Patent Publication Number: US-8524412-B2

Title: Fuel cell fabrication using photopolymer based processes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 12/341,062 filed on Dec. 22, 2008. The entire disclosure of the above application is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to a fuel cell and more particularly to fuel cell components manufactured by photopolymerization. 
     BACKGROUND OF THE INVENTION 
     A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. Individual fuel cells can be stacked together in series to form a fuel cell stack for various applications. The fuel cell stack is capable of supplying a quantity of electricity sufficient to power a vehicle. In particular, the fuel cell stack has been identified as a potential alternative for the traditional internal-combustion engine used in modern automobiles. 
     One type of fuel cell is the polymer electrolyte membrane (PEM) fuel cell. The PEM fuel cell includes three basic components: an electrolyte membrane; and a pair of electrodes, including a cathode and an anode. The electrolyte membrane is sandwiched between the electrodes to form a membrane-electrode-assembly (MEA). The MEA is typically disposed between porous diffusion media (DM), such as carbon fiber paper, which facilitates a delivery of reactants, such as hydrogen to the anode and oxygen to the cathode. In the electrochemical fuel cell reaction, the hydrogen is catalytically oxidized in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte membrane, and are instead directed as an electric current to the cathode through an electrical load, such as an electric motor. The protons react with the oxygen and the electrons in the cathode to generate water. 
     It is also known to use other reactants in the PEM fuel cell such as methanol, for example. Methanol may be catalytically oxidized to form carbon dioxide. Protons from the methanol oxidation are transported across the electrolyte membrane to the cathode where they react with oxygen, typically from air, to generate water. As with the hydrogen PEM fuel cell, electrons are transported as an electric current through the external load, such as the electric motor, from the anode to the cathode. 
     The electrolyte membrane is typically formed from a layer of ionomer. A typical ionomer is a perfluorosulfonic acid (PFSA) polymer, such as Nafion®, commercially available from the E.I. du Pont de Nemours and Company. The electrodes of the fuel cell are generally formed from a finely divided catalyst. The catalyst may be any electro-catalyst which catalytically supports at least one of an oxidation of hydrogen or methanol and a reduction of oxygen for the fuel cell electrochemical reaction. The catalyst is typically a precious metal such as platinum or another platinum-group metal. The catalyst is generally disposed on a carbon support, such as carbon black particles, and is dispersed in an ionomer. 
     The electrolyte membrane, electrodes, and DM are disposed between a pair of fuel cell plates and sealed, for example, with a gasket providing a substantially fluid-tight seal. The fuel cell plate may have a plurality of channels formed therein for distribution of the reactants and coolant to the fuel cell. The fuel cell plate is typically formed by a conventional process for shaping sheet metal such as stamping, machining, molding, or photo etching through a photolithographic mask, for example. In the case of a bipolar fuel cell plate, the fuel cell plate is typically formed from a pair of unipolar plates which are then joined. It is also known to form the fuel cell plate from a composite material, such as a graphite composite or graphite-filled polymer. Undesirably, the known methods for forming each of the electrolyte membrane, the electrodes, the diffusion media, and the fuel cell plates can be costly and time consuming. 
     It is also known to manufacture fuel cell components, including the fuel cell plates, according to conventional polymer foaming processes, such as with reticulated foam materials. However, conventional foaming processes yield non-uniform and random (not ordered) three dimensional microstructures. Certain techniques do exist to create polymer materials with ordered three dimensional microstructures, such as stereolithography techniques; however, these techniques typically rely on a bottom-up, layer-by-layer approach which prohibits production volume scalability. 
     Materials having ordered three dimensional microstructures are described by Jacobsen et al. in “Compression behavior of micro-scale truss structures formed from self-propagating polymer waveguides”, Acta Materialia 55, (2007) 6724-6733, the entire disclosure of which is hereby incorporated herein by reference. One method and system of creating polymer materials with ordered microtruss structures is disclosed by Jacobsen in U.S. Pat. No. 7,382,959, the entire disclosure of which is hereby incorporated herein by reference. The system includes at least one collimated light source selected to produce a collimated light beam; a reservoir having a photo-monomer adapted to polymerize by the collimated light beam; and a mask having at least one aperture and positioned between the at least one collimated light source and the reservoir. The at least one aperture is adapted to guide a portion of the collimated light beam into the photo-monomer to form the at least one polymer waveguide through a portion of a volume of the photo-monomer. Microtruss materials produced by the method and system are further disclosed by Jacobsen in U.S. patent application Ser. No. 11/801,908, the entire disclosure of which is hereby incorporated herein by reference. A polymer material that is exposed to radiation and results in a self-focusing or self-trapping of light by formation of polymer waveguides is also described by Kewitsch et al. in U.S. Pat. No. 6,274,288, the entire disclosure of which is hereby incorporated herein by reference. 
     There is a continuing need for a structure and method of manufacturing fuel cell components that optimizes fuel cell durability, minimizes tooling costs, minimizes production costs, and minimizes development time. Desirably, the method militates against the non-uniformity and design constraints associated with the formation of components according to the known methods. 
     SUMMARY OF THE INVENTION 
     In concordance with the instant disclosure, a structure and method for fabricating fuel cell components that optimizes fuel cell durability, minimizes tooling costs, minimizes production costs, minimizes development time, and militates against the non-uniformity and design constraints associated with the formation of components according to the known methods, is surprisingly discovered. 
     In a first embodiment, a fuel cell component includes a substrate and at least one flow field layer. The flow field layer has at least one of a plurality of radiation cured reactant flow channels and a plurality of radiation-cured coolant channels. The flow field layer may be one of disposed between the substrate and a diffusion medium layer, and disposed on a diffusion medium layer opposite the substrate. 
     In a further embodiment, a method for fabricating a repeating unit for a fuel cell includes the steps of: providing a first fuel cell component having a first substrate, a first diffusion medium layer with a microtruss structure, and a first flow field layer with a plurality of reactant flow channels, the first flow field layer disposed between the first substrate and the first diffusion medium layer; providing a second fuel cell component having a second diffusion medium layer with a microtruss structure and a second flow field layer with a plurality of reactant flow channels and a plurality of coolant channels, the second diffusion medium layer disposed between the second substrate and the second flow field layer; disposing the first fuel cell component onto the second fuel cell component, wherein the first substrate is disposed adjacent the second flow field layer; removing the second substrate from the second fuel cell component; disposing a first microporous layer onto the first diffusion layer and a second microporous layer onto the second diffusion layer; and one of applying a first electrode to the first microporous layer and a second electrode to the second microporous layer, and disposing a polymer electrolyte membrane on the first electrode; and disposing a membrane electrode assembly on one of the first and second microporous layers, thereby completing the repeating unit for the fuel cell. 
     In another embodiment, a method for fabricating a repeating unit for a fuel cell includes the steps of: providing a first substrate; applying a first radiation-sensitive material to the first substrate; placing a first mask between at least one radiation source and the first radiation-sensitive material, the mask extending along a single plane and having a plurality of apertures; exposing the first radiation-sensitive material to a plurality of radiation beams to form a plurality of negative removable cores; removing the first mask from the negative removable cores; applying a second radiation-sensitive material to the negative removable cores; removing an uncured volume of the first radiation-sensitive material; coating the first substrate and the negative removable cores with an electrically conductive coating; applying a third radiation-sensitive material to the first substrate; placing a second mask between the at least one radiation source and the second radiation-sensitive material; exposing the second radiation-sensitive material to a plurality of radiation beams to form a first diffusion medium layer having a microtruss structure; placing a third mask between the at least one radiation source and the third radiation-sensitive material; exposing the third radiation-sensitive material to a plurality of radiation beams to form a second diffusion medium layer having a microtruss structure; removing the negative removable cores and the uncured second and third radiation-sensitive; coating the first and second diffusion medium layers with an electrically conductive coating; and applying a membrane electrode assembly to one of the first and second diffusion medium layers, thereby completing the repeating unit for the fuel cell. 
    
    
     
       DRAWINGS 
       The above, as well as other advantages of the present disclosure, will become readily apparent to those skilled in the art from the following detailed description, particularly when considered in the light of the drawings described herein. 
         FIG. 1  is a fragmentary side cross-sectional view of a fuel cell component according to one embodiment of the present disclosure; 
         FIG. 2  is a fragmentary side cross-sectional view of a fuel cell component according to another embodiment of the present disclosure; 
         FIG. 3  is a fragmentary side cross-sectional view of a repeating fuel cell unit manufactured with the fuel cell components shown in  FIGS. 1 and 2  having electrically conductive coatings; 
         FIG. 4  is a fragmentary side cross-sectional view of a fuel cell component according to a further embodiment of the present disclosure; and 
         FIG. 5  is a fragmentary side cross-sectional view of a repeating fuel cell unit manufactured with the fuel cell component shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description and appended drawings describe and illustrate various embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. In respect of the methods disclosed, the steps presented are exemplary in nature, and thus, are not necessary or critical. 
     As shown in  FIGS. 1 and 2 , a photopolymer based process to fabricate one of a first fuel cell component  10  and a second fuel cell component  11  is provided. The fuel cell components  10 ,  11  include a substrate  12 , at least one diffusion medium layer  14  having a microtruss structure  15 , and at least one flow field layer  16 . In  FIG. 1 , the first fuel cell component  10  is shown with the flow field layer  16  disposed between the substrate  12  and the diffusion medium layer  14 . In  FIG. 2 , the second fuel cell component  11  is shown with the diffusion medium layer  14  disposed between the substrate  12  and the flow field layer  16 . The substrate  12  may be removable or permanently attached to one of the diffusion medium layer  14  and the flow field layer  16 , as desired. 
     At least one of the diffusion medium layer  14  and the flow field layer  16  are formed from a radiation-sensitive material. The formation of radiation-cured structures such as the diffusion medium layer  14  and the flow field layer  16  from the radiation sensitive material are described, for example, in Assignee&#39;s co-pending U.S. patent application Ser. No. 12/339,308, the entire disclosure of which is hereby incorporated herein by reference. 
     The substrate  12  may be formed from any material that permits the formation of polymeric structures thereon. The substrate  12  may be electrically nonconductive or electrically conductive, as desired. For example, the substrate  12  may be electrically nonconductive if it is intended to remove the substrate  12  after the formation of one of the diffusion medium layer  14  and the flow field layer  16 . If it is intended that the substrate  12  remain within the fuel cell, however, the substrate  12  may be electrically conductive. In one example, the substrate  12  is formed from a metal or an electrically conductive composite material that does not significantly degrade under typical photopolymerization conditions, for example. The substrate  12  may alternatively be formed from an electrically nonconductive material having holes or vias formed therein in which an electrically conductive material may be disposed, for example, by plating to cause the substrate  12  to become electrically conductive. In a particular example, the substrate  12  is formed from stainless steel strips. The stainless steel strips may be provided as one of coiled steel strips, rack loaded steel strips, or individual blanks, for example. In certain embodiments the substrate  12  is a substantially planar stainless steel strip. The substrate  12  may also be provided with a texture or a pattern formed therein, as desired. 
     The substrate  12  may further be provided with a coating or surface treatment for bonding and debonding from one of the diffusion medium layer  14  and the flow field layer  16 . As a nonlimiting example, the substrate  12  may have a coating adapted to bond with an uncured polymer material during the manufacture of the fuel cell component  10 ,  11 . The surface treatment may further facilitate a debonding of a cured polymer from the substrate  12 . In particular, a backside of the substrate  12  that is typically disposed on a flat surface during manufacture of the fuel cell component  10 ,  11  may have a coating to militate against an undesired contamination or plating of the substrate  12 . A skilled artisan may select suitable surface treatments, including coatings, as desired. 
     In a further embodiment, the substrate  12  may have through holes (not shown) formed therein. The through holes may facilitate a draining of liquid photomonomers and solvents, or a removal of solid radiation-sensitive polymers, employed during manufacture of the fuel cell components  10 ,  11 . The through holes may also provide typical end product features such as ports and header openings, for example, as described in Applicant&#39;s copending U.S. application Ser. No. 11/762,845, the entire disclosure of which is hereby incorporated herein by reference. In other embodiments, the through holes may cooperate with the flow field layer  16  (such as shown in  FIGS. 4 and 5 ). The through holes may be formed by mechanical perforation or by laser cutting, for example, prior to application of one of the diffusion medium layer  14  and the flow field layer  16  to the substrate  12 . Other suitable means for forming the through holes may also be employed. 
     The diffusion medium layer  14  according to the present disclosure has the microtruss structure  15 . The microtruss structure  15  includes a plurality of first truss elements that extend along a first direction, a plurality of second truss elements that extend along a second direction, and a plurality of third truss elements that extend along a third direction. The first, second, and third truss elements may interpenetrate each other at a plurality of nodes  18 . It should be appreciated that the first, second, and third truss elements may not interpenetrate each other, or may interpenetrate each other at the plurality of nodes  18  on an intermittent basis, as desired. The first, second, and third truss elements form a continuous, three dimensional self supporting cellular structure. 
     Although the microtruss structure  15  with the plurality of first truss elements, the plurality of second truss elements, and the plurality of third truss elements may have a 4-fold architectural symmetry as described hereinabove, a skilled artisan should appreciate that other architectures, such a 3-fold symmetry and 6-fold symmetry, may be employed within the scope of the present disclosure. The particular architecture may be selected, for example, to increase the microtruss structure  15  connectivity and reduce susceptibility to bending and buckling of the microtruss structure  15  under a load. The selected architecture may be symmetrical or asymmetrical, as desired. The architecture may also be selected to optimize strength and stiffness of the microtruss structure  15 . One of ordinary skill in the art should further understand that other architectures for microtruss structure  15  may be employed, as desired. 
     Exemplary architectures of the microtruss structure  15  are described by Jacobsen in U.S. Pat. No. 7,382,959 and U.S. patent application Ser. No. 11/801,908. For example, the plurality of first truss elements may be defined by a plurality of first self-propagating polymer truss waveguides. The plurality of second truss elements may be defined by a plurality of second self-propagating polymer truss waveguides. The plurality of third truss elements may be defined by a plurality of third self-propagating polymer truss waveguides. The first, second, and third waveguides may be formed from a liquid photomonomer that exhibits a shift in index of refraction upon photopolymerization, for example. Alternatively, it should be appreciated that the microtruss structure  15  may be formed from a material that does not form via a shift in index of refraction to form a polymer waveguide, such as by controlled radiation exposure of a substantially solid radiation-sensitive polymer as described herein. Other suitable means of forming the microtruss structure  15  may be employed as desired. 
     One of ordinary skill in the art should appreciate that the particular microtruss structure  15  can be designed as desired, for example, by at least one of: 1) selecting the angles and the patterns of the polymer truss elements with respect to one another, 2) adjusting the packing, or relative density of the resulting cellular structure, and 3) selecting the cross-sectional shapes and dimensions of the polymer truss elements. In particular, polymer truss elements having an elliptical truss cross-sectional shape may militate against degradation with differences in coefficient of thermal expansion. Other cross-sectional shapes may also be employed, as desired. 
     In certain illustrative embodiments, the polymer of the microtruss structure  15  may be converted to a metal oxide or ceramic to provide the desirable level of bending strength. For example, ceramic microtruss structures  15  and methods for preparing ceramic microtruss structures  15  are disclosed by Gross et al in U.S. patent application Ser. No. 12/074,727, the entire disclosure of which is hereby incorporated herein by reference. 
     The flow field layer  16  according to the present disclosure includes a plurality of reactant flow channels  20 . The reactant flow channels  20  are formed from a plurality of first channel walls  24  extending along a length of the fuel cell component  10 ,  11 . As shown in  FIG. 2 , the flow field layer  16  may also include a plurality of coolant channels  22 . The coolant channels  22  are formed from the plurality of first channel walls  24  and a plurality of second channel walls  26  also extending along the length of the fuel cell component  10 ,  11 . When the flow field layer  16  is formed from the liquid photomonomer, for example, the first channel walls  24  may be defined by a plurality of first self-propagating polymer wall waveguides extending along one direction and the second channel walls  26  may be defined by a plurality of second self-propagating polymer wall waveguides extending along another direction. The first and second channel walls  24 ,  26  may interpenetrate each other at a plurality of junctions  28 . As a nonlimiting example, the first and second channel walls  24 ,  26  may form substantially V-shaped coolant channels  22 . In certain embodiments, the reactant flow channels may also be formed from the first and second channel walls  24 ,  26 . It should be appreciated that forming the reactant flow channels and coolant channels  20 ,  22  as open V-shapes may allow recovery of excess radiation-sensitive materials used during manufacture of the fuel cell components  10 ,  11 . The junctions  28  may be disposed adjacent one of the substrate  12  and the diffusion medium layer  14  as shown in  FIG. 2 , as desired. 
     The flow field layer  16  may further include a plurality of spaced apart support ligaments  30  that facilitate a flow of reactants or coolant though the flow field layer  16  and militate against an undesirable flexing of the first and second channel walls  24 ,  26  under typical fuel cell operating conditions. The support ligaments  30  may further be adapted to carry current within the fuel cell. Like the truss elements of the microtruss structure  15 , the support ligaments  30  may have an elliptical cross-sectional shape or other cross-sectional shape as desired. 
     The support ligaments  30  are disposed between the first and second channel walls  24 ,  26  and one of the substrate  12  and the diffusion medium layer  14 . When the flow field layer  16  is formed from the liquid photomonomer, for example, the support ligaments  30  may be defined by a plurality of self-propagating polymer ligament waveguides. In certain embodiments, at least one of the support ligaments  30  may intersect another of the support ligaments  30  adjacent one of the substrate  12  and the diffusion medium layer  14 . In particular, the support ligaments  30  may be designed to intersect adjacent the diffusion medium layer  14  for rigidity and to avoid undesirably flexing and cracking of the first and second channel walls  24 ,  26 . 
     The radiation-sensitive materials employed to form at least one of the diffusion medium layer  14  and the flow field layer  16  according to the present disclosure includes radiation-curable materials and radiation-dissociable materials. The term “radiation-curable material” is defined herein as any material that is at least one of initiated, polymerized, and crosslinked by exposure to radiation. It should be appreciated that an increase in temperature may also be employed to at least partially complete polymerization or crosslinking of the radiation-curable materials following an initiation by the exposure to radiation. The term “radiation-dissociable material” is defined herein as any material that exhibits at least one of a scissioning of the polymer backbone and a decrosslinking by exposure to radiation. As a nonlimiting example, the radiation-dissociable material may be made solvent-soluble by a sufficient breakage of crosslinks and/or scissioning of the polymer backbone of the radiation-dissociable material. 
     As nonlimiting examples, the radiation-curable materials may include one of a liquid photomonomer and a substantially solid radiation-curable polymer. The liquid photomonomer may be a monomer as described by Jacobsen in U.S. Pat. No. 7,382,959 and U.S. application Ser. No. 11/801,908. Nonlimiting examples of suitable photomonomers include monomers that polymerize via free-radical polymerization when exposed to UV radiation (wavelength between about 250 nm and about 400 nm). The photomonomer may include any suitable free-radical photopolymer material such as urethanes (polyurethanes), acrylates, methacrylates, and cationic polymers such as photo-cured epoxies. Other suitable photomonomers may also be employed. 
     Suitable substantially solid radiation-curable polymers may include negative resist polymers. Negative resist polymers go through a photoinitiation process that leads to a curing of the negative resist polymer by polymerization or polycondensation, for example. Where the polymerization or polycondensation reaction occurs at substantially the same time, the process is referred to as “photocured”. Where only the reaction species are generated by the photoinitiation process and a subsequent step such a heating is required to generate the polymerization or polycondensation, the process is be referred to as “photoinitiated”. It should be appreciated that even though a post-cure heat treatment may be necessary to finalize the polymerization step, substantially stable radiation-cured features in the negative photoresist polymer may also be created during the initial radiation exposure. The substantially solid radiation-curable polymers can go through just the initiation process and, due to inherent stability and the limited diffusion rate of the chemical species within the solid radiation-curable polymers, the curing process may also be performed much later without significant feature degradation. It should be appreciated that most photoinitiated polymers begin the curing process at the inception of the initiation process, but the kinetics of the reaction at the exposure temperature are so slow that little, if any, polymerization or polycondensation may take place prior to heating the negative resist polymer to a desired curing temperature. 
     One particular negative resist polymer is the epoxy-based SU-8 2000™ negative resist polymer, commercially available from Microchem Corporation in Newton, Mass. The SU-8 2000™ negative resist polymer is curable by UV radiation. It should be appreciated that other substantially solid radiation-curable polymers may be employed. For example, similar to the photomonomers described above, the radiation-curable polymer selected may be cured with radiation of wavelengths other than UV radiation, as desired. The radiation-curable polymer may also be selected to have a slower cure rate than the liquid photomonomer, for example, to militate against features from appearing in the slower curing layer with exposure of the faster curing layer to a radiation source. 
     As a nonlimiting example, the radiation-dissociable materials may include positive resist polymers. Positive resist polymers begin as crosslinked polymers but may contain photoinitiators that, when exposed to a particular radiation, generate chemical species which dissociate the polymer by at least one of breaking the crosslinks and scissioning the polymer backbone. The dissociation makes the positive resist polymer soluble in the regions which have been exposed to the radiation. Regions where the positive resist polymer remains cured are masked rather than being exposed, as is the case with the negative resist polymers described hereinabove. In certain embodiments, the positive resist polymers are sensitive to radiation, e.g., ultraviolet or electron beam, without the need for photoinitiators. For example, the positive resist polymer may itself be damaged by the radiation and the remaining scissioned chains become soluble in a solvent. Other types of positive resist polymers may be employed, as desired. 
     The radiation employed to one of cure and dissociate the radiation-sensitive materials may include a UV beam from a Mercury arc lamp, for example. The radiation may be electromagnetic radiation or particle radiation, as desired. A skilled artisan understands that radiation of other wavelengths, such as infrared, visible light, and X-ray radiation, and from other sources, such as incandescent lights and lasers, may also be employed. Particle radiation such as an electron beam from a cathode ray source may also be employed. It should be further understood that the radiation may be collimated, partially collimated, or non-collimated, as desired. 
     The radiation is typically directed to the radiation-sensitive material via at least one mask having apertures or openings formed therein, through which the radiation may contact the radiation-sensitive material. The apertures may be substantially radiation-transparent apertures formed in an otherwise opaque or radiation-blocking material, for example. The mask may be formed from a plane of glass or a Mylar sheet, for example, and facilitate the directing of radiation beams at an angle relative to either the mask or the underlying substrate  12 . The mask may be lifted away after an exposure and cleaned for reuse. The apertures or openings formed in the mask have shapes that provide radiation beams to form polymer structures with a desired cross-sectional shape. The apertures may be substantially circular to create elliptical cross-sectional shapes that may better adjust to differences in coefficient of thermal expansion (CTE) within the fuel cell component  10 ,  11 . 
     In one illustrative embodiment, the mask surface has a plurality of holes. The holes may each have a diameter of about 10 microns, for example. In another embodiment, the apertures are elongate slots that provide radiation sheets to form polymer walls with substantially rectangular cross-sectional shapes, for example, to form the at least one of the reactant flow channels  20  and the coolant channels  22  of the fuel cell component  10 ,  11 . The exposure to radiation may be conducted intermittently in a series of short exposures, for example, or in longer durations to provide the desired structural design. A skilled artisan may select suitable masks materials, aperture and opening sizes and shapes, and resulting structural configurations, as desired. 
     Referring to  FIG. 3 , like structure from  FIGS. 1 and 2  have the same reference numeral and a prime (′) or double prime (″) for purpose of clarity. At least one of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″ may further be coated with an electrically conductive material  38 ′,  38 ″. For example, at least one of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″ may be plated with a metal coating that is substantially oxidation resistant, reduction resistant, and acid-resistant. The metal coating may be formed in multiple plating steps. For example, the metal coating may be formed by application of a first layer of an electroless nickel base followed by a second layer of NiCr or Cr plating for corrosion resistance and a third layer of an overcoating with Au or Pd for the minimization of contact resistance between the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″. The first layer and the second layer may be plated on, and the third layer applied by physical vapor deposition to limit coverage, for example. Other methods for coating the electrically conductive material  38 ′,  38 ″ onto the at least one of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″, may be employed as desired. 
     In certain embodiments the electrically conductive material  38 ′,  38 ″ includes a noble metal selected from the group consisting of: ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), iridium (Ir), platinum (Pt), and osmium (Os), and alloys thereof. In a particular embodiment, the electrically conductive material  38 ′,  38 ″ is gold (Au). In another particular embodiment, the electrically conductive material  38 ′,  38 ″ is tantalum (Ta). Another suitable metal coating may include nickel (Ni) alloys, such as alloys of nickel and chromium (Cr) or nickel and cobalt (Co). As should be recognized by one of ordinary skill in the art, the electrically conductive material  38 ′,  38 ″ may include mixtures or alloys of the above identified metals. Other electrically conductive metals and materials may also be employed, as desired. 
     The electrically conductive material  38 ′,  38 ″ may be deposited on at least one of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″ by one of electron beam evaporation, magnetron sputtering, physical vapor deposition, chemical vapor deposition, atomic layer deposition, electrolytic deposition, electroless deposition, flame spray deposition, brush plating, and other like processes for example. Solution based electroplating techniques that include immersing the at least one of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″ in a plating bath may also be employed. Application of electrically conductive material  38 ′,  38 ″ in the form of a slurry powder and subsequently firing the slurry powder to form the coating may also be used. A skilled artisan may select more than one deposition technique to take into account differences between line of sight and non-line of sight characteristics of the depositions techniques selected. In certain embodiments, the electrically conductive material may be substantially evenly deposited on both the interior and exterior surfaces of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″. In other embodiments, the coating of the electrically conductive material  38 ′,  38 ″ on the diffusion medium layer  14 ′,  14 ″ may have less thickness than the coating of the electrically conductive material  38 ′,  38 ″ on the flow field layer  16 ′,  16 ″. Suitable means for coating and thicknesses of the electrically conductive material  38 ′,  38 ″ may be selected as desired. 
     It should be understood that at least one of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″ may be formed from radiation-sensitive material that is electrically conductive when cured. Where the radiation-sensitive material is electrically conductive, the coating of the electrically conductive material  38 ′,  38 ″ on at least one of the diffusion medium layer  14 ′,  14 ″ and the flow field layer  16 ′,  16 ″ may not be employed, as desired. 
     The first and second fuel cell components  10 ,  11  may also have an at least one microporous layer  32 ′,  32 ″ and an at least one electrode  34 ′,  34 ″. The microporous layer  32 ′,  32 ″ may be disposed on the diffusion medium layer  14 ′,  14 ″. The electrode  34 ′,  34 ″ may be disposed on the microporous layer  32 ′,  32 ′ opposite the diffusion medium layer  14 ′,  14 ″. It should be appreciated that the electrode  34 ′,  34 ″ may also be disposed directly on the diffusion medium layer  14 ′,  14 ″, as desired. One of the electrodes  34 ′,  34 ″ may further have a polymer electrolyte membrane (PEM)  36  disposed thereon. 
     As shown in  FIG. 3 , the first and second fuel cell components  10 ,  11  may be assembled to form a single repeating unit  40  for a fuel cell. A plurality of the repeating units  40  may be stacked together to form a fuel cell stack (not shown). The repeating unit  40  includes the first diffusion medium layer  14 ′ and the second diffusion medium layer  14 ′. The first diffusion medium layer  14 ′ has the microtruss structure  15 ′ with the nodes  18 ′. The second diffusion medium layer  14 ″ includes the microtruss structure  15 ″ with the nodes  18 ″. The repeating unit  40  further includes the first flow field layer  16 ′ and the second flow field layer  16 ″. The first flow field layer  16 ′ includes the first channel walls  24 ′, intersecting and the plurality of support ligaments  30 ′. The second flow field layer  16 ″ includes the first and second channel walls  24 ″,  26 ″ intersecting at the junction  28 ″, and the plurality of support ligaments  30 ″. The substrate  12 ′ of the first fuel cell component  10  is disposed between the first and second flow field layers  16 ′,  16 ″. The substrate  12 ′ and the first and second flow field layers  16 ′,  16 ″ are further disposed between the first and second diffusion medium layers  14 ′,  14 ″. Each of the diffusion medium layers  14 ′,  14 ″ has one microporous layer  32 ′,  32 ″ disposed thereon. Each of the microporous layers  32 ′,  32 ″ has one of the electrodes  34 ′,  34 ″ applied thereto opposite the diffusion medium layer  14 ′,  14 ″. One of the electrodes  34 ′,  34 ″ has a polymer electrolyte membrane  36  disposed thereon. 
     Referring to  FIGS. 4 and 5 , like structure from  FIGS. 1 ,  2 , and  3  have the same reference numeral and a prime (′) or double prime (″) for purpose of clarity. An alternate fuel cell component  10 ′ according to the present disclosure is illustrated in  FIG. 4  and is provided for forming a single repeating unit  40 ′ for the fuel cell as shown in  FIG. 5 . 
     The fuel cell component  10 ′ includes the first and second diffusion medium layers  14 ′,  14 ″ and the single flow field layer  16 . The flow field layer  16  is formed from a plurality of electrically conductive walls  42  that define the plurality of reactant flow channels  20  and the plurality of coolant channels  22 . The electrically conductive walls  42  may be formed, for example, by disposing the coating of the electrically conductive material  38  on a plurality of removable negative cores, and subsequently removing the cores to leave the electrically conductive material  38  as the walls  42 . Other means for forming the electrically conductive walls  42  may also be used. 
     The substrate  12  may be coupled with the flow field layer  16 . As a nonlimiting example, the coating  38  forming the electrically conductive walls  42  may also be disposed over the substrate  12  to bind the substrate  12  with the flow field layer  16 . The flow field layer  16  is disposed between the first and second diffusion medium layers  14 ′,  14 ″. One of the diffusion medium layers  14 ′,  14 ″ has a membrane electrode assembly (MEA)  44  disposed thereon. The membrane electrode assembly  44  includes the microporous layers  32 ′,  32 ″, the electrodes  34 ′,  34 ″, and the PEM  36 . The flow field layer  16  provides the plurality of reactant flow channels  20  and the plurality of coolant channels  22 . As shown, the plurality of reactant flow channels  20  formed by the flow field layer in the alternate fuel cell component  10 ′ includes reactant flow channels  20  formed adjacent the first diffusion medium layer  14 ′ and reactant flow channels  20  formed adjacent the second diffusion medium layer  14 ″. A perforation, slot, or a series of holes  46  is formed though the substrate  12  and cooperates with the flow field layer  16  to provide fluid communication between the plurality of reactant flow channels  20  adjacent the second diffusion medium layer  14 ″. It should be appreciated that the alternate structure of the fuel cell  10 ′ thereby facilitates the formation of the single repeating unit  40 ′ for the fuel cell without requiring an assembly of the first and second fuel cell components  10 ,  11  as shown in  FIGS. 1 to 3 . 
     The present disclosure includes a method for fabricating the repeating unit  40  for the fuel cell. The method first includes the steps of: providing the first fuel cell component  10  and providing the second fuel cell component  11 . The first fuel cell component  10  is disposed on the second fuel cell component  11 . The first substrate  12 ′ is disposed adjacent the second flow field layer  16 ″ of the second fuel cell component  11 . The second substrate  12 ″ on the second fuel cell component  11  is removed from the second fuel cell component  11 , one of before and after the second fuel cell component  11  is disposed onto the first fuel cell component  10 , as desired. The first and second fuel cell components  10 ,  11  may be joined with a heavy plating process, for example, to close the V-shaped coolant channels  22 ″. Electrically conductive positioning adhesives may be used to join the first and second fuel cell components  10 ,  11 . Other suitable means for joining the first and second fuel cell components  10 ,  11  in a desirably sealed and electrically conductive manner may also be employed. 
     The first microporous layer  32 ′ is then disposed on the first diffusion layer  14 ′ and the second microporous layer  32 ″ is disposed on the second diffusion layer  14 ″. The first electrode  34 ′ is disposed on the first microporous layer  32 ′ and the second electrode  34 ″ is disposed on the second microporous layer  32 ″. The PEM  36  is then disposed on one of the first and second electrodes  34 ′,  34 ″ to complete the repeating unit  40  for the fuel cell. The assembly of the microporous layers  32 ′,  32 ″, electrodes  34 ′,  34 ″, and PEM  36  may occur before or after the joining of the first fuel cell component  10  with the second fuel cell component  11 , as desired. 
     The step of providing the first fuel cell component  10  may further include the steps of providing the first substrate  12 ′ and disposing a first radiation-sensitive material on the first substrate  12 ′. For example, the first substrate  12 ′ may be immersed in a volume of liquid photopolymer. A first mask is then placed between the at least one radiation source and the first radiation-sensitive material. The first mask extends along a single plane and has a plurality of elongate slots formed therein. The first mask may be selected to provide a desired flow field geometry having the plurality of first polymer walls  24 ′. The first radiation-sensitive material is then exposed to a plurality of radiation sheets directed through the elongate slots of the first mask to form the first flow field layer  16 ′ having the plurality of reactant flow channels  20 ′. 
     Following the formation of the first flow field layer  16 ′, a second radiation-sensitive material is applied to the first flow field layer  16 ′. The second radiation-sensitive material may be the same as, or different from, the first radiation-sensitive material, as desired. As a nonlimiting example, a substantially solid radiation-sensitive polymer may be laminated on the first flow field layer  16 ′. A second mask is placed between the at least one radiation source and the second radiation-sensitive material. The second mask may be selected to provide a desired diffusion medium geometry. The second mask extends along a single plane and has a plurality of apertures. The second radiation-sensitive material is exposed to a plurality of radiation beams directed through the apertures of the second mask to form the first diffusion medium layer  14 ′ having the desired microtruss structure  15 ′. 
     The step of providing the second fuel cell component  11  may further include the steps of providing the second substrate  12 ″ and applying a third radiation-sensitive material to the second substrate  12 ″. The third radiation-sensitive material may be the same as, or different from, the first and second radiation-sensitive materials. As a nonlimiting example, the second substrate  12 ″ may be laminated with a substantially solid radiation-sensitive polymer. A thickness of the third radiation-sensitive material applied to the second substrate  12 ″ may be selected based on a desired thickness of the uncompressed diffusion medium layer  14 ″ to be manufactured with the third radiation-sensitive material. A third mask is then placed between at least one radiation source and the third radiation-sensitive material. The third mask extends along a single plane and has a plurality of apertures formed therein. The third radiation-sensitive material is subsequently exposed to a plurality of radiation beams from the radiation source from one or more directions, for example, simultaneously or sequentially, as desired. The radiation beams are directed to the third radiation-sensitive material through the apertures formed in the third mask. The radiation beams cause the microtruss structure  15 ″ to form the second diffusion medium layer  14 ″. 
     Following the formation of the microtruss structure  15 ″, the third mask is removed from the second diffusion medium layer  14 ″. A fourth radiation-sensitive material is then applied to the second diffusion medium layer  14 ″. The fourth radiation-sensitive material may be the same as, or different from, the first, second, and third radiation-sensitive materials. As a nonlimiting example, the fourth radiation-sensitive material is a volume of liquid photopolymer. A fourth mask is placed between the at least one radiation source and the fourth radiation-sensitive material. The fourth mask may extend along a single plane and have a plurality of elongate slots formed therein. The fourth radiation-sensitive material is then exposed to a plurality of radiation sheets directed through the elongate slots of the fourth mask to form the second flow field layer  16 ″ having the plurality reactant flow channels  20 ″ and the plurality of V-shaped coolant channels  22 ″. 
     It should be appreciated that, to form the desired microtruss structure  15 ′,  15 ″, the radiation beams may include a first radiation beam, a second radiation beam, and a third radiation beam. The first, second, and third radiation beams are directed from the at least one radiation source toward at least one aperture to respectively form the first, second and third polymer truss elements in a portion of the volume of the radiation-sensitive material. In the case of the photomonomer, the polymer truss elements may be formed from polymer waveguides having a different refraction index than the surrounding photomonomer. 
     As a nonlimiting example, the first radiation beam may be directed at a first angle with respect to the single plane and the second radiation beam is directed at a second angle with respect to the single plane; the second angle being different from the first angle. The third radiation beam may be directed at a third angle with respect to the single plane; the third angle being different from the first angle and the second angle. The first polymer truss element forms a fourth angle with respect to the single plane; the fourth angle corresponding to the first angle. The second polymer truss element forms a fifth angle with respect to the single plane; the fifth angle corresponding to the second angle. The third polymer truss element forms a sixth angle with respect to the single plane; the sixth angle corresponding to the third angle. Each of the first, second and third polymer truss elements may intersect each other to form the microtruss structures  15 ′,  15 ″. It should be understood, however, that at least a portion of the truss elements may be selected to not intersect, as desired. 
     The method for fabricating the repeating unit  40  for the fuel cell further includes the step of coating at least one of the first diffusion medium layer  14 ′, the second diffusion medium layer  14 ″, the first flow field layer  16 ′, and the second flow field layer  16 ″ with the electrically conductive coating  38 ′,  38 ″, as described hereinabove. It should be appreciated that the electrically conductive coating  38 ′,  38 ″ may minimize contact resistance and optimize an efficiency of the fuel cell. 
     The present disclosure includes another method for fabricating the repeating unit  40 ′ for the fuel cell. The method includes the steps of providing the substrate  12  and applying a first radiation-sensitive material to the substrate  12 . For example, the substrate  12  may be immersed in a volume of liquid photopolymer. The substrate  12  may be provided with the plurality of perforations or holes  46  formed therein. The first mask is then placed between at least one radiation source and the first radiation-sensitive material. The first mask extends along a single plane and has a plurality of apertures. The first radiation-sensitive material is then exposed to a plurality of radiation beams directed through the apertures of the first mask to the first radiation-sensitive material to form a plurality of negative removable cores. The negative removable cores may also be formed by other means, such as screen printing, for example. The first mask is removed from the negative removable cores and the residual uncured first radiation-sensitive material. It should be appreciated that the term “uncured radiation-sensitive material” may also include dissociated radiation-sensitive material within the scope of the present disclosure. The remaining uncured volume of the first radiation-sensitive material may subsequently be removed, for example, by draining in the case of photomonomer, and recycled. The step of removing the residual uncured volume of first radiation-sensitive material may include rinsing the substrate and the cores with a solvent. One of ordinary skill in the art should appreciate that suitable solvents do not substantially degrade the exposed negative removable cores. 
     The removable cores have the negative shape of the walls  42  desired to form the plurality of reactant flow channels  20  and the plurality of coolant channels  22 . The step of forming the plurality of negative removable cores may include exposing the radiation-sensitive material to a plurality of inclined and vertical radiation beams, for example. 
     A second radiation-sensitive material is disposed on the negative removable cores and a third radiation-sensitive material is disposed on the substrate  12 . As nonlimiting examples, the second and third radiation-sensitive materials may be substantially solid radiation-sensitive polymers laminated on each of the negative removable cores and the substrate  12 . The substrate  12  and the negative removable cores are coated with the electrically conductive coating  38 . It should be appreciated that a mask or other physical covering, such as a physical contact with another surface, may be employed on the outer surface of the laminated radiation-sensitive polymer to militate against undesirably heavy plating from forming thereon during the application of the electrically conductive coating  38 . 
     A second mask is then placed between the at least one radiation source and the second radiation-sensitive material, and the second radiation-sensitive material is exposed to a plurality of radiation beams to form the first diffusion medium layer  14 ′ having the microtruss structure  15 ′. A third mask is placed between the at least one radiation source and the third radiation-sensitive material, and the third radiation-sensitive material is exposed to a plurality of radiation beams to form the second diffusion medium layer  14 ″ having the microtruss structure  15 ″. The first, second, and third radiation-sensitive materials may be the same, or different, as desired. The negative removable photopolymerized cores and the uncured second and third radiation-sensitive materials are removed to leave the walls  42  formed by the electrically conductive coating  38 . 
     The method further includes the steps of coating of the first and second diffusion medium layers  14 ′,  14 ″ with the electrically conductive material  38 . It should be understood, however, that the coating disposed on the diffusion medium layers  14 ′,  14 ″ may be of lesser thickness than the coating disposed on the negative removable cores that form the walls  42  of the flow field layer  16 ′. One of ordinary skill in the art may select suitable electrically conductive coating thicknesses for each of the layers, as desired. 
     To complete the repeating unit  40 ′ for the fuel cell, the MEA  44  is applied to one of the first and second diffusion medium layers  14 ′,  14 ″. The step of applying the MEA  44  includes the steps of: disposing the first microporous layer  32 ′ on the first diffusion medium layer  14 ′; applying the first electrode  34 ′ to the first microporous layer  32 ′; disposing the PEM  36  on the first electrode  34 ′; applying the second electrode  34 ″ to the PEM  36 ; and disposing the second microporous layer  32 ″ on the second electrode  34 ″. 
     In relation to the repeating units  40 ,  40 ′ described herein, further manifold apertures (not shown) typically associated with the formation of inlet and outlet manifolds of the fuel cell may also be provided. Likewise, seal and port features that facilitate the assembly and operation of the fuel cell may be provided. The manifold apertures, seals, and port features may be formed by conventional means or through use of the polymerization techniques described hereinabove. 
     It is surprisingly found that the employment of microtruss structures  15 ,  15 ′,  15 ″ allows for the cost effective production of fuel cell components  10 ,  10 ′,  11  employed to form fuel cells for automotive and other applications, such as micro-fuel cells for portable devices. The fuel cell components  10 ,  10 ′,  11  of the present disclosure employ polymers formed in the shape of the desired fuel cell part as opposed to forming negative shaped removable cores. The present disclosure expands the design features available from the known microtruss processes to produce a greater variety of features desirable for production fuel cell components such as diffusion media, separator plates, and membrane reinforcement. 
     The highly ordered structure of the fuel cell components  10 ,  10 ′,  11 ,  40 ,  40 ′ fabricated by the methods of the present disclosure provides a more uniform compression and increased durability in comparison to conventional polymer foam parts. The present fabrication methodology is also less expensive than conventional forming methods, and allows the fuel cell components to be fabricated with significantly reduced lead times. In particular, production cost reductions may result from replacement of the conventional carbon paper diffusion media, and by militating against costs typically associated with forming desirable contact resistance interfaces between conventional and separately manufactured fuel cell components. 
     While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes may be made without departing from the scope of the disclosure, which is further described in the following appended claims.