Flexible graphite composite bipolar plate for flow battery and preparation method of flexible graphite composite bipolar plate

Abstract

The invention discloses a flexible graphite composite bipolar plate for a flow battery and a preparation method thereof, and the preparation method comprises the following steps: mixing expandable graphite with a carbon material to obtain a conductive filler mixture; mixing the organic high-molecular polymer with the conductive filler mixture to prepare mixed powder; spreading and melting the mixed powder in a drying oven to obtain a melt; crushing and sieving the melt to obtain melt powder; laying the melt powder in a mold, and pressurizing the mold to obtain a prefabricated slab; the prefabricated plate is subjected to heat preservation, pressurization, cooling and curing; the composite bipolar plate prepared by the invention has high conductivity and high mechanical strength, various properties of a flow battery can be remarkably improved, the preparation process of the bipolar plate does not need harsh vacuum conditions, does not need to select a solvent for dissolving and removing the solvent, does not need to post-treat the surface of a product, and is simple in process route, low in equipment cost and low in environmental condition requirement; and the product quality is easy to control, the uniformity is good, the comprehensive cost is low, and both economy and environmental friendliness are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of the preparation of composite bipolar plates, and specifically to a flexible graphite composite bipolar plate for a flow battery and a preparation method thereof. Background Art

[0002] As one of the key materials in a flow battery stack, the bipolar plate plays roles such as supporting the electrodes and ion membranes, conducting and collecting the current generated during charge and discharge, and blocking the electrolytes on both sides. Therefore, an ideal bipolar plate needs to have good electrical conductivity, electrochemical stability, corrosion resistance, and good mechanical strength.

[0003] Currently, the flow battery bipolar plates are divided into the following four types: graphite bipolar plates, metal bipolar plates, composite bipolar plates, and integrated electrode-bipolar plates. Graphite bipolar plates have obvious advantages in terms of electrical conductivity and chemical stability, but have problems such as low mechanical strength and easy breakage, poor processability, high manufacturing cost, and difficulty in large-scale preparation. Metal bipolar plates will undergo electrochemical corrosion in the operating environment of a strongly acidic or strongly alkaline system of a flow battery, resulting in great difficulties in their development and application. Breakthrough technologies are needed to transform them, otherwise large-scale application is difficult to achieve. The integrated electrode-bipolar plate integrates the electrode and the bipolar plate, that is, the electrode is pressed into the graphite plate. It has outstanding advantages in terms of battery performance and assembly, but the preparation process is relatively complex and the cost is relatively high. The composite bipolar plate has both the advantages of high mechanical strength and easy processing, and the advantages of high electrical conductivity and corrosion resistance of graphite, and has currently become the mainstream.

[0004] The prior art, such as the invention patent with the publication number of CN119036899A, discloses using epoxy resin as A glue, curing agent and accelerator as B glue, impregnating the graphite bipolar plate under vacuum pressure conditions after mixing, and then thermally curing, and removing the resin residue after cooling; the invention patent with the publication number of CN119029234A discloses first secondarily pressing into a single plate, performing secondary resin impregnation treatment on the single plate, and pasting two processed single plate products to obtain a bipolar plate product. Most of these patents use impregnation technology to prepare bipolar plates, with a relatively long process flow, relatively harsh operating conditions, environmental protection risks in the post-treatment of the introduced solvent, and a relatively high manufacturing cost of the bipolar plate. Therefore, it is an urgent problem to be solved by the present invention to develop a flexible graphite composite bipolar plate for a flow battery and a preparation method thereof. Summary of the Invention

[0005] The object of the present invention is to solve the problems existing in the existing bipolar plates using the impregnation preparation technology, such as relatively long process flow, harsh operating conditions, environmental protection risks caused by the introduction of solvents, and relatively high manufacturing costs. A flexible graphite composite bipolar plate for a flow battery and a preparation method thereof are provided. The preparation method of the present invention is simple, requires low equipment investment, has few raw material types and is easy to obtain, has a short process flow, does not cause environmental protection problems, and the prepared composite bipolar plate has high conductivity and high mechanical strength.

[0006] To achieve the above object, the present invention is realized through the following technical solutions:

[0007] A flexible graphite composite bipolar plate for a flow battery of the present invention, the composite bipolar plate is prepared by mixing expandable graphite, carbon material, and organic polymer powder, then performing melt pulverization, then cooling and molding to obtain a prefabricated plate, and then heating, pressurizing, and cooling to cure and form.

[0008] The thickness of the composite bipolar plate of the present invention is 0.5 - 3 mm, the density is 1.5 - 2.0 g / cm 3 , the conductivity is 300 - 400 S / cm, the tensile strength ≥ 20 MPa, and the flexural strength ≥ 35 MPa.

[0009] The expandable graphite of the present invention has a size of 50 - 200 mesh, an expansion temperature of 800 - 1200 °C, an expansion time of 0.5 - 1 min, and an expansion volume of 150 - 300 mL / g.

[0010] The carbon material of the present invention is one or a mixture of several of conductive carbon black, Ketjen black, carbon nanotubes, and graphene; the organic polymer is one or a mixture of several of polypropylene resin, furan resin, polyvinylidene fluoride resin, acrylic resin, phenolic resin, and epoxy resin. The particle size of the organic polymer is 800 - 1200 mesh, and the melt index is 4 - 25 g / 10 min.

[0011] The mass ratio of the expandable graphite, carbon material, and organic polymer of the present invention is (60 - 80):(5 - 10):(20 - 30).

[0012] The present invention also provides a preparation method of a flexible graphite composite bipolar plate for a flow battery, including the following steps:

[0013] (1) Feeding expandable graphite into an expansion furnace for expansion to obtain expanded graphite;

[0014] (2) Mixing the expanded graphite and carbon material in a high-speed mixer to obtain a conductive filler mixture; then mixing the organic polymer powder and the conductive filler mixture in a high-speed mixer to obtain a mixed powder;

[0015] (3) Spread and melt the mixed powder in an oven to obtain a melt, and crush and screen the melt to obtain a powdered melt;

[0016] (4) Lay the powdered melt in a mold and apply pressure to the mold to obtain a precast slab;

[0017] (5) Obtain the product after heat preservation, pressure application, cooling and curing of the precast slab.

[0018] Preferably, in step (2) of the present invention, the mixing and stirring time of expanded graphite and carbon material is 10 - 20 min; the mixing and stirring time of the mixture of organic polymer and conductive filler is 1 - 2 h, and the rotation speed of the high-speed mixer is 500 - 1000 r / min.

[0019] Preferably, in step (3) of the present invention, the melting temperature of the mixed powder is 170 - 230 °C, the melting time is 30 - 45 min, the mesh number of the sieve is 100 - 150 meshes, and the screening rate is 90 - 100%.

[0020] Preferably, in step (4) of the present invention, the laying thickness of the powdered melt in the mold is 3 - 8 mm, the bulk density is 0.3 - 0.8 g / cm 3 , the molding pressure is 1 - 3 MPa, the molding time is 10 - 20 s, and the thickness of the powdered material after molding is 2 - 4 mm.

[0021] Preferably, in step (5) of the present invention, the molding temperature is 180 - 230 °C, the heat preservation time is 15 - 30 min, the primary molding pressure is 15 - 20 MPa, the primary molding time is 3 - 5 min, the secondary molding pressure is 25 - 30 MPa, and the secondary molding time is 1 - 3 min.

[0022] Based on the high electrical conductivity and corrosion resistance of flexible graphite materials, the present invention develops a new preparation process to improve the tensile strength and flexural strength of the materials, which can fully meet the application requirements of flow battery stacks. The principle of the present invention is as follows: The present invention uses expandable graphite after expansion as the main conductive filler to form a conductive network framework, and carbon materials as secondary conductive fillers to fill the pores between expandable graphite particles. Functional groups such as hydroxyl, carboxyl, ketone, and carbonyl groups in expandable graphite interact with functional groups such as hydroxyl and carbonyl groups in carbon materials to penetrate the conductive path, thereby improving the electrical conductivity of the composite bipolar plate. The organic polymer is fully mixed with the conductive filler, and heat preservation melting is carried out to utilize the thermal fluidity of the organic polymer to make the organic polymer enter the pores of the conductive filler, reducing the porosity of the conductive filler. The molten powder is cold-molded to form a prefabricated plate with a continuous conductive path. By using the property that the organic polymer softens when heated and re-hardens when cooled, the organic polymer is uniformly distributed and cured to obtain a composite bipolar plate. Through the selection of the above-mentioned organic polymer and the corresponding cooperation of expandable graphite and carbon materials, the mechanical strength and electrical conductivity of the bipolar plate can be balanced.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] The present invention provides a flexible graphite composite bipolar plate for a flow battery and a preparation method thereof. Graphite is used as the main conductive filler to form a conductive network framework, and the organic polymer is fully mixed with the conductive filler, and the organic polymer is melted to reduce the porosity of the conductive filler. The raw material preparation process is simple, and the preparation process does not involve any toxic, harmful, and polluting substances, which is clean, environmentally friendly, and low-cost. The molten powder is cold-molded to form a prefabricated plate with a continuous conductive path. By using the property that the organic polymer softens when heated and re-hardens when cooled, the tensile strength and flexural strength of the bipolar plate product are improved. The method of the present invention is simple to operate, has low equipment requirements, and low comprehensive cost. The obtained finished bipolar plate can simultaneously have the characteristics of high electrical conductivity, high strength, high toughness, high temperature resistance, and corrosion resistance, fully meeting the application requirements of flow battery stacks.

[0025] Description of the drawings

[0026] Figure 1 It is the process flow chart of the preparation method of the present invention. Detailed implementation manners

[0027] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and specific embodiments. However, those skilled in the art will understand that the following described embodiments are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention. For those conditions not specified in the embodiments, they shall be carried out according to the conventional conditions or the conditions recommended by the manufacturer. For those reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0028] Example 1

[0029] A flexible graphite composite bipolar plate for a flow battery in this embodiment, its preparation process flow is shown in Figure 1 , and includes the following steps:

[0030] (1) Expand expandable graphite with a particle size of 50 mesh in an expansion furnace at a temperature of 1000 °C for 1 min to obtain expanded graphite with an expansion volume of 200 mL / g;

[0031] (2) Weigh expanded graphite and conductive carbon black according to a mass ratio of 9:1, place them in a mixer with a rotation speed of 1000 r / min and mix for 20 min to prepare a conductive filler mixture; according to a mass ratio of 1:3, mix the polyvinylidene fluoride resin powder with a particle size of 1000 mesh and a melt index of 4 g / 10 min with the conductive filler mixture at a rotation speed of 1000 r / min for 1 h to prepare a mixed powder;

[0032] (3) Spread the mixed powder in an oven at 220 °C and melt it flat for 30 min to obtain a melt; crush and screen the melt to obtain melt powder, the mesh number of the screen is 100 mesh, and the screening rate is 95%;

[0033] (4) Spread the melt powder with a flat thickness of 5 mm in a mold, the bulk density is 0.5 g / cm 3 , use a hydraulic press to apply pressure to the mold at 1 MPa for 10 s to obtain a prefabricated plate, and the thickness of the powder after molding is 3 mm;

[0034] (5) Place the prefabricated plate on a flat vulcanizer at a temperature of 220 °C and keep it warm for 30 min, then keep it warm and pressurized at 20 MPa for 3 min, transfer it to a hydraulic press and keep it pressurized at 25 MPa for 1.5 min, and then cool and solidify to obtain.

[0035] Example 2

[0036] Since the melt index of the organic polymer has a great influence on the distribution of the resin and the conductive filler, in order to verify the influence of different melt indexes on the composite bipolar plate, polypropylene resin with a melt index of 20 g / 10 min was selected as the organic polymer in this embodiment, and its preparation process flow is shown in Figure 1 , including the following steps:

[0037] (1) Expand expandable graphite with a particle size of 50 mesh in an expansion furnace at a temperature of 1000 °C for 1 minute to obtain expanded graphite with an expansion volume of 200 mL / g;

[0038] (2) Weigh expanded graphite and conductive carbon black according to a mass ratio of 10:1, place them in a mixer with a rotation speed of 1000 r / min and mix for 20 minutes to prepare a conductive filler mixture; mix polypropylene resin powder with a particle size of 1000 mesh and the conductive filler mixture at a rotation speed of 1000 r / min according to a mass ratio of 1:4 for 1 hour to prepare a mixed powder;

[0039] (3) Spread the mixed powder in an oven at 220 °C and melt it for 30 minutes to obtain a melt; crush and screen the melt to obtain melt powder, with a screen mesh of 100 mesh and a screening rate of 95%;

[0040] (4) Spread the melt powder with a thickness of 5 mm in a mold, with a bulk density of 0.5 g / cm 3 , use a hydraulic press to apply a pressure of 1 MPa to the mold and press for 10 s to obtain a prefabricated board, and the thickness of the powder after pressing is 3 mm;

[0041] (5) Place the prefabricated board on a flat vulcanizer at a temperature of 220 °C and keep it warm for 30 minutes, then keep it warm and pressurized at 20 MPa for 3 minutes, transfer it to a hydraulic press and keep it pressurized at 25 MPa for 1.5 minutes, and then cool and solidify to obtain.

[0042] Example 3

[0043] Since the content of the organic polymer has a great influence on the conductivity of the composite bipolar plate, in order to verify the influence of different resin contents on the composite bipolar plate, the number of parts of polyvinylidene fluoride resin powder was reduced in this embodiment, and its preparation process flow is shown in Figure 1 , including the following steps:

[0044] (1) Expand expandable graphite with a particle size of 50 mesh in an expansion furnace at a temperature of 1000 °C for 1 minute to obtain expanded graphite with an expansion volume of 200 mL / g;

[0045] (2) Weigh expandable graphite and conductive carbon black according to a mass ratio of 9:1, place them in a mixer with a rotation speed of 1000 r / min, and mix for 20 min to obtain a conductive filler mixture; according to a mass ratio of 1:5, mix polyvinylidene fluoride resin powder with a particle size of 1000 mesh and a melt index of 20 g / 10 min with the conductive filler mixture at a rotation speed of 1000 r / min for 1 h to obtain a mixed powder;

[0046] (3) Spread the mixed powder in an oven at 220 °C and melt it flat for 30 min to obtain a melt; crush the melt and screen it to obtain melt powder, with a screen mesh of 100 mesh and a screening rate of 95%;

[0047] (4) Spread the melt powder with a thickness of 5 mm in a mold, with a bulk density of 0.5 g / cm 3 , use a hydraulic press to apply a pressure of 1 MPa to the mold and press for 10 s to obtain a prefabricated board. After pressing, the thickness of the powder is 3 mm;

[0048] (5) Place the prefabricated board on a flat vulcanizer at a temperature of 220 °C, keep it warm for 30 min, then keep it warm and under pressure at 20 MPa for 3 min, transfer it to a hydraulic press, keep it under pressure at 25 MPa for 1.5 min, and then cool and solidify to obtain it.

[0049] Example 4

[0050] Since the levels of the melting temperature and the molding temperature have a great influence on the distribution of the resin and the conductive filler, in this example, relatively low baking temperature and molding temperature are selected. The preparation process flow is shown in Figure 1 , including the following steps:

[0051] (1) Place expandable graphite with a particle size of 50 mesh in an expansion furnace at 1000 °C and expand it for 1 min to obtain expandable graphite with an expansion volume of 200 mL / g;

[0052] (2) Weigh expandable graphite and conductive carbon black according to a mass ratio of 9:1, place them in a mixer with a rotation speed of 1000 r / min, and mix for 20 min to obtain a conductive filler mixture; according to a mass ratio of 1:3, mix polyvinylidene fluoride resin powder with a particle size of 1000 mesh and a melt index of 4 g / 10 min with the conductive filler mixture at a rotation speed of 1000 r / min for 1 h to obtain a mixed powder;

[0053] (3) Spread the mixed powder in an oven at 180 °C and melt it flat for 30 min to obtain a melt; crush the melt and screen it to obtain melt powder, with a screen mesh of 100 mesh and a screening rate of 95%;

[0054] (4) Spread the melt powder with a thickness of 5 mm in a mold, with a bulk density of 0.5 g / cm 3, use a hydraulic press to apply pressure to the mold at 1 MPa for 10 s to obtain a prefabricated board, and the thickness of the powder after molding is 3 mm;

[0055] (5) Place the prefabricated board on a flat vulcanizing machine at 180 °C for heat preservation for 30 min, then keep the temperature and pressure at 20 MPa for 3 min, transfer it to a hydraulic press and keep the pressure at 25 MPa for 1.5 min, and then cool and solidify to obtain.

[0056] Comparative Example 1

[0057] Since the melt index of the organic polymer has a great influence on the distribution of the resin and the conductive filler, in order to verify the influence of different melt indexes on the composite bipolar plate, polyvinylidene fluoride resin with a melt index of 0.8 g / 10 min was selected in this example, and the rest of the preparation process flow and experimental parameters were the same as those in Example 1.

[0058] Comparative Example 2

[0059] Since both the content of the resin and the content of the conductive carbon black have an impact on the conductivity of the composite bipolar plate, polyvinylidene fluoride resin with a melt index of 4 g / 10 min was selected in this example, and the polyvinylidene fluoride resin powder was mixed with the conductive filler according to a mass ratio of 3:7, and its preparation process flow and experimental parameters were the same as those in Example 1.

[0060] Comparative Example 3

[0061] Since both the content of the resin and the content of the conductive carbon black have an impact on the conductivity of the composite bipolar plate, in this example, expanded graphite and conductive carbon black were weighed according to a mass ratio of 7:3 to prepare a conductive filler mixture; polyvinylidene fluoride resin with a melt index of 4 g / 10 min was selected, and the polyvinylidene fluoride resin powder was mixed with the conductive filler according to a mass ratio of 1:3, and the rest of the preparation process flow and experimental parameters were the same as those in Example 1.

[0062] Comparative Example 4

[0063] Since there will be a certain proportion of solvent residues in the composite bipolar plate prepared by the traditional impregnation method, which will affect the mechanical properties, conductivity and corrosion resistance of the composite bipolar plate, expanded graphite was selected as the conductive filler substrate and the DMF solution of polyvinylidene fluoride resin was used as the impregnation solution in this example, including the following steps:

[0064] (1) Place expandable graphite with a particle size of 50 mesh in an expansion furnace at 1000 °C for 1 min to obtain expanded graphite with an expansion volume of 200 mL / g;

[0065] (2) Add the expanded graphite to the cloth spreading equipment for spreading, and then send it into a calender to apply pressure at 90 MPa to obtain an expanded graphite substrate;

[0066] (3) Dissolve PVDF pellets and DMF in a 1:4 ratio under stirring conditions while heating to obtain a resin adhesive with a 20% content.

[0067] (4) Immerse the expanded graphite substrate into an impregnation kettle filled with the resin adhesive with a 20% content and impregnate for 8 h under a vacuum condition of 0.1 MPa.

[0068] (5) Keep the impregnated plate in a drying tunnel at 150 °C for 8 h and recover most of the DMF. After cooling and curing, remove the excess resin residue on the surface with a grinding machine to obtain the product.

[0069] Test the composite bipolar plates prepared in the above Examples 1-4 and Comparative Examples 1-4 respectively, and the test results are shown in Table 1 below.

[0070] Table 1

[0071]

[0072] As can be seen from Table 1 above and Examples 1-4, the composite bipolar plates prepared by using expandable graphite, conductive carbon black and combining polyvinylidene fluoride resin with a specific melt index in an appropriate ratio have both excellent electrical conductivity and mechanical strength. Their conductivity reaches 300-400 S / cm, the tensile strength can reach 20-22 MPa, and the flexural strength can also reach 35-38 MPa.

[0073] As can be seen from Examples 1-4 and Comparative Example 1, too high or too low melt index of polyvinylidene fluoride resin reflecting fluidity and the level of molding temperature have a greater impact on the distribution of resin and conductive filler. Controlling the melt index within a reasonable range can improve the mechanical strength and electrical conductivity of the flexible graphite composite bipolar plate.

[0074] As can be seen from Example 1 and Comparative Examples 2-3, the content of resin and the content of conductive carbon black both have an impact on electrical conductivity. Selecting appropriate parameters is beneficial to the orderly arrangement and stacking of conductive fillers, reducing the porosity of conductive fillers, and being able to construct more conductive paths, greatly improving the electrical conductivity and mechanical strength of the flexible graphite composite bipolar plate.

[0075] As can be seen from Example 1 and Comparative Example 4, the composite bipolar plates prepared by the traditional impregnation method have adverse effects on mechanical properties, electrical conductivity and corrosion resistance. Since the porosity of the expanded graphite after being pressed into a substrate is relatively small, the resin cannot be completely dispersed, and most of the resin is concentrated on the surface layer of the composite bipolar plate. Since the resin is not conductive, the electrical conductivity of the bipolar plate is low, the mechanical properties are poor, and grinding is required. In addition, the solvent remaining in the composite bipolar plate makes the corrosion resistance of the bipolar plate worse.

[0076] The above are only several embodiments of the present application and do not impose any form of limitation on the present application. Although the present application is disclosed above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the art, without departing from the technical solution of the present application, making some changes or modifications using the disclosed technical content is equivalent to equivalent implementation cases and all fall within the scope of the technical solution.

Claims

1. A flexible graphite composite bipolar plate for a flow battery, characterized in that: The composite bipolar plate is prepared by mixing expandable graphite, carbon material, and organic polymer powder, followed by melting and pulverizing, cooling and molding to obtain a preform, and then heating, pressurizing, and cooling and curing.

2. The flexible graphite composite bipolar plate for a flow battery according to claim 1, wherein: The thickness of the composite bipolar plate is 0.5~3 mm, the density is 1.5~2.0 g / cm 3 , the conductivity is 300~400 S / cm, the tensile strength ≥20 MPa, and the flexural strength ≥35 MPa.

3. The flexible graphite composite bipolar plate for a flow battery according to claim 1, wherein: The expandable graphite has a size of 50 - 200 mesh, an expansion temperature of 800 - 1200 °C, an expansion time of 0.5 - 1 min, and an expansion volume of 150 - 300 mL / g.

4. The flexible graphite composite bipolar plate for a flow battery according to claim 1, wherein: The carbon material is one or a mixture of conductive carbon black, Ketjen black, carbon nanotubes, and graphene; the organic polymer is one or a mixture of polypropylene resin, furan resin, polyvinylidene fluoride resin, acrylic resin, phenolic resin, and epoxy resin. The particle size of the organic polymer is 800 - 1200 mesh, and the melt index is 4 - 25 g / 10min.

5. A flexible graphite composite bipolar plate for a flow battery, characterized in that: The mass ratio of the expandable graphite, carbon material, and organic polymer is (60 - 80):(5 - 10):(20 - 30).

6. The preparation method of a flexible graphite composite bipolar plate for a flow battery according to any one of claims 1-5, characterized in that It includes the following steps: (1) Feeding the expandable graphite into an expansion furnace to obtain expanded graphite after expansion. (2) Mixing the expanded graphite and the carbon material in a high-speed mixer to obtain a conductive filler mixture; then mixing the organic polymer powder and the conductive filler mixture in the high-speed mixer to obtain a mixed powder. (3) Spreading and melting the mixed powder in an oven to obtain a melt, and pulverizing and sieving the melt to obtain a melt powder. (4) Laying the melt powder in a mold and applying pressure to the mold to obtain a preform. (5) Obtaining the product after heat preservation, pressurization, cooling, and curing of the preform.

7. The preparation method of a flexible graphite composite bipolar plate for a flow battery according to claim 6, characterized in that: In step (2), the mixing and stirring time of the expanded graphite and the carbon material is 10 - 20 min; the mixing and stirring time of the organic polymer and the conductive filler mixture is 1 - 2 h, and the rotation speed of the high-speed mixer is 500 - 1000 r / min.

8. The preparation method of a flexible graphite composite bipolar plate for a flow battery according to claim 6, characterized in that: In step (3), the melting temperature of the mixed powder is 170 - 230 °C, the melting time is 30 - 45 min, the mesh number of the sieve is 100 - 150 mesh, and the sieving rate is 90 - 100%.

9. The preparation method of a flexible graphite composite bipolar plate for a flow battery according to claim 6, wherein: In the step (4), the laying thickness of the molten powder material in the mold is 3 to 8 mm, and the bulk density is 0.3 to 0.8 g / cm 3 , the molding pressure is 1 to 3 MPa, the molding time is 10 to 20 s, and the thickness of the powder material after molding is 2 to 4 mm.

10. The preparation method of a flexible graphite composite bipolar plate for a flow battery according to claim 6, wherein: In step (5), the molding temperature is 180 - 230 °C, the heat preservation time is 15 - 30 min, the primary molding pressure is 15 - 20 Mpa, the primary molding time is 3 - 5 min, the secondary molding pressure is 25 - 30 Mpa, and the secondary molding time is 1 - 3 min.

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