Composite oxygen ion transport membrane

Abstract

A composite oxygen ion transport membrane having a dense layer, a porous support layer, an optional intermediate porous layer located between the porous support layer and the dense layer and an optional surface exchange layer, overlying the dense layer. The dense layer has electronic and ionic phases. The ionic phase is composed of scandia doped, yttrium or cerium stabilized zirconia. The electronic phase is composed of a metallic oxide containing lanthanum, strontium, chromium, iron and cobalt. The porous support layer is composed of zirconia partially stabilized with yttrium, scandium, aluminum or cerium or mixtures thereof. The intermediate porous layer, if used, contains the same ionic and electronic phases as the dense layer. The surface exchange layer is formed of an electronic phase of a metallic oxide of lanthanum and strontium that also contains chromium, iron and cobalt and an ionic phase of scandia doped zirconia stabilized with yttrium or cerium.

Claims

1. A composite oxygen ion transport membrane comprising: a dense layer having an electronic phase and an ionic phase, wherein the electronic phase comprising (La.sub.1-xSr.sub.x).sub.wCr.sub.1-y-zFe.sub.yCo.sub.zO.sub.3-δ, where x is from about 0.1 to about 0.3, w is from about 0.93 to about 1.0, y is from about 0.15 to about 0.45, z is from about 0.03 to about 0.1, and δ renders the compound charge neutral; and wherein the ionic phase comprises Zr.sub.1-x′Sc.sub.x′A.sub.y′O.sub.2-δ, where x′ is from about 0.1 to about 0.22, y′ is from about 0.01 to about 0.04, and A is Y or Ce or mixtures of Y and Ce; and a porous support layer formed of Zr.sub.1-x″A.sub.x″O.sub.2-δ, where x″ is from about 0.05 to about 0.13, A is Y or Sc or Al or Ce or mixtures of Y, Sc, Al, and Ce.

2. The composite ion transport membrane of claim 1, further comprising a porous intermediate layer between the dense layer and the porous support layer wherein the porous intermediate layer is comprised of the electronic phase and the ionic phase.

3. The composite ion transport membrane of claim 2, further comprising: a surface exchange layer overlying the dense layer so that the dense layer is located between the surface exchange layer and the porous intermediate layer and wherein the surface exchange layer comprises an electronic conductor and an ionic conductor; wherein the electronic conductor of the surface exchange layer further comprises (La.sub.1-x′″Sr.sub.x′″).sub.w′″Cr.sub.1-y′″-z′″Fe.sub.y′″Co.sub.z′″O.sub.3-δ, where x′″ is from about 0.1 to about 0.3, w′″ is from about 0.93 to about 1, y′″ is from about 0.25 to about 0.45, z′″ is from about 0.03 to about 0.1, and δ of said electronic conductor renders the compound charge neutral; and wherein the ionic conductor of the surface exchange layer further comprises Zr.sub.1-x.sup.ivSc.sub.x.sup.ivA.sub.y.sup.ivO.sub.2-δ, where x.sup.iv is from about 0.1 to about 0.22, y.sup.iv is from about 0.01 to about 0.04, and A is Y or Ce, and δ of said ionic conductor renders the compound charge neutral.

4. The composite ion transport membrane of claim 3, wherein: the ionic phase of the dense layer constitutes from about 35 percent to about 65 percent by volume of the dense layer; the ionic phase of the intermediate porous layer constitutes from about 35 percent to about 65 percent by volume of the intermediate porous layer; and the ionic conductor of the surface exchange layer constitutes from about 35 percent to about 65 percent by volume of the surface exchange layer.

5. The composite ion transport membrane of claim 4, wherein: the ionic phase of the dense layer constitutes from about 50 percent to about 65% by volume of the dense layer; the ionic phase of the intermediate porous layer constitutes between about 40 percent to 60% by volume of the intermediate porous layer; and the ionic conductor of the surface exchange layer constitutes about 40 percent by volume of the surface exchange layer.

6. The composite ion transport membrane of claim 1, wherein: the electronic phase of the dense layer is (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.25Co.sub.0.05O.sub.3-δ; and the ionic phase of the dense layer is Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-δ.

7. The composite ion transport membrane of claim 2, wherein: the electronic phase of the dense layer and a porous intermediate layer is (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.25CO.sub.0.05O.sub.3-δ; and the ionic phase of the dense layer and the porous intermediate layer is Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-δ.

8. The composite ion transport membrane of claim 1 wherein said porous support layer further comprises Zr.sub.0.923Y.sub.0.077O.sub.2-δ.

9. The composite ion transport membrane of claim 5, wherein: the electronic phase of the dense layer and the porous intermediate layer is (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.25Co.sub.0.05O.sub.3-δ; the ionic phase of the dense layer and the porous intermediate layer is Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-δ; the porous support layer further comprises Zr.sub.0.923Y.sub.0.077O.sub.2-δ; the electronic conductor of surface exchange layer is (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.25Co.sub.0.05O.sub.3-δ; and the ionic conductor of surface exchange layer is Zr.sub.0.809 Sc.sub.0.182Ce.sub.0.009O.sub.2-δ.

10. The composite ion transport membrane of claim 1, wherein: a porous intermediate layer has a thickness from about 10 microns to about 30 microns, an average pore size from about 0.1 microns to about 1 micron, and a porosity from about 25 percent to about 50 percent; the porous support layer has a thickness from about 0.7 mm to about 2.5 mm, an average pore size from about 0.5 microns to about 3 microns, and a porosity from about 25 percent to about 50 percent; and a surface exchange layer has a thickness from about 10 microns to about 25 microns, an average pore size from about 0.1 microns to about 1 micron, and a porosity from about 25 percent to about 50 percent.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a SEM micrograph of a cross-section of a composite oxygen ion transport membrane of the present invention.

DETAILED DESCRIPTION

(2) With reference to the sole FIGURE, an oxygen ion transport membrane 1 of the present invention is illustrated. Oxygen ion transport membrane 1 has a dense layer 10 supported on a porous support 12. Optional intermediate porous layer 14 and a surface exchange layer 16 can be provided.

(3) Dense layer 10 functions to separate oxygen from an oxygen containing feed exposed to one surface of the oxygen ion transport membrane 10 and contains an electronic and ionic conducting phases. As discussed above, the electronic phase of (La.sub.1-xSr.sub.x).sub.wCr.sub.1-y-zFe.sub.yCo.sub.zO.sub.3-δ, where x is from about 0.1 to about 0.3 and w is from about 0.93 to about 1 and y is from about 0.15 to about 0.45, z is from about 0.03 to about 0.1. The ionic phase is Zr.sub.1-x′-y′Sc.sub.x′A.sub.y′O.sub.2-δ, where x′ is from about 0.1 to about 0.22, y′ is from about 0.01 to about 0.04 and A is Y or Ce or a mixture of Y and Ce. The porous support layer 12 is formed of Zr.sub.1-x″A.sub.x″O.sub.2-δ, where x″ is from about 0.05 to about 0.13, A is Y or Sc or Al or Ce or mixtures thereof.

(4) Oxygen ion transport membrane 1 is specifically designed to be used in connection with oxy-fuel combustion applications as well as applications involving chemical reactions. The application of the present invention is not, however, limited to such uses. In applications involving fuel combustion, the use of intermediate porous layer 14 enhances the rate of fuel oxidation at that interface by providing a high surface area where fuel can react with oxygen or oxygen ions under the formation of partial or complete oxidation products. The oxygen ions diffuse through the mixed conducting matrix of this intermediate porous layer 14 towards the porous support 12 and react with the fuel that diffuses inward from the porous support 12 into this porous intermediate layer 14. Preferably, porous intermediate layer 14 is formed from the same electronic and ionic phases as dense layer 10.

(5) Any embodiment can incorporate advantageously a surface exchange layer 16 that overlies the dense layer opposite to the porous intermediate layer 14 if the same is used. Surface exchange layer 16 enhances the oxygen surface exchange rate by enhancing the surface area of the dense layer 10 while providing a path for the resulting oxygen ions to diffuse through the mixed conducting oxide phase to the dense layer 10 and for oxygen molecules to diffuse through the open pore space to the same. The surface exchange layer 16 therefore, reduces the loss of oxygen chemical potential driving force due to the surface exchange process and thereby increases the achievable oxygen flux. As indicated above, it can also be a two-phase mixture containing an electronic conductor composed of (La.sub.1-x′″Sr.sub.x′″).sub.w′″Cr.sub.1-y′″-z′″Fe.sub.y′″Co.sub.z′″O.sub.3-δ, where x′″ is from about 0.1 to about 0.3, w′″ is from about 0.93 to 1, y′″ is from about 0.15 to 0.45, z′″ is from about 0.03 to 0.15 and δ renders the compound charge neutral and an ionic conductor composed of Zr.sub.1-x.sup.iv.sub.-y.sup.ivSc.sub.x.sup.ivA.sub.y.sup.ivO.sub.2-δ, where x.sup.iv is from about 0.1 to about 0.22, y.sup.iv is from about 0.01 to about 0.04 and A is Y or Ce or Al or mixtures thereof.

(6) In a particularly preferred embodiment of the present invention, the dense layer 10 incorporates an electronic phase composed of (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.25Co.sub.0.05O.sub.3-δ and an ionic phase composed of Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-δ. In such embodiment, the porous support layer 12 is formed of Zr.sub.0.923Y.sub.0.077O.sub.2-δ and the surface exchange layer incorporates an ionic conductor composed of Zr.sub.0.809Sc.sub.0.182Ce.sub.0.009O.sub.2-δ and an electronic conductor composed of (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.25Co.sub.0.05O.sub.3-δ. Preferably, the porous intermediate layer 14 has a thickness from about 10 microns to about 30 microns, an average pore size from about 0.1 microns to about 1 microns and a first porosity from about 25 percent to about 50 percent. Porous support layer 12 has a thickness from about 0.7 mm to about 2.5 mm, an average pore size from about 0.5 microns to about 3 microns and a porosity from about 25 percent to about 50 percent. The surface exchange layer 16 has a thickness from about 10 microns to about 25 microns, an average pore size from about 0.1 microns to about 1 microns and a porosity from about 25 percent to about 50 percent.

(7) As an example of fabricating an oxygen transport membrane element of the present invention, a porous support layer 12 is first fabricated from yttrium stabilized zirconia powder having a chemical formula of Zr.sub.0.923Y.sub.0.077O.sub.2-δ (hereinafter, 4YSZ) The particle size of such powder is d.sub.50=0.6 μm (about a 50 percentile of the particles have a particle size of below 0.6 μm.) The powder is then mixed with carbon black having a particle size of a d.sub.50 from about 0.1 to about 1 μm and Poly(methyl methacrylate) (PMMA) having a particle size of a d.sub.50 of about 1.5 um. The mixture contains about 9 percent carbon black, 19 percent PMMA and a remainder of the yttrium stabilized zirconia powder. Binder is then added to the mixture of YSZ Powder, carbon black, and PMMA which is then poured into a high shear sigma blade mixer. Water and dispersant are then added while the mixing blades are rotating in order to form an extrudable paste.

(8) The paste is loaded into a ram extruder fitted with a die designed to form the desired tube size. The ram is moved forward and the paste is subjected to a pressure of about 1000 psi to form a green tube which exits the die. After the green tube is formed, the tube is placed on slowly rotating rollers and allowed to dry for 1-2 days. After drying, the tube is cut to size, drilled, and can then be fired from 950-1200° C. for 2-4 hours to achieve reasonable strength for further handling. After firing, the resulting tube can be checked for porosity, permeability/tortuosity and stored in a dry oven at about 60° C.

(9) After firing the green tube, intermediate porous layer 14 is then formed. A mixture of about 30 grams of powders having electronic and ionic phases with the chemical formulas, (La.sub.0.8Sr.sub.0.2).sub.0.95Cr.sub.0.7Fe.sub.0.25Co.sub.0.05O.sub.3-δ (LSCrFCo) and Zr.sub.0.802Sc.sub.0.180Y.sub.0.018O.sub.2-δ (YScZ), respectively, is prepared so that the mixture contains about 60% of LSCrFCo and about 40% of YScZ by volume. To the mixture, 25 grams of toluene, 5 grams of Ferro binder (Product ID B73210), 200 grams of 1.5 mm diameter YSZ grinding media are added. The mixture is then milled for about 6 hours to form a slurry having a particle size d.sub.50 of about 0.3 μm). About 4.5 grams of carbon black having a particle size of about d.sub.50 of 0.8 μm and 0.3 grams of surfactant KD-1 dissolved in 15 grams of toluene are then added to the slurry and milled for additional 2 hours. The slurry is then coated by meniscus coating with a coating speed of 6-10 meters per hour on the outer wall of the tube which is then fired at about 1200° C. for 4 hours in air.

(10) The dense layer 10 is then applied to the coated tube. A mixture weighing about 70 grams is prepared that contains the same powders as used in forming the intermediate porous layer 14, discussed above, except that the ratio between LSCrFCo and YScZ is about 50/50 by volume. To this mixture, 170 grams of toluene, 36 grams of the same Ferro binder mentioned above, 1100 grams of 1.5 mm diameter YSZ grinding media are added and the same is milled for about 24 hours to form a slurry having a particle size d.sub.50 of about 0.3 μm. The formed slurry is then applied as a coating on top of layer 14 also by meniscus coating process with similar coating speed. The tube is then stored dry prior to co-firing the layers 14 and 10 in a controlled environment, as described below.

(11) The coated tube is slowly heated in flowing nitrogen to about 1380° C. and held at the same temperature for about 6 hours for the cobalt containing electronic conducting perovskites to properly sinter. During sintering, the oxygen partial pressure of the atmosphere in the furnace is controlled below 20 Pa. The tube is then cooled in nitrogen to complete the sintering process. The sintered tube is checked for flow coefficient v, as defined below:

(12) $\mathrm{Cv}=\frac{q}{0.471N_2{p}_1\sqrt{\frac{1}{G_g{T}_1}}}$

(13) where q is the flow rate, N.sub.2 is a constant, p.sub.1 is the inlet pressure, G.sub.g is the gas specific gravity, and T.sub.1 is the absolute upstream temperature. The Cv of the sintered tube should not exceed 1.5×10.sup.−5.

(14) Surface exchange layer 16 is then applied. The surface exchange layer 16 has the same compositions and ratio of the electronic and ionic phases as the intermediate porous layer 14, mentioned above. To prepare the slurry, 80 grams of the electronic and ionic mixture, 28.8 grams of toluene, 19.2 grams of ethyl alcohol, 16 grams of the same Ferro binder mentioned above, 1.6 grams of surfactant KD-1, about 500 grams of 1.5 mm diameter YSZ grinding media are added and the resultant mixture is milled for about 2 hours to form a slurry having a particle size d.sub.50 of about 0.4 μm. About 12 grams of carbon black are added to the slurry and it is milled for additional 2 hours. The slurry is then applied as a coating on top of the sintered dense layer 10 again by meniscus coating with roughly the same coating speed. The coated tube is then dried and fired at 1250° C. for two hours in air. The Cv of the tube after cathode sintering is checked again to make sure no significant change has occurred.

(15) The resultant tubes have the preferred thickness, pore size and porosity within the ranges outlined above, namely, the porous intermediate layer 14 has a thickness of about 15 microns, an average pore size from about 0.1 microns to about 0.5 microns and a porosity from about 25 percent to about 50 percent. Porous support layer 12 has a thickness of about 1 mm, an average pore size from about 1 micron to about 3 microns and a porosity of about 35 percent. The surface exchange layer 16 has a thickness from about 10 microns to about 20 microns, an average pore size from about 0.1 microns to about 0.5 microns and a porosity from about 40 percent to about 60 percent. In one embodiment, dense layer 10 has a thickness from about 10 microns to 20 microns and no connected porosity.

(16) It is to be noted that in one embodiment of the present invention, the particle size of the chromite/zirconia slurry for deposition of the intermediate and dense separation layers 14 and 10 is preferably in a range from about 0.3 microns to about 0.35 microns. Although other particle sizes may be used, membranes fabricated from such slurries with particle sizes in the range from about 0.3 microns to about 0.35 microns indicated minimal reactivity between the two phases and with shrinkage matching the porous zirconia support.

(17) While the invention has been described with respect to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions may be made without departing from the spirit and scope of the present invention provided for in the appended claims.