Abstract:
There has been a problem that the cell units cannot bear the load exerted on the units while being stacked since a fuel cell stack including a refrigerant channel formed between cell units each having an even number of electrolyte/electrode structures (MEA) and metal separators which are alternated does not have any structure supporting the separators forming the refrigerant channel in a stacking direction. In order to solve the above problem, in each of a first power generating unit and a second power generating unit, projections formed at the buffer portions of the separators are disposed in the same positions in the stacking direction with the MEA interposed therebetween. Since between the first and second power generating units, the projections of the buffer portions are staggered, the projections of the first and second power generating units are thereby disposed in the same positions in the stacking direction.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a fuel cell stack including power generation units formed by stacking an even number of electrolyte electrode assemblies and metal separators alternately. Each of the electrolyte electrode assemblies includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. A fuel gas flow field for supplying a fuel gas to the anode and an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode are formed in each of the power generation units. Each of the fuel gas flow field and the oxygen-containing gas flow field has an uneven buffer at least at one of a flow field inlet and a flow field outlet of the fuel gas flow field and the oxygen-containing gas flow field. The power generation units include a first power generation unit and a second power generation unit stacked alternately such that a coolant flow field is formed between the first power generation unit and the second power generation unit. 
       BACKGROUND ART 
       [0002]    For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane, and interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between a pair of separators to form a unit cell. In use, normally, a predetermined number of the unit cells are stacked together to form a fuel cell stack. 
         [0003]    In the fuel cell, a fuel gas flow field is formed in a surface of one separator facing the anode for supplying a fuel gas to the anode, and an oxygen-containing gas flow field is formed in a surface of the other separator facing the cathode for supplying an oxygen-containing gas to the cathode. Further, a coolant flow field is formed between the separators for supplying a coolant along surfaces of the separators as necessary. 
         [0004]    In the case where metal separators are used as the separators, by providing grooves as the fuel gas flow field on one surface of the metal separator facing the anode, ridges as the back side of the grooves are formed on the other surface of the metal separator. Further, by forming grooves as the oxygen-containing gas flow field on one surface of the metal separator facing the cathode, ridges as the back side of the grooves are formed on the other surface of the metal separator. 
         [0005]    For example, a fuel cell separator disclosed in Japanese Laid-Open Patent Publication No. 08-222237 is known. According to the disclosure, in a fuel cell stack formed by stacking a plurality of fuel cells including a solid electrolyte and electrodes on both sides of the solid electrolyte, the fuel cell separator is inserted between the fuel cells. Fuel gas flow grooves for supplying a fuel gas to one of the adjacent fuel cells are formed on one surface of the fuel cell separator, and oxygen-containing gas flow grooves for supplying an oxygen-containing gas to the other of the adjacent fuel cells are formed on the other surface of the fuel cell separator. 
         [0006]    This separator is made of metal material having good workability. Material having good electrical conductivity is coated on front and back surfaces of the separator. Further, a large number of projections are provided at suitable intervals on the front and back surfaces of the separator. In the fuel cell stack, the projections contact the cell surfaces of the fuel cells. The fuel gas flow grooves and the oxygen-containing gas flow grooves between the separator and the adjacent fuel cells are formed by spaces between the projections. 
       SUMMARY OF INVENTION 
       [0007]    The fuel cell stack may adopt so called skip cooling structure where the coolant flow field is formed at intervals of a predetermined number of unit cells.  FIG. 5  shows a fuel cell where the above conventional technique is adopted in the fuel cell having the skip cooling structure of this type. The fuel cell is formed by stacking a plurality of cell units  3  each including two MEAs  1   a ,  1   b , and three metal separators  2   a ,  2   b , and  2   c.    
         [0008]    Each of the MEAs  1   a ,  1   b  includes an anode  4   b , a cathode  4   c , and a solid electrolyte membrane  4   a  interposed between the anode  4   b  and the cathode  4   c . The metal separator  2   a  has a plurality of projections  5   a  forming a fuel gas flow field  5  for supplying a fuel gas to the anode  4   b  of the MEA  1   a . The metal separator  2   b  has a plurality of projections  6   a  forming an oxygen-containing gas flow field  6  for supplying an oxygen-containing gas to the cathode  4   c  of the MEA  1   a  and a plurality of projections  5   a  forming a fuel gas flow field  5  for supplying a fuel gas to the anode  4   b  of the MEA  1   b  alternately. 
         [0009]    The metal separator  2   c  has a plurality of projections  6   a  forming an oxygen-containing gas flow field  6  for supplying an oxygen-containing gas to the cathode  4   c  of the MEA  1   b . A coolant flow field  7  for supplying a coolant is formed between the adjacent metal separators  2   c ,  2   a.    
         [0010]    The projections  5   a ,  6   a  of the metal separators  2   a ,  2   b  sandwiching the MEA  1   a  are provided at the same positions in the stacking direction. Further, the projections  5   a ,  6   a  of the metal separators  2   b ,  2   c  sandwiching the MEA  1   b  are provided at the same positions in the stacking direction. 
         [0011]    However, when the coolant flow field  7  is formed between the cell units  3 , in the coolant flow field  7 , the metal separators  2   c ,  2   a  do not support each other in the stacking direction because the projections and the recess face each other in the stacking direction. In the structure, the fuel cell stack cannot withstand the load between the cell units  3  when the cell units  3  are stacked together. Further, the fuel cell stack cannot withstand the pressure change during power generation. 
         [0012]    As a result, the fuel cell stack may be damaged undesirably due to deformation of the MEAs  1   a ,  1   b  and the metal separators  2   a  to  2   c . Further, the desired electrical conduction between the cell units  3  cannot be achieved. 
         [0013]    The present invention is based on the fuel cell having skip cooling structure of this type, and an object of the present invention is to provide a fuel cell stack having simple and economical structure in which it is possible to reliably retain the structure of components between power generation units forming a coolant flow field, and suitably prevent deformation of electrolyte electrode assemblies and metal separators. 
         [0014]    The present invention relates to a fuel cell stack including power generation units formed by stacking an even number of electrolyte electrode assemblies and metal separators alternately. Each of the electrolyte electrode assemblies includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. A fuel gas flow field for supplying a fuel gas to the anode and an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode are formed in each of the power generation units. Each of the fuel gas flow field and the oxygen-containing gas flow field has an uneven buffer at least at one of a flow field inlet and a flow field outlet of the fuel gas flow field and the oxygen-containing gas flow field. The power generation units include a first power generation unit and a second power generation unit stacked alternately such that a coolant flow field is formed between the first power generation unit and the second power generation unit. 
         [0015]    In the first power generation unit, bosses in the buffer for the fuel gas flow field and bosses in the buffer for the oxygen-containing gas flow field sandwiching each of the electrolyte electrode assemblies are arranged at the same positions in the stacking direction. In the second power generation unit, bosses in the buffer for the fuel gas flow field and bosses in the buffer for the oxygen-containing gas flow field sandwiching each of the electrolyte electrode assemblies are arranged at the same positions in the stacking direction and are staggered from the bosses in the buffers of the first power generation unit. 
         [0016]    Preferably, the first and second power generation units include at least first and second electrolyte electrode assemblies, the first electrolyte electrode assembly is stacked on a first metal separator, a second metal separator is stacked on the first electrolyte electrode assembly, the second electrolyte electrode assembly is stacked on the second metal separator, and a third metal separator is stacked on the second electrolyte electrode assembly. 
         [0017]    In the present invention, in each of the first power generation unit and the second power generation unit, the bosses of the buffers contacting each of the electrolyte electrode assemblies are in the same phase with each other. In the structure, no shearing force is applied to the electrolyte electrode assemblies, and damages of the electrolyte electrode assemblies can be prevented advantageously. 
         [0018]    Further, in the buffer of the first power generation unit and the buffer of the second power generation unit, bosses facing each other toward the coolant flow field are arranged at the same positions in the stacking direction. 
         [0019]    That is, each of the first power generation unit and the second power generation includes an even number of electrolyte electrode assemblies and an odd number of metal separators. In each of the first and the second power generation units, the bosses sandwiching each of membrane electrode assemblies arranged at both ends in the stacking direction are staggered from each other in the stacking direction. Thus, in the first power generation unit and the second power generation unit, the bosses toward the coolant flow field of each of the adjacent metal separators are arranged in the same phase. 
         [0020]    Therefore, also in the coolant flow field, the bosses face each other in the stacking direction. Thus, it becomes possible to provide structure of withstanding the load in the stacking direction, and withstanding the pressure change during power generation. In the structure, simply by stacking the first power generation unit and the second power generation unit alternately, the fuel cell stack can be produced simply and economically. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  is an exploded perspective view showing main components of a fuel cell stack according to an embodiment of the present invention; 
           [0022]      FIG. 2  is an exploded perspective view showing main components of a first power generation unit of the fuel cell stack; 
           [0023]      FIG. 3  is a cross sectional view showing the fuel cell stack, taken along a line III-III in  FIG. 2 ; 
           [0024]      FIG. 4  is an exploded perspective view showing a third metal separator of the first power generation unit and a first metal separator of a second power generation unit; 
           [0025]      FIG. 5  is a view showing a conventional fuel cell stack. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]      FIG. 1  is an exploded perspective showing main components of a fuel cell stack  10  according to an embodiment of the present invention. 
         [0027]    The fuel cell stack  10  is formed by stacking a first power generation unit  12 A and a second power generation unit  12 B alternately in a horizontal direction indicated by an arrow A. As shown in  FIG. 2 , the first power generation unit  12 A includes a first metal separator  14 A, a first membrane electrode assembly (MEA) (electrolyte electrode assembly)  16   a , a second metal separator  18 A, a second membrane electrode assembly  16   b , and a third metal separator  20 A. The first power generation unit  12 A may include an even number of, four or more MEAs. 
         [0028]    For example, the first metal separator  14 A, the second metal separator  18 A, and the third metal separator  20 A are made of steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces by surface treatment. Each of the first metal separator  14 A, the second metal separator  18 A, and the third metal separator  20 A has a concave-convex shape in cross section, by corrugating a metal thin plate under pressure. 
         [0029]    Each of the first and second membrane electrode assemblies  16   a ,  16   b  includes an anode  24 , a cathode  26  and a solid polymer electrolyte membrane  22  interposed between the anode  24  and the cathode  26 . The solid polymer electrolyte membrane  22  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. 
         [0030]    Each of the anode  24  and the cathode  26  has a gas diffusion layer (not shown) such as a carbon paper, and an electrode catalyst layer (not shown) of platinum alloy supported on porous carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layers are fixed to both surfaces of the solid polymer electrolyte membrane  22 , respectively. 
         [0031]    At an end of the first power generation unit  12 A in the longitudinal direction indicated by an arrow B, a fuel gas supply passage  30   a  for supplying a fuel gas such as a hydrogen-containing gas, a coolant supply passage  32   a  for supplying a coolant, and an oxygen-containing gas discharge passage  34   b  for discharging an oxygen-containing gas are provided. The fuel gas supply passage  30   a , the coolant supply passage  32   a , and the oxygen-containing gas discharge passage  34   b  extend through the first power generation unit  12 A in the direction indicated by the arrow A. 
         [0032]    At the other end of the first power generation unit  12 A in the longitudinal direction indicated by the arrow B, an oxygen-containing gas supply passage  34   a  for supplying the oxygen-containing gas, a coolant discharge passage  32   b  for discharging the coolant, and a fuel gas discharge passage  30   b  for discharging the fuel gas are provided. The oxygen-containing gas supply passage  34   a , the coolant discharge passage  32   b , and the fuel gas discharge passage  30   b  extend through the first power generation unit  12 A in the direction indicated by the arrow A. 
         [0033]    The first metal separator  14 A has a first oxygen-containing gas flow field  36  on its surface  14   a  facing the first membrane electrode assembly  16   a . The first oxygen-containing gas flow field  36  is connected between the oxygen-containing gas supply passage  34   a  and the oxygen-containing gas discharge passage  34   b . The first oxygen-containing gas flow field  36  includes a plurality of flow grooves  36   a  extending in the direction indicated by the arrow B. 
         [0034]    At least one of an inlet buffer  38  and an outlet buffer  40  is provided adjacent to an inlet or an outlet of the first oxygen-containing gas flow field  36 . Each of the inlet buffer  38  and the outlet buffer  40  has an uneven or embossed shape that includes bosses on the front and back surfaces of the first metal separator  14 A which defines an intermediate height. The inlet buffer  38  and the outlet buffer  40  have a plurality of bosses  38   a ,  40   a  protruding from the surface  14   a  (facing the first membrane electrode assembly  16   a ) and a plurality of bosses  38   b ,  40   b  protruding from the surface  14   b . Various shapes such as a circular shape, an oval shape, or a rectangular shape may be adopted for the bosses  38   a ,  38   b ,  40   a  and  40   b . The bosses described later may be formed in various shapes as well. 
         [0035]    A coolant flow field  44  is partially formed on the surface  14   b  of the first metal separator  14 A. The coolant flow field  44  is connected between the coolant supply passage  32   a  and the coolant discharge passage  32   b . A plurality of flow grooves (recesses)  44   a  are formed on the surface  14   b  of the first metal separator  14 A, on the back of the flow grooves  36   a  of the first oxygen-containing gas flow field  36 . Portions near an inlet and an outlet of the flow grooves  44   a  are back surfaces of the buffers having the bosses  40   b ,  38   b.    
         [0036]    The second metal separator  18 A has a first fuel gas flow field  46  on its surface  18   a  facing the first membrane electrode assembly  16   a . The first fuel gas flow field  46  is connected between the fuel gas supply passage  30   a  and the fuel gas discharge passage  30   b . The first fuel gas flow field  46  includes a plurality of flow grooves (recesses)  46   a  extending in the direction indicated by the arrow B. An inlet buffer  48  and an outlet buffer  50  are provided at positions near an inlet and an outlet of the first fuel gas flow field  46 . 
         [0037]    Each of the inlet buffer  48  and the outlet buffer  50  has an uneven or embossed shape that includes bosses on the front and back surfaces of the second metal separator  18 A which defines an intermediate height. The inlet buffer  48  and the outlet buffer  50  have a plurality of bosses  48   a ,  50   a  protruding from the surface  18   a  (facing the first membrane electrode assembly  16   a ) and a plurality of bosses  48   b ,  50   b  protruding from the surface  18   b  (facing the second membrane electrode assembly  16   b ). 
         [0038]    The second metal separator  18 A has a second oxygen-containing gas flow field  52  on its surface  18   b  facing the second membrane electrode assembly  16   b . The second oxygen-containing gas flow field  52  is connected between the oxygen-containing gas supply passage  34   a  and the oxygen-containing gas discharge passage  34   b . The second oxygen-containing gas flow field  52  includes a plurality of flow grooves (recesses)  52   a  extending in the direction indicated by the arrow B. An inlet buffer  54  and an outlet buffer  56  are provided at positions near an inlet and an outlet of the second oxygen-containing gas flow field  52 . The second oxygen-containing gas flow field  52  is provided on the back of the first fuel gas flow field  46 . The inlet buffer  54  and the outlet buffer  56  are provided on the back of the inlet buffer  54  and the outlet buffer  56 . 
         [0039]    The third metal separator  20 A has a second fuel gas flow field  58  on its surface  20   a  facing the second membrane electrode assembly  16   b . The second fuel gas flow field  58  is connected between the fuel gas supply passage  30   a  and the fuel gas discharge passage  30   b . The second fuel gas flow field  58  includes a plurality of flow grooves (recesses)  58   a  extending in the direction indicated by the arrow B. An inlet buffer  60  and an outlet buffer  62  are provided at positions near an inlet and an outlet of the second fuel gas flow field  58 . 
         [0040]    Each of the inlet buffer  60  and the outlet buffer  62  has an uneven or embossed shape that includes bosses on the front and back surfaces of the third metal separator  20 A which defines an intermediate height. The inlet buffer  60  and the outlet buffer  62  have a plurality of bosses  60   a ,  62   a  protruding from the surface  20   a  (facing the second membrane electrode assembly  16   b ) and a plurality of bosses  60   b ,  62   b  protruding from the surface  20   b  (facing the second power generation cell  12 B). 
         [0041]    The coolant flow field  44  is partially formed on the surface  20   b  of the third metal separator  20 A. A plurality of flow grooves (recesses)  44   b  are formed on the surface  20   b , on the back of the flow grooves  58   a  of the second fuel gas flow field  58 . 
         [0042]    In the first power generation unit  12 A, when the first membrane electrode assembly  16   a  is sandwiched between the first metal separator  14 A and the second metal separator  18 A, ridges between the flow grooves  36   a ,  46   a  of the first oxygen-containing gas flow field  36  and the first fuel gas flow field  46  provided oppositely are arranged at the same positions in the stacking direction. Likewise, when the second membrane electrode assembly  16   b  is sandwiched between the second metal separator  18 A and the third metal separator  20 A, ridges between the flow grooves  52   a ,  58   a  of the second oxygen-containing gas flow field  52  and the second fuel gas flow field  58  are arranged at the same positions in the stacking direction. 
         [0043]    In each embossed section, as shown in  FIG. 3 , in the first metal separator  14 A and the second metal separator  18 A, the bosses  40   a ,  48   a ,  38   a ,  50   a  protruding toward the first membrane electrode assembly  16   a  are arranged at the same positions in the stacking direction. 
         [0044]    In the second metal separator  18 A and the third metal separator  20 A, the bosses  48   b ,  60   a ,  50   b ,  62   a  protruding toward the second membrane electrode assembly  16   b  are arranged at the same positions in the stacking direction. 
         [0045]    As shown in  FIG. 2 , a first seal member  64  is formed integrally on the surfaces  14   a ,  14   b  of the first metal separator  14 A, around the outer end of the first metal separator  14 A. Further, a second seal member  66  is formed integrally on the surfaces  18   a ,  18   b  of the second metal separator  18 A, around the outer end of the second metal separator  18 A. Further, a third seal member  68  is formed integrally on the surfaces  20   a ,  20   b  of the third metal separator  20 A, around the outer end of the third metal separator  20 A. 
         [0046]    As shown in  FIG. 1 , the second power generation unit  12 B includes a first metal separator  14 B, a first membrane electrode assembly  16   a , a second metal separator  18 B, a second membrane electrode assembly  16   b , and a third metal separator  20 B. The constituent elements of the second power generation unit  12 B that are identical to those of the first power generation unit  12 A are labeled with the same reference numerals, and detailed description thereof will be omitted. 
         [0047]    The bosses in the inlet buffers  38 ,  48 ,  54 , and  60  and the outlet buffers  40 ,  50 ,  56 , and  62  of the second power generation unit  12 B are arranged in different phases, i.e., staggered from the bosses of the first power generation unit  12 A (see  FIG. 3 ). 
         [0048]    As shown in  FIGS. 3 and 4 , the coolant flow field  44  is formed between the third metal separator  20 A of the first power generation unit  12 A and the first metal separator  14 B of the second power generation unit  12 B. At both ends of the coolant flow field  44 , the bosses  60   b ,  40   b  facing each other, and the bosses  62   b ,  38   b  facing each other, protruding from the third metal separator  20 A and the first metal separator  14 B are arranged at the same positions in the stacking direction. Preferably, the ridges of the flow grooves  44   b ,  44   a  are arranged at the same positions in the stacking direction. 
         [0049]    Operation of the fuel cell stack  10  having the above structure will be described below. 
         [0050]    Firstly, as shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  34   a , and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  30   a . Further, a coolant such as pure water, ethylene glycol, or oil is supplied to the coolant supply passage  32   a.    
         [0051]    Thus, as shown in  FIG. 2 , in the first power generation unit  12 A, the oxygen-containing gas flows from the oxygen-containing gas supply passage  34   a  into the first oxygen-containing gas flow field  36  of the first metal separator  14 A and the second oxygen-containing gas flow field  52  of the second metal separator  18 A. The oxygen-containing gas moves along the first oxygen-containing gas flow field  36  in the horizontal direction indicated by the arrow B, and the oxygen-containing gas is supplied to the cathode  26  of the first membrane electrode assembly  16   a . Further, the oxygen-containing gas moves along the second oxygen-containing gas flow field  52  in the direction indicated by the arrow B, and the oxygen-containing gas is supplied to the cathode  26  of the second membrane electrode assembly  16   b.    
         [0052]    The fuel gas from the fuel gas supply passage  30   a  flows along the first fuel gas flow field  46  of the second metal separator  18 A in the horizontal direction indicated by the arrow B, and the fuel gas is supplied to the anode  24  of the first membrane electrode assembly  16   a . Further, the fuel gas moves along the second fuel gas flow field  58  of the third metal separator  20 A in the direction indicated by the arrow B, and the fuel gas is supplied to the anode  24  of the second membrane electrode assembly  16   b.    
         [0053]    Thus, in each of the first and second membrane electrode assemblies  16   a ,  16   b , the oxygen-containing gas supplied to the cathode  26  and the fuel gas supplied to the anode  24  are partially consumed in the electrochemical reactions at catalyst layers of the cathode  26  and the anode  24  for generating electricity. 
         [0054]    The oxygen-containing gas after partially consumed at the cathodes  26  of the first and second membrane electrode assemblies  16   a ,  16   b  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 after partially consumed at the anodes  24  of the first and second membrane electrode assemblies  16   a ,  16   b  is discharged to the fuel gas discharge passage  30   b.    
         [0055]    As shown in  FIGS. 3 and 4 , the coolant supplied to the coolant supply passage  32   a  flows into the coolant flow field  44  formed between the third metal separator  20 A of the first power generation unit  12 A and the first metal separator  14 A of the second power generation unit  12 B. Then, the coolant flows in the direction indicated by the arrow B. After the coolant cools the first and second membrane electrode assemblies  16   a ,  16   b , the coolant is discharged into the coolant discharge passage  32   b.    
         [0056]    Further, in the second power generation unit  12 B, in the same manner as in the case of the first power generation unit  12 A, power generation is performed by the first and second membrane electrode assemblies  16   a ,  16   b.    
         [0057]    In the embodiment, as shown in  FIG. 3 , in the first power generation unit  12 A, the bosses  40   a ,  38   a  of the first metal separator  14 A protruding toward the first membrane electrode assembly  16   a  and the bosses  48   a ,  50   a  of the second metal separator  18 A protruding toward the first membrane electrode assembly  16   a  are arranged at the same positions in the stacking direction. 
         [0058]    Further, the bosses  48   b ,  50   b  of the second metal separator  18 A and the bosses  60   a ,  62   a  of the third metal separator  20 B protruding toward the second membrane electrode assembly  16   b  are arranged at the same positions in the stacking direction. In the structure, no shearing force is applied to the first and second membrane electrode assemblies  16   a ,  16   b , and damages of the first and second membrane electrode assemblies  16   a ,  16   b  can be prevented advantageously. 
         [0059]    Further, the bosses  38   a ,  38   b ,  40   a ,  40   b ,  48   a ,  48   b ,  50   a ,  50   b ,  60   a ,  60   b ,  62   a ,  62   b  of the first power generation unit  12 A and the second power generation unit  12 B are staggered (arranged in different phases) from each other. 
         [0060]    Each of the first power generation unit  12 A and the second power generation unit  12 B includes an even number of, e.g., two MEAs, i.e., the first and second membrane electrode assemblies  16   a ,  16   b  and an odd number of, e.g., three separators, i.e., the first metal separators  14 A,  14 B, the second metal separators  18 A,  18 B, and the third metal separators  20 A,  28 B. In the structure, the bosses  40   a ,  38   a  and the bosses  48   b ,  50   b  are staggered from each other, and the bosses  48   a ,  50   a  and the bosses  60   a ,  62   a  are staggered from each other. 
         [0061]    The bosses  60   b  and the bosses  40   b  on the back of the buffers at both ends of the coolant flow field  44  formed between the first power generation unit  12 A and the second power generation unit  12 B are arranged at the same position in the stacking direction. Likewise, the bosses  62   b  and the bosses  38   b  are arranged at the same positions in the stacking direction. 
         [0062]    In the structure, in the coolant flow field  44 , the bosses  60   b ,  40   b  contact each other, and the bosses  62   b ,  38   b  contact each other in the stacking direction (see  FIG. 3 ). Thus, it becomes possible to provide structure of reliably withstanding the load in the stacking direction, and withstanding the pressure change during power generation. In the structure, electrical conductivity between the first power generation unit  12 A and the second power generation unit  12 B does not become low. Damages due to deformation of the MEAs and the separators can be prevented. Therefore, simply by stacking the first power generation unit  12 A and the second power generation unit  12 B alternately, the fuel cell stack  10  can be produced simply and economically.