A multi-dimensional conductive material, a preparation method thereof, and an application thereof in a vanadium redox flow battery

Abstract

The present invention belongs to the field of liquid flow energy storage battery materials, and particularly relates to a multi-dimensional conductive material, a preparation method thereof, and an application thereof in a vanadium redox flow battery. The present invention uses a biomass precursor to bond graphite and multi-walled carbon nanotubes, and through stepwise pyrolysis, converts the carbon-based biomass into graphitized carbon nanotubes. The amorphous carbon in the graphite and the original carbon nanotubes is also further graphitized, and finally a multi-dimensional conductive material with carbon nanotubes inserted into a graphite matrix is formed. The multi-dimensional characteristics on the surface of the multi-dimensional conductive material are beneficial to forming a longitudinal conductive path on the bipolar plate, reducing the vertical bulk resistance of the bipolar plate, and also facilitating the wetting of epoxy resin between conductive particles and the mechanical interlocking of its molecular chains, improving the mechanical strength of the bipolar plate, effectively enhancing the service performance of the bipolar plate, meeting the requirements of long-term operation of the flow battery, relatively reducing the material cost of the flow battery, strongly promoting the industrialization of the bipolar plate, and having good application value.

Description

Technical Field

[0001] The present invention belongs to the field of liquid 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, the vanadium redox flow battery (VFB) has emerged with its excellent safety characteristics and entered the commercial application stage. In order to improve the long-term operation performance of the liquid flow battery, reduce the battery performance degradation, and at the same time reduce the raw material cost, it has become the main technical problem of the vanadium redox flow battery at present. Among them, a series of problems such as improving the conductive performance and mechanical strength of the bipolar plate, and reducing the longitudinal resistance and interface contact resistance of the bipolar plate have become crucial technical breakthrough links.

[0003] The mainstream bipolar plate on the market at present is a flexible graphite bipolar plate formed by hot pressing flexible graphite and PVDF. It has good conductive performance, but its mechanical strength is relatively low. During the operation of the vanadium redox flow stack, it is found that the vertical resistance of the flexible graphite bipolar plate is relatively high, which is easy to cause battery performance degradation, and its toughness is poor, and it is easy to crack during the encapsulation process.

[0004] In order to reduce the vertical resistance, Chinese Patent CN116014161A, patent name: A bipolar plate for a vanadium redox flow battery and a preparation method thereof, application date: December 28, 2022, publication date: April 25, 2023, uses vermicular graphite and PVDF and other resins to be hot-pressed into a flexible graphite bipolar plate. Although the conductive performance of the bipolar plate has been improved, the yield of vermicular 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 a vanadium redox flow battery and a preparation process, application date: January 26, 2019, publication date: May 17, 2019, uses a grid carbon cloth, flexible graphite, carbon fiber and carbon black to be compounded to prepare a bipolar plate for a 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 lamellar shape by external force, 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 lamellar 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 that adding carbon nanotubes or graphene to the conductive filler can effectively improve the conductive performance of the bipolar plate.

[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:

[0013] A method for preparing a multidimensional conductive material comprises the following steps:

[0014] Step S-1, Preparation of biomass precursor: A metal salt solution containing vanadium salt and palladium salt is mixed uniformly with biomass and left standing to form a paste-like precursor A, where the biomass serves as both a binder and a carbon source;

[0015] 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;

[0016] Step S-3, Preparation of multi-dimensional conductive material: Conductive composite precursor B is subjected to stepwise pyrolysis under the protection of 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 with carbon nanotubes covalently inserted into the graphite matrix is finally obtained.

[0017] Furthermore, 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. 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 room temperature.

[0018] Furthermore, 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.

[0019] Furthermore, 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 condition for drying under reduced pressure is that the vacuum degree < 0.08 MPa, the drying temperature is 65℃ - 85℃, and the drying time is 6 - 8 h.

[0020] Furthermore, in the above method for preparing a multi-dimensional conductive material, during the stepwise pyrolysis process in step S-3, the roasting temperature for pyrolysis is 200 - 300℃, and the roasting time is 2 - 3 h; The roasting temperature for carbonization is 600 - 800℃, and the roasting time is 3 - 5 h; The roasting temperature for graphitization is 1200 - 1500℃, and the roasting time is 2 - 3 h; The heating rate for roasting is 5 - 10℃ / min; The inert gas is selected from one of nitrogen, argon, and helium, and the gas flow rate is 10 - 60 mL / min.

[0021] 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 minutes. 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 hours, 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 hours.

[0022] 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 and connected to the surface of the graphite matrix in an inserted manner to form a multi-dimensional conductive structure.

[0023] 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 hot-pressed to form a bipolar plate for a vanadium redox flow battery.

[0024] 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, which are mixed to form a conductive filler; the epoxy resin is a thermosetting resin of AB components, specifically acrylic epoxy resin AB components. 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 minutes - 30 minutes.

[0025] Further, in the application of the above-mentioned multi-dimensional conductive material in a vanadium redox flow battery, the bipolar plate for the vanadium redox flow battery is tested according to the standard method. Its conductivity > 200 S / cm, bending strength > 32 MPa. In the vanadium redox flow battery, at a current density of 200 mA / cm 2 Under the condition of running for 50 cycles, its energy density EE is greater than 82.4%.

[0026] Advantages and beneficial effects of the present invention:

[0027] 1. The multi-dimensional conductive material prepared by the present invention has good electrical conductivity, and its surface is conducive to 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" on the graphite matrix to form a multi-dimensional conductive structure, improving the electrical conductivity and increasing the applications. The application of precious metal palladium has a small dosage, high catalytic efficiency, and can also be recycled. The doping of vanadium and palladium improves the catalytic synergy, catalytic efficiency, and 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;

[0028] 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 attenuation 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 attenuation 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;

[0029] 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 precious metal catalyst has the characteristics of small dosage and high efficiency, and can be recycled, reducing the raw material cost. The raw materials used in the present invention are all commercially available products, with wide sources, can be produced on a large scale, and can be promoted and applied. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is the morphology of the clusters on the surface of graphite particles;

[0031] Figure 2 is the morphology of the small particle carbon powder and carbon nanotubes grown on the graphite substrate. DETAILED DESCRIPTION OF THE INVENTION

[0032] In order to illustrate the purpose, technical solution, and product performance of the present invention, the specific implementation manners 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.

[0033] 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 electrical conductivity of bipolar plates is still relatively low. Therefore, preparing conductive particles with a spatial multi-dimensional structure is the 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 nanotubes 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.

[0034] The technical idea of the present invention is to use the binding effect of biomass to agglomerate graphite (Gr) particles and multi-walled carbon nanotubes (MWCNTs). 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, through secondary graphitization of multi-walled carbon nanotubes, amorphous carbon is transformed into crystalline carbon, and it will become thin-walled carbon nanotubes, which is beneficial to improving the electrical conductivity of 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.

[0035] Unless otherwise specified, the raw materials used in the following examples are all commercially available.

[0036] In the following examples, the selection of raw materials is as follows:

[0037] The biomass selected is lignin, and the lignin is AR-grade lignosulfonate, purchased from Tianjin Huasheng Chemical Reagent Co., Ltd.;

[0038] 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.;

[0039] The multi-walled carbon nanotubes, with an inner diameter of 3-5 nm and an outer diameter of 8-15 nm, were purchased from Toray, Japan;

[0040] The epoxy resin is selected from epoxy acrylate in thermosetting resins. The epoxy acrylate is an acrylic acid-modified epoxy 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 was purchased from Liaoning Geliahao Co., Ltd.;

[0041] The hydrochloric acid, sodium hydroxide, vanadium sulfate, and palladium chloride are all commercially available chemically pure products.

[0042] Example 1

[0043] The preparation method of this example includes the following steps:

[0044] Step S-1, preparation of biomass precursor: Mix 0.5 g / 100 mL vanadium sulfate solution and 10 mg / 100 mL palladium chloride solution in an equal ratio solution, with a total of 40 ml, and mix evenly with 60 g of lignosulfonate, and let it stand for 2 h to make a paste-like precursor A-1 for standby.

[0045] 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. Under reduced pressure conditions (vacuum degree < 0.08 MPa), dry it at 65 °C for 6 h to make a conductive composite precursor B-1 for standby.

[0046] Step S-3, preparation of multi-dimensional conductive material MDCM-1: Put 100 g of conductive composite precursor B-1 into a tube furnace. Under argon protection, the gas flow rate is 10 ml / min. The first step is pyrolysis, the temperature is 200 °C, and the time is 2 h; the second step is carbonization, the temperature is 600 °C, and the time is 3 h; the third step is graphitization, the temperature is 1200 °C, and the time is 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, and filter under reduced pressure; then soak it with 200 ml of 4-fold amount of 1 M sodium hydroxide solution for 24 h, rinse multiple times, and finally wash it with deionized water until neutral, filter; under the conditions of vacuum degree < 0.08 MPa and 65 °C, dry it for 4 h to make MDCM-1 for standby.

[0047] Step S-4, Preparation of bipolar plate for flow battery: Weigh 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 prepare 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 the mixture into a mold, set the hot pressing temperature at 90 °C, the pressure at 8 MPa, and the hot pressing time at 30 min to prepare a composite bipolar plate-1 with a thickness of 0.85 mm.

[0048] Step S5: Performance test of bipolar plate:

[0049] The execution standard for the mechanical property test of the bipolar plate is NB / T 42007-2013 "Test Method for Bipolar Plate for All-Vanadium Redox Flow Battery".

[0050] Cut a composite bipolar plate with dimensions of 110 mm × 70 mm and conduct a battery performance test at a current density of 200 mA / cm 2 of the current density.

[0051] Example 2

[0052] The preparation method of this example includes the following steps:

[0053] 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, with a total volume of 30 ml, and mix 70 g of lignosulfonate evenly, then let it stand for 3 h to prepare a paste-like precursor A-2 for standby.

[0054] Step S-2, Preparation of conductive composite precursor B-2: Take 20 g of precursor A-2, mix it with 77 g of 100-mesh expanded graphite and 3 g of multi-walled carbon nanotubes to form a powder, and dry it at 75 °C for 7 h under reduced pressure (vacuum degree < 0.08 MPa) to prepare conductive composite precursor B-2 for standby.

[0055] Step S-3, Preparation of Multidimensional Conductive Material MDCM-2: Take 100 g of conductive composite precursor B-2 and place it in a tube 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 and set aside. Weigh 50 g of the conductive material, take 250 ml of 5-fold amount of 2 M sulfuric acid solution for multiple soakings for 60 min, rinse, and perform vacuum filtration; then take 300 ml of 6-fold amount of 2 M sodium hydroxide solution for multiple soakings for 24 h, rinse multiple times, and finally wash with deionized water until neutral, filter; dry at 75 °C for 6 h to prepare MDCM-2 and set aside.

[0056] Step S-4, Preparation of Bipolar Plate for 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 and set aside. 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, the hot pressing temperature is 140 °C, the pressure is 14 MPa, and the hot pressing time is 15 min to prepare a composite bipolar plate-2 with a thickness of 0.85 mm.

[0057] Step S5: Performance Test of Bipolar Plate:

[0058] The execution standard for the mechanical property test of the bipolar plate is NB / T 42007-2013 "Test Method for Bipolar Plate for All-Vanadium Redox Flow Battery".

[0059] Cut a 110 mm × 70 mm composite bipolar plate and conduct battery performance tests under the condition of 200 mA / cm 2 of the condition.

[0060] Example 3

[0061] The preparation method of this example includes the following steps:

[0062] Step S-1, Preparation of Biomass Precursor: Mix an equal-ratio solution of 0.75 g / 100 mL vanadium sulfate solution and 50 mg / 100 mL palladium chloride solution, totaling 40 ml, and mix 60 g of lignosulfonate evenly, then let it stand for 4 h to prepare a paste-like precursor A-3 and set aside.

[0063] Step S-2, Preparation of Conductive Composite Precursor B-3: Take 15 g of precursor A-3, mix it with 83 g of 100-mesh expanded graphite and 2 g of multi-walled carbon nanotubes to form a powder. Under reduced pressure (vacuum degree < 0.08 MPa), dry it at 85 °C for 8 h to prepare conductive composite precursor B-3 for standby.

[0064] Step S-3, Preparation of Multi-dimensional Conductive Material MDCM-3: Take 100 g of conductive composite precursor B-3 and put it into a tube furnace. Under argon protection, the gas flow rate is 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-fold amount of 1 M sulfuric acid solution for multiple soakings and rinses, and perform vacuum filtration; then take 250 ml of 5-fold amount of 1 M sodium hydroxide solution for multiple soakings for 24 h 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.

[0065] Step S-4, Preparation of Flow Battery Bipolar Plate: 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 respectively, 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, the hot pressing temperature is 120 °C, the pressure is 13 MPa, and the hot pressing time is 20 min to prepare a composite bipolar plate-3 with a thickness of 0.85 mm.

[0066] Step S5: Performance Test of Bipolar Plate:

[0067] The execution standard for the mechanical property test of the bipolar plate is NB / T42007-2013 "Test Method for Bipolar Plate of All-Vanadium Redox Flow Battery".

[0068] Cut a 110 mm × 70 mm composite bipolar plate, and conduct battery performance tests under the condition of 200 mA / cm 2 of current density.

[0069] Comparative Example 1

[0070] For comparison with Example 1, in this comparative example, according to the requirements of Example 1, no metal catalyst was added to prepare multi-dimensional conductive material MDCM-d, and its preparation process is as follows:

[0071] 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.

[0072] Step S-2, Preparation of conductive composite precursor B-d: Take 5 g of precursor A-d, mix it with 94 g of 100-mesh expanded graphite and 1 g of multi-walled carbon nanotubes to form a powder. Under reduced pressure (vacuum degree < 0.08 MPa), dry it at 65 °C for 6 h to prepare the conductive composite precursor B-d for standby.

[0073] 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 at a temperature of 200 °C for 2 h; the second step is carbonization at a temperature of 600 °C for 3 h; the third step is graphitization at a temperature of 1200 °C for 2 h. The heating rate for each step is 5 °C / min. Prepare MDCM-d for standby.

[0074] Step S-4, Preparation of bipolar plate for 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 respectively, mix them evenly to prepare conductive filler-d for standby. Take 15 g of the prepared epoxy resin (the dosage ratio of A to B is 3:1) and mix it evenly with 85 g of conductive filler-d, put it into a mold, the hot pressing temperature is 90 °C, the pressure is 8 MPa, and the hot pressing time is 30 min to prepare a composite bipolar plate -d with a thickness of 0.85 mm.

[0075] Step S5: Performance test of bipolar plate:

[0076] The execution standard for the mechanical property test of the bipolar plate is NB / T42007-2013 "Test Method for Bipolar Plate for Vanadium Redox Flow Battery".

[0077] Cut a composite bipolar plate with a size of 110 mm × 70 mm, and conduct battery performance tests under the condition of 200 mA / cm 2

[0078] Comparative Example 2

[0079] In Comparative Example 2, a flexible graphite plate is used for testing:

[0080] The execution standard for the mechanical property test of the bipolar plate is NB / T42007-2013 "Test Method for Bipolar Plate for Vanadium Redox Flow Battery".

[0081] Cut a flexible graphite bipolar plate with a size of 110 × 70 mm, and conduct battery performance tests under the condition of 200 mA / cm 2

[0082] Compare the performance test results of the above-mentioned examples and comparative examples. ​​

[0083] The physical property test results of the bipolar plates are shown in Table 1 as follows:

[0084] Table 1 Physical property test results of the examples and comparative examples

[0085]

[0086] 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 as follows:

[0087] 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

[0088] The data in Table 1 indicate that the multi - dimensional conductive material MDCM has good electrical conductivity and can reduce the vertical resistance in the bipolar plates. In Examples 1 - 3, due to the presence of the multi - dimensional conductive material MDCM, the vertical resistance of the prepared bipolar plates 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 are mixed and calcined, it is equivalent to a physical mixture and no multi - dimensional conductive structure is 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.

[0089] 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.

[0090] The data in Table 2 indicate 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 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.

[0091] ​In summary, the present invention uses biomass under the action of a metal salt catalyst to generate carbon nanotubes through the processes of pyrolysis, carbonization, and graphitization. At the same time, multi-dimensional conductive particles MDCM formed by covalently bonding graphite and multi-walled carbon nanotubes are obtained. The application of the multi-dimensional conductive material MDCM in the bipolar plate of a vanadium redox 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 in vanadium redox 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 bipolar plate of the flow battery prepared by the present invention using the multi-dimensional conductive material MDCM is much smaller than that of the flexible graphite bipolar plate on the market, and it is suitable for long-term operation under high current density conditions, having good market competitiveness.

[0092] 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 bipolar plate of a flow battery 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.

[0093] According to 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 protection scope 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 preparation method of a multi-dimensional conductive material, characterized in that, It includes 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 dried under reduced pressure to obtain conductive composite precursor B; Step S-3, preparation of multi-dimensional conductive material: The 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 with carbon nanotubes inserted is finally obtained; The metal salt also includes iron salt or ruthenium salt. The metal salt is a water-soluble salt and is 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 total dosage of the metal salt solution is 20-40 parts; The standing condition is 2-4 hours at room temperature; The multi-dimensional conductive material has a graphite matrix, and the multi-walled carbon nanotubes transformed from biomass are connected to the surface of the graphite matrix in a covalent bond insertion manner to form a multi-dimensional conductive structure.

2. The preparation method of a multi-dimensional conductive material according to claim 1, characterized in that In step S-1, the metal salt solution is a mixture solution of vanadium sulfate and palladium chloride. Among them, the concentration of vanadium sulfate is 0.5-1.0 g / 100 mL, and the concentration of the palladium chloride solution is 10-50 mg / 100 mL.

3. The preparation method of a multi-dimensional conductive material according to claim 1, characterized in that, In step S-2, the dosage 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 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.

4. The preparation method of a multi-dimensional conductive material according to claim 1, wherein, During the stepwise pyrolysis process in step S-3, 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.

5. The preparation method of a multi-dimensional conductive material according to claim 1, characterized in that, In step S-3, for acid treatment, one of sulfuric acid, hydrochloric acid, and acetic acid is selected as the acid solution with a concentration of 1-2 M. 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; for alkali treatment, one of sodium hydroxide aqueous solution and potassium hydroxide aqueous solution is selected as the alkali solution with a concentration of 1-2 M. 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 washed with deionized water multiple times until neutral; 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 4-6 h.

6. Use of the multi-dimensional conductive material prepared by the method according to claim 1 in a vanadium redox flow battery, characterized in that, A bipolar plate for a vanadium redox flow battery is fabricated by uniformly mixing various graphites with different particle sizes, multi-dimensional conductive materials, and epoxy resin in a kneader and then performing hot pressing.

7. Use of the multi-dimensional conductive material according to claim 6 in a vanadium redox flow battery, characterized in that, The graphites include 200-mesh expanded graphite, 500-mesh flake graphite, and 1250-mesh high-purity graphite. The amounts of the expanded graphite, flake graphite, and high-purity graphite are 50 - 60 parts, 10 - 15 parts, and 5 - 10 parts respectively, and the amount of the multi-dimensional conductive material is 5 - 10 parts. They are mixed to form a conductive filler. The epoxy resin is a thermosetting resin with A and B components, specifically acrylic epoxy resin with A and B components. The usage ratio of component A to component B is 3:1, and the total amount of the acrylic epoxy resin is 15 - 20 parts. The process conditions for hot pressing are: the hot pressing temperature is 90°C - 140°C, the pressure is 8 - 14 MPa, and the hot pressing time is 15 min - 30 min.

8. The application of the multi-dimensional conductive material according to claim 6 in a vanadium redox flow battery, characterized in that, The bipolar plate for all-vanadium redox flow battery is tested, with its conductivity > 200 S / cm, flexural strength > 32 MPa. In the all-vanadium redox flow battery, it operates for 50 cycles under the condition of current density 200 mA / cm 2 , and its energy density EE > 82.4%.

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