Patent Publication Number: US-2005118485-A1

Title: Bipolar plate and electrolyte application

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part of U.S. Applications Ser. Nos.: 10/302,558 and 10/302,559 both filed with the U.S. Patent and Trademark Office on Nov. 22, 2002 and both fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Filed of the Invention  
      The invention relates to a source of energy. In particular, the invention relates to a reactant flow channel formed on opposite surfaces of a bipolar plate and configured to improve chemical reaction between reactants.  
      2. Discussion of Related Prior Art  
      Due to an increasing demand on the earth&#39;s limited energy resources and to low conversion efficiencies of conventional power generation systems as well as environmental concerns, the need for a clean and reliable alternative source of energy has greatly escalated. Fuel cells have been considered for power generation applications for years. Many innovative improvements in operational performance capability have been achieved. Efficiencies have been increased; water-management problems have been resolved; and the use of proton exchange membranes with reduction of the thin film catalyst layers has been realized with the use of the High Velocity Oxygen Fuel (HVOF) thermal spray coating system to form the ion conducting layer and the electrodes layers.  
      Fuel cell assemblies with proton exchange membrane cells, in which a hydrogen-oxygen reaction is employed for power generation, have become a popular source of energy in an automobile industry. Unfortunately, the development of suitable stacked assemblies using the proton exchange membrane fuel cell has been subject to various problems, one of which is associated with either excessive humidity leading to flooding flow channels or excessive dryness indicating a slow-flowing chemical reaction.  
      The principle of operation of the bipolar fuel cell is based on a reaction between hydrogen-rich fuel breaking into ions at a membrane and electrons liberated to provide electric current and power. After providing power, the electric current joins the hydrogen ions and oxygen to produce water. The excess amount of the latter leads to flooding. Conversely, if the amount of water is insufficient, the environment is too dry indicating that the reaction between the reactants is slow. In either case, the fuel cell functions inefficiently characterized by the low cell&#39;s power output.  
      In a typical structure of a fuel cell stack, as reactants are guided along respective separate inlet and outlet channels, as disclosed in U.S. patent Ser. No. 6,551,736, the environment defined between these plates constantly changes. A particularly important characteristic of this environment is humidity. Not only structuring the surfaces of the bipolar plate with numerous channels may be cost-inefficient, but also the multiple-channel arrangement, particularly circularly-patterned channels, may contribute to unsatisfactory humidity distribution within the fuel cell.  
      It is, therefore, desirable to optimize the design of flow channels in bipolar plates of a fuel cell so that while a chemical reaction between reactants flows, no humidity is lost to the ambient with the exiting air-oxidant/oxygen from the fuel cell.  
     SUMMARY OF THE INVENTION  
      A bipolar plate with at least one corrosion-resistant metallic or ceramic coating layer over metallic, ceramic or composites plates structured to have a reactant guiding design and assembly, which is configured with a single, continuous flow channel, attains this objective. Structurally, the inventive concept is realized by providing the single flow channel constituted by a plurality of interdigitating inlet and outlet sub-channels. Thus, the reactant flow pattern is configured so that the humidity diffusion is maximized between the exiting gas flow characterized by high humidity concentration and entering gas flow with relatively low humidity gases entering the flow field. As a consequence, humidity is preserved within the cell.  
      In accordance with one structural modification of the inventive concept, the single flow channel has a squire-wave pattern of inlet sub-channels and a square or triangular wave pattern of outlet sub-channels folded together so that each of the inlet sub-channels extends parallel to at least one adjacent outlet sub-channel. A further structural modification of the inventive concept includes multiple inlet and outlet sub-channels alternating with one another to form a rectangular or spiral single continuous channel extending between a reactant inlet and outlet so that the inlet and outlet sub-channel alternate with one another and exchange humidity through diffusion. This diffusion mechanism, supported by the invented design, conserves considerable amount of humidity within the cell.  
      It is, therefore, a principle object of the invention to provide an improved design of flow channels formed in a bipolar plate and traversed by reactants. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages will become more readily apparent from the detailed description accompanied by the following drawings, in which:  
       FIG. 1  is an isometric view of the inventive fuel cell;  
       FIG. 2  is an exploded view of the fuel cell stack shown in  FIG. 1 ;  
       FIG. 3  is a cross-sectional view of the inventive bipolar plate;  
       FIG. 4  is an outside isometric view of the base plate configured in accordance with the invention;  
       FIG. 5  is an inside isometric view of the base plates of  FIG. 4 ;  
       FIG. 6  is a cross-sectional view of the inventive fuel cell stack of  FIG. 1 ;  
       FIG. 7  is an isometric view of an individual bipolar plate;  
       FIG. 8  is a schematic view of fuel conveying channels formed on one side of the bipolar plate of  FIG. 7 ;  
       FIG. 9  is a schematic view of oxygen conveying channels on the opposite side of the bipolar plate of  FIG. 7 ;  
       FIG. 10  is a diagrammatic representation of the principle of operation of the inventive membrane;  
       FIGS. 11A and 11B  are schematic and isometric views, respectively, of a continuous conveying flow channel configured in accordance with one embodiment of the invention;  
       FIG. 12  is a schematic view of the modified gas conveying channel shown in  FIGS. 12A and 12B ;  
       FIGS. 13A and 13B  are schematic and isometric views, respectively, of a continuous conveying flow channel configured in accordance with another embodiment of the invention;  
       FIG. 14  is a cross-sectional view of the of the conveying channel taken along lines X-X of  FIG. 9 ;  
       FIG. 15  is a sectional view of the bipolar plate of  FIG. 7  taken along lines XI-XI;  
       FIGS. 16A-16B  are top and perspective views of the bipolar plate formed with full or partial projections obstructing flow of a respective reactant gas; and  
       FIG. 17  is a schematic view of fuel conveying channels showing flow pattern designs developed to avoid water condensation and flooding of the membrane-electrode-assemblies (MEA) of the present invention. 
    
    
     DETAILED DESCRIPTION  
      An inventive fuel cell stack  10 , as shown in  FIGS. 1 and 2 , is configured to minimize and eliminate leakage of the reactant gases (H 2  and 0 2 /air) between juxtaposed bipolar plates  20  and between end bipolar plates  26  and a respective one of base plates  22 ,  24 . Primary external leakage-hazard regions of the fuel cell stack  10  are associated with inner manifolds  12 ,  14 ,  16 , and  18  traversed by reactant gases or reactants. In particular, a first pair of spaced inner manifolds  12 ,  14  are traversed by incoming and outgoing fuel, such as hydrogen, whereas another pair of inner manifolds are traversed by oxidant (0 2 /air) entering an inlet manifold  16  and exiting, as water, through an outlet manifold  18 . A further leakage-prone region of the fuel cell stack  10  includes an interface between base plates  22 ,  24  and end bipolar plates  26  each of which is adjacent to a respective one of the base plates. Accordingly, the inventive structure of the fuel cell stack  10  is configured to at least minimize, if not to completely eliminate, the possibility of external and/or internal gaseous leaks in the above-identified regions.  
      The fuel cell stack  10  includes a plurality of consecutive membrane-electrode-assemblies (MEA) each of which is assembled from a membrane  30  sandwiched by two electrodes (not shown) and by two bipolar plates  20 . Base plates  22  and  24  tend to compress the membrane-electrode assemblies upon applying a torque to the tie rods  28 .  
      Each individual bipolar plate  20  ( FIG. 3 ) has a structure including a metal substrate  32 , which is made preferably from aluminum or another low-resistance metal, and a metallic corrosion resistant layer  34 . Other low resistant metals suitable for the substrate  32  may further include, but are not limited to aluminum, stainless steel, inconnel, aluminum alloys, zinc, zinc alloys, magnesium, magnesium alloys. The corrosion resistant layer  34  is provided within a boundary region of the substrate upon impinging a plurality of metallic powdered particles onto a boundary region of the metal substrate at high velocities. As a result, the impinged metallic powdered particles splat across and embed in the boundary region of the metal substrate to metallurgically interlock therewith. It was found particularly advantageous to prepare the corrosion resistant layer  34  from nickel-, chromium- and carbon-based metallic powders deposited by a thermo-spray technique, including, but not limited to the high velocity oxygen fuel technology and detonation. However, even though metal-based bipolar plates  20  are particularly favored, the scope of the present invention does not exclude the use of graphite-based bipolar plates that can be particularly useful in highly acidic environment.  
      One of the structural advantages of using the metal bipolar plates  20 ,  26  stems from its excellent load-bearing characteristics. To reliably compress the bipolar plates together and, thus, to minimize and eliminate the external gas leakage between regions of juxtaposed bipolar plates  20  formed with inner manifolds  12 - 18  ( FIG. 2 ), a torque should be applied to the tie rods  28 . The higher the torque, the higher the pressure on the bipolar plates  20  and the gaskets located between the bipolar plates. However, these forces tend to deform the base plates  22 ,  24  so that each of the plates has an outwardly curved cross-section. As a result, the deformed base plates  22 ,  24  cause non-uniform distribution of compressing forces imposed on the end bipolar plates  26 . A particularly troubling consequence of the base plates&#39; repeated deformation is an inadequate compression between juxtaposed bipolar plates as well as membranes and gaskets in the vicinity of the manifolds  12 - 18  leading to the external leakage of reactant gases.  
      In accordance with one aspect of the invention, to minimize the external leakages, the base plates  22 ,  24  each have a raised central region  38  that can be cascaded in a stepwise fashion, as better seen in  FIGS. 2 and 4 . To convert the torque applied to the tie rods  28  into compressing forces, which cause the inner regions  36  ( FIG. 1 ) of the base plates  22 ,  24  to press against the regions with manifolds  12 - 18  of the bipolar plates  20 ,  26 , corners  40  ( FIG. 4 ) of the raised central region  38  each are aligned with a respective one of four manifolds  12 - 18 . Thus, although the compression forces still tend to bend the base plates  22 ,  24 , a stepwise structure of the latter resists this deformation and improves the transmission of compression forces from the base plates  22 ,  24  to the end bipolar plates  26  and further to the inner bipolar plates  20 . Hence, the components of stacked MEAs reliably urge against one another minimizing the risk of the external and/or internal gas leakage. While numerous shapes of the raised central region  38  ( FIG. 4 ) pf the base plates  22 ,  24  are envisioned within the scope of the invention, invariably this region should be configured to have its corner regions  40  aligned with the manifolds  12 - 18 .  
      According to another aspect of the invention, the fuel cell stack  10 , as shown in  FIGS. 2 and 6 , includes multiple fittings  42  (only two are shown in  FIG. 2 ). Each of these fittings is configured to provide flow communication between the reactant tank gas tanks (not shown) and the inner manifolds  12 - 18  of the fuel cell stack  10 . Conventionally, the fittings  42  are located on the base plates  22 ,  24 ; such a structure requires formation of additional manifolds in the plates guiding gases through the manifolds  12 - 18  formed in the bipolar plates. In contrast, the invention provides for the fittings  42  to be directly mounted to the end bipolar plates  26 . Hence, additional and potentially leak-hazard regions between the base plates  22 ,  24  and the end bipolar plates  26  are eliminated. Note that if not for the metal end bipolar plates  26 , such a structure would not be feasible, since the graphite-based plates would not have sufficient rigidity to support the mounted fittings.  
      Referring to  FIGS. 5 and 6 , to facilitate the assembly of the fuel cell stack  10  having a plurality of stacked MEAs, one of the base plates  22 ,  24  ( FIG. 5 ) has a plurality of peripheral channels  44  configured so that the width and depth of these channels  44  are sufficient to receive polygonal heads  46  ( FIG. 6 ) of the tie rods  28 . Advantageously, the channels  44  are configured to fully receive the polygonal heads  46 , which, thus, do not project beyond the outer surface of the base plate  24 , whereas the opposite sides  50 ,  52  ( FIG. 5 ) of each channel  44 , defining its width, flank the polygonal heads  46  to prevent them from rotating in response to a torque applied to the opposite ends of the tie rods  28 . To reliably guide the tie rods  28  between the base plates  24 ,  22 , bottoms  54  of the channels  44  are machined with a plurality of holes  48  dimensioned to allow the tie rods  28  to slide therethrough. As a consequence, during assembly of the fuel cell stack  10 , the tie rods  28  are easily and reliably inserted through the base plates  22 ,  24 . It is preferred to use corrosion resistant materials, such as stainless steel, for the inlet and outlet fittings  42  as well as for other fasteners securing the fuel cell pack tight.  
      Note that instead of four channels  44 , as shown in the drawings, it is conceived to have either a single peripheral channel. Alternatively, a multiplicity of channels each dimensioned to correspond to the dimension of the individual polygonal head  46  is still another modification conceived within the scope of the invention.  
      In order to increase power density of the fuel cell stack  10 , the polarities of adjacent fuel cells are combined together. The positive polarity of one cell combined with the negative polarity of the adjacent one form the bipolar plate  20 . The bipolar plate carries hydrogen, which is necessary for the negative polarity of the bipolar plate, and oxygen/air for its positive polarity. As known, water is a byproduct generated in the oxygen side of the bipolar plate  20 . Improper water management will decrease the power output of the fuel cell, or it could eventually stop the electrochemical operation of the fuel cell because of possible water flooding or drying out of the membrane that could cause small holes and/or cracks in the membrane.  
       FIGS. 8 and 9  show one of possible designs of gas conveying channels formed in the bipolar plate for the hydrogen side and for the oxygen side, respectively. As shown in  FIG. 8 , inlet channels  58  are in flow communication with the manifold  12  ( FIG. 2 ) and in flow communication with return channels  60  via a connecting channel  62 . The channels are designed in horizontal zigzag configuration to prolong its dwelling in the conduits  12 ,  14  and give more opportunity for reaction with oxygen to take place. The serpentine area on the oxygen side ( FIG. 9 ) is designed by pointing channels  64  communicating with the inlet manifold  16  downward such that water is drained by gravity, as indicated by an arrow  66 , through the outlet manifold  18 .  
      As mentioned before, one of the byproducts of the reaction between hydrogen and oxygen is water, which is typically accumulated on the cathode (oxygen) face of the membrane-electrode assemblies. The excessive amount of water is detrimental to the efficient power output of the fuel cell. Conversely, insufficient humidity is indicative of inefficiency of the fuel cell. To provide the desirable environment and to establish the optimal humidity, the membrane  30  ( FIGS. 1, 2 ) is selected to possess seemingly contradictory qualities: water-absorption and water-repellency.  
      Turning to  FIG. 10  illustrating the uniqueness of the membrane  30 , it can be seen that if one of adjacent gas-conveying channels  100  and  102  is relatively dry and the other is relatively humid, the membrane would serve as a media for water diffusion. Typically, the excess of water would tend to be conveyed through the membrane  30  from the relatively humid channel  102  to the relatively dry channel  100 .  
      In addition to the membrane  30 , the topography of gas conveying channels is as important for the efficiency of the fuel cell as the structure of the membrane.  FIGS. 11A-11B  illustrate a particularly advantageous configuration of the bipolar plates  20 ,  26  provided with a continuous gas conveying channel  120  having a plurality of inlet sub-channels  126 , a plurality of outlet sub-channels  128  and a transitional region  130 . The latter is the region along which the inflow of the gas reactants, as indicated by arrows I, reverses its direction to a counter-flow C. Thus, the continuous gas-conveying channel  120  includes an upstream portion defined by a plurality of inlet sub-channels, a downstream portion including the plurality of outlet sub-channels, and the intermediary portion formed in the transitional region  130 . The sub-channels alternate with one another so that typically relatively dry inlet sub-channels are positioned adjacent to relatively humid outlet sub-channels. Due to the membrane  30  ( FIG. 2 ) covering the sub-channels and close juxtaposition of the inlet and outlet sub-channels, the reactant humidity is conserved on each side of the membrane  30 . As shown in  FIGS. 11 and 11 B, the continuous gas-conveying channel  120  is uniformly dimensioned and shaped. However, in certain situations, this channel may have differently dimensioned and shaped sub-channels.  
      The continuous gas-conveying channel  120  is arranged in a generally polygonal pattern characterized by straight sub-channels. Alternatively to the single inlet/single outlet arrangement of  FIG. 11A -B,  FIG. 12  illustrates the continuous gas-conveying channel  120  longitudinally divided so that a few adjacent inlet sub-channels  150 ,  152 ,  154  follow a few consecutive outlet sub-channels  150 ′,  152 ′ and  154 ′ and conversely. Thus, while the principle of the direct juxtaposition between the inlet and outlet sub-channels remains the same, a number of these sub-channels are increased.  
      Still another modification of the above-discussed configuration includes a spiral pattern (not shown) of the continuous gas-conveying channel. Similarly to the configurations shown in  FIGS. 11A, 11B , and  12 , the transitional region is positioned in the center of the pattern, which substantially coincides with the central region of the bipolar plate  20 ,  26 .  
      Turning to  FIGS. 13A-13B , the continuous gas conveying channel  220  includes a plurality of inlet and outlet sub-channels  222 ,  224  each formed in a respective wave pattern, which is characterized by a plurality of subsequent troughs  230 ,  234  for inlet sub-channels and  232 ,  236  for the outlet sub-channels. Similarly to the configuration shown in  FIGS. 11A and 11B , the continuous gas-conveying channel  220  has a transitional region  238  formed in a corner region of the bipolar plate, which is spaced diagonally from a corner region  228  traversed by an inlet  240 . The wave pattern of each of the upstream portion of the continuous channel  220  defined by a plurality of inlet sub-channels  222  as well the downstream portion of this channel, as shown in  FIGS. 13A  and B, is squire-wave and, thus, is characterized by straight sub-channels. However, the wave pattern may include a sine-shaped pattern (not shown), wherein the troughs and peaks would be defined by curved regions of the sub-channels. Regardless of the type of the wave pattern, each of the troughs of the inlet sub-channel configuration of the continuous channel  220  receives a respective peak  250  of the outlet sub-channel configuration; conversely, peaks  260  of the inlet sub-channels are received within the troughs of outlet sub-channels. A short inlet sub-channels  242  each are juxtaposed with a respective short outlet sub-channel  244 , whereas each pair of long inlet sub-channels  246  alternate with a pair of long outlet sub-channels  248 . This configuration can be reversed by having sub-channels  242  and  244  longer than sub-channels  246  and  248 .  
      To reliably bond the metal corrosion-resistant layer  34  ( FIG. 7 ) on the metal substrate  34 , the gas-conveying channels  58 ,  60 ,  62  and  64  as well as the channels of  FIGS. 11-13  each have a V-shaped or U-shaped cross-section  68 , as shown in  FIG. 14 . A further improvement directed to minimizing the risk of gas leaks is illustrated in  FIG. 15  and includes a plurality of slanted channels  70  providing flow communication between the manifolds  12 - 14  ( 16 - 18 ) and the gas conveying serpentine  56  provided in each bipolar plate  20 ,  26 .  
      To further enhance the reaction between the gas reactants, the gas conveying channels or conduits  58 ,  64 ,  102 , and  220  have projections  80 , as illustrated in  FIGS. 16A and 16B . Flow obstruction provided by the projections  80  redirect gas flow towards the membrane  30  ( FIG. 2 ) and, thus, enhances the reaction of the reactant gases with the ambient air or electrolyte. A number and particular shape of the projections  80 , which can be configured to fully or partially block the flow, are subject to given conditions. As a result of the projections  80 , the power density output of the fuel cell pack is greatly improved due to the enhanced interaction between the reactant gases and reactant electrode assemblies.  
       FIG. 17  shows another of possible designs of the present invention of gas conveying channels formed in the bipolar plate for the hydrogen side and for the oxygen side. As shown, inlet channels  358  is in flow communication with the manifold  312  and in flow communication with the return channels  360  via connecting channels  362 . Connecting channels  362  comprise at least two folds creating three connecting sub-channels  362   a,    362   b,  and  362   c.  Unlike the configurations shown in  FIGS. 11A, 11B , and  12 , transitional regions are not positioned in the center of the pattern, but evenly spread out. Furthermore, similar to the configuration shown in  FIGS. 11A, 11B , and  12  and unlike that shown in  FIGS. 8 and 9 , this design comprises only one manifold  312  and the gas conveying channel  320 .  
      The inventive flow pattern design shown in  FIG. 17 , was developed to conserve humidity and minimize reactant pressure drop through the flow channel to avoid water condensation and flooding of the MEA. This flow pattern design is shifted off center, such that the inlet channel  358  is physically supported and backing an MEA membrane  30  ( FIG. 2 ) and electrodes by the next bipolar plate  20 ,  26  ( FIGS. 2 and 6 ) to minimize pin holes, cracking, and tearing of the polymer membrane  30  caused by pressure difference across the membrane combined with internal tensile and compression stresses in the membrane resulting from non-uniform shrinking and swelling of the membrane due to uneven humidity distribution on the MEA.  
      While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. The presently preferred embodiments described herein are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.