Abstract:
A direct methanol fuel cell has a proton conducting membrane (PCM), a catalyst in contact with the PCM, a gas diffusion layer in contact with the catalyst, and a conducting plate in contact with the gas diffusion membrane. The gas diffusion layer comprises a microporous membrane.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention is directed to a direct methanol fuel cell (DMFC).  
       BACKGROUND OF THE INVENTION  
       [0002]     The direct methanol fuel cell (DMFC) catalytically oxidizes methanol to generate electricity. The DMFC differs from PEM (proton exchange membrane) or solid polymer fuel cells, which use hydrogen gas for generating electricity. One major advantage of the DMFC over the PEM fuel cell is its ability to use methanol, a relatively inexpensive and easily handled material when compared to hydrogen gas. One major disadvantage of the DMFC, when compared to the PEM fuel cell, is methanol crossover. Methanol crossover occurs when methanol from the anode crosses to the cathode. This causes the loss of efficiency of the cell. Nevertheless, the DMFC appears to be a viable portable power source for devices such as cellular or mobile telephones, and handheld or laptop computers. “Types of Fuel Cells,” Fuel Cells 2000, www.fuelcells.org; Thomas, et al, “Fuel Cells-Green Power,” Los Alamos National Laboratory, LA-VR-99-3231.  
         [0003]     The DMFC is an electrochemical device. The anodic catalyzed reaction is: 
 
CH 3 OH+H 2 O→CO 2 +6H + +6e − 
 
 The cathodic catalyzed reaction is: 
 
 3/2O 2 +6H + +6e − →3H 2 O 
 
 The overall cell reaction is: 
 
CH 3 OH+ 3/2O 2 →CO 2 +2H 2 O 
 
 These cells operate at efficiencies of about 40% at temperatures of 50-100° C., the efficiencies will increase at higher operating temperatures. Fuel Cells 2000, Ibid; Thomas, Ibid. 
 
         [0004]     As with any chemical reaction, reactants, products, and unwanted products (by-products) become mixed as the reaction proceeds, and separation of these materials is an engineering challenge. So, at the anode, methanol, water, and carbon dioxide will be mixed together. One must be careful that excess methanol not accumulate at the anode because it will crossover the proton conducting membrane (PCM) and decrease the cell&#39;s efficiency. Water is good for the PCM, which needs water to maintain its proton conductivity, but if water accumulates, it can prevent methanol from reaching the catalyst, or it can be recycled back into fuel mixture where it can dilute the fuel. Both can decrease the efficiency of the cell. Carbon dioxide (or CO x s) must be removed to allow room for the fuel at the anode. Otherwise, cell efficiency can suffer.  
         [0005]     Likewise, at the cathode, oxygen typically from air, must reach the cathode and water must be removed. If oxygen cannot reach the cathode, efficiency drops because the cathode half cell reaction is impeded. If water, which can be used to moisten the PCM, is allowed to accumulate, it will prevent oxygen from reaching the cathode.  
         [0006]     One challenge related to the foregoing is managing the reactant/product issues without greatly increasing the size or weight of the DMFC. DMFC is targeted, in part, at a portable power source for cellular or mobile telephones and handheld or laptop computers.  
         [0007]     In WO 02/45196 A2, a DMFC is disclosed. Referring to  FIG. 3 , the DMFC  40  has proton conducting membrane (PCM)  80  with CO 2  conducting elements  52 . On the anode side  41 , there is a conducting plate  23  that has a flow field  25 , a gas diffusion layer  44 , and an anodic catalyst  42 . On the cathode side  31 , there is a conducting plate  33  with a flow field  35 , a gas diffusion layer  48 , and a cathode catalyst  46 . The catalyst, anode or cathode, is applied to either a surface of the PCM  80  or to the gas diffusion layers  44 ,  48 . The respective flow fields are in communication with their respective gas diffusion layers and the combined action of these flow fields and diffusion layers is intended to ensure the even distribution of reactants to the catalyst and the efficient removal of unwanted products, by-products, and unreacted reactants for the reaction. The gas diffusion layers are made of carbon fiber paper and/or carbon fiber cloth and may be “wet-proofed” with PTFE polymer. Note that the gas diffusion layer, catalyst, and PCM are in close contact to promote electrons or protons conductivity.  
         [0008]     On the anode side, fuel (methanol, methanol/water in either liquid or vapor form) is introduced at one end of the flow field  25 , and by products (water, CO 2 , and un-reacted fuel) are removed at other end of the flow field  25 . CO 2  produced at the anode is intended to cross the PCM  80  via CO 2  conductors  52 . Water produced at the anode is not meant to remain in the gas diffusion layer  42  as is apparent from the use of the PTFE. On the cathode side, air (the source of O 2 ) is introduced at one end of flow field  35 , and water, unreacted air, and CO 2  are removed at the other end of flow field  35 . Water produced at the cathode is not intended to remain in the gas diffusion layer  48  as is apparent from the use of the PTFE.  
         [0009]     In U.S. Patent Application Publication 2002/619253 A1, another DMFC is disclosed. This DMFC is similar to the foregoing DMFC, except the carbon paper or carbon cloth gas diffusion layers are replaced with a porous metal layer. See paragraphs [0022-0024].  
         [0010]     Accordingly, there is a need to improve reactant, product, and by-product management at both the anode and cathode of DMFC while not significantly increasing the size of weight of the DMFC.  
       SUMMARY OF THE INVENTION  
       [0011]     A direct methanol fuel cell has a proton conducting membrane (PCM), a catalyst in contact with the PCM, a gas diffusion layer in contact with the catalyst, and a conducting plate in contact with the gas diffusion membrane. The gas diffusion layer comprises a microporous membrane. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0012]     For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.  
         [0013]      FIG. 1  is a schematic illustration of a direct methanol fuel cell (DMFC) made according to the present invention.  
         [0014]      FIG. 2  is a schematic illustration of a plurality of DMFC&#39;s connected in series.  
         [0015]      FIG. 3  is a schematic illustration of a plurality of DMFC&#39;s connected in parallel. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0016]     Referring to the drawings, wherein like elements have like numerals, there is shown in  FIG. 1 a  direct methanol fuel cell system  10 .  
         [0017]     DMFC system  10  includes a DMFC  12 , a fuel source  14 , and an electrical circuit  16 . DMFC may include one or more DMFC. Fuel source  14  is a storage vessel that contains the fuel, methanol, or a mixture of methanol and water. Electrical circuit  16  includes a switch  18  and a load  20 . Load  20  may be any device that requires electricity, such as a cellular or mobile telephone, or a handheld or laptop computer, or the like. Fuel is supplied to DMFC  12  via line  22  from source  14  and is returned to source  14  via line  24  from DMFC  12 . Air is supplied to DMFC  12  via line  26  and vented from DMFC  12  via line  28 .  
         [0018]     DMFC  12  includes a membrane electrode assembly (MEA)  30  preferably sandwiched between a pair of collection plates  32 ,  34 . Collection plates are electrically conductive and are coupled to electrical circuit  16 . Collection plate  32  includes a fuel distribution channel  36 . One end of channel  36  is in fluid communication with line  22  and the other end of channel  36  is in fluid communication with line  24 . Collection plate  34  includes an oxidant distribution channel  38 . One end of channel  36  is in fluid communication with line  26  and the other end is in fluid communication with line  38 . The geometry of channels  36  and  38  is such that fuel or oxidant is even distributed to the catalysts of the DMFC  12 .  
         [0019]     MEA  30  includes a proton conducting membrane (PCM)  40  with an anode catalyst  42  on one side thereof and a cathode catalyst  44  on the other side thereof and all sandwiched between gas diffusion layers  46  and  48 . PCM  40  is conventional, for example NAFION® (perfluorosulfonic substituted polytetrafluorethylene (PTFE)) from DuPont, Wilmington, Del. or the materials set forth in WO 02/45196A2, incorporated herein by reference which include NAFION®-TEFLON®-phosohotungstic acid (NPTA), NAFION®-zirconium hydrogen phosphate (NZHP), polyetheretherketone, polybenzimidazole, polyvinylidene fluoride (PVDF). Anode catalyst  42  may be adhered to a face of PCM  40  or adhered to the fiber surfaces of a carbon fiber mat or cloth. Likewise, cathode catalyst  44  may be adhered to the other face of PCM  40  or adhered to fiber surfaces of a carbon fiber mat or cloth. The anode and cathode catalyst are conventional and the methods of adhering same are also conventional.  
         [0020]     The gas diffusion layers  46  and  48  may comprise a microporous membrane or a laminate of a microporous membrane and carbon fiber substrate. The microporous membrane may take on several different forms, the ultimate form being dependent upon the desire function of the membrane. Functions of the membrane will be dependent upon whether it is located on the fuel tank, or the anode and or the cathode. Functions for membranes at the fuel tank include: allowing the fuel to directionally flow to the anode and preventing the flow back of other anode components, reactants and reaction products. Functions for membranes at the anode include: allowing the fuel directionally flow to the catalyst; preventing accumulation of water at the catalyst; facilitating removal of gaseous reaction products form the electrode; maintaining adequate water level by preventing the back flow of the anode liquid to the fuel tank; helping to prevent accumulation of MeOH at the catalyst thereby reducing the chance for methanol crossover. Functions for membranes at the cathode include: allowing directional flow of the oxygen or air to the cathode catalyst for electrochemical reaction, and preventing the water loss of the fuel cell system.  
         [0021]     Membranes suitable to address these functions include microporous or nonporous membranes, skinned membranes, symmetric or asymmetric membranes, single or multi-layered membranes, and combinations thereof. Such membranes are known, see for example, Kesting, R.,  Synthetic Polymeric Membranes,  2nd Edition, John Wiley &amp; Sons, New York, N.Y. (1985), incorporated herein by reference. Such membranes are made of thermoplastic materials, such as polyolefins (polyethylene, polypropylene, polybutylene, polymethyl penetene and the like), polyamides (nylons), polyesters (PET, PBT and the like). The membranes may be made by the Celgard® process or by a TIPS (thermally induced phase separation) process or a wet (solvent extraction) process. Additionally, the membranes may have functional coatings, for example, hydrophobic or hydrophobic coatings. Such coatings are conventional. The membranes may also be combined with perm-selective gels or polymers that preferably pass one or more of the reactants, products, or by-products. Such perm-selective gels or polymers are conventional. Such a perm-selective material could coat one or more sides of the membrane or be sandwiched between membranes.  
         [0022]     As an example of the foregoing, one may use an asymmetric membrane (pores with decreasing diameters from one surface of the membranes to the other) that is coated with a hydrophobic material on the surface with the narrow pores. This membrane is preferably made from polymethylpentene (PMP). This membrane, which could be used at either the anode or cathode, would be placed in the MEA with the coated face toward the PCM. Thereby, water that is a reactant at the anode and a product at the cathode would be retained around the PCM are available to moisten the PCM so that its proton conductivity is maintained.  
         [0023]      FIGS. 2 and 3  illustrate further embodiments of the invention. In these embodiments, a plurality of DMFC&#39;s are joined together to form a stack  50 . In  FIG. 2 , the DMFC&#39;s  12  are joined in series. In  FIG. 3 , the DMFC&#39;s  12  are joined in parallel.  
         [0024]     The present invention may be embodied in other forms without departing from the spirit and the essential attributes thereof, and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicated the scope of the invention.