High-temperature fuel cell having at least one electrically insulating covering and method for producing a high-temperature fuel cell

A high-temperature fuel cell has at least one electrically insulating covering. The electrically insulating covering contains at least two layers which are disposed one on top of the other and are each formed of an electrically insulating ceramic material. The composition of the ceramic material of one layer is different from the composition of the ceramic material of the other layer. This results in optimization of the insulation characteristic and the adhesion of the electrically insulating covering. A method is also provided for producing a high-temperature fuel cell having at least one electrically insulating covering.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The invention relates to a high-temperature fuel cell having at least one 
electrically insulating covering, as well as to a method for producing a 
high-temperature fuel cell. 
It is known that live metallic components connected to one another in 
high-temperature fuel cells must be partially electrically insulated from 
one another. It is particularly difficult to satisfy that requirement in 
the case of a high operating temperature, for example an operating 
temperature of more than 800.degree. C. Frequently, an integral material 
connection between the metallic components can be produced only with a 
glass-like substance, for example with glass or glass solder, due to 
specific boundary conditions. However, such a glass-like substance is 
normally relatively highly electrically conductive at a high operating 
temperature and is consequently a poor electrical insulator. 
Furthermore, those glass-like substances have the characteristic of 
decomposing when an electrical voltage is applied to them. That 
decomposition may take place even at a voltage of considerably less than 1 
volt. 
A report entitled "Materials for Solid-Oxide High-Temperature Fuel Cells" 
by W. Drenckhahn et al., in Siemens Power Journal, Issue 4, Year 94, pages 
36 to 38, in particular, discloses the application of an electrical 
potential of, for example, 0.7 volts operating voltage to 1.3 volts 
quiescent voltage between two bipolar plates in high-temperature fuel 
cells. Even at those electrical voltages, decomposition reactions occur 
between the bipolar plates as a result of electrolysis, if a glass-like 
substance is used, that is to say ion migration takes place from one 
electrode side to the other. In the process, the intrinsic strength of the 
glass-like substance is adversely affected even after a short time, as a 
result of which the probability of a malfunction occurring is in turn 
considerably increased. 
In order to avoid that disadvantage, the bipolar plate is provided with an 
additional single-layer covering of stabilized ZrO.sub.2 which, depending 
on the requirement or the structure, has a thickness of between, for 
example, 30 .mu.m and 150 .mu.m. Optionally, for better adhesion of the 
covering on the bipolar plate, a so-called adhesion promoter composed of 
metal or ceramic is applied to the bipolar plate to improve the adhesion 
of the covering on the bipolar plate. The ZrO.sub.2 material that is used 
in general is partially or fully stabilized using known technical 
stabilization components such as Y.sub.2 O.sub.3, CaO, MgO, Al.sub.2 
O.sub.3 or CeO.sub.2. 
That single-layer covering has inadequate electrical insulation which is 
required especially if a glass-like substance is used to join the metallic 
components together, since the glass-like substance is a poor electrical 
insulator. 
U.S. Pat. No. 5,338,577 discloses a method for coating a metallic substrate 
with ceramic through the use of flame spraying, wherein an intermediate 
covering of steel is sprayed on initially, followed by a covering of 
yttrium-stabilized ZrO.sub.2 and finally an Al.sub.2 O.sub.3 covering. 
Furthermore, Japanese Published, Non-prosecuted Patent Application No. 
06-144 971 discloses a covering body which includes a ceramic substrate, 
adhesion promoter coverings of steel, a ceramic covering, and possibly an 
outer covering to close the pores, wherein the adhesion promoter coverings 
and the ceramic covering are produced through the use of flame spraying, 
and the outer covering is produced through the use of a sol-gel technique. 
In addition, US Statutory Invention Registration H 12 60 discloses a method 
for producing covering bodies for high-temperature fuel cells through the 
use of a plasma spray process. However, in the case of that process, only 
a single electrically insulating covering, composed of yttrium-stabilized 
ZrO.sub.2, is sprayed on. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a high-temperature 
fuel cell having at least one electrically insulating covering and a 
method for producing a high-temperature fuel cell, which overcome the 
hereinafore-mentioned disadvantages of the heretofore-known devices and 
methods of this general type and in which one of the coverings provides 
electrical insulation that is as complete as possible, adheres well and is 
mechanically robust. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a high-temperature fuel cell, comprising 
two bipolar plates; and at least one electrically insulating covering 
disposed between the two bipolar plates, the electrically insulating 
covering containing at least two layers disposed one on top of the other, 
the at least two layers each formed of an electrically insulating ceramic 
material, and the ceramic material of one of the at least two layers 
having a different composition than the ceramic material of the other of 
the at least two layers. 
With the objects of the invention in view, there is also provided, in a 
method for producing a high-temperature fuel cell having at least one 
electrically insulating covering disposed between two bipolar plates, the 
improvement which comprises producing the at least one electrically 
insulating covering from at least two layers disposed one on top of the 
other, each of the layers formed of an electrically insulating ceramic 
material, and the ceramic material of one of the at least two layers 
having a different composition than the ceramic material of the other of 
the at least two layers; producing the individual ceramic layers 
successively by spraying a ceramic; and applying the individual ceramic 
layers to a component. 
The use of a plurality of electrically insulating ceramic layers in an 
electrically insulating covering has the advantage of satisfying the 
various requirements for electrical insulation and adhesion of different 
ceramic layers separately, since these differing requirements cannot be 
satisfied in a single ceramic layer. Electrical insulation and good 
adhesion characteristics are thus satisfied by a plurality of ceramic 
layers of different composition within a covering. The electrically 
insulating covering thus includes at least a first layer having good 
adhesion characteristics, that is to say the linear coefficient of 
expansion of the first layer differs only slightly from the linear 
coefficient of expansion of that component to which the first layer is 
applied, and a second layer which is disposed on the first layer and is 
characterized by good electrical insulation. 
In accordance with another feature of the invention, the at least two 
layers are composed of the same electrically insulating ceramic material, 
and are separated from one another by at least one layer composed of a 
different electrically insulating ceramic material. As is known from the 
prior art, a single ceramic layer is not sufficient for adequately good 
electrical insulation. Since the covering thickness of a ceramic layer 
cannot be increased indefinit-ely due to the intrinsic mechanical stresses 
that occur, an electrically insulating covering having a plurality of 
ceramic layers, is used. In this case, covering thicknesses of, for 
example, 500 .mu.m are achieved without the intrinsic stress in the 
covering being increased, since the covering is composed of a plurality of 
ceramic layers. This leads to considerable mechanical stabilization of the 
overall high-temperature fuel cell. 
In accordance with a further feature of the invention, an adhesion promoter 
is used for application of the electrically insulating covering to a 
component of the high-temperature fuel cell. The adhesion promoter 
additionally ensures good adhesion of the electrically insulating covering 
on the component of the high-temperature fuel cell on which the 
electrically insulating covering is applied. 
In accordance with an added feature of the invention, there is provided a 
layer for closing and healing pores and cracks in the last ceramic layer 
applied to the electrically insulating covering. 
In accordance with another mode of the invention, the electrically 
insulating covering is produced by atmospheric plasma spraying, flame 
spraying, high-speed flame spraying, vacuum flame spraying or low-pressure 
flame spraying. The use of different spraying processes allows the 
respectively desired ceramic covering to be produced in different external 
conditions. 
In accordance with a further mode of the invention, there is provided a 
layer applied to the electrically insulating covering through the use of a 
sol gel for closing and healing the pores and cracks in the last ceramic 
layer. If a glass-like substance, for example glass solder, is applied to 
the last ceramic layer in a further process then, for electrical 
insulation, it is of major importance that the last ceramic layer of the 
electrically insulating coating be sealed well. If the porosity within the 
last ceramic layer is too high, the glass-like substance penetrates into 
this layer, and reduces the electrical insulation. 
In accordance with a concomitant mode of the invention, the sol gel is 
vacuum-infiltrated after application. The vacuum infiltration, which may 
also be carried out actually during the application process, ensures that 
the cracks and pores are closed and healed. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
high-temperature fuel cell having at least one electrically insulating 
covering and a method for producing a high-temperature fuel cell, it is 
nevertheless not intended to be limited to the details shown, since 
various modifications and structural changes may be made therein without 
departing from the spirit of the invention and within the scope and range 
of equivalents of the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the figures of the drawings in detail and first, 
particularly, to FIG. 1 thereof, there is seen a portion of a 
high-temperature fuel cell which includes a component 10, for example a 
bipolar plate, and an electrically insulating covering 2. In this case the 
electrically insulating covering 2 is disposed on the component 10 and the 
electrically insulating covering 2 contains a plurality of layers 4, 6 
which are disposed one on top of the other and are each composed of an 
electrically insulating ceramic material. The composition of the ceramic 
material in the layers 4 is different from the composition of the ceramic 
material in the other layers 6. 
In this configuration, in each case two layers 4 composed of the same 
electrically insulating ceramic material are separated from one another by 
a layer 6 composed of a different electrically insulating ceramic 
material. 
In a first step, an adhesion promoter 8 is applied to the component 10. In 
this case, the component 10 is composed of a metal, a metal alloy or a 
ceramic. CrFe5Y.sub.2 O.sub.3 1 is used, for example, as the metal in the 
high-temperature fuel cell. Other special alloys, such as Haynes Alloy 
230, Inconel 600 or conventional industrial stainless steels are likewise 
often used. 
The ceramic layer 4 which is composed, for example, of ZrO.sub.2 is sprayed 
onto the adhesion promoter 8, and the linear coefficient of thermal 
expansion of the ceramic layer 4 is matched to that of the component 10. 
The ceramic layer 4 is applied directly to the adhesion promoter 8 by the 
spraying process. The ceramic layer 4 thus adheres well to the component 
10, but is not suitable for ensuring good electrical insulation at the 
same time. 
In a further step, the ceramic layer 6 is applied to the ceramic layer 4. 
The ceramic layer 6 has better electrical insulation characteristic than 
that of the ceramic layer 4. If ZrO.sub.2 is used in the ceramic layer 4, 
then high-purity Al.sub.2 O.sub.3 is suitable, for example, for the 
ceramic layer 6. The Al.sub.2 O.sub.3 may be produced, for example, from 
fused corundum spinels, for example MgAl.sub.2 O.sub.4, from mullites or 
from some other electrically insulating ceramic. As a rule, these ceramics 
do not have linear coefficients of thermal expansion similar to that of 
the component 10, so that they only poorly adhere to the component 10 
without using the ceramic layer 4. A succession of different ceramic 
layers 4, 6 thus has the advantage of satisfying different requirements, 
such as electrical insulation and linear coefficient of thermal expansion, 
separately with different layers 4, 6. The ceramic layer 4 ensures good 
adhesion, while the ceramic layer 6 provides good electrical insulation. 
In order to provide adequate electrical insulation, the ceramic layer 6 
then has to have a thickness which cannot be achieved in a single layer, 
since if the thicknesses are too great, considerable mechanical intrinsic 
stresses occur which lead to destabilization of the overall 
high-temperature fuel cell. In consequence, a plurality of layers 4, 6 are 
applied one on top of the other for adequate electrical insulation, until 
the total thickness of the electrically insulating covering 2 is 
sufficient for electrical insulation. 
The spraying of the individual ceramic layers 4, 6 may be carried out, for 
example, by atmospheric plasma spraying, flame spraying, high-speed flame 
spraying, vacuum flame spraying or low-pressure flame spraying. The use of 
different spraying methods makes it possible to produce any desired 
ceramic layer 4, 6 with the desired characteristics. 
As is shown in a portion of a high-temperature fuel cell in FIG. 2, the 
electrically insulating covering 2 is disposed between two bipolar plates 
20 and 22. The high-temperature fuel cell normally operates at an 
operating temperature of more than 800.degree. C. In this case, the two 
bipolar plates 20, 22 can be connected with an integral material joint by 
only using a glass-like substance. A covering 24 of a glass-like substance 
is therefore disposed between the bipolar plate 22 and the electrically 
insulating covering 2. Such a glass-like substance may, for example, be a 
glass solder. 
Since the covering 24 of a glass-like substance is in contact with the 
ceramic layer 4 that was applied last in the electrically insulating 
covering 2, the sealing of this ceramic layer 4 is of major importance for 
the electrical insulation. The use of spraying for production, results in 
cracks 28 and pores 30 being formed in a surface 26 of the ceramic layer 
4, and they may have a negative effect on the electrical insulation 
characteristic since, for example, glass solder from the covering 24 may 
penetrate into the cracks 28 and the pores 30. That problem is solved by 
applying a layer 34 onto the last ceramic layer 4 in order to close and 
heal the cracks 28 and pores 30. This layer 34 is disposed between the 
last ceramic layer 4 and the covering 24 composed of a glass-like 
substance. 
An application of an aqueous sol gel which is used as the layer 34 and is 
composed of Al(OH).sub.3, for example, that later dehydrates to form 
A1.sub.2 O.sub.3, or a sol gel composed of MgAl.sub.2 O.sub.4 components, 
allows the cracks 28 and the pores 30 in the surface 26 of the ceramic 
layer 4 to be closed and healed. In the case of this method, the aqueous 
sol gel is first of all applied onto the surface 26 of the last ceramic 
layer 4 by spraying through the use of compressed air or electrical 
atomization, by screen printing, by brushing, by sponging or by dipping. 
In a further step of the method, the sol gel in the layer 34 is introduced 
into the cracks 28 and the pores 30 by vacuum filtration. Once the method 
has been completed, a closed smooth surface 26 is thus obtained, into 
which the glass solder in the covering 24 no longer penetrates. 
Typical sealers, which are added to the sol gel, are used and are based on 
epoxy resins and silicone resins. In the case of the high-temperature fuel 
cell, an aluminum oxide hydroxide sol with 86 to 96% aluminum oxide 
hydroxide hydrate and 4 to 14% aluminum acetate hydrate are infiltrated 
into a sprayed covering of ceroxide-stabilized zircon oxide and/or a 
covering of aluminum oxide. In this case the infiltration can also be 
carried out at room temperature and, optionally, at reduced pressure. 
The electrically insulating covering 2 is thus excellently suited for use 
in a high-temperature fuel cell due to its electrical insulation and 
chemical robustness. Gas-carrying ducts and cavities which are integrated 
in the electrically insulating covering 2 in this case can be sealed off 
from one another well.