Current collector for a SOFC fuel-cell pile

Details Current collector for a SOFC fuel-cell pile Current collector for a SOFC fuel-cell pile

1999-08-13

2001-07-31

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000

Reexamination Certificate

active

06268076

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a current collector for SOFC fuel-cell piles.
2. Background Information
A fuel-cell pile is provided with a plurality of fuel cells as essential components. A fuel cell in turn comprises a cathode, an electrolyte and an anode. An oxidizing agent such as air is supplied to the cathode and a fuel such as hydrogen to the anode. Both fuel and oxidizing agent are referred to in general as process materials hereinafter.
Various types of fuel cells exist. An example is the SOFC fuel cell, which is also known as the high-temperature fuel cell, since its operating temperature can be as high as 1000° C.
At the cathode of a high-temperature fuel cell, oxygen ions are formed in the presence of the oxidizing agent. The oxygen ions pass through the electrolyte to the anode side, where they recombine with hydrogen from the fuel to form water. The recombination reaction releases electrons and thereby generates electrical energy.
A SOFC fuel cell contains a solid electrolyte, which conducts the O
2−
ions but not electrons. Yttria-stabilized zirconia (YSZ) is usually used as material for the solid electrolyte.
Large powers are achieved by stacking a plurality of fuel cells together and connecting them electrically in series. The element which connects two fuel cells is known as an interconnector. It provides both electrical and mechanical coupling of two fuel cells. The connecting element is also used to form the cathode or anode chambers. A cathode chamber contains a cathode and an anode chamber. Such stacked fuel cells are known as fuel-cell piles.
From the prior art there is known an interconnector made from ceramic material, for example lanthanum chromite (LaCrO
3
). This interconnector indeed exhibits suitable electrical conductivity at high temperatures and can also be readily matched to the thermal expansion behavior of the cell material of the fuel-cell pile. The ceramic material is very expensive, however, in addition to which production of interconnectors therefrom is a complex process. Thereby high manufacturing costs are also incurred.
Another interconnector known from the prior art is made from a metallic material. For this purpose a heat-resistant ferritic alloy such as Cr5Fe1Y
2
O
3
is preferably used. Cr5Fe1Y
2
O
3
is a mechanical alloyed alloy which is 99% metallic and is a powder metallurgical (“PM”) alloy. Cr5Fe1Y
2
O
3
is produced using powders, high-energy milling and sintering. Cr5Fe1Y
2
O
3
contains 94% chromium, 5% iron and 1% Y
2
O
3
. Cr5Fe1Y
2
O
3
serves to improve certain properties, such as fatigue and corrosion resistance. Because of the high operating temperatures in combination with high O
2
partial pressure on the cathode side, an oxide layer is formed on the metallic interconnector material.
This oxide layer must now satisfy stringent requirements of high temperature stability and conductivity. In the prior art, these requirements are met only by chromium oxide layers. These in turn suffer from the disadvantage, however, that the cathode in particular becomes damaged by volatilization of chromium oxides under the given high-temperature operating conditions. For this reason, the prior art provides for coating the cathode side with a special full-surface barrier layer, of LaCrO
3
, for example, in order to prevent volatilization of the chromium oxides.
It is also known from the prior art that, by addition of aluminum, the alloy forms a cover layer of Al
2
O
3
. This cover layer in turn is more stable than the chromium oxide cover layer, but the cover layer of Al
2
O
3
has only vanishingly low electrical conductivity.
SUMMARY OF THE INVENTION
The goal of the present invention is therefore to provide a current collector in which the volatilization of chromium oxides is largely suppressed, while adequate electrical conductivity of the contact points between a current collector and the electrodes is simultaneously ensured.
The goal is solved by the current collector of the present invention. According to the present invention, it has been found that the requirements of conductivity of the oxide layer must be met only in the zone of the contact elements which make electrical contact. In contrast, the reverse requirements apply to the oxide layer in the zones of the remaining surface, where electrical conductivity is not particularly important, but instead the surface must be as stable as possible.
It is therefore proposed that only the alloy of the contact elements have an aluminum content of less than 2.0 wt %. Thereby it is advantageously ensured that an Al
2
O
3
layer will be formed to only a very small extent at the surface of the contact element, and thus will not impair electrical contact with one of the cathodes of the fuel cell. In contrast, the alloy of the remaining base body of the interconnector has an aluminum content of more than 2.0 wt %, and so an Al
2
O
3
layer is always formed on the surface of the base body.
This Al
2
O
3
layer is stable toward the atmosphere around it and ensures that escape of chromium oxides from the interconnector is nonexistent or very slight. Not the least consequence thereof is that the stability of the electrodes of the fuel cell is improved.
The present invention thus concerns a current collector for a SOFC fuel-cell pile including a base body made from a first heat resistant ferrite alloy which contains chromium and aluminum and has an aluminum content of more than 2 wt % and at least one contact element made from a second heat-resistant ferrite alloy which contains chromium and has an aluminum content of less than 2 wt % (which can be 0 wt %), wherein the base body defines guide ducts for feeding fuels and the at least one contact element is fastened facing away from the base body at the end of a ridge of the base body which bounds a side wall of a guide duct.
Examples of the first heat-resistant ferrite alloy include X8 CrAl 20-5; X8 CrAl 25-5; and X8 CrAl 14-4.
Examples of the second heat-resistant ferrite alloy include X10 CrAlSi 18; X10 CrAlSi 24; X10 CrAlSi 13; and X7 CrTi 12.
The meaning of the following different terms in the above described alloys are according to the DIN, which is the German Normalization Standard:
X8, X10, X7: the “X” means that the material is a steel alloy and the number following the “X” is the carbon content in {fraction (1/100)} percent. So X8, for example, is a steel with a nominal carbon content of 0.08 percent.
X8 CrAl 20-5 is an alloy steel containing 0.08 percent carbon, 20 percent chromium and 5 percent aluminum.
X8 CrAlSi 18 an alloy steel containing 0.08 percent carbon, 18 percent chromium and a certain low amount of Al and Si (about 1 percent).
X7 CrTi 12 is an alloy steel containing 0.07 percent carbon, 12 percent chromium and a certain low amount of Ti (about 1 percent).
Preferably the alloy of the contact element has an aluminum content of less than 1.5 or 1.0 wt %. This follows from the knowledge that the tendency of the alloy to form an Al
2
O
3
layer decreases as the aluminum content becomes smaller. Below 1.0 wt %, it can be assumed that an Al
2
O
3
layer will no longer be formed on the surface of the alloy.
Preferably the alloys comprise a heat-resistant ferritic alloy, which can be matched appropriately to the thermal expansion behavior of the other materials of the fuel-cell pile, and which also ensure suitable shape stability, so that mechanical integrity of the fuel-cell pile can be guaranteed.
In a further preferred embodiment, the contact elements are bonded to the base body of the current collector by means of coalesced materials. Welding, weld surfacing or brazing are examples suitable for this purpose. The bonded zone then advantageously contains barrier layers, which suppress inter diffusion of aluminum between the alloys.
A fuel-cell pile can now be constructed with the current collector described hereinabove. As also described hereinabove, such a current collector can be used not only for feeding the fuels for the fuel cells, but also for conducting the cur

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