Abstract:
A method for forming a fuel cell assembly including the pre-final-assembly step of forming a plurality of fuel cell sub-assembly modules, each module including a predetermined number of individual fuel cell repeating units, for example, ten. Each module may be leak and performance tested and certified prior to inclusion in the final fuel cell stack, thus limiting potential rework to only an individual module and only before assembly of the final stack. Preferably, each module is assembled on an assembly fixture having alignment rods, using a combination of resilient gasketing and RTV to seal between the elements. The assembled module is then placed under compression while the silicone is cured.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to fuel cells incorporating a proton exchange membrane (PEM); more particularly, to a PEM fuel cell assembly comprising a stack of individual fuel cell units; and most particularly, to method and apparatus for forming a sealed stack sub-assembly module comprising a plurality of individual fuel cell units, a plurality of such sub-assembly modules being stackable to form the fuel cell assembly.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cell assemblies employing proton exchange membranes are well known. Such assemblies typically comprise a stack of individual fuel cells, each fuel cell having an anode and a cathode separated by a catalytic proton exchange membrane (PEM). The fuel cells in the stack are connected in series electrically to provide a desired voltage output. Gaseous fuel, in the form of hydrogen or hydrogen-containing mixtures such as “reformed” hydrocarbons, flows adjacent to a first side of the membrane, and oxygen, typically in the form of air, flows adjacent to the opposite side of the membrane. Hydrogen is catalytically oxidized at the anode-membrane interface, and the resulting proton, H+, migrates through the membrane to the cathode-membrane interface where it combines with anionic oxygen, O 31 2 , to form water. Protons migrate only in those areas of the fuel cell in which the anode and cathode are directly opposed across the membrane. Electrons flow from the anode through an external circuit to the cathode, doing electrical work in a load in the circuit.  
         [0003]     A complete fuel cell assembly typically comprises a plurality of individual fuel cells connected in series to form one or more fuel cell stacks. In a preferred embodiment, a bipolar plate assembly, comprising an anode, a cathode, and having formed passages for the flow of hydrogen to the anode and air to the cathode, is disposed adjacent an element known in the art as a Membrane Electrode Assembly (MEA). A repeating pattern of alternating bipolar plate assemblies and MEA elements form a stacked fuel cell assembly.  
         [0004]     Preferably, a Gas Diffusion Layer (GDL) element is also included between each bipolar plate assembly and an adjacent MEA to promote the distribution of gas uniformly over both the anode and the cathode.  
         [0005]     At the outer edges of the stacked fuel cell assembly, the bipolar plate assemblies and MEA elements are sealed together by gasket elements to contain the reactant gases and/or coolant within the assembly. Thus, an important aspect of forming a stacked fuel cell assembly is preventing leakage between the plate assemblies.  
         [0006]     Another important consideration is precisely aligning the multitude of bipolar plate assemblies. In the prior art, a fuel cell stack typically is formed by assembling, one at a time, alternating bipolar plate assemblies and MEA elements to form a fuel cell unit. A full stack for some applications comprising about 60 individual fuel cell units, and for some other applications up to 200 units. Typically, the bipolar plate assemblies and MEAs are bonded along their outer edges with silicone rubber or other inert, curable sealant, making any subsequent disassembly difficult, time-consuming, and hazardous to the individual stack elements.  
         [0007]     It is known to provide alignment holes in the stack and to use an assembly fixture having alignment pins. A problem arises in this arrangement however, in that the assembly cannot be tested for perimeter leaks until all the elements have been assembled together and the sealant cured. If a leak is detected, the stack must be disassembled down to the point of the leak to fix the leak. Once a stack has passed the leak test, it is performance tested. Again, if a bipolar plate assembly or MEA is found defective, the stack must be disassembled and reworked.  
         [0008]     What is needed is means for intermediate testing during assembly of a fuel cell stack to limit the amount of reworking necessary when any defect is found.  
         [0009]     It is a principal object of the present invention to reduce rework labor in assembling a fuel cell stack.  
         [0010]     It is a further object of the present invention to reduce the cost of manufacturing a fuel cell assembly.  
       SUMMARY OF THE INVENTION  
       [0011]     Briefly described, a method for assembling a fuel cell stack includes the pre-final-assembly step of forming a plurality of sealed fuel cell sub-assembly modules, each module including a predetermined number of individual, sealed fuel cell repeating units, for example, ten. Each module may be leak and performance tested and certified prior to inclusion in the final fuel cell stack, thus limiting potential rework to only an individual module and only before assembly of the final stack. Preferably, each module is assembled on an assembly fixture having alignment rods, using a combination of resilient gasketing and room temperature vulcanized rubber (RTV) to seal between the elements. The assembled module is then placed under compression while the silicone is cured. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which:  
         [0013]      FIG. 1  is an elevational cross-sectional view of a portion of a PEM fuel cell of the present invention showing the various components in an uncompressed state;  
         [0014]      FIG. 2  is an elevational cross-sectional view of a portion of the PEM fuel cell shown in box A in  FIG. 1 , showing the interface between the MEA, bipolar plate assembly and gasket and seal;  
         [0015]      FIG. 3  is an elevational cross-sectional view of a portion of the PEM fuel cell shown in box B in  FIG. 1  showing the bonding interface between the MEA and bipolar plate assembly;  
         [0016]      FIG. 4  is a plan view of a fuel cell stack positioned on an alignment fixture in accordance with the invention, with the top pressure plate of the fixture removed to show the top surface of the bipolar plate assembly;  
         [0017]      FIG. 5  is an elevational cross-sectional view of a portion of a multiple cell fuel cell sub-assembly module after assembly in an alignment and compressional fixture in accordance with the invention; and  
         [0018]      FIG. 6  is a schematic elevational view of a complete fuel cell assembly comprising a plurality of fuel cell sub-assembly modules in accordance with the invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     Referring to  FIGS. 1-3 , a portion of a PEM fuel cell stack  10  of the present invention is shown at a stage prior to stack compression. Stack  10  comprises a layered series of MEA elements  12  interspersed among an alternating series of bipolar plate assemblies  14 . Anodes  16  and cathodes  18  are bonded together at interface  20  to form bipolar plate assemblies  14 . A five layer MEA element  12 , as shown in  FIGS. 1 and 2 , comprises outer gas diffusion layers  26 , catalyst layers  24 , and center membranes  22 . Alternately, a three layer MEA element could be used with separate gas diffusion layers. The three or five layer MEA element as described herein preferably consists of a central membrane made of Nafion®, available from E.I duPont de Nemours and Company of Wilmington, Del.  
         [0020]     As best shown in  FIG. 2 , Each MEA element  12  extends between cathode  18  of a first bipolar plate assembly  14  and anode  16  of an identical and adjacent second bipolar plate assembly  14 . Elements  30  and  32  of elastomeric gasket  31  ( FIGS. 2-4 ) are disposed in shallow grooves  34  in anodes  16  and bonded  33  to the anodes using a preparation and bonding process similar to that disclosed in commonly owned, pending patent application Ser. No. 10/256,770. Elastomeric gasket  31  thus provide a seal for preventing leakage of reactive gases between the plate assemblies. In addition, element  32  of elastomeric gasket  31  is also bonded to adjacent cathode  18  via portion  40  of seal gasketing element  38  to bond the bipolar plates together and, once seal gasketing element  38  is cured, to thereby form a modular assembly of MEA elements  12  and bipolar plate assemblies  14 .  
         [0021]     Referring to  FIG. 2 , for purposes of the present invention, the sequence of a bipolar plate assembly, gasketing elements and MEA element defines a “fuel cell unit”  36 . In the present invention, edge portion  28  of MEA element  12  extends beyond gas diffusion layers  26  and is disposed between elastomeric gasket element  30  and cathode  18 . Edge portion  28  does not extend between elastomeric gasketing element  32  and cathode  18 . Seal gasketing element  38  is disposed, as a thin film, on surfaces  42  and  44  of cathode  18 . A preferred and well known method for applying a thin film of the composition is screen printing, by which means complex patterns of the seal are readily provided as may be needed to accommodate complex sealing surfaces of fuel cell elements. Other methods of application, for example, roller application, are of course within the scope of the invention. The thickness of seal gasketing element  38  is preferably on the order of 0.005 inch or less, and preferably between about 0.001 inch and about 0.003 inch, and are readily formed in a single printing pass. Seal gasketing element  38  is preferably formed of a cross-linked silicone composition, for example, an organopolysiloxane such as 3140 RTV after first reducing the RTV with OS30. Both the 3140 RTV and the OS30 reducing agent are manufactured by Dow Corning Corporation of Midland, Mich. The seal gasketing element is then cured in place by atmospheric moisture and/or an incorporated activator to form a thin non-fluid elastomeric layer after curing.  
         [0022]     Seal gasketing elements  38  are applied to cathodes  18 , as described above, and are not allowed to cure prior to assembly of the of the bipolar plate assemblies to the MEA elements. After assembly of the bipolar plates to the MEA elements is completed, the plates are compressed together. Then, seal gasketing elements  38 , and particularly portion  40 , are allowed to cure while the plates are under compression to form fuel cell sub-assembly module  56 .  
         [0023]     Referring to  FIGS. 4 through 6 , an assembly fixture  46  for assembling a fuel cell stack module  56  in accordance with the invention includes a base plate  48  for receiving the stack and at least two spaced-apart alignment rods  50  (one is shown) secured at their lower ends to base plate  48 , as by threading, and extending outwards orthogonally from base plate  48 . A pressure plate  52  includes bores  54  for slidably fitting onto rods  50 . The stack module  56  comprises a plurality of bipolar plate assemblies  14 , each having bores  58  for accepting rods  50  such that upon assembly all plate assemblies  14  are highly aligned.  
         [0024]     Prior to stacking of the bipolar plate assemblies  14  and MEA elements  12  onto assembly fixture  46 , seal gasketing element  38  is applied to surfaces  42  and  44  of cathode  18  ( FIGS. 2 and 3 ). Then, once the bipolar plate assemblies are stacked and compressed by applying pressure  59  via pressure plate  52 , seal gasketing elements  38 , and particularly portion  40  of seal gasketing element  38 , are permitted to cure to form a non-fluid elastomeric layer and to bond elastomeric gasketing element  32  to cathode surface  44  to thereby form fuel cell subassembly module  56  comprised of a series of bonded together fuel cell units  36 .  
         [0025]     In a currently preferred method in accordance with the invention for forming a PEM fuel cell assembly  60  ( FIG. 6 ), a plurality of fuel cell sub-assembly modules  56  are pre-assembled, each module  56  being formed as follows (shown in  FIG. 5 ): 
        a) select n+1 number of bipolar plate assemblies  14  and n number of MEA elements  12 , n being the number of fuel cell units  36  desired in each sub-assembly module  56 , each bipolar plate assembly having pre-formed elastomeric gasket  31  on anodes  16  (in the example shown in  FIG. 5 , n=2);     b) apply a film of curable seal gasketing element  38  onto surfaces  42  and  44  of cathodes  18  of n+1 of the bipolar plate assemblies  14 ;     c) install onto base plate  48  of fixture  46  a bipolar plate assembly  14 , having cathode  18  of the bipolar plate assembly facing plate  48  with rods  50  extending though bores  58 ;     d) install an MEA element  12  onto the bipolar plate assembly  14  on the fixture  46  with edge portion  28  of MEA element  12  in contact with elastomeric gasketing element  30  of the previously installed bipolar plate assembly thereby forming a fuel cell unit  36 ;     e) install onto fixture  46  bipolar plate assembly  14  (with seal gasketing element  38  on surfaces  42  and  44 ) having cathode  18  of the bipolar plate assembly facing the previously installed bipolar plate assembly;     f) repeat step d;     g) repeat steps e) and f) for the remaining number of selected bipolar plate assemblies  14  and the remaining number of selected MEA elements  12  to form a fuel cell sub-assembly modular stack  56  having n fuel cell units;     h) install pressure plate  52  onto module  56 ;     i) exert suitable pressure  59  from a pressure source (not shown) onto pressure plate  52  while curing seal gasketing element  38 ; and     j) remove pressure  59  from plate  52 , remove plate  52  from rods  50 , and remove module  56  from assembly fixture  46 .        
 
         [0036]     Module  56  may then be submitted to leak and performance certification testing. Additional modules  56  are pre-assembled as above. A complete PEM fuel cell assembly  60  ( FIG. 6 ) containing m fuel cell units  36  may then be formed by stacking x modules  56  together, with appropriate MEA elements and seal gasketing elements therebetween, where m=xn. Typically, specialized end plates  62 ,  64  are included, bounding the stack of modules  56 , to complete the fuel cell assembly  60 . In the schematic example shown in  FIG. 6 , x=6.  
         [0037]     While the example of module  56  shown in  FIG. 5  comprises 2 fuel cell units  36  including 3 bipolar plate assemblies and 2 MEA elements, it is understood that a greater number of bipolar plate assemblies (and the required number of MEA elements) can be stacked together in assembly fixture  46  to form fuel cell sub-assembly  56 .  
         [0038]     While the method of forming module  56  as described above includes the steps of installing bipolar plate assemblies having their cathodes oriented to face the base plate of the fixture, it is understood that module  56  can be stacked in the fixture with the anodes of the bipolar plate assemblies facing the base plate.  
         [0039]     The method as recited assumes that elastomeric gasket  31  is formed and cured prior to this pre-assembly method, and that seal gasketing element  38  is applied and cured during the method. Of course, within the scope of the invention, the roles may be reversed, elastomeric gasket  31  being applied and cured during the method and seal gasketing element  38  being formed and cured prior to the method.  
         [0040]     The reliability of such an assembly formed in accordance with the invention is very high compared with a comparable assembly as formed in accordance with the prior art, because each of the x modules is tested for leaks prior to their being conjoined to form the completed fuel cell assembly.  
         [0041]     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.