Abstract:
The present invention is directed to an electrochemical fuel cell. The configuration of the fuel cell stack provides a multiple-legged current flow path therethrough. Electrically isolated zones are formed in the cells and one coupled in a serial manner. This configuration provides incremental voltage summing such that additional power converting element are not needed to match the stack output to the load.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to fuel cells, and more particularly to a multi-zone increased voltage fuel cell.  
       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. More specifically, an anode membrane and a cathode membrane form a membrane electrode assembly (MEA). 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]     One characteristic of PEM fuel cells is that power is often provided at a higher current and a lower voltage than is required by the loads they are connected to. As a result, a power conversion device is incorporated between the load and the fuel cell stack to step up the voltage supplied to the load. Such power conversion devices increase cost, weight and volume of the fuel cell system.  
       SUMMARY OF THE INVENTION  
       [0005]     Accordingly, the present invention provides a fuel cell stack having a multiple-legged current flow path defined therethrough. Electrically-isolated zones can be formed in the cell and coupled in a serial manner so as to provide a voltage summing within the cell. The fuel cell stack includes a membrane electrode assembly (MEA) and a first bipolar plate having a first side adjacent to the MEA. The first bipolar plate includes at least one cathode gas flow path and at least one anode gas flow path formed in the first side. A second bipolar plate includes a second side adjacent to the MEA and at least one anode gas flow path and at least one cathode gas flow path formed in the second side and arranged opposite from the first cathode gas flow path and the first anode gas flow path respectfully across the MEA. The first cathode and the second anode gas flow paths together with a first portion of the MEA form a first leg in the current flow path. The opposing second cathode and first anode gas flow paths together with a second portion of the MEA form a second leg in the current flow path.  
         [0006]     In one feature, the fuel cell stack further includes first and second electrode pads formed on the first bipolar plate. Third and fourth electrode pads are formed on the second bipolar plate and are respectively offset from the first and second electrode pads across the MEA. The first and third electrode pads transfer current flow of the first leg in the current path and the second and fourth electrode pads transfer current flow of the second leg in the current path.  
         [0007]     In another feature, the fuel cell stack further includes a third cathode gas flow path formed in the first side of the first bipolar plate and a third anode gas flow path formed in the second side of the second bipolar plate. The third anode gas flow path is arranged opposite from the third cathode gas flow path across the MEA. The third cathode gas flow path and the third anode gas flow path define a third leg in the current flow path together with a third portion of the MEA.  
         [0008]     In another feature, the fuel cell stack further includes opposed electrode pads that are laterally offset in a staggered manner. For example, the first electrode pad is offset from the third electrode pad across the PEM to transfer current flow from the first leg to the second leg in the current flow path.  
         [0009]     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  
       [0010]     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
         [0011]      FIG. 1  is an exploded perspective view of a multi-zone fuel cell according to the present invention;  
         [0012]      FIG. 2  is an exploded cross-section of a cell from the multi-zone fuel cell;  
         [0013]      FIG. 3  is a more detailed cross section of the multi-zone fuel cell shown in  FIG. 2 ;  
         [0014]      FIG. 4A  is a schematic illustration of a traditional fuel cell;  
         [0015]      FIG. 4B  is a schematic illustration of the multi-zone fuel cell of  FIGS. 1 through 3 ;  
         [0016]      FIG. 4C  is a schematic illustration of an alternative multi-zone fuel cell according to the present invention; and  
         [0017]      FIG. 4D  is a schematic illustration of another alternative multi-zone fuel cell according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     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.  
         [0019]     Referring now to  FIG. 1 , an exploded view of a multi-zone fuel cell  10  is shown. The multi-zone fuel cell  10  includes a membrane electrode assembly (MEA)  12  disposed between bipolar plates  14 ,  16 . Layers of diffusion media  18  are disposed between the bipolar plates  14 ,  16  and the MEA  12 . As discussed in further detail below, the bipolar plates  14 ,  16  enable flow of cathode and anode gas across the fuel cell surface and through the diffusion media  18  for reaction through the MEA  12 . Gaskets  20  are disposed between the bipolar plates  14 ,  16  and the MEA  12 . The gaskets  20  seal the various fluid paths of the multi-zone fuel cell  10  as described in detail herein.  
         [0020]     As previously mentioned, bipolar plates  14 ,  16  are divided into electrically-isolated zones I, II, III which are connected in series to provide incremental voltage summing. Specifically, conductive pads  24 ,  26  and  34 ,  36  disposed on a non-conductive substrate  22 ,  32  define multiple zones which are laterally offset in a staggered relationship such that conductive pads  26 ,  36  electrically couple sequential zones. As a result, a relatively higher voltage and lower current output is generated for a given power output. Thus, by utilizing a multiple zone design, the fuel cell  10  can be configured to match its output to a given load requirement without the use of a transformer or converter.  
         [0021]     Referring now to  FIGS. 2 and 3 , bipolar plate  14  is preferably constructed with a pair of electrode plates  14 . 1 ,  14 . 2  placed in facing relationship. Electrode plate  14 . 1  includes an electrically non-conductive substrate  22 . 1  with two sets of electrically conductive pads disposed on the opposite surfaces thereof. Specifically, upper pads  24 . 11 ,  26 . 11  are formed on upper surface  28 . 11  ( FIG. 3 ) and lower pads  24 . 12 ,  26 . 12  are formed on the lower surface  28 . 12  ( FIG. 3 ). Likewise, electrode plate  14 . 2  includes an electrically non-conductive substrate  22 . 2  with upper pads  24 . 21 ,  26 . 21  formed on upper surface  28 . 21  ( FIG. 3 ) and lower pads  24 . 22 ,  26 . 22  formed on lower surface  28 . 22  ( FIG. 3 ). Pad-to-pad continuity is established by conductors  30  extending through the substrate  22  from the upper pads to the lower pads in zone I. Specifically, conductors  30 . 1  extend through substrate  22 . 1  from upper pads  24 . 11  to lower pads  24 . 12  in zone I. Likewise, conductors  30 . 2  extend through substrate  22 . 2  from upper pads  24 . 21  to lower pads  24 . 22 . The conductive pads  26 . 11  and  26 . 22  electrically connect zones II and III along a lateral conductive path. An insulating layer  27  is disposed between zones II and III to prohibit electrical communication between the bipolar plate halves  14 . 1  and  14 . 2  in zones II and III.  
         [0022]     Bipolar plate  16  is preferably constructed with a pair of electrode plates  16 . 1 ,  16 . 2  placed in facing relationship. Electrode plate  16 . 1  includes an electrically non-conductive substrate  32 . 1  with two sets of electrically conductive pads  34 . 1 ,  36 . 1  disposed on the opposite surfaces thereof. Specifically, upper pads  34 . 11 ,  36 . 11  are formed on upper surface  38 . 11  ( FIG. 3 ) and lower pads  34 . 12 ,  36 . 12  are formed on the lower surface  38 . 12  ( FIG. 3 ). Likewise, electrode plate  16 . 2  includes an electrically non-conductive substrate  32 . 2  with upper pads  34 . 21 ,  36 . 21  formed on upper surface  38 . 21  ( FIG. 3 ) and lower pads  34 . 24 ,  36 . 22  formed on lower surface  38 . 22  ( FIG. 3 ). Pad-to-pad continuity is established by conductors  40  extending through the substrate  32  from the upper pads to the lower pads in zone III. Specifically, conductors  40 . 1  extend through substrate  32 . 1  from upper pads  34 . 11  to lower pads  34 . 12  in zone III. Likewise, conductors  40 . 2  extend through substrate  32 . 2  from upper pads  34 . 21  to lower pads  34 . 22 . The conductive pads  36 . 11  and  36 . 22  electrically connect zones I and II along a lateral conductive path. An insulating layer  37  is disposed between zones I and II to prohibit electrical communication between the bipolar plate halves  16 . 1  and  16 . 2 .  
         [0023]     Conductive pad  24  on bipolar plate  14  overlays a portion of conductive pad  36  on bipolar plate  16 , while conductive pad  26  on bipolar plate  14  overlays conductive pad  34  and a portion of conductive pad  36  on bipolar plate  16 . In this manner a current flow path is established from conductive pad  26  to conductive pad  24 , through MEA  12  to conductive pad  36  back through MEA  12  to conductive pad  26  through MEA  12  to conductive pad  34 . Thus, the current flow path passes through the MEA  12  three times to establish a three-legged current flow path.  
         [0024]     Referring now to  FIG. 4A-4D , the multiple-legged current flow plate of the present invention will be discussed by comparison to a conventional single legged current flow plate.  FIG. 4A  is a schematic cross-section of a conventional fuel cell  10 ′ including an upper bipolar plate  14 ′ and a lower bipolar plate  16 ′ having an MEA  12 ′ disposed therebetween. Anode feed gas flows through flow channels of the upper bipolar plate  14 ′ and is diffused to the MEA  12 ′ through medium  18 ′. Cathode feed gas flows through flow channels of the lower bipolar plate  16 ′ and is diffused to the MEA  12 ′ through medium  18 ′. A single zone of one-legged current flow path is established through fuel cell  10 ′. This configuration yields a voltage differential of 0.6V with a current output of 400 A for a total power output of 240 W, assuming an area of 500 cm 2 .  
         [0025]      FIG. 4B  is a schematic cross-section of the fuel cell  10  illustrated in  FIGS. 1-3 . In particular,  FIG. 4B  schematically illustrates a multiple zone fuel cell stack  10  having a three-legged current flow path therethrough. To achieve this, the upper bipolar plate  14  is divided into two regions by conductive pads  24 ,  26  that are laterally offset in staggered relationship with conductive pads  34 ,  36  which likewise divide lower bipolar plate into two regions. This configuration yields a voltage differential of 1.8V with a current output of 133 A for a total power output of 240 W, again assuming an area of 500 cm 2 .  
         [0026]      FIG. 4C  is a schematic cross-section of an alternate embodiment of the present invention in which fuel cell  110  includes a two-legged current flow path therethrough. To achieve this, the upper bipolar plate  114  is divided into two regions by conductive pads  124 ,  126  that overlay a single conductive pad  136  on lower bipolar plate  116 . For a fuel cell with the same area, this configuration yields a voltage differential of 1.2V with a current output of 200 A for a total power output of 240 W.  
         [0027]      FIG. 4D  is a schematic cross-section of another embodiment of the present invention in which fuel cell  210  includes a five-legged current flow path therethrough. To achieve this, the upper bipolar plate  214  is divided into three regions by conductive pads  224 ,  226 ,  228  that are laterally offset in staggered relationship with conductive pads  234 ,  236 ,  238  which likewise divide lower bipolar plate  21 . 6  into three regions. For a fuel cell with the same area, this configuration yields a voltage differential of 3.0V with a current output of 80 A for a total power output of 240 W.  
         [0028]     Referring again to  FIGS. 2 and 3 , the details of the multi-zone fuel cell will be described further. Each electrode plate  14 . 1 ,  14 . 2  and  16 . 1 ,  16 . 2  includes flow channels formed therein. Electrode plate  14 . 1  has reactant gas flow channels  42 . 1 ,  44 . 1  formed on upper surface  28 . 11  which define cathode and anode reactant gas flow fields, respectively. Electrode plate  14 . 2  also has reactant gas flow channels  42 . 2 ,  44 . 2  formed on lower surface  28 . 22  which define anode and cathode reactant gas flow fields, respectively. Electrode plate  14 . 1  has coolant flow channels  46 . 1  formed on lower surface  28 . 12  which, in part define a coolant flow field. Electrode plate  14 . 2  has coolant flow channels  46 . 2  formed on upper surface  28 . 21  which in part define the coolant flow field.  
         [0029]     As presently preferred, electrode plates  14 . 1 ,  14 . 2  are stacked together to form a bipolar plate  14 . More specifically, lower surface  28 . 12  of one electrode plate  14 . 1  lays adjacent to upper surface  28 . 21  of electrode plate  14 . 2  such that conductive pads formed thereon are in contact with one another to provide pad-to-pad continuity therebetween. The coolant flow paths  46 . 1 ,  46 . 2  align with one another to define the coolant flow field in bipolar plate  14 .  
         [0030]     Electrode plate  16 . 1  has reactant gas flow channels  52 . 1 ,  54 . 1  formed on upper surface  38 . 11  which define anode and cathode reactant gas flow fields, respectively. Electrode plate  16 . 2  also has reactant gas flow channels  52 . 2 ,  54 . 2  formed on lower surface  38 . 22  which define cathode and anode reactant gas flow fields, respectively. Electrode plate  16 . 1  has coolant flow channels  56 . 1  formed on lower surface  38 . 12  which, in part define a coolant flow field. Electrode plate  16 . 2  has coolant flow channels  56 . 2  formed on upper surface  38 . 21  which in part define the coolant flow field.  
         [0031]     As presently preferred, electrode plates  16 . 1 ,  16 . 2  are stacked together to form a bipolar plate  16 . More specifically, lower surface  38 . 12  of one electrode plate  16 . 1  lays adjacent to upper surface  38 . 21  of electrode plate  16 . 2  such that conductive pads formed thereon are in contact with one another to provide pad-to-pad continuity therebetween. The coolant flow paths  56 . 1 ,  56 . 2  align with one another to define the coolant flow field in bipolar plate  16 .  
         [0032]     As described herein, the configuration of the reactant gas flow fields is dependent, in part, upon the number of zones within the fuel cell  10 . As such, surfaces  28 . 11 ,  28 . 12  and  28 . 21 ,  28 . 22  can include a single anode flow field and multiple cathode flow fields, a single cathode flow field and multiple anode flow fields or multiple anode and cathode flow fields. It should also be noted that all of the flow paths are schematically illustrated in  FIG. 1 . Thus, while the reactant gas flow fields are schematically illustrated as serpentine flow fields and the coolant flow fields are schematically illustrated as a series of parallel paths, it will be appreciated that the flow field designs can vary in accordance with the specifications of a particular application.  
         [0033]     The MEA  12  is sandwiched between bipolar plates  14 ,  16  and is separated therefrom by the diffusion media  18  and the gaskets  20 . The bipolar plates  14 ,  16  are positioned such that the conductive pads  26 ,  36  are staggered. More specifically, the conductive pad  26 . 22  of the upper bipolar plate  14  are laterally offset from the conductive pad  36 . 11  of the lower bipolar plate  16 . This staggered relationship of the conductive pads  26 ,  36  across the MEA  12  is repeated throughout the fuel cell stack  10  to provide multiple series current flow paths as described above.  
         [0034]     For any given cell, the reactant gas flow paths  42 . 2 ,  44 . 2  on one side of the MEA  12  are aligned with the complementary reactant gas flow paths  54 . 1 ,  52 . 1  on the opposite side thereof such that the MEA  12  is interposed between an anode flow field and a cathode flow field. For example, as shown in  FIG. 2 , the lower surface  28 . 12  of bipolar plate  14  includes an anode flow path  42 . 2  and a pair of cathode flow paths  44 . 2  cathode flow path  36 . The upper surface  38 . 21  of bipolar plate  16  includes a pair of anode flow paths  52 . 1  and a cathode flow path  54 . 1 . Formed as such, three current zones I, II, III are defined across the MEA  12 .  
         [0035]     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.