BIPOLAR PLATE FOR A FUEL CELL OR A FLOW BATTERY HAVING A HYDROPHOBIC SURFACE AREA AND METHOD FOR MANUFACTURING THE SAME

Description

FIELD OF THE INVENTION

[0001] The present invention relates to a bipolar plate of a fuel cell or a flow battery and a method for manufacturing such a bipolar plate. BACKGROUND OF THE INVENTION

[0002] Bipolar plates are designed to fulfil a number of different functions in fuel cells, which are stacked together to form the core of a fuel cell system. On the one hand, they are designed to connect neighbouring fuel cells to one another, i.e., to physically and electrically connect the anode of one cell to the cathode of a neighbouring cell. On the other hand, gas distribution to reaction spaces within the fuel cells is designed to occur across a surface of the bipolar plate, i.e., the bipolar plate is designed to guide reaction gases into reaction zones. For this purpose, the bipolar plate typically has flow profiles (so-called flow fields) on both sides, which can be milled, formed, or reshaped, through which hydrogen flows on one side and air is supplied on the other. The bipolar plate generally also regulates the removal of water vapor and the release of thermal and electrical energy.In addition, the bipolar plate should also provide gas separation between adjacent cells, sealing to the outside and, if necessary, cooling.

[0003] Bipolar plates often have channels that extend through the interior of a bipolar plate substrate. Such channels can, for example, fulfill specific functions during bipolar plate manufacturing. For example, an adhesive can be housed within such channels, which is used to bond two sub-substrates forming the bipolar plate together. Alternatively, channels can also be provided to implement other functionalities.

[0004] It has been observed that problems can occur during the operation of fuel cells or flow batteries under certain operating conditions, particularly at or after low temperatures. These problems are suspected to be related to the bipolar plate used in the fuel cell or flow battery.

[0005] DE 10 2009 020 224 A1 describes a bipolar plate with water management features at the inlet and outlet. US 2007 / 0190401 A1 describes anode electrodes for direct oxidation fuel cells and systems operating with concentrated liquid fuel. US 6,165,634 describes a fuel cell with improved sealing between individual membrane assemblies and plate assemblies. SUMMARY OF THE INVENTION AND ADVANTAGEOUS EMBODIMENTS

[0006] There may be a need for a bipolar plate or a fuel cell or flow battery equipped therewith that can be operated reliably, particularly even under operating conditions at or after low temperatures. In particular, there may be a need for a bipolar plate that is simply constructed, can be manufactured cost-effectively, and / or can be operated particularly robustly. Furthermore, there may be a need for a method for manufacturing such a bipolar plate. In particular, there may be a need for a method by which a bipolar plate can be manufactured cost-effectively, with little effort, and / or reliably.

[0007] Such a need can be met by the subject matter of the independent claims. Advantageous embodiments are defined in the dependent claims, described in the following description, and illustrated in the figures.

[0008] A first aspect of the invention relates to a bipolar plate for a fuel cell or a flow battery, wherein the bipolar plate comprises a substrate and a channel extending from a channel entrance on a surface of the substrate into an interior of the substrate. A surface of the substrate is more hydrophobic in a first region within the channel adjacent to the channel entrance than in a second region. The first region surrounds the channel entrance in an annular manner.

[0009] A second aspect of the invention relates to a method for manufacturing a bipolar plate for a fuel cell or a flow battery, the method comprising at least the following steps: Providing a substrate having a structure such that in the fabricated bipolar plate a channel extends within the substrate, which channel extends from a channel entrance on a surface of the substrate into an interior of the substrate, and processing the surface of the substrate in a first region within the channel adjacent to the channel entrance such that the surface of the substrate in the first region is more hydrophobic than the surface of the substrate in a second region.

[0010] The first area surrounds the canal entrance in a ring shape.

[0011] Without limiting the scope of the invention in any way, ideas and possible features for embodiments of the invention can be considered to be based, among other things, on the thoughts and findings described below.

[0012] Briefly and roughly summarized, a basic idea behind the concept described herein can be seen in the recognition that problems such as those observed during the operation of fuel cells or flow batteries, particularly at or after low temperatures, could be due to water penetrating the channels of the bipolar plate or water vapor condensing in these channels and then freezing, causing forces or damage to the bipolar plate due to the thermally induced expansion that occurs. While the aim so far has been to prevent the penetration of water or water vapor by sealing measures in or on the channels, it is alternatively proposed here to deliberately make at least part of the surface of the substrate adjacent to the channel entrance and preferably also exposed areas of the surface within the channel more hydrophobic than is the case for other parts of the substrate surface.This is intended to prevent water from penetrating into exposed areas in the duct and / or accumulating there through condensation.

[0013] Possible details of the designs of the bipolar plate proposed here and the manufacturing process are explained below.

[0014] Typically, the substrate used to form the bipolar plate is plate-shaped. Accordingly, the substrate has a height and a width that are both significantly greater than the substrate's thickness, i.e., they exceed the substrate's thickness by more than ten times, for example. The substrate can have structures and / or textures on an external surface, for example, to guide reaction fluids along predetermined paths on the substrate surface and / or to enlarge the substrate surface.

[0015] The substrate typically consists of a material with very good electrical conductivity or is at least provided with such a material on its surface.

[0016] For example, according to one embodiment, the bipolar plate or its substrate can be formed with a graphite-containing material or consist of such a material.

[0017] As a carbon-containing material, graphite offers advantageous properties for many applications. For use in bipolar plates, for example, graphite offers very high electrical conductivity combined with high thermal resilience and sufficiently high mechanical strength. Materials used for bipolar plates include graphite-containing materials in which graphite particles are embedded in a polymer matrix. The graphite particles give the material the desired electrical and / or thermal properties. The polymer matrix serves, among other things, to hold the graphite particles together mechanically. The polymer matrix can contain, for example, an epoxy resin. The graphite particles thus act as a filler, and the polymer matrix as a type of binder. In addition to graphite particles and polymers, the material mixture can also contain other components, for example in the form of carbon black, other binders, or similar.Advantageously, the graphite-containing material can have a graphite content of at least 60%, preferably at least 70% or even at least 80%. The percentages can refer to the volume. Due to the high graphite content, the material can, among other things, offer very good electrical conductivity, which is particularly advantageous when used to form bipolar plates. Examples and possible properties of graphite-containing materials are described, among other things, in the applicant's earlier patent application PCT / EP2020 / 078489. The graphite-containing materials described therein can be used in embodiments of the bipolar plate described herein and can be made locally hydrophobic on their surface. The content of the earlier patent application is incorporated herein in its entirety by reference.

[0018] At least one internal channel, i.e. a preferably elongated cavity, is formed in the substrate. The channel extends from a channel inlet on the surface of the substrate into the interior of the substrate. In other words, the channel opens at the channel inlet into a volume surrounding the substrate or an atmosphere present there. The channel extends from the channel inlet into the interior of the substrate and possibly to a channel outlet at another region of the surface of the substrate. The channel can have small cross-sectional dimensions compared to the thickness of the substrate. For example, the cross-sectional dimensions can be less than half, preferably less than a third of the thickness of the substrate. The channel can be round, rectangular, or have any other geometry in cross-section. The channel can be surrounded along its entire circumference by material from the substrate of the bipolar plate.Alternatively, a portion of the circumference may be covered by another material, for example a sealant, adhesive or glue introduced within the channel.

[0019] Conventionally, attempts have been made to seal channels in the substrate of a bipolar plate in such a way that no moisture can penetrate. For example, a sealant has been introduced into the channel, which fills and seals the channel at least at its entrance, preferably along the entire volume of the channel.

[0020] However, it has been observed that it can be difficult to reliably seal the channel or its channel entrance during production of the bipolar plate. For example, it can be complex to apply a sealant in such a way that the channel is completely filled or at least its channel entrance is sufficiently sealed. In this case, only very small tolerance windows are usually permissible, which can lead to unstable manufacturing processes. Furthermore, an excessively large amount of sealant often has to be applied, allowing excess sealant to escape at the channel entrance and thus sufficiently seal the channel entrance. However, in this case the excess sealant remains ineffective and thus results in excessive sealant consumption. In addition, reworking and / or inspection of the sealed channel and / or cleaning of the tools used may be necessary.In addition, material requirements for the sealant may conflict with technical requirements for the bipolar plate. Furthermore, there may be a risk that leaks may develop between the channel and the sealant over time, for example, due to thermal stress and the associated deformation.

[0021] In order to at least partially overcome the aforementioned problems and disadvantages, an alternative approach is described herein to prevent the penetration and / or accumulation of moisture within the channel. In this case, a partial area of ​​the surface of the substrate inside the channel is specifically treated or specifically designed in such a way that a stronger hydrophobic property is established there than in other areas of the surface of the substrate, in particular than the other areas of the surface of the substrate within the channel or adjacent to the channel. This increasingly hydrophobic partial area is referred to herein as the first area. Areas with lesser hydrophobicity are referred to herein as second areas or third areas. The second area or the third area can be directly adjacent to the first area.

[0022] In the first region, the surface of the substrate can in particular be treated such that a contact angle formed by a water droplet with the surface is greater than 40°, preferably greater than 60°, or even greater than 80°. In particular, the surface of the substrate in the first region can be superhydrophobic, i.e., a contact angle can be greater than 90°. In particular, the contact angle in the first region can be, for example, more than 5°, preferably more than 10°, more than 20°, or even more than 30° greater than in the second region.

[0023] Due to the increased hydrophobic property of the surface of the substrate in the first region adjacent to the channel entrance, the risk of liquid, in particular water, penetrating the channel via the channel entrance can be massively reduced.

[0024] According to the invention, it may be advantageous for the first region to surround the channel entrance in a ring-shaped manner. In other words, the entire surface surrounding the channel entrance at its periphery can preferably be designed to be more hydrophobic. This can further minimize the risk of liquid penetration. The hydrophobic channel entrance can act as a barrier for liquid and prevent liquid from entering the channel, for example, due to capillary forces.

[0025] Furthermore, according to one embodiment, the first region can comprise parts of the surface of the substrate within the channel that are connected in gas communication with the channel inlet. In other words, the first region, in addition to a partial region in which it borders the channel inlet, can extend further into the interior of the channel and there, in particular, comprise parts of the surface of the substrate to which gas can reach from the channel inlet. Gas communication between the channel inlet and the partial region of the first region located further inside the channel can occur through gas flow and / or gas diffusion.

[0026] By including parts of the surface of the substrate within the channel in the highly hydrophobic first region, it is possible, among other things, to reduce the risk that vapor, particularly water vapor, enters the channel through the channel entrance and forms liquid droplets there by condensation.

[0027] According to one embodiment, the substrate of the bipolar plate is composed of two plate-shaped sub-substrates. The two sub-substrates are bonded together along an adhesive surface. An overflow channel extends adjacent to the adhesive surface. The first region extends at least along parts of the overflow channel. The second region extends at least along parts of the adhesive surface. During production of the bipolar plate, the two plate-shaped sub-substrates can initially be provided as separate components and then bonded together along the adhesive surface.

[0028] In other words, the bipolar plate can be constructed in two or more parts. Plate-shaped sub-substrates can initially be prefabricated as individual components. Structures such as depressions, grooves, or similar features can be formed on one surface. These structures, once the two sub-substrates are joined together, form channels in the resulting overall substrate.

[0029] These structures can be configured to form adhesive surfaces along which two adjacent, stacked sub-substrates can be bonded together. The adhesive surfaces of the stacked sub-substrates can be configured to face each other and run very close to each other, preferably parallel to each other. When bonding the two sub-substrates, an adhesive can be applied to one or both of the opposing adhesive surfaces.

[0030] The structures can further be configured such that at least one overflow channel is formed next to the adhesive surfaces. In the region of the overflow channel, a recess in the surface of a sub-substrate can be larger than in the region of the adhesive surfaces. Accordingly, a surface of the overflow channel of a sub-substrate can be spaced further apart from an opposite surface of a second sub-substrate stacked above it than is the case for opposite adhesive surfaces.

[0031] When bonding the two sub-substrates, the adhesive is preferably initially applied exclusively to one or both of the bonding surfaces, but not into the overflow channel. However, when the two sub-substrates are subsequently pressed together, the adhesive can flow laterally into the overflow channel and at least partially fill it. The overflow channel can thus absorb excess adhesive. Accordingly, the two sub-substrates can be efficiently bonded together without excess adhesive hindering the tight pressing of the sub-substrates. If necessary, overflow channels can be arranged on both opposite sides of the bonding surfaces.

[0032] Since the adhesive surfaces are intended to bond the two sub-substrates and the adhesive should therefore adhere well to these surfaces, it may be desirable to design these adhesive surfaces as second areas with relatively low hydrophobic properties or even hydrophilic properties. Accordingly, the adhesive, which is generally processed in a liquid or flowable state, can adhere to the adhesive surfaces over a large area and form a strong, adhesive bond with the adhesive surfaces.

[0033] In contrast, the aim may be to create a surface of the overflow channel with more strongly hydrophobic properties. While a portion of this surface may ultimately be covered by adhesive that oozes out laterally between the bonding surfaces, since the volume of the overflow channel is preferably selected so that it is not completely filled with adhesive, at least a portion of the overflow channel's surface can remain exposed. To prevent moisture from accumulating on this part of the surface, this portion can be designed as the first area with increased hydrophobicity.

[0034] Accordingly, according to one embodiment, in the finished bipolar plate, the channel can be partially filled with a flowable adhesive. The first region forms a portion of the surface within the channel of the substrate in which the adhesive does not contact the substrate. The second region of the substrate forms a portion of the surface within the channel of the substrate in which the adhesive does contact the substrate.

[0035] In other words, the bipolar plate composed, for example, of two sub-substrates can have a channel that is partially filled with an adhesive. The adhesive can be flowable during processing and subsequently crosslink or solidify. Accordingly, the adhesive can adhere to the adhesive surfaces of the two sub-substrates and bond the two sub-substrates to one another. A portion of the surface of the sub-substrates within the channel, in which the adhesive rests against the adhesive surfaces, can preferably be designed as a second region with low hydrophobic properties. In contrast, a portion of the surfaces of the sub-substrates that are not covered by the adhesive after such a bonding process can be designed as a first region with strong hydrophobic properties.

[0036] Preferably, the first region within the channel extends along an entire surface of the substrate in which the adhesive does not contact the substrate.

[0037] In other words, the first region within the channel is preferably dimensioned and arranged such that, after bonding the two sub-substrates using the adhesive, all surface areas of the substrate within the channel that are not covered by the adhesive are highly hydrophobic. This can, among other things, ensure that no weakly hydrophobic surfaces remain within the channel on which, for example, penetrating water vapor could condense.

[0038] In general, various ways are known to treat the surfaces of a substrate in such a way that they acquire more hydrophobic properties. For example, surfaces can be coated with a material that is highly hydrophobic, for example due to the lack of polar molecules and groups. Conversely, various ways are known to suitably treat the surfaces of a substrate in order to impart them less hydrophobic or hydrophilic properties. For example, surfaces can be coated with a material that exhibits at least a certain degree of hydrophilicity due to the polar molecules and groups it contains. Alternatively, surfaces can be specifically oxidized, for example by treating them with fluorine or sulfur trioxide. Plasma treatment or corona treatment can also lead to less hydrophobic or hydrophilic surface properties.

[0039] According to one embodiment, the first region of the substrate surface described herein can be processed by locally irradiating the surface of the substrate by means of a laser in order to give it increased hydrophobic properties.

[0040] A laser beam can be directed specifically at the first area of ​​the substrate surface. The second area of ​​the substrate surface can remain untreated or be treated in a different way to impart less hydrophobic properties. By using a laser, the first and second areas of the substrate surface can be specifically designed with different levels of hydrophobic properties. The laser beam can also be directed specifically at very small areas. In contrast, it can be technically difficult to treat small areas using other technologies, such as coating with a hydrophobic material.

[0041] Accordingly, according to one embodiment, the first region of the finished bipolar plate can be locally processed by means of a laser in order to form the desired locally increased hydrophobicity.

[0042] The first area typically exhibits characteristic microscopic properties due to the irradiation with the laser beam. For example, a region treated with the laser beam may have been briefly melted and, upon subsequent solidification, acquired a structure and / or composition characteristic of the laser treatment. Alternatively or additionally, material can be locally removed from the surface using a high-energy laser beam, i.e., ablated. This can generate a plasma near the surface, and material can then be vaporized. Additionally or alternatively, material can be chemically altered, i.e., "cracked," by thermal influences caused by the laser irradiation, and then, for example, remain as a carbon structure or pass into the gas phase.Overall, laser processing can result in a surface structure that may be rougher than in areas not laser-processed. Alternatively or additionally, a volume structure, particularly a crystal structure, of the laser-processed material can change in a manner typical for laser processing.

[0043] The laser may have properties that allow the melting or ablation of substrate material at the laser-treated surface. The laser may, for example, be a pulsed laser, particularly a short-pulse laser capable of emitting laser pulses with durations in the nanosecond range or less. The laser may, for example, be an infrared laser.

[0044] According to one embodiment, the first region within the channel can be machined such that a surface texture is formed which causes a lotus effect.

[0045] The formation of a complex microscopic and nanoscopic surface structure can result in liquid droplets forming only a very small contact area with a surface, causing them to roll off the textured surface. This effect is also known as the lotus effect and can represent a highly hydrophobic or even superhydrophobic property of a surface. The surface texture can be formed by a multitude of tiny and closely spaced depressions and / or grooves.

[0046] For example, the surface texture can be created by scanning the laser beam along regular or irregular paths over the surface to be textured, temporarily melting, raising and / or ablating material on the surface.

[0047] According to one embodiment, the same laser used to process the surface of the substrate in the first region and to make it more hydrophobic can also be used to process the surface of the substrate in the second region and / or in a third region of the surface.

[0048] It has been recognized that, with a suitably adapted process control, laser processing can both enhance or weaken the hydrophobic properties of a surface. Accordingly, it can be advantageous to specifically create the first and second regions of differing hydrophobicity to be provided on the substrate of the bipolar plate by local processing with the same laser. A third region of the substrate surface can also be processed with the same laser. For example, the substrate surface in the third region can be freed of a surface layer by laser processing. In particular, in the case of a graphite-containing material with a polymer matrix, a polymer layer formed on the surface can be opened or removed by laser processing.This can simplify the overall process and / or reduce the amount of equipment required.

[0049] In particular, according to a specific embodiment, laser parameters with which the laser is operated when processing the second and / or third region of the surface of the substrate can be selected differently than when processing the first region of the surface of the substrate.

[0050] For example, it has been recognized that when using laser parameters that lead to the formation of a surface texture producing a lotus effect, the first region can advantageously be made highly hydrophobic. When using other laser parameters, however, it has been observed that surface properties change to become less hydrophobic or even hydrophilic. For example, it has been observed that irradiation with very powerful, short laser pulses can impart hydrophilic properties to a graphite-containing material. In this regard, the applicant of the present application has filed a further patent application entitled "Method for forming a hydrophilic surface on a graphite-containing material and method for manufacturing a bipolar plate, as well as a bipolar plate and fuel cell or flow battery comprising the same."This additional patent application describes details of laser processing that can also be used in embodiments of the bipolar plate described here or in their manufacture in the same or similar manner. The content of the additional application is incorporated herein by reference.

[0051] In embodiments of the bipolar plate presented here, the penetration of liquid or moisture into the substrate channels can be prevented. This can, for example, prevent damage caused by expanding and freezing water. Shorter application times, for example, for applying an adhesive between sub-substrates to be bonded, and lower material usage can be achieved. Overall, more robust processes, expanded tolerance ranges, stable process capability, low process costs, and / or less scrap can be achieved.

[0052] It should be noted that possible features and advantages of embodiments of the invention are described herein partly with reference to a bipolar plate and partly with reference to a method for manufacturing a bipolar plate. A person skilled in the art will recognize that the features described for individual embodiments can be transferred, adapted, and / or exchanged in an analogous and suitable manner to other embodiments in order to achieve further embodiments of the invention and possibly synergistic effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Advantageous embodiments of the invention are explained in more detail below with reference to the accompanying drawings, wherein neither the drawings nor the explanations are to be interpreted as limiting the invention in any way. Figure 1 shows a plan view of a bipolar plate according to an embodiment of the present invention. Figure 2 shows a sectional view through a bipolar plate according to an embodiment of the present invention. Figure 3 shows an enlargement of the Figure 1 marked section A. Figure 4 shows an enlargement of the Figure 2 marked section B.

[0054] The figures are merely schematic and not to scale. The same reference numerals designate the same or equivalent features in the various drawings. DESCRIPTION OF ADVANTAGEOUS EMBODIMENTS

[0055] Figure 1 shows a highly schematic top view of a bipolar plate 1 as it can be used in a fuel cell or a flow battery. Figure 2 shows a schematic sectional view through the bipolar plate 1.

[0056] The bipolar plate 1 comprises a substrate 3, which can be made, for example, from a graphite-containing material. A channel 5 extends in the substrate 3. The channel 5 runs from a channel inlet 7 into an interior 9 of the substrate 3. In the example shown, the channel 5 is annular and extends near an outer circumference of the bipolar plate 3. A plurality of channel inlets 7 are provided. The channel inlets 7 can serve as openings for ventilating the annular region of the channel 5 and run towards an outer edge of the substrate 3. In the example shown, further openings are formed, which can also be referred to as channel inlets 7 or alternatively as channel outlets 11 and which are directed away from the outer edge and towards a center of the substrate 3.

[0057] A first region 19 of the surface of the substrate 3 within the channel 5 is treated or formed in such a way that it is more hydrophobic than a second region 21. The first region 19 extends, on the one hand, as far as the channel inlet 7 and preferably surrounds the channel inlet 7 in a ring shape. On the other hand, the first region 19 within the channel 5 also extends partially into the interior 9 of the substrate 3, in particular where an inner surface of the substrate is exposed within the channel 5, so that gas or vapor coming from the channel inlet 7 can reach this part of the first region 19. Furthermore, the first region 19 can also extend as far as the channel outlet 11 and likewise preferably surround it in a ring shape.

[0058] In order to manufacture the bipolar plate 1, two sub-substrates 13 can be provided. In the sub-substrates 13, near the outer circumference of the sub-substrates 13, structures can be formed by means of local depressions, which on the one hand form an adhesive surface 15 and on the other hand form an overflow channel 17 on both sides adjacent to the adhesive surface 15. The adhesive surface 15 and the overflow channels surround a central region of the bipolar plate 1 in a ring shape. The adhesive surface 15 is formed as a second region 21 with less hydrophobicity or even hydrophilicity, whereas surfaces that delimit the overflow channel 17 are formed as a first region 19 with increased hydrophobicity. On outwardly facing surfaces, the sub-substrates 13 of the bipolar plate 1 have surface structures 31 in their central region, which serve, for example, for the targeted conduction of reaction gases (only in Fig. 2 indicated and in Fig. 1not illustrated for clarity). On inward-facing surfaces, the sub-substrates 13 have other surface structures, for example, to form cooling channels 33 within the bipolar plate 1

[0059] The surface of the substrate 3 in the first region 19 can be irradiated, for example, with a laser 27. Laser parameters of this laser 27 can be adjusted such that microscopic and / or nanoscopic structures of a surface texture 25 form on the surface of the first region 19, which cause a lotus effect and / or a chemical and / or physical effect and thus a strong hydrophobicity. Fig. 3 such a surface texture 25 is schematically symbolized in a locally enlarged section.

[0060] The second region 21 can either not be irradiated with the laser 27, or other laser parameters can be set that result in this second region 21 developing less hydrophobic or hydrophilic properties. Similarly, a third region 29 can also be processed with the laser 27 by setting suitable laser parameters, for example, to specifically achieve highly hydrophilic properties there.

[0061] In order to bond the two sub-substrates 13 together, an adhesive 23, for example in the form of a flowable adhesive, is applied between the bonding surfaces 15. Due to the low hydrophobicity or hydrophilicity of the second region 21 formed there, this adhesive 23 can adhere strongly to the bonding surfaces 15. When the two sub-substrates 13 are pressed together, excess adhesive 23 can flow laterally into the overflow channels 17. However, the adhesive 23 does not need to be applied with such a large excess, as is sometimes the case with conventionally used methods, that the entire overflow channels 17 up to the channel inlet 7 are filled with adhesive 23 in order to seal the entire channel 5. Instead, parts of the overflow channels 17 can remain free, thus leaving their inner surface uncovered.Due to its strong hydrophobicity, the first region 19 formed there can then prevent moisture from accumulating on such exposed surfaces within the channel 5, even though they are connected in gas communication with the channel inlet 7. Frost damage caused by freezing moisture within the channel 5 can thus be avoided.

[0062] Finally, it should be noted that terms such as "having," "comprising," etc., do not exclude other elements or steps, and terms such as "a" or "an" do not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to one of the above embodiments may also be used in combination with other features or steps of other embodiments described above. Reference signs in the claims are not to be considered as limitations. List of reference symbols

[0063] 1Bipolar plate 3Substrate 5Channel 7Channel inlet 9Interior of the substrate 11Channel outlet 13Substrate section 15Adhesive surface 17Overflow channel 19First area 21Second area 23Adhesive 25Surface texture 27Laser 29Third area 31Surface structures 33Cooling channels

Claims

1. Bipolar plate (1) for a fuel cell or a flow battery, wherein the bipolar plate (1) comprises: a substrate (3), and a channel (5) running from a channel entrance (7) at a surface of the substrate (3) into an interior (9) of the substrate (3), characterized in that a surface of the substrate (3) being more hydrophobic in a first area (19) inside the channel (5) adjacent to the channel entrance (7) than in a second area (21), and the first area (19) annularly surrounds the channel entrance (7).

2. Bipolar plate according to claim 1, wherein the first area (19) comprises parts of the surface of the substrate (3) inside the channel (5) that are in gas communication with the channel entrance (7).

3. Bipolar plate according to any of the preceding claims, wherein the substrate (3) of the bipolar plate (1) is composed of two plate-shaped sub-substrates (13), the two sub-substrates (13) being bonded together across a bonding surface (15), and an overflow channel (17) running adjacent to the bonding surface (15), the first area (19) running at least along parts of the overflow channel (17) and the second area (21) running at least along parts of the bonding surface (15).

4. Bipolar plate according to any of the preceding claims, wherein the channel (5) is partially filled with a flowably processable adhesive (23), the first area (19) forming a portion of the surface inside the channel (5) of the substrate (3) where the adhesive (23) is not in contact with the substrate (3), and the second area (21) of the substrate (3) forming a portion of the surface inside the channel (5) of the substrate (3) where the adhesive (23) is in contact with the substrate (3).

5. Bipolar plate according to claim 4, wherein the first area (19) runs inside the channel (3) along an entire surface of the substrate (3) where the adhesive (23) is not in contact with the substrate.

6. Bipolar plate according to any of the preceding claims, wherein the first area (19) is locally treated by means of a laser (27) to provide a local increase in hydrophobicity.

7. Bipolar plate according to any of the preceding claims, wherein the first area (19) has a surface texture (25) creating a lotus effect.

8. Bipolar plate according to any of the preceding claims, wherein the bipolar plate (1) is formed with a graphite-containing material.

9. Method of manufacturing a bipolar plate (1) for a fuel cell or a flow battery, characterized in that the method comprises: providing a substrate (3) having a structure such that a channel (5) runs inside the substrate (1) in the manufactured bipolar plate (1), the channel (5) running from a channel entrance (7) at a surface of the substrate (3) into an interior of (9) the substrate (3), and treating the surface of the substrate (3) in a first area (19) inside the channel (5) adjacent to the channel entrance (7) such that the surface of the substrate (3) in the first area (19) is more strongly hydrophobic than the surface of the substrate (3) in a second area (21), wherein the first area (19) annularly surrounds the channel entrance (7).

10. Method according to claim 9, wherein the substrate (3) is provided with two plate-shaped sub-substrates (13), a bonding surface (15) being formed on the two plate-shaped sub-substrates (13) and an overflow channel (17) being formed adjacent to the bonding surface (15), the first area (19) running at least along parts of the overflow channel (17), and the second area (21) running along at least parts of the bonding surface (15), the method further comprising bonding the two plate-shaped sub-substrates (13) together across the bonding surface (15).

11. Method according to one of claims 9 and 10, wherein the first area (19) is treated by locally irradiating the surface of the substrate (3) by means of a laser.

12. Method according to claim 11, the method further comprising treating the substrate with the laser (27) in the second area (21) of the surface of the substrate (3) and / or in a third area (29) of the surface of the substrate (3).

13. Method according to claim 12, wherein the laser (27) is operated with different laser parameters when treating the second and / or third area (21, 29) of the surface of the substrate (3) than when treating the first area (19) of the surface of the substrate (3).

14. Method according to one of claims 9 to 13, wherein the first area (19) of the surface of the substrate (3) is formed by treating with a surface texture (25) producing a lotus effect.