Preparation method of carbon nanofiber rich in end face defect and application of carbon nanofiber in all-vanadium redox flow battery electrode material

Abstract

The invention belongs to the technical field of battery materials, and particularly discloses a preparation method of carbon nanofibers rich in end face defects and application of the carbon nanofibers in an all-vanadium redox flow battery electrode material. The method comprises the following steps: adding carbon black particles rich in end faces and defects into a mixed solution of PAN and DMF to obtain a precursor solution containing a carbon black functional component, and preparing a fiber surface shell structure; in addition, a blank precursor solution only containing PAN and DMF is prepared and used for preparing a core structure in the fiber; carrying out continuous spinning by adopting a coaxial electrostatic spinning technology to obtain an original core-shell composite fiber material; and carrying out pre-oxidation treatment and carbonization treatment on the obtained original composite fiber material to obtain the core-shell carbon nanofiber material of which the surface is loaded with carbon black end surface defects and the interior is mainly of a graphite microcrystalline structure. When the material is applied to an all-vanadium redox flow battery as an electrode material, the electrochemical reaction of vanadium ions can be catalyzed, and the battery efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and in particular to a method for preparing carbon nanofibers rich in end face defects and application of the carbon nanofibers in all-vanadium redox flow batteries. Background Art

[0002] Efficiently utilizing renewable energy, reducing traditional energy use, and accelerating energy transformation are key to ensuring long-term energy supply, improving the human living environment, and achieving sustainable development. However, renewable energy suffers from intermittent and volatile characteristics, necessitating the reliance on large-scale, safe, and stable energy storage technologies to achieve stable energy output and efficient utilization. Aqueous flow batteries are one of the leading technologies for large-scale, long-duration energy storage. Among them, all-vanadium redox flow batteries offer the greatest industrial potential and the highest level of technical maturity, offering advantages such as ease of scalability, long cycle life, high safety, fast response, and large capacity.

[0003] In vanadium batteries, porous electrodes provide channels for the flow of vanadium electrolyte and serve as sites for electrochemical reactions, decisively influencing battery performance. Currently, the most widely used flow battery electrode material is PAN-based carbon fiber, which offers advantages such as low cost, high conductivity, and stable properties. However, due to its poor surface hydrophilicity, low specific surface area, and limited number of active sites, its electrochemical activity is poor. By introducing catalysts onto its surface, the specific surface area, hydrophilicity, and reaction activity of the electrode can be increased to varying degrees.

[0004] Electrospinning is a mature technology for preparing functional carbon nanofibers. By manipulating the precursor solution composition and spinning parameters, the composition and structure of carbon fiber porous electrodes can be optimized. However, conventional electrospinning, when adding functional components to the precursor solution, inevitably results in most of the functional components being encapsulated within the fiber, resulting in catalyst waste and compromising the fiber's mechanical properties. Summary of the Invention

[0005] The present invention aims to provide a method for preparing end-face defect-rich carbon nanofibers and their application in all-vanadium redox flow batteries. Utilizing coaxial electrospinning technology, functional components are introduced into a shell precursor solution, while the interior is a continuous PAN fiber. This approach facilitates the uniform distribution of functional components on the electrode surface, increasing their effective utilization rate. It also ensures a single, homogeneous, and continuous structure within the fiber, resulting in excellent mechanical stability and electrochemical activity for the core-shell fiber.

[0006] To achieve the above object, the present invention adopts a technical solution: a method for preparing carbon nanofibers rich in end face defects, which comprises the following steps: 1) adding carbon black to a mixed solution of PAN and DMF, heating and stirring, and mixing uniformly to obtain a precursor solution for preparing a fiber shell layer; the carbon black has a particle size of 0.05 μm to 1 μm, the molecular weight of PAN is 90,000 to 150,000, the mass ratio of carbon black to PAN is 1:50 to 1:500, and the concentration of PAN in the mixed solution of PAN and DMF is 8 wt% to 18 wt%; 2) preparing a blank solution containing only PAN and DMF to obtain a precursor solution for preparing the fiber core; the concentration of PAN in the blank solution containing only PAN and DMF is 10 wt% to 20 wt%; 3) using a coaxial electrospinning device, injecting the two precursor solutions prepared in step 1) and step 2) into different syringes to provide fiber shell and core layer raw materials, respectively, and performing electrospinning to obtain a raw core-shell composite fiber material; 4) Flattening the original core-shell nanofiber material obtained in step 3), placing it in a tube furnace for pre-oxidation and carbonization treatment, and then cooling it to room temperature to obtain a carbon nanofiber electrode material rich in end face defects.

[0007] Furthermore: in step 3), the electrospinning conditions are as follows: the stainless steel nozzle model is G15~G22; the collector is a stainless steel roller, and the roller speed is 50~200r / min; the voltage between the nozzle and the roller is 16~23kV; the distance between the nozzle and the roller is 8~14cm; the spinning temperature is 25℃~40℃; the spinning humidity is 30%~60%RH; and the pushing speed is 10~50µL / min.

[0008] Further: In step 4), the conditions for the pre-oxidation treatment are: in an air atmosphere, heating to 260°C~290°C at a heating rate of 1°C / min~5°C / min, and keeping warm for 30min~60min; the conditions for the carbonization treatment are: in an inert atmosphere, heating at a rate of 2°C / min~5°C / min, and keeping warm at 900°C~1300°C for 1h~3h.

[0009] The present invention also discloses the use of the carbon nanofibers rich in end face defects prepared by the above preparation method as vanadium battery electrode materials in all-vanadium redox flow batteries.

[0010] The advantages of the present invention are: 1. Using coaxial electrospinning technology, the surface shell distribution of active substances is achieved, and core-shell carbon fibers with continuous carbon fibers as the core are prepared; this method is simple and easy to implement.

[0011] 2. The present invention introduces cheap, readily available carbon black material rich in end-face defects into the surface of carbon nanofibers, which can further increase the active specific surface area of the material, give full play to the role of the active material, effectively improve electrode performance, and reduce material costs.

[0012] 3. The heat treatment process further activates the end surface, improves the electrocatalytic activity and hydrophilicity of the material, and thus improves the electrode performance, effectively reduces battery polarization, and improves the energy conversion efficiency of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a scanning electron microscope photograph of the fiber; Among them: a is a scanning electron microscope image of blank carbon nanofiber electrode material ECNFs; b is a scanning electron microscope image of carbon nanofiber BC-ECNFs rich in end face defects; Figure 2 These are the cyclic voltammetry curves of the ECNFs blank carbon nanofiber electrode material prepared in the comparative example and the BC-ECNFs carbon nanofiber electrode material prepared in Example 1 in vanadium electrolyte.

[0014] Figure 3 Figures 1 and 2 are charge and discharge curves (a) and battery efficiency diagrams (bc) of two groups of vanadium batteries constructed in Example 2 at different current densities.

[0015] in: CE-ECNFs: Coulombic efficiency of the battery with blank ECNFs as electrodes; EE-ECNFs: Energy efficiency of batteries with blank ECNFs as electrodes; VE-ECNFs: voltage efficiency of cells with blank ECNFs as electrodes; CE-BC-ECNFs: Coulombic efficiency of batteries using BC-ECNFs rich in end-face defect carbon as electrodes; EE-BC-ECNFs: Energy efficiency of batteries using BC-ECNFs rich in end-face defect carbon as electrodes; VE-BC-ECNFs: Voltage efficiency of batteries using BC-ECNFs rich in end-face defect carbon as electrodes. DETAILED DESCRIPTION

[0016] In order to enable those skilled in the art to better understand the technical solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0017] The present invention discloses a method for preparing carbon nanofibers rich in end face defects, which comprises the following steps: 1) adding carbon black to a mixed solution of PAN and DMF, heating and stirring, and mixing uniformly to obtain a precursor solution for preparing a fiber shell layer; the carbon black has a particle size of 0.05 μm to 1 μm, the molecular weight of PAN is 90,000 to 150,000, the mass ratio of carbon black to PAN is 1:50 to 1:500, and the concentration of PAN in the mixed solution of PAN and DMF is 8 wt% to 18 wt%; 2) preparing a blank solution containing only PAN and DMF to obtain a precursor solution for preparing the fiber core; the concentration of PAN in the blank solution containing only PAN and DMF is 10 wt% to 20 wt%; 3) using a coaxial electrospinning device, injecting the two precursor solutions prepared in step 1) and step 2) into different syringes to provide fiber shell and core layer raw materials, respectively, and performing electrospinning to obtain a raw core-shell composite fiber material; 4) Flattening the original core-shell nanofiber material obtained in step 3), placing it in a tube furnace for pre-oxidation and carbonization treatment, and then cooling it to room temperature to obtain a carbon nanofiber electrode material rich in end face defects.

[0018] In step 3), the electrospinning conditions are as follows: the stainless steel nozzle model is G15-G22; the collector is a stainless steel roller, and the roller speed is 50-200 r / min; the voltage between the nozzle and the roller is 16-23 kV; the distance between the nozzle and the roller is 8-14 cm; the spinning temperature is 25°C-40°C; the spinning humidity is 30%-60% RH; and the pushing speed is 10-50 µL / min.

[0019] Wherein: in step 4), the conditions for the pre-oxidation treatment are: in an air atmosphere, heating to 260°C~290°C at a heating rate of 1°C / min~5°C / min, and keeping warm for 30min~60min; the conditions for the carbonization treatment are: in an inert atmosphere, heating at a rate of 2°C / min~5°C / min, and keeping warm at 900°C~1300°C for 1h~3h.

[0020] The present invention also discloses the use of the carbon nanofibers rich in end face defects prepared by the above preparation method as vanadium battery electrode materials in all-vanadium redox flow batteries.

[0021] The present invention is further described below by way of examples.

[0022] Comparative example: blank carbon nanofiber electrode material (ECNF).

[0023] The preparation method comprises the following steps: 1) Preparation of blank electrospinning precursor solution: PAN (polyacrylonitrile) powder with a molecular weight of 150,000 was dried and dissolved in DMF (N,N,-dimethylformamide) and magnetically stirred at 80°C for 6 h until completely dissolved to obtain a blank PAN / DMF electrospinning precursor solution with a mass percentage concentration of 14 wt%; 2) Preparation of blank PAN-based nanofiber materials by electrospinning: The blank PAN / DMF electrospinning precursor solution was drawn into the syringe of the electrospinning equipment for electrospinning to obtain the original blank nanofiber material. Electrospinning conditions: stainless steel nozzle model G20, stainless steel roller collector, roller speed of 100 rpm, voltage between nozzle and roller of 20 kV, distance between nozzle and roller of 12 cm, spinning temperature of 25°C, spinning humidity of 30% RH, and feed rate of 30 µL / min. 3) Preparation of blank carbon nanofiber electrode materials by pre-oxidation and carbonization: The obtained original blank PAN nanofiber material was flattened with a corundum plate, placed in a tube furnace, and heated to 290°C at a heating rate of 1°C / min in an air atmosphere and kept warm for 60 minutes to complete the pre-oxidation treatment; then it was carbonized, that is, in a N2 atmosphere, the heating rate was 5°C / min, and the temperature was kept at 1000°C for 120 minutes, and then cooled to room temperature to obtain blank carbon nanofiber electrode materials (ECNFs).

[0024] like Figure 1 As shown in (a), the diameter of the obtained blank carbon nanofiber electrode material is about 100~300nm and the surface is relatively smooth.

[0025] Example 1: Carbon nanofiber electrode material rich in end defects (BC-ECNFs).

[0026] The preparation method comprises the following steps: 1) Preparation of electrospinning precursor solution PAN (PAN) powder with a molecular weight of 150,000 was dried and dissolved in DMF (DMF) with magnetic stirring at 80°C for 6 hours until completely dissolved, resulting in a 12 wt% PAN / DMF precursor solution for the fiber core. A 14 wt% PAN / DMF precursor solution was prepared similarly, and carbon black nanoparticles were added to it at a carbon black:PAN ratio of 1:20 to prepare the fiber shell.

[0027] 2) Preparation of original composite fiber materials by electrospinning Two precursor solutions, 12 wt% PAN / DMF and 14 wt% PAN / DMF containing carbon black, were injected into separate syringes connected to the two inlets of a coaxial needle. The former served as the core component, and the latter as the shell component. Continuous electrospinning was performed using electrospinning technology to obtain the raw fiber material. The electrospinning conditions were: a G20 stainless steel nozzle, a stainless steel roller with a speed of 100 rpm, a voltage of 20 kV between the nozzle and the roller, a distance of 12 cm between the nozzle and the roller, a spinning temperature of 25°C, a spinning humidity of 30% RH, and a feed rate of 30 µL / min.

[0028] 3) Preparation of end-face defect-rich carbon nanofiber electrode materials (BC-ECNFs) by pre-oxidation and carbonization The obtained original nanofiber material was flattened with a corundum plate and placed in a tube furnace for pre-oxidation treatment, that is, in an air atmosphere, the temperature was raised to 290°C at a heating rate of 1°C / min and kept warm for 60 minutes; then it was carbonized, that is, in a N2 atmosphere, the temperature was raised at a rate of 5°C / min, kept warm at 1000°C for 120 minutes, and then cooled to room temperature to obtain a carbon nanofiber electrode material rich in end face defects (BC-ECNFs).

[0029] like Figure 1 As shown in (b), the fiber diameter of the obtained carbon nanofiber electrode material BC-ECNFs rich in end face defects is not much different from that of the original ECNFs, which is about 200~300nm; but the fiber surface is loaded with flaky carbon black particles with a size of hundreds of nanometers, and the edges of its flakes are somewhat protruding from the surface. This type of material may have more end face carbon, thus showing a higher specific surface area and electrochemical activity.

[0030] Example 2: End-defect-rich carbon nanofiber electrode material (BC-ECNFs) The preparation method comprises the following steps: 1) Preparation of electrospinning precursor solution PAN (PAN) powder with a molecular weight of 150,000 was dried and dissolved in DMF (DMF) at 80°C under magnetic stirring for 6 hours until completely dissolved, resulting in an 8wt% PAN / DMF precursor solution for the fiber core. A 10wt% PAN / DMF precursor solution was prepared similarly, and carbon black nanoparticles were added to it at a carbon black:PAN mass ratio of 1:500 to prepare the fiber shell.

[0031] 2) Preparation of original composite fiber materials by electrospinning Two precursor solutions, 8wt% PAN / DMF and 10wt% PAN / DMF containing carbon black, were injected into separate syringes connected to the two feed ports of a coaxial pillow. The former served as the core component, and the latter served as the shell component. Continuous electrospinning was performed using electrospinning technology to obtain the raw fiber material. The electrospinning conditions were: a G20 stainless steel nozzle, a stainless steel roller with a speed of 100 rpm, a voltage of 20 kV between the nozzle and the roller, a distance of 10 cm between the nozzle and the roller, a spinning temperature of 25°C, a spinning humidity of 30% RH, and a feed rate of 50 µL / min.

[0032] 3) Preparation of end-face defect-rich carbon nanofiber electrode materials (BC-ECNFs) by pre-oxidation and carbonization The obtained original nanofiber material was flattened with a corundum plate and placed in a tube furnace for pre-oxidation treatment, that is, in an air atmosphere, the temperature was raised to 280°C at a heating rate of 1°C / min and kept warm for 60 minutes; then it was carbonized, that is, in a N2 atmosphere, the temperature was raised at a rate of 5°C / min, kept warm at 1000°C for 120 minutes, and then cooled to room temperature to obtain a carbon nanofiber electrode material BC-ECNFs rich in end face defects.

[0033] The fiber diameter of the obtained end-face defect-rich carbon nanofiber electrode material BC-ECNFs is not much different from that of the original ECNFs, about 100~200nm; the fiber surface is loaded with carbon black particles and has more end-face carbon defects, which can effectively improve the active specific surface area and electrochemical performance of the electrode material.

[0034] Example 3: BC-ECNFs, a carbon nanofiber electrode material rich in end defects The preparation method comprises the following steps: 1) Preparation of electrospinning precursor solution PAN (PAN) powder with a molecular weight of 150,000 was dried and dissolved in DMF (DMF) with magnetic stirring at 80°C for 6 hours until completely dissolved, resulting in an 18 wt% PAN / DMF precursor solution for the fiber core. A 20 wt% PAN / DMF precursor solution was prepared similarly, and carbon black nanoparticles were added to it at a carbon black:PAN ratio of 1:50 to prepare the fiber shell.

[0035] 2) Preparation of original composite fiber materials by electrospinning Two precursor solutions, 18wt% PAN / DMF and 20wt% PAN / DMF containing carbon black, were injected into separate syringes connected to the two feed ports of a coaxial pillow. The former served as the core component, and the latter served as the shell component. Continuous electrospinning was performed using electrospinning technology to obtain the raw fiber material. The electrospinning conditions included a G20 stainless steel nozzle, a stainless steel roller with a speed of 100 rpm, a voltage of 20 kV between the nozzle and the roller, a distance of 14 cm between the nozzle and the roller, a spinning temperature of 25°C, a spinning humidity of 30% RH, and a feed rate of 10 µL / min.

[0036] 3) Preparation of end-face defect-rich carbon nanofiber electrode material BC-ECNFs by pre-oxidation and carbonization The obtained original nanofiber material was flattened with a corundum plate and placed in a tube furnace for pre-oxidation treatment, that is, in an air atmosphere, the temperature was raised to 300°C at a heating rate of 1°C / min and kept warm for 60 minutes; then it was carbonized, that is, in a N2 atmosphere, the temperature was raised at a rate of 5°C / min, kept warm at 1000°C for 120 minutes, and then cooled to room temperature to obtain a carbon nanofiber electrode material BC-ECNFs rich in end face defects.

[0037] The fiber diameter of the obtained end-face defect-rich carbon nanofiber electrode material BC-ECNFs is not much different from that of the original ECNFs, about 400~500nm; the fiber surface is loaded with carbon black particles and has more end-face carbon defects, which can effectively improve the active specific surface area and electrochemical performance of the electrode material.

[0038] Electrochemical performance tests of the different electrode materials prepared above: 1) Cyclic voltammetry test Method: A three-electrode system was used to prepare 1*1cm 2 ECNFs and BC-ECNFs electrode materials were used as working electrodes, saturated saturated common electrode was used as reference electrode, 2*2cm 2 The platinum sheet was used as the counter electrode and 0.1 MVOSO4+2.0 MH2SO4 was used as the electrolyte. The electrochemical properties of the electrode were investigated by cyclic voltammetry with a scan rate of 5 mV / s.

[0039] like Figure 2 As shown in Figure 2, two pairs of redox peaks are shown on both ECNFs and BC-ECNFs electrodes, corresponding to V 3+ / V 2 + and V 5+ / V 4+Compared with blank ECNFs, the CV curves measured on the BC-ECNFs electrode showed a larger peak current and a smaller peak potential difference, which demonstrated the excellent electrochemical reaction activity of the BC-ECNFs electrode material surface.

[0040] 2) Single battery charge and discharge test Method: ECNFs and BC-ECNFs prepared in the comparative example and Example 1 were used as positive and negative electrode materials of vanadium batteries, Nafion212 was used as ion exchange membrane, 1.65MV 3+ / V 4+ +3.0MH2SO4 as the electrolyte, assemble two sets of batteries and conduct charge and discharge tests. The charge and discharge cut-off voltages are 1.75V and 1V respectively, and the current density is 100mA / cm 2 ~400mA / cm 2 , 5 cycles of charge and discharge are performed at each current density. The corresponding charge and discharge curves of different current densities and energy efficiency comparisons are as follows: Figure 3 As shown in Figure 2, single cells with BC-ECNFs as positive and negative electrodes exhibit lower polarization overpotentials, higher capacities, and greater energy efficiency at all current densities. Furthermore, the difference between the two increases with increasing current density, indicating that single cells with BC-ECNFs as positive and negative electrodes in vanadium batteries have better rate performance. This is due to the greater degree of surface defects in the electrode material, which provides more active sites for the adsorption and reaction of vanadium ions.

[0041] In summary, the electrode material produced by this scheme utilizes simple and controllable coaxial electrospinning technology to achieve the distribution of active substances on the carbon fiber surface, improving the effective utilization of catalysts and saving costs. The core-shell structure ensures that the fiber core has a continuous and high-strength structural characteristic, while the outer layer exhibits a defective and highly active composition characteristic, effectively enhancing the electrochemical activity of the electrode material.

[0042] This invention introduces inexpensive, readily available carbon black material rich in end-face defects onto the surface of carbon nanofibers, further increasing the material's active specific surface area and fully utilizing the active material, effectively improving electrode performance while reducing material costs. Application of this material in flow batteries can effectively reduce battery polarization and improve energy conversion efficiency.

Claims

1. A method for preparing carbon nanofibers rich in end face defects, characterized in that: The steps include: 1) adding carbon black to a mixed solution of PAN and DMF, heating and stirring, and mixing uniformly to obtain a precursor solution for preparing a fiber shell layer; the carbon black has a particle size of 0.05 μm to 1 μm, the molecular weight of PAN is 90,000 to 150,000, the mass ratio of carbon black to PAN is 1:50 to 1:500, and the concentration of PAN in the mixed solution of PAN and DMF is 8 wt% to 18 wt%; 2) preparing a blank solution containing only PAN and DMF to obtain a precursor solution for preparing the fiber core; the concentration of PAN in the blank solution containing only PAN and DMF is 10 wt% to 20 wt%; 3) using a coaxial electrospinning device, injecting the two precursor solutions prepared in step 1) and step 2) into different syringes to provide fiber shell and core layer raw materials, respectively, and performing electrospinning to obtain a raw core-shell composite fiber material; 4) Flattening the original core-shell nanofiber material obtained in step 3), placing it in a tube furnace for pre-oxidation and carbonization treatment, and then cooling it to room temperature to obtain a carbon nanofiber electrode material with rich end face defects on the surface.

2. The method for preparing carbon nanofibers rich in end defects according to claim 1, wherein: In step 3), the electrospinning conditions are as follows: the stainless steel nozzle model is G15-G22; the fiber collector is a stainless steel roller with a roller speed of 50-200 r / min; the voltage between the nozzle and the roller is 16-23 kV; the distance between the nozzle and the roller is 8-14 cm; the spinning temperature is 25°C-40°C; the spinning humidity is 30%-60% RH; and the feed rate is 10-50 µL / min.

3. The method for preparing carbon nanofibers rich in end defects according to claim 1, wherein: In step 4), The pre-oxidation treatment conditions are: in an air atmosphere, heating to 260°C~290°C at a heating rate of 1°C / min~5°C / min, and keeping warm for 30min~60min; The conditions for the carbonization treatment are: in an inert atmosphere, a heating rate of 2°C / min~5°C / min, and keeping at 900°C~1300°C for 1h~3h.

4. The method for preparing carbon nanofibers rich in end defects according to any one of claims 1 to 3, characterized in that: The prepared carbon nanofibers with rich end face defects are used as vanadium battery electrode materials in all-vanadium redox flow batteries.

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