Field of the Invention

[0001] The present invention relates to a composite oxygen ion transport membrane in which a dense layer having electronic and ionic conducting phases is supported on a porous support layer. More particularly, the present invention relates to such a composite oxygen ion transport membrane in which the electronic phase is a cobalt containing perovskite-like metallic oxide, the ionic phase is a stabilized zirconia and the porous support layer is formed of a partially stabilized zirconia.

Background

[0002] Composite oxygen ion transport membranes have been proposed for a variety of uses that involve the production of essentially pure oxygen by separation of oxygen from an oxygen containing feed through oxygen ion transport through such membrane. For example, each membrane can be used in combustion devices to support oxy-fuel combustion or for partial oxidation reactions involving the production of a synthesis gas.

[0003] In such membranes, the oxygen ion transport principally occurs within a dense layer that allows both oxygen ions and electrons transport at elevated temperatures. The oxygen from an oxygen containing feed ionizes on one surface of the membrane and the resultant oxygen ions are driven through the dense layer and emerge on the opposite side thereof to recombine into elemental oxygen. In the recombination, electrons are liberated and are transported back through the membrane to ionize the oxygen.

[0004] Such membranes can employ two phases, an ionic phase to conduct the oxygen ions and an electronic phase to conduct the electrons. In order to minimize the resistance of the membrane to the ionic transport, such membranes are made as thin as practical and are supported on porous support layers. The resulting composite oxygen transport membrane can be fabricated as a planar element or as a tube in which the dense layer is situated either on the inside or the outside of the tube. [0005] An example of a composite oxygen ion transport membrane is disclosed in U.S. 5,240,480 that has a dense layer supported on two porous layers. The dense layer can be formed of an ionic conducting phase that contains yttrium stabilized zirconia and an electronic conducting phase that is formed from platinum or another noble metal. The porous layer adjacent to the dense layer is active and is capable of conducting oxygen ions and electrons. The other porous layer can be yttrium stabilized zirconia or calcium-stabilized zirconia.

[0006] U.S. 5,478,444 discloses a two-phase material capable of transporting oxygen ions and electrons. The oxygen ion conducting phase can be a metallic cerium oxide incorporating an yttrium stabilizer and a dopant that can be iron or cobalt. The electronic conducting phase is a perovskite that contains lanthanum, strontium, magnesium and cobalt or lanthanum, strontium cobalt and iron.

[0007] U.S. 5,306,411 discloses a dual-phase membrane having an ionic conducting phase formed from Sc ₂ 0 ₃ -stabilized zirconia. The electronic conducting phase can be a perovskite material containing, for example lanthanum, strontium, irons, chromium and cobalt. The resultant dense layer can be supported on an yttria-stabilized zirconia.

[0008] U.S. 7,556,676 discloses a dual-phase membrane having an ionic conducting fluorite phase formed of Sc-doped zirconia and an electronic conducting perovskite phase containing lanthanum, strontium, chromium, iron, and a small amount of vanadium. The dense membrane is supported on a thick 3 mol% yttria-stabilized zirconia (3YSZ) substrate. To densify the vanadium- containing perovskite, a reducing atmosphere of hydrogen and nitrogen must be used. The dense membrane also has two optional layers: a porous fuel oxidation layer to reduce the electrochemical resistance for fuel oxidation and a porous layer on the air side to facilitate oxygen reduction to oxygen ions. The main problems with this membrane are its low oxygen flux and fast degradation of oxygen flux during long-term operation. The low flux and fast degradation might be related to the membrane fabrication process in reducing environment under which the perovskite phase may react with zirconia to form an electrically insulating third phase and densification of both fuel oxidation and air reduction layers. [0009] To address these problems, U.S. 8,795,417 B2 discloses a dual-phase oxygen transport membrane consisting of a vanadium- free perovskite phase and Sc-doped zirconia phase supported on a thick 3YSZ substrate. The perovskite phase which contains lanthanum, strontium, chromium, iron and no vanadium is densified by sintering in air at temperatures from 1400 to 1430°C. The sintering process in air eliminates the formation of an electrically insulating third phase and reduces the fabrication cost. A porous fuel oxidation layer is formed from a calcium-containing perovskite and a doped zirconia. The fuel oxidation layer made of calcium-containing perovskites is more refractory and therefore, tends to have a more stable microstructure during high-temperature operation. However, one problem with this oxygen transport membrane is that the 3YSZ porous support after high temperature sintering experiences phase transformation from tetragonal to monoclinic when stored at room temperature in ambient air. The phase transformation is accompanied by about 5% volume increase and results in cracking of the porous support or delamination of the coating from the porous support.

[0010] As will be discussed the present invention provides a composite oxygen ion transport membrane that incorporates materials to enable fabrication to be accomplished in a more cost effective manner than in the prior art. Also, the present membrane is more durable than prior art membranes by avoiding the detrimental tetragonal-to-monoclinic phase transformation of the porous support. Furthermore, the materials used in all three active layers are similar in composition so the shrinkage of each layer is closely matched during membrane fabrication, results in minimal residual stress. The current oxygen transport membrane also exhibits improved oxygen flux and reduced degradation of oxygen flux during long term operation due to the inherent properties of the composition of the dense separation layer, fuel oxidation layer, and surface exchange layer and the fabrication process.

Summary of the Invention

[0011] The present invention may be characterized as a composite oxygen ion transport membrane comprising a dense separation layer having an electronic phase and an ionic phase and a porous support layer. The electronic phase and ionic phase in the dense separation layer exhibit a close thermal expansion match to each other and minimal chemical expansion so that the dense separation layer remains dense after multiple thermal cycles or after exposure to an oxygen chemical potential gradient. Furthermore, the dense separation layer and porous support layer exhibit a close thermal expansion match to each other and minimal chemical expansion so that the dense separation layer and porous support layer remain strongly adhered after multiple thermal cycles or after exposure to an oxygen chemical potential gradient. In addition, the porous support layer is formulated from a composition that does not undergo a phase transition at low temperatures during reactor cool-down or storage at ambient conditions. This eliminates the problem with prior art materials where a phase transition of the porous support layer, and concurrent volume expansion of the material, at ambient conditions causes the dense separation layer to delaminate from the porous support layer or causes the porous support layer to crack. The electronic phase of the dense layer comprises 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; whereas the ionic phase of the dense layer comprises Zri_ _X' Sc _x A _y <02-g, 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. The porous support layer is formed of Zri_ _Χ" Α _Χ "θ2-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.

[0012] There are many advantages of the materials used in the present invention over the prior art. A principal advantage of the present invention is that all the perovskite materials used in the active layers have very similar compositions and their thermal expansion is closely matched to that of the ionic conducting phase. Furthermore, all materials have limited chemical expansion and this is particularly important for the perovskite chosen for the electronic phase of the dense layer. In this regard, the use of such perovskite is particularly

advantageous as opposed to a metal in that a noble metal would have to be used to prevent oxidation. The obvious problem with the use of a noble metal is one of expense. Furthermore, the support is particularly robust due to the use of a partially stabilized zirconia composition that does not experience a phase transition from tetragonal-to-monoclinic at ambient temperature.

[0013] In some embodiments of the present invention, a porous intermediate layer can be provided between the dense layer and the porous support layer. Such porous intermediate layer can also be comprised of the electronic phase and the ionic phase similar to that of the dense layer.

[0014] Furthermore, a surface exchange layer, overlying the dense layer can be provided 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. The electronic conductor of the surface exchange layer preferably comprises (Lai_ _X "'Sr _X"' ) _W'" Cri_ y _'" - _Z"' Fey"'Co _Z'" 0 ₃ -s _, 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 renders the compound charge neutral. The ionic conductor of the surface exchange layer further comprises where x ^lv is from about 0.1 to about 0.22, y ^lv is from about 0.01 to about 0.04, and A is Y or Ce.

[0015] Preferably, the ionic phase constitutes from about 35 percent to 65 percent by volume of each of the dense layer and the intermediate porous layer, with the remainder comprising the electronic phase materials. More preferably, the ionic phase constitutes from about 50 percent to 60 percent by volume of the dense layer and about 40 percent by volume of the intermediate porous layer. Similarly, the ionic conductor comprises from about 35 percent to 65 percent by volume of the surface exchange layer, and more preferably about 40 percent by volume of the surface exchange layer.

[0016] In selected embodiments, an electronic phase of the dense layer is (Lao.8Sro. ₂ )o.95Cro.7Feo. ₂ 5Coo.o503-8 while the ionic phase is Zr ₀ .8o ₂ Sco.i8oYo.oi80 _2-8 . The porous support layer is preferably formed of Zr ₀ .923Yo.o7702-6. The surface exchange layer, if used, preferably includes an ionic conductor comprising Zr ₀ .809Sco.i82Ceo.oo902-8 and an electronic conductor comprising

(Lao. ₈ Sro.2)o. ₉ 5Cro.7Feo.25Coo.o50 ₃ _s. In a particularly preferred embodiment of the present invention, the 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. In one embodiment, the porous support layer can have 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 can have 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.

[0017] It is to be noted, that as used herein and in the claims, the term "pore size" means average pore diameter as determined by quantitative stereological line intersection analysis. In addition, the term "dense" layer means a layer in which the ceramic layer has no connected through porosity.

Brief Description of the Drawing

[0018] Figure 1 is a SEM micrograph of a cross-section of a composite oxygen ion transport membrane of the present invention.

Detailed Description

[0019] 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.

[0020] 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 (Lai _ _x Sr _x ) _w Cri_ _y _ _z Fe _y Co _z 0 ₃ -s _; 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 _1-X ' _-y 'SCxAy'0 _2- 8, 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 ΖΓ ₁ _ _χ »Α _χ ^» θ2- _δ , where x" is from about 0.05 to about 0.13, A is Y or Sc or Al or Ce or mixtures thereof. [0021] 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.

[0022] 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 _1-X'" Sr _X'" ) _W'" Cr _1- y'" _-Z '"Fey'"Co _Z "O3-8, 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 Zri_ _x ^lv _ _y ^lv Sc _x ^lv A _y ^lv 0 ₂ -8, where x ^lv is from about 0.1 to about 0.22, y ^lv is from about 0.01 to about 0.04 and A is Y or Ce or Al or mixtures thereof.

[0023] In a particularly preferred embodiment of the present invention, the dense layer 10 incorporates an electronic phase composed of

(La ₀ .8Sro.2)o.95Cro.7Feo.25Coo.o503_8 and an ionic phase composed of

Zr ₀ .802Sco.i8oYo.oi802-8. In such embodiment, the porous support layer 12 is formed of ΖΓο.923Υο.θ77θ2-δ and the surface exchange layer incorporates an ionic conductor composed of Zro.sogSco.mCeo.ocsC -e and an electronic conductor composed of (La ₀ . ₈ Sro.2)o. ₉ 5Cro.7Feo.25Coo.o50 ₃ _s. 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.

[0024] 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 ΖΓο. ₉ 2 ₃ Υο. _θ 77θ2-δ (hereinafter, 4YSZ) The particle size of such powder is dso = 0.6μιη (about a 50 percentile of the particles have a particle size of below 0.6μιη.) The powder is then mixed with carbon black having a particle size of a dso from about 0.1 to about Ιμιη and Poly(methyl methacrylate) (PMMA) having a particle size of a d5o of about 1.5um. 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.

[0025] 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. [0026] 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 ₀ . ₈ Sro.2)o. ₉ 5Cro.7Feo.25Coo.o50 ₃ _s (LSCrFCo) and Zr ₀ .802Sco.i8oYo.oi802-8 (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 dso of about 0.3μιη). About 4.5 grams of carbon black having a particle size of about dso of 0.8μιη 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.

[0027] 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 dso of about 0.3 μιη. 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.

[0028] 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:

Cv

0.471N _2Pl where q is the flow rate, N ₂ is a constant, p _t is the inlet pressure, G _g is the gas specific gravity, and T _t is the absolute upstream temperature. The Cv of the sintered tube should not exceed 1.5 x 10 ^~5 .

[0029] 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 dso of about 0.4μιη. 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.

[0030] 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.

[0031] 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.

[0032] 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.