Abstract:
A planar type fuel cell is provided. The planar type fuel cell has a membrane electrode assembly including an electrolyte membrane and an anode, and a cathode, and a plate attached to the cathode of the membrane electrode assembly to supply water to the cathode by condensing water vapor generated from the cathode.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This application claims the benefit of Korean Application No. 2006-63125, filed Jul. 5, 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 fuel cell, and more particularly, to a fuel cell having a plate which supplies water to a cathode by condensing water vapor generated by the cathode.  
         [0004]     2. Description of the Related Art  
         [0005]     Fuel cells include direct methanol fuel cells (DMFC) and polymer electrolyte fuel cells (PEMFC), among others. The DMFC is a possible replacement for the traditional battery as the supply of fuel is easily accessible and the output density is higher than that of a battery; however, the DMFC has a lower output density than the PEMFC. DMFCs are generally bipolar fuel cells, but the stacks of the replacement batteries for PDAs (personal digital assistants), mobile phones, and laptops are generally a monopolar type.  
         [0006]     A variety of monopolar type DMFCs has been introduced. Of the monopolar type DMFCs that have been introduced (hereinafter, referred to as the conventional DMFC), a planar type has a cathode in which the entire outer surface is exposed to the atmosphere. Thus, a large amount of water vapor generated from the cathode is lost. Also for the conventional DMFC, it is difficult to increase the output power density.  
       SUMMARY OF THE INVENTION  
       [0007]     To solve the above and/or other problems, aspects of the present invention provide a planar type fuel cell which can minimize the loss of water and increase the output power density of the fuel cell by condensing water evaporated from the cathode and reusing the condensed water.  
         [0008]     According to an aspect of the present invention, there is provided a planar type fuel cell comprising a membrane electrode assembly including an electrolyte membrane, an anode, and a cathode; and a plate attached to the cathode of the membrane electrode assembly, wherein the plate condenses water vapor generated by the cathode and supplies the condensed water to the cathode, and the plate resists the absorption of water.  
         [0009]     According to an aspect of the invention, a space where the water vapor generated from the cathode may be collected and condensed is provided on the plate.  
         [0010]     According to an aspect of the invention, the plate may comprise a plurality of protrusions the tips of which contact the membrane electrode assembly and the plate is separated from the membrane electrode assembly around the protrusions.  
         [0011]     According to an aspect of the invention, the protrusions may be arranged in a grid pattern.  
         [0012]     According to an aspect of the invention, the protrusions may be circular cones, polygonal cones, or pillars.  
         [0013]     According to an aspect of the invention, wrinkles or grooves may be longitudinally formed on surfaces of the protrusions in a direction from the bottom of each of the protrusions toward the top thereof.  
         [0014]     According to an aspect of the invention, the plurality of structures may be formed on the plate in a grid pattern without contacting the membrane electrode assembly.  
         [0015]     According to an aspect of the invention, the protrusions may be located around each of the structures.  
         [0016]     According to an aspect of the invention, a plurality of trenches may be formed on the plate by the protrusions and the structures.  
         [0017]     According to an aspect of the invention, the wrinkles may exist on an outer surface of the plate.  
         [0018]     According to an aspect of the invention, the wrinkles may exist on the overall or part of the outer surface of the plate.  
         [0019]     According to an aspect of the invention, a groove having the same shape as that of each protrusion may be formed at a position of an outer surface of the plate to correspond to each protrusion.  
         [0020]     According to an aspect of the invention, the structures may have surfaces facing the membrane electrode assembly which is circular or polygonal.  
         [0021]     According to an aspect of the invention, the structures may be circular cones or polygonal cones.  
         [0022]     According to another aspect of the invention, a water recirculation plate for a fuel cell having a membrane electrode assembly with a cathode is provided, including: an outer surface, and an inner surface having protrusions extending therefrom, wherein the plate resists the absorption of water, captures water vapor produced by the cathode, condenses the water vapor on the inner surface of the plate, and supplies the condensed water vapor to the membrane electrode assembly.  
         [0023]     According to an aspect of the invention, the protrusions extend from the inner surface of the plate to contact a membrane electrode assembly.  
         [0024]     According to an aspect of the invention, the protrusions have at least a groove longitudinally formed on a surface of each protrusion of the plurality of protrusions.  
         [0025]     According to an aspect of the invention, the plate may further include structures on the inner surface of the plate between the protrusions, wherein the structures extend from the inner surface of the plate but extend less than the protrusions.  
         [0026]     According to an aspect of the invention, the protrusions and the structures are arranged in a grid, each individual protrusion is surrounded by a number of the structures, and each individual structure is surrounded by a number of the protrusions.  
         [0027]     According to an aspect of the invention, each protrusion is surrounded by four structures, and each structure is surrounded by four protrusions.  
         [0028]     According to an aspect of the invention, the outer surface of the plate has cooling grooves.  
         [0029]     According to an aspect of the invention, the cooling grooves of the outer plate correspond to the protrusions of the inner plate.  
         [0030]     According to an aspect of the invention, the protrusions form trenches in which air flows between the plate and the membrane electrode assembly.  
         [0031]     According to an aspect of the invention, the protrusions and the structures for trenches in which air flows between the plate and the membrane electrode assembly.  
         [0032]     According to an aspect of the invention, the plate may further include heat removal pipes between the outer surface and the inner surface.  
         [0033]     According to aspects of the present invention, by using the fuel cell according to the present invention, the amount of water lost from the cathode can be minimized and water can be supplied from the plate to the cathode. Accordingly, the outpour power density can be increased and the hydration status of the membrane can be continuously maintained in a state proper for the transfer of the hydrogen ions (H+). Also, since the structure of the plate is simple, the manufacturing of the fuel cell is made easy.  
         [0034]     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  
       [0035]     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:  
         [0036]      FIG. 1  is a perspective view of a fuel cell according to an embodiment of the present invention;  
         [0037]      FIG. 2  is a perspective view showing a face opposite to the cathode of the plate of  FIG. 1 ;  
         [0038]      FIG. 3  is an enlarged view of part of the plate shown in  FIG. 2 ;  
         [0039]      FIG. 4  is a cross-sectional view taken along line  4 - 4 ′ of  FIG. 3 ;  
         [0040]      FIG. 5  is a cross-sectional view taken along line  5 - 5 ′ of  FIG. 3 ;  
         [0041]      FIG. 6  is a cross-sectional view showing wrinkles formed in the outer surface of the plate of  FIG. 1 ;  
         [0042]      FIG. 7  is a cross-sectional view showing wrinkles formed in the outer surface of the plate of  FIG. 1 ;  
         [0043]      FIG. 8  is a cross-sectional view showing the evaporation/condensation of water vapor generated from the cathode turning into water on the plate and supplied to the cathode in the fuel cell shown in  FIG. 1 ;  
         [0044]      FIG. 9  is a graph showing the measured power density versus operation time; and  
         [0045]      FIG. 10  is a graph showing the measured voltage and power versus current. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0046]     Reference will now be made in detail to aspects 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 aspects of the present invention by referring to the figures.  
         [0047]     Referring to  FIG. 1 , a fuel cell according to an embodiment of the present invention includes a membrane electrode assembly (MEA) A 1 , having an electrolyte membrane, an anode, and a cathode, and a plate  40  attached on the upper surface of the MEA A 1 . The plate  40  condenses water vapor generated from the cathode  18  of the MEA A 1  and supplies water to the cathode  18 . The plate  40  does not readily absorb water. The MEA A 1  may have a variety of structures. For example, as indicated by an enlarged portion A of  FIG. 1  showing the structure of a partial area a 1  of the MEA A 1 , the MEA A 1  includes an anode  10 , a first current collector  12 , an electrolyte membrane (electrolyte film)  14 , a second current collector  16 , and a cathode  18 , which are sequentially deposited. The MEA A 1  may also include, as indicated by an enlarged portion B of  FIG. 1 , a first diffusive layer  20 , a first current collector  22 , an anode  24 , a membrane  26 , a cathode  28 , a second current collector  30 , and a second diffusive layer  32 , which are sequentially deposited. The plate  40  is attached to the top layer of the MEA A 1 . As represented in enlarged portions A and B, the plate  40  may be attached to the cathode  18  or the second diffusive layer  32 .  
         [0048]     The plate  40  has an outer surface S 1  and an inner surface S 2 , which faces the cathode  18  or the second diffusive layer  32  of the MEA A 1 . The inner surface S 2  of the plate  40  includes a plurality of protrusions  40   a  formed in a grid pattern as shown in  FIG. 2 . A sharp tip of each of the protrusions  40   a  contacts the MEA A 1 . Due to the protrusions  40   a , trenches  40   c  are formed through which air can flow to the cathode  18  between the MEA A 1  and the other portion of the plate  40  around the protrusions  40   a . Although the protrusions  40   a  are illustrated as circular cones, other polygonal cones such as rectangular, triangular, or pentagonal cones can be used. Also, the protrusions  40   a  may be pillars, for example, polygonal pillars such as circular, triangular, or rectangular pillars. There may be a wrinkle or a groove (not shown) formed longitudinally on the surface of the protrusions  40   a . That is, the wrinkle or groove is formed in a direction from the bottom of each of the protrusions  40   a  toward the top thereof, or the wrinkle or groove is formed on the surface each of the protrusions  40   a  from the inner surface S 2  to the tips of the protrusions  40   a.    
         [0049]     A plurality of structures  40   b  is further provided with the protrusions  40   a  on the inner surface S 2  that faces the MEA A 1  of the plate  40 . The structures  40   b  are formed in a grid pattern with the protrusions  40   a . Although each of the structures  40   b  is illustrated as a rectangle, the structures  40   b  may have other shapes such as circular, triangular, or the same shape as the protrusions  40   a . Each protrusion  40   a  is surrounded by four of the structures  40   b , and each of the structures  40   b  is surrounded by a plurality of the protrusions  40   a , for example, four protrusions.  
         [0050]     The protrusions  40   a  provide a path by which water droplets formed on the plate  40  move toward the MEA A 1 . The condensed water moves toward the cathode  18  or the second diffusive layer  32  of the MEA A 1  along the surfaces of the protrusions  40   a . A plurality of trenches  40   c  is formed on the plate  40  and provides a space for collecting water vapor evaporated by the cathode  18  of the MEA A 1 . When the plate  40  is not provided with the structures  40   b , the water vapor can be collected in all the space between the protrusions  40   a . The water vapor collects in the trenches  40   c  and condenses on the plate  40 . As a result, water droplets are formed on the surface of the protrusions  40   a  and supplied to the MEA A 1  along the surfaces of the protrusions  40   a . The water droplets are also formed on the side surfaces of the structures  40   b . The water droplets formed on the side surfaces of the structures  40   b  move to the protrusions  40   a  along the side surfaces of the structures  40   b  and then toward the cathode  18  or the second diffusive layer  32  of the MEA A 1  along the surfaces of the protrusions  40   a . Each of the trenches  40   c  is an area made by two neighboring protrusions  40   a  and two neighboring structures  40   b , as shown in  FIG. 2 .  
         [0051]      FIG. 3  is an enlarged view of part of the plate shown in  FIG. 2 .  FIG. 4  is a cross-sectional view taken along line  4 - 4 ′ of  FIG. 3 .  FIG. 5  is a cross-sectional view taken along line  5 - 5 ′ of  FIG. 3 . Referring to  FIGS. 3 through 5 , the shapes of the protrusions  40   a , the structures  40   b , and the trenches  40   c  can be seen in detail.  
         [0052]     Referring to  FIG. 4 , the protrusions  40   a  are formed opposite the outer surface S 1  on the plate  40  extending from the plate  40  toward the cathode  18  or the second diffusive layer  32  of the MEA A 1 .  FIG. 4  illustrates aspects of this invention wherein the plate  40  only includes the protrusions  40   a  and the trenches  40   c  and excludes the structures  40   b . As such, the trenches  40   c  provide area in which water vapor may collect and then condense on the bottoms  40   cb  of the trenches  40   c  and the surfaces of the protrusions  40   a . The condensed water then returns to the cathode  18  or the second diffusive layer  32  of the MEA A 1  along the surface of the protrusions  40   a.    
         [0053]     Referring to  FIG. 5 , the protrusions  40   a  and the structures  40   b  are formed opposite the outer surface S 1  and extend from the plate  40  toward the cathode  18  or the second diffusive layer  32  of the MEA A 1 . However, as illustrated, the protrusions  40   a  extend to and contact the cathode  18  or the second diffusive layer  32  while the structures  40   b  only extend into the trenches  40   c . Thus, the structures  40   b  do not contact the cathode  18  or the second diffusive layer  32  of the MEA A 1 . Accordingly, the water vapor can move through the trenches  40   c  about the protrusions  40   a , the structures  40   b , and the cathode  18  or the second diffusive layer  32  of the MEA A 1 . The structures  40   b  may have a similar shape as the protrusions  40   b ; for example, when the protrusions  40   b  are circular cones as shown in the drawing, the structures  40   b  may be circular cones. Also, the structures  40   b  can be removed, as illustrated in  FIG. 3 . The presence of the structures  40   b  affects the time for the water droplets to form and to move toward the cathode  18  or the second diffusive layer  32  of the MEA A 1 .  
         [0054]     In order to increase the rate of condensation of the water vapor collected in the trenches  40   c  and decrease the time necessary to form water droplets on the surfaces of the protrusions  40   a , the temperatures of the plate  40 , the protrusions  40   a , and the structures  40   b  need to be lowered so as to dissipate the heat of the water vapor to the outside the plate  40 . Thus, to lower the temperatures of the plate  40 , the protrusions  40   a , and the structures  40   b , it is advantageous that the surface area of the outer surface S 1  of the plate  40  contacting the atmosphere is increased. Accordingly, the outer surfaces S 1  of the plates  40  of  FIGS. 4 and 5  can be processed to be uneven as shown in  FIG. 6 . The increased surface area of the outer surface S 1  of the plate  40  increases the area available for heat transfer from the cathode  18  or the second diffusive layer  32  through the trenches  40   c  and the plate  40  to the atmosphere.  
         [0055]     Also, as shown in  FIG. 7 , cooling grooves  50  can be formed at the position of the outer surface S 1  of the plate  40  in which the protrusions  40   a  are formed. The shape of the cooling grooves  50  may be similar to that of the protrusions  40   a . For example, when the protrusions  40   a  are circular cones, the cooling grooves  50  can also have the shape of circular cones. Again, the increased surface area of the outer surface S 1  of the plate  40  increases the area available for heat transfer, thereby decreasing the time for cooling of the plate  40 . The outer surface S 1  may be formed to contain other cooling structures such as cooling fins or may have external heatsinks attached thereto.  
         [0056]     As described above, as the area of the outer surface S 1  contacting the atmosphere is increased by changing the shape of the outer surface S 1  of the plate  40 , the time to condense the water vapor collected in the trenches  40   c  to form water droplets is decreased. Thus, the cycle of the phase changes between liquid water and water vapor occurring at the cathode  18  or the second diffusive layer  32  and then at the plate  40  is shortened.  
         [0057]     The circulation process of water occurring between the cathode  18  or the second diffusive layer  32  and the plate  40  in the fuel cell according to aspects of the present embodiment is shown in  FIG. 8 . Referring to  FIG. 8 , water vapor  52  generated by the cathode  18  contacts the surfaces of the protrusions  40   a  and the bottom  40   cb  of the trenches  40   c . When the structures  40   b  are present in the trenches  40   c , the water vapor  52  contacts and condenses on the structures  40   b . The structures  40   b  increase the surface area on which the water vapor can condense and thereby increase the circulation of the water back to the cathode  18  or the second diffusive layer  32 . The water vapor  52  is condensed and forms water droplets  54  on the surfaces of the protrusions  40   a , the trenches  40   c , and the structures  40   b , if present. The water droplets  54  formed on the surfaces of the trenches  40   c  flow toward the cathode  18  or the second diffusive layer  32  along the surfaces of the protrusions  40   a . The water supplied to the cathode  18  or the second diffusive layer  32  from the plate  40  is then supplied to the membranes  14  and  26  ( FIG. 1 ) so that the membranes  14  and  26  remain properly hydrated. Thus, hydrogen ions (H+) generated at the anodes  10  and  24  ( FIG. 1 ) pass through the membranes  14  and  26  and arrive at the cathodes  18  and  28  ( FIG. 1 ).  
         [0058]     The time necessary for the water vapor  52  to condense to the water droplets  54  decreases as the difference in temperature between the cathode  18  or the second diffusive layer  32  and the plate  40  increases. Thus, the distance between the cathode  18  or the second diffusive layer  32  and the bottoms  40   cb  of the trenches  40   c  is increased. That is, the depths of the trenches  40   c  are increased. However, when the wrinkles or grooves are in the outer surface S 1  of the plate  40  thereby increasing the surface area of the outer surface S 1 , the distance between the bottoms  40   cb  of the trenches  40   c  or the depth of the trenches  40   c  can be decreased.  
         [0059]     A monopolar fuel cell having the MEA A 1  structure as indicated by the enlarged portion B shown in  FIG. 1  (hereinafter, referred to as a test battery) and the plate  40  as shown in  FIG. 2  was tested to generate  FIGS. 9 and 10 . Pure methanol vapor was used as the fuel supplied to the anode  24  of the test battery. Also, air was supplied to the surface of the cathode  28 .  
         [0060]      FIG. 9  is a graph showing the power density versus operation time. In a graph G 1  of  FIG. 9 , a first time section T 1  indicates the output power density before water is supplied from the plate  40 . And, a second time section T 2  indicates the output power density after water starts to be supplied from the plate  40 .  
         [0061]     Referring to the graph G 1  of  FIG. 9 , it can be seen that the output power density when the operation of the fuel cell is in the second time section T 2  (hereinafter, a second power density) is higher than the output power density when the operation of the fuel cell is in the first time section T 1  (hereinafter, a first power density). The second power density peaks at about 15 mW/cm 2  and levels out at above 14 mW/cm 2 . The first power density is about 13 mW/cm 2 . The second power density is higher than the first power density by about 10%-15%. Thus, the condensation of water vapor on the plate  40  and the resultant flow of water from the plate  40  to the second diffusion layer  32  increase the power density output of the fuel cell.  
         [0062]      FIG. 10  is a graph showing the voltage and power versus current measured during the above experiments. In  FIG. 10 , a first graph G 11  indicates the voltage-current characteristics measured when the water is supplied to the cathode  32  from the plate  40 . A second graph G 22  indicates the voltage-current characteristics measured when the water is not supplied to the cathode  32  from the plate  40 . A third graph G 33  indicates the power-current characteristics measured when the water is supplied to the cathode  32  from the plate  40 . A fourth graph G 44  indicates the power-current characteristic measured when the water is not supplied to the cathode  32  from the plate  40 .  
         [0063]     When the first and second graphs G 11  and G 22  of  FIG. 10  are compared, it can be seen that as current increases, the potential of graph G 11  is greater than the potential of graph G 22 , at the same current. Thus, the monopolar fuel cell produced an increased potential at the same current when water was supplied from the plate  40  to the second diffusive layer  32 . When the third and fourth graphs G 33  and G 44  of  FIG. 10  are compared, it can be seen that the power of graph G 33  is greater than the power of graph G 44 , at the same current. Therefore, the monopolar fuel cell generates more power at the same current when water is condensed on and supplied from the plate  40  to the second diffusive layer  32 .  
         [0064]     While this invention has been particularly shown and described with reference to aspects of the embodiments thereof, it will be understood by those skilled in the art that various changes in form and details, in particular, the plate  40 , may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Also, the structure of the MEA A 1  can be configured differently from the above-described structures and other constituent elements can be added to the structure. Also, heat removal pipes may be provided such that an evaporation portion of the heat pipe is located between the outer surface S 1  of the plate  40  and the bottoms  40   cb  of the trenches  40   c  so as to accept heat from the plate.  
         [0065]     As described above, the fuel cell according to aspects of the present invention includes the plate that is attached to the cathode and condenses the water vapor by collecting the water vapor generated from the cathode and supplies water to the cathode. Thus, by using the fuel cell according to aspects of the present invention, the amount of water lost from the cathode can be minimized and water can be supplied from the plate to the cathode. Accordingly, the output power density can be increased and the membrane may be sufficiently hydrated so as to properly transfer hydrogen ions (H+) to the cathode. Also, since the structure of the plate is simple, the manufacturing of the fuel cell is made easy.  
         [0066]     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.