Consinterable ceramic interconnect for solid oxide fuel cells

Details Consinterable ceramic interconnect for solid oxide fuel cells Consinterable ceramic interconnect for solid oxide fuel cells

1997-04-10

2001-05-08

Gorgos, Kathryn (Department: 1741)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S010000

Reexamination Certificate

active

06228520

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
This invention relates to planar solid oxide fuel cell stacks comprising one or more electrodes in contact with a solid state electrolyte and in particular to a new interconnect for solid oxide fuel cells.
A solid state electrochemical cell comprises two electrodes, the anode and the cathode, and a dense solid electrolyte membrane which separates the anode and cathode regions of the cell. The anodic and cathodic reactions occur at the anode/electrolyte and cathode/electrolyte interfaces, respectively. The solid electrolyte membrane is a material capable of conducting ionic species, such as oxygen ions, sodium ions, fluoride ions, or hydrogen ions, yet has a low electrical conductivity. The electrolyte membrane must be impermeable to the electrochemical reactants.
It is known to prepare a solid oxide fuel cell comprising a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a conductive ceramic or a metal or, most commonly, a ceramic-metal composite, in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of an electronically-conductive metal oxide on the oxidant side of the cell, which generates electricity through the electrochemical reaction between a fuel and an oxidant. This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the gas phase (fuel or oxygen).
Electrode structures comprising a porous layer of electrolyte particles on a dense electrolyte membrane with electrocatalyst material on and within the porous layer of electrolyte are known. In such electrodes, the electrocatalyst material is semi-continuous on the surface of the porous electrolyte material to create a three phase boundary (TPB) where the electrolyte material, electrocatalyst, and gas are in contact. The electrode is prepared by applying an electrocatalyst precursor material as a slurry to a porous electrolyte structure, and then heating the precursor material to form the electrocatalyst. However, it is usually necessary to repeat the process of applying the electrocatalyst precursor material to the porous substrate several times in order to provide enough electrocatalyst to obtain a fuel cell with the desired performance characteristics. For fuel cell applications, this method of creating the layer of electrocatalyst in and on the porous electrolyte structure by repeated applications of the electrocatalyst slurry may create more process steps in the preparation of the fuel cell than would be desirable in a commercial manufacturing process. In addition, the performance characteristics of the electrode structure prepared by such processes, such as the voltage at a certain current density, may be less than desirable for certain applications.
U.S. Pat. No. 3,377,203 discloses a method for producing fuel cells of solid electrolyte and ceramic oxide electrode layers by sintering the electrode layers to the electrolyte. U.S. Pat. No. 4,767,518 discloses a solid oxide electrode (anode) made of metal particles that are immobilized by stabilized zirconia which may also contain praseodymium (Pr). The Pr may be added in the form of a solution. U.S. Pat. No. 4,885,078 discloses an electrochemical device which may be a solid oxide cell which comprises a porous electrode containing a deposit of metal oxide or metal salt capable of forming metal oxide upon heating, where the metal may be Pr. U.S. Pat. No. 5,021,304 discloses a method of coating a separate electronically conducted layer on a porous electrode having the steps of applying a mixture of metal salts including nitrates to the electrodes with a surfactant, and heating to form the oxides. Pr oxide is included in a list of dopant oxides which may be used.
The total amount of energy produced by a solid state fuel cell can be increased by stacking the cells anode to cathode. Interconnect layers are placed between the anode and cathode layers of adjacent cells to provide electrical connection from cell to cell and to serve as a barrier between the cells to prevent the migration of fuel or oxidant therebetween.
LaCrO
3
is difficult to keep from evaporating in an air atmosphere and must be sintered at relatively high temperatures. An interconnect material which sinters at a relatively low temperature, has a high electrical conductivity, and is impermeable to the fuel and oxidant would be desirable.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fuel cell interconnect layer composed of LaCrO
3
which can be used with solid oxide fuel cells to enhance the production of stacks of multiple cells in a single-step cofiring process.
It is a further object to provide an interconnect layer which has shrinkage properties during firing that closely match other fuel cell components.
Accordingly, a composition which is densifiable at a relatively low temperature which is suitable for sintering other cell components in an air atmosphere and suitable for use as an interconnect layer in a solid oxide fuel cell is provided. Binary alloying of SrO and CaO with LaCrO
3
is used to form a compound having the general formula La
(1−x)
(Sr,Ca)
x
CrO
3
which is a stabilized form of LaCrO
3
and has the desirable properties for a fuel cell interconnect layer. In the above formula, x is a number from 0.05 to 0.5. Preferably, x is at least 0.15, more preferably at least 0.2, most preferably at least 0.25; but is preferably no greater than 0.45, and more preferably no greater than 0.3. The molar ratio of Ca:Sr is preferably at least 1:3, more preferably at least 1:2, most preferably at least 1:1; but is preferably no greater than 3:1, more preferably no greater than 2:1.
The interconnect layer of the invention is particularly useful in the preparation of a fuel cell wherein the different components of the fuel cell (anode, cathode, electrolyte, and interconnect) are sintered as a single unit. In such cases, the use of an interconnect which will sinter at a relatively low temperature but still have acceptable properties of an interconnect (barrier for gases, good electrical conductivity; and little or no reactivity with the gases or adjacent components in the fuel cell) is preferred.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which a preferred embodiment of the invention is illustrated.


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