A graphite-based composite material bipolar plate and a production method thereof

Abstract

The present invention relates to the field of liquid flow batteries, and in particular to a graphite-based composite material bipolar plate and a production method thereof. The production method comprises the following steps: preparing graphite-based powder, which includes 72-82wt% of graphite powder, 15-23wt% of thermosetting resin powder and 3-5% of reinforcing phase powder; preparing a mixed slurry, wherein the thermosetting resin powder is stirred with an organic solvent to obtain a thermosetting resin solution; preparing graphite composite material powder; powder filling; cold pressing, wherein after the mold is closed, the mold is pressed down in a stepwise manner while the inner cavity of the mold is evacuated to the final mold closing height; locking and heating, wherein the mold is mechanically locked and transferred to a tunnel heating furnace for overall heating and insulation to solidify the bipolar plate in the mold; cooling and demolding; and cleaning the mold, wherein the inner surface of the mold is purged for subsequent continuous production of bipolar plates. The graphite bipolar plate produced by the method of the present invention has good mechanical properties, low resistivity, less surface warping, and good dimensional accuracy.

Description

Technical Field

[0001] The invention relates to the field of liquid flow batteries, and in particular to a graphite-based composite material bipolar plate and a production method thereof. Background Art

[0002] Liquid flow batteries are composed of battery stack units, electrolytes, electrolyte storage and supply units, and management and control units. Liquid flow batteries achieve the storage and release of electrical energy by controlling the cyclic conversion between different electrolytes through the reaction of active substances in the positive and negative electrolytes of the battery. The advantages of liquid flow batteries include high safety, long cycle life, flexible design, and low maintenance costs. It is an emerging field in the battery field, which is conducive to parallel design and is expected to quickly meet the needs of large and medium-sized energy storage and peak regulation. As an important component of liquid flow batteries, bipolar plates play the role of separating, connecting batteries in series, conducting current, and providing structural support for the battery stack. At present, the cost of bipolar plates accounts for about 40% of the total cost of liquid flow batteries. The bipolar plate material must be a good conductor of heat and electricity to ensure that the overall temperature of the liquid flow battery is evenly distributed when it is working, which is conducive to heat dissipation, conductivity, and improving the working efficiency of the liquid flow battery; at the same time, since the electrolyte in the liquid flow battery is corrosive, the material needs to have stable chemical properties and good corrosion resistance; in addition, the bipolar plate is responsible for supporting the entire liquid flow battery as a connection between single cells, so the bipolar plate is required to have good mechanical properties and high strength to ensure the stability of the battery stack during assembly. The commonly used materials for preparing bipolar plates are graphite, metal and composite materials. Composite bipolar plates are simple to process and can greatly reduce costs to achieve large-scale production. The raw material of the composite bipolar plate is a composite material combined with a polymer resin and carbon. Compared with metal bipolar plates, composite bipolar plates have the advantages of corrosion resistance and light weight.

[0003] In the prior art, Wang Guo, Zhao Wenbin, Jiang Hongdong and others published "A bipolar plate for all-vanadium liquid flow battery and its preparation method" (CN 117207561 A 2023.12.12). The invention relates to a bipolar plate for all-vanadium liquid flow battery and its preparation method, the method comprising the following steps: premixing and banburying the raw materials of the bipolar plate to obtain a first banburying mixture; heating and banburying the first banburying mixture to obtain a second banburying mixture; melt-banburying the second banburying mixture to obtain a third banburying mixture; after the melt-banburying is completed, stirring the third banburying mixture while cooling it to obtain a cooled banburying mixture; cooling the cooled banburying mixture, crushing it to obtain a crushed mixture; pulverizing the crushed mixture to obtain a pulverized mixture; hot pressing the pulverized mixture to obtain a hot pressed plate; cooling the hot pressed plate and demolding it to obtain a bipolar plate. However, the invention does not involve the density, compactness, hardness, electrochemical corrosion, and external dimensions of the bipolar plate, and does not take into account the continuous production of bipolar plate cold pressing + tunnel furnace curing, as well as the forming problems of the bipolar plate flow channel structure. The process method provided in this patent is not suitable for the preparation of graphite-based composite bipolar plates with high conductivity and high air tightness, and it is difficult to achieve low-cost and efficient production. The process method provided in this patent is not suitable for the preparation of graphite-based composite bipolar plates with high conductivity and high strength, and it is difficult to achieve efficient production. There are three key points in the process method in this patent that are difficult to achieve for the efficient preparation of high-performance graphite-based composite bipolar plates: 1) Preparation of graphite-based composite powder raw materials. In this patent, the conductive filler, resin, antioxidant and release agent are subjected to multiple mixing processes at different temperatures and stirring times to obtain a mixed mixture, and the final graphite-based composite powder raw material for hot pressing is obtained through cooling, crushing, pulverizing and other processes. Since the patent uses a long-term high-temperature mixing process to ensure that the conductive filler, resin, etc. are mixed evenly, for the resin materials such as polypropylene used in the patent, long-term high temperature will cause irreversible thermal decomposition, or chemical reaction with oxygen, water vapor, etc. in the air, resulting in gradual oxidation and corrosion of the surface, so the powder making process puts forward high requirements on the mixing and stirring equipment and the high-temperature mixing and preparation process of the powder. 2) The efficiency of the hot pressing process of the bipolar plate is low. Taking Example 3 of the patent as an example, the crushed mixture is placed in the hot pressing mold, and the surface raw materials are flattened by scraping the surface of the powder with a glass sheet. The pre-pressing is first performed with a pressure of 9MPa, and the mold is heated to 170°C at a heating rate of 5°C / min, and then the raw materials are pressed at 205°C and 30MPa for 10min to obtain a hot pressing plate; after the hot pressing process is completed, cooling water is passed through the mold to maintain a pressure of 22MPa, and the hot pressing plate is cooled to 80°C, demolded, and a bipolar plate is obtained. The forming process is relatively long, and the processing time of a single bipolar plate product exceeds 40 minutes, which seriously affects the final production efficiency of the product. 3) Preparation of bipolar plate graphite paper composite sheet.In order to improve the conductivity of the bipolar plate, this patent covers high-conductivity graphite paper on both sides of the bipolar plate by hot pressing + cold pressing, so that a high-conductivity covering layer is formed on the surface of the bipolar plate. However, since the graphite paper itself does not have the ability to plastically deform, the process method cannot prepare bipolar plates with flow channel structures, which limits the application of this technology in liquid flow battery products; at the same time, the process requires heating and cooling the mold to achieve a good combination of graphite paper and composite bipolar plate materials. The time required for heating and cooling the mold itself will seriously affect the final production efficiency of the product, and it is difficult to apply it to the preparation of large-size bipolar plates. Therefore, considering the three aspects of powder raw material preparation, hot pressing preparation process and bipolar plate graphite paper composite plate preparation process, the process provided by this patent is difficult to achieve industrial production of high-conductivity and high-airtightness graphite-based composite bipolar plates.

[0004] In the prior art, Wang Yong and Wang Jian published an invention patent for "a method for producing a carbon-based composite molded bipolar plate" (CN 115332484 A 2022.11.11). The invention relates to the technical field of bipolar plate production, and in particular to a method for producing a carbon-based composite molded bipolar plate. The invention discloses a method for producing a carbon-based composite molded bipolar plate, comprising the following steps: stirring graphite powder and carbon black evenly; introducing resin and stirring evenly until the color is uniform; adding carbon nanotubes and stirring evenly to obtain a mixture; pumping the mixture into the mold, pressurizing and heating the mold after the mold is closed; maintaining pressure and shape after reaching the set pressure; switching to the molding mode after reaching the set temperature; standing and waiting for the temperature to drop, taking out the finished product; placing the finished product in a jig for shape-maintaining cooling to obtain a bipolar plate. However, the patent does not involve the density, compactness, hardness, electrochemical corrosion, and external dimensions of the bipolar plate, nor does it involve the continuous production of bipolar plates. The process provided in this patent is not suitable for the preparation of graphite-based composite bipolar plates with high conductivity and high air tightness, and it is difficult to achieve low-cost and efficient production. There are three key points in the process method in this patent that make it difficult to achieve low-cost and efficient preparation of high-performance graphite-based composite bipolar plates: 1) The cost of bipolar plate raw materials is too high. The raw material components in this patent are graphite powder, carbon nanotubes, carbon black and resin, which are prepared in a ratio of 40:10:10:40. The raw material costs of carbon nanotubes and carbon black as conductive enhancement phases are much higher than graphite powder (taking the price of multi-walled carbon nanotubes and natural flake graphite powder as an example, the unit price of multi-walled carbon nanotubes is 500 yuan / kg, and the unit price of natural flake graphite powder is 10 yuan / kg), which greatly increases the total cost of raw materials and does not meet the needs of mass production. 2) The process of filling the mold with mixed raw materials is difficult. This patent pumps the mixed raw materials into the mold to complete the filling process. According to the description of the patent, the resin in the mixture is a thermosetting resin, which is not fluid at room temperature, but this type of resin is soluble in organic solvents such as ethanol. Therefore, it is reasonable to believe that the raw material pumped into the mold in this step is a thermosetting resin solution mixture containing an organic solvent. The viscosity of the resin solution at room temperature is relatively high and its fluidity is poor. The degree of filling of the complex cavity is greatly affected by the size and structure of the mold cavity. Therefore, this method is difficult to achieve full filling of large-size bipolar plate molds with flow channel structures. In addition, in order to obtain dense bipolar plate products with certain mechanical strength, the organic solvent in the raw resin solution needs to be completely discharged during the heating and forming process. The content of the patent does not involve a solution to this problem. 3) The efficiency of bipolar plate preparation is low. During the mold closing and forming process of this patent, the mold needs to be gradually heated from room temperature to the target temperature (160°C). After a period of heat preservation and pressure maintenance, the heating is stopped, and the mold temperature is waited to drop to room temperature before the mold is opened to remove the product. The whole process takes a long time, resulting in low preparation efficiency. Therefore, considering the three aspects of raw material cost, raw material filling and preparation efficiency, the process provided by this patent is difficult to achieve industrial production of graphite-based composite bipolar plates with high conductivity and high air tightness.

[0005] In the prior art, Zhai Yuewen, Yang Hao and others published a patent titled "A Graphite Composite Bipolar Plate for Liquid Flow Battery and Preparation Method and Device" (CN 117199420 A 2023.12.08). The patent discloses a graphite composite bipolar plate for liquid flow battery and a preparation method and device, wherein the method comprises the following steps: mixing worm graphite powder, chopped carbon fiber and resin solution evenly to obtain a slurry-like mixed material body; rolling the mixed material body to obtain a prefabricated graphite composite plate; and then heating and softening the prefabricated graphite composite plate and rolling it again to obtain a graphite composite bipolar plate with flow channels on both sides. The preparation method provided by the invention mainly uses worm graphite powder and chopped carbon fiber as the base material, mixes them evenly with the resin solution, and obtains a prefabricated graphite composite material plate by rolling, and then rolls the bipolar plate after heating. The mechanical properties of worm graphite powder and chopped carbon fiber are used to make the resin and worm graphite powder and chopped carbon fiber mix evenly, improve the density and mechanical properties of the bipolar plate, and realize the continuous preparation of the bipolar plate by rolling. However, the patent does not involve the control of the external dimensions of the product during the curing process. The process method provided in this patent is not suitable for the preparation of graphite-based composite bipolar plates with large size, high conductivity and high air tightness. There are three key points in the process method in this patent that make it difficult to achieve efficient preparation of large-size high-performance graphite-based composite bipolar plates: 1) The thickness of the rolled bipolar plate is limited. Due to its own technical characteristics, the rolling process has strict requirements on the spacing of the rollers. Excessive roller spacing will cause the rolling process to be unstable and the thickness of the pressed blank will fluctuate. Therefore, this process is mostly suitable for pressing plates with a thickness of less than 5 mm. For bipolar plates for liquid flow batteries, they need to meet both the functional requirements of conductivity and the requirements of structural strength. Therefore, their thickness is usually between 5 and 10 mm, especially for liquid flow batteries with large reserves. The thicker the bipolar plates, the thicker they are. Therefore, the bipolar plates prepared by the roller pressing process cannot meet the needs of various types, especially large-reserve liquid flow batteries. 2) Insufficient width control in the roller pressing process. Since the length of the roller needs to be greater than the width of the roller, the two sides of the roller are free ends, and it is difficult to control the width of the roller to match the final size of the product. In order to achieve uniform density of the product, it is necessary to design the width of the roller to be greater than the final size of the product, and cut off the excess part in the cutting step after the rolling is completed. This method will inevitably cause a lot of waste of scraps, which is not conducive to low-cost control. 3) The thermosetting process cannot control the dimensional accuracy and density. The process described in this patent is to first heat the prefabricated plate to 130°C to make the internal resin enter a thermoplastic state, and then obtain a rolled bipolar plate with structural features such as flow channels by hot rolling. The hot rolled bipolar plate is then cut to size and transferred to a tunnel furnace for drying and curing to finally obtain a bipolar plate product.The curing process of thermosetting resin is to cross-link molecules under heating and pressure to form a network structure, and finally form a cured product with high rigidity and hardness. In this process, the resin shrinks in volume. If the pressure is not continuously applied, the final product will have insufficient dimensional accuracy. At the same time, due to the different thicknesses of different parts of the bipolar plate products, the shrinkage process will also cause uneven density, especially in bipolar plate products with thicker thickness. This phenomenon is particularly obvious. Therefore, considering the three aspects of thickness limitation, waste of excess material, dimensional accuracy and uncontrollable density, the process provided by this patent is difficult to achieve industrial production of large-sized graphite-based composite bipolar plates with high conductivity and high air tightness.

[0006] In the prior art, Li Yanjun, Li Dayi and others published a patent titled "A graphite / resin composite bipolar plate and a method for preparing the same" (CN 118016922 A 2024.05.10). The invention discloses a graphite / resin composite bipolar plate and a method for preparing the same. The method comprises the following steps: weighing a thermosetting resin, expanded graphite and natural graphite powder, wherein the mass of the thermosetting resin accounts for 10% to 30% of the total mass of the three, and the mass ratio of natural graphite powder to expanded graphite is 0.5 to 5:1; dissolving the thermosetting resin in an organic solvent to obtain a thermosetting resin solution; mixing and stirring the expanded graphite, natural graphite powder and thermosetting resin solution to obtain a mixture, wherein the stirring temperature is 100-110°C to remove excess solvent; taking out the mixture, putting it into a mold, closing the mold before starting to heat up, and maintaining the molding pressure to heat up, and keeping the temperature for 30-40 minutes after reaching the preset curing temperature; stopping heating, and after the mold is cooled, taking out the bipolar plate sample to obtain a graphite / resin composite bipolar plate. However, the patent does not involve the forming of the flow channel structure of the bipolar plate, and does not design a continuous production method for the bipolar plate. The process method provided in the patent is not suitable for preparing graphite-based composite bipolar plates with high conductivity and high air tightness, and it is difficult to achieve efficient production. There are two key points in the process method in this patent that make it difficult to achieve efficient preparation of high-performance graphite-based composite bipolar plates: 1) No degassing during the raw material mixing stage. This patent mixes and stirs expanded graphite, natural graphite powder and thermosetting resin solution to obtain a mixture, wherein the stirring temperature is 100-110°C to remove excess solvent. However, this method does not take into account the problem of how to remove the bubbles introduced during the stirring of the resin solution, and there is no description of the method for exhausting the gas in the subsequent mold forming process using the mixture. Therefore, it can be considered that this process method is easy to cause the gas in the middle part of the product to be difficult to discharge when preparing larger-sized bipolar plate products, resulting in the inability to obtain bipolar plate products with uniform and dense structure. 2) The mold curing and cooling time is long. This patent takes out the mixture, puts it into the mold, closes the mold before starting to heat up, and maintains the molding pressure to heat up, and keeps warm for 30-40 minutes after reaching the preset curing temperature; stops heating, waits for the mold to cool, takes out the bipolar plate sample, and obtains a graphite / resin composite bipolar plate. This process requires a long time of heat preservation and pressure maintenance on the molding press, and it is necessary to wait for the mold to cool naturally. It takes a long time to produce a single product, and the production efficiency is low. Therefore, considering the raw material doping gas and the long molding process time, the process provided by this patent is difficult to achieve industrial production of graphite-based composite bipolar plates with high conductivity and high airtightness. Summary of the invention

[0007] In view of the above problems in the prior art, the present invention provides a new method for producing graphite-based composite material bipolar plates, which can realize continuous production and is more suitable for batch production of large-sized, high-performance graphite-based composite material bipolar plates.

[0008] In order to achieve the above-mentioned object, the present invention provides a method for producing a graphite-based composite material bipolar plate, which comprises the following steps: preparing a graphite-based powder, wherein the graphite-based powder comprises 72-82wt% of graphite powder, 15-23wt% of thermosetting resin powder and 3-5wt% of reinforcing phase powder; preparing a mixed slurry, wherein the thermosetting resin powder is stirred with an organic solvent to obtain a thermosetting resin solution, the reinforcing phase powder is mixed into the thermosetting resin solution and stirred evenly, and then the graphite powder is added and stirred evenly to obtain a mixed slurry; preparing a graphite composite material powder, wherein the mixed slurry is dried, crushed and sieved to obtain a graphite composite material powder; powder filling, wherein the mixed slurry is coated on the surface of a mold. A release agent is used to fill the graphite composite material powder into the mold and to scrape the surface of the graphite composite material powder flat; cold pressing is used to form the mold, after the mold is closed, the mold is pressed down in a step-by-step manner until the final closing height while the inner cavity of the mold is evacuated; locking heating is used to mechanically lock the mold and then transfer it to a tunnel heating furnace for overall heating and heat preservation to solidify the bipolar plate in the mold; cooling and demolding is used to transfer the mold to a cooling chamber for active cooling, and the mechanical lock is released when the mold is cooled to below the solidification temperature to take out the bipolar plate; and cleaning the mold is used to purge the inner surface of the mold for subsequent continuous production of bipolar plates.

[0009] In a possible implementation of the present invention, the size of the graphite powder ranges from 50 to 300 μm, and the mass fraction of the graphite powder is 77 to 80%. In some embodiments, the graphite powder includes one or more of artificial graphite powder, natural graphite flake powder, and expanded graphite powder.

[0010] In a possible implementation of the present invention, the thermosetting resin powder is a phenolic resin powder, the powder size of the phenolic resin powder is in the range of 10 to 50 μm, and the mass fraction of the phenolic resin powder is in the range of 18 to 21%.

[0011] In a possible implementation of the present invention, the reinforcing phase powder is selected from one or more of carbon black, carbon nanotubes, and carbon fibers. The size range of the carbon black is 10 to 50 nm, and the mass fraction of the carbon black is 0.7 to 2.2%; the size range of the carbon nanotubes is 10 to 50 nm, and the mass fraction of the carbon nanotubes is 1.2 to 2.1%; the diameter size range of the carbon fibers is 3 to 15 μm, and the mass fraction of the carbon fibers is 0.7 to 3.1%.

[0012] In a possible implementation of the present invention, the organic solvent is a low boiling point solvent, and the mass ratio of the organic solvent to the thermosetting resin is 1:2 to 5. In some embodiments of the present invention, the low boiling point solvent is selected from one or more of methanol, ethanol, tetrahydrofuran, and ethyl acetate.

[0013] The step-by-step pressing includes a pressing stage and a holding stage which are performed alternately. The holding stage lasts for 10 to 30 seconds, and the pressing stage is repeated 5 to 10 times.

[0014] In a possible implementation of the present invention, the temperature of the tunnel heating furnace is 180-220°C.

[0015] In a possible implementation of the present invention, in the film locking and heating step, the outer surface temperature of the mold is further monitored by means of a multi-point infrared thermometer to ensure that the outer surface temperature of the mold is maintained at 150-180°C, and to ensure that the mold stays in the uniform temperature zone of the tunnel heating furnace for 20-30 minutes.

[0016] In a possible implementation of the present invention, in the cooling and demolding step, the active cooling is spraying water mist, and the outer surface temperature of the mold is cooled to below 80° C. to release the mechanical lock.

[0017] In a possible implementation of the present invention, in the step of preparing the graphite composite material powder, the organic solvent separated from the mixed slurry is recovered by means of a condensation recovery device to achieve the organic recycling.

[0018] The present invention also provides a new graphite-based composite material bipolar plate, which is manufactured by any of the above production methods.

[0019] Beneficial Effects of the Invention

[0020] (1) The present invention improves the wettability of the graphite powder-resin interface by mixing thermosetting resin powder with an organic solvent to prepare a low-viscosity thermosetting resin solution. Through mechanical stirring, the conductive phase particles, mainly graphite powder particles, are evenly distributed in the thermosetting resin solution to form a mixed slurry. The mixed slurry is then dried, crushed, and screened to prepare a graphite composite material powder in which the conductive phase particles and the resin are evenly mixed, which is an important factor in the material finally obtaining a dense and uniform structure. At the same time, in some embodiments of the present invention, by adding a condensation recovery device during the drying process, the recycling of the organic solvent can be achieved.

[0021] (2) The present invention introduces reinforcing phases with long fiber structures such as carbon black and carbon nanotubes. On the one hand, due to their large surface area, at the same resin content, the bonding area between the conductive particles and the resin per unit volume can be increased, thereby obtaining better mechanical properties. On the other hand, the carbon nanotubes also play a bridging role between the graphite particles, which can effectively increase the conductive path and avoid excessive resistivity caused by the graphite particles being completely wrapped by the resin.

[0022] (3) The present invention uses the process of cold pressing + tunnel furnace curing to allow the forming and curing of graphite-based composite material powder to be carried out in steps, thereby improving the production efficiency of the press compared to hot pressing and achieving efficient production of the product. After the mold is closed, the curing is completed in the tunnel furnace to maintain the dimensional accuracy of the bipolar plate product. After the curing is completed, the mold is actively cooled in the closed mold state to avoid the surface warping problem of the bipolar plate product inside the mold due to uneven cooling conditions and lack of limiting measures.

[0023] (4) The present invention rationally designs each step so that multiple arrangements from powder filling to mold cleaning can be produced continuously, making the method more suitable for mass production of large-sized, high-performance graphite-based composite bipolar plates.

[0024] The above contents of the present invention will be more clearly understood in the following description of multiple embodiments with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings of the present invention will be provided below. These drawings are only for embodying the present invention in a more intuitive form. They are exemplary and are not intended to limit the scope of the present invention.

[0026] Figure 1 A schematic diagram showing a flow chart of a method embodiment of the present invention is shown;

[0027] Figure 2 A schematic structural diagram of a tunnel furnace and a cooling device is shown that can implement steps S106-S107 of the method for producing a graphite-based composite material bipolar plate of the present invention. DETAILED DESCRIPTION

[0028] To make the present invention easier to understand, the present invention is further described below in conjunction with specific examples. The experimental methods described in the present invention are conventional methods unless otherwise specified; the materials described are commercially available unless otherwise specified. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by technicians in the technical field of the present invention. If there is any inconsistency, the meaning described in this specification or the meaning derived from the contents recorded in this specification shall prevail. In addition, the terms used herein are only for the purpose of describing the embodiments of the present invention and are not intended to limit the present invention.

[0029] In order to accurately describe the technical contents of the present invention and to accurately understand the present invention, the following explanations or definitions are given for the words and terms used in this specification before describing the specific implementation methods.

[0030] The words "one embodiment" or "an embodiment" as used in this specification means that a particular feature, step, or characteristic described in conjunction with the embodiment is included in at least one embodiment of the present invention. Therefore, the phrases "in one embodiment" or "in an embodiment" appearing throughout this specification do not necessarily all refer to the same embodiment, but may refer to the same embodiment. Furthermore, in one or more embodiments, the particular features, steps, or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure.

[0031] Graphite-based composite bipolar plates are composite materials with double-sided flow channel structures made of graphite as the matrix and organic resin as the binder phase, and are used in energy storage technologies such as flow batteries. Compared with pure graphite bipolar plates and metal bipolar plates, composite bipolar plates have the advantages of high density, good corrosion resistance, high preparation efficiency and easy industrialization. However, the content of organic resin binder phase in such bipolar plates is inversely proportional to the conductivity and proportional to the mechanical properties. Therefore, obtaining composite bipolar plates with high mechanical properties and high conductivity at the same time is an important problem to be solved in this field. At present, there are two main methods for preparing such bipolar plates using flexible graphite sheets as raw materials. One is to form a low-density flexible graphite sheet in one step to form a flow field plate, then impregnate it with resin, and finally cure it. The other is to first mix the flexible graphite powder with an organic solvent, perform compression molding after semi-curing treatment, then perform resin impregnation, and finally cure it. Both of the above methods require resin impregnation after molding, and cleaning and other processes after impregnation, which seriously reduces production efficiency.

[0032] In view of the above-mentioned problems of the prior art, the present invention proposes a method for producing graphite-based composite material bipolar plates, wherein a new graphite-based resin composite mixed powder dedicated to bipolar plates is designed, and a continuous production method of cold pressing + tunnel furnace curing is adopted to realize the efficient production of high-performance graphite-based composite material bipolar plates. The production method of the present invention divides the forming of bipolar plates into two steps: cold pressing and tunnel furnace thermosetting. First, the mixed special graphite-based composite material powder is loaded into the mold for cold pressing, and then the upper and lower molds are mechanically locked, and the whole enters the tunnel furnace for heating and curing. After the thermosetting reaction is completed, the mold is opened, and the bipolar plate sample is taken out. After the mold is cleaned on the surface, the powder is loaded again and circulated to the press for pressing to form continuous production. Compared with the current forming method of resin impregnation and post-treatment after pressing, the production method of the present invention is more suitable for the mass production of large-size high-performance graphite-based composite material bipolar plates.

[0033] See also Figure 1 The production method of the graphite-based composite material bipolar plate of the present invention comprises the following steps:

[0034] S101: preparing a graphite-based powder, wherein the graphite-based powder comprises 72-82 wt % of graphite powder, 15-23 wt % of thermosetting resin powder and 3-5 wt % of reinforcing phase powder;

[0035] S102: preparing a mixed slurry, wherein a thermosetting resin powder and an organic solvent are stirred to obtain a thermosetting resin solution, a reinforcing phase powder is mixed into the thermosetting resin solution and stirred evenly, and then a graphite powder is added and stirred evenly to obtain a mixed slurry;

[0036] S103: preparing graphite composite material powder, wherein the mixed slurry is dried, crushed and sieved to obtain graphite composite material powder;

[0037] S104: powder filling, wherein a release agent is applied on the surface of the mold, the graphite composite material powder is filled into the mold, and the surface of the graphite composite material powder is scraped flat;

[0038] S105: cold pressing, wherein after the mold is closed, the mold is pressed down in a stepwise manner until a final closing height is reached while the inner cavity of the mold is evacuated;

[0039] S106: locking mold heating, wherein the mold is mechanically locked and transferred to a tunnel heating furnace for overall heating and heat preservation to solidify the bipolar plate in the mold;

[0040] S107: Cooling and demoulding, wherein the mold is transferred to a cooling chamber for active cooling, and the mechanical lock is released after the mold is cooled to below the solidification temperature, and the bipolar plate is taken out; and

[0041] S108: cleaning the mold, wherein the inner surface of the mold is purged, so as to return to S104 to continue to load new graphite composite material powder for subsequent continuous production of bipolar plates.

[0042] The production method of the graphite-based composite material bipolar plate of the present invention includes the following features: (1) using graphite powder as the matrix powder; (2) using thermosetting resin as the organic adhesive; (3) using carbon black, carbon nanotubes, etc. as the reinforcing phase to improve the electrical conductivity; (4) using carbon fiber as the reinforcing phase to improve the mechanical properties; (5) drying, crushing and powdering, mixing the raw materials uniformly to prepare a mixed material, drying, crushing and screening, and obtaining the final graphite-based composite material powder; (6) powder cold pressing forming stage, filling the mixed powder into a mold, and The mold is vacuumed to remove gas, and then cold pressing is performed; (7) During the mold closing and heat curing stage, the forming mold is mechanically locked and transferred to a tunnel furnace as a whole. By setting the temperature in the furnace, the mold is heated to the curing temperature as a whole and kept warm for a certain period of time to ensure that the bipolar plate in the mold completes the curing reaction; (8) During the demolding and cleaning stage, the mold is transported to a cooling room and actively cooled. The mold temperature is reduced to below the curing temperature, and the mold is opened to take out the bipolar plate product; after the mold surface is quickly cleaned, the powder is filled again for cold pressing to form a continuous production of bipolar plates.

[0043] Figure 2 The tunnel furnace and cooling device of steps S106-S107 of the method for producing the graphite-based composite material bipolar plate of the present invention are exemplarily shown. As shown in the figure, the locked mold is conveyed to the heating transition zone of the tunnel furnace via a conveyor. In the heating transition zone, the mold is heated from room temperature to the temperature required for the curing of the bipolar plate, for example, 180-220°C. Then, the heated mold is conveyed to the heating uniform temperature zone of the tunnel furnace via a conveyor. In the heating uniform temperature zone, the mold is kept warm for a period of time, for example, 20-30 minutes, so that the bipolar plate material in the mold is fully cured. In some embodiments, the heating uniform temperature zone is also provided with a multi-point infrared thermometer for monitoring the outer surface temperature of the mold to ensure that the outer surface temperature of the mold is maintained at 150-180°C, thereby ensuring that the temperature inside the mold is sufficient to fully cure the bipolar plate material. The insulated mold is conveyed to the cooling device via a conveyor. The cooling device is provided with a nozzle capable of spraying water mist to cool the outer surface temperature of the mold to below 80°C, and then release the mechanical lock of the mold.

[0044] In order to facilitate technicians to understand and implement the method for producing the graphite-based composite material bipolar plate of the present invention, several exemplary embodiments are shown below.

[0045] <Example 1>

[0046] This embodiment is a graphite-based composite bipolar plate prepared by cold pressing + hot mold curing, with dimensions of 200×300×5mm (length×width×thickness). The inner cavity dimensions of the bipolar plate forming mold are 200×300×20mm (length × width × height), the mold wall thickness is 100mm, and a one-way pressing process is used. The bipolar plate is formed and manufactured in a self-developed powder forming continuous production line. The specific steps are as follows:

[0047] Step 1: using graphite-based composite material powder: 75wt.% natural flake graphite powder, 20wt.% thermosetting resin powder, 1wt.% carbon black, 2wt.% carbon nanotubes, and 2wt.% carbon fiber are prepared;

[0048] Step 2: Thermosetting resin powder and anhydrous ethanol are stirred in a mass ratio of 1:2 to prepare a thermosetting resin solution; firstly, reinforcing phase powders such as carbon black, carbon nanotubes, and carbon fibers are mixed into the resin solution and stirred thoroughly, and then graphite powder is added and stirred evenly to obtain a uniform mixed slurry; during the stirring process, the solution is heated at a heating temperature of 65°C;

[0049] Step 3: drying and crushing the stirred mixed slurry, and sieving to obtain the final graphite composite material powder;

[0050] Step 4: Apply a release agent to the mold surface, then fill it with powder and scrape the powder surface flat;

[0051] Step 5: The mold is closed, and a step-by-step pressing method is adopted, that is, after each downward movement of 2.5 mm, stop for 30 seconds, and then continue to press down until the final mold closing height is reached; at the same time, the inside of the mold cavity is vacuumed to ensure that the residual gas in the powder gap is completely discharged; the mold is closed and the pressure is maintained for 30 seconds;

[0052] Step 6: Lock the upper and lower molds mechanically and transfer the entire mold to a tunnel heating furnace. Set the temperature in the furnace to uniformly heat the entire mold to 150°C and keep it warm for 20 minutes to ensure that the bipolar plate in the mold completes the curing reaction;

[0053] Step 7: After the insulation in the tunnel furnace is completed, the mold is transferred to the cooling room for water cooling. When the mold is cooled to below 80°C, the mold lock is released, and the mold is opened to take out the bipolar plate product;

[0054] Step 8: The mold surface is quickly purged to ensure that the mold cavity is smooth. After applying the release agent, the powder is filled again for cold pressing to form the continuous production of bipolar plates.

[0055] <Example 2>

[0056] This embodiment is a graphite-based composite bipolar plate prepared by cold pressing + hot mold curing, with a size of 200×300×5mm (length×width×thickness). The inner cavity size of the bipolar plate forming mold is 200×300×20mm (length×width×height), the mold wall thickness is 100mm, and a unidirectional pressing process is used. The bipolar plate forming is carried out in a self-developed powder forming continuous production line. The specific steps are as follows:

[0057] Step 1: using graphite-based composite material powder: 80wt.% natural flake graphite powder, 15wt.% thermosetting resin powder, 1.5wt.% carbon black, 1.5wt.% carbon nanotubes, and 2wt.% carbon fiber are prepared;

[0058] Step 2: Stirring thermosetting resin powder and ethyl acetate solution in a mass ratio of 1:2 to prepare a thermosetting resin solution; firstly mix reinforcing phase powders such as carbon black, carbon nanotubes, and carbon fibers into the resin solution and stir thoroughly, then add graphite powder and stir evenly to obtain a uniform mixed slurry; during the stirring process, heat the solution at a heating temperature of 70°C;

[0059] Step 3: drying and crushing the stirred mixed slurry, and sieving to obtain the final graphite composite material powder;

[0060] Step 4: Apply a release agent to the mold surface, then fill it with powder and scrape the powder surface flat;

[0061] Step 5: The mold is closed, and a step-by-step pressing method is adopted, that is, after each downward movement of 2.5 mm, stop for 30 seconds, and then continue to press down until the final mold closing height is reached; at the same time, the inside of the mold cavity is vacuumed to ensure that the residual gas in the powder gap is completely discharged; the mold is closed and the pressure is maintained for 30 seconds;

[0062] Step 6: Lock the upper and lower molds mechanically and transfer the entire mold to a tunnel heating furnace. Set the temperature in the furnace to uniformly heat the entire mold to 150°C and keep it warm for 20 minutes to ensure that the bipolar plate in the mold completes the curing reaction;

[0063] Step 7: After the insulation in the tunnel furnace is completed, the mold is transferred to the cooling room for water cooling. When the mold is cooled to below 80°C, the mold lock is released, and the mold is opened to take out the bipolar plate product;

[0064] Step 8: The mold surface is quickly purged to ensure that the mold cavity is smooth. After applying the release agent, the powder is filled again for cold pressing to form the continuous production of bipolar plates.

[0065] <Example 3>

[0066] This embodiment is a graphite-based composite bipolar plate prepared by cold pressing + hot mold curing, with a size of 300×400×5mm (length×width×thickness). The inner cavity size of the bipolar plate forming mold is 300×400×20mm (length×width×height), the mold wall thickness is 100mm, and a unidirectional pressing process is used. The bipolar plate forming is carried out in a self-developed powder forming continuous production line. The specific steps are as follows:

[0067] Step 1: using graphite-based composite material powder: 78wt.% natural flake graphite powder, 18wt.% thermosetting resin powder, 1wt.% carbon black, 2wt.% carbon nanotubes, and 1wt.% carbon fiber are prepared;

[0068] Step 2: The thermosetting resin powder and the methanol solvent are stirred at a mass ratio of 1:2 to prepare a thermosetting resin solution; firstly, the reinforcing phase powders such as carbon black, carbon nanotubes, and carbon fibers are mixed into the resin solution and stirred thoroughly, and then the graphite powder is added and stirred evenly to obtain a uniform mixed slurry; during the stirring process, the solution is heated at a heating temperature of 80°C;

[0069] Step 3: drying and crushing the stirred mixed slurry, and sieving to obtain the final graphite composite material powder;

[0070] Step 4: Apply a release agent to the mold surface, then fill it with powder and scrape the powder surface flat;

[0071] Step 5: The mold is closed and pressed down in a step-by-step manner, that is, after each 2 mm downward movement, stop for 30 seconds and continue pressing down until the final mold closing height is reached; at the same time, the mold cavity is vacuumed to ensure that the residual gas in the powder gap is completely discharged; the mold is closed and the pressure is maintained for 30 seconds;

[0072] Step 6: Lock the upper and lower molds mechanically and transfer the entire mold to a tunnel heating furnace. Set the temperature in the furnace to uniformly heat the entire mold to 160°C and keep it warm for 15 minutes to ensure that the bipolar plate in the mold completes the curing reaction;

[0073] Step 7: After the insulation in the tunnel furnace is completed, the mold is transferred to the cooling room for water cooling. When the mold is cooled to below 80°C, the mold lock is released, and the mold is opened to take out the bipolar plate product;

[0074] Step 8: The mold surface is quickly purged to ensure that the mold cavity is smooth. After applying the release agent, the powder is filled again for cold pressing to form the continuous production of bipolar plates.

[0075] <Comparative Example 1>

[0076] This comparative example is to prepare a graphite-based composite bipolar plate by vacuum hot pressing, with a size of 300×400×5mm (length×width×thickness). The inner cavity size of the bipolar plate forming mold is 300×400×20mm (length×width×height), the mold wall thickness is 100mm, and a unidirectional pressing process is used. The specific steps are as follows:

[0077] Step 1: using graphite-based composite material powder: 78wt.% natural flake graphite powder, 18wt.% thermosetting resin powder, 1wt.% carbon black, 2wt.% carbon nanotubes, and 1wt.% carbon fiber are prepared;

[0078] Step 2: Mechanically ball mill the thermosetting resin powder, graphite powder, and reinforcing phase powders such as carbon black, carbon nanotubes, and carbon fibers, with a ball-to-material ratio of 5:1. After 2 hours of ball milling, the initial mixed powder is obtained;

[0079] Step 3: Apply a release agent to the mold surface, then fill the powder and scrape the powder surface flat;

[0080] Step 4: The mold is closed and pressurized and heated simultaneously. A step-by-step pressing method is used, that is, after each 2mm downward movement, stop for 30 seconds and continue pressing until the final mold closing height is reached; the heating rate is 18℃ / min, and the temperature is raised to 155℃. At the same time, the mold cavity is vacuumed to ensure that the residual gas in the powder gap is completely discharged; the mold is closed and the temperature is kept at a constant pressure for 20 minutes;

[0081] Step 5: Stop heating, wait for the mold to cool down to below 80°C, open the mold and take out the bipolar plate product;

[0082] Step 6: Quickly blow the mold surface to ensure that the mold cavity is smooth. After applying the release agent, fill the powder again for hot pressing.

[0083] <Comparative Example 2>

[0084] This comparative example is to prepare a graphite-based composite bipolar plate by vacuum hot pressing, with a size of 300×400×5mm (length×width×thickness). The inner cavity size of the bipolar plate forming mold is 300×400×20mm (length×width×height), the mold wall thickness is 100mm, and a unidirectional pressing process is used. The specific steps are as follows:

[0085] Step 1: using graphite-based composite material powder: 80wt.% natural flake graphite powder, 15wt.% thermosetting resin powder, 1wt.% carbon black, 2wt.% carbon nanotubes, and 2wt.% carbon fiber are prepared;

[0086] Step 2: Mechanically ball mill the thermosetting resin powder, graphite powder, and reinforcing phase powders such as carbon black, carbon nanotubes, and carbon fibers, with a ball-to-material ratio of 5:1. After 2 hours of ball milling, the initial mixed powder is obtained;

[0087] Step 3: Apply a release agent to the mold surface, then fill the powder and scrape the powder surface flat;

[0088] Step 4: The mold is closed and pressurized and heated simultaneously. The mold is pressed downward at a uniform speed of 4 mm / min until it is pressed to the lower limit of the mold. The heating rate is 18°C / min and the temperature is raised to 155°C. At the same time, the inside of the mold cavity is evacuated to ensure that the residual gas in the powder gap is completely discharged. Keep the temperature and pressure for 20 minutes.

[0089] Step 5: Stop heating, wait for the mold to cool down to below 80°C, open the mold and take out the bipolar plate product;

[0090] Step 6: Quickly blow the mold surface to ensure that the mold cavity is smooth. After applying the release agent, fill the powder again for hot pressing.

[0091] Performance Testing

[0092] The electrical conductivity, contact resistance, bending strength, compressive strength, density and hardness of the composite material bipolar plates prepared in Examples 1-3 and Comparative Examples 1-2 were tested respectively. The test results are shown in Table 1. Table 1 Comparison of performance of composite bipolar plates in examples

[0093] It can be seen from Table 1 that the graphite-based composite material bipolar plate prepared by the production method of the present invention has better conductivity and higher mechanical strength than the graphite-based composite material bipolar plate prepared by the existing vacuum hot pressing method.

[0094] The above are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein, and that various obvious changes, readjustments and substitutions can be made by those skilled in the art without departing from the scope of protection of the present invention. Therefore, although the present invention is described in more detail through the above embodiments, the present invention is not limited to the above embodiments, and may also include more other equivalent embodiments without departing from the concept of the present invention, all of which belong to the scope of protection of the present invention.

Claims

1. A method for producing a graphite-based composite material bipolar plate, characterized in that: The following steps are involved: Prepare graphite-based powder, wherein the graphite-based powder includes 72-82 wt% of graphite powder, 15-23 wt% of thermosetting resin powder and 3-5% of reinforcing phase powder; Preparing a mixed slurry, wherein the thermosetting resin powder is stirred with an organic solvent to obtain a thermosetting resin solution, the reinforcing phase powder is mixed into the thermosetting resin solution and stirred evenly, and then the graphite powder is added and stirred evenly to obtain a mixed slurry; preparing graphite composite material powder, wherein the mixed slurry is dried, crushed and sieved to obtain graphite composite material powder; Powder filling, wherein a release agent is applied on the surface of the mold, the graphite composite material powder is filled into the mold, and the surface of the graphite composite material powder is scraped flat; Cold pressing, wherein after the mold is closed, the mold is pressed down in steps until the final closing height while the inner cavity of the mold is evacuated; Locking mold heating, wherein the mold is mechanically locked and then transferred to a tunnel heating furnace for overall heating and heat preservation to solidify the bipolar plate in the mold; Cooling and demoulding, wherein the mold is transferred to a cooling chamber for active cooling, and the mechanical lock is released after the mold is cooled to below the solidification temperature, and the bipolar plate is taken out; as well as Cleaning the mold, wherein the inner surface of the mold is purged for subsequent continuous production of bipolar plates.

2. The production method according to claim 1, characterized in that The size range of the graphite powder is 50-300 μm, and the mass fraction of the graphite powder is 77-80 wt %.

3. The production method according to claim 1, characterized in that The thermosetting resin powder is phenolic resin powder, the powder size of the phenolic resin powder is in the range of 10 to 50 μm, the mass fraction of the phenolic resin powder is 18 to 21%, and the reinforcing phase powder is selected from one or more of carbon black, carbon nanotubes, and carbon fibers.

4. The production method according to claim 1, characterized in that The reinforcing phase powder includes carbon black, carbon nanotubes and carbon fibers. The size range of the carbon black powder is 10 to 50 nm, and the mass fraction of the carbon black powder is 0.7 to 2.2%; the size range of the carbon nanotubes is 10 to 50 nm, and the mass fraction of the carbon nanotubes is 1.2 to 2.1%; the diameter size range of the carbon fibers is 3 to 15 μm, and the mass fraction of the carbon fibers is 0.7 to 3.1%.

5. The production method according to claim 1, characterized in that: The organic solvent is a low boiling point solvent, and the mass ratio of the organic solvent to the thermosetting resin when stirred is 1:2-5.

6. The production method according to claim 1, characterized in that: The step-by-step pressing includes a pressing stage and a holding stage which are performed alternately. The holding stage lasts for 10 to 30 seconds, and the pressing stage is repeated 5 to 10 times.

7. The production method according to claim 1, characterized in that: The temperature of the tunnel heating furnace is 180-220°C.

8. The production method according to claim 1, characterized in that: In the film locking and heating step, the outer surface temperature of the mold is further monitored by means of a multi-point infrared thermometer to ensure that the outer surface temperature of the mold is maintained at 150-180°C, and to ensure that the mold stays in the uniform temperature zone of the tunnel heating furnace for 20-30 minutes.

9. The production method according to claim 1, characterized in that: In the cooling and demoulding step, the active cooling is spraying water mist, and the outer surface temperature of the mold is cooled to below 80° C. to release the mechanical lock.

10. A graphite-based composite material bipolar plate, characterized in that: It is manufactured by the production method according to any one of claims 1 to 8.

Images