Patent Publication Number: US-7914937-B2

Title: Fuel cell and fuel cell stack

Description:
RELATED APPLICATIONS 
     This application is a divisional application of U.S. application Ser. No. 11/016,620 filed Dec. 17, 2004 now U.S. Pat. No. 7,625,657, which claims priority to Japanese Patent Application No. 2003-419909 filed Dec. 17, 2003. The contents of the aforementioned applications are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel cell formed by stacking an electrolyte electrode assembly and separators alternately. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. Further, the present invention relates to a fuel cell stack formed by stacking a plurality of fuel cells. 
     2. Description of the Related Art 
     Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly (unit cell). The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, a predetermined numbers of the unit cells and the separators are stacked together to form a fuel cell stack. 
     In the fuel cell, an oxygen-containing gas or air is supplied to the cathode. The oxygen in the oxygen-containing gas is ionized at the interface between the cathode and the electrolyte, and the oxygen ions (O 2− ) move toward the anode through the electrolyte. A fuel gas such as a hydrogen-containing gas or CO is supplied to the anode. Oxygen ions react with the hydrogen in the hydrogen-containing gas to produce water or react with CO to produce CO 2 . Electrons released in the reaction flow through an external circuit to the cathode, creating a DC electric energy. 
     Some of the fuel cell stacks of this type formed by stacking a plurality of fuel cells are known from, for example, Japanese Laid-Open Patent Publication No. 2002-203579, which discloses a solid oxide fuel cell. As shown  FIG. 21 , the solid oxide fuel cell is formed by stacking power generation cells  1  and separators  2  alternately. Each of the power generation cells  1  includes a fuel electrode layer  1   b , an air electrode layer  1   c , and a solid electrolyte layer  1   a  interposed between the fuel electrode layer  1   b  and the air electrode layer  1   c . A porous conductive fuel electrode current collector  3  is provided on one surface of the power generation cell  1 , and a porous conductive air electrode current collector  4  is provided on the other surface of the power generation cell  1 . The fuel electrode current collector  3 , the power generation cell  1 , and the air electrode current collector  4  are sandwiched between a pair of separators  2 . 
     The separator  2  has a fuel gas supply passage  5  and an air supply passage  6 . The fuel gas supply passage  5  is connected to a fuel gas hole  5   a  formed at a substantially central region on one surface of the separator  2 . The air supply passage  6  is connected to an air hole  6   a  formed at a substantially central region on the other surface of the separator  2 . The fuel gas hole  5   a  faces the fuel electrode current collector  3 . The air hole  6   a  faces the air electrode current collector  4 . 
     The fuel gas such as H 2  or CO flows through the fuel gas supply passage  5 , and is discharged from the substantially central region of the separator  2  toward the center of the fuel electrode current collector  3 . The fuel gas flows through holes formed in the fuel electrode current collector  3  toward the substantially central region of the fuel electrode layer  1   b . Then, the fuel gas flows along unillustrated slits to move radially outwardly toward the outer region of the fuel electrode layer  1   b.    
     Likewise, the air is supplied from the substantially central region of the separator  2  toward the center of the air electrode current collector  4  through the air supply passage  6 . The air flows through holes formed in the air electrode current collector  4  toward the substantially central region of the air electrode layer  1   c . Then, the air flows along unillustrated slits to move radially outwardly toward the outer region of the air electrode layer  1   c . In this manner, in each of the power generation cells  1 , the fuel gas is supplied to the surface of the fuel electrode layer  1   b , and the air is supplied to the surface of the air electrode layer  1   c  to carry out power generation. 
     When a large number of power generation cells  1  and separators  2  are stacked together as described above, it is necessary to apply a uniform load (pressure) to each of the power generation cells  1 . It is desirable to achieve the uniform surface pressure, high performance, and long service life. For these purposes, for example, a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 10-241707 is known. 
     As shown in  FIG. 22 , according to the disclosure of Japanese Laid-Open Patent Publication No. 10-241707, a power generation cell  7  is sandwiched between a pair of separators  8 . The power generation cell  7  includes an electrode plate  7   a  and electrolyte plates  7   b ,  7   c  provided on both surfaces of the electrode plate  7   a . A pair of current collector plates  7   d  are stacked on the outside of the electrode plates  7   b ,  7   c . The separator  8  includes a partition plate  8   a , a current collector corrugated plate  8   b , a seal frame  8   c , and a support frame  8   d . When a tightening pressure is applied to the surface of the fuel cell stack during operation of the fuel cell stack, the support frame  8   d  and the current collector corrugated plate  8   b  are deformed elastically to substantially the same extent. 
     However, according to the disclosure of Japanese Laid-Open Patent Publication No. 10-241707, when a tightening load is applied to the seal member to achieve the desired sealing performance, an excessive load may be applied to the electrolyte electrode assembly undesirably, and the electrolyte electrode assembly may be damaged. Further, since the separator  8  includes the partition plate  8   a , the current collector corrugated plate  8   b , the seal frame  8   c , and the support frame  8   d , the structure of the separator  8  is complicated, the separator  8  is expensive, and the thickness of the separator  8  is considerably large. Therefore, the power generation capacity per unit volume of the fuel cell stack is low. The number of processes required for producing the fuel cell stack is increased, and the production cost of the fuel cell stack is high. 
     SUMMARY OF THE INVENTION 
     A general object of the present invention is to provide a fuel cell and a fuel cell stack having a simple and economical structure in which the desired tightening load is applied reliably, and the sealing performance is improved. 
     According to the present invention, an electrolyte electrode assembly and separators are stacked alternately, and the electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. A fuel gas flow field for supplying a fuel gas along a surface of the anode, and an oxygen-containing gas flow field for supplying an oxygen-containing gas along a surface of the cathode are formed. 
     The separator comprises a fuel gas supply unit for allowing the fuel gas to flow into a surface of the separator, a fuel gas distribution passage connecting the fuel gas flow field and the fuel gas supply unit, an oxygen-containing gas supply unit for allowing the oxygen-containing gas to flow into a surface of the separator, and an oxygen-containing gas distribution passage connecting the oxygen-containing gas flow field and the oxygen-containing gas supply unit. Tightening means is provided at positions closer to the fuel gas supply unit and the oxygen-containing gas supply unit than the electrolyte electrode assembly for applying a tightening load to the stack of the electrolyte electrode assembly and the separators in the stacking direction. 
     It is preferable that the separator comprises an electrode stack unit on which the electrolyte electrode assembly is stacked, a first bridge connecting the electrode stack unit and the fuel gas supply unit to form the fuel gas distribution passage, and a second bridge connecting the electrode stack unit and the oxygen-containing gas supply unit to form the oxygen-containing gas distribution passage. 
     Further, it is preferable that the separator includes first and second plates which are stacked together, and a third plate interposed between the first and second plates. The fuel gas flow field is formed between the first plate and one electrolyte electrode assembly, and the oxygen-containing gas flow field is formed between the second plate and another electrolyte electrode assembly. A space in the separator is divided by the third plate into a fuel gas channel connecting the fuel gas supply unit and the fuel gas flow field and an oxygen-containing gas channel connecting the oxygen-containing gas supply unit and the oxygen-containing gas flow field. 
     Further, it is preferable that the fuel gas supply unit comprises a fuel gas supply passage extending through the electrolyte electrode assembly and the separators in the stacking direction. 
     Further, according to the present invention, a fuel cell stack is formed by stacking a plurality of fuel cells. Each of the fuel cells is formed by stacking an electrolyte electrode assembly and separators alternately. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. A fuel gas flow field for supplying a fuel gas along a surface of the anode, and an oxygen-containing gas flow field for supplying an oxygen-containing gas along a surface of the cathode are provided. 
     The separator comprises a fuel gas supply unit for allowing the fuel gas to flow into a surface of the separator, a fuel gas distribution passage connecting the fuel gas flow field and the fuel gas supply unit, an oxygen-containing gas supply unit for allowing the oxygen-containing gas to flow into a surface of the separator, and an oxygen-containing gas distribution passage connecting the oxygen-containing gas flow field and the oxygen-containing gas supply unit. Tightening means is provided at positions closer to the fuel gas supply unit and the oxygen-containing gas supply unit than the electrolyte electrode assembly for applying a tightening load to the stack of the electrolyte electrode assembly and the separators in the stacking direction. 
     Since the rigidity of the fuel gas supply unit, the rigidity of the electrode stack unit, and the rigidity of the oxygen-containing gas supply unit are separated by the bridges, the tightening loads applied to the respective positions of the separator can be determined individually. It is possible to apply the load preferentially to position where the sealing function is required. With the simple and economical structure, sealing performance is improved desirably, and no excessive load is applied to the electrolyte electrode assemblies. Therefore, the damage of the electrolyte electrode assemblies is prevented effectively. 
     The above and other objects, features and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically showing a fuel cell stack formed by stacking a plurality of fuel cells according to a first embodiment of the present invention; 
         FIG. 2  is an exploded perspective view showing the fuel cell; 
         FIG. 3  is a partial exploded perspective view showing gas flows in the fuel cell; 
         FIG. 4  is a front view showing a second plate of the fuel cell; 
         FIG. 5  is a view, with partial omission, showing one surface of a third plate of the fuel cell; 
         FIG. 6  is a view, with partial omission, showing the other surface of the third plate; 
         FIG. 7  is an enlarged cross sectional view showing a region near a fuel gas supply passage of the fuel cell; 
         FIG. 8  is an enlarged cross sectional view showing a region near an oxygen-containing gas supply passage of the fuel cell; 
         FIG. 9  is a cross sectional view schematically showing operation of the fuel cell; 
         FIG. 10  is a perspective view schematically showing a fuel cell stack formed by stacking a plurality of fuel cells according to a second embodiment of the present invention; 
         FIG. 11  is a cross sectional view showing part of a fuel cell system in which the fuel cell stack is disposed in a casing; 
         FIG. 12  is an exploded perspective view showing separators of the fuel cell; 
         FIG. 13  is a partial exploded perspective view showing gas flows of the fuel cell; 
         FIG. 14  is a view showing one surface of a third plate of the separator; 
         FIG. 15  is an enlarged cross sectional view showing a central region of the fuel cell; 
         FIG. 16  is an enlarged cross sectional view showing an outer circumferential region of the fuel cell; 
         FIG. 17  is a cross sectional view schematically showing operation of the fuel cell; 
         FIG. 18  is a perspective view schematically showing a fuel cell stack formed by stacking a plurality of fuel cells according to a third embodiment of the present invention; 
         FIG. 19  is an exploded perspective view showing the fuel cell; 
         FIG. 20  is an exploded perspective view showing operation of the fuel cell; 
         FIG. 21  is a cross sectional view showing a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2002-203579; and 
         FIG. 22  is a cross sectional view showing a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 10-241707. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a perspective view schematically showing a fuel cell stack  12  formed by stacking a plurality of fuel cells  10  according to a first embodiment of the present invention in a direction indicated by an arrow A. 
     The fuel cell  10  is a solid oxide fuel cell (SOFC) used in various applications, including stationary and mobile applications. The fuel cell  10  is mounted on a vehicle. As shown in  FIGS. 2 and 3 , the fuel cell  10  includes an electrolyte electrode assembly  26 . The electrolyte electrode assembly  26  includes a cathode  22 , an anode  24 , and an electrolyte (electrolyte plate)  20  interposed between the cathode  22  and the anode  24 . For example, the electrolyte  20  is made of ion-conductive solid oxide such as stabilized zirconia. The electrolyte electrode assembly  26  has a circular disk shape. 
     The fuel cell  10  is formed by sandwiching the electrolyte electrode assembly  26  between a pair of separators  28 . The separator  28  includes first and second plates  30 ,  32 , and a third plate  34  interposed between the first and second plates  30 ,  32 . For example, the first through third plates  30 ,  32 ,  34  are metal plates of, e.g., stainless alloy. The first plate  30  and the second plate  32  are joined to both surfaces of the third plate  34  by brazing, for example. 
     As shown in  FIG. 2 , the first plate  30  has a first small diameter end portion (a fuel gas supply unit)  38 . A fuel gas supply passage  36  for supplying a fuel gas in the direction indicated by the arrow A extends through the first small diameter end portion  38 . The first small diameter end portion  38  is integral with a first circular disk  42  having a relatively large diameter through a narrow first bridge  40 , i.e. fuel gas side bridge. The first circular disk  42  and the anode  24  of the electrolyte electrode assembly  26  have substantially the same size. 
     A large number of first protrusions  44  are formed on a surface of the first circular disk  42  which contacts the anode  24 , in a central region adjacent to an outer circumferential region. A substantially ring shaped protrusion  46  is provided on the outer circumferential region of the first circular disk  42 . A fuel gas flow field is provided between the first protrusions  44  and the anode  24 . The first protrusions  44  and the substantially ring shaped protrusion  46  jointly functions as a current collector. A fuel gas inlet  48  is provided at the center of the first circular disk  42  for supplying the fuel gas toward substantially the central region of the anode  24 . The first protrusions  44  may be formed by making a plurality of recesses in a surface which is in the same plane with the surface of the substantially ring shaped protrusion  46 . 
     The second plate  32  has a second small diameter end portion (an oxygen-containing gas supply unit)  52 . An oxygen-containing gas supply passage  50  for supplying an oxygen-containing gas in the direction indicated by the arrow A extends through the second small diameter end portion  52 . The second small diameter end portion  52  is integral with a second circular disk  56  having a relatively large diameter through a narrow second bridge  54 , i.e. oxygen-containing gas side bridge. 
     As shown in  FIG. 4 , a plurality of second protrusions  58  are formed on the entire surface of the second circular disk  56  which contacts the cathode  22  of the electrolyte electrode assembly  26 . An oxygen-containing gas flow field is provided between the second protrusions  58  and the cathode  22 , and the second protrusions  58  function as a current collector. An oxygen-containing gas inlet  60  is provided at the center of the second circular disk  56  for supplying the oxygen-containing gas toward substantially the central region of the cathode  22 . 
     As shown in  FIG. 2 , the third plate  34  includes a third small diameter end portion (the fuel gas supply unit)  62  and a fourth small diameter end portion (the oxygen-containing gas supply unit)  64 . The fuel gas supply passage  36  extends through the third small diameter end portion  62 , and the oxygen-containing gas supply passage  50  extends through the fourth small diameter end portion  64 . The third and fourth small diameter end portions  62 ,  64  are integral with a third circular disk  70  having a relatively large diameter through narrow first and second bridges  66 ,  68 , respectively. The first through third circular disks  42 ,  56 ,  70  have the same diameter. 
     As shown in  FIGS. 2 and 5 , the third plate  34  has a plurality of slits  72  radially formed in the third small diameter end portion  62 , on a surface facing the first plate  30 . The slits  72  are connected to the fuel gas supply passage  36 . Further, the slits  72  are connected to a recess  74  formed in an outer circumferential region of the third small diameter end portion  62 . The recess  74  prevents the entry of brazing material into the slits  72 , and into an area inside the recess  74 . A fuel gas channel  76  is formed in the first bridge  66  and in the surface of the third circular disk  70 . The fuel gas flows from the fuel gas supply passage  36  to the fuel gas channel  76  through the slits  72 . A plurality of third protrusions  78  are formed on the third circular disk  70 , and the third protrusions  78  are part of the fuel gas channel  76 . 
     As shown in  FIG. 6 , the third plate  34  has a plurality of slits  80  radially formed in the fourth small diameter end portion  64 , on a surface which contacts the second plate  32 . The slits  80  are connected to the oxygen-containing gas supply passage  50 . Further, the slits  80  are connected to a recess  82 . The recess  82  prevents the entry of brazing material into slits  80 , and into an area inside the recess  82 . An oxygen-containing gas channel  84  is formed in the third circular disk  70 . The oxygen-containing gas flows through the slits  80  into the third circular disk  70 . The oxygen-containing gas channel  84  is closed by the outer circumferential region of the third circular disk  70 . 
     The first plate  30  is joined to one surface of the third plate  34  by brazing to form the fuel gas channel  76  connected to the fuel gas supply passage  36  between the first and third plates  30 ,  34 . The first bridge  40  of the first plate  30  and the first bridge  66  of the third plate  34  are joined together to form a fuel gas channel member, and a fuel gas distribution passage  76   a  as part of the fuel gas channel  76  is formed in the fuel gas channel member (see  FIG. 7 ). 
     The fuel gas channel  76  is provided between the first and third circular disks  42 ,  70 , over the electrode surface of the anode  24 . The first circular disk  42  is provided between the fuel gas channel  76  and the anode  24 , and the fuel gas is supplied to the fuel gas channel  76 . That is, a fuel gas pressure chamber  86  is formed such that the first circular disk  42  tightly contacts the anode  24  under pressure (see  FIGS. 7 and 8 ). 
     The second plate  32  is joined to the third plate  34  by brazing to form the oxygen-containing gas channel  84  connected to the oxygen-containing gas supply passage  50  between the second and third plates  32 ,  34  (see  FIG. 8 ). The second bridge  54  of the second plate  32  and the second bridge  68  of the third plate  34  are joined together to form an oxygen-containing gas channel member, and an oxygen-containing gas distribution passage  84   a  as part of the oxygen-containing gas channel  84  is formed in the oxygen-containing gas channel member. 
     The oxygen-containing gas channel  84  is provided between the second and third circular disks  56 ,  70 , over the electrode surface of the cathode  22 . The second circular disk  56  is provided between the oxygen-containing gas channel  84  and the cathode  22 , and the oxygen-containing gas is supplied to the oxygen-containing gas channel  84 . That is, an oxygen-containing gas pressure chamber  88  is formed such that the second circular disk  56  tightly contacts the cathode  22  under pressure (see  FIGS. 7 and 8 ). 
     Insulating seals  89   a  for sealing the fuel gas supply passage  36  and insulating seals  89   b  for sealing the oxygen-containing gas supply passage  50  are provided between the separators  28 . For example, the insulating seals  89   a ,  89   b  are made of mica material, or ceramic material. 
     As shown in  FIG. 1 , the fuel cell stack  12  includes end plates  90   a ,  90   b  provided at opposite ends of the fuel cells  10  in the stacking direction. The end plate  90   a  or the end plate  90   b  is electrically insulated from tightening means  95 . A first pipe  92  and a second pipe  94  extend through the end plate  90   a . The first pipe  92  is connected to the fuel gas supply passage  36  of the fuel cell  10 , and the second pipe  94  is connected to the oxygen-containing gas supply passage  50  of the fuel cell  10 . The tightening means  95  applies a tightening load to the electrolyte electrode assemblies  26  and the separators  28  stacked in the direction indicated by the arrow A. The tightening means  95  is provided at positions closer to the fuel gas supply passage  36  and the oxygen-containing gas supply passage  50  than the electrolyte electrode assemblies  26 . 
     The tightening means  95  includes bolt holes  96  of the end plates  90   a ,  90   b . The fuel gas supply passage  36  and the oxygen-containing gas supply passage  50  are positioned between the bolt holes  96 , respectively. Tightening bolts  98  are inserted into the respective bolt holes  96 , and tip ends of the respective tightening bolts  98  are screwed into nuts  99  for tightening the fuel cell stack  12 . 
     Next, operation of the fuel cell stack  12  will be described below. 
     As shown in  FIG. 2 , in assembling the fuel cell  10 , firstly, the first plate  30  of the separator  28  is joined to one surface of the third plate  34 , and the second plate  32  is joined to the other surface of the third plate  34 . Thus, the third plate  34  divides a space in the separator  28  to form the fuel gas channel  76  connected to the fuel gas supply passage  36  and the oxygen-containing gas channel  84  connected to the oxygen-containing gas supply passage  50  separately (see  FIG. 3 ). 
     Further, the fuel gas pressure chamber  86  is formed between the first and third circular disks  42 ,  70 , and the oxygen-containing gas pressure chamber  88  is formed between the second and third circular disks  56 ,  70  (see  FIG. 9 ). 
     Then, the separators  28  and the electrolyte electrode assemblies  26  are stacked alternately, and the end plates  90   a ,  90   b  are provided at the opposite ends in the stacking direction. The end plate  90   a  or the end plate  90   b  is electrically insulated from the tightening bolts  98 . The tightening bolts  98  are inserted into the respective bolt holes  96  of the end plates  90   a ,  90   b , and the tip ends of the tightening bolts  98  are screwed into the nuts  99 . That is, the stacked separators  28  and the electrolyte electrode assemblies  26  are tightened together by the tightening means  95  to form the fuel cell stack  12  (see  FIG. 1 ). 
     A fuel gas (e.g., hydrogen-containing gas) is supplied to the first pipe  92  connected to the end plate  90   a , and the fuel gas flows from the first pipe  92  to the fuel gas supply passage  36 . An oxygen-containing gas (hereinafter also referred to as the air) is supplied to the second pipe  94  connected to the end plate  90   a , and the oxygen-containing gas flows from the second pipe  94  to the oxygen-containing gas supply passage  50 . 
     As shown in  FIG. 7 , after the fuel gas flows into the fuel gas supply passage  36 , the fuel gas flows in the stacking direction indicated by the arrow A, and is supplied to the fuel gas channel  76  in the separator  28  in each of the fuel cells  10 . The fuel gas flows along the fuel gas channel  76 , and flows into the fuel gas pressure chamber  86  between the first and third circular disks  42 ,  70 . The fuel gas flows between the third protrusions  78 , and flows into the fuel gas inlet  48  at the central position of the first circular disk  42 . 
     The fuel gas inlet  48  is provided at a position corresponding to the central position of the anode  24  in each of the electrolyte electrode assemblies  26 . Therefore, as shown in  FIG. 9 , the fuel gas from the fuel gas inlet  48  is supplied to the anode  24 , and flows from the central region of the anode  24  toward the outer circumferential region of the anode  24 . 
     As shown in  FIG. 8 , after the oxygen-containing gas flows into the oxygen-containing gas supply passage  50 , the oxygen-containing gas flows through the oxygen-containing gas channel  84  in the separator  28 , and is supplied to the oxygen-containing gas pressure chamber  88  between the second and third circular disks  56 ,  70 . The oxygen-containing gas flows into the oxygen-containing gas inlet  60  at the central position of the second circular disk  56 . 
     The oxygen-containing gas inlet  60  is provided at a position corresponding to the central position of the cathode  22  in each of the electrolyte electrode assemblies  26 . Therefore, as shown in  FIG. 9 , the oxygen-containing gas from the oxygen-containing gas inlet  60  is supplied to the cathode  22 , and flows from the central region of the cathode  22  to the outer circumferential region of the cathode  22 . 
     Thus, in each of the electrolyte electrode assemblies  26 , the fuel gas is supplied from the central region of the anode  24  to the outer circumferential region of the anode  24 , and the oxygen-containing gas is supplied from the central region of the cathode  22  to the outer circumferential region of the cathode  22  for generating electricity. After the fuel gas and the oxygen-containing gas are consumed in the power generation, the fuel gas and the oxygen-containing gas are discharged as an exhaust gas from the outer circumferential regions of the first through third circular disks  42 ,  56 , and  70 . 
     In the first embodiment, the separator  28  has the fuel gas supply passage  36 , the fuel gas distribution passage  76   a , the oxygen-containing gas supply passage  50 , and the oxygen-containing gas distribution passage  84   a . The fuel gas flows through the fuel gas supply passage  36  into the surface of the separator  28 . The fuel gas distribution passage  76   a  connects the fuel gas channel  76  and the fuel gas supply passage  36 . The oxygen-containing gas flows through the oxygen-containing gas supply passage  50  into the surface of the separator  28 . The oxygen-containing gas distribution passage  84   a  connects the oxygen-containing gas channel  84  and the oxygen-containing gas supply passage  50 . 
     Specifically, the first and third small diameter end portions  38 ,  62  (fuel gas supply passage  36 ) and the first and third circular disks  42 ,  70  (fuel gas channel  76 ) are connected by the narrow first bridges  40 ,  66  (fuel gas distribution passage  76   a ), and the second and fourth small diameter end portions  52 ,  64  (oxygen-containing gas supply passage  50 ) and the second and third circular disks  56 ,  70  (oxygen-containing gas channel  84 ) are connected by the narrow second bridges  54 ,  68  (oxygen-containing gas distribution passage  84   a ). 
     In the separator  28 , the rigidity of a fuel gas supply unit, the rigidity of an electrode stack unit, and the rigidity of an oxygen-containing gas supply unit are separated by the bridges. Thus, the tightening loads applied to respective positions of the separator  28  can be determined individually. It is possible to apply the load preferentially to position where the sealing function is required. 
     In the first embodiment, the tightening means  95  for applying the tightening load to the electrolyte electrode assemblies  26  and the separators  28  in the stacking direction indicated by the arrow A is provided on opposite sides of the fuel gas supply passage  36  and on opposite sides of the oxygen-containing gas supply passage  50 . Therefore, the load applied to the positions near the fuel gas supply passage  36  and the oxygen-containing gas supply passage  50  is higher than the load applied to the electrolyte electrode assemblies  26 . Thus, the sealing performance at the fuel gas supply passage  36  and the oxygen-containing gas supply passage  50  is improved, and the contact resistances of the current collectors of the electrolyte electrode assemblies  26  are reduced. 
     As described above, in the first embodiment, with the simple structure, the sealing performance is improved desirably. Further, since no excessive load is applied to the electrolyte electrode assemblies  26 , the damage of the electrolyte electrode assemblies  26  is prevented. For example, the insulating seals  89   a ,  89   b  are made of mica material, or ceramic material. Therefore, the insulating seals  89   a ,  89   b  are fabricated at low cost. 
       FIG. 10  is a perspective view schematically showing a fuel cell stack  102  formed by stacking a plurality of fuel cells  100  according to a second embodiment of the present invention in a direction indicated by an arrow A.  FIG. 11  is a cross sectional view showing part of a fuel cell system  106  in which the fuel cell stack  102  is disposed in a casing  104 . 
     The constituent elements that are identical to those of the fuel cell  10  according to the first embodiment are labeled with the same reference numeral, and description thereof will be omitted. In a third embodiment as described later, the constituent elements that are identical to those of the fuel cell  10  according to the first embodiment are labeled with the same reference numeral, and description thereof will be omitted. 
     As shown in  FIGS. 12 and 13 , a plurality of, e.g., eight electrolyte electrode assemblies  26  are interposed between a pair of separators  108  to form the fuel cell  100 . The electrolyte electrode assemblies  26  are concentric with a fuel gas supply passage  36  extending through the center of the separators  108 . 
     Each of the separators  108  includes first and second plates  110 ,  112  which are stacked together, and a third plate  114  interposed between the first and second plates  110 ,  112 . The first through third plates  110 ,  112 , and  114  are metal plates of, e.g., stainless alloy. 
     The first plate  110  has a first small diameter end portion (a fuel gas supply unit)  116 . The fuel gas supply passage  36  extends through the center of the first small diameter end portion  116 . The first small diameter end portion  116  is integral with first circular disks  120  each having a relatively large diameter through a plurality of first bridges  118 . The first bridges  118  are extending radially outwardly from the first small diameter end portion  116  at equal angles (intervals). An exhaust gas channel  122  is formed in the area around, and internal from the first circular disks  120 . 
     Each of the first circular disks  120  has a plurality of first protrusions  44  and a substantially ring shaped protrusion  46  on a surface which contacts the anode  24  of the electrolyte electrode assembly  26 . A fuel gas inlet  48  is provided at the center of the first circular disk  120 . 
     The second plate  112  has a curved outer section (an oxygen-containing gas supply unit)  124 . Respective circular arc portions of the curved outer section  124  are integral with second circular disks  128  each having a relatively large diameter through second bridges  126  extending internally from the circular arc portions. As with the first circular disks  120  of the first plate  110 , the number of the second circular disks  128  is eight, and the second circular disks  128  are provided at equal angles (intervals). Each of the second circular disks  128  has a plurality of second protrusions  58  on a surface which contacts the cathode  22  of the electrolyte electrode assembly  26 . An oxygen-containing gas inlet  60  is provided at the center in each of the second circular disks  128 . 
     The third plate  114  has a second small diameter end portion (the fuel gas supply unit)  130 . The fuel gas supply passage  36  extends through the center of the second small diameter end portion  130 . Eight first bridges  132  extend radially from the second small diameter end portion  130 , and tip ends of the first bridges  132  are integral with third circular disks  134  each having a relative large diameter. Second bridges  136  are provided on extension lines of (in alignment with) the first bridges  132 . All the second bridges  136  are integral with a curved outer section (the oxygen-containing gas supply unit)  138  of the third plate  114 . 
     A plurality of third protrusions  78  are formed on the entire surface of the third circular disk  134  facing the first plate  110 . Slits  72  and a recess  74  are formed on the second small diameter end portion  130 . Further, a fuel gas distribution passage  76   a  as part of a fuel gas channel  76  is formed in each of the first bridges  132 . 
     As shown in  FIG. 14 , the curved outer section  138  of the third plate  114  has a plurality of slits (the oxygen-containing gas supply unit)  140  as air intake passages at positions corresponding to the respective third circular disks  134 , on a surface facing the second plate  112 . Further, a recess  142  for preventing the flow of brazing material is formed along the profile of the curved outer section  138 . 
     As shown in  FIG. 15 , when the first plate  110  is jointed to the third plate  114  by brazing, the respective first bridges  118 ,  132  are joined together to form fuel gas channel members. Fuel gas distribution passages  76   a  as part of the fuel gas channel  76  are formed in the fuel gas channel members. The fuel gas channel  76  forms a fuel gas pressure chamber  86  between the first and third circular disks  120 ,  134 . 
     As shown in  FIG. 16 , when the second plate  112  is jointed to the third plate  114  by brazing, the respective second bridges  126 ,  136  are joined together to form oxygen-containing gas channel members. Oxygen-containing gas distribution passages  84   a  as part of oxygen-containing gas channel  84  are formed in the oxygen-containing gas channel members. The oxygen-containing gas channel  84  forms an oxygen-containing gas pressure chamber  88  between the second and third circular disks  128 ,  134 . 
     As shown in  FIG. 15 , insulating seals  144  for sealing the fuel gas supply passage  36  are provided between the separators  108 . Further, as shown in  FIG. 16 , insulating seals  146  are provided between the curved outer sections  124 ,  138 . For example, the insulating seals  144 ,  146  are made of mica material, or ceramic material. 
     As shown in  FIGS. 10 and 11 , the fuel cell stack  102  includes circular disk shaped end plates  150   a ,  150   b  provided at opposite ends of the fuel cells  100  in the stacking direction. The end plate  150   a  is insulated, and a fuel gas supply port  152  is formed at the center of the end plate  150   a . The fuel gas supply port  152  is connected to the fuel gas supply passage  36  extending through each of the fuel cells  100 . 
     Each of the end plates  150   a ,  150   b  has tightening means  151  at positions near the fuel gas supply passage  36  for applying a tightening load to the electrolyte electrode assemblies  26  and separators  108  stacked in the direction indicated by the arrow A. 
     The tightening means  151  includes two bolt insertion holes  154   a  and eight bolt insertion holes  156   a  of the end plate  150   a . The fuel gas supply port  152  (fuel gas supply passage  36 ) is positioned between the bolt insertion holes  154   a . The eight bolt insertion holes  156   a  are provided near outer positions of the curved outer sections  124 ,  138 . The bolt insertion holes  154   a  are provided in the exhaust gas channel  122  of the fuel cell stack  102 . 
     The end plate  150   b  is made of electrically conductive material. As shown in  FIG. 11 , the end plate  150   b  has a connection terminal  160 . The connection terminal  160  axially extends from the central region of the end plate  150   b . Further, the end plate  150   b  has two bolt insertion holes  154   b . The connection terminal  160  is positioned between the bolt insertion holes  154   b . The bolt insertion holes  154   a  are in alignment with the bolt insertion holes  154   b . Two bolts  162  are inserted through the bolt insertion holes  154   a ,  154   b , and tip ends of the bolts  162  are screwed into nuts  164 . The bolts  162  are electrically insulated from the end plate  150   b.    
     Further, the end plate  150   b  has eight bolt insertion holes  156   b  in alignment with the bolt insertion holes  156   a  of the end plate  150   a . Bolts  166  are inserted into the respective bolt insertion holes  156   a ,  156   b , and tip ends of the bolts  166  are screwed into nuts  168 . The bolts  166  are electrically insulated from the end plate  150   b . Heads of the bolts  166  are connected electrically to an output terminal  172   a  through conductive wires  170 , and the connection terminal  160  is electrically connected to an output terminal  172   a  through a conductive wire  174 . 
     The output terminals  172   a ,  172   b  are arranged in parallel, and are adjacent to each other. The output terminals  172   a ,  172   b  are fixed to the casing  104 . The casing  104  has an air supply port  176  positioned between the output terminals  172   a ,  172   b . Further, an exhaust gas port  178  is provided on the other end of the casing  104 . A fuel gas supply port  180  is provided adjacent to the exhaust gas port  178 . The fuel gas supply port  180  is connected to the fuel gas supply passage  36  through a reformer  182  as necessary. A heat exchanger  184  is provided around the reformer  182 . A dual structure section  186  is provided in the casing  104 , and the fuel cell stack  102  is disposed in the dual structure section  186 . 
     Operation of the fuel cell stack  102  will be described below. 
     As shown in  FIG. 12 , in assembling the fuel cell  100 , firstly, the first plate  110  and the second plate  112  are joined to both surfaces of the third plate  114  of the separator  108 , e.g., by brazing. Further, the ring shaped insulating seal  144  is provided on the first plate  110  or the third plate  114  around the fuel gas supply passage  36  by brazing (see  FIG. 15 ). Further, the curved insulating seal  146  is provided on the curved outer section  124  of the second plate  112  or the curved outer section  138  of the third plate  114  (see  FIG. 16 ). 
     In this manner, the separator  108  is fabricated. The third plate  114  divides a space between the first and second plates  110 ,  112  to form the fuel gas channel  76  and the oxygen-containing gas channel  84  (see  FIG. 17 ). Further, the fuel gas channel  76  is connected to the fuel gas supply passage  36  through the fuel gas distribution passage  76   a , and the oxygen-containing gas channel  84  is open to the outside through the slits  140 . The oxygen-containing gas is supplied through the slits  140  to the oxygen-containing gas channel  84 . 
     Then, the eight electrolyte electrode assembles  26  are sandwiched between the separators  108 . As shown in  FIG. 12 , the electrolyte electrode assemblies  26  are placed between the separators  108 , i.e., between the first circular disks  120  of one separator  108  and the second circular disks  128  of the other separator  108 . The fuel gas inlet  48  is positioned at the center in each of the anodes  24 , and the oxygen-containing gas inlet  60  is positioned at the center in each of the cathodes  22 . 
     The fuel cells  100  as assembled above are stacked in the direction indicated by the arrow A, and tightened together between the end plates  150   a ,  150   b  by tightening means  151  to form the fuel cell stack  102  (see  FIG. 10 ). As shown in  FIG. 11 , the fuel cell stack  102  is mounted in the casing  104 . 
     Then, the fuel gas is supplied into the fuel gas supply port  180  of the casing  104 , and the air is supplied into the air supply port  176  of the casing  104 . 
     The fuel gas flows through the reformer  182  as necessary, and supplied into the fuel gas supply passage  36  of the fuel cell stack  102 . The fuel gas flows in the stacking direction indicated by the arrow A, and flows through the fuel gas distribution passages  76   a  in the separator  108  of each fuel cell  100  (see  FIG. 15 ). 
     The fuel gas flows along the fuel gas distribution passage  76   a  into the fuel gas pressure chamber  86 . When the fuel gas flows through the small opening of the fuel gas inlet  48 , the internal pressure in the fuel gas pressure chamber  86  is increased. As shown in  FIG. 17 , the fuel gas from the fuel gas inlet  48  flows toward the central region of the anode  24  of the electrolyte electrode assembly  26 . The fuel gas flows from the central region of the anode  24  to the outer circumferential region of the anode  24 . 
     The oxygen-containing gas is supplied from the outer circumferential region in each of the fuel cell  100 . The oxygen-containing gas flows through the slits  140  formed in the outer circumferential region in each of the separator  108 , and is supplied to the oxygen-containing gas channel  84  (see  FIG. 16 ). The oxygen-containing gas supplied to the oxygen-containing gas channel  84  flows into the oxygen-containing gas pressure chamber  88 . When the oxygen-containing gas flows into the small opening of the oxygen-containing gas inlet  60 , the internal pressure of the oxygen-containing gas in the oxygen-containing gas pressure chamber  88  is increased. The oxygen-containing gas from the oxygen-containing gas inlet  60  flows toward the central region of the cathode  22 . The oxygen-containing gas flows from the central region of the cathode  22  to the outer circumferential region of the cathode  22  (see  FIG. 17 ). 
     Therefore, in the electrolyte electrode assembly  26 , the fuel gas is supplied from the central region to the outer circumferential region of the anode  24 , and the oxygen-containing gas is supplied from the central region to the outer circumferential region of the cathode  22  (see  FIG. 17 ). At this time, oxygen ions flow toward the anode  24  through the electrolyte  20  for generating electricity by the chemical reactions. 
     The fuel cells  100  are connected in series in the stacking direction indicated by the arrow A. As shown in  FIG. 11 , one of the poles is connected from the connection terminal  160  of the electrically conductive end plate  150   b  to the output terminal  172   b  through the conductive wire  174 . The other pole is connected from the bolts  166  to the output terminal  172   a  through the conductive wires  170 . Thus, the electric energy can be collected from the output terminals  172   a ,  172   b.    
     After the fuel gas and the oxygen-containing gas are consumed in the reactions, the fuel gas and the oxygen-containing gas flow toward the outer circumferential regions in each of the electrolyte electrode assembly  26 , and are mixed together. The mixed gas flows as an exhaust gas into the exhaust gas channel  122  extending through the separators  108 , and flows in the stacking direction. Then, the exhaust gas is discharged to the outside of the casing  104  from the exhaust gas port  178 . 
     In the second embodiment, the fuel gas supply passage  36  and the fuel gas channel  76  of the separator  108  are connected by the narrow first bridges  118 ,  132 , and the slit  140  as an oxygen-containing gas supply unit and the oxygen-containing gas channel  84  of the separator  108  are connected by the narrow second bridges  126 ,  136 . 
     Since the rigidity of the fuel gas supply unit, the rigidity of the electrode stack unit, and the rigidity of the oxygen-containing gas supply unit are separated by the bridges, the tightening loads applied to the respective positions of the separator  108  can be determined individually. With the simple and economical structure, sealing performance is improved desirably, and no excessive load is applied to the electrolyte electrode assemblies  26 . Therefore, the same advantages as with the first embodiment can be obtained. For example, the damage of the electrolyte electrode assemblies  26  is prevented effectively. 
     Further, the exhaust gas channel  122  is formed around the respective electrolyte electrode assemblies  26  in the separator  108 . Thus, the heat of the exhaust gas discharged into the exhaust gas channel  122  is utilized to warm the electrolyte electrode assemblies  26 . Thus, improvement in the thermal efficiency is achieved easily. 
       FIG. 18  is a perspective view schematically showing a fuel cell stack  202  formed by stacking a plurality of fuel cells  200  according to a third embodiment of the present invention in a direction indicated by an arrow A.  FIG. 19  is an exploded perspective view showing the fuel cell  200 . 
     The fuel cell  200  includes a plurality of, e.g., fifteen electrolyte electrode assemblies  26  between a pair of separators  208 . Each of the separators  208  includes first and second plates  210 ,  212  which are stacked together, and a third plate  214  interposed between the first and second plates  210 ,  212 . The first through third plates  210 ,  212 , and  214  are metal plates of, e.g., stainless alloy. 
     The first plate  210  has a first small diameter end portion (a fuel gas supply unit)  215 . The fuel gas supply passage  36  extends through the first small diameter end portion  215 . The first small diameter end portion  215  is integral with first circular disks  218  through a narrow first bridge  216 . The first circular disks  218  are arranged in directions perpendicular to the stacking direction indicated by the arrow A. Three first circular disks  218  are arranged in a direction indicated by an arrow B, and five first circular disks  218  are arranged in a direction indicated by an arrow C. In total, the number of the first circular disks  218  is 15. The first circular disks  218  are connected by bridges  220 . 
     In the embodiment, the first circular disks  218  at opposite ends in the direction indicated by the arrow B are connected to the first circular disk  218  provided at the central position indicated by the arrow B only by the bridges  220 . Alternatively, the adjacent first circular disks  218  may be connected with each other in the direction indicated by the arrow C by the bridges  220 . 
     Each of the first circular disks  218  has a plurality of first protrusions  44  and a substantially ring shape protrusion  46  on a surface facing the electrolyte electrode assembly  26 . A fuel gas inlet  48  is provided at the center in the surface of the first circular disk  218 . 
     The second plate  212  has a second small diameter end portion (an oxygen-containing gas supply unit)  222 . The oxygen-containing gas supply passage  50  extends through the second small diameter end portion  222 . The second small diameter end portion  222  is integral with second circular disks  226  through a narrow second bridge  224 . 
     The second circular disks  226  are connected by bridges  228 . As with the first circular disks  218 , the second circular disks  226  are arranged in directions perpendicular to the stacking direction indicated by the arrow A. Three second circular disks  226  are arranged in the direction indicated by the arrow B, and five second circular disks  226  are arranged in the direction indicated by the arrow C. In total, the number of the second circular disks  226  is 15. Each of the second circular disks  226  has a plurality of second protrusions  58  on a surface which contacts the cathode  22 . An oxygen-containing gas inlet  60  is provided at the center in the surface of the second circular disk  226 . 
     The third plate  214  has a third small diameter end portion (the fuel gas supply unit)  230  and a fourth small diameter end portion (the oxygen-containing gas supply unit)  232 . The fuel gas supply passage  36  extends through the third small diameter end portion  230 , and the oxygen-containing gas supply passage  50  extends through the fourth small diameter end portion  232 . The third circular disks  238  are connected to the third and fourth small diameter end portions  230 ,  232  through first and second bridges  234 ,  236 . 
     Three third circular disks  238  are arranged in the direction indicated by the arrow B, and five third circular disks  238  are arranged in the direction indicated by the arrow C. In total, the number of the third circular disks  238  is 15. The third circular disks  238  are connected by bridges  240 . Each of the third circular disks  238  has a plurality of third protrusions  78  on its surface facing the first plate  210 . 
     The first plate  210  is joined to the third plate  214 , e.g., by brazing to form a fuel gas channel  76  between the first plate  210  and the third plate  214 . The fuel gas channel  76  includes a fuel gas distribution passage  76   a  between the first bridges  216 ,  234 , and a fuel gas pressure chamber  86  between the first and third circular disks  218 ,  238  (see  FIG. 20 ). 
     The second plate  212  is joined to the third plate  214 , e.g., by brazing, to form an oxygen-containing gas channel  84  between the second plate  212  and the third plate  214 . The oxygen-containing gas channel  84  includes an oxygen-containing gas distribution passage  84   a  between the second bridges  224 ,  236 , and an oxygen-containing gas pressure chamber  88  between the second and third circular disks  226 ,  238  (see  FIG. 20 ). 
     As shown in  FIG. 18 , the fuel cell stack  202  includes substantially rectangular end plates  242   a ,  242   b  provided at opposite ends of the fuel cells  200  in the stacking direction. A first pipe  244  and a second pipe  246  extend through the end plate  242   a . The first pipe  244  is connected to the fuel gas supply passage  36 , and the second pipe  246  is connected to the oxygen-containing gas supply passage  50 . The end plates  242   a ,  242   b  have tightening means  247  at positions near the fuel gas supply passage  36  and the oxygen-containing gas supply passage  50  for applying a tightening load to the electrolyte electrode assemblies  26  and the separators  208  in the stacking direction. The electrolyte electrode assemblies  26  and the separators  208  are stacked in the direction indicated by the arrow A. 
     The tightening means  247  includes the bolt insertion holes  248  of the end plates  242   a ,  242   b . The fuel gas supply passage  36  and the oxygen-containing gas supply passage  50  are positioned between the bolt insertion holes  248 , respectively. The end plate  242   a  or the end plate  242   b  is electrically insulated from tightening bolts  250 . The tightening bolts  250  are inserted into the bolt insertion holes  248 , and tip ends of the tightening bolts  250  are screwed into nuts  252  to tighten the fuel cells  200  of the fuel cell stack  202  together. 
     In the third embodiment, the fuel gas supply passage  36 , the fuel gas channel  76 , the oxygen-containing gas supply passage  50  and the oxygen-containing gas channel  84  of the separator  208 , are connected by the narrow first bridges  216 ,  234  and the second bridges  224 ,  236 , respectively. 
     In the separator  208 , since the rigidity of the fuel gas supply unit, the rigidity of the electrode stack unit, and the rigidity of the oxygen-containing gas supply unit are separated by the bridges, the tightening loads applied to the respective positions of the separator  208  can be determined individually. With the simple and economical structure, sealing performance is improved desirably, and no excessive load is applied to the electrolyte electrode assemblies  26 . Therefore, the same advantages as with the first and second embodiments can be obtained. For example, the damage of the electrolyte electrode assemblies  26  is prevented effectively. 
     The invention has been particularly shown and described with reference to preferred embodiments, it will be understood that variations and modifications can be effected thereto by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.