Barium and cerium-based mixed oxide and preparation method thereof

Abstract

The present invention relates to barium and cerium mixed oxides, to a method for preparation thereof, and their use as electrolyte in power generation cells. Furthermore, the present invention relates to power generation cells, like solid oxide fuel cells, solid oxide electrolysis cells, or proton ceramic fuel cells, containing the mixed oxide of the invention.

Description

[0001] BARIUM AND CERIUM-BASED MIXED OXIDE AND PREPARATION METHOD THEREOF TECHNICAL FIELD

[0002] The present invention relates to barium and cerium mixed oxides, to a method for preparation thereof, and their use as electrolyte in power generation cells. Furthermore, the present invention relates to power generation cells, like solid oxide fuel cells, solid oxide electrolysis cells, or protonic ceramic fuel cells, containing the mixed oxide of the invention.

[0003] TECHNICAL BACKGROUND

[0004] Power generation cells, like solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), or proton ceramic fuel cells (PCFCs), are clean energy conversion devices and widely used due to their advantages of high flue adaptability, cost-efficiency, and their ceramic all-solid-state structure.

[0005] In SOFCs the electricity is directly produced by oxidizing fuel and metal oxides, like stabilized zirconium and / or cerium mixed oxides, used as electrolytes in the cells. These metal mixed oxides show a sufficient conductivity at operating temperatures of 800 °C or higher in SOFCs. However, the use of these metal oxides in SOFCs brings problems, for example, the cell packaging is difficult, such cells need long startup and shutdown cycles, and the operating costs are high, in particular due to the high energy costs.

[0006] Therefore, in the last years metal oxides and compositions thereof have been developed which allow the decrease of the operating temperatures.

[0007] Proton-conducting electrolyte materials used in PCFCs have been considered promising candidates due to their high proton mobility and high ionic conductivity in the low and intermediate temperature range, which is between 300 °C to 700 °C. Barium-, zirconium and / or cerium-based mixed oxides are potential candidates.

[0008] In order to provide the required conductivity in power generation cells, the metal oxides need a sufficiently high density, i.e., low porosity. For this reason, the metal oxides must be densified (sintered) before they can be used as electrolytes in power generation cells. One problem in this regard is that the temperatures which are needed to obtain the desired densification of the metal oxides are comparatively high, for example 1550 °C or 1700 °C as described in JP 2002265263. The necessary high sintering temperatures also cause undesired great energy consumption and / or segregation of barium or other metals, for example yttrium or ytterbium, which are used for doping these metal oxides to improve the electrical conductivity.

[0009] An approach to solve this problem is the use of sintering aid components, such as copper oxides, zinc oxides or lithium carbonate, during the sintering of the metal oxides, which reduces the sintering temperature (see e.g. CN 103 224394 or CN 102515750). However, the sintering aid components are incorporated in the mixed oxide during this process and cause reduction in electrical properties of the mixed oxide electrolyte (see e.g. International Journal of Hydrogen Energy 47, (2022), pages 40054-40066, Zhaoyu Zhong et al., facile method to synthesizeBaZr aiCe 0.7Y aiYb o.iOs-b (BZCYYb) nano-powders for the application on highly conductive proton-conducting electrolytes’"').

[0010] Therefore, metal oxides are desired, which have the required high density after sintering at comparatively low temperatures, and which are produced with low amounts of sintering aid components or preferably without the use of sintering aid components. Some suitable metal oxides are known in the prior art, however, even these metal oxides need a sintering temperature around 1400 °C (see e.g. scientific publication of Pooja Sawant et al. International Journal of Hydrogen Energy, 37, (2012), 3848 to 3856, “Synthesis, stability and conductivity ofBaCeo.s-xZrxY0.2O3-S as electrolyte for proton conducting SO FC.")

[0011] Consequently, there was still the need to provide mixed oxides suitable for power generation cells that can be produced by a process having reduced energy consumption.

[0012] SUMMARY OF THE INVENTION

[0013] The present invention refers to a mixed oxide comprising at least barium and cerium, and optionally at least zirconium and / or at least one rare-earth metal other than cerium (RE), characterized in that the mixed oxide:

[0014] - exhibits a relative density RDi3oo°c / 5h of at least 94% after sintering the mixed oxide at a temperature of 1300 °C for 5 hours, and

[0015] - contains at most 0.1 wt.-% of a sintering aid component or of a mixture of sintering aid components, based on the total weight of the mixed oxide.

[0016] Furthermore, the present invention provides a preparation method for producing the mixed oxide of the invention comprising the following steps:

[0017] (a) contacting an aqueous solution comprising at least a cerium salt and optionally zirconium and / or a rare-earth metal salt other than cerium salt (RE) with an aqueous precipitant solution containing a precipitant component to form a liquid medium containing a precipitate;

[0018] (b) heating the liquid medium containing a precipitate obtained in step (a) at a temperature of from 60 °C to 200 °C to obtain a heat-treated precipitate;

[0019] (c) optionally cooling and / or separating and / or washing the heat-treated precipitate obtained in step (b);

[0020] (d) drying the heat-treated precipitate obtained in step (b) or (c) at a temperature ranging from 50 °C to 700 °C;

[0021] (e) optionally milling the dried precipitate obtained in step (d);

[0022] (f) mixing the dried precipitate obtained in step (d) or (e) with a barium component; (g) calcining the mixture obtained in step (f) at a temperature of from 700 °C to 1200 °C; (h) optionally milling the calcinated material obtained in step (g).

[0023] The mixed oxide of the invention can be used in power generation cells, for example in solid oxide fuel cell (SOFC), a solid oxide electrolysis cell (SOEC), or a proton ceramic fuelcell (PCFC), whereby the power generation cell comprises one porous anode (A) and one porous cathode (C) which are separated by at least one layer (L), characterized in that at least one from (A), (C) or (L) is prepared from or comprises a mixed oxide of the invention.

[0024] BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 refers to X-ray diffraction patterns of the mixed oxide of Example 1 calcined at 960 °C for various periods.

[0026] Figure 2 refers to the relative density of the mixed oxide of Example 1 determined after sintering the produced mixed oxide at 1200 °C, 1250 °C and 1300 °C for 5 hours.

[0027] DETAILED DESCRIPTION OF THE INVENTION

[0028] Before the issues of the invention are described in detail, the following should be considered:

[0029] As used herein, the singular forms "a", "an", and "the" include both singular and plural referents unless the context clearly dictates otherwise. By way of example, "a compound" means one compound or more than one compound.

[0030] The terms "comprising", "comprises" and "comprised of' as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms "comprising", "comprises" and "comprised of' as used herein comprise the terms "consisting of', "consists" and "consists of'.

[0031] Throughout this application, the term "about" is used to indicate that a value includes the standard deviation of error for the device or method being employed to determine the value.

[0032] As used herein, the term "average" refers to number average unless indicated otherwise. As used herein, the terms "% by weight", "wt.- %", "weight percentage", or "percentage by weight", and the terms "% by volume", "vol. - %", "volume percentage", or "percentage by volume", are used interchangeably.

[0033] The recitation of numerical ranges by end points includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 2, 3, and 4 when referring to, for example, a number of elements, and can also include e.g. 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g., from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0034] Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

[0035] Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in theart to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention.

[0036] The term “rare-earth” as used herein means the chemical elements scandium, yttrium and the elements having the atomic numbers 57 to 71 in the periodic table, i.e., La, Ce, Pr, Nd, Pm. Sm, Eu, Gd, TB, Dy, Ho, Er, Tm, Yb and Lu.

[0037] The term “sintering” as used herein means, the thermal transformation of a bulk material into a compact solid at a temperature below the melting temperature of said material (see e.g. https: / / www.sciencedirect.com / topics / materials-science / sintering). The obtained sintered material is preferably homogeneous and / or has preferably a controlled porosity.

[0038] The term “impurities” as used herein refers to components which may stem from the raw materials or starting materials used in the process of preparation of the composite oxide. According to the invention, the term does not include sintering aid components.

[0039] The terms “metal mixed oxide” and “mixed oxide” as used herein are synonymous to each other.

[0040] The term “heating”, “period for heating” or “heating for a period” as used herein refers to the time of temperature ramp to reach the targeted temperature plus the time of maintaining said temperature, unless explicitly specified otherwise.

[0041] In the following passages, different alternatives, embodiments and variants of the invention are defined in more detail. Each alternative and embodiment so defined may be combined with any other alternative and embodiment, and this for each variant unless clearly indicated to the contrary or clearly incompatible when the value range of a same parameter is disjoined. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0042] Furthermore, the particular features, structures or characteristics described in the present description may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some, but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

[0043] The present invention refers to a mixed oxide comprising at least barium (B) and cerium (C), and optionally at least zirconium (Z) and / or at least one rare-earth metal other than cerium (RE), characterized in that the mixed oxide:

[0044] - exhibits a relative density RDi3oo°c / 5h of at least 94% after sintering the mixed oxide at a temperature of 1300 °C for 5 hours, and

[0045] - contains at most 0.1 wt.-% of a sintering aid component or of a mixture of sintering components, based on the total weight of the mixed oxide.

[0046] In a preferred embodiment of the invention, the mixed oxide essentially consists of, more preferably consists of, at least barium and cerium and optionally of zirconium and / or at leastone rare-earth metal other than cerium (RE) oxide and small amounts of impurities, which are usually present in a mixed oxide due to the manufacturing process of the mixed oxide. Small amounts of impurities means that the impurities are present in the mixed oxide in an amount of 0.1 wt.-% or less, based on the total amount of the mixed oxide. Additionally, the mixed oxide of this embodiment contains at most 0.1 wt.-% of a sintering aid component or of a mixture of sintering aid components, based on the total weight of the mixed oxide.

[0047] In a further preferred embodiment, the mixed oxide essentially consists of, more preferably consists of, at least barium and cerium and optionally of zirconium and / or at least one rare-earth metal other than cerium (RE) oxide and small amounts of impurities, which are usually present in a mixed oxide. The mixed oxide of this embodiment does not contain additionally a sintering aid component.

[0048] In a preferred embodiment, the mixed oxide is an oxide of barium and cerium (BC). The rare-earth metal other than cerium (RE) according to the invention is preferably selected from the group consisting of yttrium, neodymium, ytterbium, lanthanum, gadolinium, praseodymium, samarium, europium, terbium and combinations thereof, more preferably consisting of yttrium, ytterbium and combinations thereof, most preferably the RE is a combination of yttrium (Y) and ytterbium (Yb).

[0049] According to the invention, it is preferred that the mixed oxide of the invention comprises at least zirconium, more preferably comprises at least zirconium and at least one RE, which is preferably yttrium, most preferably the mixed oxide of the invention comprises zirconium, yttrium and ytterbium. In other words, in preferred embodiments of the invention the mixed oxide is a mixed oxide of barium, zirconium and cerium (BZC), or a mixed oxide of barium, zirconium, cerium and yttrium (BZCY), or a mixed oxide of barium, zirconium, cerium, yttrium and ytterbium (BZCYYb).

[0050] The molar portion of barium present in the mixed oxide of the invention is preferably between 30% and 70 %, more preferably between 30% and 55% based on the total molar ratio of barium and cerium and if present zirconium and / or RE expressed in %.

[0051] Furthermore, it is preferred that the proportion of barium present in the mixed oxide is between 30% and 70 % by weight, preferably between 30% and 55% by weight, this proportion being expressed as weight of barium expressed in form of oxide relative to the total weight of the mixed oxide.

[0052] Additionally, it is preferred that the proportion of cerium is between 10% and 70 % by weight, more preferably between 20% and 60% by weight, this proportion being expressed as weight of cerium expressed in form of oxide relative to the total weight of the mixed oxide.

[0053] Moreover, according to the invention, the proportion of zirconium, RE or a combination thereof is preferably between 0% and 40 % by weight, more preferably between 10% and 30% by weight, this proportion being expressed as weight of zirconium, RE or a combination thereof expressed in the form of oxide relative to the total weight of the mixed oxide.If the mixed oxide contains a RE, especially contains yttrium as RE, the proportion of yttrium oxide is preferably between 1% and 20% by weight, preferably between 2% and 10% by weight, this proportion being expressed as weight of yttrium expressed in the form of oxide relative to the total weight of the mixed oxide.

[0054] If the RE is ytterbium, the proportion of ytterbium oxide is between 1% and 20 % by weight, preferably between 2% and 10% by weight, this proportion being expressed as weight of ytterbium expressed in the form of oxide relative to the total weight of the mixed oxide.

[0055] The mixed oxide of the invention is usually in form of a powder.

[0056] According to the invention, the X-ray diffractogram (CuKai, =1.5406 Angstrom) of mixed oxide preferably exhibits patterns of a solid solution of at least a main crystal phase.

[0057] The mixed oxide according to the invention preferably exhibits the patterns of a solid solution of at least a main crystal phase. The main crystal phase is preferably a perovskite phase. Hence, the x-ray diffractogram of the mixed oxide of the invention preferably has a single peak P located at a 29 angle of between 28° and 32°, which is preferably the highest peak of the intensity of diffractogram. Nevertheless, the mixed oxide of the invention may contain other crystal phase like crystal phases of BaCCh.

[0058] Even though, the mixed oxide of the invention should have a high purity, as mentioned above, low amounts of impurities that might be present in the mixed oxide are acceptable. The impurities may stem from the raw materials or starting materials used in the process of preparation of the composite oxide. According to the invention, a total proportion of the impurities can be equal to or lower than 0.10 wt.-%, preferably equal to or lower than 0.05 wt.-%, or more preferably equal to or lower than 0.01 wt.%, based on the total weight of the mixed oxide, to ensure that the good electrical properties of the mixed oxide are not reduced or get lost. In preferred embodiment, the mixed oxide does not contain any impurity, which are usually present in the mixed oxides as known in the prior art.

[0059] It is also desired to lower as much as possible the amount of sintering aid component that can be used in the sintering process of the mixed oxide. According to the invention, a sintering aid component is a component that is, when used in an effective amount (this effective amount being strictly higher than 0.10 wt.-% based on the total weight of the mixed oxide), capable of lowering the sintering temperature of at least 50 °C, preferably of at least 80 °C, more preferably of at least 100 °C to obtain a determined density of the mixed oxide, whereby the sintering temperature is the temperature, which is needed to obtain a mixed oxide having the determined density without the aid of the sintering aid component. The determined density of mixed oxide is the density that is needed to ensure that the mixed oxide has a sufficient conductivity at the operation conditions of power generation cells. Preferably, the proton conductivity of the mixed oxide is at least 0.005 Scrn'1at 700 °C. The mixed oxide according to the invention has a proton conductivity of preferably at least 0.008 Scrn'1at 700 °C, more preferably of at least 0.01 Scrn'1at 700 °C.Sintering aid components are well known in the art, examples therefore are nitrates, carbonates, halides or oxides of the elements Li, Sc, B, Bi, Cr, Ca, Si, Zn, Cu, Co, Fe, Mn, or Ni.

[0060] The amount of the sintering aid component possibly present in the mixed oxide of the invention is at most 0.10 wt.-%, preferably at most 0.05 wt.-%, more preferably 0.00 wt.-%, based on the total weight of the mixed oxide. In a preferred embodiment, the mixed oxide consists of at least barium and cerium, and optionally at least zirconium and / or at least one rare-earth metal other than cerium (RE), and at most 0.1 wt.-% of a sintering aid component or of a mixture of sintering components, based on the total weight of the mixed oxide. In particular, it is preferred that the mixed oxide does not contain (be free of) a sintering aid component. More preferably the mixed oxide of the invention contains as elements only barium, cerium and optionally zirconium and / or RE as defined herein.

[0061] The determined density, which is needed to ensure that the mixed oxide shows the required conductivity in power generation cells, is the relative density of the mixed oxide. The relative density corresponds to the density of the mixed oxide over the absolute density of the mixed oxide expressed in % (RD in % = density / absolute density x 100).

[0062] In the context of the invention, the density of the mixed oxide is measured on a compacted sample of the mixed oxide after calcination of the compacted sample in air at the temperature of the test (1200 °C, 1250 °C or 1300 °C) for 5 hours. The compacted sample is obtained by applying a pressure on the composite oxide in the powder form having been introduced into a molding die. Conveniently, the die has the form of a parallelepiped since this form allows an easy measurement of the volume after the calcination in air.

[0063] More specifically, the compacted sample may be prepared by the following method: i) the powder of the composite oxide is weighed and introduced into a molding die; ii) the powder is compressed under pressure;

[0064] iii) the compacted sample obtained at the end of step ii) is taken out from the molding die;

[0065] iv) the compacted sample is then calcined in air at the targeted temperature for 5 hours;

[0066] v) the density of the compacted sample having been calcined is measured;

[0067] vi) the relative density is then calculated.

[0068] The density is measured according to any of the experimental techniques known to the skilled person.

[0069] For step v), the following method may more particularly be used:

[0070] vl) the compacted sample having been calcined is weighed (weight in g);

[0071] v2) the volume of the compacted sample is measured (volume in cm3), the compacted sample being in the form of a parallelepiped and the volume of the parallelepiped is given by:

[0072] volume = length (cm) x width (cm) x height (cm).The length, width and height of the parallelepiped can be conveniently measured with a micrometer. It is also possible to measure the volume of the parallelepiped by Archimedes' principle as described in paragraph “Measurement methods” below.

[0073] In step ii), the pressure applied to obtain the compacted sample is preferably at least 95 MPa or even at least 98 MPa.

[0074] The conditions of the test given in the examples may conveniently be used.

[0075] The absolute densities (or theoretical densities) are determined by first calculating the lattice parameters of the structure of the composite oxide from the XRD pattern and calculating the absolute density in accordance with the formula below:

[0076] absolute density (g / cm3) = SA / (a x b x c x N)

[0077] SA represents the molar mass of all the atoms in the unit lattice, a, b and c each denotes the lattice parameter, and N denotes the Avogadro’s constant.

[0078] The following absolute densities, which may be used for the calculations, are enclosed in the table below:

[0079]

[0080] The mixed oxide of the invention exhibits a relative density RDi3oo°c / 5h of at least 94%, preferably of at least 96%, determined after sintering the mixed oxide at a temperature of 1300 °C for 5 hours. In other words, for the mixed oxide of the invention a sintering temperature of 1300 °C is sufficient to obtain the required conductivity for power generation cells.

[0081] Furthermore, it is preferred that the mixed oxide of the invention exhibits a relative density RDi25o°c / 5h of at least 80%, more preferably of at least 85%, most preferably of at least 90%, after sintering the mixed oxide at a temperature of 1250 °C for 5 hours.

[0082] Additionally, it is preferred that the mixed oxide exhibits a relative density RDi2oo°c / 5h of at least 70%, more preferably of at least 74%, most preferably of at least 78%, after sintering the mixed oxide at a temperature of 1200 °C for 5 hours.The relative densities of the mixed oxide as mentioned above are determined on samples without the addition of any other material like a sintering aid component.

[0083] Furthermore, the mixed oxide of the invention exhibits several physicochemical characteristics as described below.

[0084] It is preferred that the mixed oxide has a BET specific surface area from 1 to 100 m2 / g, preferably from 1 m2 / g to 20 m2 / g, more preferably from 2 m2 / g to 15 m2 / g, determined by nitrogen adsorption using Brunauer-Emmett-Teller (BET) method.

[0085] The BET surface area is well-known in the art and measured by nitrogen adsorption using the well-known Brunauer-Emmett-Teller method. The BET method is particularly described in the journal “The Journal of the American Chemical Society, 60, 309 (1938) It is possible to comply with the recommendations of the standard ASTM D3663 - 03.

[0086] Furthermore, it is preferred that the mixed oxide has a particle diameter DBET of from 0.050 pm to 1.000 pm, preferably of from 0.070 pm to 0.500 pm, more preferably of from 0.100 pm to 0.200 pm, calculated by the following formula

[0087] DBET = 6 / pS,

[0088] wherein

[0089] p represents the theoretical density in [g / cm3] of the main mixed oxide, i.e., of the perovskite phase of the mixed oxide, and

[0090] S represents the BET specific surface area in [m2 / g] determined by the BET method as described above.

[0091] The mixed oxide of the invention may have a particle size D50 of from 0.050 pm to 0.500 pm, preferably of from 0.080 pm to 0.300 pm, more preferably of from 0.100 pm to 0.200 pm, determined by laser diffraction.

[0092] Furthermore, the mixed oxide of the invention may have a particle size D10 of from 0.010 pm to 0.200 pm, preferably of from 0.020 pm to 0.150 pm, more preferably of from 0.030 pm to 0.130 pm, determined by laser diffraction.

[0093] Additionally, the mixed oxide may have a particle size D90 of from 0.150 pm to 100 pm, preferably of from 0.200 pm to 50.0 pm, more preferably of from 0.240 pm to 1.0 pm, determined by laser diffraction.

[0094] The parameters D10, D90 and D50 are obtained from a distribution in volume determined by laser diffraction. D10 is the diameter determined from a distribution obtained by laser diffraction for which 10% of the particles have a diameter of less than D10. D90 is the diameter determined from a distribution obtained by laser diffraction for which 90% of the particles have a diameter of less than D90. D50 is the diameter determined from a distribution obtained by laser diffraction for which 50% of the particles have a diameter of less than D50. D50 is also denoted the median of the distribution.

[0095] It is further preferred that the mixed oxide of the invention has a ratio of DBET / D50 of from 0.3 to 1.5, preferably of from 0.4 to 1.4, more preferably of from 0.5 to 1.3.According to the invention, the mixed oxide preferably exhibits in the range of pores with a diameter of less than or equal to 200 nm a peak for which the maximum corresponds to a pore diameter Dp between 10 nm and 200 nm, preferably between 20 nm and 150 nm, determined by nitrogen porosimetry.

[0096] Furthermore, according to the invention the pore volume for the pores with a diameter lower or equal to 200 nm (Vp< 200 nm) may be between 0.01 ml / g and 0.5 ml / g, preferably between 0.02 ml / g and 0.1 ml / g, determined by nitrogen porosimetry.

[0097] Additionally, according to the invention, the distribution in the volume of the size of particles of the mixed oxide obtained by laser diffraction may exhibit single population Pl, optionally two populations Pl and P2 with the following characteristics:

[0098] - Population Pl centered at a size DI of between 0.05 pm and 0.5 pm, preferably between 0.1 pm and 0.3 pm,

[0099] and / or

[0100] - Population P2 centered at a size D2 of between 3.0 pm and 100 pm, preferably between 5 pm and 60 pm;

[0101] wherein the intensity of Pl is higher than of P2 and the expression “population centred at a size D” meaning that on the distribution curve, a peak is observed the maximum of which is located at D.

[0102] The mixed oxide of the invention as defined above, is preferably produced by the preparation process of the invention as described below.

[0103] The process of the invention comprises the following steps:

[0104] (a) contacting an aqueous solution comprising at least a cerium salt and optionally zirconium and / or a rare earth metal salt other than cerium salt with an aqueous precipitant solution containing a precipitant component to form a liquid medium containing a precipitate;

[0105] (b) heating the liquid medium containing a precipitate obtained in step (a) at a temperature of from 60 °C to 200 °C to obtain a heat-treated precipitate;

[0106] (c) optionally cooling and / or separating and / or washing the heat-treated precipitate obtained in step (b);

[0107] (d) drying the heat-treated precipitate obtained in step (b) or (c) at a temperature ranging from 50 °C to 700 °C;

[0108] (e) optionally milling the dried precipitate obtained in step (d);

[0109] (f) mixing the dried precipitate obtained in step (d) or (e) with a barium component; (g) calcining the mixture obtained in step (f) at a temperature of from 700 °C to 1200 °C; (h) optionally milling the calcinated material obtained in step (g).

[0110] In a preferred embodiment, the process consists of the steps as indicated above.

[0111] According to the invention it is preferred that the precipitant component of the aqueous precipitant solution used in process step (a) is selected from the group consisting of ammonium hydrogen carbonate (NH4HCO3), carbon dioxide gas, ammonium hydroxide, and a mixturethereof. More preferably, the precipitant component is NH4HCO3. The pH value of the aqueous precipitant solution is preferably between 6.5 and 9.0, more preferably between 6.9 and 7.0.

[0112] In a preferred embodiment of the invention, the molar ratio of precipitant component (preferably NH4HCO3) to the total mole number of cerium, RE and zirconium is between 3 and 6, preferably between 3 and 5.

[0113] Furthermore, it is preferred that the cerium salt used in process step (a) and if use the salt of zirconium and / or RE is a salt selected from the group consisting of carbonates, nitrates, halides and hydroxides, whereby the type of salt of cerium, zirconium and RE must not be the same. Nevertheless, in a further preferred embodiment the type of salts of cerium, zirconium and RE used in this process step (a) of the invention is the same, for example all salts are nitrates.

[0114] The contact of the solutions in step (a) can be simultaneous or successive. When the contact of the solutions in step (a) is successive, it can be carried out in any order. Step (a) can notably be carried out by adding the aqueous solution comprising at least a cerium salt and optionally zirconium and / or a rare earth metal salt other than cerium salt to the aqueous precipitant solution. Alternatively, step (a) can be carried out by adding the aqueous precipitant solution to the aqueous solution comprising at least a cerium salt and optionally zirconium and / or a rare earth metal salt other than cerium salt. According to a preferred embodiment step (a) is carried out by adding the aqueous solution comprising at least a cerium salt and optionally zirconium and / or a rare earth metal salt other than cerium salt to the aqueous precipitant solution.

[0115] Moreover, in contrast to the processes as described in the prior art, for example as described in “ Journal of the European Ceramic Society 35 (2015) 2109, A novel facile way to synthesize proton-conducting Ba(Ce,Zr,Y)Ch solid solution with improved sinterability and electrical performance”, there is no need to use in process step (a) a capping agent to obtain the mixed oxide of the invention. Capping agents are used in the preparation of nanoparticles as stabilizers that inhibit the over-growth of nanoparticles and prevent their aggregation / coagulation in colloidal synthesis, and for example are components selected from the group consisting of surfactants, polymers, dendrimers, amino acids, or polysaccharides; in particular, surfactant such as hexadecyltrimethylammonium bromide (CTAB) are usually used in the processes as described in the prior art.

[0116] Process step (b) of the invention is carried out such that the liquid medium containing a precipitate obtained in step (a) is heated at a temperature of from 60 °C to 200 °C, preferably of from 70 °C to 150 °C, more preferably of from 80 °C to 100 °C. Furthermore, it is preferred that said heating step is conducted for a period of less than 20 hour, more preferably of less than 10 hours, to obtain a heat-treated precipitate. In particular, it is preferred that heating step (b) of the invention is carried out at a temperature of 80 °C to 100 °C for less than 10 hours. The time for heating, as mentioned above, includes the time of temperature ramp to reach the targeted temperature plus the time of maintaining at said temperature.In another embodiment it is preferred that in process step (b) the liquid medium containing a precipitate obtained in step (a) is heated up to a temperature of from 60 °C to 200 °C, preferably of from 70 °C to 150 °C, more preferably of from 80 °C to 100 °C in a heating ramp period for less than 20 hours, less than 10 hours, more preferably for less than 5 hours or 1 hour, even more preferably the heating ramp period is between 20 minutes and 2 hours. In this embodiment it is further preferred that after reaching the targeted temperature said temperature is not maintained for a heating period as defined above but that the further process steps of the invention are carried out.

[0117] The drying step (d) of the process according to the invention is carried out at a temperature ranging from 50 °C to 700 °C, preferably from 150 °C to 600 °C, more preferably from 300 °C to 400 °C. This process step is preferably conducted for a period of 2 hours to 15 hours, more preferably for a period of 3 hours to 10 hours. In particular it is preferred that the drying step is carried out at temperature between 325 °C and 400 °C for 3 hours to 10 hours.

[0118] The such obtained dried precipitate preferably has a BET specific surface area of from 20 m2 / g to 150 m2 / g, preferably of from 50 m2 / g to 120 m2 / g, more preferably of from 70 m2 / g to 90 m2 / g, determined by nitrogen adsorption using Brunauer-Emmett-Teller (BET) method as mentioned above. Additionally, the pore volume of said precipitate is preferably of at least 0.18 ml / g, more preferably of at least 0.2 ml / g determined by nitrogen porosimetry.

[0119] Afterwards, the dried precipitate is combined / mixed with the barium component. This can be done by any method and equipment usually used in the art. The barium component used in this process step is preferably selected from the group consisting of barium carbonate (BaCCE), barium oxide, barium hydroxide, barium nitrate, and combinations thereof, more preferably the barium component is barium carbonate. The barium component may be in the form of a particles powder. In such a case, the barium component particle size D50 may advantageously be of 5 pm or less, in particular of 1 pm or less. D50 is the diameter determined from a distribution obtained by laser diffraction for which 50% of the particles have a diameter of less than D50.

[0120] After that said mixing step, the mixture is calcinated in a second calcination step at a temperature of from 700 °C to 1200 °C, preferably of from 800 °C to 1100 °C, more preferably of from 900 °C to 1000 °C. The second calcination step according to the invention is conducted for 2 hours to 30 hours, preferably for 10 hours to 25 hours. In particular, it is preferred that the second calcination step is conducted at a temperature of from 900 °C to 1000 °C for 10 hours to 25 hours.

[0121] The cooling, separating, washing and / or milling steps as optionally carried in the process of the invention can be done by any method and equipment usually used in the prior art, for example the cooling may be carried out by natural cooling or by introducing water in the reactor jacket; the separation and washing step may be carried out by using a nutsche filter, a press filter, or centrifuge; the milling step may be carried out by using a mortar, a ball-mill, or a hammer-mill.The mixed oxide of the invention is suitable for power generation cells, particularly for using it as electrolyte in power generation cells. Therefore, the mixed oxide of the invention is sintered at a temperature of 1300 °C or lower for 5 hours to obtain a mixed oxide that exhibits a relative density RDi3oo°c / 5h of at least 94%, preferably of at least 96%. The sintering process is conducted as usually carried out in the prior art, like uni-axial press or CIP.

[0122] The power generation cells may be solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), or proton ceramic fuel cells, more preferably in proton ceramic fuel cells (PCFCs).

[0123] According to the invention, the power generation cells comprise one porous anode (A) and one porous cathode (C) which are separated by at least one layer (L), which may be also indicated as barrier layer, and at least one from (A), (C) or (L) is prepared from or comprises a mixed oxide.

[0124] In a preferred embodiment the anode (A) comprises or is prepared from the mixed oxide of the invention, and optionally at least one other inorganic material. The inorganic material other than the mixed oxide used in the anode may be selected in the group consisting of a zirconia doped with at least one element selected in the group of Y, Sc, Ce and a combination of two or more of these three elements; a mixed oxide of cerium and gadolinium; a metal such as nickel; a lanthane, strontium, titanate; and a lanthane chromite. The anode (A) has preferably a porosity of from 30 vol.-% to 50 vol-%, more preferably of from 35 vol.-% to 45 vol.-%, expressed in volume of the pores relative to the volume of the anode.

[0125] In another preferred embodiment the cathode (C) comprises or is prepared from the mixed oxide and optionally at least one other inorganic material. The inorganic material other than the mixed oxide used in the cathode may be selected in the group consisting of a zirconia doped with at least one element selected in the group of Y, Sc, Ce and a combination of two or more of these three elements; a mixed oxide of cerium and gadolinium; a mixed oxide of cerium and samarium; a La, Sr, Co and Fe-containing perovskite; and a La, Sr, Mn-containing perovskite. The cathode (C) has preferably a porosity of from 30 vol.-% to 50 vol-%, more preferably of from 35 vol.% to 45 vol.%, expressed in volume of the pores relative to the volume of the cathode.

[0126] Furthermore, it is preferred that the layer (L) comprises or is prepared from the mixed oxide and at least one other inorganic material. The inorganic material other than the mixed oxide may be selected in the group consisting of zirconia doped with at least one element selected in the group of Y, Sc, Ce and a combination of two or more of these three elements or a mixed oxide of cerium and gadolinium. The layer (L) is preferably a dense layer, in particular having a porosity of equal or less than 10 vol.-%, preferably of equal or less than 8 vol-%, more preferably of equal or less than 5 vol.-%, expressed in volume of the pores relative to the volume of the layer.

[0127] The porosity of the electrodes (A) and (C) and layer (L) can be determined by any method known in the art, like Scanning Electron Microscopy (SEM); Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM); gas adsorption techniques e.g. Brunauer-Emmett-Teller(BET), Method Mercury Intrusion Porosimetry, or Nitrogen Porosimetry; or gas permeametry. According to the invention, it is preferred to use gas adsorption techniques for determining the porosity of the electrodes and the barrier layer, such as Brunauer-Emmett-Teller (BET), Method Mercury Intrusion Porosimetry, or Nitrogen Porosimetry, whereby the Brunauer-Emmett-Teller (BET) is preferably used for pores having a pore diameter of lower than 300 nm.

[0128] The preparation of a power generation cell may involve the composite oxide of the invention as disclosed in all its details before. The preparation may also involve the composite oxide of the invention that is modified by the addition of a sintering aid (see below) and / or by a mechanical treatment such as a grinding step.

[0129] The present invention is further illustrated by the following examples. It should be understood that the following examples are for illustration purposes only and are not used to limit the present invention thereto.

[0130] Examples

[0131] Measurement methods

[0132] 1. Particle size Distribution - DIO, D50 and D90

[0133] Laser diffraction may also be used to determine the distribution of sizes of the particles. A laser particle sizer like Horiba LA-910, 920 or 960 may be used following the guidelines of the constructor. For the measurement, a relative refractive index of 1.2 may be used. From the distribution in volume obtained by laser diffraction, the parameters DIO, D50, and D90 can be deducted.

[0134] 2. BET specific surface areas

[0135] The BET specific surface area was determined by nitrogen adsorption. In principle the BET specific surface is measured according to the Brunauer-Emmett-Teller method. The theory of the method was originally described in the periodical “The Journal of the American Chemical Society, 60, 309 (1938)”. More detailed information about the theory may also be found in chapter 4 of "Powder surface area and porosity", 2nd edition, ISBN 978-94-015-7955-1.

[0136] In particular, in the example the method for determining the BET specific surface area was the standard method ASTM D-3663 but the degassing was carried out at a temperature of 210°C for 30 min under vacuum.

[0137] 3. Nitrogen porosity

[0138] The porosity was determined by using a Tristar II 3000 device from Micromeritics. The nitrogen pore distribution measurement was carried out on 85 points using a pressure table (42 points between 0.01 and 0.995 for the adsorption and 43 points in desorption between 0.995 and 0.05). The equilibrium time for a relative pressure of between 0.01 and 0.995 exclusive is 5 s. The equilibrium time for a relative pressure of greater than or equal to 0.995 is 600 s. Thetolerances with regard to the pressures was 5 mm Hg for the absolute pressure and 5% for the relative pressure. The pO value was measured at regular intervals during the analysis (2 h). The Barett, Joyner and Halenda (BJH) method with the Harkins- Jura law was used for determining the mesoporosity. The analysis of the results was carried out on the desorption curve.

[0139] 4. X-ray diffraction

[0140] Use was made of an x-ray diffractometer Ultima IV with a copper source (CuKai, k=1.5406 Angstrom).

[0141] 5. Measurement of the density and of the relative density

[0142] The following method was used and may be used in the context of the invention:

[0143] i) the powder of the composite oxide is weighed and introduced into a molding die; ii) the powder is compressed under pressure;

[0144] iii) the compacted sample obtained at the end of step ii) is taken out from the molding die;

[0145] iv) the compacted sample is then calcined in air at the targeted temperature for 5 hours; v) the density of the compacted sample having been calcined is measured with the following steps:

[0146] vl) the compacted sample having been calcined is weighed (weight in g);

[0147] v2) the volume of the compacted sample is measured (volume in cm3), the compacted sample being in the form of a parallelepiped and the volume of the parallelepiped is given by:

[0148] volume = length [cm] x width [cm] x height [cm];

[0149] vi) the relative density is then calculated.

[0150] The specific conditions used were the following:

[0151] - in step i), dimensions of the rectangular shape: 23 x 7 mm;

[0152] - the amount of powder used: about 1 g;

[0153] - in step ii), the powder is compressed under a uniaxial pressing to obtain the compacted sample.

[0154] The strength applied during the pressing is 15.8 kN. This corresponds to a pressure of 98 MPa.

[0155] The volume of the parallelepiped used in step v2, as mentioned above, can be also determined by the Archimedes' principle. Therefore, the following steps are conducted:

[0156] i) Weighing the parallelepiped (Wl);

[0157] ii) Putting 100 ml of water in a 100 ml beaker and placing the beaker on the balance. iii) Tying the sintered body with a string (hydrophobic materials like a metal for the string are preferred and the smaller the wire diameter, the better), subsequently immersing it completely in the water and recording the increased weight (W2).iv) After wiping off the water from the surface, weight of the parallelepiped is measured (W3).

[0158] The density is calculated by the following equation:

[0159] Density [g / cm3] = Wl*p / (W2+W3-Wl), wherein

[0160] p = density of water

[0161] 6. Conductivity

[0162] It was measured by direct current 4-terminal method at between 500 °C and 700 °C with humidified 4 vol.-% H2.

[0163] Mixed Oxides

[0164] Example 1 - Mixed Oxide BaCeo.7Zro.1Yo.1Ybo.!

[0165] For the preparation of the mixed oxide BaCeo.7Zro.1Yo.1Ybo.! a molar ratio of Ba (50%)-Ce (35%)- Zr (5%) - Y (5%)-Yb (5%) (% by mole ratio) was used. In particular, a cerium (III) nitrate aqueous solution (1.95 mol / L, 67.8 mL, 21.1 g as CeO?), zirconyl nitrate aqueous solution (2.39 mol / 1, 7.9 ml, 2.2 g as ZrO?), yttrium nitrate aqueous solution (1.97 mol / 1, 9.6 mL, 2.0 g as Y2O3) and ytterbium nitrate aqueous solution (1.00 mol / 1, 18.9 mL, 3.5 g as Yl^CL) were added in water so as to form 400 ml of a mixed nitrate solution. An ammonium hydrogen carbonate aqueous solution was prepared (0.13 mol / 1, 600 ml). The mixed nitrate solution was added into the ammonium hydrogen carbonate solution over 30 minutes under stirring (stirring blade, 300 rpm) in a beaker to obtain a slurry. The molar ratio of NH4HCO3 to the total number of rare earths and zirconium was 4.2.

[0166] The obtained slurry was heated up to 90 °C (heating ramp of 30 minutes), then immediately cooled.

[0167] After filtration followed by drying at 350 °C for 4 hours in an electric furnace, Ceo.7Zro.iYo.iYbo.i oxide powder was obtained.

[0168] Then the powder (2.2 g) and BaCCL (2.6 g, BM040 commercial grade supplied from Solvay) were mixed in mortar using ethanol (95.1-96.9 vol.-%, 3 mL) and calcined at 960 °C for 20 hours in an electric furnace and then milled in a mortar to obtain BaCeo.7Zro.iYo.iYbo.i oxide powder (BZCYYb).

[0169] The obtained mixed oxide showed the following physiochemical properties:

[0170] - BET surface area: 6.3 m2 / g

[0171] - BET-particle diameter (DBET; calculated): 0.158 pm

[0172] - Particle size distribution: D10: 0.08 pm

[0173] D50: 0.13 pm

[0174] D90: 0.26 pm

[0175] - DBET / DSO ratio: 1.1- Nitrogen peak top: 64.6 nm

[0176] - Nitrogen pore volume: 0.05 cm3 / g

[0177] - Conductivity: 0.031 Scrn'1.

[0178] In Figure 1 the X-ray diffraction patterns of the mixed oxide BaCeo.7Zro.iYo.iYbo.i calcined at 960 °C for various periods (yellow line: calcined for 10 h; orange line: calcined for 20 h; and blue line: calcined for 40 h;) are shown. All three X-ray diffractograms show a single peak located at a 29 angle of between 28° and 32° and which has the highest intensity.

[0179] Furthermore, the sintering behaviour of the mixed oxide BaCeo.7Zro.iYo.iYbo.i was tested. Therefore, the mixed oxide was pressed into pellets with the diameter of 13 mm at the pressure of 100 MPa for 1 min. The pressed pellets were sintered at 1200 °C, 1250 °C, and 1300 °C for 5 hours.

[0180] The relative density of sintered pellets was calculated by the Archimedes method, using the theoretical density of 6.211 g / cm3.

[0181] The relative densities sintered at 1200, 1250, 1300 °C of the mixed oxide were 79.0, 91.8, 97.8%, respectively (see Figure 2).

[0182] Example 2 - Mixed Oxide BaCe

[0183] The mixed oxide of BaCe was prepared in the same manner as Example 1 with the exception that the liquid medium containing a precipitate obtained in step (a) was heated up to 65 °C (heating ramp of 30 minutes) and then immediately cooled. For the preparation a molar ratio of Ba (50%)-Ce (50%) (% by mole ratio) was used. The molar ratio of NH4HCO3 to the total number of cerium was 4.7.

[0184] The obtained mixed oxide showed the following physiochemical properties:

[0185] - BET surface area: 2.9 m2 / g

[0186] - BET-particle diameter (DBET; calculated): 0.333 pm.

[0187] - Particle size distribution: D10: 0.08 pm

[0188] D50: 0.16 pm

[0189] D90: 3.72 pm

[0190] - DBET / DSO ratio: 2.0

[0191] - Nitrogen peak top: 90.1 nm

[0192] - Nitrogen pore volume: 0.02 cm3 / g

[0193] The relative density of sintered pellets was calculated by the Archimedes method, using the theoretical density of 6.36 g / cm3.

[0194] The relative densities sintered at 1200, 1250, 1300 °C of the mixed oxide were 85.1, 88.0, 94.1%, respectively.

[0195] Example 3 - Mixed Oxide BaCeo.sYo.2

[0196] The mixed oxide of BaCeo.sYo.2 was prepared in the same manner as Example 1 with the exception that the liquid medium containing a precipitate obtained in step (a) was heated up to70 °C (heating ramp of 30 minutes) and then immediately cooled. For the preparation a molar ratio of Ba (50%)-Ce (40%)-Y (10%) (% by mole ratio) was used. The molar ratio of NH4HCO3 to the total number of cerium was 4.3.

[0197] The obtained mixed oxide showed the following physiochemical properties:

[0198] - BET surface area: 6.0 m2 / g

[0199] - BET-particle diameter (DBET; calculated): 0.162 pm.

[0200] - Particle size distribution: D10: 0.08 pm

[0201] D50: 0.15 pm

[0202] D90: 0.63 pm

[0203] - DBET / DSO ratio: 1.1

[0204] - Nitrogen peak top: 60.6 nm

[0205] - Nitrogen pore volume: 0.04 cm3 / g

[0206] The relative density of sintered pellets was calculated by the Archimedes method, using the theoretical density of 6.16 g / cm3.

[0207] The relative densities sintered at 1200, 1250, 1300 °C of the mixed oxide were 87.4, 94.5, 98.8 %, respectively.

[0208] Example 4 - Mixed Oxide BaCe0.7Zr0.1Y02

[0209] The mixed oxide of BaCe0.7Zr0.1Y02 was prepared in the same manner as Example 1. For the preparation a molar ratio of Ba (50%)-Ce (35%)-Zr (5%)-Y (10%) (% by mole ratio) was used. The molar ratio of NH4HCO3 to the total number of RE and zirconium was 4.2.

[0210] The obtained mixed oxide showed the following physiochemical properties:

[0211] - BET surface area: 8.4 m2 / g

[0212] - BET-particle diameter (DBET; calculated): 0.120 pm.

[0213] - Particle size distribution: D10: 0.08 pm

[0214] D50: 0.13 pm

[0215] D90: 0.27 pm

[0216] - DBET / D50 ratio: 0.90

[0217] - Nitrogen peak top: 73.1 nm

[0218] - Nitrogen pore volume: 0.06 cm3 / g

[0219] The relative density of sintered pellets was calculated by the Archimedes method, using the theoretical density of 6.14 g / cm3.

[0220] The relative densities sintered at 1200, 1250, 1300 °C of the mixed oxide were 79.2, 89.2, 97.5 %, respectively.

[0221] Example 5 - Mixed Oxide BaCeo.gZro.i

[0222] The mixed oxide of BaCeo.gZro.i was prepared in the same manner as Example 1. For the preparation a molar ratio of Ba (50%)-Ce (45%)-Zr (5%) (% by mole ratio) was used. The molar ratio of NH4HCO3 to the total number of cerium and zirconium was 4.6.Example 6 - Mixed Oxide BaCeo.4Zro.4Yo.1Ybo.!

[0223] The mixed oxide of BaCeo.4Zro.4Yo.1Ybo.! was prepared in the same manner as Example 1 with the exception that the precipitate was dried in the first calcination step at 80 °C for 20 hours. For the preparation a molar ratio of Ba (50%)-Ce (20%)-Zr (20%)-Y (5%)-Yb (5%) (% by mole ratio) was used. The molar ratio of NH4HCO3 to the total number of RE and zirconium was 3.8.

[0224] Example 7 - Mixed Oxide BaCe0.6Zr0.2Y02

[0225] The mixed oxide of BaCe0.6Zr0.2Y02 was prepared in the same manner as Example 1 with the exception that the calcination step (g) of the process according to the claimed invention was conducted at 1000 °C for 30 hours to obtain the BaCeo.6Zro.2Yo.21 oxide powder (BZCY). For the preparation a molar ratio of Ba (50%)-Ce (30%)-Zr (10%)-Y (10%) (% by mole ratio) was used. The molar ratio of NH4HCO3 to the total number of RE and zirconium was 4.0.

[0226] The obtained mixed oxide showed the following physiochemical properties:

[0227] - BET surface area: 8.8 m2 / g

[0228] - BET-particle diameter (DBET; calculated): 0.110 pm.

[0229] - Particle size distribution: D10: 0.08 pm

[0230] D50: 0.13 pm

[0231] D90: 1.91 pm

[0232] - DBET / D50 ratio: 0.82

[0233] - Nitrogen peak top: 69.2 nm

[0234] - Nitrogen pore volume: 0.07 cm3 / g

[0235] - Conductivity: 0.024 Scm'1

[0236] The relative density of sintered pellets was calculated by the Archimedes method, using the theoretical density of 6.19 g / cm3.

[0237] The relative densities sintered at 1200, 1250, 1300 °C of the mixed oxide were 76.7, 86.6, 94.9 %, respectively.

[0238] Comparative Example 1 - BaCeo.?Zro.i Yo.iYbo.i obtained by solid state reaction

[0239] The mixed oxide BaCeo.?Zro.i Yo.iYbo.i was produced by solid state reaction. In particular, cerium oxide (13.2 mmol, 2.28 g), zirconium oxide (1.9 mmol, 0.23 g), yttrium oxide (1.9 mmol, 0.21 g), ytterbium oxide (1.9 mmol, 0.37 g), and BaCCE (18.9 mmol, 2.90 g) were mixed in mortar using ethanol (95.1-96.9 vol.-%, 3 ml) and calcined at 960 °C for 20 hours in an electric furnace and then milled in a mortar to obtain BaCeo.7Zro.1Yo.1Ybo.! oxide powder (BZCYYb).

[0240] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 70.0%.

[0241] Comparative Example 2 - BaCeo.7Zro.1Yo.1Ybo.! obtained by barium co-precipitationFor the preparation of the mixed oxide BaCeo.7Zro.1Yo.1Ybo.! a molar ratio of Ba (50%)- Ce (35%)- Zr (5%) - Y (5%) - Yb (5%) (% by mole ratio) was used. In particular, a Cerium (III) nitrate aqueous solution (1.95 mol / 1, 23.2 ml, 7.83 g as CeCh), zirconyl nitrate aqueous solution (2.39 mol / L, 3.2 mL, 0.80 g as ZrCh), yttrium nitrate aqueous solution (1.97 mol / 1, 3.3 ml, 0.73 g as Y2O3), ytterbium nitrate aqueous solution (1.00 mol / L, 3.3 ml, 1.3 g as Yb2O3), and barium nitrate (17 g, 9.97 g as BaO) were added in water to form 150 ml of a mixed solution. In order to facilitate the dissolution of barium nitrate the nitrates were added in 60 °C water. An ammonium hydrogen carbonate aqueous solution was prepared (3.0 mol / 1, 60 ml). The mixed nitrate solution was added into the ammonium hydrogen carbonate solution over 30 min under stirring (magnetic stirrer, 300 rpm) to obtain a slurry. The molar ratio of NH4HCO3 to the total number of rare earths and zirconium was 1.4.

[0242] The obtained slurry was heated up to 60 °C, and kept for 3h, then immediately cooled. After drying (80°C for 10 hours) followed by calcination at 900 °C for 10 hours in an electric furnace, BaCeo.7Zro.1Yo.1Ybo.! oxide powder was obtained.

[0243] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 58.0%.

[0244] Comparative Example 3 - BaCeo.7Zro.1Yo.1Ybo.! with hydrothermal treatment of Ceo.7Zro.1Yo.1Ybo.!

[0245] Cerium (IV) nitrate aqueous solution (1.5 mol / L, 257 mL, 66.2 g as CeCh), zirconyl nitrate aqueous solution (2.39 mol / 1, 23.0 ml, 6.77 g as ZrCL), yttrium nitrate aqueous solution (1.97 mol / 1, 28.0 ml, 6.20 g as Y2O3), ytterbium nitrate aqueous solution (1.00 mol / 1, 55.0 ml, 10.8 g as Yb2O3) were added in water to form a 1800 ml mixed nitrate solution.

[0246] The nitrate mixed solution was treated at 130°C for 3 hours in autoclave. An ammonium hydrogen carbonate aqueous solution was prepared (2.3 mol / 1, 1200 ml). After cooling, the treated mixed nitrate solution was added into the ammonium hydrogen carbonate solution over 30 min under stirring (stirring blade, 300 rpm) in a beaker to obtain a slurry. The molar ratio of NH4HCO3 to the total number of rare earths and zirconium was 5.0.

[0247] After filtration followed by calcination at 700 °C for 4 hours in an electric furnace, Ceo.7Zro.iYo.iYbo.i oxide powder was obtained.

[0248] Then the powder (2.0 g) and BaCCL (2.4 g, BM040 commercial grade supplied from Solvay) were mixed in mortar using ethanol (95.1-96.9 vol.-%, 3 ml) and calcined at 960 °C for 20 hours in an electric furnace and then milled in a mortar to obtain BaCeo.7Zro.1Yo.1Ybo.! oxide powder (BZCYYb).

[0249] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 69.7%.

[0250] Comparative Example 4 - Mixed Oxide BaCeo.sYo.2 obtained by solid state reaction The mixed oxide BaCeo.sYo.2, which has the same oxide composition as the mixed oxide of Example 3, was produced by solid state reaction. In particular, cerium oxide (5.2 mmol, 0.90 g, yttrium oxide (1.3 mmol, 0.15 g), and BaCOs (6.5 mmol, 1.3 g) were mixed in mortar usingethanol (95.1-96.9 vol.-%, 3 ml) and calcined at 960 °C for 20 hours in an electric furnace and then milled in a mortar to obtain BaCeo.sYo.2 oxide powder (BCY).

[0251] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 62.9%.

[0252] Comparative Example 5 - Mixed Oxide BaCe0.7Zr0.1Y02 obtained by solid state reaction The mixed oxide BaCe0.7Zr0.1Y02, which has the same oxide composition as the mixed oxide of Example 4, was produced by solid state reaction. In particular, cerium oxide (4.6 mmol, 0.78 g), zirconium oxide (0.7 mmol, 0.080 g), yttrium oxide (1.3 mmol, 0.15 g), and BaCCh (6.5 mmol, 1.3 g) were mixed in mortar using ethanol (95.1-96.9 vol.-%, 3 ml) and calcined at 960 °C for 20 hours in an electric furnace and then milled in a mortar to obtain BaCe0.7Zr0.1Y02 oxide powder (BZCY).

[0253] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 51.2%.

[0254] Comparative Example 6 - BaCe0.6Zr0.2Y02 obtained by solid state reaction

[0255] The mixed oxide BaCe0.6Zr0.2Y02, which has the same oxide composition as the mixed oxide of Example 7, was produced by solid state reaction. In particular, cerium oxide (3.9 mmol, 0.67 g), zirconium oxide (1.3 mmol, 0.16 g), yttrium oxide (1.3 mmol, 0.15 g), and BaCCE (6.5 mmol, 1.3 g) were mixed in mortar using ethanol (95.1-96.9 vol.-%, 3 ml) and calcined at 960 °C for 20 hours in an electric furnace and then milled in a mortar to obtain BaCe0.6Zr0.2Y02 oxide powder (BZCY).

[0256] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 56.3%. Comparative Example 7 - BaCe0.6Zr0.2Y02 obtained by barium co-precipitation

[0257] For the preparation of the mixed oxide BaCe0.6Zr0.2Y02, which has the same oxide composition as the mixed oxide of Example 7, produced by barium co-precipitation. In particular, a Cerium (III) nitrate aqueous solution (1.95 mol / 1, 5.4 ml, 1.8 g as CeCE), zirconyl nitrate aqueous solution (2.39 mol / L, 1.5 ml, 0.43 g as ZrCE), yttrium nitrate aqueous solution (1.97 mol / 1, 1.8 ml, 0.403 g as Y2O3), and barium nitrate (4.6 g, 2.7 g as BaO) were added in water to form 120 ml of a mixed solution. In order to facilitate the dissolution of barium nitrate the nitrates were added in 60 °C water. An ammonium hydrogen carbonate aqueous solution was prepared (1.7 mol / 1, 30 ml). The mixed nitrate solution was added into the ammonium hydrogen carbonate solution over 30 min under stirring (magnetic stirrer, 300 rpm) to obtain a slurry.

[0258] The obtained slurry was heated up to 60 °C, and kept for 3h, then immediately cooled. After drying (80°C for 10 hours) followed by calcination at 900 °C for 10 hours in an electric furnace, BaCe0.6Zr0.2Y02 oxide powder was obtained.

[0259] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 55.4%.

[0260] Comparative Example 8 - Mixed Oxide BaCe with hydrothermal treatment of cerium oxideFor the preparation of the mixed oxide BaCe, which has the same oxide composition as the mixed oxide of Example 2, a hydrothermal treatment of cerium oxide was conducted. In particular, a cerium (IV) nitrate aqueous solution (1.5 mol / 1, 257 ml, 66.2 g as CeCh), was added in water to form a 1800 ml mixed nitrate solution.

[0261] The nitrate mixed solution was treated at 130°C for 3 hours in autoclave. An ammonium hydrogen carbonate aqueous solution was prepared (2.3 mol / 1, 1200 ml). After cooling, the treated mixed nitrate solution was added into the ammonium hydrogen carbonate solution over 30 min under stirring (stirring blade, 300 rpm) in a beaker to obtain a slurry.

[0262] After filtration followed by calcination at 700 °C for 4 hours in an electric furnace, cerium oxide powder was obtained.

[0263] Then the powder (2.0 g) and BaCCh (2.4 g, BM040 commercial grade supplied from Solvay) were mixed in mortar using ethanol (95.1-96.9 vol.-%, 3 ml) and calcined at 960 °C for 20 hours in an electric furnace and then milled in a mortar to obtain BaCe oxide powder (BC).

[0264] The relative density of the mixed oxide sintered at 1300 °C for 5 hours was 62.8%.

[0265] Hence, as can be seen from the examples if the mixed oxide is not produced according to the invention, the required relative density of the mixed oxide cannot be obtained.

[0266] Comparative Example 9

[0267] 1 g of the BaCeo.7Zro.iYo.iYbo.i of the oxide powder prepared by Example 1 and 0.025g of Li2CO3 were mixed in mortar using ethanol (95.1-96.9 vol.-%, 3 ml), pressed into pellets with the diameter of 13mm at the pressure of 100 MPa for 1 min. The pressed pellets were sintered at 1300 °C. The relative density of the sintered mixed oxide was 95.1% and the conductivity was 0.011 Scm'1, i.e., lower than the conductivity of the mixed oxide without the addition of lithium (see Example 1).

Claims

Claims1. A mixed oxide comprising at least barium and cerium, and optionally at least zirconium and / or at least one rare-earth metal other than cerium (RE), characterized in that the mixed oxideexhibits a relative density RDi3oo°c / 5h of at least 94%, preferably of at least 96%, after sintering the mixed oxide at a temperature of 1300 °C for 5 hours, andcontains at most 0.1 wt.-% of a sintering aid component or of a mixture of sintering components, based on the total weight of the mixed oxide.

2. The mixed oxide according to claim 1, characterized in that the mixed oxide essentially consists of at least barium and cerium and optionally of zirconium and / or at least one rare-earth metal other than cerium (RE) oxide.

3. The mixed oxide according to claim 1 or 2, characterized in that the rare-earth metal other than cerium (RE) is selected from the group consisting of yttrium, neodymium, ytterbium, lanthanum, gadolinium, praseodymium, samarium, europium, terbium and combinations thereof, preferably consisting of yttrium, ytterbium and combinations thereof, more preferably the RE is a combination of yttrium and ytterbium.

4. The mixed oxide according to any one of the preceding claims, characterized in that the mixed oxide comprises at least zirconium, preferably comprises at least zirconium and at least one RE, which is preferably yttrium, more preferably comprises zirconium, yttrium and ytterbium.

5. The mixed oxide according to any one of the preceding claims, characterized in that the molar proportion of barium present in the mixed oxide is between 30% and 70 %, preferably between 30% and 55% based on the total molar ratio of barium and cerium and if present zirconium and / or RE expressed in %.

6. The mixed oxide according to any one of the preceding claims, characterized in that the proportion of barium is between 30% and 70 % by weight, preferably between 30% and 55% by weight, this proportion being expressed as weight of barium expressed in form of oxide relative to the total weight of the mixed oxide.

7. The mixed oxide according to any one of the preceding claims, characterized in that the proportion of cerium is between 10% and 70 % by weight, preferably between 20% and 60% by weight, this proportion being expressed as weight of cerium expressed in form of oxide relative to the total weight of the mixed oxide.

8. The mixed oxide according to any one of the preceding claims, characterized in that the proportion of zirconium, RE or a combination thereof is between 0% and 40 % by weight, preferably between 10% and 30% by weight, this proportion being expressed as weight of zirconium, RE or a combination thereof expressed in the form of oxide relative to the total weight of the mixed oxide.

9. The mixed oxide according to any one of the preceding claims, characterized in that the mixed oxide exhibits a relative density RDi25o°c / 5h of at least 80%, preferably of at least 85%, more preferably of at least 90%, after sintering the mixed oxide at a temperature of 1250 °C for 5 hours, or a relative density RDi2oo°c / 5h of at least 70%, preferably of at least 74%, more preferably of at least 78%, after sintering the mixed oxide at a temperature of 1200 °C for 5 hours..

10. The mixed oxide according to any one of the preceding claims, characterized in that the mixed oxide has a BET specific surface area from 1 m2 / g to 100 m2 / g, preferably from 1 m2 / g to 20 m2 / g, more preferably from 2 to 15 m2 / g, determined by nitrogen adsorption using Brunauer-Emmett-Teller (BET) method.

11. The mixed oxide according to any one of the preceding claims, characterized in that the mixed oxide exhibits in the range of pores with a diameter of less than or equal to 200 nm a peak for which the maximum corresponds to a pore diameter Dp between 10 nm and 200 nm, preferably between 20 nm and 150 nm, determined by nitrogen porosimetry, or characterized in that the mixed oxide has a pore volume for the pores with a diameter lower or equal to 200 nm (Vp<2oonm) between 0.01 ml / g and 0.5 ml / g, preferably between 0.02 ml / g and 0.1 ml / g, determined by nitrogen porosimetry.

12. The mixed oxide according to any one of the preceding claims, characterized in that the mixed oxide has a proton conductivity of at least 0.008 Scm'1at 700 °C.

13. Process of the preparation of the mixed oxide as defined in any one of claims 1 to 12 comprising the following steps:(a) contacting an aqueous solution comprising at least a cerium salt and optionally zirconium and / or a rare earth metal salt other than cerium salt with an aqueous precipitant solution containing a precipitant component, which is preferably selected from the group consisting of ammonium hydrogen carbonate (NH4HCO3), carbon dioxide gas, ammonium hydroxide, and a mixture thereof, to form a liquid medium containing a precipitate;(b) heating the liquid medium containing a precipitate obtained in step (a) at a temperature of from 60 °C to 200 °C, preferably of from 70 °C to 150 °C, more preferably of from 80 °C to100 °C, preferably for a period of less than 20 hours, more preferably of less than 10 hours, to obtain a heat-treated precipitate;(c) optionally cooling and / or separating and / or washing the heat-treated precipitate obtained in step (b);(d) drying the heat-treated precipitate obtained in step (b) or (c) at a temperature ranging from 50 °C to 700 °C, preferably from 150 °C to 600 °C, more preferably from 300 °C to 400 °C, preferably for a period of 2 hours to 15 hours, preferably for a period of 3 hours to 10 hours;(e) optionally milling the dried precipitate obtained in step (d);(f) mixing the dried precipitate obtained in step (d) or (e) with a barium component, which is preferably selected from the group consisting of barium carbonate, barium oxide, barium hydroxide, barium nitrate, and combinations thereof, more preferably the barium component is barium carbonate;(g) calcining the mixture obtained in step (f) at a temperature of from 700 °C to 1200 °C, preferably of from 800 °C to 1100 °C, more preferably of from 900 °C to 1000 °C, preferably for a period of from 2 hours to 30 hours, more preferably of from 10 hours to 25 hours;(h) optionally milling the calcinated material obtained in step (g).

14. The process according to claim 13, characterized in that in step (a) no capping agent, like surfactants, polymers, dendrimers, amino acids, or polysaccharides, preferably no surfactant such as hexadecyltrimethylammonium bromide (CTAB), is used.

15. Use of the mixed oxide according to any one of claims 1 to 12 or produced by the process according to claim 13 or claim 14, in power generation cells, preferably in solid oxide fuel cells, solid oxide electrolysis cells, or proton ceramic fuel cells, more preferably in proton ceramic fuel cells.

16. A power generation cell comprising one porous anode (A) and one porous cathode (C) which are separated by at least one layer (L), characterized in that at least one from (A), (C) or (L) is prepared from or comprises a mixed oxide as defined in any one of claims 1 to 12 or produced by the process according to claim 13 or claim 14, and wherein the power generation cell is preferably a solid oxide fuel cell, a solid oxide electrolysis cell, or a proton ceramic fuel cell.

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