Abstract:
A membrane-electrode assembly (MEA) for a fuel cell includes a fuel cell electrolyte membrane, an anode disposed at a first side of the electrolyte membrane, and a cathode disposed at a second side of the electrolyte membrane, wherein the cathode has a thickness and an area, the cathode area extending in a plane substantially parallel to a major surface of the electrolyte membrane, the cathode area includes a central area and a peripheral area, the peripheral area extending to lateral edges of the cathode, the central area includes hydrophilic portions and hydrophobic portions, the peripheral area includes hydrophilic portions and hydrophobic portions, and the central area is more hydrophobic than the peripheral area.

Description:
BACKGROUND 
     1. Field 
     Embodiments relate to a membrane-electrode assembly for a fuel cell and a fuel cell stack including the same. 
     2. Description of the Related Art 
     A fuel cell is a generation system that directly converts chemical reaction energy of hydrogen contained in hydrocarbon-based fuel and separately supplied oxygen into electrical energy. 
     The fuel cell may be configured as, e.g., a polymer electrolyte membrane fuel cell (PEMFC), a direct oxidation fuel cell (DOFC), etc. 
     The polymer electrolyte fuel cell may have a fuel cell body called a stack (referred to as ‘stack’, hereinafter), and may have a structure in which electrical energy is generated according to an electrical-chemical reaction between hydrogen (which may be supplied from a reformer) and oxygen (which may be supplied by actuating an air pump or fan). 
     The direct oxidation fuel cell may receive fuel directly, i.e., without using hydrogen gas. A hydrogen component of the fuel and separately supplied oxygen may be electrically and chemically reacted to generate electrical energy. 
     The stack may have a few or scores of unit cells, each having a membrane electrode assembly (MEA) and a separator (also called a bipolar plate). 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore it may contain information that does not form the prior art that is already known to a person of skill in the art. 
     SUMMARY 
     Embodiments are therefore directed to a membrane-electrode assembly for a fuel cell and a fuel cell stack including the same, which substantially overcome one or more of the problems due to the limitations and disadvantages of the related art. 
     It is therefore a feature of an embodiment to provide a membrane-electrode assembly configured to transfer water from a central area, a fuel cell stack and fuel cell including the same, and associated methods. 
     It is therefore another feature of an embodiment to provide a membrane-electrode assembly configured to maintain sufficient water in a central area so as to provide water retention for an electrolyte membrane, a fuel cell stack and fuel cell including the same, and associated methods. 
     At least one of the above and other features and advantages may be realized by providing a membrane-electrode assembly (MEA) for a fuel cell, including fuel cell electrolyte membrane, an anode disposed at a first side of the electrolyte membrane, and a cathode disposed at a second side of the electrolyte membrane. The cathode may have a thickness and an area, the cathode area extending in a plane substantially parallel to a major surface of the electrolyte membrane, the cathode area may include a central area and a peripheral area, the peripheral area extending to lateral edges of the cathode, the central area may include hydrophilic portions and hydrophobic portions, the peripheral area may include hydrophilic portions and hydrophobic portions, and the central area may be more hydrophobic than the peripheral area. 
     An area density of the hydrophobic portions in the central area may be higher than that of the hydrophobic portions in the peripheral area. 
     The area density of the hydrophobic portions may gradually decrease from the central area to the peripheral area. 
     An area density of the hydrophobic portions in the central area is substantially identical to that of the hydrophobic portions in the peripheral area. 
     The hydrophobic portions in the central area may be more strongly hydrophobic than the hydrophobic portions in the peripheral area. 
     The hydrophilic portions in the peripheral area may be more strongly hydrophilic than the hydrophilic portions in the central area. 
     An interval between the hydrophobic portions in the central area may be smaller than an interval between hydrophobic portions in the peripheral area. 
     The hydrophobic portions in the central area may have a same width as the hydrophobic portions in the peripheral area. 
     Intervals between the hydrophobic portions may decrease monotonically from the central area to the peripheral area. 
     A width of the hydrophobic portions in the central area may increase from a first dimension to a second dimension, and the hydrophobic portions in the peripheral area may have a width substantially equal to the second dimension. 
     A width of the hydrophobic portions may monotonically increase moving along a radius of the central area toward the peripheral area, and a width of the hydrophobic portions in the peripheral area may be substantially constant. 
     A width of the hydrophobic portions may decrease from the central area to the peripheral area. 
     The width of the hydrophobic portions may decrease monotonically from the central area to the peripheral area. 
     The hydrophobic portions may form a continuous spiral from the central area to the peripheral area. 
     A plurality of hydrophobic portions may extend as continuous radial members from the central area to the peripheral area. 
     The hydrophobic portions may be discrete unit areas, and a number of the discrete unit areas per unit area of the cathode may increase from the central area to the peripheral area. 
     The cathode may further include a cathode catalyst layer in contact with the electrolyte membrane, and a cathode gas diffusion layer at an outer side of the cathode catalyst layer, and the gas diffusion layer may include a hydrophobic portion and a hydrophilic portion. 
     The cathode may further include a cathode catalyst layer in contact with the electrolyte membrane, and a cathode gas diffusion layer at an outer side of the cathode catalyst layer, the cathode gas diffusion layer may include a black layer and a micro-porous layer (MPL), and the MPL may include a hydrophobic porous layer and a hydrophilic porous layer. 
     The peripheral area may completely surround the central area. 
     At least one of the above and other features and advantages may also be realized by providing a fuel cell stack, including a plurality of electricity generating units, and a pressing plate configured to press the electricity generating units together. Each of the electricity generating units may include a fuel cell electrolyte membrane, an anode disposed at a first side of the electrolyte membrane, and a cathode disposed at a second side of the electrolyte membrane. The cathode may have a thickness and an area, the cathode area extending in a plane substantially parallel to a major surface of the electrolyte membrane, the cathode area may include a central area and a peripheral area, the peripheral area extending to lateral edges of the cathode, the central area may include hydrophilic portions and hydrophobic portions, the peripheral area may include hydrophilic portions and hydrophobic portions, and the central area may be more hydrophobic than the peripheral area. 
     At least one of the above and other features and advantages may also be realized by providing a method of fabricating a fuel cell, the method including providing a plurality of electricity generating units, mechanically and electrically coupling the electricity generating units to one another, and coupling a fuel supply for supplying a hydrogen-containing fuel and an oxidizer supply for supplying an oxygen-containing oxidizer to the electricity generating units. Each of the electricity generating units may include a fuel cell electrolyte membrane, an anode disposed at a first side of the electrolyte membrane, and a cathode disposed at a second side of the electrolyte membrane. The cathode may have a thickness and an area, the cathode area extending in a plane substantially parallel to a major surface of the electrolyte membrane, the cathode area may include a central area and a peripheral area, the peripheral area extending to lateral edges of the cathode, the central area may include hydrophilic portions and hydrophobic portions, the hydrophobic portions in the central area may be more strongly hydrophobic than the hydrophobic portions in the peripheral area, and the area density of the hydrophobic portions in the central area may be higher than that of the hydrophobic portions in the peripheral area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages will become more apparent to those of ordinary skill in the art by describing in detail example embodiments with reference to the attached drawings, in which: 
         FIG. 1  illustrates an exploded perspective view of a fuel cell stack according to a first example embodiment; 
         FIG. 2  illustrates an exploded perspective view of the configuration of an electricity generating unit of  FIG. 1 ; 
         FIG. 3  illustrates a side view of a membrane-electrode assembly (MEA) according to the first embodiment; 
         FIG. 4  illustrates a cross-sectional view of the MEA taken along line III-III in  FIG. 3 ; 
         FIG. 5  illustrates a front view of the MEA according to the first example embodiment; 
         FIG. 6A  is a photograph illustrating a distribution of moisture generated when current is produced at 100 mA per cm 2 ; 
         FIG. 6B  is a photograph illustrating a distribution of moisture generated when current is produced at 300 mA per cm 2 ; 
         FIG. 7  illustrates a front view of an MEA according to a second example embodiment; 
         FIG. 8  illustrates a front view of an MEA according to a third example embodiment; 
         FIG. 9  illustrates a front view of an MEA according to a fourth example embodiment; 
         FIG. 10  illustrates a front view of an MEA according to a fifth example embodiment; 
         FIG. 11  illustrates a side view of an MEA according to a sixth example embodiment; 
         FIG. 12  illustrates a sectional view taken along line X-X in  FIG. 10 ; and 
         FIG. 13  illustrates a front view of an MEA according to a seventh example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Korean Patent Application No. 10-2009-0018603, filed on Mar. 4, 2009, in the Korean Intellectual Property Office, and entitled: “Membrane-Electrode Assembly for Fuel Cell and Fuel Cell Stack with the Same,” is incorporated by reference herein in its entirety. 
     Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     In the drawing figures, the dimensions of layers and regions may be exaggerated for clarity of illustration. It will also be understood that when a layer or element is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Further, it will be understood that when a layer is referred to as being “under” another layer, it can be directly under, and one or more intervening layers may also be present. In addition, it will also be understood that when a layer is referred to as being “between” two layers, it can be the only layer between the two layers, or one or more intervening layers may also be present. Like reference numerals refer to like elements throughout. 
     Herein, hydrophobic treatment refers to coating a surface with a material having a hydrophobic property or forming a hydrophobic region by adding a material that is hydrophobic. Hydrophilic treatment refers to coating a surface with a material having a hydrophilic property or forming a hydrophilic region by adding a material that is hydrophilic. 
       FIG. 1  illustrates an exploded perspective view of a fuel cell stack according to a first example embodiment.  FIG. 2  illustrates an exploded perspective view of the configuration of an electricity generating unit of  FIG. 1 . 
     With reference to  FIGS. 1 and 2 , a fuel cell stack  100  according to a first embodiment may include electricity generating units  10  in units of cells that generate electrical energy by making fuel and oxygen react with each other. 
     In the present example embodiment, the plurality of electricity generating units  10  may be successively disposed, so as to form the fuel cell stack  100  with an assembly of the electricity generating units  10 . 
     Fuel used for the fuel cell stack  100  may include hydrogen-contained liquid or gas fuel such as methanol, ethanol, LPG, LNG, gasoline, butane gas, and the like. In this case, the fuel cell stack  100  may be configured as a direct oxidation fuel cell that generates electrical energy through a direct reaction between liquid or gas fuel by the electricity generating units  10  and oxygen. 
     Alternatively, the fuel cell stack  100  according to an example embodiment may use hydrogen cracked from liquid or gas fuel through a general reformer as fuel. In this case, the fuel cell stack  100  may be configured as a polymer electrolyte fuel cell that generates electrical energy through a reaction between hydrogen and oxygen by the electricity generating units  10 . 
     The fuel cell stack  100  according to the present example embodiment may use pure oxygen stored in a storage unit as oxygen reacting with fuel, or may use air that contains oxygen natively. 
     In the fuel cell stack  100 , the electricity generating unit may include a membrane-electrode assembly (MEA)  20  and separators (also called bipolar plates)  13  and  15  tightly attached to both surfaces of the MEA  20 . A plurality of electricity generating units  10  may be provided to form the laminated fuel cell stack  100  according to the present example embodiment. 
     A pressing plate  30  may be positioned at the outermost portions of the fuel cell stack  100  to tightly attach the plurality of electricity generating units  10 . In an implementation, the separators  13  and  15  positioned at the outermost portions of the plurality of electricity generating units  10  serve as the pressing plates. Also, the pressing plates  30  may be configured to have the function of the separators  13  and  15 , besides the function of tightly attaching the plurality of electricity generating units  10 . 
     The separators  13  and  15  may be disposed to be tightly attached with the MEA  20  interposed therebetween to form a hydrogen passage  13   a  and an air passage  15   a  at respective sides of the MEA  20 . The hydrogen passage  13   a  may be positioned at the side of an anode  26  (as described below) of the MEA  20 , and the air passage  15   a  is positioned at the side of a cathode  27  (as described below) of the MEA  20 . 
     The hydrogen passage  13   a  and the air passage  15   a  may be disposed in a straight line state at predetermined intervals on the separators  13  and  15 , and may be substantially formed in zigzags with both ends thereof alternately connected. However, the structures and orientations of the hydrogen passage  13   a  and the air passage  15   a  are not limited thereto. 
     The MEA  20  interposed between the both separators  13  and  15  may include an active area  201  with a certain area where an oxidation/reduction reaction occurs, and a non-active area  202  connected with edge portions of the active area  201 . A gasket (not shown) may be installed at the non-active area  202  to seal the edge portions of tightly attached faces of the separators  13  and  15  corresponding to the active area  201 . 
     When the fuel cell operates, water is generated due to oxygen reduction reaction by a cathode of the MEA. If the water is not properly discharged, a flooding phenomenon may occur, whereby oxidant gas is hindered from diffusing. However, the electrolyte membrane and the cathode catalyst layers should maintain a certain amount of water retention to improve generation efficiency. Thus, maintaining water retention of the electrolyte membrane and the cathode catalyst layers without the flooding phenomenon may help ensure effective generation. However, it will be appreciated that these requirements may be in conflict. Embodiments described herein are directed to providing a proper balance of these requirements. 
       FIG. 3  illustrates a side view of a membrane-electrode assembly (MEA) according to the first embodiment.  FIG. 4  illustrates a cross-sectional view of the MEA taken along line in  FIG. 3 . 
     With reference to  FIGS. 3 and 4 , the MEA  20  has a structure in which the anode  26  and the cathode  27  are provided on respective surfaces of the active area  201 , and an electrolyte membrane  21  is provided between the two electrodes  26  and  27 . 
     The electrolyte membrane  21  may be made of solid polymer electrolyte with a thickness of, e.g., about 5 μm to about 200 μm, enabling ion exchange of moving hydrogen ions generated from an anode catalyst layer  24  to a cathode catalyst layer  23 . In order to effectively perform the ion exchange, the electrolyte membrane  21  should have a certain amount of water retention. 
     The anode  26  forming one surface of the MEA  20  may receive hydrogen gas via the hydrogen passage  13   a  formed between the separator  13  and the MEA  20 , includes an anode gas diffusion layer (GDL)  28  and the anode catalyst layer  24 . 
     The anode GDL  28  may be made of, e.g., carbon paper or carbon cloth, and may include a plurality of holes  28   a . Also, the anode GDL  28  supplies hydrogen gas, which has been transferred via the hydrogen passage  13   a , to the anode catalyst layer  24  via the holes  28   a . The anode catalyst layer  24  oxidizes the hydrogen gas to allow converted electrons to move to the cathode  27  via the neighboring separator  15 , and the generated hydrogen ions to move to the cathode  27  via the electrolyte membrane  21 . The electricity generating units  10  generate electrical energy with the flow of the electrons. 
     The cathode  27 , to which the hydrogen ions that have been generated from the anode are moved via the electrolyte membrane  21 , is the part that receives oxygen-containing air via the air passage  15   a  formed between the separator  15  and the MEA  20 . The cathode  27  may include a cathode gas diffusion layer (GDL)  25  and the cathode catalyst layer  23 . 
     The cathode GDL  25  may be made of, e.g., carbon paper or carbon cloth, and may include a plurality of holes  25   a . The cathode GDL  25  supplies air, which is transferred via the air passage  15   a , to the cathode catalyst layer  23  via the holes  25   a.    
     The cathode catalyst layer  23  generates heat and water of a certain temperature by chemical reduction-oxidation (redox) of the hydrogen ions and electrons moved from the anode  26  with oxygen in the air. 
       FIG. 5  illustrates a front view of the MEA according to the first example embodiment. 
     With reference to  FIGS. 4 and 5 , the cathode  27  includes hydrophobic-treated hydrophobic portions  20   a  and hydrophilic-treated hydrophilic portions  20   b  configured so as to easily discharge moisture while maintaining an appropriate degree of water retention. As described in detail below, the cathode  27  may have a greater area density of hydrophobic portions  20   a  in a central area (inner area  210 ) than in a peripheral area (outer area  230 ), such that water, which a byproduct of the electricity-producing reactions, is drawn from the center of the cathode to the periphery. As is also described below, the cathode  27  may have hydrophobic portions  20   a  that are more strongly hydrophobic in the inner area  210  than in the outer area  230 , i.e., the hydrophobic portions  20   a  in the central area  210  may have a greater hydrophobicity per unit area so that they more strongly repel water than the hydrophobic portions  20   a  in the outer area  230  for a given unit area. This may be achieved by, e.g., forming the hydrophobic portions  20   a  in the central area  210  using a greater fraction of hydrophobic material to base material, or using a more hydrophobic material for the hydrophobic portions  20   a  in the central area  210 , etc. Further, in the inner area  210 , a combination of greater area density of the hydrophobic portions  20   a  and greater hydrophobicity per unit area may be used. 
     In an implementation, a hydrophilic film  253  and a hydrophobic film  251  may be coated on the cathode gas diffusion layer  25 , and the hydrophobic portions  20   a  with the hydrophobic film  251  coated thereon may be disposed between the hydrophilic portions  20   b  with the hydrophilic film  253  coated thereon. As shown in  FIG. 4 , water  29  may be attracted to the hydrophilic film  253 . As the material of the hydrophilic film  253  and the hydrophobic film  251 , generally, various materials having a hydrophilic property or a hydrophobic property may be used. 
     The hydrophobic portions  20   a  may be formed in a regular pattern. For example, as shown in  FIG. 5 , the regions connected from the center of the MEA  20  to an outer side may be radially disposed at equal intervals. 
     In this case, the density of the hydrophobic portions  20   a  at an inner area  210  adjacent to the center of the cathode  27  is higher than that at an outer area  230  positioned at an outer side of the inner area  210 . Herein, the density of the hydrophobic portions  20   a  refers to a ratio of the area occupied by the hydrophobic portions  20   a  to the entire area. The inner area  210  and the outer area  230  may be variably set according to the size and operation conditions of the fuel cell stack  100 . 
     In the present example embodiment, the hydrophobic portions  20   a  may each include a variable width portion  211  (having a width gradually increasing toward the outer side) and an equal width portion  212  (positioned at an outer side of the variable width portion  211  and having a uniform width). The variable width portion  211  may be positioned at the inner area  210  and the equal width portion  212  may be positioned at the outer area  230 . 
     The density of the hydrophobic portions  20   a  may be substantially uniform at the area where the variable width portions  211  are positioned. The density of the hydrophobic portions  20   a  may be gradually reduced at the area where the equal width portions  212  are positioned, because the entire area increases in proportion to the square of the distance as it becomes away from the center of the cathode  27 , but the hydrophobic portions  20   a  at the equal width portions  212  maintain a certain area. 
       FIG. 6A  is a photograph showing a distribution of moisture generated when current is produced at 100 mA per cm 2 , and  FIG. 6B  is a photograph showing a distribution of moisture generated when current is produced at 300 mA per cm 2 . 
     In  FIGS. 6A and 6B , air is supplied from an upper portion and fuel is supplied at a lower portion. In this respect, however, even when fuel is supplied from the upper portion and air is supplied from the lower portion, the same results may be obtained. 
     As shown in  FIGS. 6A and 6B , a relatively greater amount of moisture is generated at the central portion of the MEA. This is because air supplied from the upper portion and fuel supplied from the lower portion meet at the central portion to react with each other. Thus, if the moisture generated at the central portion is not properly discharged, a flooding phenomenon would occur to degrade generation efficiency. 
     In this case, however, in the present example embodiment, because the density of the hydrophobic portions  20   a  at the inner area  210  is higher than that of the hydrophobic portions  20   a  at the outer area  230 , the relatively greater amount of moisture generated at the inner area may be effectively discharged to allow air to easily move to the cathode catalyst layer. The moisture released from the hydrophobic portions  20   a  may be discharged to outside via the air passage  15   a  of the separator  15 . In addition, because the hydrophobic portions  20   a  are also formed at the outer area  230 , as well as at the inner area  210 , but with a relatively low density, the electrolyte membrane  21  may have appropriate water retention, thus improving the generation efficiency. 
     In the first example embodiment, both the hydrophobic portions  20   a  and the hydrophilic portions  20   b  are formed, but embodiments are not limited thereto, e.g., only the hydrophobic portions  20   a  may be formed. In this regard, the density of the hydrophobic portions  20   a  is higher at the inner area  210  than at the outer area  230 . Thus, even if only the hydrophobic portions  20   a  are formed, because the density of the hydrophobic portions  20   a  at the outer area  230  is low, appropriate moisture can be maintained. Further, at the inner area, although the density of the hydrophobic portions  20   a  is high, because the greater amount of moisture is generated, an appropriate amount of moisture can be maintained. 
       FIG. 7  illustrates a front view of an MEA according to a second example embodiment. 
     With reference to  FIG. 7 , an MEA  42  may include a hydrophobic-treated hydrophobic portion  42   a  and a hydrophilic-treated hydrophilic portion  42   b  formed between the hydrophobic portions  42   a . The hydrophobic portion  42   a  may have a pattern in a spiral shape connected from the center to an edge portion in a rotational manner as a curved line. 
     The hydrophobic portion  42   a  may be formed such that the interval ‘w’ between the hydrophobic portions  42   a  increases as it goes from the center of the cathode to an outer side. For example, where the interval between the hydrophobic portions at an inner area is ‘w 1 ’ and the interval between hydrophobic portions at an outer area formed at an outer side of the inner area is ‘w 2 ’, w 2  is larger than w 1 . Accordingly, the density of the hydrophobic portions at the central area is higher than that of the hydrophobic portions at the outer area, and thus, a relatively greater amount of moisture generated at the inner area can be easily discharged. Further, a combination of greater area density of the hydrophobic portions  42   a  and greater hydrophobicity per unit area may be used in the central area. 
       FIG. 8  illustrates a front view of an MEA according to a third example embodiment. 
     With reference to  FIG. 8 , an MEA  43  may include a hydrophobic-treated hydrophobic portion  43   a  and a hydrophilic portion  43   b  formed to be adjacent to the hydrophobic portion  43   a  and treated to be hydrophilic. 
     In the present example embodiment, the hydrophobic portion  43   a  proceeds from the center of the MEA  43  in a straight line and an angle, which is repeated several times in a rotational manner until it reaches an outer side. The width of the hydrophobic portion  43   a  may be gradually reduced toward the outside. Accordingly, the density of the hydrophobic portions  43   a  can be made higher at an inner area than at an outer area, and thus, a relatively greater amount of moisture generated at the inner area can be easily discharged. Further, a combination of greater area density of the hydrophobic portions  43   a  and greater hydrophobicity per unit area may be used in the central area. 
       FIG. 9  illustrates a front view of an MEA according to a fourth example embodiment 
     With reference to  FIG. 9 , an MEA  45  according to the fourth example embodiment may include hydrophobic-treated hydrophobic portions  45   a  and hydrophilic portions  45   b  formed to be adjacent to the hydrophobic portions  45   a  and treated to be hydrophilic. 
     In the present example embodiment, the hydrophobic portions  45   a  may be formed to be connected in an arc shape from the center of the MEA  45  to an outer side, and a plurality of hydrophobic portions  45   a  may be disposed at equal intervals. The hydrophobic portions  45   a  may be formed to be connected with the same width from the center of the MEA to the outer side. Thus, the density of the hydrophobic portions  45   a  at an inner area is higher than that of the hydrophobic portions  45   a  at an outer area positioned at an outer side of the inner area. The area of the MEA increases in proportion to the square of the distance as it goes from the inner side to the outer side, but the area of the hydrophobic portions  45   a  does not increase. Thus, the density of the hydrophobic portions  45   a  is gradually reduced toward the outer side. Accordingly, the relatively greater moisture generated at the inner area can be easily discharged and appropriate moisture can be maintained, improving the generation efficiency. Further, a combination of greater area density of the hydrophobic portions  45   a  and greater hydrophobicity per unit area may be used in the central area. 
       FIG. 10  illustrates a front view of an MEA according to a fifth example embodiment. 
     With reference to  FIG. 10 , an MEA  46  according to the fifth embodiment may include hydrophobic-treated hydrophobic portions  46   a  and hydrophilic portions  46   b  disposed to be adjacent to the hydrophobic portions  46   a  and treated to be hydrophilic. 
     The hydrophobic portions  46   a  may be formed as a plurality of discrete unit areas, i.e., unit areas that are spaced apart from each other. A larger number of hydrophobic portions  46  per unit area are positioned at an inner area of the MEA  46 , and a smaller number of hydrophobic portions  46   a  per unit area are positioned at an outer area of the MEA  46 . 
     In the present example embodiment, the hydrophobic portions  46   a  have a circular shape, but the embodiments are not limited thereto. The hydrophobic portions  46   a  may have various other shapes such as an oval or polygonal shape. Further, the size of the discrete unit areas may be varied, e.g., so that a larger size unit area is employed at the central area of the cathode and a smaller size unit area is employed at the peripheral area of the cathode. 
     In the present example embodiment, the density of the hydrophobic portions  46   a  at the inner area is higher than that of the hydrophobic portions  46   a  at the outer area, so a relatively greater amount of moisture generated at the inner area can be easily discharged and appropriate moisture can be maintained, improving the generation efficiency. Further, a combination of greater area density of the hydrophobic portions  46   a  and greater hydrophobicity per unit area may be used in the central area. 
       FIG. 11  illustrates a side view of an MEA according to a sixth example embodiment.  FIG. 12  illustrates a sectional view taken along line X-X in  FIG. 10 . 
     With reference to  FIGS. 11 and 12 , an MEA  50  according to the sixth example embodiment includes an anode  63 , a cathode  62 , and an electrolyte membrane  51  interposed between the two electrodes  62  and  63 . 
     The MEA according to the present example embodiment has the same structure as that of the MEA according to the first example embodiment, except that the gas diffusion layer includes micro-porous layers  54  and  55  and black layers  56  and  57 , so a detailed explanation of the structures described above may be omitted in order to avoid repetition. 
     The anode  63  forming one surface of the MEA  50  may include an anode catalyst layer  52  in contact with one surface of the electrolyte membrane  51 , and an anode black layer  56  and an anode micro-porous layer (MPL)  54  formed at an outer side of the anode catalyst layer  52 . 
     The anode MPL  54  may be positioned between the anode catalyst layer  52  and the anode black layer  56 , and the anode MPL  54  and the anode black layer  56  may constitute an anode gas diffusion layer (GDL). 
     The cathode  62  may include a cathode catalyst layer  53  in contact with one surface of the electrolyte membrane  51 , and a cathode black layer  57  and a cathode MPL  55  formed at an outer side of the cathode catalyst layer  53 . 
     The cathode MPL  55  may be positioned between the cathode catalyst layer  53  and the cathode black layer  57 , and the cathode MPL  55  and the cathode black layer  57  may constitute a cathode GDL. 
     The anode black layer  56  and the cathode black layer  57  may be made of, e.g., carbon paper or carbon cloth, and may include holes formed therein. 
     The anode MPL  54  and the cathode MPL  55  may be made of, e.g., graphite, carbon nano-tubes (CNT), fullerene (C60), active carbon, carbon nano-horns, or the like, and may include a plurality of holes that are smaller than those formed in the black layers  56  and  57 . The MPLs  54  and  55  may enhance gas distribution to the catalyst layers  52  and  53 . 
     The cathode black layer  57  transfers air that has been transferred via an air passage to the cathode MPL  55  via the holes  57   a , and the cathode MPL  55  greatly distributes the transferred air to supply it to the cathode catalyst layer  53 . The cathode catalyst layer  53  generates heat and water of a certain temperature by reduction-reacting hydrogen ions and electrons moved from the anode  63  with oxygen in the air. 
     In order to easily discharge the generated water, the cathode MPL  55  includes hydrophilic porous layers  55   a  and hydrophobic porous layers  55   b . The hydrophilic porous layers  55   a  are positioned between the hydrophobic porous layers  55   b , and the hydrophobic porous layers  55   b  may be formed with a regular pattern. The hydrophobic porous layers  55   b  may be formed by adding a hydrophobic material to a slurry for formation of the cathode MPL  55  and then patterning the slurry with an inkjet or according to various printing methods. Also, the hydrophilic porous layers  55   a  may be formed by adding a hydrophilic material to a slurry for formation of the cathode MPL  55 , and then patterning the slurry with an inkjet or according to various printing methods. 
     The thusly-formed hydrophilic porous layers  55   a  and the hydrophobic porous layers  55   b  have a hydrophilic property or a hydrophobic property throughout their entirely, and not only at their surfaces. In the present example embodiment, the hydrophilic porous layers  55   a  form a hydrophilic portion and the hydrophobic porous layers  55   b  form a hydrophobic portion, allowing the cathode MPL  55  to discharge or contain moisture, and thus, moisture can be more easily adjusted. Water introduced into the hydrophobic porous layers  55   b  is repelled from the interior of the hydrophobic porous layers  55   b , and is thus discharged to the outside, e.g., via the air passage formed at the separator. Accordingly, air can freely move via the hydrophobic porous layers  55   a . Water  58  introduced into the hydrophilic porous layers  55   a  remains in the hydrophilic porous layers  55   a  and is supplied to the electrolyte membrane, thus improving the water retention of the electrolyte membrane. 
     In the present example embodiment, the cathode MPL  55  is illustrated to include the hydrophilic portion and the hydrophobic portion, but the embodiments are not limited thereto. For example, the cathode black layer  57 , as well as the cathode MPL  55 , may be also treated to be hydrophilic or hydrophobic, together. In this case, the hydrophobic portion of the cathode black layer  57  may be formed at a position corresponding to the hydrophobic porous layers  55   b , and the hydrophilic portion of the cathode black layer  57  may be formed at a position corresponding to the hydrophilic porous layers  55   a.    
     In the present example embodiment, the density of the hydrophobic porous layers  55   b  may be higher at an inner area adjacent to the center of the cathode  62  than that at an outer area positioned at an outer side of the inner area. Accordingly, a larger amount of water generated at the inner area can be easily discharged. To this end, the hydrophobic porous layers  55   b  may be formed with various patterns as described above. Further, a combination of greater area density of the hydrophobic porous layers  55   b  and greater hydrophobicity per unit area may be used in the central area. 
       FIG. 13  illustrates a front view of an MEA according to a seventh example embodiment. 
     With reference to  FIG. 13 , an MEA  70  according to the seventh example embodiment includes an outer area  76  having hydrophobic portions  71  and hydrophilic portions  72 , the hydrophilic portions  72  being adjacent to the hydrophobic portions  71 . A hydrophobic property of hydrophobic portions  71  in an inner area  75  adjacent to the center of the MEA  70  may be stronger than that of the hydrophobic portions  71  in the outer area  76 . In another implementation, a hydrophilic property of hydrophilic portions  72  in the inner area  75  may be less strong than that of the hydrophilic portions  72  in the outer area  76 . Thus, the inner area  75  may be made more hydrophobic than the outer area  76 , even without using a greater area density of the hydrophobic portions  71  in the inner area  75 . 
     The cathode gas diffusion layer (GDL) may contain a hydrophilic material and a hydrophobic material. Also, a black layer or a micro-porous layer forming the cathode GDL may contain the hydrophobic material or the hydrophilic material, or both layers may contain the hydrophobic material or the hydrophilic material. 
     The hydrophobic material and the hydrophilic material may include various types of materials, without being particularly limited. 
     In the present example embodiment, the hydrophobic portions  71  may be formed to be connected from one edge to another edge of the MEA  70 , substantially having a rectangular form, i.e., stripes, and rectangular hydrophilic portions  72  may be formed between the hydrophobic portions  71 . 
     The hydrophobic portions  71  and the hydrophilic portions  72  may be disposed at equal intervals. However, a hydrophobic property of the hydrophobic portions  71  may be made stronger at an inner area  75  adjacent to the center of the MEA  70  than that of the hydrophobic portions  71  at an outer area  76  positioned at an outer side of the inner area  75 , such that water is repelled from the center of the MEA  70 . 
     In detail the hydrophobic portions  71  at the inner area  75  may contain a larger amount of hydrophobic material per unit area than those at the outer area  76 , and the amount of hydrophobic material contained in the hydrophobic portions  71  may be gradually reduced as it goes from the inner side to the outer side. Accordingly, the hydrophobic power of the hydrophobic portions  71  may be gradually reduced as it goes from the inner side to the outer side. 
     According to the present example embodiment, the density of the hydrophobic portions, i.e., a rate of the hydrophobic portions per unit area, may be constant, while the hydrophobic property of the hydrophobic portions  71  at the inner area  75  is stronger than that of the hydrophobic portions  71  at the outer area  76 . 
     Similarly, the hydrophilic property of the hydrophilic portions  72  positioned at the inner area  75  may be made weaker than that of the hydrophilic portions  72  positioned at the outer area  76 . To this end, the hydrophilic portions  72  at the inner area  75  may contain a smaller amount of hydrophilic material per unit area than that of the hydrophilic portions  72  at the outer area  76 , and the hydrophilic material contained in the hydrophilic portions  72  may gradually increase as it goes from the inner side to the outer side. Accordingly, the hydrophilic property of the hydrophilic portions  72  may be gradually reduced as it goes from the inner side to the outer side. Thus, a relatively large amount of moisture generated at the inner area  75  can be easily discharged, and the outer area  76  can maintain appropriate moisture, improving the generation efficiency. 
     As described above, embodiments may provide a fuel cell stack having advantages of maintaining a certain amount of water retention while easily discharging water generated from a membrane electrode assembly (MEA). As one example described above, the density of the hydrophobic portions at the inner area may be higher than that of the hydrophobic portions at the outer area, moisture (which is relatively more heavily generated at the center of the MEA) can be easily discharged, thereby preventing occurrence of a flooding phenomenon. Further, because the density of the hydrophobic portions at the outer area may be relatively low, the hydrophobic portions at the outer area can contain an appropriate amount of moisture to maintain water retention of the electrolyte membrane. In addition, because the hydrophilic portions may be formed adjacent to the hydrophobic portions, the electrolyte membrane may provide a proper degree of water retention, thus improving generation efficiency. 
     Also, because the hydrophobic film is coated on the cathode gas diffusion layer, or because the cathode black layer or the cathode MPL constituting the cathode gas diffusion layer may be treated to have the hydrophobic property, moisture can be easily discharged, and the hydrophobic portions can be easily patterned through printing or the like. 
     Moreover, because the hydrophobic portions may be formed such that the density is gradually reduced as it goes from the center of the cathode to the outer side, the appropriate amount of moisture can be discharged to easily adjust moisture within the MEA. 
     Further, because the hydrophobic property at the inner area may be made stronger than that at the outer area, and because the hydrophilic property at the inner area may be made weaker than that at the outer area, moisture generated at the inner side of the MEA can be easily discharged, thus preventing occurrence of flooding, and because the hydrophobic portions at the outer area may have a relatively low density, the appropriate amount of moisture may be provided to maintain water retention of the electrolyte membrane. 
     Example embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as set forth in the following claims.