Abstract:
A separator plate for use in a fuel cell stack in a fuel cell device includes a porous core with a metal layer on either side of the porous core. The metal layer has through holes formed therein such as by perforation. The metal layers are contoured to provide flow field channels, and the porous layer may have channels formed therein that are parallel to the metal layers that can be used for cooling water. A monopolar fuel stack includes twin cell units that include a center separator plate, a pair of membrane electrode assemblies, one on each side of the center separator plate, and a pair of outer plates which may have through holes formed therein, one on each side of the membrane electrode assemblies opposite the center separator plate. The outer plates cover substantially an entire electrode to which they are adjacent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Application Ser. No. 61/176,550 filed May 8, 2009. The entirety of that provisional application is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    Fuel cell-based power system technology is a promising, highly efficient electricity generation technology. This technology has been demonstrated in various applications. Among all type of fuel cells, low temperature (typically 50°-80° C.) fuel cells, such as Nafion-based proton exchange (polymer electrolyte) membrane (PEM) fuel cells, have demonstrated the broadest market potential in portable, stationary and automobile applications. 
         [0003]    The core component of low temperature fuel cell power systems is the fuel cell stack that converts the chemical energy of the fuel to electrical energy through electrochemical reactions. Each fuel cell stack comprises multiple fuel cells to deliver high power (voltage and current) for various applications. Each fuel cell include an electrolyte membrane with a cathode electrode on one side and an anode electrodes on the opposite side (collectively referred to as the membrane electrode assembly, or MEA) for electrochemical reactions, and gas diffusion layers (GDL) adjacent to each of the electrodes for gas diffusion in and out of electrodes. This combined MEA and GDL structure is sometimes referred to as the MEA/GDL. The cells in a stack are separated by separator plates. The separator plates, as the structural components of the fuel cell stack, have two major functions. The first is to serve the electrical connector between cells; the other is the fuel and air separator in the stack. 
         [0004]    The most common high power fuel cell stack design is based on a bipolar separator plate structure, wherein each separator plate is in contact with the GDL adjacent an anode (fuel electrode) on one side, and the GDL adjacent a cathode (air electrode) on the other side, as disclosed in U.S. Pat. No. 3,134,696. Electrons generated from the electrode reactions vertically pass through the body of the separator plate to connect the adjacent cells. The bipolar separator plate has flow channels for air and fuel distribution on the outer surfaces of the plate, and in some embodiments has cooling channels (which may conduct water or another thermally conductive substance) inside the plates for the thermal management of the stack, as disclosed in U.S. Pat. No. 3,392,058. 
         [0005]    The common bipolar separator plate is made of solid materials, such as graphite, or graphite/polymer composite. This type of fuel cell stack has the advantage of a simple structure. However, the challenge of this stack design is the difficulties of removing liquid water on the cathode, resulting in the flooding of cathode. The flooding blocks the airflow to the cathode, which leads to the degradation of fuel cell performance. The possible solutions to the flooding issue in solid bipolar plates can be proper, complicated flow field design, as disclosed in U.S. Pat. No. 5,773,160, and high flow rate unsaturated air to remove water from flow field channels, as disclosed in U.S. Pat. No. 5,441,819. 
         [0006]    Another type of bipolar separator plate has a porous structure. The pores wick the liquid water out of cathode by capillary action, and keep the gas flow field channels free of liquid water, as disclosed in U.S. Pat. No. 4,876,162, and U.S. Pat. No. 4,543,303. This stack design has excellent water management capability during stack operation. The challenge with this stack design is the high cost to fabricate the plates with the micro-sized porous structure necessary to keep high “bubble pressure” for the water management. 
         [0007]    Another type of fuel cell stack is the so-called mono-polar fuel cell stack, as disclosed in U.S. Pat. No. 7,585,577. Both sides of the separator plates in this type of stack are in contact with the same gas (fuel or air). Electrons generated from the electrode reaction flow through the electrode, gas diffusion layer or separator plates in planar directions relative to the edge of the cell, and are connected to the adjacent cells on the sides of the cells. The challenge of this stack design is the long passage for electron transport through the electrode, resulting the high internal electrical resistance. Therefore, this stack design is mainly used for low power fuel cell stacks. 
         [0008]    Therefore, there is a need for advanced high power fuel cell stack designs that can effectively manage water transport in the stack, and have low component fabrication and stack assembly cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIG. 1  is a schematic drawing of a bipolar fuel stack. 
           [0010]      FIG. 2A  is a schematic drawing of the cross section of a composite metal separator plate with center channels according to one embodiment. 
           [0011]      FIG. 2B  is a schematic drawing of the cross section of a composite metal separator plate without center channels according to a second embodiment. 
           [0012]      FIG. 3  is a schematic drawing of the cross section of a fuel cell stack using composite metal separator plates of  FIG. 2A  to build a bipolar plate structured stack according to a third embodiment. 
           [0013]      FIG. 4A  is a schematic drawing of the cross section of a twin-cell unit with a composite metal plate with the center channel as the center plate, and metal plates as the outside plates according to a fourth embodiment. 
           [0014]      FIG. 4B  is a schematic drawing of the cross section of the twin-cell unit with a composite metal plate without the center channel as the center plate, and metal plates as the outside plates according to a fifth embodiment. 
           [0015]      FIG. 4C  is a schematic drawing of the cross section of the twin-cell unit with a metal plate as the center plate and metal plates as the outside plates according to a sixth embodiment. 
           [0016]      FIG. 5  A is a schematic drawing of the cross section of a fuel cell stack using the twin-cell units with solid metal outside walls according to a seventh embodiment. 
           [0017]      FIG. 5B  is a schematic drawing of the cross section of a fuel cell stack using the twin-cell units with perforated metal outside walls according to an eighth embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In the following detailed description, a plurality of specific details, such as types of materials and dimensions, are set forth in order to provide a thorough understanding of the preferred embodiments discussed below. The details discussed in connection with the preferred embodiments should not be understood to limit the present inventions. Furthermore, for ease of understanding, certain method steps are delineated as separate steps; however, these steps should not be construed as necessarily distinct nor order dependent in their performance. 
         [0019]    A fuel cell stack  10  disposed in a container  19  is shown in  FIG. 1 . The fuel cell stack  10  includes three MEA/GDLs, each comprising a proton exchange membrane  11  with an anode  12  and a cathode  13  on opposite sides of the PEM  11  to form MEAs, and gas diffusion layers  14  adjacent the MEAs on opposite sides. Separator plates  15  are disposed between adjacent MEA/GDLs, and end plates  16  are present on opposite ends of the fuel stack  10  formed by the three MEA/GDLs. The separator plates  15  are referred to as bi-polar separator plates as they have an anode  12  on one side and a cathode  13  on the other. Fuel cell stacks with mono-polar separator plates in which the anode and cathode are swapped in adjoining MEAs are also known in the art as discussed above. Either of these types of fuel cell stacks may be combined with additional components (manifolds, etc., not shown in  FIG. 6 ) to form fuel cell devices as is well known in the art. 
         [0020]    An improved separator plate useful in fuel stacks such as those described above is built as a composite plate with at least one perforated outer metal layer and an inner layer of porous material. The outer surface of the metal layer is in contact with the MEA/GDL to transport electrons for electrode reaction, and gas distribution, through channels formed by the metal layer. At least one of the metal outer layers has through holes for water or moisture transport between the inner porous layer and the electrode on the metal outer surface. The through holes could be formed using perforated metal, expanded metal, or chemically etched metal techniques or by any other means. The porous layer is used to wick water out of electrode/GDL and flow field channels using capillary action, and transport extra water out of fuel cell active area through the porous sheet using gravity. The liquid water wicked in the porous layer will also evaporate back to the gas channels, keeping a high relative humidity (which could be over 50%, over 75%, over 90% or close to 100%) all cross the electrode active areas, including hot areas and hot spots, to prevent the de-hydration of the electrolyte membrane. The material in the porous layer may be polymer, ceramic, carbon, or any other low cost, non-metallic material. In preferred embodiments, the porous material is electrically non-conductive. 
         [0021]    In one embodiment, two formed metal sheets  101 , and two porous layers  102  are joined together to form a composite metal separator plate  100  as shown in  FIG. 2A . At least one of the metal sheets  101  has through holes at the center. The porous layer  102  is attached on the back sides of the perforated metal sheets  101  between the two perforated metal sheets  101 . Flow field channels  103  on the surfaces of the composite metal plates  100  facilitate fuel cell electrode reactions. The center channel  104  is for cooling water, or serves as the pre-heating/pre-humidifying channels for feeding gases. Two metal sheets  101  may be joined together at the edge by welding, clamping or any other suitable means. This composite metal plate  100  may be used in both bi-polar and mono-polar fuel cell stacks. 
         [0022]    In another embodiment, two formed metal sheets  201  and one porous layer  202  are joined together to form a composite metal separator plate  200 , as shown in  FIG. 2B . At least one of the metal sheets  201  has through holes at the center. The porous layer  202  is attached on the back sides of the metal sheets  201 , between the two metal sheets  201 . The flow-field channels  203  on the surface of the composite metal plates  200  facilitate fuel cell electrode reactions. Two metal sheets  201  may be joined together at the edge by welding, clamping or other means. This composite metal plate  200  is mainly used for monopolar structured fuel cell stacks, although it could be used as the separator plates for bipolar structured fuel cell stacks. 
         [0023]    In some embodiments, the composite metal plate  100  of  FIG. 2A  (shown as  301  in  FIG. 3 ) is used as bipolar separator plates to build a fuel cell stack  300  as shown in  FIG. 3 . The fuel stack  300  includes an MEA/GDL  302  for electrode reactions. End plates  303  are provided on each end of the stack  300 . The composite metal plate  301  includes center channels  304  for cooling water (same as  104  in  FIG. 2A ), and flow field surface channels  305  (same as  103  in  FIG. 2A ) for gas distribution. In the stack  300 , one side of the composite metal plates  301  is in contact with the cathode of one adjacent cell, and the other side of the composite plate  301  is in contact with the anode of the other adjacent cell. Water generated from the electrode reactions will be wicked away from the electrode and gas flow field channel  305 , and carried away from the stack  300  with the cooling water in center channel  304 . The small pore size of the porous layer of the composite metal plate  301  will keep the water inside the center channel  304 , without flooding the gas flow field channels  305 . 
         [0024]    In another embodiment of the invention, the composite metal plate  100  is used as the center plate to construct a twin-cell unit  400 A, as shown in  FIG. 4A . The center metal plate  401 A is the composite metal plate with the center channels (same as  100  in  FIG. 2A ). The twin cell unit  400 A includes two MEA/GDLs  402 A for electrode reactions. The center metal plate  401 A is in contact with the same electrode (cathode or anode) of the two MEAs  402 A and functions as the separator of two MEAs and the electrical current collector for electrode reactions on the electrodes. One of two outside metal plates  403 A is in contact with each of the other electrodes of the MEAs  402 A. The two outside metal plates  403 A sandwich the center composite metal plate  401 A and two MEA/GDLs  402 A to build a solid twin-cell unit  400 A. The outside plates  403 A function as the electrical current collectors and provide mechanical support for the whole twin cell structure. The outside metal plates  403 A may be solid (i.e., no through holes) or may have through holes. The outside metal plates  403 A may be joined at the edges by welding, clamping or other means or may be electrically connected by an additional member (not shown in  FIG. 4A ). The channels  404 A on both sides of the MEA/GDLs  402 A are gas (fuel and air) distribution channels. The center channel  405 A of the composite plate  401 A could be used as the pre-heating and pre-humidifying channel for the input gases. The twin cell unit  400 A is the basic unit for mono-polar separator plate structured fuel cell stacks. 
         [0025]    In another embodiment of the invention, the composite metal plate  200  is used as the center separator/current collector plate to construct a twin-cell unit  400 B, as shown in  FIG. 4B . Each twin-cell unit  400 B includes two MEA/GDLs  402 B and a center metal separator/current collector plate  401 B (which is the same as the plate  200  of  FIG. 2B ) positioned such that it is in contact with the same electrode (cathode or anode) of each of the two MEA/GDLs  402 B. Outside metal current collector plates  403 B are in contact with the other electrode of the MEA/GDLs  402 B. The two outside metal current collector plates  403 B sandwich the center composite metal plate  401 B and two MEA/GDLs  402 B to form and provide structural support the solid twin-cell unit  400 B. The outside metal plates  403 B may be joined at the edges by welding, clamping or other means, or may be electrically connected by an additional member (not shown in  FIG. 4B ). The flow field channels  404 B on both sides of the MEA/GDL  402 B are gas (air and fuel) distribution channels. The twin cell unit  400 B is the basic unit for mono-polar separator plate structured fuel cell stacks. 
         [0026]    In one further embodiment of the invention, a metal plate  401 C is used as the center separator/current collector plate to construct a twin-cell unit  400 C, as shown in  FIG. 4C . The center metal separator/current collector plate  401 C is a simple metal sheet without a porous layer. Each twin cell unit  400 C includes two MEA/GDLs. Center metal separator/current collector plate  401 C is in contact with the same electrode (cathode or anode) of the adjacent MEAs. Outside metal current collector plates  403 C are in contact with the other electrode of the MEAs  402 C, and have through holes (formed by any of the methods discussed above) at the active area for gas transport. Outside metal current collector plates  403 C also provide structural support for the fuel cell  400 C. The two outside metal current collector plates  403 C sandwich the center metal plate  401 C and the two MEA/GDLs  402 C to form a solid twin-cell unit  400 C. The outside metal current collector plates  403 B may be joined at the edges by welding, clamping or other means, or may be electrically connected by an additional member (not shown in  FIG. 4C ). The flow field channels  404 C on both sides of the MEA/GDL  402 C are gas (air and fuel) distribution channels. The twin cell unit  400 C is the basic unit for mono-polar separator plate structured fuel cell stacks. 
         [0027]    In one embodiment of the invention, the twin cell unit  400 A (or  400 B) is used to build a monopolar fuel cell stack  540 , as shown in  FIG. 5A . An electrically insulating spacer  502 A separates adjacent twin cell units  501 A (which are the same as the twin cell units  400 A of  FIG. 4A  in some embodiments or are the same as the twin cell units  400 B of  FIG. 4B  in other embodiments). An electrical conductor  503 A connects the center metal separator/current collector plate  401 A of one twin cell unit  501 A to the outer metal current collector plates  403 A of the adjacent twin cell unit  501 A. Channels  504 A (out of the twin cell units) are coolant channels. The connectors  505 A and  506 A are the electrical power outputs for the stack.  507 A is the end plate of the stack. Center channel  508 A of the twin cell unit  501 A provides for water flow. Preferably, the cathode (air electrode) of the MEA  509 A faces the center part of the twin cell unit  501 A, and the anode (fuel electrode) faces the side of the twin cell unit  501 A. During operation, the water will flow through the center channel  508 A to maintain the cathode humidity and carry produced water away. Coolant flows through cooling channel  504 A to carry heat away from the twin cell unit  501 A. The anode temperature will be lower than the cathode temperature, which will enhance the water diffusion from cathode side to the anode side keeping the electrolyte membrane properly humidified during operation. 
         [0028]    In another embodiment of the invention, the twin cell unit  400 C is used to build an open cathode mono-polar fuel cell stack  550 , as shown in  FIG. 5B . In the mono-polar fuel stack  550 , an electrically insulating spacer  502 B separates adjacent twin cell units  501 B (which are the same as twin cell units  400 C shown in  FIG. 4C ). An electrical conductor  503 B connects the center metal separator/current collector plate  501 C of one twin cell unit  501 B to the outer metal current collector plates  403 C of the adjacent twin cell unit  501 B. In this stack, the anode of the MEA  509 B faces to the center part of the twin cell unit  501 B, and the cathode faces to the outside of the twin cell unit  501 B. The outer metal plates  503  preferably cover substantially the entirety of the active area (i.e., more than 75%, preferably more than 90% and more preferably more than 95%) of the cathode, with the exception of areas not covered by the through holes formed in the outer metal plates  503 . Channels  504 B (out of the twin cell units  501 B) are used to deliver air for the cathode reaction and stack cooling. The connectors  505 B and  506 B are the electrical power outputs of the stack. End plates  507 B are provided at opposite ends of the stack. 
         [0029]    The foregoing examples are provided merely for the purpose of explanation and are in no way to be construed as limiting. While reference to various embodiments is made, the words used herein are words of description and illustration, rather than words of limitation. Further, although reference to particular means, materials, and embodiments are shown, there is no limitation to the particulars disclosed herein. Rather, the embodiments extend to all functionally equivalent structures, methods, and uses, such as are within the scope of the appended claims. 
         [0030]    Additionally, the purpose of the Abstract is to enable the patent office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature of the technical disclosure of the application. The Abstract is not intended to be limiting as to the scope of the present inventions in any way.