CN115948821B - Hollow, porous and multi-layer polyacrylonitrile-based carbon fiber and preparation method thereof - Google Patents
Hollow, porous and multi-layer polyacrylonitrile-based carbon fiber and preparation method thereof Download PDFInfo
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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
Technical Field
The invention relates to the technical field of porous carbon fiber preparation, in particular to hollow, porous and multi-level polyacrylonitrile-based carbon fiber and a preparation method thereof.
Background
The porous fiber has the advantages of large specific surface area, strong adsorptivity, high porosity and the like, and is widely applied to materials in the fields of adsorption, catalysis, filtration, electric and thermal energy storage materials and the like.
The more common methods for preparing porous fibers currently include an activation method, a template method and a phase separation method. The activation method refers to a method of etching a formed carbon fiber material by an oxidizing gas or a chemical agent to introduce a pore structure, and specifically includes both physical activation and chemical activation. The physical activation is carried out under the condition of high temperature, the energy consumption is higher, the chemical etching rule has strict requirements on the reaction temperature and the consumption of oxidant reactants, the fiber strength is obviously damaged easily due to improper control of the conditions, and the two methods have the common defects that the obtained pore structure is a micro/mesoporous structure, the mesoporous size distribution is narrower, and the rapid material transmission is not facilitated. The template method is a method of adding decomposable micro-nano particles to a fiber-forming polymer and then carrying out a subsequent treatment step such as acidolysis or high-temperature treatment to obtain a porous structure. The pore-forming effect of the template method is closely related to the dispersion condition of the micro-nano scale pore-forming agent in the fiber matrix, and the problems that the controllability of the pore structure is poor, the hierarchical pore structure is difficult to realize and the like are also existed. The phase separation rule refers to that in the process of solvent-non-solvent mass transfer solidification forming of the wet spinning solution trickle in the coagulating bath, a polymer rich phase and a polymer lean phase are formed in the spinning solution trickle through a non-solvent induced phase separation principle, the polymer rich phase forms a fiber skeleton after solidification and drying, the polymer lean phase forms a pore structure in the fiber, and the phase separation speed can be controlled by changing factors such as the solid content of the polymer, the type of non-solvent in the coagulating bath, the solvent-non-solvent mass transfer speed and the like, so that the effect of regulating and controlling the fiber pore structure and the size and level distribution of the fiber pore structure is achieved. Therefore, the method is characterized in that the fiber porous structure is formed in the solidification forming process of the nascent silk, the preparation process is relatively simple and easy to implement, and the cost is low. In theory, the porous fiber prepared by the method has better designability and controllability, but the structure of the porous fiber prepared by the method is relatively single. The porous carbon fiber has the advantages that the porous hierarchical structure of the porous carbon fiber is further enriched by regulating and controlling the technological parameters, and the specific surface area of the porous carbon fiber is further increased, so that the application efficiency of the porous carbon fiber material in the fields of adsorption filtration, thermal insulation, electrochemical energy storage and the like is improved.
Disclosure of Invention
Aiming at the problems existing in the prior art, the invention provides hollow, porous and multi-layer polyacrylonitrile-based carbon fiber and a preparation method thereof. The invention adopts a wet coaxial spinning forming method and utilizes the non-solvent induced phase separation principle to prepare hollow and porous fibers; meanwhile, the characteristic that the cross section of the fiber skeleton presents heterogeneous multi-level is realized by utilizing simultaneous phase separation and solidification on the two sides of the sheath-core.
The technical scheme of the invention is as follows:
the first object of the invention is to provide a hollow, porous, multi-layer polyacrylonitrile-based carbon fiber, the preparation method of 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 obtained in the step (1) 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 prepared in the step (2), heating to 200-250 ℃ at the speed of 2 ℃/min, and keeping for 60-180 min;
(4) Heating the pre-oxidized fiber in the step (4) to 700-900 ℃ at a speed of 4 ℃/min under the protection of inert gas (nitrogen or argon), and keeping for 60-180 min to obtain the hollow, porous and multi-level polyacrylonitrile-based carbon fiber.
In one embodiment of the present invention, in step (1), the raw material composition of the sheath dope comprises polyacrylonitrile powder and dimethyl sulfoxide;
preferably, the raw material composition of the cortex spinning solution comprises a functional conductive active material, wherein the functional conductive active material is one or more of graphene oxide, carbon nano tubes and Mxene.
Mxene is two-dimensional lamellar nano transition metal/nitrogen/carbide, and at present, research and application are relatively mature, and TiC series Mxene is selected from 400-mesh Ti 3 AlC 2 MAX phase of (a) as a raw material.
The monolithic layer Ti can be obtained by etching with hydrochloric acid and lithium fluoride 3 C 2 。
In one embodiment of the invention, the molecular weight of the polyacrylonitrile powder is 5000-150000, and the total solid content of the polyacrylonitrile and the functional conductive active material in the cortex spinning solution is 120-180 mg/mL; the functional conductive active material in the cortex spinning solution is 0.5-50% of the mass of the polyacrylonitrile.
In one embodiment of the invention, the preparation method of the cortex spinning solution comprises the following steps: adding polyacrylonitrile powder into dimethyl sulfoxide, stirring for 2-5 hours under the water bath condition of 65 ℃ until the polyacrylonitrile powder is uniformly stirred and dispersed to form a homogeneous cortex spinning solution; or adding the functional conductive active material and polyacrylonitrile powder into dimethyl sulfoxide, and stirring for 2-5 hours under the water bath condition of 65 ℃ until the functional conductive active material and the polyacrylonitrile powder are uniformly stirred and dispersed to form the homogeneous cortex spinning solution.
In one embodiment of the present invention, in the step (1), the core spinning solution is a polyvinyl alcohol aqueous solution of 50 to 150 mg/mL.
In one embodiment of the invention, the polyvinyl alcohol has an alcoholysis level of 87% to 98.8%.
In one embodiment of the present invention, in the step (1), the skin/core spinning solution is subjected to a defoaming treatment before use, and the defoaming method is as follows: and carrying out ultrasonic treatment for 10-30 minutes at room temperature.
In one embodiment of the invention, the power of the ultrasound is 80-120W.
In one embodiment of the invention, in the step (1), the advancing speed of the core spinning solution is 0.2-0.4 ml/min and the advancing speed of the sheath spinning solution is 0.7-1.2 ml/min in the coaxial/sheath-core wet spinning process.
Preferably, the inner/outer diameter of the inner tube of the coaxial spray head is 0.19-3 mm/0.4-3.2 mm respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 0.6-3.5 mm/0.9-3.8 mm.
Further preferably, the inner/outer diameter of the inner tube of the coaxial shower head is 0.41mm/0.72mm, respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 1.01mm/1.49mm.
In the step (1), in the process of coaxial/sheath-core wet spinning, the solution trickle discharged by a coaxial nozzle enters a coagulation bath at 20-70 ℃ to be coagulated and formed, and the coagulation time is 10-40 min;
the coagulation bath is any one of the following:
(1) 30-70% dimethyl sulfoxide water solution;
(2) the solid content of the calcium chloride is 5-25 wt%, and the volume ratio of the ethanol deionized water is 1:3-3:1;
(3) deionized water.
In one embodiment of the invention, the hollow, porous, multi-level polyacrylonitrile-based carbon fiber has an average outer diameter of 0.2 to 1.26mm; the average inner diameter, i.e., the diameter of the hollow portion, is 0.1 to 0.8mm.
The beneficial technical effects of the invention are as follows:
the invention adopts a wet coaxial spinning forming method, utilizes the non-solvent induced phase separation principle, designs components of core spinning solution, ensures that PAN polymer or PAN polymer/nano composite spinning solution thin streams forming a fiber skeleton are arranged on both sides of an outer surface layer and an inner surface layer, simultaneously initiates solvent-non-solvent mass transfer, and forms rich pores in the fiber skeleton along with extraction precipitation and phase separation solidification processes of non-solvent in the spinning solution, and the fiber skeleton has the multi-level distribution characteristics of interval distribution of a dense cortex layer, a loose macroporous layer and a middle dense supporting layer. And then the core layer substance is gasified and separated out after the subsequent pre-oxidation and carbonization treatment, 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 has the characteristics of hollowness, multiple holes and multiple layers structurally; the porous ceramic material has the advantages of abundant specific surface area, light weight, electric conduction and the like in performance, is convenient for adsorbing and containing abundant gaseous or liquid fluid, and is suitable for the fields of adsorption, filtration, heat insulation, heat preservation, energy storage electrodes and the like.
The preparation method of the invention does not need to add pore-forming agent or complicated treatment steps, and has the advantage of simple and feasible process. In addition, according to the non-solvent induced phase separation principle, the regulation and control of the fiber pore structure can be realized by controlling the factors such as the molecular weight of polyacrylonitrile, the solid content of spinning solution, the moisture content of the coagulating bath and the PVA hydrogel of the core layer substance, the coagulating condition and the like, and the fiber pore structure has better structural designability.
Drawings
FIG. 1 is a schematic view of the structure of a polyacrylonitrile-based carbon fiber of the present invention;
FIG. 2 is a scanning electron microscope image of the fiber product obtained in example 1 of the present invention;
FIG. 3 is a scanning electron microscope image of the fiber product obtained in example 1 of the present invention;
FIG. 4 is a scanning electron microscope image of the fiber product obtained in example 6 of the present invention;
FIG. 5 is a graph showing the electrochemical performance of the fiber electrodes of examples 1-3 and example 6 of the present invention;
FIG. 6 is a graph comparing the conductivity of the fiber electrodes used in examples 1-3 and example 6 of the present invention;
FIG. 7 is a graph showing the comparison of mechanical properties of the fiber electrodes used in examples 1-3 and example 6 of the present invention.
Detailed Description
The present invention will be described in detail below with reference to the drawings and examples.
As shown in fig. 1. The solvent in the spinning stock solution of the surface layer is preferentially extracted by the non-solvent in the coagulating bath in the curing process to be preferentially cured to form an inner and outer fiber skeleton skin layer, and the inner and outer fiber skeleton skin layer comprises a main supporting layer with fine pore size, relatively compact structure and fiber morphological structure and strength; the middle part of the fiber is mainly composed of two porous supporting layers with similar structures, the porous structures are finger-shaped pore channels which emit along the radial direction of the fiber, the size is relatively large, the density of the fiber is reduced, and a rich internal space is provided so as to absorb and store a large amount of gaseous or liquid fluid. In addition, a thin compact middle supporting layer is arranged between the two porous layers to separate the radial two-layer pore canal structure, so that the fiber hierarchical structure is enriched, and structural strong support is provided for the central macroporous loose layer.
Example 1
A hollow, porous and multi-layer polyacrylonitrile-based carbon fiber takes PAN/MXene as a skin layer, and the preparation method comprises the following specific steps:
(1) 0.5g of 400 mesh Ti is taken 3 AlC 2 Adding the powder into 10ml of mixed solution of hydrochloric acid and lithium fluoride (hydrochloric acid concentration is 9mol/L, liF:0.8 g), stirring for 24h under water bath condition at 35deg.C, etching to remove aluminum layer, centrifuging to wash acid until supernatant becomes neutral, taking out precipitate at bottom of centrifuge tube, dispersing in deionized water, centrifuging again, taking supernatant, lyophilizing to obtain dry and etched and delaminated MXene (single-layer or few-layer Ti) 3 C 2 )。
(2) 160mg of MXene subjected to the etching delamination treatment is added into 4ml of dimethyl sulfoxide solution, and ultrasonic treatment is carried out for 30min under the ice water bath condition to form MXene/DMSO dispersion;
(3) Adding polyacrylonitrile powder with molecular weight of 85000 into the MXene/DMSO dispersion, controlling the total solid content of PAN and Mxene to 160mg/mL, and stirring for 3 hours under the water bath condition of 65 ℃ to form PAN/MXene spinning solution;
(4) Adding polyvinyl alcohol master batches (with the alcoholysis degree of 88%) into deionized water, stirring for 3 hours at 95 ℃ to prepare a 100mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);
(5) Respectively carrying out ultrasonic treatment (with power of 100W) on the two spinning solutions prepared in the step (3) and the step (4) at room temperature for 15 minutes to carry out defoaming, and obtaining the porous fiber spinning solution after the defoaming is finished;
(6) Taking PVA aqueous solution as a core layer, taking PAN/MXene spinning solution as the core layer to carry out coaxial/sheath-core wet spinning, and carrying out wet spinning on the spinning solution into a coagulating bath at 25 ℃ for coagulating and forming, wherein the coagulating time is 30min, so as to form a sheath-core structure yarn;
the advancing speed of the spinning solution of the core layer is 0.3ml/min, and the advancing speed of the spinning solution of the skin layer is 0.9ml/min;
the inner/outer diameter of the inner tube of the coaxial spray head is 0.41mm/0.72mm respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 1.01mm/1.49mm.
The coagulating bath is a calcium chloride/ethanol/water solution with the solid content of calcium chloride of 5 weight percent and the volume ratio of ethanol to deionized water of 1:3.
(7) And washing the skin-core structure yarn with deionized water, and then placing the yarn in a 60 ℃ oven for drying treatment for 300min to obtain the PAN/MXene porous coaxial nascent fiber.
(8) Pre-oxidizing the prepared porous coaxial nascent fiber, heating to 250 ℃ at the speed of 2 ℃/min, and keeping for 120min;
(9) Heating the obtained pre-oxidized fiber to 800 ℃ at a speed of 4 ℃/min under the protection of nitrogen, and keeping for 120min to obtain the hollow, porous and multi-layer polyacrylonitrile-based carbon fiber (PAN/MXene@PVA). The scanning electron microscope picture is shown in figure 2.
As can be seen from fig. 2, the core PVA was removed by high temperature carbonization, leaving only the skin PAN/MXene based carbon layer. Radial aggressive finger channels formed in the cross section of the hollow fiber body can be observed and also exhibit a distinct hierarchical demarcation. In the wet spinning solidification process, the non-solvent (water) in the coagulating bath and the water in the core PVA spinning solution exchange mass transfer from the two directions of the fiber skin and the core simultaneously and the DMSO in the spinning trickle, and macromolecules in the fiber are preferentially gathered to the region where the non-solvent (water) permeates sufficiently, so that the inner and outer wall skin pores of the hollow fiber are compact in size and relatively dense, the pore channel close to the middle core is relatively large, the migration speed of PAN macromolecules at the junction of the two directions of phase separation is the slowest, and finally the fiber is condensed to form a relatively dense 'middle supporting layer', so that the fiber has the characteristics of hollowness, multiple pores and multiple layers. The existence of the Mxene nano material can lead the pore canal size of the fiber section to be relatively uniform, and the outer surface layer of the fiber is not smooth, but presents a rough appearance of folds, which is beneficial to providing high specific surface area and better adsorption capacity.
Example 2
A hollow, porous and multi-level polyacrylonitrile-based carbon fiber takes PAN/GO as a skin layer, and the preparation method comprises the following specific steps:
(1) Adding 192mg of GO into 4ml of dimethyl sulfoxide solution, and performing ultrasonic treatment for 30min under the ice water bath condition to form GO/DMSO dispersion;
(2) Adding polyacrylonitrile powder with molecular weight of 85000 into the GO/DMSO dispersion, controlling the total solid content of PAN and GO to be 180mg/mL, and then stirring for 3 hours under the water bath condition of 65 ℃ to form PAN/GO spinning solution;
(3) Adding polyvinyl alcohol master batches (with the alcoholysis degree of 88%) into deionized water, stirring for 3 hours at 95 ℃ to prepare a 100mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);
(4) Respectively carrying out ultrasonic treatment (with power of 100W) on the two spinning solutions prepared in the step (2) and the step (3) at room temperature for 15 minutes to carry out defoaming, and obtaining the porous fiber spinning solution after the defoaming is finished;
(5) Taking PVA aqueous solution as a core layer, taking PAN/GO spinning solution as the core layer for coaxial/sheath-core wet spinning, and performing wet spinning on the spinning solution in a coagulating bath at 25 ℃ for coagulating and forming, wherein the coagulating time is 30min, so as to form a sheath-core structure yarn;
the advancing speed of the spinning solution of the core layer is 0.3ml/min, and the advancing speed of the spinning solution of the skin layer is 0.9ml/min;
the inner/outer diameter of the inner tube of the coaxial spray head is 0.41mm/0.72mm respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 1.01mm/1.49mm.
The coagulating bath is a calcium chloride/ethanol/water solution with the solid content of calcium chloride of 5 weight percent and the volume ratio of ethanol to deionized water of 1:3.
(6) Washing the skin-core structure yarn with deionized water, and then placing the washed skin-core structure yarn in a 60 ℃ oven for drying treatment for 300min to obtain PAN/GO porous coaxial nascent fibers;
(7) Pre-oxidizing the prepared porous coaxial nascent fiber, heating to 250 ℃ at the speed of 2 ℃/min, and keeping for 120min;
(8) And then heating the obtained pre-oxidized fiber to 800 ℃ at a speed of 4 ℃/min under the protection of inert gas, and keeping for 120min to obtain the hollow, porous and multi-layer polyacrylonitrile-based carbon fiber (PAN/GO@PVA).
Example 3
A hollow, porous and multi-layer polyacrylonitrile-based carbon fiber takes PAN/CNT as a skin layer, and the preparation method comprises the following specific steps:
(1) Adding 192mg of CNT into 4ml of dimethyl sulfoxide solution, and performing ultrasonic treatment for 30min under the ice water bath condition to form CNT/DMSO dispersion;
(2) Adding polyacrylonitrile powder with molecular weight of 85000 into the CNT/DMSO dispersion, controlling the total solid content of PAN and CNT to be 180mg/mL, and then stirring for 3 hours under the water bath condition of 65 ℃ to form PAN/CNT spinning solution;
(3) Adding polyvinyl alcohol master batches (with the alcoholysis degree of 88%) into deionized water, stirring for 3 hours at 95 ℃ to prepare a 100mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);
(4) Respectively carrying out ultrasonic treatment (with power of 100W) on the two spinning solutions prepared in the step (2) and the step (3) at room temperature for 15 minutes to carry out defoaming, and obtaining the porous fiber spinning solution after the defoaming is finished;
(5) Taking PVA aqueous solution as a core layer, taking PAN/CNT spinning solution as the core layer to perform coaxial/sheath-core wet spinning, performing wet spinning on the spinning solution, and performing solidification forming in a solidification bath at 25 ℃ for 30min to form a sheath-core structure yarn;
the advancing speed of the spinning solution of the core layer is 0.3ml/min, and the advancing speed of the spinning solution of the skin layer is 0.9ml/min;
the inner/outer diameter of the inner tube of the coaxial spray head is 0.41mm/0.72mm respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 1.01mm/1.49mm.
The coagulating bath is a calcium chloride/ethanol/water solution with the solid content of calcium chloride of 5 weight percent and the volume ratio of ethanol to deionized water of 1:3.
(6) Washing the skin-core structure yarn with deionized water, and then placing the washed skin-core structure yarn in a 60 ℃ oven for drying treatment for 300min to obtain PAN/CNT porous coaxial nascent fibers;
(7) Pre-oxidizing the prepared porous coaxial nascent fiber, heating to 250 ℃ at the speed of 2 ℃/min, and keeping for 120min;
(8) Heating the obtained pre-oxidized fiber to 800 ℃ at a speed of 4 ℃/min under the protection of inert gas, and keeping for 120min to obtain the hollow, porous and multi-layer polyacrylonitrile-based carbon fiber (PAN/CNT@PVA).
Example 4
A hollow, porous and multi-layer polyacrylonitrile-based carbon fiber takes PAN/MXene as a skin layer, and the preparation method comprises the following specific steps:
(1) 0.5g of 400 mesh Ti is taken 3 AlC 2 Adding the powder into 10ml of mixed solution of hydrochloric acid and lithium fluoride (hydrochloric acid concentration is 9mol/L, liF:0.8 g), stirring for 24h under water bath condition at 35deg.C, etching to remove aluminum layer, centrifuging to wash acid until supernatant becomes neutral, taking out precipitate at bottom of centrifuge tube, dispersing in deionized water, centrifuging again, taking supernatant, lyophilizing to obtain dry and etched and delaminated MXene (single-layer or few-layer Ti) 3 C 2 )。
(2) 60mg of MXene subjected to the etching delamination treatment is added into 4ml of dimethyl sulfoxide solution, and ultrasonic treatment is carried out for 30min under the ice water bath condition to form MXene/DMSO dispersion;
(3) Adding polyacrylonitrile powder with the molecular weight of 100000 into the MXene/DMSO dispersion liquid, controlling the total solid content of PAN and Mxene to be 140mg/mL, and then stirring for 3 hours under the water bath condition of 65 ℃ to form PAN/MXene spinning liquid;
(4) Adding polyvinyl alcohol master batches (with the alcoholysis degree of 92%) into deionized water, stirring for 4 hours at 95 ℃ to prepare a polyvinyl alcohol aqueous solution (PVA aqueous solution) with the concentration of 80mg/mL;
(5) Respectively carrying out ultrasonic treatment (with the power of 100W) on the two spinning solutions prepared in the step (3) and the step (4) at room temperature for 20 minutes to carry out defoaming, and obtaining the porous fiber spinning solution after the defoaming is finished;
(6) Taking PVA aqueous solution as a core layer, taking PAN/MXene spinning solution as the core layer to carry out coaxial/sheath-core wet spinning, and carrying out wet spinning on the spinning solution into a coagulating bath at 25 ℃ for coagulating and forming, wherein the coagulating time is 30min, so as to form a sheath-core structure yarn;
the advancing speed of the spinning solution of the core layer is 0.2ml/min, and the advancing speed of the spinning solution of the skin layer is 1.2ml/min;
the inner/outer diameter of the inner tube of the coaxial spray head is 0.21mm/0.41mm respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 0.63mm/0.92mm.
The coagulation bath is an aqueous solution of 35% by volume of dimethyl sulfoxide.
(7) And washing the skin-core structure yarn with deionized water, and then placing the yarn in a 60 ℃ oven for drying treatment for 300min to obtain the PAN/MXene porous coaxial nascent fiber.
(8) Pre-oxidizing the prepared porous coaxial nascent fiber, heating to 250 ℃ at the speed of 2 ℃/min, and keeping for 120min;
(9) Heating the obtained pre-oxidized fiber to 800 ℃ at a speed of 4 ℃/min under the protection of nitrogen, and keeping for 120min to obtain the hollow, porous and multi-level polyacrylonitrile-based carbon fiber.
Example 5
A hollow, porous and multi-layer polyacrylonitrile-based carbon fiber takes PAN/MXene as a skin layer, and the preparation method comprises the following specific steps:
(1) 0.5g of 400 mesh Ti is taken 3 AlC 2 Adding the powder into 10ml of mixed solution of hydrochloric acid and lithium fluoride (hydrochloric acid concentration is 9mol/L, liF:0.8 g), stirring for 24h under water bath condition at 35deg.C, etching to remove aluminum layer, centrifuging to wash acid until supernatant becomes neutral, taking out precipitate at bottom of centrifuge tube, dispersing in deionized water, centrifuging again, taking supernatant, lyophilizing to obtain dry and etched and delaminated MXene (single-layer or few-layer Ti) 3 C 2 )。
(2) 120mg of MXene subjected to the etching delamination treatment is added into 4ml of dimethyl sulfoxide solution, and ultrasonic treatment is carried out for 30min under the ice water bath condition to form MXene/DMSO dispersion;
(3) Adding polyacrylonitrile powder with the molecular weight of 120000 into the MXene/DMSO dispersion liquid, controlling the total solid content of PAN and MXene to be 180mg/mL, and then stirring for 3 hours under the water bath condition of 65 ℃ to form PAN/MXene spinning liquid;
(4) Adding polyvinyl alcohol master batches (with the alcoholysis degree of 98%) into deionized water, stirring for 4 hours at 95 ℃ to prepare 140mg/mL polyvinyl alcohol aqueous solution (PVA aqueous solution);
(5) Respectively carrying out ultrasonic treatment (with the power of 100W) on the two spinning solutions prepared in the step (3) and the step (4) at room temperature for 20 minutes to carry out defoaming, and obtaining the porous fiber spinning solution after the defoaming is finished;
(6) Taking PVA aqueous solution as a core layer, taking PAN/MXene spinning solution as the core layer to carry out coaxial/sheath-core wet spinning, and carrying out wet spinning on the spinning solution into a coagulating bath at 25 ℃ for coagulating and forming, wherein the coagulating time is 30min, so as to form a sheath-core structure yarn;
the advancing speed of the spinning solution of the core layer is 0.2ml/min, and the advancing speed of the spinning solution of the skin layer is 1.1ml/min;
the inner/outer diameter of the inner tube of the coaxial spray head is 1mm/1.4mm respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 1.8mm/2.2mm.
The coagulation bath is a pure deionized water solution.
(7) And washing the skin-core structure yarn with deionized water, and then placing the yarn in a 60 ℃ oven for drying treatment for 300min to obtain the PAN/MXene porous coaxial nascent fiber.
(8) Pre-oxidizing the prepared porous coaxial nascent fiber, heating to 250 ℃ at the speed of 2 ℃/min, and keeping for 120min;
(9) Heating the obtained pre-oxidized fiber to 800 ℃ at a speed of 4 ℃/min under the protection of nitrogen, and keeping for 120min to obtain the hollow, porous and multi-level polyacrylonitrile-based carbon fiber.
Example 6
A hollow, porous and multi-layer polyacrylonitrile-based carbon fiber takes PAN as a skin layer, and the preparation method comprises the following specific steps:
(1) Adding polyacrylonitrile powder with molecular weight of 150000 into 4ml of dimethyl sulfoxide solution, controlling PAN solid content to 160mg/ml, and stirring for 3 hours under the water bath condition of 65 ℃ to form PAN spinning solution;
(2) Adding polyvinyl alcohol master batches (with the alcoholysis degree of 98%) into deionized water, stirring for 3 hours at 95 ℃ to prepare a 100mg/ml polyvinyl alcohol aqueous solution (PVA aqueous solution);
(3) Respectively carrying out ultrasonic treatment (with power of 100W) on the two spinning solutions prepared in the step (1) and the step (2) at room temperature for 15 minutes to carry out defoaming, and obtaining the porous fiber spinning solution after the defoaming is finished;
(4) Taking PVA aqueous solution as a core layer, taking PAN spinning solution as the core layer for coaxial/sheath-core wet spinning, and performing wet spinning on the spinning solution in a coagulating bath at 25 ℃ for coagulating and forming, wherein the coagulating time is 30min, so as to form a sheath-core structure yarn;
the advancing speed of the spinning solution of the core layer is 0.3ml/min, and the advancing speed of the spinning solution of the skin layer is 0.9ml/min;
the inner/outer diameter of the inner tube of the coaxial spray head is 0.41mm/0.72mm respectively; the inner diameter and the outer diameter of the outer tube of the coaxial spray head are respectively 1.01mm/1.49mm.
The coagulating bath is a calcium chloride/ethanol/water solution with the solid content of calcium chloride of 5 weight percent and the volume ratio of ethanol to deionized water of 1:3.
(5) Washing the skin-core structure yarn with deionized water, and then placing the washed skin-core structure yarn in a 60 ℃ oven for drying treatment for 300min to obtain PAN porous coaxial nascent fibers;
(6) Pre-oxidizing the prepared porous coaxial nascent fiber, heating to 250 ℃ at the speed of 2 ℃/min, and keeping for 120min;
(7) Heating the obtained pre-oxidized fiber to 800 ℃ at a speed of 4 ℃/min under the protection of inert gas, and keeping for 120min to obtain the hollow, porous and multi-layer polyacrylonitrile-based carbon fiber (PAN@PVA). The scanning electron microscope diagrams are shown in fig. 3 and 4.
As can be seen from fig. 3, PVA is carbonized and removed after high temperature treatment, leaving only a shell PAN-based carbon layer, radial aggressive finger-like channels formed on the cross section of the hollow fiber body can be observed, and a distinct hierarchical demarcation is presented. The presence of the "intermediate support layer" is evident from the high magnification picture of fig. 4. In the wet spinning solidification process, the non-solvent (water) in the coagulating bath and the water in the core PVA spinning solution exchange mass transfer from the two directions of the fiber skin and the core simultaneously and the DMSO in the spinning trickle, and macromolecules in the fiber are preferentially gathered to the region where the non-solvent (water) permeates sufficiently, so that the inner and outer wall skin pores of the hollow fiber are compact in size and relatively dense, the pore channel close to the middle core is relatively large, the migration speed of PAN macromolecules at the junction of the two directions of phase separation is the slowest, and finally the fiber is condensed to form a relatively dense 'middle supporting layer', so that the fiber has the characteristics of hollowness, multiple pores and multiple layers. The outer skin of the PAN-based hollow carbon fiber was observed to be very smooth and dense.
Test case
(1) Electrochemical performance test of fiber electrode
The carbon fibers obtained in examples 1-3 and example 6 were tested using an electrochemical workstation of the Shanghai Chenhua CHI660 type. The fiber performance is characterized by a three-electrode system, and the working voltage is-0.2-0.6V. The test content comprises a cyclic voltammogram, a constant current charge-discharge curve and an electrochemical impedance spectrum. As shown in FIG. 5a, the CV curve surrounding area of the hollow, porous, multi-level carbon fiber of PAN/MXene@PVA is the largest, which indicates that the energy storage effect is the best; as can be seen from the constant current charge-discharge curve (FIG. 5 b), the specific capacitance can reach 113.18F/g (0.5A/g), which is probably because the pore structure distribution is more reasonable, the introduction of MXene further improves the specific surface area of the material on the one hand, and the MXene sheet layer has better conductive and energy storage effects.
(2) Fiber electrode conductivity test
The resistance per unit length of the carbon fibers obtained in examples 1 to 3 and example 6 was measured using a TP-304 digital display multimeter, and the fiber diameter was measured using an optical microscope, and then the conductivity of the fibers was calculated according to formula (2):
wherein δ represents conductivity in Siemens per meter (S/m), L represents fiber length in meters (m), S represents fiber cross-sectional area in square meters (m) 2 ) R represents the resistance value in ohms (Ω) over the test length of the fiber.
As can be seen from FIG. 6, the PAN/MXene@PVA hollow, porous, multi-level carbon fiber exhibited a higher conductivity of about 546S/m.
(3) Mechanical property test of fiber electrode
The carbon fibers obtained in examples 1 to 3 and example 6 were subjected to tensile strength test with an XQ-2 monofilament strength tester, the clamping length was 1mm, the tensile speed was 1mm/min, and the strength and elongation of the fibers were calculated by sampling a plurality of times and a plurality of groups. As can be seen from fig. 7, the pan@pva hollow, porous, multi-level coaxial fiber has a post-carbonization strength of 15.23MPa, showing a higher strength.
Claims (6)
1. The hollow, porous and multi-layer polyacrylonitrile-based carbon fiber is characterized in that the preparation method 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 obtained in the step (1) in a drying 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 prepared in the step (2), heating to 200-250 ℃ at a speed of 2 ℃/min, and keeping for 60-180 min;
(4) Heating the pre-oxidized fiber in the step (4) to 700-900 ℃ at a speed of 4 ℃/min under the protection of inert gas, and keeping for 60-180 min to obtain hollow, porous and multi-layer polyacrylonitrile-based carbon fiber;
in the step (1), the raw material composition of the cortex spinning solution comprises polyacrylonitrile powder and dimethyl sulfoxide; the functional conductive active material is one or more of graphene oxide, carbon nano tube and Mxene;
the molecular weight of the polyacrylonitrile powder is 5000-150000, and the total solid content of the polyacrylonitrile and the functional conductive active material in the cortex spinning solution is 120-180 mg/mL; the functional conductive active material in the cortex spinning solution is 0.5-50% of the mass of polyacrylonitrile;
in the step (1), the core layer spinning solution is a polyvinyl alcohol aqueous solution with the concentration of 50-150 mg/mL;
the alcoholysis degree of the polyvinyl alcohol is 87% -98.8%.
2. The hollow, porous, multi-level polyacrylonitrile-based carbon fiber according to claim 1, wherein the preparation method of the sheath spinning solution is as follows: and adding the functional conductive active material and polyacrylonitrile powder into dimethyl sulfoxide, and stirring for 2-5 hours under the water bath condition of 65 ℃ until the functional conductive active material and the polyacrylonitrile powder are uniformly stirred and dispersed to form the homogeneous cortex spinning solution.
3. The hollow, porous, multi-layer polyacrylonitrile-based carbon fiber according to claim 1, wherein in step (1), the sheath/core spinning solution is subjected to a defoaming treatment before use, and the defoaming method is as follows: and carrying out ultrasonic treatment for 10-30 minutes at room temperature.
4. The hollow, porous, multi-level polyacrylonitrile-based carbon fiber of claim 3, wherein the power of the ultrasound is 80-120 w.
5. The hollow, porous, multi-layer polyacrylonitrile-based carbon fiber according to claim 1, wherein in the step (1), the advancing speed of the core spinning solution is 0.2-0.4 ml/min and the advancing speed of the sheath spinning solution is 0.7-1.2 ml/min in the coaxial/sheath-core wet spinning process.
6. The hollow, porous, multi-layer polyacrylonitrile-based carbon fiber according to claim 1, wherein in the step (1), in the coaxial/sheath-core wet spinning process, the solution trickle discharged by the coaxial nozzle enters a coagulation bath at 20-70 ℃ to be coagulated and formed, and the coagulation time is 10-40 min;
the coagulation bath is any one of the following:
(1) dimethyl sulfoxide aqueous solution with 30-70% of dimethyl sulfoxide volume content;
(2) the solid content of the calcium chloride is 5-25wt%, and the volume ratio of the ethanol deionized water is 1:3-3:1;
(3) deionized water.
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