Oxygen generating electrode and water electrolysis method

Abstract

Provided is: an oxygen generating electrode in which a high electrolytic current density can be obtained even with a content of a noble metal within a certain range, the oxygen generating electrode comprising a catalyst containing an iridium-containing manganese oxide combined with a conductive base material containing platinum; and / or a water electrolysis method using the electrode. The oxygen generating electrode comprises a conductive base material and a catalyst containing an iridium-containing manganese oxide. The conductive base material contains platinum, the total of the amount of iridium per geometric area of the oxygen generating electrode and the amount of platinum per geometric area of the oxygen generating electrode is above 0.1 mg / cm2 and 6.1 mg / cm2 or less, and the ratio of the amount of platinum per geometric area of the oxygen generating electrode to the amount of iridium per geometric area of the oxygen generating electrode is 1 or more and less than 600.

Description

Oxygen generating electrode and water electrolysis method

[0001] The present disclosure relates to an oxygen generating electrode and a water electrolysis method.

[0002] Water electrolysis can produce highly pure hydrogen, a clean energy source. Among catalysts used in oxygen generating electrodes in water electrolysis (hereinafter also referred to as "oxygen generating electrode catalysts"), iridium-based catalysts are widely known as highly active oxygen generating electrode catalysts (see, for example, Non-Patent Document 1). However, iridium reserves are extremely scarce, and sufficient supply cannot be guaranteed in the future. For this reason, iridium-containing manganese oxides (see Patent Document 1) have been reported as oxygen generating electrode catalysts with reduced iridium content.

[0003] EP4357485A1

[0004] F. Birol, World Energy Outlook 2016, International Energy Agency (IEA), Paris, 2016.

[0005] The oxygen generating electrode catalyst of Patent Document 1 is used as an oxygen generating electrode in combination with a conductive substrate containing platinum, and exhibits high catalytic activity. However, because it uses large amounts of precious metals such as iridium and platinum, it remains expensive as an electrode. However, it is believed that reducing the amount of precious metal contained in the oxygen generating electrode significantly reduces the electrolysis current density and hydrogen productivity. The present disclosure aims to provide at least one of an oxygen generating electrode that combines a catalyst containing an iridium-containing manganese oxide with a conductive substrate containing platinum, and that can achieve a high electrolysis current density even with a certain range of precious metal usage, and a water electrolysis method using the electrode.

[0006] In this disclosure, the present inventors have focused on the content of precious metals in an oxygen generating electrode that combines an iridium-containing manganese oxide catalyst and a conductive base material containing platinum, and have investigated the relationship between the content or content ratio of precious metals and electrolysis current density. As a result, they have found that a high electrolysis current density can be obtained by controlling the total amount of precious metals within a certain range and by setting the ratio of the platinum content in the oxygen generating electrode to the iridium content in the oxygen generating electrode within a certain range. That is, the present invention is as set forth in the claims, and the gist of the present disclosure is as follows.

[0007] [1] An oxygen generating electrode comprising a catalyst containing iridium-containing manganese oxide and a conductive substrate, wherein the conductive substrate contains platinum, and the sum of the amount of iridium per geometric area of ​​the oxygen generating electrode and the amount of platinum per geometric area of ​​the oxygen generating electrode is 0.1 mg / cm. ２ Super 6.1mg / cm ２ and the ratio of the amount of platinum per geometric area of ​​the oxygen generating electrode to the amount of iridium per geometric area of ​​the oxygen generating electrode is 1 or more and less than 600. [2] An oxygen generating electrode in which the amount of iridium per geometric area of ​​the oxygen generating electrode is 0 mg / cm or less. ２ Super 1mg / cm ２ [3] The oxygen generating electrode according to [1], wherein the amount of platinum per geometric area of ​​the oxygen generating electrode is 0 mg / cm or less. ２ Super 6mg / cm ２ [4] The oxygen generating electrode according to [1] or [2], wherein the total amount of iridium per geometric area and the amount of platinum per geometric area of ​​the oxygen generating electrode is 1.0 mg / cm or less. ２ Super 4.5mg / cm ２ [5] An oxygen generating electrode according to any one of [1] to [3], which is the following: [5] A water electrolysis method using the oxygen generating electrode according to any one of [1] to [4].

[0008] The present disclosure will be described in detail below. However, the following description of the constituent elements is an example of an embodiment of the present disclosure, and the present disclosure is not limited to these contents. Furthermore, the present disclosure includes any combination of the configurations and parameters disclosed in this specification, and also includes any combination of the upper and lower limits of the values ​​disclosed in this specification. In this specification, "to" means a numerical range including both ends of the numerical value.

[0009] The present embodiment relates to an oxygen generating electrode comprising a catalyst containing iridium-containing manganese oxide and a conductive substrate, wherein the conductive substrate contains platinum, and the sum of the amount of iridium per geometric area of ​​the oxygen generating electrode and the amount of platinum per geometric area of ​​the oxygen generating electrode is 0.1 mg / cm. ２ Super 6.1mg / cm ２ or less, and the ratio of the amount of platinum per geometric area of ​​the oxygen generating electrode to the amount of iridium per geometric area of ​​the oxygen generating electrode is 1 or more and less than 600 (hereinafter also referred to as the "electrode of this embodiment").

[0010] In the present disclosure, the term "geometric area" refers to the area corresponding to the projected area of ​​the largest surface of an electrode (including a conductive substrate constituting the electrode and a support constituting the conductive substrate; hereinafter the same applies unless otherwise specified), i.e., the area of ​​the projected image when the surface is projected in the normal direction. The geometric area of ​​an electrode does not take into account the thickness of the electrode or the state of the surface having the geometric area (presence or absence of irregularities, presence or absence of holes, etc.). For example, when an electrode has two opposing main surfaces, the two main surfaces have the same shape and dimensions (area), and the distance between the two main surfaces (corresponding to the thickness of the electrode) is shorter than the perimeter or the length of one side of the main surfaces (so-called flat), the geometric area of ​​the electrode may be the area of ​​the main surfaces. Furthermore, the electrode of this embodiment is typically partially covered with an electrolyte membrane (solid polymer membrane) together with a separate hydrogen generation electrode to form a membrane-electrode assembly (hereinafter also referred to as "MEA"), and the geometric area of ​​the electrode may be the area of ​​the surface facing the electrolyte membrane.

[0011] (Iridium-Containing Manganese Oxide) The electrode of this embodiment includes a catalyst containing iridium-containing manganese oxide (hereinafter also referred to as "IrMn oxide"). The electrode of this embodiment typically has a configuration in which a catalyst containing IrMn oxide covers at least a portion of the surface of a conductive substrate. The catalyst may be made of IrMn oxide. The IrMn oxide contained in the electrode of this embodiment acts as a catalyst for the reaction that produces oxygen molecules from water molecules in a water splitting reaction.

[0012] IrMn oxide is a manganese oxide containing iridium (Ir). The manganese oxide contained in IrMn oxide is manganese dioxide (MnO ２－ｘ Preferably, the x-ray diffraction pattern satisfies the following condition: 0≦x<0.5), and is more preferably manganese dioxide having an α-, β-, γ-, or ε-type crystal structure, or manganese dioxide having two or more of these crystal structures, and even more preferably manganese dioxide containing at least a β-type crystal structure. The crystal structure of the manganese oxide can be identified by comparing its powder X-ray diffraction (hereinafter also referred to as "XRD") pattern with the XRD pattern (hereinafter also referred to as "reference pattern") registered in the ICDD (International Center for Diffraction Data) Powder Diffraction File (registered trademark). The reference patterns for manganese dioxide having an α-, β-, γ-, or ε-type crystal structure are the XRD patterns registered as PDF No. 44-0141 (α-type), 24-0735 (β-type), 14-0644 (γ-type), or 30-0820 (ε-type), respectively.

[0013] In this embodiment, the XRD pattern is obtained by XRD measurement using a general powder X-ray diffractometer (for example, an Ultima IV Protectus, manufactured by Rigaku Corporation) under the following conditions: Acceleration current / voltage: 40 mA / 40 kV Radiation source: CuKα radiation (λ=1.5405 Å) Measurement mode: Continuous scan Scan conditions: 4° / min Measurement range: 2θ=10° to 80° Divergence vertical limiting slit: 10 mm Divergence / entrance slit: 1° Receiving slit: open Detector: D / teX Ultra Ni filter used

[0014] Preferable manganese oxides include at least one of electrolytic manganese dioxide (EMD) and chemical manganese dioxide (CMD), with electrolytic manganese dioxide being preferred. Note that electrolytic manganese dioxide is manganese dioxide obtained by electrolytic deposition, and chemical manganese dioxide is manganese dioxide obtained by chemical synthesis.

[0015] The iridium contained in the IrMn oxide may be contained in a state in which it can interact with the manganese oxide. Examples of states in which the interaction between iridium and manganese oxide can occur include one or more selected from the group consisting of a state in which iridium and manganese oxide are mixed, a state in which iridium is supported on manganese oxide, and a state in which iridium is solid-dissolved in manganese oxide. Preferably, at least a portion of the iridium is solid-dissolved in manganese oxide.

[0016] The form of iridium contained in the IrMn oxide may be one or more selected from the group consisting of metals, cations, and compounds, and is preferably a cation (iridium ion).Iridium is preferably contained in the manganese oxide at least as a cation (iridium ion), as this facilitates the exhibiting of high oxygen evolution electrode catalytic activity.

[0017] Examples of a state in which iridium is supported on a manganese oxide include a state in which iridium is contained on at least one of the surface and pores of the manganese oxide, preferably a state in which iridium is contained in ion-exchangeable sites of the manganese oxide. When iridium is supported on a manganese oxide, the iridium supported on the manganese oxide can be at least one of an oxide and a cation (iridium cation). When iridium is supported on a manganese oxide, the iridium can be supported as an oxide (iridium oxide) on the surface of the manganese oxide and as an iridium cation in the pores of the manganese oxide. Examples of a state in which iridium is dissolved in a manganese oxide include a state in which at least a portion of the iridium is contained by substituting manganese ions that constitute the framework of the manganese oxide.

[0018] In order to exhibit high oxygen evolution electrode catalytic activity, the IrMn oxide is preferably at least one of an iridium-supported manganese oxide and a manganese oxide in which iridium is solid-dissolved, and more preferably a manganese oxide in which part or all of iridium is solid-dissolved.

[0019] (Conductive Substrate) The electrode of this embodiment includes a conductive substrate. The conductive substrate contains platinum and typically includes platinum and a support.

[0020] The material of the support may be any material as long as it is conductive. Preferred examples of the support material include conductive carbon and / or metal, and preferred examples of the metal include titanium (Ti).

[0021] The shape of the support is not particularly limited. The shape of the support typically has two opposing main surfaces, the two main surfaces having the same shape and dimensions (area), and the distance between the two main surfaces (corresponding to the thickness of the support) is shorter than the perimeter or the length of one side of the main surfaces. The thickness of the support (thickness of the conductive substrate) is not particularly limited, but is preferably 1 mm or less from the viewpoint of facilitating the smooth supply of water, which is a reaction substrate for the oxygen generating reaction, and the smooth discharge of generated oxygen. The thickness of the support (thickness of the conductive substrate) can be measured using an optical microscope or vernier calipers.

[0022] The support may be perforated, having multiple holes communicating with the outside of the support, or non-perforated. Specific examples of non-perforated supports include conductive carbon plates and titanium plates. Perforated supports include cloth-like supports such as woven fabrics and non-woven fabrics, and mesh-like supports. The cloth-like support typically comprises an aggregate of multiple fibrous conductive materials, with the voids between the conductive materials functioning as support holes that contribute to improving catalytic activity during water electrolysis. The mesh-like support may have, for example, multiple scattered through-holes that penetrate the mesh-like support in the thickness direction. Specific examples of perforated supports include conductive carbon mesh, conductive carbon paper, sintered titanium fiber, sintered titanium powder, titanium nonwoven fabric, and titanium mesh. From the viewpoints of improving catalytic activity during water electrolysis and facilitating the discharge of gas (oxygen gas) generated during water electrolysis, the perforated support is preferred, with sintered titanium fiber being particularly preferred. The sintered titanium fiber typically comprises a heat-treated titanium nonwoven fabric composed of fibrous titanium.

[0023] When the support constituting the conductive substrate is of the perforated type, the conductive substrate has a plurality of pores. The porosity of the conductive substrate is, for example, preferably 40% or more, more preferably 50% or more, and preferably 90% or less. More specifically, the porosity of the conductive substrate is preferably 40% or more and 90% or less, more preferably 50% or more and 90% or less. When the porosity of the conductive substrate is within the above range, the mechanical strength of the electrode is increased, and water, which is a reaction substrate for the oxygen generation reaction, is more easily supplied to the electrode efficiently. The porosity of the conductive substrate is defined by the following formula (1) or (2):

[0024] Porosity (%) = {1 - (True volume of conductive substrate (cm ３ ) / apparent volume of conductive substrate (cm ３ ))} × 100 (1) In the formula (1), the "true volume of the conductive substrate" refers to the volume obtained by subtracting the "pore volume of the conductive substrate measured by mercury plethysmography" from the "apparent volume of the conductive substrate." The pore volume can be measured using a mercury porosimeter (for example, "AutoPore 9510" manufactured by Micromeritics). Examples of conditions for measuring the pore volume using a mercury porosimeter include the following. (Example of pore volume measurement conditions) Sample amount: 40 mg Mercury introduction pressure: 601.6 psia to 36,098.1 psia (4.1 MPa to 248.9 MPa) Measurement pore diameter: 5 nm to 500 μm Cell used: 5.3 cc glass cell with intrusion volume Mercury surface tension: 480 dyn Mercury contact angle: 130° Pretreatment conditions: air atmosphere, degassing treatment at 110°C for 1 hour or more) The "apparent volume of the conductive substrate" in the formula (1) is calculated by the following formula (1A). The "geometric area" in the formula (1A) is as described above. The apparent volume (cm ３ ) = geometric area of ​​the conductive substrate (cm ２ ) × thickness of conductive substrate (cm) ... (1A)

[0025] Porosity (%) = {1 - (bulk density of conductive substrate (g / cm ３ ) / skeletal density of conductive substrate (g / cm ３))} × 100 (2) In the formula (2), the "bulk density of the conductive substrate" is calculated by the following formula (2A), and the "skeletal density of the conductive substrate" is calculated by the following formula (2B). ３ ) = Mass of conductive substrate (g) / Apparent volume of conductive substrate (cm ３ ) (2A) Skeleton density of conductive substrate (g / cm ３ ) = Mass of conductive substrate (g) / {apparent volume of conductive substrate - pore volume} (cm ３ ) ... (2B) In the formulas (2A) and (2B), the "mass of the conductive substrate" can be measured using an electronic balance, and the "apparent volume of the conductive substrate" can be measured using a laser volume meter. Examples of the laser volume meter include a 3D scanner-type three-dimensional measuring machine (for example, the "VL-700 series" manufactured by Keyence Corporation).

[0026] The conductive substrate provided in the electrode of this embodiment contains platinum (Pt) in addition to the support. When the conductive substrate contains platinum, the conductivity of the conductive substrate is improved, corrosion of the support is suppressed, and the catalytic reaction proceeds efficiently. Platinum may be contained in the conductive substrate in a form that can contribute to the oxygen generation reaction, and may be at least one of being supported on the support and coating the support. The platinum contained in the conductive substrate may have any shape, and examples thereof include platinum particles. In the present disclosure, "platinum supported on a support" refers to a state other than the state in which "(platinum) coats the support" described above, i.e., a state other than a state in which platinum is attached to the surface of the support, and examples thereof include a state in which platinum is present inside the support. Preferred conductive substrates include at least one selected from the group consisting of platinum-supported conductive carbon plates, titanium plates, conductive carbon mesh, conductive carbon paper, titanium fiber sintered compact, titanium powder sintered compact, titanium nonwoven fabric, and titanium mesh, as well as platinum-coated conductive carbon plates, titanium plates, conductive carbon mesh, conductive carbon paper, titanium fiber sintered compact, titanium powder sintered compact, titanium nonwoven fabric, and titanium mesh, with platinum (Pt)-coated titanium fiber sintered compact being preferred.

[0027] (Total Amount of Iridium and Amount of Platinum per Geometric Area of ​​Oxygen Evolving Electrode) In the electrode of this embodiment, the total (hereinafter also referred to as the "total metal amount") of the amount of iridium per geometric area (hereinafter also referred to as the "unit Ir amount") and the amount of platinum per geometric area (hereinafter also referred to as the "unit Pt amount") of the oxygen evolving electrode is 0.1 mg / cm ２ Super 6.1mg / cm ２ The total metal content is 0.2 mg / cm ２ Above, 0.3mg / cm ２ or more than 1.0 mg / cm ２ More than 5.5 mg / cm ２ Below, 5.0mg / cm ２ or less than 4.5 mg / cm ２ Preferably, the density is 0.2 mg / cm or less. ２ 5.5mg / cm or more ２ Below, 0.3mg / cm ２ 5.0mg / cm or more ２ or less, or 1.0 mg / cm ２ 4.5mg / cm or more ２ If the total metal amount is outside the above range, the electrolytic current density in the electrolytic reaction decreases, resulting in a decrease in the activity of the oxygen generating electrode catalyst. Here, generally, iridium functions as an active site for the oxygen generating reaction, and platinum functions as a conductive layer for electrons generated by the oxygen generating reaction. When the total metal amount is 1.0 mg / cm or less, the electrolytic current density in the electrolytic reaction decreases, resulting in a decrease in the activity of the oxygen generating electrode catalyst. ２ 4.5mg / cm or more ２ Below, especially 1.0 mg / cm ２ 3.5mg / cm or more ２ or less, the utilization efficiency of iridium in the oxygen generation reaction is increased, and the utilization efficiency of platinum as the conductive layer is also increased, which tends to increase the electrolysis current density per total metal amount. This increase in the electrolysis current density per total metal amount allows the amount of precious metals used in the water electrolysis device to be reduced.

[0028] (Unit Ir amount) Unit Ir amount is 0 mg / cm ２ Super 1mg / cm ２ The unit Ir amount is preferably 0.01 mg / cm or less. ２or more than 0.05 mg / cm ２ More than 0.60 mg / cm ２ or less than 0.30 mg / cm ２ Preferably, the density is 0.01 mg / cm or less. ２ 0.60mg / cm or more ２ or less, or 0.05 mg / cm ２ 0.30mg / cm or more ２ When the unit Ir content is equal to or greater than the above-mentioned lower limit, the oxygen generating electrode catalytic activity is likely to be improved. On the other hand, when the unit Ir content is equal to or less than the above-mentioned upper limit, application to industrial production becomes easier.

[0029] (Unit Pt amount) For the same reason as the unit Ir amount, the unit Pt amount is 0 mg / cm ２ Super 6mg / cm ２ The unit Pt amount is preferably 0.5 mg / cm or less. ２ or more than 1.0 mg / cm ２ More than 5.0 mg / cm ２ Below, 4.0mg / cm ２ or less than 3.0 mg / cm ２ Preferably, the density is 0.5 mg / cm or less. ２ 5.0mg / cm or more ２ Below, 1.0mg / cm ２ 4.0mg / cm or more ２ or less than 1.0 mg / cm ２ 3.0mg / cm or more ２ When the unit Pt amount is equal to or greater than the above-mentioned lower limit, the conductivity of the conductive substrate is improved, and corrosion of the support constituting the conductive substrate is suppressed, making it easier for the catalytic reaction to proceed efficiently. On the other hand, when the unit Pt amount is equal to or less than the above-mentioned upper limit, application to industrial production becomes easier.

[0030] (Unit Mn Amount) In the electrode of this embodiment, the manganese amount per geometric area (hereinafter also referred to as "unit Mn amount") is 0.12 mg / cm ２ or more than 0.60 mg / cm ２ More than 6.0 mg / cm is preferable. ２ or less than 3.0 mg / cm ２Preferably, the density is 0.12 mg / cm or less. ２ 6.0mg / cm or more ２ or less than 0.60 mg / cm ２ 3.0mg / cm or more ２ When the Mn content is equal to or greater than the lower limit, the oxygen generating electrode catalytic activity is likely to be improved. On the other hand, when the Mn content is equal to or less than the upper limit, the application to industrial production is facilitated.

[0031] The contents of iridium, platinum, and manganese in the electrode of this embodiment can be measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a general ICP device. That is, the electrode of this embodiment is immersed in a mixed solution of hydrochloric acid and nitric acid to dissolve the metal components, and the resulting solution is measured by ICP-AES using an ICP device to measure the concentrations (mass%) of iridium, platinum, and manganese contained in the sample solution. From the obtained concentrations of each element, the contents per geometric area of ​​the electrode can be calculated using the following formula. W Ｉｒ = (C Ｉｒ ×W) / SW Ｐｔ = (C Ｐｔ ×W) / SW Ｍｎ = (C Ｍｎ × W) / S In the above formula, W Ｉｒ , W Ｐｔ BiW Ｍｎ are the unit amounts of Ir, Pt, and Mn [mg / cm ２ ] and C Ｉｒ , C Ｐｔ and C Ｍｎ are the concentrations [mass %] of iridium, platinum, and manganese in the sample solution, respectively; W is the mass [mg] of the sample solution; and S is the geometric area [cm ２ ].

[0032] (Pt / Ir ratio) In the electrode of this embodiment, the ratio of the unit amount of Pt to the unit amount of Ir (hereinafter also referred to as "Pt / Ir ratio") is 1 or more and less than 600. The Pt / Ir ratio is preferably 5 or more or 10 or more, and is preferably 300 or less, 150 or less, or 35 or less. Furthermore, it is preferably 5 or more and 300 or less, 10 or more and 150 or less, or 10 or more and 35 or less. If the Pt / Ir ratio is less than the above-mentioned lower limit or exceeds the above-mentioned upper limit, the oxygen generating electrode catalytic activity decreases.

[0033] (Ir / Mn molar ratio) In the electrode of this embodiment, the ratio of the number of moles of Ir to the number of moles of Mn (hereinafter also referred to as "Ir / Mn molar ratio") is preferably 0.001 or more or 0.01 or more, and preferably 0.10 or less or 0.08 or less. Also, 0.001 or more and 0.10 or less, or 0.01 or more and 0.08 or less is preferred. If the Ir / Mn molar ratio is less than the above-mentioned lower limit or exceeds the above-mentioned upper limit, the oxygen generating electrode catalytic activity is likely to decrease.

[0034] A method for producing an electrode according to this embodiment will be described below. Examples of the method for producing an electrode according to this embodiment include a production method (hereinafter also referred to as a "supporting method") that includes a step of supporting IrMn oxide on a conductive substrate and a step of annealing the conductive substrate after supporting IrMn oxide, or a production method (hereinafter also referred to as an "electrodeposition method") that includes a step of electrodepositing manganese oxide on a conductive substrate, a step of supporting iridium on the conductive substrate on which manganese oxide has been electrodeposited, and a step of annealing the conductive substrate after supporting IrMn oxide. The electrodeposition method is preferred because it is easily applicable to industrial applications.

[0035] (Supporting Method) The supporting method includes a step of supporting IrMn oxide on a conductive substrate (hereinafter also referred to as the "supporting step") and a step of annealing the conductive substrate after supporting the IrMn oxide (hereinafter also referred to as the "annealing step"). The IrMn oxide used in the supporting step may be any IrMn oxide that can be supported on a conductive substrate, but is preferably an IrMn oxide (iridium-supported manganese oxide) obtained by supporting iridium on manganese oxide obtained by electrolysis of a manganese salt solution. The electrolysis conditions for the electrolysis of the manganese salt solution include the following: Manganese salt solution: sulfuric acid-manganese sulfate solution Electrode: titanium plate Electrolysis temperature: 93°C or higher and 98°C or lower Electrolysis current density (per geometric area of ​​conductive substrate): 0.3 mA / cm ２ 20mA / cm or more ２ below

[0036] In order to achieve more stable electrolytic deposition of manganese oxide, the electrolytic current density per geometric area of ​​the conductive substrate is set to 1 mA / cm. ２ 10mA / cm or more ２ More preferably, 3 mA / cm or less ２ 8mA / cm or more ２ The following is even more preferred:

[0037] Since the efficiency of electrolytic production of deposited manganese oxide increases as the electrolysis temperature increases, the electrolysis temperature is preferably higher than 94° C. and not higher than 98° C. After electrolytic deposition, the manganese oxide may be peeled from the electrode and then pulverized into the desired shape by any method.

[0038] Once IrMn oxide is obtained, any method can be used to support iridium on manganese oxide, but one example is a method in which manganese oxide is brought into contact with an iridium salt solution. Specifically, manganese oxide may be immersed in the iridium salt solution. The type of iridium salt in the iridium salt solution is potassium hexachloroiridate (K ２ IrCl ６ ) or hexachloroiridic acid (H ２ IrCl ６) are examples. The iridium concentration of the iridium salt solution may be equal to or lower than the solubility of the iridium salt (the solute) in the solvent, and is preferably 0.1 g / L or higher and 10.0 g / L or lower, and more preferably 0.3 g / L or higher and 5.0 g / L or lower. The conditions for contacting the manganese oxide with the iridium salt solution include, for example, a temperature of 20°C or higher and 100°C or lower, and a time of 30 minutes to 24 hours or lower. Keeping the contact time or temperature within this range facilitates efficient adsorption of iridium.

[0039] In the supporting step, IrMn oxide is supported on a conductive substrate. Any method for supporting IrMn oxide on a conductive substrate may be used, including coating a slurry of IrMn oxide on the conductive substrate. The IrMn oxide slurry may contain IrMn oxide and a solvent. The solvent is not particularly limited as long as it does not chemically react with IrMn oxide, and examples thereof include one or more solvents selected from the group consisting of water, ethanol, methanol, and isopropyl alcohol.

[0040] The annealing step includes annealing the conductive substrate after supporting the IrMn oxide. The annealing brings the conductive substrate and the IrMn oxide into close contact with each other, thereby obtaining the electrode of this embodiment. The annealing temperature is preferably 300°C or higher and 550°C or lower, and more preferably 350°C or higher and 500°C or lower. The annealing time is preferably 1 hour or higher and 16 hours or lower, and more preferably 2 hours or higher and 8 hours or lower. The annealing atmosphere may be one or more atmospheres selected from the group consisting of air, nitrogen gas, and argon gas, with air being preferred.

[0041] (Electrodeposition Method) The electrodeposition method is a production method including the steps of electrodepositing manganese oxide on a conductive base material, supporting iridium on the conductive base material on which the manganese oxide has been electrodeposited, and annealing the conductive base material after supporting the IrMn oxide.

[0042] The conductive substrate containing manganese oxide to be subjected to the electrodeposition method is preferably one obtained by the electrolytic deposition method. This allows manganese oxide to be supported on the conductive substrate more simply and uniformly than when powdered manganese oxide is supported on the conductive substrate. Preferred conditions for electrolytic deposition of manganese oxide on the conductive substrate include the following: Electrolyte: sulfuric acid-manganese sulfate solution Electrode: platinum-coated titanium fiber sintered compact Electrolysis temperature: 93°C or higher and 98°C or lower Electrolysis current density: 0.3 mA / cm ２ 20mA / cm or more ２ or less (per geometric area of ​​the conductive substrate). The sulfuric acid concentration of the sulfuric acid-manganese sulfate solution is preferably 5 g / L or more and 65 g / L or less, and more preferably 20 g / L or more and 50 g / L or less. The concentration of manganese (manganese ions of manganese sulfate) in the sulfuric acid-manganese sulfate solution is not particularly limited as long as it is equal to or less than the solubility, but is preferably 5 g / L or more and 50 g / L or less, and more preferably 10 g / L or more and 30 g / L or less. In order to suppress changes in the composition of the electrolytic solution, it is effective to control the manganese ion concentration in the electrolytic solution in the electrolytic cell by at least one of mixing manganese sulfate in an amount corresponding to the amount of manganese ions consumed in the electrolytic oxidation and supplying a manganese sulfate solution. Note that the sulfuric acid concentration in the sulfuric acid-manganese sulfate solution is the concentration of divalent anions of manganese sulfate (sulfate ions: SO ４ ２－ ) is the value excluding

[0043] In order to deposit manganese oxide more stably on the conductive substrate, the electrolytic current density per geometric area of ​​the conductive substrate is 1 mA / cm ２ 10mA / cm or more ２ More preferably, 3 mA / cm or less ２ 8mA / cm or more ２ The following is even more preferred:

[0044] The higher the electrolysis temperature, the more efficient the electrolytic production of precipitated manganese oxide becomes, so the electrolysis temperature is preferably above 94°C and 98°C or less.

[0045] Before electrolytic deposition of manganese oxide, the surface of the conductive substrate to be subjected to electrodeposition may be acid-treated with one or more acids selected from the group consisting of hydrochloric acid, sulfuric acid, nitric acid, and oxalic acid to remove the passivation film on the surface of the conductive substrate and to make the surface hydrophilic. For the purpose of controlling the electrodeposition position of manganese oxide on the conductive substrate or imparting gas diffusion properties when used as an electrode for water electrolysis, the conductive substrate may be immersed in a dispersion liquid of a fluororesin to make the surface of the conductive substrate water-repellent.

[0046] The electrodeposition method includes a step of supporting iridium on a conductive substrate on which manganese oxide has been electrodeposited. Any method for supporting iridium on a conductive substrate on which manganese oxide has been electrodeposited may be used, as long as it is the same as the contact method with the iridium salt solution in the supporting step of the supporting method, except that a conductive substrate on which manganese oxide has been electrodeposited is used instead of manganese oxide. The electrodeposition method also includes a step of annealing after iridium is supported. The annealing treatment can improve the adhesion between the conductive substrate and the IrMn oxide. The annealing treatment conditions may be the same as those in the annealing step of the supporting method.

[0047] In the case of IrMn oxide in electrodeposition, for example, if one side of the conductive substrate is shielded with a resin film or the like during the electrolytic deposition of manganese oxide, IrMn oxide can be preferentially supported on only one side. Furthermore, it is presumed that the iridium-containing manganese oxide of this embodiment not only enhances the interaction between iridium and manganese oxide during annealing treatment, but also has favorable effects such as further increasing the adhesion between the IrMn oxide and conductive fibers or further increasing the crystallinity of the IrMn oxide.

[0048] (Water Electrolysis Apparatus and Water Electrolysis Method) A membrane-electrode assembly can be obtained by stacking an oxygen generating electrode including a catalyst containing the IrMn oxide of this embodiment and a conductive substrate, a polymer electrolyte membrane, and an electrode provided with a hydrogen generating catalyst. Examples of the polymer membrane include a fluororesin-based cation exchange membrane, and examples of the hydrogen generating catalyst include platinum fine particles. Hydrogen can be produced by electrolyzing water using a water electrolysis apparatus having such a membrane-electrode assembly.

[0049] The present disclosure will be described below with reference to examples, but the present disclosure is not limited to these examples.

[0050] <Analysis of Metal Contents of Electrode> A sample electrode measuring 10 mm in height and 10 mm in width was immersed in 10 mL of a mixed solution of hydrochloric acid (35% by mass, Kishida Chemical Co., Ltd.) and nitric acid (60% by mass, Kishida Chemical Co., Ltd.) in a volume ratio of 3:1, to dissolve Ir and Pt, thereby obtaining a sample solution. The obtained sample solution was subjected to inductively coupled plasma atomic emission spectroscopy (ICP-AES) using an ICP device (device name: Optima 830, PerkinElmer Co., Ltd.) to measure the concentrations (mass%) of iridium, manganese, and platinum contained in the sample solution. From the obtained concentrations of each element, the content per geometric area of ​​the electrode was calculated using the following formula. W Ｉｒ = (C Ｉｒ ×W) / SW Ｐｔ = (C Ｐｔ ×W) / SW Ｍｎ = (C Ｍｎ × W) / S In the above formula, W Ｉｒ , W Ｐｔ and W Ｍｎ are the contents of iridium, platinum, and manganese per geometric area of ​​the electrode [mg / cm ２ ] and C Ｉｒ , C Ｐｔ and C Ｍｎ are the concentrations [mass %] of iridium, platinum, and manganese in the sample solution, respectively; W is the mass [mg] of the sample solution; and S is the geometric area of ​​the sample electrode (1 cm ２ )

[0051] <Water Electrolysis Test> Using the electrodes of the present example and comparative example as working electrodes, an MEA was fabricated and a PEM-type water electrolysis cell equipped with the MEA was fabricated by the following method. (Fabrication of Counter Electrode) A solution containing pure water, ethanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich) was mixed with 20% by mass platinum-supported carbon (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich) to obtain a conductive catalyst ink. The obtained conductive catalyst ink was applied to carbon paper (product name: TGP-H-060, manufactured by Toray Industries, Inc.) and then air-dried at room temperature to obtain a counter electrode. (Preparation of electrolyte membrane) A Nafion membrane (product name: Nafion 115, manufactured by Sigma-Aldrich) was washed and protonated by boiling in 3% by mass hydrogen peroxide solution, pure water, and 1 M sulfuric acid solution in that order for 1 hour each, to obtain an electrolyte membrane. (Preparation of MEA) The electrolyte membrane was sandwiched between the catalyst-coated surfaces of the working electrode and counter electrode, and heated at 135°C with a clamping force of 400 kg / cm using a hot press machine (product name: SA-302, manufactured by Tester Sangyo Co., Ltd.). ２ The laminate was hot pressed at 100°C for 3 minutes to obtain an MEA.

[0052] (Preparation of PEM-type water electrolyzer and evaluation of water electrolysis capacity) The obtained MEA was attached to a PEM-type water electrolyzer housing (product name: WE-4S-RICW, manufactured by FC Development Co., Ltd.) to prepare a PEM-type water electrolyzer. Using the obtained PEM-type water electrolyzer, water electrolysis was performed by two-electrode linear sweep voltammetry (LSV) under the following conditions: Voltage increase rate: 10 mV / sec Water temperature: 80°C Water supply rate: 2 mL / min During the water electrolysis, the electrolysis current density when the voltage reached 2 V was measured to evaluate the water electrolysis capacity. The higher the measured value of this electrolysis current density, the higher the evaluation of the water electrolysis capacity of the working electrode being evaluated.

[0053] Example 1 A sulfuric acid-manganese sulfate mixed solution with a sulfuric acid concentration of 35 g / L and a manganese sulfate concentration of 55 g / L was used as the electrolyte, and this was filled into an electrolytic cell. Next, a conductive substrate was placed in the electrolytic cell filled with the electrolyte so as to be immersed in the electrolyte. The conductive substrate was a platinum-coated titanium fiber sintered compact (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., length 2.25 cm × width 2.25 cm × width 0.3 mm, porosity 56%, Pt content 1.0 mg / cm) obtained by sintering and rolling a platinum-coated titanium nonwoven fabric. ２ The electrolysis temperature was 95°C and the current density was 7mA / cm. ２ A current was applied at 1000 kJ / cm for 10 minutes to deposit manganese oxide on the conductive substrate, and the Mn content was 1.2 mg / cm. ２ Next, potassium hexachloroiridate (product name: K ２ IrCl ６ The conductive substrate on which manganese oxide had precipitated was immersed for 24 hours at a liquid temperature of 95°C in an aqueous solution of potassium hexachloroiridate containing 0.02 g / L of potassium hexachloroiridate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 1.0 g / L of sulfuric acid (manufactured by Kishida Chemical Co., Ltd.). The immersed conductive substrate on which manganese oxide had precipitated was recovered from the aqueous solution of potassium hexachloroiridate containing sulfuric acid and annealed in air at 450°C for 5 hours, resulting in a manganese oxide-precipitated conductive substrate with an Ir content of 0.1 mg / cm. ２ , unit Pt amount is 1.0 mg / cm ２ (Total metal amount 1.1 mg / cm ２ , Pt / Ir ratio is 10), and the unit Mn content is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.024.

[0054] (Example 2) Pt content 2.0 mg / cm ２ The same method as in Example 1 was used except that a platinum-coated titanium fiber sintered body having an Ir content of 0.1 mg / cm was used as the conductive substrate. ２ , unit Pt amount is 2.0 mg / cm ２ (Total metal amount 2.1 mg / cm ２ , Pt / Ir ratio is 20), and the unit Mn content is 1.2 mg / cm ２and an Ir / Mn molar ratio of 0.024.

[0055] (Example 3) Pt content 3.0 mg / cm ２ The same method as in Example 1 was used except that a platinum-coated titanium fiber sintered body having an Ir content of 0.1 mg / cm was used as the conductive substrate. ２ , unit Pt amount is 3.0 mg / cm ２ (Total metal amount 3.1 mg / cm ２ , Pt / Ir ratio is 30), and the unit Mn content is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.024.

[0056] (Example 4) Pt content 4.0 mg / cm ２ The same method as in Example 1 was used except that a platinum-coated titanium fiber sintered body having an Ir content of 0.1 mg / cm was used as the conductive substrate. ２ , unit Pt amount is 4.0 mg / cm ２ (Total metal amount 4.1 mg / cm ２ , Pt / Ir ratio is 40), and the unit Mn content is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.024.

[0057] (Example 5) Pt content 6.0 mg / cm ２ The same method as in Example 1 was used except that a platinum-coated titanium fiber sintered body having an Ir content of 0.1 mg / cm was used as the conductive substrate. ２ , unit Pt amount is 6.0 mg / cm ２ (Total metal amount 6.1 mg / cm ２ , Pt / Ir ratio is 60), and the unit Mn content is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.024.

[0058] Example 6 A potassium hexachloroiridate sulfate aqueous solution having a unit Ir content of 0.02 mg / cm was prepared in the same manner as in Example 5, except that a potassium hexachloroiridate sulfate aqueous solution having a concentration of 0.004 g / L was used. ２ , unit Pt amount is 6.0 mg / cm ２ (Total metal amount 6.02 mg / cm２ , Pt / Ir ratio is 300), and the unit Mn content is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.005.

[0059] (Comparative Example 1) A titanium fiber sintered body (product name: ST / Ti / 20 / 300 / 56, manufactured by Nikko Techno Co., Ltd., length 2.25 cm × width 2.25 cm × depth 0.3 mm, porosity 56%, unit Pt content 0.0 mg / cm) consisting of a sintered and rolled titanium nonwoven fabric not coated with platinum. ２ The same method as in Example 1 was used except that a conductive substrate was used. ２ , unit Pt amount is 0.0 mg / cm ２ (Total metal amount 0.1 mg / cm ２ , Pt / Ir ratio is 0), and the unit Mn amount is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.024.

[0060] (Comparative Example 2) Pt content 6.0 mg / cm ２ A manganese oxide-precipitated conductive substrate was obtained in the same manner as in Example 1, except that a platinum-coated titanium fiber sintered body of 0 mg / cm was used as the conductive substrate. ２ , unit Pt amount is 6.0 mg / cm ２ (Total metal amount 6.0 mg / cm ２ ), the unit Mn content is 1.2 mg / cm ２ , and an oxygen generating electrode of this comparative example having an Ir / Mn molar ratio of 0 was obtained.

[0061] Comparative Example 3: A potassium hexachloroiridate sulfuric acid aqueous solution having a potassium hexachloroiridate concentration of 0.0008 g / L was used in the same manner as in Example 5, except that a potassium hexachloroiridate sulfuric acid aqueous solution having a unit Ir amount of 0.004 mg / cm ２ , unit Pt amount is 6.0 mg / cm ２ (Total metal amount 6.004 mg / cm ２ , Pt / Ir ratio is 1500), and the unit Mn content is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.001.

[0062] Comparative Example 4: A potassium hexachloroiridate sulfate aqueous solution having a unit Ir content of 0.01 mg / cm was prepared in the same manner as in Example 5, except that a potassium hexachloroiridate sulfate aqueous solution having a concentration of 0.002 g / L was used. ２ , unit Pt amount is 6.0 mg / cm ２ (Total metal amount 6.01 mg / cm ２ , Pt / Ir ratio is 60), and the unit Mn content is 1.2 mg / cm ２ and an Ir / Mn molar ratio of 0.002. The oxygen generating electrodes of this comparative example were subjected to a water electrolysis test using the oxygen generating electrodes (working electrodes) of the example and comparative example, and the results are shown in the table below.

[0063]

[0064] From Table 1, the sum of the unit Ir amount and unit Pt amount (total metal amount) in the oxygen generating electrode is 0.1 mg / cm ２ Super 6.1mg / cm ２ or less, and when the Pt / It ratio was in the range of 1 or more and less than 600, it was confirmed that the oxygen generating electrodes of Examples 1 to 3 had high water electrolysis ability. In particular, the ratio of the electrolysis current density to the total metal amount was larger than those of the other Examples, that is, it was confirmed that the water electrolysis ability per unit amount of precious metal was high. Thus, the reason why the oxygen generating electrodes of Examples 1 to 3 are particularly superior is presumably that the reduction in the total metal amount increased the use efficiency of iridium and the use efficiency of platinum as the conductive layer.

[0065] The present disclosure provides at least one of an oxygen generating electrode including a catalyst containing an iridium-containing manganese oxide and a conductive substrate containing platinum, which can achieve a high electrolysis current density even when the amount of precious metal used is within a certain range, and a water electrolysis method using the electrode. The entire contents of the specification, claims, and abstract of Japanese Patent Application No. 2024-82449, filed on May 21, 2025, are incorporated herein by reference.

Claims

1. An oxygen generating electrode comprising a conductive substrate and a catalyst containing iridium-containing manganese oxide, wherein the conductive substrate contains platinum, and the sum of the amount of iridium per geometric area of ​​the oxygen generating electrode and the amount of platinum per geometric area of ​​the oxygen generating electrode is 0.1 mg / cm. ２ Super 6.1mg / cm ２ and the ratio of the amount of platinum per geometric area of ​​the oxygen generating electrode to the amount of iridium per geometric area of ​​the oxygen generating electrode is 1 or more and less than 600.

2. The amount of iridium per geometric area of ​​the oxygen generating electrode is 0 mg / cm ２ Super 1mg / cm ２ The oxygen generating electrode according to claim 1 , wherein:

3. The amount of platinum per geometric area of ​​the oxygen generating electrode is 0 mg / cm ２ Super 6mg / cm ２ The oxygen generating electrode according to claim 1 , wherein:

4. The sum of the amount of iridium per geometric area and the amount of platinum per geometric area of ​​the oxygen generating electrode is 1.0 mg / cm ２ Super 4.5mg / cm ２ The oxygen generating electrode according to claim 1 , wherein:

5. A water electrolysis method using the oxygen generating electrode according to any one of claims 1 to 4.

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