Abstract:
A fuel cell is provided with a power generation unit; the power generation unit is provided with a first metal separator, a first electrolyte membrane/electrode structure, a second metal separator, a second electrolyte membrane/electrode structure, and a third metal separator. The first electrolyte membrane/electrode structure is provided with a first resin frame member at the outer periphery, and the first resin frame member is provided with an inlet buffer section positioned outside a power generation region and coupled to a first oxidant gas flow path, and a protruding section, which is one part of an inlet coupling flow path coupling together the inlet buffer section and an oxidant gas inlet communication hole.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application PCT/JP2013/051777, filed Jan. 28, 2013, which claims priority to Japanese Patent Application No. 2012-066746 filed on Mar. 23, 2012 in Japan. The contents of the aforementioned applications are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and metal separators in a stacking direction. The membrane electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. Each of the metal separators has a reactant gas flow field for supplying a reactant gas along an electrode surface and a reactant gas passage for allowing the reactant gas to flow in the stacking direction. A resin frame member is provided at an outer circumferential portion of the membrane electrode assembly. 
     BACKGROUND ART 
     For example, a solid polymer electrolyte fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane, and the solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly make up a power generation cell (unit cell). In the fuel cell of this type, in use, typically, several tens to several hundreds of the power generation cells are stacked together to form a fuel cell stack, for example, mounted in a vehicle. 
     In many cases, the fuel cell of this type adopts so called internal manifold structure for supplying a fuel gas and an oxygen-containing gas as reactant gases, respectively, to the anode and the cathode of each of the stacked power generation cells. 
     In the internal manifold, reactant gas supply passages (fuel gas supply passage, oxygen-containing gas supply passage) and reactant gas discharge passages (fuel gas discharge passage, oxygen-containing gas discharge passage) extend through the power generation cells in the stacking direction. Each of the reactant gas supply passages is connected to the inlet of a reactant gas flow field (fuel gas flow field, oxygen-containing gas flow field) for supplying the reactant gas along the electrode surface, and each of the reactant gas discharge passages is connected to the outlet of the reactant gas flow field. 
     In this case, the reactant gas supply passage and the reactant gas discharge passage are connected to the reactant gas flow field through connection channels including parallel grooves or the like, for allowing the reactant gas to flow smoothly and uniformly. In this regard, in order to prevent entry of seal members into the connection channels, for example, metal plates are provided to cover the connection channels. However, since dedicated metal plates are used, the structure is complicated, and thus the number of production steps is increased uneconomically. 
     As a technique aimed to address the problem, for example, a fuel cell disclosed in Japanese Patent No. 4634933 is known. In the fuel cell, a membrane electrode assembly and separators are stacked together. The membrane electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes. Reactant gas flow fields are formed between the electrolyte electrode assembly and the separators for supplying reactant gases along the electrode surfaces, and reactant gas passages connected to the reactant gas flow fields extend through the fuel cell in the stacking direction. 
     Further, the separators have connection channels connecting the reactant gas passages and the reactant gas flow fields, and at least one of gas diffusion layers of the electrolyte electrode assembly includes an overlapped portion which is overlapped with the connection channel such that the overlapped portion is tightly attached on the separator to seal the connection channels. 
     SUMMARY OF INVENTION 
     In general, the gas diffusion layer of the electrolyte electrode assembly is made of carbon paper or the like. Therefore, if the connection channel is sealed by the gas diffusion layer, since the gas diffusion layer itself tends to be deformed easily, clogging may occur in the connection channel undesirably. 
     The present invention has been made to solve the problem of this type, and an object of the present invention is to provide a fuel cell having simple and economical structure in which connection channels are sealed suitably without occurrence of any clogging. 
     According to the present invention, there is provided a fuel cell formed by stacking a membrane electrode assembly and metal separators in a stacking direction. The membrane electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. Each of the metal separators has a reactant gas flow field for supplying a reactant gas along an electrode surface and a reactant gas passage for allowing the reactant gas to flow in the stacking direction. A resin frame member is provided at an outer circumferential portion of the membrane electrode assembly. 
     In the fuel cell, the outer shape of the resin frame member is configured such that the resin frame member is positioned inward relative to the reactant gas passage provided at an outer circumference of each of the metal separators. The resin frame member has a buffer positioned outside a power generation area and connected to the reactant gas flow field, and a part of a connection channel connecting the buffer and the reactant gas passage. 
     In the present invention, the resin frame member is provided at the outer circumferential portion of the membrane electrode assembly, and the resin frame member has the buffer and the part of the connection channel. In the structure, since the resin frame member having relatively high rigidity is used, the resin frame member is not deformed. Thus, with the simple and economical structure, it is possible to reliably prevent occurrence of clogging in the connection channel, while achieving a desired sealing performance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view showing main components of a power generation unit of a fuel cell according to a first embodiment of the present invention; 
         FIG. 2  is a cross sectional view showing the power generation unit taken along a line II-II in  FIG. 1 ; 
         FIG. 3  is a cross sectional view showing the power generation unit taken along a line III-III in  FIG. 1 ; 
         FIG. 4  is a cross sectional view showing the power generation unit taken along a line IV-IV in  FIG. 1 ; 
         FIG. 5  is a cross sectional view showing the power generation unit taken along a line V-V in  FIG. 1   
         FIG. 6  is a front view showing a first metal separator of the power generation unit; 
         FIG. 7  is a view showing one surface of a second metal separator of the power generation unit; 
         FIG. 8  is a view showing the other surface of the second metal separator; 
         FIG. 9  is a view showing one surface of a third metal separator of the power generation unit; 
         FIG. 10  is a view showing the other surface of the third metal separator; 
         FIG. 11  is a view showing one surface of a first membrane electrode assembly of the power generation unit; 
         FIG. 12  is a view showing the other surface of the first membrane electrode assembly; 
         FIG. 13  is a view showing one surface of a second membrane electrode assembly of the power generation unit; 
         FIG. 14  is a view showing the other surface of the second membrane electrode assembly; 
         FIG. 15  is a cross sectional view showing the first metal separator taken along a line XV-XV in  FIG. 6 ; 
         FIG. 16  is an exploded perspective view showing main components of a power generation unit of a fuel cell according to a second embodiment of the present invention; and 
         FIG. 17  is a cross sectional view showing the power generation unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIGS. 1 to 5 , a fuel cell  10  according to a first embodiment of the present invention includes a power generation unit  12 . A plurality of the power generation units  12  are stacked together in a horizontal direction indicated by an arrow A or in a vertical direction indicated by an arrow C. Each of the power generation units  12  includes a first metal separator  14 , a first membrane electrode assembly  16   a , a second metal separator  18 , a second membrane electrode assembly  16   b , and a third metal separator  20 . 
     The first metal separator  14 , the second metal separator  18 , and the third metal separator  20  are made of, e.g., laterally-elongated metal plates such as steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces by surface treatment. Each of the first metal separator  14 , the second metal separator  18 , and the third metal separator  20  has a rectangular planar surface, and is formed by corrugating a thin metal plate by press forming to have a corrugated shape (ridges and recesses) in cross section and a wavy or serpentine shape on the surface. 
     As shown in  FIG. 1 , at one end of the power generation unit  12  in the long-side direction indicated by the arrow B, specifically, at one end (outer end) of each of the first metal separator  14 , the second metal separator  18 , and the third metal separator  20  in the long-side direction, an oxygen-containing gas supply passage  22   a  for supplying an oxygen-containing gas and a fuel gas discharge passage  24   b  for discharging a fuel gas such as a hydrogen-containing gas are provided. The oxygen-containing gas supply passage  22   a  and the fuel gas discharge passage  24   b  extend through the power generation unit  12  in the direction indicated by the arrow A. 
     At the other end (outer end) of the power generation unit  12  in the long-side direction indicated by the arrow B, a fuel gas supply passage  24   a  for supplying the fuel gas and an oxygen-containing gas discharge passage  22   b  for discharging the oxygen-containing gas are provided. The fuel gas supply passage  24   a  and the oxygen-containing gas discharge passage  22   b  extend through the power generation unit  12  in the direction indicated by the arrow A. 
     At both ends of the power generation unit  12  in a short-side direction indicated by an arrow C, a pair of coolant supply passages  25   a  for supplying a coolant are provided on one side adjacent to the oxygen-containing gas supply passage  22   a . The coolant supply passages  25   a  extend through the power generation unit  12  in the direction indicated by the arrow A. At both ends of the power generation unit  12  in the short-side direction, a pair of coolant discharge passages  25   b  for discharging the coolant are provided on the other side adjacent to the fuel gas supply passage  24   a.    
     As shown in  FIG. 6 , the first metal separator  14  has a first oxygen-containing gas flow field  26  on its surface  14   a  facing the first membrane electrode assembly  16   a . The first oxygen-containing gas flow field  26  is connected to the oxygen-containing gas supply passage  22   a  and the oxygen-containing gas discharge passage  22   b.    
     The first oxygen-containing gas flow field  26  includes a plurality of wavy flow grooves (or straight flow grooves)  26   a  extending in the direction indicated by the arrow B. A plurality of inlet bosses  28   a  are provided adjacent to the inlet of the first oxygen-containing gas flow field  26 , and a plurality of outlet bosses  28   b  are provided adjacent to the outlet of the first oxygen-containing gas flow field  26 . 
     A plurality of inlet connection grooves  30   a , which are formed as part of a bridge section, are formed between the inlet bosses  28   a  and the oxygen-containing gas supply passage  22   a , and a plurality of outlet connection grooves  30   b , which are formed as part of a bridge section, are formed between the outlet bosses  28   b  and the oxygen-containing gas discharge passage  22   b.    
     As shown in  FIG. 1 , a coolant flow field  32  is formed on a surface  14   b  of the first metal separator  14 . The coolant flow field  32  is connected to the pair of coolant supply passages  25   a  and the pair of coolant discharge passages  25   b . The coolant flow field  32  is formed by stacking the back surface of the first oxygen-containing gas flow field  26  and the back surface of a second fuel gas flow field  42  to be described later. 
     As shown in  FIG. 7 , the second metal separator  18  has a first fuel gas flow field  34  on its surface  18   a  facing the first membrane electrode assembly  16   a . The first fuel gas flow field  34  is connected to the fuel gas supply passage  24   a  and the fuel gas discharge passage  24   b . The first fuel gas flow field  34  includes a plurality of wavy flow grooves (or straight flow grooves)  34   a  extending in the direction indicated by the arrow B. A plurality of supply holes  36   a  are formed adjacent to the fuel gas supply passage  24   a , and a plurality of discharge holes  36   b  are formed adjacent to the fuel gas discharge passage  24   b.    
     As shown in  FIG. 8 , the second metal separator  18  has a second oxygen-containing gas flow field  38  on its surface  18   b  facing the second membrane electrode assembly  16   b . The second oxygen-containing gas flow field  38  is connected to the oxygen-containing gas supply passage  22   a  and the oxygen-containing gas discharge passage  22   b . The second oxygen-containing gas flow field  38  includes wavy flow grooves (or straight flow grooves)  38   a  extending in the direction indicated by the arrow B. A plurality of inlet connection grooves  40   a  are formed adjacent to the oxygen-containing gas supply passage  22   a , and a plurality of outlet connection grooves  40   b  are formed adjacent to the oxygen-containing gas discharge passage  22   b.    
     As shown in  FIG. 9 , the third metal separator  20  has the second fuel gas flow field  42  on its surface  20   a  facing the second membrane electrode assembly  16   b . The second fuel gas flow field  42  is connected to the fuel gas supply passage  24   a  and the fuel gas discharge passage  24   b . The second fuel gas flow field  42  includes a plurality of wavy flow grooves (or straight flow grooves)  42   a  extending in the direction indicated by the arrow B. 
     A plurality of supply holes  44   a  are formed adjacent to the fuel gas supply passage  24   a , and a plurality of discharge holes  44   b  are formed adjacent to the fuel gas discharge passage  24   b . As shown in  FIG. 3 , the supply holes  44   a  are positioned inward relative to the supply holes  36   a  of the second metal separator  18  (closer to the fuel gas flow field), and as shown in  FIG. 4 , the discharge holes  44   b  are positioned inward relative to the discharge holes  36   b  of the second metal separator  18  (closer to the fuel gas flow field). 
     As shown in  FIG. 10 , part of the coolant flow field  32 , which is the back surface of the second fuel gas flow field  42 , is formed on a surface  20   b  of the third metal separator  20 . The surface  20   b  of the third metal separator  20  is stacked on the surface  14   b  of the first metal separator  14  adjacent to the third metal separator  20  to thereby form the coolant flow field  32  between the third metal separator  20  and the first metal separator  14 . 
     As shown in  FIG. 1 , a first seal member  46  is formed integrally with the surfaces  14   a ,  14   b  of the first metal separator  14 , around the outer circumferential end of the first metal separator  14 . A second seal member  48  is formed integrally with the surfaces  18   a ,  18   b  of the second metal separator  18 , around the outer circumferential end of the second metal separator  18 . A third seal member  50  is formed integrally with the surfaces  20   a ,  20   b  of the third metal separator  20 , around the outer circumferential end of the third metal separator  20 . 
     Each of the first seal member  46 , the second seal member  48 , and the third seal member  50  is made of seal material, cushion material, or packing material such as an EPDM, an NBR, 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. 
     As shown in  FIG. 6 , the first seal member  46  includes a first ridge seal  46   a  on the surface  14   a  of the first metal separator  14 . The first ridge seal  46   a  surrounds the oxygen-containing gas supply passage  22   a , the oxygen-containing gas discharge passage  22   b , and the first oxygen-containing gas flow field  26 , while allowing the oxygen-containing gas supply passage  22   a  and the oxygen-containing gas discharge passage  22   b  to be connected to the first oxygen-containing gas flow field  26  at outer ends thereof. As shown in  FIG. 1 , the first seal member  46  further includes a second ridge seal  46   b  on the surface  14   b  of the first metal separator  14 . The second ridge seal  46   b  surrounds the coolant supply passages  25   a , the coolant discharge passages  25   b , and the coolant flow field  32 , while allowing the coolant supply passages  25   a  and the coolant discharge passages  25   b  to be connected to the coolant flow field  32  at outer ends thereof. 
     As shown in  FIG. 7 , the second seal member  48  includes a first ridge seal  48   a  on the surface  18   a  of the second metal separator  18 . The first ridge seal  48   a  surrounds the supply holes  36   a , the discharge holes  36   b , and the first fuel gas flow field  34 , while allowing the supply holes  36   a  and the discharge holes  36   b  to be connected to the first fuel gas flow field  34 . 
     As shown in  FIG. 8 , the second seal member  48  further includes a second ridge seal  48   b  on the surface  18   b  of the second metal separator  18 . The second ridge seal  48   b  surrounds the oxygen-containing gas supply passage  22   a , the oxygen-containing gas discharge passage  22   b , and the second oxygen-containing gas flow field  38 , while allowing the oxygen-containing gas supply passage  22   a  and the oxygen-containing gas discharge passage  22   b  to be connected to the second oxygen-containing gas flow field  38  at outer ends thereof. 
     As shown in  FIG. 9 , the third seal member  50  includes a first ridge seal  50   a  on the surface  20   a  of the third metal separator  20 . The first ridge seal  50   a  surrounds the supply holes  44   a , the discharge holes  44   b , and the second fuel gas flow field  42 , while allowing the supply holes  44   a  and the discharge holes  44   b  to be connected to the second fuel gas flow field  42 . 
     As shown in  FIG. 10 , the third seal member  50  further includes a second ridge seal  50   b  on the surface  20   b  of the third metal separator  20 . The second ridge seal  50   b  surrounds the coolant supply passages  25   a , the coolant discharge passages  25   b , and the coolant flow field  32 , while allowing the coolant supply passages  25   a  and the coolant discharge passages  25   b  to be connected to the coolant flow field  32  at outer ends thereof. 
     As shown in  FIG. 2 , each of the first membrane electrode assembly  16   a  and the second membrane electrode assembly  16   b  includes a solid polymer electrolyte membrane  52 , and a cathode  54  and an anode  56  sandwiching the solid polymer electrolyte membrane  52 . The solid polymer electrolyte membrane  52  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. The surface area of the cathode  54  is smaller than the surface areas of the anode  56  and the solid polymer electrolyte membrane  52  to form a stepped-type MEA having different sizes of components. It should be noted that the cathode  54 , the anode  56 , and the solid polymer electrolyte membrane  52  may have the same surface area. Further, the surface area of the anode  56  may be smaller than the surface areas of the cathode  54  and the solid polymer electrolyte membrane  52 . 
     Each of the cathode  54  and the anode  56  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 of the cathode  54  and the electrode catalyst layer of the anode  56  are formed on both surfaces of the solid polymer electrolyte membrane  52 , respectively. 
     In the first membrane electrode assembly  16   a , a first resin frame member  58  is formed integrally with the outer circumferential edge portion of the solid polymer electrolyte membrane  52 , outside the terminal end portion of the cathode  54 , e.g., by injection molding. In the second membrane electrode assembly  16   b , a second resin frame member  60  is formed integrally with the outer circumferential edge portion of the solid polymer electrolyte membrane  52 , outside the terminal end portion of the cathode  54 , e.g., injection molding. 
     The outer shapes of the first resin frame member  58  and the second resin frame member  60  are configured such that the first resin frame member  58  and the second resin frame member  60  are positioned inward relative to the oxygen-containing gas supply passage  22   a , the oxygen-containing gas discharge passage  22   b , the fuel gas supply passage  24   a , the fuel gas discharge passage  24   b , the coolant supply passages  25   a , and the coolant discharge passages  25   b  (inwardly in the direction indicated by the arrow B and in the direction indicated by the arrow C). 
     As the resin material of the first resin frame member  58  and the second resin frame member  60 , for example, in addition to general purpose plastic, for example, engineering plastic, super engineering plastic or the like is adopted. 
     As shown in  FIGS. 11 and 12 , the first resin frame member  58  includes extensions  58   a ,  58   b  protruding respectively toward the oxygen-containing gas supply passage  22   a  and the oxygen-containing gas discharge passage  22   b , at both ends thereof in the longitudinal direction indicated by the arrow B, and further includes extensions  58   c ,  58   d  protruding respectively toward the fuel gas supply passage  24   a  and the fuel gas discharge passage  24   b , at the both ends. 
     As shown in  FIG. 11 , on a surface of the first resin frame member  58  on a side where the cathode  54  is provided, an inlet buffer  62   a  is provided between the oxygen-containing gas supply passage  22   a  and the inlet of the first oxygen-containing gas flow field  26 , and an outlet buffer  62   b  is provided between the oxygen-containing gas discharge passage  22   b  and the outlet of the first oxygen-containing gas flow field  26 . 
     The inlet buffer  62   a  includes a plurality of linear ridges  64   a  formed integrally with the first resin frame member  58 , and inlet guide grooves  66   a  are formed between the ridges  64   a . The outlet buffer  62   b  includes a plurality of linear ridges  64   b  formed integrally with the first resin frame member  58 , and outlet guide grooves  66   b  are formed between the ridges  64   b . A plurality of bosses  63   a ,  63   b  are formed in the inlet buffer  62   a  and the outlet buffer  62   b , respectively. The inlet buffer  62   a  and the outlet buffer  62   b  may include only the linear ridges or only the bosses. 
     As shown in  FIG. 12 , on a surface of the first resin frame member  58  on a side where the anode  56  is provided, an inlet buffer  68   a  is provided between the fuel gas supply passage  24   a  and the first fuel gas flow field  34 , and an outlet buffer  68   b  is provided between the fuel gas discharge passage  24   b  and the first fuel gas flow field  34 . 
     The inlet buffer  68   a  includes a plurality of linear ridges  70   a , and inlet guide grooves  72   a  are formed between the ridges  70   a . The outlet buffer  68   b  includes a plurality of linear ridges  70   b , and outlet guide grooves  72   b  are formed between the ridges  70   b . A plurality of bosses  69   a ,  69   b  are formed in the inlet buffer  68   a  and the outlet buffer  68   b , respectively. The inlet buffer  68   a  and the outlet buffer  68   b  may include only the linear ridges or only the bosses. 
     As shown in  FIGS. 13 and 14 , the second resin frame member  60  of the second membrane electrode assembly  16   b  includes extensions  60   a ,  60   b ,  60   c , and  60   d , protruding toward the oxygen-containing gas supply passage  22   a , the oxygen-containing gas discharge passage  22   b , the fuel gas supply passage  24   a , and the fuel gas discharge passage  24   b , respectively. 
     As shown in  FIG. 13 , on a surface of the second resin frame member  60  on a side where the cathode  54  is provided, an inlet buffer  74   a  is provided between the oxygen-containing gas supply passage  22   a  and the second oxygen-containing gas flow field  38 , and an outlet buffer  74   b  is provided between the oxygen-containing gas discharge passage  22   b  and the second oxygen-containing gas flow field  38 . 
     The inlet buffer  74   a  includes a plurality of linear ridges  76   a , and inlet guide grooves  78   a  are formed between the ridges  76   a . The outlet buffer  74   b  includes a plurality of linear ridges  76   b , and outlet guide grooves  78   b  are formed between the ridges  76   b . A plurality of bosses  75   a ,  75   b  are formed in the inlet buffer  74   a  and the outlet buffer  74   b , respectively. 
     As shown in  FIG. 14 , on a surface of the second resin frame member  60  on a side where the anode  56  is provided, an inlet buffer  80   a  is provided between the fuel gas supply passage  24   a  and the second fuel gas flow field  42 , and an outlet buffer  80   b  is provided between the fuel gas discharge passage  24   b  and the second fuel gas flow field  42 . 
     The inlet buffer  80   a  includes a plurality of linear ridges  82   a , and inlet guide grooves  84   a  are formed between the ridges  82   a . The outlet buffer  80   b  includes a plurality of linear ridges  82   b , and outlet guide grooves  84   b  are formed between the ridges  82   b . A plurality of bosses  81   a ,  81   b  are formed in the inlet buffer  80   a  and the outlet buffer  80   b , respectively. 
     As shown in  FIG. 3 , the fuel gas supply passage  24   a  and the first fuel gas flow field  34  are connected through an inlet connection channel  86   a  and the inlet buffer  68   a , and the fuel gas supply passage  24   a  and the second fuel gas flow field  42  are connected through an inlet connection channel  88   a  and the inlet buffer  80   a.    
     The inlet connection channel  86   a  is formed between the fuel gas supply passage  24   a  and the inlet buffer  68   a . The inlet connection channel  86   a  includes a first channel  90   a  formed between the second metal separator  18  and the third metal separator  20  that are adjacent to each other, the supply holes  36   a  formed in the second metal separator  18 , and a second channel  92   a  formed between the second metal separator  18  and the extension  58   c  of the first resin frame member  58 . One end of the first channel  90   a  is connected to the fuel gas supply passage  24   a , and the other end of the first channel  90   a  is connected to the supply holes  36   a . One end of the second channel  92   a  is connected to the supply holes  36   a , and the other end of the second channel  92   a  is connected to the inlet buffer  68   a.    
     Likewise, the inlet connection channel  88   a  includes a first channel  94   a  formed between the third metal separator  20  and the first metal separator  14  that are adjacent to each other, the supply holes  44   a  formed in the third metal separator  20 , and a second channel  96   a  formed between the third metal separator  20  and the extension  60   c  of the second resin frame member  60 . One end of the first channel  94   a  is connected to the fuel gas supply passage  24   a , and the other end of the first channel  94   a  is connected to the supply holes  44   a . One end of the second channel  96   a  is connected to the supply holes  44   a , and the other end of the second channel  96   a  is connected to the inlet buffer  80   a.    
     As shown in  FIG. 4 , the fuel gas discharge passage  24   b  and the outlet buffer  68   b  are connected through an outlet connection channel  86   b , and the fuel gas discharge passage  24   b  and the outlet buffer  80   b  are connected through an outlet connection channel  88   b . The outlet connection channel  86   b  includes a first channel  90   b  formed between the second metal separator  18  and the third metal separator  20 , the discharge holes  36   b  formed in the second metal separator  18 , and a second channel  92   b  formed between the second metal separator  18  and the extension  58   d  of the first resin frame member  58 . 
     The outlet connection channel  88   b  includes a first channel  94   b  formed between the third metal separator  20  and the adjacent first metal separator  14 , the discharge holes  44   b  formed in the third metal separator  20 , and a second channel  96   b  formed between the third metal separator  20  and the extension  60   d  of the second resin frame member  60 . 
     As shown in  FIG. 5 , the oxygen-containing gas supply passage  22   a  and the inlet buffer  62   a  are connected through an inlet connection channel  98   a , and the oxygen-containing gas supply passage  22   a  and the inlet buffer  74   a  are connected through an inlet connection channel  100   a.    
     The inlet connection channel  98   a  has a corrugated shape (see  FIG. 15 ). The inlet connection channel  98   a  includes a first channel  102   a  formed between the first metal separator  14  and the second metal separator  18 , and a second channel  104   a  formed between the first metal separator  14  and the extension  58   a  of the first resin frame member  58 . One end of the first channel  102   a  is connected to the oxygen-containing gas supply passage  22   a . One end of the second channel  104   a  is connected to the first channel  102   a , and the other end of the second channel  104   a  is connected to the inlet buffer  62   a.    
     The inlet connection channel  100   a  includes a first channel  106   a  formed between the second metal separator  18  and the third metal separator  20 , and a second channel  108   a  formed between the second metal separator  18  and the extension  60   a  of the second resin frame member  60 . One end of the first channel  106   a  is connected to the oxygen-containing gas supply passage  22   a . One end of the second channel  108   a  is connected to the first channel  106   a , and the other end of the second channel  108   a  is connected to the inlet buffer  74   a.    
     Likewise, outlet connection channels  98   b ,  100   b  are formed between the oxygen-containing gas discharge passage  22   b  and the outlet buffers  62   b ,  74   b , and description thereof is omitted. 
     When the power generation units  12  are stacked together, the coolant flow field  32  is formed between the first metal separator  14  of one of the adjacent power generation units  12  and the third metal separator  20  of the other of the adjacent power generation units  12 . 
     Operation of the fuel cell  10  will be described below. 
     Firstly, as shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  22   a , and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  24   a . Further, a coolant such as pure water, ethylene glycol, or oil is supplied to the coolant supply passages  25   a.    
     Thus, as shown in  FIG. 5 , the oxygen-containing gas flows from the oxygen-containing gas supply passage  22   a  into the inlet connection channels  98   a ,  100   a . After the oxygen-containing gas flows into the inlet connection channel  98   a , the oxygen-containing gas flows through the inlet buffer  62   a , and then the oxygen-containing gas is supplied to the first oxygen-containing gas flow field  26  of the first metal separator  14 . Further, the oxygen-containing gas flows into the inlet connection channel  100   a , the oxygen-containing gas flows through the inlet buffer  74   a , and the oxygen-containing gas is supplied to the second oxygen-containing gas flow field  38  of the second metal separator  18 . 
     As shown in  FIGS. 1, 6, and 8 , the oxygen-containing gas flows along the first oxygen-containing gas flow field  26  in the horizontal direction indicated by the arrow B, and the oxygen-containing gas is supplied to the cathode  54  of the first membrane electrode assembly  16   a . Further, the oxygen-containing gas flows along the second oxygen-containing gas flow field  38  in the direction indicated by the arrow B, and the oxygen-containing gas is supplied to the cathode  54  of the second membrane electrode assembly  16   b.    
     In the meanwhile, as shown in  FIG. 3 , the fuel gas from the fuel gas supply passage  24   a  flows into the inlet connection channels  86   a ,  88   a . In the inlet connection channel  86   a , the fuel gas from the first channel  90   a  flows through the supply holes  36   a  to the second channel  92   a , and then the fuel gas is supplied to the inlet buffer  68   a . The fuel gas flows through the inlet buffer  68   a , and the fuel gas is supplied to the first fuel gas flow field  34  of the second metal separator  18 . 
     In the inlet connection channel  88   a , the fuel gas from the first channel  94   a  flows through the supply holes  44   a  to the second channel  96   a , and then the fuel gas is supplied to the inlet buffer  80   a . The fuel gas flows through the inlet buffer  80   a , and the fuel gas is supplied to the second fuel gas flow field  42  of the third metal separator  20 . 
     As shown in  FIGS. 1, 7, and 9 , the fuel gas flows along the first fuel gas flow field  34  in the direction indicated by the arrow B, and the fuel gas is supplied to the anode  56  of the first membrane electrode assembly  16   a . Further, the fuel gas flows along the second fuel gas flow field  42  in the direction indicated by the arrow B, and the fuel gas is supplied to the anode  56  of the second membrane electrode assembly  16   b.    
     Thus, in each of the first membrane electrode assembly  16   a  and the second membrane electrode assembly  16   b , the oxygen-containing gas supplied to the cathode  54 , and the fuel gas supplied to the anode  56  are consumed in electrochemical reactions at catalyst layers of the cathode  54  and the anode  56  for generating electricity. 
     Then, the oxygen-containing gas consumed at the cathodes  54  of the first membrane electrode assembly  16   a  and the second membrane electrode assembly  16   b  flows from the outlet buffers  62   b ,  74   b  through the outlet connection channels, and the oxygen-containing gas is discharged into the oxygen-containing gas discharge passage  22   b.    
     As shown in  FIG. 4 , the fuel gas consumed at the anodes  56  of the first membrane electrode assembly  16   a  and the second membrane electrode assembly  16   b  flows from the outlet buffers  68   b ,  80   b  into the outlet connection channels  86   b ,  88   b . In the outlet connection channel  86   b , the fuel gas flows from the second channel  92   b  through the discharge holes  36   b  to the first channel  90   b . Then, the fuel gas is discharged into the fuel gas discharge passage  24   b.    
     In the outlet connection channel  88   b , the fuel gas flows from the second channel  96   b  through the discharge holes  44   b  to the first channel  94   b . Then, the fuel gas is discharged into the fuel gas discharge passage  24   b.    
     In the meanwhile, as shown in  FIG. 1 , the coolant supplied to the pair of coolant supply passages  25   a  flows into the coolant flow field  32 . The coolant from each of the coolant supply passages  25   a  is supplied to the coolant flow field  32 . The coolant temporarily flows inward in the direction indicated by the arrow C, and then the coolant moves in the direction indicated by the arrow B to cool the first membrane electrode assembly  16   a  and the second membrane electrode assembly  16   b . After the coolant moves outward in the direction indicated by the arrow C, the coolant is discharged into the pair of coolant discharge passages  25   b.    
     In the first embodiment, for example, as shown in  FIGS. 5 and 11 , in the first membrane electrode assembly  16   a , the first resin frame member  58  is provided at the outer circumferential portion of the solid polymer electrolyte membrane  52 . The strength and the rigidity of the first resin frame member  58  are considerably high in comparison with the solid polymer electrolyte membrane  52  and the gas diffusion layer made of the carbon paper. 
     Further, as shown in  FIG. 11 , the inlet buffer  62   a , the outlet buffer  62   b , and the extensions  58   a ,  58   b  as parts of the inlet connection channel  98   a  and the outlet connection channel  98   b  are provided on the surface of the first resin frame member  58  on a side where the cathode  54  is provided. In the structure, as shown in  FIG. 5 , when the extension  58   a  of the first resin frame member  58  contacts the surface between the inlet connection grooves  30   a  of the first metal separator  14  to form the bridge section, the rigid and thick extension  58   a  of the first resin frame member  58  is not deformed to enter the inlet connection grooves  30   a.    
     Accordingly, with the simple and economical structure, it is possible to reliably prevent the inlet connection grooves  30   a  from being closed, while achieving a desired sealing performance. 
     Further, since the inlet buffer  62   a  can be formed in the first resin frame member  58 , the structure of the first metal separator  14  is simplified effectively and economically. 
     Also in the outlet buffer  62   b  and the extension  58   b , the same advantages as described above are obtained. Further, also in the second resin frame member  60 , the same advantages as in the case of the first resin frame member  58  are obtained. 
     As shown in  FIGS. 16 and 17 , a fuel cell  120  according to a second embodiment of the present invention is formed by stacking a plurality of power generation units  122 . 
     The power generation unit  122  is formed by sandwiching a membrane electrode assembly  16  between a first metal separator  14  and a second metal separator  124 . The constituent elements 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. 
     The second metal separator  124  has a fuel gas flow field  34  on its surface  124   a  facing the membrane electrode assembly  16 . Part of a coolant flow field  32  is formed on the other surface  124   b  of the second metal separator  124 . The membrane electrode assembly  16  has the same structure as the first membrane electrode assembly  16   a  or the second membrane electrode assembly  16   b  according to the first embodiment. 
     In the second embodiment, the first resin frame member  58  is provided for the membrane electrode assembly  16 , and the same advantages as in the case of the first embodiment are obtained.