Abstract:
An electrolyte membrane/electrode structure constituting a fuel cell comprises a solid polymer electrolyte membrane, an anode side electrode and a cathode side electrode sandwiching the solid polymer electrolyte membrane. The anode side electrode is provided with an electrode catalyst layer and a gas diffusion layer abutting on one side of the solid polymer electrolyte membrane and exposing the outer circumference thereof in the shape of a frame, and the cathode side electrode is provided with an electrode catalyst layer and a gas diffusion layer abutting on the other side of the solid polymer electrolyte membrane. A reinforcing sheet member is arranged on the frame-shaped surface of the solid polymer electrolyte membrane projecting from the outer circumference of the gas diffusion layer.

Description:
RELATED APPLICATIONS 
     This application is a 35 U.S.C. 371 national stage filing of International Application No. PCT/JP2009/063085, filed Jul. 22, 2009, which claims priority to Japanese Patent Application No. 2008-200481 filed on Aug. 4, 2008 in Japan. The contents of the aforementioned applications are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a membrane electrode assembly (electrolyte membrane/electrode structure) including a first electrode and a second electrode, and a solid polymer electrolyte membrane interposed between the first electrode and the second electrode. Also, the present invention relates to a fuel cell including the membrane electrode assembly and first and second separators sandwiching the membrane electrode assembly. 
     BACKGROUND ART 
     Generally, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode each including a catalyst layer (electrode catalyst) and a gas diffusion layer (porous carbon) to form a membrane electrode assembly (MEA). The membrane electrode assembly is interposed between separators (bipolar plates) to form the fuel cell. Normally, in use, predetermined numbers of the fuel cells and the separators are stacked together to form a fuel cell stack. 
     In the membrane electrode assembly, a solid polymer electrolyte membrane in the form of a thin film is used. Therefore, it is required to prevent damage of the solid polymer electrolyte membrane caused by differential pressure of reactant gases or mechanical stress applied to the solid polymer electrolyte membrane. 
     In this regard, for example, a sealing structure of a solid polymer electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication No. 05-021077 is known. As shown in  FIG. 4 , a solid polymer electrolyte membrane  1  is interposed between an anode  2  and a cathode  3 . A separator  4  having a fuel gas channel  4   a  is disposed on the anode  2 , and a separator  5  having an oxygen-containing gas channel  5   a  is disposed on the cathode  3 . 
     Protective membranes  6  comprised of frame-shaped fluorine-resin sheets are inserted on both surfaces at the outer circumferential ends of the solid polymer electrolyte membrane  1 . When the solid polymer electrolyte membrane  1 , the anode  2  and the cathode  3  are combined together by thermal compression bonding process, the protective membranes  6  are formed integrally therewith. Gas seal members  7  are inserted between the separators  4 ,  5  around the anode  2  and the cathode  3  like frames of the anode  2  and the cathode  3 . 
     SUMMARY OF INVENTION 
     However, in Japanese Laid-Open Patent Publication No. 05-021077, since the protective membrane  6  is made of a thin film sheet having a thickness of 50 μm or less, when the solid polymer electrolyte membrane  1  is sandwiched between the gas seal members  7 , the sufficient strength cannot be obtained. In particular, when the gas seal members  7  like ribs are used as gaskets, a shearing force may be applied to the solid polymer electrolyte membrane  1  easily due to positional displacement of the ribs facing each other or the like. Thus, the outer marginal portion of the solid polymer electrolyte membrane  1  to which the protective membranes  6  are attached may be deformed, the solid polymer electrolyte membrane  1  may be damaged, and the sealing performance may be lowered disadvantageously. 
     The present invention has been made to solve the problems of this type, and an object of the present invention is to provide a membrane electrode assembly and a fuel cell which make it possible to reduce in thickness easily, prevent damage of the solid polymer electrolyte membrane as much as possible, and maintain a desired power generation performance. 
     The present invention relates to a membrane electrode assembly including a first electrode, a second electrode, and a solid polymer electrolyte membrane interposed between the first electrode and the second electrode. The surface area of the second electrode is larger than the surface area of the first electrode. 
     The first electrode includes a first catalyst layer and a first gas diffusion layer. The first catalyst layer contacts one surface of the solid polymer electrolyte membrane while allowing an outer circumference of the solid polymer electrolyte membrane in a frame shape to be exposed. The second electrode includes a second catalyst layer and a second gas diffusion layer. The second catalyst layer contacts the other surface of the solid polymer electrolyte membrane. 
     The plane of the second gas diffusion layer is larger than the plane of the first gas diffusion layer, and a reinforcement sheet member is provided on a frame-shaped surface of the solid polymer electrolyte membrane, the frame-shaped surface extending outwardly from an outer circumferential end of the first gas diffusion layer. 
     Further, the present invention relates to a fuel cell having a membrane electrode assembly and first and second separators sandwiching the membrane electrode assembly. The membrane electrode assembly includes a first electrode, a second electrode, and a solid polymer electrolyte membrane interposed between the first electrode and the second electrode. The surface area of the second electrode is larger than the surface area of the first electrode. 
     The first electrode includes a first catalyst layer and a first gas diffusion layer. The first catalyst layer contacts one surface of the solid polymer electrolyte membrane while allowing an outer circumference of the solid polymer electrolyte membrane in a frame shape to be exposed. The second electrode includes a second catalyst layer and a second gas diffusion layer. The second catalyst layer contacts the other surface of the solid polymer electrolyte membrane. 
     The plane of the second gas diffusion layer is larger than the plane of the first gas diffusion layer, and a reinforcement sheet member is provided on a frame-shaped surface of the solid polymer electrolyte membrane, the frame-shaped surface extending outwardly from an outer circumferential end of the first gas diffusion layer. A first ridge seal is interposed between the reinforcement sheet member and the first separator, and a second ridge seal is interposed between the first separator and the second separator. 
     In the present invention, the first electrode and the second electrode having different surface areas are provided on both surfaces of the solid polymer electrolyte membrane to form the membrane electrode assembly, and in the membrane electrode assembly of this type, so-called “stepped-type MEA”, the reinforcement sheet member is provided on the frame shaped surface exposed from the outer circumferential end of the first electrode of the solid polymer electrolyte membrane. Improvement in the strength of the outer circumferential portion of the stepped-type MEA is achieved, and it becomes possible to prevent damage of the solid polymer electrolyte membrane as much as possible. 
     Further, since the membrane electrode assembly is a stepped-type MEA, no shearing force is generated in the outer circumferential portion of the stepped-type MEA due to seal displacement. Accordingly, the desired sealing performance and the desired durability can be achieved advantageously. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exploded perspective view showing main components of a fuel cell according to an embodiment of the present invention; 
         FIG. 2  is a cross sectional view showing the fuel cell, taken along a line II-II in  FIG. 1 ; 
         FIG. 3  is a partial cross sectional view showing a membrane electrode assembly of the fuel cell; and 
         FIG. 4  is an explanatory view showing a seal structure disclosed in Japanese Laid-Open Patent Publication No. 05-021077. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As shown in  FIGS. 1 and 2 , a fuel cell  10  according to an embodiment of the present invention includes a membrane electrode assembly  12  according to the present embodiment and first and second separators  14 ,  16  sandwiching the membrane electrode assembly  12 . For example, the first and second separators  14 ,  16  are steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces formed by surface treatment. Alternatively, the first and second separators  14 ,  16  are made of carbon material or the like. 
     The membrane electrode assembly  12  includes a solid polymer electrolyte membrane  18 , an anode (first electrode)  20  and a cathode (second electrode)  22  sandwiching the solid polymer electrolyte membrane  18 . The surface area of the anode  20  is smaller than the surface area of the cathode  22 . A reinforcement sheet member  24  is provided on a frame-shaped surface of the solid polymer electrolyte membrane  18  exposed to the outside from an area around the anode  20 . 
     As shown in  FIG. 3 , the anode  20  includes an electrode catalyst layer (first catalyst layer)  20   a  that contacts one surface  18   a  of the solid polymer electrolyte membrane  18 , and a gas diffusion layer (first gas diffusion layer)  20   b . A frame-shaped outer circumferential portion of the solid polymer electrolyte membrane  18  around the electrode catalyst layer  20   a  is exposed to the outside. The cathode  22  includes an electrode catalyst layer (second catalyst layer)  22   a  contacting the other surface  18   b  of the solid polymer electrolyte membrane  18  and a gas diffusion layer (second gas diffusion layer)  22   b . Each of the electrode catalyst layers  20   a ,  22   a  may include a plurality of layers. 
     The plane of the gas diffusion layer  22   b  is larger than the plane of the gas diffusion layer  20   b . The gas diffusion layer  22   b  protrudes from the outer circumferential end of the electrode catalyst layer  22   a , and covers the entire other surface  18   b  of the solid polymer electrolyte membrane  18 . 
     An adhesive layer  26   a  is provided between the gas diffusion layer  22   b  protruding from the outer circumferential end of the electrode catalyst layer  22   a  and the solid polymer electrolyte membrane  18 . For example, fluorine based adhesive is used for the adhesive layer  26   a . The gas diffusion layer  20   b  protrudes from the outer circumferential end of the electrode catalyst layer  20   a , and an overlapping portion  24   a  is provided at an inner circumferential side of the reinforcement sheet member  24 . The overlapping portion  24   a  is interposed between the gas diffusion layer  20   b  protruding from the outer circumferential end of the electrode catalyst layer  20   a  and the solid polymer electrolyte membrane  18 . 
     The reinforcement sheet member  24  has a frame shape, and is made of engineering plastic or super engineering plastic such as PPS (polyphenylene sulfide resin) or PEEK-based (polyetheretherketone) material. 
     An adhesive layer  26   b  is provided between the reinforcement sheet member  24  and the solid polymer electrolyte membrane  18 . In the overlapping portion  24   a , an adhesive layer  26   c  is provided between the reinforcement sheet member  24  and the gas diffusion layer  20   b . The total thickness of the reinforcement sheet member  24 , the adhesive layer  26   b  and the adhesive layer  26   c  is equal to the thickness of the electrode catalyst layer  20   a . It should be noted that the adhesive layer  26   c  may be impregnated into the gas diffusion layer  20   b  for preventing gas leakage through the end of the gas diffusion layer  20   b.    
     An inner circumferential edge of the adhesive layer  26   a  contacts an outer circumferential edge of the electrode catalyst layer  22   a  without any gap. Inner circumferential edges of the adhesive layers  26   b ,  26   c  contact an outer circumferential edge of the electrode catalyst layer  20   a  without any gap. The inner circumferential end of the adhesive layer  26   a  is positioned outwardly from the inner circumferential end of the adhesive layer  26   c  (and  26   b ) by the distance H over the entire circumference in a surface direction of the solid polymer electrolyte membrane  18 . Each of the electrode catalyst layers  20   a ,  22   a  includes platinum particles supported on porous carbon particles. The platinum particles are applied onto both surfaces of the solid polymer electrolyte membrane  18  to form the electrode catalyst layers  20   a ,  22   a.    
     As shown in  FIG. 1 , at one end of the fuel cell  10  in a horizontal direction indicated by an arrow B in  FIG. 1 , an oxygen-containing gas supply passage  30   a  for supplying an oxygen-containing gas, a coolant supply passage  32   a  for supplying a coolant, and a fuel gas discharge passage  34   b  for discharging a fuel gas such as a hydrogen-containing gas are arranged in a vertical direction indicated by an arrow C. The oxygen-containing gas supply passage  30   a , the coolant supply passage  32   a , and the fuel gas discharge passage  34   b  extend through the fuel cell  10  in the direction indicated by the arrow A. 
     At the other end of the fuel cell  10  in the direction indicated by the arrow B, a fuel gas supply passage  34   a  for supplying the fuel gas, a coolant discharge passage  32   b  for discharging the coolant, and an oxygen-containing gas discharge passage  30   b  for discharging the oxygen-containing gas are arranged in the direction indicated by the arrow C. The fuel gas supply passage  34   a , the coolant discharge passage  32   b , and the oxygen-containing gas discharge passage  30   b  extend through the fuel cell  10  in the direction indicated by the arrow A. 
     The second separator  16  has an oxygen-containing gas flow field  36  on its surface  16   a  facing the membrane electrode assembly  12 . The oxygen-containing gas flow field  36  is connected between the oxygen-containing gas supply passage  30   a  and the oxygen-containing gas discharge passage  30   b.    
     The first separator  14  has a fuel gas flow field  38  on its surface  14   a  facing the membrane electrode assembly  12 . The fuel gas flow field  38  is connected between the fuel gas supply passage  34   a  and the fuel gas discharge passage  34   b . A coolant flow field  40  is formed between a surface  14   b  of the first separator  14  and a surface  16   b  of the second separator  16 . The coolant flow field  40  is connected between the coolant supply passage  32   a  and the coolant discharge passage  32   b.    
     As shown in  FIGS. 1 and 2 , a first seal member  42  is formed integrally with the surfaces  14   a ,  14   b  of the first separator  14 , around the outer circumferential end of the first separator  14 . Further, a second seal member  44  is formed integrally with the surfaces  16   a ,  16   b  of the second separator  16 , around the outer circumferential end of the second separator  16 . 
     As shown in  FIG. 2 , the first seal member  42  includes a first ridge seal  42   a  interposed between the reinforcement sheet member  24  and the first separator  14 , and a second ridge seal  42   b  interposed between the first separator  14  and the second separator  16 . The second seal member  44  is a flat seal. Instead of providing the first seal member  42  with the second ridge seal  42   b , the second seal member  44  may include a second ridge seal (not shown). 
     Each of the first and second seal members  42 ,  44  is made of seal material, cushion material, or packing material such as an EPDM, an NBR, a fluoro rubber, a silicone rubber, a fluorosilicone rubber, a butyl rubber, a natural rubber, a styrene rubber, a chloroprene rubber, or an acrylic rubber. 
     As shown in  FIG. 1 , the first separator  14  has supply holes  46  connecting the fuel gas supply passage  34   a  to the fuel gas flow field  38 , and discharge holes  48  connecting the fuel gas flow field  38  to the fuel gas discharge passage  34   b.    
     Operation of the fuel cell  10  including the membrane electrode assembly  12  will be described below. 
     Firstly, as shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  30   a , and a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas supply passage  34   a . Further, a coolant such as pure water, ethylene glycol, oil, etc. is supplied to the coolant supply passage  32   a.    
     In the structure, the oxygen-containing gas from the oxygen-containing gas supply passage  30   a  is supplied to the oxygen-containing gas flow field  36  of the second separator  16 , and flows in the direction indicated by the arrow B, and then the oxygen-containing gas is supplied to the cathode  22  of the membrane electrode assembly  12 . The fuel gas from the fuel gas supply passage  34   a  flows through the supply holes  46  into the fuel gas flow field  38 . The fuel gas flows along the fuel gas flow field  38  in the direction indicated by the arrow B, and the fuel gas is supplied to the anode  20  of the membrane electrode assembly  12 . 
     Thus, in each of the membrane electrode assemblies  12 , the oxygen-containing gas supplied to the cathode  22  and the fuel gas supplied to the anode  20  are consumed in the electrochemical reactions at the electrode catalyst layers of the cathode  22  and the anode  20  for generating electricity. 
     Then, the oxygen-containing gas consumed at the cathode  22  is discharged in the direction indicated by the arrow A along the oxygen-containing gas discharge passage  30   b . Likewise, the fuel gas after partially consumed at the anode  20  flows through the discharge holes  48 , and the fuel gas is discharged in the direction indicated by the arrow A along the fuel gas discharge passage  34   b.    
     Further, the coolant supplied to the coolant supply passage  32   a  flows into the coolant flow field  40  between the first separator  14  and the second separator  16 , and then flows in the direction indicated by the arrow B. After the coolant cools the membrane electrode assembly  12 , the coolant is discharged through the coolant discharge passage  32   b.    
     In the present embodiment, the anode  20  and the cathode  22  having different surface areas are provided on both surfaces of the solid polymer electrolyte membrane  18 . In the membrane electrode assembly (MEA)  12  having the structure of this type, so-called “stepped-type MEA”, the reinforcement sheet member  24  is provided on the frame-shaped surface of the solid polymer electrolyte membrane  18  which is exposed on the anode  20  side. 
     In the structure, improvement in the strength in the outer circumferential portion of the membrane electrode assembly  12  is achieved, and it becomes possible to prevent damage of the solid polymer electrolyte membrane  18  as much as possible advantageously. Further, since the other surface  18   b  of the solid polymer electrolyte membrane  18  is supported by the gas diffusion layer  22   b  serving as the second gas diffusion layer, it is sufficient to provide the reinforcement sheet member  24  only on the one surface  18   a  of the solid polymer electrolyte membrane  18 . 
     Further, the first seal member  42  includes the first ridge seal  42   a  interposed between the reinforcement sheet member  24  and the first separator  14 , and the second ridge seal  42   b  interposed between the first separator  14  and the second separator  16  (see  FIG. 2 ). Therefore, in the membrane electrode assembly  12 , the desired sealing performance and the desired durability are achieved without generation of any shearing force due to the positional displacement of the seals or the like. 
     Further, at the anode  20 , as shown in  FIG. 3 , the reinforcement sheet member  24  is adhered to the solid polymer electrolyte membrane  18 , and the electrode catalyst layer  20   a  and the gas diffusion layer  20   b  are adhered to the reinforcement sheet member  24 . Moreover, the outer circumferential edge of the electrode catalyst layer  20   a  contacts the inner circumferential edges of the adhesive layers  26   b ,  26   c  and the inner circumferential edge of the reinforcement sheet member  24  without any gap. In the structure, the anode  20  is fixed to the solid reinforcement sheet member  24 , and improvement in the adhesion performance is achieved easily. 
     Further, the inner circumferential end of the reinforcement sheet member  24  has the overlapping portion  24   a  which extends inward from the outer circumferential end of the gas diffusion layer  20   b  to contact the outer circumferential end of the electrode catalyst layer  20   a . In the structure, the solid polymer electrolyte membrane  18  is reinforced reliably over the entire surface other than the power generation area (other than the electrode catalyst layer  20   a ), and improvement in the durability of the solid polymer electrolyte membrane  18  is achieved. 
     Further, the inner circumferential end of the adhesive layer  26   c  of the anode  20  and the inner circumferential end of the adhesive layer  26   a  of the cathode  22  are shifted from each other with respect to the stacking direction indicated by the arrow A. More specifically, the inner circumferential end of the adhesive layer  26   a  is positioned outwardly from the inner circumferential end of the adhesive layer  26   c  by the distance H over the entire circumference in the surface direction of the solid polymer electrolyte membrane  18 . Thus, it becomes possible to prevent stress concentration suitably. Further, the inner circumferential end of the adhesive layer  26   a  may be positioned inwardly from the inner circumferential end of the adhesive layer  26   c  by the distance H over the entire circumference in the surface direction of the solid polymer electrolyte membrane  18 . 
     Further, the reinforcement sheet member  24  is made of engineering plastic or super engineering plastic. Therefore, it is possible to prevent degradation of the solid polymer electrolyte membrane  18  under the high temperature or high humidification environment at the time of operating the fuel cell  10  and also prevent deformation of the solid polymer electrolyte membrane  18  due to the sealing pressure.