Vanadium-boron-doped carbon fiber electrode, preparation method thereof and all-vanadium redox flow battery

Abstract

The invention discloses a vanadium-boron-doped carbon fiber electrode, a preparation method thereof and an all-vanadium redox flow battery, and the preparation method of the vanadium-boron-doped carbon fiber electrode comprises the following steps: taking a proper amount of nitrogen source, boron source, vanadium salt and organic chelate as preparation raw materials, dissolving in an ethanol aqueous solution, stirring and dispersing uniformly, and evaporating and drying to obtain a solid mixture; performing high-temperature pyrolysis treatment on the solid mixture, and performing ultrasonic dispersion in deionized water to obtain a vanadium-boron doped carbon nanotube; taking the vanadium-boron-doped carbon nanotube and polyacrylonitrile as matrixes, and adopting an electrostatic spinning method to obtain a nanofiber membrane; and performing pyrolysis treatment on the nanofiber membrane to obtain the vanadium-boron-doped carbon fiber electrode. According to the invention, the doping of the organic complex greatly improves the hydrophilicity of the electrode and the adsorption capacity to vanadium ions, and the doping of boron can form a V-B-C active site, and the V-B-C active site and the V-N-C active site form a synergistic effect, so that the electrochemical activity is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of liquid flow batteries, and in particular to a vanadium-boron-doped carbon fiber electrode, a preparation method thereof and an all-vanadium liquid flow battery. Background Art

[0002] All-vanadium liquid flow battery is one of the most promising technologies to solve the intermittent instability of renewable energy. It has the advantages of flexible design, long cycle life, high safety and environmental friendliness. The electrodes of all-vanadium liquid flow battery mainly provide the required active sites and electrons for the reaction, so the performance of the electrode directly affects the performance of the battery. In order to achieve better contact between the electrolyte and the electrode and faster electron transfer, high specific surface area and electrochemical activity are crucial for the electrode.

[0003] At present, the most widely used electrode is carbon felt. Although it has the advantages of good conductivity and corrosion resistance, its hydrophilicity and electrochemical activity are poor, which limits the rapid transfer of electrons and affects the large-scale use of all-vanadium liquid flow batteries. Summary of the invention

[0004] In order to overcome the deficiencies of the prior art, the object of the present invention is to provide a vanadium-boron-doped carbon fiber electrode, a preparation method thereof and an all-vanadium liquid flow battery, which have the advantages of high electrochemical activity and good hydrophilicity.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] According to a first aspect of an embodiment of the present disclosure, a method for preparing a vanadium-boron-doped carbon fiber electrode is provided, comprising:

[0007] Taking appropriate amounts of nitrogen source, boron source, vanadium salt and organic chelate as raw materials, dissolving them in ethanol aqueous solution, stirring and dispersing them uniformly, and evaporating and drying to obtain a solid mixture;

[0008] After subjecting the solid mixture to high-temperature pyrolysis treatment, ultrasonic dispersion in deionized water is performed to obtain vanadium-boron-doped carbon nanotubes;

[0009] Using the vanadium-boron-doped carbon nanotubes and polyacrylonitrile as a matrix, an electrostatic spinning method is used to obtain a nanofiber membrane;

[0010] The nanofiber membrane is subjected to pyrolysis treatment to obtain a vanadium-boron doped carbon fiber electrode.

[0011] In some exemplary embodiments, the nitrogen source is one or more of urea, dicyandiamide, melamine and graphite-like carbon nitride;

[0012] The boron source is one or both of boric acid and sodium borate;

[0013] The vanadium salt is one or more of vanadyl sulfate, vanadyl oxalate and ammonium metavanadate;

[0014] The organic chelate is one or more of phenanthroline, ammonium citrate and ethylenediamine;

[0015] The molar ratio of the vanadium salt, the boron source, the organic chelate and the nitrogen source is 1:(0.5-2):(1-6):(100-200);

[0016] The volume ratio of ethanol to water in the ethanol aqueous solution is 1:(0.5-5).

[0017] In some exemplary embodiments, the nitrogen source is urea, the boron source is boric acid, the vanadium salt is vanadyl oxalate, and the organic chelate is phenanthroline and ammonium citrate.

[0018] In some exemplary embodiments, after being stirred and dispersed uniformly, the mixture is evaporated and dried at a temperature of 90-120° C. to obtain a solid mixture.

[0019] In some exemplary embodiments, the high temperature pyrolysis treatment of the solid mixture is specifically:

[0020] The solid mixture is pyrolyzed under a nitrogen atmosphere at a temperature rising rate of 1-10° C. to a high temperature of 500-1050° C. for 1-5 hours.

[0021] In some exemplary embodiments, the vanadium-boron-doped carbon nanotubes are obtained by ultrasonic dispersion in deionized water as follows:

[0022] The solid mixture after pyrolysis is placed in deionized water at 30-38° C., and ultrasonically dispersed for 5-30 minutes under working conditions of 200-300W and 20-30kHz to obtain the vanadium-boron doped carbon nanotubes.

[0023] In some exemplary embodiments, the method of using the vanadium-boron-doped carbon nanotubes and polyacrylonitrile as a matrix to obtain a nanofiber membrane by electrospinning specifically includes:

[0024] After adding polyacrylonitrile with an average molecular weight of 100,000 into dimethylformamide, stirring continuously for 5-10 hours at a temperature of 40-80° C., an intermediate mixture is obtained, wherein the mass ratio of polyacrylonitrile to dimethylformamide is (5:95)-(20:80);

[0025] Adding vanadium-boron-doped carbon nanotubes to the intermediate mixture and continuing stirring for 5-10 hours to obtain an electrospinning solution, wherein the mass ratio of vanadium-boron-doped carbon nanotubes to polyacrylonitrile is (1-2):10;

[0026] The electrospinning solution is treated by an electrospinning method to obtain a nanofiber membrane.

[0027] In some exemplary embodiments, the electrospinning method is used to treat the electrospinning solution to obtain the nanofiber membrane, specifically:

[0028] The needle with a needle hole diameter of 0.2-4.0 mm is controlled to be 5-35 cm away from the spinning roller, a voltage of 10-45 kV is applied between the needle and the spinning roller, the injection flow rate is controlled to be 0.1-10 mL / h, the spinning roller speed is 100-2000 rpm, the spinning temperature is 10-40°C, and the ambient humidity is 20-50% RH for electrospinning treatment to obtain a nanofiber membrane with a thickness of 0.5-5 mm.

[0029] In some exemplary embodiments, the step of pyrolyzing the nanofiber membrane to obtain a vanadium-boron-doped carbon fiber electrode specifically includes:

[0030] Pyrolyzing the nanofiber membrane in an air atmosphere at a temperature of 100-300° C. for 1-5 h to obtain an intermediate membrane body;

[0031] The intermediate film body is pyrolyzed in a nitrogen atmosphere at a temperature of 500-1050° C. for 1-5 h to obtain a vanadium-boron-doped carbon fiber electrode.

[0032] According to a second aspect of an embodiment of the present disclosure, there is provided a vanadium-boron-doped carbon fiber electrode, which is prepared by the preparation process described in the first aspect.

[0033] According to a third aspect of an embodiment of the present disclosure, there is provided an all-vanadium liquid flow battery, comprising the vanadium-boron-doped carbon fiber electrode as described in the second aspect.

[0034] In summary, compared with the prior art, the present invention has the following beneficial effects:

[0035] The embodiment of the present invention provides a vanadium-boron-doped carbon fiber electrode, a preparation method thereof and an all-vanadium liquid flow battery, wherein the incorporation of an organic complex into the vanadium-boron-doped carbon fiber electrode greatly improves the hydrophilicity and adsorption capacity of the electrode for vanadium ions, and the incorporation of boron can form VBC active sites, which form a synergistic effect with the VNC active sites, thereby improving the electrochemical activity; the all-vanadium liquid flow battery using the vanadium-boron-doped carbon fiber electrode can maintain a high energy efficiency and has excellent cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 1 is a diagram showing the relationship between voltage and time in Example 1 and Example 2 of the present invention. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] A first aspect of an embodiment of the present invention provides a method for preparing a vanadium-boron-doped carbon fiber electrode, comprising:

[0039] S100: taking appropriate amounts of nitrogen source, boron source, vanadium salt and organic chelate as raw materials, dissolving them in ethanol aqueous solution, stirring and dispersing them uniformly, and evaporating and drying to obtain a solid mixture.

[0040] Specifically, the nitrogen source is one or more of urea, dicyandiamide, melamine and graphite-like carbon nitride, preferably, the nitrogen source is urea; the boron source is one or both of boric acid or sodium borate, preferably, the boron source is boric acid; the vanadium salt is one or more of vanadyl sulfate, vanadyl oxalate and ammonium metavanadate, preferably, the vanadium salt is vanadyl oxalate; the organic chelate is one or more of phenanthroline, ammonium citrate and ethylenediamine, preferably, the organic chelate is phenanthroline and ammonium citrate, phenanthroline and ammonium citrate may be added separately, or a mixture of phenanthroline and ammonium citrate may be directly added; the molar ratio of the vanadium salt, the boron source, the organic chelate and the nitrogen source is 1:(0.5-2):(1-6):(100-200).

[0041] The ethanol aqueous solution is composed of ethanol and water, wherein the volume ratio of ethanol to water in the ethanol aqueous solution is 1:(0.5-5). A nitrogen source, a boron source, a vanadium salt and an organic chelate are added into the ethanol aqueous solution, stirred and dispersed uniformly, and then evaporated and dried at a temperature of 90-120 DEG C to obtain a solid mixture, wherein the evaporation and drying is based on complete evaporation of water.

[0042] S200: subjecting the solid mixture to high-temperature pyrolysis treatment, and then ultrasonically dispersing the solid mixture in deionized water to obtain vanadium-boron-doped carbon nanotubes.

[0043] Specifically, the high-temperature pyrolysis treatment of the solid mixture is as follows: the solid mixture is pyrolyzed for 1-5 hours at a high temperature of 500-1050°C at a heating rate of 1-10°C under a nitrogen atmosphere; after the pyrolysis treatment and cooling, the solid mixture is added to deionized water for ultrasonic dispersion, and vanadium-boron-doped carbon nanotubes are formed after the solid mixture is dispersed.

[0044] The vanadium-boron-doped carbon nanotubes are obtained after ultrasonic dispersion in deionized water as follows: the solid mixture after pyrolysis is placed in deionized water at 30-38°C, and ultrasonically dispersed for 5-30 minutes under working conditions of 200-300W and 20-30kHz to obtain the vanadium-boron-doped carbon nanotubes. Preferably, the temperature of the deionized water is 35°C. The ultrasonic dispersion process can be achieved, for example, by using an ultrasonic generator.

[0045] S300: Nanofiber membrane is obtained by electrospinning using vanadium-boron-doped carbon nanotubes and polyacrylonitrile as the matrix.

[0046] Step S300 specifically includes:

[0047] S301: After adding polyacrylonitrile with an average molecular weight of 100,000 to dimethylformamide, stirring is continued for 5-10 hours at a temperature of 40-80° C. to obtain an intermediate mixture, wherein the mass ratio of polyacrylonitrile to dimethylformamide is (5:95)-(20:80).

[0048] S302: adding vanadium-boron-doped carbon nanotubes to the intermediate mixture and continuing stirring for 5-10 hours to obtain an electrospinning solution, wherein the mass ratio of vanadium-boron-doped carbon nanotubes to polyacrylonitrile is (1-2):10.

[0049] S303: The electrospinning solution is treated by electrospinning to obtain a nanofiber membrane. The thickness of the nanofiber membrane is 0.5-5mm. The specific method of the electrospinning method is to add high voltage static electricity to the electrospinning solution. The charged polymer droplets are accelerated at the top of the Taylor cone of the capillary under the action of the electric field force. When the electric field force is large enough, the polymer droplets overcome the surface tension to form a jet stream. The solvent evaporates or solidifies during the jetting process and finally falls on the receiving device to form a fiber felt similar to a non-woven fabric. The fiber felt is the nanofiber membrane. The device for electrospinning processing can adopt the existing structure.

[0050] In this embodiment, the electrospinning method is used to process the electrospinning solution to obtain the nanofiber membrane:

[0051] Take a syringe with a capacity of 50-1000mL to hold the electrospinning liquid. The front end of the syringe is a needle. The needle with a needle hole diameter of 0.2-4.0mm is controlled to be 5-35cm away from the spinning roller. A voltage of 10-45kV is applied between the needle and the spinning roller. The injection flow rate is controlled to be 0.1-10mL / h, the spinning roller speed is 100-2000rpm, the spinning temperature is 10-40℃, and the ambient humidity is 20-50%RH (Relative Humidity) for electrospinning treatment to obtain a nanofiber membrane with a thickness of 0.5-5mm.

[0052] S400: Pyrolysis treatment is performed on the nanofiber membrane to obtain a vanadium-boron doped carbon fiber electrode.

[0053] Step S400 specifically includes:

[0054] S401: Pyrolyze the nanofiber membrane in an air atmosphere at a temperature of 100-300° C. for 1-5 h to obtain an intermediate membrane body.

[0055] S402: Pyrolyze the intermediate film in a nitrogen atmosphere at 500-1050° C. for 1-5 h to obtain a vanadium-boron-doped carbon fiber electrode.

[0056] A second aspect of an embodiment of the present invention provides a vanadium-boron-doped carbon fiber electrode, which is prepared by the preparation process of the first aspect.

[0057] In the vanadium-boron-doped carbon fiber electrode prepared by the above preparation method, the incorporation of organic complexes greatly improves the hydrophilicity and adsorption capacity of vanadium ions of the electrode, and the incorporation of boron can form VBC active sites, which form a synergistic effect with the VNC active sites, thereby improving the electrochemical activity; the all-vanadium liquid flow battery using the vanadium-boron-doped carbon fiber electrode can maintain a high energy efficiency and has excellent cycle performance.

[0058] The third aspect of the embodiment of the present invention provides an all-vanadium liquid flow battery, including the vanadium-boron-doped carbon fiber electrode as in the second aspect, the all-vanadium liquid flow battery mainly includes: a positive electrode storage tank, a negative electrode storage tank, a battery stack and an infusion pump, the positive electrode storage tank is connected to the positive electrode side of the battery stack, the negative electrode storage tank is connected to the negative electrode side of the battery stack, the positive electrode storage tank stores positive electrode electrolyte, the negative electrode storage tank stores negative electrode electrolyte, an infusion pump is provided between the positive electrode storage tank and the battery stack, and between the negative electrode storage tank and the battery stack, and the positive electrode electrolyte enters the positive electrode side of the battery stack. The battery stack is electrolyzed after electrolysis and then flows back to the positive electrode storage tank, and the negative electrode electrolyte enters the battery stack from the negative electrode side of the battery stack, is electrolyzed and then flows back to the negative electrode storage tank; the positive and negative sides of the battery stack are separated by a diaphragm, and the positive electrode and the negative electrode are respectively provided on both sides of the diaphragm in the battery stack, wherein the positive electrode and the negative electrode are both vanadium-boron-doped carbon fiber electrodes. It can be understood that the specific structure of the all-vanadium liquid flow battery is the same as the prior art, the only difference is that the electrodes in the battery stack are vanadium-boron-doped carbon fiber electrodes.

[0059] The performance of the vanadium-boron-doped carbon fiber electrode of the present invention is described below in conjunction with several specific embodiments:

[0060] Example 1

[0061] A method for preparing a vanadium-boron doped carbon fiber electrode, comprising:

[0062] S100: Take 100g urea as a nitrogen source, 3g boric acid as a boron source, 2g vanadyl oxalate as a vanadium source, 8g phenanthroline and 8g ammonium citrate as organic chelates, dissolve in 10L ethanol aqueous solution, the volume ratio of ethanol to water in the ethanol aqueous solution is 1:1, stir and disperse evenly, and evaporate to dryness at 100°C to obtain a solid mixture.

[0063] S200: The solid mixture is pyrolyzed for 2 hours under a nitrogen atmosphere at a heating rate of 10°C to 900°C. After cooling, it is ultrasonically dispersed in deionized water to obtain vanadium-boron-doped carbon nanotubes.

[0064] S300: Add 100g of polyacrylonitrile with an average molecular weight of 100,000 to 900g of dimethylformamide, and continue stirring at a temperature of 50°C for 5 hours to obtain an intermediate mixture, then add 10g of vanadium-boron-doped carbon nanotubes, and continue stirring at a temperature of 50°C for 5 hours to obtain an electrospinning solution, and use an electrospinning method to treat the electrospinning solution to obtain a nanofiber membrane. The thickness of the nanofiber membrane is 3mm.

[0065] S400: Pyrolyze the nanofiber membrane in an air atmosphere at 200°C for 3 hours to obtain an intermediate membrane body, and then pyrolyze the intermediate membrane body in a nitrogen atmosphere at 950°C for 3 hours to obtain a vanadium-boron-doped carbon fiber electrode.

[0066] The vanadium-boron-doped carbon fiber electrode prepared by the preparation method was assembled into an all-vanadium liquid flow battery system for charge and discharge tests to evaluate the electrochemical performance of the vanadium-boron-doped carbon fiber electrode. -2 Under the current density conditions, the voltage efficiency of the all-vanadium liquid flow battery was tested to be 98% and the energy efficiency was 83%.

[0067] Example 2

[0068] A method for preparing a vanadium-boron doped carbon fiber electrode, comprising:

[0069] S100: Take 100g of urea as a nitrogen source, 2g of vanadyl oxalate as a vanadium source, 8g of phenanthroline and 8g of ammonium citrate as organic chelates, dissolve them in 10L of ethanol aqueous solution, the volume ratio of ethanol to water in the ethanol aqueous solution is 1:1, stir and disperse evenly, and evaporate to dryness at 100°C to obtain a solid mixture.

[0070] S200: The solid mixture is pyrolyzed for 2 hours under a nitrogen atmosphere at a heating rate of 10°C to 900°C. After cooling, it is ultrasonically dispersed in deionized water to obtain vanadium-boron-doped carbon nanotubes.

[0071] S300: Add 100g of polyacrylonitrile with an average molecular weight of 100,000 to 900g of dimethylformamide, and continue stirring at a temperature of 50°C for 5 hours to obtain an intermediate mixture, then add 10g of vanadium-boron-doped carbon nanotubes, and continue stirring at a temperature of 50°C for 5 hours to obtain an electrospinning solution, and use an electrospinning method to treat the electrospinning solution to obtain a nanofiber membrane. The thickness of the nanofiber membrane is 3mm.

[0072] S400: Pyrolyze the nanofiber membrane in an air atmosphere at 200°C for 3 hours to obtain an intermediate membrane body, and then pyrolyze the intermediate membrane body in a nitrogen atmosphere at 950°C for 3 hours to obtain a vanadium-boron-doped carbon fiber electrode.

[0073] The vanadium-boron-doped carbon fiber electrode prepared by the preparation method was assembled into an all-vanadium liquid flow battery system for charge and discharge tests to evaluate the electrochemical performance of the vanadium-boron-doped carbon fiber electrode. -2 Under the current density conditions, the voltage efficiency of the all-vanadium liquid flow battery was tested to be 91% and the energy efficiency was 77%.

[0074] Compared with Example 1, it is obvious that when no boron source is added, the voltage efficiency and energy efficiency of the all-vanadium liquid flow battery are significantly reduced, indicating that the incorporation of a boron source to form VBC active sites can effectively improve the electrochemical activity.

[0075] Example 3

[0076] A method for preparing a vanadium-boron doped carbon fiber electrode, comprising:

[0077] S100: Take 100g urea as a nitrogen source, 3g boric acid as a boron source, 2g vanadyl oxalate as a vanadium source, and 8g ammonium citrate as an organic chelate, dissolve them in 10L ethanol aqueous solution, where the volume ratio of ethanol to water in the ethanol aqueous solution is 1:1, stir and disperse evenly, and evaporate and dry at 100°C to obtain a solid mixture.

[0078] S200: The solid mixture is pyrolyzed for 2 hours under a nitrogen atmosphere at a heating rate of 10°C to 900°C. After cooling, it is ultrasonically dispersed in deionized water to obtain vanadium-boron-doped carbon nanotubes.

[0079] S300: Add 100g of polyacrylonitrile with an average molecular weight of 100,000 to 900g of dimethylformamide, and continue stirring at a temperature of 50°C for 5 hours to obtain an intermediate mixture, then add 10g of vanadium-boron-doped carbon nanotubes, and continue stirring at a temperature of 50°C for 5 hours to obtain an electrospinning solution, and use an electrospinning method to treat the electrospinning solution to obtain a nanofiber membrane. The thickness of the nanofiber membrane is 3mm.

[0080] S400: Pyrolyze the nanofiber membrane in an air atmosphere at 200°C for 3 hours to obtain an intermediate membrane body, and then pyrolyze the intermediate membrane body in a nitrogen atmosphere at 950°C for 3 hours to obtain a vanadium-boron-doped carbon fiber electrode.

[0081] The vanadium-boron-doped carbon fiber electrode prepared by the preparation method was assembled into an all-vanadium liquid flow battery system for charge and discharge tests to evaluate the electrochemical performance of the vanadium-boron-doped carbon fiber electrode. -2 Under the current density conditions, the voltage efficiency of the all-vanadium liquid flow battery was tested to be 93% and the energy efficiency was 81%.

[0082] Compared with Example 1, it can be seen that the organic chelate using phenanthroline and ammonium citrate is more conducive to improving the hydrophilicity of the electrode and the adsorption capacity of vanadium ions than using only ammonium citrate, thereby improving the voltage efficiency and energy efficiency of the all-vanadium redox flow battery.

[0083] The above embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which are equivalent modifications and improvements made to the above embodiments based on the essential technology of the present invention, and all of them belong to the protection scope of the present invention.

Claims

1. A method for preparing a vanadium-boron-doped carbon fiber electrode, characterized in that: include: Taking appropriate amounts of nitrogen source, boron source, vanadium salt and organic chelate as raw materials, dissolving them in ethanol aqueous solution, stirring and dispersing them uniformly, and evaporating and drying to obtain a solid mixture; After subjecting the solid mixture to high-temperature pyrolysis treatment, ultrasonic dispersion in deionized water is performed to obtain vanadium-boron-doped carbon nanotubes; Using the vanadium-boron-doped carbon nanotubes and polyacrylonitrile as a matrix, an electrostatic spinning method is used to obtain a nanofiber membrane; The nanofiber membrane is subjected to pyrolysis treatment to obtain a vanadium-boron doped carbon fiber electrode.

2. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 1, characterized in that: The nitrogen source is one or more of urea, dicyandiamide, melamine and graphite-like carbon nitride; The boron source is one or both of boric acid and sodium borate; The vanadium salt is one or more of vanadyl sulfate, vanadyl oxalate and ammonium metavanadate; The organic chelate is one or more of phenanthroline, ammonium citrate and ethylenediamine; The molar ratio of the vanadium salt, the boron source, the organic chelate and the nitrogen source is 1:(0.5-2):(1-6):(100-200); The volume ratio of ethanol to water in the ethanol aqueous solution is 1:(0.5-5).

3. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 2, characterized in that: The nitrogen source is urea, the boron source is boric acid, the vanadium salt is vanadyl oxalate, and the organic chelate is phenanthroline and ammonium citrate.

4. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 2, characterized in that: After being stirred and dispersed uniformly, the mixture is evaporated and dried at a temperature of 90-120° C. to obtain a solid mixture.

5. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 1, characterized in that: The high temperature pyrolysis treatment of the solid mixture is specifically as follows: The solid mixture is pyrolyzed under a nitrogen atmosphere at a temperature rising rate of 1-10° C. to a high temperature of 500-1050° C. for 1-5 hours.

6. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 5, characterized in that: The vanadium-boron-doped carbon nanotubes are obtained by ultrasonic dispersion in deionized water as follows: The solid mixture after pyrolysis is placed in deionized water at 30-38° C., and ultrasonically dispersed for 5-30 minutes under working conditions of 200-300W and 20-30kHz to obtain the vanadium-boron doped carbon nanotubes.

7. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 1, characterized in that: The method of using the vanadium-boron-doped carbon nanotubes and polyacrylonitrile as a matrix to obtain a nanofiber membrane by an electrospinning method specifically includes: After adding polyacrylonitrile with an average molecular weight of 100,000 into dimethylformamide, stirring continuously for 5-10 hours at a temperature of 40-80° C., an intermediate mixture is obtained, wherein the mass ratio of polyacrylonitrile to dimethylformamide is (5:95)-(20:80); Adding vanadium-boron-doped carbon nanotubes to the intermediate mixture and continuing stirring for 5-10 hours to obtain an electrospinning solution, wherein the mass ratio of vanadium-boron-doped carbon nanotubes to polyacrylonitrile is (1-2):10; The electrospinning solution is treated by an electrospinning method to obtain a nanofiber membrane.

8. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 7, characterized in that: The electrospinning method is used to treat the electrospinning solution to obtain the nanofiber membrane, specifically: The needle with a needle hole diameter of 0.2-4.0 mm is controlled to be 5-35 cm away from the spinning roller, a voltage of 10-45 kV is applied between the needle and the spinning roller, the injection flow rate is controlled to be 0.1-10 mL / h, the spinning roller speed is 100-2000 rpm, the spinning temperature is 10-40°C, and the ambient humidity is 20-50% RH for electrospinning treatment to obtain a nanofiber membrane with a thickness of 0.5-5 mm.

9. The method for preparing vanadium-boron-doped carbon fiber electrode according to claim 1, characterized in that: The step of pyrolyzing the nanofiber membrane to obtain the vanadium-boron-doped carbon fiber electrode specifically includes: Pyrolyzing the nanofiber membrane in an air atmosphere at a temperature of 100-300° C. for 1-5 h to obtain an intermediate membrane body; The intermediate film body is pyrolyzed in a nitrogen atmosphere at a temperature of 500-1050° C. for 1-5 h to obtain a vanadium-boron-doped carbon fiber electrode.

10. A vanadium-boron doped carbon fiber electrode, characterized in that: The method is prepared by the preparation process described in any one of claims 1 to 9.

11. An all-vanadium liquid flow battery, characterized in that: It comprises the vanadium-boron-doped carbon fiber electrode as described in claim 10.

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