CA2153736C - Solid electrolyte for a fuel cell - Google Patents
Solid electrolyte for a fuel cell Download PDFInfo
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- CA2153736C CA2153736C CA002153736A CA2153736A CA2153736C CA 2153736 C CA2153736 C CA 2153736C CA 002153736 A CA002153736 A CA 002153736A CA 2153736 A CA2153736 A CA 2153736A CA 2153736 C CA2153736 C CA 2153736C
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- solid electrolyte
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
- H01M4/90—Selection of catalytic material
- H01M4/9016—Oxides, hydroxides or oxygenated metallic salts
- H01M4/9025—Oxides specially used in fuel cell operating at high temperature, e.g. SOFC
- H01M4/9033—Complex oxides, optionally doped, of the type M1MeO3, M1 being an alkaline earth metal or a rare earth, Me being a metal, e.g. perovskites
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/10—Fuel cells with solid electrolytes
- H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
- H01M8/124—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte characterised by the process of manufacturing or by the material of the electrolyte
- H01M8/1246—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte characterised by the process of manufacturing or by the material of the electrolyte the electrolyte consisting of oxides
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M2300/00—Electrolytes
- H01M2300/0088—Composites
- H01M2300/0094—Composites in the form of layered products, e.g. coatings
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
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- Engineering & Computer Science (AREA)
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- Chemical Kinetics & Catalysis (AREA)
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- General Chemical & Material Sciences (AREA)
- Manufacturing & Machinery (AREA)
- Life Sciences & Earth Sciences (AREA)
- Sustainable Development (AREA)
- Sustainable Energy (AREA)
- Fuel Cell (AREA)
- Conductive Materials (AREA)
Abstract
The present invention relates to a solid electrolyte consisting essentially of cerium oxide and having a membrane of a perovskite oxide of ABO3 on the anode side for a fuel cell. This solid electrolyte being prevented from becoming reduced under the effect of a fuel gas of a fuel cell. The present invention also relates to a method for producing this solid electrolyte and to a solid electrolyte fuel cell utilizing this solid electrolyte. The present invention is also related to a solid electrolyte fuel cell utilizing the above mentioned solid electrolyte. A fuel cell comprising the above mentioned solid electrolyte shows excellent high-temperature characteristics.
Description
SOLID ELECTROLYTE FOR A FUEL CELL
Field of Utilization is Industry The present invention relates to a solid electrolyte for a fuel cell and its manufacturing method. This solid electrolyte comprises a solid electrolyte of oxide-ionic conduction which consists essentially of cerium oxide and a membrane of solid electrolyte of proton/oxide-ionic mixed conduction bonded on the solid electrolyte of oxide-ionic conduction.
RELATED ART
A most frequently used material of a solid electrolyte for a high-temperature-type fuel cell is zirconia stabilized with yttrium oxide. However, in order to obtain higher electric output characteristics of the cell, a solid electrolyte of higher ionic conductivity has been demanded.
A solid electrolyte consisting essentially of cerium oxide can be used in place of stabilized-zirconia electrolyte.
However, if the fuel gas supplied to its anode's side is H2, CH, or the like, the cerium oxide contained in the electrolyte may be partially reduced under the effect of the fuel gas at its operating temperature (because of its low partial pressure of oxygen), which can present a problem of decrease in terminal voltage.
The above mentioned problem can be solved by bonding a thin membrane of stabilized zirconia on the anode's side surface of the cerium oxide electrolyte. CVD, EVD, thermal spraying and the like have been proposed as a method for forming the thin membrane of stabilized zirconia (See, for example, The Extened Abstracts of The 14th Symposium on Solid State Ionics in Japan, Nov 12-13, 1987, The Solid State Ionics Society of Japan). These methods have, nevertheless, disadvantages such as high production costs due to extensive production facilities, complex production processes and the like.
SUGARY OF THE INVENTION
The object of the present invention is to provide a solid electrolyte of proton/oxide-ionic mixed conduction on the anode's side surface of a solid electrolyte consisting essentially of cerium oxide in such a manner that a high degree of adhesiveness can be achieved between these electrolytes and this bonding can be performed inexpensively and easily.
A further object of the present invention is to provide a fuel cell which utilizes the above mentioned solid electrolyte. Various experiments and researches were carried out to develop a solid electrolyte for a fuel cell which comprises a close and highly adhesive solid electrolyte of proton/oxide-ionic mixed conduction bonded on one surface of a cerium-oxide-based solid electrolyte of oxide-ionic conduction and a method for producing this solid electrolyte for a fuel Cell.
As a result, it was found that a solid electrolyte for a fuel cell which comprises a solid electrolyte of proton/oxide-ionic mixed conduction bonded on one surface of a solid electrolyte of oxide-ionic conduction can be obtained easily by coating at least one material selected from the inorganic acid salts, organic acids salts and organic metal compounds of the alkaline earth metals which are the elements composing the solid electrolyte of proton/oxide-ionic mixed conduction on one surface of a solid electrolyte consisting essentially of cerium oxide, and causing a reaction between the alkaline earth metal compounds and the solid electrolyte of oxide-ionic conduction in an oxidizing atmosphere at a temperature higher than 800°C. The present invention described hereinafter is based on this finding.
According to a first aspect of the present invention, we provide a solid electrolyte for a fuel cell comprising:
(a) a specifically-shaped solid electrolyte consisting essentially of cerium oxide, the solid electrolyte having a fluorite-type structure and oxide-ionic conduction, and (b) a membrane of a perovskite-type oxide having an AB03-type composition, which is a solid electrolyte of proton/oxide-ionic mixed conduction, being bonded on a portion of the one surface of the solid electrolyte consisting essentially of cerium oxide, in which (i) A of the "AB03" represents at least one element selected from a group consisting of alkaline earth metals (Mg, Sr, Ca, Ba), (ii) B of the "AB03cost-effectively3" represents cerium by itself or cerium and at least one element selected from a group consisting of alkaline earth metals (Mg, Sr, Ca, Ba) and rare earth elements (Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb) , the elements selected from alkaline earth metals and rare earth elements substituting the cerium by 1 to 30 moll.
According to a second aspect of the present invention, we provide a method for producing such a solid electrolyte for a fuel cell comprising the steps of:
(a) preparing a specifically-shaped solid electrolyte consisting essentially of cerium oxide which has a fluorite-type structure and oxide-ionic conduction, (b) coating at least one material selected from the inorganic salts, organic acid salts and organic metal compounds of alkaline earth metals (Mg, Sr, Ca, Ba) on a part of the one surface of the solid electrolyte consisting essentially of cerium oxide, and (c) heating the solid electrolyte thus coated to a temperature higher than 800°C in an oxidizing atmosphere to form a membrane of proton/oxide-ionic mixed conduction on a part of the one surface of the solid electrolyte consisting essentially of cerium oxide.
According to a third aspect of the present invention, we provide a solid electrolyte fuel cell comprising:
f,~, ~'.. t;'i) ;F.
y:.. .,.
(a) a specifically-shaped solid electrolyte consisting essentially of cerium oxide, the solid electrolyte having a fluorite-type structure and oxide-ionic conduction, (b) An anode side of the solid electrolyte being composed of a perovskite-type oxide which consists essentially of oxides of alkaline earth metals and cerium oxide and which has proton/oxide-ionic mixed conduction, and an anode which is made of sintered Ni paste or Pt paste formed on the fuel side of said anode side of the solid electrolyte, and (c) a cathode of the solid electrolyte being made of a perovskite-type rare earth metal oxide or Pt paste.
BRIEF DESCRIPTION OF T8E DRApIINGS
Fig.l is an X-ray diffraction pattern of the surface of a solid electrolyte which was obtained by coating saturated aqueous barium nitrate on the surface of a disk-like close sintered-body based on cerium oxide which is a solid solution containing 20 mol% of Y01.5, drying the solid electrolyte thus coated, and firing it at 1300°C for 10 hours in air.
Fig. 2 shows a sectional view of the surface structure of a solid electrolyte which was obtained by coating saturated aqueous barium nitrate on the surface of disk-like close sintered-body based on cerium oxide which is a solid solution containing 20 mol% of YOl.s, drying the solid electrolyte thus coated, and firing it at 1300°C for 10 hours in air.
Fig.3 is a schematic view of a fuel cell according to the present invention.
Fig. 4 is a graph showing voltage-current characteristics of a fuel cell according to the present invention.
In accordance with the present invention, a specifically-shaped solid electrolyte for fuel cell consisting essentially of cerium oxide is used as a solid electrolyte of oxide-ionic conduction which has suitable operating properties in a high temperature environment. The shape of this solid electrolyte may be either plate-like or cylindrical according to a required shape for a fuel cell.
It is also preferable to use a cerium-oxide solid solution which contains 1 to 30 mol% of at least one element selected from the group of alkaline earth metals (Mg, Sr, Ca, Ba) and rare earth elements (Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb), instead of the above mentioned solid electrolyte consisting of cerium oxide.
Examples of the solid electrolyte of oxide-ionic conduction includes (C202) o.e (YOl.s) 0.2. (C20z) o.s (Sm~i.s) o.i. (Ce02) o.e (Ca0) 0,2, (Ce02) o.e (Sr0) 0,2 and the like.
It is possible to form a membrane of an electrolyte of proton/oxide-ionic mixed conduction on the anode-side surface of the above mentioned solid electrolyte of oxide-ionic conduction by coating at least one material selected from the inorganic acid salts, organic acid salts and organic metal compounds of alkaline earth metals on the solid electrolyte, and then, heating the solid electrolyte thus coated to a temperature higher than 800°C in an oxidizing atmosphere, for example, ambient air. This makes it possible to prevent the cerium-oxide contained in the solid electrolyte from being reduced under the effect of fuel gas. If the heating temperature is lower than 800°C, an appropriate electrolyte of proton/oxide-ionic mixed conduction cannot be obtained.
Examples of the methods for coating at least one material selected from the inorganic acid salts, organic acids salts and organic metal compounds of alkaline earth metals include (1) a method comprising the steps of coating saturated aqueous nitrate of an alkaline earth metal uniformly on the surface of an oxide-ionic conductor consisting essentially of cerium oxide with the use of a brush or the like and drying the conductor thus coated, (2) a method comprising the steps of grinding the carbonate of an alkaline earth metal, mixing this carbonate powder with a volatile solvent (for example, ethanol) to put it into a paste form, coating it uniformly on the surface of an oxide-ionic conductor consisting essentially of cerium oxide with the use of a screen printing machine, and drying the conductor thus coated, and (3) a method comprising the steps of mixing fine powder of the nitrate, carbonate or the like of an alkaline earth metal with a solvent such as water or ethanol to yield slip, and coating the slip on an oxide-ionic conductor consisting essentially of cerium oxide by soaking the oxide-ionic conductor in the slip and soon pulling out the conductor from the slip.
These methods make it possible to coat an electrolyte membrane of proton/oxide-ionic mixed conduction on the above mentioned solid electrolyte. This electrolyte of proton/oxide-ionic mixed conduction is a perovskite-type oxide having an AB03-type composition. A of the "ABO,"
represents at least one element selected from the group consisting of alkaline earth metals (Mg, Sr, Ca, Ba), and B
of the "AB03" represents at least one element selected from rare earth elements and the like, more specifically, cerium by itself or cerium and at least one element selected from a group consisting of alkaline earth metals (Mg, Sr, Ca, Ba) and rare earth elements (Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb), the elements selected from alkaline earth metals and rare earth elements substituting the cerium by 1 to 30 mol%.
It is possible to add other substances into the above mentioned solid electrolyte as long as the substances to be added do not significantly affect the characteristics of the solid electrolyte.
Examples of this solid electrolyte of proton/oxide-ionic mixed conduction include SrCeo,9YBo.1~3-ai BaCeo,8Y0.2~3-a and the like (a = about 0 to 0.5).
., 6 Furthermore, the thickness of this membrane of solid electrolyte of proton/oxide-ionic mixed conduction is preferably ~.m or more. The thickness less than 10 ~,m cannot prevent the solid electrolyte of oxide-ionic conduction from being reduced under the influence of the fuel gas which enters into it.
An electrode material as anode such as Ni paste, Pt paste or the like is applied on the surface of the solid electrolyte of proton/oxide-ionic mixed conduction so that this electrode material can act as an anode of a fuel cell.
On the cathode-side surface of the above mentioned solid electrolye of oxide-ionic conduction, a solid electrolyte which has a high degree of adhesiveness to the solid electrolyte of oxide-ionic conduction and which forms a cathode is provided. Examples of this solid electrolye include well-known perovskite-type rare earth metal oxide and the like, and more specifically, Lao.,Sro,3Mn03, Cao.9Ceo_iMnO"
Lao.,Sro.,CoO, and the like. As the cathode Pt paste can be also used.
An interconnector or a separator which is made of platinum, Lao,.,CaOo.,CrO, or a heat resistant alloy (e.g. , Inconel*) can be connected to the anode and the cathode mentioned above. It is possible to constitute a high-power fuel cell by connecting elementary cells in series or parallel with the use of this interconnector.
It is possible to prevent a solid electrolyte consisting essentially of cerium oxide from being reduced under the influence of a fuel gas such as hydrogen, methane and the like by bonding a layer of a proton/oxide-ionic mixed conductor on the anode-side surface of the above mentioned oxide-ionic conductor consisting essentially of cerium oxide.
Furthermore, in the cathode's side, the oxide-ionic conductor consisting essentially of cerium oxide has the * Trade-mark characteristics that the polarization of its electrode reaction is small on the interface of the electrode. And at the same time, in the anode' s side, the proton/oxide-ionic mixed conductor has the characteristics that the polarization of its electrode reaction is small, which makes it possible to obtain a electric high power in the generation of electric current. Thus it becomes possible to decrease the operating temperature of a fuel cell and to reduce the necessary refractoriness and the like of the composing materials of a fuel cell. Consequently the production costs of a fuel cell can also be reduced.
Examples:
Example 1 Saturated aqueous barium nitrate was coated on the surface of disk-shaped close sintered-body based on cerium oxide which is a substitutional solid solution containing 20 moll of Y01.5. Then the sintered-body thus coated was dried, and fired at 1300°C for 10 hours in air.
Fig. 1 shows a X-ray diffraction pattern of the membrane thus obtained. This membrane had a diffraction pattern which could be identified as BaCe03 according to the X-ray diffraction data listed in JCPDS, and found to be BaCe03.
Fig.2 shows a sectional view of the membrane thus obtained.
The thickness of this membrane was 30 to 40 Vim. As a consequence of the EPMA analysis of the Ba, Ce and Y
contained in the membrane, it became apparent that the distribution of these elements was uniform and the composition of the membrane obtained was Ba . Ce . Y = 1 .
0.8 . 0.2. The adhesiveness of the BaCe03 membrane to the solid solution based on cerium oxide was appropriate and the delamination between them was not observed even after the heat cycle from room temperature to 1000°C had been repeated 30 times and more.
A
Example 2 Saturated aqueous barium nitrate was coated on the surface of disk-shaped, close sintered-body based on cerium oxide which was a substitutional solid solution containing 20 mol% of YO1.5. Then the sintered-body thus coated was dried, and fired at 1300°C for 10 hours in air. As a consequence of the X-ray diffraction analysis and the EPMA analysis of the composition of the membrane thus formed, it became apparent that the membrane was a close membrane having the composition of BaCeo.eYo.2~3-a~ The thickness of this membrane was 30 to 40 ~.m.
A fuel cell was composed with the use of the solid electrolyte of oxide-ionic conduction consisting essentially of cerium oxide mentioned above . Paste of Lao..,Sro.3Mn03 was coated on the central area (about 2 cm2) of the surface of this solid electrolyte which was plate-shaped and about 0.4 mm in thickness. Paste of Ni as anode was coated on the surface of the solid electrolyte of proton/oxide-ionic conduction mentioned above . The paste of Lao..,Sro,,Mn03 as cathode was coated on these solid electrolytes were fired.
Thus a porous electrode was obtained.
Fig.3 shows a fuel cell thus obtained. As shown in this figure, an alumina pipe 3 was connected to the surface of an electrode 6 with the use of Pyre~t glass 5 as a gas seal material. Two alumina tubes 2 each of which contained a platinum wire were inserted into the alumina pipe 3 in order to form a fuel cell. This fuel cell was placed in an electric furnace 9 and maintained at 1000°C.
Air was introduced into the space 4 located in the side of the cathode (Lao.,Sro.3Mn03) , and hydrogen gas was introduced into the space 7 located in the side of the solid electrolyte of proton/oxide-ionic conduction. A Ni electrode 8 was used as an electrode provided for the anode.
* Trade-mark Fig.4 shows the voltage-current characteristics of the fuel cell thus obtained. The measured voltage of this cell generally agreed with the theoretical electromotive force calculated from a theoretical formula and showed that the fuel cell according to the present invention displayed higher no-load voltage than that of a conventional fuel cell.
This showed that the electrolyte consisting essentially of cerium oxide was prevented from being reduced under the effect of a fuel gas and the decreasing of the non-load voltage of the cell was voided by depositing a proton/oxide-ionic mixed conductor on one surface of a solid electrolyte consisting essentially of cerium oxide and using this ionic mixed conductor on the side of the cell's anode.
Moreover, with regard to the current taken from this fuel cell, it was proved that the polarization at its electrodes was smaller than that of a conventional fuel cell of a stabilized-zirconia type and, therefore, higher power density could be obtained than that of a conventional fuel cell.
Thus it is possible to inexpensively and easily produce a highly-adhesive (and close) membrane of a solid electrolyte of proton/oxide-ionic mixed conduction as a layer bonded on a solid electrolyte consisting essentially of cerium oxide.
When the solid electrolyte membrane of proton/oxide-ionic mixed conduction is used on the side of the cell's anode, this solid electrolyte membrane can prevent the reduction of cerium oxide and can act as an appropriate solid electrolyte for a fuel cell.
A fuel cell in which the solid electrolyte layer of proton/oxide-ionic mixed conduction is used for anode side and its cathode is formed on the solid electrolyte of oxide-ionic conduction can show greatly advantageous characteristics for a fuel cell.
c,
Field of Utilization is Industry The present invention relates to a solid electrolyte for a fuel cell and its manufacturing method. This solid electrolyte comprises a solid electrolyte of oxide-ionic conduction which consists essentially of cerium oxide and a membrane of solid electrolyte of proton/oxide-ionic mixed conduction bonded on the solid electrolyte of oxide-ionic conduction.
RELATED ART
A most frequently used material of a solid electrolyte for a high-temperature-type fuel cell is zirconia stabilized with yttrium oxide. However, in order to obtain higher electric output characteristics of the cell, a solid electrolyte of higher ionic conductivity has been demanded.
A solid electrolyte consisting essentially of cerium oxide can be used in place of stabilized-zirconia electrolyte.
However, if the fuel gas supplied to its anode's side is H2, CH, or the like, the cerium oxide contained in the electrolyte may be partially reduced under the effect of the fuel gas at its operating temperature (because of its low partial pressure of oxygen), which can present a problem of decrease in terminal voltage.
The above mentioned problem can be solved by bonding a thin membrane of stabilized zirconia on the anode's side surface of the cerium oxide electrolyte. CVD, EVD, thermal spraying and the like have been proposed as a method for forming the thin membrane of stabilized zirconia (See, for example, The Extened Abstracts of The 14th Symposium on Solid State Ionics in Japan, Nov 12-13, 1987, The Solid State Ionics Society of Japan). These methods have, nevertheless, disadvantages such as high production costs due to extensive production facilities, complex production processes and the like.
SUGARY OF THE INVENTION
The object of the present invention is to provide a solid electrolyte of proton/oxide-ionic mixed conduction on the anode's side surface of a solid electrolyte consisting essentially of cerium oxide in such a manner that a high degree of adhesiveness can be achieved between these electrolytes and this bonding can be performed inexpensively and easily.
A further object of the present invention is to provide a fuel cell which utilizes the above mentioned solid electrolyte. Various experiments and researches were carried out to develop a solid electrolyte for a fuel cell which comprises a close and highly adhesive solid electrolyte of proton/oxide-ionic mixed conduction bonded on one surface of a cerium-oxide-based solid electrolyte of oxide-ionic conduction and a method for producing this solid electrolyte for a fuel Cell.
As a result, it was found that a solid electrolyte for a fuel cell which comprises a solid electrolyte of proton/oxide-ionic mixed conduction bonded on one surface of a solid electrolyte of oxide-ionic conduction can be obtained easily by coating at least one material selected from the inorganic acid salts, organic acids salts and organic metal compounds of the alkaline earth metals which are the elements composing the solid electrolyte of proton/oxide-ionic mixed conduction on one surface of a solid electrolyte consisting essentially of cerium oxide, and causing a reaction between the alkaline earth metal compounds and the solid electrolyte of oxide-ionic conduction in an oxidizing atmosphere at a temperature higher than 800°C. The present invention described hereinafter is based on this finding.
According to a first aspect of the present invention, we provide a solid electrolyte for a fuel cell comprising:
(a) a specifically-shaped solid electrolyte consisting essentially of cerium oxide, the solid electrolyte having a fluorite-type structure and oxide-ionic conduction, and (b) a membrane of a perovskite-type oxide having an AB03-type composition, which is a solid electrolyte of proton/oxide-ionic mixed conduction, being bonded on a portion of the one surface of the solid electrolyte consisting essentially of cerium oxide, in which (i) A of the "AB03" represents at least one element selected from a group consisting of alkaline earth metals (Mg, Sr, Ca, Ba), (ii) B of the "AB03cost-effectively3" represents cerium by itself or cerium and at least one element selected from a group consisting of alkaline earth metals (Mg, Sr, Ca, Ba) and rare earth elements (Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb) , the elements selected from alkaline earth metals and rare earth elements substituting the cerium by 1 to 30 moll.
According to a second aspect of the present invention, we provide a method for producing such a solid electrolyte for a fuel cell comprising the steps of:
(a) preparing a specifically-shaped solid electrolyte consisting essentially of cerium oxide which has a fluorite-type structure and oxide-ionic conduction, (b) coating at least one material selected from the inorganic salts, organic acid salts and organic metal compounds of alkaline earth metals (Mg, Sr, Ca, Ba) on a part of the one surface of the solid electrolyte consisting essentially of cerium oxide, and (c) heating the solid electrolyte thus coated to a temperature higher than 800°C in an oxidizing atmosphere to form a membrane of proton/oxide-ionic mixed conduction on a part of the one surface of the solid electrolyte consisting essentially of cerium oxide.
According to a third aspect of the present invention, we provide a solid electrolyte fuel cell comprising:
f,~, ~'.. t;'i) ;F.
y:.. .,.
(a) a specifically-shaped solid electrolyte consisting essentially of cerium oxide, the solid electrolyte having a fluorite-type structure and oxide-ionic conduction, (b) An anode side of the solid electrolyte being composed of a perovskite-type oxide which consists essentially of oxides of alkaline earth metals and cerium oxide and which has proton/oxide-ionic mixed conduction, and an anode which is made of sintered Ni paste or Pt paste formed on the fuel side of said anode side of the solid electrolyte, and (c) a cathode of the solid electrolyte being made of a perovskite-type rare earth metal oxide or Pt paste.
BRIEF DESCRIPTION OF T8E DRApIINGS
Fig.l is an X-ray diffraction pattern of the surface of a solid electrolyte which was obtained by coating saturated aqueous barium nitrate on the surface of a disk-like close sintered-body based on cerium oxide which is a solid solution containing 20 mol% of Y01.5, drying the solid electrolyte thus coated, and firing it at 1300°C for 10 hours in air.
Fig. 2 shows a sectional view of the surface structure of a solid electrolyte which was obtained by coating saturated aqueous barium nitrate on the surface of disk-like close sintered-body based on cerium oxide which is a solid solution containing 20 mol% of YOl.s, drying the solid electrolyte thus coated, and firing it at 1300°C for 10 hours in air.
Fig.3 is a schematic view of a fuel cell according to the present invention.
Fig. 4 is a graph showing voltage-current characteristics of a fuel cell according to the present invention.
In accordance with the present invention, a specifically-shaped solid electrolyte for fuel cell consisting essentially of cerium oxide is used as a solid electrolyte of oxide-ionic conduction which has suitable operating properties in a high temperature environment. The shape of this solid electrolyte may be either plate-like or cylindrical according to a required shape for a fuel cell.
It is also preferable to use a cerium-oxide solid solution which contains 1 to 30 mol% of at least one element selected from the group of alkaline earth metals (Mg, Sr, Ca, Ba) and rare earth elements (Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb), instead of the above mentioned solid electrolyte consisting of cerium oxide.
Examples of the solid electrolyte of oxide-ionic conduction includes (C202) o.e (YOl.s) 0.2. (C20z) o.s (Sm~i.s) o.i. (Ce02) o.e (Ca0) 0,2, (Ce02) o.e (Sr0) 0,2 and the like.
It is possible to form a membrane of an electrolyte of proton/oxide-ionic mixed conduction on the anode-side surface of the above mentioned solid electrolyte of oxide-ionic conduction by coating at least one material selected from the inorganic acid salts, organic acid salts and organic metal compounds of alkaline earth metals on the solid electrolyte, and then, heating the solid electrolyte thus coated to a temperature higher than 800°C in an oxidizing atmosphere, for example, ambient air. This makes it possible to prevent the cerium-oxide contained in the solid electrolyte from being reduced under the effect of fuel gas. If the heating temperature is lower than 800°C, an appropriate electrolyte of proton/oxide-ionic mixed conduction cannot be obtained.
Examples of the methods for coating at least one material selected from the inorganic acid salts, organic acids salts and organic metal compounds of alkaline earth metals include (1) a method comprising the steps of coating saturated aqueous nitrate of an alkaline earth metal uniformly on the surface of an oxide-ionic conductor consisting essentially of cerium oxide with the use of a brush or the like and drying the conductor thus coated, (2) a method comprising the steps of grinding the carbonate of an alkaline earth metal, mixing this carbonate powder with a volatile solvent (for example, ethanol) to put it into a paste form, coating it uniformly on the surface of an oxide-ionic conductor consisting essentially of cerium oxide with the use of a screen printing machine, and drying the conductor thus coated, and (3) a method comprising the steps of mixing fine powder of the nitrate, carbonate or the like of an alkaline earth metal with a solvent such as water or ethanol to yield slip, and coating the slip on an oxide-ionic conductor consisting essentially of cerium oxide by soaking the oxide-ionic conductor in the slip and soon pulling out the conductor from the slip.
These methods make it possible to coat an electrolyte membrane of proton/oxide-ionic mixed conduction on the above mentioned solid electrolyte. This electrolyte of proton/oxide-ionic mixed conduction is a perovskite-type oxide having an AB03-type composition. A of the "ABO,"
represents at least one element selected from the group consisting of alkaline earth metals (Mg, Sr, Ca, Ba), and B
of the "AB03" represents at least one element selected from rare earth elements and the like, more specifically, cerium by itself or cerium and at least one element selected from a group consisting of alkaline earth metals (Mg, Sr, Ca, Ba) and rare earth elements (Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho, Yb), the elements selected from alkaline earth metals and rare earth elements substituting the cerium by 1 to 30 mol%.
It is possible to add other substances into the above mentioned solid electrolyte as long as the substances to be added do not significantly affect the characteristics of the solid electrolyte.
Examples of this solid electrolyte of proton/oxide-ionic mixed conduction include SrCeo,9YBo.1~3-ai BaCeo,8Y0.2~3-a and the like (a = about 0 to 0.5).
., 6 Furthermore, the thickness of this membrane of solid electrolyte of proton/oxide-ionic mixed conduction is preferably ~.m or more. The thickness less than 10 ~,m cannot prevent the solid electrolyte of oxide-ionic conduction from being reduced under the influence of the fuel gas which enters into it.
An electrode material as anode such as Ni paste, Pt paste or the like is applied on the surface of the solid electrolyte of proton/oxide-ionic mixed conduction so that this electrode material can act as an anode of a fuel cell.
On the cathode-side surface of the above mentioned solid electrolye of oxide-ionic conduction, a solid electrolyte which has a high degree of adhesiveness to the solid electrolyte of oxide-ionic conduction and which forms a cathode is provided. Examples of this solid electrolye include well-known perovskite-type rare earth metal oxide and the like, and more specifically, Lao.,Sro,3Mn03, Cao.9Ceo_iMnO"
Lao.,Sro.,CoO, and the like. As the cathode Pt paste can be also used.
An interconnector or a separator which is made of platinum, Lao,.,CaOo.,CrO, or a heat resistant alloy (e.g. , Inconel*) can be connected to the anode and the cathode mentioned above. It is possible to constitute a high-power fuel cell by connecting elementary cells in series or parallel with the use of this interconnector.
It is possible to prevent a solid electrolyte consisting essentially of cerium oxide from being reduced under the influence of a fuel gas such as hydrogen, methane and the like by bonding a layer of a proton/oxide-ionic mixed conductor on the anode-side surface of the above mentioned oxide-ionic conductor consisting essentially of cerium oxide.
Furthermore, in the cathode's side, the oxide-ionic conductor consisting essentially of cerium oxide has the * Trade-mark characteristics that the polarization of its electrode reaction is small on the interface of the electrode. And at the same time, in the anode' s side, the proton/oxide-ionic mixed conductor has the characteristics that the polarization of its electrode reaction is small, which makes it possible to obtain a electric high power in the generation of electric current. Thus it becomes possible to decrease the operating temperature of a fuel cell and to reduce the necessary refractoriness and the like of the composing materials of a fuel cell. Consequently the production costs of a fuel cell can also be reduced.
Examples:
Example 1 Saturated aqueous barium nitrate was coated on the surface of disk-shaped close sintered-body based on cerium oxide which is a substitutional solid solution containing 20 moll of Y01.5. Then the sintered-body thus coated was dried, and fired at 1300°C for 10 hours in air.
Fig. 1 shows a X-ray diffraction pattern of the membrane thus obtained. This membrane had a diffraction pattern which could be identified as BaCe03 according to the X-ray diffraction data listed in JCPDS, and found to be BaCe03.
Fig.2 shows a sectional view of the membrane thus obtained.
The thickness of this membrane was 30 to 40 Vim. As a consequence of the EPMA analysis of the Ba, Ce and Y
contained in the membrane, it became apparent that the distribution of these elements was uniform and the composition of the membrane obtained was Ba . Ce . Y = 1 .
0.8 . 0.2. The adhesiveness of the BaCe03 membrane to the solid solution based on cerium oxide was appropriate and the delamination between them was not observed even after the heat cycle from room temperature to 1000°C had been repeated 30 times and more.
A
Example 2 Saturated aqueous barium nitrate was coated on the surface of disk-shaped, close sintered-body based on cerium oxide which was a substitutional solid solution containing 20 mol% of YO1.5. Then the sintered-body thus coated was dried, and fired at 1300°C for 10 hours in air. As a consequence of the X-ray diffraction analysis and the EPMA analysis of the composition of the membrane thus formed, it became apparent that the membrane was a close membrane having the composition of BaCeo.eYo.2~3-a~ The thickness of this membrane was 30 to 40 ~.m.
A fuel cell was composed with the use of the solid electrolyte of oxide-ionic conduction consisting essentially of cerium oxide mentioned above . Paste of Lao..,Sro.3Mn03 was coated on the central area (about 2 cm2) of the surface of this solid electrolyte which was plate-shaped and about 0.4 mm in thickness. Paste of Ni as anode was coated on the surface of the solid electrolyte of proton/oxide-ionic conduction mentioned above . The paste of Lao..,Sro,,Mn03 as cathode was coated on these solid electrolytes were fired.
Thus a porous electrode was obtained.
Fig.3 shows a fuel cell thus obtained. As shown in this figure, an alumina pipe 3 was connected to the surface of an electrode 6 with the use of Pyre~t glass 5 as a gas seal material. Two alumina tubes 2 each of which contained a platinum wire were inserted into the alumina pipe 3 in order to form a fuel cell. This fuel cell was placed in an electric furnace 9 and maintained at 1000°C.
Air was introduced into the space 4 located in the side of the cathode (Lao.,Sro.3Mn03) , and hydrogen gas was introduced into the space 7 located in the side of the solid electrolyte of proton/oxide-ionic conduction. A Ni electrode 8 was used as an electrode provided for the anode.
* Trade-mark Fig.4 shows the voltage-current characteristics of the fuel cell thus obtained. The measured voltage of this cell generally agreed with the theoretical electromotive force calculated from a theoretical formula and showed that the fuel cell according to the present invention displayed higher no-load voltage than that of a conventional fuel cell.
This showed that the electrolyte consisting essentially of cerium oxide was prevented from being reduced under the effect of a fuel gas and the decreasing of the non-load voltage of the cell was voided by depositing a proton/oxide-ionic mixed conductor on one surface of a solid electrolyte consisting essentially of cerium oxide and using this ionic mixed conductor on the side of the cell's anode.
Moreover, with regard to the current taken from this fuel cell, it was proved that the polarization at its electrodes was smaller than that of a conventional fuel cell of a stabilized-zirconia type and, therefore, higher power density could be obtained than that of a conventional fuel cell.
Thus it is possible to inexpensively and easily produce a highly-adhesive (and close) membrane of a solid electrolyte of proton/oxide-ionic mixed conduction as a layer bonded on a solid electrolyte consisting essentially of cerium oxide.
When the solid electrolyte membrane of proton/oxide-ionic mixed conduction is used on the side of the cell's anode, this solid electrolyte membrane can prevent the reduction of cerium oxide and can act as an appropriate solid electrolyte for a fuel cell.
A fuel cell in which the solid electrolyte layer of proton/oxide-ionic mixed conduction is used for anode side and its cathode is formed on the solid electrolyte of oxide-ionic conduction can show greatly advantageous characteristics for a fuel cell.
c,
Claims (8)
1. A solid electrolyte for a fuel cell comprising:
(a) A specifically-shaped solid electrolyte consisting essentially of cerium oxide having a fluorite-type structure and oxide-ionic conduction, and (b) a membrane of a perovskite-type oxide having an ABO3-type composition, which is a solid electrolyte of proton/oxide-ionic mixed conduction, being bonded on a portion of a surface of said solid electrolyte consisting essentially of cerium oxide, in which (i) A of the ABO3 represents at least one element selected from a group consisting of alkaline earth metals Mg, Sr, Ca and Ba, (ii) B of the ABO3 represents cerium by itself or cerium and at least one element selected from a group consisting of alkaline earth metals Mg, Sr, Ca and Ba and rare earth elements Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho and Yb, the elements selected from alkaline earth metals and rare earth elements substituting the cerium by 1 to 30 mol%.
(a) A specifically-shaped solid electrolyte consisting essentially of cerium oxide having a fluorite-type structure and oxide-ionic conduction, and (b) a membrane of a perovskite-type oxide having an ABO3-type composition, which is a solid electrolyte of proton/oxide-ionic mixed conduction, being bonded on a portion of a surface of said solid electrolyte consisting essentially of cerium oxide, in which (i) A of the ABO3 represents at least one element selected from a group consisting of alkaline earth metals Mg, Sr, Ca and Ba, (ii) B of the ABO3 represents cerium by itself or cerium and at least one element selected from a group consisting of alkaline earth metals Mg, Sr, Ca and Ba and rare earth elements Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho and Yb, the elements selected from alkaline earth metals and rare earth elements substituting the cerium by 1 to 30 mol%.
2. The solid electrolyte as defined in claim 1, wherein instead of cerium oxide, said solid electrolyte consists essentially of cerium oxide solid solution in which a part of said cerium oxide is substituted by at least one oxide of an element selected from a roup consisting of alkaline earth metals Mg, Sr, Ca, Ba and rare earth metal sSc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho and Yb by 1 to 30 mol%.
3. A method for producing a solid electrolyte for a fuel cell comprising steps of:
(a) preparing a specifically-shaped solid electrolyte consisting essentially of cerium oxide which has a fluorite-type structure and oxide-ionic conduction, (b) coating at least one material selected from the inorganic acids salts, organic acids salts and organic metal compounds of alkaline earth metals Mg, Sr, Ca and Ba [on a part of a surface] of the solid electrolyte consisting essentially of cerium oxide, and (c) heating the solid electrolyte thus coated to a temperature higher than 800°C in an oxidizing atmosphere to form a solid electrolyte membrane of proton/oxide-ionic mixed conduction on the part of the surface of the solid electrolyte consisting essentially of cerium oxide.
(a) preparing a specifically-shaped solid electrolyte consisting essentially of cerium oxide which has a fluorite-type structure and oxide-ionic conduction, (b) coating at least one material selected from the inorganic acids salts, organic acids salts and organic metal compounds of alkaline earth metals Mg, Sr, Ca and Ba [on a part of a surface] of the solid electrolyte consisting essentially of cerium oxide, and (c) heating the solid electrolyte thus coated to a temperature higher than 800°C in an oxidizing atmosphere to form a solid electrolyte membrane of proton/oxide-ionic mixed conduction on the part of the surface of the solid electrolyte consisting essentially of cerium oxide.
4. The method as defined in claim 3, wherein the specifically-shaped solid electrolyte consists essentially of cerium oxide or its substitutional solid solution in which a part of cerium is substituted by at least one element selected from a group consisting of alkaline earth metals Mg, Sr, Ca and Ba and rare earth elements Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho and Yb by 1 to 30 mol%.
5. A solid electrolyte fuel cell comprising:
(a) a specifically-shaped solid electrolyte consisting essentially of cerium oxide, said solid electrolyte having a fluorite-type structure and oxide-ionic conduction, (b) a membrane of a perovskite-type oxide which consists essentially of oxides of alkaline earth metals and cerium oxide and which has proton/oxide-ionic mixed conduction bonded on a part of a surface of said electrolyte, and an anode which is made of sintered Ni paste formed on fuel side of said membrane, and (c) a cathode of the solid electrolyte comprised of a perovskite-type rare-earth metal oxide or Pt paste.
(a) a specifically-shaped solid electrolyte consisting essentially of cerium oxide, said solid electrolyte having a fluorite-type structure and oxide-ionic conduction, (b) a membrane of a perovskite-type oxide which consists essentially of oxides of alkaline earth metals and cerium oxide and which has proton/oxide-ionic mixed conduction bonded on a part of a surface of said electrolyte, and an anode which is made of sintered Ni paste formed on fuel side of said membrane, and (c) a cathode of the solid electrolyte comprised of a perovskite-type rare-earth metal oxide or Pt paste.
6. The solid electrolyte as defined in claim 5, wherein, instead of cerium oxide, said solid electrolyte consists essentially of cerium oxide solid solution in which a part of said cerium oxide is substituted by at least one oxide of an element selected from a group consisting of alkaline earth elements Mg, Sr, Ca and Ba and rare earth elements Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho and Yb by 1 to 30 mol%.
7. The solid electrolyte fuel cell as defined in claim 6, wherein said membrane of the perovskite-type oxide of proton/oxide-ionic mixed conduction is a perovskite-type oxide having an AbO3-type composition, in which (a) A of the ABO3 represents at least one element selected from a group consisting of alkaline earth metals Mg, Sr, Ca and Ba, (b) B of the ABO3 represents cerium by itself or cerium and at least one element selected from a group consisting of alkaline earth metals Mg, Sr, Ca and Ba and rare earth elements Sc, Y, La, Nd, Sm, Eu, Gd, Dy, Ho and Yb, the elements selected from alkaline earth metals and rare earth elements substituting the cerium by 1 to 30 mol%.
8. The solid electrolyte fuel cell as defined in claim 7, wherein said membrane of the perovskite-type oxide of proton/oxide-ionic mixed conduction has a thickness of 10 mm or more.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP7043666A JPH08222242A (en) | 1995-02-09 | 1995-02-09 | Solid electrolyte for fuel cell and method for producing the same |
| JP43666/1995 | 1995-02-09 | ||
| JP7047918A JPH08222243A (en) | 1995-02-14 | 1995-02-14 | Fuel cell |
| JP47918/1995 | 1995-02-14 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2153736A1 CA2153736A1 (en) | 1996-08-10 |
| CA2153736C true CA2153736C (en) | 2000-09-05 |
Family
ID=26383469
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002153736A Expired - Fee Related CA2153736C (en) | 1995-02-09 | 1995-07-12 | Solid electrolyte for a fuel cell |
Country Status (4)
| Country | Link |
|---|---|
| US (1) | US5672437A (en) |
| EP (1) | EP0726609B1 (en) |
| CA (1) | CA2153736C (en) |
| DE (1) | DE69505784T2 (en) |
Families Citing this family (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0763864A1 (en) * | 1995-09-13 | 1997-03-19 | Kabushiki Kaisha Meidensha | High temperature solid electrolyte fuel cell |
| DE19836132B4 (en) * | 1998-08-10 | 2006-11-23 | Siemens Ag | High temperature solid electrolyte fuel cell (SOFC) for a wide operating temperature range |
| US6296687B2 (en) | 1999-04-30 | 2001-10-02 | Her Majesty The Queen In Right Of Canada, As Represented By The Minister Of Natural Resources | Hydrogen permeation through mixed protonic-electronic conducting materials |
| DE10105426A1 (en) * | 2001-02-07 | 2002-08-08 | Forschungszentrum Juelich Gmbh | Proton conductor for high temperature fuel cells |
| US20020127455A1 (en) * | 2001-03-08 | 2002-09-12 | The Regents Of The University Of California | Ceria-based solid oxide fuel cells |
| WO2003017407A1 (en) * | 2001-08-10 | 2003-02-27 | Eda, Inc. | Improved load leveling battery and methods therefor |
| DE102005003612B3 (en) * | 2005-01-26 | 2006-06-14 | Forschungszentrum Jülich GmbH | Method for producing a thin, gas-tight and proton-conducting ceramic layer and use thereof |
| JP5040123B2 (en) | 2006-02-28 | 2012-10-03 | トヨタ自動車株式会社 | Proton conducting electrolyte and electrochemical cell using the same |
| JP4946819B2 (en) * | 2007-11-20 | 2012-06-06 | 株式会社豊田自動織機 | Electrochemical device and exhaust gas purification device |
| US20110003084A1 (en) * | 2008-02-25 | 2011-01-06 | National Research Council Of Canada | Process of Making Ceria-Based Electrolyte Coating |
| FR2969395B1 (en) * | 2010-12-17 | 2013-01-11 | Ass Pour La Rech Et Le Dev De Methodes Et Processus Ind Armines | FUEL CELL WITH MONOLITHIC MEMBRANE ELECTROLYTES ASSEMBLY |
| CN108091885B (en) * | 2016-11-21 | 2020-08-04 | 中国科学院大连化学物理研究所 | High-temperature fuel cell cathode and application thereof |
| CN110092664B (en) * | 2018-01-31 | 2022-05-31 | 中国科学院青岛生物能源与过程研究所 | Self-phase-splitting mixed conductor three-phase membrane material and preparation method and application thereof |
| WO2021209380A1 (en) * | 2020-04-14 | 2021-10-21 | Solvay Sa | New lithium rare-earth halides |
Family Cites Families (11)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3583150D1 (en) * | 1984-10-23 | 1991-07-11 | Mitsubishi Heavy Ind Ltd | SOLID ELECTROLYTE FUEL CELL AND METHOD FOR THE PRODUCTION THEREOF. |
| US4562124A (en) * | 1985-01-22 | 1985-12-31 | Westinghouse Electric Corp. | Air electrode material for high temperature electrochemical cells |
| US5378345A (en) * | 1986-07-25 | 1995-01-03 | Ceramatec, Inc. | Ceramic solid electrolyte-based electrochemical oxygen concentrator cell |
| GB2206571A (en) * | 1987-07-09 | 1989-01-11 | British Petroleum Co Plc | Barium cerate doped with gadolinium |
| JP2572883B2 (en) * | 1990-09-04 | 1997-01-16 | 日本碍子株式会社 | Solid electrolyte membrane, solid electrolyte fuel cell having the same, and methods of manufacturing these |
| DK167163B1 (en) * | 1991-02-13 | 1993-09-06 | Risoe Forskningscenter | FAST OXIDE FUEL CELLS FOR OXIDATION OF CH4 |
| JP2882104B2 (en) * | 1991-07-17 | 1999-04-12 | 松下電器産業株式会社 | Proton conductor and method for producing the same |
| JP3314426B2 (en) * | 1992-12-25 | 2002-08-12 | 株式会社デンソー | Oxygen sensor |
| US5403461A (en) * | 1993-03-10 | 1995-04-04 | Massachusetts Institute Of Technology | Solid electrolyte-electrode system for an electrochemical cell |
| JPH06349503A (en) * | 1993-06-07 | 1994-12-22 | Mitsubishi Heavy Ind Ltd | Solid electrolyte type electrolytic cell |
| JP3340526B2 (en) * | 1993-08-23 | 2002-11-05 | 松下電器産業株式会社 | Reduction electrode for barium-cerium-based oxide solid electrolyte and method for producing the same |
-
1995
- 1995-04-20 DE DE69505784T patent/DE69505784T2/en not_active Expired - Fee Related
- 1995-04-20 EP EP95105914A patent/EP0726609B1/en not_active Expired - Lifetime
- 1995-06-05 US US08/461,497 patent/US5672437A/en not_active Expired - Lifetime
- 1995-07-12 CA CA002153736A patent/CA2153736C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| DE69505784D1 (en) | 1998-12-10 |
| CA2153736A1 (en) | 1996-08-10 |
| EP0726609B1 (en) | 1998-11-04 |
| EP0726609A1 (en) | 1996-08-14 |
| DE69505784T2 (en) | 1999-05-06 |
| US5672437A (en) | 1997-09-30 |
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