EP1879685A2 - Ion conducting membranes for separation of molecules - Google Patents

Abstract

Bi-continuous membranes are provided in which one phase conducts a first type of ions and a second phase conducts a second type of ions. In some embodiments, a molten phase form one of the phase of the bi-continuous membrane and a solid phase forms one of th ephase of the bi-continuous membrane. The materials comprising the membrane are effective in separation and absorption technologies and are fabricated into a structured membrane in accordance with this invention. For example, to separate carbon dioxide, alkali metal carbonates, e.g., lithium carbonate, and solid oxides, e.g., zirconia, are suitable materials for the preparation of these types of membranes and can form CO2 selective and permeable layers.

Description

Ion Conducting Membranes for Separation of Molecules

Copyright Notice

[0001] This patent disclosure may contain material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves any and all copyright rights.

Incorporation by Reference

[0002] AU patents, patent applications and publications cited herein are hereby incorporated by reference in their entirety. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described herein.

Cross-reference to Related Applications

[0003] This application claims the benefit of U.S. Patent Application No. 60/672,399, filed on April 18, 2005, the content of which is hereby incorporated by reference herein in its entirety.

Field of the Invention

[0004] The present invention relates to carbon management, and to membranes, systems, and methods for the separation of molecules.

Background of the Invention

[0005] Numerous technologies for separation of molecules exist and are currently being sought after. For example, the importance of technologies for the separation of carbon dioxide (CO₂) from gaseous process streams has gained in importance, not the least because of concerns over greenhouse gas emission. In the future, power plants that generate electricity from coal, or other carbon-based fuels may have to separate CO₂ from the gas stream. Chemical routes to hydrogen production from coal and natural gas rely on CO₂ separation that can be greatly simplified with the availability of high temperature CO₂ selective membranes. Solid oxide fuel cell designs that operate on carbon monoxide or hydrogen and carbon monoxide mixtures can benefit from the ability to separate CO₂ from the exhaust stream.

[0006] Methods conventionally used to separate carbon dioxide from a gas stream include chemical absorption using amine-based solvents, or physical sorption using liquid or solid sorbents. However, these methods require that the gas mixture be at a temperature no higher than 100°C. Further, the energetic and economic penalties, incurred mostly from sorbent regeneration, are costly {see Hendricks, C.F., Carbon dioxide removal from coal-fired power plants. Energy and Environment. Vol. 1. 1994, Amsterdam: Kluwer Academic Publisher. Chapter 2, pg. 19-49). At elevated temperatures, solid chemical absorbers like lime, lithium zirconate, or lithium silicate have been proposed but they are complicated by slow kinetics and large material handling systems for solids {see White, CM., et al., Separation and Capture of CO2 from Large Stationary Sources and Sequestration in Geological Formations - Coalbeds and Deep Saline Aquifers. Journal of Air & Waste Management Association, 2003. 53: p. 645-715).

[0007] Polymer and inorganic microporous membranes for CO₂ separation exist, but so far are limited by low selectivity or permeability, and low temperature operation {see Shekhawat, D., D.R. Luebke, and H.W. Pennline, A Review of Carbon Dioxide Selective Membranes, 2003, Department of Energy, National Energy Technology Laboratory). Particularly in gasification based systems, it would be desirable to have CO₂ separation membranes that could operate in the temperature regime in which gasification or hot gas cleanup occurs. Attempts have been made to construct membranes from solid sorbents, such as lithium zirconate {see Kawamura, H., et al., Dual-Ion Conducting Lithium Zirconate- Based Membranes for High Temperature CO₂ Separation. Journal of Chemical Engineering of Japan, 2005. 38(5), p. 322-328). However, these efforts so far have failed to produce viable membranes. Dense, dual-phase metal-carbonate membranes that operate by transporting CO₂ across as a carbonate ion in a molten carbonate phase, with a counter- current of electrons transporting in a metallic phase have also been proposed to produced membranes capable of operating at about 450 - 650 °C {see Chung, S.J., et al., Dual-Phase Metal-Carbonate Membrane for High Temperature Carbon Dioxide Separation. Industrial & Engineering Chemistry Research, 2005, 44, p. 7999-8006; see also Granite, EJ. and T. O'Brien, Review of Novel Methods for Carbon Dioxide Separation from Flue and Fuel Gases, Fuel Processing Technology, 2005, 86, p. 1423-1434). This technology, however, is limited by the requirement of having oxygen in the feed stream to convert the CO₂ to CO₃ ^", thus making it impractical for separation in fuel streams.

[0008] Mixtures of lithium carbonate and zirconia are known to react under low partial pressures of CO₂ to form lithium zirconate upon releasing gas phase carbon dioxide as shown in Equation [1] below.

Li₂CO₃ (1) + ZrO₂ <-» Li₂ZrO₃ (s) + CO₂ (g) [1]

The reaction can be reversed and its use as a chemical CO₂ absorption technology has been demonstrated (see Nakagawa, K. and T. Ohashi, A Novel Method ofCO₂ Capture from High Temperature Gases. Journal of the Electrochemical Society, 1998. 145: p. 1344-1346.; Essaki, K.N., Kazuaki; Kato, Masahiro, Acceleration Effect of Ternary Carbonate on CO ₂ Absorption Rate in Lithium Zirconate Powder. Journal of the Ceramic Society of Japan, 2001. 109(10): p. 829-833; Ida, J.-I.L., And Y.S., Mechanism of High Temperature CO₂ Sorption on Lithium Zirconate. Environmental Science Technology, 2003. 37(9): p. 1999- 2004). Lithium zirconate has recently been investigated as a membrane structure {see Kawamura, H., et al., Dual-Ion Conducting Lithium Zirconate-Based Membranes for High Temperature CO₂ Separation. Journal of Chemical Engineering of Japan, 2005. 38(5): p. 322-328). However, selectivity of carbon dioxide over other gases was very poor (selectivity of carbon dioxide over methane was about 5).

[0009] In an economic evaluation of CO₂ removal from coal-fired flue gas streams, it has been estimated that in order to make membrane separation competitive with other carbon capture technologies, selectivity for CO2 over N₂ must preferably exceed 200, and permeance must preferably be at least 2.2 x 10^"7 mol m^"2 Pa^"1 s^"1 {see Hendricks, C.F., Carbon dioxide removal from coal-fired power plants. Energy and Environment. Vol. 1. 1994, Amsterdam: Kluwer Academic Publisher. Chapter 3, pg. 53-81).

[0010] Hence, membranes, systems, and methods for separating molecules such as CO₂ that overcomes the afore-mentioned limitations of conventional separating membranes are needed. Summary of the Invention

[0011] The present invention provides for membranes that can separate one or more types of molecules by transporting ionic species from one side of the membrane to the other side. In certain embodiments, the membrane allows a neutral molecule to undergo a chemical reaction on one side of the membrane to dissociate into ions, be transported across the membrane as ions, and combine on the other side of the membrane as the neutral molecule. [0012] In certain embodiments, the membrane includes a first phase that form at least one continuous phase from one side of the membrane to another side of the membrane, and a second phase that form at least one continuous phase from one side of the membrane to another side of the membrane. The first phase can conduct a first type of ions and the second phase can conduct a second type of ions. For example, the first phase can conduct carbonate ions and the second phase can conduct oxide ions to separate carbonate dioxide. [0013] The present invention also provides membranes for use at high temperatures, e.g., at temperatures from about 200°C and higher, which allows the selective passage of carbon dioxide (CO₂). For example, the novel membranes may allow selective passage OfCO₂ as compared to molecular oxygen, nitrogen, carbon monoxide, water, methane, and the like. Carbon dioxide can selectively permeate the membrane encompassed by this invention at temperatures of about 200°C to about 1200°C, or about 400°C to about 1000°C, thus allowing many new technologies to employ the membrane.

[0014] It is an aspect of this invention to provide at least a bi-continuous ceramic membrane conducive to the mobility of ions therein. In one phase, such a bi-continuous membrane conducts a first type of ions, while a second phase conducts a second type of ions. The membrane according to this invention is permeable and selective for one molecular species relative to other molecules. In other aspects, the membrane may contain more than two continuous phases. For example, the membrane may be comprised of three, four, five, or more different and continuous phases.

[0015] The present invention also relates to a method for producing a membrane for separating a molecular species from other molecules. The method includes tapecasting a suspension that has particles of a first phase and pore formers (particles used to control a porous structure) mixed within a solvent with dissolved binders, plasticizers and other additives into a desired shape to obtain a green body (a term used to denote a roughly held together object). The green body can then be sintered to obtain a porous continuous structure where the porous structure is capable of conducting a first type of ions. The porous structure can then be filled in with a second phase that is capable of conducting a second type of ions to obtain a membrane capable of separating a molecular species from other molecules. [0016] The present invention also relates to solid oxide fuel cells capable of operating under substantially steady state conditions. The fuel cells may be driven by a fuel gas containing CO, where the CO₂ membrane of the present invention separates CO₂ from a gas stream containing CO and CO₂.

[0017] Additional aspects, features and advantages afforded by the present invention will be apparent from the detailed description and exemplification hereinbelow.

Description of the Figures

[0018] Figure 1 is a schematic diagram of a carbon dioxide (CO₂) separating membrane according to the present invention.

[0019] Figure 2 is a schematic diagram exhibiting the structure of carbon dioxide (CO₂) separating membrane according to the present invention.

[0020] Figure 3 is a diagram of one exemplary method for fabricating the membrane of the invention using tape casting and dip coating techniques.

[0021] Figure 4 is a schematic diagram of carbon fueled solid oxide fuel cell (SOFC) utilizing a carbon gasification process and the CO₂ separation membrane of the present invention.

Description of the Invention

[0022] The present invention relates to membranes, methods, and systems for separating at least one molecule from other molecules through at least two ionically conducting phases. In certain embodiments, the membrane includes a first phase that form at least one continuous phase from one side of the membrane to another side of the membrane, and a second phase that form at least one continuous phase from one side of the membrane to another side of the membrane. The first phase can conduct a first type of ions and the second phase can conduct a second type of ions. The first type of ions can be conducting from one side of the membrane to the other side in parallel or in opposite directions from the second type of ions. [0023] In certain embodiments, the membrane allows a neutral molecule to undergo a chemical reaction on one side of the membrane and dissociate into ions, allows the ions to be transported across the membrane, and combine on the other side of the membrane as a neutral molecule.

[0024] In certain embodiments, the membrane carries a net zero electric current and can be driven by partial pressure differences of the neutral species from one side of the membrane to the other side of the membrane. [0025] In certain embodiments, membranes of the present invention may selectively separate a molecular species from other molecules and have a selectivity that is greater than or equal to 5. In other embodiments, a selectivity of the molecular species to other gases that is greater than or equal to 10, 25, 50, 100, 150, 200 or 500 can be possible. [0026] In certain embodiments, the present invention provides for a bi-continuous membrane that in one phase conducts a first type of ions and a second phase that conducts a second type of ions. A bi-continuous membrane, as used herein, is a membrane having at least two different and distinct continuous phases from one side of the membrane to the other side of the membrane. In one embodiment, a bi-continuous membrane is a membrane where there are passageways for two different charges to migrate through two different materials or phases. For example, the two materials or phases can be arranged in such a way that there are continuous paths connecting the two sides of the membrane in both a first phase and a second phase.

[0027] In certain embodiments, the present invention provides membranes having a continuous but porous first phase that is solid wherein the pores are filled with a second phase that is molten. In other embodiments, the present invention provides membranes having a continuous but porous first phase that is solid wherein the pores are filled with a second phase that under operating conditions becomes molten. In another embodiments, the present invention provides membranes having a continuous but porous solid first phasea wherein the pores are filled with a solid second phase.

[0028] The present invention also relates to nano-structured membranes having a porous first phase that is filled with a second phase. The nano-structure membrane can be highly permeable to a molecule to be separated, yet prevent passage of most other materials. [0029] In one embodiment, the invention provides for custom materials made from dual- ion conducting phases that are mechanically stable in the range of 200 to 1000⁰C (without open cracks or pores) and that prohibits indiscriminate gas flow. In another embodiment, the invention provides for membrane materials that have a substantial permeability to a gas to be separated.

[0030] The invention may allow for the separation of numerous molecules such as, but not limited to, carbon dioxide, sulfur oxides such as sulfur dioxide and sulfur trioxide, nitrates such as ammonium nitrate and potassium nitrate, chlorides such as hydrochloric acid, steam, and the like.

[0031] For example, to transport steam, a first phase can conduct OH ^" ions and a second phase can conduct O^2" ions, where the first phase can be a molten hydroxide phase and the second phase can be a solid oxide phase. In another example, steam can be transported across a solid electrolyte phase as H ions and as O ^" ion across a solid oxide phase. Other suitable H⁺ conductors may be polymer electrolyte membranes such as NAFION membranes and perovskites with large metal ions such as yttria doped BaCeO_3-(1, SrCeO_3-^, and BaZrO₃.a.

See Bredesen, R., K. Jordal, and O. Bolland, "High-temperature membranes in power generation with CO₂ capture," Chemical Engineering and Processing (2004), vol. 43(9), pp.

1129-1158.

[0032] hi another example, to transport hydrochloric acid, a first phase can conduct H⁺ ions and a second and third phase can cooperatively conduct Cl^" ions, where the first phase can be a solid electrolyte, a polymer electrolyte, or perovskites with metal ions, the second phase can a chloride phase, and the third phase can be an acid.

[0033] In yet another example, to transport ammonium, a first phase can conduct NH₄ ⁺ ions and a second phase can conduct H⁺ ions, where the first phase can be a molten ammonium phase and the second phase can be a solid electrolyte, a polymer electrolyte, or perovskites with metal ions.

[0034] In still another example, to transport SO₂, a first phase can conduct SO₃ ^" ions and a second phase can conduct O^2" ions, where the first phase can be a molten sulfite phase and the second phase can be a solid oxide phase.

[0035] hi a different example, to transport SO₃, a first phase can conduct SO₂ ^2" ions and a second phase can conduct O ^" ions, where the first phase can be a molten hyposulfite phase and the second phase can be a solid oxide phase. [0036] Moreover, to transport carbon dioxide, a first phase can conduct CO₃ ^2' ions and a second phase can conduct O^2" ions, where the first phase can be a molten carbonate phase and the second phase can be a solid oxide phase.

CO? Separation

[0037] In certain preferred embodiments, the invention relates to a membrane comprising a material that absorbs CO₂ on one side and desorbs CO₂ on the other side, hi other embodiments, the membrane can have a substantial and selective CO₂ permeability. In yet other embodiments, the membrane can be selectively permeable for CO₂ over other molecules such as oxygen, nitrogen, carbon monoxide, methane, water, and hydrogen, hi certain embodiments, membranes of the invention may selectively allow permeation of CO₂ and not other gases at temperatures in excess of 200 °C. For example, the membranes of the invention may be stable and have operating temperatures from about 300 °C, 400 ⁰C, or 500 °C to about 1000 ⁰C.

[0038] In certain embodiments, membranes of the present invention may selectively separate carbon dioxide from other gases and have a selectivity that is greater than or equal to 5. In other embodiments, a selectivity of carbon dioxide to other gases that is greater than or equal to 10, 25, 50, 100, 150, 200 or 500 maybe possible.

[0039] In certain embodiments, carbon dioxide can be shuttled across a membrane as carbonate ions with a counter-current of oxide anions, hi certain embodiments, carbonate ions travel through the membrane in a molten carbonate phase while the counter-current of oxide anions travel in a solid oxide phase, as schematically illustrated in Figure 1. In an embodiment, the net flux in the membrane is neutral transport of CO₂. [0040] As shown in Figure 1, the present invention affords a structured material in which CO₂ molecules are transferred across the membrane as carbonate ions. For example, regions or layers of molten carbonate 2 alternate with regions or layers of solid oxides 4. hi order to avoid charge build-up in the transport, the negative charge can be transported back through the solid oxide as oxide ions. The net transport is that of a neutral CO₂. Charged currents flow in this membrane, but they cancel each other out. For example, the CO₂ travels across the membrane by first being converted with an oxide ion into carbonate ions (CO₃ ^2") on the feed side 1 of the membrane for transport in the molten carbonate phase. Where the partial pressure of CO₂ is low (e.g., side 2 of Figure 1), the oxide ions (O ^") return through the solid oxide phase, by first releasing its CO₂ load. As shown, the present invention provides a way to combine the oxide ion mobility of a solid oxide and the carbonate ion mobility in molten carbonates into one structured material so that the net transport is that of neutral CO2. [0041] As shown in the two-dimensional schematic of Figure 2, the membrane structure contains a porous solid oxide phase that serves to not only complete the circuit of carbon dioxide transport without an addition external electromotive force (emf), but also provide mechanical support by immobilizing the molten carbonate phase within its porous structure. The membrane is continuous with respect to each phase independently within a three dimensional structure.

[0042] The driving force for the transport across such a membrane can be due to the partial pressure difference of the CO₂ on the two sides of the membrane. Hence, CO₂ can flow in either direction of the membrane and the actual flow will follow the pressure difference.

[0043] In certain embodiments, membranes of the invention have sandwich or fiber-like structures that contain parallel current paths for oxide ions in the solid oxide phase and for carbonate ions in the molten carbonate phase. Such a morphology may provide a high conductance and high selectivity for CO2. In a representative membrane of this invention, a continuous doped zirconia phase allows for the transport of oxide ions from one side of the membrane to the other; and a continous molten carbonate phase provides a pathway for the carbonate ion in the opposite direction (see Figure 1).

[0044] In certain embodiments, the present invention provides for a bi-continuous membrane that in one phase conducts oxide ions and a second phase conducts carbonate ions. A bi-continuous membrane, as used herein, is a membrane having at least two different and distinct continuous phases from one side of the membrane to the other side of the membrane. In one embodiment, a bi-continuous membrane is a membrane where there are passageways for two different charges to migrate through two different materials or phases. For example, the two materials or phases can be arranged in such a way that there are continuous paths connecting the two sides of the membrane in both a solid oxide and a carbonate phase. Additional examples include, but are not limited to, porous media with interconnected liquid- filled pores, solid blocks with straight, tubular channels crossing the solid block matrix like thin pipes, alternating sandwich-like layers, a "Plumber's Nightmare" surface, a gyroid structure, and the like. [0045] The invention also relates to a carbon dioxide gas-selective membrane comprising a bi-continuous membrane material stable at a temperature of greater than about 200°C and comprising a porous solid oxide material that in a temperature range of interest is capable of transporting oxide ions, and a molten carbonate phase completely filling the pores capable of conducting carbonate ions, wherein (i) a partial pressure difference of carbon dioxide (from one side of the membrane to the other) drives a flux of carbonate ions across the membrane and (ii) oxide ions return the charge in the solid oxide phase.

[0046] In certain embodiments, the invention relates to a carbon dioxide gas-selective membrane comprising at least a bi-continuous structure and having at least two separate ionically conducting phases, hi certain embodiments, CO₂ may be transported across the membrane in one of the phases as a carbonate ion, converted with an oxide ion. The oxide ion may either be dissolved within the molten carbonate or transferred directly from a lattice site at the boundary of a second phase into the carbonate phase. The dissolution of an oxide ion into the carbonate phase may result in a vacant lattice site in the solid oxide phase that is filled by the conductive oxide ions compensating for the carbonate flux as shown in Equation [2]

CO₂ + O_O ^X ^ CO₃ ²- + V₀" [2]

Such a vacant lattice site that is filled by conductive oxide ions eliminates the need for molecular oxygen to contact the membrane along with the carbon dioxide. [0047] As shown above, Kroger- Vink notation is used when reactions involving a crystal lattice are shown. (See Kittel, C, Introduction to Solid State Physics, 4^th ed. 1971, New York: Wiley p. 766). A vacant lattice site normally occupied by the ion is notated as a V. A doubly-positive charged lattice site is notated as, ^", a doubly-negative charged site (not shown) is notated as ", and a neutral site is notated as, ^x. Occupied lattice positions are considered neutral due to the equal charge balance of the neighboring counter ions. Moreover, as shown in Equation [2], diffusion of charged ionic species through the lattice occurs maintaining local charge neutrality within each phase independently. [0048] In certain embodiments, the solid oxide phase and the molten carbonate phase may react with each other to form a third phase. This third phase can be deposited at the boundary of the solid oxide and carbonate phase, but other arrangements can occur as well. For example, a bicontiiiuous membrane consisting of zirconia and lithium carbonate at low partial pressures of CO₂ over the system may release CO₂ and transform itself into a new phase of lithium zirconate as shown in Equation [3].

Li₂CO₃ + ZrO₂(S) <→ Li₂ZrO₃ + CO₂ [3]

Lithium and oxide ions may incorporate themselves into the zirconium (IV) oxide crystal structure and may create lithium zirconate at the boundary of the two layers. The mobility of lithium ions can help in transferring the oxide ion (that just released a CO₂) from the carbonate phase into the zirconia face by allowing charge neutrality conditions to be more easily maintained during such transfer. Similarly, on the other side, transfer of the oxide ion from the zirconia phase back into the carbonate phase may be facilitated as described above. [0049] In other embodiments, operation may occur in a regime where the pressure of CO₂ is high enough to prohibit decomposition of the carbonate phase and conversion into a zirconate or analogous phase, while still providing a substantial partial pressure gradient of CO₂ for reasonable flux across the membrane.

[0050] In other embodiments, less miscible carbonates, wherein migration of the cations into the solid oxide phase is hindered, can be used. For example, potassium carbonate or sodium carbonate may be utilized, hi such embodiments, surface treatments can be made to facilitate the transfer of the oxide ions between the oxide and the carbonate phases. For example, catalysts, such as platinum and nickel oxide, may be suspended on the solid oxide phase. Such surface treatments need not necessarily be limited to only non-miscible carbonates.

[0051] hi certain embodiments, the present invention provides membranes having a continuous but porous solid oxide structure wherein the pores are filled with a molten carbonate material. For example, the membrane can be comprised of a solid doped zirconia having pores that are filled with a molten carbonate material.

[0052] In other embodiments, the present invention provides membranes having a continuous but porous solid oxide structure wherein the pores are filled with a carbonate material that under operating conditions is molten. For example, the membrane can be comprised of a solid ziroconia having pores that are filled with a solid carbonate material that under operating conditions become molten.

[0053] hi an embodiment, the membrane material comprises doped zirconia and molten carbonates that are mechanically stable in the range of from about 200°C to about 1000 °C, or in the range of from about 300⁰C to about 1000⁰C, or from about 500⁰C to about 1000⁰C₃ and which are devoid of open cracks or pores that allow for indiscriminate gas flow. Mechanically, the membrane is also stable at room temperature. In one embodiment, the membrane will not conduct carbonate until the carbonate is molten. In another embodiment, the membrane may not begin to separate carbon dioxide until the solid oxide becomes conductive at elevated temperatures such as around 400 °C or higher. [0054] The present invention also relates to nano-structured solid oxide membranes with carbonate filled pore spaces that are highly permeable to CO₂, yet prevent passage of most other materials. In one embodiment, the invention provides methods for making materials that achieve a continous doped zirconia phase that allows for the transport of oxide ions from one side of the membrane to the other. The continuous doped zirconia phase may further have pores filled with carbonate material, such as lithium carbonate, forming a continous carbonate phase that allows for the transport of carbonate ion from one side of the membrane to the other.

[0055] In one embodiment, the invention provides for custom materials made from doped zirconia and molten carbonates that are mechanically stable in the range of 400 to 1000°C (without open cracks or pores) and that prohibits indiscriminate gas flow. In another embodiment, the invention provides for membrane materials that have a substantial CO2 permeability. As a yardstick to gauge permeability, current flow rates of solid oxide fuels cells and molten carbonate fuel cells can be used to compare with the membranes of the invention.

[0056] In an embodiment, the invention embraces two or more of the foregoing embodiments in combination. In these embodiments, the membrane of the present invention may operate over a wide range of temperatures and CO₂ pressures.

Material

[0057] As described above, the present invention provides a structured CO₂ membrane. The membrane can work with any materials where one phase is able to conduct oxide ions, the other phase is able to conduct carbonate ions, and where the two phases can be brought into sufficient contact with each other or through one or more interevening phases so that the charge built up on the phase that is capable of conducting carbonate ions can drive the current flow in the phase that is capable of conducting oxide ions. [0058] Some exemplary suitable materials for use in fabricating the membrane include, but are not limited to, a selective CO₂ permeable composition, which is stable and long- lasting under the conditions of preparation and operation at high temperatures. [0059] By choosing the materials of the two phases appropriately, one can adjust the operational temperature range of the membrane. Moreover, the materials comprising the structured CO₂ membranes of the invention may differ by the cation mixture in the carbonate phase and by admixtures to the solid oxide phase and the carbonate phase to enhance stability, ion permeability and selectivity of the membrane. The materials comprising the membrane can be fabricated into a structured membrane in accordance with this invention, and the net neutral carbon dioxide flux may be established using dynamic pressure measurements and gas chromatography. Use of other gases or an admixture of other gases can establish the selectivity of the membrane for carbon dioxide over a range of temperatures.

Molten Alkali Metal Carbonate

[0060] Materials suitable for the carbonate phase include, but are not limited to, alkali metal carbonates such as sodium, potassium, and lithium carbonates and eutectics or admixtures with each other. Other examples include eutectics of involving calcium, barium, or magnesium carbonates. The choice can depend on the pressure and temperature operating range of the membrane. For example, the carbonate can be molten and retain enough carbonate ions to remain conductive under the prevailing conditions, (i.e.. the carbonate dissociation equilibrium (CO₃ ^2" <→ CO₂ + O^2") will not be shifted far to the right). [0061] Materials with low melting points and high carbonate ion conductivities, such as binary and ternary mixtures of alkali metal carbonates, may be suitable for use as molten carbonate materials. For example, lithium and sodium mixtures having a conductivity of about 2.5 S/cm at 700°C and a eutectic melting point at 50FC may be suitable (see Table 1). Lithium is an effective ion for depressing a mixture melting temperature and for increasing conductivity due to its small size. Moreover, mixtures that do not form eutectics can also be used. For example, sodium and potassium mixtures alone do not form a eutectic mixture, but the melting point of the mixture (71O⁰C) may be depressed below those of the individual salts (Tm OfNa₂CO₃ = 851°C and Tm OfK₂CO₃ = 891°C). Table 1. Potential Alkali Metal Carbonate Salts.

A eutectic mixture is indicated with a star, *.

{see Selman, J.R. and H.C. Mara, Physical Chemistry and Electrochemistry of Alkali Carbonate Melts: With special reference to the molten-carbonate fuel cell, in Advances in Molten Salt Chemistry, G. Mamantov, J. Braunstein, and CB. Mamantov, Editors. 1981, Plenum Press: New York. p. 159-389).

[0062] It may be possible for certain alkali metal carbonates to be susceptible to formation of a third phase, such as a different solid metal oxide, zirconate, or equivalent salt formations, at the carbonate/oxide interface. For example, as described above, zirconia (ZrO₂) and lithium carbonate (Li₂CO₃) may be reacted together at elevated temperatures (e.g., about 700°C and higher) to form a solid lithium zirconate (Li₂ZrO₃) and CO₂ gas, as shown in Equation [1] above {see Ida, J.-I.L., And Y.S., Mechanism of High Temperature CO₂ Sorption on Lithium Zirconate. Environmental Science Technology, 2003, 37(9), p. 1999- 2004).

[0063] Furthermore, alikali metal carbonates may also be doped with low portions of alkali earth carbonates to help maintain the oxobasicity of the solvent, which reduces both decomposition and volatility {see Cassir, M. and C. Belhomme, Technological applications of molten salts: the case of the molten carbonate fuel cells. Plasmas & Ions, 1999(1): p. 3- 15).

[0064] Moreover, molten carbonate phase that can wet the solid oxide surface (~ 0° contact angle) may be utilized to provide sufficient capillary force to hold the molten carbonate salt in the pores of a solid oxide phase. The Young-Laplace equation (Equation [4]) relates capillary force to the pressure differential, ΔP, across a cylindrical pore, with pore radii, R, the liquid solid surface tension, γ, and contact angle, θ. ΔP = ² * ^r *^C0S^ [4]

R

As shown above, the maximum pore size at the edges of the membrane for a given pressure drop across the membrane can be calculated using the molten carbonate surface tension and contact angle between the molten carbonate and the oxide interface. The pore structure within the bulk of the material (i.e away from the edges) may behave differently than as described above.

[0065] Immobilizing the molten carbonate salt in the solid oxide porous matrix may further allow coupling of the flux of carbonate ions to oxide vacancies. Immobilization in a porous structure may also serve to depress the possibility of gas-phase species from simply diffusing through the molten phase (see Selman, J.R. and H.C. Mara, Physical Chemistry and Electrochemistry of Alkali Carbonate Melts: With special reference to the molten-carbonate fuel cell, in Advances in Molten Salt Chemistry, G. Mamantov, J. Braunstein, and CB. Mamantov, Editors. 1981, Plenum Press: New York. p. 159-389).

Solid Oxide

[0066] Various suitable solid oxides can be mentioned. Suitable solid oxides can provide pathways for an oxide counter current as well as the structural support of the membrane. Solid oxide that have at least 0.01 S/cm conductivity at operating temperatures ranging from 600-900°C may be suitable. Solid oxides can have resistance to chemical attack by the impregnated molten carbonate mixture and can have the ability to withstand the significant chemical potential gradients of gas mixture compositions maybe suitable. Thermal shock resistance, and thermal expansion coefficients may also be considered to ensure the stability of the porous structure to allow mechanical stabilization of the membrane. [0067] It is not uncommon for solid oxide materials to exhibit electronic conductivity along with oxide conductance. This is due to the presence of cations that can undergo valency transitions depending on the reducing or oxidizing environment. The proportion of a material's total conductivity due to an electronic current is called the electronic transference number, t_el. Similarly, the proportion of the conduction due to oxide transport is the ionic transference number, tj_on. The range of oxygen chemical potential and temperature over which a material can remain predominately ionically conductive (ti_on> 0.99) is called the electrolytic domain (see Kharton, V. V., F.M.B. Marques, and A. Atkinson, Transport properties of solid oxide electrolyte ceramics: a brief review. Solid State Ionics, 2004. 174: p. 135-149). The membrane of the invention may be designed to allow the internal short- circuit of the carbonate ions to come predominantly from the oxide ions. The ability of a solid oxide material to conduct electrons may not necessary decrease or hinder the flux of oxide ions. Without a sink or source of electrons present within the membrane materials or gas phases, an electrical current should not become estabilished.

[0068] Some suitable materials that can be used for the oxygen-ion conducting phase include, but are not limited to, zirconium (IV) oxide, cerium (IV) oxide, stabilized bismuth (III) oxide, SFC (Sr-Fe-Co oxides), ABO_3-d_eit_a (general perovskite crystalline structure, exhibits oxide ionic and electronic conductivity), and the like. For example, in one embodiment, the solid oxide phase is made of zirconia or various forms of stabilized zirconia. For example, zirconia may be stabilized with MgO, Y₂O₃, CaO, or the like. In another embodiment, a more specific example of the perovskite is SrCOo.₈Fe₀.2θ_3-d_e]ta. Solid oxide membranes and sensor membranes can be used.

[0069] Some potential solid oxide materials for the dual phase membrane are listed in Table 2 below.

{see Steele, B.C., Ceramic ion conducting membranes. Current Opinion in Solid State & Materials Science, 1996. 1: p. 684-691; see also Bredesen, R., K. Jordal, and O. Bolland, High-temperature membranes in power generation with CO2 capture. Chemical Engineering and Processing, 2004. 43(9): p. 1129-1158; see also Ishihara, T., H. Matsuda, and Y. Takita, Effects of rare earth cations doped for La site on the oxide ionic conductivity ofLaGaO3- based perovskite type oxide. Solid State Ionics, 1995. 79: p. 147-151). [0070] The first class of materials, encompassing the first two species in the table, yttria stabilized zirconia (YSZ) and gadolinium stabilized ceria (CGO), are oxides with a cubic fluorite structure. The most common of these materials, used ubiquitously as SOFC electrolyte, may be stabilized zirconia (SZ). Pure zirconia has a monoclinic structure up to 1000°C (see Etsell, T.H. and S.N. Flengas, The Electrical Properties of Solid Oxide Electrolytes. Chemical Reviews, 1969. 70(3): p. 340-378). Oxides that have a similar radius and more ionic character than zirconia can be mixed at low levels to stabilize a cubic fluorite structure. An effective dopant has been yttria, Zr_1-xY_xO₍₂-χ_/2), with the highest conductivity levels attained with x = 0.8-0.11.

[0071] Doped ceria materials may offer higher conductivities at certain temperatures. Gadolinium or samarium doped ceria from 10-20% may provide high ionic conductivities (CGO-10, CGO-20). For example, at 600°C the ionic conductivity is approximately 4 x 10^"2 S/cm. (see Bredesen, R., K. Jordal, and O. Bolland, High-temperature membranes in power generation with CO 2 capture. Chemical Engineering and Processing, 2004. 43(9): p. 1129- 1158).

[0072] Additional examples of suitable solid oxide materials may be found among oxides having high electron conductivity. For example, despite the high electronic conductivity (due to the easy Bi³⁺-^ Bi²⁺ reduction) and low mechanical strength (see Kharton, V. V., F.M.B. Marques, and A. Atkinson, Transport properties of solid oxide electrolyte ceramics: a brief review. Solid State Ionics, 2004. 174: p. 135-149), a fluorite material known for its extremely high conductivity, 5-Bi₂O₃, may be a suitable oxide material. 6-Bi₂O₃ has an ionic conductivity that is greater than 1 S/cm at 750⁰C (see Bredesen, R., K. Jordal, and O. Bolland, High-temperature membranes in power generation with CO ₂ capture. Chemical Engineering and Processing, 2004. 43(9): p. 1129-1158). Bismuth oxide is an example of an intrinsic oxide conductor with every fourth oxide site vacant. /

[0073] Structures with higher oxide conductivities than those of cubic fluorite can be found in the perovskite series. Perovskite structures of the LaGaO₃ family are an example. Doping of the lanthanum with strontium and magnesium for gallium can create the oxide vacancies, Lao.₉Sro.₁Gao.9Mg₀.₂0_3-d (LSGM). (see Ishihara, T., H. Matsuda, and Y. Takita, Effects of rare earth cations doped for La site on the oxide ionic conductivity of LaGaO^- based perovskite type oxide. Solid State Ionics, 1995. 79: p. 147-151).

Fabrication and Operation of CO?. Separation Membranes

[0074] Porous membrane structures of the conducting solid oxide material may be fabricated with various different techniques. One exemplary, but non-limiting, technique may be tape casting. Tape casting can allow for careful control of porosity, thickness and density and can also tolerate co-sintering of multiple layers (see Mistier, R.E. and E.R. Twiname, Tape Casting: Theory and Practice. 2000: American Ceramic Society). Tape casting initially involves the formation of a workable film (tape) containing ceramic particles suspended inside a polymer matrix. Within this matrix, pore formers, particles of graphite, starch, polycarbonate, or polyethylene that will burn out during the sintering process, can also be included to create an engineered porous structure. Poreformers are organic or carbon particles that are mixed in with ceramic precursors that are later burnt-out during sintering. The final structure is largely independent of the sintering conditions. In certain embodiments, continuous porosity can be achieved with greater than 20% of the solids loading of the poreformers (see Moulson, AJ. and J.M. Herbert, Electroceramics: Materials, Properties, Applications. 1997, London: Chapman & Hall. 464). The final porous character of the membrane can be analyzed using mercury porosimetry and SEM. [0075] Tape casting begins with an organic or aqueous solvent dissolving a dispersant to surround individual ceramic and pore formering particles if a porous structure is desired. Examples of dispersant materials include, but are not limited to, polyisobutylene, linoleic acid, oleic acid, lanolin fatty acids, blown menhaden fish oil, and the like. Once the particles are separated, binders are added to suspend the particles in a viscous fluid that can be cast into a thin film. Examples of binder materials include, but are not limited to, polyvinyl alcohol, polyvinyl butyral, cellulose, and the like. Further additives, such as plasticizers can be added to control the flexibility and rheology of the suspension. Examples of plasticizers include, but are not limited to, poly(ethylene) glycol, poly(propylene) glycol, n-butyl phthalate, dioctyl phthalate, and the like. For a viscous slip, parameters such as solids loading, binder / dispersant loading, milling time of powder with binder / dispersant, and mill speed can be optimized. Once cast onto a mylar film, the organic solvent evaporates causing the film to shrink and bringing the particles close together. What remains is a polymer matrix suspending ceramic particles and poreformers, called a green body. The green body can be cut or punched to form a desired two dimensional geometry. Additionally, other layers can be cast upon or within the green body, to create a laminated or framed structure. When the green body is fired, the organic binders, poreformers, and other organic additives within the slip can be burnt out, and the particles can be sintered together. Sintering time and heating rates can also be tested. Those experienced to the art can change these methods as deemed fit. [0076] The tape casting process may allow for a porous solid oxide disk to be co-sintered inside a dense solid oxide frame in order to create a dense surface to form a seal against certain regions of the porous solid oxide disk (see Figure 3). This can be accomplished due to the nature of the tape casting process by either including pore forming agents to create a porous structure, or excluding them, resulting in a dense structure. To begin a slip containing the solid oxide powder, dispersant, binder, plasticizers and solvent can be cast. Upon solvent dryout, a void space can be cut out of the tape, to be next filled with a slip containing the same mixture, only now containing poreformers. Upon sintering the additives, such as the dispersant, binder, plasticizers and poreformers, can be burnt out, leaving a porous solid oxide structure surrounded by a dense solid oxide structure.

[0077] Finally, infiltration of the molten carbonate phase into the pores can be carried out. Both the molten salt and the membrane can be heated to the same temperature, and then the membrane can be dipped into the molten carbonate mixture. The porous solid oxide may be heated to be as hot as the molten carbonate to avoid cooling and solidifying of the salt once it contacts the membrane surface. Molten carbonate uptake may occur via capillary forces drawing the liquid into the solid oxide pore space. Upon cooling, the infiltrated membrane can be analyzed with SEM with EDS on both faces to examine if the carbonate infiltrated the entire thickness. XRD can also be used to detect the phases present on the surface.

Uses

[0078] The invention provides for numerous different uses of the membrane of the invention, hi certain embodiments, the present invention provides a method for carbon dioxide separation, comprising subjecting a source containing carbon dioxide to the carbon dioxide membrane of the invention described herein. For example, the present invention provides a method for carbon dioxide separation at temperatures greater than about 200 °C. [0079] In an embodiment, the source may be a fuel gas or an exhaust gas. hi certain embodiments, the invention relates to membranes, methods, and systems for separating of carbon dioxide from fuel gas mixtures and processed fuel gas mixtures. For example, membranes of the present invention may operate from about 300 to about 1200 °C. Moreover, membranes of the invention may also separate CO₂ from a fuel gas mixture containing hydrogen, water, carbon monoxide, and methane. Membranes of this invention may further separate carbon dioxide from fuel gas streams even if the fuel gas streams contains contaminants such as H₂S and NH₃.

[0080] In certain embodiments, a zero-emission coal-based electric power plant comprising the carbon dioxide membrane of the invention is provided. The invention provides for a zero-emission coal-based electric power plant comprising the membrane described herein.

[0081] In other embodiments, a fuel cell comprising the carbon dioxide membrane according to the invention is embraced. For example, the carbon dioxide separation membrane may be operating from about 600 °C to about 900 °C in a fuel cell of the present invention. The invention provides a solid oxide fuel cell comprising the membrane described herein.

[0082] Illustratively, as shown in Figure 4, such a membrane can be useful in operating a solid-oxide fuel cell (SOFC) system that can operate on a pure carbon fuel source. The fuel to be oxidized at the anode compartment of the cell stack can be carbon monoxide, generated by a gasification reaction of carbon with CO₂ (Boudouard reaction shown in Equation [5])

C + CO₂ <→ 2 CO [5]

Since it can be inefficient to drive the CO oxidation in the fuel cell to completion, a better strategy may be to remove excess product CO₂ from a reactor by letting it escape from the reaction vessel through a high temperature selective membrane that is impermeable to CO. Carbon monoxide would not be depleted in the chamber, because additional CO would be generated in the fuel chamber by gasifying a stream of injected carbon. Such a fuel cell could be maintained near the equilibrium point of the Boudouard reaction with the CO₂ selective membrane removing the net production of CO₂ and maintaining the fuel chamber at relatively steady state conditions. In contrast, conventional fuel cell designs tend to drive the fuel content to depletion. Operating near steady state entirely avoids the usually obligatory post- combustion of remnant fuel in the exhaust of the fuel cell stack. The gasification reaction can be performed in a nearly reversible fashion and thus would not consume any of the free energy originally available in the carbon fuel. Furthermore this design would naturally collect CO₂ and ready it for subsequent disposal. An SOFC fuel cell operating in this manner has the highest possible theoretical efficiency of any fuel cell, suggesting that it may be possible to exceed the 70% theoretical efficiency of a high temperature hydrogen fuel cell (see Wade, J. and K. Lackner. Development of a Coal-Based Solid-Oxide Fuel Cell System in The 30th International Technical Conference on Coal Utilization & Fuel Systems. 2005. Clearwater, FL).

[0083] Other applications for high temperature CO2 membranes include CO2 capture in novel power plants that capture CO₂ from the exhaust stream, but also for promoting water gas shift reactions by removing CO2 from the reaction zone. For example, the carbon dioxide separation membrane may be operating from about 300 ⁰C to about 600 ⁰C when used in promoting water gas shift reactions. In other embodiments, the membrane of the invention can be used in energy producing devices (such as a solid oxide fuel cell), fuel synthesis, carbon chemistry methods, steel making processes and systems, aluminum smelter and other metallurgical processes.

Advantages

[0084] Power plant designs that rely on the recirculation of only partially combusted or oxidized flue gases may benefit greatly from the availability of membranes that can perform the separation at the process temperature (e.g., in excess of 400°C and as high as 1000°C). The efficiency of recirculating the remaining gas can be greatly increased if the gas can retain its sensible heat and does not have to be subjected to a cooling and heating cycle in order to allow for the removal of CO₂. Zero emission power plants avoid smoke stacks by limiting the inputs to oxygen and carbonaceous fuels. A signature feature of all these power plant designs is that they recirculate the exhaust gases in order to gasify the input fuel or in some cases to dilute the input stream. Nevertheless, as CO₂ and steam is produced, these two components will have to be separated individually from the exhaust stream. The recirculating gas stream contains increasing amounts of CO₂ that must be removed for sequestration and steam that must be condensed out. (For an analysis of zero emission coal plants, see, for example, Lackner, K.S. and T. Yegulalp, Thermodynamic foundation of the zero emission concept. Minerals and Metallurgical Processing, 2005. 22(3): p. 161-167). An important unit process in such a power plant may be a separation unit that can remove CO₂ from the product stream into a separate sequestration-ready stream. This invention provides such a unit and thus enables the development of a zero emission power plant. [0085] The present invention may also allow a simple recovery of CO₂ in fuel gas streams. Most hydrocarbon gasification scheme to produce a hydrogen rich fuel makes use of the water gas shift reaction as shown in Equation [6]. CO + H₂O <→ CO₂ + H₂ [6]

By providing a separation membrane that can operate at higher temperatures typical of the water gas shift process (200-400°C) {see Kharton, V. V., F.M.B. Marques, and A. Atkinson, Transport properties of solid oxide electrolyte ceramics: a brief review. Solid State Ionics, 2004. 174: p. 135-149), the removal of CO₂ would shift the reaction further to the right. Further, the mechanical stability provided by the membranes of the invention may enable operating the system under high pressure conditions, resulting in an added equilibrium shift to the right since the preferential removal of CO₂ would offset the high pressure conditions. A pressurized system may also have the dual benefit of providing H₂ already at high pressures, minimizing the energy to later compress the fuel for storage. [0086] Direct steam methane reforming (SMR), as shown in Equation [7], is another application were a CO₂ membrane of the present invention may be utilized.

CH₄ + 2H₂O <→ CO₂ + 4H₂ [7]

However, in this application unlike the former, the system may be limited to low-pressure operation due to the five moles present on the hydrogen side of the reaction versus three on the left side of the reaction. Gasification of a more carbon rich fuel, such as biomass, oil, coal, or charcoal, with oxygen or steam may contain a larger concentration of CO₂ product gas that would favor the use of a high temperature and pressure membrane separation.

Theory of Operation

[0087] Without wishing to be bound by theory, the membranes of the present invention may be functioning as described below.

Ionic Conductivity

[0088] Oxide conductivity in ceramic materials arises from oxide vacancies or defects within the crystal lattice. Defects can be created by doping a compound with aliovalent counter ions which provide the charge mismatch for vacancies to form. This is common in Group IVB oxides, such as ZrO₂ and CeO₂, which are doped with alkali earth or rare earth metal cations such as Ca²⁺ or Y³⁺. It is also possible for an inorganic compound to have intrinsic oxide vacancies, which is true of 5-Bi₂O₃ or brownmillerite structures {see Yamamoto, O., Solid Oxide Fuel Cells: Fundamental aspects and prospects. Electrochimica Acta, 2000. 45: p. 2423-2435). [0089] Anion vacancies hold a net positive charge because the site may otherwise be filled with an anion maintaining the charge balance. The electrostatic interaction between the vacancies and other neighboring counter ions impedes the mobility of an ion being able to fill the vacancy. There may be an association enthalpy that must be overcome in order for the defects to become mobile in the solid (see Etsell, T.H. and S.N. Flengas, The Electrical Properties of Solid Oxide Electrolytes. Chemical Reviews, 1969. 70(3): p. 340-378). Hence, certain solid oxide conductors operate well at elevated temperatures (600-1000°C) and provide functional conductivities (σi_on > 0.01 S/cm) (see Steele, B.C., Ceramic ion conducting membranes. Current Opinion in Solid State & Materials Science, 1996. 1: p. 684- 691). Lower association energy in a defect crystal lattice may lead to higher conductivity at a given temperature. Higher conductivity may ultimately lead to greater permeability. [0090] Alkali metal carbonate salts generally have high conductivities in comparison to that of solid oxide materials at similar temperatures. This is because alkali metal carbonates, and especially eutectic mixtures thereof, have low melting points inducing liquid mobility of the carbonate ions. The electronic conductivities of most alkali metal carbonate mixtures are within the range of 1-2 S/cm (see Selman, J.R. and H.C. Mara, Physical Chemistry and Electrochemistry of Alkali Carbonate Melts: With special reference to the molten-carbonate fuel cell, in Advances in Molten Salt Chemistry, G. Mamantov, J. Braunstein, and CB. Mamantov, Editors. 1981, Plenum Press: New York. p. 159-389). Table 1 above lists the melting points and conductivities of various Li, Na and K mixtures. As shown, the conductivity of the solid oxide material is approximately an order of magnitude less than that of the carbonate. Hence, the conductivity of the solid oxide may be a limiting factor of CO₂ permeance.

Dual-Phase Chemistry

I. First regime

[0091] Gas phase CO₂ may initially undergo diffusion to the membrane surface. The

CO₂ may either adsorb onto the solid oxide surface (CO_2(acl)) or dissolve into the molten carbonate phase (CO₂₍^) (see Equation [8]).

CO_2(g) <→ CO₂(I) <→ ^C°2(ad) [8] II. Second regime

[0092] At the phase boundary of the solid oxide and molten carbonate, a surface reaction involving the reaction of an oxide ion within the solid oxide crystal lattice with a CO₂ molecule may occur. The oxide ion might either directly bind to an adsorbed CO₂ molecule (Equation [9]), or dissolve within the molten carbonate phase as shown in Equation [10]. Once an oxide anion leaves the surface of a crystal lattice an oxide vacancy is created. Oxide ions dissolved in the carbonate phase may convert CO₂ to a carbonate anion as shown in Equation [H]. These reactions will preferentially shift to the right when the partial pressure Of CO₂ is high, as in the feed stream of a membrane (e.g., side 1 of Figure 1).

C0_2(ad)* + O₀ ^x _(side i) <→ CO₃ ²-(i) + V₀ ^" _(side i) [9]

O_o ^X(sidβ 1) *→ CO₃ -Q + V₀ ^"(side I) ⁺ O ^'(i) [ 10]

CO₂(I) + O²-(_side 1) <→ CO₃ ²-(i) [11]

III. Third regime

[0093] Once the carbon dioxide has been converted to a carbonate ion, bulk diffusion of the charged species from the feed side of the membrane (side 1) to the permeate side of the membrane (side 2) may occur (see Equation [12]). The counter-current of the oxide ion may take place in the opposite phase, with oxide ions diffusing from the permeate side of the membrane towards the feed side (see Equation [13]). This can also be thought of as the migration of positively charged lattice defects from the feed side where they are created towards the peπneate side where they are filled (see Equation [14]).

Molten Carbonate:

CO₃ ^" (side l) "^ CO₃ ^"(side 2) [12]

Solid Oxide:

O₀ ^X(side 2) ^"^ O_o ^X(sidel) [13]

V₀ ^"(side l) ~^ V₀ ^"(side2) [14]

IV. Fourth regime

[0094] Carbonate ions on the permeate face of the membrane, situated on a boundary with molten carbonate and solid oxide, may undergo a surface oxide exchange reaction with an oxide vacancy in the solid oxide phase (see Equation [15]). This can be though of as being the reverse of the first surface reaction, where a carbonate ion releases an oxide ion to a vacant site in the solid oxide lattice, and becomes once again a neutral carbon dioxide molecule.

CO₃ ^"(1) + V₀ ^"(side 2) <→ CO₂(ad) + 0₀ ^X(_{side 2}) [15]

Alternatively, the carbonate ions may release CO₂ and leave the oxide ion and CO₂ dissolved in the molten melt. A separate charge transfer reaction balancing the molten carbonate oxide activity and vacancy activity may occur at the interface of the two phases (see Equation [16]).

CO₃ ² O) <→ CO₂(I) + O²-₍i) [16]

V. Fifth regime

[0095] Finally the CO₂ can desorb either into the molten carbonate (see Equation [17]) and then into the gas phase, or can escape directly into the gas phase and diffuse away from the membrane surface (see Equation [18]).

CO_2(ad) ~ CO₂₍₁₎^ CO_2(g) [17]

C0_2(ad) <→ C0_2(g) [18]

[0096] The reactions occurring on the membrane surface may not be limited to the reactions listed above. Considerations may have to be made for competing reactions of other gases in the feed mixture and the decomposition reaction of molten carbonates (see Equation

[19])-

CO₃ ²- «→ CO₂ + O^2" [19]

[0097] Molten carbonate materials maintain a natural equilibrium of dissolved carbon dioxide and oxide ions according the decomposition shown in Equation [19]. This reaction will be shifted further to the right when exposed to low partial pressures of CO₂ on the permeate side of the membrane (side 2 in Figure 1). This suggests that a critical level of the CO₂ partial pressure on the permeate face of the membrane may be needed to to avoid decomposition, while still being low enough to allow for a useful partial pressure gradient. As a further consideration, any species in the stream that is more acidic than CO₂ may also compete for oxide ions from the solid oxide or within the molten carbonate melt Transport Theory

[0098] The flux or permeance may be correlated to the driving force (which is the partial pressure difference of CO₂ across the membrane) to further elucidate the factors controlling the transport OfCO₂ across a membrane.

[0099] A first order approximation of expected flux based on the bulk diffusion of ions through the molten carbonate and solid oxide phase can be derived. Because transport within the membrane bulk occurs through ionic motion, a current within each phase is created. Further, the net current must equal zero in the absence of external circuitry as shown in Equation [20] below.

Z(side 1) + l(side 2) = 0 [20]

For a bulk diffusion approximation, it can be assumed that the two phases are coupled through an equilibrium reaction occurring on each surface of the membrane described below in Equation [21].

CO₂(_g) + O_o ^X(_side 1) = CO₃ ^"( side ₂) + V₀ ^{' "} ( side l) [21]

This model assumes a fast¹ surface reaction relative to bulk diffusion. Thus, a pseudo-steady state approximation is used. This assumption can allow us to ascertain that the bulk diffusion transport inside the membrane is indeed sufficiently fast to allow for a sensible flux within a reasonable membrane thickness.

[0100] The above approximation results in the following electrochemical potential equality, μ, at each face of the membrane:

The c and v notation are used indicate carbonate ions and vacancy species in each of their respective phases. The tilda, ~, above the chemical potential of these species indicates the extra contribution due to an electrical gradient ( VΦ ). The extra term is zero for CO₂ and O_o ^x because their charge numbers, z, are considered 0. Faraday's constant, F, is 96,500 C / mole charge.

ju, = μ, + zFΦ [23]

The chemical potential OfCO₂ can be related back to the activity a_COi of CO₂ as follows:

Mco₂ = M⁰ _CO2 ⁺ RT\na_C0% [24] where R is the gas constant 8.314 J / K mol and T is the temperature in Kelvin.

[0101] Given the net zero current and condition of electrochemical potential equality at each face of the membrane, the governing equations are as follows:

Z(side l) = - *^'(side2) [25] μ°_cθ2 + RT]na_COι (x = 0) + μ_o, (x = 0) = μ_γ (x = 0) + 2FVΦ¹ + μ_c(x = 0) ~ 2FVΦ" [26]

Equation [26] can be used similarly at the opposite edge (x = L). Further conditions assumed for this model is that within each phase the material is isotropic and isothermal, and no reactivity occurs between the phases. In other words, current in one phase cannot be transferred into the other. The only connection between the two phases is the equilibrium reaction of CO₂ occurring at the interface. Below, the flux within each phase will be considered individually.

Phase I: Solid Oxide

[0102] Oxide ions in a crystal lattice become mobile by hopping into vacant crystal lattice sites that would otherwise be occupied by oxide ions. Once an oxide ion hops into an adjacent site, a new vacant space is created where the oxide previously resided. Thus, the charge carrier can be modeled as motion of the dilute vacant sites, rather than that of the numerous oxygen anions present in the solid oxide.

[0103] Vacancies exist in solid oxide materials due to defects in the crystal lattice, either naturally occuring (e.g. bismuth oxides) or introduced through doping of a lower valent cation (e.g. yttria doped zirconia ). The positively charged vacancy neutralizes the charge deficient cation defects. The flux of the mobile charge carriers in a solid oxide material can be given as follows (see Heyne, L., Electrochemistry of Mixed Ionic-Electronic Conductors, in Solid Electrolytes. 1977, Springer- Verlag: Berlin, p. 189-197):

C D

J₁ = C_tv_t = --J-^L V(μ, + _ZiFΦ^(I)) [27]

Kl

This is the flux equation for an infinitely dilute solution, where Ji is equal to the molar flux density of species i relative to the mass-average velocity, mol / cm^{2 •} s, Q is the molar concentration of species i averaged within each phase, mol / cm³, and Di is the diffusion coefficient of species i in a dilute solution, cm /s. [0104] hi this basic model, the mobile charge carriers within the material are limited to the oxide vacancies (v). Electroneutrality is maintained within the solid material by the corresponding defects that resulted in the formation of vacancies electrons. These defects are immobile in the solid, and for this reason, the chemical potential of the mobile species is constant everywhere. Therefore, the gradient in chemical potential can be ignored in Equation [28].

[0105] The flux as described above is an average over the volume occupied by the solid oxide phase. Because the system in question is porous, the volume fraction occupied by the solid oxide and carbonate conductors must be considered. The porosity of the solid oxide material, ε, will be considered as the fractional volume occupied by the flooded molten carbonate phase. This leaves the fractional volume occupied by the oxide conductor as (1-ε). To correct for the flux averaged over the correct volume for each phase, porosity must be taken into account as follows {see Newman, J. and K.E. Thomas- Alyea, Electrochemical Methods. 2004, John Wiley & Sons: Hoboken, New Jersey, p. 823):

J'i = -^- Solid Oxide Phase (I) [29]

(1 -0

jⁿi = A. Molten Carbonate Phase (II) [30]

(O

The tilda, ~, indicates the flux averaged over the volume occupied by each of the individual phases. Furthermore, the diffusivity is going to be affected by a porous structure, and a corrected diffusivity constant will be marked with a star, *. In summary, the flux equation for the doubly charged vaccancy in the solid oxide phase are given as follows:

[0106] Often, only the total conductivity of a material is given, rather than the diffusivities of individual species. This value is given by the quotient term in Equation [31], and in future equations simplified as K¹ (Equation [32]).

[0107] Because it is assumed the only charge carrier in the solid oxide phase (I) arises from the vacancies, the total current in the oxide phase can be related to the flux as follows: i^l = 2FJ_v [33]

Z¹ = 2F(I -ε^VΦ¹ [34]

Phase H: Molten Carbonate

[0108] Within the molten carbonate phase, both the alkali metal cations and carbonate ions are mobile species. Furthermore, because this is a molten salt system, an infinitely dilute solution equation cannot be used as a starting point. Rather, to model the transport, it may be best to begin with a force balance (see Newman, J. and K.E. Thomas- Alyea, Electrochemical Methods. 2004, John Wiley & Sons: Hoboken, New Jersey, p. 823):

where vj is the velocity of species i, cm/s, and Di_j is the diffusivity of species i relative to species j. The term on the right can be considered the driving force per unit volume of species i, proportional to the electrochemical gradient of that species. The driving force is balanced by frictional interactions with other species, j, in the system. The diffusion constant, D_y, describes the interaction between the two species, and the frictional interaction is proportional to the difference in velocity of the different species.

[0109] To simplify this model, it will be assumed a pure molten carbonate solution exists where there is only one type of cation, M⁺, and one type of anion, CO₃ ^2". Further, because the flux of a species is the flux of the cation is zero because it is not created or removed at any interface; therefore the velocity of the cation is zero.

J_M = C_MV_M = 0; .\ V_M = 0 [36]

[0110] Furthermore, because electroneutrality must exist everywhere within the molten carbonate, the concentration of the alkali metal cation must be twice that of the molten carbonate. This also means the chemical potential gradient of the carbonate must be zero if the density of the material is to remain constant. ∑Z_tC_t = 0;-2C_c +C_M= 0;.-. C_M = 2C_C [37] i

V^c=O [38]

[0111] From equation [23], we have

Vμ_c = μ_c + 2FVΦ¹¹ [39] because the carbonate has a 2^" charge.

[0112] Moreover, because the total concentration must be the concentration of the metal cations and the carbonate ions,

C_T=C_M +C_C=2C_C+C_C=3C_C [40]

[0113] Combining Equations [36] - [40] with Equation [35] gives

which can be simplied as

[0114] Combining Equations [27], [30], and [42] and rearranging provides the following carbonate flux equation:

Jc=εC_cv_c=-ε^3C^™^FVΦ« _[43]

Kl

FIuXQfCO₂

[0115] The condition that couples transport of both phases is that of a net zero current. Because the two mobile carriers have equal and opposite charge, their flux must be equal. Similarly, given the stoichiometry of the surface reaction, the flux of CO₂ must be equal to the flux of the other species. This results in the following current condition given below:

T

[44] [0116] The flux of CO₂ is dependent upon the potential of each of the four boundaries. One of these potentials, (^_x=_O) will be arbitrarily set equal to zero. With the above condition and the two boundary conditions shown in Equations [46] and [47], there is a set of three equations with three unknown potentials.

Φ U = O ) = 0 [45] μ°_cθ2 + RT\na[^x;₂ ⁰⁾ + μ_Q, = μ_c - 2FΦ£₌₀₎ + μ_v +2FΦ[_x=0) [46] μ°_cθ2 + RT In a[^x;^L) + μ_QX = μ_β - 2FΦ£_=L) + μ_v + 2FΦ|_X=L) [47]

Solving for the unknowns, the flux of CO₂ can be solved as follows:

_τ 1 , t_v(l - g)K^JgK⁷⁷ ^ RT _Λ PcO₁ ^

^{Jc02 =}I ^* M-e)κ.' ₊ *°) !F^H1%> ^m

[0117] Equation [48] shows that bulk diffusion limited flux of CO₂ is dependent upon the pressure difference across the membrane thickness and an average of the conductivities. If the conductivity of one phase far exceeds the other, then flux will be limited by the weaker conductor. This dependence of flux on membrane conductivities will set a maximum thickness on the membrane in order to deliver economically useful permeance. It allows for an upfront calculation on the feasibility of a proposed membrane system. [0118] Another impedance to CO₂ transport not captured in the transport analysis may be the polarization of surface reactions. For example, if there is an excess of CO₂ built up on the surface of the feed side of the membrane, and oxide ions cannot be released from the crystal lattice due to high surface trapping energies, then the carbonate transport will be obstructed. Often times a critical length, Lc, is defined for membrane systems (see Chen, CS. , et al., Thickness dependence of oxygen permeation through erbia-stabilized bismuth oxide-silver composites. Solid State Ionics, 1997. 99: p. 215-219). Above the critical length, flux is limited by bulk phase diffusion, and can be improved with thinner membranes. Below this critical length, surface reactions become limiting.

[0119] Upon review of the description and embodiments of the present invention, those skilled in the art will understand that modifications and equivalent substitutions may be performed in carrying out the invention without departing from the essence of the invention. Thus, the invention is not meant to be limiting by the embodiments described explicitly above, and is limited only by the claims which follow.

Claims

What is claimed is:

1. A membrane for separating a molecule from other molecules, the membrane comprising: a first phase that forms at least one continuous structure from a first side of the membrane to a second side of the membrane, the first phase being capable of conducting a first type of ions; and a second phase that forms at least one continuous structure from the first side of the membrane to the second side of the membrane, the second phase being capable of conducting a second type of ions.

2. The membrane of claim 1, wherein the membrane is capable of separating at least one of carbon dioxide, steam, chlorides, nitrates, ammonium, and sulfur oxides from other molecules.

3. The membrane of claim 1, wherein the first phase is capable of conducting a carbonate ion and the second phase is capable of conducting oxide ions to separate carbon dioxide from other gaseous molecules.

4. The membrane of claim 3, wherein the first phase comprises a molten carbonate.

5. The membrane of claim 4, wherein the molten carbonate comprises lithium carbonate, potassium carbonate, sodium carbonate, any mixtures thereof.

6. The membrane of claim 3, wherein the second phase comprises a solid oxide capable of conducting oxygen ions.

7. The membrane of claim 6, wherein the solid oxide comprises zirconia, zirconia doped with magnesium oxide, calcium oxide, and ytrria, ceria, ceria doped with gadolinium or samarium, 6-Bi₂O₃, perovskites within the LaGaO₃ family, or any mixtures thereof.

8. The membrane of claim 3, wherein the membrane is capable of separating carbon dioxide at a temperature from about 200 °C to about 1200 ⁰C.

9. The membrane of claim 3, wherein the membrane is capable of separating carbon dioxide at a temperature from about 400 ⁰C to about 1000 °C.

10. The membrane of claim 3, wherein the membrane selectively separates carbon dioxide from a mixture comprising carbon dioxide and one or more of hydrogen, oxygen, carbon monoxide, nitrogen, methane, or steam.

11. The membrane of claim 3 , wherein a partial pressure difference of carbon dioxide from the first side of the membrane to the second side of the membrane drives a flux of carbonate ions in the first phase and a flux of oxide ions in the second phase.

12. The membrane of claim 11, wherein the flux of carbonate ions travels from the first side of the membrane to the second side of the membrane and the flux of oxide ions travel from the second side of the membrane to the first side of the membrane.

13. The membrane of claim 3, wherein the first phase is solid at a temperature up to about 300°C and is molten from about 300°C to about 1200°C.

14. The membrane of claim 3, wherein the first phase comprises lithium carbonate and the second phase comprises stabilized zirconia.

15. The membrane of claim 14, wherein the first phase and the second phase are in contact with each other and the first phase and the second phase react with carbon dioxide to form a layer of lithium zirconate between the first and the second phase.

16. The membrane of claim 3, wherein the second phase is surface treated with a catalyst that promotes transfer of oxide ion from the second phase to the first phase when separating carbon dioxide.

17. The membrane of claim 3, wherein the membrane is capable of separating carbon dioxide from a fuel gas or an exhaust gas.

18. The membrane of claim 3, wherein the second phase comprises a porous, continuous solid oxide and the first phase comprises a carbonate material filled inside the pores of the porous solid oxide.

19. The membrane of claim 3, wherein the membrane is in the form of a disk, plate, cylinder, cube, tube, film, or a sheet.

20. A solid oxide fuel cell comprising the membrane of claim 3.

21. The solid oxide fuel cell of claim 20, wherein the solid oxide fuel cell oxidizes carbon monoxide in a fuel chamber and wherein the carbon monoxide for the fuel cell is produced by gasifying carbon with carbon dioxide according to the Boudouard reaction.

22. A power plant comprising the membrane of claim 3.

23. The power plant of claim 22, wherein the power plant is a zero-emission coal-based electric power plant.

24. A method for separating a molecule from other molecules, the method comprising: contacting a source comprising the molecules and the other molecules to at least one side of the membrane of claim 1.

25. The method of claim 24, wherein the source is a fuel gas or an exhaust gas.

26. The method of claim 24, wherein the source is at a temperature of about 200 °C or higher.

27. The method of claim 24, wherein the source is at a temperature of about 400 ⁰C or higher.

28. The method of claim 24, wherein the source is at a temperature from about 400 ⁰C to about 1000 °C.

29. A method for producing the membrane of claim 1, the method comprising: casting a mixture comprising one or more particles of the first phase and one or more poreformers to obtain a green body; sintering the green body to obtain a porous continuous structure comprising the first phase capable of conducting the first type of ions; and filling in the pores of the porous continuous structure with a material capable of conducting the second type of ions.

30. The method of claim 29, wherein the poreformers comprises organic particles.

31. The method of claim 29, wherein the membrane comprises a bicontinuous structure and the material capable of conducting carbonate ions comprises a molten carbonate.