Patent Publication Number: US-2018040907-A1

Title: Fuel cell stack

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-152267 filed on Aug. 2, 2016, the contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a fuel cell stack including a stack body formed by stacking a plurality of power generation cells. Each of the power generation cells includes a membrane electrode assembly and metal separators on both sides of the membrane electrode assembly. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. 
     Description of the Related Art 
     For example, a solid polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) having an electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane. An anode is provided on one surface of the electrolyte membrane, and a cathode is provided on the other surface of the electrolyte membrane. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form a power generation cell. Normally, a predetermined number of the power generation cells are stacked together to form a stack body, and a fuel cell stack contains such a stack body. For example, the fuel cell stack is mounted in a fuel cell vehicle (fuel cell electric automobile, etc.). 
     In some cases, as the separators, the fuel cell stack may adopt metal separators. In this regard, seal members are provided on the metal separators for preventing leakage of an oxygen-containing gas and a fuel gas as reactant gases and a coolant (e.g., see the specification of U.S. Pat. No. 6,605,380). Elastic rubber seals such as fluorine based seals or silicone seals are used as the seal members. Therefore, the cost required for providing the seal members such as the fluorine based seals or silicone seals pushes up the production cost disadvantageously. 
     To this end, for example, as disclosed in Japanese Laid-Open Patent Publication No. 2015-191802, it has been common to adopt a structure where, instead of the elastic rubber seals, sealing beads are formed on metal separators. 
     SUMMARY OF THE INVENTION 
     Sealing beads may be formed on metal separators provided on both sides of the membrane electrode assembly. The sealing beads protrude in the stacking direction of the stack body in a manner that the sealing beads contact the frame provided at the outer circumferential portion of the membrane electrode assembly. The stack body is sandwiched between insulators at both ends of the stack body in the stacking direction in a manner that the sealing beads are deformed elastically. In this manner, leakage of the reactant gases and the coolant is prevented. 
     However, in the structure, the elastic force of the sealing beads is applied to the frame provided on the membrane electrode assembly from both sides of the frame, and the elastic force of the sealing beads is applied to the insulator only from one side of the insulator. Therefore, a desired sealing performance may not be obtained at the ends of the stack body in the stacking direction. In view of the above, there is a demand to improve the sealing performance at the ends of the stack body in the stacking direction. 
     The present invention has been made taking the above points into account, and an object of the present invention is to provide a fuel cell stack which makes it possible to improve the sealing performance at ends of a stack body in the stacking direction. 
     A fuel cell stack according to the present invention includes a stack body including a plurality of power generation cells stacked in a stacking direction. Each of the power generation cells includes a membrane electrode assembly and metal separators provided on both sides of the membrane electrode assembly. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. Sealing beads are provided on the metal separators. The sealing beads protrude in the stacking direction of the stack body in a manner that the sealing beads contact an outer circumferential portion of the membrane electrode assembly or a frame provided on the outer circumferential portion of the membrane electrode assembly. Insulators and end plates sandwiching the stack body in the stacking direction are provided on both sides of the stack body in the stacking direction in a manner that the sealing beads are deformed elastically. 
     Elastic seal members are provided on the insulators or the end plates, and the elastic seal members are configured to abut against the sealing beads of the metal separators provided at the outermost end positions in the stacking direction. 
     Further, in the fuel cell stack, preferably, recesses are provided on surfaces of the insulators or the end plates facing the stack body, and the elastic seal members are provided in the recesses. 
     Further, preferably, each of the metal separators includes a gas flow field configured to supply a reactant gas to the electrode and a plurality of passages for the reactant gas and the coolant, and the sealing beads are formed around the gas flow field and around the passages. 
     Further, in the fuel cell stack, preferably, each one of the metal separators provided at the outermost end positions in the stacking direction has the same structure as another metal separator that contacts a surface of the outer circumferential portion or the frame of the membrane electrode assembly, the surface facing the opposite side of the one of the metal separators provided at the outermost end positions in the stacking direction. 
     In the present invention, the elastic seal member which abuts against the sealing bead of the metal separator provided at the outermost end in the stacking direction of the stack body is provided on the insulator or the end plate. In the structure, the elastic force of the elastic seal member is applied to the sealing bead of the metal separator provided at the end of the stack body in the stacking direction, and the elastic force of the sealing bead is applied to the elastic seal member. Accordingly, it is possible to improve the sealing performance at the end of the stack body in the stacking direction. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a fuel cell stack according to an embodiment of the present invention; 
         FIG. 2  is a partial exploded perspective view schematically showing the fuel cell stack; 
         FIG. 3  is a cross sectional view taken along a line III-III in  FIG. 2 ; 
         FIG. 4  is an exploded perspective view showing a power generation cell of the fuel cell stack; 
         FIG. 5  is a front view showing a first metal separator of the power generation cell; 
         FIG. 6  is a front view showing one of insulators of the fuel cell stack; 
         FIG. 7  is a front view showing the other of the insulators of the fuel cell stack; 
         FIG. 8  is a cross sectional view showing a first elastic seal member and a second elastic seal member of the fuel cell stack; 
         FIG. 9  is a cross sectional view showing an example of structure of the fuel cell stack according to the present invention; and 
         FIG. 10  is a cross sectional view showing another example of structure of the fuel cell stack according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of a fuel cell stack according to the present invention will be described with reference to the accompanying drawings. 
     As shown in  FIGS. 1 and 2 , a fuel cell stack  10  according to an embodiment of the present invention includes a stack body  14  formed by stacking a plurality of power generation cells  12  in a horizontal direction (indicated by an arrow A) or in a direction of gravity (indicated by an arrow C). For example, the fuel cell stack  10  is mounted in a fuel cell vehicle such as a fuel cell electric automobile (not shown). 
     At one end of the stack body  14  in the stacking direction (indicated by the arrow A), a terminal plate  16   a  is provided. An insulator  18   a  is provided outside the terminal plate  16   a , and an end plate  20   a  is provided outside the insulator  18   a  (see  FIG. 2 ). At the other end of the stack body  14 , a terminal plate  16   b  is provided. An insulator  18   b  is provided outside the terminal plate  16   b , and an end plate  20   b  is provided outside the insulator  18   b.    
     As shown in  FIG. 1 , the end plates  20   a ,  20   b  have a laterally elongated (or longitudinally elongated) rectangular shape, and coupling bars  24  are provided between respective sides of the end plates  20   a ,  20   b . Both ends of the coupling bars  24  are fixed to inner surfaces of the end plates  20   a ,  20   b  using bolts  26  to apply a tightening load to the stacked power generation cells  12  in the stacking direction indicated by the arrow A. Alternatively, it should be noted that the fuel cell stack  10  may have a casing including the end plates  20   a ,  20   b , and the stack body  14  may be placed in the casing. 
     As shown in  FIGS. 3 and 4 , each of the power generation cells  12  is formed by sandwiching a resin film equipped MEA (membrane electrode assembly)  28  between a first metal separator  30  and a second metal separator  32 . For example, the first metal separator  30  and the second metal separator  32  are 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  30  and the second metal separator  32  is formed by corrugating the above-described metal thin plates by press forming to have a corrugated shape in cross section and a wavy or straight shape on the surface. Outer circumferential ends of the first metal separator  30  and the second metal separator  32  are joined together by welding, brazing, crimpling, etc. to form a joint separator  33 . 
     At one end of the power generation cell  12  in a long side direction of the power generation cell  12  indicated by an arrow B (horizontal direction in  FIG. 4 ), an oxygen-containing gas supply passage  34   a , a coolant supply passage  36   a , and a fuel gas discharge passage  38   b  are provided. The oxygen-containing gas supply passage  34   a , the coolant supply passage  36   a , and the fuel gas discharge passage  38   b  extend through the power generation cell  12  in the direction indicated by the arrow A. The oxygen-containing gas supply passage  34   a , the coolant supply passage  36   a , and the fuel gas discharge passage  38   b  are arranged in the direction indicated by an arrow C. An oxygen-containing gas is supplied through the oxygen-containing gas supply passage  34   a . A coolant is supplied through the coolant supply passage  36   a , and a fuel gas such as a hydrogen-containing gas is discharged through the fuel gas discharge passage  38   b.    
     At the other end of the power generation cell  12  in the direction indicated by the arrow B, a fuel gas supply passage  38   a , a coolant discharge passage  36   b , and an oxygen-containing gas discharge passage  34   b  are provided. The fuel gas supply passage  38   a , the coolant discharge passage  36   b , and the oxygen-containing gas discharge passage  34   b  extend through the power generation cell  12  in the direction indicated by the arrow A. The fuel gas supply passage  38   a , the coolant discharge passage  36   b , and the oxygen-containing gas discharge passage  34   b  are arranged in the direction indicated by the arrow C. The fuel gas is supplied through the fuel gas supply passage  38   a , the coolant is discharged through the coolant discharge passage  36   b , and the oxygen-containing gas is discharged through the oxygen-containing gas discharge passage  34   b . The positions of the oxygen-containing gas supply passage  34   a , the oxygen-containing gas discharge passage  34   b , and the fuel gas supply passage  38   a , and the fuel gas discharge passage  38   b  are not limited to the present embodiment, and may be appropriately determined according to the required specification. 
     As shown in  FIG. 3 , the resin film equipped MEA  28  has a frame shaped resin film (frame)  46  at its outer portion. For example, the resin film equipped MEA  28  includes an anode (electrode)  42 , a cathode (electrode)  44 , and a solid polymer electrolyte membrane (cation exchange membrane)  40  interposed between the anode  42  and the cathode  44 . The solid polymer electrolyte membrane  40  is a thin membrane of perfluorosulfonic acid containing water. 
     A fluorine based electrolyte may be used for the solid polymer electrolyte membrane  40 . Alternatively, an HC (hydrocarbon) based electrolyte may be used for the solid polymer electrolyte membrane  40 . The plane size (outer size) of the solid polymer electrolyte membrane  40  is smaller than the plane size (outer size) of the anode  42  and the plane size (outer size) of the cathode  44 . The solid polymer electrolyte membrane  40  includes an overlapped portion  41  overlapped with the outer ends of the anode  42  and the cathode  44 . 
     The anode  42  includes a first electrode catalyst layer  42   a  joined to one surface  40   a  of the solid polymer electrolyte membrane  40 , and a first gas diffusion layer  42   b  stacked on the first electrode catalyst layer  42   a . The outer size of the first electrode catalyst layer  42   a  is smaller than the outer size of the first gas diffusion layer  42   b , and the same as (or smaller than) the outer size of the solid polymer electrolyte membrane  40 . It should be noted that the outer size of the first electrode catalyst layer  42   a  may be the same as the outer size of the first gas diffusion layer  42   b.    
     The cathode  44  includes a second electrode catalyst layer  44   a  joined to a surface  40   b  of the solid polymer electrolyte membrane  40 , and a second gas diffusion layer  44   b  stacked on the second electrode catalyst layer  44   a . The outer size of the second electrode catalyst layer  44   a  is smaller than the outer size of the second gas diffusion layer  44   b , and the same as (or smaller than) the outer size of the solid polymer electrolyte membrane  40 . It should be noted that the outer size of the second electrode catalyst layer  44   a  may be the same as the outer size of the second gas diffusion layer  44   b.    
     The first electrode catalyst layer  42   a  is formed, for example, by depositing porous carbon particles uniformly on the surface of the first gas diffusion layer  42   b . Platinum alloy is supported on surfaces of the carbon particles. The second electrode catalyst layer  44   a  is formed, for example, by depositing porous carbon particles uniformly on the surface of the second gas diffusion layer  44   b . Platinum alloy is supported on surfaces of the carbon particles. Each of the first gas diffusion layer  42   b  and the second gas diffusion layer  44   b  comprises a carbon paper, a carbon cloth, etc. The first electrode catalyst layer  42   a  and the second electrode catalyst layer  44   a  are formed on respective both surfaces  40   a ,  40   b  of the solid polymer electrolyte membrane  40 . 
     A resin film  46  having a frame shape is sandwiched between an outer edge portion of the first gas diffusion layer  42   b  and an outer edge portion of the second gas diffusion layer  44   b . An inner end surface of the resin firm  46  is positioned close to, or contacts an outer end surface of the solid polymer electrolyte membrane  40 . As shown in  FIG. 4 , the oxygen-containing gas supply passage  34   a , the coolant supply passage  36   a , and the fuel gas discharge passage  38   b  are provided at one end of the resin film  46  in the direction indicated by the arrow B. The fuel gas supply passage  38   a , the coolant discharge passage  36   b , and the oxygen-containing gas discharge passage  34   b  are provided at the other end of the resin film  46  in the direction indicated by the arrow B. 
     For example, the resin film  46  is made of PPS (polyphenylene sulfide), PPA (polyphthalamide), PEN (polyethylene naphthalate), PES (polyether sulfone), LCP (liquid crystal polymer), PVDF (polyvinylidene fluoride), a silicone resin, a fluorine resin, m-PPE (modified polyphenylene ether resin), PET (polyethylene terephthalate), PBT (polybutylene terephthalate), or modified polyolefin. It should be noted that the solid polymer electrolyte membrane  40  may protrude outward without using the resin film  46 . Further, a pair of frame shaped films may be provided on respective both sides of the solid polymer electrolyte membrane  40  which protrudes outward. 
     As shown in  FIG. 4 , the first metal separator  30  has an oxygen-containing gas flow field  48  on its surface  30   a  facing the resin film equipped MEA  28 . For example, the oxygen-containing gas flow field  48  extends in the direction indicated by the arrow B. As shown in  FIG. 5 , the oxygen-containing gas flow field  48  is in fluid communication with the oxygen-containing gas supply passage  34   a  and the oxygen-containing gas discharge passage  34   b . The oxygen-containing gas flow field  48  includes straight flow grooves (or wavy flow grooves)  48   b  between a plurality of ridges  48   a  extending in the direction indicated by the arrow B. 
     An inlet buffer  50   a  having a plurality of bosses is provided between the oxygen-containing gas supply passage  34   a  and the oxygen-containing gas flow field  48 . An outlet buffer  50   b  having a plurality of bosses is provided between the oxygen-containing gas discharge passage  34   b  and the oxygen-containing gas flow field  48 . 
     The oxygen-containing gas flow field  48 , the inlet buffer  50   a , the outlet buffer  50   b , and a first seal line (metal bead seal)  52  each having a corrugated shape in cross section by press forming, are formed on the surface  30   a  of the first metal separator  30 . The oxygen-containing gas flow field  48 , the inlet buffer  50   a , the outlet buffer  50   b , and the first seal line are expanded toward the resin film equipped MEA  28 . The first seal line  52  includes an outer bead (sealing bead)  52   a  formed around the outer marginal portion of the surface  30   a . As shown in  FIG. 3 , the first seal line  52  has a tapered shape in cross section toward the front end of the first seal line  52 . The front end of the first seal line  52  has a flat shape or an R shape. Further, the first seal line  52  includes an inner bead (sealing bead)  52   b  formed around the oxygen-containing gas flow field  48 , the oxygen-containing gas supply passage  34   a , and the oxygen-containing gas discharge passage  34   b , while allowing the oxygen-containing gas flow field  48 , the oxygen-containing gas supply passage  34   a , and the oxygen-containing gas discharge passage  34   b  to communicate with each other. 
     Further, the first seal line  52  includes passage beads (sealing bead)  52   c  formed around the fuel gas supply passage  38   a , the fuel gas discharge passage  38   b , the coolant supply passage  36   a , and the coolant discharge passage  36   b . The outer bead  52   a , the inner bead  52   b , and the passage bead  52   c  protrude from the surface  30   a . The outer bead  52   a  should be provided as necessary, i.e., the outer bead  52   a  may not be provided. 
     As shown in  FIG. 5 , the first metal separator  30  includes a plurality of inlet channels  54   a  and a plurality of outlet channels  54   b . The inlet channels  54   a  connect a coolant flow field  66  (described later) formed on a surface  30   b  of the first metal separator  30  with the coolant supply passage  36   a . The outlet channels  54   b  connect the coolant flow field  66  with the coolant discharge passage  36   b . Each of the inlet channels  54   a  and the outlet channels  54   b  extends in the direction indicated by the arrow B. Part of the first metal separator  30  is expanded from the surface  30   a  to thereby form the inlet channels  54   a  and the outlet channels  54   b . The number and shape of each of the inlet channels  54   a  and the outlet channels  54   b  can be determined arbitrarily. 
     The inlet channels  54   a  are connected to the inner bead  52   b  and the passage bead  52   c  between the coolant flow field  66  and the coolant supply passage  36   a . The outlet channels  54   b  are connected to the inner bead  52   b  and the passage bead  52   c  between the coolant flow field  66  and the coolant discharge passage  36   b.    
     In the first seal line  52 , as shown in  FIG. 3 , a resin material  56   a  is fixed to each of protruding front end surfaces of the outer bead  52   a  and the inner bead  52   b  by printing, coating, or the like. For example, polyester is used as the resin material  56   a . As shown in  FIG. 5 , the resin material  56   a  is fixed to a protruding front surface of the passage bead  52   c  by printing, coating, or the like. Alternatively, as the resin material  56   a , punched-out sheets having the plane surface shapes corresponding to the shapes of the outer bead  52   a , the inner bead  52   b , and the passage bead  52   c  may be attached to the surface  30   a  of the first metal separator  30 . The resin material  56   a  should be provided as necessary, i.e., the resin material  56   a  may not be provided. 
     As shown in  FIG. 4 , the second metal separator  32  has a fuel gas flow field  58  on its surface  32   a  facing the resin film equipped MEA  28 . For example, the fuel gas flow field  58  extends in the direction indicated by the arrow B. The fuel gas flow field  58  is in fluid communication with the fuel gas supply passage  38   a  and the fuel gas discharge passage  38   b . The fuel gas flow field  58  includes straight flow grooves (or wavy flow grooves)  58   b  between a plurality of ridges  58   a  extending in the direction indicated by the arrow B. 
     An inlet buffer  60   a  having a plurality of bosses is provided between the fuel gas supply passage  38   a  and the fuel gas flow field  58 . An outlet buffer  60   b  having a plurality of bosses is provided between the fuel gas discharge passage  38   b  and the fuel gas flow field  58 . 
     The fuel gas flow field  58 , the inlet buffer  60   a , the outlet buffer  60   b , and a second seal line (metal bead seal)  62  each having a corrugated shape in cross section by press forming, are formed on the surface  32   a  of the second metal separator  32 . The fuel gas flow field  58 , the inlet buffer  60   a , the outlet buffer  60   b , and the second seal line  62  are expanded toward the resin film equipped MEA  28 . The second seal line  62  includes an outer bead (sealing bead)  62   a  formed around the outer marginal portion of the surface  32   a . As shown in  FIG. 3 , the second seal line  62  has a tapered shape in cross section toward the front end of the second seal line  62 . The front end of the second seal line  62  has a flat shape or an R shape. Further, the second seal line  62  includes an inner bead (sealing bead)  62   b  formed around the fuel gas flow field  58 , the fuel gas supply passage  38   a , and the fuel gas discharge passage  38   b , while allowing the fuel gas flow field  58 , the fuel gas supply passage  38   a , and the fuel gas discharge passage  38   b  to communicate with each other. 
     Further, the second seal line  62  includes passage bead (sealing bead)  62   c  formed around the oxygen-containing gas supply passage  34   a , the oxygen-containing gas discharge passage  34   b , the coolant supply passage  36   a , and the coolant discharge passage  36   b . The outer bead  62   a , the inner bead  62   b , and the passage bead  62   c  protrude from the surface  32   a . The outer bead  62   a  should be provided as necessary, i.e., the outer bead  62   a  may not be provided. 
     As shown in  FIG. 4 , the second metal separator  32  includes a plurality of inlet channels  64   a  and a plurality of outlet channels  64   b . The inlet channels  64   a  connect a coolant flow field  66  (described later) formed on a surface  32   b  of the second metal separator  32  with the coolant supply passage  36   a . The outlet channels  64   b  connect the coolant flow field  66  with the coolant discharge passage  36   b . Each of the inlet channels  64   a  and the outlet channels  64   b  extends in the direction indicated by the arrow B. Part of the second metal separator  32  is expanded from the surface  32   a  to thereby form the inlet channels  64   a  and the outlet channels  64   b . The number and shape of each of the inlet channels  64   a  and the outlet channels  64   b  can be determined arbitrarily. 
     The inlet channels  64   a  are connected to the inner bead  62   b  and the passage bead  62   c  between the coolant flow field  66  and the coolant supply passage  36   a . The outlet channel  64   b  is connected to the inner bead  62   b  and the passage bead  62   c  between the coolant flow field  66  and the coolant discharge passage  36   b.    
     In the second seal line  62 , as shown in  FIG. 3 , a resin material  56   b  is fixed to each of protruding front end surfaces of the outer bead  62   a  and the inner bead  62   b  by printing, coating, or the like. For example, polyester is used as the resin material  56   b . As shown in  FIG. 4 , the resin material  56   b  is fixed to a protruding front surface of the passage bead  62   c  by printing, coating, or the like. Alternatively, as the resin material  56   b , punched-out sheets having the plane surface shapes corresponding to the shapes of the outer bead  62   a , the inner bead  62   b , and the passage bead  62   c  may be attached to the surface  32   a  of the second metal separator  32 . The resin material  56   b  should be provided as necessary, i.e., the resin material  56   b  may not be provided. 
     The coolant flow filed  66  is formed between adjacent metal separators  30 ,  32  that are joined together, i.e., between the surface  30   b  of the first metal separator  30  and the surface  32   b  of the second metal separator  32 . The coolant flow field  66  fluidically communicates with the coolant supply passage  36   a  and the coolant discharge passage  36   b . The coolant flow field  66  is formed by stacking the back surface of the oxygen-containing gas flow field  48  of the first metal separator  30  and the back surface of the fuel gas flow field  58  of the second metal separator  32  together. 
     The terminal plates  16   a ,  16   b  shown in  FIG. 2  are made of electrically conductive material. For example, the terminal plates  16   a ,  16   b  are made of metal such as copper, aluminum or stainless steel. Terminal units  68   a ,  68   b  extending outward in the stacking direction are provided at substantially the centers of the terminal plates  16   a ,  16   b.    
     The terminal unit  68   a  is inserted into an electric insulating tubular body  70   a . The terminal unit  68   a  then passes through a hole  72   a  of the insulator  18   a  and a hole  74   a  of the end plate  20   a , and protrudes to the outside of the end plate  20   a . The terminal unit  68   b  is inserted into an electric insulating tubular body  70   b . The terminal unit  68   b  then passes through a hole  72   b  of the insulator  18   b  and a hole  74   b  of the end plate  20   b , and protrudes to the outside of the end plate  20   b.    
     As shown in  FIG. 2 , the insulators  18   a ,  18   b  are made of electric insulating material such as polycarbonate (PC) or phenolic resin. Recesses  76   a ,  76   b  are formed at the centers of the insulators  18   a ,  18   b , respectively. The recesses  76   a ,  76   b  are opened to the stack body  14 . The holes  72   a ,  72   b  are formed at the bottom surfaces of the recesses  76   a ,  76   b , respectively. 
     The oxygen-containing gas supply passage  34   a , the coolant supply passage  36   a , and the fuel gas discharge passage  38   b  extend through one end of each of the insulator  18   a  and the end plate  20   a  in the direction indicated by the arrow B. The fuel gas supply passage  38   a , the coolant discharge passage  36   b , and the oxygen-containing gas discharge passage  34   b  extend through the other end of each of the insulator  18   a  and the end plate  20   a  in the direction indicated by the arrow B. 
     As shown in  FIGS. 3 and 6 , a first recess  82  is formed on a surface  19   a  of the insulator  18   a  facing the stack body  14 . A first elastic seal member  80  is provided in the first recess  82 . The first elastic seal member  80  abuts against the second seal line  62  of the second metal separator  32  provided at the outermost end of the stack body  14  in the stacking direction (on the insulator  18   a  side). In the following description, the second metal separator  32  provided at the outermost end in the stacking direction of the stack body  14  on the insulator  18   a  side will also be referred to as the “second end metal separator  32   e ”, and the second seal line  62  of the second end metal separator  32   e  will also be referred to as the “second end seal line  62   e”.    
     A predetermined gap Sa is formed between the first elastic seal member  80  and a side surface  83   a  of the first recess  82  so as to allow the first elastic seal member  80  to be deformed elastically in a direction perpendicular to the stacking direction (i.e., in a direction indicated by the arrow B or C). Specifically, the width of the first recess  82  is larger than the width of the first elastic seal member  80 . The first elastic seal member  80  is spaced from the side surface  83   a  of the first recess  82 . The first elastic seal member  80  is spaced from the side surface  83   a  of the first recess  82  by a substantially constant distance. The gap Sa is provided on each of both sides of the first elastic seal member  80  in the width direction. 
     For example, the first elastic seal member  80  has a rectangular shape in lateral cross section, and made of elastic polymer material. For example, such polymer material includes a silicone rubber, an acrylic rubber, a nitrile rubber, etc. The first elastic seal member  80  is attached (by adhesive) or fused to a bottom surface  83   b  of the first recess  82 . 
     A surface  81  of the first elastic seal member  80  facing the second end seal line  62   e  is positioned inside the first recess  82  for allowing the second end metal separator  32   e  to tightly contact the terminal plate  16   a . Stated otherwise, the surface  81  of the first elastic seal member  80  is arranged at a position shifted from a surface  17   a  of the terminal plate  16   a  facing the second end metal separator  32   e , toward the bottom surface  83   b  of the first recess  82 . Further, the surface  81  of the first elastic seal member  80  has a flat shape in parallel to the solid polymer electrolyte membrane  40  (i.e., in parallel to a surface perpendicular to the stacking direction of the stack body  14 ). 
     The first recess  82  includes an outer recess  82   a  formed at a position facing the outer bead  62   a  of the second end seal line  62   e , an inner recess  82   b  formed at a position facing the inner bead  62   b  of the second end seal line  62   e , and a passage recess  82   c  formed at a position facing the passage bead  62   c  of the second end seal line  62   e.    
     The first elastic seal member  80  includes an outer seal  80   a  provided inside the outer recess  82   a , an inner seal  80   b  provided inside the inner recess  82   b , and a passage seal  80   c  provided inside the passage recess  82   c.    
     That is, the outer seal  80   a  is formed around the outer marginal portion of the surface  19   a  of the insulator  18   a , and abuts against the outer bead  62   a  of the second end seal line  62   e . The inner seal  80   b  is formed around the recess  76   a , and abuts against the inner bead  62   b  of the second end seal line  62   e . The passage seal  80   c  is formed around the fuel gas supply passage  38   a , the fuel gas discharge passage  38   b , the coolant supply passage  36   a , and the coolant discharge passage  36   b , and abuts against part of the inner bead  62   b  that surrounds the fuel gas supply passage  38   a  and the fuel gas discharge passage  38   b , and the passage bead  62   c  of the second end seal line  62   e.    
     In the embodiment of the present invention, as can be seen from  FIG. 6 , the outer seal  80   a  and the inner seal  80   b  are provided separately. A portion of the passage seal  80   c  around the coolant supply passage  36   a  and the coolant discharge passage  36   b  is formed separately from the outer seal  80   a , but formed integrally with the inner seal  80   b . Part of the passage seal  80   c  that surrounds the oxygen-containing gas supply passage  34   a , the oxygen-containing gas discharge passage  34   b , the fuel gas supply passage  38   a , and the fuel gas discharge passage  38   b  is formed separately from the outer seal  80   a  and the inner seal  80   b.    
     Alternatively, the outer recess  82   a , the inner recess  82   b , and the passage recess  82   c  may be formed so as to connect with each other, and the outer seal  80   a , the inner seal  80   b , and the passage seal  80   c  may be formed integrally. The outer seal  80   a  and the outer recess  82   a  should be provided as necessary, i.e., the outer seal  80   a  and the outer recess  82   a  may not be provided. 
     As shown in  FIGS. 3 and 7 , a second recess  86  is formed on a surface  19   b  of the insulator  18   b  facing the stack body  14 . A second elastic seal member  84  is provided in the second recess  86 . The second elastic seal member  84  abuts against the first seal line  52  of the first metal separator  30  provided at the outermost end of the stack body  14  in the stacking direction on the insulator  18   b  side. In the following description, the first metal separator  30  provided at the outermost end in the stacking direction of the stack body  14  on the insulator  18   b  side will also be referred to as the “first end metal separator  30   e ”, and the first seal line  52  of the first end metal separator  30   e  will also be referred to as the “first end seal line  52   e”.    
     A predetermined gap Sb is formed between the second elastic seal member  84  and a side surface  87   a  of the second recess  86  so as to allow the second elastic seal member  84  to be deformed elastically in a direction perpendicular to the stacking direction (i.e., in a direction indicated by the arrow B or C). Specifically, the width of the second recess  86  is larger than the width of the second elastic seal member  84 . The second elastic seal member  84  is spaced from the side surface  87   a  of the second recess  86 . The second elastic seal member  84  is spaced from the side surface  87   a  of the second recess  86  by a substantially constant distance. The gap Sb is provided on each of both sides of the second elastic seal member  84  in the width direction. 
     For example, the second elastic seal member  84  has a rectangular shape in lateral cross section, and made of elastic polymer material. For example, such polymer material includes a silicone rubber, an acrylic rubber, a nitrile rubber, etc. The second elastic seal member  84  is attached (by adhesive) or fused to a bottom surface  87   b  of the second recess  86 . 
     A surface  85  of the second elastic seal member  84  facing the first end seal line  52   e  is positioned inside the second recess  86  for allowing the first end metal separator  30   e  to tightly contact the terminal plate  16   b . Stated otherwise, the surface  85  of the second elastic seal member  84  is arranged at a position shifted from a surface  17   b  of the terminal plate  16   b  facing the first end metal separator  30   e , toward the bottom surface  87   b  of the second recess  86 . Further, the surface  85  of the second elastic seal member  84  has a flat shape in parallel to the solid polymer electrolyte membrane  40  (i.e., in parallel to a surface perpendicular to the stacking direction of the stack body  14 ). 
     The second recess  86  includes an outer recess  86   a  formed at a position facing the outer bead  52   a  of the first end seal line  52   e , an inner recess  86   b  formed at a position facing the inner bead  52   b  of the first end seal line  52   e , and a passage recess  86   c  formed at a position facing the passage bead  52   c  of the first end seal line  52   e.    
     The second elastic seal member  84  includes an outer seal  84   a  provided inside the outer recess  86   a , an inner seal  84   b  provided inside the inner recess  86   b , and a passage seal  84   c  provided inside the passage recess  86   c.    
     That is, the outer seal  84   a  is formed around the outer marginal portion of the surface  19   b  of the insulator  18   b , and abuts against the outer bead  52   a  of the first end seal line  52   e . The inner seal  84   b  is formed around the recess  76   b , and portions facing the oxygen-containing gas supply passage  34   a  and the oxygen-containing gas discharge passage  34   b  of the first end metal separator  30   e , and abuts against the inner bead  52   b  of the first end seal line  52   e . The passage seal  84   c  is formed around portions facing the fuel gas supply passage  38   a , the fuel gas discharge passage  38   b , the coolant supply passage  36   a , and the coolant discharge passage  36   b  of the first end metal separator  30   e  and abuts against the passage bead  52   c  of the first end seal line  52   e.    
     In the embodiment of the present invention, as can be seen from  FIG. 7 , the outer seal  84   a  and the inner seal  84   b  are provided separately. Part of the passage seal  84   c  around the portions facing the coolant supply passage  36   a  and the coolant discharge passage  36   b  of the first end metal separator  30   e , is formed separately from the outer seal  84   a , but formed integrally with the inner seal  84   b . Another part of the passage seal  84   c  around the portions facing the fuel gas supply passage  38   a  and the fuel gas discharge passage  38   b  of the first end metal separator  30   e , is formed separately from the outer seal  84   a  and the inner seal  84   b.    
     Alternatively, the outer recess  86   a , the inner recess  86   b , and the passage recess  86   c  may be formed so as to connect with each other, and the outer seal  84   a , the inner seal  84   b , and the passage seal  84   c  may be formed integrally. The outer seal  84   a  and the outer recess  86   a  should be provided as necessary, i.e., the outer seal  84   a  and the outer recess  86   a  may not be provided. 
     As can be seen from  FIG. 3 , in the fuel cell stack  10 , the first end metal separator  30   e  has the same structure as each of the first metal separators  30  provided at intermediate positions of the stack body  14  in the stacking direction (hereinafter also referred to as the “first intermediate metal separators  30   i ”). Stated otherwise, the first end metal separator  30   e  has the same structure as each of the first intermediate metal separators  30   i  which contacts a surface of the resin film  46  that is on the opposite side of the first end metal separator  30   e . That is, all of the first metal separators  30  have the same structure. 
     Further, the second end metal separator  32   e  has the same structure as each of the second metal separators  32  provided at intermediate positions of the stack body  14  in the stacking direction (hereinafter also referred to as the “second intermediate metal separators  32   i ”). Stated otherwise, the second end metal separator  32   e  has the same structure as each of the second intermediate metal separators  32   i  which contact a surface of the resin film  46  that is on the opposite side of the second end metal separator  32   e . That is, all of the second metal separators  32  have the same structure. 
     In the fuel cell stack  10 , the coupling bars  24  are fixed to the inner surfaces of the end plates  20   a ,  20   b  using the bolts  26  in a manner that the first seal line  52  and the second seal line  62  are deformed elastically. In this manner, a tightening load is applied to the stack body  14  in the stacking direction. Therefore, the resin film  46  is sandwiched between the first seal line  52  and the second seal line  62  in the stacking direction in a manner that the first seal line  52  and the second seal line  62  are deformed elastically. That is, since the elastic force of the first seal line  52  and the elastic force of the second seal line  62  are applied to the resin film  46 , leakage of the oxygen-containing gas, the fuel gas, and the coolant is prevented. 
     Next, operation of the fuel cell stack  10  having the above structure will be described below. 
     Firstly, as shown in  FIG. 1 , an oxygen-containing gas such as the air is supplied to the oxygen-containing gas supply passage  34   a  at the end plate  20   a . A fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  38   a  at the end plate  20   a . A coolant such as pure water, ethylene glycol, oil, or the like is supplied to the coolant supply passage  36   a  at the end plate  20   a.    
     As shown in  FIG. 4 , the oxygen-containing gas flows from the oxygen-containing gas supply passage  34   a  to the oxygen-containing gas flow field  48  at the first metal separator  30 . The oxygen-containing gas flows along the oxygen-containing gas flow field  48  in the direction indicated by the arrow B, and the oxygen-containing gas is supplied to the cathode  44  of the membrane electrode assembly  28 . 
     In the meanwhile, the fuel gas is supplied from the fuel gas supply passage  38   a  to the fuel gas flow field  58  of the second metal separator  32 . The fuel gas flows along the fuel gas flow field  58  in the direction indicated by the arrow B, and the fuel gas is supplied to the anode  42  of the membrane electrode assembly  28 . 
     Thus, in each of the membrane electrode assemblies  28 , the oxygen-containing gas supplied to the cathode  44  and the fuel gas supplied to the anode  42  are consumed in the electrochemical reactions in the second electrode catalyst layer  44   a  and the first electrode catalyst layer  42   a  of the cathode  44  and the anode  42  for generating electricity. 
     Then, the oxygen-containing gas consumed at the cathode  44  flows along the oxygen-containing gas discharge passage  34   b , and is discharged in the direction indicated by the arrow A. Likewise, the fuel gas consumed at the anode  42  flows along the fuel gas discharge passage  38   b , and is discharged in the direction indicated by the arrow A. 
     Further, the coolant supplied to the coolant supply passage  36   a  flows into the coolant flow field  66  formed between the first metal separator  30  and the second metal separator  32 . Then, the coolant flows in the direction indicated by the arrow B. After the coolant cools the membrane electrode assembly  28 , the coolant is discharged from the coolant discharge passage  36   b.    
     In the embodiment of the present invention, the first elastic seal member  80  is provided on the insulator  18   a , and the first elastic seal member  80  abuts against the second end seal line  62   e  of the second end metal separator  32   e . In the structure, the elastic force of the first elastic seal member  80  is applied to the second end seal line  62   e , and the elastic force of the second end seal line  62   e  is applied to the first elastic seal member  80 . Further, the second elastic seal member  84  is provided on the insulator  18   b , and the second elastic seal member  84  abuts against the first end seal line  52   e  of the first end metal separator  30   e . In the structure, the elastic force of the second elastic seal member  84  is applied to the first end seal line  52   e , and the elastic force of the first end seal line  52   e  is applied to the second elastic seal member  84 . Therefore, improvement in the sealing performance at both ends of the stack body  14  in the stacking direction is achieved. 
     Further, the first recess  82  is formed in the surface  19   a  of the insulator  18   a  to provide the first elastic seal member  80  in the first recess  82 , and the second recess  86  is formed in the surface  19   b  of the insulator  18   b  to provide the second elastic seal member  84  in the second recess  86 . In the structure, it is possible to reduce the size of the stack body  14  in the stacking direction. 
     Further, the first seal line  52  is provided around the oxygen-containing gas flow field  48 , and around the oxygen-containing gas supply passage  34   a , the oxygen-containing gas discharge passage  34   b , the fuel gas supply passage  38   a , the fuel gas discharge passage  38   b , the coolant supply passage  36   a , and the coolant discharge passage  36   b . Further, the second seal line  62  is provided around the fuel gas flow field  58 , and around the oxygen-containing gas supply passage  34   a , the oxygen-containing gas discharge passage  34   b , the fuel gas supply passage  38   a , the fuel gas discharge passage  38   b , the coolant supply passage  36   a , and the coolant discharge passage  36   b . In the structure, it is possible to reliably prevent leakage of the reactant gases (oxygen-containing gas and the fuel gas) and the coolant. 
     In the embodiment of the present invention, all of the first metal separators  30  have the same structure, and all of the second metal separators  32  have the same structure. That is, since no dedicated component parts are required for the first end metal separator  30   e  and the second end metal separator  32   e , it is possible to reduce the types of component parts of the fuel cell stack  10 , and achieve reduction in the number of production steps of the fuel cell stack  10 . 
     For example, if power generation of the fuel cell stack  10  is started, the temperature of the fuel cell stack  10  is increased. If power generation of the fuel cell stack  10  is stopped, the temperature of the fuel cell stack  10  is decreased. In general, the difference between the linear expansion coefficient of the joint separator  33  and the linear expansion coefficient of the insulators  18   a ,  18   b  is relatively large. 
     However, in the embodiment of the present invention, the second seal line  62  does not contact the insulator  18   a , but contacts the first elastic seal member  80 . Therefore, for example, as shown in  FIG. 8 , even in the case where the positional relationship between the insulator  18   a  and the second seal line  62  is shifted in the direction indicated by the arrow C by heat expansion or heat contraction, since the first elastic seal member  80  is deformed elastically, it is possible to suppress displacement of the contact position between the second seal line  62  and the first elastic seal member  80 . 
     Likewise, the first seal line  52  does not contact the insulator  18   b , but contacts the second elastic seal member  84 . Therefore, for example, even in the case where the positional relationship between the insulator  18   b  and the first seal line  52  is shifted in the direction indicated by the arrow C by heat expansion or heat contraction, since the first elastic seal member  80  is deformed elastically, it is possible to suppress displacement of the contact position between the second seal line  62  and the first elastic seal member  80 . Accordingly, it is possible to suppress degradation of the sealing performance at the ends of the stack body  14  in the stacking direction which may occur as a result of the change in the temperature of the fuel cell stack  10 . 
     Further, the predetermined gap Sa is formed between the first elastic seal member  80  and the side surface  83   a  of the first recess  82 , and the predetermined gap Sb is formed between the second elastic seal member  84  and the side surface  87   a  of the second recess  86 . In the structure, it is possible to ensure that the first elastic seal member  80  and the second elastic seal member  84  are easily deformed elastically. 
     Further, since the surface  81  of the first elastic seal member  80  facing the stack body  14  has the flat shape, it is possible to efficiently ensure that the second end seal line  62   e  contacts the surface  81  of the first elastic seal member  80  tightly. Further, since the surface  85  of the second elastic seal member  84  facing the stack body  14  has the flat shape, it is possible to ensure that the first end seal line  52   e  efficiently contacts the surface  85  of the second elastic seal member  84  tightly. 
     The present invention is not limited to the above structure. For example, the first elastic seal member  80  may be provided on the flat surface  19   a  of the insulator  18   a  where the first recess  82  is not formed, and the second elastic seal member  84  may be provided on the flat surface  19   b  of the insulator  18   b  where the second recess  86  is not formed. In this case, since there is no need to provide the first recess  82  and the second recess  86 , it is possible to simplify the structure of the insulators  18   a ,  18   b.    
     Further, in the above described embodiment, the first elastic seal member  80  is provided on the insulator  18   a , and the second elastic seal member  84  is provided on the insulator  18   b . However, as shown in  FIG. 9 , in the case where the insulators  18   a ,  18   b  are slightly smaller than the joint separator  33 , the first elastic seal member  80  may be provided in a first recess  21  of the end plate  20   a , and the second elastic seal member  84  may be provided in a second recess  25  of the end plate  20   b.    
     In this case, the gap Sa is formed between the first elastic seal member  80  and a side surface  23   a  of the first recess  21 , and in this state, the first elastic seal member  80  is attached or fused to a bottom surface  23   b  of the first recess  21 . Specifically, an outer seal  80   a  (first elastic seal member  80 ) is provided in an outer recess  21   a  (first recess  21 ) of the end plate  20   a , and an inner seal  80   b  (first elastic seal member  80 ) is provided in an inner recess  21   b  (first recess  21 ) of the end plate  20   a.    
     The gap Sb is formed between the second elastic seal member  84  and a side surface  27   a  of the second recess  25 , and in this state, the second elastic seal member  84  is attached or fused to a bottom surface  27   b  of the second recess  25 . Further, the outer seal  84   a  (second elastic seal member  84 ) is provided in an outer recess  25   a  (second recess  25 ) of the end plate  20   b , and the inner seal  84   b  (second elastic seal member  84 ) is provided in an inner recess  25   b  (second recess  25 ) of the end plate  20   b.    
     It should be noted that the first elastic seal member  80  may be provided on the surface  29   a  of the end plate  20   a  and the second elastic seal member  84  may be provided on the surface  29   b  of the end plate  20   b . In this case, since there is no need to provide the first recess  21  and the second recess  25 , it is possible to simplify the structure of the end plates  20   a ,  20   b.    
     In the above described embodiment, the seal line  52  is formed on the first metal separator  30 , and the seal line  52  protrudes in the stacking direction of the stack body  14  in a manner to contact the resin film  46 . The seal line  62  is formed on the second metal separator  32 , and the seal line  62  protrudes in the stacking direction of the stack body  14  in a manner to contact the resin film  46 . However, in the present invention, as shown in  FIG. 10 , the seal lines  52 ,  62  may be provided to contact the outer circumferential portion of the membrane electrode assembly  28  which does not have the resin film  46 . In this case, in order to effectively suppress leakage of the fuel gas and the oxygen-containing gas, preferably, the seal lines  52 ,  62  are formed by impregnating the outer circumferential portion of the membrane electrode assembly  28  therewith. 
     In the embodiment of the present invention, the resin film equipped MEA  28  is sandwiched between the first metal separator  30  and the second metal separator  32  to thereby form the power generation cell  12 , and the coolant flow field  66  is formed in each space between the adjacent power generation cells  12 , whereby a cooling structure for cooling each of the power generation cells  12  is provided. Alternatively, for example, three or more metal separators and two or more membrane electrode assemblies (MEAs) may be provided, and the metal separators and the membrane electrode assemblies may be stacked alternately to thereby form a cell unit. In this case, so called a skip cooling structure where a coolant flow field is formed between the adjacent cell units is provided. 
     In the skip cooling structure, a fuel gas flow field is formed on one surface of a single metal separator, and an oxygen-containing gas flow field is formed on the other surface of the single metal separator. Therefore, one metal separator is provided between membrane electrode assemblies. 
     The fuel cell stack according to the present invention is not limited to the above described embodiments. It is a matter of course that various structures can be adopted without deviating from the scope of the present invention.