Abstract:
A fuel diffusion unit including: a fuel diffusion plate; a diffusion sheet disposed on fuel diffusion plate, to evenly distribute a fuel to the fuel diffusion plate; a primary transportation unit disposed on the diffusion sheet; secondary transportation units connected to the primary transportation unit, to distribute the fuel to the fuel from the primary transportation unit to the diffusion sheet. The diffusion sheet has a wetting direction that allows the fuel to flow in a predetermined direction. The fuel diffusion unit can be included in a fuel supply unit and a fuel cell system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Korean Application No. 2007-140669, filed Dec. 28, 2007, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Aspects of the present invention relate to a fuel supply unit, and more particularly, to a fuel supply unit that can vaporize a fuel, and a fuel cell system that employs the fuel supply unit. 
         [0004]    2. Description of the Related Art 
         [0005]    A fuel cell is an electricity generator that chemically reacts an oxidant and a fuel, to produce electricity. A fuel cell can continuously generate electricity, as long as a fuel is supplied. 
         [0006]    A direct methanol fuel cell (DMFC) is a fuel cell that generates electricity, through a reaction between methanol supplied to an anode, and oxygen supplied to a cathode. In a DMFC, the anode and the cathode face each other across an electrolyte membrane. An assembly of the anode, the cathode, and the electrolyte membrane is conventionally referred to as a membrane and electrode assembly (MEA). 
         [0007]    The cathode is exposed to an oxidant, such as oxygen in air. The anode is surrounded by a spacer, and vaporized methanol is supplied to the anode, through a plurality of holes formed in the spacer. Electrons are generated at the anode, and the electrons move through an electrical circuit, to the cathode. At this point, when a load is applied to the circuit, work can done using the generated electricity. 
         [0008]    A fuel cell is generally part of a fuel cell system. A fuel cell system can be classified as an active-type, or a passive-type, according to how a fuel is supplied to an anode. In the active-type, a low concentration liquid methanol is pumped to the anode. 
         [0009]    Passive-type fuel cell systems can include a fuel supply unit. The fuel supply unit vaporizes liquid methanol stored in a cartridge, and then the vapor is supplied to the anode, at room temperature, by diffusion. Passive-type fuel cell systems are often used in mobile devices, because passive-type fuel cell systems generally are smaller in size. 
         [0010]    It is generally beneficial for a fuel supply system to evenly supply fuel to an entire surface of an anode. In this way, a rapid initial start-up can be realized, by uniformly supplying vaporized fuel to an MEA. 
         [0011]    If the fuel is supplied through a single supply path, it takes time to achieve a uniform concentration of fuel across the anode. Thus, an initial start-up may be delayed. Therefore, there is a need to develop a configuration of fuel supply system that can uniformly supply fuel across an anode of an MEA, in a short time. 
       SUMMARY OF THE INVENTION 
       [0012]    To address the above and/or other problems, aspects of the present invention provide a fuel diffusion unit, and a fuel supply unit, which can realize a rapid initial start-up, by uniformly supplying fuel to a membrane electrode assembly (MEA), and a fuel cell system employing the same. 
         [0013]    According to an aspect of the present invention, there is provided a fuel diffusion unit comprising: a fuel diffusion plate; a diffusion sheet disposed on the fuel diffusion plate, to evenly distribute fuel to the fuel diffusion plate; a primary transportation unit disposed on the diffusion sheet; and secondary transportation units connected to the primary transportation unit, to distribute the fuel from the primary transportation unit to the diffusion sheet. The fuel flows through the diffusion sheet in a wetting direction of the diffusion sheet. 
         [0014]    According to an aspect of the present invention, there is provided a fuel supply unit for a fuel cell, comprising: a fuel storage unit; a fuel diffusion plate to evaporate fuel supplied from the fuel storage unit; a diffusion sheet disposed on the fuel diffusion plate, to uniformly distribute the fuel to the fuel diffusion plate, having a wetting direction along which the fuel is distributed; a primary transportation unit disposed on the diffusion sheet, connected to the fuel storage unit; an actuator to pump the fuel through the primary transportation unit, and secondary transportation units that are connected to the primary transportation unit, to distribute the fuel to the diffusion sheet. 
         [0015]    According to an aspect of the present invention, there is provided a fuel cell system comprising: a unit cell to generate electricity using a fuel; a fuel storage unit to store the fuel; a fuel diffusion plate to vaporize the fuel, and to supply the vaporized fuel to the unit cell; a diffusion sheet to uniformly distribute the fuel to the fuel diffusion plate, having a wetting direction along which the fuel is distributed; a primary transportation unit disposed on the diffusion sheet, and connected to the fuel storage unit; secondary transportation units connected to the primary transportation unit, to distribute the fuel to the diffusion sheet; and an actuator to pump the fuel through the primary transportation unit. 
         [0016]    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 
         [0017]    These and/or other aspects and advantages of the invention will become apparent, and more readily appreciated, from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, of which: 
           [0018]      FIG. 1  is a plan view of a fuel supply unit for a fuel cell, according to an exemplary embodiment of the present invention; 
           [0019]      FIG. 2  is a cross-sectional view taken along line A-A of  FIG. 1 ; 
           [0020]      FIG. 3  is a drawing showing wetting directions in diffusion sheets of  FIG. 2 ; 
           [0021]      FIG. 4  is a cross-sectional view of a fuel cell system that employs the fuel supply unit of  FIG. 1 , according to an exemplary embodiment of the present invention; 
           [0022]      FIG. 5  is a cross-sectional view of a configuration of a cell of  FIG. 4 ; and 
           [0023]      FIG. 6  is a graph showing a cell voltage, according to time and temperature measured in multiple positions of cells that use a fuel supply unit, according to an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Reference will now be made in detail to the exemplary 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 exemplary embodiments are described below, in order to explain the aspects of the present invention, by referring to the figures. 
         [0025]      FIG. 1  is a plan view of a fuel supply unit  100  for a fuel cell, according to an exemplary embodiment of the present invention.  FIG. 2  is a cross-sectional view taken along line A-A of  FIG. 1 , and  FIG. 3  is a drawing for explaining wet directions in diffusion sheets of  FIG. 2 . 
         [0026]    Referring to  FIGS. 1 through 3 , the fuel supply unit  100  includes: a fuel storage unit  110  that stores fuel; a fuel diffusion unit  140 ; and an actuator  120  that is connected between the fuel storage unit  110  and the fuel diffusion unit  140 . The fuel is stored in the fuel storage unit  110 , as a liquid. The fuel may be, for example, methanol MeOH, ethanol EtOH, or sodium boromohydride NaBH 4 . 
         [0027]    The fuel diffusion unit  140  includes a fuel diffusion plate  130  to vaporize the fuel, a primary transportation unit  141 , secondary transportation units  142 , and a diffusion sheet  144 . The primary transportation unit  141  can be a tube that extends in a lengthwise direction along the center of the fuel diffusion plate  130 . An end of the primary transportation unit  141  is connected to the actuator  120 , and the other end is blocked. The primary transportation unit  141  may have a diameter φ 1 , of 0.1 to 1 mm, in consideration of the portability and mobility of a mobile device in which a fuel cell is used. That is, the above range of diameters achieve a stable fuel supply to the fuel diffusion plate  130 , from the fuel storage unit  110 , without being significantly affected by the orientation of the primary transportation unit  141 . The primary transportation unit  141  may have a larger diameter than the secondary transportation units  142 . 
         [0028]    The secondary transportation units  142  can be small tubes, such as capillaries, and can be disposed on opposing sides of the primary transportation unit  141 . The secondary transportation units  142  have attached ends that are connected to the primary transportation unit  141 , and open ends through which the fuel can exit the secondary transportation units  142 . The attached ends can be offset from one another, along the length of the primary transportation unit  141 , such that the attached ends do not face one another, across the primary transportation unit  141 . 
         [0029]    The secondary transportation units  142  include first tubes  142   a  and second tubes  142   b . The first tubes  142   a  can be straight, such that the open ends of the first tubes  142   a  face away from the primary transportation tube  141 . The second tubes  142   b  can be curved, such that the open ends of the second tubes  142   b  generally face toward the primary transportation unit  141 . The first tubes  142   a  and the second tubes  142  can be alternately disposed along each side of the transportation unit  141 . In other words, different types of the secondary transportation units  142  are adjacent to one other, on each side of the primary transportation unit  141 . 
         [0030]    The different secondary transportation units  142  facilitate a rapid and uniform delivery of fuel, across the entire fuel diffusion plate  130 . The structures of the secondary transportation units  142  are not limited to the shapes depicted in  FIG. 1 . In other words, the secondary transportation units  142  can have various shapes, orientations, and/or lengths. For example, the second tubes  142   b  can be straight, and can be shorter than the first tubes  142   a.    
         [0031]    The secondary transportation units  142  may have increasing diameters, the further each secondary transportation unit  142  is from the actuator  120 . The change in diameter facilitates a uniform supply of fuel to the entire fuel diffusion plate  130 , since a fuel supply pressure decreases as a distance from the actuator  120  increases. The diameter φ 2  of the secondary transportation units  142  may be in a range from 5 to 250 μm. 
         [0032]    The secondary transportation units  142  can be connected to the primary transportation unit  141 , in various ways. For example, if the primary transportation unit  141  is formed from a flexible material, and the secondary transportation units  142  are formed from a harder material, the secondary transportation unit  142  can be stabbed into the primary transportation unit  141 . The secondary transportation units  142  can be welded or glued to the primary transportation unit  141 , for example. 
         [0033]    The number of the secondary transportation units  142  can be determined, according to the area of the fuel diffusion plate  130 . For example, if the fuel diffusion plate  130  is relatively large, the number of the secondary transportation units  142  can be increased, to increase the amount of fuel flowing there through. If the number of the secondary transportation units  142  is increased in the same area, the amount of fuel supply per unit area can also be increased. Thus, the fuel diffusion plate  130  can be more rapidly supplied with fuel, as compared to a conventional fuel diffusion plate, in which fuel is supplied through a single path, since the fuel supply unit  100  supplies the fuel simultaneously through the secondary transportation units  142 . 
         [0034]    The diffusion sheet  144  is formed to completely cover a surface of the fuel diffusion plate  130 . The diffusion sheet  144  can, in some exemplary embodiments, include a first diffusion sheet  144   a  and a second diffusion sheet  144   b . The diffusion sheet  144  may have a wetting direction. Due to the wetting direction, the diffusion sheet  144  can rapidly transport fuel from the secondary transportation units  142 , to the entire surface of the fuel diffusion plate  130 , by absorbing and transporting the fuel. For example, the diffusion sheet  144  can transport fuel by capillary action, wicking, or the like. 
         [0035]    The second diffusion sheet  144   b  is positioned under the first diffusion sheet  144   a . As depicted in  FIG. 3 , the first diffusion sheet  144   a  and the second diffusion sheet  144   b  have wetting directions that are different from each other. The different wetting directions, of the first diffusion sheet  144   a  and the second diffusion sheet  144   b , facilitate the rapid distribution of fuel to the entire surface of the fuel diffusion plate  130 . The wetting direction of the first diffusion sheet  144   a  may be generally perpendicular to that of the second diffusion sheet  144   b , or may be otherwise angled from the wetting direction of the second diffusion sheet  144   b . Accordingly, the diffusion sheet  144  can rapidly distribute fuel supplied from the secondary transportation units  142 , to the entire surface of the fuel diffusion plate  130 , by absorbing and transporting the fuel. 
         [0036]      FIG. 4  is a cross-sectional view of a fuel cell system  200  having the fuel supply unit  100 , according to an exemplary embodiment of the present invention.  FIG. 5  is a cross-sectional view of a configuration of a unit cell  150 , of  FIG. 4 . Referring to  FIGS. 4 and 5 , the fuel cell system  200  includes the fuel supply unit  100  of  FIG. 1 , and the unit cell  150 , which is connected to the fuel supply unit  100 , to generate a current. 
         [0037]    The unit cell  150  includes an anode  152 , a cathode  154 , and an electrolyte membrane  153  interposed between the anode  152  and the cathode  154 . The cathode  154  is exposed to the outside, so that the cathode  154  can be supplied with an oxygen source (air). The anode  152  is surrounded by a spacer  155 . Fuel is evaporated from the fuel diffusion plate  130 , and is supplied to the anode  152 , through a plurality of supply holes  155   a  formed in the spacer  155 . 
         [0038]    As shown below, the Chemical Equation 1 occurs at the anode  152 , to generate electrons, and the electrons move to the cathode  154 , along a circuit  156 , to participate in the Chemical Equation 2. A load  157  can applied to the circuit  156 . An assembly of the anode  152 , the cathode  154 , and the electrolyte membrane  153  is conventionally referred to as a membrane and electrodes assembly (MEA). 
         [0000]      CH 3 OH+H 2 ⇄CO 2 +6H++6 e−   [Chemical Equation 1] 
         [0000]       3/2O 2 +6H++6 e−⇄ 3H 2 O  [Chemical Equation 2] 
         [0039]    Generally, the current generated from a single unit cell  150  is not large enough to use, and thus, a plurality of the unit cells  150  are connected in a series, on the fuel diffusion plate  130 . Thus, a desired voltage/current can be obtained, by using multiple unit cells  150 . 
         [0040]      FIG. 6  is a graph showing unit cell voltages, according to time, and temperatures of the unit cells, when the unit cells are supplied with fuel using the fuel supply unit  100 . Referring to  FIG. 6 , the horizontal axis indicates time in minutes, the vertical axis on the left side indicates voltages V of the unit cells, and the vertical axis on the right side indicates cathode surface temperatures, in ° C.  FIG. 6  shows the voltage increase in each of the unit cells, when sixteen unit cells are installed on the fuel diffusion plate  130  of the fuel supply unit  100 . 
         [0041]    Referring to  FIG. 6 , the voltages of the sixteen unit cells uniformly increase, without a large deviation. This indicates that fuel is uniformly supplied to each of the unit cells, since the voltages increase in nearly the same manner. Thus, the fuel supply unit  100  uniformly distributes fuel to the entire surface of the fuel diffusion plate  130 , in a short time. 
         [0042]      FIG. 6  also shows temperatures measured by three temperature detecting devices that are installed in three different positions. It can be said that temperature nearly uniformly rises, although there is a slight temperature deviation (approximately within 2° C.). This result denotes that the amount of fuel cross-over is similar in each of the cells, and fuel was uniformly supplied to all of the sixteen cells. Thus, it is seen that the fuel supply unit  100  uniformly distributes fuel to the entire surface of the fuel diffusion plate  130 , in a short time. 
         [0043]    Although a few exemplary 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 these exemplary embodiments, without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.