Abstract:
Disclosed is a membrane electrode assembly provided with a polymer electrolyte membrane; a catalyst layer (A) which is laminated onto one surface of the polymer electrolyte membrane; a gas diffusion layer (A) which is laminated onto the catalyst layer (A); a catalyst layer (B); and a gas diffusion layer (B). The outer circumferential section of the catalyst layer (A) is the membrane electrode assembly with an integrated frame which comprises a membrane electrode assembly that protrudes from the gas diffusion layer (A) and a frame adhered to the outer circumferential section of the catalyst layer (A), whereby said frame surrounds the edge of the membrane electrode assembly. The surface that is adhered to the frame in the outer circumferential section of the catalyst layer (A) comprises a plurality of cracks.

Description:
TECHNICAL FIELD 
     The present invention relates to a frame-integrated membrane electrode assembly and a fuel cell. 
     BACKGROUND ART 
     A solid polymer fuel cell is basically composed of a polymer electrolyte membrane which selectively transports hydrogen ion and a pair of catalyst electrodes (fuel electrode and air electrode) which sandwich the polymer electrolyte membrane. The fuel cell which has the above structure can continuously take out electric energy by supplying fuel gas (hydrogen is contained) to the fuel electrode, and supplying oxidizing gas (oxygen is contained) to the air electrode. 
     The polymer electrolyte membrane is composed of an electrolyte which contains a polymer ion-exchange membrane or the like, such as a sulfonic acid group-containing fluorine resin ion-exchange membrane or hydrocarbon resin ion-exchange membrane. Further, in order for the polymer electrolyte membrane to have an ion transport function, it needs to contain a given quantity of water. 
     The catalyst electrode is composed of a catalyst layer that is arranged on the polymer electrolyte membrane side and promotes a redox reaction therein and of a gas diffusion layer that is arranged on top of the catalyst layer and has both air permeability and electric conductivity. The catalyst layer is mainly composed of carbon powder carrying a platinum group metal catalyst. A polymer electrolyte membrane integrated with a pair of catalyst electrodes (catalyst layer and gas diffusion layer) is called a membrane electrode assembly (hereinafter also referred to as “MEA”). 
     Further, a technique is known in which the edge of an MEA is surrounded by a frame with high rigidity in order to facilitate easier handling of an MEA with low rigidity (for example, see Patent Literatures 1 and 2). A member composed of an MEA and a frame surrounding the edge of the MEA is called a frame-integrated MEA. 
       FIG. 1A  is a plan view of frame-integrated MEA  1  disclosed in Patent Literature 1.  FIG. 1B  is a cross-sectional view of frame-integrated MEA  1  of  FIG. 1A , taken along line AA. 
     As shown in  FIG. 1A , frame-integrated MEA  1  includes membrane electrode assembly (MEA)  5  and frame  6  surrounding the edge of MEA  5 . Further, as shown in  FIG. 1B , MEA  5  is composed of polymer electrolyte membrane  5 A and a pair of catalyst electrodes  5 D (fuel electrode and air electrode) sandwiching polymer electrolyte membrane  5 A. Catalyst electrode  5 D is composed of catalyst layer  5 B disposed on polymer electrolyte membrane  5 A and gas diffusion layer  5 C disposed on catalyst layer  5 B. Frame  6  is adhered to polymer electrolyte membrane  5 A. 
     Further, a technique is known in which the amount of cracks to be formed in a surface of a catalyst layer is adjusted (for example, see Patent Literature 3). 
     Further, a technique is known in which a reinforcement member is connected to an MEA by hot pressing so as to cover the edge of a catalyst layer in order to facilitate easier handling of the MEA (for example, see Patent Literatures 4 to 6). 
     CITATION LIST 
     Patent Literature 
     PTL 1
     WO2009/072291   

     PTL 2
     Japanese Patent Application Laid-Open No. 5-234606   

     PTL 3
     WO2003/077336   

     PTL 4
     Japanese Patent Application Laid-Open No. 2009-193860   

     PTL 5
     U.S. Patent Application Publication No. 2009/0208805   

     PTL 6
     U.S. Patent Application Publication No. 2004/0091767   

     SUMMARY OF INVENTION 
     Technical Problem 
     However, a frame-integrated MEA such as that disclosed in Patent Literature 1 has a problem that adhesion between the MEA and the frame is low. For this reason, with a conventional frame-integrated MEA, there is a likelihood that the MEA is detached from the frame or that gas leaks from a connection between the MEA and the frame. 
     The present invention has been made in view of the above, and it is therefore an object of the present invention to provide a frame-integrated MEA that achieves high adhesion between the MEA and the frame. 
     Solution to Problem 
     The inventors established that adhesion between a frame and an MEA can be improved by providing multiple cracks in a catalyst layer to achieve the anchor effect by a virtue of the cracks. The inventors conducted additional studies and completed the present invention. That is, a first aspect of the present invention relates to a frame-integrated MEA given below. 
     According to a first aspect, a frame-integrated membrane electrode assembly includes:
         a membrane electrode assembly including a polymer electrolyte membrane, a catalyst layer A disposed on one surface of the polymer electrolyte membrane, a gas diffusion layer A disposed on the catalyst layer A, a catalyst layer B disposed on the other surface of the polymer electrolyte membrane, and a gas diffusion layer B disposed on the catalyst layer B, an outer edge of the catalyst layer A protruding outwardly beyond the gas diffusion layer A; and   a frame surrounding an edge of the membrane electrode assembly and adhered to the outer edge of the catalyst layer A;   wherein the catalyst layer A includes multiple cracks in a surface of the outer edge, the surface being adhered to the frame.       

     According to a second aspect, a frame-integrated membrane electrode assembly according to the first aspect is provided, wherein an area occupied by the multiple cracks in the surface of the outer edge of the catalyst layer A is 10-25%, the surface being adhered to the frame. 
     According to a third aspect, a frame-integrated membrane electrode assembly according to any one of the first or second aspects is provided, wherein a thickness of the catalyst layer A is 5-20 μm. 
     According to a fourth aspect, a frame-integrated membrane electrode assembly according to any one of the first to third aspects is provided, wherein:
         the catalyst layer A includes carbon powder carrying a metal catalyst, and   a weight ratio of the carbon powder to the metal catalyst is 19:1 to 1:1.       

     According to a fifth aspect, a frame-integrated membrane electrode assembly according to any one of the first to fourth aspects is provided, wherein:
         an outer edge of the catalyst layer B protrudes outwardly beyond the gas diffusion layer B,   the frame is adhered to the outer edge of the catalyst layer B, and   the catalyst layer B includes multiple cracks in a surface of the outer edge, the surface being adhered to the frame.       

     According to a sixth aspect, a frame-integrated membrane electrode assembly according to any one of the first to fifth aspects is provided, wherein the frame is separated from the gas diffusion layer A and the gas diffusion layer B. 
     The present invention further relates to a method of manufacturing a frame-integrated MEA given below. 
     According to a seventh aspect, a method of manufacturing the frame-integrated membrane electrode assembly according to the first aspect is provided, the method including:
         providing the polymer electrolyte membrane having the catalyst layer A on one surface and the catalyst layer B on the other surface;   inserting the polymer electrolyte membrane having the catalyst layer A and the catalyst layer B in a mold having a cavity, and exposing in the cavity the outer edge of the catalyst layer A that has the cracks; and   filling the cavity with a resin to form the frame adhered to the outer edge of the catalyst layer A.       

     According to an eight aspect, a method of manufacturing the frame-integrated membrane electrode assembly according to the seventh aspect is provided, wherein the catalyst layer A is made by a method including:
         preparing a paste material containing carbon powder carrying a metal catalyst and a solvent, a weight ratio of the carbon powder to the metal catalyst being 19:1 to 1:1;   applying the paste material; and   drying the applied paste material.       

     According to a ninth aspect, a method of manufacturing the frame-integrated membrane electrode assembly according to the eighth aspect is provided, wherein a drying rate of the paste material is 2.5 to 20 mg/cm 2 ·min. 
     According to a tenth aspect, a method of manufacturing the frame-integrated membrane electrode assembly according to any one of the first to eighth or ninth aspects is provided, wherein a drying temperature for the paste material is 40-100° C. 
     According to an eleventh aspect, a method of manufacturing the frame-integrated membrane electrode according to any one of the eighth to tenth aspects is provided, wherein a drying period for the paste material is 1-5 minutes. 
     Advantageous Effects of Invention 
     According to the present invention, adhesion between an MEA and a frame is high, so that there is less likelihood that the MEA is detached from the frame or that gas leaks from a connection between the MEA and the frame. Therefore, according to the present invention, it is possible to provide a frame-integrated MEA with high reliability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  show a conventional frame-integrated MEA; 
         FIGS. 2A to 2C  show a flow of a manufacturing process of a frame-integrated membrane electrode assembly according to the present invention; 
         FIGS. 2D and 2E  show a flow of a manufacturing process of a frame-integrated membrane electrode assembly according to the present invention; 
         FIGS. 2F and 2G  show a flow of a manufacturing process of a frame-integrated membrane electrode assembly according to the present invention; 
         FIGS. 3A and 3B  show a frame-integrated membrane electrode assembly according to Embodiment 1; 
         FIGS. 4A to 4E  show a flow of a manufacturing process of a frame-integrated membrane electrode assembly according to Embodiment 1; 
         FIGS. 5A and 5B  show a frame-integrated membrane electrode assembly according to Embodiment 2; 
         FIGS. 6A to 6D  show a flow of a manufacturing process of a frame-integrated membrane electrode assembly according to Embodiment 2; 
         FIGS. 6E to 6G  show a flow of a manufacturing process of a frame-integrated membrane electrode assembly according to Embodiment 2; and 
         FIG. 7  is an electron microscopic image of a catalyst layer made in Experimental Example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     1. Frame-Integrated Membrane Electrode Assembly of the Present Invention 
     The present invention relates to a frame-integrated membrane electrode assembly which constitutes a component of a solid polymer fuel cell. The frame-integrated membrane electrode assembly of the present invention includes (1) a membrane electrode assembly (MEA) and (2) a frame surrounding the edge of the MEA. 
     (1) Membrane Electrode Assembly 
     A membrane electrode assembly is composed of a polymer electrolyte membrane and a pair of catalyst electrodes (fuel electrode and air electrode) sandwiching the polymer electrolyte membrane. Each of the catalyst electrodes is composed of a catalyst layer disposed on the polymer electrolyte membrane and a gas diffusion layer disposed on the catalyst layer. Therefore, the membrane electrode assembly includes the catalyst layer disposed on the polymer electrolyte membrane and the gas diffusion layer disposed on the catalyst layer on both surfaces of the polymer electrolyte membrane. 
     As used herein, the catalyst layer and the gas diffusion layer that are disposed on one surface of the polymer electrolyte membrane are also referred to as catalyst layer A and gas diffusion layer A, respectively. Further, the catalyst layer and the gas diffusion layer that are disposed on the other surface of the polymer electrolyte membrane are also referred to as catalyst layer B and gas diffusion layer B, respectively. 
     Hereinafter, i) the polymer electrolyte membrane, ii) the catalyst layer, and iii) the gas diffusion layer will be described. 
     i) Polymer Electrolyte Membrane 
     The polymer electrolyte membrane is a polymer membrane which selectively transports protons in a humidified state. Materials of the polymer electrolyte membrane are not specifically limited as long as protons can be selectively transported. Examples thereof include fluorine polymer electrolyte membranes and hydrocarbon polymer electrolyte membranes. Specific examples of fluorine polymer electrolyte membranes include Nafion® membranes (DuPont), Flemion® membranes (Asahi Glass Co., Ltd.), Aciplex® membranes (Asahi Kasei Corporation), and GORE-SELECT® membranes (Japan Gore-Tex Inc.). 
     The outer edge of the polymer electrolyte membrane preferably protrudes outwardly beyond the catalyst layer. By arranging the outer edge of the polymer electrolyte membrane so as to protrude outwardly beyond the catalyst layer, a short circuit is unlikely to occur between the pair of catalyst electrodes. 
     ii) Catalyst Layer 
     The catalyst layer contains a catalyst which promotes a redox reaction of hydrogen or oxygen. Normally, the catalyst layer is mainly composed of carbon powder carrying a metal catalyst. Examples of the metal catalyst include platinum group metal catalysts. A feature of the present invention lies in the structure of catalyst layer A. Hereinafter, the structure of catalyst layer A will be described in detail. 
     The thickness of catalyst layer A is preferably 5-20 μm. Further, the weight ratio of carbon powder to metal catalyst (carbon powder:metal catalyst) in the catalyst layer is preferably 19:1 to 1:1. By adjusting the thickness of the catalyst layer and the weight ratio of carbon powder to metal catalyst in the catalyst layer in this way, desired cracks are likely to be formed in catalyst layer A. 
     The outer edge of catalyst layer A protrudes outwardly beyond gas diffusion layer A. That is, the size of catalyst layer A is larger than that of gas diffusion layer A. A frame is adhered to the outer edge of catalyst layer A that protrudes outwardly beyond gas diffusion layer A. 
     Further, catalyst layer A includes multiple cracks in a surface of the outer edge that is adhered to the frame (hereinafter also referred to as “surface that is adhered to a frame”). Here, “crack” means a rift that has reached the surface of the polymer electrolyte membrane among rifts formed in the catalyst layer. Therefore, the polymer electrolyte membrane is exposed at the bottom of a crack. Further, the depth of a crack equals to the thickness of the catalyst layer. The width of a crack is preferably 5-300 μm. When the width of a crack is less than 5 μm, the crack is hard to exhibit the anchor effect (described later). On the other hand, when the width of a crack is greater than 300 μm, the catalyst layer can deteriorate easily. 
     In the surface of catalyst layer A that is adhered to the frame, the ratio of an area occupied by cracks (hereinafter, also referred to as “area occupied by cracks”) is preferably 10-25%. When the ratio of the area occupied by cracks is less than 10%, the anchor effect by a virtue of cracks (described later) will be weakened, and therefore there is a likelihood that sufficient adhesion between the frame and catalyst layer A cannot be ensured. On the other hand, when the ratio of the area occupied by cracks is over 25%, life of the membrane electrode assembly is likely to be short (see Patent Literature 3). 
     The ratio of the area occupied by cracks can be determined by binarizing a microscopic photograph of the surface of the catalyst layer. Examples of a microscope for obtaining microscopic photographs include electron microscopes, confocal laser scanning microscopes, and optical microscopes. Magnification of a microscopic photograph may be set to 50 to 400-fold and visual field area may be set to 10 4 -10 6  μm 2 . Examples of binary coded processing include fixed threshold processing, variable threshold processing, adaptive binary coded processing, and constant variance enhancement processing. These methods are described in, for example, “Digital gazou shori nyumon (Introduction to Digital Image Processing),” CQ Publishing Co., Ltd., pages 63-67 and “Kagaku keisoku no tameno gazou data shori (Image Data Processing for Scientific Measurement),” CQ Publishing Co., Ltd., pages 111-117. 
     For example, when performing fixed threshold processing, it is only necessary to convert a microscopic photograph of the surface of the catalyst layer into 256 levels of gray (0-255) to binarize the converted data with a threshold value being set to 120, and to determine the ratio of an area of the microscopic photograph to an area of pixels corresponding to cracks (see  FIG. 7 ). 
     Catalyst layer A may have cracks in the surface adhered to the frame, but normally has multiple cracks over the entire surface. In order to form such a catalyst layer A having multiple cracks in the surface, it is only necessary to adjust the thickness of catalyst layer A, drying condition of a paste material, a material of catalyst layer A, the weight ratio of carbon powder to metal catalyst in catalyst layer A, and the like. A method for forming catalyst layer A having cracks in the surface will be described later in detail in the section titled “Manufacturing process of frame-integrated membrane electrode assembly.” 
     By providing cracks in the surface of catalyst layer A that is adhered to the frame in this way, cracks exhibit the anchor effect, improving the adhesion between catalyst layer A and the frame. By this means, adhesion between the frame and the membrane electrode assembly is improved, so that it is possible to prevent the membrane electrode assembly from being detached from the frame and to prevent gas from leaking from a connection between the frame and the membrane electrode assembly. 
     Further, the structure of catalyst layer B is not particularly limited, but, as with catalyst layer A, it is preferable that catalyst layer B protrude outwardly beyond gas diffusion layer B in the direction of the edge of the membrane electrode assembly and that catalyst layer B have multiple cracks in the surface. 
     iii) Gas Diffusion Layer 
     The gas diffusion layer is electrically conductive and exhibits permeability of fuel gas or oxidizing gas. The gas diffusion layer may be woven or unwoven fabrics of carbon fibers, or may be a porous sheet made from carbon powder and binder. Further, the gas diffusion layer may be in contact with the frame (described later), but preferably be separated from the frame. Further, as described above, the size of gas diffusion layer A is smaller than that of catalyst layer A. 
     (2) Frame 
     A frame surrounds the edge of a membrane electrode assembly to support the membrane electrode assembly (see  FIG. 3A ). By surrounding the edge of the membrane electrode assembly with the frame, it is possible to facilitate easier handling of the membrane electrode assembly and to prevent the membrane electrode assembly from being ruptured. 
     The frame is preferably made of resin material. Examples of the material of the frame include polyphenylene sulfide (PPS), polypropylene containing glass (PP-G), polystyrene (PS), and silicone (SI). From the viewpoint of heat-resistant property, cost, and durability, PPS and PP-G are preferably used as a material of the frame. 
     The frame includes a coolant feed manifold for supplying a coolant and a coolant discharge manifold for discharging a coolant. Further, manifolds for intaking and discharging fuel gas and manifolds for intaking and discharging oxidizing gas are formed in the frame. Further, sealing members made of rubber may be formed in the frame for sealing in a coolant, oxidizing gas, or fuel gas. 
     According to the present invention, the frame is adhered to the outer edge of catalyst layer A. In order to adhere the frame to catalyst layer A, it is only necessary to fill a cavity with a material of the frame, with the outer edge of catalyst layer A being exposed in the cavity, as will be later described. As described above, catalyst layer A has multiple cracks in the surface adhered to the frame. Therefore, the material of the frame intrudes into the cracks. By this means, cracks exhibit the anchor effect, improving adhesion between the frame and catalyst layer A. Accordingly, adhesion between the frame and the membrane electrode assembly is improved, preventing the membrane electrode assembly from being detached from the frame and preventing gases from leaking from a connection between the frame and the membrane electrode assembly. 
     Further, the frame may be adhered to the outer edge of catalyst layer B (see Embodiment 2). 
     The frame-integrated membrane electrode assembly of the present invention may be sandwiched by a pair of separators (fuel electrode separator and air electrode separator) to constitute a fuel cell. The separator is a member that is electrically connected to the membrane electrode assembly and prevents mixing of reaction gases. 
     2. Manufacturing Process of Frame-Integrated Membrane Electrode Assembly 
     Next, a manufacturing process of the frame-integrated membrane electrode assembly of the present invention will be described with reference to  FIGS. 2A to 2G . 
     The manufacturing process of the frame-integrated membrane electrode assembly of the present invention includes (1) a first step of providing a polymer electrolyte membrane having catalyst layer A on one surface and catalyst layer B on other surface ( FIG. 2A ), (2) a second step of inserting the polymer electrolyte membrane having catalyst layer A and catalyst layer B into a mold with a cavity ( FIG. 2B ), (3) a third step of filling the cavity with a resin to form a frame adhered to the outer edge of catalyst layer A ( FIGS. 2C to 2F ), and (4) a fourth step of disposing gas diffusion layer A on catalyst layer A and gas diffusion layer B on catalyst layer B ( FIG. 2G ). Each step will be described in detail below. 
     (1)  FIG. 2A  shows the first step. 
     As shown in  FIG. 2A , in the first step, polymer electrolyte membrane  111  having catalyst layer  113 A on one surface and catalyst layer  113 B on the other surface is provided. In order to provide such a polymer electrolyte membrane  111 , it is only necessary to form catalyst layer  113  on polymer electrolyte membrane  111 . In order to form catalyst layer  113  on polymer electrolyte membrane  111 , a paste material (described later) may be applied on polymer electrolyte membrane  111  and be dried, or catalyst layer  113  made in advance may be disposed on polymer electrolyte membrane  111  by hot pressing or the like. 
     A feature of the present invention lies in that catalyst layer A has cracks in the surface. 
     The method of making such a catalyst layer A having cracks includes, for example, A) step A of preparing a paste material of catalyst layer A, B) step B of applying the paste material, and C) step C of drying the applied paste material. The paste material may be applied on a separate substrate or may be directly applied on the polymer electrolyte membrane. 
     A) In step A, the paste material of catalyst layer A is prepared. The paste material includes a solvent and carbon powder carrying metal catalyst. The paste material may contain a binder. According to the present invention, in order to form desired cracks in catalyst layer A, the weight ratio of carbon powder to metal catalyst in the paste material is adjusted. 
     Examples of the metal catalyst include platinum group metal catalysts. Examples of the carbon powder include Ketjen Black and acetylene black. The weight ratio of carbon powder to metal catalyst (carbon powder:metal catalyst) is preferably 19:1 to 1:1. 
     Examples of a solvent contained in the paste include water, ethyl alcohol, methyl alcohol, isopropyl alcohol, ethylene glycol, methylene glycol, propylene glycol, methyl ethyl ketone, acetone, toluene, xylene, n-methyl-2-pyrolidone, and their mixtures. 
     Examples of the binder include perfluoro sulfonic acid, polytetrafluoroethylene, and polyvinyliden fluoride resin. The weight ratio of carbon powder to binder (carbon powder:binder) is preferably 5:1 to 1:2. 
     The concentration of solid content (metal catalyst, carbon powder, and binder) in the paste is preferably 15 to 25 wt %. 
     B) In step B, the paste material is applied. 
     The paste material may be applied on a separate substrate or may be directly applied on the polymer electrolyte membrane. Examples of means for applying the paste material include a comma coater, a kiss coater, a roll coater, a doctor blade, a spray coater, a die coater, and a gravure coater. 
     According to the present invention, in order to form desired cracks in catalyst layer A, the amount of the paste material to be applied is adjusted. Specifically, the amount of the paste material to be applied on a separate substrate is adjusted so that the thickness of catalyst layer A to be formed is 5-20 μm. More specifically, the thickness of the paste material immediately after application is preferably 30-200 μm. 
     C) In step C, the applied paste material is dried. Examples of means for drying the paste material include infrared drying and hot-air drying. With the present invention, in order to form desired cracks in catalyst layer A, drying rate, drying temperature, and drying period are adjusted. Specifically, during drying, the drying rate is adjusted to the range of 2.5 to 20 mg/cm 2 ·min, the drying temperature to the range of 40-100° C., and the drying period to the range of 1-5 minutes. During drying, the drying temperature may be elevated gradually. 
     By adjusting the weight ratio of carbon powder to metal catalyst in the paste material, the amount of the paste material to be applied, and drying condition in this way, catalyst layer A can be formed to have desired cracks (see  FIG. 7 ). In particular, when the applied paste material is quickly dried, cracks are formed easily in catalyst layer A. That is, desired cracks are easily formed in catalyst layer A when the boiling point of a solvent for the paste material is low, when the drying rate is high, when the drying temperature is high, and when the drying period is short. 
     A manufacturing method of catalyst layer B is not particularly limited, but catalyst layer B is preferably made using the same method as catalyst layer A. 
     (2)  FIG. 2B  shows the second step. 
     As shown in  FIG. 2B , in the second step, polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  113 B is inserted into the mold having cavity  130 , with the outer edge of catalyst layer  113 A being exposed in cavity  130 . It is preferable that the edge of polymer electrolyte membrane  111  be also exposed in cavity  130 . 
     The mold includes core plate  133  and cavity plate  131  having cavity  130  for frame  120 . 
     (3)  FIGS. 2C to 2F  show the third step. As shown in  FIGS. 2C to 2F , in the third step, cavity  130  is filled with resin  125 , which is a material of the frame, with a predetermined injection pressure to mold frame  120  adhered to the outer edge of catalyst layer  113 A. The injection pressure applied for filling in resin  125  is preferably 10-200 MPa. 
     As described above, the outer edge of catalyst layer  113 A is exposed in cavity  130 . Therefore, frame  120  is adhered to catalyst layer  113 A. 
       FIG. 2D  is an enlarged view of square X indicated by dashed lines in  FIG. 2C . As shown in  FIG. 2D , catalyst layer  113 A includes multiple cracks  114  that have reached the surface of polymer electrolyte membrane  111  at their bottom as described above. Further, catalyst layer  113 A may include in addition to cracks  114  rifts  114 ′ that do not reach the surface of polymer electrolyte  111  at their bottom. Further,  FIG. 2E  is an enlarged view of square X′ indicated by dashed lines in  FIG. 2D . As shown in  FIG. 2E , the catalyst layer further includes concaved and convexed features on the surface including wall surfaces of crack  114 . For this reason, concaved portions  116  formed on wall surfaces of crack  114  form undercuts. 
     Resin  125  that has been injected into cavity  130  intrudes into multiple cracks  114  and concaved portions  116  formed in wall surfaces of cracks  114  on catalyst layer  113 A. By this means, cracks  114  exhibit the anchor effect, improving adhesion between frame  120  and catalyst layer  113 A. 
     By separating a molded article from the mold after die time has passed, frame  120  adhered to the outer edge of catalyst layer  113 A can be obtained ( FIG. 2F ). 
     (4) In the fourth step, gas diffusion layer A is disposed on catalyst layer A and gas diffusion layer B is disposed on catalyst layer B ( FIG. 2G ). As described above, according to the present invention, it is preferable that gas diffusion layer  115  be formed after formation of frame  120 . By forming gas diffusion layer  115  after formation of frame  120 , gas diffusion layer  115  can be prevented from being deformed by a pressure which is generated when the cavity is filled with a material of the frame. Further, when gas diffusion layer  115  is formed after formation of frame  120 , a resin material of the frame will not intrude into gas diffusion layer  115 , which is porous. Further, when gas diffusion layer  115  is formed after formation of frame  120 , normally, frame  120  will be separated from gas diffusion layer  115 . 
     On the other hand, when gas diffusion layer  115  has sufficient strength, gas diffusion layer  115  may be disposed on catalyst layer  113  before formation of frame  120 . 
     As described above, according to the present invention, adhesion between the frame and catalyst layer A, which is one member of the membrane electrode assembly, is high, so that there is less likelihood that the membrane electrode assembly is detached from the frame or that gas leaks from a connection between the membrane electrode assembly and the frame. For this reason, according to the present invention, it is possible to provide a frame-integrated membrane electrode assembly with high reliability. 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The present invention is not limited to the embodiments below. 
     Embodiment 1 
       FIG. 3A  is a plan view of frame-integrated membrane electrode assembly  100  of Embodiment 1.  FIG. 3B  is a cross-sectional view of frame-integrated membrane electrode assembly  100  of  FIG. 3A , taken along dashed dotted line AA. As shown in  FIGS. 3A and 3B , frame-integrated membrane electrode assembly  100  of Embodiment 1 includes membrane electrode assembly  110  and frame  120 . 
     Membrane electrode assembly  110  includes polymer electrolyte membrane  111 , catalyst layer  113 A disposed on one surface of polymer electrolyte membrane  111 , gas diffusion layer  115 A disposed on catalyst layer  113 A, catalyst layer  113 B disposed on the other surface of polymer electrolyte membrane  111 , and gas diffusion layer  115 B disposed on catalyst layer  113 B (see  FIG. 3B ). 
     The outer edge of catalyst layer  113 A protrudes outwardly beyond gas diffusion layer  115 A. Further, the outer edge of catalyst layer  113 B protrudes outwardly beyond gas diffusion layer  115 B. Frame member  121 A (described later) is adhered to the outer edge of catalyst layer  113 A. Further, catalyst layer  113 A includes multiple cracks in the surface adhered to frame member  121 A. Width W of the surface of catalyst layer  113 A that is adhered to frame member  121 A is preferably 0.5-5.0 mm. 
     Frame  120  surrounds the edge of membrane electrode assembly  110  and includes multiple manifold holes  101  and sealing members  103 . Frame  120  is composed of frame member  121 A sandwiching the edge of membrane electrode assembly  110  and of frame member  121 B. Materials of frame member  121 A and frame member  121 B may be different, but preferably are the same. Frame member  121 A is adhered to frame member  121 B. 
     As described above, frame member  121 A is adhered to catalyst layer  113 A. On the other hand, frame member  121 B may be in contact with catalyst layer  113 B, but is not adhered to catalyst layer  113 B. 
     Next, a manufacturing process of frame-integrated membrane electrode assembly  100  of Embodiment 1 will be described with reference to  FIGS. 4A to 4D . 
     The manufacturing process of frame-integrated membrane electrode assembly  100  of Embodiment 1 includes: 
     (1) a first step of providing polymer electrolyte membrane  111  having catalyst layer  113 A on one surface and catalyst layer  113 B on the other surface ( FIG. 4 ); 
     (2) a second step of sandwiching polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  113 B by core plate  133  and cavity plate  131  ( FIG. 4B ); 
     (3) a third step of filling cavity  130  with material  125  of frame member  121 A ( FIGS. 4C and 4D ); and 
     (4) a fourth step of disposing gas diffusion layer  115 A on catalyst layer  113 A and disposing gas diffusion layer  115 B on catalyst layer  113 B ( FIG. 4E ). 
     (1)  FIG. 4  is a cross-sectional view of polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  113 B, which has been provided in the first step. Polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  113 B may be provided by, for example, applying catalyst layers  113 A and  113 B on polymer electrolyte membrane  111 . 
     (2)  FIG. 4B  shows the second step. 
     As shown in  FIG. 4B , in the second step, polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  113 B is sandwiched by core plate  133  on which frame member  121 B is arranged in advance and cavity plate  131  having cavity  130  for frame member  121 A. At this time, the edge of polymer electrolyte membrane  111 , frame member  121 B, and the outer edge of catalyst layer  113 A are exposed in cavity  130 . 
     Frame member  121 B may be injection molded or may be formed by other molding methods. Further, in the second step, the outer edge of polymer electrolyte membrane  111  or the outer edge of catalyst layer  113 B is in contact with frame member  121 B. 
     (3)  FIG. 4C  shows the third step. 
     As shown in  FIG. 4C , in the third step, cavity  130  is filled with material  125  of frame member  121 A. As described above, the outer edge of catalyst layer  113 A is exposed in cavity  130 . Therefore, frame member  121 A is adhered to catalyst layer  113 A. Further, as described above, catalyst layer  113 A has multiple cracks in the outer edge. For this reason, material  125  of frame member  121 A also intrudes into cracks in catalyst layer  113 A. By this means, adhesion between frame member  121 A and catalyst layer  113 A will be improved. 
     Further, as described above, frame member  121 B is exposed in cavity  130 . Accordingly, frame member  121 A is adhered to frame member  121 B. It is preferable to use the same material for frame member  121 B and frame member  121 B as described above, in order to improve adhesion between frame member  121 A and frame member  121 B. 
     By separating a molded article from cavity plate  131  and core plate  133  after die time has passed, frame  120  surrounding the edge of polymer electrolyte membrane  111  can be obtained ( FIG. 4D ). 
     (4)  FIG. 4E  shows the fourth step. 
     As shown in  FIG. 4E , in the fourth step, by hot pressing or the like, gas diffusion layer  115 A is disposed on catalyst layer  113 A and gas diffusion layer  115 B is disposed on catalyst layer  113 B. 
     As described above, according to the present embodiment, adhesion between frame member A which is one member of the frame and catalyst layer A which is one member of the membrane electrode assembly, is high, so that there is less likelihood that the membrane electrode assembly is detached from the frame or that gas leaks from a connection between the membrane electrode assembly and the frame. 
     Embodiment 2 
     Embodiment 1 has described an embodiment where frame member A is adhered to catalyst layer A, but frame member B is not adhered to catalyst layer B. Embodiment 2 will describe an embodiment where frame member B is also adhered to catalyst layer B. 
       FIG. 5A  is a plan view of frame-integrated membrane electrode assembly  200  of Embodiment 2.  FIG. 5B  is a cross-sectional view of frame-integrated membrane electrode assembly  200  in  FIG. 5A , taken along dashed dotted line AA. Frame-integrated membrane electrode assembly  200  shown in  FIGS. 5A and 5B  is the same as frame-integrated membrane electrode assembly  100  of Embodiment 1 except that frame member  221 B is adhered to catalyst layer  213 B. Components identical to those of frame-integrated membrane electrode assembly  100  of Embodiment 1 are given the same reference signs and descriptions are not provided. 
     The outer edge of catalyst layer  213 B protrudes outwardly beyond gas diffusion layer  115 B. Frame member  221 B (described later) is adhered to the outer edge of catalyst layer  213 B. Further, catalyst layer  213 B has multiple cracks in the surface adhered to frame member  221 B. 
     Next, a manufacturing process of frame-integrated membrane electrode assembly  200  of Embodiment 2 will be described with reference to  FIGS. 6A to 6G . 
     The manufacturing process of frame-integrated membrane electrode assembly  200  of Embodiment 2 includes:
         (1) a first step of providing polymer electrolyte membrane  111  having catalyst layer  113 A on one surface and catalyst layer  213 B on the other surface ( FIG. 6A );   (2) a second step of sandwiching polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  213 B by core plate  133 A and cavity plate  131 A ( FIG. 6B );   (3) a third step of filling cavity  130 A with material  125 A of frame member  121 A ( FIG. 6C );   (4) a fourth step of sandwiching polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  213 B and frame member  121 A adhered to the outer edge of catalyst layer  113 A by core plate  133 B and cavity plate  131 B ( FIG. 6D );   (5) a fifth step of filling cavity  130 B with material  125 B of frame member  221 B ( FIG. 6E ); and   (6) a sixth step of disposing gas diffusion layer  115 A on catalyst layer  113 A and gas diffusion layer  115 B on catalyst layer  213 B ( FIG. 6G ).       

     (1)  FIG. 6A  shows a cross-sectional view of polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  113 B, which has been provided in the first step. 
     (2)  FIG. 6B  shows the second step. 
     As shown in  FIG. 6B , in the second step, polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  213 B is sandwiched by core plate  133 A and cavity plate  131 A, with the outer edge of catalyst layer  113 A being exposed in cavity  130 A. Cavity plate  131 A has cavity  130 A for frame member  121 A. 
     (3)  FIG. 6C  shows the third step. 
     As shown in  FIG. 6C , in the third step, cavity  130 A is filled with material  125 A of frame member  121 A to form frame member  121 A adhered to the outer edge of catalyst layer  113 A. As described above, the outer edge of catalyst layer  113 A is exposed in cavity  130 A. Accordingly, frame member  121 A is adhered to catalyst layer  113 A. 
     Further, as described above, catalyst layer  113 A has multiple cracks in the outer edge. For this reason, material  125 A of frame member  121 A intrudes into cracks in catalyst layer  113 A. By this means, adhesion between frame member  121 A and catalyst layer  113 A will be improved. 
     By separating a molded article from cavity plate  131 A and core plate  133 A after die time has passed, frame member  121 A adhered to the outer edge of catalyst layer  113 A can be obtained. 
     (4)  FIG. 6D  shows the fourth step. 
     As shown in  FIG. 6D , in the fourth step, polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  213 B and frame member  121 A adhered to the outer edge of catalyst layer  113 A are sandwiched by core plate  133 B and cavity plate  131 B, with the outer edge of catalyst layer  213 B and frame member  121 A being exposed in cavity  130 B. Cavity plate  131 B has cavity  130 B for frame member  221 B. 
     (5)  FIG. 6E  shows the fifth step. 
     As shown in  FIG. 6E , in the fifth step, cavity  130 B is filled with frame material  125 B of member  221 B to form frame member  221 B adhered to the outer edge of catalyst layer  213 B. As described above, the outer edge of catalyst layer  213 B is exposed in cavity  130 B. Accordingly, frame member  221 B is adhered to catalyst layer  213 B. 
     As described above, catalyst layer  213 B has multiple cracks in the outer edge. For this reason, material  125 B of frame member  221 B intrudes also into cracks in catalyst layer  213 B. By this means, adhesion between frame member  221 B and catalyst layer  213 B will be improved. 
     Further, as described above, frame member  121 A is exposed in cavity  130 B. Accordingly, frame member  221 B is adhered also to frame member  121 A. 
     By separating a molded article from cavity plate  131 B and core plate  133 B after die time has passed, frame  120  surrounding the edge of polymer electrolyte membrane  111  having catalyst layer  113 A and catalyst layer  213 B can be obtained ( FIG. 6F ). 
     (6)  FIG. 6G  shows the sixth step. 
     As shown in  FIG. 6G , in the sixth step, by hot pressing or the like, gas diffusion layer  115 A is disposed on catalyst layer  113 A and gas diffusion layer  115 B is disposed on on catalyst layer  213 B. 
     As described above, according to the present embodiment, not only frame member A but also frame member B is adhered to catalyst layer B. For this reason, adhesion between the frame and the membrane electrode assembly will be improved. 
     Experimental Example 1 
     In order to show that catalyst layer A of the present invention has cracks, a catalyst layer is made using the following method and the surface is observed with an electron microscope. 
     First, a paste material is prepared by mixing Ketjen Black (carbon powder) carrying 40 wt % of platinum as a metal catalyst, perfluorocarbon sulfonic acid (a binder), and a mix solvent of water/isopropyl alcohol/ethylene glycol (1:1:1). The weight ratio among carbon powder, binder, and solvent (carbon powder:binder:solvent) is 1:1:8. 
     The prepared paste material is applied on a film (release substrate) made of polyethylene terephthalate and is dried to make a catalyst layer. The drying period is set at for 2 minutes, and drying temperature is gradually elevated during the drying process. Specifically, during the drying process, the drying temperatures for the first 30 seconds, the subsequent 60 seconds, and the last 30 seconds are set at 50° C., 80° C., and 100° C., respectively. The average drying rate is 2.5 mg/cm 2 ·min. The surface of the obtained catalyst layer is observed with an optical microscope (QUICK VISION, Mitutoyo Corporation). 
       FIG. 7  shows a photograph that is obtained by converting the microscopic photograph (100-fold magnification, visual field area: 1.6 mm 2 ) of the surface of the obtained catalyst layer into 256 levels of gray (0-255) and binarizing the converted data by fixed threshold value processing, with a threshold value being set to 120. In  FIG. 7 , each of white parts C indicates a region in which a crack is formed. The area of white pixels accounts for about 15% of the entire microscopic photograph in  FIG. 7 . 
     Experimental Example 2 
     A catalyst layer is made using the same method as Experimental Example 1 except that the drying condition for a paste material to be applied is changed. Specifically, a catalyst layer is made using the same method as Experimental Example 1 except that, during a drying process, drying temperatures for the first 20 seconds, the subsequent 50 seconds, and the last 50 seconds are set at 60° C., 80° C., and 100° C., respectively. In other words, in Experimental Example 2, the average drying temperature is set higher than that of Experimental Example 1. Therefore, in Experimental Example 2, the drying rate of the paste material is higher than that of Experimental Example 1. 
     The surface of the obtained catalyst layer is observed with an optical microscope (QUICK VISION, Mitutoyo Corporation). Then, in the same way as Experimental Example 1, the area of white regions indicating cracks is determined by converting a microscopic photograph of the surface of the obtained catalyst layer (100-fold magnification, visual field area: 1.6 mm 2 ) into 256 levels of gray (0-255) and binarizing the converted data using fixed threshold value processing method with a threshold value being set to 120 (not shown). The determined area of cracks is 23%. 
     The results of Experimental Examples 1 and 2 show that a catalyst layer made by the method of the present invention has cracks that can contribute to the anchor effect. Further, the results of Experimental Examples 1 and 2 suggest that the amount of cracks can be increased by increasing the drying rate of a paste material. 
     This application is entitled and claims the benefit of Japanese Patent Application No. 2010-066282, filed on Mar. 23, 2010, the disclosure of which including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The frame-integrated membrane electrode assembly of the present invention is suitable for use as a component of polymer electrolyte fuel cells for portable power sources, power sources for electric vehicles, household cogeneration systems and the like. 
     REFERENCE SIGNS LIST 
     
         
           100 ,  200  frame-integrated membrane electrode assembly 
           101  manifold hole 
           103  sealing member 
           110  membrane electrode assembly (MEA) 
           111  polymer electrolyte membrane 
           113 ,  213  catalyst layer 
           114  crack 
           115  gas diffusion layer 
           120  frame 
           121 ,  221  frame member 
           125  material of frame 
           130  cavity 
           131  cavity plate 
           133  core plate