WO1997041612A1 - Electrolytes et membranes solides stables, a stratification compositionnelle, a gradient fonctionnel et a haute conductivite - Google Patents

Electrolytes et membranes solides stables, a stratification compositionnelle, a gradient fonctionnel et a haute conductivite Download PDF

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Publication number
WO1997041612A1
WO1997041612A1 PCT/US1996/006065 US9606065W WO9741612A1 WO 1997041612 A1 WO1997041612 A1 WO 1997041612A1 US 9606065 W US9606065 W US 9606065W WO 9741612 A1 WO9741612 A1 WO 9741612A1
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Prior art keywords
conducting layer
oxygen
electronic
mixed ionic
ion
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PCT/US1996/006065
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English (en)
Inventor
Eric D. Wachsman
Palitha Jayaweera
David M. Lowe
Bruce C. Pound
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Gas Research Institute, Inc.
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Priority to PCT/US1996/006065 priority Critical patent/WO1997041612A1/fr
Priority to AU56348/96A priority patent/AU5634896A/en
Publication of WO1997041612A1 publication Critical patent/WO1997041612A1/fr

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0072—Inorganic membrane manufacture by deposition from the gaseous phase, e.g. sputtering, CVD, PVD
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B01D71/024—Oxides
    • B01D71/0271—Perovskites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/22—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion
    • B01D53/228—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by diffusion characterised by specific membranes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0041—Inorganic membrane manufacture by agglomeration of particles in the dry state
    • B01D67/00411—Inorganic membrane manufacture by agglomeration of particles in the dry state by sintering
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D67/00—Processes specially adapted for manufacturing semi-permeable membranes for separation processes or apparatus
    • B01D67/0039—Inorganic membrane manufacture
    • B01D67/0046—Inorganic membrane manufacture by slurry techniques, e.g. die or slip-casting
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D71/00—Semi-permeable membranes for separation processes or apparatus characterised by the material; Manufacturing processes specially adapted therefor
    • B01D71/02—Inorganic material
    • B01D71/024—Oxides
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/406—Cells and probes with solid electrolytes
    • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
    • G01N27/4073—Composition or fabrication of the solid electrolyte
    • 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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2323/00—Details relating to membrane preparation
    • B01D2323/08—Specific temperatures applied
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2325/00—Details relating to properties of membranes
    • B01D2325/26—Electrical properties
    • 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
    • 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

Definitions

  • This invention relates to stable high conductivity functionally gradient compositionally layered solid state electrolytes and membranes providing improved oxygen-ion conductivity for electrolytes and improved mixed oxygen-ion and electronic conductivity for membranes.
  • the electrolytes of this invention are useful in solid oxide fuel cells and as sensors.
  • the membranes of this invention are useful in gas separation and in membrane reactors.
  • Multi-layered solid electrolytes and membranes of this invention have a mixed conducting anodic portion with high oxygen ionic conduction and electronic conduction on one side, preferably of the n-type, which may be exposed to a reducing atmosphere, and interface on the opposite side with a high oxygen ionic conducting and low electronic conducting portion for the electrolyte and high p-type ionic conducting for the membrane, and exposed to an oxidizing atmosphere on the opposite side.
  • Oxygen ion conductors based upon Bi 2 O 3 have been known for solid electrolytes and oxygen separation due to their high oxygen ion conductivity and their ability to operate at lower temperatures. There have been various attempts to improve their phase stability and their thermodynamic stability against reduction.
  • Patent Numbers 5,006,494 and 5,183,801 teach the phase stability of Bi 2 O 3 in the cubic form stabilized by 10 to 40 mole percent of a rare earth oxide, such as yttria, is enhanced by inclusion of up to 10 mole percent of an oxide of a cation having a valence of 4 or greater, such as zirconia, hafnia, thoria, stannic oxide, tantalum oxide, and niobium oxide as a dopant. It has been shown that Bi 2 O, stabilized in the ⁇ -phase by 20 mol% Er 2 O 3 has ionic conductivity in the order of 1 to 2 orders of magnitude greater than that of yttria stabilized zirconia at comparable temperatures.
  • a rare earth oxide such as yttria
  • an oxide of a cation having a valence of 4 or greater such as zirconia, hafnia, thoria, stannic oxide, tantalum oxide, and niobium oxide
  • U.S. Patent Number 5,213,911 teaches a solid oxide low temperature electrolyte with very low electron conduction and stable in H 2 formed by combination of a compound having weak intramolecular metal-oxygen interactions with one having stronger metal-oxygen interactions.
  • a graded zirconia-bismuth oxide electrolyte is taught by U.S. Patent Number 5,171 ,645 having a Bi 2 O 3 doped for increased oxygen ion transport rich layer on one side and a ZrO 2 doped for increased oxygen ion transport rich layer on the opposite side with gradations of the components between, enabling low temperature operation and better thermal expansion stability in a solid oxide fuel cell.
  • both surfaces are doped ZrO 2 rich layers graded to a relatively thick doped Bi 2 O 3 rich central layer.
  • Aliovalent doped ceria in the fluorite structure is also known to exhibit ionic conductivity significantly greater than yttria stabilized zirconia at comparable temperatures.
  • ceria electrolytes have a high electronic conduction in a reducing environment.
  • U. S. Patent Number 5,0011,021 teaches limiting electronic conduction in ceria electrolytes by double metal doping.
  • a mixed oxygen ion and electronic conductor may favor n-type electronic conduction in a reducing environment while in an oxidizing environment it may favor p- type conduction.
  • a mixed conductor may exhibit pure ionic conduction over a central region between the n-type and p-type regions.
  • U.S. Patent Numbers 4,793,904; 4,933,054; and 4,802,958 teach a solid electrolyte of Bi 2 O 3 stabilized by a lanthanide or calcium oxide having a conductive coating on one side capable of reducing oxygen to oxygen ions and a conductive coating on the opposite side capable of desired oxidative catalysis.
  • Use of mixed ionic-electronic conductors for electrocatalytic chemical reactions is taught by U.S. Patent Number 5,273,628 which teaches thin membranes of homogeneous solid solutions and non-homogeneous mixtures of Bi 2 O 3 stabilized with a stabilizer, such as Y, and containing a variable valence metal, such as Ti.
  • U.S. Patent Number 5,306,411 teaches multi-phase mixtures of an electronically conductive material and an oxygen ion conductive material based upon ABO 3 perovskites preferably containing small amounts or no bismuth.
  • U.S. Patent Number 5,069,987 teaches a solid oxide fuel cell having an electrolyte with repeating array of ductile ordered, continuous metallic fibers imbedded in the ceramic matrix, such as expanded Ni foil.
  • U. S. Patent Number 5,240,480 teaches an oxygen ion conducting membrane having a porous layer with average pore sizes of less than about 10 microns and a contiguous dense non-porous layer, both materials being a mixed ionic-electronic conducting multicomponent metallic oxide, such as perovskites.
  • Oxygen ion conducting electrolytes of this invention are suitable for a wide variety of uses, including oxygen sensors and solid oxide fuel cells.
  • Cells using electrolytes have conductive regions of n-type
  • the intrinsic region has essentially exclusive high oxygen conductivity.
  • the material must have higher mobility of and/or a greater concentration of oxygen-ion vacancies.
  • t is the transport number
  • ⁇ is the oxygen ion conductivity
  • ⁇ is the total conductivity.
  • Numerous oxide structures, such as fluorite, pyrochlore, perovskite, and brownmillerite, are being considered as potential intermediate temperature electrolytes, but perovskites and similar structures having high conductivities have unacceptably high electronic conductivity, such that t, is less than 0.9.
  • Suitable for use as an essentially exclusive high oxygen ion conductor in this invention are those materials having t, greater than about 0.95, and preferably greater than 0.99.
  • ionic conducting we mean having an ionic conductivity of t, >0.95, and preferably t, > 0.99.
  • Bismuth oxide-based materials having the above mentioned ionic conductivity are suitable in addition to Bi 2 O 3 itself, such as bismuth-metal(s)-oxide as disclosed in U.S. Patent No. 5,213,911 , as well as rare earth-metal (s)-oxide and lanthanum-metal(s)-oxide, both as disclosed in U.S. Patent No. 5,213,911 , and multiple metal doped bismuth oxides as disclosed in U.S. Patent Numbers 5,006,494 and 5, 183,801. Any other metal oxide having t, >0.95, and preferably t, >0.99, is suitable.
  • the stability of the oxygen-ion conducting material may be reduced.
  • the major oxygen ion conductor is protected from the reducing environment by a dense layer of mixed ionic-electronic conductor which is tolerant to the reducing environment and raises the oxygen partial pressure to greater than that providing stability at its interface with an adjacent essentially exclusive high oxygen ion conductor.
  • Any material which provides such mixed ionic electronic conductivity under reducing conditions and provides sufficiently high partial pressure of oxygen at the interface is suitable. We have found ceria-based materials to be suitable.
  • Oxygen ion conducting membranes according to this invention are suitable for a wide variety of uses, including oxygen separation and in membrane reaction systems for chemical conversions.
  • Systems using membranes have conductive regions of either n-type or p-type, or preferably, n-type
  • the n-type conductor which is tolerant of the reducing environment protects the stability of the adjacent mixed oxygen ion conductor.
  • Suitable n-type conductors are the same as those set forth above for electrolytes.
  • Suitable mixed ionic-electronic conductors for the p-type conductor include Bi 2 O 3 and bismuth oxide-based materials, such as mixed metal bismuth oxides as disclosed in U.S. Patent No. 5,273,628, and perovskite materials as disclosed in U.S. Patent No. 5,306,411 which have the required mixed ionic-electronic conduction.
  • This invention will be primarily described and demonstrated with respect to oxygen ion conductance in solid oxide fuel cells, but it will be recognized that the stable high conductivity functionally gradient compositionally layered solid state electrolytes and membranes of this invention may utilize other conducting materials for other ion conduction for a variety of other uses.
  • Solid oxide fuel cells offer a desirable process for conversion of chemical energy directly to electricity since they produce negligible NO x , and due to their high fuel efficiency, produce less CO 2 per kilowatt hour than most alternative processes.
  • Current solid oxide fuel cells based upon zirconia electrolytes must operate at about 1000 "C and above to avoid unacceptably high ohmic losses. These high operating temperatures reduce selection of suitable electrode and interconnect materials and induce adverse structural changes in the cell as well as constraints on fabrication, thus increasing costs.
  • Yttria-stabilized zirconia has been used as the electrolyte in solid oxide fuel cells due to its most favorable compromise in chemical and thermal stability, ionic conductivity, and cost, but the limitations imposed by its high operating temperatures render a low commercial feasibility of such cells.
  • Thin film on the order of about 5 microns, yttria-stabilized zirconia electrolytes which would reduce the operating temperature required to obtain ohmic losses comparable to current thick film electrolytes, about 100 microns, have been considered.
  • thin films of an electrolyte with a higher ionic conductance would further reduce the temperature requirement or increase the cell power density at the same temperature.
  • zirconia based solid oxide fuel cells still require high fabrication temperatures, in the order of about 1200 ° to 1700 ° C, which sinters attached electrodes resulting in lower specific surface area, decreased electrocatalytic activity, and higher electrode polarization losses, making an intermediate temperature electrolyte of a more advantageous material highly desirable.
  • Another object of this invention is to provide electrolytes and membranes having high oxygen ion conductance at temperatures lower than 800 'C, and down to about 300 ° to about 500 °C.
  • Yet another object of this invention is to provide low cost, intermediate temperature, solid oxide fuel cells using a multi-layered electrolyte having a dense mixed ionic and electronic conducting ceria-based layer and a dense oxygen-ion conducting bismuth oxide based layer providing increased open circuit potential, increased power density, and greater electrolyte stability than fuel cells using prior ceria-based or bismuth oxide-based electrolytes.
  • Another object of this invention is to provide a multi-layered ceria-based
  • the superior ionic conductors based upon bismuth oxide and ceria have not been used as electrolytes in reducing environments, such as in solid oxide fuel cells, because they are readily reduced in the presence of reducing fuel gases at the anode.
  • Bismuth oxide electrolytes have the advantage, in addition to a lower operating temperature, that their required processing temperatures during fabrication is about 850 " C, only about half that required by yttrium-stabilized zirconia, resulting in a much wider selection of compatible materials and ease of fabrication. While the lower limit of stability of bismuth oxide is subject to some controversy involving a kinetically limited decomposition to metallic Bi, Bi 2 O 3 -based electrolytes have been found to exhibit purely ionic conduction down to 10
  • Ceria by contrast, is not reduced to metallic cerium, but Ce +4 is reduced to Ce + ⁇ resulting in mixed ionic and electronic conduction with decreasing P 02 .
  • the usefulness of ceria as an electrolyte is limited by t ; being less than 1.0 with decreasing P 02 resulting in lower open-circuit potential and power loss by electronic conduction.
  • This invention involves use of stable high conductivity functionally gradient compositionally layered solid state electrolytes, which, in the case of oxygen ion conductance, result in the synergistic combination of the superior ionic conductors Bi 2 O 3 and CeO 2 as the electrolyte in an intermediate temperature solid oxide fuel cell.
  • a dense, mixed ionic/electronic conducting metal oxide layer facing the reducing atmosphere acts as a protective layer for a high oxygen ion conducting electrolyte layer and, in principle, can serve as the anode itself.
  • a mixed ionic/electronic conducting layer means that the electrode can be non-porous since the electron transfer reaction does not require the presence of a three-phase boundary between the electrode, electrolyte, and reactant, as is necessary when the electrode is purely an electronic conductor.
  • the mixed conducting metal oxide layer also allows charge transfer to occur over the entire electrode surface due to the mobility of both electrons and oxygen ions.
  • a dense mixed conducting layer therefore, protects the stabilized Bi 2 O 3 from the reducing environment and avoids materials problems associated with maintaining a porous structure for gas diffusion to the electrolyte-electrode interface, allowing good utilization of bismuth oxide's high oxygen ion conductivity in a solid oxide fuel cell.
  • the bismuth oxide also blocks electronic conductance through the ceria-based mixed ionic/electronic conducting layer due to increase its desired higher ionic conductance.
  • the relative electronic conductivity contribution of a mixed oxygen ion/electron conductor depends upon the local oxygen activity. Under a reducing environment, electron conductivity may exceed ionic conductivity, forming an n-type conductor, and in an oxidizing environment, electronic conductivity may exceed ionic conductivity, forming a p-type conductor. The relative electronic contribution to conductivity decreases with distance from the active gas interface. As a result of the mixed ionic and electronic conducting properties and their dependence upon local oxygen activity, a dense layer of a mixed conductor can act as an anode and also protect an adjacent layer of high oxygen ion conductor, such as Bi 2 O 3 , from reduction by a reducing atmosphere, such as fuel in a fuel cell.
  • a dense layer of a mixed conductor can act as an anode and also protect an adjacent layer of high oxygen ion conductor, such as Bi 2 O 3 , from reduction by a reducing atmosphere, such as fuel in a fuel cell.
  • FIG. 1 schematically illustrates the effect of the relative thickness of the mixed ionic-electronic conducting anodic electrolyte layer and the ionic conducting electrolyte layer upon the partial pressure of oxygen at their interface.
  • the interface of the mixed ionic-electronic conducting anodic electrolyte layer and the ionic conducting electrolyte layer is X E A -
  • the left hand portion illustrates a relatively thin anodic electrolyte layer with a thicker ionic conducting electrolyte layer, while the right hand portion illustrates a relatively thick anodic electrolyte layer with a thin ionic conducting electrolyte layer.
  • the oxygen partial pressure at the exposed face of the anodic-electrolyte layer is P 02 ° while the oxygen partial pressure at the exposed face of the ionic conducting electrolyte layer is a higher value P ⁇ 2 L .
  • the P 02 gradient across the entire electrolyte is shown as Hnear for simplicity. It is seen that the partial pressure of oxygen at the mixed conductor anodic electrolyte
  • P 02 E/A (2) obtained with the relatively thick mixed conducting anodic electrolyte layer, is much higher than P 02 E/A (1) obtained with the relatively thin mixed conducting anodic electrolyte layer.
  • the mixed ionic-electronic conducting layer should have both comparable ionic conductivity, so that an increase in relative thickness of a mixed conducting anode has minimal effect on resistance, and a structure similar to the thin ionic conducting layer to reduce interfacial stresses due to lattice mismatch as well as mismatch due to different thermal expansion characteristics.
  • CeO 2 stabilized in the fluorite type structure fills both of these requirements.
  • a CeO 2 layer can act as both an anode, n-type, and electrolyte, depending upon the local oxygen partial pressure.
  • the electrolyte in an intermediate temperature solid oxide fuel cell, can be considered as a bi-layered structure of Bi 2 O 3 and CeO 2 , with the CeO 2 layer exposed to the fuel environment.
  • This structure provides an intermediate temperature solid oxide fuel cell having many benefits in terms of materials compatibility, selection and fabrication, avoiding processing and fabrication limitations associated with current high temperature electrolytes which have limited development and commercialization of solid oxide fuel cells.
  • the bi-layered electrolyte structure based upon highly conductive CeO 2 and Bi 2 O 3 according to this invention, exhibits the synergistic effect of a higher open circuit potential than either material separately as a single layered electrolyte. Further, the bi-layered electrolyte structure using these materials according to this invention exhibits high stability against reduction by hydrogen at the anode of a fuel cell.
  • One preferred bi-layered electrolyte structure according to this invention is the bi-layered structure of MO x -CeO 2
  • bi-layered structures are preferred to be as thin as possible consistent with fabrication methods and physical stability.
  • bi-layered electrolytes are generally thin films, about 1 to about 30 microns, of M' x O 3 -Bi 2 0 3 on relatively thick, about lO ⁇ m to about 1.0 mm thick layer of M y O 3 -CeO 2 .
  • the mixed ionic-electronic conducting electrolyte layer should be sufficiently thick to provide sufficiently high oxygen partial pressure at its interface with the oxygen ion conducting electrolyte layer to provide high stability against chemical reduction.
  • FIG. 1 are two graphs illustrating increase in local oxygen activity at the oxygen ion conducting electrolyte interface of a dense bi-layered electrolyte by increase in the thickness of mixed ionic-electronic conducting anode portion;
  • FIG. 2 compares plots of open circuit potential versus temperature for fuel cells having a mono-layered electrolyte and one having a bi-layered electrolyte as described in
  • FIG. 3 compares plots of open circuit potential versus temperature for fuel cells having a mono-layered electrolyte and four having bi-layered electrolytes as described in Example VIII;
  • FIG. 4 compares plots of potential versus current density for two fuel cells having mono-layered electrolytes and one having a bi-layered electrolyte as described in Example IX;
  • FIG. 5 compares plots of power density versus current density for the same fuel cells for which data is shown in FIG. 4 and described In Example IX;
  • FIG. 6 shows reproducibility of plots of potential and power density versus current density for fuel cells having bi-layered electrolytes as described in Example IX;
  • FIG. 7. shows plots of potential and power density versus current density at temperatures of 650° to 800 'C for a fuel cell having a bi-layered electrolyte as described in Example IX; and
  • FIG. 8 shows plots of potential versus time showing stability of fuel cells having bi-layered electrolytes as described in Example X.
  • This invention utilizes the functional gradient across conductive layers to make advantageous use of superior conducting materials which individually would not be chemically stable in a desired use environment.
  • an interface where the materials are stable may be achieved, due to the chemical gradient across the two layers.
  • an advantageous conductive pair of materials may be used as an electrolyte or membrane .
  • this invention provides a stable, high conductivity, functionally gradient, compositionally layered, solid membrane or electrolyte which comprises a dense ionic conducting layer capable of ionic conduction of an ion derived from a gas in contact with one face.
  • the opposite face of the ionic conducting layer is in contact with one face of a mixed ionic-electronic conducting layer which has its opposite face in contact with and tolerant to an environment rendering the ion conducting layer chemically unstable.
  • the mixed ionic-electronic conducting layer has sufficient thickness to raise the partial pressure of the gas from which the ions were derived to a sufficiently high level at the interface of the mixed ionic-electronic conducting layer and the ionic conducting layer to provide high chemical stability to the ionic conducting layer. In this manner, materials providing high ionic conduction at lower temperatures than previously used materials may be utilized in environments which have previously rendered these materials unsuitable.
  • the ionic conducting layer is protected from the adverse gaseous environment in contact with the mixed ionic-electronic conducting layer by the thickness of the dense mixed ionic-electronic layer, tolerant to the adverse gaseous environment, providing at the interface of the mixed ionic-electronic conductor and the ionic conductor sufficiently high partial pressure of the gas from which the ions conducted by the ionic conducting layer were derived to assure chemical stability and ionic conductance of the ionic conducting layer. It is apparent that many mixed ionic-electronic conductors and essentially exclusive ionic conductors may be matched to form functionally gradient and compositionally layered electrolytes according to this invention.
  • Ions for conductance by the essentially exclusive ionic conductor may be derived from different gaseous sources, but oxygen ion conductance is described with reference to solid oxide fuel cells.
  • the essentially exclusive ionic conductor may be protected from different adverse gaseous environments by different mixed ionic-electronic conductors which are tolerant, or even, advantageously affected by such gaseous environments.
  • An electrolyte comprising the combination of a dense bismuth oxide-based essentially exclusive oxygen ion conducting layer and a dense ceria-based mixed oxygen ion-electronic conducting layer has been found to be synergistic in providing high oxygen ion conductivity with minimal electronic conductivity, and thus high potential and power density, with the ceria-based mixed ionic-electronic conductor in contact with a reducing fuel environment.
  • suitable temperature ranges for operation of solid oxide fuel cells using the electrolytes of this invention are those less than about 800 ° C, preferably about 300 ° to about 800 °C, and most preferably about 500° to about 800 °C.
  • the ceria-based mixed ionic-electronic conducting layer is sufficiently thick to provide an oxygen partial pressure at the interface with the bismuth oxide-based essentially exclusive oxygen ion conducting layer of greater than about IO "20 and preferably greater than about 10 ⁇ 13 atm. In practice, this layer is less than about 1.0mm and down to about lO ⁇ m thick.
  • the bismuth oxide-based oxygen ion conducting layer is suitably about 1 to about 30 ⁇ m thick. Thinner electrolytes are desirable because of their lower resistance.
  • the ceria is suitably doped with known dopants to enhance desired conductance.
  • known dopants A number of dopants have been described above. We have found ceria doped with about
  • a lanthanide to be suitable particularly suitable is samaria and gadolinia doped ceria.
  • the bismuth oxide is stabilized with known stabilizers to maintain high ionic conductance.
  • stabilizers A number of stabilizers have been described above.
  • the bi-layered electrolyte having the formulation (Er 2 O 3 ) 0 , .(1 4 (Bi 2 O 3 ) 0 9 _ 0 6 I (Sm 2 O 3 ) ⁇ ⁇ 5 . 025 (CeO 2 ) 0 95 . 0.75 is particularly suitable for use as an electrolyte in a solid oxide fuel cell.
  • the dense oxygen ion conducting layer maintains high essentially exclusive oxygen ion conduction at about 300 ° to about 800 ° C when the mixed ionic-electronic conducting layer is sufficiently thick to raise the oxygen partial pressure at the interface of the two layers to greater than about 10 "20 , and preferably, greater than about 10" 13 atm.
  • a porous metal and/or metal oxide anode may be provided adjacent the open face of the mixed ionic-electronic conducting layer and a porous metal and/or metal oxide cathode provided adjacent the open face of the ionic conducting layer.
  • Suitable current collectors and electrical leads may be provided. Conventional means are provided for passing fuel gas in contact with the anode and Tor passing oxidant gas in contact with the cathode.
  • electrical leads may be attached to the mixed ionic-electronic conducting layer and that layer of the electrolyte may function as an anode.
  • the bi-layered electrolyte based upon ceria and bismuth oxide may be fabricated at lower temperatures broadening the selection of cell components, reducing warpage, and making fabrication much easier.
  • a thin layer of doped bismuth oxide is applied to an appreciably thicker pre-sintered ceria layer, prepared by any suitable process known to the art.
  • the bismuth oxide layer may be applied by an aqueous slurry, but more desirable methods to obtain a thinner bismuth oxide layer may be sputter deposition, electrochemical vapor deposition, and various dip or spin coating methods.
  • the bismuth oxide coated pre-sintered ceria layer is then sintered at temperatures of about 835 ' to about 1000° C, dependent upon the nature and amount of stabilizer used, to form the bi-layered electrolyte. Suitable sintering times are about 1 to about 10 hours, the longer times providing a denser bismuth oxide layer.
  • the bismuth oxide content of the ionic conducting layer may be enhanced by coating the sintered bi-layered electrolyte with pure bismuth oxide and then sintering at a temperature of about 835 ° to about 875 ° , the bismuth oxide serving as a low melting temperature flux to fill the pores in the ionic conducting layer for enhanced ionic conductivity.
  • the slurries were cast onto a glass plate using an EPH tape caster with a doctor blade height of 2mm.
  • the cast tapes were dried at room temperature, removed from the glass plate, and cut into 3cm diameter disks using a machined stainless steel punch.
  • the green disks were placed into a Lindberg tube furnace with flowing O 2 and the temperature raised to 1000 ° C at a temperature ramp of about 1 °C/min to avoid warping and cracking.
  • the disks were then cooled to room temperature and placed in a Thermolyne MoSi 2 furnace where they were sintered to 1650 °C for 15 hours.
  • the disks were removed from the furnace and cooled to room temperature.
  • the final sintered disks were 2.5cm in diameter and about 0.8mm thick.
  • YDC yttria-doped ceria
  • SDC samaria-doped ceria
  • SDC disks was 91.0% . Higher density of the SDC disks would be desirable.
  • Bi-layered electrolytes were prepared by first preparing doped ceria sintered disks,
  • SUBSTITUTE SHEET (RULE 2 ⁇ ) the bi-layer electrolyte M' 2 O 3 -Bi 2 O 3
  • Bi 2 O 3 melt-sintered, resulting in a dense thin film.
  • the layered structure was confirmed by scanning electron micrographs.
  • the migration of the dopant metal into the Bi 2 O 3 layer by cation diffusion at the sintering temperature was confirmed with ED AX mapping of the cross section of the sintered bi-layered disks.
  • the amount of dopant in the M' y O 3 -Bi 2 O 3 layer was less than necessary for good conductivity and stabilization of the cubic phase which requires about 20 mol % .
  • the SDC disks were first coated and sintered with ESB followed by coating and sintering the Bi 2 O 3 which served as a low melting temperature flux to fill the porosity of the ESB layer forming a bi-layered SDC j ESB electrolyte, denoted as SDC
  • Example VI Laboratory cells were fabricated for solid oxide fuel cell evaluation of the electrolytes. Porous Pt anodes and Au cathodes were used for these evaluations. Electrodes were deposited and sintered on each side of electrolyte disks to form metal
  • the electrochemical cells were fabricated with the electrolyte disks having porous metal electrodes deposited on each side with attached external electrical leads, sealed gas-tight both to the external atmosphere and between opposite sides of the disk, all within a cell housing providing a fuel and oxidant chamber.
  • the electrolyte cell was sealed using a low-melting-temperature glass, Corning 7059, seal around the circumference of the disk, which, upon heating, formed a positive pressure seal.
  • the seals were confirmed by measuring inlet and outlet flows, with O 2 and H 2 flowing at 30cm 3 /min on the cathode and anode, respectively, and for small diffusion leaks, open circuit potential was measured as function of temperature and time.
  • Current-potential (I-E) data were obtained galvanostatically, potential as a function of applied current, for most of the cells, in the temperature range of 500 ° to 800 ° C.
  • Example VII Open circuit potentials E Q for the cells was measured to demonstrate the higher value for E_ obtained with bi-layered electrolytes according to this invention, approaching the theoretical E 0 while exhibiting stability against reduction of the electrolyte by H 2 at the anode.
  • the theoretical Er, for a solid oxide fuel cell at 800 ° C with H 2 /3%H 2 O at the anode and pure O 2 at the cathode is:
  • R is the gas constant (J/mol K)
  • T is the temperamre (K)
  • z equivalents/mol
  • F is the Faraday constant (Coulomb/equivalent)
  • P' ()2 is the oxygen partial pressure on side a and b of the cell. Due to thermodynamics of the H 2 /H 2 O equilibria, E 0 increases slightly with decreasing temperamre, 40mV from 800 ° to 500 ° C. Theoretical EQ can only occur with no electronic conduction across the electrolyte and no gas leak between the anode and cathode or to the external atmosphere.
  • Example VI Cells were prepared in the manner described in Example VI using the YDC electrolyte as prepared in Example I, for comparison, and using the bi-layered YDC j YSB electrolyte as prepared in Example II according to this invention.
  • the E réelle was measured at 500 " , 600 ° and 800 ° C for the YDC cell and at 700 ° and 800 ' C for the YDC
  • Example VIII Five test cells were fabricated, as described in Example VI, based upon the
  • E 0 of the SDC electrolyte cell is greater than that of the YDC electrolyte cell at all temperatures tested. This can be attributed to the higher t, of SDC as compared to YDC.
  • the drop in E ( , with increasing temperature exhibited by the YDC and SDC electrolyte cells is primarily due to the Ce + /Ce +3 equilibrium and resulting decrease in ionic domain of the ceria electrolytes with increasing temperature.
  • FIG. 3 Also shown in Fig. 3 are the E 0 (T) measurements for cells using bi-layered SDC I ESB electrolytes:
  • the bi-layered electrolytes according to this invention show significant increases in E Q at all temperatures compared with the cell using the mono-layer SDC electrolyte.
  • the increase in E (l increased successively with electrolytes prepared in Examples III, IV, and V, due to improvement in the density of the ESB layer and resultant ability of the electrolyte to block electronic conduction.
  • the cell fabricated from the bi-layered electrolyte prepared in Example v obtained an Eo of 1003mV at 500°C. and overall a 90 to 160 mV increase in E , depending upon temperamre, as compared to the mono-layer
  • Example IX Test cells fabricated as described in Example VI were tested for current-potential data using porous metal electrodes, except SDC j ESB-V which had a LaSrCoO 3 cathode and Ni-Ceria anode, with O 2 -cathode and H 2 -anode at 800°C. The results are summarized in Table 1.
  • the cell resistance was determined by a linear fit (slope) of the I-E data and is shown in Table 1.
  • the actual I-E data had a slight positive curvature, typical of mixed conductors, and dominated by non-electrolyte, e.g. electrode, polarization.
  • the resistance of the 0.8mm thick SDC electrolyte was only 0.88 ⁇ cm2 at 800 'C. Electrolyte performance may still be compared, based upon cell performance, since the cells were identically fabricated.
  • the lower resistance of the SDC cell relative to the YDC cell is attributed to its higher conductivity.
  • the bi-layered electrolyte fabrication according to this invention did not increase the cell resistance, but in fact decreased cell resistance by 23 ⁇ 7% , for the SDC cells.
  • SSDDCC 1 I EESSBB VV 888800 * *
  • the potential is plotted as a function of current density for cells YDC, SDC, and SDC ! ESB III-A at 800 °C in Fig. 4.
  • the current density was calculated from the current and the geometric surface area of the electrodes. The data were curve fit and extrapolated to higher current densities shown by the dashed lines.
  • the I-E data were replotted in terms of power density for cells YDC, SDC, and
  • SDC ! ESB III-A electrolyte of this invention to be 33 % greater than the SDC mono-layer electrolyte under identical conditions.
  • the data were curve fit and extrapolated to higher current densities shown by the dashed lines.
  • Example X All SDC-based cells prepared in Examples I, III, IV, and V were thermally cycled several time for electrode deposition and sealing and were found to operate well without any sign of mechanical failure.
  • the stability of the bi-layered electrolytes of this invention under open circuit and maximum power conditions is shown in Fig. 8.
  • a dashed line at 840mV indicates the Eiller of the cell using the single layered SDC electrolyte for comparison under identical conditions of gas composition and temperamre.
  • ESB III-B electrolyte was operated for one day at various temperatures for I-E and E 0 data and then run continuously for an additional 250 hours at 650° C under open circuit conditions with no decrease in E () , as shown in Fig. 8. Over this time period the E 0 was quite constant at 882 ⁇ lmV.
  • the current-potential curves before and after the stability test were essentially superimposable, further demonstrating the stability of the cell.
  • ESB IV electrolyte was operated continuously for 1250 hours.
  • the cell was primarily under open circuit condition at 650° C. except where noted for thermal cycling and I-E data acquisition.
  • the cell was operated at maximum power conditions of 65mA for 300 hours and then remrned to open circuit for 10 hours, at which time a crack developed due to water buildup at the anode. Over the entire time period, the cell readings were relatively noise-free and demonstrated a continuous increase in potential under both open circuit and maximum power conditions.
  • E 0 was 910mV, and after 1250 hours of operation, it had increased to 935m V.
  • the I-E data also demonstrated a continuous increase in power density with time.
  • ESB V electrolyte was operated continuously for 1400 hours until a power outage caused shut down of the cell.
  • the cell had a continuously high E 0 which appeared to increase near the end of the test.
  • the scatter of the data was due to a contact problem associated with the LaSrCoO 3 cathode and Ni-ceria anode.
  • the H 2 was temporarily shut off during the test and E n dropped and recovered, as expected.

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Abstract

L'invention concerne des corps solides stables à stratification compositionnelle, à gradient fonctionnel et à haute conductivité, destinés à être utilisés comme électrolytes et membranes, et présentant une conductivité des ions oxygène améliorée lorsqu'ils sont utilisés comme électrolytes, ainsi qu'une conductivité mixte des ions oxygène et des électrons améliorée lorsqu'ils sont utilisés comme membranes. Les électrolytes constitués de tels corps solides permettent la création de piles à combustible oxide solide fonctionnant avec un grand débit, à une température comprise entre 300 et 800 °C.
PCT/US1996/006065 1996-05-01 1996-05-01 Electrolytes et membranes solides stables, a stratification compositionnelle, a gradient fonctionnel et a haute conductivite WO1997041612A1 (fr)

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AU56348/96A AU5634896A (en) 1996-05-01 1996-05-01 Stable high conductivity functionally gradient compositionally layered solid state electrolytes and membranes

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GB2411043A (en) * 2004-02-10 2005-08-17 Ceres Power Ltd A method and apparatus for operating an intermediate-temperature solid-oxide fuel cell stack
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DE10317976B4 (de) * 2003-04-17 2013-05-29 Deutsches Zentrum für Luft- und Raumfahrt e.V. Festelektrolyt-Brennstoffzelle und Verfahren zu ihrer Herstellung sowie Verwendung der Festelektrolyt-Brennstoffzelle als Elektrolyseur
DE10317976A1 (de) * 2003-04-17 2004-10-28 Bayerische Motoren Werke Aktiengesellschaft Festoxid-Brennstoffzelle und Verfahren zu ihrer Herstellung
GB2411043A (en) * 2004-02-10 2005-08-17 Ceres Power Ltd A method and apparatus for operating an intermediate-temperature solid-oxide fuel cell stack
GB2411043B (en) * 2004-02-10 2007-09-19 Ceres Power Ltd A method and apparatus for operating an intermediate-temperature solid-oxide fuel cell stack
US10283792B2 (en) 2004-02-10 2019-05-07 Ceres Intellectual Property Company Limited Method and apparatus for operating a solid-oxide fuel cell stack with a mixed ionic/electronic conducting electrolyte
EP2338201A4 (fr) * 2008-10-14 2014-04-23 Univ Florida Conception et matériaux avancés pour sofc à basse température
KR20110086016A (ko) * 2008-10-14 2011-07-27 유니버시티 오브 플로리다 리서치 파운데이션, 인크. 저온 고체 산화물형 연료전지(ｓｏｆｃ)에 사용되는 개선된 물질 및 설계
US9343746B2 (en) 2008-10-14 2016-05-17 University Of Florida Research Foundation, Inc. Advanced materials and design for low temperature SOFCs
KR101699091B1 (ko) * 2008-10-14 2017-01-23 유니버시티 오브 플로리다 리서치 파운데이션, 인크. 저온 고체 산화물형 연료전지(ｓｏｆｃ)에 사용되는 개선된 물질 및 설계
EP2338201A2 (fr) * 2008-10-14 2011-06-29 University of Florida Research Foundation, Inc. Conception et matériaux avancés pour sofc à basse température
WO2014187559A1 (fr) * 2013-05-21 2014-11-27 Plansee Composite Materials Gmbh Ensemble multicouche pour électrolyte solide
JP2016524282A (ja) * 2013-05-21 2016-08-12 プランゼー コンポジット マテリアルズ ゲーエムベーハー 固体電解質用多重層配置構成
US10312540B2 (en) 2013-05-21 2019-06-04 Plansee Composite Materials Gmbh Multi-layered layer arrangement for a solid electrolyte
FR3039652A1 (fr) * 2015-07-29 2017-02-03 Bosch Gmbh Robert Element de capteur pour saisir au moins une propriete d'un gaz de mesure dans un espace de gaz de mesure et son procede de fabrication

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