Abstract:
A fuel cell stack. A fuel cell stack, an example of the fuel cell stack, is configured by alternately overlaying first electricity generating units and second electricity generating units in the horizontal direction. The first electricity units are each provided with a first fuel gas flow path, a first oxidant gas flow path, a second fuel gas flow path, and a second oxidant gas flow path, and the flow paths are set to the same phase in the overlaying direction. The second electricity generating units are each provided with a first fuel gas flow path, a first oxidant gas flow path, a second fuel gas flow path, and a second oxidant gas flow path which are set to the same phase in the overlaying direction and are set to a phase different from the phase of the flow paths of the first electricity generating units.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/JP2009/060224, filed Jun. 4, 2009, which claims priority to Japanese Patent Application No. 2008-157872 filed on Jun. 17, 2008 in Japan. The contents of the aforementioned applications are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fuel cell stack including power generation units each formed by sandwiching an electrolyte electrode assembly between metal separators. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. Each of the power generation units has a corrugated fuel gas flow field for supplying a fuel gas to the anode and a corrugated oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode. 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 
     For example, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer 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 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. 
     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. 
     In the case where metal separators of thin corrugated plates 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. 
     In the structure, by providing corrugated grooves in a serpentine pattern to form the fuel gas flow field and the oxygen-containing gas flow field, the back surfaces of the grooves are stacked together between unit cells to form a coolant flow field where a coolant flows in a direction different from the flow directions of the fuel gas and the oxygen-containing gas. 
     For example, according to the disclosure of Japanese Laid-Open Patent Publication No. 2007-141553, as shown in  FIG. 11 , a plurality of unit cells  1  are stacked together to form the fuel cell stack. Each of the unit cells  1  includes metal separators  3 ,  4  on both sides of a membrane electrode assembly  2 . 
     The membrane electrode assembly  2  includes an anode  2   b,  a cathode  2   c , and a solid polymer electrolyte membrane  2   a  interposed between the anode  2   b  and the cathode  2   c . A plurality of fuel gas flow grooves  5  extending vertically in a serpentine pattern are formed on a surface of a metal separator  3  facing the anode  2   b . A plurality of oxygen-containing gas flow grooves  6  extending vertically in a serpentine pattern are formed on a surface of a metal separator  4  facing the cathode  2   c.    
     Grooves  7  are formed on the back of the fuel gas flow grooves  5  of the metal separator  3 . Grooves  8  are formed on the back of the oxygen-containing gas flow grooves  6  of the metal separator  4 . Therefore, when the unit cells  1  are stacked together, the grooves  7 ,  8  are overlapped together to form a coolant flow field extending in a horizontal direction between the unit cells  1 . 
     SUMMARY OF INVENTION 
     In the above fuel cell stack, in order to form the coolant flow field extending in the horizontal direction in each space between the unit cells  1 , it is required to form the grooves  7 ,  8  in different phases, and mutually overlap the grooves  7 ,  8 . Therefore, in the state where the membrane electrode assembly  2  is sandwiched between the metal separators  3 ,  4 , the fuel gas flow grooves  5  and the oxygen-containing gas flow grooves  6  are arranged in serpentine patterns in different phases. 
     In the structure, when the membrane electrode assembly  2  is sandwiched between the ridges forming the fuel gas flow grooves  5  in the serpentine pattern and the ridges forming the oxygen-containing gas flow grooves  6  in the serpentine pattern, the serpentine ridges are deviated from each other (in different phases) in the stacking direction. Therefore, a shearing force may be applied to the membrane electrode assembly  2  undesirably. 
     The present invention has been made to solve the problems of this type, and an object of the present invention is to provide a fuel cell stack having simple and economical structure in which it is possible to reliably prevent a shearing force from being applied to an electrolyte electrode assembly from metal separators. 
     A fuel cell stack includes power generation units each formed by sandwiching an electrolyte electrode assembly between metal separators. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. Each of the power generation units has a corrugated fuel gas flow field for supplying a fuel gas to the anode and a corrugated oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode. The power generation units includes 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. 
     The fuel gas flow field and the oxygen-containing gas flow field of the first power generation unit are in the same phase with each other. The fuel gas flow field and the oxygen-containing gas flow field of the second power generation unit are in the same phase with each other. The fuel gas flow field and the oxygen-containing gas flow field of the first power generation unit and the fuel gas flow field and the oxygen-containing gas flow field of the second power generation unit are in different phases from each other, respectively. 
     Further, 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. 
     Further, preferably, the first and second power generation units are formed by stacking the electrolyte electrode assemblies and the metal separators alternately such that the metal separators are provided at both ends of the fuel cell stack in the stacking direction. 
     According to the present invention, in each of the first power generation unit and the second power generation unit, the fuel gas flow field and the oxygen-containing gas flow field are in the same phase with each other. In the structure, no shearing force is applied to the membrane electrode assemblies, and damages of the membrane electrode assemblies can be prevented advantageously. Further, simply by stacking the first power generation unit and the second power generation unit alternately, the fuel cell stack having simple and economical structure can be produced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view showing main components of a fuel cell stack according to a first embodiment of the present invention; 
         FIG. 2  is an exploded perspective view showing main components of a first power generation unit of the fuel cell stack; 
         FIG. 3  is a cross sectional view showing the fuel cell stack, taken along a line III-III in  FIG. 2 ; 
         FIG. 4  is a partial cross sectional view showing the fuel cell stack; 
         FIG. 5  is an exploded perspective view showing main components of a second power generation unit of the fuel cell stack; 
         FIG. 6  is a perspective view showing a coolant flow field formed between the first power generation unit and the second power generation unit; 
         FIG. 7  is an exploded perspective view showing main components of a fuel cell stack according to a second embodiment of the present invention; 
         FIG. 8  is an exploded perspective view showing main components of a first power generation unit of the fuel cell stack; 
         FIG. 9  is a partial cross sectional view showing the fuel cell stack; 
         FIG. 10  is an exploded perspective view showing main components of a second power generation unit of the fuel cell stack; and 
         FIG. 11  is a view showing a fuel cell stack disclosed in Japanese Laid-Open Patent Publication No. 2007-141553. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is an exploded perspective view showing main components of a fuel cell stack  10  according to a first embodiment of the present invention. 
     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  FIGS. 2 and 3 , the first power generation unit  12   a  includes a first metal separator  14 , a first membrane electrode assembly (electrolyte electrode assembly)  16   a , a second metal separator  18 , a second membrane electrode assembly  16   b , and a third metal separator  20 . The first power generation unit  12   a  may include three or more MEAs. 
     For example, the first metal separator  14 , the second metal separator  18 , and the third metal separator  20  are steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces by surface treatment. Each of the first metal separator  14 , the second metal separator  18 , and the third metal separator  20  has a concave-convex shape in cross section, by corrugating a metal thin plate under pressure. 
     The surface area of the first membrane electrode assembly  16   a  is smaller than the surface area of the second membrane electrode assembly  16   b . 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. The surface area of the anode  24  is smaller than the surface area of the cathode  26 . 
     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 formed on both surfaces of the solid polymer electrolyte membrane  22 , respectively. 
     As shown in  FIG. 2 , at an upper end of the first power generation unit  12   a  in the longitudinal direction indicated by an arrow C, an oxygen-containing gas supply passage  30   a  for supplying an oxygen-containing gas and a fuel gas supply passage  32   a  for supplying a fuel gas such as a hydrogen-containing gas are provided. The oxygen-containing gas supply passage  30   a  and the fuel gas supply passage  32   a  extend through the first power generation unit  12   a  in the direction indicated by the arrow A. 
     At a lower end of the first power generation unit  12   a  in the longitudinal direction indicated by the arrow C, a fuel gas discharge passage  32   b  for discharging the fuel gas and an oxygen-containing gas discharge passage  30   b  for discharging the oxygen-containing gas are provided. The fuel gas discharge passage  32   b  and the oxygen-containing gas discharge passage  30   b  extend through the first power generation unit  12   a  in the direction indicated by the arrow A. 
     At one end of the first power generation unit  12   a  in a lateral direction indicated by an arrow B, a coolant supply passage  34   a  for supplying a coolant is provided, and at the other end of the first power generation unit  12   a  in the lateral direction indicated by the arrow B, a coolant discharge passage  34   b  for discharging the coolant is provided. The coolant supply passage  34   a  and the coolant discharge passage  34   b  extend through the first power generation unit  12   a  in the direction indicated by the arrow A. 
     The first metal separator  14  has a first fuel gas flow field  36  on its surface  14   a  facing the first membrane electrode assembly  16   a . The first fuel gas flow field  36  is connected between the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b . The first fuel gas flow field  36  includes a plurality of corrugated flow grooves (recesses)  36   a  extending in the direction indicated by the arrow C. An inlet buffer  38  and an outlet buffer  40  are provided at positions near an inlet and an outlet of the first fuel gas flow field  36 , and a plurality of bosses are provided in the inlet buffer  38  and the outlet buffer  40 . 
     A coolant flow field  44  is partially formed on a surface  14   b  of the first metal separator  14 . The coolant flow field  44  is connected between the coolant supply passage  34   a  and the coolant discharge passage  34   b . A plurality of corrugated flow grooves (recesses)  44   a  are formed on the back of the corrugated flow grooves  36   a  of the first fuel gas flow field  36  on the surface  14   b.    
     The second metal separator  18  has a first oxygen-containing gas flow field  50  on its surface  18   a  facing the first membrane electrode assembly  16   a . The first oxygen-containing gas flow field  50  is connected between the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b . The first oxygen-containing gas flow field  50  includes a plurality of corrugated flow grooves (recesses)  50   a  extending in the direction indicated by the arrow C. An inlet buffer  52  and an outlet buffer  54  are provided at positions near an inlet and an outlet of the first oxygen-containing gas flow field  50 . 
     The second metal separator  18  has a second fuel gas flow field  58  on its surface  18   b  facing the second membrane electrode assembly  16   b . The second fuel gas flow field  58  is connected between the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b . The second fuel gas flow field  58  includes a plurality of corrugated flow grooves (recesses)  58   a  extending in the direction indicated by the arrow C. 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 . The second fuel gas flow field  58  is formed on the back of the first oxygen-containing gas flow field  50 , and the inlet buffer  60  and the outlet buffer  62  are formed on the back of the inlet buffer  52  and the outlet buffer  54 . 
     The third metal separator  20  has a second oxygen-containing gas flow field  66  on its surface  20   a  facing the second membrane electrode assembly  16   b . The second oxygen-containing gas flow field  66  is connected between the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b . The second oxygen-containing gas flow field  66  includes a plurality of corrugated flow grooves (recesses)  66   a  extending in the direction indicated by the arrow C. An inlet buffer  68  and an outlet buffer  70  are provided at positions near an inlet and an outlet of the second oxygen-containing gas flow field  66 . 
     The coolant flow field  44  is partially formed on the surface  20   b  of the third metal separator  20 . A plurality of corrugated flow grooves (recesses)  44   b  are formed on the back of the corrugated flow grooves  66   a  of the second oxygen-containing gas flow field  66 . 
     In the first power generation unit  12   a , corrugations of the first fuel gas flow field  36 , the first oxygen-containing gas flow field  50 , the second fuel gas flow field  58 , and the second oxygen-containing gas flow field  66  are in the same phase with each other along the stacking direction. Further, the corrugations have the same pitch and the amplitude. 
     As shown in  FIG. 4 , when the first membrane electrode assembly  16   a  is sandwiched between the first metal separator  14  and the second metal separator  18 , ridges  36   c  forming the corrugated flow grooves  36   a  of the first fuel gas flow field  36  and ridges  50   c  forming the corrugated flow grooves  50   a  of the first oxygen-containing gas flow field  50  are arranged at the same positions in the stacking direction. 
     When the second membrane electrode assembly  16   b  is sandwiched between the second metal separator  18  and the third metal separator  20 , ridges  58   c  forming the corrugated flow grooves  58   a  of the second fuel gas flow field  58  and ridges  66   c  forming the corrugated flow grooves  66   a  of the second oxygen-containing gas flow field  66  are arranged at the same positions in the stacking direction. 
     As shown in  FIGS. 2 and 3 , a first seal member  74  is formed integrally on the surfaces  14   a ,  14   b  of the first metal separator  14 , around the outer end of the first metal separator  14 . Further, the second seal member  76  is formed integrally on the surfaces  18   a ,  18   b  of the second metal separator  18 , around the outer end of the second metal separator  18 . A third seal member  78  is formed integrally on the surfaces  20   a ,  20   b  of the third metal separator  20 , around the outer end of the third metal separator  20 . 
     The first metal separator  14  has a plurality of outer supply holes  80   a  and inner supply holes  80   b  connecting the fuel gas supply passage  32   a  to the first fuel gas flow field  36 , and a plurality of outer discharge holes  82   a  and inner discharge holes  82   b  connecting the fuel gas discharge passage  32   b  to the first fuel gas flow field  36 . 
     The second metal separator  18  has a plurality of supply holes  84  connecting the fuel gas supply passage  32   a  and the second fuel gas flow field  58 , and a plurality of discharge holes  86  connecting the fuel gas discharge passage  32   b  and the second fuel gas flow field  58 . 
     As shown in  FIG. 1 , the second power generation unit  12   b  includes a first metal separator  90 , a first membrane electrode assembly  16   a , a second metal separator  92 , a second membrane electrode assembly  16   b , and a third metal separator  94 . 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 such detailed description will be omitted. 
     As shown in  FIG. 5 , a first fuel gas flow field  36  including a plurality of corrugated flow grooves  36   b  is formed on the surface  14   a  of the first metal separator  90 , and corrugated flow grooves  44   c  are formed on the surface  14   b  of the first metal separator  90 . 
     A first oxygen-containing gas flow field  50  including a plurality of corrugated flow grooves  50   b  is formed on the surface  18   a  of the second metal separator  92 , and a second fuel gas flow field  58  including a plurality of corrugated flow grooves  58   b  is formed on the surface  18   b  of the second metal separator  92 . 
     A second oxygen-containing gas flow field  66  including a plurality of corrugated flow grooves  66   b  is formed on the surface  20   a  of the third metal separator  94 , and a plurality of corrugated flow grooves  44   d  are formed on the surface  20   b.    
     In the second power generation unit  12   b , the first fuel gas flow field  36 , the first oxygen-containing gas flow field  50 , the second fuel gas flow field  58 , and the second oxygen-containing gas flow field  66  are in the same phase with each other in the stacking direction. The first fuel gas flow field  36 , the first oxygen-containing gas flow field  50 , the second fuel gas flow field  58 , and the second oxygen-containing gas flow field  66  of the first power generation unit  12   a  and the first fuel gas flow field  36 , the first oxygen-containing gas flow field  50 , the second fuel gas flow field  58 , and the second oxygen-containing gas flow field  66  of the second power generation unit  12   b  are in different phases from each other (in opposite phases to each other). Further, the corrugations have the same pitch and the same amplitude (see  FIG. 1 ). 
     The first power generation unit  12   a  and the second power generation unit  12   b  are stacked together. Thus, the coolant flow field  44  extending in the direction indicated by the arrow B is formed between the first metal separator  14  of the first power generation unit  12   a  and the third metal separator  94  of the second power generation unit  12   b.    
     In the coolant flow field  44 , the corrugated flow grooves  44   a  and the corrugated flow grooves  44   d  are in different phases. In the structure, by mutually overlapping the corrugated flow grooves  44   a  and the corrugated flow grooves  44   d , a plurality of grooves  44   e  extending in a horizontal direction indicated by the arrow B are formed between the corrugated flow grooves  44   a  and the corrugated flow grooves  44   d  (see  FIGS. 4 and 6 ). 
     Operation of the fuel cell stack  10  having the structure will be described below. 
     Firstly, as shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  30   a,  and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  32   a . Further, a coolant such as pure water, ethylene glycol, or oil is supplied to the coolant supply passage  34   a.    
     Thus, in the first power generation unit  12   a , as shown in  FIG. 2 , the oxygen-containing gas from the oxygen-containing gas supply passage  30   a  flows into the first oxygen-containing gas flow field  50  of the second metal separator  18  and the second oxygen-containing gas flow field  66  of the third metal separator  20 . The oxygen-containing gas moves along the first oxygen-containing gas flow field  50  in the direction of gravity indicated by the arrow C, 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  66  in the direction indicated by the arrow C, and the oxygen-containing gas is supplied to the cathode  26  of the second membrane electrode assembly  16   b.    
     As shown in  FIG. 3 , the fuel gas from the fuel gas supply passage  32   a  flows through the outer supply holes  80   a  toward the surface  14   b  of the first metal separator  14 . Further, the fuel gas flows from the inner supply holes  80   b  toward the surface  14   a , and then, the fuel gas is supplied to the inlet buffer  38 . The fuel gas moves along the first fuel gas flow field  36  in the direction of gravity indicated by the arrow C, and the fuel gas is supplied to the anode  24  of the first membrane electrode assembly  16   a  (see  FIG. 2 ). 
     Further, as shown in  FIG. 3 , the fuel gas flows through the supply holes  84  toward the surface  18   b  of the second metal separator  18 . Thus, as shown in  FIG. 2 , after the fuel gas is supplied to the inlet buffer  60  on the surface  18   b , the fuel gas moves along the second fuel gas flow field  58  in the direction indicated by the arrow C, and the fuel gas is supplied to the anode  24  of the second membrane electrode assembly  16   b.    
     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 electrode catalyst layers of the cathode  26  and the anode  24  for generating electricity. 
     The oxygen-containing gas supplied to and partially consumed at the cathodes  26  of the first and second membrane electrode assemblies  16   a ,  16   b  is discharged to the oxygen-containing gas discharge passage  30   b , and flows in the direction indicated by the arrow A. 
     The fuel gas supplied to and partially consumed at the anode  24  of the first membrane electrode assembly  16   a  flows from the outlet buffer  40  to the inner discharge holes  82   b  toward the surface  14   b  of the first metal separator  14 . The fuel gas discharged to the surface  14   b  flows through the outer discharge holes  82   a , and moves toward the surface  14   a  again. Then, the fuel gas is discharged into the fuel gas discharge passage  32   b.    
     Further, the fuel gas supplied to and partially consumed at the anode  24  of the second membrane electrode assembly  16   b  flows from the outlet buffer  62  through the discharge holes  86  toward the surface  18   a . The fuel gas is discharged into the fuel gas discharge passage  32   b.    
     As shown in  FIGS. 4 and 5 , the coolant supplied to the coolant supply passage  34   a  flows into the coolant flow field  44  formed between the first metal separator  14  of the first power generation unit  12   a  and the third metal separator  94  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 to the coolant discharge passage  34   b.    
     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.    
     In the first embodiment, in the first power generation unit  12   a , as shown in  FIG. 2 , the first fuel gas flow field  36 , the first oxygen-containing gas flow field  50 , the second fuel gas flow field  58 , and the second oxygen-containing gas flow field  66  are in the same phase with each in the stacking direction. Therefore, as shown in  FIG. 4 , the first membrane electrode assembly  16   a  is sandwiched between the ridges  36   c  forming the first fuel gas flow field  36  and the ridges  50   c  forming the first oxygen-containing gas flow field  50  at the same positions in the stacking direction. 
     Likewise, the second membrane electrode assembly  16   b  is sandwiched between the ridges  58   c  forming the second fuel gas flow field  58  and the ridges  66   c  forming the second oxygen-containing gas flow field  66  at the same positions in the stacking direction. In the structure, when the first and second membrane electrode assemblies  16   a ,  16   b  are fastened and retained together in the stacking direction, 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. 
     Further, the first fuel gas flow field  36 , the first oxygen-containing gas flow field  50 , the second fuel gas flow field  58 , and the second oxygen-containing gas flow field  66  of the first power generation unit  12   a  and the first fuel gas flow field  36 , the first oxygen-containing gas flow field  50 , the second fuel gas flow field  58 , and the second oxygen-containing gas flow field  66  of the second power generation unit  12   b  are in different phases from each other. In the structure, simply by stacking the first power generation unit  12   a  and the second power generation unit  12   b  alternately, the coolant flow field  44  having the grooves  44   e  extending in the direction indicated by the arrow B is formed between the first and second power generation units  12   a ,  12   b.    
     Thus, simply by alternately stacking the first power generation unit  12   a  and the second power generation unit  12   b , the fuel cell stack  10  having simple and economical structure is produced advantageously. 
       FIG. 7  is an exploded perspective view showing main components of a fuel cell stack  100  according to a second embodiment of the present invention. The constituent elements of the fuel cell stack  100  that are identical to those of the fuel cell stack  10  according to the first embodiment are labeled with the same reference numerals, and such detailed description will be omitted. 
     The fuel cell stack  100  is formed by stacking a first power generation unit  102   a  and a second power generation unit  102   b  alternatively in a horizontal direction. As shown in  FIGS. 8 and 9 , a first metal separator  104 , a membrane electrode assembly  16 , and a second metal separator  106  are provided in the first power generation unit  102   a.    
     The first metal separator  104  has a fuel gas flow field  108  on its surface  104   a  facing the membrane electrode assembly  16 . The fuel gas flow field  108  includes a plurality of corrugated flow grooves  108   a  extending in the direction indicated by the arrow C. A plurality of corrugated flow grooves  44   a  forming a coolant flow field  44  are formed on a surface  104   b  of the first metal separator  104 . 
     The second metal separator  106  has an oxygen-containing gas flow field  110  on its surface  106   a  facing the membrane electrode assembly  16 . The oxygen-containing gas flow field  110  includes a plurality of corrugated flow grooves  110   a  extending in the direction indicated by the arrow C. A plurality of corrugated flow grooves  44   b  forming part of the coolant flow field  44  are formed on a surface  106   b  of the second metal separator  106 . The fuel gas flow field  108  and the oxygen-containing gas flow field  110  are in the same phase with each other in the stacking direction. 
     The first metal separator  104  has supply holes  112   a  connecting the fuel gas supply passage  32   a  and the fuel gas flow field  108 , and discharge holes  112   b  connecting the fuel gas discharge passage  32   b  and the fuel gas flow field  108 . 
     As shown in  FIGS. 7 and 10 , the second power generation unit  102   b  includes a first metal separator  114 , a membrane electrode assembly  16 , and a second metal separator  116 . The first metal separator  114  has a fuel gas flow field  108  on its surface  114   a  facing the membrane electrode assembly  16 . The fuel gas flow field  108  includes a plurality of corrugated flow grooves  108   b  extending in the direction indicated by the arrow C. 
     A plurality of corrugated flow grooves  44   c  forming a coolant flow field  44  are formed on a surface  114   b  of the first metal separator  114 . The second metal separator  116  has an oxygen-containing gas flow field  110  on its surface  116   a  facing the membrane electrode assembly  16 . The oxygen-containing gas flow field  110  includes corrugated flow grooves  110   b  extending in the direction indicated by the arrow C. A plurality of corrugated flow grooves  44   d  forming the coolant flow field  44  are formed on a surface  116   b  of the second metal separator  116 . 
     In the second power generation unit  102   b , the fuel gas flow field  108  and the oxygen-containing gas flow field  110  are in the same phase with each other in the stacking direction. The fuel gas flow field  108  and the oxygen-containing gas flow field  110  of the second power generation unit  102   b  and the fuel gas flow field  108  and the oxygen-containing gas flow field  110  of the first power generation unit  102   a  are in different phases. 
     The first power generation unit  102   a  and the second power generation unit  102   b  are stacked together alternately to form the coolant flow field  44  including a plurality of grooves  44   e  extending in the direction indicated by the arrow B between the first power generation unit  102   a  and the second power generation unit  102   b.    
     In the second embodiment having the above structure, in the first power generation unit  102   a , corrugations of the fuel gas flow field  108  and the oxygen-containing gas flow field  110  are in the same phase with each other in the stacking direction (see  FIG. 9 ). Further, corrugations of the fuel gas flow field  108  and the oxygen-containing gas flow field  110  have the same pitch and amplitude. In the structure, the membrane electrode assembly  16  is sandwiched between ridges  108   c  forming the corrugated flow grooves  108   a  and ridges  110   c  forming the corrugated flow grooves  110   a  at the same positions in the stacking direction. Therefore, no shearing force is applied to the membrane electrode assembly  16 , and damages of the membrane electrode assembly  16  can be prevented advantageously. 
     Likewise, in the second power generation unit  102   b , the fuel gas flow field  108  and the oxygen-containing gas flow field  110  are in alignment with each other in the stacking direction. Therefore, since the membrane electrode assembly  16  is sandwiched between ridges  108   d  forming the corrugated flow grooves  108   b  and ridges  110   d  forming the corrugated flow grooves  110   b  at the same positions along the stacking direction, no shearing force is applied to the membrane electrode assembly  16 , and damages of the membrane electrode assembly  16  can be prevented advantageously. 
     Further, the embodiment can be carried out simply by alternately stacking the first power generation unit  102   a  and the second power generation unit  102   b , and the same advantages as in the case of the first embodiment are obtained.