Abstract:
A bipolar plate for use in a fuel cell stack includes a first plate having a first coolant face with a first set of coolant channels formed therein. A second plate has a second coolant face with a second set of coolant channels formed therein. The first and second coolant faces are adjacent to one another to intermittently cross-link the first and second sets of coolant channels over a region of the first and second coolant faces.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to bipolar plates for fuel cell stacks, and more particularly to a bipolar plate having cross-linked channels.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cell systems are increasingly being used as a power source in a wide variety of applications. Fuel cell propulsion systems have also been proposed for use in vehicles as a replacement for internal combustion engines. The fuel cells generate electricity that is used to charge batteries and/or to power an electric motor. A solid-polymer-electrolyte fuel cell includes a polymer electrolyte membrane (PEM) that is sandwiched between an anode and a cathode. To produce electricity through an electrochemical reaction, a fuel, commonly hydrogen (H 2 ), but also either methane (CH 4 ) or methanol (CH 3 OH), is supplied to the anode and an oxidant, such as oxygen (O 2 ) is supplied to the cathode. The source of the oxygen is commonly air.  
         [0003]     In a first half-cell reaction, dissociation of the hydrogen (H 2 ) at the anode generates hydrogen protons (H + ) and electrons (e − ). The membrane is proton conductive and dielectric. As a result, the protons are transported through the membrane. The electrons flow through an electrical load (such as the batteries or the electric motor) that is connected across the membrane. In a second half-cell reaction, oxygen (O 2 ) at the cathode reacts with protons (H + ), and electrons (e − ) are taken up to form water (H 2 O).  
         [0004]     Bipolar plates are implemented between fuel cells in the fuel cell stack to facilitate the flow of the gaseous reactants for reaction in the fuel cells. The bipolar plates also facilitate coolant flow through the fuel cell stack to regulate a temperature of the fuel cell stack. Traditional bipolar plates include discrete or inter-digitated channels that define the reactant and coolant flow fields. Such channel configurations can induce coolant maldistribution, which results in a non-uniform temperature profile (i.e., temperature variations) across the fuel cell stack. Additionally, bipolar plates are subject to compressive loads and vibration which results in stress. Accordingly, there is a need to provide improved plate design with enhanced performance over the conventional art.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, the present invention provides a bipolar plate for use in a fuel cell stack. The bipolar plate includes a first plate having a first coolant face with a first set of coolant channels formed therein. A second plate has a second coolant face with a second set of coolant channels formed therein. The first and second coolant faces are adjacent to one another to intermittently cross-link the first and second sets of coolant channels over a region of the first and second coolant faces.  
         [0006]     In one feature, the first set of cooling channels define a first wave pattern having a first amplitude and a first wavelength. The second set of cooling channels define a second wave pattern having a second amplitude and a second wavelength. The first wave pattern of the first set of coolant channels is interwoven with respect to the second wave pattern of the second set of cooling channels.  
         [0007]     In another feature, a first set of lands of the first coolant face intermittently contact a second set of lands of the second coolant face to define intermittent interface areas. The first plate and the second plate are bonded at the intermittent interface areas.  
         [0008]     In still another feature, the first plate includes a first flow field formed in a face opposite the first coolant face.  
         [0009]     In yet another feature, the second plate includes a second flow field formed in a face opposite the second coolant face.  
         [0010]     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0012]      FIG. 1  is an exploded view of a fuel cell stack including bipolar plates according to the present invention;  
         [0013]      FIG. 2  is a plan view of a reactant gas side of a cathode plate of the bipolar plate;  
         [0014]      FIG. 3  is a plan view of a reactant gas side of an anode plate of the bipolar plate;  
         [0015]      FIG. 4  is a plan view of a coolant side of the cathode plate of the bipolar plate;  
         [0016]      FIG. 5  is a plan view of a coolant side of the anode plate of the bipolar plate;  
         [0017]      FIG. 6  is a schematic illustration of cross-linked cooling channels of the anode plate and cathode plate; and  
         [0018]      FIG. 7  is a perspective view of a portion of the bipolar plate having a circular section cut from the center of the cathode plate 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
         [0020]     Referring now to  FIG. 1 , a portion of a fuel cell stack is illustrated. The fuel cell stack includes a fuel cell  10  having a polymer electrolyte membrane (PEM)  12  sandwiched between bipolar plates  14 . Diffusion media  16  are included and are disposed between the PEM  12  and the bipolar plates  14 . The bipolar plates  14  each include a cathode side  18  and an anode side  20 . An anode reactant flows across the anode side  20  and is diffused through the diffusion media  16  for reaction across the PEM  12 . A cathode reactant flows across the cathode side  18  and is diffused through the diffusion media  16  for reaction across the PEM  12 . A gasket (not shown) is disposed between the bipolar plates  14  and the PEM  12  to seal the fluid flow across the cathode side  18  and anode side  20 .  
         [0021]     Referring now to  FIGS. 2 through 7 , the bipolar plate  14  will be discussed in detail. The bipolar plate  14  includes a cathode plate  22  and an anode plate  24 . With particular reference to  FIGS. 2 and 4 , the cathode plate  22  includes the cathode side  18  having a cathode flow field defined by zigzagging cathode flow channels  26 . The cathode flow channels are separated by zigzagging lands  28 . The cathode plate  22  also includes a coolant side  30  having a coolant flow field defined by zigzagging coolant flow channels  32 . The coolant flow channels are similarly separated by zigzagging lands  34 . The cathode plate  22  includes cathode inlet and outlet headers  36  formed on either side. The cathode inlet and outlet headers  36  direct cathode reactant flow into the cathode flow channels  26 . The cathode plate  22  further includes coolant inlet and outlet headers  38 . The coolant inlet and outlet headers  38  direct coolant flow into the coolant flow channels  32 .  
         [0022]     With particular reference to  FIGS. 3 and 5 , the anode plate  24  includes the anode side  20  having an anode flow field defined by zigzagging anode flow channels  40 . The anode flow channels  40  are separated by zigzagging lands  42 . The anode plate  24  also includes a coolant side  44  having a coolant flow field defined by zigzagging coolant flow channels  46 . The coolant flow channels  46  are similarly separated by zigzagging lands  48 . The anode plate  24  includes anode inlet and outlet headers  50  formed on either side. The anode inlet and outlet headers  50  direct anode reactant flow into the anode flow channels  40 . The anode plate  24  further includes coolant inlet and outlet headers  52 . The coolant inlet and outlet headers  52  direct coolant flow into the coolant flow channels  46 .  
         [0023]     As illustrated, the cathode and anode plates  22 , 24  are stamped stainless steel plates such that the reactant gas channels  26 , 40  define the coolant lands  34 , 48  and the reactant gas lands  28 , 42  define the coolant channels  32 , 46 . The cathode and anode plates  22 , 24  are stacked adjacently and are bonded together to form the bipolar plate  14 . Bonding of the cathode and anode plates  22 , 24  can be achieved in a number of manners including, but not limited to, adhesive bonding, brazing, soldering and welding. The cathode and anode plates  22 , 24  are stacked such that the coolant sides  30 , 44  are immediately adjacent to and facing one another. Although the cathode and anode plates  22 , 24  are traditionally provided as stamped stainless steel plates, it is anticipated that the flow field designs described herein can be applied to various methods and materials for producing the cathode and anode plates  22 , 24 . Therefore, the present invention is not limited to stainless steel sheet forming.  
         [0024]     With particular reference to  FIGS. 6 and 7 , the coolant sides  30 , 44  of the cathode and anode plates  22 , 24 , respectively, are aligned such that the coolant channels  32 , 46  are interweaved across one another.  FIG. 7  illustrates a portion of the bipolar plate  14  having a circular section cut from the cathode plate  22 . The coolant channels  32  of the cathode plate  22  are shown with a solid line and the coolant channels  46  of the anode plate  24  are shown in phantom (see  FIG. 6 ).  
         [0025]     Interweaving results in the coolant channels  32 , 46  cutting across one another at a resultant angle α. The resultant angle α can vary based on the wave length (L) or amplitude (A) of the zigzagging coolant channels  32 , 46 . Interweaving is provided by zigs in the coolant channels  32  of the cathode plate  22  crossing zags in the coolant channels  46  of the anode plate  24  and vice-versa. In this manner, the lands  34  between the coolant channels  32  of the cathode plate  22  are intermittently adjacent to the lands  48  between the coolant channels  46  of the anode plate  24  in an interface area  60 , indicated as hatched areas. The cathode and anode plates  22 , 24  are bonded together at the intermittent interface areas  60 . The interface areas  60  define conductivity points across the cathode and anode plates  22 , 24  to carry electrical current through the fuel cell stack.  
         [0026]     As illustrated, the channels of the cathode and anode plates  22 , 24  have the same geometric configuration (i.e., wavelength, amplitude and resultant angle). However, the present invention contemplates using different geometric configurations for the channels of the cathode and anode plates  22 , 24 . It is further anticipated that the wavelength and/or amplitude for the channels can vary across the cathode and anode plates  22 , 24 . That is to say, a particular channel or group of channels may have a first amplitude and wavelength in one region of the cathode and/or anode plates  22 , 24  and have a second amplitude and/or a second wavelength in another region. As a result, the resultant angle α will also vary from region to region.  
         [0027]     Because the coolant channels  32 , 46  of the cathode and anode plates  22 , 24  are interweaved there are intermittent cross-link areas  62  where the coolant channels  32  are open to the coolant channels  46 . In the intermittent cross-link areas  62 , fluid flow is enabled between the coolant channels  32 , 46  of the cathode and anode plates  22 , 24 . Also, as a result of interweaving, there are intermittent non-cross-link areas  64 , where the coolant channels  32  of the cathode plate  22  are open to the lands  48  of the anode plate  24  and the coolant channels  46  of the anode plate  24  are open to the lands  34  of the cathode plate  22 . In the intermittent non-cross-link areas  64 , fluid flow is limited to the respective coolant channels  32 , 46  and no cross-linking occurs. Thus, a three-dimensional or multi-layer coolant flow field is defined and includes two elevations. The first elevation is defined by the coolant channels  32  of the cathode plate  22  and the second elevation is defined by the coolant channels  46  of the anode plate  24 . Generally, two-dimensional or planar flow exists in the non-cross-link areas  64  at one of the two elevations. Coolant flow in the third dimension exists in the cross-link area  62  where coolant may flow between the first and second elevations. The three-dimensional multi-layer flow field enables gaseous, acqueous or two-phase temperature regulation of the fuel cell stack.  
         [0028]     The bipolar plates  14  are positioned on either side of the PEM  12  such that adjacent reactant flow fields are mostly aligned. That is to say, the bipolar plates  14  are positioned such that the zigzagging cathode flow channels  26  of the cathode side  18  are mostly aligned with the zigzagging anode flow channels  40  of the anode side  20 . Thus, there is no or limited interweaving between the cathode flow channels  26  and anode flow channels  40  across the PEM  12 . This is achieved by rotating every other bipolar plate  14  180° in the plane of the plate surface relative to the bipolar plate  14  on the other side of the PEM  12 .  
         [0029]     Humidification of outlet reactant streams to inlet reactant streams is improved when inlet and outlet headers  36 , 50  are oriented such that the reactant fluids flow in a counter-flow manner. As used herein, the term counter-flow indicates the anode fluid flowing across the PEM  12  in a direction opposite to the cathode fluid flow. It is also anticipated that the reactant fluid flow can be provided in a co-flow manner. As used herein, the term co-flow indicates the anode fluid flowing across the PEM  12  in the same direction as the cathode fluid flow. The fuel cell stack can be arranged either as co-flow or counter-flow depending on which headers  36 , 50  are chosen as inlets and outlets.  
         [0030]     Similarly, the coolant flow can be directed in one of either a counter-flow manner, co-flow manner or a cross-flow manner, relative to the reactant fluid flows. As used herein, cross-flow indicates coolant fluid flowing in a direction transverse to the direction of the reactant fluid flow. Cross-flow can include a transverse direction anywhere between greater than 0° (i.e., parallel) up to 90° (i.e., perpendicular) relative to the reactant fluid flow direction. In order to achieve a cross-flow coolant fluid direction with either the co-flow or counter-flow reactant fluid direction, the amplitude (i.e., interweaving) of the coolant channels  32 , 46  is increased, thereby effectively increasing the resultant angle α. Increasing the amplitude of the coolant channels  32 , 46  increases the number of cross-link areas  62  across a greater number of coolant channels  32 , 46 .  
         [0031]     Interweaving of the channels of the cathode and anode plates  22 , 24  results in a uniform stiffness in the compression sensitive areas of the bipolar plate  14 . The adjacent channels are oriented such that each land acts as a brace for multiple adjacent channels. The wavelength and amplitude of the channels can be modified to optimize plate stiffness in a given area. Adjacent alignment of the reactant flow fields across the PEM  12  improves distribution of compressive loads across the fuel cell stack. More particularly, the force that the bipolar plate  14  imparts on the PEM  12  via the flow channel lands are optimized to ensure that the lands have a more uniform response or deflection when load is applied. This uniformity improves vibration performance and reduces the local stress on the PEM  12  and other components of the fuel cell stack. In this manner, durability of the fuel cell stack is increased. Additionally, the current density through the contact points can also be reduced to provide reduced contact resistance losses.  
         [0032]     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.