WO2014059992A1

WO2014059992A1 - High performance ceria based oxygen membrane

Info

Publication number: WO2014059992A1
Application number: PCT/DK2013/050323
Authority: WO
Inventors: Christodoulos CHATZICHRISTODOULOU; Peter Vang Hendriksen; Søren Preben Vagn FOGHMOES; Sandrine RICOTE; Andreas Kaiser; Julie CLASSCOCK; Shiyang CHENG
Original assignee: Danmarks Tekniske Universitet
Current assignee: Danmarks Tekniske Universitet
Priority date: 2012-10-19
Filing date: 2013-10-11
Publication date: 2014-04-24
Anticipated expiration: 2015-04-19

Abstract

The invention describes a new class of highly stable mixed conducting materials based on acceptor doped cerium oxide (CeO_2-8 ) in which the limiting electronic conductivity is significantly enhanced by co-doping with a second element or co- dopant, such as Nb, W and Zn, so that cerium and the co-dopant have an ionic size ratio between 0.5 and 1. These materials can thereby improve the performance and extend the range of operating conditions of oxygen permeation membranes (OPM) for different high temperature membrane reactor applications. The invention also relates to the manufacturing of supported thin film membrane devices using these materials.

Description

HIGH PERFORMANCE CERIA BASED OXYGEN MEMBRANE

FIELD OF THE INVENTION

The present invention relates to the field of oxygen permeation membranes (OPMs). The invention also relates to the manufacturing of supported thin film membrane devices.

BACKGROUND OF THE INVENTION

Oxygen is used in important industrial processes like cement, steel and synthesis gas (syngas) production. Syngas can also be used directly as a fuel in solid-oxide fuel cells (SOFC), or act as an intermediate for the production of hydrogen gas, ammonia, hydrocarbons or oxygenates.

Syngas (a mixture of CO and H₂), is produced either by partial oxidation of methane or by gasification of biomass, organic waste or even plastic.

Partial oxidation of natural gas to syngas, followed by Fischer-Tropsch synthesis allows for the liquefaction of natural gas, thereby providing a method to produce liquid fuels, which are easier to transport than gaseous fuels.

All the oxygen required for the partial oxidation is presently being supplied by cryogenic distillation, now the common technology for large scale oxygen production.

This is an energy consuming process (210 kWh/t), where the efficiency has already been maximized.

OPMs can produce pure 0₂, which is an important feedstock for the steel and cement industries, for medical purposes, or for the gasification of coal in oxyfuel power plants in conjunction with C0₂ capture and sequestration. OPMs may also be used within catalytic membrane reactors for the production of syngas. Catalytic oxygen membrane reactors loaded with appropriate catalysts can be used for a variety of partial oxidation reactions other than transformation of methane to syngas, yielding value added chemicals.

OPMs have the potential to reduce substantially the energy required for the production of oxygen gas (down to 147 kWh/t) and provide an economic alternative at the large and intermediate scale. Especially for cement production a large emission reduction potential exists, as this process contributes 5% of the global anthropogenic CO2 emissions. The most cost effective emission reduction is via the use of pure oxygen to combust biomass/waste fuels. OPMs are likely to be a cost competitive way for producing the oxygen for such an application.

However, no high temperature ceramic OPM systems currently exist for large scale industrial applications, despite efforts in developing demonstrators for specific applications, e.g. for syngas.

Replacing the cryogenic unit with an oxygen membrane reactor is estimated to significantly reduce the capital cost of the Gas to Liquid (GTL) plant as well as producing higher purity oxygen.

Hence, an improved OPM would be advantageous, and in particular a more efficient and stable OPM would be advantageous. OBJECT OF THE INVENTION

It is an object of the invention to replace large cryogenic units used for large scale industrial oxygen gas production.

It is also an object of the invention to provide an oxygen membrane with increased electron conductivity. It is a further object of the invention to provide an oxygen membrane having reduced chemical expansion and reduced thermal expansion coefficients.

It is a further object of the present invention to provide an alternative to the prior art. In particular, it may be seen as an object of the present invention to provide an oxygen permeation membrane that solves the above mentioned problems of the prior art by providing an increased oxygen flux.

SUMMARY OF THE INVENTION

The above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a metal oxide having fluorite structure, for use in OPMs, doped with : at least one electron acceptor; and/or at least one co-dopant, such as an electron donor, a further electron acceptor or equi-valent substitutional or substitutional of similar valency. Fluorite is referred herein to fluorite structures or similar. Following the inclusion of dopants the atomic structure may be partially changed.

In a second aspect of the invention an OPM comprising a metal oxide according to the first or other aspect of the invention is provided.

Doping is a process where elements of the fluorite structure are partly substituted by foreign elements which may act as donors of electrons, acceptors of electrons or substitutionals of similar valency.

The inventors observed that the combination of electron acceptor doping, i.e. a first element, and a co-dopant dopant, i.e. a second element, of a metal oxide, fulfilling a specific size requirement as described in the third aspect of invention, i.e. wherein the metal of the metal oxide and the second element have an ionic size ratio between 0.5 and 1, provides a synergistic effect that improves the performance, in terms of achievable oxygen flux, through the OPM produced with this co-doped metal oxide.

Acceptor doping in the fluorite structure makes the oxide ion vacancy

concentration increase leading to a net increase of oxide ion conductivity. In some embodiemts co-doping by electron donor doping makes the electron concentration increase, which in turn increases the electronic conductivity. When oxide ion vacancies are present in the fluorite structure (e.g. due to acceptor doping), donor doping would act to annihilate those vacancies.

Thus, a combination of electron donor doping and electron acceptor doping should theoretically cancel out the effects of each other.

Surprisingly the inventors observed that a combination of electron donor doping and electron acceptor doping provides a synergistic effect that improves the performance, in terms of achievable oxygen flux, through the OPM produced with this material.

The invention is particularly, but not exclusively, advantageous for obtaining an oxygen membrane with increased electron conductivity, reduced chemical expansion coefficient and reduced thermal expansion coefficient, e.g. in

comparison to state of the art fluorite compound such as Gd-doped ceria (CGO). The co-doped metal oxide has also the advantage of improving the performance and extending the range of operating conditions of OPMs for different applications in high temperature membrane reactors. Electron acceptors may be also referred to as elements having the function of electron acceptor.

In some embodiments the at least one electron acceptor is an element of group IIIA of the periodic table.

In some other embodiments the at least one electron acceptor is a rare earth. For example, the at least one electron acceptor may be gadolinium.

In some further embodiments the at least one electron acceptor is an element of group IIA.

For example, the at least one electron acceptor may be calcium.

Co-dopants may be electron donors that are also referred to as elements having the function of electron donor.

In some embodiments the co-dopants are not electron donors.

In general co-dopants may be small sized ions acting as donors of electrons, acceptors of electrons or substitutionals of similar valency. In some embodiments the at least one co-dopant is a transition metal.

For example, the at least one co-dopant may be an element of group VA, VIA, IIB or an actinide.

For example the at least one co-dopant may be Mg, Sc, In, Ga, Zr, Sn.

In some further embodiments the at least one co-dopant is niobium or tungsten or uranium.

In some further embodiments the at least one co-dopant is Zn.

The metal oxide having fluorite structure may be ceria (Ce02-5) wherein δ is the oxygen non-stoichiometry which lies in the range between 0 and 0.5.

Generally δ depends on the temperature conditions and depends on the mixed valence state of Ce, thus it may vary in the range between 0 and 0.5.

The doped metal oxide according to the first aspect of the invention may have the general formula :

(CexAy)i zDz02- 5, wherein D is the at least one co-dopant, such as an electron donor, and A is the at least one electron acceptor and wherein x is in the range between 0 and 1 (preferably between 0.6 and 1), y is in the range between 0 and 1 (preferably between 0 and 0.4), z is in the range between 0 and 0.2 and δ is in the range between 0 and 0.5. In some embodiments the metal oxide according to the first aspect of the invention may comprise a mixture of metal oxides.

The metal oxide doped with at least one electron acceptor and at least one co- dopant such as an electron donor may be or comprise a composite consisting of one or more metal oxide mixtures.

For example the metal oxide may be a mixture of metal oxides having the general formulas: Cei xD_x02- 5, wherein D is the at least one co-dopant such as an electron donor and wherein x is in the range between 0 and 1 (preferably 0 and 0.2), and δ is in the range between -0.5 and 0.5; and Cei-_yA_y02- 5, wherein A is the at least one electron acceptor and wherein y is in the range between 0 and 1 (preferably 0 and 0.4), and δ is in the range between -0.5 and 0.5.

The production of composites of acceptor doped ceria and co-doped or donor doped ceria powders appears to be a very promising route to improve the performance of OPMs. It is expected that composites of acceptor doped ceria and co-doped or donor doped ceria powders lead to greater improvement of the electronic conductivity than the one observed for the co-doped compositions, while at the same time minimizing the adverse effect of reduced ionic

conductivity.

Generally composite membranes have the disadvantage that very similar thermo- mechanical properties are required for the components of the composite membrane to allow for mechanical stability. Furthermore, the two components should not react forming undesirable phases during any step of the fabrication process or during operation.

The proposed combination of acceptor doped ceria and co-doped or donor doped ceria compositions satisfy these requirements, thus represent a good candidate for OPMs with improved performance.

The membrane materials proposed herein are very stable and enable operation at high temperatures under harsh chemical conditions (e.g. CO, CO2, H2S, water vapour and other aggressive chemicals). This is a great advantage as compared to perovskite materials also applied as OPMs.

For example a fluorite type based material, such as CGO, is much more stable towards harsh, high temperature operating conditions in membrane reactors than other material classes, such as perovskites. By co-doping the electronic

conduction/performance of the membrane layer over a broad range of operating conditions, i.e. extended range of oxygen partial pressures (p0₂), is improved. The lower total conductivity of the new co-doped ceria materials compared to perovskite materials is addressed by the manufacturing and use of supported thin film membrane architectures (asymmetric membranes), resulting in high performance and high stability OPMs.

A number of compositions with potentially improved transport properties have been identified on the basis of the gained knowledge relating to this invention . In particular it has been found that co-doping Gd-doped ceria with Nb or W, results in materials with enhanced electronic conductivity. Preferred compositions having an improved electronic conductivity are for example:

Ceo.89Gdo.1Nbo.01O1.95-x;

Ceo.865Gdo.095Nbo.04O1.95-x;

Ce0.865Gd0.095W0.04O1.95-x.

A comparison of the electronic conductivity of Ceo.865Gdo.095Nbo.04O1.95-x and Ceo.865Gdo.095Wo.04O1.95-x with that of CGO shows that the electronic conductivity of the CGO co-doped with Nb or W is enhanced relative to that of Gd-doped ceria by ca. one order of magnitude.

In some other examples it has been found that co-doping Gd-doped ceria with Zn, results in materials with enhanced electronic conductivity. Preferred compositions having an improved electronic conductivity are for example:

Ceo.9Gdo.1Zno.02O1.95-x;

Ceo.9Gdo.1Zno.05O1.95-x.

Ceo.9Gdo.1Zno.1O1.95-x;

Other examples of tested co-doped Gd-doped ceria are:

Ceo.9Gdo.1Zno.05Pro.02O1.95-x;

Ceo.9Gdo.1Zno.05Pro.05O1.95-x.

In a third aspect of the invention a metal oxide having fluorite structure, for use in oxygen permeation membranes is provided; the metal oxide being doped with a first element, such as an electron acceptor and a second element, such as and electron donor or co-dopant, wherein the metal of the metal oxide and the second element have an ionic size ratio between 0.5 and 1.

The introduction of a co-dopant, may induce changes in the lattice parameter. The introduction of a co-dopant having a smaller ionic size (IS) than the metal of the metal oxide, also referred to as host, decreases the lattice parameter in respect to the un-doped metal oxide, due to the difference in ionic size. IS may be also referred to as ionic radius.

The ionic size of the co-dopant smaller than the one of the metal of the metal oxide, introduces elastic strain in the host lattice, which may modify the redox properties of Ce, leading to facilitated reduction of Ce⁴⁺ to Ce³⁺ and thus enhanced electronic conductivity, due to the increased concentration of electronic carriers (Ce³⁺). As mentioned in some embodiments the first element is an electron acceptor, such as Gd, and the second element is an electron donor, such as Nb.

The introduction of an electron donor or co-dopant, such as Nb or Zn, may induce changes in the lattice parameter as mentioned above. Thus introduction of a second element, also referred to as co-dopant, such as a donor, having a smaller IS than the metal of the metal oxide, also referred to as host, e.g. Nb^v IS=0.66 and Ce^IV IS=0.97, decreases the lattice parameter in respect to the un-doped metal oxide, due to the difference in ionic size.

In a fourth aspect of the invention a metal oxide having fluorite structure, for use in OPMs is provided; the metal oxide being doped with a first element and a second element, wherein the first element and the second element have an ionic size ratio in the range between 1 and 2.5.

In a fifth aspect of the invention a metal oxide having fluorite structure, for use in OPMs is provided; the metal oxide being doped with a first element and a second element wherein the metal of the metal oxide and the first element have an ionic size ratio between 0.5 and 1.5.

In a sixth aspect the invention an electrochemical device for oxygen production, comprising an OPM according to the fifth and all other aspects of the invention is provided.

In a seventh aspect of the invention an electrolysis cell electrode comprising a metal oxide according to any of the preceding aspects of the invention is provided. In another aspect of the invention a solid oxide fuel cell electrode comprising a metal oxide according to any of the preceding aspects of the invention is provided.

In a further aspect of the invention a catalytic membrane reactor comprising a metal oxide according to any of the preceding aspects of the invention is provided.

The co-doped metal oxide of the invention may be used in catalytic membrane reactors, such as in reactors for partial oxidation reactions. In another aspect of the invention an apparatus for permeating oxygen from a gaseous mixture comprising oxygen is provided. The apparatus comprises: a) a first chamber comprising an inlet for the gaseous mixture that is maintained at a pressure PI > latm; b) a second chamber comprising : i) an inlet for a gaseous mixture comprising a reducing or inert gas that is maintained at a pressure P2≤ PI, wherein the oxygen partial pressure difference between the first and the second chamber is maintained higher than 1 atm; and ii) an outlet; c) a gas impermeable barrier separating the two chambers, wherein the barrier comprises an OPM, and wherein the OPM comprises a metal oxide or mixture of metal oxides having fluorite structure, doped with : at least one electron acceptor element and at least one co-dopant element, wherein the metal of the metal oxide and the at least one co-dopant have an ionic size ratio between 0.5 and 1; and wherein the metal oxide having fluorite structure is ceria (Ce0₂-5) wherein δ is in the range between 0 and 0.5. In another aspect a method of doping a metal oxide according to any of the preceding aspects is provided, the method comprising : doping the metal oxide with at least one electron acceptor, thereby producing a doped metal oxide;

doping the doped metal oxide with at least one co-dopant such as an electron donor thereby producing a co-doped metal oxide.

Generally the process of doping a metal oxide with at least one electron acceptor thereby producing an ion conductive oxide such as CGO lies within the person skilled in the art.

For example by mixed oxide method in which ceria and gadolinia powders are mixed together with powders containing at least one co-dopant such as an electron donor, sintered at high temperature, such as 1400 °C and crushed and milled to an homogeneous powder subsequently.

In some other embodiments the co-doping process may be performed in a single step. In some further embodiments the co-doping process may be performed in two separate steps. For example, this doping can be achieved by solid state reactions such as mixing ceria and gadolinia powders and sintering at high temperature, such as 1200 °C, the mixture. To this first step doping with at least one co-dopant such as an electron donor may follow. Doping of a doped metal oxide with at least one co-dopant such as an electron donor may lay within the person skilled in the art. However, doping of a doped metal oxide with at least one co-dopant such as an electron donor may also be achieved by novel methods such as infiltration or impregnation from pre-cursor solutions containing co-dopant such as an electron donor elements.

The oxygen permeation membrane comprising the co-doped metal oxide of the invention may preferably have an asymmetric architecture, e.g. with a porous support of a low cost material such as MgO, YSZ and a thin or ultrathin active membrane layer. The thin membrane layers with a thickness of 15-150μηι

(preferable 15-50 μηι) may be realized by any coating method (described in the prior art), starting from the co-doped powder (dip coating, lamination, screen printing etc.). Ultrathin membranes with a thickness below 15μηι may be realised by using physical or chemical methods (PVD, CVD) with other pre-cursors (than powders). The application of ultra-thin ceria membrane layers by low temperature methods is preferably realized on metal supports with the advantage of reduced cost and robustness.

The main idea of the invention is co-doping of a metal oxide with an electron acceptor and co-dopant such as an an electron donor. In this way an oxygen membrane with increased electron conductivity, reduced chemical expansion and reduced thermal expansion is obtained. Surprisingly, despite the decreased ionic conductivity that is an undesirable effect for oxygen membranes, the overall performance of the oxygen membrane of the invention is improved as a higher oxygen flux can be achieved.

Thus the invention provide a new class of mixed conducting materials for asymmetric oxygen membranes allowing sufficiently high mixed conductivity over a wider range of operation conditions including high oxygen partial pressures. This new class of mixed conducting materials produces high oxygen fluxes, when used to produce OPMs and have the advantage of moderate chemical expansion and low thermal expansion compared to other MIEC membrane materials.

Further this new class of mixed conducting materials has a high stability towards aggressive gas impurities and strongly reducing environments.

In search for a chemically stable metal oxide having high electrical conductivity and low thermal expansion, the inventors devise the invention by improving electrical conductivity and reducing the thermal expansion of the chemically stable CGO through the doping of CGO with a co-dopant such as an electron donor. The first, second and other aspects or embodiments of the present invention may each be combined with any of the other aspects or embodiments. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The metal oxide for use in OPM according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figure 1 is a schematic drawing of an OPM according to some embodiments of the invention.

Figure 2 is a schematic drawing of a catalytic membrane reactor according to some embodiments of the invention.

Figure 3 is a data representation of the ionic conductivity of CGO co-doped with Nb or W at 1 and 4 at% in comparison with CGO as a function of temperature in air.

Figure 4 is a data representation of the electronic conductivity of CGO co-doped with Nb or W in comparison with CGO as a function of the oxygen activity at different temperatures.

Figure 5 is a data representation of the simulated oxygen permeation flux for a 30μΓη thick OPM made of CGO co-doped at different at% of Nb or W in comparison to CGO as a function of the oxygen activity at the anode (permeate) side at different temperatures.

Figure 6 shows a comparison of the electronic conductivity of Ceo.9Gdo.1Zno.02O1.95- x (Fig. 6a) and Ceo.9Gdo.1Zno.05O1.95-x (Fig. 6b) with the one of Gd-doped ceria as a function of the oxygen activity at 600, 700, 800 and 900 °C.

Figure 7 shows the ionic conductivity as a function of reciprocal temperature in the temperature range 900°C- 600°C.

Figure 8 shows an apparatus for permeating oxygen from a gaseous mixture comprising oxygen according to some aspects of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

The OPM 1 is sandwiched between a cathode 2 and an anode 3. Air may flow through cathode 2 as shown by the arrow 4. The gradient in the chemical potential of oxygen across the membrane provides the driving force for oxygen diffusion as shown by arrow 11. The oxide ionic current flowing through the membrane is counterbalanced by an electronic current of the same magnitude flowing in the opposite direction, as shown by arrow 12, thereby supporting a continuous flux of oxygen through the membrane.

Oxygen depleted air exits the cathode 2 following arrow 4. At the anode 3 different reactions may occur depending on the function of device 5. For example if device 5 aims at the production of syngas, methane may be fed to the anode 3 and may flow through anode 3 following arrow 6. Syngas is thus produced and exits the anode 3 following arrow 6.

In some other embodiment pure oxygen is produced that exits the anode 3 following arrow 6. The driving force for oxygen permeation is provided 1) by connecting a pump at the permeate side that establishes vacuum conditions or 2) by flowing elevated pressure air/oxygen containing gas at the feed side or 3) by both methods 1 and 2.

In further applications the outgas of an oxyfuel plant (comprising mainly CO2) is fed to the anode 3 and may flow through the anode 3 following the arrow 6.

Oxygen rich CO2 gas is thus produced and exits the anode 3 following the arrow 6. This gas is then fed to the inlet of the oxyfuel plant, for combustion of coal.

Figure 2 is a schematic drawing of a catalytic membrane reactor 7 according to some embodiments of the invention. The oxygen membrane 8 is a thin mixed oxide ionic and electronic conducting (MIEC) layer between the porous cathode 9, where air is flowing and the porous anode support 10, where methane or other hydrocarbons are fed and syngas or oxygenates are produced.

The performance of an OPM is characterised by the oxygen flux that it can deliver. This is given by the Wagner equation :

where σ_θι and σ\ are the electronic and ionic conductivities of the MIEC membrane and are functions of the oxygen activity. R is the ideal gas constant, T the temperature, L is the thickness of the membrane, F is the Faraday constant, and po2^an' ^s and po2^cat' ^s are the values of the oxygen activity just inside the MIEC at the permeate (anode) and feed (cathode) side, respectively.

One of the best ionic conductors known today is CGO. This material has a low electronic conductivity at oxygen activities above 10 ¹⁵ atm (at relevant temperatures of ca. 700 °C), but it's electronic conductivity increases with decreasing oxygen activity, rendering this material very interesting as an OPM for applications where a low oxygen activity is established at the permeate side (e.g. syngas production and catalytic partial oxidation of hydrocarbons).

OPM components with CGO have achieved very high fluxes, approaching a value of 10 Nml min ¹ cnr² at 800 °C, and exceeding 16 Nml min ¹ cnr² at 900 °C.

In search for higher oxygen fluxes in CGO, the inventors devise the current invention.

The performance of CGO as an OPM is limited by its electronic conductivity.

Larger oxygen fluxes in Gd-doped ceria may be achievable if one could enhance the electronic conductivity of this material.

Figure 3 is a data representation of ionic conductivity of Ceo.865Gdo.095Nbo.04O1.95-x (4Nb), Ce0.865Gd0.095W0.04O1.95-x (4W), Ceo.89Gdo.1Nbo.01O1.95-x (INb),

Ceo.89Gdo.1Wo.01O1.95-x (1W) in comparison with CGO (CGO) as a function of temperature.

It can be clearly seen that co-doping with Nb or W results in reduced ionic conductivity as compared to CGO, which is an undesirable effect with respect to the performance of an oxygen permeation membrane. Figure 4 is a data representation of the electron conductivity of

Ceo.865Gdo.095Nbo.04O1.95-x (4%Nb) and Ce0.865Gd0.095W0.04O1.95-x (4% W) in comparison to Gd-doped ceria (CGOIO) as a function of the oxygen activity at 600, 700, 800 and 900 °C.

Despite the reduced ionic conductivity it can be seen that the doping with Nb or W leads to a significant enhancement of the electronic conductivity as compared to CGO.

As mentioned the metal oxide of the invention has also an increased dimensional stability as shown in table 1. It can be noticed that the thermal expansion coefficient from RT to the

temperature of 700, 800 and 900 °C of CGO is reduced in 4Nb and even further reduced in 4W.

Table 1 also shows that the chemical expansion at a temperature such as 800 and 5 900 °C is also reduced in 4Nb and 4W in comparison with CGO. The reduction of chemical expansion arises from a reduced chemical expansion coefficient. Indeed the change in oxygen nonstoichiometry is larger for the co-doped metal oxides 4Nb and 4W than for CGO under similar conditions. Thus the reduced chemical expansion arises from a reduced chemical expansion coefficient.

0

Table 1

Figure 5 is a data representation of simulated oxygen permeation flux for a 30μηι thick OPM made of Ceo.865Gdo.095N bo.04O1.95-x (4%Nb), Ce0.865Gd0.095W0.04O1.95-x5 (4% W) and Ceo.89Gdo.1N bo.01O1.95-x (l%Nb) in comparison to CGO as a function of the oxygen activity at the anode (permeate) side at different temperatures.

It appears that the performance of ceria based OPMs is limited by their electronic conductivity as co-doping Gd-doped ceria with Nb or W have a positive effect on the OPM performance.

0 Thus figure 5 shows the simulated performance of the hereby claimed

compositions as OPMs in comparison to that simulated for the state of the art material, i.e. CGO.

As it can be seen from figure 5, improved performance is expected for the Nb or W co-doped compositions, especially at temperatures of 800 °C or lower.

5 Further improvement may be expected for compositions containing greater

amounts of oxide ion vacancies, i.e. compositions more heavily doped in acceptors.

Table 2 shows the data related to other co-doped metal oxide according to some embodiments of the invention. Table 2

Figure 6 shows a comparison of the electronic conductivity of Ceo.9Gdo.1Zno.05O1.95- x and Ceo.9Gdo.1Zno.02O1.95-x. with that of Gd-doped ceria. As can be seen, the electronic conductivity of Ceo.9Gdo.1Zno.02O1.95-x and Ceo.9Gdo.1Zno.05O1.95-x is enhanced relative to that of Gd-doped ceria by ca. 1-2 orders of magnitude below 700°C.

The co- doping with Zn results also in a slightly reduced ionic conductivity as compared to Gd-doped ceria. Nevertheless, because the performance of ceria based OPMs is limited by their electronic conductivity, co-doping Gd-doped ceria with Zn has a positive effect on the performance of the OPM comprising the co- doped metal oxide.

Improvements in the electronic conductivity may be forseen for compositions containing greater amounts of oxide ion vacancies, i.e. compositions more heavily doped in electron acceptors.

In order to improve electronic conductivity composites of acceptor doped ceria and co-doped such as donor doped ceria powders may be used. In this way electronic conductivity improvements greater than the one shown in this example may be foreseen as the adverse effect of the reduced ionic conductivity would be minimized.

Figure 8 shows an apparatus 20 for permeating oxygen from a gaseous mixture comprising oxygen, the apparatus 20 comprising : a first chamber 21, a second chamber 22 and a gas impermeabile barrier comprising an OPM 23. The apparatus 20, first chamber 21, second chamber 22 and OPM 23 are adapted according to some aspects of the invention,

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A metal oxide having fluorite structure, for use in oxygen permeation membrane, doped with : at least one electron acceptor element and at least one co-dopant element, wherein the metal of said metal oxide and said at least one co-dopant have an ionic size ratio between 0.5 and 1; and wherein said metal oxide having fluorite structure is ceria (Ce02-5) wherein δ is in the range between 0 and 0.5; thereby producing a co-doped metal oxide having an electron conductivity higher than said metal oxide.

2. A metal oxide according to claim 1, wherein said at least one electron acceptor is a rare earth.

3. A metal oxide according to any of the preceding claims wherein said at least one electron acceptor is an element of group IIIA.

4. A metal oxide according to any of the preceding claims wherein said at least one electron acceptor is gadolinium.

5. A metal oxide according to any of the preceding claims 1-2 wherein said at least one electron acceptor is an element of group IIA.

6. A metal oxide according to claim 5 wherein said at least one electron acceptor is calcium.

7. A metal oxide according to any of the preceding claims wherein said at least one co-dopant element is a transition metal.

8. A metal oxide according to any of the preceding claims wherein said at least one co-dopant element is an element of group VA, VIA, IIB or an actinide.

9. A metal oxide according to any of the preceding claims wherein said at least one co-dopant element is niobium or tungsten or uranium.

10. A metal oxide according to any of the preceding claims 1-8 wherein said at least one co-dopant element is Mg, Sc, In, Ga, Zr or Sn.

11. A metal oxide according to any of the preceding claims 1-8 wherein said at least one co-dopant element is Zn.

12. A metal oxide according to any of the preceding claims wherein said doped metal oxide has the general formula :

(CexAy)i zDz02- 5, wherein D is said at least one co-dopant according to any of the preceding claims and A is said at least one electron acceptor according to any of the preceding claims and wherein x is in the range between 0 and 1, y is in the range between 0 and 1, Z is in the range between 0 and 0.2 and δ is the range between 0 and 0.5.

13. A metal oxide mixture comprising a mixture of metal oxides according to any of the claims 1-12.

14. An oxygen permeation membrane (OPM) comprising a metal oxide or metal oxide mixture according to any of the preceding claims.

15. A catalytic membrane reactor comprising a metal oxide or metal oxide mixture according to any of the preceding claims 1-13.

16. An electrolysis cell electrode comprising a metal oxide or metal oxide mixture according to any of the preceding claims 1-13.

17. A solid oxide fuel cell electrode comprising a metal oxide or metal oxide mixture according to any of the preceding claims 1-13.

18. An electrochemical device for oxygen production, comprising an OPM according to claim 14.

19. An apparatus for permeating oxygen from a gaseous mixture comprising oxygen, said apparatus comprising :

a) a first chamber comprising an inlet for said gaseous mixture that is maintained at a pressure PI > latm ; b) a second chamber comprising : i) an inlet for a gaseous mixture comprising a reducing or inert gas that is maintained at a pressure P2≤ PI, wherein the oxygen partial pressure difference between the first and the second chamber is maintained higher than 1 atm; and ii) an outlet;

c) a gas impermeable barrier separating the two chambers, wherein the barrier comprises an OPM, and wherein the OPM comprises a metal oxide or mixture of metal oxides having fiuorite structure, doped with: at least one electron acceptor element and at least one co-dopant element, wherein the metal of said metal oxide and said at least one co-dopant have an ionic size ratio between 0.5 and 1; and wherein said metal oxide having fiuorite structure is ceria (Ce02-5) wherein δ is in the range between 0 and 0.5.

20. A method of doping a metal oxide according to any of the preceding claims 1- 12, the method comprising :

- doping said metal oxide with at least one electron acceptor;

- doping said metal oxide with at least one co-dopant;

thereby increasing the electron conductivity of said metal oxide.