Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-07-17
2002-11-05
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000, C429S047000, C429S047000
Reexamination Certificate
active
06475657
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of solid electrochemistry.
The elementary electrochemical cell used for separating oxygen from the air or from a gas mixture containing it generally consists of a ternary system comprising solid electrolyte/electrodes/current collectors.
The solid electrolytes used for separating oxygen from a gas mixture are doped ceramic oxides which, at the operating temperature, are in the form of a crystal lattice having oxide ion vacancies. The associated crystal structures may, for example, be fluorite, perovskite or brown-millerite cubic phases called Aurivillius phases; J. C. Boivin and G. Mairesse have referred to all the crystal phases which are O
2−
anionic conductors in a general article (Chem. Mat., 1998, pp 2870-2888; “Recent Material Developments in Fast Oxide Ion Conductors”).
The electrode materials associated with the solid electrolyte are generally perovskites. These are materials having a crystal structure based on the structure of natural perovskite, CaTiO
3
, and exhibit good hybrid (ionic and electronic) conductivity properties by virtue of this cubic crystal structure in which the metal ions are located at the corners and at the center of an elementary cube and the oxygen ions are in the middle of the edges of this cube. The electrode materials may also be mixtures of perovskite materials and of a purely ionic conductor or else mixtures based on materials having other crystal phases, for example of the Aurivillius, brown-millerite or pyrochlore type.
The current collecting is provided either by a metal or a metal lacquer or by a metal/“inert oxide” ceramic (such as alumina) mixture, by a metal/carbide (such as silicon carbide) mixture or by a metal
itride (such as silicon nitride) mixture, in which the main role of the oxide, the carbide or the nitride is to mechanically block the segregation/sintering phenomena that occur because of the high operating temperatures (700° C.<T<900° C.), especially when silver is used as the current collector metal, or by a metal/“hybrid conductor” oxide ceramic (such as an oxide having a perovskite structure of the family of strontium-doped lanthanum manganites) mixture or by a metal/“ion conductor” oxide ceramic (such as yttrium-stabilized zirconia) mixture.
However, the Applicant has found that when a tubular electrochemical cell in which the solid electrolyte is zirconium oxide stabilized with 8% yttria (8% YSZ), the electrodes are made of La
0.9
Sr
0.1
MnO
3−&dgr;
(LSM) and the current collectors are a silver lacquer is operated at a temperature of between 700 and 900° C., whether at atmospheric pressure or at an internal oxygen pressure of 20×10
5
Pa (120 bars) or as at an external oxygen pressure of 120×10
5
Pa (120 bar), accelerated ageing of this cell is observed, resulting in a 70% increase in the cell voltage in 40 h of operation; by replacing the current collectors made of silver lacquer with current collectors made of 50/50 vol % Ag/(8% YSZ) or 50/50 vol % Ag/LSM “cermets” or metal/ceramic mixtures, the ageing is slowed down. However, the degradation phenomenon is not completely eliminated since a 6-15% increase in the total voltage is observed for 100 h of operation. When the cell is operated with an internal oxygen pressure of 20×10
5
Pa (20 bar) at 780° C., a reduction in the coulombic efficiency and a drop in the voltage are also observed.
In the case of current collectors based on silver lacquer, it has been possible to attribute the ageing (with 1<P<20×10
5
Pa) and the drop in coulombic efficiency at high pressure (P>20×10
5
Pa) and at high temperature (800° C.) to three concomitant phenomena:
a silver sintering/segregation phenomenon for temperatures greater than 750° C.;
a silver evaporation phenomenon accentuated by the flushing of the cell with hot air, for temperatures greater than 700° C.; and
a silver diffusion phenomenon at pressure (20×10
5
Pa) through the solid electrolyte at high temperature (>780° C.).
L. S. Wang and S. A. Barnett have described the use of LaCoO
3
for covering stabilzied-zirconia-based cells which are covered with an Ag/YSZ mixture. This work has shown that, after 150 h at 750° C., the YSZ/Ag—YSZ(50/50)/LaCoO
3
layer (1 &mgr;m) system did not lose silver, unlike the system without the “protective” layer of LaCoO
3
for which there was, over time, segregation and loss of silver mass by evaporation. However, perovskite LaCoO
3
does not have good hybrid conductivity properties.
The Applicant has therefore sought a means of limiting, or indeed stopping, the degradation described above
SUMMARY OF THE INVENTION
This is why the subject of the invention is a ceramic membrane, which is an oxide ion conductor, characterized in that it comprises a non-zero volume, of non-zero total thickness E, of an assembly consisting of:
a) a dense layer, having opposed faces of areas S and S′ and having a non-zero thickness e, of a solid electrolyte having, at the electrolysis temperature, a crystal structure which is an oxide ion conductor;
b) two porous electrodes, which are hybrid conductors and have non-zero thicknesses e
1
and e′
1
, which are identical or different, coated on non-zero areas s
1
and s′
1
, which are identical or different, of the two opposed faces of areas S and S′ of the said solid electrolyte;
c) two porous current collectors, of non-zero thicknesses e
2
and e′
2
, which are identical or different, coated on non-zero areas s
2
and s′
2
, which are identical or different, of the said two porous electrodes; and
d) at least one porous covering layer, of non-zero thickness e
3
, coated on a non-zero area s
3
, of at least one of the said collectors, made of a material, or of a mixture of materials, which is chemically compatible with the materials, or the mixture of materials, of the said electrodes, the said collectors and the said solid electrolyte, and the sintering temperature of which is very close to the sintering temperatures of the materials, or of the mixtures of materials, of which the said electrodes, the said collectors and the said solid electrolyte are composed, and characterized in that the thickness E of the said membrane is equal to the sum of the thicknesses of each of the elements mentioned.
The expression “crystal structure which is an oxide ion conductor” should be understood within the context of the present invention to mean any crystal structure which, at the operating temperature, is in the form of a crystal lattice having oxide ion vacancies. The associated crystal structures may, for example, be fluorite, perovskite, brown-millerite cubic phases called Aurivillius phases or else those mentioned in: J. C. Boivin and G. Mairesse, Chem. Mat., 1998, pp 2870-2888; “Recent Material Developments in Fast Oxide Ion Conductors”.
The expression “material or mixture of materials, which is chemically compatible with that of the current collector or collectors” should be understood in the present description to mean any material or mixture of materials which, at a sintering temperature of between approximately 600° C. and 1000° C., does not undergo any chemical reaction with that material or those materials of the layer which it covers, namely in the present case, the material or mixture of materials of which the current collector(s) is(are) composed. Such a chemical reaction would possibly be revealed by the appearance of one or more chemical compounds absent in the initial materials or mixtures of materials.
The expression “porous layers” means, in the present description, that the layers of materials in question must be capable of allowing dioxygen to diffuse. More generally, their porosity index is between 10% and 70%, more precisely between 30 and 60%.
The expression “hybrid conductors” in the present description means that the layers of materials in question are both ion and electron conductors.
The expression “very similar sintering temperatures” means that the difference between the sintering te
Del Gallo Pascal
Gouriou Guylaine
Chaney Carol
L'Air Liquide, Societe Anonyme a Directoire et Conseil de S
Young & Thompson
Yuan Dah-Wein D.
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