Abstract:
A new reactant delivery system for delivering reactants to the membrane electrode assembly of a fuel cell. The invention uses a plurality of small holes to propel high-velocity streams of reactant gases (“microjets”) against an impingement plate. The microjets assist in catalyzing the reactant gases and forcing them toward the proton exchange membrane. Reactant gas flow is primarily perpendicular to the orientation of the proton exchange membrane, thereby enhancing diffusion rates. In addition, each microjet acts like an expansion valve, which significantly cools the flowing gas and provides internal heat absorption. This internal heat absorption permits higher energy densities in the fuel cell.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This is a nonprovisional application claiming the benefit of the filing date of Provisional Application No. 60/808,836 which was filed on May 26, 2006 and names the same inventor. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     MICROFICHE APPENDIX 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of fuel cells. More specifically, the invention comprises a novel reactant delivery system in which microjets direct a stream of gas against an impingement plate. 
     2. Description of the Related Art 
     Proton Exchange Membrane (“PEM”) fuel cells have undergone extensive development since the 1950&#39;s. The first practical application occurred during the Gemini space flights of the 1960&#39;s. While a complete explanation of the operation of PEM fuel cells is beyond the scope of this disclosure, the reader may benefit from a simple explanation. 
       FIG. 1  shows a prior art PEM fuel cell in schematic form. This particular fuel cell uses gaseous hydrogen and gaseous oxygen as its reactants. Proton exchange membrane (“PEM”)  14  lies at the center of the device and is the key to its operation. When properly conditioned, this membrane will allow hydrogen ions to pass through, but will not allow the passage of electrons. In operation, hydrogen inlet  18  supplies hydrogen gas (a “reactant”) to hydrogen manifold  30 . Catalysts such as platinum or palladium strip the electrons from the hydrogen atoms to form hydrogen ions and free electrons. The catalysts are typically located on the exterior of the PEM. 
     The hydrogen ions flow through porous anode  12 , through PEM  14 , and into porous cathode  16 . At that point the hydrogen ions combine with oxygen supplied by oxygen manifold  32  to produce water. Oxygen inlet  20  supplies a suitable flow of gaseous oxygen. 
     The free electrons are unable to pass through PEM  14  because the membrane is electrically insulating. They are forced instead to flow through an electrical circuit including electrical load  22 . Electron flow  24  therefore provides electrical power to an external load, which is the primary purpose of the fuel cell. The “waste product” is water, which obviously poses no environmental concerns. 
       FIG. 2  shows a section through anode  12 , proton exchange membrane  14 , and cathode  16 . These three components are typically laminated together to form membrane electrode assembly (“MEA”)  26 . The anode and cathode are typically very thin (less than a millimeter). They may actually be formed by vapor deposition or electro deposition processes. Because the entire MEA must allow the passage of hydrogen ions, the anode and cathode (collectively referred to as the “electrodes”) must be porous. 
     The catalyst or catalysts are often also formed on the exterior of the MEA itself. Gas diffusion layer (GDL) is also typically added to the MEA&#39;s exterior in order to evenly distribute the fuel and oxidizer over the catalyst. The inclusion of the GDL can reduce the amount of catalyst needed. 
     The reader having an interest in further details regarding the nature of MEA&#39;s is referred to U.S. Pat. No. 3,134,697 to Niedrach (1964) and U.S. Pat. No. 6,099,984 to Rock (2000). Both these patents are hereby incorporated by reference in this disclosure. 
       FIGS. 1 and 2  provide a basic explanation of PEM fuel cell operation. However, as one might reasonably expect, the physical realization of the device is much more complex. Because fuel cells were critical to long term operations in space, fuel cell development was a critical obstacle in the moon race of the 1960&#39;s. PEM and MEA development took many thousands of man-hours. 
       FIG. 3  shows a simplified depiction of a PEM fuel cell. Membrane exchange assembly  26  includes a proton exchange membrane  14  sandwiched between anode  12  and cathode  16 . The anodes and cathodes are depicted as hatched lines to indicate their thin and porous nature. The depiction is not intended to show what the anode and cathode actually look like. MEA  26  would naturally include other components as well. Current collection grids and conduits would be attached to the anode and cathode. Sealing gaskets are also used. For purposes of visual clarity, these components have not been illustrated. 
     Membrane exchange assembly  26  is sandwiched between hydrogen manifold  30  and oxygen manifold  32 . Hydrogen inlet  18  supplies hydrogen gas to the hydrogen manifold while oxygen inlet  20  supplies oxygen gas to the oxygen manifold. Hydrogen and oxygen are considered the “reactants” for this type of fuel cell. 
     Mechanical features are often included in the prior art to facilitate clamping the assembly together. A series of mounting holes  42  pass in alignment through all the components. Bolts can be passed through these holes and nuts will then be tightened to clamp the assembly firmly together. 
     Of course, if one merely feeds gaseous reactants into the manifold, the fuel cell will not operate. The reactants must be ionized, and this is typically done by a catalyst placed on the anode and cathode. Palladium is a typical catalyst which can be deposited as a thin layer on the anode and cathode. 
     Serpentine passage  34  is cut into the face of the oxygen manifold which bears against MEA  26 . The serpentine passage allows the oxygen to flow smoothly over the catalyst, which may be deposited on the surface of the serpentine passage, the cathode surface, or both. The serpentine passage is needed to hold the gas and the catalyst in contact for a time sufficient to allow ionization. 
     A similar serpentine passage is cut into the surface of hydrogen manifold  30  which faces the MEA. The serpentine passages have been used in fuel cells for many years. However, their efficiency is limited. The catalyst is often exhausted in the proximity of the gas inlet long before it is exhausted in the “tail” of the serpentine passage. 
     In state-of-the-art fuel cells, the hydrogen and oxygen manifolds are often made of graphite. The manifolds themselves may therefore be used as electrodes, eliminating the need for separate components. The manifolds are placed in direct contact with opposing sides of the MEA so that they can conduct the electrical current created by the reaction within the fuel cell. The mounting holes may then be used to house electrical conductors, with external clamping means being used to assemble the fuel cell. 
     Those skilled in the art will know that the operation of a prior art fuel cell is limited by several factors. First, the appropriate amount of water must be maintained in the membrane electrode assembly in order to keep the MEA “soaked” (critical to its operation) yet not “flooded” (which will destroy the membrane&#39;s operation). This is true for most existing MEA&#39;s, with Nafion being the most commonly used PEM material. 
     Second, power output is often limited by internal heat generation. The fuel cell generates internal heat across the MEA and this must be dissipated. Too much heat will damage the fuel cell. Thus, increasing power is not simply a matter of pumping in more reactants. The reactant flow must be limited in order to limit the generation of heat. Thus, a fuel cell construction which reduces or absorbs heat generation would be highly beneficial. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a new reactant delivery system for delivering reactants to the membrane electrode assembly of a fuel cell. The invention uses a plurality of small holes to propel high-velocity streams of reactant gases (“microjets”) against an impingement plate. The microjets assist in diffusing the reactant gases evenly toward the proton exchange membrane. The PEM is impregnated with catalyst on both sides, so that the flowing gases come in contact with the catalyst. 
     Reactant gas flow is primarily perpendicular to the orientation of the proton exchange membrane, thereby enhancing diffusion rates. In addition, each microjet acts like an expansion valve, which significantly cools the flowing gas. The cooled gas returns to near-ambient conditions as it strikes the impingement plate and flows toward the MEA. However, the near-ambient temperature gas is injected directly against the MEA, the primary point of heat generation within the fuel cell. Thus, cooling is provided precisely where it is needed. The cooling effect is also quite uniform, eliminating potentially harmful localized temperature spikes. This internal heat absorption permits higher energy densities in the fuel cell. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic view, showing a prior art fuel cell. 
         FIG. 2  is a detailed elevation view, showing a membrane electrode assembly. 
         FIG. 3  is an exploded perspective view, showing a simplified physical embodiment of a prior art fuel cell. 
         FIG. 4  is an exploded perspective view, showing the oxidant delivery system of a fuel cell made according to the present invention. 
         FIG. 4B  is an exploded perspective view, showing the fuel cell of  FIG. 4  with the fuel delivery system added. 
         FIG. 5  is a detail view, showing details of the microjet plate. 
         FIG. 6  is a hidden line view, showing the impingement plate laid over the microjet plate. 
         FIG. 6B  is a hidden line view, showing details of how the microjets align with the webs in the impingement plate. 
         FIG. 7  is a perspective view with a cutaway, showing the interaction of the microjet plate with the impingement plate. 
         FIG. 7B  is a section elevation view, showing the operation of the microjets. 
         FIG. 8  is an exploded perspective view, showing the stacking of the retaining plate, the impingement plate, and the microjet plate. 
         FIG. 8B  is an exploded perspective view, showing the stacking of the retaining plate, the impingement plate, and the microjet plate from the opposite side. 
         FIG. 9  is an exploded perspective view, showing how to create a stacked fuel cell using the present invention. 
         FIG. 10  is an exploded perspective view, showing how to create a stacked fuel cell using the present invention. 
         FIG. 11  is an exploded perspective view, showing an alternate embodiment of the present invention. 
         FIG. 12  is a section view, showing an alternate embodiment of the present invention. 
     
    
    
     
       
         
               
             
               
               
               
               
             
           
               
                   
               
               
                 REFERENCE NUMERALS IN THE DRAWINGS 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 fuel cell 
                 12 
                 anode 
               
               
                 14 
                 proton exchange membrane 
                 16 
                 cathode 
               
               
                 18 
                 hydrogen inlet 
                 20 
                 oxygen inlet 
               
               
                 22 
                 electrical load 
                 24 
                 electron flow 
               
               
                 26 
                 membrane electrode assembly 
                 30 
                 hydrogen manifold 
               
               
                 32 
                 oxygen manifold 
                 34 
                 serpentine passage 
               
               
                 36 
                 retaining plate 
                 38 
                 impingement plate 
               
               
                 41 
                 waste product hole 
                 40 
                 microjet plate 
               
               
                 42 
                 mounting hole 
                 43 
                 conduit hole 
               
               
                 44 
                 oxygen stagnation chamber 
                 45 
                 end plate 
               
               
                 46 
                 microjet hole 
                 47 
                 hydrogen stagnation 
               
               
                   
                   
                   
                 chamber 
               
               
                 48 
                 exhaust hole 
                 49 
                 center-to-center line 
               
               
                 50 
                 web 
                 52 
                 gas flow 
               
               
                 54 
                 impingement plate recess 
                 61 
                 thinned section 
               
               
                 63 
                 microjet 
                 73 
                 waste product channel 
               
               
                 80 
                 oxygen manifold 
                 82 
                 oxygen supply line 
               
               
                 84 
                 stagnation chamber 
                 86 
                 microjet plate 
               
               
                 88 
                 impingement plate 
                 90 
                 load 
               
               
                 92 
                 oxygen electrode 
                 94 
                 proton exchange membrane 
               
               
                 96 
                 hydrogen electrode 
                 98 
                 hydrogen manifold 
               
               
                 100 
                 hydrogen supply line 
                 102 
                 aqueous chamber 
               
               
                 104 
                 threaded connector 
                 108 
                 alkaline solution 
               
               
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  shows a fuel cell incorporating the present invention. Membrane electrode assembly  26  is essentially the same as for the prior art. The example uses hydrogen (fuel) and oxygen (oxidizer) as the reactants. As for the prior art, the MEA must be sandwiched between two manifolds delivering ionized gaseous reactants. However, the method of delivering the reactants is substantially different. 
       FIG. 4  shows the MEA and the oxygen delivery components. The components are shown in an exploded view. When they are assembled they are all tightly clamped together using external clamping plates (not shown). 
     Retaining plate  36  clamps against the far side of the MEA. It retains impingement plate  38  in the correct position again microjet plate  40  when microjet plate  40  is clamped against the retaining plate. Oxygen manifold  32  is then clamped against the back side of the microjet plate. End plate  45  seals the far side of oxygen manifold  32  to form oxygen stagnation chamber  44 . In operation, pressurized oxygen is fed in through oxygen inlet  20 . The pressurized oxygen within oxygen stagnation chamber  44  is then forced through a plurality of microjet holes  46  in microjet plate  40 . 
     The flow assumes the form of high velocity microjets. These impinge upon impingement plate  38 , in an arrangement to be described in more detail subsequently. The gas flow then passes through exhaust holes in the impingement plate and then to the MEA. Once the oxygen leaves the oxygen stagnation chamber, it generally flows in a direction which is perpendicular to the plane of the proton exchange membrane. 
     The catalyst is evenly distributed on the outward-facing surfaces of the MEA. A gas diffusion layer (GDL) may also be included on the MEA, though the use of the microjet-based delivery approach will likely eliminate the need for this component. 
     Numerous additional components have been omitted for visual clarity. Each of the components includes a conduit hole  43  in the middle of each side and an exhaust hole  41  proximate each corner. The conduit holes house appropriately positioned electrical conductors which carry the electrical current to an external load (as well as possibly hooking up successive cells in a series connection). Waste product holes  41  house components for carrying away waste products. 
     Gaskets would be employed between the various layers to prevent gas leakage. Other hardware is needed to channel the water formed and to retain the proper saturation level for the MEA. All these components are well known to those skilled in the art and—accordingly—they have not been illustrated. 
     In  FIG. 4B , the MEA and the oxygen delivery components have been clamped together. The hydrogen delivery components are shown in exploded form. The reader will observe that the hydrogen components are simply the oxygen components rotated 180 degrees and attached to the opposite side of the MEA. Hydrogen manifold  30  is closed by an end plate  45 , which forms hydrogen stagnation chamber  47 . The hydrogen manifold clamps against the back of microjet plate  40 . The microjet plate clamps against impingement plate  38 , which is retained in the proper position by retaining plate  36 . All the hydrogen components are then clamped against the side of the MEA which faces the viewer. The result is a completed fuel cell having one set of electrodes. The fuel cell can be stacked to form a multi-cell unit as for the prior art. The type of stacking will be explained subsequently. 
     Of course, those skilled in the art will realize that the microjet sizes may need to be adjusted for different reactants. Thus, while the oxygen and hydrogen microjet plates may appear to be the same, the hole sizes and separations may in fact be different. 
       FIG. 5  shows a detail view of a portion of microjet plate  40 . The reader will observe how the thinned section in the middle of the plate is pierced by many microjet holes  46 .  FIG. 6  is a hidden line view showing how impingement plate  38  lies over microjet plate  40 . Impingement plate  38  includes a plurality of exhaust holes  48 . The array of microjets  46  are positioned to be directly beneath the webs between the exhaust holes in the impingement plate.  FIG. 6  clearly shows this alignment. 
       FIG. 6B  shows the alignment in greater detail. The exhaust holes are preferably placed in a geometric pattern. Center-to-center line  49  connects the centers of adjacent exhaust holes (12 of these are shown as phantom lines in the view). Each microjet hole  46  preferably lies beneath the middle of a center-to-center line, which places each microjet hole in the center of a web  50  between adjacent exhaust holes. This alignment ensures that the microjet produced will strike a solid portion of the impingement plate, but that the “strike” will also be close to at least two exhaust holes. 
       FIG. 7  shows a perspective view of impingement plate  38  lying over microjet plate  40  with a cutaway added to show internal details. The cutaway is angled to pass through several microjet holes  46 . Pressurized oxygen within oxygen stagnation chamber  44  is forced through these holes and against impingement plate  38 . A gap exists between the microjet and impingement plates, since the microjet holes are formed in thinned section  61 . 
       FIG. 7B  shows a section view through two adjacent microjet holes  46 . The pressurized oxygen flowing through each of these small holes creates a microjet  63 . The microjet strikes the impingement plate and diffuses the gas at high velocity. Gas flow  52  then flows through exhaust holes  48  toward the membrane electrode assembly which lies adjacent to the impingement plate. The flowing gas then reacts with the catalyst on the MEA. 
     Each microjet hole acts like a small expansion valve. The gas within oxygen stagnation chamber  44  is essentially stagnant. As the gas flows through the microjet hole, it accelerates to high velocity. A pressure and temperature drop results from the Bernoulli effect. Thus, if the gas enters the stagnation chamber at room temperature, it will be substantially cooled by passing through the microjets. The temperature of the flowing gas recovers to near-ambient conditions after striking the impingement plate and then flowing into the MEA. Thus, the gas entering the MEA is back to near-ambient conditions. However, this near-ambient gas is flowing directly into the point of maximum heat generation (where the reaction is occurring in the outer regions of the MEA). This fact means that cooling is provided exactly where it is needed. 
     In addition, the cooling is provided in a very even and controlled manner across the entire face of the MEA. Localized “hot spots” are thereby minimized. This cooling phenomenon allows the operation of fuel cells having a higher energy density. This allows a faster reactant delivery (and more extracted power) without exceeding the fuel cell&#39;s temperature limit. 
     The cooling mechanism also allows inexpensive materials to be used for many of the components, since high temperatures will no longer be a problem. Some or all of the components in the reactant delivery systems can be made of relatively inexpensive thermoplastics. 
     The even gas distribution provided by the microjet delivery system also ensures a uniform gas distribution to the catalyst on the MEA. The catalyst is thereby uniformly consumed, which means that a lesser total amount of catalyst will be needed than for the prior art devices. 
       FIG. 8  shows an exploded view of retaining plate  36 , impingement plate  38 , and microjet plate  40 . Many different geometric arrangements can be made between the microjet array and the impingement plate. The alignment of the microjet holes with the webs on the impingement plate is only one effective example. However, whatever alignment is chosen, it is significant that the alignment be maintained by fixing the position of the impingement plate relative to the microjet plate. In  FIG. 8 , the reader will observe that the side of the retaining plate which faces the impingement plate includes impingement plate recess  54 . This fits around the perimeter of the impingement plate and secures it in position. 
       FIG. 8B  shows the same assembly from the other side. Four waste product channels  73  are contained in the side of retaining plate  36  which faces the MEA. These can be used to carry waste products to the four waste product holes  41  which carry away water in the case of a hydrogen/oxygen fuel cell. 
     Those skilled in the art will know that PEM fuel cells produce a relatively low electrical potential across each individual membrane electrode assembly. Thus, prior art cells are typically stacked in series in order to increase the voltage which is ultimately extracted from the stacked cells. The assembly of several mated fuel cells is actually referred to as a “stack.” The nature of the prior art fuel cells, such as shown in  FIG. 3 , allows the stacking to proceed positive-negative-positive-negative by simply connecting adjoining cells together. 
     The structure of the present invention suggests a different arrangement.  FIG. 9  shows a single oxygen manifold  32  being used to feed two microjet plates  40  (facing in opposite directions). Each microjet plate feeds through an impingement plate and to a membrane electrode assembly  26 . 
     In  FIG. 10 , all the components shown in  FIG. 9  have been clamped together into the unified “block” shown in the middle. The two MEAs  26  then form the outward facing portions.  FIG. 10  shows a pair of exploded hydrogen supply components in position and ready to be clamped onto the central block. 
     Once all the components are clamped together, the pair of hydrogen manifolds  30  will feed hydrogen through microjet and impingement plates through to the two MEAs. The polarity of this assembly will then be negative-positive-positive-negative, so it is not possible to simply connect adjacent portions together in series. Instead, alternating bus bar arrangements must be made in order to create the positive-negative-positive-negative arrangement needed to obtain higher voltages. These bus bars are housed within the conduit holes. 
     Since the stack will not have the conventional positive-negative-positive-negative arrangement, it may be undesirable to use graphite for the manifolds (which would make the manifolds themselves act as the electrodes. Separate electrodes may be preferable. These will be porous components likely formed on the outer surfaces of the MEA itself. The bus bars will then be used to conduct the electricity out of each cell. 
     Of course, the stack shown in  FIG. 10  is not limited to two cells. Two more microjet and MEA assemblies can be substituted for the end plates and the stack can be extended in both directions for many additional cells. These principles are well understood by those skilled in the art and are therefore not further illustrated. 
     Another embodiment of the present invention is illustrated in  FIGS. 11 and 12 . In this embodiment an electrolytic, alkaline solution is used on one side of the assembly to carry positively charged hydrogen protons to the proton exchange membrane. As illustrated in  FIG. 11 , the assembly includes oxygen manifold  80  having stagnation chamber  84  which is fluidly connected to oxygen supply line  82 . Microjet plate  86 , impingement plate  88 , oxygen electrode  92 , and proton exchange membrane  94  are arranged in the same configuration as previously described. 
     Hydrogen manifold  98  has aqueous chamber  102  which contains the electrolytic, alkaline solution. Hydrogen electrode  96  is attached to hydrogen supply line  100  by threaded connector  104 . Hydrogen electrode  96  and oxygen electrode  92  are connected to an electrical circuit including load  90 . 
       FIG. 12  illustrates the assembly of  FIG. 11  in an assembled state. It is sectioned in half to aid visualization. Microjet plate  86 , impingement plate  88 , oxygen electrode  92  and proton exchange membrane  94  are sandwiched together between oxygen manifold  80  and hydrogen manifold  98  (as for the previously-described embodiments). Alkaline solution  108  fills the aqueous chamber. It is important that the alkaline solution have a pH of at least about 12.0 or greater for effective operation of the fuel cell. A 1 molar solution of potassium hydroxide in water is particularly effective. 
     Oxygen is fed to stagnation chamber  84  through oxygen supply line  82 . The oxygen is forced through microjet holes in microjet plate  86  and diffused by impingement plate  88  as previously described. 
     Hydrogen is fed through hydrogen supply line  100  to hydrogen electrode  96 . The hydrogen electrode includes a hollow interior which feeds the hydrogen gas out through perforations extending through its wall (shown best in  FIG. 12 ). Electrons are stripped off of the hydrogen molecules and pass through the electrical circuit and load  90 . The positively charged hydrogen protons are then able to pass through proton exchange membrane  94  where they react with oxygen to form water. The water is then transmitted out of the assembly through an exhaust port. Because the reaction occurs on the oxygen side of the membrane, the water does not dilute alkaline solution  108 . 
     The readers should bear in mind that the structure shown for the hydrogen electrode is meant to be representative rather than an actual working version. In reality, the diameter of the perforations would likely be much smaller (and therefore harder to see in the illustrations). The size and shape of the hydrogen electrode might be substantially different as well. None of these modifications would alter the invention&#39;s structural nature. 
     The preceding description contains significant detail regarding the novel aspects of the present invention. It should not be construed, however, as limiting the scope of the invention but rather as providing illustrations of the preferred embodiments of the invention. As one example, new proton exchange membranes are being developed which do not require humidification in order to be effective. The present invention could function with these newer “dry” membranes just as well as for the “wet” membranes described in the examples. Accordingly, the scope of the invention should be fixed by the following claims, rather than by the examples given.