Method and bipolar plate for coating a bipolar plate

Abstract

The present invention relates to a method for coating a bipolar plate (1), which comprises at least the following process steps: - applying an epoxy resin-carbon mixture (2) to at least a portion of the surface (S) to be coated of at least one bipolar plate (1), - placing a stamper (3) on the surface (S) of the bipolar plate (1) coated with the epoxy resin-carbon mixture (2) and fixing the stamper (3) against the bipolar plate with a defined distance (x) between the bipolar plate (1) and the stamper (3); - curing the epoxy resin-carbon mixture (2) and removing the mold (3); - roughening the cured epoxy resin-carbon mixture coating (4) at least in some areas; - hydrophilizing at least the roughened areas (4.1) of the epoxy resin-carbon mixture coating (4) by exposing said at least roughened areas (4.1) to a low-pressure plasma (5).

Description

Technical Field

[0001] The present invention relates to a method for coating a bipolar plate, a bipolar plate, a fuel cell, and a vehicle.

Background Art

[0002] The bipolar plate is a central component of a fuel cell stack, also known as a so-called stack. As an important assembly, the bipolar plate has the following functions: establishing an electrical connection between the anode of the first cell of the fuel cell stack and the cathode of the second cell adjacent to the first cell of the fuel cell stack, supplying and distributing reaction gases into the reaction zone of the cells of the fuel cell stack, discharging the generated reaction products, such as liquid or gaseous water, and absorbing or releasing thermal energy. In this regard, for the supply of reaction gases and the discharge of reaction products, a flow profile, typically a flow channel, is typically provided on the bipolar plate, for example, by milling or pressing. Furthermore, the bipolar plate can have cooling channels inside, and a refrigerant for discharging the dissipated heat is passed through these cooling channels. For this purpose, the bipolar plate is often combined with two halves back to back.

[0003] Metal bipolar plates are typically coated in order to enhance their electrical conductivity and thus increase the power output from the fuel cell. In addition, the coating enhances the chemical resistance of the bipolar plate, thereby protecting the bipolar plate from corrosion. Such coatings are typically applied by a so-called physical vapor deposition (PVD) method. This requires the generation of a vacuum. In addition, the bipolar plate is exposed to a high thermal load. Therefore, the production of such coatings is relatively expensive and time-consuming.

Summary of the Invention

Problems to be Solved by the Invention

[0004] The object of the present invention is to present a method for coating a bipolar plate, with which the bipolar plate can be coated with relatively high process reliability, more simply and more inexpensively. A further object of the present invention is to present a bipolar plate coated by such a method.

Means for Solving the Problems

[0005] According to the present invention, these problems are solved by a method for coating a bipolar plate having the features of claim 1 and a bipolar plate having the features of claim 6. Advantageous forms and modifications, as well as a fuel cell provided with such a bipolar plate and a vehicle provided with such a fuel cell, are apparent from its dependent claims.

[0006] In the method for coating a bipolar plate, according to the present invention, at least the following process steps, namely - a step of applying an epoxy resin-carbon mixture to at least a part of the area on at least one surface to be coated of the bipolar plate, - a step of placing a pressing mold on the surface of the bipolar plate coated with the epoxy resin-carbon mixture and fixing the pressing mold to the bipolar plate at a specified interval between the bipolar plate and the pressing mold, - a step of curing the epoxy resin-carbon mixture and removing the pressing mold, - a step of roughening the coating of the cured epoxy resin-carbon mixture in at least a part of the area, - a step of hydrophilizing at least the roughened area of the coating of the epoxy resin-carbon mixture by exposing at least the roughened area to low-pressure plasma, are carried out.

[0007] The method according to the invention enables a simple and inexpensive coating of bipolar plates, in particular with a high process reliability. That is, the coating can be applied onto the bipolar plate like a paint in the form of an epoxy resin-carbon mixture. For this purpose, it is not necessary to generate a vacuum as in the case of the PVD method or to expose the bipolar plate to a high thermal load. In this case, the epoxy resin is a protective layer for protecting the bipolar plate from corrosion. In this regard, the conductivity of the coating can be ensured by mixing carbon. By means of hydrophilization using low-pressure plasma, the chemical structure of the surface of the coating is changed, so that polar functional groups are constructed within the coating. Thereby, the conductivity of the surface of the coating is ensured, and in addition, the wettability and water adhesion of the coated surface are improved. The low-pressure plasma can already be generated at room temperature, which enables a gentle but nevertheless effective surface treatment. According to the method according to the invention, only the roughened areas of the epoxy resin-carbon mixture coating are hydrophilized. The areas of the coating that are not surface-treated remain in the untreated state of the coating and thus have hydrophobic properties based on the epoxy resin. Thereby, on the bipolar plate with the completed coating, there are partial areas with hydrophilic properties and partial areas with hydrophobic properties. This can enable the targeted removal of the generated water that occurs and can improve the gas flow on the bipolar plate. This can increase the power released from the individual fuel cells of the fuel cell stack.

[0008] In an advantageous variant of this method, at least the surface of the bipolar plate to be coated has at least one raised portion and at least one recess in the direction of the normal vector facing away from the bipolar plate, the raised portion is hydrophilized in at least some sections, and the recess remains in an untreated surface state in at least some sections. As mentioned at the beginning, bipolar plates often have a flow profile for guiding gases and liquids as desired. By hydrophilizing the raised portions facing away from the bipolar plate and leaving the recesses extending into the bipolar plate in a hydrophobic state, the fluid guiding characteristics of the bipolar plate can be further improved. That is, the liquid remains on the raised portions of the bipolar plate, and these raised portions typically form a contact surface for the gas diffusion layer. This improves, on the one hand, the conductivity to the gas diffusion layer and, on the other hand, the evacuation action for the evacuation of the generated water.

[0009] According to a further advantageous form of this method, an epoxy resin curable by UV radiation is used, the pressing mold has permeability to UV radiation in at least some regions, and the epoxy resin-carbon mixture is cured by irradiation with UV radiation in a state where the pressing mold is placed on the coated surface of the bipolar plate. Generally, for curing the epoxy resin-carbon mixture, typical methods such as mixing a curing agent can be applied. However, since the curing of the epoxy resin-carbon mixture is realized by irradiation with UV radiation, the step of mixing the curing agent can be omitted. In addition, with the pressing mold, the desired layer thickness can be adjusted on the bipolar plate. For this purpose, the pressing mold can be adjusted at a defined interval with respect to the bipolar plate. Such a layer thickness, that is, the interval between the pressing mold and the bipolar plate, can be, for example, 100 μm. However, thicker or thinner layers are also possible. The pressing mold remains on the bipolar plate during the curing of the epoxy resin, thereby protecting the still soft epoxy resin-carbon mixture from damage and / or contamination. In this regard, the pressing mold can be made of a material that is generally permeable to UV radiation, or the pressing mold can have several openings that are permeable to UV radiation. At this time, these openings overlap the surface area of the bipolar plate where the epoxy resin-carbon mixture is applied to coat the bipolar plate. It is ideal that the entire surface of at least one surface of the bipolar plate is coated.

[0010] To fix the pressing mold on the bipolar plate, during the curing process, the bipolar plate and the pressing mold can remain in a common press. However, the pressing mold may be fixed on the bipolar plate by, for example, at least one fixing element such as a screw, a clip, rubber, or the like. This makes it possible to take out the bipolar plate together with the fixed pressing mold from the press and transport it. In this regard, a spacer may be inserted between the bipolar plate and the pressing mold to ensure the defined interval between the bipolar plate and the pressing mold while taking out and transporting the bipolar plate and the pressing mold from the corresponding machine.

[0011] In a further advantageous embodiment of this method, furthermore, in order to apply an epoxy resin-carbon mixture to the surface of the bipolar plate to be coated, the epoxy resin-carbon mixture is sprayed or applied to the surface to be coated, especially by using a squeegee. This ensures a particularly uniform and gap-free coating of the surface of the bipolar plate to be coated. In this regard, the epoxy resin-carbon mixture can be applied onto the bipolar plate like a paint. Furthermore, using a squeegee, agglomerates of the protruding epoxy resin-carbon mixture can be scraped off.

[0012] It is preferable that the cured epoxy resin-carbon mixture is roughened by irradiation with a laser beam or by an abrasive jet, especially glass bead blasting. Roughening of some areas of the coating of the bipolar plate serves for subsequent hydrophilization preparation. In this regard, it is typical that some areas of the coating are removed at a height of about 5 to 20 μm. Furthermore, for sufficient hydrophilization preparation, it is sufficient that only the areas to be hydrophilized are partially roughened. By means of a laser beam, the coating can be roughened particularly precisely and thus reliably. This is also possible using glass bead blasting. However, generally, roughening using an ordinary abrasive jet can also be carried out.

[0013] According to the present invention, the bipolar plate has a coating in at least some areas on at least one surface to be coated, a first section of the coating has hydrophilic properties, and a second section of the coating has hydrophobic properties. By having several coating sections with hydrophilic and hydrophobic properties, the water distribution on the bipolar plate according to the present invention can be adjusted precisely, which enables an increase in the output of a fuel cell or a fuel cell stack provided with such a bipolar plate. In this regard, especially, the hydrophilic regions of the coating are brought onto the raised portions of the bipolar plate, and the hydrophobic regions of the coating are brought into the depressions of the bipolar plate.

[0014] In this regard, it is preferable that the coating in at least a part of the region of the bipolar plate is performed with an epoxy resin-carbon mixture, and this epoxy resin-carbon mixture is applied to the bipolar plate by the aforementioned method. By such a method, a particularly reliable, simple, and inexpensive coating of the bipolar plate is possible.

[0015] According to the present invention, a fuel cell includes at least one such bipolar plate. Thereby, based on the improved liquid conveyance characteristics of the bipolar plate and the improved conductivity of the contact surface of the bipolar plate with the gas diffusion layer, the power discharged from the fuel cell can be increased.

[0016] According to the present invention, a vehicle includes at least one such fuel cell. In this regard, a plurality of such fuel cells may be assembled into one fuel cell stack. At that time, the vehicle can be any vehicle, for example, a passenger car, a truck, a van, a bus, or the like. The vehicle can be manufactured as a hybrid vehicle equipped with an internal combustion engine and at least one electric motor for driving the vehicle. The vehicle may be purely electrically driven. Generally, it is also conceivable that the vehicle is a railway vehicle, an airplane, or a ship.

[0017] The method according to the present invention and further advantageous forms of the bipolar plate according to the present invention are also apparent from the exemplary embodiments described in more detail below with reference to the drawings.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0019] Figure 1 shows in part a) a bipolar plate 1 coated with an epoxy resin-carbon mixture 2 in process step 110. In this case, the epoxy resin-carbon mixture 2 is applied to the surface S to be coated of the bipolar plate 1 by means of a nozzle 10. To coat the bipolar plate 1, the nozzle 10 is moving in the direction of the feed direction V along the bipolar plate 1. Generally, it is also conceivable that the bipolar plate 1 moves relative to a stationary or movable nozzle 10.

[0020] This bipolar plate 1 has a plurality of elevations 6 and depressions 7 extending in the direction of the normal vector N away from the plate plane E. These elevations 6 and depressions 7 form flow channels for guiding gases and / or liquids.

[0021] As not shown here, the epoxy resin-carbon mixture 2 may be applied onto the bipolar plate 1, for example, using a squeegee.

[0022] Figure 1 b) shows process step 120, in which a pressing die 3 is placed on the bipolar plate 1 at a defined distance x between the bipolar plate 1 and the pressing die 3 in order to adjust the desired coating thickness. Not only is the desired layer thickness adjusted, but the entire pressing die 3 serves to protect the coating 4 formed by the curing of the epoxy resin-carbon mixture 2 on the bipolar plate 1 during the curing process. This pressing die 3 can have any shape, and on the side facing the surface S to be coated of the bipolar plate 1, the pressing die 3 has a shape that fits exactly to the bipolar plate 1. On the opposite side of the pressing die 3, the pressing die 3 can have a flat shape, for example, as suggested by the solid line, or can also have a shape that fits exactly to the bipolar plate 1, as shown by the dashed line.

[0023] Figure 1c) shows process step 130, in which the epoxy resin-carbon mixture 2 is cured. In this exemplary embodiment in this regard, the curing of the epoxy resin-carbon mixture 2 is performed by irradiating the epoxy resin-carbon mixture 2 with UV radiation 8. The UV radiation 8 can be generated, for example, by a UV lamp 11. The pressing die 3 has at least partially or entirely a permeability to the UV radiation 8, and thus the UV radiation 8 can pass through the pressing die 3 and hit the epoxy resin-carbon mixture 2.

[0024] Figure 1d) shows the roughening of a partial region of the coating 4 formed by curing the epoxy resin-carbon mixture 2 on the bipolar plate 1 in process step 140. For this purpose, the partial region of the coating 4 is being treated with a laser beam 9. The region 4.1 to be roughened may be roughened by an abrasive jet (not shown) instead of the laser beam 9, for example by glass bead blasting.

[0025] Figure 1e) shows process step 150, which shows the surface treatment of the roughened region 4.1 with the low-pressure plasma 5. The low-pressure plasma 5 converts the chemical structure of the coating 4, thereby constructing polar functional groups in the coating 4, whereby the region of the coating 4 treated with the low-pressure plasma 5 is hydrophilized. In this regard, the roughened region 4.1 is treated with the low-pressure plasma. The regions outside the roughened region 4.1 remain untreated, whereby these regions have hydrophobic properties. In this regard, it is desirable that the raised portions 6 are hydrophilized in at least some regions and the recessed portions 7 continue to be in their untreated hydrophobic state in at least some regions. Thereby, the moisture distribution on the bipolar plate 1 can be adjusted as desired. Thus, more liquid remains attached to the raised portions 6, which improves the conductivity at that location. In the recessed portions 7, it is difficult for liquid, especially water, to accumulate, whereby ice cannot be formed even in the recessed portions 7, for example, at sub-zero temperatures. Therefore, reliable discharge of the reaction liquid from the individual fuel cells of the fuel cell stack is ensured. That is, not only the power is increased, but also the fuel cell or the fuel cell stack can operate reliably.

[0026] Figure 2 shows the bipolar plate 1 according to the invention with an alternative geometry. This bipolar plate 1 may also have, for example, cooling channels 12 for guiding a refrigerant. Figure 2 serves to specifically illustrate that generally the bipolar plate 1 can have any plate-like geometry. In this regard, in particular, the raised portions 6 and the recessed portions 7 extend away from the bipolar plate 1 or extend into the bipolar plate 1, respectively. In the example of Figure 2, the raised portion 6 or the recessed portion 7 is formed in a rectangle. Generally, the raised portions and / or the recessed portions 6, 7 can have any geometry. For example, the raised portions and / or the recessed portions 6, 7 can have a triangular, elliptical, circular, or any polygonal cross-sectional shape. The raised portions 6 and the recessed portions 7 may be made in various sizes. Particularly advantageous in this regard is that a first section A1 having hydrophilic properties is formed at least on a partial region of the raised portion 6, and a second section A2 having hydrophobic properties is formed at least partially on or in the recessed portion 7. At this time, the individual sections A1 and A2 on each raised portion 6 and / or within each recessed portion 7 can be shaped individually, that is, they can have different widths.

[0027] The bipolar plate 1 may be coated at least in part on at least two opposing surfaces S.

[0028] The bipolar plate 1 is typically made from two separate bipolar plate halves 1.1 and 1.2, which are joined to each other on the sides where they do not face their respective raised portions 6 and recessed portions 7. In this regard, the sides having the raised portions 6 and the recessed portions 7 form the anode side and the cathode side to which the coating is applied. On the side where the bipolar plate halves 1.1 and 1.2 can be joined, recesses for forming the cooling channels 12 can be provided.

Claims

1. A method for coating a bipolar plate (1), comprising at least the following process steps, namely - applying an epoxy resin-carbon mixture (2) to at least a partial area on at least one surface (S) to be coated of the bipolar plate (1); - placing a pressing mold (3) on the surface (S) of the bipolar plate (1) coated with the epoxy resin-carbon mixture (2), and fixing the pressing mold (3) to the bipolar plate at a defined distance (x) between the bipolar plate (1) and the pressing mold (3); - curing the epoxy resin-carbon mixture (2) and removing the pressing mold (3); - roughening the coating (4) of the cured epoxy resin-carbon mixture in at least a partial area; - hydrophilizing at least the roughened area (4.1) of the coating (4) of the epoxy resin-carbon mixture by exposing at least the roughened area (4.1) to a low-pressure plasma (5). The method is characterized by the above steps.

2. At least the surface (S) to be coated of the bipolar plate (1) has at least one raised portion (6) and at least one recessed portion (7) in the direction of the normal vector (N) facing away from the bipolar plate, at least a partial section of the raised portion (6) is hydrophilized, and at least a partial section of the recessed portion (7) remains in an untreated surface state. The method according to claim 1 is characterized by this.

3. An epoxy resin curable by UV radiation (8) is used, the pressing mold (3) has permeability to UV radiation (8) in at least a partial area, and the epoxy resin-carbon mixture (2) is cured by irradiation with UV radiation (8) in a state where the pressing mold (3) is placed on the coated surface (S) of the bipolar plate (1). The method according to claim 1 or 2 is characterized by this.

4. To apply the epoxy resin-carbon mixture (2) to the surface (S) to be coated of the bipolar plate (1), the epoxy resin-carbon mixture (2) is sprayed or applied to the surface (S) to be coated, especially applied using a squeegee. The method according to any one of claims 1 to 3 is characterized by this.

5. ​ The method according to any one of claims 1 to 4, characterized in that the cured epoxy resin-carbon mixture (2) is roughened by irradiation with a laser beam (9) or by an abrasive jet, in particular by glass bead blasting.

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