Abstract:
A fluid flow plate for a fuel cell is provided with hydration channels along the reactant channel lands of the plate. Water is injected into a gas diffusion layer facing the hydration channels in order to promote hydration of the fuel cell membrane.

Description:
This invention relates generally to fuel cells and more specifically to humidifying the ion exchange membrane in such cells. 
     BACKGROUND OF THE INVENTION 
     A fuel cell is a device which converts chemical energy of a fuel into electrical energy, typically by oxidizing the fuel. In general a fuel cell includes an anode and a cathode fluid flow plate separated by an electrolyte. When fuel is supplied to the anode and oxidant is supplied to the cathode, the electrolyte electrochemically generates a useable electric current which is passed through an external load. The fuel typically supplied is hydrogen and the oxidant typically supplied is oxygen. In such cells, the electrolyte combines the oxygen and hydrogen to form water and to release electrons. The chemical reaction of a fuel cell using hydrogen as the fuel and oxygen as the oxidant is shown in equation (1). 
     
       
         H 2 +½O 2 →H 2 O  (1) 
       
     
     This process occurs through two redox or separate half-reactions which occur at the electrodes: 
     Anode Reaction 
     
       
         H 2 →2H + +2e −   (2) 
       
     
     Cathode Reaction 
     
       
         ½O 2 +2H + +2e − →H 2 O  (3) 
       
     
     In the anode half-reaction, hydrogen is consumed at the fuel cell anode releasing protons and electrons as shown in equation (2). The protons are injected into the fuel cell electrolyte and migrate to the cathode. The electrons travel from the fuel cell anode to cathode through an external electrical load. In the cathode half-reaction, oxygen, electrons from the load, and protons from the electrolyte combine to form water as shown in equation (3). The directional flow of protons, such as from anode to cathode, serves as a basis for labeling an “anode” side and a “cathode” side of the fuel cell. 
     The anode and cathode fluid flow plates are made of an electrically conductive material, typically metal or compressed carbon, in various sizes and shapes. Fluid flow plates act as current collectors, provide paths for access of the fuels and oxidants to the cell, and provide a path for removal of waste products formed during operation of the cell. Each fuel cell includes a catalyst, such as platinum, for promoting the chemical reaction(s) that take place on the electrodes in the fuel cells. Additionally, the fluid flow plates include a fluid flow field of channels for directing fluids within the cell. 
     Fluid flow plates are commonly produced by any of a variety of processes. For example, one technique for plate construction, referred to as “monolithic” style, includes compressing carbon powder into a coherent mass which is subjected to high temperature processes to bind the carbon particles together, and to convert a portion of the mass into graphite for improved electrical conductivity. The mass is then cut into slices, which are formed into the fluid flow plates. Typically, each fluid flow plate is subjected to a sealing process (e.g., resin impregnation) in order to decrease gas permeation therethrough and reduce the risk of uncontrolled reactions. 
     Fuel cells are classified into several types according to the electrolyte used to accommodate ion transfer during operation. Examples of electrolytes include aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate, stabilized zirconium oxide, and solid polymers, e.g., a solid polymer ion exchange membrane. 
     An example of a solid polymer ion exchange membrane is a Proton Exchange Membrane (hereinafter “PEM”) which is used in fuel cells to convert the chemical energy of hydrogen and oxygen directly into electrical energy. A PEM is a solid polymer electrolyte which when used in a PEM-type fuel cell permits the passage of protons (i.e.,H + ions) from the anode side of a fuel cell to the cathode side of the fuel cell while preventing passage of reactant fluids such as hydrogen and oxygen gases. 
     Typically, a PEM-type fuel cell includes an electrode assembly disposed between two fluid flow plates. The electrode assembly usually includes five components: two gas diffusion layers; two catalysts; and an electrolyte. The electrolyte is located in the middle of the five-component electrode assembly. On one side of the electrolyte (the anode side), a gas diffusion layer (the anode gas diffusion layer) is disposed adjacent the anode layer, and a catalyst (the anode catalyst) is disposed between the anode gas diffusion layer and the electrolyte. On the other side of the electrolyte (the cathode side), a gas diffusion layer (the cathode gas diffusion layer) is disposed adjacent the cathode layer, and a catalyst (the cathode catalyst) is disposed between the cathode gas diffusion layer and the electrolyte. 
     Several PEM-type fuel cells usually are arranged as a multi-cell assembly or “stack.” In a multi-cell stack, multiple single PEM-type cells are connected together in series. The number and arrangement of single cells within a multi-cell assembly are adjusted to increase the overall power output of the fuel cell. Typically, the cells are connected in series with one side of an fluid flow plate acting as the anode for one cell and the other side of the fluid flow plate acting as the cathode for an adjacent cell. 
     Fluid flow plates also have holes therethrough for alignment and for formation of fluid manifolds that service fluids for the stack. Some of the fluid manifolds distribute fuel (such as hydrogen) and oxidant (such as air or oxygen) to, and remove unused fuel and oxidant as well as product water from, the fluid flow fields of the plates. Additionally, other fluid manifolds circulate coolant. Furthermore, other cooling mechanisms such as cooling plates are commonly installed within the stack between adjacent single cells to remove heat generated during fuel cell operation. 
     Typically, the PEM works more effectively if it is wet. Conversely, once any area of the PEM dries out, the electrochemical reaction in that area stops. Eventually, the dryness can progressively march across the PEM until the fuel cell fails completely. As a result, the fuel and oxidant fed to each fuel cell are usually humidified, e.g., with steam. 
     SUMMARY OF THE INVENTION 
     In general, in one aspect, the invention is a fluid flow element for a fuel cell. The fluid flow element includes a plate made at least in part of a conductive material. The plate has a front surface in which there is formed a first plurality of open-faced channels and a second plurality of open-faced channels. The second plurality of channels is interleaved among the first plurality of channels. Each of the channels of said first plurality of channels has an inlet end and an outlet end and each of the channels of the second plurality of channels has an inlet end. The plate further includes a first supply opening, a first exhaust opening, and a second supply opening which is separate from the first supply opening. The inlet ends of the first plurality of channels are connected to the first supply opening and the inlet ends of the second plurality of channels are connected to the second supply opening. 
     In another aspect, a fuel cell plate has reactant channels, lands, and hydration channels formed in at least a portion of the lands. 
     In another aspect, a solid polymer fuel cell includes a solid polymer electrolyte, an anode fluid flow plate, and a cathode fluid flow plate. At least one of the plates has a front surface in which there is formed a first plurality of open-faced channels and a second plurality of open-faced channels interleaved among the first plurality of channels. Each of the channels of the first plurality of channels has an inlet end and an outlet end and each of the channels of the second plurality of channels have an inlet end. Each plate further includes a first supply opening, a first exhaust opening, and a second supply opening which is separate from the first supply opening. The inlet ends of the first plurality of channels are connected to the first supply opening and the inlet ends of the second plurality of channels are connected to the second supply opening. 
     In other embodiments, the second supply opening receives water for humidifying the PEM. Each channel of the second plurality of channels further includes an outlet end connected to a second exhaust opening of the plate. The fluid flow element also can include microchannels which connect the first plurality of channels to the second plurality of channels. 
     The plurality of first and second channels also can include single or multiple channels formed in serpentine patterns or non-serpentine patterns, such as by forming straight channels. The second plurality of channels may traverse only part of the serpentine pattern formed by the first plurality of channels. 
     Additionally, in the solid polymer fuel cell, both the anode plate and cathode plate can include a first plurality of open-faced channels and a second plurality of open-faced channels. 
     The invention has various advantages among one or more of the following. The fluid flow plates which include the coolant distribution channels increase the evenness of hydration water distribution within the PEM active area by physically supplying water or water vapor over the entire PEM active area. The fluid flow plates also provide more uniform cooling of the fluid flow field which results in a more uniform temperature distribution in the stack. Additionally, the fluid flow plate decreases the fuel assembly cooling load by providing hydration water which will provide evaporative cooling. Also, the plate provides higher stack performance by reducing the volume of liquid water that appears in and which tends to cause flooding in the channels carrying fuel or oxidant and waste products. 
     As used herein the term “PEM active area” refers to the area of the MEA adjacent to the gas diffusion layers. 
     Other features and advantages of the invention will be apparent from the following detailed description, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a sectional, elevated, side view of a fuel cell assembly; 
     FIG. 2 is a cross-sectional view of a single PEM-type fuel cell of the fuel cell assembly of FIG. 1; 
     FIG. 3 is an off-axis side view of a fluid flow plate; 
     FIG. 4 is a top view of the top face of the fluid flow plate of FIG. 3; 
     FIG. 5 is a cross-sectional view about section A—A of the fluid flow plate of FIG. 5; 
     FIG. 6 is an expanded view of section B of the fluid flow plate of FIG. 3; 
     FIG. 7 is a cross-sectional view about section C—C of the fluid flow plate of FIG. 5; and 
     FIG. 8 is the cross-section view of the PEM-type cell of FIG. 2 in operation. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a fuel cell assembly  100  including a fuel cell stack  114  located between end assembly  101  and end assembly  103 . End assembly  101  includes an insulation layer  106  sandwiched between an end plate  102  and a current collector/conductor plate  110 . End assembly  103  includes an insulation layer  108  sandwiched between an end plate  104  and a current collector/conductor plate  112 . A number of structural members  116 , such as tie-bolt(s), join the end plates and are used to compress the fuel cell assembly. Typically, fuel cell assembly  100  is compressed with enough pressure to create both gas tight seals and good electrical contact within the cell stack assembly  114 . For example, the compression pressure applied to layers of fuel cell assembly  100  can be anywhere between fifty to one thousand pounds per square inch depending on the design of the cell. 
     Fuel cell stack  114  includes a number of layers  118  which are assembled together to form several individual PEM-type fuel cells  300 . Layers  118  include, for example, fluid flow plates, cooling plates, and solid electrolytes such as PEMs. 
     As shown in FIG. 2, PEM-type fuel cell  300  includes an anode gas diffusion layer  312 , a membrane electrode assembly  310  (hereinafter “MEA”) and a cathode gas diffusion layer  312 ′(hereafter “GDLs”) sandwiched between an anode fluid flow plate  200  and a cathode fluid flow plate  200 ′. MEA  310  includes a membrane or solid electrolyte  306 , and an anode catalyst  308  and a cathode catalyst  308 ′. 
     A detailed view of anode fluid flow plate  200  is shown in FIG.  3 . Anode fluid flow plate  200  includes a back face  201 , a front face  202 , and holes  205 ,  207 ,  207 ′,  209 ,  209 ′, and  211 . Typically, each layer  118  (See FIG. 1) within fuel cell stack  114  includes holes which are identical to the holes of anode fluid flow plate  200 . Layers  118  are assembled into the fuel cell stack  114  such that the identical holes for each layer form fluid manifolds which extend along the length of the fuel cell stack  114 . The fuel cell fluid manifolds supply fluids to and remove fluids from the fuel cell stack  114 . A cooling manifold, for example, is formed by identical holes  205  of each layer  118 . The cooling manifold supplies water through holes  205  into an inlet end portion  303  of a series of parallel channels  203  on back face  201 . Anode fluid flow plate is cooled as water flows through the parallel channels  203  towards an outlet end portion  303 ′. Similarly, a humidifying manifold formed by holes  211  supplies deionized water for humidifying the PEM; holes  209  form a fuel manifold for supplying fuel to the anode side of the PEM-type fuel cell; holes  209 ′ form a waste manifold for removing unused fuel from the anode side of the PEM-type fuel cell; holes  207  form an oxidant manifold for supplying oxidant to the cathode side of the PEM-type fuel cell; and holes  207 ′ form a waste manifold for removing oxidant and product water from the cathode side of the PEM-type fuel cell. 
     Referring now to FIG. 4, anode fluid flow plate  200  has a fluid flow face  212  including many flow channel(s)  204  for supplying fuel to the PEM-type cell. For example, fluid flow face  212  includes  16  channels  204  which traverse fluid flow face  212  in a serpentine pattern. Each channel  204  is formed by lands  314  which serve as electrical contact positions on anode face  202 . Additionally, in an expanded cross-sectional view as shown in FIG. 5, fluid flow face  212  includes several humidifying channels  206  for supplying water to humidify the PEM of the fuel cell. Humidifying channels  206  are formed in lands  314 . For example,  8  of the lands between channels  204  contain humidifying channels  206 . Channels  204  are connected to an inlet port  213  and an outlet port  213 ′ (See FIG.  3 ). Humidifying channels  206  are connected to inlet port  215  (See FIG.  3 ). Inlet ports  213 ,  215  and outlet port  213 ′ are used to transmit fluids such as fuel or water to and from the channels on front face  212 . As shown in FIG. 6, inlet port  213  includes several port channels  313  extending from hole  209  to bore holes  315 . Typically, the number of port channels  313  directly corresponds to the number of channels  204  on fluid flow face  202  and each bore hole  315  connects one port channel  313  on back face  201  to one channel  204  on front face  202 . Note that outlet port  213 ′, shown in FIG. 3, is similar to inlet port  213 . Inlet port  215  includes a recess  320  extending from hole  211  to bore holes  325  and, as shown in FIG. 6, each bore hole  325  of port  215  connects recess  320  to humidifying channels  206  on front face  202 . 
     Bore holes  315  have a diameter equal to the width of channels  204 , and bore holes  325  have a diameter equal to the width of humidifying channels  206 . Typically, bore holes  315  have a diameter of less than or equal to 0.040 inches, and bore holes  325  have a diameter less than or equal to 0.020 inches. In one embodiment, bore hole  315  may be oblong having a width of about 0.040 inches and a length of 0.060 inches. 
     In operation, referring to FIG. 8, PEM-type cell  300  includes fluid flow plates  202 ,  200 ′ which serve, respectively, as an anode side and as a cathode side of the fuel cell. That is, face  212  is an anode face, and face  212 ′ is a cathode face. Face  212  includes channels  204  formed by lands  314  and receive fuel from the fuel manifold (formed by holes  209 ) through inlet port  213 . Fuel flows along channels  204  and diffuses through GDL  312  into MEA  310  where the fuel is oxidized. Excess fuel flows out of channels  204  through an outlet port and into a waste manifold formed by holes  209 ′. Humidifying channels  206  are contained within lands  314  and receive deionized water from the humidifying manifold (formed by holes  211 ) through inlet port  215 . Cathode face  212 ′ includes channels  204 ′ formed by lands  314 ′ and receives oxidant from the oxidant manifold (formed by holes  207 ) through an inlet port. Oxidant flows along channels  204 ′ and diffuses into MEA  310  where the oxidant is reduced. Excess oxidant and product water flow out of channels  204 ′ through an outlet port and into a waste manifold (formed by holes  207 ′). Optionally, depending upon fuel cell design, cathode lands  314 ′ also include humidifying channels. 
     Humidifying channels  206  distribute (See Arrows) deionized water through the GDL  312  to humidify both the fuel flowing within channels  206  and membrane  306 . Specifically, deionized water within humidifying channels  206  wicks into an area  500  of GDL  312  opposite lands  314  and adjacent channels  204 . Once wicked into the GDL, deionized water humidifies the PEM either by wicking directly into the MEA or by humidifying the fuel flowing within channels  204 . Typically, the pressure of deionized water in the humidifying channels is adjusted to achieve a desired level of PEM and MEA humidification. For example, when using low flow rates of fuels, the pressure of water is decreased so that the catalysts, GDL, and flow channels are not flooded with deionized water. Alternatively, when using higher flow rates of fuels, the amount of water is increased to compensate for any water that evaporates into the fuel channels. Typically, the amount of deionized water is adjusted to maintain a 100 percent relative humidity of the fuel at 70° C. 
     Humidifying channels also provide cooling for each PEM-type fuel cell. For example, due to the flow of reactant or oxidant gas, water wicked into the GDL evaporates into and is carried away by the reactant or oxidant gas. As water evaporates, localized cooling may occur resulting in cooling of the PEM-type fuel cell. 
     The anode GDL  312  and cathode GDL  312 ′ serve as electrochemical conductors between corresponding catalyzed sites of solid polymer electrolyte  306  and the fuel and oxidant flowing in anode channels  204  and cathode channels  204 ′. GDLs are formed, for example, with a resilient and conductive material such as carbon fabric, carbon fiber paper, carbon cloth, or carbon paper. Additionally, the porous carbon cloth or paper can be infused with TEFLON® to inhibit the collection of water in the GDL. 
     The GDLs also exhibit a combination of microscopic porosity and macroscopic porosity. Microscopic porosity allows reactant gas molecules to pass generally longitudinally from the flow channels to a surface of the MEA. Macroscopic porosity allows product water formed at the cathode surface of the MEA to be removed by flowing generally longitudinally into the cathode channels  204 ′, to prevent flooding of the catalyst particles. 
     In the described embodiment, fluid flow plates may be formed of any material, e.g., non-magnetic, austenitic stainless steel, titanium, or compressed carbon. The plate can also include conductive, non-conductive, injection-moldable, and compliant portions. Typically, channels  204  are engraved or milled into a face of an electrically conductive material. Alternatively, the plates can be injection or compression molded. The width, depth, and length of each channel can be varied depending upon the design of the fuel cell. Typically, channels  204  have a width of about 0.040 inches and a depth of about 0.050 inches. Additionally, humidifying channels  206  are engraved or milled into the lands. The width, depth, and length of the humidifying channels are adjusted so that the PEM is adequately humidified. For instance, humidifying channels that are too small won&#39;t supply enough humidification to the PEM without using high pressures of water; and humidifying channels that are too big will flood the fuel cell. Typically, humidifying channels  206  have a width of about 0.020 inches and a depth of about 0.025 inches. The cross-sectional profile of the channels can be square, rounded or tapered. When rounded, the bottom of the channel has an effective radius between about 0.020 to 0.005 inches. 
     As illustrated in FIG. 4, the multiple fluid flow and humidifying channels can be formed in a serpentine pattern on the fluid flow plate. It should be understood, however, that the arrangement of the fluid flow channels and humidifying channels relative both to each other and to the fluid flow plate can be varied based upon the design of the fuel cell. For example, the fluid flow plate may include multiple fluid flow channels and a single humidifying channel both of which form a non-serpentine pattern, such as that formed by straight channels. The channels also may be continuous or discontinuous, such as one channel branching into several channels. 
     Fluid flow plates may be formed in accordance with the principles of U.S. application Ser. No. 09/054,670 by Carlstrom (entitled “Easily-Formable Fuel Cell Assembly Fluid Flow Plate Having Conductivity and Increased Non-Conductive Material,” filed Apr. 3, 1998, and assigned to Plug Power, L.L.C.), which is hereby incorporated herein by reference in its entirety. 
     Fluid flow plates include bipolar, monopolar, combined monopolar (e.g., anode cooler or cathode cooler), or cooling plates. For instance, when a fluid flow field plate is an anode or a cathode cooler plate, a back face of the plate supplies coolant to the stack and a front face of the plate acts either as an anode or a cathode flow field, supplying reactant gases to the PEM. Alternatively, a bipolar plate includes channels on both a front face and a back face. For example, the front face acts as a cathode for one PEM-type cell and the back face acts as an anode for an adjacent PEM-type cell. In this arrangement channels on the front face conduct oxidant and waste product and the channels on the back face conduct fuel. Additionally, the lands on both sides of the bipolar plate may include humidifying channels. 
     In the embodiments described above, the solid electrolyte may include a solid polymer electrolyte made with a polymer such as a material manufactured by E. I. DuPont de Nemours Company and sold under the trademark NAFION®. In another example, the solid polymer electrolyte might be formed with a product manufactured by W.L. Gore &amp; Associates (Elkton, Md.) and sold under the trademark GORE-SELECT®. The MEA might be formed with a product manufactured by W.L. Gore &amp; Associates and sold under the trade designation PRIMEA 5510-HS. 
     In alternative embodiments, the humidifying channels only partially extend into the PEM active area. In this situation, the humidifying channels indirectly humidify the remaining portion of the PEM active area by humidifying the fuel and oxidant gases as they flow past the humidifying channels. 
     In another alternative embodiment, the lands of the fluid flow field plate include humidifying channels and microchannels which allow deionized water to flow directly from the humidifying channel into the fuel/oxidant flow channels. 
     It is understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. Other aspects, advantages, and modifications are within the scope of the following claims.