Abstract:
Disclosed is a fuel cell in which an electrolyte membrane-electrode structure is held between the first separator and a second separator. The electrolyte membrane-electrode structure comprises a solid polymer electrolyte membrane, a cathode-side electrode and an anode-side electrode. An end portion of the solid polymer electrolyte membrane projects outwardly beyond end portions of gas diffusion layers, and the both surfaces of the end portion of the solid polymer electrolyte membrane are held between the first protective film and a second protective film. The thickness of the first protective film is set to be thinner than the thickness of the second protective film.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and separators. The membrane electrode assembly includes an electrolyte membrane and a pair of gas diffusion layers provided on both sides of the electrolyte membrane. 
       BACKGROUND ART 
       [0002]    Generally, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode each including a catalyst layer (electrode catalyst) and a gas diffusion layer (porous carbon) to form a membrane electrode assembly (MEA). The membrane electrode assembly is interposed between separators (bipolar plates). Normally, in use, predetermined numbers of the fuel cells are stacked together to form a fuel cell stack mounted in a vehicle, for example. 
         [0003]    In the membrane electrode assembly, a thin solid polymer electrolyte membrane is used. Therefore, the solid polymer electrolyte membrane may be damaged undesirably due to the mechanical stress resulting from, e.g., the difference between pressures of reactant gases supplied to the solid polymer electrolyte membrane. 
         [0004]    In this regard, for example, a fuel cell as disclosed in Japanese Laid-Open Patent Publication No. 2006-318940 is known. As shown in  FIG. 7 , the fuel cell includes a unit cell  1 , and first and second separators  2 ,  3  sandwiching the unit cell  1 . The unit cell  1  includes a cathode  5   a,  an anode  6   a,  and a solid polymer electrolyte membrane  4  interposed between the cathode  5   a  and the anode  6   a.    
         [0005]    A first gas diffusion layer  5   b  is provided on the cathode  5   a,  and a second gas diffusion layer  6   b  is provided on the anode  6   a.  The surface area of the anode  6   a  and the surface area of the second gas diffusion layer  6   b  are smaller than the surface area of the solid polymer electrolyte membrane  4 . The surface area of the cathode  5   a  and the surface area of the first gas diffusion layer  5   b  are smaller than the surface area of the anode  6   a  and the surface area of the second gas diffusion layer  6   b.    
         [0006]    A first seal S 1  is interposed between the first separator  2  and the solid polymer electrolyte membrane  4  around the cathode  5   a.  A second seal S 2  is interposed between the first separator  2  and the second separator  3  around the anode  6   a.    
         [0007]    In the unit cell  1 , steps, buffers, gas inlets, and gas outlets are not used as power generation areas. In these portions, no gas diffusion function is required. However, the first gas diffusion layer  5   b  and the second gas diffusion layer  6   b  extend to a buffer, and the second gas diffusion layer  6   b  extends to a step. 
         [0008]    Normally, the first and second gas diffusion layers  5   b ,  6   b  are porous layers of expensive material such porous carbon clothes and porous carbon papers. Therefore, the unit cell  1  is not economical, and the overall cost of the fuel cell is high. 
         [0009]    Further, in an attempt to achieve the desired gas diffusion function, the first and second gas diffusion layers  5   b,    6   b  are relatively thick. In the structure, it is difficult to achieve the sufficient flow field height in the buffers, the gas inlets, and the gas outlets. Thus, the pressure loss is increased, and the gas cannot be distributed smoothly to the flow fields at the electrodes. 
       SUMMARY OF INVENTION 
       [0010]    The present invention has been made to solve the problem of this type, and an object of the present invention is to provide a fuel cell in which electrolyte membranes are protected reliably, and the sufficient height is achieved in reactant gas flow fields. 
         [0011]    The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and separators. The membrane electrode assembly includes an electrolyte membrane and a pair of gas diffusion layers provided on both sides of the electrolyte membrane. 
         [0012]    In the membrane electrode assembly, an end of the electrolyte membrane protrudes outward beyond ends of the gas diffusion layers, and both surfaces at the end of the electrolyte membrane are sandwiched between a first protection film and a second protection film, and the thickness of the first protection film is smaller than the thickness of the second protection film. 
         [0013]    In the present invention, both surfaces at the end of the electrolyte membrane, which protrude outside from the end of the gas diffusion layers and are sandwiched between the first protection film and the second protection film, are protected reliably. Further, since the thickness of the first protection film is smaller than the thickness of the second protection film, the sufficient reactant gas flow field height is achieved in the buffer, the reactant gas inlet, and the reactant gas outlet on the side where the first protection film is provided. Further, the reactant gas can be supplied to the power generation area in the flow field almost at the uniform flow rate, and improvement in the power generation performance is achieved. Thus, the desired power generation performance is maintained, and the overall thickness of the fuel cell is reduced easily. 
         [0014]    Further, the gas diffusion layers are only provided in areas used for power generation. Since the expensive gas diffusion layers are not formed in the portions where no gas diffusion function is required, the gas diffusion layers are economical, and efficient. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is an exploded perspective view showing main components of a fuel cell according to a first embodiment of the present invention; 
           [0016]      FIG. 2  is a cross sectional view showing the fuel cell, taken along a line II-II in  FIG. 1 ; 
           [0017]      FIG. 3  is a front view showing a membrane electrode assembly of the fuel cell; 
           [0018]      FIG. 4  is a front view showing a second separator of the fuel cell; 
           [0019]      FIG. 5  is a view illustrating a method of producing the membrane electrode assembly; 
           [0020]      FIG. 6  is a cross sectional view showing a fuel cell according to a second embodiment of the present invention; 
           [0021]      FIG. 7  is a cross sectional view showing a fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-318940. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]    As shown in  FIG. 1 , a fuel cell  10  according to a first embodiment of the present invention is formed by sandwiching a membrane electrode assembly  12  between a first separator  14  and a second separator  16 . 
         [0023]    At an upper end of the fuel cell  10  in a longitudinal direction indicated by an arrow C, an oxygen-containing gas supply passage  20   a  for supplying an oxygen-containing gas and a fuel gas supply passage  22   a  for supplying a fuel gas such as a hydrogen-containing gas are provided. The oxygen-containing gas supply passage  20   a  and the fuel gas supply passage  22   a  extend through the fuel cell  10  in the direction indicated by the arrow A. 
         [0024]    At a lower end of the fuel cell  10  in the longitudinal direction indicated by the arrow C, a fuel gas discharge passage  22   b  for discharging the fuel gas and an oxygen-containing gas discharge passage  20   b  for discharging the oxygen-containing gas are provided. The fuel gas discharge passage  22   b  and the oxygen-containing gas discharge passage  20   b  extend through the fuel cell  10  in the direction indicated by the arrow A. 
         [0025]    At one end of the fuel cell  10  in a lateral direction indicated by an arrow B, a coolant supply passage  24   a  for supplying a coolant, and at the other end of the fuel cell  10  in the lateral direction, a coolant discharge passage  24   b  for discharging the coolant are provided. The coolant supply passage  24   a  and the coolant discharge passage  24   b  extend through the fuel cell  10  in the direction indicated by the arrow A. 
         [0026]    As shown in  FIGS. 1 to 3 , the membrane electrode assembly  12  is elongated in the longitudinal direction. For example, the membrane electrode assembly  12  includes a cathode  28 , an anode  30 , and a solid polymer electrolyte membrane  26  interposed between the cathode  28  and the anode  30 . The solid polymer electrolyte membrane  26  is formed by impregnating a thin membrane of perfluorosulfonic acid with water, for example. 
         [0027]    As shown in  FIG. 3 , a pair of upper extensions  12   a  protruding toward the oxygen-containing gas supply passage  20   a  and the fuel gas supply passage  22   a  are provided on one short side (upper side) of the membrane electrode assembly  12 , and a pair of lower extensions  12   b  protruding toward the oxygen-containing gas discharge passage  20   b  and the fuel gas discharge passage  22   b  are provided on the other short side (lower side) of the membrane electrode assembly  12 . The number of the upper extensions  12   a  and the number of the lower extensions  12   b  may be increased as necessary depending on the number of fluid passages. 
         [0028]    As shown in  FIG. 2 , the cathode  28  has a gas diffusion layer  28   a  such as a carbon paper, and an electrode catalyst layer  28   b  of platinum alloy supported on porous carbon particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer  28   a.  The anode  30  has a gas diffusion layer  30   a  such as a carbon paper, and an electrode catalyst layer  30   b  of platinum alloy supported on carbon porous particles. The carbon particles are deposited uniformly on the surface of the gas diffusion layer  30   a.    
         [0029]    The electrode catalyst layers  28   b,    30   b  are formed in the same area on both surfaces of the solid polymer electrolyte membrane  26 . The gas diffusion layers  28   a,    30   a  are formed in the same area on both surfaces of the solid polymer electrolyte membrane  26 , and positions of end surfaces of the gas diffusion layers  28   a,    30   a  are aligned with positions of end surfaces of the electrode catalyst layers  28   b,    30   b  in the stacking direction. 
         [0030]    The solid polymer electrolyte membrane  26  includes outer marginal surfaces  26   a,    26   b  protruding outward from ends of the gas diffusion layers  28   a,    30   a.  A first protection film  32  is joined to the outer marginal surface  26   a,  and a second protection film  33  is joined to the outer marginal surface  26   b.  Each of the first and second protection films  32 ,  33  has a frame shape, and is made of engineering plastic or super engineering plastic such as polyphenylene sulfide (PPS) resin, polyetheretherketone (PEEK) based material, or polyether nitrile (PEN). 
         [0031]    An outer end surface  32   a  of the first protection film  32  is provided inside the outer end of the solid polymer electrolyte membrane  26 , and outer marginal surface  26   a  of the solid polymer electrolyte membrane  26  is partially exposed. An outer end surface  33   a  of the second protection film  33  may be aligned with the outer end surface of the solid polymer electrolyte membrane  26  at the same position. The thickness T 1  of the first protection film  32  is smaller than the thickness T 2  of the second protection film  33  (T 1 &lt;T 2 ). 
         [0032]    Preferably, the thickness of the second protection film  33  is substantially the same as the thickness of the cathode  28 . That is, the second protection film  33  and the cathode  28  have the same height without any steps over the surface of the second protection film  33  to the surface of the gas diffusion layer  28   a  of the cathode  28  (see  FIG. 2 ). The end surface  32   a  of the first protection film  32  may be aligned with the outer end surface  33   a  of the second protection film  33  at the same position. 
         [0033]    For example, the first separator  14  and the second separator  16  are carbon separators. It should be noted that the first separator  14  and the second separator  16  may be metal plates elongated in the longitudinal direction such as steel plates, stainless steel plates, aluminum plates, plated steel sheets, or metal plates having anti-corrosive surfaces by surface treatment. 
         [0034]    As shown in  FIG. 1 , the first separator  14  has an oxygen-containing gas flow field  34  on its surface  14   a  facing the membrane electrode assembly  12 . The oxygen-containing gas flow field  34  is connected to the oxygen-containing gas supply passage  20   a  and the oxygen-containing gas discharge passage  20   b.  The oxygen-containing gas flow field  34  includes a plurality of flow grooves  34   a  extending in the direction indicated by the arrow C. An inlet buffer  36   a  is provided adjacent to the inlet of the oxygen-containing gas flow field  34 , and an outlet buffer  36   b  is provided adjacent to the outlet of the oxygen-containing gas flow field  34 . 
         [0035]    A plurality of inlet connection channels  38   a  are formed between the inlet buffer  36   a  and the oxygen-containing gas supply passage  20   a.  Likewise, a plurality of outlet connection channels  38   b  are formed between the outlet buffer  36   b  and the oxygen-containing gas discharge passage  20   b.    
         [0036]    As shown in  FIG. 4 , the second separator  16  has a fuel gas flow field  40  on its surface  16   a  facing the membrane electrode assembly  12 . The fuel gas flow field  40  is connected to the fuel gas supply passage  22   a  and the fuel gas discharge passage  22   b.  The fuel gas flow field  40  includes a plurality of flow grooves  40   a  extending in the direction indicated by the arrow C. An inlet buffer  42   a  is provided adjacent to the inlet of the fuel gas flow field  40 , and an outlet buffer  42   b  is provided adjacent to the outlet of the fuel gas flow field  40 . A plurality of bosses  41   a,    41   b  are formed in the inlet buffer  42   a  and the outlet buffer  42   b,  respectively. 
         [0037]    The second separator  16  has a plurality of supply holes  44   a  connecting the fuel gas supply passage  22   a  and the fuel gas flow field  40 , and a plurality of discharge holes  44   b  connecting the fuel gas discharge passage  22   b  and the fuel gas flow field  40 . 
         [0038]    A coolant flow field  46  is formed between a surface  14   b  of the first separator  14  and a surface  16   b  of the second separator  16 . The coolant flow field  46  is connected to the coolant supply passage  24   a  and the coolant discharge passage  24   b.  The coolant flow field  46  includes a plurality of flow grooves  46   a  extending in the direction indicated by the arrow B. An inlet buffer  48   a  is provided adjacent to the inlet of the coolant flow field  46 , and an outlet buffer  48   b  is provided adjacent to the outlet of the coolant flow field  46 . A plurality of bosses are formed in the inlet buffer  48   a  and the outlet buffer  48   b,  respectively. 
         [0039]    As shown in  FIGS. 1 and 2 , a first seal member  50  is formed integrally with the surfaces  14   a,    14   b  of the first separator  14 , around the outer end of the first separator  14 . Alternatively, a member separate from the first separator  14  may be provided as the first seal member  50  on the surfaces  14   a,    14   b  of the first separator  14 . A second seal member  52  is formed integrally with the surfaces  16   a ,  16   b  of the second separator  16 , around the outer end of the second separator  16 . Alternatively, a member separate from the second separator  16  may be provided as the second seal member  52  on the surfaces  16   a,    16   b  of the second separator  16 . 
         [0040]    The first seal member  50  includes a ridge  50   a.  The ridge  50   a  is formed around the outer end of the membrane electrode assembly  12 , while allowing the oxygen-containing gas flow field  34  to be connected to the oxygen-containing gas supply passage  20   a  and the oxygen-containing gas discharge passage  20   b.    
         [0041]    As shown in  FIGS. 2 and 4 , the second seal member  52  includes an inner seal (inner seal member)  52   a  which contacts the solid polymer electrolyte membrane  26  of the membrane electrode assembly  12  along the outer marginal surface  26   a  of the solid polymer electrolyte membrane  26 , and an outer seal (outer seal member)  52   b  provided around the membrane electrode assembly  12  between the first separator  14  and the second separator  16 . 
         [0042]    In the fuel cell  10 , a method of producing the membrane electrode assembly  12  will be described with reference to  FIG. 5 . 
         [0043]    Firstly, as shown in (a) of  FIG. 5 , the frame shaped first protection film  32  and the frame shaped second protection film  33  are provided on both surfaces of the solid polymer electrolyte membrane  26 . The first protection film  32  is joined to one outer marginal surface  26   a  of the solid polymer electrolyte membrane  26 , e.g., using acrylic based (or fluorine based) adhesive. The second protection film  33  is joined to the other marginal surface  26   b  of the solid polymer electrolyte membrane  26 , e.g., using acrylic based (or fluorine based) adhesive (see (b) of  FIG. 5 ). 
         [0044]    Then, as shown in (c) of  FIG. 5 , the electrode catalyst layers  30   b,    28   b  are formed on both surfaces of the solid polymer electrolyte membrane  26  by applying coating of catalyst paste at an opening of the film. Specifically, for example, ion conductive component and catalyst particles which consist of carbon particles supporting Pt are mixed at certain proportions to make catalyst paste. Then, screen printing is applied to both surfaces of the solid polymer electrolyte membrane  26  using this catalyst paste. Then, the catalyst paste is dried to form the electrode catalyst layers  30   b,    28   b.    
         [0045]    Then, the process proceeds to the step in (d) of  FIG. 5 , and the gas diffusion layers  30   a,    28   a  are joined to the electrode catalyst layers  30   b,    28   b.  Specifically, the gas diffusion layers  30   a,    28   a  and the electrode catalyst layers  30   b,    28   b  are combined together by hot pressing. The acrylic adhesive may be applied to the outer marginal portions of the gas diffusion layers  30   a,    28   a  to join the gas diffusion layers  30   a,    28   a  to the electrode catalyst layers  30   b,    28   b . In this manner, the membrane electrode assembly  12  is produced (see (e) of  FIG. 5 ). 
         [0046]    Then, operation of the fuel cell  10  will be described below. 
         [0047]    As shown in  FIG. 1 , an oxygen-containing gas is supplied to the oxygen-containing gas supply passage  20   a , and a fuel gas such as a hydrogen containing gas is supplied to the fuel gas supply passage  22   a.  Further, a coolant such as pure water, ethylene glycol, or oil is supplied to the coolant supply passage  24   a.    
         [0048]    In the structure, the oxygen-containing gas from the oxygen-containing gas supply passage  20   a  is supplied through the inlet connection channels  38   a  into the oxygen-containing gas flow field  34 . The oxygen-containing gas flows along the oxygen-containing gas flow field  34  in the direction of gravity indicated by the arrow C, and the oxygen-containing gas is supplied to the cathode  28  of the membrane electrode assembly  12 . 
         [0049]    In the meanwhile, the fuel gas from the fuel gas supply passage  22   a  flows through the supply holes  44   a  toward the surface  16   a  of the second separator  16 . As shown in  FIG. 4 , the fuel gas flows along the fuel gas flow field  36  in the direction of gravity indicated by the arrow C, and the fuel gas is supplied to the anode  30  of the membrane electrode assembly  12  (see  FIG. 1 ). 
         [0050]    Thus, in the membrane electrode assembly  12 , the oxygen-containing gas supplied to the cathode  28  and the fuel gas supplied to the anode  30  are partially consumed in the electrochemical reactions at electrode catalyst layers of the cathode  28  and the anode  30  for generating electricity. 
         [0051]    Then, the oxygen-containing gas supplied and partially consumed at the cathode  28  of the membrane electrode assembly  12  is discharged along the oxygen-containing gas discharge passage  20   b  in the direction indicated by the arrow A. The fuel gas supplied to the anode  30  of the membrane electrode assembly  12  and partially consumed flows through the discharge holes  44   b  toward the surface  16   b  of the second separator  16 . After the fuel gas reaches the surface  16   b,  the fuel gas is discharged into the fuel gas discharge passage  22   b.    
         [0052]    Further, as shown in  FIG. 1 , the coolant supplied to the coolant supply passage  24   a  flows into the coolant flow field  46 . The coolant flowing from the coolant supply passage  24   a  through the coolant flow field  46  moves in the direction indicated by the arrow B to cool the fuel cell  10 , and then, the coolant is discharged into the coolant discharge passage  24   b.    
         [0053]    In the first embodiment, as shown in  FIG. 2 , in the membrane electrode assembly  12 , the end of the solid polymer electrolyte membrane  26  protrudes outward beyond the ends of the gas diffusion layers  30   a,    28   a.  The first protection film  32  and the second protection film  33  are joined to the outer marginal surfaces  26   a,    26   b  as both surfaces at the end of the solid polymer electrolyte membrane  26 . 
         [0054]    In the structure, the outer marginal surfaces  26   a,    26   b  of the solid polymer electrolyte membrane  26  sandwiched between the first protection film  32  and the second protection film  33  are protected reliably. The outer end surface  33   a  of the second protection film  33  and the outer end surface of the solid polymer electrolyte membrane  26  extend to the same position. Thus, the solid polymer electrolyte membrane  26  is reinforced further reliably. 
         [0055]    Further, the thickness T 1  of the first protection film  32  is smaller than the thickness T 2  of the second protection film  33 . It is because, since the second protection film  33  has the reinforcement function for the solid polymer electrolyte membrane  26 , the thickness of the first protection film  32  can be minimized. 
         [0056]    Thus, it is possible to ensure that the sufficient flow field height is achieved in each of the inlet buffer  42   a  and the outlet buffer  42   b,  and the fuel gas flow field  40  on the side where the first protection film  32  is provided. Further, the fuel gas can be supplied to the power generation area in the fuel gas flow field  40  almost at the uniform flow rate, and improvement in the power generation performance is achieved. Thus, the desired power generation performance is maintained, and the overall thickness of the fuel cell  10  is reduced easily. 
         [0057]    Depending on the layout, for example, in the structure where reactant gas flow fields (fuel gas flow field and oxygen-containing gas flow field) are provided on both sides of the separator (structure where the coolant flow field is provided in a skipped manner), the flow field on the side where the second protection film is provided, adjacent to the first protection film through the separator, is expanded toward the first protection film to achieve the desired reactant flow field height (depth) on the side where the second protection film is provided. 
         [0058]    Further, the gas diffusion layers  30   a,    28   a  are provided only in the power generation area, and the first protection film  32  and the second protection film  33  are provided in the inlet buffers  42   a,    36   a,  the gas inlets, and the gas outlets, where no gas diffusion function is required. Therefore, the amount of relatively expensive material such as carbon papers or carbon clothes used in the fuel cell  10  is significantly reduced, and the production cost of the fuel cell  10  is reduced suitably. 
         [0059]    Further, since the gas diffusion layers  30   a,    28   a  have a rectangular shape, for example, after coating of a base layer is applied to the entire carbon paper having a roll shape continuously, by cutting it into pieces each having a plurality of length, the gas diffusion layers  30   a,    28   a  can be produced easily. Thus, improvement in the productivity of the gas diffusion layer  30   a,    28   a  is achieved. Further, the shape of the gas diffusion layers  30   a,    28   a  is simplified, and yield rate of the material is improved suitably and economically. 
         [0060]      FIG. 6  is a cross sectional view showing a fuel cell  60  according to a second embodiment of the present invention. The constituent elements that are identical to those of the fuel cell  10  according to the first embodiment are labeled with the same reference numerals, and description thereof is omitted. 
         [0061]    The fuel cell  60  includes a membrane electrode assembly  62 . An end surface of the anode  30  of the membrane electrode assembly  62  protrudes outward beyond the end surface of the cathode  28  by a distance S. 
         [0062]    Thus, in the second embodiment, the end surface of the anode  30  and the end surface of the cathode  28  on both sides of the solid polymer electrolyte membrane  26  are shifted from each other in the stacking direction indicated by the arrow A. In the structure, in particular, it becomes possible to reliably suppress shear stress concentration in the solid polymer electrolyte membrane  26 . 
         [0063]    Instead of adopting the above structure, the end surface of the cathode  28  may protrude outward beyond the end surface of the anode  30  by the distance S.