Flexible graphite bipolar plate as well as preparation method and application thereof

Abstract

The invention relates to the technical field of flow battery key components, and particularly discloses a flexible graphite bipolar plate as well as a preparation method and application thereof. The flexible graphite bipolar plate comprises a flexible graphite substrate and a nano composite impregnant filled in pores of the flexible graphite substrate, the nano composite impregnant comprises the following components: epoxy resin, surface functionalized carbon nanotubes, a curing agent, a dispersing agent and a reactive diluent, the surface functionalized carbon nanotube is subjected to carboxylation treatment. Through the synergistic enhancement effect of the functionalized carbon nanotubes and the epoxy resin, the bending strength of the bipolar plate is greatly improved, the fracture toughness is remarkably enhanced, and the problem of assembly brittle rupture of the graphite bipolar plate is thoroughly solved; the functionalized carbon nanotubes form a three-dimensional continuous conductive network in the substrate, so that the surface resistance of the bipolar plate is greatly reduced, and the volume conductivity is obviously increased, thereby reducing the ohmic loss of the cell stack; the vacuum-pressure combined impregnation process ensures that graphite pores are deeply filled with the nano-complexing agent, so that the helium permeability is remarkably reduced, and the cross permeation of electrolyte is thoroughly blocked.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of key components of flow battery, in particular to a flexible graphite bipolar plate and a preparation method and application thereof. BACKGROUND

[0002] The flow battery bipolar plate needs to have high conductivity, strong corrosion resistance and reliable sealing. Although the traditional graphite plate has conductivity and corrosion resistance, the inherent brittleness leads to a cracking rate of more than 30% during assembly, and the ohmic loss is significantly increased when the thickness is greater than 3mm; the flexible graphite paper or felt can partially alleviate the brittleness problem, but the high porosity of 20-60% will cause electrolyte mutual stringing, and the actual helium permeability is greater than 10 -4 cm 2 / s, and it must rely on resin impregnation to achieve sealing.

[0003] In order to balance the sealing and conductivity requirements, the prior art adopts epoxy resin modification impregnation: adding conductive fillers such as carbon black can reduce the surface resistance to 15-25mΩ·cm, but the uneven impregnation caused by filler settlement leads to a porosity residual rate of more than 5%; using graphene or carbon fiber reinforcement can increase the bending strength to 50Mpa, but the surface resistance is still higher than 20mΩ·cm; and the pure resin impregnation scheme makes the surface resistance deteriorate to more than 80mΩ·cm, and the resin cracking after cycling causes sealing failure.

[0004] Carbon nanotubes can theoretically simultaneously enhance mechanical and electrical properties, but there are three bottlenecks in actual application: first, the specific surface area of carbon nanotubes is more than 200m 2 / g, and it is easy to agglomerate into bundles, and the dispersion stability in epoxy resin is less than 24 hours; second, when more than 2 parts by weight of carbon nanotubes are added, the viscosity of the system increases to more than 2000mPa·s, and it cannot fully penetrate the graphite matrix with a thickness of more than 1mm; third, the interfacial bonding strength between non-functionalized carbon nanotubes and resin is less than 10Mpa, and it is easy to delaminate under thermal cycle stress.

[0005] In summary, the current technology still has significant gaps: there is no solution that can simultaneously achieve surface resistance ≤10mΩ·cm (to ensure energy efficiency of more than 85%), bending strength ≥45Mpa (to adapt to compact battery stack assembly), and helium permeability ≤10 - 10 cm 2 / s (to completely block electrolyte mutual stringing), and breakthrough innovation in materials and processes is urgently needed, therefore, it is necessary to provide a flexible graphite bipolar plate and a preparation method and application thereof. SUMMARY

[0006] The present application aims to at least solve one of the technical problems existing in the prior art, and for this purpose, the present application provides a flexible graphite bipolar plate and a preparation method and application thereof, which solves the technical contradiction that strength-conductivity-sealing difficulty is difficult to be optimized simultaneously.

[0007] The first aspect of the present application provides a flexible graphite bipolar plate.

[0008] Specifically, the flexible graphite bipolar plate comprises a flexible graphite matrix and a nano-composite impregnant filled in the pores of the flexible graphite matrix.

[0009] The components of the nano-composite impregnant comprise: an epoxy resin, surface-functionalized carbon nanotubes, a curing agent, a dispersant, and an active diluent.

[0010] The surface-functionalized carbon nanotubes are carboxylated.

[0011] Preferably, the components of the nano-composite impregnant comprise, by weight fraction: 90-110 parts of the epoxy resin, 1-4 parts of the surface-functionalized carbon nanotubes, 20-40 parts of the curing agent, 0.1-2 parts of the dispersant, and 5-15 parts of the active diluent.

[0012] Further preferably, the components of the nano-composite impregnant comprise, by weight fraction: 95-105 parts of the epoxy resin, 2-4 parts of the surface-functionalized carbon nanotubes, 25-35 parts of the curing agent, 1-2 parts of the dispersant, and 10-15 parts of the active diluent.

[0013] Still further preferably, the components of the nano-composite impregnant comprise, by weight fraction: 100 parts of the epoxy resin, 3 parts of the surface-functionalized carbon nanotubes, 30 parts of the curing agent, 1 part of the dispersant, and 10 parts of the active diluent.

[0014] Preferably, the epoxy resin comprises a bisphenol A type epoxy resin.

[0015] Preferably, the viscosity of the epoxy resin at 25°C is 100-2000 mPa·s.

[0016] Preferably, the surface-functionalized carbon nanotubes have a length of 10-50 μm, a diameter of 10-20 nm, and an aspect ratio ≥500.

[0017] Preferably, the curing agent comprises at least one of an alicyclic amine curing agent and an acid anhydride curing agent.

[0018] Preferably, the dispersant comprises a non-ionic polyether-modified siloxane.

[0019] Preferably, the active diluent comprises at least one of a phenyl glycidyl ether and a butyl glycidyl ether.

[0020] The second aspect of the present application provides a preparation method of a flexible graphite bipolar plate.

[0021] Specifically, the preparation method of the flexible graphite bipolar plate comprises the following steps:

[0022] Step S1, pretreatment of the flexible graphite matrix: the flexible graphite matrix is subjected to plasma cleaning or acid cleaning to activate the surface, thereby obtaining a pretreated flexible graphite matrix;

[0023] Step S2, preparation of a nano-composite impregnating agent: carbon nanotubes subjected to surface functionalization are added to an active diluent containing a dispersant, and subjected to ultrasonic treatment and high-speed shearing dispersion, thereby obtaining a pre-dispersed slurry; the pre-dispersed slurry is mixed with an epoxy resin, and the mixed liquid after grinding is added to a curing agent and stirred, and vacuum degassing is performed, thereby obtaining a nano-composite impregnating agent;

[0024] Step S3, combined impregnation: the pretreated flexible graphite matrix obtained in step S1 is immersed in the nano-composite impregnating agent obtained in step S2, vacuum is first extracted to ≤10 kPa and maintained for 20-40 minutes in the vacuum stage, and then pressurized to 0.5-2 MPa and maintained for 40-80 minutes in the pressure stage, thereby obtaining an impregnated matrix;

[0025] Step S4, curing and forming: the impregnated matrix is placed in a mold and subjected to stepwise temperature curing under a pressure of 5-10 MPa: the first stage is 80-90℃ for 1-2 h, and the second stage is 120-130℃ for 2-4 h.

[0026] Preferably, in step S2, the ultrasonic treatment has a power of 300-500 W and a time of 20-40 min.

[0027] Further preferably, in step S2, the ultrasonic treatment has a power of 450-500 W and a time of 25-35 min.

[0028] More preferably, in step S2, the ultrasonic treatment has a power of 500 W and a time of 30 min.

[0029] Preferably, in step S2, the high-speed shearing dispersion has a rotational speed of 4000-6000 rpm and a time of 40-80 min.

[0030] Further preferably, in step S2, the high-speed shearing dispersion has a rotational speed of 5000-6000 rpm and a time of 60-80 min.

[0031] More preferably, in step S2, the high-speed shearing dispersion has a rotational speed of 6000 rpm and a time of 60 min.

[0032] Preferably, in step S2, the mixed liquid after grinding has a viscosity of ≤500 mPa·s.

[0033] Further preferably, in step S2, the viscosity of the ground mixture is ≤480 mPa·s.

[0034] Further preferably, in step S2, the viscosity of the ground mixture is 480 mPa·s.

[0035] Preferably, in step S3, the combined impregnation is as follows: the pretreated flexible graphite matrix in step S1 is immersed in the nanocomposite impregnant obtained in step S2, the vacuum stage is first vacuumed to ≤8 kPa and maintained for 25-35 minutes, and the pressure stage is then pressurized to 1.5-2 MPa and maintained for 50-70 minutes, to obtain the impregnated matrix.

[0036] Further preferably, in step S3, the combined impregnation is as follows: the pretreated flexible graphite matrix in step S1 is immersed in the nanocomposite impregnant obtained in step S2, the vacuum stage is first vacuumed to 5 kPa and maintained for 30 minutes, and the pressure stage is then pressurized to 1.5 MPa and maintained for 60 minutes, to obtain the impregnated matrix.

[0037] Preferably, in step S3, the switching interval between the vacuum stage and the pressure stage is ≤5 min.

[0038] Preferably, in step S4, the solidification molding is as follows: the impregnated matrix is placed in a mold and subjected to stepwise temperature rising solidification under a pressure of 8-10 MPa: the first stage is 85-90℃ for 1.5-2 h, and the second stage is 125-130℃ for 2-3 h.

[0039] Further preferably, in step S4, the solidification molding is as follows: the impregnated matrix is placed in a mold and subjected to stepwise temperature rising solidification under a pressure of 8 MPa: the first stage is 85℃ for 1.5 h, and the second stage is 125℃ for 3 h.

[0040] The third aspect of the present application provides a flexible graphite bipolar plate for use in a vanadium redox flow battery.

[0041] Specifically, the flexible graphite bipolar plate is used for isolating the positive and negative electrolytes.

[0042] Compared with the prior art, the present application has the following advantages:

[0043] 1. The bending strength of the bipolar plate is greatly improved, and the fracture toughness is significantly enhanced, thus completely solving the brittle fracture problem of the graphite bipolar plate assembly;

[0044] 2. The functionalized carbon nanotubes form a three-dimensional continuous conductive network in the matrix, which greatly reduces the surface resistance of the bipolar plate and significantly increases the volume conductivity, thereby reducing the ohmic loss of the battery stack;

[0045] 3. Vacuum-pressure combined impregnation process ensures that the nanocomposite agent deeply fills the graphite pores, significantly reduces the helium permeability, and completely blocks the cross penetration of electrolyte. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 Flow chart of the preparation method of the flexible graphite bipolar plate in Example 1. DETAILED DESCRIPTION

[0047] In order to make the skilled in the art more clearly understand the technical solutions described in the present application, the following examples are listed for illustration. It should be pointed out that the following examples do not constitute a limitation on the scope of protection required by the present application.

[0048] The raw materials, reagents or devices used in the following examples, unless otherwise specified, can be obtained from conventional commercial channels, or can be obtained by existing known methods.

[0049] Example 1

[0050] The components of the nanocomposite impregnating agent include, by weight fraction: epoxy resin E51 100 parts, surface functionalized carbon nanotubes 3 parts (diameter 10-20 nm, length 10-50 μm), curing agent T31 30 parts, dispersing agent BYK-2150 1 part, and phenyl glycidyl ether 10 parts.

[0051] The carboxylation treatment of the surface functionalized carbon nanotubes includes the following steps:

[0052] 1. In a dry three-necked flask, concentrated sulfuric acid is added to concentrated nitric acid (volume ratio of concentrated nitric acid: concentrated sulfuric acid = 3:1), and stirred with a magnetic stirrer while adding, to make the mixture uniform and promote heat dissipation.

[0053] 2. Weigh the dry carbon nanotube powder and slowly add it to the cooled mixed acid. Continue stirring to disperse the carbon nanotubes in the acid.

[0054] 3. Place the three-necked flask in an oil bath. Install a condensation reflux tube and pass cooling water to prevent acid loss due to volatilization. Turn on the magnetic stirrer and set the appropriate stirring speed. Slowly heat to 80°C. Keep refluxing at 80°C for 8 hours for oxidation reaction.

[0055] 4. After the reaction is complete, turn off the heating and let the reaction mixture cool naturally to room temperature.

[0056] 5. Slowly pour the cooled reaction mixture into cold deionized water to dilute the strong acid.

[0057] 6. Using strong acid-resistant polytetrafluoroethylene (PTFE) membrane, vacuum filtration is performed on a Buchner funnel. The filter cake is repeatedly washed with deionized water until the pH of the filtrate is close to neutral. The filter cake is carefully stirred with a glass rod during the washing process to help washing.

[0058] The method for preparing the flexible graphite bipolar plate comprises the following steps:

[0059] Step S1, pretreatment of the flexible graphite substrate: a flexible graphite plate with a porosity of 40% is selected as the substrate, which is cut to the target size and then subjected to surface activation treatment by oxygen plasma to enhance the interfacial bonding force with the impregnant, thereby obtaining the pretreated flexible graphite substrate;

[0060] Step S2, preparation of the nano-composite impregnant: surface-functionalized carbon nanotubes are added to phenyl glycidyl ether containing a dispersant, and subjected to ultrasonic treatment at 500 W for 30 min and high-speed shearing dispersion at 6000 rpm for 60 min, thereby obtaining a pre-dispersed slurry; the pre-dispersed slurry is mixed with epoxy resin E51, and subjected to three-roll grinding until the viscosity is 480 mPa·s, and then the curing agent T31 is added and stirred, and vacuum degassing is performed, thereby obtaining the nano-composite impregnant;

[0061] Step S3, combined impregnation: the pretreated flexible graphite substrate obtained in step S1 is immersed in the nano-composite impregnant obtained in step S2, vacuum is first applied to 5 kPa and maintained for 30 min to completely remove the gas in the pores; then the pressure is quickly switched to the pressure impregnation stage, and the pressure is increased to 1.5 MPa and maintained for 60 min to ensure that the nano-composite agent is deeply filled into the internal pores of the substrate, thereby obtaining the impregnated substrate;

[0062] Step S4, curing and forming: the impregnated substrate is placed in a mold and subjected to stepwise temperature curing under a constant pressure of 8 MPa: in the first stage, 85°C is maintained for 1.5 h to complete the preliminary crosslinking, and in the second stage, 125°C is maintained for 3 h to achieve complete polymerization, thereby finally forming a dense and integrated flexible graphite bipolar plate.

[0063] Comparative Example 1

[0064] The components of the nano-composite impregnant include, by weight fraction: epoxy resin E51 100 parts, non-surface-functionalized carbon nanotubes 3 parts (diameter 10-20 nm, length 10-50 μm), curing agent T31 30 parts, dispersant BYK-2150 2 parts, and phenyl glycidyl ether 10 parts.

[0065] The method for preparing the flexible graphite bipolar plate comprises the following steps:

[0066] Step S1, flexible graphite substrate pretreatment: select a flexible graphite plate with a porosity of 40% as the substrate, cut it to the target size, and then perform surface activation treatment using oxygen plasma to enhance the interfacial bonding force with the impregnant, thereby obtaining a pretreated flexible graphite substrate;

[0067] Step S2, preparation of nanocomposite impregnant: add the non-surface functionalized carbon nanotubes to the phenyl glycidyl ether containing dispersant, perform 500W ultrasonic treatment for 30min, and perform high-speed shearing dispersion at 6000rpm for 60min to obtain a pre-dispersed slurry; mix the pre-dispersed slurry with the epoxy resin E51, perform three-roll grinding until the viscosity is 620mPa·s, add the curing agent T31 and stir, and vacuum degassing to obtain a nanocomposite impregnant;

[0068] Step S3, combined impregnation: immerse the pretreated flexible graphite substrate obtained in step S1 in the nanocomposite impregnant obtained in step S2, first vacuum to 5kPa and maintain for 30min in the vacuum stage to completely remove the gas in the pores; then quickly switch to the pressure impregnation stage, pressurize to 1.8MPa and maintain for 70min, to ensure that the nanocomposite agent is deeply filled into the internal pores of the substrate, thereby obtaining an impregnated substrate;

[0069] Step S4, curing and forming: place the impregnated substrate in a mold and perform stepwise temperature curing under a constant pressure of 8MPa: the first stage is 85℃ for 1.5h to complete the preliminary crosslinking, and the second stage is 125℃ for 3h to realize complete polymerization, thereby finally forming a dense and integrated flexible graphite bipolar plate.

[0070] Comparative Example 2

[0071] The components of the nanocomposite impregnant include, by weight fraction: epoxy resin E51 100 parts, surface functionalized carbon nanotubes 0.5 parts (diameter 10-20nm, length 10-50μm), curing agent T31 30 parts, dispersant BYK-2150 1 part, and butyl glycidyl ether 15 parts.

[0072] The carboxylation treatment of the surface functionalized carbon nanotubes includes the following steps:

[0073] 1. In a dry three-necked flask, add concentrated sulfuric acid to concentrated nitric acid (volume ratio of concentrated nitric acid: concentrated sulfuric acid = 3:1), and stir with a magnetic stirrer while adding, to make the mixture uniform and promote heat dissipation.

[0074] 2. Weigh the dry carbon nanotube powder and slowly add it to the cooled mixed acid. Continue stirring to disperse the carbon nanotubes in the acid solution.

[0075] 3. Place the three-necked flask on the oil bath. Install the condenser and run cold water to prevent acid loss by volatilization. Turn on the magnetic stirrer and set the appropriate stirring speed. Slowly raise the temperature to 80°C. Keep the reaction at 80°C for 8h to complete the oxidation reaction.

[0076] 4. After the reaction is completed, turn off the heating and let the reaction mixture cool to room temperature naturally.

[0077] 5. Slowly pour the cooled reaction mixture into cold deionized water to dilute the strong acid.

[0078] 6. Use a strong acid-resistant polytetrafluoroethylene (PTFE) membrane to perform vacuum filtration on a Buchner funnel. Wash the filter cake repeatedly with deionized water until the pH of the filtrate approaches neutrality. Gently stir the filter cake with a glass rod during the washing process to help with the washing.

[0079] The method for preparing the flexible graphite bipolar plate comprises the following steps:

[0080] Step S1, pretreatment of the flexible graphite substrate: select a flexible graphite plate with a porosity of 40% as the substrate, cut it to the target size, and then perform surface activation treatment using oxygen plasma to enhance the interfacial bonding force with the impregnant, thereby obtaining the pretreated flexible graphite substrate;

[0081] Step S2, preparation of the nanocomposite impregnant: add the surface-functionalized carbon nanotubes to the butyl glycidyl ether containing the dispersant, perform 500W ultrasonic treatment for 30min, and perform high-speed shearing dispersion at 6000rpm for 60min to obtain a pre-dispersed slurry; mix the pre-dispersed slurry with the epoxy resin E51, perform three-roll grinding until the viscosity is 320mPa·s, add the curing agent T31 and stir, and perform vacuum degassing to obtain the nanocomposite impregnant;

[0082] Step S3, combined impregnation: immerse the pretreated flexible graphite substrate obtained in step S1 in the nanocomposite impregnant obtained in step S2, first perform vacuuming to 5kPa and maintain for 30min in the vacuum stage to completely remove the gas in the pores; then quickly switch to the pressure impregnation stage, pressurize to 0.8MPa and maintain for 50min to ensure that the nanocomposite agent is deeply filled into the internal pores of the substrate, thereby obtaining the impregnated substrate;

[0083] Step S4, curing and forming: place the impregnated substrate in a mold and perform stepwise temperature curing under a constant pressure of 8MPa: the first stage is 85°C for 1.5h to complete the preliminary crosslinking, and the second stage is 125°C for 3h to achieve complete polymerization, thereby finally forming a dense and integrated flexible graphite bipolar plate.

[0084] Comparative Example 3

[0085] The components of the nanocomposite impregnant include, by weight fraction: epoxy resin E51 100 parts, surface functionalized carbon nanotubes 5 parts (diameter 10-20 nm, length 10-50 μm), curing agent T31 30 parts, dispersant BYK-2150 1 part.

[0086] The carboxylation treatment of the surface functionalized carbon nanotubes includes the following steps:

[0087] 1. In a dry three-necked flask, concentrated sulfuric acid is added to concentrated nitric acid (volume ratio of concentrated nitric acid: concentrated sulfuric acid = 3:1), and stirred with a magnetic stirrer while adding, to make the mixture uniform and promote heat dissipation.

[0088] 2. The weighed dry carbon nanotube powder is slowly added to the cooled mixed acid. Continue stirring to disperse the carbon nanotubes in the acid solution.

[0089] 3. The three-necked flask is placed on an oil bath. Install a condensation reflux tube and pass cooling water to prevent acid volatilization loss. Turn on the magnetic stirrer and set the appropriate stirring speed. Slowly heat to 80°C. Keep refluxing at 80°C for 8 h to perform the oxidation reaction.

[0090] 4. After the reaction is completed, turn off the heating and let the reaction mixture cool to room temperature naturally.

[0091] 5. Slowly pour the cooled reaction mixture into cold deionized water to dilute the strong acid.

[0092] 6. Use a strong acid-resistant polytetrafluoroethylene (PTFE) membrane to perform vacuum filtration on a Buchner funnel. Wash the filter cake repeatedly with deionized water until the pH of the filtrate approaches neutrality. Gently stir the filter cake with a glass rod during the washing process to help washing.

[0093] The method for preparing the flexible graphite bipolar plate includes the following steps:

[0094] Step S1, pretreatment of the flexible graphite substrate: select a flexible graphite plate with a porosity of 40% as the substrate, cut it to the target size, and then perform surface activation treatment using oxygen plasma to enhance the interfacial bonding force with the impregnant, to obtain a pretreated flexible graphite substrate;

[0095] Step S2, preparation of the nanocomposite impregnant: add the surface functionalized carbon nanotubes to the dispersant BYK-2150 without active diluent, ultrasonically treat for 30 min at 500 W, and high-speed shear disperse for 60 min at 6000 rpm, to obtain a pre-dispersed slurry; mix the pre-dispersed slurry with the epoxy resin E51, grind with three rollers until the viscosity is 950 mPa·s, add the curing agent T31 and stir, and vacuum degassing, to obtain the nanocomposite impregnant;

[0096] Step S3, combined impregnation: the flexible graphite matrix after step S1 pretreatment is immersed in the nano-composite impregnant obtained in step S2, vacuum stage first vacuum to 5 kPa and maintain 30 min, completely remove the gas in the pore; Then quickly switch to the pressure impregnation stage, pressurized to 2 MPa and keep 80 min, ensure the nano-composite agent depth filling to the internal pore of the matrix, get the impregnated matrix;

[0097] Step S4, solidification forming: the impregnated matrix is placed in the mold, under the constant pressure of 0.8 MPa, the temperature is increased in stages: the first stage is 85℃ for 1.5 h to complete the preliminary crosslinking, the second stage is 125℃ for 3 h to realize complete polymerization, finally forming a dense integrated flexible graphite bipolar plate.

[0098] Performance test:

[0099] Table 1 physical property test results of each flexible graphite bipolar plate of example 1 and comparative examples 1-3

[0100] Parameters Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Bending strength (MPa) 46 35 39 51 Surface resistance (mΩ-cm) 8.5 21.7 12.1 6.2 Helium permeability (cm2 / s) 5.3 x 10 -11 ]] 2.5 x 10 -9 ]] 8.7 x 10 -11 ]] 3.1 x 10 -10 ]]>

[0101] Through the system test of example 1 and comparative examples 1-3, it is shown that the content range of carbon nanotubes and surface functionalization treatment are the key factors to determine the performance of the bipolar plate. When the content of carbon nanotubes is 0.5 parts (comparative example 2), the bending strength decreases to 39 MPa (decreased by 15.2% compared with example 1), and the surface resistance increases to 12.1 mΩ·cm (increased by 42.4%), which proves that low content of carbon nanotubes is difficult to build an effective reinforcing-conductive network; while the content of carbon nanotubes increases to 5 parts (comparative example 3), although the bending strength increases to 51 MPa (increased by 10.9% compared with example 1) and the surface resistance decreases to 6.2 mΩ·cm (decreased by 27.1%), the impregnation is not sufficient due to the rapid increase of the viscosity of the system, and the helium permeability deteriorates to 3.1×10 -10 cm2 / s (increased by 484.9%), which reveals the essential contradiction of excessive carbon nanotubes sacrificing the sealing performance. In contrast, example 1 with 3 parts of carbon nanotubes achieves a synergistic optimization of bending strength of 46 MPa and surface resistance of 8.5 mΩ·cm, which verifies the necessity of 1-4 parts of surface functionalized carbon nanotubes.

[0102] The experimental results of comparative example 1 using non-functionalized carbon nanotubes further show that surface functionalization treatment is an absolute prerequisite for performance. Its bending strength (35 MPa) and surface resistance (21.7 mΩ·cm) are respectively 23.9% and 155% worse than example 1, and the helium permeability increases to 2.5×10 -9cm2 / s, the root cause of which is that the severe agglomeration of non-functionalized carbon nanotubes hinders the mechanical enhancement effect and the formation of conductive network. This phenomenon confirms the irreplaceability of the carboxylation treatment to achieve strong coupling of the carbon nanotube-resin-graphite three-phase interface through covalent bonding.

[0103] The preferred embodiments of the present application have been described in detail above. It should be understood that modifications and variations of the preferred embodiments of the present application are possible without actually departing from the scope and spirit of the present application as defined in the accompanying claims. Accordingly, any and all modifications, variations or improvements, as well as equivalents, of the preferred embodiments of the present application are intended to fall within the scope and spirit of the present application as defined by the following claims.

Claims

1. A flexible graphite bipolar plate, characterized in that, Includes a flexible graphite matrix and a nanocomposite impregnating agent filling its pores; The components of the nanocomposite impregnating agent include: epoxy resin, surface-functionalized carbon nanotubes, curing agent, dispersant, and reactive diluent; The surface-functionalized carbon nanotubes are carboxylated.

2. The flexible graphite bipolar plate according to claim 1, characterized in that, The components of the nanocomposite impregnating agent, by weight, include: 90-110 parts epoxy resin, 1-4 parts surface-functionalized carbon nanotubes, 20-40 parts curing agent, 0.1-2 parts dispersant, and 5-15 parts reactive diluent.

3. The flexible graphite bipolar plate according to claim 1, characterized in that, The epoxy resin includes bisphenol A type epoxy resin.

4. The flexible graphite bipolar plate according to claim 3, characterized in that, The epoxy resin has a viscosity of 100–2000 mPa·s at 25°C.

5. The flexible graphite bipolar plate according to claim 1, characterized in that, The surface-functionalized carbon nanotubes have a length of 10–50 μm, a diameter of 10–20 nm, and an aspect ratio ≥500.

6. The flexible graphite bipolar plate according to claim 1, characterized in that, The curing agent includes at least one of alicyclic amine curing agents and acid anhydride curing agents.

7. The flexible graphite bipolar plate according to claim 1, characterized in that, The dispersant includes nonionic polyether-modified siloxane.

8. The method for preparing the flexible graphite bipolar plate according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step S1, flexible graphite substrate pretreatment: The flexible graphite substrate is activated by plasma cleaning or acid washing to obtain a pretreated flexible graphite substrate. Step S2, preparation of nanocomposite impregnating agent: surface-functionalized carbon nanotubes are added to an active diluent containing a dispersant, and then ultrasonically treated and dispersed by high-speed shearing to obtain a pre-dispersed slurry; the pre-dispersed slurry is mixed with epoxy resin, ground, and then a curing agent is added and stirred, followed by vacuum degassing to obtain the nanocomposite impregnating agent; Step S3, combined impregnation: The flexible graphite matrix pretreated in step S1 is immersed in the nanocomposite impregnating agent obtained in step S2. In the vacuum stage, the vacuum is first drawn to ≤10kPa and maintained for 20 to 40 minutes. In the pressure stage, the pressure is increased to 0.5 to 2MPa and maintained for 40 to 80 minutes to obtain the impregnated matrix. Step S4, Curing and Molding: Place the impregnated substrate in a mold and cure it under a stepwise temperature increase at a pressure of 5-10 MPa: the first stage is at 80-90℃ for 1-2 hours, and the second stage is at 120-130℃ for 2-4 hours.

9. The preparation method according to claim 8, characterized in that, In step S3, the switching interval between the vacuum stage and the pressure stage is ≤5 min.

10. The application of the flexible graphite bipolar plate according to any one of claims 1 to 7 in an all-vanadium redox flow battery, characterized in that, The flexible graphite bipolar plate is used to isolate the positive and negative electrode electrolytes.

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