Abstract:
A fuel cell includes a plate system including a porous media having a surface that defines a plurality of channels configured to distribute gas throughout the plate system, and a catalyst layer in contact with the porous media. The porous media is configured to permit the gas to move from the channels, through the porous media, and to the catalyst layer.

Description:
BACKGROUND 
       [0001]      FIG. 1  illustrates a portion of a conventional fuel cell  10  in cross-section. The fuel cell  10  includes a non-porous plate  12 , a gas diffusion layer  14  in contact with the plate  12 , a catalyst layer  16  in contact with the gas diffusion layer  14  (together forming an anode), and a proton exchange membrane  18  in contact with the catalyst layer  16 . 
         [0002]    Channels  20  formed in the plate  12  are configured to direct gas, such as hydrogen, to the gas diffusion layer  14 . The gas diffuses through the gas diffusion layer (as indicated by arrow) to the catalyst layer  16 . The catalyst layer  16  promotes separation of the hydrogen into protons and electrons. The protons migrate through the membrane  18 . The electrons travel through an external circuit (not shown). 
         [0003]    Oxygen may flow to a cathode portion (not shown) of the fuel cell  10 . The protons that migrate through the membrane  18  combine with the oxygen and electrons returning from the external circuit to form water and heat. 
         [0004]      FIG. 2  illustrates a portion of another conventional fuel cell  22  in cross-section. The fuel cell  22  includes a corrugated, non-porous plate  24  with opposing surfaces  26 ,  28 , a contact plate  30  in contact with portions of the surface  26 , a gas diffusion layer  32  in contact with portions of the surface  28 , a catalyst layer  34  in contact with the gas diffusion layer  32 , and a proton exchange membrane  36  in contact with the catalyst layer  34 . 
         [0005]    Portions of the surface  26  and plate  30  define channels  33  configured to direct coolant through the fuel cell  22 . Portions of the surface  28  and the gas diffusion layer  32  define channels  35  configured to direct gas to the gas diffusion layer  32 . The gas diffuses through the gas diffusion layer  32  (as indicated by arrow) to the catalyst layer  34 . 
       SUMMARY 
       [0006]    A fuel cell includes a plate having a flow field formed therein, a catalyst layer in contact with the plate, and a proton exchange membrane in contact with the catalyst layer. The flow field is configured to distribute gas throughout the plate. The plate is configured to permit the gas to at least one of convect and diffuse from the flow field, through the plate, and to the catalyst layer. 
         [0007]    A fuel cell includes a plate at least partially defining a flow field configured to distribute gas throughout the plate, a porous matrix deposited on the plate, a catalyst layer in contact with the porous matrix, and a proton exchange membrane in contact with the catalyst layer. The porous matrix is configured to permit the gas to convect from the flow field, through the porous matrix, and to the catalyst layer. 
         [0008]    A fuel cell includes a plate system including a porous media having a surface that defines a plurality of channels configured to distribute gas throughout the plate system, and a catalyst layer in contact with the porous media. The porous media is configured to permit the gas to move from the channels, through the porous media, and to the catalyst layer. 
         [0009]    While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is an end view, in cross-section, of a portion of a conventional fuel cell. 
           [0011]      FIG. 2  is an end view, in cross-section, of a portion of another conventional fuel cell. 
           [0012]      FIG. 3  is an end view, in cross-section, of a portion of an embodiment of a fuel cell. 
           [0013]      FIG. 4  is an end view, in cross-section, of a portion of another embodiment of a fuel cell. 
           [0014]      FIG. 5  is an end view, in cross-section, of a portion of yet another embodiment of a fuel cell. 
           [0015]      FIG. 6  is a plot of example polarization curves for cathodes with and without gas diffusion layers based on geometric land area. 
           [0016]      FIG. 7  is a plot of example polarization curves for cathodes with and without gas diffusion layers based on actual land area. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    In certain proton exchange membrane fuel cells, anode and cathode gas diffusion layers allow hydrogen and air/oxygen respectively to reach catalyst layers within electrodes. Electrons and heat conduct through the gas diffusion layers, which form a link between the catalyst layers and cooling plates/collector plates. Water may also be removed via gas diffusion layers. 
         [0018]    Gas diffusion layers (which are typically made from carbon fibers or cloth) may introduce significant Ohmic resistance, have low heat conductivity, and be subjected to mechanical stresses. Ohmic resistance may contribute to electrical losses within the fuel cell circuit. Low heat conductivity may make heat management difficult within the fuel cell. Mechanical stresses may change properties, such as porosity, of the gas diffusion layers. Additionally, reactants typically have to diffuse under the gas diffusion layers to reach active areas under landing/current collectors. This may limit channel and landing/current collector width. 
         [0019]    Certain embodiments of fuel cells described herein lack gas diffusion layers. Instead, flow fields formed in porous materials support catalyst layers and/or manage water. Several benefits may result: (i) improved electrical and heat conductivity within the fuel cell—porous metals/graphite/etc. may be capable of conducting electricity and heat better than carbon based gas diffusion layers, thereby reducing Ohmic resistances and improving heat management; (ii) shortened diffusion paths of protons to catalyst layers—by removing gas diffusion layers, the path that protons traverse to reach active areas may be reduced, therefore, reducing mass transport limitations due to the flow of protons/ions; (iii) increased structural stability of the cell—having a rigid structure and high resistance to tensile and compressive stresses, porous electrodes could be made from metals or other materials to maintain their porous structure regardless of mechanical stresses they are subjected to during installation and operation; (iv) improved distribution of reactants and catalyst utilization—because reactants may no longer need to diffuse through gas diffusion layers to reach active areas under lands, landing areas may be made larger and thus can support more catalyst; and (v) reduced manufacturing cost and complexity—eliminating gas diffusion layers reduces the number of parts to be purchased and assembled. 
         [0020]    Referring now to  FIG. 3 , an embodiment of a fuel cell  38  includes a porous plate  40  (graphite, porous carbon, porous metal, etc.) having landing areas  41 , a catalyst layer  42  in contact with the landing areas  41 , and a proton exchange membrane  44  in contact with the catalyst layer  42 . A non-porous cover, layer, coating, etc.  44  (such as metallic plates, conductive glue, etc.) may be applied to outer surfaces of the plate  40 . 
         [0021]    Channels  44  formed in the plate  40  (defining a flow field) are configured to direct gas, such as hydrogen or air, through the plate  40 . In the embodiment of  FIG. 3 , the channels  44  are rectangular in cross-section and form a serpentine passageway through the plate  40 . In other embodiments, the channels  44  may take any suitable shape in cross-section and may form an interdigitated, noninterdigitated, fractal, straight-flow, etc. passageway through the plate  40 . 
         [0022]    The porosity of the plate  40  is such that the gas in the channels  44  convects and/or diffuses through the plate to the catalyst layer  42  (as indicated by arrow) and also between the channels  44 . (As known in the art, pressure gradients drive convection whereas concentration gradients drive diffusion.) The porosity of the plate  40  may range from 0.01 to 0.99 and need not be uniform. For example, the porosity of the plate  40  near the landing areas  41  may be less than elsewhere. The tortuosity of the plate  40  may be at least 1. Optimum plate porosity (distribution) and tortuosity for a given fuel cell design may be determined based on testing, simulation, etc. 
         [0023]    Because the plate  40  (instead of a gas diffusion layer) distributes reactants to the catalyst layer  42 , channels having relatively large dimensions are not necessary. As a result, smaller channels and larger landing/current collector areas may be achieved. For example, landing areas may be increased by a factor of 2 (or larger) in some configurations. Additionally, these smaller channels may remain free from flooding as the porous plate  40  may absorb any water droplets that form. 
         [0024]    Referring now to  FIG. 4 , another embodiment of a fuel cell  46  includes a porous plate  48  having landing areas  49  and channels  50  formed therein, a catalyst layer  52  deposited on the landing areas  49  and within the channels  50 , and a proton exchange membrane  54  in contact with the catalyst layer  52 . In another embodiment, some/all of the channels  50  may be formed completely within the plate  48 . That is, for a channel having a rectangular cross-section, all four walls of the channel may be defined by a surface of the plate  48 . Other configurations are also possible. 
         [0025]    In other embodiments, only portions of the channels  50  may have the catalyst layer  52  deposited thereon. Additionally, portions of the catalyst layer  52  (e.g., the catalyst layer  52  deposited within the channels  50 ) may include an ionomer to facilitate the transport of protons to and from the membrane  54 . 
         [0026]    Referring now to  FIG. 5 , yet another embodiment of a fuel cell  56  includes a corrugated, non-porous plate  58  having opposing surfaces  60 ,  62 , a contact plate  64  in contact with portions of the surface  60 , a porous matrix/coating  66  (e.g., graphite, porous carbon, porous metal, conductive plastic, etc.) deposited on the surface  62 , a catalyst layer  68  in contact with portions of the porous coating  66 , and a proton exchange membrane  70  in contact with the catalyst layer  68 . 
         [0027]    Portions of the surface  60  and plate  64  define channels  72  configured to direct coolant through the fuel cell  56 . Portions of the porous coating  66  and membrane  70  define channels  74  configured to direct gas through the fuel cell  56 . The gas may convect (and diffuse) through the porous coating  66  to the catalyst layer  68 . 
         [0028]    The porous coating  66  in the embodiment of  FIG. 5  has a thickness of 120 μm. Of course, the porous coating  66  may have any suitable thickness (e.g., a thickness ranging from 10 μm to 2 mm, etc.). The porosity of the porous coating  66  may range from 0.01 to 0.99. The tortuosity of the porous coating  66  may be at least 1. Optimum coating thickness, porosity, and tortuosity for a given fuel cell design may be determined based on testing, simulation, etc. 
         [0029]    In other embodiments, different coatings may be applied to different portions of the surface  62 . As an example, a coating having a relatively low porosity may be applied to those portions of the surface  62  that are adjacent to the catalyst layer  68  (i.e., the landing areas), while a coating having a relatively high porosity may be applied to those portions of the surface  62  that define the channels  74 . This configuration may improve current collection at the landing areas. Other configurations are also possible. For example, the porous coating  66  may be applied to only certain portions of the surface  62  (e.g., those portions adjacent to the catalyst layer  68 ). 
       Experimental Analysis 
       [0030]    Anode-side portions of fuel cells were assembled from commercially available, serpentine flow fields with 5 cm 2  active areas, 12-W series gas diffusion electrodes with 5 g Pt/m 2 , and Nafion 117 membranes. 
         [0031]    Cathode-side portions of fuel cells were of two varieties: conventional non-porous plates with conventional gas diffusion and catalyst layers, and porous graphite plates with catalyst layers supported directly on landing areas of the graphite plates. The graphite plates had dimensions of 1.9″×1.9″×⅜″, with 61% total porosity and 95% open porosity. The serpentine flow fields of the anode-side plates were replicated and machined into the cathode-side plates. 
         [0032]    A catalyst ink with a combination of 150 mg, 40% Pt/C and 1200 mg, 5% Nafion solution was prepared and sonicated to ensure better dispersion. The ink was applied to the porous plates by either brushing/spraying the ink on the landing areas or dipping the plates into the ink container. The ink on the porous plates was left to dry under a hood for 24 hours. 
         [0033]    The fuel cells were assembled and pre-conditioned by running them for 24 hours subject to 70° C. at 0.2 V with 1000 sccm air/300 sccm hydrogen with 100% RH. 
         [0034]    The effective current collector area of porous plates is significantly less than the current collector area for conventional non-porous plates. To account for this difference, active areas were normalized with plate porosity. 
         [0035]    Referring now to  FIG. 6 , example polarization curves, based on geometric land area, are plotted for (i) fuel cells having cathode-side porous graphite plates lacking gas diffusion layers and (ii) fuel cells having cathode-side conventional non-porous plates with gas diffusion layers. The fuel cells with cathode-side porous plates lacking gas diffusion layers did not include an impermeable cover, such as the cover  44  illustrated with reference to  FIG. 3 . As such, better performance would be expected in circumstances where such a cover is provided. 
         [0036]    Referring now to  FIG. 7 , example polarization curves, based on actual land area, are plotted for the fuel cells of  FIG. 6 . The fuel cells with cathode-side porous plates lacking gas diffusion layers appear to demonstrate superior performance compared with the fuel cells with cathode-side conventional non-porous plates with gas diffusion layers. 
         [0037]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.