Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Aluminous Interconnector for Fuel Cells
The present invention relates to an aluminous interconnector for the
electrodes of fuel cells.
A fuel cell comprises a cathode, an electrolyte, and an anode. An oxidizing
agent, e.g., air, is routed to the cathode, and fuel, e.g. hydrogen, is routed
to the anode.
There are various types of fuel cells; these include the SOFC described in DE
44 30 958 Cl, and the PEM fuel cell that is described in DE 195 31 852 Cl.
The SOFC is also referred to as a high-temperature fuel cell since its
operating temperature can reach 1000 C. Oxygen ions form on the cathode
of a high-temperature fuel cell in the presence of the oxidizing agent. The
oxygen ions diffuse through the electrolyte and recombine with the hydrogen
that originates from the fuel to form water on the anode side. Electrons are
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liberated during this recombination, and electrical energy is generated
thereby.
As a rule, a plurality of fuel cells are connected together electrically and
mechanically by connecting elements-also referred to as inter-connectors-
in order to generate large amounts of electrical power. One example of a
connecting element is the bipolar plate. Fuel cells that are stacked one
above the other and electrically connected in series are formed using bipolar
plates. This arrangement is referred to as a fuel-cell stack. The fuel-cell
stacks comprise the interconnectors and the electrode-electrolyte units.
In addition to their electrical and mechanical characteristics,
interconnectors
incorporate gas-distribution structures. In the case of the bipolar plate,
these are realized in the form of bars with electrode contact that separate
the gas channels used to supply the electrodes from each other (DE 44 10
711 Cl). Gas-distribution structures ensure that the operating medium is
distributed evenly into the electrode spaces (the spaces in which the
electrodes are located).
The following problems can arise with fuel cells and fuel-cell stacks:
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- Metal bipolar plates that have a high chromium content form
conductive chromium oxide covering layers; during operation,
vapourization of the chromium causes aging within the fuel cell.
- Metal bipolar plates with high aluminum contents form A1203 covering
layers that act as undesirable electrical insulators.
- A plurality of fuel cells can be stacked by means of bipolar plates,
using a joining process. When this is done, the fuel cells are joined
together under pressure. During the joining process, low-conductivity
contact points can result between the known, rigid bipolar plates and
the electrode-electrolyte units. These are caused, amongst other
things, by manufacturing tolerances during the production of bipolar
plates or electrode-electrolyte units.
- thermal stresses can occur during cyclical temperature loads; these
result from the various expansion behaviours associated with the
materials that are used during operation.
Thus, it is the objective of some embodiments of the present invention to
create an interconnector
that ensures long-term, stable contact with the anode. A method for
manufacturing such an interconnector is also described.
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In some embodiments, this objective has been achieved by an interconnector as
defined herein.
The interconnector is aluminous and incorporates contact surfaces and gas-
distribution structures for electrodes of high-temperature fuel cells, said
contact surfaces having-at least in part-nickel-aluminun alloys. These can
be nickel alumindes (e.g., NiAl, NiAIZ, Ni3AI). Such nickel-aluminum alloys
offer the following advantages in some embodiments:
- Nickel-aluminum alloys act as diffusion barriers for alloy components of
the steels used for interconnectors and prevent the formation of low-
conductivity corrosion products (e.g., aluminum oxide) on the surfaces
between the anode interconnector and the nickel coating of the foil.
- Nickel-aluminum alloys are resistant to high temperatures (e.g., the
melting point of NiAI is 1638 C).
- Nickel-aluminum alloys are electrically conductive to a sufficient
degree.
- Low material and machining costs.
In one embodiment, the contact surfaces are formed by bars
that separate the interconnector gas channels from each other.
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The interconnector is then in the form of a bipolar plate, the bars of which
have nickel-aluminum alloys, at least in part.
In one embodiment, at least one nickel foil is connected to the contact
surfaces of the interconnector by the formation of an alloy.
This means that there are nickel-aluminum alloys on the
boundary surface between the contact surfaces of the interconnector and the
nickel foil. Some embodiments describe the form of the foil(s), and thereby
the
form of the nickel-aluminum alloys that are present on the contact surfaces
of the interconnector. A nickel foil in the form of a perforated plate
can be joined to the contact surfaces of the interconnector by the
formation of an alloy; the openings of this lie on the gas channels of the
interconnector; this ensures that fuel is moved right to the electrodes of the
fuel cell.
However, strip-like or punctiform nickel foils can be
joined to the contact surfaces of the interconnector by the formation of an
alloy. The contact surfaces of the interconnector can be exactly as wide as
the nickel foils, aithough they can also be wider than the foils.
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A high-temperature fuel cell may include such an interconnector,
the contact surfaces of the interconnector contacting the
anode of the high-temperature fuel cell . Long-term, stable contact
is ensured because of the properties of the nickel-aluminum alloys, such as
resistance to high temperatures. From this it follows that because of the
nickel-aluminum alloys on the anode side of the interconnector, a reduction
of contact resistance in the high-temperature fuel cell is achieved, since the
formation of insulating aluminum oxide layers is prevented. If the
interconnector is in the form of a bipolar plate, the nickel-aluminum alloys
lie
on the bars with electrode contact, when the bars can be covered more or
less completely with the nickel-aluminum alloys.
In a further embodiment of the present invention, an elastic nickel grid is
arranged between the anode and the interconnector. This nickel
grid serves as an additional means to ensure even electrical contact between
the anode and the interconnector through the mesh points of the grid, and
thereby even out of the above-discussed manufacturing tolerances in the
interconnector or the anode.
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A fuel-cell stack comprises at least two such high-temperature fuel cells
in some embodiments. This achieves greater power outputs.
A method for manufacturing an interconnector is described herein. At least one
nickel foil is
applied to the contact
surfaces of an aluminous interconnector. This is followed by thermal
processing, in particular a hot-pressing process, that results in the
formation
of nickel-aluminum alloys. The nickel foils consist of 99-% nickel
and NiAI, NiA12, Ni3A1, and other alloys. If the foil is in the form of a
perforated plate, the openings can have been punched out of the nickel very
simply beforehand. It has been shown that hot-pressing processes at up to
1150 C are particularly well suited for producing nickel-aluminum alloys that
are stable for long periods. Welding methods carried out in an atmosphere
of a protective gas as well as, to some extent, plasma spraying methods can
also be used.
A further possibility for producing nickel-aluminum alloys involves the use of
nickel electroplating. Subsequently, the nickel-plated surface is annealed in
a vacuum with the formation of nickel-aluminum alloys. -
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In some embodiments, the advantage entailed in this method
is that large surfaces can be covered with nickel-aluminum
alloys in this way.
In accordance with a broad aspect of the present invention,
there is provided an aluminous interconnector with contact
surfaces and gas-distribution structures for electrodes of
high-temperature fuel cells, the contact surfaces having
nickel-aluminum alloys, at least in part, wherein at least
one nickel foil is joined to the contact surfaces of the
interconnector by formation of an alloy.
Using such methods it is possible to form a fuel cell or a fuel cell stack.
Illustrative embodiments of the present invention will be described in
greater detail below on the basis of the drawings appended hereto.
Figure la is a diagrammatic cross-section through part of a high-
temperature fuel cell in which the anode is contacted through an aluminous
interconnector that is in the form of a bipolar plate. The bipolar plate is
joined to a nickel foil 13 in the form of a perforated plate by the formation
of
an alloy. The nickel foil can be of any thickness from 50-1000Nni, in
particular 500Nm. Nickel-aluminum alloys are present on the border surface
between a bipolar plate 5 and the nickel foil 13. A nickel grid 2 is arranged
between the anode 1 and the bipolar plate 5. Figure lb is a plan view
showing the arrangement of the perforated nickel foil 13 on the bipolar plate
5. The cross section of Figure la is taken on the plane A-A' or B-B'. Figure
lb is thus a plan of the object shown in Figure la, although without the
nickel grid 2 and the anode 1. The perforated nickel foil 13 is joined to the
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bars 6 of the bipolar plate 5 by the formation of an alloy, so that nickel-
aluminum alloys are present. In total, Figure 1 has seven bars 6, only one
of which bears the reference number 6. Six gas channels 14 are separated
from each other by the seven bars 6. The openings 7 of the perforated
nickel foil 13 are located on the gas channels 14 of the bipolar plate 5.
Figure lb shows three rows of six openings each. The openings 7 are in the
form of rectangular gaps that are separated from each other in the planes A-
A' and B-B'. At the locations where the perforated nickel foil 13 lie over the
gas channels 14 of the bipolar plate 5 there is only the nickel foil or its
openings. On the one hand, this ensures the flow of gas from the gas
channels 14 of the bipolar plate 5 through the openings 7 in the perforated
nickel foil 13 to the anode 1. Because of the position of the opening 7 above
the gas channels 14, the thermal stresses that are generated between the
perforated nickel foil 13 and the bipolar plate 5 during operation at high
temperatures are minimized. The openings 7 in the perforated nickel foil 13
are interrupted in the planes A-A' and B-B' in Figure lb. This enhances the
stability of the nickel foil under operating conditions.
In Figure la, between the anode 1 and a perforated nickel foil 13 there is an
elastic nickel grid 2. This grid is 250pm thick and has a mesh size of 200pm.
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The diameter of the wire is 125pm. The nickel grid bridges the poor
electrical contact between anode 1 and the bars 6 of the bipolar plate 5,
which can occur because of manufacturing tolerances during production of
the materials.
Figure 2 and Figure 3 are diagrammatic plan views of two other
embodiments of nickel foils 1 on the contact surfaces of an interconnector.
In Figure 2, seven strip-shaped nickel foils 23, and in Figure 3 a total of 42
punctiform nickel foils 33 are joined to the contact surfaces of the
interconnector by the formation of an alloy. In Figure 3, only the punctiform
nickel foil in the top left-hand corner bears the reference number 33. In
both of these figures, six gas distribution structures 24, 34 extend
vertically
between the nickel foils 23 or 33, respectively. In both of these figures, the
width of the strip-shaped or punctiform nickel foils 23, 33 corresponds to the
width of the interconnector contact surfaces. In Figure 3, the width of the
contact surfaces is indicated by two dashed lines that bear the reference
number 8.
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Figure 4 shows series of measurements of the contact resistance between
the anode and the interconnector of two high-temperature fuel cells as a
function of operating time at an operating temperature of 800 C. In each
instance, the steel used for the interconnector was an iron-chromium-
aluminum alloy containing approximately 5% aluminum (Material No.
1.4767). Between the anode and the interconnector there was a nickel grid,
250pm thick, with a mesh size of 200pm and a wire diameter of 125pm.
The curve made up of the square symbols refers to a fuel-cell stack in which
the interconnector has no nickel-aluminum alloys. The curve made up of the
triangular symbol relates to a fuel-cell stack in which the interconnector has
been plated with a nickel foil during a hot-pressing process and thus contains
nickel-aluminum alloys. The operating temperature during the hot-pressing
process amounted to 1150 C, and this was maintained for a period of 60
minutes. The operating temperature in the fuel cell stack was 800 C. Within
approximately 30 hours of operation, the contact resistance in the
embodiment without the nickel-aluminum alloys rose from an initial 25 to 37
mg cm2. In the case of a fuel cell with an interconnector that contains nickel-
aluminum alloys, there was still no increase of the contact resistance at the
anode, even after 500 hours of operation. Even after cycling of the
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operating temperature, as indicated by the arrow in Figure 4, the contact
resistance rose only insignificantly, from 4 to 6 mg cm2 and remained at this
low value even after 1000 hours of operation. The nickel-aluminum alloys
thus brought about a constant low and thus long-term stable contact
resistance. After 300 hours of operation, the contact resistance of such a
high-temperature fuel cell is almost 1000% lower as compared to a cell with
interconnectors that contain nickel-aluminum alloys.
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