Abstract:
A method for producing a fuel cell core including: providing two identical subassemblies each including a substrate and a current collector removably arranged thereon, depositing an ionic liquid or pasty polymerizable membrane on at least one of the subassemblies in such a way that the collector thereof is completely covered, applying the subassemblies one against the other so as to obtain an assembly having a solidified membrane with the two collectors incorporated, face to face, in this membrane, and detaching the two substrates from the collectors.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of fuel cells. It relates, more particularly, to a method for producing a miniature cell core. It also relates to a cell core and a cell obtained by this method. 
     BACKGROUND 
     A fuel cell is described in document FR 03 07 967. It consists of a large number of elementary cells disposed in series and each having a stack comprising, as shown diagrammatically in  FIG. 1 , an anode  13   a  and a cathode  13   b  separated by an electrolytic membrane  10 . The stack is positioned between two current collecting plates  11  and  12  through a diffuser ( 14   a  and  14   b ). 
     The patent application EP 04 405063.1 presents a method according to which the current collector is made by galvanic deposition on a substrate and then transferred onto the membrane so as to be held there by being inlaid or by adhesive bonding. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method enabling the core of a fuel cell to be further miniaturized and enabling the performance to be improved and production costs to be reduced. 
     More precisely, the invention relates to a method for producing a fuel cell core that may comprise the following operations in sequence:
         providing two identical subassemblies formed of a substrate and a current collector positioned on it in a detachable manner,   depositing a polymerizable ionic membrane in the liquid or pasty state on at least one of said subassemblies so as to cover its collector completely,   applying the two subassemblies obtained one against the other so as to obtain an assembly comprising a solidified membrane with the two collectors incorporated, face to face, in this membrane, and   detaching the two substrates from the collectors.       

     The collectors may be either formed in situ on their substrate or formed separately and then added to their substrate. 
     According to a first embodiment of the invention, a membrane may be deposited on the two subassemblies. 
     According to a second embodiment of the invention, a membrane may be deposited on only one of the two subassemblies. 
     The invention also relates to a fuel cell core made by the above method wherein the assembly obtained is inserted between two rigid frames in the manner of a transparency. 
     Finally, the invention relates to a fuel cell of which the core is made by the above method and wherein the assembly obtained may be inserted between two rigid covers provided with connecting end pieces and providing, either side of the assembly, spaces for necessary reagents. 
     Advantageously, the frames and covers may also serve as supports for electrical contacts connecting the two collectors to the outside. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features of the invention will become apparent subsequently on reading the following description, made with regard to the appended drawings in which: 
         FIG. 1  shows diagrammatically a fuel cell as described in document FR 03 07 967, 
         FIG. 2  and  FIG. 3  illustrate two different ways of implementing the method, 
         FIG. 4  shows the incorporation of the cell core between two frames, and 
         FIG. 5  shows the incorporation of the cell core between two covers. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will first of all be made to  FIG. 2  which shows diagrammatically the main operating principles of a first exemplary embodiment of the invention. 
     
       FIG. 2 
       a  
     
     At the start of the method, two identical subassemblies  20   a - 20   b  are provided, each formed of a substrate  21  and a metal current collector in the form of a mesh  22  deposited on the substrate. The collector may have, in section, a mushroom or harpoon profile and, typically, a thickness of 5 to 10 μm. The face of the substrate which receives the collector may be such that it can be detached therefrom by mechanical, chemical or thermal action. 
     Each collector  22  may advantageously be made in situ by galvanic deposition of gold with the aid of a mask formed on the substrate according to the method described in detail in document EP 04 405063.1 already mentioned. 
     As a variant, collectors  22  could also be made separately and then added to the substrates and fixed to these by adhesive bonding. 
     
       FIG. 2 
       b  
     
     A polymerizable ionic semi-membrane of the Nafion® (cationic) or of the ADP-Morgane® (anionic) type is deposited in the liquid or pasty state on each subassembly  20   a - 20   b  so as to cover collectors  22  completely. Typically, this layer may be spread out by a technique known by the name of “spin coating” and may have a thickness of 10 to 20 μm. 
     
       FIG. 2 
       c  
     
     After prepolymerization of semi-membranes  23 , an operation which is not indispensable, resulting subassemblies  24   a - 24   b  may be fixed respectively on the work plates of a machine, called “flip chip bonding” machine, not shown in the drawing, and well known to a person skilled in the art, two semi-membranes  23  facing each other. 
     
       FIG. 2 
       d  
     
     The alignment and flatness of two subassemblies  24   a - 24   b  having preferably been adjusted, they are applied one against the other by the machine under pressure at a temperature and for a period such that semi-membranes  23  are welded to each other and solidified by polymerization. 
     
       FIG. 2 
       e  
     
     When the two plates of the machine are separated, collectors  22  have to be detached from their respective substrates  21 . In the case of collectors formed galvanically, separation may be made by a simple mechanical action. If the collectors have been brought together and fixed by adhesive bonding, separation may be made by chemical and/or thermal action. 
     The result is an assembly  25  comprising a solidified membrane  26  and two collectors  22  incorporated, face to face, in the membrane. Typically, the assembly may have a thickness of 20 to 40 μm. 
     
       FIG. 2 
       f  
     
     The two faces of assembly  25  may be covered, above collectors  22 , with a catalyst layer  27  essentially comprising catalyst elements properly so called, such as platinum and ruthenium, and electrical and ionic conducting elements such as carbon and the same material as that which constitutes membrane  26 . 
     A variant of this method is illustrated in  FIG. 3  on which the elements identical to those in  FIG. 2  carry the same reference numbers. 
     
       FIG. 3 
       a  
     
     The first operation is identical to that of  FIG. 2   a.    
     
       FIG. 3 
       b  
     
     A polymerizable ionic membrane  28  of the same type as semi-membranes  23 , but with a double thickness, is deposited in a liquid or pasty state on assembly  20   a  in order to constitute subassembly  29 . 
     
       FIG. 3 
       c  
     
     After prepolymerization of membrane  28 , an operation that is not indispensable, assemblies  20   b  and  29  may be fixed respectively onto the work plates of a “flip chip bonding” machine, membrane  28  facing collector  22  of assembly  20   b.    
     
       FIG. 3 
       d  
     
     The alignment and flatness of assemblies  20   b  and  29  having been adjusted, they may be applied against each other by the machine under pressure at a temperature and for a duration such that collector  22  of assembly  20   b  may be inlaid in the membrane which solidifies by polymerization. 
     
       FIG. 3 
       e  
     
     The operation is identical to that of  FIG. 2   e.    
     
       FIG. 3 
       f  
     
     The operation is identical to that of  FIG. 2   f.    
     Whatever the method used, the structure obtained may suffer from the fact that thin membrane  26  risks being deformed under the action of moisture, which may present a problem when the assembly has to be handled in order to incorporate it in a fuel cell. 
     According to the invention, as illustrated in  FIG. 4 , the above problem may be solved by inserting assembly  25  between two rigid frames  30 , which may advantageously be made of PVC and fixed to each other with the aid of points  31  passing through membrane  26 . It will be noted that these frames may also serve as supports for electrical contacts  32  connecting two collectors  22  to the outside. 
     This “packaging” of the assembly, in the manner of a transparency, makes it possible to stabilize the shape of the membrane and makes it easier to handle. 
     Finally, reference will be made to  FIG. 5  showing that simple frames  30  of  FIG. 4  are replaced by rigid covers  33 , which may advantageously be made of PVC and also fixed to each other by points (not shown). These covers may be provided with connecting end pieces  34  which also ensure stiffening of the assembly but moreover provide, either side of catalysts  27 , spaces  35  for the necessary reagents. Seals  36  ensure the leakproofness of these spaces. The structure of  FIG. 5  thus constitutes a complete fuel cell. 
     The present invention has been described with reference to isolated assemblies. In practice, as is conventional in the field of microelectronics, several assemblies provided with their frames or covers may be produced by forming a single membrane on a matrix of collectors. The assemblies may finally be separated by cutting the membrane around the frames or covers. 
     It is therefore proposed to produce a miniature fuel cell core which, by virtue of the integration of current collectors into the membrane, greatly improves the membrane-collectors-catalysts contact and, by virtue of the use of frames, makes it possible to prevent deformations of the membranes without excessive supplementary costs. The invention also makes it possible, by virtue of the use of covers, to provide a ready-to-operate miniature fuel cell. Finally, it will be noted that the harpoon shape of the current collectors appreciably reinforces their strength in the membrane.