Abstract:
A separator plate for a PEM fuel cell and a method of making the same includes providing a sheet of material having through plane passages formed therein. A sheet of graphite is placed on each of a first face and a second face of the sheet of material to form a laminated member. Compressive force is applied onto the laminated member. First portions of the graphite are extruded to flow into the through plane passages. An array of electrically conductive pathways through the sheet are created. Second portions of graphite are bonded to each of the first face and the second face.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to PEM fuel cells and more particularly to a separator plate having reduced contact resistance.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or separator plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts.  
         [0003]     The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster.  
         [0004]     With any electrical circuit element, the element&#39;s ability to carry current is always reduced from ideal by a loss factor, i.e. the element&#39;s resistance. In a typical separator plate, or bi-polar plate as referred to in a back to back orientation, there are two losses- one due to the bulk resistance of the plate and another due to the contact resistance with the adjacent current collector/MEA. The electrically conductive separator plates are typically made of metal such as stainless steel to serve as current collectors. While such metallic materials present favorable conductive properties, they also present unfavorable contact resistance across the plane of the plate.  
       SUMMARY OF THE INVENTION  
       [0005]     A separator plate for a PEM fuel cell and a method of making the same includes providing a sheet of material having through plane passages are formed therein. A sheet of graphite is placed on each of a first face and a second face of the sheet of material to form a laminated member. Compressive force is applied onto the laminated member. First portions of the graphite are extruded to flow into the through plane passages. An array of electrically conductive pathways through the sheet are created. Second portions of graphite are bonded to each of the first face and the second face.  
         [0006]     According to other features, adhesive is applied to each of the first and second faces of the sheet. The adhesive includes thermally-activated adhesive bonding the graphite to each of the first and second faces upon application of the compressive force. The sheet of material includes a polymeric substrate such as polyimide. Forming the passages through the sheet of material includes removing about 40% of the sheet of material. Placing the graphite includes placing sheets of graphite, each of which are between about five to ten times as thick as the sheet of material. Applying compressive force includes roll bonding the respective sheets of graphite to the sheet of material. The method further includes forming flow fields in said laminated member.  
         [0007]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0009]      FIG. 1  is an isometric exploded view of a fuel cell in a PEM fuel cell stack;  
         [0010]      FIG. 2  is a sectional view of a separator plate according to the present teachings;  
         [0011]      FIG. 3  is a perspective view of a substrate used in accordance to the present teachings;  
         [0012]      FIG. 4  is a perspective view of the substrate of  FIG. 3  shown with thermally activated adhesive applied to opposite faces;  
         [0013]      FIG. 5  is a perspective view of the substrate of  FIG. 4  shown perforated across its plane;  
         [0014]      FIG. 6  is a perspective view of the substrate of  FIG. 5  shown with graphite applied to the opposite faces and a compression force applied thereto; and  
         [0015]      FIG. 7  is a process diagram illustrating steps for making a separator plate according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0017]      FIG. 1  schematically depicts a partial PEM fuel cell stack  10  having membrane-electrode-assemblies (MEAs)  14 ,  16  separated from each other by a non-porous, electrically-conductive bipolar plate  20 . The MEAs  14  and  16  and bipolar plate  20  are stacked together between non-porous, electrically-conductive, bipolar plates  22  and  24 . Porous, gas permeable, electrically conductive sheets or diffusion media  26 ,  28 ,  30  and  32  press up against the electrode faces of the MEAs  14  and  16  and serve as primary current collectors for the electrodes. The diffusion media  26 ,  28 ,  30  and  32  also provide mechanical supports for the MEAs  14  and  16 , especially at locations where the MEAs are otherwise unsupported in the flow field. Suitable diffusion media include carbon/graphite paper/cloth, fine mesh noble metal screens, open cell noble metal foams, and the like which conduct current from the electrodes while allowing gas to pass therethrough.  
         [0018]     Bipolar plates  22  and  24  press up against the primary current collector  26  on the cathode face  14   c  of the MEA  14  and the primary current collector  32  on the anode face  16   a  of the MEA  16 . The bipolar plate  20  presses up against the primary current collector  28  on the anode face  14   a  of the MEA  14  and against the primary current collector  30  on the cathode face  16   c  of the MEA  16 . An oxidant gas such as oxygen or air is supplied to the cathode side of the fuel cell stack  10  from a storage tank  38  via appropriate supply plumbing  40 . Similarly, a fuel such as hydrogen is supplied to the anode side of the fuel cell stack  10  from a storage tank  48  via appropriate plumbing  50 .  
         [0019]     In a preferred embodiment, the oxygen tank  38  may be eliminated, and air supplied to the cathode side from the ambient. Likewise, the hydrogen tank  48  may be eliminated and hydrogen supplied to the anode side from a reformer which catalytically generates hydrogen from methanol or a liquid hydrocarbon (e.g., gasoline). Exhaust plumbing  52  for the H 2  and O 2 /air sides of the MEAs is also provided for removing H 2 -depleted anode gas from the anode flow field and O 2 -depleted cathode gas from the cathode flow field. Although the exhaust plumbing  52  is shown as a single pipe, it will be appreciated that a distinct pipe may be provided for exhausting each gas.  
         [0020]     With reference now to  FIGS. 2-6 , a separator plate  60  according to the present invention will be described in greater detail. The separator plate  60  is configured to carry one of the reactant gases to a respective face of the MEA  16 . It will be appreciated that each bipolar plate  20 ,  22  and  24  comprise two separator plates  60  lying in a back-to-back orientation. The separator plate  60  according to the present teachings provides a laminated graphite polymer substrate having discrete conductive pathways through it. More specifically, the separator plate  60  includes a gas impermeable, polymeric substrate  64  such as polyimide. The polymeric substrate is preferably about 0.002″ thick. A suitable polyimide material includes Kapton® manufactured by the E.I. DuPont Corporation. The polymeric substrate  64  is laminated with first and second sheets of graphite  66  and  70  on opposite faces. The graphite layers  66 ,  70  are preferably on the order of approximately five to ten times thicker than polyimide substrate  64 . Thus, the polymeric substrate provides a sheet of material which is non-conductive but more importantly functions as a support substrate in the separator plate  60  having adequate mechanical strength for ease in handling during manufacture and assembly. In this manner, the present invention takes advantage of the use of graphite to achieve the necessary conductivity without the brittleness associated with a pure graphite sheet compared with a graphite/polymer laminate. It will also be appreciated that the separator plate  60  illustrated in  FIGS. 2 and 6 , is shown prior to forming flow field channels as shown on the bipolar plates  20 ,  22  and  24  in  FIG. 1 .  
         [0021]     As will be described in detail below, the graphite  66 ,  70  is initially located onto opposite faces of the polymeric substrate  64  and subsequently subjected to a pressure application. The material properties of graphite allow the graphite to also flow into passages or perforations  72  across the plane of the polymeric substrate  64  during the pressure application. The graphite extending through the perforations  72  forms the discrete conductive pathways or pillars ( FIG. 2 )  74  through the polymeric substrate  64  to provide electrical communication between adjacent MEAs  14  and  16 . The end result provides a separator plate  60  having high strength, provided in part by the polymeric substrate  64 , and low contact resistance, provided in part by the graphite layers  66 ,  70 .  
         [0022]     With continued reference to  FIGS. 2-6 , and further reference to  FIG. 7 , a method of making the separator plate  60  will be described. A method of making the separator plate  60  according to the present teachings is illustrated in a process chart shown in  FIG. 7  and referred generally at reference  80 . In step  84 , the polymeric substrate  64  is provided ( FIG. 3 ). In step  90 , a thermally activated, dry adhesive  92  is applied to opposite surfaces of the polymeric substrate  64  ( FIG. 4 ). In step  96 , the substrate  64  is perforated across its plane to form the perforations  72  ( FIG. 5 ). The perforations  72  may be formed by any suitable machining operation. The perforations  72  may remove about 30% to 50% of the material from the substrate  64 , and preferably 40% of the substrate  64  is removed. The perforations  72  are shown as having a generally cylindrical configuration which simplifies the machining operations necessary for forming. However, it will be appreciated that the size, shape, density, distribution and location of the perforations  72  (and resulting graphite pillars  74  extending therethrough) within the separator plate  60  may be selected in accordance with the specification and operational parameters of a given fuel cell application.  
         [0023]     The graphite sheets of material  66 ,  70  are then placed across the opposite faces of the substrate  64  in step  112  ( FIG. 6 ). The graphite sheets  66 ,  70  are preferably about 0.010″ thick. However, as described above, the graphite sheets  66 ,  70  may be on the order of five to ten times thicker than the polymeric substrate  64 , but may also have other thicknesses as dictated by the requirements of a give application and in particular the bulk resistance requirements. Of note, because the dry adhesive  92  is thermally activated, the graphite sheets  66 ,  70  do not adhere to the substrate  64  at this time. In step  116 , the polymeric substrate  64  having the graphite sheets  66 ,  70  on opposite faces is placed in compression (designated at arrows F in  FIG. 6 ) such as by a roller press assembly. The compressive force exerted onto the respective graphite sheets  66 ,  70  causes the graphite to flow or extrude into the perforations  72  across the substrate  64  ( FIG. 2 ). Furthermore, the thermally activated adhesive  92  bonds the remaining graphite to the opposite faces of the polymeric substrate  64  absent the perforations (designated at  122 ,  FIG. 2 ). The desired flow pattern is finally formed in step  120  (not specifically shown), such as by a stamping operation for example, across the plane of the material.  
         [0024]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.