Abstract:
A separator comprises a first and second metal plates laid over each other. A cooling medium flow passage is integrally provided between the first and second metal plates. The cooling medium flow passage has inlet buffer portions communicating with a cooling medium inlet communication hole, outlet buffer portions communicating with a cooling medium outlet communication hole, and linear flow passage grooves linearly extending in the direction of arrow B and that of arrow C.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/JP2003/13755, filed 28 Oct. 2003, which claims priority to Japanese Patent Application No. 2002-313242 filed on 28 Oct. 2002, and Japanese Patent Application No. 2002-333742 filed 18 Nov. 2002. The contents of the aforementioned applications are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a fuel cell formed by alternatively stacking an electrolyte electrode assembly and separators. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. 
     2. Background Art 
     For example, a solid polymer fuel cell employs a polymer ion exchange membrane as a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly. Each of the anode and the cathode is made of electrode catalyst and porous carbon. The membrane electrode assembly is sandwiched between separators (bipolar plates) to form the fuel cell. In use, generally, a predetermined number of the fuel cells are stacked together to form a fuel cell stack. 
     In the fuel cell, a fuel gas (reactant gas) such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy. An oxidizing gas (reactant gas) such as a gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water. 
     In the fuel cell, a fuel gas flow field (reactant gas flow field) is formed on a surface of the separator facing the anode for supplying the fuel gas to the anode. An oxygen-containing gas flow field (reactant gas flow field) is formed on a surface of the separator facing the cathode for supplying the oxygen-containing gas to the cathode. Further, a coolant flow field is provided between the anode side separator and the cathode side separator such that a coolant flows along the surfaces of the separators. 
     Normally, the separators of this type are formed of carbon material. However, it has been found that it is not possible to produce a thin separator using the carbon material due to factors such as the strength. Therefore, recently, attempts to reduce the overall size and weight of the fuel cell using a separator formed of a thin metal plate (hereinafter also referred as the metal separator) have been made (see Japanese Laid-Open Patent Publication No. 8-222237). In comparison with the carbon separator, the metal separator has the higher strength, and it is possible to produce a thin metal separator easily. The desired reactant flow field can be formed on the metal separator under pressure to achieve the reduction in thickness of the metal separator. 
     For example, a fuel cell  1  shown in  FIG. 28  includes a membrane electrode assembly  5  and a pair of metal separators  6   a ,  6   b  sandwiching the membrane electrode assembly  5 . The membrane electrode assembly  5  includes an anode  2 , a cathode  3 , and an electrolyte membrane  4  interposed between the anode  2  and the cathode  3 . 
     The metal separator  6   a  has a fuel gas flow field  7   a  for supplying a fuel gas such as a hydrogen-containing gas on its surface facing the anode  2 . The metal separator  6   b  has an oxygen-containing gas flow field  7   b  for supplying an oxygen-containing gas such as the air on its surface facing the cathode  3 . The metal separators  6   a,    6   b  have planar regions  8   a,    8   b  in contact with the anode  2  and the cathode  3 . Further, coolant flow fields  9   a,    9   b  as passages of a coolant is formed on back surfaces (surfaces opposite to the contact surfaces) of the planar regions  8   a,    8   b.    
     However, in the metal separators  6   a,    6   b,  the shapes of the coolant flow fields  9   a,    9   b  are determined inevitably based on the shapes of the fuel gas flow field  7   a  and the oxygen-containing gas flow field  7   b.  In particular, in an attempt to achieve the long grooves, assuming that the fuel gas flow field  7   a  and the oxygen-containing gas flow field  7   b  comprise serpentine flow grooves, the shapes of the coolant flow fields  9   a,    9   b  are significantly constrained. Therefore, it is not possible to supply the coolant along the entire surfaces of the metal separators  6   a,    6   b . Thus, it is difficult to uniformly cool the electrode surfaces, and achieve the stable power generation performance. 
     In view of the above, for example, Japanese Laid-Open Patent Publication 2002-75395 discloses a separator of a fuel cell. The separator is a metal separator, and includes two corrugated metal plates having gas flow fields, and a corrugated metal intermediate plate sandwiched between the two metal plates. The metal intermediate plate has coolant water flow fields on both surfaces. 
     However, according to the disclosure of Japanese Laid-Open Patent Publication 2002-75395, the metal separator has three metal plates including the two metal plates having gas flow fields, and the one intermediate metal plate having the coolant flow fields on its both surfaces. Therefore, in particular, when a large number of metal separators are stacked to form the fuel cell stack, the number of components of the fuel cell stack is large, and the dimension in the stacking direction of the metal separators is large. Thus, the overall size of the fuel cell stack is large. 
     SUMMARY OF THE INVENTION 
     The present invention solves this type of problem, and an object of the present invention is to provide a fuel cell having a simple structure in which a coolant flows in a surface of a separator uniformly, and the desired power generation performance is achieved. 
     In a fuel cell of the present invention, an electrolyte electrode assembly and separators are stacked alternately, and the separator at least includes first and second metal plates stacked together. The first metal plate has an oxygen-containing gas flow field including a curved flow passage for supplying an oxygen-containing gas along a cathode, and the second metal plate has a fuel gas flow field including a curved flow passage for supplying a fuel gas along the anode. 
     Further, a coolant flow field including two or more inlet buffers connected to the coolant supply passage, two or more outlet buffers connected to the coolant discharge passage, and straight flow grooves connected between the two or more inlet buffers and the two more outlet buffers is provided between the first and second metal plates. 
     The coolant between the first and second metal plates flows separately from the coolant supply passage into the two or more inlet buffers, flows through the straight flow grooves into the two or more outlet buffers, and is discharged into the coolant discharge passage. 
     Therefore, the coolant flows in the surface of the separator uniformly, and cools electrode surfaces uniformly. Thus, the stable power generation performance of the fuel cell can be achieved. 
     Further, in a fuel cell of the present invention, an electrolyte electrode assembly and separators are stacked alternately, and the separator at least includes first and second metal plates stacked together, and a coolant flow field is formed between the first and second metal plates. The coolant flow field includes two or more inlet buffers connected to the coolant supply passage through inlet connection passages, two or more outlet buffers connected to the coolant discharge passage through outlet connection passages, and flow grooves connected between the two or more inlet buffers and the two or more outlet buffers. 
     Further, at least the number of grooves in a first inlet connection passage connecting the first inlet buffer to the coolant supply passage and the number of grooves in a second inlet connection passage connecting the second inlet buffer to the coolant supply passage are different, and at least the number of grooves in a first outlet connection passage connecting the first outlet buffer to the coolant discharge passage and the number of grooves in a second outlet connection passage connecting the second outlet buffer to the coolant discharge passage are different. 
     Therefore, the stagnation of the flow of the coolant due to the pressure equilibrium in the coolant flow field is prevented. Thus, the desired flow rate and the desired flow condition of the coolant in the coolant flow field are achieved. Accordingly, the coolant flows in the separator surface uniformly, and cools the entire electrode surface uniformly. Thus, the stable power generation performance can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view showing main components of a fuel cell according to a first embodiment of the present invention; 
         FIG. 2  is a cross sectional view showing part of the fuel cell; 
         FIG. 3  is a front view showing one surface of a first metal plate; 
         FIG. 4  is a perspective view showing a coolant flow field formed in a separator; 
         FIG. 5  is a front view showing the other surface of the first metal plate; 
         FIG. 6  is a front view showing a second metal plate; 
         FIG. 7  is a cross sectional view taken along a line VII-VII in  FIG. 4 ; 
         FIG. 8  is a cross sectional view taken along a line VIII-VIII in  FIG. 4 ; 
         FIG. 9  is a front view showing the other surface of the second metal plate; 
         FIG. 10  is an exploded perspective view showing main components of a fuel cell according to a second embodiment of the present invention; 
         FIG. 11  is a front view showing a first metal plate of the fuel cell; 
         FIG. 12  is a front view showing a second metal plate of the fuel cell; 
         FIG. 13  is a front view showing a coolant flow field formed in a separator of the fuel cell; 
         FIG. 14  is a front view showing a first metal plate of a fuel cell according to a third embodiment of the present invention; 
         FIG. 15  is a front view showing a second metal plate of the fuel cell; 
         FIG. 16  is a front view showing a coolant flow field formed between the first and second metal plates; 
         FIG. 17  is a front view of first metal plate of a fuel cell according to a fourth embodiment of the present invention; 
         FIG. 18  is a front view showing a second metal plate of the fuel cell; 
         FIG. 19  is a front view showing a coolant flow field formed between the first and second metal plates; 
         FIG. 20  is a front view showing a first metal plate of a fuel cell according to a fifth embodiment of the present invention; 
         FIG. 21  is a front view of a second metal plate of the fuel cell; 
         FIG. 22  is a front view showing a coolant flow field formed between the first and second metal plates; 
         FIG. 23  is an exploded perspective view showing main components of a fuel cell according to a sixth embodiment of the present invention; 
         FIG. 24  is a perspective view showing a coolant flow field formed in a separator; 
         FIG. 25  is a view showing measured positions in the coolant flow field; 
         FIG. 26  is a view showing the relationship between the measured positions and the flow rate in the sixth embodiment and a comparative example; 
         FIG. 27  is a view showing the relationship between the measured positions and the temperature in the sixth embodiment and the comparative example; and 
         FIG. 28  is a cross sectional view showing part of a conventional fuel cell. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is an exploded perspective view showing main components of a fuel cell  10  according to a first embodiment of the present invention.  FIG. 2  is a cross sectional view showing part of the fuel cell  10 . 
     The fuel cell  10  is formed by stacking a membrane electrode assembly  12  and separators  13  alternately in a horizontal direction. Each of the separators  13  includes first and second horizontally long rectangular metal plates  14 ,  16  which are stacked together. 
     As shown in  FIG. 1 , at one end of the fuel cell  10  in a direction indicated by an arrow B, an oxygen-containing gas supply passage  20   a  for supplying an oxidizing gas such as an oxygen-containing gas, a coolant supply passage  22   a  for supplying a coolant, and a fuel gas discharge passage  24   b  for discharging a fuel gas such as a hydrogen-containing gas are arranged vertically in a direction indicated by an arrow C. The oxygen-containing gas supply passage  20   a,  the coolant supply passage  22   a,  and the fuel gas discharge passage  24   b  extend through the fuel cell  10  in the stacking direction (horizontal direction) indicated by an arrow A. 
     At the other end of the fuel cell  10  in the direction indicated by the arrow B, a fuel gas supply passage  24   a  for supplying the fuel gas, a coolant discharge passage  22   b  for discharging the coolant, and an oxygen-containing gas discharge passage  20   b  for discharging the oxygen-containing gas are arranged vertically in the direction indicated by the arrow C. The fuel gas supply passage  24   a,  the coolant discharge passage  22   b,  and the oxygen-containing gas discharge passage  20   b  extend through the fuel cell  10  in the direction indicated by the arrow A. 
     The membrane electrode assembly  12  comprises an anode  28 , a cathode  30 , and a solid polymer electrolyte membrane  26  interposed between the anode  28  and the cathode  30 . The solid polymer electrolyte membrane  26  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. 
     Each of the anode  28  and the cathode  30  has a gas diffusion layer such as a carbon paper, and an electrode catalyst layer of platinum alloy supported on carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer of the anode  28  and the electrode catalyst layer of the cathode  30  are fixed to both surfaces of the solid polymer electrolyte membrane  26 , respectively. 
     As shown in  FIGS. 1 and 3 , the first metal plate  14  has an oxygen-containing gas flow field  32  on its surface  14   a  facing the membrane electrode assembly  12 . The oxygen-containing gas flow field  32  is connected to the oxygen-containing gas supply passage  20   a  and the oxygen-containing gas discharge passage  20   b.  The oxygen-containing gas flow field  32  includes an inlet buffer  34  near the oxygen-containing gas supply passage  20   a,  and an outlet buffer  36  near the oxygen-containing gas discharge passage  20   b.  A plurality of bosses  34   a,    36   a  are formed in the inlet buffer  34  and the outlet buffer  36 , respectively. 
     The inlet buffer  34  and the outlet buffer  36  are connected by three oxygen-containing gas flow grooves  38   a,    38   b,    38   c.  The oxygen-containing gas flow grooves  38   a  through  38   c  extend in parallel with each other in a serpentine pattern for allowing the oxygen-containing gas to flow back and forth in the direction indicated by the arrow B, and flows in the direction indicated by the arrow C. Specifically, the oxygen-containing gas flow grooves  38   a  through  38   c  have two turn regions T 1 , T 2 , and three straight regions extending in the direction indicated by the arrow B, for example. 
     A line seal  40  is provided on the surface  14   a  of the first metal plate  14  around the oxygen-containing gas supply passage  20   a,  the oxygen-containing gas discharge passage  20   b,  and the oxygen-containing gas flow field  32  for preventing leakage of the oxygen-containing gas. 
     A surface  14   b  of the first metal plate  14  faces a surface  16   a  of the second metal plate  16 , and a coolant flow field  42  is formed between the surface  14   b  of the first metal plate  14  and the surface  16   a  of the second metal plate  16 . As shown in  FIG. 4 , the coolant flow field  42  includes, e.g., two inlet buffers  44 ,  46  near the coolant supply passage  22   a,  and includes, e.g., two outlet buffers  48 ,  50  near the coolant discharge passage  22   b.  The inlet buffers  44 ,  46  are provided at opposite sides of the coolant supply passage  22   a  in the direction indicated by the arrow C, and the outlet buffers  48 ,  50  are provided at opposite sides of the coolant discharge passage  22   b  in the direction indicated by the arrow C. A plurality of bosses  44   a,    46   a,    48   a,  and  50   a  are formed in the inlet buffers  44 ,  46  and the outlet buffers  48 ,  50 , respectively. 
     The coolant supply passage  22   a  and the inlet buffers  44 ,  46  are connected by two inlet flow grooves  52 ,  54 , respectively, and the coolant discharge passage  22   b  and the outlet buffers  48 ,  50  are connected by two outlet flow grooves  56 ,  58 , respectively. 
     The inlet buffer  44  and the outlet buffer  48  are connected by straight flow grooves  60 ,  62 ,  64 , and  66  extending in the direction indicated by the arrow B. The inlet buffer  46  and the outlet buffer  50  are connected by straight flow grooves  68 ,  70 ,  72 , and  74  extending in the direction indicated by the arrow B. Straight flow grooves  76 ,  78  extending in the direction indicated by the arrow B for a predetermined distance are provided between the straight flow groove  66  and the straight flow groove  68 . 
     The straight flow grooves  60  through  74  are connected by straight flow grooves  80 ,  82  which are extending in the direction indicated by the arrow C. The straight flow grooves  62  through  72  are connected with each other by straight flow grooves  84 ,  86  which are extending discontinuously in the direction indicated by the arrow C. The straight flow grooves  64 ,  66 , and  76  and the straight flow grooves  68 ,  70 , and  78  are connected with each other by straight flow grooves  88 ,  90  which are extending discontinuously in the direction indicated by the arrow C, respectively. 
     The coolant flow field  42  is partially defined by the first metal plate  14 , and partially defined by the second metal plate  16 . The coolant flow field  42  is formed between the first metal plate  14  and the second metal plate  16  when the first metal plate  14  and the second metal plate  16  are stacked together. As shown in  FIG. 5 , part of the coolant flow field  42  is formed on the surface  14   b  where the oxygen-containing gas flow field  32  is not formed on the surface  14   a.  Protrusions on the surface  14   b  formed by the grooves of the oxygen-containing gas flow field  32  on the surface  14   a  are not shown for ease of understanding. Likewise, in  FIG. 6 , protrusions on the surface  16   b  formed by the grooves of the fuel gas flow field  96  on the surface  16   a  are not shown. 
     As shown in  FIG. 5 , the inlet buffer  44  connected to the coolant supply passage  22   a  through the two inlet flow grooves  52  is provided on the surface  14   b.  Further, the outlet buffer  50  connected to the coolant discharge passage  22   b  through the two outlet connection grooves  58  is provided on the surface  14   b.    
     Grooves  60   a,    62   a,    64   a,  and  66   a  connected to the inlet buffer  44  extend discontinuously in the direction indicated by the arrow B for a predetermined distance. The grooves  60   a,    62   a,    64   a,  and  66   a  are formed where the turn region T 2  of the oxygen-containing gas flow grooves  38   a  through  38   c  and the outlet buffer  36  is not formed. Grooves  68   a,    70   a,    72   a,  and  74   a  connected to the outlet buffer  50  extend in the direction indicated by the arrow B. The grooves  68   a,    70   a,    72   a,  and  74   a  are formed where the turn region T 1  of the oxygen-containing gas flow grooves  38   a  through  38   c  and the inlet buffer  34  is not formed. 
     The grooves  60   a  through  78   a  are part of the straight flow grooves  60  through  78 , respectively. Grooves  80   a  through  90   a  of the straight flow grooves  80  through  90  extend in the direction indicated by the arrow C for a predetermined distance where the serpentine oxygen-containing gas flow grooves  38   a  through  38   c  are not formed. 
     As shown in  FIG. 6 , part of the coolant flow field  42  is formed on the surface  16   a  of the second metal plate  16  where the fuel gas flow field  96  as described later is not formed. Specifically, the inlet buffer  46  connected to the coolant supply passage  22   a,  and the outlet buffer  48  connected to the coolant discharge passage  22   b  are provided. 
     Grooves  68   b  through  74   b  of the straight flow grooves  68  through  74  connected to the inlet buffer  46  extend discontinuously in the direction indicated by the arrow B for a predetermined distance. Grooves  60   b  through  66   b  of the straight flow grooves  60  through  66  connected to the outlet buffer  48  extend in a predetermined pattern. On the surface  16   a,  grooves  80   b  through  90   b  of the straight flow grooves  80  through  90  extend in the direction indicated by the arrow C. 
     In the coolant flow field  42 , at part of the straight flow grooves  60  through  78  extending in the direction indicated by the arrow B, the grooves  60   a  through  78   a  and the grooves  60   b  through  78   b  face each other to form a main flow field. The sectional area of the main flow field in the coolant flow field  42  is twice as large as the sectional area of the other part of the coolant flow field  42  (see  FIGS. 4 and 7 ). The straight flow grooves  80  through  94  are partially defined by grooves on both surfaces  14   b,    16   a  of the first and second metal plate  14 ,  16 , partially defined on one surface  14   b  of the first metal plate  14 , and partially defined on one surface  16   a  of the second metal plate  16  (see  FIG. 8 ). A line seal  40   a  is formed between the surface  14   a  of the first metal plate  14  and the surface  16   a  of the second metal plate  16 . 
     As shown in  FIG. 9 , the second metal plate  16  has the fuel gas flow field  96  on its surface  16   b  facing the membrane electrode assembly  12 . The fuel gas flow field  96  includes an inlet buffer  98  provided near the fuel gas supply passage  24   a,  and an outlet buffer  100  provided near the fuel gas discharge passage  24   b.    
     A plurality of bosses  98   a,    100   a  are formed in the inlet buffer  98  and the outlet buffer  100 , respectively. For example, the inlet buffer  98  and the outlet buffer  100  are connected by three fuel gas flow grooves  102   a,    102   b,    102   c.  The fuel gas flow grooves  102   a  through  102   c  extend in a serpentine pattern for allowing the fuel gas to flow back and forth in the direction indicated by the arrow B, and flows in the direction indicated by the arrow C. The fuel gas flow grooves  102   a  through  102   c  are substantially serpentine flow grooves having two turn regions T 3 , T 4 , and three straight regions, for example. On the surface  16   b,  a line seal  40   b  is provided around the fuel gas flow field  96 . 
     Next, operation of the fuel cell  10  according to the first embodiment will be described. 
     As shown in  FIG. 1 , an oxidizing gas such as an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  20   a,  a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  24   a,  and a coolant such as pure water, an ethylene glycol or an oil is supplied to the coolant supply passage  22   a.    
     The oxygen-containing gas flows from the oxygen-containing gas supply passage  20   a  into the oxygen-containing gas flow field  32  of the first metal plate  14 . As shown in  FIG. 3 , the oxygen-containing gas flows through the inlet buffer  34 , and is distributed into the oxygen-containing gas flow grooves  38   a  through  38   c.  The oxygen-containing gas flows through the oxygen-containing gas flow grooves  38   a  through  38   c  in a serpentine pattern along the cathode  30  of the membrane electrode assembly  12 . 
     The fuel gas flows from the fuel gas supply passage  24   a  into the fuel gas flow field  96  of the second metal plate  16 . As shown in  FIG. 9 , the fuel gas flows through the inlet buffer  98 , and is distributed into the fuel gas flow grooves  102   a  through  102   c.  The fuel gas flows through the fuel gas flow grooves  102   a  through  102   c  in a serpentine pattern along the anode  28  of the membrane electrode assembly  12 . 
     In the membrane electrode assembly  12 , the oxygen-containing gas supplied to the cathode  30 , and the fuel gas supplied to the anode  28  are consumed in the electrochemical reactions at catalyst layers of the cathode  30  and the anode  28  for generating electricity. 
     After the oxygen-containing gas is consumed at the cathode  30 , the oxygen-containing gas flows into the oxygen-containing gas discharge passage  20   b  through the outlet buffer  36 . Likewise, after the fuel gas is consumed at the anode  28 , the fuel gas flows into the fuel gas discharge passage  24   b  through the outlet buffer  100 . 
     The coolant supplied to the coolant supply passages  22   a  flows into the coolant flow field  42  between the first and second metal plates  14 ,  16 . As shown in  FIG. 4 , the coolant from the coolant supply passage  22   a  flows through the inlet flow grooves  52 ,  54  in the direction indicated by the arrow C, and flows into the inlet buffers  44 ,  46 . 
     The coolant is distributed from the inlet buffers  44 ,  46  into the straight flow grooves  60  through  66 , and  68  through  74 , and flows horizontally in the direction indicated by the arrow B. The coolant also flows through the straight flow grooves  80  through  90 ,  76 , and  78 . Thus, the coolant is supplied to the entire power generation surface of the membrane electrode assembly  12 . Then, the coolant flows through the outlet buffers  48 ,  50 , and flows into the coolant discharge passages  22   b  through the outlet flow grooves  56 ,  58 . 
     In the first embodiment, the coolant flow field  42  between the first and second metal plates  14 ,  16  includes the two inlet buffers  44 ,  46  connected to the coolant supply passage  22   a  and the two outlet buffers  48 ,  50  connected to the coolant discharge passage  22   b.  Therefore, the coolant flows separately from the coolant supply passage  22   a  in the direction indicated by the arrow C, and flows into the inlet buffers  44 ,  46 . Then, the coolant flows through the straight flow grooves  60  through  90  along the power generation surface, flows into the outlet buffers  48 ,  50 , and is discharged into the coolant discharge passage  22   b.    
     Therefore, the coolant flows along the entire surface of the separator  13  uniformly, and cools the power generation surface of the membrane electrode assembly  12  uniformly. Thus, the overall power generation performance of the fuel cell  10  is stable. 
     In the first metal plate  14 , part of the coolant flow field  42  is formed where the oxygen-containing gas flow field  32  is not formed on the surface  14   a  (the oxygen-containing gas flow field  32  is fabricated by pressure forming). Specifically, as shown in  FIG. 3 , the inlet buffer  44  is formed at a position below the coolant supply passage  22   a  where the inlet buffer  34  is not formed, and the outlet buffer  50  is formed at a position above the coolant discharge passage  22   b  where the outlet buffer  36  is not formed. Further, the grooves  60   a  through  90   a  each having a predetermined shape are formed where the serpentine oxygen-containing gas flow grooves  38   a  through  38   c  are not formed (see  FIGS. 3 and 5 ). Thus, the oxygen-containing gas flow field  32  and the coolant flow field  42  are formed on both surfaces  14   a,    14   b  of the first metal plate  14 , respectively. 
     Further, part of the coolant flow field  42  is formed on the surface  16   a  of the second metal plate  16  where the fuel gas flow field  96  on the surface  16   b  is not formed. Specifically, as shown in  FIG. 9 , the inlet buffer  46  is formed at a position above the coolant supply passage  22   a  where the outlet buffer  100  is not formed. Further, the outlet buffer  48  is formed at a position below the coolant discharge passage  22   b  where the inlet buffer  98  is not formed. Further, the grooves  60   b  through  90   b  each having a predetermined shape are formed where the serpentine oxygen fuel gas flow grooves  102   a  through  102   c  are not formed (see  FIGS. 6 and 9 ). Thus, the fuel gas flow field  96  and the coolant flow field  42  are formed on both surfaces  16   a,    16   b  of the second metal plate  16 , respectively. 
     In this manner, even though the shape of the coolant flow field  42  on the first metal plate  14  is constrained by the oxygen-containing gas flow field  32 , and the shape of the coolant flow field  42  on the second metal plate  16  is constrained by the fuel gas flow field  96 , the coolant flow field  42  on the first metal plate  14  and the coolant flow field  42  on the second metal plate  16  compensate with each other. Therefore, with the simple structure, the coolant flow field  42  having the desired shape is reliably formed in the separator  13 . 
       FIG. 10  is an exploded perspective view showing a fuel cell  110  according to a second embodiment of the present invention. In  FIG. 10 , the constituent elements that are identical to those of the first embodiment are labeled with the same reference numeral, and description thereof will be omitted. Likewise, in third through sixth embodiments as described later, the constituent elements that are identical to those of the first embodiment are labeled with the same reference numeral, and description thereof will be omitted. 
     The fuel cell  110  is formed by stacking the membrane electrode assembly  12  and separators  112  alternately. Each of the separators  112  includes first and second metal plates  114 ,  116 , which are stacked together. At one end of the fuel cell  110  in the direction indicated by an arrow B, an oxygen-containing gas supply passage  20   a,  a coolant supply passage  22   a,  and an oxygen-containing gas discharge passage  20   b  are formed. At the other end of the fuel cell  110  in the direction indicated by the arrow B, a fuel gas supply passage  24   a,  a coolant discharge passage  22   b,  and a fuel gas discharge passage  24   b  are formed. 
     As shown in  FIG. 11 , the first metal plate  114  has an oxygen-containing gas flow field  118  on its surface  114   a  facing a cathode  30  of the first metal plate  114 . The oxygen-containing gas flow field  118  includes an inlet buffer  34  connected to the oxygen-containing gas supply passage  20   a  through two inlet flow grooves  120  and an outlet buffer  36  connected to the oxygen-containing gas discharge passage  20   b  through two outlet flow grooves  122 . The inlet buffer  34  and the outlet buffer  36  are adjacent to each other. The inlet buffer  34  and the outlet buffer  36  are connected through oxygen-containing gas flow grooves  124   a,    124   b,    124   c  each curved in a substantially U-shape. 
     A coolant flow field  126  is formed between the first and second metal plates  114 ,  116 . The second metal plate  116  has a fuel gas flow field  125  on its surface  116   a  facing an anode  28 . 
     As shown in  FIG. 12 , the fuel gas flow field  125  includes an inlet buffer  98  connected to the fuel gas supply passage  24   a  through two inlet flow grooves  127  and an outlet buffer  100  connected to the fuel gas discharge passage  24   b  through two outlet flow grooves  129 . The inlet buffer  98  and the outlet buffer  100  are adjacent to each other. The inlet buffer  98  and the outlet buffer  100  are connected through fuel gas flow grooves  131   a,    131 b,  131   c  each curved in a substantially U-shape. 
     As shown in  FIG. 13 , the coolant flow field  126  includes inlet buffers  44 ,  46  near the coolant supply passage  22   a,  and outlet buffers  48 ,  50  near the coolant discharge passage  22   b.  The inlet buffer  44  and the outlet buffer are connected by straight flow grooves  128 ,  130  extending in the direction indicated by the arrow B. Likewise, the inlet buffer  46  and the outlet buffer  50  are connected by straight flow grooves  132 ,  134  extending in the direction indicated by the arrow B. 
     Straight flow grooves  136 ,  138  are formed outside the straight flow grooves  128 ,  134  in the direction indicated by the arrow C. Further, a straight flow groove  140  is formed between the straight flow grooves  130 ,  132 . 
     The straight flow grooves  128  through  140  are connected by straight flow grooves  142 ,  144  extending in the direction indicated by the arrow C. The straight flow grooves  128  through  134 , and  140  are connected by straight flow grooves  146 ,  148  extending in the direction indicated by the arrow C. The straight flow grooves  130 ,  132 , and  140  are connected by straight flow grooves  150 ,  152  extending in the direction indicated by the arrow C. 
     As shown in  FIG. 11 , the first metal plate  114  has outlet buffers  48 ,  50  connected to the coolant discharge passage  22   b  on its surface  114   b  facing the second metal plate  116 . On the surface  114   b,  grooves  128   a  through  140   a  of the straight flow grooves  128  through  140  are formed where curved portions of the oxygen-containing containing gas flow grooves  124   a  through  124   c  of the oxygen-containing gas flow field  118  are not formed. On the surface  114   b,  straight flow grooves  146 ,  148  and  152  are formed in the direction indicated by the arrow C. 
     As shown in  FIG. 12 , the second metal plate  116  has inlet buffers  44 ,  46  near the coolant supply passage  22   a  on its surface  116   b  facing the first metal plate  114 . On the surface  116   b,  grooves  128   b  through  140   b  of the straight flow grooves  128  through  140  are formed where curved portions of the fuel gas flow grooves  131   a  through  131   c  are not formed. On the surface  116   b,  straight flow grooves  142 ,  146  and  150  are formed in the direction indicated by the arrow C. Line seals  40   c,    40   d  are formed on the surfaces  114   a,    116   a,  and unillustrated line seals are formed between the surfaces  114   b,    116   b.    
     In the second embodiment, the first metal plate  114  has an oxygen-containing gas flow field  118  on the surface  114   a.  In the oxygen-containing gas flow field  118 , the inlet buffer  34  and the outlet buffer  36  are connected through the oxygen-containing gas flow grooves  124   a  through  124   c  each curved in a substantially U-shape. The second metal plate  116  has the fuel gas flow field  125  on the surface  116   a.  In the fuel gas flow field  125 , the inlet buffer  98  and the outlet buffer  100  are connected by the fuel gas flow grooves  131   a  through  131   c  each curved in a substantially U-shape. 
     In this manner, even though the shape of the grooves for the coolant on the surface  114   b  of the first metal plate  114  and the shape of the grooves for the coolant on the surface  116   b  of the second metal plate  116  are constrained, the coolant flow field  126  is formed between the first metal plate  114  and the second metal plate  116  such that the grooves on the first metal plate  114  and the grooves on the second metal plate  116  compensate with each other. 
     In the coolant flow field  126 , the two inlet buffers  44 ,  46  connected to the coolant supply passage  22   a,  and the outlet buffers  48 ,  50  connected to the coolant discharge passage  22   b  are formed. Therefore, the coolant flows along the surface of the separator  112  uniformly. Thus, the same advantages as with the first embodiment can be obtained. For example, it is possible to uniformly cool the electrode surface of the membrane electrode assembly  12  to obtain the stable performance of the fuel cell. 
       FIG. 14  is a front view showing a first metal plate  160  of a fuel cell according to a third embodiment of the present invention. The first metal plate  160  has an oxygen-containing gas flow field  162  on its surface  160   a  facing the cathode  30 . The oxygen-containing gas flow field  162  is connected to the oxygen-containing gas supply passage  20   a  and the oxygen-containing gas discharge passage  20   b.  The oxygen-containing gas flow field  162  includes three oxygen-containing gas flow grooves  164   a  through  164   c  connected between the inlet buffer  34  and the outlet buffer  36 . The oxygen-containing gas flow grooves  164   a  through  164   c  extend in a serpentine pattern for allowing the oxygen-containing gas to flow back and forth in the direction indicated by the arrow B, and flow in the direction indicated by the arrow C. Each of the oxygen-containing gas flow grooves  164   a  through  164   c  have four turn regions and five straight regions extending in the direction indicated by the arrow B. 
       FIG. 15  is a front view showing a second surface  166   a  of a second metal plate  166  facing the anode  28 . The second metal plate  166  is stacked on the first metal plate  160 . 
     A fuel gas flow field  168  is formed on the surface  166   a.  The fuel gas flow field  168  is connected between the fuel gas supply passage  24   a  and the fuel gas discharge passage  24   b.  The fuel gas flow field  168  includes three fuel gas flow grooves  170   a  through  170   c  connected between the inlet buffer  98  and the outlet buffer  100 . The fuel gas flow grooves  170   a  through  170   c  extend in a serpentine pattern for allowing the fuel gas to flow back and forth in the direction indicated by the arrow B, and flow in the direction indicated by the arrow C. Each of the fuel gas flow grooves  170   a  through  170   c  have four turn regions and five straight regions extending in the direction indicated by the arrow B. 
     A coolant flow field  172  is formed between the first and second metal plates  160 ,  166 . As shown in  FIG. 16 , the coolant flow field  172  includes inlet buffers  44 ,  46  connected to the coolant supply passage  22   a  and outlet buffers  48 ,  50  connected to the coolant discharge passage  22   b.  The inlet buffer  44  and the outlet buffer  48  are connected by four straight flow grooves  174  extending in the direction indicated by the arrow B. Further, the inlet buffer  46  and the outlet buffer  50  are connected by four straight flow grooves  176  extending in the direction indicated by the arrow B. 
     Eight straight flow grooves  178  are formed in parallel in the direction indicated by the arrow B between the straight flow grooves  174 ,  176 . The straight flow grooves  174  through  178  are connected together by two straight flow grooves  180  extending in the direction indicated by the arrow C. Further, the straight flow grooves  174  through  178  are connected by the two straight flow grooves  182  which are shorter than the straight flow groove  174 , and two straight flow grooves  184  which are extending discontinuously, and shorter than the straight flow grooves  182 . 
     The coolant flow field  172  is partially defined by the first metal plate  160 , and partially defined by the second metal plate  166 . Specifically, as shown in  FIG. 14 , on the surface  160   b  of the first metal plate  160 , the inlet buffer  44  and the outlet buffer  50  are formed at positions where the inlet buffer  34  and the outlet buffer  36  are not formed. On the surface  160   b,  grooves  174   a  through  178   a  of the straight flow grooves  174  through  178  extending in the direction indicated by the arrow B are formed, and grooves  180   a  through  184   a  of the straight flow grooves  180  through  184  extending in the direction indicated by the arrow C are formed. The grooves  174   a  through  184   a  are formed at predetermined positions where the serpentine oxygen-containing gas flow grooves  164   a  through  164   c  are not formed. 
     As shown in  FIG. 15 , the inlet buffer  46  and the outlet buffer  48  are formed on the surface  166   b  of the second metal plate  166  where the outlet buffer  100  and the inlet buffer  98  are not formed. On the surface  166   b,  the grooves  174   b  through  184   b  as part of the straight flow grooves  174  through  184  are formed at positions where the fuel gas flow grooves  170   a  through  170   c  are not affected. Unillustrated line seals are formed between the surfaces  160   b,    166   b.    
     In this manner, the same advantages as with the first and second embodiments can be obtained. For example, even though the shapes of the grooves are constrained, the grooves compensate with each other to form the coolant flow field  172  having the desired flow field structure as a whole. 
     Further, each of the oxygen-containing gas flow field  162  and the fuel gas flow field  168  have the flow field structure in which four turn regions and five straight regions are formed along the electrode surface. Therefore, the grooves in the flow field are long, the gas flow rate is high, and the gas pressure loss is large. Thus, the water produced in the power generation is discharged efficiently. 
       FIG. 17  is a front view of a first metal plate  190  of a fuel cell according to a fourth embodiment of the present invention.  FIG. 18  is a front view of a second metal plate  192  stacked on the first metal plate  190 . 
     The first metal plate  190  has an oxygen-containing gas flow field  194  on its surface  190   a  facing the cathode  30 . The oxygen-containing gas flow field  194  includes an inlet buffer  196  connected to the oxygen-containing gas supply passage  20   a  and an outlet buffer  198  connected to the oxygen-containing gas discharge passage  20   b.  A plurality of bosses  196   a,    198   a  are formed in the inlet buffer  196  and the outlet buffer  198 , respectively. Each of the inlet buffer  196  and the outlet buffer  198  is elongated in the direction indicated by the arrow C. 
     The inlet buffer  196  is connected to six oxygen-containing gas flow grooves  200 . After the oxygen-containing gas flow grooves  200  extend in the direction in the arrow B, the oxygen-containing gas flow grooves  200  are curved in the direction in the arrow C. Then, every two of the oxygen-containing gas grooves  200  are joined together into the oxygen-containing gas flow grooves  202 , and extend in the direction indicated by the arrow B. Every oxygen-containing gas flow grooves  202  is separated into two grooves, i.e., the oxygen-containing gas flow grooves  202  are separated into six oxygen-containing gas flow grooves  204 . The oxygen-containing gas flow grooves  204  are curved from the direction indicated by the arrow C to the direction indicated by the arrow B. Then, the oxygen-containing gas flow grooves  204  are connected to the outlet buffer  198 . 
     As shown in  FIG. 18 , the second metal plate  192  has a fuel gas flow field  206  on its surface  192   a  facing the anode  28 . The fuel gas flow field  206  includes an inlet buffer  208  connected to the fuel gas supply passage  24   a  and an outlet buffer  210  connected to the fuel gas discharge passage  24   b.  A plurality of bosses  208   a,    210   a  are formed in the inlet buffer  208 , and the outlet buffer  210 , respectively. Each of the inlet buffer  208  and the outlet buffer  210  is elongated in the direction indicated by the arrow C. 
     The inlet buffer  208  is connected to six fuel gas flow grooves  212 . After the fuel gas flow grooves  212  extend in the direction indicated by the arrow B, the fuel gas flow grooves  212  are curved in the direction indicated by the arrow C. Every two of the fuel gas flow grooves  212  are joined together to form three fuel gas flow grooves  214 . After the fuel gas flow grooves  214  extend in the direction indicated by the arrow B, every fuel gas flow groove  214  is separated into two grooves to form six fuel gas flow grooves  216 . After the fuel gas flow grooves  216  extend in the direction indicated by the arrow C, the fuel gas flow grooves  216  are curved in the direction indicated by the arrow B, and connected to the outlet buffer  210 . 
     A coolant flow field  218  is formed between a surface  190   b  of the first metal plate  190  and a surface  192   b  of the second metal plate  192 . As shown in  FIG. 19 , the coolant flow field  218  includes two inlet buffers  220 ,  222  connected to the coolant supply passage  22   a  and elongated in the direction indicated by the arrow C, and outlet buffers  224 ,  226  connected to the coolant discharge passage  22   b  and elongated in the direction indicated by the arrow C. A plurality of bosses  220   a,    222   a,    224   a    226   a  are formed in the inlet buffers  220 ,  222 , and the outlet buffers  224 ,  226 , respectively. 
     The inlet buffer  220  is directly connected to the outlet buffer  224  by six straight flow grooves  228  extending in the direction indicated by the arrow B, and the inlet buffer  222  is directly connected to the outlet buffer  226  by six straight flow grooves  228  extending in the direction indicated by the arrow B. Four straight flow grooves  230  are provided on the surface  190   a.  Each of the straight flow grooves  230  has open ends, and extends in the direction indicated by the arrow B. 
     Two straight flow grooves  236  elongated in the direction arrow C are formed near the inlet buffers  220 ,  222 , and the outlet buffers  224 ,  226 . Eight straight flow grooves  238  each having a predetermined length are formed between the straight flow grooves  236 . 
     The coolant flow field  218  is partially defined by the first metal plate  190 , and partially defined by the second metal plate  192 . As shown in  FIG. 17 , on the surface  190   b  of the first metal plate  190 , the inlet buffer  220  and the outlet buffer  226  are formed, and the grooves  228   a,    230   a,    236   a,    238   a  as part of the straight flow grooves  228 ,  230 ,  236 ,  238  are formed. 
     As shown in  FIG. 18 , on the surface  192   b  of the second metal plate  192 , the inlet buffer  222  and the outlet buffer  224  are formed, and the grooves  228   b ,  230   b ,  236   b ,  238   b  as part of the straight flow grooves  228 ,  230 ,  236 ,  238  are formed. Line seals  40   g ,  40   h  are formed on the surfaces  190   a ,  192   a , and unillustrated line seals are provided between the surfaces  190   b ,  192   b.    
     In the fourth embodiment, the number of grooves in the oxygen-containing gas flow field  194  and the number of grooves in the fuel gas flow field  206  change from six to three, and three to six. Therefore, the inlet buffer  196  and the outlet buffer  198  for the oxygen-containing gas and the inlet buffer  208  and the outlet buffer  210  for the fuel gas, and the inlet buffers  220 ,  222  and the outlet buffers  224 ,  226  for the coolant are elongated respectively in the direction indicated by the arrow C. Thus, it is possible to supply the oxygen-containing gas, the fuel gas, and the coolant more uniformly and smoothly along the electrode surfaces. 
       FIG. 20  is a front view showing a first metal plate  240  of a fuel cell according to a fifth embodiment.  FIG. 21  is a front view showing a second metal plate  242  stacked on the first metal plate  240 . 
     The first metal plate  240  has an oxygen-containing gas flow field  244  on its surface  240   a  facing the cathode. The oxygen-containing gas flow field  244  includes four oxygen-containing gas flow grooves  246 . Each of the oxygen-containing gas flow grooves  246  has a serpentine pattern having two turn regions and three straight regions for allowing the oxygen-containing gas to flow back and forth in the direction indicated by the arrow B. The oxygen-containing gas flow grooves  246  are connected between the inlet buffer  34  and the outlet buffer  36 . 
     As shown in  FIG. 21 , the second metal plate  242  has a fuel gas flow field  248  on its surface  242   a  facing the anode  28 . The fuel gas flow field  248  includes three fuel gas flow grooves  250 . The fuel gas flow grooves  250  has a serpentine pattern having four turn regions and five straight regions for allowing the fuel gas to flow back and forth in the direction indicated by the arrow B. The fuel gas flow grooves  250  are connected between an inlet buffer  98  and an outlet buffer  100 . 
     A coolant flow field  252  is connected between the first and second metal plates  240 ,  242 . As shown in  FIG. 22 , the coolant flow field  252  includes inlet buffers  254 ,  256  connected to the coolant supply passage  22   a,  and outlet buffers  258 ,  260  connected to the coolant discharge passage  22   b.  A plurality of bosses  254   a,    256   a  are formed in the inlet buffers  254 ,  256 , respectively, and a plurality of bosses  258   a,    260   a  are formed in the outlet buffers  258 ,  260 , respectively. 
     The inlet buffer  254  and the outlet buffer  258  are connected directly by four straight grooves  262  extending in the direction indicated by the arrow B, and the inlet buffer  256  and the outlet buffer  260  are connected by four straight grooves  262  extending in the direction indicated by the arrow B. Ends of two straight flow grooves  264  are connected to the inlet buffer  256 , and the other ends of the straight flow grooves  264  terminate at an area adjacent to the outlet buffer  260 . Ends of two straight flow grooves  266  are connected to the outlet buffer  258 , and the other ends of the straight flow grooves  266  terminate at an area adjacent to the inlet buffer  254 . Further, four straight flow grooves  268  extending in the direction indicated by the arrow B are provided. Both ends of the straight flow grooves  268  are open. 
     Straight flow grooves  270  elongated in the direction indicated by the arrow C are provided near the inlet buffers  254 ,  256 , and near the outlet buffers  258 ,  260 . Further, eight straight flow grooves  272  each having a predetermined length in the direction indicated by the arrow C are formed between the straight flow grooves  270 . 
     The surface  240   b  of the first metal plate  240  and the surface  242   b  of the second metal plate  242  face each other. The coolant flow field  252  is partially defined on the surface  240   b  of the first metal plate  240 , and partially defined on the surface  242   b  of the second metal plate  242 . As shown in  FIG. 20 , the inlet buffer  254  and the outlet buffer  260  are formed on the surface  240   b  of the first metal plate  240 . Further, grooves  262   a  through  272   a  as part of the straight flow grooves  262  through  272  are formed on the surface  240   b  of the first metal plate  240 . 
     As shown in  FIG. 21 , the inlet buffer  256  and the outlet buffer  258  are formed on the surface  242   b  of the second metal plate  242 . Further, grooves  262   b  through  272   b  as part of the straight flow grooves  262  through  272  are formed on the surface  242   a  of the second metal plate  242 . Line seals  40   i,    40   j  are formed on the surfaces  240   a,    242   a.  Unillustrated line seals are provided between the surfaces  240   b,    242   b.    
     The fifth embodiment is advantageous in that even if the oxygen-containing gas flow field  244  and the fuel gas flow field  248  formed on the first and second metal plates  240 ,  242  have different shapes, it is possible to form the coolant flow field  252  having a predetermined shape between the first and second metal plates  240 ,  242 . 
       FIG. 23  is an exploded perspective view showing main components of a fuel cell  300  according to a sixth embodiment of the present invention. 
     The fuel cell  300  is formed by stacking the membrane electrode assembly  12  and separators  302  alternately. The separator  302  includes first and second metal plates  304 ,  306  which are stacked together. 
     As shown in  FIG. 24 , the coolant supply passage  22   a  is connected to the first inlet buffer  44  through a first inlet connection passage  308 , and connected to the second inlet buffer  46  through a second inlet connection passage  310 . The coolant discharge passage  22   b  is connected to the first outlet buffer  48  through a first outlet connection passage  312 , and connected to the second outlet buffer  50  through a second outlet connection passage  314 . The first inlet connection passage  308  comprises, for example, two flow grooves, and the second inlet connection passage  310  comprises, for example, six flow grooves. Likewise, the first outlet connection passage  312  comprises six flow grooves, and the second outlet connection passage  314  comprises two flow grooves. 
     The number of flow grooves in the first inlet connection passage  308  is not limited to “two”, and the number of flow grooves in the second inlet connection passage  310  is not limited to “six”. Likewise, the number of flow grooves in the first outlet connection passage  312  is not limited to “six”, and the number of flow grooves in the second outlet connection passage  314  is not limited to “two”. 
     In the sixth embodiment, the first and second inlet connection passages  308 ,  310  connecting the coolant supply passage  22   a  and the inlet buffers  44 ,  46  are provided. For example, the first inlet connection passage  308  comprises two flow grooves, and the second inlet connection passage  310  includes six flow grooves. Likewise, the first and second outlet connection passages  312 ,  314  connecting the coolant discharge passage  22   b  and the outlet buffers  48 ,  50  are provided. For example, the first outlet connection passage  312  comprises six flow grooves, and the second outlet connection passage  314  comprises two flow grooves. 
     Therefore, as shown in  FIG. 25 , assuming that a position near the inlet buffer  44  is defined as the position P 1 , and a position near the inlet buffer  46  is defined as the position P 2 , the flow resistance from the coolant supply passage  22   a  to the position P 1  is larger than the flow resistance from the coolant supply passage  22   a  to the position P 2 . Therefore, the pressure of the coolant applied to the position P 2  is larger than the pressure of the coolant applied to the position P 1 . Thus, the coolant flows from the position P 2  to the position P 1 . It is possible to prevent stagnation of the coolant, and produce the flow of the coolant from the position P 2  to the position P 1  in the coolant flow field  42 . 
     The flow rate and temperature distribution in the coolant flow field  42  were confirmed in a comparative example and the sixth embodiment. In the comparative example, the number of flow grooves of the first inlet connection passage  308  and the number of flow grooves of the second inlet connection passage  310  are the same, and the number of the flow grooves of the first outlet connection passage  312  and the number of flow grooves of the second outlet connection passage  314  are the same. The confirmation was made around positions Pa, Pb, Pc, and Pd along a central line T connecting the coolant supply passage  22   a  and the coolant discharge passage  22   b.  As shown in  FIG. 25 , the positions Pa, Pd are end positions of the coolant flow field  42 . The distance (H) between the position Pb and the position Pa and the distance (H) between the position Pc and the position Pd were set to ½ of the entire flow field width (2H) of the coolant flow field  42 . 
     As a result, in the comparative example, since the pressure at the position P 1  and the pressure at the position P 2  were substantially the same, as shown in  FIG. 26 , near the position Pa, the pressure of the coolant supplied from the inlet buffer  44  was in equilibrium with the pressure of the coolant supplied from the inlet buffer  46 . Therefore, the flow rate of the coolant was small near the positions Pa to Pd on the central line T. In contrast, in the sixth embodiment, since the pressure at the position P 2  is higher than the pressure at the position P 1 , the pressure difference created the flow of the coolant from the position P 2  to the position P 1 . 
     Further, as shown in  FIG. 27 , in the comparative example, the temperature was high near the positions Pa, Pb and the positions Pc, Pd since the coolant did not flow smoothly. In contrast, in the sixth embodiment, the coolant flowed smoothly by the pressure difference. Therefore, in the temperature distribution of the sixth embodiment, the temperature increased from the coolant supply passage  22   a  to the coolant discharge passage  22   b.    
     Thus, in the sixth embodiment, the coolant flows smoothly and reliably in the coolant flow field  42 . It is possible to uniformly and reliably cool the entire power generation surface of the membrane electrode assembly  12 . 
     In the sixth embodiment, the number of the flow grooves of the first inlet connection passage  308  is smaller than the number of the flow grooves of the second inlet connection passage  310 . Conversely, the number of the flow grooves of the first inlet connection passage  308  may be larger the number of the flow grooves of the second outlet connection passage  310 . Likewise, the number of the flow grooves of the second outlet connection passage  314  may be larger the number of the flow grooves of the first outlet connection passage  312 . In the sixth embodiment, the numbers of the flow grooves are two and six. However, the present invention is not limited in this respect. As long as the numbers of the flow grooves are different, various combinations of numbers may be adopted. 
     The present invention is not limited to the above first through sixth embodiments. For example, three or more inlet buffers connected to the coolant supply passage  22   a,  and three or more outlet buffers connected to the coolant discharge passage  22   b  can be provided. 
     In the fuel cell according to the present invention, the coolant flows between the first and second metal plates of the separator. The coolant is supplied from the coolant supply passage separately into two or more inlet buffers. After the coolant flows through the straight flow grooves into two or more outlet buffers, the coolant is discharged into the coolant discharge passage. Therefore, the coolant flows along the surface of the separator uniformly, and cools the separator surface uniformly. Thus, the stable power generation performance can be achieved. 
     Further, in the present invention, the numbers of the flow grooves of the respective inlet connection passages are different, and the number of the flow grooves of the respective outlet connection passages are different. Thus, the desired flow rate and the desired flow condition of the coolant in the coolant flow field are achieved. Accordingly, the coolant flows in the separator surface uniformly, and cools the entire electrode surface uniformly. Thus, the stable power generation performance can be achieved.