Multi-dimensional conductive material, preparation method thereof and application of multi-dimensional conductive material in all-vanadium redox flow battery

Abstract

The invention belongs to the field of flow energy storage battery materials, and particularly relates to a multi-dimensional conductive material, a preparation method thereof and application of the multi-dimensional conductive material in an all-vanadium flow battery. Graphite and multi-walled carbon nanotubes are bonded together by adopting a biomass precursor, a carbon base of biomass is converted into graphitized carbon nanotubes through step-by-step pyrolysis, amorphous carbon in the graphite and the original carbon nanotubes is further graphitized, and finally the multi-dimensional conductive material with the carbon nanotubes inserted on a graphite matrix is formed. The multidimensional characteristic of the surface of the multidimensional conductive material is beneficial to formation of a longitudinal conductive path of the bipolar plate and reduction of vertical body resistance of the bipolar plate, and is also beneficial to wetting of epoxy resin among conductive particles and mechanical interlocking of molecular chains of the epoxy resin, so that the mechanical strength of the bipolar plate is improved, the use performance of the bipolar plate is effectively enhanced, and the service life of the bipolar plate is prolonged. The requirement of long-time operation of the flow battery is met, the material cost of the flow battery is relatively reduced, the industrialization of the bipolar plate is powerfully promoted, and the application value is very good.

Description

Technical Field

[0001] The present invention belongs to the field of flow energy storage batteries, and particularly relates to a multi-dimensional conductive material, a preparation method thereof, and an application thereof in a vanadium redox flow battery. Background Art

[0002] In recent years, vanadium redox flow batteries (VFBs) have emerged and entered the commercial application stage with their excellent safety characteristics. In order to improve the long-term operation performance of flow batteries, reduce battery performance degradation, and simultaneously reduce raw material costs, these have become the main technical problems of current vanadium redox flow batteries. Among them, a series of problems such as improving the electrical conductivity and mechanical strength of bipolar plates, and reducing the longitudinal resistance and interfacial contact resistance of bipolar plates have become crucial technical breakthrough links.

[0003] The mainstream bipolar plates on the current market are flexible graphite bipolar plates formed by hot pressing flexible graphite and PVDF. They have good electrical conductivity, but their mechanical strength is relatively low. During the operation of vanadium redox flow stacks, it has been found that the vertical resistance of flexible graphite bipolar plates is relatively high, which easily causes battery performance degradation, and they have poor toughness, and are prone to cracking during the encapsulation process.

[0004] In order to reduce the vertical resistance, Chinese Patent CN116014161A, patent name: A Bipolar Plate for Vanadium Redox Flow Battery and a Preparation Method Thereof, application date: December 28, 2022, publication date: April 25, 2023, uses worm graphite and resins such as PVDF for hot pressing to form a flexible graphite bipolar plate. Although the electrical conductivity of the bipolar plate has been improved, the yield of worm graphite is relatively low, and the conductive structure is still in a two-dimensional transverse structure, and the battery attenuation performance caused by the high vertical resistance has not been fundamentally changed.

[0005] In order to construct a multi-dimensional conductive structure and reduce the vertical resistance, Chinese Patent CN109768296B, patent name: A Bipolar Plate for Vanadium Redox Flow Battery and a Preparation Process Thereof, application date: January 26, 2019, publication date: May 17, 2019, uses grid carbon cloth, flexible graphite, carbon fiber, and carbon black for compounding to prepare a bipolar plate for vanadium redox flow battery bonded with epoxy resin. Although two-dimensional graphite, one-dimensional carbon fiber, and zero-dimensional carbon black can form a multi-dimensional conductive structure, they are only mechanically compounded and are still easily squeezed into a sheet-like shape by external forces, and the vertical resistance is still relatively high, and the performance degradation problem of the bipolar plate has not been fundamentally solved.

[0006] The structure of carbon nanotubes is the same as the sheet structure of graphite, and the P electrons of carbon atoms form P-π conjugate bonds, so carbon nanotubes have some special electrical properties. Many literature reports have shown that adding carbon nanotubes or graphene to conductive fillers can effectively improve the electrical conductivity of bipolar plates.

[0007] The catalytic cracking method for preparing carbon nanotubes usually uses a gaseous carbon source at a temperature of 600-1000°C and a catalyst to prepare carbon nanotubes. Since this method cracks carbon-containing compounds into carbon atoms at a relatively high temperature, the carbon atoms adhere to the surface of the catalyst particles under the action of the transition metal-catalyst to form carbon nanotubes. However, the cost of carbon nanotubes is relatively high and is not suitable for large-scale use.

[0008] In order to reduce the production and process costs of carbon nanotubes, Chinese patent CN112973625A, patent name: A lignin-based carbon nanotube and its preparation method and application, application date: February 5, 2021, publication date: June 18, 2021, uses lignin and a catalyst to mix, freeze-dry, sinter at 500~800℃ and pyrolysis at 800~1100℃ to prepare carbon nanotubes, and use them in gas adsorption materials. However, no description of conductive fillers was seen.

[0009] In order to increase the dimension of the conductive material, Chinese patent CN111170310A, patent name: a three-dimensional graphene / carbon nanotube composite material and its preparation method, application date: January 15, 2020, publication date: May 19, 2020, uses a gas carbon source to graphitize iron salts and polyvinyl pyrrolidone at high temperature to produce a three-dimensional graphene / carbon nanotube conductive composite material, which is used in the fields of energy storage and catalysis, solving the complexity and high cost of existing processes. The process includes three steps: sintering at 150~250℃, preliminary carbonization at 700~800℃, secondary carbonization at 850~880℃, and finally graphitization at 2800~3000℃. Although the high temperature burns off the amorphous carbon, the product contains iron / iron carbide particles because it has not been cleaned, which is not suitable for the preparation of liquid flow battery bipolar plates.

[0010] Therefore, preparing multidimensional conductive materials and forming multidimensional conductive structures are the key technologies to solve the vertical conductivity of the current flexible graphite bipolar plates, which can reduce the battery attenuation performance and has important guiding significance and economic value for the technological progress and market promotion of liquid flow batteries. Summary of the invention

[0011] In view of the problem of performance degradation of vanadium redox flow batteries in the prior art, the present invention proposes a multi-dimensional conductive material and a preparation method thereof and application in all-vanadium redox flow batteries, focusing on solving the problem of reducing the vertical resistance in the existing flexible graphite bipolar plate technology.

[0012] The technical solution of the present invention is: A method for preparing a multidimensional conductive material comprises the following steps: Step S-1, Preparation of biomass precursor: A metal salt solution containing vanadium salt and palladium salt is mixed evenly with biomass and left standing to form a paste-like precursor A, where the biomass serves as both a binder and a carbon source; Step S-2, Preparation of conductive composite precursor B: Precursor A is mixed with expanded graphite and multi-walled carbon nanotubes to form a powder, which is then dried under reduced pressure to obtain conductive composite precursor B; Step S-3, Preparation of multi-dimensional conductive material: Conductive composite precursor B is subjected to stepwise pyrolysis under the protection of an inert gas. The stepwise pyrolysis includes pyrolysis, carbonization, and graphitization processes; subsequently, acid treatment, alkali treatment, and cleaning are carried out step by step. After drying under reduced pressure, a multi-dimensional conductive material in which carbon nanotubes are covalently inserted into a graphite matrix is finally obtained.

[0013] Further, in the above method for preparing a multi-dimensional conductive material, in step S-1, the metal salt further includes iron salt or ruthenium salt, and the metal salt is a water-soluble salt selected from one of hydrochloride, sulfate, and nitrate; the biomass is selected from at least one of soluble lignosulfonate, cellulose, straw, and chitin; the dosage of the biomass is 60 - 80 parts, and the dosage of the metal salt solution is 20 - 40 parts; the standing condition is 2 - 4 hours at normal temperature.

[0014] Further, in the above method for preparing a multi-dimensional conductive material, the metal salt solution is a mixture solution of vanadium sulfate and palladium chloride. Among them, the concentration of the vanadium sulfate solution is 0.5 - 1.0 g / 100 mL, and the concentration of the palladium chloride solution is 10 - 50 mg / 100 mL.

[0015] Further, in the above method for preparing a multi-dimensional conductive material, in step S-2, the dosage of precursor A is 10 - 20 parts, the expanded graphite is 77 - 89 parts, and the carbon nanotubes are 1 - 3 parts; the expanded graphite is 100-mesh expanded graphite; the conditions for drying under reduced pressure are a vacuum degree < 0.08 MPa, a drying temperature of 65°C - 85°C, and a drying time of 6 - 8 h.

[0016] Further, in the stepwise pyrolysis process of step S-3 in the above method for preparing a multi-dimensional conductive material, the roasting temperature for pyrolysis is 200 - 300°C, and the roasting time is 2 - 3 h; the roasting temperature for carbonization is 600 - 800°C, and the roasting time is 3 - 5 h; the roasting temperature for graphitization is 1200 - 1500°C, and the roasting time is 2 - 3 h; the heating rate for roasting is 5 - 10°C / min; the inert gas is selected from one of nitrogen, argon, and helium, and the gas flow rate is 10 - 60 mL / min.

[0017] Further, in the preparation method of the above-mentioned multi-dimensional conductive material, in step S-3, one of sulfuric acid, hydrochloric acid, and acetic acid is selected as the acid solution for acid treatment, with a concentration of 1-2M. The usage amount of the acid solution is 3-5 times that of the conductive material, and the soaking time is 30-60 min. After acid washing, it is washed with deionized water; the alkali treatment selects one of sodium hydroxide aqueous solution and potassium hydroxide aqueous solution as the alkali solution, with a concentration of 1-2M. The usage amount of the alkali solution is 4-6 times that of the conductive material. After soaking at room temperature for 24 h, it is then washed with deionized water multiple times until neutral; the conditions for vacuum drying are a vacuum degree <0.08 MPa, a drying temperature of 65°C - 85°C, and a drying time of 4-6 h.

[0018] For the multi-dimensional conductive material prepared by the above method, the multi-dimensional conductive material uses graphite as the matrix, and multi-walled carbon nanotubes converted from biomass are covalently bonded to the surface of the graphite matrix in an inserted manner to form a multi-dimensional conductive structure.

[0019] Further, in the application of the above-mentioned multi-dimensional conductive material in a vanadium redox flow battery, various graphites with different particle sizes, the multi-dimensional conductive material, and epoxy resin are mixed evenly in a kneader and then hot-pressed into a bipolar plate for a vanadium redox flow battery.

[0020] Further, in the application of the above-mentioned multi-dimensional conductive material in a vanadium redox flow battery, the graphite includes 200-mesh expanded graphite, 500-mesh flake graphite, and 1250-mesh high-purity graphite. The usage amount of the expanded graphite is 55-70 parts, the usage amount of the flake graphite is 5-10 parts, the usage amount of the high-purity graphite is 5-10 parts, and the usage amount of the multi-dimensional conductive material is 5-10 parts, and they are mixed to form a conductive filler; the epoxy resin is a thermosetting resin of AB components, specifically an acrylic epoxy resin AB component, and the usage ratio of component A to component B is 3:1, and the total usage amount of the acrylic epoxy resin is 15-20 parts. The process conditions for hot pressing are: a hot pressing temperature of 90°C - 140°C, a pressure of 8-14 MPa, and a hot pressing time of 15 min - 30 min.

[0021] Further, in the application of the above-mentioned multi-dimensional conductive material in a vanadium redox flow battery, the bipolar plate for a vanadium redox flow battery is tested according to the standard method, and its conductivity > 200 S / cm, flexural strength > 32 MPa. In the vanadium redox flow battery, under the condition of a current density of 200 mA / cm 2 it operates for 50 cycles, and its energy density EE is greater than 82.4%.

[0022] Advantages and beneficial effects of the present invention: 1. The multi-dimensional conductive material prepared by the present invention has good electrical conductivity and a surface that facilitates the wetting effect of epoxy resin. The present invention uses biomass as a binding medium to bond graphite and multi-walled carbon nanotubes into clusters. Under the action of a catalyst, through stepwise pyrolysis, carbonization, and graphitization, the graphitized medium, carbon nanotubes, and multi-walled carbon nanotubes generated from biomass are "inserted" onto the graphite matrix to form a multi-dimensional conductive structure, improving the electrical conductivity and increasing the applications. The application of noble metal palladium uses less amount, has high catalytic efficiency, and can also be recycled. The doping of vanadium and palladium improves the catalytic synergy, catalytic efficiency, and the bonding effect of carbon nanotubes. At the same time, during the operation of the vanadium redox flow battery, vanadium can precipitate from the bipolar plate, preventing vanadium ions from penetrating into the bipolar plate, playing a certain inhibitory role; 2. The multi-dimensional conductive material prepared by the present invention breaks through the two-dimensional conductive structure of graphite, effectively constructs the conductive path in the bipolar plate, reduces the vertical resistance of the composite bipolar plate, reduces the performance decay of the flow battery, and can extend the service life of the battery. In addition, the surface of the multi-dimensional conductive particles has good wetting with epoxy resin, increasing the toughness of the bipolar plate and facilitating the encapsulation process of the bipolar plate. The present invention solves the problem of high vertical resistance of the bipolar plate from the conductive structure, reduces the performance decay of the flow battery, provides technical support for improving the performance of the bipolar plate of the flow battery, and can be extended to the fields of lithium batteries, electrodes, and hydrogen fuel cells, having profound theoretical guiding significance; 3. The preparation method and process of the multi-dimensional conductive material are simple, the production cost is low, and it is suitable for large-scale production. The noble metal catalyst has the characteristics of less amount and high efficiency, and can also be recycled, reducing the raw material cost. The raw materials used in the present invention are all commercially available products, with wide sources, and can be produced on a large scale and popularized and applied. Description of the Drawings

[0023] Figure 1 is the morphology of the clusters on the surface of graphite particles; Figure 2 is the morphology of the small particle carbon powder and carbon nanotubes grown on the graphite substrate. Detailed Embodiments

[0024] In order to illustrate the purpose, technical solutions, and product performance of the present invention, the specific embodiments of the present invention will be further described in detail in combination with the embodiments and the drawings of the specification. The following embodiments are used to illustrate the present invention, but cannot be used to limit the scope of the present invention.

[0025] In the prior art, although two-dimensional graphite particles and one-dimensional carbon nanotubes can form a so-called multi-dimensional conductive path through physical mixing to improve the electrical conductivity of bipolar plates, their effect on the vertical conductivity of bipolar plates is still relatively low. Therefore, preparing conductive particles with a spatial multi-dimensional structure is the primary technical means to solve the problem at present. The present invention uses biomass that can be graphitized as an adhesive to bond graphite and carbon nanotubes. After sintering, carbonization, and graphitization, the biomass finally forms carbon black, graphitized medium, and carbon nanotube CNTs, and bonds graphite and carbon nanotubes through covalent bonds. The carbon nanotubes are like being inserted into the graphite matrix, forming multi-dimensional conductive particles, and effectively constructing a multi-dimensional conductive material in the bipolar plate. In addition, through high-temperature graphitization treatment, the amorphous carbon in multi-walled carbon nanotubes will be further graphitized, possibly forming single-walled carbon nanotubes and promoting the improvement of electrical conductivity. Moreover, carbon nanotubes can also play a strengthening role in the bipolar plate, relatively improving the mechanical properties of the bipolar plate. When this multi-dimensional conductive material is applied in the bipolar plate of a flow battery, the usage amount is small, a continuous conductive path can be formed, effectively reducing the vertical resistance formed between graphite particles, effectively reducing the vertical resistance of the bipolar plate, and also reducing the attenuation of the electrical conductivity of the bipolar plate.

[0026] The technical idea of the present invention is to use the adhesive effect of biomass to bond graphite (Gr) particles and multi-walled carbon nanotubes (MWCNTs) into clusters. Under the action of a noble metal palladium catalyst, the biomass undergoes a process of stepwise pyrolysis, carbonization, and graphitization, and is transformed into a graphitized medium and carbon nanotubes (CNTs). At the same time, the multi-walled carbon nanotubes and the newly generated carbon nanotubes are inserted onto the graphite particles through the generated covalent bonds, and their morphology is as shown in the appendix Figure 1-2 shown, forming a multi-dimensional structure conductive material (MDCM). At the same time, the multi-walled carbon nanotubes undergo secondary graphitization, and the amorphous carbon is transformed into crystalline carbon, and will become thin-walled carbon nanotubes, which is beneficial to improving the electrical conductivity of the particles. When the multi-dimensional structure conductive material (MDCM) is applied in the bipolar plate of a flow battery, it can reduce the vertical resistance of the bipolar plate, improve the electrical conductivity, and reduce the battery attenuation of the bipolar plate, meeting the requirements of long-term energy storage.

[0027] Unless otherwise specified, the raw materials used in the following examples are all commercially available.

[0028] In the following examples, the selection of raw materials is as follows: The biomass selects lignin, and the lignin is AR-grade lignosulfonate, purchased from Tianjin Huasheng Chemical Reagent Co., Ltd.; The 100-mesh expanded graphite, 200-mesh expanded graphite, 500-mesh flake graphite, and 1250-mesh high-purity graphite are all purchased from Qingdao Dongkai Graphite Co., Ltd.; The multi-walled carbon nanotubes, with an inner diameter of 3 - 5 nm and an outer diameter of 8 - 15 nm, are purchased from Toray, Japan; The epoxy resin is selected from epoxy acrylate in thermosetting resins. The epoxy acrylate is an acrylic acid modified epoxy type structural adhesive, which is composed of synthetic epoxy acrylate (component A) and modified curing agent (component B), and the mixing ratio of A:B is 3:1. It is purchased from Liaoning Geliaohao Company; The hydrochloric acid, sodium hydroxide, vanadium sulfate, and palladium chloride are all commercially available chemically pure products.

[0029] Example 1

[0030] The preparation method of this example includes the following steps: Step S-1, preparation of biomass precursor: Take 0.5 g / 100 mL vanadium sulfate solution and 10 mg / 100 mL palladium chloride solution and mix them in an equal ratio solution, totaling 40 ml, and mix them evenly with 60 g of lignosulfonate, and let it stand for 2 h to make a paste-like precursor A-1 for standby.

[0031] Step S-2, preparation of conductive composite precursor B-1: Take 10 g of precursor A-1 and mix it with 89 g of 100-mesh expanded graphite and 1 g of multi-walled carbon nanotubes to make a powder, and dry it at 65 °C for 6 h under reduced pressure (vacuum degree < 0.08 MPa) to make a conductive composite precursor B-1 for standby.

[0032] Step S-3, preparation of multi-dimensional conductive material MDCM-1: Put 100 g of conductive composite precursor B-1 into a tube furnace, start under argon protection, with a gas flow rate of 10 ml / min. The first step is pyrolysis, with a temperature of 200 °C and a time of 2 h; the second step is carbonization, with a temperature of 600 °C and a time of 3 h; the third step is graphitization, with a temperature of 1200 °C and a time of 2 h. The heating rate for each step is 5 °C / min. Then cool it for standby. Weigh 50 g of the multi-dimensional conductive material, soak it multiple times with 150 ml of 3-fold amount of 1 M sulfuric acid solution for 30 min, rinse it, and filter it under reduced pressure; then soak it with 200 ml of 4-fold amount of 1 M sodium hydroxide solution for 24 h and rinse it multiple times, and finally wash it with deionized water until neutral and filter it; dry it at 65 °C for 4 h under a vacuum degree < 0.08 MPa to make MDCM-1 for standby.

[0033] Step S-4, preparation of the bipolar plate of the flow battery: Take 50 g of 200-mesh expanded graphite, 15 g of 500-mesh flake graphite, 10 g of 1250-mesh high-purity graphite, and 10 g of MDCM-1, mix them evenly to make conductive filler-1 for standby. Take 15 g of the prepared acrylic epoxy resin (the dosage ratio of A to B is 3:1) and mix it evenly with the 85 g of conductive filler-1, put it into a mold, with a hot pressing temperature of 90 °C, a pressure of 8 MPa, and a hot pressing time of 30 min to make a composite bipolar plate-1 with a thickness of 0.85 mm.

[0034] Step S5: Bipolar plate performance test: The execution standard for the mechanical property test of the bipolar plate is NB / T 42007-2013 "Test Method for Bipolar Plates for All-Vanadium Redox Flow Batteries".

[0035] Cut a composite bipolar plate of 110 mm × 70 mm and conduct a battery performance test at a current density of 200 mA / cm 2 of the current density.

[0036] Example 2

[0037] The preparation method of this example includes the following steps: Step S-1, Preparation of biomass precursor: Mix 1.0 g / 100 mL of vanadium sulfate solution and 50 mg / 100 mL of palladium chloride solution in equal proportion, totaling 30 ml, and mix 70 g of lignosulfonate evenly, then let it stand for 3 h to make a paste-like precursor A-2 for standby.

[0038] Step S-2, Preparation of conductive composite precursor B-2: Take 20 g of precursor A-2 and mix it with 77 g of 100-mesh expanded graphite and 3 g of multi-walled carbon nanotubes to make a powder. Under reduced pressure conditions (vacuum degree < 0.08 MPa), dry it at 75 °C for 7 h to make a conductive composite precursor B-2 for standby.

[0039] Step S-3, Preparation of multi-dimensional conductive material MDCM-2: Put 100 g of conductive composite precursor B-2 into a tubular furnace. Under argon protection, the gas flow rate is 60 ml / min. The first step is pyrolysis at a temperature of 300 °C for 3 h; the second step is carbonization at a temperature of 800 °C for 5 h; the third step is graphitization at a temperature of 1500 °C for 3 h. The heating rate for each step is 10 °C / min. Then cool it for standby. Weigh 50 g of the conductive material, take 250 ml of 5-fold amount of 2M sulfuric acid solution for multiple soakings for 60 min, rinse, and filter under reduced pressure; then take 300 ml of 6-fold amount of 2M sodium hydroxide solution for multiple soakings for 24 h, rinse multiple times, and finally wash it with deionized water until neutral, filter; dry it at 75 °C for 6 h to make MDCM-2 for standby.

[0040] Step S-4, Preparation of the bipolar plate for the flow battery: Take 60 g of 200-mesh expanded graphite, 10 g of 500-mesh flake graphite, 5 g of 1250-mesh high-purity graphite, and 5 g of MDCM-2, mix them evenly to prepare conductive filler-2 for standby. Take 20 g of the prepared acrylic epoxy resin (the dosage ratio of A to B is 3:1) and 80 g of conductive filler-2, mix them evenly, put them into a mold, with a hot pressing temperature of 140 °C, a pressure of 14 MPa, and a hot pressing time of 15 min to prepare a composite bipolar plate-2 with a thickness of 0.85 mm.

[0041] Step S5: Performance testing of the bipolar plate: The execution standard for the mechanical property testing of the bipolar plate is NB / T 42007-2013 "Test Method for Bipolar Plates for All-Vanadium Redox Flow Batteries".

[0042] Cut a composite bipolar plate with dimensions of 110 mm × 70 mm and conduct battery performance testing under the condition of 200 mA / cm 2 of the current density.

[0043] Example 3

[0044] The preparation method of this example includes the following steps: Step S-1, Preparation of the biomass precursor: Take an equal-proportion solution mixture of 0.75 g / 100 mL of vanadium sulfate solution and 50 mg / 100 mL of palladium chloride solution, totaling 40 ml, mix 60 g of lignosulfonate evenly, and let it stand for 4 h to prepare a paste-like precursor A-3 for standby.

[0045] Step S-2, Preparation of the conductive composite precursor B-3: Take 15 g of precursor A-3, 83 g of 100-mesh expanded graphite, and 2 g of multi-walled carbon nanotubes, mix them to form a powder, and dry it at 85 °C for 8 h under reduced pressure (vacuum degree < 0.08 MPa) to prepare the conductive composite precursor B-3 for standby.

[0046] Step S-3, Preparation of the multi-dimensional conductive material MDCM-3: Put 100 g of the conductive composite precursor B-3 into a tubular furnace, start under argon protection with a gas flow rate of 30 ml / min. The first step is pyrolysis at a temperature of 250 °C for 3 h; the second step is carbonization at a temperature of 700 °C for 4 h; the third step is graphitization at a temperature of 1250 °C for 3 h. The heating rate for each step is 10 °C / min. Then cool it for standby. Weigh 50 g of the conductive material, take 200 ml of 4 times the amount of 1 M sulfuric acid solution for multiple soakings and rinses, and perform vacuum filtration; then take 250 ml of 5 times the amount of 1 M sodium hydroxide solution for multiple 24-h soakings and multiple rinses, and finally wash it with deionized water until neutral and filter; dry it at 85 °C for 5 h to prepare MDCM-3 for standby.

[0047] Step S-4, Preparation of bipolar plates for flow batteries: Take 54 g of 200-mesh expanded graphite, 13 g of 500-mesh flake graphite, 7 g of 1250-mesh high-purity graphite, and 8 g of MDCM-3, mix them evenly to prepare conductive filler-3 for standby. Take 18 g of the prepared epoxy resin (the dosage ratio of A to B is 3:1) and 82 g of conductive filler-3, mix them evenly, put them into a mold, with a hot pressing temperature of 120 °C, a pressure of 13 MPa, and a hot pressing time of 20 min to prepare a composite bipolar plate-3 with a thickness of 0.85 mm.

[0048] Step S5: Performance testing of bipolar plates: The execution standard for the mechanical property testing of bipolar plates is NB / T42007-2013 "Test Methods for Bipolar Plates for All-Vanadium Redox Flow Batteries".

[0049] Cut a composite bipolar plate with a size of 110 mm × 70 mm and conduct battery performance testing under the condition of 200 mA / cm 2 .

[0050] Comparative Example 1 For comparison with Example 1, in this comparative example, according to the requirements of Example 1, a multi-dimensional conductive material MDCM-d was prepared without adding a metal catalyst. The preparation process is as follows: Step S-1, Preparation of biomass precursor: Take 40 ml of deionized water, mix 60 g of lignin evenly, and let it stand for 2 h to prepare a paste-like precursor A-d for standby.

[0051] Step S-2, Preparation of conductive composite precursor B-d: Take 5 g of precursor A-d, 94 g of 100-mesh expanded graphite, and 1 g of multi-walled carbon nanotubes, mix them into a powder, and dry it at 65 °C for 6 h under reduced pressure (vacuum degree < 0.08 MPa) to prepare conductive composite precursor B-d for standby.

[0052] Step S-3, Preparation of multi-dimensional conductive material MDCM-d: Put 100 g of conductive composite precursor B-d into a tube furnace, under argon protection (flow rate 10 ml / min), the first step is pyrolysis, with a temperature of 200 °C and a time of 2 h; the second step is carbonization, with a temperature of 600 °C and a time of 3 h; the third step is graphitization, with a temperature of 1200 °C and a time of 2 h. The heating rate for each step is 5 °C / min. Prepare MDCM-d for standby.

[0053] Step S-4, Preparation of the bipolar plate of the flow battery: Take 50 g of 200-mesh expanded graphite, 15 g of 500-mesh flake graphite, 10 g of 1250-mesh high-purity graphite, and 10 g of MDCM-d, mix them evenly to make conductive filler-d for standby. Take 15 g of the prepared epoxy resin (the dosage ratio of A to B is 3:1) and 85 g of conductive filler-d, mix them evenly, put them into a mold, with a hot pressing temperature of 90 °C, a pressure of 8 MPa, and a hot pressing time of 30 min to make a composite bipolar plate-d with a thickness of 0.85 mm.

[0054] Step S5: Performance testing of the bipolar plate: The execution standard for the mechanical property test of the bipolar plate is NB / T42007-2013 "Test Method for Bipolar Plates for All-Vanadium Redox Flow Batteries".

[0055] Cut a composite bipolar plate with dimensions of 110 mm × 70 mm, and conduct battery performance testing under the condition of 200 mA / cm 2 For the battery performance test, the conditions are as follows.

[0056] Comparative Example 2 In Comparative Example 2, a flexible graphite plate is used for testing: The execution standard for the mechanical property test of the bipolar plate is NB / T42007-2013 "Test Method for Bipolar Plates for All-Vanadium Redox Flow Batteries".

[0057] Cut a flexible graphite bipolar plate with dimensions of 110 × 70 mm, and conduct battery performance testing under the condition of 200 mA / cm 2 For the battery performance test, the conditions are as follows.

[0058] Compare the performance test results of the above-mentioned examples and comparative examples.

[0059] The physical property test results of the bipolar plate are shown in Table 1: Table 1 Physical Property Test Results of Examples and Comparative Examples

[0060] For Example 3 and Comparative Example 2, the battery performance results after 50 discharge cycles at a current density of 150 - 250 mA / cm 2 are shown in Table 2: Table 2 Battery Performance Results of Example 3 and Comparative Example 2 after 50 Discharge Cycles at a Current Density of 150 - 200 mA / cm 2 Current Density

[0061] The data in Table 1 show that the multi-dimensional conductive material MDCM has good electrical conductivity and can reduce the vertical resistance in the bipolar plate. In Examples 1-3, due to the presence of the multi-dimensional conductive material MDCM, the vertical resistance of the prepared bipolar plate is less than that of the flexible graphite plate in Comparative Example 2. In Comparative Example 1, without the action of a catalyst, although lignin, graphite, and carbon nanotubes were mixed and calcined, it was equivalent to physical mixing and no multi-dimensional conductive structure was formed. Therefore, the vertical resistance of the bipolar plate in Comparative Example 1 is much larger than that in Examples 1-3, and the volume conductivity is much smaller.

[0062] In addition, after curing, epoxy resin has rigidity, so the tensile strength and flexural strength of the examples are greater than those of the flexible graphite plate in Comparative Example 2.

[0063] The data in Table 2 show that the multi-dimensional conductive material MDCM has good electrical conductivity, has a small vertical resistance in the bipolar plate, can improve the performance of the flow battery, and reduce the attenuation of the flow battery performance. At a current density of 200 mA / m 2 Under the current density condition, after 50 discharge cycles, the energy efficiency EE of Example 3 is higher than that of the flexible graphite plate in Comparative Example 2, indicating that the flow battery performance of Example 3 is better; the battery capacity retention rate CR of Example 3 is better than that of Comparative Example 2, indicating that the attenuation degree of the flow battery performance in Example 3 is less than that of the flexible graphite plate in Comparative Example 2.

[0064] In summary, in the present invention, biomass is pyrolyzed, carbonized, and graphitized under the action of a metal salt catalyst to finally generate carbon nanotubes. At the same time, graphite and multi-walled carbon nanotubes are covalently bonded to form a multi-dimensional conductive particle MDCM. The application of the multi-dimensional conductive material MDCM in the bipolar plate of a vanadium flow battery can effectively reduce the vertical resistance of the bipolar plate, improve the electrical conductivity, reduce the attenuation of battery performance, and is suitable for long-term energy storage of vanadium flow batteries; due to the multi-dimensional structure of the conductive particles and the strengthening effect of carbon nanotubes, the mechanical strength of epoxy resin can be relatively improved. The experimental results show that the vertical resistance of the flow battery bipolar plate prepared by using the multi-dimensional conductive material MDCM in the present invention is much smaller than that of the flexible graphite bipolar plate on the market, is suitable for long-term operation under high current density conditions, and has good market competitiveness.

[0065] The multi-dimensional conductive material MDCM of the present invention and its application in the bipolar plate of a flow battery improve the technical problems of the current flow battery bipolar plate in terms of conductive structure, mechanical structure, and large-scale production process. Further detailed research work still needs to be done, which can provide effective technical support for the popularization of technologies such as flow battery energy storage and fuel cells.

[0066] Based on the disclosure and description of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments disclosed and described above, and some modifications and changes to the invention should also fall within the scope of protection of the claims of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A method for preparing a multidimensional conductive material, characterized in that: The following steps are involved: Step S-1, preparation of biomass precursor: uniformly mixing a metal salt solution containing vanadium salt and palladium salt with biomass, and allowing to stand to prepare a paste precursor A, wherein the biomass serves as a binder and a carbon source; Step S-2, preparation of conductive composite material precursor B: Precursor A is mixed with expanded graphite and multi-walled carbon nanotubes to prepare a powder, and the powder is dried under reduced pressure to prepare conductive composite material precursor B; Step S-3, preparation of multidimensional conductive material: conducting step-by-step pyrolysis of conductive composite material precursor B under inert gas protection, wherein the step-by-step pyrolysis comprises pyrolysis, carbonization and graphitization processes; followed by step-by-step acid treatment, alkali treatment and cleaning, and finally obtaining a multidimensional conductive material with carbon nanotubes inserted therein after reduced pressure drying.

2. The method for preparing a multidimensional conductive material according to claim 1, characterized in that: In step S-1, the metal salt further comprises an iron salt or a ruthenium salt, the metal salt is a water-soluble salt selected from one of hydrochloride, sulfate and nitrate; the biomass is selected from at least one of soluble lignin sulfonate, cellulose, straw and chitin; the amount of the biomass is 60 to 80 parts, and the total amount of the metal salt solution is 20 to 40 parts; The static condition is 2 to 4 hours at room temperature.

3. The method for preparing a multidimensional conductive material according to claim 2, characterized in that: In step S-1, the metal salt solution is selected from a mixture solution of vanadium sulfate and palladium chloride, wherein the concentration of vanadium sulfate is 0.5-1.0 g / 100 mL, and the concentration of palladium chloride solution is 10-50 mg / 100 mL.

4. The method for preparing a multidimensional conductive material according to claim 1, characterized in that: In step S-2, the amount of precursor A is 10-20 parts, the expanded graphite is 77-89 parts, and the multi-walled carbon nanotubes are 1-3 parts; the expanded graphite is 100-mesh expanded graphite; the conditions for the reduced pressure drying are vacuum degree <0.08MPa, drying temperature is 65°C-85°C, and drying time is 6-8h.

5. The method for preparing a multidimensional conductive material according to claim 1, characterized in that: In the step S-3 step-by-step pyrolysis process, the calcination temperature of pyrolysis is 200-300°C, and the calcination time is 2-3h; the calcination temperature of carbonization is 600-800°C, and the calcination time is 3-5h; the calcination temperature of graphitization is 1200-1500°C, and the calcination time is 2-3h; the calcination heating rate is 5-10°C / min; the inert gas is selected from one of nitrogen, argon, and helium, and the gas flow rate is 10-60mL / min.

6. The method for preparing a multidimensional conductive material according to claim 1, characterized in that: In step S-3, the acid treatment uses one of sulfuric acid, hydrochloric acid, and acetic acid as the acid solution, the concentration is 1~2M, the amount of acid solution used is 3~5 times that of the conductive material, the soaking time is 30~60min, and the acid is washed with deionized water after washing; the alkali treatment is selected from one of sodium hydroxide aqueous solution and potassium hydroxide aqueous solution as the alkali solution, the alkali solution concentration is 1~2M, the amount of alkali solution used is 4~6 times that of the conductive material, after soaking at room temperature for 24h, it is washed with deionized water multiple times until neutral; the reduced pressure drying conditions are vacuum degree <0.08MPa, drying temperature is 65℃~85℃, and drying time is 4~6h.

7. A multidimensional conductive material prepared by the method according to any one of claims 1 to 6, characterized in that: The multi-dimensional conductive material uses graphite as a matrix, and multi-walled carbon nanotubes converted from biomass are covalently bonded and connected to the surface of the graphite matrix in an intercalation manner to form a multi-dimensional conductive structure.

8. Use of the multidimensional conductive material according to claim 7 in an all-vanadium redox flow battery, characterized in that: Various graphites of different particle sizes, multi-dimensional conductive materials and epoxy resins are mixed evenly in a kneading machine, and then hot-pressed to form bipolar plates for all-vanadium liquid flow batteries.

9. The use of the multi-dimensional conductive material in an all-vanadium redox flow battery according to claim 8, characterized in that: The graphite includes 200 mesh expanded graphite, 500 mesh flake graphite and 1250 mesh high-purity graphite, wherein the amount of the expanded graphite is 50-60 parts, the amount of the flake graphite is 10-15 parts, the amount of the high-purity graphite is 5-10 parts, and the amount of the multi-dimensional conductive material is 5-10 parts, which are mixed to form a conductive filler; the epoxy resin is a thermosetting resin of AB components, specifically an acrylic epoxy resin AB component, the amount ratio of component A to component B is 3:1, and the total amount of acrylic epoxy resin is 15-20 parts; the process conditions of hot pressing molding are: hot pressing temperature is 90°C-140°C, pressure is 8-14MPa, and hot pressing time is 15min-30min.

10. The use of the multi-dimensional conductive material in an all-vanadium redox flow battery according to claim 8, characterized in that: The bipolar plates for all-vanadium flow batteries were tested and found to have a conductivity of >200S / cm and a bending strength of >32MPa. 2 After running for 50 cycles under the same conditions, the energy density EE is greater than 82.4%.

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