Abstract:
A fuel cell unit configuring a fuel cell is provided with a first separator, a first electrolyte film/electrode body, a second separator, a second electrolyte film/electrode body, and a third separator. Resin guide members are provided on the outer periphery of the first separator, the second separator, and the third separator. The resin guide members have outer peripheral ends which protrude outwards, and in the aforementioned resin guide members are formed concave reliefs which are spaced inwards from the aforementioned outer peripheral ends.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/JP2011/053822, filed Feb. 22, 2011, which claims priority to Japanese Patent Application No. 2010-044493 filed on Mar. 1, 2010 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 a plurality of fuel cell units each including one or more membrane electrode assemblies and a plurality of separators. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. 
     BACKGROUND ART 
     For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane comprises a polymer ion exchange membrane. The membrane electrode assembly is sandwiched between separators. In use, generally, a predetermined number of fuel cells are stacked together to form a fuel cell stack, which is mounted in a vehicle. 
     In general, several tens to hundreds of fuel cells are stacked together to form the fuel cell stack. In this regard, it is required to accurately position each of the fuel cells themselves relative to each other. For example, a fuel cell stack of this type is disclosed in Japanese Laid-Open Patent Publication No. 2009-283469. 
     As shown in  FIG. 16 , the fuel cell stack is formed by stacking a plurality of fuel cell units  1  together. Each of the fuel cell units  1  includes first and second membrane electrode assemblies  2   a ,  2   b , and first, second, and third separators  3   a ,  3   b , and  3   c . The fuel cell units  1  are positioned together by a positioning mechanism  4 . The positioning mechanism  4  includes positioning members  5 . Each of the positioning members  5  is formed integrally with an end of the second separator  3   b , and both ends of the positioning members  5  are engaged with the first separator  3   a  and the third separator  3   c.    
     SUMMARY OF INVENTION 
     In the fuel cell stack, resin guides  6  are expanded on one side of the second separators  3   b , each of which is positioned at the center of each fuel cell unit  1 . Therefore, simply by guiding the resin guides  6  along a guide rail  7 , the fuel cell units  1  can be stacked together easily and accurately. 
     In some cases, the fuel cell stack is placed, for example, in a casing. In such cases, it is desirable to use the resin guides  6  as shock absorbing resin members for protecting electrode surfaces of the fuel cell stack when shocks are applied to the fuel cell stack. 
     The present invention has been made to meet demands of this type. An object of the present invention is to provide a fuel cell having a simple and compact structure in which shock absorbing performance of the fuel cell is improved effectively, and wherein desired positioning performance is achieved. 
     The present invention relates to a fuel cell formed by stacking a plurality of fuel cell units each including one or more membrane electrode assemblies and a plurality of separators. The membrane electrode assembly includes a pair of electrodes and an electrolyte membrane interposed between the electrodes. 
     In the fuel cell, resin guide members are provided at outer circumferential portions of the separators, or at outer circumferential portions of the membrane electrode assemblies, at the same position in the stacking direction. The resin guide members provided in all but one of the separators of the fuel cell unit or the resin guide members provided in all but one of the membrane electrode assemblies of the fuel cell unit have recessed portions, which are spaced inwardly from outer ends of the resin guide members. 
     In the present invention, the outer end of the resin guide member, which is provided in one separator or in one membrane electrode assembly, is exposed to the outside through the recessed portion in the outer ends of the resin guide members of all of the other separators or all of the other membrane electrode assemblies. In this structure, positioning operations can be performed for each of the fuel cell units by the outer end that is exposed to the outside. Thus, desired performance in positioning of the fuel cell unit can be achieved. 
     Further, in the separators or in the membrane electrode assemblies of the fuel cell unit, the outer ends of the resin guide members are overlapped in the stacking direction in portions thereof excluding the recessed portion. Thus, the resin guide members can function as shock absorbers, and the amount of weight and shocks that can be supported is increased effectively. Thus, with a simple structure, shock resistance of the fuel cell is improved effectively, and desired performance in positioning of the fuel cell can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view of a fuel cell unit of a fuel cell according to a first embodiment of the present invention; 
         FIG. 2  is a front view showing a first separator of the fuel cell unit; 
         FIG. 3  is a front view showing a second separator of the fuel cell unit; 
         FIG. 4  is a front view showing a third separator of the fuel cell unit; 
         FIG. 5  is a cross sectional view of the fuel cell, taken along line V-V in  FIG. 1 ; 
         FIG. 6  is a perspective view showing resin guide members of the fuel cell unit; 
         FIG. 7  is a perspective view showing a state in which the fuel cell is placed in a casing; 
         FIG. 8  is an exploded perspective view showing a fuel cell unit of a fuel cell according to a second embodiment of the present invention; 
         FIG. 9  is an exploded perspective view showing a fuel cell according to a third embodiment of the present invention; 
         FIG. 10  is a cross sectional view of the fuel cell, taken along line X-X in  FIG. 9 ; 
         FIG. 11  is a front view showing a first separator of the fuel cell; 
         FIG. 12  is a front view showing a first membrane electrode assembly of the fuel cell; 
         FIG. 13  is a front view showing a second membrane electrode assembly of the fuel cell; 
         FIG. 14  is a cross sectional view of the fuel cell, taken along line XIV-XIV in  FIG. 13 ; 
         FIG. 15  is a cross sectional view of the fuel cell, taken along line XV-XV in  FIG. 13 ; and 
         FIG. 16  is a cross sectional view showing the fuel cell stack disclosed in Japanese Laid-Open Patent Publication No. 2009-283469. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIG. 1 , a fuel cell  10  according to a first embodiment of the present invention is formed by stacking a plurality of fuel cell units  12  in a horizontal direction, as indicated by the arrow A, or in the direction of gravity, as indicated by the arrow C. Each of the fuel cell units  12  includes a first separator  14 , a first membrane electrode assembly (MEA)  16   a , a second separator  18 , a second membrane electrode assembly  16   b , and a third separator  20 . 
     For example, the first separator  14 , the second separator  18 , and the third separator  20  are metal plates such as steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces formed by application of surface treatment thereto. The first separator  14 , the second separator  18 , and the third separator  20  have ridges and grooves in cross section, which are formed by corrugating metal thin plates under pressure. Instead of metal separators, for example, carbon separators may be used as the first separator  14 , the second separator  18 , and the third separator  20 . 
     The surface area of the first membrane electrode assembly  16   a  is smaller than the surface area of the second membrane electrode assembly  16   b . Each of the first and second membrane electrode assemblies  16   a ,  16   b  includes an anode  24 , a cathode  26 , and a solid polymer electrolyte membrane  22  interposed between the anode  24  and the cathode  26 . The solid polymer electrolyte membrane  22  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. 
     The surface area of the anode  24  is smaller than the surface area of the cathode  26 . The surface areas of the anode  24  and the cathode  26  are reduced by forming cutouts at upper and lower positions at both ends in the direction indicated by the arrow B, respectively. 
     Each of the anode  24  and the cathode  26  has a gas diffusion layer (not shown) such as carbon paper, and an electrode catalyst layer (not shown) of platinum alloy supported on porous particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer. The electrode catalyst layer of the anode  24  and the electrode catalyst layer of the cathode  26  are fixed to both surfaces of the solid polymer electrolyte membrane  22 , respectively. 
     At an upper end of the fuel cell unit  12  in the longitudinal direction indicated by the arrow C, an oxygen-containing gas supply passage  30   a  for supplying an oxygen-containing gas, and a fuel gas supply passage  32   a  for supplying a fuel gas such as a hydrogen-containing gas are provided. The oxygen-containing gas supply passage  30   a  and the fuel gas supply passage  32   a  extend through the fuel cell unit  12  in the direction indicated by the arrow A. 
     At a lower end of the fuel cell unit  12  in the longitudinal direction indicated by the arrow C, a fuel gas discharge passage  32   b  for discharging the fuel gas, and an oxygen-containing gas discharge passage  30   b  for discharging the oxygen-containing gas are provided. The fuel gas discharge passage  32   b  and the oxygen-containing gas discharge passage  30   b  extend through the fuel cell unit  12  in the direction indicated by the arrow A. 
     At one end of the fuel cell unit  12  in a lateral direction indicated by the arrow B, a coolant supply passage  34   a  for supplying a coolant is provided, and at the other end of the fuel cell unit  12  in the lateral direction, a coolant discharge passage  34   b  for discharging the coolant is provided. The coolant supply passage  34   a  and the coolant discharge passage  34   b  extend through the fuel cell unit  12  in the direction indicated by the arrow A. 
     As shown in  FIG. 2 , the first separator  14  has a first fuel gas flow field  36  on a surface  14   a  thereof facing the first membrane electrode assembly  16   a . The first fuel gas flow field  36  is connected to the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b . The first fuel gas flow field  36  includes a plurality of corrugated flow grooves extending in the direction indicated by the arrow C. An inlet buffer  38  is provided adjacent to the outlet (upper end) of the first fuel gas flow field  36 , and an outlet buffer  40  is provided adjacent to the inlet (lower end) of the first fuel gas flow field  36 . Plural bosses are formed in the inlet buffer  38  and the outlet buffer  40 , respectively. 
     A coolant flow field  44  is formed on a surface  14   b  of the first separator  14 . The coolant flow field  44  is connected to the coolant supply passage  34   a  and the coolant discharge passage  34   b . The coolant flow field  44  is formed on the back surface of the first fuel gas flow field  36 . 
     As shown in  FIG. 3 , the second separator  18  has a first oxygen-containing gas flow field  50  on a surface  18   a  thereof facing the first membrane electrode assembly  16   a . The first oxygen-containing gas flow field  50  is connected to the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b . The first oxygen-containing gas flow field  50  includes a plurality of corrugated flow grooves extending in the direction indicated by the arrow C. An inlet buffer  52  is provided adjacent to the outlet (upper end) of the first oxygen-containing gas flow field  50 , and an outlet buffer  54  is provided adjacent to the inlet (lower end) of the first oxygen-containing gas flow field  50 . Plural bosses are formed in the inlet buffer  52  and the outlet buffer  54 , respectively. 
     As shown in  FIG. 1 , the second separator  18  has a second fuel gas flow field  58  on a surface  18   a  thereof facing the second membrane electrode assembly  16   b . The second fuel gas flow field  58  is connected to the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b . The second fuel gas flow field  58  includes a plurality of corrugated flow grooves extending in the direction indicated by the arrow C. An inlet buffer  60  is provided adjacent to the inlet (upper end) of the second fuel gas flow field  58 , and an outlet buffer  62  is provided adjacent to the outlet (lower end) of the second fuel gas flow field  58 . 
     As shown in  FIG. 4 , the third separator  20  has a second oxygen-containing gas flow field  66  on a surface  20   a  thereof facing the second membrane electrode assembly  16   b.    
     An inlet buffer  68  is provided adjacent to the inlet (upper end) of the second oxygen-containing gas flow field  66 , and an outlet buffer  70  is provided adjacent to the outlet (lower end) of the second oxygen-containing gas flow field  66 . Plural bosses are formed in the inlet buffer  68  and the outlet buffer  70 , respectively. 
     As shown in  FIG. 1 , the coolant flow field  44  is formed on a surface  20   b  of the third separator  20 . The coolant flow field  44  is connected to the coolant supply passage  34   a  and the coolant discharge passage  34   b . The coolant flow field  44  is formed by stacking corrugated back surfaces of the first fuel gas flow field  36  and the second oxygen-containing gas flow field  66 . 
     A first seal member  74  is formed integrally with the surfaces  14   a ,  14   b  of the first separator  14 , around the outer circumferential end of the first separator  14 . A second seal member  76  is formed integrally with the surfaces  18   a ,  18   b  of the second separator  18 , around the outer circumferential end of the second separator  18 . A third seal member  78  is formed integrally with surfaces  20   a ,  20   b  of the third separator  20 , around the outer circumferential end of the third separator  20 . 
     Each of the first to third seal members  74 ,  76 ,  78  is made of a sealing material, a cushion material, or a packing material such as EPDM rubber (ethylene propylene diene monomer), NBR (nitrile butadiene rubber), fluoro rubber, silicone rubber, fluorosilicone rubber, butyl rubber, natural rubber, styrene rubber, chloroprene rubber, or acrylic rubber. 
     As shown in  FIGS. 1 and 2 , a plurality of outer supply holes  80   a  and a plurality of inner supply holes  80   b  connecting the fuel gas supply passage  32   a  and the first fuel gas flow field  36  are formed in the first separator  14 . Further, a plurality of outer discharge holes  82   a  and a plurality of inner discharge holes  82   b , which connect the fuel gas discharge passage  32   b  and the first fuel gas flow field  36 , are formed in the first separator  14 . 
     As shown in  FIG. 3 , on the surface  18   a  of the second separator  18 , a plurality of inlet connection channels  84   a  and a plurality of outlet connection channels  84   b  are formed in portions where the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b  are connected to the first oxygen-containing gas flow field  50 . Further, a plurality of supply holes  86 , which connect the fuel gas supply passage  32   a  and the second fuel gas flow field  58 , and a plurality of discharge holes  88 , which connect the fuel gas discharge passage  32   b  and the second fuel gas flow field  58 , are formed in the second separator  18 . 
     As shown in  FIG. 4 , on the surface  20   a  of the third separator  20 , a plurality of inlet connection channels  89   a  and a plurality of outlet connection channels  89   b  are formed in portions where the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b  are connected to the second oxygen-containing gas flow field  66 . 
     As shown in  FIG. 1 , a plurality of resin guide members  90   a  are provided on the outer circumferential end of the first separator  14 , a plurality of resin guide members  90   b  are provided on the outer end of the second separator  18 , and a plurality of resin guide members  90   c  are provided on the outer end of the third separator  20 . Each of the resin guide members  90   a ,  90   b ,  90   c , for example, is made of polyphenylene sulfide (PPS), polyacetal (POM), polybutylene terephthalate (PBT), polyetheretherketone (PEEK), liquid crystal polymer (LCP), or ABS resin. 
     The resin guide members  90   a ,  90   b , and  90   c  are formed by fixing molded pieces, which are formed by molding insulating resin, into cutout portions provided in the metal plates of the first separator  14 , the second separator  18 , and the third separator  20 . The resin guide members  90   a ,  90   b ,  90   c  may be fixed by crimping, adhesion or the like. Alternatively, the resin guide members  90   a ,  90   b , and  90   c  may be formed integrally with the cutout portions of the metal plates by injection molding. 
     Holes  92   a ,  92   b  are formed in parallel with each other in each of the resin guide members  90   c . Also, holes  94   a ,  94   b  are formed in each of the resin guide members  90   a , and holes  96   a ,  96   b  are formed in each of the resin guide members  90   b . The holes  94   a ,  94   b  of the resin guide member  90   a  and the holes  96   a ,  96   b  of the resin guide member  90   b  are connected to the holes  92   a ,  92   b  of the resin guide member  90   c  in the direction indicated by the arrow A. 
     As shown in  FIG. 5 , the diameter of the holes  92   a ,  92   b  is smaller than the diameter of the holes  94   a ,  94   b ,  96   a , and  96   b . In every other fuel cell unit  12  along the stacking direction, for example, a connection member such as an insulating resin clip  98  having a plurality of slits formed in radial directions is inserted into the holes  92   a ,  94   a ,  96   a . In every other fuel cell unit  12  along the stacking direction except for the aforementioned fuel cell units  12 , likewise, insulating resin clips  98  serving as connection members are inserted into the holes  92   b ,  94   b , and  96   b.    
     Each of the insulating resin clips  98  includes a neck  98   a  and a flange  98   b  having a large diameter. The neck  98   a  is engaged with the third separator  20 , and the flange  98   b  contacts the first separator  14 . Thus, the first separator  14 , the second separator  18 , and the third separator  20  are fixed together in the stacking direction. 
     As shown in  FIG. 6 , the resin guide members  90   a ,  90   b , and  90   c  have outer ends  100   a ,  100   b , and  100   c  that protrude outwardly from outer circumferential end surfaces EF of the first separator  14 , the second separator  18 , and the third separator  20 . Among the first separator  14 , the second separator  18 , and the third separator  20  (i.e., among the plurality of separators), except for the second separator  18 , the resin guide members  90   a ,  90   c , which are provided in the first separator  14  and the third separator  20 , have recessed portions  102   a ,  102   b  that are spaced inwardly from the outer ends  100   a ,  100   c . Preferably, the recessed portions  102   a ,  102   b  are provided at substantially central positions of the resin guide members  90   a ,  90   b.    
     As shown in  FIG. 7 , the fuel cell  10  is placed in a casing  110 . The casing  110  includes end plates  112   a ,  112   b  provided at opposite ends of the fuel cell units  12  in the stacking direction, four side panels  114   a  to  114   d  provided on sides of the fuel cell units  12 , and hinge mechanisms  116  that couple the end plates  112   a ,  112   b  and the side panels  114   a  to  114   d  together. The side panels  114   a  to  114   d  are made of stainless steel (e.g., SUS 304) or another metal material. Alternatively, the side panels  114   a  to  114   d  may be made from a carbon material. 
     As shown in  FIG. 5 , in the casing  110 , outer ends  100   a ,  100   b , and  100   c  of the resin guide members  90   a ,  90   b , and  90   c  of the first separator  14 , the second separator  18 , and the third separator  20  of each fuel cell unit  12  can come into contact with the inner surface of the casing  110  (i.e., inner surfaces of the side panels  114   a  to  114   d ). Instead of the casing  110 , a bar (not shown) may be provided that extends across the end plates  112   a ,  112   b . In this case, the outer ends  100   a ,  100   b , and  100   c  of the resin guide members  90   a ,  90   b , and  90   c  come into contact with the inner surface of the bar. 
     Operations of the fuel cell  10  will be described below. 
     First, as shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  30   a , and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  32   a . Further, pure water, ethylene glycol, or oil is supplied to the coolant supply passage  34   a.    
     Thus, the oxygen-containing gas flows from the oxygen-containing gas supply passage  30   a  to the first oxygen-containing gas flow field  50  of the second separator  18  and the second oxygen-containing gas flow field  66  of the third separator (see  FIGS. 3 and 4 ). The oxygen-containing gas flows along the first oxygen-containing gas flow field  50  in the direction of gravity, as indicated by the arrow C, and the oxygen-containing gas is supplied to the cathode  26  of the first membrane electrode assembly  16   a . Further, the oxygen-containing gas also flows along the second oxygen-containing gas flow field  66  in the direction indicated by the arrow C, and the oxygen-containing gas is supplied to the cathode  26  of the second membrane electrode assembly  16   b  (see  FIG. 1 ). 
     As shown in  FIGS. 1 and 2 , the fuel gas from the fuel gas supply passage  32   a  flows through the outer supply holes  80   a  toward the surface  14   b  of the first separator  14 . Further, the fuel gas flows from the inner supply holes  80   b  toward the surface  14   a . Thus, as shown in  FIG. 2 , the fuel gas is supplied to the inlet buffer  38 . Then, the fuel gas moves along the first fuel gas flow field  36  in the direction of gravity, as indicated by the arrow C, and the fuel gas is supplied to the anode  24  of the first membrane electrode assembly  16   a.    
     Further, as shown in  FIG. 3 , the fuel gas flows through the supply holes  86  and moves toward the surface  18   b  of the second separator  18 . Thus, as shown in  FIG. 1 , the fuel gas is supplied to the inlet buffer  60  on the surface  18   b . Then, the fuel gas moves along the second fuel gas flow field  58  in the direction indicated by the arrow C, and the fuel gas is supplied to the anode  24  of the second membrane electrode assembly  16   b.    
     Thus, in each of the first and second membrane electrode assemblies  16   a ,  16   b , the oxygen-containing gas, which is supplied to the cathode  26 , and the fuel gas, which is supplied to the anode  24 , are consumed in electrochemical reactions that take place at electrode catalyst layers of the cathode  26  and the anode  24  for thereby generating electricity. 
     Then, the oxygen-containing gas, which is consumed at the cathodes  26  of the first and second membrane electrode assemblies  16   a ,  16   b , is discharged along the oxygen-containing gas discharge passage  30   b  in the direction indicated by the arrow A. 
     As shown in  FIG. 2 , the fuel gas, which is consumed at the anode  24  of the first membrane electrode assembly  16   a , flows from the outlet buffer  40  through the inner discharge holes  82   b  toward the surface  14   b  of the first separator  14 . As shown in  FIG. 1 , after the fuel gas has been supplied to the surface  14   b , the fuel gas flows into the outer discharge holes  82   a , whereupon the fuel gas moves again toward the surface  14   a . Thus, as shown in  FIG. 2 , the fuel gas is discharged from the outer discharge holes  82   a  into the fuel gas discharge passage  32   b.    
     Further, the fuel gas, which is consumed at the anode  24  of the second membrane electrode assembly  16   b , flows from the outlet buffer  62  through the discharge holes  88  toward the surface  18   a . As shown in  FIG. 3 , the fuel gas is discharged into the fuel gas discharge passage  32   b.    
     As shown in  FIG. 1 , the coolant that is supplied to the coolant supply passage  34   a  flows into the coolant flow field  44  formed between the first separator  14  and the third separator  20 , and then, the coolant flows in the direction indicated by the arrow B. After the coolant has cooled the first and second membrane electrode assemblies  16   a ,  16   b , the coolant is discharged into the coolant discharge passage  34   b.    
     In the first embodiment, in a predetermined number of fuel cell units  12 , the insulating resin clips  98  are inserted into the holes  94   a ,  96   a , and  92   a  of the resin guide members  90   a ,  90   b , and  90   c . In another predetermined number of fuel cell units  12 , the insulating resin clips  98  are inserted into the holes  94   b ,  96   b , and  92   b . Then, as shown in  FIG. 6 , in the fuel cell units  12  which have been assembled together, only the outer end  100   b  of the resin guide member  90   b  of the second separator  18  is exposed to the outside at the center thereof. This is because the inwardly spaced recessed portions  102   a ,  102   b  are formed in the outer ends  100   a ,  100   c  of the resin guide members  90   a ,  90   c  sandwiching the resin guide member  90   b  therebetween. The width of the recessed portions  102   a ,  102   b  is larger than the width Wa of the guide rail  120 . 
     In this structure, simply by guiding the outer end  100   b  of the resin guide member  90   b  of each of the fuel cell units  12  along the guide rail  120 , the fuel cell units  12  can be stacked together easily and accurately. 
     Further, according to the first embodiment, in the first separator  14 , the second separator  18 , and the third separator  20  of the fuel cell unit  12 , the outer ends  100   a ,  100   b , and  100   c  of the resin guide members  90   a ,  90   b , and  90   c  are overlapped in the stacking direction, in portions thereof excluding the recessed portions  102   a  and  102   b  (i.e., on both sides of the recessed portions  102   a ,  102   b ). 
     Thus, as shown in  FIG. 5 , the resin guide members  90   a ,  90   b , and  90   c  all come into contact together with the inner surface of the casing  110 . In this structure, the resin guide members  90   a ,  90   b , and  90   c  are capable of functioning as shock absorbers, such that the weight and shocks that can be supported are increased effectively. With such a simple structure, shock resistance of the fuel cell  10  is improved effectively, and a desired performance in positioning of the fuel cell  10  can be achieved. 
       FIG. 8  is an exploded perspective view showing a fuel cell unit  132  of a fuel cell  130  according to a second embodiment of the present invention. 
     Constituent elements, which are identical to those of the fuel cell  10  according to the first embodiment, are designated with the same reference numerals, and descriptions of such features are omitted. Also, in a third embodiment to be described later, constituent elements thereof, which are identical to those of the fuel cell  10  according to the first embodiment, are designated with the same reference numerals, and descriptions of such features are omitted. 
     The fuel cell unit  132  includes a first separator  134 , a membrane electrode assembly  16 , and a second separator  136 . Plural resin guide members  90   b ,  90   c  are provided on outer circumferential ends of the first separator  134  and the second separator  136 , respectively. The resin guide members  90   b ,  90   c  have outer ends  100   b ,  100   c  that protrude outwardly. In addition, recessed portions  102   b  are formed in each of the outer ends  100   c  of the resin guide members  90   c.    
     In the second embodiment, after the fuel cell units  132  have been assembled together, only the outer ends  100   b  of the resin guide members  90   b , which are provided in each of the first separators  134 , are used as positioning references for mutually positioning the fuel cell units  132 . 
     The outer ends  100   b ,  100   c , excluding the recessed portions  102   b  of the resin guide members  90   b ,  90   c  provided in the first separator  134  and the second separator  136 , are located at the same position in the stacking direction and are overlapped with each other. Thus, the resin guide members  90   b ,  90   c  are capable of functioning as shock absorbers. With such a simple structure, the same advantages as those of the first embodiment can be obtained. For example, the shock absorbing performance of the fuel cell  130  is improved effectively, and a desired positioning performance can be achieved. 
       FIG. 9  is an exploded perspective view showing main components of a fuel cell unit  142  of a fuel cell  140  according to a third embodiment of the present invention. 
     The fuel cell unit  142  includes a first separator  144 , a first membrane electrode assembly (MEA)  146   a , a second separator  148 , a second membrane electrode assembly  146   b , and a third separator  150 . Each of the first membrane electrode assembly  146   a  and the second membrane electrode assembly  146   b  includes a cathode  154 , an anode  156 , and a solid polymer electrolyte membrane  152  interposed between the cathode  154  and the anode  156 . The solid polymer electrolyte membrane  152  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example (see  FIG. 10 ). 
     The surface area of the solid polymer electrolyte membrane  152  is larger than the surface area of the cathode  154  and the surface area of the anode  156 . A resin frame (outer circumferential resin frame)  158  is formed, e.g., by injection molding, integrally with the outer circumferential end of the solid polymer electrolyte membrane  152 . As the resin material, for example, in addition to a general purpose plastic, engineering plastic, super engineering plastic or the like may be adopted. 
     As shown in  FIG. 9 , at one end of the frame  158  in the direction indicated by the arrow B, an oxygen-containing gas supply passage  30   a , a coolant discharge passage  34   b , and a fuel gas discharge passage  32   b  are arranged in the vertical direction, as indicated by the arrow C. At the other end of the frame  158  in the direction indicated by the arrow B, a fuel gas supply passage  32   a , a coolant supply passage  34   a , and an oxygen-containing gas discharge passage  30   b  are arranged in the direction indicated by the arrow C. 
     Outer circumferential ends of the first separator  144 , the second separator  148 , and the third separator  150  are positioned respectively inside the oxygen-containing gas supply passage  30   a , the coolant supply passage  34   a , the fuel gas discharge passage  32   b , the fuel gas supply passage  32   a , the coolant discharge passage  34   b , and the oxygen-containing gas discharge passage  30   b  (hereinafter also simply referred to as “fluid passages”). 
     As shown in  FIGS. 9 and 11 , at both ends of the first separator  144  in the direction indicated by the arrow B, extensions  160   a ,  160   b  are provided, which protrude toward the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b . Corrugated inlet channels  162   a  are formed in the extension  160   a . The inlet channels  162   a  interconnect the oxygen-containing gas supply passage  30   a  and the first oxygen-containing gas flow field  50 . Corrugated outlet channels  162   b  are formed in the extension  160   b . The outlet channels  162   b  interconnect the oxygen-containing gas discharge passage  30   b  and the first oxygen-containing gas flow field  50 . 
     Extensions  164   a ,  164   b  that protrude outwardly are formed at central positions at both ends of the first separator  144  in the direction indicated by the arrow C. Knock holes  166   a ,  166   b  extend through the extensions  164   a ,  164   b.    
     As shown in  FIG. 9 , at both ends of the second separator  148  in the direction indicated by the arrow B, extensions  168   a ,  168   b  are provided, which protrude toward the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b , and extensions  170   a ,  170   b  are provided, which protrude toward the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b.    
     Corrugated inlet channels  172   a  are formed in the extension  168   a  on the surface  148   a  of the second separator  148 . The inlet channels  172   a  interconnect the fuel gas supply passage  32   a  and the first fuel gas flow field  36 . Corrugated outlet channels  172   b  are formed in the extension  168   b . The outlet channels  172   b  interconnect the fuel gas discharge passage  32   b  and the first fuel gas flow field  36 . 
     Corrugated inlet channels  174   a  are formed in the extension  170   a  on the surface  148   b  of the second separator  148 . The inlet channels  174   a  interconnect the oxygen-containing gas supply passage  30   a  and the second oxygen-containing gas flow field  66 . Corrugated outlet channels  174   b  are formed in the extension  170   b . The outlet channels  174   b  interconnect the oxygen-containing gas discharge passage  30   b  and the second oxygen-containing gas flow field  66 . 
     Outwardly protruding extensions  176   a ,  176   b  are formed at central positions on both ends of the second separator  148  in the direction indicated by the arrow C. Knock holes  178   a ,  178   b  extend through the extensions  176   a ,  176   b.    
     At both ends of the third separator  150  in the direction indicated by the arrow B, extensions  180   a ,  180   b  are provided that protrude toward the fuel gas supply passage  32   a  and the fuel gas discharge passage  32   b , and extensions  182   a ,  182   b  are provided that protrude toward the coolant supply passage  34   a  and the coolant discharge passage  34   b.    
     Corrugated inlet channels  184   a  are formed in the extension  180   a . The inlet channels  184   a  interconnect the fuel gas supply passage  32   a  and the second fuel gas flow field  58  on the side of the surface  150   a . Corrugated outlet channels  184   b  are formed in the extension  180   b . The outlet channels  184   b  interconnect the fuel gas discharge passage  32   b  and the second fuel gas flow field  58 . Corrugated inlet channels  186   a  are formed in the extension  182   a . The inlet channels  186   a  interconnect the coolant supply passage  34   a  and the coolant flow field  44  on the side of the surface  150   b . Corrugated outlet channels  186   b  are formed in the extension  182   b . The outlet channels  186   b  interconnect the coolant discharge passage  34   b  and the coolant flow field  44 . 
     Outwardly protruding extensions  188   a ,  188   b  are formed at central positions on both ends of the third separator  150  in the direction indicated by the arrow C. Knock holes  190   a ,  190   b  extend through the extensions  188   a ,  188   b.    
     A seal member  192  is formed integrally with the frame  158  of the first membrane electrode assembly  146   a . As shown in  FIG. 10 , the seal member  192  has a first seal  192   a  on a surface thereof adjacent to the first separator  144 . The first seal  192   a  is formed around and slidably contacts the outer circumferential end of the first separator  144 . 
     As shown in  FIGS. 10 and 12 , the seal member  192  has a second seal  192   b  and a third seal  192   c  on a surface thereof adjacent to the second separator  148 . The second seal  192   b  is formed along and slidably contacts the outer circumferential end of the second separator  148 . The third seal  192   c  is formed on the outer side of the outer circumference of the second separator  148 , and slidably contacts the frame  158  of the adjacent second membrane electrode assembly  146   b.    
     As shown in  FIG. 12 , the third seal  192   c  includes portions that bypass the central positions, and which extend to the outside at both ends of the first membrane electrode assembly  146   a  in the direction indicated by the arrow C. The third seal  192   c  also includes knock holes  194   a ,  194   b  that extend through the seal member  192  between the bypassing portions of the third seal  192   c  and the second seal  192   b.    
     Resin guide members  196   a  are formed integrally with the frame  158  on both sides of the knock holes  194   a ,  194   b , in each of long sides of the first membrane electrode assembly  146   a . The resin guide members  196   a  may be provided separately from the frame  158 . Recessed portions  200  are formed in the resin guide members  196   a . The recessed portions  200  are spaced inwardly from the outer ends  198   a  of the resin guide members  196   a.    
     As shown in  FIGS. 10 and 13 , a second seal member  202  is formed integrally with the frame  158  of the second membrane electrode assembly  146   b . The second seal member  202  includes a first seal  202   a  and a second seal  202   b  on a surface thereof adjacent to the third separator  150 . The first seal  202   a  is formed along and slidably contacts the outer circumferential end of the third separator  150 . The second seal  202   b  is formed on the outer side of the outer circumference of the third separator  150 , and slidably contacts the frame  158  of the adjacent first membrane electrode assembly  146   a.    
     As shown in  FIG. 13 , the second seal  202   b  includes portions that bypass the central positions, and which extend to the outside at both ends in the direction indicated by the arrow C. The second seal  202   b  also includes knock members  204 , which are formed integrally with the frame  158  between the bypassing portions of the second seal  202   b  and the first seal  202   a.    
     As shown in  FIG. 14 , each of the knock members  204  includes an outer expansion  206   a , which is expanded toward the second separator  148 . The outer expansion  206   a  is inserted into a knock hole  178   a  of the second separator  148 , a knock hole  194   a  of the first membrane electrode assembly  146   a , and a knock hole  166   a  of the first separator  144 . A hole  206   c  is formed on the inner side of the outer expansion  206   a  through a step  206   b.    
     The knock member  204  includes an inner expansion  206   d , which expands oppositely to the outer expansion  206   a . The inner expansion  206   d  is positioned at the step  206   b  of the knock member  204  of the adjacent second membrane electrode assembly  146   b.    
     As shown in  FIG. 13 , resin guide members  196   b  are formed integrally with the frame  158  of the second membrane electrode assembly  146   b . Each of the resin guide members  196   b  includes an outer end  198   b , which is exposed to the outside from the recessed portion  200  provided in the resin guide member  196   a  of the first membrane electrode assembly  146   a  (see  FIG. 15 ). 
     Operations of the fuel cell  140  will briefly be described below. 
     As shown in  FIG. 9 , the oxygen-containing gas, which is supplied to the oxygen-containing gas supply passage  30   a , is supplied to the first oxygen-containing gas flow field  50  through the inlet channels  162  formed in the extension  160   a  of the first separator  144 , and is supplied to the second oxygen-containing gas flow field  66  through the inlet channels  174   a  formed in the extension  170   a  of the second separator  148 . 
     After the oxygen-containing gas has flowed through the first oxygen-containing gas flow field  50 , the oxygen-containing gas is discharged through the outlet channels  162   b  formed in the extension  160   b  of the first separator  144  into the oxygen-containing gas discharge passage  30   b . Meanwhile, after the oxygen-containing gas has flowed through the second oxygen-containing gas flow field  66 , the oxygen-containing gas is discharged into the oxygen-containing gas discharge passage  30   b  through the outlet channels  174   b  formed in the extension  170   b  of the second separator  148 . 
     The fuel gas, which is supplied to the fuel gas supply passage  32   a , is supplied to the first fuel gas flow field  36  through the inlet channels  172   a  formed in the extension  168   a  of the second separator  148 , and is supplied to the second fuel gas flow field  58  through the inlet channels  184   a  formed in the extension  180   a  of the third separator  150 . 
     After the fuel gas has flowed through the first fuel gas flow field  36 , the fuel gas is discharged into the fuel gas discharge passage  32   b  through the outlet channels  172   b  formed in the extension  168   b  of the second separator  148 . Meanwhile, after the fuel gas has flowed through the second fuel gas flow field  58 , the fuel gas is discharged into the fuel gas discharge passage  32   b  through the outlet channels  184   b  formed in the extension  180   b  of the third separator  150 . 
     Further, the coolant supplied to the coolant supply passage  34   a  is supplied to the coolant flow field  44  through the inlet channels  186   a  formed in the extension  182   a  of the third separator  150 . After the coolant has flowed through the coolant flow field  44 , the coolant is discharged into the coolant discharge passage  34   b  through the outlet channels  186   b  formed in the extension  182   b.    
     In the third embodiment, the recessed portions  200  are formed in the outer ends  198   a  of the resin guide members  196   a  of the resin frame  158  of the first membrane electrode assembly  146   a . Thus, when the fuel cell  140  is assembled, the outer ends  198   b  of the resin guide members  196   b  of the resin frame  158  of the second membrane electrode assembly  146   b  are exposed to the outside through the recessed portions  200 . In this structure, the outer ends  198   b  of the resin guide members  196   b  can be used for guiding the guide rail, and thus the same advantages as those of the first embodiment are obtained. 
     Further, as shown in  FIG. 14 , the knock member  204  is formed integrally with the resin frame  158  of the second membrane electrode assembly  146   b . The outer expansion  206   a  of the knock member  204  is inserted into the knock hole  194   a  of the second separator  148  and into the knock hole  166   a  of the first separator  144 . In this structure, the load from the first membrane electrode assembly  146   a  and the second membrane electrode assembly  146   b  can be received effectively by the first separator  144  and the second separator  148 , and thus the rigidity of the fuel cell units  142  as a whole can be improved effectively. 
     Although a combination of two MEAs and three separators is adopted in the first embodiment, a combination of one MEA and two separators is adopted in the second embodiment, and a combination of two MEAs and two separators is adopted in the third embodiment, the present invention is not limited in this respect. For example, a fuel cell unit, which is made up of a combination of three or more MEAs and four or more separators, may be used.