Abstract:
A method for mounting a seal in a fuel cell comprising: a membrane electrode assembly formed by holding an electrolyte membrane between a first electrode and a second electrode and having a seal mounting portion; a separator plate layered on both surfaces of the membrane electrode assembly so as to form gas passage; and a frame-shaped separator plate held between the membrane electrode assembly and the separator plate so as to seal the gas passage in air tight. The method comprises: preforming the seal into a predetermined shape; setting the seal at the mounting portion of the membrane electrode assembly; and integrally forming the seal with the membrane electrode assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a method for mounting seals used for gas sealing at a predetermined portion in a polymer electrolyte fuel cell, and relates to fuel cells having such seals. 
     2. Background Art 
     In polymer electrolyte fuel cells, a separator plate is layered on both sides of a plate-shaped membrane electrode assembly to form a unit of the layered structure, and the plural units are layered to form a fuel cell stack. The membrane electrode assembly is a layered structure, in which an electrolyte membrane is held by gas-diffusion electrode plates at a positive side and a negative side. The separator plate is made from a material having electron transmitting characteristics, and has plural grooved gas passages in which a fuel gas such as hydrogen gas, an oxidizing gas such as oxygen or air, and a coolant flow individually. The separator plate is layered on the membrane electrode assembly such that linear protrusions between the gas passages are contacted with the gas-diffusion electrode plates. 
     In the fuel cell, a fuel gas is provided to the gas passage of the separator plate at the negative electrode side, and an oxidizing gas is provided to the gas passage of the separator plate at the positive electrode side, whereby electricity is generated by electrochemical reaction. During the operation of the fuel cell, the gas-diffusion electrode plates transmit the electrons generated by the electrochemical reaction between the gas-diffusion electrode plates and the separator plates, and diffuse the fuel gas and the oxidizing gas. The electrode plate at the negative electrode side produces a chemical reaction for the fuel gas so as to generate protons and electrons. The electrode plate in the positive electrode side generates water from oxygen, the protons, and the electrons, and the electrolyte membrane facilitates ionic migration for the proton, whereby the electric power is provided via the positive and negative electrode plates. 
     In the above-described fuel cell, the fuel gas, the oxidizing gas, and the coolant must be flowed in the individual gas passages, so that the gas passages are separated from each other by a seal. The sealing portion varies according to the structure of the fuel cell stack. For example, a seal is provided around communicating openings of the gas passages penetrating the fuel cell stack, around the membrane electrode assembly, around a coolant passage provided on the outer surface of the separator plate, and around the circumference of the outer surface of the separator plate. 
     According to conventional sealing technology, in general, an elastic material made from an organic rubber of the fluorine type, silicone type, ethylene propylene type, or the like, is formed into a shape of a sheet or an O-ring, and is mounted to a sealing portion. The sealing member seals the sealing portion by a reaction force generated by being compressed in a stacked condition. As other sealing structures, a seal in which an inorganic material formed of carbon or ceramics is compressed, a mechanical seal using caulking, and the like have been provided. 
     Fuel cells are often carried or installed in automobiles for use. In these cases, the cells are stringently required to be small and thin. Since separator plates are usually made from brittle carbon, they are readily broken during assembly of a fuel cell stack. Therefore, seals made from organic rubbers are widely used, since they are flexible and have suitable reaction force, thereby preventing breakage of the separator plate in the assembly of a fuel cell stack. 
     In order to mount such a seal between an electrolyte membrane of a membrane electrode assembly and a separator plate, heretofore, the membrane electrode assembly was installed in a die, a vulcanized rubber as a material for the seal was charged into a cavity formed in the die, and the material for the seal was hardened to be integrally formed with the electrolyte membrane. 
     Electrolyte membranes are easily deformed and wrinkled due to humidity, so that sealing properties cannot be ensured, and problems in which sufficient clamping thickness for the seal cannot be obtained occur due to variation of the thickness of the membrane electrode assembly. The mounting method in which seals are integrally formed with the membrane electrode assembly has been mentioned to be effective for overcoming such problems. However, in such a method, the vulcanization for the seal material is generally performed at a high temperature and at a high pressure, so that an excess amount of heat load is exerted on the electrolyte membrane and the electrode plate. As a result, the electrolyte membrane and the electrode plate are deteriorated, and they are contaminated and damaged during the handing thereof in the vulcanization in some cases. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a method for suitably mounting seals in a membrane electrode assembly without adverse effects thereto, and to provide a fuel cell having such a seal. 
     The present invention provides a method for mounting a seal in a fuel cell comprising: a membrane electrode assembly formed by holding an electrolyte membrane between a first electrode and a second electrode and having a seal mounting portion; a separator plate layered on both surfaces of the membrane electrode assembly so as to form gas passage; and a frame-shaped separator plate held between the membrane electrode assembly and the separator plate so as to seal the gas passage in air tight; the method comprising: preforming the seal into a predetermined shape; setting the seal at the mounting portion of the membrane electrode assembly; and integrally forming the seal with the membrane electrode assembly. 
     According to the invention, since the seal has been preformed, and is integrally formed with the membrane electrode assembly, excess heat load is not exerted on the membrane electrode assembly in comparison with the conventional method in which a seal is adhered to a membrane electrode assembly by vulcanization. Therefore, problems such as deterioration, contamination, and damage to electrode plates and the electrolyte membrane can be avoided, and the seal can be normally mounted on the membrane electrode assembly. Since the seal is mounted on the membrane electrode assembly, operation for handling separated seals can be omitted when the membrane electrode assemblies and seals are alternately layered to form a fuel cell stack, and the seal does not easily become twisted and misaligned, whereby the sealing properties can be improved. 
     The present invention further provides a method for mounting a seal in a fuel cell comprising: a membrane electrode assembly formed by holding an electrolyte membrane between a first electrode and a second electrode; a separator plate layered on both surfaces of the membrane electrode assembly so as to form gas passage; and a frame-shaped separator plate held between the membrane electrode assembly and the separator plate so as to seal the gas passage in air tight; the method comprising: using a hot pressing die having a first die and a second die; setting the first electrode in the first die; preforming the seal into a predetermined shape and coating an adhesive on a portion thereof with which the electrolyte membrane is contacted; setting the seal at a circumference of the first electrode in the first die; layering the electrolyte membrane on the adhesive coated on the seal and the first electrode; layering the second electrode on the electrolyte membrane; and close-contacting the first and second electrodes, the electrolyte membrane, and the seal by holding them with the first and second dies, and integrally forming them by hot pressing. This invention includes embodiments in which a reinforcement member is inserted into the seal in a condition in which a portion thereof is exposed, and an adhesive is coated on the exposed portion of the reinforcement member, and in which a reinforcement member is inserted into the seal, and an adhesive is coated on the seal. 
     The present invention further provides a method for mounting a seal in a fuel cell comprising: a membrane electrode assembly formed by holding an electrolyte membrane between a first electrode and a second electrode; a separator plate layered on both surfaces of the membrane electrode assembly so as to form gas passage; and a frame-shaped separator plate held between the membrane electrode assembly and the separator plate so as to seal the gas passage in air tight; the method comprising: using a hot pressing die having a first die and a second die; setting the first electrode in the first die; preforming the seal into a predetermined shape in a condition in which a reinforcement member is inserted into the seal and an inner portion of the reinforcement member projects inwardly; layering the seal at a circumference of the first electrode in the first die in a condition in which the inner projected portion of the reinforcement member overlaps with a portion of the first electrode; layering the electrolyte membrane on the first electrode in a condition in which the inner projected portion of the reinforcement member is held between the first electrode and the electrolyte membrane; layering the second electrode on the electrolyte membrane; and close-contacting the first and second electrodes, the electrolyte membrane, the seal, and the reinforcement member by holding them with the first and second dies, and integrally forming them by hot pressing. 
     The present invention further provides a method for mounting a seal in a fuel cell comprising: a membrane electrode assembly formed by holding an electrolyte membrane between a first electrode and a second electrode; a separator plate layered on both surfaces of the membrane electrode assembly so as to form gas passage; and a frame-shaped separator plate held between the membrane electrode assembly and the separator plate so as to seal the gas passage in air tight; the method comprising: using a hot pressing die having a first die and a second die; setting the first electrode in the first die; preforming the seal into a predetermined shape in which a inner portion thereof projects inwardly; layering the seal at a circumference of the first electrode in the first die in a condition in which the inner projected portion of the seal overlaps with a portion of the first electrode; layering the electrolyte membrane on the first electrode in a condition in which the inner projected portion of the seal is held between the first electrode and the electrolyte membrane; layering the second electrode on the electrolyte membrane; and close-contacting the first and second electrodes, the electrolyte membrane, and the seal by holding them with the first and second dies, and integrally forming them by hot pressing. 
     In the above three specific features, the seal can be mounted on the electrolyte membrane of the membrane electrode assembly at the same time as the membrane electrode assembly consisting of the electrode plates and the electrolyte membrane is formed. 
     The present invention further provides a method for mounting a seal in a fuel cell comprising: a membrane electrode assembly formed by holding an electrolyte membrane between a first electrode and a second electrode; a separator plate layered on both surfaces of the membrane electrode assembly so as to form gas passage; and a frame-shaped separator plate held between the membrane electrode assembly and the separator plate so as to seal the gas passage in air tight; the method comprising: using a hot pressing die having a first die and a second die; preforming the seal into a predetermined shape and coating an adhesive on a portion thereof with which the electrolyte membrane is contacted; setting the seal in the first die; preforming the membrane electrode assembly so as to expose a portion of the electrolyte membrane toward a surface of the membrane electrode assembly; setting the membrane electrode assembly in the first die in a condition in which the exposed portion of the electrolyte membrane overlaps with the adhesive coated on the seal; close-contacting seal and the membrane electrode assembly by holding them with the first and second dies, and integrally forming them by hot pressing. 
     In the invention, the seal may be formed from materials of the elastomer type which require heating for vulcanizing or hardening, or materials of the thermoplastic elastomer type which do not require heating. As other materials for the seal, liquid materials of the cold setting type or the thermosetting type may be mentioned. The reinforcement member may be a sheet formed from a resin or a metal, or a wire made from a resin or a metal. 
     The present invention also provides a fuel cell formed by layering plural membrane electrode assemblies via a separator plate respectively, wherein the membrane electrode assembly is mounted with a seal by the above method for mounting a seal in a fuel cell. 
    
    
     
       BRIEF EXPLANATION OF THE DRAWINGS 
         FIGS. 1A to 1E  are vertical cross sections showing sequential steps in a method for mounting seals according to a first embodiment of the invention. 
         FIGS. 2A and 2B  are vertical cross sections showing sequential steps in a method for mounting seals according to a second embodiment of the invention. 
         FIGS. 3A and 3B  are vertical cross sections showing sequential steps in a method for mounting seals according to a third embodiment of the invention. 
         FIGS. 4A and 4B  are vertical cross sections showing sequential steps in a method for mounting seals according to a fourth embodiment of the invention. 
         FIGS. 5A and 5B  are vertical cross sections showing sequential steps in a method for mounting seals according to a fifth embodiment of the invention. 
         FIGS. 6A and 6B  are vertical cross sections showing sequential steps in a method for mounting seals according to a sixth embodiment of the invention. 
         FIGS. 7A and 7B  are vertical cross sections showing sequential steps in a method for mounting seals according to a seventh embodiment of the invention. 
         FIGS. 8A to 8E  are vertical cross sections showing sequential steps in a method for mounting seals according to a eighth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will be explained hereinafter with reference to the figures. 
     (1) First Embodiment 
       FIGS. 1A to 1E  are vertical cross sections showing sequential steps in a method for mounting seals according to a first embodiment. Reference numeral  10 A is a seal which is preformed, and  20  is membrane electrode assembly. The seal  10 A is formed into a rectangular frame in plane view, and is applied to be held between circumferences of a layered structure consisting of the membrane electrode assembly  20  and a separator plate (not shown) so as to air tight seal gas passages formed between the membrane electrode assembly  20  and the separator plate. The lower surface of the seal  10 A is formed in duplicate with an outer linear protrusion  11  having a trapezoidal cross section and an inner linear protrusion  12  having a smaller width than the outer linear protrusion  11 . The inner linear protrusion  12  can be omitted when the separator plate has a linear protrusion at a position corresponding to the inner linear protrusion  12 . The upper surface of the seal  10 A is formed with a linear protrusion  13  symmetrically with the outer linear protrusion  11  at the lower side with respect to the vertical direction. The inner portion of the linear protrusion  13  on the upper surface of the seal  10 A is formed with a step portion  14  having a rectangular cross section. The inner portion of the step portion  14  is formed with a plane close-contacting surface  15  for close-contacting with a electrolyte membrane  23  forming the membrane electrode assembly  20 . The seal  10 A is formed from materials of the elastomer type which require heating for vulcanizing or hardening, materials of the thermoplastic elastomer type which do not require heating, and liquid materials of the cold setting type or the thermosetting type. 
     Numeral  30  and  40  in  FIG. 1  are rectangular upper and lower dies for forming a hot pressing die. As shown in  FIG. 1A , the circumference of the upper surface of the lower die  4  is formed in duplicate with an outer groove  41  and an inner groove having a trapezoidal cross section. These grooves  41  and  42  are rectangular along the circumference of the lower die  40 , into which the outer linear protrusion  11  and the inner linear protrusion  12  on the lower side of the seal  10 A are fitted. 
     As shown in  FIG. 1E , the circumference of the lower surface of the upper die  30  is formed with a groove  31  similarly with that of the outer groove  41  of the lower die  40 . The groove  31  corresponds to the outer groove  41  of the lower die  40 , into which the outer linear protrusion  13  of the seal  10 A is fitted. The center portions of the upper and lower dies  30  and  40  are formed with rectangular recesses  33  and  43 , into which a positive electrode plate  21  and a negative electrode plate  22 A, which form the membrane electrode assembly  20 , are fitted respectively. In this case, the area of the recess  43  of the lower die  40  is larger than that of the recess  33  of the upper die  30 , so that the overall circumference of the recess  33  of the upper die  30  projects outward from the recess  43  of the lower die  40  when the upper and lower dies  30  and  40  are brought together. 
     As shown in  FIG. 1D , the membrane electrode assembly  20  is a three-layered structure in which the electrolyte membrane  23  is held by a pair of the gas-diffusion electrode plates (positive electrode plate  21  and negative electrode plate  22 A). The electrolyte membrane  23  is made from, for example, fluorine-type membranes with a sulfonic acid group at a side-chain thereof (for example, Flemion (trade name) produced by Asahi Glass Co., Ltd., and Nafion (trade name) produced by DuPont). In this case, the area of the negative electrode plate  22 A is smaller than that of the positive electrode plate  21 , and the electrolyte membrane  23  has the same area as the positive electrode plate  21 . When these are layered with each other, the overall circumferences of the lower surfaces of the positive electrode plate  21  and electrolyte membrane  23  project outward from the negative electrode plate  22 A, and the circumference of the lower surface of the electrolyte membrane  23  is exposed. 
     Next, the procedure for mounting the seal  10 A to the membrane electrode assembly  20  will be explained with reference to  FIGS. 1A to 1E . 
     As shown in  FIG. 1A , the negative electrode plate  22 A is fitted into the recess  43  of the lower die of the hot pressing die. In this condition, approximately half the height of the negative electrode plate  22 A projects from the recess  43 . Then, as shown in  FIG. 1B , an adhesive is coated on the close-contacting surface  15  of the seal  10 A, and the seal  10 A is set to the lower die  40  by fitting the linear protrusions  11  and  12  on the lower surface side into the grooves  41  and  42 . In this condition, the close-contacting surface  15  coincides with the upper surface of the negative electrode plate  22 A. Then, as shown in  FIG. 1C , the electrolyte membrane  23  is layered on the negative electrode plate  22 A and the close-contacting surface  15  of the seal  10 A. The electrolyte membrane  23  has a size so as to be contained within the inside of the step portion  14  of the seal  10 A without clearance. Then, as shown in  FIG. 1D , the positive electrode plate  21  is layered on the electrolyte membrane  23 . 
     As shown in  FIG. 1E , the upper and lower dies  30  and  40  are brought together in fitting the linear protrusion  13  and the positive electrode plate  21  into the groove  31  and the recess  33  respectively. The seal  10 A and the membrane electrode assembly  20  are clamped by the dies  30  and  40 , and hot pressing is performed. The conditions for the hot pressing are, for example, a temperature of 150 to 160° C. a time of 1 to 2 minutes, and a pressure of 1 to 2 MPa. By the hot pressing, the electrolyte membrane  23  is close-contacted with the positive electrode plate  21  and the negative electrode plate  22 A, so that the membrane electrode assembly  20  is assembled, and the seal  10 A is integrally adhered to the electrolyte membrane  23 , that is, the membrane electrode assembly, via the adhesive  50 . 
     According to the above embodiment, since the seal  10 A has been preformed, the seal is integrally formed with the membrane electrode assembly  20 , excess heat load is not exerted on the membrane electrode assembly  20  in comparison with the conventional method in which a seal is adhered to a membrane electrode assembly by vulcanization. Therefore, problems such as deterioration, contamination, and damage to the positive electrode plate  21 , the negative electrode plate  22 A, and the electrolyte membrane  23  can be avoided, and the seal  10 A can be normally mounted on the membrane electrode assembly  20 . Since the seal  10 A is mounted on the membrane electrode assembly  20 , operation for handling separated seals can be omitted when the membrane electrode assemblies  20  and seals are alternately layered to form a fuel cell stack, and the seal  10 A does not easily become twisted and misaligned, whereby the sealing properties can be improved. Furthermore, since the assembly of the membrane electrode assembly  20  and the mounting the seal  10 A to the membrane electrode assembly  20  can be performed at the same time as the hot pressing, the process can be simplified. 
     Next, second through eighth embodiments of the invention will be explained hereinafter. In the explanations of these embodiments, numerals corresponding to those in the first embodiment are attached to the elements corresponding to those in the first embodiment, and explanations for these elements will be omitted. The hot pressing die is not shown in the figures except for  FIG. 8 . 
     (2) Second Embodiment 
       FIGS. 2A and 2B  show a mounting method for seals according to a second embodiment. In the second embodiment, a frame-shaped reinforcement member  60 A is inserted in the inner circumference of the seal  10 A. The reinforcement member  60 A is a thin-plate sheet made from a resin or a metal, the outer circumference thereof is inserted in the lower portion of the step portion  14  of the seal  10 A, and the upper surface of the inner circumference thereof is exposed so as to form close-contacting surface  61  similar to the close-contacting surface  15 . The inner end surfaces of the reinforcement member  60 A and the seal  10 A coincide with each other. 
     The procedure for mounting seals in the second embodiment is the same as in the first embodiment. As shown in  FIG. 2A , an adhesive  50  is coated on the close-contacting surface  61  of the reinforcement member  60 A. As shown in  FIG. 2B , in hot pressing, the close-contacting surface  61  is adhered to the electrolyte membrane  23  via the adhesive  50 , and the seal  10 A is integrally mounted on the membrane electrode assembly  20 . 
     (3) Third Embodiment 
       FIGS. 3A and 3B  show a mounting method for seals according to a third embodiment. In the third embodiment, the entire portion of the reinforcement member  60 A except for the inner end surface is inserted in the inner circumference of the seal  10 A. The reinforcement member  60 A is located at a position lower than that of the second embodiment, and only the inner end surface is exposed at the inner end surface of the seal  10 A. It should be noted that the inner end surface of the reinforcement member  60 A need not be exposed. The close-contacting surface  15  is designed similarly to that in the first embodiment. 
     The procedure for mounting seals in the third embodiment is the same as that in the first embodiment. An adhesive  50  is coated on the close-contacting surface  15  of the seal  10 A, and as shown in  FIG. 3B , in hot pressing, the close-contacting surface  15  is adhered to the electrolyte membrane  23  via the adhesive  50 , and the seal  10 A is integrally mounted to the membrane electrode assembly  20 . 
     (4) Fourth Embodiment 
       FIGS. 4A and 4B  show a mounting method for seals according to a fourth embodiment. As shown in  FIG. 4A , in the fourth embodiment, a reinforcement member  60 B is inserted in the inner circumference of the seal  10 A. The inner end portion of the reinforcement member  60 B projects inwardly from the seal  10 A. The reinforcement member  60 B is inserted in the seal  10 A in the same manner as the reinforcement member  60 A in the second embodiment. That is, the reinforcement member  60 B has a width wider than that of the reinforcement member  60 A in the second and third embodiments, and the wider portion projects inwardly as a projected portion  62 . 
     The procedure for mounting seals in the fourth embodiment is generally the same as that in the first embodiment. However, an adhesive  50  is not coated on the close-contacting surface  63  of the reinforcement member  60 B, and the seal  10 A is set in the lower die of the hot pressing die. As result, the projected portion  62  of the reinforcement member  60 B overlaps with the circumference of the negative electrode plate  22 A. Then, the electrolyte membrane  23  is layered on the negative electrode plate  22 A, so that the projection  23  of the reinforcement member  60 B is held between the negative electrode plate  22 A and the electrolyte membrane  23 . Then, the positive electrode plate  21  is layered on the electrolyte membrane  23 , and hot pressing is performed, whereby the seal  10 A is integrally mounted to the membrane electrode assembly  20  as shown in  FIG. 4B . In this procedure, the seal  10 A is mounted to the membrane electrode assembly  20  by compression bonding or fusion bonding of the projected portion  62  of the reinforcement member  60 B to the negative electrode plate  22  and the electrolyte membrane  23 . The negative electrode  22 A overlaps with the projected portion  62  of the reinforcement member  60 B, and the overlapped portion thereof is slightly bent according to the thickness of the reinforcement member  60 B. 
     (5) Fifth Embodiment 
       FIGS. 5A and 5B  show a mounting method for seals according to a fifth embodiment, which is a rearrangement of the fourth embodiment. As shown in  FIG. 5A , a negative electrode plate  22 B, in which a step portion  44  is formed at the overlapping portion with the projected portion  62  of the reinforcement member  60 B to avoid bending of the projected portion  62 , is used. That is, the step portion  44  is formed at the outer circumference of the upper surface of the negative electrode plate  22 B, and the depth thereof is the same as the thickness of the reinforcement member  60 B. In the fifth embodiment, when the seal  10 A is set in the lower die  40  of the hot pressing die, the projected portion  62  of the reinforcement member  60 B is fitted into the step portion  44  of the negative electrode plate  22 B.  FIG. 5B  shows the condition in which the seal  10 A is integrally mounted to the membrane electrode assembly  20  by hot pressing. The negative electrode plate  22 B has a planar lower surface which is not influenced by the projected portion  62  of the reinforcement member  60 B. 
     (6) Sixth Embodiment 
       FIGS. 6A and 6B  show a mounting method for seals according to a sixth embodiment, in which a seal  10 B is used instead of the seal  10 A. The seal  10 B has a projected portion  16  at the inner surface thereof, which projects in the same manner as the projected portion  62  of the reinforcement members  60 B in the fourth and fifth embodiments. The thickness of the projected portion  16  is the same as that of the reinforcement member  60 B. In hot pressing, as shown in  FIG. 6B , the projected portion  16  of the seal  10 B is held and mounted between the negative electrode plate  22 A and the electrolyte membrane  23  instead of the projected portion  62  of the reinforcement member  60 B. The negative electrode plate  22 A overlaps with the projected portion  16  of the seal  10 B, so that the overlapping portion thereof is slightly bent according to the thickness of the projected portion  16 . 
     (7) Seventh Embodiment 
       FIGS. 7A and 7B  show a mounting method for seals according to a seventh embodiment, which is a combination of the fifth and sixth embodiments. The seventh embodiment includes the negative electrode plate  22 B formed with the step portion  44  in the fifth embodiment and the seal  10 B formed with the projected portion  16 . When the seal  10 B is set in the lower die  40  of the hot pressing die, the projected portion  16  of the seal  10 B is fitted into the step portion  44  of the negative electrode plate  22 B, and these are mounted each other. As shown in  FIG. 7B , the negative electrode plate  22 B has a planar lower surface which is not influenced by the projected portion  16  of the seal  10 B. 
     (8) Eighth Embodiment 
       FIGS. 8A to 8D  show a mounting method for seals according to an eighth embodiment. In the eighth embodiment, the seal  10 A in the first embodiment is used, and the membrane electrode assembly  20  consisting of the positive electrode plate  21 , the negative electrode plate  22 A, and electrolyte membrane  23  is integrally formed and prepared beforehand. 
     In order to mount the seal  10 A, first, as shown in  FIG. 8A , the seal  10 A in which the adhesive  50  is coated on the close-contacting surface  15  is set in the lower die  40  of the hot pressing die. Then, as shown in  FIG. 8B , the negative electrode plate  22 A of the negative electrode plate  22 A is faced downward, and as shown in  FIG. 8C , the negative electrode plate  22 A is fitted into the recess  43  of the lower die  40 , and the exposed lower surface of the electrolyte membrane  23  is overlapped with the close-contacting surface  15  of the seal  10 A, on which the adhesive is coated, whereby the membrane electrode assembly  20  is set in the lower die  40 . Then, similarly to the first embodiment, as shown in  FIG. 8D , the upper die  30  is coincided with the lower die  40 , and the seal  10 A and the membrane electrode assembly  20  are held between the upper and lower dies  30  and  40 , and hot pressing is performed so as to integrally mount the seal  10 A with the membrane electrode assembly  20 . 
     According to the second through eighth embodiments, the same effects and advantages as in the first embodiment can be obtained. That is, adverse effects due to excess heat load exerted on the membrane electrode assembly  20  can be avoided, labor for mounting separated seals can be omitted when a fuel cell stack is assembled, and the seal does not easily become twisted and misaligned, whereby the sealing properties can be improved. Specifically in the fourth through seventh embodiments, the projected portion  62  of the reinforcement member  60 B inserted into the seal  10 A is held between the negative electrode plate  22 A (or negative electrode plate  22 B) and the electrolyte membrane  23 , or alternatively, the projected portion  16  of the seal  10 B is held between the negative electrode plate  22 A (or negative electrode plate  22 B) and the electrolyte membrane  23 , and the seal  10 A ( 10 B) is mounted by compression bonding or fusion bonding. Therefore, adhesive is not needed and securing strength can be improved. 
     The membrane electrode assembly mounted with the seal by the method for mounting seals according to the first through eighth embodiments can form a fuel cell stack by integrally layering it via a separator plate.