Polycrystalline metal oxide, proton-conductive solid electrolyte, electrolyte layer, and electrochemical device

Abstract

A polycrystalline metal oxide represented by a general formula BaiScjM1kM2lO3-δ, wherein an atom of M1 comprises at least one of tungsten (W) and molybdenum (Mo), an atom of M2 comprises at least one of copper (Cu) and nickel (Ni), and l / i>0.01 and i+j+k+l=2 are satisfied.

Description

Polycrystalline metal oxides, proton-conducting solid electrolytes, electrolyte layers, and electrochemical devices 【0001】 The present invention relates to polycrystalline metal oxides, proton-conducting solid electrolytes, electrolyte layers, and electrochemical devices. 【0002】 Materials with high proton conductivity are known (see, for example, Patent Document 1). Patent Document 1 describes several proton-conducting solid electrolytes composed of various metallic elements. Non-Patent Documents 1 and 2 describe materials with the general formula Ba ｉ Sc ｘ M １－ｘ O ３－δ This document describes the density of a material represented by (where M is a metallic element) when the firing conditions are changed. 【0003】 International Publication No. 2024 / 053651 【0004】 "High proton conduction by full hydration in highly oxygen deficient perovskite", Kei Saito, Kensei Umeda, Kotaro Fujii, Kazuhiro Mori, and Masatomo Yashima, Journal of Materials Chemistry A, Issue 22, 2024 "High proton conductivity within the 'Norby gap' by stabilizing a perovskite with disordered intrinsic oxygen vacancies", Kei Saito, Masatomo Yashima, Nature Communications volume 14, Article number: 7466 (2023), Published: 17 November 2023 【0005】 Patent Document 1 provides various numerical ranges for various types of proton-conducting solid electrolytes, but the provided numerical ranges include materials that do not have sufficient proton conductivity, and there is room for improvement in improving proton conductivity. Non-Patent Document 1 provides Baｉ Sc ｘ M １－ｘ O ３－δ When M of this is W (tungsten) and the coefficient X is 0.2, even if firing at 1600 degrees Celsius (°C) for 10 hours (h) is repeated twice, the relative density is described as being as low as 70 to 74 percent (%). In Non-Patent Document 2, Ba ｉ Sc ｘ M １－ｘ O ３－δ When M of this is Mo (molybdenum) and the coefficient X is 0.25, even if fired at 1500 °C for 12 hours and at 1550 °C for 24 hours, the relative density was described as being about 80 to 85%. Ba ｉ Sc ｘ M １－ｘ O ３－δ In order to apply an electrode to the electrolyte formed by this and operate it as a device, a higher relative density is required as a material. 【0006】 The present invention has been made to solve at least a part of the above-described problems, and an object thereof is to provide a material having proton conductivity and an improved relative density. 【0007】 The present invention has been made to solve at least a part of the above-described problems, and can be realized in the following forms. 【0008】 (1) According to one form of the present invention, a polycrystalline metal oxide is provided. This polycrystalline metal oxide has a general formula of Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ and is represented by, the atoms of M1 are composed of at least one of W (tungsten) and Mo (molybdenum), the atoms of M2 are composed of at least one of Cu (copper) and Ni (nickel), and satisfy the following formulas (1), (2). l / i > 0.01... (1) i + j + k + l = 2... (2) 【0009】 According to this configuration, the general formula is Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δThe metal atom M1, represented by the formula (1), is either W, Mo, or both W and Mo, and the metal atom M2 is either Cu, Ni, or both Cu and Ni. Furthermore, as shown in formula (1) above, the proportion of the metal element M2, determined by the coefficient l, is greater than 0.01 relative to the proportion of Ba (barium), determined by the coefficient i. This promotes sintering of the material and improves the relative density of the material. Also, as shown in formula (2) above, the total number of moles of each metal element, Ba, Sc (scandium), M1, and M2, is constant. In this configuration, Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ By satisfying these conditions, a material is provided that possesses proton conductivity and has improved relative density. 【0010】 (2) In the polycrystalline metal oxide of the above form, the following formula (3) may be satisfied: 0.01 < l / i ≤ 0.05 ... (3) With this configuration, the proportion of the metal element M2 determined by the coefficient l relative to the proportion of Ba determined by the coefficient i is set to a lower limit by formula (1) above, and further set to an upper limit of 0.05 or less. When the proportion of M2 to Ba increases, densification progresses, but the proton transport rate (electrolyte properties) decreases. In other words, the contribution of conduction of conductive species other than protons increases, and the resulting energy loss tends to increase. In this configuration, by setting an upper limit on the proportion of M2 to Ba, the densification of the material is improved, and the decrease in the proton transport rate is suppressed. 【0011】 (3) In the polycrystalline metal oxide of the above form, the atoms of M2 may consist only of Ni. With this configuration, the metal atoms constituting M2 are limited to Ni only. The difference in ionic radius between Ni and Sc, and between W and Mo which can become M1, is within 15%. When the difference in ionic radius is within 15%, solid solution formation is easily performed, and the material is easily densified. On the other hand, although the difference in ionic radius between Cu and Sc is within 15%, the difference in ionic radius between Cu and W and Mo which can become M1 is greater than 15%. Therefore, by limiting the metal element constituting M2 to Ni only, the density of the material is further improved. 【0012】 Furthermore, the present invention can be realized in various forms, for example, in the form of a polycrystalline metal oxide, a proton-conducting solid electrolyte, an electrolyte layer, an electrochemical device, and a system comprising these, as well as a method for producing a polycrystalline metal oxide. 【0013】 This is a schematic cross-sectional view of an electrode equipped with an electrolyte according to this embodiment. ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ This is a flowchart of the manufacturing method. Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ These are explanatory diagrams for Examples 1-7 and Comparative Examples 1-4, which are samples. This is an explanatory diagram of the XRD analysis results for single-phase Example 3. This is an explanatory diagram of the XRD analysis results for Example 3 including a subphase. This is an explanatory diagram of the dispersion state of Ni element in Example 3. This is an explanatory diagram of the cross-sectional SEM image of Example 3. This is an explanatory diagram of the method for calculating the electrical conductivity of each sample. 【0014】 <Embodiment> Figure 1 is a schematic cross-sectional view of an electrode 10 comprising an electrolyte 1 of an embodiment. The electrode 10 shown in Figure 1 comprises a proton-conducting electrolyte 1, a fuel electrode (anode) 2 formed on one side of the electrolyte 1, and an air electrode (cathode) formed on the other side of the electrolyte 1. The electrolyte 1 of this embodiment has the general formula Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ It is represented by and formed from a polycrystalline metal oxide having a perovskite structure. The coefficients i, j, k, l, and δ are real numbers greater than or equal to 0. Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δIn this equation, M1 is an atom composed of at least one of W (tungsten) and Mo (molybdenum). M2 is an atom composed of at least one of Cu (copper) and Ni (nickel). That is, M1 may consist only of W, only of Mo, or both of W and Mo. If M1 consists of both W and Mo, the coefficient k is the sum of the mol ratio of W and the mol ratio of Mo. Similarly, M2 may consist only of Cu, only of Ni, or both of Cu and Ni. If M2 consists of both Cu and Ni, the coefficient l is the sum of the mol ratio of Cu and the mol ratio of Ni. 【0015】 Ba forming electrolyte 1 ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ Furthermore, the following equations (4) and (5) are satisfied: l / i > 0.01 ... (4) i + j + k + l = 2 ... (5) 【0016】 Figure 2 shows the Ba of this embodiment ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ This is a flowchart of the manufacturing method. In the manufacturing flow shown in Figure 2, first, Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ BaCO2 is the raw material for ３ , Sc ２ O ３ MoO ３ WO ３First, raw material powders such as NiO and CuO are weighed to the desired composition ratio (Step S1). After these raw material powders are mixed, ethanol is added and wet mixing is performed in a ball mill for 15 hours to produce a slurry (Step S2). The wet mixing time can be changed as appropriate. The slurry is dried and the resulting mixed powder is calcined in an alumina crucible under air at 1000°C to 1300°C (Step S3). The calcined powder is crushed, sieved, and then subjected to uniaxial pressing and CIP (Cold Isostatic Pressing) to produce a molded body (Step S4). The molded body is fired at 1450°C to 1550°C for 12 to 24 hours to obtain a sintered body (Step S5). The sintered body is polished to produce pellets of electrolyte 1 of the desired shape (Step S6), and the manufacturing flow is completed. Furthermore, as a method for producing the polycrystalline metal oxide in this embodiment, methods other than the solid-phase method described above may be employed. For example, various methods known as methods for producing complex oxides, such as the citrate complex polymerization method, the coprecipitation method, and the sol-gel method, can be used. 【0017】 Figure 3 shows Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ This is an explanatory diagram of Examples 1 to 7 and Comparative Examples 1 to 4, which are samples. Figure 3 shows the mole ratio of the metal elements contained in each sample of Examples 1 to 7 and Comparative Examples 1 to 4, the relative density, the ratio obtained by dividing the mole amount of the element M2 represented on the left side of the above formula (4) by the mole amount of Ba (barium), and the proton conductivity index representing the proton conductivity. 【0018】As shown in Figure 3, the elemental compositions of the metals in Examples 1-7 and Comparative Examples 1-4 all satisfy the relationship in formula (5) above. On the other hand, regarding the relationship in formula (4) above, Examples 1-7 satisfy it, while Comparative Examples 1-4 do not. The elemental composition of each sample can be confirmed by performing emission spectroscopy (ICP-OES or ICP-AES) on the sintered body of each sample using ICP (inductively coupled plasma) as the light source. The wt% ratio obtained from the analysis can be converted to the mol ratio. Furthermore, the crystal structure of each sample was obtained by performing structural analysis of the sintered body using XRD (X-ray diffraction). A SmartLab manufactured by Rigaku Corporation was used as the XRD measurement device, and the analysis was performed using CuKα rays in the range of 2θ = 10° to 60°. 【0019】 Figures 4 and 5 are explanatory diagrams of the XRD analysis results for Example 3. Figure 4 shows the XRD pattern of Example 3, which is formed as a single phase. Figure 5 shows the XRD pattern of Example 3, which has a subphase present. The subphase identified from the XRD pattern shown in Figure 5 is BaSc. ２ O ４ , Ba ３ Sc ４ O ９ , and BaCO ３ Furthermore, even in Example 3, which includes a sub-phase, the performance was almost the same as in Example 3, which is formed from a single phase. 【0020】 The relative density of the samples shown in Figure 3 was measured, and it was determined whether each sample was dense or not. In this embodiment, a sample was determined to be dense if the relative density was 90% or higher, and not dense if it was less than 90%. The relative density was measured using the Archimedes method described in JIS R1634:1998. As shown in Figure 3, the relative densities of Examples 1 to 7, which satisfy both equations (4) and (5) above, were all 90% or higher, and were determined to be dense. 【0021】Furthermore, among Examples 1 to 7, the relative density of Examples 3 to 7, in which the M2 metal element is composed solely of Ni, is higher than the relative density of Examples 1 to 2, which contain Cu as the M2 metal element. In other words, the samples of Examples 3 to 7 are denser because the M2 metal element is composed solely of Ni. This is thought to be because, according to the Hume-Rosaly law, the ionic radius difference between Ni and both Sc (scandium) and W and Mo, which are used as the M1 element, is within 15%. It is thought that the ionic radius difference of within 15% facilitates solid solution formation, resulting in a denser sample. On the other hand, while Cu, used as the M2 element, has an ionic radius difference of within 15% with Sc, its ionic radius difference with W and Mo exceeds 15%. Note that while the Hume-Rosaly law itself is an empirical rule concerning the formation of intermetallic compounds (alloys), it can also be applied to metal oxides. [1] . [1] CBAlcock, Annu. Rev. Mater. Sci., 1, 219 (1971). 【0022】 Figure 6 is an explanatory diagram of the dispersion state of Ni element in Example 3. Figure 6 shows an image taken by EPMA (Electron Probe Microanalyzer) in Example 3. When performing analysis using EPMA, the above cross-section of the electrode 10 should be mirror-polished, and then the cross-section of the sample should be treated with a cross-section polisher (CP) using an ion beam such as argon ions, before taking an image using EPMA. The magnification scale is indicated by "5 μm" shown in the EPMA image, which represents the actual size of 5 μm. As shown in Figure 6, the Ni element is well dispersed in the sample. Although Figure 6 is an example of an image taken with EPMA, EDS (Energy Dispersive X-ray Spectroscopy) may also be used. 【0023】Figure 7 is an explanatory diagram of the cross-sectional SEM image of Example 3. In Figure 7, an enlarged view of the cross-sectional SEM image of Example 3 where CP processing was performed is shown. The scale of enlargement is such that "10 μm" shown in the SEM image represents an actual dimension of 10 μm. As shown in Figure 7, it can be seen that the sample of this embodiment is dense. 【0024】 The proton conduction index shown in Figure 3 is the value obtained by measuring the electrical conductivity of each sample by the direct current four-terminal method. In a Dry gas atmosphere (Air (20% O ２ )) and a Wet gas atmosphere (Air + 5% H ２ O), by performing chronopotentiometry evaluation, the direct current resistance is evaluated and the electrical conductivity is calculated. Specifically, the resistivity is calculated from the direct current resistance and the dimensions of the sample, and the reciprocal of the resistivity is calculated as the electrical conductivity. As a measurement sample, each sample was formed into a pellet of 10 mm × 30 mm × 3 mm by uniaxial pressing, and the measurement sample was produced by the same method as the manufacturing flow shown in Figure 2. Four Pt (platinum) wires were wound around the obtained measurement sample, and a Pt paste was applied to make the measurement sample and the Pt wire adhere closely. In order to remove the organic component contained in the Pt paste, it was heated at 700 °C for 1 hour using an electric furnace. 【0025】 Figure 8 is an explanatory diagram of the method for calculating the electrical conductivity of each sample. In Figure 8, the temperature changes of the direct current electrical conductivity σ ＤＣ (Dry) and σ ＤＣ (Wet) are represented. In Figure 8, the direct current electrical conductivity σ ＤＣ (Dry) indicated by a square is the conductivity in a Dry gas atmosphere. On the other hand, the direct current electrical conductivity σ ＤＣ (Wet) indicated by a circle is the conductivity in a Wet gas atmosphere. In a Dry gas atmosphere, O ２－ and e － (h ＋ ) conduct. On the other hand, in a Wet gas atmosphere, in addition to O ２－ and e － (h ＋ ), H ＋Protons are conducted. The oxygen partial pressure is almost the same in the dry gas atmosphere and the wet gas atmosphere. Therefore, the difference between the direct current electrical conductivity σ ＤＣ (Wet) and the direct current electrical conductivity σ ＤＣ (Dry) can be considered as the conductivity of H ＋ . In the present embodiment, if the value obtained by dividing the difference by the direct current electrical conductivity σ ＤＣ (Wet) is 0.5 or more, it is evaluated that the proton conductivity is high. Also, when the value (0.5) is 0.4 or more, which is 80% of the value, it is evaluated that the sample has proton conductivity. In the present embodiment, the temperature condition for determining the proton conductivity is in the range of 470 °C or higher and 530 °C or lower. 【0026】 Among Examples 1 to 7, the proton conductivities of Examples 1 to 5 and 7 that satisfy the following relational expression (6) are as high as 0.5 or more as shown in FIG. 3. The proton conductivity of Example 6 that does not satisfy the relational expression (6) is 0.4. On the other hand, in Comparative Examples 1 to 4, sintering was not sufficient and the samples were damaged during processing and evaluation, so the proton conductivity could not be evaluated. 0.01 < l / i ≤ 0.05... (6) 【0027】 As described above, the electrolyte 1 of the present embodiment has a general formula Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ . Note that the coefficients i, j, k, l, and δ are real numbers of 0 or more. In Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ , M1 is an atom composed of at least one of W and Mo. Also, M2 is an atom composed of at least one of Cu and Ni. According to this configuration, as shown in the above formula (4), the ratio of the metal element of M2 determined by the coefficient l is more than 0.01 with respect to the ratio of Ba (barium) determined by the coefficient i. As a result, the sintering of the material of the electrolyte 1 progresses, and the relative density improves as in Examples 1 to 7 shown in FIG. 3. Also, as shown in the above formula (5), the total number of moles of each metal element of Ba, Sc, M1, and M2 is constant. As a result, in the present embodiment, Baｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ By satisfying these conditions, an electrolyte 1 is provided that has proton conductivity and improved relative density. 【0028】 Furthermore, in this embodiment, of the Examples 1 to 7 shown in Figure 3, Examples 1 to 5 and 7 satisfy the above formula (6). In Examples 1 to 5 and 7, the ratio of the metal element M2 determined by the coefficient l to the ratio of Ba determined by the coefficient i is set to a lower limit by the above formula (4), and further set to an upper limit of 0.05 or less. When the ratio of M2 to Ba increases, densification progresses, but the proton transport rate (electrolyte properties) decreases. In other words, the contribution of conduction of conductive species other than protons increases, and the resulting energy loss tends to increase. In Examples 1 to 5 and 7, by setting an upper limit on the ratio of M2 to Ba, the densification of the material is improved, and the decrease in the proton transport rate is suppressed. 【0029】 Furthermore, in this embodiment, of Examples 1 to 7 shown in Figure 3, Examples 3 to 7 consist solely of Ni as the metal element M2. The ionic radius difference between Ni and Sc, and between W and Mo which can become M1, is within 15%. When the ionic radius difference is within 15%, solid solution formation is easily achieved, and the material is easily densified. On the other hand, while Cu has an ionic radius difference of within 15% with Sc, the ionic radius difference with W and Mo which can become M1 is greater than 15%. Therefore, by limiting the metal element constituting M2 to Ni only, the relative density of Examples 3 to 7 is improved as shown in Figure 3, and the density of electrolyte 1 is further improved. 【0030】 <Modifications of this Embodiment> The present invention is not limited to the above-described embodiments, and can be implemented in various forms without departing from the spirit of the invention. For example, the following modifications are also possible. 【0031】 In the above embodiment, the general formula is Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δThe electrolyte 1 represented by was explained. However, the general formula is Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ It is represented as such, where the atoms of M1 are composed of at least one of W and Mo, and the atoms of M2 are composed of at least one of Cu and Ni, and it is deformable within the range of polycrystalline metal oxides that satisfy the above formulas (4) and (5). Examples 1 to 7 shown in Figure 3 are examples, and the general formula is Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ The polycrystalline metal oxide is represented by the formula (4) and (5) above, and the molar ratio of the metal elements contained in the sample is deformable within the range that satisfies formulas (4) and (5). The polycrystalline metal oxide may form a component other than electrolyte 1, for example, a proton-conducting solid electrolyte used in fuel cells, steam electrolytic cells, sensors, and electrodes. The polycrystalline metal oxide may also be contained in the electrolyte layer. This electrolyte may be used in various electrochemical devices. The fuel cell and the like using electrolyte 1 shown in Figure 1 can be described as an example of an electrochemical device. 【0032】 Examples 3 to 7 shown in Figure 3 are Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ In Example 1, the metal element M2 was Ni, but the polycrystalline metal oxide may be composed of Ni and Cu as the metal element M2, or it may be composed of Cu only as the metal element M2, as in Example 2. It is preferable that the metal element M2 be composed of Ni only in order to improve the relative density (denseness) of the polycrystalline metal oxide. 【0033】 Examples 1 to 5 and 7 shown in Figure 3 were polycrystalline metal oxides that satisfied formula (6) above, but as in Example 6, the formula (6) may not be satisfied and l / i may be greater than 0.05. In order to improve the proton conductivity of the polycrystalline metal oxide, it is preferable that l / i be within the range that satisfies formula (6) above. 【0034】The embodiments of this specification have been described above based on the embodiments and modifications described above. The embodiments described above are for the purpose of facilitating understanding of this specification and do not limit it. This specification may be modified and improved without departing from its spirit and the scope of the claims, and equivalents thereof are included in this specification. Furthermore, any technical features that are not described as essential in this specification may be deleted as appropriate. 【0035】 The present invention can also be realized in the following form: [Application Example 1] A polycrystalline metal oxide having the general formula Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ A polycrystalline metal oxide characterized by being represented as follows, wherein the atoms of M1 are composed of at least one of W (tungsten) and Mo (molybdenum), and the atoms of M2 are composed of at least one of Cu (copper) and Ni (nickel), and satisfying the following formulas (7) and (8): l / i > 0.01 ... (7) i + j + k + l = 2 ... (8) [Application Example 2] A polycrystalline metal oxide according to Application Example 1, characterized by satisfying the following formula (9): 0.01 < l / i ≤ 0.05 ... (9) [Application Example 3] A polycrystalline metal oxide according to Application Example 1 or Application Example 2, characterized in that the atoms of M2 are composed only of Ni. [Application Example 4] A proton-conducting solid electrolyte, characterized in that the polycrystalline metal oxide according to any one of Application Examples 1 to 3 is used in a fuel cell, a steam electrolytic cell, a sensor, an electrolyte, or an electrode. [Example 5] An electrolyte layer characterized by comprising a polycrystalline metal oxide described in any one of Examples 1 to 3. [Example 6] An electrochemical device characterized by comprising an electrolyte layer described in Example 5. 【0036】 1...Electrolyte 2...Fuel electrode (anode) 3...Air electrode (cathode) 10...Electrode σ ＤＣ...DC electrical conductivity

Claims

1. A polycrystalline metal oxide whose general formula is Ba ｉ Sc ｊ M1 ｋ M2 ｌ O ３－δ A polycrystalline metal oxide characterized by being represented as follows, wherein the atoms of M1 are composed of at least one of W (tungsten) and Mo (molybdenum), and the atoms of M2 are composed of at least one of Cu (copper) and Ni (nickel), and satisfying the following formulas (1) and (2): l / i > 0.01 ... (1) i + j + k + l = 2 ... (2) 2. A polycrystalline metal oxide according to claim 1, characterized in that it satisfies the following formula (3): 0.01 < l / i ≤ 0.05 ... (3) 3. A polycrystalline metal oxide according to claim 2, characterized in that the M2 atoms are composed solely of Ni.

4. A proton-conducting solid electrolyte, characterized in that the polycrystalline metal oxide described in any one of claims 1 to 3 is used in a fuel cell, a steam electrolysis cell, a sensor, an electrolyte, or an electrode.

5. An electrolyte layer comprising a polycrystalline metal oxide as described in any one of claims 1 to 3.

6. An electrochemical device comprising the electrolyte layer described in claim 5.

Images