Abstract:
A fuel cell plate having a first plate having an inlet aperture and a second plate disposed against the first forming a conduit. The fuel cell plate, well suited for use in a vehicle fuel cell stack, for reducing water retention in a fuel cell without increasing the number of required components and fabrication cost of the fuel cell plate is disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to fuel cells, and more particularly to a fuel cell plate having features that militate against liquid retention at bipolar plate inlets and outlets. 
       BACKGROUND OF THE INVENTION 
       [0002]    Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of the fuel (such as hydrogen) and the oxidant (such as air or oxygen) to generate electricity. Water is a byproduct of the electrochemical reaction. The PEM is a solid polymer electrolyte that facilitates transfer of protons from an anode electrode to a cathode electrode in each individual fuel cell of a stack of fuel cells normally deployed in a fuel cell power system. 
         [0003]    In the typical fuel cell stack, the individual fuel cells have fuel cell plates with channels, through which various reactants and cooling fluids flow. Fuel cell plates may be unipolar, for example. A bipolar plate may be formed by combining a plurality of unipolar plates. Movement of water from the channels to an outlet header and through a tunnel region formed by the fuel cell plates is caused by the flow of the reactants through the fuel cell assembly. Boundary layer shear forces and the reactant pressure aid in transporting the water through the channels and the tunnel region until the water exits the fuel cell through the outlet header. 
         [0004]    A membrane-electrolyte-assembly (MEA) is disposed between successive plates to facilitate the electrochemical reaction. The MEA includes the anode electrode, the cathode electrode, and an electrolyte membrane disposed therebetween. Porous diffusion media (DM) are positioned on both sides of the MEA, facilitating delivery of reactants, typically hydrogen and oxygen from air, for an electrochemical fuel cell reaction. 
         [0005]    Water must not be allowed to accumulate within the tunnel regions of the fuel cell because of a resulting poor performance of the fuel cell. Water accumulation causes reactant flow maldistribution in individual fuel cell plates and within the fuel cell stack. Additionally, water remaining in a fuel cell after operation may solidify in sub-freezing temperatures, creating difficulties when the fuel cell needs to be restarted. Prior solutions for effectively removing water from a fuel cell have led to increased manufacturing costs and the use of additional components. 
         [0006]    Numerous techniques have been employed to remove water from the tunnel regions of the fuel cell. These techniques include pressurized purging, gravity flow, and evaporation. A pressurized gas purge at shutdown may be used to effectively remove water from the tunnel regions of fuel cells. Conversely, this purge increases required shutdown time of the stack and wastes fuel. Positioning of the stack appropriately may allow gravitational forces to remove water from the tunnel regions. Gravitational removal of water may be limited to tunnels having at least a certain diameter. Capillary forces of the conduit and corner wetting by the well known Concus-Finn condition militate against gravitational removal of water. Water removal by evaporation is an insufficient technique as well. Evaporation may require costly heaters and may lead to an undesirable drying of the electrolyte membrane. A dry fuel cell stack militates against proton conduction and prompt starting. 
         [0007]    The use of water transport structures and surface coatings are two methods that also allow the tunnel region of a fuel cell plate to transport water into a header region of the fuel cell stack. 
         [0008]    Water transport structures, typically in the form of hydrophilic or hydrophobic foam, may be incorporated within the bipolar plate. Water transport structures may be disposed between an active region of the fuel cell and the outlet header. Water transport structures improve removal of liquid water from a fuel cell through a capillary action. While beneficial to the operation and a restart time of the fuel cell, adding water transport structures to the fuel cell stack increases the number of components required to form the bipolar plate. Fabrication and assembly costs of the fuel cell stack subsequently increase when components are added. 
         [0009]    Surface coatings may also be used to facilitate a removal of water from the fuel cell. Hydrophobic or hydrophilic surface coatings may be used to increase or decrease the surface contact angle of the bipolar plate, aiding the ability of reactant boundary layer shear forces and pressure to remove water from within the fuel cell. Hydrophobic surface coatings may also be used to militate against a film of water from forming. Coating precursors may be applied to the bipolar plate by spraying, dipping, or brushing, and formed into a hydrophobic or hydrophilic surface coating by secondary operations. Alternately, the coatings may be directly applied. While being less complex and expensive than water transport structures, surface coatings increase the fabrication costs of the bipolar plate. 
         [0010]    There is a continuing need for a cost effective fuel cell plate that facilitates a transport of water through the tunnel region of a fuel cell that is inexpensive, minimizes the number of required components, and simplifies plate manufacture. 
       SUMMARY OF THE INVENTION 
       [0011]    Presently provided by the invention, a cost effective fuel cell plate that facilitates a transport of water through the tunnel region of the fuel cell plate that minimizes the number of required components, and simplifies plate manufacture, is surprisingly discovered. 
         [0012]    In one embodiment, the fuel cell plate comprises a first plate having an inlet aperture, and a second plate abutting the first plate and forming a conduit therebetween, the conduit in fluid communication with the inlet aperture and an outlet aperture of the fuel cell plate, the conduit having a continuous seam formed between the first plate and the second plate to facilitate a transport of water to the outlet aperture. 
         [0013]    In another embodiment, the fuel cell comprises a pair of fuel cell plates, one of the fuel cell plates including a first plate having an inlet aperture, and a second plate abutting the first plate and forming a conduit therebetween, the conduit in fluid communication with the inlet aperture and an outlet aperture of the fuel cell plate, the conduit having a continuous seam formed between the first plate and the second plate to facilitate a transport of water to the outlet aperture, and an electrolyte membrane and a pair of electrodes disposed between the pair of fuel cell plates. 
         [0014]    In another embodiment, the fuel cell stack comprises a plurality of fuel cells, one of the cells having a pair of fuel cell plates, one of the fuel cell plates including a first plate having an inlet aperture, and a second plate abutting the first plate and forming a conduit therebetween, the conduit in fluid communication with the inlet aperture and an outlet aperture of the fuel cell plate, the conduit having a continuous seam formed between the first plate and the second plate to facilitate a transport of water to the outlet aperture, and an electrolyte membrane and a pair of electrodes disposed between the pair of fuel cell plates. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0015]    The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
           [0016]      FIG. 1  is a cross-sectional view of a fuel cell stack according to an embodiment of the invention; 
           [0017]      FIG. 2  is a fragmentary perspective view of a fuel cell plate from the fuel cell stack shown in  FIG. 1 ; 
           [0018]      FIG. 3  is an enlarged fragmentary perspective view of the fuel cell plate illustrated in  FIG. 2  showing a portion in section taken along section line  3 - 3 ; 
           [0019]      FIG. 4  is an enlarged fragmentary top plan view of the fuel cell plate illustrated in  FIG. 2 ; 
           [0020]      FIG. 5  is a fragmentary perspective view of the fuel cell plate illustrated in  FIG. 2 , showing a continuous seam between a first plate and a second plate; and 
           [0021]      FIG. 6  is an enlarged fragmentary cross-sectional view of a conduit of the fuel cell plate illustrated in  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]    The following detailed description and appended drawings describe and illustrate various exemplary 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. 
         [0023]      FIG. 1  shows a fuel cell assembly  10  according to an embodiment of the present disclosure. The fuel cell assembly  10  includes a plurality of stacked fuel cell plates  12 . Each of the fuel cell plates  12  includes an inlet port  14  and an outlet port  16 . Collectively, the inlet ports  14  of each of the fuel cell plates  12  form an inlet header  18  and the outlet ports  16  of each of the plates form an outlet header  20 . An inlet  22  is in fluid communication with the inlet header  18  and an outlet  24  is in fluid communication with the outlet header  20 . The fuel cell assembly  10  shown in  FIG. 1  is illustrative of an anode inlet header and anode outlet header, a cathode inlet header and cathode outlet header, and a coolant inlet header and coolant outlet header. 
         [0024]      FIG. 2  shows one of the fuel cell plates  12 , which includes a first plate  26  and a second plate  28 . The first plate  26  and the second plate  28  both include active regions  30  and inactive regions  32 . The first plate  26  and the second plate  28  may be formed from any conventional material such as stamped metal, graphite, or a carbon composite, for example. It is understood that the material of construction, size, shape, quantity, and type of fuel cell plates  12  in the fuel cell assembly  10 , and the configuration of the fuel cell plates  12  within the assembly  10 , may vary based on design parameters such as the amount of electricity to be generated, the size of the machine to be powered by the fuel cell assembly  10 , the desired volumetric flow rate of gases through the fuel cell assembly  10 , and other similar factors, for example. 
         [0025]    The second plate  28  is disposed adjacent the first plate  26  and bonded thereto by any conventional means, such as welding, an adhesive, and the like to form the fuel cell plate  12 . Disposing a membrane electrode assembly and a diffusion media between two successive fuel cell plates  12  forms an individual fuel cell. One of the first plate  26  and second plate  28  may be used for an anode side or for a cathode side of the fuel cell assembly  10 . The first plate  26  and second plate  28  may be spaced apart, and a coolant channel existing therebetween may be used for liquid cooling of the fuel cell or for creating a tunnel region  34 . 
         [0026]    The tunnel region  34  allows for reactants and water produced during the electrochemical reaction to enter the fuel cell from the inlet header  18  and exit the cell to the outlet header  20 . Features defining the tunnel regions  34  are integrally formed in the first plate  26  and the second plate  28 . The tunnel regions  34  are separated from the coolant channels through the use of welds or adhesives selectively joining the plates  26 ,  28 . As shown in  FIGS. 3 ,  4 ,  5 , and  6 , the fuel cell plate  12  may have a number of conduits  36  formed between the first plate  26  and the second plate  28  to collectively form the tunnel region  34 . The conduits  36  may be formed along a linear edge of the outlet aperture  16  or may be formed annularly around a substantially circular aperture, for example. The tunnel regions  34  may be formed around one of the inlet header  18  and outlet header  20 . 
         [0027]      FIGS. 3 and 4  show the first plate  26  to include a plurality of inlet apertures  38 . The first plate  26  may also include first plate protuberances  40  and first planar portions  41 . As shown in  FIG. 2 , the active region  30  includes a plurality of flow channels  42  which effectively distribute reactants across the active region  30 . Additionally, the flow channels  42  guide water created during the electrochemical reaction towards the inactive region  32 . Liquid water is moved through the active region  30  and inactive region  32  by reactant drag forces. The reactants and water enter the tunnel region  34  of the plate  12  through the inlet apertures  38 . The inlet apertures  38  may be formed during plate stamping or through other secondary processes, for example. A gasket may be disposed on the first planar portion  41  of the first plate  26  or other planar portion of the plate  12 . The gasket may follow a periphery of the outlet header  20  or a periphery of the plate  12 . The gasket militates against leakage of the reactants and the water from the fuel cell and electrically insulates the fuel cell plate  12  from an adjacent fuel cell plate  12 . 
         [0028]    The second plate  28  includes a plurality of second plate indentations  44 . The second plate indentations  44  may be formed along a linear edge of the outlet aperture  16  or may be formed annularly around a substantially circular aperture, for example. A leading edge  45  of the second plate indentation  44  may be substantially aligned with an edge of the inlet aperture  38 . Upon disposing the second plate  28  adjacent the first plate  26 , the second plate indentations  44  are spaced apart from the first plate  26 . A plurality of contact ridges  46  between the second plate indentations  44  defines a second plate mating surface  48  in the vicinity of the second plate indentations  44 . The second plate  28  may also include a plurality of second plate protuberances  50 , formed adjacent the second plate indentations  44  and substantially aligned with the first plate protuberances  40 . The second plate protuberances  50  provide for a cross sectional area of the conduit  36  to remain substantially constant, militating against velocity changes of a fluid moving therein. 
         [0029]    The conduits  36  are formed by a cooperation of the second plate indentations  44 , the first plate  26 , inlet apertures  38 , and a plurality of outlet apertures  52 . The conduits  36  may also include the first plate protuberances  40  and the second plate protuberances  50 . It should be understood that a contact between the first plate  26  and the second plate  28  defines a continuous seam  54  from the inlet aperture  38  to the outlet aperture  52 . The continuous seam  54  between the first plate  26  and the second plate  28  can be accomplished in various ways. For example, a planar surface of the first plate  26  can meet an edge of the second plate indentations  44 , a planar surface of the second plate  28  can meet an edge of the first plate protuberances  40 , an edge of the first plate protuberances  40  can meet an edge of the second plate indentations  44 , and the leading edge  45  of one of the second plate indentations  44  can meet an edge of one of the inlet apertures  38 . The continuous seam  54  isolates each of the conduits  36  from other of the conduits  36 . The outlet apertures  52  may be formed between the first plate  26  and the second plate  28 . As illustrated, a terminal end of the conduit  36  forms at least a portion of the outlet aperture  52 . Alternatively, the outlet aperture  52  may be singularly formed in the second plate  28 . An edge of the outlet aperture  52  maintains contact with the continuous seam  54 . Likewise, the inlet apertures  38  may be formed in the first plate protuberance  40 , provided the inlet aperture  38  edge maintains contact with the continuous seam  54 . 
         [0030]    The conduits  36  provide fluid communication between the inlet apertures  38  and the outlet apertures  52 . For example, fluid flow may include one of reactants and water movement to and from the fuel cell. The reactants may enter or exit the fuel cell during operation of the fuel cell. Water may be drained from the fuel cell by capillary flow after fuel cell operation. The discrete conduits  36  are advantageous over non-discrete conduits by preventing flow maldistribution in the tunnel region  34  and encouraging capillary flow. 
         [0031]    Capillary flow is facilitated through the use of the discrete conduits  36  of the tunnel regions  34 . The continuous seam  54  formed between the first plate  26  and the second plate  28  is an exemplary location for a process termed spontaneous wetting or spontaneous imbibition. This process, as it relates to open capillaries produced by V-shaped or triangular surface grooves, is described, in Rye et al., Langmuir, 12:555-565 (1996), hereby incorporated herein by reference in its entirety. The physical requirements to support spontaneous wetting in the corners of a flow channel are characterized by the Concus-Finn condition, β+α/2&lt;90°, where β is a static contact angle formed between a liquid surface and a solid surface. An angle α is the conduit  36  corner angle, and in particular embodiments the angle formed by the intersection of the first plate  26  with the second plate  28 . The static contact angle β is a property specific to a particular surface and material that is experimentally determined, for example, by placing a liquid droplet on the surface and recording when an equilibrium condition is met where no further spreading of the droplet occurs. 
         [0032]    As a nonlimiting example, a rectangular channel has an α/2 of 45°, which dictates that spontaneous wetting will occur when the static contact angle is less than 45°. As illustrated in  FIGS. 3  through  FIGS. 6 , the intersection between the first plate  26  and the second plate  28  have a β+α/2&lt;90°, thereby satisfying the Concus-Finn condition. Illustratively, the intersection between the first plate  26  and the second plate  28  may have an acute angle. As shown in  FIG. 6 , spontaneous wetting along the continuous seam  54  collectively forms a continuous ligament  56  of water running from the inlet aperture  38  to the outlet aperture  52 . The continuous ligament  56  facilitates a constant and uninterrupted flow of water through the conduit  36 , from a pooling surface  58  of the first plate  26  to the outlet header  20 . The continuous ligaments  56  may form on both sides of the conduit  36 . 
         [0033]    In use, the continuous ligament  56  facilitates removal of water from the tunnel region  34  and the pooling surface  58  after operation of the fuel cell stack  10 . Upon stack shutdown, water within the fuel cell may collect in the pooling surface  58  or within the tunnel region  34  due to a removal of the drag forces and pressure of reactant flow. Water within the tunnel region  34  spontaneously wets the continuous seam  54  and forms the continuous ligament  56 . Alternately, the continuous ligament  56  may be formed during operation of the fuel cell stack  10 . Water on the pooling surface  58  enters the inlet aperture  38  due to one of a formation of a liquid film and a gravitational force. The water contacts the leading edge  45  and spontaneously wets the continuous seam  54 . Capillary action continues to collect the water, spreading it along the continuous seam  54 , forming the continuous ligament  56 . Water protrudes from the outlet aperture  52 , into one of the inlet header  18  and the outlet header  20  as the volume of water forming the continuous seam  54  increases. Water protruding from the outlet aperture  52  is substantially free from the capillary forces that form the continuous ligament  56 . Water moves into one of the inlet header  18  and the outlet header  20  by one of boundary layer shear forces, gravitational forces, and capillary mechanisms. Water is continuously removed from the tunnel region  34  and the pooling surface  58  until the amount of water left cannot support the capillary based removal of water. Accordingly, water remaining after completion of the capillary based removal is an amount incapable of affecting fuel cell performance in sub-freezing conditions. 
         [0034]    Flow maldistribution may occur in non-discrete conduits due to water pooling within one or more conduits. Non-discrete conduit tunnel regions may include a conduit header, bridged conduits, or other shared plate features located before the conduit terminates in the outlet aperture  52 . These features, which may be relatively large compared to the size of the conduit itself, cause the flow velocity of reactants through the conduits to be significantly reduced. Liquid water may pool in areas having a reduced flow velocity, causing reactant gasses to bypass portions of conduits, whole conduits, or reduced flow velocity areas. Discrete conduit tunnel regions provide a substantially constant reactant flow velocity, militating against water pooling, reactant bypassing, and flow variation between the fuel cells. 
         [0035]    A cross sectional area of the conduits  36  may be selected to facilitate the removal of liquid water from the tunnel region  34 . The conduits  36  may have a substantially constant cross-sectional area for example, allowing flow velocity of the fluids moving therein to remain relatively constant. A relatively constant flow velocity in the conduit  36  militates against water from pooling. Alternately, the conduits  36  may have a decreasing cross-sectional area. A first cross-sectional area near the inlet aperture  38  may have a greater cross-sectional area than a second cross-sectional area near the outlet apertures  52 . The conduits  36  having a decreasing cross-sectional area likewise facilitate removal of liquid water therefrom by increasing the flow velocity in the conduit  36  as the fluid moves along a length of the conduit  36  towards the outlet aperture  52 . 
         [0036]    A plurality of transition portions  60  of the conduits  36  may be formed by an overlap of the first plate protuberances  40  and the second plate indentations  44 , as shown in  FIGS. 3 and 4 . The first plate protuberances  50  and the second plate indentations  44  are formed accordingly to maintain the substantially constant or decreasing cross-sectional area of the conduits  36  in the transition portions  60 . 
         [0037]    It should be appreciated that the present fuel cell plate  12  is cost-effective due to elimination of a need for additional components, such as water transport structures, surface coatings, and the like. It is surprisingly found that the fuel cell plate  12  is effective in militating against water accumulation in the tunnel region  34  of the fuel cell and reactant maldistribution. The fuel cell plate  12  thereby maximizes starting performance of the fuel cell in sub-freezing temperatures. 
         [0038]    From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.