Multi-layer seal for demanding fuel cell applications

Abstract

A fuel cell stack comprises end plate units, each having a metal plate, a dielectric plate, and a circumferential groove defined by the dielectric plate and an edge of the metal plate. The stack also includes a gas inlet and a coolant inlet configured to receive a reactant gas and a coolant, respectively, into the fuel cell stack, fuel cells with a pair of bipolar plates, and a multilayer seal located within the fuel cell stack at the end plate units or the bipolar plates. The multilayer seal comprises a first layer of a material that is substantially impermeable to the reactant gas and a second layer of a material that is more corrosion-resistant than the first.

Description

Technical FieldThe present disclosure relates to electrochemical fuel cell systems that are for converting a gaseous reactant, such as hydrogen, into electricity. More particularly, aspects of the disclosure relate to internally sealing the fuel cell system.IntroductionModern hybrid electric vehicles and all-electric vehicles may use a fuel cell system to generate an electric current suitable for operating one or more electric traction motors. As known in the art, a hydrogen (H 2)- fuel cell is an electrochemical device that consists of a negative electrode / anode supplied with gaseous H 2 a positive electrode / cathode supplied with ambient air as an oxidant, and an electrolyte barrier disposed between anode and cathode. H 2- molecules are oxidized at the anode by an induced electrochemical reaction. The H 2- gas is catalytically cleaved to produce free electrons and protons. The free protons pass through the electrolyte barrier to the cathode side of the fuel cell and react with oxygen molecules (O 2). In this reaction, steam and heat are formed as inert by-products. The free electrons from the anode are then conducted to a connected load, e.g., the electric traction motor(s) and accessories mentioned above.In fuel cell stacks for motor vehicles and other high current applications, a solid polymer electrolyte membrane (PEM) is often used. The PEM provides for ion transport between the aforementioned anode and cathode. The combination of the anode catalytic layer, the cathode catalytic layer, and an electrolyte membrane forms a membrane electrode assembly (MEA). The MEA is disposed between gas diffusion layers (GDLs). The GDLs, in turn, are arranged between bipolar plates (BPPs) and form a fuel cell. Multiple fuel cells are assembled into a fuel cell stack to generate the required current and voltage for a particular application. The BPPs together form circular flow channels for distributing the H 2 and O 2 reactant gases through the stack. Elastomeric seals or other types of seals, e.g., metal bead seals, are provided on the edges of the MEA, on the surfaces of the BPP, and on the end plate units of the stack to provide effective separation of the reactant and coolant streams while preventing leaks and mixing of the various gases.JP 2021-072 250 A describes a fuel cell and a fuel cell supply system for increasing the reliability of the fuel gas seal. A fuel cell is described that generates electricity by chemical reaction of fuel gas that includes a seal for sealing the fuel gas in the fuel cell, and the seal is made of a self-healing elastomer material.US 2009 / 0 092 886 A1 describes a composite seal with a multilayer elastomeric construction and a method for its production. More specifically, the present invention provides a composite gasket comprised of a compliant layer of a low durometer elastomer coated with or alternatively encapsulated by a thin protective layer to securely seal a bipolar plate and a membrane electrode assembly of a fuel cell. The compliant elastomer layer is preferably a silicone component and the thin protective layer is preferably a fluoroelastomer or fluoropolymer component suitable for bonding to the compliant elastomer layer. The aforementioned layers forming the composite gasket are preferably directly applied to the aforementioned fuel cell components along a predetermined circumference.US 2006 / 0 024 561 A1 describes a connection plate, an insulating plate and an end plate which are stacked one on top of the other. A rectangular recess is formed in the center of the insulating plate. The terminal plate is placed in the recess. A supply channel for oxygen-containing gas, a supply channel for coolant, a discharge channel for fuel gas, a supply channel for fuel gas, a discharge channel for coolant and a discharge channel for oxygen-containing gas run as fluid channels through the insulating plate. These fluid passages do not pass through the port plate.DESCRIPTION OF THE INVENTIONThe invention is defined by the claims.Here, a multi-layer gasket for use in a fuel cell stack of the type described above is presented. While the solutions described herein may be of particular benefit when applied to a relatively small size, complicated geometry fuel cell system (FCS) and / or with a variety of design requirements where press-in-place (PIP) seals and other types of seals do not fit properly, the present teachings may be extended to a larger or less complex FCS without limitation.The multi-layer seal contemplated herein includes multiple sealing layers and is therefore suitable for use with an FCS under demanding application conditions, e.g., in acidic or corrosive operating environments. The multi-layer seal could be used in some implementations to seal a non-repeating hardware (NRHW), e.g., an end unit of the fuel cell stack, or to seal repeating hardware such as a bipolar plate (BPP). Although adjacent sealing layers may be made of the same material, different materials are used in other embodiments to achieve additional advantages (see below). The associated methods of applying the various sealing layers may also be the same or different, e.g., one layer may be applied by a molded in place gasket (FIPG) method, while other layers may be applied by a room temperature vulcanizing sealer (RTV) method or a dispensing method. Each sealing layer could be applied by screen printing or dispensing, e.g. for the above mentioned BPP seal, among other possible implementations.Between the sealant layers, the constructed layer-to-layer interfaces may have strong or weak adhesion, or a gap may be formed between the layers to avoid interference between adjacent layers. The geometry, shape and / or thickness of the individual layers may vary depending on the application, various possible constructions of the multilayer seal being described in detail below.More particularly, according to a first aspect of the invention, a fuel cell stack is disclosed that includes a pair of end plate units. Each end plate unit includes a metal plate, a dielectric plate, and a circumferential groove defined by the dielectric plate and an edge of the metal plate. The fuel cell stack also includes a gas inlet and a coolant inlet configured to receive a reactant gas and a coolant, respectively, into the fuel cell stack, a plurality of fuel cells each having a pair of bipolar plates, and a multi-layer gasket disposed within the fuel cell stack at the end plate units or the bipolar plates. The multi-layer gasket comprises a first and a second layer. The first layer in this embodiment is comprised of a first material that is substantially impermeable to the reactant gas and the refrigerant. The second layer, in turn, consists of a second material that is more corrosion resistant than the first material. The multi-layer gasket further includes a third layer disposed between the first layer and the third layer. The third layer is made of a third material different from the first and second materials. The third material is air such that the third layer includes an air gap defined between the first layer and the second layer.According to an embodiment, the multi-layer gasket is disposed between the metal plate and the dielectric plate of each of the end plate units.In one or more embodiments, the adhesion strength to the dielectric sheet and the maximum tensile strain of the second layer is higher than the adhesion strength to the dielectric sheet and a maximum tensile strain of the first material.The bipolar plates may include a bead bond in one or more embodiments. In such a construction, the multi-layer seal may comprise a thin layer seal disposed on a surface of the bead joint.According to one embodiment, the first material of the multi-layer gasket comprises a foamed fluorinated carbon-based synthetic rubber (FKM). The second layer is applied as a solid FKM to the surface of the bead joint.According to a second aspect of the invention, a fuel cell system (FCS) is disclosed. The fuel cell system includes a reactant supply tank containing a reactant gas, a fuel cell stack according to any of the embodiments described herein in fluid communication with the reactant supply tank and configured to receive coolant and the reactant gas, and a load. The load is configured to be driven by an electric current from the fuel cell stack.A method of sealing a fuel cell stack having a plurality of fuel cells disposed between end plate assemblies includes applying a first material to an end plate assembly or a bipolar plate of the fuel cells as a first layer. The first layer has low permeation for a reactant gas and the coolant used in the fuel cell stack, e.g., water. The method also includes applying a second material to the end plate assembly or the bipolar plate as a second layer, thereby forming a multi-layer seal, the second material being more corrosion resistant than the first material. The method further includes curing the first layer and the second layer to thereby form the multi-layer seal.In one or more possible embodiments, the method comprises applying a third material as a third layer between the first and second layers. The third material in this approach is different from the first and second materials and has a low adhesion strength with them. The low adhesion is sufficient to reduce chemical and adhesion related disturbances between the first and second layers.The application of the first material could optionally comprise using a form-in-place-gasket method, while the application of the second material could be done with a manual dispensing method.The above-summarized and other features and advantages of the present teachings will be readily apparent from the following detailed description of some of the best modes and other embodiments for carrying out the present teachings as defined in the appended claims when considered in connection with the accompanying drawings.Brief Description of the DrawingsFIG. 1 shows a representative motor vehicle having a powertrain system with a fuel cell system (FCS), the FCS constructed using a multi-layer sealing strategy in accordance with the present disclosure. FIG. 2 shows a representative fuel cell stack with a multi-layer gasket according to the present disclosure.FIGS. 3A and 3B are a top view and a partial perspective view, respectively, of a non-repeating hardware (NRHW) portion of the fuel cell stack shown in FIGS. 1 and 2. FIG. 3C is a top view of an example bipolar plate (BPP) of the fuel cell stack shown in FIGS. 1 and 2.FIGS. 4A, 4B, and 4C are partial cross-sectional side views of a portion of the NRHW portion of the representative FCS shown in FIG. 1, wherein the multi-layer seal is disposed in a circumferential connection between a steel plate and a representative dielectric plate of the NRHW portion. FIG. 5 is a side cross-sectional view of a representative metal bead that may be used as part of a bipolar plate (BPP) of the example fuel cell stack shown in FIGS. 1 and 2.FIGS. 6A and 6B are schematic diagrams of a multi-layer thin sealing material applied to a metal bead to form a BPP seal on the BPP of FIG. 3C within the fuel cell stack of FIGS. 1 and 2.FIGS. 7A and 7B show exemplary pressure distributions on a bead seal of different construction. FIG. 8 illustrates a two-layer elastomeric seal according to an aspect of the disclosure.The present disclosure may be modified or embodied in alternative forms, with representative embodiments shown in the drawings and described in detail below. The inventive aspects of the present disclosure are not limited to the disclosed embodiments. Rather, the present disclosure is intended to cover alternatives falling within the scope of the disclosure as defined by the appended claims.Detailed DescriptionReferring now to the drawings, wherein like reference numerals correspond to similar components throughout the several figures, there is shown in FIG. 1 an automobile 10 having a fuel cell system (FCS) 12. The motor vehicle 10 includes a vehicle body 14 connected to a set of road wheels 16. As shown, the motor vehicle 10 is configured as a passenger vehicle, e.g., sport utility vehicle, sedan, truck, motorcycle, etc. However, the FCS 12 may be installed in a variety of vehicles, e.g., boats, airplanes, rail vehicles, agricultural equipment, etc., mobile platforms that are not vehicles, or stationary systems such as power plants, hoists, and the like. The motor vehicle 10 of FIG. 1 is used below as an example host system for the FCS 12, without the present teaching being limited to such use.The FCS 12 of FIG. 1 includes a fuel cell stack 18 having a number of fuel cells 20 disposed therein suitable for use, an example configuration of the fuel cell stack 18 being shown in FIG. 2. A multi-layer seal 15 is disposed within the fuel cell stack 18 at one or more locations, possibly including on or along connected end plate units 30A and 30B or other non-repeating hardware portions 30 as shown in FIGS. 2-4C, between adjacent beads 60-1 of bipolar plates 38 (FIGS. 2 and 3C ) as shown in FIGS. 6A and 6B, and / or on other inner or outer surfaces 80 of the fuel cell stack 18 as shown in FIG. 8, e.g., as a press-in-place seal (PIP) or elastomeric seal. The use of the described multi-layer seal 15 thus increases the seal integrity and overall life of the fuel cell stack 18 and thereby makes the fuel cell stack 18 more suitable for use in relatively difficult operating environments.The multi-layer seal 15 is in turn applied according to various methods described herein, which facilitates manufacture. As provided herein, each layer of the multi-layer gasket 15 serves a specific sealing function to meet multiple sealing requirements. For example, as shown in the non-repetitive hardware embodiment of FIGS. 3A and 3B, which is depicted in more detail in FIGS. 4A, 4B, and 4C along section line 4- 4 of FIG. 3B, a first layer 45 could be made of a first material (A) having a high density to meet the requirements of hydrogen and coolant permeation, i.e., low permeation / effectively impermeable to H 2 and coolant permeation as described herein, and a second layer 47 (B) could be used for high acid and coolant corrosion resistance, e.g., for sealing an end plate of the fuel cell stack. As used herein, "low permeation" and "effectively impermeable" refer to a gas diffusion rate through a fluoropolymer material such as SIFEL ® or VITON ®, which is less than a diffusion rate for a suitable silicon such as DOWSIL™. The substantially impermeable material may have a gas permeation coefficient (diffusivity) of less than 0.5-1.5×10 -10( m 2 / s) for H 2 which could be less than about 25% of the permeation coefficient of the second layer 47.Similarly, material A could be used to form a primary seal, while material B serves as a protective layer for material A. Various materials may be used to make the respective first and second layers 45 and 47 from material A or B, e.g., polyolefin elastomer, silicone rubber, ethylene propylene diene monomer (EPDM), nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), vinylmethyl silicone, phenylmethyl, various fluoroelastomers / rubbers (moldable or non-moldable), polytetrafluoroethylene (PTFE), etc. These sealants may be cured at room temperature or at elevated temperature, e.g., in an oven.Except for the formation of peripheral seals in the end plate or other NRHW portion 30 of FIGS. 4A-C, the multi-layer seal 15 described below could be used with bead seals 60-1 (see FIGS. 6A and 6B ) to increase the seal thickness and reduce the sensitivity of the seal to contaminants. Such use could help achieve a greater load range of the seal and an optimal pressure distribution profile as shown in FIG. 7B. In some embodiments, less expensive materials could be used for the inner layers or other protected or unexposed areas of the gasket. The use of a multi-layer gasket can also reduce the permeation of the gasket and increase the set load with harder and denser inner layers. These features and advantages will be described in more detail below with reference to FIGS. 4A-8.The motor vehicle 10 is only one possible accommodation system for the FCS 12 and its fuel cell stack 18 (see FIG. 1 ). Other types of vehicles and mobile systems could use a similarly constructed FCS 12, e.g., boats, rail vehicles, aircraft, agricultural equipment, mobile platforms, and the like. Stationary systems, such as power plants, hoists, emergency power units, etc., could also use the FCS 12 and fuel cell stack 18. The motor vehicle 10 is therefore an example of a representative type of host system that could benefit from the sealing techniques described herein.The FCS 12 of FIG. 1 uses the fuel cell stack 18 to generate on-board power. As is known in the art, air and hydrogen (H 2) or other suitable reactant gas 17R are supplied from a reactant supply tank 17 that is in fluid communication with the fuel cell stack 18, e.g., via a series of valves, pressure regulators, and fittings (not shown). A suitable coolant 19 is also circulated through the fuel cell stack 18 to regulate the temperature of the fuel cell stack 18. The fuel cell stack 18 is in turn constructed from a number of fuel cells 20 suitable for use. The fuel cell stack 18 ultimately generates electrical current, with water vapor and heat being produced as byproducts.An electrified drive system 11 using the fuel cell stack 18 may include a direct current to direct current (DC / DC) converter 22 to convert an output direct current voltage (V 1) from the fuel cell stack 18 to an application-appropriate voltage (V 2). This operation may be via boost operation facilitated by high speed switching of semiconductor power switches and transformers (not shown) within the DC / DC converter 22, as is known in the art. The electrified drive system 11 may include an alternating current (AC) traction motor (M E) 25 depicted as part of a driven load, in this case an electric traction motor coupled to one or more of the road wheels 16 via a shaft 160.In this configuration, a direct current to alternating current (DC / AC) inverter 24 is disposed between the DC / DC converter 22 and the travel motor 25. The internal switching operation of the inverter 24 ultimately converts the DC input voltage (V2) to an AC voltage (VAC) suitable for energizing the phase windings of the traction motor 25, thereby causing the rotation of the engine and the transmission of the motor output torque (T O) to the road wheels 16. Other possible components of the electrified powertrain 11 may include an auxiliary power module 26, i.e., another DC / DC converter, that lowers the voltage (V 2) of the DC bus to an auxiliary voltage (V 3) that is nominally about 12-15V. A low-voltage auxiliary battery (B AUX) 28 is thus supplied with the auxiliary voltage (V 3) in order to operate low-voltage functions on board the motor vehicle 10.FIG. 2 shows the fuel cell stack 18, the sealing of which is effected here with the aid of the multilayer seal 15, in a simplified manner. The NRHW portion 30 of FIGS. 3A-3B and 4A-4C sealed via the multilayer gasket 15 as described below includes a pair of end plate units 30A and 30B between which a plurality of fuel cells 20 and electrodes E 1 and E 2 are disposed. Each fuel cell 20 includes a pair of bipolar plates (BPP) 38 between which a membrane electrode assembly (MEA) 36 is disposed. One or more gas inlets 32 are configured to receive a flow of a suitable reactant gas, such as hydrogen (H 2) into the fuel cell stack 18. One or more gas outlets 34 also allow the dissipation of water vapor and heat to the environment. Other connections for supplying coolant and air are omitted for simplicity, but are illustrated in the non-limiting embodiment of FIGS. 3A and 3B. Positive and negative electrodes with a suitable dielectric coating are disposed at opposite ends of the fuel cell stack 18, as shown. The multi-layer seal 15 could be deployed within the fuel cell stack 18 at various locations to optimize the seal integrity of the fuel cell stack 18 under oxidizing, acidic, or other difficult operating conditions, as will now be explained with reference to the remaining figures.In FIGS. 3A and 3B, the NRHW portion 30 is shown in plan view and in partial perspective view, respectively, and includes a current collector plate 31 disposed within a coated metal plate 40 ("SUS plate") that is in turn joined to a dielectric plate 42 as shown in FIG. 3B. The current collector 31 is surrounded by a circumferential groove 400 which can be filled by a press-in-place seal (PIP), as is conventional in the art. In the representative configuration of FIG. 3A, the metal plate 40 defines a plurality of liquid openings or channels, including the gas inlet 32 and the gas outlet 34, shown in the other configuration of FIG. 2. Also shown in FIG. 3A are a coolant inlet port 33C, an air inlet port 33A, an air outlet port 133A, and a coolant outlet port 133C. The particular number, location, and shape of the various fluid ports or passages may vary depending on the design of the NRHW portion 30, and thus the particular configuration of FIG. 3A is illustrative of the present teachings and is not limiting.Thus, in accordance with the disclosure, the NRHW portion 30 has various bonded surfaces that would benefit from the multi-layer sealing concept described herein. For example, the gas inlet 32 and the coolant inlet opening 133C, visible in FIG. 3B, are surrounded by a circumferential groove 35 that is in turn defined by the dielectric plate 42 and an edge of the metal plate 40. Thus, a flange portion 142 of the dielectric plate 42 protrudes through the gas inlet 32, the coolant inlet port 133C, and other ports of FIG. 3A as illustrated, with the circumferential groove 35 surrounding the flange portion 142. Other locations for the disclosed sealing approach will be described below with reference to FIGS. 3C and 5A-8.As shown in FIGS. 4A, 4B, and 4C, the multi-layer seal 15 described above may be used within the NRHW portion 30 of FIGS. 3A and 3B to take advantage of the associated advantages in a variety of ways. As shown, the NRHW portion 30 includes, for example, the end plate units 30A and 30B of FIG. 2, a partial cross-sectional view of which is shown. As known in the art, such a structure is representative of joined portions of the fuel cell stack 18 in which different materials are disposed immediately adjacent to one another. In this case, the metal plate 40, e.g., 304 or 316 stainless steel (about 1 mm to about 10 mm thick), may be pressed against the dielectric plate 42, e.g., glass fiber reinforced polyphenylene sulfide (PPS), or other electrically insulating material. When the respective metal and dielectric plates 40 and 42 are brought into close proximity to each other by this compression, a bead of sealing material originally located between the metal plate 40 and the dielectric plate 42 is pushed away or squeezed into the circumferential groove 35 defined by the dielectric plate 42 and an edge and surfaces of the metal plate 40, the circumferential groove having a width dimension (DD) of about 1 mm to about 5 mm or more. The circumferential groove 35 is sealed by the multilayer gasket 15 as described below.The materials in this portion of the fuel cell stack 18 differ in both composition (i.e., metallic or dielectric) and surface roughness, with the dielectric plate 42 nominally "rough" and the metal plate 40 nominally "smooth", in a relative relationship. These materials have very different thermal expansion coefficients, so that sites such as the representative NRHW portion 30 are difficult to seal. Thus, in such or similar sealing application, the use of the multi-layer seal 15 of the present disclosure would provide various advantages and could be manufactured as a molded-in-place gasket (FIPG) or by a dispensing, room temperature vulcanization (RTV), or injection molding process for the various locations of the multi-layer seal 15.For example, as shown in FIG. 4A, after the above-described compression of the metal plate 40 and the dielectric plate 42, a thin layer of the sealing material remains therebetween, most of which is ejected into the circumferential groove 35. In this case, a plurality of sealing materials could be used as layers of the multi-layer gasket 15. The multi-layer gasket 15 would then be disposed between the metal plate 40 and the dielectric plate 42 of each end plate unit 30A and 30B.Here, nominal first and second materials A and B could be used to form respective first and second layers 45 and 47. As mentioned above, in one or more embodiments, materials A and B may have different material compositions and / or properties. For example, the first material A could be substantially impermeable to H 2 or other reactant gas and coolant as used in the fuel cell stack 18, with "low permeation" and "effectively impermeable" as used herein meeting the required H 2- and coolant permeation specifications for the selected embodiments. For example, the first material A could have a high density to prevent the permeation of H 2 when H 2 is used as the reactant gas 17R of FIG. 1, or the permeation for the corresponding refrigerant. The second material B, in turn, could be more corrosion resistant than the first material A to resist corrosion by refrigerants and acids that could be present in the operating environment of the FCS 12 of FIG. 1. In this way, the multi-layer seal 15 would improve the robustness and longevity of the fuel cell stack 18 of FIG. 2 in relatively harsh or demanding operating environments.In this or other embodiments, the second material B used to form the second layer 47 could have a higher adhesion strength and maximum allowed shear / compression strains or tensile strains than the first layer 45 and its first material A, but possibly with a higher H 2- permeation. The synergy of materials A and B would therefore provide the desired sealing properties at various locations on and within the fuel cell stack 18 of FIG. 2.FIG. 4B shows the multi-layer gasket 15 with an additional third layer 51, referred to as material C. The material C, which may be different from the first material / material A of the first layer 45 and the second material / material B of the second layer 47 in some embodiments, is disposed between the aforementioned first and second layers 45 and 47. In this optional three layer approach, the third layer 51 is thinly deposited between the first and second layers 45 and 47. In this case, the third layer 51 made of a third material C has a low adhesion strength with the first material A or the second material B or both A and B. The introduction of the third layer 51 in forming an end seal as shown can reduce chemical and adhesion-related disturbances between the first and second materials A and B of the respective first and second layers 45 and 47. Thus, failure of the first layer 45 would not affect the sealing performance of the second layer 47, and vice versa. In this embodiment, material A and material B may be made of the same material.Again, FIG. 4C shows substantially the same structure as FIG. 4B, with the third layer 51 / the third material C replaced by an air gap. The "material" of the third layer 51 is thus air in this case, i.e. an air-filled gap or cavity pocket located between the first and second layers 45 and 47. Such an approach would achieve a similar effect as the third layer 51 in FIG. 4B. Among the potential difficulties in implementing the solution in FIG. 4C is maintaining the separation of the first and second layers 45 and 47, which is likely to require the use of devices, tools, or sacrificial elements therebetween.Referring briefly to FIG. 3C, the fuel cell stack 18 of FIGS. 1 and 2 may also include one or more bead connections 60 disposed on the bipolar plate (BPP) 38 (see FIG. 2 ). As known in the art, the BPP 38 is a hardware repetitive element, as opposed to the NRHW portion 30 shown in FIGS. 4A-C. The BPP 38 separates the individual fuel cells 20 of the fuel cell stack 18 shown in FIG. 2 and connects the fuel cells 20 in series to form the fuel cell stack 18. A BPP 38 may also provide various flow channels 62 for the flow of hydrogen (H 2), oxygen (O 2) and coolant 19 within the fuel cell stack 18 to distribute the reactant gases 17R of FIG. 1 over the surfaces of the fuel cells 20.The BPP 38 may include the bead connections 60 made of a metal material suitable for the application, e.g., stainless steel, titanium, aluminum, or another metal or metals. As known in the art, the bead joints 60 may be made of non-metallic materials such as graphite, and thus the bead joints 60 are not limited to metal designs. To prevent undesired leakage of liquids between adjacent BPPs 38 or between the BPP and metal plate 40 of FIGS. 3A and 3B, the bead joints 60 may be sealed via the multi-layer gaskets 150, 250, or 350 as described below. Such bead joints 60 may be disposed along a peripheral edge of the BPP 38 and / or a periphery of the various openings 33C, 33A, 34, 133A, 32, and 133C.As shown in FIG. 5, each bead joint 60 may be formed in one or more constructions by joining metal plates 600A and 600B together to form the flow channels 62 therebetween. The flat ends 66 are joined (e.g., laser welding) or sealed (e.g., bonded) together to further define the flow channels 62. The surfaces 64A and 64B of the bead joint 60 may be sealed with a multi-layer seal 150 of the present disclosure. The multi-layer seal 150, such as an embodiment of the multi-layer seal 15 of FIGS. 1-4C, could therefore be formed as a metal bead seal disposed on the surfaces 64A and / or 64B of the metal bead joint 60.In FIGS. 6A and 6B, such a bead seal is shown, for example, as bead seal 60-1 of the bead joint 60 of FIG. 5. The bead seal 60-1 includes a multi-layer seal 150 disposed on the bead seal surface 64 (i.e., surface 64A and / or 64B). When properly shaped, the multi-layer seal 150 prevents the escape of the reactant gas 17R of FIG. 1. When the fuel cell stack 18 is finally assembled, the multi-layer gaskets 150 (bead gaskets) of adjacent BPPs 38 are compressed, which in turn compresses the multi-layer gaskets 150. As a result, H 2 cannot escape from the fuel cell stack 18. The flow channels 62 have openings (not shown) therein through which reactants / refrigerants can enter the active area of the cell (e.g., 62 in FIG. 3C ) as is conventional in the art.The representative multilayer gasket 150 of FIGS. 6A and 6B includes the first and second layers 45 and 47 described above, with the embodiment of FIG. 6B also including the optional third layer 51. As is conventional in the art, typical single layer gaskets used at the location indicated are screen printed to a desired thickness, e.g., about 30-40 micrometers. This limits the allowable impurity sizes that must be less than the thickness of the seal, and a representative impurity 63 having a thickness greater than a single layer seal is shown in the multi-layer seal 250 of FIG. 6B.It is herein recognized that a single layer approach does not provide the required sealing pressures in the center of the seal 150 when the sealing load is high and the top surface of the metal bead deforms. An uneven or "M-shaped" pressure distribution profile 70 in FIG. 7A illustrates this potential problem of a typical single-ply bead seal 15*, which profile indicates that the pressure is lowest at the center of the single-ply bead seal 15*. Seal integrity or sealability is therefore lower in this middle region.The multilayer gaskets 150 and 250 in Figures 6A and 6B help solve this problem. A thicker seal is applied along the central portion of surface 64 by multi-path / multi-layer screen printing or by a combination of screen printing and application. In screen printing, a fine mesh screen is stretched over a frame and certain areas which are not to be printed are shut off. During application, the sealing material is applied in a controlled manner in a continuous bead or line along the surfaces to be sealed. The materials of the first, second and third layers 45, 47 and 51 may be the same or different to achieve the desired sealing properties.Illustratively, the second layer 47 could be formed of a foamed fluorinated carbon-based synthetic rubber (FKM) to reduce pressure profile variations as shown in the pressure curve 70 of FIG. 7A as well as sensitivity to contaminants. For example, with respect to the pressure profile of the transverse bead, the flatter or more uniform pressure distribution 700 of FIG. 7B achieved here with an optional three-layer version of the multi-layer seal 250 of FIG. 6B illustrates the potential result of incorporating the multi-layer seal 150 or 250 of FIG. 6A or 6B into the structure of the BPP 38 (see FIG. 2 ) for better tightness. The first layer 45 and the third layer 51 could be made of a solid FKM to reduce the permeation of H 2 or refrigerant (see above). Other screen printable / essential materials could be used for similar end results within the scope of the disclosure.Referring to FIG. 8, the fuel cell stack 18 of FIG. 1 may also include a multi-layer seal 350, or alternatively a press-in-place or PIP seal, on the surface 80 of the fuel cell stack 18 of FIG. 2, where the surface 80 may be located anywhere on or within the fuel cell stack 18. Such a multi-layer seal 350 may be used between various components of the fuel cell stack 18, e.g., the BPPs 38, the MEAs 36, and the end plate units 30A, 30B. Upon assembly of the fuel cell stack 18, a force is applied to such a seal of elastomeric material which is in turn compressed to form a leak-proof seal. However, typical approaches use a single layer seal for this purpose which is either too soft or too stiff for use and may also have suboptimal gas permeation properties due to the thick elastomer.As shown in FIG. 8, the present approach solves this potential problem by using the multi-layer seal 350 as a thin-layer seal. In this representative embodiment, the first layer 45 and the second layer 47 are used, with the optional layer 51 also being shown therebetween. In this case, by using two or more elastomer layers for the respective materials A and B, the above problems are alleviated. For example, the first layer 45 / material A may be selected as a relatively hard and relatively inexpensive material to reduce cost and H 2- permeation and increase the stiffness of the seal. The second layer 47 / material B could be softer and more corrosion resistant than material B in this example. "More corrosion resistant" refers to the material B of the second layer 47 maintaining a higher adhesion to both the dielectric (e.g., PPS) and the metal plates 40 of FIGS. 4A-C than the first layer 45 in the presence of coolant and acidic liquid / vapor.As before, materials A and B could be manufactured by injection molding, which would facilitate the manufacture of the fuel cell stack. Also, the combination of the first and second materials would provide a required load variation curve, permeation rate and corrosion resistance at optimum cost. Optional third layer 51 could be interposed between first and second layers 45 and 47 in another embodiment to achieve the additional sealing advantages described above.Related methods of sealing the fuel cell stack 18 of FIG. 1, or other such stack having a plurality of fuel cells 20 disposed between end plate units 30A, 30B (commonly referred to as NRHW 30), may generally include depositing the first material A on the NRHW portion 30 as the first layer 45. As mentioned above, the first layer 45 is impermeable to H 2 or other reactant gas 17R or coolant 19 used in the fuel cell stack 18. For example, the H 2- transmittance of the first layer 45 may be less than about 2e-13moles-m / (m 3- s-Pa) at 25°C. The coolant permeability, e.g., when the coolant 19 is water, may be less than about 2 g / m 2 per twenty four hours at 25° C. Other values may be used in other implementations.The method also includes applying the second material B to the NRHW portion 30 as the second layer 47. the second material B could be more corrosion resistant than the first material A as described above. in such a method, the first layer 45 and the second layer 47 could then be cured, e.g., in an oven (or in air) at an appropriate curing temperature, to form the multi-layer seal 15.Optionally, such a method could include depositing or forming the third material C as a third layer 51 between the first layer 45 and the second layer 47 (FIG. 4C ). The third material C, which may be different from the first material 45 and the second material 47, also has a low adhesion strength, wherein the low adhesion strength provided herein is sufficient to reduce chemical and adhesion related disturbances between the first layer 45 and the second layer 47, as mentioned above. The application of the first material A could be by a form-in-place-gasket (FIPG) method, while the application of the second material or the third material C could be by a manual dispensing method, among other things.As set forth above, the present disclosure enables the use of the multi-layer gasket 15 and its embodiments 150, 250, and 350 of FIGS. 4A-8 at various locations within the fuel cell stack 18 of FIG. 1. For an NRHW portion 30 or an end unit 30A, 30B of FIGS. 4A-C, for example, one could use the FIPG method to apply the first layer 45 / material A, and then apply the second layer 47 / material B manually, e.g., using a syringe, and then cure at low temperature.For a bead seal formed as multilayer seals 150 or 250 of the types illustrated in Figures 6A and 6B, the first layer 45 / material A and possibly the optional third layer 51 / material C may be screen printed followed by low temperature curing and drying. Then, the screen printing process could apply the second layer 47 / material B, again followed by low temperature curing / drying. After these steps, the final curing may be carried out at a temperature higher than the temperature used for the pre-curing of the previously applied layers.The present solutions may also be extended to elastomer / PIP gaskets as shown by multilayer gasket 350 in FIG. 8, e.g., by applying first layer 45 / material A and pre-treating it, followed by applying second layer 47 / material B and pre-treating it as described above. An optional layer 51 / material C may be deposited between material A and material B to further optimize sealing properties and sealability. The final curing can then take place at higher temperatures. The advantages of improved resistance to gas permeation and corrosion are thereby realizable at several different locations and types of gaskets within the representative FCS 12 of FIG. 1.

Claims

A fuel cell stack (18) comprising: a pair of end plate units (30A, 30B) each having a metal plate (40), a dielectric plate (42), and a circumferential groove (35) defined by the dielectric plate (42) and the metal plate (40); a coolant inlet (133C) and a gas inlet (32) configured to receive coolant (19) and a reactant gas (17R) into the fuel cell stack (18), respectively; a plurality of fuel cells (20), each individual fuel cell (20) of the plurality of fuel cells (20) comprising a pair of bipolar plates (38); and a multilayer gasket (15) disposed within the fuel cell stack (18) on the end plate units (30A, 30B) or the bipolar plates (38), the multilayer gasket (15) comprising: a first layer (45) made of a first material (A) substantially impermeable to the reactant gas and the coolant; and a second layer (47) made of a second material (B) more corrosion resistant than the first material (A); wherein the multilayer gasket (15) comprises a third layer (51) disposed between the first layer (45) and the second layer (47), the third layer (51) made of a third material (C) different from the first material (A) and the second material (B); wherein the third material (C) is air such that the third layer (51) comprises an air gap defined between the first layer (45) and the second layer (47).The fuel cell stack (18) according to claim 1, wherein the multilayer gasket (15) is disposed between the metal plate (40) and the dielectric plate (42) of each of the end plate units (30A, 30B).The fuel cell stack (18) according to claim 2, wherein an adhesive strength and a maximum tensile elongation of the second layer (47) to the dielectric plate (42) are higher than an adhesive strength and a maximum tensile elongation of the first material (A) to the dielectric plate (42).The fuel cell stack (18) of claim 1, wherein the bipolar plates (38) each have a metal bead joint (60), and wherein the multi-layer seal (15) has a thin-film seal (350) disposed on a surface (80) of the metal bead joint (60).The fuel cell stack (18) of claim 4, wherein the first material (A) of the multi-layer gasket (15) includes a foamed fluorinated carbon based synthetic rubber (fluororubber, FKM), and wherein the second layer (47) is deposited on the surface (80) of the metal bead joint (60) and has a solid FKM.A fuel cell system (12) comprising: a reactant storage tank (17) containing a reactant gas (17R); a fuel cell stack (18) according to any preceding claim in fluid communication with the reactant storage tank (17) and configured to receive a coolant (19) and the reactant gas (17R); and a load configured to be driven by an electrical current from the fuel cell stack (18).

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