Patent Publication Number: US-10312529-B2

Title: Fuel cell stack

Description:
TECHNICAL FIELD 
     The present invention relates to a fuel cell stack constituted by sticking fuel cells having a separator. 
     BACKGROUND ART 
     JP2015-22802A discloses that, in fuel cells that are adjacent, an anode separator of one fuel cell is welded to a cathode separator of the other fuel cell to join both separators. The separator includes a plurality of fluid flow passages formed such that a bottom portion that abuts on a membrane electrode assembly, and a protrusion that protrudes from this bottom portion are repeatedly arranged. The adjacent two separators are joined such that the protrusions are welded to one another. 
     SUMMARY OF INVENTION 
     Such joining of the separators is performed for preventing positional deviation or the like of the separators when a fuel cell stack is manufactured. Therefore, all the protrusions of the separators are not welded, and some protrusions among the plurality of existing protrusions will be selected as separator welding positions. 
     When the separators are welded using only several protrusions, in stacking of a plurality of fuel cells, a protrusion height of the protrusion at a welding position will be higher than a protrusion height of another protrusion caused by a thickness of a welding bead. Thus, if a part of the fluid flow passage of the separator is higher than another part, in constituting of the fuel cell stack, a contact surface pressure that acts on the separator varies. Then, the separator cannot contact the membrane electrode assembly uniformly within a contact surface, thus increasing contact resistance inside the fuel cell stack. The high part in the separator will be strongly pressed to the membrane electrode assembly, thus having a concern that the membrane electrode assembly deteriorates. 
     An object of the present invention is to provide a fuel cell stack configured to reduce variation of a contact surface pressure of a separator and a membrane electrode assembly. 
     According to an aspect of this invention, a fuel cell stack constituted by stacking fuel cells including a membrane electrode assembly constituted by sandwiching an electrolyte membrane with a pair of electrodes and a pair of separators that have flow passages through which gas to be supplied to the membrane electrode assembly flows, the pair of separators being arranged across the membrane electrode assembly, is provided. The fuel cell stack includes a welded portion where the separators adjacent to one another in a stacking direction of the fuel cell are welded. The separator in the stacking direction at the welded portion has a height lower than a height of the separator other than the welded portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a fuel cell stack according to a first embodiment of the present invention. 
         FIG. 2  is a front view of a membrane electrode assembly that constitutes a fuel cell. 
         FIG. 3  is a front view of an anode separator that constitutes the fuel cell. 
         FIG. 4  is a front view of a cathode separator that constitutes the fuel cell. 
         FIG. 5  is a partial vertical cross-sectional view of the fuel cell stack. 
         FIG. 6A  is a view illustrating a separator assembly such that two separators are welded. 
         FIG. 6B  is a view illustrating the separator assembly in stacking of the fuel cell. 
         FIG. 6C  is a view illustrating a state of the separator assembly in constituting of the fuel cell stack. 
         FIG. 7  is a view illustrating an exemplary welding bead formed in welding of the separator. 
         FIG. 8  is a view illustrating an exemplary welding bead formed in welding of the separator. 
         FIG. 9A  is a view illustrating a separator assembly such that separators are welded according to a second embodiment of the present invention. 
         FIG. 9B  is a view illustrating a state of the separator assembly in advance compression forming. 
         FIG. 9C  is a view illustrating a state of the separator assembly in constituting of a fuel cell stack. 
         FIG. 10  is a view illustrating a cross-sectional surface in a stacking direction of a fuel cell stack according to one modification of the first and second embodiments. 
         FIG. 11  is a view illustrating a cross-sectional surface in a stacking direction of a fuel cell stack according to another modification of the first and second embodiments. 
         FIG. 12  is an explanatory view of another modification in a separator structure of a fuel cell stack. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present invention with reference to the drawings and the like. 
     First Embodiment 
     A fuel cell is constituted such that an anode electrode as a fuel electrode and a cathode electrode as an oxidant electrode sandwich an electrolyte membrane. The fuel cell generates electric power using anode gas containing hydrogen supplied to the anode electrode and cathode gas containing oxygen supplied to the cathode electrode. Electrode reaction that progresses in both electrodes: the anode electrode and the cathode electrode is as follows.
 
Anode electrode: 2H 2 →4H + +4 e   −   (1)
 
Cathode electrode: 4H + +4 e   − +O 2 →2H 2 O  (2)
 
With these electrode reactions (1) and (2), the fuel cell generates an electromotive force with about 1 V (volt).
 
       FIG. 1  is an exploded view illustrating a schematic configuration of a fuel cell stack  100  according to a first embodiment. 
     The fuel cell stack  100  illustrated in  FIG. 1  is a fuel cell stack used for a vehicle such as an electric vehicle and a hybrid vehicle. However, the fuel cell stack  100  may be used as a power source of various electrical devices, not limited to the use in the automobile and the like. 
     The fuel cell stack  100  is a stacked battery constituted such that a plurality of fuel cells  10  as unit cells are stacked. 
     The fuel cell  10  that constitutes the fuel cell stack  100  includes a membrane electrode assembly (MEA)  20 , an anode separator  30  arranged at one surface of the MEA  20 , and a cathode separator  40  arranged at the other surface of the MEA  20 . Thus, in the fuel cell stack  100 , a pair of separators  30 ,  40  are arranged across the MEA  20 . 
     As illustrated in  FIG. 2  and  FIG. 3 , the MEA  20  is constituted of an electrolyte membrane  21 , an anode electrode  22  arranged at one surface of the electrolyte membrane  21 , and a cathode electrode  23  arranged at the other surface of the electrolyte membrane  21 . It should be noted that  FIG. 2  illustrates a front view of the MEA  20  that constitutes the fuel cell  10 , and FIG.  5  illustrates a partial vertical cross-sectional view of the fuel cell  10  at a position V-V in  FIG. 2 . 
     As illustrated in  FIG. 5 , the electrolyte membrane  21  is a proton-conductive ion exchange membrane formed of fluorine-based resin. The anode electrode  22  is constituted such that, in an order from a side of the electrolyte membrane  21 , an electrode catalyst layer made of an alloy such as platinum, a water-repellent layer made of fluorine resin or the like, and a gas diffusion layer made of a carbon cloth or the like are arranged. The cathode electrode  23 , similarly to the anode electrode  22 , is constituted such that, in an order from the electrolyte membrane  21  side, the electrode catalyst layer, the water-repellent layer, and the gas diffusion layer are arranged. 
     On the MEA  20 , a frame portion  50  made of resin is disposed along an assembly outer periphery. The frame portion  50  is a frame body made of a synthetic resin or the like, and integrally formed on the MEA  20 . The frame portion  50  may be constituted as a plate-shaped material having rigidity, or may be constituted as a sheet-shaped member having flexibility. 
     As illustrated in  FIG. 2 , at one end side (a left side in  FIG. 2 ) of the frame portion  50 , in an order from above, an anode gas supply manifold  51 A, a cooling water supply manifold  52 A, and a cathode gas supply manifold  53 A are formed. At the other end side (a right side in  FIG. 2 ) of the frame portion  50 , in an order from above, an anode gas exhaust manifold  51 B, a cooling water exhaust manifold  52 B, and a cathode gas exhaust manifold  53 B are formed. 
     As illustrated in  FIG. 3  and  FIG. 5 , the anode separator  30  is a plate-shaped material formed of a conductive material such as metal. The anode separator  30  has an anode gas flow passage  34  that flows the anode gas on a surface at an MEA side, and has a cooling water flow passage  35  that flows cooling water on an opposite side surface of the MEA side. 
     As illustrated in  FIG. 3 , at one end side (a left side in  FIG. 3 ) of the anode separator  30 , in an order from above, an anode gas supply manifold  31 A, a cooling water supply manifold  32 A, and a cathode gas supply manifold  33 A are formed. At the other end side (a right side in  FIG. 3 ) of the anode separator  30 , in an order from above, an anode gas exhaust manifold  31 B, a cooling water exhaust manifold  32 B, and a cathode gas exhaust manifold  33 B are formed. 
     The anode gas supplied from the anode gas supply manifold  31 A passes through the anode gas flow passage  34  to flow out to the anode gas exhaust manifold  31 B. The cooling water supplied from the cooling water supply manifold  32 A passes through the cooling water flow passage  35  to flow out to the cooling water exhaust manifold  32 B. 
     As illustrated in  FIG. 4  and  FIG. 5 , the cathode separator  40  is a plate-shaped material formed of a conductive material such as metal. The cathode separator  40  has a cathode gas flow passage  44  that flows the cathode gas on a surface at the MEA side, and has a cooling water flow passage  45  that flows the cooling water on an opposite side surface of the MEA side. 
     As illustrated in  FIG. 4 , at one end side (a left side in  FIG. 4 ) of the cathode separator  40 , in an order from above, an anode gas supply manifold  41 A, a cooling water supply manifold  42 A, and a cathode gas supply manifold  43 A are formed. At the other end side (a right side in  FIG. 4 ) of the cathode separator  40 , in an order from above, an anode gas exhaust manifold  41 B, a cooling water exhaust manifold  42 B, and a cathode gas exhaust manifold  43 B are formed. 
     The cathode gas supplied from the cathode gas supply manifold  43 A passes through the cathode gas flow passage  44  to flow out to the cathode gas exhaust manifold  43 B. The cooling water supplied from the cooling water supply manifold  42 A passes through the cooling water flow passage  45  to flow out to the cooling water exhaust manifold  42 B. 
     When the fuel cell stack  100  is constituted such that the fuel cell  10  having the MEA  20 , the anode separator  30 , and the cathode separator  40  is stacked, the anode gas supply manifolds  31 A,  41 A, and  51 A are aligned in the stacking direction to function as one anode gas supply passage. At this time, the cooling water supply manifolds  32 A,  42 A, and  52 A function as one cooling water supply passage, and the cathode gas supply manifolds  33 A,  43 A, and  53 A function as one cathode gas supply passage. Similarly, the anode gas exhaust manifolds  31 B,  41 B, and  51 B, the cooling water exhaust manifolds  32 B,  42 B, and  52 B, and the cathode gas exhaust manifolds  33 B,  43 B, and  53 B function as an anode gas exhaust passage, a cooling water exhaust passage, and a cathode gas exhaust passage respectively. 
     It should be noted that, as illustrated in  FIG. 5 , in adjacent two fuel cells  10 , the respective cooling water flow passages  35 ,  45  disposed on the anode separator  30  of one fuel cell  10  and the cathode separator  40  of the other fuel cell  10  are arranged to face one another. The cooling water flow passages  35 ,  45  thus arranged constitute one cooling passage. 
     The fuel cell  10  is formed such that the anode separator  30  and the cathode separator  40  are bonded to the frame portion  50  with an adhesive  60 . In a state where the anode separator  30  and the cathode separator  40  are bonded to the frame portion  50 , the anode separator  30  is arranged such that a surface at a side of the anode gas flow passage  34  is adjacent to one side surface of the MEA  20 , and the cathode separator  40  is arranged such that a surface at a side of the cathode gas flow passage  44  is adjacent to the other side surface of the MEA  20 . 
     In these separators  30 ,  40 , the adhesive  60  is arranged to surround separator outer edges and peripheral areas of the respective manifolds. The adhesive  60  has not only a function that bonds the respective members, but also a function as a sealing material that seals between the respective members. Accordingly, as the adhesive  60 , an olefin-based adhesive, a silicon-based adhesive, or the like having an adhesion function and a seal function is employed. These adhesives are in a gel state before hardening, and in a solid state having elasticity after hardening. 
     The following describes the configurations of the anode separator  30  and the cathode separator  40  in the fuel cell stack  100  according to this embodiment in more detail, with reference to  FIG. 6A  to  FIG. 6C . 
     In the adjacent two fuel cells  10 , as illustrated in  FIG. 6A , the anode separator  30  of one fuel cell  10  is joined to the cathode separator  40  of the other fuel cell  10  by welding. The two separators  30 ,  40  are thus welded and connected to form a separator assembly, thus mutually positioning both separators  30 ,  40 . The separator assembly thus formed and the MEA  20  are alternately built up to stack them, thus constituting the fuel cell stack  100 . 
     The cathode separator  40  is constituted as an uneven-shaped member such that a flat-plate-shaped bottom portion  46  that abuts on the MEA  20 , and a rectangular-shaped protrusion  47  that protrudes from the bottom portion  46  in the fuel cell stacking direction are sequentially arranged in a short side direction (a vertical direction in  FIG. 4 ) within a separator surface. The cathode separator  40  has an uneven-shaped structure, thus including a plurality of cathode gas flow passages  44  on one side surface, and including a plurality of cooling water flow passages  45  on the other side surface. 
     The anode separator  30  is also constituted as an uneven-shaped member such that a flat-plate-shaped bottom portion  36  that abuts on the MEA  20 , and a rectangular-shaped protrusion  37  that protrudes from the bottom portion  36  in the fuel cell stacking direction are sequentially arranged in the separator short side direction (a vertical direction in  FIG. 3 ). With such a configuration, a plurality of anode gas flow passages  34  are formed on one side surface of the anode separator  30 , and a plurality of cooling water flow passages  35  are formed on the other side surface of the anode separator  30 . 
     The above-described anode separator  30  and cathode separator  40  are welded to be mutually connected, in a state where the protrusion  37  and the protrusion  47  are positioned to be matched up. When the separator assembly is formed, all the protrusions  37 ,  47  are not selected as welding positions, and several protrusions  37 ,  47  among the plurality of existing protrusions  37 ,  47  are selected as the welding positions. In this embodiment, the protrusions  37 ,  47  in contact with both sides of the center bottom portions  36 ,  46  are the welding positions. 
     The welding for forming the separator assembly is performed such that, in a state where the anode separator  30  and the cathode separator  40  are stuck, the protrusions  37 ,  47  are irradiated with laser from one separator side. This joins both separators  30 ,  40  via a welding bead  70  (a welded portion). 
     It should be noted that, in the anode separator  30  and the cathode separator  40 , the center bottom portions  36 ,  46  in contact with the two protrusions  37 ,  47  that are the welding positions are formed lower than the other bottom portions  36 ,  46  other than the center. That is, the cathode separator  40  is constituted such that an amount of protrusion h 1  of the protrusion  47  on which the welding bead  70  is formed is lower than an amount of protrusion h 2  of the protrusion  47  at other than the welding position. The anode separator  30  is similarly constituted such that an amount of protrusion of the protrusion  37  on which the welding bead  70  is formed is lower than an amount of protrusion of the protrusion  37  at other than the welding position. The protrusions  37 ,  47  at the welding position thus set low function as surface pressure adjustment portions that reduce variation of a surface pressure that acts on the separators  30 ,  40  in stacking of the fuel cell. 
     When stacking the fuel cell, to a stacked body formed such that the separator assembly and the MEA  20  are alternately built up, a predetermined pressing force is added in the stacking direction. In the separator assembly illustrated in  FIG. 6A , a gap exists between both separators  30 ,  40  caused by the existence of the welding bead  70 . With the pressing force in stacking, the anode separator  30  and the cathode separator  40  will overlap one another such that the protrusion  37  abuts on the protrusion  47  as illustrated in  FIG. 6B . 
     When the amounts of protrusion of all the protrusions  37 ,  47  are set equal, the center bottom portions  36 ,  46  protrude in the stacking direction caused by the existence of the welding bead  70 , as indicated by the dashed line in  FIG. 6B . 
     However, in this embodiment, the amounts of protrusion of the protrusions  37 ,  47  at the welding position are constituted lower than the amounts of protrusion of the other protrusions  37 ,  47 . Thus, even when constituting the fuel cell stack  100 , a protrusion height H 1  of the protrusions  37 ,  47  at the welding position is lower than a protrusion height H 2  of the other protrusions  37 ,  47 . As a result, when constituting the fuel cell stack  100 , as illustrated in  FIG. 6C , all the bottom portions  36 ,  46  will be approximately aligned on an identical planar surface. It should be noted that the protrusion height of the protrusions  37 ,  47  when constituting the fuel cell stack  100  means a distance from the MEA  20  to distal end surfaces of the protrusions  37 ,  47 . 
     In the fuel cell stack  100 , in order to align the bottom portions  36  of the anode separator  30  on the identical planar surface, and to align the bottom portions  46  of the cathode separator  40  on the identical planar surface, the protrusion height H 1  of the protrusions  37 ,  47  at the welding position is preferably set lower than the protrusion height H 2  of the protrusions  37 ,  47  other than the welding position by a height (a thickness) of the welding bead  70 . 
     As described above, the welding bead  70  that joins the separators  30 ,  40  is, for example, as illustrated in  FIG. 7 , formed along an extending direction of the gas flow passages  34 ,  44  (fluid flow passages) between the protrusions  37 ,  47 . Thus, the welding bead  70  is constituted as a line-shaped bead (a line welded portion). 
     It should be noted that, as illustrated in  FIG. 8 , the welding bead  70  may be constituted as a welding bead (a spot welded portion) formed in spots having a predetermined, distance d in the extending direction of the gas flow passages  34 ,  44  (the fluid flow passages). In this case, a plurality of welding beads  70  are disposed in the gas-flow-passage extending direction. 
     The predetermined distance d is set to a distance such that the protrusions  37 ,  47  between the welding beads  70  arranged in the gas-flow-passage extending direction do not bend in the stacking direction by the pressing force that acts on the separators  30 ,  40  when stacking the fuel cell. For example, the predetermined distance d is preferably set to fulfill the following formula (1). 
     
       
         
           
             
               
                 
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             δ: Allowable deformation amount determined according to membrane electrode assembly 
             E: Young&#39;s modulus of material of separator 
             I: Second moment of area determined by shapes of bottom portion and protrusion 
             W: Surface pressure that acts on separator 
           
         
       
    
     With the above-described fuel cell stack  100  in the first embodiment, the following effect can be obtained. 
     In the fuel cell stack  100 , at least a pair of protrusions  37 ,  47  of the anode separator  30  and the cathode separator  40  is the welding position, and the protrusion height (a separator height in the stacking direction) of the protrusions  37 ,  47  at the welding position is lower than the protrusion height of the protrusions  37 ,  47  other than the welding position. 
     With such configurations of the separators  30 ,  40 , as illustrated in  FIG. 6C , when constituting the fuel cell stack  100 , all the bottom portions  36  of the anode separator  30  will be approximately aligned on the identical planar surface, and all the bottom portions  46  of the cathode separator  40  will be approximately aligned on the identical planar surface. This can reduce the variation of the contact surface pressure of the separators  30 ,  40  and the MEA  20  to ensure reduction of the contact resistance inside the fuel cell stack  100 . Furthermore, the bottom portions  36 ,  46  of the separators  30 ,  40  are not strongly pressed to the MEA  20  to ensure reduction of deterioration of the MEA  20 . 
     In the anode separator  30  and the cathode separator  40 , the protrusion height of the protrusions  37 ,  47  at the welding position is set lower than the protrusion height of the other protrusions  37 ,  47  by the height of the welding bead  70 . Such a consideration of the height of the welding bead  70  ensures the reduction of the variation (variability) of the contact surface pressure of the separators  30 ,  40  and the MEA  20  in the fuel cell stack  100  with more certainty. 
     The welding bead  70  that connects the anode separator  30  to the cathode separator  40  is constituted as the line-shaped bead formed along the extending direction of the gas flow passages  34 ,  44  between the protrusions  37 ,  47 . Such a line shape of the welding bead  70  can join the anode separator  30  to the cathode separator  40  with more certainty. 
     It should be noted that the welding bead  70  that connects the anode separator  30  to the cathode separator  40  may be constituted as a spot welding bead formed having the predetermined distance d in the extending direction of the gas flow passages  34 ,  44 . In such welding, the anode separator  30  and the cathode separator  40  are joined at minimal welding positions, thus ensuring reduction of welding work hours. The predetermined distance d is set to the distance such that the protrusions  37 ,  47  between the welding beads  70  do not bend in the stacking direction by the pressing force that acts on the separators  30 ,  40  when stacking of the fuel cell, thus ensuring reduction of deformation of the separators  30 ,  40  in stacking of the fuel cell. 
     Second Embodiment 
     The following describes a fuel cell stack  100  according to a second embodiment of the present invention with reference to  FIG. 9A  to  FIG. 9C . A technical idea of this embodiment can be combined with a technical idea of the first embodiment as necessary. The following embodiments use identical reference numerals to components that have functions identical to those of the first embodiment, and therefore such elements may not be further elaborated here. 
     In the second embodiment, in the anode separator  30  and the cathode separator  40  before the constitution of the fuel cell stack  100 , all the protrusions  37 ,  47  are set to have an identical amount of protrusion. When these separators  30 ,  40  are connected by welding, as illustrated in  FIG. 9A , a separator assembly having a gap by the height of the welding bead  70  between the protrusions  37 ,  47  other than the welding position is formed. 
     Observing the cathode separator  40 , the welding beads  70  are formed on the protrusions  47  different in an arranging direction of the gas flow passage  44 . As illustrated in  FIG. 9A , in this embodiment, two protrusions  47  positioned next to the center protrusion  47  are set as the welding positions. Thus, between the protrusions  47 ,  47  that will be the welding positions, at least one or more (one in this embodiment) protrusion  47  on which the welding is not performed in the arranging direction of the gas flow passage  44  will exist. It should be noted that the same applies to the anode separator  30 . 
     When the fuel cell stacking is performed using directly thus constituted separator assembly, the bottom portions  36 ,  46  positioned inside the welding positions protrude in the stacking direction caused by the existence of the welding bead  70  to vary the contact surface pressure of the separators  30 ,  40  and the MEA  20 . 
     Therefore, in the anode separator  30  and the cathode separator  40  in this embodiment, the protrusions  37 ,  47  at the welding position have deformed portions  37 A,  47 A elastically deformed by presswork. The deformed portions  37 A,  47 A are sidewall parts that connect the bottom portions  36 ,  46  to the end surfaces of the protrusions  37 ,  47 . These deformed portions  37 A,  47 A function as the surface pressure adjustment portions. 
     In this embodiment, in a step before the stacking of the separator assembly and the MEA  20 , an advance compression forming (the presswork) is performed on the separator assembly. This advance compression forming will be described with reference to  FIG. 9B . 
     As illustrated in  FIG. 9B , the advance compression forming is performed such that flat plate molds  80  are abutted on the end surface of the protrusion  37  of the anode separator  30  and the end surface of the protrusion  47  of the cathode separator  40  to compress these separators  30 ,  40  in the stacking direction. In the advance compression forming, a compressive load that acts on the separators  30 ,  40  is set higher than a stacking load (the pressing force) that acts on the separators  30 ,  40  and the like in stacking of the fuel cell. The compressive load in the advance compression forming is determined considering a separator shape, a welding bead shape, and the like. 
     When the advance compression forming is thus performed, stress concentrates on the deformed portions  37 A,  47 A of the protrusions  37 ,  47  at the welding position, and these deformed portions  37 A,  47 A elastically deform to be depressed inside. By thus deforming the deformed portions  37 A,  47 A, in the separator assembly, the protrusion  37  of the anode separator abuts on the protrusion  47  of the cathode separator  40  where the welding bead  70  does not exist. As a result, in the cathode separator  40 , the protrusion height H 1  of the protrusion  47  on which the welding bead  70  is formed becomes lower than the protrusion height. H 2  of the protrusion  47  at other than the welding position. In the anode separator  30 , similarly, the protrusion height of the protrusion  37  on which the welding bead  70  is formed becomes lower than the protrusion height of the protrusion  37  at other than the welding position. 
     In the separator assembly thus shaped by the advance compression forming, in constituting of the fuel cell stack  100 , as illustrated in  FIG. 9C , the protrusion height H 1  of the protrusions  37 ,  47  at the welding position is lower than the protrusion height H 2  of the other protrusions  37 ,  47 . Thus, all the bottom portions  36 ,  46  will be approximately aligned on the identical planar surface. Accordingly, when the fuel cell stack  100  is constituted using the separator assembly shaped by the advance compression forming, the reduction of the variation of the contact surface pressure of the separators  30 ,  40  and the MEA  20  is ensured. 
     With the above-described the fuel cell stack  100  in the second embodiment, the following effect can be obtained. 
     In the fuel cell stack  100 , in the anode separator  30  and the cathode separator  40 , the protrusion height (the separator height in the stacking direction) of the protrusions  37 ,  47  at the welding position is lower than the protrusion height of the protrusions  37 ,  47  at other than the welding position. The sidewall parts of the protrusions  37 ,  47  at the welding positions of the separators  30 ,  40  are constituted as the deformed portions  37 A,  47 A configured to elastically deform. These deformed portions  37 A,  47 A are preliminarily deformed to set the protrusion height of the protrusions  37 ,  47  at the welding position lower than the protrusion height of the protrusions  37 ,  47  at other than the welded portion. 
     With such constitution of the fuel cell stack  100  using the separators  30 ,  40  shaped by the advance compression forming or the like, as illustrated in  FIG. 9C , in constituting of the fuel cell stack  100 , all the bottom portions  36  of the anode separator  30  will be approximately aligned on the identical planar surface, and all the bottom portions  46  of the cathode separator  40  will be approximately aligned on the identical planar surface. This can reduce the variation of the contact surface pressure of the separators  30 ,  40  and the MEA  20  to ensure reduction of the contact resistance inside the fuel cell stack  100 . Furthermore, the bottom portions  36 ,  46  of the separators  30 ,  40  are not strongly pressed to the MEA  20  to ensure reduction of deterioration of the MEA  20 . 
     In the fuel cell stack  100 , between the protrusions  37 ,  47  and the protrusions  37 ,  47  as the welding positions, at least one or more protrusions  37 ,  47  that are not welded exist in the arranging direction of the gas flow passages  34 ,  44 . Thus, welding position distances are appropriately separated from one another in the arranging direction of the gas flow passages  34 ,  44  to ensure enhancement of bending moment at the deformed portions  37 A,  47 A in the advance compression forming. Accordingly, without so increasing the compressive load in the advance compression forming, the deformed portions  37 A,  47 A of the protrusions  37 ,  47  are ensured to elastically deform, thus preventing damage of the separators  30 ,  40  in the advance compression forming. 
     The mold used in the advance compression forming has been described as the flat plate mold  80 , but may be a mold other than the flat plate mold. For example, the mold may be a mold shaped into an uneven shape corresponding to the separator shape to be configured to deform the deformed portions  37 A,  47 A of the protrusions  37 ,  47  in processing. 
     Modification of First and Second Embodiments 
     A fuel cell stack  100  according to a modification of the first and second embodiments will be described with reference to  FIG. 10 . 
     The fuel cell stack  100  according to this modification is a fuel cell stack constituted using the separator assembly described in the first or second embodiment, and has a feature in an arrangement of the welding bead  70  that joins the separators  30 ,  40 . 
     The fuel cell stack  100  is constituted as a stacked body formed such that a plurality of fuel cells  10  are stacked. In two separators  30 ,  40  arranged across the MEA  20  of one certain fuel cell  10 , the welding beads  70  at a side of the anode separator  30  and the welding beads  70  at a side of the cathode separator  40  are formed shifted in the arranging direction of the gas flow passages  34 ,  44  as illustrated in  FIG. 10 . 
     When the welding beads  70  are formed in spots as illustrated in  FIG. 8 , the welding beads  70  of the anode separator  30  and the welding beads  70  of the cathode separator  40  may be shifted not only in the arranging direction of the gas flow passages  34 ,  44 , but also in the extending direction of the gas flow passages  34 ,  44 . 
     With the above-described fuel cell stack  100  according to the modification, the following effect can be obtained. 
     In the fuel cell stack  100 , the anode separator  30  and the cathode separator  40  are arranged across the MEA  20  of the fuel cell  10 . In the two separators  30 ,  40  thus arranged, the welding beads  70  at the anode separator  30  side and the welding beads  70  at the cathode separator  40  side are arranged shifted at least in one direction of the extending direction of the gas flow passages  34 ,  44  and the arranging direction of the gas flow passages  34 ,  44 . 
     Formation positions of the welding beads  70  are thus dispersed to ensure reduction of overlapping in the stacking direction of the welding beads  70  (the welding positions) in the stacked fuel cell  10 . As a result, in constituting of the fuel cell stack  100 , the variation of the contact surface pressure of the separators  30 ,  40  and the MEA  20  can be more effectively reduced. 
     In the fuel cell stack  100  illustrated in  FIG. 10 , when viewed from the stacking direction, all the welding beads  70  are not formed at an identical position, but several welding beads  70  are formed to overlap. Then, as illustrated in  FIG. 11 , the fuel cell stack  100  may be constituted such that, when viewed from the stacking direction, all the welding beads  70  are formed at different positions. 
     Thus, when viewed from the stacking direction, all the formation positions of the welding beads  70  do not overlap to more ensure the reduction of the variation of the contact surface pressure of the separators  30 ,  40  and the MEA  20 . 
     The embodiment of the present invention described above is merely illustration of a part of application example of the present invention and not of the nature to limit the technical scope of the present invention to the specific constructions of the above embodiment. 
     In the second embodiment, the advance compression forming is performed on all the surfaces of the anode separator  30  and the cathode separator  40  to elastically deform the deformed portions  37 A,  47 A of the protrusions  37 ,  47 . However, the advance compression forming may be performed on only a region where the variation of the contact surface pressure is likely to become a problem in the anode separator  30  and the cathode separator  40 . 
     For example, in the fuel cell stack  100 , an anode gas pressure is high at a side of the anode gas exhaust manifold  31 B, compared with a side of the anode gas supply manifold  31 A. Thus, in driving of a fuel cell system, the contact pressure that acts on the anode separator  30  tends to increase at a position near the anode gas exhaust manifold  31 B. Accordingly, in the anode separator  30 , the advance compression forming is performed on only the position near the anode gas exhaust manifold  31 B, and the deformed portion  37 A of the protrusion  37  in a region on which this compression forming is performed is elastically deformed. Thus, the variation of the contact surface pressure of the anode separator  30  and the MEA  20  can be reduced. 
     It should be noted that, also for the cathode separator  40 , from an identical aspect, the advance compression forming is preferably performed on a position near the cathode gas exhaust manifold  43 B. Thus, the deformed portion  47 A of the protrusion  47  in the region on which the advance compression forming is performed is elastically deformed to ensure the reduction of the variation of the contact surface pressure of the cathode separator  40  and the MEA  20 . 
     The following describes another modification in the separator structure of the fuel cell stack  100  with reference to  FIG. 12 . 
     In the fuel cell stack  100  according to this modification illustrated in  FIG. 12 , the anode separator  30  and the cathode separator  40  are constituted as flat-plate-shaped materials having grooves as the gas flow passages  34 ,  44 . A surface that abuts on the cathode separator  40  of the anode separator  30  is formed as a flat surface. A surface that abuts on the anode separator  30  of the cathode separator  40  is formed as a flat surface. Especially, on this flat surface of the cathode separator  40 , a depressed portion  48  (a welded portion) is formed. Inside the depressed portion  48 , the welding bead  70  for joining the separators  30 ,  40  is disposed. 
     By thus disposing the depressed portion  48 , in the cathode separator  40 , the separator height H 1  in the stacking direction at the welding position is lower than the separator height H 2  other than the welding position. Thus, even if the adjacent cathode separator  40  and anode separator  30  are joined by the welding bead  70  formed inside the depressed portion  48 , without forming a gap between both separators  30 ,  40 , the flat surfaces of these separators  30 ,  40  can be abutted on one another. As a result, the variation of the contact surface pressure of the separators  30 ,  40  and the MEA  20  can be reduced to reduce the contact resistance inside the fuel cell stack  100 . Furthermore, the bottom portions  36 ,  46  of the separators  30 ,  40  are not strongly pressed to the MEA  20  to ensure the reduction of the deterioration of the MEA  20 . 
     In the fuel cell stack  100  illustrated in  FIG. 12 , the depressed portion  48  is formed on the flat surface of the cathode separator  40 . However, a depressed portion may be formed on the flat surface of the anode separator  30 . Depressed portions may be formed on both flat surfaces of the separators  30 ,  40 .