Method for preparing composite provided with catalyst material on support including carbon nanotube molded body

Abstract

[Problem] To provide a composite that can be used in an electrode for electrochemical water decomposition and is configured such that the carried amount of a catalyst material such as platinum or iridium oxide is reduced as compared with the prior art. [Solution] Provided is a method for preparing a composite in which a catalyst material is formed on a support including a carbon nanotube molded body, the method comprising: (1) a step for bringing a support including a carbon nanotube molded body into contact with a solution containing a catalyst precursor for the catalyst material; (2) a step for drying the support brought into contact with the solution to obtain a support carrying the catalyst precursor; and (3) a step for reducing the catalyst precursor to convert the catalyst precursor to a metal, the method optionally including a step, following step (3), for oxidizing the metal.

Description

Method for preparing a composite comprising a catalyst material on a support including a carbon nanotube molded body

[0001] The present invention relates to a method for preparing a composite in which a catalyst material is formed on a support including a carbon nanotube molded body, and to a composite obtained by said method. The present invention also relates to an electrode for electrochemical water splitting comprising a composite obtained by the method of the present invention, and to an electrochemical device provided with said electrode.

[0002] As a method for producing hydrogen, electrolysis of water (hereinafter also referred to as "electrochemical water splitting" or "water electrolysis") is considered a promising method. In electrode catalysts for hydrogen production by electrochemical water splitting, platinum (Pt) and iridium oxide (IrO ｘ ) are used as cathode catalysts and anode catalysts, respectively. Since both platinum and iridium are rare resources, how to reduce the loading amount of the catalyst on electrodes has become an important research subject.

[0003] Many methods and devices for reducing the loading amount of platinum and iridium oxide have been devised so far, but in the hydrogen production process by electrochemical water splitting, a current density of 1 A / cm ２ or higher is required. Therefore, in order to maintain the activity and durability of the catalyst, the loading amounts of platinum and iridium oxide are respectively 500 µg / cm ２ and 1000 µg / cm ２ are the minimum required.

[0004] An object of the present invention is to provide a composite that can be used for an electrode for electrochemical water splitting, in which the loading amount of a catalyst material such as platinum or iridium oxide is reduced compared to conventional techniques.

[0005] In order to solve the above problem, the present inventors have conducted intensive studies and found that by using a nanotextile-type nanostructure composed of carbon nanotubes (CNTs) as a template (support) and forming a catalyst material on this support by a wet method, it is possible to provide a composite with a reduced loading amount of the catalyst material, and thus completed the present invention.

[0006] That is, the present invention has the following configurations.

[0007] [1] A method for preparing a composite in which a catalyst material is formed on a support containing a carbon nanotube molded body, comprising: (1) a step of contacting a support containing a carbon nanotube molded body with a solution containing a catalyst precursor for the catalyst material; (2) a step of drying the support that has been in contact with the solution to obtain a support carrying the catalyst precursor; and (3) a step of converting the catalyst precursor into a metal by reduction of the catalyst precursor, wherein the step of oxidizing the metal may be included following step (3). [2] The method for preparing according to [1], wherein the carbon nanotube molded body is a carbon nanotube sheet. [3] The method for preparing according to [1] or [2], wherein the film thickness of the carbon nanotube molded body is 30 μm or less. [4] The method for preparing according to claim 1, wherein the catalyst precursor is a metal salt or a metal acid. [5] The method for preparing according to [4], wherein the metal salt or metal acid comprises one or more selected from the group consisting of a metal salt or metal acid that yields iridium(III) ions, platinum(IV) ions, or platinum ions. [6] The preparation method according to [5], wherein the metal salt or metal acid that yields iridium(III) ions comprises one or more selected from the group consisting of iridium chloride, iridium nitrate, and iridium acetate. [7] The preparation method according to [5], wherein the metal salt or metal acid that yields platinum(IV) ions or platinum ions comprises one or more selected from the group consisting of chloroplatinic acid, hexachloroplatinate, potassium tetrachloroplatinate, dinitrodiammineplatin, dinitrodiammineplatinate, cis-diamminediaquaplatinate, trans-diamminediaquaplatinate, tetranitroplatinic acid, tetra(oxalato)platinic acid, cis-dinitrodiaquaplatin, tetraammineplatinate hydroxylate, hexaammineplatinate hydroxylate, tetraammineplatinate chloride, hexa-hydroxyplatinic acid, platinum oxide, platinum I chloride, and platinum I chloride. [8] The preparation method according to [5], wherein the metal salt or metal acid comprises iridium chloride or chloroplatinic acid. [9] The preparation method according to any one of [1] to [8], wherein the carbon nanotube molded body is a carbon nanotube molded body prepared by a method comprising the following steps.A composite obtained by the method described in any one of [1] to [9].

[11] The composite according to

[10] , wherein the intensity ratio (G / D) of the G band to the D band observed in the Raman spectrum is 30 or more.

[12] The composite according to

[10] , wherein the RBM peak in the Raman spectrum is reduced to 50% or less compared to a support containing a carbon nanotube molded body in which the catalyst material is not formed.

[13] The composite according to any one of

[10] to

[12] , which is used in an electrochemical water splitting device.

[14] An electrode for electrochemical water splitting comprising the composite described in any one of

[10] to

[12] .

[15] The electrode according to

[14] , wherein the catalyst material is platinum or iridium oxide.

[16] An electrochemical water splitting device comprising the electrode according to

[14] .

[0008] The present invention makes it possible to provide a composite in which the amount of catalyst material such as platinum or iridium oxide supported is reduced compared to the conventional technology. Such a composite can be suitably used in electrodes for electrochemical water splitting.

[0009] This document shows the procedure for preparing the CCM (Catalyst coated membrane) used in the examples, and a schematic diagram of the resulting CCM. It also shows a schematic diagram of the electrochemical water splitting apparatus used in the examples, a photograph of the electrochemical water splitting apparatus prepared in the examples, and a comparison of the oxygen evolution reaction performance of various catalysts by three-electrode electrochemical analysis. A: IrO ２ SEM image of single-layer carbon nanotubes (SWCNTs) using [a specific method], B: IrO ２Backscatter SEM images of SWCNTs using [the specified method], C and D: SEM images of commercially available iridium oxide. The results of the electrochemical analysis (analysis under acidic conditions) of Example 1 (2) of the PEM (proton exchange membrane) water electrolysis cell are shown (performance comparison of commercially available CCM and the CCM of the present invention). The results of stability confirmation by I-T analysis are shown. A: 1st time, B: 2nd time, C: Stability confirmation results by I-V analysis before and after I-T measurement. The results of comparing the performance of water electrolysis with different catalyst loads are shown (comparison of PEM water electrolysis performance with various catalyst loads). The test results of PEM water electrolysis performance with various catalyst packing and conditions are shown. The intensity ratio of the G band to the D band (G / D) observed in the Raman spectra of each sample measured in Example 4 are shown. The RBM peak intensity observed in the Raman spectra of each sample measured in Example 4 is shown.

[0010] 1. Method for Preparing a Composite One embodiment of the present invention is a method for preparing a composite in which a catalyst material is formed on a support containing a carbon nanotube molded body, comprising: (1) a step of contacting a support containing a carbon nanotube molded body with a solution containing a catalyst precursor for the catalyst material; (2) a step of drying the support that has been in contact with the solution to obtain a support carrying the catalyst precursor; and (3) a step of converting the catalyst precursor into a metal by reduction, wherein the step of oxidizing the metal may be included following step (3) (hereinafter also referred to as "the preparation method of the present invention").

[0011] Each component of the method for preparing the composite of the present invention will be described in detail below.

[0012] (a) Support containing a carbon nanotube molded body In the preparation method of the present invention, it is important to use a support containing a carbon nanotube (CNT) molded body as a template (support) for forming the catalyst material. Carbon nanotubes can be suitably used as electrode supports (carriers) because they have excellent properties such as high conductivity, excellent mechanical strength and thermal conductivity, and a large specific surface area.

[0013] (a-1) Properties of carbon nanotube molded bodies As the carbon nanotube molded body, it is preferable to use a textile-type nanostructure composed of carbon nanotubes, and it is particularly preferable to use a textile-type nanostructure composed of single-walled carbon nanotubes (SWCNTs).

[0014] The carbon nanotube molded body preferably has 700 m ２ / g or more, more preferably 800 m ２ / g or more, still more preferably 1000 m ２ / g or more specific surface area. If the specific surface area of the carbon nanotube molded body is less than 700 m ２ / g, the molded body contains a large amount of large-sized bundles (aggregates of carbon nanotubes), which may cause problems such as inhibiting the loading of atomic cluster-level catalysts. The higher the specific surface area of the carbon nanotube molded body above 700 m ２ / g, the better. However, to increase the specific surface area, carbon nanotubes must be subjected to oxidation treatment, which also causes damage to carbon nanotubes resulting from the oxidation treatment. For the reason of maintaining the excellent electron conductivity inherent to carbon nanotubes, the specific surface area of the carbon nanotube molded body is preferably 1500 m ２ / g or less, more preferably 1300 m ２ / g or less.

[0015] The carbon nanotube molded body preferably has a pore distribution in the range of 3 to 15 nm. If the pore distribution of the carbon nanotube molded body is less than 3 nm, the total number of pores is small, so the carbon nanotube sheet exhibits the properties of a flat sheet, resulting in no pores or low porosity, low or no water penetration, and a high amount of polymer surfactant, which leads to relatively low electrical conductivity. On the other hand, if the pore distribution of the carbon nanotube molded body exceeds 15 nm, it results in a low active surface area. In terms of improving water penetration and active surface area, the pore distribution of the carbon nanotube molded body is preferably in the range of 7 to 15 nm, and more preferably in the range of 9 to 14 nm.

[0016] The carbon nanotube molded body preferably has a tensile strength of 45 MPa or more, more preferably 50 MPa or more, and even more preferably 60 MPa or more. If the tensile strength of the carbon nanotube molded body is less than 45 MPa, it may be prone to bending and breaking, which can make it difficult to handle. While a higher tensile strength is desirable for the carbon nanotube molded body, this would require reducing the carbonization and oxidation treatments, which would leave behind polymer surfactants and reduce surface area and electrical conductivity. Therefore, the tensile strength of the carbon nanotube molded body is preferably 70 MPa or less, and more preferably 65 MPa or less. Furthermore, the carbon nanotube molded body preferably has a Young's modulus of 1600 MPa or more, more preferably 1800 MPa or more, and even more preferably 2000 MPa or more. If the Young's modulus of the carbon nanotube molded body is less than 1600 MPa, it may have poor elasticity, which can lead to damage and deformation when pulled. While a higher Young's modulus is preferable for a carbon nanotube molded body, it would require reducing the carbonization and oxidation treatments. In that case, polymer surfactants would remain, reducing the surface area and electrical conductivity. Therefore, the Young's modulus of the carbon nanotube molded body is preferably 2400 MPa or less, and more preferably 2000 MPa or less.

[0017] The film thickness of the carbon nanotube molded body is preferably 30 μm or less, and more preferably 20 μm or less. When the film thickness of the carbon nanotube molded body is 30 μm or less, the diffusion resistance of substances such as protons can be reduced.

[0018] The shape of the carbon nanotube molded body is not particularly limited, but may be sheet-shaped, arch-shaped, ring-band-shaped, strip-shaped, or thread-shaped, for example. Among these, the carbon nanotube molded body is preferably a carbon nanotube sheet because it can be suitably used as an electrode for electrochemical water splitting, as described later.

[0019] In the present invention, the "support containing a carbon nanotube molded body" includes cases where it consists only of a carbon nanotube molded body (preferably a carbon nanotube sheet), and cases where the carbon nanotube molded body (preferably a carbon nanotube sheet) is supported on the surface of another structure. Examples of such other structures include metal structures, carbon fibers, or carbon fiber structures made of carbon fibers. In the preparation method of the present invention, a composite may be prepared using only a carbon nanotube molded body (preferably a carbon nanotube sheet) as the "support containing a carbon nanotube molded body" (i.e., without being supported on another structure), and the obtained composite may be supported on the surface of another structure to produce an electrode for electrochemical water splitting.

[0020] (a-2) Method for manufacturing a carbon nanotube molded body The carbon nanotube molded body can be manufactured by any conventionally known manufacturing method, as long as it is a carbon nanotube molded body that can be used in the preparation method of the present invention.

[0021] In one preferred aspect of the present invention, the carbon nanotube molded article is a carbon nanotube molded article prepared by a method comprising the following steps: mixing a dispersion containing carbon nanotubes, a dispersant and a dispersion medium with a solution containing cellulose nanofibers to obtain a mixture; supplying the obtained mixture to a mold and drying it in a temperature environment of 25 to 80°C to obtain a solid containing carbon nanotubes; washing the solid to remove the dispersant from the solid; firing the solid to carbonize the cellulose nanofibers; and applying an activation treatment to the surface of the fired solid to obtain a carbon nanotube molded article containing carbon nanotubes.

[0022] The above method for manufacturing a carbon nanotube molded article is also referred to as "Method for manufacturing a carbon nanotube molded article 1".

[0023] In the first method for producing a carbon nanotube molded body, a mixture is first obtained by mixing a dispersion containing carbon nanotubes, a dispersant, and a dispersion medium with a solution containing cellulose nanofibers.

[0024] The carbon nanotubes used may be single-walled or multi-walled, but single-walled carbon nanotubes (SWCNTs) are preferred because they have excellent electrical and thermal conductivity.

[0025] The diameter of carbon nanotubes is preferably in the range of 0.5 to 2 nm, and more preferably in the range of 1 to 1.5 nm, because smaller diameters are recommended due to their physical, electrical, and structural properties. The diameter of carbon nanotubes can be measured using a transmission electron microscope (TEM). The diameters of 10 carbon nanotubes are measured, and the arithmetic mean is taken as the diameter of the carbon nanotube.

[0026] The length of carbon nanotubes is preferably in the range of 0.5 to 3 μm, and more preferably in the range of 1 to 2 μm, because longer carbon nanotubes with smaller diameters exhibit higher aspect ratios, thereby improving the properties of the carbon nanotubes and maintaining their network structure. If they are too long, it is difficult to obtain a high dispersion effect. The length of carbon nanotubes can be measured using an atomic force microscope (AFM), by measuring the lengths of 10 carbon nanotubes and taking the arithmetic mean value as the length of the carbon nanotube.

[0027] Suitable dispersion media include water, water-soluble organic solvents, or mixtures thereof. Of these, water is preferred from the viewpoint of reducing environmental impact. Examples of water include distilled water, deionized water, and ultrapure water. Examples of water-soluble organic solvents include alcohols (methanol, ethanol, isopropanol, isobutanol, sec-butanol, tert-butanol, methyl cellosolve, ethyl cellosolve, ethylene glycol, glycerin, etc.), ethers (ethylene glycol dimethyl ether, 1,4-dioxane, tetrahydrofuran, etc.), ketones (acetone, methyl ethyl ketone), N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide.

[0028] Dispersants are used to disperse carbon nanotubes. Examples of dispersants include ionic surfactants such as cationic surfactants, anionic surfactants, and amphoteric surfactants, nonionic surfactants, disaccharides such as sucrose, maltose, lactose, cellobiose, and trehalose, oligosaccharides such as cyclodextrin, steroid derivatives such as bile acids, cholesterol, and cholic acid, DNA, π-conjugated polymers, and phthalocyanine derivatives. Among these, steroid derivatives, especially bile acids, are preferred for the following reason: Since carbon nanotubes are hydrophobic, when a steroid derivative is added as a dispersant, the hydrophobic groups of the steroid derivative bind to the surface of the carbon nanotubes, pulling the carbon nanotubes and making them easily isolated and dispersed.

[0029] The dispersant content is preferably in the range of 2 to 5% by mass relative to the carbon nanotube content, and more preferably in the range of 3 to 4% by mass, because optimization is necessary to minimize the amount of dispersant used so as not to affect the electrical conductivity of the carbon nanotubes (increasing the amount of dispersant improves dispersion efficiency but decreases electrical conductivity).

[0030] The diameter of cellulose nanofibers is preferably in the range of 0.7 to 4 nm, and more preferably in the range of 1 to 2 nm, for reasons of maintaining functionality and mechanical and chemical properties. The diameter of cellulose nanofibers can be measured using an atomic force microscope (AFM), by measuring the diameters of 10 cellulose nanofibers and taking the arithmetic mean value as the diameter of the cellulose nanofiber.

[0031] The length of the cellulose nanofibers is preferably in the range of 1 to 5 μm, and more preferably in the range of 2 to 3 μm, because a certain length is necessary to maintain the dispersion effect in the solvent, but if they are too short, they cannot maintain the property of protecting carbon nanotubes. The length of the cellulose nanofibers can be measured using an atomic force microscope (AFM), and the lengths of 10 cellulose nanofibers are measured and their arithmetic mean is taken as the length of the cellulose nanofibers.

[0032] The ratio of length to diameter (length / diameter, aspect ratio) of cellulose nanofibers is preferably in the range of 1000 to 5000, due to their mechanical and structural properties, as well as to maintain the optimal dispersion effect of carbon nanotubes and cellulose nanofibers, and to preserve their network structure.

[0033] Solvents used in solutions containing cellulose nanofibers include, for example, dimethyl sulfoxide, n-methyl-2-pyrrolidone, and water. Among these, water is preferred because it easily provides a uniform dispersion effect, is inexpensive, and is also environmentally friendly.

[0034] In the method for producing a carbon nanotube molded article 1, preferably, both the dispersion containing carbon nanotubes and the solution containing cellulose nanofibers do not contain monosaccharides. Examples of monosaccharides include arabinose, ribose, xylose, glucose, mannose, galactose, fructose, sorbose, rhamnose, fucose, and ribodesose. By mixing these dispersions and solutions, a mixture that does not contain monosaccharides is obtained.

[0035] The ratio of cellulose nanofiber mass to carbon nanotube mass in the mixture needs to be optimized to minimize the amount of cellulose nanofiber used so as not to affect the electrical conductivity of the carbon nanotubes (increasing the amount of cellulose nanofiber improves the quality of the carbon nanotube network, but decreases electrical conductivity). Furthermore, because the amount of amorphous carbon increases after carbonization, the ratio is preferably in the range of 0.05 to 1, and more preferably in the range of 0.2 to 0.5.

[0036] Next, the obtained mixture is supplied to a mold and dried to obtain a solidified product containing carbon nanotubes. The mold can be appropriately selected according to the shape of the carbon nanotube molded product to be manufactured; for example, when manufacturing a carbon nanotube sheet, a tray can be used as the mold. Drying is carried out, for example, in a temperature environment of 25 to 80°C.

[0037] Next, the solidified material is washed to remove the dispersant. Specifically, the solidified material is first washed with a polar organic solvent such as alcohol. This removes the dispersant used when dispersing the carbon nanotubes. Subsequently, the solidified material is washed with an acid. This removes impurities (such as catalysts) that originate from the carbon nanotube manufacturing process. The acid concentration is preferably adjusted to within the range of 45 to 60% by mass. Examples of acids include hydrochloric acid and nitric acid. Finally, the solidified material is washed with deionized water until it becomes neutral. After washing the solidified material, it is preferable to dry it. This drying is also carried out in a temperature environment of, for example, 25 to 80°C.

[0038] Next, the solidified material is calcined to carbonize the cellulose nanofibers. The calcination is carried out, for example, under a nitrogen atmosphere at a temperature in the range of 600 to 800°C. In this process, the cellulose nanofibers are carbonized into amorphous carbon nanoparticles, which can suppress the re-aggregation of carbon nanotubes. The calcination time can be, for example, 60 to 120 minutes.

[0039] Finally, the surface of the solidified material after firing is subjected to an activation treatment. The activation treatment involves, for example, heating the carbon nanotube molded body to a temperature in the range of 800 to 900°C in a carbon dioxide atmosphere to activate the carbon nanotube molded body. The heating time for the activation treatment can be, for example, 30 to 60 minutes.

[0040] (b) Catalyst material The composite obtained by the preparation method of the present invention comprises a catalyst material formed on a support containing a carbon nanotube molded body. The catalyst material includes any catalyst material commonly used in electrodes for electrochemical water splitting, but also platinum (Pt), iridium oxide (IrO ２ ), Pd, Fe, Ni, Rb, Co, Ru, Rh, and alloys containing two or more of these are preferred, as are platinum (Pt) and iridium oxide (IrO ２ ) is preferable.

[0041] (c) Preparation method of the present invention In the preparation method of the present invention, first, in step (1), a support containing a carbon nanotube molded body is brought into contact with a solution containing a catalyst precursor for the catalyst material. That is, it is important to use a wet method in the preparation method of the present invention. Conventionally, physical loading methods such as vapor deposition have been used to load catalyst materials onto supports such as carbon nanotubes, but these methods have problems such as high costs. In contrast, the preparation method of the present invention uses a wet method, which is cost-effective and has the advantage of being applicable to mass production. Furthermore, while vapor deposition methods can only load catalyst materials onto the vapor-deposited surface, using a wet method, unlike vapor deposition methods, makes it possible to load the catalyst into the interior of the sheet.

[0042] The catalyst precursor for the catalyst material includes a metal salt or metal acid, which is a metal precursor that is converted to a metal by reduction in step (3) described later.

[0043] Preferably, the metal salt or metal acid is one that yields iridium(III) ions, or one that yields platinum(IV) ions or platinum ions.

[0044] In one preferred embodiment of the preparation method of the present invention, the metal salt or metal acid includes one or more selected from the group consisting of a metal salt or metal acid that yields iridium(III) ions, a metal salt or metal acid that yields platinum(IV) ions, and a metal salt or metal acid that yields platinum ions.

[0045] The metal salts or metal acids from which iridium(III) ions can be obtained are preferably iridium chloride, iridium nitrate, and iridium acetate, with iridium chloride being particularly preferred.

[0046] Examples of metal salts or metal acids from which platinum(IV) acid ions or platinum ions can be obtained include chlorplatinic acids such as hexachloroplatinic acid (including hydrates such as hexachloroplatinic acid hexahydrate), chlorplatinic acid salts such as hexachloroplatinate (sodium hexachloroplatinate, potassium hexachloroplatinate, etc.) and potassium tetrachloroplatinate, dinitrodiammineplatin, dinitrodiammineplatinate nitrate, cis-diamminediaquaplatinate nitrate, trans-diamminediaquaplatinate nitrate, tetranitroplatinic acid, tetra(oxalato)platinic acid, cis-dinitrodiaquaplatin, tetraammineplatinate hydroxylate, hexaammineplatinate hydroxylate, tetraammineplatinate chloride, hexaammineplatinate chloride, hexahydroxyplatinic acid, platinum oxide, platinum-1 chloride, and platinum-2 chloride. Among these, chlorplatinic acids such as hexachloroplatinic acid are preferred.

[0047] In one preferred aspect of the preparation method of the present invention, the metal salt or metal acid includes iridium chloride or platinum chloride.

[0048] The solution containing the catalyst precursor is a solution obtained by dissolving the catalyst precursor (preferably a metal salt or metal acid) in water, an organic solvent, or a mixed solvent of water and an organic solvent, and preferably an aqueous solution obtained by dissolving the catalyst precursor (preferably a metal salt or metal acid) in water (preferably pure water).

[0049] The concentration of the catalyst precursor in a solution containing the catalyst precursor can be arbitrarily determined depending on the type of catalyst precursor used and the final amount of catalyst material supported. For example, for a solution containing iridium(III) ions, the concentration is, for example, 10 μg / mL to 100 mg / mL, and for a solution containing hexachloride platinum(IV) ions, the concentration is, for example, 10 μg / mL to 100 mg / mL.

[0050] To bring a support containing a carbon nanotube molded body into contact with a solution containing a catalyst precursor, methods include dipping the support into the solution containing the catalyst precursor, or applying the solution containing the catalyst precursor to the surface of the support (e.g., spraying, coating). Alternatively, the support containing the carbon nanotube molded body may be cut to a desired size before contact with the solution containing the catalyst precursor.

[0051] Next, in step (2), the support that has been in contact with the solution containing the catalyst precursor is dried to obtain a support equipped with the catalyst precursor. When immersing or impregnating the support in the solution containing the catalyst precursor, the support is first recovered from the solution before drying. Alternatively, the support may be washed with water or the like before drying to remove excess catalyst precursor.

[0052] Drying can be carried out at atmospheric pressure, usually at 60 to 90°C, preferably around 70°C.

[0053] Step (2) yields a support on which a catalyst precursor is supported (hereinafter also referred to as "support A"). Support A includes states in which the catalyst precursor is formed or adsorbed on the surface of the support, and states in which the catalyst precursor is contained within the pores of the support. Furthermore, when the catalyst precursor is a metal salt or metal acid, when dissolved in water, an organic solvent, or a mixed solvent of water and an organic solvent, the metal salt or metal acid usually dissociates into a metal ion or metal complex ion and a counterion such as an anion in the solution. However, the state in which the catalyst precursor is supported on the support includes both the state in which it is supported on the support as a metal salt or metal acid, and the state in which the metal ion or metal complex ion is supported on the support.

[0054] Next, in step (3), the catalyst precursor supported on the support is reduced. The reduction treatment after supporting the catalyst precursor is preferably carried out by adding a reducing agent. Suitable reducing agents include hydrogen, boron compounds such as sodium borohydride, dimethylamine borane, and trimethylamine borane, alcohols such as methanol, ethanol, propanol, and butanol, hydrazine, formic acid, and formalin. Hydrogen gas is preferred for reduction because it leaves no impurities and facilitates post-treatment.

[0055] In one preferred aspect of the preparation method of the present invention, the reduction of the catalyst precursor is carried out under a hydrogen atmosphere (H ２ This is done using / Ar). In this case, the hydrogen gas concentration is H ２ The concentration is 1 to 7 vol%, preferably 5 vol%. The reduction under a hydrogen atmosphere is usually carried out at 200 to 450°C for about 2 to 10 hours.

[0056] In the preparation method of the present invention, the catalyst precursor is converted to a metal by reduction. For example, if the catalyst precursor is iridium chloride, iridium nitrate, iridium acetate, etc., from which iridium(III) ions can be obtained, it is converted to iridium by reduction. Also, if the catalyst precursor is chloroplatinic acid, etc., from which chloroplatinate(IV) ions can be obtained, it is converted to platinum by reduction.

[0057] In the preparation method of the present invention, after the catalyst precursor is converted to a metal, the metal may be subsequently oxidized. Oxidation is preferably carried out with air (oxygen) gas. Oxidation with air (oxygen) gas ensures that no impurities remain, facilitating post-treatment. Oxidation with air (oxygen) gas can be carried out by heating in an oxygen atmosphere, for example, at 200 to 400°C for 2 to 10 hours. Alternatively, the metal can be oxidized with hydrogen peroxide or the like.

[0058] In step (3), if iridium is obtained by reduction, iridium oxide (IrO) is obtained by oxidizing the iridium. ｘ(where x is, for example, 2) is obtained and used as a catalyst material.

[0059] 2. Composite of the Present Invention Another embodiment of the present invention is a composite obtained by the preparation method of the present invention (hereinafter also referred to as "the composite of the present invention"). In the composite of the present invention, a catalytic material is formed on a support containing a carbon nanotube molded body.

[0060] In the composite of the present invention, the amount of catalyst material supported is preferably 200 μg / cm³ for platinum. ２ More preferably, 150 μg / cm³ ２ More preferably, 100 μg / cm³ ２ The following applies. Furthermore, while it is desirable to use as little platinum as possible, a preferred amount is 30 μg / cm³ in order to maintain catalyst activity and durability. ２ The above is a comfortable 50 μg / cm³ ２ That's all.

[0061] Furthermore, regarding the amount of catalyst material supported in the composite of the present invention, the amount of iridium oxide supported is preferably 500 μg / cm³. ２ More preferably, 300 μg / cm³ ２ More preferably, 200 μg / cm³ ２ The following applies. Furthermore, while it is desirable to use as little iridium oxide as possible, a preferred amount is 100 μg / cm³ in order to maintain catalyst activity and durability. ２ More preferably, 150 μg / cm³ ２ That's all.

[0062] The composite material of the present invention is suitably used in electrochemical water splitting devices. Furthermore, electrochemical water splitting electrodes containing the composite material of the present invention have good water electrolysis performance. Although not intended to be theoretically bound, we believe that the excellent water electrolysis performance of the composite material of the present invention is due to its structure. Specifically, one of the structural characteristics of the composite material of the present invention is a large G / D intensity ratio observed in the Raman spectrum. The G / D intensity ratio corresponds to crystallinity, and the larger the ratio, the fewer defects there are in the structure. In addition, defects in the carbon nanotube support can serve as a scaffold for interacting with metal ions, but on the other hand, they lead to a decrease in the conductivity of the carbon nanotube. The composite material obtained by the preparation method of the present invention is thought to have a large G / D intensity ratio because it supports the metal catalyst without introducing excessive defects.

[0063] Specifically, the composite of the present invention preferably has a G-band to D-band intensity ratio (G / D) of 30 or more, and more preferably 40 or more, as observed in the Raman spectrum. Here, the Raman spectrum is measured under conditions where the laser wavelength is 532 nm. The peak ranges of the G-band and D-band are 1500 to 1650 cm, respectively. －１ , 1270-1450cm －１ The intensity ratio is calculated as the ratio of the peak intensity of the G band to the peak intensity of the D band, excluding the background.

[0064] Furthermore, the composite of the present invention also has the characteristic of a reduced RBM (corresponding to diametrical stretching vibration), which is a peak in the Raman spectrum characteristic of single-walled carbon nanotubes. This corresponds to the fact that the catalytic material is densely supported on the carbon nanotubes in the composite of the present invention. Here, the Raman spectrum is measured under the condition of a laser wavelength of 532 nm. The peak range corresponding to RBM is 100-300 cm. －１ That is the case.

[0065] That is, one aspect of the composite of the present invention is a composite in which the RBM peak in the Raman spectrum is reduced by 50% or less, preferably 40% or less, and particularly preferably 30% or less, compared to a support containing a carbon nanotube molded body on which the catalyst material is not formed. Another preferred aspect of the composite of the present invention is a composite that substantially does not have an RBM peak in the Raman spectrum.

[0066] 3. Electrochemical Hydrolysis Electrode Another embodiment of the present invention is an electrochemical hydrolysis electrode comprising the composite of the present invention (hereinafter also referred to as the "electrode of the present invention"). That is, the electrode of the present invention comprises a composite in which a catalyst material is formed on a support containing a carbon nanotube molded body, obtained by the preparation method of the present invention.

[0067] Details of the support containing the carbon nanotube molded body are described in detail in the preparation method of the present invention.

[0068] In the electrode of the present invention, the catalyst material is preferably platinum or iridium oxide.

[0069] In the electrode of the present invention, the amount of catalyst material supported is preferably 200 μg / cm³ for platinum. ２ More preferably, 150 μg / cm³ ２ More preferably, 100 μg / cm³ ２ The following applies. Furthermore, while it is desirable to use as little platinum as possible, a preferred amount is 30 μg / cm³ in order to maintain catalyst activity and durability. ２ The above is a comfortable 50 μg / cm³ ２ That concludes the explanation. Furthermore, in the electrode of the present invention, the amount of iridium oxide supported is preferably 500 μg / cm³. ２ More preferably, 300 μg / cm³ ２ More preferably, 200 μg / cm³ ２ The following applies. Furthermore, while it is desirable to use as little iridium oxide as possible, a preferred amount is 100 μg / cm³ in order to maintain catalyst activity and durability. ２ More preferably, 150 μg / cm³ ２ That's all.

[0070] In the electrode of the present invention, the carbon nanotube molded body is preferably a carbon nanotube sheet, and the carbon nanotube sheet is preferably supported on the surface of a metal structure or on the surface of carbon fibers or a carbon fiber structure made of carbon fibers. By having such a configuration, electrical conductivity is increased, mechanical strength is increased, long-term stability is achieved, and the overall effect of electrochemical water splitting can be enhanced.

[0071] The metals that make up the metal structure are preferably titanium, platinum, gold, nickel, molybdenum, silver, cobalt, and iron, with titanium being particularly preferred, because they increase current density and minimize free electron loss. When carbon nanotube sheets are supported on the surface of the metal structure, there are advantages such as lower internal resistance and higher mechanical strength compared to when they are supported on the surface of carbon fibers or carbon fiber structures made of carbon fibers.

[0072] When carbon nanotube sheets are supported on the surface of carbon fibers or carbon fiber structures made of carbon fibers, they are more stable in acidic environments over the long term and the weight of the water electrolysis cell is reduced compared to when they are supported on the surface of metal structures. In addition, carbon fibers or carbon fiber structures made of carbon fibers are porous and have high porosity, which has the advantage of high water and gas permeability.

[0073] When applied to the electrodes of the present invention, the shape of the carbon nanotube molded body is preferably a sheet (i.e., a carbon nanotube sheet) as described above, but it is not limited to this, and for example, taking advantage of its outstanding flexibility and durability, it may be made into an arch shape, a ring band shape, a strip shape, a thread shape, etc.

[0074] The electrodes of the present invention are particularly suitable for use in the electrochemical water splitting apparatus of the present invention described later, but they can also be applied without particular limitation to various conventional applications using electrodes with platinum or iridium oxide as catalysts, such as electrolytic water purifiers, hydrogen water production devices, portable hydrogen water production devices, strongly acidic water generators, drinking water supply devices, ionized beverage drinkers, liquid activation / electrolysis devices, electrolytic water production evaluation devices, various batteries (secondary batteries, lead-acid batteries, dye-sensitized solar cells, fuel cells, biofuel cells, etc.), and various sensors (water quality sensors, hydrogen sensors, gas sensors, biosensors, alcohol sensors, condensation sensors, etc.).

[0075] 4. Electrochemical Water Splitting Apparatus Another embodiment of the present invention is an electrochemical device (electrochemical water splitting apparatus) for electrolyzing water, equipped with the electrodes of the present invention (hereinafter also referred to as "the water splitting apparatus of the present invention").

[0076] In the water splitting apparatus of the present invention, the amount of catalyst material supported in the electrode of the present invention is preferably 200 μg / cm³ for platinum. ２ More preferably, 150 μg / cm³ ２ More preferably, 100 μg / cm³ ２ The following applies. Furthermore, while it is desirable to use as little platinum as possible, a preferred amount is 30 μg / cm³ in order to maintain catalyst activity and durability. ２ The above is a comfortable 50 μg / cm³ ２ That concludes the explanation. Furthermore, in the water splitting apparatus of the present invention, the amount of iridium oxide supported in the electrode of the present invention is preferably 500 μg / cm³. ２ More preferably, 300 μg / cm³ ２ More preferably, 200 μg / cm³ ２ The following applies. Furthermore, while it is desirable to use as little iridium oxide as possible, a preferred amount is 100 μg / cm³ in order to maintain catalyst activity and durability. ２ More preferably, 150 μg / cm³ ２ That's all.

[0077] In the electrochemical water splitting apparatus of the present invention, the carbon nanotube molded body in the electrode of the present invention is preferably a carbon nanotube sheet, and it is preferable that the carbon nanotube sheet is supported on the surface of a metal structure or on the surface of carbon fibers or a carbon fiber structure made of carbon fibers. By having such a configuration, electrical conductivity is increased, mechanical strength is increased, long-term stability is achieved, and the overall effect of electrochemical water splitting can be enhanced.

[0078] Furthermore, as described above, when the carbon nanotube molded body (preferably a carbon nanotube sheet) in the electrode of the present invention is supported on the surface of a metal structure or on the surface of carbon fibers or a carbon fiber structure made of carbon fibers, the method for supporting the carbon nanotube sheet is as follows: in the case of the surface of a metal structure, the Nafion dispersion is sprayed onto the surface of the metal structure, the carbon nanotube sheet is placed on top, and it is dried; in the case of carbon fibers, the Nafion dispersion is sprayed onto the surface of the metal structure, the carbon nanotube sheet is placed on top, and it is dried; in the case of the surface of a carbon fiber structure, the Nafion dispersion is sprayed onto the surface of the carbon fiber structure, the carbon nanotube sheet is placed on top, and it is dried; or the carbon nanotube sheet can be sandwiched on both sides with carbon fibers, or hot-pressed.

[0079] Furthermore, the following method can also be used to fabricate an electrochemical hydrolysis cell by supporting carbon nanotube sheets: 1. Droplet Nafion solution onto a metal-supported CNT sheet. 2. Place the sheet on a PEM (in this case, a Nafion film). 3. Press and fix it in place. 4. Similarly, fix a metal-supported CNT sheet on the opposite side (CCM (Catalyst coated membrane)). Here, the cathode side (platinum-supported CNT sheet) and the anode side (iridium oxide-supported CNT sheet) can be fixed in either order. 5. When assembling the electrochemical hydrolysis cell, the CCM is fixed by sandwiching it between metal structures or carbon fiber structures. The above method for fabricating an electrochemical hydrolysis cell is also used in the examples described later. Figure 1 shows the procedure for fabricating the above CCM and a schematic diagram of the resulting CCM.

[0080] The present invention will be described below based on examples, but the present invention is not limited to these examples.

[0081] 1. The materials iridium(III) chloride, hexachloroplatin(IV) hexahydrate, and ultrapure water were all purchased from Fujifilm Wako Pure Chemical Industries, Ltd.

[0082] For the preparation of the SWCNT sheet, ultra-long TUBALL® SWCNTs purchased from OCSiAl Ltd were used as carbon nanotubes. Reagent-grade 0.5M sulfuric acid and 60% nitric acid were purchased from Fujifilm Wako Pure Chemical Industries, Ltd. As cellulose nanofibers, TEMPO-CNF (2.0 wt%) manufactured by the Isogai method and purchased from Nippon Paper Industries Co., Ltd. was used.

[0083] 7.5 g of TUBALL SWCNT powder, 15 g of sodium cholate (dispersant), and 10 g of polyvinylpyrrolidone (stabilizer) were pre-dispersed in 1000 mL of deionized water for 12 hours using a ball mill (Masuda Universal Ball Mill Model UBM 2). Next, this mixture was processed in a bead mill (Multi Lab DYNO Mill) containing 0.6 mm zirconium beads until good dispersion was achieved.

[0084] SWCNT solution was mixed with cellulose nanofiber solution in various ratios. The resulting mixture was poured onto a tefront layer and dried at room temperature. The dried sheets were immersed in 80% ethanol and peeled off. The surfactants from the sheets were removed by repeated washing with ethanol, followed by digestion with concentrated nitric acid. Next, the SWCNT sheets were carbonized under a nitrogen atmosphere and then activated in a tubular furnace under carbon dioxide at 900°C.

[0085] [Synthesis Example 1] Preparation of SWCNT sheets containing iridium and iridium oxide nanoparticles. Iridium(III) chloride solution was prepared by dissolving an appropriate amount of reagent (2.5-10 mg) in ultrapure water. The activated SWCNT sheet was cut to the desired size and mixed with iridium(III) chloride solution (1 mL / cm³). ２ The sheets were then held overnight in a hydrogen atmosphere (H). Next, the SWCNT sheets were recovered from the iridium chloride solution and washed several times with ultrapure water to remove excess iridium from the surface. The iridium-containing SWCNT sheets were dried in an oven for 24 hours. The dried samples were then placed in a tube furnace and heated in a hydrogen atmosphere (H). ２ It was heated at 400°C for 4 hours in (5%) / Ar (95%) and named CNT-Ir. Then the furnace temperature was lowered to a low temperature of 300°C and held in an air atmosphere for 4 hours to produce CNT-IrO ２ I obtained it.

[0086] [Synthesis Example 2] Preparation of SWCNT Sheets Containing Platinum Nanoparticles A hexachloroplatinic acid hexahydrate solution was prepared by dissolving an appropriate amount of reagent (0.025-0.25 mg) in ultrapure water. The activated SWCNT sheet was cut to the desired size and placed in the hexachloroplatinic acid hexahydrate solution overnight. Next, the SWCNT sheet was recovered from the platinum solution and washed several times with ultrapure water to remove excess platinum from the surface. The platinum-containing SWCNT sheet was dried in an oven for 24 hours. Next, the dried sample was placed in a tube furnace and heated in a hydrogen atmosphere (H ２ The mixture was heated at 400°C for 4 hours in (5%) / Ar (95%).

[0087] [Example 1] (1) Electrochemical analysis (analysis under alkaline conditions) A glassy carbon electrode (GCE, 3 mm in diameter) obtained from CHI Instruments was used in this study to support an iridium-based catalyst for electrochemical analysis. 2 μl of 0.5 wt% Nafion in ethanol was dropped onto a cleaned GCE. The SWCNT sheet with the deposited catalyst was carefully cut into a 3 mm diameter circle and placed on the GCE before the Nafion dried. The electrode was dried overnight under ambient conditions for further use. Commercial iridium oxide (TEC77110) was purchased from Tanaka Kikinzoku Kogyo Co., Ltd., and the calculated amount of iridium oxide was dispersed in 0.5 wt% Nafion in ethanol by sonication for one hour. Next, the iridium oxide dispersion was deposited onto a clean GC electrode and dried overnight. Electrocatalytic activity testing was performed using a standard three-electrode system with CH Instruments' Model CHI 760E electrochemical analyzer. 1M sodium hydroxide, porous carbon, and Ag / AgCl electrodes were used as the electrolyte, counter electrode, and reference electrode. Stability testing was performed over 2000 cycles by cyclic voltammetry.

[0088] (2) Electrochemical analysis (analysis under acidic conditions) The electrode catalyst activity test conditions were changed as follows, and electrochemical analysis was performed using the same electrode fabrication conditions as in Example 1 (1). <Conditions> SWCNT-IrO ２ (IrO ２ : 150 μg / cm³ ２ ) Electrolyte: 0.1M H ２ SO ４ Reference electrode: Ag / AgCl Counter: Pt Scan Rate: 5mV / S

[0089] (3) Fabrication of Water Electrolysis Cells A complete PEM (Proton Exchange Membrane) and AEM (Anion Exchange Membrane) water electrolysis system was constructed. Figures 2 and 3 show schematic diagrams and photographs of the water electrolysis apparatus that was actually constructed. The newly developed catalyst was used for the positive and negative electrodes, and the area for the PEM water electrolysis cell was 4 cm². ２ A catalyst coating film (CCM) was fabricated.

[0090] Using a 5% Nafion solution as a binder, cathode (negative electrode) and anode (positive electrode) catalyst samples were immobilized onto a Nafion film (Nafion 117 or Nafion 115), and then hot-pressed. For the membrane electrode assembly (MEA), platinum-coated titanium mesh and carbon cloth were used as porous transport layers for the anode and cathode sides, respectively.

[0091] (4) Water electrolysis analysis First, the cell temperature was spiked to the desired value, and we waited 4 to 6 hours to activate the membrane. The initial activation of the cell was 5 mVS. －１ The process was performed for 12 cycles (2 hours) within a potential range of 1V to 2.5V. After activation, the step speed was 5mVS within a potential range of 1V to 2.19V. －１ The i-V curve was obtained.

[0092] (5) Results and Discussion First, the oxygen evolution reaction performance of various catalysts was compared by three-electrode electrochemical analysis. As a result, as shown in Figure 4, it was found that the SWCNT sheet catalyst was superior to conventional catalysts. The CNT-Ir (metal) catalyst showed an overpotential of 36.1 mV, and the CNT-IrO ２ The catalyst operates at an overpotential of 33.4 mV and a current density of 50 mA / cm². ２ It reaches [a certain value]. On the other hand, pure IrO ２ (TEC77110) provides 50mA / cm² only at an overvoltage of 52.6mV. ２The results reach [value]. The variability in the results is mainly due to the morphology and distribution of iridium oxide, as shown in the SEM images (Figure 5). Activation of SWCNT sheets in carbon dioxide provides a large surface area and gentle oxidation of the CNTs. Carbon dioxide can provide nanocavities by slightly opening the CNTs. Therefore, activated carbon nanotubes can be excellent supports for nano-sized catalysts. SEM images of SWCNT sheets with iridium oxide catalyst (Figure 5A) and the corresponding backscatter images (Figure 5B) confirm that the particles are nano-sized and uniformly distributed on the individual CNTs. Washing the CNT sheets after immersion in iridium chloride solution helped remove any unadhered catalyst from the CNTs. In contrast, commercially available iridium oxide samples do not have uniformly sized particles. Nano or micro-sized iridium oxide aggregates into particles ranging from 0.1 to 60 μm, as shown in Figures 5C and 5D. The results of the electrochemical analysis (analysis under acidic conditions) are also shown in Figure 6.

[0093] [Example 2] PEM (Proton Exchange Membrane) Water Electrolysis Cell After initial setup of the PEM (Proton Exchange Membrane) water electrolysis cell, its performance was calibrated using a commercially available CCM (purchased from JFE Techno Research Corporation). 1 mg / cm³ on the cathode side. ２ Pt / C (500 μg Pt / cm ２ ), 1 mg / cm² on the anode side ２ We purchased iridium oxide-supported CCM. Nafion 117 was used as the proton exchanger. In the PEM system, high-temperature ultrapure water at 80°C was used, passing through the cell at a rate of 2 ml / min.

[0094] In the next step, the performance of the catalyst was compared with that of a commercially available catalyst (Figure 7). SWCNT-IrO was used as the anode. ２ (IrO ２ : 500 μg / cm² ２ ), SWCNT-Pt (Pt: 200 μg / cm²) as the cathode. ２ ) use IrO ２ The load of Pt was compared with that of a commercially available product. The performance of the developed CCM was 2.0 A / cm² compared to the commercially available product.２ Approximately 95%, 3.0 A / cm ２ They showed nearly identical voltages.

[0095] [Example 3] The stability of the new CCM was confirmed by a constant voltage test. The voltage was fixed at 1.84V. The first operation was continued for approximately 5 hours, and the initial current fluctuation was observed as shown in Figure 8A. During the 5 hours of operation, the cell performance decreased by approximately 10%. The operation was stopped for several hours and then restarted. The initial current density of the second operation was the same as the first. From this observation, it can be seen that the decrease in current density was not due to catalyst decay, but rather an increase in internal resistance due to the generated gas being trapped in the electrodes. The second operation was continued for approximately 18 hours, and the current density decreased by 5% in the first 4 hours, and by 4% in the remaining 14 hours (Figure 8B). The I-V curves before and after the I-T test (Figure 8C) confirmed the long-term stability of the CCM.

[0096] In the next step, the performance of water electrolysis with different catalyst loads was compared. The iridium oxide load on the anode was approximately 500 μg–113 μg. Similarly, the platinum load on the cathode was 200 μg–87 μg. The comparison results are shown in Figure 9 and the corresponding data in Table 1. As the inventors anticipated, the anode plays a crucial role in water electrolysis. By using the novel method of the present invention, the amount of platinum and iridium used was significantly reduced without greatly degrading the overall performance.

[0097]

[0098] The experiment was conducted using Nafion 115, which is thinner than N117, as the cation exchange membrane. Furthermore, the water temperature and flow rate were increased. Applied voltage 2V, current 2A / cm ２ We were able to achieve this. We were able to reduce the amount of catalyst by five times compared to commercially available CCM (Figure 10, Table 2).

[0099]

[0100] Thus, the present invention has enabled the development of a promising manufacturing technology for SWCNT sheets and a method for incorporating an active catalyst thereon. Using the manufacturing method of the present invention, a catalyst load of 50% compared to commercially available CCMs, consisting of 500 μg of iridium oxide and 200 μg of platinum, was achieved at 2 V and 1.96 A / cm². ２ We were able to achieve this. In addition, by reducing the amount of catalyst supported by 80% compared to commercially available CCMs, we achieved 2V, 1.44A / cm². ２ This was achieved. Using Nafion 115 as a cation exchange membrane, the target current density of 2.06 A / cm² was obtained with an applied voltage of 2 V. ２ This was achieved. As a result, the amount of catalyst supported was reduced by five times compared to commercially available CCMs.

[0101] [Example 4] Raman spectroscopy measurement under the following conditions: SWCNT, SWCNT-Pt (loading amount: 110 μg / cm³) ２ ), SWCNT-IrO ２ (Loading amount: 300 μg / cm³) ２ The Raman spectrum was measured for the sample. <Conditions> Laser wavelength 532 nm Renishaw Raman spectroscopy

[0102] Figure 11 shows the intensity ratio (G / D) of the G band to the D band observed in the Raman spectrum of each sample. Figure 12 shows the RBM peak intensity of each sample.

[0103] From Figure 11, SWCNT-Pt, SWCNT-IrO ２ Both exhibit a high G / D ratio (40 or higher). Furthermore, Figure 12 shows that when a catalyst material is supported on SWCNTs, the RBM peak intensity decreases (becomes almost undetectable). RBM is a peak characteristic of SWCNTs and corresponds to diametrical stretching vibrations. The decrease in this peak is thought to indicate an interaction between the metal / metal oxide nanoparticles and the SWCNTs.

Claims

1. A method for preparing a composite in which a catalyst material is formed on a support containing a carbon nanotube molded body, comprising: (1) a step of contacting a support containing a carbon nanotube molded body with a solution containing a catalyst precursor for the catalyst material; (2) a step of drying the support that has been in contact with the solution to obtain a support carrying the catalyst precursor; and (3) a step of converting the catalyst precursor into a metal by reduction, wherein the method for preparation may further include a step of oxidizing the metal following step (3).

2. The preparation method according to claim 1, wherein the carbon nanotube molded body is a carbon nanotube sheet.

3. The preparation method according to claim 1, wherein the film thickness of the carbon nanotube molded body is 30 μm or less.

4. The preparation method according to claim 1, wherein the catalyst precursor is a metal salt or a metal acid.

5. The preparation method according to claim 4, wherein the metal salt or metal acid comprises one or more selected from the group consisting of a metal salt or metal acid that yields iridium(III) ions, platinum(IV) ions, or platinum ions.

6. The preparation method according to claim 5, wherein the metal salt or metal acid from which iridium(III) ions are obtained comprises one or more selected from the group consisting of iridium chloride, iridium nitrate, and iridium acetate.

7. The preparation method according to claim 5, wherein the metal salt or metal acid from which platinum(IV) acid ions or platinum ions are obtained includes one or more selected from the group consisting of chloroplatinic acid, hexachloroplatinate, potassium tetrachloroplatinate, dinitrodiammineplatin, dinitrodiammineplatinate nitrate, cis-diamminediaquaplatinate nitrate, trans-diamminediaquaplatinate nitrate, tetranitroplatinic acid, tetra(oxalato)platinic acid, cis-dinitrodiaquaplatin, tetraammineplatinate hydroxylate, hexaammineplatinate hydroxylate, tetraammineplatinate chloride, hexaammineplatinate chloride, hexahydroxyplatinic acid, platinum oxide, platinum I chloride, and platinum I chloride.

8. The preparation method according to claim 5, wherein the metal salt or metal acid comprises iridium chloride or chloroplatinic acid.

9. The preparation method according to claim 1, wherein the carbon nanotube molded body is a carbon nanotube molded body prepared by a method comprising the following steps: a step of mixing a dispersion containing carbon nanotubes, a dispersant and a dispersion medium with a solution containing cellulose nanofibers to obtain a mixture; a step of supplying the obtained mixture to a mold and drying it in a temperature environment of 25 to 80°C to obtain a solidified product containing carbon nanotubes; a step of washing the solidified product to remove the dispersant from the solidified product; a step of calcining the solidified product to carbonize the cellulose nanofibers; and a step of applying an activation treatment to the surface of the calcined solidified product to obtain a carbon nanotube molded body containing carbon nanotubes.

10. A composite obtained by the method described in any one of claims 1 to 9.

11. The composite according to claim 10, wherein the intensity ratio of the G band to the D band observed in the Raman spectrum (G / D) is 30 or more.

12. The composite according to claim 10, wherein the RBM peak in the Raman spectrum is reduced to 50% or less compared to a support containing a carbon nanotube molded body on which no catalyst material is formed.

13. A composite obtained by the method of any one of claims 1 to 9, used in an electrochemical water splitting device.

14. An electrode for electrochemical water splitting comprising a composite obtained by the method described in any one of claims 1 to 9.

15. The electrode according to claim 14, wherein the catalyst material is platinum or iridium oxide.

16. An electrochemical water splitting device comprising the electrode described in claim 14.

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