Abstract:
Fuel cell gaskets are employed to seal around an individual cell of a fuel cell assembly. The individual cell includes a membrane electrode assembly where some of the components do not extend as close to the perimeter of the cell as others, thereby creating a gap. The gaskets, which may be formed of laminated layers, each include a pair of sealing beads extending therefrom. The sealing beads are formed to extend toward and seal against separator plates, and have different heights to account for the gap caused by the discontinuous layers of the membrane electrode assembly.

Description:
BACKGROUND OF INVENTION  
       [0001]     This invention relates in general to static seals and more particularly to a gasket employed for sealing between components in a fuel cell.  
         [0002]     A fuel cell is an electrochemical energy converter that includes two electrodes placed on opposite surfaces of an electrolyte. In one form, an ion-conducting polymer electrolyte membrane is disposed between two electrode layers (also sometimes called gas diffusion layers), with layers of a catalyst material between the membrane and the electrode layers, to form a membrane electrode assembly (MEA). The MEA is used to promote a desired electrochemical reaction from two reactants. One reactant, oxygen or air, passes over one electrode while hydrogen, the other reactant, passes over the other electrode. The oxygen and hydrogen combine to produce water, and in the process generate electricity and heat.  
         [0003]     An individual cell within a fuel cell assembly includes a MEA placed between a pair of separator plates (also sometimes called flow field plates). The separator plates are typically fluid impermeable and electrically conductive. Fluid flow passages or channels are formed adjacent to each plate surface at an electrode layer to facilitate access of the reactants to the electrodes and the removal of the products of the chemical reaction.  
         [0004]     In such fuel cells, resilient gaskets or seals are typically provided between the faces of the MEA and the perimeter of each separator plate to prevent leakage of the fluid reactant and product streams. Since the fuel cell operates with oxygen and hydrogen, it is important to provide a seal that not only seals well against hydrogen, oxygen and water, but that will seal well as the temperature changes due to the heat that is given off during fuel cell operation. To assure a good seal, the seals need to be formed accurately as well as have the proper sealing force applied along the seal after being assembled into an individual cell. In particular, the appropriate sealing force can be difficult to attain since different layers within the cell may only extend out a portion of the way to the perimeter of the cell. An adhesive (and in particular, a pressure sensitive adhesive) can be employed to aid in the assembly and sealing of components, but it is not always desirable to use an adhesive in a cell assembly. The assembly cycle time may be more than is desirable because one must wait for the adhesive to cure. Moreover, the gasket may need to be thicker than is otherwise necessary in the area of the pressure sensitive adhesive in order to obtain the proper adhesion of the adhesive during assembly.  
         [0005]     Thus, it is desirable to have a gasket of an individual cell of a fuel cell that is relatively easy to align and secure to the other components during an assembly operation, while assuring the proper sealing force in the finished assembly.  
       BACKGROUND AND SUMMARY OF INVENTION  
       [0006]     In its embodiments, the present invention contemplates a seal for use in an individual cell of a fuel cell comprising: a first gasket including a first surface adapted to be adjacent to a first separator plate, a first sealing bead extending beyond the first surface a first distance, and a second sealing bead extending beyond the first surface a second distance, with the second distance being greater than the first distance.  
         [0007]     The present invention further contemplates an apparatus for use in an individual cell of a fuel cell assembly comprising: a membrane electrode assembly having a plurality of layers, with a portion of the plurality of layers being discontinuous relative to the other layers; and a first gasket mounted to a first side of the membrane electrode assembly and including a first surface adapted to be adjacent to a first separator plate, a first sealing bead extending beyond the first surface a first distance, and a second sealing bead extending beyond the first surface a second distance, with the second distance being greater than the first distance.  
         [0008]     The present invention also contemplates assembling an individual cell of a fuel cell by: providing a membrane electrode assembly; assembling a first side of a first gasket to a first side of the membrane electrode assembly; assembling a first side of a second gasket to a second side of the membrane electrode assembly; assembling a first separator plate to a second side of the first gasket, wherein the first gasket includes a first sealing bead and a second sealing bead extending outward from the second side toward the first separator plate, with the second sealing bead being closer to a perimeter of the individual cell and extending outward farther from the second surface than the first sealing bead; assembling a second separator plate to a second side of the second gasket; and compressing the first and second separator plates toward each other to create a sealing force on the first and second sealing beads.  
         [0009]     It should be noted that because any of the above-mentioned sealing beads can have a “height”, or protrude outwardly, a distance that is different from that of any one or more of the other sealing beads, the invention assures that there is sufficient sealing force on each sealing bead, while assuring that none of the sealing beads are over-compressed. This is true even though the different layers of the fuel cell assembly do not all extend the same distance out toward the perimeter of the fuel cell. Moreover, the various sealing forces on the various sealing beads will cause the gaskets and the gas diffusion layers to compress together, eliminating any undesirable gap.  
         [0010]     An advantage of the present invention is that gaskets allow for a multi-layer laminate to have non-continuous layers while still providing the appropriate sealing forces around a MEA in a cell. Thus, the single gasket will allow a non-continuous MEA to mate with and seal to a plate without requiring the over-compression one seal section or under-compression on another.  
         [0011]     Another advantage of the present invention is that the sealing can be accomplished without requiring the use of an adhesive around the perimeter of the MEA. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a partially exploded, section cut through a portion of an individual cell of a fuel cell assembly prior to assembling separator plates thereto; and  
         [0013]      FIG. 2  is a partially exploded, section cut through a portion of an individual cell according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]      FIG. 1  illustrates a portion of an individual cell  20  for use in a fuel cell assembly. The individual cell  20  preferably includes a gasket unitized membrane electrode assembly (MEA)  22 , (although the gasket may be separate rather than unitized, if so desired). The MEA  22  is made up of a membrane  24 , with a layer of catalyst material  26  on both sides of the membrane  24 . The MEA  22  also includes a first gas diffusion layer (GDL)  30  and second GDL  32  on either side of the layers of catalyst material  26 , and a first gasket  34  and a second gasket  36 , secured around the perimeters of the first GDL  30  and the second GDL  32 , respectively. The gaskets  34 ,  36  may be secured to the GDLs  30 ,  32  by adhesive, although other means of securing may be used if so desired, such as molding each gasket to its GDL. The first gasket  34  is shown as much larger and a different shape than the second gasket  36  to illustrate that different shapes and thicknesses may be employed. However, the actual relative thickness of a gasket and shape of its sealing beads depends upon the particulars of the individual cell being sealed. Typically, though, the components of the cell  20  are generally symmetric about the membrane  24 .  
         [0015]     A first separator plate  38  mounts against the first gasket  34  and the first GDL  30 , and a second separator plate  40  mounts against the second gasket  36  and the second GDL  32 , in order to form the individual cell  20 . Since the relative thicknesses of the various components are very thin, the thicknesses are only depicted schematically in the figures in order to aid in describing the invention. The actual thicknesses of the components may vary according to the particular application of the fuel cell and are known to those skilled in the art.  
         [0016]     The membrane  24  is preferably an ion-conducting, polymer, electrolyte membrane, as generally employed in this type of fuel cell application. The catalyst material  26  is preferably platinum or other suitable catalyst material for a typical polymer electrode membrane type of fuel cell application. The first and second GDLs  30 ,  32  are preferably a carbonized fiber, or may be another suitable gas permeable material for use as an electrode in a fuel cell. The MEA  22  can include a catalyzed membrane with GDLs assembled thereto, or a membrane assembled between two catalyzed GDLs, each of which is known to those skilled in the art.  
         [0017]     In this embodiment, the membrane  24  and catalyst layers  26  do not extend outward toward the perimeter as far as the gas diffusion layers  30 ,  32 . This non-continuity of the membrane layer  24  and catalysts  26  creates a gap  50  with a thickness G. A thin electrically insulating filler material  60  fills a portion of the thickness G of the gap  50 , and serves to electrically insulate the first gas diffusion layer  30  from the second gas diffusion layer  32 .  
         [0018]     The gaskets  34 ,  36 , are each preferably a laminated gasket with a thin, flexible carrier  52 ,  54  upon which an elastomeric seal  56 ,  58 , respectively, is secured. The carriers  52 ,  54  each mount adjacent its respective gas diffusion layer  30 ,  32 , between which is formed the gap  50 . Each carrier  52 ,  54  preferably has a thickness of less than  1 . 0  millimeter and is preferably made from a polymer substrate, such as, for example polyimide or polyester. Each elastomeric seal  56 ,  58  is preferably molded to its carrier  52 ,  54 , although other means of securing the two may also be employed if so desired.  
         [0019]     The first gasket  34  includes a first sealing bead  64  and a second sealing bead  66 , which each extend around substantially the entire perimeter of the cell  20  in order to accomplish the sealing function when compressed against the first separator plate  38  under a predetermined sealing force. A first set of channels  68 ,  70 , 72  are formed adjacent these sealing beads  64 ,  66 . These channels  68 ,  70 ,  72  provide space for the sealing beads  64 ,  66  to expand into when compressed with the sealing force during assembly of the cell  20 . The first sealing bead  64 , in its uncompressed state, extends beyond the outer surface  74  of the first gasket  34  a distance H 1 , while the second sealing bead  66 , in its uncompressed state, extends beyond the outer surface  74  of the first gasket  34  a distance H 2 . The distance H 2  is greater than distance H 1  by about one half of the difference between the thickness of the gap G minus one half the thickness of the filler  60 .  
         [0020]     The second gasket  36  includes a third sealing bead  76  and a fourth sealing bead  78 , which each extend around substantially the entire perimeter of the cell  20  in order to accomplish the sealing function when compressed against the second separator plate  40  under the sealing force. A second set of channels  80 ,  82 ,  84  are formed adjacent these sealing beads  76 ,  78 . The channels  80 ,  82 ,  84  provide space for the sealing beads  76 ,  78  to expand into when compressed with the sealing force. The third sealing bead  76 , in its uncompressed state, extends beyond the outer surface  86  of the second gasket  36  a distance H 3 , while the fourth sealing bead  78 , in its uncompressed state, extends beyond the outer surface  86  of the second gasket  36  a distance H 4 . The distance H 4  is greater than distance H 3  by about one half of the difference between the thickness of the gap G minus one half of the thickness of the filler  60 .  
         [0021]     Each sealing bead  64 ,  66 ,  76 ,  78  is designed to be compressed against the surface of its corresponding separator plate  38 ,  40  and held with sufficient sealing force to prevent migration of fluid past the seals  34 ,  36  along the surface of the separator plates  38 ,  40 . During assembly of a cell  20 , as the separator plates  38 ,  40  are brought toward the gaskets  34 ,  36 , they will first contact the second sealing bead  66  and the fourth sealing bead  78 , respectively. As the separator plates  38 ,  40  continue to be moved toward one another, the sealing beads  66 ,  78  begin to compress, and the gaskets  34 ,  36  and GDLs  30 ,  32  bend toward the filler  60 . The plates  38 ,  40  then contact and begin compressing the first  64  and third  76  sealing beads as well. Since the heights of the second  66  and fourth  78  sealing beads are greater than the first  64  and third  76  sealing beads, when in the fully assembled position, the four sealing beads  64 ,  66 ,  76 ,  78 , will be compressed and the GDL&#39;s  30 ,  32  will be compressed against the filler  60 . The extra height of the two beads  66 ,  78  that are nearer to the perimeter of the cell  20  accounts for the open spaces in the gap  50  between the GDLs  30 ,  32  and the filler  60 .  
         [0022]     Consequently, by providing for the difference in relative height between the beads on each gasket, this assures that there is sufficient sealing force on each bead  64 ,  66 ,  76 ,  78 , while assuring that none of the sealing beads are over-compressed. This is true even though the different layers do not all extend the same distance out toward the perimeter of the cell  20 . Moreover, the sealing force on sealing beads  66 ,  78  will cause the gaskets  34 ,  36  and gas diffusion layers  30 ,  23  to compress together, eliminating the gap  50 .  
         [0023]      FIG. 2  illustrates another embodiment of the present invention. In this embodiment, similar elements to the first embodiment will be similarly designated, but with a 100 series number. The first gas diffusion layer  130  and the second gas diffusion layer  132  now only extend toward the perimeter about as far as the membrane  124  and catalyst layers  126 , while the first and second carriers  152 ,  154  and the first and second elastomeric seals  156 ,  158  extend closer to the perimeter of the individual cell  120 . This creates a gap  150  with a thickness of G′. With this embodiment, since the first GDL  130  cannot come into contact with the second GDL  132 , no filler material is needed to electrically insulate them from each other. Consequently, the thickness G′ of the gap  150  is about equal to the combined thicknesses of the membrane  124 , catalyst layers  126 , and the GDLs  130 ,  132 . The differences in heights of the four sealing beads  164 ,  166 ,  176 ,  178  above their respective surfaces  174 ,  186  will be represented by the following equation: (H 2 ′-H 1 +)+(H 4 ′-H 3 ′)=G′. Similar to the first embodiment, as the separator plates  138 ,  140  are compressed to the gaskets  134 ,  136 , the gap  150  will be closed and sealed, and the sealing beads  164 ,  166 ,  176 ,  178  will be compressed with the appropriate amount of sealing force against the plates  138 ,  140 .  
         [0024]     As mentioned above, in any of the embodiments of the invention, any of the above-mentioned sealing beads can have a “height”, or protrude outwardly away, a distance that is different from that of any one or more of the other sealing beads. Thus, the invention assures that there is sufficient sealing force on each sealing bead, while assuring that none of the sealing beads are over-compressed. This is true even though the different layers of the fuel cell assembly do not all extend the same distance out toward the perimeter of the fuel cell. Moreover, the various sealing forces on the various sealing beads will cause the gaskets and the gas diffusion layers to compress together, eliminating any undesirable gap.  
         [0025]     While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.