Abstract:
A method of making a bipolar plate includes molding a non-conductive housing defining a plurality of receiving areas. A first plurality of conductive charges are located into each of the plurality of receiving areas. Compressive force is applied to each of the first plurality of conductive charges thereby defining a first plurality of conductive plates bonded on outer edges to the non-conductive housing. A second plurality of conductive plates are located onto the first series of conducive plates. The first and second plurality of conductive plates are bonded together defining a coolant flow field therebetween.

Description:
FIELD OF THE INVENTION 
     The present invention relates to PEM fuel cells and more particularly to a method of making a bipolar plate for use within a fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     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 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. 
     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. 
     In PEM fuel cells, hydrogen (H 2 ) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O 2 ) or air (a mixture of O 2  and N 2 ). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. As such these MEAs are relatively expensive to manufacture and require certain conditions, including proper water management and humidification and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation. 
     The electrically conductive plates sandwiching the MEAs may contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell&#39;s gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels. 
     In general, it is desirable to provide a fuel cell stack having high voltage. One way to provide high voltage is to implement several fuel cell stacks electrically connected in series. The cost associated with handling and assembling a large volume of fuel cells for automotive applications is cost prohibitive. In addition to providing high voltage, packaging constraints within a vehicle require a fuel cell stack to occupy a reduced area. As a result, it is desirable to provide a high-voltage fuel cell stack while satisfying related packaging constraints. 
     SUMMARY OF THE INVENTION 
     A method of making a bipolar plate includes molding a non-conductive housing defining a plurality of receiving areas. A first plurality of conductive charges are located into each of the plurality of receiving areas. Compressive force is applied to each of the first plurality of conductive charges thereby defining a first plurality of conductive plates bonded on outer edges to the non-conductive housing. A second plurality of conductive plates are located onto the first series of conducive plates. The first and second plurality of conductive plates are bonded together defining a coolant flow field therebetween. 
     According to other features compressive force is applied to a second plurality of conductive charges thereby defining the second plurality of conductive plates. Applying compressive force to each of the first plurality of conductive charges includes defining first reactant flow fields on respective first plurality of conductive plates. Applying compressive force to a second plurality of conductive charges includes defining second reactant flow fields on respective second plurality of conductive plates. Bonding the first and second plurality of conductive plates together includes applying adhesive onto contact surfaces defined between the first and second plurality of conductive plates. Compressive force is applied onto the first and second plurality of conductive plates thereby thermally activating the adhesive and forming a bond at the contact surfaces. 
     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 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a partial sectional view of a fuel cell stack according to the present teachings; 
         FIG. 2  is a partial exploded view of the fuel cell stack of  FIG. 1 ; 
         FIG. 3  is a plan view of a bipolar plate assembly according to the present teachings; 
         FIG. 4  is a perspective view of a non-conductive housing employed for constructing the bipolar plate assembly of  FIG. 3 ; 
         FIG. 5  is a perspective view of a mold process used to form a first plurality of conductive plates in the non-conductive housing of  FIG. 4 ; and 
         FIG. 6  is a perspective view of a second plurality of conductive plates being attached to the first plurality of conductive plates. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
       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 assembly  20 . The MEAs  14  and  16  and bipolar plate assembly  20  are stacked together between non-porous, electrically-conductive, bipolar plate assemblies  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 may 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. 
     The bipolar plate assemblies  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 assembly  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 . 
     With continued reference to  FIG. 1  and further reference to  FIGS. 2 and 6 , the bipolar plate assembly  20  will be described. The bipolar plate assembly  20  generally includes a series of distinct anode plates  40  and cathode plates  44  ( FIG. 6 ) as will be described more fully below. An anode flow field  46  ( FIGS. 1 and 2 ) is defined across each of the anode plates  40  of the bipolar plate assembly  20 , a cathode flow field  48  ( FIG. 1 ) is defined across each of the cathode plates  44  of the bipolar plate assembly  20 , and a coolant flow field  50  is defined between the anode and cathode plates  40 ,  44 . An oxidant gas such as oxygen or air is supplied to the cathode side of the fuel cell stack  10  from a storage tank  56  via appropriate supply plumbing  58 . Similarly, a fuel such as hydrogen is supplied to the anode side of the fuel cell stack  10  from a storage tank  60  via appropriate plumbing  62 . Coolant is supplied between adjacent anode and cathode plates  40  and  44  from a coolant tank  64 . 
     In a preferred embodiment, the oxygen tank  56  may be eliminated, and air supplied to the cathode side from the ambient. Likewise, the hydrogen tank  60  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 (not specifically shown) for the H 2  and O 2  air sides of the MEAs is also provided for removing H 2  depleted anode gas from the respective anode flow field  46  and O 2  depleted cathode gas from the respective cathode flow fields  48 . 
     With specific reference now to  FIGS. 1-3 , the porting of the respective reactants in the fuel cell stack  10  will be further described.  FIG. 3  illustrates a top view of the bipolar plate assembly  20  (anode side faced up). As will be described in further detail below, the bipolar plate assembly  20  generally includes the first plurality of anode plates  40 , a complementary series of cathode plates  44  (disposed under the anode plates as viewed from  FIG. 3 ) and a non-conductive housing  66 . The anode and cathode plates  40  and  44  respectively, form a plurality (four as illustrated in  FIG. 2 ) of individual bipolar plates  70  that are electrically insulated laterally from each other by the nonconductive housing  66 . In addition, the anode and cathode plates  40  and  44  are electrically connected in series within the fuel cell stack  10  to provide a high output voltage in reduced space. While the respective anode and cathode plates  40  and  44  respectively are shown as circular, it is appreciated that the geometry is merely exemplary and other shapes may similarly be employed such as, but not limited to rectangle. Additionally, the respective flow fields  44 ,  46  and  50  are merely exemplary and may comprise other flow patterns. Likewise, while the housing  66  takes on a generally clover-like shape to accommodate the circular anode and cathode plates  40  and  44 , other suitable shapes may similarly be employed to contain the respective anode and cathode plates  40  and  44  comprising the bipolar plates  70 . 
     The bipolar plate assembly  20  generally includes an intake header  74  arranged vertically through the middle of the bipolar plate assembly  20  and an exhaust header  78  arranged horizontally through the middle of the bipolar plate assembly  20 . The intake header  74  generally defines a series of inlet apertures I 1 -I 6 . Similarly, the exhaust header generally defines a series of exhaust apertures E 1 -E 6 . In one implementation, the inlet apertures I 1  and I 6  cooperate to deliver a first reactant gas (e.g. H 2 ) to anode plates A 1 , A 2  and A 3 , A 4  respectively. During operation, once the first reactant gas flows across the respective anode flow fields  46 , it is exhausted by way of exhaust apertures E 3  and E 4 . The remaining inlet apertures and exhaust apertures are utilized to deliver the second reactant gas (e.g. O 2 ) and the coolant respectively. In the exemplary configuration shown, the inlet apertures I 2  and I 5  cooperate to deliver the second reactant gas (e.g. H 2 ) to respective cathode flow fields  48  along the cathode plates  44  (while not specifically illustrated in  FIG. 2 , the cathode plates  44  are operatively positioned under each of the anode plates  40 ). 
     During operation, once the second reactant gas flows across the respective cathode flow fields  48 , it is exhausted by way of exhaust apertures E 2  and E 5 . Finally, the inlet apertures I 3  and I 4  cooperate to deliver a coolant to the respective coolant flow fields  50  defined between the anode and cathode plates  40  and  44 . The coolant is exhausted by way of exhaust apertures E 1  and E 6 . It is appreciated that while the respective inlet and exhaust apertures I 1-6  and E 1-6  have been specifically assigned to communicate a given fluid, the apertures are interchangeable and may be configured to deliver a given fluid as desired. It will become apparent that such configurations are defined according to the porting configuration provided by the non-conductive housing  66 . Moreover, while a single inlet and outlet aperture is described as supplying a pair of flow fields, other configurations may be similarly employed. 
     With particular reference now to  FIGS. 3-6 , a method of making a bipolar plate according to the present teachings will be described. At the outset, a housing  66  ( FIG. 4 ) is formed from a suitable non-conductive, insulating material. The housing  66  may be formed by any suitable process, such as injection molding for example. As shown in  FIG. 4 , the housing  66  defines a plurality of receiving portions  82 . Similarly, the necessary porting is defined in the housing  66  to communicate appropriate fluids from the intake header  74 , across respective flow fields  46 ,  48  and  50 , and out the exhaust header  78 . In addition to the porting, necessary gaskets and seals may be molded into the non-conductive housing  66  (not specifically shown). A central bore  84  is arranged for accommodating an alignment rod (not shown) during the molding process. 
     Next, the non-conductive housing  66  is placed into a compression mold  90  ( FIG. 5 ). A plurality of conductive charges  100  are located into respective receiving areas. The conductive charges  100  define a molding compound and may take the form of spheres or disks for example. The conductive charges  100  comprise any suitable material having favorable bipolar plate properties. One suitable material is BMC 940 manufactured by Bulk Molding Compounds, Inc. Once the charges  100  are placed into the respective receiving areas  82 , a compressive force is applied to the conductive charges  100 . As a result, the respective conductive charges  100  are driven toward the respective inner diameters  104  of the receiving areas  82  defined in the housing  66 . Concurrently, the desired flow fields  48  and  50  ( FIG. 1 ), having associated lands and channels, is defined on opposite faces of the newly formed plates. The pattern of the flow fields are defined by die portions  110  and  112  of the mold  90 . Furthermore, the compression mold  90  is arranged to align appropriate inlets and outlets of the flow fields  48  and  50  on the respective plates  44  with appropriate porting. I 1-6  and E 1-6 . While the plates shown are cathode plates  44 , it is appreciated that the anode plates  40  may alternatively be formed in this step. The compression molding process creates an integral chemical seal at an interface between the housing  66  and the cathode plates  44  (at the receiving area inside diameter  104 ) ( FIG. 5 ). 
     Turning now to  FIG. 6 , the cathode plates  44  are shown after the molding process of  FIG. 5 . At this point, a thermally activated, conductive adhesive  120  is applied to the upper face of each cathode plate  44 . Next, a complementary plurality of anode plates  40  are located onto the cathode plate  44 . The anode plates  40  may be formed as standalone pieces in a complementary compression mold as a preliminary step. Next, the assembly is placed into a press  130  suitable to hold the anode plates  40  against the cathode plates  44  while exposing the assembly to necessary levels of heat to activate the adhesive bond (facilitated by the adhesive  120 ) between the anode and cathode plates  40  and  44 . It is noted that the coolant flow field  50  is defined collectively by the flow on the upper face of cathode plates  44  and lower face of anode plates  40 . As a result, proper alignment must be maintained while placing the anode plates  40  atop the cathode plates  44  in this step. It is contemplated that while not specifically shown, that the anode plates  40  may have keys extending from a perimeter for locating into a groove defined on the inside diameter&#39;s  104 . Alternatively, as mentioned previously, the anode plates  40  (and cathode plates  44 ) may define other geometric shapes, such as rectangular, to facilitate proper alignment. In general, the adhesive bond encourages land to land contact between opposing anode and cathode plates  40  and  44  facilitating electrical conduction. This completes construction of the bipolar plate assembly  20 . 
     Preferably a series of bipolar plate assemblies  20  are made according to the above method. Next, the fuel cell stack  10  is completed by incorporating respective MEAs  14  and  16  and diffusion media  26 ,  28 ,  30  and  32  between adjacent bipolar plate assemblies  20  as illustrated in  FIG. 1A . 
     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. For example, while the necessary gaskets and seals have been described herein as molded concurrently with the non-conductive housing, the gaskets and seals may be incorporated in a supplemental molding step. In addition, the porting defined on the intake header  74  and the exhaust header  78 , including the respective anode, cathode, and coolant pathways defined through the housing  66 , is exemplary and other arrangements may be employed, such as through the molding process. 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.