Solid oxide fuel cell interconnector

Details Solid oxide fuel cell interconnector Solid oxide fuel cell interconnector

1998-02-04

2001-12-04

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000

Reexamination Certificate

active

06326096

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to solid oxide fuel cells, in particular, interconnects comprising a superalloy and a metal which does not oxidize in the atmosphere of the fuel side of a solid oxide fuel cell, for example, nickel or copper for use in solid oxide fuel cells.
2. Description of Prior Art
It is well known that nickel is perfectly satisfactory as a constituent of an anode electrode in solid oxide fuel cells in which hydrogen, as well as reformed methane, is used as a fuel. It can be readily shown that, on the one hand, in the fuel atmosphere at the anode electrode, neither nickel oxide nor nickel carbide can form. Most superalloys, that is alloys which are resistant to oxidation at high temperatures, such as austenitic stainless steel and inconel, on the other hand, contain a significant amount of chromium. The partial pressure of oxygen at the anode electrode is usually high enough to form chromium oxide, Cr
2
O
3
. Although chromium oxide scale does not grow rapidly, its resistance is rather high and, thus, it is desirable that its thickness be as small as possible. In our work on fuel cell stack testing, we have observed that the oxide coating is usually thicker on the fuel side (anode side) than on the air side (cathode side) of the interconnector between cell units. We believe that this may be due to the fact that water formed by the electrochemical reaction of fuel and oxygen adversely affects the kinetics of oxide growth. We have, in fact, observed in our work that the resistance of the interconnector on the fuel side is actually greater than the resistance on the air side, both immediately after testing and at room temperature. The solid oxide fuel cell interconnectors of this invention address this issue by preventing an increase in net interconnector resistance by minimizing the formation of an oxide coating on the fuel side of the interconnector.
Solid oxide fuel cells, like other fuel cells, comprise an anode electrode, a cathode electrode, and an electrolyte disposed between the anode electrode and the cathode electrode. In contrast to other types of fuel cells, for example molten carbonate fuel cells, solid oxide fuel cells operate at relatively high temperatures, typically greater than about 800° C. Accordingly, the interconnector materials must be able to withstand said temperatures.
One solution to the problem of metallic interconnector oxidation in solid oxide fuel cells is taught, for example, by U.S. Pat. No. 4,950,562 which teaches a solid electrolyte type of fuel cell having an interconnector comprising a heat resistant alloy substrate coated on its surface with a composite metal oxide of the perovskite-type structure, that is La
1-x
M
1
x
M
2
O
3
wherein M
1
is an alkaline earth metal, M
2
is Co, Fe, Mn, Ni or Cr and x is greater than or equal to zero and less than one. U.S. Pat. No. 5,411,767 teaches a method for producing interconnectors for electrically connecting unit cells of a solid electrolyte type fuel cell in which the interconnector material, a perovskite-complexed oxide, is thermally sprayed onto the surface of an electrode of a solid electrolyte type fuel cell by plasma thermal spraying. An interconnector made of lanthanum chromite or lanthanum oxide and chromium oxide doped with copper, zinc, calcium or strontium for a solid oxide fuel cell is taught by U.S. Pat. No. 5,480,739. See, also, U.S. Pat. No. 4,874,678 and U.S. Pat. No. 4,888,254, both of which teach interconnects of lanthanum chromite doped with calcium, strontium, or magnesium for use in connection with solid oxide electrolyte fuel cell stacks; U.S. Pat. No. 5,034,288 which teaches a solid electrolyte fuel cell stack comprising a metallic bipolar plate comprising a nickel-based alloy and coated on the oxygen side with a lanthanum/manganese perovskite applied by plasma spraying; U.S. Pat. No. 4,997,727 which teaches an interconnect for a solid electrolyte fuel cell stack constructed of Inconel X; and U.S. Pat. No. 5,496,655 which teaches a bipolar interconnector manufactured from NiAl or Ni
3
Al coated with strontium or calcium-doped lanthanum chromite.
In contrast thereto, the interconnects for solid oxide fuel cells in accordance with this invention are substantially lower in cost while providing high conductivity relative to other known interconnects.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a solid oxide fuel cell which utilizes a heat-resistant, electrically conductive part suitable for use as an interconnector between adjacent fuel cell units in a fuel cell stack.
It is another object of this invention to provide an interconnector for a solid oxide fuel cell stack which substantially avoids the formation of surface oxides on the fuel side of said interconnectors.
These and other objects of this invention are achieved by a solid oxide fuel cell system comprising a plurality of fuel cell units, each said fuel cell unit comprising an anode, a cathode, and an electrolyte separating the anode from the cathode, and an interconnector structure separating the anode of one of the fuel cell units from the cathode of an adjacent fuel cell unit. The interconnector structure in accordance with this invention comprises a superalloy layer and a metal layer, the metal layer being disposed on an anode-facing face of the superalloy layer and comprising a metal which does not oxidize to any significant extent in the atmosphere of the anode side of the interconnector. Such metals are preferably selected from the group consisting of copper, nickel, silver, gold, platinum, palladium, iridium, and rhodium. Due to the relative costs of these metals, nickel and copper, being the least expensive, are particularly preferred with copper being preferred over nickel due to the fact that it is more “noble” than nickel, that is, more resistant to oxidation than nickel.


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