Metal composite oxide, composite body, catalyst ink, oxygen generation catalyst and electrode, and method for producing metal composite oxide and composite body

Abstract

A metal composite oxide according to the present invention contains potassium, iridium, ruthenium, and oxygen. The metal composite oxide contains a composite oxide of iridium and ruthenium as an essential component, and contains KRu4O8 having a hollandite structure.

Description

Metal composite oxide, composite, catalytic ink, oxygen generating catalyst and electrode, and method for producing metal composite oxide and composite

[0001] The present invention relates to a metal composite oxide, a composite, a catalytic ink, an oxygen generating catalyst and an electrode, and a method for producing the metal composite oxide and the composite. This application claims priority based on Japanese Patent Application No. 2024-102078, filed on June 25, 2024, the contents of which are incorporated herein by reference.

[0002] Water electrolysis technology is attracting attention as a technology that can produce hydrogen from water using renewable energy electricity, with the aim of solving environmental and energy resource problems.

[0003] Patent Document 1 describes a catalyst composition containing tin oxide particles and an iridium-ruthenium oxide layer covering the tin oxide particles as an efficient electrode catalyst for the oxygen evolution reaction by water electrolysis.

[0004] Japanese Patent Application Laid-Open No. 2022-169613

[0005] However, there is still room for further study on providing a novel catalyst material that has sufficient catalytic activity while reducing the content of precious metals such as iridium contained in the catalyst composition.The present invention aims to provide a metal composite oxide, composite, catalytic ink, oxygen generating catalyst and electrode, as well as a method for producing a metal composite oxide and composite, which can be used as a catalyst even when the content of precious metals is low.

[0006] The present invention includes the following aspects: [1] A metal composite oxide containing potassium, iridium, ruthenium, and oxygen, comprising a composite oxide of iridium and ruthenium as an essential component, and having a hollandite structure, ４ O ８ [2] The metal composite oxide according to [1], wherein the iridium content is 50 mass% or less and the potassium content is 1 mass% or more and 10 mass% or less. [3] The metal composite oxide according to [1] or [2], wherein the primary particles have a particle size of 150 nm or less. [4] A median diameter D determined by dynamic light scattering ５０[5] The metal composite oxide according to any one of [1] to [3], wherein the specific surface area measured by the BET method is 10 m ２ / g or more. [6] The metal composite oxide according to any one of [1] to [5], wherein the ratio of the total content of the iridium and the ruthenium to the total mass of the metal composite oxide, as determined by XRF analysis, is 50 mass % or more and 80 mass % or less. [7] A composite comprising the metal composite oxide according to any one of [1] to [6] and a support, wherein the metal composite oxide is supported on the support, and the support is an inorganic material. [8] The composite is a composite comprising the support made of tin oxide (SnO ２) and carbon. [9] An oxygen generating catalyst comprising the metal composite oxide according to any one of [1] to [6].

[10] The oxygen generating catalyst according to [9], further containing a conductive material.

[11] An oxygen generating catalyst comprising the composite according to [7] or [8].

[12] The oxygen generating catalyst according to

[11] , further containing a conductive material.

[13] A catalyst ink comprising the metal composite oxide according to any one of [1] to [6] and a medium.

[14] The catalyst ink according to

[13] , further containing a conductive material.

[15] The catalyst ink according to

[13] or

[14] , further containing a polymer electrolyte.

[16] A catalyst ink comprising the composite according to [7] or [8] and a medium.

[17] The catalyst ink according to

[16] , further containing a conductive material.

[18] The catalyst ink according to

[16] or

[17] , further containing a polymer electrolyte.

[19] An electrode having a surface comprising the metal composite oxide according to any one of [1] to [6].

[20] An electrode having, on its surface, the composite according to [7] or [8].

[21] A method for producing a metal composite oxide, comprising the steps of: preparing a mixed solution containing an iridium compound, a ruthenium compound, and an aqueous medium; adjusting the pH of the mixed solution obtained in the step to between 8 and 13 using potassium hydroxide to produce a precipitate; isolating the precipitate from the aqueous medium; and calcining the precipitate obtained in the step.

[22] A method for producing a composite, comprising the steps of: dispersing an inorganic substance in an aqueous medium to prepare a first aqueous dispersion containing the inorganic substance; dissolving an iridium compound and a ruthenium compound in the first aqueous dispersion to prepare a second aqueous dispersion; adjusting the pH of the second aqueous dispersion obtained in the step to between 8 and 13 using potassium hydroxide to produce a precipitate; isolating the precipitate from the aqueous medium; and calcining the precipitate obtained in the step.

[0007] According to the present invention, it is possible to provide a metal composite oxide, a composite, a catalytic ink, an oxygen generating catalyst and an electrode, and a method for producing a metal composite oxide and a composite, which can be used as a catalyst even when the iridium content is low.

[0008] 1 shows XRD spectra of the metal composite oxides prepared in Examples 1 to 3. 2 shows a transmission electron microscope image of the metal composite oxide obtained in Example 3. 3 shows an SEM-EDS image of the metal composite oxide obtained in Example 3, in which mapping of iridium, ruthenium, and potassium elements is superimposed on the SEM image. 4 shows an SEM-EDS image of the metal composite oxide obtained in Example 3, in which the upper left is an SEM image, the upper right is a potassium element mapping image, the lower left is a ruthenium element mapping image, and the lower right is an iridium element mapping image.

[0009] (Metal composite oxide) A metal composite oxide according to one embodiment of the present invention is a metal composite oxide containing potassium, iridium, ruthenium, and oxygen. The metal composite oxide according to this embodiment contains a composite oxide of iridium and ruthenium as an essential component and is a KRu oxide having a hollandite structure. ４ O ８ The metal composite oxide of this embodiment may further contain iridium oxide or ruthenium oxide.

[0010] [Composition Ratio of Metal Composite Oxide] In the metal composite oxide of this embodiment, the composition ratio (mass ratio) of potassium (K), iridium (Ir), ruthenium (Ru), and oxygen (O) elements, relative to a total of 100 mass% of potassium (K), iridium (Ir), ruthenium (Ru), and oxygen (O), is preferably within the following ranges, for example. Potassium (K): From the viewpoint of reducing precious metals, preferably 1 mass% or more, and more preferably 3 mass% or more. From the viewpoint of improving activity, preferably 10 mass% or less, and more preferably 7 mass% or less. Iridium (Ir): From the viewpoint of maintaining the crystal structure, preferably 1 mass% or more, and more preferably 5 mass% or more. From the viewpoint of reducing costs, preferably 50 mass% or less, and more preferably 30 mass% or less. Ruthenium (Ru): From the viewpoint of maintaining the crystal structure, preferably 10 mass% or more, and more preferably 30 mass% or more. From the viewpoint of reducing costs, preferably 60 mass% or less, and more preferably 40 mass% or less. Oxygen (O): from the viewpoint of maintaining the crystal structure, preferably 10% by mass or more, more preferably 20% by mass or more. From the viewpoint of improving performance, preferably 50% by mass or less, more preferably 40% by mass or less.

[0011] The composition ratio of each metal component in the metal composite oxide of this embodiment can be measured, for example, by X-ray fluorescence spectroscopy (XRF), inductively coupled plasma (ICP) atomic emission spectroscopy, or the like. Furthermore, when the metal composite oxide of this embodiment contains metals other than potassium (K), iridium (Ir), and ruthenium (Ru), these other metals may include manganese (Mn), chromium (Cr), or the like. The total content of metals other than potassium, iridium, and ruthenium may be, for example, 0.1% by mass or more, 1% by mass or more, or 10% by mass or less, or 5% by mass or less, relative to 100% by mass of the total of potassium (K), iridium (Ir), and ruthenium (Ru). These metals other than potassium, iridium, and ruthenium may be contained as a single oxide or as a composite oxide with one or more of potassium, iridium, and ruthenium.

[0012] The metal composite oxide of this embodiment can be represented by, for example, the following general formula (A):ｘ Ru ｙ Ir ｚ O ｗ　　 (A) In formula (A), x, y, z, and w are the atomic ratio (molar ratio) of each element in the metal composite oxide. For example, when x + y + z + w = ​​1, the following ranges are preferable. x: May be 0.01 or more, or 0.02 or more. May be 0.10 or less, or 0.07 or less. y: May be 0.10 or more, or 0.13 or more. May be 0.25 or less, or 0.20 or less. z: May be 0.01 or more, or 0.02 or more. May be 0.20 or less, or 0.13 or less. w: May be 0.50 or more, or 0.60 or more. May be 0.90 or less, or 0.85 or less.

[0013] [Metal Composite Oxide Particles] The metal composite oxide of this embodiment may be a particulate structure (metal composite oxide particles). In the metal composite oxide of this embodiment, the particle diameter of the primary particles is preferably 150 nm or less, more preferably 130 nm or less, and even more preferably 110 nm or less. While the lower limit is not particularly limited, it is preferably 1 nm or more. The method for measuring the particle diameter of the primary particles will be described in detail below. <Method for Measuring the Particle Diameter of Primary Particles> The particle diameter of the primary particles is the average particle diameter of the primary particles. The metal composite oxide powder is photographed with a scanning electron microscope (SEM) or a transmission electron microscope (TEM), and the major axis (the Feret diameter of the longest observed part) and minor axis (the shorter Feret diameter perpendicular to the Feret diameter of the longest part) of the smallest unit particle constituting the aggregate in the two-dimensional image (i.e., the primary particle) are measured, and the average value is taken as the particle diameter of the primary particles. The average value was the average particle diameter of 50 randomly selected primary particles.

[0014] In the metal composite oxide of this embodiment, a plurality of primary particles may aggregate to form secondary particles. ５０ The median diameter D of the metal composite oxide of this embodiment obtained by dynamic light scattering is usually observed together with aggregates such as secondary particles. ５０The median diameter D is preferably 800 nm or less, more preferably 500 nm or less, and even more preferably 300 nm or less. ５０ A metal composite oxide having a t value equal to or less than the upper limit can more effectively exhibit catalytic efficiency.

[0015] The median diameter D of the metal composite oxide of this embodiment determined by dynamic light scattering ５０ For example, the thickness may be 50 nm or more, 100 nm or more, 150 nm or more, or 200 nm or more.

[0016] The median diameter D of the metal composite oxide of this embodiment determined by dynamic light scattering ５０ An example of the range of the above numerical value may be 50 nm or more and 800 nm or less, 50 nm or more and 500 nm or less, 100 nm or more and 500 nm or less, or 150 nm or more and 300 nm or less.

[0017] The median diameter D of the metal composite oxide of this embodiment calculated by dynamic light scattering ５０ can be determined as the particle size at which the volume cumulative percentage is 50% in a particle size distribution measured in a wet state using a dynamic light scattering particle size distribution analyzer (e.g., Nanotrac Wave II, manufactured by Microtrac-Bell) with acetone as a medium.

[0018] The specific surface area of ​​the metal composite oxide of this embodiment measured by the BET method is 10 m ２ / g or more, and ２ / g or more is more preferable, and 30m ２ / g or more is more preferable, and 50m ２ A metal composite oxide having a specific surface area equal to or greater than the above lower limit has a large specific surface area, which increases catalytically active sites and facilitates mass transfer in the reaction system, thereby enabling the catalytic efficiency to be improved more effectively.

[0019] The specific surface area of ​​the metal composite oxide of this embodiment measured by the BET method is, for example, 200 m ２ / g or less, and ２ / g or less, ２ / g or less, ２ / g or less.

[0020] An example of the range of the specific surface area of ​​the metal composite oxide of this embodiment measured by the BET method is 10 m ２ / g or more 200m ２ / g or less, ２ / g or more 100m ２ / g or less, ２ / g or more 100m ２ / g or less, ２ / g or more 100m ２ / g or less, ２ / g or more 80m ２ / g or less.

[0021] The specific surface area is measured using a specific surface area meter (for example, BELSORP-mini manufactured by Microtrac-Bell Co., Ltd.), and the surface area per 1 g of sample measured from the amount of nitrogen gas adsorbed by the BET method (Brunauer-Emmett-Teller method) is defined as the specific surface area (m ２ / g).

[0022] [Structure of Metal Composite Oxide] The metal composite oxide of this embodiment contains a composite oxide of iridium and ruthenium as an essential component, and is a KRu composite oxide having a hollandite structure. ４ O ８ The metal composite oxide of this embodiment may further contain iridium oxide or ruthenium oxide. The metal composite oxide of this embodiment can be represented by the following general formula (1): KRu ４ O ８ / IrO ２ / RuO ２　　　 (1) In the above formula (1), "IrO ２ / RuO ２" means that the metal composite oxide essentially contains a composite oxide of iridium and ruthenium, and may additionally contain an iridium oxide or a ruthenium oxide. That is, the metal composite oxide of this embodiment is a KRu oxide having a hollandite structure. ４ O ８ The presence of the iridium and ruthenium composite oxide can be confirmed, for example, by mapping in elemental analysis using energy dispersive X-ray spectroscopy (EDS). ４ O ８ The form of the composite oxide of iridium and ruthenium is not particularly limited, and examples thereof include a randomly dispersed structure, a uniformly dispersed structure, a core-shell structure, a sea-island structure, or a mixed structure thereof.

[0023] [K.R.U. ４ O ８ KRu contained in the metal composite oxide of this embodiment ４ O ８ essentially contains a portion having a hollandite structure. The structure can be evaluated by powder X-ray diffraction (XRD). KRu having a hollandite structure ４ O ８ In the profile obtained by XRD using Cu-Kα radiation as the X-ray source, KRu has peaks near 2θ=12.6°, 2θ=17.8°, and 2θ=35.1°. ４ O ８ The crystallite size of KRu in the metal composite oxide of this embodiment is preferably 20 nm to 30 nm. ４ O ８ The crystallite size can be measured, for example, by calculating the average crystallite size of particles from the half-width of the peak appearing at 2θ=14.4°±0.5° using the Scherrer equation.

[0024] [IrO ２ / RuO ２In the composite oxide of iridium and ruthenium contained in the metal composite oxide of this embodiment, the oxidation state of iridium and / or ruthenium is preferably +IV. The composite oxide of iridium and ruthenium may contain iridium and / or ruthenium in an oxidation state of +III. The structure of the composite oxide of iridium and ruthenium contained in the metal composite oxide of this embodiment is not particularly limited. The composite oxide of iridium and ruthenium has peaks near 2θ = 28°, 2θ = 35°, 2θ = 40°, 2θ = 57.9°, and 2θ = 59.4° in a profile obtained by XRD using Cu-Kα radiation as an X-ray source. The crystallite size of the composite oxide of iridium and ruthenium in the metal composite oxide of this embodiment is preferably 20 nm to 30 nm. Furthermore, the crystallite size of the composite oxide of iridium and ruthenium in the metal composite oxide of this embodiment can be measured, for example, by a method in which the average crystallite size of particles is calculated using the Scherrer equation from the half-width of the peak appearing at 2θ = 39.4 ± 0.5°.

[0025] [Method for Producing Metal Composite Oxide] The method for producing a metal composite oxide of this embodiment includes the steps of: preparing a mixed solution containing an iridium compound, a ruthenium compound, and an aqueous medium; adjusting the pH of the mixed solution obtained in the above step to a range of 8 to 13 using potassium hydroxide to produce a precipitate; isolating the precipitate from the aqueous medium; and calcining the precipitate obtained in the above step.

[0026] According to the method for producing a metal composite oxide of this embodiment, it is possible to easily produce the above-described metal composite oxide of this embodiment.

[0027] When a potassium salt such as potassium carbonate is used instead of the potassium hydroxide, KRu ４ O ８ cannot be generated.

[0028] The iridium compounds include, for example, iridium salts and iridium-containing acids. The oxidation state of iridium is +III or +IV. The salts of iridium include, for example, halide salts, chloro complexes, nitrates, or acetates. The iridium-containing acids include, for example, H ２ IrCl ６ Examples of the ruthenium compound include ruthenium salts. The oxidation state of ruthenium is +III or +IV. Examples of ruthenium salts include halide salts, chloro complexes, nitrates, and acetates. The total concentration of the iridium compound and ruthenium compound in the mixed solution is preferably in the range of 2 mmol / L to 50 mmol / L, and more preferably 5 mmol / L to 30 mmol / L, relative to the aqueous medium. The molar ratio of the amounts of the iridium compound and ruthenium compound charged (iridium compound:ruthenium compound) in terms of metals is preferably 1:1 to 1:10, and more preferably 1:1.5 to 1:8.

[0029] The aqueous medium may be any medium capable of dissolving the iridium compound and ruthenium compound, and may include water, aqueous media containing organic media, etc. Examples of organic media include alcohols such as methanol, benzene, dichloromethane, acetone, and chloroform.

[0030] The temperature of the mixed solution may be any temperature that allows the iridium compound and the ruthenium compound to be dissolved, and may be, for example, 10 to 100°C, or 20 to 80°C.

[0031] The pH of the mixed solution is ４ O ８ In order to precipitate and deposit KRu, the pH is adjusted to 8 or more and 13 or less. ４ O ８ From the viewpoint of ensuring a minimum production amount of the compound oxide of iridium and ruthenium, the pH is preferably 9 or higher. Furthermore, from the viewpoint of ensuring a production amount of the compound oxide of iridium and ruthenium, the pH must be 13 or lower.

[0032] After forming a precipitate from the mixture, it is subjected to treatments such as centrifugation and drying. Furthermore, by calcining in air at a calcination temperature of 200 to 800°C, preferably 300 to 600°C, for 1 to 10 hours, preferably 1 to 5 hours, the metal composite oxide of this embodiment can be easily obtained. When obtaining a metal composite oxide containing another metal element, a method can be used in which a compound containing that metal element is dissolved in an aqueous medium in combination with an iridium compound and a ruthenium compound, and steps similar to those described above are carried out.

[0033] (Composite) A composite according to one embodiment of the present invention includes the above-described metal composite oxide and a support. The metal composite oxide is supported on the support. The support is an inorganic material. Examples of the inorganic material include tin oxide (SnO ２ ), transition metal oxides, carbon, etc. The carrier in the composite of this embodiment is tin oxide (SnO ２ The carrier content in the composite of the present embodiment is preferably 10% by mass to 90% by mass.

[0034] [Tin oxide (SnO ２ ) ] Examples of the tin oxide include undoped tin oxide particles and tin oxide particles doped with a metal dopant. The undoped tin oxide particles and the tin oxide particles doped with a metal dopant may be, for example, the undoped tin oxide particles and the tin oxide particles doped with a metal dopant described in Patent Document 1. The tin oxide is preferably undoped tin oxide particles, more preferably titanium-doped tin oxide particles, and even more preferably niobium-doped tin oxide particles.

[0035] [Carbon] Examples of the carbon (carbon material) include graphite, carbon nanotubes, carbon fibers, carbon microbeads, and combinations thereof. Examples of the shape of the carbon include mesh, foam, porous, and combinations thereof. The carbon (carbon material) is preferably carbon nanotubes, more preferably carbon microbeads, and even more preferably graphite.

[0036] [Transition Metal Oxide] Examples of the transition metal oxide include titanium oxide (e.g., TiO ２ ), zirconium oxide (e.g., ZrO ２ ), niobium oxide (e.g., Nb ２ O ５ ), tantalum oxide (e.g., Ta ２ O ５ ) or cerium oxide. The transition metal oxide may be in the form of a powder, particle, or the like. The transition metal oxide may be doped with a suitable element. The transition metal oxide is preferably tantalum oxide, more preferably niobium oxide, and even more preferably titanium oxide.

[0037] (Method for Producing Composite) The method for producing the composite of this embodiment may include, for example, the steps of dispersing the inorganic material as the carrier in an aqueous medium to prepare a dispersion (first aqueous dispersion); adding and dissolving an iridium compound and a ruthenium compound to the dispersion obtained above to prepare a mixed dispersion (second aqueous dispersion); adjusting the pH of the mixed dispersion obtained in the step above to 8 or more and 13 or less using potassium hydroxide to produce a precipitate; isolating the precipitate from the aqueous medium; and calcining the precipitate obtained in the step above (calcination step). In addition, when obtaining a metal composite oxide containing another metal element, a method can be used in which a compound containing the metal element is added and dissolved in combination with an iridium compound and a ruthenium compound to the dispersion (first aqueous dispersion) and steps similar to those described above are performed. In the step of preparing the mixed dispersion (second aqueous dispersion), it is preferable to adjust the proportion of the carrier in the resulting precipitate to be in the range of 10% by mass to 90% by mass. The total concentration of the iridium compound and the ruthenium compound in the mixed dispersion (second aqueous dispersion) is preferably in the range of 2 mmol / L to 50 mmol / L, and particularly preferably 5 mmol / L to 30 mmol / L. When other metal elements are used in combination, it is also preferable to adjust the total concentration thereof to be in the above range.

[0038] The method for producing the composite of this embodiment may include, for example, the steps of dispersing the inorganic material as the carrier in an aqueous medium to prepare a dispersion (first aqueous dispersion); mixing an iridium compound, a ruthenium compound, and an aqueous medium to prepare an aqueous solution; mixing the dispersion and the aqueous solution to prepare a mixed solution (second aqueous dispersion); adjusting the pH of the mixed solution (second aqueous dispersion) obtained in the step (second aqueous dispersion) to 8 or more and 13 or less using potassium hydroxide to produce a precipitate; isolating the precipitate from the aqueous medium; and calcining the precipitate obtained in the step (calcination step). In addition, when obtaining a metal composite oxide containing another metal element, a method can be used in which a compound containing the metal element is dissolved in the aqueous medium in combination with an iridium compound and a ruthenium compound, and the same steps as those described above are performed. The ratios and concentrations of the iridium compound and ruthenium compound used in the step of preparing the aqueous solution are the same as those described above. In addition, when preparing the second aqueous dispersion, it is preferable that the proportion of the carrier in the resulting precipitate be 10 to 90% by mass.

[0039] The order in which the dispersion and the aqueous solution are mixed is not particularly limited. The procedure of adding the aqueous solution to the dispersion is preferred because it is easy to control the mixing speed and adjust the temperature. The conditions for the calcination step are the same as those for obtaining the metal composite oxide.

[0040] (Oxygen generating catalyst of first embodiment) The metal composite oxide of this embodiment can be suitably used as a catalyst in the oxygen generating reaction (OER) (sometimes referred to as "oxygen generating catalyst" in this specification). The metal composite oxide of this embodiment can also be used as an oxygen generating catalyst when used in combination with a conductive material. By using the metal composite oxide of this embodiment in combination with a conductive material, the catalytic activity of the oxygen generating catalyst in the oxygen generating reaction (OER) becomes higher.

[0041] The conductive material may be a known material. Examples of the conductive material include carbon and metals, which have high conductivity. The oxygen generating catalyst may contain only one type of conductive material, or two or more types of conductive materials.

[0042] Examples of the conductive material include carbon black such as acetylene black, Cabot carbon black, and Ketjen black, graphite, carbon fiber, and metal powder.

[0043] Examples of the metal that is the conductive material include gold, silver, copper, aluminum, rhodium, molybdenum, tungsten, iron, nickel, cobalt, iridium, etc. The metal that serves as the conductive material differs from the iridium in the metal composite oxide of this embodiment, and in the oxygen generating catalyst, it is simply mixed with the metal composite oxide of this embodiment.

[0044] The metal contained as the conductive material in the oxygen generating catalyst of the first embodiment may be only one type, or two or more types.

[0045] The oxygen generating catalyst of the first embodiment may contain, for example, no metal but carbon as the conductive material, may contain no carbon but metal, or may contain both metal and carbon.

[0046] In the oxygen generating catalyst of the first embodiment, the content of the conductive material is preferably 0.1 to 100 parts by mass, and more preferably 0.5 to 50 parts by mass, relative to 100 parts by mass of the content of the metal composite oxide of the present embodiment.

[0047] In the oxygen generating catalyst of the first embodiment, the ratio of the total content of the metal composite oxide of this embodiment and the conductive material to the total mass (100 mass%) of the oxygen generating catalyst is preferably 80 mass% or more, more preferably 90 mass% or more, and even more preferably 95 mass% or more. For example, it may be either 97 mass% or more or 99 mass% or more, or it may be 100 mass%. When this ratio is equal to or greater than the lower limit, the catalytic activity of the oxygen generating catalyst in the oxygen generating reaction (OER) becomes higher. The ratio may be 100 mass% or less.

[0048] (Oxygen generating catalyst of second embodiment) The composite of this embodiment can be suitably used as a catalyst in the oxygen generating reaction (OER) (sometimes referred to as "oxygen generating catalyst" in this specification). The composite of this embodiment can also be used as an oxygen generating catalyst when used in combination with a conductive material. When the composite of this embodiment is used in combination with a conductive material, the catalytic activity of the oxygen generating catalyst in the oxygen generating reaction (OER) becomes higher.

[0049] The conductive material may be the conductive material of the oxygen generating catalyst of the first embodiment.

[0050] In the oxygen generating catalyst of the second embodiment, the content of the conductive material is preferably 0.1 to 100 parts by mass, and more preferably 0.5 to 50 parts by mass, relative to 100 parts by mass of the content of the composite of this embodiment.

[0051] In the oxygen generating catalyst of the second embodiment, the ratio of the total content of the composite of this embodiment and the conductive material to the total mass (100 mass%) of the oxygen generating catalyst is preferably 80 mass% or more, more preferably 90 mass% or more, and even more preferably 95 mass% or more. For example, it may be either 97 mass% or more or 99 mass% or more, or it may be 100 mass%. When this ratio is equal to or greater than the lower limit, the catalytic activity of the oxygen generating catalyst in the oxygen generating reaction (OER) becomes higher. The ratio may be 100 mass% or less.

[0052] (Catalyst Ink) A catalyst ink according to one embodiment of the present invention contains the metal composite oxide according to the present embodiment or the composite according to the present embodiment, and a medium. The catalyst ink according to the present embodiment may contain the metal composite oxide according to the present embodiment or the composite according to the present embodiment, a polymer electrolyte, and a medium.

[0053] The catalyst ink of this embodiment may further contain a conductive material. The type and content of the conductive material may be those exemplified above.

[0054] The catalyst ink can be applied to, for example, a substrate for a working electrode to form a catalyst layer.

[0055] The polymer electrolyte may be one generally used in forming a catalyst layer, such as a perfluorocarbon polymer having a sulfonic acid group (e.g., Nafion (registered trademark)), a hydrocarbon polymer compound having a sulfonic acid group, a polymer compound doped with an inorganic acid such as phosphoric acid, an organic / inorganic hybrid polymer partially substituted with a proton-conductive functional group, or a proton conductor obtained by impregnating a polymer matrix with a phosphoric acid solution or a sulfuric acid solution.

[0056] The medium may be any medium that can disperse the metal composite oxide of this embodiment or the composite of this embodiment and that can be applied to a working electrode substrate or the like to form a catalyst layer. The medium preferably contains an alcohol such as 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol, 2-pentanol, 1-hexanol, 2-hexanol, 1-heptanol, or 2-heptanol.

[0057] (Electrode) An electrode according to one embodiment of the present invention comprises the metal composite oxide or composite on its surface. For example, the catalyst ink in the form of the dispersion obtained above is applied to the surface of an electrode, the surface liquid is allowed to dry naturally, and then the electrode is transferred to a vacuum dryer and dried at 30 to 90°C for at least 2 hours, thereby producing an electrode comprising the metal composite oxide or composite on its surface. The oxygen generating activity of the produced electrode can be evaluated using a three-electrode method. The evaluation conditions are, for example, an LSV method, with a scan voltage range of -0.3 V to 0 V and a scan speed of 5 mV / s.

[0058] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0059] (Evaluation) [Measurement of composition ratio by XRF] [XRF analysis] Using a Primus IV X-ray fluorescence analyzer (manufactured by Rigaku Corporation), approximately 20 to 30 mg of the sample was placed on filter paper and covered with a PP film for composition analysis. The amounts of iridium, ruthenium, potassium, and oxygen determined from the XRF analysis results were calculated as the content of each element relative to 100% by mass of the sample particles.

[0060] [Measurement of primary particle diameter] The particle diameter of the primary particles of the sample powder was measured using the above-described <Method for measuring primary particle diameter>. A transmission electron microscope, JEM-1400 manufactured by JEOL Ltd., was used.

[0061] [EDS Mapping Measurement] In order to evaluate the composition distribution of the sample powder obtained in the examples, EDS (energy dispersive X-ray spectroscopy) mapping measurement was carried out. Apparatus used: JCM-7000 tabletop scanning electron microscope equipped with EDS function, manufactured by JEOL Ltd.

[0062] [Median diameter D ５０ Measurement of the median diameter D, which is the particle diameter at which the volume cumulative percentage is 50%, is obtained by adding 0.1 g of sample powder to 20 cc of acetone, subjecting the powder to ultrasonic treatment in an ice bath for 4 hours, and then adjusting the concentration with acetone appropriately to a measurable range using a dynamic light scattering particle size distribution analyzer (Microtrac-Bell, Nanotrac Wave II) to obtain a measurement sample. ５０ The median diameter D ５０ For those having a particle diameter of more than 10 μm (Comparative Example 2), a solution was prepared in the same manner, and the particle diameter distribution in the particle diameter range of 0.015 μm to 500 μm was measured using a laser diffraction particle size distribution measuring device (Shimadzu Corporation, SALD-7000). ５０ was calculated.

[0063] [Measurement of crystallite size] A SmartLab 9kW (manufactured by Rigaku Corporation) was used as an X-ray diffractometer, a scintillation counter detector was used as a detector, and PDXL2 was used as analysis software to carry out the measurement. The measurement method was the 2θ / θ method, and the average crystallite size of the particles was calculated using the Scherrer formula from the half-width of the peak appearing at 2θ = 14.4 ° ± 0.5 °. The average crystallite size of the particles was also calculated using the Scherrer formula from the half-width of the peak appearing at 2θ = 39.4 ± 0.5 °. The measurement conditions were: scan speed (2θ) 2.0 ° / min, scan range (2θ) 10 to 70 °, step (2θ) 0.02 °, and no standard instrument width.

[0064] [Measurement of BET Specific Surface Area] Measurement was performed using a specific surface area meter (Microtrac-Bell, BELSORP-mini), and the surface area per 1 g of sample measured from the amount of nitrogen gas adsorbed by the BET method was calculated as the specific surface area.

[0065] [Measurement of Overvoltage (Evaluation of Oxygen Evolution Activity)] The overvoltage of the prepared electrode was measured using a three-electrode method. The measurement conditions were an LSV method, a scan voltage range of −0.3 V to 0 V, and a scan speed of 5 mV / s. The current density was 10 mA / cm. ２ The overvoltage (mV) at

[0066] [Crystal Structure Analysis of Metal Composite Oxide: XRD Method] A sample of the metal composite oxide obtained in each example was filled into a measurement sample holder with a depth of 0.5 mm, and the holder was set in an X-ray diffraction (XRD) device (Ultima IV manufactured by Rigaku Corporation, an optical system using a parallel beam method + scintillation counter detector, and a rotary stage), and measurement was performed under the conditions of Cu / Kα radiation, 40 kV / 40 mA, a scan speed (2θ) of 2° / min, a step (2θ) of 0.02°, and a scan range (2θ) of 10° to 70°.

[0067] Example 1 RuCl ３ ・3H ２ O (Kanto Chemical Co., Ltd. reagent) 70 mg, and IrCl ３ ・3H ２A solution was obtained by dissolving 60 mg of KOH (Kanto Chemical Co., Ltd. reagent) in 30 mL of pure water. While stirring the resulting solution, an aqueous solution of KOH (Kanto Chemical Co., Ltd. reagent) (concentration: 1 mol / L) was gradually added, and the pH of the solution was adjusted to 12. After leaving it for about a day, the precipitate in the solution was recovered using a centrifuge, washed three times with water using ultrasonic waves, and then vacuum dried at 80°C to obtain a solid precipitate. The precipitate was crushed in a mortar, placed in a crucible, and fired at 400°C for 1 hour under a nitrogen atmosphere to obtain a powder.

[0068] The composition of the obtained powder and the content ratio of each element converted to each element were measured by the above-mentioned X-ray fluorescence analysis (XRF). In addition, using the content ratio, the atomic ratio (molar ratio) of each element was calculated relative to x + y + z + w = ​​1 in the case of the above-mentioned general formula (A). The results are shown in Table 1. The particle size of the primary particles of the obtained powder and the median diameter D ５０ , K.R.U. ４ O ８ The crystallite size of the composite oxide of iridium and ruthenium ("RuO" in Table 2) ２ / IrO ２ The crystallite size and specific surface area were measured by the above-mentioned measuring method, and the results are shown in Table 2.

[0069] The obtained powder was subjected to XRD measurement by the above-mentioned method. It was found to have peaks at around 2θ=12.6°, 2θ=17.8°, and 2θ=35.1°, which indicated that the powder had a hollandite structure. ４ O ８ Furthermore, EDS mapping confirmed the presence of a composite oxide of iridium and ruthenium.

[0070] 4 mg of the obtained metal composite oxide, 2 mg of Ketjen Black EC300J (Lion Specialty Chemicals Co., Ltd.), and 40 μL of 5% Nafion dispersion (Fujifilm Wako Pure Chemical Industries, Ltd. Nafion dispersion solution) were added to 1 mL of ethanol and treated with an ultrasonic disperser for 1 hour to prepare a catalyst ink. 20 μL of the obtained catalyst ink was applied to the surface of an electrode, and the liquid on the surface was allowed to dry naturally. After that, the electrode was transferred to a vacuum dryer and dried at 60°C for 6 hours to obtain an electrode with a metal composite oxide on its surface. The current density of the produced electrode was 10 mA / cm. ２ The overvoltage (mV) at 231 mV was measured and found to be 231 mV. The results are shown in Table 2.

[0071] (Examples 2 and 3) In Examples 2 and 3, RuCl ３ ・3H ２ Powders were obtained in the same manner as in Example 1, except that the amounts of HCl, HCl, and HCl were changed to 133 mg and 350 mg, respectively, and the amounts of pure water were changed to 50 ml and 100 ml, respectively. The composition was calculated in the same manner as in Example 1, and the particle diameter of the primary particles and the median diameter D ５０ , K.R.U. ４ O ８ The crystallite size, the crystallite size of the iridium and ruthenium composite oxide, and the specific surface area were evaluated, and the results are shown in Tables 1 and 2. Then, except that the obtained powder was used, a catalyst ink was prepared and an electrode was fabricated in the same manner as in Example 1, and the oxygen generation activity (overvoltage) of the electrode was evaluated. The results are shown in Table 2.

[0072] (Comparative Example 1) RuCl ３ ・3H ２ 60 mg of IrCl was used. ３ ・3H ２ The same procedure as in Example 1 was carried out except that no O was used. ４ O ８ According to the XRD measurement, the obtained powder contained KRu. ４ O ８The catalyst ink was prepared in the same manner as in Example 1, except that the obtained powder was used, and then an electrode was fabricated using the ink. The oxygen generation activity of the electrode was evaluated. The results are shown in Table 2.

[0073] (Comparative Example 2) "Commercially available IrO ｘ IrO purchased from Tanaka Kikinzoku Kogyo Co., Ltd. ｘ (Ir content: 76% by mass) was used as the powder of Comparative Example 2. A catalytic ink was prepared in the same manner as in Example 1, except that the IrOx powder was used, and then an electrode was fabricated, and the oxygen generation activity of the electrode was evaluated.

[0074] Comparative Example 3: A powder containing a metal composite oxide was obtained in the same manner as in Example 1, except that the pH of the solution was adjusted to 7. The composition was calculated in the same manner as in Example 1, and XRD measurement was performed. As a result, potassium element was not contained, and no hollandite structure was observed. In addition, the particle diameter of the primary particles, the median diameter D ５０ , RuO ２ / IrO ２ The crystallite size and specific surface area were evaluated, and the results are shown in Tables 1 and 2. A catalyst ink was then prepared in the same manner as in Example 1, except that the obtained powder was used, and an electrode was then fabricated using this, and the oxygen generation activity (overvoltage) of the electrode was evaluated. The results are shown in Table 2.

[0075] The results of the above measurements are shown in Tables 1 and 2.

[0076]

[0077]

[0078] 1 shows the XRD spectra of the metal composite oxides prepared in Examples 1 to 3. As shown in FIG. 1, the XRD spectra show that KRu with a hollandite structure ４ O ８ The peaks derived from the above (12.6°, 17.8° and 35.1°) were observed.

[0079] FIG. 2 shows an observation image of the particles of Example 3 obtained with a transmission electron microscope (TEM, manufactured by JEOL Ltd., JEM-1400). The magnifications were 50K and 250K. As shown in FIG. 2, a metal composite oxide having a particle diameter of about several nanometers was observed. FIGS. 3 and 4 are views showing observation images of the metal composite oxide obtained in Example 3 by SEM-EDS. As shown in FIGS. 3 and 4, it was confirmed that the potassium element, iridium element, and ruthenium element were each distributed almost uniformly.

[0080] From Tables 1 and 2, KRu containing a composite oxide of iridium and ruthenium and having a hollandite structure ４ O ８ The samples of Examples 1 to 3 containing KRu with a hollandite structure ４ O ８ In particular, the sample of Example 3, which had an iridium content of 9 mass% and a total content of ruthenium and iridium of 53 mass%, exhibited a significantly reduced overvoltage compared to the sample of Comparative Example 3, which had an iridium content of 63 mass%, and thus exhibited excellent catalytic activity. ４ O ８ In Comparative Example 1, which contained only PEG, no catalytic activity was observed.

Claims

1. A metal composite oxide containing potassium, iridium, ruthenium, and oxygen, which contains a composite oxide of iridium and ruthenium as an essential component and has a hollandite structure. ４ O ８ A metal composite oxide comprising:

2. The metal composite oxide according to claim 1, wherein the iridium content is 50% by mass or less and the potassium content is 1% by mass or more and 10% by mass or less.

3. The metal composite oxide according to claim 1, wherein the primary particles have a particle size of 150 nm or less.

4. Median diameter D determined by dynamic light scattering ５０ The metal composite oxide according to claim 1, wherein the average particle size is 500 nm or less.

5. The specific surface area measured by the BET method is 10 m ２ The metal composite oxide according to claim 1, wherein the metal composite oxide has a molecular weight of 1 / g or more.

6. The metal composite oxide according to claim 1, wherein the ratio of the total content of said iridium and said ruthenium to the total mass of said metal composite oxide, as determined by XRF analysis, is 50 mass % or more and 80 mass % or less.

7. A composite comprising the metal composite oxide according to any one of claims 1 to 6 and a support, wherein the metal composite oxide is supported on the support, and the support is an inorganic material.

8. The support is tin oxide (SnO ２ 8. The composite according to claim 7, wherein the composite is at least one selected from the group consisting of tungsten, tungsten carbide ...

9. An oxygen generating catalyst comprising the metal composite oxide according to any one of claims 1 to 6.

10. The oxygen generating catalyst according to claim 9, further comprising a conductive material.

11. An oxygen generating catalyst comprising the composite of claim 7.

12. The oxygen generating catalyst according to claim 11, further comprising a conductive material.

13. A catalyst ink comprising the metal composite oxide according to any one of claims 1 to 6 and a medium.

14. The catalyst ink of claim 13, further comprising a conductive material.

15. The catalyst ink of claim 13, further comprising a polyelectrolyte.

16. A catalyst ink comprising the composite of claim 7 and a medium.

17. The catalyst ink of claim 16, further comprising a conductive material.

18. The catalyst ink of claim 16, further comprising a polyelectrolyte.

19. An electrode having a surface comprising the metal composite oxide according to any one of claims 1 to 6.

20. An electrode having the composite of claim 7 on its surface.

21. A method for producing a metal composite oxide, comprising: preparing a mixed solution containing an iridium compound, a ruthenium compound, and an aqueous medium; adjusting the pH of the mixed solution obtained in the above step to between 8 and 13 using potassium hydroxide to produce a precipitate; isolating the precipitate from the aqueous medium; and calcining the precipitate obtained in the above step.

22. A method for producing a composite, comprising the steps of: dispersing an inorganic substance in an aqueous medium to prepare a first aqueous dispersion containing the inorganic substance; dissolving an iridium compound and a ruthenium compound in the first aqueous dispersion to prepare a second aqueous dispersion; adjusting the pH of the second aqueous dispersion obtained in the first step to between 8 and 13 using potassium hydroxide to produce a precipitate; isolating the precipitate from the aqueous medium; and calcining the precipitate obtained in the first step.