Patent Publication Number: US-2015072262-A1

Title: Membrane electrode assembly, fuel cell, fuel cell stack, and method for manufacturing membrane electrode assembly

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application Nos. 2012-085610, filed Apr. 4, 2012 and 2012-087456, filed Apr. 6, 2012, each incorporated herein in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a membrane electrode assembly used for example in a solid polymer electrolyte fuel cell, a fuel cell, a fuel cell stack, and a method for manufacturing a membrane electrode assembly. 
     BACKGROUND 
     As a conventional technique concerning a fuel cell, there is a configuration disclosed in Japanese Patent Application Laid-Open Publication No. 2009-245871. In a fuel cell described in Japanese Patent Application Laid-Open Publication No. 2009-245871, a gas-permeable electrode region MPL is formed with coarse regions formed of conductive particles of a large particle size and dense regions formed of conductive particles of a particle size smaller than those of the coarse regions. The gas-permeable electrode region MPL is in contact with a gas diffusion layer at an upper surface thereof and with a catalyst layer at a lower surface thereof. 
     The particles used in the dense regions have such a particle size as to make a saturated water vapor pressure in voids, which is determined according to the Kelvin equation, be higher than that in an open space. Thereby, in the fuel cell, the saturated water vapor pressure in the coarse regions is made lower than that in the dense regions so that condensation of water vapor produced at the catalyst layer is suppressed in the dense regions. 
     In the fuel cell described in Japanese Patent Application Laid-Open Publication No. 2009-245871, the porosity in the dense regions determined according to the Kelvin equation is in nano-order, and if liquid water is condensed in the coarse regions, gas diffusivity drastically decreases. 
     SUMMARY 
     In view of the above problem, the present invention has an objective of providing a membrane electrode assembly, a fuel cell, and a method for manufacturing a membrane electrode assembly, which can facilitate discharge of liquid water produced upon power generation and improve oxygen transport and consequently the power generation performance. 
     A membrane electrode assembly according to a first aspect of the present invention is a membrane electrode assembly in which a first porous body is stacked on a surface of a catalyst layer and a second porous body is stacked on the first porous body. In this membrane electrode assembly, the first porous body has a low porosity at portions in contact with solid-phase portions of the second porous body, and has a relatively high porosity at portions facing gas-phase portions of the second porous body. 
     A fuel cell according to a second aspect of the present invention comprises: a membrane electrode assembly having a structure in which an electrolyte membrane is sandwiched by paired electrode layers; and a separator configured to form a gas flow channel between the separator and the membrane electrode assembly. Each of the electrode layers includes a first porous body and a second porous body which is formed of a metal porous body and which forms an electrode surface, and the first porous body and the second porous body engage with each other such that the first porous body partly digs into voids in the second porous body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the outer appearance of a fuel cell stack  10  according to a first embodiment. 
         FIG. 2  is a dismantled perspective view showing main components of the fuel cell stack  10  shown in  FIG. 1 . 
         FIG. 3  is a plan view of a cell unit A 1  shown in  FIG. 2 . 
         FIG. 4  is a sectional view of the cell unit A 1  taken along line I-I in  FIG. 3 . 
         FIG. 5(A)  is an enlarged plan view of a surface of an air electrode  32  shown in  FIG. 4 , and  FIG. 5(B)  is an enlarged sectional view showing an electrolyte membrane  31 , the air electrode  32 , and a fuel electrode  33  shown in  FIG. 4 . 
         FIG. 6  is an explanatory diagram schematically illustrating the structure of the air electrode  32  shown in  FIG. 4 . 
         FIGS. 7A and 7B  are diagrams illustrating the degree of bend in a space inside a porous body (a second porous body). Specifically,  FIG. 7(A)  is a schematic diagram showing a shortest transport distance L1 in a free space, and  FIG. 7(B)  is a schematic diagram showing a shortest transport distance L2 in a space inside a porous body (the second porous body). 
         FIGS. 8A and 8B  show a first embodiment of a method for manufacturing a membrane electrode assembly  30 . Specifically,  FIG. 8(A)  shows a first porous body  32   b  before being compressed by a second porous body  32   a , and  FIG. 8(B)  shows the first porous body  32   b  after being compressed by the second porous body  32   a.    
         FIGS. 9A and 9B  show a second example of the method for manufacturing the membrane electrode assembly  30 . Specifically,  FIG. 9(A)  shows the first porous body  32   b  before being compressed by the second porous body  32   a , and  FIG. 9(B)  shows the first porous body  32   b  after being compressed by the second porous body  32   a.    
         FIG. 10  is an explanatory diagram schematically illustrating the structure of the air electrode  32  according to a second embodiment. 
         FIG. 11  is an explanatory diagram specifically illustrating the structure of the air electrode shown in  FIG. 10 . 
         FIG. 12  is an explanatory diagram schematically illustrating the structure of the air electrode  32  according to another embodiment. 
         FIGS. 13A and 13B  are explanatory diagrams schematically illustrating the structure of the air electrode  32  according to yet another embodiment. Specifically,  FIG. 13(A)  shows the first porous body  32   b  before being compressed by the second porous body  32   a , and  FIG. 13(B)  shows the first porous body  32   b  after being compressed by the second porous body  32   a.    
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments of the present invention are described below with reference to the drawings. 
     First Embodiment 
     With reference to  FIGS. 1 and 2 , a description is given of the overall configuration of a fuel cell stack  10  according to a first embodiment. The fuel cell stack  10  is a solid polymer electrolyte fuel cell stack installed for example in a vehicle. 
     The fuel cell stack  10  has paired end plates  11  and  12 , paired power collection plates  13  and  14  placed between the paired end plates  11  and  12 , and multiple cell units (fuel cells) A 1  placed between the paired power collection plates  13  and  14 . The end plate  12  is provided at one end of the cell units A 1  in their stacking direction (an X direction) (at the right end in  FIGS. 1 and 2 ) with the power collection plate  14  and a spacer  19  interposed therebetween. Also, the end plate  11  is provided at the other end of the cell units A 1  in the X direction (at the left end in  FIGS. 1 and 2 ) with the power collection plate  13  interposed therebetween. The end plates  11  and  12  sandwich the paired power collection plates  13  and  14  and the stacked cell units A 1 . The fuel cell stack  10  further includes fastening plates  15  and  16  and reinforcement plates  17  and  17  to fasten the sandwiched power collection plates  13  and  14  and cell units A 1 . The fastening plates  15  and  16  are provided, respectively, on front and rear surfaces of the cell units A 1  on their long sides (the upper and lower surfaces in  FIGS. 1 and 2 ), and the reinforcement plates  17  and  17  are provided, respectively, on front and rear surfaces of the cell units A 1  on their short sides. The fastening plates  15  and  16  and the reinforcement plates  17  and  17  are connected to the end plates  11  and  12  with bolts  18 . In this way, a stack of the cell units A 1  is made into a structure with an integral case as shown in  FIG. 1 , and the stack of the cell units A 1  are tied and pressed in the X direction such that each cell unit A 1  receives a predetermined contact surface pressure. Favorable gas sealing and conductivity are thus maintained. 
     As shown in  FIG. 2 , each cell unit A 1  has a cell frame  20  for fuel cell and paired separators  40  and  41  in contact with front and rear surfaces of the cell frame  20  for fuel cell, respectively. Note that the cell frame for fuel cell is simply called a “cell frame” in the embodiments. The cell frame  20  has a horizontal rectangular shape in a front view seen in the stacking direction of the cell units A 1  (the X direction), and has: a frame  21  made of a resin and having an almost constant plate thickness; and a membrane electrode assembly  30  located in a center portion of the frame  21 . 
     The plan structure of the cell unit A 1  is described with reference to  FIG. 3 . Manifold portions H for supplying coolant water, a hydrogen-containing gas, and an oxygen-containing gas and manifold portions H for discharging them are formed at both side portions of the cell unit A 1 , respectively. 
     The manifold portions H at the one side include supply manifold holes H1 to H3. The supply manifold holes H1 to H3 are specifically a manifold hole for supplying oxygen-containing gas (H1), a manifold hole for supplying coolant fluid (H2), and a manifold hole for supplying hydrogen-containing gas (H3), and form flow channels for an oxygen-containing gas, a coolant fluid, and a hydrogen-containing gas, respectively, in the X direction shown in  FIGS. 1 and 2 . 
     The manifold portions H at the other side include discharge manifold holes H4 to H6. The discharge manifold holes H4 to H6 are specifically a manifold hole for discharging hydrogen-containing gas (H4), a manifold hole for discharging coolant fluid (H5), and a manifold hole for discharging oxygen-containing gas (H6), and form flow channels for the hydrogen-containing gas, the coolant fluid, and the oxygen-containing gas, respectively, in the X direction shown in  FIGS. 1 and 2 . Note that the supply manifold holes and the discharge manifold holes may be reversed in position partly or entirely. 
     The sectional structure of the cell unit A 1  is described with reference to  FIG. 4 . Gas flow channels G through each of which a power generation gas (the hydrogen-containing gas or the oxygen-containing gas) flows are defined by the paired separators  40  and  41  in contact with both surfaces of the cell frame  20  ( 31  to  33  and  21 ), respectively. 
     The cell frame  20  ( 31  to  33  and  21 ) is also called a membrane electrode assembly (MEA), and includes a membrane electrode assembly  30  ( 31  to  33 ) and the frame  21  having a quadrangular shape in a plan view. The membrane electrode assembly  30  ( 31  to  33 ) has: an electrolyte membrane  31  made for example of a solid polymer; and an air electrode  32  and a fuel electrode  33  in contact with respective surfaces of the electrolyte membrane  31 . 
     As shown in  FIG. 3 , the frame  21  has the supply manifold holes H1 to H3 along one of the short sides of the frame  21 , and has the discharge manifold holes H4 to H6 along the other short side thereof. The air electrode  32  and the fuel electrode  33  will be further described later with reference to  FIG. 5 . 
     The separators  40  and  41  are each made for example of stainless steel, have a quadrangular shape that matches the frame  21  and the electrolyte membrane  31 , and have the supply manifold holes H1 to H3 and the discharge manifold holes H4 to H6 like the frame  21 . The separators  40  and  41  form the gas flow channels G by being superimposed on the cell frame  20  ( 31  to  33  and  21 ). In this state, the supply manifold holes H1 to H3 and the discharge manifold holes H4 to H6 of the separators  40  and  41  and of the frame  21  communicate with each other in the X direction. 
     A gas seal  36  is provided between an edge portion of the frame  21  and an edge portion of each of the separators  40  and  41 , as well as around each of the supply manifold holes H1 to H3 and the discharge manifold holes H4 to H6. The gas seal  36  is also provided between every adjacent ones of the stacked cell units A 1 , i.e., between the adjacent separators  40  and  41 . This enables the coolant liquid to flow between the adjacent separators  40  and  41 . The gas seal  36  forms gas flow channels for the oxygen-containing gas, the hydrogen-containing gas, and the coolant fluid between the layers in an air-tight manner. The gas seal  36  provides an opening to an edge portion of appropriate ones of the supply holes H1 to H3 and the discharge holes H4 to H6 so that a fluid may flow between the layers. 
     With reference to  FIGS. 5(A) and 5(B) , a description is given of the configurations of the air electrode  32  and the fuel electrode  33  shown in  FIG. 4 .  FIG. 5(A)  is a plan view showing a surface of the air electrode  32  in  FIG. 4  in an enlarged manner, and  FIG. 5(B)  is a sectional view showing the membrane electrode assembly  30  ( 31  to  33 ) in  FIG. 4  in an enlarged manner. 
     The air electrode  32  has a catalyst layer  32 A in contact with one of the surfaces of the electrolyte membrane  31  and a gas diffusion layer  32 B stacked on a surface of the catalyst layer  32 A on the separator  40  side. The gas diffusion layer  32 B has a first porous body  32   b  in contact with the catalyst layer  32 A and a second porous body  32   a  stacked thereon on the separator  40  side. 
     The second porous body  32   a  is for example wire mesh formed by weaving metal wire materials (several tens micro meter) alternately, and has solid-phase portions  32   a ′ where the metal wire material exists and gas-phase portions  32   a ″ where no metal wire material exists. The first porous body  32   b  has a low porosity at portions in contact with the solid-phase portions  32   a ′ of the second porous body  32   a , and has a relatively high porosity at portions facing the gas-phase portions  32   a ″ of the second porous body  32   a . Specifically, the first porous body  32   b  has a lower porosity at the portions in contact with the solid-phase portions  32   a ′ than at the portions facing the gas-phase portions  32   a″.    
     Further, the first porous body  32   b  has a large particle size at the portions in contact with the solid-phase portions  32   a ′ of the second porous body  32   a , and has a relatively small particle size at the portions facing the gas-phase portions  32   a ″ of the second porous body  32   a.    
     The fuel electrode  33  has the same structure as the air electrode  32  described above. To be more specific, the fuel electrode  33  has a catalyst layer  33 A in contact with the other surface of the electrolyte membrane  31  and a gas diffusion layer  33 B stacked on a surface of the catalyst layer  33 A on the separator  41  side. The gas diffusion layer  33 B has a first porous body  33   b  in contact with the catalyst layer  33 A and a second porous body  33   a  stacked thereon on the separator  41  side. The second porous body  33   a  is for example wire mesh formed by weaving metal wire materials (several tens micro meter) alternately, and has solid-phase portions  33   a ′ where the metal wire material exists and gas-phase portions  33   a ″ where no metal wire material exists. The first porous body  33   b  has a low porosity at the portions in contact with the solid-phase portions  33   a ′ of the second porous body  33   a , and has a relatively high porosity at the portions facing the gas-phase portions  33   a ″ of the second porous body  33   a.    
     Further, the first porous body  33   b  has a large particle size at the portions in contact with the solid-phase portions  33   a ′ of the second porous body  33   a , and has a relatively small particle size at the portions facing the gas-phase portions  33   a ″ of the second porous body  33   a.    
     The structure of the air electrode  32  shown in  FIG. 4  is described with reference to  FIG. 6 . Although only the air electrode  32  is described as an example, the fuel electrode  33  has a similar structure. In  FIG. 6 , “EL” denotes a flow of electrons, “H 2 O” denotes a flow of liquid water produced upon power generation, and “Ox” denotes a flow of an oxygen-containing gas. 
     The membrane electrode assembly  30  generates power when the hydrogen-containing gas flowing through one of the gas flow channels G flows to and comes into contact with the fuel electrode  33  and also when the oxygen-containing gas flowing through the other one of the gas flow channels G flows to and comes into contact with the air electrode  32 . The first porous body  32   b  ( 33   b ) has a low porosity at the portions in contact with the solid-phase portions  32   a ′ ( 33   a ′) of the second porous body  32   a  ( 33   a ), and has a relatively high porosity at the portions facing the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ). For this reason, as shown with arrows EL in  FIG. 6 , transport paths for the electrons are secured between the first porous body  32   b  ( 33   b ) and the solid-phase portions  32   a ′ ( 33   a ′) of the second porous body  32   a  ( 33   a ). Thus, favorable transport of electrons can be achieved. 
     As for the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ), as shown with arrows Ox in  FIG. 6 , transport paths for the oxygen-containing gas are secured between the gas flow channel G and the first porous body  32   b  ( 33   b ). Thus, favorable transport of the oxygen-containing gas can be achieved. Moreover, in the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ), as shown with arrows H 2 O in  FIG. 6 , liquid water produced upon power generation is easily discharged toward the gas flow channel G by capillary action, and the produced water is thereby prevented from spreading within the first porous body  32   b  ( 33   b ). 
     The above-described improvement in the transport of the electrons and of the oxygen-containing gas is related to a “degree of bend.” 
     With reference to  FIGS. 7(A) and 7(B) , a description is given of the degree of bend in the second porous body  32   a . In a free space shown in  FIG. 7(A) , a shortest transport path L1 from position FA (plane FA) to position FB (plane FB) is a straight line. On the other hand, in the second porous body  32   a  shown in  FIG. 7(B) , a shortest transport path L2 is not a straight line but bendy because of the presence of the solid-phase portions  32   a ′ in the second porous body  32   a . Thus, the shortest transport path L2 is a bendy line which is longer than that in the free space. 
     The degree of bend in the second porous body  32   a  is represented by L2/L1. Thus, in the second porous body  32   a , the smallest value of the degree of bend is “1.” The same applies to the second porous body  33   a.    
     Next, a description is given of a method for manufacturing the membrane electrode assembly  30  according to the first embodiment. A first example of the method for manufacturing the membrane electrode assembly  30  is described with reference to  FIG. 8 , and a second example of the method for manufacturing the membrane electrode assembly  30  is described with reference to  FIG. 9 . 
     In the method for manufacturing the membrane electrode assembly  30  according to the first embodiment, the structure of the first porous body  32   b  ( 33   b ) is changed according to the arrangement of the gas-phase portions (voids)  32   a ″ ( 33   a ″) and the solid-phase portions  32   a ′ ( 33   a ′) of the second porous body  32   a  ( 33   a ). Specifically, in the method for manufacturing the membrane electrode assembly  30 , the voids in the first porous body  32   b  ( 33   b ) are crushed by part of the second porous body  32   a  ( 33   a ). Thereby, the first porous body  32   b  ( 33   b ) has a low porosity at the portions in contact with the solid-phase portions  32   a ′ ( 33   a ′) of the second porous body  32   a  ( 33   a ), and has a relatively high porosity at the portions facing the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ). 
     As shown in  FIGS. 8(A) and 8(B) , when a pressing force is applied by the second porous body  32   a  ( 33   a ) to the MPL (first porous body)  32   b  ( 33   b ), voids in the first porous body  32   b  ( 33   b ) at the portions in contact with the solid-phase portions  32   a ′ ( 33   a ′) are crushed. This is because carbon particles forming the first porous body  32   b  ( 33   b ) do not crush. The crushing of the gas-phase portions  32   a ″ ( 33   a ″) relatively decreases the porosity of the first porous body  32   b  ( 33   b ) and increases the occupancy of the solid (carbon particles) in the first porous body  32   b  ( 33   b ). As a result, an electron transport path Pas is shortened, which decreases the resistance against electron transport. If carbon particles originally not in contact with each other are brought into contact completely, the solid-phase portions  32   a ″ ( 33   a ″) are crushed no more unless they are fractured by compression. Thus, the pressing force can be easily controlled. Moreover, the second porous body  32   a  ( 33   a ) is not limited to the wire mesh, but can be a metal porous body. When the metal porous body is used, the manufacturing method can be facilitated. 
     The pressing force is not applied to the first porous body  32   b  ( 33   b ) at the portions facing the gas-phase portions (voids)  32   a ″ ( 33   a ″). Hence, voids in the first porous body  32   b  ( 33   b ) at the portions facing the gas-phase portions (voids)  32   a ″ ( 33   a ″) are not crushed, and therefore little structural change occurs. However, carbon particles are often bound with a binder such as polytetrafluoroethylene (PTFE), and they follow the compressed carbon particles as shown with encircling line II in  FIG. 8(B) . As a result, the increase in the density of carbon particles under the solid-phase portions  32   a ′ ( 33   a ′) tends to increase the porosity under the gas-phase portions  32   a ″ ( 33   a ″), too. 
     In the first example shown in  FIGS. 8(A) and 8(B) , the wire mesh as the second porous body  32   a  ( 33   a ) has a quadrate sectional shape. In contrast, in the second example shown in  FIGS. 9(A) and 9(B) , the wire mesh as the second porous body  32   a  ( 33   a ) has a round sectional shape (including a perfect circle and an ellipse). Also in this case, the structure of the first porous body  32   b  ( 33   b ) is changed according to the arrangement of the gas-phase portions (voids)  32   a ″ ( 33   a ″) and the solid-phase portions  32   a ′ ( 33   a ′) of the second porous body  32   a  ( 33   a ). Moreover, since the sectional shape of the wire mesh is round, there is a smooth area division between a lower portion of the solid-phase portion  32   a ′ ( 33   a ′) and a lower portion of the gas-phase portion  32   a ″ ( 33   a ″). However, since the porosity of the first porous body  32   b  ( 33   b ) changes according to the solid-phase portions  32   a ′ ( 33   a ′), the same effect as that of the first example can be offered. 
     According to the membrane electrode assembly  30  configured as above and the method for manufacturing the membrane electrode assembly  30 , the following effects can be attained. Condensation of liquid water in the coarse region is prevented, and thereby decrease in the gas diffusivity can be prevented. Carbon particles are brought into more contact with each other under the solid-phase portions  32   a ′ ( 33   a ′), so that the transport paths for electrons increase. On the other hand, particle contact is relatively low under the gas-phase portions  32   a ″ ( 33   a ″), which allows securement of transport paths for oxygen. In addition, capillary pressure promotes discharge of liquid water from the solid-phase portions  32   a ′ ( 33   a ′) to the gas-phase portions  32   a ″ ( 33   a ″). Thereby, transport of not only electrons but also oxygen can be improved, and consequently the power generation performance can be improved. 
     Second Embodiment 
     In a second embodiment, the cell plate (fuel cell) A 1  and the fuel cell stack  10  formed by stacking multiple cell plates A 1  are described. In the cell plate A 1 , the first and second porous bodies  32   a  and  32   b  ( 33   a  and  33   b ) engage with each other such that the first porous body  32   b  ( 33   b ) is partly embedded in the gas-phase portions (voids)  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ). 
     The overall configuration of the fuel cell stack  10  ( FIGS. 1 and 2 ), the configuration of the cell plate A 1  ( FIGS. 3 and 4 ), and the configuration of the membrane electrode assembly  30  ( FIG. 5 ) of the second embodiment are the same as those of the first embodiment, and therefore their illustrations and descriptions are omitted. 
     The structure of the air electrode  32  is described with reference to  FIG. 10 . Although only the air electrode  32  is described here as an example, the fuel electrode  33  has a similar structure. The air electrode  32  has the first porous body  32   b  and the second porous body  32   a . The first and second porous bodies  32   a  and  32   b  engage with each other such that the first porous body  32   b  is partly embedded in the gas-phase portions (voids)  32   a ″ of the second porous body  32   a.    
     A height H by which the first porous body  32   b  and the second porous body  32   a  engage with each other is equal to or smaller than a depth D of the gas-phase portions  32   a ″ of the second porous body  32   a.    
     The first porous body  32   b  is a so-called porous solid, and is made for example of a carbon material. Specifically, the first porous body  32   b  is formed by binding randomly-stacked fiber with a binder and giving the stack a water-repellent treatment such as PTFE, or by sintering an aggregate of carbon black or the like with a binder such as PTFE. 
     The second porous body  32   a  is a metal porous body and is distinct from the first porous body  32   b . At least one metal selected from the group consisting of iron, stainless steel, aluminum, aluminum alloys, chromium, chromium alloys, nickel, nickel alloys, magnesium, and magnesium alloys can be used for the second porous body  32   a . A specific mode of the metal porous body includes wire mesh, punched metal, etched metal, expanded metal, and the like, and is wire mesh in this embodiment as shown in  FIG. 5  or  FIG. 11 . 
     The fuel cell stack  10  is formed by stacking multiple cell plates (fuel cells) A 1  according to the second embodiment. When an oxygen-containing gas and a hydrogen-containing gas are supplied to the air electrode  32  and the fuel electrode  33 , respectively, the cell plate (fuel cell) A 1  generates electric energy by electrochemical reaction. In this event, since the first porous body  32   b  ( 33   b ) is partly embedded in the gas-phase portions (voids)  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ), transport paths for electrons are secured between the first porous body  32   b  ( 33   b ) and the solid-phase portions  32   a ′ ( 33   a ′) of the second porous body  32   a , as shown with arrows EL in  FIG. 11 . Thereby, favorable transport of electrons can be achieved. 
     Under the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ), as shown with arrows Ox in  FIG. 11 , transport paths for the oxygen are secured between the gas flow channel G and the first porous body  32   b  ( 33   b ), and thereby favorable transport of oxygen can be achieved. Moreover, under the gas-phase portions (voids)  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ), as shown with arrows H 2 O in  FIG. 11 , liquid water produced upon power generation is discharged toward the gas flow channel G easily by capillary action, and the produced water is prevented from spreading with the first porous body  32   b  ( 33   b ). 
     As described, with the cell plate (fuel cell) A 1  and the fuel cell stack  10  according to the second embodiment, discharge of liquid water from the air electrode  32  and the fuel electrode  33  is facilitated, and at the same time, oxygen transport (gas diffusivity) is improved to consequently improve the power generation performance. 
     The second porous body  32   a  ( 33   a ) is at least one metal selected from the group consisting of iron, stainless steel, aluminum, aluminum alloys, chromium, chromium alloys, nickel, nickel alloys, magnesium, and magnesium alloys. Thereby, electron transportability can be improved while maintaining high oxygen transportability. 
     The height H by which the first porous body  32   b  ( 33   b ) and the second porous body  32   a  ( 33   a ) engage with each other is equal to or smaller than the depth D of the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ). Thereby, the first porous body  32   b  ( 33   b ) does not partly protrudes toward the gas flow channel G. This enables the second porous body  32   a  ( 33   a ) having good conductivity to be in contact with the separator  40  ( 41 ) without fail, which allows securement of favorable conductive paths with low resistance. 
     Other embodiments are described with reference to  FIGS. 12 and 13 . Note that the same components as those in the prior embodiments are given the same reference numerals and are not described in detail. 
       FIG. 12  is an explanatory diagram schematically illustrating the structure of the air electrode  32  according to one embodiment. Although only the air electrode  32  is described here as an example, the fuel electrode  33  has a similar structure. In this embodiment, the first porous body  32   b  ( 33   b ) and the second porous body  32   a  ( 33   a ) are brought into pressure contact with each other to plastically deform the first porous body  32   b  ( 33   b ) in the stacking direction (the X direction) such that the first porous body  32   b  ( 33   b ) is partly embedded in the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ). Thereby, the first porous body  32   b  ( 33   b ) and the second porous body  32   a  ( 33   a ) engage with each other. 
     Under the solid-phase portions  32   a ′ ( 33   a ′) of the second porous body  32   a  ( 33   a ), the first porous body  32   b  ( 33   b ) is compressed to increase contact among carbon particles forming the first porous body  32   b  ( 33   b ). Thus, more electron transport paths (arrows EL in  FIG. 12 ) are secured to make the electron transport more favorable. 
     At the gas-phase portions  32   a ″ ( 33   a ″) of the second porous body  32   a  ( 33   a ), the oxygen transport paths (arrows Ox in  FIG. 12 ) are secured between the gas flow channel G and the first porous body  32   b  ( 33   b ) to make the oxygen transport favorable. Further, under the gas-phase portions (voids)  32   a ″ ( 33   a ″), as shown with arrows H 2 O in  FIG. 12 , liquid water produced upon power generation is discharged easily toward the gas flow channel G by capillary action, and the produced water is prevented from spreading within the first porous body  32   b  ( 33   b ). 
       FIG. 13  is an explanatory diagram schematically illustrating the structure of the air electrode  32  according to one embodiment. Although only the air electrode  32  is described here as an example, the fuel electrode  33  has a similar structure. In this embodiment, the first porous body  32   b  ( 33   b ) has a reinforcement layer  32   c  ( 33   c ) as an interlayer thereof. The reinforcement layer  32   c  ( 33   c ) is formed of reinforcement fiber such as, for example, carbon fiber. The second porous body  32   a  ( 33   a ) is wire mesh being a metal porous body, and has a round sectional shape as shown in  FIG. 13 . 
     Like the embodiment shown in  FIG. 12 , the first porous body  32   b  ( 33   b ) and the second porous body  32   a  ( 33   a ) are brought into pressure contact with each other to plastically deform the first porous body  32   b  ( 33   b ) in the stacking direction (the X direction) such that the first porous body  32   b  ( 33   b ) is partly embedded in the voids of the second porous body  32   a  ( 33   a ). Thereby, the first porous body  32   b  ( 33   b ) and the second porous body  32   a  ( 33   a ) engage with each other. 
     In the embodiment shown in  FIG. 13 , upon the plastic deformation of the first porous body  32   b  ( 33   b ), the reinforcement layer  32   c  ( 33   c ) allows the first porous body  32   b  ( 33   b ) not to be fractured but to be partly embedded in the voids of the second porous body  32   a  ( 33   a ). 
     Although the embodiments of the present invention have been described, the invention is not limited to the foregoing embodiments, and various modifications may be made within the scope of the invention. 
     For example, although the wire mesh is used as an example for the second porous body in the embodiments described above, the present invention is not limited this. For example, punched metal or the like can of course be used instead. 
     According to the embodiments of the present invention, discharge of liquid water produced upon power generation is facilitated, and at the same time, oxygen transport (gas diffusivity) is improved to consequently improve the power generation performance. Therefore, the present invention is industrially applicable.