Iridium-containing manganese oxide

Abstract

Provided are at least one of: an iridium-containing manganese oxide exhibiting higher oxygen evolution electrocatalytic activity compared to conventional iridium-containing manganese oxides; a catalyst comprising the same; an electrode comprising the catalyst; a water electrolysis cell equipped with the electrode; and a water electrolysis method using the water electrolysis cell.　An iridium-containing manganese oxide having a crystal structure of β-type MnO2, wherein the full width at half maximum of the XRD peak corresponding to the (211) plane of β-type MnO2 is 0.65° or more and 1.05° or less.

Description

Iridium-containing manganese oxide 【0001】 This disclosure relates to an iridium-containing manganese oxide, an oxygen-evolving electrode catalyst, an oxygen-evolving electrode, and a water electrolysis apparatus, as well as a method for producing hydrogen by electrolyzing water using the oxygen-evolving electrode. 【0002】 Due to the depletion of fossil fuels and environmental pollution, the use of hydrogen as a clean energy source and its production technology are being investigated. One effective method for producing high-purity hydrogen gas is water electrolysis, and among these, the Proton Exchange Membrane (PEM) type water electrolysis method is attracting attention. In PEM type water electrolysis, iridium-based catalysts are widely known as highly active oxygen-evolving electrode catalysts (for example, Non-Patent Document 1). However, iridium reserves are extremely small compared to other precious metals. Therefore, there is a resource problem in that a sufficient amount of catalyst cannot be secured for the future widespread adoption of water electrolysis technology. Against this backdrop, iridium-containing manganese oxide (Patent Document 1), in which a small amount of iridium is introduced into manganese oxide, has been reported as an oxygen-evolving electrode catalyst with reduced iridium usage. 【0003】 International release 2022 / 264960 【0004】 F. Birol, World Energy Outlook 2016, International Energy Agency (IEA), Paris, 2016. 【0005】 Although the iridium-containing manganese oxide described in Patent Document 1 has high oxygen-evolving electrode catalytic activity, further improvement of catalytic activity was necessary to meet the increasing demand for hydrogen. The present disclosure aims to provide at least one of the following: an iridium-containing manganese oxide exhibiting higher oxygen-evolving electrode catalytic activity compared to conventional iridium-containing manganese oxides; a catalyst containing the same; an electrode containing the catalyst; a water electrolysis apparatus equipped with the electrode; and a water electrolysis method using the water electrolysis apparatus. 【0006】This disclosure focuses on iridium-containing manganese oxides, examining their crystallinity and oxygen evolution electrode catalytic activity. As a result, it was found that specific crystal planes in iridium-containing manganese oxides affect oxygen evolution electrode catalytic activity, and furthermore, that controlling the crystallinity of these specific crystal planes improves oxygen evolution electrode catalytic activity. That is, the present invention is as described in the claims, and its gist is as follows: [1] β-type MnO ２ It is an iridium-containing manganese oxide having the following crystal structure, β-type MnO ２ Iridium-containing manganese oxide having a full width at half maximum of 0.65° or more and 1.05° or less for the XRD peak corresponding to the (211) plane. [2] β-type MnO ２ [1] The iridium-containing manganese oxide described above, wherein the full width at half maximum of the XRD peak corresponding to the (220) plane is 0.55° or more and 2.00° or less. [3] The iridium-containing manganese oxide described above, wherein the Ir / Mn molar ratio is 0.001 or more and 0.100 or less. [4] An oxygen-evolving electrode catalyst comprising the iridium-containing manganese oxide described above, which is one of [1] to [3]. [5] An oxygen-evolving electrode comprising the oxygen-evolving electrode catalyst described above, and a conductive substrate. [6] The content of the oxygen-evolving electrode catalyst is 0.1 mg / cm² per geometric area of ​​the oxygen-evolving electrode. ２ 12.0mg / cm or more ２ The oxygen-generating electrode described in [5] above. [7] The iridium content is 0.001 mg / cm² per geometric area of ​​the oxygen-generating electrode. ２ 1.000mg / cm or more ２ The oxygen generating electrode according to [5] or [6] above, which is as follows: [8] The oxygen generating electrode according to any one of [5] to [7] above, wherein the conductive substrate contains titanium. [9] A water electrolysis apparatus comprising the oxygen generating electrode according to any one of [5] to [8] above.

[10] A method for producing hydrogen by electrolyzing water using the oxygen generating electrode according to any one of [5] to [8] above. 【0007】According to the present disclosure, there can be provided at least any one of an iridium-containing manganese oxide showing high oxygen generation electrode catalytic activity as compared with a conventional iridium-containing manganese oxide, a catalyst containing the same, an electrode containing the catalyst, a water electrolysis cell including the electrode, and a water electrolysis method using the water electrolysis cell. 【0008】 Hereinafter, an embodiment of the present disclosure will be described. The present disclosure includes any combination of the configurations and numerical values disclosed in this specification, and also includes a range consisting of any combination of the upper and lower limits of the numerical values disclosed in this specification. 【0009】 The iridium-containing manganese oxide of this embodiment (hereinafter, also referred to as "Ir-Mn oxide") is an oxide having a crystal structure of β-type MnO ２ and β-type MnO ２The full width at half maximum of the XRD peak corresponding to the (211) plane is 0.65° or more and 1.05° or less. The Ir-Mn oxide of this embodiment contains iridium (Ir). The iridium only needs to be 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 includes one or more states selected from the group consisting of a state in which it is mixed with the manganese oxide, a state in which it is supported on the manganese oxide, and a state in which it is solid-dissolved in the manganese oxide, and it is preferable that at least a portion of the iridium is solid-dissolved in the manganese oxide. The form of iridium contained in the Ir-Mn oxide of this embodiment is arbitrary, and the iridium only needs to be 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. In this embodiment, the iridium contained in the Ir-Mn oxide is preferably contained in the manganese oxide as iridium ions, and more preferably, at least a portion of the iridium ions are substituted for manganese ions in the manganese oxide, i.e., at least a portion of the iridium is solid-dissolved in the manganese oxide. The state in which iridium is supported on the manganese oxide is sufficient if 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 the manganese oxide, the iridium may be supported in the form of an oxide, or more precisely, as iridium oxide. When iridium is supported in the pores of the manganese oxide, the iridium may be contained as a cation, or more precisely, as an iridium cation in an ion-exchangeable site of the manganese oxide. When iridium is solid-dissolved in the manganese oxide, at least a portion of the iridium cation is substituted for manganese ions in the manganese oxide. The shape of Ir-Mn oxide is not particularly limited, but can be any shape such as thin film, granules, or powder. Ir-Mn oxide has a crystal structure of β-type MnO ２ It has a crystal structure of β-type MnO ２It is preferable to have a single-phase crystal structure, but γ-type MnO ２ δ-type MnO ２ , and α-type MnO ２ One or more crystal structures selected from the group, and β-type MnO ２ It may have a crystal structure consisting of a mixed phase. The crystal structure of the Ir-Mn oxide in 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 Diffraction Data Center) PDF (Powder Diffraction File). γ-type MnO ２ β-type MnO ２ δ-type MnO ２ or α-type MnO ２ The reference patterns for the crystal structure can be found in PDF No. 14-0644 (γ-type), 24-0735 (β-type), 80-1098 (δ-type), or 44-0141 (α-type), respectively. 【0010】 In this embodiment, the XRD pattern can be obtained by XRD measurement using a general powder X-ray diffractometer (e.g., Ultima IV Protectus, manufactured by Rigaku Corporation) under the following conditions: Acceleration current / voltage: 40 mA / 40 kV Radiation source: CuKα rays (λ = 1.5405 Å) Measurement mode: Continuous scan Scan conditions: 4° / min Measurement range: 2θ = 10° to 80° Divergence longitudinal limiting slit: 10 mm Divergence / entry slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used 【0011】In this embodiment, "full width at half maximum" is the width connecting the two ends of the peak corresponding to half the maximum peak intensity in the XRD peak. In this embodiment, the measurement of the full width at half maximum can be performed by detecting each XRD pattern by peak search of the obtained XRD pattern and determining the full width at half maximum of the detected XRD peak. Peak search can be performed by second derivative method on an XRD pattern that has undergone smoothing processing using B-Spline with a smoothing parameter of 10.00, background processing using the Sonneveld-Visser method with a peak width threshold of 1.00 and an intensity threshold of 10.00, and Kα2 line removal processing with an intensity ratio of 0.4970. Analysis of the XRD pattern, including smoothing processing, background processing, profile fitting, and calculation of full width at half maximum, can be performed using general analysis software (e.g., SmartLab Studio II, manufactured by Rigaku Corporation). 【0012】 The Ir-Mn oxide of this embodiment has at least β-type MnO in its XRD pattern. ２ It has an XRD peak corresponding to the (211) plane (hereinafter also referred to as "P(211)"), and P(211) and β-type MnO ２ It is preferable to have an XRD peak corresponding to the (220) plane (hereinafter also referred to as "P(220)"). The Ir-Mn oxide of this embodiment has a full width at half maximum (FMAX) of P(211) (hereinafter also referred to as "FMAX 1") of 1.05° or less. The FMAX 1 is preferably 1.00° or less, 0.98° or less, or 0.95° or less. In order to exhibit high oxygen evolution electrode catalytic activity (hereinafter also simply referred to as "catalytic activity"), the FMAX 1 is 0.65° or more, preferably 0.70° or more, or 0.80° or more, or 0.85° or more. Preferred FMAX 1 for the Ir-Mn oxide of this embodiment include 0.70° or more and 0.98° or less, or 0.80° or more and 0.95° or less. One reason why an electrode with high catalytic activity can be obtained by satisfying the above-mentioned FMAX 1 of the Ir-Mn oxide of this embodiment is the following reason. In other words, the full width at half maximum (FMAX) of 1 is β-type MnO ２ It is one of the indicators showing the crystallinity of the (211) plane. β-type MnO２ The (211) plane is considered to be a crystal plane where oxygen evolution reactions preferentially occur because water adsorption is likely to occur there. Since the full width at half maximum (FMAX) is within the aforementioned range, structural defects that inhibit oxygen evolution are reduced, and a sufficient reaction field for oxygen evolution can be provided. 【0013】 In this embodiment, the Ir-Mn oxide preferably has a full width at half maximum (FMAX) of P(220) of 0.55° or more, 0.60° or more, 0.65° or more, 0.70° or more, or 0.80° or more. This increases the specific surface area and makes it easier to exhibit superior catalytic activity. On the other hand, it is preferable that the FMAX 2 is 2.00° or less, 1.95° or less, 1.90° or less, 1.85° or less, or 1.80° or less. Having a FMAX 2 below this range makes it easier to exhibit even higher catalytic activity. Examples of FMAX 2 include 0.55° or more and 2.00° or less, 0.60° or more and 1.95° or less, 0.65° or more and 1.95° or less, 0.70° or more and 1.90° or less, 0.70° or more and 1.80° or less, and 0.80° or more and 1.80° or less. In this embodiment, the full width at half maximum 1 and the full width at half maximum 2 are obtained from the XRD pattern described above. In the XRD pattern, P(211) can be considered as a peak having its peak top at 2θ = 56.4 ± 1°, and P(220) can be considered as a peak having its peak top at 2θ = 58.5 ± 1°. 【0014】In this embodiment, the molar ratio of iridium to manganese in the Ir-Mn oxide (hereinafter also referred to as the "Ir / Mn molar ratio") is preferably 0.001 or higher, 0.002 or higher, or 0.010 or higher in order to facilitate the expression of high catalytic activity. Within the range in which the effects of the Ir-Mn oxide of this embodiment are achieved, it is not necessary to include an excessive amount of iridium, and the Ir / Mn molar ratio may be 0.100 or lower, 0.075 or lower, or 0.050 or lower. Examples of Ir / Mn molar ratios for the Ir-Mn oxide of this embodiment include 0.001 to 0.100, 0.002 to 0.075, or 0.010 to 0.050. The Ir / Mn molar ratio of the Ir-Mn oxide in this embodiment can be measured by ICP emission spectroscopy using a general ICP instrument (instrument name: Optima 830, manufactured by Perkin Elmer). It can be determined by measuring the molar concentrations of iridium (Ir) and manganese (Mn) in the Ir-Mn solution obtained by dissolving the Ir-Mn oxide, and calculating the iridium [mol] relative to the obtained manganese [mol]. The Ir-Mn solution can be prepared by dissolving the Ir-Mn oxide of this embodiment in a mixed acid of hydrochloric acid and nitric acid as a dissolving agent at a temperature of 80°C to 95°C for 2 to 4 hours. The Ir-Mn oxide of this embodiment can be used as an oxygen-evolving electrode catalyst, and by combining it with a conductive substrate, it can be used as an oxygen-evolving electrode (hereinafter also referred to as "Ir-Mn electrode"). The Ir-Mn electrode of this embodiment only needs to include the Ir-Mn oxide of this embodiment and a conductive substrate. Furthermore, it is preferable that the Ir-Mn electrode of this embodiment has a structure in which a part or all, or even at least a part of, of the conductive substrate is coated with an oxygen-evolving electrode catalyst. 【0015】 The content of Ir-Mn oxide per geometric area of ​​the Ir-Mn electrode in this embodiment (hereinafter also referred to as "unit catalyst content") is 0.1 mg / cm². ２ Above, 0.5mg / cm ２ Above, or 0.8 mg / cm³ ２ Preferably, the concentration is 12.0 mg / cm³. ２ Below, 11.0mg / cm ２ The following, or 10.0 mg / cm³ ２The following is preferred: 0.1 mg / cm³ 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 following are examples: Unit catalyst content of 0.1 mg / cm³ ２ As a result, the coating rate of Ir-Mn oxide on the conductive substrate is improved. On the other hand, the catalyst content is 12.0 mg / cm³. ２ The following conditions reduce the electrical resistance of the electrodes. As a result, high catalytic activity is more easily achieved. 【0016】 The unit catalyst content in the electrode of this embodiment can be determined from the following equation (1): Unit catalyst content (mg / cm³) ２ ) = [Amount of Mn contained in the electrode (mg) + Amount of Ir contained in the electrode (mg)] / Geometric area of ​​the electrode (cm²) ２ ) (1) 【0017】In the above formula, the "amount of Mn contained in the electrode" and the "amount of Ir contained in the electrode" are values ​​obtained by multiplying the Mn concentration (mg / L) and Ir concentration (mg / L) of the Ir-Mn electrode solution, respectively, by the volume (L) of the Ir-Mn electrode solution. The Ir-Mn electrode solution is a solution obtained by dissolving the Ir-Mn oxide contained in the electrode. This is obtained by immersing the oxygen-evolving electrode in a mixed acid of hydrochloric acid and nitric acid as the dissolving solution at a temperature of 80°C to 95°C for 2 to 4 hours. The amount of dissolving solution should be expressed as the ratio of the mass of Ir-Mn oxide to the volume of dissolving solution, with a ratio of 1 mL to 20 mL of dissolving solution per 1 mg of Ir-Mn oxide, provided that the total amount of Ir-Mn oxide is dissolved. The Mn concentration and Ir concentration can be determined by ICP emission spectroscopy using a general ICP instrument (for example, instrument name: Optima 830, manufactured by Perkin-Elmer). The "geometric area of ​​an electrode" is the area of ​​the projection of the electrode onto a plane, and is defined by the external shape of the electrode. Therefore, irregularities and defects are not considered in the geometric area. For example, in the case of an electrode defined by length × width × thickness, the area of ​​the plane obtained from length × width is the geometric area of ​​the electrode, and is the projected area of ​​the plane (geometric plane) that corresponds to the surface facing the electrolyte membrane when forming a membrane-electrode assembly (hereinafter also referred to as "MEA"). 【0018】 In the Ir-Mn electrode of this embodiment, the iridium content per unit geometric area of ​​the electrode (hereinafter also referred to as "iridium content") is 0.001 mg / cm². ２ Above, 0.005mg / cm ２ Above, 0.01mg / cm ２ Above or equal to 0.02 mg / cm³ ２ Preferably, the concentration is 1.0 mg / cm³ or higher, and also 1.0 mg / cm³. ２ Below, 0.75mg / cm ２ The following, or 0.5 mg / cm³ ２ Preferably, the following is the case: 0.001 mg / cm³ ２ 1.0mg / cm or more ２ Below, 0.005mg / cm ２ 0.75mg / cm or more ２ The following, or 0.01 mg / cm³ ２ 0.5mg / cm or more２ The following can be cited: When the upper or lower limit of the iridium content is set to this value, the dispersibility of iridium in the Ir-Mn electrode is increased. As a result, the utilization rate of iridium as an active site in the oxygen evolution reaction is increased. Therefore, when combined with a conductive substrate as described later, it is easier to achieve higher catalytic activity. The conductive substrate in the Ir-Mn electrode can be any substrate made of a conductive material, and it is preferable to use a substrate containing titanium, and more preferably a substrate made of titanium. 【0019】 The conductive substrate can take the shape of one or more selected from the group consisting of mesh, cloth, and plate shapes. A mesh-like conductive substrate is preferable because it results in higher catalytic activity. Specific examples of conductive substrates include titanium mesh composed of fibrous or powdered conductive titanium, and sintered titanium mesh obtained by heat treatment. These conductive substrates are preferably coated with platinum on their surface to easily achieve high catalytic activity, platinum-coated titanium mesh is more preferable, and platinum-coated sintered titanium mesh is even more preferable. 【0020】 When a conductive substrate is coated with platinum on its surface, it exhibits higher conductivity; therefore, the amount of platinum per geometric area is 0.5 mg / cm². ２ Above, 0.7mg / cm ２ More than or equal to 1.0 mg / cm² ２ The above is preferable, and also 10.0 mg / cm³. ２ Below, 8.0mg / cm ２ The following or 6.0 mg / cm³ ２ The following is preferred: 0.5 mg / cm² ２ 10.0mg / cm or more ２ Below, 0.7mg / cm ２ 8.0mg / cm or more ２ The following, or 1.0 mg / cm³ ２ 6.0mg / cm or more ２ The following are listed: 【0021】To facilitate the expression of excellent oxygen-evolving electrode catalytic activity, the thickness of the conductive substrate is preferably 50 μm or more, 100 μm or more, or 150 μm or more, and also preferably 500 μm or less, 400 μm or less, or 300 μm or less, with examples including 50 μm or more and 500 μm or less, 100 μm or more and 400 μm or less, or 150 μm or more and 300 μm or less. "Thickness of the conductive substrate" refers to the length corresponding to the length perpendicular to the geometric plane of the conductive substrate, and is the length corresponding to the thickness when the shape of the electrode is defined by length × width × thickness. It is the projected length of the plane perpendicular to the plane (geometric plane) corresponding to the plane facing the electrolyte membrane when forming the MEA. 【0022】 The porosity of the conductive substrate is preferably 30% or more, or 40% or more, and preferably 80% or less, or 70% or less, with examples including 30% to 80% or 40% to 70% or more. Satisfying this porosity increases the mechanical strength of the electrode and facilitates the efficient supply of water, which is the reaction substrate for the oxygen evolution reaction. In this embodiment, the porosity can be calculated from the following formula: Porosity (%) = {1 - (bulk density of the conductive substrate (g / cm³) ３ ) / Skeleton density of conductive substrate (g / cm³) ３ ))} × 100 The bulk density and skeletal density of a conductive substrate can be determined from the pore volume measured by the mercury pressure method using a general mercury porosimeter (name: AutoPore 9510, manufactured by micromeritics) and the volume of the conductive substrate using the following formula: Bulk density of conductive substrate (g / cm³ ３ ) = Mass of conductive substrate (g) / Volume of conductive substrate (cm³) ３ ) Skeleton density of conductive substrate (g / cm³) ３ ) = Mass of conductive substrate (g) / {Volume of conductive substrate - Pore volume} (cm ３ ) 【0023】In the above two equations, the volume of the conductive substrate is determined by measurement using a laser volumetric meter (for example, device name: 3D scanner type three-dimensional measuring machine VL-700 series (controller VL-700, stage VL-750 / VL-C35, measuring unit VL-770), manufactured by Keyence Corporation), and the mass of the conductive substrate is determined by mass measurement using an electronic balance. 【0024】 The following are specific measurement conditions for a mercury porosimeter: Sample volume: 40 mg Mercury introduction pressure: 601.6 psi to 36,098.1 psi (4.1 MPa to 248.9 MPa) Measurement pore size: 5 nm to 500 μm Cell used: 5.3 cc glass cell Mercury surface tension: 480 dyn Mercury contact angle: 130° Pretreatment conditions: Degassing at 110°C for 1 hour or more in an air atmosphere 【0025】 The method for producing Ir-Mn oxide according to this embodiment will be described below. The method for producing Ir-Mn oxide according to this embodiment is not particularly limited as long as it yields Ir-Mn oxide having the above-described configuration. A preferred method for producing iridium-containing manganese oxide includes electrolysis of an electrolyte containing an ammonium salt and a manganese salt to precipitate manganese oxide (hereinafter also referred to as the "electrolysis step"), contact of manganese oxide with an iridium salt solution to obtain a precursor compound (hereinafter also referred to as the "contact step"), and heat treatment of the precursor compound (hereinafter also referred to as the "heat treatment step"). 【0026】In the electrolysis process, an electrolyte containing ammonium salt and manganese salt is electrolyzed to precipitate manganese oxide, preferably by electrolysis of the electrodes of the electrolytic device. This yields manganese oxide, which will serve as the base material for Ir-Mn oxide. Any electrolysis method is acceptable as long as the manganese oxide is electrolytically produced; any method involving immersing the electrodes of the electrolytic device in the electrolyte and electrolyzing it is sufficient. By electrolytically producing manganese oxide from an electrolyte containing manganese salt and ammonium salt, manganese oxide crystals can be grown with good orientation. As a result, the structural change of the manganese oxide in the heat treatment described later becomes uniform, and the full width at half maximum (FMAX) of the resulting Ir-Mn oxide is 1, which is that of the Ir-Mn oxide of this embodiment. 【0027】 The electrolyte contains a manganese salt, preferably manganese sulfate. The manganese concentration of the electrolyte is preferably 0.05 mol / L or more and 1.00 mol / L or less. This manganese concentration suppresses the oxidation reaction of water at the electrodes of the electrolytic device, thereby increasing the efficiency of electrolytic deposition of manganese oxide. The electrolyte also contains an ammonium salt, preferably ammonium sulfate. The inclusion of an ammonium salt facilitates the precipitation of δ-type manganese dioxide as manganese oxide. One reason for this is the ammonium ions (NH₄) contained in the ammonium salt. ４ ＋ It is thought that ammonium ions have a template effect for forming the layered structure of delta-type manganese dioxide. Furthermore, it is thought that the incorporation of ammonium ions between the layers of delta-type manganese dioxide can direct the crystal growth of delta-type manganese dioxide in a specific plane direction. This makes it easier to obtain Ir-Mn oxides with a small total width at half maximum in the heat treatment process described later. 【0028】 Ammonium salt concentration is ammonium nitrogen (NH ４ ＋The ammonium salt concentration is preferably 0.1 mol / L or more or 0.5 mol / L or more, and preferably 3.0 mol / L or less or 2.5 mol / L or less, for example, 0.1 mol / L or more and 3.0 mol / L or less. Having the ammonium salt concentration in this range makes it easier to obtain the template effect of ammonium ions. As a result, the proportion of δ-type manganese dioxide in the manganese oxide produced by electrolysis increases, and as a result, it becomes easier to obtain Ir-Mn oxide with a small full width at half maximum in the heat treatment step described later. As the ammonium salt, inorganic ammonium salts and one or more selected from the group consisting of ammonium sulfate, ammonium nitrate and ammonium chloride are mentioned, and ammonium sulfate is preferred. 【0029】 The electrolyte is preferably a mixed solution containing manganese sulfate, and more preferably an aqueous solution of manganese sulfate, and more preferably contains sulfuric acid. A preferred electrolyte is an aqueous solution containing manganese sulfate, sulfuric acid, and ammonium sulfate. In this case, the sulfuric acid concentration is preferably 0.05 mol / L or more and 1.00 mol / L or less. A sulfuric acid concentration within this range suppresses excessive crystal growth of manganese oxide. If the sulfuric acid concentration is higher within this range, the total width at half maximum of the resulting Ir-Mn oxide tends to decrease. 【0030】 In the electrolysis process, manganese oxide is deposited by electrolysis of the electrolyte. The current density in electrolysis should be such that manganese oxide can be electrolyzed, but the current density per geometric area of ​​the conductive substrate should be 1.0 mA / cm². ２ Above or above, or 5.0 mA / cm² ２ Preferably, it is 20 mA / cm² or higher, and also 20 mA / cm². ２ The following, or 10 mA / cm² ２ Preferably, it is 1.0 mA / cm². ２ 20mA / cm or more ２ The following, or 5.0 mA / cm² ２ 10mA / cm or more ２The following can be cited: When the current density is within the above range, the crystal growth of manganese oxide is more likely to be directed in a specific plane direction. As a result, the proportion of δ-type manganese dioxide in the manganese oxide tends to increase in the electrolytic extraction of manganese oxide. The energizing time in electrolytic extraction (hereinafter also referred to as "electrolysis time") is preferably 1 minute or more, 5 minutes or more, or 10 minutes or more, and also preferably 60 minutes or less, 40 minutes or less, or 30 minutes or less, and can be 1 minute or more and 60 minutes or less, 5 minutes or more and 40 minutes or less, or 10 minutes or more and 30 minutes or less. When the electrolysis time is within the above range, manganese oxide tends to be electrolytically extracted uniformly. 【0031】 The electrolyte temperature in electrolysis (hereinafter also referred to as "electrolysis temperature") is preferably 60°C or higher, 70°C or higher, or 80°C or higher, and also preferably 98°C or lower, 97°C or lower, or 96°C or lower, with examples including 60°C to 98°C, 70°C to 97°C, and 80°C to 96°C. When the electrolysis temperature is within the above range, the proportion of δ-type manganese dioxide in the electrolyzed manganese oxide increases. When the electrolysis temperature is lower within these ranges, the total width at half maximum of the Ir-Mn oxide obtained in the heat treatment process described later tends to decrease. 【0032】 The manganese oxide obtained by the electrolytic process is preferably a manganese oxide containing δ-type manganese dioxide, more preferably a manganese oxide with δ-type manganese dioxide as the main phase, and even more preferably δ-type manganese dioxide. On the other hand, it may also be a manganese oxide consisting of at least one of γ-type manganese dioxide and α-type manganese dioxide and δ-type manganese dioxide, or even a manganese oxide consisting of γ-type manganese dioxide and δ-type manganese dioxide. 【0033】 The manufacturing method of this embodiment may include washing the obtained manganese oxide (hereinafter also referred to as the "washing step"). In the washing step, the washing method should be such that the electrolyte components on the surface of the manganese oxide are removed, for example, by rinsing the manganese oxide with pure water. 【0034】The manufacturing method of this embodiment may include drying the manganese oxide (hereinafter also referred to as the "drying step"). The drying method in the drying step can be any method that removes moisture from the surface of the manganese oxide, for example, by air drying. 【0035】 The iridium salt solution used in the contact process can be any solution containing an iridium salt, and a solution containing an iridium salt is preferred, and an aqueous solution containing an iridium salt is more preferred. Examples of iridium salts include iridium(III) chloride (IrCl). ３ ), iridium(IV) chloride (IrCl ４ ) and iridium nitrate (Ir(NO) ３ ) ４ One or more selected from the group of iridium(III) are mentioned, and it is preferable that at least one of iridium(III) chloride and iridium nitrate be used, with iridium(III) chloride being more preferable. Compared with iridium nitrate and the like, using iridium(III) chloride tends to result in a smaller total width at half maximum of the resulting Ir-Mn oxide. 【0036】 The iridium concentration in the iridium salt solution is preferably between 0.001 mmol / L and 5.000 mmol / L. The iridium concentration in the iridium salt solution is preferably between 0.001 mmol / L or 0.005 mmol / L, and also preferably between 5.000 mmol / L or 1.000 mmol / L, or between 0.001 mmol / L and 5.000 mmol / L, or between 0.005 mmol / L and 1.000 mmol / L, because this allows iridium and manganese oxide to efficiently contact and facilitate iridium incorporation into the manganese oxide. A higher iridium concentration within this range tends to result in a higher iridium content in the resulting Ir-Mn oxide. 【0037】The method for contacting manganese oxide with an iridium salt solution is not particularly limited, as long as the conditions allow for the manganese oxide to contain a desired amount of iridium. Examples include impregnating the iridium salt solution with the manganese oxide deposited on the electrodes of an electrolytic device, or detaching the manganese oxide from the electrodes of the electrolytic device, crushing it, and mixing it with the iridium salt solution in powder form. The contact temperature in this method is preferably 20°C or higher, or 50°C or higher, preferably 100°C or lower, and can range from 20°C to 100°C, or 50°C to 100°C. The higher the contact temperature, the easier it is for iridium to be incorporated into the Ir-Mn oxide. Also, the higher the contact temperature, the easier it is for manganese atoms in the crystal structure of the manganese oxide to be replaced by iridium atoms. Therefore, the higher the contact temperature, the easier it is for iridium to replace manganese in the manganese oxide and form a solid solution, making it easier for at least a portion of the iridium to be solidly dissolved in the manganese oxide. While longer contact times tend to increase the iridium content of manganese oxide, it is preferable that the contact time be between 30 minutes and 200 hours. 【0038】 The manufacturing method of this embodiment may include a drying step in which the manganese oxide is dried after contact with the iridium salt. Drying helps to ensure a uniform heat distribution in the manganese oxide during the heat treatment step described later. The drying method can be any condition that removes moisture from the surface of the manganese oxide, for example, by air drying. 【0039】 In the heat treatment process, δ-type MnO ２ From β-type MnO ２The method is not particularly limited as long as it provides a thermal history that causes a structural change, but one example is heating in an electric furnace. The precursor compound obtained in the contact step can be filled into a firing container and heat-treated. Alternatively, the precursor compound obtained in the contact step may be deposited on a conductive substrate or coated and then heat-treated together with the conductive substrate. Through heat treatment, manganese atoms inside the crystal structure of the manganese oxide are replaced by iridium atoms contained on the surface or in the pores of the manganese oxide, and as a result, even more iridium can be dissolved in the manganese oxide. 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, or 300°C or higher, and preferably 600°C or lower, or 500°C or lower, and can be 100°C to 600°C or 300°C to 500°C. The heat treatment time can be appropriately set depending on the amount of manganese oxide etc. subjected to heat treatment, but for example, it can be 10 minutes to 24 hours. 【0040】 The method for manufacturing the oxygen-generating electrode comprising the Ir-Mn oxide of this embodiment is not particularly limited, but examples include a manufacturing method that includes a step of electrolytically evolving manganese oxide on a conductive substrate, a step of contacting the obtained manganese oxide-coated substrate with an iridium salt, and a heat treatment step of heat-treating the obtained Ir-Mn oxide-coated substrate. In this method, a uniform Ir-Mn oxide layer is easily formed on the conductive substrate, and it is easy to exhibit high catalytic activity when used as an oxygen-generating electrode. Another example is a method in which the Ir-Mn oxide obtained by the above method for manufacturing Ir-Mn oxide is made into a catalytic ink, and an Ir-Mn electrode is obtained by applying the catalytic ink to a conductive substrate. The above catalytic ink only needs to be in a state in which the Ir-Mn oxide is dispersed in a solvent, and the solvent can be water, alcohol, or a mixed solvent of water and alcohol. 【0041】The oxygen-evolving electrode catalyst containing Ir-Mn oxide according to this embodiment can be used as a catalyst-coated membrane (CCM) comprising the catalyst and an electrolyte membrane. The catalyst-coated membrane only needs to include the oxygen-evolving electrode catalyst of this embodiment and an electrolyte membrane, and it is preferable that at least a portion of the electrolyte membrane is coated with the oxygen-evolving electrode catalyst. 【0042】 The oxygen-evolving electrode catalyst of this embodiment can be used as a membrane-electrode assembly (MEA) comprising the catalyst-coated membrane and a conductive substrate. Alternatively, it may be a membrane-electrode assembly comprising an electrolyte membrane and the oxygen-evolving electrode described above. The membrane-electrode assembly only needs to comprise an electrolyte membrane, the oxygen-evolving electrode catalyst containing the Ir-Mn oxide of this embodiment, and a conductive substrate. The oxygen-evolving electrode catalyst of this embodiment can be used as a water electrolysis apparatus comprising the membrane-electrode assembly. Furthermore, it can be used as a water electrolysis method using the water electrolysis apparatus, or as a method for producing hydrogen using the water electrolysis apparatus. 【0043】 The present disclosure will be described in detail below with reference to examples and comparative examples, but the present disclosure is not limited to these examples. <Analysis of Metal Content> A sample solution was obtained by dissolving 0.5 mg to 10 mg of oxygen-evolving electrode catalyst in 10 mL of a solution of hydrochloric acid and nitric acid mixed in a volume ratio of 3:1. The composition of the sample solution was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a general ICP instrument (instrument name: Optima 830, manufactured by Perkin Elmer). 【0044】<Identification of Crystal Structure> An XRD pattern was obtained using a general powder X-ray diffractometer (instrument name: Ultima IV Protectus, manufactured by Rigaku Corporation) under the following conditions: Acceleration current / voltage: 40 mA / 40 kV Radiation source: CuKα rays (λ = 1.5405 Å) Measurement mode: Continuous scan Scan conditions: 4° / min Measurement range: 2θ = 10° to 80° Divergence longitudinal limiting slit: 10 mm Divergence / entry slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used 【0045】 The crystalline phase of the sample was identified by comparing the obtained XRD patterns with the reference patterns. The reference patterns for manganese dioxide with γ-type, β-type, δ-type, or α-type crystalline structures were PDF No. 14-0644 (γ-type), 24-0735 (β-type), 80-1098 (δ-type), or 44-0141 (α-type), respectively. Next, the width of the diffraction lines at half the height of the diffraction line intensity of the XRD peak corresponding to the (211) plane of β-type manganese dioxide (XRD peak with peak top at 2θ = 56.4 ± 1°) and the XRD peak corresponding to the (220) plane (XRD peak with peak top at 2θ = 58.5 ± 1°) was determined, and these were designated as full width at half maximum 1 and full width at half maximum 2, respectively. 【0046】 <Construction of a PEM-type water electrolytic cell> A PEM-type water electrolytic cell was constructed using a conductive substrate coated with iridium-containing manganese oxide as the working electrode (anode) and equipped with a MEA prepared by the following method. A conductive catalyst ink was prepared by mixing 20% ​​by mass of platinum-supported carbon catalyst (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich) with a solution containing water, ethanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich). This was applied to carbon paper (product name: TGP-H-060, manufactured by Toray Industries), air-dried, and used as the counter electrode. 【0047】Next, a Nafion membrane (product name: Nafion 115, manufactured by Sigma-Aldrich) was washed and protonated by boiling it successively for 1 hour in 3% hydrogen peroxide solution, 1 hour in pure water, 1 hour in 1 M sulfuric acid aqueous solution, and 1 hour in pure water, and this was used as the electrolyte membrane. The electrolyte membrane was sandwiched between the catalytic surfaces of the working electrode (anode) and the counter electrode, and hot pressing was performed at 135 °C and a molding pressure of 400 kg / cm ２ for 3 minutes using a hot press machine (product name: SA-302, manufactured by Tester Sangyo Co., Ltd.) to obtain a MEA. The obtained MEA was attached to the casing of a PEM type water electrolyzer (product name: WE-4S-RICW, manufactured by FC Development Co., Ltd.) to fabricate a PEM type water electrolyzer. 【0048】 <Measurement of Oxygen Generation Electrode Catalytic Activity> A PEM type water electrolyzer using the oxygen generation electrode catalyst of the examples and comparative examples as the catalyst at the working electrode (anode) was used, and the oxygen generation electrode catalytic activity was evaluated by linear sweep voltammetry (LSV) of a two-electrode system under the following conditions. Voltage increase rate: 10 mV / sec Water temperature: 80 °C Water supply rate: 2 mL / min From this evaluation, the current density at a voltage of 2 V (hereinafter also simply referred to as "current density") was determined. 【0049】 Example 1 An electrolytic cell was filled with a sulfuric acid-manganese sulfate-ammonium sulfate mixed solution having a sulfuric acid concentration of 0.30 mol / L, a manganese sulfate concentration of 0.50 mol / L, and an ammonium sulfate concentration of 1.50 mol / L as an electrolyte solution, and a conductive substrate made of a Ti fiber sintered body coated with platinum (product name: Pt-plated Ti fiber sintered body, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.; thickness 0.2 mm, porosity 56%, Pt content 5 mg / cm ２ ) was immersed. The temperature of the mixed solution was maintained at 95 °C, and a current was applied to the conductive substrate at a current density of 7 mA / cm ２ for 10 minutes to electrochemically deposit manganese oxide on the conductive substrate. From the XRD pattern, it was identified that the manganese oxide was δ-type manganese dioxide. Next, iridium(IV) nitrate (Ir(NO ３ )) ４An electroconductive substrate on which the obtained manganese oxide was deposited was immersed in an iridium salt aqueous solution containing 0.1 mmol / L and 0.01 mol / L of sulfuric acid at 95°C for 24 hours, and then annealed in an air atmosphere at 450°C for 5 hours to obtain an oxygen evolution electrode having the iridium-containing manganese oxide of this example. The iridium-containing manganese oxide has a crystal structure of β-type MnO ２ and had a full width at half maximum 1 of 0.95° and a full width at half maximum 2 of 1.08°. Also, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.024 (the amount of Mn and the amount of Ir per geometric area of the electroconductive substrate were 1.2 mg / cm ２ and 0.1 mg / cm ２ ), respectively). 【0050】 Example 2 An oxygen evolution electrode having an iridium-containing manganese oxide was obtained in the same manner as in Example 1, except that iridium(IV) chloride (IrCl ４ ) was used as the iridium salt. The iridium-containing manganese oxide had a crystal structure of β-type MnO ２ from its XRD pattern, and had a full width at half maximum 1 of 0.84° and a full width at half maximum 2 of 1.06°. Also, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.024 (the amount of Mn and the amount of Ir per geometric area of the electroconductive substrate were 1.2 mg / cm ２ and 0.1 mg / cm ２ ), respectively). 【0051】 Example 3 Manganese oxide was electrochemically deposited on an electroconductive substrate in the same manner as in Example 1, except that the sulfuric acid concentration of the electrolytic solution was 0.10 mol / L. From the XRD pattern, the manganese oxide was identified as δ-type manganese dioxide. Next, an electroconductive substrate on which the manganese oxide obtained above was deposited was immersed in an iridium salt aqueous solution containing 0.1 mmol / L of iridium(III) chloride (IrCl ３ ) and 0.01 mol / L of sulfuric acid at 95°C for 24 hours, and then annealed in an air atmosphere at 450°C for 5 hours to obtain an oxygen evolution electrode having the iridium-containing manganese oxide of this example. The iridium-containing manganese oxide had a crystal structure of β-type MnO ２It had the following crystal structure, with a full width at half maximum (FMAX) 1 of 0.92° and a full width at half maximum (FMAX) 2 of 1.13°. Furthermore, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.024 (the amount of Mn and Ir per geometric area of ​​the conductive substrate was 1.2 mg / cm²). ２ and 0.1 mg / cm ２ ). 【0052】 Example 4 Manganese oxide was electrochemically deposited on a conductive substrate in the same manner as in Example 3, except that the electrolyte temperature was set to 60°C. From the XRD pattern, the manganese oxide was identified as δ-type manganese dioxide. Next, iridium(III) chloride (IrCl) ３ The conductive substrate on which the manganese oxide obtained above had precipitated was immersed in an iridium salt aqueous solution containing 0.1 mmol / L of iodine and 0.01 mol / L of sulfuric acid at 95°C for 24 hours, and then annealed in an air atmosphere at 450°C for 5 hours to obtain an oxygen-generating electrode having the iridium-containing manganese oxide of this embodiment. From its XRD pattern, the iridium-containing manganese oxide was found to be β-type MnO ２ It had the following crystal structure, with a full width at half maximum (FMAX) 1 of 0.95° and a full width at half maximum (FMAX) 2 of 0.82°. Furthermore, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.024 (the amount of Mn and Ir per geometric area of ​​the conductive substrate was 1.2 mg / cm²). ２ and 0.1 mg / cm ２ ). 【0053】 Example 5 Iridium(IV) chloride (IrCl ４ An oxygen-evolving electrode having an iridium-containing manganese oxide was obtained in the same manner as in Example 2, except that the concentration of ) was set to 0.2 mmol / L. From its XRD pattern, the iridium-containing manganese oxide was found to be β-type MnO ２ It had the following crystal structure, with a full width at half maximum (FMAX) 1 of 0.86° and a full width at half maximum (FMAX) 2 of 0.69°. Furthermore, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.048 (the amount of Mn and Ir per geometric area of ​​the conductive substrate was 1.2 mg / cm²). ２ and 0.2 mg / cm ２ ). 【0054】Example 6 Iridium(IV) chloride (IrCl ４ An oxygen-evolving electrode having an iridium-containing manganese oxide was obtained in the same manner as in Example 2, except that the concentration of ) was set to 0.3 mmol / L. From its XRD pattern, the iridium-containing manganese oxide was found to be β-type MnO ２ It had the following crystal structure, with a full width at half maximum (FMAX) 1 of 0.97° and a full width at half maximum (FMAX) 2 of 0.81°. Furthermore, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.072 (the amount of Mn and Ir per geometric area of ​​the conductive substrate was 1.2 mg / cm²). ２ and 0.3 mg / cm ２ ). 【0055】 Comparative Example 1 Manganese oxide was electrochemically deposited on a conductive substrate in the same manner as in Example 1, except that a sulfuric acid-manganese sulfate mixed solution with a sulfuric acid concentration of 0.36 mol / L and a manganese sulfate concentration of 0.50 mol / L was used as the electrolyte. From the XRD pattern, the manganese oxide was identified as γ-type manganese dioxide. Next, potassium hexachloroiridiate (K ２ IrCl ６ The conductive substrate on which the manganese oxide obtained above had precipitated was immersed in an iridium salt aqueous solution containing 0.1 mmol / L of iodine and 0.01 mol / L of sulfuric acid at 95°C for 24 hours, and then annealed in an air atmosphere at 450°C for 5 hours to obtain an oxygen-generating electrode having the iridium-containing manganese oxide of this comparative example. From its XRD pattern, the iridium-containing manganese oxide was found to be β-type MnO ２ It had the following crystal structure, with a full width at half maximum (FMAX) 1 of 1.73° and a full width at half maximum (FMAX) 2 of 1.84°. Furthermore, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.024 (the amount of Mn and Ir per geometric area of ​​the conductive substrate was 1.2 mg / cm²). ２ and 0.1 mg / cm ２ ). 【0056】 Comparative Example 2: Hexachloroiridiic acid (H) as the iridium salt ２ IrCl ６An oxygen-evolving electrode having the iridium-containing manganese oxide of this comparative example was obtained in the same manner as in Comparative Example 1, except that the iridium-containing manganese oxide of β-type MnO was used. From its XRD pattern, the iridium-containing manganese oxide was found to be β-type MnO ２ It had the following crystal structure, with a full width at half maximum (FMAX) 1 of 1.26° and a full width at half maximum (FMAX) 2 of 1.42°. Furthermore, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.024 (the amount of Mn and Ir per geometric area of ​​the conductive substrate was 1.2 mg / cm²). ２ and 0.1 mg / cm ２ ). 【0057】 Comparative Example 3: Iridium salt with iridium(IV) chloride (IrCl ４ An oxygen-evolving electrode having the iridium-containing manganese oxide of this comparative example was obtained in the same manner as in Comparative Example 1, except that the iridium-containing manganese oxide of β-type MnO was used. From its XRD pattern, the iridium-containing manganese oxide was found to be β-type MnO ２ It had the following crystal structure, with a full width at half maximum (FMAX) 1 of 1.10° and a full width at half maximum (FMAX) 2 of 1.33°. Furthermore, the iridium-containing manganese oxide had an Ir / Mn molar ratio of 0.024 (the amount of Mn and Ir per geometric area of ​​the conductive substrate was 1.2 mg / cm²). ２ and 0.1 mg / cm ２ ). 【0058】 【0059】 In both the examples and comparative examples, it was confirmed that heat treatment yielded Ir-Mn oxides having a β-type crystal structure. However, in the example using an electrolyte containing an ammonium salt, it was confirmed that δ-type manganese dioxide was electrolytically extracted as manganese oxide by electrolysis, and that heat treatment of the Ir-Mn oxide yielded Ir-Mn oxides with a full width at half maximum of 1.05° or less, further 1.00° or less, and further 0.80° to 0.98°. In PEM-type water electrolysis using these Ir-Mn oxides as oxygen-evolving electrode catalysts, the current density in the examples was 3.3 A / cm². ２ Furthermore, 3.4 A / cm ２ It was confirmed that the electrode exhibited high oxygen-evolving catalytic activity, enabling highly efficient water electrolysis. 【0060】 Furthermore, the entire contents of the specification, claims, and abstract of Japanese Patent Application No. 2024-216094, filed on December 11, 2024, are incorporated herein by reference as part of the disclosure of this specification.

Claims

1. β-type MnO ２ It is an iridium-containing manganese oxide having the following crystal structure, β-type MnO ２ Iridium-containing manganese oxide having a full width at half maximum of 0.65° or more and 1.05° or less for the XRD peak corresponding to the (211) plane.

2. β-type MnO ２ The iridium-containing manganese oxide according to claim 1, wherein the full width at half maximum of the XRD peak corresponding to the (220) plane is 0.55° or more and 2.00° or less.

3. The iridium-containing manganese oxide according to claim 1 or claim 2, wherein the Ir / Mn molar ratio is 0.001 or more and 0.100 or less.

4. An oxygen-evolving electrode catalyst comprising the iridium-containing manganese oxide described in any one of claims 1 to 3.

5. An oxygen-evolving electrode comprising the oxygen-evolving electrode catalyst according to claim 4 and a conductive substrate.

6. The content of the oxygen-evolving electrode catalyst is 0.1 mg / cm² per geometric area of ​​the oxygen-evolving electrode. ２ 12.0mg / cm or more ２ The oxygen-generating electrode according to claim 5.

7. The iridium content is 0.001 mg / cm² per geometric area of ​​the oxygen-evolving electrode. ２ 1.000mg / cm or more ２ The oxygen-generating electrode according to claim 5 or 6, which is as follows:

8. The oxygen-generating electrode according to any one of claims 5 to 7, wherein the conductive substrate contains titanium.

9. A water electrolysis apparatus comprising an oxygen generating electrode according to any one of claims 5 to 8.

10. A method for producing hydrogen by electrolyzing water using an oxygen-generating electrode according to any one of claims 5 to 8.

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