Abstract:
A fuel cell assembly ( 110, 210 ) has a plurality of fuel cell component elements ( 112 ) extending between a pair of end plates ( 114, 115 ) to form a stack ( 116 ), and plural reactant gas manifolds ( 120, 220; 122, 222; 124, 224; 126, 226 ) mounted externally of and surrounding the stack, in mutual, close sealing relationship to prevent leakage of reactant gas in the manifolds to the environment external to the manifolds. The reactant gas manifolds are configured and positioned to maximize sealing contact with smooth surfaces of the stack and the manifolds. One embodiment is configured for an oxidant reactant manifold ( 120, 124 ) to overlie the region where the fuel reactant manifold ( 122, 126 ) engages the stack. Another embodiment further subdivides an oxidant reactant manifold to include a liquid flow channel ( 270, 274 ), which liquid flow channel overlies the region where the fuel reactant manifold ( 122, 126 ) engages the stack.

Description:
BACKGROUND 
       [0001]    The disclosure relates generally to manifolds for stack assemblies, and more particularly to external manifolds for fuel cell stack assemblies. More particularly still, the disclosure relates to such external manifolds for fuel cell stack assemblies that may be located in leak-sensitive environments. 
         [0002]    In cell stack assemblies, it is known to use internal reactant and coolant manifolds as well as external reactant and coolant manifolds. Internal manifolds generally comprise passageways made within the various plates that constitute the cells of the cell stack assemblies. This renders the plates themselves more expensive than plates which are fabricated for use with external manifolds. Internal manifolds have potential leakage paths between the plates of every cell, the leakage thereby being “overboard” to the external environment. While external manifolds may also leak gasses to the external environment, the avoidance of such leaks is more easily accomplished. The operating location of the cell stack assemblies and associated manifolds is often determinative of the level of gas (typically reactant) leakage that can be tolerated to the environment. Indeed, in some instances, relatively greater leakage can be tolerated if the leakage region can be purged with air or the like. Typically, various closed spaces, as for example vehicular environments containing humans, are relatively more leak-sensitive, i.e., less leak-tolerant, and the ability to purge the leakage may be very limited or impractical. 
         [0003]    External manifolds consist of manifold shells, manifold seal gaskets, and a mechanical loading or restraint system to hold the manifolds in compression, tightly against the edges of the cell stack. 
         [0004]    A fuel cell module  10  of U.S. Pat. No. 4,345,009, shown in  FIG. 1  and in cross section in  FIG. 2 , is one example of the external manifold and containment systems known in the art. The lower right corner of  FIG. 2  is the corner of the module  10  pointing toward the viewer in  FIG. 1 . The module  10  includes a stack  12  of fuel cells  14 . As shown in  FIG. 3 , each fuel cell  14  comprises a gas porous anode electrode  16  and a gas-porous cathode electrode  18  spaced apart with a layer  20 , such as a liquid electrolyte-retaining matrix or a proton exchange membrane (PEM), disposed there between. Each electrode  16 ,  18  includes a very thin catalyst layer  19 ,  21 , respectively on the surfaces thereof adjacent the layer  20 . An electrically conductive, gas impervious plate  22  may separate adjacent fuel cells in the stack  12 . Each fuel cell in the stack may include one separator plate  22  such that the phrase “fuel cell” will encompass a repeating unit of the stack which includes one separator plate. The fuel cells of this exemplary embodiment may be the same as shown in U.S. Pat. No. 4,115,627 in which the electrolyte is phosphoric acid. However, fuel cell stacks with proton exchange membrane electrolytes, as in U.S. Pat. No. 6,024,848, have similar needs with respect to manifolds and their integrity. 
         [0005]    In this embodiment, every third fuel cell  14 ′ ( FIG. 3 ) includes a coolant carrying layer  24  disposed between the electrode  16  and the separator plate  22 . Passing in-plane through this layer  24  are coolant carrying passages  26 . The coolant flow through these passages carries away the heat generated by the fuel cells. The number of coolant layers  24  and passages  26  required by a stack is dictated by a variety of factors which are not relevant here. Although the coolant passages  26  are shown as extending to the surface  32  for clarity, in an actual fuel cell stack they would only do so in regions adjacent to two corners of the cell. The stack  14  is completed by top and bottom flat graphite current collector blocks  27 ,  28 , respectively, bonded to the separator plates  22  at each end of the stack, and pressure plates  66 ,  68 . 
         [0006]    As shown in the drawing, the outer edges  29  of the stack components  16 ,  18 ,  20 ,  22 ,  24 ,  27  and  28  form four outwardly facing, approximately planar surfaces which are the external surfaces of the stack  12 . Portions of two of these surfaces  30 ,  32  are shown in  FIG. 3 . Each of the four surfaces is substantially completely covered by a reactant gas manifold. An air or oxygen gas inlet manifold  34  covers the surface  30  while a fuel or hydrogen gas inlet manifold  36  covers the surface  32 . The opposing surfaces are covered by an air outlet manifold  38  and a fuel outlet manifold  40  ( FIG. 2 ). 
         [0007]    The manifolding arrangement just described incorporates an outlet manifold on each side of the stack opposite an inlet manifold. However, as shown in in U.S. Pat. No. 3,994,748, a fuel manifold covering one surface of the stack may be divided into two compartments to serve as both the inlet and the outlet manifold, while the manifold on the opposite surface of the stack serves as a mixing manifold; the same configuration may be used for the air. 
         [0008]    The anode electrode  16  and the cathode electrode  18  both comprise relatively thick substrates with ribs formed on one side thereof defining reactant gas channels  42 ,  44 , respectively. The fuel gas channels  42  carry hydrogen or a hydrogen-rich gas across the cells from the fuel inlet manifold  36  to the fuel outlet manifold  40 . The air channels  44  carry air across the cells from the air inlet manifold  34  to air outlet manifold  38 . The flat surface of each substrate, which is opposite to the surface having the ribs (and thus the gas channels), has a layer  19 ,  21  of catalyst disposed thereon. 
         [0009]    The graphite blocks  27 ,  28  have the same outer dimensions as the other stack components, and their outwardly facing surfaces (two of which,  50  and  52 , can be seen in  FIG. 3 ) provide smooth sealing surfaces for the top and bottom sealing flanges  54 ,  56  of each manifold. A thick block at one end of the stack is required to accommodate the possible differences in stack height (or length, depending on stack orientation) which could result from the buildup of the very small tolerances in the thickness of the many hundreds of components in the stack  12 . For example, a stack of 400 cells each having a thickness of about 0.64 cm (0.25 inch) with a tolerance of 0.01 cm (±0.004 inch) could have an overall height of anywhere from 250 to 258 cm (98.4 to 101.6 inches). The manifolds, on the other hand, have a fixed height (length) A large block thickness is thus required to ensure that both the top and bottom flanges  54 ,  56  are located somewhere on the smooth sealing surfaces of the blocks  27 ,  28  after the desired compressive force has been applied to the stack, as hereinafter explained. 
         [0010]    As best shown in  FIG. 2 , side flanges  58  seal against the vertically extending external surfaces of the stack  12  near the corners of the stack which do not have reactant gas channels. A sealing material, such as a porous polytetrafluoroethylene, is disposed between the manifold flanges  54 ,  58  and the surfaces of the stack. Steel bands  60  ( FIGS. 1 and 2 ) surround the stack manifolds, and hold them in sealing relationship with the stack and graphite blocks. Fasteners  62  connecting the ends of each band permit tightening the bands to the extent necessary to ensure adequate sealing. 
         [0011]    To obtain good electrical, thermal, and sealing contact between the various components of the fuel cells and the stack  12 , the module  10  includes a constraint system  64 . In this exemplary embodiment, the constraint system  64  comprises inflexible top and bottom steel end or pressure plates  66 ,  68 , respectively, and tie rods  70  connecting the plates. The plates  66 ,  68  rest flat against the graphite blocks  27 ,  28 , respectively. In assembling a module  10 , the pressure plates  66 ,  68 , the blocks  48 ,  49 , and the various stack components are arranged one atop (or adjacent, depending on orientation) the other in proper sequence. This assembly is hydraulically loaded whereupon a preselected axial (i. e., perpendicular to the plane of the cells) load is applied to the plates  66 ,  68  to compress the stack  12 . The tie bolts  70  are then tightened down to an extent that, when the assembly is removed from the press, the compressive force on the stack  12  is of approximately the desired magnitude. The manifolds  34 ,  36 ,  38 , and  40  are then positioned against the sides of the stack and secured by the bands  60 . 
         [0012]    Since the constraint system  64  and the manifolds  34 ,  36 ,  38 , and  40  are made from similar materials (carbon steel), they have the same or approximately the same coefficient of thermal expansion. Therefore, when the stacks heat up during operation, these items expand to approximately the same extent. Although the stack  12  has a lower coefficient of thermal expansion, as the plates  66 ,  68  move apart the elasticity or spring rate of the compressed stack  12  results in the height of the stack  12  increasing by the same rate with an accompanying loss in axial load. Thus, there is virtually no relative movement between the graphite blocks  27 ,  28  and their respective manifold sealing flanges  54 ,  56  during thermal expansion. Likewise, there is relatively little motion between the stack external surfaces, such as  30  and  32 , and the vertical manifold sealing flanges  58 . Once steady state is reached the constraint system  64  holds the stack height constant. 
         [0013]    The external manifold system described with respect to  FIGS. 1-3  presents difficulty in assuring the avoidance of leakage overboard to the external environment. Although the four outwardly facing surfaces formed by the sides of the assembled stack components  16 ,  18 ,  20 ,  22 ,  24 ,  27  and  28  of that U.S. Pat. No. 6,764,787 were characterized above as being “approximately planar”, in fact they are typically somewhat irregular due to minor inconsistencies in the sizes and/or alignments of those assembled stack components. This may be better understood with reference to U.S. Pat. No. 6,660,422, which describes and depicts the irregularity of such surfaces. These irregularities in those outwardly facing surfaces of the stack further complicate efforts to obtain a good seal between the manifolds and at least those surfaces of the stack. The possible leakage of fuels, such as hydrogen, in prior art fuel cell assembly designs may be particularly undesirable in closed spaces, as for example in human-occupied vehicles and the like, where the elimination or dilution of such leaked fuel is not possible or is inadequate. 
         [0014]    However, the use of external manifolds may be advantageous for various reasons. External manifolds can be lower in product cost to manufacture, and provide a protective shield around the fuel cells top protect them from physical damage and contamination. External manifolds can also provide an electrical short protection when combined with dielectric seal materials, plastic coated metals, and/or made from plastic materials. They can provide structural support for the cell stack. External manifolds can also provide the flexibility of integrating reactant flow and coolant distribution features, such as baffles and/or chambers to allow the fuel cell stack to operate with better reactant and coolant usage and to make the fuel cell stack more durable. 
       SUMMARY 
       [0015]    A fuel cell assembly has a plurality of fuel cell component elements extending under compressive pressure between a pair of end plates to form a stack, and plural reactant gas manifolds mounted externally of and surrounding the stack, in mutual, close sealing relationship to prevent leakage of reactant gas in the manifolds to the environment external to the manifolds, i. e., overboard. The fuel cell stack may include, for example, a plurality of polymer electrolyte membrane (PEM) fuel cells. The reactant gas manifolds are configured and positioned to maximize sealing contact with smooth surfaces of the stack and the manifolds. At least some of the plural reactant gas manifolds include smooth surfaces in close sealing engagement with smooth surfaces of others of the plural reactant gas manifolds. The end plates also have smooth peripheral surfaces for close sealing engagement with the reactant gas manifolds. As used herein, the phrase “smooth surface” or “smooth face” as applied to various surfaces, typically means that the particular surface has a surface roughness of less than about 128 micro inch (or less than approximately 3 microns). 
         [0016]    One example embodiment includes a pair of opposed fuel reactant manifolds on opposite sides of the stack, and a pair of opposed oxidant reactant manifolds on further opposite sides of the stack in orthogonal relation with the fuel reactant manifolds. The reactant manifolds are provided with inwardly-facing sealing surfaces, or flanges, which are smoothly contoured for good sealing engagement with a smooth surface, and may include similarly smoothly contoured external surfaces for mated sealing engagement with the inwardly-facing sealing surfaces of other manifolds. The reactant manifolds are typically rectilinear, and the inwardly-facing sealing surfaces at opposite ends of the manifolds are in mated sealed engagement with the smooth peripheral surfaces of the end plates. The inwardly-facing sealing surfaces of the oxidant reactant manifolds and the fuel reactant manifolds are typically coextensive with one another with respect to the (axial) length of the stack. However, whereas the inwardly-facing sealing surfaces of the fuel reactant manifold that extend transversely of the stack are coextensive with the corresponding transverse dimension of the end plate, the inwardly-facing sealing surfaces of the oxidant reactant manifold that extend transversely of the stack extend beyond the corresponding transverse dimension of the end plate and overlap a smooth external surface of the fuel reactant manifolds for good sealing engagement therewith. This assures that any leakage from the fuel reactant manifold in regions of irregular mating surfaces will find its way to the oxidant reactant manifold, where the inwardly-facing sealing surfaces are in engagement only and entirely with mating smooth sealing surfaces. 
         [0017]    Another example embodiment, similar to that described above, additionally includes channels for another fluid, typically the coolant liquid, within at least a peripheral region of a reactant manifold. The liquid is typically a coolant such as water, and the positioning of the liquid channels is such that gaseous reactant leakage, particularly of fuel, but also oxidant, will find its way to the liquid for safe containment and/or removal. The liquid channels are typically in the oxidant reactant manifold and are positioned to overlie at least the region along which the fuel reactant manifold engages the fuel cell stack. Gasketing that typically exists between the mating sealing surfaces of the manifolds and the end plates and/or manifold exteriors may be structured to facilitate flow of coolant in the peripheral regions, or channels, outwardly of the oxidant and fuel reactant flow channels. Moreover, the relative operating pressures of the fuel reactant, the oxidant reactant, and the coolant may be selected such that the fuel reactant pressure is greatest and the coolant pressure least so that any reactant leakage is toward and into, the coolant. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a perspective view, partly broken away, showing a fuel system of the prior art. 
           [0019]      FIG. 2  is a sectional view taken along line  2 - 2  in  FIG. 1 . The section is taken parallel to the plane of the cells, cutting through the reactant gas channels of a cathode electrode. 
           [0020]      FIG. 3  is a perspective view of a portion of the stack of fuel cells of  FIG. 1  with the manifolds and constraint system of  FIG. 1  removed. 
           [0021]      FIG. 4  is a simplified, stylized schematic perspective view of an embodiment of the external manifold system of the disclosure, disposed about a cell stack. 
           [0022]      FIG. 5  is a partly exploded view of the external manifold system of  FIG. 4 . 
           [0023]      FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 4 . 
           [0024]      FIG. 7  is a simplified, stylized schematic perspective view, partly broken away, of another embodiment of the external manifold system of the disclosure, disposed about a cell stack and providing a coolant channel. 
           [0025]      FIG. 8  is a partly exploded view of the external manifold system of  FIG. 7 . 
           [0026]      FIG. 9  is a sectional view taken along line  9 - 9  of  FIG. 4 . 
           [0027]      FIG. 10  is an enlarged view of the coolant channel(s) shown encircled by broken line  10  in  FIG. 7 . 
           [0028]      FIG. 11  is an exploded view of the external manifold system of  FIG. 7 , further depicting gaskets and fluid flow paths. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The prior art fuel cell module of  FIGS. 1-3  was depicted in a form generally representative of one having a liquid electrolyte, such as phosphoric acid, and was depicted in a vertical orientation and having the graphite current collector blocks and the end plates as separate members. While the aspects of the embodiments to be disclosed hereinafter are similarly applicable to a configuration as discussed in the preceding sentence, the example embodiments hereinafter described are presented in the context of a PEM fuel cell module, typically oriented horizontally and having the collector block and end plate at an end of the stack formed as a unitary member. 
         [0030]    Referring to  FIGS. 4 through 6 , there is depicted, in simplified, stylized, perspective fashion, a fuel cell assembly  110  having a plurality of fuel cell component elements  112  extending under compressive pressure between a pair of end plates  114 ,  115  to form a fuel cell stack  116 . The fuel cell stack  116  illustrated is of the PEM type, though other electrolyte configurations may be used as well. The fuel cell assembly  110  also includes external reactant manifolds  120 ,  122 ,  124  and  126 . In the illustrated embodiment, top and bottom reactant manifolds  120 ,  124  provide for the inlet and outlet, respectively, of oxidant reactant, such as air or O 2  to the fuel cell stack  116 , and the opposite side reactant manifolds  122 ,  126  provide for inlet and outlet, respectively, of fuel reactant, such as H 2  or a H 2 -rich gas, to the fuel cell stack  116 . 
         [0031]    The fuel cell stack  116 , the end plates  114 ,  115 , and the reactant manifolds  120 ,  122 ,  124 , and  126  are maintained in compressive engagement, both axially and circumferentially. As noted earlier, in this embodiment the end plates  114 ,  115  each combine the end plate and the collector block of the described prior art into a unitary, or integral, structure. The stack  116  with end plates  114 ,  115  is held in compressive axial engagement by suitable means, such as tie rods  130  that extend between mounting flanges  132  at opposite ends of the assembly. The mounting flanges  132  may be formed as integral “ears” extending from corners of the end plates  114 ,  115 , or they may be part of a separate “X-shaped” cross member  132 ′ as shown herein. The tie rods  130  include threaded ends and have retaining nuts, or any other suitable fastening arrangement. The fuel cell stack  116  and the surrounding reactant manifolds  120 ,  122 ,  124 , and  126  are retained circumferentially in compressive engagement by, for example, stainless steel compression bands  134 , only one being partly shown in  FIG. 4 . 
         [0032]    Referring to the disclosed external manifold arrangement in greater detail, each of the reactant manifolds  120 ,  122 ,  124 , and  126  is typically rectilinear in shape and covers the entirety of a side of at least the fuel cell component elements  112  of the fuel cell stack  116 . The reactant manifolds  120 ,  122 ,  124 , and  126  each include a respective cover wall member  120 A,  122 A,  124 A and  126 A having an outwardly facing, smooth planar surface, and respective side flanges  120 B,  122 B,  124 B, and  126 B and end flanges  120 C,  122 C,  124 C, and  126 C continuously connected and extending inwardly toward the fuel cell stack  116  to define respective manifold chambers  140 ,  142 ,  144 , and  146 . The side and end flanges  120 B,  122 B,  124 B, and  126 B and  120 C,  122 C,  124 C, and  126 C are typically orthogonal to the respective cover wall members  120 A,  122 A,  124 A and  126 A, and include smooth end faces for good sealing engagement. Those smooth end faces are not separately numbered in the Figures, but are at the distal ends of the respective side and end flanges with which they are associated. As used herein, the phrase “smooth surface” or “smooth face” as applied to the end faces of the manifold side and end flanges, and to the manifold cover walls and the side perimeter surfaces of the end plates, typically means that the particular surface has a surface roughness of less than about 128 micro inch (or less than approximately 3 microns). 
         [0033]    The manifold chambers  140  and  144  respectively supply oxidant reactant to, and remove oxidant reactant from, the fuel cell stack  116 . Similarly, the manifold chambers  142  and  146  respectively supply fuel reactant to, and remove fuel reactant from, the fuel cell stack  116 . The length of the fuel reactant manifolds  122  and  126  is such that the respective end flanges  122 C and  126 C may be, and are, positioned in engagement with the respective and plates  114 ,  115 . Correspondingly, the width (in this depiction, height) of the fuel reactant manifolds  122  and  126  is such that the respective side flanges  122 B and  126 B may be, and are, substantially flush, or even, with that same dimension of the end plates  114 ,  115  and the fuel cell stack  116 , which are substantially equal. This is seen most clearly in  FIG. 6 . 
         [0034]    The length of the oxidant reactant manifolds  120  and  124  is such that, like manifolds  122  and  126 , the respective end flanges  120 C and  122 C may be, and are, positioned in engagement with the respective and plates  114 ,  115 . However, in accordance with the disclosure, the width of the oxidant reactant manifolds  120  and  124  is such that the respective side flanges  120 B and  124 B are outboard of that dimension of the end plates  114 ,  115  and the fuel cell stack  116 , and are aligned in substantially perpendicular, butting engagement, perhaps via a thin gasket, with the smooth external surfaces of flanges  122 B and  126 B of the manifolds  122  and  126  to provide a good seal. This also is seen most clearly in  FIG. 6 . 
         [0035]    The ends of the various flanges  120 B, 120 C,  122 B,  122 C,  124 B,  124 C,  126 B, and  126 C are each formed and/or machined to be smooth and linear to provide close sealing engagement with the similarly smooth surfaces of the end plates  114 ,  115  and the manifold flanges  122 B,  122 C,  126 B, and  126 C. It should be noted that as used herein with reference to the sealing engagements described above, the phrase “sealing engagement” is meant to include not only direct contact between the abovementioned metal, graphite , and plastic elements, but to also include the provision of a thin sealing agent or gasket between those members as well. In the example described, thin gaskets  150  are positioned at the end faces of the flanges of the oxidant reactant manifolds  120  and  124 , and similar thin gaskets  152  are positioned at the end faces of the flanges of the fuel reactant manifolds  122 ,  126 , as seen in  FIG. 6 . The gaskets  150 ,  152  are generally shaped to conform to the perimeters of the manifolds  120 ,  122 ,  124 ,  126 , as represented by the end faces of their respective flanges. The gaskets  150  and  152  are formed of a material suitably resilient for sealing purposes and resistant to the fluids in that environment. The gaskets are typically solid or foam elastomer-type materials, of flat or shaped profile. The gaskets are normally either adhesively attached to the stack and/or manifold, or they are retained in a groove of appropriate geometry. The gaskets can also be single or multiple “formed in place” layers of cured-in-place elastomer materials as well, or a combination of those previously mentioned techniques. One example of a cured-in-place elastomer is silicone RTV rubber sealants. 
         [0036]    Because the width (in this depiction, height) of the fuel reactant manifolds  122 ,  126  is such that the respective side flanges  122 B and  126 B may be, and are, substantially flush, or even, with that same dimension of the end plates  114 ,  115  and the fuel cell stack  116 , the ends of those flanges may abut a somewhat irregular surface along the fuel cell stack because of slightly varying dimensions and thermal expansion of the individual fuel cell component elements  112  (as described and depicted in the aforementioned U.S. Pat. No. 6,660,422). While the gaskets  152  aid in filling and sealing any voids between these abutting surfaces, experience has shown the seal to be less than complete. However, because the width of the oxidant reactant manifolds  120  and  124  is such that the respective side flanges  120 B and  124 B are outboard of that dimension of the end plates  114 ,  115  and the fuel cell stack  116 , and are aligned in substantially perpendicular, butting engagement with the smooth surfaces of flanges  122 B and  126 B of the manifolds  122  and  126 , there is a greatly diminished requirement for the gaskets  150  to be of complex form in order to provide a good seal. Moreover, because the width of the oxidant reactant manifolds  120  and  124  places side flanges  120 B and  124 B outboard of the locations where the fuel reactant manifold side flanges  122 B and  126 B abut the fuel cell stack  116 , any fuel leakage occurring at those latter junctures is scavenged by, or delivered into, the oxidant reactant in manifolds  120  and  124 , without undesired leakage overboard to the local environment external to the fuel cell assembly  110 . 
         [0037]    Reference is now made to an embodiment of a fuel cell assembly  210  depicted in  FIGS. 7-11  in which there exists strong similarities to the fuel cell assembly  110  of the  FIG. 4-6  embodiment. For the sake of brevity, elements for the axial and circumferential compression and retention of the fuel cell assembly  210  are present but not shown. Further, elements of this embodiment which are the same as those in the  FIG. 4-6  embodiment have been given similar numbers, such as the fuel cell component elements  112  forming the fuel cell stack  116 , and the end plates  114  and  115 . The elements of this embodiment which are broadly analogous to, but differ somewhat from, the  FIG. 4-6  embodiment, are given similar “suffix” numbers in the “200” series, and the several components which are newly identified are given new “suffix” numbers in the “200” series. 
         [0038]    While the  FIG. 7-11  embodiment includes the broader external seal aspects of the  FIG. 4-6  embodiment, it further includes provision for one or more liquid flow channels, typically for coolant such as water, positioned in the external manifolds to further contain any reactant leakage that might occur. The presence of liquid in the liquid flow channel(s) may serve to supply and circulate coolant not only for conventional purposes, but also to scavenge or collect any reactant gas that may leak from the reactant manifold compartments or cell element corners. The liquid flow channel(s) in the external manifolds is/are positioned to overlie and/or directly communicate with most, or all, of the regions where the reactant manifolds are in engagement with the fuel cell stack  116  and might be otherwise subject to reactant leakage externally to the stack. 
         [0039]    Referring to  FIGS. 7-11  in greater detail, the fuel cell stack  116  is bounded on each end by end plates  114 ,  115 , and along the respective sides by external manifolds  220 ,  222 ,  224 , and  226 . The reactant manifolds  220 ,  222 ,  224 , and  226  each include a respective cover wall member  220 A,  222 A,  224 A and  226 A having an outwardly facing planar surface, and respective outboard side flanges  220 B,  222 B,  224 B, and  226 B and outboard end flanges  220 C,  222 C,  224 C, and  226 C continuously connected and extending inwardly toward the fuel cell stack  116  to define associated manifold chambers to be described. Additionally, the reactant manifolds  220  and  224  include further inboard side flanges  220 B′ and  224 B′ spaced inward of the respective outboard side flanges  220  B and  224 B, and further inboard end flanges  220 C′ and  224 C′ (not separately visible) spaced inward of the respective outboard end flanges  220 C and  224 C, with the respective inboard side and end flanges being joined and continuous to define new chambers. Further still, the reactant manifold  226  may include an inboard side flange  226 B′ located inward of the outward side flange  226 B to subdivide a chamber. The various side flanges  220 B,  220 B′,  222 B,  224 B,  224 B′,  226 B and  226 B′ and various end flanges  220 C,  220 C′,  222 C,  224 C,  224 C′, and  226 C are typically orthogonal to the respective cover wall members  220 A,  222 A,  224 A, and  226 A, and include smooth end faces for good sealing engagement. 
         [0040]    The side manifolds  222  and  226  comprise, respectively, manifold chambers  242 , and  246 A and  246 B respectively. As depicted herein, the manifold chamber  246 A is for the entry of fuel reactant to the fuel cell stack  116 , the manifold chamber  242  is for reversing the direction of the fuel reactant at the opposite side of the stack, and the manifold chamber  246 B is for the discharge of fuel reactant from the stack. The inboard side flange  226 B′ serves to subdivide manifold  226  into the chambers  246 A and  246 B. The remaining, or top and bottom, manifolds  220  and  224  comprise, respectively, manifold chambers  240  and  270 , and  244  and  274  respectively, with manifold chambers  270  and  274  being laterally outward of manifold chambers  240  and  270  respectively. The inboard side flanges  220 B′ and  224 B′, and the corresponding inboard end flanges  220 C′ and  224 C′ joined therewith, serve to subdivide the corresponding manifolds  220  and  224  into the respective manifold chambers  240 ,  270 , and  244 ,  274 . The manifold chambers  240  and  244  serve to contain and direct the entry and exhaust of oxidant reactant to and from the fuel cell stack  116 , generally as described with respect to the  FIG. 4-6  embodiment. Typically oxidant will enter at manifold  220  and exhaust at manifold  224 , and coolant entry and exhaust will be the reverse of that, but that arrangement may be reversed. The manifold chambers  270  and  274  are added in the present embodiment and comprise the liquid flow channels in which a liquid, such as coolant water or the like, is contained and flows. The lateral, our outward, positioning of those manifold chambers  270  and  274  in the manifolds  220  and  224  of which they are a part, is such that those manifold chambers overlie the regions along which the fuel reactant manifolds  222 ,  226  engage the fuel cell stack  116 , which regions might give rise to possible leakage of fuel reactant. In this way, the manifold chambers  270  and  274  which form the liquid flow channels are capable of receiving, or intercepting, any such leakage of fuel reactant. 
         [0041]    As with the previous embodiment, the  FIG. 7-11  embodiment includes thin gaskets  250  positioned at the end faces of the flanges of the oxidant reactant/coolant manifolds  220  and  224 , and similar thin gaskets  252  are positioned at the end faces of the flanges of the fuel reactant manifolds  222  and  126 , as seen in  FIGS. 9-11 . The gaskets  250 ,  252  are generally shaped to conform to the perimeters of the manifolds  220 ,  222 ,  224 ,  226 , as represented by the end faces of their respective flanges. More specifically, the gaskets  250 ,  252  are generally structured and configured to provide gasket material between the end faces of the various manifold flanges and the surfaces which thy engage to provide the desired seal. Because there are now inboard flanges such as  220 B′ and  224 B′ and  220 C′ and  224 C′ which are spaced from the respective outboard flanges to define the liquid channels  270  and  274 , it is appropriate and desirable that the gasket material generally be absent in/from the space and region between the flanges which define the liquid channels  270  and  274 . Moreover, this spacing between adjacent arms or runs of the gasket material in this area should be great enough, e.g., 0.6 mm, to allow flow of a gas relative to the liquid through which it may flow, without undue resistance by surface tension. In this way, liquid coolant may easily circulate into and out of the fuel cell stack  116  via the liquid channels  270 ,  274 , and any leakage of fuel reactant or even oxidant reactant, beyond the respective seals will readily find its way into the liquid in the liquid channels  270 ,  274 . Thus, the geometry of the gaskets  250  and  252  is selected to provide integral structures where possible, yet to also contain flow passages that facilitate and/or control the flow of fluids in the channels or manifold chambers to which they are adjacent. The gaskets  250  and  252  are formed of a material suitably resilient for sealing purposes and resistant to the fluids in that environment and may be, conveniently, a foam rubber or the like. 
         [0042]    Referring further to  FIGS. 7-11  and particularly to the exploded view of the fuel cell assembly  210  in  FIG. 11 , there is depicted the external manifolds and associated gasketing to provide a disclosed arrangement for minimizing or preventing leakage of reactant, and particularly fuel reactant, to the environment external to the fuel cell assembly. As noted, the various manifolds  220 ,  222 ,  224 , and  226  are formed and configured to provide smooth mating surfaces for good sealing, as well as to provide and position liquid flow channels  270 ,  274  in the manifolds in a manner that facilitates collection of any reactant gas leakage into the contained liquid/coolant and prevents its unwanted release to the environment. Flow arrows in  FIG. 11  are intended to very generally show one possible path of a liquid, such as coolant water, in the region of the fuel cell stack  116  serving to entrain and/or scavenge any reactant gas that might leak from the respective reactant manifold chambers  240 ,  242 ,  244 ,  246 A, and/or  246 B. One possible configuration of the gaskets  250  and  252  is illustrated in greater detail in  FIG. 11 . Although not depicted herein, it will be understood that the liquid in flow channels  270  and  274  is admitted to and exhausted from the fuel cell assembly  210  in a conventional manner and may be directed through an external accumulator/scrubber to remove any entrained reactant gasses in an acceptable known manner. Further, the fuel reactant, the oxidant reactant, and the liquid coolant within the system are each supplied to and removed from the fuel cell stack  116  via the respective manifolds as described above, and their respective pressures are regulated in a known manner (not shown in detail) such that the fuel reactant pressure is relatively the greatest and the coolant pressure is relatively the least, such that any reactant leakage is toward, and into, the coolant. 
         [0043]    Although the disclosure has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made without departing from the spirit and scope of the disclosure.