Abstract:
A method for producing a fuel cell unit including a membrane electrode assembly formed by a solid polymer electrolyte membrane and a pair of electrodes located at both sides of the solid polymer electrolyte membrane, and a pair of separators which hold the membrane electrode assembly. The method includes the steps of applying liquid sealant to one of a marginal portion of the solid polymer electrolyte membrane, the marginal portion being not covered by the pair of electrodes when assembled, and a surface of each of the pair of separators, the surface corresponding to the marginal portion of the solid polymer electrolyte membrane; holding the solid polymer electrolyte membrane with the pair of separators to perform temporary assembling; and solidifying the liquid sealant while maintaining a temporary assembling state.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods for producing fuel cell units and for producing fuel cell stacks. More specifically, the present invention relates to a method for producing a fuel cell unit which is formed by a membrane electrode assembly (MEA) including a solid polymer electrolyte membrane held by a pair of electrodes, the outside of the membrane electrode assembly being held by a pair of separators, and to a method for producing a fuel cell stack which is formed by stacking a plurality of the fuel cell units. In particular, the present invention relates to a technique by which tightening margin at a sealing portion in the membrane electrode assembly may be made constant without being influenced by the non-uniformity in the thickness of the membrane electrode assembly. 
     2. Description of Related Art 
     In conventional solid polymer type fuel cell units, a solid polymer electrolyte membrane, which functions as a cation exchange membrane, is sandwiched by a pair of electrodes, and the outside of each of the electrodes is held by a pair of separators. 
     Generally, a certain number of the fuel cell units having the above-mentioned structure are stacked and used as a fuel cell stack. 
       FIG. 11  is a diagram showing an enlarged cross-sectional view of main parts of an example of the fuel cell unit. 
     In the fuel cell unit  1  shown in  FIG. 11 , a passage  4  for an oxidant gas (for instance, air including oxygen) is provided on a surface of a cathode side separator  3   a  which is disposed so as to face a cathode  2   a.    
     On the other hand, a passage  5  for a fuel gas (for instance, hydrogen) is provided on a surface of an anode side separator  3   b , which is disposed as to face an anode  2   b , and a passage  6  for a cooling medium (for instance, water or ethylene glycol) is provided on the other surface of the anode side separator  3   b.    
     Since it is necessary that the oxidant gas, the fuel gas, (hereinafter these gases may be abbreviated as “reaction gas(es)”, and the cooling medium be independently passed through the passages  4 - 6 , respectively, the technique used for sealing between each of the passages  4 - 6  becomes important. 
     Examples of portions to be sealed include in the vicinity of a communication hole (not shown in the figure) which penetrates through the separators  3   a  and  3   b  in order to distribute and supply the reaction gases and the cooling medium to each of the fuel cell units  1 , an outer periphery of a membrane electrode assembly  8  formed by a solid polymer electrolyte membrane  7  and the electrodes  2   a  and  2   b  disposed so as to sandwich the solid polymer electrolyte membrane  7 , an outer periphery of a cooling medium passage of the separators  3   a  and  3   b , and an outer periphery of the both sides of the separators  3   a  and  3   b.    
     As a sealing technique used for the fuel cell unit  1  and the fuel cell stack, one is known in which a solid seal  9  made of a soft material having a suitable resilience, such as an organic rubber, is disposed at sealing portions, and a load is applied to the solid seal  9  in a stacking direction (i.e., the longitudinal direction in  FIG. 11 ) to compress the solid seal  9  so that the sealing portions are sealed using the surface pressure generated thereby. 
     In the above-mentioned technique, the seal compression amount Δh, which is the tightening margin for the solid seal  9 , may be defined by the following formula:
 
Δ h=Δh   1 + Δh   2   (1)
 
Δ h   1 = h seal − h   MEA   (2)
 
     hseal: the height of the solid seal  9 ; 
     h MEA : the thickness of the membrane electrode assembly  8 ; and 
     Δh 2 : the compression amount of the membrane electrode assembly  8  when load is applied. 
     Here, at each stacking surface of the fuel cell stack, it is necessary that the surface pressure, which is sufficient for an appropriate contact in or between the fuel cell unit(s)  1 , be applied in order to suppress the increase in the internal resistance or the contact resistance of the fuel cell unit  1 . 
     However, as it is clear from the above formulae of (1) and (2), if the thickness h MEA  of each of the membrane electrode assembly  8  is not uniform, the non-uniformity Δh MEA  is directly reflected to the seal compression amount Δh, which is the lightening margin for the solid seal  9 . 
     As shown in the graph of  FIG. 12 , the seal compression amount Δh may be expressed by the distance between points of intersection, which are present on the threshold value of a surface load F of the membrane electrode assembly  8  required for obtaining the above-mentioned degree of the surface pressure, formed by the surface load curve (expressed by a two-dotted line in the graph) of the membrane electrode assembly  8  having a predetermined thickness h MEA  (hereinafter referred to as “a standard thickness”), and by the surface load curve (expressed by a one-dotted line in the graph) of the membrane electrode assembly  8  having a thickness h MEA  which is different from the predetermined thickness h MEA  by Δh MEA . Accordingly, if the non-uniformity Δh MEA  is directly reflected on the seal compression amount Δh, the non-uniformity ΔFs of the seal load Fs (expressed by a dashed line in the graph) is also increased. 
     Also, if the thickness h MEA  is not uniform in in-phase directions of the same membrane electrode assembly  8 , the seal surface pressure which acts on the solid seal  9 , which in turn acts on the sealing portions, and on the separators  3   a  and  3   b  and the membrane electrode assembly  8 , is also made non-uniform. Accordingly, the power generation performance of the fuel cell may decrease due to the deterioration of the sealing property, and the fuel cell unit  1  may be bent and deformed due to the non-uniformity in the surface load between the fuel cell units  1 . 
     Although the generation of the bent-deformation may be prevented by increasing the thickness of the separators  3   a  and  3   b , the resultant fuel cell is not suitable for mounting on a vehicle, for instance, since the size and the weight of the fuel cell stack are increased. 
     Other than the technique relating to the solid seal  9  described above, as a sealing technique relating to the fuel cell unit  1  and the fuel cell stack, one is known in which an adhesive, etc., is filled in a sealing portion in a load applied state in the stacking direction and the sealing portion is sealed by using the adhesive strength at boundary surfaces as disclosed in, for example. Japanese Unexamined Patent Application, First Publication No. Hei 7-249417. 
     However, in the above technique relating to the adhesive seal, there are problems, such as a low reliablity in the durability of the adhesive strength at the boundary surfaces. 
     SUMMARY OF THE INVENTION 
     The present invention takes into consideration the above-mentioned circumstances, and has an a object to provide methods for producing fuel cell units and for producing fuel cell stacks by which tightening margin at a sealing portion in a membrane electrode assembly may be made constant without being influenced by the non-uniformity in the thickness of the membrane electrode assembly if such a non-uniformity is present. The other objects and features of the invention will be understood from the following description with reference to the accompany drawings. 
     Accordingly, the present invention provides a method for producing a fuel cell unit (for instances, a fuel cell unit  10  in embodiments described below) including a membrane electrode assembly (for instance, a membrane electrode assembly  12  in the embodiments described below) formed by a solid polymer electrolyte membrane (for instance, a solid polymer electrolyte membrane  18  in the embodiments described below) and a pair of electrodes (for instance, a cathode  25  and an anode  27  in the embodiments described below) located at both sides of the solid polymer electrolyte membrane, and a pair of separators (for instance, a cathode side separator  14  and an anode side separator  16  in the embodiments described below) which hold the membrane electrode assembly, comprising the steps of: applying liquid sealant to at least one of a marginal portion (for instance, a marginal portion  18   a  in the embodiments described below) of the solid polymer electrolyte membrane, the marginal portion being not covered by the pair of electrodes when assembled, and a surface (for instance, a groove portion  28  in the embodiments described below) of each of the pair of separators, the surface corresponding to the marginal portion of the solid polymer electrolyte membrane; holding the solid polymer electrolyte membrane with the pair of separators to perform temporary assembling; and solidifying the liquid sealant while maintaining a temporary assembling state (i.e., steps shown in, for instances,  FIG. 3 through 6  in the embodiments described below). 
     According to the method described above, since the liquid sealant applied onto the sealing portion is squeezed during the temporary assembly step and thereby absorbing or eliminating the non-uniformity in the thickness of the membrane electrode assembly, the compression amount of the liquid sealant, i.e., the tightening margin, at the sealing portion is made constant, if the liquid sealant is solidified in that state, even when the thickness of the membrane electrode assembly is not uniform in the in-plane direction or in each of the membrane electrode assembly. 
     The present invention also provides a method for producing a fuel cell stack having a plurality of stacked fuel cell units (for instance, a fuel cell unit  10  in the embodiments described below) including a membrane electrode assembly (for instance, a membrane electrode assembly  12  in the embodiments described below) formed by a solid polymer electrolyte membrane (for instance, a solid polymer electrolyte membrane  18  in the embodiments described below) and a pair of electrodes (for instances, a cathode  25  and an anode  27  in the embodiments described below) located at both sides of the solid separator  14  and an anode side separator  16  in the embodiments described below) which hold the membrane electrode assembly, comprising the steps of: applying liquid sealant to at least one of a marginal portion (for instance, a marginal portion  18   a  in the embodiments described below) of the solid polymer electrolyte membrane, the marginal portion being not covered by the pair of electrodes when assembled, and a surface (for instance, a groove portion  28  in the embodiments described below) of the each of the pair of separators, the surface corresponding to the marginal portion of the solid polymer electrolyte membrane; holding the solid polymer electrolyte membrane with the pair of separators to perform temporary assembling; solidifying the liquid sealant while maintaining a temporary assembling state (i.e., steps shown in, for instance,  FIG. 3 through 6  in the embodiments described below) to obtain a fuel cell unit; stacking a predetermined number of the fuel cell units so as to be placed between a pair of end plates (for instance, end plates  90  in the embodiments described below), and applying a compression load in a direction reducing the distance between the end plates to produce a fuel cell stack (i.e., steps shown in, for instance,  FIGS. 7 through 9  in the embodiments described below). 
     According to the method described above, since the liquid sealant applied onto the sealing portion in the process of obtaining the fuel cell unit is squeezed during the temporary assembly step and thereby absorbing or eliminating the non-uniformity in the thickness of the membrane electrode assembly, the compression amount of the liquid sealant, i.e., the tightening margin, at each of the sealing portion is also made constant when a predetermined number of the obtained fuel cell units is stacked and the compression load is applied in the stacking direction to produce a fuel cell stack. 
     The present invention also provides a fuel cell unit (for instance, a fuel cell unit  10  in embodiments described below) including a membrane electrode assembly (for instance, a membrane electrode assembly  12  in the embodiments described below) formed by a solid polymer electrolyte membrane (for instance, a solid polymer electrolyte membrane  18  in the embodiments described below) and a pair of electrodes (for instance, a cathode  25  and an anode  27  in the embodiments described below) located at both sides of the solid polymer electrolyte membrane, and a pair of separators (for instance, a cathode side separator  14  and an anode side separator  16  in the embodiments described below) which hold the membrane electrode assembly, obtained by the process comprising the steps of: applying liquid sealant to at least one of a marginal portion (for instance, a marginal portion  18   a  in the embodiments described below) of the solid polymer electrolyte membrane, the marginal portion being not covered by the pair of electrodes when assembled, and a surface (for instance, a groove portion  28  in the embodiments described below) of each of the pair of separators, the surface corresponding to the marginal portion of the solid polymer electrolyte membrane; holding the solid polymer electrolyte membrane with the pair of separators to perform temporary assembling; and solidifying the liquid sealant while maintaining a temporary assembling state (i.e., steps shown in, for instance,  FIGS. 3 through 6  in the embodiments described below). 
     The present invention also provides a fuel cell stack having a plurality of stacked fuel cell units (for instance, a fuel cell unit  10  in the embodiments described below) including a membrane electrode assembly (for instance, a membrane electrode assembly  12  in the embodiments described below) formed by a solid polymer electrolyte membrane (for instance, a solid polymer electrolyte membrane  18  in the embodiments described below) and a pair of electrodes (for instance, a cathode  25  and an anode  27  in the embodiments described below) located at both sides of the solid polymer electrolyte membrane, and a pair of separators (for instance, a cathode side separator  14  and an anode side separator  16  in the embodiments described below) which hold the membrane electrode assembly, obtained by the process comprising the steps of: applying liquid sealant to at least one of a marginal portion (for instance, a marginal portion  18   a  in the embodiments described below) of the solid polymer electrolyte membrane, the marginal portion being not covered by the pair of electrodes when assembled, and a surface (for instance, a groove portion  28  in the embodiments described below) of each of the pair of separators, the surface corresponding to the marginal portion of the solid polymer electrolyte membrane; holding the solid polymer electrolyte membrane with the pair of separators to perform temporary assembling; solidifying the liquid sealant while maintaining a temporary assembling state (i.e., steps shown in, for instance,  FIGS. 3 through 6  in the embodiments described below) to obtain a fuel cell unit; stacking a predetermined number of the fuel cell units so as to be placed between a pair of end plates (for instance, end plates  90  in the embodiments described below), and applying a compression load in a direction reducing the distance between the end plates to produce a fuel cell stack (i.e., steps shown in, for instance,  FIGS. 7 through 9  in the embodiments described below). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some of the features and advantages of the invention have been described, and others will become apparent from the detailed description which follows and from the accompanying drawings, in which: 
         FIG. 1  is a diagram showing an exploded perspective view of a fuel cell unit manufactured by using a method according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of the fuel cell unit shown in  FIG. 1  cut along the line A—A; 
         FIG. 3  is a diagram showing a part of the process, i.e., liquid sealant is applied to a separator, of the method for producing fuel cell units according to the embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of the fuel cell unit shown in  FIG. 3  cut along the line B—B; 
         FIG. 5  is a diagram showing a part of the process, i.e., temporary assembly is carried out by holding both sides of a membrane electrode assembly by a pair of separators, of the method for producing fuel cell units according to the embodiment of the present invention; 
         FIG. 6  is a diagram showing an enlarged cross-sectional view of main portions of a fuel cell unit which is formed by curing the liquid sealant after the temporary assembly shown in  FIG. 5 ; 
         FIG. 7  is a diagram showing a part of the process, i.e., liquid sealant is applied to one of the separators, of the method for producing fuel cell units according to the embodiment of the present invention; 
         FIG. 8  is a diagram showing a part of the process, i.e., a plurality of the fuel cell units are stacked, of the method for producing fuel cell units according to the embodiment of the present invention; 
         FIG. 9  is a diagram showing an enlarged cross-sectional view of main portions of a fuel cell stack which is manufactured by tightening the stacked structure shown in  FIG. 8  by using bolts so as to shorten the distance between the end plates; 
         FIG. 10  is a graph showing the relationship between the non-uniformity in thickness of the membrane electrode assembly and the seal compression amount and the seal load by comparing a fuel cell stack manufactured by the method according to the embodiment of the present invention with a conventional fuel cell stack using a solid seal; 
         FIG. 11  is a diagram showing an enlarged cross-sectional view of main portions of the conventional fuel cell stack; and 
         FIG. 12  is a graph showing the relationship between the non-uniformity in thickness of the membrane electrode assembly of the conventional fuel cell stack and the seal compression amount and the seal load. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention summarized above and defined by the enumerated claims may be better understood by referring to the following detailed description, which should be read with reference to the accompanying drawings. This detailed description of particular preferred embodiments, set out below to enable one to build and use particular implementations of the invention, is not intended to limit the enumerated claims, but to serve as particular examples thereof. 
       FIG. 1  is a diagram showing an exploded perspective view of a fuel cell unit manufactured by a method in accordance with the first embodiment of the present invention. 
     The fuel cell unit  10  includes a membrane electrode assembly  12 , and a cathode side separator  14  and an anode side separator  16 , which hold the membrane electrode assembly  12 . A plurality of the fuel cell units  10  may be stacked to produce, for example, a fuel cell stack used for a vehicle. 
     The membrane electrode assembly  12  includes a solid polymer electrolyte membrane  18 , and a cathode side catalytic layer  20  and an anode side catalytic layer  22  which are disposed so as to sandwich the solid polymer electrolyte membrane  18 . Also, a cathode side gas diffusion layer  24  and an anode side gas diffusion layer  26  are disposed outside of the cathode side catalytic layer  20  and the anode side catalytic layer  22 , respectively. 
     The cathode side catalytic layer  20  together with the cathode side gas diffusion layer  24  forms a cathode  25 . Similarly, the anode side catalytic layer  22  together with the anode side gas diffusion layer  26  forms an anode  27 . 
     As shown in  FIG. 1 , the solid polymer electrolyte membrane  18  has a marginal portion  18   a , i.e., an area outside of the two-dotted line in the solid polymer membrane portion  18   a , i.e., an area outside of the two-dotted line in the solid polymer membrane anode side catalytic layer  22  when disposed so as to sandwich the solid polymer electrolyte membrane  18 . 
     A liquid sealant S, which is applied to the outer peripheral portion of the cathode side separator  14  and that of the anode side separator  16 , makes direct contact with a side (i.e., a surface) of the marginal portion  18   a . The liquid sealant S will be with a side (i.e., a surface) of the marginal portion  18   a . The liquid sealant S will be 
     The cathode side separator  14 , as shown in  FIG. 1 , includes an inlet side fuel gas opening  36   a  for allowing a fuel gas, such as a gas containing hydrogen, to pass through, at the upper left end close to the edge thereof, and an inlet side oxidant gas opening  38   a  for allowing an oxidant gas, such as a gas containing oxygen, or air, to pass through, at the upper right end close to the edge thereof. 
     Also, the cathode side separator  14  includes an inlet side cooling medium opening  40   a  which allows a cooling medium, such as pure water, ethylene glycol, or oil, to pass through at the left end at the middle in the vertical direction, and an outlet side cooling medium opening  40   b  which allows the used cooling medium to pass through at the right end at the middle in the vertical direction. 
     Moreover, the cathode side separator  14  includes an outlet side fuel gas opening  36   b  for allowing the fuel gas to pass through at the lower right end close to the edge thereof, and an outlet side oxidant gas opening  38   b  for allowing the oxidant gas to pass through at the lower left end close to the edge thereof. In this embodiment, the outlet side fuel gas opening  36   b  and the outlet side oxidant gas opening  38   b  are disposed so as to be diagonal with respect to the inlet side fuel gas opening  36   a  and the inlet side oxidant gas opening  38   a , respectively. 
     As shown in  FIG. 1 , a plurality of independent first oxidant gas channels  42  are formed on a surface  14   a  of the cathode side separator  14  opposite the cathode side catalytic layer  20 . The first oxidant gas channels  42  start in the vicinity of the inlet side oxidant gas opening  38   a , and run horizontally while meandering downward in the direction of gravity. 
     The first oxidant gas channels  42  merge into a plurality of second oxidant gas channels  44 , and the second oxidant gas channels  44  end in the vicinity of the outlet side oxidant gas opening  38   b.    
     The cathode side separator  14  includes first oxidant gas connecting passages  46  which passes through the cathode side separator  14 , whose ends are connected to the inlet side oxidant gas opening  38   a  on a surface  14   b  opposite the surface  14   a , and whose other ends are connected to the first oxidant gas channels  42  on the surface  14   a . Also, the cathode side separator  14  includes second oxidant gas connecting passages  48  which passes through the cathode side separator  14 , whose ends are connected to the outlet side oxidant gas opening  38   b  on the surface  14   b , and whose other ends are connected to the second oxidant gas channels  44  on the surface  14   a.    
     In addition, the anode side separator  16  also includes an inlet side fuel gas opening  36   a , an inlet side oxidant gas opening  38   a , an inlet side cooling medium opening  40   a , an outlet side cooling medium opening  40   b , an outlet side fuel gas opening  36   b , and an outlet side oxidant gas opening  38   b , at both ends close to the edges thereof, in a manner similar to the cathode side separator  14 . 
     As shown in  FIG. 2 , a plurality of independent first fuel gas channels  60  are formed on a surface  16   a  of the anode side separator  16  in the vicinity of the inlet side fuel gas opening  36   a.    
     The first fuel gas channels  60  run horizontally while meandering downward in the direction of gravity, and merge into three second fuel gas channels (not shown in the figure). The second fuel gas channels end in the vicinity of the outlet side fuel gas opening  36   b.    
     The anode side separator  16  includes first fuel gas connecting passages  64  which connect the inlet side fuel gas opening  36   a  on the surface  16   b  to the first fuel gas channels  60 , and second fuel gas connecting passages (not shown in the figure) which connect the outlet side fuel gas opening  36   b  on the surface  16   b  to the second fuel gas channels  62 . The first fuel gas connecting passages  64  and the second fuel gas connecting passages are formed so as to pass through the anode side separator  16 . 
     A plurality of main channels  72   a  and  72   b  which function as cooling medium channels are formed on the surface  16   b  of the anode side separator  16 , within the area enclosed by the liquid sealant S, which will be described later, and close to the inlet side cooling medium opening  40   a  and the outlet side cooling medium opening  40   b.    
     Also, a plurality of branch channels  74  branch off from the main channels  72   a  and  72   b  are disposed so as to extend in the horizontal direction. 
     The anode side separator  16  includes first cooling medium connecting passages  76  which connect the inlet side cooling medium opening  40   a  to the main channels  72   a , and second cooling medium connecting passages  78  which connect the outlet side cooling medium opening  40   b  to the main channels  72   b . The first cooling medium connecting passages  76  and the second cooling medium connecting passages  78  pass through the anode side separator  16 . 
     In this embodiment, a groove portion  28  is formed on the surface  16   a  of the anode side separator  16   a  (i.e., an area of the separator  16   a  corresponding to the marginal portion  18   a ), which holds the solid polymer electrolyte membrane  18 , opposite the anode side catalytic layer  22  at a position corresponding to the marginal portion  18   a  of the solid polymer electrolyte membrane  18 . The liquid sealant S is applied to the groove portion  28 . 
     Also, a groove portion  30  is formed around each of the inlet side fuel gas opening  36   a , the inlet side oxidant gas opening  38   a , the inlet side cooling medium opening  40   a , the outlet side cooling medium opening  40   b , the outlet side fuel gas opening  36   b , and the outlet side oxidant gas opening  38   b , which are formed on the surface  16   a  of the separator  16 . The liquid sealant S is also applied to the groove portion  30 . 
     Moreover, the groove portions  28  and  30  are formed on the surface  14   a  of the cathode side separator  14 , which holds the membrane electrode assembly  12  together with the anode side separator  16 , opposite the cathode side catalytic layer  20 , at a position corresponding to the groove portions  28  and  30 , respectively, on the surface  16   a  of the anode side separator  16 . The liquid sealant S is also applied to each of the groove portions  28  and  30 . 
     Accordingly, as shown in  FIG. 2 , the liquid sealant S applied to the groove portions  28  and  30  on the cathode side separator  14  and the anode side separator  16  which hold the membrane electrode assembly  12 , respectively, seal around the membrane electrode assembly  12  by directly contacting with the marginal portion  18   a  at positions sandwiching the marginal portion  18   a  as for the liquid sealant S used for the groove portion  28 , and seal around the openings  36   a ,  36   b ,  38   a ,  38   b ,  40   a , and  40   b  by directly contacting with each other as for the liquid sealant S used for the groove portion  30 . 
     A groove portion  34  which surrounds the branch channels  74  is formed on the surface  16   b  of the anode side separator  16  at a position opposite the surface  14   b  of the cathode side separator  14  when a plurality of the fuel cell units  10  are stacked. The liquid sealant S is also applied to the groove portion  34 . 
     Also, a groove portion  35  is formed around each of the inlet side fuel gas opening  36   a , the inlet side oxidant gas opening  38   a , the inlet side cooling medium opening  40   a , the outlet side cooling medium opening  40   b , the outlet side fuel gas opening  36   b , and the outlet side oxidant gas opening  38   b  on the surface  16   b  of the anode side separator  16 . The liquid sealant S is also applied to the groove portion  35 . 
     The groove portions  35  around the inlet side fuel gas opening  36   a  and the outlet side fuel gas opening  36   b  are formed so as to surround the first fuel gas connecting passage  64  and the second fuel gas connecting passage, respectively. 
     Also, the groove portions  35  around the inlet side oxidant gas opening  38   a  and the outlet side oxidant gas opening  38   b  are formed so as to surround the inlet side oxidant gas opening  38   a  and the outlet side oxidant gas opening  38   b , respectively, on the surface  14   b  of the cathode side separator  14 . 
     Accordingly, when the fuel cell units  10  are stacked and the surface  14   b  of the cathode side separator  14  contacts the surface  16   b  of the anode side separator  16 , the liquid sealant S of the anode side separator  16  arranged around the inlet side fuel gas opening  36   a , the inlet side oxidant gas opening  38   a , the inlet side cooling medium opening  40   a , the outlet side cooling medium opening  40   b , the outlet side fuel gas opening  36   b , the outlet side oxidant gas opening  38   b , and the branch channels  74  are in contact with the surface  14   b  of the cathode side separator  14 , thereby ensuring watertightness between the cathode side separator  14  and the anode side separator  16 . 
     In this embodiment, the above-mentioned liquid sealant S may be made of a thermosetting type fluoride material or a thermosetting type silicone, and has a viscosity of a certain degree by which the cross-sectional shape thereof will not change in an applied state, and be cured (or solidified) after the application while maintaining a certain degree of elasticity. In addition, the liquid sealant S is made of a material which is capable of absorbing dimensional errors at sealing portions, i.e., non-uniformity in the thickness HMEA of the membrane electrode assembly  12  and in the thickness of the cathode side and the anode side separators  14  and  16 , by being squeezed in the groove portions  28 ,  30 ,  34 , and  35  after the application, and makes uniform the compression amount in a load applied state after being cured. 
     Next, main steps of a method for manufacturing the fuel cell unit  10  having the above-mentioned structure, and a method for manufacturing a fuel cell stack which is produced by stacking a plurality of the fuel cell units  10  according to an embodiment of the present invention will be described with reference to  FIGS. 3 through 9 . 
     First, the cathode side separator  14  and the anode side separator  16  having the above-mentioned configuration are prepared, and the liquid sealant S is applied to each of the groove portions  28  and  30  formed on the separators  14  and  16  (refer to FIG.  3 ). 
     Note that the cross-sectional shape of the liquid sealant S applied is substantially circular as shown in FIG.  4  and this shape is maintained. 
     Next, the membrane electrode assembly  12  constructed in advance is prepared and the membrane electrode assembly  12  is disposed between the cathode side separator  14  and the anode side separator  16 , and then placed between a compression jig  82  as shown in FIG.  5 . 
     In  FIG. 5 , the numeral  86  indicates supporting jigs which support the outer periphery portion of the membrane electrode assembly  12  while positioning the membrane electrode assembly  12  with respect to the cathode side and anode side separators  14  and  16  in the in-plane direction. 
     After this, a temporary assembly of a fuel cell is carried out by closing an upper and a lower part of the compression jig  82  to hold the membrane electrode assembly  12  by the cathode side and the anode side separators  14  and  16  so that the liquid sealant S applied to the groove portions  28  on both separators  14  and  16  makes direct contact to the marginal portion  18   a  of the solid polymer electrolyte membrane  18  at a position the marginal portion  18   a  of the solid polymer electrolyte membrane  18  at a position direct contact with each other. 
     The term temporary assembly means an assembly of a fuel cell to reach a state in which a low degree of load is applied thereon so that the thickness HMEA of the membrane electrode assembly  12  is made uniform in the in-plane direction. 
     During the temporary assembly, the liquid sealant S is squeezed in the groove portions  28  and  30 , and absorbs errors in the sealing portions, i.e., the non-uniformity in thickness HMEA of the membrane electrode assembly  12  and in the thickness of the cathode side and anode side separators  14  and  16 . 
     In this manner, the compression amount, i.e., the tightening margin, of the liquid sealant S at each of the sealing portions is made uniform over the entire fuel cell stack even when compressed in the stacking direction by using bolts  92  after the fuel cell units are stacked. This will be described later. 
     After this, the temporary assembly in which the membrane electrode assembly  12  is sandwiched by the cathode side and the anode side separators  14  and  16 , is heated together with the compression jig  82  in an oven so that the liquid sealant S is cured while the above-mentioned load is applied thereto. 
     After the compression jig  82  is separated from the temporary assembly and the assembly is cooled, a fuel cell unit  10  having the above-mentioned configuration and constant tightening margin of the liquid sealant S is obtained even if the thickness of the membrane electrode assembly  12  is not uniform in the in-plane direction (refer to FIG.  6 ). 
     Then, the liquid sealant S is applied to the groove portions  34  and  35  formed on the surface  16   b  of the anode side separator  16  of the fuel cell unit  10  obtained by the procedure explained above (refer to FIG.  7 ), and a process is repeated in which the surface  14   b  of the cathode side separator  14  of another fuel cell unit  10  also obtained via the above procedure is placed on the surface  16   b  so that the fuel cell units  10  are sequentially stacked on an end plate  90  (refer to FIG.  8 ). 
     After a predetermined number of the fuel cell units  10  are stacked, another end plate (not shown in the figure) is placed on top and the plates are tightened by using the bolts  92  to produce a fuel cell stack. 
     More specifically, when the end plates are tightened by using the bolts  92 , a compression load is applied in the stacking direction, i.e., the direction reducing the distance between the end plates  90 . Accordingly, surface pressure which is sufficient for suppressing the increase in the internal resistance of the contact resistance is generated at each of the stacking surfaces in and between the fuel cell units  10 . 
     During that time, the liquid sealant S in the groove portions  28 ,  30 ,  34 , and  35  are squeezed, and the seal compression amount Δh at each sealing portion is made constant regardless of the non-uniformity in thickness h MEA  of the membrane electrode assembly  12  and in the thickness of the cathode side and the anode side separators  14  and  16  (refer to FIG.  9 ). 
     That is, as shown in  FIG. 10 , when the thickness h MEA  of the membrane electrode assembly  12  differs by Δh MEA  with respect to the standard thickness h MEA , the non-uniformity Δh MEA  of the thickness h MEA  is directly reflected on the seal compression amount Δh if a solid seal is used as in conventional techniques. However, when liquid sealant S is used as in the embodiments of the present invention, the non-uniformity Δh MEA  is not reflected on the seal compression amount Δh′ since the non-uniformity Δh MEA  is absorbed during the temporary assembly. Accordingly, the tightening margins are made constant according to the embodiments of the present invention. 
     In this manner, as shown in the graph in  FIG. 10 , the non-uniformity A Fs′ of the seal load Fs (expressed by a dashed line in the graph), which may be expressed by the distance between points of intersection, which are present on the threshold value of a surface load F of the membrane electrode assembly  12  required for obtaining the above-mentioned degree of the seal surface pressure, formed by the surface load curve (expressed by a two-dotted line in the graph) of the membrane electrode assembly  12  having a standard thickness h MEA , and by the surface load curve (expressed by a solid line in the graph) of the membrane electrode assembly  8  having a thickness h MEA  which is different from the standard thickness by Δh MEA , can be significantly reduced as compared with the non-uniformity ΔFs obtained when a solid seal is used as in conventional techniques. 
     As explained above, by using the methods for producing fuel cell units  10  and for producing fuel cell stacks according to the embodiments of the present invention, the seal surface pressure acting on sealing portions is made uniform and an excellent sealing property is maintained due to excellent follow-up of the liquid sealant S for the dimensional errors. Accordingly, it becomes possible to produce the fuel cell unit  10  and the fuel cell stack which are capable of exerting a desired power generation property. 
     Also, according to the present invention, since a strict control of the dimension, especially in the thickness direction, becomes unnecessary for the membrane electrode assembly  12 , and the cathode side and the anode side separators  14  and  16  due to the excellent follow-up property of the liquid sealant S as mentioned above, it becomes possible to significantly reduce the manufacturing cost. 
     Moreover, since the surface load between the fuel cell units  10  is made constant, the thickness of each of the separators  14  and  16  may be decreased. Accordingly, the size and weight of the fuel cell unit  10  and the fuel cell stack can be reduced, and a fuel cell stack which is especially suitable for a vehicle in which the size of available space is restricted and the thickness of each of the separators  14  and  16  must be minimized as much as possible, can be produced. 
     Having thus described exemplary embodiments of the invention, it will be apparent that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements, though not expressly described above, are nonetheless intended and implied to be within the spirit and scope of the invention. Accordingly, the foregoing discussion is intended to be illustrative only; the invention is limited and defined only by the following claims and equivalents thereto.