Preparation method of flexible composite bipolar plate and fuel cell

Abstract

The application discloses a preparation method of a flexible composite bipolar plate and a fuel cell, and relates to the technical field of fuel cells.The preparation method comprises the following steps: S1, oxidizing, intercalating, cleaning and drying treatment of flaky graphite; S2, uniformly mixing an organic binder with the dried flaky graphite and heating in an inert gas atmosphere, so as to form a high-strength expanded graphite polymer; S3, pressing and cutting the expanded graphite polymer to form a flexible graphite original plate; S4, pressing the flexible graphite original plate into a semi-finished product bipolar plate with a runner feature; and S5, performing impregnation, cleaning, solidification and drying treatment on the semi-finished product bipolar plate to obtain a finished product bipolar plate.The three-dimensional amorphous carbon film formed by synchronous carbonization of the flaky graphite and the organic binder when the flaky graphite is expanded by heating can interact with each other, so that the high-strength expanded graphite polymer is formed, and the mechanical strength of the finished product bipolar plate is improved, so that the finished product bipolar plate can meet the application requirements of a fuel cell high-power stack.

Description

Technical Field

[0001] This invention relates to the field of fuel cell technology, and in particular to a method for preparing a flexible composite bipolar plate and a fuel cell. Background Technology

[0002] A fuel cell is a power generation device that produces electricity through an electrochemical reaction between hydrogen and oxygen (from the air). Due to its advantages such as high conversion efficiency, zero pollution, and strong environmental adaptability, proton exchange membrane fuel cells are considered a promising new energy technology.

[0003] Bipolar plates are the core components of fuel cell stacks, serving to provide mechanical support, separate oxidant and reductant, and transfer heat and electricity. This necessitates that they possess excellent mechanical properties, low permeability, high electrical conductivity, high corrosion resistance, and high thermal conductivity. As the power requirements of fuel cell stacks continue to increase, the active area of ​​bipolar plates is growing, placing higher demands on the mechanical strength of large-area bipolar plates.

[0004] In the existing technology, the preparation method of flexible composite bipolar plates usually involves first oxidizing and intercalating flake graphite, expanding it into expanded graphite at high temperature, and then rolling it into a flexible graphite plate. The flexible graphite plate is then molded and impregnated with polymer resin to form a flexible composite bipolar plate.

[0005] However, the increasing power and power density requirements of fuel cell stacks are driving bipolar plates to become larger and thinner. Currently, flexible composite bipolar plates are difficult to meet the needs of high-power fuel cell stack applications. Therefore, it is urgent to develop new technologies to improve the mechanical strength of flexible composite bipolar plates. Summary of the Invention

[0006] This invention provides a method for preparing a flexible composite bipolar plate and a fuel cell. Its main purpose is to solve the problem that the existing flexible composite bipolar plates have insufficient mechanical strength and cannot meet the application requirements of high-power fuel cell stacks.

[0007] The present invention adopts the following technical solution:

[0008] A method for preparing a flexible composite bipolar plate includes the following steps:

[0009] S1. Oxidize, intercalate, clean and dry the flake graphite;

[0010] S2. The organic binder is uniformly mixed with the dried flake graphite and heated in an inert gas atmosphere to carbonize the organic binder and form a viscous three-dimensional amorphous carbon film. At the same time, the flake graphite expands and interacts with the three-dimensional amorphous carbon film to form a high-strength expanded graphite polymer.

[0011] S3. Expanded graphite polymer is pressed and cut to form flexible graphite plates with different surface densities;

[0012] S4. The flexible graphite plate is placed in a vacuum mold and pressed into a semi-finished electrode plate with flow channel characteristics.

[0013] S5. The semi-finished electrode plate is impregnated, cleaned, cured and dried to obtain the finished electrode plate.

[0014] Furthermore, in step S2, the organic binder is an epoxy resin solution or an acrylic resin solution.

[0015] Furthermore, the solid content of the organic binder is 5%-20%, and the organic binder accounts for 10%-40% of the mass fraction of the flake graphite.

[0016] Furthermore, in step S2, the heating temperature is 300℃-800℃.

[0017] Furthermore, in step S5, the semi-finished electrode plate is first initially cured using a hot air curing method, and then the semi-finished electrode plate is cured a second time using a water bath curing method.

[0018] Furthermore, the curing temperature of the hot air curing method is 110℃-150℃, and the curing time is not less than 20 minutes.

[0019] Furthermore, the curing temperature of the water bath curing method is 85℃-95℃, and the curing time is not less than 20 minutes.

[0020] Furthermore, in step S5, the resin solution used in the impregnation process is a polyurethane system or an acrylic system.

[0021] Furthermore, in step S3, the expanded graphite polymer needs to be cleaned and dried before pressing and cutting.

[0022] A fuel cell includes several finished electrode plates obtained by the above-described preparation method.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] 1. This invention adds an organic binder so that the three-dimensional amorphous carbon film formed by the synchronous carbonization of flake graphite and organic binder when the graphite expands due to heat interacts with each other, thereby forming a high-strength expanded graphite polymer. This improves the mechanical strength of the finished electrode plate, enabling it to meet the application requirements of high-power fuel cell stacks.

[0025] 2. In order to ensure that the organic binder carbonizes to form a three-dimensional amorphous carbon film and to ensure that the expanded graphite can further react with the three-dimensional amorphous carbon film to form a high-strength expanded graphite polymer, this invention clearly defines the details such as the type, content and heating temperature of the organic binder, thereby making the preparation method more instructive and more versatile.

[0026] 3. In order to solve the problem of insufficient cross-linking and curing of the deep resin inside the electrode plate due to the low temperature in the existing single water bath heating curing method, resulting in low mechanical strength, this invention adopts a combined curing method of hot air curing and water bath curing to fully cure the resin that has penetrated into the electrode plate. This effectively improves the cross-linking and curing effect of the deep resin inside the electrode plate and significantly improves the mechanical strength of the finished electrode plate. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the preparation method of the present invention. Detailed Implementation

[0028] Specific embodiments of the present invention will now be described with reference to the accompanying drawings. Many details are described below to provide a comprehensive understanding of the invention; however, those skilled in the art will be able to implement the invention without these details.

[0029] Reference Figure 1 This invention discloses a method for preparing a flexible composite bipolar plate, comprising the following steps:

[0030] S1. The flake graphite is subjected to oxidation, intercalation, cleaning, and drying. Preferably, potassium permanganate solution can be used as the oxidant, and a mixed solution of concentrated sulfuric acid and concentrated nitric acid can be used as the intercalating agent.

[0031] S2. The organic binder is uniformly mixed with the dried flake graphite and heated in an inert gas atmosphere, causing the organic binder to carbonize and form a viscous three-dimensional amorphous carbon film. Simultaneously, the flake graphite expands and interacts with the three-dimensional amorphous carbon film to form a high-strength expanded graphite polymer. Preferably, the organic binder is a liquid epoxy resin solution or an acrylic resin solution.

[0032] Specifically, when heated in an inert gas atmosphere, the organic binder carbonizes, releasing hydrogen and oxygen to form a viscous three-dimensional amorphous carbon film. Meanwhile, the flake graphite expands, forming expanded graphite. Due to its small size and loose structure, the expanded graphite particles adhere to each other under the adhesive effect of the three-dimensional amorphous carbon film, effectively enhancing its mechanical strength. Furthermore, since the flake graphite undergoes intercalation in step S1, interlayer or surface sites are activated to form graphite intercalation compounds. Upon heating, these compounds rapidly decompose, generating a large amount of gas that causes the flake graphite to expand axially into expanded graphite. Simultaneously, carbon-carbon bonds may form at the contact points between the heated, carbonized three-dimensional amorphous carbon film and the expanded graphite. The carbon-carbon bonds formed in this step include carbon-carbon single bonds and carbon-carbon double bonds. The bond energy of a carbon-carbon single bond is approximately 347 kJ / mol, and the bond energy of a carbon-carbon double bond is approximately 611 kJ / mol. Therefore, the mechanical strength of the expanded graphite polymer is further improved, making it less susceptible to damage in the high-temperature, acidic environment inside a fuel cell. It can be seen that by adding an organic binder, the three-dimensional amorphous carbon film formed by the simultaneous carbonization of flake graphite and the organic binder during expansion can further react, thereby forming a high-strength expanded graphite polymer.

[0033] Theoretically, the higher the content of organic binder, the higher the mechanical strength of the expanded graphite polymer. However, after testing, it was found that if the content of organic binder is too high, the resin in subsequent steps will not be able to penetrate into the expanded graphite polymer under pressure. Therefore, this embodiment limits the solid content of organic binder to 5%-20%, and the organic binder accounts for 10%-40% of the mass fraction of flake graphite. That is, when ensuring that the solid content of organic binder is 5%-20%, the weight ratio of organic binder to flake graphite is 0.1-0.4:1.

[0034] To ensure the organic binder carbonizes to form a three-dimensional amorphous carbon film and that the expanded graphite further reacts with this film to form a high-strength expanded graphite polymer, this step requires the correct selection of a suitable type of organic binder. It is crucial that the mixed organic binder and flake graphite are heated in an inert gas atmosphere, and that the heating temperature is strictly controlled. The emphasis on the need for a three-dimensional amorphous carbon film after organic binder carbonization stems from its stronger adhesion, enabling it to form an adhesive expanded graphite polymer with loose expanded graphite particles. Therefore, after repeated testing, this invention has selected epoxy resin solution or acrylic resin solution as the preferred organic binder. The heating temperature in this step depends on the material properties of the organic binder; based on either epoxy resin or acrylic resin, this embodiment explicitly limits the heating temperature to 300℃-800℃.

[0035] To more clearly illustrate the important role of heating temperature in the carbonization of organic binders to form a three-dimensional amorphous carbon film, the following examples are provided in detail:

[0036] When the organic binder is an acrylic resin solution, the characteristic structure of acrylic resin is a carbon-carbon single bond main chain and ester side chains. When heated in an inert gas atmosphere, by strictly controlling the heating temperature, the acrylic resin can undergo carbonization, causing hydrogen and oxygen to be released from the molecules while the main chain structure remains essentially unchanged, thus forming a viscous three-dimensional amorphous carbon film. The interlayer or surface sites of the acidified flake graphite are activated to form graphite intercalation compounds. Upon heating, these graphite intercalation compounds rapidly decompose, forming a large number of unsaturated carbon atoms, which further combine with the unsaturated side chains in the three-dimensional amorphous carbon film to form carbon-carbon bonds. Therefore, in this step, the heating temperature cannot be too low, otherwise the hydrogen and oxygen in the acrylic resin cannot be released; however, the heating temperature also cannot be too high, otherwise the main chain of the acrylic resin will break, preventing the formation of the three-dimensional amorphous carbon film.

[0037] S3. The expanded graphite polymer is pressed and cut to form flexible graphite plates with different areal densities. Before pressing and cutting, the expanded graphite polymer obtained in step S3 must be cleaned and dried to remove impurities such as sulfur and nitrogen from the surface of the expanded graphite polymer.

[0038] S4. The flexible graphite substrate is placed in a vacuum mold and pressed into a semi-finished electrode plate with flow channel characteristics. Preferably, this step uses a hydraulic press to press the flexible graphite substrate.

[0039] S5. The semi-finished electrode plate is impregnated, cleaned, cured and dried to obtain the finished electrode plate.

[0040] Specifically, the impregnation process involves mixing and pressurizing a resin solution to impregnate the semi-finished electrode plate. The resin solution includes thermosetting resin and additives such as curing agents, wherein the thermosetting resin is a polyurethane system or an acrylic system.

[0041] Unlike existing technologies that use only a single water bath heating curing method, this embodiment uses a combination of hot air curing and water bath curing. First, hot air curing is used to pre-cur the semi-finished electrode plate, and then water bath curing is used to perform secondary curing. This solves the problem of insufficient cross-linking and curing of the deep resin inside the electrode plate due to the low temperature in the single water bath heating curing method used in the prior art, resulting in low mechanical strength.

[0042] It is important to emphasize that the order of the two curing methods provided in this embodiment cannot be changed. This is because the curing temperature of the water bath curing method will inevitably not exceed 100℃, while the curing temperature of the hot air curing method can exceed 100℃. If the water bath curing method is used alone, the surface groups of the resin that have penetrated into the electrode plate will cure, but the deep resin will not cure completely. At this point, the mechanical strength of the electrode plate is insufficient. If hot air curing is continued, the internal stress of the electrode plate will increase, leading to severe deformation, which is extremely detrimental to the bonding of the anode and cathode in subsequent processes. However, if the hot air curing method is used for preliminary curing first, it can ensure that the deep resin inside the electrode plate is completely cross-linked and cured, thus ensuring sufficient mechanical strength. Then, the water bath curing method is used for secondary curing, which can have an effect similar to metal "tempering," resulting in lower internal stress and a flatter electrode plate, thereby ensuring the quality of the finished electrode plate.

[0043] As a preferred embodiment, the curing temperature of the hot air curing method is limited to 110℃-150℃, and the curing time is not less than 20 minutes; the curing temperature of the water bath curing method is 85℃-95℃, and the curing time is not less than 20 minutes.

[0044] The present invention also discloses a fuel cell comprising a plurality of finished electrode plates obtained by the above-described preparation method.

[0045] The above are merely specific embodiments of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantial modifications made to the present invention using this concept shall be considered as infringing upon the protection scope of the present invention.

Claims

1. A method for preparing a flexible composite bipolar plate, characterized in that: Includes the following steps: S1. Oxidize, intercalate, clean and dry the flake graphite; S2. The organic binder is uniformly mixed with the dried flake graphite and heated in an inert gas atmosphere, causing the organic binder to carbonize and form a viscous three-dimensional amorphous carbon film. Simultaneously, the flake graphite expands and interacts with the three-dimensional amorphous carbon film to form a high-strength expanded graphite polymer. The organic binder is an epoxy resin solution or an acrylic resin solution, with a solid content of 5%-20% and a mass fraction of 10%-40% of the flake graphite. S3. Expanded graphite polymer is pressed and cut to form flexible graphite plates with different surface densities; S4. The flexible graphite plate is placed in a vacuum mold and pressed into a semi-finished electrode plate with flow channel characteristics. S5. The semi-finished electrode plate is impregnated, cleaned, cured and dried to obtain the finished electrode plate. During curing, the semi-finished electrode plate is first cured by hot air curing and then cured by water bath curing. The curing temperature of hot air curing is 110℃-150℃ and the curing time is not less than 20 minutes. The curing temperature of water bath curing is 85℃-95℃ and the curing time is not less than 20 minutes.

2. The method for preparing a flexible composite bipolar plate as described in claim 1, characterized in that: In step S2, the heating temperature is 300℃-800℃.

3. The method for preparing a flexible composite bipolar plate as described in claim 1, characterized in that: In step S5, the resin solution used in the impregnation process is a polyurethane system or an acrylic system.

4. The method for preparing a flexible composite bipolar plate as described in claim 1, characterized in that: In step S3, before pressing and cutting, the expanded graphite polymer needs to be cleaned, washed and dried to remove impurities.

5. A fuel cell, characterized in that, It includes several finished electrode plates obtained by the preparation method described in any one of claims 1-4.

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