BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a fuel cell for a fuel cell stack and, more particularly, to a fuel cell for a fuel cell stack, where the fuel cell includes nested flow channels in an active region of the fuel cell and non-nested flow channels in inactive feed regions of the fuel cell, and where the diffusion media layers in the cells are removed in the inactive feed regions to provide more space for the non-nested channels. 
     2. Discussion of the Related Art 
     Hydrogen is an attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today&#39;s vehicles employing internal combustion engines. 
     A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle. 
     Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid-polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO). 
     Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. 
     The fuel cell stack includes a series of flow field plates or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided in the anode side of the bipolar plates that allow the anode gas to flow to the anode side of each MEA. Cathode gas flow channels are provided in the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of each MEA. The bipolar plates are made of a conductive material, such as stainless steel, so that they conduct the electricity generated by the fuel cells from one cell to the next cell as well as out of the stack. 
     It has previously been proposed by the inventors in U.S. patent application Ser. No. 10/661,195, titled Nested Stamped Plates for a Compact Fuel Cell, filed Sep. 12, 2003, that the thickness or repeat distance of a fuel cell stack can be reduced by nesting the flow channels in the active region of the fuel cells.  FIG. 1  is a cross-sectional view of a portion of a fuel cell stack  10  showing this proposed design. The fuel cell stack  10  includes two MEAs  12  and  14  for adjacent fuel cells in the stack  10 . Each MEA  12  and  14  includes a membrane of the type referred to above and an anode side catalyst layer and a cathode side catalyst later. An anode side gas diffusion media layer  16  is positioned adjacent to the MEA  12  and a cathode side gas diffusion media layer  18  is positioned adjacent to the MEA  14 . The diffusion media layers  16  and  18  are porous layers that provide for input gas transport to and water transport from the MEAs  12  and  14 . Various techniques are known in the art for depositing the catalyst layers on the membranes in the MEAs  12  and  14  or on the diffusion media layers  16  and  18 . 
     A bipolar plate assembly  20  is positioned between the diffusion media layers  16  and  18 . The bipolar plate assembly  20  includes two stamped metal bipolar plates  22  and  24  that are assembled together in the nested configuration as shown. The nested plates  22  and  24  define parallel anode gas flow channels  28  and parallel cathode gas flow channels  30 , where the anode flow channels  28  provide a hydrogen flow to the anode side of the MEA  12  and the cathode flow channels  30  provide airflow to the cathode side of the MEA  14 . Additionally, the plates  22  and  24  define coolant flow channels  32  through which a cooling fluid flows to cool the fuel cell stack  10 , as is well understood in the art. In this design, the size of the coolant flow channels  32  is reduced from the size of the cooling channels provided in the non-nested stamped plates of the prior art, which provides the reduction in the repeat distance of the fuel cell stack  10 . Reducing the size of the coolant flow channels  32  over the known cooling channels does not significantly affect the cooling capability of the cooling channels because the larger channels were more than adequate to provide the necessary cooling. The reduction in coolant volume also reduces the thermal mass that must be heated during system start-up. 
     The anode flow channels  28  are in fluid communication with an anode flow channel header at each end of the fuel cell stack  10 , where one header receives the anode gas flow to distribute it to the anode gas flow channels  28  and the other anode header receives the anode exhaust gas from the anode flow channels. Likewise, the cathode gas flow channels  30  are in fluid communication with a cathode flow channel header at each end of the stack  10 , and the cooling flow channels  32  are in fluid communication with a coolant flow channel header at each end of the stack  10 . However, in order to couple the anode flow channels  28  to the anode channel headers, the cathode flow channels  30  to the cathode channel headers and the coolant flow channels  32  to the coolant channel headers, it is necessary to separate and un-nest the plates  22  and  24  in the non-active feed regions of the stack. 
     Because the non-nested configuration of the flow channels  28 ,  30  and  32  requires more space than the nested configuration of the channels  28 ,  30  and  32 , the reduction in thickness of the stack  10  provided by the nested configuration would be eliminated by using the known non-nested configuration in the inactive regions. It is possible to reduce the size of the flow channels  28 ,  30  and  32  in the non-nested inactive regions so that the flow channels  28 ,  30  and  32  do not use more space than they use in the nested configuration. However, such a reduction in the size of the channels  28 ,  30  and  32  would cause a pressure drop across the channels that would adversely affect the flow rate and performance of the stack  10 . 
     The present invention proposes a solution to a transition from the nested configuration to the non-nested configuration of the bipolar plates without reducing the size of the channels or increasing the thickness of the stack. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a fuel cell in a fuel cell stack is disclosed that provides a transition from nested bipolar plates in the active region of the stack to non-nested bipolar plates in the inactive feed regions of the stack without giving up the reduced stack thickness provided by the nested plates or changing the size of the flow channels. Particularly, the diffusion media layers in the fuel cells of the stack are removed in the inactive feed regions where the bipolar plates are non-nested so that the volume necessary to maintain the size of the flow channels is provided without the need to increase the distance between adjacent MEAs. Additionally, the membrane of the MEAs would not be catalyzed in the inactive regions. A thin shim can be provided between the membranes and the plates in the inactive regions to support the membrane where the diffusion media layer has been removed to prevent the membrane from intruding into the flow channels and blocking the reactive flow. 
     Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross-sectional view of an active region of a fuel cell stack employing nested stamped bipolar plates; 
         FIG. 2  is a partial cross-sectional view of an inactive feed region of a fuel cell stack employing non-nested stamped bipolar plates where the gas diffusion media layers have been removed, according to an embodiment of the present invention; 
         FIG. 3  is a partial cross-sectional view of an inactive feed region of a fuel cell stack employing non-nested stamped bipolar plates where the gas diffusion media layers have been removed and shims have been added, according to another embodiment of the present invention; 
         FIG. 4  is a partial cross-sectional view of the transition between an inactive feed region and an active region of a fuel cell stack, according to the invention; 
         FIG. 5  is a top view of a plate in a fuel cell stack, according to an embodiment of the present invention; and 
         FIG. 6  is a solid model of a fuel cell stack including an active region having nested stamped bipolar plates and an inactive feed region having non-nested stamped bipolar plates. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a fuel cell design is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     According to the present invention, a fuel cell design is described that includes nested stamped bipolar plates in an active region of the fuel cell and non-nested stamped bipolar plates in an inactive feed region of the fuel cell.  FIG. 2  is a partial cross-sectional view through an inactive feed region of a fuel cell stack  40 . The stack  40  includes adjacent membranes  42  and  44  that are part of two adjacent MEAs in the stack  40 . The fuel cell stack  40  also includes a bipolar plate assembly  46  having two stamped non-nested bipolar plates  48  and  50 . The plates  48  and  50  are stamped so that they define anode flow channels  52 , cathode flow channels  54  and coolant flow channels  56 . 
     It is necessary that the plates  48  and  50  be non-nested in the feed regions of the stack  40  so that the input gasses and the cooling fluid can be separated and coupled to appropriate manifold headers. The fuel cell stack  40  would include a transition region, discussed below, between the active region and the inactive regions of the fuel cell stack  40  where the anode flow channels  52  are in fluid communication with the anode flow channels  28 , the cathode flow channels  54  are in fluid communication with the cathode channels  30  and the coolant flow channels  56  are in fluid communication with the coolant flow channels  32 . 
     According to the invention, the size of the non-nested channels  52  and  54  are the same, or nearly the same, as the size of the nested channels  28  and  30 , respectively, by eliminating the diffusion media layers  16  and  18  in the inactive feed regions of the fuel cell stack  40 . In the inactive feed regions, the catalyst layers of the MEAs  12  and  14  would also be eliminated leaving sub-gasketed membranes  42  and  44 . Note that the MEAs  12  and  14  would typically include a sub-gasket (not shown) outside of the active region. The sub-gasket prevents direct contact of the ionomer membrane to the plates  48  and  50  or the seals. The sub-gasket would typically a 0.25 μm film of Kapton or other suitable plastic. Therefore, the volume that was used by the diffusion media layers  16  and  18  in the active region of the fuel cell stack  40  can be used to accommodate the non-nested bipolar plates  48  and  50  in the inactive regions so that the size of the flow channels can be maintained without increasing the repeat distance of the stack  40 . The diffusion media layers  16  and  18  are generally about 0.2 mm thick, which is enough to provide the necessary space. 
     The size of the coolant flow channels  56  does increase to about twice the size from the nested configuration to the non-nested configuration, but the pressure drop provided by the coolant channel transition does not adversely affect the performance of the stack  40 . Further, the inactive feed regions with non-nested plates may increase the plate footprint for the active region, but the overall volume of the stack is reduced because of the decrease in stack height provided by the nested plates. 
     Because the membranes  42  and  44  are not supported by the diffusion media layers  16  and  18  in the feed regions of the stack  40 , they may have a tendency to intrude into the flow channels  52  and  54 . As the MEA typically includes sub-gaskets beyond the active region, with sufficient thickness, the sub-gaskets could provide adequate membrane support in the feed regions.  FIG. 3  is a cross-sectional view of a fuel cell stack  60  that is similar to the fuel cell stack  40 , where like elements are identified by the same reference numeral. The fuel cell stack  60  includes a thin shim  62  positioned between the membrane  42  and the plate  48  and a thin shim  64  positioned between the membrane  44  and the plate  50 . The shims  62  and  64  prevent the membranes  42  and  44 , respectively, from intruding into the flow channels  52  and  54 , respectively. The shims  62  and  64  can be located in place or can be either bonded to the membranes  42  and  44 , respectively, or to the plates  48  and  50 , respectively. The shims  62  and  64  may also function as a gasket carrier. The shims  62  and  64  can be made of any suitable material, such as metal or plastic, and can have a suitable thickness, such as 0.025 μm, to provide the desired support. 
       FIG. 4  is a cross-sectional view of a portion of a fuel cell stack  70  showing an example of a transition region  72  between nested bipolar plates  74  and  76  in an active region  78  of the fuel cell stack  70  and non-nested bipolar plates  80  and  82  in an inactive feed region  84  of the fuel cell stack  70 . The fuel cell stack  70  includes membranes  86  and  88  extending across the active region  78  and the inactive region  84 . Gas diffusion media layers  90  and  92  are provided adjacent to the membranes  86  and  88 , respectively, in the active region  78 . Shims  94  and  96  are positioned between the non-nested plates  80  and  82  and the membranes  86  and  88 , respectively, in the inactive region  84 . The relative size of anode and cathode flow channels  98  and  100  in the inactive region  84  and the active region  78  are substantially the same. Flow channel  102  in the active region  78  can represent any of the anode flow channel, the cathode flow channel or the coolant flow channel. 
       FIG. 5  is top view of a bipolar plate assembly  110  in a fuel cell stack  112 . The fuel cell stack  112  includes an active region  114  having stamped bipolar plates that are nested, and inactive feed regions  116  and  118 , at opposite ends of the active region  114 , having stamped bipolar plates that are non-nested, consistent with the discussion above. The stamped bipolar plates include the various flow channels discussed above. A cathode inlet header  120  at one end of the fuel cell stack  112  directs the cathode air into the cathode flow channels in the inactive region  116 . The cathode air flows through the cathode flow channels in the inactive feed region  116 , through the cathode flow channels in the active region  114  and through the cathode flow channels in the inactive region  118 . The cathode exhaust gas is collected by a cathode outlet header  122 . 
     An anode inlet header  126  at one end of the fuel cell stack  112  directs the hydrogen gas into the anode flow channels in the inactive region  118 . The hydrogen gas flows through the anode flow channels in the inactive feed region  118 , through the anode flow channels in the active region  114  and through the anode flow channels in the inactive region  116 . The anode exhaust gas is collected by an anode outlet header  128 . In this non-limiting embodiment, the anode gas and the cathode gas are counter-flow. 
     A coolant inlet header  132  at one end of the fuel cell stack  112  directs the cooling fluid into the coolant flow channels in the inactive region  116 . The cooling fluid flows through the coolant flow channels in the inactive feed region  116 , through the coolant flow channels in the active region  114  and through the coolant flow channels in the inactive region  118 . The cooling fluid is collected by a coolant outlet header  134 . 
       FIG. 6  is a solid model perspective view of a fuel cell stack  140  including an active region  142  having the nested bipolar plates and an inactive feed region  144  having the non-nested bipolar plates. A transition region  146  between the region  142  and the region  144  provides the transition of the channels from the nested configuration to the non-nested configuration. The cooling fluid from the coolant header (not shown in  FIG. 6 ) is directed into flow channels  148  in the inactive region  144 , the hydrogen gas flow from the anode header (not shown in  FIG. 6 ) is directed into flow channels  150  in the inactive region  144  and the cathode gas from the cathode header (not shown in  FIG. 6 ) is directed into flow channels  152  in the inactive region  144 . In this embodiment, the anode gas and the cathode gas are co-flow. 
     Table 1 below provides a comparison of various parameters discussed above for a nested plate design, a non-nested plate design and a nested plate design including half height channels. This data is from a fuel cell stack including a 360 cm 2  active area, 200 cells, 66 kW output power, 1.5 Acm 2  current density and a low pressure. The nested designs are smaller (higher kW/l) and have an even greater reduction in thermal mass from 27 to 19-20 kJ/K due to the reduced coolant volume. The half height feed region provides a smaller stack than the nested present invention because the feed regions can be active regions. However, the pressure drop due to these very shallow feed channels leads to an unacceptably high pressure drop (85 kPa vs 30 kPa on the cathode side). 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Nested 
                   
                 Nested 
               
               
                   
                 (present 
                 Non- 
                 (half height feed 
               
               
                   
                 invention) 
                 nested 
                 channels) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Channel depth (mm) 
                 0.34 
                 0.35 
                 0.34 
               
               
                 An ch depth (mm) 
                 — 
                 0.31 
                 — 
               
               
                 Channel depth (mm) 
                 0.37 
                 — 
                 0.37 
               
               
                 (no region GDM) 
               
               
                 repeat distance (mm) 
                 0.97 
                 1.29 
                 0.97 
               
               
                 An dP (kPa) 
                 13 
                 13 
                 30 
               
               
                 Ca dP (kPa) 
                 30 
                 30 
                 85 
               
               
                 Coolant dp (kPa) 
                 57 
                 22 
                 106 
               
               
                 Power density (kW/l) 
                 6.0 
                 4.8 
                 6.3 
               
               
                 Thermal mass (kJ/K) 
                 20 
                 27 
                 19 
               
               
                 (with coolant) 
               
               
                   
               
             
          
         
       
     
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
A fuel cell in a fuel cell stack that provides a transition from nested bipolar plates in the active region of the stack to non-nested bipolar plates in the inactive regions of the stack without giving up the reduced stack thickness provided by the nested plates or changing the size of the flow channels. Particularly, the diffusion media layers in the fuel cells are removed in the inactive regions where the bipolar plates are non-nested so that the volume necessary to maintain the size of the flow channels is provided without the need to increase the distance between adjacent MEAs. A thin shim can be provided between the membranes and the plates in the inactive regions to support the membrane where the diffusion media layer has been removed to prevent the membrane from intruding into the flow channels and blocking the reactive flow.