Abstract:
A cell unit for use in fuel cells of the type using an electrode portion having an anode formed of a plate or film on one surface of an electrolyte and a cathode formed of a plate or film on the other surface. An anode side plate of the invention has stepped portions to form recesses which accommodate the electrode and provide an anode side chamber facing the anode. A cathode side plate having a cathode side chamber facing the cathode is also accommodated in the recessed portions of the anode side plate. A fuel supply manifold and a fuel discharge manifold can extend through the stepped portions. Use of such cell units reduces the number of seal members required between fuel or oxidant manifolds when cell units are stacked to form a fuel cell stack. Such reduction in seal members simplifies fabrication of the units and renders the cell unit easier to maintain and repair.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the structure of cell units for use in fuel cells. 
     BACKGROUND OF THE INVENTION 
     As shown in FIG. 7, cell units for use in fuel cells generally comprise a cell  2  having an electrode portion  24  which is composed of an anode  21  formed on one surface of an electrolyte  20  in the form of a plate or film and a cathode  22  formed on the other surface of the electrolyte  20 , an anode side plate  3  having an anode side chamber  30  facing the anode for passing hydrogen gas or like fuel therethrough, and a cathode side plate  4  having cathode side chambers  40  facing the cathode for passing air or like oxidant therethrough. A multiplicity of cell units  10  of the structure described are arranged in layers for use in the fuel cell. 
     The fuel cell has fuel manifolds for supplying the fuel from outside uniformly to the anode side chambers  30  of the respective anode side plates  3  and discharging the fuel flowing through the anode side chambers  30  to the outside. The fuel manifolds are formed inside the cell units  10 , or alternatively externally of the cell units. Similarly, the fuel cell has oxidant manifolds for supplying the oxidant from outside uniformly to the cathode side chambers  40  of the respective cathode side plates  4  and discharging the oxidant flowing through the cathode side chambers  40  to the outside. The oxidant manifolds are formed inside the cell units  10 , or alternatively externally of the cell units. 
     FIG. 7 shows fuel manifolds formed inside the cell unit  10 . More specifically, FIG. 7 shows fuel supply manifolds  31 ,  90  extending through the anode side plate  3  and the cathode side plate  4 , respectively, and communicating with the anode side chamber  30  of the anode side plate  3  for supplying the fuel to the anode side chamber  30 , and fuel discharge manifolds  32 ,  91  extending through the anode side plate  3  and the cathode side plate  4 , respectively, and communicating with the anode side chamber  30  of the anode side plate  3  for discharging the fuel passing through the anode side chamber  30 . 
     As shown in FIG. 7, a seal member  5  for preventing the fuel from leaking is disposed between the anode side plate  3  and the cathode side plate  4 . 
     Provision of seal members requires many steps of work for forming seal grooves and installing the seal members and is therefore costly, so that it is desired to minimize the number of locations where the seal member needs to be provided. 
     To diminish the number of seal members  5  to be installed for the fuel manifolds, the anode side plate  3  of one cell unit  10  and the cathode side plate  4  of another cell unit  10  adjacent thereto are integrally made into a bipolar plate  8  as shown in FIG.  8 . Since this structure requires no seal member  5  between the anode side plate  3  and the cathode side plate  4  of the adjacent cell units  10 , the number of seal members  5  needed for the fuel manifolds can be reduced. 
     However, in fabricating a fuel cell with use of bipolar plates  8 , it is necessary to arrange cells  2  and bipolar plates  8  alternately in layers. At this time, the cell  2  adheres to the bipolar plate  8  adjacent thereto. Accordingly, when there arises a need to replace the cell  2 , the cell  2  must be peeled off the bipolar plate  8  and is therefore difficult to repair or maintain. Moreover, the cell substituted for the removed cell is likely to alter the performance depending on the state of the cell as attached to the bipolar plate  8 , failing to assure the fuel cell of stabilized performance in its entirety. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a cell unit for use in fuel cells which is reduced in the number of seal members required for fuel manifolds or oxidant manifolds to simplify the fabrication process of the unit and to render the cell unit easy to maintain or repair. 
     To fulfill the above object, the present invention provides a cell unit for use in fuel cells which comprises an anode side plate having a pair of first and second stepped portions approximately parallel to each other and projecting from one surface of a generally rectangular base plate, the stepped portions and the base plate surface defining a recessed portion, the base plate surface of the recessed portion being recessed to provide an anode side chamber; a cell including an electrode portion having an anode formed on one surface of an electrolyte in the form of a plate or film and a cathode formed on the other surface of the electrolyte, the cell being accommodated in the recessed portion of the anode side plate with the anode facing the anode side chamber of the anode side plate; and a cathode side plate having a cathode side chamber formed in one surface of a generally rectangular base plate, the cathode side plate being accommodated in the recessed portion of the anode side plate with the cathode side chamber facing the cathode of the cell. The stepped portions of the anode side plate have respective top faces substantially flush with the other surface of the cathode side plate, with the cell and the cathode side plate accommodated in the recessed portion of the anode side plate. 
     Alternatively, the anode side plate can be formed with a stepped portion on one surface of its base plate along the entire outer periphery thereof. 
     A fuel supply manifold for passing a fuel therethrough and a fuel discharge manifold for passing therethrough the fuel to be discharged can be made to extend through the respective stepped portions and the base plate, with the supply manifold held in communication with the anode side chamber and with the anode side chamber held in communication with the discharge manifold by respective internal manifolds. 
     A stepped portion or stepped portions can be formed on the cathode side plate instead of the stepped portion or portions formed on the anode side plate. 
     A bipolar plate is usable in the cell unit of the invention for use in fuel cells, the bipolar plate comprising a generally rectangular base plate and being provided with a stepped portion on each of opposite surfaces of the base plate to form an anode side chamber in one of the surfaces and a cathode side chamber in the other surface. 
     The cell unit of the present invention can be smaller in the number of seal members required than in conventional cell units since the seal members conventionally used between the anode side plate and the cathode side plate can be dispensed with. 
     With the cell unit of the invention, the cell is sandwiched between the anode side plate and the cathode side plate, so that the components of fuel cells can be fabricated in the form of cell units. The fuel cell can therefore be checked for performance or maintained from unit to unit. 
     Since the fuel cell is fabricated by arranging a plurality of cell units of the invention into an assembly of layers, a malfunctioning cell unit, if found, can be removed singly after use for replacement. This results in facilitated repair or maintenance, assuring the fuel cell of more stabilized performance. 
     When comprising the bipolar plate of the invention, the cell unit also has these advantages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded sectional view showing an embodiment of cell unit of the invention for use in common fuel cells; 
     FIG. 2 is an exploded perspective view showing an embodiment of cell unit of the invention for use in solid polymer electrolyte fuel cells; 
     FIG. 3 is an exploded sectional view showing the embodiment of cell unit of the invention for use in solid polymer electrolyte fuel cells; 
     FIG. 4 is an exploded perspective view of the embodiment of FIG. 2 to show movement restraining means formed therein; 
     FIG. 5 is an exploded sectional view showing an embodiment wherein stepped portions are formed on a cathode side plate; 
     FIG. 6 is an exploded sectional view showing an embodiment wherein stepped portions are formed on a bipolar plate; 
     FIG. 7 is an exploded sectional view showing a conventional cell unit; 
     FIG. 8 is an exploded sectional view showing a conventional cell unit comprising bipolar plates; 
     FIG. 9 is a perspective view partly broken away and showing a manifold for dividedly supply a fuel to anode side chambers; 
     FIG. 10 is a perspective view partly broken away and showing a manifold for discharging the fuel as collected from the anode side chambers; 
     FIG. 11 is an exploded perspective view showing an embodiment wherein stepped portions are formed on both an anode side plate and a cathode side plate; 
     FIG. 12 is an exploded perspective view showing an embodiment wherein a stepped portion is formed on the base plate of an anode side plate along the entire outer periphery thereof; 
     FIG. 13 is an exploded perspective view showing an embodiment wherein a stepped portion is formed on the base plate of a cathode side plate along the entire outer periphery thereof; 
     FIG. 14 is an exploded perspective view showing an embodiment wherein a stepped portion is formed on each of opposite surfaces of a bipolar plate; and 
     FIG. 15 is an exploded view in section taken along the line A-A′ in FIG.  14 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described below in detail with reference to the embodiments shown in the drawings. 
     FIG. 1 is an exploded sectional view of cell units  1  for use in common fuel cells. 
     The cell unit  1  for use in fuel cells comprises a cell  2  including an electrode portion  24  which has an anode  21  formed on one surface of an electrolyte  20  in the form of a plate or film and a cathode  22  formed on the other surface of the electrolyte  20 , an anode side plate  3  disposed on the anode ( 21 ) side of the cell  2 , and a cathode side plate  4  disposed on the cathode ( 22 ) side of the cell  2 . 
     The anode side plate  3  has an anode side chamber  30  for passing hydrogen gas or like fuel therethrough. The cathode side plate  4  has cathode side chambers  40  for passing air or like oxidant therethrough. A multiplicity of such cell units  1  are arranged in layers for use in the fuel cell. 
     The fuel cell has fuel manifolds through which the fuel fed from outside is uniformly supplied to the anode side chambers  30  of the anode side plate  3  and discharged to the outside after passing through the chambers  30 . The fuel manifolds are formed inside the cell unit, or alternatively externally of the cell unit. 
     Similarly, the fuel cell has oxidant manifolds for uniformly supplying the oxidant from outside to the cathode side chambers  40  of the cathode side plates  4  and discharging the oxidant passing through the chambers  40  to the outside. The oxidant manifolds are formed inside the cell unit, or alternatively externally of the cell unit. 
     With the cell unit  1  shown in FIG. 1, fuel manifolds are formed inside the cell unit  1 . The anode side plate  1  is formed in its upper end with a fuel supply manifold  311  for passing therethrough the fuel supplied from outside and in its lower end with a fuel discharge manifold  321  for passing therethrough the fuel to be discharged from the anode side plate  3 . 
     The anode side plate  3  comprises a generally rectangular base plate and has a pair of first and second stepped portions  301 ,  302  approximately parallel to each other and projecting from one surface of the base plate at respective opposite edge portions thereof. The stepped portions  301 ,  302  and the base plate surface define a recessed portion  33 . The base plate surface of the recessed portion  33  is recessed to provide the anode side chamber  30 . The anode side plate  3  has a fuel supply passageway  313  formed between the fuel supply manifold  311  and the anode side chamber  30 , and a fuel discharge passageway  323  between the anode side chamber  30  and the fuel discharge manifold  321 . The supply manifold  311 , chamber  30  and discharge manifold  321  communicate with one another through these passageways. 
     The cell unit  1  is completed by placing the cell  2  into the recessed portion  33  with the anode  21  exposed to the anode side chamber  30  and subsequently placing the cathode side plate  4  into the recessed portion  33  with the cathode  22  of the cell  2  exposed to the cathode side chambers  40 . In this state, the top faces of the stepped portions  301 ,  302  of the anode side plate  3  become substantially flush with the surface of the cathode side plate  4  opposite to their chambers  40 . 
     Cell units  1  are arranged into an assembly of cell layers to provide a fuel cell, with seal members  5 ,  5  disposed respectively between the fuel supply manifolds  311 ,  311  of each pair of adjacent cell units  1 ,  1  and between the fuel discharge manifolds  321 ,  321  thereof. 
     FIGS. 2 and 3 show cell units  1  for use in solid polymer electrolyte fuel cells. 
     The cell unit  1  includes an anode side plate  3  which, as in FIG. 1, comprises a generally rectangular base plate and has a pair of first and second stepped portions  301 ,  302  approximately parallel to each other and projecting from one surface of the base plate at respective opposite edge portions thereof. The stepped portions  301 ,  302  and the base plate surface define a recessed portion  33 . A plurality of ridges are formed in the base plate surface of the recessed portion  33  to provide anode side chambers  30  for passing the fuel therethrough. Fuel supply manifolds  311 ,  311  for passing therethrough the fuel (i.e., hydrogen gas) supplied from outside extend through the first stepped portion  301  and the base plate. Fuel discharge manifolds  321 ,  321  for passing therethrough the hydrogen gas to be discharged from the plate  3  extend through the second stepped portion  302  and the base plate. 
     With solid polymer electrolyte fuel cells, it is necessary to hold a solid polymer electrolyte film  200  wet. Accordingly, the anode side plate  3  has water supply manifolds  351 ,  351  adjacent to the fuel supply manifolds  311 ,  311  and extending through the first stepped portion  301  and the base plate for passing therethrough the water supplied from outside. 
     The hydrogen gas and water to be supplied are mixed together in fuel supply passageways  313 , and the mixture is passed through the anode side chambers  30  and then discharged. Accordingly, the gas-liquid mixture to be discharged from the anode side plate  3  is passed through the fuel discharge manifolds  321 ,  321 . 
     It is desired that a manifold  331  be provided between the fuel supply manifolds  311  and the anode side chambers  30  for supplying the fuel uniformly. The manifold is shown in FIG. 9 in detail. The manifold  331  comprises a fuel supply groove  312  formed in communication with the fuel supply manifolds  311 ,  311 , and a plurality of fuel supply passageways  313  communicating with the supply groove  312  and the anode side chambers  30 . The fuel supply passageways  313  include a plurality of small ports as illustrated, and the supply of hydrogen gas from the supply manifolds  311  to the anode side chambers  30  is restricted by the small ports to ensure equivalent supply of hydrogen gas to the anode side chambers  30 . 
     Preferably, a manifold  332  for collectively discharging the fuel from the anode side chambers is provided between the anode side chambers  30  and the fuel discharge manifolds  321 . The manifold  332 , which is shown in detail in FIG. 10, comprises a fuel discharge groove  322  formed in communication with the fuel discharge manifolds  321 , and a plurality of fuel discharge passageways  323  communicating with the groove  322  and the anode side chambers  30 . 
     The water supply manifolds  351  can be provided with a similar manifold  333 , which comprises a water supply groove  352  communicating with the water supply manifolds  351 , and the fuel supply passageways  313 . Accordingly, the hydrogen gas flowing into the fuel supply groove  312  from the fuel supply manifolds  311 ,  311  is admitted to the fuel supply passageways  313 , in which the gas is mixed with the water flowing into the water supply groove  352  from the water supply manifolds  351 ,  351  to make a gas-liquid mixture. The mixture is sent into the anode side chambers  30 . The mixture passing through the anode side chambers  30  flows through the fuel discharge passageways  323  and the fuel discharge groove  322  and is sent into the fuel discharge manifolds  321 ,  321 . 
     A platelike gasket  50  serving as a seal member  5  is provided along the periphery of the recessed portion  33  of the anode side plate  3 . Similarly, a platelike gasket  51  serving as a seal member  5  is provided for a cathode side plate  4  along the periphery of the surface thereof to be opposed to a cell  2 . 
     The cell  2  is placed into the recessed portion  33  of the anode side plate  3 , with the anode  21  thereof in contact with the base plate surface  300  of recessed portion  33  of the anode side plate  3  and with the peripheral edge of the cell in contact with the gasket  50 . 
     Next, the cathode side plate  4  is placed into the recessed portion  33 , with a base plate surface  400  in contact with the cathode  22  of the cell  2  and with the gasket  51  in contact with the peripheral edge of the cell  2 , whereby a cell unit  1  is completed. In this state, the top faces of the stepped portions  301 ,  302  of the anode side plate  3  are approximately flush with the surface of the cathode side plate  4  opposite to the cathode side chambers  40  thereof. 
     The cell unit  1  thus constructed has an O-ring  52  serving as a seal member  5  and provided around the fuel supply manifolds  311 ,  311  and the fuel supply groove  312  and around the water supply manifolds  351 ,  351  and the water supply groove  352 , and an O-ring  53  serving as a seal member  5  and provided around the fuel discharge manifolds  321 ,  321  and the fuel discharge groove  322 . A partition plate  6  is placed over the cell unit  1  for preventing mixing of hydrogen gas and water, another cell unit  1  is placed over the cell unit  1 , and this procedure is repeated to form an assembly of cell layers for a fuel cell. In the case where the base plates are made of a gas impermeable material, such partition plates  6  can be dispensed with. 
     The recessed portion  33  of the anode side plate  3  of the present embodiment is left free at its lateral opposite sides, so that there is the likelihood of the cell  2  and the cathode side plate  4  becoming displaced laterally relative to the anode side plate  3 . 
     As shown in FIG. 4, therefore, it is desirable to provide lateral movement restraining means, i.e., projections  39 ,  39  at corners of the recessed portion  33  of the anode side plate  3 , and cutouts  49 ,  49  in the cathode side plate  4  at locations to be opposed to the respective projections  39 ,  39 . The projections  39 ,  39  and the cutouts  49 ,  49  formed make it possible to accommodate the cathode side plate  4  in the recessed portion  33  of the anode side plate  3  with accuracy and ease. 
     FIG. 11 shows an embodiment comprising a cathode side plate  4  which is also formed with first and second stepped portions  401 ,  402 . The thickness T including the stepped portion and base plate of the cathode side plate  4  is approximately equal to the thickness T including the stepped portion and base plate of the anode side plate. 
     As is the case with the anode side plate  3 , it is desired that the cathode side plate  4  have a first manifold  431  for holding oxidant supply manifolds  411  for passing an oxidant therethrough in communication with cathode side chambers  40 , and a second manifold  432  for holding oxidant discharge manifolds  421  for passing therethrough the oxidant to be discharged in communication with the cathode side chambers  40 . 
     The first manifold  431  comprises an oxidant supply groove  412  communicating with the oxidant supply manifolds  411 , and a plurality of oxidant supply passageways  413  communicating with the oxidant supply groove  412  and the cathode side chambers  40 . The second manifold  432  comprises an oxidant discharge groove  422  communicating with the oxidant discharge manifolds  421 , and a plurality of oxidant discharge passageways  423  communicating with the oxidant discharge groove  422  and the cathode side chambers  40 . 
     The embodiment of FIG. 11 is free of the likelihood that the cell  2  and the cathode side plate  4  will be displaced laterally relative to the anode side plate  3 . 
     With the foregoing embodiment for use in solid polymer electrolyte fuel cells, usable as materials for the components of the cell  2  are perfluorocarbonsulfonic acid (e.g., brand name “Nafion,” product of Du Pont) for the solid polymer electrolyte film, and platinum-supported carbon (the carbon is, for example, carbon black with the brand name “Vulcan XC-72R,” product of CABOT) for the anode  21  and cathode  22 . 
     Usable as component materials for the anode side plate  3  are a carbon material or porous carbon material for the base plate, and a high-molecular-weight material (such as phenol, epoxy, Teflon or polyphenylene sulfide) for the stepped portions. The portion of the base plate for forming the anode side chambers  30  can be prepared from the carbon material or porous carbon material, and the other portion thereof from the high-molecular-weight material. 
     Usable as component materials for the cathode side plate  4  are a carbon material or porous carbon material for the base plate, and a high-molecular-weight material (such as phenol, epoxy, Teflon or polyphenylene sulfide) for the stepped portions. The portion of the base plate for forming the cathode side chambers  40  can be prepared from the carbon material or porous carbon material, and the other portion thereof from the high-molecular-weight material. 
     Fluororubber (e.g., brand name “Viton,” product of Du Pont) is usable for the seal materials  5 . 
     A carbon material is usable for the partition plates  6 . 
     FIG. 5 shows an embodiment wherein stepped portions are formed on a cathode side plate  4  instead of forming the stepped portions on an anode side plate  3 . With this embodiment, the cathode side plate  4  comprises a base plate and has a pair of first and second stepped portions  401 ,  402  approximately parallel to each other and projecting from one surface of the base plate at respective opposite edge portions thereof. The stepped portions  401 ,  402  and the base plate surface define a recessed portion  43 . The base plate surface of the recessed portion  43  is recessed to provide a cathode side chamber  40 . The same cell  2  as already described and the anode side plate  3  which has anode side chambers  30  formed in one surface of a base plate are placed into the recessed portion  43  to assemble a cell unit. 
     It is desired that the embodiment shown in FIG. 5, like the preceding embodiment, have a manifold formed between the cathode side chamber  40  and oxidant supply manifolds  411  for uniformly supplying the oxidant to the chamber  40 . This manifold is nearly the same as the manifold  431  of the cathode side plate of FIG. 11, and comprises an oxidant supply groove  412  communicating with the oxidant supply manifolds  411 , and a plurality of oxidant supply passageways  413  communicating with the oxidant supply groove  412  and the cathode side chamber  40 . Similarly, it is desired to provide a manifold between the cathode side chamber  40  and oxidant discharge manifolds  421  for collectively discharging therethrough the oxidant to be discharged from the cathode side chamber. This manifold is also nearly the same as the manifold  432  of the cathode side plate of FIG. 11, and comprises an oxidant discharge groove  422  communicating with the oxidant discharge manifolds  421 , and a plurality of oxidant discharge passageways  423  communicating with the oxidant discharge groove  422  and the cathode side chamber  40 . 
     FIG. 12 shows an embodiment wherein an anode side plate  3  has a stepped portion  341  extending along the entire outer periphery of the base plate thereof. 
     It is desired that this embodiment also have manifolds in its interior. Stated more specifically, the stepped portion  341  includes a first part  341   a  formed with a first manifold  331  for holding fuel supply manifolds  311  for passing the fuel therethrough in communication with anode side chambers  30 , a second part  341   b  formed with a second manifold  332  for holding fuel discharge manifolds  321  for passing therethrough the fuel to be discharged in communication with the anode side chambers  30 , a third part  341   c  formed with a third manifold  433  for holding oxidant supply manifolds  411  for passing the oxidant therethrough in communication with a cathode side chamber  40  in a cathode side plate  4 , and a fourth part  341   d  formed with a fourth manifold  434  for holding oxidant discharge manifolds  421  for passing therethrough the oxidant to be discharged in communication with the cathode side chamber  40  of the cathode side plate  4 . 
     This embodiment is also free of the likelihood that the cell  2  and the cathode side plate  4  will be displaced laterally relative to the anode side plate  3 . 
     Like the preceding embodiment, the embodiment of FIG. 12 can be provided with a stepped portion formed on the cathode side plate  4  in place of the stepped portion on the anode side plate  3 . 
     FIG. 13 is a embodiment wherein a cathode side plate  4  has a stepped portion  441  extending along the entire outer periphery of the base plate thereof. 
     It is desired that this embodiment also have manifolds In its interior. Stated more specifically, the stepped portion  441  includes a first part  441   a  formed with a first manif old  331  for holding fuel supply manifolds  311  for passing the fuel therethrough in communication with anode side chambers  30  in an anode side plate  3 , a second part  441   b  formed with a second manifold  332  for holding fuel discharge manifolds  321  for passing therethrough the fuel to be discharged in communication with the anode side chambers  30  of the anode side plate  3 , a third part  441   c  formed with a third manifold for holding oxidant supply manifolds  411  for passing the oxidant therethrough in communication with cathode side chambers  40 , and a fourth part  441   d  formed with a fourth manifold for holding oxidant discharge manifolds  421  for passing therethrough the oxidant to be discharged in communication with the cathode side chambers  40 . Although not shown in FIG. 13, the third manifold comprises an oxidant supply groove  412 , and oxidant supply passageways (not shown) communicating with the cathode side chambers  40 , and the fourth manifold comprises an oxidant discharge groove  422 , and oxidant discharge passageways (not shown) communicating with the cathode side chambers  40 . 
     FIGS. 6,  14  and  15  show embodiments comprising a bipolar plate  7  formed with a stepped portion on each surface of a base plate. 
     The bipolar plate  7  shown in FIG. 6 has a pair of first and second stepped portions  701 ,  702  approximately parallel to each other and projecting from one surface (the left surface in the drawing) of a generally rectangular base plate. The stepped portions  701 ,  702  and the base plate surface define a first recessed portion  72 , and the base plate surface of the recessed portion  72  is recessed to provide an anode side chamber  70 . The bipolar plate  7  has a pair of second and third stepped portions  703 ,  704  approximately parallel to each other and formed at the back of the respective first and second stepped portions  701 ,  702  on the other surface (the right surface in the drawing) of the base plate. The stepped portions  703 ,  704  and the base plate other surface define a second recessed portion  73 , and the base plate surface of the recessed portion  73  is recessed to provide a cathode side chamber  71 . The cell unit  1  of this embodiment comprises the bipolar plate  7 , first and second cells  2   a,    2   b  which are so sized as to fit into the respective recessed portions  72 ,  73  of the bipolar plate  7 , an anode side plate  3  so sized as to fit into the second recessed portion  73  of the bipolar plate  7 , and a cathode side plate  4  so sized as to fit into the first recessed portion  72  of the plate  7 . 
     The bipolar plate  7  of the embodiment shown in FIG. 14 has first and second stepped portions  705 ,  706  formed on the respective surfaces of a base plate and extending along the entire outer periphery of the plate. More specifically, the bipolar plate  7  has a first stepped portion  705  projecting from one surface (the front side in the drawing) of a generally rectangular base plate and extending along an entire outer periphery of the base plate. The stepped portion  705  and the base plate surface define a first recessed portion  72 , and the base plate surface of the recessed portion  72  is recessed to provide anode side chambers  70 . The bipolar plate  7  has a second stepped portion  706  formed on the other surface (the rear side in the drawing) of the base plate along the entire outer periphery thereof. The stepped portion  706  and the base plate other surface define a second recessed portion  73 , and the base plate surface of the recessed portion  73  is recessed to provide cathode side chambers  71 . 
     Like the preceding embodiment, the embodiment comprising this bipolar plate  7  preferably has manifolds in the interior for equivalently supplying the fuel and oxidant and collectively discharging them. 
     These manifolds In the bipolar plate  7  will be described with reference to FIGS. 14 and 15. The first stepped portion  705  has first, second, third and fourth parts  705   a,    705   b,    705   c,    705   d,  and the second stepped portion  706  has first, second, third and fourth parts  706   a,    706   b,    706   c,    706   d  corresponding respectively to the first, second, third and fourth parts  705   a,    705   b,    705   c,    705   d  of the first stepped portion  705 . 
     The bipolar plate  7  has manifolds: fuel supply manifolds  711  for passing a fuel therethrough extending through the first part  705   a  of the first stepped portion  705 , the base plate and the first part  706   a  of the second stepped portion  706 ; fuel discharge manifolds  721  for passing therethrough the fuel to be discharged extending through the second part  705   b  of the first stepped portion  705 , the base plate and the second part  706   b  of the second stepped portion  706 ; oxidant supply manifolds  741  for passing an oxidant therethrough extending through the third part  705   c  of the first stepped portion  705 , the base plate and the third part  706   c  of the second stepped portion  706 ; and oxidant discharge manifolds  751  for passing therethrough the oxidant to be discharged extending through the fourth part  705   d  of the first stepped portion  705 , the base plate and the fourth part  706   d  of the second stepped portion  706 . 
     The fuel supply manifolds  711  are held in communication with the anode side chambers  70  of the bipolar plate by a first manifold  731  and with the anode side chambers  30  of the anode side plate  3  by a second manifold  732 . 
     The fuel discharge manifolds  721  are held in communication with the anode side chambers  70  of the bipolar plate by a third manifold  733  and with the anode side chambers  30  of the anode side plate  3  by a fourth manifold  734 . 
     The oxidant supply manifolds  741  are held in communication with the cathode side chambers  40  of the cathode side plate  4  by a fifth manifold  735  and with the cathode side chambers  71  of the bipolar plate by a sixth manifold (not shown). 
     The oxidant discharge manifolds  751  are held in communication with the cathode side chambers  40  of the cathode side plate  4  by a seventh manifold  737  and with the cathode side chambers  71  of the bipolar plate by an eighth manifold (not shown). 
     The first manifold  731  comprises a fuel supply groove  712  communicating with the fuel supply manifolds  711 , and fuel supply passageways  713  communicating with the groove  712  and the anode side chambers  70  of the bipolar plate. 
     The second manifold  732  comprises a fuel supply groove  714  communicating with the fuel supply manifolds  711 , and fuel supply passageways  715  communicating with the groove  714  and the anode side chambers  30  of the anode side plate  3 . 
     The third manifold  733  comprises a fuel discharge groove  722  communicating with the fuel discharge manifolds  721 , and fuel discharge passageways  723  communicating with the groove  722  and the anode side chambers  70  of the bipolar plate. 
     The fourth manifold  734  comprises a fuel discharge groove  724  communicating with the fuel discharge manifolds  721 , and fuel discharge passageways  725  communicating with the groove  724  and the anode side chambers  30  of the anode side plate  3 . 
     The fifth manifold  735  comprises an oxidant supply groove  742  communicating with the oxidant supply manifolds  741 , and oxidant supply passageways  743  communicating with the groove  742  and the cathode side chambers  40  of the cathode side plate  4 . 
     The sixth manifold (not shown) comprises an oxidant supply groove (not shown) communicating with the oxidant supply manifolds  741 , and oxidant supply passageways (not shown) communicating with the groove and the cathode side chambers  71  of the bipolar plate. 
     The seventh manifold  737  comprises an oxidant discharge groove  752  communicating with the oxidant discharge manifolds  751 , and oxidant discharge passageways  753  communicating with the groove  752  and the cathode side chambers  40  of the cathode side plate  4 . 
     The eighth manifold (not shown) comprises an oxidant discharge groove (not shown) communicating with the oxidant discharge manifolds  751 , and oxidant supply passageways (not shown) communicating with the groove and the cathode side chambers  71  of the bipolar plate. 
     The cell unit  1  comprising a bipolar plate having a recessed portion in each of its opposite surfaces serves to further diminish the number of seal members required. 
     The embodiments described above are intended to illustrate the present invention and should not be construed as restricting the invention defined in the appended claims or reducing the scope thereof. The cell unit of the invention is not limited to the foregoing embodiments in construction but can of course be modified variously within the technical scope as set forth in the claims.