Abstract:
A polymer electrolyte membrane fuel cell stack ( 52 ) includes a plurality of membrane electrode assemblies ( 12 ) having evenly distributed fuel concentration across an anode ( 24 ), and efficient cooling across a cathode ( 26 ). A first plate ( 14 ) has a fuel side ( 28 ) with a plurality of serpentine channels ( 40 ) formed therein for distributing fuel across the anode ( 24 ), and a second plate ( 16 ) has an oxidant side ( 30 ) with oxidant channels ( 42 ) formed therein for distributing an oxidant across the cathode ( 26 ). The membrane electrode assembly ( 12 ) has an even fuel concentration thereacross, and the oxidant is routed through the cell ( 10 ) at least twice for absorbing heat prior to being distributed across the cathode ( 26 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to fuel cells, and more particularly to a polymer electrolyte membrane fuel cell stack having improved fuel concentration across an anode, and efficient cooling across the cathode.  
       BACKGROUND OF THE INVENTION  
       [0002]     Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. A typical fuel cell comprises a fuel electrode (anode) and an oxidant electrode (cathode) separated by an ion-conducting electrolyte. The electrodes are connected electrically to a load (such as an electronic circuit) by an external circuit conductor. In the circuit conductor, electric current is transported by the flow of electrons, whereas in the electrolyte it is transported by the flow of ions, such as the hydrogen ion (H + ) in acid electrolytes, or the hydroxyl ion (OH − ) in alkaline electrolytes. In theory, any substance capable of chemical oxidation that can be supplied continuously (as a gas or fluid) can be oxidized galvanically as the fuel at the anode of a fuel cell. Similarly, the oxidant can be any material that can be reduced at a sufficient rate. Gaseous hydrogen has become the fuel of choice for most applications, because of its high reactivity in the presence of suitable catalysts and because of its high power density. Similarly, at the fuel cell cathodes the most common oxidant is gaseous oxygen, which is readily and economically available from air for fuel cells used in terrestrial applications. When gaseous hydrogen and oxygen are used as fuel and oxidant, the electrodes are porous to permit the gas-electrolyte junction area to be as great as possible. The electrodes must be electronic conductors, and possess the appropriate reactivity to give significant reaction rates. At the anode, incoming hydrogen gas is oxidized to produce hydrogen ions (protons) and electrons. Since the electrolyte is a non-electronic conductor, the electrons flow away from the anode via an external electrical circuit. At the cathode, oxygen gas is reduced and reacts with the hydrogen ions migrating through the electrolyte and the incoming electrons from the external circuit to produce water as a byproduct. The byproduct water is typically expelled as vapor at elevated temperatures. The overall reaction that takes place in the fuel cell is the sum of the anode and cathode reactions, with part of the free energy of reaction released directly as electrical energy. The difference between this available free energy and the heat of reaction is produced as heat at the temperature of the fuel cell. It can be seen that as long as hydrogen and oxygen are supplied to the fuel cell, the flow of electric current will be sustained by electronic flow in the external circuit and ionic flow in the electrolyte.  
         [0003]     In practice, a number of these unit fuel cells are normally stacked or ‘ganged’ together to form a fuel cell assembly. A number of individual cells are electrically connected in series by abutting the anode current collector of one cell with the cathode current collector of its nearest neighbor in the stack. Fuel and oxidant are introduced through manifolds into respective cells. The fuel and oxidant flow across the anode and cathode, respectively. One known fuel cell disclosed in U.S. Patent Publication 2004/0038112 A1 shows the fuel and oxidant flowing in serpentine channels across the anode and cathode. However, fuel is consumed as it progresses along the anode, creating an uneven fuel concentration and distribution of power across the anode. Furthermore, the oxidant tends to cool the cells it first contacts much more that the remainder of cells in the stack, causing uneven cooling of the fuel cell assembly resulting in uneven power distribution across the stack. Ideally, the temperature of the cells at the ends of the stack are the same as the cells in the center of the stack  
         [0004]     Accordingly, it is desirable to provide a polymer electrolyte membrane fuel cell stack having evenly distributed fuel concentration from fuel flowing across an anode and improved heat distribution within the fuel cell stack. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     A polymer electrolyte membrane fuel cell stack comprises a plurality of membrane electrode assemblies having evenly distributed fuel concentration across an anode, and efficient cooling from the oxidant that flows across a cathode. A first plate has a fuel side with a plurality of serpentine channels formed therein for distributing fuel across the anode, and a second plate has an oxidant side with oxidant channels formed therein for distributing an oxidant across the cathode. The membrane electrode assembly has an even fuel concentration thereacross and the oxidant is routed through the cell for absorbing heat prior to being distributed across the cathode. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0007]      FIG. 1  is an exploded perspective view of a fuel cell in accordance with an exemplary embodiment of the present invention;  
         [0008]      FIG. 2  is a cross sectional view of a portion of the fuel cell taken along the line  2 - 2  of  FIG. 1 ;  
         [0009]      FIG. 3  is a plane side view of a plate including fuel channels of the fuel cell of  FIG. 1 ;  
         [0010]      FIG. 4  is a plane side view of a plate including oxidant channels of the fuel cell of  FIG. 1 ;  
         [0011]      FIG. 5  is an exploded schematic side view of a stack of the fuel cells shown in  FIG. 1 ;  
         [0012]      FIG. 6  is a plane view of one backing plate of the fuel cell taken along the line  6 - 6  of  FIG. 1 ; and  
         [0013]      FIG. 7  is a plane view of the other backing plate of the fuel cell taken along the line  7 - 7  of  FIG. 1 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
         [0015]     A polymer electrolyte membrane fuel cell is disclosed that operates at high temperature using membranes that don&#39;t require humidification to work. More specifically, the preferred embodiment of the present invention comprises a stack for operating elevated temperature (120-250° C.) polymer electrolyte membrane fuel cells that is compact and lightweight. Low and medium temperature polymer electrolyte membrane stacks (&lt;120° C.) are complicated by water management and cooling issues. Water management is not an issue in polymer electrolyte membrane fuel cells operating at temperatures above 120° C., as long as the membrane is capable of proton conduction independent of humidity. Cooling issues are significantly different when using elevated temperature membrane electrode assemblies compared to “standard” Nafion-type membrane electrode assemblies. The problems that need to be resolved in an elevated temperature polymer electrolyte membrane fuel cell stack include: ensuring even fuel and thermal distribution across the stack, and sufficient thermal management such that operation at current densities &lt;600 mA/cm 2  requires no additional stack-related “balance of plant” (other components required to cool or maintain the temperature in a fuel cell stack, e.g., fans, pumps, cooling channels, coolant fluid/gas, and control circuitry). Also, the stack should have very low pressure drop so that demand on system balance of plant is minimized, and also compact and lightweight to meet certain power density specs. In the stack disclosed herein, stack temperature is controlled by the rate, or stoichiometry, at which the cathode oxidant is supplied. During startup, or when the stack is operated at low load, cathode oxidant is supplied at rates corresponding to a fuel stoichiometry in the range of 1-3. During operation at higher temperatures/load, the rate of the oxidant flow is increased, which withdraws heat more quickly from the stack and reduces temperature. Typically, cathode oxidant flow rates corresponding to reactant stoichiometry 3-10 are sufficient to control the stack temperature when operating at the parameters anticipated for a portable power system. It is important for the temperature of the cells to be even across the length of the stack in order to ensure even power distribution, particularly when operating the stack using reformate fuel containing carbon monoxide impuritites. In practice, it is preferred that the temperature gradient along the length of the stack be less than 30° C. In the stack disclosed herein, the oxidant is passed through the edges of the stack assembly prior to passage across the cathode side of the fuel cell. This method has the advantage of preventing the ends of the stack from becoming excessively cooled during operation, and thus maintain an even stack temperature.  
         [0016]      FIG. 1  is an exploded isometric view showing a single fuel cell  10  according to an exemplary embodiment of the present invention. The fuel cell  10  includes a membrane electrode assembly  12  separated from end plates  14 ,  16  by gaskets  18 ,  20 , respectively. Gaskets  18 ,  20  seal gases between the membrane electrode assembly  12  and both the end plates  14 ,  16  within the fuel cell  10 . The gaskets  18 ,  20  preferably comprise Teflon® materials such as Teflon coated fiberglass sheet, but may also comprise, for example, Viton® FEP (fluorinated ethylene propylene), and Nowoflon® PFA (perfluoroalkoxy). The end plates  14 ,  16  may comprise, for example, a metal.  
         [0017]     The membrane electrode assembly  12  comprises an ion exchange membrane  22  of a solid polymer electrolyte interposed between an anode  24 , and a cathode  26  on a side opposed to the anode  24 . The ion exchange membrane  22  is any ionically conductive and not electron conductive material capable of operating up to 250° C. independent of external humidification, for example, Celtec® membrane electrode assembles from PEMEAS, Inc., preferably between 2 and 8 mils in thickness. The gasket  18  forms an opening  23  wherein the anode  24  is positioned against a side  28  of the end plate  14  and the gasket  20  forms an opening  25  wherein the cathode  26  is positioned against a side  30  of the end plate  16 . The gasket preferably would have a thickness for which a sufficient compression and sealing is achieved.  
         [0018]     Backing plate  13  is positioned adjacent to the end plate  14  and backing plate  15  is positioned adjacent the end plate  16  on opposed ends of the cell (or a stack of cells as described hereinafter). The backing plates  13 ,  15  have internal passages for the flow of fuel and an oxidant, respectively, as described hereinafter with reference to  FIGS. 6 and 7 . The oxidant preferably comprises air, but could be any gas containing sufficient oxidant for reacting with protons from the anode  24 .  
         [0019]     The backing plates  13 ,  15 , end plates  14 ,  16 , gaskets  18 ,  20 , and ion exchange membrane  12  are held together, for example, by tightening bolts (not shown) inserted through holes  54  formed in opposed corners and secured by nuts (not shown).  
         [0020]     Referring to  FIG. 2 , a cross section taken along the line  2 - 2  of  FIG. 1  shows the anode  24  and cathode  26  each having a gas diffusion layer  32  and  34 , respectively, comprising carbon cloth, non-woven fabric, or paper of typically between 14 and 16 mils in thickness (z direction), and each have an electrode catalyst layer  36  and  38 , respectively comprising electrocatalysts such as platinum or alloys thereof, of between 10 and 20 microns in thickness (z direction). The material of the gas diffusion layer  32 ,  34  is laminated or pressed uniformly on the surface of the electrode catalyst layers  36 ,  38 , respectively. The electrode catalyst layers  36 ,  38  are affixed to opposite sides of the ion exchange membrane  22 . A plurality of fuel flow channels  40  and a plurality of air flow channels  42  are formed in the end plates  14  and  16 , respectively, and are described subsequently in more detail.  
         [0021]      FIG. 3  is a side view (the view is reversed from that of  FIG. 1 ) of the side  28  of plate  14  that is positioned against the anode  24  of the ion exchange membrane  22 . The fuel flow channels  40  comprise a plurality of grooves extending in a serpentine pattern that allows the fuel to flow back and forth across the anode  24  in both the x and y direction. The fuel enters the fuel flow channels  40  at a fuel inlet passage  44  and exits at a fuel outlet passage  46 .  
         [0022]     Referring to  FIG. 2 , the fuel diffuses into the gas diffusion layer  32  as it is distributed through the fuel flow channels  40 . Also, the air diffuses into the gas diffusion layer  34  as it is distributed through the air flow channels  42 . Protons from the fuel transverse the electrode catalyst layer  36 , the ion exchange membrane  22 , and the electrode catalyst layer  38  to the gas diffusion layer  34  in a manner known to those in the industry.  
         [0023]     The ideal performance of a fuel cell is defined by its Nernst potential, E, or the ideal cell voltage. The overall reactions for a hydrogen fuel cell is as follows: 
 
Anode: H 2 →2H + +2e − 
 
Cathode: ½O 2   +2H   + +2e − →H 2 O 
 
Overall Cell Reaction: H 2 +½O 2 →H 2 O 
 
Nernst Equation: E=E°+(RT/2F) In [P H2 /P H2O ]+(RT/2F) In [P 62 ]
 
         [0024]     At the anode, the reaction releases hydrogen ions (protons) and electrons whose transport is crucial to energy production. The protons build up on the anode creating a positive potential which promotes their transfer through the electrolyte (membrane) either by remaining connected through an attraction to a water or phosphoric acid molecule which travels through the electrolyte, or by transferring between water or phosphoric acid molecules. The oxygen side of the water molecule contains a slight negative charge which attracts the protons and may become attached to it, but the attraction is weak so any forces made are easily broken. The actual method of transfer varies depending on the type of electrolyte, but is based on the thickness of the membrane, the amount of water or phosphoric acid in the membrane, and the number of protons transported. Thus, the anode contains a net positive charge while the cathode, towards which the ions drift, contains a negative potential.  
         [0025]     The acid functional groups in the electrolyte serve to provide structure for the electrolyte as well as a barrier to electrons. It is conducive for electrons to flow through materials whose electrons are held loosely (conductive materials) because of the process of electron transport. Thus, electrons move from the reactions sites on the anode through the gas diffusion section of the electrode, through the anode current collector, through a load to do work, across the cathode current collector, through the gas diffusion section of the electrode on the cathode and then to the catalyzed reaction sites on the cathode. The electrons do not move through the electrolyte because the acid chains hold their electrons tightly and thus constitute an electric insulator. Other criteria for selecting an electrolyte are its structural stability, low resistance to ionic movement and low porosity.  
         [0026]     As the fuel is distributed and consumed through the fuel flow channels  40  ( FIG. 3 ), the protons available for conduction to the cathode  26  decrease in number. Just after the fuel first enters the fuel flow channels  40  (at section  70 ), availability of fuel is as high as 95-100%, for example. After the fuel has flowed along the fuel flow channels  40 , the fuel stream would be depleted to as low as 5% fuel availability as the fuel stream exits through the fuel outlet passage  46  (at section  72 ). By forming the serpentine fuel flow channels  40  in the manner shown in  FIG. 3 , an even distribution (concentration) of reactant (fuel) of about 50% is created across the anode. By positioning the fuel inlet passage  44  adjacent to the fuel outlet passage  46 , the 100% and 5% proton availability averages out to about 50%. At section  74 , the proton availability of the fuel would be about 75% while the fuel at section  76  would be about 25%, giving an average of about 50-55%. At section  78  the proton availability of the fuel would be about 50% while the fuel at section  80  would be about 50%, giving an average of 50%. The result is a more even distribution of fuel across the anode  24  than previously known.  
         [0027]      FIG. 4  is a side view of the side  30  of the end plate  16  that is positioned against the cathode  26  of the ion exchange membrane  22 . The air flow channels  42  comprise a plurality of short grooves extending in a preferably parallel pattern for allowing oxidizing and cooling gas to flow across the cathode  26 . The air enters the air flow channels  42  at an air inlet passage  66  and exits at a air outlet passage  50 .  
         [0028]     Referring to  FIG. 5 , a schematic block view illustrates a stack  52  of fuel cells  10  wherein, except for the cells  10  on the ends of the stack  52  (and more specifically end plates  14 ,  16 ), each cell  10  shares a bipolar plate  53  (positioned therebetween) with fuel being distributed on one side and an oxidizing gas being distributed on the other side. In this exemplary embodiment, the cell  10  on one end of the stack  52  has an end plate  14  for distributing fuel that is not shared with an adjacent cell  10  in the stack  52 , and the cell  10  on the other end of the stack  52  has an end plate  16  for distributing the oxidizing gas that is not shared with an adjacent cell  10  in the stack  52 .  
         [0029]     A fuel, such as hydrogen, is fed into the fuel inlet passage  44  at the backing plate  13 , traversing each of the cells  10  in the stack  52  through the fuel inlet passage  44  in the z direction. At end plate  14  and each bipolar plate  53 , some of the fuel is diverted through fuel flow channels  40  to the fuel outlet passage  46  and out of the fuel cell stack  52  at the backing plate  15 .  
         [0030]     Referring to  FIGS. 6 and 7 , a cut away view taken along lines  6 - 6  and  7 - 7  of the backing plates  13 ,  15 , respectively, are shown. The grooves  58 ,  62  and fan groove  64  are internal to the metal backing plates  13 ,  15 . An oxidizing agent, such as air preferably, or any gas containing oxidant, is pumped into air inlet passage  48  in backing plate  13  and transverses the stack  52  in the z direction. When the air reaches the backing plate  15  ( FIG. 7 ), the air migrates along groove  58  within the backing plate  15  to oxidation passage  60  and transverses the stack  52  in the −z direction. When the oxidizing agent reaches backing plate  13  again ( FIG. 6 ), it migrates along groove  62  and is dispersed by fan grove  64  to air inlet passage  66 . The oxidizing agent transverses the stack  52  through the air inlet passage  66  in the z direction. When the air reaches bipolar plates  52  and end plate  16 , the oxidizing agent is dispersed through parallel grooves  42  across the cathode  26 . After passing through groves  42 , the air exits through air outlet passage  50  to fan groove  67  and out of the fuel cell  10  through air outlet passage  68 .  
         [0031]     The distribution of air through the air inlet passage  48  in the z direction and then back through the air passage  60  in the −z direction absorbs heat from the cells  10  of the stack  52 . This both cools the stack  52  and preheats the air so the cells first exposed to the air are not cooled significantly more than the rest of the cells. By using the air to cool the stack  52  as well as for cathode oxidant, an additional cooling plate previously used in known fuel cells is avoided.  
         [0032]     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.