Abstract:
A fuel cell stack includes: a plurality of membrane-electrode assemblies; first and second end plates respectively positioned outside outermost ones of the membrane-electrode assemblies; and a plurality of separators respectively positioned between the membrane-electrode assemblies and between the outermost ones of the membrane-electrode assemblies and the first and second end plates. The first end plate includes an oxidizing agent inlet, an oxidizing agent outlet, and a moisture supplying flow path connecting the oxidizing agent inlet and the oxidizing agent outlet. The moisture supplying flow path includes a first end portion adjacent to the oxidizing agent outlet and a second end portion adjacent to the oxidizing agent inlet, the first end portion being larger than the second end portion and being a different distance away from a surface of the first end plate facing away from the second end plate than the second end portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0055628 filed in the Korean Intellectual Property Office on Jun. 11, 2010, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (a) Field 
     The present invention relates to a fuel cell stack. More particularly, the present invention relates to an end plate for a fuel cell stack. 
     (b) Description of Related Art 
     A fuel cell system includes a fuel cell stack that generates electrical energy using the electrochemical reaction between a fuel (hydrocarbon-based fuel, pure hydrogen, or reformed gas rich in hydrogen) and an oxidizing agent (air or pure oxygen). A direct oxidation type fuel cell uses a liquid or gas hydrocarbon-based fuel, and a polymer electrolyte type fuel cell uses pure hydrogen or a reformed gas rich in hydrogen. 
     The fuel cell stack includes membrane-electrode assemblies (MEAs), separators positioned between the membrane-electrode assemblies and supplying a fuel and an oxidizing agent to the membrane-electrode assemblies, and a pair of end plates positioned outside the outermost separator and pressing the membrane-electrode assemblies and the separators together to fix them as one body. 
     The end plate generally includes a fuel inlet, an oxidizing agent inlet, a fuel outlet, and an oxidizing agent outlet. The fuel introduced into the fuel inlet is provided to the anode of the membrane-electrode assemblies while passing through a fuel channel of the separator, and non-reacted fuel is discharged through the fuel outlet. The oxidizing agent introduced into the oxidizing agent inlet is provided to the cathode of the membrane-electrode assemblies while passing through an oxidizing agent channel of the separator, and the non-reacted oxidizing agent is discharged through the oxidizing agent outlet. 
     The oxidizing agent is humidified to a predetermined humidity by using a humidifying device before the introduction of the oxidizing agent to the fuel cell stack, or external air may be introduced as is, without humidifying. However, when using the humidifying device, the volume of the fuel battery system is increased, manufacturing costs are increased, and a water supply source and a pump for supplying the water to the humidifying device are required. Meanwhile, when air that is not humidified is introduced as is, the region that is initially supplied with the oxidizing agent becomes very dry with respect to the cathodes of the membrane-electrode assemblies, such that the membrane-electrode assemblies may be deteriorated. 
     That is, moisture is generated as a byproduct of the electrochemical reaction inside the fuel cell stack, however the electrochemical reaction is not active in the region that is initially supplied with the oxidizing agent with respect to the cathodes, such that a dry state is maintained. The dry state can cause radicals generated during the operation of the membrane-electrode assembly to be sustained in the membrane-electrode assembly for a long time, such that deterioration of the membrane-electrode assembly may be accelerated. 
     The above information disclosed in the Background section is only for enhancement of understanding of the background of the invention, and therefore may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fuel cell stack for preventing or reducing deterioration of the membrane-electrode assembly by increasing a moisture content of an oxidizing agent provided to the fuel cell stack, without substantially increasing a volume of the fuel battery system or manufacturing costs. 
     A fuel cell stack according to an exemplary embodiment of the present invention includes: a plurality of membrane-electrode assemblies; first and second end plates respectively positioned outside outermost ones of the membrane-electrode assemblies; and a plurality of separators respectively positioned between the membrane-electrode assemblies and between the outermost ones of the membrane-electrode assemblies and the first and second end plates, for supplying at least one of a fuel or an oxidizing agent to the membrane-electrode assemblies. The first end plate includes an oxidizing agent inlet, an oxidizing agent outlet, and a moisture supplying flow path connecting the oxidizing agent inlet and the oxidizing agent outlet. The moisture supplying flow path includes a first end portion adjacent to the oxidizing agent outlet and a second end portion adjacent to the oxidizing agent inlet, the first end portion being larger than the second end portion and being a different distance away from a surface of the first end plate facing away from the second end plate than the second end portion. 
     The first end plate may be substantially parallel to a ground surface, and a distance between the ground surface and the first end portion may be greater than a distance between the second end portion and the ground surface. The moisture supplying flow path may be include two flow paths having different sizes connected along a length direction of the moisture supplying flow path. 
     The moisture supplying flow path may include a first flow path having a substantially same size as the first end portion, and a second flow path connected to the first flow path and having a substantially same size as the second end portion. The first flow path and the second flow path may have substantially circular cross-sections. The first flow path and the second flow path may have hydrophobic surfaces. 
     The moisture supplying flow path may be formed along a substantially straight line at a slope. Lower surfaces of the first flow path and the second flow path may be substantially aligned, and a latch jaw may be formed on an opposite surface of a boundary region where the first flow path and the second flow path are connected to one another. The latch jaw may be substantially perpendicular to a length direction of the first flow path and the second flow path, or the latch jaw may have a slope of less than or equal to 45° with respect to a length direction of the first flow path and the second flow path. 
     The moisture supplying flow path may be formed such that a central axis of the first flow path and a central axis of the second flow path are substantially aligned with one another, such that a latch jaw may be formed on at least two opposite surfaces of a boundary region where the first flow path and the second flow path are connected. The latch jaw may be substantially perpendicular to a length direction of the first flow path and the second flow path, or the latch jaw may have a slope of less than or equal to 45° with respect to a length direction of the first flow path and the second flow path. 
     The first end plate may be positioned closer to the ground surface than the second end plate, and the oxidizing agent inlet and the oxidizing agent outlet may be arranged to be diagonal to one another on the first end plate. 
     A fuel cell stack according to another exemplary embodiment of the present invention includes: a plurality of membrane-electrode assemblies; first and second end plates respectively positioned outside outermost ones of the membrane-electrode assemblies; and a plurality of separators respectively positioned between the membrane-electrode assemblies and between the outermost ones of the membrane-electrode assemblies and the first and second end plates, for supplying at least one of a fuel or an oxidizing agent to the membrane-electrode assemblies. The first end plate includes an oxidizing agent inlet, an oxidizing agent outlet, and a moisture supplying flow path connecting the oxidizing agent inlet and the oxidizing agent outlet. The moisture supplying flow path includes at least two flow paths continuously connected along a length of the moisture supplying flow path and having different sizes, wherein a flow path adjacent to the oxidizing agent outlet from among the flow paths is larger than a flow path adjacent to the oxidizing agent inlet from among the flow paths. 
     A first flow path may include the flow path adjacent to the oxidizing agent outlet, and a second flow path connected to the first flow path may include the flow path adjacent to the oxidizing agent inlet, wherein the first flow path and the second flow path may have circular cross-sections. The first flow path and the second flow path may have hydrophobic surfaces. 
     The moisture supplying flow path may include a latch jaw at a boundary region where the first flow path and the second flow path are connected, and the latch jaw may have a slope of less than or equal to 45° with respect to a length direction of the first flow path and the second flow path. 
     The fuel cell stack according to exemplary embodiments of the present invention recycles moisture of the discharged oxidizing agent by using a moisture supplying flow path, such that a humidity of the supplying oxidizing agent may be increased. As a result, the whole membrane-electrode assembly may be maintained with appropriate humidity, such that deterioration of the membrane-electrode assembly may be suppressed or reduced, and the generating efficiency and life-span of the fuel cell stack may be improved. Also, the fuel cell stack does not include an additional humidifying device, such that a volume of the fuel battery system is small, manufacturing costs may be decreased, and an increase in a number of components may be minimized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a fuel cell stack according to a first exemplary embodiment of the present invention; 
         FIG. 2  is an exploded perspective view showing one membrane-electrode assembly and two separators in the fuel cell stack shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of the membrane-electrode assembly taken along I-I of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a first end plate taken along line II-II of  FIG. 1 ; 
         FIG. 5  is a perspective view of a moisture supplying flow path shown in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view of a first end plate of a fuel cell stack according to a second exemplary embodiment of the present invention; 
         FIG. 7  is a cross-sectional view of a first end plate of a fuel cell stack according to a third exemplary embodiment of the present invention; and 
         FIG. 8  is a cross-sectional view of a first end plate of a fuel cell stack according to a fourth exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF SYMBOLS 
     
       
         
               
               
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 100: fuel cell stack 
                 10: membrane-electrode assembly 
               
               
                   
                 11: electrolyte- membrane 
                 12: anode 
               
               
                   
                 13: cathode 
                 20: separator 
               
               
                   
                 210: anode separator 
                 220: cathode separator 
               
               
                   
                 21: fuel channel 
                 22: oxidizing agent channel 
               
             
          
           
               
                   
                 31, 32: first and second current collecting plates 
               
               
                   
                 41, 42: first and second end plates 
               
             
          
           
               
                   
                 51: fuel inlet 
                 52: fuel outlet 
               
               
                   
                 61: oxidizing agent inlet 
                 62: oxidizing agent outlet 
               
             
          
           
               
                   
                 70, 710, 720, 730: moisture supplying flow path 
               
             
          
           
               
                   
                 71: first end portion 
                 72: second end portion 
               
               
                   
                 73, 731: first flow path 
                 74, 741: second flow path 
               
               
                   
                 75, 751, 752, 753: latch jaw 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art will recognize, the described embodiments may be modified in various different ways without departing from the spirit or scope of the present invention. 
       FIG. 1  is an exploded perspective view of a fuel cell stack according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a fuel cell stack  100  according to the first exemplary embodiment includes a plurality of membrane-electrode assemblies  10 , a plurality of separators  20  positioned between the membrane-electrode assemblies  10 , first and second current collecting plates  31  and  32  respectively positioned outside the outermost separators, and first and second end plates  41  and  42  respectively positioned outside the first and second current collecting plates  31  and  32 . 
     The fuel cell stack  100  is integrally fixed, for example, by a bonding means that is not shown, and the membrane-electrode assembly  10 , the separator  20 , the first and second current collecting plates  31  and  32 , and the first and second end plates  41  and  42  are pressed and held tightly close to each other by the bonding means. The bonding means may include bonding bolts penetrating at least four edges of the fuel cell stack  100  and fixing nuts coupled to the ends of the bonding bolts to tighten the bonding bolts. 
     The plurality of membrane-electrode assemblies  10  and the plurality of separators  20  form an electrical generator. The membrane-electrode assembly  10  is supplied with fuel and an oxidant through the separators  20 , and generates electrical energy by using an electrochemical reaction between the fuel and the oxidizing agent. The separators  20  pressurize and support the membrane-electrode assembly  10 , which has relatively weak mechanical strength, and are made of a conductive material, thereby electrically connecting the membrane-electrode assemblies  10  to one another. 
     The fuel cell stack  100  may be supplied with a hydrocarbonaceous fuel (methanol, ethanol, liquefied petroleum gas, liquefied natural gas, gasoline, butane gas, etc.) in a liquid phase or a gas phase, or may be supplied with cracked hydrogen that is cracked by a reformer from the hydrocarbonaceous fuel or a hydrogen-enriched reformate gas. The fuel cell stack  100  may further be supplied with pure oxygen stored in a separate storage unit as the oxidant, or it may be supplied with external air containing oxygen as is. 
     The first and second end plates  41  and  42  and the members positioned therebetween may be disposed perpendicular to a ground surface or parallel to the ground surface.  FIG. 1  shows an example in which the first and second end plates  41  and  42  and the members positioned therebetween are parallel to the ground surface. In this case, a fuel inlet  51 , a fuel outlet  52 , an oxidizing agent inlet  61 , and an oxidizing agent outlet  62  may be formed on the end plate that is disposed or positioned at a lower side of the stack, from among the first and second end plates  41  and  42  (e.g., the first end plate  41  in  FIG. 1 ). 
     The fuel and the oxidizing agent are respectively provided to the fuel inlet  51  and the oxidizing agent inlet  61 , and the non-reacted fuel and the non-reacted oxidizing agent that passes through the electrical generator are discharged through the fuel outlet  52  and the oxidizing agent outlet  62 . The fuel inlet  51  and the fuel outlet  52  are arranged to be diagonal to one another on the first end plate  41 , and the oxidizing agent inlet  61  and the oxidizing agent outlet  62  are also arranged to be diagonal to one another. 
     A moisture supplying flow path  70  connecting the oxidizing agent inlet  61  and the oxidizing agent outlet  62  is formed inside the first end plate  41 . The moisture supplying flow path  70  is formed with a very small diameter so as not to influence or affect the flow of the oxidizing agent injected through the oxidizing agent inlet  61  and the non-reacted oxidizing agent discharged through the oxidizing agent outlet  62 . The first and second end plates  41  and  42  may be manufactured with an engineering plastic having a thickness of about 20 mm. 
       FIG. 2  is an exploded perspective view showing one membrane-electrode assembly and two separators in the fuel cell stack shown in  FIG. 1 , and  FIG. 3  is a cross-sectional view of the membrane-electrode assembly taken along line I-I of  FIG. 2 . 
     Referring to  FIGS. 2 and 3 , the membrane-electrode assembly  10  includes an electrolyte membrane  11 , an anode electrode  12  that is formed at one side or surface of the electrolyte membrane  11 , a cathode electrode  13  that is formed at an opposing side or surface of the electrolyte membrane  11 , and a supporting film  14  that is fixed along an edge of the electrolyte membrane  11 . 
     The anode electrode  12  is a portion supplied with fuel, and includes a catalyst  121  that converts hydrogen in fuel into electrons and hydrogen ions by an oxidation reaction, and a gas diffusion layer  122  that covers the catalyst layer  121 . The cathode electrode  13  is a portion supplied with an oxidant, and includes a catalyst  131  that converts oxygen in the oxidant into electrons and oxygen ions by a reduction reaction, and a gas diffusion layer  132  that covers the catalyst layer  131 . The electrolyte membrane  11  has an ion exchange function that moves protons generated in the anode electrode  12  to the cathode electrode  13 . 
     The anode electrode  12  and the cathode electrode  13  are formed to be a smaller size than the electrolyte membrane  11 , and the supporting film  14  may be attached to the edge of the electrolyte membrane  11  where the anode electrode  12  and the cathode electrode  13  are not formed. The supporting film  14  suppresses expansion and crease generation of the electrolyte membrane  11  due to moisture adsorption, and it may be mechanically connected with the separators  20 . 
     The separators  20  may be divided into an anode separator  210  that faces the anode electrode  12  and a cathode separator  220  that faces the cathode electrode  13 . The anode separator  210  has a fuel channel  21  on one surface toward the anode electrode  12 , and the cathode separator  220  has an oxidizing agent channel  22  on one surface toward the cathode electrode  13 . The fuel channel  21  and the oxidizing agent channel  22  may be formed as concave grooves. The anode separator  210  and the cathode separator  220  may be integrally bonded to each other, and together are called a bipolar plate. 
     The anode separator  210 , the cathode separator  220 , and the supporting film  14  form a fuel inlet manifold  53 , a fuel outlet manifold  54 , an oxidizing agent inlet manifold  63 , and an oxidizing agent outlet manifold  64 . 
     The fuel inlet manifold  53  and the fuel outlet manifold  54  are respectively positioned at the same positions as (e.g., are substantially aligned with) the fuel inlet  51  and the fuel outlet  52 . The oxidizing agent inlet manifold  63  and the oxidizing agent outlet manifold  64  are respectively positioned at the same positions as (e.g., are substantially aligned with) the oxidizing agent inlet  61  and the oxidizing agent outlet  62 . The fuel channel  21  is connected to the fuel inlet manifold  53  and the fuel outlet manifold  54 , and the oxidizing agent channel  22  is connected to the oxidizing agent inlet manifold  63  and the oxidizing agent outlet manifold  64 . 
     A cooling channel  23  may be formed on an inner surface of the anode separator  210  and the cathode separator  220 . The cooling channel  23  may be connected with a blowing unit (not shown), and external air may be introduced to the cooling channel  23  via the blowing unit. With the heat exchange between external air and the electrical generator, the temperature of the electrical generator may be decreased. Alternatively, the anode separator  210  and the cathode separator  220  may include a cooling water manifold (not shown), in which cooling water is circulated instead of the above-described air cooling method. 
     Referring to  FIG. 1  and  FIG. 2 , the fuel that is supplied to the fuel inlet  51  is dispersed into the fuel channel  21  of the anode separators  210  through the fuel inlet manifold  53 , and is supplied substantially simultaneously to the anode electrodes  12  of the membrane-electrode assemblies  10 . The oxidant that is supplied to the oxidizing agent inlet  61  is dispersed into the oxidant channel  22  of the cathode separators  220  through the oxidizing agent inlet manifold  63 , and is supplied substantially simultaneously to the cathode electrodes  13  of the membrane-electrode assemblies  10 . Thereby, electrical energy is generated by the electrochemical reaction between the fuel and the oxidizing agent in the membrane-electrode assembly  10 . 
     Non-reacted fuel that is not used in the electrochemical reaction of the membrane-electrode assemblies  10  is discharged to the outside of the fuel cell stack  100  through the fuel outlet  52  via the fuel outlet manifold  54 . Non-reacted oxidant that is not used in the electrochemical reaction of the membrane-electrode assemblies  10  and moisture generated as a by-product of the electrochemical reaction are discharged to the outside of the fuel cell stack  100  through the oxidizing agent outlet  62  via the oxidizing agent outlet manifold  64 . 
     The oxidizing agent provided to the oxidizing agent inlet  61  that is not passed through a humidifying device may be external air. Most of this oxidizing agent is in a dry state, such that humidifying may be beneficial. Meanwhile, the non-reacted oxidizing agent that is passed through the electrical generator as a by-product of the electrochemical reaction includes a large amount of moisture. 
     The moisture supplying flow path  70  of the first end plate  41  in the fuel cell stack  100  of the first exemplary embodiment functions as a moisture moving path toward the oxidizing agent inlet  61  from the oxidizing agent outlet  62 . That is, the moisture supplying flow path  70  moves the moisture of the non-reacted oxidizing agent (hereinafter referred to as “discharged oxidizing agent”) passing through the oxidizing agent outlet  62 , to the oxidizing agent passing through the oxidizing agent inlet  61  (hereinafter referred to as the “supplied oxidizing agent”), thereby functioning to humidify the supplied oxidizing agent. 
       FIG. 4  is a cross-sectional view of the first end plate taken along the line II-II of  FIG. 1 , and  FIG. 5  is a perspective view of a moisture supplying flow path shown in  FIG. 4 . 
     Referring to  FIG. 4  and  FIG. 5 , the moisture supplying flow path  70  is formed inside the first end plate  41 , includes a first end portion  71  contacting the oxidizing agent outlet  62  and a second end portion  72  contacting the oxidizing agent inlet  61 , and runs diagonally through the first end plate  41  from outlet  62  to inlet  61 . The first end portion  71  contacting the oxidizing agent outlet  62  is larger than the second end portion  72  contacting the oxidizing agent inlet  61 . Also, when the first and second end plates  41  and  42  are parallel to a ground surface, the height of the first end portion  71  and the second end portion  72  are different from each other. That is, the first end portion  71  and the second end portion  72  have different heights with respect to the ground surface. 
     The moisture supplying flow path  70  may be made of at least two flow paths having different diameters. For example, the moisture supplying flow path  70  includes of a first flow path  73  having the same size as the first end portion  71  and a second flow path  74  connected to the first flow path  73  and having the same size as the second end portion  72 . 
     The cross-sections of the first flow path  73  and the second flow path  74  may be circular. If, for example, the cross-sections of the first flow path  73  and the second flow path  74  are shaped to have corners such as with a quadrangle or triangle, moisture may be collected at the corners by the surface tension of the water. Accordingly, the first flow path  73  and the second flow path  74  in the embodiment are circular, such that the movement of the moisture may be easier or more fluid. 
     The moisture supplying flow path  70  may have a hydrophobic surface. In this case, the movement of the moisture may be more smooth or fluid by decreasing a contact angle of the water with respect to the surface of the moisture supplying flow path  70 . A hydrophobic surface may be realized by coating a hydrophobic material on an interior wall of the first flow path  73  and the second flow path  74 , or by forming a protruded column on a nanometer (nm) scale on the interior wall of the first flow path  73  and the second flow path  74  through a surface treatment process. 
     The first flow path  73  and the second flow path  74  may have the same length. On the other hand, the length ratio of the first flow path  73  and the second flow path  74  may be changed according to a pressure and a flow amount of the oxidizing agent. That is, the first flow path  73  may be longer than the second flow path  74 , or the second flow path  74  may be longer than the first flow path  73 , according to the pressure and the flow amount of the oxidizing agent passing through the oxidizing agent inlet  61  and the oxidizing agent outlet  62 . 
     The center of the moisture supplying flow path  70  may not be curved in any direction, but may be formed in a straight line. Accordingly, the moisture supplying flow path  70  forms the shortest path connecting the oxidizing agent outlet  62  and the oxidizing agent inlet  61 , and may be arranged with a substantially uniform slope, where its height with respect to the ground surface is gradually decreased from the first end portion  71  toward the second end portion  72 . 
     In  FIG. 5 , the first flow path  73  and the second flow path  74  have the same length, and the cross-section of the moisture supplying flow path  70  is circular. However, the cross-section of the moisture supplying flow path  70  and the length of the first and second flow paths  73  and  74  are not limited thereto, and may be any of various different configurations. 
     In an operation process of the fuel cell stack  100 , the supplied oxidizing agent of higher pressure is supplied to and flows in the oxidizing agent inlet  61 , and the discharged oxidizing agent of lower pressure and including moisture flows in the oxidizing agent outlet  62 . At least some of the moisture included in the discharged oxidizing agent is absorbed in the moisture supplying flow path  70  by the pressure difference and the diameter difference of the first flow path  73  the second flow path  74  such that the moisture is provided to the supplied oxidizing agent. 
     That is, the moisture included in the discharged oxidizing agent having lower pressure flows easily into the first flow path  73  having a relatively larger diameter, and the collected moisture in the first flow path  73  is absorbed into the second flow path  74  by the higher pressure of the supplied oxidizing agent, such that the moisture is provided or discharged into the supplied oxidizing agent. Here, the second flow path  74  is lower than the first flow path  73  with respect to the ground surface, such that gravity force also has an influence when the moisture of the first flow path  73  is absorbed into the second flow path  74 . 
     The first flow path  73  is larger than the second flow path  74 , such that a latch jaw  75  is formed at a boundary region where the first flow path  73  and the second flow path  74  contact each other. This latch jaw  75  has a function of increasing the speed of the moisture that flows into the second flow path  74  from the first flow path  73 . Accordingly, the movement of the moisture passing through the moisture supplying flow path  70  may be accelerated by the influence of the gravity force and the speed increase from the latch jaw. The influence by the gravity force may be increased as the slope of the moisture supplying flow path  70  is increased. 
     Along a lower surface or end of the first and second flow paths  73  and  74  toward a lower surface of the first end plate  41 , the moisture supplying flow path  70  may be formed to be straight without a step. In this case, the latch jaw  75  is positioned on an upper surface or end of the boundary region where the first flow path  73  and the second flow path  74  contact each other. The lower end of the first flow path  73  and the second flow path  74  is formed as a straight line such that the moisture of the first flow path  73  is not stagnated at the boundary region and may be instantly or more easily discharged to the second flow path  74 . 
     As described above, the fuel cell stack  100  of the first exemplary embodiment reuses the moisture of the discharged oxidizing agent by using the moisture supplying flow path  70 , such that the humidity of the supplied oxidizing agent may be increased. As a result, the whole membrane-electrode assembly  10  may be maintained with a more appropriate humidity, such that deterioration of the membrane-electrode assembly  10  may be suppressed or reduced and the generating efficiency and life-span of the fuel cell stack  100  may be improved. Also, the fuel cell stack  100  does not include an additional humidifying device such that a volume or size of the fuel battery system is small, the manufacturing costs may be decreased, and an increase in the number of components may be minimized. 
       FIG. 6  is a cross-sectional view of a first end plate of a fuel cell stack according to a second exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , a fuel cell stack in the second exemplary embodiment is the same as the fuel cell stack of the first exemplary embodiment, except in a first end plate  411 , a latch jaw  751  of a moisture supplying flow path  710  has a smooth slope along a length direction of the first flow path  73  and the second flow path  74 . The same members as in the first exemplary embodiment are indicated by the same reference numerals. 
     The latch jaw  75  of the moisture supplying flow path  70  in the first exemplary embodiment is substantially perpendicular to the length direction of the first flow path  73  and the second flow path  74 , while the latch jaw  751  of the moisture supplying flow path  710  in the second exemplary embodiment forms a smooth angle of less than approximately 45° with respect to the length direction of the first flow path  73  and the second flow path  74 . In this case, a phenomenon in which water gathers near the latch jaw may be suppressed or reduced such that the latch jaw  751  having the smooth slope serves to guide the moisture movement. 
       FIG. 7  is a cross-sectional view of a first end plate of a fuel cell stack according to a third exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , the fuel cell stack of the third exemplary embodiment is the same as the fuel cell stack of the first exemplary embodiment, except that in a first end plate  412 , a moisture supplying flow path  720  is formed on a central line or axis of a first flow path  731  and a central line or axis of a second flow path  741  that are substantially aligned with one another. The same members as in the first exemplary embodiment are indicated by the same reference numerals. In  FIG. 7 , the central line of the first flow path  731  and the second flow path  741  is indicated by a chain line (e.g., broken line). A latch jaw  752  of the moisture supplying flow path  720  in the third exemplary embodiment is formed on a lower portion of the moisture supplying flow path  720  toward a lower surface of the first end plate  412 , as well as on an upper portion of the moisture supplying flow path  720 . 
       FIG. 8  is a cross-sectional view of a first end plate of a fuel cell stack according to a fourth exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , the fuel cell stack of the fourth exemplary embodiment is the same as the fuel cell stack of the second exemplary embodiment, except that in a first end plate  413 , a moisture supplying flow path  730  is formed on a central line or axis of the first flow path  731  and a central line or axis of the second flow path  741  that are substantially aligned with one another. The same members as in the second exemplary embodiment are indicated by the same reference numerals. A latch jaw  753  of the moisture supplying flow path  730  in the fourth exemplary embodiment is formed on a lower portion of the moisture supplying flow path  730  toward a lower surface of the first end plate  413 , as well as on an upper portion of the moisture supplying flow path  730 . 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but instead is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.