Metallic bipolar plate for an electrochemical system

Abstract

Metallic bipolar plate (2) for an electrochemical system, wherein the bipolar plate (2) comprises two separator plates (2a, 2b) and each of the separator plates (2a, 2b) has a flow field (17), wherein at least one of the flow fields (17) has a coating (26) on at least one surface of the associated separator plate (2a, 2b) at least in sections, wherein the coating (26) comprises: - at least 50% by weight and / or at most 95% by weight of a graphite-carbon black mixture, and - at least 5% by weight and / or at most 31% by weight of one or more binders in total.

Description

[0001] The invention relates to a metallic bipolar plate for an electrochemical system.

[0002] Known electrochemical systems in which the metallic bipolar plate disclosed here can be used include, for example, fuel cell systems, flow batteries, or electrochemical compressor systems, in particular electrolyzers. Known electrolyzers are designed, for example, in such a way that hydrogen and oxygen are generated from water by applying a potential, with at least the hydrogen being present in compressed form. In addition, electrochemical compressor systems are also known, such as electrochemical hydrogen compressors, to which gaseous molecular hydrogen is supplied and in which this is electrochemically compressed by applying a potential. Furthermore, known electrochemical systems include electrochemical separator systems in which, for example, hydrogen is extracted from one reaction system and enriched in another part of the electrochemical system.

[0003] Known electrochemical systems often comprise a stack of electrochemical cells, each of which is separated from one another by bipolar plates. Such bipolar plates can, for example, be used to indirectly electrically contact the electrodes of the individual electrochemical cells (e.g., fuel cells) and / or to indirectly electrically connect neighboring cells (series connection of the cells). The bipolar plates can, in particular, have two separator plates. The separator plates or the bipolar plate can have or form a web-channel structure that is designed to supply the cells with one or more media and / or to transport away reaction products. The media can be fuels (e.g., hydrogen or methanol), reaction gases (e.g., air or oxygen), or coolants. Such a web-channel structure is usually arranged in an electrochemically active region (so-called flow field or"Flowfield") as well as in the distribution and collection areas leading to and from it. Furthermore, the bipolar plates can be sealed against one another and / or to the outside in order to conduct the waste heat generated during the conversion of electrical or chemical energy in the electrochemical cell and to seal the various media channels, including the cooling channels. For example, the bipolar plates can have openings through which the media to be supplied and / or the reaction products can be guided to the electrochemical cells arranged between adjacent bipolar plates of the stack or away from them. All of the above statements regarding bipolar plates can in principle also apply to the bipolar plates disclosed here or be provided for them.

[0004] The electrochemical cells can, for example, each comprise one or more membrane electrode assemblies (MEAs). The MEAs consist of several layers inside, which will not be discussed further here. The MEAs can have one or more electrically conductive gas diffusion layers (GDLs), which are usually oriented toward the bipolar plates. Due to this proximity, they are considered explicitly here. The MEAs are usually designed as electrically conductive fleece, particularly as metal or carbon fleece. The bipolar plates are usually constructed from two separator plates, one of which is adjacent to each membrane electrode assembly. Bipolar plates for electrochemical systems generally have a multitude of webs and channels arranged between the webs, which serve as the aforementioned web-channel structure or as the flow field.To improve their electrical properties, these webs can be coated with graphite, for example. Graphite can be applied to the webs as a dispersion or suspension. This is already known, for example, from DE 10 2004 009 869 A1 and DE 10 2007055 222 A1.

[0005] However, it has been shown that unexpected and, in particular, premature performance losses of the electrochemical cell(s) or the electrochemical system can occur during operation of electrochemical cells. In particular, it has been shown that the graphite-based coatings currently used do not exhibit sufficiently stable adhesion to the separator plate substrate. For example, increasingly stringent requirements are being placed on the adhesion of the coatings, even in systems with varying MEA component materials. Furthermore, longer operating times, for example, pose further risks of substances being introduced via the reagents, which can lead to premature performance losses if the coatings are unsuitable.

[0006] Based on this, the object of the present invention was to provide a metallic bipolar plate in which the risk of premature power losses can be limited.

[0007] This object is achieved by the subject matter of independent claim 1. Advantageous further developments are specified in this description, in the figures and in the dependent claims.

[0008] According to the invention, it was particularly recognized that previously occurring premature power losses can be at least partially attributable to premature loss of the previous graphite coating on the webs. For example, the graphite coating can detach prematurely from a bipolar plate, whereby a previously essentially closed surface can form capillaries; likewise or alternatively, the contact conditions between the coating and the bipolar plate substrate can change. The capillaries can contribute to substances carried in the reaction medium penetrating towards the bipolar plate substrate surface, i.e. into the interface between the coating and the substrate. The overall system, but in particular the bipolar plates (starting at an outer coating surface), can then have reduced electrical conductivity, which leads to corresponding power losses.

[0009] Accordingly, a metallic bipolar plate for an electrochemical system is proposed, wherein the electrochemical system can be designed in particular according to any of the above examples. The bipolar plate comprises two separator plates, and each of the separator plates has a flow field, wherein at least one of the flow fields has a coating on at least one surface of the associated separator plate, at least in sections, and wherein the coating comprises: - at least 50 wt% and / or at most 95 wt% of a graphite-carbon black mixture, and - at least 5% by weight and / or at most 31% by weight of one or more binders in total.

[0010] It has been shown that a coating with such a composition has improved durability and, in particular, improved resistance to premature detachment from the bipolar plate during operation of the electrochemical system. One possible explanation is that the binder(s) impede the penetration and / or infiltration of the coating by water or other substances that may arise during operation of the electrochemical system. The provision of the graphite-soot mixture further achieves the desired improvement in electrical properties, and in particular in conductivity. The addition of carbon black to graphite has proven particularly effective, as explained in more detail below. A total binder content of 15 to 28 wt.% is particularly preferred.

[0011] Each of the separator plates can have two surfaces, in particular a first surface facing the corresponding other separator plate, and a second surface facing away from the corresponding other separator plate. The first surface can form an inner side, which in particular delimits an optionally fluid-conducting and / or at least partially fluidically sealed interior of the bipolar plate. The second surface can form an outer side, which, for example, bears against an MEA of the type explained in the introduction; in particular, it can border a GDL. In a manner known per se, a flow field formed on a first of the surfaces of a separator plate, and in particular a channel-web structure encompassed thereby, can form a complementary channel-web structure on the corresponding other surface of the separator plate.The coating disclosed here can be formed in particular or exclusively on a surface of the separator plate which forms one of the outer sides of the bipolar plate.

[0012] The separator plates preferably each comprise or consist of metal. They can, for example, be stamped and / or deep-drawn parts. The coating can, for example, be applied to the separator plate initially in fluid form and solidify there. According to embodiments, the coating can be applied in several layers, with each layer being at least partially dried prior to the application of a corresponding subsequent layer. After all layers have been applied, the entire coating can be cured.

[0013] According to one embodiment, the coating comprises at least two different binders and has a single-layer structure. For example, these binders can be distributed substantially homogeneously within the coating and / or the coating can have the same composition regardless of location. However, due to demixing processes and the like, it is also possible that the at least two different binders are distributed inhomogeneously within one coating layer. In such a case, the coating can be applied and cured, in particular, as a single layer, which accordingly limits the manufacturing effort for the coated bipolar plate.

[0014] In general, both separator plates of a bipolar plate can be coated according to any of the variants described here, particularly on their respective outer sides or second surfaces. The coatings of the separator plates can be identical or different from one another.

[0015] According to one embodiment, the coating comprises at least two different binders and is constructed in at least two layers. A binder can be present in at least one of the layers and not present in one of the other layers. For example, in a two-layer coating, at least a first binder can be provided in the first layer but not in the second layer, while a second binder can be provided only in the second layer but not in the first layer. Furthermore, in this context, it can be provided that each of the layers comprises exactly one binder. Such a two-layer coating can therefore have an inhomogeneous material composition at least insofar as its layers can have different components and in particular binders.In principle, it is also possible for a first binder to be present in the first layer and in the second layer, but for different second binders to be present in both layers.

[0016] Using a two-layer coating of the type described above, the respective layers can be produced, for example, under conditions optimized for a particular binder. The provision of different binders can enable a combination of the specific advantages associated with each binder to improve the durability and, in particular, the adhesion of the coating to the separator plate.

[0017] The layers can be arranged substantially congruently to one another, whereby, for example, side surfaces or even end faces of the respective layers can be exposed. According to another embodiment, in particular, a lower layer that is in direct physical contact with the separator plate is covered by the at least one further layer on all of its initially exposed surfaces. In other words, this lower layer can be completely shielded from the environment by the at least one further layer and / or embedded in it; in this case, the further layer can also form contact surfaces with the separator plate.

[0018] According to one embodiment, one of the binders is a thermoset or an amorphous first thermoplastic, and the other binder is a second thermoplastic. In particular, the second thermoplastic can be semi-crystalline.

[0019] It has been shown that the inventive object can be reliably achieved with such a combination of different binders. One possible explanation is that by providing, in particular, the second thermoplastic, any fluid paths in the coating can be narrowed or closed to an increased extent, especially if the coating is heated to temperatures higher than the melting point of the thermoplastic before the bipolar plate is installed. As a result, water reaching the coating can migrate through it to a lesser extent and, at most, can reach an interface with the separator plate substrate to a correspondingly reduced extent. This could otherwise accelerate detachment of the coating from the separator plate.The thermoset and / or the first amorphous thermoplastic may, for example, be selected such that it / they improve adhesion to the metal and / or the adhesion between the graphite and carbon black particles, in particular to an increased extent compared to the corresponding other binder.

[0020] According to one embodiment, one of the binders comprises polyamide, PA, and the other binder polyamide-imide, PAI. In particular, the binders can comprise exclusively PA or PAI. Generally, PA or PAI can each be understood as a group of substances, so that the binders can also each comprise mixtures of different polyamides or mixtures of different polyamide-imides. Specifically, one of the binders can comprise or consist of at least one type of polyamide or a mixture of different polyamides. The corresponding other binder can in particular comprise or consist of at least one type of polyamide-imide or a mixture of different polyamide-imides. A mixture of different polyamides or a mixture of different polyamide-imides each counts as a binder in the context of the present document.

[0021] In a structure as at least two-layer coating, the uppermost or outermost layer, which comprises a large-area contact surface with the environment and in particular with any adjacent gas diffusion layer, can comprise predominantly or exclusively PA as a binder, at least in the region of this contact surface. The lower layer, which comprises a contact surface with the separator plate, can, in contrast, comprise predominantly or exclusively PAI as a binder, at least in the region of this contact surface. It has been shown that this can improve the electrical conductivity between the coating and the separator plate substrate, but also suppress detachment of the coating from the separator plate substrate, particularly during operation of the electrochemical system. However, a reverse sequence with PAI in the outermost layer and PA in the lowermost layer can also be provided.In this case, a layer containing a PA binder can be melted directly onto the separator plate substrate. In the contact area between the layers, localized mixing of the respective binders can occur, for example, due to partial dissolution of the first layer during the application of the second layer or during the melting of the entire coating.

[0022] According to one embodiment, the coating on the separator plate substrate contains at least 3 wt% and at most 30 wt% PAI. Alternatively or additionally, it contains at least 1 wt% and at most 12 wt% PA. Additional binders may, but do not have to, be provided, and / or the remaining portion of the coating can be formed by the graphite-carbon black mixture.

[0023] According to one embodiment, the coating comprises PAI as the thermoset or the amorphous first thermoplastic and PA as the second thermoplastic. The above statements specifically regarding the PAI / PA combination can also apply to other combinations of thermoset or amorphous first thermoplastic on the one hand and second thermoplastic on the other.

[0024] For example, the proportion of the binder formed from a thermoset or an amorphous first thermoplastic - for example PAI - can amount to % of the total binder content in the entire coating, while the proportion of the second thermoplastic binder - for example PA - can amount to ¼ to %.

[0025] According to one embodiment, the graphite-soot mixture has a soot-to-graphite ratio of at least 1:1 up to and including 1:25, preferably up to and including 1:20. It has been shown that this can reliably improve the electrical conductivity of the coating, for example, because the conductive soot material can adhere to the surfaces of the likewise conductive graphite in its structural voids, but without filling them.

[0026] According to one embodiment, the graphite particles of the graphite-soot mixture are platelet-shaped. Unlike platelet-shaped graphite particles, elongated fibrous carbon structures and / or very thin individual graphene layers have proven unsuitable for achieving the desired increase in electrical conductivity. The soot particles of the graphite-soot mixture are preferably substantially spherical.

[0027] According to one embodiment, the at least one flow field has a plurality of channels and webs running between the channels and in particular separating the channels, wherein the coating is applied at least to the webs and in particular at least to web crests that are encompassed by the webs or form the webs; and / or wherein at least channel bottoms of the channels are free of the coating. In this way, a defined channel geometry can be maintained in which the coating does not form any disruptive contours that impede fluid flow and in particular does not form any constrictions. In principle, a corresponding coating in the region of the channels also offers no specific advantages with regard to electrical conductivity, since these regions are generally spaced apart from an adjacent MEA and its GDL.

[0028] In this context, it can be provided in particular that predominantly or exclusively the webs and in particular their outward-facing surfaces and / or surfaces that can be brought into contact with an adjacent structure, in particular an MEA or its GDL, are coated. This can particularly relate to web crests, which can form the highest or outermost regions of the webs. Alternatively, the coating can extend from the webs and in particular from the web crests to a maximum of three-quarters of the height, up to a maximum of half the height of the flow field and preferably up to a maximum of one quarter of the height, when measured from the webs and in particular their web crests. In other words, in the latter case, it can extend to a region that is at least one quarter, at least half, or at least three-quarters of the height away from a channel floor or its lowest point.

[0029] The height of the flow field can correspond to the height of a respective web, which can, for example, consist of a web crest and two web flanks connected to it on both sides. The web flanks can merge into an adjacent channel, and in particular the channel floor, at their ends facing away from the web crest. The channel floor can form a lowest region of the flow field. The height can be measured orthogonally to a plate plane disclosed here.

[0030] The coating can, in particular, be arranged on a flow field extending across a surface of a separator plate facing away from the other separator plate of the bipolar plate. Advantageously, the coated separator plate surface is an outer surface of the bipolar plate. It is also possible for both outer surfaces of the bipolar plate to be coated in the region of their flow field. In other words, the coating is advantageously arranged in regions adjacent to a gas diffusion layer and, via this, to the MEA.

[0031] According to a further embodiment, the maximum thickness of the coating is at least 8 µm and / or at most 40 µm. For two- or multi-layer systems, an upper limit of 40 µm is advantageous; however, in individual cases, a larger total layer thickness of up to 60 µm may be preferable with regard to application. It has been shown that within this value range, the desired increase in electrical conductivity can be reliably achieved with improved coating durability, while still maintaining cost-effectiveness due to the limited maximum layer thickness. Furthermore, significantly thicker coatings can exhibit increased electrical resistance.

[0032] The substrate of the separator plates usually has a thickness of less than 100 µm, preferably less than 80 µm, in some cases even essentially only 50 µm.

[0033] According to a further embodiment, the at least one flow field on at least one surface of the associated separator plate has, at least in some regions, periodic surface structures with an average spatial period of less than 10 µm. Such surface structures can be formed according to any of the examples known from the applicant's DE 10 2021 202 214 A1. They can be accompanied by any of the advantages described therein, in particular a further increase in electrical conductivity.

[0034] In this context, the coating can be applied, in particular, to a first surface of the associated separator plate, and the periodic surface structures can be incorporated into a second surface opposite the first surface of the associated separator plate. The opposite surface can be, for example, the inner side of the bipolar plate described above or the back side of the separator plate.

[0035] In the following, exemplary embodiments of the invention are explained with reference to the accompanying schematic figures. The same reference symbols can be used for similar features across the figures. Within a given figure, only selected instances of a feature can be provided with the reference symbol associated with that feature. Fig. 1 shows a schematic perspective view of an electrochemical system comprising a plurality of bipolar plates arranged in a stack, each formed according to an embodiment of the invention; Fig. Figure 2 shows schematically in a perspective view two bipolar plates of the system according to Fig. 1 with a membrane electrode assembly (MEA) arranged between the bipolar plates; Fig. Figure 3A shows a schematic section through a region of an electrochemical system of Fig. 1 and Fig. 2; Fig. Figure 3B shows an enlarged view of section IV of Fig. 3A; Fig. Figure 4 shows a schematic cross-sectional view through a region of an electrochemical system similar to Fig. 3A; Fig. 5 shows a schematic sectional view of a coating applied to the bipolar plates according to the embodiment of the Fig. 1-4 can be used; Fig. 6 shows a schematic sectional view of a further coating which can alternatively be applied to the bipolar plates according to the embodiment of the Fig. 1-4 can be used; Fig. Figure 7 shows a schematic sectional view of yet another coating that can be applied alternatively to the bipolar plates according to the embodiment of Fig. 1-4 can be used;

[0036] Fig. 1 shows an electrochemical system 1 with a plurality of identical metallic bipolar plates 2 arranged in a stack 6 and stacked along a z-direction 7. The bipolar plates 2 are formed according to embodiments of the invention, as explained in more detail below, and in particular are coated according to the invention.

[0037] The bipolar plates 2 of the stack 6 are clamped between two end plates 3, 4. The z-direction 7 is also called the stack direction. In the present example, the system 1 is a fuel cell stack. Two adjacent bipolar plates 2 or the mutually facing separator plates 2a, 2b of each of these bipolar plates 2 of the stack 6 define an electrochemical cell, which serves, for example, to convert chemical energy into electrical energy. To form the electrochemical cells of the system 1, a membrane electrode assembly (MEA) is arranged between adjacent bipolar plates 2 of the stack (see, for example, Fig. 2). The MEA typically includes at least one membrane, e.g., an electrolyte membrane. Furthermore, a gas diffusion layer (GDL) can be arranged on one or both surfaces of the MEA, which adjoins and, in particular, touches the surface of a separator plate 2a, 2b.

[0038] In alternative embodiments, the system 1 can also be designed, for example, as an electrolyzer, an electrochemical compressor, or a redox flow battery. Bipolar plates can also be used in these electrochemical systems. The structure of these bipolar plates can then correspond to the structure of the bipolar plates 2 explained in more detail here, particularly with regard to the coatings disclosed here. This applies even though the media conducted on or through the bipolar plates in an electrolyzer, an electrochemical compressor, or a redox flow battery can differ from the media used for a fuel cell system.

[0039] The z-axis 7, together with an x-axis 8 and a y-axis 9, spans a right-handed Cartesian coordinate system. The bipolar plates 2 each define a plate plane, wherein the plate planes of the separator plates 2a, 2b of the bipolar plates 2 are each aligned parallel to the xy-plane and thus perpendicular to the stacking direction or to the z-axis 7. The end plate 4 has a plurality of media connections 5 via which media can be supplied to the system 1 and via which media can be removed from the system 1. These media that can be supplied to the system 1 and removed from the system 1 can include, for example, fuels such as molecular hydrogen or methanol, reaction gases such as air or oxygen, reaction products such as water vapor or depleted fuels, or coolants such as water and / or glycol.

[0040] Fig. 2 shows in perspective two adjacent bipolar plates 2 of the electrochemical system of Fig. 1 and a membrane electrode assembly 10 arranged between these adjacent bipolar plates 2 and known from the prior art, wherein the MEA 10 in Fig. 2 is largely covered by the bipolar plate 2 facing the viewer and is only shown schematically, here in the form of its circumferential reinforcing edge 15. The bipolar plate 2 is formed from two materially joined separator plates 2a, 2b, of which in Fig. 2, only the first separator plate 2a facing the viewer is visible, concealing the second separator plate 2b. The separator plates 2a, 2b can each be made of a metal sheet, e.g., a stainless steel sheet. The separator plates 2a, 2b can be welded together, e.g., along and spaced from their edges, e.g., by laser welding.

[0041] In an electrochemically active region 18, the separator plates 2a, 2b have a flow field 17 with structures for guiding a reaction medium along the outer side of the respective separator plate 2a, 2b. These structures are shown in Fig. 2 by a plurality of webs 21 and channels 22 running between the webs 21 and delimited by the webs 21.

[0042] The separator plates 2a, 2b have through-openings aligned with one another, which form the through-openings 11a-c of the bipolar plate 2. When stacking a plurality of bipolar plates of the type of bipolar plate 2, the through-openings 11a-c form lines that extend in the stacking direction (or z-direction 7) through the stack 6 (see Fig. 1). Typically, each of the lines formed by the through-openings 11a-c is in fluid communication with one of the media connections 5 in the end plate 4 of the system 1.

[0043] The separator plates 2a, 2b also each have a distribution or collection area 20. The distribution or collection area 20 comprises structures that are configured to distribute a medium introduced into the distribution or collection area 20 from a first of the two through-openings 11b over the active area 18 and / or to collect or bundle a medium flowing from the active area 18 to the second of the through-openings 11b. The distribution structures of the distribution or collection area 20 are Fig. 2 are also provided by webs and channels extending between the webs and delimited by the webs. In general, elements 17, 18, 20, 21, 22 can therefore be considered media-conducting embossed structures formed integrally with the separator plates 2a, 2b.

[0044] To seal the through-openings 11a-c from the interior of the stack 6 and from the environment, the first separator plates 2a usually each have sealing arrangements in the form of sealing beads 12a-c, which are each arranged around the through-openings 11a-c and which completely enclose the through-openings 11a-c. The second separator plates 2b have, on the side visible from the viewer of the Fig. 2 opposite the rear side of the bipolar plates 2 corresponding sealing beads for sealing the through openings 11a-c (not shown).

[0045] The sealing beads 11a-c each have passages 13a-c for fluid passage. For example, coolant can be introduced into the stack 6 or discharged from the stack 6 via the passages 13a, in particular into or from the intermediate space 19 between the separator plates 2a, 2b, via the lines formed by the passages 11a. The lines formed by the passages 11b, 11c, on the other hand, can be configured to supply the electrochemical cells of the fuel cell stack 6 of the system 1 with fuel and reaction gas via the passages 13b, 13c, as well as to discharge the reaction products from the stack. The media-carrying passages 11a-c are configured essentially parallel to the plate plane.

[0046] The first separator plates 2a typically each further comprise a further sealing bead 12d in the form of a perimeter bead, which surrounds the flow field 17 of the active region 18, the distribution or collection region 20, and the through-openings 11b, 11c and seals them from the through-opening 11a, i.e., from the coolant circuit, and from the environment of the system 1. The second separator plates 2b each comprise corresponding perimeter beadings.

[0047] Fig. 3A and Fig. 3B each schematically show a region through a section of the electrochemical system 1 from Fig. 1 and Fig. 2, wherein the cutting plane is aligned in the z-direction and thus perpendicular to the plate planes of the bipolar plates 2. Furthermore, the cutting plane runs through or in the area of ​​the flow fields 17 of the bipolar plate 2.

[0048] In the Fig. 3A and Fig. 3B again shows the previously described first metallic separator plate 2a and the previously described second metallic separator plate 2b, which are joined together to form a respective bipolar plate 2. Each metallic separator plate 2a, 2b can have a metallic substrate thickness of approximately 75 µm. The structures for media conduction along the outer surfaces and in the flow field 17 of the bipolar plates 2 can be seen in the form of the webs 21 and the channels 22 delimited by the webs 21. The two separator plates 2a, 2b rest on one another in a contact area 23 and can be connected to one another there, for example by means of laser welds. The metallic separator plates 2a, 2b can be formed, for example, from stainless steel and / or a titanium alloy, for example from a stainless steel core with at least one surface made of a titanium alloy.

[0049] In the Fig. The section shown in Figure 3A shows a central bipolar plate 2 arranged between two membrane electrode assemblies 10. The MEA 10 typically comprises a membrane 14, e.g., an electrolyte membrane, and a fluid-sealing edge region 15 connected to the membrane, as well as additional layers. For example, the fluid-sealing edge region 15 can be integrally bonded to the membrane 14 and, if appropriate, additional layers, e.g., by an adhesive bond or by lamination.

[0050] The membrane 14 of the MEA 10 extends at least over or along the active region(s) 18 of the adjacent bipolar plates 2 and enables proton transfer over or through the membrane 14. The membrane 14 does not extend into the distribution or collection region 20. The edge region 15 of the MEA 10 serves to position, secure, and seal the membrane 14 between the adjacent bipolar plates 2.

[0051] Furthermore, additional gas diffusion layers 16 of the MEA 10 can be arranged in the active region 18. The gas diffusion layers 16 enable the flow to the membrane 14 over the largest possible area of ​​the surface of the membrane 14 and can thus improve the proton transfer across the membrane 14. The gas diffusion layers 16 can, for example, be arranged on both sides of the membrane 14 in the active region 18 between the adjacent separator plates 2. The gas diffusion layers 16 can, for example, be formed from an electrically conductive nonwoven or comprise an electrically conductive nonwoven. The electrically conductive nonwoven is, in particular, a metal nonwoven or a carbon nonwoven.

[0052] In Fig. Figure 3B is an enlarged view of section IV of Fig. 3A. This comprises one of the webs 21 as well as parts of the adjacent channels 22 of the respective separator plates 2a, 2b forming the middle bipolar plate 2. The reference numerals are primarily entered for the upper separator plate 2a, with the lower separator plate 2b being of essentially the same design. Likewise, the webs 21 and channels 22 of a respective bipolar plate 2 within the flow field 17 are essentially of the same design, so that the web 21 and the channels 22 of Fig. 4 are accordingly representative of the entire flow field 17.

[0053] It can be seen that the bridges 21 are made of Fig. 3B, each web 21 has a web crest 24 extending along the main direction of extension of the respective web 21. The main direction of extension runs perpendicular to the plane of the sheet. Furthermore, the webs 21 have a first and a second web flank (or side wall) 25a and 25b, each of which adjoins the web crest 24 and also extends along the main direction of extension.

[0054] The Fig. 3A and 3B have no curvature (or are flat), but can alternatively have a curvature with a radius of, for example, at least 0.5 mm, preferably at least 1 mm, as will be explained below in Fig. 4 is shown.

[0055] Preferably, the web flanks 25a and 25b of a respective web 21 each have a minimum angle to the zero plane in the range of 15° to 75°, preferably 25° to 65°. The zero plane is understood to be the main extension plane and / or plate plane of the metallic separator plates 2a and 2b, respectively.

[0056] Preferably, the web flanks 25a and 25b of a respective web 21 are connected tangentially to the web tip 24 of the respective web 21 with a radius which is at least 0.05 mm, preferably at least 0.15 mm, wherein this radius is shown in the schematic representation of the Fig. 3A and Fig. 3B is not visible. This radius can be considered part of the web flank 25a, 25b.

[0057] Preferably, the web flanks 25a and 25b of a respective web 21 are each connected tangentially to the bottom 22a of the channel 22 adjacent to the respective web flank 25a, 25b or delimited by these with a radius which is at least 0.05 mm, preferably at least 0.15 mm, wherein this radius in the schematic representation of the Fig. 3A and Fig. 3B is not visible. This radius can also be considered part of the web flank 25a, 25b.

[0058] A height axis H of a web 21 and the flow field 17 is in Fig. 3B and runs orthogonal to the zero plane or main extension plane of the metallic separator plates 2a, 2b and thus parallel to the z-axis. The height of a respective web 21 can be measured along the height axis H, starting from the bottom 22a of the channel 22 to the web crest 24. The channel bottom 22a and the web flank 25a, 25b define the channel 22, which can guide a fluid flow along the flow field 17. At least the web crest 24 can form a structure separating two channels 22 and / or can be understood as a correspondingly separating web. Alternatively, the flanks 25a, 25b or at least a height section thereof can be understood and referred to as components of the channel 22, e.g., as channel flanks. Likewise, the web crest 24 could be understood and referred to as a web.

[0059] At least the web tips 24 are each provided with a coating 26, which can be designed according to any variants disclosed here, see in particular the following discussion of the Fig. 5-7. In the example shown, only the web crests 24 are provided with the coating 26. A targeted or process-related coating of the web flanks 25a, 25b, however, can be omitted or is at least limited to one of the previously disclosed regions and, in particular, to one of the previously disclosed height levels.

[0060] Opposite the channel bottoms 22a, i.e., on an inner side of the separator plate 2a, an exemplary region 27 is marked in which an optional surface structuring of the type mentioned above can be formed. Such a surface structuring can also be present in the case of the lower separator plate 2b.

[0061] The illustration shows that the coating 26 is in direct contact with the adjacent GDL 16. This results in a defined interaction between the coating 26 and the fibers of the gas diffusion layer 16, particularly in the form of establishing an electrically conductive connection. Furthermore, it is possible for sections of the gas diffusion layer 16 to penetrate into the coating 26.

[0062] Fig. Figure 4 shows schematically an area through a section of an electrochemical system 1 comparable Fig. 1 and Fig. 2, wherein the cutting plane is aligned in the z-direction and thus perpendicular to the plate planes of the bipolar plates 2. Furthermore, the cutting plane runs through or in the area of ​​the flow fields 17 of the bipolar plate 2. Unlike in the embodiment of the Fig. 3A and Fig. 3B, the web tips 21 are not flat, but have a slight curvature, which nevertheless allows good contact between the coating 26 and the GDL 16. In Fig. 4, the transition radii between the web flanks 25a, 25b and the web crests 24 or the bottom 22a of the channels 22 are also clearly visible.

[0063] Based on the Fig. 5-7, coatings 26 according to embodiments of the invention are explained, which, in addition to graphite and carbon black, also comprise at least one binder to improve the resistance of the coatings 26. The Fig. 5-7 show analogous to the view of Fig. 3B sections of a bipolar plate 2 and more precisely of a single web tip 24, whereby all further web tips 24 within the flow field 17 can also be designed analogously.

[0064] The coating 26 is applied with a thickness D and essentially completely covers the web tip 24, and more specifically, its outer side. The thickness D can have any values ​​disclosed in this document.

[0065] In the example of Fig. 5, the coating 26 comprises a graphite-soot mixture and at least one binder distributed therein, for example, one or two binders. The proportions of these components can be selected according to any of the examples disclosed herein. The binder does not comprise graphite or soot. The at least one binder is formed according to one of the examples disclosed herein, for example, comprising PA or PAI. Preferably, a binder contains only a single material and not a material mixture, or only a mixture of the materials of a single substance group, for example, a mixture of different types of PA (for example, a mixture of PA 6 and PA 12) or of different types of PAI. The coating 26 can be applied according to any of the prior art examples, for example, by roller printing or by spraying. It can dry and / or cure in the applied state.

[0066] It has been shown that adding the binder(s) can improve the adhesion of the coating 26 to the web tip 24 and prevent or at least significantly limit the formation of capillaries within the coating 26. In particular, this can reduce the risk of water and / or other substances penetrating an interface or contact surface between the coating 26 and the web tip 24 of the separator plate substrate.

[0067] Fig. 6 shows a further exemplary embodiment in which the coating 26 is constructed in two layers. A first layer 30 forms an outermost layer, which can, for example, come into contact with a GDL (not shown). The first layer 30 is not in direct physical contact with the web tip 24 of the separator plate substrate. A second layer 32 is located between the first layer 30 and the web tip 24 of the separator plate substrate and is in direct physical contact with the latter. The coating 26 is accordingly applied in two layers, with the second layer 32 first being applied to the web tip 24 of the separator plate substrate and there being at least intermediately dried and / or at least partially cured. The first layer 30 is then applied to the second layer 32, whereupon the entire coating 26 can be heated, whereby the thermoplastic binder(s) can be melted.a thermosetting binder can be cured.

[0068] The first layer 30 and the second layer 32 differ in their material composition. In particular, they each comprise a binder that is not present in the corresponding other layer 30, 32. For example, the first layer 30 comprises PA but not PAI, and the second layer comprises PAI but not PA. The PA or PAI material can also be provided as a respective mixture of different PA or PAI types.

[0069] Even after melting or curing, the first and second layers 30, 32 can be distinguishable as such, even if they at least partially merge into one another and / or may be mixed with one another at their interface. For example, in an interface to a GDL (not shown), even after melting or curing, PA may be predominantly or exclusively present as a binder, and in the interface to the metallic web tip 24, PAI may be predominantly or exclusively present as a binder. It has been shown that the electrical conductivity of the coating 26 and separator plate 2a, 2b can be particularly effectively improved by the internal PAI, whereas the external PA can effectively prevent the penetration of water or other substances and thus the premature detachment of the coating 26.

[0070] It should be noted that a reversed arrangement of predominantly PAI in the outer layer 30 and predominantly PA in the inner layer 32 can also be provided. In such a case, for example, the adhesion of the coating 26 to the metallic web tip 24 can be specifically increased by the PA.

[0071] In an alternative embodiment, the first layer 30 may comprise a first binder mixture in addition to a graphite-carbon black mixture, and the second layer 32 may comprise a second binder mixture. For example, the first layer 30 may comprise a first ratio of PA and PAI, and the second layer may comprise a different second ratio of PA and PAI.

[0072] In the example from Fig. 6, the layers 30, 32 are arranged essentially congruently one above the other. Fig. 5 Left- and right-facing end faces, which, for example, extend substantially orthogonally to a plane of the web crest 24 and / or along the thickness dimension D, are each exposed. Thus, water can come into direct contact with at least these end faces of the lower second layer 32.

[0073] The example from Fig. 7 is in principle analogous to that of Fig. 6, with the only difference that the first layer 30 completely encloses the outer surfaces of the second layer 32 that are initially exposed after its application. This also includes the Fig.6 exposed end faces. In other words, the second layer 32 is completely shielded from the environment by the first layer 30 in cooperation with the web tip 24 and / or embedded in the first layer 30. This deliberately suppresses contact of this second layer 32 with water and / or other substances in order to further reduce the risk of detachment of the coating 26. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2004 009 869 A1

[0004] DE 10 2007055 222 A1

[0004] DE 10 2021 202 214 A1

[0033]

Claims

[1] Metallic bipolar plate (2) for an electrochemical system, wherein the bipolar plate (2) comprises two separator plates (2a, 2b) and each of the separator plates (2a, 2b) has a flow field (17), wherein at least one of the flow fields (17) has a coating (26) on at least one surface of the associated separator plate (2a, 2b) at least in sections, wherein the coating (26) comprises: - at least 50% by weight and / or at most 95% by weight of a graphite-carbon black mixture, and - at least 5% by weight and / or at most 31% by weight of one or more binders in total. [2] Bipolar plate (2) according to claim 1, wherein the coating (26) comprises at least two different binders and is constructed in a single layer. [3] Bipolar plate (2) according to claim 1, wherein the coating (26) comprises at least two different binders and is constructed in at least two layers; and wherein one binder is present in at least one of the layers (30) and is not present in one of the corresponding other layers (32). [4] Bipolar plate (2) according to claim 3, wherein each of the layers (30, 32) comprises exactly one binder. [5] Bipolar plate (2) according to one of claims 2 to 4, wherein one of the binders is a thermoset or an amorphous first thermoplastic and wherein the other binder is a second thermoplastic. [6] Bipolar plate (2) according to claim 5, wherein the second thermoplastic is semi-crystalline. [7] Bipolar plate (2) according to claim 5 or 6, wherein one of the binders comprises polyamide, PA, and the other binder comprises polyamide-imide, PAI. [8] Bipolar plate (2) according to claim 7, wherein the coating (26) contains at least 3 wt% and at most 30 wt% PAI and / or at least 1 wt% and at most 12 wt% PA. [9] Bipolar plate (2) according to claim 7 or 8, wherein the coating (26) comprises PAI as the thermoset or the amorphous first thermoplastic and PA as the second thermoplastic. [10] Bipolar plate (2) according to one of the preceding claims, wherein the graphite-soot mixture has a soot-graphite ratio of at least 1:1 up to and including 1:25, preferably up to and including 1:

20. [11] Bipolar plate (2) according to one of the preceding claims, wherein the graphite particles of the graphite-soot mixture are platelet-shaped. [12] Bipolar plate (2) according to one of the preceding claims, wherein the at least one flow field (17) has a plurality of channels (22) and webs (21) extending between the channels (22), wherein the coating (26) is applied at least to the webs (21); and / or wherein at least channel bottoms (22a) of the channels (22) are free of the coating (26). [13] Bipolar plate (2) according to claim 12, wherein the at least one flow field (17) is arranged on a surface of a separator plate (2a, 2b) facing away from the other separator plate (2b, 2a) of the bipolar plate (2). [14] Bipolar plate (2) according to one of the preceding claims, wherein the maximum thickness (D) of the coating (26) is at least 8 µm and / or at most 40 µm. [15] Bipolar plate (2) according to one of the preceding claims, wherein the at least one flow field (17) on at least one surface of the associated separator plate (2a, 2b) has, at least in some regions, periodic surface structures with an average spatial period of less than 10 µm. [16] Bipolar plate (2) according to claim 15, wherein the coating (26) is applied to a first surface of the associated separator plate (2a, 2b) and the periodic surface structures are introduced into a second surface opposite the first surface of the associated separator plate (2a, 2b).

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