Abstract:
The present invention is intended to enable efficient assembly of a fuel cell stack without causing any damage to the conductive separator. In a fuel cell including an anode, a cathode, an electrolyte membrane interposed between the anode and the cathode, and conductive separators each having manifold apertures  12  and a flow channel  16  for supplying a gas to the anode or the cathode, manifold aperture connecting portions  15  are formed at an inlet-side end and an outlet-side end of the flow channel  16 , respectively. Each of the manifold aperture connecting portions  15  is recessed below the upper surface of the conductive separator  11 . Cover plates are fitted and fixed to the recessed portions, respectively.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a fuel cell including conductive separators having a gas flow channel for supplying a gas to an electrode and manifold apertures connecting to the gas flow channel.  
       BACKGROUND OF THE INVENTION  
       [0002]     A polymer electrolyte fuel cell generates electricity and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. Typically, the polymer electrolyte fuel cell comprises a proton conductive polymer electrolyte membrane, electrodes and conductive separators. The electrodes each comprise a combination of a diffusion layer and a catalyst reaction layer. The catalyst reaction layer, which is composed mainly of a carbon powder carrying a platinum group metal catalyst thereon, is formed on each surface of the polymer electrolyte membrane that selectively transports hydrogen ions. On the outer surface of the catalyst reaction layer is formed the diffusion layer having electron conductivity and gas permeability for fuel gas or oxidant gas.  
         [0003]     In order to prevent supplied fuel gas and oxidant gas from leaking out and to prevent them from mixing with each other, gas sealants or gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane sandwiched therebetween. The gas sealants or gaskets are combined in advance with the electrodes and the polymer electrolyte membrane. This is called as an MEA (membrane electrode assembly).  
         [0004]     On the outer surfaces of the MEA are arranged the conductive separators for mechanically fixing the MEA and electrochemically connecting adjacent MEAs in series. On the surface of each conductive separator facing the MEA is formed a gas flow channel for supplying a fuel gas or oxidant gas to an electrode and exhausting a produced gas or excess gas. The gas flow channel may be formed by providing a separate member to the separator. Usually, a groove or rib is formed on the surface of the separator by means of cutting or pressing the separator surface to serve as a gas flow channel.  
         [0005]     In order to supply a fuel gas or oxidant gas to the groove, a pipe for supplying a fuel gas or oxidant gas must be branched into individual separators. Further, a plumbing jig is necessary in which the branched pipes are connected to the grooves on the separators. Such a jig is called as an “external manifolds”. The conduit through which the fuel gas or oxidant gas passes is called as an “manifold aperture”.  
         [0006]     There is another type of manifold called “internal manifold”, which has a simpler structure. In an internal manifold type, a through hole is formed through a cell stack comprising an MEA and separators each having a gas flow channel, in the stacking direction thereof, and is used as a manifold aperture. Around each through hole in the MEA and the separators, a rib is formed or an O-ring is arranged therearound, to provide sufficient sealing. The manifold aperture is connected to a pipe for supplying a fuel gas or oxidant gas so that a fuel gas or oxidant gas is supplied directly from the aperture.  
         [0007]     Since a fuel cell generates heat during its operation, it should be cooled down with cooling water or the like to maintain the cell at an appropriate temperature. Normally, a cooling unit for flowing cooling water is provided between the separators for every 1 to 3 unit cells. The cooling unit is usually a cooling water flow channel formed on the back surface of the separator.  
         [0008]     In a typical fuel cell, 100 to 200 unit cells, each comprising an MEA, conductive separators and a cooling unit, are stacked alternately to form a cell stack, which is fixed with bolts from both ends.  
         [0009]     In the internal manifold type stack, the gas sealants or gaskets disposed sandwiching the polymer electrolyte membrane may sometimes enter the ends of the gas flow channel, which are connected to the manifold apertures, respectively. As a consequence, a gap is created between the separators and the MEA, through which the gas leaks. In order to prevent the gas sealants or gaskets from entering the ends of the gas flow channel, for example, Japanese Laid-Open Patent Publication No. Hei 9-35726 proposes to form a tunnel structure by covering each end of gas flow channel, which is connected to corresponding manifold aperture, with a flat cover plate.  
         [0010]     In the above separator structure, the ends of gas flow channel are covered with cover plates, respectively, but the cover plates are not fixed. This creates loss of time in the production of a cell stack. For example, it will be difficult to assemble a cell stack with the surface of the separator having a gas flow channel formed thereon facing downward.  
         [0011]     In order to cope with this problem, attempts were made to fix the separator and the cover plate with an adhesive. However, the use of an adhesive may cause displacement beyond acceptable dimensional tolerance. Moreover, there is a concern that the plasticizer contained in the adhesive and unreacted lower molecular components might leach out, causing damage to the MEA.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     In view of the above, the present invention has been made to efficiently assemble a fuel cell stack without causing any damage to the MEA.  
         [0013]     More specifically, the present invention relates to a fuel cell comprising: an anode; a cathode; an electrolyte membrane interposed between the anode and the cathode; an anode-side conductive separator having manifold apertures and a flow channel for supplying a fuel to the anode; and a cathode-side conductive separator having manifold apertures and a flow channel for supplying an oxidant to the cathode, wherein, manifold aperture connecting portions formed at an inlet-side end and an outlet-side end of the flow channel, respectively, in each of the conductive separators are recessed below the upper surface of the separator, and cover plates for covering each end of the flow channel are fitted and fixed to the recessed portions, respectively.  
         [0014]     The cover plates preferably have a shape capable of fitting to the recessed portion while sandwiching the manifold aperture connecting portion. Alternatively, a rubber material may be disposed between a side face of the cover plate and a wall of the recessed portion that faces the side face so as to fix the cover plate to the recessed portion.  
         [0015]     The cover plates may comprise a magnetic material so that the cover plates can be fixed to the manifold aperture connecting portions by the magnetic force. Further, a minute through hole may be formed on the surface of each manifold aperture connecting portion to be in contact with the lower side of the cover plate so as to fix the cover plate to the manifold aperture connecting portion by a suction force created by suction through the through hole.  
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0016]      FIG. 1  is a schematic perspective view of a conductive separator according to an example of Embodiment 1 in the present invention.  
         [0017]      FIG. 2  is a schematic sectional view of the conductive separator taken on line II-II in  FIG. 1 .  
         [0018]      FIG. 3  is a schematic sectional view of a conductive separator according to an example of Embodiment 2 in the present invention viewed from a manifold aperture-side.  
         [0019]      FIG. 4  is a schematic sectional view of a conductive separator according to an example of Embodiment 3 in the present invention viewed from a manifold aperture-side.  
         [0020]      FIG. 5  is a schematic sectional view of a conductive separator according to an example of Embodiment 4 in the present invention viewed from a manifold aperture-side. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     Embodiment 1  
       [0021]      FIG. 1  is a schematic perspective view of a conductive separator according to an example of Embodiment 1 in the present invention. In the drawing, a fuel gas flow channel is formed on the surface.  
         [0022]     As can be seen in  FIG. 1 , a conductive separator  11  comprises: a base plate  18  having a fuel gas flow channel  16   a , fuel gas manifold apertures  12   a , oxidant gas manifold apertures  12   b  and cooling water manifold apertures  19 ; and cover plates  13 . Each of the cover plates  13  serves to cover each end of the fuel gas flow channel  16   a  that communicates with each fuel gas manifold aperture  12   a.    
         [0023]     The fuel gas flow channel  16   a  for supplying a fuel gas and exhausting a produced gas is formed on the surface of the conductive separator  11  to be in contact with an MEA. The both ends of the fuel gas flow channel  16   a  are located diagonally opposite to each other on the base plate  18 . Each of the fuel gas manifold apertures  12   a  is formed such that it adjoins to each end of the fuel gas flow channel  16   a.    
         [0024]     At the ends of the fuel gas flow channel  16   a  are formed manifold aperture connecting portions  15 , respectively, and the manifold aperture connecting portions  15  are recessed below the upper surface of the base plate  18  having the fuel gas flow channel  16   a  formed thereon such that each cover plate  13  is fitted to each recessed portion.  
         [0025]     The cover plates  13  are rectangularly concave-shaped, and each comprises a pair of supporting members facing an upper surface and a bottom surface of the manifold aperture connecting portion  15 , respectively, and the cover plate  13  is fitted and fixed to the manifold aperture connecting portion  15  by sandwiching the manifold aperture connecting portion  15  with the supporting members. In other words, the cover plate  13  is fixed directly to the base plate  18  without the use of an adhesive. The upper surface of the cover plate  13  and the surface of the base plate  18  are flush with each other.  
         [0026]     On the other surface of the conductive separator  11 , i.e., the surface of the conductive separator  11  opposite to the surface thereof having the fuel gas flow channel  16   a  formed thereon, are formed grooves which serve as a fuel gas flow channel for supplying a fuel gas or a cooling water flow channel for flowing cooling water.  
         [0027]      FIG. 2  is a schematic sectional view of the conductive separator of  FIG. 1  taken on line II-II.  
         [0028]     As shown in  FIG. 2 , the cross section of the cover plate  13  is rectangularly concave-shaped, and comprises a pair of supporting members  21  facing an upper surface and a bottom surface of the manifold aperture connecting portion and a wall member  22  for connecting the ends of the pair of supporting members  21 . In the wall member  22  is formed a gas flow through hole  14  that allows a gas flow groove  17  and the fuel gas manifold aperture  12   a  to communicate with each other. A fuel gas can be supplied or exhausted through the gas flow through hole  14 .  
         [0029]     Although, in this embodiment, description has been given for the case where the cover plate  13  has a rectangularly concave-shaped cross section, the shape of the cover plate  13  is not specifically limited as long as the cover plate  13  has a shape capable of sandwiching the manifold aperture connecting portion  15 . The shape of the cover plate  13  can vary according to the shape of the manifold aperture connecting portion  15 . For example, the cover plate  13  can have a U-shaped or V-shaped cross section. Further, the cover plate  13  can be fixed by engaging a groove formed in the manifold aperture connecting portion  15  with a rib formed on the cover plate  13 .  
       Embodiment 2  
       [0030]      FIG. 3  is a schematic sectional view of a conductive separator according to an example of Embodiment 2 in the present invention viewed from a manifold aperture-side.  
         [0031]     As can be seen in  FIG. 3 , a cover plate  33  is disposed on the upper surface of the manifold aperture connecting portion  15  (i.e., the upper surface of ribs). The cover plate  33  may be a flat plate capable of covering at least the end of the gas channel, or an L-shaped plate capable of covering the end of the gas channel as well as the end face of the manifold aperture connecting portion  15 . In the case of the cover plate  33  being L-shaped, a gas flow through hole should be formed in a part of the L-shaped cover plate to be in contact with the end face of the manifold aperture connecting portion  15 , so as to allow the gas flow grooves  17  and the fuel gas manifold aperture  12   a  to communicate with each other.  
         [0032]     On the surface of the manifold aperture connecting portion  15  to be in contact with the underside of the cover plate  33  is formed a minute through hole  38 . Accordingly, the cover plate  33  can be fixed to the manifold aperture connecting portion  15  by a suction force created by suction from an end of the through hole  38 . Further, by clogging the through hole  38  after the suction, the cover plate  33  stays fixed to the manifold aperture connecting portion  15  even after the suction process.  
         [0033]     Although, in this embodiment, description has been given for the case where the cover plate  33  is fixed to the base plate  18  by the suction, the cover plate  33  may be equipped with a pin that fits the through hole  38 . By inserting the pin into the through hole  38 , the cover plate  33  can be fixed to the base plate  18 . Alternatively, an appropriate amount of an adhesive may be injected into the through hole  38  to fix the cover plate  33  with the base plate  18 .  
       Embodiment 3  
       [0034]      FIG. 4  is a schematic sectional view of a conductive separator according to an example of Embodiment 3 in the present invention viewed from a manifold aperture-side.  
         [0035]     A cover plate  43  shown in  FIG. 4  is made of a magnetic material and fixed to the manifold aperture connecting portion  15  by the magnetic force. In this embodiment also, the cover plate  43  may be a flat plate capable of covering at least the end of the gas channel, or an L-shaped plate capable of covering the end of the gas channel as well as the end face of the manifold aperture connecting portion  15 . Alternatively, the cover plate  43  may have a rectangularly concave-shaped cross section as described in Embodiment 1. In the case of the cover plate  43  being L-shaped or rectangularly concave-shaped, a gas flow through hole should be formed so as to allow the gas flow grooves  17  and the fuel gas manifold aperture  12   a  to communicate with each other.  
         [0036]     In the case where the manifold aperture connecting portion  15  is made of a magnetic material or the base plate  18  comprises a magnetized substance, by just placing the cover plate made of a magnetic material on the manifold aperture connecting portion  15 , the cover plate can be fixed to the manifold aperture connecting portion  15 . In the case where the manifold aperture connecting portion  15  is not made of a magnetic material, a magnet may be disposed opposite to the cover plate  43  with the manifold aperture connecting portion  15  therebetween so as to fix the cover plate to the manifold aperture connecting portion  15 . More specifically, as illustrated in  FIG. 4 , a magnet  48  is disposed on the underside of the base plate  18 , i.e., on the surface of the base plate  18  opposite to the surface thereof where the cover plate  43  is disposed, whereby the cover plate  43  can be fixed to the base plate  18 . During the assembly process of a cell stack, the magnet  48  can be detached after the separator is stacked. According to this manner, the magnet  48  does not affect the size and shape of the cell stack.  
       Embodiment 4  
       [0037]      FIG. 5  is a schematic sectional view of a conductive separator according to an example of Embodiment 4 in the present invention viewed from a manifold aperture-side.  
         [0038]     As can be seen in  FIG. 5 , a cover plate  53  has sealing rubbers  58  attached to the side faces thereof to be in contact with the walls of the base plate  18 . The cover plate  53  may be a flat plate capable of covering at least the end of the gas channel, or an L-shaped plate capable of covering the end of the gas channel as well as the end face of the manifold aperture connecting portion  15 . Alternatively, the cover plate  53  may have a rectangularly concave-shaped cross section as described in Embodiment 1. In the case of the cover plate  53  being L-shaped or rectangularly concave-shaped, a gas flow through hole should be formed so as to allow the gas flow grooves  17  and the fuel gas manifold aperture  12   a  to communicate with each other.  
         [0039]     The sealing rubber  58  may be provided to the entire side face of the cover plate  53  to be in contact with the base plate  18 , or to a part of the side face of the same. Instead of the sealing rubber  58 , a material having a high friction coefficient may be attached. Further, the side faces of the cover plate  53  may be treated or processed to increase the friction coefficient.  
         [0040]     The present invention will be described below in more detail with reference to examples, but it is to be understood that the present invention is not limited thereto.  
       EXAMPLES  
       [0000]     (1) Production of Conductive Separator  
         [0041]     Fifty wt % of artificial graphite powder having an average particle size of about 10 μm and 38 wt % of fibrous graphite having an average diameter of 50 μm and an average length of 0.5 mm were baked at about 500° C. in air for 10 minutes. The heat-treated artificial graphite powder and fibrous graphite were kneaded with 12 wt % of thermosetting phenol resin using an extrusion kneader. The resulting kneaded powder was injected into a metal mold having machined to have grooves for gas flow channel/cooling water flow channel and manifold apertures, which was then hot-pressed under a pressure of 100 kg/cm 2  for 10 minutes with a metal mold temperature of 150° C.  
         [0042]     A conductive separator thus obtained had a shape as shown in  FIG. 1  with an outer dimension of 18 cm×20 cm and a thickness of 1.3 mm. The gas flow channel/cooling water flow channel had a depth of 0.5 mm.  
       Example 1  
       [0043]     The cover plates  13  in accordance with Embodiment 1 were fitted to the manifold aperture connecting portions  15 , respectively. Thereby, conductive separators  11  were formed. On the cathode side of an MEA sheet was disposed a conductive separator having an oxidant gas flow channel for supplying an oxidant gas to the cathode formed thereon. Likewise, on the anode side of the MEA sheet was disposed another conductive separator having a fuel gas flow channel for supplying a fuel gas to the anode formed thereon. Thereby, a unit cell was constructed. Subsequently, a cell stack was formed by stacking 120 unit cells, which was then clamped with clamping bolts from both ends to produce a fuel cell stack.  
         [0044]     Because, in the fuel cell stack thus produced, the cover plates were fitted and fixed to the base plate, the production time was dramatically shortened compared to the case of using conventional conductive separators. In the conventional conductive separators, the cover plates were merely placed on the upper portions of the manifold aperture connecting portions, in short, the cover plates were not fixed, creating loss of time in the production of a cell stack. The construction as described above reduced the production time by about 15% compared to the production time using conventional conductive separators.  
       Example 2  
       [0045]     The cover plates  33  in accordance with Embodiment 2 were disposed on the upper surfaces of the manifold aperture connecting portions, respectively. Then, the cover plates  33  were fixed by suction from an end of the through hole  38 , after which the through hole  38  was clogged. Thereby, conductive separators were formed. On the cathode side of an MEA sheet was disposed a conductive separator having an oxidant gas flow channel for supplying an oxidant gas to the cathode formed thereon. Likewise, on the anode side of the MEA sheet was disposed another conductive separator having a fuel gas flow channel for supplying a fuel gas to the anode formed thereon. Thereby, a unit cell was constructed. Subsequently, a cell stack was formed by stacking 120 unit cells, which was then clamped with clamping bolts from both ends to produce a fuel cell stack.  
         [0046]     Because, in the fuel cell stack thus produced, the cover plates was fixed to the base plate by the suction force, the production time was dramatically shortened. In the conventional conductive separators, the cover plates were merely placed on the upper portions of the manifold aperture connecting portions, in short, the cover plates were not fixed, creating loss of time in the production of a cell stack. The construction as described above reduced the production time by about 12% compared to the production time using conventional conductive separators.  
       Example 3  
       [0047]     The cover plates  43  in accordance with Embodiment 3 were disposed on the upper surfaces of the manifold aperture connecting portions, respectively. Each of the cover plates  33  was fixed using a magnet. Thereby, conductive separators were formed. On the cathode side of an MEA sheet was disposed a conductive separator having an oxidant gas flow channel for supplying an oxidant gas to the cathode formed thereon. Likewise, on the anode side of the MEA sheet was disposed another conductive separator having a fuel gas flow channel for supplying a fuel gas to the anode formed thereon. Thereby, a unit cell was constructed. Subsequently, a cell stack was formed by stacking 120 unit cells. During the stacking process of the unit cells, the magnets were detached one by one. Then, the obtained cell stack was clamped with clamping bolts from both ends to produce a fuel cell stack.  
         [0048]     Because, in the fuel cell stack thus produced, the cover plates were fixed to the base plate by the magnetic force during the assembly process of the cell stack, the production time was dramatically shortened. In the conventional conductive separators, the cover plates were merely placed on the upper portions of the manifold aperture connecting portions, in short, the cover plates were not fixed, creating loss of time in the production of a cell stack. The construction as described above reduced the production time by about 13% compared to the production time using conventional conductive separators.  
       Example 4  
       [0049]     The cover plates  53  in accordance with Embodiment 4 having rubber materials were fitted to the manifold aperture connecting portions, respectively. Thereby, conductive separators were formed. On the cathode side of an MEA sheet was disposed a conductive separator having an oxidant gas flow channel for supplying an oxidant gas to the cathode formed thereon. Likewise, on the anode side of the MEA sheet was disposed another conductive separator having a fuel gas flow channel for supplying a fuel gas to the anode formed thereon. Thereby, a unit cell was constructed. Subsequently, a cell stack was formed by stacking 120 unit cells, which was then clamped with clamping bolts from both ends to produce a fuel cell stack.  
         [0050]     Because, in the fuel cell stack thus produced, the cover plates were fixed to the base plate by the rubber material, the production time was dramatically shortened. In the conventional conductive separators, the cover plates were merely placed on the upper portions of the manifold aperture connecting portions, in short, the cover plates were not fixed, creating loss of time in the production of a cell stack. The construction as described above reduced the production time by about 12% compared to the production time using conventional conductive separators.  
         [0051]     As described above, according to the present invention, the cover plates are fixed to the manifold aperture connecting portions of the base plate by fitting the cover plates to the manifold aperture connecting portions, respectively, so that the easy handling of the conductive separator is achieved and the time for assembling the fuel cell stack can be shortened.