Electrode containing oxygen-evolving electrode catalyst

Abstract

This invention provides an oxygen generation electrode that achieves both higher catalytic activity and durability compared to conventional oxygen generation electrodes for water electrolysis equipped with an oxygen generation electrode catalyst made of ruthenium-containing manganese oxide. [Solution] An oxygen-evolving electrode comprising an oxygen-evolving electrode catalyst and a conductive substrate, wherein the conductive substrate has the oxygen-evolving electrode catalyst on at least its surface, the oxygen-evolving catalyst contains ruthenium-containing manganese oxide, and the amount of manganese per geometric area of ​​the oxygen-evolving electrode is 0.2 mg / cm². 2 More than 5.0mg / cm 2 An oxygen-evolving electrode characterized by the following, and furthermore, having a Tafel gradient of 50 mV / dec. to 140 mV / dec.

Description

Technical Field

[0001] The present invention relates to an oxygen generation electrode, a method for manufacturing the same, a water electrolysis apparatus including the oxygen generation electrode, and a method for producing hydrogen using the oxygen generation electrode.

Background Art

[0002] Due to the problems of depletion of fossil fuels and environmental pollution, the use of hydrogen as a clean energy and its production method have attracted attention. As a means for producing high-purity hydrogen, there is a water electrolysis method. Among the catalysts used for an oxygen generation electrode (anode) in the water electrolysis method (hereinafter, also referred to as an "oxygen generation electrode catalyst"), iridium-based catalysts are known as oxygen generation electrode catalysts having high catalytic activity and high durability (for example, Non-Patent Document 1). However, since the reserves of iridium are extremely small, the use of iridium-based catalysts is accompanied by resource constraints. On the other hand, ruthenium-based catalysts have fewer resource constraints compared to iridium-based catalysts and exhibit oxygen generation electrode catalytic activity equal to or higher than that of iridium-based catalysts. However, ruthenium-based catalysts mainly composed of ruthenium oxide have extremely low durability (Non-Patent Document 2). In contrast, it has been reported that the durability of ruthenium-based catalysts is improved by an oxygen generation electrode catalyst in which ruthenium is contained in manganese oxides having various crystal structures. Further, among oxygen generation electrode catalysts in which ruthenium is contained in manganese oxides, it has been disclosed that when the crystal structure of the manganese oxide is β-type manganese dioxide, the effect of improving durability is large (Non-Patent Document 3).

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

[0004] The oxygen-evolving electrode catalyst described in Non-Patent Document 3 has catalytic activity comparable to that of conventional ruthenium-based catalysts made of ruthenium oxide. Therefore, further improvement of catalytic activity was necessary when using it in water electrolysis. This disclosure aims to provide at least one of the following: an oxygen generation electrode that achieves both higher catalytic activity and durability compared to conventional oxygen generation electrodes for water electrolysis equipped with an oxygen generation electrode catalyst made of ruthenium-containing manganese oxide, and a method for manufacturing the same. [Means for solving the problem]

[0005] In this disclosure, in order to improve the catalytic activity and durability of an oxygen-evolving electrode, we focused not only on the oxygen-evolving electrode catalyst itself, but also on the relationship between the conductive substrate used as the anode of a water electrolytic cell and the oxygen-evolving electrode catalyst. As a result, we found that by using an oxygen-evolving electrode with an oxygen-evolving electrode catalyst made of ruthenium-containing manganese oxide on a conductive substrate in a state that does not inhibit the diffusion of the substance, durability can be improved without a decrease in the catalytic activity of the oxygen-evolving electrode.

[0006] In other words, the claims of the present invention are as stated, and the gist thereof is as follows. [1] An oxygen-evolving electrode comprising an oxygen-evolving electrode catalyst and a conductive substrate, The conductive substrate has the oxygen-evolving electrode catalyst on at least its surface, The oxygen evolution catalyst contains ruthenium-containing manganese oxide, The amount of manganese per geometric area in the oxygen-evolving electrode is 0.2 mg / cm². 2 More than 5.0mg / cm 2The following, and moreover, The Tafel gradient of the oxygen generation electrode is characterized by being 50 mV / dec. or more and 140 mV / dec. or less. Oxygen-generating electrode. [2] The oxygen-evolving electrode according to [1], wherein the ruthenium-containing manganese oxide includes a manganese oxide having the structure of β-type manganese dioxide. [3] The amount of ruthenium per geometric area in the oxygen-evolving electrode is 0.005 mg / cm². 2 More than 0.500mg / cm 2 The oxygen-generating electrode described in [1] or [2] above, which is as follows: [4] An oxygen-evolving electrode according to any one of [1] to [3] above, wherein the molar ratio of ruthenium to manganese is 0.001 or more and 0.250 or less. [5] The oxygen-generating electrode according to any one of [1] to [4], wherein the conductive substrate contains titanium. [6] A method for producing an oxygen-generating electrode according to any one of [1] to [5], comprising: an electrolysis step of electrolytically generating manganese oxide from a conductive substrate to obtain a manganese oxide-containing substrate; a contact step of contacting the manganese oxide-containing substrate with a ruthenium salt solution; and a heat treatment step of heat-treating the manganese oxide-containing substrate that has undergone the contact step. [7] A water electrolysis apparatus comprising an oxygen generating electrode as described in any one of [1] to [5] above. [8] A method for producing hydrogen using an oxygen-generating electrode described in any one of [1] to [5] above. [Effects of the Invention]

[0007] This disclosure provides an oxygen generation electrode that achieves both higher catalytic activity and durability compared to conventional oxygen generation electrodes for water electrolysis equipped with an oxygen generation electrode catalyst made of ruthenium-containing manganese oxide, and at least one of the above, as well as a method for manufacturing the same. [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. The oxygen-generating electrode of this embodiment (hereinafter also simply referred to as "electrode") is In an oxygen-evolving electrode comprising an oxygen-evolving electrode catalyst and a conductive substrate, The conductive substrate has the oxygen-evolving electrode catalyst on at least its surface, The oxygen evolution catalyst contains ruthenium-containing manganese oxide (hereinafter also referred to as "Ru-Mn oxide"), The amount of manganese per geometric area (hereinafter also referred to as the amount of Mn) in the oxygen-evolving electrode is 0.2 mg / cm³. 2 More than 5.0mg / cm 2 The following, and moreover, The Tafel gradient of the oxygen generation electrode is characterized by being 50 mV / dec. or more and 140 mV / dec. or less. This is an oxygen-evolving electrode. By fulfilling this configuration, the electrode becomes more durable compared to conventional electrodes equipped with oxygen-evolving electrode catalysts containing ruthenium-containing manganese oxide.

[0009] The oxygen generation electrode of this embodiment relates to an oxygen generation electrode comprising an oxygen generation electrode catalyst and a conductive substrate. In the electrode of this embodiment, the oxygen-evolving electrode catalyst is a catalyst containing Ru-Mn oxide. This allows the electrode of this embodiment to function as an oxygen-evolving electrode. In the electrode of this embodiment, the Ru-Mn oxide is a manganese oxide containing ruthenium. The Ru-Mn oxide only needs to contain ruthenium in a state in which it can interact with the manganese oxide, and the conductive substrate only needs to have the oxygen-evolving electrode catalyst on at least its surface, and is preferably coated with the oxygen-evolving electrode catalyst. The state in which the manganese oxide contains ruthenium includes one or more states selected from the group consisting of a state supported on the manganese oxide and a state in which ruthenium is solid-dissolved in the manganese oxide. The form of ruthenium contained in Ru-Mn oxides is arbitrary; ruthenium only needs to be included in one or more forms selected from the group consisting of metals, cations, and oxides. To obtain higher oxygen-evolving electrode catalytic activity, it is preferable that the Ru-Mn oxide in the electrode of this embodiment contains a manganese oxide having a β-type manganese dioxide crystal structure.

[0010] In the electrode of this embodiment, the crystal structure of manganese oxide contained in the Ru-Mn oxide can be identified by comparing the powder X-ray diffraction (hereinafter also referred to as "XRD") pattern of the Ru-Mn oxide with the XRD pattern (hereinafter also referred to as "reference pattern") registered in the ICDD (International Diffraction Data Center) PDF (Powder Diffraction File). For the reference pattern of manganese dioxide having a β-type crystal structure, PDF No. 24-0735 (β-type) can be used.

[0011] 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 The XRD peak is the peak at the top of the XRD pattern detected when analyzing the XRD pattern using common analysis software (SmartLab Studio II, manufactured by Rigaku Corporation).

[0012] The amount of manganese per geometric area of the oxygen generation electrode of the present embodiment (hereinafter also referred to as "unit Mn amount") is 0.2 mg / cm 2 or more and 5.0 mg / cm 2 or less. When the unit Mn amount is less than 0.2 mg / cm 2 , the adhesion between the conductive substrate and the oxygen generation electrode catalyst decreases, and the durability decreases. On the other hand, when the unit Mn amount exceeds 5.0 mg / cm 2 , the thickness of the oxygen generation electrode catalyst increases, so the electrical resistance increases and the oxygen generation electrode catalyst activity decreases. From the viewpoint of achieving both higher oxygen generation electrode catalyst activity and durability, the unit Mn amount is 0.2 mg / cm 2 or more, 0.5 mg / cm 2 or more, or 1.0 mg / cm 2 or more, which is preferable. Also, it is preferably 5.0 mg / cm 2 or less, 4.0 mg / cm 2 or less, or 3.0 mg / cm 2 or less. The unit Mn amount is 0.2 mg / cm 2 or more and 5.0 mg / cm 2 or less, 0.50 mg / cm 2 or more and 4.0 mg / cm 2 or less, or 1.0 mg / cm 2 or more and 3.0 mg / cm 2 or less.

[0013] The "geometric area" of the electrode in the present embodiment is the area corresponding to the projected area without considering unevenness and voids. The "geometric area of the electrode" in the electrode of the present embodiment is the projected area of the electrode, which is the area of the plane obtained from the length × width when the shape of the electrode is defined by length × width × thickness. The plane is the surface (geometric surface) facing the electrolyte membrane when forming a membrane-electrode assembly (hereinafter also referred to as "MEA").

[0014] The unit Mn amount in the electrode of the present embodiment is obtained from the following formula (1). Unit Mn amount (mg / cm 2 ) = Amount of Mn contained in the electrode (mg) / Geometric area of the electrode (cm 2 ) (1) In the above equation, the "amount of Mn contained in the electrode" can be determined by analyzing the Mn concentration of the Mn solution prepared by the dissolution method described below. The amount of Mn contained in the electrode can be determined by multiplying the Mn concentration by the volume of the Mn solution. The concentration of the Mn solution can be determined using a general ICP instrument (for example, instrument name: Optima 830, manufactured by PerkinElmer). One dissolution method is to use a mixed acid of hydrochloric acid and nitric acid as the dissolving solution and immerse the oxygen-evolving electrode under dissolution conditions of 80°C to 95°C for 2 to 4 hours.

[0015] The Tafel gradient of the electrode in this embodiment is 50 mV / dec. to 140 mV / dec. If the Tafel gradient falls below 50 mV / dec., the progress of the oxygen evolution reaction is inhibited. On the other hand, if the Tafel gradient exceeds 140 mV / dec., the adsorption of water on the oxygen evolution electrode catalyst is inhibited, making it difficult for the oxygen evolution reaction to occur. In order to exhibit excellent oxygen evolution catalytic activity, the Tafel gradient is preferably 50 mV / dec. or higher or 60 mV / dec. or higher, and preferably 140 mV / dec. or lower or 120 mV / dec. Examples of the Tafel gradient of the oxygen evolution electrode in this embodiment include 50 mV / dec. to 140 mV / dec. or 60 mV / dec. to 120 mV / dec.

[0016] The electrodes of this embodiment exhibit high catalytic activity and high durability by incorporating the above-mentioned Tafel gradient as one of their configurations. One possible reason for this is as follows: In this embodiment, the oxygen evolution reaction is thought to proceed by the following elementary reaction. In the following reaction equation, · represents the active site of the oxygen evolution reaction. (Step 1) · + H2O → ·OH + H + + e - (Step 2) OH + OH - → ·O - + H2O (Step 3) - → ·O +e - (Step 4) ·O + H2O → ·OOH + H + + e - (Step 5) · OOH → · + O2 + H + + e -

[0017] In the case of an oxygen evolution reaction in which one of steps 1 to 5 is the rate-determining reaction, the theoretical values ​​of the Tafel gradient at an electrolysis temperature of 80°C are, for example, (step 1) 140 mV / dec., (step 2) 70 mV / dec., (step 3) 47 mV / dec., (step 4) 28 mV / dec., and (step 5) 20 mV / dec. When the Tafel gradient is as described above, it is thought that the elementary reactions in steps 1 to 3 will mainly proceed as the rate-determining reaction. As a result, appropriate water molecule adsorption and oxygen molecule desorption are obtained in the oxygen evolution reaction, and catalytic activity is thought to be higher compared to conventional ruthenium-based catalysts using Ru-Mn oxide. In contrast, when a Ru-Mn oxide with many grain boundaries is used as an oxygen evolution electrode catalyst, grain boundary resistance is thought to increase and catalytic activity is thought to decrease. On the other hand, by reducing the grain boundaries of the Ru-Mn oxide, it is thought that catalyst degradation caused by grain boundaries can be suppressed, and good catalytic activity and durability can be obtained. While there are no particular limitations on the method for obtaining Ru-Mn oxides with few grain boundaries, one method involves electrolytically eluting manganese oxide on a conductive substrate to obtain Ru-Mn oxides with few grain boundaries. In this embodiment, the Tafel gradient is determined by electrochemical measurement using a solid polymer (hereinafter also referred to as "PEM") type water electrolytic cell, and is defined as the voltage value (overvoltage) [V] and current value (A / cm²). 2This value is obtained from the following: ) That is, a plot (Tafel plot) is created with the logarithm of current density i (log current density i) on the horizontal axis and the overvoltage η on the vertical axis, and this is the slope of the region (Tafel region) where a linear function approximation is possible. The electrochemical measurement described above can be performed using the linear sweep voltammetry (hereinafter also referred to as "LSV") method with a general measuring device (for example, device name: Electrochemical Measurement System HZ-7000 HAH1232m, manufactured by Hokuto Denko Co., Ltd.) that can acquire voltage values ​​(overvoltage) and current values, under the following conditions.

[0018] (Configuration of a PEM-type water electrolysis cell) Anode: Electrode of this embodiment Cathode: Platinum-supported carbon coated carbon paper (20% platinum by weight) Electrolyte membrane: Nafion 115 (film thickness 127 μm) (Conditions for using PEM-type water electrolysis cells) Water temperature: 80℃ Water supply rate: 2mL / min (LSV condition) Voltage increase rate: 10mV / sec Voltage sweep range: 2.0V from open-circuit voltage

[0019] The conductive substrate included in the electrode of this embodiment may be any substrate made of a conductive material, and it is preferable that the substrate contains titanium. The conductive substrate can take one or more shapes selected from the group consisting of mesh, cloth, and plate shapes, and a mesh shape is preferred because it tends to increase the oxygen-evolving electrode catalytic activity. Examples of specific conductive substrates include titanium mesh composed of fibrous or powdered conductive titanium metal, and sintered titanium mesh obtained by heat treatment. In order to exhibit excellent oxygen-evolving electrode catalytic activity, the surface of these conductive substrates is preferably coated with platinum, and a platinum-coated titanium mesh is even more preferable.

[0020] If the conductive substrate is one whose surface is coated with platinum, the amount of platinum per geometric area should be 0.5 mg / cm² to achieve high conductivity.2 More than 10.0mg / cm 2 The following is preferred: 1.0 mg / cm³ 2 More than 8.0mg / cm 2 The following is more preferable: 1.0 mg / cm³ 2 More than 6.0mg / cm 2 The following is even more preferable.

[0021] The thickness of the conductive substrate is preferably 50 μm to 500 μm, more preferably 100 μm to 400 μm, and even more preferably 150 μm to 300 μm, in order to exhibit excellent oxygen-evolving electrode catalyst activity. "Thickness" refers to the length corresponding to the length perpendicular to the geometric plane. "Thickness of the conductive substrate" refers to the length corresponding to the length perpendicular to the geometric plane of the conductive substrate.

[0022] The porosity of the conductive substrate is preferably 30% to 80%, and more preferably 40% to 70%. Here, porosity is defined as the volume of space without conductive substrate within the volume of the conductive substrate, and a mercury pressure method is one method for measuring porosity. When the porosity is within the above range, excellent effects are obtained in that the mechanical strength of the electrode is increased and water, which is the reaction substrate for oxygen generation, can be supplied smoothly. The amount of ruthenium per unit geometric area in the electrode of this embodiment (hereinafter also referred to as "unit Ru amount") is 0.005 mg / cm² in order to exhibit superior oxygen evolution electrode catalytic activity. 2 More than 0.075mg / cm 2 Above or equal to 0.010 mg / cm³ 2 Preferably, the concentration is 0.500 mg / cm³ or higher, and also 0.500 mg / cm³. 2 The following, or 0.4000 mg / cm³ 2 The following is more preferable: In this embodiment, the unit amount of Ru is 0.05 mg / cm³. 2 More than 0.500mg / cm 2 The following, or 0.010 mg / cm³ 2 More than 0.400mg / cm 2 The following are listed: In the Ru-Mn oxide contained in the electrode of this embodiment, the molar ratio of ruthenium to manganese (hereinafter also referred to as the "Ru / Mn molar ratio") is preferably 0.001 or higher, or 0.003 or higher, and more preferably 0.250 or lower, or 0.050 or lower, in order to exhibit excellent oxygen evolution electrode catalytic activity. More preferably, the Ru / Mn molar ratio of the Ru-Mn oxide contained in the electrode of this embodiment is 0.001 or higher and 0.250 or lower, or 0.003 or higher and 0.050 or lower.

[0023] The oxygen generation electrode of this embodiment is preferably used as an oxygen generation electrode in a water electrolysis apparatus. The oxygen generation electrode of this embodiment can be used in a hydrogen production method that involves electrolysis of water.

[0024] The method for manufacturing the oxygen generation electrode of this embodiment will be described below. The method for manufacturing the oxygen-generating electrode of this embodiment is not particularly limited as long as it can produce an oxygen-generating electrode having the above configuration, but a preferred manufacturing method includes an electrolytic step of electrolytically generating manganese oxide from a conductive substrate to obtain a manganese oxide-containing substrate, a contact step of contacting the manganese oxide-containing substrate with a ruthenium salt solution, and a heat treatment step of heat-treating the manganese oxide-containing substrate that has undergone the contact step.

[0025] In the electrolysis process, manganese oxide is electrolytically deposited onto a conductive substrate. This yields a manganese oxide-containing substrate. It is preferable to uniformly deposit the manganese oxide on the conductive substrate in this process. This reduces the number of grain boundaries of the Ru-Mn oxide, which promotes oxygen generation when used as an oxygen generation electrode and allows for a higher Tafel gradient. It is also preferable that the deposited manganese oxide has voids. This makes it easier for water, a raw material for oxygen, to adsorb to the active sites, preventing the Tafel gradient from becoming too high. The method of electrodeposition of manganese oxide onto the surface of a conductive substrate is arbitrary; any method involving immersing the conductive substrate in an electrolyte and electrolyzing it is acceptable. Electrodeposition allows for uniform and constant deposition on the conductive substrate, making it easier to achieve more uniform manganese oxide deposition. Furthermore, it allows for control of the crystal growth direction from the conductive substrate, making it easier to create a state with voids.

[0026] The electrolyte can be any solution containing manganese. Preferably, the manganese concentration of the electrolyte is between 0.05 mol / L and 1.00 mol / L. 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. In this case, the sulfuric acid concentration is preferably 0.05 mol / L or more and 1.00 mol / L or less, and the manganese concentration is preferably 0.05 mol / L or more and 1.00 mol / L or less. The mixed solution containing sulfuric acid and manganese sulfate may also contain an ammonium salt. In this case, the ammonium sulfate concentration is typically between 0.1 mol / L and 3.0 mol / L.

[0027] The conductive substrate used in the electrolytic process can be any of the conductive substrates described above. Electrolysis in the electrolytic process can be carried out by immersing a conductive substrate in an electrolyte solution and then electrolyzing it. The current density during electrolysis (hereinafter also referred to as "deposition current density") is sufficient as long as the conditions for electrolysis of manganese oxide are met, but the deposition current density must be 1.0 mA / cm² per geometric area of ​​the conductive substrate. 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 2The following are some examples of the advantages: When the deposition current density is within the above range, manganese oxide with fewer grain boundaries is more easily deposited. The 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. When the electrolysis time is within the above range, the manganese oxide can be uniformly electrolytically deposited on the conductive substrate, and it is easier to obtain manganese oxide with fewer grain boundaries and voids. Therefore, when it is used as Ru-Mn oxide, it is more likely to exhibit high oxygen evolution catalytic activity. Furthermore, in the heat treatment process described later, the crystal structure of Ru-Mn oxide at the electrode is more likely to change to the structure of β-type manganese dioxide. The electrolysis temperature is typically between 90°C and 100°C, and preferably between 93°C and 98°C.

[0028] The ruthenium salt solution used in the contact process can be any solution containing a ruthenium salt, and a solution containing a ruthenium salt and sulfuric acid, or more preferably an aqueous solution containing a ruthenium salt and sulfuric acid, is preferred. Examples of ruthenium salts include at least one of ruthenium(III) chloride and ruthenium(IV) chloride. The ruthenium concentration in the ruthenium salt solution is preferably between 0.01 mol / L and 10 mol / L.

[0029] Contact between the manganese oxide-containing substrate and the ruthenium salt solution can be achieved by immersing the manganese oxide-containing substrate in the ruthenium salt solution. The contact is not particularly limited as long as the conditions for ruthenium impregnation of the surface of the manganese oxide-containing substrate are met, but the contact temperature can be said to be between 20°C and 100°C. The contact time can be appropriately adjusted depending on the size of the manganese oxide-containing substrate, but for example, it can be between 30 minutes and 24 hours. In the heat treatment process, the manganese oxide-containing substrate obtained in the contact process is heat-treated. This causes the manganese atoms inside the crystalline structure of the manganese oxide to be replaced by ruthenium atoms contained on the surface and in the pores of the manganese oxide, allowing a large amount of iridium to be dissolved in the manganese oxide. Examples of heat treatment atmospheres in the heat treatment process include an air atmosphere or an inert atmosphere, with an air atmosphere being preferred.

[0030] The heat treatment temperature is preferably between 100°C and 600°C, as this facilitates the transformation of the Ru-Mn oxide crystal structure in the electrode to the β-type manganese dioxide structure. The heat treatment time can be, for example, between 10 minutes and 24 hours. [Examples]

[0031] 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 electrode or sample electrolyte membrane measuring 10 mm x 10 mm was immersed in 10 mL of a solution prepared by mixing hydrochloric acid and nitric acid in a volume ratio of 3:1 to dissolve the oxygen-evolving electrode catalyst contained in the sample and obtain a sample solution. 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).

[0032] <Identification of crystal structure> A standard powder X-ray diffractometer (device name: Ultima IV Protectus, manufactured by Rigaku Corporation) was used to obtain the XRD pattern 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 The crystal phase of the sample was identified by comparing the obtained XRD pattern with a reference pattern. For the reference patterns of manganese dioxide having a γ-type, β-type, ε-type or α-type crystal structure, PDF No. 14-0644 (γ-type), 24-0735 (β-type), 30-0820 (ε-type) or 44-0141 (α-type) was used respectively.

[0033] <Calculation of Tafel slope> Using the electrode of the example and the electrode of the comparative example or the electrolyte membrane with catalyst as the measurement sample, a PEM type water electrolyzer with the surface coated with the oxygen evolution electrode catalyst as the working electrode (anode) was used, and linear sweep voltammetry (LSV) measurement of a two-electrode system was performed under the following conditions. Voltage increase rate: 10 mV / sec Water temperature: 80 °C Water supply rate: 2 mL / min The voltage value [V] obtained by this evaluation was taken as the overvoltage η [V], and the obtained current value was taken as the current density i [A / cm 2 . A Tafel plot was created by plotting the overvoltage η against log (current density i). In the Tafel region of the obtained Tafel plot, the logarithmic formula of the Tafel formula shown in the following formula (2) was applied to calculate the Tafel slope b [mV / dec.] of the measurement sample. Tafel slope b [mV / dec.] = (Overvoltage η [V] - Tafel constant a [V]) / log (current density i) [A / cm 2 × 1000 (2)

[0034] <Fabrication of PEM type water electrolyzer> A PEM-type water electrolytic cell was fabricated using the electrode of this embodiment as the working electrode (anode) and equipped with a membrane-electrode assembly (hereinafter also referred to as "MEA") prepared by the following method. A catalyst ink was prepared by mixing a 20% by mass platinum-supported carbon catalyst (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich) with a solution containing water, ethanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich). This was applied to carbon paper (product name: TGP-H-060, manufactured by Toray Industries), air-dried, and used as the counter electrode. Next, the electrolyte 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. Using the sample electrode as the working electrode (anode), the electrolyte membrane was sandwiched between the catalyst surfaces of the working electrode (anode) and the counter electrode, and the process was performed 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.

[0035] <Measurement of Oxygen Evolution Electrode Catalytic Activity> Linear sweep voltammetry (LSV) measurements were performed using the electrodes from the examples and the electrodes or catalyst-equipped electrolyte membranes from the comparative examples as measurement samples, in the same manner as for calculating the Tafel gradient. From this evaluation, the current density at a voltage of 2V (hereinafter also simply referred to as "2V-current density") was determined.

[0036] <Durability Test> The electrodes of the example and the electrodes or catalyst-coated electrolyte membranes of the comparative example were used as measurement samples. A PEM-type water electrolytic cell was used, with the surface coated with the oxygen-evolving electrode catalyst as the working electrode (anode), and a constant current was applied to the two-electrode system under the following conditions. Applied current density: 1A / cm 2 Water temperature: 80℃ Water supply rate: 2mL / min From this evaluation, the voltage at 0 seconds of electrolysis (hereinafter also referred to as "voltage 1") [V] and the voltage at 24 hours of electrolysis (hereinafter also referred to as "voltage 2") [V] were determined, and the degradation rate of the measured sample was calculated using the following formula (3). Degradation rate [%] = (Voltage 2[V] - Voltage 1[V]) / Voltage 1[V] × 100 (3)

[0037] Example 1 An electrolytic cell filled with a sulfuric acid-manganese sulfate mixed solution with sulfuric acid concentration of 0.36 mol / L and manganese sulfate concentration of 0.36 mol / L is used to insert plate-shaped Pt-coated Ti fibers measuring 1 cm x 1 cm (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 / cm³). 2 A conductive substrate made of ) was immersed in it. The conductive substrate had a deposition current density of 7 mA / cm². 2 Then, an electric current was applied for 10 minutes to electrolytically precipitate manganese oxide onto the conductive substrate, thereby obtaining a manganese precipitate substrate. Next, the manganese precipitate substrate was immersed in a ruthenium salt solution of 0.2 mmol / L ruthenium(III) chloride (RuCl3) and 0.01 mol / L sulfuric acid at 95°C for 24 hours, and then heat-treated at 450°C for 5 hours in an air atmosphere to obtain a ruthenium-containing manganese oxide composite electrode material, which was used as the electrode in this example. The electrode in this embodiment is an electrode having an oxygen-evolving electrode catalyst made of manganese oxide having a crystalline structure of β-type manganese dioxide and containing ruthenium, on a conductive substrate made of Pt-coated Ti fibers, with a manganese content of 1.20 mg / cm³. 2 And the ruthenium content is 0.01 mg / cm³ 2 The result was (Ru / Mn molar ratio was 0.005). Measurements were performed using a PEM-type water electrolytic cell with the electrode of this embodiment as the working electrode (anode), and the Tafel gradient of this embodiment was 78 mV / dec. The catalyst activity and durability of the oxygen-evolving electrode were evaluated using the aforementioned PEM-type water electrolytic cell. The results are shown in Table 1.

[0038] Example 2 The electrode of this example was obtained in the same manner as in Example 1, except that the concentration of RuCl3 in the ruthenium salt solution was 0.4 mmol / L. The electrode in this embodiment is an electrode having an oxygen-evolving electrode catalyst made of manganese oxide having a crystalline structure of β-type manganese dioxide and containing ruthenium, on a conductive substrate made of Pt-coated Ti fibers, with a manganese content of 1.20 mg / cm³. 2 And the ruthenium content is 0.02 mg / cm³ 2 The result was (Ru / Mn molar ratio was 0.009). A PEM-type water electrolytic cell was fabricated in the same manner as in Example 1, except that the electrode in this example was the working electrode (anode). LSV measurements were performed using this PEM-type water electrolytic cell, and the Tafel gradient was 77 mV / dec. The oxygen-evolving electrode catalyst activity and durability were evaluated using the PEM-type water electrolytic cell obtained above. The results are shown in Table 1.

[0039] Example 3 Manganese oxide was electrochemically deposited on a conductive substrate in the same manner as in Example 1, except that the electrolytic cell was filled with a sulfuric acid-manganese sulfate-ammonium sulfate mixed solution with a sulfuric acid concentration of 0.30 mol / L, a manganese sulfate concentration of 0.50 mol / L, and an ammonium sulfate concentration of 1.5 mol / L. Next, a ruthenium-containing manganese oxide composite electrode material was obtained in the same manner as in Example 1, except that a conductive substrate was electrolytically produced from the manganese oxide obtained above, and this material was used as the electrode for this example. The electrode in this embodiment is an electrode having an oxygen-evolving electrode catalyst made of manganese oxide having a crystalline structure of β-type manganese dioxide and containing ruthenium, on a conductive substrate made of Pt-coated Ti fibers, with a manganese content of 1.20 mg / cm³. 2 And the ruthenium content is 0.05 mg / cm³ 2 The result was (Ru / Mn molar ratio was 0.023). A PEM-type water electrolytic cell was fabricated in the same manner as in Example 1, except that the electrode in this example was the working electrode (anode). LSV measurements were performed using this PEM-type water electrolytic cell, and the Tafel gradient was 69 mV / dec. The oxygen-evolving electrode catalyst activity and durability were evaluated using the PEM-type water electrolytic cell obtained above. The results are shown in Table 1.

[0040] Example 4 Except for setting the current application time to 5 minutes, manganese oxide was electrolytically extracted onto a conductive substrate in the same manner as in Example 1. Next, a ruthenium-containing manganese oxide composite electrode material was obtained in the same manner as in Example 1, except that a conductive substrate was electrolytically analyzed from the obtained manganese oxide, and this material was used as the electrode for this example. The electrode in this embodiment is an electrode having an oxygen-evolving electrode catalyst made of manganese oxide having a crystalline structure of β-type manganese dioxide and containing ruthenium, on a conductive substrate made of Pt-coated Ti fibers, with a manganese content of 0.60 mg / cm³. 2 And the ruthenium content is 0.01 mg / cm³ 2 The result was (Ru / Mn molar ratio was 0.009). A PEM-type water electrolytic cell was fabricated in the same manner as in Example 1, except that the electrode in this example was the working electrode (anode). LSV measurements were performed using this PEM-type water electrolytic cell, and the Tafel gradient was 79 mV / dec. The oxygen-evolving electrode catalyst activity and durability were evaluated using the PEM-type water electrolytic cell obtained above. The results are shown in Table 1.

[0041] Example 5 Except for the current application time being set to 20 minutes, manganese oxide was electrolytically extracted onto a conductive substrate in the same manner as in Example 1. Next, a ruthenium-containing manganese oxide composite electrode material was obtained in the same manner as in Example 1, except that a conductive substrate was electrolytically analyzed from the obtained manganese oxide, and this material was used as the electrode for this example. The electrode in this embodiment is an electrode having an oxygen-evolving electrode catalyst made of manganese oxide having a crystalline structure of β-type manganese dioxide and containing ruthenium, on a conductive substrate made of Pt-coated Ti fibers, with a manganese content of 2.40 mg / cm³. 2 And the ruthenium content is 0.02 mg / cm³ 2 The result was (Ru / Mn molar ratio was 0.005). A PEM-type water electrolytic cell was fabricated in the same manner as in Example 1, except that the electrode in this example was the working electrode (anode). LSV measurements were performed using this PEM-type water electrolytic cell, and the Tafel gradient was 79 mV / dec. The oxygen-evolving electrode catalyst activity and durability were evaluated using the PEM-type water electrolytic cell obtained above. The results are shown in Table 1.

[0042] Comparative Example 1 A catalyst ink was prepared by mixing 6.6 mg of ruthenium oxide powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a catalyst into a solution containing 610 μL of water, 610 μL of ethanol, and 31 μL of ionomer (product name: 10 wt% Nafion dispersion solution, manufactured by Sigma-Aldrich). 25 μL of this ink was then applied to an electrolyte membrane (product name: Nafion 115, manufactured by Sigma-Aldrich) to cover an area of ​​1 cm × 1 cm, and air-dried to obtain the catalyst-coated electrolyte membrane of this comparative example. The resulting catalyst-equipped electrolyte membrane has an oxygen-evolving electrode catalyst made of ruthenium oxide on one side of the electrolyte membrane, with a ruthenium content of 0.10 mg / cm³. 2 (The manganese content was 0 mg / cm³) 2 ). Using the obtained catalyst-equipped electrolyte membrane, a PEM-type water electrolytic cell equipped with a MEA prepared by the following method was fabricated. A catalyst ink was prepared by mixing 20% ​​by mass of platinum-supported carbon catalyst (product name: 20% Platinum on Vulcan XC-72, manufactured by Sigma-Aldrich) with a solution containing water, ethanol, and ionomer (product name: Nafion dispersion solution, manufactured by Sigma-Aldrich). This was coated onto carbon paper (product name: TGP-H-060, manufactured by Toray Industries), air-dried, and used as the counter electrode. In a catalyst-equipped electrolyte membrane, the side coated with catalyst ink is designated as the working electrode (anode), and the opposite side as the counter electrode. On the working electrode side, Pt-coated Ti fibers (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 / cm³) are used. 2 ) and the catalyst surface of the counter electrode obtained above are placed opposite each other, and the catalyst-coated electrolyte membrane is sandwiched between them, and a hot press machine (product name: SA-302, manufactured by Tester Sangyo Co., Ltd.) is used to press 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. Using the obtained PEM-type water electrolysis cell, LSV measurements were performed, and the Tafel gradient for this comparative example was 46 mV / dec. Furthermore, the catalytic activity and durability of the oxygen-evolving electrode were evaluated using the obtained PEM-type water electrolytic cell. The results are shown in Table 1.

[0043] Comparative Example 2 A 30 mL aqueous solution containing 0.1 mol / L manganese sulfate, 0.4 mol / L potassium permanganate, and 30.02 mol / L RuCl was packed into an autoclave with a 50 mL capacity Teflon cylinder and subjected to hydrothermal treatment at 180°C for 10 hours. Next, the autoclave was cooled to room temperature, and the contents were separated into solid and liquid by suction filtration. The resulting cake was washed with pure water and then dried at 60°C to obtain ruthenium-containing manganese oxide powder. A catalyst-coated electrolyte membrane was obtained in the same manner as in Comparative Example 1, except that the obtained ruthenium-containing manganese oxide powder was used as the catalyst. The resulting catalyst-equipped electrolyte membrane has a crystalline structure of β-type manganese dioxide on one side of the electrolyte membrane and an oxygen evolution electrode catalyst made of manganese oxide containing ruthenium, with a manganese content of 0.66 mg / cm³. 2 And the ruthenium content is 0.10 mg / cm³ 2 (The Ru / Mn molar ratio was 0.082.) A PEM-type water electrolytic cell was prepared in the same manner as in Comparative Example 1, except that the catalyst-equipped electrolyte membrane obtained above was used. Using the obtained PEM-type water electrolysis cell, LSV measurements were performed, and the Tafel gradient for this comparative example was 151 mV / dec. Furthermore, the catalytic activity of the oxygen-evolving electrode was evaluated using the obtained PEM-type water electrolytic cell. The results are shown in Table 1. Note that the catalytic activity of the catalyst-equipped electrolyte membrane in this comparative example was extremely low, making it impossible to evaluate its durability.

[0044] Comparative Example 3 Except for setting the current application time to 1 minute, manganese oxide was electrochemically deposited on a conductive substrate in the same manner as in Example 1. Next, a ruthenium-containing manganese oxide composite electrode material was obtained in the same manner as in Example 1, except that a conductive substrate on which the manganese oxide obtained above was electrochemically deposited was used, and this was used as the electrode for this comparative example. The electrode in this comparative example is an electrode having an oxygen-evolving electrode catalyst made of manganese oxide having a crystalline structure of β-type manganese dioxide and containing ruthenium, on a conductive substrate made of Pt-coated Ti fibers, with a manganese content of 0.12 mg / cm³. 2 And the ruthenium content is 0.01 mg / cm³ 2 The result was (Ru / Mn molar ratio was 0.045). A PEM-type water electrolytic cell was prepared in the same manner as in Example 1, except that the electrode in this comparative example was the working electrode (anode). LSV measurements were performed using this PEM-type water electrolytic cell, and the Tafel gradient was 97 mV / dec. The oxygen-evolving electrode catalyst activity and durability were evaluated using the obtained PEM-type water electrolytic cell. The results are shown in Table 1.

[0045] [Table 1]

[0046] The electrodes in the examples all exhibit high durability, with degradation rates of 10% or less, and even 5% or less, and a 2V-current density of 1.8A / cm². 2Furthermore, 2.0 A / cm 2 The above confirms that it exhibits high activity. In contrast, the electrode of Comparative Example 1, which does not contain manganese, showed catalytic activity, but its 2V-current density was 1.2 A / cm². 2 The catalytic activity was lower compared to the electrode in the example. Furthermore, Comparative Example 2, which had a Tafel gradient of 151 mV / dec., showed significantly lower durability, and the unit Mn content was 0.12 mg / cm³. 2 Comparative Example 3 showed a low 2V current density. From these results, it was confirmed that the electrode using ruthenium-containing manganese oxide as the oxygen-evolving electrode catalyst in the example exhibits higher oxygen-evolving electrode catalyst activity and durability compared to conventional electrolyte membranes with catalysts using ruthenium oxide as the oxygen-evolving electrode catalyst.

Claims

1. In an oxygen-evolving electrode comprising an oxygen-evolving electrode catalyst and a conductive substrate, The conductive substrate has the oxygen-evolving electrode catalyst on at least its surface, The oxygen evolution catalyst contains ruthenium-containing manganese oxide, The amount of manganese per geometric area in the oxygen-evolving electrode is 0.2 mg / cm². ２ 5.0mg / cm or more ２ The following, and moreover, The Tafel gradient of the oxygen generation electrode is characterized by being 50 mV / dec. to 140 mV / dec. Oxygen-generating electrode.

2. The oxygen-evolving electrode according to claim 1, wherein the ruthenium-containing manganese oxide includes a manganese oxide having the structure of β-type manganese dioxide.

3. The amount of ruthenium per geometric area in the oxygen-evolving electrode is 0.005 mg / cm². ２ 0.500mg / cm or more ２ The oxygen generating electrode according to claim 1 or claim 2, which is as follows:

4. The oxygen-evolving electrode according to claim 1 or claim 2, wherein the molar ratio of ruthenium to manganese is 0.001 or more and 0.250 or less.

5. The oxygen generating electrode according to claim 1 or claim 2, wherein the conductive substrate contains titanium.

6. A method for manufacturing an oxygen-generating electrode according to claim 1 or claim 2, comprising: an electrolysis step of electrolytically generating manganese oxide from a conductive substrate to obtain a manganese oxide-containing substrate; a contact step of contacting the manganese oxide-containing substrate with a ruthenium salt solution; and a heat treatment step of heat-treating the manganese oxide-containing substrate that has undergone the contact step.

7. A water electrolysis apparatus comprising the oxygen generating electrode described in claim 1 or claim 2.

8. A method for producing hydrogen using the oxygen generating electrode described in claim 1 or claim 2.

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