Abstract:
A novel flat panel DMFC (direct methanol fuel cell) includes an integrated cathode electrode plate, a membrane electrode assembly (MEA) unit, an intermediate bonding layer, an integrated anode electrode plate, and a fuel container. The integrated cathode and anode electrode plates are manufactured by using PCB compatible processes. The integrated cathode and anode electrode plates have embedded metal layout and improved heat dissipation capability.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to the field of fuel cells, and more particularly, to a flat panel Direct Methanol Fuel Cell (DMFC) and method of making the same.  
         [0003]     2. Description of the Prior Art  
         [0004]     A fuel cell is an electrochemical cell in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. Fuel cells utilizing methanol as fuel are typically called Direct Methanol Fuel Cells (DMFCs), which generate electricity by combining gaseous or aqueous methanol with air. DMFC technology has become widely accepted as a viable fuel cell technology that offers itself to many application fields such as electronic apparatuses, vehicles, military equipment, the aerospace industry, and so on.  
         [0005]     DMFCs, like ordinary batteries, provide DC electricity from two electrochemical reactions. These reactions occur at electrodes (or poles) to which reactants are continuously fed. The negative electrode (anode) is maintained by supplying methanol, whereas the positive electrode (cathode) is maintained by the supply of air. When providing current, methanol is electrochemically oxidized at the anode electrocatalyst to produce electrons, which travel through the external circuit to the cathode electrocatalyst where they are consumed together with oxygen in a reduction reaction. The circuit is maintained within the cell by the conduction of protons in the electrolyte. One molecule of methanol (CH3OH) and one molecule of water (H2O) together store six atoms of hydrogen. When fed as a mixture into a DMFC, they react to generate one molecule of CO2, 6 protons (H+), and 6 electrons to generate a flow of electric current. The protons and electrons generated by methanol and water react with oxygen to generate water. The methanol-water mixture provides an easy means of storing and transporting hydrogen, and is much better than storing liquid or gaseous hydrogen in storage tanks. Unlike hydrogen, methanol and water are liquids at room temperature and are easily stored in thin walled plastic containers. Therefore, DMFCs are lighter than their most closely related fuel cells, hydrogen-air fuel cells.  
         [0006]     In terms of the amount of electricity generated, a DMFC can generate 300-500 milliwatts per centimeter squared. The area of the cell size and the number of cells stacked together will provide the necessary power generation for whatever the watt and kilowatt needs are for vehicular and stationary applications.  
         [0007]      FIG. 1  and  FIG. 2  illustrates a conventional DMFC  10 , wherein  FIG. 1  is a plan view of the conventional DMFC  10  and  FIG. 2  is a cross-sectional view of the conventional DMFC  10  along line I-I of  FIG. 1 . As shown in  FIG. 1  and  FIG. 2 , the conventional DMFC  10  comprises a bipolar platelet assembly  12  and a fuel container  14 . The bipolar platelet assembly  12  comprises an upper frame  51 , lower frame  52 , cathode wire lath  121 , a plurality of bended bipolar wire laths  122 ,  123 ,  124 ,  125 , an anode wire lath  126 , and membrane electrode assembly (MEA)  131 ,  132 ,  133 ,  134 ,  135  interposed between corresponding wire laths. The upper frame  51 , the lower frame  52 , the cathode wire lath  121 , the plural bended bipolar wire laths  122 ,  123 ,  124 ,  125 , the anode wire lath  126 , and the MEA  131 ,  132 ,  133 ,  134 ,  135  are adhesively stacked together to produce the stack structure as shown in  FIG. 2 . Typically, epoxy resin  53  or the like is used in between adjacent MEA, thereby forming five basic cell units  21 ,  22 ,  23 ,  24  and  25 . As known in the art, the cathode wire lath  121 , bended bipolar wire laths  122 ,  123 ,  124 ,  125 , and the anode wire lath  126  are titanium meshes treated by gold plating, and are therefore costly.  
         [0008]     The basic cell unit  21  of the prior art DMFC  10  consists of the cathode wire lath  121 , MEA  131 , and the bended bipolar wire lath  122 . The basic cell unit  22  consists of the bended bipolar wire lath  122 , which functions as a cathode of the cell unit  22 , MEA  132 , and the bended bipolar wire lath  123 , which functions as an anode of the cell unit  22 . The basic cell unit  23  consists of the bended bipolar wire lath  123 , which functions as a cathode of the cell unit  23 , MEA  133 , and the bended bipolar wire lath  124 , which functions as an anode of the cell unit  23 . The basic cell unit  24  consists of the bended bipolar wire lath  124 , which functions as a cathode of the cell unit  24 , MEA  134 , and the bended bipolar wire lath  125 , which functions as an anode of the cell unit  24 . The basic cell unit  25  consists of the bended bipolar wire lath  125 , which functions as a cathode of the cell unit  25 , MEA  135 , and the bended bipolar wire lath  126 , which functions as an anode of the cell unit  25 . Typically, each of the basic cell units  21 ,  22 ,  23 ,  24  and  25  provides a voltage of 0.6V, such that DMFC  10  comprising five serially connected basic cell units  21 ,  22 ,  23 ,  24  and  25  can provide a total voltage of 3.0V (0.6V×5=3.0V).  
         [0009]     However, the above-described conventional DMFC  10  has several drawbacks. First, the bipolar platelet assembly  12  is too thick and thus unwieldy to carry. Furthermore, as mentioned, the cost of producing the conventional DMFC  10  is high since the cathode wire lath  121 , bended bipolar wire laths  122 ,  123 ,  124 ,  125 , and the anode wire lath  126  are titanium meshes treated by gold plating. Besides, the throughput of the conventional DMFC  10  is low because the bipolar wire laths  122 ,  123 ,  124 ,  125  are bended manually before mounting on the upper and lower frames. In light of the above, there is a need to provide a thin, inexpensive, and highly integrated DMFC that is capable of achieving the scale of mass production.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore an objective of the present invention to provide a novel flat direct methanol fuel cell (DMFC) for improving the above-mentioned problems.  
         [0011]     Another objective of the present invention is to provide a method of fabricating a novel flat DMFC for increasing mass production.  
         [0012]     Another objective of the present invention is to provide a novel DMFC with improved heat radiating ability.  
         [0013]     According to the preferred embodiment of the present invention, an electrode plate of a flat panel direct methanol fuel cell (DMFC) comprises: a multilevel substrate that comprises a copper clad laminate (CCL), wherein the CCL comprises at least a radiating copper layer, and the radiating copper layer forms predetermined patterns via an etching process; a bonding sheet pressed on the radiating copper layer; and an electrode copper layer disposed on the bonding sheet, wherein the electrode copper layer is processed by an etching process for forming a predetermined electrode area and a drilling process for forming a plurality of through holes by penetrating the electrode copper layer, the bonding sheet, and the CCL.  
         [0014]     According to another embodiment of the present invention, a method for fabricating a flat panel DMFC comprises: providing a multilevel substrate having copper clad laminate (CCL) thereon, a first copper layer disposed over the upper surface of the CCL, and a second copper layer disposed over the bottom surface of the CCL; performing a drilling process on a predetermined electrode area of the multilevel substrate for forming a plurality of through holes by penetrating the first copper layer, the CCL, and the second copper layer; depositing a chemical copper layer on the multilevel substrate and within the plurality of through holes; using a photoresistance for defining the predetermined electrode area on the multilevel substrate; using the photoresistance as an electroplating resist to perform an electroplating process for forming an electroplating copper layer in the area uncovered by the photoresistance, and forming a tin/lead layer thereon; removing the photoresistance; performing an etching process for removing the chemical copper layer not yet covered by the tin/lead layer, the first copper layer and the second copper layer; and etching the tin/lead layer for exposing the electroplating copper layer.  
         [0015]     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a plain view of the conventional Direct Methanol Fuel Cell.  
         [0017]      FIG. 2  is a cross-sectional view of the conventional Direct Methanol Fuel Cell along line I-I of  FIG. 1 .  
         [0018]      FIG. 3  is a perspective, exploded diagram illustrating a flat panel Direct Methanol Fuel Cell with five serially connected basic cell units in accordance with one preferred embodiment of the present invention.  
         [0019]      FIG. 4  to  FIG. 14  illustrate a method for fabricating an integrated thin cathode electrode sheet and an integrated thin anode electrode sheet of the DMFC according to this invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     Please refer to  FIG. 3 .  FIG. 3  is a perspective, exploded diagram illustrating a flat panel DMFC  20  with five serially connected basic cell units in accordance with one preferred embodiment of the present invention. It is to be understood that the flat panel DMFC  20  with five serially connected basic cell units is merely an exemplary embodiment. Depending on the requirements of the applied apparatuses, other numbers of basic cell units such as ten or twenty may be used. As shown in  FIG. 3 , the present invention flat panel DMFC  20  generally comprises an integrated thin cathode electrode sheet  200 , Membrane Electrode Assembly (MEA) unit  300 , intermediate bonding layer  400 , integrated thin anode electrode sheet  500 , and a fuel container  600 .  
         [0021]     The integrated thin cathode electrode sheet  200  comprises a substrate  210 , cathode electrode areas  201 ,  202 ,  203 ,  204 , and  205 , and conductive via through holes  211 ,  212 ,  213 ,  214 , and  215 . Preferably, on the surface area of the substrate  210  outside the cathode electrode areas  201 ,  202 ,  203 ,  204 , and  205 , and the conductive via through hole  211 ,  212 ,  213 ,  214 , and  215 , a layer of solder resist is coated thereon. At the corners of the substrate  210 , mounting through holes  221 ,  222 ,  223 , and  224  are provided. It is noteworthy that the integrated thin cathode electrode sheet  200  is fabricated by using PCB compatible processes. The substrate  210  may be made of ANSI-grade glass fiber reinforced polymeric materials such as FR- 1 , FR- 2 , FR- 3 , FR- 4 , FR- 5 , CEM- 1  or CEM- 3 , but not limited thereto. Each of the cathode electrode areas  201 ,  202 ,  203 ,  204 , and  205 , on which a plurality of through holes are formed, is defined by a patterned copper foil. The opening ratio of each of the cathode electrode areas  201 ,  202 ,  203 ,  204 , and  205 , which is the ratio of the surface area of the through holes to the area of each of the cathode electrode areas, is preferably no less than 50%.  
         [0022]     The conductive via through hole  212  is electrically connected to the cathode electrode area  201  with the conductive wire  250 . The conductive via through hole  213  is electrically connected to the cathode electrode area  202  with the conductive wire  251 . The conductive via through hole  214  is electrically connected to the cathode electrode area  203  with the conductive wire  252 . The conductive via through hole  215  is electrically connected to the cathode electrode area  204  with the conductive wire  253 . The cathode electrode area  205  is electrically connected to a positive (cathode) electrode node  261 , which, in operation, is further electrically connected with an external circuit. The conductive via through hole  211 , which acts as a negative (anode) electrode node of the DMFC  20 , is electrically connected with the external circuit in operation.  
         [0023]     The MEA unit  300  comprises a first proton exchange membrane  301 , a second proton exchange membrane  302 , a third proton exchange membrane  303 , a fourth proton exchange membrane  304 , and a fifth proton exchange membrane  305 , corresponding to the cathode electrode areas  201 ,  202 ,  203 ,  204 , and  205 . Each of the proton exchange membranes  301 ,  302 ,  303 ,  304 , and  305  may use commercially available proton conducting polymer electrolyte membranes, for example, NAFION™, but not limited thereto.  
         [0024]     The intermediate bonding layer  400  comprises at least one bonding sheet, which may be made of Prepreg B-stage resin, which is an ordinary material in PCB processes. The Prepreg B-stage resin may be completely cured at about 140° C. for a process time period of about 30 minutes. Corresponding to the proton exchange membranes  301 ,  302 ,  303 ,  304 , and  305 , five openings  401 ,  402 ,  403 ,  404 , and  405  are provided on the intermediate bonding layer  400  for accommodating respective proton exchange membranes. At a side of the opening  401  corresponding to the conductive via through hole  211  of the substrate  210 , as specifically indicated in  FIG. 3 , a conductive via through hole  411  is provided. At a side of respective openings  402 ,  403 ,  404 , and  405  corresponding to the conductive via through holes  212 ,  213 ,  214 , and  215 , conductive via through holes  412 ,  413 ,  414 , and  415  are provided. In another case, the intermediate bonding layer  400  may further include a thin supporting layer that is made of glass fiber reinforced polymeric materials such as FR- 1 , FR- 2 , FR- 3 , FR- 4 , FR- 5 , CEM- 1  or CEM- 3 . At the corners, corresponding to the mounting through holes  221 ,  222 ,  223 , and  224  of the substrate  210 , there are mounting through holes  421 ,  422 ,  423 , and  424  provided.  
         [0025]     The integrated thin anode electrode sheet  500  comprises a substrate  510 , anode electrode areas  501 ,  502 ,  503 ,  504 , and  505 , and conductive pads  511 ,  512 ,  513 ,  514 , and  515 . It is noteworthy that the anode electrode areas  501 ,  502 ,  503 ,  504 ,  505  are defined simultaneously with the conductive pads  511 ,  512 ,  513 ,  514 ,  515 . At the corners of the substrate  510 , corresponding to the mounting through holes  221 ,  222 ,  223 , and  224  of the substrate  210 , there are mounting through holes  521 ,  522 ,  523 , and  524  provided. The integrated thin anode electrode sheet  500  is fabricated by using PCB compatible processes. Likewise, the substrate  510  may be made of ANSI-grade glass fiber reinforced polymeric materials such as FR- 1 , FR- 2 , FR- 3 , FR- 4 , FR- 5 , CEM- 1 , CEM- 3  or the like. Each of the anode electrode areas  501 ,  502 ,  503 ,  504 , and  505 , on which a plurality of through holes are formed, is defined by a patterned copper foil. The opening ratio of each of the anode electrode areas is preferably no less than 50%.  
         [0026]     The fuel container  600  has fuel channel  601  and mounting through holes  621 ,  622 ,  623 , and  624  corresponding to the mounting through holes  221 ,  222 ,  223 , and  224  of the substrate  210 . The fuel container  600  may be made of polymeric materials such as epoxy resin, polyimide, or acrylic. The fuel channel  601  may be fabricated by using conventional mechanical grinding methods or plastic extrusion methods.  
         [0027]     When assembling, the proton exchange membranes  301 ,  302 ,  303 ,  304 , and  305  are installed within the respective openings  401 ,  402 ,  403 ,  404 , and  405  of the intermediate bonding layer  400 . The intermediate bonding layer  400 , together with the installed proton exchange membranes  301 ,  302 ,  303 ,  304 , and  305 , is then sandwiched by the integrated thin cathode electrode sheet  200  and the integrated thin anode electrode sheet  500 . The resultant laminate stack having (in order) the integrated thin cathode electrode sheet  200 , Membrane Electrode Assembly (MEA) unit  300 , the intermediate bonding layer  400  (and installed proton exchange membranes), and the integrated thin anode electrode sheet  500  is then mounted on the fuel container  600 .  
         [0028]     The conductive via through holes  211 ,  212 ,  213 ,  214  and  215  of the integrated thin cathode electrode sheet  200  are aligned, and in contact, with the respective conductive via through holes  411 ,  412 ,  413 ,  414  and  415  of the intermediate bonding layer  400 , which are aligned with the conductive pads  511 ,  512 ,  513 ,  514  and  515  of the integrated thin anode electrode sheet  500 . Conventional soldering process may be used to electrically connect and fix the aligned conductive through holes such as conductive via through holes  211 ,  411 , and conductive pad  511 , and so on. By doing this, the cathode electrode area  201  of the integrated thin cathode electrode sheet  200  is electrically connected to the anode electrode area  502  of the integrated thin anode electrode sheet  500  through the conductive path constituted by the conductive wire  250 , the soldered conductive via through holes  212  and  412 , and the conductive pad  512  of the integrated thin anode electrode sheet  500 . The cathode electrode area  202  of the integrated thin cathode electrode sheet  200  is electrically connected to the anode electrode area  503  of the integrated thin anode electrode sheet  500  through the conductive path constituted by the conductive wire  251 , the soldered conductive via through holes  213  and  413 , and the conductive pad  513  of the integrated thin anode electrode sheet  500 , and so on. The conductive via through hole  211  of the integrated thin cathode electrode sheet  200 , which acts as the negative electrode of the DMFC  20 , is electrically connected to the anode electrode area  501  and conductive pad  511  of the integrated thin anode electrode sheet  500  through the conductive via through hole  411  of the intermediate bonding layer  400 .  
         [0029]     It is advantageous to use the present invention because the DMFC  20  has integrated thin cathode electrode sheet  200  and integrated thin anode electrode sheet  500 , which reduce the thickness as well as the production cost of the DMFC  20 . No bended bipolar wire lath is needed. The integrated thin cathode electrode sheet  200  and integrated thin anode electrode sheet  500  are fabricated by using PCB compatible processes, thus can achieve the scale of mass production. Another benefit is that the control circuit layout for controlling the DMFC and external circuit can be integrated on the substrate  210  or  510 .  
         [0030]     Nevertheless, heat is commonly produced when the fuel cell is generating electricity. According to the preferred embodiment of the present invention, when the overall temperature of the fuel cell is above 70° C., the efficiency of the MEA unit  300  will decrease and the amount of electricity generated by the fuel cell will be affected. A method for fabricating the integrated thin cathode electrode sheet  200  and the integrated thin anode electrode sheet  500  of the DMFC  20  is now described in detail with reference to  FIG. 4  to  FIG. 12 . According to this invention, the method for fabricating the integrated thin cathode electrode sheet  200  and the integrated thin anode electrode sheet  500  of the DMFC  20  is compatible with standard PCB processes.  
         [0031]     First, as shown in  FIG. 4 , a CCL (Copper Clad Laminate) substrate  30   a  is provided. The CCL substrate  30   a  is commercially available and has a thickness of few millimeters. The CCL substrate  30   a  comprises a base layer  32   a,  a copper layer  34   a  laminated to an upper surface of the base layer  32   a,  and a copper layer  36   a  laminated to a lower surface of the base layer  32   a.    
         [0032]     As shown in  FIG. 5 , a photolithography and an etching process are performed on the copper layers  34   a  and  36   a.  First, photoresistance  37  and  38  with patterns are formed on the copper layers  34   a  and  36   a,  in which the photoresistance  37  and  38  also include openings  37   a  and  38   a  for exposing the copper layers  34   a  and  36   a.  Next, a wet etching process is performed by using the photoresistance  37  and  38  as an etch-stopping mask to etch the exposed copper layers  34   a  and  36   a  through the openings  37   a  and  38   a  for forming the required patterns within the copper layers  34   a  and  36   a.  The photoresistance  37  and  38  is removed thereafter.  
         [0033]     According to the preferred embodiment of the present invention, the patterns formed within the copper layers  34   a  and  36   a  can be dummy area patterns used for radiating heat, and the copper layers  34   a  and  36   a  can be any layer within a multi-layer substrate except for the surface layer. In order to radiate heat, the copper layers should be situated in proximity to the surface of the substrate and the thickness of the copper layer  34   a  or  36   a  should be at least 17 μm. The thickness of the copper layers  34   a  or  36   a  according to the preferred embodiment is approximately 35 μm. Moreover, the patterns formed within the copper layer  34   a  and  36   a  can also be utilized as an embedded active circuit for integrating with the energy management system (EMS) that controls the DMFC  20 . Preferably, the layout of the circuit can be adjusted according to the functional demands of the fuel cell.  
         [0034]     As shown in  FIG. 6 , a multi-layer substrate  30  is formed by laminating the CCL  30   a  in between bonding sheets  39  and metal plates  34  and  36 , in which the metal plates  34  and  36  can be comprised of copper clad, and the bonding sheets  39  can be made of B-stage prepreg resin. Alternatively, the multi-layer substrate  30  can be utilized to combine with a plurality of bonding sheets  39  and metal plates  34  or  36  to form a multi-layer circuit board.  
         [0035]     Before the lamination process is performed, a surface blackening process is performed on the copper layers  34   a  and  36   a  of the CCL  30   a  to increase the roughness of the surface. Preferably, the surface blackening process is performed by utilizing a strong oxidizing agent, such as solutions containing sulfuric acid.  
         [0036]     As shown in  FIG. 7 , a drilling process is performed on the predetermined area of the substrate  30  to penetrate the copper layer  34 , the bonding sheet  39 , the substrate  30   a,  and the copper layer  36  to form a plurality of through holes  42 .  
         [0037]     Next, a chemical copper layer  46  is formed on the substrate  30  and within the through holes  42 , as shown in  FIG. 8 . By forming the chemical copper layer  46  via a chemical deposition method, the chemical copper layer  46  is deposited evenly on the substrate  30  and within the through holes  42 .  
         [0038]     As shown in  FIG. 9 , a predetermined electrode area  49  is defined on the substrate  30  by using a photoresistance  48 . Relating to the integrated thin cathode electrode sheet  200  of  FIG. 3 , the predetermined electrode area  49  corresponds to the cathode electrode areas  201 - 205 , and the photoresistance  48  also defines the conductive wires  250 - 254  and the positive electrode node  261  (not shown). Preferably, the fabrication of the integrated thin cathode electrode sheet  200  and the conductive via through holes  211 - 215  are completed at the same time as the through holes  42  within the predetermined electrode area  49 . Relating to the integrated thin anode electrode sheet  500  of  FIG. 3 , the predetermined electrode area  49  corresponds to the anode electrode areas  501 - 505 , and the photoresistance  48  further defines the conductive pads  511 - 515  and the connecting area (not shown) between the conductive pads and the anode electrode areas.  
         [0039]     Next, an electroplating process is performed by using the photoresistance  48  as an electroplating barrier to form a copper layer  62  on the area uncovered by the photoresistance  48 , including the predetermined electrode area  49 , and a tin/lead layer  64  on the copper layer  62 , as shown in  FIG. 10 .  
         [0040]     As shown in  FIG. 11 , the photoresistance  48  is removed thereafter.  
         [0041]     A copper etching process is then performed to remove the chemical copper layer  46  not yet covered by the tin/lead layer  64  and the copper layers  34  and  36  on the substrate  30 , as shown in  FIG. 12 . Next, another etching process is performed to remove the tin/lead layer  64  to expose the copper layer  62  and complete the initial fabrication of the integrated thin anode electrode sheet  500 .  
         [0042]     In order to prevent any damage to the substrate during the soldering process, a solder resist  72  is applied, as shown in  FIG. 13 . Used widely in circuit board industries, the solder resist is comprised of photosensitive materials, in which the solder resist can also be used to define the required protective area on the integrated thin cathode electrode sheet  200  by a standard photolithography process.  
         [0043]     Next, an electroplating process is performed to form a protective layer  74  over the surface of the electrodes to prevent the oxidation of the integrated thin cathode electrode sheet  200  after long exposure in the air, as shown in  FIG. 14 . Preferably, the protective layer  74  is comprised of nickel/gold, tin/lead, graphite carbon, or chemical silver.  
         [0044]     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.