Preparation method of copper sheet-graphite composite bipolar plate, bipolar plate and flow battery

Abstract

The invention provides a preparation method of a copper sheet-graphite composite bipolar plate, the bipolar plate and a flow battery. The bipolar plate is formed by compounding a copper sheet and a graphite composite layer through an injection molding process. The preparation method comprises the following steps: carrying out pretreatment, protonation, silane hydrolysate reaction and other treatments on the surface of a copper sheet to obtain a surface-grafted silanization copper sheet; preparing a graphite resin composite material; and putting the copper sheet with the surface grafted with the silane into a mold cavity, injecting the graphite resin composite material into a mold, and carrying out injection molding to obtain the copper sheet-graphite composite integrated bipolar plate. The bipolar plate prepared by the method effectively solves the technical problem that the balance of strength improvement and contact resistance reduction of a metal graphite composite bipolar plate in the prior art is difficult to realize at the same time, has excellent electrolyte corrosion resistance, and can be widely applied to the technical field of precise manufacturing of flow battery bipolar plates.

Description

Technical Field

[0001] This invention relates to the field of flow battery manufacturing technology, and in particular to a method for preparing a copper-graphite composite bipolar plate, the bipolar plate, and a flow battery. Background Technology

[0002] As a key component in flow battery systems, bipolar plates primarily collect and conduct the current generated during battery charging and discharging. Ideal bipolar plate materials typically need to simultaneously meet core requirements such as high strength, low resistance, and resistance to electrolyte corrosion. Currently, most mainstream bipolar plates utilize graphite composite systems, but these are prone to cracking or deformation under assembly pressure and long-term operational vibration, increasing the risk of electrolyte leakage. Furthermore, the contact interfaces between bipolar plates and electrodes / current collectors are susceptible to high contact resistance due to surface unevenness or pressure fluctuations, thereby increasing internal battery losses and reducing energy efficiency.

[0003] To address the aforementioned issues, the industry has attempted to introduce a conductive metal layer to utilize its high conductivity and mechanical support to reduce resistance and enhance overall strength. Copper sheets are a commonly used composite metal material, but existing copper-graphite composite bipolar plate processes have significant limitations: (1) In traditional molding composite processes, the copper sheet and graphite composite layer rely solely on the physical adhesion of the resin, resulting in weak interfacial bonding and easy delamination in the electrolyte environment over a long period; (2) In diffusion welding processes, although metallurgical bonding can be achieved, the high temperature and high pressure process can easily lead to resin carbonization failure, and the internal stress generated by the difference in thermal expansion coefficients between copper and graphite further damages the interfacial contact stability; (3) In the process of surface coating with conductive adhesive, the conductive adhesive is prone to aging and swelling in the electrolyte, causing a significant increase in contact resistance in the short term, which cannot meet the long-term operation requirements of flow batteries.

[0004] Therefore, developing an integrated fabrication technology that can achieve a stable combination of copper sheet and graphite composite bipolar plate, balance high strength, stable contact resistance and electrolyte environment adaptability, and is suitable for large-scale production has become a key challenge to promote the development of flow batteries. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the above-mentioned technologies and provide a method for preparing a copper-graphite composite bipolar plate, a bipolar plate and a flow battery. While achieving a stable combination of copper sheet and graphite composite bipolar plate, it can also balance high strength, stable contact resistance and electrolyte environment adaptability, and is an integrated preparation technology suitable for large-scale production.

[0006] Therefore, the present invention provides a method for preparing an integral copper-graphite composite bipolar plate, comprising the following steps: S1. Pre-treat the surface of the copper sheet to obtain a pre-treated copper sheet; S2. Protonize the pretreated copper sheet obtained in step S1 to obtain a protonated copper sheet; S3. Immerse the protonated copper sheet obtained in step S2 in a silane hydrolysate to obtain a copper sheet with silane grafted onto its surface. S4. Fabrication of graphite resin composite materials; S5. Place the copper sheet with silane grafted on its surface obtained in step S3 into the mold cavity, and inject the graphite resin composite material into the mold through an injection molding machine to obtain a copper sheet-graphite composite integrated bipolar plate. The order of step S4 and the three steps S1, S2 and S3 can be interchanged.

[0007] Preferably, in step S1, the pretreatment method for the copper sheet includes: polishing the copper sheet with 800-1200 grit sandpaper, ultrasonically cleaning and degreasing it in an ethanol-acetone mixed solution for 10-15 minutes, and immersing it in a 5%-8% dilute sulfuric acid solution at room temperature for 5-8 minutes for acid washing and activation treatment, wherein the volume ratio of the ethanol-acetone mixed solution is 1:1.

[0008] Preferably, in step S2, the soaking in the protonation solution is for 20-30 minutes, and the vacuum drying is carried out in a vacuum drying oven set at 50-60°C for 1-2 hours. The protonation solution is a 0.05-0.1 mol / L citric acid-sulfuric acid mixed solution with a molar ratio of citric acid to sulfuric acid of 2:1.

[0009] Preferably, in step S3, the copper sheet is immersed in the silane hydrolysate for 1 to 1.5 hours at room temperature, and then cured in an oven at 80 to 90°C for 2 to 3 hours.

[0010] Preferably, in step S3, the silane hydrolysate is a silane solution with a mass fraction of 1% to 3% prepared by dissolving the silane coupling agent in an ethanol-water mixed solvent, and 0.1 mol / L hydrochloric acid is added dropwise to adjust the pH to 4 to 5, and the solution is stirred at room temperature for 30 to 40 minutes to obtain the silane hydrolysate.

[0011] Preferably, the silane coupling agent is one or more of aminosilane (KH550) and epoxysilane (KH560), and the volume ratio of ethanol to water in the ethanol-water mixed solvent is 3:1.

[0012] Preferably, in step S4, the resin matrix is ​​30-40 parts, the graphite powder is 60-70 parts, and the toughening agent is 1.5-4 parts. The resin matrix is ​​one or more of modified phenolic resin and epoxy resin, and the toughening agent is nitrile rubber powder. Melt plasticizing includes adding the graphite resin mixture to a twin-screw extruder and plasticizing and granulating it at 160-190°C and a screw speed of 150-180 rpm.

[0013] Preferably, in step S5, the temperature of the feed inlet in the segmented temperature control of the injection molding machine barrel is 150-160℃ and the temperature of the nozzle is 180-200℃; the pressure holding is to inject at an initial pressure of 50-60MPa during injection, increase the pressure to 80-120MPa after 1-2 seconds, hold the pressure for 5-15 seconds, and cool and solidify naturally to room temperature.

[0014] The present invention also provides a copper-graphite composite bipolar plate prepared by the above-described preparation method.

[0015] Furthermore, the present invention also provides a flow battery comprising the aforementioned copper-graphite composite bipolar plate.

[0016] Compared with existing technologies, this invention provides a method for preparing a copper-graphite composite bipolar plate, the bipolar plate, and a flow battery, which can bring the following technical effects: The preparation method of the copper-graphite composite bipolar plate, the bipolar plate, and the flow battery of this invention have the following significant advantages: (1) Strong interfacial bonding: On the one hand, physical bonding is achieved through injection molding; on the other hand, the introduction of silane coupling agent forms a stable chemical bridge between the protonated copper sheet and the phenolic resin, significantly enhancing the interfacial bonding force.

[0017] (2) Simultaneous improvement of electrical and mechanical properties: The injection-molded integrated structure not only significantly reduces the interfacial contact resistance, but also effectively improves the overall structural strength.

[0018] (3) Excellent corrosion resistance: The bipolar plate exhibits excellent stability in acidic electrolytes and does not show significant corrosion after long-term immersion. It can be widely used in various electrochemical devices. Detailed Implementation

[0019] The embodiments of the present invention are described in detail below. The described embodiments are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0020] Example 1 S100: Polish the surface of the copper sheet with 1000-grit sandpaper to achieve a uniform metallic luster and remove the thick oxide layer. Place the polished copper sheet in an ethanol-acetone mixture (volume ratio 1:1) and ultrasonically clean for 15 minutes to remove surface oil. Then rinse with deionized water and dry. Next, immerse it in an 8% (w / w) dilute sulfuric acid solution and react at room temperature for 5 minutes to remove the residual oxide layer and expose the fresh copper surface. Then rinse with deionized water and dry for later use. S200. The copper sheet pretreated in step S100 is placed in a 0.1 mol / L citric acid-sulfuric acid mixed solution (molar ratio of citric acid to sulfuric acid 2:1), soaked at room temperature for 30 min, and then dried in a vacuum drying oven at 60℃ for 2 h to obtain a protonated copper sheet.

[0021] S300: Dissolve the silane coupling agent in an ethanol-water mixed solvent (volume ratio 3:1) to prepare a 2% (w / w) silane solution. Add 0.1 mol / L hydrochloric acid dropwise to the silane solution to adjust the pH to 4.5. Stir at room temperature for 30 min to obtain a silane hydrolysate. Then, immerse the protonated copper sheet obtained in step S200 in the silane hydrolysate and react at room temperature for 1 h. Then, cure it in a 90°C drum oven for 3 h to obtain a copper sheet with silane grafted on its surface.

[0022] S400, mix raw materials in the following proportions: 26 parts resin, 70 parts graphite powder, and 4 parts toughening agent. Add the graphite-resin mixture to a twin-screw extruder and plasticize and granulate at 180°C and 160 rpm. After granulation, dry for later use.

[0023] It is worth mentioning that the steps for making graphite-resin composite preforms can be performed in parallel with the steps S100, S200, and S300 mentioned above, without any specific order.

[0024] S500: First, preheat the mold and place the copper sheet obtained in step S300 into the mold for positioning and fixing. Then, add the graphite-resin granules obtained in step S400 into the injection molding machine. The injection molding machine barrel is temperature-controlled in sections, with the inlet temperature at 160℃ and the nozzle temperature at 200℃. During injection, first inject at an initial pressure of 60MPa, then increase the pressure to 120MPa after 1-2 seconds, hold the pressure for 10 seconds, and after cooling, open the mold and take out the initial composite bipolar plate. Then, cure it and let it cool naturally to room temperature to complete the chemical bonding between the copper sheet and the graphite composite bipolar plate.

[0025] In the above reaction, during the protonation process in step S200, fresh Cu atoms on the copper sheet surface react with O2 in the solution to form a thin Cu2O / CuO oxide layer. The oxide layer surface hydrolyzes to generate hydroxyl groups (-OH), which further react with H in the solution. +The silane coupling agent in step S300 combines to form protonated sites (Cu-OH2+). The hydrolyzable group -OCH3 of the silane coupling agent hydrolyzes to generate silanol groups (-Si-OH), which deprotonate Cu-OH2+ on the surface of the protonated copper sheet in step S200 to generate Cu-OH. Cu-OH and -Si-OH undergo dehydration condensation to form Si-O-Cu covalent bonds. At the same time, unreacted -Si-OH self-polymerizes to form a Si-O-Si crosslinked network. Furthermore, during the curing process in step S500, the exposed organic functional groups (such as amino and epoxy groups) of the silane graft layer react chemically with the hydroxymethyl (-CH2OH) groups of the phenolic resin in the phenolic resin-based graphite composite bipolar plate: the amino groups and hydroxymethyl groups undergo a condensation reaction to generate -NH-CH2- covalent bonds, and the epoxy groups and hydroxymethyl groups undergo a ring-opening reaction to generate -O-CH2-CH(OH)-CH2- covalent bonds; ultimately, a continuous chemical bond network of "Cu-O-Si-RY-resin" (R is the hydrocarbon skeleton of silane, Y is the organic functional group of silane) is formed between the copper sheet and the graphite composite layer, which significantly improves the interfacial bonding force between the copper sheet and the composite bipolar plate. Previously, the copper sheet and the graphite composite bipolar plate only had physical contact; the integrated injection molding process further effectively improves the contact resistance and increases the strength. Experiments show that the prepared copper-graphite composite bipolar plate has excellent resistance to electrolyte corrosion, and shows no significant corrosion after long-term immersion in an acidic environment, making it suitable for various electrochemical devices.

[0026] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

[0027] The progressiveness of the present invention will be further demonstrated below with several comparative examples.

[0028] Comparative Example 1 D101. Polish the surface of the copper sheet with 1000-grit sandpaper to achieve a uniform metallic luster and remove the thick oxide layer. Place the polished copper sheet in an ethanol-acetone mixture (volume ratio 1:1) and ultrasonically clean for 15 minutes to remove surface oil. Then rinse with deionized water and dry. Next, immerse the copper sheet in an 8% (w / w) dilute sulfuric acid solution and react at room temperature for 5 minutes to remove the residual oxide layer and expose the fresh copper surface. Then rinse with deionized water and dry for later use.

[0029] D102. Place the pretreated copper sheet from step D101 into a 0.1 mol / L citric acid-sulfuric acid mixed solution (citric acid to sulfuric acid molar ratio 2:1), soak at room temperature for 30 min, and then dry in a vacuum drying oven at 60℃ for 2 h to obtain protonated copper sheet.

[0030] D103. Dissolve the silane coupling agent in an ethanol-water mixed solvent (volume ratio 3:1) to prepare a 2% (w / w) silane solution. Add 0.1 mol / L hydrochloric acid dropwise to the silane solution to adjust the pH to 4.5. Stir at room temperature for 30 min to obtain a silane hydrolysate. Then, immerse the protonated copper sheet obtained in step D102 in the silane hydrolysate and react at room temperature for 1 h. Then, cure it in a 90°C drum oven for 3 h to obtain a copper sheet with silane grafted on its surface.

[0031] D104. Mix raw materials in the following proportions: 26 parts resin, 70 parts graphite powder, and 4 parts toughening agent. Add the graphite-resin mixture to a twin-screw extruder and plasticize and granulate at 180°C and 160 rpm. After granulation, dry for later use.

[0032] D105. Add the graphite-resin granules obtained in step D104 to the injection molding machine. The injection molding machine barrel is temperature-controlled in sections, with the inlet temperature at 160℃ and the nozzle temperature at 200℃. During injection, first inject at an initial pressure of 60MPa, then increase the pressure to 120MPa after 1-2 seconds, hold the pressure for 10 seconds, and after cooling, place the copper sheet from step D103 into the mold for pressing. The pressure is 120MPa, and the pressure is held for 10 seconds. After cooling, remove and solidify, and allow it to cool naturally to room temperature to complete the bonding between the copper sheet and the graphite composite bipolar plate.

[0033] In Comparative Example 1, the copper sheet relies solely on physical adsorption and mechanical intercalation generated by molding. The copper sheet lacks active functional groups on its surface and exhibits a lack of chemical interaction with the resin matrix, resulting in microscopic gaps at the interface. The thermal expansion coefficient of the copper sheet differs significantly from that of the graphite resin substrate, leading to irreversible internal stress at the interface during the molding and cooling process, which easily causes cracking with prolonged use. These interface gaps, along with the natural oxide film on the copper sheet surface, hinder electron conduction, resulting in persistently high contact resistance.

[0034] Comparative Example 2 D201. Polish the surface of the copper sheet with 1000-grit sandpaper to achieve a uniform metallic luster and remove the thick oxide layer. Place the polished copper sheet in an ethanol-acetone mixture (volume ratio 1:1) and ultrasonically clean for 15 minutes to remove surface oil. Then rinse with deionized water and dry. Next, immerse it in an 8% (w / w) dilute sulfuric acid solution and react at room temperature for 5 minutes to remove the residual oxide layer and expose the fresh copper surface. Then rinse with deionized water and dry for later use.

[0035] D202. Dissolve the silane coupling agent in an ethanol-water mixed solvent (volume ratio 3:1) to prepare a 2% (w / w) silane solution. Add 0.1 mol / L hydrochloric acid dropwise to the silane solution to adjust the pH to 4.5. Stir at room temperature for 30 min to obtain a silane hydrolysate. Then, immerse the copper sheet from step D201 in the silane hydrolysate and react at room temperature for 1 h. Then, cure it in a 90°C drum oven for 3 h to obtain a copper sheet with silane grafted on its surface.

[0036] D203, mix the raw materials in the following proportions: 26 parts resin, 70 parts graphite powder, and 4 parts toughening agent. Add the graphite-resin mixture to a twin-screw extruder and plasticize and granulate it at 180°C and 160 rpm. After granulation, dry it for later use.

[0037] D204. First, preheat the mold and place the copper sheet obtained in step D202 into the mold for positioning and fixation. Then, add the graphite-resin granules obtained in step D203 into the injection molding machine. The injection molding machine barrel is temperature-controlled in sections, with the inlet temperature at 160℃ and the nozzle temperature at 200℃. During injection, first inject at an initial pressure of 60MPa, then increase the pressure to 120MPa after 1-2 seconds, hold the pressure for 10 seconds, and after cooling, open the mold and remove the initial composite bipolar plate. Then, cure it and allow it to cool naturally to room temperature to complete the bonding between the copper sheet and the graphite composite bipolar plate.

[0038] Comparative Example 2 lacks protonation treatment. The number of hydroxyl groups (-OH) generated by the hydrolysis of the Cu2O / CuO oxide layer formed after acid washing on the copper sheet surface is less than that in Example 1, and the hydroxyl groups are less active. The conversion rate of the dehydration condensation reaction with the silanol groups (-Si-OH) in the silane hydrolysate is low. Only a small number of dispersed Si-O-Cu covalent bonds are formed, failing to construct a continuous chemical bridge between the copper sheet and silane and resin. The interfacial bonding force relies on physical adsorption and local chemical bonds, resulting in uneven distribution. This leads to high overall contact resistance of the bipolar plate and reduced mechanical strength.

[0039] Comparative Example 3 D301. Use 1000-grit sandpaper to polish the surface of the copper sheet until it has a uniform metallic luster, in order to remove the thick oxide layer on the surface. Place the polished copper sheet in an ethanol-acetone mixed solution (volume ratio 1:1) and ultrasonically clean it for 15 minutes to remove surface oil. Then rinse it with deionized water and blow it dry. Then immerse it in an 8% (w / w) dilute sulfuric acid solution and react it at room temperature for 5 minutes to remove the residual oxide layer on the surface, exposing the fresh copper surface. Then rinse it with deionized water and blow it dry for later use.

[0040] D302. Place the pretreated copper sheet from step D301 into a 0.1 mol / L citric acid-sulfuric acid mixed solution (citric acid to sulfuric acid molar ratio 2:1), soak at room temperature for 30 min, and then dry in a vacuum drying oven at 60℃ for 2 h to obtain protonated copper sheet.

[0041] D303, mix raw materials in the following proportions: 26 parts resin, 70 parts graphite powder, and 4 parts toughening agent. Add the graphite-resin mixture to a twin-screw extruder and plasticize and granulate at 180°C and 160 rpm. After granulation, dry for later use.

[0042] D304. First, preheat the mold and place the copper sheet obtained in step D302 into the mold for positioning and fixation. Then, add the graphite-resin granules obtained in step D303 into the injection molding machine. The injection molding machine barrel is temperature-controlled in sections, with the inlet temperature at 160℃ and the nozzle temperature at 200℃. During injection, first inject at an initial pressure of 60MPa, then increase the pressure to 120MPa after 1-2 seconds, hold the pressure for 10 seconds, and after cooling, open the mold and remove the initial composite bipolar plate. Then, cure it and let it cool naturally to room temperature to complete the bonding between the copper sheet and the graphite composite bipolar plate.

[0043] In the comparative example, the protonated sites (Cu-OH2+) on the surface of the triprotonated copper sheet cannot directly react chemically with the modified phenolic resin in the graphite resin. Physical contact and weak hydrogen bonding are only achieved through injection pressure, resulting in weak interfacial adhesion. Furthermore, the hydroxymethyl groups (-CH2OH) in the modified phenolic resin have low reactivity with the hydroxyl groups (-OH) on the copper sheet surface, preventing the formation of stable covalent bonds and making the interface susceptible to electrolyte corrosion.

[0044] The performance test results of the bipolar plates obtained in the above embodiments are compared in the following table: As can be seen from the table above, the "protonation + silane grafting" combined treatment of this invention is the optimal solution for constructing a stable interfacial chemical bond network. The absence of either step will significantly degrade interfacial bonding strength, conductivity, and corrosion resistance. The protonation and silane grafting steps work synergistically for the bipolar plate forming performance, and their combined effect is indispensable. This invention achieves a balance between high strength, low contact resistance, and corrosion resistance through the dual action of "chemical bridging + physical composite," solving the core pain points of existing technologies. The technical effect is significantly superior to comparative examples of single-process improvements.

Claims

1. A method for preparing an integrated copper-graphite composite bipolar plate, characterized in that, Includes the following steps: S1. Pre-treat the surface of the copper sheet to obtain a pre-treated copper sheet; S2. Protonize the pretreated copper sheet obtained in step S1 to obtain a protonated copper sheet; S3. Immerse the protonated copper sheet obtained in step S2 in a silane hydrolysate to obtain a copper sheet with silane grafted onto its surface. S4. Fabrication of graphite resin composite materials; S5. Place the copper sheet with silane grafted on its surface obtained in step S3 into the mold cavity, and inject the graphite resin composite material into the mold through an injection molding machine to obtain a copper sheet-graphite composite integrated bipolar plate. The order of step S4 and the three steps S1, S2 and S3 can be interchanged.

2. The method according to claim 1, characterized in that, In step S1, the pretreatment method of the copper sheet includes: polishing the copper sheet with 800-1200 grit sandpaper, ultrasonically cleaning and degreasing it in an ethanol-acetone mixed solution for 10-15 minutes, and immersing it in a 5%-8% (w / w) dilute sulfuric acid solution at room temperature for 5-8 minutes for acid washing and activation treatment, wherein the volume ratio of the ethanol-acetone mixed solution is 1:

1.

3. The method according to claim 1, characterized in that, In step S2, the soaking in the protonation solution is for 20-30 minutes, and the vacuum drying is performed in a vacuum drying oven set at 50-60°C for 1-2 hours. The protonation solution is a 0.05-0.1 mol / L citric acid-sulfuric acid mixed solution, and the molar ratio of citric acid to sulfuric acid is 2:

1.

4. The method according to claim 1, characterized in that, In step S3, the copper sheet is immersed in silane hydrolysate for 1-1.5 hours at room temperature, and the curing is carried out in an oven at 80-90°C for 2-3 hours.

5. The method according to claim 1 or 4, characterized in that, The silane hydrolysate is a silane solution with a mass fraction of 1% to 3% prepared by dissolving the silane coupling agent in an ethanol-water mixed solvent, adjusting the pH to 4 to 5 by adding 0.1 mol / L hydrochloric acid dropwise, and stirring at room temperature for 30 to 40 minutes to obtain the silane hydrolysate.

6. The method according to claim 5, characterized in that, The silane coupling agent is one or more of aminosilane (KH550) and epoxysilane (KH560), and the volume ratio of ethanol to water in the ethanol-water mixed solvent is 3:

1.

7. The method according to claim 1, characterized in that, In step S4, the resin matrix comprises 30-40 parts, graphite powder comprises 60-70 parts, and toughening agent comprises 1.5-4 parts. The resin matrix is ​​one or more of modified phenolic resin and epoxy resin, and the toughening agent is nitrile rubber powder. The melt plasticizing process includes adding the graphite resin mixture to a twin-screw extruder and plasticizing and granulating it at 160-190°C and a screw speed of 150-180 rpm.

8. The method according to claim 1, characterized in that, In step S5, the temperature at the inlet of the injection molding machine barrel is 150-160℃ and the temperature at the nozzle is 180-200℃ in the segmented temperature control of the injection molding machine barrel; the pressure increase and holding pressure are performed by first injecting at an initial pressure of 50-60MPa, then increasing the pressure to 80-120MPa after 1-2 seconds, and holding the pressure for 5-15 seconds; the cooling and curing are performed by naturally cooling to room temperature.

9. A copper-graphite composite bipolar plate prepared by the method described in any one of claims 1-8.

10. A flow battery, characterized in that, Including the copper-graphite composite bipolar plate as described in claim 9.

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