PATENT ABSTRACT
A fuel cell electrode includes a plate having a front surface and a back surface and also having a plurality of gas delivery holes and a plurality of gas exhaust holes formed through the plate. The front surface of the plate has a plurality of open gas distributions channels, a first portion of which is connected at one end to a first one of the plurality of gas delivery holes and at another end to a first one of the plurality of gas exhaust holes, a second portion of which is connected at one end to a second one of the plurality of gas delivery holes and at another end to a second one of the plurality of gas exhaust holes, and a third portion of which is connected at one end to said second one of the plurality of gas delivery holes and at another end to said first one of the plurality of gas exhaust holes.

PATENT DESCRIPTION
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 USC §119 to U.S. provisional application Ser. No. 60/257,849, “VARIABLE PRESSURE DROP PLATE DESIGN,” filed Dec. 21, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to fuel cells and to fluid flow plates within fuel cells. 
     A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases. 
     One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell system made up of multiple fuel cells also typically includes one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plates and/or the exterior of the cathode flow field plates. 
     Each flow field plate has an inlet region, an outlet region, and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly. 
     The membrane electrode assembly usually includes a solid electrolyte, e.g., a proton exchange membrane (PEM), between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate. 
     During operation of the fuel cell, a reactant gas, e.g., hydrogen, enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas, e.g., air, enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region. 
     As the reactant gas flows through the channels of the anode flow field plate, the reactant gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the other gas flows through the channels of the cathode flow field plate, the other gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst. 
     The anode catalyst interacts with the reactant gas to catalyze the conversion of the reactant gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the other gas and the reaction intermediates to catalyze the conversion of the other gas to the chemical product of the fuel cell reaction. 
     The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate. 
     The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly. 
     Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate. 
     Electrons are formed at the anode side of the membrane electrode assembly, indicating that the reactant gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the other gas undergoes reduction during the fuel cell reaction. 
     For example, when hydrogen and oxygen are the two gases that are used in the fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in the following Eqs. 1-3:
 
H 2 →2H + +2e −   (1)
 
½O 2 +2H + +2e − →H 2 O  (2)
 
H 2 +½O 2 →H 2 O  (3)
 
     As shown in Eq. 1, the hydrogen forms protons (H + ) and electrons (e − ). The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in Eq. 2, the electrons and protons react with the oxygen to form water. Eq. 3 shows the overall fuel cell reaction. 
     In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and maintain appropriate stack temperatures. 
     Each coolant flow field plate has an inlet region, an outlet region, and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant, e.g., liquid de-ionized water or other low conductivity fluids, at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell. 
     To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a fuel cell electrode includes a plate having a front surface and a back surface and also having a plurality of gas delivery holes and a plurality of gas exhaust holes formed through the plate, the front surface of the plate having a plurality of open gas distributions channels, a first portion of which is connected at one end to a first one of the plurality of gas delivery holes and at another end to a first one of the plurality of gas exhaust holes, a second portion of which is connected at one end to a second one of the plurality of gas delivery holes and at another end to a second one of the plurality of gas exhaust holes, and a third portion of which is connected at one end to said second one of the plurality of gas delivery holes and at another end to said first one of the plurality of gas exhaust holes. 
     In another aspect of the invention, a fuel cell system includes a plurality of fuel cells stacked together, each having a first electrode, a second electrode, and a membrane sandwiched between the first and second electrodes, wherein each first electrode includes a plurality of gas distribution channels on a surface thereof. The fuel cell system also includes a plurality of gas delivery manifolds, each of which is connected to the plurality of channels of each of the plurality of first electrodes, and a plurality of gas exhaust manifolds, each of which is connected to the plurality of channels of each of the plurality of first electrodes, wherein on the first electrode of each of the plurality of fuel cells, a first portion of the plurality of gas distribution channels is connected at one end to a first one of the plurality of gas delivery manifolds and at another end to a first one of the plurality of gas exhaust manifolds, a second portion of the plurality of gas distribution channels is connected at one end to a second one of the plurality of gas delivery manifolds and at another end to a second one of the plurality of gas exhaust manifolds, and a third portion of the plurality of gas distribution channels is connected at one end to said second one of the plurality of gas delivery manifolds and at another end to said first one of the plurality of gas exhaust manifolds. 
     One or more of the following advantages may be provided by one or more aspects of the invention. 
     The invention enables one to operate a fuel cell over a broad range of fluid flow rates while still keeping the pressure drop across fluid flow plates in the fuel cell within an acceptable operating range. When operating at high flow rates, the multiple input manifolds are operated as a single supply manifold and the multiple output manifolds are operated as a single output manifold. In this configuration, the gas flows from the single input manifold, through the fluid flow field, and out the single output manifold. When operating at low flow rates, the multiple input manifolds are operated as separate manifolds, as are the multiple output manifolds. In this configuration, the gas flows from one of the input manifolds, through the fluid flow field, and into one of the output manifolds. This output manifold is closed at both ends, so the gas flows back through the fluid flow field to another one of the input manifolds. This input manifold is closed at both ends, so the gas flows back through the fluid flow field to another output manifold. The gas continues flowing through the fluid flow field between different input and output manifolds until the gas enters an output manifold open at one end, allowing the gas to exit the fuel cell system. 
     Providing multiple input and output manifolds also presents an opportunity for water to drop out of the flow through the fuel cell plates. When the fuel becomes redirected back into a fuel cell plate&#39;s flow channels at an input or output manifold, water tends to drop out of the fuel and into the manifold before the fuel returns to the fuel cell stack. Thus, the amount of water passing through the fuel cell stack decreases, lowering the chance of water clogging the flow channels. 
     The same design can be used in the air input and output manifolds of the fuel cell stack. 
     Other features and advantages of the invention will be apparent from the detailed description and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a fuel cell system; 
         FIG. 2  is a cross-sectional partial view of a single fuel cell; 
         FIG. 3  shows an anode fuel cell plate; 
         FIG. 4  shows a cathode fuel cell plate; 
         FIG. 5  is a schematic representation of flow paths in a fuel cell plate with an open manifold configuration; and 
         FIG. 6  is a schematic representation of flow paths in a fuel cell plate with a closed manifold configuration. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a fuel cell system  100  includes left and right end plates  102  and  104 , left and right insulation layers  106  and  108 , left and right current collector/conductor plates  110  and  112 , with a working section  114  in-between. Left and right structural members  116  and  118 , two tie-bolts on either side of the working area  114 , are used to join the left and right end plates  102  and  104 . 
     The working section  114  includes eighty-eight fuel cells  120 , although there may be more or less fuel cells  120  depending on design considerations. Input and output fuel manifolds  122  and  124  supply fuel to, remove fuel from, and otherwise communicate and/or service fuels as desired within the working section  114 . Fuel flows into the input fuel manifold  122  from an inlet pipe  134 . The fuel then enters the working area  114  from the input fuel manifold  122 , flows through each of the fuel cells  120  at least once, and exits the working area  114  through the output fuel manifold  124 . The fuel exits the output fuel manifold  124  and into an outlet pipe  136 . The fuel may encounter various elements such as a blower after entering the outlet pipe  136 . 
     The input manifold  122  actually includes two manifolds: first and second input manifolds  126   a  and  126   b . Opening and closing an inlet valve  130  redirects a flow path of fuel in the input fuel manifold  122  through the working area  114 . With the inlet valve  130  open, fuel flows from the inlet pipe  134  into both the first and second input manifolds  126   a  and  126   b  and therefore into the working area  114  from both the first and second input manifolds  126   a  and  126   b . With the inlet valve  130  closed, fuel flows from the inlet pipe  134  into only the first input manifold  126   a  and therefore into the working area  114  from only the first input manifold  126   a.    
     Similarly, the output manifold  124  also includes two manifolds: first and second output manifolds  128   a  and  128   b , with an outlet valve  132  configured to close the first output manifold  128   a.    
     With both the inlet valve  130  and the outlet valve  132  closed, fuel flows into the first input manifold  126   a , through the working area  114 , and in to the first output manifold  128   a , which is closed at both ends. Thus, the fuel flows from the first output manifold  128   a  back through the working area  114  and into the second input manifold  126   b . The second input manifold  126   b  is also closed at both ends, so the fuel again enters the working area  114  from the second input manifold  126   b  and flows in to the second output manifold  128   b , from which the fuel flows out of the stack through output pipe  136 . 
     Before discussing the operation of the multiple manifold configuration, we will first provide a few useful details about the design of the individual fuel cells. Then after discussing the operation of the multiple manifold configuration we will provide more details about a particular type of fuel cell. 
       FIG. 2  illustrates a cross-section of a single fuel cell  200  within the working section  114 . It includes an anode fluid flow plate  202 , a cathode fluid flow plate  204 , anode and cathode flow channels  206  and  208 , anode and cathode lands  220  and  222 , and a center area  224 , each described in more detail below. 
     The anode fluid flow plate  202  includes a number of flow channels  206  that receive and transmit fuel, e.g., hydrogen gas, and humidification water in vapor and/or liquid form. The cathode fluid flow plate  204  includes a number of flow channels  208  that receive and transmit air, e.g., oxygen gas as oxidant, and product water in vapor and/or liquid form. Adjacent flow channels  206  and  208  are separated by the lands  220  and  222 . The lands  220  and  222  serve as electrical contact positions on the corresponding anode and cathode fluid flow plates  202  and  204 . The lands  220  and  222  and the fluid flow plates  202  and  204  can be formed with a material such as non-magnetic, austenitic stainless steel or titanium. The fluid flow plates  202  and  204  are described in more detail below with reference to  FIGS. 3-4 , respectively. 
     Referring to  FIG. 3 , the anode fluid flow plate  202  includes one or more substantially parallel and/or generally serpentine fuel flow channel(s)  300  and land(s)  318  (corresponding to the flow channels  206  and the lands  220  in FIG.  2 ). The fuel flow channels  300  run between inlet fuel and outlet fuel holes  302  and  304 . The fuel flow channels  300  carry the fuel and the humidification water through the fuel stack. Twelve flow channels  300  are shown, but the anode fluid flow plate  202  can include more or fewer flow channels  300  depending on design considerations. The anode fluid flow plate  202  also includes inlet and outlet water holes  310  and  312  and inlet and outlet air holes  314  and  316 . 
     The inlet fuel hole  302  and the outlet fuel hole  304  are each actually made up of an equal number of separate inlet holes  306   a  and  306   b  and outlet holes  308   a  and  308   b . Here there are two inlet holes  306   a  and  306   b  and two outlet holes  308   a  and  308   b . Each of the separate inlet holes  306   a  and  306   b  and outlet holes  308   a  and  308   b  connects to a corresponding different group of flow channels  300 . In this example, the first inlet fuel hole  306   a  connects to three flow channels  300 . The first outlet fuel hole  308   a  also connects to these three flow channels  300  at the opposite end along with three additional flow channels  300 . The second inlet hole  306   b  connects to the remaining nine flow channels  300  at the inlet fuel hole  302 , while a second outlet fuel hole  308   b  connects to the remaining six flow channels  300  at the outlet fuel hole  304 . 
     Referring to  FIG. 4 , the cathode fluid flow plate  204  includes one or more substantially parallel and/or generally serpentine air flow channel(s)  400  and land(s)  418  (corresponding to the flow channels  208  and the lands  222  in FIG.  2 ). The air flow channels  400  run between inlet and outlet air holes  402  and  404 . The air flow channels  400  carry the oxidant gas and the product water through the fuel stack. The inlet air hole  402  and the outlet air hole  404  are each actually made up of two separate inlet holes  406   a  and  406   b  and two separate outlet holes  408   a  and  408   b  as described above with reference to the anode fluid flow plate&#39;s separate inlet holes  306   a  and  306   b  and outlet holes  308   a  and  308   b  (see FIG.  3 ). Twelve flow channels  400  are shown, but the cathode fluid flow plate  204  can, depending on design considerations, include more or less channels (and corresponding lands) equal to the number of flow channels on the anode fluid flow plate  202  (see FIG.  2 ). The cathode fluid flow plate  204  also includes inlet and outlet water holes  410  and  412  and inlet and outlet fuel holes  414  and  416 . 
     When a plurality of the anode fluid flow plates  202  (see  FIG. 3 ) and a plurality of the cathode fluid flow plates  204  (see  FIG. 4 ) are stacked on one another, the inlet fuel holes  302  and  414  align to form the input fuel manifold  122  (see  FIG. 1 ) and the outlet fuel holes  304  and  416  align to form the output fuel manifold  124  (see FIG.  1 ). Similarly, the inlet air holes  314  and  402  align to form an input air manifold and the outlet air holes align to form an output air manifold. The inlet water holes  310  and  410  and the outlet water holes  312  and  412  align to form input and output water manifolds, respectively. (A coolant fluid flow plate may be located between the anode and cathode fluid flow plates  202  and  204 , and the coolant fluid flow plate includes holes in the appropriate places to accommodate the fuel, air, and water manifolds.) 
     Referring to  FIG. 5 , flow paths through an anode fluid flow plate  500  are shown in an open manifold configuration.  FIG. 5  is a simplified schematic drawing meant to illustrate the operation of the flow paths of the anode fluid flow plate  500 . The anode fluid flow plate  500  has the inlet valve  130  and the outlet valve  132  for the input fuel manifold  122  and the output fuel manifold  124 , respectively, in open positions (see FIG.  1 ). Thus, inlet holes  502   a  and  502   b  form part of a single inlet fuel manifold  504  and outlet holes  508   a  and  508   b  form part of a single outlet manifold  510 . Fuel enters the anode fuel cell plate  500  through the inlet fuel manifold  504  (all inlet holes  502   a  and  502   b ) and flows through flow channels  506   a - 506   c  in the anode fuel cell plate  500 . Then, the fuel exits the anode fuel cell plate  500  through all outlet holes  508   a  and  508   b  of the exit fuel manifold  510 . In this way, the anode fuel cell plate  500  performs as a three channel, single pass design. 
     Referring to  FIG. 6 , flow paths through an anode fluid flow plate  600  are shown in a closed manifold configuration. The anode fluid flow plate  600  has the inlet valve  130  and the outlet valve  132  for the input fuel manifold  122  and the output fuel manifold  124 , respectively, in closed positions (see FIG.  1 ). Thus, inlet holes  602   a  and  602   b  form part of separate inlet manifolds and outlet holes  606   a  and  606   b  form part of separate outlet manifolds. Fluid enters the anode fuel cell plate  600  through the first inlet hole  602   a  and flows through a first flow channel  604   a  to the first outlet hole  606   a . With the outlet valve  132  closed, the manifold formed in part by the first outlet hole  606   a  is closed at both ends, i.e., it is a sealed plenum through which the fuel cannot exit from the fuel cell stack. Instead, the fuel flows back into a second flow channel  604   b  across the fuel cell and into the second inlet hole  602   b . With the inlet valve  130  also in a closed position, the manifold formed in part by the second inlet hole  602   b  is closed at both ends, i.e., it is a sealed plenum through which the fuel cannot exit the fuel cell stack, so the fluid flows through a third flow channel  604   c  to the second outlet hole  606   b , through which the fluid exits the anode fuel cell plate  600 . In this way, the anode fuel cell plate  600  acts as a single channel, three pass design. 
     The closed manifold configuration of the anode fluid flow plate  600  produces a higher pressure drop than the pressure drop produced when fuel flows through the open manifold configuration of the anode fluid flow plate  500  (see FIG.  5 ). The higher pressure drop is about three to five times higher than the pressure drop produced in the anode fluid flow plate  500  given that the flow path is about three times longer and has more bends. 
     In addition, at each of the sealed plenums formed in part by the first outlet hole  606   a  and the second inlet hole  602   b , water tends to drop out of the flow path through the fuel cells. Therefore, an added advantage of this configuration is that less water passes through a subsequent flow channel, e.g., third flow channel  604   c , than in a previous flow channel, e.g., first flow channel  604   a  or second flow channel  604   b.    
     Each one of the flow channels  506   a - 506   c  (see  FIG. 5 ) and  604   a - 604   c  (see  FIG. 6 ) can in fact represent multiple individual flow channels in an actual implementation. Similarly, there may be more inlet fuel holes  502   a  and  502   b  (see  FIG. 5 ) and  602   a  and  602   b  (see  FIG. 6 ) and more outlet holes  508   a  and  508   b  (see  FIG. 5 ) and  608   a  and  608   b  (see  FIG. 6 ) in an actual implementation, thereby increasing the number of times that fuel may pass through the fuel cell stack. 
     The discussion of  FIGS. 5-6  illustrated flow through anode fluid flow plates, but the discussion can also apply to cathode fluid flow plates (with air flowing instead of fuel). 
     Having discussed the operation of the multiple manifold configuration, we will now provide more details about a particular type of fuel cell. 
     Referring back to the partial view  200  in  FIG. 2  of the fuel cell  120 , the center area  224  includes a membrane or solid electrolyte  210 , anode and cathode catalysts  212  and  214 , and anode and cathode gas diffusion layers  216  and  218 , each described in more detail below. The solid electrolyte  210  includes a solid polymer, e.g., a solid polymer ion exchange membrane, such as a solid polymer proton exchange membrane, e.g., a solid polymer containing sulfonic acid groups. Such membranes are commercially available from E. I. DuPont de Nemours Company (Wilmington, Del.) under the trademark NAFION. Alternatively, the electrolyte  210  can be prepared from the commercial product GORE-SELECT, available from W. L. Gore &amp; Associates (Elkton, Md.). 
     Together, the solid electrolyte  210  and the catalysts  212  and  214  form a membrane electrode assembly (MEA). The anode catalyst  212  includes material capable of interacting with molecular hydrogen to form protons and electrons. Such materials include, for example, platinum, platinum alloys, and platinum dispersed on carbon black. The catalytic material can be dispersed in one or more solvents, e.g., isopropanol, to form a suspension. The suspension is then applied to the surfaces of the solid electrolyte  210  that face the gas diffusion layers  216  and  218 , and the suspension is then dried. Alternatively, the suspension can be applied to the surfaces of the gas diffusion layers  216  and  218  that face the solid electrolyte  210 , and the suspension is then dried. The method of preparing the catalyst  212  may further include the use of heat, temperature, and/or pressure to achieve bonding. 
     The catalyst  214  on the cathode side is formed of a material capable of interacting with molecular oxygen, electrons, and protons to form water. Examples of such materials include platinum, platinum alloys, and noble metals dispersed on carbon black. The catalyst  214  can then be prepared as described above with respect to the catalyst  212 . 
     The gas diffusion layers  216  and  218  are formed of a material that is both gas and liquid permeable material so that the reactant gases, e.g., molecular hydrogen and molecular oxygen, and products, e.g., water, can pass therethrough. In addition, the gas diffusion layers  216  and  218  should be electrically conductive so that electrons can flow from the catalyst  212  to the flow field plate  202  and from the flow field plate  204  to the catalyst  214 . 
     While certain embodiments of the invention, as well as their principals of operation, have been disclosed herein, the invention is not limited to these embodiments or these principals of operation. Other embodiments are in the claims.