Abstract:
A hydrogen-oxygen fuel cell including an electrolyte sandwiched between two catalyst layers or sheets, each catalyst layer or sheet being in contact with a porous electrode, in which one or several catalyst layers or sheets and one or several electrode layers or sheets interpenetrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of French patent application number 09/50211, filed on Jan. 15, 2009, entitled “FUEL CELL ELECTRODE,” which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to fuel cells and in particular, to hydrogen and oxygen fuel cells, and more specifically relates to the electrode structure of such cells. 
     2. Discussion of the Related Art 
     As illustrated in  FIG. 1 , a hydrogen-oxygen fuel cell comprises a layer or a sheet of an electrolyte  1  sandwiched between two catalyst layers or sheets  3  and  4  coated with conductive layers  6  and  7  intended for the contacting. The upper cell surface is in contact with oxygen, for example, air, and the lower cell surface is in contact with hydrogen. 
     Under such conditions, when the cell is connected to a load  8 , a positive voltage appears on the side of the upper surface or cathode and a negative voltage appears on the side of the lower surface or anode and a current flows through the load. On the anode side, the catalyst transforms gaseous hydrogen molecules into two protons and two electrons, the protons flow from the anode catalyst layer, through the electrolyte layer to the cathode catalyst layer where reaction 2H + +½O 2 +2e − →H 2 O occurs, with both electrons flowing through the load. 
     Presently, electrolyte  1  is Nafion and catalyst  3 ,  4  is a mixture of platinum carbon, for example comprising a few percents of platinum, and preferably contains an amount of Nafion, for example, from 20 to 40%. 
     Conductors  6  and  7  for example are very thin gold layers to be both conductive and permeable to hydrogen or oxygen. Conductors  6  and  7  may also be formed of gold grids. In the case where conductors  6  and  7  are very thin gold layers, for example, of a thickness below 1 μm, to be sufficiently permeable to gas, they are poorly conductive since the catalyst layers are porous layers and the gold layers do not form even planar surfaces, but deposit discontinuously by penetrating into the catalyst pores. A problem of contact resistance (or current collection) thus arises. In the case where conductors  6  and  7  form a gold grid with meshes of a thickness from 2 to 3 μm, an obvious disadvantage is that, under the solid portions of the grid, the gas cannot react with the underlying catalyst. 
     Another parameter which adversely affects a satisfactory current collection is that the material of the catalyst, as indicated previously, preferably is a mixture of platinum carbon and electrolyte (for example, Nafion). The presence of Nafion in the catalyst layers enhances a good operation of the cell but since Nafion is a good proton conductor but a poor electron conductor, this increases the internal resistance of the cell. As a result, the proportion of Nafion has to be decreased in the catalyst, which adversely affects the efficiency of this catalyst. Indeed, when the amount of Nafion in the electrode is decreased, the proton conduction across the electrode bulk, and thus the amount of exchanged protons, is decreased. 
       FIG. 2  shows an embodiment of a hydrogen and oxygen fuel cell. The cell is formed from a support made of a portion of a silicon wafer  10 , preferably thinned down in the portion where it supports the actual cell. In this thinned-down portion, openings  11  letting through oxygen are made in silicon wafer  10 . It should be understood that, generally, all the wafer surfaces are coated with an insulator formed at least of native silicon oxide. An active cell area is delimited, substantially above the thinned-down region comprising the openings, by an insulator layer  12 , for example, a silicon oxide layer. Inside of the opening formed in layer  12 , a catalyst layer  3  is formed by any process, for example by inkjet deposition. An electrolyte  1 , such as a Nafion layer, is, for example, spun on and covered with a second catalyst layer  4 . Previously, a conductor  6 , such as a gold anode layer, for example having a thickness on the order of one micrometer, has been formed on the surface of a support such as silicon wafer  10 . Conductor  7  may be a gold cathode layer for example having a thickness on the order of one half micrometer formed on the upper surface of the structure. The foregoing is an example only of a possible embodiment of a hydrogen-oxygen fuel cell, this embodiment having the advantage of using technologies well known in silicon-based component manufacturing for the support. In such a structure, the contact between conductor  6  corresponding to a conductive anode layer and catalyst layer  3  is not a major issue but the quality of the contact between conductor  7  corresponding to a thin gold cathode layer and catalyst layer  4  is difficult to achieve if, as indicated previously, both a good collection of charge carriers and a good “transparency” of the layers for a proper diffusion of the oxygen in catalyst layer  4  are desired to be obtained. Currently, the useful surface area of the fuel cell ranges from 1 to 3 cm 2 . 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention aims at solving at least some of the problems of prior art fuel cell contact electrodes. 
     An embodiment of the present invention aims at providing a novel hydrogen-oxygen fuel cell electrode structure enabling to improve the collection of electrons. 
     To achieve these and other objects, an embodiment of the present invention provides a hydrogen-oxygen fuel cell comprising an electrolyte sandwiched between two catalyst layers or sheets, each catalyst layer or sheet being in contact with a porous electrode, in which one or several catalyst layers or sheets and one or several electrode layers or sheets interpenetrate. 
     According to an embodiment of the present invention, the fuel cell comprises an alternation of catalyst layers and of electrode layers, the electrode layers being interconnected. 
     According to an embodiment of the present invention, the electrode snakes across the catalyst bulk, with a corrugated or rippled shape. 
     According to an embodiment of the present invention, the porous electrode layer is a layer of a conductive material of a thickness smaller than 1 μm, preferably on the order of from 0.2 to 0.6 μm. 
     According to an embodiment of the present invention, the porous electrode layer(s) is (are) a grid(s) of a conductive material. 
     According to an embodiment of the present invention, the conductive material is gold. 
     The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section view very schematically illustrating a fuel cell structure; 
         FIG. 2  is a cross-section view illustrating an embodiment of a fuel cell; 
         FIG. 3  is a cross-section view of a fuel cell electrode according to an embodiment of the present invention; and 
         FIG. 4  is a cross-section view of a fuel cell electrode structure according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is an enlarged view of the upper portion of an electrolyte  1  of a hydrogen-oxygen fuel cell. It may be an electrolyte sheet or a deposited layer. The following description will consider the case of an electrolyte layer on which are deposited catalyst and electrode layers. This should not be considered as a limitation and the present invention encompasses the case where the electrolyte is a sheet on which catalyst and electrode layers are deposited, the case where the electrolyte is a sheet against which are arranged catalyst and/or electrode sheets, and combinations of these various cases. 
     In  FIG. 3 , a layer  21  of a mixture of a material forming a catalyst, for example mainly comprising carbon powder, platinum, and a proportion of electrolyte, for example, Nafion, is deposited on electrolyte layer  1 , for example by inkjet-type projection. A very thin gold layer  22  is deposited on catalyst layer  21 , for example, by sputtering. After this, a second catalyst layer  23 , a second very thin gold layer  24 , and a final catalyst layer  25  are formed. The two gold layers  22  and  24  superpose at the periphery to form a thicker gold layer  27  on which a contact can be taken. 
     With this structure, the oxygen reaching the upper surface side reacts well with catalyst layer  25  and may cross very thin gold layers  24  and  22  to also react with catalyst layers  23  and  21 . The very thin gold layers may have a thickness on the order of a few tenths of a μm, for example, from 0.2 to 0.6 μm, preferably on the order of 0.3 μm. On the other hand, given the interpenetration between the conductive layers and the catalyst layers, the catalyst layers can be more resistant without adversely affecting the quality of the current collection and the internal resistance of the cell. Thus, the catalyst layers can contain more Nafion than usual, and instead of containing from 20 to 40% of Nafion, they may for example contain on the order of from 35 to 45% of Nafion, which favors the reaction between oxygen, protons, and electrons and increases the amount of current that the cell may supply for a given voltage. 
     The present invention contemplates any other structure in which conductive layers interpenetrate catalyst layers. For example, in the case of the example of  FIG. 3 , instead of very thin gold layers, one or several gold grids of a thickness from 2 to 5 μm or grids of any other conductive material compatible with a contacting on or inside of the catalyst layer. 
       FIG. 4  shows an embodiment of the present invention in which, above electrolyte layer  1  is formed a first catalyst layer  31  having an uneven upper surface, so that a deposited gold layer  33  has an uneven shape, for example, the illustrated corrugated or wavy shape. This gold layer is then coated with a second catalyst layer  35 . With such a structure, the same type of advantages as those described in relation with  FIG. 3  is obtained. It should be noted that several thicknesses of gold layers with a corrugated surface such as layer  33  may alternate with catalyst layers. Once again, in this embodiment, the gold layer(s) may be replaced with gold grids. 
     The present invention has been more specifically described in relation with embodiments relating to a specific fuel cell embodiment in which various materials are deposited in successive layers. The present invention also applies to the case where the cell is formed of superposed sheets of the various materials, which are for example assembled under pressure. 
     Similarly, the present invention has been described in the context of a specific embodiment where a specific electrode structure is used for the contact with the cathode catalyst. The present invention may similarly also be implemented in relation with the anode layer, or only with it in some specific embodiments. 
     Various embodiments with different variations have been described hereabove. Those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step. Further, materials other than Nafion may be used for the electrolyte and materials other than carbon/platinum may be used for the catalyst, for example, carbon/platinum-cobalt or carbon/platinum-nickel. 
     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.