Abstract:
The present invention provides a polymer electrolyte fuel cell having a small-sized, light-weighted mechanism for fastening a stack of unit cells assembly. The polymer electrolyte fuel cell of the present invention includes a stack of unit cells obtained by laying a plurality of unit cells one upon another; a first end plate disposed on one end of the stack of unit cells; a second end plate arranged on the other end of the stack of unit cells; an auxiliary plate disposed at least outside the first end plate; at least one set of restraining means, each of which has a band-like shape and restrains a first member located on one end of an assembly, which includes the stack of unit cells, the first and the second end plates, and the auxiliary plate, and a second member located on the other end of the assembly to restrict separation of the first member and the second member from each other; a screw fitted in a threaded hole formed in the auxiliary plate in such a manner that an end of the screw comes into contact with the first end plate; and compressive means that generates a repulsive force to compress the stack of unit cells when the screw is fitted in the threaded hole of the auxiliary plate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a polymer electrolyte fuel cell that works at ordinary temperature and is used for portable power sources, electric vehicle power sources, and domestic cogeneration systems. 
     The polymer electrolyte fuel cell causes a gaseous fuel, such as gaseous hydrogen, and an oxidant gas, such as the air, to be subjected to electrochemical reactions at gas diffusion electrodes, thereby generating the electricity and the heat simultaneously. A pair of catalytic reaction layers, which are mainly composed of carbon powder with a platinum metal catalyst carried thereon, are closely attached to opposite faces of a polymer electrolyte membrane, which selectively transports hydrogen ions. A pair of diffusion layers having both the gas permeability and the electric conductivity are further arranged on the respective outer faces of the catalytic reaction layers. The catalytic reaction layer and the diffusion layer constitute each electrode. 
     A pair of conductive separator plates are arranged across the membrane electrode assembly so as to mechanically fix the assembly and cause the assembly to electrically connect with the assembly in series. A specific part of the separator plate that is in contact with the electrode of the assembly has a gas flow path, which feeds a supply of reaction gas to the electrode and flows out the gas evolved by the reaction and the remaining excess gas. 
     The structure of each unit cell included in such a fuel cell is described below with the drawings. 
     FIG. 11 shows a unit cell in which a pair of electrodes  1 , each having a catalytic layer  2 , are arranged across a polymer electrolyte membrane  3  to yield a membrane electrode assembly, the circumferential part of the polymer electrolyte membrane  3  is interposed between a pair of sealing members  17 , and a pair of separator plates  4  are arranged across the membrane electrode assembly. The separator plate  4  has a gas flow path  5  for feeding a supply of the gaseous fuel or a supply of the oxidant gas to the electrode  1 . The sealing member  17  prevents the gaseous hydrogen as the gaseous fuel and the air as the oxidant gas from leaking out of the fuel cell or from being mixed with each other. A separator plate having the gas flow path formed on its one surface and a flow path of cooling water formed on its other surface is applied for every two unit cells. An  0  ring is interposed between the separator plates having the flow path of cooling water, in order to prevent a leak of the cooking water. 
     FIG. 12 shows another sealing technique for preventing leaks of the gases and the cooling water. This technique arranges gaskets  19 , which are composed of an appropriate resin or metal and have a substantially identical thickness with that of the electrode  1 , around the electrodes  1 . In this structure, the clearance between a separate plate  4  and the gasket  19  is sealed with a grease  20  or an adhesive. The clearance between the separator plates having the flow path of the cooling water is also sealed with the grease or the adhesive. 
     FIG. 13 shows another example, in which membrane electrode assemblies (hereinafter referred to as MEAs), each of which is obtained by interposing a polymer electrolyte membrane between a pair of electrodes having an identical size with that of the polymer electrolyte membrane, and separator plates are alternately laid one upon another. This technique causes specific parts of the MEA that require the gas sealing property, to be previously impregnated with a resin  21 , which has sealing effect and subsequently solidifies. The solidified resin ensures the gas sealing property between the MEA and the separator plate. 
     Most of the fuel cells have a laminate structure in which a large number of unit cells having the above configuration are laid one upon another. In the course of operation of the fuel cells, heat is produced with generation of the electric power. In the stack of unit cells, a cooling plate is provided for every one or two unit cells, in order to keep the cell temperature at a substantially fixed level and simultaneously enable the generated thermal energy to be unitized, for example, in the form of warm water. The cooling plate is generally a thin metal plate which a heat transfer medium, such as cooling water, flows through. Another possible application forms a flow path of cooling water on the rear face of the separator plate included in the unit cell, so as to make the separator plate function as the cooling plate as discussed above. In this case, a cooling water flow path is formed on the rear face of the separator plate, which is included in each unit cell, to make a flow of cooling water. In this structure, O rings and gaskets are required to seal the heat transfer medium, such as cooling water. The  0  rings in the seal should be compressed to ensure the sufficient electric conductivity across the cooling plate. 
     The stack of unit cells generally has a so-called internal manifold arrangement having gas inlets, gas outlets, and inlets and outlets of cooling water to and from the respective unit cells, which are generally called manifolds, inside the stack of unit cells. In the case where the reformed city gas is used as the gaseous fuel to drive the cells, however, the CO concentration rises in the downstream area of the flow path of the gaseous fuel. This may cause the electrode to be poisoned with CO, which results in lowering the temperature and thereby further accelerating the poisoning of the electrode. In order to relieve the deterioration of the cell performance, the external manifold type is noted as the structure that increases the length of the gas supply and exhaust unit between the manifold and each unit cell. 
     In either of the internal manifold type and the external manifold type, the required process lays a plurality of unit cells including the cooling units one upon another in one direction to provide a stack of unit cells, arranges a pair of end plates outside the stack of unit cells, and fixes the stack of unit cells between the pair of end plates with tie rods. It is naturally desirable to urge the whole face of each unit cell as uniformly as possible. In other words, it is desirable that the substantially uniform compressive force is applied to the whole laminating faces of the stack of unit cells. By taking into account the mechanical strength, the end plates and the tie rods are generally made of a metal material, such as stainless steel. These end plates and tie rods are electrically insulated from the stack of unit cells by insulator plates, so that the electric current does not run outside through the end plates. One improved technique of fastening makes the tie rods pierce the through holes formed in the separator plates. Another improved technique binds the whole stack of unit cells via the end plates with metal belts. 
     In any of the sealing methods shown in FIGS. 11 through 13, the constant compressive force is required to maintain the sufficient sealing property. One adopted structure inserts a coiled spring or a disc spring between the tie rod and the end plate. The compressive force ensures the electric contact between the respective constituents of the cells including the separator plates, the electrodes, and the electrolyte membranes. 
     In the structure that disposes the sealing members or O rings around the electrodes for the purpose of the seal of the gas, for example, the gaseous hydrogen or the air, a relatively large plane pressure is required. The adopted arrangement accordingly presses the sealing member or the sealing part between the pair of separator plates, so as to maintain the sufficient sealing effect. It is thus required to apply a relatively large compressive force constantly. This, however, makes the fastening mechanism including the end plates and the tie rods bulky and heavy in weight, while the fuel cell is required to have less total weight. 
     The long-term application of a pressure to the seals and the electrodes causes distortion of the constituents and thereby lowers the plane pressure required for the seals and the electrodes. A mechanism for absorbing the distortion is thus required in the fastening mechanism. One adopted mechanism for that purpose installs a spring on the end of the tie rod. This is another factor of making the whole fuel cell undesirably bulky. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is thus to solve the above problems and provide a polymer electrolyte fuel cell having a small-sized, simply-constructed fastening mechanism of a stack of unit cells. 
     Another object of the present invention is to provide a polymer electrolyte fuel cell having excellent long-term stability and a fastening mechanism of a stack of unit cells that reduces creep deformation due to the long-time application of a pressure. 
     The present invention is accordingly directed to a polymer electrolyte fuel cell having a stack of unit cells, which is provided by laying a plurality of unit cells one upon another, and a fastening mechanism. Each of the unit cells includes a polymer electrolyte membrane, an anode and a cathode arranged across the polymer electrolyte membrane, an anode-side conductive separator plate having a gas flow path for feeding a supply of gaseous fuel to the anode, and a cathode-side conductive separator plate having a gas flow path for feeding a supply of oxidant gas to the cathode. 
     The fastening mechanism of the present invention includes: a first end plate disposed on one end of the stack of unit cells; a second end plate arranged on the other end of the stack of unit cells; an auxiliary plate disposed at least outside the first end plate; at least one set of restraining means, each of which has a band-like shape and restrains a first member located on one end of an assembly, which includes the stack of unit cells, the first and the second end plates, and the auxiliary plate, and a second member located on the other end of the assembly to restrict separation or unfastening of the first member and the second member from each other; a screw that is to be fitted in a threaded hole formed in the auxiliary plate in such a manner that an end of the screw comes into contact with the first end plate; and compressive means that generates a repulsive force to compress the stack of unit cells when the screw is fitted in the threaded hole of the auxiliary plate. 
     In accordance with one preferable mode of the present invention, the auxiliary plate includes a metal plate having elasticity and also functions as the compressive member. 
     In accordance with another preferable mode of the present invention, the fastening mechanism further includes a second auxiliary plate arranged outside the second end plate, and the compressive means is interposed between the second end plate and the second auxiliary plate. 
     In accordance with still another preferable mode of the present invention, the restraining means includes a band that surrounds the assembly and has an end fixed to the auxiliary plate. 
     In accordance with one preferable configuration of the present invention, the restraining means includes a pair of bands that are disposed on opposite side faces of the assembly and are fixed respectively to an end of the auxiliary plate and an end of the second end plate. 
     In accordance with another preferable configuration of the present invention,.which is applicable when the fastening mechanism has the second auxiliary plate, the restraining means includes a pair of bands that are disposed on opposite side faces of the assembly and are fixed respectively to an end of the auxiliary plate and an end of the second auxiliary plate. 
     In either one of the above configurations, the auxiliary plate linked with the set of restraining means is divided into a plurality of parallel parts, and each divisional auxiliary plate has a threaded hole, in which a screw is fitted. 
     The compressive means interposed between the second end plate and the second auxiliary plate is preferably a disc spring. 
     The restraining means preferably have heat insulating effects. 
     In one aspect of the present invention, a polymer electrolyte fuel cell comprises: 
     a stack of unit cells, each of the unit cells comprising a polymer electrolyte membrane, an anode and a cathode arranged across said polymer electrolyte membrane, an anode-side conductive separator plate having a gas flow path for feeding a supply of gaseous fuel to said anode, and a cathode-side conductive separator plate having a gas flow path for feeding a supply of oxidant gas to said cathode; 
     first and second end plates disposed on both ends of the stack of unit cells; 
     first and second auxiliary plates disposed on both ends of an assembly, which includes the stack of unit cells, the first and the second end plates, and the first and second auxiliary plates; 
     at least one set of restraining means, each of which has a band-like shape and restrains the first auxiliary plate and the second auxiliary plate from separating each other; 
     a screw fitted in a threaded hole formed in the auxiliary plate in such a manner that an end of the screw comes into contact with the first end plate; and 
     compressive means that generates a repulsive force to compress the stack of unit cells when the screw is fitted in the threaded hole of the first auxiliary plate. 
     The present invention also comprehends a polymer electrolyte fuel cell which comprises: 
     a stack of unit cells, each of the unit cells comprising a polymer electrolyte membrane, an anode and a cathode arranged across said polymer electrolyte membrane, an anode-side conductive separator plate having a gas flow path for feeding a supply of gaseous fuel to said anode, and a cathode-side conductive separator plate having a gas flow path for feeding a supply of oxidant gas to said cathode; 
     a first end plate disposed on one end of the stack of unit cells; 
     a second end plate arranged on the other end of the stack of unit cells; 
     an auxiliary plate disposed outside the first end plate; 
     at least one set of restraining means, each of which has a band-like shape and restrains the auxiliary plate located on one end of an assembly, which includes the stack of unit cells, the first and the second end plates, and the auxiliary plate, and the second end plate located on the other end of the assembly to restrict separation of the auxiliary plate and the second end plate from each other; 
     a screw fitted in a threaded hole formed in the auxiliary plate in such a manner that an end of the screw comes into contact with the first end plate; and 
     compressive means that generates a repulsive force to compress the stack of unit cells when the screw is fitted in the threaded hole of the auxiliary plate. 
     While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWING 
     FIG. 1 is a plan view illustrating a polymer electrolyte fuel cell in one embodiment of the present invention. 
     FIG. 2 is a front view illustrating the fuel cell of FIG.  1 . 
     FIG. 3 is a sectional view, taken on the line III-III′ of FIG.  2 . 
     FIG. 4 is a plan view illustrating another polymer electrolyte fuel cell in another embodiment of the present invention. 
     FIG. 5 is a front view illustrating the fuel cell of FIG.  4 . 
     FIG. 6 is a sectional view, taken on the line VI-VI′ of FIG.  4 . 
     FIG. 7 is a plan view illustrating another polymer electrolyte fuel cell in still another embodiment of the present invention. 
     FIG. 8 is a sectional view illustrating still another polymer electrolyte fuel cell in another embodiment of the present invention. 
     FIG. 9 is a partly omitted perspective view illustrating constituents of the polymer electrolyte fuel cell in the embodiment of the present invention. 
     FIG. 10 is a perspective view illustrating separator plates used in the embodiment of the present invention. 
     Fig. 11 is a sectional view illustrating one configuration of a prior art polymer electrolyte fuel cell. 
     FIG. 12 is a sectional view illustrating another configuration of the prior art polymer electrolyte fuel cell. 
     FIG. 13 is a sectional view illustrating still another configuration of the prior art polymer electrolyte fuel cell. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following describes preferred embodiments of the present invention with reference to the accompanied drawings. 
     First Embodiment 
     FIGS. 1 through 3 show a fastening mechanism of a fuel cell adopted in a first embodiment of the present invention. 
     A pair of end plates  31  and  32  are arranged across a stack of unit cells  30 , which includes a plurality of unit cells. The stack of unit cells  30  with output terminals  40  and  41  is electrically insulated from the end plates  31  and  32  by insulator members, although not specifically illustrated. The assembly of the end plate  31 , the stack of unit cells  30 , and the end plate  32  are fastened together at four different positions. Namely the stack of unit cells  30  and the end plates  31  and  32  are fastened with four sets of fastening members. Each fastening member includes a restraining means  39 , which includes a band surrounding the assembly, a first auxiliary plate  33  to which one end of the band is fixed, a second auxiliary plate  34  with a plurality of projections  37 , which are formed corresponding to a plurality of recesses  36  formed in the bottom face of the end plate  32  and have disc springs  38  set thereon, and a plurality of screws  35  fitted in threaded holes formed in the first auxiliary plate  33 . 
     When the screws  35  are bolted to the first auxiliary plate  33 , the first auxiliary plate  33  moves in a direction apart from the end plate  31 , so that the band  39  causes the second auxiliary plate  34  to move in a direction of compressing the springs  38 . This arrangement accordingly fastens the stack of unit cells  30  via the end plates  31  and  32  and applies the required plane pressure to seals and electrodes. Inlets  42 ,  43 , and  44  for supplies of gases fed to the electrodes and a flow of cooling water are arranged on the top side of the stack of unit cells  30 , whereas their outlets are arranged on the opposite side, that is, on the bottom side of the stack of unit cells  30 . 
     The following describes a concrete example of the first embodiment. 
     The process first soaked carbon powder having the particle diameter of not greater than several microns in an aqueous solution of chloroplatinic acid and caused the platinum catalyst to be carried on the surface of the carbon powder by reduction. The weight ratio of carbon to platinum carried thereon was one to one. The process then dispersed the carbon powder with the platinum catalyst carried thereon in an alcohol solution of a polymer electrolyte to yield a slurry. 
     The process, on the other hand, caused carbon paper having a thickness of 400 μm, which was the material of electrodes, to be impregnated with an aqueous dispersion of a fluorocarbon resin (Neoflon ND 1  manufactured by Daikin Industries, Ltd.) The process then dried the impregnated carbon paper and heated at 400° C. for 30 minutes to give the water repellency to the carbon paper. As shown in FIG. 9, the process homogeneously applied the slurry containing the carbon powder on one face of a water-repelled carbon paper electrode  1  to form a catalytic layer  2 . The process laid a pair of the carbon paper electrodes  1  across a polymer electrolyte membrane  3  in such a manner that the respective catalytic layers  2  of the carbon paper electrodes  1  were in contact with the polymer electrolyte membrane  3 , and dried the layered structure to yield a membrane electrode assembly (MEA). Each of the two carbon paper electrodes  1  had both a length and a width of 10 cm and was disposed on the center of the larger polymer electrolyte membrane  3  having both a length and a width of 12 cm. The MEA was interposed between a pair of carbon separator plates  4  having air tightness to yield a unit cell. The separator plate  4  is 4 mm in thickness and has a large number of gas flow paths  5 , which have a width of 2 mm and a depth of 1 mm and have been cut in its surface. The separator plate  4  also has a plurality of gas manifold holes  6  and a plurality of cooling water manifold holes  7  formed on its circumferential part. In the process of interposing the MEA between the pair of separator plates  4 , polyethylene terephthalate (PET) sheets  8  having the same dimensions as those of the carbon separator plates  4  are arranged around the electrodes  1 . The PET sheet, which is hard and does not have the sealing property, was used as a spacer between the carbon separator plate  4  and the electrolyte membrane  3 . After lamination of two such unit cells, the process disposed a pair of separator plates each having a cooling water flow path, through which the cooling water flows, across the laminated unit cells to give a unit stack of unit cells. Repetition of this pattern completed a stack of unit cells. In this example, no O ring for sealing was used between the separator plates having the cooling water flow path. 
     The process laid 50 unit stack of unit cells one upon another and disposed metal current collectors and insulator plates composed of an electrically insulating material on both ends of the layered structure, so as to complete a stack of unit cells. 
     The pair of end plates are arranged across the stack of unit cells as part of the fastening mechanism according to the first embodiment discussed above with FIGS. 1 through 3. The fastening mechanism of this embodiment attains the sufficient compressive force by means of the four metal bands  39  (SUS304-CSP) having a thickness of 1 mm and a width of 22 mm, the end plates  31  and  32 , the first and second auxiliary plates  33  and  34 , and the disc springs  38 . The spring constant of the springs  38  was 500 kgf/mm. A compressive force of 400 kg per position was applied under the compression of 0.8 mm. The compressive force in assembly was 13 kgf/cm 2 . The pressure distribution of the separator plate was measured with a pressure sensitive paper. The result showed a substantially uniform pressure distribution over the whole surface of the separator plate. Compared with the conventional arrangement that disposes tie rods outside the separator plates to apply the compressive force, the technique of this embodiment desirably applies the compressive force inside the separator plate and thus significantly reduces deformation of the end plates. This enables the thickness of each end plate to be thinner of not greater than 5 mm. 
     Second Embodiment 
     FIGS. 4 through 6 show another fastening mechanism of the fuel cell applied in a second embodiment of the present invention. 
     A pair of end plates  51  and  52  are arranged across a stack of unit cells  50 , which includes a plurality of unit cells. A first auxiliary plate  53  with screws  55  fitted in threaded holes is disposed above the end plate  51 , whereas a second auxiliary plate  54  having a plurality of projections  57 , which correspond to recesses  56  formed in the bottom face of the end plate  52  and have disc springs  58  set thereon, is disposed below the end plate  52 . The first auxiliary plate  53  and the second auxiliary plate  54  are joined with each other via restraining means, which include bands  59  having hooks to engage with recesses  61  formed on the periphery of the auxiliary plates  53  and  54 . When the screws  55  are bolted to the first auxiliary plate  53 , the springs  57  are compressed to fasten the stack of unit cells  50  via the end plates  51  and  52  and apply the required plane pressure to seals and electrodes. 
     In this embodiment, the two sets of the fastening members, each including the first and second auxiliary plates  53  and  54  and the band  59 , are used to fasten the stack of unit cells  50  and the end plates  51  and  52 . The four screws  55  are attached to each auxiliary plate  53 . 
     Output terminals  62  and  63  of this fuel cell are disposed on the top and bottom of the fuel cell, respectively. These output terminals  62  and  63  are respectively connected to current collectors, which are arranged inside the end plates  51  and  52  via insulator plates. Metal plates  66  and  67  respectively insulated from the output terminals  62  and  63  by insulating collars  68  and  69  are attached to the end plates  51  and  52 , for example, by welding. The metal plates  66  and  67  prevent the first and the second auxiliary plates  53  and  54  from being shifted laterally due to the compressive force. A manifold  64  having gas inlets for feeding supplies of gases to the electrodes and a cooling water inlet and a mainifold  65  having gas outlets and a cooling water outlet are disposed on opposite side faces of the stack of unit cells  50 . The manifolds  64  and  65  also have the same functions as those of the metal plates  66  and  67 . 
     The following describes a concrete example of the second embodiment. 
     In the example of the first embodiment discussed above, the spacers of the PET sheets are arranged around the electrodes in the MEA. The example of the second embodiment does not use the PET sheets but forms the carbon paper electrodes with the catalytic layers applied thereon to the same outer dimensions as those of the carbon separator plates. The ends of the electrodes are accordingly exposed to the side faces of the stack of unit cells. The carbon separator plates used here are for the external manifold arrangement as shown in FIG. 10. A separator plate  4   a  used for every two unit cells has a cooling water flow path  22  and its openings  12  on one face thereof and has, for example, a gas flow path for the gaseous fuel and its openings  10  on the other face thereof. A separator plate  4   b  is disposed to be in contact with a counter electrode, which mates the electrode that is in contact with the face of the separator plate  4   a  with the openings  10  of the gas flow path. The separator plate  4   b  has a gas flow path  5  for the oxidant gas and its openings  11  on the face in contact with the counter electrode, and has a gas flow path for the gaseous fuel and its openings  10  on the other face thereof. A separator plate  4   c  facing the separator plate  4   b  via a unit cell has a gas flow path  5  for the oxidant gas and its openings  11  on one face thereof, and has a cooling water flow path and its openings  12  on the other face thereof. 
     A manifold for feeding supplies of the gaseous fuel, the oxidant gas, and the cooling water to the openings  10  and  11  of the gas flow paths and the opening of the cooling water flow path is disposed on one side face of the stack of unit cells. A manifold for discharging exhausts of the gaseous fuel, the oxidant gas, and the cooling water from the openings  10  and  11  of the gas flow paths and the opening of the cooling water flow path is disposed on the opposite side face of the stack of unit cells. 
     The pair of end plates are arranged across the stack of unit cells as part of a fastening mechanism according to the second embodiment discussed above with FIGS. 4 through 6. The fastening mechanism of this embodiment attains the sufficient compressive force by means of the two metal bands  59  (SUS304-CSP) having a thickness of 1 mm and a width of 75 mm, the end plates  51  and  52 , the first and second auxiliary plates  53  and  54 , the disc springs  58 , and the screws  55 . The spring constant of the springs  58  was 500 kgf/mm. A compressive force of 400 kg per position was applied under the compression of 0.8 mm. The compressive force in assembly was 13 kgf/cm 2 . Hooks  60  are welded to the ends of the band  59 , in order to enable the band  59  to be freely attached to and detached from the stack of unit cells. This improves the performance of assembling, compared with the arrangement of the first embodiment. 
     Third Embodiment 
     FIG. 7 shows still another fastening mechanism of the fuel cell adopted in a third embodiment of the present invention. 
     The second embodiment uses the two sets of the fastening members and fixes the auxiliary plates  53  in the respective fastening members with two sets of screws. The third embodiment, on the other hand, uses four sets of fastening members and fixes auxiliary plates  53   a  in the respective fastening members with one set of screws. The other configuration of the third embodiment is identical with that of the second embodiment. 
     The following describes a concrete example of the third embodiment. 
     The pair of end plates are arranged across the stack of unit cells of the second embodiment as part of a fastening mechanism according to the third embodiment shown in FIG.  7 . The fastening mechanism of this embodiment attains the sufficient compressive force by means of the four metal bands  59  (SUS304-CSP) having a thickness of 1 mm and a width of 75 mm, the end plates  51  and  52 , the first and second auxiliary plates  53   a  and  54 , the disc springs  58 , and the screws  55 . The spring constant of the springs  58  was 500 kgf/mm. A compressive force of 400 kg per position was applied under the compression of 0.8 mm. The compressive force in assembly was 13 kgf/cm 2 . The third embodiment divides the auxiliary plate  53  used in the second embodiment into two parts. In the second embodiment, each auxiliary plate  53  is fastened with four screws. If the band  59  has poor mechanical accuracy, the load is not evenly applied to the four screws. The structure of this embodiment, on the other hand, fastens each auxiliary plate  53   a  with two screws, thereby ensuring even application of the load to the two screws. The pressure distribution of the separator plate was measured with a pressure sensitive paper. The result showed a substantially uniform pressure distribution over the whole surface of the separator plate. 
     Although this embodiment divides the auxiliary plate with the screws into two parts, the similar effects can be exerted by dividing the auxiliary plate with the springs into two parts. 
     Fourth Embodiment 
     FIG. 8 shows another fastening mechanism of the fuel cell adopted in a fourth embodiment according to the present invention. In this embodiment, an auxiliary plate  73  also functions as the compressive member. 
     A pair of end plates  71  and  72  are arranged across a stack of unit cells  70 , which includes a plurality of unit cells, via a pair of current collectors  84  and  86  and a pair of insulator plates  85  and  87 . An auxiliary plate  73  with threaded holes, in which screws  75  are fitted, is disposed above the end plate  71 . The auxiliary plate  73  and the end plate  72  are joined with each other via a band  79  having hooks  80  that engage with recesses  81  formed at the corners of the auxiliary plate  73  and the end plate  72 . When the screws  75  are fitted in the threaded holes of the auxiliary plates  73 , the ends of the screws  75  press the end plate  71 . The auxiliary plate  73  and the end plate  72  are connected with each other via the restraining means  79 , so that the stack of unit cells  70  is compressed with the screw-in motion of the screws  75 . The auxiliary plate  73  functions as the compressive member, and the stack of unit cells  70  is compressed by the compressive force of the compressive member. The compressed stack of unit cells  70  is subjected to creep deformation with an elapse of time, because of the characteristics of the material. The leaf spring mechanism of the auxiliary plate  73 , however, absorbs the creep deformation and enables a stable compressive force to be constantly applied to the stack of unit cells  70 . 
     The output of the fuel cell is supplied to external equipment (not shown) via output terminals  82  and  83  respectively connected to the current collectors  84  and  86 . The other configuration of the fourth embodiment is identical with that of the third embodiment. 
     The arrangement of the present invention makes the mechanism for applying the fastening force to a stack of unit cells assembly desirably small-sized and light-weighted, thereby effectively decreasing the total weight of the fuel cell. 
     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains, after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.