Porous titanium suboxide, anode catalyst, membrane electrode assembly, and method for producing membrane electrode assembly

Abstract

Provided is a porous titanium suboxide which has: a crystallite diameter of 60 nm or more; and a ratio of the volume of mesopores having a diameter of more than 2 nm and less than 50 nm to the total pore volume, of 70 vol % or more.

Description

Porous titanium suboxide, anode catalyst, membrane electrode assembly, and method for manufacturing membrane electrode assembly

[0001] The present disclosure relates to a porous titanium suboxide, an anode catalyst, a membrane electrode assembly, and a method for manufacturing the membrane electrode assembly.

[0002] In recent years, development of polymer electrolyte membrane (PEM) water electrolysis devices capable of generating hydrogen as a clean energy source has progressed. PEM water electrolysis devices include an anode catalyst, a solid polymer electrolyte membrane, and a cathode catalyst. In PEM water electrolysis devices, a water decomposition reaction progresses on the anode catalyst side, and protons generated by the water decomposition reaction migrate to the cathode catalyst side via the solid polymer electrolyte membrane. The protons are then reduced by the cathode catalyst to generate hydrogen.

[0003] In such a PEM-type water electrolysis device, the anode catalyst is known to include a platinum group element as an active metal and titanium suboxide as a support for supporting the active metal (see, for example, Patent Document 1). In Example 1 of Patent Document 1, a method for producing a titanium suboxide support used in the anode catalyst is described, in which titanium dioxide powder (TiO ２ The method involves calcining the powder at high temperature in a hydrogen atmosphere.

[0004] Japanese Patent Application Laid-Open No. 2021-188095

[0005] However, when such a titanium suboxide support is used as an anode catalyst, the surface area of ​​the active metal decreases due to particle growth of the active metal, and the active metal becomes liberated. Furthermore, during use, the load on the active metal near the titanium suboxide support increases. As a result, there is a problem of reduced catalytic performance.

[0006] An object of the present disclosure is to provide a porous titanium suboxide, an anode catalyst, a membrane electrode assembly, and a method for manufacturing a membrane electrode assembly, which are capable of improving catalyst durability.

[0007] The present disclosure [1] includes porous titanium suboxide having a crystallite diameter of 60.0 nm or more, and a ratio of the volume of mesopores having a pore diameter of more than 2 nm and less than 50 nm to the total pore volume of 70 vol % or more.

[0008] The present disclosure [2] includes the porous titanium suboxide according to the above [1], in which the ratio of the volume of micropores having a pore diameter of 2 nm or less to the total pore volume is 20 vol % or less.

[0009] The present disclosure [3] includes the porous titanium suboxide according to the above [1] or [2], in which the ratio of the volume of macropores having a pore diameter of 50 nm or more to the total pore volume is less than 30% by volume.

[0010] The present disclosure [4] is directed to the porous titanium suboxide (TiO Ｘ The porous titanium suboxide according to any one of [1] to [3] above, wherein the average oxidation degree (X) of the porous titanium suboxide is greater than 1.70 and less than 1.85.

[0011] The present disclosure [5] includes an anode catalyst comprising the porous titanium suboxide according to any one of the above [1] to [4] and an active metal supported on the porous titanium suboxide, wherein the active metal comprises a platinum group metal and / or an oxide thereof.

[0012] The present disclosure [6] includes the anode catalyst according to the above [5], wherein the active metal includes iridium and / or iridium oxide.

[0013] The present disclosure [7] includes a membrane electrode assembly including an anode containing the anode catalyst according to the above [5] or [6], a cathode containing a cathode catalyst, and an electrolyte membrane disposed between the anode and the cathode.

[0014] The present disclosure [8] includes the membrane electrode assembly according to the above [7], in which the cathode catalyst comprises a second active metal and a carbon support supporting the second active metal.

[0015] The present disclosure [9] includes the membrane electrode assembly according to the above [7] or [8], wherein the electrolyte membrane is a fluorine-based polymer having sulfonic acid groups.

[0016] The present disclosure

[10] includes the membrane electrode assembly according to any one of the above [7] to [9], wherein the anode further contains an ionomer.

[0017] The present disclosure

[11] includes a method for producing the membrane electrode assembly according to any one of the above [7] to [9], the method including the steps of preparing the anode and the cathode, and disposing an electrolyte membrane between the anode and the cathode and applying heat and pressure.

[0018] In the porous titanium suboxide of the present disclosure, the ratio of the volume of mesopores with pore diameters of 2 to 50 nm to the total pore volume is 70% by volume or more. Therefore, when such porous titanium suboxide is used as a catalyst support to support an active metal and used as an anode catalyst, it is possible to suppress a decrease in the surface area of ​​the active metal and the liberation of the active metal due to particle growth of the active metal.

[0019] Furthermore, the porous titanium suboxide has a crystallite diameter of 60.0 nm or more. This improves the electrical conductivity of the porous titanium suboxide. When such porous titanium suboxide is used as a catalyst support to support an active metal and used as an anode catalyst, the entire active metal supported on the porous titanium suboxide can be utilized, thereby reducing the load per active metal particle during use.

[0020] As a result, the durability of the catalyst can be improved.

[0021] The anode catalyst of the present disclosure comprises the porous titanium suboxide and an active metal supported on the porous titanium suboxide. This prevents a decrease in the surface area of ​​the active metal and the release of the active metal due to particle growth. Furthermore, the load per active metal particle during use can be reduced. As a result, catalyst durability can be improved.

[0022] The membrane electrode assembly disclosed herein includes an anode containing the above-described anode catalyst, a cathode containing a cathode catalyst, and an electrolyte membrane disposed between the anode and the cathode. Therefore, when subjected to water electrolysis or the like, it is possible to suppress a decrease in the surface area of ​​the active metal and the liberation of the active metal due to particle growth of the active metal. Furthermore, it is possible to reduce the load per active metal particle during use. As a result, it is possible to improve catalyst durability.

[0023] The method for manufacturing a membrane electrode assembly according to the present disclosure includes the steps of preparing an anode and a cathode, and disposing an electrolyte membrane between the anode and the cathode and applying heat and pressure to the membrane electrode assembly, thereby enabling the membrane electrode assembly to be manufactured through a relatively simple process.

[0024] Figure 1 shows X-ray diffraction pattern spectra of the porous titanium suboxides of Preparation Example 1 and Comparative Preparation Example 2. Figure 2 shows graphs of nitrogen adsorption isotherms of the porous titanium suboxides of Preparation Example 1 and Comparative Preparation Example 2. Figure 3 shows graphs of pore size distributions of the porous titanium suboxides of Preparation Example 1 and Comparative Preparation Example 2. Figure 4 shows graphs plotting the number of cycles on the horizontal axis and the change in average voltage (ΔV) on the vertical axis for the membrane electrode assemblies of Examples 1 and 2 and Comparative Examples 1 and 2.

[0025] 1. Porous titanium suboxide Porous titanium suboxide (TiO Ｘ ) is used as a support for supporting active metals in the anode catalyst of a PEM-type water electrolysis device.

[0026] The porous titanium suboxide is porous. Specifically, the porous titanium suboxide has mesopores with a pore size of more than 2 nm and less than 50 nm. The porous titanium suboxide may further have micropores with a pore size of 2 nm or less and macropores with a pore size of 50 nm or more.

[0027] The total pore volume per 1 g of porous titanium suboxide is, for example, 0.050 cc / g or more, preferably 0.100 cc / g or more, more preferably 0.120 cc / g or more, and even more preferably 0.140 cc / g or more, and from the viewpoint of durability, for example, 1.00 cc / g or less.

[0028] The total pore volume is the total volume of the micropore volume, the mesopore volume, and the macropore volume.

[0029] A porous titanium suboxide having a total pore volume per gram of porous titanium suboxide equal to or greater than the above lower limit is defined as porous.

[0030] The mesopore volume per 1 g of porous titanium suboxide is, for example, 0.050 cc / g or more, preferably 0.070 cc / g or more, more preferably 0.090 cc / g or more, even more preferably 0.100 cc / g or more, particularly preferably 0.110 cc / g or more, and most preferably 0.120 cc / g or more, and is, for example, 1.00 cc / g or less.

[0031] The micropore volume per 1 g of porous titanium suboxide is, for example, 0.070 cc / g or less, preferably 0.050 cc / g or less, more preferably 0.030 cc / g or less, even more preferably 0.020 cc / g or less, and for example, 0.001 cc / g or more.

[0032] The macropore volume per 1 g of porous titanium suboxide is, for example, 0.070 cc / g or less, preferably 0.060 cc / g or less, more preferably 0.050 cc / g or less, even more preferably 0.040 cc / g or less, particularly preferably 0.030 cc / g or less, and is, for example, 0.001 cc / g or more.

[0033] The total pore volume and the volume of each pore (micropores, mesopores, macropores) can be measured by the method described in Examples. Specifically, the total pore volume and the volume of each pore (micropores, mesopores, macropores) can be calculated by analyzing a nitrogen adsorption isotherm at 77 K by BET analysis using a specific surface area / pore size distribution measuring device.

[0034] The ratio of the mesopore volume to the total pore volume of the porous titanium suboxide is, for example, 70 vol% to 100 vol%, preferably 73 vol% to 100 vol%, more preferably 75 vol% to 99 vol%, even more preferably 78 vol% to 98 vol%, and particularly preferably 80 vol% to 97 vol%.

[0035] The ratio of the volume of mesopores to the total pore volume of the porous titanium suboxide is 70 vol% or more, preferably 73 vol% or more, more preferably 75 vol% or more, even more preferably 78 vol% or more, particularly preferably 80 vol% or more, and for example, less than 100 vol%, preferably 99 vol% or less, more preferably 98 vol% or less, particularly preferably 97 vol% or less.

[0036] If the ratio of the mesopore volume to the total pore volume of porous titanium suboxide is equal to or greater than the above lower limit, when the porous titanium suboxide is used as a catalyst support, the active metal can be accommodated in the mesopores, and a decrease in the surface area of ​​the active metal due to particle growth of the active metal can be suppressed. Furthermore, by accommodating the active metal in the mesopores, liberation of the supported active metal can be suppressed. As a result, catalyst durability can be improved.

[0037] Catalyst durability is an index showing the degree to which the deterioration of catalytic performance is suppressed when porous titanium suboxide is used as a catalyst support, an active metal is supported, and the catalyst is used as an anode catalyst. In other words, high catalyst durability indicates that the deterioration of catalytic performance with use can be suppressed, while low catalyst durability indicates that the catalytic performance will deteriorate with use.

[0038] The ratio of the volume of micropores to the total pore volume of the porous titanium suboxide is, for example, 0 vol % to 30 vol %, preferably 0 vol % to 25 vol %, more preferably 1 vol % to 20 vol %, even more preferably 1 vol % to 15 vol %, and particularly preferably 2 vol % to 10 vol %.

[0039] The ratio of the volume of micropores to the total pore volume of the porous titanium suboxide is, for example, more than 0 vol%, preferably 1 vol% or more, more preferably 2 vol% or more, and for example, less than 30 vol%, preferably 25 vol% or less, more preferably 20 vol% or less, even more preferably 15 vol% or less, and particularly preferably 10 vol% or less.

[0040] If the ratio of the micropore volume to the total pore volume of the porous titanium suboxide is excessively high, the active metal cannot be accommodated in the micropores, and the amount of active metal supported near the surface of the porous titanium suboxide increases, leading to liberation of the active metal during production and use, resulting in loss of the active metal. Therefore, if the ratio of the micropore volume to the total pore volume of the porous titanium suboxide is not more than the above upper limit, the volume ratio of mesopores can be secured, and catalyst durability can be improved.

[0041] The ratio of the volume of macropores to the total pore volume of the porous titanium suboxide is, for example, 0 vol % to 30 vol %, preferably 0 vol % to 25 vol %, more preferably 1 vol % to 20 vol %, even more preferably 2 vol % to 15 vol %, and particularly preferably 2 vol % to 10 vol %.

[0042] The ratio of the volume of macropores to the total pore volume of the porous titanium suboxide is, for example, more than 0 vol%, preferably 1 vol% or more, more preferably 2 vol% or more, and for example, less than 30 vol%, preferably 25 vol% or less, more preferably 20 vol% or less, even more preferably 15 vol% or less, and particularly preferably 10 vol% or less.

[0043] If the ratio of the macropore volume to the total pore volume of the porous titanium suboxide is excessively high, the active metal contained in the macropores will grow into particles, reducing the surface area of ​​the active metal. Therefore, if the ratio of the macropore volume to the total pore volume of the porous titanium suboxide is within the above range, the volume ratio of mesopores can be ensured, and catalyst durability can be improved.

[0044] The ratio of the micropore volume to the mesopore volume of the porous titanium suboxide (micropore volume / mesopore volume) is, for example, 0.45 or less, preferably 0.40 or less, more preferably 0.35 or less, even more preferably 0.30 or less, and particularly preferably 0.25 or less.

[0045] The ratio of the macropore volume to the mesopore volume of the porous titanium suboxide (macropore volume / mesopore volume) is, for example, 0.45 or less, preferably 0.40 or less, more preferably 0.35 or less, even more preferably 0.30 or less, and particularly preferably 0.25 or less.

[0046] The most common pore size of the porous titanium suboxide is in the mesopore range of more than 2 nm and less than 50 nm. Specifically, the most common pore size of the porous titanium suboxide is, for example, more than 2 nm, preferably 3 nm or more, more preferably 4 nm or more, and for example, less than 50 nm, preferably 40 nm or less, more preferably 30 nm or less, even more preferably 25 nm or less, and particularly preferably 20 nm or less.

[0047] The most frequent pore size of the porous titanium suboxide can be measured by the method described in the Examples. Specifically, the most frequent pore size of the porous titanium suboxide can be determined from the pore size distribution measured using a specific surface area / pore size distribution measuring device.

[0048] In addition, porous titanium suboxide (TiO Ｘ ) is titanium dioxide (TiO ２ ) contains titanium oxide with a lower oxidation number.

[0049] Specifically, the main component of the porous titanium suboxide is, for example, Ti ４ O ７ (oxidation number: 1.75). The main component is Ti in the total amount of porous titanium suboxide. ４ O ７ This indicates that the content of is 50 mass % or more.

[0050] Porous titanium suboxide is Ti ４ O ７ The titanium oxide (other component) may contain titanium oxide having an oxidation number other than TiO. ２ (oxidation number: 2), Ti ３ O ５ (oxidation number: 1.67), Ti ５ O ９ (oxidation number: 1.80), Ti ６ O １１(oxidation number: 1.83), and Ti ８ O １５ (oxidation number: 1.86).

[0051] Ti in the total amount of porous titanium suboxide ４ O ７ The content is, for example, 50% by mass or more, preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0052] The components contained in the porous titanium suboxide can be confirmed by powder X-ray diffraction. Specifically, if there are peaks in the diffraction pattern at around 2θ=22.8° and 2θ=26.4°, the components are Ti. ６ O １１ If peaks exist near 2θ=22.0° and 2θ=26.4°, it can be determined that the sample contains titanium suboxide. ５ O ９ If peaks exist around 2θ=20.8° and 2θ=26.4°, it can be determined that the sample contains titanium suboxide. ４ O ７ If peaks are present around 2θ=18.9° and 2θ=26.4°, it can be determined that the sample contains titanium suboxide. ３ O ５ If peaks exist near 2θ=27.5° and 2θ=36.1°, it can be determined that the sample contains titanium dioxide (TiO ２ ) can be determined to be included.

[0053] Porous titanium suboxide (TiO Ｘ The average oxidation degree (X) of the hydroxybenzoates is, for example, 1.65 or more, preferably 1.70 or more, more preferably more than 1.70, even more preferably 1.73 or more, particularly preferably 1.75 or more, and for example, less than 2.00, preferably 1.90 or less, more preferably 1.85 or less, even more preferably less than 1.85, particularly preferably 1.80 or less, and most preferably less than 1.80.

[0054] Porous titanium suboxide (TiO ＸWhen the average oxidation degree (X) of the porous titanium suboxide is within the above range, when the porous titanium suboxide is used as a catalyst support, the porous titanium suboxide has good electrical conductivity, improving the charge transfer efficiency and, as a result, improving the catalytic performance.

[0055] In addition, porous titanium suboxide (TiO Ｘ The average oxidation degree (X) of porous titanium suboxide (TiO) can be measured by the method described in the Examples. Ｘ The average oxidation degree (X) of the cellulose acetate solution can be calculated based on the carbon content measured using a total organic carbon analyzer and the weight change due to the heat treatment measured using a thermogravimetric analyzer.

[0056] The crystallite diameter of the porous titanium suboxide is, for example, 60.0 nm to 100 nm, preferably 60.0 nm to 90.0 nm, more preferably 60.0 nm to 80.0 nm, and even more preferably 60.5 nm to 75.0 nm.

[0057] The crystallite diameter of the porous titanium suboxide is 60.0 nm or more, preferably 60.5 nm or more, and for example, 100 nm or less, preferably 90.0 nm or less, more preferably 80.0 nm or less, and even more preferably 75.0 nm or less.

[0058] When the crystallite size of the porous titanium suboxide is equal to or greater than the lower limit, the conductivity of the porous titanium suboxide can be improved. Therefore, when used as an anode catalyst, the active metal supported on the porous titanium suboxide can be fully utilized, reducing the load per active metal particle during use. As a result, the durability of the catalyst can be improved.

[0059] The crystallite refers to the smallest unit of a crystal grain that can be regarded as a single crystal. The crystallite diameter of porous titanium suboxide can be calculated using the position of the diffraction peak in X-ray diffraction and the half-width of the peak according to the Scherrer formula below. In the examples described below, the crystallite diameter of the main component Ti ４ O ７The crystallite size is calculated based on the peak information around 2θ=20.8°, which is a peak specific to the structure. Scherrer formula: D=Kλ / Bcosθ D: crystallite size (nm) K: Scherrer constant (0.90) λ: X-ray wavelength (nm) B: diffraction line width (rad) θ: Bragg angle (rad)

[0060] The carbon content in the porous titanium suboxide is, for example, 10% by mass or less, preferably 8.0% by mass or less, more preferably 6.0% by mass or less, even more preferably 5.0% by mass or less, particularly preferably 3.0% by mass or less, and most preferably 2.5% by mass or less, and for example, 0% by mass or more.

[0061] Porous titanium suboxide contains carbon due to the manufacturing method. As will be described in detail later, when a titanium suboxide precursor contains an organic substance, the carbon in the organic substance remains in the porous titanium suboxide manufactured by reducing the titanium suboxide precursor. Therefore, the porous titanium suboxide contains carbon.

[0062] If the carbon content in the porous titanium suboxide is excessively high, when the porous titanium suboxide is used as a catalyst support, the carbon in the porous titanium suboxide may be decomposed, and the supported active metal may be liberated, depending on the environment in which the catalyst is used. Therefore, if the carbon content in the porous titanium suboxide is not more than the above upper limit, the liberation of the active metal can be suppressed, and the catalyst durability can be improved.

[0063] The specific surface area of ​​the porous titanium suboxide is, for example, 10 m ２ / g or more, preferably 20m ２ / g, more preferably 25m ２ / g or more, more preferably 30m ２ / g or more, particularly preferably 40m ２ / g or more, for example, 200m ２ / g or less, preferably 180m ２ / g or less, more preferably 160m ２ / g or less, more preferably 150m ２ / g or less.

[0064] If the specific surface area of ​​the porous titanium suboxide is equal to or greater than the lower limit, the amount of active metal supported can be increased. If the specific surface area of ​​the porous titanium suboxide is equal to or less than the upper limit, it is considered that the amount of residual carbon can be controlled, and catalyst durability can be improved.

[0065] The specific surface area of ​​the porous titanium suboxide can be measured by the method described in the Examples. Specifically, the specific surface area of ​​the porous titanium suboxide can be calculated by analyzing the nitrogen adsorption isotherm at 77 K by BET analysis using a specific surface area / pore size distribution measuring device.

[0066] 2. Method for Producing Porous Titanium Suboxide A method for producing the porous titanium suboxide will now be described.

[0067] The method for producing porous titanium suboxide includes a step of preparing a precursor in which titanium dioxide and a cationic surfactant are combined (preparation step), and a step of firing the precursor in a reducing atmosphere (reduction step).

[0068] (Preparation Step) In the preparation step, a precursor in which titanium dioxide and a cationic surfactant are composited is prepared.

[0069] Specifically, a titania source is added to a solution in which a cationic surfactant is dissolved in a solvent to prepare a mixed solution. The resulting mixed solution (containing the titania source and the cationic surfactant) is aged to form a composite of titanium dioxide and the cationic surfactant. If necessary, the titania source may be mixed with a chelating agent in advance and then mixed with the solution in which the cationic surfactant is dissolved in a solvent.

[0070] The titania source is not particularly limited as long as it is a titanium compound that forms titanium dioxide upon hydrolysis. Examples of the titania source include organic titanium compounds and inorganic titanium compounds. The titania source is preferably water-soluble.

[0071] Examples of organic titanium compounds include titanium alkoxides. Examples of titanium alkoxides include titanium ethoxide, titanium propoxide, titanium isopropoxide, titanium butoxide, titanium tetrabutoxide, titanium tetrapropoxide, titanium tetraisopropoxide, titanium tetraisobutoxide, and titanium tetraisopropenyl oxide. Preferably, titanium tetraisopropoxide is used.

[0072] Inorganic titanium compounds include, for example, titanium tetrachloride, titanium tetranitrate, and titanyl sulfate.

[0073] Examples of cationic surfactants include surfactants having an amino group in the molecular structure and surfactants having a quaternary ammonium salt in the molecular structure.

[0074] Furthermore, from the viewpoint of reducing the carbon content remaining in the porous titanium suboxide, it is preferable that the carbon chain length of the cationic surfactant is not excessively long. The carbon chain length of the cationic surfactant is, for example, 20 or less, preferably 16 or less, more preferably 14 or less, and even more preferably 12 or less, and for example, 4 or more, preferably 6 or more, more preferably 8 or more, and even more preferably 10 or more.

[0075] When the carbon chain length of the cationic surfactant is equal to or less than the upper limit, the carbon content of the porous titanium suboxide can be reduced, and catalyst durability can be improved.When the carbon chain length of the cationic surfactant is equal to or greater than the lower limit, the titanium dioxide and the cationic surfactant are appropriately composited, and porous titanium suboxide can be obtained.

[0076] Examples of surfactants having an amino group in their molecular structure include monoamines such as hexadecylamine, tetradecylamine, dodecylamine, decylamine, and octaneamine, and diamines such as 1,12-dodecanediamine, 1,10-decanediamine, 1,8-octanediamine, and 1,6-hexanediamine. Preferred are 1,12-dodecanediamine, 1,10-decanediamine, and dodecylamine.

[0077] Examples of surfactants having a quaternary ammonium salt in their molecular structure include hexadecyltrimethylammonium salt, tetradecyltrimethylammonium salt, dodecyltrimethylammonium salt, decyltrimethylammonium salt, n-octyltrimethylammonium salt, and halides thereof, with dodecyltrimethylammonium bromide being preferred.

[0078] The molar mass of the cationic surfactant is, for example, 80 g / mol or more, preferably 100 g / mol or more, more preferably 120 g / mol or more, even more preferably 140 g / mol or more, particularly preferably 160 g / mol or more, and for example, 400 g / mol or less, preferably 350 g / mol or less, more preferably 320 g / mol or less.

[0079] When the molar mass of the cationic surfactant is equal to or less than the upper limit, the carbon content of the porous titanium suboxide can be reduced, and catalyst durability can be improved.When the molar mass of the cationic surfactant is equal to or more than the lower limit, the titanium dioxide and the cationic surfactant are appropriately composited, and porous titanium suboxide can be obtained.

[0080] Examples of the solvent include water and organic solvents. The solvents may be used alone or in combination of two or more.

[0081] Examples of water include ion-exchanged water, distilled water, and ultrapure water.

[0082] The organic solvent is preferably a water-soluble organic solvent, such as an alcohol, a ketone, or a nitrogen-containing solvent.

[0083] Examples of alcohols include monohydric alcohols, polyhydric alcohols, and polyhydric alcohol alkyl ethers. Examples of monohydric alcohols include methanol, ethanol, 1-propanol, 2-propanol, tert-butanol, and diacetone alcohol. Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, propylene glycol (1,2-propanediol), 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, triethylene glycol, glycerin, polyethylene glycol, and polypropylene glycol. Examples of polyhydric alcohol alkyl ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monoisopropyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, tetraethylene glycol monomethyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, and dipropylene glycol monoethyl ether.

[0084] Examples of ketones include acetone and 4-hydroxy-4-methyl-2-pentanone. Examples of nitrogen-containing solvents include N,N-dimethylformamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone.

[0085] The solvent may, for example, contain water. Preferably, the solvent consists of water or contains water and an organic solvent. When the solvent contains water and an organic solvent, it more preferably contains water and a water-soluble organic solvent. Even more preferably, it contains water and an alcohol. Especially preferably, it contains water and a monohydric alcohol. Most preferably, it contains water and 2-propanol.

[0086] An aqueous alkaline solution may be added to the solvent to adjust the pH, such as aqueous ammonia and aqueous sodium hydroxide.

[0087] Examples of chelating agents include acetylacetone and triethanolamine. By premixing the titania source and the chelating agent, the rates of hydrolysis and polycondensation can be controlled, enabling uniform complexation with the surfactant.

[0088] The amount of the titania source mixed per 1 mol of the chelating agent is, for example, 0.1 mol or more, preferably 0.2 mol or more, more preferably 0.3 mol or more, and for example, 3 mol or less, preferably 1 mol or less, more preferably 0.8 mol or less.

[0089] In the preparation step, aging promotes the formation of a complex between titanium dioxide and the cationic surfactant. During aging, the mixed solution may be left to stand or may be stirred. The aging temperature is, for example, 20°C to 150°C, preferably 50°C to 100°C. The aging time is 1 hour to 120 hours.

[0090] After aging, the resulting gel-like solid is collected, and the washing and collection procedure is repeated. The collected sample is then dried to obtain a precursor in which titanium dioxide and a cationic surfactant are combined.

[0091] (Reduction Step) In the reduction step, the precursor is fired in a reducing atmosphere.

[0092] Specifically, a precursor in which titanium dioxide and a cationic surfactant are combined is fired in a state in which a reducing gas is passed through, thereby preparing porous titanium suboxide.

[0093] When a precursor composite of titanium dioxide and a cationic surfactant is calcined in a state where a reducing gas is passed through it, part of the cationic surfactant functions as a reducing agent while thermal decomposition proceeds, which causes a templating effect and enables the production of porous titanium suboxide with a high volume fraction of mesopores.

[0094] Examples of reducing gases include hydrogen gas and carbon monoxide gas. The flow rate of the reducing gas is, for example, 1 mL / min to 1000 mL / min per 1 g of precursor. Note that an inert gas (e.g., argon gas) may be added to the reducing gas. This allows the average oxidation degree of the porous titanium suboxide to be adjusted.

[0095] A heating device such as an electric furnace can be used to calcinate the precursor. The calcination temperature is, for example, 800°C to 1200°C. The calcination time is, for example, 0.5 hours to 10 hours, preferably 4 hours to 10 hours. The calcination time indicates the holding time in the examples. The time required to raise the temperature of the precursor to the calcination temperature is preferably short.

[0096] By adjusting conditions such as the composition of the reducing gas, the flow rate of the reducing gas, the firing temperature, and the firing time, it is possible to adjust the volume fraction of mesopores and the crystallite diameter of the porous titanium suboxide.

[0097] In this manner, porous titanium suboxide can be obtained.

[0098] In this embodiment, the method for producing porous titanium suboxide includes a preparation step and a reduction step, but may also include other steps, such as a step of pulverizing the obtained porous titanium suboxide (pulverization step), a step of washing (washing step), and a step of classifying (classification step), as necessary.

[0099] 3. Anode Catalyst Anode catalysts promote oxidation reactions in polymer electrolyte membrane (PEM) water electrolysis devices. Specifically, the anode catalyst in a PEM water electrolysis device promotes the following water decomposition reaction: ２ O → O ２ +4H ＋ +4e －

[0100] The anode catalyst comprises the porous titanium suboxide and an active metal supported on the porous titanium suboxide. That is, in the anode catalyst, the porous titanium suboxide serves as a support for supporting the active metal.

[0101] [Support] The support in the anode catalyst includes porous titanium suboxide, and is preferably porous titanium suboxide.

[0102] If the support in the anode catalyst contains porous titanium suboxide, the active metal can be accommodated in the mesopores, thereby improving the amount of the active metal supported. Furthermore, by accommodating the active metal in the mesopores, it is possible to prevent the supported active metal from being liberated, thereby improving the durability of the catalyst.

[0103] The support in the anode catalyst may contain a support other than the porous titanium suboxide, but preferably does not contain any other support.

[0104] Examples of other carriers include silica, alumina, titania, and zeolite. Titania refers to titanium dioxide. The other carriers may be used alone or in combination of two or more.

[0105] [Active Metal] The active metal is supported on the catalyst support and promotes the water decomposition reaction, in other words, the active metal has activity in the water decomposition reaction.

[0106] The active metal is not particularly limited as long as it is a metal and / or metal oxide that promotes the water decomposition reaction. The active metal preferably includes a platinum group metal and / or an oxide thereof. The active metal may be used alone or in combination of two or more kinds.

[0107] Examples of platinum group metals include iridium, ruthenium, and palladium. Preferably, iridium is used. That is, the active metal preferably includes iridium and / or iridium oxide.

[0108] The active metal is, for example, a particle. The crystallite size of the active metal particle is, for example, 0.1 nm to 50 nm, preferably 0.5 nm to 40 nm, more preferably 1.0 nm to 30 nm, even more preferably 1.5 nm to 20 nm, and particularly preferably 2.0 nm to 10 nm.

[0109] The crystallite diameter of the active metal particles is, for example, 0.1 nm or more, preferably 0.5 nm or more, more preferably 1.0 nm or more, even more preferably 1.5 nm or more, particularly preferably 2.0 nm or more, and, for example, less than 50 nm, preferably 40 nm or less, more preferably 30 nm or less, even more preferably 20 nm or less, particularly preferably 10 nm or less.

[0110] The crystallite diameter of the active metal, like the crystallite diameter of porous titanium suboxide, can be calculated using the Scherrer equation from the position of the diffraction peak in X-ray diffraction and the half-width of the peak. The active metal typically exhibits a peak in the range of 2θ = 20° to 50°. Specifically, when the active metal is iridium, a peak appears near 2θ = 40.6°, and when the active metal is tetravalent iridium oxide, a peak appears near 2θ = 27.3°. If the active metal peak is not located at a fixed position, the diffraction peaks before and after the active metal is loaded on the porous titanium suboxide are compared, and the peak with the highest detected intensity among the peaks that newly appear after the active metal is loaded on the porous titanium suboxide is considered to be the active metal peak. The active metal particles are crystalline nanoparticles, and the crystallite diameter of the active metal can be approximated to the average primary particle diameter of the active metal.

[0111] When an active metal is subjected to electrolysis of water, particle growth usually occurs. That is, the crystallite size of the active metal particles usually increases when the active metal is subjected to electrolysis of water.

[0112] The crystallite diameter of the active metal particles after the durability evaluation test is larger than the crystallite diameter of the active metal particles before (initial) the durability evaluation test. The change rate (%) of the crystallite diameter of the active metal particles before and after the durability evaluation test is, for example, 70% or less, preferably 60% or less, more preferably 50% or less, and for example, 0% or more. The change rate (%) of the crystallite diameter of the active metal particles before and after the durability evaluation test is calculated by the following formula: Change rate (%) of the crystallite diameter of the active metal particles = (crystallite diameter of the active metal particles after the durability evaluation test - crystallite diameter of the active metal particles before the durability evaluation test) / crystallite diameter of the active metal particles before the durability evaluation test × 100

[0113] Details of the durability evaluation test will be described in the Examples.

[0114] The initial active metal content per 1 g of the anode catalyst is, for example, 0.01 g or more, preferably 0.10 g or more, more preferably 0.20 g or more, still more preferably 0.27 g or more, and particularly preferably 0.30 g or more.

[0115] The initial active metal content per gram of anode catalyst refers to the active metal content per gram of anode catalyst after production, before use as a catalyst.

[0116] From the viewpoint of improving catalytic function, the initial active metal content in the anode catalyst is, for example, 1 mass % or more, preferably 10 mass % or more, more preferably 20 mass % or more, even more preferably 27 mass % or more, and particularly preferably 30 mass % or more. From the viewpoint of cost reduction, the initial active metal content in the anode catalyst is, for example, 70 mass % or less, preferably 60 mass % or less, more preferably 50 mass % or less.

[0117] [Additives] Additives may be added to the anode catalyst as needed.

[0118] The additives are not particularly limited as long as they are additives that are commonly used in anode catalysts, and examples thereof include diluents and binders.

[0119] The diluent component is a substance that is inert to the water splitting reaction, and adding the diluent component to the anode catalyst makes it easier to control the temperature and also increases the surface area of ​​the anode catalyst.

[0120] Examples of the diluent component include alumina (e.g., α-alumina, θ-alumina, γ-alumina, etc.) and titania (e.g., rutile-type titania, anatase-type titania, etc.). Titania refers to titanium dioxide. The diluent component may be used alone or in combination of two or more types.

[0121] The binder is, for example, a binding component for binding the anode catalyst together.

[0122] Examples of binders include silicates, titanates, aluminates, etc. The binders may be used alone or in combination of two or more.

[0123] The ratio of the diluent and binder to be added may be selected arbitrarily.

[0124] 4. Method for Producing Anode Catalyst Next, a method for producing the anode catalyst of the present disclosure will be described.

[0125] The method for producing an anode catalyst includes a step of supporting an active metal on the porous titanium suboxide.

[0126] The step of supporting an active metal on porous titanium suboxide is not particularly limited as long as it is a method commonly used in the production of anode catalysts.

[0127] Specifically, the method for producing an anode catalyst (a step of supporting an active metal on porous titanium suboxide) includes a step (first step) of mixing a slurry containing porous titanium suboxide with a solution containing an active metal to prepare a mixed slurry, and a step (second step) of supporting the active metal on the porous titanium suboxide and then drying the mixture to obtain an anode catalyst.

[0128] (First Step) In the first step, a slurry containing porous titanium suboxide and a solution containing an active metal are mixed to prepare a mixed slurry.

[0129] The slurry containing porous titanium suboxide can be prepared, for example, by dispersing porous titanium suboxide in a solvent.

[0130] The solvent is not particularly limited as long as it can disperse porous titanium suboxide, and examples thereof include water (e.g., ion-exchanged water and ultrapure water) and organic solvents. Examples of organic solvents include alcohol, acetone, dimethyl sulfoxide, dimethylformamide, tetrahydrofuran, and dioxane. Examples of alcohols include monohydric water-soluble alcohols (e.g., methanol, ethanol, and propanol) and dihydric or higher water-soluble alcohols (e.g., ethylene glycol and glycerin). The solvent is preferably an organic solvent, more preferably an alcohol, and even more preferably ethylene glycol.

[0131] The slurry containing porous titanium suboxide may contain additives to improve dispersibility, such as a pH adjuster (e.g., sodium acetate), a chelating agent, and a dispersant.

[0132] The content (concentration) of porous titanium suboxide in the slurry containing porous titanium suboxide is, for example, 0.1 mass% or more, preferably 0.3 mass% or more, more preferably 0.5 mass% or more, and for example, 10 mass% or less, preferably 5 mass% or less, more preferably 3 mass% or less.

[0133] The solution containing the active metal can be prepared, for example, by mixing a salt of the active metal with a solvent.

[0134] Examples of salts of active metals include organic acid salts of active metals and inorganic salts of active metals. Examples of organic acid salts include acetates and oxalates. Examples of inorganic salts include sulfates, nitrates, chlorides, and phosphates. Examples of salts of active metals include preferably inorganic salts of active metals. More preferably, examples of salts of active metals include chlorides of active metals.

[0135] The active metal salt includes a salt of a platinum group metal, preferably an iridium salt, and more preferably iridium chloride. The active metal salt may also include a salt of an active metal other than a platinum group metal.

[0136] The solvent is not particularly limited as long as it is one that is normally used in preparing a solution containing an active metal, and examples thereof include the solvents used in preparing the above-mentioned slurry containing porous titanium suboxide. The solvent used in the solution containing an active metal is preferably water, and more preferably ion-exchanged water.

[0137] A mixed slurry can be prepared by mixing the above-mentioned slurry containing porous titanium suboxide with the above-mentioned solution containing active metal. In preparing the mixed slurry, ultrasonic treatment is preferably carried out in order to improve the dispersibility of the porous titanium suboxide.

[0138] The slurry containing the porous titanium suboxide and the solution containing the active metal are mixed so that the amount of active metal salt per 1 g of porous titanium suboxide is, for example, 0.01 g or more, preferably 0.1 g or more, more preferably 0.3 g or more, even more preferably 0.5 g or more, and for example, 5 g or less, preferably 3 g or less, more preferably 2 g or less, even more preferably 1 g or less.

[0139] In this way, a mixed slurry containing porous titanium suboxide and active metal is obtained.

[0140] (Second Step) In the second step, the active metal is supported on the porous titanium suboxide, and then the porous titanium suboxide is dried to obtain an anode catalyst.

[0141] Specifically, the mixed slurry is heated while being stirred, then cooled to room temperature, and the solid content is recovered by centrifugation, washed, and then dried.

[0142] The heating temperature of the mixed slurry is, for example, 80° C. to 200° C. The heating time of the mixed slurry is, for example, 1 hour to 15 hours.

[0143] Before heating, a reducing agent such as formic acid can be added to the mixed slurry. The reducing agent reduces the active metal ions in the mixed slurry and promotes particle formation.

[0144] The drying temperature is, for example, 80° C. to 150° C. The drying time is, for example, 1 hour to 24 hours.

[0145] In this manner, an anode catalyst can be produced in which an active metal is supported on the porous titanium suboxide.

[0146] In the present embodiment, the method for producing an anode catalyst includes the first step and the second step, but may also include other steps, such as a step of pulverizing the obtained anode catalyst (pulverizing step), a step of classifying the anode catalyst (classifying step), or a step of compression molding the anode catalyst (molding step), as necessary.

[0147] Examples of the shape of the anode catalyst include spherical, pellet, and tablet shapes. A spherical shape is preferred. When the anode catalyst has a spherical shape, the median diameter (D50) of the anode catalyst (the median diameter (D50) of the secondary particles of the anode catalyst) is, for example, 100 μm or less, preferably 80 μm or less, and more preferably 50 μm or less. The median diameter D50 of the anode catalyst is the particle diameter at which the volume cumulative frequency reaches 50% from the small diameter side in the volume-based particle size distribution. The median diameter (D50) of the anode catalyst is determined based on the particle size distribution obtained by laser diffraction / scattering.

[0148] 5. Membrane Electrode Assemblies Membrane electrode assemblies are used in polymer electrolyte membrane (PEM) water electrolysis devices and the like.

[0149] The membrane electrode assembly includes an anode containing the above-described anode catalyst, a cathode containing a cathode catalyst, and an electrolyte membrane disposed between the anode and the cathode.

[0150] [Anode] The anode contains the above-described anode catalyst. The anode further contains an ionomer, if necessary. Preferably, the anode contains the above-described anode catalyst and ionomer.

[0151] The ionomer is not particularly limited. The ionomer has ionic conductivity. Examples of the ionomer include a fluorine-based polymer having a sulfonic acid group and a hydrocarbon-based polymer having a sulfonic acid group. Examples of the fluorine-based polymer having a sulfonic acid group include a perfluorocarbon polymer having a sulfonic acid group. Commercially available fluorine-based polymers having sulfonic acid groups may be used, such as Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Corporation), and Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.). Examples of hydrocarbon-based polymers having sulfonic acid groups include polysulfonic acid, polystyrene sulfonic acid, polyaryl ether ketone sulfonic acid, polyphenyl sulfonic acid, polybenzimidazole sulfonic acid, polybenzimidazole phosphonic acid, and polyimide sulfonic acid.

[0152] The content of the anode catalyst in the anode is, for example, 50 mass% or more, preferably 60 mass% or more, more preferably 70 mass% or more, and even more preferably 80 mass% or more, and for example, 99 mass% or less, preferably 95 mass% or less, and more preferably 90 mass% or less.

[0153] The ionomer content in the anode is, for example, 1 mass% or more, preferably 5 mass% or more, more preferably 10 mass% or more, and for example, 50 mass% or less, preferably 40 mass% or less, more preferably 30 mass% or less, and even more preferably 20 mass% or less.

[0154] The anode has, for example, a sheet shape. In other words, the anode is a catalyst sheet containing the anode catalyst. The dimensions of the anode are adjusted appropriately depending on the application.

[0155] [Cathode] The cathode is not particularly limited as long as it is one that is normally used for water electrolysis.

[0156] The cathode includes, for example, a cathode catalyst, and optionally further includes the above-described ionomer.

[0157] The cathode catalyst includes, for example, a second active metal and a carbon support that supports the second active metal.

[0158] Examples of the second active metal include platinum, gold, silver, palladium, iridium, rhodium, ruthenium, tin, iron, cobalt, nickel, molybdenum, tungsten, vanadium, alloys thereof, and oxides thereof. Examples of the carbon support include graphite carbon, glassy carbon, carbon black, graphene, and carbon nanotubes.

[0159] The cathode has, for example, a sheet shape. In other words, the cathode is a catalyst sheet containing the cathode catalyst. The dimensions of the cathode are adjusted appropriately depending on the application.

[0160] [Electrolyte Membrane] The electrolyte membrane is disposed between the anode and the cathode. The electrolyte membrane is in contact with the anode and also with the cathode. Note that the anode and the cathode do not come into contact with each other because the electrolyte membrane is present between them.

[0161] The electrolyte membrane has ion conductivity.

[0162] Examples of the electrolyte membrane include fluorine-based polymers having sulfonic acid groups and hydrocarbon-based polymers having sulfonic acid groups. Examples of the fluorine-based polymers having sulfonic acid groups include perfluorocarbon polymers having sulfonic acid groups. Commercially available fluorine-based polymers having sulfonic acid groups may be used, such as Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Corporation), and Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.). Examples of the hydrocarbon-based polymers having sulfonic acid groups include polysulfonic acid, polystyrene sulfonic acid, polyaryl ether ketone sulfonic acid, polyphenyl sulfonic acid, polybenzimidazole sulfonic acid, polybenzimidazole phosphonic acid, and polyimide sulfonic acid.

[0163] The electrolyte membrane has a thickness of, for example, 10 μm or more, preferably 20 μm or more, and for example, 300 μm or less, preferably 200 μm or less.

[0164] The membrane electrode assembly may also be used in a water electrolysis cell together with other components such as a gas diffusion layer and a gasket. Specifically, the membrane electrode assembly, the gas diffusion layer, and the gasket are sandwiched between cell components to form a water electrolysis cell.

[0165] The gas diffusion layer is disposed on the opposite side of the electrolyte membrane from the anode. The gas diffusion layer is also disposed on the opposite side of the electrolyte membrane from the cathode. The gasket is disposed around the anode and cathode, not in contact with the electrolyte membrane. In other words, the gasket has a frame shape that surrounds the anode and cathode. The gas diffusion layer allows gas to pass from the anode or cathode. The gasket also prevents gas leakage from the portions of the anode and cathode that are not in contact with the electrolyte membrane.

[0166] The cell member has a metal current collector plate. A commercially available cell member can be used, for example, a JARI standard cell (manufactured by FC Development Co., Ltd.). The cell member sandwiches the membrane electrode assembly, gas diffusion layer, and gasket arranged as described above.

[0167] The gas diffusion layer is, for example, a porous membrane. Different gas diffusion layers may be used on the anode and cathode sides. Examples of the anode-side gas diffusion layer include porous metals (specifically, metal mesh, metal foam, and sintered metal fiber). Examples of the cathode-side gas diffusion layer include carbon paper, carbon cloth, porous ceramic, and nonwoven fabric.

[0168] Examples of gaskets include polyethylene terephthalate sheets, polytetrafluoroethylene sheets, silicone rubber sheets, nitrile rubber sheets, ethylene propylene rubber sheets, and acrylic rubber sheets.

[0169] 6. Method for Producing Membrane Electrode Assembly Next, a method for producing the membrane electrode assembly of the present disclosure will be described.

[0170] The method for producing a membrane electrode assembly includes a step of preparing the above-mentioned anode and cathode (preparation step), and a step of placing an electrolyte membrane between the anode and cathode and applying heat and pressure (lamination step).

[0171] (Preparation Step) In the preparation step, an anode catalyst ink containing an anode catalyst and a cathode catalyst ink containing a cathode catalyst are prepared, and the anode catalyst ink and cathode catalyst ink are applied to a substrate, respectively, and dried to obtain an anode (anode catalyst sheet) and a cathode (cathode catalyst sheet).

[0172] Specifically, first, an anode catalyst ink is prepared by mixing an anode catalyst, an ionomer, and a solvent and dispersing the anode catalyst, and then a cathode catalyst ink is prepared by mixing a cathode catalyst, an ionomer, and a solvent and dispersing the cathode catalyst.

[0173] The solvent is not particularly limited as long as it can disperse the anode catalyst and the ionomer. Examples of the solvent include the solvents described in the preparation step of the method for producing porous titanium suboxide. A preferred example of the solvent is 2-propanol.

[0174] The content (concentration) of the anode catalyst in the anode catalyst ink is, for example, 5.0 mass% or more, preferably 10 mass% or more, more preferably 15 mass% or more, and for example, 50 mass% or less. The content (concentration) of the cathode catalyst in the cathode catalyst ink is, for example, 5.0 mass% or more, preferably 10 mass% or more, more preferably 15 mass% or more, and for example, 50 mass% or less.

[0175] The method for dispersing the anode catalyst and the cathode catalyst is not particularly limited, but from the viewpoint of improving dispersibility, ultrasonic treatment is preferred.

[0176] In this way, an anode catalyst ink and a cathode catalyst ink are obtained.

[0177] Next, the anode catalyst ink and the cathode catalyst ink are each applied to a substrate and dried to obtain an anode (anode catalyst sheet) and a cathode (cathode catalyst sheet).

[0178] The substrate is not particularly limited, but examples thereof include a resin sheet and a metal film. A resin sheet is preferred, and a polytetrafluoroethylene sheet is more preferred.

[0179] There are no particular limitations on the method for applying the anode catalyst ink and the cathode catalyst ink, and the coating thickness is adjusted appropriately depending on the desired thickness of the anode (anode catalyst sheet) and cathode (cathode catalyst sheet).

[0180] The drying temperature and drying time for the anode catalyst ink and the cathode catalyst ink are adjusted appropriately depending on the coating thickness. Drying temperatures are, for example, 30°C to 120°C. Drying times are, for example, 0.5 hours to 24 hours. Drying may be performed in a vacuum environment.

[0181] In this manner, an anode (anode catalyst sheet) and a cathode (cathode catalyst sheet) are prepared. The substrate may be removed before the lamination step, or may be used in the lamination step together with the anode (anode catalyst sheet) and the cathode (cathode catalyst sheet).

[0182] (Laminating Step) In the laminating step, the electrolyte membrane is disposed between the anode and the cathode, and the mixture is heated and pressurized.

[0183] Specifically, a stack is prepared by stacking in order a cushioning material, a heat-resistant material, an anode with a substrate (anode catalyst sheet), an electrolyte membrane, a cathode with a substrate (cathode catalyst sheet), a heat-resistant material, and a cushioning material. The stack is then sandwiched between metal plates and placed in a press machine, where it is heated and pressurized. The electrolyte membrane is in contact with the anode and cathode. After heating and pressing, the cushioning material, heat-resistant material, substrate, and metal plates are removed.

[0184] The cushioning material prevents excessive pressure from being applied to a portion of the laminate due to differences in thickness when pressure is applied (specifically, because the dimensions of the anode and cathode in the planar direction are smaller than the dimensions of the electrolyte membrane, the thickness of the portion where the anode and cathode are arranged is greater than the thickness of the portion where they are not arranged). Examples of the cushioning material include a cushioning material made of polytetrafluoroethylene. Examples of the heat-resistant material include a material that prevents the electrolyte membrane from coming into contact with a metal plate or the like. Examples of the heat-resistant material include a polytetrafluoroethylene sheet. Examples of the metal plate include an aluminum plate.

[0185] The heating and pressurizing conditions are not particularly limited as long as they allow the formation of a membrane electrode assembly. Examples of the heating temperature include 80° C. to 250° C., and examples of the pressure include 0.5 kN to 10 kN.

[0186] In this way, a membrane electrode assembly is obtained which includes an anode, a cathode, and an electrolyte membrane disposed between the anode and the cathode.

[0187] The resulting membrane electrode assembly is sandwiched between cell members together with a gas diffusion layer and a gasket to form a water electrolysis cell.

[0188] 7. Effects and Benefits The porous titanium suboxide of the present disclosure has a ratio of the volume of mesopores with pore diameters of 2 to 50 nm to the total pore volume of 70% or more by volume. Therefore, when such porous titanium suboxide is used as a catalyst support to support an active metal and used as an anode catalyst, it is possible to suppress a decrease in the surface area of ​​the active metal and the release of the active metal due to particle growth of the active metal.

[0189] Furthermore, the porous titanium suboxide has a crystallite diameter of 60.0 nm or more. This improves the conductivity of the porous titanium suboxide. When such porous titanium suboxide is used as a catalyst support to support an active metal and used as an anode catalyst, the entire active metal supported on the porous titanium suboxide can be utilized, thereby reducing the load per active metal particle during use.

[0190] As a result, the durability of the catalyst can be improved.

[0191] The anode catalyst of the present disclosure comprises the porous titanium suboxide and an active metal supported on the porous titanium suboxide. This prevents a decrease in the surface area of ​​the active metal and the release of the active metal due to particle growth. Furthermore, the load per active metal particle during use can be reduced. As a result, catalyst durability can be improved.

[0192] The membrane electrode assembly disclosed herein includes an anode containing the above-described anode catalyst, a cathode containing a cathode catalyst, and an electrolyte membrane disposed between the anode and the cathode. Therefore, when subjected to water electrolysis or the like, it is possible to suppress a decrease in the surface area of ​​the active metal and the liberation of the active metal due to particle growth of the active metal. Furthermore, it is possible to reduce the load per active metal particle during use. As a result, it is possible to improve catalyst durability.

[0193] The method for manufacturing a membrane electrode assembly according to the present disclosure includes the steps of preparing an anode and a cathode, and disposing an electrolyte membrane between the anode and the cathode and applying heat and pressure to the membrane electrode assembly, thereby enabling the membrane electrode assembly to be manufactured through a relatively simple process.

[0194] The porous titanium suboxide of the present disclosure can be used as a support for various catalysts, and is particularly suitable as a support for the anode catalyst of a PEM-type water electrolysis device.

[0195] The present disclosure will be described in more detail below with reference to examples and comparative examples. It should be noted that the present disclosure is not limited to these examples and comparative examples. The specific numerical values ​​of the blending ratios (content ratios), physical property values, parameters, etc. used in the following description can be replaced with the upper limit (a numerical value defined as "equal to or less than") or lower limit (a numerical value defined as "equal to or more than") of the corresponding blending ratios (content ratios), physical property values, parameters, etc. described in the above-mentioned "Modes for Carrying Out the Invention."

[0196] Preparation Example 1 <Preparation of porous titanium suboxide> (Preparation step) 3.38 mol of triethanolamine was mixed with 1.69 mol of titanium tetraisopropoxide while being ice-cooled and stirred to prepare a solution A. In addition, 0.48 mol of 12-dodecanediamine (molecular formula: C) was added to 480 g of ion-exchanged water. １２ H ２８ N ２ (a cationic surfactant having two amino groups in its molecular structure, a carbon chain length of 12, a molar mass of 200.37 g / mol) was mixed to prepare solution B. Next, solutions A and B were mixed and stirred at room temperature for 10 minutes. The resulting mixed solution was transferred to a sealable reaction vessel and allowed to stand in a thermostatic bath at 80°C (aging). After 48 hours, the gel-like solid content in the reaction vessel was removed. The gel-like solid content was washed with ion-exchanged water, and the washed solid content was recovered (centrifugation, 6000 rpm, 20 minutes). Note that the washing and recovery after washing were repeated approximately three times. The recovered sample was then dried at 110°C, yielding 333 g of a light yellowish-brown powder sample (titanium suboxide precursor).

[0197] (Reduction step) 20 g of titanium suboxide precursor was placed in a tubular furnace and reduced at 950° C. while flowing hydrogen gas at 0.5 L / min. The temperature was increased at a rate of 20° C. / min, the holding time was 4 hours, and the material was naturally cooled.

[0198] In this way, porous titanium suboxide was obtained for Preparation Example 1. The obtained porous titanium suboxide was in the form of black particles and weighed 11.0 g.

[0199] Preparation Example 2 (Preparation Step) 2.53 mol of triethanolamine was mixed with 1.27 mol of titanium tetraisopropoxide while being ice-cooled and stirred to prepare Solution A. 0.32 mol of 12-dodecanediamine (molecular formula: C) was added to a mixed solution of 180 g of ion-exchanged water and 180 g of 2-propanol. １２ H ２８ N ２(a cationic surfactant having two amino groups in its molecular structure, a carbon chain length of 12, a molar mass of 200.37 g / mol) was mixed to prepare solution B. Next, solutions A and B were mixed and stirred at room temperature for 10 minutes. The resulting mixed solution was transferred to a sealable reaction vessel and allowed to stand in a thermostatic bath at 80°C (aging). After 24 hours, the gel-like solid content in the reaction vessel was removed. The gel-like solid content was washed with ion-exchanged water, and the washed solid content was recovered (centrifugation, 6000 rpm, 20 minutes). Note that the washing and recovery after washing were repeated approximately three times. The recovered sample was then dried at 110°C, yielding 144 g of a light yellowish-brown powder sample (titanium suboxide precursor).

[0200] (Reduction step) 20 g of titanium suboxide precursor was placed in a tubular furnace and reduced at 950° C. while flowing hydrogen gas at 0.5 L / min. The temperature was increased at a rate of 20° C. / min, the holding time was 4 hours, and the material was naturally cooled.

[0201] In this way, porous titanium suboxide was obtained for Preparation Example 2. The obtained porous titanium suboxide was in the form of black particles and weighed 11.0 g.

[0202] Comparative Preparation Example 1 Porous titanium suboxide of Comparative Preparation Example 1 was obtained in the same manner as Preparation Example 1, except that the retention time in the reduction step was changed to 3 hours. The porous titanium suboxide of Comparative Preparation Example 1 was in the form of black particles and weighed 10.3 g.

[0203] Comparative Preparation Example 2 Porous titanium suboxide of Comparative Preparation Example 2 was obtained in the same manner as Preparation Example 2, except that the retention time in the reduction step was changed to 3 hours. The porous titanium suboxide of Comparative Preparation Example 2 was in the form of black particles and weighed 10.4 g.

[0204] Comparative Preparation Example 3 Commercially available titanium oxide nanoparticles (trade name: P25, TiO ２ 10.0 g of nanoparticles (manufactured by Evonic) were placed in a tubular furnace and subjected to reduction treatment at 950° C. while flowing hydrogen gas at 0.5 L / min. The heating rate was 20° C. / min, the holding time was 3 hours, and cooling was carried out naturally.

[0205] In this way, porous titanium suboxide was obtained as Comparative Preparation Example 3. The obtained porous titanium suboxide was in the form of black particles and weighed 9.49 g.

[0206] Comparative Preparation Example 4 (Preparation Step) 0.14 mol of triethanolamine was mixed with 0.07 mol of titanium tetraisopropoxide while being ice-cooled and stirred to prepare Solution A. Also, polyethylene glycol 200 (molecular formula H(OCH ２ CH ２ ) ｎ (OH, molar mass 180 g / mol to 220 g / mol, compound having hydrophilic hydroxyl groups and hydrophobic polyethylene chains in its molecular structure) were mixed to prepare Solution B. Next, Solutions A and B were mixed and stirred at room temperature for 10 minutes. The resulting mixed solution was transferred to a sealable reaction vessel and allowed to stand in a thermostatic bath at 80°C (aging). After 24 hours, the gel-like solid content in the reaction vessel was removed. The gel-like solid content was washed with ion-exchanged water, and the washed solid content was recovered (centrifugation, 6000 rpm, 20 minutes). Note that the washing and recovery after washing were repeated approximately three times. The recovered sample was then dried at 110°C, yielding 12.52 g of a light yellowish-brown powder sample (titanium suboxide precursor).

[0207] (Reduction step) The obtained titanium suboxide precursor was packed into a tubular furnace and subjected to reduction treatment at 950° C. while flowing hydrogen gas at 0.5 L / min. The temperature was increased at a rate of 20° C. / min, the holding time was 3 hours, and cooling was carried out by natural cooling.

[0208] In this way, porous titanium suboxide was obtained as Comparative Preparation Example 4. The obtained porous titanium suboxide was in the form of black particles and weighed 6.27 g.

[0209] Example 1 A membrane electrode assembly of Example 1 was produced according to the following procedure.

[0210] [Preparation of Anode Catalyst] An anode catalyst was prepared using the porous titanium suboxide of Preparation Example 1. Specifically, 85.0 g of ethylene glycol was placed in a beaker, 1.0 g of sodium acetate was mixed, and 0.75 g of the porous titanium suboxide of Preparation Example 1 was added. Next, an iridium solution was prepared by mixing 0.5 g of iridium chloride with 5.75 g of ion-exchanged water. This iridium solution was added to the beaker and subjected to ultrasonic treatment for 90 minutes. After ultrasonic treatment, 103.7 g of formic acid was added to the beaker, and the mixture was heated at 120°C for 5 hours while stirring. After heating, the reaction solution was cooled to room temperature, and the solid content was recovered using a centrifuge. Next, the solid content was washed with 30 g of deionized water and 30 g of ethanol and dried overnight in a dryer at 110°C. In this way, iridium oxide (IrO ２ ) was supported on the anode catalyst.

[0211] [Preparation of Cathode Catalyst] A fuel cell / water electrolysis (PEM type) electrode catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Co., Ltd.) was used as the cathode catalyst. The cathode catalyst was a carbon support carrying 50 mass % of platinum.

[0212] (Preparation Step) 0.094 g of Nafion (registered trademark) dispersion solution (solid content 20% by mass, DE2020 CS type, manufactured by Fujifilm Wako Chemical Co., Ltd.) and 0.350 g of 2-propanol were mixed in a beaker, and 0.131 g of the above anode catalyst was further added and stirred at 700 rpm for 10 minutes. Next, ultrasonic treatment (40 kHz) was performed for 30 minutes, stirring at 700 rpm for 10 minutes, ultrasonic treatment (40 kHz) for 15 minutes, and stirring at 700 rpm for 10 minutes to prepare an anode catalyst ink.

[0213] Next, a polytetrafluoroethylene sheet (Nitoflon, manufactured by Nitto Denko Corporation) was placed as a base material on a substrate for a bar coater, and the above-mentioned anode catalyst ink was applied in a linear pattern. Next, a doctor blade (slit 101 μm) was drawn at a speed of 50 mm / s to apply the anode catalyst ink. Thereafter, the sheet was left to stand at room temperature for 5 minutes, and then vacuum-dried in a vacuum drying oven at 80°C for 1 hour to obtain an anode (anode catalyst sheet).

[0214] Furthermore, 0.245 g of a Nafion (registered trademark) dispersion solution (solid content 20% by mass, DE2020 CS type, manufactured by Fujifilm Wako Chemical Co., Ltd.) and 1.161 g of 2-propanol were added to a beaker and mixed, and 0.101 g of the above cathode catalyst was further added, and the mixture was dispersed in the same manner as for the above anode catalyst to prepare a cathode catalyst ink. Next, a cathode (cathode catalyst sheet) was obtained in the same manner as for preparing the anode (anode catalyst sheet), except that the cathode catalyst ink was used.

[0215] (Lamination Step) Two 8.0 cm x 8.0 cm Gore gaskets (GORE (registered trademark) Hypersheet (registered trademark) Gasket, manufactured by Gore) serving as cushioning materials, two 9.0 cm x 9.0 cm polytetrafluoroethylene sheets (NITOFLON, manufactured by Nitto Denko Corporation) serving as heat-resistant materials, an anode (1.0 cm x 1.0 cm) with a substrate, a cathode (1.0 cm x 1.0 cm) with a substrate, and an 8.0 cm x 8.0 cm electrolyte membrane (N115, manufactured by Chemours).

[0216] Next, a laminate was prepared by stacking a cushioning material, a heat-resistant material, an anode with a substrate, an electrolyte membrane, a cathode with a substrate, a heat-resistant material, and a cushioning material in this order, and the laminate was sandwiched between aluminum plates. In the planar direction perpendicular to the thickness direction, the anode and cathode were positioned at the center of the electrolyte membrane, and the anode and cathode were in contact with the electrolyte membrane. This was placed in a hot press and heated and pressed at 160°C and 0.98 kN for 11 minutes, and then heated and pressed at 160°C and 6.37 kN for 5 minutes. The cushioning material, heat-resistant material, substrate, and aluminum plate were then removed to obtain a membrane electrode assembly of Example 1, which included an anode, a cathode, and an electrolyte membrane disposed between the anode and cathode.

[0217] Example 2 As shown in Table 1, a membrane electrode assembly of Example 2 was obtained in the same manner as in Example 1, except that the porous titanium suboxide of Preparation Example 2 was used.

[0218] Comparative Examples 1 to 4 As shown in Table 1, membrane electrode assemblies of Comparative Examples 1 to 4 were obtained in the same manner as in Example 1, except that the porous titanium suboxides of Comparative Preparation Examples 1 to 4 were used, respectively.

[0219] <Evaluation> [Evaluation of X-ray diffraction pattern] For the porous titanium suboxide of each Preparation Example and each Comparative Preparation Example, the powder X-ray diffraction pattern was measured using an X-ray diffractometer (product name: Ultima IV, manufactured by Rigaku Corporation). The conditions were as follows. {Measurement conditions} X-ray source: CuKα radiation Scanning range: 10° to 60° (2θ range) Scanning speed: 5.0° / min Voltage: 40 kV Current: 40 mA

[0220] In each of the porous titanium suboxides in the preparation examples and the comparative preparation examples, Ti ４ O ７ The diffraction peaks due to Ti were clearly observed. ４ O ７ It was confirmed that Ti ４ O ７ The crystallite diameters calculated from the half-width of the diffraction peaks derived from the TiAlN alloy are shown in Table 1, and it was confirmed that the crystallites were aggregates of fine crystals. ４ O ７ It was confirmed that the substance contained titanium suboxide TiOx (x = 1.5 to 2.0) with different oxidation degrees, rather than the structure. ４ O ７ The crystallite diameters calculated based on the peak information at around 2θ = 20.8°, which is a peak specific to the structure, are shown in Table 1. As an example of the analysis results, the X-ray diffraction patterns of the porous titanium suboxides prepared in Preparation Example 1 and Comparative Preparation Example 2 are shown in Figure 1.

[0221] [Specific Surface Area, Pore Volume, and Moderate Pore Diameter] The specific surface area, pore volume, and moderate pore diameter of the porous titanium suboxide of each Preparation Example and each Comparative Preparation Example were evaluated using a specific surface area / pore distribution measuring device (product name: BELSORP MAX, manufactured by Microtrac-Bell). Specifically, 100 to 200 mg of porous titanium suboxide was placed in a measurement cell, and the nitrogen adsorption isotherm at 77 K was analyzed to calculate the specific surface area, pore volume, and moderate pore diameter. The values ​​of the specific surface area, pore volume, and moderate pore diameter were calculated using BET analysis. The volume ratio of each pore (micropores, mesopores, and macropores) to the total pore volume was also calculated.

[0222] Specifically, the specific surface area and pore size distribution analyzer measures the specific surface area of ​​a sample by exposing porous titanium suboxide to nitrogen gas and observing changes in the amount of nitrogen adsorption while varying the pressure or temperature. It also measures the pore size distribution based on the condensation of nitrogen gas molecules. When nitrogen gas is exposed to cooled porous titanium suboxide, nitrogen molecules first adsorb onto the pore surfaces. Even after the entire surface is covered with a monolayer of nitrogen molecules, further increasing the nitrogen gas pressure causes adsorption of nitrogen molecules, filling the pores. As a result of the adsorption of a large number of nitrogen molecules within the pores, condensation of the nitrogen molecules from gas to liquid occurs at a certain pressure. The pressure and adsorption amount at which condensation occurs can be used to evaluate the pore volume and pore size. In this study, the region where the relative pressure of nitrogen gas, p / P, at 77 K, is 0 to 0.20 or less was defined as the micropore region. The amount of nitrogen adsorption at p / P = 0.20 was converted to liquid density to calculate the micropore volume. The mesopore volume was calculated by subtracting the micropore volume from the pore volume calculated from the nitrogen adsorption amount at p / P0 = 0.96, which was defined as the mesopore region. Similarly, the macropore region was defined as the relative pressure p / P0 = 0.96 to 0.99, which was defined as the macropore region. The mesopore volume was calculated by subtracting the micropore volume and mesopore solution volume from the pore volume calculated from the nitrogen adsorption amount at p / P0 = 0.99. The results are shown in Table 1. Figure 2 shows graphs of nitrogen adsorption isotherms for Preparation Example 1 and Comparative Preparation Example 2, and Figure 3 shows graphs of pore size distributions for Preparation Example 1 and Comparative Preparation Example 2.

[0223] [Evaluation of Initial Electrolysis Performance] The initial electrolysis performance of the membrane electrode assemblies of each example and each comparative example was evaluated using an electrochemical measurement device (Hz-Pro, manufactured by Meiden Hokuto Corporation).

[0224] Specifically, the membrane electrode assembly of each example and each comparative example, an anode-side gas diffusion layer (metal fiber sintered body, Currento (registered trademark), manufactured by Bekaert), a cathode-side gas diffusion layer (carbon paper, TGP-H-060, manufactured by Toray Industries, Inc.), two polytetrafluoroethylene sheets (Nitoflon, manufactured by Nitto Denko Corporation) as gaskets, and a JARI cell (electrode area 1 cm ２ The gasket was in the shape of a frame having a through hole in the center in the plane direction perpendicular to the thickness direction.

[0225] On the anode side of the membrane electrode assembly, an anode-side gas diffusion layer was placed in contact with the anode, and a gasket was placed so that the through-holes of the gasket overlapped the anode. On the cathode side of the membrane electrode assembly, a cathode-side gas diffusion layer was placed in contact with the cathode, and a gasket was placed so that the through-holes of the gasket overlapped the cathode. These were sandwiched between JARI cells and tightened with a tightening torque of 4 N m to obtain a water electrolysis cell.

[0226] The obtained water electrolysis cell was connected to the above-mentioned electrochemical measurement device. Next, pure water at 80°C was supplied to the anode side of the water electrolysis cell, and after the cell temperature was stabilized at 80°C, an electrolysis performance test (0 A / cm ２ From 2 A / cm ２ A test was conducted in which a constant current was applied stepwise up to 2 A / cm, the cell voltage was measured, and the current-voltage characteristics were obtained. The initial electrolysis performance obtained was evaluated according to the following criteria. The results are shown in Table 1. {Criteria} A: 2 A / cm ２ B: The cell voltage is less than 2.5 V when a current of 2 A / cm is applied. ２ When a current of 2.5V is applied, the cell voltage is 2.5V or more.

[0227] [Durability Evaluation Test] The durability of the membrane electrode assembly (durability of the anode catalyst) was evaluated using an electrochemical measurement device (Hz-Pro, manufactured by Meiden Hokuto Co., Ltd.) for the membrane electrode assembly of each Example and Comparative Examples 1 and 2. The evaluation of the durability of the membrane electrode assembly (durability of the anode catalyst) is an index for confirming the suppression of deterioration in catalyst performance due to use.

[0228] Specifically, a water electrolysis cell was prepared and connected to the electrochemical measurement device using the same procedure as in the initial electrolysis performance evaluation described above. Next, pure water at 80°C was supplied to the anode side of the water electrolysis cell. After the cell temperature stabilized at 80°C, a start-stop cycle test was performed. The start-stop cycle test was performed with a current of 4.0 A / cm. ２ The test consisted of a start-up step in which the voltage was controlled to 0.1 V and held for 10 seconds, and a stop step in which the voltage was controlled to drop from 2 V to 0.1 V over 4 seconds and then held at 0.1 V for 10 seconds. The cycle from the start-up step to the stop step was repeated 40,000 times. The average voltage during the start-up step was measured for each cycle.

[0229] 4 shows a graph plotting the number of cycles on the horizontal axis and the change in average voltage (ΔV) on the vertical axis for the membrane electrode assemblies of Examples 1 and 2 and Comparative Examples 1 and 2. The change in average voltage (ΔV) is calculated using the following formula: Change in average voltage (ΔV) = average voltage at the startup step in each cycle - average voltage at the startup step in the first cycle

[0230] 4 shows the slope from 20,000 cycles to 40,000 cycles for the membrane electrode assemblies of each Example and Comparative Examples 1 and 2. The slope from 20,000 cycles to 40,000 cycles indicates the rate of voltage increase per cycle from 20,000 cycles to 40,000 cycles. Table 1 shows the slope from 20,000 cycles to 40,000 cycles as the rate of increase in electrolysis voltage.

[0231] A high rate of voltage rise per cycle indicates a rapid increase in voltage during long-term operation. Such a high rate of electrolysis voltage rise suggests the progression of degradation of the membrane electrode assembly. In other words, when a membrane electrode assembly with a high rate of electrolysis voltage rise is used, it is judged that the PEM water electrolysis device may reach the end of its life earlier than its normal service life.

[0232] Furthermore, Comparative Examples 3 and 4 did not meet the criteria in the initial electrolysis performance evaluation, and therefore no durability evaluation test was carried out.

[0233] [Crystallite diameter of active metal] In the membrane electrode assemblies of each Example and Comparative Examples 1 and 2, the crystallite diameter of the active metal of the anode catalyst was calculated by crystallite diameter analysis using X-ray diffraction method.

[0234] Specifically, before being subjected to the durability evaluation test, the crystallite size of the active metal in the anode catalyst was measured for the membrane electrode assemblies of each Example and Comparative Examples 1 and 2. The results are shown in Table 1 ("Initial active metal crystallite size" in Table 1).

[0235] Next, after the durability evaluation test, the crystallite diameter of the active metal in the anode catalyst was measured for each of the membrane electrode assemblies of Examples and Comparative Examples 1 and 2. The results are shown in Table 1 ("Crystallite diameter of active metal after durability evaluation test" in Table 1).

[0236] Furthermore, the percentage change in crystallite size of the active metal was calculated from the initial particle size of the active metal and the particle size of the active metal after the durability evaluation test according to the following formula. The results are shown in Table 1. Percent change in crystallite size of active metal = (crystallite size of active metal after durability evaluation test - initial crystallite size of active metal) / initial crystallite size of active metal × 100

[0237] In the membrane electrode assemblies of Comparative Examples 3 and 4, the crystallite diameter of the active metal of the anode catalyst could not be calculated by crystallite diameter analysis using X-ray diffraction because the peaks in the XRD spectrum could not be distinguished. Therefore, the average primary particle diameter of 50 or more particles was measured by observation using a transmission electron microscope (TEM), and the average value was calculated as the crystallite diameter. Furthermore, since the membrane electrode assemblies of Comparative Examples 3 and 4 were not subjected to a durability evaluation test, the crystallite diameter of the active metal and the rate of change in the crystallite diameter of the active metal after the durability evaluation test were not calculated.

[0238]

[0239] <Discussion> As shown in Table 1 and Figures 2 and 3, the porous titanium suboxide used in Examples 1 and 2 had a crystallite diameter of 60 nm or more and a mesopore volume ratio of 70 vol% or more to the total pore volume. Therefore, in Examples 1 and 2, the rate of change in the active metal particle diameter was small, and the electrolysis voltage rise rate was low. In other words, it can be inferred that the reduction in the active metal surface area and the release of the active metal due to active metal particle growth can be suppressed, and the improved conductivity can reduce the load per active metal particle during use. This suggests that the catalyst durability was improved. In contrast, the porous titanium suboxide used in Comparative Examples 1, 2, and 4 had a crystallite diameter of less than 60 nm. Furthermore, the porous titanium suboxide used in Comparative Examples 3 and 4 had a mesopore volume ratio of less than 70 vol% to the total pore volume. Therefore, Comparative Examples 3 and 4 did not meet the criteria for the initial electrolysis evaluation, and Comparative Examples 1 and 2 had a high electrolysis voltage rise rate. That is, it was suggested that Comparative Examples 3 and 4 did not have catalytic performance at a level that would allow practical use, and Comparative Examples 1 and 2 were inferior in catalytic durability.

[0240] The above invention is provided as an exemplary embodiment of the present disclosure, but this is merely an example and should not be interpreted as limiting. Modifications of the present disclosure that are obvious to those skilled in the art are intended to be included in the scope of the following claims.

[0241] The porous titanium suboxide can be used as a support for various catalysts, and is particularly suitable as a support for an anode catalyst in a PEM-type water electrolysis device.

Claims

1. Porous titanium suboxide having a crystallite diameter of 60.0 nm or more, and a ratio of the volume of mesopores with a pore diameter of more than 2 nm and less than 50 nm to the total pore volume of 70 vol % or more.

2. The porous titanium suboxide according to claim 1, wherein the ratio of the volume of micropores having a pore diameter of 2 nm or less to the total pore volume is 20% by volume or less.

3. The porous titanium suboxide according to claim 1, wherein the ratio of the volume of macropores having a pore diameter of 50 nm or more to the total pore volume is less than 30% by volume.

4. The porous titanium suboxide (TiO Ｘ 2. The porous titanium suboxide according to claim 1, wherein the average oxidation degree (X) of the porous titanium suboxide is greater than 1.70 and less than 1.

85.

5. An anode catalyst comprising: the porous titanium suboxide according to any one of claims 1 to 4; and an active metal supported on said porous titanium suboxide, wherein said active metal comprises a platinum group metal and / or an oxide thereof.

6. The anode catalyst according to claim 5, wherein the active metal comprises iridium and / or iridium oxide.

7. A membrane electrode assembly comprising: an anode containing the anode catalyst according to claim 5; a cathode containing a cathode catalyst; and an electrolyte membrane disposed between the anode and the cathode.

8. The membrane electrode assembly according to claim 7, wherein the cathode catalyst comprises a second active metal and a carbon support supporting the second active metal.

9. The membrane electrode assembly according to claim 7, wherein the electrolyte membrane is a fluorine-based polymer having sulfonic acid groups.

10. The membrane electrode assembly of claim 7, wherein the anode further comprises an ionomer.

11. A method for producing the membrane electrode assembly according to claim 7, comprising the steps of: preparing the anode and the cathode; and disposing an electrolyte membrane between the anode and the cathode, and applying heat and pressure.

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