Abstract:
A method for production of a solid oxide fuel cell (SOFC) ( 1 ), having an electrolyte body ( 10 ) with a tubular structure, wherein at least one internal electrode ( 11 ) and one external electrode ( 12 ) are applied to the tubular electrolyte body, with the method having at least the following steps: provision of an injection molding core ( 13 ) on which at least one interconnector material ( 14 ) and the internal electrode ( 11 ) are mounted, arrangement of the injection molding core ( 13 ) in an injection mold ( 25   a,    25   b ), injection molding of an electrolyte compound ( 10   a ) in order to form the electrolyte body ( 10 ), and removal of the injection molding core ( 13 ) in the form of a casting process with a lost core.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a method for production of a solid oxide fuel cell (SOFC) having an electrolyte body in tubular form, wherein at least one internal electrode and one external electrode are applied to the electrolyte body. 
         [0002]    Solid oxide fuel cells (SOFC) with a ceramic electrolyte body form a high-temperature variant of fuel cells. They are operated at 600° C. to 1000° C., and in the process provide very high electrical efficiencies of up to about 50%. In principle, solid oxide fuel cells are subdivided into two main variants: one variant is formed by a tubular shape of the electrolyte body which, according to a further variant, can be bounded, by a flat, planar form. In this case, it is necessary to fit the internal and external electrodes to the wall of the electrolyte body during the method for production of a solid oxide fuel cell based on the tubular variant. For this purpose, it is known for the electrode which is arranged on the inside of the electrolyte body, generally in the form of the anode, around which the fuel gas flows, to be applied as a coating on the inner wall of the electrolyte body. In this case, the electrolyte body is preferably extruded. 
         [0003]    In addition to the mounting of the electrodes on the finished electrolyte body, so-called interconnectors are applied flat to the electrodes in order to make contact with them, in which case fitting on the inside of the electrolyte body frequently leads to problems. Metallic interconnectors are known, which are composed of a material with a high chromium content, in order to obtain adequate corrosion resistance, combined with adequate electrical conductivity, on the basis of the high operating temperatures. The chromium-oxide layer which is formed during operation of the fuel cell in this case has a negative effect on the cathode material, and can lead to premature aging of the fuel cell. In contrast, ceramic interconnectors for making electrical contact with the electrodes are known in the planar form, but have not previously been used for tubular solid oxide fuel cells. Because the electrolyte bodies are very thin, there are major problems associated with fitting ceramic interconnectors to the inner wall of the electrolyte body. The problem results in particular from the thin walls since, for example, the electrolyte body preferably has a wall thickness of about 200 μm, in which case the porous anode on the inside of the electrolyte body must be coated with a ceramic interconnector layer. For cost reasons, the electrodes themselves must likewise be made thin, for example with a thickness of 50 μm. Furthermore, the interconnector layer must likewise be highly porous, with further advantages being obtained from the interconnector material on the surface of the electrolyte body having a thickness which varies over the longitudinal extent of the electrolyte body. 
       SUMMARY OF THE INVENTION 
       [0004]    The object of the present invention is therefore to overcome the disadvantages mentioned above relating to the production of a solid oxide fuel cell, and to provide a method for producing an electrolyte body, as well as the electrodes and corresponding interconnector layers which are required on the inside and/or outside of the electrolyte body, in a simple manner. 
         [0005]    The invention includes the technical teaching that the method for production of a solid oxide fuel cell comprises at least the steps of provision of an injection molding core on which at least one interconnector material and the internal electrode are mounted, with the method furthermore comprising the arrangement of the injection molding core in an injection mold, the injection molding of an electrolyte compound in order to form the electrolyte body, and the removal of the injection molding core in the form of a casting process with a lost core. 
         [0006]    The invention is in this case based on the idea that a ceramic injection molding process is used to produce the electrolyte body, in which the interconnector material and the internal electrode are already arranged in the injection molding step such that they can form a connection to the electrolyte body. In this process, an injection molding core is arranged centrally in an injection mold such that a tubular cavity is formed into which the electrolyte compound is injected in order to form the electrolyte body. Since both the interconnector material and the internal electrode have already been applied to the injection molding core, the electrolyte compound can form a connection to the inner electrode, such that is subsequently necessary to remove the injection molding core from the electrolyte body and from the internal electrode. The interconnector material is applied to the injection molding core before the application of the material to form the internal electrode, such that the electrolyte compound can form a connection to the material of the internal electrode. 
         [0007]    The injection molding core can advantageously be removed by means of a thermal process, and preferably by means of a lost core technique when the electrolyte body is being injection molded. The principle of destroying the shaping components for casting of a workpiece during removal from the mold is referred to as a lost core process, which can be used in the present case. The thermal removal of the injection molding core can be carried out during a sintering process, to which the electrolyte body together with the interconnector material and the internal electrode are passed after the injection molding process. 
         [0008]    The injection molding core can advantageously be formed from a plastic material, wherein the thermal process for removal of the injection molding core is preferably carried out by burning out the plastic material from the electrolyte body. 
         [0009]    The injection molding core may be formed with a conical external contour, such that a first end with a large diameter and a second end with a small diameter of the injection molding core are formed, wherein the second end with the small diameter constitutes the side of the electrolyte body which forms the holding end of the tubular electrolyte body on a base body. If the interconnector material and/or the internal electrode are/is now applied to the injection molding core by means of a printing process, in particular by means of a web-fed printing process or a screen printing process, then the application can be carried out such that a cylindrical overall body is produced after application of the interconnector material and the material of the internal electrode. This results in an area of the interconnector material which has a greater wall thickness, with this area being used to subsequently make electrical contact with the internal electrode, because the highest current density occurs at the contact end of the interconnector material. This allows the thickness of the interconnector material to be matched to the current density over the length of the electrolyte body. 
         [0010]    According to a further advantageous embodiment of the method according to the invention, the interconnector material and/or the internal electrode are/is applied to a carrier film, which is arranged on the injection molding core before the injection molding of the electrolyte compound, such that the electrolyte compound is connected to the internal electrode on the carrier film when the electrolyte compound is injection molded. 
         [0011]    The interconnector material can advantageously be fitted to the carrier film in layers, with the internal electrode being fitted only after this has been done. If the layers of the interconnector material are fitted with a different width over the length of the injection molding core, then free areas are formed which can be filled with filling layers, in particular with glassy carbon layers. A large number of layers fitted one on top of the other and with a different width create a variable-thickness interconnector, whose thickness increases in the direction for making contact with the interconnector, since the current density is highest here. The electrolyte body can be held on a base body via a flange, via which contact is made with the interconnector. In consequence, more layers of interconnect material are applied to the carrier film in the direction facing the flange when the fuel cell is complete, and in consequence in the direction for making contact. Filling layers, in particular glassy carbon layers, can be applied to the carrier film in order to fill the free areas which result from a reduced number of layers applied. Since layers of interconnector material are initially not applied to the carrier film over the entire length corresponding to the length of the injection molding core, the filling layers are applied adjacent to the individual layers of the interconnector material. In consequence, a next layer of interconnector material can also be applied above a filling layer. Once the layers of interconnector material have all been applied, the material to form the internal electrode is then applied. 
         [0012]    According to a further advantageous embodiment, the injection molding core may have grooves into which the interconnector material is introduced, in order to form an interconnector layer which has a structure, preferably a web structure or a grid structure. By way of example, the interconnector material can be introduced into the grooves in the injection mold by a wiper. In consequence, the interconnector forms a matrix structure on the electrode, in order not to cover it completely. The number of grooves can advantageously be increased over the length of the injection molding core in one direction in the structure, in order to form a larger line cross section in the direction in which contact will later be made with the interconnector, in order to carry the greater current density. 
         [0013]    The interconnector material may be applied with a small thickness in the area of the first end and with a large thickness in the area of the second end, such that the interconnector material has a conical internal shape, and preferably compensates for the conical shape of the injection molding core, such that the interconnector material has an approximately cylindrical external shape. In this case, the grooves in the injection molding core may also have a different depth, which increases in the direction in which contact will later be made with the interconnector. 
         [0014]    According to a further method step, after the electrolyte body together with the internal electrode and the interconnector material have been removed from the mold, the external electrode is fitted, and is preferably subsequently burnt in. The external electrode can also be provided with an interconnector material which can be applied after the electrolyte body has been removed from the injection mold, in which case the external electrode and an interconnector which is provided on the outside can also be applied in the same manner to that on the inside of the electrolyte body, based on the principle of film insert molding. In particular, the principle of film insert molding can be provided both from the direction of the injection molding core and from the direction from the inside of the injection mold for spraying the electrolyte body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Further measures which improve the invention will be described in more detail in the following text together with the description of one preferred exemplary embodiment of the invention, with reference to the figures, in which: 
           [0016]      FIG. 1  shows a schematic view of one exemplary embodiment of the present invention, 
           [0017]      FIG. 2  shows a further view of one exemplary embodiment of the present invention with a conical injection molding core, 
           [0018]      FIG. 3  shows a schematic view of interconnector material being applied in layers on a carrier film, 
           [0019]      FIG. 4  shows the schematic view of an electron with interconnector material applied in strips, in a developed form, 
           [0020]      FIG. 5  shows a perspective view of an electrolyte body with an internal electrode and an external electrode, in each case with interconnector material applied, 
           [0021]      FIG. 6   a  shows one exemplary embodiment of an injection molding core provided with grooves, 
           [0022]      FIG. 6   b  shows the injection molding core as shown in  FIG. 6   a , with interconnector material having been introduced into the grooves in the injection molding core by means of a wiping process, and 
           [0023]      FIG. 6   c  shows the view as shown in  FIG. 6   b , with material for forming an internal electrode having been applied externally onto the injection molding core, and with the interconnector material having been introduced. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  shows a schematic view of one exemplary embodiment of the method for production of a solid oxide fuel cell  1 , as is illustrated in perspective form in  FIG. 5 . First of all,  FIG. 1  shows a cross section through an injection mold, which has a first mold element  25   a  and a second mold element  25   b.  The mold elements  25   a  and  25   b  can move with respect to one another, and can be moved apart from one another in order to remove an injection-molded electrolyte body  10  from the mold. The electrolyte body  10  is in the form of an electrolyte body that is closed at the ends, and therefore has a cap  24 . According to one possible embodiment for production of the electrolyte body  10 , this may be in the form of a single part, such that the tubular, cylindrical section of the electrolyte body  10  can be produced by injection of an electrolyte compound  10   a  into the injection mold, in which case the cap  24  can also be produced by the electrolyte compound  10   a  itself in the injection mold. On the inside, the injection mold has an injection molding core  13  which is illustrated merely by way of example as a hollow core, in order to minimize the mass of the injection molding core  13  which has to be removed by burning out once the injection molding process has been completed. On the outside, an interconnector material  14  is first of all applied to the injection molding core  13 , with the lower face having a large thickness which decreases in the direction of the cap  24  of the electrolyte body  10 . An internal electrode  11  is applied to the interconnector material  14  and forms a direct connection with an electrolyte mass  10  when the latter is injected. When the electrolyte body  10  that has been formed in this way is removed from the injection mold, and the injection molding core  13  has been removed by a thermal process from the inside of the electrolyte body  10 , preferably by a thermal process, then an electrolyte body  10  is produced having an internal electrode  11  and an interconnector material  14  which makes contact with it. At the same time, the interconnector material  14  is already thicker at the second end  16  of the electrolyte body  10  since the subsequent contact will be made at this end, and a higher current density must therefore be taken into account. 
         [0025]      FIG. 2  shows schematic views of the configuration of the coatings on a carrier film  20 , with the carrier film  20  being adjacent to the internal electrode  11  in the upper illustration while, in contrast, the carrier film  20  is illustrated in the lower illustration as being adjacent to layers  21  of interconnector material. The principle of film insert molding can therefore be implemented, with the film being applied with the layer structure to the injection molding core  13 . 
         [0026]    According to the upper illustration, the carrier film  20  is first of all coated with the internal electrode  11 , with layers  21  of interconnector material then being applied to the internal electrode  11 . The layers  21  of the interconnector material are shown with different extents, thus resulting in a free area  22  which is then filled with a filling layer  23 , for example composed of glassy carbon layers  23 . The electrolyte compound  10   a  is then sprayed on in order to form the electrolyte body  10 , such that the electrolyte compound  10   a  can make a firm, integral connection to the internal electrode  11 . 
         [0027]    The carrier film  20  may be filled over the area, such that it can be placed around the cylindrical injection molding core  13 . It is particularly advantageous for the carrier film  20  to be preformed in the form of a sleeve or flexible tube, such that the sleeve or the flexible tube can be placed, with the layers  11  and  21  or  23  applied to it, over the injection molding core  13 . 
         [0028]    The carrier film  20  can likewise also be removed by means of a thermal process by removing the injection molding core  13  after the injection molding step. This results in an electrolyte body  10  with an applied internal electrode  11  and an interconnector formed from layers  21 . In order to obtain a cylindrical overall shape, the filling layers  23  are provided at the points which the remaining layer thicknesses of the layers  21  fill before the interconnector material. 
         [0029]      FIG. 4  shows a development of a carrier film  20  which may already be fitted with an internal electrode ( 11 , not illustrated) on the upper face. The interconnector material  14  is applied in a strip form, with the strips of the interconnector material  14  preferably extending in the longitudinal direction of the electrolyte body  10 , and therefore in the longitudinal direction of the injection molding core  13 .  FIG. 4  shows a development in the form of a flattened-out illustration, in which the carrier film  20  can preferably form a flexible tube with a circular cross section. The strips of the interconnector material in consequence run at regular distances from one another in the longitudinal direction of the electrolyte body  10 . 
         [0030]      FIG. 5  shows a perspective view of a solid oxide fuel cell  1  which has an electrolyte body  10  which forms the basic structure of the fuel cell  1  and is closed at the top by a cap  24 , in order to form the solid oxide fuel cell  1  as a fuel cell which is closed at one end. The solid oxide fuel cell  1  is mounted on a base body  17 , at the same time showing contact elements  26  for making contact with the internal electrode  11 , and a contact element  27  for making contact with the external electrode  12 . Interconnector material  14  is furthermore shown both on the inside, having a web structure  19  which is formed in strips by the interconnector material  14 , as is illustrated in  FIG. 4 . The external electrode  12  is likewise shown with a web structure. Because the solid oxide fuel cell  1  has a closed structure, it has an opening  28  for the fuel gas flow on the side of the base body  17 . 
         [0031]      FIG. 6   a  shows one exemplary embodiment of an injection molding core  13  which is illustrated in the form of a cross section, and has grooves  18  distributed uniformly over the circumference. As can be seen from  FIG. 6   b , the grooves  18  can be filled with interconnector material  14 , in which case, by way of example, the interconnector material  14  can be introduced into the grooves  18  by means of a wiping process.  FIG. 6   c  shows that the internal electrode  11  has been applied after the application of the interconnector material  14 , for example by a web-fed printing process or a screen printing process.  FIG. 6   c  therefore shows an injection molding core  13  with the respective coating which, without the principle of film insert molding, and in consequence without a carrier film  20 , allows electrolyte compound  10   a  to be injection molded into a mold, in which case the electrolyte compound  10   a  can form an integral connection to the internal electrode  11 . During a subsequent burning process, both the ceramic electrolyte body  10  and the respective coatings  11  and  14  can be burnt in and hardened. 
         [0032]    The embodiment of the invention is not restricted to the preferred exemplary embodiment indicated above. In fact, a number of variants are feasible which also make use of fundamentally different types of embodiment from the described solution. All features and/or advantages which result from the claims, the description or the drawings, including design details, physical arrangements and method steps, may be significant to the invention both in their own right and in widely differing combinations.