Mixed metal oxide material

Abstract

A material is disclosed comprising a first mixed metal oxide comprising a first metal species and a second metal species in a first atomic ratio, and a second mixed metal oxide comprising the first metal species and the second metal species in a second atomic ratio. The first mixed metal oxide and the second mixed metal oxide have the same crystal structure, and the first atomic ratio and second atomic ratio are different. Also disclosed is a material comprising a first mixed metal oxide that may be represented as the composition A1-xBxO2-δ, and a second mixed metal oxide may be represented as the composition A1-yByO2-δ. The first mixed metal oxide and the second mixed metal oxide have the same crystal structure, and 0.01 < x ≤ 0.5, 0.01 < y ≤ 0.5. A and B are each selected from Ln, Sc, Y, Zr, or a combination thereof; Ln is a lanthanide, and δ is the degree of oxygen deficiency.

Description

[0001] MIXED METAL OXIDE MATERIAL

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to materials, and in particular, to mixed metal oxide materials. The present invention also relates to electrochemical cells comprising at least one layer comprising such materials, to ceramic inks, to methods of manufacturing such ceramic inks, and to methods of forming dense ceramic layers on the surface of a substrate.

[0004] BACKGROUND OF THE INVENTION

[0005] Mixed metal oxides are metal oxides that contain cations of more than one metallic element and / or cations of one metallic element in more than one oxidation state. Mixed metal oxides find uses in many areas of technology. One important use is in electrochemical cells formed of oxide layers (often known as solid oxide cells: SOC). SOCs may be used as oxygen separators, electrochemical sensors, fuel cells, electrolyser / electrolysis cells or in other electrochemical devices.

[0006] SOC fuel cell units produce electricity using an electrochemical conversion process that oxidises fuel. SOC cell units can also, or instead, operate as regenerative fuel cell (or reverse fuel cell) units, often known as solid oxide electrolyser fuel cell units, for example to separate hydrogen and oxygen from water, or carbon monoxide and oxygen from carbon dioxide. SOC units generally use an oxygen-ion conducting mixed metal oxide containing ceramic as an electrolyte. Many ceramic oxygen ion conductors (for instance, doped zirconium oxide or doped cerium oxide) have useful ion conductivities at temperatures in excess of 500°C (for cerium-oxide based electrolytes) or 650°C (for zirconium oxide-based ceramics), so SOCs tend to operate at elevated temperatures.

[0007] A solid oxide fuel cell (SOFC) generates electrical energy through the electrochemical oxidation of a fuel gas (usually hydrogen-based). In operation, the electrolyte of the SOFC conducts oxygen ions from a cathode to an anode located on opposite sides of the electrolyte. A fuel, for example a fuel derived from the reforming of a hydrocarbon or alcohol, contacts the anode (usually known as the “fuel electrode”) and an oxidant, such as air or an oxygen rich fluid, contacts the cathode (usually known as the “air electrode”).A solid oxide electrolyse! cell (SOEC) may have the same structure as an SOFC but is, in practice, an SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water and / or carbon dioxide.

[0008] Conventional ceramic-supported (e.g. anode- supported) SOCs have low mechanical strength and are vulnerable to fracture. Hence, metal-supported SOCs have recently been developed which have the active fuel cell component layers supported on a metal substrate. In these cells, the ceramic layers can be very thin since they only perform an electrochemical function: that is to say, the ceramic layers are not self-supporting but rather are thin coatings / films laid down on and supported by the metal substrate. Such metal supported SOC stacks are more robust, lower cost, have better thermal properties than ceramic-supported SOCs and can be sealed using conventional metal welding techniques.

[0009] Applicant’s earlier patent application WO-A-2015 / 136295 discloses metal-supported SOFCs in which the electrochemically active layer (or active fuel cell component layer) comprises anode, electrolyte and cathode layers respectively deposited (e.g. as thin coatings / films) on, and supported by, a metal support plate (e.g. foil). The metal support plate has a porous region (formed by holes drilled through the metal foil substrate) surrounded by a non-porous region with the active layers being deposited upon the porous region so that gases may pass through the pores from one side of the metal support plate to the opposite side to access the active layers coated thereon. Applicant’s earlier patent application GB-A-2456445 discloses depositing a layer of metal oxide crystalline ceramic on a substrate.

[0010] An important consideration for oxide layers in electrochemical cells (and in other uses) is the porosity of some of the oxide layers. Electrolyte layers of relatively high porosity may allow diffusion of gases from one side of the cell to the other, resulting in reduced efficiency of the electrochemical device. For example, in a fuel cell, the fuel gas may diffuse through the electrolyte layer(s) to the air side or vice versa. In the manufacture of oxide layers in electrochemical cells, each layer of the oxide material may be deposited on the substrate or on a lower layer, dried (if necessary) and sintered at elevated temperature to densify the layer and thereby reduce porosity. Sintering of mixed metal oxide may also occur in other uses. There is a need to ensure that for some layers of an SOC (e.g. the electrolyte) the porosity of mixed metal oxides is as low as possible after sintering. It is an aim of the present invention to address such a need.SUMMARY OF THE INVENTION

[0011] The present invention accordingly provides, in a first aspect, a material comprising

[0012] a first mixed metal oxide comprising a first metal species and a second metal species in a first atomic ratio, and

[0013] a second mixed metal oxide comprising the first metal species and the second metal species in a second atomic ratio,

[0014] wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, and

[0015] wherein the first atomic ratio and second atomic ratio are different.

[0016] This is advantageous because the inventors have discovered that, surprisingly, sintering of metal oxide materials (for example, ceria or zirconia) may be enhanced by mixing two powders with different doping levels, in particular when the dopant (e.g. second metal species) is aliovalent to the first metal species. The mixture of dopant levels may be chosen such that the average dopant level is around the desired final dopant level in the final product. The final product may be a sintered electrolyte, electrode or any other layer or structure where a degree of densification is important. Without wishing to be bound, it is thought that, surprisingly, sintering and densification is enhanced by the concentration gradient of dopant and host cations between adjacent particles, creating a driving force for cation diffusion, which is generally considered to be the rate-limiting step in sintering since anion diffusion is very rapid, particularly in fast oxygen ion conductors such as electrolyte materials.

[0017] Generally, the first metal species and a second metal species may be different.

[0018] In the crystal structure of the first metal oxide and the second metal oxide, the first metal species and the second metal species may occupy equivalent crystallographic sites in the structure.

[0019] The crystal structure may be selected from an orthorhombic, a tetragonal or a cubic crystal structure, preferably a cubic crystal structure. Usually, ceria and stabilised zirconia have a cubic crystal structure.

[0020] The crystal structure may be selected from a fluorite crystal structure, a perovskite crystal structure, or a spinel crystal structure.Suitably, the first atomic ratio and / or the second atomic ratio are the corresponding atomic ratios of the first metal species to the second metal species. The first atomic ratio of the first metal species to the second metal species may be between 1 : 1 and 1 :0.01, and / or the second atomic ratio of the first metal species to the second metal species may be between 1 : 1 and 1 :0.01. The first and / or second atomic ratios may optionally be in the range 1 : 1 to 1 :0.03, optionally 1:1 to 1:0.05. Optionally, the first and / or second atomic ratios of the first metal species to the second metal species may be between 1 : 1 and 1 :0.07, or the first and / or second atomic ratio of the first metal species to the second metal species may be between 1 : 1 and 1:0.1. Thus, the first and / or second atomic ratio of the first metal species to the second metal species (i.e. atomic ratio of primary to secondary cation) should preferably be such that the amount of the second metal species is not greater than the first because this may affect the crystal structure. Higher amounts of the second metal species may result in a two phase material.

[0021] The first metal species and the second metal species may each be selected from Ln, Sc, Y, Zr, or a combination thereof, wherein Ln is a lanthanide.

[0022] If the second metal species of the first metal oxide or the second metal species of the second metal oxide is a combination of two, three, four or more different metal species, the first and / or second metal oxide may be referred to as multiply doped.

[0023] In one embodiment, the first metal oxide may be represented as the composition AI-XBXO2-5, and the second metal oxide may be represented as the composition Ai-yByCh-s, wherein: A and B are each selected from Ln, Sc, Y, Zr, or a combination thereof, wherein Ln is a lanthanide; 0.01 < x < 0.5, 0.01 < y < 0.5

[0024]

[0025] y, and 5 is the degree of oxygen deficiency. Generally, the first metal species and the second metal species may be aliovalent cations. In a second aspect, the present invention provides a material comprising

[0026] a first mixed metal oxide that may be represented as the composition Ai-XBXO2-5, and

[0027] a second mixed metal oxide may be represented as the composition Ai-yByC - 5,

[0028] wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, andwherein: 0.01 < x < 0.5, 0.01 < y < 0.5, x y, and

[0029] wherein A and B are each selected from Ln, Sc, Y, Zr, or a combination thereof, wherein Ln is a lanthanide, and 5 is the degree of oxygen deficiency.

[0030] In an embodiment, A may be Ce; and B may be selected from Pr, Gd, Sm or mixtures thereof. Thus, the first or second mixed metal oxide and the material may comprise ceria doped with at least one rare earth element selected from Y, Sc or a lanthanide (Ln).

[0031] The doped ceria may be selected from samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), samaria- gadolinia doped ceria (SGDC) and mixtures thereof. Gd and Sm are advantageous and may yield higher ionic conductivities. The doped ceria may be of formula Ce(i-X)MxO(2-o.5x-5) where 0<x<0.5, (M = Sc, Y, Ln or a mixture). The dopant concentration in the ceria may be in the range 3 to 45 atom %, optionally 5 to 40 atom %, optionally 10 to 20 atom %.

[0032] When A is Ce, preferably B may be selected from Pr, Gd, Sm or mixtures thereof. Thus, doped ceria may comprise, for example, Ce(i-X)PrxO(2-o.5x-5), Ce(i-X)GdxO(2-o.5x-5), Ce(i-x)SmxO(2- o.5x-5), or doped ceria with mixed dopants (e.g. Gd / Sm). In an embodiment B may be Gd, and the doped ceria may therefore be gadolinium doped ceria also known as cerium gadolinium oxide (CGO). Examples of particular materials may be Ceo.9Gdo.iO(i.95-5) (CGOIO), Ceo.95Gdo.o50(i.975-5) (CGO5), Ceo.sGdo.20(i.9o-5) (CGO20), or Ceo.6Gdo.40(i.8-5) (CGO40). In an embodiment, A may be Zr.

[0033] Thus, the first or second mixed metal oxide and the material may comprise zirconia doped with at least one rare earth element selected from Y, Sc or a lanthanide (Ln). An example of rare earth element may be Y, or Yb.

[0034] The doped zirconia may be selected from scandia stabilised zirconia (ScSZ), yttria stabilised zirconia (YSZ), scandia ceria co-stabilised zirconia (ScCeSZ), ytterbia stabilised zirconia (YbSZ), scandia yttria co-stabilised zirconia (ScYSZ) and mixtures thereof. Doped zirconia may be of formula Zr(i.X)MxO(2-o.5x-5) where 0<x<0.2, and M is a rare earth element (M = Sc, Y, Ln or a mixture). The dopant concentration in the zirconia may be in the range 3 to 25 atom %, optionally in the range 3 to 25 atom %, optionally in the range 4 to 25 atom %, optionally in the range 5 to 25 atom %, optionally in the range 5 to 22 atom %, optionally in the range 5 to 20 atom %, optionally in the range 5 to 17 atom %, optionally in the range 5 to 15 atom %, optionally in the range 5 to 12 atom %, optionally in the range 6 to 12 atom %.Where the dopant is Y, the Y dopant concentration in the zirconia may be in the range 5 to 15 atom %, optionally about 8 atom%. Where the dopant is Sc, the Sc dopant concentration in the zirconia may be in the range 5 to 15 atom %, optionally about 10 atom%. Where the dopant is Yb, the Yb dopant concentration in the zirconia may be in the range 5 to 25 atom %, optionally about 5 atom% to 16 atom%, optionally about 5 atom% to 15 atom%.

[0035] When A is Zr, preferably B may be selected from Y, Yb, Sc, alkaline earth metal (e.g. Mg, Ca, Sr, Ba), or mixtures thereof. Optionally, Ce may be added as a stabilizing element (but preferably is not the only stabilizing element). If present, amounts of Ce should be less than the other B components (or components) (e.g. preferably less than about 5 atom%).

[0036] The first mixed metal oxide and / or the second mixed metal oxide may be double or multiply doped so that B comprises two (or more) metal species, the second dopant may be the same or different to the first dopant.

[0037] Thus, the first mixed metal oxide may be represented as the composition Ai.xi-x2BxiB’x2O2-5, and wherein the second mixed metal oxide may be represented as the composition Ai-yi-y2ByiB”y2O2-5, wherein B, B’ and B” are independently selected from Ln, Sc, Y, or Zr, wherein Ln is at least one lanthanide.

[0038] In materials according to the second (or first) aspect preferably |x-y| > 0.05, optionally > 0.1, optionally > 0.2 (where the first or second mixed metal oxide are double doped, x=xl+x2, y=yl+y2).

[0039] In a further embodiment, the invention provides a material comprising:

[0040] a first mixed metal oxide that may be represented as the composition Ai- wA wBi-xB xCL-s, and

[0041] a second mixed metal oxide may be represented as the composition Ai-yA’yBi. zB’zCL-s,

[0042] wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, and

[0043] wherein 0 < w, x, y, z < 1,

[0044]

[0045] A, A’, B, and B’ are metal species, and 5 is the degree of oxygen deficiency.Generally, the weight ratio of the first mixed metal oxide to the second mixed metal oxide may be in the range 1:10 to 10:1, optionally 1:5 to 5:1, optionally 1:3 to 3:1, optionally 1:2 to 2:1, optionally 1:1.

[0046] Usually, the material may be in particulate form, the first mixed metal oxide may be in particulate form, and / or the second mixed metal oxide may be in particulate form. The first mixed metal oxide and / or the second mixed metal oxide independently may have a particle size in the range between Inm and 1pm.

[0047] To further improve sintering and densification, the material may further comprise a sintering aid. The sintering aid may comprise, for example, Fe, Mn, Co, Cu, Ni, or mixtures thereof. The sintering aid preferably comprises Fe. The material may comprise 5weight% or more sintering aid. Once sintered, the material may have a density of 95% or greater of theoretical density, optionally wherein the layer has a density of 96% or greater of theoretical density. Generally, the material may be a ceramic material, the first mixed metal oxide may be a ceramic material, and / or the second mixed metal oxide may be a ceramic material.

[0048] Materials according to the invention find uses in many fields. One important use is in electrochemical cells / devices.

[0049] Thus, in a third aspect, the present invention provides an electrochemical cell (preferably a solid oxide electrochemical cell) comprising at least one layer comprising a material according to the first or second aspect.

[0050] The layer may be an electrolyte layer, an electron blocking layer or a reaction preventing layer.

[0051] Preferably, the layer may have a density of 95% or greater of theoretical density, optionally wherein the layer has a density of 98% or greater of theoretical density.

[0052] In preferred embodiments, the layer may comprise doped ceria or doped zirconia.

[0053] When the layer comprises doped ceria, ceria may be doped with one or more dopants selected from Gd, Sm, or Pr, and wherein when the layer comprises doped zirconia, zirconia may be doped with one or more dopants selected from Y, Yb, Ce, or Sc.

[0054] The electrochemical cell may be supported or disposed on a metallic or ceramic substrate.The substrate may comprise a metallic foil (i.e. solid metal) in which openings are provided. That has an advantage that the porosity can be tailored and positioned in specific areas of the substrate. Alternatively, or in addition, a substrate may have inherent porosity (e.g. isotropic porosity) formed for example as tape cast by powder depositing a film that is then sintered to form a porous substrate. References herein to metal supports or a porous steel sheet may refer to either of these.

[0055] The substrate may comprise steel, preferably stainless steel. Usually, the substrate may comprise a drilled metal support, optionally a laser drilled metal support.

[0056] In some circumstances (e.g. where it is desired to further protect the metal support from corrosion) the porous metal support may comprise a barrier layer on the surface thereof and the electrode layer may be on the barrier layer.

[0057] The electrochemical cell may comprise other layers.

[0058] Electrochemical cells according to the third aspect may be arranged in a stack of electrochemical cell units, electrically connected in series.

[0059] Materials according to the disclosure may be deposited as layers using, for example, a ceramic ink. Thus, in a further aspect, the present invention provides a ceramic ink comprising a dispersion of:

[0060] a first mixed metal oxide comprising a first metal species and a second metal species in a first atomic ratio,

[0061] a second mixed metal oxide comprising the first metal species and the second metal species in a second atomic ratio, and

[0062] one or more of a solvent, dispersant, binder, wetting agent and / or plasticiser, wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, and

[0063] wherein the first atomic ratio and second atomic ratio are different.

[0064] As discussed above in relation to the material, sintering aids may improve the sintering of the material. Thus, the ceramic ink may further comprise a sintering aid. The sintering aid may comprise a metal or metal oxide comprising Co, Cu, Ni, Fe or mixtures thereof.

[0065] Alternatively, the ceramic ink may not comprise a sintering aid.The ceramic ink may be manufactured by milling. Thus, in a fifth aspect, the present invention provides a method of manufacturing a ceramic ink, the method comprising:

[0066] milling a first mixed metal oxide comprising a first metal species and a second metal species in a first atomic ratio, a second mixed metal oxide comprising the first metal species and the second metal species in a second atomic ratio, and one or more of a solvent, dispersant, binder, wetting agent and / or plasticiser,

[0067] wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, and

[0068] wherein the first atomic ratio and second atomic ratio are different.

[0069] The material may be deposited to form a dense material layer on the surface of a substrate either by printing or using other methods. Thus, in a sixth aspect the present invention provides a method of forming a dense ceramic layer on the surface of a substrate, the method comprising,

[0070] applying a material or precursor of a material according to the first or second aspect on to the surface of the substrate,

[0071] optionally drying the layer, and

[0072] heating the layer at a temperature in the range 400 °C to 1400 °C to sinter the layer.

[0073] Heating the layer (i.e. the sintering step) may be at a temperature in the range 600 °C to 1550 °C. Sintering may optionally be performed at a temperature in the range 750 °C to 1400 °C, optionally from 800 °C to 1250 °C, optionally from 800 °C to 1100 °C, optionally from 800 °C to 970 °C. Sintering may be performed in an air atmosphere.

[0074] In an embodiment, the precursor of a material may comprise a ceramic ink according to the fifth aspect.

[0075] Applying the ceramic ink may be by printing, optionally screen printing, roller printing or ink jet printing. Other methods of applying the ceramic ink include spraying (for example atomised spraying).

[0076] As discussed herein, the substrate may comprise a metallic or ceramic substrate, optionally having layers thereon.Preferably, the dense ceramic layer may have a density of 95% or greater of the theoretical density of the material of the dense ceramic layer once deposited and sintered.

[0077] The layer may be a layer in an electrochemical cell. Thus, the dense ceramic layer may comprise an electrolyte layer, electron blocking layer or reaction prevention layer of an electrochemical cell.

[0078] Definitions

[0079] In this specification, the term ‘metal’ refers to any of the alkali metals, alkaline earth metals, transition metals, post transition metals, lanthanides and actinides.

[0080] In this specification, the terms “rare earth metal” or “rare earth element” refer to metals selected from Y, Sc, and lanthanoid.

[0081] “Lanthanoid”, “lanthanide” and “Ln” are used interchangeably and mean the metallic chemical elements with atomic numbers 57-71.

[0082] The term "dopant" as used herein is not intended to be restricted to a maximum percentage of elements, ions or compounds added to chemical structures. Similarly, the term "doping" is intended to mean the addition of a certain amount of elements, ions or compounds to a material. It is not limited to a maximum quantity of material, after which, further addition of material no longer constitutes doping.

[0083] 5 is the degree of oxygen deficiency and may vary depending on the environment and history of the relevant composition or material. Generally, 5 may be 0.25 or lower, suitably 0.2 or lower and more suitably < 0.15. 5 may have a lower limit of 0.0001, optionally 0.001, optionally 0.005, optionally 0.01, optionally 0.05.

[0084] The term "fluorite structure" as used herein refers to a single network of chemically bonded crystal structures which have a generally fluorite (AB2) structure.

[0085] The term "perovskite structure" as used herein refers to a single network of chemically bonded crystal structures which have a generally perovskite (ABX3) structure.

[0086] The term “solid oxide cell” (SOC) is intended to encompass both solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs).The term “source of’ an element, compound or other material refers to a material comprising the element, compound or other material whether or not chemically bonded in the source. The source of the element, compound or other material may be an elemental source (e.g. Ln, Ni or O2) or may be in the form of a compound or mixture comprising the element, compound or other material including one or more of those elements, compounds or materials.

[0087] In this specification references to electrochemical cell, SOC, SOFC and SOEC may refer to tubular or planar cells. Electrochemical cell units may be tubular or planar in configuration. Planar fuel cell units may be arranged overlying one another in a stack arrangement, for example 100-200 fuel cell units in a stack, with the individual fuel cell units arranged electrically in series. References to “a stack of electrochemical cells” therefore refer to a plurality of electrochemical cells units arranged electrically in series.

[0088] Electrochemical cells may be fuel cells, reversible fuel cells or electrolyser cells. Generally, these cells may have the same structure and reference to electrochemical cells may refer (unless the context suggests otherwise) to any of these types of cell.

[0089] “Oxidant electrode” or “air electrode” and “fuel electrode” are used herein and may be used interchangeably to refer to cathodes and anodes respectively of SOFCs because of potential confusion between fuel cells or electrolyser / electrolysis cells.

[0090] Electrochemical cells as encompassed by the invention may comprise:

[0091] a) two planar components welded together with fluid volume in between (e.g. substrate with electrochemical layers and interconnecter (separate plate))

[0092] b) three planar components welded together with fluid volume in between (e.g. substrate with electrochemical layers and interconnecter (separate plate) and spacer providing fluid volume).

[0093] The various features of aspects of the disclosure as described herein may be used in combination with any other feature in the same or other aspect of the disclosure, if needed with appropriate modification, as would be understood by the person skilled in the art.

[0094] Furthermore, although all aspects of the invention or disclosure preferably “comprise” the features described in relation to that aspect, it is specifically envisaged that they may “consist” or “consist essentially” of those features outlined in the claims.

[0095] The invention will now be described with reference to accompanying figures and examples.BRIEF DESCRIPTION OF THE FIGURE

[0096] Figure 1 shows in (a) a plot of pushrod dilatometer results of relative change (%) in length for samples of CGO20 / CGO5 (1 part weight to 2 parts weight; curve 102) and CGOIO (standard, curve 100) as a function of temperature, and in (b) the rate of change of the relative change as a function of temperature.

[0097] Figure 2 shows in (a) a plot of pushrod dilatometer results of relative change (%) in length for samples of CG040 / CG010 (curve 104) and CGOIO (standard, curve 100) as a function of temperature, and in (b) the rate of change of the relative change as a function of temperature. Figure 3 shows in (a) a plot of pushrod dilatometer results of relative change (%) in length for various samples of YbSZ as a function of temperature , and in (b) the rate of change of the relative change as a function of temperature.

[0098] Figure 4 shows a cross section of an electrochemical cell unit.

[0099] DETAILED DESCRIPTION OF THE INVENTION

[0100] Figure 4 shows, schematically and not to scale (for reasons of clarity) a cross section of an electrochemical cell unit 2 that may be a SOFC or SOEC. A ferritic stainless steel metal support 4 has a peripheral, non-porous portion 6 and a central, porous portion 8 where holes have been drilled through the metal support 4. A barrier layer (not shown) to reduce corrosion is located on the surface of the metal support 4. A layer of a fuel electrode 10 of Ni:CG0 of thickness 15 pm to 35 pm is located on the porous portion 8 of the metal support 4. A first electrolyte layer 12 of rare earth doped ceria (RE=Y, Sc or any Ln) of thickness 4 pm or greater (optionally 6 pm to 12 pm) is located on the fuel electrode layer 10. A second electrolyte layer 14 of rare earth (RE) stabilised zirconia (RE = Y, Sc or any Ln, e.g. Yb) of thickness 0.5 pm or greater (e.g. 1 pm to 4 pm) is located on the first electrolyte layer 12. The first electrolyte layer 12 surrounds the fuel electrode layer 10 to prevent gas flowing through the fuel electrode layer 10 from the fuel side 22 to the air (oxidant) side 20 or vice versa. In some other embodiments the second electrolyte layer 14 may also surrounds the fuel electrode layer 10 to prevent gas flowing through the fuel electrode layer 10 from the fuel side 22 to the air (oxidant) side 20 or vice versa.The cell unit is completed with a second (air) electrode assembly located on the second electrolyte layer 14. The second electrode 16 may be formed of an active air electrode layer 16 and a bulk air electrode layer 18. The active second electrode layer and bulk second electrode layer may comprise a suitable material, the bulk layer material selected for example from lanthanum cobaltite, lanthanum ferrite, lanthanum nickel ferrite, Lao.99Coo.4Nio.60(3-5) (LCN60) and mixtures thereof. Densification of the electrolyte layers 12 and 14 may be achieved by use of material according to this disclosure which may result in enhanced density and corresponding lower porosity.

[0101] It has been discovered that sintering of electrolyte materials (e.g. doped ceria and doped zirconia) of aliovalently-doped materials may be enhanced by mixing two powders with different aliovalent doping levels and sintering these materials together. The mixture of dopant levels and weight ratio of the oxides with differing doping levels may be chosen such that the average dopant level is around the desired final dopant level in the sintered electrolyte. It appears, without wishing to be bound, that sintering and densification is enhanced by the concentration gradient of dopant and host cations between adjacent particles, creating a large driving force for cation diffusion, which is generally considered to be the rate-limiting step in sintering since anion diffusion is very rapid in these fast oxygen ion conductors.

[0102] Good sintering of CGO has been observed when mixing powders of cerium gadolinium oxide (CGO) having different doping levels of Gd.

[0103] Two parts per weight cerium gadolinium oxide with 5% doping (i.e. Ceo.95Gdo.o50(i.975-5) (CG05)) was mixed with 1 part per weight cerium gadolinium oxide with 20% doping (Ceo.sGdo.20(i.9-5) (CGO20)) to give an average of 10% doping. Pellets of this material were prepared and the relative length change measured as a function of sintering temperature using a pushrod dilatometer. The relative change (%) is shown in Figure la and the rate of change of the relative change is shown in Figure lb compared to CGO 10 as standard (10% doping). The peak of the CGO5 / CGO20 curve 102 (see in particular Figure 1 b) is at lower temperature that the CGOIO curve 100, indicating lower temperature i.e. enhanced sintering.One part per weight cerium gadolinium oxide with 10% doping (i.e. Ceo.9Gdo.iO(i,95- 5) (CGOIO)) was mixed with 2 part per weight cerium gadolinium oxide with 40% doping (Ceo.6Gdo.40(i.8-5) (CGO40)) to give an average of 30% doping. Pellets of this material were prepared and the relative length change measured as a function of sintering temperature using a pushrod dilatometer. The relative change (%) is shown in Figure 2a and the rate of change of the relative change is shown in Figure 2b compared to CGOIO as standard (10% doping). The peak of the CG040 / CG010 curve 104 (see Figure 2b) is at a significantly lower temperature that the CGOIO curve 100, indicating lower temperature i.e. enhanced sintering. The density of two samples of the CGO40 / CGO10 (average 30% doping) material were measured at 7.14 g / cm3and 7.15 g / cm3. The theoretical density of CGO30 is 7.27g / cm3so the densification of the CG040 / CG010 material is 98.4%. For comparison, the density of the CGOIO material was measured at 7.05 g / cm3(theory 7.23 g / cm3) giving a densification of 97.6%.

[0104] Zirconia tends to be inherently harder to sinter at low temperatures, but examples using Yb doped zirconia shows advantageous effects.

[0105] Yb doped zirconia with 3 weight % Ytterbium oxide ((Zr02)o.97(Yb203)o.o3; 3YbSZ), and 10 weight % Ytterbium oxide ((Zr02)o.9(Yb203)o.i; lOYbSZ) were prepared. Mixed materials of 50:50 (by weight) 3 YbSZ and lOYbSZ and 70:50 (by weight) lOYbSZ and 3 YbSZ were prepared and sintered (in air at a temperature of up to 1070°C). Median density (Archimedes method) results for the materials are shown in Table 1, below

[0106]

[0107] Table 1: Density of YbSZ MaterialsPellets of as made lOYbSZ, 3YbSZ, the 50 / 50 and 70 / 30 materials (pellets of 3.89 mm to 4.43 mm length and 5.17 mm to 5.59 mm diameter) were prepared and the relative length change measured as a function of sintering temperature using a pushrod dilatometer. The relative change (%) is shown in Figure 3 a, and the rate of change of the relative change is shown in Figure 3b.

[0108] The temperatures and dy / dx for the peaks for each of the materials as shown in Figure 3b are as described in Table 2, below:

[0109]

[0110] Table 2.

[0111] Reference Numerals

[0112] 2 electrochemical cell

[0113] 4 metal support

[0114] 6 non-porous portion of metal support

[0115] 8 porous portion of metal support

[0116] 10 fuel electrode layer

[0117] 12 first electrolyte layer

[0118] 14 second electrolyte layer

[0119] 16 second (air) electrode active layer

[0120] 18 second (air) electrode bulk layer

[0121] 20 oxidant (air) side

[0122] 22 fuel side

[0123] 100 CGO standard curve (CGO10)

[0124] 102 CGO20 / CGO5 (weight ratio 2 / 1) curve104 CG040 / CG010 (weight ratio 2 / 1) curve

[0125] All publications mentioned in the above specification are herein incorporated by reference. Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be performed therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

Claims

Claims1. A material comprisinga first mixed metal oxide comprising a first metal species and a second metal species in a first atomic ratio, anda second mixed metal oxide comprising the first metal species and the second metal species in a second atomic ratio,wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, andwherein the first atomic ratio and second atomic ratio are different.

2. A material as claimed in claim 1, wherein in the crystal structure of the first metal oxide and the second metal oxide, the first metal species and the second metal species occupy equivalent crystallographic sites in the structure.

3. A material as claimed in either claim 1 or claim 2, wherein the crystal structure is selected from an orthorhombic, a tetragonal or a cubic crystal structure.

4. A material as claimed in any one of the preceding claims, wherein the crystal structure is selected from a fluorite crystal structure, a perovskite crystal structure, or a spinel crystal structure.

5. A material as claimed in any one of the preceding claims, wherein the first atomic ratio of the first metal species to the second metal species is between 1 : 1 and 1 :0.01, and / or the second atomic ratio of the first metal species to the second metal species is between 1 : 1 and 1:0.01.

6. A material as claimed in claim 5, wherein the first atomic ratio of the first metal species to the second metal species is between 1 : 1 to 1 :0.05, and / or the second atomic ratio of the first metal species to the second metal species is between 1 : 1 to 1 :0.05.

7. A material as claimed in any one of the preceding claims, wherein the first metal species and the second metal species are each selected from Ln, Sc, Y, Zr, or a combination thereof, wherein Ln is a lanthanide.

8. A material as claimed in any one of the preceding claims, wherein the first metal oxide may be represented as the composition AI-XBXO2-5, and the second metal oxide may be represented as the composition Ai-yByCh-s, wherein: A and B are each selected from Ln, Sc, Y, Zr, or a combination thereof, wherein Ln is a lanthanide; 0.01 < x < 0.5, 0.01 < y < 0.5, x y, and 5 is the degree of oxygen deficiency9. A material as claimed in any one of the preceding claims, wherein the first metal species and the second metal species are aliovalent cations.

10. A material comprisinga first mixed metal oxide that may be represented as the composition Ai-XBXO2-5, anda second mixed metal oxide may be represented as the composition Ai-yByC - 5,wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, andwherein: 0.01 < x < 0.5, 0.01 < y < 0.5,y, andwherein A and B are each selected from Ln, Sc, Y, Zr, or a combination thereof, wherein Ln is a lanthanide, and 5 is the degree of oxygen deficiency.

11. A material as claimed in any one of claims 8 to 10, wherein A is Ce.

12. A material as claimed in claim 11, wherein B is selected from Pr, Gd, Sm or mixtures thereof.

13. A material as claimed in any one of the preceding claims 8 to 10, wherein A is Zr.

14. A material as claimed in claim 13, wherein B is selected from Y, Yb, Sc, Ce, Ca, Mg, or mixtures thereof.

15. A material as claimed in any one of claims 8 to 10, wherein the first mixed metal oxide may be represented as the composition Ai-xi-xiBxiB^Ch-s, and wherein the second mixed metal oxide may be represented as the composition Ai-yi.y2ByiB”y2O2-5, wherein B, B’ and B” are independently selected from Ln, Sc, Y, or Zr, wherein Ln is at least one lanthanide.

16. A material as claimed in any one of the preceding claims 8 to 15, wherein |x-y| > 0.05, optionally > 0.1, optionally > 0.2.

17. A material as claimed in any one of the preceding claims, wherein the weight ratio of the first mixed metal oxide to the second mixed metal oxide is in the range 1 : 10 to 10:1, optionally 1:5 to 5:1, optionally 1:3 to 3:1, optionally 1:2 to 2:1, optionally 1:1.

18. An electrochemical cell comprising at least one layer comprising a material according to any one of claims 1 to 17.

19. An electrochemical cell as claimed in claim 18, wherein the layer is an electrolyte layer, an electron blocking layer or a reaction preventing layer.

20. An electrochemical cell as claimed in either claim 18 or claim 19, wherein the layer has a density of 95% or greater of theoretical density, optionally wherein the layer has density of 98% or greater of theoretical density.

21. An electrochemical cell as claimed in any one of claims 18 to 20, wherein the layer comprises doped ceria or doped zirconia.

22. An electrochemical cell as claimed in claim 21, wherein when the layer comprises doped ceria, ceria is doped with one or more dopants selected from Gd, Sm, or Pr, and wherein when the layer comprises doped zirconia, zirconia is doped with one or more dopants selected from Y, Yb, or Sc.

23. A ceramic ink comprising a dispersion of:a first mixed metal oxide comprising a first metal species and a second metal species in a first atomic ratio,a second mixed metal oxide comprising the first metal species and the second metal species in a second atomic ratio, andone or more of a solvent, dispersant, binder, wetting agent and / or plasticiser, wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, andwherein the first atomic ratio and second atomic ratio are different.

24. A method of forming a dense ceramic layer on the surface of a substrate, the method comprising,applying a material or precursor of a material according to any one of claims 1 to 17 on to the surface of the substrate,optionally drying the layer, andheating the layer at a temperature in the range 400 °C to 1400 °C to sinter the layer.

25. A material comprisinga first mixed metal oxide that may be represented as the composition Ai- wA wBi-xB xOs-g, anda second mixed metal oxide may be represented as the composition Ai-yA’yBi. zB’zCh-s,wherein the first mixed metal oxide and the second mixed metal oxide have the same crystal structure, andwherein 0 < w, x, y, z < 1,A, A’, B, and B’ are metal species, and 5 is the degree of oxygen deficiency.

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