Abstract:
A fuel cell includes: a membrane electrode assembly including an electrolyte membrane, catalyst layers stacked on both sides of the electrolyte membrane, and two or more porous bodies having different moduli of elasticity and provided on a surface of one of the catalyst layers; a separator defining a gas flow passage between the separator and the membrane electrode assembly; and a frame body surrounding an outer periphery of the electrolyte membrane. A porous body adjacent to the separator out of the two or more porous bodies includes an outer edge portion including an outer extending portion extending to overlap with the frame body. An elastic body is provided between the outer extending portion and the frame body.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a fuel cell such as a polymer electrolyte fuel cell (hereinafter called “PEFC”). 
       BACKGROUND ART 
       [0002]    Technologies related to this type of fuel cell include what is disclosed in Patent Literature 1, titled a “membrane-electrode assembly.” 
         [0003]    The membrane-electrode assembly disclosed in Patent Literature 1 includes a membrane-membrane reinforcing member assembly, an anode catalyst layer (or a first catalyst layer), a cathode catalyst layer (or a second catalyst layer), an anode gas diffusion layer (or a first gas diffusion layer), and a cathode gas diffusion layer (or a second gas diffusion layer). 
         [0004]    The membrane-membrane reinforcing member assembly includes a polymer electrolyte membrane, one or more flaky first membrane reinforcing members disposed on top of a principal surface of the polymer electrolyte membrane in such a way as to extend along the periphery of the polymer electrolyte membrane as a whole, and one or more flaky second membrane reinforcing members disposed on top of the first membrane reinforcing member in such a way as to extend along the periphery of the polymer electrolyte membrane as a whole and to have its inner periphery shifted from the inner periphery of the first membrane reinforcing member, as seen from a thickness direction of the polymer electrolyte membrane. Incidentally, the first membrane reinforcing member and the second membrane reinforcing member are mainly made of synthetic resin. 
         [0005]    The anode catalyst layer is formed to cover the principal surface of the polymer electrolyte membrane while filling in an opening formed in the first membrane reinforcing member, and likewise, the cathode catalyst layer is formed to cover the principal surface of the polymer electrolyte membrane. The anode gas diffusion layer is disposed to cover the anode catalyst layer and a portion of a principal surface of the first membrane reinforcing member, and the cathode gas diffusion layer is disposed to cover the cathode catalyst layer and a portion of the principal surface of the first membrane reinforcing member. 
         [0006]    The above-described configuration is intended to increase durability by preventing damage to the polymer electrolyte membrane by contact with an end portion of the gas diffusion layer, and by more reliably suppressing damage to the polymer electrolyte membrane by an end portion of the first membrane reinforcing member. 
       CITATION LIST 
     Patent Literature 
       [0007]    [PTL 1] International Patent Application Publication No. WO/2008/126350 
       SUMMARY OF INVENTION 
       [0008]    In this connection, studies have recently been made on the use of a porous metallic material rather than a carbon material as a material for the gas diffusion layer for the purpose of size reduction of a fuel cell. When the porous metallic material is used for the gas diffusion layer and the gas diffusion layer is disposed to cover the first membrane reinforcing member as disclosed in Patent Literature 1, an excessive surface pressure acts on an overlapping portion of the gas diffusion layer and the first membrane reinforcing member (or a peripheral portion of the polymer electrolyte membrane) and hence it is difficult to ensure a proper surface pressure on the polymer electrolyte membrane on which the catalyst layer is formed, and this problem remains unsolved. 
         [0009]    An object of the present invention is to provide a fuel cell capable of ensuring a proper surface pressure on a polymer electrolyte membrane by preventing an excessive surface pressure from being exerted on a peripheral portion of the polymer electrolyte membrane. 
         [0010]    A fuel cell in accordance with some embodiments includes: a membrane electrode assembly including an electrolyte membrane, catalyst layers stacked on both sides of the electrolyte membrane, and two or more porous bodies having different moduli of elasticity and provided on a surface of one of the catalyst layers; a separator defining a gas flow passage between the separator and the membrane electrode assembly; and a frame body surrounding an outer periphery of the electrolyte membrane. A porous body adjacent to the separator out of the two or more porous bodies includes an outer edge portion including an outer extending portion extending to overlap with the frame body An elastic body is provided between the outer extending portion and the frame body. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a fuel cell stack formed by stacking fuel cells according to a first embodiment of the present invention. 
           [0012]      FIG. 2A  is a plan view of a frame body and a membrane electrode assembly which form part of the fuel cell according to the first embodiment of the present invention. 
           [0013]      FIG. 2B  is a cross-sectional view taken along arrowed line I-I of  FIG. 2A . 
           [0014]      FIG. 3  is an enlarged partial view illustrating details of a portion of the fuel cell corresponding to an area encircled by chain line II in  FIG. 2B . 
           [0015]      FIG. 4  is an enlarged partial view of a fuel cell according to a second embodiment of the present invention, corresponding to the area encircled by chain line II in  FIG. 2B . 
           [0016]      FIG. 5  is an enlarged partial view of a fuel cell according to a third embodiment of the present invention, corresponding to the area encircled by chain line II in  FIG. 2B . 
           [0017]      FIG. 6  is an enlarged partial view of a fuel cell according to a fourth embodiment of the present invention, corresponding to the area encircled by chain line II in  FIG. 2B . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    Embodiments of the present invention will be described below with reference to the drawings.  FIG. 1  is a perspective view of a fuel cell stack formed by stacking fuel cells according to a first embodiment of the present invention;  FIG. 2A  is a plan view of a frame body and a membrane electrode assembly which form part of the fuel cell according to the first embodiment of the present invention;  FIG. 2B  is a cross-sectional view taken along arrowed line I-I of  FIG. 2A ; and  FIG. 3  is an enlarged partial view illustrating details of a portion of the fuel cell corresponding to an area encircled by chain line II in  FIG. 2B . Incidentally, a separator and a sealing compound are additionally illustrated in  FIG. 3 . 
         [0019]    A fuel cell stack A is formed by stacking plural fuel cells B 1  according to the first embodiment of the present invention, and the fuel cell stack A is configured so that the fuel cells B 1  are stacked one on top of another between a pair of end plates  10 ,  10  and the fuel cells B 1  are pressed together by the end plates  10 ,  10  with the fuel cells B 1  sandwiched in between the end plates  10 ,  10 . 
         [0020]    The fuel cell B 1  is provided with a membrane electrode assembly C and a frame body  20  (hereinafter called a “frame”) interposed between a pair of separators  15 ,  15  illustrated in  FIG. 3  in such a manner as to define gas flow passages through which gases for power generation flow. Incidentally, a gasket may be adopted as the frame body in place of the frame. The “gases for power generation” include a hydrogen-containing gas and an oxygen-containing gas. 
         [0021]    The frame  20  is made of resin, and, in the first embodiment, the frame  20  is formed in an oblong rectangular shape in a front view as seen from a stacking direction X of the fuel cells B 1  illustrated in  FIG. 1 , and with a certain plate thickness t greater than that of the membrane electrode assembly C to be described later, and the membrane electrode assembly C is disposed in a central portion of the frame  20 . Also, an inner wall surface of the frame  20  which abuts against an outer wall surface of the membrane electrode assembly C is formed flat. 
         [0022]    As illustrated in  FIG. 2A , the frame  20  is provided with manifold portions M 1 , M 2  for supply and discharge of the hydrogen-containing gas or the oxygen-containing gas or a cooling fluid, which are formed in side portions, respectively, of the frame  20 . The manifold portion M 1  in one of the side portions is formed of manifold holes H 1  to H 3 . The manifold holes H 1  to H 3  are for supply of the oxygen-containing gas (H 1 ), for supply of the cooling fluid (H 2 ), and for supply of the hydrogen-containing gas (H 3 ), respectively, and form flow paths, respectively, in the stacking direction X illustrated in  FIG. 1 . 
         [0023]    The other manifold portion M 2  is formed of manifold holes H 4  to H 6 . The manifold holes H 4  to H 6  are for discharge of the hydrogen-containing gas (H 4 ), for discharge of the cooling fluid (H 5 ), and for discharge of the oxygen-containing gas (H 6 ), respectively, and form flow paths, respectively, in the stacking direction X described above. Incidentally, the relative positions of the manifold holes for supply and the manifold holes for discharge may be partially or wholly in reverse order. 
         [0024]    As illustrated in  FIG. 3 , sealants  11 ,  11  are extendedly formed on upper and lower surfaces, respectively, of the frame  20  between the upper and lower surfaces and lower surfaces  15   a  of the separators  15 , and also, elastic bodies  40 ,  40  are extendedly formed on the upper and lower surfaces, respectively, in an inner peripheral portion of the frame  20 . 
         [0025]    The membrane electrode assembly C is sometimes called MEA (Membrane Electrode Assembly), and, as illustrated in  FIG. 3 , the membrane electrode assembly C has a structure in which an electrolyte membrane  50  formed of a solid polymer, for example, is held between a pair of catalyst layers  60 ,  70  with the electrolyte membrane  50  sandwiched between the catalyst layers  60 ,  70  and gas diffusion layers  100  are formed on the catalyst layers  60 ,  70  in such a way as to coat their surfaces. An outer periphery of the electrolyte membrane  50  is surrounded by the frame  20 . 
         [0026]    The gas diffusion layer  100  includes first and second porous bodies  80 ,  90  having different moduli of elasticity. As illustrated in  FIGS. 2B and 3 , the first and second porous bodies  80 ,  90  are stacked one on top of another with the second porous body  90  located adjacent to the catalyst layer  60  or  70  and with the first porous body  80  located adjacent to the separator  15 . 
         [0027]    The second porous body  90  is formed in such a manner that a side surface  90   b  of the second porous body  90  is flush with a sidewall surface Ca (or a boundary surface) of the catalyst layers  60 ,  70  and an upper surface  90   a  of the second porous body  90  is higher in level than an upper surface  20   a  of the frame  20 . 
         [0028]    As illustrated in  FIG. 3 , an outer edge portion of the first porous body  80  extends out toward an inner edge portion of the frame  20 , and this extending portion forms an outer extending portion  80   a.  In other words, as illustrated in  FIG. 3 , the outer extending portion  80   a  extends out with a gap between the outer extending portion  80   a  and the upper surface  20   a  of the frame  20 . The elastic body  40  described above is arranged between the outer extending portion  80   a  extending out and the upper surface  20   a  of the frame  20 . 
         [0029]    The elastic body  40  is formed of a material or materials selected from the group consisting of a carbon material, a spring, an elastomer, rubber, an adhesive, and a composite of these. The elastic body  40  has a lower modulus of elasticity than moduli of compressive elasticity of the first porous body  80  and the frame  20 . In the first embodiment, the elastic body  40  is formed integrally with a lower surface of the outer extending portion  80   a  of the first porous body  80  with an adhesive a interposed in between. Also, the sealant  11  is arranged outside the elastic body  40  and between the upper surface  20   a  of the frame  20  and the lower surface  15   a  of the separator  15 . 
         [0030]    In the first embodiment, the modulus of compressive elasticity of the first porous body  80  is higher than a modulus of compressive elasticity of the second porous body  90 . The first porous body  80  is formed of a material or materials selected from the group consisting of iron, stainless steel, aluminum, aluminum alloys, titanium, titanium alloys, chromium, chromium alloys, nickel, nickel alloys, magnesium, magnesium alloys, and combinations of these. Also, the first porous body  80  is constructed of metal mesh, punching metal, etching metal, expanded metal, or the like. Also, the second porous body  90  is formed of a carbon material. 
         [0031]    According to the fuel cell B 1  having the above-described configuration, the following effects can be achieved. 
         [0032]    The elastic body  40  accommodates a surface pressure acting between the outer extending portion  80   a  of the porous body and the frame  20  and prevents an excessive surface pressure from acting on a portion therebetween, thereby ensuring a proper surface pressure on the electrolyte membrane  50  on which the catalyst layers  60 ,  70  are formed. 
         [0033]    The outer extending portion  80   a  of the first porous body  80  extends out toward the inner edge portion of the frame  20 , thus preventing an end portion of the first porous body  80  from contacting the electrolyte membrane  50  and causing damage to the electrolyte membrane  50 . Also, the outer extending portion  80   a  of the first porous body  80  suppresses a differential pressure between a cathode and an anode or variations in the frame  20  due to swelling and shrinkage of the electrolyte membrane  50 , thus relieving stress concentration on the electrolyte membrane  50  at the boundary surface between the catalyst layers  60 ,  70  and the frame  20 . 
         [0034]    Also, the modulus of compressive elasticity of the first porous body  80  is higher than the modulus of compressive elasticity of the second porous body  90 , and thus, the elastic body  40  can accommodate a surface pressure more effectively. 
         [0035]    The elastic body  40  has a lower modulus of elasticity than the moduli of compressive elasticity of the first porous body  80  and the frame  20  and thus enables preventing an excessive surface pressure from acting between the outer extending portion  80   a  of the porous body and the frame  20  at the time of stacking. 
         [0036]    The elastic body  40  is formed integrally with the first porous body  80 , which in turn facilitates positioning at the time of stacking and also enables the elastic body  40  to improve in its effect of surface pressure accommodation or variation accommodation. 
         [0037]    Next, description will be given with reference to  FIGS. 4 to 6  with regard to fuel cells according to second to fourth embodiments.  FIGS. 4 to 6  are enlarged partial views of the fuel cells according to the second to fourth embodiments, respectively, corresponding to the area encircled by chain line II in  FIG. 2B . Incidentally, corresponding parts to those described for the above-mentioned embodiment are designated by the same reference numerals, and description of the corresponding parts will be omitted. 
         [0038]    In a fuel cell B 2  according to the second embodiment illustrated in  FIG. 4 , the elastic body  40  is integrally fixed to the frame  20  by an adhesive s. 
         [0039]    Also, a configuration may be adopted in which the equivalent of the elastic body  40  is formed integrally with a side edge portion of the frame  20 . The elastic body  40  is formed integrally with the frame  20  to thus facilitate positioning at the time of stacking. 
         [0040]    In a fuel cell B 3  according to the third embodiment illustrated in  FIG. 5 , the elastic body  40  is integrally fixed to the side surface  90   b  of the second porous body  90  by an adhesive s. The elastic body  40  is formed integrally with the second porous body  90  to thus facilitate positioning at the time of stacking. 
         [0041]    In a fuel cell B 4  according to the fourth embodiment illustrated in  FIG. 6 , flange portions  20   b,    20   c  protruding inwardly are formed throughout the periphery in upper and lower inner edge portions, respectively, of the frame  20  between which the electrolyte membrane  50  is sandwiched in, and flange portions  90   c,    90   c  are formed on the side surfaces  90   b,    90   b  of the second porous bodies  90 ,  90 , respectively. 
         [0042]    In other words, elastic body fitting recess portions Y are defined between the frame  20  and the side surfaces  90   b  of the second porous bodies  90  by the flange portions  90   c ,  90   c  of the second porous bodies  90 ,  90 , and the flange portions  20   b,    20   c  of the frame  20  abutting and facing the flange portions  90   c,    90   c,  respectively, and the elastic bodies  40 ,  40  are fitted in the elastic body fitting recess portions, respectively. 
         [0043]    According to the present invention, the elastic body inserted between the outer extending portion of the porous body adjacent to the separator and the frame body accommodates a surface pressure acting between the outer extending portion of the porous body and the frame body and prevents an excessive surface pressure from acting on a portion therebetween, thereby ensuring a proper surface pressure on the electrolyte membrane on which the catalyst layers are formed. 
         [0044]    The entire content of Japanese Patent Application No. 2012-058096 (filed on Mar. 15, 2012) is herein incorporated by reference. 
         [0045]    Although the present invention has been described above by reference to the embodiments and the example, the present invention is not limited to those, and it will be apparent to these skilled in the art that various modifications and improvements can be made. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           15  separator 
           20  frame body (frame) 
           40  elastic body 
           50  electrolyte membrane 
           60 ,  70  catalyst layers 
           80 ,  90  porous bodies (first and second porous bodies) 
           80   a  outer extending portion 
         C membrane electrode assembly