Abstract:
A bipolar plate includes a plurality of flow channels for fuel flow, wherein the flow channels are divided into a plurality of sections along a direction of the fuel flow. The total cross-sectional area of the flow channels across the sections becomes smaller from a fuel inlet toward a fuel outlet. A plurality of protrusions are formed between the sections, and the protrusions mix a fuel that passes through the flow channels. A fuel cell includes membrane electrode assemblies interposed between a plurality of the bipolar plates.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 2006-9008, filed on Jan. 27, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Aspects of the present invention relate to a structure of a bipolar plate used for a fuel cell. 
         [0004]    2. Description of the Related Art 
         [0005]    A fuel cell is an electrical generation system that transforms chemical energy directly into electrical energy through a chemical reaction between hydrogen that is contained in a hydrocarbon group material, such as methanol, ethanol, or natural gas, and oxygen. 
         [0006]    A proton exchange membrane fuel cell (PEMFC) has advantages of superior output, low operating temperature, rapid starting, and speedy response time compared to other fuel cells, and is the preferred fuel cell for automotive, portable, residential and small commercial applications. An example of a proton exchange membrane fuel cell is a direct liquid feed fuel cell. 
         [0007]      FIG. 1  is a cross-sectional view of a basic configuration of a conventional PEMFC, specifically, a direct liquid feed fuel cell. As depicted in  FIG. 1 , a conventional PEMFC has a structure that includes an anode electrode  2 , a cathode electrode  3 , and an electrolyte membrane  1  interposed between the two electrodes  2  and  3 . The anode electrode  2  and the cathode electrode  3  respectively include diffusion layers  22  and  32  that supply and diffuse a fuel, catalyst layers  21  and  31  at which oxidation and reduction reactions of the fuel occur, and electrode supporting layers  23  and  33 . A theoretical voltage output from a unit cell of a direct methanol fuel cell (DMFC) is approximately 1.2 V. However, an open circuit voltage at ambient temperature and atmospheric pressure falls below 1 V due to a voltage drop caused by an active surcharge and a resistance surcharge. In practice, an actual operating voltage of the unit cell lies in the range of 0.4˜0.7 V. Therefore, to obtain higher voltages, a plurality of unit cells connected in series is required. 
         [0008]    A fuel cell stack is formed by stacking a plurality of unit fuel cells that are electrically connected in series with each other. A conductive bipolar plate  4  is interposed between adjacent unit cells to electrically connect the unit cells to each other. 
         [0009]    The bipolar plate  4  may be formed, for example, of a graphite block having high mechanical strength, high electrical conductivity, and good workability. A block of a composite material containing a metal or a conductive polymer can also be used as the bipolar plate  4 . Flow channel  41  and flow channel  42 , which independently supply fuel and oxidant (typically, air) to an anode  2  and a cathode  3  contacting the bipolar plate  4  are formed on respective surfaces of the bipolar plate  4 . In other words, the bipolar plate  4 , when placed in the fuel stack, has one surface that faces an anode  2  of a unit cell and includes flow channel  41  and has an opposite surface that faces the cathode  3  of another unit cell and includes the flow channel  42 . On an uppermost and a lowermost end of the fuel stack, end plates (not shown), which are monopolar plates that respectively supply fuel or air to the anode electrode  2  or the cathode electrode  3 , are disposed. The end plates respectively include the flow channel  41  or the flow channel  42  (see  FIG. 1 ) for supplying fuel or air to the contacting unit cells. 
         [0010]      FIG. 2  is a plan view of a surface of a conventional bipolar plate  4  for a conventional PEMFC In particular,  FIG. 2  shows a surface where flow channels for a cathode are formed. 
         [0011]    Referring to  FIG. 2 , in the conventional bipolar plate  4 , a plurality of flow channels  42 , of which upper parts thereof are opened, are formed in an electrode region  47  where a membrane electrode assembly (MEA) is disposed. Between the channels are lands  48  that contact the MEA. A region outside of the electrode region  47  includes manifolds  46  and  46 ′ connected to an inlet or an outlet, respectively, of the flow channels  41  and fuel path holes  43   a ,  43   b ,  44   a , and  44   b  that are through holes for supplying or discharging hydrogen fuel or oxidant by connecting to the manifolds  46  and  46 ′ and that perforate the bipolar plate  4 . The fuel path holes  43   a ,  43   b ,  44   a , and  44   b  constitute an inlet  43   a  and an outlet  43   b  of the hydrogen fuel and an inlet  44   a  and an outlet  44   b  of the oxidant. 
         [0012]    The flow channels  42  in  FIG. 2  can be formed to have a simple structure in which the flow channels have the same cross-sectional area (same width and same depth). In the conventional bipolar plate  4  having the flow channels  42 , the concentration of oxygen and hydrogen in a gas (air or a reformed hydrogen gas) that flows in the flow channels  42  is reduced as it flows. Accordingly, the current density of the fuel cell is not uniform, and reaction heat may be locally increased. Also, oxygen concentration (or hydrogen concentration at the anode electrode) at a portion of a vertical cross-section of the flow channels  42  that contacts the membrane  1  (see  FIG. 1 ) is lower than the concentration of oxygen at a bottom portion of the flow channels  41 , thereby reducing the efficiency of the fuel cell. 
       SUMMARY OF THE INVENTION 
       [0013]    Aspects of the present invention provide a bipolar plate of a fuel cell, such as a direct liquid fuel cell, that maintains concentration uniformity of a fluid that flows in channels of the bipolar plate. 
         [0014]    Aspects of the present invention also provide a fuel cell having the above bipolar plate. 
         [0015]    According to an aspect of the present invention, there is provided a bipolar plate of a fuel cell, the bipolar plate having a fuel inlet, a fuel outlet and a plurality of flow channels for a fluid flow, wherein the flow channels are divided into multiple sections along a direction of the fuel flow, wherein the total cross-sectional area of the flow channels across the sections becomes smaller from the fluid inlet toward the fluid outlet and wherein a plurality of protrusions are formed between the sections, and the protrusions mix a fuel that passes through the flow channels. 
         [0016]    The flow channels may be formed in approximately straight lines. 
         [0017]    A total opening area of the flow channels of the sections may increase from the fuel inlet toward the fuel outlet. 
         [0018]    The flow channels may be divided into two or three sections. 
         [0019]    According to another aspect of the present invention, there is provided a fuel cell in which membrane electrode assemblies (MEAs) each having an anode and a cathode respectively provided on each surface of an electrolyte membrane are interposed between a plurality of bipolar plates, wherein each of the bipolar plates comprises a first surface having a hydrogen fuel inlet, a hydrogen fuel outlet and a plurality of flow channels that supply a hydrogen fuel to the anode of one of the MEAs and a second surface having an oxidant inlet, an oxidant outlet and a plurality of flow channels that supply an oxidant to the cathode of an adjacent one of the MEAs, wherein the flow channels that supply the hydrogen fuel or oxidant to at least one of the anode and the cathode are divided into a plurality of sections along a direction of hydrogen fuel or oxidant flow, wherein the total cross-sectional area of the flow channels across the sections becomes smaller from a hydrogen fuel or oxidant inlet toward a hydrogen fuel or oxidant outlet, and wherein a plurality of protrusions are formed between the sections, and the protrusions mix the hydrogen fuel or the oxidant that passes through the flow channels. 
         [0020]    Additional aspects and/or advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
           [0022]      FIG. 1  is a cross-sectional view of a basic configuration of a direct liquid feed fuel cell; 
           [0023]      FIG. 2  is a plan view of a surface of a bipolar plate for a conventional PEMFC; 
           [0024]      FIG. 3  is a plan view of a surface of a bipolar plate according to an embodiment of the present invention; 
           [0025]      FIGS. 4 and 5  are graphs showing oxygen concentrations (mol/m 3 ) in air flow channels respectively in a conventional bipolar plate and a bipolar plate according to an embodiment of the present invention; 
           [0026]      FIG. 6  is a partial perspective view of a bipolar plate according to another embodiment of the present invention; 
           [0027]      FIG. 7  is a partial perspective view of a bipolar plate according to another embodiment of the present invention; 
           [0028]      FIG. 8  is a plan view of a surface of a bipolar plate, for example, a surface where air is supplied, according to another embodiment of the present invention; and 
           [0029]      FIG. 9  is a cross-sectional view of a direct liquid feed fuel cell having the bipolar plate of  FIG. 3 , according to an embodiment the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
         [0031]      FIG. 3  is a plan view of a surface of a bipolar plate according to an embodiment of the present invention. As described herein, the surface is a surface that supplies an oxidant such as, for example, air and contacts a cathode side of a membrane electrode assembly (MEA). However, it is to be understood that the structural features of embodiments described herein may also be incorporated in a surface that supplies hydrogen fuel, such as a surface that contacts an anode side of an MEA. 
         [0032]    As used herein, the term “fuel” may refer to either a hydrogen fuel or an oxidant. More generally, the term “fluid” refers to any liquid or gas that is directed to flow in a bipolar plate. The term “hydrogen fuel” refers to any fluid that is capable of reacting at an anode side of a fuel cell to produce hydrogen ions and may refer to, for example, hydrogen-containing gas, such as a hydrogen-rich gas, methanol, ethanol, reformed methanol or ethanol, etc. The term “oxidant” refers to any fluid that is capable of reacting at a cathode side of a fuel cell. For example, the oxidant may be oxygen that combines with hydrogen ions and electrons generated by the fuel cell to produce water. Specifically, the oxidant may be an oxygen-containing gas, such as air. 
         [0033]    Referring to  FIG. 3 , oxidant flow channels  121  and  122  through which an oxidant (air) flows are formed on one surface of a bipolar plate  100 , and fuel flow channels (not shown) through which a fuel flows are formed on the other surface of the bipolar plate  100 . The oxidant flow channels  121  and  122  are formed in an electrode region  110  that contacts a membrane electrode assembly (MEA). The oxidant flow channels  121  and  122  are divided into a first section  111  and a second section  112 , and a mixed space  114 , in which the bipolar plate  100  does not contact the MEA, is formed between the first and second sections  111  and  112 . Upper parts of the oxidant flow channels  121  and  122  and the mixed space  114  are opened, and lands  131  and  132  between the oxidant flow channels  121  and  122 , respectively, contact the MEA. 
         [0034]    Regions of the bipolar plate  100  outside of the electrode region  110  include a manifold  130  connected to an inlet of the first section  111  and a manifold  130 ′ connected to an outlet of the second section  112  and fuel path holes  141  through  144  for supplying or discharging a fuel (air and hydrogen fuel) by connecting to the manifolds  130  and  130 ′ and that perforate the bipolar plate  100 . The fuel path holes  141  and  142  constitute an inlet  141  and an outlet  142  of the oxidant. Reference numerals  143  and  144  indicate hydrogen fuel path holes. 
         [0035]    The oxidant flow channels  121  of the first section  111  have a different size from the oxidant flow channels  122  of the second section  112 . Table 1 shows an exemplary design ratio of the oxidant flow channels  121  and  122  of the first and second sections  111  and  112 . 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 First section 
                 Second section 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Width 
                 1 
                 0.8 
               
               
                   
                 Depth 
                 0.7 
                 0.5 
               
               
                   
                 Length 
                 1 
                 1 
               
               
                   
                 Number of fuel channels 
                 2 
                 3 
               
               
                   
                   
               
             
          
         
       
     
         [0036]    Referring to Table 1, a cross-sectional area (width×depth×number of channels) of each of the oxidant flow channels  121  in the first section  111  is 1.4, but a cross-sectional area of each of the oxidant flow channels  122  in the second section  112  is 1.2. Accordingly, the flow rate in the second section  112  is higher than in the first section  111 . Also, the ratio of total opening area (width×number of channels) (ratio of area that contacts the MEA) of the oxidant flow channels  121  in the first section  111  is 1, while the ratio of total opening area of the oxidant flow channels  121  in the second section  112  is 1.2. Accordingly, the ratio of the contact area with the MEA in the second section  112  is higher than in the first section  111 . Therefore, this design increases the flow rate of air and the ratio of contact area with the MEA in the second section  112  to compensate for the lower oxygen concentration in the second section  112  than in the first section  111 . 
         [0037]      FIG. 4  is a graph showing oxygen concentrations (mol/m 3 ) in a conventional bipolar plate  4 .  FIG. 5  is a graph showing oxygen concentrations (mol/m 3 ) in the bipolar plate  100  having oxidant flow channels  121  and  122  according to an embodiment of the present invention. 
         [0038]    Referring to  FIGS. 4 and 5 , the rate of oxygen concentration reduction in the first section  111  of the bipolar plate  100  according to the present embodiment is almost the same as in the corresponding portion of the conventional bipolar plate  4 . However, in the second section  112 , the rate of oxygen concentration reduction of the bipolar plate  100  according to the present embodiment is lower than in the conventional bipolar plate  4 . This is due to the relatively shallow depth of the oxidant flow channels  122  in the second section  112 , which eventually reduces the oxygen concentration difference at the vertical cross-section of the oxidant flow channels  122 . 
         [0039]    Table 2 summarizes the performances of a fuel cell having a conventional bipolar plate  4  and a fuel cell having the bipolar plate  100  according to an embodiment of the present embodiment. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Conventional 
                 Present 
               
               
                   
                 art 
                 embodiment 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Average current density, A/cm 2   
                 0.3367 
                 0.3371 
               
               
                   
                 Pressure loss (cathode), Pa 
                 189 
                 268 
               
               
                   
                 Pressure loss (anode), Pa 
                 85 
                 85 
               
               
                   
                   
               
             
          
         
       
     
         [0040]    Table 3 shows operating conditions of the fuel cells. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 Mass flow rate (anode), kg/s 
                 4.0 × 10 −8   
               
               
                   
                 Mass flow rate (cathode), kg/s 
                 2.0 × 10 −6   
               
               
                   
                 Operating pressure, atm 
                 1.1 
               
               
                   
                 Anode gas 
                 H 2  = 100% 
               
               
                   
                 Cathode gas 
                 O 2 :N 2  = 0.2:0.8 
               
               
                   
                 Cell voltage, V 
                 0.7 
               
               
                   
                   
               
             
          
         
       
     
         [0041]    Referring to Tables 2 and 3, the fuel cell that uses the bipolar plate  100  according to the present embodiment has an increased average current density. The increased pressure loss in the oxidant flow channels  121  and  122  that contact the cathode is caused by the increased flow rate due to the reduction of the cross-sectional areas of the oxidant flow channels  122  in the second section  112 . The increase in the flow rate and the contact area between the MEA and the uniformity of concentration at the vertical cross-sections of the oxidant flow channels  122  in the second section  112  help the supply of oxygen in the second section  112 . As a result, favorable current characteristics are obtained. On the other hand, no difference in the pressure loss was observed at a surface of the bipolar plates that contacts the anode since identical flow channels at the surface facing the anode are formed in the conventional bipolar plate and in the bipolar plate  100  according to the present embodiment. 
         [0042]      FIG. 6  is a partial perspective view of a bipolar plate  100 ′ according to another embodiment of the present invention. Like reference numerals are used for substantially identical elements in  FIG. 3 , and detailed descriptions thereof will not be repeated. 
         [0043]    Referring to  FIG. 6 , obstacles  150 , which are protrusion portions, are formed in a mixed space  114  between a first section  111  and a second section  112 . The obstacles  150  are formed such that a fluid that passes through oxidant flow channels  121  in the first section  111  flows over the obstacles  150  to enter into the oxidant flow channels  122  in the second section  112 . At this time, the fluid becomes mixed, and accordingly, a uniform fuel concentration can be achieved. The obstacles  150  may be formed facing the outlets of the oxidant flow channels  121 . 
         [0044]      FIG. 7  is a partial perspective view of a bipolar plate  100 ″ according to another embodiment of the present invention. Like reference numerals are used for substantially identical elements in  FIG. 3 , and detailed descriptions thereof will not be repeated. 
         [0045]    Referring to  FIG. 7 , disturbers  160 , which are protrusions, are formed in a mixed space  114  between a first section  111  and a second section  112 . The disturbers  160  may have a cylindrical shape. The disturbers  160  are formed such that a fluid that passes through oxidant flow channels  121  in the first section  111  flows around the disturbers  160  to enter into oxidant flow channels  122  in the second section  112 . At this time, the fluid becomes mixed, and accordingly, a uniform fuel concentration can be achieved. 
         [0046]    Table 4 summarizes the performances of the fuel cells respectively having a conventional bipolar plate  4  and the bipolar plates  100 ′ (embodiment 1) and  100 ″ (embodiment 2) according to the present embodiments. The same operating conditions indicated in Table 3 were applied. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Conventional art 
                 Embodiment 1 
                 Embodiment 2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Average current 
                 0.3367 
                 0.3373 
                 0.3419 
               
               
                 density, A/cm 2   
               
               
                 Pressure loss 
                 189 
                 275 
                 303 
               
               
                 (cathode), Pa 
               
               
                 Pressure loss 
                 85 
                 82 
                 76 
               
               
                 (anode), Pa 
               
               
                   
               
             
          
         
       
     
         [0047]    Referring to Table 4, the fuel cells that use the bipolar plates  100 ′ and  100 ″ according to the embodiments of the present invention have higher average current densities than a fuel cell that uses the conventional bipolar plate  4 . In particular, the fuel cell that uses the bipolar plate  100 ″ having the disturbers  160  shows a large increase in the current density. The increased pressure loss in the oxidant flow channels  121  and  122  that contact the cathode is caused by the reduction of the cross-sectional areas of the oxidant flow channels  122  across the second section  112  and the protrusions  150  and  160 , thereby increasing the flow rate in the second section  112 . The increase in the flow rate and the contact area between the MEA and the uniformity of concentration at the vertical cross-sections of the oxidant flow channels  122  in the second section  112  help the supply of oxygen to the cathode in the second section  112 . As a result, favorable current characteristics are obtained. 
         [0048]      FIG. 8  is a plan view of a surface of a bipolar plate  200 , for example, a surface where air is supplied, according to another embodiment of the present invention. 
         [0049]    Referring to  FIG. 8 , oxidant flow channels  221 ,  222 , and  223  are formed on a surface of the bipolar plate  200 , and fuel flow channels (not shown) are formed on the other surface of the bipolar plate  200 . The oxidant flow channels  221 ,  222 , and  223  are formed in an electrode region  210  that contacts the MEA. The electrode region  210  is divided into first, second, and third sections  211 ,  212 , and  213 . Mixed spaces  214  and  215  that do not contact the MEA are respectively formed between the first section  211  and the second section  212  and between the second section  212  and the third section  213 . Upper parts of the oxidant flow channels  221 ,  222 , and  223  and the mixed spaces  214  and  215  are opened to function as flow channels, and lands  231 ,  232  and  233  between the oxidant flow channels  221 ,  222  and  223 , respectively, contact the MEA. Disturbers  160  (see  FIG. 7 ) may be formed in the mixed spaces  214  and  215 . 
         [0050]    Regions outside of the electrode region  210  include a manifold  230  that is connected to an inlet of the first section  211  and a manifold  230 ′ connected to an outlet of the third section  213  and fuel path holes  241  through  244  for supplying or discharging a fuel (air and hydrogen fuel) by connecting to the manifolds  230  and  230 ′ and that perforate the bipolar plate  200 . The fuel path holes  241  and  242  constitute an inlet  241  and an outlet  242  of the oxidant. Reference numerals  243  and  244  respectively indicate hydrogen fuel path holes. 
         [0051]    Table 5 shows a design ratio of the oxidant flow channels  221 ,  222 , and  223  of the first through third sections  211 ,  212 , and  213 . 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 First section 
                 Second section 
                 Third section 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Width 
                 1 
                 0.7 
                 0.6 
               
               
                 Depth 
                 0.7 
                 0.6 
                 0.5 
               
               
                 Length 
                 0.4 
                 0.3 
                 0.3 
               
               
                 Number of channels 
                 2 
                 3 
                 4 
               
               
                   
               
             
          
         
       
     
         [0052]    Referring to Table 5, cross-sectional areas of the oxidant flow channels  221 ,  222 , and  223  in the first through third sections  211 ,  212 , and  213  are respectively 1.4, 1.32, and 1.2. Accordingly, the flow rate of the fluid increases as it goes to the third section  213  from the first section  211 . 
         [0053]    Also, the ratio of the total opening area (width×number of channels) (ratio of area that contacts the MEA) of the oxidant flow channels  221 ,  222 , and  223  in the first through third sections  211 ,  212 , and  213  is 2:2.1:2.4. The ratio of contact area with the MEA in the second section  212  and the third section  213  is higher than in the first section  211 . The design increases the flow rate of air and the ratio of contact area with the MEA in the second section  212  and the third section  213  to compensate for the fact that the second section  212  and the third section  213  have a lower oxygen concentration than the first section  211 . 
         [0054]    Table 6 summarizes the performances of a fuel cell having the bipolar plate  200  according to the present embodiment. The same operating conditions indicated in Table 3 were applied. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Conventional 
                 Present 
               
               
                   
                 art 
                 embodiment 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Average current density, A/cm 2   
                 0.3367 
                 0.3488 
               
               
                   
                 Pressure loss (cathode), Pa 
                 189 
                 453 
               
               
                   
                 Pressure loss (anode), Pa 
                 85 
                 76 
               
               
                   
                   
               
             
          
         
       
     
         [0055]    Referring to Table 6, the fuel cell that uses the bipolar plate  200  according to the current embodiment of the present invention has a higher average current density than a fuel cell that uses the conventional bipolar plate  4 . The increased pressure loss in the oxidant flow channels  221 ,  222 , and  223  that contact the cathode is caused by the reduction of cross-sectional area in the second section  212  and the third section  213 , and accordingly, the flow rate of air in the second section  212  and the third section  213  is increased. The increase in the flow rate of air and the contact area between the MEA and the uniformity of concentration at the vertical cross-sections of the oxidant flow channels  222  and  223  in the second section  212  and the third section  213  help the supply of oxygen in the second section  212  and the third section  213 . As a result, favorable current characteristics are obtained. On the other hand, no substantial difference in pressure loss was observed at surfaces of the bipolar plates that contact an anode since identical flow channels are formed at the surface facing the anode in the conventional bipolar plate and in the bipolar plate  200  according to the present embodiment. 
         [0056]      FIG. 9  is a cross-sectional view of a direct liquid feed fuel cell having the bipolar plate  100 , illustrated in  FIG. 3 , according to an embodiment of the present invention. Like reference numerals are used for substantially identical elements in  FIG. 3 , and detailed descriptions thereof will not be repeated. Although  FIG. 9  shows the bipolar plate  100 , it is to be understood that the bipolar plates  100 ′ ( FIG. 6 ),  100 ″ (( FIG. 7 ), or  200  ( FIG. 8 ) may also be used in a direct liquid feed fuel cell. Although  FIG. 9  shows a direct liquid feed fuel, it is to be understood that the present invention is not limited to direct liquid fuel cells and that the bipolar plate according to the aspects of the present invention may be used in any type of proton exchange membrane fuel cell, or more generally, in any type of fuel cell that includes bipolar plates placed between MEAs. 
         [0057]    Referring to  FIG. 9 , a plurality of MEAs are stacked in a fuel cell stack, and conductive bipolar plates  100  are placed between the MEAs. Each MEA has an electrolyte membrane  170  having an anode  172  on a surface thereof and a cathode  174  on an opposite surface thereof. Conductive end plates  180   a  and  180   b  constitute uppermost and lowermost layers of the fuel cell stack, respectively. Only one surface of the conductive end plates  180   a  and  180   b  contacts an MEA, and thus the contacting surface of the conductive end plates  180   a  and  180   b  has the same shape as a surface of the bipolar plate  100  and functions in the same manner as the bipolar plate  100 . The MEA, the bipolar plates  100  between the MEAs, the conductive end plates  180   a  and  180   b , and current collection plates  190   a  and  190   b  can be fixed using both fixing end plates  195   a  and  195   b  by screw. 
         [0058]    Reference numeral  150  indicates a sealing member, such as, for example, a gasket, and prevents a hydrogen fuel or an oxidant (air) supplied from the fuel path holes  141  through  144  from connecting to the anode  172  or the cathode  174 . 
         [0059]    As described above, a bipolar plate according to an embodiment of the present invention increases flow rate of air in a section where an oxygen concentration is reduced, reduces a vertical height of flow channels, and increases a supply of oxygen to an MEA by increasing a total width of the flow channels. 
         [0060]    In a PEMFC stack according to an embodiment of the present invention, a fuel can be easily supplied by mounting the bipolar plate, thereby increasing current density. 
         [0061]    Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.