Abstract:
An electro-conductive plate assembly for a fuel cell has a pair of stamped plates joined together to define a coolant volume therein. Each of the pair of stamped plates has a flow field on a major outer surface arranged to maximize the contact area between major inner surfaces of the plates while allowing coolant to distribute and flow readily within the coolant volume. The flow fields formed on the major outer surfaces provide corresponding sets of lands on the major inner surfaces that contact to form a third flow field of the coolant volume. The third flow field formed by the lands includes a plurality of longitudinal channels and an array of flow disruptors. The bipolar plate assembly further includes a seal arrangement and integral manifolds to direct reactant gas and coolant flow through the fuel cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/418,536 filed on Apr. 18, 2003. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to a fuel cell stack assembly and more particularly to a bipolar plate assembly having a pair of stamped metal plates bonded together to provide coolant volume therebetween. 
     BACKGROUND OF THE INVENTION 
     Fuel cells have been proposed as a power source for many applications. One such fuel cell is the proton exchange member or PEM fuel cell. PEM fuel cells are well known in the art and include an each cell thereof a so-called membrane-electrode-assembly or MEA having a thin, proton conductive, polymeric membrane-electrolyte with an anode electrode film formed on major face thereof and a cathode electrode film formed on the opposite major face thereof. Various membrane electrolytes are well known in the art and are described in such U.S. Pat. Nos. 5,272,017 and 3,134,697, as well as in the  Journal of Power Sources , vol. 29 (1990) pgs. 367-387, inter alia. 
     The MEA is interdisposed between sheets of porous gas-permeable, conductive material known as a diffusion layer which press against the anode and cathode faces of the MEA and serve as the primary current collectors for the anode and cathode as well as provide mechanical support for the MEA. This assembly of diffusion layers and MEA are pressed between a pair of electronically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors and for conducting current between adjacent cells internally of the stack (in the case of bipolar plates) and externally of the stack (in the case of monopolar plates at the end of the stack). Secondary current collector plates each contain at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions also known as flow fields typically include a plurality of lands which engage the primary current collector and define therebetween a plurality of grooves or flow channels through which the gaseous reactant flow between a supply header and a header region of the plate at one of the channel and an exhaust header in a header region of the plate at the other end of the channel. In the case of bipolar plates, an anode flow field is formed on a first major face of the bipolar plate and a cathode flow field is formed on a second major face opposite the first major face. In this manner, the anode gaseous reactant (e.g., H 2 ) is distributed over the surface of the anode electric film and the cathode gaseous reactant (e.g., O 2 /air) is distributed over the surface of the cathode electrode film. 
     Various concepts have been employed to fabricate a bipolar plate having flow fields formed on opposite major faces. For example, U.S. Pat. No. 6,099,984 discloses bipolar plate assembly having a pair of thin metal plates with an identical flow field stamped therein. These stamped metal plates are positioned in opposed facing relationships with a conductive spacer interposed therebetween. This assembly of plates and spacers are joined together using conventional bonding technology such as brazing, welding, diffusion bonding or adhesive bonding. Such bipolar plate technology has proved satisfactory in its gas distribution function, but results in a relatively thick and heavy bipolar plate assembly and thus impacts the gravimetric and volumetric efficiency of the fuel cell stack assembly. 
     In another example, U.S. Pat. No. 6,503,653 discloses a single stamped bipolar plate in which the flow fields are formed in opposite major faces thereof to provide a non-cooled bipolar plate. A cooled bipolar plate using this technology again requires a spacer element interposed between a pair of stamped plates, thereby increasing the thickness and weight of the cooled plate assembly. U.S. Pat. No. 6,503,653 takes advantage of unique reactant gas porting and staggered seal arrangements for feeding the reactant gases from the header region through the port in the plate to the flow field formed on the opposite side thereof. This concept is very desirable in terms of cost but its design constraints on flow fields may rule out some application. Furthermore, this design concept does not lend itself readily to providing an internal cooling flow. 
     Applications with high powered density requirements need cooling in about every other fuel cell. Thus, there is an ever present desire to refine the design of a bipolar plate assembly to be efficiently used in a fuel cell stack to provide a high gravimetric power density, high volumetric power density, low cost and higher, reliability. The present invention is directed to a stamped fuel cell bipolar plate that offers significant flow field design flexibility while minimizing the weight and thickness thereof. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a bipolar plate assembly having two thin metal plates formed with conventional stamping processes and then joined together. In another aspect, the centerlines of the flow fields must be arranged to align the channels for plates on opposite sides of the MEA wherever possible to further provide uniform compression of the diffusion media. In another aspect, the configuration of the flow fields formed in each of the two stamped metal plates are such that the contact area therebetween is maximized to enable the bipolar plate assembly to carry compressive loads present in a fuel cell stack. Thus, the centerlines of the flow fields formed in the two thin metal plates of a bipolar plate assembly need to be coincident in many places to carry the compressive loads. However, since the interior volume defined between the plates and their context areas form an interior cavity for coolant flow, it is necessary to have sufficient instances where the centerlines are not coincident in order to allow adequate coolant flow. The present invention achieves these two apparently opposing objections with a unique flow field design in which adjoining areas of the flow channels adjacent the inlet and exhaust margins provide a geometric configurations to provide the desired flow field and contact area requirements. 
     The present invention provides a bipolar plate assembly which includes a pair of plates having reactant gas flow fields defined by a plurality of channels formed the outer faces of the plates. The plates are arranged in a facing relationship to define an interior volume therebetween. A coolant flow field is formed in an interior volume defined between the pair of plates at the contact interface therebetween. The coolant flow field has an array of discrete flow disruptors adjacent a coolant header inlet and a plurality of parallel channels interposed between the array and the coolant exhaust header. Fluid communication is provided from the coolant inlet header through the coolant flow field to the coolant exhaust header. 
     The present invention also provides a separator plate which includes a thin plate having an inlet margin with a pair of lateral inlet headers and a medial inlet header formed therethrough, an exhaust margin including a pair of lateral exhaust headers and a medial exhaust header formed therethrough and a reactant gas flow field formed on a major face of the thin plate. The reactant gas flow field includes a first set of flow channels, each having an inlet leg with a first longitudinal portion in fluid communication with one of the pair of lateral inlet headers and a first transverse portion, a serpentine leg having a first end in fluid communication with the first transverse portion and a second end and an exhaust leg having a second transverse portion in fluid communication with the second end of the serpentine leg and a second longitudinal portion in fluid communication with one of the pair of lateral exhaust headers. Either of the transverse portion of the inlet leg adjacent the medial inlet header and the transverse portion of the exhaust leg adjacent the medial exhaust header may be defined by an undulating flow channel. 
     These and other aspects of the present invention provide a bipolar plate assembly which increases the design flexibility in terms of flow field options, while achieving the cooling requirements as well as providing a relatively high gravimetric power density and high volumetric power density from a fuel cell stack incorporating the bipolar plate assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood when considered in the light of the following detailed description of a specific embodiment thereof which is given hereafter in conjunction with the several figures in which: 
         FIG. 1  is a schematic isometric exploded illustration of a fuel cell stack; 
         FIG. 2  is an isometric exploded illustration of a bipolar plate assembly and seal arrangement in accordance with the present invention; 
         FIG. 3  is a plan view of the flow field formed in the major face of an anode plate in the bipolar plate assembly shown in  FIG. 2 ; 
         FIG. 4  is a plan view of the flow field formed in the major face of a cathode plate in the bipolar plate assembly shown in  FIG. 2 ; 
         FIG. 5  is a plan view showing the contact areas at the interface between the anode and cathode plates; 
         FIG. 6  is an isometric view of multiple cells within the fuel cell stack and further showing a section taken through the cathode header; 
         FIG. 7  is a cross-section taken through the coolant header and showing the coolant flow path; 
         FIG. 8  is a cross-section taken through the anode header and showing the anode gas flow path; and 
         FIG. 9  is a cross-section taken through the cathode header and showing the cathode flow path. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. With reference to  FIG. 1 , a two-cell stack (i.e., one bipolar plate) is illustrated and described hereafter, it being understood that a typical stack will have many more such cells and bipolar plates.  FIG. 1  depicts a two-cell bipolar PEM fuel cell stack  2  having a pair of membrane-electrode-assemblies (MEAs)  4 ,  6  separated from each other by an electrically conductive, liquid-cooled bipolar plate  8 . The MEAs  4 ,  6  and bipolar plate  8  are stacked together between clamping plates  10 ,  12  and monopolar end plates  14 ,  16 . The clamping plates  10 ,  12  are electrically insulated from the ends plate  14 ,  16 . The working face of each monopolar end plates  14 ,  16 , as well as both working faces of the bipolar plate  8  contain a plurality of grooves or channels  18 ,  20 ,  22 ,  24  defining a so-called “flow field” for distributing fuel and oxidant gases (i.e., H 2  and O 2 ) over the faces of the MEAs  4 , 6 . Nonconductive gaskets  26 ,  28 ,  30  and  32  provide seals and electrical insulation between the several components of the fuel cell stack. Gas-permeable diffusion media  34 ,  36 ,  38 ,  40  press up against the electrode faces of the MEAs  4 ,  6 . The end plates  14 ,  16  press up against the diffusion media  34 ,  40  respectfully, while the bipolar plate  8  presses up against the diffusion media  36  on the anode face of MEA  4 , and against the diffusion media  38  on the cathode face of MEA  6 . 
     With reference to  FIG. 2 , the bipolar plate assembly  8  includes two separate metal plates  100 ,  200  which are bonded together so as to define a coolant volume therebetween. The metal plates  100 ,  200  are made as thin as possible (e.g., about 0.002-0.02 inches thick) and are preferably formed by suitable forming techniques as is known in the art. Bonding may, for example, be accomplished by brazing, welding diffusion bonding or gluing with a conductive adhesive as is well known in the art. The anode plate  100  and cathode plate  200  of a bipolar plate assembly  8  are shown having a central active region that confronts the MEAs  36 ,  38  (shown in  FIG. 1 ) and bounded by inactive regions or margins. 
     The anode plate  100  has a working face with an anode flow field  102  including a plurality of serpentine flow channels for distributing hydrogen over the anode face of the MEA that it confronts. Likewise, the cathode plate  200  has a working face with a cathode flow field  202  including a plurality of serpentine flow channels for distributing oxygen (often in the form of air) over the cathode face of the MEA that it confronts. The active region of the bipolar plate  8  is flanked by two inactive border portions or margins  104 ,  106 ,  204 ,  206  which have openings  46 - 56  formed therethrough. When the anode and cathode plates  100 ,  200  are stacked together, the openings  46 - 56  in the plates  100 ,  200  are aligned with like openings in adjacent bipolar plate assemblies. Other components of the fuel cell stack  2  such as gaskets  26 - 32  as well as the membrane of the MEAs  4  and  6  and the end plates  14 ,  16  have corresponding openings that align with the openings in the bipolar plate assembly in the stack, and together form headers for supplying and removing gaseous reactants and liquid coolant to/from the stack. 
     In the embodiment shown in the figures, opening  46  in a series of stacked plates forms an air inlet header, opening  48  in series of stacked plates forms an air outlet header, opening  50  in a series of stacked plates forms a hydrogen inlet header, openings  52  in a series of stacked plates forms a hydrogen outlet header, opening  54  in a series of stacked plates forms a coolant inlet header, and opening  56  in a series of stacked plates forms a coolant outlet header. As shown in  FIG. 1 , inlet plumbing  58 ,  60  for both the oxygen/air and hydrogen are in fluid communication with the inlet headers  46 ,  50  respectively. Likewise, exhaust plumbing  62 ,  64  for both the hydrogen and the oxygen/air are in fluid communication with the exhaust headers  48 ,  52  respectively. Additional plumbing  66 ,  68  is provided for respectively supplying liquid coolant to and removing coolant from the coolant header  54 ,  56 . 
       FIG. 2  illustrates a bipolar plate assembly  8  and seals  28 ,  30  as they are stacked together in a fuel cell. It should be understood that a set of diffusion media, an MEA, and another bipolar plate (not shown) would underlie the cathode plate  200  and seal  30  to form one complete cell. Similarly, another set of diffusion media and MEAs (not shown) will overlie the anode plate  100  and seal  28  to form a series of repeating units or cells within the fuel cell stack. It should also be understood that an interior volume or coolant cavity  300  is formed directly between anode plate  100  and cathode plate  200  without the need of an additional spacer interposed therebetween. 
     Turning now to  FIG. 3 , a plan view of the anode plate  100  is provided which more clearly shows the anode flow field  102  formed in the working face of anode plate  100 . As can also be clearly seen in  FIG. 3 , the inlet margin  104  of anode plate  100  has a pair of lateral inlet headers  46  and  50  to transport cathode gas and anode gas, respectively, through the fuel cell stack and a medial inlet header  54  to transport a coolant through the stack. Similarly, the exhaust margin  106  has a pair of lateral exhaust headers  48 ,  52  for transporting anode affluent and cathode affluent, respectively through the fuel cell stack, and a medial exhaust header  56  for transporting coolant through the fuel cell stack. 
     The anode flow field  102  is defined by a plurality of channels formed to provide fluid communication along a tortuous path from the anode inlet header  50  to the anode exhaust header  52 . In general, the flow channels are characterized by an inlet leg  108  having a longitudinal portion  110  with a first end in fluid communication with the anode inlet header  50  and a second end in fluid communication with a transverse portion  112 . As presently preferred, the transverse portion  112  of the inlet leg  108  branches to provide a pair of transverse inlet legs associated with each longitudinal portion  110 . Furthermore, the path of these transverse inlet portions  112  undulate within the plane of the anode plate  100  to provide an undulating flow channel adjacent the coolant inlet header  54  as represented in the area designated  114 . The transverse portion  112  of inlet leg  108  is in fluid communication with a serpentine leg  116 . The flow channel  108  further includes an exhaust leg  118  having transverse portions  120  and a longitudinal portion  122  to provide fluid communication from the serpentine leg  116  to the anode exhaust header  52 . The exhaust leg portion  118  is configured similar to the inlet leg portion  108  in that each longitudinal portion  122  is associated with a pair of transverse portions  120 . The path of the transverse exhaust portions  120  undulate within the plane of the anode plate  100  to provide an undulating flow channel adjacent the coolant exhaust header  56  as represented in the area designated  124 . 
     Turning now to  FIG. 4  a plan view of the cathode plate  200  is provided which more clearly shows the cathode flow field  202  formed in the working face of cathode plate  200 . As can also be clearly seen in  FIG. 4 , the inlet margin  204  of cathode plate  200  has a pair of lateral inlet headers  46 ,  50  to transport cathode gas and anode gas, respectively, through the fuel cell stack and a medial inlet header  54  to transport a coolant through the stack. Similarly, the exhaust margin  206  has a pair of lateral exhaust headers  48 ,  52  for transporting anode affluent and cathode affluent, respectively through the fuel cell stack, and a medial exhaust header  56  for transporting coolant through the fuel cell stack. 
     The cathode flow field  202  is defined by a plurality of channels formed to provide fluid communication along a tortuous path from the cathode inlet header  46  to the cathode exhaust header  48 . In general, the flow channels are characterized by an inlet leg  208  having a longitudinal portion  210  with a first end in fluid communication with the cathode inlet header  46  and a second end in fluid communication with a transverse portion  212 . A single transverse portion  212  is associated with each longitudinal portion  210 . Thus, the transverse portion  212  of the inlet leg  208  does not branch off to provide a pair of transverse inlet portions as the transverse portion  112  of anode inlet leg  108 . The path of the transverse inlet portions  212  undulate within the plane of the cathode plate to provide an undulating flow channel adjacent the coolant inlet header  54  as represented in the area designated  214 . The flow channel further includes a serpentine leg  216  which is in fluid communication with the end of transverse inlet portion  212 . The flow channel further includes an exhaust leg  218  having a transverse portion  220  and a longitudinal portion  222 . The exhaust leg portion  218  is configured similar to the inlet leg portion  208  to provide fluid communication from the serpentine leg  216  to the cathode exhaust header  48 . The path of the transverse exhaust portions  220  undulate within the plane of the cathode plate to provide an undulating flow channel adjacent the coolant exhaust header  56  as represented in the area designated  224 . 
     Referring now to  FIGS. 2 and 6 , the anode plate  100  and the cathode plate  200  are positioned in an opposed facing relationship such that the various inlet and exhaust headers are in alignment. The anode plate  100  and the cathode plate  200  are then joined together using conventional techniques. The centerlines of the anode flow fields  102  and cathode flow fields  202  are arranged to align the flow channels on opposing plates (e.g. on opposite sides of the MEA as shown in  FIG. 6 ) wherever possible to provide uniform compression of the diffusion media and the MEA. Likewise, the contact area between the adjacent, joined anode plate  100  and cathode plate  200  (as shown in  FIG. 2 ) are coincident in many places so as to carry the compressive loads imposed on the fuel cell stack. Specifically, the flow channels of anode flow field  102  formed in the working face of anode plate  100  provide a complimentary contact surface on an inner face opposite the working face. Similarly, the flow channels of the cathode flow field  202  formed in the working face of the cathode plate  200  define a contact surface on an inner face of the cathode plate  200 . Thus, when the anode plate  100  and cathode plate  200  are joined together, an interference or contact area is defined therebetween. 
     With reference now to  FIG. 5 , the contact area between the anode plate  100  and the cathode plate  200  defines a coolant flow field  302  between an inlet margin  304  and an exhaust margin  306  within coolant cavity  300 . The coolant flow field  302  includes an array of discrete flow disruptors  308  adjacent the coolant inlet manifold  54  formed at the interface of the anode inlet legs  108  and the cathode inlet legs  208 . Similarly, a set of flow disrupters  310  are formed adjacent the coolant exhaust header  56  at the interface of the anode exhaust leg  118  and the cathode exhaust legs  218 . The coolant flow field  302  further includes a plurality of parallel flow channels  312  interposed between the inlet margin  304  and the exhaust margin  306  which are defined at the interface of the serpentine legs  116  and the serpentine legs  216 . In accordance with the configuration of the anode flow field  102  and cathode flow field  202 , the array of discrete flow disruptors  308  extend obliquely from the area of the coolant flow field  302  adjacent the coolant inlet header  54  as indicated by directional arrow  314  into the parallel flow channels  312 . Likewise, the array of discrete flow disruptors  310  extend from the parallel flow channels  312  obliquely towards the coolant exhaust header  56  as indicated by directional arrow  316 . 
     Turning now to  FIGS. 6-9 , the present invention incorporates a staggered seal and an integral manifold configuration for directing fluid communication from the header into the appropriate flow field. For example, the location of the seal beads between the inlet margin  104 ,  204  and the flow field structure  102 ,  202  step left and right (as seen in  FIGS. 7-9 ) for each successive layer. Thus, the seal position shifts to provide fluid communication therebetween. Ports in the form of holes or slots penetrate vertically through the anode plate  100  or cathode plate  200  to provide means for fluid communication from the header to the flow field. In this manner, the present invention employs a staggered seal concept similar to that disclosed in U.S. Pat. No. 6,503,653, which is commonly owned by the assignee of the present invention and whose disclosure is expressly incorporated by reference herein. This approach allows the combined seal thicknesses to equal the repeat distance minus the thickness of the anode plate and cathode plate. This approach also provides an advantage over other conventional fuel cell stack design in which the thickness available for seals is reduced by the height required for the fluid passage from the header region to the active area region. By utilizing a staggered seal concept, the present invention affords the use of thicker seals which are less sensitive to tolerance variations. 
     The present invention further improves upon the staggered seal concept disclosed in U.S. Pat. No. 6,503,653 with the use of separate anode plate  100  and cathode plate  200  in each bipolar plate assembly. Specifically, a second plate enables the use of an integral manifold with the space between the plates. Reactant gases or coolant fluid can now enter on the top side of the upper plate, travel between the upper and lower plate through such integral manifolds and then enter the lower side of the upper plate to feed the bottom side of the MEA. As a result, the width of the region where the reactant gases enter the flow field is twice as wide as that disclosed in U.S. Pat. No. 6,503,653, thereby lowering the overall pressure drop across a given flow field. This aspect of the present invention is best illustrated in  FIGS. 7-9 . Specifically, as illustrated in  FIG. 7 , the coolant flow path is indicated by the arrows A showing flow from the coolant header (not shown) between the anode plate  100  and the cathode plate  200  and into the coolant flow field  302  defined therebetween. Similarly, in  FIG. 8  the anode gas flow path is indicated by the arrows B showing flow from the anode header (not shown) between the cathode plate  200  and the anode plate  100  and into the anode flow field  102 . Similarly, in  FIG. 9  the cathode gas flow path is indicated by the arrows C showing flow from the cathode header (not shown) between the anode plate  100  and the cathode plate  200  and into the cathode flow field  202 . In this manner, a wider manifold region is provided between the header region and the flow field region for each of the fluids passed through the fuel cell stack. 
     As presently preferred, the design of the bipolar plate assembly further includes an additional feature to support the seal loads given the effect of widening the inlet manifold region between the headers and the active flow fields. Specifically, as best seen in  FIGS. 4 and 6  an in-situ support flange  226  extends transversely across the inlet margin through the cathode inlet header  46 , the coolant inlet header  54  and the anode header  50 . This support flange  226  is formed with a wavy or corrugated configuration to allow inlet fluids to freely pass from the header region through the manifold region into the flow field region while at the same time providing through-plane support for the bipolar plate assembly. For example, as best seen in  FIG. 6 , the support flange  226  for the cathode plate  200  of the bipolar plate  8  occurs directly over the support flange  126  for the anode plate  100  of the neighboring cell. In this manner, compressive loads are readily transmitted through the fuel cell stack. Alternately, the support function could be provided with grooved blocks of a non-conductive material or similar features which could be formed in the seals to replace the in-situ configuration provided by the transverse support flange. 
     When using this configuration, these adjacent regions must be insulated since they are at different electrical potentials. Various suitable means are available such as the use of a non-conductive coating such as that disclosed in U.S. application Ser. No. 10/132,058 entitled “Fuel Cell Having Insulated Coolant Manifold” filed on Apr. 25, 2002 which is commonly owned by the assignee of the present invention and the disclosure of which is expressly incorporated by reference. Alternately, a film of non-conductive plastic tape may be interposed for providing electrical isolation therebetween. 
     The present invention provides a two piece bipolar plate assembly having a coolant flow field formed therebetween. The configuration of the various flow fields are such that the bipolar plate assembly may be a formed of relatively thin material, and still support the required compressive loads of the fuel cell stack. Furthermore, the present invention provides much greater design flexibility in terms of flow field options. In this regard, the present invention provides an improvement in the gravimetric and volumetric power densities of a given fuel cell stack as well as significant material and cost savings. 
     The description of the invention set forth above is merely exemplary in nature and, thus, variations that do not depart from the jest of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.