Patent Publication Number: US-2007099063-A1

Title: Solid polymer fuel cell

Description:
FIELD OF THE INVENTION  
      This invention relates to separators of a solid polymer fuel cell and a configuration of passages provided therein.  
     BACKGROUND OF THE INVENTION  
      A solid polymer fuel cell stack comprises a plurality of fuel cells stacked together in one direction. Each fuel cell is composed of a membrane electrode assembly and separators sandwiching the membrane electrode assembly. A membrane electrode assembly comprises an electrolyte membrane with an anode and a cathode respectively arranged on either side thereof. The separator in contact with the anode has a passage for an anode gas whose main component is hydrogen, and the separator in contact with the cathode has a passage for a cathode gas containing oxygen.  
      The fuel cell generates power through electrochemical reaction through the electrolyte membrane between hydrogen supplied to the anode and oxygen supplied to the cathode. To cause this electrochemical reaction, the electrolyte membrane requires a moist condition. Thus, it is desirable to moisten the anode gas and the cathode gas beforehand. On the other hand, as a result of the electrochemical reaction, the fuel cell generates water at the cathode. This water is also re-utilized to moisten the electrolyte membrane.  
      In each fuel cell, the anode gas flows down through a groove-like passage formed in the separator so as to face the anode. The cathode gas flows down through a groove-like passage formed in the separator so as to face the cathode.  
      The fuel cell stack is provided with an anode gas supply manifold that distributes an anode gas to each anode gas passage and an anode effluent exhaust manifold that recovers anode effluent exhausted from each anode gas passage, the both manifolds extending through the fuel cells.  
      Similarly, the fuel cell stack is provided with a cathode gas supply manifold that distributes a cathode gas to each cathode gas passage and a cathode effluent exhaust manifold that recovers cathode effluent exhausted from each cathode gas passage, the both manifolds extending through the fuel cells.  
      As stated above, as a result of the electrochemical reaction between hydrogen and oxygen, water is generated at the cathode. Part of this water permeates through the cathode to moisten the electrolyte membrane, whereas the rest of the water is discharged from the cathode gas passage to the cathode effluent exhaust manifold as water vapor together with cathode effluent. The more downstream, the larger the amount of water vapor in the cathode gas passage. As a result, water vapor is condensed in the downstream portion of the cathode gas passage to become liquid water, which may hinder the circulation of the cathode gas. When the cathode gas passage has a meandering configuration with portions bent at substantially 180 degrees, the bent portions tend to be subject to flooding. Regarding the anode gas also, when moistening is performed prior to supplying it to the fuel stack, a similar flooding may occur in the anode gas passage.  
     SUMMARY OF THE INVENTION  
      Regarding the prevention of flooding, JP 2000-100458 A and JP 2000-82482 A, published in the year 2000 by the Japan Patent Office, propose forming a through-hole in the fuel cell stack that communicates between the bent portions of the gas passages of the fuel cells. The through-hole helps to attain evenness of water contained in the anode gas and the cathode gas so that the surplus moisture in the cathode gas passages and the anode gas passages may not become excessive in particular fuel cells, which is desirable from the viewpoint of preventing flooding.  
      In JP 2000-100458 A, in consideration for the arrangement of the manifolds and the through-holes, the flow of the anode gas and the flow of the cathode gas on either side of the membrane electrode assembly are made perpendicular to each other. However, to uniformize water distribution by water movement between the anode and cathode of a fuel cell through the membrane electrode assembly, it is desirable to form the anode gas passage and the cathode gas passage on either side of the membrane electrode assembly so as to be parallel to each other, and, at the same time, cause the anode gas and the cathode gas to flow in opposite directions.  
      The fuel cell disclosed in JP 2000-82482 A satisfies the above condition regarding the gas flow. However, in this fuel cell, the anode gas supply manifold and the cathode effluent exhaust manifold are arranged so as to be spaced apart from each other. Thus, the portion of the anode gas passage in the vicinity of the inlet thereof, which is mostly liable to cause water shortage, is not superimposed on the portion of the cathode gas passage in the vicinity of the outlet thereof, which is mostly liable to cause flooding. Further, the anode effluent exhaust manifold and the cathode gas supply manifold are spaced apart from each other. Thus, the portion of the anode gas passage in the vicinity of the outlet thereof, which is most subject to flooding, is not superimposed on the portion of the cathode gas passage in the vicinity of the inlet thereof, which is most subject to water shortage. In this way, in this fuel cell, the water movement between the cathode and anode cannot be effected efficiently.  
      Further, in this fuel cell, in consideration for the arrangement of the manifold and the through-holes, the through-holes are provided in the middle flow portions of the anode gas passage and the cathode gas passage. Thus, in this fuel cell construction, it is difficult to prevent flooding in the downstream portions of the passages, which are most subject to flooding.  
      It is therefore an object of this invention to uniformize the water distribution in the fuel cell to thereby achieve an improvement in terms of flooding prevention performance.  
      In order to achieve the above object, this invention provides a fuel cell stack comprising fuel cells effecting power generation upon supply of an anode gas and a cathode gas, each of the fuel cells comprising; an anode separator comprising an anode gas passage which has a meandering configuration with two or more bent portions; a cathode separator comprising a cathode gas passage which has a meandering configuration with bent portions, the number of the bent portions of the cathode gas passage being equal to the number of the bent portions of the anode gas passage, the cathode gas passage and the anode gas passage forming gas flows that are in parallel and in opposite directions to each other; and a through-hole which is provided in a most downstream bent portion in at least one of the anode gas passage and the cathode gas passage, the through-hole allowing movement of moisture through the fuel cells.  
      The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of a fuel cell system according to a first embodiment of this invention.  
       FIG. 2  is a schematic diagram of a fuel cell according to the first embodiment of this invention.  
       FIGS. 3A and 3B  are a plan view of an anode gas and cathode gas separators according to the first embodiment of this invention.  
       FIG. 4  is a diagram showing water distribution in a cathode gas passage.  
       FIG. 5  is a diagram showing water distribution in an anode gas passage.  
       FIGS. 6A and 6B  are a plan view of an anode gas and cathode gas separators according to a second embodiment of this invention.  
       FIGS. 7A and 7B  are a plan view of an anode gas and cathode gas separators according to a third embodiment of this invention.  
       FIGS. 8A and 8B  are a plan view of an anode gas and cathode gas separators according to a fourth embodiment of this invention.  
       FIGS. 9A and 9B  are a plan view of an anode gas and cathode gas separators according to a fifth embodiment of this invention.  
       FIGS. 10A and 10B  are a plan view of an anode gas and cathode gas separators according to a sixth embodiment of this invention.  
       FIGS. 11A and 11B  are a plan view of an anode gas and cathode gas separators according to a seventh embodiment of this invention.  
       FIGS. 12A and 12B  are a plan view of a cathode gas passage and LLC passage on a cathode gas separator according to an eighth embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 1  of the drawings, a construction of a fuel cell system having a fuel cell stack  1  according to a, first embodiment of this invention will be described.  
      The fuel cell system comprises a fuel cell stack  1  that generates power through electrochemical reaction between an anode gas containing hydrogen and a cathode gas containing oxygen. Further, the fuel cell system comprises an LLC circulation system  2  that circulates an LLC (long life coolant) as a coolant through the fuel cell stack  1  to thereby keep the fuel cell stack  1  at an appropriate temperature. The LLC circulation system  2  circulates, as the LLC, an antifreeze obtained by mixing ethylene glycol and water with each other. The LLC circulation system  2  comprises an LLC tank  12 , an LLC pump  13 , a temperature sensor  14 , a bypass valve  15 , and a radiator  16 .  
      The opening of the bypass valve  15  is controlled according to the output of the temperature sensor  14  to adjust the flow rate of the LLC circulating through the radiator  16 . This helps to keep the LLC at a temperature suitable for the cooling of the fuel cell stack  1 .  
      The fuel cell system further comprises an anode gas supply system  3  for supplying the anode gas to the fuel cell stack  1 , and a cathode gas supply system  4  for supplying the cathode gas thereto.  
      The anode gas supply system  3  supplies the anode gas to the fuel cell stack  1  through an anode gas supply passage  3 A, and recovers anode effluent from the fuel cell stack  1  through an anode effluent recovery passage  3 B. The anode gas supply system  3  comprises a moisture exchanger  17  that moistens the anode gas by using the water contained in the anode effluent.  
      The cathode gas supply system  4  supplies the cathode gas to the fuel cell stack  1  through a cathode gas supply passage  4 A, and recovers cathode effluent from the fuel cell stack  1  through a cathode effluent recovery passage  4 B. The cathode gas supply system  4  comprises a moisture exchanger  18  that moistens the cathode gas by using the water contained in the cathode effluent.  
      It is also possible, for example, to moisten the anode gas by using the moisture of the cathode effluent.  
      Next, the construction of each fuel cell  20  constituting the fuel cell stack  1  will be described.  
      Referring to  FIG. 2 , the fuel cell  20  comprises a membrane electrode assembly (MEA)  21  formed by an electrolyte membrane with catalyst layers, and gas diffusion layers (GDLs)  22  bonded to either side of the MEA  21 . The catalyst layers of the MEA  21  and the GDLs  22  consist of porous materials with pores. It is also possible to integrate the GDLs  22  with the MEA  21 .  
      The fuel cell  20  further comprises an anode separator  23  and a cathode separator  24  sandwiching the GDLs  22  from outside. The anode separator  23  has an anode gas passage  32  facing the anode side GDL  22 . The cathode separator  24  has a cathode gas passage  36  facing the cathode side GDL  22 . The separator  24  further has a coolant passage (an LLC passage)  27  formed on the side opposite to the cathode gas passage  36  so as to face the adjacent fuel cell  20 . It is also possible to provide the LLC passage  27  in the anode separator  23  or provide the LLC passage  27  in both the anode separator  23  and the cathode separator  24 .  
      Next, the configurations of the anode gas passage  32  and the cathode gas passage  36  will be described.  
      Referring to  FIG. 3A , the anode gas passage  32  consists of a plurality of parallel meandering grooves provided in the anode separator  23 . Referring to  FIG. 3B , the cathode gas passage  36  consists of a plurality of parallel meandering grooves provided in the cathode separator  24 . The anode gas passage  32  comprises two bent portions  511  and  512  of substantially 180 degrees. The cathode gas passage  36  comprises two bent portions  521  and  522  of substantially 180 degrees.  
      The upstream end of the anode gas passage  32  is connected to an anode gas supply manifold  31  through a distributing groove  41 . The downstream end of the anode gas passage  32  is connected to an anode effluence exhaust manifold  34  through a recovery groove  42 . The anode gas supply manifold  31  and the anode effluent exhaust manifold  34  are passages extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked. The anode gas supply manifold  31  is connected to the anode gas supply passage  3 A, and the anode effluent exhaust manifold  34  is connected to the anode effluent recovery passage  3 B.  
      The portions of the anode gas passage  32  between the distributing groove  41  and the bent portion  511 , between the bent portions  511  and  512 , and between the bent portion  512  and the recovery groove  42  are in the form of a linear groove.  
      The upstream end of the cathode gas passage  36  is connected to a cathode gas supply manifold  35  through a distributing groove  43 . The downstream end of the cathode gas passage  36  is connected to a cathode effluence exhaust manifold  38  through a recovery groove  44 . The cathode gas supply manifold  35  and the cathode effluent exhaust manifold  38  are passages extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked. The cathode gas supply manifold  35  is connected to the cathode gas supply passage  4 A, and the cathode effluent exhaust manifold  38  is connected to the cathode effluent recovery passage  4 B.  
      The portions of the anode gas passage  36  between the distributing groove  43  and the bent portion  521 , between the bent portions  521  and  522 , and between the bent portion  522  and the recovery groove  44  are in the form of a linear groove.  
      The anode gas passage  32  and the cathode gas passage  36  are of the same specifications as much as possible regarding the number of grooves, the groove intervals, the groove width, the length of the linear portions, the configuration and size of the bent portions  511  and  521 , and the configuration and size of the bent portions  512  and  522 . It should be noted, however, that it is not always necessary for the anode gas passage  32  and the cathode gas passage  36  to be of the same specifications regarding the groove width, the length of the linear portions, the configuration and size of the bent portions  511  and  521 , and the configuration and size of the bent portions  512  and  522 .  
      As can be seen from  FIG. 3A  and  FIG. 3B , the separators  23  and  24  are formed in a squares shape of the same size, and the anode gas passage  32  and the cathode gas passage  36  overlap each other in most portions in the stacking direction. Further, as shown in these drawings, the anode gas supply manifold  31  and the cathode effluent exhaust manifold  38  are formed in parallel along a first side  29  of the separators  23  and  24 , and the anode effluent exhaust manifold  34  and the cathode gas supply manifold  35  are formed in parallel along a second side  30  opposed to the first side  29  of the separators  23  and  24 . Further, the anode gas supply manifold  31  and the cathode effluent exhaust manifold  38  are formed diagonally with respect to the anode effluent exhaust manifold  34  and the cathode gas supply manifold  35 .  
      In other words, the upstream portion of the anode gas passage  32  overlaps the downstream portion of the cathode gas passage  36 , and the downstream portion of the anode gas passage  32  overlaps the upstream portion of the cathode gas passage  36 . Further, the distributing groove  41  overlaps the recovery groove  44 , and the recovery groove  42  overlaps the distributing groove  43 , so the portion of the anode gas passage  32  in the vicinity of the inlet thereof overlaps the portion of the cathode gas passage  36  in the vicinity of the outlet thereof, and the portion of the cathode gas passage  36  in the vicinity of the inlet thereof overlaps the portion of the anode gas passage  32  in the vicinity of the outlet thereof. As a result, the flow of anode gas in the anode gas passage  32  is reversed with respect to the flow of cathode gas in the cathode gas passage  36 . In the fuel cell  20 , the anode gas and the cathode gas circulate in opposite directions in the passage formed in parallel in the linear portions including the portions in the vicinity of the inlets and outlets thereof.  
      The bent portion  512  on the downstream side of the anode gas passage  32  is connected to a through-hole  332  extending through the fuel cell  20  in the stacking direction. The bent portion  522  on the downstream side of the cathode gas passage  36  is connected to a through-hole  372  extending through the fuel cell  20  in the stacking direction.  
      The through-hole  332  re-distributes the anode gas distributed to the anode gas passage  32  of each fuel cell  20 . At this time, the anode gas is re-distributed not only in a single fuel cell  20 , but throughout the anode gas passages  32  of all the fuel cells  20  stacked together. Thus, it is possible to uniformize the amount of water vapor in the anode gas passages  32  over the entire fuel cell stack  1 . Similarly, the through-hole  372  uniformizes the amount of water vapor in the cathode gas passages  36  over the entire fuel cell stack  1 .  
      It is also possible to divide the fuel cell stack  1  into a plurality of units, and to form the through-holes  332  and  372  in each unit, thereby uniformizing the amount of water vapor unit by unit.  
      As can be seen from  FIG. 3A  and  FIG. 3B , the sectional area of the through-hole  332  is larger than the sectional area of each groove forming the anode gas passage  32 , and the sectional area of the through-hole  372  is larger than the sectional area of each groove forming the cathode gas passage  36 . Thus, if condensed water remains in the through-hole  332 , the flow of anode gas is less hindered as compared with the case in which condensed water remains in the anode gas passage  32 . Similarly, if condensed water remains in the through-hole  372 , the flow of cathode gas is less hindered as compared with the case in which condensed water remains in the cathode gas passage  36 .  
      The bent portion  512 , which is situated most downstream and likely to allow condensed water to remain there, is connected to the through-hole  332 , so that the condensed water generated in the bent portion  512  remains in the through-hole  332  with a large sectional area. As a result, it is possible to remove the surplus moisture in the anode gas, thereby preventing flooding on the downstream portion of the anode gas passage  32 .  
      Similarly, The bent portion  522 , which is situated most downstream and likely to allow condensed water to remain there, is connected to the through-hole  372 , so that the condensed water generated in the bent portion  522  remains in the through-hole  372  with a large sectional area. As a result, it is possible to remove the surplus moisture in the cathode gas, thereby preventing flooding on the downstream portion of the cathode gas passage  36 .  
      Further, as can be seen from these drawings, the bent portion  512  of the anode gas passage  32  is situated in the vicinity of the bent portion  521  of the cathode gas passage  36 , and the bent portion  522  of the cathode gas passage  36  is situated in the vicinity of the bent portion  511  of the anode gas passage  32 . Further, the bent portion  512  is situated on the outer side of the bent portion  521 , and the bent portion  522  is situated on the outer side of the bent portion  511 .  
      Due to this construction, the through-hole  332  does not interfere with the bent portion  521 , and the through-hole  372  does not interfere with the bent portion  511 . Therefore, it is possible to cause the anode gas passage  32  and the cathode gas passage  36  to overlap each other in many portions, thereby moving water between the anode gas passage  32  and the cathode gas passage  36  through the MEA  21  and uniformizing the water distribution inside the fuel cell  20 .  
      The LLC passage  27  communicates with an LLC supply manifold  39  and an LLC exhaust manifold  40 . As shown in  FIG. 3A  and  FIG. 3B , both the LLC supply manifold  39  and the LLC exhaust manifold  40  extend through the fuel cell  20  at locations where they do not interfere with the anode gas passage  32  or the cathode gas passage  36 .  
      Next, the water distributing condition inside the fuel cell  20  will be described.  
      First, referring to  FIG. 4 , the water distributing condition in the cathode gas passage  36  will be described.  
      In the cathode gas passage  36 , water generated at the cathode is evaporated into the cathode gas from the pores of the cathode side catalyst layer of the MEA 21  through the cathode side GDL  22 . As a result, the relative humidity (RH) increases along the cathode gas flow, and condensed water is generated in the downstream of the cathode gas passage  36  due to saturation of water vapor.  
      Next, referring to  FIG. 5 , the water distribution in the anode gas passage  32  will be described. As stated above, the anode gas is moistened beforehand.  
      Regarding the anode gas passage  32 , Water vapor having permeated through the electrolyte membrane of the MEA  21  is mixed with the anode gas through the pores of the anode side catalyst layer of the MEA  21  through the anode side GDL  22 . On the other hands, as the electrochemical reaction proceeds, the amount of anode gas greatly decreases. As a result, in the anode gas passage  32  also, the RH of the anode gas increases along the flow, and, on the downstream side of the anode gas passage  32 , condensed water due to saturation of water vapor is likely to be generated.  
      In other words, the RH increases on the respective downstream sides of the anode gas passage  32  and the cathode gas passage  36 , and condensed water is likely to be generated. When concentrated on a particular fuel cell  20 , and further, when concentrated on a particular groove of the anode gas passage  32  or the cathode gas passage  36 , such condensed water leads to flooding.  
      In this embodiment, condensed water generated in the anode gas remains in the through-hole  332 , and liquid water in the vicinity of the outlet of the anode gas passage  32  permeates through the electrolyte membrane to move to the portion in the vicinity of the inlet of the cathode gas passage  36 , whereby it is possible to prevent flooding in the anode gas passage  32 . Further, condensed water generated in the cathode gas remains in the through-hole  372 , and liquid water in the vicinity of the outlet of the cathode gas passage  36  permeates through the electrolyte membrane to move to the portion in the vicinity of the inlet of the anode gas passage  32 , whereby it is possible to prevent flooding in the cathode gas passage  36 .  
      Further, throughout the fuel cell stack  1 , the through-hole  372  uniformizes the RH in the downstream portions of the cathode gas passages  36 , and the through-hole  332  uniformizes the RH in the downstream portions of the anode gas passages  32 . Thus, it is possible to prevent flooding due to concentration of condensed water on a particular portion.  
      It is also possible to set the fuel cell stack  1  such that the fuel cells  20  are stacked in the vertical direction, and to provide, at the lowermost ends in the vertical direction of the through-holes  332  and  372 , spaces for recovering condensed water from the through-holes  332  and  372 . This helps to prevent the condensed water collected into the through-hole  332  from re-entering the anode gas passage  32 , and to prevent the condensed water collected into the through-hole  372  from re-entering the cathode gas passage  36 .  
      Next, referring to  FIG. 6A  and  FIG. 6B , a second embodiment of this invention will be described. In the second embodiment, the anode gas passage  32  and the cathode gas passage  36  are constructed as follows.  
      Referring to  FIG. 6A , the anode gas passage  32  comprises four bent portions  511 ,  512 ,  513 , and  514 , and five linear portions. The five linear portions are provided between the anode gas supply manifold  31  and the bent portion  511 ; between the bent portions  511  and  512 ,  512  and  513 , and  513  and  514 ; and between the bent portion  514  and the anode effluent exhaust manifold  34 . Of the bent portions  511  through  514 , the bent portion  514  situated most downstream with respect to the anode gas flow is connected to a through-hole  332  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together.  
      Referring to  FIG. 6B , the cathode gas passage  36  comprises four bent portions  521 ,  522 ,  523 , and  524 , and five linear portions. The five linear portions are provided between the cathode gas supply manifold  35  and the bent portion  521 ; between the bent portions  521  and  522 ,  522  and  523 , and  523  and  524 ; and between the bent portion  524  and the cathode effluent exhaust manifold  38 . Of the bent portions  521  through  524 , the bent portion  524  situated most downstream with respect to the cathode gas flow is connected to a through-hole  372  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together.  
      The bent portions  512  and  523  overlap each other in the stacking direction, and the bent portions  513  and  522  overlap each other in the stacking direction. The bent portion  514  is situated on the outer side of the bent portion  521 , and the bent portion  524  is situated on the outer side of the bent portion  511 .  
      Due to this construction, the through-hole  334  does not interfere with the bent portion  521 , and the through-hole  374  does not interfere with the bent portion  511 , whereby it is possible to cause the anode gas passage  32  and the cathode gas passage  36  to overlap each other in most portions.  
      The configurations of the anode gas passage  32  and the cathode gas passage  36  are not restricted to those of the first embodiment and the second embodiment. It is only necessary for the anode gas passage  32  to comprise two or more bent portions  511 ,  512 , . . . including the bent portion  51   m  situated most downstream, which is connected to the through-hole  33   m . Further, it is only necessary for the cathode gas passage  36  to comprise two or more bent portions  521 ,  522 , . . . including the bent portion  52   m  situated most downstream, which is connected to the through-hole  37   n . In this case, it is possible for the anode gas passage  32  and the cathode gas passage  36  to overlap each other in many portions.  
      Next, referring to  FIG. 7A  and  FIG. 7B , a third embodiment of this invention will be described. In the third embodiment, the anode gas passage  32  and the cathode gas passage  36  are constructed as follows.  
      Referring to  FIG. 7A , the anode gas passage  32  comprises four bent portions  511 ,  512 ,  513 , and  514 , and five linear portions. Of the bent portions  511  through  514 , the bent portion  514  situated most downstream with respect to the anode gas flow is connected to a through-hole  334  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together. Further, of the bent portions  511  through  514 , an even-numbered bent portion as counted from the inlet side of the anode gas passage  32 , in this case the second bent portion  512 , is connected to a through-hole  332  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together.  
      Referring to  FIG. 7B , the cathode gas passage  36  comprises four bent portions  521 ,  522 ,  523 , and  524 , and five linear portions. Of the bent portions  511  through  514 , the bent portion  524  situated most downstream with respect to the cathode gas flow is connected to a through-hole  374  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together. Further, of the bent portions  521  through  524 , an even-numbered bent portion as counted from the inlet side of the cathode gas passage  36 , in this case the second bent portion  522 , is connected to a through-hole  372  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together.  
      As can be seen from  FIG. 7A  and  FIG. 7B , the bent portion  512  is arranged on the outer side of the bent portion  523 , the bent portion  514  is arranged on the outer side of the bent portion  521 , the bent portion  522  is arranged on the outer side of the bent portion  513 , and the bent portion  524  is arranged on the outer side of the bent portion  511 . Due to this arrangement, the through-hole  332  does not interfere with the bent portion  523 , the through-hole  334  does not interfere with the bent portion  521 , the through-hole  372  does not interfere with the bent portion  513 , and the through-hole  374  does not interfere with the bent portion  511 , whereby it is possible to cause the anode gas passage  32  and the cathode gas passage  36  to overlap each other in most portions.  
      The configurations of the anode gas passage  32  and the cathode gas passage  36  are not restricted to those described above as long as they satisfy the following conditions. The anode gas passage  32  comprises an even number of bent portions  511 ,  512 , . . . ,  51   m , which are not less than four, and through-holes  33   x  are connected to an even-numbered bent portions  51   x  as counted from the inlet side. Further, it is not necessary for all the even-numbered bent portions  51   x  as counted from the inlet side of the anode gas passage  32  to be connected to the through-holes  33   x . It is only necessary to connect, in addition to the bent portion  51   m  situated most downstream, at least one even-numbered bent portion  51   x  to the through-hole  33   x . This also applies to the cathode gas passage  36 .  
      Due to this construction, it is possible to restrain flooding in the anode gas passage  32  having a number of bent portions  511 ,  512 , . . . and the cathode gas passage  36  having a number of bent portions  521 ,  522 , . . .  
      Next, referring to  FIG. 8A  and  FIG. 8B , a fourth embodiment of this invention will be described. In the fourth embodiment, the anode gas passage  32  and the cathode gas passage  36  are constructed as follows.  
      Referring to  FIG. 8A , the anode gas passage  32  comprises three bent portions  511 ,  512 , and  513 , and four linear portions. Of the bent portions  511  through  513 , the bent portion  513  situated most downstream with respect to the anode gas flow is connected to a through-hole  333  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together.  
      Referring to  FIG. 8B , the cathode gas passage  36  comprises three bent portions  521 ,  522 , and  523 , and four linear portions. Of the bent portions  521  through  523 , the bent portion  523  situated most downstream with respect to the cathode gas flow is connected to a through-hole  373  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together.  
      The bent portions  512  and  522  overlap each other in the stacking direction. The bent portion  513  is situated on the outer side of the bent portion  521 , and the bent portion  523  is situated on the outer side of the bent portion  511 . Due to this construction, it is possible to cause the anode gas passage  32  and the cathode gas passage  36  to overlap each other in most portions.  
      Here, the anode gas supply manifold  31  and the cathode effluent exhaust manifold  38  adjacent to each other, and the anode effluent exhaust manifold  34  and the cathode gas supply manifold  35  adjacent to each other, are arranged on the same side with respect to the region where the passages are provided.  
      The configurations of the anode gas passage  32  and the cathode gas passage  36  are not restricted to those described above. It is only necessary for the anode gas passage  32  to comprise an odd number of bent portions  511 ,  512 , . . . , which are not less than three, with the bent portion  51   m  situated most downstream being connected to the through-hole  33   m . This also applies to the cathode gas passage  36 . Here, the anode gas passage  32  and the cathode gas passage  36  are of the same specifications regarding the number of grooves, the groove interval, the groove width, the length of the linear portions, and the configurations and sizes of the bent portions  511  through  51   m  and  521  through  52   m . This makes it possible for the anode separator  23  and the cathode separator  24  to be formed of plates of the same configuration.  
      Next, referring to  FIG. 9A  and  FIG. 9B , a fifth embodiment of this invention will be described. In the fifth embodiment, the anode gas passage  32  and the cathode gas passage  36  are constructed as follows.  
      Referring to  FIG. 9A , the anode gas passage  32  comprises seven bent portions  511  through  517 , and eight linear portions. Of the bent portions  511  through  517 , the bent portion  517  situated most downstream with respect to the anode gas flow is connected to a through-hole  337  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together. Further, of the bent portions  511  through  516 , the bent portion  515  situated halfway on the downstream side of the anode gas passage  32  is connected to a through-hole  335 , and the bent portion  516  situated between the bent portions  515  and  517  is connected to a through-hole  336 .  
      Referring to  FIG. 9B , the cathode gas passage  36  comprises seven bent portions  521  through  527 , and eight linear portions. Of the bent portions  521  through  527 , the bent portion  527  situated most downstream with respect to the anode gas flow is connected to a through-hole  377  extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together. Further, of the bent portions  521  through  526 , the bent portion  525  situated halfway on the downstream side of the cathode gas passage  36  is connected to a through-hole  375 , and the bent portion  526  situated between the bent portions  525  and  527  is connected to a through-hole  376 .  
      As can be seen from  FIG. 9A  and  FIG. 9B , the bent portion  515  is arranged on the outer side of the bent portion  523 , the bent portion  516  is arranged on the outer side of the bent portion  522 , and the bent portion  517  is arranged on the outer side of the bent portion  521 . The bent portion  525  is arranged on the outer side of the bent portion  513 , the bent portion  526  is arranged on the outer side of the bent portion  512 , and the bent portion  527  is arranged on the outer side of the bent portion  511 . Due to this arrangement, it is possible to cause the anode gas passage  32  and the cathode gas passage  36  to overlap each other in the stacking direction in most portions.  
      The configurations of the anode gas passage  32  and the cathode gas passage  36  are not restricted to those described above as long as they satisfy the following conditions.  
      The anode gas passage  32  comprises an odd number of bent portions  511 ,  512 , . . . , which are not less than five, and through-holes  33   z  are connected to bent portions  51   z  situated further downstream than halfway. Further, it is not necessary for all the bent portions  51   z  situated further downstream than halfway to be connected to the through-holes  33   z . It is only necessary to connect, in addition to the bent portion  51   m  situated most downstream, at least one bent portion  51   z to the through-hole  33   z . This also applies to the cathode gas passage  36 .  
      Next, referring to  FIG. 10A  and  FIG. 10B , a sixth embodiment of this invention will be described. In the sixth embodiment, the anode gas passage  32  and the cathode gas passage  36  are constructed as follows.  
      Referring to  FIG. 10A , as in the first embodiment, the anode gas passage  32  comprises two bent portions  511  and  512 , and the bent portion  512  on the downstream side is connected to a through-hole  332 . In addition, the fuel cell stack  1  comprises a drain manifold  45  which is further connected to the through-hole  332  and consists of a passage extending through the fuel cell stack  1  in the direction in which the fuel cells  20  are stacked together. The drain manifold  45  constitutes a passage that discharges condensed water remaining in the through-hole  332  to the exterior of the fuel cell stack  1 . When the fuel cell stack  1  is installed in a working environment, the drain manifold  45  is situated at the lowest part of the through-hole  332 .  
      Referring to  FIG. 10B , as in the first embodiment, the cathode gas passage  36  comprises two bent portions  521  and  522 , and the bent portion  522  on the downstream side is connected to a through-hole  372 . In addition, the fuel cell stack  1  comprises a drain manifold  46  which is further connected to the through-hole  372  and which consists of a passage extending through the fuel cell stack  1  in the direction in which the fuel cells.  20  are stacked together. The drain manifold  46  constitutes a passage that discharges condensed water remaining in the through-hole  372  to the exterior of the fuel cell stack  1 . When the fuel cell stack  1  is installed in a working environment, the drain manifold  46  is situated at the lowest part of the through-hole  372 .  
      When the fuel cell stack  1  is not operating, a purge gas is selectively introduced into the drain manifold  45 , whereby the water remaining in the through-hole  332  is discharged from the fuel cell stack  1 . Further, air is selectively introduced into the drain manifold  46 , whereby the water remaining in the through-hole  372  is discharged from the fuel cell stack  1 . It is also possible, during operation, to cause anode gas to selectively flow through the drain manifold  45 , and to cause cathode gas to selectively flow through the drain manifold  46 , thereby discharging remaining water.  
      Due to this construction, it is possible to prevent condensed water from re-entering the anode gas passage  32  from the through-hole  332 , or prevent condensed water from re-entering the cathode gas passage  36  from the through-hole  372 , thereby preventing flooding.  
      Next, referring to  FIG. 11A  and  FIG. 11B , a seventh embodiment of this invention will be described. In the seventh embodiment, the anode gas passage  32  and the cathode gas passage  36  are constructed as follows.  
      Referring to  FIG. 11A , as in the first embodiment, the anode gas passage  32  comprises two bent portions  511  and  512 , with the bent portion  512  on the downstream side being connected to the through-hole  332 . The anode gas passage  32  are directly connected to the anode gas supply manifold  31 , without any intermediation of the distributing groove  41 , and are directly connected to the anode effluent exhaust manifold  34 , without any intermediation of the recovery groove  42 . In the vicinity of the portion connected to the anode gas supply manifold  31 , the anode gas passage  32  comprises a right-angle portion  531  that changes the flowing direction of the anode gas by 90 degrees. Further, in the vicinity of the portion connected to the anode effluent exhaust manifold  34 , the anode gas passage  32  comprises a right-angle portion  532  that changes the flowing direction of the anode gas by 90 degrees.  
      The anode gas supply manifold  31  is arranged at a position attained by rotating the cathode effluent manifold  38  by 90 degrees around the right-angle portion  531 , and the anode effluent exhaust manifold  34  is arranged at a position attained by rotating the cathode gas supply manifold  35  by 90 degrees around the right-angle portion  532 . Further, as can be seen from  FIG. 11A , the entire width of the anode gas passage  32  is substantially equal to the width of the anode gas supply manifold  31  and the width of the anode effluent exhaust manifold  34 . Thus, it is possible to diminish the pressure loss when anode gas flows from the anode gas supply manifold  31  to the anode gas passage  32 , and the pressure loss when anode gas flows from the anode gas passage  32  to the anode effluent exhaust manifold  34 .  
      Further, the anode gas changes its flowing direction from the horizontal to the vertical direction at the right-angle portion  532  of the anode gas passage  32 . The right-angle portion  532  provided in the downstream region of the anode gas passage  32  is subject to generation of condensed water. However, by changing the flowing direction of the anode gas to the vertical direction, condensed water is more easily discharged into the anode effluent exhaust manifold  34 .  
      Further, referring to  FIG. 11B , as in the first embodiment, the cathode gas flow passage  36  comprises two bent portions  521  and  522 , with the bent potion  522  on the downstream side being connected to the through-hole  372 . The entire width of the cathode gas passage  36  is substantially equal to the width of the cathode gas supply manifold  35  and the width of the cathode effluent exhaust manifold  38 . Accordingly, it is possible to diminish the pressure loss when cathode gas is supplied from the cathode gas supply manifold  35  to the cathode gas passage  36 , and the pressure loss when cathode gas is discharged from the cathode gas passage  36  to the cathode effluent exhaust manifold  38 .  
      Instead of the anode gas passage  32 , the cathode gas passage  36  may comprise the right-angle portions  531  and  532 .  
      Next, referring to  FIG. 12A  and  FIG. 12B , an eighth embodiment of this invention will be described. In the eighth embodiment, the cathode gas passage  36 , anode gas passage  32  and LLC passage  27  are constructed as follows.  
      Referring to  FIG. 12A , the cathode gas passage  36  is constructed in the same manner as in the first embodiment. The anode gas passage  32  is formed in the same configuration as in the first embodiment. It should be noted, however, that the width of the cathode gas supply manifold  35  and the width of the cathode effluent exhaust manifold  38  are smaller than those in the first embodiment. Further, the width of the anode gas supply manifold  31  and the width of the anode effluent exhaust manifold  34  are smaller than those in the first embodiment.  
      An LLC supply manifold  39  is aligned with the anode effluent exhaust manifold  34  and the cathode gas supply manifold  35 , and an LLC exhaust manifold  40  is aligned with the anode gas supply manifold  31  and the cathode effluent exhaust manifold  38 .  
      Referring to  FIG. 12B , the LLC passage  27  is of the same configuration as the cathode gas passage  36 . In other words, the LLC passage  27  is formed as meandering grooves comprising two bent portions substantially at 180 degrees. The LLC passage  27  overlap the anode gas passage  32  and the cathode gas passage  36  in many portions with respect to the stacking direction. It should be noted, however, that the bent portions of the LLC passage  27  are not connected to the through-hole, but are formed by grooves provided in the surface of the cathode separator  24 .  
      The LLC circulating through the LLC circulation system  2  is supplied to the LLC supply manifold  39 , and is distributed to the fuel cells  20 . In each fuel cell  20 , LLC is distributed to the grooves of the LLC passage  27  through the distributing groove  49 . The LLC undergoes heat exchange with the fuel cell  20  in the LLC passage  27 , and is recovered through the recovery groove  50  to the LLC exhaust manifold  40  before being discharged from the fuel cell stack  1 .  
      At this time, the LLC flows in the LLC passage  27  in a direction opposite to the flowing direction of the anode gas, and in the same direction as the cathode gas. As a result of the heat exchange between the LLC and the fuel cell  20 , the temperature of the LLC gradually increases as it flows downstream through the LLC passage  27 , so the temperature of the cathode gas gradually increases as it flows downstream through the cathode gas passage  36 . As a result, in the cathode gas passage  36 , the amount of water that can be contained by the cathode gas is larger in the downstream portion, thereby making it possible to restrain flooding.  
      The contents of Tokugan 2003-384039 with filing data of Nov. 13, 2003 in Japan are hereby incorporated by reference.  
      Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variation of the embodiments described above will occur to those skilled in the air, within the scope of the claims.  
      While in  FIG. 3  and  FIGS. 6A through 12B  the number of grooves forming the anode gas passage  32  and the cathode gas passage  36  ranges from 2 to 5, it is also possible for the passages to be formed by a larger number of grooves.  
      Further, while in  FIG. 3  and  FIGS. 6A through 12B  both the anode gas passage  32  and the cathode gas passage  36  comprise through-holes, it is also possible for such a through-hole to be provided solely for one category of passage.  
     INDUSTRIAL FIELD OF APPLICATION  
      This invention, which relates to water management in a fuel cell system, provides a particularly desirable effect when applied to a vehicle-mounted fuel cell system, which is subject to great fluctuations in load.  
      The embodiment of this invention in which an exclusive property or privilege is claims are defined as follow: