Abstract:
A cell unit constituting a fuel cell is provided with a first electrolyte membrane/electrode structure, a first separator, a second electrolyte membrane/electrode structure, and a second separator. The first and second electrolyte membrane/electrode structures respectively have a frame section on the outer periphery, and the frame sections are formed with a fluid communicating hole extending in the stacking direction. The first and second separators are disposed towards the inside of the fluid communication hole and are respectively provided with two metal plates which have the same shape and which are stacked on one another.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/JP2011/070130, filed Sep. 5, 2011, which claims priority to Japanese Patent Application No. 2010-235425, filed on Oct. 20, 2010, Japanese Patent Application No. 2010-235427, filed on Oct. 20, 2010, Japanese Patent Application No. 2010-235718, filed on Oct. 20, 2010, Japanese Patent Application 2010-235721, filed on Oct. 20, 2010, and Japanese Patent Application No. 2010-279976, filed on Dec. 16, 2010 in Japan. The contents of the aforementioned applications are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fuel cell including a cell unit formed by sandwiching an electrolyte electrode assembly between a first separator and a second separator. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. 
     BACKGROUND ART 
     For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (electrolyte electrode assembly) which includes an anode, a cathode, and a solid polymer electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. Each of the anode and the cathode includes an electrode catalyst layer and a porous carbon layer. The membrane electrode assembly and separators (bipolar plates) sandwiching the membrane electrode assembly make up a unit cell. In use, generally, a predetermined number of unit cells are stacked together to form a fuel cell stack, which is mounted in a vehicle, for example. 
     In general, the fuel cell adopts the so-called internal manifold structure where supply passages and discharge passages extend through separators in a stacking direction. The fuel gas, the oxygen-containing gas, and the coolant are supplied from the respective supply passages respectively to a fuel gas flow field, an oxygen-containing gas flow field, and a coolant flow filed along electrode surfaces, and then, the fuel gas, the oxygen-containing gas, and the coolant are discharged into the respective discharge passages. 
     For example, in a fuel cell separator disclosed in Japanese Laid-Open Patent Publication No. 08-222237, as shown in  FIG. 32 , a separator plate  1  is provided. The separator plate  1  is a metal plate, and a large number of projections  2   a ,  2   b  are formed on front and back surfaces of the separator plate  1  by embossing or dimpling. Manifold loading ports  3   a ,  3   b ,  3   c , and  3   d  for being loaded with respective gas manifolds, extend through the separator plate  1  outside an area having the projections  2   a ,  2   b.    
     For example, the gas manifold loading ports  3   a ,  3   b ,  3   c , and  3   d  are used as a fuel gas inlet manifold, an oxygen-containing gas inlet manifold, a fuel gas discharge manifold, and an oxygen-containing gas discharge manifold. 
     SUMMARY OF INVENTION 
     However, since the manifold loading ports  3   a ,  3   b ,  3   c , and  3   d  extend through the separator plate  1 , the area of the separator plate  1  is considerably large. As a consequence, in particular, a large amount of expensive material such as stainless steel is required for the separator plate  1 , and the unit cost for the component is high undesirably. 
     The present invention has been made to solve the problems of this type, and an object of the present invention is to provide a fuel cell which makes it possible to reduce the size of relatively expensive separators, and achieve cost reduction. 
     The present invention relates to a fuel cell including a cell unit formed by sandwiching an electrolyte electrode assembly between a first separator and a second separator. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. 
     In the fuel cell, a frame member made of polymer material is provided integrally with an outer circumference of the electrolyte electrode assembly, fluid passages including a reactant gas supply passage, a reactant gas discharge passage, a coolant supply passage, and a coolant discharge passage extend through the frame member in the stacking direction, and a seal member for hermetically surrounding the fluid passages and an outer circumference of a reaction surface is provided between the frame members that are adjacent to each other in the stacking direction. 
     At least the first separator or the second separator includes two plates having the same outer shape and which are joined together, and outer circumferential ends of the first separator and the second separator are positioned on an inward side relative to the fluid passages. 
     Further, in the present invention, outer circumferential ends of the first separator and the second separator are positioned on an inward side relative to the fluid passages, and the first separator and the second separator have a first reactant gas flow field and a second reactant gas flow field for allowing different reactant gases to flow along separator surfaces, respectively, on both sides of the electrolyte electrode assembly. 
     Further, connection channels are formed to connect the reactant gas supply passage and the reactant gas discharge passage to the first reactant gas flow field, and the connection channels include grooves formed in a surface of the frame member and extending along the separator surface. 
     Further, in the present invention, connection channels are formed to connect the reactant gas supply passage and the reactant gas discharge passage to the first reactant gas flow field. The connection channels include grooves formed in a surface of the frame member and extending along the separator surface and holes connected to the grooves and extending through the first separator or the second separator in the stacking direction. 
     Further, in the present invention, a frame member made of polymer material is provided integrally with an outer circumference of the electrolyte electrode assembly, and fluid passages including a reactant gas supply passage, a reactant gas discharge passage, a coolant supply passage, and a coolant discharge passage extend through the frame member in the stacking direction. Outer circumferential ends of the first separator and the second separator are positioned on an inward side relative to the fluid passages, and at least the first separator or the second separator includes two plates to form a coolant flow field inside the first separator or the second separator for allowing a coolant to flow along a separator surface. 
     A seal member for hermetically surrounding the fluid passages and an outer circumference of a reaction surface is provided between the frame members that are adjacent to each other in the stacking direction, and connection channels connecting the coolant supply passage and the coolant discharge passage to the coolant flow field are formed between the frame members that are adjacent to each other in the stacking direction. 
     In the present invention, the fluid passages extend in the stacking direction through the frame members provided around the electrolyte electrode assembly. Therefore, no fluid passages are required in the first separator and the second separator. 
     The outer dimensions of the first separator and the second separator can be determined in such a manner that the outer dimensions of the first separator and the second separator correspond to the power generation area. Thus, reduction in the size and weight of the first separator and the second separator can be achieved easily, and it becomes possible to reduce the production cost of the first separator and the second separator. Accordingly, the first separator and the second separator can be produced efficiently, and it is possible to obtain the entire fuel cell economically. Further, in each cell unit, the seal members can be provided only on one surface. In the structure, the size of the fuel cell in the stacking direction is reduced as a whole. 
     Further, at least the first separator or the second separator includes two plates having the same outer shape and which are stacked together. In the structure, the production cost of the separator is reduced effectively, and economically. 
     Further, in the present invention, a seal member for hermetically surrounding the fluid passages is provided between the frame members that are adjacent to each other in the stacking direction. Further, connection channels are formed in a surface of the frame member and a surface of the first separator to connect the reactant gas supply passage and the reactant gas discharge passage to the reactant gas flow field. Thus, the structure of the fuel cell is simplified, and it becomes possible to effectively reduce the size of the fuel cell in the stacking direction as a whole. 
     Further, in the present invention, a seal member for hermetically surrounding the fluid passages is provided between the frame members that are adjacent to each other in the stacking direction. Connection channels are formed to connect the reactant gas supply passage and the reactant gas discharge passage to the first reactant gas flow field. The connection channels include grooves formed in the frame member and extending along the separator surface, and holes connected to the grooves and extending through a first separator or a second separator in the stacking direction. Thus, the structure of the fuel cell is simplified, and it becomes possible to reduce the size of the fuel cell in the stacking direction as a whole. 
     Further, in the present invention, a seal member for hermetically surrounding the fluid passages is provided between the frame members that are adjacent to each other in the stacking direction, and connection channels connecting the coolant supply passage and the coolant discharge passage to the coolant flow field are formed between the frame members that are adjacent to each other in the stacking direction. Thus, the structure of the fuel cell is simplified, and it becomes possible to effectively reduce the size of the fuel cell in the stacking direction as a whole. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view showing a fuel cell according to a first embodiment of the present invention; 
         FIG. 2  is a cross sectional view showing the fuel cell, taken along a line II-II in  FIG. 1 ; 
         FIG. 3  is a view showing a cathode surface of a first membrane electrode assembly of the fuel cell; 
         FIG. 4  is a view showing an anode surface of the first membrane electrode assembly; 
         FIG. 5  is a view showing a cathode surface of a second membrane electrode assembly of the fuel cell; 
         FIG. 6  is a view showing an anode surface of the second membrane electrode assembly; 
         FIG. 7  is a view showing a cathode surface of a first separator of the fuel cell; 
         FIG. 8  is a view showing an anode surface of the first separator; 
         FIG. 9  is a view showing a cathode surface of a second separator of the fuel cell; 
         FIG. 10  is a view showing an anode surface of the second separator; 
         FIG. 11  is a cross sectional view showing the fuel cell, taken along a line XI-XI in  FIG. 1 ; 
         FIG. 12  is a cross sectional view showing the fuel cell, taken along a line XII-XII in  FIG. 1 ; 
         FIG. 13  is a cross sectional view showing the fuel cell, taken along a line XIII-XIII in  FIG. 1 ; 
         FIG. 14  is a cross sectional view showing the fuel cell, taken along a line XIV-XIV in  FIG. 1 ; 
         FIG. 15  is an exploded perspective view showing a fuel cell according to a second embodiment of the present invention; 
         FIG. 16  is a cross sectional view showing the fuel cell, taken along a line XVI-XVI in  FIG. 15 ; 
         FIG. 17  is a view showing a cathode surface of the first membrane electrode assembly of the fuel cell; 
         FIG. 18  is a view showing an anode surface of the first membrane electrode assembly; 
         FIG. 19  is a view showing a cathode surface of a second membrane electrode assembly of the fuel cell; 
         FIG. 20  is a view showing an anode surface of the second membrane electrode assembly; 
         FIG. 21  is a view showing a cathode surface of a first separator of the fuel cell; 
         FIG. 22  is a view showing an anode surface of the first separator; 
         FIG. 23  is a view showing a cathode surface of a second separator of the fuel cell; 
         FIG. 24  is a view showing an anode surface of the second separator; 
         FIG. 25  is a cross sectional view showing the fuel cell, taken along a line XXV-XXV in  FIG. 15 ; 
         FIG. 26  is a cross sectional view showing the fuel cell, taken along a line XXVI-XXVI in  FIG. 15 ; 
         FIG. 27  is a cross sectional view showing the fuel cell, taken along a line XXVII-XXVII in  FIG. 15 ; 
         FIG. 28  is an exploded perspective view showing a fuel cell according to a third embodiment of the present invention; 
         FIG. 29  is a view showing a cathode surface of a first separator of the fuel cell; 
         FIG. 30  is a cross sectional view showing the fuel cell; 
         FIG. 31  is a cross sectional view showing a fuel cell according to a fourth embodiment of the present invention; and 
         FIG. 32  is a view showing a fuel cell separator disclosed in Japanese Laid-Open Patent Publication No. 08-222237. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIGS. 1 and 2 , a fuel cell  10  according to a first embodiment of the present invention is formed by stacking a plurality of cell units  12  in a horizontal direction indicated by an arrow A. 
     The cell unit  12  includes a first membrane electrode assembly (electrolyte electrode assembly) (MEA)  14 , a first separator  16 , a second membrane electrode assembly (electrolyte electrode assembly) (MEA)  18 , and a second separator  20 . By stacking the cell units  12 , the first membrane electrode assembly  14  is sandwiched between the second and first separators  20 ,  16 , and the second membrane electrode assembly  18  is sandwiched between the first and second separators  16 ,  20 . 
     As described later, the first separator  16  and the second separator  20  are formed by corrugating metal thin plates by pressure forming. Alternatively, the carbon separators may be used as the first separator  16  and the second separator  20 . 
     Each of the first membrane electrode assembly  14  and the second membrane electrode assembly  18  includes a cathode  24 , an anode  26 , and a solid polymer electrolyte membrane (electrolyte)  22  interposed between the cathode  24  and the anode  26  (see  FIG. 2 ). For example, the solid polymer electrolyte membrane  22  is formed by impregnating a thin membrane of perfluorosulfonic acid with water. 
     In the first membrane electrode assembly  14 , the surface area of the solid polymer electrolyte membrane  22  is identical with the surface area of the cathode  24  and the surface area of the anode  26 . It should be noted that the outer circumferential end of the solid polymer electrolyte membrane  22  may protrude beyond the cathode  24  and the anode  26 . The surface area of the cathode  24  may be different from the surface area of the anode  26 . 
     In the first membrane electrode assembly  14 , a frame  28   a  (e.g., a first frame member) made of insulating polymer material is formed integrally with the outer circumferential ends of the solid polymer electrolyte membrane  22 , the cathode  24 , and the anode  26 , e.g., by injection molding. Likewise, in the second membrane electrode assembly  18 , a frame  28   b  (e.g., a second frame member) made of polymer material is formed integrally with the outer circumferential ends of the solid polymer electrolyte membrane  22 , the cathode  24 , and the anode  26 , e.g., by injection molding. For example, engineering plastics and super engineering plastics as well as commodity plastics may be adopted as the polymer material. 
     Each of the cathode  24  and the anode  26  has a gas diffusion layer (not shown) such as a carbon paper, and an electrode catalyst layer (not shown) of platinum alloy supported on porous carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer contacts the solid polymer electrolyte membrane  22 . 
     As shown in  FIG. 1 , at one end (upper end) of the frames  28   a ,  28   b  in a vertical direction indicated by an arrow C, an oxygen-containing gas supply passage  30   a  for supplying an oxygen-containing gas, a coolant supply passage  32   a  for supplying a coolant, and a fuel gas supply passage  34   a  for supplying a fuel gas such as a hydrogen-containing gas are arranged in a horizontal direction indicated by an arrow B. 
     At the other end (lower end) of the frames  28   a ,  28   b  in the direction indicated by the arrow C, a fuel gas discharge passage  34   b  for discharging the fuel gas, a coolant discharge passage  32   b  for discharging the coolant, and an oxygen-containing gas discharge passage  30   b  for discharging the oxygen-containing gas are arranged in the direction indicated by the arrow B. 
     As long as the oxygen-containing gas supply passage  30   a , the coolant supply passage  32   a , the fuel gas supply passage  34   a , the oxygen-containing gas discharge passage  30   b , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b  are provided in the frames  28   a ,  28   b , the positions of the oxygen-containing gas supply passage  30   a , the coolant supply passage  32   a , the fuel gas supply passage  34   a , the oxygen-containing gas discharge passage  30   b , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b  are not limited. 
     As shown in  FIG. 3 , the frame  28   a  has a plurality of inlet ridges  36   a  and a plurality of inlet grooves  37   a  at upper positions of a cathode surface (the surface where the cathode  24  is provided)  14   a  of the first membrane electrode assembly  14 , adjacent to the lower portion of the oxygen-containing gas supply passage  30   a . Further, the frame  28   a  has a plurality of inlet grooves  38   a  at upper positions of the cathode surface  14   a , adjacent to the lower portion of the coolant supply passage  32   a , and adjacent to the oxygen-containing gas supply passage  30   a . Further, a plurality of inlet holes  40   a  extend through the frame  28   a , at positions adjacent to the lower portion of the coolant supply passage  32   a , and adjacent to the fuel gas supply passage  34   a.    
     The frame  28   a  has a plurality of outlet ridges  36   b  and a plurality of outlet grooves  37   b  at lower positions of the cathode surface  14   a  of the first membrane electrode assembly  14 , adjacent to the upper portion of the oxygen-containing gas discharge passage  30   b . Further, the frame  28   a  has a plurality of outlet grooves  38   b  at lower positions of the cathode surface  14   a , adjacent to the upper portion of the coolant discharge passage  32   b , and adjacent to the oxygen-containing gas discharge passage  30   b . Further, a plurality of outlet holes  40   b  extend through the frame  28   a , at positions adjacent to the upper portion of the coolant discharge passage  32   b , and adjacent to the fuel gas discharge passage  34   b.    
     As shown in  FIG. 4 , the frame  28   a  has a plurality of inlet grooves  42   a  at upper positions of an anode surface (the surface where the anode  26  is provided)  14   b  of the first membrane electrode assembly  14 , adjacent to the lower portion of the coolant supply passage  32   a , and adjacent to the fuel gas supply passage  34   a . A plurality of inlet holes  40   a  extend through the frame  28   a , at positions adjacent to the lower portions of the inlet grooves  42   a . The frame  28   a  has a plurality of inlet grooves  46   a  below the fuel gas supply passage  34   a.    
     The frame  28   a  has a plurality of outlet grooves  42   b  at lower positions of the anode surface  14   b  of the first membrane electrode assembly  14 , adjacent to the upper portion of the coolant discharge passage  32   b , and adjacent to the fuel gas discharge passage  34   b . A plurality of outlet holes  40   b  extend through the frame  28   a , at positions adjacent to the upper portions of the outlet grooves  42   b . The frame  28   a  has a plurality of outlet grooves  46   b  above the fuel gas discharge passage  34   b.    
     An outer seal member (outer seal line)  48  and an inner seal member (inner seal line)  50  are provided integrally with the anode surface  14   b  of the frame  28   a . Alternatively, the outer seal member  48  and the inner seal member  50  may be formed separately from the frame  28   a , and provided on the anode surface  14   b  of the frame  28   a . Each of the outer seal member  48  and the inner seal member  50  is an elastic seal made of seal material, cushion material, or packing material such as an EPDM rubber (ethylene propylene diene monomer), an NBR (nitrile butadiene rubber), a fluoro rubber, a silicone rubber, a fluorosilicone rubber, a butyl rubber, a natural rubber, a styrene rubber, a chloroprene rubber, or an acrylic rubber. Seal members as described later have the same structure as that of the outer seal member  48  and the inner seal member  50 , and description thereof will be omitted. 
     The outer seal member  48  is provided along the outer circumferential end of the frame  28   a , around all of the fluid passages, i.e., the oxygen-containing gas supply passage  30   a , the coolant supply passage  32   a , the fuel gas supply passage  34   a , the oxygen-containing gas discharge passage  30   b , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b , and around the reaction surface (power generation surface). The outer seal member  48  surrounds the coolant supply passage  32   a , the fuel gas supply passage  34   a , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b . The outer seal member  48  surrounds the inlet grooves  42   a , the inlet holes  40   a , and the coolant supply passage  32   a  together, and surrounds the outlet grooves  42   b , the outlet holes  40   b , and the coolant discharge passage  32   b  together. 
     The inner seal member  50  is positioned inside the outer seal member  48 , and surrounds the anode  26 , the inlet grooves  46   a , and the outlet grooves  46   b  together. 
     The inner seal member  50  is provided along a profile line corresponding to the outer shape of the first separator  16 , and contacts the entire outer circumferential end surface of the first separator  16  (within the separator surface). The outer seal member  48  is provided around the outer circumferential end of the first separator  16  (outside the separator surface). All of the fluid passages are hermetically surrounded by the outer seal member  48  and the inner seal member  50 . 
     As shown in  FIG. 3 , on the cathode surface  14   a  of the frame  28   a , a ring-shaped inlet seal member  52   a  surrounding the inlet holes  40   a  and a ring-shaped outlet seal member  52   b  surrounding the outlet holes  40   b  are provided. 
     As shown in  FIG. 5 , the frame  28   b  has a plurality of inlet ridges  54   a  and a plurality of inlet grooves  56   a  at upper positions of a cathode surface (the surface where the cathode  24  is provided)  18   a  of the second membrane electrode assembly  18 , adjacent to the lower portion of the oxygen-containing gas supply passage  30   a.    
     The frame  28   b  has a plurality of inlet grooves  58   a  at upper positions of the cathode surface  18   a , adjacent to the lower portion of the coolant supply passage  32   a , and adjacent to the fuel gas supply passage  34   a . A plurality of inlet holes  60   a  are formed adjacent to the lower portion of the coolant supply passage  32   a , and adjacent to the oxygen-containing gas supply passage  30   a . The inlet holes  60   a  of the second membrane electrode assembly  18  are offset from the inlet holes  40   a  of the first membrane electrode assembly  14  such that the inlet holes  60   a  and the inlet holes  40   a  are not overlapped with each other as viewed from the stacking direction. 
     The frame  28   b  has a plurality of inlet grooves  62   a  at upper positions of the cathode surface  18   a , adjacent to the lower portion of the fuel gas supply passage  34   a . A plurality of inlet holes  64   a  extend through the frame  28   b  at the lower ends of the inlet grooves  62   a . A plurality of inlet holes  66   a  extend through the frame  28   b  below the inlet holes  64   a , at positions spaced at predetermined distances from the inlet holes  64   a.    
     The frame  28   b  has a plurality of outlet grooves  58   b  at lower positions of the cathode surface  18   a  of the frame  28   b , adjacent the upper portion of the coolant discharge passage  32   b , and adjacent to the fuel gas discharge passage  34   b . Further, a plurality of outlet holes  60   b  are formed adjacent to the upper portion of the coolant discharge passage  32   b , and adjacent to the oxygen-containing gas discharge passage  30   b . The outlet holes  60   b  of the second membrane electrode assembly  18  are offset from the outlet holes  40   b  of the first membrane electrode assembly  14  such that the outlet holes  60   b  and the outlet holes  40   b  are not overlapped with each other as viewed from the stacking direction. 
     The frame  28   b  has a plurality of outlet grooves  62   b  at lower positions of the cathode surface  18   a , adjacent to the upper portion of the fuel gas discharge passage  34   b . A plurality of outlet holes  64   b  extend through the frame  28   b  at the upper ends of the outlet grooves  62   b . A plurality of outlet holes  66   b  extend through the frame  28   b  above the outlet holes  64   b , at positions spaced at predetermined distances from the outlet holes  64   b.    
     As shown in  FIG. 6 , the frame  28   b  has a plurality of inlet grooves  68   a  at upper positions of an anode surface (the surface where the anode  26  is provided)  18   b  of the second membrane electrode assembly  18 , adjacent to the lower portion of the coolant supply passage  32   a , and adjacent to the oxygen-containing gas supply passage  30   a . A plurality of inlet holes  60   a  extend through the frame  28   b , adjacent to the lower portions of the inlet grooves  68   a . The frame  28   b  has a plurality of inlet grooves  72   a  below the fuel gas supply passage  34   a . The inlet grooves  72   a  connect the inlet holes  64   a ,  66   a.    
     The frame  28   b  has a plurality of outlet grooves  68   b  at lower positions of the anode surface  18   b , adjacent to the upper portions of the coolant discharge passage  32   b , and adjacent to the oxygen-containing gas discharge passage  30   b . A plurality of outlet holes  60   b  extend through the frame  28   b , adjacent to the upper portions of the outlet grooves  68   b . The frame  28   b  has a plurality of outlet grooves  72   b  above the fuel gas discharge passage  34   b . The outlet grooves  72   b  connect the outlet holes  64   b ,  66   b.    
     An outer seal member (outer seal line)  74  and an inner seal member (inner seal line)  76  are provided integrally with the anode surface  18   b  of the frame  28   b . Alternatively, the outer seal member  74  and the inner seal member  76  may be formed separately from the frame  28   b , and provided on the anode surface  18   b  of the frame  28   b . The outer seal member  74  is provided along the outer circumferential end of the frame  28   b , around all of the fluid passages, i.e., the oxygen-containing gas supply passage  30   a , the coolant supply passage  32   a , the fuel gas supply passage  34   a , the oxygen-containing gas discharge passage  30   b , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b.    
     The outer seal member  74  surrounds the coolant supply passage  32   a , the fuel gas supply passage  34   a , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b . The outer seal member  74  surrounds the inlet grooves  68   a , the inlet holes  60   a , and the coolant supply passage  32   a  together, and surrounds the outlet grooves  68   b , the outlet holes  60   b , and the coolant discharge passage  32   b  together. 
     The inner seal member  76  is positioned inside the outer seal member  74 , and surrounds the anode  26 , the inlet holes  64   a ,  66   a , the inlet grooves  72   a , the outlet holes  64   b ,  66   b , and the outlet grooves  72   b  together. 
     The inner seal member  76  is provided along a profile line corresponding to the outer shape of the second separator  20 , and contacts the entire outer circumferential end surface of the second separator  20 . The outer seal member  74  is provided around the outer circumferential end of the second separator  20 . All of the fluid passages are hermetically surrounded by the outer seal member  74  and the inner seal member  76 . 
     As shown in  FIG. 5 , on the cathode surface  18   a  of the frame  28   b , ring-shaped inlet seal members  78   a ,  80   a  surrounding the inlet holes  60   a ,  66   a  and ring-shaped outlet seal members  78   b ,  80   b  surrounding the outlet holes  60   b ,  66   b  are provided. 
     The first and second separators  16 ,  20  are dimensioned such that the first and second separators  16 ,  20  are provided on an inward side relative to the oxygen-containing gas supply passage  30   a , the coolant supply passage  32   a , the fuel gas supply passage  34   a , the oxygen-containing gas discharge passage  30   b , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b  (all of the fluid passages). 
     As shown in  FIG. 2 , the first separator  16  includes two metal plates (e.g., stainless plates)  82   a ,  82   b  having the same outer shape. The metal plates  82   a ,  82   b  are stacked together. The outer circumferential ends of the metal plates  82   a ,  82   b  are welded or bonded together to form a hermetical internal space between the metal plates  82   a ,  82   b . An oxygen-containing gas flow field  84  facing the cathode  24  is formed on the metal plate  82   a , and a fuel gas flow field  86  facing the anode  26  is formed on the metal plate  82   b . A coolant flow field  88  is formed in the internal space between the metal plates  82   a ,  82   b.    
     As shown in  FIG. 7 , the first separator  16  has the oxygen-containing gas flow field  84  on the surface of the metal plate  82   a . The oxygen-containing gas flow field  84  includes a plurality of flow grooves extending in the vertical direction indicated by the arrow C. An inlet buffer  85   a  is provided on the upstream side of the oxygen-containing gas flow field  84 , and an outlet buffer  85   b  is provided on the downstream side of the oxygen-containing gas flow field  84 . A plurality of inlet grooves  87   a  are formed above the inlet buffer  85   a  and below the oxygen-containing gas supply passage  30   a . A plurality of outlet grooves  87   b  are formed below the outlet buffer  85   b  and above the oxygen-containing gas discharge passage  30   b.    
     A plurality of holes  90   a  and a plurality of holes  92   a  are formed at upper positions of the metal plate  82   a . The holes  90   a  are connected to the inlet holes  60   a  of the second membrane electrode assembly  18 , and the holes  92   a  are connected to the inlet holes  66   a  of the second membrane electrode assembly  18 . The holes  92   a  are also formed in the metal plate  82   b , and extend through the first separator  16 . 
     A plurality of holes  90   b  and a plurality of holes  92   b  are formed at lower positions of the metal plate  82   a . The holes  90   b  are connected to the outlet holes  60   b  of the second membrane electrode assembly  18 , and the holes  92   b  are connected to the outlet holes  66   b  of the second membrane electrode assembly  18 . The holes  92   b  are also formed in the metal plate  82   b , and extend through the first separator  16 . 
     The first separator  16  includes an upper recess  94   a  in order to avoid the inlet holes  40   a  of the first membrane electrode assembly  14 , and a lower recess  94   b  in order to avoid the outlet holes  40   b  of the first membrane electrode assembly  14 . 
     As shown in  FIG. 8 , the first separator  16  has the fuel gas flow field  86  on the surface of the metal plate  82   b . The fuel gas flow field  86  includes a plurality of flow grooves extending in the vertical direction indicated by the arrow C. An inlet buffer  96   a  is provided on the upstream side of the fuel gas flow field  86 , and an outlet buffer  96   b  is provided on the downstream side of the fuel gas flow field  86 . A plurality of inlet grooves  98   a  are formed above the inlet buffer  96   a  and below the oxygen-containing gas supply passage  30   a , and a plurality of inlet grooves  100   a  are provided above the inlet buffer  96   a  and below the coolant supply passage  32   a . The inlet grooves  100   a  have a ridge-and-groove structure to form coolant channels inside the first separator  16 . 
     A plurality of outlet grooves  98   b  are formed below the outlet buffer  96   b  and above the oxygen-containing gas discharge passage  30   b , and a plurality of outlet grooves  100   b  are provided below the outlet buffer  96   b  and above the coolant discharge passage  32   b . The outlet grooves  100   b  have a ridge-and-groove structure to form a coolant channel inside the first separator  16 . 
     As shown in  FIG. 2 , the second separator  20  includes two metal plates (e.g., stainless plates)  102   a ,  102   b  having the same outer shape. The metal plates  102   a ,  102   b  are stacked together. The outer circumferential ends of the metal plates  102   a ,  102   b  are welded or bonded together, and the internal space between the metal plates  102   a ,  102   b  is closed hermetically. An oxygen-containing gas flow field  84  is formed on the metal plate  102   a  to face the cathode  24 , and a fuel gas flow field  86  is formed on the metal plate  102   b  to face the anode  26 . A coolant flow field  88  is formed between the metal plates  102   a ,  102   b.    
     As shown in  FIG. 9 , the second separator  20  has an oxygen-containing gas flow field  84  on the surface of the metal plate  102   a . The oxygen-containing gas flow field  84  includes a plurality of flow grooves extending in the vertical direction indicated by the arrow C. An inlet buffer  104   a  is provided on the upstream side of the oxygen-containing gas flow field  84 , and an outlet buffer  104   b  is provided on the downstream side of the oxygen-containing gas flow field  84 . A plurality of holes  106   a  are formed at upper positions of the metal plate  102   a . The holes  106   a  are connected to the inlet holes  40   a  of the first membrane electrode assembly  14 . Further, a plurality of holes  106   b  are formed at lower positions of the metal plate  102   a . The holes  106   b  are connected to the outlet holes  40   b  of the first membrane electrode assembly  14 . 
     The second separator  20  includes an upper recess  108   a  in order to avoid the inlet holes  60   a  of the second membrane electrode assembly  18 , and a lower recess  108   b  in order to avoid the outlet holes  60   b  of the second membrane electrode assembly  18 . 
     As shown in  FIG. 10 , the second separator  20  has the fuel gas flow field  86  on a surface of the metal plate  102   b . The fuel gas flow field  86  includes a plurality of flow grooves extending in the vertical direction indicated by the arrow C. An inlet buffer  110   a  is provided on the upstream side of the fuel gas flow field  86 , and an outlet buffer  110   b  is provided on the downstream side of the fuel gas flow field  86 . 
     A plurality of inlet grooves  112   a  are formed at upper positions of the metal plate  102   b  and below the coolant supply passage  32   a , and a plurality of outlet grooves  112   b  are formed at lower positions of the metal plate  102   b  and above the coolant discharge passage  32   b . Both of the inlet grooves  112   a  and the outlet grooves  112   b  have a ridge-and-groove structure to form coolant channels in the second separator  20 . 
     As shown in  FIG. 11 , an oxygen-containing gas connection channel  113   a  and an oxygen-containing gas connection channel  113   b  are formed between the frames  28   a ,  28   b  that are adjacent to each other in the stacking direction. The oxygen-containing gas connection channel  113   a  connects the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas flow field  84  of the first membrane electrode assembly  14 , and the oxygen-containing gas connection channel  113   b  connects the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas flow field  84  of the second membrane electrode assembly  18 . Though not shown, oxygen-containing gas connection channels connecting the oxygen-containing gas discharge passage  30   b  and the oxygen-containing gas flow field  84  are formed between the frames  28   a ,  28   b  in the same manner. 
     The oxygen-containing gas connection channel  113   a  and the oxygen-containing gas connection channel  113   b  are formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  28   a , and the outer seal member  74  and the inner seal member  76  of the frame  28   b  at different positions as viewed from the stacking direction. 
     The oxygen-containing gas connection channel  113   b  includes the inlet ridges  54   a  formed on the surface of the frame  28   b  and extending along the separator surface, the inlet grooves  56   a  (e.g., a first set of grooves) formed in the frame  28   b , and the inlet grooves  87   a  formed in the surface of the metal plate  82   a  of the first separator  16 . The inlet grooves  87   a  are connected to grooves between the inlet ridges  54   a , and extend along the separator surface. Ends of the inlet grooves  56   a  are connected to ends of the inlet grooves  87   a.    
     The oxygen-containing gas connection channel  113   a  includes the inlet ridges  36   a  formed on the surface of the frame  28   a  and extending along the separator surface, and the inlet grooves  37   a . formed on the surface of the frame  28   a  and extending along the separator surface, and the inlet grooves  37   a  (e.g., a second set of grooves). 
     As shown in  FIG. 12 , a fuel gas connection channel  114  is formed between the frames  28   a ,  28   b  that are adjacent to each other in the stacking direction. The fuel gas connection channel  114  connects the fuel gas supply passage  34   a  and the fuel gas flow field  86 . Though not shown, a fuel gas connection channel connecting the fuel gas discharge passage  34   b  and the fuel gas flow field  86  is formed between the frames  28   a ,  28   b  in the same manner. 
     The fuel gas connection channels are formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  28   a , and the outer seal member  74  and the inner seal member  76  of the frame  28   b  at different positions as viewed from the stacking direction. 
     The fuel gas connection channel  114  includes the inlet grooves  62   a  (e.g., a first set of grooves),  72   a  (e.g., a second set of grooves) formed in the frame  28   b  of the second membrane electrode assembly  18  and extending along the separator surface, and the holes  92   a  extending through the outer circumferential end of the first separator  16  in the stacking direction. It should be noted that the inlet grooves  62   a  may be provided in the frame  28   a  of the first membrane electrode assembly  14 . 
     More specifically, the frame  28   b  has the inlet holes (first through holes)  64   a  and the inlet holes (second through holes)  66   a , and the inlet grooves  62   a ,  72   a  formed on both surfaces of the frame  28   b  are connected to each other through the inlet holes  64   a . The inlet holes  66   a  are provided coaxially with, or offset from the holes  92   a  in the stacking direction. The inlet grooves  62   a ,  72   a  are connected from the holes  92   a  to the fuel gas flow field (first reactant gas flow field)  86  of the first separator  16  through the inlet holes  66   a . The inlet grooves  72   a  are directly connected to the fuel gas flow field  86  of the second separator  20 . 
     As shown in  FIGS. 13 and 14 , a coolant connection channel  116   a  and a coolant connection channel  116   b  are formed between the frames  28   a ,  28   b  that are adjacent to each other in the stacking direction. The coolant connection channel  116   a  connects the coolant supply passage  32   a  and the coolant flow field  88  of the second separator  20 . The coolant connection channel  116   b  connects the coolant supply passage  32   a  and the coolant flow field  88  of the first separator  16 . Though not shown, a coolant connection channels connecting the coolant discharge passage  32   b  and the coolant flow field  88  are formed between the frames  28   a ,  28   b.    
     The coolant connection channels  116   a ,  116   b  are formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  28   a , and the outer seal member  74  and the inner seal member  76  of the frame  28   b  at different positions as viewed from the stacking direction. It should be noted that the coolant connection channels  116   a ,  116   b  may be formed in one of the frame  28   a  and the frame  28   b.    
     As shown in  FIG. 13 , the coolant connection channel  116   a  includes the inlet grooves  42   a  (e.g., a first set of grooves),  58   a  (e.g., a second set of grooves) provided along the separator surface, the inlet holes (first holes)  40   a  formed in the frame  28   a  in the stacking direction, and the holes (second holes)  106   a  formed in the metal plate  102   a  of the second separator  20  in the stacking direction. Ends of the inlet grooves  42   a  and ends of the inlet grooves  58   a  are connected together. 
     As shown in  FIG. 14 , the coolant connection channel  116   b  includes the inlet grooves  68   a ,  38   a  provided along the separator surface, the inlet holes (first holes)  60   a  formed in the frame  28   b  in the stacking direction, and the holes (second holes)  90   a  formed in the metal plate  82   a  of the first separator  16  in the stacking direction. Ends of the inlet grooves  68   a  and ends of the inlet grooves  38   a  are connected together. 
     The inlet holes  40   a  of the frame  28   a  and the holes  106   a  are not overlapped with the inlet holes  60   a  of the frame  28   b  and the holes  90   a  as viewed from the stacking direction. 
     Operation of this fuel cell  10  will be described below. 
     As shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  30   a , and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  34   a . Further, a coolant such as pure water, ethylene glycol, or the like is supplied to the coolant supply passage  32   a.    
     In each of the cell units  12 , as shown in  FIGS. 1 and 11 , the oxygen-containing gas supplied to the oxygen-containing gas supply passage  30   a  flows in between the inlet ridges  36   a  of the first membrane electrode assembly  14 , and between the inlet ridges  54   a  of the second membrane electrode assembly  18  into the inlet grooves  56   a.    
     The oxygen-containing gas flowing between the inlet ridges  36   a  is supplied through the inlet grooves  37   a  to the oxygen-containing gas flow field  84  of the second separator  20 . Then, the oxygen-containing gas is supplied from the oxygen-containing gas flow field  84  to the cathode  24  of the first membrane electrode assembly  14 . Thereafter, the oxygen-containing gas is consumed in the power generation reaction, the remaining oxygen-containing gas flows between the outlet ridges  36   b , and then the oxygen-containing gas is discharged into the oxygen-containing gas discharge passage  30   b.    
     In the meanwhile, the oxygen-containing gas flowing between the inlet grooves  56   a  is supplied through the inlet grooves  87   a  between the second membrane electrode assembly  18  and the first separator  16 , and then, the oxygen-containing gas is supplied to the oxygen-containing gas flow field  84  of the first separator  16 . The oxygen-containing gas from the oxygen-containing gas flow field  84  is supplied to the cathode  24  of the second membrane electrode assembly  18 . Thereafter the oxygen-containing gas is consumed in the power generation reaction, the remaining oxygen-containing gas flows from the outlet grooves  87   b ,  56   b  and between the outlet ridges  54   b , and then the oxygen-containing gas is discharged into the oxygen-containing gas discharge passage  30   b.    
     Further, as shown in  FIGS. 1 and 12 , the fuel gas supplied to the fuel gas supply passage  34   a  flows into the inlet grooves  62   a  of the second membrane electrode assembly  18 . The fuel gas from the inlet grooves  62   a  moves toward the anode  26  through the inlet holes  64   a , and then, part of the fuel gas is supplied from the inlet grooves  72   a  to the fuel gas flow field  86  of the second separator  20 . 
     The remaining part of the fuel gas flows through the inlet holes  66   a  and the holes  92   a  of the first separator  16 , and then flows between the first separator  16  and the first membrane electrode assembly  14 . Thereafter, the fuel gas is supplied to the fuel gas flow field  86  of the first separator  16 . 
     After the fuel gas is consumed in the power generation reaction in the fuel gas flow field  86  of the second separator  20 , the consumed fuel gas is discharged into the outlet grooves  72   b . Then, the fuel gas is discharged from the outlet holes  64   b  through the outlet grooves  62   b  into the fuel gas discharge passage  34   b . In the meanwhile, after the fuel gas is consumed in the power generation reaction in the fuel gas flow field  86  of the first separator  16 , the consumed fuel gas is discharged from the holes  92   b  through the outlet holes  66   b  into the outlet grooves  72   b . Then, likewise, the fuel gas is discharged into the fuel gas discharge passage  34   b.    
     Thus, in each of the first membrane electrode assembly  14  and the second membrane electrode assembly  18 , the oxygen-containing gas supplied to the cathode  24  and the fuel gas supplied to the anode  26  are consumed in electrochemical reactions at electrode catalyst layers of the cathode  24  and the anode  26  for generating electricity. 
     Further, as shown  FIGS. 1 and 13 , part of the coolant supplied to the coolant supply passage  32   a  flows into the inlet grooves  42   a  of the first membrane electrode assembly  14 , and then, the coolant is supplied from the inlet grooves  58   a  to the inlet holes  40   a . The coolant from the inlet holes  40   a  flows through the holes  106   a  of the second separator  20  into the second separator  20 . 
     The coolant flows inside the second separator  20  along the inlet grooves  112   a , and is supplied to the coolant flow field  88 . Then, the coolant flows from the outlet grooves  112   b  through the holes  106   b , and then is discharged from the second separator  20 . Further, the coolant flows from the outlet holes  40   b  to the outlet grooves  58   b ,  42   b , and then is discharged into the coolant discharge passage  32   b.    
     In the meanwhile, as shown in  FIGS. 1 and 14 , another part of the coolant supplied to the coolant supply passage  32   a  flows into the inlet grooves  68   a  of the second membrane electrode assembly  18 , and then, the coolant flows through the inlet grooves  38   a  to the inlet holes  60   a . The coolant from the inlet holes  60   a  flows through the holes  90   a  of the first separator  16 , and then, the coolant flows into the first separator  16 . 
     The coolant flows along the inlet grooves  100   a  in the first separator  16 , and then, the coolant is supplied to the coolant flow field  88 . Thereafter the coolant flows from the outlet grooves  100   b  through the holes  90   b , and then, the coolant is discharged from the first separator  16 . Further, the coolant from the outlet holes  60   b  flows through the outlet grooves  38   b ,  68   b , and then is discharged into the coolant discharge passage  32   b.    
     Thus, the first membrane electrode assembly  14  and the second membrane electrode assembly  18  are cooled by the coolant flowing through the coolant flow field  88  in the first separator  16  and the coolant flow field  88  in the second separator  20 . 
     In the first embodiment, all of the fluid passages, i.e., the oxygen-containing gas supply passage  30   a , the coolant supply passage  32   a , the fuel gas supply passage  34   a , the oxygen-containing gas discharge passage  30   b , the coolant discharge passage  32   b , and the fuel gas discharge passage  34   b  extend through the frame  28   a  of the first membrane electrode assembly  14 , and the frame  28   b  of the second membrane electrode assembly  18  in the stacking direction. 
     In the structure, no fluid passages are required in the first separator  16  and the second separator  20 . The outer dimensions of the first separator  16  and the second separator  20  can be determined in such a manner that the outer dimensions of the first separator  16  and the second separator  20  correspond to the power generation area. Thus, reduction in the size and weight of the first separator  16  and the second separator  20  can be achieved easily, and it becomes possible to reduce the production cost of the first separator  16  and the second separator  20 . 
     Accordingly, the first separator  16  and the second separator  20  can be produced efficiently, and it becomes possible to obtain the entire fuel cell  10  economically. 
     Further, the unit cell (two separators and one MEA) in each cell unit  12  has the outer seal member  48  and the inner seal member  50 , and the outer seal member  74  and the inner seal member  76  alternately, as shown in  FIG. 2 . In effect, the seal members are provided only on one surface. In the structure, the size of the fuel cell  10  in the stacking direction is reduced as a whole suitably, and the size reduction of the fuel cell  10  is achieved. 
     Further, the first separator  16  includes the two metal plates  82   a ,  82   b  having the same outer shape. The metal plates  82   a ,  82   b  are stacked together. The outer circumferential ends of the metal plates  82   a ,  82   b  are welded or bonded together to form a hermetical internal space between the metal plates  82   a ,  82   b . Likewise, the second separator  20  includes the two metal plates  102   a ,  102   b  having the same outer shape. The metal plates  102   a ,  102   b  are stacked together. The outer circumferential ends of the metal plates  102   a ,  102   b  are welded or bonded together to form a hermetical internal space between the metal plates  102   a ,  102   b.    
     In the first separator  16 , no seal is required between the metal plates  82   a ,  82   b , and in the second separator  20 , no seal is required between the metal plates  102   a ,  102   b . Therefore, in the first embodiment, the production cost of the first separator  16  and the second separator  20  is reduced effectively, and the fuel cell  10  can be produced economically as a whole. 
     Further, in the first embodiment, as shown in  FIG. 11 , the oxygen-containing gas connection channel  113   a  and the oxygen-containing gas connection channel  113   b  are formed between the frames  28   a ,  28   b  that are adjacent to each other in the stacking direction. The oxygen-containing gas connection channel  113   a  connects the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas flow field  84  of the first membrane electrode assembly  14 , and the oxygen-containing gas connection channel  113   b  connects the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas flow field  84  of the second membrane electrode assembly. 
     The oxygen-containing gas connection channel  113   a  and the oxygen-containing gas connection channel  113   b  are formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  28   a , and the outer seal member  74  and the inner seal member  76  of the frame  28   b  at different positions as viewed from the stacking direction. 
     More specifically, the oxygen-containing gas connection channel  113   b  includes the inlet ridges  54   a  formed on the surface of the frame  28   b  and extending along the separator surface, the inlet grooves  56   a  formed in the frame  28   b , and the inlet grooves  87   a  formed in the surface of the metal plate  82   a  of the first separator  16 . The inlet grooves  87   a  are connected to grooves between the inlet ridges  54   a , and extend along the separator surface. Ends of the inlet grooves  56   a  are connected to ends of the inlet grooves  87   a . Thus, the structure of the fuel cell  10  is simplified, and it becomes possible to reduce the size of the entire fuel cell  10  in the stacking direction. 
     Further, in the first embodiment, as shown in  FIG. 12 , the fuel gas connection channel  114  is formed between the frames  28   a ,  28   b  that are adjacent to each other in the stacking direction. The fuel gas connection channel  114  connects the fuel gas supply passage  34   a  and the fuel gas flow field  86 . 
     The fuel gas connection channels  114  are formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  28   a , and the outer seal member  74  and the inner seal member  76  of the frame  28   b  at different positions as viewed from the stacking direction. 
     The fuel gas connection channel  114  includes the inlet grooves  62   a ,  72   a  formed in the frame  28   b  of the second membrane electrode assembly  18  and extending along the separator surface, and the holes  92   a  extending through the outer circumferential end of the first separator  16  in the stacking direction. 
     More specifically, the frame  28   b  has the inlet holes  64   a  and the inlet holes  66   a , and the inlet grooves  62   a ,  72   a  formed on both surfaces of the frame  28   b  are connected to each other through the inlet holes  64   a . The inlet holes  66   a  are provided coaxially with the holes  92   a  in the stacking direction. The inlet grooves  62   a ,  72   a  are connected to the fuel gas flow field  86  of the first separator  16  through the inlet holes  66   a  and the holes  92   a . The inlet grooves  72   a  are directly connected to the fuel gas flow field  86  of the second separator  20 . 
     Thus, the structure of the fuel cell  10  is simplified, and it becomes possible to reduce the size of the fuel cell  10  in the stacking direction as a whole. 
     Further, in the first embodiment, as shown in  FIGS. 13 and 14 , the coolant connection channel  116   a  connecting the coolant supply passage  32   a  and the coolant flow field  88  of the second separator  20 , and the coolant connection channel  116   b  connecting the coolant supply passage  32   a  and the coolant flow field  88  of the first separator  16  are formed between the frames  28   a ,  28   b  that are adjacent to each other in the stacking direction. 
     The coolant connection channels  116   a ,  116   b  are formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  28   a , and the outer seal member  74  and the inner seal member  76  of the frame  28   b  at different positions as viewed from the stacking direction. 
     More specifically, as shown in  FIG. 13 , the coolant connection channel  116   a  includes the inlet grooves  42   a ,  58   a  provided along the separator surface, the inlet holes  40   a  formed in the frame  28   a  in the stacking direction, and the holes  106   a  formed in the metal plate  102   a  in the stacking direction. Ends of the inlet grooves  42   a  and ends of the inlet grooves  58   a  are connected together. 
     As shown in  FIG. 14 , the coolant connection channel  116   b  includes the inlet grooves  68   a ,  38   a  provided along the separator surface, the inlet holes  60   a  formed in the frame  28   b  in the stacking direction, and the holes  90   a  formed in the metal plate  82   a  in the stacking direction. Ends of the inlet grooves  68   a  and ends of the inlet grooves  38   a  are connected together. 
     In this regard, the inlet holes  40   a  of the frame  28   a  and the holes  106   a  are not overlapped with the inlet holes  60   a  of the frame  28   b  and the holes  90   a  in the stacking direction. Thus, the structure of the fuel cell  10  is simplified, and it becomes possible to reduce the size of the fuel cell  10  in the stacking direction as a whole. 
     In the first embodiment, the channels for oxygen-containing gas may adopt the channel structure for the fuel gas, and the channels for the fuel gas may adopt the channel structure for the oxygen-containing gas. Further, both of the channels for the fuel gas and the channels for the oxygen-containing gas may adopt bridges having the same structure. 
       FIG. 15  is an exploded perspective view showing a fuel cell  120  according to a second embodiment of the present invention. The constituent elements of the fuel cell  120  that are identical to those of the fuel cell  10  according to the first embodiment are labeled with the same reference numerals, and description thereof will be omitted. Also in third and fourth embodiments described later, the constituent elements of the fuel cell that are identical to those of the fuel cell  10  according to the first embodiment are labeled with the same reference numerals, and description thereof will be omitted. 
     As shown in  FIGS. 15 and 16 , the fuel cell  120  is formed by stacking a plurality of cell units  122 , and each of the cell units  122  includes a first membrane electrode assembly (electrolyte electrode assembly) (MEA)  124 , a first separator  126 , a second membrane electrode assembly (electrolyte electrode assembly) (MEA)  128 , and a second separator  130 . 
     The first membrane electrode assembly  124  and the second membrane electrode assembly  128  include a frame  132   a  and a frame  132   b , respectively. As shown in  FIG. 17 , at upper positions of the cathode surface  124   a  of the frame  132   a , no inlet grooves  38   a  are provided adjacent to the lower portion of the coolant supply passage  32   a , and a plurality of inlet holes  134   a  are formed along the width direction of the coolant supply passage  32   a . The inlet holes  134   a  are surrounded by a ring-shaped inlet seal member  136   a.    
     At lower positions of the cathode surface  124   a  of the frame  132   a , no outlet grooves  38   b  are provided adjacent to the upper portion of the coolant discharge passage  32   b , and a plurality of outlet holes  134   b  are formed along the width direction of the coolant discharge passage  32   b . The outlet holes  134   b  are surrounded by a ring-shaped outlet seal member  136   b.    
     As shown in  FIG. 18 , at upper positions of the anode surface  124   b  of the frame  132   a , a plurality of inlet grooves  138   a  corresponding to the inlet holes  134   a  are provided, and at lower positions of the anode surface  124   b , a plurality of outlet grooves  138   b  corresponding to the outlet holes  134   b  are provided. 
     As shown in  FIG. 19 , at upper positions of the cathode surface  128   a  of the frame  132   b , no inlet holes  60   a  are provided adjacent to the lower portion of the coolant supply passage  32   a , and a plurality of inlet grooves  140   a  are formed along the width direction of the coolant supply passage  32   a.    
     At lower positions of the cathode surface  128   a  of the frame  132   b , no outlet holes  60   b  are provided adjacent to the upper portion of the coolant discharge passage  32   b , and a plurality of outlet grooves  140   b  are formed along the width direction of the coolant discharge passage  32   b.    
     As shown in  FIG. 20 , neither the inlet grooves  68   a  nor the outlet grooves  68   b  are provided on the anode surface  128   b  of the frame  132   b.    
     The first separator  126  is formed of a single metal plate member. As shown in  FIG. 21 , a plurality of holes  92   a  and a plurality of inlet grooves  87   a  are formed above the oxygen-containing gas flow field  84  provided on one surface (e.g., a second surface of the first separator) of the first separator  126 , but no holes  90   a  are provided. A plurality of holes  92   b  and a plurality of grooves  87   b  are formed below the oxygen-containing gas flow field  84 , but no holes  90   b  are provided. 
     As shown in  FIG. 22 , a plurality of inlet grooves  98   a  are provided above the fuel gas flow field  86  formed on the other surface (e.g., a first surface of the first separator) of the first separator  126 , but no inlet grooves  100   a  are provided. A plurality of outlet grooves  98   b  are provided below the fuel gas flow field  86 , but no outlet grooves  100   b  are provided. 
     As shown in  FIG. 23 , the second separator  130  includes two metal plates (e.g., stainless plates)  142   a ,  142   b  having the same outer shape. The metal plates  142   a ,  142   b  are stacked together. The outer circumferential ends of the metal plates  142   a ,  142   b  are welded or bonded together, and the internal space between the metal plates  142   a ,  142   b  is closed hermetically. The metal plate  142   a  has an oxygen-containing gas flow field  84  facing the cathode  24 , and the metal plate  142   b  has a fuel gas flow field  86  facing the anode  26 . A coolant flow field  88  is formed between the metal plates  142   a ,  142   b.    
     As shown in  FIG. 23 , at the upper positions of the metal plate  142   a , a plurality of holes  144   a  are formed below the coolant supply passage  32   a , over the width direction of the coolant supply passage  32   a . A plurality of holes  144   b  are formed below the oxygen-containing gas flow field  84  and above the coolant discharge passage  32   b , over the width direction of the coolant discharge passage  32   b.    
     As shown in  FIG. 24 , at the upper positions of the metal plate  142   b , a plurality of inlet grooves  146   a  are formed below the coolant supply passage  32   a , over the width direction of the coolant supply passage  32   a . A plurality of outlet grooves  146   b  are formed below the fuel gas flow field  86  and above the coolant discharge passage  32   b , over the width direction of the coolant discharge passage  32   b.    
     As shown in  FIG. 25 , an oxygen-containing gas connection channel  150   a  connecting the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas flow field  84  of the first membrane electrode assembly  124  and an oxygen-containing gas connection channel  150   b  connecting the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas flow field  84  of the second membrane electrode assembly  128  are formed between the frames  132   a ,  132   b  that are adjacent to each other in the stacking direction. Though not shown, oxygen-containing gas connection channels connecting the oxygen-containing gas discharge passage  30   b  and the oxygen-containing gas flow fields  84  are formed between the frames  132   a ,  132   b  in the same manner. 
     The oxygen-containing gas connection channel  150   a  and the oxygen-containing gas connection channel  150   b  are formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  132   a , and the outer seal member  74  and the inner seal member  76  of the frame  132   b  at different positions as viewed from the stacking direction. 
     The oxygen-containing gas connection channel  150   b  includes inlet ridges (first grooves)  54   a  formed on the surface of the frame  132   b  and extending along the separator surface, inlet grooves  56   a  formed in the surface of the frame  132   b , and inlet grooves (second grooves)  87   a  formed in the surface of the first separator  126 . The inlet grooves  87   a  are connected to the grooves between the inlet ridges  54   a , and extend along the separator surface. Ends of the inlet grooves  56   a  are connected to ends of the inlet grooves  87   a.    
     The oxygen-containing gas connection channel  150   a  includes inlet ridges  36   a  formed on the surface of the frame  132   a  along the separator surface, and inlet grooves  37   a.    
     As shown in  FIG. 26 , a fuel gas connection channel  152  connecting the fuel gas supply passage  34   a  and the fuel gas flow field  86  is formed between the frames  132   a ,  132   b  that are adjacent to each other in the stacking direction. Though not shown, fuel gas connection channels connecting the fuel gas discharge passage  34   b  and the fuel gas flow field  86  are formed between the frames  132   a ,  132   b  in the same manner. 
     The fuel gas connection channel  152  is formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  132   a , and the outer seal member  74  and the inner seal member  76  of the frame  132   b  at different positions as viewed from the stacking direction. 
     The fuel gas connection channel  152  includes the inlet grooves  62   a ,  72   a  formed in the frame  132   b  of the second membrane electrode assembly  128  and extending along the separator surface, and the holes  92   a  extending through the outer circumferential end of the first separator  126  in the stacking direction. It should be noted that the inlet grooves  62   a  may be provided in the frame  132   a  of the first membrane electrode assembly  124 . 
     More specifically, the frame  132   b  has the inlet holes  64   a  and the inlet holes  66   a , and the inlet grooves  62   a ,  72   a  formed on both surfaces of the frame  132   b  are connected to each other through the inlet holes  64   a . The inlet holes  66   a  are provided coaxially with, or offset from the holes  92   a  in the stacking direction. The inlet grooves  62   a ,  72   a  are connected to the fuel gas flow field  86  of the first separator  16  through the inlet holes  66   a  and the holes  92   a . The inlet grooves  72   a  are directly connected to the fuel gas flow field  86  of the second separator  130 . 
     As shown in  FIG. 27 , a coolant connection channel  154  connecting the coolant supply passage  32   a  and the coolant flow field  88  of the second separator  130  is formed between the frames  132   a ,  132   b  that are adjacent to each other in the stacking direction. Though not shown, a coolant connection channel connecting the coolant discharge passage  32   b  and the coolant flow field  88  is formed between the frames  132   a ,  132   b  in the same manner. 
     The coolant connection channel  154  is formed by arranging the outer seal member  48  and the inner seal member  50  of the frame  132   a  and the outer seal member  74  and the inner seal member  76  of the frame  132   b  at different positions as viewed from the stacking direction. 
     The coolant connection channel  154  includes the inlet grooves  138   a ,  140   a  provided along the separator surface, the inlet holes (first holes)  134   a  formed in the frame  132   a  in the stacking direction, and the holes (second holes)  144   a  formed in the metal plate  142   a  in the stacking direction. 
     Ends of the inlet grooves  138   a  and ends of the inlet grooves  140   a  are connected together. 
     Operation of the fuel cell  120  will be described briefly below. 
     In each of the cell units  122 , as shown in  FIGS. 15 and 25 , the oxygen-containing gas supplied to the oxygen-containing gas supply passage  30   a  flows in between the inlet ridges  36   a  of the first membrane electrode assembly  124 , and between the inlet ridges  54   a  of the second membrane electrode assembly  128  into the inlet grooves  56   a.    
     The oxygen-containing gas flowing between the inlet ridges  36   a  is supplied through the inlet grooves  37   a  to the oxygen-containing gas flow field  84  of the second separator  130 . Then, the oxygen-containing gas is supplied from the oxygen-containing gas flow field  84  to the cathode  24  of the first membrane electrode assembly  124 . The remaining oxygen-containing gas after consumption in the power generation reaction flows between the outlet ridges  36   b , and then is discharged into the oxygen-containing gas discharge passage  30   b.    
     The oxygen-containing gas supplied to the inlet grooves  56   a  flows through the inlet grooves  87   a  between the second membrane electrode assembly  128  and the first separator  126 , and the oxygen-containing gas is supplied into the oxygen-containing gas flow field  84  of the first separator  126 . The oxygen-containing gas is supplied from the oxygen-containing gas flow field  84  to the cathode  24  of the second membrane electrode assembly  128 . The remaining oxygen-containing gas after consumption in the power generation reaction flows from the outlet grooves  87   b ,  56   b  and between the outlet ridges  54   b , and then is discharged into the oxygen-containing gas discharge passage  30   b.    
     Further, as shown in  FIGS. 15 and 26 , the fuel gas supplied to the fuel gas supply passage  34   a  flows into the inlet grooves  62   a  of the second membrane electrode assembly  128 . The fuel gas flows from the inlet grooves  62   a  through the inlet holes  64   a  toward the anode  26 , and some of the fuel gas is supplied from the inlet grooves  72   a  to the fuel gas flow field  86  of the second separator  130 . 
     The remaining fuel gas flows through the inlet holes  66   a  and the holes  92   a  of the first separator  126 , and then flows in between the first separator  126  and the first membrane electrode assembly  124 . Then, the fuel gas is supplied to the fuel gas flow field  86  of the first separator  126 . 
     The fuel gas that has been consumed in the power generation reaction in the fuel gas flow field  86  of the second separator  130  is discharged into the outlet grooves  72   b . Then, the fuel gas flows from the outlet holes  64   b , and is discharged through the outlet grooves  62   b  into the fuel gas discharge passage  34   b . In the meanwhile, the fuel gas that has been consumed in the power generation reaction in the fuel gas flow field  86  of the first separator  126  flows from the holes  92   b , and then is discharged through the outlet holes  66   b  into the outlet grooves  72   b . Likewise, the fuel gas is discharged into the fuel gas discharge passage  34   b.    
     Thus, in the first membrane electrode assembly  124  and the second membrane electrode assembly  128 , the oxygen-containing gas supplied to the cathode  24  and the fuel gas supplied to the anode  26  are consumed in electrochemical reactions at electrode catalyst layers of the cathode  24  and the anode  26  for generating electricity. 
     Further, as shown in  FIGS. 15 and 27 , the coolant supplied to the coolant supply passage  32   a  flows into the inlet grooves  138   a  of the first membrane electrode assembly  124 , and then the coolant is supplied from the inlet grooves  140   a  to the inlet holes  134   a . The coolant from the inlet holes  134   a  flows through the holes  144   a  of the second separator  130  into the second separator  130 . 
     The coolant flows inside the second separator  130  along the inlet grooves  146   a , and then is supplied to the coolant flow field  88 . The coolant flows from the outlet grooves  146   b  through the holes  144   b , and then is discharged from the second separator  130 . The coolant flows from the outlet holes  134   b  through the outlet grooves  140   b ,  138   b , and then is discharged into the coolant discharge passage  32   b.    
     In the structure, the first membrane electrode assembly  124  and the second membrane electrode assembly  128  are cooled by skip cooling by the coolant flowing through the coolant flow field  88  of the second separator  130 . 
     In the second embodiment, the same advantages as in the case of the first embodiment are obtained. For example, reduction in the size and weight of the first separator  126  and the second separator  130  is achieved easily, the production cost is reduced effectively, and it becomes possible to produce the fuel cell  120  economically as a whole. 
       FIG. 28  is an exploded perspective view showing a fuel cell  160  according to a third embodiment of the present invention. 
     The fuel cell  160  is formed by stacking a plurality of cell units  162  together. Each of the cell units  162  includes a first membrane electrode assembly  14 , a first separator  164 , a second membrane electrode assembly  18 , and a second separator  20 . The first separator  164  includes two metal plates  82   a ,  82   b . The outer circumferential ends of the metal plates  82   a ,  82   b  are welded or bonded together to form a hermetical internal space between the metal plates  82   a ,  82   b.    
     As shown in  FIGS. 29 and 30 , a plurality of holes  92   a ,  92   b  pass through the metal plates  82   a ,  82   b . The metal plates  82   a ,  82   b  are welded or bonded together around the holes  92   a ,  92   b  to form joint portions  166   a ,  166   b  between the two metal plates  82   a ,  82   b , the joint portions  166   a ,  166   b  functioning as seals between the holes  92   a ,  92   b  and the internal space (coolant flow field  88 ). 
     In the third embodiment, the same advantages as in the cases of the first and second embodiments are obtained. Further, the joint portions  166   a ,  166   b  are provided around the holes  92   a ,  92   b  as seals. Therefore, entry of the fuel gas from the holes  92   a ,  92   b  into the space between the metal plates  82   a ,  82   b  (internal space of the first separator  164 ) is prevented as much as possible. 
       FIG. 31  is a cross sectional view showing a fuel cell  170  according to a fourth embodiment of the present invention. 
     In the fuel cell  170 , the first separator  16  does not have any inlet grooves  87   a  and outlet grooves  87   b . Therefore, in particular, the structure of the first separator  16  is simplified economically.