Abstract:
A novel flat panel DMFC (direct methanol fuel cell) includes an integrated cathode electrode sheet; a membrane electrode assembly (MEA) unit; an intermediate bonding layer; an integrated anode electrode sheet; and a fuel container. The integrated cathode/anode electrode sheets are manufactured by using PCB compatible processes.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to the field of fuel cells. More particularly, the present invention relates 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 named as 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 equipments, 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 (CH 3 OH) and one molecule of water (H 2 O) together store six atoms of hydrogen. When fed as a mixture into a DMFC, they react to generate one molecule of CO 2 , 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, 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 nearest rival hydrogen-air fuel cells.  
         [0006]     In terms of the amount of electricity generated, a DMFC can currently 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 . 6 V, 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 INVENTION  
       [0010]     It is therefore the primary object of the present invention to provide an improved thin flat panel type DMFC to solve the above-mentioned problems.  
         [0011]     It is another object of the present invention to provide a method for fabricating a thin and highly integrated DMFC, thereby achieving the scale of mass production and thus saving cost, wherein the method for fabricating the highly integrated DMFC is compatible with standard PCB (printed circuit board) processes.  
         [0012]     According to the claimed invention, a flat-panel direct methanol fuel cell (DMFC) is provided. The present invention DMFC comprises an integrated cathode electrode sheet, a membrane electrode assembly (MEA) unit, an intermediate bonding layer, an integrated anode electrode sheet, and a fuel container. The integrated cathode electrode sheet comprises a first substrate, a plurality of cathode electrode areas, a plurality of first conductive via through holes, wherein the cathode electrode areas is electroplated on a front side and backside of the first substrate and has a plurality of apertures therein, wherein the first conductive via through holes are disposed outside the cathode electrode areas and is electrically connected to respective cathode electrode areas with a conductive wire. The membrane electrode assembly (MEA) unit comprises a plurality of proton exchange membranes corresponding to the plurality of cathode electrode areas.  
         [0013]     The intermediate bonding layer comprises at least one bonding sheet, wherein the intermediate bonding layer comprises a plurality of openings for respectively accommodating the plurality of proton exchange membranes, and a plurality of second conductive via through holes that are aligned with the first conductive via through holes. The integrated anode electrode sheet comprises a second substrate, a plurality of anode electrode areas corresponding to the plurality of cathode electrode areas, and a plurality of third conductive via through holes corresponding to the second conductive via through holes.  
         [0014]     According to one aspect of the present invention, a method for fabricating an integrated cathode electrode sheet of a flat-panel direct methanol fuel cell is provided.  
         [0015]     The method comprises the steps of: 
        (1) providing a CCL (copper clad laminate) substrate comprising a base layer, a first copper layer laminated on an upper surface of the base layer, and a second copper layer laminated on a lower surface of the base layer;     (2) drilling the CCL substrate within pre-selected electrode areas to form a plurality of apertures through the first copper layer, the base layer and the second copper layer;     (3) chemically depositing a third copper layer on the CCL substrate and interior sidewalls of inside the apertures;     (4) forming a patterned resist layer on the CCL substrate to expose the pre-selected electrode areas;     (5) using the patterned resist layer as a plating mask, per-forming an electroplating process to electroplate a fourth copper layer within the expose the pre-selected electrode areas and area not covered by the patterned resist layer, and then electroplating a Sn/Pb layer on the fourth copper layer;     (6) stripping the patterned resist layer;     performing a copper etching process to etch away the third copper layer and the first and second copper layer that are not covered by the Sn/Pb layer; and     (7) removing the Sn/Pb layer to expose the fourth copper layer.        
 
         [0024]     Other objects, advantages and novel features of the invention will become more clearly and readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0025]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0026]      FIG. 1  is a plain view of the conventional Direct Methanol Fuel Cell;  
         [0027]      FIG. 2  is a cross-sectional view of the conventional Direct Methanol Fuel Cell along line I-I of  FIG. 1 ;  
         [0028]      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; and  
         [0029]      FIG. 4  to  FIG. 12  illustrate a method for fabricating integrated thin cathode electrode sheet and integrated thin anode electrode sheet of the DMFC according to this invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]     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 .  
         [0031]     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%.  
         [0032]     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.  
         [0033]     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.  
         [0034]     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 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 fitly 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 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.  
         [0035]     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%.  
         [0036]     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.  
         [0037]     When assembling, the proton exchange membranes  301 ,  302 ,  303 ,  304 , and  305  are fitly installed within the 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 consisting in the order of the integrated thin cathode electrode sheet  200 , 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 .  
         [0038]     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 connected 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  of the integrated thin anode electrode sheet  500  through the conductive via through hole  411  of the intermediate bonding layer  400 .  
         [0039]     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 .  
         [0040]     A method for fabricating integrated thin cathode electrode sheet  200  and 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 integrated thin cathode electrode sheet  200  and integrated thin anode electrode sheet  500  of the DMFC  20  is compatible with standard PCB processes.  
         [0041]     First, as shown in  FIG. 4 , a CCL (Copper Clad Laminate) substrate  30  is provided. The CCL substrate  30  is commercially available and has a thickness of few millimeters.  
         [0042]     The CCL substrate  30  comprises a base layer  32 , a copper layer  34  laminated on an upper surface of the base layer  32 , and a copper layer  36  laminated on a lower surface of the base layer  32 .  
         [0043]     As shown in  FIG. 5 , a conventional drilling process is carried out to drill a plurality of through holes  42  in the CCL substrate  30  within pre-selected electrode areas (not explicitly shown). In accordance with the preferred embodiment, the surface area of the through holes  42  within a pre-selected electrode area is preferably more than 50% of the surface area of the pre-selected electrode area.  
         [0044]     Subsequently, as shown in  FIG. 6 , a thin copper layer  46  is chemically deposited on the CCL substrate  30  and on the exposed interior sidewalls of the through holes  42 . It is noted that the copper layer  46  is deposited in a nonselective manner.  
         [0045]     As shown in  FIG. 7 , a patterned resist (dry film)  48  is formed on the CCL substrate  30  to define the electrode area  49 . Taking the integrated cathode electrode sheet  200  of  FIG. 3  as an example, the electrode area  49  defined by the patterned resist  48  is one of the cathode electrode areas  201 ˜ 205 . Not shown in  FIG. 7 , the patterned resist  48  also defines the conductive wires  250 ˜ 254  and the positive electrode node  261 . It is noted that the conductive via through holes  211 ˜ 215  of the integrated cathode electrode sheet  200  are formed simultaneously with the through holes  42  in the same drilling process. Taking the integrated anode electrode sheet  500  of  FIG. 3  as an example, the electrode area  49  defined by the patterned resist  48  is one of the anode electrode areas  501 ˜ 505 , and the patterned resist  48  also defines the conductive pads  511 ˜ 515  (not shown in  FIG. 7 ).  
         [0046]     As shown in  FIG. 8 , using the patterned resist  48  as a plating mask, an electroplating process is carried out to form a copper layer  62  on the CCL substrate  30  where is not covered by the patterned resist  48  including the electrode area  49 . A tin/lead (Sn/Pb) composite layer  64  is then electroplated on the copper layer  62 .  
         [0047]     As shown in  FIG. 9 , the patterned resist  48  is stripped to expose the rest of the copper layer  46 .  
         [0048]     As shown in  FIG. 10 , a copper etching process such as conventional wet etching is then carried out to etch away the copper layer  46  and the copper layers  34  and  36  that are not covered by the Sn/Pb layer  64 . After this, another etching process is carried out to etch away the Sn/Pb layer  64 , thereby exposing the remaining copper layer  62 . At this stage, the fabrication of the integrated anode electrode sheet  500  of  FIG. 3  is complete.  
         [0049]     To complete the fabrication of the integrated cathode electrode sheet  200  of  FIG. 3 , there are still few steps to go. As shown in  FIG. 11 , to prevent short-circuiting caused during the subsequent soldering process and potential damages to the substrate, a solder resist layer  72  is coated. The solder resist layer  72  may be made of materials that are commercially available and are commonly used in conventional PCB processes. Preferably, the solder resist layer  72  is made of photosensitive materials that can be patterned by using conventional lithographic process to define the protected area on the electrode sheet  200 .  
         [0050]     As shown in  FIG. 12 , optionally, to further protect the integrated cathode electrode sheet  200  from oxidation due to long-term contact with air, a conductive protection layer  74  is coated on the electrode. The conductive protection layer  74  may be made of nickel/gold (Ni/Au), tin/lead (Sn/Pb), or chemical silver.  
         [0051]     To sum up, the present invention flat panel type DMFC encompasses at least the following advantages.  
         [0052]     (1)The cost per cell is reduced since the starting material, CCL substrate, is cheaper. Besides, the process of fabricating the germane parts, the integrated thin cathode electrode sheet  200  and integrated thin anode electrode sheet  500  of the DMFC  20 , is compatible with mature PCB processes.  
         [0053]     (2)The process of fabricating the germane parts, the integrated thin cathode electrode sheet  200  and integrated thin anode electrode sheet  500  of the DMFC  20 , is compatible with mature PCB process. The production cost is therefore reduced.  
         [0054]     (3)No bended bipolar wire lath is needed. The manufacture of the integrated thin cathode electrode sheet  200  and integrated thin anode electrode sheet  500  can therefore achieve the scale of mass production. Direct stack assembly is more precise.  
         [0055]     (4)The control circuit layout for controlling the lithium battery of portable apparatus, the DMFC and the external circuit can be simultaneously fabricated on the laminate substrate, thus reducing the size of the DMFC and increasing the integrity of the DMFC.  
         [0056]     Those skilled in the art will readily observe that numerous modification and alterations of the present invention 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.