Patent Publication Number: US-2021167402-A1

Title: Fuel cell sub-assembly and method of making it

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. Nonprovisional patent application Ser. No. 16/190,279, filed on Nov. 14, 2018, which is a division of U.S. Nonprovisional patent application Ser. No. 15/026,441, filed Mar. 31, 2016, which is a U.S. National Stage Entry Application of International Application No. PCT/CA2014/050947, filed Oct. 1, 2014, which claims the benefit of U.S. Provisional Patent Application No. 61/885,652, filed on Oct. 2, 2013, the disclosures of all of which are hereby expressly incorporated herein by reference. 
    
    
     FIELD 
     This specification relates to electrochemical cells, such as fuel cells, and in particular to a sub-assembly of plates and seals for use in making in a cell stack, and to methods of making the sub-assembly. 
     BACKGROUND 
     A proton exchange membrane (PEM) fuel cell (PEMFC), alternatively called a polymer electrolyte membrane fuel cell, typically comprises an anode plate and a cathode plate separated by a membrane electrode assembly (MEA), typically with a gas diffusion layer (GDL) between each side of the MEA and its adjacent plate. The surfaces of the anode plate and cathode plate that face the MEA are shaped to provide a flow field for the reactant gasses, typically hydrogen and air. A PEM fuel cell stack comprises an assembly of fuel cells clamped between and end plates, end plate insulator and current collector at each end of the stack. In the stack, the anode plate and cathode plate of adjacent fuel cells are electrically connected and may be provided by a bipolar. The bipolar plate may be a unitary structure or an anode plate and cathode plate bonded together. Coolant flow fields may be provided between adjacent fuel cells, either between every pair of successive fuel cells or at some lesser interval, for example after every second to fifth fuel cell. The coolant flow fields may be provided within a bipolar plate, between abutting anode and cathode plates, or in a separate plate. Typically, there are also various holes through the thickness of the plates. These holes collectively define conduits through the stack (perpendicular to the plates) to transport reactants, reaction products, or coolant to or from the individual fuel cells. Seals are required between each flow field and the adjacent MEA. Seals are also required around the holes in the plates, and between the holes and their associated flow fields. Seals may also be required around coolant flow fields. Optionally, seals may also electrically insulate the anode plate and cathode plate of a fuel cell, or between adjacent bipolar plates. Due to the large number of seals and plates in a fuel cell stack, methods of making and assembling these components are constantly in need of alternatives to provide improvements or to be suited to selected manufacturing techniques and materials. 
     In U.S. Pat. No. 6,599,653, anode and cathode plates are molded from plastic composites that include graphite. The anode and cathode plates are made into a sub-assembly called a fuel cell unit. Each fuel cell unit also includes an insulation layer on the bottom of the anode plate, a bead of sealant between the anode plate and the cathode plate, and another bead of sealant on the top of the cathode plate. 
     The anode and cathode plate have aligned gates to facilitate the flow of a curable liquid silicone through the plates and grooves to receive the beads of sealant. A fuel cell unit is made by placing an anode plate and cathode plate on the floor of a mold with the anode plate spaced from the floor of the mold. Liquid silicone is then forced through the gates and into the space between the anode plate and the floor of the mold. When the silicone cures, the insulation layer and the two beads of sealant are formed as a unitary, contiguous mass. This mass bonds the anode plate and cathode plate together and provides an insulation layer and seal on opposed sides of the bonded plates. 
     U.S. Pat. No. 7,210,220 describes a sealing technique for fuel cells and other electrochemical cells. To provide a seal, a groove network is provided through various elements of a fuel cell assembly. One fuel cell assembly includes anode and cathode plates, MEAs and GDLs for several fuel cells, all clamped together between end plates, end plate insulators and current collectors. Insulating material is provided between the anode and cathode plates of each fuel cell to prevent shorts across the fuel cells. The insulation may be provided as part of an adjacent MEA (for example as a non-conductive flange bonded to the MEA), by a GDL which extends to the edge of the plate, or by using plates that are made non-conductive or covered with an insulator in these areas. A source of seal material is then connected to an external filling port and injected into the groove network. When the sealing material cures, it forms a “seal in place” that bonds and seals the fuel cell assembly elements. In an alternative embodiment, a Membrane Electrode Unit (MEU) is made which comprises 1 to 5 sealed in place fuel cells. At least one of the outer faces of the MEU has an outer seal. This outer seal is adapted to seal to another MEU. Typically, an outer face of the MEU is adapted to form a cooling chamber with the other MEU. A fuel cell stack is produced by assembling any number of MEUs with end plates, end plate insulators and current collectors. 
     SUMMARY OF THE INVENTION 
     The following summary is intended to introduce the reader to the detailed description to follow and not to limit or define any claimed invention. 
     A sub-assembly for an electrochemical stack described in this specification has a bipolar plate with flow fields on its upper and lower faces, and a sealing material bonded to the bipolar plate. The sealing material extends from the upper face of the bipolar plate, around the edge of the bipolar plate, and onto the lower face of the plate. Preferably, the sealing material also forms a bead around the periphery of one or both of the flow fields. Preferably, the sealing material also forms beads around one or more holes for reactant, combustion product, or coolant flow through the bipolar plate. 
     The bipolar plate may be a unitary structure or, preferably, a combination of an anode plate and a cathode plate bonded together and having an internal coolant flow field. In this case, the anode plate and the cathode plate may be bonded together by sealing material which also provides a seal around the coolant flow field. One or both of the plates preferably has one or more gates through its thickness, or extending inwards from its edge, to allow liquid sealing material to be injected between the plates. Optionally, all of the sealing material in the sub-assembly may be one contiguous mass. 
     In a method of making a sub-assembly described in this specification, a single anode plate and a single cathode plate are loaded into a mold in a liquid injection molding machine such that reactant flow fields on the plates face away from each other. A liquid sealing material, for example liquid silicone rubber, is injected into the mold and fills a gap between the edge of the plates, and portions of the outer faces of the plates, and the mold. The liquid sealing material may also flow through various grooves or gates, or both, of the plates. Preferably, sealing material extending around the edges of the plates, sealing material bonding the anode and cathode plates together, and sealing material sealing around a coolant flow field between the plates, are all applied while the plates are in a single mold. Preferably, all of the sealing material applied to the plates merges into a single mass. 
     An electrochemical cell stack, for example a PEM fuel cell stack, described in this specification has a plurality of sub-assemblies as described above, or sub-assemblies made by the method described above. Within the stack, a GDL is located on the upper face of a lower sub-assembly, preferably within the sealing material on the upper face of the lower sub-assembly. An MEA is located over the GDL and at least partially overlaps with the sealing material on the upper face of the lower sub-assembly. A second GDL is located over the MEA, preferably within the sealing material on the lower face of an upper sub-assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic plan view of a face of a sub-assembly. 
         FIG. 2  is a schematic plan view of another face of the sub-assembly of  FIG. 1 . 
         FIGS. 3 and 6  are schematic cross sections of portions of the sub-assembly of  FIGS. 1 and 2 . 
         FIGS. 4 and 5  are schematic cross sections of portions of alternative sub-assemblies. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show a sub-assembly  10  for an electrochemical cell, for example a PEM fuel cell. The sub-assembly  10  has an anode plate  12  and a cathode plate  14  located back to back. The anode plate  12  is visible in  FIG. 1  and the cathode plate is visible in  FIG. 2 . The plates  12 ,  14  have faces  30 , which are visible in  FIGS. 1 and 2 , and edges  32 , which are shown as lines bordering the faces  30  in  FIGS. 1 and 2 . Either of the outer faces  30 , visible in  FIGS. 1 and 2 , may be called an upper or a lower face  30  depending on the orientation of the sub-assembly  10 . Inner faces  30  of the plates  12 ,  14  contact each other and are not visible in  FIGS. 1 and 2  but appear as lines in the other Figures. The plates  12 ,  14  are made of a conductive material. For example, the plates  12 ,  14  may be made of a metal such as stainless steel or, preferably, a molded composite of a plastic or other resin mixed with graphite. 
     The sub-assembly  10  also has one or more masses of cured sealing material  16 . The sealing material  16  may be made by curing any suitable curable liquid such as liquid silicone rubber (LSR), a polysiloxane elastomeric material as described in U.S. Pat. No. 7,210,220, an ethylene acrylic polymer, an ethylene propylene terpolymer, an epoxy resin or a thermoplastic elastomer. The sub-assembly  10  does not include a gas diffusion layer (GDL) or membrane electrode assembly (MEA) and instead consists essentially of the plates  12 ,  14  and cured sealing material  16 . Preferably, the sub-assembly  10  consists only of the plates  12 ,  14  and sealing material  16 . 
     Preferably, the plates  12 ,  14  are not bonded together other than by the sealing material  16 . Optionally, the plates  12 ,  14  may be separately bonded together, for example by an epoxy resin mixed with graphite and applied to the inside face of one or both of the plates  12 ,  14 . However, this requires an extra step, and additional manufacturing equipment and space, all of which can be avoided by using the sealing material  16  to bond the plates  12 ,  14 . 
     One or both of the plates  12 ,  14 , for example the anode plate  12 , preferably defines a coolant flow field  18  between the plates  12 ,  14 . The outer face of the anode plate  12  also defines an anode flow field  20  and the outer face of the cathode plate  14  defines a cathode flow field  22 . The flow fields  18 ,  20 ,  22  are typically more complex than what is shown in  FIGS. 1 and 2 . 
     Aligned openings  24  through the plates  12 ,  14  define parts of conduits. The openings  24  may be collected at the ends of the plates  12 ,  14  as shown, or provided in different locations. In the sub-assembly  10  shown, one opening  24   a  is provided to supply a reactant, typically air, to the cathode flow field  22  and then to a second opening  24   b  provided to remove excess air, or nitrogen, and water. A third opening  24   c  is provided to input another reactant, for example hydrogen, to the anode flow field  20  and then to a fourth opening  24   d  to remove excess hydrogen. A fifth opening  24   e  is provided to supply a coolant, for example water or water mixed with an anti-freezing agent, to the coolant flow field  18  and then out through a sixth opening  24   f . Optionally, more or less openings  24  may be used. For example, in an air cooled cell stack, the coolant flow field  18  is open at two opposed sides of the plates  12 ,  14  and openings  24  for coolant flow are not required. 
     The plates  12 ,  14  provide a bipolar plate with an internal coolant flow field  18 . To create a PEM fuel cell stack, two or more sub-assemblies  10  are stacked together. A gas diffusion layer (GDL), a membrane electrode assembly (MEA) and a second GDL are placed between successive sub-assemblies  10 . The gas diffusion layers extend generally across the anode flow field  20  and cathode flow field  22 , but preferably end within sealing material  16  on the faces  30  of the plates  12 ,  14 . The MEA extends across the anode flow field  20  and the cathode flow field  22 , and at least overlaps with sealing material  16  on the faces  30  of the plates  12 ,  14 . Optionally, the MEA may also extend from the reactant flow fields  20 ,  22  and overlap with sealing material surrounding one or more openings  24  that define reactant conduits. In this way, the reactants are sealed on opposite sides of the MEA. Passages  34  in the plates  12 ,  14  between the openings  24  and the flow fields  18 ,  20 ,  22  are shown in a simplified form in  FIGS. 1 and 2  but can also be provided in other configurations known in the art. For example, whereas  FIGS. 1 and 2  show a “back side feed” configuration in which passages  34  for reactants are provided on inner faces  30  of the plates, the passages  34  may alternatively be located in the outer faces  30  of the plates  12 ,  14 . 
     Sealing material  16  is applied to the plates  12 ,  14  in a liquid form and then cured on, and preferably between, the plates  12 ,  14 . The plates  12 ,  14  are placed in a mold having recesses to define outer surfaces of the sealing material  16  on the outer faces  30  and edges  32  of the plates  12 ,  14 . This mold is placed in a liquid injection molding (LIM) press or other suitable molding machine. The liquid sealing material, preferably liquid silicone rubber, is then injected into the mold and cured. Vents are provided in the mold or the plates  12 ,  14  to allow air to escape as the sealing material  16  is injected into the mold. The size, number and location of mold injection points and vents can be determined by methods known in the art of injection molding. Injected liquid sealing material  16  flows quickly around the periphery of the plates in the injection mold which advantageously reduces the number of injection points to the mold that are required and can also reduce, or optionally eliminate, the need for injection molding gates through the thickness of the plates  12 ,  14 . 
     When using composite molded plates  12 ,  14 , some water (or another coolant fluid) may diffuse from the coolant flow field  18  through the plates  12 ,  14  themselves. Water (or vapor) that diffuses into the reactant flow fields  20 ,  22  is carried away with the flows of the reactants or reaction products and typically causes no harm. However, some water can also appear at the edges of the plates  12 ,  14 . This water can cause problems, such as shorting between adjacent fuel cells or interference with balance of plant elements around a stack. For this reason, the sealing material  16  preferably includes an edge sealing portion  26  that wraps around the edges  32  of the plates  12 ,  14 . The edge sealing portion  26  also electrically isolates the edges  32  of the plates  12 ,  14 , which is useful even with metal plates  12 ,  14 . The edge sealing portion  26  is preferably contiguous around the entire periphery of the plates  12 ,  14 . The MEAs preferably do not extend to the edges  32  of the plates. In this way, the side of a complete stack is practically insulated in that a solid conductor touching the outside of a stack is unlikely to cause a short. 
     The sealing material  16  preferably provides various bead portions  28 . The bead portions  28  seal the reactants on either side of the MEA and may also help seal between the openings  24  of adjacent sub-assemblies  10  in a stack. The shape of the bead portions  28  is selected to produce a sufficient pressure against the MEA when a stack is clamped together. Preferably, the bead portions are located over grooves  42  in the plates  12 ,  14 . The bead portions  28  may be provided on one or both sides of the plates  12 ,  14  around the peripheries of the reactant flow fields  20 ,  22 . Preferably, the edge sealing portion  26  of the sealing material  16  extends from a bead portion  28  on one outer face  30  of the sub-assembly  10  to the bead portion  28  on the other outer face  30  of the sub-assembly  10  to form one continuous mass of sealing material. The bead portions  28  are made thicker than adjacent parts of the edge sealing portion  26  on the outer face of a plate  12 ,  14 . This is avoids needlessly increasing the total force that would be required to provide sufficient compression in the bead portions  28 . It also helps allow the deformation of the sealing material  16  to be consistent as between bead portions  28  located near the edges  32  of the plates  12 ,  14  and bead portions  28  displaced from the edges  32  by openings  24 . Optionally, additional bead portions  28  may be located near at or the edges  32  of the plates  12 ,  14 , beyond the area that will be overlapped by the MEA, to better insulate the edges of the MEA from the sides of a stack. 
       FIG. 3  shows a portion of an alternative sub-assembly  10   a  in cross section. In this portion of the sub-assembly  10   a  there is no opening  24  and a coolant flow field  18  extends to near the edge  32  of the plates  12 ,  14 . The thickness of the plates  12 ,  14  is exaggerated in  FIG. 3  (and in  FIGS. 4 to 6 ) and each may be on the order of 1 mm. 
     Although it is possible for the edges  32  of the plates  12 ,  14  to form a single plane as in  FIGS. 1 and 2 , the resulting edge sealing portion  26  alone might not provide an adequate seal around an internal coolant field  18 . In  FIG. 3 , the edges  32  of the plates  12 ,  14  have a step  40  to provide additional sealing material  16  near the inner faces  30  of the plates  12 ,  14 . Preferably, the step  40  is provided around the entire periphery of the plates  12 ,  14 . Optionally, the step  40  could be provided in only the anode plate  12  or only the cathode plate  14 . Alternatively, the step  40  may have another profile rather than the generally rectangular notch shown. 
       FIG. 4  shows a portion of another alternative sub-assembly  10   b  in cross section. In this case, a key  44  is provided in the anode plate  12 . Optionally, the key  44  could be provided in the cathode plate  14  or keys  44  could be provided in both plates  12 ,  14 . The key  44  again provides additional sealing material near the inner faces of the plates  12 ,  14 . In addition, the key  44  mechanically locks the edge sealing portion  26  of the sealing material  16  to the edges  32  of the plates. For this purpose, the key  44  is preferably provided around the entire periphery of the plates  12 ,  14 . The key  44  may have a profile other than the profile shown. 
       FIG. 5  shows a portion of another alternative sub-assembly  10   c  in cross section. In this case, a groove  42  is located on the inner face  30  of the anode plate  12 . Optionally, a groove  42  may be located on the inner face  30  of the cathode plate  12 , or on the inner faces  30  of both plates  12 ,  14 . This groove  42  may be located directly below, or overlapping with, a groove  42  on an outer face  30  of the same plate  12 ,  14 . However, it is preferable for a groove  42  on an inner face  30  of a plate  12 ,  14  to be located either inside (away from the edge  32 ) or outside (towards the edge  32 ) of a groove  42  on an outer face  30  of the same plate  12 ,  14  to avoid having a very thin section in the plate  12 ,  14 . Liquid sealing material  16  may be fed to a groove  42  on an inner face  30  of a plate  12 ,  14  through one or more gates  46 . The gates  46  may, for example, pass through the thickness of a plate  12 ,  14 . Alternatively, or additionally, gates  46  may be provided in the form of channels molded into the inner face  30  of a plate  12 ,  14  and contiguous with the edge  32  of the plate  12 ,  14 . 
       FIG. 6  shows another portion of the alternative sub-assembly  10   a  of  FIG. 3  in cross section. In this portion of the sub-assembly  10   a  there is an opening  24  between the coolant flow field  18  and the edges  32  of the plates  12 ,  14 . The sealing material  16  surrounds the opening  24  on the outer faces  30  of the plates  12 ,  14 , preferably by way of beaded sections  28  located over grooves  42 . The sealing material  16  also surrounds the opening  24  in the inside face  30  of one, or optionally both, plates  12 ,  14 . The internal sealing material  16  required to surround the opening  24  flows inwards from the edge  32  of a plate  12 ,  14  through grooves  42  or gates  46  formed in the inside face  30  of the plate  12 ,  14 . 
     Alternatively or additionally, sealing material  16  required to surround the opening  24  may also be provided through one or more gates  46  through the thickness of a plate  12 ,  14 . Sealing material  16  may be provided around an opening  24  in the sub-assemblies  10  of the  FIG. 1, 2, 4 or 5  in a similar manner. 
     In further alternative structures, a coolant flow field may be provided in a separate plate rather than as part of the cathode plate  14  or anode plate  12 . The coolant field plate may be connected to an opening  24  in the plate or to an external coolant jacket or to the atmosphere. In this case, some of the sub-assemblies  10  in a stack may be made as described above but without a coolant field  18  by omitting the coolant field plate. In sub-assemblies  10  with a coolant field  18 , the coolant field plate is placed between the cathode plate  14  and anode plate  12  in a mold and sealing material  16  is injected around the edges  32  of all three plates as described above. The coolant field plate can be sealed to either, or both, of the cathode plate  14  and anode plate  12  by the edge sealing portion  26  alone or as shown for seals between the cathode plate  14  and anode plate  12  in any of  FIGS. 3 to 6 . 
     The sealing material  16  both seals to the MEA when compressed in a stack and separate adjacent sub-assemblies  10  in a stack. Although many individual sub-assemblies must be made, the bead portions  28  assist in locating the GDLs and MEAs while forming a stack. The various methods described in U.S. Pat. No. 7,210,220 to avoid shorting the fuel cells when using a seal in place are not required. The stack may be disassembled and the MEAs examined if the stack is defective. Yet, the edges of the plates  12 ,  14  are sealed against coolant leakage without requiring additional steps. In this way, the sub-assemblies  10  at least provide a useful alternative to the seal in place method. As discussed above, in some cases gates through the thickness of the plates  12 ,  14  can be reduced or eliminated. 
     Although the sub-assembly  10  has been described above for use in a PEM fuel cell, a sub-assembly  10  as described above may also be used in another type of fuel cells, in a PEM or other type of electrolyser, or in electrolytic cells generally. The sub-assembly  10 , and the method of making it, may also be modified in various ways within the scope of the invention, which is defined by the following claims.