Patent Publication Number: US-9406948-B2

Title: Electroformed bipolar plates for fuel cells

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 12/765,049 filed Apr. 22, 2010, hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates to fuel cell stacks and, more particularly, to a bipolar plate assembly and methods for preparing bipolar plates for fuel cell stacks. 
     BACKGROUND OF THE INVENTION 
     Fuel cells can be used as a power source in many applications. For example, fuel cells have been proposed for use in automobiles as a replacement for internal combustion engines. In proton exchange membrane (PEM) type fuel cells, a reactant such as hydrogen is supplied as a fuel to an anode of the fuel cell, and a reactant such as oxygen or air is supplied as an oxidant to the cathode of the fuel cell. The PEM fuel cell includes a membrane electrode assembly (MEA) having a proton transmissive, non-electrically conductive, proton exchange membrane. The proton exchange membrane has an anode catalyst on one face and a cathode catalyst on the opposite face. The MEA is often disposed between “anode” and “cathode” diffusion media or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. The diffusion media serve as the primary current collectors for the anode and cathode as well as providing mechanical support for the MEA and facilitating a delivery of the reactants. 
     In a fuel cell stack, a plurality of fuel cells is aligned in electrical series, while being separated by gas impermeable, electrically conductive bipolar plates. Each MEA is typically sandwiched between a pair of the electrically conductive plates that serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the fuel cell stack in the case of bipolar plates and conduct current externally of the stack in the case of unipolar plates at the ends of the stack. 
     The bipolar plates typically include two thin, facing conductive sheets. One of the sheets defines a flow path on one outer surface thereof for delivery of the fuel to the anode of the MEA. An outer surface of the other sheet defines a flow path for the oxidant for delivery to the cathode side of the MEA. When the sheets are joined, a flow path for a dielectric cooling fluid is defined. The plates are typically produced from a formable metal that provides suitable strength, electrical conductivity, and corrosion resistance, such as 316 alloy stainless steel, for example. 
     The bipolar plates have a complex array of grooves or channels that form flow fields for distributing the reactants over the surfaces of the respective anodes and cathodes. Tunnels are also internally formed in the bipolar plate and distribute appropriate coolant throughout the fuel cell stack, in order to maintain a desired temperature. 
     The typical bipolar plate is a joined assembly constructed from two separate unipolar plates. Each unipolar plate has an exterior surface having flow channels for the gaseous reactants and an interior surface with the coolant channels. Plates are known to be formed from a variety of materials, including, for example, a metal, a metal alloy, or a composite material. The metals, metal alloys, and composite materials must have sufficient durability and rigidity to function as sheets in the bipolar plate assembly, as well as to withstand clamping forces when assembled into a fuel cell stack without collapsing. It is known to form the plates using various processes such as, for example, machining, molding, cutting, carving, stamping, or photo-etching. In each known method of forming the plates, a substrate material, typically a metal or composite sheet, is required. It is possible to achieve a desired minimal thickness of the substrate, but at a tradeoff to cost and to undesirable material properties. For example, as a composite sheet is molded to a thinner dimension, it becomes more brittle and harder to work. Additionally, a thinner composite sheet is often less desirable because high carbon content may cause a thinner sheet to become porous. Similarly, as a metal sheet is thinned in multiple steps by drawing or rolling the sheet, it also becomes brittle or work hardened after each step, and requires annealing prior to further working. Thus, a higher manufacturing cost is associated with a thinner substrate material. Also, more care is required to form the complex surface features of the plates, such as the flow field pattern, from a thinner metal substrate material to avoid localized areas of high stress and the resulting cracks or tears in the plates due to thinner material. A thinner metal substrate also limits the depth of any flow channel due to metal stretch limitations. As a result, metal sheet plates are optimally formed having a thickness of about 3 to 6 mils (0.003 to 0.006 inches, or approximately 0.075 to 0.15 millimeters thick). It is understood, however, that thicker metal plates may be employed thicker in order to reduce cost and to improve workability of the plate material. 
     Additionally, conventional processes of forming the plates from the metal sheet material result in nearly half of the material being discarded as scrap. Some of the scrap is generated as apertures are punched in the non-active portion of the plates to create flow areas and manifolds for delivery and exhaust of reactants and coolant when a plurality of bipolar plates is aligned in the fuel cell stack. A larger portion of the scrap results from a clamping area that is required about the perimeter of the sheet material during the processes that form plates from the sheet material, which is then trimmed or cut off after processing. 
     Finally, in order to conduct electrical current between the anodes and cathodes of adjacent fuel cells in the fuel cell stack, the paired unipolar plates forming each bipolar plate assembly are mechanically and electrically joined. A variety of bipolar plate assemblies and methods for preparing bipolar plate assemblies are known in the art. For example, it is reported by Neutzler in U.S. Pat. No. 5,776,624, incorporated herein by referenced in its entirety, that a bipolar plate including corrosion-resistant metal sheets may be brazed together to provide a passage between the sheets through which a dielectric coolant flows. Further, U.S. Pat. No. 6,887,610 to Abd Elhamid, et al., incorporated herein by reference in its entirety, discloses a bipolar plate assembly without welding or brazing that includes an electrically conductive layer deposited over the coolant channels and lands and a fluid seal disposed between the inside facing surface about a perimeter of the coolant channels. Also, U.S. Pat. No. 6,942,941 to Blunk et al., incorporated herein by reference in its entirety, recites a bipolar plate having a first and second surface that are coated with an electrically conductive primer coating and joined to one another by an electrically conductive adhesive. Schlag in U.S. Pat. No. 7,009,136, incorporated herein by reference in its entirety, describes a method of fabrication adapted to weld bipolar plates together using a partial vacuum that holds paired unipolar plates together during the welding process. Commonly owned U.S. Pat. Appl. Pub. No. 2008/0292916, incorporated by reference herein in its entirety, discloses a bipolar plate assembly that includes a first unipolar plate disposed adjacent a second unipolar plate, where the first and second unipolar plates are bonded together by a plurality of localized electrically conductive nodes. The bonds may be formed as a weld, a solder joint, a braze joint, and an adhesive. 
     There is a continuing need for a cost-effective bipolar plate assembly having an efficient and robust structure that provides an optimized electrical contact between the plates of the assembly while minimizing material usage and waste and maximizing the structural integrity of the plates. A method for rapidly producing the bipolar plate assembly applicable to optimized flowfield designs is also desired. 
     SUMMARY OF THE INVENTION 
     In concordance with the instant disclosure, a cost-effective bipolar plate assembly having an efficient and robust structure that provides an optimized electrical contact between the plates of the assembly while minimizing material usage and waste and maximizing the structural integrity of the plate is surprisingly discovered. 
     The bipolar plate assembly includes a first electroformed unipolar plate disposed adjacent a second electroformed unipolar plate. The first and second unipolar plates are bonded by a plurality of localized electrically conductive plugs by electroplated material deposited within apertures formed in the substrate onto which the unipolar plates are formed. The localized electrically conductive plugs, the first unipolar plate, and the second unipolar plate form a unitary structure. 
     In another embodiment, a fuel cell stack is provided. The fuel cell stack includes a plurality of membrane electrode assemblies arranged in a stacked configuration, each of the plurality of membrane electrode assemblies having a cathode and an anode. The fuel cell stack further includes a bipolar plate assembly disposed between adjacent membrane electrode assemblies, the bipolar plate assembly including a first unipolar plate having a first inner surface disposed adjacent a second inner surface of a second unipolar plate, the first unipolar plate and the second unipolar plate bonded by an electroforming process. 
     In another embodiment, a method for preparing the bipolar plate assembly is provided. The method includes providing a first substrate having a predetermined external surface pattern and at least one aperture formed therethrough. The first substrate is then immersed in a plating bath. The method further includes plating a predetermined thickness of a plating material on the external surface of the first substrate to form a bipolar plate, the at least one via filled by the plating material when the predetermined thickness is achieved to provide thermal and electrical conductivity between each plate of the bipolar plate assembly. The first substrate may be removed after the metal plating step. 
    
    
     
       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 schematic exploded perspective view of a PEM fuel cell stack as is known in the art; 
         FIG. 2  is a perspective view of a joined bipolar plate assembly according to an embodiment of the present disclosure; 
         FIG. 3A  is a fragmentary cross-sectional elevational view of a substrate used to form a bipolar plate assembly according to the embodiment of the invention shown in  FIG. 2 . 
         FIG. 3B  is a fragmentary perspective view of a portion of the joined bipolar plate assembly depicted by circle  3 B of  FIG. 2  showing a plug interconnecting the plate assembly; 
         FIG. 4A  is an exploded cross-sectional view of substrates used to form a bipolar plate assembly according to another embodiment of the present disclosure; 
         FIG. 4B  is an elevational cross-sectional view of a joined bipolar plate assembly according the embodiment of  FIG. 4A ; and 
         FIG. 5  is a flow chart summarization of the steps for producing a bipolar plate assembly according to the present disclosure. 
     
    
    
     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, the order of the steps is not necessary or critical. 
       FIG. 1  illustrates a PEM fuel cell stack  10  according to the prior art. For simplicity, only a two-cell stack (i.e. one bipolar plate) is illustrated and described in  FIG. 1 , it being understood that a typical fuel cell stack will have many more such cells and bipolar plates. The fuel cell stack  10  includes a pair of membrane electrode assemblies (MEAs)  12 ,  14  separated by an electrically conductive bipolar plate  16 . The MEAs  12 ,  14  and the bipolar plate  16  are stacked between a pair of clamping plates  18 ,  20  and a pair of unipolar end plates  22 ,  24 . The clamping plates  18 ,  20  are electrically insulated from the end plates  22 ,  24  by a gasket or a dielectric coating (not shown). Respective working faces  26 ,  28  of each of the unipolar end plates  22 ,  24 , as well as the working faces  30 ,  32  of the bipolar plate  16 , respectively include a plurality of grooves or channels  34 ,  40 ,  36 ,  38  adapted to facilitate the flow of a fuel such as hydrogen and an oxidant such as oxygen therethrough. Nonconductive gaskets  42 ,  44 ,  46 ,  48  provide seals and an electrical insulation between the components of the fuel cell stack  10 . Gas-permeable diffusion media  50 ,  52 ,  54 ,  56  such as carbon or graphite diffusion papers substantially abut each of an anode face and a cathode face of the MEAs  12 ,  14 . The end plates  22 ,  24  are disposed adjacent the diffusion media  50 ,  56  respectively. The bipolar plate  16  is disposed adjacent the diffusion media  52  on the anode face of the MEA  12  and adjacent the diffusion media  54  on the cathode face of the MEA  14 . 
     As shown, each of the MEAs  12 ,  14 , the bipolar plate  16 , the end plates  22 ,  24 , and the gaskets  42 ,  44 ,  46 ,  48  include a cathode supply aperture  58 , a cathode exhaust aperture  60 , a coolant supply aperture  62 , a coolant exhaust aperture  64 , an anode supply aperture  66 , and an anode exhaust aperture  68 . A cathode supply manifold is formed by the alignment of adjacent cathode supply apertures  58  formed in the MEAs  12 ,  14 , the bipolar plate  16 , the end plates  22 ,  24 , and the gaskets  42 ,  44 ,  46 ,  48 . A cathode exhaust manifold is formed by the alignment of adjacent cathode exhaust apertures  60  formed in the MEAs  12 ,  14 , the bipolar plate  16 , the end plates  22 ,  24 , and the gaskets  42 ,  44 ,  46 ,  48 . A coolant supply manifold is formed by the alignment of adjacent coolant supply apertures  62  formed in the MEAs  12 ,  14 , the bipolar plate  16 , the end plates  22 ,  24 , and the gaskets  42 ,  44 ,  46 ,  48 . A coolant exhaust manifold is formed by the alignment of adjacent coolant exhaust apertures  64  formed in the MEAs  12 ,  14 , the bipolar plate  16 , the end plates  22 ,  24 , and the gaskets  42 ,  44 ,  46 ,  48 . An anode supply manifold is formed by the alignment of adjacent anode supply apertures  66  formed in the MEAs  12 ,  14 , the bipolar plate  16 , the end plates  22 ,  24 , and the gaskets  42 ,  44 ,  46 ,  48 . An anode exhaust manifold is formed by the alignment of adjacent anode exhaust apertures  68  formed in the MEAs  12 ,  14 , the bipolar plate  16 , the end plates  22 ,  24 , and the gaskets  42 ,  44 ,  46 ,  48 . 
     A hydrogen gas is supplied to the fuel cell stack  10  through the anode supply manifold via an anode inlet conduit  70 . An oxidant gas is supplied to the fuel cell stack  10  through the cathode supply manifold of the fuel cell stack  10  via a cathode inlet conduit  72 . An anode outlet conduit  74  and a cathode outlet conduit  76  are provided for the anode exhaust manifold and the cathode exhaust manifold, respectively. A coolant inlet conduit  78  and a coolant outlet conduit  80  are in fluid communication with the coolant supply manifold and the coolant exhaust manifold to provide a flow of a liquid coolant therethrough. It is understood that the configurations of the various inlets  70 ,  72 ,  78  and outlets  74 ,  76 ,  80  in  FIG. 1  are for the purpose of illustration, and other configurations may be chosen as desired. 
       FIG. 2  shows a bipolar plate  16  according to an embodiment of the invention. The bipolar plate  16  is formed from a first unipolar plate  90  and a second unipolar plate  92 , joined together by plugs  94 . A first active surface  96  of the first unipolar plate  90  corresponds to the working face  32  ( FIG. 1 ), and acts as the cathode side of the bipolar plate  16 . A second active surface  98  of the second unipolar plate  92  corresponds to the working face  30  ( FIG. 1 ), and acts as the anode side of the bipolar plate. 
     The first active surface  96  of the first unipolar (cathode) plate  90  includes a plurality of grooves  36  adapted to distribute the reactant gases across the first active surface  96  of the cathode plate  90 . The plurality of grooves  36  define a plurality of lands  100  disposed therebetween. Similarly, the second active surface  98  of the second unipolar (anode) plate  92  includes a plurality of grooves  38  separated by lands  102 . The grooves  38  act as reactant flow paths along the lower surface  98  of the anode plate  92 . The first unipolar plate  90  and the second unipolar plate  92  cooperate upon assembly into the bipolar plate assembly  16  to form coolant channels  104 . The bipolar plate  16  shown is analogous to a stamped plate flow field pattern, whereby the upper wall  106  of the coolant channel  104  is formed by the lands  100  of the cathode plate  90 , while the lower wall  108  of the coolant channel  104  is formed by the lands  102  of the anode plate  92 . The side walls  110 ,  112  of the coolant channel  104  are formed by the sides of the grooves  36 ,  38 , as desired. The bipolar plate assembly  16  further includes a plurality of gas ports (not shown) and coolant ports (not shown), to provide inlet and outlet passages for the fuel, the oxidant, and the coolant to flow through the bipolar plate assembly  16 . A skilled artisan should understand that various configurations of the grooves  36 ,  38 , the coolant flow channels  104 , and the ports in the bipolar plate assembly  16  may be used as desired. However, straight anode, cathode, and coolant flow channels are commonly used, such as those described in commonly owned U.S. Pat. Publ. No. 2007/0275288, hereby incorporated by reference in its entirety. It should also be recognized that the present disclosure is not limited to a particular flow field pattern, but has application to bipolar plate assemblies independent of the flow field pattern. 
     The first and second unipolar plates  90 ,  92  are formed from an electrically conductive material deposited in an electroforming process. As shown in  FIG. 3A , a mold or substrate  120  is formed of a suitable material that can be removed after the electroforming process. Suitable compositions for the substrate  120  include at least one of a wax, a polymer, or a metal, although other materials may also be used. The substrate  120  is made with surface features corresponding to the grooves  36 ,  38 , and includes solid portions corresponding to the coolant flow paths  104 . It is understood that the substrate  120  may also include any other desirable flow field pattern as required in the bipolar plate assembly  16 . The substrate  120  is also formed to include a plurality of apertures or vias  122 . It is understood that the vias  122  may be circular as shown, or they may be any other desired geometric shape, such as slots. The vias  122  may be formed with chamfered inner surfaces  126  and have a minimum diameter D 1 . 
       FIG. 3B  shows a portion of the bipolar plate assembly  16  from  FIG. 2  to show one of the vias  122  and surrounding areas of the substrate  120  after an electroforming operation. To form the bipolar plate assembly  16 , the substrate  120  is placed in a metal plating bath, where a desired plating material is plated on the exterior surface  124  of the substrate  120 . The plating material is preselected to include appropriate and desired physical properties, including durability, rigidity, gas permeability, conductivity, density, thermal conductivity, corrosion resistance, definition, thermal and pattern stability, availability and cost. The substrate  120  is allowed to remain in the metal plating bath until a sufficient thickness of plating material is deposited thereon. Favorable results have been found when the plating thickness t 1  is between 10 and 50 micrometers. However, it is understood that different thicknesses t 1  may be applied as desired. 
     During the period when the substrate  120  remains in the metal plating bath, plating material also deposits along the inner chamfered surface  126  of the vias  122 . The vias  122  are sized such that the vias  122  are completely filled with the plating material during the electroforming process, so that the vias  122  are completely closed to reactant flow by plugs  94 , thereby forming a hermetic seal between the unipolar plates  90 ,  92 . The plugs  94  also serve to interconnect the unipolar plates  90 ,  92 , thereby providing both thermal and electrical conductivity therebetween. Favorable results have been found when the diameter D 1  of the vias  122  formed in the substrate  120  is approximately twice the desired plating thickness t 1  to ensure that the vias  120  are completely filled by the plating material. Favorable results have been found where a thickness t 2  of the plugs  94  is between t 1  and twice the value of t 1  (i.e. t 1 ≦t 2 ≦(2×t 1 )). 
     Once a sufficient and desirable thickness t 1  of plating material has been deposited on the substrate  120 , the substrate  120 , now including a bipolar plate assembly  16 , is removed from the metal plating bath for further processing. In one processing step, the substrate  120  is removed from between the unipolar plates  90 ,  92  by melting and draining, by oxidation, or by chemical dissolution, as appropriate. Once the substrate  120  is removed, only the bipolar plate assembly  16  remains, including both unipolar plates  90 ,  92  interconnected by plugs  94 . The distribution and number of plugs  94  may be chosen as desired. However, favorable results have been obtained using a substantially even distribution of the plugs  94 . Electroforming the bipolar plate assembly  16  on the substrate  120  advantageously allows for design flexibility of the unipolar plates  90 ,  92  that is not afforded by other manufacturing processes, such as stamping or forming. In particular, the electroforming process allows for deeper grooves  36 ,  38  and a lower reactant pressure drop across each unipolar plate  90 ,  92 , and avoids metal tearing issues accompanying a metal plate stamping process. Moreover, the electroforming process requires on the order of 10% to 50% (depending on the thickness t 1  desired) less plating material than required in a stamped plate process, and eliminates waste material in the peripheral regions of the plate assembly  16 . 
     In one embodiment, the bipolar plate assemblies  16  may be formed as individual and discrete assemblies using separate and individually crafted substrates  120 . As discrete assemblies, the plating material may then be allowed to deposit about the perimeter  130  ( FIG. 2 ,  FIG. 3A ) of the substrate  120  and form perimeter edges  132 , thereby hermetically sealing the perimeter of the bipolar plate assembly  16 . By allowing the plating material to form the sealed perimeter edges  132  of the bipolar plate assembly  16 , the need for a separate perimeter seal between the unipolar plates  90 ,  92  is eliminated. The perimeter edge  132  may be cut within the coolant headers (not shown) both to facilitate removal of the substrate  120  and to permit a coolant flow path through the bipolar plate assembly  16 , which is necessary for proper operation of the fuel cell. However, removal of the substrate  120  may occur from any location as desired. 
     Alternatively, the bipolar plate assemblies may be formed on a substrate  120  that includes repeating surface patterns of the grooves  36 ,  38 , the solid portions corresponding to the coolant flow paths  104 , or any other desirable flow field features, and the vias  122 . Repeating and adjacent bipolar plate assemblies  16  would be separated, for example, by a cutting process, after the electroforming process. Once separated, the substrate  120  would be removed by previously mentioned methods. However, the perimeter edges  132  of bipolar plate assemblies formed by a continuous substrate process may require a separate sealing action, such as by crimping, welding, bonding or any other desirable process. In this way, many bipolar plate assemblies  16  may be efficiently manufactured using the electroforming process of the present disclosure. 
     In another embodiment, shown in  FIGS. 4A and 4B , a bipolar plate assembly  16 ′ may be formed from two separately formed laminate unipolar assemblies  140 ,  142  that are subsequently bonded together during the plating process. Each of the unipolar assemblies  140 ,  142  includes a substrate  144 , 146 , respectively, formed from a preferred substrate material, such as at least one of a polymer, a composite, or a metal. Favorable results have been obtained by utilizing a polymer having a similar thermal expansion coefficient as the plating material. 
     Each substrate  144 ,  146  is prepared having desirable flow field configurations. For purposes of illustration in  FIGS. 4A and 4B , the substrate  144  of the unipolar assembly  140  corresponds to an anode plate assembly, while the substrate  146  of the unipolar assembly  142  corresponds to a cathode plate assembly. The flow field pattern of the bipolar plate assembly  16 ′ shown in  FIGS. 4A and 4B  is analogous to a composite plate flow field pattern, whereby coolant channels are formed in a pattern that is independent of the reactant flow field pattern. In particular, with reference to the anode unipolar plate  140 , a first face  148  of the anode substrate  144  includes reactant flow channels  150  that are separated by lands  152 . The lands  152  are generally wider than the reactant flow channels  150 , thereby allowing formation of coolant channels  154  on a second side  156  of the anode substrate within the lands  152 . Moreover, the coolant channels  154  are provided only within the anode second side  156 , and are not provided in the cathode unipolar plate  142 . Instead, a first side  158  of the cathode substrate  146  includes cathode reactant flow channels  160 , while a second side  162  of the cathode substrate  142  is substantially flat. 
     At least one of the substrates  144 ,  146  includes joining apertures or vias  164  to facilitate joining of the two substrates together during an electroplating process. Favorable results have been found when the joining vias  164  are located along the bottom wall  166  of the anode reactant flow channels  150 , as shown in  FIG. 4A , such that when the substrates  144 ,  146  are abutted, the vias  164  lay adjacent the flat second side  162  of the cathode substrate  144 . The vias  164  are shown as circular in cross section, but may have any desired shape, including slots to assist with alignment. Further, depending upon the material used to construct the substrates  144 , 146 , one or both of the substrates may include additional, smaller sized vias  168  that, once filled with the plating material, allow for electrical conductivity and heat rejection from the plates  140 ,  142  during operation of the fuel cell. In particular, because of the size of the small sized vias  168 , they may be omitted due to improve manufacturability of the bipolar plate assembly  16 ′. For example, as shown particularly in  FIGS. 4A and 4B , the left side lands  152 ′ omit the smaller sized vias  168 , while the right side lands  152  include the smaller sized vias  168 . Similarly, the left side flat second side  162 ′ is shown without the smaller sized vias  168 , while the right side flat second side  162  includes the smaller sized vias  168 . Therefore, the substrates  144 ,  146  include either small vias  168  or joining vias  164  to provide electrical conductivity through the plates  140 ,  142 . Additionally, as previously noted, the joining vias  164  are located adjacent the flat second side  162  of the adjacent substrate  144  to form a bond thereto to militate against coolant leakage from coolant channels  154  through the joining vias  164  in the event that the joining vias  164  are not completely filled by the plating process. 
     In practice, one of two methods is utilized to construct the bipolar plate assembly  16 ′. In a first method, the substrates  144 ,  146  are formed and then are provided with a first layer of plating material in an electroforming process. The first layer of plating material is relatively thin, on the order of 3 to 10 micrometers in thickness, and covers the entire outer surfaces  170 ,  172  of the substrates  144 ,  146 , respectively, including the inner surface  174  of the joining vias  164 . If desired, the first plating process may continue until the smaller vias  168  are filled in with plating material. Once a desired thickness of plating material is deposited, the substrates  144 ,  146  are abutted and held together, and a second plating operation is performed, wherein the joining vias  164  are entirely filled in with plating material to bond the unipolar plates  140 ,  142  together with plugs  194 . Additionally, the external perimeter  176  of the interface between the unipolar plates  140 ,  142  may also be bonded together due to the deposition of plating material, thereby sealing the external perimeter  176  against coolant leakage. Favorable results have been found when the second plating operation deposits an additional 3 to 10 micrometers of plating material, such that the entire thickness t 3  of plating material  180  deposited on each unipolar plate is between about 5 and 20 micrometers thick. However, it is understood that different thickness of material may be applied. 
     In a second method, only a single plating operation is utilized. The first and second substrates  144 , 146  are prepared as above, and are immediately abutted and held together. The abutted substrates  144 ,  146  are then subjected to a single plating operation to deposit the entire thickness of plating material  180  to the outer surfaces  170 ,  172  of the substrates  144 ,  146 . The plating operation continues until the joining vias  164  and the smaller vias  168  are entirely filled with plating material, and the perimeter  176  is bonded together to seal the interface between unipolar plates  140 ,  142 . Favorable results have been found when the larger joining vias  164  of the anode substrate  144  are aligned with and overlap a plurality of the smaller vias  168  formed on the cathode substrate  146  to create both plate-to-plate sealing and through plate conduction and sealing through the plugs  194 . As noted above, the final thickness t 3  of the plating material  180  is preferably between about 5 and 20 micrometers thick. 
       FIG. 5  summarizes the process steps necessary to form the embodiments of the present disclosure. According to the method, a first step  210  requires formation of at least one substrate having desirable flow field properties. The substrate may be formed from a removable material, or it may be formed of a material having desirable thermal characteristics intended to remain within the bipolar plate assembly. Once formed, the substrate is immersed in a metal plating bath in a second step  212  until a desirable amount of plating material is deposited at  214 . 
     Once a desirable amount of plating material is deposited, the assembly is removed from the bath in a fourth step  216 . Depending upon the embodiment, the substrate is optionally removed in a fifth step  218  as previously described by opening apertures within coolant ports or by separating continuous parts. Lastly, a sixth step  220  allows for further processing as required, such as by crimping, welding or bonding the separated plates, for example. 
     It is surprisingly found that the plurality of electrically and thermally conductive plugs  94 ,  194  provides a stable, low-electrical resistance pathway between the first unipolar plates  90 ,  140  and the second unipolar plates  92 ,  142 . One of ordinary skill should appreciate that such a pathway is now provided with an optimized quantity of material used to respectively bond the unipolar plates  90 ,  92  and  140 ,  142  together. The methods of the disclosure may also be more rapidly performed in comparison to conventional processes for preparing fully-bonded bipolar plate assemblies, and utilize significantly less material than conventional forming processes. As well, a large amount of waste material is eliminated, while the complex flow field patterns on the unipolar plates may be repetitively manufactured. Finally, extremely thin unipolar plate assemblies may be manufactured, at reduced costs over conventional plates, which minimize the overall size and cost of a fuel cell assembly. 
     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.