Iridium cluster, production method of iridium cluster, oxygen-generating electrode, water electrolysis apparatus, and water electrolysis method

Abstract

To provide: an iridium cluster composed of a small number of constituent atoms and exhibiting excellent activity as a catalyst for an oxygen evolution reaction; a production method of the iridium cluster; an oxygen-generating electrode using the iridium cluster; a water electrolysis apparatus using the oxygen-generating electrode; and a water electrolysis method.SOLUTION: The number of constituent atoms is 2 or more and 100 or less in an iridium cluster of the invention. The iridium cluster preferably has a ligand. The ligand preferably contains an organic ligand such as triphenylphosphine, diphenyl (p-tolyl) phosphine and phenylethanethiol, or a ligand containing CO.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an iridium cluster, a method for manufacturing the iridium cluster, an oxygen generation electrode, a water electrolysis device, and a method for electrolyzing water.

Background Art

[0002] As a countermeasure against environmental problems and energy resource problems, technologies for producing hydrogen by electrolyzing water have been studied. The electrolysis of water is caused by a hydrogen generation reaction (HER) and an oxygen generation reaction (OER). In order to efficiently produce hydrogen in the electrolysis of water, it is necessary to improve the efficiency of the oxygen generation reaction.

[0003] Iridium is known as a catalyst for the oxygen generation reaction that can improve the efficiency of the oxygen generation reaction. For example, Non-Patent Document 1 discloses a technique for producing iridium nanoparticles having a diameter of about 1.6 nm by a colloid method as a catalyst for the oxygen generation reaction.

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In order to make the oxygen generation reaction more efficient, it is desirable to improve the activity of the oxygen evolution reaction catalyst. By reducing the size of iridium particles, for example, by reducing the number of constituent atoms in the iridium cluster, the specific surface area can be increased and the activity of the oxygen evolution reaction catalyst can be improved.

[0006] Therefore, an object of the present invention is to provide an iridium cluster having a small number of constituent atoms and excellent activity as a catalyst for the oxygen generation reaction, a method for producing the iridium cluster, an oxygen generation electrode using the iridium cluster, a water electrolysis apparatus using the oxygen generation electrode, and a method for electrolyzing water.

Means for Solving the Problems

[0007] The present inventors have found that an iridium cluster having 2 to 100 constituent atoms can be produced by a method for producing an iridium cluster including a reduction step of obtaining an iridium-containing solution by reducing an iridium compound by a liquid phase reduction method, and a ligand mixing step of obtaining an iridium cluster having an organic ligand by mixing the iridium-containing solution and an organic ligand, and have found that the above problems can be solved thereby, leading to the completion of the present invention. More specifically, the present invention is as follows.

[0008] (1) An iridium cluster having 2 to 100 constituent atoms.

[0009] (2) The iridium cluster according to (1), having a ligand.

[0010] (3) The iridium cluster according to (2), wherein the ligand includes an organic ligand.

[0011] (4) The iridium cluster according to (3), wherein the organic ligand is at least one selected from triphenylphosphine, diphenyl(p-tolyl)phosphine, and phenylethanethiol.

[0012] (5) The iridium cluster according to any one of (2) to (4), wherein the ligand contains CO.

[0013] (6) The iridium cluster according to any one of (1) to (5), wherein the number of constituent atoms is 4 or more and 15 or less.

[0014] (7) A method for producing an iridium cluster having 2 or more and 100 or less constituent atoms, comprising: a reduction step of obtaining an iridium-containing liquid by reducing an iridium compound by a liquid-phase reduction method; an organic ligand mixing step of obtaining an iridium cluster having the organic ligand by mixing the iridium-containing liquid and an organic ligand.

[0015] (8) The method for producing an iridium cluster according to (7), further comprising a washing step of washing the iridium cluster having the organic ligand obtained in the organic ligand mixing step with a solvent two or more times.

[0016] (9) The method for producing an iridium cluster according to (7) or (8), wherein the liquid-phase reduction method is a polyol reduction method in which the iridium compound is heated in the presence of a polyol.

[0017] (10) An oxygen evolution electrode for electrolysis of water, comprising: an electrode substrate and an iridium cluster provided on the electrode substrate; the iridium cluster has 2 or more and 100 or less constituent atoms.

[0018] (11) The oxygen evolution electrode according to (10), wherein the iridium cluster is supported on a porous body.

[0019] (12) A water electrolysis device comprising the oxygen evolution electrode according to (10) or (11) and a hydrogen evolution electrode.

[0020] A method for electrolyzing water, comprising the step of electrolyzing water using the oxygen generation electrode according to (13), (10) or (11).

Advantages of the Invention

[0021] According to the present invention, it is possible to provide an iridium cluster having a small number of constituent atoms and excellent activity as a catalyst for an oxygen generation reaction, a method for producing the iridium cluster, an oxygen generation electrode using the iridium cluster, a water electrolysis apparatus using the oxygen generation electrode, and a method for electrolyzing water.

Brief Description of the Drawings

[0022]

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BEST MODE FOR CARRYING OUT THE INVENTION

[0023] ≪Iridium Cluster≫ The iridium cluster has 2 or more and 100 or fewer constituent atoms. That is, the iridium cluster is a metal cluster in which 2 or more and 100 or fewer iridium atoms are bonded. Conventionally, it has not been possible to produce an iridium cluster with as few as 2 to 100 constituent atoms. For example, the number of constituent atoms of the iridium nanoparticles with a diameter of about 1.6 nm in Non-Patent Document 1 is about 150 by theoretical calculation from the covalent radius. However, by the method for producing an iridium cluster described below, it has become possible to produce an iridium cluster with as few as 2 to 100 constituent atoms.

[0024] The iridium cluster has as few as 2 to 100 constituent atoms. Therefore, the iridium cluster has a larger specific surface area and is superior in activity as a catalyst for the oxygen evolution reaction (OER) than conventional iridium clusters. In other words, the iridium cluster has high OER activity. Therefore, by using the iridium cluster as a catalyst for the oxygen evolution reaction, the oxygen generation reaction can be made highly efficient. For example, the efficiency of water electrolysis can be increased. Also, by using the iridium cluster as a catalyst for the oxygen evolution reaction, the oxygen evolution reaction can be made highly efficient in various devices using the oxygen evolution reaction, such as secondary batteries. In addition, since the iridium cluster has high OER activity, the amount of OER catalyst used can also be reduced in water electrolysis and other devices.

[0025] The number of constituent atoms of the iridium cluster may be 2 or more and 100 or less, preferably 2 or more and 50 or less, more preferably 4 or more and 15 or less, and may also be 4 or more and 13 or less. The number of constituent atoms of the iridium cluster can be determined by electrospray ionization mass spectrometry (ESI-MS).

[0026] The iridium cluster may have a ligand. The iridium cluster may have one type or two or more types of ligands. Examples of the ligand include an organic ligand and CO. Examples of the organic ligand include triphenylphosphine, diphenyl(p-tolyl)phosphine, and phenylethanethiol. Further, the iridium cluster may have a halogen atom such as a chlorine atom.

[0027] ≪Method for Producing Iridium Cluster≫ The iridium cluster having 2 or more and 100 or less of the above-described constituent atoms can be produced, for example, by a production method of an iridium cluster including a reduction step of obtaining an iridium-containing liquid by reducing an iridium compound by a liquid-phase reduction method, and an organic ligand mixing step of obtaining an iridium cluster having an organic ligand by mixing the iridium-containing liquid and the organic ligand. The method for producing an iridium cluster may further include a washing step of washing the iridium cluster having the organic ligand obtained in the organic ligand mixing step two or more times with a solvent. Hereinafter, each step will be described.

[0028] [Reduction Step] In the reduction step, an iridium-containing liquid is obtained by reducing an iridium compound by a liquid-phase reduction method.

[0029] Examples of the iridium compound include iridium halides such as iridium chloride, and a specific example is iridium(III) chloride. The iridium compound may be a hydrate.

[0030] Examples of the liquid-phase reduction method include a polyol reduction method in which an iridium compound is heated in the presence of a polyol. Examples of the polyol include polyols used as a reducing agent, such as ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, diethylene glycol, triethylene glycol, dipropylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2-methyl-1,3-propanediol, and glycerin. Note that the polyol reduction method is preferably carried out in the presence of a hydroxide such as sodium hydroxide.

[0031] The heating temperature of the polyol reduction method is preferably 80°C or higher and 160°C or lower, more preferably 100°C or higher and 140°C or lower. The heating time of the polyol reduction method is preferably 3 minutes or longer and 20 minutes or shorter, more preferably 5 minutes or longer and 15 minutes or shorter. When heating in the polyol reduction method, it is preferable to stir. In the polyol reduction method, after heating the iridium compound in the presence of a polyol to reduce the iridium compound to obtain an iridium-containing solution, the iridium-containing solution may be cooled as necessary before the organic ligand mixing step.

[0032] In the polyol reduction method, in the obtained iridium-containing solution, it is preferable that CO derived from a polyol such as ethylene glycol or propylene glycol is coordinated to iridium. That is, in the iridium-containing solution, it is preferable that iridium is stabilized by a carbonyl ligand derived from a polyol such as ethylene glycol or propylene glycol.

[0033] Examples of the liquid phase reduction method include a method of reducing an iridium compound with sodium borohydride. The temperature during reduction with sodium borohydride is preferably 0°C or higher and 40°C or lower, more preferably 1°C or higher and 30°C or lower. The reaction time during reduction with sodium borohydride is preferably 1 second or longer and 24 hours or shorter, more preferably 5 minutes or longer and 1 hour or shorter. When reacting, it is preferable to stir. In the reduction with sodium borohydride, after reducing the iridium compound in the presence of sodium borohydride to obtain an iridium-containing solution, the iridium-containing solution may be cooled as necessary before the organic ligand mixing step.

[0034] [Organic Ligand Mixing Step] In the organic ligand mixing step, an iridium cluster having an organic ligand is obtained by mixing the iridium-containing solution obtained in the reduction step with the organic ligand. By mixing the iridium-containing solution obtained in the reduction step with the organic ligand, the organic ligand coordinates to iridium. That is, iridium is stabilized by the organic ligand. For this reason, it is presumed that the growth of the iridium cluster is suppressed, and an iridium cluster having as few as 2 to 100 constituent atoms can be produced. Further, it is presumed that the growth of the iridium cluster is more suppressed by adding the organic ligand immediately after the reduction step and stirring.

[0035] Examples of the organic ligand include compounds having an aromatic ring such as triphenylphosphine, diphenyl(p-tolyl)phosphine, and phenylethanethiol.

[0036] The method of mixing the organic ligand with the iridium-containing solution is not particularly limited. For example, a solution in which the organic ligand is dissolved in a solvent may be mixed with the iridium-containing solution, or the organic ligand may be directly added to the iridium-containing solution. Examples of the solvent for dissolving the organic ligand include acetone and toluene.

[0037] The temperature of the organic ligand mixing step is not particularly limited, and room temperature (for example, 15°C or higher and 25°C or lower) may be used.

[0038] [Washing Step] In the washing step, the iridium cluster having the organic ligand obtained in the organic ligand mixing step is washed with a solvent two or more times. For example, the washing step includes a first washing step of washing the iridium cluster having the organic ligand obtained in the organic ligand mixing step with a first solvent, and a second washing step of washing with a second solvent after the first washing step.

[0039] In the first washing step, the iridium cluster having the organic ligand obtained in the organic ligand mixing step is washed with a first solvent to remove the excess polyol used in the polyol reduction method, the unreacted iridium compound, and the excess organic ligand used in the organic ligand mixing step. Examples of the first solvent include water, methanol, and a mixed solvent of water and methanol.

[0040] In the second washing step, the iridium cluster having the organic ligand after the first washing step is washed with a second solvent to obtain an iridium cluster having a desired number of constituent atoms. Examples of the second solvent include a mixed solvent of methanol and chloroform.

[0041] By such a production method, an iridium cluster having 2 or more and 100 or less of the above-described constituent atoms can be produced.

[0042] ≪Oxygen generation electrode≫ The iridium cluster having 2 or more and 100 or less of the above-described constituent atoms can be used as a catalyst for the oxygen generation reaction of an oxygen generation electrode for water electrolysis. Such an oxygen generation electrode for water electrolysis includes an electrode substrate and an iridium cluster provided on the electrode substrate, and the iridium cluster has 2 or more and 100 or less of the constituent atoms. The iridium cluster having 2 or more and 100 or less of the constituent atoms has a large specific surface area and is excellent in activity as a catalyst for the oxygen generation reaction (OER). Therefore, by using an oxygen generation electrode using the iridium cluster as a catalyst for the oxygen generation reaction, the efficiency of water electrolysis can be increased.

[0043] The electrode substrate of the oxygen generation electrode is not particularly limited, and a known conductive substrate can be used. Examples of the electrode substrate of the oxygen generation electrode include known substrates used as electrodes for water electrolysis, and specific examples include metal substrates, carbon substrates, glass substrates, and the like.

[0044] Examples of the metal substrate include a single metal substrate such as nickel, titanium, iron, and copper, and an alloy substrate. Examples of the carbon substrate include glassy carbon (GC), carbon paper, carbon fiber paper, and carbon rods. Examples of the glass substrate include conductive glass. The electrode substrate may be a porous body.

[0045] In the oxygen generation electrode, the iridium cluster is preferably supported on a porous body. By supporting the iridium cluster on the porous body, aggregation of the iridium cluster is suppressed during firing in the production of the oxygen generation electrode.

[0046] In the oxygen generation electrode, when the iridium cluster is supported on a porous body, the iridium cluster may be supported on the porous body, and the one in which the iridium cluster is supported on the porous body may be provided on the surface of the electrode substrate, or the iridium cluster may be supported on the electrode substrate made of a porous body. Examples of the porous body supporting the iridium cluster include carbon black and metal oxides.

[0047] The method for manufacturing the oxygen generation electrode is not particularly limited. For example, after producing a catalyst (OER catalyst) by impregnating a porous body such as carbon black with the above-described iridium cluster solution and firing, the oxygen generation electrode can be produced by applying a liquid containing the catalyst (for example, catalyst slurry) to the electrode substrate. When the iridium cluster used has a ligand, it is preferable that at least a part of the ligand is removed by firing. When manufacturing the oxygen generation electrode, the electrochemical characteristics of the oxygen generation electrode can be adjusted by adjusting the firing temperature. The firing temperature is, for example, 200°C or higher, preferably 250°C or higher and 400°C or lower, and more preferably 300°C or higher and 350°C or lower. The firing time is, for example, 1 hour or more and 3 hours or less.

[0048] ≪Water electrolysis device≫ The above oxygen generation electrode can be used as the oxygen generation electrode of a water electrolysis device. Such a water electrolysis device includes the above oxygen generation electrode and a hydrogen generation electrode. Since the water electrolysis device uses an iridium cluster having 2 or more and 100 or less constituent atoms as a catalyst for the oxygen generation reaction, it can perform efficient electrolysis of water.

[0049] The oxygen generation electrode included in the water electrolysis device is as described above. As the hydrogen generation electrode included in the water electrolysis device, a known hydrogen generation electrode can be used, for example, a platinum electrode, a carbon electrode, etc.

[0050] The water electrolysis device has, for example, an electrolytic cell for containing an electrolytic solution, and the oxygen generation electrode and the hydrogen generation electrode are immersed in the electrolytic solution. The electrolytic solution is an aqueous solution in which an electrolyte is dissolved in water. Such a water electrolysis device can decompose water efficiently using light such as sunlight and generate hydrogen and oxygen. Further, the water electrolysis device including the above oxygen generation electrode may be a water electrolysis device that performs solid polymer (PEM) type water electrolysis without using an electrolytic solution.

[0051] ≪Method for electrolyzing water≫ The above oxygen generation electrode can be used for electrolyzing water. Such a method for electrolyzing water includes a step of electrolyzing water using the above oxygen generation electrode. Since the method for electrolyzing water uses an iridium cluster having 2 or more and 100 or less constituent atoms as a catalyst for the oxygen generation reaction, it can perform efficient electrolysis of water.

Example

[0052] Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0053] 〔Example 1〕 (Synthesis) <Reduction process and organic ligand mixing process> Iridium(III) chloride hydrate (59.7 mg) and sodium hydroxide (135 mg) were dissolved in ethylene glycol (15 ml) to obtain a reaction solution. This solution was stirred (1000 rpm) while heating at 120 °C for 7 minutes using a chemical station (manufactured by EYELA, model number: PPS-CTRL1), and then ice-cooled to room temperature. Thereafter, a solution prepared by dissolving triphenylphosphine (524.5 mg) in acetone (10 ml) was quickly added to the reaction solution, and the mixture was stirred at room temperature for 60 minutes. Ultra-pure water (>18 MΩ×cm) (12 ml) and toluene (8 ml) were added to the reaction solution, and centrifugation was performed (3300 rpm, 2 minutes), and the upper layer was extracted. This extraction solution was evaporated until it became viscous.

[0054] <First washing process> After the above evaporation, the crude product was washed in order with a total of 30 ml of an ultra-pure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10), centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the obtained iridium cluster (crude), centrifuged (3300 rpm, 2 minutes), and the supernatant was extracted.

[0055] <Second washing process> This extraction solution was evaporated, dissolved in 1 ml of chloroform, then methanol (19 ml) was added, centrifugation was performed (3300 rpm, 2 minutes), and the supernatant was discarded. This operation was performed 3 times. Approximately 5 ml of toluene was added, centrifuged (3300 rpm, 2 minutes), and the supernatant was extracted. This extraction solution was evaporated and dissolved in toluene. This solution was designated as iridium cluster solution 1.

[0056] (Identification) For the iridium cluster solution 1, the obtained iridium clusters were confirmed using a transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), electrospray ionization mass spectrometry (ESI-MS), and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Figure 1 shows the transmission electron micrograph (Figure 1(a)) and particle size (Figure 1(b)), Figure 2 shows the FT-IR spectrum, and Figure 3 shows the ESI-MS spectrum. As a result, the iridium cluster solution 1 obtained in Example 1 contained an iridium cluster having triphenylphosphine (TPP) and CO as ligands and 13 constituent atoms, and an iridium cluster having triphenylphosphine and CO as ligands and 15 constituent atoms. In addition, the iridium cluster solution 1 also slightly contained an iridium cluster having triphenylphosphine and CO as ligands and 18 constituent atoms. The composition of the iridium cluster is presumed to be Ir 13 (CO) 15 (TPP)8, Ir 15 (CO) 19 (TPP)8, Ir 18 (CO) 19 (TPP)9, respectively. In addition, the iridium cluster had a particle size of 0.8 ± 0.1 nm.

[0057] (Electrochemical Measurement 1) <Catalyst Preparation> To 100 mg of carbon black (manufactured by Fuel Cell Earth, product name: VULCAN XC-72), the iridium cluster solution 1 measured by ICP-MS so that Ir was 1 mg was added, and impregnation was carried out in a mortar. This was evacuated in a desiccator overnight and calcined in a firing furnace under reduced pressure (<5.0×10 3 Pa) at 250 °C for 2 hours (7 °C min -1 ) to prepare an Ir-supported catalyst in which Ir clusters were supported on carbon black.

[0058] <Slurry Preparation, Coating> The prepared Ir-supported catalyst was added to a mixed solution consisting of ultrapure water (19.1 mL), 2-propanol (6 mL), and a polymer electrolyte (Nafion (registered trademark) solution, manufactured by Fujifilm Wako Pure Chemical Corporation) (100 μL). The obtained mixed solution was sonicated in an ice water bath for 30 minutes to disperse the Ir-supported catalyst in the mixed solvent, thereby preparing a catalyst slurry. The catalyst slurry (10 μL) was cast over the entire surface of a glassy carbon (GC) electrode and dried at 750 rpm for 40 minutes.

[0059] <Measurement> Using the GC coated with the prepared catalyst slurry as the working electrode, a three-electrode system was constructed with a platinum (Pt) coil counter electrode and a silver / silver chloride (Ag / AgCl) reference electrode. Electrochemical measurements were performed using a potentiostat / galvanostat in an aqueous solution of 0.1 M perchloric acid (HClO4). In the measurement, after bubbling N2 gas for 30 minutes, cyclic voltammetry (CV) was performed 100 times in the region from 0 to 1.00 V (versus reversible hydrogen electrode; RHE) at a scan rate of 200 mV s -1 to clean the electrode. After CV, linear sweep voltammetry (LSV) was performed under N2 at 20 mV s -1 in the region from 1.00 to 1.85 V (vs. RHE). The results are shown in Fig. 4. Fig. 4(a) shows a linear sweep voltammogram, and Fig. 4(b) shows the result of obtaining the oxygen evolution reaction activity (OER mass activity) per Ir mass from the current value at a voltage of 1.6 V vs. RHE in the linear sweep voltammogram. Note that instead of the iridium cluster solution 1, iridium(III) chloride hydrate was supported on carbon black by an impregnation method and calcined at 250 °C for 2 hours under reduced pressure (<5.0×10 3 Pa) (7 °C min -1 ) to obtain an Ir-supported catalyst (Comparative Example 1) in which Ir nanoparticles with a particle size of 2.1 nm were supported on carbon black. Electrochemical measurements were also performed in the same manner as in Example 1, and the results are also shown in Fig. 4. As shown in Fig. 4, Example 1 had 2.63 times higher OER activity than Comparative Example 1, and an improvement in OER activity was confirmed.

[0060] (Electrochemical measurement 2) For iridium cluster solution 1 (Example 1), except that the calcination temperature in <Catalyst Preparation> was set to 200 °C, 250 °C, 300 °C, or 350 °C, or no calcination (No cal) was performed, the same operations as in Electrochemical measurement 1 were carried out. The results are shown in FIGS. 5 to 7. FIG. 5 shows a linear sweep voltammogram. FIG. 6 shows a linear sweep voltammogram when the calcination temperature in <Catalyst Preparation> is 250 °C, and FIG. 7 shows the result of obtaining the oxygen evolution reaction activity (OER mass activity) per Ir mass from the current value at a voltage of 1.6 V vs. RHE in the linear sweep voltammogram when the calcination temperature is 250 °C. In addition, except that an Ir-supported catalyst (Comparative Example 2) in which a toluene solution of a commercially available catalyst (amorphous iridium oxide catalyst, manufactured by Tanaka Precious Metals Industry, product number: TEC77100) was supported on carbon black by an impregnation method was used instead of iridium cluster solution 1, the OER activity evaluation was also carried out in the same manner as in Example 1, and the results are also shown in FIGS. 6 and 7. As shown in FIG. 5, the electrochemical properties can be adjusted by adjusting the calcination temperature in <Catalyst Preparation>, and it was confirmed that the OER mass activity was particularly high when the calcination temperature was 250 °C or higher and 350 °C or lower. Also, as shown in FIGS. 6 and 7, it was confirmed that Example 1 can achieve 1.5 times higher OER activity than Comparative Example 2.

[0061] (Examination of the washing process) For the extraction solution obtained in <First washing step>, MALDI-MS was used to confirm the obtained iridium clusters. The results are shown in FIG. 8. The results of iridium cluster solution 1 (that is, the results after the second washing step) are also shown in FIG. 8 for comparison. As shown in FIG. 8, it can be seen that by performing the first washing step and the second washing step, an iridium cluster with a narrower distribution of the number of constituent atoms and more uniform number of constituent atoms can be obtained than when the second washing step is not performed.

[0062] (Examination of the heating time in the reduction step) In the reduction step, an iridium cluster solution was obtained in the same manner as the iridium cluster solution 1 (Example 1), except that the heating at 120°C for 7 minutes was changed to heating at 120°C for 3 minutes, 120°C for 10 minutes, or 120°C for 15 minutes. For the obtained extraction solution, the resulting iridium clusters were confirmed using MALDI-MS. The results are shown in Fig. 9. The results of heating at 120°C for 7 minutes are also shown in accordance with Fig. 9. Fig. 9(a) shows the results of heating at 120°C for 3 minutes, Fig. 9(b) shows the results of heating at 120°C for 7 minutes, Fig. 9(c) shows the results of heating at 120°C for 10 minutes, and Fig. 9(d) shows the results of heating at 120°C for 15 minutes. As shown in Fig. 9, it can be seen that by adjusting the heating time in the reduction step, the distribution of the number of constituent atoms of the obtained iridium clusters can be adjusted.

[0063] [Example 2] (Synthesis) <Reduction step and organic ligand mixing step> Iridium(III) chloride hydrate (59.7 mg) and sodium hydroxide (135 mg) were dissolved in ethylene glycol (15 ml) to obtain a reaction solution. This solution was stirred (1000 rpm) while heating at 120°C for 3 minutes using a chemical station (manufactured by EYELA, model number: PPS), and then ice-cooled to room temperature. Thereafter, a solution prepared by dissolving triphenylphosphine (524.5 mg) in 10 ml of acetone was quickly added to the reaction solution, and the mixture was stirred at room temperature for 60 minutes. A mixture of ultrapure water (>18 MΩ×cm) (12 ml) and toluene (8 ml) was added to the reaction solution, and the mixture was centrifuged (3300 rpm, 2 minutes), and the upper layer was extracted. This extraction solution was evaporated until it became viscous.

[0064] <First washing step> After the above evaporation, the solid was washed in this order with a total of 30 ml of ultrapure water / methanol mixed solvents (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10), centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification. About 5 ml of toluene was added to the obtained solid, centrifuged (3300 rpm, 2 minutes), and the supernatant was extracted.

[0065] <Second washing step> This extraction solution was evaporated, dissolved in 1 ml of chloroform, then methanol (19 ml) was added, centrifuged (3300 rpm, 2 minutes), and the supernatant was extracted. After evaporating this extraction solution, it was washed with 5 ml of methanol, then centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification and then dissolved in toluene. This solution was designated as iridium cluster solution 2.

[0066] (Identification) Regarding iridium cluster solution 2, the obtained iridium clusters were confirmed using TEM, FT-IR, ESI-MS and MALDI-MS. Figure 10 shows a transmission electron micrograph (Figure 10(a)) and particle size (Figure 10(b)), and Figure 11 shows an ESI-MS spectrum. As a result, the iridium cluster solution 2 obtained in Example 2 contained iridium clusters having triphenylphosphine (TPP) and CO as ligands and a constituent atom number of 6 to 8. The composition of the iridium cluster was speculated to be Ir 6-8 (CO) 3-7 (TPP)8Cl1. Also, the iridium clusters had a particle size of 0.8 ± 0.1 nm.

[0067] (Electrochemical measurement 3) Instead of the iridium cluster solution 1 (Example 1), the iridium cluster solution 2 (Example 2) was used, and the same operations as in Electrochemical Measurement 1 were performed except that the calcination temperature was set to 250 °C, 300 °C, or 350 °C in <Catalyst Preparation>. The results are shown in FIGS. 12 and 13. FIG. 12 shows a linear sweep voltammogram, and FIG. 13 shows the result of obtaining the oxygen evolution reaction activity (OER mass activity) per Ir mass from the current value at a voltage of 1.6 V vs. RHE of the linear sweep voltammogram when the calcination temperature in <Catalyst Preparation> was 300 °C. In addition, the toluene solution of a commercially available catalyst (amorphous iridium oxide catalyst, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., product number: TEC77100) (Comparative Example 2) was used instead of the iridium cluster solution 1, and the OER activity evaluation was carried out in the same manner as in Example 1 except that no calcination (No cal) was performed in <Catalyst Preparation>. The result is also shown in FIG. 13. As shown in FIG. 12, the electrochemical properties can be adjusted by adjusting the calcination temperature in <Catalyst Preparation>, and it was confirmed that the OER mass activity was particularly high when the calcination temperature was 250 °C or higher and 350 °C or lower. Also, as shown in FIG. 13, it was confirmed that Example 2 had 1.5 times higher OER activity than Comparative Example 2.

[0068] (Examination of the washing process) Regarding the extraction solution obtained in <First Washing Step>, the obtained iridium cluster was confirmed using MALDI-MS. The results are shown in FIG. 14. The results of the iridium cluster solution 2 (that is, the results after the second washing step) are also shown in FIG. 14 for comparison. As shown in FIG. 14, it can be seen that by performing the first washing step and the second washing step, an iridium cluster with a narrower distribution of the number of constituent atoms and more uniform number of constituent atoms can be obtained than when the second washing step is not performed.

[0069] [Example 3] (Synthesis) <Reduction Step and Organic Ligand Mixing Step> Iridium(III) chloride hydrate (59.7 mg) and sodium hydroxide (135 mg) were dissolved in ethylene glycol (15 ml) to obtain a reaction solution. This solution was stirred (1000 rpm) while heating at 120 °C for 7 minutes using a chemical station (manufactured by EYELA, model number: PPS-CTRL1), and then ice-cooled to room temperature. Thereafter, a solution prepared by dissolving diphenyl(p-tolyl)phosphine (552.6 mg) in 5 ml of toluene was quickly added to the reaction solution, and the mixture was stirred at room temperature for 60 minutes. A mixture of ultrapure water (>18 MΩ×cm) (12 ml) and toluene (8 ml) was added to the reaction solution, followed by centrifugation (3300 rpm, 2 minutes), and the upper layer was extracted. This extraction solution was evaporated until it became viscous.

[0070] <First washing step> After the above evaporation, the residue was washed in this order with a total of 30 ml of an ultrapure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10), followed by centrifugation (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the obtained Ir cluster (crude), followed by centrifugation (3300 rpm, 2 minutes), and the supernatant was extracted.

[0071] <Second washing step> This extraction solution was evaporated, dissolved in 1 ml of chloroform, then methanol (19 ml) was added, followed by centrifugation (3300 rpm, 2 minutes), and the supernatant was discarded. This operation was performed 3 times. Approximately 5 ml of toluene was added, followed by centrifugation (3300 rpm, 2 minutes), and the supernatant was extracted. This extraction solution was evaporated and dissolved in toluene. This solution was designated as iridium cluster solution 3.

[0072] (Identification) For the iridium cluster solution 3, the obtained iridium clusters were confirmed using TEM, FT-IR, ESI-MS, and MALDI-MS. Figure 15 shows the MALDI-MS spectrum, Figure 16 shows the ESI-MS spectrum, and Figure 17 shows the transmission electron micrograph (Figure 17(a)) and particle size (Figure 17(b)). As a result, the iridium cluster solution 3 obtained in Example 3 contained iridium clusters having diphenyl(p-tolyl)phosphine (DPTP) and CO as ligands and having 13 to 18 constituent atoms. The composition of the iridium cluster is Ir 13-18 (CO) 14-20 (DPTP) 7-9 and is presumed to be so. Also, the iridium clusters had a particle size of 0.8 ± 0.1 nm.

[0073] (Electrochemical measurement 4) Using the iridium cluster solution 3 (Example 3) instead of the iridium cluster solution 1 (Example 1), the same operations as in Electrochemical measurement 1 were performed except that the firing temperature was set to 300 °C in <Catalyst preparation>. The results are shown in Figures 18 and 19. Figure 18 shows the linear sweep voltammogram, and Figure 19 shows the result of obtaining the oxygen evolution reaction activity (OER mass activity) per Ir mass from the current value at a voltage of 1.6 V vs. RHE in the linear sweep voltammogram. In addition, the results of OER activity evaluation performed in the same manner as in Example 1 are also shown in Figures 18 and 19, except that a toluene solution of a commercially available catalyst (amorphous iridium oxide catalyst, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., product number: TEC77100) (Comparative Example 2) was used instead of the iridium cluster solution 1.

[0074] [Example 4] <Reduction step and organic ligand mixing step> Iridium(III) chloride hydrate (59.7 mg) and sodium hydroxide (135 mg) were dissolved in ethylene glycol (15 ml) to obtain a reaction solution. This solution was stirred (1000 rpm) while heating at 120 °C for 30 minutes using a chemical station (manufactured by EYELA, model number: PPS-CTRL1), and then ice-cooled to room temperature. Thereafter, a solution prepared by adding phenylethyl mercaptan (268 μl) to 5 ml of toluene was quickly added to the reaction solution, and the mixture was stirred at room temperature for 60 minutes. A mixture of ultrapure water (15 ml) and toluene (2 ml) was added to the reaction solution, followed by centrifugation (3300 rpm, 2 minutes), and the upper layer was extracted. This extraction solution was evaporated until it became viscous.

[0075] <First washing step> After the above evaporation, washing was performed in this order using a total of 30 ml of an ultrapure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10), followed by centrifugation (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the obtained Ir cluster (crude), followed by centrifugation (3300 rpm, 2 minutes), and the supernatant was extracted. This solution was designated as iridium cluster solution 4.

[0076] (Identification) For iridium cluster solution 4, the obtained iridium cluster was confirmed using TEM, FT-IR, ESI-MS, and MALDI-MS. The ESI-MS spectrum is shown in Fig. 20, and the transmission electron micrograph (Fig. 21(a)) and particle size (Fig. 21(b)) are shown in Fig. 21. As a result, iridium cluster solution 4 obtained in Example 4 contained an iridium cluster having phenylethyl mercaptan (PET) and CO as ligands and a constituent atom number of 9 to 13. The composition of the iridium cluster is speculated to be Ir 9-13 (CO) 8-11 (PET) 8-12 as inferred. Also, the iridium cluster had a particle size of 0.8 ± 0.1 nm.

[0077] (Electrochemical measurement 5) Instead of the iridium cluster solution 1 (Example 1), the iridium cluster solution 4 (Example 4) was used, and the same operations as in the electrochemical measurement 1 were performed. The results are shown in FIGS. 22 and 23. FIG. 22 shows a linear sweep voltammogram, and FIG. 23 shows the result of obtaining the oxygen evolution reaction activity (OER mass activity) per Ir mass from the current value at a voltage of 1.6 V vs. RHE in the linear sweep voltammogram. In addition, the results of OER activity evaluation performed in the same manner as in Example 1, except that a toluene solution (Comparative Example 2) of a commercially available catalyst (amorphous iridium oxide catalyst, manufactured by Tanaka Precious Metals Industry, product number: TEC77100) was used instead of the iridium cluster solution 1, are also shown in FIGS. 22 and 23.

[0078] [Example 5] <Reduction process and organic ligand mixing process> Iridium(III) chloride hydrate (25 mg) was put into methanol (10 ml) and dissolved by stirring at 60 ° C. for 3 hours. A solution prepared by dissolving tetraoctylammonium bromide (48.6 mg) and triphenylphosphine (105 mg) in toluene (5 ml) was added to this solution. After this solution was cooled to room temperature in an ice bath, a solution prepared by dissolving sodium borohydride (64 mg) in methanol (5 ml) was added. It was also stirred at room temperature for 3 hours. A mixture of ultrapure water (> 18 MΩ×cm) (12 ml) and toluene (3 ml) was added to this reaction solution, centrifuged (3300 rpm, 2 minutes), and the upper layer was extracted. This extraction solution was evaporated until it became viscous.

[0079] <First washing process> After the above evaporation, washing was performed in this order using a total of 30 ml of an ultrapure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10), centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the obtained solid, centrifuged (3300 rpm, 2 minutes), and the supernatant was extracted. This solution was designated as the iridium cluster solution 5.

[0080] Fixation For the iridium cluster solution 5, the obtained iridium cluster was confirmed using MALDI-MS. The MALDI-MS spectrum is shown in Fig. 24. As a result, the iridium cluster solution 5 obtained in Example 5 contained an iridium cluster having triphenylphosphine (TPP) as a ligand and having 4 to 5 constituent atoms. The composition of the iridium cluster was presumed to be Ir 4-5 (TPP) 3―6 Cl1.

Claims

1. An iridium cluster having 2 or more and 100 or fewer constituent atoms.

2. The iridium cluster according to claim 1, having a ligand.

3. The iridium cluster according to claim 2, wherein the ligand includes an organic ligand.

4. The iridium cluster according to claim 3, wherein the organic ligand is at least one selected from triphenylphosphine, diphenyl(p-tolyl)phosphine, and phenylethanethiol.

5. The iridium cluster according to claim 2, wherein the ligand includes CO.

6. The iridium cluster according to claim 1, having 4 or more and 15 or fewer constituent atoms.

7. A method for producing an iridium cluster having 2 or more and 100 or fewer constituent atoms, comprising: a reduction step of obtaining an iridium-containing liquid by reducing an iridium compound by a liquid-phase reduction method; and an organic ligand mixing step of obtaining an iridium cluster having the organic ligand by mixing the iridium-containing liquid and an organic ligand.

8. The method for producing an iridium cluster according to claim 7, further comprising a washing step of washing the iridium cluster having the organic ligand obtained in the organic ligand mixing step two or more times with a solvent.

9. The method for producing an iridium cluster according to claim 7, wherein the liquid-phase reduction method is a polyol reduction method of heating the iridium compound in the presence of a polyol.

10. An oxygen evolution electrode for electrolysis of water, comprising: an electrode substrate; and an iridium cluster provided on the electrode substrate, wherein the iridium cluster has 2 or more and 100 or fewer constituent atoms.

11. The oxygen evolution electrode according to claim 10, wherein the iridium cluster is supported on a porous body.

12. A water electrolysis device comprising the oxygen evolution electrode according to claim 10 or 11 and a hydrogen evolution electrode.

13. A method for electrolysis of water, comprising a step of electrolyzing water using the oxygen evolution electrode according to claim 10 or 11.

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