Sealing arrangement, plate assembly, electrochemical system, and method for producing a sealing arrangement

The present disclosure relates to a sealing arrangement, comprising: an elastomeric sealing element, which comprises a foamed material containing microspheres, and a metal layer having a surface structuring, the surface structuring comprising a plurality of depressions, wherein the sealing element is configured as a coating of the metal layer and is arranged at least in some areas on the surface structuring, wherein a concentration of the microspheres in the sealing element, measured perpendicular to the surface of the metal layer, is inhomogeneous. The disclosure additionally relates to a plate assembly, to an electrochemical system, and to a method for producing the sealing arrangement.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to German Patent Application Serial No. 10 2020 205 902.0, entitled “SEALING ARRANGEMENT, PLATE ASSEMBLY, ELECTROCHEMICAL SYSTEM, AND METHOD FOR PRODUCING A SEALING ARRANGEMENT,” and filed on May 11, 2020. The entire contents of the above-listed application are hereby incorporated by reference for all purposes.

TECHNICAL FIELD

The present disclosure relates to a sealing arrangement, which comprises an elastomeric sealing element and a metal layer. The disclosure further relates to a plate assembly for an electrochemical system, and to such an electrochemical system comprising such a sealing arrangement. The disclosure additionally relates to a method for producing the sealing arrangement.

BACKGROUND AND SUMMARY

Known electrochemical systems are, for example, fuel cell systems, electrochemical compressor systems or electrolysers. Known electrolysers are configured for example in such a way that, by applying a potential besides producing hydrogen and oxygen from water, these gases will simultaneously be compressed under pressure. Also known in addition are electrochemical compressor systems, such as electrochemical hydrogen compressors for example, to which gaseous molecular hydrogen is fed and in which the latter is electrochemically compressed by applying a potential. This electrochemical compression is suitable for small quantities of hydrogen to be compressed, since a mechanical compression of the hydrogen would be much more laborious here.

Also known are electrochemical systems comprising a stack of electrochemical cells, each separated from one another by bipolar plates. Such bipolar plates may serve for example for indirectly electrically contacting the electrodes of the individual electrochemical cells (for example fuel cells) and/or for indirectly electrically connecting adjacent cells (series connection of the cells). The bipolar plates may also have a channel structure or may form a channel structure, which is configured to supply the cells with one or more media and/or to remove reaction products. The media may be fuels (for example hydrogen or methanol), reaction gases (for example air or oxygen) or coolants. Such a channel structure is usually arranged in an electrochemically active region (flow field) as well as in the distribution and collection regions leading to and away from the latter. Furthermore, the bipolar plates may be configured to transmit the waste heat that arises when converting electrical and/or chemical energy in the electrochemical cell, and also to seal the various media channels, including the cooling channels, with respect to one another and/or with respect to outside. By way of example, the bipolar plates may have openings, through which the media to be fed and/or the reaction products can be routed towards or away from the electrochemical cells arranged between adjacent bipolar plates of the stack.

The electrochemical cells may for example each comprise one or more membrane electrode assemblies (MEAs). The MEAs may have one or more electrically conductive gas diffusion layers, which are usually oriented towards the bipolar plates and are configured for example as an electrically conductive fleece, such as a metal or carbon fleece. The membrane electrode assemblies usually have a frame-like seal at their outer edge, said seal being formed in some embodiments of polymer-based material, such as polymer-based films.

The sealing between the bipolar plates and the membrane electrode assembly usually takes place outside of the electrochemically active region and usually comprises both at least one port seal and one outer seal. The bipolar plates are usually composed of two separator plates, each of which adjoins a membrane electrode assembly. The separator plates may have seals for sealing with respect to the membrane electrode assembly, for example with respect to the frame-like seal of the membrane electrode assembly; if the separator plates are configured as metal plates, for example made of stainless steel, said seals may be integrally formed as sealing beads in the separator plates, for example by embossing, deep drawing or hydroforming. To improve the micro-sealing, such sealing beads usually have polymer-based sealing coatings on at least one side.

In some applications, the sealing device is designed to perform its function equally reliably in a temperature range between a minimum temperature of for example −40° C. and a maximum temperature of for example +100° C. Such temperature changes may occur when starting to operate a fuel cell system at ambient temperature or during a cold start in winter at minus degrees towards the maximum operating temperature of the stack. As already mentioned, the frame-like sealing element of the membrane electrode assembly is usually made of polymer-based material, and the separator plates of the bipolar plate may be made of a metal material. These materials have different thermal expansion coefficients. In the case of a fast cold start, the separator plate and the membrane electrode assembly may expand differently, and the frame-like sealing element may be displaced with respect to the sealing beads. This may lead to a detachment of a coating that may be present on the sealing bead. Usually, the sealing coating on the frame-like sealing element does not slide off, but rather the polymer-based sealing coating comes to adhere at least temporarily to the polymer-based frame-like sealing element. The effects of detachment and adhesion of the coating become obvious when the stack is disassembled, with the coating being pulled off on account of the previous detachment from the bipolar plate.

To further improve the micro-sealing, metal sealing beads are often provided with a microsphere-containing elastomeric coating at least in the area of the beads surrounding the through-openings. Such coatings have an approximately regular distribution of the microspheres in the coating mass, so that a bond between the elastomer and the metal surface does not exist across the entire surface of the coating facing towards the metal surface. Such microsphere-containing elastomeric coatings may therefore detach easily over time, which may lead to leaks in the respective system. It would be desirable if the adhesion of these coatings can be improved.

The object of the present disclosure was to provide a sealing arrangement, a plate assembly and an electrochemical system which at least partially solve the problems mentioned above. In addition, it would be advantageous to develop a method for producing a sealing arrangement.

This object is achieved by the sealing arrangement, the plate assembly and the electrochemical system as well as the method for producing a sealing arrangement according to the independent claims. Further developments form the subject matter of the dependent claims and of the description below.

According to one aspect of the present disclosure, a sealing arrangement is provided. The sealing arrangement comprises:an elastomeric sealing element, which comprises a foamed material containing microspheres, anda metal layer having a surface structuring, the surface structuring comprising a plurality of depressions.

The sealing element is configured as a coating of the metal layer and is arranged at least in some areas on the surface structuring. A concentration of the microspheres in the sealing element, measured perpendicular to the surface of the metal layer, is inhomogeneous. Due to the inhomogeneous concentration of the microspheres, the sealing element may have an improved adhesion to the metal layer.

The inhomogeneous concentration should be understood here to mean not that only the sizes of the microspheres are different, but may also mean that the volume content of the gas in the polymer matrix is different, that is to say the sum of the volumes of the microspheres relative to a volume of the coating mass consisting of polymer matrix and microspheres.

It may be provided that the sealing element has a first interface adjoining the metal layer, and the concentration of the microspheres in the sealing element is lower within the first interface than outside of the first interface. Given suitable temperature control of the coating, crosslinking and/or expansion process, the sealing element may have a second interface directed away from the metal layer, wherein the concentration of the microspheres in the sealing element is lower within the first and second interface than outside of the first and second interface. The first and the optionally present second interface may individually or together have a thickness of no more than 35%, no more than 30%, or no more than 23% relative to a total maximum layer thickness of the sealing element measured perpendicular to the surface of the metal layer. If only a first interface has a lower concentration of microspheres, the thickness proportion thereof in the maximum layer thickness of the sealing element may also be only at most 18%.

Conversely, the concentration of the polymer-based coating material, that is to say the polymer matrix minus the microspheres, may also be inhomogeneous. The weight content of the polymer matrix may be greater in the first and optionally also the second interface than outside of the first and optionally the second interface. This can be checked for example by a measurement using terahertz radiation or computed tomography.

One possible explanation for the improved adhesion is that the surface of the metal layer that is provided with the surface structuring is usually rougher than the untreated surface. The microspheres may have a lower affinity for the rougher surface than the elastomeric base material, as a result of which the elastomeric base material adheres better to the metal layer in the area of the surface structuring than to smooth, unstructured metal layers. Overall, an increase in the elastomer content and/or polymer matrix content in the interface to the metal layer can be brought about by the surface structuring. Due to the lower concentration of microspheres in the interface, the adhesion of the sealing element to the metal layer can be improved in this area. This greatly improved adhesion of the sealing element to the metal layer has been confirmed in various tests, in which the sealing arrangement proposed in this application was compared with similar sealing arrangements without surface structuring. To this end, the sealing arrangements to be compared were drizzled with organic solvents and then mechanically stressed. It was found in these tests that the sealing arrangement according to the disclosure has an adhesion that is up to ten times stronger.

The sealing arrangement may have an elastically deformable bead integrally formed in the metal layer, said bead sometimes also being referred to as a sealing bead. The surface structuring may be formed on just one surface or on both surfaces of the bead. The sealing element may be arranged at least in some areas on the bead. The bead may have for example a bead top and at least one bead flank adjoining the bead top, or else only a curved bead top with no explicit bead flanks. The surface structuring may extend at least in some areas over the bead top and/or the at least one bead flank. However, the surface structuring may also be extended laterally beyond the area of the bead flank. It may be provided that the surface structuring covers the surface of the bead on at least one side of the metal layer at least in some areas or completely. The bead may be provided at least in some areas or completely with a surface structuring on at least the side of the metal layer that faces towards the component to be sealed, that is to say a sealing edge region of a membrane electrode assembly. The surface structuring and the sealing element may therefore be formed on the convex, outwardly curved surface of the bead. The bead usually has a transverse direction which points from one bead flank towards the bead top and/or towards the optionally present second bead flank or extends along the curvature of the curved bead top. The longitudinal direction of the bead usually corresponds to the direction of extension thereof, that is to say for example around a port in a separator plate.

In the sealing arrangement, a mean diameter of the microspheres may be at least 20 μm, at least 30 μm and/or at most 80 μm, or at most 60 μm. The microspheres are usually filled with a gaseous medium, which may be an expanding agent, such as a gaseous hydrocarbon, which may be a saturated hydrocarbon such as n-pentane, isopentane or isobutane. The gaseous medium is usually enclosed in the microspheres. The compressible gaseous medium in the microspheres may increase the elasticity of the sealing element and thus improve the sealing function of the sealing element.

In some embodiments, the microspheres have a mean diameter which is larger than a width of the depressions. In some embodiments, the microspheres have a mean diameter which is smaller than a width of the depressions. In this case, a further mechanism for better adhesion may be proposed. Due to the fact that the mean diameter of the microspheres is larger than a lateral extent of the depressions, the microspheres cannot accumulate or can only partially accumulate in the area of the interface. On the whole, therefore, the concentration of the microspheres is lower in the interface than outside of the interface.

The layer thickness of the sealing element of the sealing arrangement may for example be in total at most 500 μm, at most 300 μm, at most 200 μm, or at most 150 μm. In some embodiments, the metal layer is provided over its entire surface or in part with the sealing element configured as a coating. Various elastomers are considered for the elastomeric sealing element. By way of example, the sealing element contains FPM (fluoroelastomer), silicone rubber or NBR rubber (nitrile butadiene rubber), PUR (polyurethane), NR (natural rubber), FFKM (perfluoro rubber), SBR (styrene butadiene rubber), BR (butyl rubber), FVSQ (fluorosilicone), CSM (chloro-sulphonated polyethylene), silicone resin, epoxy resin, or mixtures of the aforementioned substances, or pressure sensitive adhesive and/or physically binding adhesive. The sealing element may also contain layers which each comprise one or more of the aforementioned materials.

The depressions may have a width and/or a diameter, which may be measured at the mid-height of the depressions and/or parallel to the untreated and/or unstructured metal surface, of at most 150 μm, at most 100 μm, or at most 70 μm. The depressions may have a depth, which may be measured from the untreated surface of the metal layer in the area around the depression to the lowest point of the depression, of at least 2 μm and/or at most 40 μm. The depressions may have a depth of no more than 20% of the thickness of the metal layer. This ensures that the metal layer has no material weakenings that lead to leaks or breaks in the material during operation.

To achieve a sufficient effect of the surface treatment, an embodiment of maximum spacing of adjacent depressions may be at most five times the diameter of a depression. On the other hand, the depressions may not be arranged too close to one another since this could lead to destruction of the material. Adjacent depressions may have a minimum spacing which corresponds to half the diameter of the depressions. The spacing between adjacent depressions may be different in the longitudinal direction and in the transverse direction of the sealing element. In the transverse direction, the minimum spacing may correspond to at least half the diameter. Furthermore, in the transverse direction, the maximum spacing may correspond to at most three times the diameter. In the longitudinal direction, the minimum spacing may correspond to at least the diameter of a depression. In the longitudinal direction, the maximum spacing may be five times the diameter. If such a sealing element is combined with a sealing bead, the longitudinal and transverse direction of the sealing element correspond respectively to the longitudinal and transverse direction of the sealing bead.

A circumferential edge of the depressions may for example be substantially round, elliptical or oval. The shape of the circumferential edge may depend on the respective position of the depressions on the metal layer. By way of example, the circumferential edge of the depressions arranged on the bead top may differ from the circumferential edge of the depressions arranged on the at least one bead flank (see below).

The depressions may be created by means of laser radiation. In other embodiments, the depressions are created mechanically, for example by engraving or scoring the metal layer. The depressions may be at least partially surrounded by elevations. The elevations may be formed at the respective circumferential edge of the depressions. The respective elevation may protrude above the untreated surface of the metal layer. Said elevation may be configured for example as a crater edge. The crater edge is typically formed when forming the depressions, for example by laser radiation, the crater edge being formed by solidification of molten material. The crater edge or the elevation may thus be formed of the material that has migrated from the depressions.

The metal layer may have further embossed structures, which are created for example by means of deep drawing, embossing and/or hydroforming. These further embossed structures may comprise, besides the sealing beads already mentioned above, structures for guiding media along the metal layer, such as a flow field and/or channel structures. The sealing element may surround the embossed structures, for example in an intrinsically closed manner. The sealing arrangement may have for example a perimeter bead, that is to say a sealing bead which extends around the flow field and seals off the latter with respect to the surrounding area of the metal layer and has the aforementioned surface structuring at least in some portions and is covered by an elastomeric sealing element.

In certain embodiments, the sealing arrangement may surround, in an intrinsically closed manner, at least one through-opening for gas or liquid which is formed in the metal layer. In some embodiments, said bead surrounds the through-opening in an intrinsically closed manner. By way of example, the sealing arrangement may have a port bead, that is to say a sealing bead which seals off the through-opening for media.

The metal layer is often configured as a separator plate or part of a bipolar plate for an electrochemical system.

According to a further aspect of the present disclosure, a plate assembly for an electrochemical system is proposed. This comprises a sealing arrangement having an elastomeric sealing element, which comprises a foamed material containing microspheres. The system further comprises two separator plates configured as metal layers, which like the metal layers mentioned above are provided with a surface structuring at least in some areas. As in the sealing arrangements mentioned above, the surface structuring comprises a plurality of depressions. The plate assembly further comprises a membrane electrode assembly arranged between the separator plates. The sealing element is configured as a coating on at least one side, or on precisely one side, of each of the separator plates and is arranged at least in some areas on the surface structuring. The sealing arrangement may be arranged on the surface of the separator plates facing towards the membrane electrode assembly. As in the sealing arrangements mentioned above, a concentration of the microspheres in the sealing element, measured perpendicular to the surface of the metal layer, is inhomogeneous.

Each plate assembly may comprise only precisely two separator plates, which are each separated from one another by the membrane electrode assembly. Each separator plate may be a constituent part of a bipolar plate, the bipolar plate typically comprising two separator plates bearing against one another. One bipolar plate thus may belong to two different plate assemblies.

According to a further aspect of the present disclosure, an electrochemical system is proposed. The electrochemical system comprises for example a plurality of plate assemblies of the type described above. As an alternative or in addition, the electrochemical system comprises a plurality of sealing arrangements of the type described above.

The stack comprising the bipolar plates and the electrochemical cells, that is to say the electrochemical system, is usually terminated by an end plate at each end of the stack. At least one of the end plates typically has one or more ports. Lines for feeding the media and/or for removing the reaction products can be connected to said ports. In addition, at least one of the end plates usually has electrical connections, via which the cell stack can be electrically connected to a consumer or to a voltage source. Correspondingly, the electrochemical system may have end plates.

Also proposed by the disclosure is a method for producing a sealing arrangement of the type described above. The method comprises at least the following steps:providing a metal layer,providing the metal layer with a surface structuring, the surface structuring comprising a plurality of depressions,applying a foamable material containing expandable microspheres to the surface structuring,forming an elastomeric sealing element on the metal layer by expanding the microspheres, wherein a distribution of the expanded microspheres in the sealing element, measured perpendicular to the surface of the metal layer, is inhomogeneous.

Once again, the inhomogeneous concentration should be understood here to mean not that only the sizes of the microspheres are different, but rather that the volume content of the gas in the polymer matrix is different, that is to say the sum of the volumes of the microspheres relative to a volume of the coating mass consisting of polymer matrix and microspheres. Conversely, the concentration of the polymer-based coating material, that is to say the polymer matrix minus the microspheres, may also be inhomogeneous here.

The microspheres may have a mean diameter of at least 5 and/or at most 50 μm in the non-expanded state. In addition, the microspheres may have a mean diameter of at least 20 and/or at most 80 μm in the expanded state. This value may depend for example on a crosslinking rate and/or a degree of crosslinking of the elastomer. The more rapid the crosslinking of the elastomer, the lower the degree of expansion of the microspheres.

By expanding the microspheres, the maximum thickness of the sealing element applied as a layer typically expands to a multiple. By way of example, the finished sealing element has a maximum layer thickness which is at least two times, at least three times, or at least four times the layer thickness of the foamable material.

In one embodiment, the depressions of the surface structuring are created by laser radiation. For example, a pulsed laser can be used for this. The depressions are then formed by a melting of the material of the metal layer. Due to the heat generated as a result of the laser irradiation, very fine particles of metal or metal compounds are detached from the surface and are vaporized. It appears that, in the area of the depressions, a change in the physical surface structure at least partially takes place in the material of the metal layer (for example an electronic and/or geometric structural change). In some cases, a chemical change in the material of the metal layer also occurs (for example altered chemical composition). If, for example, the metal layer is made of stainless steel, the chromium/iron ratio or the oxygen content may be different in the area of the depressions, that is to say in the surface of a depression and/or in the surface in the immediate vicinity of a depression, that is to say in the area close to the surface of the crater edge, than in the region of the unstructured, untreated surface. The surface treatment by means of laser radiation is advantageously carried out in such a way that the mechanical properties of the material of the metal layer are not damaged and the metal layer is not undesirably weakened.

According to one alternative variant of the method, the depressions are created mechanically, for example by engraving to produce a microstructure. In this case, the depressions may also be formed by scoring the metal layer.

In a further optional method step, a sealing bead is integrally formed in the at least one metal layer, which may be done by embossing, by deep drawing and/or by hydraulic methods. This may take place before, at the same time as or after the surface structuring. If the surface structuring is formed by engraving, a simultaneous embossing of the sealing bead may take place. If the surface structuring takes place by engraving to produce a microstructure or by laser irradiation, the sealing bead may be integrally formed beforehand.

The foamable material may be applied by spraying, brush electroplating, screen printing, roller printing, stencil printing or metering processes.

Both the surface structuring and the application of the foamable material may take place only on the convex surface of the sealing bead.

The method is suitable for producing the above-described sealing arrangement, the plate assembly and/or the electrochemical system. Features which are described in connection with the sealing arrangement, the plate assembly and/or the electrochemical system can therefore be combined or claimed with the method, and vice versa.

FIGS.1-18and20are shown approximately to scale.

Here and below, features which recur in different figures are in each case denoted by the same or similar reference signs.

DETAILED DESCRIPTION

The present disclosure relates to a sealing arrangement. The sealing arrangement can be used in an electrochemical system1(seeFIGS.1-4).

FIG.1shows an electrochemical system1comprising a plurality of structurally identical metal bipolar plates2which are arranged in a stack6and are stacked along a z-direction7. The bipolar plates2of the stack6are clamped between two end plates3,4. The z-direction7will also be referred to as the stacking direction. In the present example, the system1is a fuel cell stack. Each two adjacent bipolar plates2or the mutually facing separator plates X of each of these bipolar plates2of the stack therefore bound an electrochemical cell, which serves for example to convert chemical energy into electrical energy. To form the electrochemical cells of the system1, a membrane electrode assembly (MEA) is arranged in each case between adjacent bipolar plates2of the stack (see for exampleFIG.2). The MEA typically contains at least one membrane, for example an electrolyte membrane. Furthermore, a gas diffusion layer (GDL) may be arranged on one or both surfaces of the MEA.

In alternative embodiments, the system1may also be configured as an electrolyser, as an electrochemical compressor, or as a redox flow battery. Bipolar plates can likewise be used in these electrochemical systems. The structure of these bipolar plates may then correspond to the structure of the bipolar plates2explained in detail here, although the media guided on and/or through the bipolar plates in the case of an electrolyser, an electrochemical compressor or a redox flow battery may differ in each case from the media used for a fuel cell system.

The z-axis7, together with an x-axis8and a y-axis9, spans a right-handed Cartesian coordinate system. The bipolar plates2each define a plate plane, each of the plate planes of the separator plates2a,2bof the bipolar plates2being oriented parallel to the x-y plane and thus perpendicular to the stacking direction or to the z-axis7. The end plate4has a plurality of media ports5, via which media can be fed to the system1and via which media can be discharged from the system1. Said media, which can be fed to the system1and discharged from the system1, may comprise 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.

FIG.2shows, in a perspective view, two adjacent bipolar plates2of an electrochemical system of the same type as the system1fromFIG.1, as well as a membrane electrode assembly (MEA)10known from the prior art, which is arranged between said adjacent bipolar plates2, the MEA10inFIG.2being largely obscured by the bipolar plate2facing towards the viewer. An embodiment of bipolar plate2is formed of two separator plates2a,2bwhich are joined together by a material bond (see for exampleFIG.3), of which only the first separator plate2afacing towards the viewer is visible inFIG.2, said first separator plate obscuring the second separator plate2b. The separator plates2a,2bmay each be manufactured from a metal sheet, for example from a stainless-steel sheet. The separator plates2a,2bmay for example be welded to one another, for example by laser welds.

The separator plates2a,2bhave through-openings, which are aligned with one another and form the through-openings11a-cof the bipolar plate2. When a plurality of bipolar plates of the same type as the bipolar plate2are stacked, the through-openings11a-cform lines which extend through the stack6in the stacking direction7(seeFIG.1). Typically, each of the lines formed by the through-openings11a-cis fluidically connected to one of the ports5in the end plate4of the system1. For example, coolant can be introduced into the stack or discharged from the stack via the lines formed by the through-openings11a. In contrast, the lines formed by the through-openings11b,11cmay be configured to supply fuel and reaction gas to the electrochemical cells of the fuel cell stack6of the system1and to discharge the reaction products from the stack. The media-guiding through-openings11a-11care substantially parallel to the plate plane.

In order to seal off the through-openings11a-cwith respect to the interior of the stack6and with respect to the surrounding environment, an embodiment of the first separator plates2ausually have sealing arrangements in the form of sealing beads12a-c, which are each arranged around the through-openings11a-cand in each case completely surround the through-openings11a-c. On the rear side of the bipolar plates2, facing away from the viewer ofFIG.2, the second separator plates2bhave corresponding sealing beads for sealing off the through-openings11a-c(not shown).

In an electrochemically active region18, the first separator plates2ahave, on the front side thereof facing towards the viewer ofFIG.2, a flow field17with structures for guiding a reaction medium along the front side of the separator plate2a. InFIG.2, these structures are defined by a plurality of webs and channels extending between the webs and delimited by the webs. On the front side of the bipolar plates2facing towards the viewer ofFIG.2, the first separator plates2aadditionally have a distribution or collection region20. The distribution or collection region20comprises structures which are configured to distribute over the active region18a medium that is introduced into the distribution or collection region20from a first of the two through-openings11b, and/or to collect or to pool a medium flowing towards the second of the through-openings11bfrom the active region18. InFIG.2, the distributing structures of the distribution or collection region20are likewise defined by webs and channels extending between the webs and delimited by the webs. In general, therefore, the elements17,18,20can be understood as media-guiding embossed structures.

The sealing beads12a-12cusually have passages13a-13c, which are embodied here at least partially as local elevations of the bead, of which the passages13aare formed both on the underside of the upper separator plate2aand on the upper side of the lower separator plate2b, while the passages13bare formed in the upper separator plate2aand the passages13care formed in the lower separator plate2b. By way of example, the passages13aenable a passage of coolant between the through-opening12aand the distribution region, so that the coolant reaches the distribution region between the separator plates and is guided out therefrom. Furthermore, the passages13benable a passage of hydrogen between the through-opening12band the distribution region on the upper side of the upper separator plate2a; these passages13bare characterized by perforations facing towards the distribution region and extending at an angle to the plate plane. Therefore, hydrogen for example flows through the passages13bfrom the through-opening12bto the distribution region on the upper side of the upper separator plate2a, or in the opposite direction. The passages13cenable a passage of air for example between the through-opening12cand the distribution region, so that air reaches the distribution region on the underside of the lower separator plate2band is guided out therefrom. The associated perforations are not visible here.

The first separator plates2aalso typically each have a further sealing arrangement in the form of a perimeter bead12d, which extends around the flow field17of the active region18, the distribution or collection region20and the through-openings11b,11cand seals these off with respect to the through-opening11a, that is to say with respect to the coolant circuit, and with respect to the environment surrounding the system1. The second separator plates2beach comprise corresponding perimeter beads. The structures of the active region18, the distributing structures of the distribution or collection region20and the sealing beads12a-dare each formed in one piece with the separator plates2aand are integrally formed in the separator plates2a, for example in an embossing process, deep-drawing process and/or by means of hydroforming. The same applies to the corresponding distributing structures and sealing beads of the second separator plates2b. Outside of the region surrounded by the perimeter bead12d, a predominantly unstructured outer edge region22is formed in each separator plate2a,2b.

The two through-openings11bor the lines through the plate stack of the system1that are formed by the through-openings11bare each fluidically connected to one another via passages13bin the sealing beads12b, via the distributing structures of the distribution or collection region20and via the flow field17in the active region18of the first separator plates2afacing towards the viewer ofFIG.2. Analogously, the two through-openings11cor the lines through the plate stack of the system1that are formed by the through-openings11care each fluidically connected to one another via corresponding bead passages, via corresponding distributing structures and via a corresponding flow field on an outer side of the second separator plates2bfacing away from the viewer ofFIG.2. In contrast, the through-openings11aor the lines through the plate stack of the system1that are formed by the through-openings11aare each fluidically connected to one another via a cavity19that is enclosed or surrounded by the separator plates2a,2b. This cavity19serves in each case to guide a coolant through the bipolar plate2, for cooling the electrochemically active region18of the bipolar plate2.

FIG.3schematically shows a section through a portion of the plate stack6of the system1ofFIG.1, the sectional plane being oriented in the z-direction and thus perpendicular to the plate planes of the bipolar plates2; it may extend for example along the kinked section line A-A inFIG.2.

The structurally identical bipolar plates2of the stack each comprise the above-described first metal separator plate2aand the above-described second metal separator plate2b. Each metal separator plate2a,2bhas a thickness of around 75 μm. Structures for guiding media along the outer faces of the bipolar plates2are visible, here in the form of webs and channels delimited by the webs. Shown here are channels29on the surfaces of adjoining separator plates2a,2bdirected away from one another, as well as cooling channels19between adjoining separator plates2a,2b. Between the cooling channels19, the two separator plates2a,2bbear against one another in a contact region24and are connected to one another at that point, in the present example by means of laser welds.

A membrane electrode assembly (MEA)10, known for example from the prior art, is arranged in each case between adjacent bipolar plates2of the stack. The MEA10typically comprises a membrane14, for example an electrolyte membrane, and a sealing edge region15connected to the membrane. By way of example, the sealing edge region may be materially connected to the membrane, for example by an adhesive bond or by lamination.

The membrane of the MEA10extends in each case at least over the active region18of the adjoining bipolar plates2and at that location enables a proton transfer via or through the membrane. The membrane does not extend into the distribution or collection region20. The sealing edge region15of the MEA10serves in each case for positioning, fastening and sealing off the membrane between the adjoining bipolar plates2. If the bipolar plates2of the system1are clamped between the end plates3,4in the stacking direction (seeFIG.1), the sealing edge region15of the MEA10may be compressed for example in each case between the port beads12a-cof the respective adjoining bipolar plates2and/or in each case at least between the perimeter beads12dof the adjoining bipolar plates2, in order in this way to fix and seal off the MEA10between the adjoining bipolar plates2.

The sealing edge region15in each case covers the distribution or collection region20of the adjoining bipolar plates2. Towards the outside, the edge portion15may also extend beyond the perimeter bead12dand may adjoin the outer edge region22of the separator plates2a,2b(cf.FIG.2).

Gas diffusion layers16may additionally be arranged in the active region18. The gas diffusion layers16enable a flow across the membrane14over the largest possible area of the surface of the membrane14and may thus improve the proton transfer via the membrane14. The gas diffusion layers16may be arranged for example on both sides of the membrane14in the active region18between the adjoining separator plates2. The gas diffusion layers16may for example be formed of an electrically conductive nonwoven or may comprise an electrically conductive nonwoven. The electrically conductive nonwoven may be a metal fleece or a carbon fleece.

FIG.4schematically shows a further section through a portion of the plate stack6of the system1ofFIG.1. Here, the section has been selected such that it extends through two adjacent bipolar plates2. For space reasons, the section is shown in broken off towards both outer sides of the beads12bextending around the through-opening11b. In the area of the peripheral bead12b, sealing arrangements50′,50″,50′″,50″″ according to the disclosure, each comprising a sealing element52′,52″,52′″,52″″, which will be described in greater detail below, are arranged between each bipolar plate2and the sealing edge region15of the membrane electrode assembly10.

The first sealing arrangement50′ comprises the separator plate2bconfigured as a metal layer60, which is provided with a surface structuring62′ of the type described above on the bead top68thereof, that is to say the convex side thereof. An elastomeric sealing element52′ containing microspheres54is arranged on the surface structuring62′. The second sealing arrangement50″ comprises the separator plate2aconfigured as a metal layer60, which is likewise provided with a surface structuring62″ on the bead top thereof. A further elastomeric sealing element52″ containing microspheres54is arranged on the surface structuring62″. The sealing elements52′ and52″ may be made of the same elastomeric material or of different elastomeric materials. The first sealing arrangement50′ faces towards the second sealing arrangement50″. On the concave sides thereof, the sealing arrangements50′,50″ are each formed without surface structuring and without an elastomeric coating. The first sealing arrangement50′ and the second sealing arrangement50″ bear against both sides of the sealing edge region15of the membrane electrode assembly; they seal around the port bead12b. The metal layers60of the separator plates2a,2bon the one hand and the sealing edge regions15formed of polymer-based materials have different thermal expansion coefficients. In the case of a fast cold start, the separator plate2a,2band the sealing edge regions15may expand differently and the frame-like sealing element may be displaced with respect to the sealing beads. Due to the high microsphere content in the respective elastomeric sealing element52′,52″ the latter may at least partially follow the displacement. Furthermore, the surface structuring62,62′ of the metal layers60with the lower microsphere concentration in the first interface57of the elastomeric sealing elements52′,52″ (seeFIGS.6-9) ensures a better adhesion of the elastomeric sealing elements52′,52″ to the respective separator plate2a,2b.

The embodiments ofFIG.3andFIG.4show that in each case only one separator plate2band one separator plate2abelongs to a plate assembly70. In both examples, the separator plate2barranged on the upper side of the membrane electrode assembly10and the separator plate2aarranged on the underside of the membrane electrode assembly10belong to the plate assembly70explicitly provided with a reference sign. The separator plate2aof the bipolar plate2arranged on the upper side of the membrane electrode assembly10already belongs to the next plate assembly.

As indicated above, the electrochemical system1comprises sealing arrangements50, which will be described in greater detail below.

FIG.5shows a section through a sealing arrangement50according to the prior art, which comprises an elastomeric sealing element52and a metal layer60. The elastomeric sealing element52is configured as a coating of the metal layer60and comprises a foamed material containing microspheres54. Here, a concentration of the microspheres54in the sealing element52, measured perpendicular to the surface61of the metal layer60, is substantially homogeneous. Although the microspheres54inFIGS.5-9are shown in idealized form as elements which are round and of equal size in each case, this does not mean that they are all of the same size. A homogeneous distribution of the concentration of the microspheres54is accordingly to be understood to mean that the volume content of the gas based on a sufficiently long portion of the sealing arrangement50, for example along a portion corresponding to at least ten times the width of the sealing arrangement50, in a direction perpendicular to the surface61of the metal layer60substantially does not change. The volume or weight content of the polymer matrix likewise remains constant in this portion of the sealing arrangement50in the same direction.

It has been found that the sealing arrangement50shown inFIG.5cannot always withstand to the desired extent the mechanical and/or thermal stresses that occur during the operation of the electrochemical system1. The adhesion of the sealing element52to the metal layer60may sometimes be insufficient, as a result of which the elastomeric sealing element52may detach from the metal layer60.

After carrying out various tests, the inventors of the present disclosure have discovered that the adhesion of the elastomeric sealing element52to the metal layer60can be significantly improved if the metal layer60has a surface structuring62.

FIGS.6-9show sections through sealing arrangements50according to the disclosure, wherein the metal layers60thereof have such surface structuring62. As in the case of the sealing arrangement50ofFIG.5, the sealing arrangements50shown inFIGS.6-9each comprise an elastomeric sealing element52and a metal layer60. The elastomeric sealing element52is configured as a coating of a surface61of the metal layer60and comprises a foamed material containing microspheres54.

In contrast toFIG.5, however, the metal layer60has a surface61with a surface structuring62. The surface structuring62of the metal layer60comprises a plurality of depressions64. The sealing element52is arranged at least in some areas on the surface structuring62. It has been found that a concentration of the microspheres54, that is to say the volume content of gas, in the sealing element52, measured perpendicular to the surface61of the metal layer60, is inhomogeneous. The sealing element52may have a first interface57adjoining the metal layer60, wherein the concentration of the microspheres54, that is to say the volume content of gas, in the sealing element52is lower within the first interface57than outside of the first interface57. Conversely, the weight content of the polymer matrix is higher in the first interface57than outside of the first interface57. The first interface57may for example have a thickness of at most 35%, at most 30%, at most 23%, or at most 18% relative to a total maximum layer thickness of the sealing element52measured perpendicular to the surface61of the metal layer60. In addition, the sealing element52may also have a second interface59directed away from the metal layer60, wherein the concentration of the microspheres54, that is to say the volume content of gas, in the sealing element52is lower within the second interface59than outside of the first and second interface57,59. Conversely, the weight content of the polymer matrix is higher in the first and second interface57,59than outside of the first and second interface57,59. Together, that is to say in sum along a line perpendicular to the surface of the metal layer, the first and second interface57,59may have for example a thickness of no more than 35%, no more than 30%, or no more than 23% relative to a total maximum layer thickness of the sealing element52measured perpendicular to the surface61of the metal layer60. The formation of such a second interface59of the sealing element52requires suitable process control, such as suitable temperature control of the coating, crosslinking and/or expansion process. Advantageously, the crosslinking takes place prior to the expansion in the first and optionally second interface.

One possible first mechanism (or explanation) for the inhomogeneous concentration of the microspheres54in the elastomeric sealing element52would be that the coating material of the sealing element52without microspheres has a higher affinity for the surface structuring62than the coating material containing the microspheres54. The reason for the modified (higher) affinity may possibly be that the chemical surface composition of the metal layer60is different in the area of the surface structuring62than in the untreated, non-structured area, and/or that the roughness of the metal layer60is different in the area of the surface structuring62than in the untreated, non-structured area. This may be true in the case of laser treatment of the metal layer60(see production method below). One possible second mechanism for this would be that the depressions64of the surface treatment have dimensions which are smaller than the dimensions of the microspheres54. The microspheres54do not fit geometrically into the depressions64and therefore the concentration of the microspheres54is lower in the interface59than above the interface59. It should be noted that the two mechanisms presented are not mutually exclusive and may possibly take place simultaneously.

The sealing elements52of the sealing arrangements50shown inFIGS.6and7are arranged only on a surface area of the metal layer60that has the surface structuring62. In other words, the sealing arrangements50are not situated at untreated locations on the metal layer60.

The sealing element52shown inFIG.8is arranged partially on the surface structuring62. The sealing element52is also arranged partially on an untreated, smooth area63of the metal layer60. The surface structuring62is therefore present only in some portions on the surface61of the metal layer60that is coated by the sealing element52. The sealing element52shown inFIG.9extends slightly beyond the surface structuring62on both sides into the untreated area63; here, too, the surface structuring62is therefore present only in some portions on the surface61of the metal layer60that is coated by the sealing element52.

The layer thickness of the sealing element52may for example be in total no more than 500 μm, no more than 300 μm, no more than 200 μm, or no more than 150 μm. The layer thickness may be determined here perpendicular to the surface61of the metal layer60. The elastomeric sealing element52may contain the following substances: FPM (fluoroelastomer), silicone rubber or NBR rubber (nitrile butadiene rubber), PUR (polyurethane), NR (natural rubber), FFKM (perfluoro rubber), SBR (styrene butadiene rubber), BR (butyl rubber), FVSQ (fluorosilicone), CSM (chlorosulphonated polyethylene), silicone resin, epoxy resin, or mixtures of the aforementioned substances, or pressure sensitive adhesive and/or physically binding adhesive. Fillers or other additives may also be provided in the elastomeric material of the sealing element52.

In the expanded state, the microspheres are usually filled with a gaseous medium58, wherein the gas58may be for example a saturated hydrocarbon, such as n-pentane, isopentane or isobutane. The gas58is usually enclosed in the microspheres54and may not be able to escape from the sealing element52.

A mean diameter of the microspheres54may be at least 20 μm and/or at most 80 μm in the expanded state. According to one example, the microspheres54have a mean diameter in the range 35 to 55 μm. The depressions64may have a width or a diameter, which may be measured at the mid-height of the depressions64and/or parallel to the untreated metal surface61, of at most 150 μm, at most 100 μm, or at most 70 μm. Due to the size of the depressions64in the micrometre range, the surface structuring can also be referred to as surface microstructuring. Adjacent depressions64may be spaced apart from one another and therefore do not merge into one another.

In the embodiment ofFIG.6, the depressions64have a diameter which is larger than a mean diameter of the microspheres54. In the embodiment ofFIG.7, the diameter of the depressions64is smaller than a mean diameter of the microspheres54.FIGS.5to9are simplified in that all the microspheres are shown with the same diameter, but in fact they have a range of diameters.

In this connection, reference should also be made toFIG.20, which shows a microscope image of the metal layer60with the surface structuring62in the form of depressions64.FIG.20will be discussed again below.

In general, the depressions64may have a depth of at least 2 μm and/or at most 40 μm. The depth of the depressions64may be no more than 20% of the thickness of the metal layer60. The depth of the depressions64may be measured from the untreated surface61to the lowest point of the depression64.

A size of the individual depressions64lies for example in a range from 0.0001 to 0.05 mm2, 0.001 to 0.02 mm2, or 0.0008 to 0.01 mm2. According to one variant, there are around 500 to 100,000, or around 4000 to 20,000 depressions64per square centimetre. It may be provided that 10 to 90%, 20 to 50%, or 25 to 50% of the surface61of the metal layer60that is provided with a surface structuring62is taken up by the depressions64.

The depressions64may be created by means of laser radiation. In other embodiments, the depressions64are created mechanically, for example by engraving or scoring the metal layer60.

FIGS.10-18shows sections through examples of beads65of metal layers60.

FIG.10shows a metal layer60according to the prior art, which has a bead65. The metal layer60and the bead65has a relatively smooth, unstructured surface63and thus has no surface structuring62.

In contrast, the metal layers60and the beads65thereof shown inFIGS.11-18have a surface structuring62at least in some areas. The surface structuring62is formed on a surface of the bead65, namely on the surface of the layer60on which the convex side of the bead65projects. For the sake of clarity, the sealing element52has not been shown here. It is clear to a person skilled in the art that the sealing element52in the sealing arrangement50is situated on the bead65; the width (lateral extent) of the sealing element is indicated in each case by a bracket52.

Each bead65according to one embodiment of the disclosure, shown inFIGS.11-18, is integrally formed in the metal layer60, for example by embossing, deep drawing and/or hydroforming. In some of the embodiments, shown inFIGS.11-14and16-18, each bead65has at least one bead flank66,66′ and a bead top68, the bead top68adjoining the bead flanks66,66′ at both sides. The bead top68forms an elevation compared to a plate plane defined by the metal layer60, for example in the outer edge region22thereof. In the region of the bead flanks66,66′, the material of the layer60rises at an angle out of the plate plane. The bead top68may be a region of the bead65that extends substantially parallel to the plate plane, as shown inFIGS.11-14and18, or may have a curvature, as shown inFIGS.16-17. In the embodiment ofFIG.15, on the other hand, the bead65has only a strongly curved bead top68′, so that the flanks are integrated in this curvature, that is to say no explicit bead flanks are present.

As can be seen fromFIGS.11-18, the surface structuring62extends at least in some areas over the bead top68,68′ and/or the optionally present at least one bead flank66,66′.

It is therefore indicated in the variants ofFIGS.11,13and15-18that the entire bead top68is provided in full with the surface structuring62, while the bead top68ofFIGS.12and14has a surface structuring62in some areas but has at least one unstructured area63in the middle or at least spaced apart from the bead flanks66,66′.

In the variants ofFIGS.11,12and14, around one-third of each bead flank66,66′ is provided with the surface structuring62. The surface structuring62of the bead flanks66,66′ in this case adjoins the surface structuring62of the bead top68.

In the variant ofFIG.13, only the bead flank66′ comprises a surface structuring62, whereas the bead flank66is unstructured or smooth. In other variants, the structuring62may change in the course of the bead65from bead flank66′ to bead flank66and back again, it being possible for the change to take place multiple times. This is the case for example in the variant ofFIG.18, in which a section through a bead65is shown, which to the left merges into a passage13, and the next passage13′, which is offset in relation to the passage13shown in section, is visible in the background on the right. The passages13,13′ are free of surface structuring and coatings. In the case of such a surface structuring62which changes between the bead flanks66,66′, the total width of the surface structuring62along the course or change thereof may vary or remain the same.

However, the surface structuring62may alternatively also be extended laterally beyond the area of the respective bead flank66,66′ and may cover part of the bead foot67,67′, as shown in the variant ofFIG.14.

The bead65shown inFIGS.11-18may for example be or comprise one of the beads12a-dmentioned above. Accordingly, the bead65may surround, in an intrinsically closed manner, one of the through-openings11a-cfor fluids or the flow field17and may be configured as a sealing bead12a-d.

The metal layer60may be configured for example as one of the layers or plates2,2a,2bmentioned above, for example as a separator plate2a,2b(cf.FIGS.1-4).

FIG.19shows a flowchart of a method according to the disclosure for producing a sealing arrangement50, such as the sealing arrangement described above.

The method comprises at least the following steps.providing S1a metal layer60,providing S2the metal layer60with a surface structuring62, the surface structuring62comprising a plurality of depressions64,applying S3a foamable material containing expandable microspheres54to the surface structuring62,forming S4an elastomeric sealing element52on the metal layer60by expanding the microspheres54, wherein a distribution of the expanded microspheres54in the sealing element52, measured perpendicular to the surface61of the metal layer60, is inhomogeneous.

In certain embodiments, the depressions64in step S2are created by laser radiation. For example, a pulsed laser can be used for this.

In an optional method step P, a sealing bead12a-d,65is integrally formed in the metal layer60, for example by embossing, deep drawing and/or hydroforming. This may take place before, at the same time as or after the creation of the surface structuring62in the metal layer60.

FIG.20shows a microscope image of a metal layer60which has been provided with a surface structuring62by means of a laser. The laser system used here has a power of 100 Watt of laser at a wavelength of 1064 nm, a focal length of 254 mm and an ablation rate of around 25 cm2/s. Of course, other laser systems or lasers are also suitable for creating the surface structuring62or depressions64.

With regard to the properties of the depressions64of the surface structuring62, reference is made to what has been stated above. As can also be seen fromFIG.20, the depressions64may be arranged for example in rows or paths69extending parallel to one another; this may be the case when providing surface structuring on straight portions of a perimeter bead. The row arrangement69of the depressions64may be a consequence of producing the depressions64by means of the laser. It is also possible to arrange the depressions on concentric circles, as may be used for example in the case of port beads. In this case, the concentric circles form the paths69. Many other forms are conceivable; for example, the depressions may be arranged next to one another on paths69such that the paths follow the course of the surface to be structured, that is to say at least in an approximately wave-like manner in the case of beads extending in a wave-like manner. The depressions64may each be at least partially surrounded by elevations75, which are formed at the respective circumferential edge of the depressions. The respective elevation75may project above the untreated surface63of the metal layer60. Said elevation75is typically formed when forming the depressions64, for example by laser radiation, the elevation75being formed as a crater edge by solidification of molten material. The crater edge or the elevation75may thus be formed of the material that has migrated from the depressions64.

It can also be seen inFIG.20that the circumferential edge of the depressions64is substantially round. The substantially round depressions64have a diameter of around 65 μm. The shape of the circumferential edge of the depressions may depend on the lateral extent of the laser beam used and the angle of incidence of the laser beam onto the metal layer60. The shape of the circumferential edge may thus also depend on the respective position of the depressions64on the metal layer60. By way of example, the circumferential edge of the depressions64arranged on the bead top68may be round, while the circumferential edge of the depressions64arranged on the bead flank66,66′ may be elliptical or oval. The reason for this dependence may lie in the aforementioned dependence on the angle of incidence of the laser light, since the bead top68and the bead flank66,66′ usually have different surface normals.

To achieve a sufficient effect of the surface structuring62, a maximum spacing of adjacent depressions64may be at most five times the diameter of a depression64. On the other hand, the depressions64are not arranged too close to one another since this could lead to destruction of the material. Adjacent depressions64may have a minimum spacing which corresponds to half the diameter of the depressions64. The spacing between adjacent depressions64may be different in the longitudinal direction (that is to say in the longitudinal direction of the row or path69) and in the transverse direction of the sealing element52. In the transverse direction, the minimum spacing may correspond to at least half the diameter. Furthermore, in the transverse direction, the maximum spacing may correspond to at most three times the diameter. In the longitudinal direction, the minimum spacing may correspond to at least the diameter of a depression. In the longitudinal direction, the maximum spacing may be five times the diameter. In the example ofFIG.12, spacings are considered only within a contiguous surface-structured area, that is to say, on the one hand in the area of the bead flank66and the left-hand area of the bead top68and on the other hand in the area of the bead flank66′ and the right-hand area of the bead top68.

In alternative variants of the method, the depressions64are created for example by engraving or scoring to produce a microstructure. In this case, the depressions64may be configured either as round or as elliptical or oval depressions; in some embodiments, these shapes can be combined with one another.

In step S3, the foamable material may be applied to the metal layer60or the surface structuring62thereof by spraying, brush electroplating, screen printing, roller printing, stencil printing or metering processes. Once the foamable material has been applied to the surface structuring62, a solvent contained in the foamable material can evaporate. The evaporation of the solvent can be encouraged by applying a negative pressure or vacuum or by increasing to a first temperature. In a further step, the elastomeric material can be crosslinked or partially crosslinked. This (partial) crosslinking can be encouraged for example by a jump in temperature, for example by increasing to a second temperature which may be higher than the first temperature, or by UV radiation.

As a result of the increase in temperature, the microspheres54expand due to a phase change of the medium contained therein from the liquid phase to the gas phase. The expanded shape of the microspheres is maintained even on cooling due to the crosslinking of the polymer and the incorporation of the shell of the microspheres into the surrounding polymer.

In some embodiments of the method, the microspheres54in the non-expanded state have a mean diameter of at least 5 μm and/or at most 50 μm. In the expanded state, the microspheres54may have a mean diameter of at least 20 and/or at most 80 μm. By expanding the microspheres54, the maximum thickness of the sealing element52applied as a layer typically expands to a multiple. By way of example, the finished sealing element52has a maximum layer thickness which is four times the layer thickness of the foamable material.

The method is suitable for producing the above-described sealing arrangement50, the plate assembly70and/or the electrochemical system1. Features which are described in connection with the sealing arrangement50, the plate assembly70and/or the electrochemical system1can therefore be combined or claimed with the method, and vice versa.

LIST OF REFERENCE SIGNS