Preparation method of a high-strength composite bipolar plate

Abstract

The present invention relates to a preparation method of a high-strength composite bipolar plate, belonging to the field of electrochemistry technology. This method uses green and natural polymer lignin as raw material, and an alkaline-resistant surfactant as a structure-directing agent to carbonize lignin by a one-step hydrothermal method to obtain biomass porous carbon spheres. By blending with one-dimensional and two-dimensional carbon materials, a rich conductive network is formed inside the bipolar plate to improve the electrical conductivity of the bipolar plate. At the same time, based on the large specific surface area and the rich pore structure on the surface of the porous carbon spheres, effective adsorption of fluoropolymer powder can be achieved during the air flow mixing process. During the molding process, the molten polymer penetrates into the pores of the porous carbon spheres, thereby reducing the internal gaps / holes of the bipolar plate and improving the mechanical strength of the bipolar plate, realizing the preparation of a high-strength composite bipolar plate.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical technology, and particularly relates to a method for preparing a high-strength composite bipolar plate. Background Art

[0002] The bipolar plate is one of the key materials for flow batteries and fuel cell stacks, playing the roles of collecting current and supporting electrodes. The carbon / polymer composite bipolar plate combines the good mechanical properties and processability of polymer resins and the excellent electrical conductivity of carbon materials, becoming an alternative material to metal bipolar plates and graphite bipolar plates.

[0003] Composite bipolar plates are usually prepared by mixing various carbon materials, using resin as a binder, and adopting injection molding or compression molding. Carbon materials can effectively improve the electrical conductivity of bipolar plates, and resins enhance the mechanical strength and airtightness of bipolar plates. However, due to the poor interfacial compatibility between carbon materials and resins, gaps / holes will appear at the interface during the blending process, which is prone to gas / electrolyte penetration, affecting the safe and stable operation of the battery. At the same time, continuous gaps or holes are likely to form stress concentration points, affecting the mechanical strength of the bipolar plate.

[0004] To solve this problem, currently, some work has focused on modifying carbon materials or resins. The Chinese patent document with the publication number CN117325491A discloses a composite bipolar plate, its preparation method and a fuel cell. Graphite materials are reacted with nitrogen and hexamethyldisilazane vapor to obtain modified graphite. Resin and modified graphite are mixed by a wet method to obtain a composite material, and the composite material is pressed and cured to obtain a composite bipolar plate. This method enhances the compatibility and bonding force between graphite and resin by modifying the graphite material, thereby improving the mechanical strength and airtightness of the composite bipolar plate.

[0005] The Chinese patent document with the publication number CN114976086A discloses a composite graphite bipolar plate for fuel cells and its preparation method. Thermosetting resin polymers are directly grafted onto graphene to prepare modified graphene, and the powder of the modified graphene is hot-pressed to form a bipolar plate. The above method improves the interfacial compatibility between carbon materials and resins by modifying carbon materials or resins, reduces the formation of gaps / holes inside the bipolar plate, and enhances the mechanical strength of the bipolar plate. However, the above modification methods have cumbersome steps and great operation difficulty, increasing the complexity of the bipolar plate manufacturing process and being not conducive to industrial production and application. Summary of the Invention

[0006] The object of the present invention is to provide a preparation method of a high-strength composite bipolar plate. This method prepares porous carbon spheres through a one-step hydrothermal method, uses air flow to improve the adsorption capacity of the porous carbon spheres for polymer resin powder, enhances the interfacial compatibility between the polymer resin and the porous carbon spheres, and further reduces the internal gaps / holes in the bipolar plate during the forming process of the bipolar plate, improves the mechanical strength of the bipolar plate, and realizes the preparation of the high-strength composite bipolar plate.

[0007] To achieve this object, the technical solution of the present invention is as follows:

[0008] A preparation method of a high-strength composite bipolar plate, the steps are as follows:

[0009] 1) Mix lignin, an aqueous surfactant solution, and an aqueous alkaline solution, perform hydrothermal treatment to obtain a suspension, filter, wash, and dry the suspension to prepare porous carbon spheres;

[0010] (2) Place a fluoropolymer, porous carbon spheres, and a carbon material in an air flow mixer, and mix them using air flow to obtain a blend;

[0011] (3) Subject the blend to molding pressing to prepare a high-strength composite bipolar plate.

[0012] Preferably, in the step (1), the surfactant is one of sodium dodecyl sulfate, fatty alcohol polyoxyethylene ether sulfate, or secondary alkyl sulfonate; the mass fraction of the aqueous surfactant solution is 0.18% - 0.4%; the aqueous alkaline solution is one of an aqueous sodium hydroxide or potassium hydroxide solution; the mass fraction of the aqueous alkaline solution is 5% - 7%; the mass ratio of lignin, surfactant, and the alkaline compound in the aqueous alkaline solution is 1:0.03 - 0.1:1 - 2.

[0013] Preferably, in the step (1), the hydrothermal treatment temperature is 140 - 180 °C, and the hydrothermal treatment time is 6 - 10 h.

[0014] Preferably, in the step (2), the fluoropolymer is one of ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride, or polyvinylidene fluoride-hexafluoropropylene copolymer; the particle size of the fluoropolymer is 5 nanometers to 2 micrometers; the carbon material is one or a combination of two of flake graphite, carbon nanotubes, or carbon nanofibers; the mass ratio of the fluoropolymer, the porous carbon spheres, and the carbon material is 1:0.2 to 0.4:0.33 to 1.25. Compared with conventional thermoplastic polymers such as high-density polyethylene and polypropylene, the composite bipolar plate prepared from the fluoropolymer as a raw material has a higher electrical conductivity, which is attributed to the higher polarity of the fluoropolymer, which can enhance the dispersion performance of the carbon material in the PVDF matrix, form a rich conductive network, and thus improve the electrical conductivity of the bipolar plate. Based on the large specific surface area and rich mesoporous structure of the porous carbon spheres, the ability to adsorb or embed the fluoropolymer powder on the surface of the porous carbon spheres can be achieved during the blending process. During the molding process of the blended material by compression molding, the molten polymer will penetrate into the pores of the porous carbon spheres, thereby improving the interfacial bonding force of the composite material and reducing the internal pores.

[0015] Preferably, in the step (2), the air flow mixing pressure is 0.5 to 1 MPa, and the mixing time is 30 to 60 s. The porous carbon spheres have a rich mesoporous structure on the surface. When using compressed air to jet the air flow to drive the movement of the blended material, the rich pore structure on the surface of the porous carbon spheres provides a large number of adsorption sites, which can improve the ability of the fluoropolymer powder to adsorb or embed on the surface of the porous carbon spheres. During the molding process of the blended material by compression molding, the molten polymer will penetrate into the pores of the porous carbon spheres, thereby effectively improving the interfacial bonding force of the composite material and reducing the internal pores.

[0016] More preferably, the carbon material is a mixture of flake graphite and carbon nanotubes or a mixture of flake graphite and carbon nanofibers, and the mass ratio of flake graphite to carbon nanotubes or carbon nanofibers is 1:0.2 to 0.3.

[0017] Preferably, in the step (3), the compression molding temperature is 180 to 230 °C, the pressure is 5 to 20 MPa, and the time is 10 to 15 min.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) The present invention uses lignin, a green and natural polymer, as the raw material and an alkali-resistant surfactant as the structure-directing agent to carbonize lignin by a one-step hydrothermal method to obtain biomass porous carbon spheres. During this reaction process, the surfactant molecules self-assemble in the aqueous solution. When the concentration of the surfactant aqueous solution reaches or exceeds the critical micelle concentration, spherical micelles will be formed, inducing the formation of spherical nanoparticles of lignin during the hydrothermal carbonization process. In a strongly alkaline environment, functional groups in lignin, such as phenolic hydroxyl groups, will undergo a removal reaction. In addition, alkaline substances such as potassium hydroxide can act as activators during the hydrothermal carbonization process to promote the formation of the surface pore structure.

[0020] (2) The present invention uses air flow to mix fluoropolymer powder, porous carbon spheres, and carbon materials. Based on the large specific surface area and rich pore structure of the porous carbon spheres, effective adsorption of the fluoropolymer powder on the surface of the porous carbon spheres can be achieved, thereby improving the interfacial compatibility between the fluoropolymer powder and the porous carbon spheres, reducing the internal gaps / holes in the bipolar plate, and improving the mechanical strength of the bipolar plate. Compared with the prior art, the present invention has the characteristics of simple preparation process, short process, and high efficiency.

[0021] (3) The present invention blends zero-dimensional porous carbon spheres with one-dimensional and two-dimensional carbon materials such as flake graphite and carbon nanofibers to synergistically increase the conductive network inside the bipolar plate, and the conductivity of the bipolar plate is greatly improved.

[0022] (4) The preparation method of a high-strength composite bipolar plate provided by the present invention, compared with the existing preparation methods, does not require modification of the carbon material or polymer. The composite bipolar plate is prepared by a one-step hydrothermal method, the operation steps are simple, and the product has good conductivity and mechanical strength. Description of the Drawings

[0023] Figure 1 It is a scanning electron microscope picture of the porous carbon spheres prepared in step (1) of Example 1 of the present invention. A is the scanning electron microscope picture of the porous carbon spheres, and B is the enlarged scanning electron microscope picture of the porous carbon spheres.

[0024] Figure 2 It is a scanning electron microscope picture of the blend obtained in step (2) of Example 2 of the present invention. Detailed Embodiments

[0025] The present invention will be further described below in conjunction with the drawings and embodiments, but is not limited thereto.

[0026] At the same time, the experimental methods described in the following embodiments are all conventional methods unless otherwise specified; the reagents and materials are all commercially available products unless otherwise specified. Example 1

[0027] (1) Weigh 6 g of lignin, 150 mL of sodium dodecyl sulfate with a mass fraction of 0.4%, and 240 mL of potassium hydroxide aqueous solution with a mass fraction of 5% into a beaker, stir at 300 rpm for 30 min to obtain a mixed solution; pour the mixed solution into a hydrothermal reaction kettle, react at 180 °C for 8 h, and after the reaction kettle cools to room temperature, filter the obtained suspension, wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres;

[0028] (2) Weigh 4.8 g of polyvinylidene fluoride - hexafluoropropylene copolymer powder with a particle size of 5 nm, 1.8 g of porous carbon spheres, 2.4 g of flake graphite, and 0.6 g of carbon nanofibers, add them into a mixing pipeline, and mix the above substances by using air flow. Set the air flow pressure during feeding to 0.1 MPa, the air flow pressure during mixing to 0.7 MPa, and the mixing time to 40 s. Degas the mixed sample by using a cyclone dust collector, and set the air flow pressure to 0.1 MPa to obtain a blend;

[0029] (3) Place the blend in a molding die with a thickness of 0.3 mm, perform molding under pressure at 190 °C and 20 MPa for 10 min. After the molding under pressure is completed, place the bipolar plate in a water - cooling area, take out the mold after water - cooling for 5 min to obtain a high - strength composite bipolar plate.

[0030] Figure 1 is a scanning electron microscope picture of the porous carbon spheres prepared by the above method. As Figure 1 can be seen, using lignin as the raw material, the porous carbon spheres prepared by one - step hydrothermal method have a size between 300 - 500 nm, are spherical as a whole, and have a relatively uniform size distribution. And through magnification, it can be seen that there are abundant mesoporous structures with a size of 20 - 30 nm on the surface of the carbon spheres. Therefore, during the process of mixing the fluorine - containing polymer powder by air flow, the large specific surface area and abundant mesoporous structures of the porous carbon spheres can effectively adsorb the polymer resin powder. During the subsequent molding under pressure process, the molten polymer penetrates into the pores of the porous carbon spheres, thereby reducing the interfacial pores between the carbon material and the polymer resin, enhancing the interfacial wetting performance with the porous carbon spheres, improving the interfacial bonding strength, and realizing the preparation of the high - strength composite bipolar plate. Example 2

[0031] (1) Weigh 3 g of lignin, 45 mL of sodium laureth sulfate aqueous solution with a mass fraction of 0.2%, and 50 mL of potassium hydroxide aqueous solution with a mass fraction of 6% into a beaker, stir at 300 rpm for 20 min to obtain a mixed solution; pour the mixed solution into a hydrothermal reaction kettle, react at 140 °C for 10 h, and after the reaction kettle cools to room temperature, filter the obtained suspension, wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres;

[0032] (2) Weigh 3.6 g of ethylene-tetrafluoroethylene copolymer powder with a particle size of 15 nm, 1.2 g of porous carbon spheres, 1 g of flake graphite, and 0.2 g of carbon nanotubes, add them into the mixing pipeline, and use air flow to mix the above substances. Set the air flow pressure during feeding to 0.1 MPa, the air flow pressure during mixing to 1 MPa, and the mixing time to 30 s; Remove the gas from the mixed sample using a cyclone dust collector, set the air flow pressure to 0.2 MPa, and obtain the blend;

[0033] (3) Place the blend in a molding die with a thickness of 0.3 mm, and perform molding under pressure at 230 °C and 5 MPa for 10 min. After the molding under pressure is completed, place the bipolar plate in the water cooling area, take out the mold after water cooling for 5 min, and obtain a high-strength composite bipolar plate.

[0034] Figure 2 It is a scanning electron microscope picture of the fluoropolymer, porous carbon spheres, flake graphite, and carbon nanotubes in Example 2 after air flow mixing. From Figure 2 It can be seen that based on the rich pore structure on the surface of the porous carbon spheres, after air flow mixing, due to the high polarity of the fluoropolymer, the fluoropolymer powder can form an effective coating on the surface of the porous carbon spheres, reducing the pore structure between the porous carbon spheres and the fluoropolymer matrix, which is beneficial to improving the mechanical strength of the bipolar plate and realizing the preparation of a high-strength composite bipolar plate. At the same time, the carbon nanotubes are relatively uniformly dispersed in the blend matrix, bridging with the porous carbon spheres to form a rich network, and the flaky flake graphite intersperses between the porous carbon spheres and the carbon nanotubes, providing a rich contact surface and greatly improving the electrical conductivity of the bipolar plate. Example 3

[0035] (1) Weigh 5 g of lignin, 100 mL of secondary alkyl sulfonate with a mass fraction of 0.3%, and 200 mL of sodium hydroxide aqueous solution with a mass fraction of 5% in a beaker, stir at 300 rpm for 30 min to obtain a mixed solution; Pour the mixed solution into a hydrothermal reaction kettle, react at 160 °C for 6 h. After the reaction kettle cools to room temperature, filter the obtained suspension, wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres;

[0036] (2) Weigh 13 g of polyvinylidene fluoride powder with a particle size of 2 μm, 2.6 g of porous carbon spheres, and 10.4 g of carbon nanofibers, add them into the mixing pipeline, and use air flow to mix the above substances. Set the air flow pressure during feeding to 0.2 MPa, the air flow pressure during mixing to 0.9 MPa, and the mixing time to 60 s; Remove the gas from the mixed sample using a cyclone dust collector, set the air flow pressure to 0.1 MPa, and obtain the blend;

[0037] (3) Place the blend in a molding die with a thickness of 0.3 mm, and perform compression molding at 190 °C and 8 MPa for 15 min. After the compression molding is completed, place the bipolar plate in the water cooling area. Take out the mold after water cooling for 5 min to obtain a high-strength composite bipolar plate. Example 4

[0038] (1) Weigh 4 g of lignin, 120 mL of sodium lauryl polyoxyethylene ether sulfate with a mass fraction of 0.2%, and 100 mL of potassium hydroxide aqueous solution with a mass fraction of 6% into a beaker, and stir at 300 rpm for 50 min to obtain a mixed solution; pour the mixed solution into a hydrothermal reaction kettle, and react at 160 °C for 8 h. After the reaction kettle cools to room temperature, filter the obtained suspension. Wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres;

[0039] (2) Weigh 8 g of polyvinylidene fluoride - hexafluoropropylene copolymer powder with a particle size of 500 nm, 2 g of porous carbon spheres, and 10 g of flake graphite, add them into the mixing pipeline, and use air flow to mix the above substances. Set the air flow pressure during feeding to 0.1 MPa, the air flow pressure during mixing to 0.8 MPa, and the mixing time to 45 s; degas the mixed sample using a cyclone dust collector, and set the air flow pressure to 0.1 MPa to obtain a blend;

[0040] (3) Place the blend in a molding die with a thickness of 0.3 mm, and perform compression molding at 180 °C and 15 MPa for 12 min. After the compression molding is completed, place the bipolar plate in the water cooling area. Take out the mold after water cooling for 5 min to obtain a high-strength composite bipolar plate. Example 5

[0041] (1) Weigh 9 g of lignin, 150 mL of secondary alkyl sulfate with a mass fraction of 0.18%, and 200 mL of sodium hydroxide aqueous solution with a mass fraction of 7% into a beaker, and stir at 300 rpm for 40 min to obtain a mixed solution; pour the mixed solution into a hydrothermal reaction kettle, and react at 180 °C for 10 h. After the reaction kettle cools to room temperature, filter the obtained suspension. Wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres;

[0042] (2) Weigh 3 g of polyvinylidene fluoride powder with a particle size of 1 μm, 1.2 g of porous carbon spheres, 1 g of flake graphite, and 0.3 g of carbon nanofibers, add them into the mixing pipeline, and use air flow to mix the above substances. Set the air flow pressure during feeding to 0.1 MPa, the air flow pressure during mixing to 1 MPa, and the mixing time to 60 s; degas the mixed sample using a cyclone dust collector, and set the air flow pressure to 0.0 MPa to obtain a blend;

[0043] (3) Place the blend in a molding die with a thickness of 0.3 mm, and perform compression molding at 200 °C and 10 MPa for 15 min. After the compression molding is completed, place the bipolar plate in the water cooling area. Take out the mold after water cooling for 5 min to obtain a high-strength composite bipolar plate. Comparative Example 1

[0044] (1) Weigh 4.8 g of polyvinylidene fluoride-hexafluoropropylene copolymer powder with a particle size of 5 nm, 1.8 g of graphite powder, 2.4 g of flake graphite, and 0.6 g of carbon nanotubes, add them into the mixing pipeline, and use air flow to mix the above substances. Set the air flow pressure during feeding to 0.1 MPa, the air flow pressure during mixing to 0.7 MPa, and the mixing time to 40 s. Degas the mixed sample using a cyclone dust collector, and set the air flow pressure to 0.1 MPa to obtain a blend.

[0045] (2) Place the blend in a molding die with a thickness of 0.3 mm, and perform compression molding at 190 °C and 20 MPa for 10 min. After the compression molding is completed, place the bipolar plate in the water cooling area. Take out the mold after water cooling for 5 min to obtain a composite bipolar plate. Comparative Example 2

[0046] (1) Weigh 6 g of lignin, 150 mL of sodium dodecyl sulfate with a mass fraction of 0.4%, and 240 mL of potassium hydroxide aqueous solution with a mass fraction of 5% in a beaker, and stir at 300 rpm for 30 min to obtain a mixed solution; pour the mixed solution into a hydrothermal reaction kettle, and react at 180 °C for 8 h. After the reaction kettle cools to room temperature, filter the obtained suspension, wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres.

[0047] (2) Weigh 4.8 g of polyethylene powder with a particle size of 5 nm, 1.8 g of porous carbon spheres, 2.4 g of flake graphite, and 0.6 g of carbon nanotubes, add them into the mixing pipeline, and use air flow to mix the above substances. Set the air flow pressure during feeding to 0.1 MPa, the air flow pressure during mixing to 0.7 MPa, and the mixing time to 40 s. Degas the mixed sample using a cyclone dust collector, and set the air flow pressure to 0.1 MPa to obtain a blend.

[0048] (3) Place the blend in a molding die with a thickness of 0.3 mm, and perform compression molding at 160 °C and 20 MPa for 10 min. After the compression molding is completed, place the bipolar plate in the water cooling area. Take out the mold after water cooling for 5 min to obtain a composite bipolar plate. Comparative Example 3

[0049] (1) Weigh 6 g of lignin, 150 mL of sodium dodecyl sulfate with a mass fraction of 0.4%, and 240 mL of potassium hydroxide aqueous solution with a mass fraction of 5% into a beaker, stir at 300 rpm for 30 min to obtain a mixed solution; pour the mixed solution into a hydrothermal reactor, react at 180 °C for 8 h, after the reactor cools to room temperature, filter the obtained suspension, wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres;

[0050] (2) Weigh 24 g of polyvinylidene fluoride - hexafluoropropylene copolymer powder with a particle size of 5 nm, 9 g of porous carbon spheres, 12 g of flake graphite, and 3 g of carbon nanotubes, add the above substances into a torque rheometer, set the melt blending temperature to 200 °C, the mixing speed to 60 rpm, and the mixing time to 10 min to obtain a blend;

[0051] (3) Place the blend in a molding die with a thickness of 0.3 mm, perform molding under pressure at 190 °C and 20 MPa for 10 min, after the molding under pressure is completed, place the bipolar plate in a water cooling area, take out the die after water cooling for 5 min to obtain a composite bipolar plate. Comparative Example 4

[0052] (1) Weigh 6 g of lignin, 150 mL of sodium dodecyl sulfate with a mass fraction of 0.4%, and 240 mL of potassium hydroxide aqueous solution with a mass fraction of 5% into a beaker, stir at 300 rpm for 30 min to obtain a mixed solution; pour the mixed solution into a hydrothermal reactor, react at 180 °C for 8 h, after the reactor cools to room temperature, filter the obtained suspension, wash the filtered solid matter with deionized water until the pH value is neutral, and dry at 80 °C to obtain porous carbon spheres;

[0053] (2) Weigh 4.8 g of polyvinylidene fluoride - hexafluoropropylene copolymer powder with a particle size of 5 nm and 4.8 g of porous carbon spheres, add them into a mixing pipeline, mix the above substances using air flow, set the air flow pressure during feeding to 0.1 MPa, the air flow pressure during mixing to 0.7 MPa, and the mixing time to 40 s, degas the mixed sample using a cyclone dust collector, set the air flow pressure to 0.1 MPa to obtain a blend;

[0054] (3) Place the blend in a molding die with a thickness of 0.3 mm, perform molding under pressure at 190 °C and 20 MPa for 10 min, after the molding under pressure is completed, place the bipolar plate in a water cooling area, take out the die after water cooling for 5 min to obtain a composite bipolar plate.

[0055]

[0056] As can be seen from Table 1, the composite bipolar plates prepared in Examples 1 to 5 exhibit high electrical conductivity, excellent tensile strength and flexural strength. The electrical conductivity is 92 - 133 S / cm, the tensile strength is 30.2 - 36.8 MPa, and the flexural strength is 56.3 - 71.9 MPa. This is mainly attributed to the large specific surface area and abundant mesoporous structure of the porous carbon spheres. After gas flow mixing, due to the high polarity of the fluoropolymer, the affinity between the fluoropolymer and the carbon material is improved, thus enabling effective coating of the polymer on the surface of the porous carbon spheres. During the molding process, the molten fluoropolymer is more likely to penetrate into the pores of the porous carbon spheres, thereby reducing the pore structure between the porous carbon spheres and the fluoropolymer matrix, which is beneficial to improving the mechanical strength of the bipolar plate and realizing the preparation of a high-strength composite bipolar plate. In addition, by blending zero-dimensional porous carbon spheres with one-dimensional and two-dimensional carbon materials, carbon materials of multiple dimensions can synergistically enhance the effect, constructing a rich conductive network, thereby greatly improving the electrical conductivity of the bipolar plate.

[0057] When graphite powder is used instead of porous carbon spheres, as can be seen from Comparative Example 1, the tensile strength and flexural strength of the bipolar plate are significantly reduced. This is because the graphite powder has fewer pore structures on its surface. During the blending process with the polymer, due to the poor interfacial compatibility between the two, continuous gaps or holes will appear at the interface during the blending process, resulting in poor mechanical strength and electrical conductivity of the bipolar plate.

[0058] When the polymer matrix is changed to polyethylene, as can be seen from Comparative Example 2, the electrical conductivity, tensile strength and flexural strength of the bipolar plate all decrease significantly. This is because polyethylene is a non-polar polymer and has a weak interaction with the carbon material. During the mixing process, the carbon material is prone to form aggregates, affecting the electrical conductivity and mechanical strength of the bipolar plate.

[0059] When the blending method is changed to melt blending, as can be seen from Comparative Example 3, the electrical conductivity, tensile strength and flexural strength of the bipolar plate all decrease. This is because in the molten state, the carbon material is prone to agglomeration, thus forming large aggregates, which affects the formation of the conductive network inside the bipolar plate. In addition, in the molten state, a higher temperature and a longer blending time will cause the polymer to age, resulting in a decrease in the mechanical strength of the bipolar plate.

[0060] When blending the fluoropolymer with a single porous carbon sphere, as can be seen from Comparative Example 4, the bipolar plate exhibits a low electrical conductivity. This is because the contact points of the zero-dimensional porous carbon spheres are limited, and it is difficult to form a continuous conductive path in the bipolar plate matrix. A higher filling amount is required to reach the percolation threshold, resulting in a lower electrical conductivity of the composite material. At the same time, the contact area between the zero-dimensional porous carbon spheres and the polymer matrix is small, and the stress transfer efficiency is low, having a small effect on improving the mechanical strength of the bipolar plate. Therefore, the tensile strength and flexural strength of the bipolar plate are slightly lower than those of the bipolar plate prepared by blending carbon materials of multiple dimensions.

Claims

1. A method for preparing a high-strength composite bipolar plate, characterized in that: The steps include: (1) mixing lignin, a surfactant aqueous solution, and an alkaline aqueous solution, and subjecting the mixture to a hydrothermal treatment to obtain a suspension, and filtering, washing, and drying the suspension to obtain porous carbon balls; (2) placing the fluorine-containing polymer, porous carbon balls and carbon material in an air flow mixer and mixing them by air flow to obtain a blend; (3) molding the blend to obtain a high-strength composite bipolar plate; In the step (2), the fluorine-containing polymer is one of ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride, or polyvinylidene fluoride-hexafluoropropylene copolymer; the particle size of the fluorine-containing polymer is 5 nanometers to 2 micrometers; the carbon material is one or a combination of flake graphite, carbon nanotubes, or carbon nanofibers; the mass ratio of the fluorine-containing polymer, porous carbon balls, and carbon material is 1:0.2-0.4:0.33-1.25; the air flow mixing pressure is 0.5-1 MPa, and the mixing time is 30-60 seconds; the carbon material is a mixture of flake graphite and carbon nanotubes or a mixture of flake graphite and carbon nanofibers, and the mass ratio of flake graphite to carbon nanotubes or carbon nanofibers is 1:0.2-0.3; In the step (1), the surfactant is one of sodium dodecyl sulfate, sodium fatty alcohol polyoxyethylene ether sulfate or sodium secondary alkyl sulfonate; the mass fraction of the surfactant aqueous solution is 0.18% to 0.4%; the alkaline aqueous solution is one of sodium hydroxide or potassium hydroxide aqueous solution; the mass fraction of the alkaline aqueous solution is 5% to 7%; the mass ratio of the lignin, the surfactant and the alkaline compound in the alkaline aqueous solution is 1:0.03 to 0.1:1 to 2; the hydrothermal treatment temperature is 140 to 180°C, and the hydrothermal treatment time is 6 to 10 hours; the surfactant molecules self-assemble in the aqueous solution, and when the concentration of the surfactant aqueous solution reaches or exceeds the critical micelle concentration, spherical micelles are formed, inducing the lignin to form spherical nanoparticles during the hydrothermal carbonization process.

2. The method for preparing a high-strength composite bipolar plate according to claim 1, characterized in that: In the step (3), the molding temperature is 180-230° C., the pressure is 5-20 MPa, and the time is 10-15 min.

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