Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-01-20
2001-12-25
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S296000, C429S006000, C429S006000
Reexamination Certificate
active
06332964
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally solid electrolyte ion transport membranes and to methods for preparing such membranes and, more particularly, to such membranes having at least two continuous phases wherein one of the phases comprises an oxygen ion conductive material, or a mixed conductor, and wherein the second phase comprises an electronically-conductive metal and occupies a minor percentage of the membrane volume. The second phase can be incorporated into the membrane by deposition of the metal from a chelated metal dispersion in an organic polymer. Alternately the membrane is composed of a powder containing a mixture of the first phase material and the second material and a second powder containing the first phase material only. The objective of the invention is to maximize the volume of the ionic phase for high ion transport and minimize the volume of the electronic conducting phase to levels below those achieved by the prior art while retaining continuity in both phases. The resultant structure comprises two interpenetrating continuous networks, one network for oxygen ion transport and one network for electron transport. The invention is useful in fabricating ion transport membranes having porous catalytic surface exchange enhancements, and for making electrodes for solid oxide fuel cells.
BACKGROUND OF THE INVENTION
Solid electrolyte ion transport membranes have significant potential for the separation of oxygen from gas streams containing oxygen. Of particular interest are mixed conductor materials that conduct both oxygen ions and electrons and hence can be operated in a pressure driven mode without the use of external electrodes.
In an ionic or mixed conducting membrane reactor, a solid electrolyte membrane that can conduct oxygen ions with infinite selectivity is disposed between an oxygen-containing feed stream and an oxygen-consuming, typically methane-containing, product or purge stream. The membrane elements have “oxygen selectivity,” which means that oxygen ions are exclusively transported across the membrane without transport of other elements, and ions of other elements. Such membranes may also be used in gas purification applications as described in European Patent Application Publication No. 778,069 entitled “Reactive Purge for Solid Electrolyte Membrane Gas Separation,” issued to Prasad et al.
Composite ceramic mixed conductor membranes comprised of multi-phase mixtures of an oxygen ion conductive material and an electronically-conductive material are known. Exemplary multi-phase ceramic compositions of this type are disclosed in U.S. Pat. No. 5,306,411 (Mazanec et al.) and U.S. Pat. No. 5,478,444 (Liu et al.). Such compositions are also taught by C. S. Chen et al. in
Microstructural Development, Electrical Properties and Oxygen Permeation of Zirconia-Palladium Composites,
Solid State Ionics 76: 23-28 (1995). These patents and this technical journal article are all incorporated herein by reference in their entireties. In order to develop a membrane suitable for use in pressure driven oxygen separation, an electronic conductivity characteristic has to be added to pure ionic conductors, thereby creating multiphase mixed conductors. This is typically accomplished by adding an electronically-conductive phase, such as Pt or Pd, to the ionic conductor in volume percentages above the percolation limit (typically greater than 30%) to obtain a continuous electronically conducting phase that exists as a continuous interpenetrating network with the oxide ion conducting phase.
In contrast to multi-phase mixed conductors, true mixed conductors, which are exemplified by perovskites such as La
0.2
Sr
0.8
CoO
x
, La
0.2
FeO
x
, La
0.2
Sr
0.8
Fe
0.8
Co
0.1
Cr
0.1
O
x
and others, are materials that possess intrinsic conductivity for both electrons and ions in a single phase. Some of these materials possess some of the highest oxygen ion conductivities known, as well as rapid surface exchange kinetics. U.S. Pat. No. 5,702,999 (Mazanec et al.) and U.S. Pat. No. 5,712,220 (Carolan, et al.) disclose mixed oxide perovskites of this type that are useful for oxygen separation. However, while there is great potential for these materials in gas separation applications, there are some drawbacks in their use.
A common problem among most ceramic mixed conductors, including perovskites, is their fragility and low mechanical strength in tension, which makes it difficult to fabricate large elements, such as tubes, and deploy them in commercial systems requiring high reliability. These problems have been recognized and reported in technical journal publications, such as, for instance, Yamamoto et al. in Perovskite-Type Oxides as Oxygen Electrodes for High Temperature Oxide Fuel Cells, Solid State Ionics 22: 241-46 (1987); and B. Fu et al. in (Y
1−x
Ca
x
) FeO
3
: A Potential Cathode Material for Solid Oxide Fuel Cells, Proc. 3rd Intl. Symp. on Solid Oxide Fuel Cells, S. C. Singhal, Ed., The Electrochem. Soc. Vol. 93-4: 276-282 (1993).
U.S. Pat. No. 5,911,860 discloses dual phase solid electrolyte ion transport materials comprised of a mixed conductor such as perovskite and a second phase such as Ag, Pd or an Ag/Pd alloy. This patent discloses that the introduction of a metallic second phase to a ceramic mixed or pure ion conductor such as perovskite prevents microcracking during fabrication of the membrane, and enhances the mechanical properties and/or surface exchange rates, as compared to those provided by a mixed conductor phase alone.
The introduction of a metallic second phase into ceramic mixed conductors is thus desirable for solid electrolyte ion transport membrane manufacture, not only for ceramic conductors, where the metallic phase is needed to achieve electronic conductivity, but also for true mixed conductors such as perovskites, where the metallic phase enhances mechanical properties and/or catalytic performance, as well as possibly enhancing the desired electronic conductivity. The most common technique disclosed in the prior art for introducing a metallic second phase into a solid electrolyte ion transport membrane is powder mixing. Illustrative of powder mixing techniques are the following patents:
(A) U.S. Pat. No. 5,306,411 (Mazanec et al.) discloses a typical powder mixing process to fabricate solid electrolyte ion transport membranes comprising gas impervious, multi-phase mixtures of an electronically-conductive material and an ion-conductive material and/or gas impervious, single phase mixed metal oxides of a perovskite structure. A mixture of La (C
2
H
3
O
2
)
3
.1.5H
2
O, Sr(C
2
H
3
O
2
)
2
and Co
3
O
4
was placed into a polyethylene jar mill, together with ZrO
2
media and acetone, and rolled for 70 hours. The resulting slurry was decanted and vacuum distilled at room temperature until dry. The solids were then calcined in air in an evaporating dish for 12 hours at 900° C. and 6 hours at 1100° C.
(B) U.S. Pat. No. 5,712,220 (Carolan et al.), discloses a membrane containing a dense multicomponent metallic oxide layer formed from La
0.2
Ba
0.8
Co
0.62
Fe
0.21
O
3−z
. This composition was prepared by a powder preparation technique wherein various applicable weighed quantities of La
2
O
3
, BaCo
3
, CoO, Fe
2
O
3
and CuO were mixed and ball milled for 12 hours. The mixture was then fired in air to 1000° C. for 24 hours followed by cooling to room temperature. The mixture was then ground by ball milling, remixed and refired. The resulting perovskite powder was milled in air to about 1-5 micron particle sizes and combined with a plasticizer, binder and toluene solvent to form a slip, suitable for tape casting.
(C) U.S. Pat. No. 5,624,542 (Shen et al.) is primarily concerned with improving the mechanical strength of an ion conducting dense membrane by inclusion of a second metallic phase within the matrix described therein. The patent's independent claims disclose continuous electronic conductivity for a range of volume percent of the electronic conducting metal phase within the range of between 10 and 50 percent. However, t
Besecker Charles J.
Chen Chieh Cheng
Mazanec Terry J.
Prasad Ravi
Bell Bruce F.
Praxair Technology Inc.
Rosenblum David M.
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