High-temperature fuel cell

Details High-temperature fuel cell High-temperature fuel cell

2001-09-26

2003-09-16

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S047000

Reexamination Certificate

active

06620541

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVNETION
The invention relates to a high-temperature fuel cell, in which an electrical conductor electrically connects an interconnector to the anode of an electrolyte/electrode unit.
It is known that when water is electrolyzed the electrical current breaks down the water molecules to hydrogen (H
2
) and oxygen (O
2
) A fuel cell reverses this procedure. Electrochemical combination of hydrogen (H
2
) and oxygen (O
2
) to give water is a very effective generator of electric current. This occurs without any emission of pollutants or carbon dioxide if the fuel gas used is pure hydrogen (H
2
). Even with an industrial fuel gas, such as natural gas or coal gas, and with air (which may also have been enriched with oxygen (O
2
)) instead of pure oxygen (O
2
), a fuel cell produces markedly lower levels of pollutants and less carbon dioxide than other energy generators in which the energy is introduced from different sources. The fuel cell principle has been implemented industrially in various ways, and indeed with various types of electrolyte and with operating temperatures of from 80° C. to 1,000° C.
Depending on their operating temperature, fuel cells are divided into low, medium, and high-temperature fuel cells, and these in turn have a variety of technical configurations.
In the case of a high-temperature fuel cell stack composed of a large number of high-temperature fuel cells, there is an upper interconnector, which covers the high-temperature fuel cell stack, and under this plate there are, in order, at least one contact layer, an electrolyte/electrode unit, a further contact layer, a further interconnector, etc.
The electrolyte/electrode unit here contains two electrodes—an anode and a cathode—and a solid electrolyte configured as a membrane disposed between the anode and the cathode. Each electrolyte/electrode unit here situated between two adjacent interconnectors forms, with the contact layers situated immediately adjacent to the electrolyte/electrode unit on both sides, a high-temperature fuel cell, which also includes those sides of each of the two interconnectors which are situated on the contact layers. This type of fuel cell, and others types, are known from the reference titled “Fuel Cell Handbook” by A. J. Appleby and F. R. Foulkes, 1989, pp. 440-454, for example.
A single high-temperature fuel cell provides an operating voltage of less than one volt. Connecting a large number of adjacent high-temperature fuel cells in series can give an operating voltage of hundreds of volts from a fuel cell system. Since the current provided by a high-temperature fuel cell is high—up to 1,000 amperes in the case of large high-temperature fuel cells—the electrical connection between the individual cells should preferably be one that gives rise to particularly low series electrical resistance under the above-mentioned conditions.
The electrical connection between two high-temperature fuel cells is provided by an interconnector, via which the anode of one high-temperature fuel cell is connected to the cathode of the next high-temperature fuel cell. The interconnector therefore has an electrical connection to the anode of one high-temperature fuel cell and to the cathode of the next high-temperature fuel cell.
The electrical connection between the anode and the interconnector, which is configured as a plate, is provided by an electrical conductor, which may take the form of a nickel grid (see, for example, German Patent DE 196 49 457 C1). It has been found that the series electrical resistance between the anode and the interconnector, when the high-temperature fuel cell is operating, is high. This has a serious adverse effect on the electrical performance of the high-temperature fuel cell stack.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a high-temperature fuel cell which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which avoids any relatively high series electrical resistance, even when operating under high temperatures, and to ensure high conductivity, even over prolonged periods.
With the foregoing and other objects in view there is provided, in accordance with the invention, a high-temperature fuel cell. The fuel cell contains an electrolyte/electrode unit having an anode, an interconnector having a fuel gas side, and at least two metallic functional layers applied one above another on the fuel-gas side of the interconnector. The two metallic functional layers include a first functional layer containing nickel and a second functional layer containing copper disposed below the first functional layer. An electrical conductor connects the anode to the first functional layer.
According to the invention, the object is achieved by the high-temperature fuel cell of the type with at least two metallic functional layers that are applied one above the other on the fuel-gas side of the interconnector. One of the functional layers contains nickel and the functional layer below it contains copper.
Experiments with the high-temperature fuel cell stack and appropriate modeling experiments have shown that an increase in the electrical resistance between the electrical conductor and the interconnector formed of CrFe5Y
2
O
3
1 is established, even after a short operating period at operating temperatures of between 850° C. and 950° C. The designation CrFe5Y
2
O
3
1 represents a chromium alloy that contains 5% by weight of Fe and 1% by weight of Y
2
O
3
. The increase in the electrical resistance is caused by an oxide layer that contains chromium oxide and is formed on the surface of that side of the interconnector that faces the chamber that carries the fuel gas. It also forms where the electrical conductor, for example the nickel grid, rests on the interconnector or, for example, is joined to the interconnector by a spot weld or a soldering point. If the nickel grid has been spot-welded to the interconnector, during operation, amazingly, chromium oxide even creeps beneath these contact points, which are in the form of weld spots. Chromium has a higher electrical resistance than the unoxided metals of the interconnector.
Therefore, there is an oxide layer of poor conductivity between the electrical conductor and the interconnector, which has an unfavorable influence on the series resistance of series-connected high-temperature fuel cells. The formation of chromium oxide takes place even at oxygen partial pressures of less than 10
−18
bar. The oxygen partial pressures are also generally present in the chamber that carries the fuel gas—known as the fuel-gas chamber for short—while the high-temperature fuel cell is operating.
In a first step, the invention is based on the idea that suppressing the formation of the oxide layer on the anode side of the interconnector avoids any relatively high series electrical resistance and ensures high conductivity even over prolonged periods. This is reliably achieved during the operation of the high-temperature fuel cell by the fact that the interconnector is protected from oxidation by a functional layer. Naturally, a functional layer of this type should not be permeable to oxygen under operating conditions. It must not have an adverse effect on the electrical connection between conductor and interconnector. Furthermore, it should be inexpensive and easy to handle.
All these conditions are met by a thin metallic functional layer that closes off the interconnector in a gas-tight manner around the contact point. However, with a functional layer of this type the problem exists that it is oxidized during the initial heating of the high-temperature fuel cell to its operating temperature. During the initial “start-up”, there is generally also sufficient air in the fuel-gas chamber of the high-temperature fuel cell to oxidize an inexpensive metallic functional layer. In this case, the oxygen also reaches the interconnector. The oxygen then forms the above-described, disruptive chromium oxide layer on the interconnector.
In a second step, th

Affiliated with

Also associated with

Rating

High-temperature fuel cell does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with High-temperature fuel cell, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High-temperature fuel cell will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3050942