AU713015B2 - Process for coating a component of a high temperature fuel cell with a vitreous layer, use of the process for the production of a high temperature fuel cell stack, and high temperature fuel cell stack - Google Patents

Description

GR 96 P 3168 P Description Process for coating a component of a high temperature fuel cell with a vitreous layer, use of the process for the production of a high temperature fuel cell stack, and high temperature fuel cell stack The invention relates to a process for coating a component of a high temperature fuel cell with a vitreous layer, to a use of the process for the production of a high temperature fuel cell stack, and to a high temperature fuel cell stack.

In various components, for example the planar components of a high temperature fuel cell stack, it is necessary to provide the planar components with a vitreous layer, for example for electrical or gas-tight insulation of the planar components from one another.

The planar components are generally connected to one another using glass solders or with composite glass solders. A suitable composite glass solder consists, for example, of a B 2 0 3 -SiO 2 -CaO or B 2 0 3 -SiO 2 -BaO glass base and corresponding ceramic composite components of, for example, Zr02, A1 2 0 3 or MgO. One advantage of glass solders consists in the fact that it is possible to solder both under an atmosphere and in a vacuum.

From the "Fuel Cell Handbook" by A.J. Appelby and F.R. Foulkes, 1989, pages 440 to 454, it is known that, in a high temperature fuel cell stack, a fuel cell stack also being abbreviated to "stack" in the specialist literature, various planar components are stacked on one another. Below an upper interconnecting conducting plate, which has the function of an upper cover plate, GR 96 P 3168 P 2 a contact layer, a solid electrolyte/electrode element, a further contact layer, a further interconnecting conducting plate which, inside the high temperature fuel cell stack, is designed as a bipolar plate, etc. are arranged in this order. In this case, each solid electrolyte/electrode element lying between two neighboring interconnecting conducting plates, including the contact layer directly adjoining this solid electrolyte/electrode element on both sides, and those sides of each of the two interconnecting conducting plates which adjoin the contact layer, together form a high temperature fuel cell.

One technique commonly used for assembling the various planar components involves the use of so-called green sheets. In such green sheets, the composite glass solder is mixed with an organic binder and drawn, for example by means of tape casting, into an about 200 to 400 gm thick sheet. The binder proportion is in this case usually about 20% by weight. These green sheets are then structured and applied to the desired surface of the planar component.

In sheets of this type, the total thickness of the green sheet is relatively low owing to the volume fraction of the binder. After the components have been assembled and the binder has been burnt off, the green sheet undergoes shrinkage, which may amount to up to Since gas-carrying channels and spaces, which need to be isolated in gas-tight fashion from one another, are formed between the individual planar components, gas-tight isolation of the various channels and spaces can be achieved only with difficulty and a high degree of unreliability when green sheets are used, on account of the shrinkage.

Further, the binder must be removed from the green sheet without leaving a residue. Between the various planar components, this can lead to additional problems, since the substances which are released, for example binder, dispersing agent, solvent and plasticizer, can only pass to the outside with great difficulty through the seal made when the planar components are assembled. It is therefore not guaranteed that these released substances will be burnt off without leaving a residue inside the high temperature fuel cell stack.

s Said disadvantages lead to a reduction in the power density, or even to malfunction of the high temperature fuel cell stack.

The object of the invention is therefore to provide a process for coating a component of a high temperature fuel cell, in which the technical disadvantages mentioned above are substantially avoided. A further objection of the invention is to provide a use of the process for the production of a high temperature fuel cell stack, in which the power density is not reduced on assembly of the components forming the high temperature fuel cell stack. A further object is to provide a high temperature fuel cell stack for carrying out the process.

The first object is achieved according to the invention by a process for introducing 1i.. vitreous material into a gap when connecting a high temperature fuel cell, characterised in i that, before the connection, a component of the high temperature fuel cell is coated with at least one vitreous layer which is produced by enamelling.

The second object is achieved according to the invention by a use of the process for *o o: the production of a high temperature fuel cell stack.

The third obj et is achieved according to the invention by a high temperature fuel cell stack having a plurality of components, in which the components are connected to one another by 0, 0, 0: [/libff]01064:SSD GR 96 P 3168 P 4 at least one vitreous layer, the vitreous layer being produced by enameling.

In the process for coating the component of a high temperature fuel cell with a vitreous layer, the vitreous layer is produced by enameling. As a result, the disadvantages known from the prior art are substantially avoided. The product resulting from the enameling is a vitreous layer with quite good chemical stability. The enamels may be defined as fusible mixtures, which harden at comparatively low temperature, of silicates, borates and fluorides of the glass-forming elements, in particular Na, K, Pb and Al, which harden by expelling solid or gaseous substances, for example gas bubbles. The enameling may be carried out either with wet enamel or with powder enamel as enamel base material. The term enamel base material is used to denote the base material with which the enameling is carried out.

By the use of spray processes for producing the vitreous layer, the technical disadvantages mentioned above are straight away essentially avoided. Examples of spray processes which may be used include atmospheric plasma spraying, high speed flame spraying or vacuum plasma spraying.

However, a disadvantageous effect which is found in this case is, depending on the composition of the vitreous layer which is to be applied to the component by one of these thermal spray processes, increased bubble formation taking place in the vitreous layer after application to the component. The increased porosity which then occurs in the vitreous layer has a detrimental effect on the subsequent soldering process when the components are being assembled. The power during later use of the high temperature fuel cell stack is consequently reduced.

I

GR 96 P 3168 P 5 Further, the high cost factor due to the process when these thermal spraying processes are carried out is bound to be a disadvantage. All thermal spraying processes require considerable technical and equipment outlay.

When the enameling process is employed, the bubble formation in the vitreous layer is almost fully prevented. This process for producing a vitreous layer, in which the enamel base material is preferably applied to the component by wet powder spraying, is in addition more cost-effective at least by a factor of 15 to 20 than the thermal spraying processes known from the prior art.

Further advantageous refinements are given in the subclaims To explain the invention further, reference will be made to the illustrated embodiments in the drawing, in which: FIG 1 shows a schematic representation of a detail of a coated component of a high temperature fuel cell; FIG 2 shows a schematic representation of a detail of a high temperature fuel cell stack.

According to Fig. 1, a plurality of layers 6, 8, 8, 10, 12 are applied to a planar component 4 of a high temperature fuel cell 2. In this case, the planar component 4 is, for example, an interconnecting conducting plate. It preferably consists of the metal CrFeY 2 031, Haynes Alloy 230, Inconel 600 or a usual industrial steel. Via this planar component 4, an electrode (not represented in further detail) is supplied with a working medium and, further, the GR 96 P 3168 P 6 current is drawn off from the high temperature fuel cell 2 in order to be used.

The ceramic layer 6 is here arranged first on the planar component 4 of the high temperature fuel cell 2.

The ceramic layer 6 consists, for example, of A1 2 0 3 spinel or ZrO 2 In practice, however, a plurality of ceramic layers 6 with different compositions are arranged above one another, which is not represented in further detail in this embodiment. The ceramic layer 6, which is matched in terms of its linear coefficient of thermal expansion to the linear coefficient of thermal expansion of the component 4 of the high temperature fuel cell 2, guarantees that the vitreous layers 8, 10, 12 arranged subsequently adhere well to the component 4. Electrical insulation is also achieved by the ceramic layer 6.

The vitreous layer 8 is produced by enameling, for example from a glass solder or a glass ceramic composite solder, on the ceramic layer 6. The term glass solder is in this case used to denote a readily melting soldering glass which is characterized by low viscosity and low surface tensions and whose melting temperature lies in a temperature range between 600 and 1000 0

C.

On the other hand, glass ceramic denotes a polycrystalline solid which is produced by ceraming, that is to say controlled devitrification of a glass. These vitreous substances can be applied very densely and with good adhesion to the component in the pulverulent state, i.e. in other words as an enamel base material.

The enamel base material for the enameling is applied to the component 4 by wet powder spraying or by a screen printing process before the actual enameling process.

GR 96 P 3168 P 7 In this case, the enamel base material contains a binder, the proportion of the binder amounting to between 5 and 20% by weight of the enamel base material.

The cost saved by these cold processes in comparison with the thermal spraying processes represents a factor of at least 15 to 20. The total cost saving in the present process is therefore achieved as early as during the application of the enamel base material.

After the enamel base material has been applied, it is dried in a specific way. This is done, for example at a temperature T between 50 and 100*C over a period of from 2 hours to 3 days.

On account of its powdered structure, the enamel base material adheres poorly to the planar component 4.

The enameling provides an integral connection between the vitreous layer 8, consisting of the enamel base material treated by enameling, and the planar component 4. The enameling, i.e. in other words the production of the vitreous, solid, binder-free and low-porosity layer 8, takes place at a predetermined temperature T. of, for example, about 8000C. This is above the softening point for the components of the enamel base material and below the soldering temperature for assembling the components 4 of the high temperature fuel cell 2. The heating rate is in this case a function of the components of the enamel base material and is, for example, In this illustrated embodiment, a further vitreous layer 10 is arranged on the vitreous layer 8. In this case, the vitreous layer 10 may have the same composition as the vitreous layer 8, or a different composition. A succession of different vitreous layers 8, has the advantage that various requirements, for example electrical insulation, linear coefficient of thermal GR 96 P 3168 P 8 expansion and gas-tightness, can be met separately by different vitreous layers 8, The thickness of the individual vitreous layers 8, 10 has an upper limit placed on it by the occurrence of mechanical stresses. According to the desired total layer thickness, the sequence of vitreous layers 8, 10 is repeated periodically. In this case, it is possible to achieve total layer thicknesses of, for example, 500 gm, without increasing the resultant internal stress of the arrangement of a plurality of layers 8, 10. This leads to substantial mechanical stabilization of the high temperature fuel cell as a whole.

In order to ensure good mechanical adhesion of the top vitreous layers 8, 10 to a further component (not represented in this illustrated embodiment), a further vitreous layer 12, for which no enameling is carried out, is applied to the last vitreous layer 10 produced by the enameling process. This embodiment of the vitreous layer 12 is optional.

Use of this process is particularly suitable for the production of a high temperature fuel cell stack.

According to Fig. 2, the detail of a high temperature fuel cell stack 20 comprises two components 22 and 24, which are connected to one another in gas-tight fashion by a vitreous layer 26 and are electrically insulated from one another, the vitreous layer 26 being produced by enameling.

Vitreous substances which have proved suitable in this case include, in particular, composite glass solders which, for example, consist of 75% by weight of a glass solder, for example B 2 0 3 -SiO 2 -CaO and/or BaO, and 25% by weight GR 96 P 3168 P 9 of a ceramic, such as for example Zr02, MgO or A1 2 0 3 in the form of a mixture.

The process presented in conjunction with Fig. 1 for coating the component 4 of the high temperature fuel cell 2 is therefore very highly suitable for connecting the components 22, 24 of a high temperature fuel cell stack Advantageously, the vitreous layer 26 can be replaced by an arrangement which consists of the sequence of layers 6, 8, 10, 8, 10, 12, which is shown in the illustrative embodiment in Fig. 1.

Claims (9)

1. A process for introducing vitreous material into a gap when connecting a high temperature fuel cell, characterised in that, before the connection, a component of the high temperature fuel cell is coated with at least one vitreous layer which is produced by enameling.

2. The process as claimed in claim 1, in which an enamel base material is applied by a screen printing process before the enameling.

3. The process as claimed in claim 1, in which an enamel base material is applied by wet powder spraying before enameling.

4. The process as claimed in any one of the preceding claims, in which a plurality of vitreous layers are applied successively.

The process as claimed in claim 4, in which the sequence of the vitreous layers is periodic.

6. The process as claimed in any one of the preceding claims, in which a ceramic layer is arranged on the component before the production of the vitreous layers.

7. The process as claimed in any one of the preceding claims, in which a further S vitreous layer, not produced by enameling, is arranged on the last vitreous layers produced 0 S. by enameling.

8. A process for introducing vitreous material into a gap when connecting a high 00Cm 20 temperature fuel cell, substantially as hereinbefore described with reference to the accompanying drawings. Use of the process as claimed in any one of the preceding claims, for the production of a high temperature fuel cell stack. A high temperature fuel cell stack having a plurality of components, in which the components are connected to one another by at least one vitreous layer, the vitreous layer S. "o being produced by enameling. 0 I 11. A high temperature fuel cell stack, substantially as hereinbefore described with reference to the accompanying drawings. aese Dated

9 September, 1998 Siemens Aktiengesellschaft 0**n Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON [/libff]Ol064:MEF