Patent Publication Number: US-6989214-B2

Title: Unitized fuel cell assembly

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to fuel cells and, more particularly, to a unitized fuel cell assembly and packaging methodology. 
   BACKGROUND OF THE INVENTION 
   A typical fuel cell power system includes a power section in which one or more stacks of fuel cells are provided. The efficacy of the fuel cell power system depends in large part on the integrity of the various contacting and sealing interfaces within individual fuel cells and between adjacent fuel cells of the stack. 
   Presently, the process of building a stack of fuel cells using conventional approaches is tedious, time-consuming, and not readily adaptable for mass production. By way of example, a typical 5 k kW fuel cell stack can include some 80 membrane electrode assemblies (MEAs), some 160 flow field plates, and some 160 sealing gaskets. These and other components of the stack must be carefully aligned and assembled. Misalignment of even a few components can lead to gas leakage, hydrogen crossover, and performance/durability deterioration. 
   Moreover, fuel cell MEAs are very fragile and need to be handled very carefully to prevent electrical shorting, pinholes, and wrinkles formed on the membrane, for example. MEA contamination is another significant concern during fuel cell stack assembly. Presently known stack assembling processes are so labor intensive that cost effective manufacturing of fuel cell systems may not be achievable using conventional approaches. 
   There is a need for an improved fuel cell assembly and packaging methodology. There is a further need for a fuel cell assembly that facilitates efficient assembling and disassembling of fuel cell stacks. There is a further need for recycling useful components in fuel cell stacks and systems. The present invention fulfills these and other needs. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a unitized fuel cell assembly (UCA) and packaging methodology. A unitized fuel cell system is unitary module or unit that comprises one or more cells that can work as a functioning fuel cell alone or in conjunction with other UCA&#39;s in a stack. According to an embodiment of the present invention, a UCA includes a first flow field plate and a second flow field plate. A membrane electrode assembly (MEA) is provided between the first and second flow field plates. The MEA includes first and second fluid transport layers (FTLs) and a membrane provided between anode and cathode catalytic layers. A hard stop arrangement is provided between the first and second flow field plates. A thermoplastic material is provided between at least a portion of the first and second FTLs and within a gap defined between the first and second FTLs and the hard stop arrangement. 
   According to another embodiment, a UCA includes a first flow field plate, a second flow field plate, and an MEA provided between the first and second flow field plates. A hard stop arrangement is provided between the first and second flow field plates. The hard stop arrangement is dimensioned to control compressive forces imparted to the MEA upon establishment of contact between the first and second flow field plates under pressure. A sealing arrangement is provided between the first and second flow field plates and peripheral to the MEA. 
   In accordance with a further embodiment, a unitized fuel cell assembly is configured for recyclable use. A recyclable unitary fuel cell assembly according to this embodiment includes a first flow field plate, a second flow field plate, and an MEA provided between the first and second flow field plates. A sealing arrangement is provided between the first and second flow field plates. The assembly further includes an engagement arrangement that releasably couples together the first and second flow field plates. The engagement arrangement is configured to permit repeated coupling and decoupling of the first and second flow field plates, whereby at least the first and second flow field plates are recoverable for reuse with a replacement component of the unitary fuel cell assembly. 
   In yet another embodiment of the present invention, a unitary fuel cell assembly includes a first flow field plate, a second flow field plate, and an MEA provided between the first and second flow field plates. The MEA includes first and second fluid transport layers (FTLs) and a membrane provided between anode and cathode catalytic layers. A thermoplastic material is provided between at least a portion of the first and second FTLs, and further between the first and second flow field plates for bonding together the first and second flow field plates to define a unitary structure. 
   The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1   a  is an illustration of a fuel cell and its constituent layers; 
       FIG. 1   b  illustrates a unitized cell assembly having a monopolar configuration in accordance with an embodiment of the present invention; 
       FIG. 1   c  illustrates a unitized cell assembly having a monopolar/bipolar configuration in accordance with an embodiment of the present invention; 
       FIG. 2   a  is a sectional view of a unitized cell assembly employing an external hard stop arrangement and an in-situ formed sealing gasket in accordance with an embodiment of the present invention; 
       FIG. 2   b  is a sectional view of a unitized cell assembly employing an internal hard stop arrangement and an in-situ formed sealing gasket in accordance with an embodiment of the present invention; 
       FIGS. 3   a  and  3   b  are sectional views of a unitized cell assembly employing a built-in hard stop arrangement and an in-situ formed sealing gasket in accordance with an embodiment of the present invention; 
       FIGS. 4   a  and  4   b  are schematic sectional views of a unitized cell assembly employing an internal hard stop arrangement and an in-situ formed sealing gasket in accordance with another embodiment of the present invention; 
       FIGS. 5   a  and  5   b  are schematic sectional views of a unitized cell assembly before and after a bonding process, respectively, the unitized cell assembly employing an internal hard stop arrangement and an in-situ formed thermoplastic sealing gasket in accordance with an embodiment of the present invention; 
       FIGS. 5   c  and  5   d  are schematic sectional views of a unitized cell assembly before and after a bonding process, respectively, the unitized cell assembly employing an internal hard stop arrangement and an in-situ formed thermoplastic sealing gasket in accordance with another embodiment of the present invention; 
       FIGS. 5   e  and  5   f  are schematic sectional views of a unitized cell assembly before and after a bonding process, respectively, the unitized cell assembly employing an in-situ formed thermoplastic sealing gasket and excluding a hard stop arrangement in accordance with a further embodiment of the present invention; 
       FIGS. 6   a - 6   c  show a unitized cell assembly system which includes a monopolar unitized cell assembly and a separable cooling structure in accordance with an embodiment of the present invention; 
       FIG. 6   d  shows a unitized cell assembly system which includes a monopolar/bipolar unitized cell assembly and a separable cooling structure in accordance with another embodiment of the present invention; 
       FIGS. 7   a  and  7   b  illustrate a stack of unitized cell assemblies disposed within a compression system in accordance with an embodiment of the present invention; 
       FIGS. 8   a - 8   c  illustrate various sectional views of a unitized cell assembly which employs a locking or engagement capability in accordance an embodiment of the present invention; 
       FIGS. 9   a - 9   e  illustrate various views of a unitized cell assembly which incorporates an integral cooling arrangement in accordance with an embodiment of the present invention; 
       FIG. 10  is an illustrative depiction of a simplified fuel cell stack that facilitates an understanding of the manner in which fuels pass into and out of a stack of fuel cells, wherein the fuel cells are preferably configured as unitized cell assemblies in accordance with the principles of the present invention; and 
       FIG. 11  illustrates a fuel cell system within which one or more fuel cell stacks employing unitized cell assemblies of the present invention can be employed. 
   

   While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It is to be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
   In the following description of the illustrated embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration, various embodiments in which the invention may be practiced. It is to be understood that the embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
   The present invention is directed to an improved fuel cell assembly. Various embodiments of the present invention are directed to a unitized fuel cell assembly which provides for ease of fuel cell assembling and disassembling. A unitized fuel cell package implemented in accordance with the present invention can further provide for recycling of fuel cells configured for arrangement in a stack during fabrication, repair, and maintenance of individual fuel cells and the fuel cell stack. 
   Certain embodiments are directed to a unitized fuel cell assembly implemented in a monopolar or bipolar configuration. In other embodiments, a unitized fuel cell is provided with a thermal management arrangement. In such embodiments, the thermal management arrangement can be implemented integral to a unitized fuel cell assembly or as a structure separate from the unitized fuel cell assembly. Further embodiments of the present invention are directed to fuel cell stacks and systems implemented using unitized fuel cell assemblies. 
   A fuel cell is an electrochemical device that combines hydrogen fuel and oxygen from the air to produce electricity, heat, and water. Fuel cells do not utilize combustion, and as such, fuel cells produce little if any hazardous effluents. Fuel cells convert hydrogen fuel and oxygen directly into electricity, and can be operated at much higher efficiencies than internal combustion electric generators, for example. 
   A typical fuel cell is depicted in  FIG. 1   a . The fuel cell  10  shown in  FIG. 1  includes a first fluid transport layer (FTL)  12  adjacent an anode  14 . Adjacent the anode  14  is an electrolyte membrane  16 . A cathode  18  is situated adjacent the electrolyte membrane  16 , and a second fluid transport layer  19  is situated adjacent the cathode  18 . In operation, hydrogen fuel is introduced into the anode portion of the fuel cell  10 , passing through the first fluid transport layer  12  and over the anode  14 . At the anode  14 , the hydrogen fuel is separated into hydrogen ions (H) and electrons (e). 
   The electrolyte membrane  16  permits only the hydrogen ions or protons to pass through the electrolyte membrane  16  to the cathode portion of the fuel cell  10 . The electrons cannot pass through the electrolyte membrane  16  and, instead, flow through an external electrical circuit in the form of electric current. This current can power an electric load  17 , such as an electric motor, or be directed to an energy storage device, such as a rechargeable battery. 
   Oxygen flows into the cathode side of the fuel cell  10  via the second fluid transport layer  19 . As the oxygen passes over the cathode  18 , oxygen, protons, and electrons combine to produce water and heat. 
   Individual fuel cells, such as that shown in  FIG. 1   a , can be packaged as unitized fuel cell assemblies as will be described in detail hereinbelow. The unitized fuel cell assemblies, referred to herein as unitized cell assemblies or UCAs for convenience, can be combined with a number of other UCAs to form a fuel cell stack. The number of UCAs within the stack determines the total voltage of the stack, and the active surface area of each of the cells determines the total current. The total electrical power generated by a given fuel cell stack can be determined by multiplying the total stack voltage by total current. 
   A number of different fuel cell technologies can be employed to construct UCAs in accordance with the principles of the present invention. For example, a UCA packaging methodology of the present invention can be employed to construct proton exchange membrane (PEM) fuel cell assemblies. PEM fuel cells operate at relatively low temperatures (about 175° F./80° C.), have high power density, can vary their output quickly to meet shifts in power demand, and are well suited for applications where quick startup is required, such as in automobiles for example. 
   The proton exchange membrane used in a PEM fuel cell is typically a thin plastic sheet that allows hydrogen ions to pass through it. The membrane is typically coated on both sides with highly dispersed metal or metal alloy particles (e.g., platinum or platinum/ruthenium) that are active catalysts. The electrolyte used is typically a solid organic polymer such as poly-perfluorosulfonic acid. Use of a solid electrolyte is advantageous because it reduces corrosion and management problems. 
   Hydrogen is fed to the anode side of the fuel cell where the catalyst promotes the hydrogen atoms to release electrons and become hydrogen ions (protons). The electrons travel in the form of an electric current that can be utilized before it returns to the cathode side of the fuel cell where oxygen has been introduced. At the same time, the protons diffuse through the membrane to the cathode, where the hydrogen ions are recombined and reacted with oxygen to produce water. 
   A membrane electrode assembly (MEA) is the central element of PEM fuel cells, such as hydrogen fuel cells. As discussed above, typical MEAs comprise a polymer electrolyte membrane (PEM) (also known as an ion conductive membrane (ICM)), which functions as a solid electrolyte. 
   One face of the PEM is in contact with an anode electrode layer and the opposite face is in contact with a cathode electrode layer. Each electrode layer includes electrochemical catalysts, typically including platinum metal. Fluid transport layers (FTLs) facilitate gas transport to and from the anode and cathode electrode materials and conduct electrical current. 
   In a typical PEM fuel cell, protons are formed at the anode via hydrogen oxidation and transported to the cathode to react with oxygen, allowing electrical current to flow in an external circuit connecting the electrodes. The FTL may also be called a gas diffusion layer (GDL) or a diffuser/current collector (DCC). The anode and cathode electrode layers may be applied to the PEM or to the FTL during manufacture, so long as they are disposed between PEM and FTL in the completed MEA. 
   Any suitable PEM may be used in the practice of the present invention. The PEM typically has a thickness of less than 50 μm, more typically less than 40 μm, more typically less than 30 μm, and most typically about 25 μm. The PEM is typically comprised of a polymer electrolyte that is an acid-functional fluoropolymer, such as Nafion® (DuPont Chemicals, Wilmington DE) and Flemion® (Asahi Glass Co. Ltd., Tokyo, Japan). The polymer electrolytes useful in the present invention are typically preferably copolymers of tetrafluoroethylene and one or more fluorinated, acid-functional comonomers. 
   Typically, the polymer electrolyte bears sulfonate functional groups. Most typically, the polymer electrolyte is Nafion®. The polymer electrolyte typically has an acid equivalent weight of 1200 or less, more typically 1100 or less, more typically 1050 or less, and most typically about 1000. 
   Any suitable FTL may be used in the practice of the present invention. Typically, the FTL is comprised of sheet material comprising carbon fibers. The FTL is typically a carbon fiber construction selected from woven and non-woven carbon fiber constructions. Carbon fiber constructions which may be useful in the practice of the present invention may include: Toray Carbon Paper, SpectraCarb Carbon Paper, AFN non-woven carbon cloth, Zoltek Carbon Cloth, and the like. The FTL may be coated or impregnated with various materials, including carbon particle coatings, hydrophilizing treatments, and hydrophobizing treatments such as coating with polytetrafluoroethylene (PTFE). 
   Any suitable catalyst may be used in the practice of the present invention. Typically, carbon-supported catalyst particles are used. Typical carbon-supported catalyst particles are 50-90% carbon and 10-50% catalyst metal by weight, the catalyst metal typically comprising Pt for the cathode and Pt and Ru in a weight ratio of 2:1 for the anode. The catalyst is typically applied to the PEM or to the FTL in the form of a catalyst ink. The catalyst ink typically comprises polymer electrolyte material, which may or may not be the same polymer electrolyte material which comprises the PEM. 
   The catalyst ink typically comprises a dispersion of catalyst particles in a dispersion of the polymer electrolyte. The ink typically contains 5-30% solids (i.e. polymer and catalyst) and more typically 10-20% solids. The electrolyte dispersion is typically an aqueous dispersion, which may additionally contain alcohols, polyalcohols, such a glycerin and ethylene glycol, or other solvents such as N-methylpyrilidon (NMP) and dimethylformaldehyde (DMF). The water, alcohol, and polyalcohol content may be adjusted to alter rheological properties of the ink. The ink typically contains 0-50% alcohol and 0-20% polyalcohol. In addition, the ink may contain 0-2% of a suitable dispersant. The ink is typically made by stirring with heat followed by dilution to a coatable consistency. 
   The catalyst may be applied to the PEM or the FTL by any suitable means, including both hand and machine methods, including hand brushing, notch bar coating, fluid bearing die coating, wire-wound rod coating, fluid bearing coating, slot-fed knife coating, three-roll coating, or decal transfer. Coating may be achieved in one application or in multiple applications. 
   Direct methanol fuel cells (DMFC) are similar to PEM cells in that they both use a polymer membrane as the electrolyte. In a DMFC, however, the anode catalyst itself draws the hydrogen from liquid methanol fuel, eliminating the need for a fuel reformer. DMFCs typically operate at a temperature between 120-190° F./49-88° C. A direct methanol fuel cell can be subject to UCA packaging in accordance with the principles of the present invention. 
   Referring now to  FIG. 1   b , there is illustrated an embodiment of a UCA implemented in accordance with a PEM fuel cell technology. As is shown in  FIG. 1   b , a membrane electrode assembly (MEA)  25  of the UCA  20  includes five component layers. A PEM layer  22  is sandwiched between a pair of fluid transport layers  24  and  26 , such as diffuse current collectors (DCCs) or gas diffusion layers (GDLs) for example. An anode  30  is situated between a first FTL  24  and the membrane  22 , and a cathode  32  is situated between the membrane  22  and a second FTL  26 . 
   In one configuration, a PEM layer  22  is fabricated to include an anode catalyst coating  30  on one surface and a cathode catalyst coating  32  on the other surface. This structure is often referred to as a catalyst-coated membrane or CCM. According to another configuration, the first and second FTLs  24 ,  26  are fabricated to include an anode and cathode catalyst coating  30 ,  32 , respectively. In yet another configuration, an anode catalyst coating  30  can be disposed partially on the first FTL  24  and partially on one surface of the PEM  22 , and a cathode catalyst coating  32  can be disposed partially on the second FTL  26  and partially on the other surface of the PEM  22 . 
   The FTLs  24 ,  26  are typically fabricated from a carbon fiber paper or non-woven material or woven cloth. Depending on the product construction, the FTLs  24 ,  26  can have carbon particle coatings on one side. The FTLs  24 ,  26 , as discussed above, can be fabricated to include or exclude a catalyst coating. 
   In the particular embodiment shown in  FIG. 1   b , MEA  25  is shown sandwiched between a first edge seal system  34  and a second edge seal system  36 . Adjacent the first and second edge seal systems  34  and  36  are flow field plates  40  and  42 , respectively. Each of the flow field plates  40 ,  42  includes a field of gas flow channels  43  and ports through which hydrogen and oxygen feed fuels pass. In the configuration depicted in  FIG. 1   b , flow field plates  40 ,  42  are configured as monopolar flow field plates, in which a single MEA  25  is sandwiched there between. The flow field in this and other embodiments may be a low lateral flux flow field as disclosed in co-pending application Ser. No. 09/954,601, filed Sep. 17, 2001, and incorporated herein by reference. 
   The edge seal systems  34 ,  36  provide the necessary sealing within the UCA package to isolate the various fluid (gas/liquid) transport and reaction regions from contaminating one another and from inappropriately exiting the UCA  20 , and may further provide for electrical isolation and hard stop compression control between the flow field plates  40 ,  42 . The term “hard stop” as used herein generally refers to a nearly or substantially incompressible material that does not significantly change in thickness under operating pressures and temperatures. More particularly, the term “hard stop” refers to a substantially incompressible member or layer in an membrane electrode assembly (MEA) which halts compression of the MEA at a fixed thickness or strain. A “hard stop” as referred to herein is not intended to mean an ion conducting membrane layer, a catalyst layer, or a gas diffusion layer. 
   In one configuration, the edge seal systems  34 ,  36  include a gasket system formed from an elastomeric material. In other configurations, as will be described below, one, two or more layers of various selected materials can be employed to provide the requisite sealing within UCA  20 . Other configurations employ an in-situ formed seal system. 
   In certain embodiments, the gasket may be a closed-cell foam rubber gasket as disclosed in co-pending application Ser. No. 10/294,098, filed Nov.14, 2002 incorporated herein by reference. In other embodiments, the gasket may be formed with a contact face having a raised-ridge microstructured sealing pattern as disclosed in co-pending application Ser. No. 10/143,273, filed May 10, 2002, and incorporated herein by reference. 
     FIG. 1   c  illustrates a UCA  50  which incorporates multiple MEAs  25  through employment of one or more bipolar flow field plates  56 . In the configuration shown in  FIG. 1   c , UCA  50  incorporates two MEAs  25   a  and  25   b  and a single bipolar flow field plate  56 . MEA  25   a  includes a cathode  62   a /membrane  61   a /anode  60   a  layered structure sandwiched between FTLs  66   a  and  64   a . FTL  66   a  is situated adjacent a flow field end plate  52 , which is configured as a monopolar flow field plate. FTL  64   a  is situated adjacent a first flow field surface  56   a  of bipolar flow field plate  56 . 
   Similarly, MEA  25   b  includes a cathode  62   b /membrane  61   b /anode  60   b  layered structure sandwiched between FTLs  66   b  and  64   b . FTL  64   b  is situated adjacent a flow field end plate  54 , which is configured as a monopolar flow field plate. FTL  66   b  is situated adjacent a second flow field surface  56   b  of bipolar flow field plate  56 . It will be appreciated that N number of MEAs  25  and N−1 bipolar flow field plates  56  can be incorporated into a single UCA  50 . It is believed, however, that, in general, a UCA  50  incorporating one or two MEAs  56  (N=1, bipolar plates=0 or N=2, bipolar plates=1) is preferred for more efficient thermal management. 
   The UCA configurations shown in  FIGS. 1   b  and  1   c  are representative of two particular arrangements that can be implemented for use in the context of the present invention. These two arrangements are provided for illustrative purposes only, and are not intended to represent all possible configurations coming within the scope of the present invention. For example, the seal system  34  shown in  FIG. 1   b  can be replaced or supplemented with other sealing systems, such as those disclosed herein. Rather,  FIGS. 1   b  and  1   c  are intended to illustrate various components that can be selectively incorporated into a unitized fuel cell assembly packaged in accordance with the principles of the present invention. 
   By way of further example, a variety of sealing methodologies implemented in accordance with the present invention can be employed to provide the requisite sealing of a UCA comprising a single MEA disposed between a pair of monopolar flow field plates, and can also be employed to seal a UCA comprising multiple MEAs, a pair of monopolar flow field plates and one or more bipolar flow field plates. For example, a UCA having a monopolar or bipolar structure can be constructed to incorporate an in-situ formed solid gasket, such as a flat solid silicone gasket. 
   In particular embodiments, a UCA, in addition to including a sealing gasket, can incorporate a hard stop arrangement. The hard stop(s) can be built-in, disposed internal to the UCA, or integrated into the monopolar and/or bipolar flow field plates. Other features can be incorporated into a UCA, such as an excess gasket material trap channel and a micro replicated pattern provided on the flow field plates. Incorporating a hard stop into the UCA packaging advantageously limits the amount of compressive force applied to the MEA during fabrication (e.g., press forces) and during use (e.g., external stack pressure system). For example, the height of a UCA hard stop can be calculated to provide a specified amount of MEA compression, such as 30%, during UCA construction, such compression being limited to the specified amount by the hard stop. Incorporating a hard stop into the flow field plates can also act as a registration aid for the two flow field plates. 
   Moreover, a variety of UCA configurations can be implemented with a thermal management capability in accordance with other embodiments of the present invention. By way of example, a given UCA configuration can incorporate an integrated thermal management system. Alternatively, or additionally, a given UCA can be configured to mechanically couple with a separable thermal management structure, embodiments of which will be describe below. Accordingly, a fuel cell assembly of the present invention is not to be limited to a specific UCA configuration or to a particular thermal management system as described herein. 
   It is appreciated by one skilled in the art that advancements in fuel cell technology are needed in order to mass produce fuel cells and systems at marketable prices. Conventional fuel cell packaging approaches presently limit the ability to achieve high levels of fuel cell stack assembling efficiency. Moreover, current packaging and stacking approaches are presently not amenable to fuel cell component recycling, which results in wasteful scrapping of an entire fuel cell assembly once the fuel cell has been identified as a poor performer. Fuel cell recycling permits reuse of certain fuel cell assembly components once a defective fuel cell has been removed and subject to disassembly. A UCA packaging approach consistent with the principles of the present invention provides for efficient assembling and disassembling of fuel cell stacks and, further, provides for recycling of various UCA components. 
   Turning now to  FIG. 2   a , there is illustrated a cross-sectional view of a UCA in accordance with one embodiment of the present invention. According to this embodiment, UCA  80  incorporates in-situ formed flat, solid silicone gaskets and a hard stop arrangement. In the embodiment shown in  FIG. 2   a , and in other embodiments described herein, a liquefied silicone sealant can be employed. It is understood that silicone sealant material represents one of several types of materials suitable for use in the construction of a UCA in accordance with the present invention. Other sealing materials can alternatively be employed, assuming such materials exhibit appropriate elastic properties for sealing and are sufficiently durable for fuel cell environments. 
   The UCA  80  shown in  FIG. 2   a  can be constructed according to the following illustrative process. Flow field plate  84  is placed on a flat surface with the flow channels  85  facing upwardly. The flow field plate  84 , for purposes of example, is a 13 cm×13 cm plate having a 10 cm×10 cm flow channel area. It is noted that the flow field plates  84 ,  82  can be fabricated from a carbon/polymer composite material, graphite, metal or metal coated with conductive material. 
   A liquefied silicone sealant material is dispensed at a predetermined rate, such as a rate of about 0.35 mg/min, onto the surfaces of the flow field plate  84  where the gasket of the MEA will be formed. A suitable silicone material is D98-55, parts A and B, available from Dow Corning. The flow channel area  85  is covered by an 11 cm×11 cm FTL  88 . A catalyst-coated membrane (CCM)  90 , which represents a PEM coated with an anode catalyst material on one surface and a cathode catalyst material on the other surface, is placed on the lower FTL  88  with the CCM  90  center aligned to the FTL center. 
   An upper 11 cm×11 cm FTL  86  is placed on the CCM  90  with alignment of the respective centers. The FTLs  86 ,  88  are slightly larger than the CCM  90  to provide a space into which the silicone can flow and infiltrate into the porous carbon fiber of the FTLs  86 ,  88  to create an edge seal. This oversizing of the FTLs  86 ,  88  relative to the CCM  90  also prevents silicone from flowing into the flow channels  85 , which would otherwise plug up the outer flow channels. 
   As shown, a membrane  91  of CCM  90  or the entire CCM  90  extends outwardly from the MEA to a position proximate a hard stop  92 . This extended membrane or CCM portion provides for enhanced electrical isolation between the flow field plates  84  and  82 . 
   It is understood, however, that membrane  91  or CCM  90  need not extend from the MEA as is illustrated in  FIG. 2   a  and other figures. Further, is it understood that membrane  91  or CCM  90  can extend from the MEA to a position at some desired distance between the MEA and hard stop  92 . 
   An external hard stop  92  is used within the UCA  80  as a shim to control MEA compression. The hard stop  92  can be fabricated from a variety of materials, including, for example, polyethylene napthalate (PEN), polyethylene terephthalate (PET), Teflon, or other incompressible material or a combination of such materials. In the embodiment shown in  FIG. 2   a , the external hard stop  92  is fabricated from PEN and coated with Teflon to ensure non-stickiness and removability after the UCA has formed. The thickness of hard stop  92  can be selected to achieve a desired amount of MEA compression. In  FIG. 2   a , the thickness of hard stop  92  is selected to ensure 30% compression of the MEA. 
   Liquefied silicone with two parts (A and B) premixed is dispensed at a rate of about 0.35 mg/min onto the surfaces of the upper flow field plate  82  and the lower flow field plate  84  where the gaskets of the MEA will be formed. The MEA components and external hard stops  92  are sandwiched between the two flow field plates  82  and  84  with the dispensed silicone. The entire sandwich structure  80  is then placed into a press. The sandwich structure  80  is preferably subject to press conditions of 270° F. at 3 tons for 10 minutes, which results in formation of UCA  80  with in-situ formed flat, solid gaskets. During the UCA forming process, FTLs  88 ,  86  and CCM  90  are bonded to form an MEA with good interfaces. It is noted that a full 10 minutes of bonding time is typically needed if the MEA has not previously been bonded. It is further noted that the silicone material may cure after a time much shorter than 10 minutes, and that the typical press/bonding time of 10 minutes cart be reduced in cases where the subject MEA is a previously bonded MEA. 
     FIG. 2   b  illustrates another embodiment of a UCA in accordance with the principles of the present invention. In this embodiment, an internal hard stop arrangement is employed, in addition to use of an in-situ formed silicone gasket. A 13 cm×13 cm flow field plate  84  with a 10 cm×10 cm flow channel area  85  is placed on a flat surface with the flow channels facing upwardly. This UCA configuration includes a trap channel  95  provided on each of the flow field plates  82 ,  84  within the silicone gasket formation region. As shown, the trap channel  95  is located between the hard stop arrangement  93   a / 93   b  and the outer periphery of the respective flow field plates  82 ,  84 . The trap channels  95  provide a space for the excess liquefied silicone to flow into so as not to plug the flow channels. This can also provide an internal locking mechanism that enhances UCA packaging integrity, in addition to the requisite MEA sealing. 
   A liquefied silicone is dispensed at a rate of about 0.35 mg/min onto the surfaces of the flow field plate  84  where the gasket of the MEA will be formed. The amount of silicone dispensed on the plate surfaces can be reduced by about 50% of the amount calculated for  FIG. 2   a  due to the presence of the integral hard stop arrangement. 
   The hard stop arrangement of the instant embodiment includes frames  93   a  and  93   b  formed from a suitable material such as PEN, PET, polyethylene, polypropylene, polyester, fiberglass, nylon, Delrin, Lexan, Mylar, Kapton, Teflon, or the like. Blends of these materials or composite materials of these with fillers such as carbon, glass, ceramic, etc. may also be used as hard stops. It is understood that the hard stop arrangement need not be a single continuous member, but may instead be defined by a number of unconnected or loosely connected discrete hard stop elements. 
   The frames  93   a  and  93   b  shown in  FIG. 2   b  are fashioned from PEN. The PEN frame  93   b  in this embodiment has an outer dimension of 12.5 cm×12.5 cm and an 11 cm×11cm window. The frame  93   b  is placed on the flow field plate  84 , such that the frame  93   b  covers much of the liquefied silicone  94 . The thickness of the PEN frame  94  is selected to ensure 30% compression of the MEA. 
   An 11 cm×11 cm FTL  88  is placed into the inner window of the PET frame  93   b.  A CCM  90  is placed on the FTL  88 , with the CCM center aligned to the FTL center. Another PET frame  93   a  with the same dimensions as frame  93   b  is placed on the CCM  90  with centers respectively aligned. The second 11 cm×11 cm FTL  86  is placed into the window of PEN frame  93   a.    
   A liquefied silicone  94  is dispensed at a rate of about 0.35 mg/min onto the surfaces of The second flow field plate  82  where the gasket of the MEA will be formed. The second flow field plate  82  is placed on top of the flow field plate  84 /FTL  88 /CCM  9 O/FTL  86  structure, and placed into a press, preferably under press conditions of 270° F., 3 tons for 10 minutes. 
     FIG. 3   a  illustrates another embodiment in which a built-in hard stop is employed in addition to an in-situ formed silicone gasket. The basic construction of UCA  80  shown in  FIG. 3   a  is similar to that shown in  FIG. 2   b , with the exception of the hard stop configuration. In the embodiment shown in  FIG. 3   a , the hard stop feature is built into the flow field plates  82 ,  84 . As shown, each of the flow field plates  82 ,  84  has a protruding peripheral edge  82   a ,  84   a , best seen in  FIG. 3   b . The edges  82   a ,  84   a  are formed to register with one another and to provide a gap of a predetermined size between internal flow field plate surfaces sufficient to accommodate the silicone seal  94 . The heights of the protruding peripheral edges  82   a ,  84   a  are selected to provide an appropriate degree of MEA compression. 
   As shown in  FIG. 3   b , the peripheral edge  82   a  includes a protruding interface and the peripheral edge  84   a  includes a recessed interface. The protruding interface of edge  82   a  is received by the recessed interface of edge  84   a  when the two flow field plates  82 ,  84  are brought together under pressure within the press. An insulating layer  89 , such as an insulating film, is disposed between the peripheral edges  82   a ,  84   a  to provide the requisite electrical isolation between the two flow field plates  82 ,  84 . 
   In accordance with another sealing approach, surfaces of the flow field plates can be machined to include a micro replicated pattern, often referred to as a microstructured surface. Various microstructure patterns and methods of producing same are known in the art. The microstructured patterns can be machined into particular regions of the flow field plates to provide mechanical coupling between flow field plates of the UCA upon engagement of the patterns provided on opposing flow field plate surfaces. The patterns, for example, can have a ridge having a width which can vary between 5 and 25 mils, and a height that can vary between about 1.5 and 2.5 mils. 
   For example, microstructured patterns can be machined into the flow field surfaces within the gasket region to form many small semi-ridges on the surface of the gasket. Microstructured patterns can also be machined into the flow field lands. As will be discussed in greater detail below, UCA sealing can be accomplished by the combined use of microstructured patterns and polymeric gaskets (e.g., in-situ formed silicone gaskets or separate elastomeric gaskets) or by sole use of microstructured patterns or other mechanical arrangements (e.g., locator pins, screws, bolts/nuts, interlocking surface features). 
     FIGS. 4   a  and  4   b  illustrate a further embodiment of a UCA which employs an internal hard stop and in-situ formed silicone seal or gasket. In accordance with this embodiment, the UCA  100  includes an upper flow field plate  102  that represents the cathode side of the fuel cell, and a lower flow field plate  104  that represents the anode side of the fuel cell. The hard stop arrangement  110 , as best seen in  FIG. 4   b , includes a one-piece hard stop core or coil  112  positioned within a slot  114  provided in the lower flow field plate  104 . 
   The slot  114  can be pre-machined or molded in place during the plate making process. The depth of the slot  114  can be varied according to the diameter of the hard stop core  112 . A curved recess  116  is provided in the upper flow field plate  102  and has a radius matching that of the hard stop core  112 . The lower flow field plate  104  can include a trap channel  105  for accommodating excess sealant material that may flow during gasket formation. 
   The hard stop coil  112 , as with other hard stop embodiments described herein, can be formed from an incompressible material, such as PET, PEN, or Teflon. The thickness of the hard stop coil  112  typically ranges between 0.5 mm and 2.0 mm. In general, the thickness of the hard stop coil  112  should be about 70% of the MEA&#39;s thickness, which is typically about 0.012 inch thick. 
   A silicone gasket is formed by dispensing liquid silicone on top of the hard stop coil  112  prior to positioning the coil  112  within the slot  114 . The hard stop coil  112  will then sink into the slot  114  and remain orientated along the centerline of the slot  114 . This helps to maintain the same thickness of silicone layer proximate the hard stop coil  112 . The MEA  106  and upper flow field plate  102  are properly situated, and the sandwich structure  100  is placed in a press under appropriate temperature and pressure conditions for a predetermined duration of time, as discussed previously. 
   It is noted that the size of the membrane can be the same as the FTLs. Even if the catalyst were unexpectedly exposed, this would not be a problem since the silicone forms to protect against exposing the catalysts to the fuels. If it is intended that the UCA be subject to recycling, an additional release coating can be applied on the surface of the flow field plates  102 ,  104  which will come into direct contact with the silicone gasket/sealing material. As such, the MEA and seal/gasket of a failed UCA can be readily separated from the reusable flow field plates  102 ,  104 . 
   Turning now to  FIGS. 5   a - 5   f , there is illustrated a portion of a UCA which employs a sealing arrangement in accordance with another embodiment of the present invention. The embodiments depicted in  FIGS. 5   a - 5   f  incorporate a thermoplastic sealing material, which is typically dispensed in the form of a film, tape, or other solid form. The thermoplastic can be a fluoroplastic like THV (terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene difluoride); polyethylene; copolymers of polyethylene such as those of ethylene and acrylic acid; Thermo-Bond 845 (manufactured by 3M, e.g., a polyethylene maleic anhydride copolymer) and Thermo-Bond 668 (manufactured by 3M, e.g., a polyester). Blends of these materials or composite materials of these with fillers such as carbon, glass, ceramic, etc. may also be used as thermoplastics. Preferably, the melt range is 50-180° C., and more preferably 100-150° C., which should be similar to the MEA bonding temperature. The thermoplastic material should also adhere to the hard stop and flow field plate. 
   In certain UCA/MEA configurations, the thermoplastic sealing material provides for enhanced membrane edge protection, in addition to UCA sealing. Among other benefits, use of a solid thermoplastic sealing film significantly reduces or eliminates the risk of flow channel blockage that can occur with the use of liquefied sealants. Further, the FTLs can be positioned within the UCA/MEA to prevent the thermoplastic sealing film from flowing into the gas feed holes and channels. 
   One particular advantage to using a solid thermoplastic sealing film concerns the characteristic that such a sealing film melts into the FTL so there is no thickness variation on the edges of the FTL where the sealing film overlaps the FTL. Conventional methods of building MEAs, in contrast, can result in a small thickness variation on the edge of the MEA, thus producing a location that is subject to significantly more pressure then the rest of the MEA. If the MEA is subject to too much pressure along its edges, the MEA becomes prone to failure in those areas. Because a thermoplastic sealing film according to this embodiment has no effective thickness variation relative to the FTL surface once diffused therein, the MEA will absorb the pressure in the UCA stack equally across the entire MEA surface. 
   Another advantage of using a thermoplastic sealing film in accordance with this embodiment, as briefly discussed above, is membrane edge protection. In durability experiments, it has been found that a major cause of failure for MEAs is stress developing in the area between the FTL and gasket hard stop, which can cause tearing and produce massive hydrogen crossover. This crossover completely renders the MEA useless, and makes the rest of the UCA stack fail because it cannot deliver the hydrogen fuel to other MEAs in the stack. 
   This edge tearing phenomena can occur in certain MEA structures because there is often a pressure difference between the anode and cathode layers, and the membrane is weak in that area because there is little or nothing to support it. In this case, the thermoplastic sealing film melts over the membrane and provides a strong support that is not easily torn. The resulting membrane protected at the edge of the FTL has a higher potential for lifetime and reliable performance because the chances of failure on the edge are significantly reduced. 
   A further advantage of this UCA construction is membrane protection from sharp corners of the FTL. It has been seen in many cases, especially with a more rigid FTL, that sometimes the edges of the FTL can poke through the membrane and cause a short, thus causing the MEA to fail. Conventional approaches can only partially control this problem. According to the instant embodiment, a thermoplastic sealing film is preferably situated underneath the edges of the FTLs prior to bonding. The thermoplastic sealing film protects the membrane from falling victim to sharp edges during the bonding process. Employment of a thermoplastic sealing film according to this embodiment, in contrast to conventional approaches, completely eliminates FTL promoted membrane puncture from occurring. 
   The addition of a hard stop, in certain embodiments, provides the further advantage of precisely controlling compression within the UCA and stopping compression of the fuel cell when FTL compression has reached an optimal level. The thickness of the hard stop can be varied depending on the thickness of the FTL to provide an optimal compression value. 
     FIG. 5   a  illustrates an edge portion of a UCA prior to being placed in a bonding press, while  FIG. 5   b  illustrates the UCA portion of  FIG. 5   a  after completion of the bonding process. The UCA  200  includes first and second flow field plates  202 ,  204 , each having a feed gas port  206 ,  210  and a number of gas flow channels  208 ,  212 . Provided between the first and second flow field plates  202 ,  204  is an MEA  212 . MEA  212  is shown to include a pair of FTLs  220 ,  222  between which a CCM  214  is situated. CCM  214  includes cathode and anode catalyst layers  224 ,  226  and a membrane  228  which extends outwardly from the MEA  212 . As shown, the membrane  228  extends beyond the edges of the FTLs  220 ,  222  and terminates proximate the edges of the first and second flow field plates  202 ,  204 . 
   The UCA  200  incorporates a hard stop arrangement which includes first and second hard stop frames  234 ,  240  situated between the membrane  228  and respective first and second flow field plates  202 ,  204 . The hard stop frames  234 ,  240  can be fabricated from a suitable hard stop material, such as PET, PEN, Teflon or the like. The first hard stop frame  234  is retained in position on the membrane  228  by use of an adhesive film or layer  236  provided between the first hard stop frame  234  and membrane  228 . Similarly, the second hard stop frame  240  is retained in position on the membrane  228  by use of an adhesive film or layer  238  provided between the second hard stop frame  240  and membrane  228 . 
   A first end of a first thermoplastic film  230  is situated between the first FTL  220  and a portion of the membrane  228  that extends beyond the cathode catalyst layer  224 . The first thermoplastic film  230  passes within a gap  211   a  formed between the end of the first FTL  220  and the first hard stop frame  234 . In the configuration of  FIG. 5   a , the first thermoplastic film  230  is shown situated between the first hard stop frame  234  and the first flow field plate  202 . A second end of the first thermoplastic film  230  terminates at the edge of the first flow field plate  202 . 
   A first end of a second thermoplastic film  232  is situated between the second FTL  222  and a portion of the membrane  228  that extends beyond the anode catalyst layer  226 . The second thermoplastic film  232  passes within a gap  211   b  formed between the end of the second FTL  222  and the second hard stop frame  240 . In the configuration of  FIG. 5   a , the second thermoplastic film  232  is shown situated between the second hard stop frame  240  and the second flow field plate  204 . A second end of the second thermoplastic film  232  terminates at the edge of the second flow field plate  204 . 
   Each of the thermoplastic films  230 ,  232  is typically about 2.5 mils in thickness, and a single hard stop frame  234 ,  240  is typically about 5 mils in thickness. In a UCA configuration in which a single hard stop frame is employed, such as in the embodiment shown in  FIGS. 5   c - 5   d , the hard stop frame  235  in this case is about 10 mils in thickness. It is noted that the FTLs  220 ,  222  are typically about 8 mils in thickness. It will be appreciated that these dimensions will vary depending on a particular UCA design. 
   In accordance with another configuration, a first thermoplastic film  230  is situated between the first FTL  220  and the portion of the membrane  228  that extends beyond the cathode catalyst layer  224 . The first thermoplastic film  230 , according to this configuration, passes within the gap  211   a  and terminates at the edge of the first hard stop frame  234 . An adhesive film or layer, similar to that of layer  236 , is provided between the first hard stop frame  234  and the first flow field plate  202 . 
   According to this embodiment, a second thermoplastic film  232  is situated between the second FTL  222  and the portion of the membrane  228  that extends beyond the anode catalyst layer  226 . The second thermoplastic film  232 , according to this configuration, passes within the gap  211   b  and terminates at the edge of the second hard stop frame  240 . An adhesive film or layer, similar to that of layer  238 , is provided between the second hard stop frame  240  and the second flow field plate  204 . 
     FIG. 5   b  illustrates an edge protected UCA after completion of the bonding process. As can be seen in  FIG. 5   b , the various components of UCA  200  are held together by the melted thermoplastic films  230 ,  232 . The thermoplastic material has impregnated the FTLs  220 ,  222 , but has not seeped into the feed gas ports  206 ,  210 . Moreover, strategic placement of the FTLs  220 ,  222  relative to the gas channels  208 ,  212  prevents gas channel blockage from developing during the bonding process. The membrane  228  has a healthy layer of melted thermoplastic material surrounding it at the vulnerable edge between the first and second stop frames  234 ,  240  and the first and second FTLs  220 ,  222 . 
     FIGS. 5   c  and  5   d  illustrate another embodiment of a UCA packaging configuration which incorporates a thermoplastic sealant material in combination with a hard stop arrangement.  FIG. 5   c  illustrates an edge portion of a UCA prior to being placed in a bonding press, while  FIG. 5   d  illustrates the UCA portion of  FIG. 5   c  after completion of the bonding process. According to this embodiment, MEA  212  includes a membrane  228  that terminates at or near the edge of the first and second FTLs  220 ,  222 . As in the embodiment of  FIGS. 5   a  and  5   b , first and second thermoplastic films  230 ,  232  in  FIGS. 5   c  and  5   d  are situated between the membrane  228  that extends beyond the catalyst layers  224 ,  226  and the first and second FTLs  220 ,  222 , respectively. 
   The first and second thermoplastic films  230 ,  232  respectively pass within the gap  211  formed between the end of the first and second FTLs  220 ,  222  and a hard stop frame  235 . Because the membrane  228  does not extend beyond the hard stop arrangement, a single hard stop frame  235  can be employed. It is noted that the membrane  228  can extend into the gap  211  and to the hard stop frame  235  to provide for enhanced electrical isolation between the first and second flow field plates  202 ,  204 . 
   The first and second thermoplastic films  230 ,  232  are shown situated between the hard stop frame  235  and the first and second flow field plates  202 ,  204 , respectively. As in the configuration shown in  FIGS. 5a and 5b , the first and second thermoplastic films  230 ,  232  can extend to, and terminate at, the hard stop frame  235 , in which case an adhesive film or layer can be disposed between the hard stop frame  235  and the first and second flow field plates  202 ,  204 , respectively. 
     FIGS. 5   e  and  5   f  illustrate a further embodiment of a UCA which incorporates a thermoplastic sealant material in accordance with the present invention. In this embodiment, a hard stop arrangement is not employed, as is the case in the embodiments shown in  FIGS. 5   a - 5   d .  FIG. 5   e  illustrates an edge portion of a UCA  200  prior to being placed in a bonding press, while  FIG. 5   f  illustrates the UCA portion of  FIG. 5   e  after completion of the bonding process. 
   According to this embodiment, MEA  212  includes a membrane  228  that extends outwardly from the MEA  212  and terminates at or near the edge of the respective first and second flow field plates  202 ,  204 . A first thermoplastic film  230  is situated between the first FTL  220  and the portion of the membrane  228  that extends beyond the cathode catalyst layer  224 . The first thermoplastic film  230 , according to this configuration, is situated on the extended portion of the membrane  228  and terminates at the edge of the first flow field plate  202 . 
   A second thermoplastic film  232 , according to this embodiment, is situated between the second FTL  222  and the portion of the membrane  228  that extends beyond the anode catalyst layer  226 . The second thermoplastic film  232  is situated on the extended portion of the membrane  228  and terminates at the edge of the second flow field plate  202 . It is understood that the membrane  228  need not extend beyond the edge of the MEA  212  or all the way to the edge of the respective first and second flow field plates  202 ,  204 . 
   Moving now to another aspect of the present invention, further embodiments are directed to a UCA assembly provided with a thermal management feature. In certain embodiments, the thermal management feature includes a cooling structure that is separable with respect to the UCA. In other embodiments, the thermal management feature includes a cooling structure that is integrally incorporated into the UCA package. In further embodiments, the UCA cooling structure, which can be integral or separable with respect to the UCA, is implemented to facilitate efficient assembling and disassembling of a stack of UCAs. 
   In accordance with other embodiments, various locking/engagement arrangements are employed to facilitate easy insertion and removal of UCAs assembled in UCA stacks. In further embodiments, various locking/engagement arrangements are employed to facilitate easy insertion and removal of an MEA with respect to a pair of flow field plates. These and other features will now be described in greater detail. 
   In general, a fuel cell stack comprising flow field plates, MEAs, and cooling structures is generally assembled by carefully aligning all components, and pressing these components together so each fuel cell is subjected to a specific amount of compression. Conventional fuel cell stack building utilizes a tie rod approach with fixed holes that pass inside the flow field plates to compress the stack. Should a cell fail, the faulty cell would need to be removed and possibly replaced in order for the stack or module to continue operation. 
   With conventional fuel cell stack assemblies, the process of removing or replacing a bad cell or a bad section of the stack is complex and time consuming. In order to remove one defective cell from a fuel stack assembled using conventional approaches, for example, the entire stack has to be taken apart and subsequently completely rebuilt. This involves removing all tie rods and each cell, followed by rebuilding of the whole stack after removing the failed cell from the stack. 
   A fuel cell stacking approach consistent with the principles of the present invention provides for efficient removal and replacement of defective cells within a stack assembly, which advantageously reduces the complexity and time expenditure associated with stack dissembling and reassembling. Further, a fuel cell stacking approach of the present invention provides for enhanced recycling of fuel cell components, thus allowing for reuse of certain fuel cell components (e.g., flow field plates, hard stop components, elastomeric seals, cooling components, etc.) of a defective fuel cell assembly removed from the stack. 
   In accordance with one thermal management configuration, and as illustrated in  FIGS. 6   a - 6   c , a UCA assembly  300  is shown as including a UCA  302  and a separable cooling plate  304 . UCA  302  in this embodiment is configured to have a rectangular or square block shape, it being understood that other shapes and configurations are possible. The cooling plate  304  includes a recessed surface  308  which is dimensioned to receive the UCA  302 . One or more surfaces, such as a back surface and/or a side surface(s) of cooling plate  304 , are provided with a cooling arrangement  306 , such as cooling channels or fins. A fluid heat transfer medium, such as air, water, or other gaseous or fluidic coolant, can be passed through or over the cooling arrangement  306  to control the temperature of the UCA  302  (i.e., heating and/or cooling or UCA  302 ). 
   As can be seen in  FIGS. 6   b  and  6   c , a first UCA  302  can be fit into the recess  308  provided on a first surface of the cooling plate  304 . In addition, a second surface of the cooling plate  304 , such as the surface which includes the cooling arrangement  306 , can include a recess  307  which is dimensioned to receive a second UCA  302 . In this manner, a single cooling plate  304  can be used to provide cooling and aligned engagement with two UCAs  302 . 
   Accordingly, the stack of UCA assemblies  300  can be interlocked by use of the recessed fit relationship between UCAs  302  and cooling plates  304 . It is noted that, in an alternative configuration, a recess can be provided on one or more surfaces of the UCAs  302 , and that the cooling plates  304  can be configured to include one or more protruding surfaces that fit into the associated recessed surfaces of the UCAs  302 . 
   The recessed fit as between the UCA  302  and cooling plate  304  provides for both ease of alignment and ease of insertion/removal there between. According to this implementation, and as best seen in  FIGS. 7   a  and  7   b , a stack of UCA assemblies  300  (i.e., UCAs  302  with associated cooling plates  304 ) can be compressed using a compression apparatus  320  in which the tie rods  326  are situated completely outside of the UCA assemblies  300 . With this design, no special alignment is needed for stack assembly. Removing or replacing a particular cell within the stack system requires significantly less work than conventional approaches that use tie rods that pass through the flow field plates. 
   The compression apparatus  320  shown in  FIGS. 7   a  and  7   b  includes a pair of end plates  322 ,  324  between which a number of tie rods  326  extend. The fuel and coolant manifolds and alignment pins are not shown for simplicity of explanation. Initially, each UCA  302  is placed within the recess  308  of its associated cooling plate  304 , the combination of which defines a UCA assembly  300  within the context of this embodiment. The tie rods  326  are threaded into holes provided within the respective end plates  322 ,  324 . 
   As shown, one of the tie rods  326   a  can initially be left uninstalled in order to facilitate insertion of the UCA assemblies  300  into the compression apparatus  320 . After all UCA assemblies  300  are inserted, nuts  325  are threaded onto associated tie rods  326  and tightened to generate an appropriate amount of stack compression (e.g., about 150 psi). A torque wrench can be used to tighten the nuts  325  by the desired amount. It can be seen that the recess fit as between UCAs  302  and cooling plates  304  precisely aligns the UCA  302  within the stack and prevents the UCAs  302  from slipping out during stack assembling and disassembling. 
   As shown in  FIGS. 7   a  and  7   b , a single tie rod  326   a  can be removed to facilitate removal of a defective UCA  302  from the stack. As is illustrated, a tie rod  326   a  is removed and all other tie rods  326  are loosened. The failed UCA  302  is removed. A replacement UCA  302  can then be inserted into the cooling plate recess from which the failed UCA  302  was removed. Alternatively, the cooling plate  304  associated with the removed UCA  302  can itself be removed from the stack, resulting in one less UCA assembly  300  within the stack. The previously removed tie rod  326   a  is replaced and all tie rods  326  are retightened by the appropriate amount. 
   With a recess fit design according to this embodiment, the fuel cell stack need only be loosened and the bad cells (UCAs) removed and either replaced or retightened with one less cell (UCA) in the stack. The recess fit design advantageously provides precise alignment of all the cells (UCAs) in a module so they are in the exact same position. The cells (UCAs) are not permitted to shift or slide around, which can otherwise create high compression gradients or bad seals. Since the tie rod holes are no longer located within the flow field plates, the flow field plates are less complicated and costly to produce. In addition, there are fewer seals to be made because there are no tie rod holes that require sealing on both sides of each flow field plate. By reducing the number of seals, a corresponding reduction in crossovers and leaks can be achieved. 
     FIG. 6   d  illustrates another embodiment of a recyclable UCA assembly in accordance with the principles of the present invention. As in the embodiment illustrated in  FIGS. 6   a - 6   c , the UCA configuration shown in  FIG. 6   d  employs separable cooling plates  404   a ,  404   b  and a similar recessed fit interlocking mechanism. The UCA assembly design shown in  FIG. 6   d  can similarly be employed to construct a fuel cell stack that employs a compression apparatus as shown in  FIGS. 7   a  and  7   b.    
   The UCA assembly illustrated in  FIG. 6   d  includes a bipolar UCA  402  which effectively incorporates two UCAs  402   a ,  402   b . The bipolar UCA  402  is situated between a pair of cooling plates  404   a ,  404   b . The UCA  402  includes a first monopolar flow field plate  410 , a bipolar flow field plate  414 , and a second monopolar flow field plate  420 . A first MEA  412  is situated between the first flow field plate  410  and the biplolar flow field plate  414 , and a second MEA  416  is situated between the second flow field plate  420  and the biplolar flow field plate  414 . Cooling of the first MEA  412  is provided primarily by cooling plate  404   a , and the second MEA  416  is cooled primarily by cooling plate  404   b.    
     FIG. 6   d  illustrates a UCA packing configuration in which various components of the UCA can be recycled in cases where a defective MEA is identified. Assuming that the UCA&#39;s sealing gasket arrangement is provided by a removable elastomeric seal arrangement or a thermoplastic seal arrangement as previously described, a defective UCA can be removed from the cell stack and subject to disassembly. In one approach, a defective bipolar UCA assembly  402  as shown in  FIG. 6   d , for example, can be removed from its associated cooling plates  404   a ,  404   b  and replaced by an operable bipolar UCA assembly  402  in a manner described previously with respect to a monopolar UCA implementation. 
   According to another approach, a defective bipolar UCA assembly can be removed and further disassembled to remove each of the two MEAs from the bipolar UCA package. For example, the bipolar UCA assembly can be heated to soften or re-flow the in-situ formed thermoplastic seal arrangement. The flow field plates  410 ,  414 , and  420  can then be separated to expose the two MEAs  412 ,  416 . The defective MEA or MEAs can then be removed. The flow field plates  410 ,  414 ,  420  can then be cleaned and prepared for reuse. As mentioned previously, a release coating can be applied to the surfaces of the flow field plates where the in-situ gasket is to be formed to facilitate easy disassembling of the UCA components. 
   Referring now to  FIGS. 8   a - 8   c , there is illustrated an embodiment of a UCA assembly which employs a locking or engagement capability in accordance with the present invention. The UCA  500  includes a first flow field plate  502  and a second flow field plate  504 . The first flow field plate  502  further includes a recessed surface  512  dimensioned to receive an MEA. The second flow field plate  504  also includes a recessed surface  514  dimensioned to receive an MEA. 
   The first flow field plate  502  incorporates a cooling arrangement  510  which, in this particular embodiment, is integral to the first flow field plate  502 . The cooling arrangement  510  can, for example, include cooling channels, fins, or other structures that facilitate the transport of a thermal transfer medium over or through the rear surface of the first flow field plate  502 . 
   The UCA  500  shown in  FIGS. 8   a - 8   c  incorporates a locking or engagement arrangement  506  that facilitates precise alignment and easy assembling of UCAs when constructing a stack  501  of UCAs. In the embodiment shown in  FIGS. 8   a - 8   c , the locking arrangement  506  includes mechanical locking structures  520  and  524  provided at opposing ends of the first and second flow field plates  502  and  504 , respectively. The locking structure  524  includes a protruding surface of the second flow field plate  504 , preferably located near the peripheral edge of the second flow field plate  504 . The locking structure  520  includes a recessed surface of the first flow field plate  502 , also preferably located near the peripheral edge of the first flow field plate  502 . 
   The respective locking structures  520 ,  524  provide for an aligned recessed fit between the first and second flow field plates  502  and  504 , and between the assembled UCA  500  and an adjacent UCA equipped with respective locking structures  520 ,  524 . It is understood that the locking structures  520 ,  504  are electrically isolated from one another by use of a suitable insulating material. 
   This mechanical locking arrangement provides for easy assembling and disassembling of a stack  501  of UCAs  500 , as is shown in  FIG. 8   c . It will be appreciated that other recessed and protruding surface configurations can be employed to implement a mechanical locking capability according to this embodiment of the present invention. For example, first flow field plate  502  can incorporate a recessed surface which receives a protruding surface provided on the second flow field plate  504 . 
   Other mechanical locking arrangements can be employed to permit mechanical coupling and decoupling of opposing flow field plates of a UCA. Such arrangements provided with the flow plates include the use of locator pins, hook and loop material, microstructured patterns, screws, bolts, snap-together coupling features, and other types of mechanical fasteners. 
   Turning now to  FIGS. 9   a - 9   e , there is illustrated a UCA assembly which incorporates an integral cooling arrangement in accordance with an embodiment of the present invention. This embodiment includes a number of advantageous features, including registration and alignment features, mechanical locking structures, and an integral cooling arrangement, among other features. Some or all of these features can be incorporated into a UCA assembly in accordance with the present invention. 
     FIG. 9   a  shows two plates  602 ,  604  that, together with an MEA situated there between (not shown), define a UCA  600 . Plate  602  includes a first surface  606 , which incorporates an integral cooling arrangement  630 , and a second surface  608 , which includes a flow field. The first surface  606  of plate  602  is shown in  FIG. 9   e  and the second surface  608  of plate  602  is shown in  FIG. 9   b . Plate  604  shown in  FIG. 9   a  incorporates a flow field  650  on a first surface  610  and has a smooth region on a second surface  612 . The first surface  610  of plate  604  is shown in  FIG. 9   c , and the second surface  612  is shown in  FIG. 9   d.    
   Plates  602  and  604  are provided with locking structures that facilitate a recessed fit as between repeating pairs of plates  602 ,  604  that define the UCAs of a fuel cell stack. As best shown in  FIG. 9   a , the first and second plates  602 ,  604  matingly engage one another in the orientation shown, such that the second surface  608  of plate  602  matingly engages the first surface  610  of plate  604 . Recessed and protruding surfaces provided along the edges of the first and second plates  602 ,  604  define the primary engagement or locking structures of the UCA  600 . When brought together, these surfaces engage to provide a mechanically sound recessed fit. 
   After two such UCAs  600  are assembled, the two UCAs  600  are mechanically coupled to one another vis-à-vis the recessed fit between the first surface  606  of plate  602  of a first UCA  600  and the second surface  612  of the second plate  604  of a second UCA  600 . In this manner, any number of UCAs  600  can be assembled to construct a given fuel cell stack. 
   In addition to providing registration, alignment, and interlocking capabilities, various recessed, smooth, and protruding surfaces of UCA plates  602  and  604  are configured to facilitate sealing of the various regions of the UCA, such as the cooling region  620 , fuel manifold regions  622 ,  624 ,  628 , and  626 , and peripheral edge regions of the UCA. 
   The first surface  606  of plate  602 , as shown in  FIGS. 9   a  and  9   e , includes a cooling region  620  within which a coolant dispersion field  630  is provided between coolant manifold ports  634  and  632 . Fuel inlet and outlet ports  690 ,  688 ,  682 , and  692  are defined within fuel manifold regions  622 ,  624 ,  628 , and  626 , respectively. The fuel manifold regions  622 ,  624 ,  628 ,  626  and cooling region  620  are raised surfaces relative to the base surface of plate  606 . These raised surfaces are configured to be received by corresponding recessed and/or smooth surfaces provided on the second surface  612  of plate  604 . 
   In particular, the raised cooling region  620  provided on surface  606  of plate  602  of a first UCA  600  is configured to engage, and establish a seal, with smooth surface  700  provided on the second surface  612  of plate  604  of a second UCA  600 . Raised fuel manifold regions  622 ,  624 ,  628 , and  626  provided on the first surface  606  of plate  602  of a first UCA  600  are configured to engage, and establish a seal, with smooth surfaces proximate fuel manifold regions  652 ,  662 ,  660 , and  658  provided on the second surface  612  of plate  604  of a second adjacent UCA  600 . Fuel ports  635  and  637  provided in fuel manifold regions  622  and  626  allow for the passage of fuels to pass through the flow fields of the UCA  600 . 
   The second surface  608  of the first plate  602  includes a flow field  680 , as is shown in  FIG. 9   b . The flow field  680  includes a fuel inlet  684  and a fuel outlet  686 . The fuel inlet  684  is fluidly coupled to fuel port  635  provided on the first surface  606  of plate  602 , as can be seen in  FIG. 9   e . The fuel outlet  686  is fluidly coupled to fuel port  637  provided on the first surface  606  of plate  602 . 
   In a similar manner, the first surface  610  of the second plate  604  includes a flow field  650 , as is shown in  FIG. 9   c . The flow field  650  includes a fuel inlet  656  and a fuel outlet  654 . The fuel inlet  656  is fluidly coupled to fuel port  701  provided on the second surface  612  of plate  604 , as can be seen in  FIG. 9   d . The fuel outlet  654  is fluidly coupled to fuel port  703  provided on the second surface  612  of plate  604 . 
   During construction of the UCA  600 , an MEA is properly positioned on one of the flow fields  680 ,  650  of the first or second plates  602 ,  604 . One or more hard stop frames can also be positioned on the plate  602  or  604 . A preformed seal or an in-situ formed seal (e.g., liquefied silicone or thermoplastic seal) can be provided in a manner previously discussed. 
   Various other mechanical coupling approaches may alternatively or additionally be employed, such as locator pins, hook and loop material, microstructured patterns, screws, bolts, snap-together coupling features, and other types of mechanical fasteners as previously discussed. The plates  602 ,  604  can be machined or formed from a metal, carbon or a composite material, such as a conductive graphite or carbon/polymer composite material, for example. 
   After construction, the UCA  600  can be arranged with other such UCAs  600  during fuel cell stack assembly. As in other embodiments, the UCA  600  shown in  FIGS. 9   a - 9   e  can be subject to recycling in a manner previously discussed should the UCA  600  operate poorly. 
     FIG. 10  is a depiction of a simplified fuel cell stack that facilitates an understanding of the manner in which fuels pass into and out of the stack. It is understood that several UCAs having a construction described hereinabove are intended to be employed in a stack of the type generally depicted in  FIG. 10 , and that the particular components and configuration of the stack shown in  FIG. 10  are provided for illustrative purposes only. Those skilled in the art will readily appreciate that a fuel cell stack of the type contemplated in the instant application can be assembled using UCAs constructed in accordance with the principles of the present invention. 
   The fuel cell stack  800  shown in  FIG. 10  includes a first end plate  802  and a second end plate  804 . Each of the end plates  802 ,  804  includes a flow field plate, which is configured as a monopolar flow field plate. A number of MEAs  820  and bipolar flow field plates  830  are situated between the first and second end plates  802 ,  804 . These MEA and flow field components are preferably of a type described hereinabove, it being understood that cooling arrangements can also be incorporated into the stack  800 . 
   The first end plate  802  includes a first fuel inlet port  806 , which can accept oxygen, for example, and a second fuel outlet port  808 , which can discharge hydrogen, for example. The second end plate  804  includes a first fuel outlet port  809 , which can discharge oxygen, for example, and a second fuel inlet port  810 , which can accept hydrogen, for example. The fuels pass through the stack in a specified manner via the various ports provided in the endplates  802 ,  804  and manifold ports  825  provided on each of the MEAs  820  and flow field plates  825  (e.g., UCAs) of the stack  800 . 
     FIG. 11  illustrates a fuel cell system within which one or more fuel cell stacks employing UCAs of the present invention can be employed. The fuel cell system  900  shown in  FIG. 11  illustrates one of many possible systems in which UCA-based fuel cell stacks can find utility. 
   The fuel cell system  900  includes a fuel processor  904 , a power section  906 , and a power conditioner  908 . The fuel processor  904 , which includes a fuel reformer, receives a source fuel, such as natural gas, and processes the source fuel to produce a hydrogen rich fuel. The hydrogen rich fuel is supplied to the power section  906 . Within the power section  906 , the hydrogen rich fuel is introduced into the stack of UCAs of the fuel cell stack(s) contained in the power section  906 . A supply of air is also provided to the power section  906 , which provides a source of oxygen for the stack(s) of fuel cells. 
   The fuel cell stack(s) of the power section  906  produce DC power, useable heat, and clean water. In a regenerative system, some or all of the byproduct heat can be used to produce steam which, in turn, can be used by the fuel processor  904  to perform its various processing functions. The DC power produced by the power section  906  is transmitted to the power conditioner  908 , which converts DC power to AC power for subsequent use. It is understood that AC power conversion need not be included in a system that provides DC output power. 
   The foregoing description of the various embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.