Abstract:
A bipolar plate includes at least two channels respectively arranged in first and second opposite surfaces of the bipolar plate, the at least two channels adapted to allow respective fluids to flow therethrough; the first surface of the opposite surfaces of the bipolar plate has hydrophobicity and the second surface of the opposite surfaces of the bipolar plate has hydrophilicity; a unit cell includes such a bipolar plate; and a fuel cell system also includes such a bipolar plate. With this configuration, carbon dioxide and water produced by a chemical reaction of a unit cell is easily discharged to the outside, to enhance the power generation efficiency of the unit cell and to enhance the power generation efficiency of the fuel cell system.

Description:
CLAIM OF PRIORITY  
       [0001]     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for FUEL CELL SYSTEM AND UNIT CELL AND BIPOLAR PLATE USING THEREFOR, earlier filed in the Korean Intellectual Property Office on the 31 of Aug. 2005 and there duly assigned Serial No. 10-2005-0080992.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a fuel cell system to generate electrical energy by an electrochemical reaction between hydrogen and oxygen, and more particularly, to a bipolar plate having one surface facing an anode electrode being treated to have hydrophobicity and another surface facing a cathode electrode being treated to have hydrophilicity, and to a unit cell and a fuel cell system including the bipolar plate.  
         [0004]     2. Discussion of Related Art  
         [0005]     In general, a fuel cell system is a power generator that transforms chemical reaction between hydrogen and oxygen into electric energy. The fuel cell system has been researched and developed as an alternative power source to satisfy an increased demand of power and to solve environmental problems. Here, hydrogen is extracted from hydrogen containing fuel that includes an alcoholic fuel such as methanol, ethanol, etc.; a hydro-carbonaceous fuel such as methane, propane, butane, etc.; or a natural gas fuel such as liquefied natural gas, etc.  
         [0006]     The fuel cell system is classified into a phosphoric acid fuel cell (PAFC), a molten carbon fuel cell (MCFC), a solid oxide fuel cell (SOFC), a polymer electrolyte membrane fuel cell (PEMFC), an alkaline fuel cell (AFC), a direct methanol fuel cell (DMFC), etc. according to kinds of fuel. Furthermore, the fuel cell system can be applied to various fields such as a mobile device, transportation, a distributed power source, etc. according to kinds of fuel, a driving temperature, an output range, etc.  
         [0007]     Among various fuel cell systems, the PEMFC and the DMFC have been widely researched for the mobile device. Each of such fuel cell systems basically includes a stack in which unit cells are stacked to generate electricity. The stack has a structure that a plurality of unit cells stacked between end plates are fastened by bolts and nuts. The unit cell includes a membrane electrode assembly (MEA) having an electrolyte membrane between an anode electrode and a cathode electrode; and separators (e.g., bipolar plates) placed in opposite sides of the electrolyte membrane and formed with channels for fluid.  
         [0008]     The bipolar plates supply the anode electrode and the cathode electrode with hydrogen containing fuel and an oxidant, respectively. Furthermore, the bipolar plates discharge carbon dioxide and water from the anode electrode and the cathode electrode to the outside, respectively.  
         [0009]     If water and carbon dioxide are not smoothly discharged, the power generation efficiency of the fuel cell system is decreased. However, the discharging function of the bipolar plates has not been actively researched.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, it is an object of the present invention to provide bipolar plates having surfaces treated to respectively effectively discharge carbon dioxide and water from an anode electrode and a cathode electrode, a unit cell including the bipolar plates, and a fuel cell system having the bipolar plates.  
         [0011]     The foregoing and/or other objects of the present invention are achieved by providing a bipolar plate including: at least two channels respectively arranged in first and second opposite surfaces of the bipolar plate, the at least two channels adapted to allow respective fluids to flow therethrough; the first surface of the opposite surfaces of the bipolar plate has hydrophobicity and the second surface of the opposite surfaces of the bipolar plate has hydrophilicity.  
         [0012]     The first surface of the opposite surfaces of the bipolar plate has a coating of a hydrophobic material, and the second surface of the opposite surfaces of the bipolar plate has a coating of a hydrophilic material. The hydrophobic material includes either a phenol or epoxy compound, and the hydrophilic material includes an acrylic compound.  
         [0013]     The foregoing and/or other objects of the present invention are also achieved by providing a unit cell including: a membrane electrode assembly including a polymer membrane, and anode and cathode electrodes arranged in opposite sides of the polymer membrane; and a bipolar plate including a first surface facing the anode electrode and having hydrophobicity, and a second surface facing the cathode electrode and having hydrophilicity.  
         [0014]     The first surface of the bipolar plate preferably has a coating of a hydrophobic material, and the second surface preferably has a coating of a hydrophilic material. The hydrophobic material preferably includes either a phenol or epoxy compound, and the hydrophilic material preferably includes an acrylic compound.  
         [0015]     The first surface of the bipolar plate preferably includes a fuel supplying channel adapted to supply a hydrogen containing fuel to the anode electrode, and the second surface of the bipolar plate preferably includes an oxygen supplying channel adapted to supply oxygen to the cathode electrode.  
         [0016]     The foregoing and/or other objects of the present invention are further achieved by providing a fuel cell system including: a stack having an electric generator adapted to generate electricity by a chemical reaction between hydrogen and oxygen; a fuel feeder adapted to supply the stack with a hydrogen containing fuel; and an oxygen feeder adapted to supply the stack with oxygen; the electric generator includes a bipolar plate having first and second opposite surfaces having respective fluid flow channels, the first surface of the opposite surfaces having hydrophobicity and the second surface of the opposite surfaces having hydrophilicity.  
         [0017]     The fluid flow channels respectively preferably include a fuel supplying channel adapted to supply a hydrogen containing fuel, and an oxygen supplying channel adapted to supply oxygen. The first surface including the fuel supplying channel preferably has hydrophobicity, and the second surface having the oxygen supplying channel preferably has hydrophilicity. The first surface preferably has a coating of a hydrophobic material, and the second surface preferably has a coating of a hydrophilic material. The hydrophobic material preferably includes either a phenol or epoxy compound, and the hydrophilic material preferably includes an acrylic compound.  
         [0018]     The electric generator preferably includes a membrane electrode assembly including a polymer membrane and anode and cathode electrodes arranged in opposite sides of the polymer membrane.  
         [0019]     The anode electrode preferably faces the first surface of the bipolar plate, and the cathode electrode preferably faces the second surface of the bipolar plate.  
         [0020]     The foregoing and/or other objects of the present invention are still further achieved by providing a bipolar plate including: at least two channels respectively arranged in first and second opposite surfaces of the bipolar plate, the at least two channels adapted to allow respective fluids to flow therethrough; the first and second opposite surfaces respectively have different contact angles between a liquid surface and a solid surface.  
         [0021]     One contact angle is preferably in a range of 60° to 135° on one of the first and second opposite surfaces and other contact angle is preferably less than 60° on the other of the first and second opposite surfaces.  
         [0022]     One of the first and second opposite surfaces preferably includes a fuel supplying channel adapted to supply a hydrogen containing fuel and the other of the first and second opposite surfaces preferably includes an oxygen supplying channel adapted to supply oxygen.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     A more complete appreciation of the present invention, and many of the attendant advantages thereof, will be readily apparent as the present invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:  
         [0024]      FIG. 1  is a schematic view of a fuel cell system with a stack according to an embodiment of the present invention;  
         [0025]      FIG. 2  is a schematic view of the stack according to an embodiment of the present invention;  
         [0026]      FIG. 3  is a sectional view partially showing a bipolar plate according to an embodiment of the present invention;  
         [0027]      FIG. 4  is a sectional view partially showing a unit cell including the bipolar plates and a membrane-electrode assembly; and  
         [0028]      FIG. 5  illustrates the concepts of hydrophilicity and hydrophobicity. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     Hereinafter, exemplary embodiments of the present invention are described with reference to the accompanying drawings.  
         [0030]     Referring to  FIG. 1 , a fuel cell system according to an embodiment of the present invention includes a stack  10  in which one and more unit cells are stacked, a fuel feeder  20  to supply the stack  10  with a hydrogen containing fuel, and an oxygen feeder  30  to supply the stack  10  with oxygen.  
         [0031]     The hydrogen containing fuel includes raw fuel or hydrogen, for example, an alcohol fuel, such as methanol, ethanol, etc.; a hydro-carbonaceous fuel, such as methane, propane, butane, etc.; or a natural gas fuel, such as liquefied natural gas, etc. Preferably, hydrogen can be obtained by reforming the raw fuel. Therefore, the fuel feeder  20  can include only a fuel storage (not shown) to store the raw fuel including an alcohol fuel, such as methanol, ethanol, etc.; a hydro-carbonaceous fuel, such as methane, propane, butane, etc.; or a natural gas fuel, such as liquefied natural gas, etc., or can include a reformer (not shown) to reform the raw fuel into hydrogen in addition to the fuel storage.  
         [0032]     Referring to  FIG. 2 , the stack  10  includes a Membrane Electrode Assembly (MEA)  12  having a polymer membrane  12   a , and anode and cathode electrodes  12   b  and  12   c  provided on opposite sides of the polymer membrane  12   a . In the MEA  12 , the electrodes are formed by applying a catalyst material onto a porous support such as a carbon paper, and include the anode electrode  12   b  and the cathode electrode  12   c . In the anode electrode  12   b , hydrogen gas contained in the hydrogen containing fuel is oxidized to generate hydrogen ions (H + ) and electrons (e − ), and carbon dioxide is produced as a byproduct of the oxidization and discharged to the outside. In the cathode electrode  12   c , the hydrogen ions transferred from the anode electrode  12   b  through the polymer membrane  12   a  is chemically reacted with oxygen supplied from the oxygen feeder  30 , and water produced by this chemical reaction is discharged to the outside.  
         [0033]     Furthermore, the stack  10  includes bipolar plates  14 , interposed between adjacent membrane electrode assemblies  12 , to respectively supply the anode and cathode electrodes  12   b  and  12   c  with hydrogen and oxygen. The stack  10  includes a plurality of unit cells  11   a ˜ 11   n  each including its respective membrane electrode assembly  12  and bipolar plates  14  arranged on opposite sides of the membrane electrode assembly  12 .  
         [0034]     In the bipolar plates  14  interposed between the adjacent membrane electrode assemblies  12 , as shown in  FIGS. 3 and 4 , a plate main body  14   a  has a first surface formed with a fuel supplying channel A to supply the hydrogen containing fuel, and a second surface formed with an oxygen supplying channel B to supply oxygen.  
         [0035]     According to an embodiment of the present invention, the bipolar plate  14  has one surface, i.e., a hydrophobic surface  24  treated to have hydrophobicity. For example, the hydrophobic surface  24  is formed by being coated with a material having hydrophobicity, such as a phenol or epoxy compound. On the other hand, the bipolar plate  14  has the other surface, i.e., a hydrophilic surface  22  treated to have hydrophilicity. For example, the hydrophilic surface  22  is formed by being coated with a material having hydrophilicity, such as an acrylic compound. However, the surface treatments are not limited to the foregoing descriptions.  
         [0036]     The hydrophobic surface  24  is also prepared interior of the fuel supplying channel A formed on the first surface of the plate main body  14   a  and the hydrophilic surface  22  is also prepared interior of the oxygen supplying channel B formed on the second surface of the plate main body  14   a.    
         [0037]     The hydrophobicity is a characteristic tending not to combine with liquid, but the hydrophilicity is a characteristic having an affinity for liquid. In general, when liquid is thermodynamically balanced on a solid surface, a predetermined angle is formed between the liquid surface and the solid surface as shown in  FIG. 5 , which will be called a contact angle (κ). In this embodiment, hydrophobicity means that the contact angle (θ) is maintained in a range from about 60° to about 135°. On the other hand, hydrophilicity means that the contact angle (θ) is maintained to be less than about 60°.  
         [0038]     In result, the hydrogen containing fuel, particularly methanol, can be smoothly transferred from one surface of the bipolar plate  14  to the anode electrode  12   b  in the fuel supplying channel A having the hydrophobic surface  24 , and water can be smoothly transferred from the cathode electrode  12   c  to the other surface of the bipolar plate  14  in the oxygen supplying channel B having the hydrophilic surface  22 .  
         [0039]     Referring to  FIG. 4 , the external surfaces of the anode and cathode electrodes  12   b  and  12   c  face the bipolar plates  14  to respectively supply the hydrogen containing fuel and oxygen. The plate main body  14   a  has the first surface facing the anode electrode  12   b  and formed with the fuel supplying channel A through which the hydrogen containing fuel flows, and the second surface facing the cathode electrode  12   c  and formed with the oxygen supplying channel B into which oxygen in air is introduced.  
         [0040]     The anode electrode  12   b  facing the first surface of the plate main body  14   a  includes a catalyst layer to facilitate changing the hydrogen containing fuel supplied through the fuel supplying channel A formed in the first surface into the hydrogen ions and the electrons, and a Gas Diffusion Layer (GDL) to uniformly diffuse the hydrogen containing fuel to the catalyst layer and to discharge carbon dioxide to the outside. Likewise, the cathode electrode  12   c  facing the second surface of the plate main body  14   a  includes a catalyst layer to facilitate a chemical reaction between the hydrogen ions and oxygen supplied through the oxygen supplying channel B formed in the second surface, and a gas diffusion layer to uniformly diffuse oxygen to the catalyst layer and to discharge water produced due to the chemical reaction to the outside.  
         [0041]     Since the first surface of the plate main body  14   a  has the hydrophobic surface  24 , the hydrogen containing fuel in the fuel supplying channel A can be smoothly supplied to the gas diffusion layer of the anode electrode  12   b , so that carbon dioxide produced as a byproduct in the anode electrode  12   b  can be easily discharged through the fuel supplying channel A. On the other hand, the second surface of the plate main body  13   a  has the hydrophilic surface  22 , so that water produced as a byproduct in the cathode electrode  12   c  can be easily transferred from the cathode electrode  12   c  to the second surface of the plate main body  14   a  and discharged to the outside.  
         [0042]     The polymer membrane  12   a  is a conductive polymer electrolyte membrane that has not only an ion exchange function to transfer the hydrogen ions generated in the catalyst layer of the anode electrode  12   b  to the catalyst layer of the cathode electrode  12   c  but also a function to prevent the hydrogen containing fuel from passing therethrough. Preferably, the polymer membrane  12   a  has a thickness of about 50 μm through 200 μm. Furthermore, the polymer membrane  12   a  includes a perfluorosulfonate resin film made of a perfluorosulfonate resin (Nafion), a film having a porous polytetrafluoroethylene thin film support coated with perfluorinated sulfonic acid or the like resin solution, a film having a porous nonconductive polymer support coated with positive ion exchange resin and inorganic silicate, etc.  
         [0043]     In the stack  10 , a part in which the plurality of unit cells  11   a  through  11   n  is stacked will be called the electric generator  11  for convenience. The stack  10  includes end plates  18  provided on opposite sides of the electric generator  11 . The end plate  18  placed in one side of the stack  10  includes a fuel inlet in which the hydrogen containing fuel is introduced, and an output terminal to supply Direct Current (DC) electricity generated by the unit cells  11   a  through  11   n  of the stack  10  to the outside. On the other hand, the end plate  18  placed on the other side of the stack  10  includes an air inlet in which air is introduced, and a discharging part to discharge carbon dioxide (CO 2 ) and water (H 2 O) to the outside.  
         [0044]     The outmost bipolar plate  14  of the electric generator  11  faces and is electrically connected to the end plate  18 . Furthermore, the supplying channel is provided on the surface opposite to the surface of the bipolar plate  14  facing the end plate  18 . The supplying channel is used as the fuel supplying channel or the oxygen supplying channel according to the facing electrodes. For example, the supplying channel adjacent to the anode electrode  12   b  is used as the fuel supplying channel for supplying the hydrogen containing fuel, and the supplying channel adjacent to the cathode electrode  12   c  is used as the oxygen supplying channel for supplying oxygen.  
         [0045]     In the stack  10  with the foregoing configuration, a fastening means (not shown) is provided for preventing the hydrogen containing fuel and air supplied to the electric generator  11  from leaking and for applying a predetermined pressure to assemble the plurality of unit cells forming the electric generator  11  into the fuel cell system  10 .  
         [0046]     For example, the fastening means includes a plurality of penetrating bars (not shown) penetrating a plurality of through holes (not shown) respectively formed in a circumferential edge of the end plate  18 ; and a plurality of nuts (not shown) coupled to threaded parts provided in opposite ends of the penetrating bar. Therefore, the nuts are coupled to the ends of the penetrating bars and that the penetrating bars penetrate the through holes, so that the electric generator  11  is maintained as being air-tightly pressed by the end plates  18 .  
         [0047]     The operation of the fuel cell system according to the embodiment of the present invention described above is as follows.  
         [0048]     The hydrogen containing fuel is smoothly supplied from the fuel feeder  20  to the electric generator  11 , in particular, to the anode electrode  12   b  of the membrane electrode assembly  12  via the fuel supplying channel A of the bipolar plate  14  in the stack  10 . Furthermore, oxygen is smoothly supplied from the oxygen feeder  30  to the electric generator  11 , in particular, to the cathode electrode  12   c  of the membrane electrode assembly  12  via the oxygen supplying channel B of the bipolar plate  14  in the stack  10 .  
         [0049]     Due to hydrogen oxidation in the anode electrode  12   b , the hydrogen ions and carbon dioxide are produced. The hydrogen ions are transferred to the cathode electrode  12   c  through the electrolyte membrane  12   a , and carbon dioxide is discharged to the outside through the fuel supplying channel A of the bipolar plate  14 . Furthermore, water produced by oxygen reduction in the cathode electrode  12   c  is smoothly transferred from the cathode electrode  12   c  to the bipolar plate  14  and discharged to the outside through the oxygen supplying channel B. Also, the electrons generated in the anode electrode  12   b  are transferred to the cathode electrode  12   c , thereby generating electricity. The electricity generated in the respective unit cells  11   a  through  11   n  is supplied to an external load through the output terminal provided in the end plates  18  via the bipolar plates  14  that are electrically connected to each other.  
         [0050]     According to an embodiment of the present invention, the bipolar plate has one surface treated to have hydrophobicity and the other surface treated to have hydrophilicity, so that carbon dioxide and water produced by the chemical reaction in the unit cells are smoothly discharged to the outside, thereby enhancing the power generation efficiency of the unit cells. Furthermore, the power generation efficiency of the fuel cell system is enhanced.  
         [0051]     Although exemplary embodiments of the present invention have been shown and described, it can be appreciated by those skilled in the art that modifications can be made to these embodiment without departing from the principles and spirit of the present invention, the scope of which is defined by the following claims.