Abstract:
Disclosed is a method for manufacturing a membrane electrode assembly wherein a fuel cell electrode layer is formed on a material and is transferred to a fuel cell electrolyte membrane. The method includes the steps of: forming a fuel cell electrode layer on a first substrate layer; cutting from the fuel cell electrode layer side using cutting means so as to reach a second substrate layer, and forming a cut of a predetermined shape in the fuel cell electrode layer and the first substrate layer; and a removal step for peeling off an outer side portion of the predetermined shape from the second substrate layer.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for manufacturing a membrane electrode assembly used in a fuel cell. 
       BACKGROUND OF THE INVENTION 
       [0002]    Engines in which gasoline is used as a fuel, motors driven by electricity, and hybridized engine/motor combinations are used as power sources in automotive vehicles. Recently, fuel cell vehicles, in which a motor that is driven using electricity generated by a fuel cell, have been attracting attention due to environmental considerations. A fuel cell is an apparatus that supplies air and hydrogen to a membrane electrode assembly configured having an anode and a cathode with an electrolyte membrane interposed therebetween to generate electrical energy and water. The following method has been proposed for manufacturing membrane electrode assemblies employed in such fuel cells. 
         [0003]    As shown in  FIG. 5A , a mask  101  that has been cut to a predetermined shape is disposed on a substrate  100 , and as shown in  FIG. 5B , an electrode layer paste  102  to become an electrode layer is applied to the substrate  100 . The substrate  100  and mask  101  covered with the electrode layer paste  102  are placed inside a dryer, and dried at a predetermined temperature. The drying causes the electrode layer paste  102  to solidify. 
         [0004]    Next, the mask  101  is peeled from the substrate  100 . As shown in  FIG. 5C , a solid electrode layer  103  of predetermined shape is obtained. At this point, when the mask  101  is peeled from the substrate  100  after the electrode layer paste  102  has solidified, sharply peaked protuberant configurations  104  are formed on the outer peripheral edge of the electrode layer  103 . 
         [0005]    As shown in  FIG. 5D , the electrode layer  103  is transferred to one surface of an electrolyte membrane  105  using thermocompression bonding. A membrane electrode assembly is obtained by transferring an electrode layer in the same way to the other surface of the electrolyte membrane  105  using thermocompression bonding, and forming gas diffusion layers on the electrode layers on the both surfaces [of the electrolyte membrane  105 . However, when the electrode layer  103  on which the protuberant configurations  104  are formed is transferred to the electrolyte membrane  105 , the protuberant configurations  104  penetrate the electrolyte membrane  105  and damage the electrolyte membrane  105 . Various methods for manufacturing membrane electrode assemblies that address such drawbacks have been proposed (e.g. Japanese Patent Application Laid-Open Publication No. 2014-67483). 
         [0006]    As shown in  FIG. 6A , a rectangular substrate  110  is prepared, and as shown in  FIG. 6B , a mask  111  that has been cut in a predetermined shape is disposed on the substrate  110 . As shown in  FIG. 6C , an electrode layer paste  112  is applied to the substrate  110 , and as shown in  FIG. 6D , partial drying is performed by leaving the substrate  110  in a hot chamber to make the electrode layer paste  112  a semi-solid. Partial drying is performed so that that electrode layer paste  112  will have sufficient viscosity to retain its shape when the mask is removed. As shown in  FIG. 6E , the mask  111  is peeled from the substrate  110  while the electrode layer paste  112  impregnated with a solvent is wet. By removing the mask  111 , the outer peripheral edge part of the electrode layer paste  112  is also removed, and the electrode layer paste  112  assumes the desired shape. 
         [0007]    As shown in  FIG. 7 , the outer peripheral edge part  113  of the electrode layer paste  112 , being in a partially dry state, flows toward the outer periphery under gravity, and slants to a taper so as to decrease in thickness toward the outer periphery. 
         [0008]    Next, as shown in  FIG. 6F , the electrode layer paste  112  is placed in a dryer  114  together with the substrate  110  and fully dried, and, as shown in  FIG. 6G , an electrode layer  115  that has formed into a solid on the substrate  110  is obtained. As shown in  FIG. 6H , substrates  110 ,  110  having the electrolyte membrane  116  therebetween are thermocompression bonded by a press machine  118  so that the electrode layer  115  on the anode side and an electrode layer  117  on the cathode side make contact with the electrolyte membrane  116 . As shown in  FIG. 6I , peeling the substrates  110 ,  110  from the electrode layers  115 ,  117  yields an electrolyte membrane  116  on which the electrode layers  115 ,  117  are disposed on both surfaces. 
         [0009]    However, because the mask  111  is removed while the electrode layer paste  112  is wet (semi-solid), the outer peripheral edge of the electrode layers  115 ,  117  may change slightly in shape. Accordingly, a method is required for manufacturing a membrane electrode assembly that enables the shape of the electrode layer to be formed accurately without mechanically damaging the electrolyte membrane. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention addresses the problem of providing a method for manufacturing a membrane electrode assembly that enables the shape of the electrode layer to be formed accurately without causing mechanical damage to the fuel cell electrolyte membrane. 
         [0011]    According to the present invention, there is provided a method for manufacturing a membrane electrode assembly in which a fuel cell electrode layer is formed on a substrate, and the fuel cell electrode layer is transferred to a fuel cell electrolyte membrane, which the method comprises: a substrate preparation step for preparing the substrate, in which a first substrate layer and a second substrate layer are laminated with a primary adhesive layer interposed therebetween; an electrode layer formation step for forming the fuel cell electrode layer on the first substrate layer; a cutting step for cutting, using cutting means, from the fuel cell electrode layer side so as to reach the second substrate layer, and forming a cut of a predetermined shape in the fuel cell electrode layer and the first substrate layer; a removal step for peeling off an outer side portion of the predetermined shape from the second substrate layer; and a transfer step for transferring the fuel cell electrode layer of predetermined shape from the substrate to the fuel cell electrolyte membrane. 
         [0012]    In the thus-arranged invention, a substrate is prepared in which a first substrate layer and a second substrate layer are laminated with a primary adhesive layer interposed therebetween, and a fuel cell electrode layer is formed on the first substrate layer. Cutting is performed from the fuel cell electrode layer side using cutting means so as to reach the second substrate layer to form a cut of a predetermined shape in the fuel cell electrode layer and the first substrate layer. When an outer side portion of the predetermined shape is peeled off from the second substrate layer, the fuel cell electrode layer and the first substrate layer are peeled together from the second substrate layer, and accordingly there is no incidence of just the fuel cell electrode layer being sheared by the edge of the mask and protuberant configurations being formed on the outer peripheral edge of the electrode layer, as in the prior art. As a result, when the fuel cell electrode layer is transferred to the fuel cell electrolyte membrane, the outer peripheral edge of the electrode layer does not penetrate the fuel cell electrolyte membrane, and no mechanical damage is done to the fuel cell electrolyte membrane. Further, because the cutting is performed using cutting means after the fuel cell electrode layer has been dried and formed into a solid, the shape of the electrode layer can be formed accurately without the outer peripheral edge part of the partially dried electrode layer paste gravitationally flowing toward the outer periphery as in the prior art. 
         [0013]    Preferably, a first auxiliary substrate layer and a second auxiliary substrate layer of the first substrate layer are pasted together with auxiliary adhesive layer interposed therebetween, and the adhesive strength of the auxiliary adhesive layer is greater than the adhesive strength of the primary adhesive layer. Since the adhesive strength of the auxiliary adhesive layer is greater than the adhesive strength of the primary adhesive layer between the second auxiliary substrate layer and the second substrate layer, the second auxiliary substrate layer can be easily peeled from the backup second substrate layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic cross-sectional view illustrating a membrane electrode assembly according to the present invention; 
           [0015]      FIGS. 2A-2D  are views illustrating a substrate preparation step to a transfer step; 
           [0016]      FIG. 3  is an exploded perspective view illustrating features of a unit cell constituting a fuel cell; 
           [0017]      FIG. 4A  to  FIG. 4D  are views illustrating another mode of  FIG. 2 ; 
           [0018]      FIG. 5A  to  FIG. 5D  are views illustrating conventional substrate preparation to transfer steps; 
           [0019]      FIGS. 6A-6I  schematically illustrate a flow of a conventional method for manufacturing a membrane electrode assembly; and 
           [0020]      FIG. 7  is a cross-section illustrating major elements of a conventional substrate and electrode layer with a mask removed. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    As shown in  FIG. 1 , the membrane electrode assembly  10  employed in a fuel cell comprises a fuel cell electrolyte membrane  11 , an anode  21  disposed on one surface  12  of the fuel cell electrolyte membrane  11 , and a cathode  31  disposed on the other surface  13  of the fuel cell electrolyte membrane  11 . The fuel cell electrolyte membrane  11  is, for example, a polymer electrolyte membrane in which a perfluorosulfonic acid thin film is impregnated with water. 
         [0022]    The area of the fuel cell electrolyte membrane  11  is set larger than the areas of the anode  21  and the cathode  31 , but may be set to be the same as the areas of the anode  21  and the cathode  31 . Further, the area of the anode  21  and the area of the cathode  31  may be different. 
         [0023]    The anode  21  is configured from a first fuel cell electrode layer  22  disposed on the one surface  12  of the fuel cell electrolyte membrane  11 , and a first gas diffusion layer  23  disposed so as to cover the first fuel cell electrode layer  22 . The cathode  31  is configured from a second fuel cell electrode layer  32  disposed on the other surface  13  of the fuel cell electrolyte membrane  11 , and a second gas diffusion layer  33  disposed so as to cover the second fuel cell electrode layer  23 . 
         [0024]    The first fuel cell electrode layer  22  and the second fuel cell electrode layer  32  are obtained, for example, by preparing an electrode layer slurry comprising carbon that supports a catalyst made of platinum particles or the like, a polymer electrolyte, a solvent (water, alcohol, or a mixture thereof), and carbon fiber; applying the slurry to a transfer substrate; and thermocompression bonding the coated substrate to the polymer electrolyte membrane layer (described in detail below). The slurry is obtained by mechanically mixing the solids and liquids and obtaining a fluidized body. The first gas diffusion layer  23  and the second gas diffusion layer  33  comprise, for example, carbon paper or carbon cloth. 
         [0025]    An outer peripheral edge  24  of the first fuel cell electrode layer  22  is positioned on the one surface  12  without penetrating the fuel cell electrolyte membrane  11 . An outer peripheral edge  34  of the second fuel cell electrode layer  32  is positioned on the other surface  13  without penetrating the fuel cell electrolyte membrane  11 . The shapes of the outer peripheral edge  24  of the first fuel cell electrode layer  22  and the fuel cell electrolyte membrane  11  are exaggerated for the sake of convenience. Further, hereinafter as well, descriptions will be given using diagrams in which the shapes of the fuel cell electrolyte membrane  11 , the first fuel cell electrode layer  22 , and so forth have been exaggerated. 
         [0026]    Next, a description of the substrate preparation step to the transfer step will be given. 
         [0027]    As shown in  FIG. 2A , a first substrate layer  41  that serves as the transfer substrate and a second substrate layer  42  that serves as a backup substrate are laminated with a primary adhesive layer  43  interposed therebetween, and a substrate  40  is obtained. The substrate  40  is prepared on a base  51  (substrate preparation step). 
         [0028]    In the first substrate layer  41 , a first auxiliary substrate layer  41   a  and a second auxiliary substrate layer  41   b  are pasted together with an auxiliary adhesive layer  44  interposed therebetween. The adhesive strength of the auxiliary adhesive layer  44  is greater than the adhesive strength of the primary adhesive layer  43 . Therefore, in the removal step, which will be described below, the second auxiliary substrate layer  41   b  can be easily peeled from the backup second substrate layer  42 . 
         [0029]    As shown in  FIG. 2B , an electrode layer slurry  22   a  comprising carbon that supports a catalyst made of platinum particles or the like, a polymer electrolyte, a solvent, and carbon fiber is prepared and applied on the first substrate layer  41  of the substrate  40 . The electrode layer slurry  22   a  is dried together with the substrate  40  using drying means  52 . Thus, the first fuel cell electrode layer  22  is formed on the first substrate layer  41  (electrode layer formation step). 
         [0030]    Furthermore, the electrode layer slurry  22   a  that has been dried to a solid is used as the fuel cell electrode layer  22 . 
         [0031]    As shown in  FIG. 2C , cutting means  53 , which is disposed so as to be capable of moving relatively with respect to the base  51 , is prepared. The cutting means  53  is made to move as indicated by arrow ( 1 ), and cuts from the first fuel cell electrode layer  22  side so as to reach partway through the second substrate layer  42 . As a result, predetermined cuts are formed in the first fuel cell electrode layer  22  and the first substrate layer  41  (cutting step). 
         [0032]    By cutting from the first fuel cell electrode layer  22  side using the cutting means  53 , top parts of the cutting surface of the first fuel cell electrode layer  22  are penetrated and caused to curve slightly downwardly in the drawing. When the first fuel cell electrode layer  22  is to be transferred to the fuel cell electrolyte membrane  11  (refer to  FIG. 1 ), the curved part side of the first fuel cell electrode layer  22  come in contact with the fuel cell electrolyte membrane  11 , making it possible to further reduce the mechanical damage caused to the fuel cell electrolyte membrane  11 . 
         [0033]    As shown in  FIG. 2D , the cutting means  53  is retracted as indicated by arrow ( 2 ). Outer side portions  45 ,  45  of a predetermined shape are peeled off from the second substrate layer  42  as indicated by arrows ( 3 ) (removal step). 
         [0034]    Thus, the predetermined-shape outer side portions  45 ,  45  are peeled from the second substrate layer  42 . Since the first fuel cell electrode layer  22  and the first substrate layer  41  are peeled off together from the second substrate layer  42 , there is no incidence of just the fuel cell electrode layer being sheared by the edge of the mask and protuberant configurations being formed on the outer peripheral edge of the electrode layer, as in the prior art. As a result, when the first fuel cell electrode layer  22  is transferred to the fuel cell electrolyte membrane  11  (refer to  FIG. 1 ), the outer peripheral edge  24  of the first fuel cell electrode layer  22  does not penetrate the fuel cell electrolyte membrane  11 , making it possible to prevent the fuel cell electrolyte membrane  11  from experiencing mechanical damage. 
         [0035]    Further, because the cutting is performed using the cutting means  53  after the first fuel cell electrode layer  22  has been dried and formed into a solid, the shape of the outer peripheral edge of the first fuel cell electrode layer  22  can be accurately formed without the outer peripheral edge part of the partially dried electrode layer paste gravitationally flowing toward the outer periphery as in the prior art. A substrate on which the second fuel cell electrode layer  32  (refer to  FIG. 1 ) is formed is obtained in the same manner. 
         [0036]    Next, likewise with respect to the prior art shown in  FIG. 6( h ) , substrates  40  are positioned on either side of the fuel cell electrolyte membrane  11 , and the assembly is thermocompression bonded by a press machine so that the first fuel electrode layer  22  of the anode side and the second fuel cell electrode layer  32  of the cathode side make contact with the fuel cell electrolyte membrane  11 . When the first substrate  41  is peeled from the first fuel cell electrode layer  22  and the second fuel cell electrode layer  32  after the press machine has been retracted, the fuel cell electrolyte membrane  11  having the first fuel cell electrode layer  22  and the second fuel cell electrode layer  32  disposed on both surfaces is obtained (transfer step). 
         [0037]    In addition, the membrane electrode assembly  10  (refer to  FIG. 1 ) is obtained by forming the first gas diffusion layer  23  on the first fuel cell electrode layer  22  side, and forming the second gas diffusion layer  33  on the second fuel cell electrode layer  32  side. In the present invention, the workload can be reduced since drying requires only one step, not the two drying steps as in the prior art; i.e. partial drying and full drying. Further, since the mechanical damage to the fuel cell electrolyte membrane  11  is reduced, it is possible to manufacture an exceptionally durable fuel cell exhibiting minimal incidence of “cross leaking.” 
         [0038]    Next, the operation of a fuel cell in which the above-described membrane electrode assembly  10  is employed will be described. 
         [0039]    As shown in  FIG. 3 , the unit cell  60  that constitutes the fuel cell includes the fuel cell electrolyte membrane  11 , and a separator  61  and a second separator  71  disposed on both sides of the fuel cell electrolyte membrane  11 . For the sake of convenience, the unit cell  60  will be explained using an exploded perspective view. An oxidant gas, such as air, is supplied to an oxidant gas inlet communication hole  62 , and a fuel gas, such as hydrogen, is supplied to a fuel gas inlet communication hole  63 . In addition, a cooling medium, such as pure water, ethylene glycol, oil, or the like, is supplied to a cooling medium inlet communication hole  64 . 
         [0040]    The fuel gas is introduced from the fuel gas inlet communication hole  63  into a fuel gas flow channel  66  of the first separator  61  through a supply hole part  46 , and thereafter, moves along the fuel gas flow channel  66  in the direction of the Y arrow and is supplied to the anode  21  of the membrane electrode assembly  10 . In the anode  21 , the fuel gas passes through the first gas diffusion layer  23  (refer to  FIG. 1 ) while being diffused, and reaches the first fuel cell electrode layer  22 . 
         [0041]    Thereafter, the hydrogen in the fuel gas is ionized in the first fuel cell electrode layer  22  and a reaction in which protons are generated occurs. The protons move to the second fuel cell electrode layer  32  of the cathode  31  (refer to  FIG. 1 ) by the proton conduction of the fuel cell electrolyte membrane  11 . Electrons are used as a source of electrical energy for energizing an external load electrically connected to a solid polymer fuel cell. 
         [0042]    Meanwhile, the oxidant gas is introduced from the oxidant gas inlet communication hole  62  to an oxidant gas flow channel  72  of the second separator  71 , moves in the direction of the Y arrow, and is supplied to the cathode  31  of the membrane electrode assembly  10 . 
         [0043]    In the cathode  31 , the oxidant gas passes through the second gas diffusion layer  33  (refer to  FIG. 1 ) while being diffused, and reaches the second fuel cell electrode layer  23 . Thereafter, in the second fuel cell electrode layer  23 , the oxygen in the oxidant gas, the protons that have moved through the fuel cell electrolyte membrane  11 , and the electrons that have reached the cathode  31  by energizing the external load undergo a reaction and produce water. 
         [0044]    The fuel gas that was supplied to the anode  21  and consumed passes through an exhaust hole part  67  and exhausts in the direction of the X arrow along a fuel gas outlet communication hole  68 . Similarly, the oxidant gas supplied to the cathode  31  and consumed exhausts in the direction of the X arrow along an oxidant gas outlet communication hole  69 . 
         [0045]    While electricity is being generated as described above, a cooling medium is supplied through a cooling medium inlet communication hole  64  to a cooling medium flow channel  73  between the first separator  61  and the second separator  71 . The cooling medium, after cooling the membrane electrode assembly  10  by flowing in the direction of the Y arrow, exhausts via a cooling medium outlet communication hole  74 . 
         [0046]    Next, another mode of  FIG. 2  will be described on the basis of the drawings. The same reference signs are assigned to structures that are the same as the structures shown in  FIG. 2 , and detailed descriptions thereof will be omitted. 
         [0047]    As shown in  FIG. 4A , a first substrate layer  41  that serves as a transfer substrate and a second substrate layer  42  that serves as a backup substrate are laminated with a primary adhesive layer  43  interposed therebetween, and a substrate  40  is obtained. The substrate  40  is prepared on a base  51  (substrate preparation step). 
         [0048]    As shown in  FIG. 4B , an electrode layer slurry  22   a  comprising carbon that supports a catalyst made of platinum particles or the like, a polymer electrolyte, a solvent, and carbon fiber is prepared and applied on the first substrate layer  41  of the substrate  40 . The electrode layer slurry  22   a  is dried in a dryer  52  together with the substrate  40 . Thus, the first fuel cell electrode layer  22  is formed on the first substrate layer  41  (electrode layer formation step). 
         [0049]    As shown in  FIG. 4C , cutting means  53 , which is disposed so as to be capable of moving relatively with respect to the base  51 , is prepared. The cutting means  53  is made to move as indicated by arrow ( 4 ), and cuts from the first fuel cell electrode layer  22  side so as to reach partway through the second substrate layer  42 . As a result, predetermined cuts are formed in the first fuel cell electrode layer  22  and the first substrate layer  41  (cutting step). 
         [0050]    As shown in  FIG. 4D , the cutting means  53  is retracted as indicated by arrow ( 5 ). Outer side portions  45 ,  45  of the predetermined shape are peeled off from the second substrate layer  42  as indicated by arrows ( 6 ) (removal step). Making the first substrate layer  41  a single layer enables the overall substrate  40  to assume a simple structure. 
         [0051]    In the embodiment, the first fuel cell electrode layer  22  and the second fuel cell electrode layer  32  are covered by the first gas diffusion layer  23  and the second gas diffusion layer  33 , respectively, but no limitation is presented thereby; the electrode layers  22 ,  32  may be covered by an adhesive layer formed from a fluorine-containing adhesive. 
         [0052]    Obviously, various minor changes and modifications of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.