Abstract:
A solid polymer fuel cell comprising a membrane electrode assembly that is in adhesive contact with a first flow field plate around the circumferential edge of the membrane electrode assembly and in non-adhesive contact with a second flow field plate, and an elastomeric manifold seal member that circumscribes at least one manifold opening of the first flow field plate and the second flow field plate. In this configuration, the adhesive substantially seals a first reactant gas while the manifold seal member substantially seals a second reactant gas, thereby improving sealing reliability and simplifying the seal design without overly compressing and damaging the circumferential edge of the membrane electrode assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a solid polymer electrolyte membrane fuel cell assembly and a fuel cell stack configuration and, more particularly, to an integrated seal for the same. 
     2. Description of the Related Art 
     Electrochemical fuel cells convert fuel and oxidant into electricity. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (hereinafter referred to as the “MEA”) which includes an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane electrode interface to induce the desired electrochemical reaction. In operation, the electrodes are electrically coupled for conducting electrons between the electrodes through an external circuit. Typically, a number of MEAs are electrically coupled in series and/or in parallel to form a fuel cell stack having a desired power output. 
     The MEA is typically interposed between two electrically conductive bipolar flow field plates or separator plates wherein the bipolar flow field plates may comprise polymeric, carbonaceous, graphitic, or metallic materials. These bipolar flow field plates act as current collectors, provide support for the electrodes, and provide passages for the reactants and products. Such bipolar flow field plates may comprise flow fields to direct the flow of the fuel and oxidant reactant gases to the anode and cathode electrodes of the MEA, respectively, and to remove excess reactant gases and reaction products, such as water formed during fuel cell operation. 
     Fuel cells need to be sealed in order to isolate the anode and cathode electrodes and to prevent leakage of the reactant gas and product streams either internally inside the fuel cell or externally into the environment. The fuel cell stack typically comprises supply or inlet manifolds for directing the flow of reactant gas streams into the fuel cell stack, as well as exhaust or outlet manifolds for directing the flow of product and excess reactant streams out of the fuel cell stack. Alternatively, the fuel cell stack may comprise coolant inlet and outlet manifolds wherein the coolant is circulated to absorb heat from the exothermic reactions of the fuel cell during operation to maintain the fuel cell stack at a desired operating temperature. These manifolds can be internal manifolds wherein the manifold openings are formed in an extended area of the bipolar flow field plate, or can be external manifolds wherein the manifolds are attached to the edge of the bipolar flow field plate. In a fuel cell stack, the manifold openings of each bipolar flow field plate are in fluid communication with corresponding manifold openings of adjacent bipolar flow field plates to form manifolds thereof for the various fluid streams. 
     To increase power density of the fuel cell stack, there is a consistent trend to decrease fuel cell stack volume by decreasing the thickness of individual fuel cell components. As the thickness of individual fuel cell components decreases to the micron range, thickness tolerances of individual fuel cell components tend to increase due to manufacturing variability of thin components. Furthermore, when the fuel cell components are assembled to form a fuel cell, the thickness tolerance will increase. Thus, seal design is becoming of greater importance because the seals must be able to withstand a wide range of compression pressure to compensate for the large thickness tolerance of the fuel cell. 
     MEAs may be individually edge-sealed by sealing around the perimeter of the MEA prior to fuel cell stack assembly. One method is to attach a sheet-type gasket frame around a perimeter of the MEA. Elastomeric seals are formed on the bipolar flow field plates and compressed against the gasket frame under a compression pressure, thus providing a substantially fluid leak-tight seal and thereby isolating the reactant gases and product streams and their corresponding inlet and outlet manifolds. However, this sealing method is not cost-effective because it requires a number of materials to form a substantially fluid leak-tight seal. Moreover, variations in MEA thickness around the perimeter of the MEA may result in non-uniform pressure exertion by the reaction force produced by the elastomeric gaskets, and therefore non-uniform sealing may occur around the perimeter of the MEA. 
     Another method of edge-sealing MEAs is disclosed in U.S. Pat. No. 6,699,613. A liquid sealant is directly in contact with the projecting portion provided at the periphery of the solid polymer electrolyte membrane, is pressed between the solid polymer electrolyte membrane and the separators, fitting the varying sizes of the seal sections, and maintains gas-tightness between the solid polymer electrolyte membrane and the separators (hereinafter referred to interchangeably with “flow field plates”). However, this approach is problematic because the MEA is adhesively attached to both separators and cannot easily be removed from them without damaging the MEA and/or the separators. Thus, if an MEA is degraded and needs to be replaced, the separators will also need to be replaced, thereby increasing replacement costs. 
     A similar approach is described in U.S. Pat. App. No. 2004/0168306, which discloses a method of laminating a separator and a membrane/electrode assembly for fuel cells and an apparatus for laminating the same. This method corrects a warp in a separator applied with a sealant during production of fuel cells. The correction is performed at a correcting device. With the warp being corrected at the correcting device, a membrane/electrode assembly is superimposed on the separator. Since the membrane/electrode assembly is superimposed on the separator while the separator is corrected with the correcting device being operated, the sealant applied to the separator can be spread out to an even thickness, providing good sealing. However, this method also results in an MEA that is glued to both plates and, thus, entire fuel cell assemblies would need to be replaced when replacing degraded MEAs. 
     MEAs can also be individually edge-sealed with silicone-based elastomers that are injection-molded to encapsulate and/or impregnate the perimeter of the electrochemically active area of the MEA. However, silicone-based elastomers have been shown to degrade under certain fuel cell operating conditions and exhibit creep and compression set under prolonged stack compression, which can lead to seal thinning with extended fuel cell operation as well as internal and external leakage of the reactant gas and/or coolant. Furthermore, the polymer electrolyte membrane at the perimeter edge of the MEA may experience membrane thinning, thereby increasing the occurrence of premature membrane failures. 
     U.S. Pat. App. No. 2004/0161655 discloses a method for assembling electrochemical cells for monopolar arrays or bipolar stacks using an adhesive to bond and seal the interfaces of the stack components. Accordingly, no gaskets, o-rings or similar devices are required to seal between the components. However, this method is also undesirable because it would be difficult and expensive to replace individual fuel cell assembly components that have degraded because all the fuel cell assemblies and components are adhesively attached together in the fuel cell stack. 
     Given these problems, there remains a need to improve the sealing design of fuel cells to improve durability, and to decrease cost and complexity. The present invention addresses these issues and provides further related advantages. 
     BRIEF SUMMARY OF THE INVENTION 
     In brief, a solid polymer electrolyte membrane fuel cell assembly and fuel cell stack configuration and, more particularly, a sealing design for the same are disclosed. 
     In one embodiment, an extended region around a perimeter of an MEA is adhesively attached to a first planar surface of a first flow field plate and not adhesively attached to a second planar surface of a second flow field plate to form a unitized fuel cell. In this configuration, a first electrode of the MEA faces the first surface of the first flow field plate and a second electrode of the MEA faces the second surface of the second flow field plate. The second surface of the second flow field plate comprises an adhesive joint recess that provides a space for the adhesive joint resulting from the adhesive bond of the extended region of the MEA to the first surface of the first flow field plate. 
     At least one manifold seal member is provided around the perimeter of at least one manifold opening wherein the manifold seal member is situated between the first surface of the first flow field plate and the second surface of the second flow field plate. The first surface of the first flow field plate allows the flow of a first fluid stream and the second surface of the second flow field plate allows the flow of a second fluid stream. In a fuel cell, the first and second fluid streams may comprise gaseous reactants, such as hydrogen and air, as well as any reaction products and inert gases, such as water, steam, and nitrogen. 
     Alternatively, at least one of the first surface of the first flow field plate and the second surface of the second flow field plate further comprises a manifold seal groove such that the manifold seal member is substantially aligned therein. In this configuration, the adhesive bond prevents external leaks of a first fluid stream as well as intermixing of the first fluid stream with a second fluid stream because the adhesive bond substantially isolates the first fluid stream. Likewise, the manifold seal member prevents intermixing of the second fluid stream and the first fluid stream. Furthermore, the manifold seal member can prevent intermixing of a coolant stream with the first and second fluid streams, if the coolant stream is present in the fuel cell stack. One of ordinary skill in the art will recognize that the adhesive joint does not need to be fully compressed against the surface of the adhesive joint recess of the second bipolar flow field plate because the manifold seal member prevents the second fluid stream from intermixing with the first fluid stream and coolant stream. Moreover, a small gap is allowed between the adhesive joint and the adhesive joint recess if the gap is sufficiently small and if the pressure drop in the oxidant gas stream is small. 
     In a second embodiment, the adhesive joint recess comprises ridges to provide a small contact pressure between the adhesive joint and the adhesive joint recess, to provide support for the glue joint and to prevent separation of the adhesive joint from the first surface of the first flow field plate under a fuel cell stack compression pressure during operation. 
     In a third embodiment, the second electrode and the membrane may have a larger surface area than the first electrode to form an extended region around the perimeter of the first electrode. The adhesive material may be applied on the membrane in the extended region to adhesively attach the membrane to the first surface of the first flow field plate while the second electrode of the MEA faces and contacts the second surface of the second flow field plate. In this case, the surface of the second electrode is flush with the second surface of the second flow field plate in the extended region around the perimeter of the MEA because the thickness of the adhesive joint is the same as the thickness of the MEA and thus no adhesive joint recess is necessary on the second surface of the second flow field plate. Alternatively, the MEA is flush-cut (in other words, the edges of the electrodes and the membrane are substantially aligned) and the adhesive infiltrates the circumferential edge of the first electrode to adhesively bond the MEA to the first surface of the first flow field plate. 
     In still another embodiment, multiple units of such fuel cells can be stacked together to form a fuel cell stack. For example, a first and a second fuel cell may be stacked together such that the second surface of the first flow field plate of the first fuel cell contacts the first surface of the second flow field plate of the second fuel cell to form a fuel cell stack. In this case, the first flow field plate of the first fuel cell and the second flow field plate of the second fuel cell are adhesively joined together to form a bipolar flow field plate. The adhesive seal, the manifold seal member and the flow field plate perimeter seal member in each fuel cell isolate each of the manifold openings, fluids, and MEA wherein the seal members are substantially aligned in their respective seal grooves. The inlet and outlet manifold openings for the first fluid stream, the second fluid stream, and the coolant stream are formed on an extended area of the bipolar flow field plates. Each fuel cell may also comprise a flow field plate seal member and a flow field plate seal groove that is formed on at least one of the first surface of the first flow field plate and the second surface of the second flow field plate wherein the flow field plate perimeter seal member and the flow field plate perimeter seal groove circumscribe the MEA. Alternatively, the manifold seal member may be attached to the flow field plate perimeter seal member to form an integrated seal member and, likewise, the manifold seal groove may be connected with the flow field plate perimeter seal groove wherein the integrated seal member is substantially aligned therein. 
     In yet another alternative for a fuel cell stack configuration, the second fluid stream can be sealed globally when the fuel cell stack is placed in a stack enclosure that is substantially fluid leak-tight. Internal inlet and outlet manifolds and manifold openings for the second fluid stream are not necessary because the second fluid stream is allowed to flow and vent to those areas defined by the substantially fluid leak-tight stack enclosure. Reactant gas shorting of the second fluid into individual cells can be minimized if the gap between the adhesive joint and the adhesive joint recess is small. Minor external leaks of the first fluid stream may vent into the second fluid stream as long as the leaks have an insignificant effect on fuel cell performance. In addition, the substantially fluid leak-tight stack enclosure can provide a flat surface that may be used to align the individual fuel cells inside the fluid leak-tight stack enclosure during fuel cell stack assembly. In addition, the fluid leak-tight stack enclosure may be configurable to compress the fuel cell stack. 
     These and other aspects of the invention will be evident upon review of the attached figures an following detailed discussion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the figures are not necessarily drawn to scale. For example, the shapes of various elements and angles are not drawn to scale, and some of these elements are arbitrarily enlarged and positioned to improve figure legibility. Further, the particular shapes of the elements, as drawn, are not intended to convey any information regarding the actual shape of the particular elements, and have been solely selected for ease of recognition in the figures. 
         FIG. 1  shows a cross-sectional view of the fuel cell stack according to one illustrated embodiment. 
         FIG. 2  shows a cross-sectional view of the fuel cell stack according to a second illustrated embodiment. 
         FIG. 3  shows a cross-sectional view of the fuel cell stack according to a third illustrated embodiment. 
         FIG. 4   a  shows a planar view of the oxidant flow field plate according to one configuration of the fuel cell stack. 
         FIG. 4   b  shows a planar view of the fuel flow field plate according to one configuration of the fuel cell stack. 
         FIG. 4   c  shows a planar view of the coolant flow field plate according to one configuration of the fuel cell stack. 
         FIG. 5   a  shows a planar view of the oxidant flow field plate and possible manifold locations in the fluid leak-tight stack enclosure. 
         FIG. 5   b  shows a planar view of the fuel flow field plate and possible manifold locations in the fluid leak-tight stack enclosure. 
         FIG. 5   c  shows a planar view of the coolant flow field plate and possible manifold locations in the fluid leak-tight stack enclosure. 
         FIG. 6  shows a three-dimensional view of the fluid leak-tight stack enclosure. 
         FIG. 7   a  shows a cross-sectional view of the bipolar flow field plate wherein the coolant flow fields are on the backside of one of the anode or cathode flow field plates. 
         FIG. 7   b  shows a cross-sectional view of the bipolar flow field plate wherein the coolant flow fields are on both the backside of the anode and the cathode flow field plates. 
         FIG. 7   c  shows a cross-sectional view of the bipolar flow field plate that comprises one plate with anode and cathode flow fields on each side of the bipolar flow field plate. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Unless the context requires otherwise, throughout the specification and claims which follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is as “including but not limited to”. Further, while generally disclosed in the context of solid polymer electrolyte (SPE) fuel cell stacks, those of ordinary skill in the art will appreciate that the present invention may be employed with other types of fuel cell stacks as well. 
     In  FIGS. 1 through 3  discussed in further detail below, it should be understood that the MEA is depicted as being adhesively attached to the fuel flow field surface of the first flow field plate such that the anode electrode of the MEA faces the fuel flow field surface of the first flow field plate, and the oxidant flow field surface of the second flow field plate is in non-adhesive contact with the cathode electrode of the MEA. Alternatively, the MEA may also be adhesively attached to the oxidant flow field surface of the second flow field plate such that the cathode electrode of the MEA faces the oxidant flow field surface of the second flow field plate, and the fuel flow field surface of the first fuel cell plate is in non-adhesive contact with the anode electrode of the MEA. In addition, in  FIGS. 1 through 3 , coolant flow fields (not shown) are interposed between the flow field plates of adjacent fuel cells. Furthermore, the fuel cell stacks of  FIGS. 1 through 3  may comprise of current collector plates placed at each end of the fuel cell stack. 
       FIG. 1  shows a fuel cell stack according to an embodiment of the present invention. Fuel cell assembly  2  comprises oxidant flow field plate  14  of bipolar flow field plate  16 , MEA  5 , and fuel flow field plate  12  of bipolar flow field plate  4 , wherein MEA  5  comprises anode electrode  6 , cathode electrode  8 , and membrane  10 . In this embodiment, bipolar flow field plate  4  comprises fuel flow field plate  12  and oxidant flow field plate  14 . During operation, a fuel gas stream enters fuel flow field  16  through internal manifold  18  and exits through a corresponding internal manifold for fuel gas stream exhaust (not shown). In this case, the fuel stream enters flow field  16  by flowing underneath adhesive joint  20  wherein adhesive joint  20  enables the peripheral region of MEA  5  to be adhesively attached to fuel flow field plate  12  of first bipolar flow field plate  4 . Adhesive joint recess  21  on oxidant flow field plate  14  of bipolar flow field plate  16  comprises a space to accommodate adhesive joint  20  of MEA  5 . Manifold seal member  24  is situated in manifold seal groove  26  and comprises an elastomeric material that circumscribes fuel manifold opening  18 . Under a fuel cell stack compression pressure, manifold seal member  24  provides a substantially fluid leak-tight seal so that each of the reactant and/or coolant streams is substantially isolated. Optionally, gap  22  is allowable to provide compliance for minor creep and compression set of manifold seal member, provided that gap  22  is sufficiently small and does not allow for pressurized gases to blow out manifold seal  24 . 
     A fuel cell stack is formed by stacking a plurality of fuel cell assemblies  2  such that fuel flow field plate  12  of bipolar flow field plate  16  contacts oxidant flow field plate  14  of bipolar flow field plate  16 . MEA  15  is adhesively attached to fuel flow field plate  12  of bipolar flow field plate  16  such that anode electrode  6  of MEA  15  faces fuel flow field plate  12  in the same manner as MEA  11  is adhesively attached to fuel flow field plate  12  of bipolar flow field plate  4 . An additional oxidant flow field plate (not shown) is placed against cathode electrode  8  of MEA  15  to form a fuel cell stack. The manifold openings of each fuel cell are in fluid communication with the respective manifold openings of adjacent fuel cells to form manifolds for supplying and exhausting the reactant and/or coolant streams. For example, the fuel inlet manifold opening of a first fuel cell is fluidly connected to the fuel inlet manifold opening of a second fuel cell to form a fuel inlet manifold for supplying fuel to the first and second fuel cells. 
       FIG. 2  shows a fuel cell stack according to another embodiment of the present invention incorporating ridges  28  located in adhesive joint recess  21 . Ridges  28  provide mechanical support between adhesive joint  20  and adhesive joint recess  21  to help minimize separation of adhesive joint  20  from fuel flow field plate  12  of bipolar flow field plate  4  when the fuel cell stack is subjected to a compression pressure. 
       FIG. 3  shows a fuel cell stack according to a further embodiment where adhesive joint  20  is thinner than the MEA. In one alternative, MEA  5  is flush-cut (i.e., edges of the MEA are substantially aligned) and the adhesive infiltrates anode electrode  6 , which is porous, and adhesively attaches anode electrode  6  and membrane  10  of MEA  5  to anode flow field plate  12  of bipolar flow field plate  4 . In another alternative, membrane  10  and cathode electrode  8  are larger and jut out beyond anode electrode  6  to form an extended region around the circumference of MEA  5 . The adhesive is applied to the extended region on membrane  5  of MEA  5  to form adhesive joint  20 . In this configuration, no adhesive joint recess is necessary on oxidant flow field plate  14  of flow field plate  16  because adhesive joint  20  is thinner than MEA  5 . Therefore, cathode electrode  8  of MEA  5  is flush with oxidant flow field plate  14  of flow field plate  16  in the extended region of MEA  5 . At least one of cathode electrode  8  and membrane  10  is larger in perimeter than anode electrode  6  in the extended region of MEA  5 . 
     In all the cases above, elastomeric manifold seal  20  provides compliance to variations in MEA thickness. Suitable manifold seal member materials include, for example, santoprene, ethylene-propylene-diene terpolymer (also known as EPDM) and other types of liquid elastomers. Although the material for manifold seal member  24  may experience the disadvantages of creep and compression set, the material for manifold seal member  24  does not need to be silicone-based injection-moldable materials as in the prior art because these materials have been shown to cause premature membrane degradation when in contact with the edge of the MEA. However, since an epoxy-based material is used to seal around the peripheral edge of the MEA, premature membrane degradation can be minimized. 
     The thickness of elastomeric manifold seal  20  may be optimized in order to ensure that the pressure applied to adhesive joint  17  in adhesive joint recess  18  is small to prevent high mechanical stress on bipolar flow field plates  4 , 16  when manifold seal member  24  is subjected to a compressive force. Adhesive joint  20  may comprise a substantially rigid thermoset material that does not need to exhibit substantial elastic properties. Suitable thermoset materials may be, for example, polyimides, polyesters and epoxies. In some embodiments, epoxies may be employed because they exhibit desired durability, such as high resistance to creep and compression set after prolonged stress and high resistance to chemical attack. Furthermore, epoxy resins do not release volatiles or shrink in volume during curing, and exhibit excellent tensile-shear strength upon curing. Epoxies are also relatively cheap compared to conventional gasket materials and only a small amount is required per fuel cell assembly. On the other hand, elastomeric seal materials, such as silicone-based seal materials, exhibit oxidative degradation at operating temperatures above 70° C., seal thinning due to creep under prolonged stress, and delamination from the MEA as result of excessive shearing stresses under high seal compression load. 
     As illustrated in  FIGS. 1 through 3 , bipolar flow field plates are typically used in fuel cell stacks. Bipolar flow field plates generally comprise two opposing active surfaces, one surface that comprises fuel flow fields and an opposing surface that comprises oxidant flow fields. In a series arrangement, the surface of the bipolar flow field plate with fuel flow fields faces the anode electrode of a first MEA while the surface of the bipolar flow field plate with oxidant flow fields faces the cathode electrode of an adjacent second MEA. In one alternative, bipolar flow field plates may comprise two plates, namely the fuel flow field plate and the oxidant flow field plate. The fuel flow field plate and the oxidant flow field plate may each comprise two planar surfaces: an active surface that faces and contacts the reactant gases and the corresponding electrodes, and a non-active surface that faces a non-active surface of the adjoining plate. Each flow field plate typically comprises a plurality of channels that allow the flow of reactant gases separated by landings that contact the electrode of the MEA. In some cases, the fuel flow field plate and the oxidant flow field plate can be attached to each other by an adhesive bond, chemical bond, or mechanical bond to form a single bipolar flow field plate such that the non-active surfaces of the two plates face each other and may be in contact with a coolant fluid. 
       FIG. 4   a  shows the active surface of a fuel flow field plate with fuel flow fields  16  that guide the fuel gas stream from fuel inlet manifold opening  18  to fuel outlet manifold opening  22 . Fuel flow field plate  12  further comprises oxidant inlet manifold opening  30  and oxidant outlet manifold opening  32  to allow for supply and exhaust of the oxidant gas stream, respectively, as well as coolant inlet manifold opening  34  and coolant outlet manifold opening  36  to allow for supply and exhaust of the coolant stream, respectively. Manifold seal groove  26  provides a space to accommodate manifold seals (not shown) that isolate oxidant inlet manifold opening  30 , oxidant outlet manifold opening  32 , coolant inlet manifold opening  34  and coolant outlet manifold opening  36  from fuel flow fields  16 . Similarly,  FIG. 4   b  shows the active surface of an oxidant flow field plate with oxidant flow fields  13  that guide the oxidant gas stream from oxidant inlet manifold opening  30  to oxidant outlet manifold opening  32 . Oxidant flow field plate  14  further comprises fuel inlet manifold opening  18  and fuel outlet manifold opening  22  to allow for supply and exhaust of the fuel gas stream, respectively, as well as coolant inlet manifold opening  34  and coolant outlet manifold opening  36  to allow for supply and exhaust of the coolant stream, respectively. Again, manifold seal groove  26  provides a space to accommodate manifold seals (not shown) that isolate fuel inlet manifold opening  18 , fuel outlet manifold opening  22 , coolant inlet manifold opening  34  and coolant outlet manifold opening  36  from oxidant flow fields  13 . 
     In both  FIGS. 4   a  and  4   b , at least one of fuel flow field plate  12  and oxidant flow field plate  14  may comprise flow field plate seal groove  41  to accommodate a flow field plate seal member (not shown) that prevents leakage of air from the fuel cell. In one alternative, the flow field plate seal member may be connected to the manifold seal member to form an integrate flow field plate seal member thereof and, thus, flow field plate seal groove  41  will be connected to manifold seal groove  26 . 
     The non-active surface of either or both the anode and cathode flow field plates may further comprise coolant flow fields, as shown in  FIG. 4   c , which may also comprise a plurality of landings and channels. Coolant flow fields  42  guide the coolant stream from coolant inlet manifold opening  34  to coolant outlet manifold opening  36 . The non-active surface may also comprise adhesive groove  44  to provide a space for disposing an adhesive therein for adhesively joining the fuel flow field plate and the oxidant flow field plate to form a bipolar flow field plate, and for isolating each manifold individually to prevent fluid streams from mixing with each other. 
     In this configuration, the surface of the bipolar flow field plate with fuel flow fields faces the anode electrode of a first MEA while the surface of the bipolar flow field plate with oxidant flow fields faces the cathode electrode of an adjacent second MEA. Alternatively, the bipolar flow field plate may comprise one plate wherein both opposing surfaces of the bipolar flow field plate are active, for example, one surface comprises fuel flow fields and the opposing surface comprises oxidant flow fields. 
       FIG. 5   a  shows a planar view of a possible oxidant flow field plate configuration according to another embodiment of the present invention. Oxidant inlet manifold opening  30  and oxidant outlet manifold opening  32  are formed by fluid leak-tight stack enclosure  40  around the flow field plate wherein the oxidant gas enters and exits the fuel cell stack directly from the manifold defined by fluid leak-tight stack enclosure  40  surrounding the fuel cell stack. Oxidant flow fields  13  deliver oxidant gas from oxidant inlet manifold opening  18  to oxidant outlet manifold opening  22 . Manifold seal groove  26  circumscribes fuel gas inlet manifold opening  18  and fuel gas outlet manifold opening  22 , as well as coolant inlet manifold opening  34  and coolant outlet manifold opening  36  to provide a space for the manifold seal member. Furthermore, adhesive joint recess  21  circumscribes the electrochemically active area of the oxidant flow field plate to provide a space for the adhesive seal joint of the MEA. In this case, the flow field landings of the oxidant plates comprise an adhesive joint recess to support the adhesive joint. One of ordinary skill in the art will recognize that the depth of the flow field channels on the oxidant plate should be greater than the depth of the adhesive joint recess in order to allow the flow of oxidant gas from oxidant inlet manifold opening  30  to oxidant outlet manifold opening  32 . 
       FIG. 5   b  shows a schematic of a representative fuel flow field plate configuration in fluid leak-tight stack enclosure  40 . Fuel flow fields  16  deliver fuel gas from fuel inlet manifold opening  18  to fuel outlet manifold opening  22 . Again, manifold seal groove  26  surround fuel gas inlet manifold opening  18  and fuel gas outlet manifold opening  22 , as well as coolant inlet manifold opening  34  and coolant outlet manifold opening  36  to provide a space for the manifold seal member to substantially align therein. 
       FIG. 5   c  shows a schematic of a coolant flow field plate in fluid leak-tight stack enclosure  40 . Coolant flow fields  42  deliver coolant from the coolant inlet manifold opening  34  to coolant outlet manifold opening  36 . The coolant stream is isolated from the oxidant gas stream and fuel gas stream using a manifold seal member or an adhesive around fuel manifold openings  18 ,  22 , as well as along each edge of coolant flow field plate  38  such that the manifold seal member or adhesive is aligned or disposed in adhesive groove  44  to prevent leakage of the coolant fluid into the fuel and oxidant gas streams. 
       FIG. 6  shows a three-dimensional view of a substantially fluid leak-tight stack enclosure. Oxidant inlet port  41 , oxidant outlet port  43 , fuel inlet port  46 , fuel outlet port  48 , coolant inlet port  50 , and coolant outlet port  52  are formed on one end of stack enclosure  40 . Each port is in fluid connection with its corresponding manifold. For example, the fuel inlet manifold opening for each fuel cell is fluidly connected to form a fuel inlet manifold, which is fluidly connected to fuel inlet port  46 . In this configuration, fluid leak-tight stack enclosure  40  is a dielectric material to prevent fuel cell stack shorting. 
     There are many advantages to forming the oxidant manifolds using the walls of the fluid leak-tight stack enclosure. For example, fuel cell stack ventilation of minor fuel and coolant leaks can be accomplished by venting oxidant outlet manifold opening  32  because any minor fuel and coolant leaks will leak into oxidant inlet manifold opening  30  and oxidant outlet manifold opening  32 , thus eliminating the need for an extra mechanism or mechanical device to vent the stack enclosure. Optionally, fluid leak-tight stack enclosure  40  is configurable to compress the fuel cell stack, for example with tie rods (not shown) that extend along a length of fluid leak-tight stack enclosure  40  or by welding the edges of stack enclosure  40  thereby eliminating the need of additional compression hardware. In addition, stack enclosure  40  may comprise thermal insulation (not shown) that is placed on at least one of the inner and outer surfaces of stack enclosure  40  to prevent freezing of the fuel cell stack under freezing conditions. 
       FIGS. 7   a ,  7   b , and  7   c  illustrate representative bipolar plate configurations. In  FIG. 7   a , bipolar flow field plate  4  is formed such that second surface of fuel flow field plate  12  is attached to the second surface of oxidant flow field plate  14 . In this example, coolant flow fields  42  are formed on the second surface of oxidant flow field plate  14  while the second surface of fuel flow field plate  12  may be flat. Alternatively, coolant flow fields  42  may be formed on the second surface of fuel flow field plate  12  while the second surface of oxidant flow field plate  14  may be flat. In  FIG. 7   b , bipolar flow field plate  4  is formed such that fuel flow field plate  12  is attached to oxidant flow field plate  7  wherein coolant flow fields  42  are formed on the second surface of both fuel flow field plate  12  and oxidant flow field plate  14 .  FIG. 7   c  is a schematic of a bipolar flow field plate such that bipolar flow field plate  4  comprises one plate wherein fuel flow fields  13  are formed on the first surface of bipolar flow field plate  4  and oxidant flow fields  16  are formed on the second surface of bipolar flow field plate  4 . 
     While particular elements, embodiments, and applications of the present invention have been shown and described, it will be understood that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings.