Abstract:
An example method of forming a fuel cell sheet includes flattening a screen to form a sheet that has a plurality of apertures operative to communicate a fluid within a fuel cell.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0001]    This invention was made with United States Government support under contract NNC06CA45C awarded by the National Aeronautics and Space Administration. The United States Government may have certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to fuel cells and, more particularly, to a porous sheet for a fuel cell. 
       DESCRIPTION OF RELATED ART 
       [0003]    Fuel cell assemblies are well known. One type of fuel cell is a solid oxide fuel cell (SOFC). Known SOFCs include a tri-layer cell having an electrolyte layer positioned between a cathode electrode layer and an anode electrode layer. An interconnector near the anode electrode layer and another interconnector near the cathode electrode layer facilitate electrically connecting the cell to an adjacent cell within a fuel cell stack. 
         [0004]    Fluids, such a fuel and oxidant, often communicate within the fuel cell through holes in porous sheets. For example, some SOFCs include supportive porous sheets between the anode interconnector and the anode electrode layer. Fuel flows between the anode electrode later and the anode interconnector through the sheet. International Publication No. WO2007/044045 to Yamanis, the contents of which are incorporated herein by reference, describes one such supportive porous sheet. 
         [0005]    One example porous sheet is 20-30% porous and includes multiple 10 micrometer diameter holes. Other example fuel cells utilize porous sheets with different porosities and hole diameters. As known, manufacturing porous sheets is often difficult. Drilling and punching operations can create individual holes, but required clearances for drilling and punching tools hamper machining multiple, closely positioned holes. These operations are also costly. Fabricating the porous sheets using powder metallurgy processes can enable closely positioning the holes, but often results in thick and heavy porous sheets that are often cumbersome to incorporate within the SOFC. 
       SUMMARY 
       [0006]    An example method of forming a fuel cell sheet includes flattening a screen to form a sheet that has a plurality of apertures operative to communicate a fluid within a fuel cell. In one example, the sheet is a porous fuel cell supporting sheet that communicates fluid to a fuel cell electrode. 
         [0007]    An example fuel cell stack assembly includes a cell and a supporting sheet formed from a flattened screen. The sheet includes a plurality of apertures configured to allow passage of a fuel cell fluid through the sheet. The sheet is a supporting sheet in one example. 
         [0008]    The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1A  shows a schematic view a fuel cell stack assembly. 
           [0010]      FIG. 1B  shows a schematic view of a solid oxide fuel cell within the  FIG. 1A  assembly. 
           [0011]      FIG. 2A  shows an example screen. 
           [0012]      FIG. 2B  shows an end view of the  FIG. 2A  screen. 
           [0013]      FIG. 3  shows an example sheet formed from the  FIG. 2A and 2B  screen. 
           [0014]      FIG. 4A  shows a top view of the porous sheet from  FIG. 3 . 
           [0015]      FIG. 4B  shows an edge view of the porous sheet from  FIG. 3   
           [0016]      FIG. 5  shows a sectional view of the  FIG. 3A  sheet within a portion of the fuel cell. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIGS. 1A and 1B , an example thick-film solid oxide fuel cell assembly (SOFC)  10  is positioned within a fuel cell stack assembly  50  between a SOFC  10   a  and a SOFC  10   b.  A first metal plate  12  and a second metal plate  14  are secured at opposing ends of the fuel cell stack assembly  50 . Electrons travel from the SOFC  10   a,  to the SOFC  10 , to the SOFC  10   b  and to the second metal plate  14  to provide electric power from the fuel cell stack assembly  50  along path  16  in a known manner. 
         [0018]    The example thick-film solid oxide fuel cell assembly (SOFC)  10  includes a tri-layer cell portion  18 , a type of cell, having an electrolyte layer  20  positioned between a cathode electrode layer  22  and an anode electrode layer  24 . The cathode electrode layer  22  is mounted adjacent a cathode interconnector  28 , which abuts a separator sheet  32   a  of the SOFC  10   a.  A separator sheet  32  of the SOFC  10  separates fuel fluid in an anode interconnector  36  from an oxidant fluid in a cathode interconnector  28   b  of the SOFC  10   b.    
         [0019]    A porous sheet  44  separates the anode electrode layer  24  of the tri-layer cell portion  18  from the anode interconnector  36 . Fuel, a type of fluid comprised of hydrogen or mixtures of hydrogen, carbon monoxide, and other gases, moves between the fluid channel corresponding to the anode interconnector  36  and the anode electrode layer  24  through a plurality of apertures in the porous sheet  44 . In this example, the porous sheet  44  also supports the tri-layer cell portion  18 . Open spaces, or fluid channels, between the porous sheet  44  and the separator sheet  32  are available for fluid flow. These open spaces are also known as the anode interconnect channels  46 . 
         [0020]    The porous sheet  44  in this example is incorporated within the SOFC  10  that has the fuel fluid contained by reliably sealed boundaries. In another example, however, the porous sheet  44  could serve as the support for the cathode electrode  22  or the electrolyte layer  20 . 
         [0021]    Referring now to example of  FIGS. 2A and 2B , a plurality of first wires  70  woven with a plurality of second wires  72  form an example screen  66 . In one example, the first plurality of wires  70  and the second plurality of wires  72  are metal wires, such as nickel wires or nickel-based alloy wires or stainless steel wires, drawn to diameters of about 25 micrometers or greater, and the wires  70 ,  72  have a generally circular cross-section. 
         [0022]    The screen  66  includes a plurality of openings  76  each having a generally rectangular geometry. The example screen  66  is a 400 mesh plain weave. That is, the example screen includes 400 wires per inch (about 180 wires per centimeter). Other example weave patterns include square, twill, Dutch, twill-Dutch, etc. As known, altering the diameter of the wires  70 ,  72 , modifying the weave pattern of the screen  66 , or both can change the profile of the openings  76 . 
         [0023]    Referring to  FIG. 3 , a first roller  80  and a second roller  84  rotate in opposite directions. The rollers  80 ,  84  are spaced such that they compress the screen  66  and flatten it as it is fed between the rollers  80 ,  84 . Flattening the screen  66  forms the porous sheet  44  by moving material to reduce the open area of the screen  66 . The screen  66  has a higher porosity than the porous sheet  44  because of the reduced open area. Other examples suitable for flattening the screen  66  include rolling the screen  66  and the porous sheet  44  multiple times with or without intermediate heat treatments to anneal cold work stresses, stamping the screen  66 , etc. Temperature, exposure time, and atmosphere for the intermediate heat treatments depend on the type and size of the wires  70 ,  72  in some examples. 
         [0024]    The rollers  80 ,  84  exert pressure on the wires,  70 ,  72 , which plastically deforms the wires  70 ,  72  and cold welds the wires  70 ,  72  together to form the porous sheet  44 . Accordingly, the porous sheet  44  is substantially monolithic. As known, ductile materials, such as those comprising the wires  70 ,  72  are especially suited for such plastic deformation. In this example, the wires  70 ,  72  are metal wires, thus, the porous sheet  44  is also metal. 
         [0025]    Referring now to  FIGS. 4A and 4B  with continuing reference to  FIGS. 2A and 2B , the flattening reduces the thickness t1 of the screen  66  to the thickness t2 of the porous sheet  44 . The apertures  62  in the porous sheet  44  have a smaller diameter d2 than the diameter d1 of the openings  76  in the screen  66  due to the material movement during the flattening. In this example, the apertures  62  are wider near a surface  78  of the porous sheet  44  due to the flattening operation. As can be appreciated from  FIG. 4B , the apertures  62  have a somewhat “hour glass” shape. 
         [0026]    A person skilled in the art and having the benefit of this disclosure would be able to adjust parameters (such as the size of the openings  76  in the screen, weave patterns, thickness t1, etc.) to produce a desired diameter d2 . . . In one example, the diameter d2 is 10 micrometers or less. Opening  62  can be tailored to have a diameter that can be bridged by sinter-reactive ceramic powders, metal powders and mixtures thereof, during the process of depositing the anode electrode layer  24  of the fuel cell assembly  10  shown in  FIG. 1B  while keeping the resistance to fuel flow through the opening  62  substantially unaffected. 
         [0027]    Referring again to  FIG. 1A , in this example, the porous sheet  44  is used as a support structure within the SOFC  10 . In some examples, strengthening the porous sheet  44  for such a supportive use includes bonding, such as by diffusion bonding, brazing or welding, the porous sheet  44  to an expanded metal sheet  86  to further strengthen the porous sheet  44 . 
         [0028]    Although the porous sheet  44  is generally described as suitable for use as a support within the SOFC  10  and for communicating fluid between the anode interconnector  36  and, the anode electrode layer  24 , other areas of the SOFC  10  and other types of fuel cells would benefit from such a sheet. 
         [0029]    Referring now to  FIG. 5 , the example porous sheet  44  and separator sheet  32  are sealed at their periphery, which encloses the anode interconnector  36  and prevents the fuel and oxidant fluids from freely mixing at their periphery. Sealing thus facilitates limiting wasteful and potentially destructive fuel combustion. 
         [0030]    In this example, the separator sheet  32  is shaped into a shallow dish  33  of a desired geometry. Other examples include other shapes, such as rectangular, square, circular and the like. The dish  33  is of sufficient depth to accommodate the anode interconnector  36  in this example. The stamped separator sheet  33 , the anode interconnector  36 , and the porous sheet  44  are assembled and bonded at  34  both at the periphery as well as at the interfaces between the anode interconnector  36  and the porous sheet  44 , and also bonded at  35  between the anode interconnector  36  and the stamped sheet  33 . Bonds  34  and  35  could be effected by means of welding, brazing, diffusion bonding or any combination thereof. 
         [0031]    Features of the disclosed example include a lightweight porous sheet having a desired porosity that is manufactured from a woven screen. 
         [0032]    Although a preferred embodiment has been disclosed, a worker of ordinary skill in this art may recognize that certain modifications are possible and come within the scope of this disclosure. For that reason, the following claims should be studied to determine the true scope of legal protection coverage.