Method for producing proton-containing oxide, dense body of proton-containing basic composite oxide, solid electrolyte, fuel cell, hydrogen production cell, hydrogen sensor or ammonia synthesis cell, and methods for producing these

Abstract

Provided are: a method for producing a proton-containing oxide, the method comprising reacting a basic oxide with a carboxylic acid melt having at least pKa4 and introducing a proton into the basic oxide to obtain a proton-containing oxide; a dense body of a proton-containing basic composite oxide; a fuel cell; a hydrogen production cell; a hydrogen sensor; or an ammonia synthesis cell; and methods for producing the same.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a proton-containing oxide, and a method for producing a fuel cell, a hydrogen production cell, a hydrogen sensor, or an ammonia synthesis cell using the proton-containing oxide obtained by the production method. The present invention also relates to a dense body of a proton-containing basic composite oxide, a solid electrolyte containing the same, a fuel cell, a hydrogen production cell, a hydrogen sensor or an ammonia synthesis cell. [Background technology]

[0002] Oxides that are thermally and chemically stable in the medium temperature range of approximately 300 to 600°C and exhibit high proton conductivity are required for various electrochemical devices such as fuel cells, hydrogen production cells, hydrogen sensors, and ammonia synthesis cells. However, most of the known proton-conducting oxides are synthesized through a high-temperature (approximately 1000°C) sintering process, and are therefore limited to specific material systems such as perovskite-type or related crystal systems, or phosphate-based glasses as described in Patent Document 1, which limits the degree of freedom in terms of shape, including crystal structure, and composition.

[0003] Although they decompose at high temperatures, proton-conducting oxides that are stable up to the medium temperature range of 300 to 600°C are thought to be available in a wide variety of materials and crystal systems, and research and development into their manufacturing methods is underway. For example, Patent Document 2 and Non-Patent Documents 1 and 2 describe that a dense body of a proton-conducting oxide stable in the medium temperature range was obtained by electrochemically replacing alkali ions in a specific alkali-containing sample with protons by ion exchange using hydrogen gas and a direct current voltage. However, Patent Document 2 describes that the sample is a phosphate glass containing an alkali oxide and an oxide of a polyvalent electropositive element that has multiple oxidation states, and Non-Patent Document 1 describes that the sample is a phosphate glass containing an alkali oxide and an oxide of a polyvalent electropositive element that has multiple oxidation states, such as NaNbWO6, Na3Zr2Si3PO 12 , NaMgPO4, NaLa(PO4)3, Li3Sc2(PO4)3, Li5La3Nb2O 12Using each crystal as a sample, Non-Patent Document 2 only describes using Na3Zr2Si3PO 12 as a sample. As described in Non-Patent Document 1, when NaNbWO6 is used as a sample, not only is the substitution of sodium ions with protons involved, but also W 6+ +e - →W 5+ reactions proceed simultaneously, and ion exchange only occurs in the immediate vicinity of the anode and cannot be applied to oxides such as NaNbWO6 that are easily reduced. Also, in the above electrochemical ion exchange method, it is necessary to apply a proton introduction electrode and an alkali ion absorption electrode to an alkali-containing sample and cannot be applied to porous or powdered samples.

[0004] Also, Patent Document 3 and Non-Patent Documents 3 and 4 describe a method of substitution with protons using a chemical potential difference. For example, Patent Document 3 describes that a powder of a substituted garnet-type lithium ion conductive oxide having excellent lithium ion conductivity and no aluminum contamination is subjected to ion exchange between lithium ions and protons at 80 °C or higher in a solution of a substance having a hydroxy group or a carboxy group, whereby a proton conductive composite oxide, Li 7-x-y H x La3Zr 2-y M y O 12 (M is Ta and / or Nb, 3.2 < x ≦ 7 - y, 0.25 < y < 2), and it is described that a powder of a composite oxide which is a single phase having a garnet-type structure belonging to the cubic system can be obtained. Also, Non-Patent Document 3 describes that Li 13.9 Sr 0.1 Zn(GeO4)4 powder is immersed in an acetic acid aqueous solution to exchange lithium ions with protons, a pressure of 2000 MPa is applied to the obtained proton-substituted powder, and a dense body is obtained by applying a DC voltage in hydrogen. Also, Non-Patent Document 4 describes Li7La3Nb2O 12It is described that a completely ion-exchanged dense body having a thickness of 3 mm was obtained by immersing the dense body in water at room temperature for 14 days. However, the method described in Patent Document 3 is only capable of ion exchange for powder samples and cannot be applied to dense body samples. Furthermore, the method described in Non-Patent Document 3 cannot be applied to oxide samples that are easily reduced, and the process for producing dense bodies into which protons have been introduced is impractical. Furthermore, it is necessary to apply a proton-introducing electrode and a Li-absorbing electrode to the sample obtained by applying pressure, making the process complicated. Furthermore, the method described in Non-Patent Document 4 is also considered impractical because it is only applicable to limited material systems and requires a long time. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 3905899 [Patent Document 2] Patent No. 6041606 [Patent Document 3] International Publication No. 2017 / 033865 [Non-patent literature]

[0006] [Non-Patent Document 1] Development of solid electrolytes for intermediate-temperature fuel cells: Development of new materials by high-temperature alkali-proton substitution method, Grant-in-Aid for Scientific Research, Research Report, Japan, May 25, 2017, Project No. 15K14126 [Non-patent document 2] S. Tsukuda et al., Inorg. Chem. 2017, Vol. 56, No. 22, pp. 13949-13954 [Non-patent document 3] T. Wei et al., Chem. Mater. 2017, Vol. 29, No. 4, pp. 1490-1495 [Non-patent document 4] L. Truong et al., J. Mater. Chem. A, 2013, Vol. 1, No. 43, pp. 13469-13475 Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, research has been conducted into the synthesis of proton-conducting oxides that are stable up to the medium temperature range of 300 to 600°C. However, Patent Documents 1 to 3 and Non-Patent Documents 1 to 4 only describe methods for producing proton-introduced oxides (hereinafter referred to as "proton-containing oxides") that are limited in the shape (powder, porous, dense, etc.) and / or composition (crystal structure, etc.) of the sample for introducing protons. There is a demand for the development of an efficient production method that is less limited in the shape and composition of the applicable sample and that allows for the easier production of proton-containing oxides.

[0008] Therefore, an object of the present invention is to provide a production method that can be applied to raw materials in any form, such as powder, porous body, or dense body, and that can easily obtain a proton-containing oxide. Another object of the present invention is to provide a novel dense body of a proton-containing basic composite oxide that can be obtained by the production method of the present invention. Another object of the present invention is to provide a solid electrolyte, a fuel cell, a hydrogen production cell, a hydrogen sensor, or an ammonia synthesis cell, which includes a proton-containing oxide or a dense body of the proton-containing basic composite oxide obtained by the production method of the present invention, and to apply the production methods thereof. [Means for solving the problem]

[0009] The above-mentioned problems of the present invention have been solved by the following means. [1] A method for producing a proton-containing oxide, comprising reacting a basic oxide with a molten carboxylic acid having a pKa of 4 or more to introduce protons into the basic oxide, thereby obtaining a proton-containing oxide. [2] The method for producing a proton-containing oxide according to [1], wherein the basic oxide is a basic composite oxide containing at least one element selected from B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from Si, Ti, Zr, Hf, Lu, Y, and Sc. [3] The method for producing a proton-containing oxide according to [1] or [2], wherein the basic oxide is a garnet type or a Caswell silverite-related type. [4] The method for producing a proton-introduced oxide according to any one of [1] to [3], wherein the carboxylic acid melt is a fatty acid melt. [5] The method for producing a proton-containing oxide according to any one of [1] to [4], wherein the reaction temperature is 150 to 350°C. [6] The method for producing a proton-containing oxide according to any one of [1] to [5], wherein the reaction time of the reaction is 1 to 20 hours. [7] The method for producing a proton-containing oxide according to any one of [1] to [6], which comprises washing after the reaction. [8] The method for producing a proton-containing oxide according to any one of [1] to [7], wherein the washing is carried out using oil or fat. [9] A method for producing a fuel cell, a hydrogen production cell, a hydrogen sensor, or an ammonia synthesis cell, comprising incorporating, as a solid electrolyte, a proton-containing oxide obtained by the method for producing a proton-containing oxide according to any one of [1] to [8] above.

[10] A dense body of a proton-containing basic composite oxide containing at least one element selected from the group consisting of B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from the group consisting of Si, Ti, Zr, Hf, Lu, Y, and Sc, and containing oxide ions.

[11] A solid electrolyte comprising a dense body of the proton-containing basic composite oxide according to

[10] .

[12] A fuel cell, a hydrogen production cell, a hydrogen sensor, or an ammonia synthesis cell, comprising the solid electrolyte according to

[11] above. [Effects of the Invention]

[0010] The production method of the present invention can be applied to raw materials in any form, such as powder, porous body, or dense body, and can easily produce a proton-containing oxide. Furthermore, the dense body of the proton-containing basic composite oxide of the present invention is a novel proton-containing basic composite oxide that can be obtained by the production method of the present invention, and has a high degree of design freedom in terms of shape, composition, and electronic conductivity, and can be used as a solid electrolyte in fuel cells, hydrogen production cells, hydrogen sensors, or ammonia synthesis cells. [Brief explanation of the drawings]

[0011] [Figure 1] The graph below shows the free energy change ΔG°Oxide→Hydroxide when a hydroxide is produced from a simple oxide, plotted against temperature. [Figure 2] The graph shows the change in free energy of formation of simple oxides, ΔG°Oxide, plotted against temperature. [Figure 3] FIG. 1 is a schematic explanatory view of one embodiment of an apparatus for carrying out an ion exchange reaction between a basic oxide and a carboxylic acid melt in the production method of the present invention. [Figure 4] These are EDS (energy dispersive X-ray spectroscopy) spectra of Na2ZrO3 before and after the ion exchange reaction. The lower spectrum is Na2ZrO3 before proton introduction, and the upper spectrum is the spectrum after proton introduction into Na2ZrO3. [Figure 5] The X-ray diffraction spectra of Na2ZrO3 before and after the ion exchange reaction are shown. The lower spectrum is Na2ZrO3 before proton introduction, and the upper spectrum is the spectrum after proton introduction into Na2ZrO3. [Figure 6]The changes in the crystal structure of Na2ZrO3 that are predicted by the ion exchange reaction are shown in Figure 6. Figure 6(a) shows the crystal structure of Na2ZrO3 before proton introduction, and Figure 6(b) shows the crystal structure of Na2ZrO3 after proton introduction. [Figure 7] X-ray diffraction spectra of Al-doped LLZ before and after the ion exchange reaction are shown. The lower spectrum is the Al-doped LLZ before proton introduction, and the upper spectrum is the spectrum after proton introduction into the Al-doped LLZ. [Figure 8] The Raman spectra of Al-doped LLZ before and after the ion exchange reaction are shown. The lower spectrum is the Al-doped LLZ before proton introduction, and the upper spectrum is the spectrum after proton introduction into the Al-doped LLZ. [Figure 9] The graph shows the results of depth profile analysis by Raman spectroscopy of the Al-doped LLZ and Ta-doped LLZ sintered pellets after the ion exchange reaction. ▼ indicates the plot for Ta-doped LLZ, and ▲ indicates the plot for Al-doped LLZ. [Figure 10] This shows a fracture surface image near the surface of a pellet-shaped sintered body of Al-added LLZ taken by SEM (scanning electron microscope) after ion exchange. [Figure 11] This is a plot showing the ion exchange reaction characteristics of acidic compounds. The horizontal axis represents the ion exchange reaction temperature, and the vertical axis represents the pKa of the acidic compound, and the relationship between these is plotted. For tests in which protons were introduced to a depth of 10 μm or more, the maximum depth at which proton introduction was confirmed is also shown. [Figure 12] The graph shows the results of depth profile analysis by Raman spectroscopy of a pellet-shaped sintered body of Al-added LLZ after an ion exchange reaction. ◆ indicates the plot when behenic acid was used at a reaction temperature of 190°C, ▲ indicates the plot when adipic acid was used at a reaction temperature of 180°C, and ● indicates the plot when behenic acid was used at a reaction temperature of 250°C. [Figure 13] X-ray diffraction spectra of Al-doped LLZ before and after the ion exchange reaction are shown. The lower spectrum is the Al-doped LLZ before proton introduction, and the upper spectrum is the spectrum after proton introduction into the Al-doped LLZ. [Figure 14]Scanning electron microscope (SEM) images of the fracture surface near the thickness of the Al-doped LLZ before and after the ion exchange reaction are shown. The image on the left shows the Al-doped LLZ before proton doping, and the image on the right shows the fracture surface near the thickness of the Al-doped LLZ after proton doping. [Figure 15] These are the micro-Raman spectra of the Al-doped LLZ before and after the ion exchange reaction. The bottom spectrum is the Al-doped LLZ before proton introduction, and the other spectra are the spectra after proton introduction into the Al-doped LLZ. In the spectrum of the Al-doped LLZ after proton introduction, the spectra shown from top to bottom are at 0 μm (surface), 2.3 μm, 25.3 μm, 48.3 μm, 71.3 μm, 85.1 μm, and 87.4 μm (other surface), relative to the surface in the thickness direction of the thin film. [Figure 16] The results of depth profile analysis by micro-Raman spectroscopy of Al-doped LLZ after ion exchange reaction are shown below. The error bars indicate the standard deviation, and the black circles indicate the median. [Figure 17] This is a plot showing the results of measuring the electrical conductivity of Al-added LLZ before and after the ion exchange reaction. The horizontal axis represents temperature, and the vertical axis represents the common logarithm of electrical conductivity, and the relationship between these is plotted. △ indicates the plot for Al-added LLZ before proton introduction, and ▲ indicates the plot for Al-added LLZ after proton introduction. DETAILED DESCRIPTION OF THE INVENTION

[0012] In the present invention, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0013] [Method of producing proton-containing oxide] In the method for producing a proton-containing oxide of the present invention (hereinafter also referred to as "the production method of the present invention"), a basic oxide is reacted with a molten carboxylic acid having a pKa of 4 or higher through an ion exchange reaction, whereby cations of the basic oxide are exchanged with protons of the carboxylic acid having a pKa of 4 or higher, and protons are introduced into the basic oxide, thereby producing a proton-containing oxide. The basic oxide and the carboxylic acid melt having a pKa of 4 or more, the ion exchange reaction, and the washing treatment used in the production method of the present invention will be described below.

[0014] <Basic oxides> In the manufacturing method of the present invention, the term "basic oxide" refers to an oxide that reacts with a carboxylic acid melt having a pKa of 4 or higher to form a carboxylate. Therefore, the basic oxide is an oxide of at least one of alkali metals, alkaline earth metals, and transition metals with a low oxidation state (+1 or +2). The basic oxide may contain elements other than oxygen, alkali metals, alkaline earth metals, and transition metals with a low oxidation state (+1 or +2). In particular, the basic oxide is preferably an oxide of at least one of alkali metals and alkaline earth metals. The basic oxide may be a simple oxide, which is an oxide of a single metal element, or a composite oxide, which is an oxide of two or more metal elements, and among these, a composite oxide (basic composite oxide) is preferred. Of the cations and anions that constitute the basic oxide, the anion may be an anion containing an oxygen atom. Examples of an anion containing an oxygen atom include an oxide ion composed only of oxygen atoms, [SiO4] 4- , [PO4] 3- or [GeO4] 3- In the present invention, the oxide ion is a polyanion formed of a polyhedron composed of multiple ions, such as O 2- The term "oxide ion" is not limited to the above, but includes those whose valence is more positive than -2 by forming a covalent bond. In the present invention, oxide ion does not mean an oxide ion that constitutes a polyanion. In the present invention, it is preferable that the anion contains an oxide ion. In the production method of the present invention, it is preferable that the basic oxide is hydrophilic in order to allow the ion exchange reaction to proceed more smoothly. In the production method of the present invention, the basic oxide may have a crystalline structure or may be amorphous, but preferably has a crystalline structure. In the present invention, "having a crystalline structure" means that the presence of regions having translational symmetry can be confirmed by X-ray diffraction. Furthermore, in the production method of the present invention, the shape of the basic oxide is not particularly limited, and any basic oxide in the form of a powder, a porous body, a dense body, or a laminate of a porous body and a dense body can be used to obtain the desired proton-containing oxide. A porous body is a bulk body having many pores (voids), and a dense body is a bulk body that is not porous. A dense body usually has the physical property of being impermeable to liquids and gases. In the present invention, a dense body is defined as one having a relative density of 70% or more. The above-mentioned relative density refers to the percentage of the actual density relative to the theoretical density calculated from the crystal structure, lattice constant, and composition. The actual density is measured by dividing the mass measured with an electronic balance with built-in weights by the average volume measured with a standard digital caliper (calculated from the average of three measurements of diameter and height). The relative density of the dense body in the present invention is preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more. There is no particular upper limit, and in practice it is 100% or less.

[0015] In the present invention, in order to maintain the skeleton of the crystal structure after proton introduction into the basic oxide, it is preferable that the basic composite oxide contains at least one element with high basicity and at least one element with lower basicity. For highly basic elements, the free energy change ΔG° when a hydroxide is produced from a simple oxide is Oxide→Hydroxide As shown in Figure 1, ΔG° Oxide→Hydroxide These elements include B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, which have a valence of -30 kJ / mol or less at room temperature (25°C). The low basicity element species are Oxide→HydroxideThese ΔG° Oxide→Hydroxide There is no known thermodynamic data, so it cannot be shown. Therefore, the free energy change of formation of simple oxides, ΔG° Oxide As shown in Figure 2, the oxide phase can be selected based on its chemical stability. Oxide Among elements having a basicity of −850 kJ / mol or less at room temperature (25° C.), Si, Ti, Zr, Hf, Lu, Y, and Sc, which do not fall under the category of the above-mentioned highly basic elements, fall under these categories. Therefore, the basic oxide is preferably a basic composite oxide containing at least one element selected from B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from Si, Ti, Zr, Hf, Lu, Y, and Sc.

[0016] Of the elements other than oxygen constituting the basic composite oxide, the proportion of B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na and K is preferably 20 to 95 at%, more preferably 30 to 90 at%, and even more preferably 30 to 85 at%, and the proportion of Si, Ti, Zr, Hf, Lu, Y and Sc is preferably 5 to 80 at%, more preferably 10 to 70 at%, and even more preferably 15 to 70 at%. The basic composite oxide may contain elements other than B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, as well as Si, Ti, Zr, Hf, Lu, Y, and Sc (hereinafter referred to as "other elements"), such as Ta and Ga. Of the elements other than oxygen constituting the basic composite oxide, the proportion of the other elements is preferably 0 to 33 at %, more preferably 0 to 15 at %. In the above, "at %" means a ratio based on the number of elements. From the perspective of obtaining a proton-containing oxide in which ion exchange (proton introduction) reaches deeper inside by the production method of the present invention, the basic complex oxide is preferably composed of O, at least one element selected from B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from Si, Ti, Zr, Hf, Lu, Y, and Sc. That is, it is preferable not to contain elements other than O, other than B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and other than Si, Ti, Zr, Hf, Lu, Y, and Sc.

[0017] In the examples described later, as the basic oxide, a garnet-type Li7La3Zr2O 12 (hereinafter also abbreviated as "LLZ") derivative or a caswellsilverite-like Na2ZrO3 is used as an example, but the basic oxide used in the present invention is not limited to these, and various basic oxides can be used. For example, bronze-based crystals such as K2Ti8O 17 and merlite-type crystals such as Ca2Al2SiO7 can be mentioned. The above basic oxide may be a commercially available one or a synthetic one by a conventional method. When preparing a basic oxide by synthesis, there are no particular restrictions on raw materials, mixing, calcination, sintering methods, etc., and it can be synthesized according to a conventional method. For example, as raw materials, general oxides and carbonates can be used, and examples include mixing by a ball mill, etc., calcination by an electric furnace, etc., and sintering.

[0018] <Carboxylic acid melt with pKa of 4 or more> In the production method of the present invention, a carboxylic acid melt with a pKa of 4 or more (hereinafter also simply referred to as "carboxylic acid melt") means a melt of a Bronsted acid having an acid dissociation constant (pKa) of 4 or more at room temperature (25°C) and having one or more carboxy groups. The melting point of the carboxylic acid used is not particularly limited as long as the carboxylic acid melt is supplied as a melt (liquid carboxylic acid) during heating in the ion exchange reaction. Preferably, the carboxylic acid is an organic carboxylic acid that can melt without thermal decomposition during heating in the ion exchange reaction. Furthermore, the production method of the present invention uses a carboxylic acid melt as a proton source, which is different from conventional methods that use an aqueous carboxylic acid solution as a proton source. That is, in the production method of the present invention, the fact that water is substantially absent in the reaction system of the ion exchange reaction works as an advantage, as will be described later. The carboxylic acid melt may be a melt of one type of carboxylic acid or a melt of a mixture of two or more types of carboxylic acids. In the case of a melt of a mixture of two or more types of carboxylic acids, at least one type needs to be melted, and the remaining carboxylic acids may be melted or dissolved in the melted carboxylic acid. In the case of a melt of a mixture of two or more types of carboxylic acids, the pKa of the carboxylic acid melt means the pKa of the carboxylic acid showing the lowest pKa. In addition, for carboxylic acids showing two or more pKas, the lowest pKa of the two or more pKas is taken as the pKa of the carboxylic acid melt. The pKa of the carboxylic acid melt used in the production method of the present invention is 4 or more (4.0 or more), preferably more than 4.2, more preferably 4.3 or more, and even more preferably 4.4 or more. There is no particular upper limit to the pKa of the carboxylic acid melt, but it is usually 5.5 or less, preferably 5.0 or less. In the present invention, pKa refers to the negative common logarithm (-logKa) of the acid dissociation constant (Ka) in water at room temperature (25°C). pKa can be calculated by adding 0.01 mol / L of aqueous sodium hydroxide solution to an aqueous solution of a measurement sample (carboxylic acid) and reading the amount of aqueous sodium hydroxide solution added up to the half-equivalent point. For acids that do not dissolve in water, the acid dissociation constant determined by the above-mentioned titration using another solvent in which the acid is soluble (e.g., dimethyl sulfoxide) is dissolved is multiplied by a conversion constant calculated using the pKa of an acid that dissolves in both the solvent and water to calculate the pKa.

[0019] The carboxylic acid melt is preferably a fatty acid melt. In the present invention, the fatty acid melt refers to a melt of a carboxylic acid having a solubility in water (number of grams dissolved in 100 g of water at 25° C.) of less than 10 g / 100 g. Examples of the fatty acid melt include unsaturated fatty acids having 12 to 22 carbon atoms, such as oleic acid; saturated fatty acids having 12 to 22 carbon atoms, such as stearic acid and behenic acid; and compounds in which two carboxy groups are bonded to a hydrocarbon having 4 to 8 carbon atoms (which may be saturated or unsaturated), such as adipic acid (solubility in water: 2.4 g / 100 g). The blending ratio of the carboxylic acid melt to the basic oxide is not particularly limited as long as the ion exchange reaction described below proceeds, and it is preferable to use a carboxylic acid melt containing 20 or more equivalents of protons (H in carboxy groups) relative to the content of elements in the basic oxide that can be replaced by protons, and more preferably 100 or more equivalents of protons (H in carboxy groups). There is no particular upper limit, and it is sufficient that the amount is such that the basic oxide can be immersed in the carboxylic acid melt and the reaction can proceed.

[0020] <Ion exchange reaction between basic oxide and carboxylic acid melt> The production method of the present invention provides a proton-containing oxide in which at least a portion of the cations of at least one of alkali metals, alkaline earth metals, and transition metals in a low oxidation state (+1 or +2) in the basic oxide are exchanged with protons in the carboxylic acid melt. The ion exchange reaction between a basic oxide and a carboxylic acid melt is described below. 12 The following describes an ion exchange reaction using (LLZ) and a melt of a monocarboxylic acid compound R-COOH (R represents a hydrocarbon group; the same meaning is used hereinafter) as the carboxylic acid melt. However, the ion exchange reaction in the production method of the present invention is not limited to the reaction using these compounds. Li7La3Zr2O 12(LLZ) and the ion exchange reaction with R-COOH are represented by the following chemical reaction formula. x is a number satisfying 0 < x ≦ 7. Li7La3Zr2O 12 +xR-COOH→ H x Li 7-x La3Zr2O 12 +xR-COOLi In the above chemical reaction formula, at least a part of the lithium ions in Li7La3Zr2O 12 is exchanged with the protons in R-COOH, and a proton-containing oxide represented by H x Li 7-x La3Zr2O 12 is obtained.

[0021] Based on utilizing the above ion exchange reaction, the production method of the present invention has the following excellent advantages (1) and (2). (1) Since it is a reaction using the carboxylic acid melt itself as a proton source and a solvent, there is no need to use a low-boiling solvent such as water or alcohol. Therefore, compared with the conventional technology that performs ion exchange using a low-boiling solvent such as water or alcohol, for example, a high-temperature reaction at 100 °C or higher can be carried out, and the ion exchange reaction rate can be increased. Therefore, the shape of the basic oxide is not limited to powder, and a proton-containing oxide can be obtained even if it is a porous body or a dense body. In particular, even when a dense body is used, protons can be introduced (injected) deep into the dense body. (2) Also, since there is no need to use water, hydrated proton species (H3O + or H9O4 + ) that have low mobility do not occur in principle, and it is considered that the ion exchange reaction is not rate-limited by these hydrated proton species. Furthermore, when the basic oxide is hydrophilic and the carboxylic acid melt is hydrophilic, the ion exchange reaction is also less likely to be inhibited. That is, since the by-product organic carboxylic acid metal salt (R-COOLi) is hydrophobic, the proton-containing oxide (H x Li 7-x La3Zr2O 12) and the by-products can be easily removed from the surface of the target object by stirring or the like.

[0022] In the production method of the present invention, the reaction between the basic oxide and the carboxylic acid melt may be carried out in any manner as long as an ion exchange reaction occurs between the basic oxide and the carboxylic acid melt to produce a proton-containing oxide in which protons are introduced into the basic oxide. Specifically, by placing the basic oxide in contact with the carboxylic acid melt, the ion exchange reaction occurs on the surface of the basic oxide (the surface in contact with the carboxylic acid melt), and the protons introduced by ion exchange on the surface of the basic oxide are diffused by heat into the interior of the basic oxide, thereby obtaining a proton-containing oxide in which protons are introduced into the interior of the basic oxide, as will be described in detail later with reference to the results shown in Figure 11 and elsewhere. From the viewpoint of injecting protons into the basic oxide at a high concentration, it is preferable that the organic carboxylic acid metal salt, which is a by-product produced by ion exchange on the surface of the basic oxide, is removed from the surface of the proton-containing oxide so that the surface of the proton-containing oxide can be kept in contact with the carboxylic acid melt.

[0023] The reaction temperature in the ion exchange reaction may be any temperature at which the carboxylic acid melt can be maintained in a molten state or higher, and can be appropriately adjusted depending on the acid strength (pKa) of the carboxylic acid melt, etc. From the viewpoint of the rate of the ion exchange reaction, the reaction temperature is preferably 150 to 350°C, more preferably 180 to 300°C, even more preferably 190 to 300°C, and particularly preferably 200 to 300°C. The reaction time for the ion exchange reaction can be adjusted appropriately depending on the acid strength (pKa) of the carboxylic acid melt, etc., but from the viewpoint of the balance between the rate of the ion exchange reaction and productivity, it is preferably 1 to 12 hours, more preferably 2 to 10 hours, even more preferably 2 to 8 hours, and particularly preferably 6 to 8 hours. The reaction time for the ion exchange reaction can be adjusted appropriately depending on the acid strength (pKa) of the carboxylic acid melt, etc., but from the viewpoint of further increasing the ion exchange rate in addition to balancing the rate and productivity of the ion exchange reaction, it is preferably 1 to 20 hours, and more preferably 2 to 18 hours.

[0024] An example of an apparatus for carrying out an ion exchange reaction between a basic oxide and a carboxylic acid melt will be described below using FIG. 3. However, the present invention is not limited to this example, and the size, shape, number, etc. of the apparatus can be adjusted or changed as appropriate. In the ion exchange reaction apparatus 100 shown in Fig. 3, the basic oxide 1 is placed on a mesh 3a of a holder 3 having a mesh 3a and immersed in a reaction vessel 7 containing a carboxylic acid melt 5. The reaction vessel 7 is closed with a lid 9 capable of fixing the holder 3, and is placed on a mantle heater 11. In the ion exchange reaction apparatus 100 shown in Fig. 3, the parts of the basic oxide 1 and the holder 3 that are visible through the lid 9 and that are immersed in the carboxylic acid melt 5 are indicated by dashed lines. The holder 3 is preferably made of a chemically inert noble metal, and is preferably made of silver from the viewpoint of being relatively inexpensive. When the basic oxide 1 is porous or dense, it can be placed directly on the mesh 3a. However, when it is in powder form, it can be made into a powder compact by applying a pressure of about 0.1 to 50 MPa, and then placed on the mesh 3a and used in the manufacturing method of the present invention. The powder compact will be porous or dense depending on the hardness of the particles constituting the powder. Generally, when the particles are hard, the powder compact will be porous, and when the particles are soft, the powder compact will be dense. Since the particles of the LLZ used in the examples described below are hard, the resulting powder compact will be porous. The reaction vessel 7 and the lid 9 may be made of any material as long as they are chemically inert, for example, quartz. The lid 9 has a structure that can suppress the volatilization of the carboxylic acid melt 5. The gas inside the reaction vessel 7 is preferably replaced with an inert gas such as nitrogen in order to prevent oxidation of the carboxylic acid melt 5 . To perform the ion exchange reaction by heating, as shown in Fig. 3, the reaction vessel 7 is placed on a temperature-controllable mantle heater 11 as a heating device for the carboxylic acid melt 5. For precise temperature control, it is preferable to place a thermocouple (not shown in Fig. 3) for measuring the temperature near the basic oxide 1 in the carboxylic acid melt 5. In order to improve the efficiency of the ion exchange reaction, it is preferable that the ion exchange reaction apparatus 100 has a stirring function that constantly stirs the carboxylic acid melt 5. For example, it is preferable that the reaction vessel 7 is placed on a mantle heater with a stirrer to heat and stir the carboxylic acid melt 5 (not shown in Figure 3). Since the by-product organic carboxylic acid metal salt has a larger specific gravity than the carboxylic acid melt 5, by placing the basic oxide 1 above the carboxylic acid melt 5, it becomes easy to separate the organic carboxylic acid metal salt from the proton-containing oxide in which protons have been introduced into the basic oxide 1.

[0025] <Cleaning> The melt after the ion exchange reaction (proton introduction treatment) contains high-melting-point organic carboxylic acid metal salts, which adhere to the surface of the proton-containing oxide, and therefore must be thoroughly washed. Even with acetone, which is known for its strong degreasing power, washing may not be sufficient. In the production method of the present invention, cleaning can be performed by using oil or fat. Specifically, cleaning can be performed by replacing the melt after the ion exchange reaction (proton introduction treatment) with oil or fat and heating it again at about 150°C for a certain period of time while stirring. The fat or oil may be any fat or oil containing a highly unsaturated, low-viscosity fatty acid (commonly known as a saturated fatty acid), such as linoleic acid or linolenic acid, such as salad oil. These linoleic and linolenic acids are composed of unsaturated hydrophobic carboxylic acids that are compatible with organic carboxylic acid metal salts due to their similar polarity, and have the effect of increasing fluidity, making them suitable for use as inexpensive detergents. Note that linoleic acid and linolenic acid have low acidity (pKa 4.8 and 5.0 at room temperature (25°C) respectively), and proton-containing oxides do not dissolve in this washing treatment. In addition, in the heating and stirring washing in fats and oils, the heating temperature and stirring time can be appropriately adjusted without particular limitation as long as the washing is carried out. After this washing treatment, by rinsing the proton-containing oxide with acetone, a clean oxide after proton introduction treatment (proton-containing oxide) can be obtained.

[0026] <Proton-containing oxide> The "proton-containing oxide" obtained by the production method of the present invention means a proton-containing oxide in which at least a part of at least one of the cations of an alkali metal, an alkaline earth metal, and a transition metal in a low oxidation state (+1 or +2) in a basic oxide has been exchanged with a proton by the above ion exchange reaction. For example, when Li7La3Zr2O 12 (LLZ) is used, a proton-containing oxide represented by H x Li 7-x La_{3}Zr_{2}O 12 (x is a number satisfying 0 < x ≤ 7) is obtained, and when Na2ZrO3 is used as the basic oxide, a proton-containing oxide represented by H y Na 2-y ZrO3 (y is a number satisfying 0 < y < 1.5) is obtained respectively. [[ID=二十六]]Note that the statement that the production method of the present invention can be applied to raw materials in any of the forms of powder, porous body, and dense body means that even when a basic oxide in the form of a dense body, which is the most difficult to introduce protons into the interior, is used as the raw material, a proton-containing oxide into which protons have been introduced to a depth of at least 10 μm or more from the surface can be obtained.

[0027] (Dense body of proton-containing basic composite oxide) By the production method of the present invention, a dense body of a proton-containing basic composite oxide containing at least one element selected from B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from Si, Ti, Zr, Hf, Lu, Y, and Sc, and containing oxide ions (hereinafter also referred to as "the dense body of the proton-containing basic composite oxide of the present invention") can be obtained as a novel proton-containing oxide. The term "dense body of proton-containing basic composite oxide" refers to a dense body of proton-containing oxides containing at least one element selected from B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from Si, Ti, Zr, Hf, Lu, Y, and Sc, and in which at least one cation of at least one of alkali metals and alkaline earth metals in the basic composite oxide containing oxide ions has been substituted with protons. Therefore, the dense body of a proton-introduced body of phosphate-based glass, Li 13.9 Sr 0.1 A dense body of a proton-introduced Zn(GeO4)4 does not contain oxide ions and is not included in the dense body of the proton-containing basic composite oxide of the present invention. The dense body of the proton-containing basic composite oxide of the present invention is a dense body and therefore has low gas permeability, and further contains an element with high basicity (at least one element selected from B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K) and an element with low basicity (at least one element selected from Si, Ti, Zr, Hf, Lu, Y, and Sc), and is a basic composite oxide containing oxide ions, so that its crystal structure is resistant to change even after proton introduction and is stable. For this reason, the dense body of the proton-containing basic composite oxide of the present invention is superior to a dense body of a proton-containing body of a phosphate-based glass, Li 13.9 Sr 0.1 Compared to dense bodies of proton-introduced Zn(GeO4)4, it is a rigid body that does not deform even at high temperatures of 200°C or higher.

[0028] [Solid electrolyte, fuel cell, hydrogen production cell, hydrogen sensor or ammonia synthesis cell] The proton-containing oxide obtained by the production method of the present invention is expected to exhibit stable proton conductivity in the medium temperature range of 300 to 600°C. Therefore, the proton-containing oxide obtained by the production method of the present invention is expected to be used as a solid electrolyte material, and the solid electrolyte obtained from the proton-containing oxide obtained by the production method of the present invention is expected to be used in fuel cells, hydrogen production cells, hydrogen sensors, or ammonia synthesis cells. [Example]

[0029] The present invention will be described in more detail below based on examples, but the present invention is not limited to these embodiments except as defined in the present invention.

[0030] [Example 1] (Synthesis of basic oxides) It was synthesized by a general firing method as follows. (1) Synthesis of Na2ZrO3 powder First, the raw material oxides, ZrO2 and Na2CO3, which was dried at 300°C for 5 hours prior to ball milling due to its hygroscopicity, were milled together with hexane in a planetary ball mill at 400 rpm for 10 hours. The compounding ratio of each raw material oxide was adjusted to ensure that Na was in 10 mol% excess relative to the target composition, taking into account sublimation during firing. Hydrophobic hexane was also used to suppress reactions between the raw materials and water (moisture in the air). The resulting mixed powder was pelletized using a uniaxial press to a shape with a diameter of 10 mm and a thickness of 3 mm. It was then cold isostatically pressed at 20 MPa and calcined (fired) in a magnesia crucible at 950°C for 12 hours, with the temperature increased at a rate of 5°C / min. To prevent contamination of the aluminum, a magnesia crucible was used instead of a typical alumina crucible. The pellets were covered with a buffer powder of the same composition before calcination to prevent sublimation and unintended reactions. After polishing both sides of the calcined pellets, they were again ground and pelletized using a ball mill in the same manner as above, and then calcined in the same manner as above. Through the above process, dense pellets of Na2ZrO3 (dense bodies with a relative density of 82% or more) with a diameter of approximately 9 mm and a thickness of 1 mm were produced. The pellets were again pulverized in a ball mill in the same manner as above to prepare powder samples.

[0031] (2) Li 6.16 Al 0.28 La3Zr2O 12 (Al-doped LLZ) and Li6La3Zr1Ta1O 12 Synthesis of dense bodies and powders of (Ta-doped LLZ) The Al-doped LLZ and Ta-doped LLZ powders were prepared in the same manner as in the preparation of the NaZrO powder sample described above in (1), except that the raw oxide materials used were LiCO and ZrO, AlO or TaO, and LaO, which was calcined at 700°C for 5 hours prior to ball milling due to its hygroscopicity. The compounding ratio of each raw oxide was adjusted to ensure that Li was in excess of 10 mol% relative to the target composition, taking into account sublimation during calcination. The dense bodies of Al-added LLZ and Ta-added LLZ were obtained in the process of producing the powder, and were dense pellets of Al-added LLZ and Ta-added LLZ with a diameter of approximately 9 mm and a thickness of 1 mm (dense bodies with a relative density of 82% or more).

[0032] (3) Li 6.16 Al 0.28 La3Zr2O 12 Preparation of thin film (dense body) of (Al-added LLZ) The pellets of Al-added LLZ prepared in (2) above were pulverized using a ball mill in the same manner as in (1) above to prepare pulverized powder of Al-added LLZ. 3 g of the ground powder of Al-added LLZ prepared as described above, 0.5 g of polyvinyl butyral (average degree of polymerization: 630), 250 μL of dibutyl phthalate, 100 μL of ethylene glycol, 100 μL of polyoxyethylene (average degree of polymerization: 10) oxyphenyl ether, 2 mL of isopropanol, and 2 mL of toluene were mixed in a mortar to prepare an Al-added LLZ slurry. Using the Al-doped LLZ slurry prepared as described above, a green sheet was prepared using a doctor blade with a 500 μm gap. The sheet was heated to 500°C at a heating rate of 2°C / min, held at 500°C for 3 hours, then heated to 1200°C at a heating rate of 10°C / min, held at 1000°C for 10 hours, and cooled to room temperature at a heating rate of 10°C / min to produce a dense Al-doped LLZ thin film (a dense body with a relative density of 92±5% or more) approximately 90 μm thick. Note that the relative density of the thin film prepared by the doctor blade method is reported as a value with a measurement error of ±5% to take into account the measurement error of the volume measured and calculated using standard digital calipers.

[0033] (Immersion of basic oxide in carboxylic acid melt (ion exchange reaction)) The Al-doped LLZ pellets and the Ta-doped LLZ pellets were immersed in the carboxylic acid melt using a quartz container in an ion exchange apparatus 100 shown in a schematic explanatory diagram in FIG. This vessel (reaction vessel 7) was placed in a mantle heater 11 with a stirrer to control the temperature and remove the by-product lithium carboxylate from the surface of the basic oxide sample. The LLZ pellet (basic oxide 1) was suspended above the inside of the reaction vessel 7 using a silver holder 3 with a silver mesh 3a for a more uniform reaction. A thermocouple was also installed near the LLZ pellet (basic oxide 1) in the carboxylic acid melt 5 for accurate temperature control. To prevent deterioration due to oxidation of the carboxylic acid melt 5 and to allow observation of the sample during the reaction, the entire ion exchange apparatus 100 system was placed in a transparent chamber filled with nitrogen. A carboxylic acid containing more than 100 equivalents of protons (H in the carboxyl group) relative to the Li content of the LLZ pellet (basic oxide 1) was added to the reaction vessel 7. A carboxylic acid melt 5 was prepared using a mantle heater, and the position of the holder 3 was adjusted so that the LLZ pellet (basic oxide 1) was immersed in the carboxylic acid melt 5, and an ion exchange reaction was carried out. During the reaction, a quartz lid 9 having a through-hole for the holder 3 was placed over the carboxylic acid melt 5 to prevent evaporation of the carboxylic acid melt 5. The immersion of the powders of Al-doped LLZ, Ta-doped LLZ, and NaZrO into the carboxylic acid melt was carried out in the same manner as above, except that the powders were lightly uniaxially pressed at approximately 100 kPa to form a green compact with a diameter of 10 mm and a thickness of 1 mm, which was then placed on a mesh 3a. The type of carboxylic acid melt used in the ion exchange reaction, the reaction temperature, and the reaction time are as described in the respective evaluations below.

[0034] (Washing) The LLZ pellets after the ion exchange reaction were washed with cooking oil heated to about 150°C to remove any lithium carboxylate salts potentially remaining on the surface. Specifically, the carboxylic acid melt (containing lithium carboxylate salts) after the reaction in reaction vessel 7 was replaced with cooking oil heated to about 150°C, and the mixture was heated and stirred at 150°C for about 20 minutes, after which the LLZ pellets after the ion exchange reaction were removed from the cooking oil. The LLZ pellets were then ultrasonically washed with acetone to obtain a clean sample. After the ion exchange reaction, the powder compact was washed in the same manner as above to remove any sodium carboxylate potentially remaining on the surface. The proton-containing oxide thus obtained was evaluated as follows.

[0035] [Evaluation 1] Caswell Silverite-like Na2ZrO3 derivatives (Reaction conditions) Basic oxide: Compacted Na2ZrO3 powder, Carboxylic acid melt: Oleic acid (strong pKa 5) melt, Reaction temperature: 300°C, Reaction time: 2 hours Figure 4 shows the results of qualitative and quantitative elemental analysis of Na2ZrO3 by EDS (energy dispersive X-ray spectroscopy) before and after the ion exchange reaction (proton introduction reaction). In Figure 4, the horizontal axis represents energy (unit: keV) and the vertical axis represents the number of counts. From the ratio of the integrated values ​​of the number of counts in the Na Kα line (0.99-1.09 keV) and the Zr Lα line (1.99-2.09 keV), it was found that while the elemental composition ratio of the raw material Na2ZrO3 before proton introduction was approximately Na:Zr = 4:1 (see the lower spectrum in Figure 4), this had changed to approximately Na:Zr = 2:1 in the oxide after proton introduction (see the upper spectrum in Figure 4). As shown in Figure 5, the results of X-ray diffraction before and after the ion exchange reaction showed that the (002) diffraction peak of Caswell silverite-related Na2ZrO3 shifted from 2θ = 16.4° (see the lower spectrum in Figure 5) to 2θ = 18.2° (see the upper spectrum in Figure 5), which, according to Bragg's reflection equation, corresponds to an 11% contraction in the interlayer distance. These results indicate that in the Na2ZrO3 crystal structure before proton introduction shown in Fig. 6(a), Na 0.5 Na present between the ZrO3 block layers 1.5 However, as shown in Figure 6(b), Na 0.75 +H 0.75 This confirms that protons have been replaced (ion-exchanged) with oxygen atoms, resulting in a proton-containing oxide. In Figure 6, dark circles (●) represent oxygen atoms, and light circles (◯) represent sodium atoms. This indicates that protons have been sufficiently introduced into the Na2ZrO3 powder compact (up to 500 μm from the surface).

[0036] [Evaluation 2] Garnet-type Li7La3Zr2O 12 (LLZ) derivatives -1 (1) (Reaction conditions) Basic oxide: compacted powder of Al-added LLZ powder prepared in (2) above, carboxylic acid melt: oleic acid, reaction temperature: 150°C, reaction time: 2 hours As a result of X-ray diffraction analysis before and after the ion exchange reaction, a shift in the X-ray diffraction peak was observed, as shown in Figure 7. This confirmed that, similar to the above-mentioned Caswell silverite-related Na2ZrO3, an oxide was obtained in which the crystal structure of Al-doped LLZ was maintained while changing to a structure in which protons were introduced (ion exchanged). Furthermore, as a result of Raman spectroscopy, as shown in Figure 8, the OH stretching vibration peak, which was hardly observed in the raw material Al-added LLZ before proton introduction (before the ion exchange reaction), increased to approximately 3520 cm after proton introduction (after the ion exchange reaction). -1 was clearly observed nearby. These results suggest that the Al-doped LLZ crystal structure before proton introduction is + Part of H + In other words, protons were sufficiently introduced into the compact of the Al-added LLZ powder up to the center of the compact (500 μm from the surface).

[0037] (2) (Reaction conditions) Basic oxide: dense body of Al-added LLZ or Ta-added LLZ prepared in (2) above (relative density 82% or more, porosity approximately 15%), carboxylic acid melt: adipic acid (pKa 4.4), reaction temperature: 180°C, reaction time: 8 hours The oxides of the Al-doped LLZ and Ta-doped LLZ pellet-shaped sintered bodies (porosity approximately 15%) after the ion exchange reaction were analyzed in the depth direction by Raman spectroscopy, and the relationship between the depth from the pellet surface on the horizontal axis and the OH stretching vibration peak intensity on the vertical axis is plotted in Figure 9. In the Raman spectroscopy analysis of the present invention, analysis was performed every 10 μm in the depth direction from the surface (depth 0 μm), and the deepest depth at which the OH stretching vibration peak was no longer observed was determined to be the maximum depth at which protons were introduced. As shown in Figure 9, it was confirmed that protons were introduced from the surface (0 μm) to a depth of 20 μm in the Ta-added LLZ (indicated by ▼ in Figure 9), and that protons were introduced from the surface (0 μm) to a depth of approximately 250 μm in the Al-added LLZ (indicated by ▲ in Figure 9). Furthermore, as shown in FIG. 10, no cracks or the like were observed under a scanning electron microscope on the fracture surface near the surface of the pellet-shaped sintered body of Al-added LLZ after the ion exchange reaction. These results confirmed that after ion exchange on the surface of the Al-doped LLZ, protons diffused inward, resulting in an oxide with protons introduced all the way to the inside.

[0038] [Evaluation 3] Garnet-type Li7La3Zr2O 12 (LLZ) derivatives -2 (Reaction conditions) Basic oxide: dense body of Al-added LLZ prepared in (2) above (relative density 82% or more, porosity approximately 15%), reaction time: 8 hours The ion exchange reaction was carried out at various temperatures using aqueous acetic acid or various carboxylic acid melts as proton sources. The resulting oxides were analyzed by Raman spectroscopy to measure the surface and depth profiles of the OH stretching vibration peak intensities. The relationship between the pKa (room temperature: 25°C) of the compound used as the proton introduction source and the reaction temperature is plotted in Figure 11, along with the maximum depth at which proton introduction was confirmed in each test. In Figure 11, "ion exchange only on the surface (<10 μm)" means that an OH stretching vibration peak was observed in the surface analysis, but no OH stretching vibration peak was observed at the surface (depth 0 μm) in the depth direction analysis, and "dissolved in acid" means that the basic oxide was dissolved by acid. As shown in Figure 11, when an aqueous solution of acetic acid (pKa 4.7) was used at a reaction temperature of 80°C, ion exchange only occurred within a depth of less than 10 µm from the surface. Furthermore, because it was an aqueous solution, the reaction temperature could not be raised above 100°C, making it difficult to introduce protons further into the interior. Furthermore, when an isoleucine adipic acid melt (pKa 2.3) was used at a reaction temperature of 180°C, when an ethylene-acrylic acid copolymer melt (acrylic acid component 15 wt%, pKa 4.2) was used at a reaction temperature of 250°C or 350°C, or when an adipic acid melt (pKa 4.4) was used at a reaction temperature of 250°C, the basic oxide was dissolved by the acid, and the desired proton-containing oxide could not be obtained. In contrast, when using adipic acid (pKa 4.4) and behenic acid (pKa 4.7), which are carboxylic acid melts with pKa values ​​above 4, ion exchange was achieved to a depth of more than 60 μm by setting the reaction temperature at a level that did not dissolve the basic oxides in the acid. In particular, as shown in Figures 11 and 12, proton introduction was achieved up to a depth of 250 μm with adipic acid at a reaction temperature of 180 °C, and up to a depth of 60 μm with behenic acid at a reaction temperature of 190 °C, and up to a depth of more than 300 μm with a reaction temperature of 250 °C. Furthermore, the presence of La2Zr2O7 as a second phase was confirmed in the reaction using behenic acid at a reaction temperature of 250 °C. Considering these results, when an oleic acid melt (pKa 5.0) was used at a reaction temperature of 200 °C, ion exchange was achieved only to a depth of less than 10 μm from the surface. However, as is clear from the plot in Figure 11, ion exchange can be achieved to a greater depth by increasing the reaction temperature. Thus, it was found that protons can be introduced from the surface to deeper inside by using a weakly acidic carboxylic acid melt with a pKa of 4 or higher at high temperature.

[0039] [Rating 4] Garnet-type Li7La3Zr2O 12 (LLZ) derivatives -3 (Reaction conditions) Basic oxide: thin film of Al-doped LLZ prepared in (3) above (dense body with relative density of 92±5% or more), carboxylic acid melt: behenic acid (pKa 4.7), reaction temperature: 250°C, reaction time: 15 hours The Al-doped LLZ thin film samples before and after the ion exchange reaction were analyzed by ICP-MS (inductively coupled plasma mass spectrometry), X-ray diffraction, SEM (scanning electron microscope), micro-Raman spectroscopy, and AC impedance spectroscopy. [ICP-MS analysis] Each sample was crushed in a mortar before and after the ion exchange reaction (proton introduction), and then 1 mL of aqua regia was added to 1 mg of the crushed powder, which was then hydrolyzed at 105°C for 2 hours to form a solution, which was then introduced into an ICP-MS device for analysis. ICP-MS analysis confirmed that the Li concentration in the sample after the ion exchange reaction was 1 / 10 of the Li concentration in the sample before the ion exchange reaction, i.e., the ion exchange rate in the ion exchange reaction was 90%. As shown in Figure 13, X-ray diffraction revealed a shift in the X-ray diffraction peak, confirming that the ion exchange reaction resulted in an oxide in which the crystal structure of the sample before the ion exchange reaction was maintained while a proton-introduced (ion-exchanged) structure was obtained. SEM observation confirmed that both the samples before and after the ion exchange reaction were dense bodies without cracks, open pores, etc., as shown in Figure 14. Note that in Figure 14, the difference in sample thickness in the SEM images before and after the ion exchange reaction is due to uneven thickness of the sample itself. As shown in Figure 15, the OH stretching vibration peak, which was hardly observed in the sample before proton introduction (before the ion exchange reaction), increased from the surface to the interior of the sample (at relative positions 0 μm to 87.4 μm) after proton introduction (after the ion exchange reaction). -1 It was observed at about 2840-2940 cm -1 A peak thought to be derived from OH stretching vibration was also observed at approximately 3520 cm. -1 The OH stretching vibration peak intensity is plotted on the vertical axis, and the relationship between these is shown in Figure 16. As shown in Figure 16, -1 It was confirmed that the intensity of the OH stretching vibration peak was almost constant on the surface and inside of the sample from one surface (relative position 0 μm) to the other surface (relative position 87.4 μm). These results confirm that after ion exchange on the surface of the Al-doped LLZ thin film, protons diffused inward, resulting in an oxide with an ion exchange rate of 90% and protons uniformly introduced to the inside.

[0040] [AC impedance spectroscopy] Platinum paste was applied to the thin film of Al-added LLZ for current collection, and platinum electrodes were connected. The thin film was then placed in a tube furnace with synthetic air circulating under an AC voltage of 100 mV and an AC frequency of 2 × 10 7 The impedance spectrum was measured at a frequency of 100 Hz or less while increasing the temperature from 150°C to 400°C. The point where the impedance spectrum intersects with the real axis on the Nyquist plot was taken as the real resistance value, which was converted to electrical conductivity using the current collection area and sample thickness. Figure 17 shows a plot of the relationship between the measurement temperature on the horizontal axis and the common logarithm of the calculated electrical conductivity on the vertical axis. Note that the main electrical conduction in the Al-added LLZ before ion exchange is lithium ion conduction, while the main electrical conduction in the Al-added LLZ after ion exchange is proton conduction. As shown in Figure 17, AC impedance spectroscopy revealed that the sample after the ion exchange reaction had a conductivity of approximately 1 × 10 at a temperature range of 325°C. -4 S cm -1 It was confirmed that the present invention made it possible to easily obtain a proton-containing oxide exhibiting high proton conductivity.

[0041] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims. [Explanation of symbols]

[0042] 1 Basic oxides 3 Holding device 3a mesh 5. Carboxylic acid melt 7. Reaction vessel 9 Lid 11 Mantle heater 100 Ion exchange reactor

Claims

1. A method for producing a proton-containing oxide, comprising reacting a basic oxide with a molten carboxylic acid having a pKa of 4 or more to introduce protons into the basic oxide, thereby obtaining a proton-containing oxide.

2. 2. The method for producing a proton-containing oxide according to claim 1, wherein the basic oxide is a basic composite oxide containing at least one element selected from the group consisting of B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from the group consisting of Si, Ti, Zr, Hf, Lu, Y, and Sc.

3. 3. The method for producing a proton-containing oxide according to claim 1, wherein the basic oxide is a garnet type or a Caswell silverite-related type.

4. The method for producing a proton-containing oxide according to claim 1 or 2, wherein the carboxylic acid melt is a fatty acid melt.

5. 3. The method for producing a proton-containing oxide according to claim 1, wherein the reaction temperature is 150 to 350°C.

6. 3. The method for producing a proton-containing oxide according to claim 1, wherein the reaction time is 1 to 20 hours.

7. The method for producing a proton-containing oxide according to claim 1 or 2, further comprising washing after the reaction.

8. The method for producing a proton-containing oxide according to claim 7 , wherein the cleaning is carried out using oil or fat.

9. A method for producing a fuel cell, a hydrogen producing cell, a hydrogen sensor or an ammonia synthesis cell, comprising incorporating the proton-containing oxide obtained by the method for producing a proton-containing oxide according to claim 1 or 2 as a solid electrolyte.

10. A dense body of a proton-containing basic composite oxide containing at least one element selected from the group consisting of B, Mg, Al, Li, Ca, S, La, Sr, P, Ba, Na, and K, and at least one element selected from the group consisting of Si, Ti, Zr, Hf, Lu, Y, and Sc, and containing oxide ions.

11. A solid electrolyte comprising a dense body of the proton-containing basic composite oxide according to claim 10.

12. A fuel cell, a hydrogen production cell, a hydrogen sensor or an ammonia synthesis cell, comprising the solid electrolyte according to claim 11.

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