Iridium cluster, method for producing iridium cluster, oxygen-generating electrode, water electrolysis device, and method for electrolyzing water

Abstract

Provided are: an iridium cluster composed of a small number of constituent atoms and exhibiting excellent activity as a catalyst for an oxygen generation reaction; a method for producing the iridium cluster; an oxygen-generating electrode using the iridium cluster; a water electrolysis device using the oxygen-generating electrode; and a method for electrolyzing water using the oxygen-generating electrode.　This iridium cluster includes 2 to 100 constituent atoms. The iridium cluster preferably has a ligand. The ligand is preferably an organic ligand such as triphenylphosphine, diphenyl (p-tolyl)phosphine or phenylethanethiol, or a ligand containing CO.

Description

Iridium cluster, method for producing iridium cluster, oxygen generating electrode, water electrolysis device and method for electrolyzing water

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

[0002] As a countermeasure to environmental and energy resource issues, technology for producing hydrogen by electrolysis of water has been researched. Water electrolysis occurs through the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). To efficiently produce hydrogen through water electrolysis, it is necessary to increase the efficiency of the oxygen evolution reaction.

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

[0004] Francesco Bizzotto, Jonathan Quinson, Alessandro Zana, Jacob JK Kirkensgaard, Alexandra Dworzak, Mehtap Oezaslan and Matthias Arenz “Ir nanoparticles with ultrahigh dispersion as oxygen evolution reaction (OER)catalysts: synthesis and activity benchmarking“, Catal. Sci. Technol., 2019, 9, 6345-6356

[0005] To make the oxygen generation reaction more efficient, it is desirable to improve the activity of the oxygen generation reaction catalyst. By reducing the size of iridium particles, for example by reducing the number of atoms constituting the iridium cluster, the specific surface area can be increased, and the activity of the oxygen generation 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 catalytic activity in the oxygen generation reaction, a method for producing the iridium cluster, an oxygen generation electrode using the iridium cluster, and a water electrolysis device and a method for water electrolysis using the oxygen generation electrode.

[0007] The present inventors have discovered that an iridium cluster having 2 or more and 100 or less constituent atoms can be produced by a method for producing an iridium cluster, the method including a reduction step of obtaining an iridium-containing solution 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 solution with an organic ligand, and have found that this can solve the above-mentioned problems, leading to the completion of the present invention. More specifically, the present invention is as follows.

[0008] (1) An iridium cluster having 2 or more and 100 or less constituent atoms.

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

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

[0011] (4) The iridium cluster according to (3), wherein the organic ligand is at least one selected from the group consisting of 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), which has 4 to 15 constituent atoms.

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

[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 generating electrode for water electrolysis, comprising: an electrode substrate; and an iridium cluster provided on the electrode substrate, wherein the iridium cluster has a number of constituent atoms of 2 or more and 100 or less.

[0018] (11) The oxygen generating electrode according to (10), wherein the iridium clusters are supported on a porous body.

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

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

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

[0022] 1 is a transmission electron microscope photograph of the iridium cluster obtained in Example 1. FIG. 2 is an FT-IR spectrum of the iridium cluster obtained in Example 1. FIG. 3 is an ESI-MS spectrum of the iridium cluster obtained in Example 1. FIG. 4 is a diagram showing the linear sweep voltammogram and OER mass activity of Example 1. FIG. 5 is a linear sweep voltammogram of Example 1. FIG. 6 is a linear sweep voltammogram of Example 1 and Comparative Example 2. FIG. 7 is a diagram showing the OER mass activity of Example 1 and Comparative Example 2. FIG. 8 is a MALDI-MS spectrum showing the results of an investigation into the cleaning step. FIG. 9 is a MALDI-MS spectrum showing the results of an investigation into the heating time in the reduction step. FIG. 10 is a transmission electron microscope photograph of the iridium cluster obtained in Example 2. FIG. 11 is an ESI-MS spectrum of the iridium cluster obtained in Example 2. FIG. 12 is a linear sweep voltammogram of Example 2. FIG. 13 is a diagram showing the OER mass activity of Example 2 and Comparative Example 2. FIG. 14 is a MALDI-MS spectrum showing the results of an investigation into the cleaning step. 1 is a MALDI-MS spectrum of the iridium cluster obtained in Example 3. FIG. 2 is an ESI-MS spectrum of the iridium cluster obtained in Example 3. FIG. 3 is a transmission electron microscope photograph of the iridium cluster obtained in Example 3. FIG. 4 is a linear sweep voltammogram of Example 3. FIG. 5 is a diagram showing OER mass activities of Example 3 and Comparative Example 2. FIG. 6 is an ESI-MS spectrum of the iridium cluster obtained in Example 4. FIG. 7 is a transmission electron microscope photograph of the iridium cluster obtained in Example 4. FIG. 8 is a linear sweep voltammogram of Example 4. FIG. 9 is a diagram showing OER mass activities of Example 4 and Comparative Example 2. FIG. 10 is a MALDI-MS spectrum of the iridium cluster obtained in Example 5.

[0023] <Iridium Cluster> An iridium cluster has a number of constituent atoms of 2 or more and 100 or less. That is, an iridium cluster is a metal cluster in which 2 or more and 100 or less iridium atoms are bonded. Conventionally, it has not been possible to produce an iridium cluster having such a small number of constituent atoms of 2 or more and 100 or less. For example, the number of constituent atoms of iridium nanoparticles with a diameter of approximately 1.6 nm in Non-Patent Document 1 is theoretically calculated to be about 150 based on the covalent bond radius. However, the method for producing an iridium cluster described below makes it possible to produce an iridium cluster having a small number of constituent atoms of 2 or more and 100 or less.

[0024] Iridium clusters have a small number of constituent atoms, between 2 and 100. Therefore, the iridium clusters have a larger specific surface area and superior catalytic activity for the oxygen evolution reaction (OER) than conventional iridium clusters. In other words, iridium clusters have high OER activity. Therefore, by using iridium clusters as catalysts for the oxygen evolution reaction, the efficiency of the oxygen generation reaction can be increased, and for example, the efficiency of water electrolysis can be increased. Furthermore, by using iridium clusters as catalysts for the oxygen evolution reaction, the efficiency of the oxygen evolution reaction can also be increased in various devices that use the oxygen evolution reaction, such as secondary batteries. Furthermore, because iridium clusters have high OER activity, the amount of OER catalyst used can be reduced in water electrolysis and other devices.

[0025] The number of constituent atoms of the iridium cluster may be from 2 to 100, preferably from 2 to 50, more preferably from 4 to 15, and may be from 4 to 13. 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 or more types of ligand. Examples of the ligand include organic ligands and CO. Examples of the organic ligand include triphenylphosphine, diphenyl(p-tolyl)phosphine, and phenylethanethiol. The iridium cluster may also have a halogen atom such as a chlorine atom.

[0027] <<Method for Producing Iridium Clusters>> The above-mentioned iridium clusters having 2 or more and 100 or less constituent atoms can be produced by a method for producing iridium clusters, including, for example, a reduction step of reducing an iridium compound by a liquid-phase reduction method to obtain an iridium-containing solution, and an organic ligand mixing step of mixing the iridium-containing solution with an organic ligand to obtain an iridium cluster having an organic ligand. The method for producing iridium clusters may further include a washing step of washing the iridium clusters having an organic ligand obtained in the organic ligand mixing step with a solvent two or more times. Each step will be described below.

[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] The iridium compound may be an iridium halide such as iridium chloride, a specific example of which is iridium(III) chloride. The iridium compound may also be a hydrate.

[0030] An example of a liquid phase reduction method is a polyol reduction method in which an iridium compound is heated in the presence of a polyol. Examples of polyols include polyols used as reducing agents, 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. The polyol reduction method is preferably carried out in the presence of a hydroxide such as sodium hydroxide.

[0031] The heating temperature in 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 in the polyol reduction method is preferably 3 minutes or higher and 20 minutes or lower, more preferably 5 minutes or higher and 15 minutes or lower. Stirring is preferably performed during heating in the polyol reduction method. In the polyol reduction method, an iridium compound is heated in the presence of a polyol to reduce the iridium compound, thereby obtaining an iridium-containing solution. After that, if necessary, the iridium-containing solution may be cooled before the organic ligand mixing step.

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

[0033] An example of a liquid phase reduction method is a method in which an iridium compound is reduced 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 higher and 24 hours or lower, more preferably 5 minutes or higher and 1 hour or lower. Stirring is preferred during the reaction. In reduction with sodium borohydride, after an iridium-containing solution is obtained by reducing an iridium compound in the presence of sodium borohydride, the iridium-containing solution may be cooled, if necessary, before the organic ligand mixing step.

[0034] [Organic Ligand Mixing Step] In the organic ligand mixing step, the iridium-containing liquid obtained in the reduction step is mixed with an organic ligand to obtain an iridium cluster having an organic ligand. By mixing the iridium-containing liquid obtained in the reduction step with the organic ligand, the organic ligand coordinates with the iridium. In other words, the iridium is stabilized by the organic ligand. For this reason, it is presumed that the increase in size of the iridium cluster is suppressed, and an iridium cluster having a small number of constituent atoms, 2 to 100, can be produced. Furthermore, it is presumed that the increase in size of the iridium cluster is further suppressed by adding the organic ligand and stirring immediately after the reduction step.

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

[0036] The method for mixing the organic ligand with the iridium-containing liquid 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 liquid, or the organic ligand may be added directly to the iridium-containing liquid. Examples of solvents for dissolving the organic ligand include acetone and toluene.

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

[0038] [Washing Step] In the washing step, the iridium cluster having an 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 in which the iridium cluster having an organic ligand obtained in the organic ligand mixing step is washed with a first solvent, and a second washing step in which the first washing step is followed by a second washing step.

[0039] In the first washing step, the iridium clusters having organic ligands obtained in the organic ligand mixing step are washed with a first solvent to remove the excess polyol used in the polyol reduction method, unreacted iridium compound, and the excess organic ligands 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 ligands obtained in the first washing step is washed with a second solvent, such as a mixed solvent of methanol and chloroform, to obtain an iridium cluster having the desired number of constituent atoms.

[0041] By using this production method, it is possible to produce the above-mentioned iridium clusters having 2 or more and 100 or less constituent atoms.

[0042] <Oxygen Evolving Electrode> The above-mentioned iridium cluster having 2 or more and 100 or less constituent atoms can be used as a catalyst for the oxygen generating reaction in an oxygen generating electrode for water electrolysis. Such an oxygen generating 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 constituent atoms. The iridium cluster having 2 or more and 100 or less constituent atoms has a large specific surface area and excellent activity as a catalyst for the oxygen generating reaction (OER). Therefore, by using an oxygen generating electrode that uses the iridium cluster as a catalyst for the oxygen generating reaction, the efficiency of water electrolysis can be increased.

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

[0044] Examples of metal substrates include substrates of simple metals such as nickel, titanium, iron, and copper, and substrates of alloys. Examples of carbon substrates include glassy carbon (GC), carbon paper, carbon fiber paper, and carbon rods. Examples of glass substrates include conductive glass. The electrode substrate may be a porous material.

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

[0046] In the oxygen generating electrode, when the iridium clusters are supported on a porous body, the iridium clusters may be supported on a porous body and the porous body may be provided on the surface of an electrode substrate, or the iridium clusters may be supported on an electrode substrate made of a porous body. Examples of porous bodies supporting the iridium clusters include carbon black and metal oxides.

[0047] The method for producing the oxygen generating electrode is not particularly limited. For example, a catalyst (OER catalyst) is produced by impregnating a porous body such as carbon black with a solution of the iridium cluster and calcining the resulting material, and then a liquid containing the catalyst (e.g., catalyst slurry) is applied to an electrode substrate to produce the oxygen generating electrode. When the iridium cluster used has a ligand, it is preferable that at least a portion of the ligand is removed by calcination. When producing the oxygen generating electrode, the electrochemical properties of the oxygen generating electrode can be adjusted by adjusting the calcination temperature. The calcination 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 calcination time is, for example, 1 hour or higher and 3 hours or lower.

[0048] <Water Electrolysis Device> The above-described oxygen generating electrode can be used as the oxygen generating electrode for a water electrolysis device. Such a water electrolysis device includes the above-described oxygen generating electrode and a hydrogen generating electrode. The water electrolysis device uses an iridium cluster having 2 to 100 constituent atoms as a catalyst for the oxygen generating reaction, and therefore can perform water electrolysis with high efficiency.

[0049] The oxygen generating electrode included in the water electrolysis apparatus is as described above. As the hydrogen generating electrode included in the water electrolysis apparatus, any known hydrogen generating electrode can be used, for example, a platinum electrode, a carbon electrode, or the like.

[0050] The water electrolysis device has, for example, an electrolytic cell containing an electrolyte solution, and an oxygen generating electrode and a hydrogen generating electrode are immersed in the electrolyte solution. The electrolyte solution is an aqueous solution in which an electrolyte is dissolved in water. Such a water electrolysis device can highly efficiently decompose water to produce hydrogen and oxygen using light such as sunlight. Furthermore, the water electrolysis device equipped with the above-mentioned oxygen generating electrode may be a water electrolysis device that performs polymer electrolyte membrane (PEM)-type water electrolysis without using an electrolyte solution.

[0051] <Water electrolysis method> The above-described oxygen generating electrode can be used for water electrolysis. Such a water electrolysis method includes a step of electrolyzing water using the above-described oxygen generating electrode. The water electrolysis method uses an iridium cluster having 2 or more and 100 or less constituent atoms as a catalyst for the oxygen generating reaction, and therefore can perform water electrolysis with high efficiency.

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

[0053] Example 1 (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 prepare a reaction solution. This solution was heated and stirred (1000 rpm) at 120°C for 7 minutes using a chemical station (EYELA, model number: PPS-CTRL1), and then ice-cooled to room temperature. A solution of triphenylphosphine (524.5 mg) dissolved in acetone (10 ml) was then quickly added to the reaction solution, followed by stirring 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 extracted solution was evaporated until viscous.

[0054] After the evaporation, the iridium clusters were washed with 30 ml of ultrapure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10) in this order, centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the obtained iridium clusters (crude), and the mixture was centrifuged (3300 rpm, 2 minutes) to extract the supernatant.

[0055] <Second washing step> This extracted solution was evaporated and dissolved in 1 ml of chloroform, then methanol (19 ml) was added, centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded. This operation was repeated three times, and then about 5 ml of toluene was added, centrifuged (3300 rpm, 2 minutes), and the supernatant was extracted. This extracted solution was evaporated and dissolved in toluene. This solution was designated iridium cluster solution 1.

[0056] (Identification) The iridium clusters obtained from iridium cluster solution 1 were identified using transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), electrospray ionization mass spectrometry (ESI-MS), and matrix-assisted laser desorption / ionization mass spectrometry (MALDI-MS). FIG. 1 shows a transmission electron microscope photograph (FIG. 1(a)) and particle size (FIG. 1(b)), FIG. 2 shows the FT-IR spectrum, and FIG. 3 shows the ESI-MS spectrum. As a result, iridium cluster solution 1 obtained in Example 1 contained an iridium cluster having triphenylphosphine (TPP) and CO as ligands and having 13 constituent atoms, and an iridium cluster having triphenylphosphine and CO as ligands and having 15 constituent atoms. Iridium cluster solution 1 also contained a small amount of iridium cluster having triphenylphosphine and CO as ligands and having 18 constituent atoms. The composition of the iridium cluster is Ir １３ (CO) １５ (TPP) ８ , Ir １５ (CO) １９ (TPP) ８ , Ir １８ (CO) １９ (TPP) ９ The iridium clusters had a particle size of 0.8±0.1 nm.

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

[0058] <Slurry preparation and application> The prepared Ir-supported catalyst was added to a mixed solution consisting of ultrapure water (19.1 mL), 2-propanol (6 mL), and polymer electrolyte (Nafion (registered trademark) solution, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (100 μL). The resulting mixed solution was ultrasonically treated 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 onto a glassy carbon (GC) electrode so that it was evenly distributed, and then dried at 750 rpm for 40 minutes.

[0059] <Measurement> A three-electrode system was constructed using a GC coated with the prepared catalyst slurry as the working electrode, a platinum (Pt) coil counter electrode, and a silver-silver chloride (Ag / AgCl) reference electrode. ４ Electrochemical measurements were carried out in aqueous solutions using a potentio / galvanostat. ２ After bubbling the gas for 30 minutes, cyclic voltammetry (CV) was performed at 200 mVs in the range of 0 to 1.00 V (vs. reversible hydrogen electrode; RHE). －１ The electrode was cleaned by scanning 100 times at a scanning speed of 100 times. ２ Linear sweep voltammetry (LSV) at 20 mVs －１ The measurements were carried out in the range of 1.00 to 1.85 V (vs. RHE) at 1.6 V. The results are shown in Figure 4. Figure 4(a) shows a linear sweep voltammogram, and Figure 4(b) shows the oxygen evolution reaction activity (OER mass activity) per Ir mass calculated from the current value at a voltage of 1.6 V vs. RHE on the linear sweep voltammogram. Note that instead of iridium cluster solution 1, iridium (III) chloride hydrate was supported on carbon black by impregnation and the reaction mixture was heated under reduced pressure (<5.0 × 10 ３ Pa), baked at 250°C for 2 hours (7°C min －１ 4 also shows the results of electrochemical measurements performed in the same manner as in Example 1, except that an Ir-supported catalyst (Comparative Example 1) was used, in which Ir nanoparticles with a particle size of 2.1 nm were supported on carbon black by performing the above-mentioned procedure. As shown in FIG. 4, Example 1 had 2.63 times higher OER activity than Comparative Example 1, confirming the improvement in OER activity.

[0060] (Electrochemical Measurement 2) For iridium cluster solution 1 (Example 1), the same operation as in Electrochemical Measurement 1 was performed, except that in <Catalyst Preparation> the calcination temperature was set to 200°C, 250°C, 300°C, or 350°C, or no calcination was performed (No cal). The results are shown in Figures 5 to 7. Figure 5 shows a linear sweep voltammogram. Figure 6 shows a linear sweep voltammogram when the calcination temperature in <Catalyst Preparation> was 250°C, and Figure 7 shows the results of calculating 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 was 250°C. 6 and 7 show the results of an OER activity evaluation performed in the same manner as in Example 1, except that an Ir-supported catalyst (Comparative Example 2) was used, in which a toluene solution of a commercially available catalyst (amorphous iridium oxide catalyst, manufactured by Tanaka Kikinzoku Kogyo Kogyo, product number: TEC77100) was supported on carbon black by impregnation instead of iridium cluster solution 1. As shown in FIG. 5, it was confirmed that the electrochemical characteristics could be adjusted by adjusting the calcination temperature in <Catalyst Preparation>, and that the OER mass activity was particularly high when the calcination temperature was 250°C or higher and 350°C or lower. Furthermore, as shown in FIGS. 6 and 7, it was confirmed that Example 1 could achieve an OER activity 1.5 times higher than that of Comparative Example 2.

[0061] (Investigation of the cleaning step) The extracted solution obtained in the first cleaning step was analyzed using MALDI-MS to confirm the iridium clusters obtained. The results are shown in Figure 8. The results for iridium cluster solution 1 (i.e., the results after the second cleaning step) are also shown in Figure 8. As shown in Figure 8, by performing the first and second cleaning steps, it is possible to obtain iridium clusters with a narrower distribution of the number of constituent atoms and a more uniform number of constituent atoms than when the second cleaning step is not performed.

[0062] (Investigation of Heating Time in the Reduction Step) An iridium cluster solution was obtained in the same manner as in the above-described iridium cluster solution 1 (Example 1), except that in the reduction step, heating at 120°C for 7 minutes was changed to 120°C for 3 minutes, 10 minutes, or 15 minutes. The iridium clusters obtained from the obtained extracted solution were confirmed using MALDI-MS. The results are shown in Figure 9. The results of heating at 120°C for 7 minutes are also shown in Figure 9. Figure 9(a) shows the results of heating at 120°C for 3 minutes, Figure 9(b) shows the results of heating at 120°C for 7 minutes, Figure 9(c) shows the results of heating at 120°C for 10 minutes, and Figure 9(d) shows the results of heating at 120°C for 15 minutes. As shown in Figure 9, it can be seen that the distribution of the number of constituent atoms of the obtained iridium clusters can be adjusted by adjusting the heating time in the reduction step.

[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 prepare a reaction solution. This solution was heated and stirred (1000 rpm) at 120°C for 3 minutes using a chemical station (EYELA, model number: PPS), and then ice-cooled to room temperature. A solution of triphenylphosphine (524.5 mg) dissolved in 10 ml of acetone was then quickly added to the reaction solution, followed by stirring 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 extracted solution was evaporated until viscous.

[0064] After the evaporation, the solid was washed with 30 ml of ultrapure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10) in this order, centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the resulting solid, and the mixture was centrifuged (3300 rpm, 2 minutes) to extract the supernatant.

[0065] <Second washing step> This extract solution was evaporated and dissolved in 1 ml of chloroform, then methanol (19 ml) was added, and centrifuged (3300 rpm, 2 minutes) to extract the supernatant. This extract solution was evaporated, then washed with 5 ml of methanol, and then centrifuged (3300 rpm, 2 minutes). The supernatant was discarded, and the purified product was dissolved in toluene. This solution was designated iridium cluster solution 2.

[0066] (Identification) The iridium clusters obtained from iridium cluster solution 2 were identified using TEM, FT-IR, ESI-MS, and MALDI-MS. Figure 10 shows a transmission electron microscope photograph (Figure 10(a)) and particle size (Figure 10(b)), and Figure 11 shows the ESI-MS spectrum. As a result, it was found that iridium cluster solution 2 obtained in Example 2 contained iridium clusters having triphenylphosphine (TPP) and CO as ligands and having 6 to 8 constituent atoms. The composition of the iridium clusters was Ir ６－８ (CO) ３－７ (TPP) ８ Cl １ It is estimated that the iridium clusters had a particle size of 0.8±0.1 nm.

[0067] (Electrochemical Measurement 3) The same operation as in Electrochemical Measurement 1 was performed, except that iridium cluster solution 2 (Example 2) was used instead of iridium cluster solution 1 (Example 1), and the calcination temperature in <Catalyst Preparation> was changed to 250°C, 300°C, or 350°C. The results are shown in Figures 12 and 13. Figure 12 shows a linear sweep voltammogram, and Figure 13 shows the result of determining 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 in <Catalyst Preparation> was 300°C. FIG. 13 also shows the results of an 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 Kikinzoku Kogyo Kogyo, product number: TEC77100) was used instead of iridium cluster solution 1 and no calcination (No cal) was performed in <Catalyst Preparation>. As shown in FIG. 12, it was confirmed that the electrochemical properties could be adjusted by adjusting the calcination temperature in <Catalyst Preparation>, and that the OER mass activity was particularly high when the calcination temperature was 250°C or higher and 350°C or lower. Furthermore, as shown in FIG. 13, it was confirmed that Example 2 had an OER activity 1.5 times higher than that of Comparative Example 2.

[0068] (Investigation of the cleaning step) The extracted solution obtained in the first cleaning step was analyzed using MALDI-MS to confirm the iridium clusters obtained. The results are shown in Figure 14. The results for iridium cluster solution 2 (i.e., the results after the second cleaning step) are also shown in Figure 14. As shown in Figure 14, by performing the first and second cleaning steps, it is possible to obtain iridium clusters with a narrower distribution of the number of constituent atoms and a more uniform number of constituent atoms than when the second cleaning 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 prepare a reaction solution. This solution was heated and stirred (1000 rpm) at 120°C for 7 minutes using a chemical station (EYELA, model number: PPS-CTRL1), and then ice-cooled to room temperature. Diphenyl(p-tolyl)phosphine (552.6 mg) dissolved in 5 ml of toluene was then quickly added to the reaction solution, followed by stirring 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 extracted solution was evaporated until viscous.

[0070] After the evaporation, the Ir clusters were washed with 30 ml of ultrapure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10) in this order, centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the obtained Ir clusters (crude), and the mixture was centrifuged (3300 rpm, 2 minutes) to extract the supernatant.

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

[0072] (Identification) The iridium clusters obtained from iridium cluster solution 3 were identified 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 a transmission electron microscope photograph (Figure 17(a)) and particle size (Figure 17(b)). As a result, it was found that 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 was Ir １３－１８ (CO) １４－２０ (DPTP) ７－９ It is estimated that the iridium clusters had a particle size of 0.8±0.1 nm.

[0073] (Electrochemical Measurement 4) The same operation as in Electrochemical Measurement 1 was performed, except that iridium cluster solution 3 (Example 3) was used instead of iridium cluster solution 1 (Example 1), and the calcination temperature in <Catalyst Preparation> was set to 300°C. The results are shown in Figures 18 and 19. Figure 18 shows a linear sweep voltammogram, and Figure 19 shows the results of determining the oxygen evolution reaction activity (OER mass activity) per Ir mass from the current value at a voltage of 1.6 V vs. RHE on the linear sweep voltammogram. 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 Kikinzoku Kogyo Kogyo, product number: TEC77100) was used instead of iridium cluster solution 1, are also shown in Figures 18 and 19.

[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 prepare a reaction solution. This solution was stirred (1000 rpm) while heating at 120°C for 30 minutes using a chemical station (EYELA, model number: PPS-CTRL1), and then cooled to room temperature on ice. Phenylethanethiol (268 μl) dissolved in 5 ml of toluene was then quickly added to the reaction solution, followed by stirring 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 extracted solution was evaporated until viscous.

[0075] <First washing step> After the evaporation, the mixture was washed with a total of 30 ml of ultrapure water / methanol mixed solvent (10:0, 8:2, 6:4, 4:6, 2:8, 0:10, 0:10, 0:10) in that order, centrifuged (3300 rpm, 2 minutes), and the supernatant was discarded for purification. Approximately 5 ml of toluene was added to the obtained Ir cluster (crude), and the mixture was centrifuged (3300 rpm, 2 minutes) to extract the supernatant. This solution was designated as iridium cluster solution 4.

[0076] (Identification) The iridium clusters obtained from iridium cluster solution 4 were identified using TEM, FT-IR, ESI-MS, and MALDI-MS. Figure 20 shows the ESI-MS spectrum, and Figure 21 shows a transmission electron microscope photograph (Figure 21(a)) and particle size (Figure 21(b)). As a result, it was found that iridium cluster solution 4 obtained in Example 4 contained iridium clusters having phenylethanethiol (PET) and CO as ligands and having 9 to 13 constituent atoms. The composition of the iridium clusters was Ir ９－１３ (CO) ８－１１ (PET) ８－１２ It is estimated that the iridium clusters had a particle size of 0.8±0.1 nm.

[0077] (Electrochemical Measurement 5) The same operation as in Electrochemical Measurement 1 was performed using iridium cluster solution 4 (Example 4) instead of iridium cluster solution 1 (Example 1). The results are shown in Figures 22 and 23. Figure 22 shows a linear sweep voltammogram, and Figure 23 shows the results of determining the oxygen evolution reaction activity (OER mass activity) per Ir mass from the current value at a voltage of 1.6 V vs. RHE on the linear sweep voltammogram. The results of OER activity evaluation performed in the same manner as in Example 1 are also shown in Figures 22 and 23, except that a toluene solution (Comparative Example 2) of a commercially available catalyst (amorphous iridium oxide catalyst, manufactured by Tanaka Kikinzoku Kogyo Kogyo, product number: TEC77100) was used instead of iridium cluster solution 1.

[0078] Example 5 Reduction Step and Organic Ligand Mixing Step Iridium(III) chloride hydrate (25 mg) was added to methanol (10 ml) and stirred at 60°C for 3 hours to dissolve. To this solution, a solution of tetraoctylammonium bromide (48.6 mg) and triphenylphosphine (105 mg) dissolved in toluene (5 ml) was added. This solution was cooled to room temperature in an ice bath, and then a solution of sodium borohydride (64 mg) dissolved in methanol (5 ml) was added. Stirring was continued 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, followed by centrifugation (3300 rpm, 2 minutes), and the upper layer was extracted. This extracted solution was evaporated until viscous.

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

[0080] (Identification) The iridium clusters obtained from iridium cluster solution 5 were identified using MALDI-MS. Figure 24 shows the MALDI-MS spectrum. As a result, it was found that iridium cluster solution 5 obtained in Example 5 contained iridium clusters having triphenylphosphine (TPP) as a ligand and having 4 to 5 constituent atoms. The composition of the iridium cluster was Ir ４－５ (TPP) ３―６ Cl １ It is speculated that...

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, the method comprising: a reduction step of obtaining an iridium-containing solution 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 solution and the 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 generation 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 generation electrode according to claim 10, wherein the iridium cluster is supported on a porous body.

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

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

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