Patent Publication Number: US-6663995-B2

Title: End plates for a fuel cell stack structure

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cells and, more particularly, to fuel cells arranged in a stack and held in compression. 
     BACKGROUND OF THE INVENTION 
     Fuel cell stacks typically comprise a plurality of fuel cells stacked one upon the other and held in compression with respect to each other. The plurality of stacked fuel cells form a fuel cell assembly which is compressed to hold the plurality of fuel cells in a compressive relation. Typically, each fuel cell comprises an anode layer, a cathode layer, and an electrolyte interposed between the anode layer and the cathode layer. The fuel cell assembly requires a significant amount of compressive force to squeeze the fuel cells of the stack together. The need for the compressive force comes about from the internal gas pressure of the reactants within the fuel cells plus the need to maintain good electrical contact between the internal components of the cells. Generally, the per area unit force is about 195-205 psi total which is distributed evenly over the entire active area of the cell (typically 77-155 square inches for automotive size stacks). Thus, for a fuel cell with an area of about 80 square inches, the typical total compressive force of these size stacks is about 15,500 to 16,500 pounds. 
     Typical prior art fuel cell stack structures focused on the use of rigid end plates to apply and maintain a compressive force on the fuel cell assembly. The fuel cell assembly to be compressed is interposed between a pair of rigid end plates. The end plates are then compressed together and held in a spaced relation to maintain the compressive force. The end plates can be held in a spaced relation by a variety of means. For example, tie rods that extend through the end plates can be used to impart a compressive force on the end plates and hold the end plates in a spaced relation. The tie rods are typically external to the fuel cell assembly and are situated along the periphery of the end plates. Side plates that extend along a length of the fuel cell assembly and are attached to the end plates have also been used to hold the end plates in a spaced relation and maintain a compressive force on the fuel cell assembly. 
     Due to the high compressive force that must be imparted on the fuel cell assembly and the size of the active area of the fuel cell assembly over which the compressive force must be imparted, rigid end plates that are maintained in a spaced relation by means along the periphery of the rigid end plates have a tendency to deflect and not impart a generally uniform compressive force over the entire active area of the fuel cell assembly. That is, the central portion of the rigid end plates deflect and the force applied to the active area below the central portions of the rigid end plates is not as great as the force applied to the active area along the periphery of the rigid end plates. 
     Prior art attempts to provide a generally uniform compression distribution over the active area have included very thick rigid end plates with external tie rods, rigid end plates with internal tie rods passing through the fuel cell assembly, semi-rigid end plates with a cavity for a gas bladder, and the use of discreet force exerting members, such as screws, that are positioned above the central portions of the end plates and can be selectively moved relative to the end plates to impart a compressive force along the central portions of the end plates. 
     In the rigid end plate with external tie rods, threaded tie rods extend from the periphery of an upper end plate along the outside of the fuel cell assembly to the periphery of the lower end plate so the total compressive force is carried by the tie rods. The end plate must be thick enough so that a small (about less than 1 mil per cell) total deflection is achieved. The disadvantage of this system is that the end plate must be very thick as compared with all other options since the total end plate span is the largest and no other method is employed to generate an even force over the entire active area. 
     In the rigid end plate with internal tie rods through the fuel cell assembly, the tie rods extend through the center of the fuel cells to allow the placement of the tie rods nearer to the central portion of the end plates. Now the total span of the bending force is not extended over the entire width of the upper end plates but rather a shorter span is achieved. This arrangement has the advantage of reducing span length of the end plates resulting in the ability to use a thinner end plate but has the disadvantage that it requires complex bipolar plate sealing mechanisms to enable the tie rods to pass through the fuel cell assembly. 
     In the semi-rigid end plate with a cavity for a gas bladder, the lower face of the upper end plate is hollowed out, a bladder is positioned in the end plate cavity, and the bladder is pressurized to provide the desired compressive loading of the stack. The upper end plate itself is now allowed to bend somewhat while the bladder maintains uniform force distribution over the entire active area. This arrangement has the advantage that the structural component of the upper end plate can be made thinner since it is allowed to flex considerably but has the disadvantage that it requires a cavity in the end plate with the result that the overall thickness of the end plate is significantly increased. 
     In the use of discreet force exerting members, the discreet force exerting members are positioned above the central portions of the end plates and are selectively moved relative to the end plates to impart a compressive force along the central portions of the end plates. This arrangement has the advantage of fine tuning the compressive force applied to the various locations of the end plates above which a discreet force exerting member is positioned but has the disadvantage that it requires extra mechanisms to maintain the discreet force exerting members and requires an iterative process of tightening the various discreet force exerting members to attain a generally uniform force distribution on the active area of the fuel cell assembly. 
     Therefore, what is needed is a fuel cell stack structure that has end plates that apply a generally uniform compressive force along the active area of the fuel cell assembly without requiring excessively thick end plates or the use of extra means to apply a compressive force to the central portions of the end plates. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an apparatus for providing a fuel cells stack structure that compresses the fuel cell assembly and imparts a generally uniform compressive force over the active area of the fuel cell assembly. More specifically, this invention is directed to variations on the design of the end plates that improve the distribution of the compressive force over the active area of the fuel cell assembly. 
     An electro-chemical fuel cell stack of the present invention comprises a plurality of fuel cells arranged in a stacked configuration to form a fuel cell assembly. The fuel cell assembly has opposite first and second ends. First and second end plates with opposite inner and outer surfaces are disposed adjacent the respective first and second ends of the fuel cell assembly. The inner surfaces of the end plate face the ends of the fuel cell assembly. The first and second end plates are held in a spaced relation so that the first and second end plates impart a compressive force on the fuel cell assembly. The inner surface of at least one of the first or second end plates is contoured so that a generally uniform compressive force is imparted on the fuel cell assembly. The contoured inner surface can be contoured so that the inner surface extends from the at least one end plate toward the end of the fuel cell assembly. Preferably, the end plate with the contoured inner surface has contours so that the end plate has a thickness that increases from a periphery of the end plate to a center of the end plate with a maximum thickness being in the center of the end plate. Optionally, the inner surfaces of both the first and second end plates can be contoured to extend from the first and second end plates toward the respective first and second ends of the fuel cell assembly so that a generally uniform compressive load is imparted on the fuel cell assembly. 
     In an alternate embodiment of the present invention, an electro-chemical fuel cell stack comprises a plurality of fuel cells arranged in a stacked configuration to form a fuel cell assembly. The fuel cell assembly has opposite first and second ends. First and second spacer plates with opposite inner and outer surfaces are disposed adjacent the respective first and second ends of the fuel cell assembly. The inner surfaces of the spacer plates face the ends of the fuel cell assembly. First and second end plates with opposite inner and outer surfaces are disposed adjacent the respective first and second spacer plates with the spacer plates being between the end plates and the ends of the fuel cell assembly. The inner surfaces of the end plates face the outer surfaces of the spacer plates. The first and second end plates are held in a spaced relation so that the first and second end plates impart a compressive force on the spacer plates and on the fuel cell assembly. At least one of the surfaces of at least one of the spacer plates or end plates is contoured so that a generally uniform compressive force is imparted on the fuel cell assembly. 
     In a different alternate embodiment of the present invention, an electro-chemical fuel cell stack comprises a plurality of fuel cells arranged in a stacked configuration to form a fuel cell assembly. The fuel cell assembly has opposite first and second ends. First and second terminal plates are disposed adjacent the respective first and second ends of the fuel cell assembly. First and second end plates are disposed adjacent the respective first and second terminal plates with the terminal plates being between the end plates and the ends of the fuel cell assembly. At least one terminal plate of the first and second terminal plates is attached to one of the first or second end plates so that stiffness of the at least one terminal plate contributes to stiffness of the attached end plate. The first and second end plates are held in a spaced relation so that the first and second end plates impart a compressive force on the fuel cell assembly. Optionally, the fuel cell stack can also comprise at least one spacer plate. The at least one spacer plate is interposed between the at least one terminal plate attached to the one of the first or second end plates. The at least one spacer plate is attached to the at least one terminal plate and to one of the first or second end plates so that stiffness of the at least one spacer plate contributes to the stiffness of the attached end plate. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of an electro-chemical fuel cell stack of the present invention; 
     FIG. 2 is a simplified cross-sectional view of the electro-chemical fuel cell stack of FIG. 1 taken along line  2 — 2 ; 
     FIG. 3 is a partial exploded perspective view of the electro-chemical fuel cell stack of FIG. 1 showing the attachment of a side plate to the electro-chemical fuel cell stack; 
     FIG. 4 is a simplified fragmentary view showing details of a fuel cell; 
     FIGS. 5A-G are cross-sectional views of various configurations for the end plate and spacer plate of the electro-chemical fuel cell stack of the present invention; 
     FIG. 6A is a plan view of a contoured inner surface of an end plate according to the principles of the present invention; 
     FIG. 6B is a cross-sectional view of the end plate of FIG. 6A taken along line B—B; 
     FIG. 6C is a cross-sectional view of the end plate of FIG. 6A taken along line C—C; 
     FIGS. 7A-B are fragmentary cross-sectional views of an end assembly of an electro-chemical fuel cell stack of the present invention showing various ways of attaching the components of the end assemblies; 
     FIG. 8 is a perspective view of a spacer plate used in an electro-chemical fuel cell stack of the present invention showing the use of holes to decrease the weight of the spacer plate; 
     FIGS. 9A-B are simplified cross-sectional views of the electro-chemical fuel cell stack of FIG. 1 illustrating the respective compressing of the fuel cell assembly and fuel cell stack with a compressive force of a predetermined magnitude F; 
     FIGS. 10A-B are simplified cross-sectional views of the electro-chemical fuel cell stack of FIG. 1 illustrating the compressing of the fuel cell assembly and fuel cell stack a predetermined distance D; 
     FIG. 11 is flow chart showing the steps of the Predetermined Force Compression Method of making a fuel cell stack according to the principles of the present invention; 
     FIG. 12 is a flow chart showing the steps of the Predetermined Compressive Distance Method of making a fuel cell stack according to the principles of the present invention; and 
     FIG. 13 is a flow chart showing the steps of using spacer plates to make a fuel cell stack of a predetermined or uniform length. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to FIGS. 1 and 2, there is shown an electro-chemical fuel cell stack  20  in accordance with a preferred embodiment of the present invention. The fuel cell stack  20  includes a plurality of fuel cells  22  arranged in a stacked configuration to form a fuel cell assembly  24  having opposite upper and lower ends  26 ,  28  with a compressed length  30  and an uncompressed length  31 , is shown in FIG. 10A, therebetween. The fuel cell assembly  24  is interposed between upper and lower end assemblies  32 ,  34 . The upper and lower end assemblies  32 ,  34  are held in a fixed spaced relation by a side wall. In the presently preferred embodiment, the side wall includes at least one side plate  36 . The side plates  36  hold the upper and lower end assemblies  32 ,  34  in a spaced relation so that the upper and lower end assemblies  32 ,  34  impart a compressive force on the fuel cell assembly  24 . The fuel cell stack  20 , in accordance with known fuel stack technology, includes inlets  37 , outlets  38 , and passageways (not shown) for supplying and exhausting reactant and coolant fluid streams to/from the fuel cell assembly  24 . 
     The fuel cell assembly  24 , as can be seen in FIG. 4, includes multiple repeating units or fuel cells  22  having an MEA  40  and a pair of bipolar plate assemblies  42  disposed on opposite sides of the MEA  40 . Each bipolar plate assembly  42  consists of a coolant distribution layer  42   c  interposed between two gas distribution layers  42   g.  Interposed between the coolant distribution layer  42   c  and gas distribution layer  42   g  is an impermeable separator plate  44  which contains the coolant and separates the anode and cathode gas streams. A fuel cell  22  is formed when an MEA  40  is interposed between an anode gas distribution layer  42   ga  of one cell and the cathode gas distribution layer  42   gc  of the adjacent cell. The MEA  40  can take a variety of forms, as is known in the art. For example, the MEA  40  can be a polymer electrolyte membrane. Preferably, the polymer electrolyte membrane is a thin reinforced membrane having a thickness on the order of approximately 0.018 microns. The thin reinforced polymer electrolyte membrane is much thinner than the polymer electrolyte membrane used in prior art fuel cells that had a thickness of approximately 0.007 inches. The thin and reinforced polymer electrolyte membrane used in the present invention represents a smaller percentage of the length  30  of the fuel cell assembly  24  and exhibits significantly less slip or stress relaxation than the thicker polymer electrolyte membrane used in the prior art fuel cell stacks. 
     The fuel cells  22  are arranged in a stacked configuration to form the fuel cell assembly  24 . The number of fuel cells  22  that are stacked adjacent one another to form the fuel cell assembly  24  can vary. The number of fuel cells  22  that are utilized to form the fuel cell assembly  24  is dependent upon the needs of the fuel cell stack  20 . That is, when a larger or more powerful fuel cell stack  20  is desired, the number of fuel cells  22  in the fuel cell assembly  24  will be increased. As is known in the art, the fuel cells  22  need to be compressed so that the fuel cells  22  are more efficient and generate more power. Therefore, the fuel cell assembly  24  is compressed between the upper and lower end assemblies  32 ,  34 . Preferably, the active area (not shown) of the fuel cell assembly  24  is uniformly compressed to maximize the efficiency of the fuel cell assembly  24  and each of the fuel cells  22  within the fuel cell assembly  24 . 
     With reference again to FIGS. 2 and 3, the upper end assembly  32  is positioned adjacent the upper end  26  of the fuel cell assembly  24 . The upper end assembly  32  includes an upper end plate  45  having opposite inner and outer surfaces  46 ,  48 . The inner surface  46  of the upper end plates  45  faces the upper end  26  of the fuel cell assembly  24 . The upper end plate  45  has numerous openings  50  that allow the various inlets  37  and outlets  38  connected to the fluid passageways to extend from the fuel cell assembly  24  to an exterior of the fuel cell stack  20 . The end of the fuel cell stack  20  having the inlets  37  and outlets  38  that connect to the passageways is also referred to as the “wet end”. 
     The lower end assembly  34  is positioned adjacent the lower end  28  of the fuel cell assembly  24 . The lower end assembly  34  includes a lower end plate  58  having opposite inner and outer surfaces  60 ,  62 . The lower end plate  58  is oriented so that the inner surface  60  of the lower end plate  58  faces the lower end  28  of the fuel cell assembly  24 . When there are no inlets and outlets going through the lower end assembly  34  that connect to the fluid passageways, the lower end  28  of the fuel cell stack  20  is also known as the “dry end”. 
     Optionally, but preferably, one or more spacer plates  52  may be located between the fuel cell assembly  24  and the upper and/or lower end plates  45 ,  58 . The spacer plate  52  is positioned between the end plate  45 ,  58  and the end  26 ,  28  of the fuel cell assembly  24  with the inner surface  54  of the spacer plate  52  facing the end  26 ,  28  of the fuel cell assembly  24  and the outer surface  55  of the spacer plate  52  facing the inner surface  54 ,  60  of the end plate  45 ,  58 . When a terminal plate  56  is positioned on the end  26 ,  28  of the fuel cell assembly  24 , the spacer plate  52  is positioned between the terminal plate  56  and the end plate  45 ,  58  with the inner surface  54  of the spacer plate  52  facing the terminal plate  56 . The spacer plate  52  separates the end plate  45 ,  58  from the terminal plate  56 . The spacer plates  52  are oriented in the end assemblies  32 ,  34  so that the thickness  57  of the spacer plate  52  is aligned with the length  30  of the fuel cell assembly  24 . While the preferred embodiment illustrates a spacer plate  52  associated with the upper and lower end assemblies  32 ,  34 , a skilled practitioner will recognize that the number and location of the spacer plate  52  may vary depending on the design and application of the fuel cell stack  20 . 
     The upper and lower end plates  45 ,  58  each have a peripheral side wall  64  that separates the inner surfaces  46 ,  60  from the outer surfaces  48 ,  62 . The peripheral side wall  64  on the upper and lower end plates  45 ,  58  are aligned with the length  30  of the fuel cell assembly  24 . Preferably, as shown in the figures, the fuel cell stack  20  is generally rectangular in shape and the upper and lower end plates  45 ,  58  are also rectangular in shape. The peripheral side wall  64  of the rectangular shaped upper and lower end plates  45 ,  58  is comprised of first and second pairs of opposite side walls  66 ,  68  that are generally perpendicular to one another. The first and second pairs of opposite side walls  66 ,  68  each have one or more threaded bores  70  which receive threaded fasteners  80  for securing the side plates  36  to the upper and lower end plates  45 ,  58 . 
     As was mentioned above, the upper and lower end assemblies  32 ,  34  impart a compressive force on the fuel cell assembly  24 . The compressive force imparted on the fuel cell assembly  24  is generated by the upper and lower end plates  45 ,  58  being held in a fixed spaced relation. Preferably, the upper and lower end plates  45 ,  58  are held in a fixed spaced relation by the side plates  36 . Each side plate  36  has opposite first and second ends  72 ,  74  and a length  76  therebetween. Each side plate  36  is oriented on the fuel cell stack  20  so that the first end  72  is adjacent the upper end plate  45  and the second end  74  is adjacent the lower end plate  58  with the length  76  of the side plate  36  aligned with the length  30  of the fuel cell assembly  24 . Optionally, but preferably, the side plates  36  extend along the entire peripheral side walls  64  of the end plates  45 ,  58 . The first and second ends  72 ,  74  of each side plate  36  have one or more openings  78  that align with the threaded bores  70  in the peripheral side walls  64  of the upper and lower end plates  45 ,  58  when the fuel cell assembly  24  is compressed. Preferably, the openings  78  in either the first and/or second ends  72 ,  74  of each side plate  36  are in the form of a slot so that the upper and lower end plates  45 ,  58  can be held in a fixed spaced relation. The slots allow for variations in the size of the various components of the fuel cell stack  20  while still being capable of holding the upper and lower end plates  45 ,  58  in a fixed spaced relation. While threaded mechanical fasteners  80  are preferably used to attach the side plates  36  to the upper and lower end plates  45 ,  58 , a skilled practitioner will recognize that other means of attaching the side plate  36  to the upper and lower end plates  45 ,  58  can be employed without departing from the scope of the invention as defined by the claims. In this regard, the joint formed by the side plates  36  and the end plates  45 ,  58  should be sufficient to resist relative rotation at the interface therebetween. For example, the first and/or second ends  72 ,  74  of the side plates  36  can be secured to the respective upper and/or lower end plates  45 ,  58  through other mechanical fastening means such as rivets or pins, or through various bonding means such as welding, brazing or adhesive bonding and still be within the spirit of the invention. Furthermore, it should be understood that one of the ends  72 ,  74  of the side plates  36  can be bent to form a retaining element (not shown) that can be positioned on one of the end plates  45 ,  48  to retain the end plate  45 ,  48  while the opposite end  72 ,  74  of the side plates  36  are attached to an opposite end plate  45 ,  48  and hold the end plates in a fixed spaced relation. 
     Each side plate  36 , as needed, can have one or more openings  82  that allow a terminal block  83  on the terminal plate  56  to extend to an exterior of the fuel cell stack  20 . Preferably, each side plate  36  is electrically grounded and protects the fuel cell assembly  24  against electro-magnetic interference. Also preferably, each side plate  36  is made of metal. The side plates  36  that are used to hold the upper and lower end plates  45 ,  58  in a fixed spaced relation are dimensioned to maintain the upper and lower end plates  45 ,  58  in the fixed spaced relation while the upper and lower end plates  45 ,  58  impart and maintain a compressive force on the fuel cell assembly  24 . Because the width of the side plate  36  is relatively large, a relatively small thickness is required to provide the necessary tensile strength for carrying the compressive load. This aspect of the invention represents a weight-savings over the conventional use of axial rods around and/or through the fuel cell assembly. 
     Preferably, the one or more side plates  36  enclose at least a portion of the fuel cell assembly  24  to provide protection for the fuel cell assembly  24  against inadvertent damage. Even more preferably, the side plates  36  enclose the entire fuel cell assembly  24  and provide a protective cover for the fuel cell assembly  24  and the fuel cell stack  20 . Accordingly, the side plates  36  are dimensioned so that the side plates  36  can withstand impacts, blows and other assaults of various natures while protecting the fuel cell assembly  24  and the fuel cell stack  20  from damage as a result of the impact, blow or other assaults. In this manner, the side plates  36  not only act to retain the upper and lower end plates  45 ,  58  in a fixed spaced relation that imparts and maintains a compressive load on the fuel cell assembly  24 , but also provides a protective enclosure for the fuel cell assembly  24  and the fuel cell stack  20 . The use of side plates  36  to perform the protective function eliminates the need of having an additional structure placed around the fuel cell stack  20  to provide protection from inadvertent blows, impacts or other assaults on the fuel cell stack  20 , as is done in conventional fuel cell stacks. 
     The spacer plates  52  that are optionally included in the upper end assembly  32  and/or the lower end assembly  34 , serve a variety of purposes. That is, the spacer plates  52  can be included in the fuel cell stack  20  for one or more reasons. For example, the spacer plates  52  can be used to separate the upper and/or lower end plates  45 ,  58  from the terminal plates  56 . The terminal plates  56 , as mentioned above, are electrically conductive and used to extract current flow from the fuel cell stack  20  through terminal block  83 . When the upper and/or lower end plate  45 ,  58  is electrically conductive, the spacer plate  52  positioned between the upper and/or lower end plates  45 ,  58  and the terminal plates  56  can electrically insulate the upper end and/or lower end plate  45 ,  58  from the terminal plates  56 . The spacer plates  52  can also be used to control the overall dimensions of the fuel cell stack  20 . That is, one or more spacer plates  52  can be positioned between the fuel cell assembly  24  and the upper and/or lower end plates  45 ,  58  to provide a fuel cell stack  20  having a predetermined length while the end assemblies  32 ,  34  still impart a compressive force on the fuel cell assembly  24 , as will be discussed in more detail below. As presently preferred, the spacer plate(s)  52  have a thickness  57  in the range of about 8-18 millimeters to provide adequate electrical insulation and uniform dimension of the fuel cell stack  20 . However, one skilled in the art will recognize that the particular application and design specification will dictate the ranges of thickness  57  for the spacer plate(s)  52 . The spacer plates  52  can also be used in combination with the upper and/or lower end plates  45 ,  58  for imparting a generally uniform compressive load on the fuel cell assembly  24 , as will be discussed in more detail below. 
     Preferably, the spacer plates  52  are non-conductive and can serve to electrically insulate various components of the fuel cell stack  20 . Therefore, the spacer plates  52  are preferably made from a non-conductive material such as plastic. Even more preferably, the spacer plates  52  are made from an engineering grade high performance plastic. The engineering grade high performance plastic used to make the one or more spacer plates  52  is relatively noncompressible (i.e., insignificant stress relaxation) under the magnitude of compressive loading applied to the fuel cell assembly  24  so as to transfer the compressive load from the upper and/or lower end plates  45 ,  58  to the respective upper and lower ends  26 ,  28  of the fuel cell assembly  24 . In particular, polythenylene sulfide has proven to be an especially effective material from which to make spacer plates  52 . Polythenylene sulfide is available under the RYTON PPS brand sold by Chevron Phillips Chemical Company, L. P. and under the FORTRON brand sold by Celanese A G, of Frankfurt, Germany. Preferably, as can be seen in FIG. 7, the spacer plates  52  have one or more apertures  84  that reduce the weight of the spacer plates  52 . 
     As was mentioned above, the upper and lower end plates  45 ,  58  are held in a fixed spaced relation by the side plates  36  and impart a compressive load on the fuel cell assembly  24 . As previously described, the upper and lower end plates  45 ,  58  are retained in the fixed spaced relation by the side plates  36 . The compressive loading generated in the upper and lower ends  26 ,  28  of the fuel cell assembly  24  will vary depending on the distance from the peripheral side walls  64  with the compressive loading being at a maximum along the peripheral side walls  64  and at a minimum in a center of the upper and lower end plates  45 ,  58 . That is, because the upper and lower end plates  45 ,  58  are only retained along their peripheral side walls  64 , the upper and lower end plates  45 ,  58  will deform or deflect in response to the compressive loading on the fuel cell assembly  24  and the inability of the peripheral side walls  64  of the upper and lower end plates  45 ,  58  to move further apart. Because the efficiency of the fuel cell stack  20  is partially dependent upon a uniform compressive load being applied across the active area of the fuel cell assembly  24 , it is desirable to maintain a generally uniform compressive load over the entire active area of the fuel cell assembly  24 . 
     One means for obtaining a generally uniform load is to make the upper and lower end plates  45 ,  58  rigid by increasing the thickness thereof so that the deflection that occurs in the upper and lower end plates  45 ,  58  has a deminimus effect on the efficiency of the fuel cell assembly  24 . However, providing this thickness can make for upper and lower end plates  45 ,  58  that are excessively thick and add excess weight to the fuel cell stack  20  thereby decreasing the gravimetric and volumetric efficiency of the fuel cell stack. To avoid the necessity of providing end plates  45 ,  58  that are relatively rigid, the end plates  45 ,  58  can optionally be attached to the spacer plates  52  and to the terminal plates  56  so that the stiffness of the spacer plates  52  and the stiffness of the terminal plates  56  contribute to the overall stiffness of the end assemblies  32 ,  34  and reduces the thickness of the end plates  45 ,  58  required to apply a generally uniform compressive load across the active area of the fuel cell assembly  24 . That is, as can be seen in FIGS. 7A-B, the terminal plate  56 , the spacer plates  52  and the end plates  45 ,  58  can be fastened together to combine their stiffnesses and form end assemblies  32 ,  34  that can impart a generally uniform compressive load on the active area of the fuel cell assembly  24 . As can be seen in FIG. 7A, the terminal plate  56  can be connected to the spacer plate  52  by means of a mechanical fastener  86 , such as a threaded bolt or screw, and the combined terminal plate  56  and spacer plate  52  can then be attached to one of the end plates  45 ,  58  via mechanical fasteners  87 . Alternatively, the terminal plate  56 , the spacer plates  52 , and one of the end plates  45 ,  58  can all be attached by means of adhesive layers  88  interposed between the respective components. The stiffness of the terminal plate  56  and the stiffness of the spacer plates  52  thereby combine with the stiffness of the end plates  45 ,  58  to provide end assemblies  32 ,  34  that can apply a generally uniform compressive load to the active area of the fuel cell assembly  24  with thinner end plates  45 ,  58  then would be necessary without the attachment of the terminal plate  56  or spacer plates  52  to the end plates  45 ,  58 . 
     Alternatively, and/or additionally, the end plates  45 ,  58  and/or the spacer plates  52  can have shaped surfaces that compensate for the deflection of the end plates  45 ,  58  and impart a generally uniform compressive load across the active area of the fuel cell assembly  24  without requiring the use of excessively thick end plates  45 ,  58 . That is, as can be seen in FIGS. 5A-G, which only show the upper end plate  45  and a single spacer plate  52 , the inner surface  46  of the upper end plate  45  can be dimensioned to curve away from the upper end plate  45  and toward the upper end  26  of the fuel cell assembly  24  so that the upper end plate  45  has a thickness that is a minimum along a peripheral side wall  64  and is at a maximum in a center of the upper end plate  45 . The shape of the inner surface  46  of the upper end plate  45  is contoured to account for the deflection that will occur in the upper end plate  45  as a result of the upper end plate  45  being retained along its peripheral side wall  64  in a fixed spaced relation from a lower end plate  58 , while imparting a compressive load of a desired magnitude on the active area of the fuel cell assembly  24 . FIGS. 6A-C show an exemplary contouring of the inner surface  46  of the upper end plate  45 . As can be seen, the upper end plate  45  has a maximum thickness in the approximate center of the upper end plate  45 . 
     Alternatively, and/or additionally, the spacer plate  52  can have the inner and/or outer surfaces  54 ,  55  contoured to account for the deflection that will occur in the upper end plate  45 . That is, the spacer plate  52  can be configured to have a thickness that is at a minimum along a periphery of the spacer plate  52  and at a maximum in the center of the spacer plate  52 . For example, as shown in FIG. 5G, the inner surface  54  of the spacer plate  52  can be contoured to extend from the spacer plate  52  toward the upper end  26  of the fuel cell assembly  24 , or, as can be seen in FIG. 5E, the outer surface  55  of the spacer plate  52  can be contoured to extend from the spacer plate  52  toward the upper end plate  45  so that a generally uniform compressive load can be imparted on the active area of the fuel cell assembly  24  by the end plate  45 . Alternatively, as can be seen in FIG. 5F, both the inner and outer surfaces  54 ,  55  of the spacer plate  52  can be contoured to extend from the spacer plate  52  toward the respective upper end  26  of the fuel cell assembly  24  and the inner surface  46  of the upper end plate  45  so that a generally uniform compressive load can be imparted on the active area of the fuel cell assembly  24 . 
     Various permutations on the shaping of the inner and outer surfaces  54 ,  55  of the spacer plate  52  and of the inner surface  46  of the upper end plate  45  are shown in FIGS. 5A-G. The contoured shape of the surfaces of the upper end plate  45  and/or the spacer plate  52  can be dimensioned to not only account for the deflection of the upper end plate  45  but also for the deflection of the lower end plate  58  so that both the upper and lower ends  26 ,  28  of the fuel cell assembly  24  receive a generally uniform compressive loading. Likewise, it should be understood that the inner surface  60  of the lower end plate  58  and the inner and outer surfaces  54 ,  55  of a spacer plate  52  in the lower end assembly  34  can also be contoured or shaped in the same manner so that the components of the lower end assembly  34  impart a generally uniform compressive load on the active area of the fuel cell assembly  24 . A skilled practitioner will recognize that the inner surface  46  may have various localized features formed therein for obtaining a more uniform compressive loading over the active area of the fuel cell assembly  24 . Therefore, it should be understood that the components of the upper end assembly  32  and/or the components of the lower end assembly  34  can have their surfaces contoured and shaped, either singularly or simultaneously, to apply a generally uniform compressive load on the active area of the fuel cell assembly  24 . It should further be understood that the dimensions shown in the various figures are exaggerated for exemplary purposes and should not be taken as being sized relative to each component of the fuel cell stack  20 . That is, it should be understood that the deflection of the end plates  45 ,  58  and the correction by shaping the surfaces of the end plates  46 ,  58  and/or the spacer plates  52  are exaggerated to better exemplify the principles of the invention. It should also be understood that use of the terms upper and lower to describe the various components of the fuel cell stack  20  are not be construed as being an absolute reference, but rather are to be construed as providing a relative relationship of the components of the fuel cell stack  20 . 
     While the fuel cell stack  20  is described and illustrated as being generally a rectangular shaped configuration, it should be understood that the shape of the fuel cell stack  20  may take a variety of configurations and still be within the scope of the invention as defined by the claims. For example, the fuel cell stack  20  can be cylindrical and the fuel cell assembly  24  along with the upper and lower end assemblies  32 ,  34  would also be cylindrical. When the fuel cell stack  20  is cylindrical, the side plate  36  can be a single cylindrical sleeve within which the upper and lower end assemblies  32 ,  34  and the fuel assembly  24  are inserted. The side plates  36  could also be portions of a cylindrical sleeve and enclose the components of the fuel cell stack  20 . Therefore, the use of the term side plate should not be limited to a flat plate but rather should be construed as being a plate that can be flat or curved or take a variety of shapes as dictated by the particular shape of the fuel cell stack  20 . 
     As was stated earlier, the fuel cell stack  20  has a fuel cell assembly  24  that is maintained with a compressive loading so that the fuel cell assembly  24  is more efficient. The present invention further includes various methods of making a fuel cell stack  20  having a fuel cell assembly  24  under a compressive loading. In a first method, the Predetermined Compressive Load Method, as can be seen in FIGS. 9A-B and  11 , the fuel cell assembly  24  and/or the fuel cell stack  20  is compressed with an external compressive load which generates an internal compressive load of a predetermined magnitude F on the fuel cell assembly  24 . The side plates  36  are then secured to the upper and lower end plates  45 ,  58  to maintain the upper and lower end plates  45 ,  58  in a fixed spaced relation when the external compressive loading is removed from the fuel cell assembly  24  and/or fuel cell stack  20 . Because the upper and lower end plates  45 ,  58  are maintained in a fixed spaced relation after the external compressive loading is removed, an internal compressive loading remains imparted on the fuel cell assembly  24  by the upper and lower end plates  45 ,  58 , as will be discussed in more detail below. 
     In a second method, the Predetermined Compressive Distance Method, as can be seen in FIGS. 10A-B and  12 , the fuel cell assembly  24  and/or the fuel cell stack  20  is compressed a predetermined distance D by an external compressive load C. In other words, the magnitude of the external compressive load is sufficient to compress the fuel cell assembly  24  a predetermined distance D. The side plates  36  are then attached to the upper and lower end plates  45 ,  58  (as will be described in more detail below). The external compressive load is then removed. The upper and lower end plates  45 ,  58  remain in their fixed spaced relation. The fuel cell assembly  24  remains compressed generally the predetermined distance D to impart an internal compressive loading thereon. 
     As was stated above, the Predetermined Compressive Load Method of making a fuel cell stack  20  having a fuel cell assembly  24  under a compressive loading of a predetermined magnitude F involves applying an external compressive load to the fuel cell stack  20 . The Predetermined Compressive Load Method includes the steps of: 1) positioning the fuel cell assembly  24  between the upper and lower end plates  45 ,  58  with the upper end  26  of the fuel cell assembly  24  adjacent the upper end plate  45  and the lower end  28  of the fuel cell assembly  24  adjacent the lower end plate  58 ; 2) applying an external compressive force to at least one of the end plates  45 ,  58  so that the fuel cell assembly  24  is compressed and experiences an internal compressive force of a predetermined magnitude F; 3) attaching the side plates  36  to the end plates  45 ,  58  with the first and second ends  72 ,  74  of the side plates  36  being attached to the respective upper and lower end plates  45 ,  58 ; and 4) removing the external compressive force being applied to at least one of the end plates  45 ,  58 , whereby the upper and lower end plates  45 ,  58  remain in a fixed spaced relation to maintain a compressive force generally equal to the predetermined magnitude F on the fuel cell assembly  24 . The Predetermined Compressive Load Method thereby provides a fuel cell stack  20  that has a compressive force generally equal to the predetermined magnitude F imparted on the fuel cell assembly  24 . 
     In contrast, when the Predetermined Compressive Distance Method is used to assemble a fuel cell stack  20 , the fuel cell stack  20  and/or the fuel cell assembly  24  are compressed a predetermined distance D as opposed to being compressed with a compressive force of a predetermined magnitude F. The reference point for the predetermined distance D could be an overall length of the fuel cell assembly  24  itself. Therefore, further reference will only be made to compressing the fuel cell assembly  24  the predetermined distance D and not to compressing the fuel cell stack  20 . However, it should be understood that the compressing of the fuel cell assembly  24  the predetermined distance D could also be done by compressing the fuel cell stack  20  the predetermined distance D. Preferably, the predetermined distance D corresponds to applying a compressive force to the fuel cell assembly  24  that results in efficient operation of the fuel cell stack  20 . The predetermined distance D to compress the fuel cell assembly  24  can be determined in a variety of ways. For example, the predetermined distance D can be computationally based upon a fixed distance compression for each fuel cell  22  that comprises the fuel cell assembly  24  or can be based upon empirical data from past experience with compressing fuel cell assemblies  24  having a known number of fuel cells  22 , as will be discussed in more detail below. Once the predetermined distance D has been determined, an external compressive load is applied to the fuel cell stack  20  so that the fuel cell stack  20  and/or fuel cell assembly  24  is compressed the predetermined distance D. The side plates  36  are then attached to the upper and lower end plates  45 ,  58  and the external compressive load is removed. The resulting fuel cell stack  20  has a fuel cell assembly  24  that is compressed the predetermined distance D and has an internal compressive loading that corresponds to efficient operation of the fuel cell stack  20 . 
     When the predetermined distance D is computationally based (i.e., based upon a fixed distance compression for each fuel cell), each fuel cell  22  is compressed a given distance. The predetermined distance D to compress the fuel cell assembly  24  is calculated by multiplying the number of fuel cells  22  n that are in the fuel cell assembly  24  by the fixed distance d that each fuel cell  22  is to be compressed. In other words, by the equation D=n×d. The fixed distance to compress each fuel cell  22  is chosen to provide a compressive force on the fuel cell  22  that has a magnitude that generally corresponds to providing efficient operation of the fuel cell assembly  24 . That is, the fixed distance d that each fuel cell  22  is to be compressed is based upon the physical properties of the fuel cells  22  and the amount of compression required for the fuel cells  22  to operate efficiently. The resulting fuel cell stack  20  has a fuel cell assembly  24  that is compressed the predetermined distance D and has a compressive loading that corresponds to efficient operation of the fuel cell assembly  24 . 
     When based on empirical data, the predetermined distance D to compress the fuel cell assembly  24  is determined from past experiences with compressing fuel cell assemblies  24  by a known compressive load as opposed to compressing each fuel cell  22  a fixed distance. The resulting predetermined distance D may be the same for both methods. Because of general uniformity in the composition of the fuel cells  22  that comprise a fuel cell assembly  24 , a general correlation can be established, for each type of fuel cell  22 , between the number of fuel cells  22  and the compressed distance of the fuel cell assembly  24  and/or fuel cell stack  20  that occurs when the fuel cell assembly  24  is subject to a compressive force of a known magnitude. The correlation can be used to determine, based upon the number of fuel cells  22  that comprise the fuel cell assembly  24 , the predetermined distance D to compress the fuel cell assembly  24  to impart a compressive force of a desired magnitude on the fuel cell assembly  24 . For example, empirical data shows that fuel cell assemblies having fifty and two hundred fuel cells are compressed a distance of X and 4X, respectively, imparts a compressive force of the desired magnitude. A fuel cell stack  20  having a fuel cell assembly  24  that is comprised of one hundred similar fuel cells  22  would be compressed a distance of 2X and based on the correlation should impart a compressive force of generally the same desired magnitude on the fuel cell assembly  24 . 
     Because there is some variability in the composition of any given type of fuel  22 , the resulting compressive force imparted on the fuel cell assembly  24  may vary. The amount of variation in the resulting compressive force will depend on the accuracy of the correlations and the variability of the fuel cells  22 . Preferably, the resulting compressive force will vary within an acceptable range around the desired magnitude such that the variation has a negligible effect on the efficiency of the fuel cell stack  20 . The empirical data method thereby provides a fuel cell stack  20  having a fuel cell assembly  24  that is subjected to a compressive force generally equal to a desired magnitude, that corresponds to efficient operation of the fuel cell assembly  24 , when the fuel cell assembly  24  is compressed the predetermined distance D. 
     As was mentioned above, spacer plates  52  can be used to provide a fuel cell stack  20  of a predetermined or uniform length L. That is, spacer plates  52  can be used in the fuel cell stack  20  to occupy space so that the fuel cell stack  20  is a predetermined or uniform length L. A uniform length L provides many advantages. For example, a uniform length L allows for fuel cell stacks to be easily interchanged and also allows apparatuses in which the fuel cell stack  20  is utilized to have standardized spaces for the fuel cell stack  20 . 
     The present invention provides various assembly sequences for a fuel cell stack having a uniform length L as shown in FIGS. 13 a - 13   b.  The desired predetermined or uniform length L of the fuel cell stack  20  can be either a known length, such as an industry standard, or a chosen length. In either case, the overall length L is a known quantity. The thicknesses of the upper and lower end plates  45 ,  58 , any terminal plates  56  that are used in the fuel cell stack  20 , and any other components of the end assemblies  32 ,  34  can be measured and thus are also known quantities. Based on these known quantities/dimensions, the space within the fuel cell stack  20  in which the fuel cell assembly  24  is to be placed can be calculated and is therefore also a known quantity. That is, the length of the space within the fuel cell stack  20  in which the fuel cell assembly  24  is to be placed is equal to the predetermined or uniform length L of the fuel cell stack  20  minus the dimensions of the end plates  45 ,  58 , any terminal plates  56  and any other components that make up the end assemblies  32 ,  34 . The only unknown dimension is the compressed length  30  of the fuel cell assembly  24 . The compressed length  30  of the fuel cell assembly  24  can vary depending upon which method, as discussed above, is used to make the fuel cell stack  20  and the number of fuel cells  22  that comprise the fuel cell assembly  24 . 
     As was stated above, spacer plates  52  can be used with the Predetermined Compressive Load Method to make a fuel cell stack  20  of a predetermined or uniform length L with the fuel cell assembly  24  being imparted a compressive loading generally equal to the predetermined magnitude F. To do this, either the compressed length  30  of the fuel cell assembly  24  or the compressed length of the fuel cell stack  20  needs to be determined so that the required combined thicknesses of the one or more spacer plates  52  can be ascertained. 
     The compressed length  30  of the fuel cell assembly  24  can be determined either by (1) compressing the fuel cell assembly  24  with an external compressive load such that an internal compressive load of the predetermined magnitude F is obtained and measuring the compressed length  30 , as can be seen in FIG. 9A; or by (2) compressing the fuel cell stack  20  with an external load so that an internal compressive load of predetermined magnitude F is imparted on the fuel cell assembly  24 , as can be seen in FIG. 9B, and either (A) measuring the compressed length  30  of the fuel cell assembly  24 ; or (B) measuring the compressed length of the fuel cell stack  20  and calculating the compressed length  30  of the fuel cell assembly  24  by subtracting the known dimensions of the end plates  45 ,  58 , the terminal plates  56  and any other components of the end assemblies  32 ,  34 . Once the compressed length  30  of the fuel cell assembly  24  has been determined, the external compressive load can be removed from the fuel cell assembly  24  or fuel cell stack  20 . The compressed length  30  of the fuel cell assembly  24  is used to calculate the required combined thickness of the spacer plates  52  to make the fuel cell stack  20  of the predetermined or uniform length L. The required combined thickness of the spacer plates  52  equals the difference between the length of the space within which the fuel cell assembly  24  is to be placed (as was discussed above) and the compressed length  30  of the fuel cell assembly  24 . Therefore, the required combined thickness of the spacer plates  52  can be calculated. 
     Alternatively, the compressed length of the fuel cell stack  20  having an internal compressive loading on the fuel cell assembly  24  of the predetermined magnitude F can be used. The compressed length of the fuel cell stack  20  can be determined by compressing the fuel cell stack  20  with an external compressive load so that an internal compressive load of the predetermined magnitude F is imparted on the fuel cell assembly  24  and then measuring the compressed length of the fuel cell stack  20 . The external compressive load on the fuel cell stack is then removed. The difference between the predetermined or uniform length L of the fuel cell stack  20  and the measured compressed length of the fuel cell stack  20  is calculated. The calculated difference is the required combined thickness of the spacer plates  52 . 
     Once the required combined thickness of the spacer plates  52  has been determined, one or more spacer plates  52  having the required combined thickness are selected. The selected spacer plates  52  are positioned between the upper and/or lower end plates  45 ,  58  and the respective upper and/or lower ends  26 ,  28  of the fuel cell assembly  24 . The spacer plates  52  are oriented so that the combined thicknesses of the spacer plates  52  are aligned with the length  30  of the fuel cell assembly  24 . The fuel cell stack  20  is then compressed by applying an external compressive load to the fuel cell stack  20  so that the fuel cell stack  20  is generally at the predetermined or uniform length L. The resulting internal compressive load of the fuel cell stack  20  having the predetermined or uniform length L should generally be equal to the predetermined magnitude F. The side plates  36  are then secured to the upper and lower end plates  45 ,  58  so that the upper and lower end plates  45 ,  58  retain the fuel cell stack  20  generally at the predetermined or uniform length L. Finally, the external compressive load is removed from the fuel cell stack  20 . The resulting fuel cell stack  20  has a length that is generally equal to the predetermined or uniform length L with the fuel cell assembly  24  compressed generally at the predetermined magnitude F. 
     The Predetermined Compressive Distance Method of making a fuel cell stack  20  can also utilize spacer plates  52  to make a fuel cell stack  20  of a predetermined or uniform length L. The required combined thickness of the spacer plates  52  is based upon the desired predetermined or uniform length L for the fuel cell stack  20 , the compressed length  30  of the fuel cell assembly  24 , and the thickness of the components that comprise the end assemblies  32 ,  34 . The compressed length  30  of the fuel cell assembly  24  is calculated by subtracting the predetermined distance D from the uncompressed length  31  of the fuel cell assembly  24 . The compressed length  30  of the fuel cell assembly  24  and the thicknesses of the end plates  45 ,  58 , the terminal plates  56  and any other components that comprise the end assemblies  32 ,  34  are subtracted from the predetermined or uniform length L of the fuel cell stack  20  to yield the required combined thickness of the spacer plates  52 . Spacer plates  52  are then selected so that the combined thickness of the spacer plates  52  are generally equal to the required overall thickness. The selected spacer plates  52  are then added to the fuel cell stack  20  as discussed above. The resulting fuel cell stack  20  generally has the desired predetermined or uniform length L, a fuel cell assembly  24  generally compressed the predetermined distance D, and an internal compressive loading that corresponds to efficient operation of the fuel cell assembly  24 . 
     When a term is quantified with the adverb “generally” herein, it should be understood to mean that the magnitude of the factor described is within an acceptable range of tolerance to the desired magnitude. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.