CN116745923A - Perovskite structure, preparation method thereof and application of perovskite structure in electrode and solid oxide battery - Google Patents

Abstract

Disclosed is a perovskite structure comprising: first element X, strontium, iron, cobalt, oxygen, magnesium and tungsten, the first element X may be barium and/or a lanthanide; the structure includes a single perovskite region and a double perovskite region. Methods for forming the structure, electrodes comprising the structure, and solid oxide cells using the structure are also disclosed.

Description

Perovskite structure, preparation method thereof and application of perovskite structure in electrode and solid oxide battery

Technical Field

The present application relates to a structure for use in a solid oxide cell, an electrode, a solid oxide cell and a method of forming a structure. In particular, the present application relates to perovskite structures for use as electrodes and methods of making the same.

Background

A Solid Oxide Fuel Cell (SOFC) is a type of Solid Oxide Cell (SOC). Which is an electrochemical device that generates electrical energy through electrochemical oxidation of a fuel gas, typically a hydrogen-based fuel gas. The device is typically ceramic-based, using an oxygen ion conducting metal oxide derived ceramic as its electrolyte. Since most ceramic oxygen ion conductors (e.g., doped zirconia or doped ceria) exhibit technology dependent ionic conductivity only at temperatures in excess of 500 ℃ (for ceria-based electrolytes) or 650 ℃ (for zirconia-based ceramics), SOFCs operate at elevated temperatures.

Like other fuel cells, SOFCs include an anode where the fuel is oxidized and a cathode where the oxygen is reduced. The electrodes must be capable of catalyzing the electrochemical reaction, stable in their respective atmospheres at the operating (reducing on the anode side, oxidizing on the cathode side) temperature, and capable of conducting electrons so that the current generated by the electrochemical reaction can be drawn away from the electrode-electrolyte interface.

Various materials have been explored for use as cathodes in SOFCs, including perovskite cobalt crystals. Barium and lanthanide containing materials such as BSCF and LSCF (barium/lanthanum, strontium and iron containing cobalt oxides) are examples of such materials and perform well as SOFC cathodes due to their high oxygen ion conductivity and specific area resistance (ASR).

However, many such materials (such as conventional "undoped" BSCF) have poor thermal and chemical stability. In particular BSCF reacts with various electrolyte materials upon sintering [ in terms of SOFC operating temperature, with ceria-based electrolytes (the most common electrolyte type of BSCF) at > 900 ℃ and undergoes a phase change from cubic to hexagonal at < 900 ℃ (which is the typical operating temperature of the material), which is detrimental to its transport and catalytic properties, whereby ASR increases over time, thus eliminating it from practical use in SOFC applications.

Accordingly, it is desirable to develop materials that have ASR comparable to or lower than BSCF and LSCF in low and medium temperature applications; but also the material is more stable, in particular shows reduced phase transitions and thus has the ability to maintain a lower ASR over time.

Some work has been done to improve the properties of these materials to improve oxygen ion conductivity, to increase thermal stability, and to enhance resistance to degradation. For example, it has been found that heavily doping BSCF with molybdenum can increase conductivity, and can also improve material stability while maintaining ASR values comparable to BSCF.

Unfortunately, many dopant materials suffer from the phenomenon of "leaching" (leeching) when used in SOFCs, in which the dopant comes out of the cathode material (e.g., thereby forming (Ba/Sr) MoO ₄ ) And cathode performance is reduced. Furthermore, if excessive dopant is allowed to leach from the cathode material, structural rearrangements may occur within the crystal structure, which may lead to breakage of the electrode material and reduced performance.

Demont, A.et al, "charge-inhibited driven Shan Yajing lattice internal axial phase separation in composite oxides" (Single Sublattice Endotaxial Phase Separation Driven by Charge Frustration in a Complex Oxide), journal of American chemistry, 2013, 135, pages 10114-10123, disclose the use of molybdenum as a dopant material for the manufacture of perovskite structures. Popov et al, "modification of Ba by partial substitution of cobalt with tungsten _0.5 Sr _0.5 Co _0.8 Fe _0.2 O _3-δ Functional Properties "(improvement of Ba) _0.5 Sr _0.5 Co _0.8 Fe _0.2 O _3-δ functional properties by partial substitution of cobalt with tungsten), journal of membrane science, 2014, 469, pages 89-94, relates to tungsten substituted SOFCs.

WO 2016/083780A1 describes a dual phase perovskite structure for a solid oxide fuel cell comprising a tungsten dopant. This provides enhanced performance compared to some other materials and resists leaching of the dopant. US-se:Sup>A-2016/0329370 discloses perovskite structures for use as electrodes in Solid Oxide Fuel Cells (SOFCs). US-se:Sup>A-2010/0018394 discloses an inorganic/organic complex which may comprise perovskite.

Despite these advances, it is desirable to find materials that exhibit enhanced performance. The present application aims to solve or at least ameliorate some of the problems outlined above.

Disclosure of Invention

In a first aspect of the application there is provided a perovskite structure comprising: first element X, strontium, iron, cobalt, oxygen and tungsten; wherein the first element X is barium and/or a lanthanide, the structure comprising a Single Perovskite (SP) region and a Double Perovskite (DP) region; characterized in that the perovskite structure further comprises magnesium.

The inventors have surprisingly found that tungsten-containing perovskite doped with magnesium shows a significant improvement in performance compared to conventional SOC air electrode materials and other doped perovskite materials. In particular, the presence of magnesium appears to not only improve the ASR of the material, but also greatly improve ASR consistency over time.

Without being bound by theory, it is believed that magnesium deposits itself at the boundary between the single perovskite and the double perovskite by incorporation into the B-site region of the material. It is not clear why this would enhance the resistance of a material to a decrease in ASR.

The term "dopant" as used herein is not intended to be limited to the maximum percentage of an element, ion, or compound added to a chemical structure. Similarly, the term "doping" refers to the addition of certain amounts of elements, ions or compounds to a material. It is not limited to the maximum number of materials after which further additional additions of materials no longer constitute doping.

The term "perovskite structure" as used herein means a structure having a general perovskite (ABO ₃ ) A single network of chemically bonded crystal structures of the structure. This does not mean that the single webThe complex must have a single, uniform crystal structure throughout the structure. However, when different crystal structures are present between different regions of the network, typically these regions will have complementary structures that make chemical bonds more likely to form between them. Examples of this are single perovskite crystal regions and double perovskite crystal regions.

The term "region" as used herein in reference to the single perovskite region and the double perovskite region is intended to mean a region or portion that forms part of and is integrated with a single network that forms the perovskite structure. This is in contrast to regions that simply are adjacent to each other and/or in physical contact.

The term "solid oxide cell" (SOC) is intended to cover both Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs). In general, the application is implemented with respect to SOFCs.

The term "atomic percent" or "atomic percent" (abbreviated herein as "atomic percent") refers to the atomic percent relative to a given perovskite dopant site. As will be appreciated by those skilled in the art, the perovskite has ABO ₃ A shaped structure. Thus, there are three sites that can be doped: site a, site B and an oxygen site. For example, when a tungsten dopant is used to enhance the BSCF perovskite, tungsten is incorporated into the B site (i.e., it replaces some of the cobalt and iron in the natural B site). Thus, a 10 atomic% concentration of tungsten dopant corresponds to a case in which 10% of the B atoms (i.e., cobalt and/or iron atoms) have been replaced with tungsten. Similarly, a 3 atomic% magnesium dopant concentration in BSCFW corresponds to the case where 3% of the B atoms (i.e., cobalt, iron, and/or tungsten atoms) have been replaced with magnesium.

The first element X may be a lanthanide, such as lanthanum. The addition of magnesium may enhance various perovskite materials to meet specific objectives.

Typically, the first element X is barium. Barium, when used with strontium, iron, oxygen, tungsten and cobalt, forms a particularly effective air electrode material and is significantly improved by the presence of magnesium. The air electrode is typically the cathode.

Typically, the perovskite structure of the present application will contain tungsten in an atomic percentage of 20 atomic% to 50 atomic%. Doping a material (e.g., BSCF or LSCF) with an amount of dopant (e.g., tungsten) greater than about 20 atomic percent can result in a mixed perovskite structure comprising a large amount of double and single perovskite. It is believed that the presence of such concentrations of dopants (e.g., tungsten) promotes the occurrence of internal growth, resulting in the formation of significant amounts of both single and double perovskite structures.

As used herein, the term "ingrowth" (endotaxial growth) is intended to refer to the formation of more than one complementary crystal structure, such as a single perovskite and a double perovskite, such that they coexist. Typically, this refers to the propagation of two complementary crystal structures (typically a single perovskite and a double perovskite).

The perovskite structure defined herein (having single perovskite regions and double perovskite regions), e.g. obtained by internal growth, is advantageous because mixtures or "alloys" of regions in the perovskite structure improve the properties of the overall material. Without being bound by theory, it is believed that when the double perovskite structure and the single perovskite structure lock together in a conventional perovskite structure, this enables the single perovskite to resist structural changes due to surrounding interlocking double perovskites that have a more stable crystal structure.

The total concentration of tungsten may range from 20 atomic% to 50 atomic%. The inventors have found that this particular concentration of dopant results in an optimal perovskite structure with a good balance between structural stability and conductivity.

Furthermore, the concentration of magnesium is typically up to 20 atomic% (i.e., in the range of >0 atomic% to 20 atomic%). More typically, magnesium is present at 1 atomic% to 18 atomic%; even more typically, from 2 atomic% to 16 atomic%; more typically from 3 to 14 at%; even more typically from 4 to 12 at%; and most typically is present at 5 atomic% to 10 atomic%.

The structure of the composition according to the application is generally of formula (la):

(Ba _1-x Sr _x )(Co _1-y Fe _y ) _a W _b Mg _c O _d

wherein x and y are independently 0.1 to 0.9; a. the sum of b and c is equal to 1; c is >0; and d is 2 to 3. This structure is intended to be the average chemical formula of a typical perovskite material of the application. The different regions of a typical perovskite structure differ in composition and structure. The ratios of elements in this formula and the corresponding values described (including the values of a, b, and c or a sum thereof) should not be construed as exact or integer values. Defects, interstitial ions (interstitial ions), impurities and other changes in the crystal structure occur naturally in all ion lattices, and furthermore, the perovskite structures described herein have at least both single and double perovskite regions.

The value of "c" is >0, which may be >0 to 0.2, more suitably 0.05 to 0.2, and typically 0.05 to 1.0.

The value "d" is from 2 to 3, typically from 2.5 to 3. Suitably, the value "d" is about 3. The drastic conditions under which electrochemical systems, such as SOFCs, operate, mean that oxygen present in the crystal structure and oxygen present in the oxidant source can be used as oxygen ion sources. In general, oxygen travels from a region of higher oxygen partial pressure to a region of lower oxygen partial pressure, for example, from the oxidant side (air side) to the reduction side (fuel side) of the fuel cell. Oxygen present in the perovskite material may be released from the ionic matrix to which it is bound and travel through the material. Therefore, the amount of oxygen present in the perovskite structure may vary, and within the above ranges, depending on the reaction conditions and the particular crystal composition. Suitably, the value "d" is approximately equal to 3, as this has been found to provide the best results. This change in oxygen content is generally described as "delta", e.g. "ABO _3-δ "。

It is often the case that x may be from 0.2 to 0.8, alternatively from 0.3 to 0.7, or more often from 0.4 to 0.6. In most cases, x may typically be 0.5. Furthermore, it is often the case that y may be from 0.1 to 0.8, or more typically from 0.1 to 0.7, or more typically from 0.2 to 0.6, or even more typically from 0.2 to 0.4. Typically, y may be 0.3.

The inventors have found that choosing these values for x and y results in a perovskite structure with an optimal balance between oxygen ion conduction and stability. As described above, the change in crystal structure is common and natural. These values should not be construed as exact and exact. All of these values should be considered as modified by the term "about".

The ratio of single perovskite to double perovskite in the present application may be varied to suit a particular purpose. Typically, the weight ratio of single perovskite to double perovskite may be from 1:9 to 9:1. More typically, the ratio of mono-perovskite to di-perovskite is from 1:5 to 5:1, even more typically from 1:1 to 1:9. Typically, the ratio of single perovskite to double perovskite is 2:8. It is often the case that double perovskite is present more than single perovskite, as this increases the stability of the perovskite structure, which is important for many electrochemical systems (such as SOFCs) that need to operate under harsh conditions for a long period of time.

The second aspect of the application also provides an electrode for an electrochemical system (e.g. a fuel cell) comprising a perovskite structure according to the first aspect of the application. Typically, the electrode is an air electrode (e.g., cathode).

The third aspect of the application also provides an electrochemical cell (typically a fuel cell) comprising an electrode according to the perovskite structure of the first aspect of the application or according to the second aspect of the application. Typically, the electrochemical cell is a solid oxide cell, e.g., a SOFC.

In a fourth aspect of the application there is also provided a stack comprising one or more solid oxide cells according to the third aspect of the application, typically a solid oxide fuel cell stack.

In addition, in a fifth aspect of the present application, there is provided a method of forming the perovskite structure of the first aspect of the present application, the method comprising: mixing starting materials to form a mixture, wherein the starting materials comprise a first element X, strontium, iron, cobalt, oxygen, magnesium, and tungsten; heating the mixture to a first temperature for a first period of time to form a single perovskite; and heating the mixture to a second temperature for a second period of time to form a double perovskite; wherein the first element X is barium and/or a lanthanide such as lanthanum.

As used herein, elements such as barium, lanthanum, strontium, iron, cobalt, oxygen, magnesium, and tungsten refer to materials comprising the elements. This may be an element (e.g., pure tungsten), or may be a compound (e.g., co) comprising a series of elements including one or more of those described herein ₃ O ₄ Or CO ₂ ). The elements are typically provided in oxide form, as these are one of the most common and stable forms of elements that naturally occur. Typically, magnesium is provided in the form of magnesium oxide (MgO).

The inventors have found that when tungsten is used in the above method, this results in a perovskite structure in which almost all tungsten is incorporated into the double perovskite region. This appears to result in a particularly stable conductive material.

In particular, the inventors have found that the use of tungsten as a dopant results in a perovskite structure having a low oxygen content (high oxygen vacancies).

Typically, the first element X is barium. Those materials produced by the process using barium have been found to be particularly effective in electrochemical cells (e.g., SOFCs).

Typically, the method of the application further comprises a comminution step prior to the heating step. It is advantageous to reduce the starting materials to a fine particulate form so that the starting materials can be blended into a homogeneous mixture having a high surface area. This results in a more uniform perovskite structure upon heating.

Although there are various pulverizing methods and techniques, a method commonly used for pulverizing the starting material is ball milling. The inventors have found that ball milling provides a fast and efficient method of breaking and reducing the size of the starting material.

After the starting material is crushed, it is often the case that the crushed starting material is pressed to increase the density of the densified form prior to the heating step (compact form density). This is advantageous because it ensures that air is forced out of any gaps in the blended mixture and improves the contact between the particles. This helps ensure that the resulting perovskite structure is free of defects, cracks and other weak spots. Typically, the mixture of crushed starting materials is compressed into pellets. The pressing step may be repeated in multiple stages throughout the synthesis. Typically, this is done before the sintering step three, i.e. after the first and second steps are performed. Although the pressing is typically performed only once, the pressing process may be performed multiple times and prior to the steps of the method.

The first and second temperatures to which the starting material is heated are sufficient to form a single perovskite and a double perovskite, respectively. The absolute temperature at which these formations occur depends on the ratio of starting materials to the particular dopants and additives contained in the starting materials. Those skilled in the art will be familiar with crystal classification techniques such as x-ray diffraction, neutron scattering experiments, and spectroscopic techniques such as musburger spectroscopy (Mossbauer spectroscopy), and can determine whether a given perovskite structure has been formed.

Typically, the first temperature ranges from 600 ℃ to 800 ℃, more typically from 650 ℃ to 750 ℃, even more typically about 700 ℃. The inventors have found that these temperatures are most effective in promoting single perovskite formation and result in little to no formation of double perovskite.

Further, the second temperature range is typically 800 ℃ to 1100 ℃, more typically 850 ℃ to 1000 ℃, even more typically about 900 ℃. The inventors have found that these temperatures are most effective in promoting double perovskite formation.

Typically, the first period of exposure of the starting material to the first temperature is greater than 20 minutes, more typically greater than 1 hour. Typically, the first temperature is for a period of 4 to 8 hours. The second period of time is typically greater than 20 minutes, more typically greater than 1 hour. Typically, the second period of time is from 1 hour to 10 hours, often from 6 hours to 10 hours.

Although not required, the method may further comprise: a sintering step in air at a third temperature for a third period of time after the second heating step. The inventors have found that this results in an improved property of the perovskite material obtained. In particular, the inventors have found that the further sintering step improves the crystallinity and the identity (uniformity) of the SP/DP perovskite structure. This high crystallinity improves stability and oxygen ion conductivity properties.

Typically, the third temperature range is 900 ℃ to 1300 ℃; more typically 1100 ℃ to 1300 ℃; more typically 1200 ℃ to 1250 ℃; more typically about 1250 deg.c. If the temperature is raised well above 1300 ℃, the perovskite or composition may melt. Furthermore, the third period of time during which the sintering step occurs is typically at least 20 minutes, more typically at least 1 hour. Typically, the third period of time ranges from 1 hour to 12 hours, and in particular may range from 8 hours to 12 hours.

The inventors have found that if the starting material is heated to very high temperatures in a period of time shorter than these periods of time, the resulting perovskite structure typically contains defects. Therefore, in order to allow the perovskite structure to be gradually formed, each of the heating steps is expected to have the shortest period of time. There is no practical disadvantage in exposing the starting material to heating conditions for a longer period of time, but this generally does not lead to any significant improvement in properties, and it is costly to maintain high temperature conditions for a nearly negligible improvement in performance. This time period also depends to some extent on the specific temperature used in the process. Thus, these time periods represent a typical compromise in order to obtain an optimal perovskite structure.

Furthermore, it may be the case that the method is repeated at least once. This means that once the perovskite structure is formed, the product is used as at least part of the starting material for the repetition and the same method is again employed. This improves the properties and uniformity of the final perovskite structure. There is no limitation on the number of times the method can be repeated in this way, however, it is usually 3 times or 4 times. Repeating the method beyond that number appears to provide only incremental or negligible improvement in performance.

Unless otherwise indicated, each integer described herein may be used in combination with any other integer understood by those skilled in the art. In addition, while all aspects of the application preferably "include" features associated with that aspect, it is specifically contemplated that all aspects of the application may consist of or "consist essentially of the features recited in the claims.

The application will be described with reference to the drawings and embodiments.

Drawings

Fig. 1 shows basic properties of BSCFW-xMg, in particular (a) XRD patterns of BSCFW-xMg (x=0, 0.05,0.1, 0.15); (b) total conductivity; (c) Thermal stability and CO resistance of BSCFW-0.05Mg ₂ Performance; and (d) a coefficient of thermal expansion. As shown in fig. 1b, the curve is 10BSCFW;20BSCFW-0.05Mg;30BSCFW-0.1Mg and 40BSCFW-0.15Mg. In FIG. 1d, the curve is 70BSCFW;60BSCFW-0.05Mg;50BSCFW-0.1Mg and 80BSCFW-0.15Mg.

FIGS. 2a-2e show ASR and ASR stability of BSCFW-xMg, particularly (a) ASR-T; (b) For BSCFW-xMg, ASR and ASR decay rate at operating temperature (650 ℃) are a function of x; (c) Comparison of ASR stability for BSCF, BSCFW and BSCFW-0.05Mg, and impedance semi-circles before and after 7200 minutes stability test in static air at 650℃as shown in (d) and (e). In fig. 2a, the curve is 110BSCFW;100BSCFW-0.05Mg;120BSCFW-0.1Mg;130BSCFW-0.15Mg; and 90BSCFW-0.2Mg.

Examples

EXAMPLE 1 Synthesis of BSCFW-xMg

Ba _0.5 Sr _0.5 (Co _0.7 Fe _0.3 ) _0.69-x Mg _x W _0.31 O _3-δ (x＝0,0.02,0.03,0.04,0.05,0.06,

0.07,0.08,0.1,0.15,0.2,0.3, abbreviated as BSCFW-xMg) was prepared by the solid state reaction route. Stoichiometric amount of BaCO ₃ (99.995％)、SrCO ₃ (99.994％)、Co ₃ O ₄ (99.7％)、Fe ₂ O ₃ (99.99％)、WO ₃ (99.9%) and MgO (99.95%) were dried at 200℃and weighed. ZrO is used as raw material ₂ Milling media (10 mm balls) and isopropanol were ball milled at 350r.p.m. (revolutions per minute) for 12 hours. The milled mixture was dried and calcined at 700 ℃ for 6 hours, then at 900 ℃ for 8 hours, and both heating and cooling rates were 5 ℃/min. The calcined powder was further milled using the same conditions as the first ball milling, the powder was pressed into pellets having a diameter of 10mm, and the addition was at 5 c/minThe heat and cooling rates were sintered at 1200-1250 ℃ for 12 hours.

Example 2-manufacture of symmetrical cells and ASR measurement

Samarium doped ceria (abbreviated as "SDC") was chosen as the electrolyte material for symmetrical BSCFW-xMg cells. The SDC powder was pressed into 10mm pellets and sintered in air at 1400℃for 14 hours. BSCFW-xMg ink was prepared by mixing BSCFW-xMg powder and binder (V-600, heraeus) in a weight ratio of 1:0.7, and ball milling the mixture for 3 hours. The BSCFW-xMg ink was then screen printed 6 times on both sides of the SDC pellet. The cells were fired at 950 ℃ in air for 1 hour with heating and cooling rates of 1.8 ℃/min and 3 ℃/min, respectively. Gold paste was applied to both sides of the pellets on the annealed ink before firing in air at 600 ℃ for a further 1 hour. ASR measurements were performed on BSCFW-xMg symmetrical cells with a 10mV alternating voltage applied in the frequency range of 0.01Hz to 1 MHz.

EXAMPLE 3 comparison between BSCFW and BSCFW-xMg

The Area Specific Resistance (ASR) and ASR stability of BSCFW-xMg cathodes were measured by a symmetric cathode/electrolyte (samarium doped ceria)/cathode cell and the data are shown in fig. 2a-2 e. Fig. 2 (a) shows ASR versus inverse temperature for BSCFW-xMg (x= 0,0.05,0.1,0.15,0.2). For BSCFW-xMg where x.ltoreq.0.1, ASR values are similar to undoped BSCFW over a temperature range of 500℃to 700 ℃. The activation energy calculated by the linear fit in FIG. 2 (a) increases from 1.34eV (BSCFW) to 1.52eV (BSCFW-0.05 Mg) and then decreases to 1.46eV (BSCFW-0.1 Mg).

ASR at typical SOFC operating temperatures (650 ℃) is plotted in FIG. 2 (b) for different BSCFW-xMg compositions (0.ltoreq.x.ltoreq.0.15). 2 atomic% Mg doping slightly increases ASR to 0.0584 (8) Ω cm ² For further Mg-doped compositions, the ASR values were lower, reaching 0.028 (3) Ω cm for BSCFW-0.15Mg ² . For 5% Mg doping, the lowest ASR decay rate was observed, but as more Mg was added, the ASR decay rate increased and BSCFW-0.15Mg (1.6 (1). Times.10-6Ω cm) ² Minute (min) ^-1 ) The result is close to undoped BSCFW.

FIG. 2 (c) shows the evolution of ASR over time for BSCFW, BSCFW-0.05Mg and commercial BSCF cells. All cells were maintained at 650 ℃ for 3600 minutes; initial ASR of BSCFW-0.05Mg was 0.0468 (2) Ω cm ² Slightly lower than BSCFW (0.0480 (1) Ω & cm) ² ) But the ASR attenuation rate is much lower within 60 hours, which is 0.18 (1). Times.10-6Ω cm ² Minute (min) ^-1 Corresponds to about 10% of BSCFW (1.74 (3). Times.10-6Ω.cm) ² Minute (min) ^-1 ) And 4% of the commercially available BSCF.

FIGS. 2 (d) and 2 (e) show a direct comparison of BSCFW and BSCFW-0.05Mg impedance arc diagrams before and after 7200 minutes of aging test at 650 ℃. Nyquist spectrum (Nyquist) data is plotted with the high frequency (10 MHz) intercept set to zero to more clearly show the change in polarization response. After 7200 minutes of testing, ASR of BSCFW-0.05Mg was 0.0488 (4) Ω cm ² And BSCFW of 0.0632 (1) Ω & cm ² ASR decay was shown to be almost completely inhibited by Mg doping.

The measurement cell was evaluated by SEM after measurement. The cross-sectional image of the BSCFW-0.05Mg cell showed that the aged cell appeared identical to the unaged cell and that there was no sign of connection problems (including significant interfacial chemical reactions, melting or delamination between electrolyte and cathode).

All publications mentioned in the above specification are herein incorporated by reference. Although illustrative embodiments of the present application have been disclosed in detail herein with reference to the accompanying drawings, it is to be understood that the application is not limited to the precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the application as defined by the appended claims and their equivalents. The disclosures of the publications cited herein are incorporated by reference in their entirety.

The application claims priority from GB2016089.1 submitted on 10/9/2020 and GB 2015136.1 submitted on 24/9/2020: the entire contents of both documents are incorporated herein by reference.

Claims (24)

1. A perovskite structure, the perovskite structure comprising: first element X, strontium, iron, cobalt, oxygen and tungsten; wherein the first element X is barium and/or a lanthanide, said structure comprising a single perovskite region and a double perovskite region, characterized in that the perovskite further comprises magnesium.

2. The perovskite structure of claim 1, wherein the concentration of tungsten ranges from 20 to 50 atomic percent.

3. A perovskite structure as claimed in claim 1 or claim 2, wherein the concentration of magnesium is at most 20 atomic%.

4. A perovskite structure as claimed in any one of claims 1 to 3, wherein the concentration of magnesium is in the range 1 to 18 atomic%.

5. A perovskite structure according to any one of the preceding claims, wherein the concentration of magnesium is in the range 3 to 14 atomic%.

6. A perovskite structure according to any one of the preceding claims, wherein the concentration of magnesium is in the range 5 to 10 atomic%.

7. The perovskite structure of any one of the preceding claims, wherein the lanthanide is lanthanum.

8. A perovskite structure according to any one of the preceding claims, wherein the element X is barium.

9. A perovskite structure according to any one of the preceding claims, wherein the perovskite has the formula according to formula (I):

(Ba _1-x Sr _x )(Co _1-y Fe _y ) _a W _b Mg _c O _d (I)

wherein,,

x and y are each independently 0.1 to 0.9;

a. the sum of b and c is equal to 1;

c is >0; and is also provided with

d ranges from 2 to 3.

10. The perovskite structure of claim 9, wherein c ranges from 0.05 to 0.2.

11. An electrode comprising a perovskite structure according to any one of the preceding claims.

12. The electrode of claim 11, wherein the electrode is an air electrode.

13. A solid oxide cell comprising the perovskite structure of any one of claims 1 to 10.

14. The solid oxide cell of claim 13, wherein the solid oxide cell is a solid oxide fuel cell or a solid oxide electrolysis cell.

15. A method of forming a perovskite structure according to any one of claims 1 to 10, the method comprising the steps of:

mixing starting materials to form a mixture, wherein the starting materials comprise: first element X, strontium, iron, cobalt, oxygen, tungsten and magnesium;

heating the mixture to a first temperature for a first period of time to form a single perovskite; and

heating the mixture to a second temperature for a second period of time to form a double perovskite;

wherein the first element X is barium and/or a lanthanide.

16. The method of claim 15, further comprising a comminution step prior to the heating step.

17. A method according to claim 15 or claim 16, wherein the first temperature is in the range 650 ℃ to 750 ℃.

18. The method of any one of claims 15 to 17, wherein the second temperature range is 850 ℃ to 1000 ℃.

19. The method of any one of claims 15 to 18, wherein the first period of time is 4 to 8 hours.

20. The method of any one of claims 15 to 19, wherein the second period of time is 6 to 10 hours.

21. The method of any one of claims 15 to 20, further comprising: a sintering step in air at a third temperature for a third period of time after the second heating step.

22. The method of claim 21, wherein the third temperature ranges from 1100 ℃ to 1300 ℃.

23. The method of claim 21 or claim 22, wherein the third period of time is 8 to 12 hours.

24. The method of any one of claims 15 to 23, wherein the method is repeated at least once.