U.S. patent application number 10/760666 was filed with the patent office on 2004-11-04 for seal construction for a fuel cell electrolyser and process for making a fuel cell with same.
This patent application is currently assigned to Bayerische Motoren Aktiengesellschaft. Invention is credited to Finkenwirth, Olav, Kuhn, Bernd.
Application Number | 20040219417 10/760666 |
Document ID | / |
Family ID | 32602821 |
Filed Date | 2004-11-04 |
United States Patent
Application |
20040219417 |
Kind Code |
A1 |
Finkenwirth, Olav ; et
al. |
November 4, 2004 |
Seal construction for a fuel cell electrolyser and process for
making a fuel cell with same
Abstract
A sealing structure in a fuel cell and/or an electrolyzer
(particularly a solid-oxide fuel cell and/or a solid-oxide
electrolyzer) is arranged between neighboring separator plates of a
cell stack. The sealing structure is constructed in at least two
layers, including at least one insulating layer and at least one
sealing layer.
Inventors: |
Finkenwirth, Olav;
(Muenchen, DE) ; Kuhn, Bernd; (Leinfelden,
DE) |
Correspondence
Address: |
CROWELL & MORING LLP
INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
Bayerische Motoren
Aktiengesellschaft
|
Family ID: |
32602821 |
Appl. No.: |
10/760666 |
Filed: |
January 21, 2004 |
Current U.S.
Class: |
429/433 ;
427/115; 429/495; 429/508; 429/509; 429/514; 429/535 |
Current CPC
Class: |
Y02P 70/50 20151101;
Y02E 60/50 20130101; H01M 8/0286 20130101; H01M 8/2432 20160201;
H01M 8/2483 20160201; H01M 8/0271 20130101; H01M 8/0282
20130101 |
Class at
Publication: |
429/035 ;
427/115 |
International
Class: |
H01M 002/08; B05D
005/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 21, 2003 |
DE |
103 021 24.8 |
Claims
What is claimed is:
1. A sealing structure for a fuel cell or an electrolyzer, having
at least first and second neighboring separator plates with a
sealing structure arranged between said separator plates, wherein:
said sealing structure comprises at least two layers, including at
least one insulating layer and at least one sealing layer; and the
insulating layer is arranged on a carrier element.
2. A sealing structure according to claim 1, wherein the fuel cell
or electrolyzer is one of a solid-oxide fuel cell and a solid-oxide
electrolyzer.
3. A sealing structure according to claim 1, wherein the insulating
layer is a metal oxide.
4. A sealing structure according to claim 1, wherein the insulating
layer is made of Al.sub.2O.sub.3.
5. A sealing structure according to claim 4, wherein the
Al.sub.2O.sub.3 is present in the structure of the
.gamma.-modification.
6. A sealing structure according to claim 1, wherein the sealing
layer comprises an inorganic material.
7. A sealing structure according to claim 6, wherein the inorganic
material is a glass-ceramic solder.
8. A sealing structure according to claim 1, wherein the sealing
layer has additions which ensure that the sealing layer is adapted
to the thermal expansion behavior of the material of a separator
plate.
9. A sealing structure according to claim 1, wherein the carrier
element is a carrier layer.
10. A sealing structure according to claim 9, wherein the carrier
layer is a steel plate with an aluminum content greater than
2%.
11. A sealing structure according to claim 9, wherein the carrier
layer is a steel plate with an aluminum content greater than
4.5%.
12. A sealing structure according to claim 9, wherein the carrier
layer is constructed of a ferritic steel with a chrome content of
approximately 20%.
13. A sealing structure according to claim 9, wherein the carrier
layer is constructed of a ferritic steel with a chrome content of
from about 15% to about 28%.
14. A sealing structure according to claim 9, wherein the carrier
layer is composed of Material Number 1.4765 or 1.4767.
15. A sealing structure according to claim 1, wherein the carrier
element is a separator plate, the insulating layer being arranged
in the sealing areas.
16. A sealing structure according to claim 1, wherein one or more
carrier elements are provided with an insulating layer formed by
pre-oxidation in at least one sealing area.
17. A sealing structure according to claim 1, wherein the sealing
structure is arranged in a fuel cell stack, the fuel cell stack
being constructed of a plurality of individual fuel cells which are
stacked above one another in a tower-type manner.
18. A sealing structure according to claim 17, wherein the
plurality of individual fuel cells have an electrolyte layer, a
cathode layer and an anode layer, the anode layer being arranged on
a carrying substrate layer.
19. A sealing structure according to claim 17, wherein the
insulating layer is arranged between two neighboring individual
fuel cells on a top side of a separator plate, wherein the top side
faces an oxidation space.
20. A sealing structure according to claim 1, wherein the
insulating layer is arranged between two neighboring separator
plates of an individual fuel cell in the region of the sealing
areas on at least one of the separator plates.
21. A sealing structure according to claim 1, wherein the sealing
layer is arranged on a free surface of the insulating layer.
22. A sealing structure according to claim 1, wherein the sealing
layer is constructed of an inorganic material containing additions
that is adapted to the coefficients of thermal expansion of the
separator plates.
23. A sealing structure according to claim 1, wherein the
insulating layer on the carrier element covers a larger surface
than is required of the sealing layer.
24. A sealing structure according to claim 1, wherein a first
sealing device is arranged between the insulating layer and a
neighboring separator plate, and a second sealing layer is arranged
between the carrier layer and another neighboring separator
plate.
25. A method of producing a sealing structure for a fuel cell or an
electrolyzer, comprising: producing at least one insulating layer
on a carrier element; and producing at least one sealing layer made
of a sealing material, the sealing structure being arranged in a
sealing area of a fuel cell stack.
26. A method according to claim 25, wherein the fuel cell or
electrolyzer is one of a solid-oxide fuel cell and a solid-oxide
electrolyzer.
27. A method according to claim 25, wherein the insulating layer is
produced by oxidizing the carrier element in at least one area.
28. A method according to claim 27, wherein the oxidizing takes
place at a temperature above 900.degree. C.
29. A method according to claim 27, wherein the oxidizing takes
place at a temperature above 1,050.degree. C.
30. A method according to claim 25, wherein after the production of
the insulating layer, the sealing layer is fitted on in the form of
a sealing material strand.
31. A method according to claim 25, wherein the sealing area is
arranged between separator plates, and wherein producing at least
one sealing layer made of a sealing material comprises: applying a
sealing medium strand for forming a first sealing layer; fitting
the carrier layer having the insulating layer onto the first
sealing layer; and applying a sealing medium strand for forming the
second sealing layer to the fitted-on carrier layer in the sealing
areas.
32. A fuel cell or electrolyzer comprising a sealing structure
according to claim 1.
33. The fuel cell or electrolyzer of claim 32, wherein the fuel
cell or electrolyzer is one of a solid-oxide fuel cell and a
solid-oxide electrolyzer.
Description
[0001] This application claims the priority of German Patent
document DE 103 021 24.8, filed 21 Jan. 2003.
FIELD OF THE INVENTION
[0002] This invention relates to a sealing structure for a fuel
cell or an electrolyzer, to a method of producing the sealing
structure, and to a fuel cell or an electrolyzer.
BACKGROUND OF THE INVENTION
[0003] A conventional fuel cell stack 1, shown in FIG. 3, has two
or more individual fuel cells 2 which are stacked above one another
in the manner of a tower. Each fuel cell 2 has an electrolyte layer
3, a cathode layer 4 arranged on one flat side of the electrolyte
layer 3, and an anode layer 5 arranged on the other flat side of
the electrolyte layer 3. For contacting a neighboring fuel cell 2,
a contacting layer 6 is disposed on the cathode layer 4.
[0004] In addition, each individual fuel cell 2 has first and
second separator plates 7, 8 that bound a combustible-gas space 9,
into which the anode layer 5 projects. The combustible-gas space 9
is connected with the anode layer 5 such that combustible gas,
which flows through the combustible-gas space 9 (direction of the
arrow 10), can come in contact with the free surface of the anode
layer 5.
[0005] Between the second separator plate 8 of one fuel cell 2 and
a first separator plate 7 of the neighboring fuel cell 2, an
oxidation gas space 11 is constructed, through which oxidation gas
can flow (direction of the arrow 12), so that oxidation gas can
flow against the free surface of the cathode layer 4, which
projects into the oxidation gas space 11.
[0006] One flat side of the contacting layer 6 is in contact with
the cathode layer 4 while the other flat side contacts a flat side
of a first separator plate 7 of the neighboring individual fuel
cell 2 (the latter facing the oxidation gas space 11). By way of
corresponding openings 13 in the first and second separator plates
7 and 8, all combustible-gas spaces 9 are connected with one
another. In the area between a second separator plate 8 and a first
separator plate 7 of a neighboring individual fuel cell 2, the
combustible-gas spaces 9 are separated in a gas-tight manner from
the oxidation gas space 11 by means of a sealing layer 14, so that
a fuel feeding duct 15 and a removal duct 16 for the reaction
products are formed. Thus, combustible gas can be fed to the
combustible-gas spaces 9 in the direction of the arrow 18 and flows
through these in the direction of the arrow 10. In this case, the
combustible gas is oxidized in a fuel cell 2 along the anode layer
5, and the reaction product can leave the fuel cell stack 1 again
in the direction of the arrow 19. By way of correspondingly
constructed feeding and removal ducts, the oxidation gas, analogous
to the combustible gas, is guided through the oxidation gas spaces
11.
[0007] The separator plates 7 and 8 of an above-described fuel cell
stack 1 therefore, on the one hand, have the function of
electrically connecting the individual fuel cells 2, which are
disposed in series. On the other hand, they ensure the separation
of combustible and oxidation gas. For this purpose, the separator
plates 7 and 8 (also called bipolar plates or interconnector
plates) are constructed of a combustible-gas-tight,
oxidation-gas-tight and electronically conductive material, such as
a chrome-containing alloy, ferritic steel or perovskite. In order
to ensure a reliable separation of the oxidation gases and the
combustible gases, it is required that, in each case, between the
second separator plate 8 of a first fuel cell 2 and the first
separator plate 7 of a neighboring fuel cell 2, the feeding duct 15
as well as the product removal duct 16 be reliably sealed off from
the oxidation gas space 11.
[0008] It is known from the state of the art to construct the
sealing layer 14, for example, of glass-ceramic solders, which are
normally applied as pastes or etched foils before the assembling of
a fuel cell stack 1 onto the relevant sealing surfaces of the
separator plates 7, 8.
[0009] These sealing materials (glass-ceramic solders) normally
used in the case of solid-electrolyte fuel cells have two
characteristics influencing one another in opposite directions. The
coefficient of thermal expansion of the sealing material is clearly
lower in comparison to the coefficients of expansion of most
materials used for the bipolar plates 7 and 8. During the rapid
heating of the fuel cell stack 1, this may result in thermally
induced tension cracks in the sealing layer 14 and thus in a
failure of its sealing effect. This is particularly critical in the
case of solid-electrolyte fuel cells (the so-called SOFC's--solid
oxide fuel cells) which operate in the high-temperature range.
Particularly for solid-electrolyte fuel cells, which are stressed
by a frequent starting and switching-off of the operation, this
represents a problem which has not been satisfactorily solved.
[0010] From the state of the art, it is conventional to increase
the coefficient of expansion of the sealing materials by means of
additions. However, these additions frequently lead to a reduction
of the electric resistance of the sealing material at the typically
high operating temperatures of a solid-electrolyte fuel cell. By
way of the sealing layer 14 between a second separator plate 8 and
a first separator plate 7 of two neighboring individual fuel cells
2, this results in undesirable leak currents which impair the
electric efficiency of a fuel cell stack 1.
[0011] Another disadvantage of the sealing device known according
to FIG. 3 of the state of the art is that the materials for the
sealing layer 14 have a compression behavior and/or shrinking
behavior which differs in comparison to the contacting layer 6,
whereby, during the mounting of the fuel cell stack 1, undesirable
inaccuracies occur which may make a reliable contacting between the
contacting layer 6 and an adjoining separator plate 7 doubtful.
Furthermore, it is disadvantageous that the providing of a suitable
sealing layer 14 before the assembling of the fuel cell stack 1
requires high expenditures and cost because, for example, a sealing
agent strand has to be established or, in the case of a foil-type
construction of the sealing layer 14, the latter has to be produced
separately and has to be positioned or inserted before the
assembling process.
[0012] The above-mentioned glass-ceramic solders have two serious
disadvantages:
[0013] 1. The coefficient of thermal expansion of glass ceramics is
clearly lower in comparison to the coefficients of expansion of
most materials (chrome alloys, ferritic steel, perovskite) used for
the bipolar plates. During the rapid heating of the fuel cell
stack, this may result in thermally induced tension cracks in the
sealings and thus in a failure of the sealing effect. This is
particularly critical in the case of a mobile use of the fuel cell
stack, for example, in an auxiliary energy supply unit in an
automobile.
[0014] 2. Glass-ceramic solders shrink during the joining process,
that is, during the pressing-together and the first heating to the
operating temperature of 750-900.degree. C., to approximately
40%-70% of their initial volume. The entire stack therefore sinks
together during the joining process. In order to ensure the
tightness of the stack, the porous electric contacting layer 6 of
the fuel cell (see FIG. 3) also has to shrink by the same
thickness. The difficulty now consists of coordinating the
shrinkage behavior of the sealing layer and of the contacting
layer. The pasty ceramic suspensions normally used for the electric
contacting shrink even at low temperatures and compact at
temperatures higher than 400.degree. C. In the case of
glass-ceramic solders, the shrinking process starts only at
temperatures >500.degree. C. and is concluded only at
temperatures >750.degree. C. The two processes therefore do not
take place simultaneously and frequently result in gas leakages,
lack of electric contacting or a fracture of the SOFC (solid oxide
fuel cell) because of locally excessive contact pressure
forces.
[0015] Based on the above-mentioned disadvantages of the
glass-ceramic solders, the development of an alternative inorganic
sealing mass was carried out. With respect to its coefficient of
expansion, it is better adapted to the used bipolar plate materials
and has only minimal shrinkage during the joining process, so that
the necessity of using electric contacting materials especially
adapted in their shrinkage behavior is eliminated. However, the
disadvantage of this sealing paste is an electric insulating
capacity which is insufficient at the operating temperature. When
solid-electrolyte fuel cell stacks are used, this results in
electric leak currents (short circuits) between the individual
bipolar plates and thus results in power losses in the system.
[0016] German Patent Document DE 19515457 C1 describes a sealing
structure for a fuel cell. The fuel cell has an electrolyte layer
that consists of an electrolyte matrix saturated with an
electrolyte and, in the sealing area, the electrolyte matrix is
constructed to be extended beyond the electrodes. In the sealing
area, the electrolyte matrix is saturated by means of a material
chemically related to the electrolyte, which material is firm at
the working temperature of the fuel cell. However, the suggested
solution relates to a so-called molten-carbonate fuel cell which
has a molten electrolyte which is present in liquid form in an
electrolyte matrix. In the case of this type of fuel cell, one
usually speaks of a wet-sealing area because the electrolyte which
is molten in its operating condition, forms a wet area in the edge
region which is to be sealed off. However, this solution cannot be
transferred to a solid-electrolyte fuel cell since, in the case of
such a solid-electrolyte fuel cell (SOFC: solid-oxide fuel cell),
no so-called wet electrodes or wet electrolytes exist, and thus the
problem on which German Patent Document DE 19515457 C1 is based
does not occur as a result of the type of construction.
[0017] German Patent Document DE 19960516 A1 describes a sealing
device for a fuel cell. The fuel cell has an electrolyte membrane
that is extended into the edge sealing area between two separator
plates and a two-layer rubber seal that is arranged on the
electrolyte membrane. For the sealing structure, it is suggested
that one layer be constructed of a soft sponge rubber and the
second layer be constructed of a harder rubber, such as silicone
rubber or butyl rubber. This document relates to a so-called
low-temperature fuel cell with a polymer membrane electrolyte.
These so-called low-temperature fuel cells have operating
temperatures which are in the range of between 60.degree. C. and
800C. Because of their operating temperatures, such fuel cells
cannot be compared with a solid-electrolyte fuel cell because
normally solid-electrolyte fuel cells are operated in temperatures
range of between 700 and 1,100.degree. C. Because of the high
operating temperatures of a solid-electrolyte fuel cell, the
sealing device suggested in German Patent Document DE 19960516 A1
can therefore not be transferred to a solid-electrolyte fuel
cell.
[0018] Japanese Patent Document JP 10092450 shows a fuel cell stack
with insulating layers and sealing layers, arranged in layers and
formed as separate components.
SUMMARY OF THE INVENTION
[0019] It is an object of the invention to provide a sealing
structure for a fuel cell or an electrolyzer, particularly a
solid-electrolyte fuel cell, which is insensitive to
thermo-mechanical tensions and simultaneously ensures an electric
(particularly an electronic) insulation; that is, an impermeability
for electrons. Furthermore, the sealing structure according to the
invention is to be producible in a simple and cost-effective
manner, particularly in comparison to the state of the art, without
additional working steps. In addition, the compressibility and/or
the shrinkage characteristic of the sealing structure is to be
adapted to that of the contacting layer and thus provide a
facilitated and particularly more process-secure mounting.
[0020] This and other objects and advantages are achieved by a
sealing structure in a fuel cell or electrolyzer according to the
invention. In an embodiment, the sealing structure according to the
invention is arranged between neighboring separator plates of a
cell stack, the sealing structure being constructed in at least two
layers and having at least one insulating layer and at least one
sealing layer, and wherein the insulating layer is arranged on a
carrier element. Additional objects and advantages are achieved by
a method of producing a sealing structure for a fuel cell or an
electrolyzer according to the invention. In an embodiment, the
method comprises:
[0021] producing at least one insulating layer on a carrier
element; and
[0022] producing at least one sealing layer made of a sealing
material, the sealing structure being arranged in a sealing area of
a fuel cell stack.
[0023] Still other objects and advantages are achieved by a fuel
cell or electrolyzer according to the invention that comprises a
sealing structure as described above. Further advantageous
embodiments of the invention are indicated in the specification and
claims below.
[0024] In one embodiment of the invention, in order to counter the
lack of electric insulation capacity of certain sealing materials,
an outer skin made of aluminum oxide (Al.sub.2O.sub.3) in a
.gamma.-modification is produced by means of a targeted oxidation
process, either on the sealing surfaces of the bipolar plates
themselves or on insulation plates additionally inserted between
the sealing surfaces of the bipolar plates. .gamma.-Al.sub.2O.sub.3
has a very high electric resistance and an excellent corrosion
stability in oxidizing as well as in reducing media. In another
embodiment, additional insulation elements can be used when the use
of .gamma.-Al.sub.2O.sub.3-forming steel types as a bipolar
material is not desirable, for example, because of the restriction
on the electric current conduction between the bipolar plate and
the cells.
[0025] In an embodiment, the outer skin of aluminum oxide is
produced by the targeted oxidizing of steel plates with a high
aluminum content (>2%, preferably >4.5%) at temperatures
>900.degree. C., preferably >1,050.degree. C. In order to
ensure that the coefficients of thermal expansion of the bipolar
plates, the sealing devices and possibly the insulation plate
correspond, ferritic steel types with chrome contents of
approximately 20% can mainly be used (for example, the Material
Numbers 1.4765 and 1.4767). More generally, ferritic steel types
with a chrome content from about 15% to about 28% can be used.
Since these materials can be commercially obtained as steel strips
in many different thicknesses, the insulation plates, which may be
used, can simultaneously have the function of a spacer between the
individual bipolar plates which was taken over in the state of the
art by the sealing device itself. The strips are to be machined
easily in a shaping manner (stamping, punching, cutting) and, in
principle, can be shaped into any form--adapted to the bipolar
plate. During the joining of the fuel cell stack, they are sealed
in on both sides between the bipolar plates.
[0026] In preferred embodiments of the invention, electric short
and leak currents between the individual cell elements in the fuel
cell stack are prevented. The application of an electrically
insulating element between the bipolar plates of solid-electrolyte
fuel cell stacks permits the use of sealing materials that are less
than completely electrically insulating for separating and
distributing the combustible and oxidation gases. The option of
using these sealing devices, which are conductive at the SOFC
operating temperature, permits a novel joining concept of the fuel
cell stack for which a high-expenditure adaptation, which is
difficult to implement, of the shrinking behavior of the sealing
device and a porous electric contacting layer of the
solid-electrolyte fuel cell can be eliminated. As a result, the
joining process is significantly simplified.
[0027] Furthermore, the use of electrically conductive sealing
materials permits the use of materials which are better adapted to
the coefficients of thermal expansion of the bipolar plate
materials, so that the probability of a failure of the sealing
function because of faster thermal cycles is reduced, such as in
the use of the solid-electrolyte fuel cell in a mobile auxiliary
energy supply unit.
[0028] The use of the above-mentioned electric insulation layers
will be particularly advantageous when the latter can be produced
in a cost-effective manner from commercially available materials.
This applies to the use of pre-oxidized ferritic steel types, but
not, for example, to sintered ceramic insulation elements.
[0029] The two possible application sites of the
Al.sub.2O.sub.3-insulatio- n layer--directly onto the bipolar plate
or onto additional insulation plates--each have specific
advantages. The direct oxidation of the bipolar surface requires no
additional components in the stack, and thus also the number of
working operations during the joining of the fuel cell stack does
not increase. Alternatively, when additional insulation elements
are inserted, the combination of electric and mechanical tasks of
the insulation elements has an advantageous effect. This
combination is achieved when the insulation element simultaneously
takes over the spacer function between neighboring bipolar plates
from the sealing material and the sealing can thus be reduced to a
minimal thickness--defined only by the sealing function.
[0030] Other objects, advantages and novel features of the present
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a schematic cross-sectional view of a fuel cell
stack according to the invention having two individual fuel cells
which have a sealing structure according to the invention;
[0032] FIG. 2 is a schematic cross-sectional view of a second
embodiment of a fuel cell stack according to the invention having
two individual fuel cells which are equipped with a second
embodiment of a sealing structure according to the invention;
and
[0033] FIG. 3 is a schematic cross-sectional view of a conventional
fuel cell stack.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0034] A fuel cell stack 1 (FIG. 1) according to the invention has
at least two individual fuel cells 2, preferably a plurality of
individual fuel cells 2, which are stacked above one another in a
tower-type manner. The individual fuel cells 2 have an electrolyte
layer 3, a cathode layer 4 and an anode layer 5 and are preferably
constructed as so-called solid-electrolyte fuel cells (SOFC
solid-oxide fuel cells). The anode layer 5 is optionally arranged
on a supporting substrate layer (not shown). The free flat side of
the cathode layer 4 is connected via a contacting layer 6 with a
first separator plate 7 (also called bipolar plate or
interconnector plate) of a neighboring individual fuel cell 2.
[0035] The fuel cell stack 1 according to the invention has
combustible gas spaces 9 through which combustible gas can flow by
way of a combustible gas feeding duct 15 in the direction of the
arrows 18, 10, 19. By way of a removal duct 16, excess combustible
gas and reaction products can be removed. Likewise, the fuel cell
stack 1 according to the invention has oxidation gas spaces 11
through which oxidation gas can flow by way of suitable feeding and
removal ducts (not shown). The contacting layer 6 has an
electrically conductive and porous construction so that the
oxidation gas can flow through the contacting layer 6 in the
direction of the arrow 12.
[0036] For separating the combustible gases from the oxidation
gases, a fuel cell stack 1 according to the invention has a sealing
structure 20 which has a multi-layer, particularly at least
two-layer construction.
[0037] According to a first embodiment of the sealing structure 20
according to the invention, the latter has a sealing layer 21 and
an insulating layer 22. The insulating layer 22 consists of a metal
oxide, particularly an aluminum oxide (Al.sub.2O.sub.3) which, in a
particularly preferred manner, is constructed in the so-called
.quadrature.-modificati- on. Al.sub.2O.sub.3 in the
.quadrature.-modification has a particularly high electric
resistance and an excellent corrosion stability in oxidizing as
well as in reducing media.
[0038] In an embodiment corresponding to FIG. 1, the insulating
layer 22 is arranged in all required sealing areas 25 between two
neighboring individual fuel cells 2 on a top side 26 of one of the
bipolar plates 7, 8, which top side 26 faces an oxidation space 11.
The arrangement of the sealing layer 21 on a free surface of a
bipolar plate 7, 8 is advantageous particularly when the bipolar
plates 7, 8 are produced from a steel material with a high aluminum
content (>2%). In this case, the aluminum oxide insulating layer
can be produced by a targeted, particularly local oxidizing of the
bipolar plates 7, 8 in the sealing areas 25, preferably above
900.degree. C. The sealing layer 21 is arranged on a free surface
22a of the insulating layer 22, in which case it should be ensured
that the sealing layer 21 is connected with no more than one of the
bipolar plates 7 or 8 and, at the other end, comes in contact only
with the insulating layer 22.
[0039] The sealing layer 21 is constructed, for example, of a
glass-ceramic solder which, by means of additions, is adapted to
the coefficients of thermal expansion of the separator plates 7, 8.
Advantageously, the shrinkage behavior of the sealing layer 21
(when heated) is adapted to the shrinkage behavior of the
contacting layer 6. An increased conductibility of the sealing
layer 21, optionally caused by suitable additions, at the operating
temperature of the fuel cells 2, particularly in the temperature
range of from 750.degree. C. to 900.degree. C., in which
solid-electrolyte fuel cells (so-called SOFC--solid-oxide fuel
cells) are normally operated, is easily acceptable because of the
reliable electric insulation by the insulation layer 22.
[0040] For the gas-tight sealing-off of the feeding and removal
ducts for the oxidation gas, which are not shown in FIGS. 1 and 2,
a sealing structure 20 according to the invention is
correspondingly arranged between a first separator plate 7 and a
second separator plate 8 of an individual fuel cell 2, so that the
burnable-gas spaces 9 are separated from the oxidation-gas carrying
ducts.
[0041] Preferably, the insulating layer 22 covers a larger area on
the separator plate 7 or 8, on which it is mounted, than is
required by the sealing layer 21, so that it is ensured that no
"electric bridge" can be formed by the material of the sealing
layer 21 when the fuel cell stack is joined together.
[0042] In embodiments corresponding to FIG. 1, the invention
provides the advantage that an improved sealing and insulating
effect can be achieved between two individual fuel cells 2 without
requiring additional mounting steps in comparison to previous
mounting sequences during the mounting of the fuel cell stack 1.
The formation of the insulating layer 22, for example, of
Al.sub.2O.sub.3, can take place in a simple manner by an oxidation
process during the production of the separator plates in a fully
automatic fashion. Thus, a process-secure and reliable mounting of
a fuel cell stack 1 corresponding to FIG. 1 of the invention is
ensured.
[0043] In a second embodiment of the sealing structure of the
invention corresponding to FIG. 2, the insulating layer 22 is
arranged on a carrier layer 23, in which case a first sealing layer
21a is arranged between the insulating layer 22 and a neighboring
separator plate 7, and a sealing layer 21b is arranged between the
carrier layer 23 and its neighboring separator plate 8. Thus, in
this embodiment, the sealing structure 20 has at least four layers,
having at least one carrier layer 23, at least one insulating layer
22 and at least two sealing layers 21a, 21b.
[0044] In this embodiment, the insulating layer 22 is constructed
of the same material as the insulating layer 22 of an embodiment
corresponding to FIG. 1. The sealing layers 21a, 21b are preferably
constructed of the same material as the sealing layer 21 of an
embodiment corresponding to FIG. 1. The carrier layer 23 is, for
example, a steel plate with a high aluminum content (>2%). In
the case of such steel plates, at temperatures above 900.degree.
C., the insulating layer 22 can be produced of aluminum oxide by a
targeted oxidizing.
[0045] Ferritic steel types with chrome-contents of approximately
20% (for example, Material No. 1.4765, particularly with an
aluminum content of 5-6%; Material No. 1.4767, particularly with an
aluminum content of 4.5-5.5%) are particularly preferred. These
materials are particularly suitable for the construction of the
carrier layer 23 when the uniformity or the correlation of the
coefficients of thermal expansion of the bipolar plates 7, 8, the
sealing devices of the sealing layer 21 and the insulation layer 22
is to be achieved. The above-mentioned materials are also
particularly preferred because they are commercially available as
strips in many different thicknesses and can easily be processed in
a shaping manner, for example, stamped, punched and cut.
[0046] If the carrier layer 23 is constructed of a steel plate, by
means of the suitable selection of the plate thickness, the carrier
layer 23 can advantageously carry out an additional function,
specifically the function of a spacer between two neighboring
individual fuel cells 2.
[0047] Thus, in a simple manner, the shrinkage of the entire
sealing structure 20 can be adapted to the shrinkage of the
contacting layer 6. For example, when a sealing material for the
sealing layers 21a, 21b is used which has a high shrinkage, the
thickness of the carrier layer 23 can be selected to be relatively
large, so that the sum of the shrinkages of the sealing layers 21a,
21b corresponds the total shrinkage of the contacting layer 6. In
the case of a sealing material which has only a very slight
shrinkage, by selecting a thinner carrier layer 23, the remaining
sealing layer thickness of the sealing layers 21a, 21b can be
selected to be so large that the sum of the shrinkages of the
sealing layers 21a, 21b corresponds to the total shrinkage of the
contacting layer 6.
[0048] The coordination of the individual layer thicknesses of the
first sealing layer 21a, of the second sealing layer 21b and of the
carrier layer 23 can be defined in a limited number of tests by a
person skilled in the art in such a manner that, with respect to
its shrinkage behavior, the sealing structure 20 corresponds to
that of the contacting layer 6. When the shrinkage of the material,
of which the contacting layer 6 consists, is particularly low, for
example, also the layer thickness of the sealing layers 21a, 21b
can be minimized such that the sealing-material-specific minimal
thickness defined only by the sealing function is adjusted. The
electric insulation capacity of the sealing device therefore no
longer has to be considered when selecting the layer thickness.
[0049] The carrier layer 23 is particularly preferably constructed
of a pre-oxidized ferritic steel because the operation of the
oxidizing of the carrier layer 23 can therefore be eliminated
during the production of the fuel cells.
[0050] The characteristics of embodiments corresponding to FIGS. 1
and 2 can also be combined. Particularly, also in the case of a
sealing structure 20 corresponding to FIG. 1, an adaptation of the
shrinkage characteristic can be used by inserting a carrier layer
23, which optionally has no Al.sub.2O.sub.3-layer, only for the
function of a spacer.
[0051] The method according to the invention will be explained in
detail in the following by means of examples.
[0052] For producing a sealing structure 20 according to the
invention, an insulating layer 22 is applied to a carrier 7, 8, 23.
In particularly preferred embodiments, the carrier may, on the one
hand, be one of the separator plates 7, 8 and, on the other hand,
the carrier layer 23.
[0053] In an embodiment, an insulating layer 22, particularly made
of Al.sub.2O.sub.3, preferably Al.sub.2O.sub.3 in the
.gamma.-modification, is mounted on the carriers 7, 8, 23 in the
sealing areas 25. In this case, the material of the carrier 7, 8,
23 is provided with the insulating layer 22 by targeted
oxidation.
[0054] It is particularly advantageous to use a material for the
carrier 7, 8, 23 which contains aluminum in a sufficiently large
quantity, particularly a quantity >2%. Suitable materials are,
for example, materials of the Numbers 1.4765 and 1.4767.
[0055] In this case, the targeted oxidation preferably takes place
at >900.degree. C., particularly at temperatures
>1,050.degree. C.
[0056] According to a particularly preferred embodiment, after the
production of the insulating layer 22 in the sealing areas 25 of
the separator plates 7, 8, the sealing layer 21, particularly in
the form of a sealing material strand, is fitted onto the
insulating layer 22.
[0057] If the insulating layer 22 is mounted on a carrier layer 23,
particularly a carrier plate, the sealing structure is produced in
that a sealing device strand for forming a first sealing layer 21a
is applied in the sealing area 25 of the bipolar plates 7 and 8
respectively. Subsequently, the carrier layer 23 having the
insulating layers 22 will be fitted onto the first sealing layer
21a. In this case, a sealing medium strand for forming is second
sealing layer 21b is applied to the fitted-on carrier layer 23
again in the sealing areas 25. The above-described layer sequence
is arranged between two neighboring bipolar plates 7, 8 of two
neighboring individual fuel cells 2 such that either feeding and
removal ducts respectively for burnable gas or feeding and removal
ducts respectively for oxidation gas are constructed, the burnable
gas ducts each being connected with burnable gas spaces 9, and the
oxidation gas ducts being connected with oxidation gas spaces
11.
[0058] It is an advantage of the first described embodiment that,
for the mounting of a fuel cell stack, in comparison to the state
of the art, no additional parts exist which have to be used and
thus the mounting is not made difficult, although an improved
adaptation of the insulating characteristics and of the expansion
or shrinkage characteristics of the sealing structure, particularly
a possible adaption of the shrinkage and of the coefficient of
thermal expansion of the sealing mass to the corresponding
parameters of the contacting layer 6 or of the separator plates 7,
8 is achieved.
[0059] In the second embodiment of the method according to the
invention, it is advantageous that, despite an additional mounting
part (carrier layer 23 with the insulating layer 22), which
additionally has to be inserted during the mounting of the fuel
cell stack 1, it can be achieved that the carrier layer 23,
together with the insulating layer 22, is incompressible and can
therefore take over a spacer function. In addition, by varying the
thickness ratios of the carrier layer 23 and of the sealing layers
21a, 21, the shrinkage of the sealing structure 20 can be adapted
to the shrinkage of the contacting layer 6 with respect to the
absolute end measurement as well as in its progress during the
shrinkage. In a particularly preferred case, the thicknesses of the
sealing layers 21a, 21b can be reduced so far that only the minimum
thickness defined for the sealing function is present and thus a
saving of the relatively expensive sealing medium used for
constructing the sealing layers 21a, 21b can be achieved. It is
particularly advantageous that the gap width of the sealing gap,
which is to be filled with sealing material, can be reduced
considerably and the risk of a failing of sealing device is
therefore considerably reduced.
[0060] The foregoing disclosure has been set forth merely to
illustrate the invention and is not intended to be limiting. Since
modifications of the disclosed embodiments incorporating the spirit
and substance of the invention may occur to persons skilled in the
art, the invention should be construed to include everything within
the scope of the appended claims and equivalents thereof.
* * * * *