Abstract:
A system and method for aligning and reducing the relative movement between adjacent fuel cells within a fuel cell stack. The inter-cell cooperation between fuel cells along a stacking dimension is enhanced by one or more datum placed along the edge of a bipolar plate that makes up a part of a cell-containing assembly. The datum is shaped along a thickness dimension that substantially coincides with the cell stacking dimension to promote a nested fit with a comparable datum on an adjacently-stacked bipolar plate. This nesting facilitates an interference fit that enhances the resistance to sliding movement between respective cells that may otherwise arise out of the occurrence of a significant acceleration along the dimension that defines the major surfaces of the plates, cells and their respective assemblies. In one form, the use of welding, bonding or related attachment of the datum to the plate promotes enhanced metallic support without the need for increasing the plate footprint and without having to overmold the datum directly onto the plate.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present disclosure relates generally to an improved design for assembling a fuel cell stack, as well as to distribute an acceleration load over a fuel cell stack to secure and maintain the relative position of the fuel cells within the stack after exposure to impacts and other high acceleration loads. 
         [0002]    A significant benefit to using fuel cells to convert a fuel into usable electricity via electrochemical reaction 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 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 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 hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load (such as an electric motor) 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 in series along a common stacking dimension—much like a deck of cards—to form a fuel cell stack. 
         [0003]    The delivery of the reactants to the MEA—as well as the removal of the byproduct water and the delivery of the cell-generated electrical current to the load—is facilitated through a gas-permeable diffusion medium (also called a gas diffusion medium (GDM)) and a bipolar plate the latter of which is sized to be placed about the periphery of a corresponding fuel cell. One way to improve alignment of each plate and cell assembly during stack formation is to include one or more datum structures (also called datum) that are built in, secured to or otherwise cooperative with one or more of the bipolar plates. In one common form, the datum may accept a datum pin to promote such alignment in the stacking (for example, Y-axis in a Cartesian coordinate system) dimension. 
         [0004]    Fuel cell stacks placed within vehicles must be able to withstand severe load changes from acceleration and deceleration of the vehicle, as well as from crashes, accidents and related impacts. In particular, in order to continue to perform after exposure to high acceleration loads (for example, up to 160 g or more) during disruptive events such as a vehicle crash, the position of the fuel cells that make up the stack must be retained relative to one another. In the event of high acceleration, deceleration, or impact of the vehicle, a high shearing force may cause sliding between adjacent cells of the stack (especially in the X-Z plane of the aforementioned Cartesian coordinate system). Small displacements between cells can result in large cell block displacements when all cells are considered (e.g. a 100 micron cell shift can result in a 30 mm cell block shift for a 300 cell stack assembly). Such problems may be exacerbated by cold start conditions where thermally-induced contraction may reduce the Y-axis compressive retention load that was placed on the cells during stack assembly, as well as by reduced inter-cell friction brought about by the use of surface treatments or inserts that may have low coefficient of friction attributes. The use of adhesives or supplemental support structure may tend to meliorate this problem to some degree, but in the process tends to add weight and complexity and—in the case of adhesives—is not conducive to subsequent stack disassembly for repair or diagnostic analysis. 
       SUMMARY OF THE INVENTION 
       [0005]    According to one aspect of the present disclosure, a fuel cell stack is disclosed. The stack includes numerous fuel cells arranged in an adjacently facing relationship along a stacking dimension (for example, the Y-axis in a conventional Cartesian coordinate system), and a respective bipolar plate for each of the cells. At least one datum is disposed adjacent an edge of each bipolar plate, and is shaped to define a nesting relationship with another datum on an adjacently-stacked plate to provide an increased resistance to inter-cell misalignment or movement (i.e., that movement which is in a plane that is substantially orthogonal to the stacking dimension) along a dimension that is substantially orthogonal to the stacking dimension. 
         [0006]    According to another aspect of the present disclosure, a method of assembling a fuel cell stack that has an improved resistance to inter-cell shifting in response to a disruptive event is disclosed. The method includes arranging fuel cells along a stacking dimension and securing the arranged cells within a housing such that one or more datum placed along the edge of each of the bipolar plates that form part of a respective fuel cell assembly is shaped to provide the increased resistance. The datum is possessive of an enhanced thickness dimension along the cell stacking dimension and is configured to cooperate with an alignable datum on an adjacently-stacked bipolar plate such that resistance to sliding movement between adjacent cells increases. This cooperation among stacked data minimizes or removes the inter-cell movement that may arise out of the occurrence of a significant acceleration along the dimension that defines the major surfaces of the plates, cells and their respective assemblies. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    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: 
           [0008]      FIG. 1  is a simplified exploded view of a fuel cell stack; 
           [0009]      FIG. 2  is a perspective cutaway view of a vehicle with the fuel cell stack of  FIG. 1 ; 
           [0010]      FIG. 3  is a simplified exploded view of a bipolar plate that is used in the fuel cell stack of  FIG. 1 ; 
           [0011]      FIG. 4  is a detail of the bipolar plate of  FIG. 3  showing a datum affixable to an edge thereof according to an aspect of the present invention; 
           [0012]      FIG. 5  is an elevation cutaway view of several bipolar plates in a registered stacking arrangement according to an aspect of the present invention; 
           [0013]      FIG. 6  is a perspective view of the datum of  FIG. 4  with an optional datum pin extending therethrough for supplemental inter-cell sliding resistance; 
           [0014]      FIG. 7  shows the use of a continuous metal strip from which multiple data can be overmolded onto a metal tab; 
           [0015]      FIG. 8  shows the use of an extruded housing shaped to cooperate with the datum of  FIG. 4  in order to provide another form of supplemental inter-cell sliding resistance; and 
           [0016]      FIG. 9  shows the use of a cast housing with supplemental load brackets shaped to cooperate with the datum of  FIG. 4  in order to provide another form of supplemental inter-cell sliding resistance. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring first to  FIG. 1 , a fuel cell stack  1  is shown, and includes a dry end unit plate  5 , a wet end unit plate  10  and a block of fuel cells  15  placed in stacking alignment between the end unit plates  5 ,  10 . Although not shown in detail, each fuel cell  15  within the block generally includes the anode, cathode, and membrane arranged together to form the generally planar sandwich structure of the MEA that is pressed between a pair of the gas-permeable electrically conductive diffusion media that serve to both deliver reactants (i.e., H 2  on the anode side of the MEA and O 2  (typically in the form of air) on the cathode side of the MEA as well as collect electrical current that is catalytically produced at the anode and cathode. Within the present context, the stacking dimension that corresponds to the height of the assembled stack  1  is defined along the Y-axis as shown, although it will be appreciated by those skilled in the art that such is a matter of convenience, and that any suitable linear dimension is equally suitable, depending on the orientation of the stack  1  during the assembly process. 
         [0018]    The numerous individual cells  15  within stack  1  are kept in place via housing  20  made up of a pair of opposing end caps  25 ,  30 , sidewalls  35 ,  40  and rigid bracketing elements  45  disposed vertically along each of the sidewalls  35 ,  40  for securing the wet end unit plate  10  to the dry end unit plate  5 . In one form, the wet end unit plate  10  is fixedly secured to the opposing end caps  25 ,  30  while the dry end unit plate  5  is adjustably secured. This latter connection is particularly useful in stack forming situations, as the end-point secured position is not known until final assembly; this in turn allows the securing to be adapted to accommodate a range of possible end-point securing positions. By contrast, the fixed securing of the wet end plate  10  means that the positional relationship of the secured components to one another is substantially invariant. 
         [0019]    Referring next to  FIG. 2 , vehicle  100  (for example, a car, van, bus, truck, or motorcycle) includes a fuel-cell based propulsion system  110  made up of an electric motor  120  that receives its electric power from the fuel cell stack  1  of  FIG. 1  that includes numerous individual fuel cells  15 . The propulsion system  110  may include one or more fuel storage gas vessels  130 ,  140 , as well as power converters or related electronics  150 , electrical storage devices (e.g., batteries, ultra-capacitors or the like)  160  and controllers that provide operational management, and any number of valves, compressors, tubing, temperature regulators, and other ancillary equipment. 
         [0020]    Any number of different types of fuel cells  15  may be used to make up the stack  1  of the propulsion system  110 ; these cells  15  may be of the metal hydride, alkaline, electrogalvanic, or other variants. In one preferred (although not necessary) form, the cells  15  are PEM fuel cells as discussed above, and it is with this configuration that the remainder of the present disclosure is based. In one preferred form, the cells  15  within stack  1  are combined in series, parallel or a combination of the two in order to produce a higher voltage or current yield, depending on the needs of vehicle  100 . It will be understood that stack  1  may be used for purposes other than motor vehicles. 
         [0021]    Referring next to  FIG. 3 , an exploded perspective view of a bipolar plate assembly  200  associated with the fuel cell  15  in accordance with one embodiment of the present invention is shown. As with the MEAs to which they are attached, each bipolar plate of the assembly  200  defines a generally planar face portion  210  and a generally rectangular edge portion  220  that forms a periphery around the face portion  210 . Disposed on the face portion  210  is a flow field characterized by numerous serpentine flow channels  230  through which the reactant gases flow from a supply manifold  240  formed adjacent one side  220 A of the edge portion  220  to an exhaust manifold  250  formed adjacent the opposite edge  220 C. In an alternate configuration (not shown), the supply and exhaust manifolds  240 ,  250  can be situated side-by-side adjacent the same edge of the plate assembly  200 . In one form, the bipolar plate assembly  200  may be made of separate sheets  200 A,  200 B (typically between about 75 and 100 microns along the thickness (i.e., stacking) dimension) that can be joined together through brazing, laser welding or related operation; in such configuration, the flow channels  230  formed with each face portion  210  may define generally mirror images of one another such that upon stacking of two adjacent cells  15 , the face portions  210  of the two are in a back-to-back arrangement. In addition to these sheets that include the flow channels  230  for the reactant gas flow field, similarly-shaped sheets (not shown) may be stackingly interspersed to provide a separate coolant-flow circuit; such coolant flow circuits may define any suitable flow channel shape (such as the serpentine shape depicted in flow channels  230 . As with the sheets  200 A,  200 B, any such additional sheet coolant-flow circuit may also be joined through brazing or some other suitable joining technique. In the present context, the bipolar plates of the plate assembly  200  may or may not have the serpentine flow channels  230  formed on both opposing planar faces; for example, when such plate forms the last plate in a stack  1 , it would not be necessary to have channels  230  formed in the side that faces the end unit plates (such as end unit plates  5 ,  10  of  FIG. 1 ). 
         [0022]    Referring next to  FIGS. 4 through 7 , details associated with the placement of a datum  300  along a side  220 D of edge portion  220  that is formed by one or more sheets  200 A,  200 B of bipolar plate assembly  200  are shown. As shown, the datum  300  defines a significant out-of-plane thickness along the Y-axis dimension of  FIG. 1 . In the particular form shown, datum  300  defines a conical feature  310  that by being substantially hollow within the region defined by the cone allows for nesting of two or more data  300  upon stacking of adjacently-facing cells  15  with bipolar plate assemblies  200 . In the present context, like components (for example, data) of two adjacently-stacked objects (for example, cells, bipolar plates or the like) are deemed to be nested when their geometric features are such that a substantial portion of one may be placed within a volumetric portion of the other; an example of such nesting can be visualized through the way that disposable drinking cups are stacked in a columnar container. The nesting between adjacent data  300  helps promote plate-to-plate locating; as such, the conical features are particularly well-adapted to keeping the stacked cells  15  aligned without having to rely upon a separate datum pin. In addition to being useful to align the cells  15  in the stack  1  during compression, sealing and other assembly operations associated with the stack  1 , the out-of-plane (i.e., Y-axis) projection of the datum  300  offer improved inter-cell sliding resistance; such resistance is an important contributor to maintaining the stack  1  integrity during the aforementioned events that give rise to high acceleration loads. In a preferred embodiment, the out-of-plane thickness defined by each conical-shaped datum  300  is sufficient to ensure inter-cell sliding resistance up to three cells  15  above and three cells  15  below the cell  15  in question; this is shown with particularity in  FIG. 5 . Another important feature of the shaped datum  300  is that no permanent connection (such as that which is attendant to the use of epoxy, glue or the like) is formed between adjacently-stacked cells  15 ; this allows for serviceability of each of the individual cells  15  within stack  1 . 
         [0023]    Importantly, datum  300  can be used to alleviate concerns over recent design increases in inter-cell sliding resistance (for example, being able to withstand up to about 160 g loads whereas previous designs only required about 40 g) as a way to provide out-of-plane support. In one particular form, datum  300  may be made from a rigid, load-bearing plastic that may be overmolded onto a thin, generally rectangular portion of a metal tab  320 . In this way, full metal reinforcement of the datum  300  may be achieved with no increase in footprint of the bipolar plate assembly  200 . Although the figures presently only show one datum  300  per bipolar plate, it will be understood that multiple (for example, secondary, tertiary or the like) data  300  may be coupled to any given plate; not only will this help promote better alignment during stack  1  assembly, but it may also provide additional shearing movement resistance between adjacent cell  15  layers within the stack  1  in situations where higher levels of the sliding resistance may be required. In a preferred embodiment, datum pins  330  (discussed in more detail below) may be removed after cell  15  stacking is complete and a load is applied to the cell block that makes up stack  1 . 
         [0024]    Referring with particularity to  FIG. 7 , to facilitate repeatable, rapid formation, such numerous data  300  may be overmolded into a continuous linear metal strip  400  such that each datum  300  can be subsequently removed individually along its tab (such as tab  320  discussed above) for attachment (such as by welding) onto one of sides  220 A,  220 B,  220 C or  220 D of edge portion  220  of plate assembly  200 . In this separate metal strip-based approach, overmolding each bipolar plate is avoided. In a preferred form, the conical portion of the datum  300  is overmolded over a portion of the tab  320  that unitarily extends from the rectangular portion that is configured to be welded to or otherwise embedded into the bipolar plate assembly  200 ; the other portion may in one form define a generally larger rectangle with an aperture therein that defines the peripheral dimension along a cut line of the conical datum  300 . The resulting overmolded datum  300  produced from the linear metal strip  400  can be “net-build” to the plate; this additional datum  300  placement means that its location on the bipolar plate and plate assembly  200  will be dependent only by weld tooling, and not by part geometry. Regardless of whether the linear metal strip  400  is used to form the individual datum  300 , the laser welding approach mentioned above as a way to join individual sheets  200 A,  200 B of the bipolar plate is beneficial in that it readily accommodates individualized placement of the datum  300  in between the surfaces of the sheets  200 A,  200 B along any portion of the sheet edge. 
         [0025]    Significantly, the use of datum  300  ensures that no integrally-formed datum pins are required, although as shown with particularity in  FIG. 6 , optional datum pins  330  may be used as part of a temporary or permanent stack  1  structure to (in the case of temporary structure) provide enhanced alignment during stack  1  formation, and (in the case of permanent structure) to provide supplemental resistance to inter-cell sliding that may arise out of the aforementioned disruptive event; such may be particularly useful when extremely high acceleration load requirements (for example, the aforementioned 160 g loading) are imposed on the design of stack  1 . In one form, pins  330  are threaded through apertures  300 A formed in the datum  300 . Such pins  330  may be placed during the stacking process, and either left after the stack has been assembled, or taken out prior to its final assembly. In a preferred form, the pins  330  are not be required for stacking, although in the uncompressed state factors such as GDM thickness, seal thickness and material geometry inconsistencies in general that could all cause deviations from plate assembly flatness; in such circumstance, the conical feature  310  of datum  300  may not be sufficient on its own to allow for proper cell-to-cell alignment. As such, pins  330  may be employed on a case-by-case basis to further improve alignment during stacking, if needed. 
         [0026]    The shape of the aperture  300 A may be made to promote manufacturing tolerance in a preferred direction within the plane (for example, the X-Z plane, as shown) that is susceptible to sliding movement between adjacent cells  15  or plate assemblies  200 ; such shaping (presently shown as oval-shaped) may help facilitate pin  330  insertion. In another shape (not shown), aperture  300 A may be circular, with multiple (for example, four, evenly-spaced) outwardly radial-extending slotted cutouts; in this form, the circular portion provides a slight interference fit with a datum pin  330 , while the elongate cutouts permit tolerances in manufacturing, as the additional breaks in the conical shape of the datum  300  promotes dimensional variation. 
         [0027]    Referring with particularity to  FIGS. 8 and 9 , other supplemental means may be used to promote improved resistance to shearing and the related inter-cell sliding. For example,  FIG. 8  shows with particularity indentations  510  may be molded into a substantially integral housing  500  (which may in overall shape and dimension may resemble that of the housing  20  of  FIG. 1 ). In such event, the molded indentations  510  are sized to allow a close-tolerance fit along at least one sliding direction S within the X-Z plane of cell  15 , and are spaced to coincide with the edgewise placement of the stack of data  300 . In a preferred form, the indentations  510  may define a generally C-shaped profile such that any tendency of inter-cell sliding movement along sliding direction S is additionally resisted by the columnar shape that extends a substantial entirety of the height (which coincides with the stacking dimension of the Y-axis of  FIG. 1 ) of the housing  500 . As such, the shape of the molded indentations  510  help them act as a bulkhead against movement in the X-direction of  FIG. 1 , as well as provide such additional resistance along one movement in the Z-direction. In one form, the housing is made of an extruded plastic material. In a preferred form, the material for integral housing  500  and the molded indentations  510  is extruded aluminum (or one of its alloys) in order to provide support to the cell block  501  (which in general mimics the block of cells  15  depicted in  FIG. 1 ) during collision and to maintain stack  501  made from cell block  501  in compression. The use of the indentations  510  may be in conjunction with secondary or tertiary datum  300  discussed above, and may obviate the need for datum pins  330 . Insulation  520  is formed around the periphery of the stack  501  to provide electrical isolation. 
         [0028]    Likewise,  FIG. 9  shows with particularity a cast enclosure  600  placed around a cell block  601 . As with the extruded aluminum housing  500 , the cast enclosure  600  (which is preferably aluminum or one of its alloys) defines a columnar portion where the stacked data  300  may be placed. For the cast enclosure  600 , two or more additional bracket sets  610  are installed to contain the datum  300  as a primary load retention mechanism. As with the stack  1  in  FIG. 1 , the cell block  601  is surrounded by opposing end caps and sidewalls (not labeled). Each bracket set  610  includes a vertical columnar member  610 A that is shaped to accept the aligned, stacked data  300  and an upper horizontal member  610 B that affixes the top of the vertical columnar member  610 A to the enclosure  600 . As such, the enclosure  600  is used predominantly for environmental protection and secondary load retention. 
         [0029]    Severe load changes due to a disruptive event, which includes high acceleration or deceleration of the vehicle  100 , an impact involving the vehicle  100 , or similar impact to the fuel cell stack  1  itself, such as a vertical fall, can damage the fuel cell stack  1  or disassemble the stack  1  by causing the individual fuel cells  15  to move relative to one another. The mechanical properties of the datum  300  must be sufficient to carry the maximum acceleration that is attendant to such load changes. In one exemplary form, the datum  300  for each corresponding bipolar plate within the assembly  200  would have a shear strength of about 150 N as a way to absorb the 160 g loading. 
         [0030]    It is noted that terms like “preferably,” “commonly,” 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, for the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term 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. 
         [0031]    For the purposes of describing and defining the present invention it is noted that the terms “fuel cell” or the like are utilized herein to represent a one or more individual cells used to provide electric current, preferably for vehicular, propulsive or related purposes. Furthermore, variations on the terms “automobile”, “automotive”, “vehicular” or the like are meant to be construed generically unless the context dictates otherwise. As such, reference to an automobile will be understood to cover cars, trucks, buses, motorcycles and other similar modes of transportation unless more particularly recited in context. 
         [0032]    Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.