Iridium-containing manganese oxide, catalyst, electrode, and water electrolysis method

Abstract

The present disclosure provides at least one of an iridium-containing manganese oxide that exhibits high oxygen-generating electrode catalytic activity in a water electrolysis method, a catalyst that contains the same, an electrode that contains the catalyst, and a water electrolysis method that uses the electrode. With respect to the iridium-containing manganese oxide according to the present invention, the molar ratio of iridium to manganese is not less than 0.001 but 0.250 or less. In one embodiment, the manganese oxide is manganese dioxide that has a β-type crystal structure. In another embodiment, the ratio of the lattice constant in the a-axis direction to the lattice constant in the c-axis direction is not less than 1.420 but less than 1.521.

Description

Iridium-containing manganese oxide, catalyst, electrode, and water electrolysis method

[0001] The present disclosure relates to an iridium-containing manganese oxide, a catalyst, an electrode, and a water electrolysis method.

[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.

[0003] A polymer electrolyte membrane (PEM) 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 also simply referred to as an "electrolyte membrane"), a catalyst layer, and a conductive substrate. A typical MEA is fabricated by bonding a conductive substrate to the surface of a 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 1).

[0004] CCMs can be easily produced on an industrial scale by applying an ink containing an oxygen-evolving electrode catalyst dispersed in a solvent to an electrolyte membrane. Iridium-based catalysts are widely known to have very high activity as oxygen-evolving electrode catalysts (see, for example, Non-Patent Document 1).

[0005] On the other hand, iridium is very expensive due to its limited reserves and uneven distribution in certain regions, so the development of alternative catalysts using inexpensive transition metals is underway. In recent years, transition metal materials such as manganese (Mn) have been considered as alternative catalysts. In Non-Patent Document 2, α-MnO ２ Patent Document 2 discloses an iridium-manganese oxide composite electrode material in which iridium is introduced into manganese oxide that is electrodeposited directly onto a conductive substrate, rather than by applying an oxygen generating electrode catalyst powder to an electrolyte membrane.

[0006] JP 2014-22140 A International Publication No. 2022 / 264960

[0007] F. Birol, World Energy Outlook 2016, International Energy Agency (IEA), Paris, 2016. Nature Communications (2024) 15:95

[0008] Iridium-containing manganese oxides such as those described in Non-Patent Document 2 and Patent Document 2 have been investigated as oxygen-generating electrode catalysts in water electrolysis, but their catalytic activity needs to be further improved before they can be put to practical use.

[0009] An object of the present disclosure is to provide at least one of an iridium-containing manganese oxide that exhibits high oxygen evolution electrode catalytic activity, a catalyst containing the same, an electrode containing the catalyst, and a water electrolysis method using the electrode.

[0010] This disclosure investigates the physical properties of iridium-containing manganese oxides, and has found that by controlling the manganese oxide preparation method and the iridium loading method, it is possible to obtain iridium-containing manganese oxides with specific iridium and manganese molar compositions, which exhibit significantly higher oxygen evolution electrode catalytic activity in water electrolysis than conventional iridium-containing manganese oxides.

[0011] That is, the present invention is as set forth in the claims, and the gist of the present disclosure is as follows. [1] An iridium-containing manganese oxide having a molar ratio of iridium to manganese of 0.001 or more and 0.250 or less. [2] The iridium-containing manganese oxide according to [1] above, wherein the manganese oxide is manganese dioxide having a β-type crystal structure. [3] The iridium-containing manganese oxide according to [2] above, wherein the ratio of the lattice constant in the a-axis direction to the lattice constant in the c-axis direction is 1.420 or more and less than 1.521. [4] The iridium-containing manganese oxide according to any one of [1] to [3] above, wherein the D50 diameter in the volume particle size distribution is 0.1 μm or more and 50.0 μm or less. [5] An iridium-containing manganese oxide having a BET specific surface area of ​​1 m ２ / g or more 200m ２ / g or less. [6] The iridium-containing manganese oxide according to any of [1] to [5] above, wherein in a powder X-ray diffraction pattern, when CuKα radiation is used as a radiation source, the full width at half maximum of a peak appearing at 2θ = 28 ± 1.5° is more than 1.90° and 4.00° or less. [7] A catalyst comprising the iridium-containing manganese oxide according to any of [1] to [6] above. [8] An electrode comprising the catalyst according to [7] above. [9] A water electrolysis method using the electrode according to [8] above.

[0012] The present disclosure can provide at least one of an iridium-containing manganese oxide that exhibits high oxygen evolution electrode catalytic activity, a catalyst containing the same, an electrode containing the catalyst, and a water electrolysis method using the electrode.

[0013] 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.

[0014] The present embodiment is an iridium-containing manganese oxide that contains iridium and has a molar ratio of iridium to manganese of 0.001 or more and 0.250 or less.

[0015] The manganese oxide may be any oxide of manganese (Mn), and may be any of manganese oxide (MnO), manganese sesquioxide (Mn ２ O ３ ), manganese dioxide (MnO ２ ) and manganese tetroxide (Mn ３ O ４ In order to have high oxygen evolution electrode catalytic activity, the manganese oxide in the iridium-containing manganese oxide of this embodiment preferably contains manganese dioxide, and is preferably electrolytic manganese dioxide.

[0016] Manganese dioxide is manganese dioxide having a γ-, β-, ε-, or α-type crystal structure, and is preferably manganese dioxide having a β-type crystal structure (hereinafter also referred to as "β-type manganese dioxide") because it is likely to exhibit high oxygen generating electrode catalytic activity when containing iridium. Electrolytic manganese dioxide is manganese dioxide having a γ- or δ-type crystal structure, but by containing iridium and subjecting it to heat treatment (e.g., at 200°C or higher), it becomes manganese dioxide having a β-type crystal structure. The manganese dioxide contained in the iridium-containing manganese oxide of this embodiment may contain two or more manganese dioxides having different crystal structures. That is, the iridium-containing manganese oxide of this embodiment preferably contains iridium-containing β-type manganese dioxide, and more preferably iridium-containing β-type manganese dioxide.

[0017] 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) PDF (Powder Diffraction File, registered trademark). For example, reference patterns for manganese dioxide having a γ-type, β-type, ε-type, δ-type, or α-type crystal structure can be PDF Nos. 14-0644 (γ-type), 24-0735 (β-type), 30-0820 (ε-type), 80-1098 (δ-type), or 44-0141 (α-type), respectively.

[0018] In this embodiment, the XRD pattern was obtained by XRD measurement using a general powder X-ray diffractometer (for example, an Ultima IV Protectus, manufactured by Rigaku Corporation) under the following conditions: Acceleration current / voltage: 40 mA / 40 kV Radiation source: CuKα radiation (λ=1.5405 Å) Measurement mode: Continuous scan Scan conditions: 4° / min Measurement range: 2θ=10° to 80° Divergence vertical limiting slit: 10 mm Divergence / entrance slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used

[0019] In this embodiment, the XRD peak is a peak detected by identifying the 2θ of the peak top in an XRD pattern analysis using general analysis software (for example, SmartLab Studio II, manufactured by Rigaku). The analysis conditions for the XRD pattern include the following: Fitting conditions: automatic, background refinement, and distributed pseudo-Voigt function (peak shape).

[0020] The crystal structure of the manganese oxide of this embodiment 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) PDF (Powder Diffraction File, registered trademark).

[0021] In the iridium-containing manganese oxide of this embodiment, in an XRD pattern obtained using CuKα radiation (λ=1.5405 Å) as a radiation source, the full width at half maximum (hereinafter also referred to simply as "half width") of an XRD peak (hereinafter also referred to as "Mn peak") having a peak top at 2θ=28.00±1.50° preferably has a lower limit of 0.50° or more, 1.00° or more, or more than 1.90°, and preferably has an upper limit of 4.00° or less or 3.50° or less. A preferred range for the full width at half maximum of the peak appearing at 2θ=28±1.5° is more than 1.90° and 4.00° or less. When the half width of the Mn peak is within the above range, excellent oxygen evolution electrode catalytic activity is exhibited.

[0022] When the iridium-containing manganese oxide of this embodiment has a β-type manganese dioxide (tetragonal) crystal structure, the ratio of the lattice constant in the a-axis direction (hereinafter also referred to as the "lattice constant a") to the lattice constant in the c-axis direction (hereinafter also referred to as the "lattice constant c") (hereinafter also referred to as the "a / c ratio") is preferably 1.420 or more and less than 1.521. When the a / c ratio is within the above range, iridium is uniformly dissolved in the manganese oxide, and more iridium contributes to the reaction, making it easier to achieve high oxygen generating electrode catalytic activity. In terms of making it easier to achieve higher oxygen generating electrode catalytic activity, the a / c ratio is preferably 1.420 or more, 1.450 or more, or 1.480 or more, and less than 1.521, or 1.520 or less. Specifically, the combination of the upper and lower limits of the a / c ratio is preferably 1.420 or more and less than 1.521, and more preferably 1.480 or more and 1.520 or less.

[0023] When the iridium-containing manganese oxide of this embodiment has a β-type manganese dioxide (tetragonal) crystal structure, the lattice constant a is preferably 4.20 Å or more and 4.51 Å or less, and more preferably 4.30 Å or more and 4.45 Å or less. When the iridium-containing manganese oxide of this embodiment has a β-type manganese dioxide (tetragonal) crystal structure, the lattice constant c is preferably 2.70 Å or more and 3.18 Å or less, and more preferably 2.80 Å or more and 3.10 Å or less.

[0024] The lattice constant a and lattice constant c in the tetragonal crystal can be calculated from the peak position of the XRD peak (hereinafter also referred to as the "(110) peak") corresponding to the tetragonal (110) plane having a peak top at a lattice spacing d = 3.11 ± 0.11 Å in the XRD pattern, and the peak position of the XRD peak (hereinafter also referred to as the "(101) peak") corresponding to the tetragonal (101) plane having a peak top at a lattice spacing d = 2.40 ± 0.10 Å. The a / c ratio can be calculated from the obtained lattice constants a and c using the following formula: a / c ratio = lattice constant a [Å] / lattice constant c [Å] (1)

[0025] The iridium-containing manganese oxide of this embodiment contains iridium (Ir), and it is sufficient that the iridium is contained in a state in which it can interact with the manganese oxide, and it is preferable that the manganese oxide contains iridium. The state in which the manganese oxide contains iridium can be one or more states selected from the group consisting of a state in which the manganese oxide is mixed with the iridium, a state in which the manganese oxide supports the iridium, and a state in which the iridium is solid-solved in the manganese oxide, and it is preferable that at least a portion of the iridium is solid-solved in the manganese oxide.

[0026] The form of iridium contained in the iridium-containing manganese oxide of this embodiment is arbitrary, as long as it is contained in one or more forms selected from the group consisting of metals, cations, and compounds, and it is preferable that it is contained in one or more forms selected from the group consisting of metals, cations, and oxides. The iridium contained in the iridium-containing manganese oxide of this embodiment is preferably contained in the manganese oxide as at least iridium ions, and more preferably in a state where at least a portion of the iridium ions are substituted for manganese ions in the manganese oxide, that is, in a state where at least a portion of the iridium is solid-solved in the manganese oxide.

[0027] 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 in solid solution in manganese oxide, at least a portion of the iridium cations may be substituted for manganese ions of the manganese oxide.

[0028] In order to exhibit high oxygen evolution electrode catalytic activity, the iridium-containing manganese oxide 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 a part or all of the iridium in a state in which it is solid-solved in manganese oxide.

[0029] In the iridium-containing manganese oxide 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 its XRD pattern, when the area intensity of the XRD peak with the greatest area intensity is taken as 100, is 3 or less. For example, when the manganese oxide is β-type manganese dioxide, the XRD peak with the greatest area intensity is an XRD peak having a peak top at 2θ = 28 ± 1° (hereinafter also referred to as the "Mn peak").

[0030] The XRD peak having a peak top at 2θ = 35.00 ± 1.00° is considered to be the main peak of the iridium compound. Since the iridium-containing manganese oxide of this embodiment preferably contains iridium in a state of solid solution in the manganese oxide, the Ir intensity ratio is preferably 0 to 3, more preferably 0 to 1. Furthermore, the crystal structure of the manganese oxide on which iridium is supported and in which it is solid solution is prone to distortion. In this case, the half-width of the Mn peak of the manganese oxide containing iridium in a solid solution tends to be larger than the Mn peak of the manganese oxide on which iridium is supported and in which it is solid solution, and the half-width of the Mn peak of the manganese oxide containing iridium in a solid solution tends to be larger. For example, the half-width of the Mn peak in the manganese oxide on which iridium is supported and in which it is solid solution may be greater than 1.90°, 2.30° or more, or 2.50° or more, and examples thereof include greater than 1.90° and 4.0°, 2.30° or more and 3.50° or less, or 2.50° or more and 3.50° or less.

[0031] The iridium-to-manganese molar ratio (mol / mol; hereinafter also referred to as "Ir / Mn molar ratio") of the iridium-containing manganese oxide of this embodiment is 0.001 or more and 0.250 or less. If the Ir / Mn molar ratio is less than 0.001, the properties of the manganese oxide become stronger, and the oxygen generating electrode catalytic activity is significantly reduced. On the other hand, if the Ir / Mn molar ratio exceeds 0.250, iridium becomes more likely to be eluted, and the oxygen generating electrode catalytic activity is likely to be reduced. The Ir / Mn molar ratio is preferably 0.004 or more or 0.008 or more, and is preferably 0.200 or less or 0.150 or less. Also, 0.004 or more and 0.200 or less, or 0.008 or more and 0.150 or less is preferred.

[0032] The Ir / Mn 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 iridium-containing manganese oxide are measured by the ICP method, and the Ir / Mn molar ratio of the iridium-containing manganese oxide is determined from the obtained iridium content relative to the manganese content.

[0033] The "manganese content" and "iridium content" refer to the mass ratio of manganese and iridium to the mass of the iridium-containing manganese oxide, and are calculated using the following formula: Manganese content [mass %] = (W Ｍｎ / W) × 100 (2) Iridium content [mass%] = (W Ｉｒ / W)×100 (3)

[0034] In the above equation, W Ｍｎ is the mass of manganese in the iridium-containing manganese oxide [g], W Ｉｒ is the mass [g] of iridium in the iridium-containing manganese oxide, and W is the mass [g] of the iridium-containing manganese oxide determined by weighing.

[0035] In the iridium-containing manganese oxide of this embodiment, the iridium content is preferably 0.2% by mass or more or 0.8% by mass or more, and is preferably 35% by mass or less or 30% by mass or less, and is preferably 0.2% by mass or more and 35% by mass or less, or 0.8% by mass or more and 30% by mass or less, so that sufficient catalytic activity for the oxygen generation reaction is easily exhibited.

[0036] Because this makes it easier for iridium to contribute to the catalytic reaction and also makes it easier to uniformly apply the resulting anode catalyst ink to an electrolyte membrane or a conductive substrate, the D50 diameter of the iridium-containing manganese oxide of this embodiment is preferably 0.1 μm or more and 50.0 μm or less, more preferably 0.3 μm or more or 0.5 μm or more, and is also preferably 30.0 μm or less, 10.0 μm or less, or 3.0 μm or less, and more preferably 0.1 μm or more and 30.0 μm or less, 0.3 μm or more and 10.0 μm or less, or 0.5 μm or more and 3.0 μm or less.

[0037] The standard deviation in the volumetric particle size distribution of the iridium-containing manganese oxide 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.

[0038] The D16 diameter of the iridium-containing manganese oxide of this embodiment is preferably 0.05 μm or more, 0.15 μm or more, or 0.25 μm or more, and is preferably 30.00 μm or less, 20.00 μm or less, or 10.00 μm or less, and is preferably 0.05 μm or more and 30.00 μm or less, 0.15 μm or more and 20.00 μm or less, or 0.25 μm or more and 10.00 μm or less.

[0039] The D84 diameter of the iridium-containing manganese oxide of this embodiment is preferably 0.5 μm or more, 1.0 μm or more, or 1.5 μm or more, and is preferably 60.0 μm or less, 50.0 μm or less, or 30.0 μm or less, and is preferably 0.5 μm or more and 60.0 μm or less, 1.0 μm or more and 50.0 μm or less, or 1.5 μm or more and 30.0 μm or less.

[0040] In this embodiment, the "D16 diameter," "D50 diameter," and "D84 diameter" are the particle diameters [μm] corresponding to a cumulative amount of particle diameter frequency of 16%, a particle diameter [μm] corresponding to a cumulative amount of particle diameter frequency of 50%, and a particle diameter [μm] corresponding to a cumulative amount of particle diameter frequency of 84%, respectively, in an integrated volume particle size distribution obtained by a laser diffraction / scattering method. Note that D50 is used interchangeably with "median diameter."

[0041] The D16 diameter, D50 diameter, and D84 diameter of the iridium-containing manganese oxide of this embodiment may be measured using a common particle size distribution measuring device (for example, device name: MT-3100II, manufactured by Microtrac Bell) under the following conditions: Measurement range: 0.02 to 2000 μm Particle refractive index: 2.2 Particle transparency: transparent Particle shape: aspherical Solvent refractive index: 1.333 Ultrasonic pretreatment: 10 minutes

[0042] The standard deviation can be calculated from the obtained D16 diameter and D84 diameter values ​​using the following formula: Standard deviation [μm] = (D84 diameter [μm] - D16 diameter [μm]) / 2 (4)

[0043] The BET specific surface area of ​​the iridium-containing manganese oxide of this embodiment is 1 m ２ / g or more 200m２ / 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 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 / g or less is more preferable.

[0044] The BET specific surface area can be measured using a general measuring device (for example, Macsorb, manufactured by MOUNTECH) and a mixed gas of 30% by volume of nitrogen and 70% by volume of helium as the adsorption gas, by the one-point method defined 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.

[0045] The iridium-containing manganese oxide of this embodiment can be used as a catalyst containing it. When using the iridium-containing manganese oxide of this embodiment as a catalyst, it may be used as is, or it may be used as a catalyst composition containing the iridium-containing manganese oxide of this embodiment (hereinafter, these will also be collectively referred to as the "catalyst of this embodiment"). The catalyst of this embodiment can preferably be used as an oxygen generating electrode catalyst. The catalyst of this embodiment can be used as an anode catalyst (oxygen generating electrode catalyst) in a water electrolysis method using a PEM-type electrolytic cell, and is expected to have high oxygen generating electrode catalytic activity.

[0046] The catalyst composition may be an anode catalyst ink containing the iridium-containing manganese oxide of this embodiment, for example, a composition obtained by mixing the iridium-containing manganese oxide 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.

[0047] The method for producing the iridium-containing manganese oxide of this embodiment will be described below.

[0048] [Method for Producing Iridium-Containing Manganese Oxide of the Present Embodiment] The method for producing the iridium-containing manganese oxide of the present embodiment is not particularly limited as long as it can produce an iridium-containing manganese oxide having the above-mentioned configuration, but any method can be used as long as it can produce manganese oxide in which at least a portion of the manganese oxide can contain iridium. Preferably, the method includes a production method (hereinafter also referred to as the "powder method") comprising 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; and a production method (hereinafter also referred to as the "electrodeposition method") comprising an electrolysis step of electrolytically depositing manganese oxide on a conductive substrate, an iridium-containing step of contacting the manganese oxide source electrolytically deposited on the conductive substrate with an iridium salt solution to obtain a mixture and a conductive substrate, and a heat treatment step of heat-treating the mixture and the conductive substrate.

[0049] <Powder Method> A preferred method for producing the iridium-containing manganese oxide 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. By the powder method, the iridium-containing manganese oxide of this embodiment can be obtained in powder form.

[0050] Mixing step: In the powder method, the mixing step involves bringing the manganese oxide source into contact with an iridium salt solution, thereby allowing iridium to be contained in the manganese oxide source.

[0051] Manganese oxide source: The manganese oxide source is manganese dioxide (MnO ２ ), manganese tetroxide (Mn ３ O ４ ), manganese trioxide (Mn ２ O ３) group, and at least one of manganese dioxide and trimanganese tetraoxide, with manganese dioxide being more preferred. The manganese oxide source is preferably manganese dioxide having a γ-type crystal structure (hereinafter also referred to as "γ-type manganese dioxide"), since high oxygen evolution electrode catalytic activity is likely to be exhibited when iridium is contained therein.

[0052] When the manganese oxide source is γ-manganese dioxide, the crystallite diameter of the γ-manganese dioxide is preferably 2.4 nm or more and 4.0 nm or less. When the crystallite diameter of the γ-manganese dioxide is within the above range, the structure of the β-manganese dioxide is more likely to be distorted in the c-axis direction in the iridium-containing manganese oxide obtained in the heat treatment step described below, which tends to reduce the a / c ratio and, as a result, is expected to result in higher oxygen generating electrode catalytic activity. The crystallite diameter of the γ-manganese dioxide is more preferably 2.6 or more and 3.5 or less, and even more preferably 2.7 or more and 3.2 or less.

[0053] In this embodiment, the "crystallite diameter of γ-type manganese dioxide" can be determined using the following Scherrer equation from the half-width of the XRD peak having a peak top at 2θ=22.5±1.5° in an XRD pattern of γ-type manganese dioxide when the radiation source is CuKα radiation (λ=1.5405 Å): Crystallite diameter [nm]=Scherrer constant [−]×wavelength of radiation source λ [nm]×0.1 / (half-width [rad]×cos θ [rad]) (5) Here, the Scherrer constant is 0.879, and θ is the Bragg angle.

[0054] The D50 diameter of the manganese oxide source is preferably 0.05 μm or more and 50.00 μm or less, preferably 0.07 μm or more or 0.10 μm or more, and also preferably 30.00 μm or less, 10.00 μm or less, or 5.00 μm or less, and preferably 0.05 μm or more and 30.00 μm or less, 0.07 μm or more and 10.00 μm or less, or 0.10 μm or more and 3.00 μm or less. When the D50 diameter of the manganese oxide source is within the above range, it becomes easier to obtain an iridium-containing manganese oxide having the above-mentioned D50 diameter in the heat treatment step described below.

[0055] The D16 diameter of the manganese oxide source is preferably 0.05 μm or more, 0.15 μm or more, or 0.25 μm or more, and is preferably 30.00 μm or less, 20.00 μm or less, or 10.00 μm or less, and more preferably 0.05 μm or more to 30.00 μm or less, 0.15 μm or more to 20.00 μm or less, or 0.25 μm or more to 10.00 μm or less.

[0056] The D84 diameter of the manganese oxide source is preferably 0.5 μm or more, 1.0 μm or more, or 1.5 μm or more, and is preferably 60.0 μm or less, 50.0 μm or less, or 30.0 μm or less, and more preferably 0.5 μm or more and 60.0 μm or less, 1.0 μm or more and 50.0 μm or less, or 1.5 μm or more and 30.0 μm or less.

[0057] The powder method may include a grinding step of grinding the manganese oxide source prior to the mixing step in order to adjust the D50 diameter, D16 diameter, and D84 diameter of the manganese oxide source to the above-mentioned values. The manganese oxide source can be ground by a known method, such as wet grinding or dry grinding.

[0058] 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, more preferably an aqueous solution containing an iridium salt, and more preferably an aqueous solution of an iridium salt. Examples of iridium salts include iridium (III) chloride (IrCl ３ ), iridium(IV) chloride (IrCl ４ ) and iridium nitrate (Ir(NO ３ ) ４ ), and iridium(III) chloride (IrCl ３ The iridium salt aqueous solution may contain one kind of iridium salt or two or more kinds of iridium salts.

[0059] The iridium concentration of the iridium salt solution is preferably 0.001 g / L or more and 20 g / L or less.

[0060] 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. It is believed that the iridium content on the surface of the manganese oxide can be controlled by keeping the contact time and contact temperature within the above ranges. That is, the higher the contact temperature and the longer the contact time, the more likely the iridium content of the manganese oxide tends to 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, it is believed that the higher the contact temperature, the more easily iridium substitutes for manganese in the manganese oxide and forms a solid solution, and at least a portion of the iridium is more likely to 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.

[0061] Heat Treatment Step: In the powder method, the mixture obtained in the mixing step is heat-treated in the heat treatment step. 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 atmosphere in the heat treatment step is preferably an air atmosphere or an inert atmosphere, more preferably an air atmosphere, and the heat treatment temperature is preferably 100°C or higher and 600°C or lower. The heat treatment time may be set appropriately depending on the amount of manganese oxide, etc., to be subjected to the heat treatment, and may be, for example, 10 minutes to 24 hours.

[0062] By the powder method, it is possible to obtain an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, preferably an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, wherein the manganese oxide is a β-type manganese dioxide, and more preferably an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, wherein the manganese oxide is a β-type manganese dioxide and wherein the a / c ratio is 1.420 or more and less than 1.521.

[0063] The iridium-containing manganese oxide obtained by the powder method is preferably in the form of a powder.

[0064] <Electrodeposition Method> Another preferred method for producing the iridium-containing manganese oxide of this embodiment includes an electrolysis step of electrolytically depositing manganese oxide onto a conductive substrate, an iridium-containing step of contacting the manganese oxide source electrolytically deposited on the conductive substrate with an iridium salt solution to obtain a mixture and a conductive substrate, and a heat treatment step of heat-treating the mixture and the conductive substrate. This allows the iridium-containing manganese oxide of this embodiment to be obtained in a state where it coats at least a portion of the conductive substrate.

[0065] Electrolysis step: In the electrodeposition method, manganese oxide is electrolytically deposited on a conductive substrate in the electrolysis step, thereby obtaining manganese oxide electrolytically deposited on the conductive substrate (hereinafter also referred to as "electrodeposited manganese oxide source").

[0066] Electrolytic Method Any electrolytic deposition method may be used as long as the manganese oxide source is electrolytically deposited on the surface of the conductive substrate, and any method may be used as long as the conductive substrate is immersed in an electrolyte and electrolyzed.

[0067] The conductive substrate to be subjected to the electrolysis step may have a thickness of less than 500 μm, preferably 300 μm or less. The conductive substrate may be made of a conductive material, preferably containing titanium.

[0068] The shape of the conductive substrate can be one or more selected from the group consisting of mesh, cloth, and plate, and mesh is preferred because it tends to increase the oxygen generating electrode catalytic activity. Specific examples of the conductive substrate include a titanium mesh made of fibrous or powdered conductive titanium metal, and a sintered titanium mesh obtained by heat-treating the titanium mesh. In order to easily achieve high oxygen generating electrode catalytic properties, it is preferred that the surface of these conductive substrates be coated with platinum, and a platinum-coated titanium mesh, or even a platinum-coated sintered titanium mesh, is more preferred.

[0069] The electrodeposited manganese oxide source is preferably manganese dioxide having a δ-type structure (hereinafter also referred to as "δ-type manganese dioxide"). This makes it easier to obtain, in the iridium-containing step described below, iridium-containing manganese oxide containing iridium and γ-type manganese dioxide having a crystallite diameter of 2.4 nm to 4.0 nm.

[0070] The electrolytic solution may be a solution containing manganese, as long as an electrolytic manganese oxide source is electrolytically deposited on the surface of the conductive substrate. The manganese concentration of the electrolytic solution is preferably 0.05 mol / L or more and 1.00 mol / L or less. The electrolytic solution is preferably a solution containing manganese sulfate, or more preferably a manganese sulfate aqueous solution. In this case, the sulfuric acid concentration is preferably 0.05 mol / L or more and 1.00 mol / L or less, and the manganese concentration is preferably 0.05 mol / L or more and 1.00 mol / L or less.

[0071] The electrolytic solution preferably contains an ammonium salt. By including an ammonium salt, δ-type manganese dioxide is easily obtained as an electrolytic manganese oxide source. Ammonium ions have a template effect for forming a layered structure of δ-type manganese dioxide. Furthermore, by incorporating ammonium ions between the layers of δ-type manganese dioxide, crystal growth of δ-type manganese dioxide can be directed in a specific plane direction. As a result, in the iridium-containing step described below, the iridium-containing manganese oxide contains iridium and γ-type manganese dioxide having a crystallite diameter of 2.4 nm to 4.0 nm is easily obtained. In this case, the ammonium salt concentration of the electrolytic solution may be 0.1 mol / L to 3.0 mol / L. The ammonium salt may be one or more selected from the group consisting of ammonium sulfate, ammonium nitrate, and ammonium chloride, with ammonium sulfate being preferred.

[0072] In the electrolytic deposition, the conductive substrate is immersed in an electrolyte and electrolyzed. The electrolytic deposition conditions may be any conditions that allow the electrolytic deposition of manganese oxide, and the current density per geometric area of ​​the conductive substrate is 0.3 mA / cm. ２ 20mA / cm or more ２ and the electrolysis temperature is 93°C or higher and 98°C or lower.

[0073] Iridium-containing step: In the electrodeposition method, the iridium-containing step involves contacting the conductive substrate and the manganese oxide electrolytically deposited on the conductive substrate (hereinafter referred to as the "manganese oxide-containing substrate") with an iridium salt solution. The manganese oxide (electrodeposited manganese oxide source) in the manganese oxide-containing substrate obtained in the electrolysis step is in a state of being adhered to and deposited on the conductive substrate, and therefore the iridium-containing step provides a mixture in which iridium is contained in the electrodeposited manganese oxide source (hereinafter also referred to as the "iridium-containing electrolytic manganese oxide source"). This results in the mixture and the conductive substrate (hereinafter also referred to as the "iridium-manganese oxide-containing substrate").

[0074] The iridium salt solution used in the iridium-containing step may be any solution containing an iridium salt, and is preferably a solution containing an iridium salt and sulfuric acid, or more preferably an aqueous solution containing an iridium salt and sulfuric acid. Examples of iridium salts include iridium(III) chloride (IrCl), iridium(IV) chloride (IrCl), iridium nitrate (Ir(NO)), and the like. ３ ) ４ ), potassium hexachloroiridate (K ２ IrCl ６ ) and hexachloroiridic acid (H ２ IrCl ６ ) The group of the present invention can be exemplified by one or more selected from the following.

[0075] When the electrolytic manganese oxide source is δ-type manganese dioxide, δ-type manganese dioxide has cation exchange properties and is easily reactive with δ-type manganese dioxide. Therefore, the iridium salt is preferably an iridium salt in which iridium dissociates as a cation in the solution, and specifically, IrCl3, IrCl4, and Ir(NO ３ ) ４ Preferably, one or more selected from the group consisting of the above is used, and IrCl3 is more preferred.

[0076] The iridium concentration of the iridium salt solution is preferably 0.001 mmol / L or more and 5.000 mmol / L or less.

[0077] The manganese oxide-containing substrate can be brought into contact with the iridium salt solution by immersing the manganese oxide-containing substrate in the iridium salt solution. The contact conditions are not particularly limited as long as the surface of the manganese oxide-containing substrate is impregnated with iridium, and examples of the contact temperature include 20° C. or higher and 100° C. or lower. The contact time can be adjusted appropriately depending on the size of the manganese oxide-containing substrate, and can be, for example, 30 minutes or longer and 24 hours or shorter.

[0078] The mixture in which iridium is contained in the electrodeposited manganese oxide source (hereinafter also referred to as "iridium-containing electrolytic manganese oxide source") is preferably γ-type manganese dioxide containing iridium, and more preferably γ-type manganese dioxide containing iridium and having a crystallite diameter of 2.4 nm to 4.0 nm. By using δ-type manganese dioxide as the electrodeposited manganese oxide source, when iridium is added to it, the crystal structure can be changed to γ-type manganese dioxide containing iridium.

[0079] Heat Treatment Step: In the electrodeposition method, the iridium-manganese oxide-containing substrate is heat-treated in the heat treatment step. This increases the adhesion between the iridium-containing manganese oxide and the conductive substrate. The heat treatment atmosphere in the heat treatment step can be an air atmosphere or an inert atmosphere, or even an air atmosphere, and the heat treatment temperature can be 100°C or higher and 600°C or lower. The heat treatment time can be adjusted appropriately depending on the size of the conductive substrate to be subjected to the heat treatment, and can be, for example, 10 minutes to 24 hours.

[0080] By the electrodeposition method, it is possible to obtain an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.300 or less, preferably an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.300 or less, a β-type crystal structure, and an a / c ratio of 1.420 or more and less than 1.521, and further an electrode including a catalyst containing the same (the catalyst of the present embodiment) and a conductive base material.

[0081] The iridium-containing manganese oxide obtained by electrodeposition is a production method in which iridium-containing manganese oxide is electrolytically deposited on a conductive substrate, and therefore it is possible to directly produce an electrode containing the iridium-containing manganese oxide of this embodiment and a conductive substrate (hereinafter referred to as the "electrode of this embodiment"), and further an electrode containing the catalyst of this embodiment and a conductive substrate.

[0082] The iridium-containing manganese oxide of this embodiment obtained by the powder method and the electrodeposition method can both be used as a catalyst containing it (the catalyst of this embodiment), and can preferably be used as an oxygen generating electrode catalyst.

[0083] The catalyst of this embodiment can be used as an anode catalyst for an MEA in a water electrolysis method using a PEM-type electrolytic cell. An MEA may be constructed using the iridium-containing manganese oxide obtained by the powder method as the anode catalyst for a CCM, or an MEA may be constructed using an electrode obtained by the electrodeposition method and a CCM consisting only of a cathode catalyst and an electrolyte membrane. In either case, the MEA obtained by either manufacturing method can exhibit significantly higher oxygen evolution electrode catalytic activity than conventional iridium-containing manganese oxides.

[0084] By the electrolysis method, it is possible to obtain, on a conductive substrate, an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, preferably an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, wherein the manganese oxide is β-type manganese dioxide, and more preferably an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, wherein the manganese oxide is β-type manganese dioxide and wherein the a / c ratio is 1.420 or more and less than 1.521.

[0085] The iridium-containing manganese oxide obtained by the electrolysis method is preferably in the form of a catalyst containing the iridium-containing manganese oxide and an electrode containing a conductive substrate.

[0086] Electrode: The electrode of this embodiment can be used as the anode catalyst and conductive substrate of an MEA in a water electrolysis method using a PEM-type electrolytic cell. In this case, an MEA can be constructed by combining the electrode of this embodiment with a CCM consisting only of a cathode catalyst and an electrolyte membrane (i.e., a CCM without an anode catalyst).

[0087] The electrode of the present embodiment comprises a catalyst including an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, preferably an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, wherein the manganese oxide is a β-type manganese dioxide, more preferably an iridium-containing manganese oxide having an Ir / Mn molar ratio of 0.001 or more and 0.250 or less, wherein the manganese oxide is a β-type manganese dioxide and wherein the a / c ratio is 1.420 or more and less than 1.521, and a conductive substrate.

[0088] The metal content per geometric area of ​​the electrode of this embodiment (hereinafter also referred to as the "metal content of the electrode") is 0.1 mg / cm ２ Above, 0.5mg / cm ２ or more, or 0.8 mg / cm ２ It is preferable that the concentration is 12.0 mg / cm or more. ２ Below, 11.0mg / cm ２ or less, or 10.0 mg / cm ２ Preferably, 0.1 mg / cm or less ２ 12.0mg / cm or more ２ Below, 0.5mg / cm ２ 11.0mg / cm or more ２ Below, 0.8mg / cm ２ 10.0mg / cm or more ２ The metal content of the electrode is 0.1 mg / cm ２ By satisfying the above conditions, the coverage of the iridium-containing manganese oxide on the conductive substrate is improved, and high oxygen evolution electrode catalytic activity is more likely to be exhibited.

[0089] On the other hand, the metal content of the electrode is 12.0 mg / cm ２ When the metal content of the electrode is less than or equal to 1000 W, the electrical resistance of the electrode is reduced, and as a result, high oxygen evolution electrode catalytic activity is easily exhibited. The metal content of the electrode of this embodiment can be calculated using the following formula: ２ ] = (manganese content of electrode [mg] + iridium content of electrode [mg]) / geometric area of ​​electrode [cm ２ ] (6)

[0090] The manganese content and iridium content of the electrode can be determined by the ICP method in the same way as the measurement of the manganese content and iridium content of the iridium-containing manganese oxide. ２ The electrode is impregnated with the catalyst to dissolve the catalyst contained in the electrode. The mixed solution in which the catalyst is dissolved is used as a measurement sample, and the manganese content and iridium content of the measurement sample are determined by ICP method, and these are taken as the manganese content and iridium content of the electrode.

[0091] The "geometric area" is the area equivalent to the projected area without taking into consideration irregularities or voids on the surface, and is the projected area of ​​the surface (geometric surface) that faces the electrolyte membrane when constituting a membrane-electrode assembly (hereinafter also referred to as "MEA"). The "geometric area of ​​an electrode" is the projected area of ​​an electrode, and is the area of ​​a plane calculated by multiplying the length and width when the shape of the electrode is defined as length, width, and width.

[0092] The molar ratio of iridium to manganese per geometric area of ​​the electrode of this embodiment (hereinafter also referred to as the "Ir / Mn molar ratio of the electrode") may be the same as the Ir / Mn molar ratio of the iridium-containing manganese oxide of this embodiment.

[0093] Furthermore, the crystal structure, a / c ratio, lattice constant a, lattice constant c, Ir / Mn molar ratio, manganese content, and iridium content of the iridium-containing manganese oxide contained in the electrode of this embodiment are similar to those of the iridium-containing manganese oxide of this embodiment, and therefore description thereof will be omitted.

[0094] The conductive substrate may be any substrate made of a conductive material, and is preferably a substrate containing titanium.

[0095] The shape of the conductive substrate can be one or more selected from the group consisting of mesh, cloth, and plate, and mesh is preferred because it tends to increase the oxygen generating electrode catalytic activity. Specific examples of the conductive substrate include a titanium mesh made of fibrous or powdered conductive titanium metal, and a sintered titanium mesh obtained by heat-treating the titanium mesh. In order to easily achieve high oxygen generating electrode catalytic properties, it is preferred that the surface of these conductive substrates be coated with platinum, and a platinum-coated titanium mesh, or even a platinum-coated sintered titanium mesh, is more preferred.

[0096] When the surface of the conductive substrate is coated with platinum, the amount of platinum per geometric area is 0.5 mg / cm 2 in order to exhibit high conductivity. ２ 10.0mg / cm or more ２ Preferably, 1.0 mg / cm or less ２ 8.0mg / cm or more ２ More preferably, 1.0 mg / cm or less ２ 6.0mg / cm or more ２ The following is even more preferred:

[0097] The thickness of the conductive substrate is preferably 50 μm or more and 500 μm or less, more preferably 100 μm or more and 400 μm or less, and even more preferably 150 μm or more and 300 μm or less, in order to exhibit excellent oxygen evolution electrode catalytic activity.

[0098] The "thickness" refers to the length corresponding to the length in the direction perpendicular to the geometric surface. The "thickness of the conductive substrate" refers to the length corresponding to the length in the direction perpendicular to the geometric surface of the conductive substrate, and is the length corresponding to the width when the shape of the electrode is defined as length x width x depth.

[0099] The porosity of the conductive substrate is preferably 30% to 80%, more preferably 40% to 70%. Here, the porosity is defined as the volume of the space occupied by the conductive substrate and the conductive substrate is not present in the volume of the conductive substrate. The porosity can be measured by, for example, mercury plethysmography. When the porosity is within the above range, excellent effects can be obtained in that the mechanical strength of the electrode is increased and water, which is a reactive gas for oxygen generation, can be smoothly supplied.

[0100] In this embodiment, the porosity can be calculated by the following formula: Porosity (%) = {1 - (bulk density of conductive substrate (g / cm ３ ) / skeletal density of conductive substrate (g / cm ３ ))) × 100

[0101] The bulk density and skeletal density of the conductive substrate are values ​​that can be calculated using the following formulas from the pore volume measured by a mercury pressure method using a general mercury porosimeter (device name: AutoPore 9510, manufactured by Micromeritics) and the volume of the conductive substrate. ３ ) = Mass of conductive substrate (g) / Volume of conductive substrate (cc) Skeleton density of conductive substrate (g / cm ３ ) = mass of conductive substrate (g) / {volume of conductive substrate - pore volume} (cc)

[0102] In the above two equations, the volume of the conductive substrate is a value that can also be determined by measurement with a laser volume meter (device name: 3D scanner type three-dimensional measuring machine VL-700 series (controller VL-700, stage VL-750 / VL-C35, measurement unit VL-770), manufactured by Keyence Corporation), and the mass of the conductive substrate is a value that can be determined by mass measurement using an electronic balance.

[0103] Specific measurement conditions for the mercury porosimeter include the following: Sample amount: 40 mg Mercury introduction pressure: 601.6 psia to 36,098.1 psia (4.1 MPa to 248.9 MPa) Measurement pore size: 5 nm to 500 μm Cell used: 5.3 cc glass cell with indentation volume Mercury surface tension: 480 dyn Mercury contact angle: 130° Pretreatment conditions: Degassing treatment in air at 110°C for 1 hour or more

[0104] [Method for manufacturing the electrode of the present embodiment] When the iridium-containing manganese oxide of the present embodiment is an iridium-containing manganese oxide obtained by electrodeposition, the iridium-containing manganese oxide is integrated with the conductive substrate, and therefore, this may be used as the electrode of the present embodiment as is.

[0105] On the other hand, when the iridium-containing manganese oxide of this embodiment is obtained by a powder method, the electrode of this embodiment can be obtained by forming the iridium-containing manganese oxide into a catalyst ink and applying the catalyst ink to a conductive substrate. The catalyst ink can be in a state where the Ir—Mn oxide is dispersed in a solvent, and the solvent can be water, alcohol, or a mixture of water and alcohol.

[0106] The present invention will be described in detail below with reference to examples, but the present invention is not limited to the examples shown below.

[0107] <Composition Analysis> The prepared sample was dissolved in a mixed solution of hydrochloric acid and nitric acid, and the manganese content and iridium content of the prepared sample were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a general ICP device (device name: Optima 830, manufactured by PerkinElmer). The Ir / Mn molar ratio of the prepared sample was calculated from the obtained manganese content and iridium content.

[0108] <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 measuring device (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 transparency: transparent Particle shape: non-spherical Solvent refractive index: 1.333 Ultrasonic pretreatment: 10 minutes

[0109] From the obtained volumetric particle size distribution, the D16 diameter [μm], D50 diameter (median diameter) [μm], and D84 diameter [μm] of the prepared sample were obtained. In addition, the standard deviation was calculated from the obtained D16 diameter and D84 diameter values ​​using the above formula (4).

[0110] <BET Specific Surface Area> The BET specific surface area of ​​the prepared sample was determined by measurement in accordance with JIS Z 8830:2013. That is, 0.3 g of the prepared sample was placed in a glass cell for BET specific surface area measurement, and dehydration treatment was performed in a nitrogen flow atmosphere at 150°C for 20 minutes as pretreatment. Using a general BET specific surface area measurement device (product name: Macsorb (registered trademark), manufactured by MOUNTECH), the BET specific surface area of ​​the prepared sample after pretreatment was measured by the single-point method using a mixed gas of 30% by volume nitrogen and 70% by volume helium as the adsorption gas.

[0111] <Full width at half maximum (FWHM)> An XRD pattern was obtained using a general powder X-ray diffractometer (device name: Ultima IV Protectus, manufactured by Rigaku Corporation) under the following conditions: Acceleration current / voltage: 40 mA / 40 kV Radiation source: CuKα radiation (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 / entrance slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used

[0112] The crystalline phase of the prepared sample was identified by comparing the obtained XRD pattern with a reference pattern. The reference patterns were PDF No. 14-0644 (γ-type MnO ２ ), 24-0735 (β-type MnO ２ ), 30-0820 (ε-type MnO ２ ), 80-1098 (δ type) or 44-0141 (α type MnO ２ ) was used.

[0113] The XRD pattern of the prepared sample was analyzed using 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 ± 1.5° was determined and used as the half width (°).

[0114] The background of the obtained XRD pattern was removed using the analysis software (product name: PDXL2, manufactured by Rigaku Corporation) attached to the powder X-ray diffractometer. Then, the interplanar spacing d [Å] was calculated using the diffraction angles [°] of the peak tops of the (110) peak and the (101) peak and the Bragg equation, and the lattice constants a [Å] and c [Å] in the tetragonal crystal were calculated. The a / c ratio was calculated from the obtained lattice constants a and c using the above formula (1).

[0115] Example 1: <Preparation of iridium-containing manganese oxide powder> Electrolytic manganese dioxide (product name: HMR-AF, manufactured by Tosoh Corporation) was pulverized in a jet mill to produce a manganese oxide powder having a median diameter of 0.5 μm and a BET specific surface area of ​​47 m ２ / g of manganese dioxide powder (manganese oxide source) was obtained. The manganese dioxide was γ-type manganese dioxide with a crystallite diameter of 3.0 nm.

[0116] Potassium hexachloroiridate (K ２ IrCl ６ ) K concentration is 1.60 g / L ２ IrCl ６ Aqueous solution 33.2mL (K ２ IrCl ６ 500 mg of the manganese dioxide powder was immersed in an iridium salt solution bath filled with iridium (53 mg in total) at 95°C for 48 hours, followed by solid-liquid separation to obtain a mixture. The obtained mixture was dried in the air at 90°C for 2 hours, then allowed to cool to room temperature by natural cooling, and then annealed in the air at 450°C for 5 hours to obtain an iridium-containing manganese oxide powder.

[0117] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-value width of the Mn peak of 2.12, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0118] Example 2: K ２ IrCl ６ K with a concentration of 3.98 g / L ２ IrCl６ Aqueous solution 33.2mL (K ２ IrCl ６ Content: 132 mg ２ IrCl ６ An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that the solution was used as an aqueous solution.

[0119] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-width of the Mn peak of 2.40, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0120] Example 3: K ２ IrCl ６ K with a concentration of 9.97 g / L ２ IrCl ６ Aqueous solution 33.2mL (K ２ IrCl ６ Content: 331 mg ２ IrCl ６ An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that the solution was used as an aqueous solution.

[0121] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-value width of the Mn peak of 2.99, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0122] Example 4: K ２ IrCl ６ K with a concentration of 19.91 g / L ２ IrCl ６ Aqueous solution 33.2mL (K ２ IrCl ６ Content: 661 mg ２ IrCl ６ An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that the solution was used as an aqueous solution.

[0123] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-value width of the Mn peak of 2.99, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0124] Example 5: Instead of the crushed manganese dioxide, a powder having a γ-type crystal structure, a crystallite diameter of 2.5 nm, a median diameter of 2.3 μm, and a BET specific surface area of ​​45 m ２ / g electrolytic manganese dioxide powder (product name: FM, manufactured by Tosoh Corporation) was used. ２ IrCl ６ K with a concentration of 2.504 g / L ２ IrCl ６ 99.8 mL of aqueous solution (K ２ IrCl ６ An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that the manganese dioxide powder was immersed for 24 hours in an iridium salt solution bath filled with iridium salt (250 mg in total).

[0125] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-width of the Mn peak of 2.20, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0126] Example 6: Instead of the crushed manganese dioxide, a powder having a γ-type crystal structure, a crystallite diameter of 2.5 nm, a median diameter of 2.3 μm, and a BET specific surface area of ​​45 m ２ / g electrolytic manganese dioxide powder (product name: FM, manufactured by Tosoh Corporation) was used. ２ IrCl ６ K with a concentration of 2.504 g / L ２ IrCl ６ 99.8 mL of aqueous solution (K ２ IrCl ６An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that the manganese dioxide powder was immersed for 96 hours in an iridium salt solution bath filled with iridium salt (250 mg in total).

[0127] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-width of the Mn peak of 2.91, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0128] Example 7: Manganese dioxide having a median diameter of 7.9 μm and a BET specific surface area of ​​36 m ２ / g electrolytic manganese dioxide powder (product name: LM-10P, manufactured by Tosoh Corporation) was used. ２ IrCl ６ K with a concentration of 20.00 g / L ２ IrCl ６ Aqueous solution 13.9mL (K ２ IrCl ６ An iridium-containing manganese oxide powder was obtained in the same manner as in Example 1, except that the manganese dioxide powder was immersed for 96 hours in an iridium salt solution bath filled with iridium salt (278 mg in total).

[0129] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-value width of the Mn peak of 2.52, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0130] Example 8: 2 kg of electrolytic manganese dioxide raw material powder (product name: HMR-AF, manufactured by Tosoh Corporation) was mixed with pure water to prepare a slurry with a solids concentration of 20% by mass. The slurry was then pulverized for 3 hours using zirconia media with a diameter of 0.5 mm (device name: Dyno Mill, manufactured by Shinmaru Enterprises) at a filling rate of 80% by volume and a peripheral speed of 14 m / sec. After pulverization, the mixture was dried overnight at 90°C in an air atmosphere to obtain electrolytic manganese dioxide powder. The electrolytic manganese dioxide was a γ-type manganese dioxide with a crystallite diameter of 3.0 nm and a D50 diameter of 0.4 μm.

[0131] IrCl ３ Aqueous solution (IrCl ３ 500 mg of electrolytic manganese dioxide powder was immersed at 95°C for 24 hours in an iridium salt solution bath filled with 68.6 mL of iridium salt (concentration: 0.005 mol / L), followed by solid-liquid separation to obtain a mixture. The obtained mixture was dried in an air atmosphere at 90°C for 2 hours and then allowed to cool naturally to room temperature. Annealing treatment was then performed in an air atmosphere at 450°C for 5 hours to obtain the iridium-containing manganese oxide powder of this example. The XRD pattern of this powder revealed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.51, indicating that it was iridium-containing β-type manganese dioxide containing at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0132] Example 9: Ir(NO ３ ) ４ Aqueous solution (Ir(NO) ３ ) ４ An iridium-containing manganese oxide powder of this example was obtained in the same manner as in Example 8, except that 68.6 mL of an iridium salt solution (concentration: 0.005 mol / L) was used. The XRD pattern of this powder revealed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.45, indicating that it was iridium-containing β-type manganese dioxide, containing at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0133] Example 10: Ir(NO ３ ) ４ Aqueous solution (Ir(NO) ３ ) ４ An iridium-containing manganese oxide powder of this example was obtained in the same manner as in Example 8, except that 114.3 mL of an iridium salt solution (concentration: 0.005 mol / L) was used. The XRD pattern of this powder revealed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.77, indicating that it was iridium-containing β-type manganese dioxide, containing at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0134] Example 11: IrCl ４ Aqueous solution (IrCl ４ The iridium-containing manganese oxide powder of this example was obtained in the same manner as in Example 8, except that 45.7 mL of an iridium salt solution (concentration: 0.004 mol / L) was used. The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.70, indicating that it was iridium-containing β-type manganese dioxide, containing at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0135] Example 12: IrCl ４ Aqueous solution (IrCl ４ An iridium-containing manganese oxide powder of this example was obtained in the same manner as in Example 8, except that 96.0 mL of an iridium salt solution (concentration: 0.004 mol / L) was used. The XRD pattern of this powder revealed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.35, indicating that it was iridium-containing β-type manganese dioxide, containing at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0136] Example 13: IrCl ４ Aqueous solution (IrCl４ The iridium-containing manganese oxide powder of this example was obtained in the same manner as in Example 8, except that 160.0 mL of an iridium salt solution (concentration: 0.004 mol / L) was used. The XRD pattern of this powder revealed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a Mn peak half-width of 2.19, indicating that it was iridium-containing β-type manganese dioxide, containing at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0137] Example 14: An electrolytic cell was filled with a sulfuric acid-manganese sulfate-ammonium sulfate mixed solution having a sulfuric acid concentration of 0.3 mol / L, a manganese sulfate concentration of 0.5 mol / L, and an ammonium sulfate concentration of 1.5 mol / L, and a conductive substrate made of a platinum-coated Ti fiber sintered body (product name: Pt-plated Ti fiber sintered body, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was immersed in the solution. The conductive substrate was subjected to a current density of 7 mA / cm. ２ A current was applied for 10 minutes at 40°C to electrochemically deposit manganese oxide onto the conductive substrate. The manganese oxide was δ-type manganese dioxide.

[0138] Next, iridium(III) chloride (IrCl ３ The conductive substrate on which the manganese oxide obtained above had been precipitated was immersed in an iridium salt solution containing 0.1 mmol / L of sodium hydroxide and 0.01 mol / L of sulfuric acid at 95°C for 24 hours to obtain a mixture and a conductive substrate. The mixture was a γ-type manganese dioxide containing iridium with a crystallite diameter of 3.1 nm. The mixture and the conductive substrate were annealed in an air atmosphere at 450°C for 5 hours to obtain the iridium-containing manganese oxide of this example precipitated on the conductive substrate.

[0139] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-value width of the Mn peak of 2.03, and therefore it was found to be iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a state of solid solution in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0140] Example 15: Manganese oxide was electrochemically deposited on a conductive substrate in the same manner as in Example 14, except that the electrolytic cell was filled with a sulfuric acid-manganese sulfate mixed solution having a sulfuric acid concentration of 0.36 mol / L and a manganese sulfate concentration of 0.36 mol / L. The manganese oxide was γ-type manganese dioxide.

[0141] Next, potassium hexachloroiridate (K ２ IrCl ６ The conductive substrate on which the manganese oxide obtained above had been precipitated was immersed in an iridium salt solution containing 0.1 mmol / L of sodium hydroxide and 0.01 mol / L of sulfuric acid at 95°C for 24 hours to obtain a mixture and a conductive substrate. The mixture was γ-type manganese dioxide containing iridium and had a crystallite diameter of 3.8 nm. The mixture and the conductive substrate were annealed in air at 450°C for 5 hours to obtain the iridium-containing manganese oxide of this example precipitated on the conductive substrate.

[0142] The XRD pattern of this powder showed that it had a β-type crystal structure, an Ir intensity ratio of 0, and a half-value width of the Mn peak of 2.29, indicating that it was iridium-containing β-type manganese dioxide, which contains at least a portion of iridium in a solid solution state in manganese oxide. The evaluation results of this iridium-containing β-type manganese dioxide are shown in the table below.

[0143] Comparative Example 1: Electrolytic manganese dioxide (product name: HMR-AF, manufactured by Tosoh Corporation) was pulverized to a median diameter of 0.5 μm and a BET specific surface area of ​​47 m ２ / g of manganese dioxide powder (manganese oxide source) was obtained.

[0144] 500 mg of the obtained manganese dioxide powder (manganese oxide source) was annealed in an air atmosphere at 450°C for 5 hours to obtain a β-type manganese dioxide powder, which was the powder of this comparative example. The half-width of the Mn peak was 2.06. The powder also had a D16 diameter of 0.5 μm, a D50 diameter of 0.7 μm, and a D84 diameter of 1.9 μm, with a standard deviation of 0.7 μm. The evaluation results of the powder are shown in the table below.

[0145]

[0146]

[0147]

[0148] Measurement Example 1: <Evaluation of oxygen generating electrode catalyst> A membrane-electrode assembly (hereinafter also referred to as "MEA") including the iridium-containing manganese oxide of each of the Examples and Comparative Examples as an anode catalyst, and a PEM-type water electrolyzer including the MEA were produced by the following method.

[0149] Preparation of PEM-type water electrolysis cells using Examples 1 to 13 and Comparative Example 1: A solution containing water, ethanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich) was mixed with a 20 mass % platinum-supported carbon catalyst (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich) to prepare a cathode catalyst ink. 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 had a pH of 10.0 or higher, and then 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, pure water for 1 hour, and 1 M sulfuric acid solution for 1 hour, and then protonated to prepare an electrolyte membrane.

[0150] Furthermore, the iridium-containing manganese oxides of Examples 1 to 13 and Comparative Example 1 were mixed with a solution containing water, isopropanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich) to prepare anode catalyst inks. The mass ratio of water:isopropanol:ionomer in the anode catalyst inks was 55.9:43.8:0.3. This was used to prepare anode catalysts with 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.

[0151] 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 the laminate was then pressed using a hot press (product name: SA-302, manufactured by Tester Sangyo Co., Ltd.) at 140°C and a clamping force of 80 kg / cm. ２ The PTFE sheet was then peeled off to obtain a catalyst-coated electrolyte membrane.

[0152] The anode current collector, catalyst-coated electrolyte membrane, and cathode electrode were stacked in this order so that the platinum-coated Ti fiber sintered body (product name: Pt-plated Ti fiber sintered body, manufactured by Tanaka Kikinzoku Kogyo K.K.; hereinafter also referred to as "anode current collector") faced the anode catalyst-coated surface of the catalyst-coated electrolyte membrane, and the catalyst-free surface of the catalyst-coated electrolyte membrane faced the cathode catalyst-coated surface of the cathode electrode. The resulting stack was then heated at 130°C and a clamping force of 50 kg / cm using a hot press. ２ The resulting MEA was attached to a housing of a PEM water electrolysis cell (product name: WE-4S-RICW, manufactured by FC Development Co., Ltd.) to prepare a PEM water electrolysis cell.

[0153] Fabrication of PEM-type water electrolytic cells using Examples 14 and 15: The conductive substrates provided with the iridium-containing manganese oxides of Examples 14 and 1 were used as working electrodes (anodes), respectively.

[0154] A conductive catalyst ink was prepared by mixing a 20% by mass platinum-supported carbon catalyst (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich) with a solution containing water, ethanol, and an ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich). This ink was then applied to carbon paper (product name: TGP-H-060, manufactured by Toray Industries, Inc.) and air-dried to form a counter electrode.

[0155] Next, a Nafion membrane (product name: Nafion 115, manufactured by Sigma-Aldrich) was washed and protonated by boiling in 3 mass % hydrogen peroxide solution for 1 hour, pure water for 1 hour, 1 M sulfuric acid aqueous solution for 1 hour, and pure water for 1 hour in this order, to prepare an electrolyte membrane.

[0156] The electrolyte membrane was sandwiched between the catalytic surfaces of the working electrode (anode) and the counter electrode, and the mixture was heated at 135°C with a clamping force of 400 kg / cm using a hot press (product name: SA-302, manufactured by Tester Sangyo Co., Ltd.). ２ The resulting MEA was attached to a housing of a PEM water electrolysis cell (product name: WE-4S-RICW, manufactured by FC Development Co., Ltd.) to prepare a PEM water electrolysis cell.

[0157] <Evaluation of oxygen evolution electrode catalyst performance> Using a PEM-type water electrolytic cell, 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 oxygen evolution electrode catalyst performance was evaluated. Voltage increase rate: 10 mV / sec Water temperature: 80°C Water supply rate: 2 mL / min

[0158] MEAs each containing the powders of the Examples and Comparative Examples as anode catalysts and PEM-type water electrolyzers each containing the MEAs were fabricated, and the oxygen generating electrode catalysts were evaluated. The results are shown in the table below.

[0159]

[0160] From Table 1, it was confirmed that, compared to Comparative Example 1 which did not contain iridium, the iridium-supported manganese oxide powder which contained iridium and had an Ir / Mn molar ratio of 0.001 or more and 0.25 or less exhibited high oxygen evolution electrode catalytic activity.

[0161] The current density increased with increasing Ir / Mn molar ratio, and in the sample with an Ir / Mn molar ratio of 0.065 mol / mol, the current density was 2 A / cm ２ It was confirmed that the above current density could be obtained.

[0162] In Examples 1 to 4 and Comparative Example 1, the D16 diameter was 0.5 μm, the D50 diameter was 0.7 μm, the D84 diameter was 1.9 μm, and the standard deviation was 0.7 μm. ２ IrCl ６ It was confirmed that the treatment with the aqueous solution and the subsequent heat treatment had no effect on the particle size.

[0163] It was confirmed that the current density increased as the a / c ratio decreased, and that iridium-containing manganese oxides with a / c ratios of less than 1.521 exhibited high oxygen evolution electrode catalyst properties.

[0164] The present disclosure provides at least one of an iridium-containing manganese oxide that exhibits high oxygen evolution electrode catalytic activity, a catalyst containing the same, an electrode containing the catalyst, and a water electrolysis method using the electrode.

Claims

1. An iridium-containing manganese oxide having a molar ratio of iridium to manganese of 0.001 or more and 0.250 or less.

2. The iridium-containing manganese oxide according to claim 1, wherein the manganese oxide is manganese dioxide having a β-type crystal structure.

3. The iridium-containing manganese oxide according to claim 2, in which the ratio of the lattice constant in the a-axis direction to the lattice constant in the c-axis direction is 1.420 or more and less than 1.

521.

4. The iridium-containing manganese oxide according to any one of claims 1 to 3, having a D50 diameter in a volumetric particle size distribution of 0.1 µm or more and 50.0 µm or less.

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

6. The iridium-containing manganese oxide according to any one of claims 1 to 3, in which in a powder X-ray diffraction pattern, when a CuKα ray is used as a radiation source, the full width at half maximum of a peak appearing at 2θ = 28 ± 1.5° is more than 1.90° and not more than 4.00°.

7. A catalyst comprising the iridium-containing manganese oxide according to any one of claims 1 to 3.

8. An electrode comprising the catalyst according to claim 7.

9. A method for electrolyzing water using the electrode according to claim 8.

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