Abstract:
A fuel cell stack with a bipolar plate assembly and a method of assembling a fuel cell stack such that reactant or coolant leakage is reduced. Bipolar plates within the system include reactant channels and coolant channels that are fluidly coupled to inlet and outlet flowpaths, all of which are formed within a coolant-engaging or reactant-engaging surface of the plate. One or more thin or low aspect-ratio microseals are also formed on a metal bead that is integrally-formed on a surface of the plate and is used to help reduce leakage by maintaining fluid isolation of the reactants and coolant as they flow through their respective channels and flowpaths that are defined between adjacently-placed plates. By delaying the activation of the adhesive bond formed between the microseal and an adjacent surface within the fuel cell until after the aligned cell assemblies have been compressively supported in a stack housing, the ability of the microseal and its adjacent surface to avoid reactant or coolant leakage is enhanced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to an apparatus and method for improved reactant and coolant flow sealing within joined or fluidly-cooperating fluid-delivery plates used in a fuel cell assembly, and more particularly to the use of a microseal disposed on top of a metal bead that is integrally formed on a cooperating surface of one or both of the plates to provide more effective fluid isolation for the reactant or coolant that is conveyed through channels defined within the plate surfaces. 
         [0002]    Fuel cells convert a fuel into usable electricity via electrochemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) for propulsion and related motive applications. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™) in what is commonly referred to as a membrane electrode assembly (MEA). The electrochemical reaction occurs when a first reactant in the form of a gaseous reducing agent (such as hydrogen, H 2 ) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a second reactant in the form of a gaseous oxidizing agent (such as oxygen, O 2 ) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a byproduct. The electrons that were liberated in the ionization of the first reactant proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor, as well as various pumps, valves, compressors or other fluid delivery components) where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells into a larger current-producing assembly. In one such construction, the fuel cells are connected along a common stacking dimension—much like a deck of cards—to form a fuel cell stack. 
         [0003]    In such a stack, adjacent MEAs are separated from one another by a series of reactant flow channels, typically in the form of a gas impermeable bipolar plate (also referred to herein as a flow field plate) that—in addition to promoting the conveyance of reactants, coolant and byproducts—provides structural support for the MEA, as well as electrical current collection or conveyance. In one common form, the channels are of a generally serpentine layout that covers the majority of the opposing generally planar surfaces of each plate. The juxtaposition of the plate and MEA promotes reactant flow to or from the fuel cell, while additional channels (that are fluidly decoupled from the reactant channels) may also be used for coolant delivery. In one configuration, the bipolar plate is itself an assembly formed by securing a pair of thin metal sheets (called half plates) that have the channels stamped or otherwise integrally formed on their surfaces. The various reactant and coolant flowpaths formed by the channels on each side typically convene at a manifold (also referred to herein as a manifold region or manifold area) defined on one or more opposing edges of the plate. Examples of all of these features—as well as a typical construction of such bipolar plate assemblies that may be used in PEM fuel cells—are shown and described in commonly-owned U.S. Pat. Nos. 5,776,624 and 8,679,697 the contents of which are hereby incorporated by reference in their entirety. 
         [0004]    It is important to avoid leakage and related fluid crosstalk within a PEM fuel cell stack. To overcome such leakage, the Assignee of the present invention has applied a relatively thick elastomeric sealant onto discrete portions of the relatively planar surface of the bipolar plate. While useful in establishing the requisite degree of sealing, the thick nature of the sealants makes such an approach unfeasible in actual fuel cell stacks that are made up of more than one hundred bipolar plate and MEA assemblies, as volumetric concerns—especially in the confined spaces of an automobile engine compartment—become paramount. Moreover, the difficulty of ensuring a consistent, repeatable placement of the seal makes this approach cost-prohibitive. 
         [0005]    In an alternate to using thick elastomeric sealants, the Assignee of the present invention has developed integrally-formed bipolar plate sealing where stampings formed in the plate surfaces in a manner generally similar to those used to form the reactant and coolant channels produce gasket-like outward-projecting metal beads to establish discrete contact points between adjacent plate surfaces. These beads (which may be formed to define a cross sectional rectangular, trapezoidal, semi-spherical or other related shape) are more compatible with high-volume production needs than that of the deposition of a thick elastomeric sealant such as mentioned above. In particular, the Assignee applied a thin, relatively soft, compliant sealing layer where in an ideal situation there is no variation in the thickness or structural stiffness along the length of the sealant such that the nominal sealing pressure (which is based on the applied stacking force per sealant length divided by the sealant width) should be substantially uniform. Nevertheless, proper sealing and avoidance of pressure variations along the length of the bead is difficult to achieve, especially in view of the inherent vagaries of fuel cell stack manufacturing where both dimensional tolerances of the formed beads as well as the misalignment of one hundred or more individual cells within the stack are present such that variation of seal effective pressure and concomitant leakage along the length of the bead around one or more regions of the plate is unavoidable. 
         [0006]    An additional difficulty stems from how the sealant is adhered within the bipolar plate assembly. In the previously investigated approach by the Assignee discussed above, the sealant first forms an adhesive bond between itself and the bead substrate. As mentioned above, while conventional thick sealants tend to be relatively insensitive to such bonding, the present inventors have discovered that any attempt to reduce the thickness of the sealant results in significant sealing pressure sensitivity to how the sealant is constrained at the interface between it and the underlying substrate. For example, in the case of a 1.1 mm wide sealant that is relatively thin (i.e., about 0.15 mm high), the locations that lose adhesion or have no adhesion to begin with may exhibit pressures significantly lower than that of the same seal with perfect adhesion. A further difficulty arises out of the fact that the long service life associated with an operating fuel cell stack in a harsh automotive environment often leads to some debonding along the length of the cured sealant. Previous studies conducted by the present inventors have shown that if a spot or section loses adhesion during the fuel cell stack lifetime, that area can lose 75% of the sealing pressure, which can lead to unacceptably high levels of reactant or coolant leakage. 
       SUMMARY OF THE INVENTION 
       [0007]    In view of the above difficulties, the present inventors have discovered a robust way to guard against the loss or non-uniform adhesion of a sealant that is being used within a bipolar plate assembly or fuel cell stack that employs such bipolar plates, as well as a way to use such a sealant in conjunction with integrally-formed metal beads. According to an aspect of the present invention, a method of forming a fuel cell stack is disclosed. The method includes providing a pair of plates that are used in a bipolar plate assembly, where each of the plates includes one or more of a reactant channel, reactant manifold, coolant channel and coolant manifold defined on its surface, as well as an integrally-formed metal bead projecting from the surface. The topmost part of the metal bead defines a generally planar gasket-like engaging portion that is configured to facingly cooperate with one of a compatible surface of a facingly-adjacent plate, MEA or related surface that has a microseal disposed thereon. In situations where the microseal is formed on the metal bead and bonded thereto, the combination is referred to as a metal bead seal (MBS). In one form, the assembly is stacked by placing a first micro seal on the engaging portion of the bead of a first of the plates such that the microseal is cured prior to any stacking or related engagement between adjacent facing surfaces. Because of such curing, the microseal becomes temporarily bonded or tacked—such as through relatively weak van der Waals forces or the like—to the relevant surface (i.e., subgasket, metal bead engaging portion, MEA or a second cured microseal). After this curing, these assemblies are aligned with corresponding MEAs along a stacking dimension and then placed into a compressed state in a housing as a way to put the stack into its final and proper heightwise dimension. Activation of the adhesion that is disposed between the microseal and plate within each assembly (as well as optionally between adjacent assemblies) takes place only after the stack has been substantially assembled and properly aligned and compressively supported in the housing. Such post hoc adhesive activation between the microseals and their adjacent substrate has the effect of permitting the microseals to be more thoroughly and evenly distributed within the final stack. In addition, it helps make the sealing pressure insensitive to any subsequent loss in adhesion that may take place over the operating life of the stack. 
         [0008]    Within the present context, the curing of the bulk microseal material and activation of adhesion at the interfaces should be understood as two separate steps where the curing corresponds to the use of heat or a related agent to facilitate crosslinking within the microseal polymer network as a way to produce the desired structure, while the activation of adhesion is to create a substantially permanent chemical bonding between the microseal material and the substrate (specifically, either or both the metal bead and subgasket) to which it is attached. In one preferred form, the engaging portion of the bead of the second of the pair of plates is in contact with the second microseal so that upon cooperative engagement between the pair of plates, the two MBSs contact to provide substantial fluid isolation of a reactant or coolant that upon stack operation will be conveyed through a respective one of the channels or manifolds. In another preferred form, the microseal may be applied to a subgasket that is used to provide leakage reduction at the periphery of the MEA. 
         [0009]    In the present context, the term “microseal” is meant to distinguish the thin, low aspect ratio (i.e., less than one) seal of the present invention from those that employ thick (i.e., high aspect ratio of equal to or greater than one) constructions. As mentioned above, relatively thick seals are not economically viable with the large-scale production of fuel cell stacks that include a large number of bipolar plates, MEAs and related components, and are deemed to be outside the scope of the present invention. Further in the present context, an effective seal pressure (or effective sealing pressure or effective pressure, in all cases P eff ) differs from that of a conventional sealing pressure in that the former takes into consideration deviation in the stiffness or compliance of a deposited microseal that attend the use of such seals in very thin forms. The present inventors have determined that conventional bulk material properties do not apply under these very thin microseal structures due to geometric confinement. For example, the elastomeric material that makes up the normally compliant seal starts to behave in an stiffening manner when the seal is very thin relative to its width; thus, in situations where the seal is relatively wide relative to its height, spatial constraints on the ability of the seal to compress in response to applied loads start to arise. These spatial constraints are more pronounced when the microseal is adhesively bonded to one or more substrates. These effects in turn tend to cause an effective modulus of elasticity (E eff ) to be significantly higher than that of the bulk property of the elastomeric material that makes up the microseal. Details associated with this increase in E eff  and upon sealing pressure P eff  may be found in an article entitled  The Effect of Compressibility on the Stress Distributions in Thin Elastomeric Blocks  by Yeh-Hung Lai, D. A. Dillard and J. S. Thornton in  The Journal of Applied Mechanics  (1992) the contents of which are incorporated by reference in their entirety. This higher effective modulus manifests itself as requiring a correspondingly higher amount of compressive load to affect the same degree of microseal compressive displacement. Because fuel cell stacks are typically assembled under a sealing load of between 1 and 6 MPa, limiting microseal displacement (such as through the premature activation of an adhesive placed (or otherwise formed) compressively-engaged components) during the stack assembly process is tantamount to inhibiting the ability of the microseal to conform to irregularities in the substrate (i.e., metal bead, subgasket or MEA) surfaces, which in turn undesirably leads to an increased incidence of leaking. It is this type of inhibited movement that the present invention avoids by freeing up the spatial constraints associated with the seal-to-substrate bonding as discussed herein. 
         [0010]    According to another aspect of the present invention, a bipolar plate assembly for a fuel cell system includes a pair of plates each with one or more reactant channels, reactant manifolds, coolant channels and coolant manifolds defined on a surface thereof is disclosed. At least one of the plates defines an integrally-formed metal bead seal that projects from the surface in a manner generally similar to the plate projections that define the reactant or coolant channels. 
         [0011]    According to yet another aspect of the present invention, a fuel cell stack is disclosed. The stack includes numerous individual fuel cells aligned and compressibly contained along a stacking axis within a housing. Each of the cells includes a pair of plates in a facingly-adjacent placement of their surfaces, where each surface defines one or more of a reactant channel, reactant manifold, coolant channel and coolant manifold. The surfaces also include an integrally-formed metal bead that projects therefrom to define an engaging portion thereon. MEAs are disposed between at least some of plate pairs; in such case, each of the reactant channels from such pair of plates is placed in fluid communication with a respective anode or cathode within the MEA. A microseal is placed on the engaging portion of at least a first of the pair of plates such that the microseal is substantially cured at a time prior to alignment but not substantially adhesively bonded until after the numerous cells have been compressively contained within the housing. Depending on the nature of the cell construction, such contact from the microseal can be to any one of an adjacent subgasket, MEA, engaging portion of an adjacent bipolar plate assembly and cured microseal that has been deposited on the engaging portion of the adjacent bipolar plate assembly. 
         [0012]    These and other aspects or embodiments will become apparent to those of ordinary skill in the art from a reading of the following detailed description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which the various components of the drawings are not necessarily illustrated to scale: 
           [0014]      FIG. 1  depicts a schematic exploded view of a fuel cell stack that can be assembled according to an aspect of the present invention; 
           [0015]      FIG. 2  is a simplified illustration of a partially exploded, sectional view of a portion of a fuel cell with surrounding bipolar plates; 
           [0016]      FIG. 3  is a top detailed view of a bipolar plate from  FIG. 2  that includes a metal bead that can accommodate a microseal according to an aspect of the present invention; 
           [0017]      FIG. 4  is graph showing how avoiding the formation of an adhesive bond between the microseal and a bipolar plate assembly substrate prior to individual cell stacking, alignment and compression according to an aspect of the present invention improves leakage prevention; and 
           [0018]      FIG. 5  shows a simplified elevation view of the relative placement of the metal beads, microseals and subgaskets within an adjacently-placed bipolar plate assemblies according to an aspect of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0019]    Referring initially to  FIGS. 1 through 3 , a simplified view of fuel cell stack  1  in exploded form ( FIG. 1 ), a PEM fuel cell  30  ( FIG. 2 ) and a bipolar plate assembly  65  ( FIG. 3 ) are shown. The stack  1  includes a housing  5  made up of a dry end unit plate  10  and a wet end unit plate  15 ; these (as well as others, not shown) may help perform the compressive clamping action of the compression retention system of the housing  5 ; such compression retention system includes numerous bolts (not shown) that extend through the thickness of the stack  1 , as well as various side panels  20  and rigid bracketing elements  25  disposed vertically along the stacking direction (the Y axis) for securing the wet end unit plate  15  to the dry end unit plate  10 . Stacks of numerous fuel cells  30  are securely held in a compressive relationship along the stacking direction by the action of the bolts, bracketing elements  25  and other components within housing  5 . Thus, in the present context, while the stacking axis of the fuel cell  1  may be along a substantially vertical (i.e., Y) Cartesean axis, the majority of the generally planar surfaces of each of the fuel cells  30  resides in the X-Z plane. Regardless, it will be appreciated by those skilled in the art that the particular orientation of the cells  30  within the stack  1  isn&#39;t critical, but rather provides a convenient way to visualize the landscape that is formed on the surfaces of the individual plates that are discussed in more detail below. 
         [0020]    Referring with particularity to  FIGS. 2 and 3 , the fuel cell  30  includes a substantially planar proton exchange membrane  35 , anode catalyst layer  40  in facing contact with one face of the proton exchange membrane  35 , and cathode catalyst layer  45  in facing contact with the other face. Collectively, the proton exchange membrane  35  and catalyst layers  40  and  45  are referred to as the MEA  50 . An anode diffusion layer  55  is arranged in facing contact with the anode catalyst layer  40 , while a cathode diffusion layer  60  is arranged in facing contact with the cathode catalyst layer  45 . Each of diffusion layers  55  and  60  are made with a generally porous construction to facilitate the passage of gaseous reactants to the catalyst layers  40  and  45 . Collectively, anode catalyst layer  40  and cathode catalyst layer  45  are referred to as electrodes, and can be formed as separate distinct layers as shown, or in the alternate (as mentioned above), as embedded at least partially in diffusion layers  55  or  60  respectively, as well as embedded partially in opposite faces of the proton exchange membrane  35 . 
         [0021]    In addition to providing a substantially porous flowpath for reactant gases to reach the appropriate side of the proton exchange membrane  35 , the diffusion layers  55  and  60  provide electrical contact between the electrode catalyst layers  40 ,  45  and a bipolar plate assembly  65  that in turn acts as a current collector. Moreover, by its generally porous nature, the diffusion layers  55  and  60  also form a conduit for removal of product gases generated at the catalyst layers  40 ,  45 . Furthermore, the cathode diffusion layer  60  generates significant quantities of water vapor in the cathode diffusion layer. Such feature is important for helping to keep the proton exchange membrane  35  hydrated. Water permeation in the diffusion layers can be adjusted through the introduction of small quantities of polytetrafluoroethylene (PTFE) or related material. 
         [0022]    Although shown notionally as having a thick-walled structure in  FIG. 2 , the individual plates  65 A and  65 B (also referred to herein as half-plates) that make up the assembly  65  preferably employ thin sheet-like or foil-like structure (as will be shown and described in more detail below in conjunction with  FIG. 3 ); as such,  FIG. 2  should not be used to infer the relative assembly  65  thickness. Simplified opposing surfaces defined by the facingly-adjacent half-plates  65 A and  65 B are provided to separate each MEA  50  and accompanying diffusion layers  55 ,  60  from adjacent MEAs and layers (neither of which are shown) in the stack  1 . One half-plate  65 A engages the anode diffusion layer  55  while a second half-plate  65 B engages the cathode diffusion layer  60 . The two thin, facing metal sheets that make up the half-plates  65 A,  65 B define—upon suitable compression and related joining techniques—the plate assembly  65 . Each half-plate  65 A and  65 B (which upon assembly as a unitary whole would make up the bipolar plate  65 ) defines numerous reactant gas flow channels  70  along a respective plate face. Although bipolar plate  65  is shown (for stylized purposes) defining purely rectangular reactant gas flow channels  70  and surrounding structure, it will be appreciated by those skilled in the art that a more accurate (and preferable) embodiment will employ generally serpentine-shaped channels  70 . The tops of the channels define lands  72  that act as engaging surfaces with complementary-shaped lands  72  of facing plates. 
         [0023]    In operation, a first gaseous reactant, such as H 2 , is delivered to the anode side of the MEA  50  through the channels  70  from half-plate  65 A, while a second gaseous reactant, such as O 2  (typically in the form of air) is delivered to the cathode side of the MEA  50  through the channels  70  from half-plate  65 B. Catalytic reactions occur at the anode  40  and the cathode  45  respectively, producing protons that migrate through the proton exchange membrane  35  and electrons that result in an electric current that may be transmitted through the diffusion layers  55  and  60  and bipolar plate  65  by virtue of contact between it and the layers  55  and  60 . Related channels (not shown) may be used to convey coolant to help control temperatures produced by the fuel cell  1 . In situations where the half-plates  65 A,  65 B are configured for the flow of coolant, their comparable features to their reactant-conveying plate counterparts are of similar construction and will not be discussed in further detail herein. 
         [0024]    Subgaskets  75  (a portion of which is shown in cutaway view) may be disposed in many places within the stack  1  for enhanced sealing. In a preferred form, they are made from a non-conductive and gas impermeable material (such as plastic) that is attached at the perimeter of the MEA  50  to separate the various electronically-conductive layers (such as electrode  40  and gas diffusion layer  55  on the anode side and the electrode  45  and gas diffusion layer  60  on the cathode side). Another key function of the subgasket  75  is to prevent the crossover leak and related mixing of reactants around the edge of MEA  50 . In one form, subgasket  75  defines a generally planar frame-like member that is placed peripherally to protect the edge of the MEA  50 . As such, the subgasket  75  is preferably placed where the elastomeric seal (discussed below) comes into contact with either the MEA  50  or the facing surface of one or more metal beads (also discussed below). This helps reduce overboard leaks of reactant gases and coolant, as well as their inter-mixing at the manifold area  85 . Moreover, subgasket  75 —which is preferably between about 50 μm and 250 μm in thickness—is often used to extend the separation of gases and electrons between the catalyst layers  40  and  45  to the edge of MEA  50  as a way to increase the membrane  35  active surface area. 
         [0025]    Referring with even particularity to  FIG. 3 , an exploded view showing two adjacently-stacked half-plates  65 A,  65 B to form the bipolar plate assembly is shown in more detail. In particular, the individual half-plates  65 A,  65 B each include both an active area  80  and a manifold area  85 , where the former establishes a planar facing relationship with the electrochemically active area that corresponds to the MEA  50  and diffusion layers  55  and  60  and the latter corresponds an edge (as shown) or peripheral (not shown) area where apertures formed through the plates  65 A,  65 B may act as conduit for the delivery and removal of the reactants, coolant or byproducts to the stacked fuel cells  30 . As can be seen from the exploded view of  FIG. 3 , these two half-plates  65 A,  65 B may be used to form a sandwich-like structure with the MEA  50  and anode and cathode diffusion layers  55 ,  60  and then repeated as often as necessary to form the fuel cell stack  1 . In one form, one or both of the anode half-plate  65 A and cathode half-plate  65 B are made from a corrosion-resistant material (such as 304L SS or the like). The generally serpentine gas flow channels  70  form a tortuous path from near one edge  90  that is adjacent one manifold area  85  to near the opposite edge  95  that is adjacent the opposing manifold area  85 . As can be seen, the reactant (in the case of a plate  65 A,  65 B placed in facing relationship with the MEA  50 ) or coolant (in the case of a plate  65 A placed in facing relationship with the back of another plate  65 B where coolant channels are formed) is supplied to channels  70  from a series of repeating gates or grooves that form a header  100  that lies between the active area  80  and the manifold area  85  of one (for example, supply) edge  90 ; a similar configuration is present on the opposite (for example, exhaust) edge  95 . In an alternate embodiment (not shown), the supply and exhaust manifold areas can lie adjacent the same edge (i.e., either  90  or  95 ). In situations where the individual plates  65 A,  65 B are made from a formable material (such as the aforementioned stainless steel) the various surface features (including the grooves, channels, lands or the like) are preferably stamped through well-known techniques, thereby ensuring that both the channels  70 , lands  72  and their respective structure, in addition to the metal beads (which will be discussed in more detail below) are integrally formed out of a single sheet of material. 
         [0026]    Referring next to  FIG. 5 , in one embodiment, a generally planar portion of a metal bead  105  that makes up the respective plates  65 A,  65 B is facially engaged with the subgasket  75 ; this planar engaging portion acts as a gasket to which the subgasket  75  may be joined through a microseal  110 . In another embodiment (not shown), a generally planar portion of a metal bead  105  that makes up the respective plates  65 A,  65 B is facially engaged with the metal bead  105  of the opposing bipolar plates  65 B,  65 A; both such embodiments are deemed to be within the scope of the present invention. In either configuration, the gasket-like structure of the metal bead  105  and the microseal  110  together define the MBS  115 . The gasket-like nature of the metal bead  105  arises out of it being shaped as an upstanding rectangular, trapezoidal (as shown) or slightly curved projection that is formed by stamping from the thin metal material that makes up the respective plates  65 A,  65 B. The metal beads  105  preferably define height of about 300 μm to 600 μm and a width of between about 1 mm and 4 mm. The top surface defines an engaging portion  107  that is generally similar in construction and function to the lands  72  that may also be integrally formed within one or both of the plates  65 A,  65 B. As such, the engaging portion  107  corresponds to the region of the metal bead  105  that is designed to be placed into facing contact with the microseal  110 , subgasket  75 , MEA  50  or adjacent metal bead  105 . Significantly, the microseal  110  functions to (a) fill in the surface imperfections of the metal bead  105  or subgasket  75  in the engaging portion  107 , (b) induce a more uniform seal force per length along the metal bead  105  length by providing a compliant cushion to make up the non-uniform compressed height of the metal bead  105 , (c) prevent fluid (such as reactant) permeation through its bulk and (d) prevent leakage through the interface formed between either (i) the subgasket  75  and microseal  110  or (ii) metal bead  105  and microseal  110 , depending on the precise engagement during stack  1  formation. The elastomeric microseal  110  is shown attached to the engaging portion  107 , although it will be appreciated by those skilled in the art that the microseal  110  may also be formed onto the surfaces of the subgaskets  75  as well as (or instead of) directly on the metal bead  105 ; all such variants are deemed to be within the scope of the present invention, as are variants where the microseal  110  is directly mounted to the plate  65 A,  65 B or other structure. 
         [0027]    In one preferred embodiment, microseal  110  is between about 30 μm and 300 μm in thickness and between about 1 mm and 3 mm in width. With such dimensions, the microseal  110  may become deformed under the high compressive loading that accompanies formation of stack  1 . Spatial confinement and the inherent incompressibility of the material that makes up the microseal  110  can cause stresses in the microseal, particularly at its interface with the corresponding substrates of the engaging portion  107  of the metal bead  105  or the subgasket  75  to which the microseal  110  is adhered. The inventors have discovered that by substantially delaying the formation of the adhesive bond until after the stack  1  is assembled and compressed, significant stress reduction through mitigating boundary constraints and related effects can be realized. This in turn reduces the likelihood of stress-induced premature microseal  110  failure. In an idealized sense where latent adhesion may be employed according to the present invention, there is no adhesive bonding taking place between the microseal  110  and an adjacent substrate prior to stack  1  assembly and compression; even in situations in the present invention where small, relatively inconsequential deviations from the ideal take place, the degree is limited such that the boundary constraints that would otherwise be associated with a robust degree of relatively prompt adhesion are substantially abated. As such, in the present context, descriptions that pertain to the formation of an adhesive bond (such as the phrase “substantially activating adhesion” between the subgasket  75  and an adjacent substrate) will be understood to encompass those situations where a slight amount of adhesive bonding may develop at the interfacial region between the joined surfaces prior to stack  1  assembly and compression, so long as a substantial majority of such bonding is avoided until such assembly and compression activities have been completed. 
         [0028]    In fact, a small residual amount of non-latent adhesion prior to stack  1  assembly may even be beneficial in promoting improved handling of the individual parts prior to assembly and compression. In this way, the residual adhesion that arises out of the microseal  110  bonding is not so great that it acts to set up a permanent face-to-face alignment between adjacent assemblies  65  (or individual components within an assembly  65 ) prior to stack  1  formation, but enough to avoid relative in-plane sliding between adjacent surfaces as a way to facilitate such handling. To that end, the present inventors have determined that it may be desirable to have some weak form of adhesion between microseals  110  and metal beads  105  after the microseal  110  is cured but before the more permanent adhesion step is activated. As such, during the assembly of stack  1 , the interfacial mechanical stress arising from the compression force is expected to break these relatively weak bonds so the microseal  110  can spread along the interfacial region. In one form, relatively weak bonds (such as through van der Waals forces or related interactions) between the microseal  110  and metal bead  105  may be promoted after the microseal  110  is applied and cured as a way to effect this temporary degree of adhesion. In the present context, such weak (or temporary) forms adhesion are to be distinguished from more permanent variants, such as those due to covalent bonds that produce strong chemical bonding. As such, substantially all of the adhesive activation takes place only after all of the fuel cells within the stack  1  have been aligned and compressed together, with the possible exception of the residual adhesion. 
         [0029]    The material used to form the microseal  110  is made from resilient plastic or elastomer (including polyacrylate, alhydrated chlorosulphonated polyethylene, ethylene acrylic, chloroprene, chlorosulphonated polyethylene, ethylene propylene, ethylene vinyl acetate, perfluoroelastomer, fluorocarbon, fluorosilicone, hydrogenated nitrile, polyisoprene, microecllular polyurethane, nitrile rubber, natural rubber, polyurethane, styrene-butadiene rubber, TFE/propylene, silicone, carboxylated nitrile or the like), and is preferably applied by a screen printing process known in the art, although other approaches, such as pad printing, injection molding or other deposition techniques may also be used. As mentioned above, in a preferred form, the layer formed by the microseal  110  has a thickness of between about 30 and 300 μm, while a preferred width across the engaging portion  107  is between about 1 mm and 3 mm. In a more particular form, the material used in the microseal  110  includes least silicone (for example, in the form of a vinyl polydimethylsiloxane, PDMS), a structural reinforcement (such as silica, SiO 2 ), a linkage catalyst (such as a Pt-bearing catalyst for vinyl-SiH linkage) and an adhesion promoter (such as 1,2 Bis(triethoxysilyl)ethane). By using one of these preferred formulations, the microseal  110  exhibits a two-part property the first of which promotes prompt curing and structural setup, while the second delays the formation of the interfacial adhesive bond until after assembly and compression of the fuel cell stack  1 . Details associated with these materials—as well as the use of screen printing to deposit them on a suitable metal bead  105  or subgasket  75  substrate—may be found in concurrently-filed U.S. patent application Ser. No. 15/019,100 (hereinafter the &#39;100 application) entitled SEAL MATERIAL WITH LATENT ADHESIVE PROPERTIES AND A METHOD OF SEALING FUEL CELL COMPONENTS WITH SAME that is owned by the Assignee of the present invention and the contents of which are incorporated herein by reference in their entirety. Additional screen printing features unique to the formation of seals are disclosed in an exemplary form in U.S. Pat. No. 4,919,969 to Walker entitled METHOD OF MANUFACTURING A SEAL, the contents of which are incorporated by reference in their entirety herein. 
         [0030]    Referring next to  FIG. 4 , a graph depicting predicted seal pressure versus displacement is shown as an example to demonstrate the improvement of the present invention over the prior art. As mentioned above, nominal seal pressures in a fuel cell stack are between about 1 to 6 MPa. In both the prior art and present invention cases shown in the chart, the microseal width and thickness values were set to 1.1 mm and 0.15 mm, respectively. In the prior art case, the adhesion between the microseal and the metal bead is activated before the cell is assembled and reaches a 3 MPa nominal sealing pressure at a displacement of 0.046 mm (which corresponds to location A), whereas in the case of the present invention, the adhesion between the microseal  110  and metal bead  105  is activated after the microseal  110  is cured; this too was subjected to a 3 MPa nominal sealing pressure, this time to a displacement of 0.092 mm (which corresponds to location C). As such, the present invention requires more displacement for the same 3 MPa sealing pressure; this in turn beneficially allows the microseal  110  the opportunity to move around during the alignment, stacking and (at least) portions of the compressing process such that it fills in all of the gaps and irregularities between the adjacent metal beads  105  of the joined bipolar plates (such as plates  65 A and  65 B from  FIG. 2 ). Another advantage of the present invention is in its ability to maintain the sealing pressure in the event of adhesion loss (which corresponds to arrow going from location A to location B) during stack  1  lifetime. It is known in a fuel cell stack that the distance between the adjacent bipolar plates is typically maintained as a constant, which would result in a constant displacement being applied to the metal bead  105  and microseal  110  of the present invention. When a conventional sealant loses its adhesion, it will spread along the interfacial region, which in turn results in a decrease of the sealing pressure from 3 MPa to less than 1 MPa, as depicted moving from location A to location B. On the other hand, the sealing pressure of the sample from the present invention does not decrease in the event of adhesion loss since the relatively adhesionless interface that is still present during the alignment and compression steps allows the microseal  110  to spread laterally along the interface during the stack assembly process, thereby settling into its final shape and dimension at the time of stack formation and subsequent adhesive bond formation. Moreover, excessive sealing pressure (which may arise, for example, when trying to achieve the same nominal pressure in prior art case by compressing to a large displacement) has detrimental effects on not just the microseal  110 , but other components within the stack  1  as well, particularly in the formation of high material stresses, creep or the like. 
         [0031]    Although not shown, one particular application for a system based on a stack of PEM fuel cells  1  could be an automobile or related vehicle. Within the present context, it will be appreciated that the term “vehicle” may apply to car, truck, van, sport utility vehicle (SUV) or other such automotive forms such as buses, aircraft, watercraft, spacecraft and motorcycles; all are deemed to be made cooperative with the present invention for the purposes of generating propulsive or motive power. 
         [0032]    It is noted that terms like “preferably”, “generally” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. Likewise, the terms “substantially” and “approximately” and their variants are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement or other representation. As discussed above with particularity to ways to ensure that no more than weak, temporary forms of adhesion are used between adjacent microseals  110  and their associated substrates, the term “substantially” when used to modify the assembly of the fuel cell stack  1  is utilized herein to represent that some of these temporary or residual adhesive means may be used herein without resulting in a change in the basic function of the subject matter at issue; as such, by including some of these weaker, more temporary ways to hold the various stack  1  components in place during assembly do not detract from the fact that the significant (i.e., more permanent) form of adhesion is not used until such time as the various stacked cells have been aligned, pressed together and secured within the stack  1 . 
         [0033]    Having described the invention in detail and by reference to specific embodiments, it will nonetheless be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. In particular it is contemplated that the scope of the present invention is not necessarily limited to stated preferred aspects and exemplified embodiments, but should be governed by the appended claims.