Iridium-containing manganese oxide

Abstract

The present invention provides 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 electrolytic cell equipped with the electrode; and a water electrolysis method using the water electrolytic cell. An iridium-containing manganese oxide having a β-type MnO2 crystal structure, wherein the full width at half maximum of the XRD peak corresponding to the (211) plane of β-type MnO2 is between 0.65° and 1.05°.

Description

Technical Field

[0001] The present disclosure relates to an iridium-containing manganese oxide, an oxygen generation electrode catalyst, an oxygen generation electrode and a water electrolysis device, and a method for producing hydrogen by water electrolysis using the oxygen generation electrode.

Background Art

[0002] Due to the depletion problem of fossil fuels and environmental pollution problems, the use of hydrogen as a clean energy and its production technology are being studied. Water electrolysis is one of the effective means for producing high purity hydrogen gas, and among them, the solid polymer membrane (PEM: Proton Exchange Membrane) type water electrolysis method has attracted attention. In the PEM type water electrolysis method, an iridium-based catalyst is widely known as a highly active oxygen generation electrode catalyst (for example, Non-Patent Document 1). However, iridium has an extremely small reserve compared to other precious metals. Therefore, there is a resource problem that a sufficient amount of catalyst cannot be secured for the future spread of water electrolysis technology. Under such a background, an 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 generation electrode catalyst with a reduced amount of iridium used.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[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. [Means for solving the problem]

[0006] This disclosure focuses on iridium-containing manganese oxides, examining their crystallinity and oxygen evolution electrode catalytic activity. The results show that specific crystal planes in iridium-containing manganese oxides influence oxygen evolution electrode catalytic activity, and that controlling the crystallinity of these specific crystal planes improves oxygen evolution electrode catalytic activity. In other words, the present invention is as described in the claims, and its gist is as follows. [1] An iridium-containing manganese oxide having a crystalline 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 between 0.65° and 1.05°. [2] The iridium-containing manganese oxide described in [1], wherein the full width at half maximum of the XRD peak corresponding to the (220) plane of β-type MnO2 is 0.55° or more and 2.00° or less. [3] The iridium-containing manganese oxide according to [1] or [2] 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 in any one of [1] to [3] above. [5] An oxygen-evolving electrode comprising the oxygen-evolving electrode catalyst described in [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 More than 12.0mg / cm 2 The oxygen-generating electrode described in [5] above. [7] The iridium content is 0.001 mg / cm² per geometric area of ​​the oxygen-evolving electrode. 2 More than 1.000mg / cm 2 The oxygen-generating electrode described in [5] or [6] above, which is as follows: [8] The oxygen-generating electrode according to any one of [5] to [7], wherein the conductive substrate contains titanium. [9] A water electrolysis apparatus comprising an oxygen generating electrode as described in any one of [5] to [8] above.

[10] A method for producing hydrogen by electrolyzing water using an oxygen-generating electrode described in any one of [5] to [8] above. [Effects of the Invention]

[0007] This disclosure provides 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 electrolytic cell equipped with the electrode; and a water electrolysis method using the water electrolytic cell. [Modes for carrying out the invention]

[0008] An embodiment of this disclosure is described below. This disclosure includes any combination of the configurations and numerical values ​​disclosed herein, and also includes a range consisting of any combination of the upper and lower limits of the numerical values ​​disclosed herein.

[0009] The iridium-containing manganese oxide of this embodiment (hereinafter also referred to as "Ir-Mn oxide") is an oxide having a crystalline structure of β-type MnO2, and 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. The Ir-Mn oxide of this embodiment contains iridium (Ir). The iridium only needs to be present 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; iridium may be contained in one or more forms selected from the group of metals, cations, and compounds, and it is preferable that it is contained in one or more forms selected from the group of metals, cations, and oxides. The iridium contained in the Ir-Mn oxide of this embodiment is preferably contained in the manganese oxide as iridium ions, and it is more preferable that at least a portion of the iridium ions are substituted for manganese ions in the manganese oxide, i.e., that at least a portion of the iridium is solid-dissolved in the manganese oxide. The state in which iridium is supported on manganese oxide is sufficient if iridium is present in one or more locations 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, it may be supported in the form of an oxide, or more specifically, as iridium oxide. When iridium is supported within the pores of manganese oxide, it may be contained as a cation, or more specifically, as iridium cations in ion-exchangeable sites of the manganese oxide. When iridium is dissolved in manganese oxide, at least a portion of the iridium cations may be substituted for manganese ions in the manganese oxide. The shape of the Ir-Mn oxide is not particularly limited, but can be any shape, such as a thin film, granules, or powder. The Ir-Mn oxide has a crystal structure that is that of β-type MnO2, and preferably has a crystal structure consisting of a single phase of β-type MnO2. However, it may also have a crystal structure consisting of a mixed phase of β-type MnO2 and one or more crystal structures selected from the group of γ-type MnO2, δ-type MnO2, and α-type MnO2. 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 registered in the ICDD (International Diffraction Data Center) PDF (Powder Diffraction File) (hereinafter also referred to as "reference pattern"). The reference patterns for the crystal structures of γ-type MnO2, β-type MnO2, δ-type MnO2, or α-type MnO2 can be 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 any pattern obtained by XRD measurement using a general powder X-ray diffractometer (for example, instrument name: Ultima IV Protectus, manufactured by Rigaku Corporation) under the following conditions. Acceleration current / voltage: 40mA / 40kV Radiation source: CuKα radiation (λ=1.5405Å) Measurement mode: Continuous scan Scanning conditions: 4° / min Measurement range: 2θ = 10° to 80° Divergence vertical limiting slit: 10mm Divergence / Induction Slit: 1° Light-receiving slit: open Detector: D / teX Ultra Nifilter 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 full width at half maximum can be measured 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 using the second derivative method on an XRD pattern that has undergone smoothing using a 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 removal of Kα2 lines with an intensity ratio of 0.4970. XRD pattern analysis, including smoothing, 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] In this embodiment, the Ir-Mn oxide preferably has an XRD peak in its XRD pattern that corresponds to at least the (211) plane of β-type MnO2 (hereinafter also referred to as "P(211)"), and also has an XRD peak that corresponds to both P(211) and the (220) plane of β-type MnO2 (hereinafter also referred to as "P(220)"). In this embodiment, the Ir-Mn oxide has a full width at half maximum (FMAX) of P(211) of 1.05° or less (hereinafter also referred to as "FMAX 1"). A FMAX 1 of 1.00° or less, 0.98° or less, or 0.95° or less is preferred. To exhibit high oxygen-evolving electrode catalytic activity (hereinafter also simply referred to as "catalytic activity"), a FMAX 1 of 0.65° or more is preferred, and 0.70° or more, or 0.80° or more, or 0.85° or more is preferred. Preferred FMAX 1 values ​​for the Ir-Mn oxide in this embodiment include 0.70° or more and 0.98° or less, or 0.80° or more and 0.95° or less. One possible reason why the Ir-Mn oxide in this embodiment satisfies the above-mentioned full width at half maximum (FMAX) of 1, resulting in an electrode with high catalytic activity, is as follows: That is, the FMAX of half maximum (FMAX) of 1 is one of the indicators of the crystallinity of the (211) plane of β-type MnO2. The (211) plane of β-type MnO2 is prone to water adsorption, and is therefore considered to be a crystal plane where oxygen evolution reactions preferentially occur. It is thought that by having the FMAX of half maximum (FMAX) of 1 within the above-mentioned range, structural defects that inhibit oxygen evolution are reduced, and a sufficient reaction field can be provided for the oxygen evolution reaction.

[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 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 with its peak top at 2θ = 56.4 ± 1°, and P(220) can be considered as a peak with 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 Ir-Mn oxide of this embodiment is effective, 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 PerkinElmer). 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 manganese [mol] obtained. 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 the 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 the "Ir-Mn electrode"). The Ir-Mn electrode of this embodiment may comprise 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 part or all, or at least part of, of the conductive substrate is coated with an oxygen-evolving electrode catalyst.

[0015] The Ir-Mn oxide content 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 More than 0.5mg / cm 2 Above or equal to 0.8 mg / cm³ 2 Preferably, the concentration is 12.0 mg / cm³ or higher. 2 Below, 11.0mg / cm2 The following, or 10.0 mg / cm 2 or less is preferable, 0.1 mg / cm 2 or more and 12.0 mg / cm 2 or less, 0.5 mg / cm 2 or more and 11.0 mg / cm 2 or less, 0.8 mg / cm 2 or more and 10.0 mg / cm 2 or less can be mentioned. When the unit catalyst content is 0.1 mg / cm 2 or more, the coating rate of the Ir-Mn oxide on the conductive substrate is improved. On the other hand, when the catalyst content is 12.0 mg / cm 2 or less, the electrical resistance of the electrode becomes small. As a result of these, it becomes easier to exhibit high catalytic activity.

[0016] The unit catalyst content in the electrode of this embodiment is obtained from the following formula (1). Unit catalyst content (mg / cm 2 ) = [Amount of Mn contained in the electrode (mg) + Amount of Ir contained in the electrode (mg)] / Geometric area of the electrode (cm 2 ) (1)

[0017] The "amount of Mn contained in the electrode" and the "amount of Ir contained in the electrode" in the above formula are values obtained by multiplying the Mn concentration (mg / L) and the Ir concentration (mg / L) of the Ir-Mn electrode solution described later by the volume (L) of the Ir-Mn electrode solution, respectively. The Ir-Mn electrode solution is a solution obtained by dissolving the Ir-Mn oxide contained in the electrode. This can be obtained by immersing the oxygen evolution electrode at 80°C or higher and 95°C or lower for 2 hours or more and 4 hours or less using a mixed acid of hydrochloric acid and nitric acid as a dissolution solution. The amount of the dissolution solution may be 1 mL or more and 20 mL or less per 1 mg of the Ir-Mn oxide substance mass as the ratio of the Ir-Mn oxide substance mass to the volume of the dissolution solution, as long as the entire amount of the Ir-Mn oxide can be dissolved. The Mn concentration and the Ir concentration can be determined by ICP emission spectroscopic analysis using a general ICP device (for example, device name: Optima 830, manufactured by PerkinElmer). 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 / cm³. 2 More than 0.005mg / cm 2 More than 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, it is 0.001 mg / cm³ 2 More than 1.0mg / cm 2 Below, 0.005mg / cm 2 More than 0.75mg / cm 2 The following, or 0.01 mg / cm³ 2 More than 0.5mg / cm 2 The following can be cited: When the upper or lower limit of the iridium content reaches this value, the dispersibility of iridium in the Ir-Mn electrode increases. As a result, the utilization rate of iridium as an active site in the oxygen evolution reaction increases. Therefore, when combined with the conductive substrate described later, it is more likely to exhibit higher catalytic activity. The conductive substrate in the Ir-Mn electrode can be any substrate made of a conductive material, and is preferably 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, more preferably platinum-coated titanium mesh, and even more preferably platinum-coated sintered titanium mesh, as they tend to exhibit high catalytic activity.

[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 More than 0.7mg / cm 2 or more, or 1.0 mg / cm³ 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 / cm³ 2 More than 10.0mg / cm 2 Below, 0.7mg / cm 2 More than 8.0mg / cm 2 The following, or 1.0 mg / cm³ 2 More than 6.0mg / cm 2 The following are listed:

[0021] To facilitate the expression of excellent oxygen-evolving electrode catalyst 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 of the conductive substrate perpendicular to its geometric plane, and is the length corresponding to the thickness when the electrode shape is defined by length × width × thickness. It is the projected length of the plane perpendicular to the plane (geometric plane) that faces the electrolyte membrane when constructing 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. The porosity in this embodiment can be determined from the following formula. Porosity (%) = {1 - (bulk density of 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 (model 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 ) Conductive substrate skeleton density (g / cm³) 3 ) = Mass of conductive substrate (g) / {Volume of conductive substrate - Pore volume} (cm³) 3 )

[0023] In the two equations above, 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 amount: 40 mg Mercury introduction pressure: 601.6 psia to 36,098.1 psia (4.1 MPa to 248.9 MPa) Measuring pore diameter: 5nm~500μm Cell used: Glass cell with an indentation volume of 5.3cc Mercury surface tension: 480dyn Mercury contact angle: 130° Pretreatment conditions: Degassing treatment at 110°C for at least 1 hour 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 in this embodiment is not particularly limited as long as it yields Ir-Mn oxide having the above-described structure. 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 on the electrodes of the electrolytic device. This yields manganese oxide, which serves as the base material for Ir-Mn oxide. The electrolysis method is arbitrary as long as manganese oxide can be electrolyzed; any method involving immersing the electrodes of an electrolytic device in an electrolyte and electrolyzing it is acceptable. By electrolyzing 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 becomes uniform in the heat treatment described later, and the total 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. + It is thought that ) has 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] The ammonium salt concentration is ammonium nitrogen (NH4) + 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 between 0.05 mol / L and 1.00 mol / L. This sulfuric acid concentration suppresses excessive crystal growth of manganese oxide. Higher sulfuric acid concentrations within this range tend to result in a smaller total width at half maximum of the resulting Ir-Mn oxide.

[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, the current level is 20 mA / cm². 2 The following, or 10mA / cm² 2 Preferably, it is 1.0 mA / cm². 2 More than 20mA / cm 2 The following, or 5.0 mA / cm² 2 More than 10mA / cm 2 The following can be cited: When the current density is within the above range, the crystal growth of manganese oxide tends 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 emission of manganese oxide. The energizing time in electrolysis (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 examples include 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. By having the electrolysis time within the above range, manganese oxide can be uniformly electrolyzed more easily.

[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 step can be any solution containing an iridium salt, with a solution containing an iridium salt being preferred, and an aqueous solution containing an iridium salt being more preferred. The iridium salt can be one or more selected from the group consisting of iridium(III) chloride (IrCl3), iridium(IV) chloride (IrCl4), and iridium nitrate (Ir(NO3)4), and it is preferable that it be at least one of iridium(III) chloride and iridium nitrate, with iridium(III) chloride being more preferred. 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, as this allows for efficient contact between iridium and manganese oxide, making it easier for iridium to be incorporated into the manganese oxide. Examples include between 0.001 mmol / L and 5.000 mmol / L, or between 0.005 mmol / L and 1.000 mmol / L. A higher iridium concentration within this range tends to result in a higher iridium content in the obtained 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 solid-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] The heat treatment process is not particularly limited as long as it provides a thermal history that causes a structural change from δ-type MnO2 to β-type MnO2, but one possible method is heating in an electric furnace. The precursor compound obtained in the contact process can be filled into a firing container and heat-treated. Alternatively, the precursor compound obtained in the contact process 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 and 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 set appropriately depending on the amount of manganese oxide and other materials subjected to the heat treatment, but examples include 10 minutes to 24 hours.

[0040] The method for manufacturing the oxygen-evolving 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 resulting manganese oxide-coated substrate with an iridium salt, and a heat treatment step of heat-treating the resulting Ir-Mn oxide-coated substrate. In this method, a uniform Ir-Mn oxide layer is easily formed on the conductive substrate, and it is easier to exhibit high catalytic activity when used as an oxygen-evolving electrode. Another method involves obtaining an Ir-Mn electrode by converting the Ir-Mn oxide obtained by the above-mentioned method for producing Ir-Mn oxide into a catalytic ink, and then applying the catalytic ink to a conductive substrate. The catalyst ink described above only needs to be in a state where Ir-Mn oxide is dispersed in a solvent, and the solvent can be water, alcohol, or a mixture of water and alcohol.

[0041] The oxygen-evolving electrode catalyst containing Ir-Mn oxide of 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 include an electrolyte membrane, the oxygen-evolving electrode catalyst containing the Ir-Mn oxide of this embodiment, and a conductive substrate. The oxygen-generating electrode catalyst of this embodiment can be used as a water electrolysis apparatus equipped with 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. [Examples]

[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, PerkinElmer).

[0044] <Identification of crystal structure> A standard powder X-ray diffractometer (instrument name: Ultima IV Protectus, manufactured by Rigaku Corporation) was used to obtain the XRD pattern under the following conditions. Acceleration current / voltage: 40mA / 40kV X-ray source: CuKα ray (λ = 1.5405 Å) Measurement mode: continuous scan Scan condition: 4° / min Measurement range: 2θ = 10° to 80° Divergent vertical limiting slit: 10 mm Divergent / incident slit: 1° Receiving slit: open Detector: D / teX Ultra Using Ni filter <Omitted content>[

[0045] <Omitted content>[ The crystal phase of the sample was identified by comparing the obtained XRD pattern with the reference pattern. The reference patterns of manganese dioxide having γ-type, β-type, δ-type or α-type crystal structures were PDF No. 14-0644 (γ-type), 24-0735 (β-type), 80-1098 (δ-type) or 44-0141 (α-type), respectively. Next, the widths 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 having a peak top at 2θ = 56.4 ± 1°) and the XRD peak corresponding to the (220) plane (XRD peak having a peak top at 2θ = 58.5 ± 1°) were determined and designated as full width at half maximum 1 and full width at half maximum 2, respectively. <Omitted content>[

[0046] <Omitted content>[ <Construction of PEM water electrolyzer>[ A PEM water electrolyzer equipped with a MEA prepared by the following method was fabricated using a conductive substrate coated with iridium-containing manganese oxide as the working electrode (anode). A conductive catalyst ink was prepared by mixing 20 mass% platinum-supported carbon catalyst (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich) into a solution containing water, ethanol and an ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich). This was applied to carbon paper (product name: TGP-H-060, manufactured by Toray) and air-dried to obtain the counter electrode. <Omitted content>[

[0047] <Omitted content>[ Next, a Nafion membrane (product name: Nafion 115, manufactured by Sigma-Aldrich) was washed and protonated by boiling in the following order: 3% by mass hydrogen peroxide solution for 1 hour, pure water for 1 hour, 1M sulfuric acid solution for 1 hour, and pure water for 1 hour, to obtain the electrolyte membrane. The electrolyte membrane is sandwiched between the catalyst surfaces of the working electrode (anode) and the counter electrode, and pressed using a hot press machine (product name: SA-302, manufactured by Tester Sangyo Co., Ltd.) at 135°C with a clamping force of 400 kg / cm². 2 MEA was obtained by hot pressing for 3 minutes. The obtained MEA was attached to the housing of a PEM-type water electrolysis cell (product name: WE-4S-RICW, manufactured by FC Development Co., Ltd.) to fabricate a PEM-type water electrolysis cell.

[0048] <Measurement of Oxygen Evolution Electrode Catalytic Activity> Using a PEM-type water electrolytic cell with the oxygen-evolving electrode catalysts of the examples and comparative examples as the working electrode (anode), the oxygen-evolving electrode catalyst activity was evaluated by linear sweep voltammetry (LSV) of a two-electrode system under the following conditions. Voltage increase rate: 10mV / sec Water temperature: 80℃ Water supply rate: 2mL / min From this evaluation, the current density at a voltage of 2V (hereinafter also simply referred to as "current density") was determined.

[0049] Example 1 A mixed solution of sulfuric acid, manganese sulfate, and ammonium sulfate with an electrolyte concentration of 0.30 mol / L, sulfuric acid, 0.50 mol / L, and ammonium sulfate, is packed into an electrolytic cell. A platinum-coated Ti fiber sintered body is then formed (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 / cm²). 2 A conductive substrate made of ) was immersed in the mixture. The temperature of the mixed solution was maintained at 95°C, and a current density of 7 mA / cm² was applied to the conductive substrate. 2 A current was applied for 10 minutes to electrolyze manganese oxide onto the conductive substrate. From the XRD pattern, the manganese oxide was identified as δ-type manganese dioxide. Next, the conductive substrate on which the manganese oxide had precipitated was immersed at 95°C for 24 hours in an iridium salt aqueous solution containing iridium(IV) nitrate (Ir(NO3)4) 0.1 mmol / L and sulfuric acid 0.01 mol / L. After that, it was annealed in an air atmosphere at 450°C for 5 hours to obtain an oxygen-evolving electrode having the iridium-containing manganese oxide of this example. The iridium-containing manganese oxide had a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1 being 0.95° and a full width at half maximum (FMAX) of 2 being 1.08°. 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 / cm³ 2 ).

[0050] Example 2 An oxygen-evolving electrode having an iridium-containing manganese oxide was obtained using the same method as in Example 1, except that iridium(IV) chloride (IrCl4) was used as the iridium salt. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1 being 0.84° and a full width at half maximum (FMAX) of 2 being 1.06°. 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, respectively). 2 and 0.1 mg / cm³ 2 ).

[0051] Example 3 Manganese oxide was electrochemically deposited on a conductive substrate using the same method as in Example 1, except that the sulfuric acid concentration of the electrolyte was set to 0.10 mol / L. From the XRD pattern, the manganese oxide was identified as δ-type manganese dioxide. Next, the conductive substrate on which the manganese oxide obtained above had precipitated was immersed in an iridium salt aqueous solution containing iridium(III) chloride (IrCl3) 0.1 mmol / L and sulfuric acid 0.01 mol / L at 95°C for 24 hours. After that, it was annealed in an air atmosphere at 450°C for 5 hours to obtain an oxygen-evolving electrode having the iridium-containing manganese oxide of this example. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1 being 0.92° and a full width at half maximum (FMAX) of 2 being 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, respectively). 2 and 0.1 mg / cm³ 2 ).

[0052] Example 4 Manganese oxide was electrochemically deposited on a conductive substrate using the same method 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, the conductive substrate on which the manganese oxide obtained above had precipitated was immersed in an iridium salt aqueous solution containing iridium(III) chloride (IrCl3) 0.1 mmol / L and sulfuric acid 0.01 mol / L at 95°C for 24 hours. After that, it was annealed in an air atmosphere at 450°C for 5 hours to obtain an oxygen-evolving electrode having the iridium-containing manganese oxide of this example. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1 being 0.95° and a full width at half maximum (FMAX) of 2 being 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, respectively). 2 and 0.1 mg / cm³ 2 ).

[0053] Example 5 An oxygen-evolving electrode containing iridium-containing manganese oxide was obtained using the same method as in Example 2, except that the concentration of iridium(IV) chloride (IrCl4) was set to 0.2 mmol / L. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1 being 0.86° and a full width at half maximum (FMAX) of 2 being 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 / cm³ 2 ).

[0054] Example 6 An oxygen-evolving electrode containing iridium-containing manganese oxide was obtained using the same method as in Example 2, except that the concentration of iridium(IV) chloride (IrCl4) was set to 0.3 mmol / L. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1 being 0.97° and a full width at half maximum (FMAX) of 2 being 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, respectively). 2 and 0.3 mg / cm³ 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, 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 potassium hexachloroiridiate (K2IrCl6) and 0.01 mol / L sulfuric acid at 95°C for 24 hours. After that, it was 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. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1.73° and a full width at half maximum (FMAX) of 2.84°. 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 / cm³ 2 ).

[0056] Comparative Example 2 An oxygen-evolving electrode containing iridium-containing manganese oxide was obtained in the same manner as in Comparative Example 1, except that hexachloroiridiic acid (H2IrCl6) was used as the iridium salt. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1.26° and a full width at half maximum (FMAX) of 2.42°. 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 / cm³ 2 ).

[0057] Comparative Example 3 An oxygen-evolving electrode containing iridium-containing manganese oxide was obtained in the same manner as in Comparative Example 1, except that iridium(IV) chloride (IrCl4) was used as the iridium salt. The iridium-containing manganese oxide, based on its XRD pattern, possessed a β-type MnO2 crystal structure, with a full width at half maximum (FMAX) of 1.10° and a full width at half maximum (FMAX) of 2.33°. 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, respectively). 2 and 0.1 mg / cm³ 2 ).

[0058] [Table 1]

[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 this 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 and included as part of the disclosure of the 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 claim 1 or claim 2.

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, which is as follows:

8. The oxygen-generating electrode according to claim 5, wherein the conductive substrate contains titanium.

9. A water electrolysis apparatus comprising the oxygen generating electrode described in claim 5.

10. A method for producing hydrogen by electrolyzing water using the oxygen-generating electrode described in claim 5.

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