Patent Document

FIELD OF THE INVENTION 
       [0001]    The invention relates to fuel cells, and more particularly to a bipolar plate for a fuel cell having features that militate against water accumulation within fuel cell stack headers. 
       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 the anode to the cathode 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 created by combining a pair of unipolar plates. Movement of water from the channels to an outlet header 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]    Numerous techniques have been employed to remove water from the tunnel regions and headers 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 and headers of fuel cells. However, the purge increases shutdown time of the stack and does not remove any water formed from condensation after the purge. Positioning of the stack appropriately may allow gravitational forces to remove water from the tunnel regions and headers. However, gravitational removal of water may be limited to substantially flat surfaces, surfaces having at least a minimum diameter, and surfaces of low energy. Capillary forces of the tunnel region and self wetting of the plurality of seams in the bipolar plates also militate against gravitational removal of water. Water removal by evaporation has been an undesirable technique as well. Evaporation may require costly heaters to be placed in the headers and may lead to an undesirable drying of the fuel cell stack. Additionally, evaporation may only be performed during operation of the fuel cell stack. A dry fuel cell stack militates against proton conduction and prompt starting. 
         [0005]    Water that accumulates in the tunnel regions of the fuel cell in sub-freezing temperatures may freeze after the fuel cell is shut down. Frozen water in the tunnel regions and the headers of a fuel cell may prevent the fuel cell from restarting or result in poor performance of the fuel cell until a desired operating temperature is reached. 
         [0006]    In addition to water produced from the fuel cell itself, water may enter the tunnel region of the fuel cell from an inlet or the outlet header. During fuel cell operation, liquid water may collect on the edges of the fuel cell plates that form the inlet and outlet headers. Also, the humid environment necessary for the operation of the fuel cell promotes water condensation in the headers after fuel cell shutdown. As the water accumulates on the edges of the fuel cell plates, the water also wicks along the edges forming the headers. The condensed water may wick into the tunnel region, causing one of a self wetting the tunnel region of the bipolar plates of the fuel cell and the formation of a plurality of menisci along an edge of the tunnel region. 
         [0007]    There is a continuing need for a cost effective bipolar plate for a fuel cell that facilitates the removal of water from the header of the fuel cell stack, militates against water entering the tunnel regions of the bipolar plate, and militates against the tunnel regions of the fuel cell stack from becoming blocked with frozen water. 
       SUMMARY OF THE INVENTION 
       [0008]    Presently provided by the invention, a cost effective bipolar plate for a fuel cell that facilitates the removal of water from the header of the fuel cell stack, militates against water entering the tunnel regions of the bipolar plate, and militates against the tunnel regions of the fuel cell stack from becoming blocked with frozen water is surprisingly discovered. 
         [0009]    In one embodiment, the bipolar plate for a fuel cell comprises a first plate having a first header edge defining a first header aperture, the first header edge having a first break and a second plate disposed adjacent the first plate having a second header edge defining a second header aperture, the second header edge having a second break, the bipolar plate having a plurality of tunnel outlets along one of the first header edge and the second header edge, a portion of the first break abutting a portion of the second break, the first break and first header aperture substantially aligned with the second break and second header aperture. 
         [0010]    In another embodiment, the fuel cell comprises a pair of bipolar plates for a fuel cell, each of the bipolar plates for a fuel cell having a first plate having a first header edge defining a first header aperture, the first header edge having a first break, and a second plate disposed adjacent the first plate having a second header edge defining a second header aperture, the second header edge having a second break, the bipolar plate having a plurality of tunnel outlets along one of the first header edge and the second header edge, a portion of the first break abutting a portion of the second break, the first break and first header aperture substantially aligned with the second break and second header aperture and a membrane electrode assembly disposed between the pair of bipolar plates for a fuel cell, having a third header edge defining a third header aperture, the third header edge having a third break, the third break and third header aperture substantially aligned with the first break and the second break and the first header aperture and the second header aperture. 
         [0011]    In another embodiment, the fuel cell stack comprises a plurality of fuel cells, one of the fuel cells including a pair of bipolar plates for a fuel cell, each of the bipolar plates for a fuel cell having a first plate having a first header edge defining a first header aperture, the first header edge having a first break, and a second plate disposed adjacent the first plate having a second header edge defining a second header aperture, the second header edge having a second break, the bipolar plate having a plurality of tunnel outlets along one of the first header edge and the second header edge, a portion of the first break abutting a portion of the second break, the first break and first header aperture substantially aligned with the second break and second header aperture, and a membrane electrode assembly disposed between the pair of bipolar plates for a fuel cell, having a third header edge defining a third header aperture, the third header edge having a third break, the third break and third header aperture substantially aligned with the first break and the second break and the first header aperture and the second header aperture. 
     
    
     
       DRAWINGS 
         [0012]    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: 
           [0013]      FIG. 1  is an exploded perspective view of a fuel cell stack according to an embodiment of the invention; 
           [0014]      FIG. 2  is a fragmentary perspective view of a bipolar plate for a fuel cell illustrated in  FIG. 1 ; 
           [0015]      FIG. 3  is an exploded fragmentary perspective view of a portion of an exhaust header including a water removal guide according to an embodiment of the invention; and 
           [0016]      FIG. 4  is a fragmentary perspective view of a portion of an exhaust header including a non-conductive material according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    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. 
         [0018]      FIG. 1  depicts a fuel cell stack  10  having a pair of membrane electrode assemblies  12  separated from each other by an electrically conductive bipolar plate  14 . For simplicity, only a two-cell stack (i.e. one bipolar plate) is illustrated and described in  FIG. 1 , it being understood that the typical fuel cell stack  10  will have many more cells and bipolar plates. 
         [0019]    The membrane electrode assemblies  12  and bipolar plate  14  are stacked together between a pair of clamping plates  16 ,  18  and a pair of unipolar end plates  20 ,  22 . The clamping plates  16 ,  18  are electrically insulated from the end plates  20 ,  22  by a seal or a dielectric coating (not shown). The unipolar end plate  20 , both working faces of the bipolar plate  14 , and the unipolar end plate  22  include respective active areas  24 ,  26 ,  28 ,  30 . The active areas  24 ,  26 ,  28 ,  30  are typically flow fields for distributing gaseous reactants such as hydrogen gas and air over an anode and a cathode, respectively, of the membrane electrode assemblies  12 . 
         [0020]    The bipolar plate  14  is typically formed by a conventional process for shaping sheet metal such as stamping, machining, molding, or photo etching through a photolithographic mask, for example. In one embodiment, the bipolar plate  14  is formed from unipolar plates which are then joined by any conventional process such as welding or adhesion. It should be further understood that the bipolar plate  14  may also be formed from a composite material. In one particular embodiment, the bipolar plate  14  is formed from a graphite or graphite-filled polymer. 
         [0021]    A plurality of nonconductive gaskets  32 , which may be a component of the membrane electrode assemblies  12 , militates against fuel cell leakage and provides electrical insulation between the several components of the fuel cell stack  10 . Gas-permeable diffusion media  34  are disposed adjacent the membrane electrode assemblies  12 . The end plates  20 ,  22  are also disposed adjacent the diffusion media  34 , respectively, while the active areas  26 ,  28  of the bipolar plate  14  are disposed adjacent the diffusion media  34 . 
         [0022]    The bipolar plate  14 , unipolar end plates  20 ,  22 , and the membrane electrode assemblies  12  each include a cathode supply aperture  36  and a cathode exhaust aperture  38 , a coolant supply aperture  40  and a coolant exhaust aperture  42 , and an anode supply aperture  44  and an anode exhaust aperture  46 . Supply headers  48  and exhaust headers  50  of the fuel cell stack  10  are formed by an alignment of the respective apertures  36 ,  38 ,  40 ,  42 ,  44 ,  46  in the bipolar plate  14 , unipolar end plates  20 ,  22 , and the membrane electrode assemblies  12 . The hydrogen gas is supplied to an anode supply header via an anode inlet conduit  52 . The air is supplied to a cathode supply header of the fuel cell stack  10  via a cathode inlet conduit  54 . An anode outlet conduit  56  and a cathode outlet conduit  58  are also provided for an anode exhaust header and a cathode exhaust header, respectively. A coolant inlet conduit  60  is provided for supplying liquid coolant to a coolant supply header. A coolant outlet conduit  62  is provided for removing coolant from a coolant exhaust header. It should be understood that the configurations of the various inlets  52 ,  54 ,  60  and outlets  56 ,  58 ,  62  in  FIG. 1  are for the purpose of illustration, and other configurations may be chosen as desired. 
         [0023]    The bipolar plate  14  for the fuel cell stack  10  is illustrated in  FIG. 2  The bipolar plate  14  includes a first plate  64  and a second plate  65 . The first plate  64  and the second plate  65  respectively include a first header aperture  66  and a second header aperture  67 , which may be one of a cathode supply aperture  36 , cathode exhaust aperture  38 , coolant supply aperture  40 , coolant exhaust aperture  42 , anode supply aperture  44 , and anode exhaust aperture  46 . The apertures  66 ,  67  are produced by a manufacturing process performed on the first plate  64  and the second plate  65 , such as stamping. As shown in  FIG. 3 , the first header aperture  66  of the first plate  64  is defined by a first header edge  68 . The first header edge  68  includes a first break  69 . The second header aperture  67  of the second plate  65  is defined by a second header edge  70 . The second header edge  70  includes a second break  71 . 
         [0024]    The first plate  64  includes a first header edge  68 . The second plate  65  includes a second header edge  70 . The header edges  68 ,  70  may form a bead on a primary surface  72  of both the first plate  64  and the second plate  65 . When a plurality of the plates  14  is arranged to form the stack  10 , header edges  68 ,  70  in adjacent plates  14  may act to secure one of the membrane electrode assemblies  12  and the gaskets  32  disposed on the primary surface  72 . As illustrated in  FIG. 3 , the header edges  68 ,  70  include features such as protuberances and indents that define a tunnel region  76  of the plates  14 . The breaks  69 ,  71  are located at opposing sides of the tunnel region  76 . The tunnel region  76  may include a plurality of tunnel outlets  78  formed between the first plate  64  and the second plate  65 . An edge cavity  80  having a substantially “V” shaped meeting of the first plate  64  and the second plate  65  may be formed between the header edges  68 ,  70 . During stack  10  assembly, substantial alignment of the header edges  68 ,  70  from adjacent plates  14  form the supply headers  48  and the exhaust headers  50 . 
         [0025]    The breaks  69 ,  71  form a portion of the header edges  68 ,  70 . The header apertures  66 ,  67  are substantially defined by the header edges  68 ,  70 . The breaks  69 ,  71  are defined by a secondary edge  84  extending beyond the edge cavity  80 . At least a portion of the first break  69  abuts a portion of the second break  71 . As illustrated, at least a portion of the secondary edge  84  of the first break  69  abuts a portion of the secondary edge  84  of the second break  71 . An abutment of the secondary edges  84  form a secondary edge abutment  85 . The edge cavity  80  is interrupted by the breaks  69 ,  71 , eliminating the substantially “V” shaped meeting of the first plate  64  and the second plate  65  present within the edge cavity  80 . The breaks  69 ,  71  may be substantially rectangular in shape as shown, but other break shapes such as triangular, semi-circular, and the like may be used. The breaks  69 ,  71  may be substantially rectangular in shape to optimize a length of the secondary edge abutment  85 . A rectangular shape of the breaks  69 ,  71  is preferred to eliminate an acute corner and to minimize a number of corners formed by the breaks  69 ,  71 . In particular embodiments of the invention, the length of the secondary edge abutment  85  is greater than a distance between the header edges  68 ,  70  to militate against a plurality of water droplets bridging the breaks  69 ,  71 . The plates  14  may have a plurality of breaks  69 ,  71 , separating the header edge into a first portion having the plurality of tunnel outlets  78  and a second portion having the edge cavity  80 . The breaks  69 ,  71  from adjacent plates  14  may be substantially aligned to form a water removal guide  92 . In the embodiment shown in  FIG. 3 , breaks  69 ,  71  are located at opposing sides of the tunnel region  76 . 
         [0026]    A portion of the membrane electrode assemblies  12  may be formed from a non-conductive material  86  to electrically insulate successive plates  14 . The non-conductive material  86  may be a polymeric film and in the form of a layer. The non-conductive material  86  may substantially follow an outer periphery  94  of the bipolar plate  14  and the unipolar plate  20 ,  22  as seen in  FIG. 2 . In the embodiment shown in  FIG. 3 , the non-conductive material  86  includes a plurality of third header apertures  88 , which may be one of a cathode supply aperture  36 , cathode exhaust aperture  38 , coolant supply aperture  40 , coolant exhaust aperture  42  anode supply aperture  44 , and anode exhaust aperture  46 . When a plurality of the plates  14  is arranged to form the stack  10 , header edges  68 ,  70  in adjacent plates  14  having the bead secure the non-conductive material  86 . The non-conductive material  86  disposed between the beads in adjacent plates  14  is secured when the stack  10  is compressed. The third header aperture  88  of the non-conductive material  86  includes a third break  90 . The third header aperture  88  and the third break  90  are substantially aligned with the header apertures  66 ,  67  and breaks  69 ,  71 . 
         [0027]    In the embodiment shown in  FIG. 4 , the water removal guide  92  is formed by the alignment of successive breaks  69 ,  71 ,  90  in the plates  14 . The fuel cell stack  10  may have a plurality of water removal guides  92  formed in of one of the supply headers  48  and the exhaust headers  50 . The quantity and placement of the water removal guides  92  may be tailored to suit the needs of the fuel cell stack  10 . As a non-limiting example, the supply headers  48  may not require water removal guides  92  whereas the exhaust headers  50  may incorporate a plurality of water removal guides  92 . The water removal guides  92  are substantially oriented downward, optimizing water drainage from one of the supply headers  48  and the exhaust headers  50  by the use of gravitational forces. The water removal guides  92  have a width W and a depth D. The width W, the depth D, and a size of the edge cavity  80  may be adjusted to optimize the water removal needs of the fuel cell stack  10 . The width W may be adjusted to militate against the plurality of water droplets bridging across the water removal guides  92 . As a non-limiting example, favorable water removal results have been obtained where the width W is from about 1 millimeters to about 5 millimeters and the depth D is from about 1 millimeter to about 3 millimeters. 
         [0028]    Water in the edge cavities  80  is spread along the surface in 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, for example, 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 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, and α is the channel corner angle, and in particular embodiments the angle formed by the joining of the first plate  64  having a first header edge  68  with the second plate  65  having the second header edge  70 . 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. 
         [0029]    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 shown in  FIGS. 3 and 4  of the present disclosure, the edge cavities  80  are substantially triangular and have a β+α/2&lt;90°, thereby satisfying the Concus-Finn condition. Thus, the edge cavities  80  will exhibit spontaneous wetting when exposed to water. 
         [0030]    During fuel cell stack  10  operation, water produced may be forcefully ejected from the tunnel region  76  into the exhaust headers  50  by a flow of reactants through the stack  10 . The water is collected by capillary action in the edge cavities  80 , and is spread by a self wetting of a plurality of seams existing along the header edges  68 ,  70  of the exhaust headers  50 . The water reaches one of the water removal guides  92 , where the secondary edges  84  forming the breaks  69 ,  71  militate against a self wetting and water dispersion along the edge cavities  80 . As shown in  FIGS. 3 and 4  of the present disclosure, the secondary edge abutment  85  is flat and has a β+α/2&gt;90°, thereby not satisfying the Concus-Finn condition. Thus, the secondary edge abutment  85  will not exhibit spontaneous wetting when exposed to water. The amount of water collected in the edge cavities  80  increases to form the plurality of droplets protruding from the edge cavities  80  into the water removal guides  92 . Additionally, reactant flow through the exhaust headers  50  acts to shear the droplets from the edge cavities  80  by a reactant gas shear force. The droplets, freed from the capillary forces of the edge cavities  80 , flow by gravity through the water removal guides  92 , where the water is subsequently removed from the fuel cell stack  10 . 
         [0031]    Upon shutdown of the fuel cell stack  10 , water vapor present in one of the supply headers  48  and the exhaust headers  50  may condense in the headers  48 ,  50 . The water is collected by capillary action in the edge cavities  80 , and is spread by the self wetting of the plurality of seams existing along the header edges  68 ,  70  of one of the supply headers  48  and the exhaust headers  50 . The water eventually reaches one of the water removal guides  92 , where the secondary edges  84  forming the breaks  69 ,  71  militate against the self wetting and militate against water spreading along the edge cavities  80  and into the tunnel region  76 . The water collected in the edge cavities  80  increases to form a plurality of droplets protruding from the edge cavities  80  into the water removal guides  92 . The droplets, freed from the capillary forces of the edge cavities  80 , flow by gravity through the water removal guides  92 , where the water is subsequently removed from the fuel cell stack  10 . 
         [0032]    The water removal guides  92  militate against water spreading within one of the supply headers  48  and the exhaust headers  50  and subsequently militate against water from one of entering the tunnel regions  76  and forming of a plurality of menisci along an edge of the tunnel outlets  78 . Water in the tunnel regions  76  after fuel cell stack  10  shut down may freeze, preventing the fuel cell stack  10  from restarting or result in poor performance of the fuel cell stack  10 . Water collected in the headers  48 ,  50  during operation of the fuel cell stack  10  or by condensation may be effectively removed from the stack  10  by gravitational forces. The fuel cell stack  10  incorporating water removal guides  92  includes tunnel regions  76  that are substantially water free, resulting in increases in cold start performance of the fuel cell stack  10 . 
         [0033]    It should be appreciated that the present bipolar plate  14  is cost-effective by eliminating a need for additional components that facilitate water removal from the headers  48 ,  50  of a fuel cell stack  10 . It is surprisingly found that the bipolar plate  14  is effective in militating against water entering the tunnel regions  76  of the bipolar plate  14 . The bipolar plate  14  thereby militates against the tunnel regions of the fuel cell stack  10  from becoming blocked with frozen water. 
         [0034]    In use, the water removal guides  92  provide a cost effective method of minimizing water from collecting in one of the supply headers  48  and the exhaust headers  50 , and subsequently the tunnel region  76  of the bipolar plate  14 . 
         [0035]    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.

Technology Category: 5