Abstract:
A fuel cell system and a method of assembling a fuel cell system. The system is made up of numerous fluid-conveying plate assemblies stacked such that seals are placed between adjacent plates. Seal stabilizers are placed adjacent at least one of the seals on each plate to reduce the tendency of a moment produced by the compressive force of the stack onto the seals to cause an angular orientation to between adjacent ones of the plates within the stacked assemblies. In one form, the stabilizers produce an interference fit to counteract the moment.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to an apparatus and method for improved reactant and coolant flow sealing within fluid-delivery plates used in a fuel cell assembly, and more particularly to the use of a seal stabilizer to counteract the tendency of misaligned plate stacks to form gaps in such seals. 
         [0002]    Fuel cells convert a fuel into usable energy via electrochemical reaction. A significant benefit to such an approach 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 in the assembly—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, electrically conductive bipolar plates. In one common form, the channels are of a generally serpentine layout, although other forms—including those with generally straight or sinusoidal patterns—may also be used. Regardless of the channel shape, it covers the majority of the opposing generally planar surfaces of each plate. The juxtaposition of the plate and MEA promotes the conveyance of one of the reactants 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, while in another, the assembly includes an additional interstitial sheet with channels to put coolant into thermal communication with the adjacent anode and cathode channels of the outer sheets. Regardless of whether the assembly is two-sheet or three-sheet variety, the various reactant and coolant flowpaths formed by the channels on each of these sheets 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. 
         [0004]    It is important to avoid leakage and related disparate fluid crosstalk within a PEM fuel cell stack. In particular, the cross-sectional area of a flow channels formed on the surface of the plates is significantly smaller than that of the fluidly-coupled manifold. As such, the reactants or coolant entering the flow channels from the corresponding manifold experience a significant pressure rise. To mitigate against leakage of such high pressure fluids, seals (typically in the form of an elastomeric seal bead, gasket or the like) may be placed between adjacent bipolar plates. In one form, the plates may include grooves or related channels in the form of sub-gaskets that are formed within the surface. In such construction, these grooves that can accept a seal therein are generally similar to that of the previously-mentioned flowpath channels, and are especially prevalent around the various apertures that are formed within the headers or manifolds, as well as around the plate center that corresponds to the active area defined by the MEA. In other configurations, the grooves are not required. In such construction, variations in the dimension of the seals would have additional flexibility. Regardless of whether the seals are configured to cooperate with grooved or non-grooved surfaces, by virtue of their constituent materials (for example, resilient elastomer-based compounds), the gasket and seals may also provide electrical insulation between the anode and cathode sides of the plates. 
         [0005]    Ideally, these seals form good fluid-tight connections between adjacent plates once compression and subsequent stack assembly has been completed. In practice, even slight plate misalignment leads to out-of-plane rotation between adjacent plates that leads to variations in pressure applied to the corresponding seals, resulting in gaps being formed in the seals, while more severe misalignment may result in seal failure (such as through buckling or the like). It would be desirable to reduce seal misalignment and resulting plate rotation that harms the ability of the seals to perform their fluid-containment functions. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the disclosure to provide a seal stabilizer that will mitigate the impact of seal-to-seal misalignment that may be due to plate-to-plate misalignment in configurations where the seal is attached to the plate, MEA-to-MEA misalignment in configurations where the seal is attached to an MEA-based sub-gasket, as well as simply seal-to-seal misalignment in configurations where the seal is not attached to either a plate or the MEA. The present inventor observed that control of seal misalignment and free rotation between plates may help to achieve such seal stabilization. For example, buckling of the seal is more likely to happen when the seal-to-seal misalignment along the stacking axis is greater, as this creates plate tilting (i.e., rotation) near its unsupported/cantilevered edges. In one form, this can take place around an outer edge, while in another, around the headers or along the long edge around the cell active area. This in turn tends to form an angle for the seal compressive force that then leads to a horizontal (i.e., in-plane) force that has a tendency to push the seal out of its intended placement, especially when the friction or adhesion between the seal and groove is low. The present inventor has further determined that this creates an eccentricity that can lead to an incompressible deformation and subsequent buckling of the seal. 
         [0007]    According to one aspect of the present invention, a fuel cell system includes a stack made up of numerous cells each of which includes an MEA cooperative with a plate that may be part of a bipolar plate or related multi-plate assembly. The plate defines one or more fluid channels formed on its surface for the flow of coolant or reactant thereacross. One or more seals are placed on surfaces of the plate along the plate stacking axis such that the repeating seal arrangement is generally aligned, while a seal stabilizer is included to meliorate the effect of sealing offsets due to any misalignment between the stacked assemblies. The seal stabilizer is placed near to the seal in order to resist any tendency by a moment couple (also called force couple) produced by the compressive force of the stack onto the seals to cause an angular (i.e., non-parallel) orientation to between adjacent ones of the plates within the stacked configuration. In one preferred embodiment, the seal is placed substantially coplanar with the stabilizer, while in another preferred embodiment, the seal and the stabilizer are formed on the same plate surface. In yet another preferred embodiment, a sub-gasket may form or be cooperative with the MEA such that the stabilizer is attached to it to achieve the aforementioned reduction or elimination of MEA-to-MEA misalignment. 
         [0008]    According to another aspect of the present invention, a method of assembling a fuel cell system includes placing numerous fuel cells on top of one another in a stacked configuration and placing a seal stabilizer in cooperative engagement with the seal such that the stabilizer inhibits misalignment-induced rotation between adjacent plates within the stack. In particular, the plates that provide structural support and fluid delivery to a respective MEA have a tendency upon stacking misalignment to impart in-plane forces as well as out-of-plane rotational forces to adjacent bipolar plates; the inclusion of the stabilizers in substantially coplanar locations that are adjacent the seals—while not preventing the misalignment itself—at least mitigates the effects of such misalignment by providing enough of an interference fit to resist such in-plane and rotational tendencies. 
         [0009]    According to yet another aspect of the present invention, a method of preventing seal buckling within an assembled fuel cell system includes placing numerous fuel cells on top of one another in a stacked configuration and placing a seal stabilizer in cooperative engagement with the seal such that the misalignment-based rotation between adjacent ones of the bipolar plates within the stacked configuration is reduced at least along an axial dimension defined by the seal to an extent that avoids a fluid-liberating deformation therein. In the present context, buckling is understood to be a phenomena that manifests itself predominantly along the elongate dimension of a structure. With particular regard to the seals used between bipolar plates, the aspect ratio of the seal is such that it is far longer (often by orders of magnitude) along its axial dimension than they along the radial or lateral dimension. As such, the occurrence of buckling is likely to manifest itself as a bend, hump or related undulation being formed along at least a portion of the length of the seal such that a gap that in turn permits the relatively unimpeded leakage of the fluid that the seal was designed to contain. 
         [0010]    These and other objects, features, embodiments, and advantages 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 
         [0011]    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: 
           [0012]      FIG. 1  is a illustration of a partially exploded, sectional view of a portion of a simplified fuel cell with surrounding bipolar plates; 
           [0013]      FIG. 2  is a simplified isometric, exploded view of a bipolar plate assembly that can be used in accordance with the present invention; 
           [0014]      FIGS. 3A and 3B  show simplified edge-on and perspective views respectively of an ideal alignment between adjacent seals; 
           [0015]      FIGS. 4A and 4B  show the effects of slight misalignments within a planar dimension of the seals of  FIGS. 3A and 3B ; 
           [0016]      FIGS. 5A and 5B  show the placement of stabilizers to meliorate the impact of misalignment on the seals of  FIGS. 4A and 4B ; and 
           [0017]      FIG. 6  shows how a misaligned seal can buckle without the stabilizers of  FIGS. 5A and 5B . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0018]    Referring initially to  FIG. 1 , a simplified view of a PEM fuel cell  1  in exploded form is shown. The fuel cell  1  includes a substantially planar proton exchange membrane  10 , anode catalyst layer  20  in facing contact with one face of the proton exchange membrane  10 , and cathode catalyst layer  30  in facing contact with the other face. Collectively, the proton exchange membrane  10  and catalyst layers  20  and  30  are referred to as the MEA  40 . An anode diffusion layer  50  is arranged in facing contact with the anode catalyst layer  20 , while a cathode diffusion layer  60  is arranged in facing contact with the cathode catalyst layer  30 . Each of diffusion layers  50  and  60  are made with a generally porous construction to facilitate the passage of gaseous reactants to the catalyst layers  20  and  30 . Collectively, anode catalyst layer  20  and cathode catalyst layer  30  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  50  or  60  respectively, as well as embedded partially in opposite faces of the proton exchange membrane  10 . As mentioned above, sub-gaskets may be used to promote seal attachment or related cooperation with the plate (discussed in more detail below); in one form, sub-gasket  45  may be in the form of a plastic frame and plaed peripherally to protect the edge of the MEA  40 . This sub-gasket  45  is often used to extend the separation of gases and electrons between the catalyst layers  20  and  30  to the edge of MEA  40 , and is often placed where the elastomeric seal (discussed below) comes into contact with the MEA  40 . This helps reduce overboard leaks of reactant gases and coolant, as well as their inter-mixing at the manifold region. In some cases, the elastomeric seal can be attached or directly formed onto the sub-gasket  45  as part or extension of the MEA  40 ; either variant is deemed to be within the scope of the present invention, as are variants where the seal is directly mounted to the plate or other structure. 
         [0019]    In addition to providing a substantially porous flowpath for reactant gases to reach the appropriate side of the proton exchange membrane  10 , the diffusion layers  50  and  60  provide electrical contact between the electrode catalyst layers  20 ,  30  and a bipolar plate  70  that in turn acts as a current collector. Moreover, by its generally porous nature, the diffusion layers  50  and  60  also form a conduit for removal of product gases generated at the catalyst layers  20 ,  30 . 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  10  hydrated. Water permeation in the diffusion layers can be adjusted through the introduction of small quantities of polytetrafluoroethylene (PTFE) or related material. 
         [0020]    Although shown notionally as having a thick-walled structure, bipolar plates  70  preferably employ a thin-walled structure (as will be shown and described in more detail below); as such,  FIG. 1  should not be used to infer the relative thickness between the channels  72  and the plate structure that gives definition to such channels. Simplified opposing surfaces  70 A and  70 B of a pair of bipolar plates  70  are provided to separate each MEA  40  and accompanying diffusion layers  50 ,  60  from adjacent MEAs and layers (neither of which are shown) in a stack that may be subsequently compressed along the stacking axis and placed into a housing or related enclosure. One plate  70 A engages the anode diffusion layer  50  while a second plate  70 B engages the cathode diffusion layer  60 . Each plate  70 A and  70 B (which upon assembly as a unitary whole would make up the bipolar plate  70 ) defines numerous reactant gas flow channels  72  along a respective plate face. Three-dimensional (i.e., out-of-plane) structure  74  is made up of walls  74 A and lands  74 B that separate adjacent sections of the reactant gas flow channels  72  by projecting toward and making direct contact with the respective diffusion layers  50 ,  60 . Although bipolar plate  70  is shown (for stylized purposes) defining purely rectangular reactant gas flow channels  72  and structure  74 , it will be appreciated by those skilled in the art that a more accurate (and preferable) embodiment will be shown below, where generally serpentine-shaped channels  72  (along with their respective generally planar apexes that correspond to the lands  74 B) are formed. As mentioned above, the flow channels  72  need not be serpentine, and as such may embody other shapes, including generally straight or sinusoidal-like profiles. In another form (not shown) the sub-gaskets  45  discussed above may be formed as a groove or related indentation directly into the surface of plate  70  to accept the placement of a seal or seal bead (as discussed below) therein; such portion of the plate  70  that includes such a sub-gasket is known as a seal bead region. 
         [0021]    In operation, a first gaseous reactant, such as H 2 , is delivered to the anode  20  side of the MEA  40  through the channels  72  from plate  70 A, while a second gaseous reactant, such as O 2  (typically in the form of air) is delivered to the cathode  30  side of the MEA  40  through the channels  72  from plate  70 B. Catalytic reactions occur at the anode  20  and the cathode  30  respectively, producing protons that migrate through the proton exchange membrane  10  and electrons that result in an electric current that may be transmitted through the diffusion layers  50  and  60  and bipolar plate  70  by virtue of contact between the lands  74 B and the layers  50  and  60 . 
         [0022]    Referring next to  FIG. 2 , the bipolar plate  70  of  FIG. 1  is shown in more detail. In particular, the plate  70  includes both an active area  70   ACT  and a header area  70   H , where the former establishes a planar facing relationship with the electrochemically active area that corresponds to the MEA  40  and diffusion layers  50  and  60  and the latter corresponds an edge (as shown) or peripheral (not shown) area where apertures formed through the plate  70  may act as conduit for the delivery and removal of the reactants, coolant or byproducts to the stacked fuel cells. As shown in  FIG. 1 , these two plates  70   A ,  70   B  may be used to form a sandwich-like structure with the MEA  40  and anode and cathode diffusion layers  50 ,  60  and then repeated as often as necessary to form a fuel cell stack (not shown). In one form, one or both of the anode plate  70   A  and cathode plate  70   B  are made from a corrosion-resistant material (such as  304  SS or the like). The generally serpentine gas flow channels  72  form a tortuous path from near one edge E 1  that is adjacent one header area  70   H  of the bipolar plate  70  to near the opposite edge E 2  that is adjacent the opposing header area  70   H . As can be seen in  FIG. 2 , the reactant is supplied to channels  72  from a series of repeating gates or grooves that form a port area  70   P  that lies between the active area  70   ACT  and the header area  70   H  of one (for example, supply) edge E 1 ; a similar configuration is present on the opposite (for example, exhaust) edge E 2 . In an alternate embodiment (not shown), the supply and exhaust manifold areas can lie adjacent the same edge (i.e., either E 1  or E 2 ) of the bipolar plate  70 . In situations where the bipolar plate  70  is made from a formable material (such as the aforementioned stainless steel) the various surface features (including the grooves, channels or the like) may be stamped or otherwise formed through well-known techniques. 
         [0023]    Seals  80  (which are preferably made from a resilient plastic or elastomers (including polyacrylate, alhydrated chlorosulphonated polyethylene, ethylene acrylic, chloroprene, chlorosulphonated polyethylene, ethylene propylene, ethylene vinyl acetate, perfluoroelastomer, flourocarbon, flourosilicone, hydrogenated nitrile, polyisoprene, microecllular polyurethane, nitrile rubber, natural rubber, polyurethane, styrene-butadiene rubber, TFE/propylene, silicone, carboxylated nitrile or the like) are seated on portions of the plates  70  that are adjacent fluid conduit (such as the apertures in the header area  70   H , as well as around the active area  70   ACT ). In the present context, the stacking dimension of the fuel cell  1  may be along a substantially vertical (i.e., Y) Cartesean axis so that the majority of the surface of each of the bipolar plates  70  is in the X-Z plane that is substantially orthogonal to the stacking axis. Regardless, it will be appreciated by those skilled in the art that the particular orientation of the cells  1 , plates  70  and stack isn&#39;t critical, but rather provides a convenient way to visualize the placement of the seals  80  and seal stabilizers  90  of the present invention (both shown and described in more detail below), as well as situations where the seals  80  and their respective bipolar plates  70  are or are not ideally aligned with one another. 
         [0024]    As mentioned above, one or more groove-like sub-gaskets may be formed into the surface of the bipolar plate  70  to define a seal bead region (not shown). As with the grooves, channels and other features mentioned above, the seal bead regions may be formed by stamping or other forming operations (in configurations where the seal bead region is formed as a groove in the plate surface). In one preferred form, the seal bead regions may define a trough-like shape with a semicircular cross-sectional profile to accommodate a comparably-sized seal  80 . The seal bead regions allow placement of a seal  80  therein, where the seal  80  is preferably made from an elastic, compliant material to promote deformability upon compressing two adjacent plates  70  together. As shown seals  80  may be placed around some or all of the active area  70   ACT , header area  70   H  or both to promote fluid-tightness in the regions adjacent the reactant, coolant or byproduct conduit within the stack. As will be shown next, 
         [0025]    Referring next to  FIGS. 3A and 3B  in conjunction with the detail region highlighted in  FIG. 2 , an edge-on view ( FIG. 3A ) and a perspective view ( FIG. 3B ) of two plates  70  and their seals  80  are shown in an ideal stacked configuration. In the present context, the stacking between adjacent plates  70  and their seals  80  is deemed to be “ideal” when there is no misalignment between them when stacked in their as-designed orientation. Details associated with an actual bipolar plate  70  (including channels, grooves or related surface undulations) that are shown in  FIG. 2  have been omitted from the remaining figures for simplicity. Similarly, a single T-joint (also called a T-shaped joined region) within the highlighted region is shown in  FIG. 3B  rather than the entire portion of the sealed relationship. 
         [0026]    Referring next to  FIGS. 4A, 4B and 6 , the effect of a misalignment of 1 mm in each of the X and Z axes of the seals  80  is shown. In particular, the misalignment M X , M Z  shows how the ordinarily parallel spacing of the adjacent plates  70  becomes angled by an amount in response to the compressive forces from the aligning and stacking process that impart moments via pivot points P formed by partially offset contact between adjacently-compressed seals  80 . The rotated plates  70  form the opening angle θ for the seal  80  compressive forces; this in turn creates a horizontal force component (i.e., along the X axis) to push the seal  80  out of place. The resulting in-plane (i.e., X-Z plane as shown) movement of the seal  80  creates compression within it which in turn imparts an axial force along the length direction of the seal  80  due to relative incompressibility of the seal  80 ,; such compression and resulting forces are present in configurations where the seal is situated on the surface of plate  70 , as well as in configurations where the seal  80  is seated within a trough-like groove (not shown) that is formed into the surface of the plate  70  in a manner generally similar to the reactant or coolant channels  72  that were discussed above in conjunction with  FIG. 1 . This incompressibility can be correlated to the eccentricity condition in an Euler beam buckling problem, where when the axial force in the seal  80  reaches a critical value in conjunction with the eccentricity condition mentioned above, seal  80  instability occurs. As shown with particularity in  FIG. 6 , this instability leads to a popping out of the seal  80  from its intended location at a buckling point  80 B, which in turn results in breaches in the seal  80  perimeter and concomitant overboard leakage of the fluid through such breach. Without wishing to be bound by theory, the inventor belies that the seal  80  buckling problem at buckling point  80 B is a function of many parameters, including the overall load, compressive strain, the cross-sectional profile, the nature of the intersection (i.e., T-shaped junctures, regions or the like), length, material/surface properties, adhesion/friction, hyperelasticity, viscoelasticity or the like; some of these factors are discussed in more detail herein. 
         [0027]    Referring next to  FIGS. 5A and 5B , edge-on ( FIG. 5A ) and perspective ( FIG. 5B ) views show how the placement of seal stabilizer  90  helps to reduce the misalignment-induced force couple of three adjacently-stacked bipolar plates  70 . Significantly, the stabilizer  90 —by removing at least a portion of the freedom of rotation of adjacent plates  70 —reduces the likelihood that the seal  80  will experience the Euler beam buckling problem. In general, buckling is more likely to occur when the stack compression is higher, the friction/adhesion between the seal  80  and the plate  70  is lower, misalignment is larger (which as mentioned above allows the plate  70  to tilt to form a larger opening angle θ). In one preferred form, the height of the seal stabilizer  90  is between 50% and 150% of the height of the seal  80 . 
         [0028]    With this cooperative placement between the seals  80  and stabilizers  90 , a countering force is set up to reduce the tendency of the plates  70  to rotate. In one form, seal stabilizers  90  may not need to be placed along the entire length or periphery of the plate  70 ; as such, it may be possible to judiciously place the seal stabilizers  90  (whether in bead or strip form) in locations where plate  70  rotation is expected to be particularly large. In addition to the discussion above where the seal stabilizer  90  can be made of elastomers, plastics or the like, they may also be made from metals that are attached to or formed as part of the bipolar plates  70 . In a similar manner (not shown), the seal stabilizers  90  may be attached to the sub-gaskets that themselves can be attached to or even formed as part of the bipolar plates  70 . Moreover, the seal stabilizers  90  can take various forms such as beads and strips. 
         [0029]    Changes in the coefficients of friction and thermal expansion of the seal  80  material may have an impact on overall seal  80  stability. For example, higher friction is better to maintain the seal  80  in its place. In one form, the material for seal  80  is Henkel  651 , while that of the plate  70  material is 304 stainless steel. To determine the suitability of the seal 80 material, assumed coefficients were varied between 0.01, 0.05, 0.75, 0.1 and 0.2. Compression values were set at 20%, 30% (F91), 40% (MRC107), and 50%. Likewise, the tensile strength of the material used in the seals  80  should be high enough to further delay the onset of Euler beam buckling. Moreover, the shape of the seals  80  is important as well, with the inventor discovering that seals  80  that define a circular or semicircular cross-sectional profile tend to perform better than those with rectangular or trapezoidal profiles. 
         [0030]    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. 
         [0031]    For the purposes of describing and defining the present invention, it is noted that 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. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0032]    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.