Abstract:
The application relates to a method of fabricating micro fuel cells and membrane electrode assemblies by thin film deposition techniques using a dimensionally stable proton exchange membrane as a substrate. The application also relates to membrane electrode assemblies and fuel cells fabricated in accordance with the method. The method includes the steps of successively depositing catalyst, current collector and flow management layers on the membrane substrate in predetermined patterns. Since the fuel cell is formed layer by layer, the need for assembly and sealing of discrete components is avoided. The method improves the contact resistance between the current collectors and catalyst layers and reduce ohmic losses, thereby avoiding the need for end plates or other compressive elements. This in turn reduces the overall thickness of the manufactured fuel cell. Since the fuel cell layers are optionally flexible, the devices may be fabricated using a continuous roller process or other automated means. The method minimizes production costs and costs of non-essential materials and is particularly suitable for low power battery replacement applications.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit of U.S. provisional patent application No. 60/410,001 filed Sep. 12, 2002. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This application relates to a method of fabricating micro fuel cells and membrane electrode assemblies by thin film deposition techniques using a dimensionally stable proton exchange membrane as a substrate. The application also relates to membrane electrode assemblies and fuel cells fabricated in accordance with the method.  
         BACKGROUND  
         [0003]    Fuel cells are electrochemical devices that convert the chemical energy of a fuel (e.g. hydrogen or hydrocarbons) directly to electrical energy. They offer an environmentally friendly means to generate power with high efficiencies. They are modular in design and flexible with respect to size and fuel requirements. In general, a fuel cell functions by combining hydrogen and oxygen to form water, and the use of an electrode-electrolyte assembly ensures that this reaction is carried out electrochemically, without combustion, to generate electricity. A fuel cell generates a potential difference (i.e. electrical power) from two electrochemical half reactions, namely the oxidation of hydrogen at the anode and the reduction of oxygen at the cathode, to produce water. For a hydrogen fuel cell, the electrochemical half reactions are as follows:  
           Anode: H 2 →2H + +2 e   −   
           Cathode: 0.5 O 2 +2H + +2 e   − →H 2 O  
           [0004]    The net reaction is as follows:  
           Net: H 2 +0.5 O 2 →H 2 O  
           [0005]    Some fuel cells operate by directly oxidizing methanol at the anode to produce hydrogen ions and carbon dioxide. The mechanism for this reaction is not well understood; however, the net anode electrochemical reaction is as follows:  
           Anode: CH 3 OH+H 2 O→CO 2 +6H + +6 e   −   
           [0006]    Proton exchange membrane (PEM) fuel cells are characterized by an ion or proton conducting membrane separating the two half reactions. This membrane is permeable to positive ions, preferably protons only, and is impervious to liquids and gasses. The membrane catalyst and gas diffusion layers are collectively known as a membrane electrode assembly (MEA).  
           [0007]    [0007]FIG. 1 illustrates a conventional PEM fuel cell  10  of the prior art comprising a MEA. Such fuel cells  10  are usually built around a polymer membrane  12  comprising a solid polymer electrolyte, such as Nafion® manufactured by Dupont. The fuel, usually hydrogen, flows through a top plate  14  which is commonly made from graphite or some other chemically inert material having the required electrical and heat conductivity characteristics. PEM fuel cells  10  have catalysts  16  at both the anode and cathode to enhance the reaction rate, usually platinum on activated carbon. Different platinum alloys have been investigated for reducing light hydrocarbons directly, increasing the reaction rate and alleviating sensitivity to contaminant gasses. A gas diffusion layer  18  consisting of a carbon cloth is typically provided to better distribute the fuel and oxidant across the catalyst  16  and to conduct electrons. Seals  19  are typically provided at the end portions of the fuel cell assembly.  
           [0008]    Significant public and private sector research has been conducted recently on micro fuel cell development. Micro fuel cells are generally defined as fuel cells producing less than 100 W of power, intended for portable applications. Typical portable electronics applications include laptop computers, cellular phones, hand-held communicators, pagers, video recorders, and portable power tools. Portable power devices are also becoming increasingly common in military and medical applications. For example, devices such as radios, navigation aids, night vision goggles and air conditioned protective suits require reliable portable power supplies. Embedded electronic devices such as pacemakers and diagnostic sensors may also potentially be powered by micro fuel cells. Microelectromechanical system (MEMS) devices are another area of active research which demand the development of smaller, lighter and longer lasting power sources.  
           [0009]    If traditional fuel cells can be reduced in size and cost, then they could potentially compete with lithium ion batteries for use in such portable power applications. In terms of power density, a micro fuel cell can provide between 6 and 7 times the energy per unit mass as lithium ion batteries. For example, in cellular phone applications, the talk time of a cellular phone using a lithium ion battery is typically between 4 to 5 hours whereas a micro fuel cell would enable approximately 17-27 hours of talk time. The use of methanol as a fuel supply would also enable instant recharging whereas recharging conventional lithium ion batteries typically requires several hours. Further, direct methanol fuel cells are particularly suitable for portable power applications because of the high volumetric energy density of methanol.  
           [0010]    Fuel cells are traditionally manufactured in a step-by-step fashion and then assembled from discrete components. This assembly is difficult since many of the component parts are not rigid and require complex sealing regimes which are prone to failure. The assembly process increases the complexity and reduces the reliability of fuel cell products. Particular problems arise in the fabrication of micro fuel cells. Most micro fuel cell fabrication processes employ traditional serial machining techniques, which are expensive to miniaturize, or MEMS techniques which are inherently batch processes and require expensive vacuum based steps. These processes dramatically increase the cost of the fuel cell system and make competition with established solutions like lithium ion batteries unlikely.  
           [0011]    U.S. Patent Publication No. 2002/0076589 A1, Bostaph et al., dated Jun. 20, 2002 exemplifies prior art micro fuel cell designs which require substantial assembly. While fluid and electron flow are controlled by micromachined structures in the Bostaph et al. design, the MEA is a discrete component having a conventional configuration.  
           [0012]    Published PCT application No. WO 02/41433, Ren et al., dated May 23, 2002, similarly describes a micro fuel cell design requiring extensive assembly. The Ren et al. fuel cell employs methanol fuel and is designed for low power battery replacement applications. In this invention the MEA is formed by applying anode and cathode ink directly on a polymer proton conducting membrane.  
           [0013]    U.S. Pat. No. 5,759,712, Hockaday, dated Jun. 2, 1998, describes a miniature fuel cell system using porous plastic membranes as substrates for fuel cells. The fuel cells may be deployed in a flexible membrane package that may be wrapped around a protective container or the like. Hockoday has also described systems using vapor deposition techniques for depositing catalyst film layers on a central membrane. The Hockaday fuel cell system of the &#39;712 patent does, however, employ seals requiring some mechanical compression.  
           [0014]    Some other methods are known in the prior art for fabricating micro fuel cells or components thereof using deposition rather than assembly steps. Published PCT application No. WO 00/45457, Janowski et al., dated Aug. 3, 2000 describes a MEMS-based compact fuel cell fabricated by thin film deposition technologies. In one embodiment a MEA laminate structure is attached, bonded or mechanically sealed to a micromachined manifold host structure.  
           [0015]    U.S. Patent Publication No. US 2002/0045082 A1, Marsh, dated Apr. 18, 2002, relates to a miniature fuel based power source. According to the Marsh fuel cell topology, a wide channel is etched into a substrate and the MEA is formed in a central column within the channel by successive deposition of a proton conducting material.  
           [0016]    European patent application EP 1 078 408 B1, Dong, describes a fuel cell flow field structure formed by layered deposition. Dong describes the use of silk-screening techniques to build-up channels for flow fields on a substrate, such as an ion-exchange membrane. Deposition may be effected by screen-printing machines in a production line arrangement. Dong, however, focuses on the manufacture of electrochemical fuel cell strata or plates in which are formed flow field channels and does not describe the formation of an integrated fuel cell having current collectors directly deposited on a membrane substrate.  
           [0017]    While significant advances have been made in micro fuel cell fabrication techniques, most prior art systems exhibit one or more of the following drawbacks:  
           [0018]    Assembly: As component parts become smaller, they usually become more difficult to manipulate and their functional effectiveness may also be reduced, as is the case with miniature gaskets. Mating rigid ceramic or silicon-based components with flexible components is a difficult task by hand, and is extremely difficult to automate. These problems are exacerbated at small scales.  
           [0019]    Sealing: Manipulating small gaskets poses extreme technical problems. Managing the appropriate balance between over-compressing the gaskets and providing sufficient compression to minimize the contact resistance between the electrodes and bulk current collecting plate is very difficult to achieve, especially with brittle materials such as silicon or graphite. Some designs have opted for adhesive based sealing rather than gasket based sealing. However, controlling the distribution of adhesive is difficult.  
           [0020]    Size: If compression is required for either sealing or minimizing contact resistance, endplates or similar compressive elements will be required. Such compressive elements add a significant amount of weight and bulk for no gain in active area.  
           [0021]    Material Cost While costs associated with noble metal catalysts and patented polymer ion conductors are unavoidable, costs associated with graphite, silicon and other favored substrates are. Reducing or eliminating the need for use of such materials in flow fields and bi-polar plates, for example, can result in significant cost savings.  
           [0022]    The need has therefore arisen for an improved method for economically fabricating fuel cells and MEA devices without assembly using thin film deposition techniques.  
         SUMMARY OF INVENTION  
         [0023]    The present invention overcomes the limitations of conventional fuel cell fabrication processes by enabling fuel cells and MEAs to be fabricated in a continuous process without assembly. The method minimizes production costs and costs of non-essential materials. In accordance with the invention, a proton exchange membrane is used as a substrate and layers of catalyst, current collector and flow management channels are successively deposited on the substrate. By building up the fuel cell from a stable substrate, the following advantages can be achieved:  
           [0024]    1. Contact resistance between the bulk current collectors and catalyst becomes negligible.  
           [0025]    2. Assembly of discrete pieces is no longer required, increasing the automatibility of the system.  
           [0026]    3. Sealing is achieved inherently, removing the need for gaskets and compression.  
           [0027]    4. End-plates are not required reducing the thickness of the fuel cell by an order of magnitude.  
           [0028]    5. Because all of the components are preferably flexible, the fuel cell is more durable, and can be optionally fabricated using a continuous roller process.  
           [0029]    6. Non-essential component costs are reduced to a minimum.  
           [0030]    Applicant&#39;s fuel cell fabrication method generally involves four steps: membrane preparation, catalyst deposition, current collector deposition and flow field formation. The method eliminates the need for a MEA gas diffusion layer and requires no compression for either sealing or minimizing contact resistance. While micromachining techniques may be used to fabricate molds, jigs and templates used in conjunction with the invention, the fabrication method itself is more akin to high speed printing, decreasing production costs and increasing throughput. The result is a smaller, less expensive, easily manufactured fuel cells and MEA components suitable for low power battery replacement applications.  
           [0031]    According to one embodiment of the invention the method includes the steps of providing a dimensionally stable membrane having a first surface and a second surface; depositing a first catalyst layer on the first surface according to a first predetermined pattern; and depositing a first current collector layer on the first surface according to a second predetermined pattern. Preferably the catalyst layer and the current collector layer are aligned so that they are in contact with one another on the membrane. Both the catalyst layer and the current collector layer may be applied to the membrane in a generally common plane of deposition.  
           [0032]    The catalyst layer may be subdivided according to the first predetermined pattern into a plurality of discrete catalyst regions. The current collector layer may also be subdivided according to the second predetermined pattern into a plurality of discrete conductive regions. Preferably the conductive regions are formed immediately adjacent the catalyst regions on the membrane. Each of the conductive regions comprises a distinct electrode and such electrodes may be electrically connected together in series or parallel.  
           [0033]    The method may further include the steps of depositing a second catalyst layer on the second surface of the membrane according to the first predetermined pattern and depositing a second current collector layer on the second surface of the membrane according to the second predetermined pattern. The first predetermined pattern on the first surface of the membrane is preferably aligned with the first predetermined pattern on the second surface of the membrane, and the second predetermined pattern on the first surface of the membrane is likewise aligned with the second predetermined pattern on the second surface of the membrane.  
           [0034]    The dimensionally stable membrane may constitute a proton exchange membrane. The membrane may be formed by providing a porous substrate composed of an inert material, such as glass, polytetrafluoroethylene, polyethylene, and/or polypropylene, and impregnating the substrate with an ionomer, such as Nafion®.  
           [0035]    The step of depositing the catalyst layer on the first surface according to the first predetermined pattern may include providing a first template having openings corresponding to the first predetermined pattern; temporarily coupling the first template to the membrane; and spraying a catalyst through the openings in the first template to deposit the catalyst on the membrane in the first predetermined pattern. The template may be temporarily coupled to the membrane during the spraying process by interposing the membrane between the first template and a magnet, for example. Alternatively, the catalyst may be deposited and/or patterned on the membrane by other means, such as microspraying, photolithography, printing or other direct mechanical application. The membrane electrode assembly may then be subjected to one or more hot pressing steps.  
           [0036]    The step of depositing the current collector layer on the first surface according to the second predetermined pattern may also be accomplished by various means including printing, stamping, spraying, photolithography and the like. In particular embodiments the current collector layer comprises a sputtered gold film or a conductive polymer.  
           [0037]    The fuel cell may be manufactured by fabricating a membrane electrode assembly as described above and further forming a first flow field layer on the membrane according to a third predetermined pattern, wherein at least a portion of the flow field layer is bonded to the membrane. The flow field layer may be deposited by applying a curable epoxy, such as SU-8, to the membrane and allowing the epoxy to cure in the third predetermined pattern. The flow field layer includes a plurality of flow field channels formed adjacent the catalyst regions. Further, at least a portion of the flow field layer may overlap the conductive regions. The flow field layer may alternatively be pre-formed in the third predetermined pattern, such as casting the layer on a mold, and then adhering the layer to the membrane, for example by using silicone rubber.  
           [0038]    In an alternative embodiment of the invention, a fuel cell may be fabricated by forming first and second membrane electrode assemblies as described above and annealing the second surface of the first membrane assembly to the second surface of the second membrane assembly. The second surfaces may optionally be coated with Nafion® prior to the annealing step.  
           [0039]    The application also relates to membrane electrode assemblies, fuel cells and fuel cell stacks fabricated according to the above method. Preferably such devices are flexible and have a thickness not exceeding 1 mm in the case of membrane electrode assemblies and 5 mm in the case of micro fuel cells, including the flow field layer.  
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0040]    In drawings which describe embodiments of the invention but which should not be construed as restricting the spirit or scope thereof,  
         [0041]    [0041]FIG. 1 is a cross-sectional view of a conventional PEM fuel cell of the prior art.  
         [0042]    [0042]FIG. 2( a ) is a cross-sectional view of a dimensionally stable PEM membrane used as a substrate for fuel cell fabrication in accordance with the invention.  
         [0043]    [0043]FIG. 2( b ) is a cross-sectional view of the membrane of FIG. 2( a ) with a catalyst layer deposited thereon.  
         [0044]    [0044]FIG. 2( c ) is a cross-sectional view of the membrane of FIG. 2( b ) with a bulk current collector layer deposited thereon.  
         [0045]    [0045]FIG. 2( d ) is a cross-sectional view of the membrane of FIG. 2( c ) with flow field layer posts deposited thereon.  
         [0046]    [0046]FIG. 2( e ) is a cross-sectional view showing a cap applied to the posts of FIG. 2( d ) to cap the flow field layer on the anode side of the membrane.  
         [0047]    [0047]FIG. 2( f ) is a cross-sectional view of an alternative embodiment of the invention showing a pre-formed flow field layer bonded to the membrane substrate.  
         [0048]    [0048]FIG. 2( g ) is a fuel cell stack comprising a pair of micro fuel cells as shown in FIG. 2( f ).  
         [0049]    [0049]FIG. 3( a ) is a plan view of a membrane electrode assembly fabricated in accordance with the invention comprising co-planar catalyst and bulk current collector layers deposited on a PEM membrane substrate.  
         [0050]    [0050]FIG. 3( b ) is a plan view of the catalyst layer of FIG. 3( a )  
         [0051]    [0051]FIG. 3( c ) is a plan view of the current collector layer of FIG. 3( a )  
         [0052]    [0052]FIG. 3( d ) is an isometric view of a micro fuel cell comprising a membrane electrode assembly and a flow field layer.  
         [0053]    [0053]FIG. 4( a ) is a plan view of an alternative embodiment of a membrane electrode assembly fabricated in accordance with the invention comprising co-planar catalyst and bulk current collector layers deposited on a PEM membrane substrate.  
         [0054]    [0054]FIG. 4( b ) is a plan view of the catalyst layer of FIG. 4( a ).  
         [0055]    [0055]FIG. 4( c ) is a plan view of the current collector layer of FIG. 4( a ).  
         [0056]    [0056]FIG. 5( a ) is an exploded view of a template and magnet assembly for applying a catalyst layer pattern on to a membrane substrate interposed therebetween  
         [0057]    [0057]FIG. 5( b ) is an exploded view of a template and magnet assembly for applying a current conductor layer pattern on to a membrane substrate interposed therebetween.  
         [0058]    [0058]FIG. 6 is an isometric view of a mold for producing a pre-formed flow field layer.  
         [0059]    [0059]FIG. 7 is a graph showing polarization and power data for a fuel cell fabricated in accordance with one embodiment the invention.  
         [0060]    [0060]FIG. 8 is a graph showing polarization and power output data for a fuel cell fabricated in accordance with a second embodiment of the invention.  
     
    
     DESCRIPTION  
       [0061]    Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.  
         [0062]    FIGS.  2 ( a )- 2 ( f ) illustrate Applicant&#39;s method for fabricating fuel cells  20  using thin film deposition techniques. As described below, the fabrication method may be automated in a continuous process to reduce fuel cell production costs. The method employs a dimensionally stable proton exchange membrane  22  as a substrate for receiving successive layers of material, namely a catalyst layer  24 , a current collector layer  26  and a flow field layer  28 . The method enables the production in an assembly-less fashion of very thin fuel cells  20  suitable for micro power applications.  
         [0063]    The first step in the Applicant&#39;s method is to provide a membrane  22  as shown in FIG. 2( a ) composed of a solid proton or oxide conducting material or combination of materials. Membrane  22  has a first exposed surface  29  and a second exposed surface  30 . Since membrane  22  is used as a substrate for deposition of layers  24 - 26 , it must be dimensionally stable over the range of chemical exposure and operating temperatures expected for a fuel cell. As used in this patent specification “dimensionally stable” means that membrane  22  is mechanically robust and will not substantially expand or contract, such as when hydrated or dehydrated. Suitable dimensionally stable membranes may be composed, for example, of ceramics, polymers, plastics and supported composite membranes, or combinations thereof, and may include flexible materials.  
         [0064]    PEM fuel cells typically employ a solid polymer electrolyte such as Nafion® from DuPont or Flemion® from Asahi Glass Company, Limited. While such polymers provide good proton conductivity and ionic selectivity, they are not dimensionally stable, and expand and contract substantially when hydrated or dehydrated. This shortcoming may be overcome by impregnating the polymer within a stable substrate. In one embodiment of the invention, membrane  22  comprises a polymer electrolyte such as Nafion® supported within a porous glass network. Porous glass has the advantage that it is hydrophilic and therefore exhibits excellent polymer uptake characteristics.  
         [0065]    As discussed further below, Nafion® ionomer or resin may be applied to a porous glass substrate through a droplet or spray. Alternatively the glass substrate may be immersed in Nafion® ionomer. Several coats or applications may be required to ensure membrane  22  is saturated with Nafion® and is devoid of pinholes. As will be apparent to a person skilled in the art, other means for forming a membrane  22  may also be employed, such as using threads or meshes pre-coated with Nafion®.  
         [0066]    The next step in the Applicant&#39;s method is to apply catalyst layer  24  to membrane  22  according to a predetermined pattern. The patterned catalyst layer  24  is preferably applied to both first surface  29  (which will become the anode side of membrane  22 ) and second surface  30  (which will become the cathode side of membrane  22 ). Unlike the conventional prior art fuel cell of FIG. 1, catalyst layer  24  is applied directly to membrane  22  and no intervening gas diffusion layers are provided. Catalyst layer  24  forms a three-phase boundary with membrane  22  and provides the medium on which the fuel cell electrochemical reaction takes place. Catalyst layer  24  may consist of a conventional catalyst, such as platinum on carbon black.  
         [0067]    As shown in FIGS.  2 ( b ) and  4 , catalyst layer  24  may be applied to membrane  22  in a pattern consisting of a plurality of spaced-apart catalyst regions  32  to thereby generate a plurality of distinct electrodes. These electrodes may then be electrically connected in parallel to create a single cell with a high peak current, or in series to create several cells with high peak voltages (FIGS.  3 ( a ) and  4 ( a )).  
         [0068]    Catalyst regions  32  may comprise a plurality of spaced-apart lines or squares to facilitate in-plane current collection as described below. Various means may be employed to apply catalyst layer  24  on membrane  22  in the desired pattern, including spraying, printing, photolithography or mechanical application. FIG. 5( a ) illustrates one possible means for spray depositing catalyst layer  24  on membrane  22  employing a mask or template  34 . Template  34  includes a plurality of openings  36  configured in the pattern of regions  32 . Template  34  may be formed from a metal such as steel or nickel and may be temporarily held in close contact relative to membrane  22  with a magnetic chuck comprising a magnet  38  and a steel base plate  39  (FIG. 5( a )). Catalyst may then be sprayed on template  34  using an airbrush operated with a compressed airstream. Catalyst passing through openings  36  forms the catalyst layer  24  on an exposed surface  29 ,  30  of membrane  22  in the desired pattern. Once the desired amount of catalyst is deposited, template  34  and membrane  22  are removed from magnet  38 , membrane  22  is reversed and the spraying procedure is repeated on the opposite surface  29 ,  30  of membrane  22 . Care must be taken to align template  34  and membrane  22  with respect to the catalyst pattern already deposited on the opposite surface  29 ,  30 . This alignment may be achieved, for example, with the aid of a light table.  
         [0069]    The next step in the fabrication procedure is to apply current collector layer  26  to membrane  22  as shown in FIG. 2( c ) according to a predetermined pattern. Preferably current collector layer  26  is applied directly to membrane  22  in a pattern matching catalyst layer  24  so that both layers extend in the same plane in contact with one another. For example, current collecting layer  26  may be applied to membrane  22  in a pattern consisting of a plurality of spaced-apart bulk current collection regions  40  which are each disposed between or otherwise adjacent to catalyst regions  32  (FIGS.  2 ( c ),  3 ( a ) and  4 ( a )). Regions  40  are patterned so as to minimize the unused regions  41  on membrane  22  between current collection regions  40  and to facilitate linking in series or parallel. Thus the active area of membrane  22  is maximized while avoiding the potential for short circuits between adjacent electrodes. Further, in an alternative embodiment of the invention membrane  22  could be coated with an insulator in regions  41 . In the embodiment illustrated in the drawings, a small portion  45  of each conducting region  40  may overlap a corresponding catalyst region  32  to ensure effective electrical conduction. Current collector layer  26  may be composed of any electrically conducting material which is temperature and chemically compatible with the fuel cell system, such as a sputtered gold or a conductive polymer.  
         [0070]    Deposition of current collector layer  26  directly on membrane  22  avoids the prior art requirement for compression to reduce contact resistance between the current collectors and catalyst layers  24 ,  26 . This allows for the fabrication of a much thinner fuel cell  20  in comparison with prior art designs. As with catalyst layer  24 , various means may be employed to apply current collecting layer  26  on membrane  22  in the desired pattern, including spraying, printing, photolithography or mechanical application. One possible means for depositing layer  26  on membrane  22  is by using a sputtering process deposited through a metallic template  42  having a plurality of openings  44  (FIG. 5( b )). Templates  42  are fashioned in the same manner as templates  34  described above to provide a minimum reliable contact between layers  24  and  26  and a minimum thickness profile. Template  42  may be pre-aligned with coated membrane  22  under a microscope on a flat magnet  38 . The assembly comprising magnet  38 , membrane  22  and template  42  is then placed inside a sputter-coater (not shown) with a pre-loaded gold target. After the gold is deposited, membrane  22  is disassembled from template  42  and magnet  38  to reveal the current collection regions  40 . The combination of membrane  22  and catalyst and current conductor layers  24 ,  26  comprises a novel membrane electrode assembly  43  (FIGS.  3 ( a ) and  4 ( a )).  
         [0071]    The cell electrodes may then be electrically connected in any parallel or series combination required for the application. Several fabrication techniques including soldering, conductive epoxies, wire bonding or further conductor deposition step(s) can be used to perform the necessary interconnections.  
         [0072]    The final step in the fabrication procedure is to apply flow field layer  28  to membrane  22  and/or to catalyst and current collector layers  24 ,  26  deposited thereon. Flow field layer  28  may either be deposited on membrane  22  (FIGS.  2 ( d ) and  2 ( e )) or may be preformed and adhered to membrane  22  with an adhesive (FIG. 2( f )). In either case, flow field layer  28  comprises at least one channel  46  for supplying fuel or other reactants to catalyst layer  24  and for removing reaction products therefrom. In the illustrated embodiment a plurality of channels  46  are shown which may be physically separated or in fluid communication, such as connected in a serpentine pattern. In the case of very small fuel cells  20  (e.g. watch battery size) a single small channel  46  could be provided.  
         [0073]    Layer  28  may be formed from any material having suitable thermal and chemically stability for use in fuel cells, such as metals, ceramics, polymers and plastics. In one embodiment of the invention molded silicone rubber may be employed in view of its low cost, ease of manufacture and suitable thermal and chemical properties. Flow field layer  28  covers membrane  22  and confers mechanical support to fuel cell  20 .  
         [0074]    In a first embodiment of the invention illustrated in FIG. 2( d ), flow field layer  28  is formed on membrane  22  by direct deposit of flow field posts  48  in regions overlapping current collector regions  40  generally between catalyst regions  32 . For example, as described further below, flow field posts  48  may be formed directly on the membrane electrode assembly  43  by casting a high aspect UV curable epoxy, such as SU-8. The SU-8 is spun on membrane  22  at ˜700 rpm for 30 seconds. After a short period where the film is allowed to cool and relax, it is placed in an oven at 100° C. for approximately two hours. The film should be hard to the touch after cooling. The film is then exposed to UV light through an emulsion mask to pattern flow field posts  48 . Areas exposed through the mask are cured. Posts  48  are positioned to leave the catalyst regions  32  undeveloped. The developed area could cover the current collectors  40 , an insulating zone, or both, as mentioned above. The film is exposed four consecutive times for 45 seconds, with a 15 second break between exposures. The film is then baked again at 100° C. for 15 minutes. The film is developed in SU-8 developer at room temperature for approximately one hour with gentle agitation. The film is then cleaned with new developer.  
         [0075]    As shown in FIG. 2( e ), flow field posts  48  may be capped with an outer cap layer  50  on the anode side of membrane  22  for controlled reactant or product flow through channels  46 , or optionally left uncapped on the cathode side of membrane  22  for air breathing operation (FIG. 3( d )). As is the case in respect of catalyst regions  32  and current collector regions  40  described above, the deposition and patterning of flow field regions  48  may be accomplished by injection molding, photolithography or mechanical means.  
         [0076]    As an alternative to the step of FIG. 2( d ), or in conjunction with it, a pre-formed flow field layer  28  may be formed which is secured to membrane  22  with an adhesive (FIG. 2( f )). Layer  28  may be pre-formed in a mold  47  (FIG. 6). Sealing layer  28  to membrane  22  could be accomplished using silicone rubber adhesive (which the inventors have determined bonds particularly well to Nafion®). Both membrane  22  and flow field layer  28  could be flexible to facilitate lamination of one to the other using rotating rollers or the like in an automated process to avoid the need for assembly. The pre-formed flow field layer  28  of FIG. 2( f ) has the advantage that significant quantities of solvent are not required to develop layer  28  in the desired pattern.  
         [0077]    As will be apparent to a person skilled in the art, multiple fuel cells  20  fabricated in accordance with the invention may be readily connected together as shown in FIG. 2( g ) to form a fuel cell stack  52 . For example, the capped anode surfaces of respective fuel cells  20  of FIG. 2( f ) could be bonded together to form stack  52 .  
         [0078]    As mentioned above, the Applicant&#39;s fuel cell fabrication method may be optimized for mass production of micro fuel cells. Since membrane  22  and membrane electrode assembly  43  derived therefrom may be flexible, the fabrication method could implemented in a continuous fashion, such as by passing membrane  22  through sequential deposition, molding, patterning and/or embossing stations in a calendaring process akin to papermaking. Since the fuel cell  20  end product is also preferentially flexible, it may be formed into non-planar shapes for versatility of packaging. For example, fuel cell  20  could be formed in a tubular shape in which case catalyst and current collector layers  24 ,  26  would extend in a generally common cylindrical orientation rather than the generally common horizontal plane of FIG. 2( c ). Other shapes and orientations could be readily envisaged by a person skilled in the art.  
       EXAMPLES  
       [0079]    The following examples will further illustrate the invention in greater detail, although it will be appreciated that the invention is not limited to the specific examples.  
         [0080]    Membrane formation  
         [0081]    Both porous glass and Teflon® supported Nafion® membranes  22  have been researched. Both provide the mechanical support necessary to create dimensionally stable membranes  22 . As described below, Nafion® ionomer or resin is applied to the porous glass or Teflon substrate, through a droplet or spray, or the porous substrate is immersed in Nafion® ionomer. Several coats are generally required to create a membrane without pinholes.  
         [0082]    Glass substrates may exhibit superior performance because they are hydrophilic, and thus absorb the ionomer better. Dipping appears to yield better performance than dropping or spraying, especially with the glass substrate. Nafion® saturation can be reached in four dipping operations instead of nine dropping or spraying operations.  
         [0083]    One particular method for fabricating membranes  22  is by means of an immersion-hot press system. According to this method, the porous substrate is weighed to determine the initial conditions. The porous substrate is then placed on a stainless steel mesh and dipped in a Nafion® ionomer solution. The dipping time is approximately 30 seconds for the first coat. Every subsequent coat requires an additional 30 seconds of immersion to compensate for the reduction in pore size. The composite membrane  22  is removed from the solution on its steel mesh to ensure that it does not tear. Membrane  22  is then placed on another stainless steel mesh and left to dry at room temperature for approximately 10 to 20 minutes. Subsequently, membrane  22  is placed in an oven for 25 minutes at a temperature of 75° C. to ensure that solvent has been driven off.  
         [0084]    During this time a hot press is set to a temperature of 140° C. One or more membranes  22  are placed between clean, chemically inert sheets (e.g. composed of Teflon®) and the combination is placed between two flat and leveled steel plates. The sandwich is placed in the press and pressure is applied. The following Table 1 shows pressure versus coating number.  
                           TABLE 1                                   Coating   Pressure                           1   0.5 ton           2   1.0 ton             3+   2.0 ton                      
 
         [0085]    Each membrane  22  is then weighed to evaluate the Nafion® loading. The above steps are generally repeated 3 to 5 times to ensure that membrane  22  is completely saturated with Nafion® and all pinholes are removed.  
         [0086]    After soaking membrane  22  for several hours in a 10% H 2 SO 4  solution, membrane  22  is rinsed with deionized water and soaked in water for another hour. The result of the membrane preparation step is a high conductivity, mechanically robust, dimensionally stable proton exchange membrane  22  that is suitable for subsequent deposition steps. Conductivities of between 1 and 10 mS/cm for Teflon supported membranes  22  and  20  and 52 mS/cm for glass-supported membranes  22  have been measured using standard AC impedance techniques. These results compare favorably to the 78 mS/cm measured for bulk Nafion® during the same experiment.  
         [0087]    By way of example, a sample of approximate 12-14 mm length and approximately 10 mm width was removed from a membrane  22  and introduced to an AC impedance test station. To calculate the conductivity the following formula is used:  
       σ   =     L     W   ·   T   ·   R                             
 
         [0088]    where  
         [0089]    L=distance between the two platinum electrodes in the AC impedance test station  
         [0090]    W=width of the piece of membrane being tested  
         [0091]    T=thickness of the piece of membrane being tested  
         [0092]    R=resistance of the piece of membrane being tested at the lowest value for Z  
         [0093]    The following results are indicative for membranes  22  created using the above procedure. Nafion®  117  was characterized using the same apparatus and its conductivity is included for reference.  
                                                                     TABLE 2                           L               σ   Ratio       Item No.   (cm)   W(cm)   T(cm)   R(Ω)   (Scm −1 )   (%)                                NAFION ® 117   0.96   1.174   0.02   524.51   0.078   100       030503VTA   0.96   1.164   0.01763   3844.35   0.012   16       030503VTAa   0.96   1.204   0.01303   3049.85   0.020   26       030503VTB   0.96   1.2075   0.0166   4488.45   0.011   14       030503VTBb   0.96   1.0225   0.01538   3327   0.018   24       030503VTC   0.96   1.112   0.014125   2310.3   0.026   34       030503VTD   0.96   1.278   0.008   2241.55   0.042   54       030503VTD1   0.978   0.875   0.01005   4090   0.027   35       030503VTD3A   0.96   0.999   0.007975   3050.1   0.040   51       030503VTD3B   0.96   0.894   0.005075   4043   0.052   67       040103VTE   0.96   0.970   0.01835   3039.5   0.018   23       040103VTF   0.96   1.064   0.01382   2236.6   0.029   37       041603VTH2   0.965   1.119   0.01320   1222.9   0.053   69                  
 
         [0094]    Catalyst Deposition  
         [0095]    A standard platinum on carbon black catalyst supplied by E-Teck Inc. has been tested. One means for depositing a catalyst layer  24  on membrane  22  is by spraying catalyst ink through a metallic template  34 . Steel or nickel templates  34  are suitable for this purpose. Both are magnetic, facilitating good template-membrane contact through a magnetic chuck (FIG. 5( a )). By way of example, 2 mil thick nickel and 1 mil thick steel shim stock may be used. The templates  34  are patterned using micromachining photolithographic techniques. A UV sensitive polymer, known as photoresist, spun onto the templates  34  and patterned with the desired pattern. The pattern is reduced in size by slightly more than the thickness of the metal film to accommodate the expansion of the hole during isotropic etching. The nickel can be etched in 30% FeCl 3  at 60 C for approximately 12 minutes. The steel can also be etched in FeCl 3 , and etches completely in 3-4 minutes.  
         [0096]    To transfer the catalyst pattern onto a Nation®-impregnated membrane  22 , membrane  22  is placed between the nickel template  34  and a flat magnet  38  (FIG. 5( a )). A homogenized Pt/C catalyst in butylacetate solution (10 wt % Pt/C, 20 wt % Nafion®) is subsequently applied on the mask/membrane/magnet setup using an airbrush operated with a compressed air-stream at approximately 50 psi. Spraying is alternated with drying in a room temperature forced air stream to prevent smearing of the catalyst pattern. The setup is rotated periodically with respect to the airbrush to ensure uniformity in the catalyst loading. Once the desired amount of catalyst is deposited, the mask/membrane/magnet system is disassembled, membrane  22  reversed and the setup reassembled for applying the catalyst on the opposite side of membrane  22 . Care must be taken to align template  34  on membrane  22  with respect to the catalyst pattern already deposited on the opposite side of membrane  22 . This can be easily done with the aid of a light table, for example.  
         [0097]    Membrane  22  is weighed before and after applying the catalyst on each side thereof to determine the overall amount of catalyst deposited. By way of typical example, approximately 30 mg of catalyst may be applied to an area of  ˜ 450 mm 2  area (i.e. one side of membrane  22 ). In order to achieve such catalysts loading, approximately 20 ml of catalyst is sprayed over the entire area of template  34  (i.e. for each side  29 ,  30  of membrane  22 ). After applying the catalyst solution to both sides of membrane  22 , membrane  22  can be hot pressed (as discussed above), typically at  ˜ 130 degrees Celsius at 6 metric tonnes (90 mm diameter membrane) to facilitate a better three phase interface.  
         [0098]    Current Collection  
         [0099]    One possible means for deposition of current collector layer  26  on membrane  22  is by a sputtering process through a metallic template  42 . As discussed above, templates  42  are fashioned in the same manner as the catalyst templates  34 , with a matched design to provide a minimum reliable overlap between the catalyst and current collector layers  24 , 26 , and a minimum profile for current collector layer  26 .  
         [0100]    According to this example, template  42  is pre-aligned on coated membrane  22  under a microscope on a flat magnet  38  (FIG. 5( b )). The magnet-MEA-template assembly is then placed inside a sputter coater with a pre-loaded gold target. The gold is deposited on membrane  22  using the following sputterer settings.  
                                                       Target:   Gold           Thickness:    200 nm           Voltage:   2500 VDC           Plasma Current:    20 mA           Time:    16 min           Sputter Rates:    13 nm/mim                      
 
         [0101]    The cell electrodes can then be electrically connected in any parallel or series combination, such as by using a conductive epoxy. The conductive epoxy can be painted between traces to wire the cell in parallel, or in conjunction with short pieces of wire, be used to wire the cell in series. The two-part silver epoxy is mixed in a small quantity with a one-to-one ratio then painted on the cell. The epoxy is then cured for 15 minutes at 70 C, or left to cure overnight at room temperature.  
         [0102]    Flow Channel Fabrication  
         [0103]    Many metals, ceramics and plastics have the necessary thermal and chemical stability to serve as flow channels. The materials can be cast, injection molded or embossed. By way of example, silicone rubber has been shown to be a suitable material for formation of flow channel layer  28 . Our preferred method for small runs is casting into a single sided mold  47  (FIG. 6), but injection molding or embossing would likely be preferred for mass production. Injection molds or embossing irons can be etched photolithographically, or machined using a combination of laser micromachined graphite, and plunge electrodischarge machining.  
         [0104]    Single-sided molds  47  for casting have been produced using photolithographic techniques. In this example, SU-8, a high aspect ratio UV curable epoxy, was used to form the casting mold. As described above, the SU-8 was spun on a flat substrate such as a silicon wafer or glass plate at  ˜ 700 rpm for 30 seconds. After a short period where the film is allowed to relax, it is placed in an oven at 100° C. for approximately 2 hours. The film should be hard to the touch after cooling. The film is then exposed to UV light through an emulsion mask with the desired pattern. The film is exposed four consecutive times for 45 seconds, with a 15 second break between exposures. The film is then baked again at 100° C. for 15 minutes. The film is developed in SU-8 developer at room temperature for approximately 1 hour with gentle agitation. The mold  47  is then cleaned with new developer.  
         [0105]    The flow fields are cast directly onto mold  47  using Dow Corning mold making silicone rubber. Other castable materials are possible, but Dow Corning  3110  RTV Rubber with Catalyst  1  has been shown to be effective. The catalyst and compound are mixed using the suggested process of the manufacturer, using a 20 to 1 ratio. Gentle mixing is required to avoid embedded bubbles in the mixture. The mixture is poured over the mold approximately 2 mm deep on a clean level surface. After 12 hours of curing time, the cast flow fields can be removed by hand, and any excess rubber cut away.  
         [0106]    Sealing the flow field layer  28  to membrane  22  is accomplished using standard silicone rubber adhesive. The adhesive can be painted directly onto flow field layer  28 , or can be spread in a thin layer on a flat substrate, and roll the flow fields over the film like a stamp. Once the adhesive has been applied, the flow field layer is affixed on membrane  22  by applying modest pressure. The silicone is then allowed to dry for 12 hours.  
         [0107]    Alternatively, the flow fields can be created directly on membrane  22  using SU-8 as described above.  
         [0108]    A micro fuel cell  20  fabricated as described above is shown, for example, in FIG. 3( d ).  
         [0109]    Fuel Cell Test Polarization and Power Results  
         [0110]    [0110]FIGS. 7 and 8 are graphs showing polarization and power data for fuel cells fabricated in accordance with the invention. In one design, a fuel cell as in the embodiment of FIG. 3( a ) was fabricated with 13 electrodes electrically connected in series. Each electrode had dimensions of 1.2 mm width by 30 mm length, and the electrodes were spaced 1.2 mm apart. The gold current collectors had widths of 0.4 mm, and they overlapped the electrodes by 0.2 mm. Preliminary testing and evaluation of this fuel cell at room temperature with 1 atm H 2  and 1 atm air yielded the polarization and power data as illustrated in FIG. 7. The open-cell voltage was 4.5 V, and the peak power was approx. 0.8 mW.  
         [0111]    In another design, a fuel cell was fabricated as in FIG. 4( a ) with 15 electrodes electrically connected in parallel. Each electrode had dimensions of 1 mm width by 30 mm length, and the electrodes were spaced 1 mm apart. The gold current collectors had widths of 1 mm, and they overlap the electrodes by 0.2 mm. Preliminary testing and evaluation of this fuel cell at R.T. with 1 atm H2 and 1 atm air yielded the polarization and power data as depicted in FIG. 8. The open-cell voltage was 0.6 V, and the peak power was approximately 37 mW.  
         [0112]    As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.