile-based carbon fiber (PAN/GO@PVA).

Example 3

A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber with PAN/CNT as the skin layer, and its preparation method includes the following specific steps:

(1) Add 192 mg of CNT to 4 ml of dimethyl sulfoxide solution and conduct ultrasonic treatment for 30 minutes in an ice-water bath to form a CNT/DMSO dispersion;

(2) Add polyacrylonitrile powder with a molecular weight of 85,000 to the CNT/DMSO dispersion, control the total solid content of PAN and CNT to 180 mg/mL, and then stir for 3 hours under 65°C water bath conditions to form PAN/CNT spinning liquid;

(3) Add polyvinyl alcohol masterbatch (degree of alcoholysis: 88%) into deionized water and stir at 95°C for 3 hours to prepare a 100 mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);

(4) Treat the two spinning liquids prepared in step (2) and step (3) with ultrasonic treatment (power: 100W) at room temperature for 15 minutes for defoaming. After the degassing is completed, the porous fiber spinning solution is obtained. silk liquid;

(5) Use PVA aqueous solution as the core layer and PAN/CNT spinning solution as the core layer for coaxial/skin-core wet spinning. The spinning solution is wet-spun into a coagulation bath at 25°C for coagulation and shaping. The coagulation time is 30 minutes. , forming a skin-core structure filament;

The core layer spinning solution advancement speed is 0.3ml/min, and the skin layer spinning solution advancement speed is 0.9ml/min;

The inner/outer diameters of the inner tube of the coaxial nozzle are 0.41mm/0.72mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 1.01mm/1.49mm respectively.

The coagulation bath is a calcium chloride/ethanol/water solution with a calcium chloride solid content of 5 wt% and an ethanol-deionized water volume ratio of 1:3.

(6) Rinse the sheath-core structure filaments with deionized water and then dry them in a 60°C oven for 300 minutes to obtain PAN/CNT porous coaxial primary fibers;

(7) Pre-oxidize the prepared porous coaxial nascent fibers, raise the temperature to 250°C at a rate of 2°C/min, and keep it for 120 minutes;

(8) The obtained pre-oxidized fiber is heated to 800°C at a rate of 4°C/min under the protection of inert gas and kept for 120 minutes to obtain hollow, porous, multi-layered polyacrylonitrile-based carbon fiber (PAN/CNT@PVA).

Example 4

A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber with PAN/MXene as the skin layer, and its preparation method includes the following specific steps:

(1) Take 0.5g of 400 mesh Ti ₃ AlC ₂ powder and add it to 10 ml of hydrochloric acid + lithium fluoride mixed solution (hydrochloric acid concentration is 9 mol/L, LiF: 0.8g), stir for 24 hours in a 35°C water bath, and remove aluminum by etching layer, then wash the acid through multiple centrifugations until the supernatant is neutral, then take out the precipitate at the bottom of the centrifuge tube, evenly disperse it in deionized water, centrifuge again and take the supernatant and freeze-dry it to obtain a dry, etched and peeled layer Processed MXene (monolithic or few lamellar Ti ₃ C ₂ ).

(2) Add 60 mg of the etched and stripped MXene to 4 ml of dimethyl sulfoxide solution, and conduct ultrasonic treatment for 30 minutes in an ice-water bath to form an MXene/DMSO dispersion;

(3) Add polyacrylonitrile powder with a molecular weight of 100,000 to the MXene/DMSO dispersion, control the total solid content of PAN and Mxene to 140 mg/mL, and then stir for 3 hours under 65°C water bath conditions to form PAN/MXene spinning liquid;

(4) Add polyvinyl alcohol masterbatch (degree of alcoholysis: 92%) into deionized water and stir at 95°C for 4 hours to prepare an 80 mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);

(5) Treat the two spinning solutions prepared in step (3) and step (4) with ultrasonic treatment (power: 100W) at room temperature for 20 minutes for degassing. After the degassing is completed, the porous fiber spinning solution is obtained. silk liquid;

(6) Use PVA aqueous solution as the core layer and PAN/MXene spinning solution as the core layer for coaxial/skin-core wet spinning. The spinning solution is wet-spun into a coagulation bath at 25°C for coagulation and shaping. The coagulation time is 30 minutes. , forming a skin-core structure filament;

The core layer spinning solution advancement speed is 0.2ml/min, and the skin layer spinning solution advancement speed is 1.2ml/min;

The inner/outer diameters of the inner tube of the coaxial nozzle are 0.21mm/0.41mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 0.63mm/0.92mm respectively.

The coagulation bath is a dimethyl sulfoxide aqueous solution with a dimethyl sulfoxide volume content of 35%.

(7) Rinse the sheath-core structure filaments with deionized water and dry them in an oven at 60°C for 300 minutes to obtain PAN/MXene porous coaxial primary fibers.

(8) Pre-oxidize the prepared porous coaxial nascent fibers, raise the temperature to 250°C at a rate of 2°C/min, and keep it for 120 minutes;

(9) Heat the obtained pre-oxidized fiber to 800°C at a rate of 4°C/min under nitrogen protection and keep it for 120 minutes to obtain hollow, porous, multi-layer polyacrylonitrile-based carbon fiber.

Example 5

A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber with PAN/MXene as the skin layer, and its preparation method includes the following specific steps:

(1) Take 0.5g of 400 mesh Ti ₃ AlC ₂ powder and add it to 10 ml of hydrochloric acid + lithium fluoride mixed solution (hydrochloric acid concentration is 9 mol/L, LiF: 0.8g), stir for 24 hours in a 35°C water bath, and remove aluminum by etching layer, then wash the acid through multiple centrifugations until the supernatant is neutral, then take out the precipitate at the bottom of the centrifuge tube, evenly disperse it in deionized water, centrifuge again and take the supernatant and freeze-dry it to obtain a dry, etched and peeled layer Processed MXene (monolithic or few lamellar Ti ₃ C ₂ ).

(2) Add 120 mg of the etched and stripped MXene to 4 ml of dimethyl sulfoxide solution, and conduct ultrasonic treatment for 30 minutes in an ice-water bath to form an MXene/DMSO dispersion;

(3) Add polyacrylonitrile powder with a molecular weight of 120,000 to the MXene/DMSO dispersion, control the total solid content of PAN and MXene to 180 mg/mL, and then stir for 3 hours under 65°C water bath conditions to form PAN/MXene spinning liquid;

(4) Add polyvinyl alcohol masterbatch (degree of alcoholysis: 98%) into deionized water and stir at 95°C for 4 hours to prepare a 140 mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);

(5) Treat the two spinning solutions prepared in step (3) and step (4) with ultrasonic treatment (power: 100W) at room temperature for 20 minutes for degassing. After the degassing is completed, the porous fiber spinning solution is obtained. silk liquid;

(6) Use PVA aqueous solution as the core layer and PAN/MXene spinning solution as the core layer for coaxial/skin-core wet spinning. The spinning solution is wet-spun into a coagulation bath at 25°C for coagulation and shaping. The coagulation time is 30 minutes. , forming a skin-core structure filament;

The core layer spinning solution advancement speed is 0.2ml/min, and the skin layer spinning solution advancement speed is 1.1ml/min;

The inner/outer diameters of the inner tube of the coaxial nozzle are 1mm/1.4mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 1.8mm/2.2mm respectively.

The coagulation bath is a pure deionized water solution.

(7) Rinse the sheath-core structure filaments with deionized water and dry them in an oven at 60°C for 300 minutes to obtain PAN/MXene porous coaxial primary fibers.

(8) Pre-oxidize the prepared porous coaxial nascent fibers, raise the temperature to 250°C at a rate of 2°C/min, and keep it for 120 minutes;

(9) Heat the obtained pre-oxidized fiber to 800°C at a rate of 4°C/min under nitrogen protection and keep it for 120 minutes to obtain hollow, porous, multi-layer polyacrylonitrile-based carbon fiber.

Example 6

A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber with PAN as the skin layer, and its preparation method includes the following specific steps:

(1) Add polyacrylonitrile powder with a molecular weight of 150,000 to 4 ml of dimethyl sulfoxide solution, control the PAN solid content to 160 mg/ml, and then stir for 3 hours under 65°C water bath conditions to form a PAN spinning liquid;

(2) Add polyvinyl alcohol masterbatch (degree of alcoholysis: 98%) into deionized water and stir at 95°C for 3 hours to prepare a 100 mg/ml polyvinyl alcohol aqueous solution (PVA aqueous solution);

(3) Treat the two spinning solutions prepared in step (1) and step (2) with ultrasonic treatment (power: 100W) at room temperature for 15 minutes for defoaming. After the degassing is completed, the porous fiber spinning solution is obtained. silk liquid;

(4) Use PVA aqueous solution as the core layer and PAN spinning solution as the core layer for coaxial/skin-core wet spinning. The spinning solution is wet-spun into a coagulation bath at 25°C for coagulation and shaping. The coagulation time is 30 minutes to form Sheath-core structure filament;

The core layer spinning solution advancement speed is 0.3ml/min, and the skin layer spinning solution advancement speed is 0.9ml/min;

The inner/outer diameters of the inner tube of the coaxial nozzle are 0.41mm/0.72mm respectively; the inner and outer diameters of the outer tube of the coaxial nozzle are 1.01mm/1.49mm respectively.

The coagulation bath is a calcium chloride/ethanol/water solution with a calcium chloride solid content of 5 wt% and an ethanol-deionized water volume ratio of 1:3.

(5) Rinse the sheath-core structure filaments with deionized water and then dry them in a 60°C oven for 300 minutes to obtain PAN porous coaxial primary fibers;

(6) Pre-oxidize the prepared porous coaxial primary fibers, raise the temperature to 250°C at a rate of 2°C/min, and keep it for 120 minutes;

(7) The obtained pre-oxidized fiber is heated to 800°C at a rate of 4°C/min under inert gas protection and kept for 120 minutes to obtain hollow, porous, multi-layered polyacrylonitrile-based carbon fiber (PAN@PVA). The scanning electron microscope images are shown in Figures 3 and 4.

As can be seen from Figure 3, PVA is carbonized and removed after high-temperature treatment, leaving only the shell PAN-based carbon layer. The radial erosive finger-shaped pores formed on the main section of the hollow fiber can be observed, and there are obvious layer boundaries. From the high-magnification picture in Figure 4, the existence of the "middle support layer" can be clearly seen. In this structure, during the coagulation process of wet spinning, the non-solvent (water) in the coagulation bath and the moisture in the PVA spinning liquid in the core layer carry out mass transfer and spinning simultaneously from both directions of the fiber's cortex and core layer. The DMSO in the thin stream is exchanged, and the macromolecules inside the fiber preferentially gather in the area where the non-solvent (water) has fully penetrated, so that the hole size of the inner and outer walls of the hollow fiber is fine and relatively dense, while the holes near the core are relatively large. , the migration speed of PAN macromolecules is the slowest at the junction of two-way phase separation, and finally condenses to form a relatively dense "middle support layer", thus making the fiber hollow, porous, and multi-layered. It can be observed that the outer skin of the PAN-based hollow carbon fiber is very smooth and dense.

test case

(1) Fiber electrode electrochemical performance test

Shanghai Chenhua CHI660 electrochemical workstation was used to test the carbon fibers obtained in Examples 1-3 and 6. The fiber properties were characterized through a three-electrode system with an operating voltage of -0.2-0.6V. Test content includes cyclic voltammetry curves, galvanostatic charge and discharge curves, and electrochemical impedance spectroscopy. As shown in Figure 5a, the CV curve of PAN/MXene@PVA hollow, porous, multi-layered carbon fiber has the largest surrounding area, indicating that it has the best energy storage effect; from the galvanostatic charge and discharge curve (Figure 5b), it can be seen that its specific capacitance can reach 113.18F/g (0.5A/g), which may be due to its more reasonable distribution of pore structure. The introduction of MXene on the one hand further increases the specific surface area of the material, and the MXene sheet also has better conductivity and energy storage effects.

(2) Fiber electrode conductive performance test

Use a TP-304 digital multimeter to test the resistance within the unit length of the carbon fibers obtained in Examples 1-3 and 6, and use an optical microscope to measure the fiber diameter, and then calculate the conductivity of the fiber according to formula (2):

Among them, δ represents the electrical conductivity in Siemens per meter (S/m), L represents the fiber length in meters (m), S represents the fiber cross-sectional area in square meters (m ² ), and R represents the fiber test length. The resistance value within is in ohms (Ω).

As can be seen from Figure 6, PAN/MXene@PVA hollow, porous, and multi-layered carbon fibers show higher electrical conductivity, approximately 546 S/m.

(3) Mechanical properties test of fiber electrodes

The tensile strength test of the carbon fibers obtained in Examples 1-3 and 6 was carried out using the The strength and elongation of the fiber are revealed. As can be seen from Figure 7, the strength of PAN@PVA hollow, porous, multi-layered coaxial fibers after carbonization is 15.23MPa, showing higher strength.

Claims (6)

1. A hollow, porous, multi-layered polyacrylonitrile-based carbon fiber, characterized in that its preparation method includes the following specific steps: (1) The skin/core layer spinning liquid forms a skin-core structure filament through coaxial/skin-core wet spinning; (2) Dry the sheath-core structural filament obtained in step (1) in an oven at 30~80°C for 180~400 minutes to obtain porous coaxial primary fibers; (3) Pre-oxidize the porous coaxial primary fibers prepared in step (2), raise the temperature to 200~250°C at a rate of 2°C/min, and keep it for 60~180min; (4) Heat the pre-oxidized fiber in step (4) to 700~900°C at a rate of 4°C/min under the protection of inert gas, and keep it for 60~180 minutes to obtain hollow, porous, multi-layered polyacrylonitrile-based carbon fiber; In step (1), the raw material composition of the cortex spinning solution includes polyacrylonitrile powder and dimethyl sulfoxide; it also includes functional conductive active materials, and the functional conductive active materials are graphene oxide, carbon nanotubes, and Mxene. one or more of; The molecular weight of the polyacrylonitrile powder is 5000~150000, the total solid content of polyacrylonitrile and functional conductive active materials in the cortex spinning solution is 120~180 mg/mL; the functional conductive active material in the cortex spinning solution is polypropylene 0.5~50% of nitrile mass; In step (1), the core layer spinning liquid is a polyvinyl alcohol aqueous solution of 50 to 150 mg/mL; The alcoholysis degree of the polyvinyl alcohol is 87% to 98.8%. 2. The hollow, porous, multi-level polyacrylonitrile-based carbon fiber according to claim 1, characterized in that the preparation method of the cortex spinning liquid is: adding functional conductive active materials and polyacrylonitrile powder to dimethyl sulfide In sulfone, stir in a 65°C water bath for 2 to 5 hours until the mixture is evenly dispersed to form a homogeneous skin spinning solution. 3. The hollow, porous, multi-layered polyacrylonitrile-based carbon fiber according to claim 1, characterized in that in step (1), the skin/core layer spinning solution undergoes deaeration treatment before use. The method is: ultrasonic treatment at room temperature for 10 to 30 minutes. 4. The hollow, porous, multi-layered polyacrylonitrile-based carbon fiber according to claim 3, characterized in that the power of the ultrasound is 80~120W. 5. The hollow, porous, multi-layered polyacrylonitrile-based carbon fiber according to claim 1, characterized in that in step (1), during the coaxial/skin-core wet spinning process, the core layer spinning liquid advances speed The speed of the cortical spinning liquid is 0.2~0.4 ml/min, and the advancement speed of the cortex spinning solution is 0.7~1.2 ml/min. 6. The hollow, porous, multi-layered polyacrylonitrile-based carbon fiber according to claim 1, characterized in that in step (1), during the coaxial/sheath-core wet spinning process, the solution spit out by the coaxial nozzle is fine. The flow enters the coagulation bath at 20~70℃ to solidify and form, and the solidification time is 10~40 minutes; The coagulation bath is any of the following: ① Dimethyl sulfoxide aqueous solution with a volume content of 30 to 70%; ② Calcium chloride/ethanol/water solution with solid content of 5~25wt% and ethanol-deionized water volume ratio of 1:3~3:1; ③Deionized water.