Polypyrrole modified graphite / resin composite bipolar plate and preparation method thereof

Abstract

The invention discloses a polypyrrole modified graphite / resin composite bipolar plate and a preparation method thereof. The preparation method mainly comprises the following main steps: in-situ synthesis of polypyrrole, preparation of composite powder and hot press molding. Aiming at the problems that an existing composite bipolar plate is weak in interface bonding force and conductivity and mechanical property are difficult to consider at the same time, a polypyrrole nano-film is synthesized on the surface of natural graphite in situ through a solution polymerization method, and the interface conductivity is improved by using a conjugated double-bond structure of polypyrrole; meanwhile, an oxygen-containing group and resin form a hydrogen bond to enhance the interface bonding strength, and the rapid preparation of the composite bipolar plate is realized by combining a dry-method spheroidal graphite mixing process and a short-time hot pressing process. The polypyrrole modified graphite / resin composite bipolar plate prepared by the invention has high conductivity, and the conductivity can reach 223.2 S / cm; the prepared bipolar plate is high in material strength which can reach 46.3 MPa, and excellent in corrosion resistance, and is suitable for large-scale production of the bipolar plate of the fuel cell.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of proton exchange membrane fuel cell composite bipolar plates, and specifically relates to a polypyrrole-modified graphite / resin composite bipolar plate and a preparation method thereof. Background Art

[0002] The performance of the bipolar plate of a proton exchange membrane fuel cell directly affects the energy conversion efficiency and service life of the cell. In the prior art, due to the large polarity difference between graphite and resin in the resin-graphite composite bipolar plate, the interface between the two mainly relies on physical adsorption to combine, resulting in weak interfacial binding force, which makes it difficult to effectively improve the mechanical properties of the composite bipolar plate and limits its reliability and stability in practical applications. At the same time, as an insulating layer, the resin will hinder the transmission of electrons, making the interfacial resistance account for more than 40% of the total resistance of the composite bipolar plate, seriously affecting the electrical conductivity of the bipolar plate and thus reducing the energy conversion efficiency of the fuel cell. Traditional interfacial modification methods, such as oxidation treatment, etc., are not only complex in process but also high in cost, and it is difficult to meet the requirements of large-scale industrial production.

[0003] The present invention uses solution polymerization to achieve uniform deposition of polypyrrole on the surface of natural graphite, effectively avoiding the agglomeration problem. In traditional preparation methods, polypyrrole is prone to agglomeration, which will destroy the continuity of the conductive network and affect the performance of the bipolar plate. In the present invention, by immersing graphite in a specific solution and under the conditions of low-temperature stirring and slow dropping of the initiator, polypyrrole grows in-situ on the surface of graphite to form a uniform conductive film. At the same time, the dry ball milling mixing and short-time hot pressing process significantly shortens the production cycle. In traditional processes, the materials are not fully mixed and the hot pressing time is long. The dry ball milling of the present invention can quickly achieve uniform mixing of the materials. The short-time hot pressing, while ensuring the performance of the bipolar plate, shortens the production cycle by more than 50%, greatly improving the production efficiency.

[0004] CN118016922A discloses a graphite / resin composite bipolar plate and a preparation method thereof. The main raw materials of this invention are thermosetting resin, expanded graphite, and natural graphite, and finally a graphite / resin composite bipolar plate is obtained through mixing, stirring, and hot pressing. However, the conductivity and flexural strength of this graphite / resin composite bipolar plate are not very excellent. Summary of the Invention

[0005] The present invention provides a polypyrrole-modified graphite / resin composite bipolar plate and a preparation method thereof to solve the performance defects existing in the existing graphite / resin-based composite bipolar plates. In the present invention, a polypyrrole thin film is in-situ synthesized on the surface of natural graphite, and the interface between graphite and resin is modified by using the unique structure and properties of polypyrrole. Polypyrrole has a conjugated double bond structure, which can form a π-π conjugated conductive network at the interface between graphite and resin, effectively improving the conductive performance of the interface and reducing the interface resistance at the same time, thereby significantly improving the overall conductivity of the composite bipolar plate; at the same time, the oxygen-containing groups on the polypyrrole molecule can form hydrogen bonds with the resin molecules, enhancing the interfacial adhesion between graphite and resin, and greatly improving the mechanical properties of the composite bipolar plate; in addition, the present invention combines dry ball milling mixing and short-time hot pressing processes to achieve the rapid preparation of the composite bipolar plate. This not only improves production efficiency, shortens the production cycle, and reduces production costs, but also is suitable for the large-scale production of fuel cell bipolar plates, promoting the further development and application of proton exchange membrane fuel cell technology.

[0006] The present invention also provides the following technical solutions: A preparation method of a polypyrrole-modified graphite / resin composite bipolar plate mainly includes the following steps: Step 1. In-situ synthesis of polypyrrole; Step 2. Preparation of composite powder: Mix the polypyrrole-modified graphite powder obtained in the above step 1 with phenolic resin powder according to a mass ratio of (3-9):1, continue to add an appropriate amount of curing agent, and stir for 25-40 minutes at a stirring speed of 200-400 revolutions per minute; Step 3. Hot pressing and forming: Put the mixture obtained by stirring in the above step 2 into a mold, close the mold before starting to heat up, and heat up while maintaining the forming pressure. After reaching the preset curing temperature, keep it for 25-35 minutes. The hot pressing temperature is 160-180 °C, and the pressure is 10-30 MPa; The polypyrrole-modified graphite powder in the above step 2 is further preferably natural graphite powder; The phenolic resin in the above step 2 is further preferably linear thermoplastic phenolic resin; The addition amount of the curing agent in the above step 2 is 20-60% of the mass of the resin added; The mold used in the above step 3 is a stainless steel mold; The present invention also discloses the preparation of the in-situ synthesis of polypyrrole in the above step 1, including the following steps: a. Prepare a pyrrole monomer solution: Dissolve 1-3 ml of pyrrole monomer in 30-50 ml of hydrochloric acid solution with a concentration of 0.8-1.1 mol / L; b. Prepare an initiator solution: Dissolve 2-5 g of ammonium persulfate with a concentration of 0.4-0.6 mol / L in 46-53 ml of deionized water solution; c. Immerse the graphite flakes into the pyrrole solution and stir for 5 - 30 minutes under an ice - water bath condition; d. Slowly add dropwise the initiator solution and continuously stir for polymerization for 5 - 30 minutes; e. Wash alternately with ethanol / deionized water three times and vacuum - dry for 25 - 35 minutes In the above step a of preparing the pyrrole monomer solution, the concentration of hydrochloric acid is 0.8 - 1.1 mol / L, and further preferably 1.0 mol / L; In the above step b of preparing the initiator solution, the concentration of ammonium persulfate is 0.4 - 0.6 mol / L, and further preferably 0.47 mol / L; In the above step c of immersing the graphite flakes into the pyrrole solution and stirring for 5 - 30 minutes under an ice - water bath condition, it is further preferably 10 - 20 minutes; The graphite is preferably natural graphite; In the above step d of slowly adding dropwise the initiator solution and continuously stirring for polymerization for 5 - 30 minutes, it is further preferably 10 - 20 minutes; Advantageous technical effects of the present invention: A polypyrrole - modified graphite / resin composite bipolar plate prepared by the present invention adopts the polypyrrole interface modification technology, effectively overcoming the problems of poor interfacial bonding, difficulty in balancing conductivity and mechanical properties of traditional graphite / resin composite bipolar plates. By directly synthesizing a polypyrrole film on the graphite surface, the characteristics of the graphite - resin interface are optimized, significantly improving the interfacial conductivity and reducing the resistance. The interfacial resistance is reduced by 60%, greatly enhancing the conductive ability of the composite bipolar plate, and the conductivity reaches 223.2 S / cm. In addition, this method enhances the adhesion between the graphite and the resin, greatly improving the mechanical strength of the composite bipolar plate. The strength is increased by 43%, and the flexural strength reaches 46.3 MPa. The corrosion current density is 0.003 μA / cm² and the contact resistance is 9.25 mΩ·cm². Combining the dry - method ball - milling mixing and rapid hot - pressing technologies, efficient preparation is achieved, improving the production efficiency, shortening the manufacturing cycle, reducing the cost, being suitable for the batch production of fuel - cell bipolar plates, and promoting the progress and application of proton - exchange membrane fuel - cell technology. Brief Description of the Drawings

[0007] Figure 1 is the FTIR infrared test result of a polypyrrole - modified graphite composite material prepared in an embodiment of the present invention; Figure 2 is the scanning electron microscope images of polypyrrole and polypyrrole films formed on the graphite surface at different times of the present invention; Figure 2 a. Polypyrrole, b. Polypyrrole film, c. Polypyrrole film with a formation time of 20 minutes, d. Polypyrrole film with a formation time of 60 minutes Detailed Embodiments

[0008] To further understand the present invention, a polypyrrole-modified graphite / resin composite bipolar plate and its preparation method provided by the present invention will be described in detail below in conjunction with embodiments.

[0009] In the following descriptions: Natural graphite, product model: HQGraphene, sourced from Shanghai Juna Technology Co., Ltd.; Artificial graphite, particle size 5 microns, sourced from Qingdao Tianheda Graphite Co., Ltd.; Expanded graphite, particle size 44 microns, sourced from Qingdao Tianheda Graphite Co., Ltd.; Micronized graphite, model: FS-1, particle size 38 microns, sourced from Qingdao Tianheda Graphite Co., Ltd.; Flake graphite, specification: 200 mesh, sourced from Qingdao Tianheda Graphite Co., Ltd.; Phenolic resin, model: SP-6700, sourced from Saint-Lecourt Chemical Industry (Shanghai) Co., Ltd.; Example 1 The present invention discloses a preparation method of a polypyrrole-modified graphite / resin composite bipolar plate, including the following steps: Step 1. In-situ synthesis of polypyrrole; Step 2. Preparation of composite powder: Mix the polypyrrole-modified natural graphite powder obtained in the above step 1 with phenolic resin powder, and continue to add 12.5 grams of curing agent hexamethylene alcoholamine, stir for 30 minutes, and the stirring speed is 300 revolutions per minute; Step 3. Hot pressing and forming: Put the mixture obtained by stirring in the above step 2 into a stainless steel mold, close the mold before starting to heat up, and keep heating up while maintaining the forming pressure. After reaching the preset curing temperature, keep it for 30 minutes. The hot pressing temperature is 160 °C, and the pressure is 20 MPa; The present invention also discloses the preparation of the in-situ synthesis of polypyrrole in the above step 1, including the following steps: a. Prepare a pyrrole monomer solution: Dissolve 3 milliliters of pyrrole monomer in 40 milliliters of hydrochloric acid solution with a concentration of 1.0 mol / L; b. Prepare an initiator solution: Dissolve 3 grams of ammonium persulfate with a concentration of 0.47 mol / L in 50 milliliters of deionized water solution; c. Immerse the graphite sheet in the pyrrole solution and stir for 10 minutes under an ice-water bath condition; d. Slowly dropwise add the initiator solution and continuously stir and polymerize for 10 minutes; e. Wash alternately with ethanol / deionized water 3 times and vacuum dry for 30 minutes The mass of the above graphite sheet is 75 grams and the mass of the phenolic resin is 25 grams; Example 2 The difference between Example 2 and Example 1 is that the matrix graphite powder used in Example 2 is artificial graphite powder; Example 3 The difference between Example 3 and Example 1 is that the matrix graphite powder used in Example 3 is micropowder graphite; Example 4 The difference between Example 4 and Example 1 is that the matrix graphite powder used in Example 4 is flake graphite; Example 5 The difference between Example 5 and Example 1 is that the matrix graphite powder used in Example 5 is expanded graphite; Example 6 The difference between Example 6 and Example 1 is that the in-situ synthesis time of polypyrrole in Example 6 is 10 minutes; Example 7 The difference between Example 7 and Example 1 is that the in-situ synthesis time of polypyrrole in Example 7 is 30 minutes; Example 8 The difference between Example 8 and Example 1 is that the in-situ synthesis time of polypyrrole in Example 8 is 40 minutes; Example 9 The difference between Example 9 and Example 1 is that the in-situ synthesis time of polypyrrole in Example 9 is 50 minutes; Example 10 The difference between Example 10 and Example 1 is that the in-situ synthesis time of polypyrrole in Example 10 is 60 minutes; Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that in Comparative Example 1, the graphite was not modified at the interface with polypyrrole; Test item 1: Conductivity test The conductivity of the bipolar plate was tested using a four-probe resistance meter (ST2258C). Several groups were tested and the average value was taken.

[0010] Table 1 Conductivity test results of a polypyrrole-modified graphite / resin composite bipolar plate Test item Conductivity / (S / cm) Example 1 223.2 Example 2 201.2 Example 3 157.5 Example 4 182.8 Example 5 177.2 Example 6 165.2 Example 7 183.9 Example 8 174.5 Example 9 182.2 Example 10 161.4 Comparative Example 1 156.4 From Table 1 above, it can be concluded that by comparing Examples 1-10 and Comparative Example 1, for the polypyrrole-modified graphite / resin composite bipolar plate prepared by the present invention, when the graphite material is natural graphite and the polypyrrole synthesis time is 20 minutes, the conductivity of the prepared polypyrrole-modified graphite / resin composite bipolar plate is the best, reaching 223.2 S / cm; Comparing Examples 1-5, it can be seen that the conductivity of a polypyrrole-modified graphite / resin composite bipolar plate prepared by using different base graphite materials is different, and the gap is still relatively large. The possible reason is that the conductivity of graphite is closely related to its crystal structure, surface characteristics and interface bonding ability with polymers. Different types of graphite will lead to significantly different conductivity properties of composite bipolar plates due to differences in their structure and physical and chemical properties. Natural graphite has high crystallinity, complete layered structure, large grain size, micron level, and highly delocalized π electron cloud between layers. The impact on conductivity is that the electron migration path is long and continuous, and the conductivity is excellent; for expanded graphite, the layers are stretched open by intercalants to form a porous worm-like structure, the interlayer spacing increases, and the crystal integrity is destroyed. The significant impact is the π electron conjugation break and the decrease in conductivity; analysis of micro-powder graphite shows that its particle size is small, from nanometer to micron level, with many grain boundaries and large specific surface area. Due to the high contact resistance between particles, electron migration is hindered and the conductive network is discontinuous; for artificial graphite, after high-temperature graphitization treatment, the crystals are ordered but there are impurities, such as binder residues, and the grain size is small, which leads to the introduction of impurities and the conductivity is lower than that of natural graphite; for flake graphite, although it is also a layered structure, the stacking method and force between layers are different from those of natural graphite.

[0011] On the other hand, from the perspective of surface properties and polypyrrole interface. The surface of natural graphite is smooth and contains a small amount of oxygen-containing groups, such as carboxyl and hydroxyl groups, which have a strong π-π conjugation effect with polypyrrole; in comparison, the surface of expanded graphite is porous and rough. Due to the residual intercalation oxidation, it contains a large number of oxygen-containing groups, which makes it easy for polypyrrole to fill the gaps, but local accumulation leads to discontinuous conductive networks and hindered interfacial electron transmission; for micro-powdered graphite, the surface activity is high and it is easy to agglomerate. After resin wrapping, the conductive channels between particles are reduced, resulting in the difficulty of forming a conductive path through the network after polypyrrole is coated; for artificial graphite, the residual binder on the surface, such as asphalt, affects the direct contact between polypyrrole and graphite, causing the binder to hinder π-π conjugation and increase the interface resistance.

[0012] By comparing Example 1 with Examples 6-10, it can be seen that the polymerization time of polypyrrole will also affect the conductivity of the polypyrrole modified graphite / resin composite bipolar plate. The possible reason is that the synthesis time of polypyrrole directly affects its morphology, thickness and conductive network formation on the graphite surface, which significantly affects the conductivity and interface bonding strength of the composite bipolar plate. The possible mechanism is as follows: too short polymerization time will lead to incomplete polymerization and the formation of a discontinuous film. The thickness of the polypyrrole conductive film is insufficient, which will lead to a discontinuous conductive network and high interface resistance. When the polymerization time is relatively appropriate, the polymerization will be sufficient to form a uniform and dense film with a smooth surface and a complete π-π conjugated structure, thereby reducing the interface resistance. Too long a polymerization time of polypyrrole will lead to excessive growth of the film, a rough surface or cracks, oxidation degradation in some areas, and excessively thick layers that hinder the bonding of graphite and resin, increasing the risk of interface delamination.

[0013] Comparing Example 1 with Comparative Example 1, it can be seen that the conductivity of the composite bipolar plate formed by the combination of polypyrrole-modified graphite and resin is much greater than that of the unmodified graphite / resin composite bipolar plate. The possible reason is that polypyrrole has a conjugated double bond structure, which can form a π-π conjugated conductive network at the interface between graphite and resin, effectively improving the conductive performance of the interface, reducing the interface resistance, and thus significantly improving the overall conductivity of the composite bipolar plate.

[0014] Test Example 2: Flexural Strength Test Cut a 10 cm × 1 cm sample strip, and use a universal material testing machine to apply bending pressure to the sample at a speed of 2 mm / min, record the load value when the sample breaks, and take the average value after testing several times.

[0015] Table 2 Flexural Strength Test Results of a Polypyrrole-Modified Graphite / Resin Composite Bipolar Plate Test item Flexural strength / (MPa) Example 1 46.3 Example 2 46.1 Example 3 37.6 Example 4 44.8 Example 5 41.3 Example 6 38.1 Example 7 44.7 Example 8 40.3 Example 9 42.2 Example 10 36.3 Comparative Example 1 32.1 It can be concluded from Table 1 above that when comparing Examples 1-10 with Comparative Example 1, for a polypyrrole-modified graphite / resin composite bipolar plate prepared by the present invention, when the graphite material is natural graphite and the synthesis time of polypyrrole is 20 minutes, the prepared polypyrrole-modified graphite / resin composite bipolar plate has the best flexural strength, which can reach 46.3 MPa; Comparing Examples 1-5, it can be seen that the flexural strength of a polypyrrole-modified graphite / resin composite bipolar plate prepared with different matrix graphite materials is different, and the gap is relatively large. The possible reason is that natural graphite is highly crystalline, with a complete layered structure, large sheet sizes, a smooth surface, and contains a small amount of oxygen-containing functional groups; polypyrrole is tightly combined with the graphite surface through π-π conjugation, and at the same time its amino / imino groups form hydrogen bonds with the hydroxyl groups in the resin, forming a "graphite-polypyrrole-resin" ternary strong interface, enhancing the flexural strength; the large-size sheet structure enables the load to be evenly distributed along the graphite plane, avoiding local stress concentration.

[0016] Comparing Comparative Example 1 with Examples 6-10, it can be seen that the polymerization time of polypyrrole also affects the flexural strength of the polypyrrole-modified graphite / resin composite bipolar plate. The possible reasons are as follows: If the polymerization time is too short, only part of the pyrrole monomers are oxidized and cross-linked to form short-chain polypyrrole, which cannot effectively bridge the gaps between graphite flakes, resulting in incomplete polymerization, the formation of a discontinuous film with insufficient thickness, an increase in the direct contact area between graphite and resin, and a decrease in mechanical strength; If the polymerization time is appropriate, a uniform and dense polypyrrole conductive film will be formed, with a smooth surface and a complete π-π conjugate structure. The polypyrrole forms stable chemical bonds with graphite / resin, and at the same time forms a secondary conjugate with the benzene rings in the resin, enhancing the interfacial bonding strength; If the synthesis time of polypyrrole is too long, it will lead to excessive growth of the polypyrrole conductive film, making its surface rough or cracked, and partial oxidation and degradation in some areas. The degradation products introduce defects, resulting in a decrease in the overall conductivity of the polypyrrole-modified graphite / resin composite bipolar plate of the present invention.

[0017] Meanwhile, in the subsequent hot pressing process, if the polymerization time of polypyrrole is too short, it is easily squeezed and broken by the resin during hot pressing, and microcracks are generated at the interface; If the polymerization time is appropriate, the polypyrrole layer has both flexibility and rigidity, and undergoes co-deformation with the resin during the hot pressing process to form a strong interfacial bond, with hydrogen bonding and physical anchoring; If the polymerization time of polypyrrole is too long, it leads to too high rigidity of the polypyrrole layer, with a large difference in the coefficient of thermal expansion from the resin, and stress concentration at the interface after cooling will cause the composite bipolar plate to be easily delaminated.

[0018] Comparing Comparative Example 1 with Comparative Example 1, it can be seen that the conductivity of the composite bipolar plate formed by the combination of polypyrrole-modified graphite and resin is much greater than that of the unmodified graphite / resin composite bipolar plate. The possible reasons are as follows: Polypyrrole has a conjugated double bond structure, which can form a π-π conjugate conductive network at the interface between graphite and resin, effectively improving the interfacial conductivity, reducing the interfacial resistance, and thus significantly increasing the overall conductivity of the composite bipolar plate. At the same time, the oxygen-containing groups on the polypyrrole molecule can form hydrogen bonds with the resin molecules, enhancing the interfacial adhesion between graphite and resin, and greatly improving the mechanical properties of the composite bipolar plate.

[0019] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. Modifications, substitutions, and improvements made within the principles of the present invention should all be included within the protection scope of the present invention.

Claims

1. A polypyrrole-modified graphite / resin composite bipolar plate, characterized in that, It mainly includes the following components: matrix graphite material, resin.

2. The polypyrrole-modified graphite / resin composite bipolar plate according to claim 1, wherein: The main components, in addition to the matrix graphite material and resin, also include a curing agent.

3. A polypyrrole-modified graphite / resin composite bipolar plate according to claim 2, characterized in that, By mass fraction: the matrix graphite material is 60 - 90 wt%, the resin is 10 - 40 wt%, and the curing agent is 5 - 8 wt%.

4. A polypyrrole-modified graphite / resin composite bipolar plate according to any one of claims 1-3, characterized in that: The matrix graphite material is at least one of natural graphite, artificial graphite, expanded graphite, micro-powder graphite, and flake graphite.

5. A polypyrrole-modified graphite / resin composite bipolar plate according to any one of claims 1-3, characterized in that: The curing agent is further preferably hexamethylene alkanolamine.

6. A polypyrrole-modified graphite / resin composite bipolar plate according to any one of claims 1-3, characterized in that: The resin is preferably a thermoplastic phenolic resin.

7. The polypyrrole-modified graphite / resin composite bipolar plate according to claim 6, wherein: The thermoplastic phenolic resin is further preferably a linear thermoplastic phenolic resin.

8. A preparation method of a polypyrrole-modified graphite / resin composite bipolar plate according to any one of claims 1-7, characterized in that, It mainly includes the following steps: Step 1. In-situ synthesis of polypyrrole; Step 2. Preparation of composite powder: Mix the polypyrrole-modified graphite powder obtained in the above Step 1 and phenolic resin powder in a mass ratio of (3 - 9):1, continue to add an appropriate amount of curing agent, and stir for 25 - 40 minutes at a stirring speed of 200 - 400 revolutions per minute; Step 3. Hot pressing and forming: Put the mixture obtained by stirring in the above Step 2 into a mold, close the mold before starting to heat up, and keep heating up while maintaining the forming pressure. After reaching the preset curing temperature, keep it for 25 - 35 minutes. The hot pressing temperature is 160 - 180 °C, and the pressure is 10 - 30 MPa.

9. The preparation method of a polypyrrole-modified graphite / resin composite bipolar plate according to claim 8, characterized in that, The in-situ synthesis of polypyrrole in Step 1 mainly includes the following steps: Prepare a pyrrole monomer solution: Dissolve 1 - 3 mL of pyrrole monomer in 30 - 50 mL of hydrochloric acid solution with a concentration of 0.8 - 1.1 mol / L; Prepare an initiator solution: Dissolve 2 - 5 g of ammonium persulfate with a concentration of 0.4 - 0.6 mol / L in 46 - 53 mL of deionized water solution; Immerse the graphite sheet in the pyrrole solution and stir for 5 - 30 minutes under an ice-water bath condition; Slowly dropwise add the initiator solution and continuously stir and polymerize for 5 - 30 minutes; Wash it alternately with ethanol / deionized water 3 times and vacuum dry for 25 - 35 minutes.

10. A polypyrrole-modified graphite / resin composite bipolar plate according to any one of claims 1-9, characterized in that: The thickness of the polypyrrole graphite conductive film is 10 - 20 nm.

Images