Flexible graphite bipolar plate with adjustable contact angle and preparation method thereof

Abstract

The invention belongs to the field of fuel cells, and discloses a flexible graphite bipolar plate with an adjustable contact angle and a preparation method thereof.The preparation method comprises the steps that expandable graphite with different mesh numbers is mixed in proportion, and then expansion, mold pressing, infiltration, surface cleaning, water bath curing and drying are conducted to obtain the flexible graphite bipolar plate. By regulating and controlling the etching time and the types of etching gas, the surface of the bipolar plate is impacted by utilizing a large number of free electrons, ions, metastable state particles and other high-energy particles contained in plasmas to form new chemical bonds, and regulation and control of the number and types of functional groups in the polar plate are realized, so that the hydrophilic / hydrophobic characteristics of the surface of the polar plate are changed.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fuel cells, in particular to a flexible graphite bipolar plate with adjustable contact angle and a preparation method thereof. BACKGROUND

[0002] Fuel cell technology is a new hydrogen energy utilization device that directly converts the chemical energy of hydrogen and oxygen into electricity through chemical reactions in the stack, and has the characteristics of environmental friendliness, high energy conversion efficiency, fast start-up speed, etc., and is widely used in mobile power sources, automobiles, ships, etc.

[0003] As one of the core components of fuel cells, the bipolar plate is mainly responsible for distributing reaction gases, conducting electricity and heat, and supporting the membrane electrode in the fuel cell stack, and is the "skeleton" of the fuel cell. Currently reported bipolar plates mainly include metal bipolar plates, graphite plates, composite plates, and flexible graphite bipolar plates. Among them, flexible graphite bipolar plates are widely concerned due to their low cost, corrosion resistance, and high strength. Generally, flexible graphite bipolar plates are prepared by vacuum impregnation of resin, curing of flexible graphite die plates. Under the actual working conditions of hydrogen fuel cells, a large amount of water is generated on the cathode side in the high current density area, which needs to be quickly removed by the bipolar plate to ensure the water balance inside the cell. If it is not removed in time, it will cause the "flood" phenomenon of the stack, resulting in a sharp decline in the performance of the stack. Therefore, the wettability of liquid water on the ridge or flow channel wall of the bipolar plate has a significant impact on the dynamic behavior of liquid droplets in the flow channel.

[0004] Under the same working conditions, the more hydrophilic the flow channel ridge, the more conducive to the separation of liquid droplets from the GDL surface of the membrane electrode and the spreading in the flow channel. However, too much hydrophilicity makes it difficult for liquid droplets to be quickly removed in the flow channel, and the transmission resistance is too large. Therefore, in actual engineering applications, the appropriate hydrophilic / hydrophobic bipolar plate needs to be selected according to the membrane electrode, diffusion layer and actual working conditions of the cell. However, the hydrophilicity / hydrophobicity of the flexible graphite bipolar plate is difficult to control in actual industrial mass production, so the flexible graphite plate is difficult to achieve high performance in the stack. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a flexible graphite bipolar plate with adjustable contact angle and a preparation method thereof, aiming to solve the problem of imbalance of the surface contact angle of the flexible graphite bipolar plate, which leads to the decline of its electrochemical performance.

[0006] To solve the above technical problems, the present application provides a preparation method of a flexible graphite bipolar plate with adjustable contact angle, which comprises the following steps: S1, expandable graphite is subjected to high-temperature thermal expansion, and a flexible graphite expansion plate is obtained by rolling, and then the flexible graphite expansion plate is molded by a flow channel mold, vacuum-pressurized impregnation, surface cleaning, water bath curing and drying to obtain a flexible graphite bipolar plate; S2, place the flexible graphite bipolar plate in the plasma cavity, input etching gas, and etch the surface of the graphite bipolar plate by plasma, control the etching time and the type of etching gas, and obtain the flexible graphite bipolar plate with adjustable contact angle; The mass ratio of the 80-mesh powder to the 100-mesh powder in the expandable graphite is (80-90):(20-10), and the sum of the mass ratio of the two is 100.

[0007] In some embodiments, the impregnation solution in step S1 comprises at least one of an acrylate resin, a phenolic resin, an epoxy resin, and a vinyl ester resin.

[0008] In some embodiments, the expandable graphite in step S1 has an expansion temperature of 800-1000 DEG C and an expansion time of 20-40 s.

[0009] In some embodiments, the vacuum-pressurized impregnation in step S1 has a pressure of 1-2 MPa, an impregnation time of 60-180 min, a water bath curing temperature of 90-98 DEG C, and a water bath curing time of 20-40 min.

[0010] In some embodiments, the etching gas in step S2 is one of oxygen, hydrogen, and argon, and the etching time in step S2 is 5-20 min.

[0011] In addition, a flexible graphite bipolar plate is also disclosed, which is prepared by the above-mentioned method for preparing the flexible graphite bipolar plate with adjustable contact angle. The surface contact angle of the flexible graphite bipolar plate is 82.6-113.2 DEG.

[0012] The present application has the following advantages: The plasma contains a large number of free electrons, ions, and metastable particles, and other high-energy particles. These high-energy particles impact the surface of the bipolar plate, and can quickly form new chemical bonds. By using different etching gases and etching time, the number and type of functional groups in the bipolar plate can be controlled, thereby changing the hydrophilic / hydrophobic properties of the bipolar plate surface, obtaining the flexible graphite bipolar plate with adjustable contact angle. At the same time, different mesh numbers of graphite are mixed and matched, and the substrate can be more closely connected, and a tight conductive network is constructed, thereby effectively reducing the contact resistance. The method for controlling the hydrophilic / hydrophobic properties of the bipolar plate has the characteristics of simple operation, rapidness, and high efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 The figure is a flowchart of the method for preparing the flexible graphite bipolar plate with adjustable contact angle in an embodiment of the present application. Figure 2 The figure is a schematic diagram of the cavity structure of the plasma equipment. DETAILED DESCRIPTION

[0014] In the description of the present application, it should be noted that, in the examples, no specific conditions are specified, and the conventional conditions or the conditions recommended by the manufacturer are used. The reagents or instruments used are not specified by the manufacturer, and are conventional products that can be purchased on the market.

[0015] Please refer to Figure 1 The application provides a preparation method of a flexible graphite bipolar plate with adjustable contact angle, which comprises the following steps: S1, expandable graphite is heated and expanded to obtain a flexible graphite expansion plate, and then the flexible graphite expansion plate is molded by a flow channel mold, vacuum-pressurized impregnation, cleaning, water bath curing and drying to obtain a flexible graphite bipolar plate; The mass ratio of the 80-mesh powder to the 100-mesh powder in the expandable graphite is (80-90):(20-10), and the sum of the mass ratio of the two is 100.

[0016] The impregnating liquid in step S1 comprises at least one of an acrylate resin, a phenolic resin, an epoxy resin and a vinyl ester resin.

[0017] The expansion temperature of the expandable graphite in step S1 is 800-1000 DEG C, and the expansion time is 20-40 s.

[0018] The pressure of the vacuum-pressurized impregnation in step S1 is 1-2 MPa, the impregnation time is 60-180 min, the water bath curing temperature is 90-98 DEG C, and the water bath curing time is 20-40 min.

[0019] The 80-mesh and 100-mesh expandable graphite powders are graded in a mass ratio of (80-90):(20-10), the 80-mesh powder of the coarse particles constitutes a structural framework, ensuring that the bipolar plate has sufficient mechanical strength and pore smoothness, and the 100-mesh powder of the fine particles effectively fills the gaps between the coarse particles, which significantly improves the density uniformity and surface flatness of the green body during subsequent molding, providing a good foundation for the flow channel mold pressing molding. On the basis of this optimized powder grading, combined with the molding and vacuum-pressurized impregnation process, the impregnating liquid can be more uniformly and fully penetrated into the micro-pores of the graphite framework, and after water bath curing and drying, a flexible graphite bipolar plate with high density, low porosity and excellent electrical conductivity, mechanical strength and anti-medium penetration performance is finally obtained.

[0020] The expandable graphite is expanded into flexible graphite expanded plate with abundant porous structure by high temperature expansion at 800-1000℃ and short time treatment for 20-40s, which not only endows the substrate with good flexibility, but also provides sufficient physical space for subsequent impregnation; the curing of thermosetting resins such as acrylic resin, phenolic resin, epoxy resin and vinyl ester resin can significantly enhance the mechanical strength, gas barrier property and dimensional stability of the graphite plate, and finally obtain ideal bipolar plate material with excellent flexibility, high density and reliable electrical conductivity; The vacuum-pressurized impregnation is carried out at a pressure of 1-2MPa, which can not only remove the gas in the pores of the graphite expanded plate, but also push the impregnating liquid to fill the small pores by moderate positive pressure, and the sufficient impregnation time of 60-180min can ensure the deep and uniform penetration of the resin into the porous graphite substrate, which lays the foundation for forming a dense composite structure; then, the warm water bath curing condition of 90-98℃ provides a mild and controllable crosslinking reaction environment for the thermosetting resin, which can not only avoid the cracking or deformation of the plate caused by excessive curing and large shrinkage stress, but also effectively promote the completion of the resin curing within 20-40min, so as to firmly bond the loose graphite worms into a whole, and finally endow the bipolar plate with excellent mechanical strength, low gas permeability and stable electrical conductivity network.

[0021] S2, placing the flexible graphite bipolar plate in the cavity of the plasma equipment, introducing etching gas, and etching the surface of the graphite bipolar plate by plasma to obtain a flexible graphite bipolar plate with adjustable contact angle.

[0022] The etching gas in step S2 is one of oxygen, hydrogen and argon, and the etching time is 5-20min.

[0023] When the oxidizing gas oxygen is used for plasma etching, the high active oxygen species in the plasma will bombard and chemically oxidize the surface of the graphite, introducing oxygen-containing functional groups such as carboxyl and hydroxyl groups at the edge of the graphite lattice, which significantly enhances the hydrophilicity of the surface, thereby effectively reducing the contact angle of water; on the contrary, when the reducing gas argon or hydrogen is used, the hydrogen plasma can remove the existing oxygen-containing functional groups on the surface of the graphite by chemical reduction, and repair part of the graphite lattice structure, so that the surface is more pure and shows hydrophobic characteristics, thereby significantly increasing the contact angle of water, which is beneficial to prevent water flooding and improve the transmission of reaction gas. When argon is used as the etching gas, its main role is physical bombardment rather than chemical reaction. In the plasma, the accelerated argon ions will bombard the surface of the graphite bipolar plate like small cannonballs, which mainly removes the surface contamination layer, weak boundary layer and part of amorphous carbon through physical sputtering, thereby exposing the underlying more pure graphite crystal structure, increasing the surface energy of the bipolar plate and causing the contact angle of water to decrease moderately.

[0024] In summary, by precisely selecting the oxidizing or reducing gas and controlling the etching process within 5-20 min, the surface property from highly hydrophilic to highly hydrophobic can be targeted to meet the specific requirements of different application scenarios for the surface wettability of the bipolar plate.

[0025] In addition, the application also provides a flexible graphite bipolar plate with adjustable contact angle, which is prepared by the above-mentioned method.

[0026] The surface contact angle of the flexible graphite bipolar plate is 82.6-113.2°.

[0027] For example, the application provides the following specific embodiments to illustrate the specific preparation method: Example 1 S1, expand the expandable graphite at 870℃ for 30s, roll it, get the flexible graphite expansion plate, then mold it through the flow channel mold, vacuum-pressurized impregnate with acrylate resin, clean it, water bath curing, drying, get the flexible graphite bipolar plate, wherein the mass ratio of 80 mesh powder to 100 mesh powder in the expandable graphite is 85:15, the vacuum-pressurized impregnation pressure is 1.5MPa, the impregnation time is 120min, the water bath curing temperature is 95℃, and the water bath curing time is 30min; S2, put the graphite bipolar plate into the cavity of the plasma equipment, introduce oxygen, etch the surface of the graphite bipolar plate by plasma, the etching time is 5min, get the flexible graphite bipolar plate with adjustable contact angle, wherein the vacuum background pressure is 5x10 -3 Pa, the working pressure is 30Pa, the plasma power is 180W, and the etching gas flow is 100sscm.

[0028] Example 2 The same as example 1, the difference is that the etching time of step S2 is 13min.

[0029] Example 3 The same as example 1, the difference is that the etching time of step S2 is 20min.

[0030] Example 4 The same as example 1, the difference is that the etching gas of step S2 is hydrogen.

[0031] Example 5 The same as example 1, the difference is that the etching gas of step S2 is argon.

[0032] Example 6 The same as Example 1, except that the mass ratio of 80 mesh powder to 100 mesh powder in the expandable graphite of step S1 is 80:20.

[0033] Example 7 The same as Example 1, except that the mass ratio of 80 mesh powder to 100 mesh powder in the expandable graphite of step S1 is 90:10.

[0034] Comparative Example 1 S1, the expandable graphite was expanded at 870℃ for 30s, rolled, and a flexible graphite expansion plate was obtained, which was then molded by a flow channel mold, vacuum-pressurized impregnated with an acrylate resin, cleaned, water bath cured, and dried to obtain a flexible graphite bipolar plate, wherein the mass ratio of 80 mesh powder to 100 mesh powder in the expandable graphite was 85:15, the vacuum-pressurized impregnation pressure was 1.5 MPa, the impregnation time was 120 min, the water bath curing temperature was 94℃, and the water bath curing time was 30 min.

[0035] Performance test of flexible graphite bipolar plate Contact angle: pure water was added dropwise on the surface of the cleaned flexible graphite bipolar plate, and a contact angle measuring instrument was used to measure.

[0036] Contact resistance (referring to national standard test GB / T 20042.6-2024): (1) Sample preparation: cut 3cm x 3cm, area of 9cm 2 square bipolar plate sample, sample number is 5; (2) Contact resistance test: measure the resistance value with a low resistance meter, the test electrode is a gold-plated copper electrode, and carbon paper is placed on both sides of the sample to improve the contact condition, during the test, record a resistance value every time the pressure increases by 0.1 MPa, until the change rate of the current resistance test value and the previous resistance test value is ≤5%, then it is considered that the minimum resistance value is reached.

[0037] The contact angle and contact resistance performance test results of the flexible graphite bipolar plates prepared in Examples 1-5 and Comparative Example 1 are shown in Table 1: Table 1 Contact angle and contact resistance performance test results From Table 1, in combination with the experimental data of Examples 1-5 and Comparative Example 1, it can be seen that by adjusting the etching gas and the etching time, the contact angle of the flexible graphite bipolar plate can be effectively reduced from 106.5° to 82.6° or increased from 106.5° to 113.2°, thereby adjusting the contact resistance of the flexible graphite bipolar plate; further observing Examples 1 and 6-7, adjusting the mass ratio between the 80-mesh powder and the 100-mesh powder in the expandable graphite can also adjust the contact angle and the contact resistance of the flexible graphite bipolar plate.

[0038] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited by the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, which are all included in the protection scope of the present application.

Claims

1. A method for preparing a flexible graphite bipolar plate with adjustable contact angle, characterized in that, The preparation method includes the following steps: S1. Expandable graphite is thermally expanded at high temperature, and then rolled to obtain a flexible graphite expansion plate. It is then molded, impregnated with impregnating liquid under vacuum and pressure, cleaned, cured in a water bath, and dried to obtain a flexible graphite bipolar plate. S2. Place the flexible graphite bipolar plate in the plasma equipment cavity, introduce etching gas, and perform plasma etching on the surface of the graphite bipolar plate. Control the etching time and the type of etching gas to obtain a flexible graphite bipolar plate with adjustable contact angle. The mass ratio of 80-mesh powder to 100-mesh powder in expandable graphite is (80~90):(20~10), and the sum of their mass ratios is 100.

2. The method for preparing a flexible graphite bipolar plate with adjustable contact angle according to claim 1, characterized in that, The impregnation solution in step S1 includes at least one of acrylate resin, phenolic resin, epoxy resin, and vinyl ester resin.

3. The method for preparing a flexible graphite bipolar plate with adjustable contact angle according to claim 1, characterized in that, In step S1, the expansion temperature of the expandable graphite is 800~1000℃ and the expansion time is 20~40s.

4. The method for preparing a flexible graphite bipolar plate with adjustable contact angle according to claim 1, characterized in that, In step S1, the vacuum-pressure impregnation pressure is 1~2MPa, the impregnation time is 60~180min, the water bath curing temperature is 90~98℃, and the water bath curing time is 20~40min.

5. The method for preparing a flexible graphite bipolar plate with adjustable contact angle according to claim 1, characterized in that, In step S2, the etching gas is one of oxygen, hydrogen, or argon, and the etching time is 5 to 20 minutes.

6. A flexible graphite bipolar plate, characterized in that, The flexible graphite bipolar plate is prepared by the method for preparing a flexible graphite bipolar plate with adjustable contact angle as described in any one of claims 1-5. The surface contact angle of the flexible graphite bipolar plate is 82.6~113.2°.

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