Powder including iridium-containing manganese oxide powder, catalyst, electrode, and water electrolysis method

Abstract

To provide at least one of iridium-containing manganese oxide powder exhibiting high oxygen generation electrode catalyst activity with a reduced used amount of iridium, having a low aspect ratio and being suitable for industrial production, a catalyst including the powder, an electrode including the catalyst, and a water electrolysis method using the electrode.SOLUTION: The powder contains iridium-containing manganese oxide particles according to the present invention, in which an average molar ratio of iridium relative to manganese in the iridium-containing manganese oxide particles is 0.001 or higher and 0.1 or lower, an average aspect ratio of the iridium-containing manganese oxide particles is 1.0 or higher and 2.5 or lower, the iridium-containing manganese oxide particle has a rutile crystal structure, and the ratio of the iridium-containing manganese oxide particles with the molar ratio of iridium of 1 or higher relative to manganese, is 2.5% or lower, among the iridium-containing manganese oxide particles.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a powder, a catalyst, an electrode, and a water electrolysis method that include iridium-containing manganese oxide particles. [Background technology]

[0002] Due to the problems of fossil fuel depletion and environmental pollution, attention is being focused on the use of hydrogen as a clean energy source and methods for producing it. One effective method for producing high-purity hydrogen gas is water electrolysis. In water electrolysis, iridium-based catalysts are widely known as highly active oxygen-evolving electrode catalysts (e.g., Non-Patent Document 1). Furthermore, Patent Document 1 discloses an iridium-manganese oxide composite electrode material as an oxygen-evolving electrode catalyst, in which iridium is introduced into manganese oxide that is directly electrodeposited onto a conductive substrate. In addition, an iridium-containing manganese oxide in which a small amount of iridium is introduced into α-MnO2 has been reported as an oxygen-evolving electrode catalyst with a reduced amount of iridium used (Non-Patent Document 2).

[0003] On the other hand, a water electrolysis device is typically configured by serially stacking cells each including a membrane electrode assembly (MEA) and separators sandwiching the MEA. The MEA includes a proton-conductive solid polymer electrolyte membrane (hereinafter simply referred to as "electrolyte membrane"), a catalyst layer, and a conductive substrate. A typical MEA is fabricated by bonding a conductive substrate to the surface of the catalyst layer of a catalyst-coated membrane (CCM), which has catalyst layers formed on both sides of the electrolyte membrane, by heat and pressure bonding (see, for example, Patent Document 2). CCMs can be easily manufactured on an industrial scale by applying an ink containing an oxygen-evolving electrode catalyst dispersed in a solvent to an electrolyte membrane, which requires the oxygen-evolving electrode catalyst to be in powder form. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2022 / 264960 [Patent Document 2] International Publication No. 2024 / 004857 [Non-patent literature]

[0005] [Non-Patent Document 1] F. Birol, World Energy Outlook 2016, International Energy Agency (IEA), Paris, 2016. [Non-patent document 2] Nature Communications (2024)15:95 Summary of the Invention [Problem to be solved by the invention]

[0006] The powdered iridium-containing manganese oxide disclosed in Non-Patent Document 2 is more suitable for industrial production of CCMs than the iridium-manganese oxide composite electrode material disclosed in Non-Patent Document 1. However, the iridium-containing manganese oxide is in the form of acicular particles with a high aspect ratio, which presents significant limitations, such as making it difficult to handle during production. An object of the present disclosure is to provide at least one of an iridium-containing manganese oxide powder that exhibits high oxygen evolution electrode catalytic activity, uses a reduced amount of iridium, has a low aspect ratio, and is suitable for industrial production; a catalyst containing the powder; an electrode containing the catalyst; and a water electrolysis method using the electrode. [Means for solving the problem]

[0007] The present disclosure has investigated the physical properties of iridium-containing manganese oxide powder, and has found that by controlling the method of supporting iridium on manganese oxide with specific composition and physical properties, it is possible to obtain an iridium-containing manganese oxide powder suitable for industrial production that exhibits remarkably high oxygen evolution electrode catalytic activity in water electrolysis despite having a low iridium content. That is, the present invention is as described in the claims, and the gist of the present disclosure is as follows.

[0008] [1] A powder containing iridium-containing manganese oxide particles, the iridium-containing manganese oxide particles have an average molar ratio of iridium to manganese of 0.001 or more and 0.10 or less; the iridium-containing manganese oxide particles have an average aspect ratio of 1.0 or more and 2.5 or less; the iridium-containing manganese oxide particles have a rutile crystal structure, The powder contains iridium-containing manganese oxide particles, of which the proportion of iridium-containing manganese oxide particles having a molar ratio of iridium to manganese of 1 or more is 2.5% or less. [2] The iridium-containing manganese oxide powder according to [1], having a D50 diameter in the volume particle size distribution of 0.1 μm or more and 50 μm or less. [3] BET specific surface area is 1m 2 / g or more 200m 2 The iridium-containing manganese oxide powder according to [1] or [2], wherein the iridium content is 1 / g or less. [4] The iridium-containing manganese oxide powder according to any one of [1] to [3], wherein in a powder X-ray diffraction pattern, when CuKα radiation is used as a radiation source, the full width at half maximum of the peak appearing at 2θ=28±1° is greater than 2.06° and not greater than 4.00°. [5] A catalyst comprising the iridium-containing manganese oxide powder according to any one of [1] to [4] above. [6] An electrode comprising the catalyst according to [5] above. [7] A water electrolysis method using the electrode according to [6] above. [Effects of the Invention]

[0009] The present disclosure can provide at least one of an iridium-containing manganese oxide powder that exhibits high oxygen evolution electrode catalytic activity, uses a reduced amount of iridium, and is suitable for industrial production; a catalyst containing the powder; an electrode containing the catalyst; and a water electrolysis method using the electrode. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing a method for measuring the major axis diameter and minor axis diameter of a particle. [Figure 2] 1 is a backscattered electron image of the iridium-containing manganese oxide particles of Example 1. [Figure 3] 1 is a backscattered electron image of iridium-containing manganese oxide particles of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present disclosure will be described below with reference to an example embodiment. The main terms used in this embodiment are as follows: The configurations and parameters disclosed herein may be combined in any desired manner, and the upper and lower limits of the values ​​disclosed herein may be combined in any desired manner.

[0012] The present embodiment relates to a powder containing iridium-containing manganese oxide particles, in which the iridium-to-manganese oxide particles have an average molar ratio of iridium to manganese of 0.001 or more and 0.1 or less, an average aspect ratio of 1.0 or more and 2.5 or less, the iridium-containing manganese oxide particles have a rutile crystal structure, and the proportion of iridium-containing manganese oxide particles with an iridium-to-manganese molar ratio of 1 or more among the iridium-containing manganese oxide particles is 2.5% or less.

[0013] (Manganese oxide) Known manganese oxides include manganese oxide (MnO), manganese trioxide (Mn2O3), manganese dioxide (MnO2), and manganese tetraoxide (Mn3O4). However, because of its high oxygen generating capacity, the manganese oxide in the powder of this embodiment preferably includes manganese dioxide, and more preferably electrolytic manganese dioxide.

[0014] Manganese dioxide is known to have α-type (hollandite type), β-type (rutile type), γ-type, δ-type, λ-type (spinel type), and ramsdellite-type crystal structures, but manganese dioxide having a β-type (rutile type) crystal structure (hereinafter also referred to as "rutile type manganese dioxide") is preferred. Note that electrolytic manganese dioxide is manganese dioxide having a γ-type crystal structure, but by heat treating it (for example, at 200°C or higher), it becomes manganese dioxide having a β-type crystal structure. The manganese oxide contained in the powder of this embodiment may also include manganese oxides other than manganese dioxide.

[0015] The crystal structure of manganese oxide can be identified by comparing its powder X-ray diffraction (hereinafter also referred to as "XRD") pattern with the XRD pattern (hereinafter also referred to as "reference pattern") registered in the ICDD (International Center for Diffraction Data) Powder Diffraction File (registered trademark). For example, the reference patterns for manganese dioxide having a γ-type, β-type, ε-type, or α-type crystal structure can be PDF Nos. 14-0644 (γ-type), 24-0735 (β-type), 30-0820 (ε-type), or 44-0141 (α-type), respectively.

[0016] In this embodiment, the XRD pattern may be obtained by XRD measurement using a general powder X-ray diffractometer (for example, device name: Ultima IV Protectus, manufactured by Rigaku Corporation) under the following conditions. Acceleration current / voltage: 40mA / 40kV Radiation source: CuKα radiation (λ=1.5405Å) Measurement mode: Continuous scan Scan condition: 4° / min Measurement range: 2θ=10° to 80° Divergence vertical limit slit: 10mm Divergence / entrance slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used

[0017] The XRD peak is a peak whose peak top 2θ is detected in an XRD pattern analysis using general analysis software (for example, IGOR Pro 8 manufactured by WaveMetrics, or PDXL2 manufactured by Rigaku Corporation).

[0018] In the XRD pattern of the powder of this embodiment, the full width at half maximum (hereinafter simply referred to as "half width") of the XRD peak (hereinafter also referred to as "Mn peak") having a peak top at 2θ=28.00±1.00° is preferably 0.50 or more, 1.50 or more, or 2.10 or more, and is preferably 4.00 or less or 3.50 or less, and more preferably 0.50 to 4.00, 1.50 to 3.50, or 2.10 to 3.50. When the half width of the Mn peak is within the above range, excellent oxygen evolution electrode catalytic activity is exhibited.

[0019] (iridium) The powder of this embodiment contains iridium (Ir), and it is sufficient that the iridium is contained in a state that allows it to interact with manganese oxide, and it is preferable that the manganese oxide contains iridium. The state in which manganese oxide contains iridium can be one or more states selected from the group consisting of a state in which it is mixed with manganese oxide, a state in which it is supported on manganese oxide, and a state in which it is solid-solved in manganese oxide, and it is preferable that at least a portion of the iridium is solid-solved in manganese oxide.

[0020] The iridium contained in the powder of this embodiment may be contained in one or more forms selected from the group consisting of metal, cation, and compound, but is preferably contained in the form of a cation, as this is more likely to exhibit high oxygen evolution electrode catalytic activity. The iridium contained in the powder of this embodiment is preferably contained in the manganese oxide as at least iridium ions, and more preferably in a state in which at least a portion of the iridium ions are substituted for manganese ions in the manganese oxide, i.e., in a state in which at least a portion of the iridium is solid-solved in the manganese oxide.

[0021] The state in which iridium is supported on manganese oxide may be such that iridium is contained in one or more selected from the group consisting of the surface, pores, and skeletal structure of the manganese oxide. For example, when iridium is supported on the surface of manganese oxide, the iridium may be in the form of an oxide, or may be supported as iridium oxide. When iridium is supported in the pores of manganese oxide, the iridium may be contained as a cation, or may be contained as iridium cations in ion-exchangeable sites of the manganese oxide. When iridium is dissolved in manganese oxide, at least a portion of the iridium cations may be substituted for manganese ions of the manganese oxide.

[0022] In order to exhibit high oxygen evolution electrode catalytic activity, the powder of this embodiment preferably contains iridium in at least one of a state in which it is supported on manganese oxide and a state in which it is solid-solved in manganese oxide, and more preferably contains some or all of the iridium in a state in which it is solid-solved in manganese oxide.

[0023] (Ir intensity ratio in XRD pattern) In the powder of this embodiment, at least a portion of the iridium can be considered to be dissolved in the manganese oxide if the ratio of the area intensity of the XRD peak having a peak top at 2θ=35.00±1.00° (hereinafter also referred to as the "Ir intensity ratio") in the XRD pattern is 3 or less, where the area intensity of the XRD peak with the greatest area intensity is taken as 100. For example, when the manganese oxide is β-type manganese dioxide, the XRD peak with the greatest area intensity is the XRD peak having a peak top at 2θ=28±1° (hereinafter also referred to as the "Mn peak").

[0024] The XRD peak with a peak top at 2θ=35.00±1.00° is considered to be the main peak of the iridium compound. Since the powder of this embodiment preferably contains iridium in a state of solid solution in manganese oxide, the Ir intensity ratio is preferably 0 to 3, more preferably 0 to 1. Furthermore, the crystal structure of manganese oxides that support and dissolve iridium is prone to distortion. In this case, the half-width of the Mn peak of manganese oxides that contain iridium in a solid solution tends to be larger than the Mn peak of manganese oxides that do not contain iridium in a solid solution. For example, the half-width of the Mn peak of manganese oxides that support and dissolve iridium may be greater than 2.06°, 2.30° or more, or 2.50° or more. Examples of the half-width of the Mn peak of manganese oxides that support and dissolve iridium include greater than 2.06°, 2.30° or more, or 2.50° or more. Examples include greater than 2.06° and 4.00°, 2.30° or more and 3.50° or less, or 2.50° or more and 3.50° or less.

[0025] (Ir / Mn average molar ratio) The iridium-containing manganese oxide particles of this embodiment have an average molar ratio of iridium to manganese (mol / mol; hereinafter, also referred to as the "Ir / Mn average molar ratio") of 0.001 to 0.10. The Ir / Mn average molar ratio refers to the ratio of the molar amount of Ir compound contained per mole of manganese oxide. The Ir / Mn average molar ratio is preferably 0.004 to 0.008, and more preferably 0.095 to 0.093. Furthermore, a range of 0.004 to 0.095, or 0.008 to 0.093, is also preferred. When the Ir / Mn average molar ratio is equal to or greater than the above-mentioned lower limit, oxygen generating electrode catalytic activity is improved, which is preferable. On the other hand, when the Ir / Mn average molar ratio is equal to or less than the above-mentioned upper limit, iridium elution is suppressed, and oxygen generating electrode catalytic activity is improved, which is preferable.

[0026] The Ir / Mn average molar ratio can be measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a common ICP apparatus (apparatus name: Optima 830, manufactured by PerkinElmer). That is, the manganese content and iridium content of the powder are measured by the ICP method, and the Ir / Mn average molar ratio of the powder can be calculated from the obtained iridium content relative to the manganese content.

[0027] (Manganese content, Iridium content) The "manganese content" and "iridium content" are the mass proportions of manganese and iridium, respectively, relative to the mass of the powder, and are calculated using the following formula. Manganese content [mass%] = (W Mn / W)×100 Iridium content [mass%] = (W Ir / W)×100

[0028] In the above equation, W Mn is the mass of manganese in the powder [g], W Ir is the mass [g] of iridium in the powder, and W is the mass [g] of the powder determined by weighing.

[0029] In the powder of this embodiment, the iridium content is preferably 0.2 mass% or more or 0.8 mass% or more, and is preferably 35 mass% or less or 30 mass% or less, and is preferably 0.2 mass% or more and 35 mass% or less, or 0.8 mass% or more and 30 mass% or less, so that sufficient catalytic activity for the oxygen generation reaction is more likely to be exhibited.

[0030] (powder particle size) Because this makes it easier for iridium to contribute to the catalytic reaction and because, when made into an anode catalyst ink, it is easier to uniformly apply the iridium to an electrolyte membrane or a conductive substrate, the D50 diameter of the powder of this embodiment is preferably 0.1 μm or more and 50 μm or less, and preferably 0.3 μm or more or 0.5 μm or more, and is also preferably 30 μm or less, 10 μm or less, or 3 μm or less, and more preferably 0.1 μm or more and 30 μm or less, 0.3 μm or more and 10 μm or less, or 0.5 μm or more and 3 μm or less.

[0031] The standard deviation in the volume particle size distribution of the powder of this embodiment is preferably 0.05 μm or more, and is preferably 5.0 μm or less, 3.0 μm or less, or 2.0 μm or less, and is preferably 0.05 μm or more and 5.0 μm or less, 0.05 μm or more and 3.0 μm or less, or 0.05 μm or more and 2.0 μm or less.

[0032] The D16 diameter of the powder of this embodiment is preferably 0.05 μm or more, 0.15 μm or more, or 0.25 μm or more, and is preferably 30 μm or less, 20 μm or less, or 10 μm or less, and is preferably 0.05 μm or more and 30 μm or less, 0.15 μm or more and 20 μm or less, or 0.25 μm or more and 10 μm or less.

[0033] The D84 diameter of the powder of this embodiment is preferably 0.5 μm or more, 1.0 μm or more, or 1.5 μm or more, and is preferably 60 μm or less, 50 μm or less, or 30 μm or less, and is preferably 0.5 μm or more to 60 μm or less, 1.0 μm or more to 50 μm or less, or 1.5 μm or more to 30 μm or less.

[0034] In this embodiment, the "D16 diameter," "D50 diameter," and "D84 diameter" are the particle diameters [μm] corresponding to a cumulative frequency of 16%, 50%, and 84%, respectively, in the cumulative volume particle size distribution obtained by laser diffraction / scattering. D50 is used interchangeably with "median diameter."

[0035] The D16 diameter, D50 diameter, and D84 diameter of the powder of this embodiment may be measured using a general particle size distribution measuring device (for example, device name: MT-3100II, manufactured by Microtrack Bell) under the following conditions. Measurement range: 0.02 to 2000 μm Particle refractive index: 2.2 Particle permeability: permeation Particle shape: non-spherical Solvent refractive index: 1.333 Ultrasonic pretreatment: 10 minutes

[0036] The standard deviation can be calculated from the obtained D16 diameter and D84 diameter values ​​using the following formula. Standard deviation [μm]=(D84[μm]-D16[μm]) / 2

[0037] (BET specific surface area) The BET specific surface area of ​​the powder of this embodiment is 1 m 2 / g or more 200m 2 / g or less is preferable, and 10m 2 / g or more or 19m 2 / g or more, and 100m 2 / g or less, 50m 2 / g or less or 30m 2 / g or less, and 1m 2 / g or more 100m 2 / g or less, 10m 2 / g or more 50m 2 / g or less or 19m 2 / g or more 30m 2 It is more preferable that the saturation coefficient is 1 / g or less.

[0038] The BET specific surface area can be measured using a general measuring device (for example, Macsorb (registered trademark), manufactured by Mountec Co., Ltd.) and a mixed gas of 30% by volume nitrogen and 70% by volume helium as the adsorption gas, by the one-point method specified in JIS Z8830: 2013. The sample to be measured can be pretreated by placing the sample in a glass cell for BET specific surface area measurement and dehydrating it in a nitrogen flow atmosphere at 150°C for 20 minutes.

[0039] (average aspect ratio) The average aspect ratio of the iridium-containing manganese oxide particles of this embodiment is 1.0 or more and 2.5 or less. The average aspect ratio is preferably 1.0 or more, and is preferably 2.3 or less, or 2.0 or less. Also, 1.0 or more and 2.3 or 1.0 or more and 2.0 or less are preferred. By using an iridium-containing manganese oxide having an average aspect ratio within this range, production with excellent handleability can be achieved. The aspect ratio defined in this embodiment is the ratio of the major axis diameter to the minor axis diameter when the particle is viewed in plan. A method for measuring the major axis diameter and the minor axis diameter will be explained using a schematic diagram ( FIG. 1 ). A particle 1 placed on a flat surface is observed with a scanning electron microscope (hereinafter also referred to as “SEM”). The outline 2 of the backscattered electron image of the particle 1 is sandwiched between two parallel lines 3 tangent to the outline 2, and the maximum distance between the parallel lines 3 is defined as the major axis diameter 4 of the particle 1, and the minimum distance is defined as the minor axis diameter 5 of the particle 1. The ratio of the major axis diameter to the minor axis diameter (major axis diameter / minor axis diameter) is the aspect ratio. Specifically, the aspect ratios are calculated for 50±5 particles per field of view, for a total of 250±25 particles in five fields of view, and the average value is taken as the average aspect ratio.

[0040] (iridium agglomerated particles) In this embodiment, the proportion of iridium-containing manganese oxide particles having an iridium to manganese molar ratio of 1 or more among the iridium-containing manganese oxide particles is 2.5% or less. In this specification, iridium-containing manganese oxide particles having an iridium to manganese molar ratio of 1 or more are referred to as iridium agglomerated particles. In this specification, the fine grains that make up the powder are referred to as particles. A collection of particles is a powder. The proportion of iridium agglomerated particles can be determined by the following procedure.

[0041] First, a backscattered electron image of the powder sample to be measured is obtained using a general scanning electron microscope (for example, JSM-IT500, manufactured by JEOL Ltd.) under the following conditions using a backscattered electron detector. The observation surface is preferably perpendicular to the electron beam. When comparing multiple samples, the measurement conditions and observation area should be the same. Magnification: 1500x Observation area: 85.33 x 64 μm Number of pixels: 1280 pixels wide x 960 pixels high Accelerating voltage: 15kV Working distance: 10mm Contrast: 3168 Brightness: 2033 Number of fields of view: 5 (no overlapping observation fields)

[0042] Using image analysis software Image J (manufactured by the National Institutes of Health), the obtained backscattered electron image is divided into 1280 pixels horizontally by 960 pixels vertically, and a histogram of pixel frequency versus brightness value is obtained. From the obtained histogram, the total number of pixels with a brightness value of 41 or more but less than 235 (hereinafter referred to as "number of medium-brightness pixels") and the total number of pixels with a brightness value of 235 or more but less than 255 (hereinafter referred to as "number of high-brightness pixels") are calculated. The ratio of high-brightness pixels to the total number of high-brightness pixels and medium-brightness pixels (hereinafter referred to as "high-brightness pixel ratio") is calculated using the following formula. High-brightness pixel ratio [%] = High-brightness pixel count / (High-brightness pixel count + Medium-brightness pixel count) x 100

[0043] Backscattered electrons depend on the material that makes up the sample, and it is known that the higher the atomic number, the greater the amount of backscattered electrons emitted. In other words, the amount of backscattered electrons emitted from iridium, with atomic number 77, is greater than that from manganese, with atomic number 25. Therefore, it is presumed that the bright pixels contain particles with a relatively high content of Ir. In fact, it has been confirmed that high brightness pixels with a brightness number of 235 or more and 255 or less become brighter when they contain particles with a molar ratio of iridium to manganese of at least 1. The molar ratio of iridium to manganese in the particles can be determined by the following method.

[0044] A scanning electron microscope-energy dispersive X-ray analysis (hereinafter also referred to as "SEM-EDS") is performed on the particles of the powder sample to be measured using a general scanning electron microscope (for example, device name: JSM-IT500, manufactured by JEOL Ltd.) under the following conditions to obtain the ratio of manganese atoms (hereinafter "manganese atomic ratio") and the ratio of iridium atoms (hereinafter "iridium atomic ratio") in the measured elements of the particles of the powder sample.

[0045] Observation magnification: 1000-15000x Accelerating voltage: 15kV Working distance: 10mm Measurement elements: manganese (K line), iridium (M line), oxygen (K line), Carbon (K line), chlorine (K line), nitrogen (K line), potassium (K line) From the obtained results of the manganese atomic ratio and the iridium atomic ratio, the molar ratio of iridium to manganese in the particles of the powder sample is calculated using the following formula. Molar ratio of iridium to manganese = atomic ratio of iridium / atomic ratio of manganese

[0046] (Ratio of iridium agglomerated particles) Five backscattered electron images are acquired for the sample to be measured, the percentage of high-brightness pixels is calculated for each, and the average value is taken as the percentage (%) of iridium agglomerated particles in the sample. In this embodiment, the proportion of iridium agglomerated particles is preferably 2.5% or less, more preferably 1.0% or less, and even more preferably 0.2% or less. There are no particular limitations on the lower limit of the proportion of iridium agglomerated particles, but examples of practical lower limits include 0% or more, more than 0%, and even 0.1% or more. Specific ranges for the proportion of iridium agglomerated particles include 0% or more and 2.5% or less, more than 0% and 1.0% or less, and even 0.1% or more and 0.2% or less.

[0047] (catalyst) The powder of this embodiment can be used as a catalyst, preferably an oxygen evolution electrode catalyst.

[0048] When the powder of this embodiment is used as a catalyst, it may be used as is, or it may be used as a catalyst composition containing the powder of this embodiment (hereinafter, these are also collectively referred to as "the catalyst of this embodiment").

[0049] The catalyst composition may be an anode catalyst ink containing the powder of this embodiment, for example, a composition obtained by mixing the powder of this embodiment, water, ethanol, and an ionomer (for example, product name: Nafion Dispersion Solution, manufactured by Sigma-Aldrich). The composition may contain isopropyl alcohol for the purpose of adjusting viscosity, etc.

[0050] (electrode) An electrode containing the present catalyst (hereinafter also referred to as "the present electrode") can be used as an anode in a water electrolysis method.

[0051] The electrode can be configured to suit the intended application. When the electrode is used as an anode in a water electrolysis method, the electrode is preferably a membrane-electrode assembly in which a proton-conductive solid polymer electrolyte membrane, a catalyst layer containing the catalyst, and a conductive substrate are laminated. A water electrolysis device equipped with such a membrane-electrode assembly as an anode can be used in a method for producing hydrogen by electrolyzing water. In this case, the amount of Mn supported per geometric area of ​​the electrolyte membrane or conductive substrate is 0.1 mg / cm. 2 More than 5.0mg / cm 2 It is preferable that:

[0052] The "geometric area" is the area equivalent to the projected area, without taking into account surface irregularities or voids. The "geometric area of ​​an electrolyte membrane" is the projected area of ​​an electrolyte membrane, which is the area of ​​a plane calculated by multiplying the length and width when the shape of the electrolyte membrane is specified as length x width x depth.

[0053] (Manufacturing method) The method for producing the powder of this embodiment is not particularly limited as long as it can produce a powder having the above-described configuration. Examples of the method include a production method including a mixing step of contacting a manganese oxide source with an iridium salt solution to obtain a mixture and a heat treatment step of heat-treating the mixture, a production method in which a manganese oxide source and an iridium source are mixed, and a production method in which an iridium-containing manganese oxide is precipitated on a conductive substrate by electrolysis of a mixed solution containing sulfuric acid, manganese sulfate, and an iridium salt.

[0054] A preferred method for producing the powder of this embodiment includes a mixing step of contacting a manganese oxide source with an iridium salt solution to obtain a mixture, and a heat treatment step of heat-treating the mixture. This production method will be described below as an example.

[0055] In the mixing step, the manganese oxide source is brought into contact with the iridium salt solution, thereby allowing iridium to be contained in the manganese oxide source.

[0056] The manganese oxide source can be exemplified by one or more selected from the group consisting of manganese dioxide (MnO2), trimanganese tetraoxide (Mn3O4), and manganese trioxide (Mn2O3), with at least one of MnO2 and Mn3O4, and MnO2 being more preferred.

[0057] (iridium) The iridium salt solution to be subjected to the mixing step may be any solution containing an iridium salt, and is preferably a solution containing an iridium salt, or even more preferably an aqueous solution containing an iridium salt, with an aqueous iridium salt solution being more preferred. Examples of iridium salts include iridium chloride (III) (IrCl), iridium chloride (IV) (IrCl), and iridium nitrate (Ir(NO)). Of these, iridium chloride (IrCl) and iridium nitrate are preferred. The aqueous iridium salt solution may contain one type of iridium salt, or two or more types.

[0058] The iridium concentration of the iridium salt solution is preferably 0.001 g / L or more and 20 g / L or less.

[0059] The method for contacting a manganese oxide source with an iridium salt solution is not particularly limited as long as the conditions are such that the desired amount of iridium can be contained in the manganese oxide. Specific conditions for this method include a contact temperature of 20°C or higher and 100°C or lower, and a contact time of 30 minutes or higher and 100 hours or lower. By keeping the contact time and temperature within the above ranges, the iridium content on the surface of the manganese oxide can be controlled. The higher the contact temperature and the longer the contact time, the more likely it is that the iridium content of the manganese oxide will increase. Furthermore, when the contact temperature is high, manganese atoms within the crystal structure of the manganese oxide are more likely to be replaced by iridium atoms. Therefore, the higher the contact temperature, the more likely it is that iridium will replace manganese in the manganese oxide and form a solid solution, and at least a portion of the iridium will be solid-dissolved in the manganese oxide. From this perspective, the contact temperature is preferably 70°C or higher, and, for example, 70°C or higher and 100°C or lower is preferred.

[0060] In the heat treatment step, the mixture obtained in the mixing step is heat-treated. As a result, manganese atoms inside the crystal structure of the manganese oxide are replaced with iridium atoms contained on the surface and in the pores of the manganese oxide, resulting in even more iridium being dissolved in the manganese oxide. The heat treatment in the heat treatment step can be carried out under any conditions that allow the manganese oxide to maintain the above-mentioned crystal structure. The heat treatment atmosphere is preferably an air atmosphere or an inert atmosphere, and more preferably an air atmosphere. The heat treatment temperature is preferably 100°C or higher and 600°C or lower. The heat treatment time can be set appropriately depending on the amount of manganese oxide, etc., to be subjected to the heat treatment, and can be, for example, 10 minutes to 24 hours. [Example]

[0061] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to the examples. <Composition analysis> The prepared sample was dissolved in a mixed solution of hydrochloric acid and nitric acid, and the manganese and iridium contents of the prepared sample were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a general ICP apparatus (apparatus name: Optima 830, manufactured by PerkinElmer). The average Ir / Mn molar ratio of the prepared sample was calculated from the obtained manganese and iridium contents.

[0062] <Particle size distribution> The volume particle size distribution was determined by measuring the volume particle size distribution using a laser diffraction / scattering particle size distribution analyzer (device name: Microtrac MT3300EXII, manufactured by Microtrac Bell). The measurement conditions are as follows: Measurement range: 0.02 to 2000 μm Particle refractive index: 2.2 Particle permeability: permeation Particle shape: non-spherical Solvent refractive index: 1.333 Ultrasonic pretreatment: 10 minutes From the obtained volume particle size distribution, the D16 diameter [μm], D50 diameter (median diameter) [μm], and D84 diameter [μm] of the prepared sample were obtained. Also, from the obtained values of the D16 diameter and D84 diameter, the standard deviation was calculated using the following formula. Standard deviation [μm] = (D84 [μm] - D16 [μm]) / 2

[0063] <BET specific surface area> The BET specific surface area of the sample was determined by measurement according to JIS Z 8830:2013. That is, 0.3 g of the sample was placed in a glass cell for BET specific surface area measurement, and dehydration treatment was performed at 150 °C for 20 minutes in a nitrogen flow atmosphere as a pretreatment. Using a general BET specific surface area measuring device (product name: Macsorb (registered trademark), manufactured by MOUNTECH Co., Ltd.), the BET specific surface area of the pretreated sample was measured by the one-point method using a mixed gas of 30% by volume of nitrogen - 70% by volume of helium as the adsorption gas.

[0064] <Full width at half maximum (FWHM)> Using a general powder X-ray diffractometer (device name: Ultima IV Protectus, manufactured by Rigaku Corporation), a powder X-ray diffraction pattern (XRD pattern) was obtained under the following conditions. Accelerating current · voltage: 40 mA · 40 kV X-ray source: CuKα ray (λ = 1.5405 Å) Measurement mode: Continuous scan Scan condition: 4° / min Measurement range: 2θ = 10° to 80° Divergence vertical limiting slit: 10 mm Divergence / incidence slit: 1° Receiving slit: open Detector: D / teX Ultra Using a Ni filter The XRD pattern of the sample was analyzed for the peaks that appeared using the analysis software (product name: PDXL2, manufactured by Rigaku Corporation) attached to the powder X-ray diffractometer, and the full width at half maximum (°) of the XRD peak having a peak top at 2θ = 28 ± 1° was determined and taken as the half width (°) of the Mn peak.

[0065] <Calculation of the percentage of iridium agglomerated particles> A general scanning electron microscope (device name: JSM-IT500, manufactured by JEOL Ltd.) was used to obtain a backscattered electron image of the sample using a backscattered electron detector under the following conditions. (Backscattered electron image acquisition conditions) Magnification: 1500x Observation area: 85.33 x 64 μm Number of pixels: 1280 pixels wide x 960 pixels high Accelerating voltage: 15kV Working distance: 10mm Contrast: 3168 Brightness: 2033 Number of fields of view: 5 (no overlapping observation fields) The backscattered electron image obtained using image analysis software Image J (manufactured by the National Institutes of Health, USA) was divided into 1280 pixels horizontally by 960 pixels vertically, and a histogram of pixel frequency versus brightness value was obtained. From the obtained histogram, the total number of pixels with a brightness value of 41 or more but less than 235 (hereinafter referred to as "mid-brightness pixel count") and the total number of pixels with a brightness value of 235 or more but less than 255 (hereinafter referred to as "high-brightness pixel count") were calculated, and the ratio of high-brightness pixels to the total number of high-brightness pixels and mid-brightness pixels (hereinafter referred to as "high-brightness pixel ratio") was calculated using the following formula. High-brightness pixel ratio [%] = High-brightness pixel count / (High-brightness pixel count + Medium-brightness pixel count) x 100 The percentage of high-brightness pixels was calculated for each of the five backscattered electron images, and the average value was taken as the percentage [%] of iridium agglomerated particles in the sample.

[0066] <Calculating the average aspect ratio> A common scanning electron microscope (JSM-IT500, manufactured by JEOL Ltd.) was used to obtain backscattered electron images of the sample in the same manner as in calculating the proportion of iridium agglomerated particles, and the major and minor axis diameters of the sample particles were determined. The major axis diameter and minor axis diameter were measured based on the definitions given in the literature "Particle Size Measurement Technology, compiled by the Society of Powder Technology," from section 5, line 3 to section 6, line 2. That is, a particle placed on a flat surface was observed using an SEM from a direction perpendicular to the plane, and the outline of the backscattered electron image of the particle was sandwiched between two parallel lines. The maximum length between the parallel lines was taken as the major axis diameter of the particle, and the minimum length was taken as the minor axis diameter of the particle. The ratio of the long axis diameter to the short axis diameter (long axis diameter [μm] / short axis diameter [μm]) of 50±5 particles per field of view, for a total of 250±25 particles in five fields of view, was calculated, and the average value was used as the average aspect ratio of the sample.

[0067] <Fabrication of membrane-electrode assembly and PEM-type water electrolyzer> A membrane-electrode assembly (hereinafter also referred to as "MEA") equipped with the sample as an anode catalyst, and a PEM-type water electrolyzer equipped with the MEA were fabricated by the following methods.

[0068] A cathode catalyst ink was prepared by mixing a solution containing water, ethanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich) with a 20% by mass platinum-supported carbon catalyst (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich). The mass ratio of water:ethanol:ionomer in the cathode catalyst ink was 55.5:43.5:1.0. This resulted in a platinum loading per geometric area of ​​0.3 mg / cm. 2 The solution was applied to carbon paper (product name: TGP-H-060H, manufactured by Toray Industries, Inc.) so that the solution satisfies the following formula: and air-dried to prepare a cathode electrode. Next, a cation exchange membrane (product name: Nafion 115, manufactured by Sigma-Aldrich) was boiled and washed in 3% by mass hydrogen peroxide solution for 1 hour, purified water for 1 hour, and 1 M sulfuric acid solution for 1 hour, and then protonated to prepare an electrolyte membrane. Furthermore, the sample (iridium-containing manganese oxide powder) was mixed with a solution containing water, isopropanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich) to prepare an anode catalyst ink. The mass ratio of water:isopropanol:ionomer in the anode catalyst ink was 55.9:43.8:0.3. This resulted in an Ir loading per geometric area of ​​0.1 mg / cm. 2 The solution was applied to a PTFE sheet (manufactured by Tokyo Glass Instruments Co., Ltd.) having a thickness of 0.05 mm so as to give the following composition, and then air-dried to obtain a sheet with a catalyst layer. The catalyst layer-attached sheet was laminated on the electrolyte membrane so that the surface on which the anode catalyst was applied faced the electrolyte membrane, and then heated at 140°C with a clamping force of 80 kg / cm using a hot press (product name: SA-302, manufactured by Tester Sangyo Co., Ltd.). 2 The PTFE sheet was then peeled off to obtain a catalyst-coated electrolyte membrane. The anode current collector, catalyst-coated electrolyte membrane, and cathode electrode were stacked in this order, with the platinum-coated Ti fiber sintered compact (product name: Pt-plated Ti fiber sintered compact, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.; hereinafter also referred to as the "anode current collector") facing the anode catalyst-coated surface of the catalyst-coated electrolyte membrane, and with the catalyst-free surface of the catalyst-coated electrolyte membrane facing the cathode catalyst-coated surface of the cathode electrode. The resulting stack was then heated in a hot press at 130°C with a clamping force of 50 kg / cm. 2 The MEA was obtained by hot pressing for 3 minutes at 100°C. The obtained MEA was attached to the housing of a PEM water electrolyzer (product name: WE-4S-RICW, manufactured by FC Development Co., Ltd.) to prepare a PEM water electrolyzer.

[0069] <Evaluation of oxygen evolution electrode catalyst performance> Using a PEM-type water electrolyzer, the current density at a voltage of 2 V was measured by linear sweep voltammetry (LSV) of a two-electrode system under the following conditions, and the performance of the oxygen evolution electrode catalyst was evaluated. Voltage increase rate: 10mV / sec Water temperature: 80℃ Water supply rate: 2mL / min

[0070] <Preparation of iridium-containing manganese oxide powder> Example 1 100 mg of electrolytic manganese dioxide powder (product name: FM, manufactured by Tosoh Corporation) was immersed in an iridium salt solution bath filled with 33.5 mL of an IrCl4 aqueous solution (6 mg of Ir content) with an IrCl4 concentration of 0.001 mol / L (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) at 95°C for 24 hours, followed by solid-liquid separation to obtain a mixture. The resulting mixture was dried in an air atmosphere at 90°C for 2 hours and then allowed to cool to room temperature by natural cooling. Next, it was annealed in an air atmosphere at 450°C for 5 hours to obtain an iridium-containing manganese oxide powder. The XRD pattern of this powder showed that it had a β-type (rutile) crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.24, indicating that it contained at least a portion of iridium dissolved in manganese oxide. Furthermore, the powder had a D16 diameter of 1.0 μm, a D50 diameter of 3.1 μm, a D84 diameter of 7.1 μm, and a standard deviation of 3.0 μm, and a BET specific surface area of ​​24 m. 2 / g, the average Ir / Mn molar ratio was 0.025 mol / mol, the average aspect ratio was 1.4, and the proportion of Ir aggregated particles was 0.0%. A backscattered electron image of the iridium-containing manganese oxide powder of this example is shown in Figure 2.

[0071] Example 2 An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that 33.5 mL of an Ir(NO3)4 solution (manufactured by Furuya Metal Co., Ltd.) with a concentration of 0.001 mol / L (6 mg of Ir content) was used as the iridium salt solution. The XRD pattern of this powder showed that it had a β-type (rutile) crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.23, indicating that it contained at least a portion of iridium dissolved in manganese oxide. Furthermore, the powder had a D16 diameter of 0.9 μm, a D50 diameter of 2.9 μm, a D84 diameter of 6.5 μm, and a standard deviation of 2.6 μm, and a BET specific surface area of ​​30 m 2 / g, the average Ir / Mn molar ratio was 0.025 mol / mol, the average aspect ratio was 1.4, and the proportion of Ir aggregated particles was 0.0%.

[0072] (Comparative Example 1) An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that 33.5 mL of a K2IrCl6 aqueous solution (6 mg of Ir content) with a K2IrCl6 (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) concentration of 0.001 mol / L was used as the iridium salt solution. The XRD pattern of this powder showed that it had a β-type (rutile) crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.20, indicating that it contained at least a portion of iridium dissolved in manganese oxide. Furthermore, the powder had a D16 diameter of 1.3 μm, a D50 diameter of 3.2 μm, a D84 diameter of 6.5 μm, and a standard deviation of 2.7 μm, and a BET specific surface area of ​​20 m 2 The average Ir / Mn molar ratio was 0.021 mol / mol, the average aspect ratio was 1.5, and the proportion of Ir aggregated particles was 2.9%. A backscattered electron image of the iridium-containing manganese oxide powder of this comparative example is shown in FIG.

[0073] MEAs each comprising the powders of Examples 1 and 2 and Comparative Example 1 as anode catalysts, and PEM-type water electrolyzers each comprising the MEAs were fabricated, and the oxygen generating electrode catalysts were evaluated. The results are shown in Table 1.

[0074] [Table 1]

[0075] From Table 1, it was confirmed that Examples 1 and 2, in which the proportion of Ir aggregated particles was 0%, exhibited high current density.

Claims

1. A powder containing iridium-containing manganese oxide particles, the iridium-containing manganese oxide particles have an average molar ratio of iridium to manganese of 0.001 or more and 0.10 or less; the iridium-containing manganese oxide particles have an average aspect ratio of 1.0 or more and 2.5 or less; the iridium-containing manganese oxide particles have a rutile crystal structure, The powder contains iridium-containing manganese oxide particles, wherein the proportion of iridium-containing manganese oxide particles having a molar ratio of iridium to manganese of 1 or more is 2.5% or less.

2. 2. The iridium-containing manganese oxide powder according to claim 1, wherein the D50 diameter in the volume particle size distribution is 0.1 μm or more and 50 μm or less.

3. BET specific surface area is 1m ２ / g or more 200m ２ The iridium-containing manganese oxide powder according to claim 1 or 2, wherein the iridium content is 1 / g or less.

4. 3. The iridium-containing manganese oxide powder according to claim 1, wherein in a powder X-ray diffraction pattern, when CuKα radiation is used as the radiation source, the full width at half maximum of a peak appearing at 2θ = 28 ± 1° is greater than 2.06° and not greater than 4.00°.

5. A catalyst comprising the iridium-containing manganese oxide powder according to claim 1 or 2.

6. An electrode comprising the catalyst of claim 5.

7. A water electrolysis method using the electrode according to claim 6.

Images