Abstract:
An oxidant gas conduit communicating with both an oxidant gas inlet communication hole and an oxidant gas outlet communication hole is formed in a surface of a cathode-side metallic separator which forms a fuel cell. Continuous linear guide ridges which protrude from intermediate height sections to the oxidant gas conduit side and form continuous guide conduits are provided on the cathode-side metallic separator. The linear guide ridges are continuously connected to ends of rectilinear conduit ridges which form rectilinear conduits, are provided with bend portions, and are set to lengths which are different from each other in a step-like manner.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/JP2010/059313, filed Jun. 2, 2010, which claims priority to Japanese Patent Application No. 2009-151229 filed on Jun. 25, 2009 in Japan. The contents of the aforementioned applications are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and a metal separator in the form of a corrugated plate in a stacking direction. The electrolyte electrode assembly includes electrodes and an electrolyte interposed between the electrodes. A reactant gas flow field as a passage of a fuel gas or an oxygen-containing gas is formed on one surface of the metal separator. A reactant gas passage for the fuel gas or the oxygen-containing gas extends through the fuel cell in the stacking direction. 
     BACKGROUND ART 
     For example, a solid polymer electrolyte fuel cell employs an electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane. The electrolyte membrane is interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly is sandwiched between a pair of separators to form a unit cell for generating electricity. In use, normally, a predetermined number of 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. 
     In this regard, the fuel cell may adopt internal manifold structure in which fuel gas passages for flowing a fuel gas therethrough, oxygen-containing gas passages for flowing an oxygen-containing gas therethrough, and coolant passages for flowing a coolant therethrough are formed in the fuel cell and extend through the fuel cell in the stacking direction. 
     As a fuel cell of this type, for example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-172924 is known. As shown in  FIG. 10 , a separator  1  disclosed in Japanese Laid-Open Patent Publication No. 2006-172924 includes a fuel gas flow field  2 . The fuel gas flow field  2  includes a main flow field  3  connected to an inlet manifold  6   a  and an outlet manifold  6   b  through a distribution section  4  and a merge section  5 . 
     The main flow field  3  is divided by a plurality of ribs  7   a , and the distribution section  4  and the merge section  5  are divided by a plurality of ribs  7   b ,  7   c . The ribs  7   b ,  7   c  are divided respectively by disconnected portions  8   a ,  8   b  in the middle in the longitudinal direction. The disconnected portions  8   a ,  8   b  of the ribs  7   b ,  7   c  are shifted from disconnected portions  8   a ,  8   b  of the adjacent ribs  7   b ,  7   c  in the longitudinal direction of the separator  1 . 
     SUMMARY OF INVENTION 
     However, in the separator  1 , since each of the ribs  7   b ,  7   c  is divided into a plurality of pieces by the disconnected portions  8   a ,  8   b , water produced in the power generation reaction tends to stagnate at the disconnected portions  8   a ,  8   b . In this case, the fuel gas and the oxygen-containing gas flow around the produced water, and flows between the ribs  7   b ,  7   c . Therefore, the water cannot be discharged from the fuel cell. As a result, the fuel gas and the oxygen-containing gas may not flow smoothly, and thus the power generation performance may be lowered undesirably. 
     Further, in the case where water flows into the fuel cell stack from the outside, the water may stagnate therein, and cannot be discharged from the fuel cell stack. As a result, the power generation performance may be lowered undesirably. 
     Further, since the ribs  7   b ,  7   c  are divided into a plurality of pieces by the disconnected portions  8   a ,  8   b , the sizes of the distribution section  4  and the merge section  5  that are, in effect, not used in power generation become large. As a result, the entire separator  1  is large in size. 
     The present invention has been made to solve the problems of these types, and an object of the present invention is to provide a fuel cell which is capable of improving the performance of discharging water produced by the power generation reaction in reactant gas flow fields, and suitably achieving size reduction of the fuel cell. 
     The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and a metal separator in the form of a corrugated plate in a stacking direction. The electrolyte electrode assembly includes electrodes and an electrolyte interposed between the electrodes. A reactant gas flow field as a passage of a fuel gas or an oxygen-containing gas is formed on one surface of the metal separator. A reactant gas passage for the fuel gas or the oxygen-containing gas extends through the fuel cell in the stacking direction. 
     The metal separator includes a buffer provided between an end of the reactant gas flow field and the reactant gas passage. A plurality of continuous linear guide ridges are provided on the buffer, and the linear guide ridges include bent portions, and have different lengths in a stepwise manner. 
     In the present invention, the continuous linear guide ridges are provided in the buffer. The linear guide ridges include the bent portions, and have different lengths in a stepwise manner. Thus, the reactant gas does not flow around water produced in the power generation reaction. In the structure, by the reactant gas, the water produced in the power generation reaction is easily and reliably discharged. Also, the reactant gas can be supplied uniformly, and a desired power generation performance can be maintained suitably. Further, the areas of the buffer can be reduced effectively, and the overall size of the fuel cell can be reduced easily. 
    
    
     
       BRIEF DESCRIPTION OF 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 view showing one surface of a cathode-side metal separator of the fuel cell; 
         FIG. 3  is an enlarged view showing main components of the cathode-side metal separator; 
         FIG. 4  is a view showing the other surface of the cathode-side metal separator; 
         FIG. 5  is a partial perspective view showing an inlet buffer of the cathode-side metal separator; 
         FIG. 6  is a cross sectional view showing the cathode-side metal separator, taken along a line VI-VI in  FIG. 5 ; 
         FIG. 7  is a front view showing an anode-side metal separator of the fuel cell; 
         FIG. 8  is an exploded perspective view showing main components of a fuel cell according to a second embodiment of the present invention; 
         FIG. 9  is a front view showing an intermediate metal separator of the fuel cell; and 
         FIG. 10  is a view showing a separator disclosed in Japanese Laid-Open Patent Publication No. 2006-172924. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIG. 1 , a fuel cell  10  according to a first embodiment of the present invention includes a cathode-side metal separator  12 , a membrane electrode assembly (electrolyte electrode assembly) (MEA)  14 , and an anode-side metal separator  16 . 
     For example, the cathode-side metal separator  12  and the anode-side metal separator  16  are made of steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces by surface treatment. The cathode-side metal separator  12  and the anode-side metal separator  16  are formed by pressing metal thin plates into corrugated plates to have ridges and grooves in cross section. 
     For example, the membrane electrode assembly  14  includes a cathode  20 , an anode  22 , and a solid polymer electrolyte membrane (electrolyte)  18  interposed between the cathode  20  and the anode  22 . The solid polymer electrolyte membrane  18  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. 
     Each of the cathode  20  and the anode  22  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 layer of the cathode  20  and the electrode catalyst layer of the anode  22  are fixed to both surfaces of the solid polymer electrolyte membrane  18 , respectively. 
     At one end of the fuel cell  10  in a longitudinal direction indicated by the arrow B, a fuel gas supply passage  24   a  for supplying a fuel gas such as a hydrogen containing gas, a coolant discharge passage  26   b  for discharging a coolant, and an oxygen-containing gas discharge passage  28   b  for discharging an oxygen-containing gas are provided. The fuel gas supply passage  24   a , the coolant discharge passage  26   b , and the oxygen-containing gas discharge passage  28   b  extend through the fuel cell  10  in the direction indicated by the arrow A. 
     At the other end of the fuel cell  10  in the longitudinal direction indicated by the arrow B, an oxygen-containing gas supply passage  28   a  for supplying the oxygen-containing gas, a coolant supply passage  26   a  for supplying the coolant, and a fuel gas discharge passage  24   b  for discharging the fuel gas are provided. The oxygen-containing gas supply passage  28   a , the coolant supply passage  26   a , and the fuel gas discharge passage  24   b  extend through the fuel cell  10  in the direction indicated by the arrow A. 
     The oxygen-containing gas supply passage  28   a  has a substantially triangular shape, and includes two sides in parallel to two sides of a corner of the fuel cell  10 . The oblique side connected to these two sides of the triangle is in parallel to an outer line  37   c  of an inlet buffer  36   a  as described later. The oxygen-containing gas discharge passage  28   b , the fuel gas supply passage  24   a , and the fuel gas discharge passage  24   b  have the same structure as the oxygen-containing gas supply passage  28   a.    
     As shown in  FIGS. 1 and 2 , the cathode-side metal separator  12  has an oxygen-containing gas flow field (reactant gas flow field)  30  on its surface  12   a  facing the membrane electrode assembly  14 . The oxygen-containing gas flow field  30  is connected between the oxygen-containing gas supply passage  28   a  and the oxygen-containing gas discharge passage  28   b . On the other surface  12   b  of the cathode-side metal separator  12 , there is formed a coolant flow field  32 , which has a shape corresponding to the back side of the oxygen-containing gas flow field  30 . 
     The oxygen-containing gas flow field  30  includes a plurality of straight flow grooves  34   a  along the power generation surface extending in the direction indicated by the arrow B, and also includes an inlet buffer (distribution section)  36   a  and an outlet buffer (merge section)  36   b . The straight flow grooves  34   a  are arranged in the direction indicated by the arrow C. The inlet buffer  36   a  and the outlet buffer  36   b  are provided adjacent to the inlet and the outlet of the straight flow grooves  34   a , respectively. The straight flow grooves  34   a  are formed between straight flow field ridges (linear flow field ridges)  34   b  protruding from the surface  12   a . Instead of the straight flow field ridges  34   b , curved, bent, or wavy ridges (not shown) may be adopted. 
     It should be noted that the present invention is at least applicable to the inlet buffer  36   a  or the outlet buffer  36   b . Hereinafter, it is assumed that the present invention is applied to both of the inlet buffer  36   a  and the outlet buffer  36   b.    
     The inlet buffer  36   a  includes outer lines  37   a ,  37   b , and  37   c  forming a substantially trapezoidal (polygonal) shape in a front view. The outer line  37   a  is in parallel to the inner wall surface of the fuel gas discharge passage  24   b , the outer line  37   b  is in parallel to the inner wall surface (vertical surface) of the coolant supply passage  26   a , and the outer line  37   c  is in parallel to the inner wall surface of the oxygen-containing gas supply passage  28   a . The outer lines  37   a  to  37   c  may form a triangle, a rectangle or the like. 
     The inlet buffer  36   a  includes a plurality of continuous linear guide ridges  40   a  protruding from an intermediate height area  38   a  toward the oxygen-containing gas flow field  30  side. The linear guide ridges  40   a  form a continuous guide flow field  42   a.    
     As shown in  FIGS. 2 and 3 , the linear guide ridges  40   a  are continuously connected to ends of the straight flow field ridges  34   b  of the straight flow grooves  34   a  at predetermined positions. Further, each of the linear guide ridges  40   a  has a bent portion  41   a , and the linear guide ridges  40   a  have different lengths in a stepwise fashion. The linear guide ridges  40   a  have the same width. The width of the linear guide ridges  40   a  is narrower than, or equal to the width of the straight flow field ridges  34   b.    
     The linear guide ridge  40   a  connected to the straight flow field ridge  34   b  near the oxygen-containing gas supply passage  28   a  is shorter than the linear guide ridge  40   a  connected to the straight flow field ridge  34   b  remote from the oxygen-containing gas supply passage  28   a . The linear guide ridge  40   a  includes a straight line segment  40   aa  in parallel to the outer line  37   a . Further, the linear guide ridge  40   a  includes a straight line segment  40   ab  in parallel to the outer line  37   b.    
     As shown in  FIG. 3 , the linear guide ridges  40   a  are arranged such that intervals between connections of the linear guide ridges  40   a  with the straight flow field ridges  34   b  are the same distance L 1 , intervals between the bent portions  41   a  are the same distance L 2 , intervals between vertical segments thereof are the same distance L 3 , and intervals between ends thereof near the oxygen-containing gas supply passage  28   a  are the same distance L 4 . It is preferable that the linear guide ridges  40   a  are equally arranged at the same distance L 1 , the same distance L 2 , the same distance L 3 , and the same distance L 3  at respective positions. However, the linear guide ridges  40   a  may be arranged at different distances. 
     The inlet buffer  36   a  is connected to the oxygen-containing gas supply passage  28   a  through a bridge section  44   a . For example, the bridge section  44   a  is formed by corrugating a seal member to have ridges and grooves. Other bridge sections as described later have the same structure. 
     As shown in  FIG. 2 , the outlet buffer  36   b  and the inlet buffer  36   a  are symmetrical with respect to a point. The outlet buffer  36   b  includes outer lines  37   d ,  37   e , and  37   f  forming a substantially trapezoidal (polygonal) shape in a front view. The outer line  37   d  is in parallel to the inner wall surface of the fuel gas supply passage  24   a , the outer line  37   e  is in parallel to the inner wall surface (vertical surface) of the coolant discharge passage  26   b , and the outer line  37   f  is in parallel to the inner wall surface of the oxygen-containing gas discharge passage  28   b.    
     The outlet buffer  36   b  includes linear guide ridges  40   b  protruding from an intermediate height area  38   b  toward the oxygen-containing gas flow field  30  side. The linear guide ridges  40   b  form a continuous guide flow field  42   b . The outlet buffer  36   b  is connected to the oxygen-containing gas discharge passage  28   b  through a bridge section  44   b . The outlet buffer  36   b  has the same structure as the inlet buffer  36   a , and detailed description of the outlet buffer  36   b  is omitted. 
     As shown in  FIG. 4 , the coolant flow field  32  is formed on the other surface  12   b  of the cathode-side metal separator  12 , the coolant flow field  32  having a shape corresponding to the back side of the oxygen-containing gas flow field  30 . The coolant flow field  32  includes a plurality of straight flow grooves  46   a  along the power generation surface extending in the direction indicated by the arrow B, and also includes an inlet buffer  48   a  and an outlet buffer  48   b . The straight flow grooves  46   a  are arranged in the direction indicated by the arrow C. The inlet buffer  48   a  and the outlet buffer  48   b  are provided adjacent to the inlet and the outlet of the straight flow grooves  46   a , respectively. 
     The straight flow grooves  46   a  are formed between straight flow field ridges (linear flow field ridges)  46   b  protruding from the surface  12   b . The straight flow grooves  46   a  have a shape corresponding to the back side of the straight flow field ridges  34   b , and the straight flow field ridges  46   b  have a shape corresponding to the back side of the straight flow grooves  34   a . The inlet buffer  48   a  has a shape corresponding to the back side of the inlet buffer  36   a , and the outlet buffer  48   b  has a shape corresponding to the back side of the outlet buffer  36   b  (see  FIG. 5 ). 
     As shown in  FIGS. 5 and 6 , the inlet buffer  48   a  includes bosses  50   a  protruding from the intermediate height area  38   a  toward the coolant flow field  32  side. The bosses  50   a  form an embossed flow field  52   a . The depth of the continuous guide flow field  42   a  from the intermediate height area  38   a  is the same as the depth of the embossed flow field  52   a  from the intermediate height area  38   a . The inlet buffer  48   a  is connected to the coolant supply passage  26   a  through a bridge section  53   a  (see  FIG. 4 ). 
     As shown in  FIG. 4 , the outlet buffer  48   b  includes bosses  50   b  protruding from the intermediate height area  38   b  toward the coolant flow filed  32  side. The bosses  50   b  form an embossed flow field  52   b . The outlet buffer  48   b  is connected to the coolant discharge passage  26   b  through a bridge section  53   b.    
     As shown in  FIG. 7 , the anode-side metal separator  16  has a fuel gas flow field (reactant gas flow field)  54  on its surface  16   a  facing the membrane electrode assembly  14 . The coolant flow field  32  is formed on a surface  16   b  of the anode-side metal separator  16 , the coolant flow field  32  having a shape corresponding to the back side of the fuel gas flow field  54 . 
     The fuel gas flow field  54  includes a plurality of straight flow grooves  56   a  along the power generation surface and which extend in the direction indicated by the arrow B. Also, the fuel gas flow field  54  includes an inlet buffer  58   a  and an outlet buffer  58   b . The straight flow grooves  56   a  are arranged in the direction indicated by the arrow C. The inlet buffer  58   a  and the outlet buffer  58   b  are provided adjacent to the inlet and the outlet of the straight flow grooves  56   a , respectively. The straight flow grooves  56   a  are formed between straight flow field ridges (linear flow field ridges)  56   b  protruding on the surface  16   a . Instead of the straight flow field ridges  56   b , curved, bent, or wavy ridges (not shown) may be adopted. 
     The inlet buffer  58   a  includes outer lines  37   a ,  37   b , and  37   c  forming a substantially trapezoidal (polygonal) shape in a front view. The outer line  37   a  is in parallel to the inner wall surface of the oxygen-containing gas discharge passage  28   b , the outer line  37   b  is in parallel to the inner wall surface (vertical surface) of the coolant discharge passage  26   b , and the outer line  37   c  is in parallel to the inner wall surface of the fuel gas supply passage  24   a . The outer lines  37   a  to  37   c  may form a triangle, a rectangle or the like. 
     The inlet buffer  58   a  includes a plurality of continuous linear guide ridges  62   a  protruding from an intermediate height area  60   a  toward the fuel gas flow field  54  side. The linear guide ridges  62   a  form a continuous guide flow field  64   a.    
     The linear guide ridges  62   a  are continuously connected to ends of the straight flow field ridges  56   b  forming the straight flow grooves  56   a . Further, each of the linear guide ridges  62   a  has a bent portion  41   a , and the linear guide ridges  62   a  have different lengths in a stepwise fashion. The linear guide ridges  62   a  have the same width. The width of the linear guide ridges  62   a  is narrower than, or equal to the width of the straight flow field ridges  56   b . The linear guide ridges  62   a  have the same structure as the linear guide ridges  40   a , and detailed description of the linear guide ridges  62   a  is omitted. The inlet buffer  58   a  is connected to the fuel gas supply passage  24   a  through a bridge section  65   a.    
     The outlet buffer  58   b  and the inlet buffer  58   a  are symmetrical with respect to a point. The outlet buffer  58   b  includes outer lines  37   d ,  37   e , and  37   f  forming a substantially trapezoidal (polygonal) shape in a front view. The outer line  37   d  is in parallel to the inner wall surface of the oxygen-containing gas supply passage  28   a , the outer line  37   e  is in parallel to the inner wall surface (vertical surface) of the coolant supply passage  26   a , and the outer line  37   f  is in parallel to the inner wall surface of the fuel gas discharge passage  24   b.    
     The outlet buffer  58   b  includes a plurality of continuous linear guide ridges  62   b  protruding from an intermediate height area  60   b  toward the fuel gas flow field  54  side. The linear guide ridges  62   b  form a continuous guide flow field  64   b.    
     The linear guide ridges  62   b  are continuously connected to the ends of the straight flow field ridges  56   b  forming the straight flow grooves  56   a . Further, each of the linear guide ridges  62   b  has a bent portion  41   b , and the linear guide ridges  62   b  have different lengths in a stepwise fashion. The linear guide ridges  62   b  have the same structure as the linear guide ridges  40   b , and detailed description of the linear guide ridges  62   b  is omitted. The outlet buffer  58   b  is connected to the fuel gas discharge passage  24   b  through a bridge section  65   b.    
     As shown in  FIG. 1 , the coolant flow field  32  is formed on the other surface  16   b  of the anode-side metal separator  16 , the coolant flow field  32  having a shape corresponding to the back side of the fuel gas flow field  54 . The coolant flow field  32  has the same structure as that of the cathode-side metal separator  12 . The constituent elements that are identical to those of the cathode-side metal separator  12  are labeled with the same reference numerals, and detailed description thereof is omitted. 
     A first seal member  70  is formed integrally with the surfaces  12   a ,  12   b  of the cathode-side metal separator  12 , around the outer circumferential end of the cathode-side metal separator  12 . A second seal member  72  is formed integrally with the surfaces  16   a ,  16   b  of the anode-side metal separator  16 , around the outer circumferential end of the anode-side metal separator  16 . 
     Operation of the fuel cell  10  will be described below. 
     Firstly, as shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  28   a , and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  24   a . Further, a coolant such as pure water, ethylene glycol, oil or the like is supplied to the coolant supply passage  26   a.    
     In the structure, in the fuel cell  10 , the oxygen-containing gas is supplied from the oxygen-containing gas supply passage  28   a  to the oxygen-containing gas flow field  30  of the cathode-side metal separator  12 . The oxygen-containing gas moves from the inlet buffer  36   a  along the straight flow grooves  34   a  in the horizontal direction indicated by the arrow B, and the oxygen-containing gas is supplied to the cathode  20  of the membrane electrode assembly  14 . 
     The fuel gas flows from the fuel gas supply passage  24   a  to the fuel gas flow field  54  of the anode-side metal separator  16 . As shown in  FIG. 7 , the fuel gas moves from the inlet buffer  58   a  along the straight flow grooves  56   a  in the horizontal direction indicated by the arrow B, and the fuel gas is supplied to the anode  22  of the membrane electrode assembly  14 . 
     Thus, in the membrane electrode assembly  14 , the oxygen-containing gas supplied to the cathode  20 , and the fuel gas supplied to the anode  22  are consumed in the electrochemical reactions at the electrode catalyst layers of the cathode  20  and the anode  22  for generating electricity. 
     Then, the oxygen-containing gas supplied to and consumed at the cathode  20  of the membrane electrode assembly  14  is discharged from the outlet buffer  36   b  along the oxygen-containing gas discharge passage  28   b  in the direction indicated by the arrow A. Likewise, the fuel gas supplied to and consumed at the anode  22  of the membrane electrode assembly  14  is discharged from the outlet buffer  58   b  into the fuel gas discharge passage  24   b.    
     In the meanwhile, the coolant supplied to the coolant supply passage  26   a  flows into the coolant flow field  32  formed between the cathode-side metal separator  12  and the anode-side metal separator  16  of the fuel cell  10 , and then, the coolant flows in the direction indicated by the arrow B. After the coolant flows from the inlet buffer  48   a  along the straight flow grooves  46   a  to cool the membrane electrode assembly  14 , the coolant is discharged from the outlet buffer  48   b  into the coolant discharge passage  26   b.    
     In the first embodiment, for example, as shown in  FIG. 2 , a plurality of continuous linear guide ridges  40   a  are provided in the inlet buffer  36   a  of the oxygen-containing gas flow field  30 . The linear guide ridges  40   a  have the bent portions  41   a , and have different lengths in a stepwise fashion. Likewise, the continuous linear guide ridges  40   b  are provided in the outlet buffer  36   b . The linear guide ridges  40   b  have the bent portions  41   b , and have different lengths in a stepwise fashion. 
     Thus, in the oxygen-containing gas flow field  30 , since the inlet buffer  36   a  and the outlet buffer  36   b  have the continuous guide flow fields  42   a ,  42   b , the oxygen-containing gas does not flow around the water produced in the power generation reaction. In the structure, by the oxygen-containing gas, the water produced in the power generation reaction is easily and reliably discharged from the inlet buffer  36   a  and the outlet buffer  36   b . The oxygen-containing gas can be supplied uniformly, and desired power generation performance can be maintained suitably. 
     Further, the areas of the inlet buffer  36   a  and the outlet buffer  36   b  can be reduced effectively, and the overall size of the fuel cell  10  can be reduced easily. 
     Further, the straight line segment  40   aa  of the linear guide ridge  40   a  is in parallel to the outer line  37   a , and the straight line segment  40   ab  of the linear guide ridge  40   a  is in parallel to the outer line  37   b.    
     Further, as shown in  FIG. 3 , the linear guide ridges  40   a  are arranged such that intervals between connections between the linear guide ridges  40   a  and the straight flow field ridges  34   b  are the same distance L 1 , intervals between the bent portions  41   a  are the same distance L 2 , intervals between the vertical segments thereof are the same distance L 3 , and intervals between the ends thereof near the oxygen-containing gas supply passage  28   a  are the same distance L 4 . The linear guide ridges  40   b  have the same structure as the linear guide ridges  40   a.    
     In the structure, the oxygen-containing gas is supplied smoothly and uniformly along the entire power generation surface in the oxygen-containing gas flow field  30 , and suitable power generation performance can be obtained reliably. Further, in the fuel gas flow field  54 , the same advantages as in the case of the oxygen-containing gas flow field  30  are obtained. 
     Further, in the coolant flow field  32 , the inlet buffer  48   a  and the outlet buffer  48   b  have the embossed flow fields  52   a ,  52   b . In the structure, improvement in the performance of distributing the coolant is achieved advantageously. The membrane electrode assembly  14  is held between the inlet buffer  36   a , the outlet buffer  36   b , and the inlet buffer  58   a , the outlet buffer  58   b.    
     Thus, in the fuel cell  10 , degradation of the power generation performance due to insufficient supply of the oxygen-containing gas and the fuel gas can be prevented. Further, a desired cooling function can be obtained, and the power generation of the fuel cell  10  can be performed suitably. 
       FIG. 8  is an exploded perspective view showing main components of a fuel cell  80  according to a second embodiment of the present invention. The constituent elements of the fuel cell  80  that are identical to those of the fuel cell  10  according to the first embodiment are labeled with the same reference numerals, and description thereof is omitted. 
     The fuel cell  80  includes a cathode-side metal separator  12 , a first membrane electrode assembly  14   a , an intermediate metal separator  82 , a second membrane electrode assembly  14   b , and an anode-side metal separator  16 . 
     As shown in  FIG. 9 , the intermediate metal separator  82  has a fuel gas flow field (reactant gas flow field)  84  on its surface  82   a  facing the first membrane electrode assembly  14   a , and an oxygen-containing gas flow field (reactant gas flow field)  86  on its surface  82   b  facing the second membrane electrode assembly  14   b , the oxygen-containing gas flow field  86  having a shape corresponding to the back side of the fuel gas flow field  84 . 
     The fuel gas flow field  84  includes a plurality of straight flow grooves  88   a  extending along the power generation surface in the direction indicated by the arrow B. The straight flow grooves  88   a  are arranged in the direction indicated by the arrow C. Further, the fuel gas flow field  84  includes an inlet buffer  90   a  and an outlet buffer  90   b  provided respectively adjacent to the inlet and the outlet of the straight flow grooves  88   a . The straight flow grooves  88   a  are formed between straight flow field ridges (linear flow field ridges)  88   b  protruding on the surface  82   a.    
     The inlet buffer  90   a  includes outer lines  37   a ,  37   b , and  37   c  forming a trapezoidal shape (polygonal shape) in a front view. The inlet buffer  90   a  has a plurality of continuous linear guide ridges  94   a  protruding from an intermediate height area  92   a  toward the fuel gas flow field  84  side, and the linear guide ridges  94   a  form a continuous guide flow field  96   a.    
     The outlet buffer  90   b  has linear guide ridges  94   b  protruding from an intermediate height area  92   b  toward the fuel gas flow field  84  side, and the linear guide ridges  94   b  form a continuous guide flow field  96   b . The linear guide ridges  94   a ,  94   b  have the same structure as the linear guide ridges  62   a ,  62   b.    
     As shown in  FIG. 8 , the oxygen-containing gas flow field  86  includes a plurality of straight flow grooves  98   a  extending along the power generation surface in the direction indicated by the arrow B. The straight flow grooves  98   a  are arranged in the direction indicated by the arrow C. Further, the oxygen-containing gas flow field  86  includes an inlet buffer  100   a  and an outlet buffer  100   b  provided respectively adjacent to the inlet and outlet of the straight flow grooves  98   a . The straight flow grooves  98   a  are formed between straight flow field ridges (linear flow field ridges)  98   b  protruding on the surface  82   b.    
     The inlet buffer  100   a  includes bosses  102   a  protruding from the intermediate height area  92   b  toward the oxygen-containing gas flow field  86  side, and the bosses  102   a  form an embossed flow field  104   a . The outlet buffer  100   b  includes bosses  102   b  protruding from the intermediate height area  92   a  toward the oxygen-containing gas flow field  86  side, and the bosses  102   b  form an embossed flow field  104   b.    
     In the second embodiment, the continuous guide flow fields  96   a ,  96   b  protruding toward the fuel gas flow field  84  side are formed in the inlet buffer  90   a  and the outlet buffer  90   b  on the surface  82   a  of the intermediate metal separator  82 . Therefore, the fuel gas does not flow around the water produced in the power generation reaction. 
     Further, the embossed flow fields  104   a ,  104   b  protruding toward the oxygen-containing gas flow field  86  side are formed in the inlet buffer  100   a  and the outlet buffer  100   b , on the surface  82   b  of the intermediate metal separator  82 . Thus, in the oxygen-containing gas flow field  86 , the oxygen-containing gas flows smoothly without any influence by the shapes of the back side of the continuous guide flow fields  96   a ,  96   b.