Catalyst for water electrolysis, membrane-electrode assembly, water electrolysis module, and water electrolyzer

Abstract

Provided: are a catalyst for water electrolysis comprising a mixture of iron-containing oxide particles having a powder volume resistivity of 1.0 × 103 Ω·cm or above and conductive particles having a powder volume resistivity of 5.0 Ω·cm or below; and a membrane-electrode assembly, a water electrolysis module, and a water electrolyzer using the catalyst for water electrolysis.

Description

Catalysts for water electrolysis, membrane electrode assemblies, modules for water electrolysis, and water electrolysis devices

[0001] The present invention relates to a water electrolysis catalyst, a membrane electrode assembly, a water electrolysis module, and a water electrolysis device.

[0002] Hydrogen is a clean energy source that does not emit carbon dioxide and is used, for example, as fuel for fuel cell vehicles and household fuel cells. Water electrolysis (electrolysis) is a well-known method for producing hydrogen. Hydrogen can be produced without carbon dioxide emissions by electrolyzing water using a renewable energy power generation system as a power source. Therefore, hydrogen is increasingly attracting attention as a fundamental energy source for a sustainable society.

[0003] Alkaline water electrolysis (AWE), which uses a highly concentrated alkaline aqueous solution as the electrolyte, is a known practical water electrolysis technology. In alkaline water electrolysis, hydrogen bubbles (2H ２ O + 2e － →H ２ +2OH － ) to the anode side, and also prevents the bubble-like oxygen (4OH) generated at the anode (positive electrode). － →O ２ +2H ２ O+4e － In order to prevent OH from migrating to the cathode side, a gas barrier separator (membrane) is disposed between the cathode and the anode. － Anion-conducting membrane water electrolysis (AEMWE) has also been proposed, which uses an anion-conducting membrane to improve the efficiency of the conduction (transfer) of hydroxy ions. AEMWE can conduct hydroxy ions from the cathode to the anode with high efficiency without using a highly concentrated alkaline aqueous solution as the electrolyte.

[0004] The electrodes (anode catalyst layer and cathode catalyst layer) of an AEMWE are generally formed by mixing a catalyst in which a noble metal, metal oxide, or the like is optionally supported on a carrier such as carbon, an anionic ionomer that functions as both a binder and an ion conductor, and a liquid medium, applying the mixture to a gas-permeable and electronically conductive conductive substrate (gas diffusion layer), and drying the coating to remove the liquid medium. Next, an anion conductive membrane containing a hydroxy ion-conducting polymer is sandwiched between the formed anode catalyst layer and cathode catalyst layer, and the anion conductive membrane is joined to the anode catalyst layer and the cathode catalyst layer by thermocompression bonding under pressure, thereby obtaining a membrane electrode assembly (MEA) for an AEMWE.

[0005] As described above, catalysts in which a noble metal, metal oxide, or the like is supported on a support such as carbon as necessary have been used as catalysts for water electrolysis. For example, Patent Document 1 discloses that a first catalyst layer (anode) of a solid electrolytic element is formed from a first supported catalyst formed from a first support of a conductive metal oxide and a first catalyst of a platinum group metal, and a titanium porous substrate, and that a second catalyst layer (cathode) is formed from a second support of carbon powder, a second catalyst of a platinum group metal, and a porous substrate. The technology described in Patent Document 1 is said to prevent passivation of the anode and anode-side catalyst due to oxygen generated when current is applied on the anode side, and to prevent ionization and elution of the cathode when current is not applied on the cathode side.

[0006] Japanese Patent Application Publication No. 9-291384

[0007] Conventionally, catalysts used in water electrolysis often contain expensive noble metals in order to achieve high catalytic activity. For example, in the technology described in Patent Document 1, both the anode and the cathode contain platinum group metals, which limits cost reduction. Furthermore, catalysts are required to have not only excellent catalytic activity but also durability that allows them to maintain catalytic activity even when water electrolysis is repeated. Catalysts containing iron element have been investigated as inexpensive catalysts, but sufficient catalytic activity and durability have not yet been achieved.

[0008] An object of the present invention is to provide a water electrolysis catalyst that contains an inexpensive iron-containing oxide instead of a precious metal, yet has excellent catalytic activity and is resistant to deterioration in catalytic activity even with repeated use. Another object of the present invention is to provide a membrane electrode assembly, a water electrolysis module, and a water electrolysis device that use the water electrolysis catalyst.

[0009] The above-mentioned problems of the present invention are solved by the following means: [1] A volume resistivity of 1.0×10 ３ A water electrolysis catalyst comprising a mixture of iron-containing oxide particles having a volume resistivity of 5.0 Ω cm or less and conductive particles having a volume resistivity of 5.0 Ω cm or less. [2] The water electrolysis catalyst according to [1], wherein the conductive particles comprise at least one of carbon particles, metal particles, and metal oxide particles. [3] The water electrolysis catalyst according to [1] or [2], wherein the conductive particles comprise carbon particles. [4] The water electrolysis catalyst according to [2] or [3], wherein the carbon particles are at least one of carbon nanotubes and carbon particles having a hollow shell structure. [5] The water electrolysis catalyst according to any one of [1] to [4], wherein the ratio of the content of the conductive particles to the content of the iron-containing oxide particles in the water electrolysis catalyst is 0.01 to 30.00 by volume. [6] The water electrolysis catalyst according to any one of [1] to [4], wherein the ratio of the content of the conductive particles to the content of the iron-containing oxide particles in the water electrolysis catalyst is 0.01 to 30.00 by volume. [7] The water electrolysis catalyst according to any one of [1] to [4], wherein the specific surface area of ​​the iron-containing oxide particles is 1.0 m ２ / g or more. [7] The water electrolysis catalyst according to any one of [1] to [6], wherein the primary particle size of the iron-containing oxide particles is 10.000 μm or less. [8] The water electrolysis catalyst according to any one of [1] to [7], wherein the iron-containing oxide particles contain at least one of barium, calcium, strontium, nickel, titanium, sodium, magnesium, cobalt, copper, manganese, and zinc. [9] The water electrolysis catalyst according to any one of [1] to [8], wherein the iron-containing oxide particles have a hexagonal close-packed structure, a spinel structure, or a perovskite structure.

[10] The water electrolysis catalyst according to any one of [1] to [9], wherein the iron-containing oxide particles contain at least one of barium ferrite, calcium ferrite, and strontium ferrite.

[11] The water electrolysis catalyst according to any one of [1] to [9], wherein the specific surface area of ​​the iron-containing oxide particles is 80.0 m ２ / g or more.

[12] A membrane / electrode assembly having a catalyst layer in contact with at least one surface of an anion conductive membrane, the catalyst layer comprising the water electrolysis catalyst according to any one of [1] to

[11] .

[13] The membrane / electrode assembly according to

[12] , further comprising a gas diffusion layer on the catalyst layer opposite to the anion conductive membrane.

[14] A water electrolysis module comprising the membrane / electrode assembly according to

[12] or

[13] , an electrolyte solution flow path for supplying and / or recovering an electrolyte solution to the membrane / electrode assembly via the gas diffusion layer of the membrane / electrode assembly, and a gas-liquid separation mechanism connected to the electrolyte solution flow path.

[15] A water electrolysis device comprising the membrane / electrode assembly according to

[12] or

[13] or the water electrolysis module according to

[14] .

[0010] The water electrolysis catalyst of the present invention has excellent catalytic activity despite containing elemental iron, and can form a highly durable catalyst layer that is resistant to deterioration in catalytic activity even with repeated use. The water electrolysis catalyst of the present invention is suitable as a catalyst for the membrane electrode assembly, water electrolysis module, and water electrolysis device of the present invention.

[0011] FIG. 1 is a longitudinal sectional view showing a schematic diagram of a basic layer structure of one embodiment of a membrane electrode assembly according to the present invention.

[0012] [Water electrolysis catalyst] The water electrolysis catalyst of the present invention is suitable as a catalyst for use in water electrolysis. The water electrolysis catalyst of the present invention can be widely used, for example, as a water electrolysis catalyst in alkaline water electrolysis. The water electrolysis catalyst of the present invention has a volume resistivity of 1.0 × 10 ３ The water electrolysis catalyst of the present invention comprises a mixture of iron-containing oxide particles having a volume resistivity of 1.0×10 or more and conductive particles having a volume resistivity of 5.0 Ω·cm or less. The water electrolysis catalyst of the present invention is a mixture of the iron-containing oxide particles and the conductive particles, and is not a composite in which the iron-containing oxide particles and the conductive particles are integrated together. The water electrolysis catalyst of the present invention may further contain other components (e.g., polymers, inorganic particles, etc.) in addition to the iron-containing oxide particles and the conductive particles. The water electrolysis catalyst of the present invention comprises a mixture of iron-containing oxide particles and conductive particles having a volume resistivity of 1.0×10 or more and conductive particles having a volume resistivity of 5.0 Ω·cm or less. ３The content of the mixture of iron-containing oxide particles with a volume resistivity of Ω cm or more and conductive particles with a volume resistivity of 5.0 Ω cm or less is preferably 60 mass % or more, and more preferably 70 mass % or more. When the content of this mixture is less than 100 mass %, the remainder may contain polymers, inorganic particles, etc., as described below. The components contained in the water electrolysis catalyst of the present invention will be described in more detail.

[0013] <Volume resistivity is 1.0 × 10 ３ Iron-containing oxide particles with a volume resistivity of 1.0 × 10 ３ The iron-containing oxide particles (hereinafter referred to as "iron-containing oxide particles (a)") having a resistivity of Ω cm or more are not particularly limited as long as they are particles of an oxide containing iron element. In the present invention, the "volume resistivity" refers to the powder volume resistivity at 25°C, and can be measured by the method described in the Examples. The volume resistivity of the iron-containing oxide particles (a) is 1.0 x 10 ３ Ω cm or more, and 3.0 × 10 ３ Ω cm or more is preferable, and 1.0 × 10 ４ More preferably, 1.0×10 ５ More preferably, it is 1.0×10 ６ The upper limit is not particularly limited, and is preferably 1.0×10 １０ Therefore, the volume resistivity of the iron-containing oxide particles (a) is 1.0×10 ３ ~1 x 10 １０ Ω cm is preferred, and 3.0×10 ３ ~1.0 x 10 ９ Ω cm is more preferable, and 1.0×10 ４ ~1.0 x 10 ８ Ω cm is more preferable, and 1.0×10 ５ ~1.0 x 10 ８ Ω cm is more preferable, and 1.0×10 ６ ~1.0 x 10 ８Ω·cm is more preferable. The iron-containing oxide particles (a) may be used singly or in combination of two or more types. When the iron-containing oxide particles (a) are composed of two or more types of iron-containing oxide particles, the mixture of the two or more types of iron-containing oxide particles exhibits the above-mentioned volume resistivity. The volume resistivity of the iron-containing oxide particles (a) can be controlled by adjusting the elemental composition, particle size, specific surface area, etc. of the iron-containing oxide particles (a).

[0014] The specific surface area (BET specific surface area) of the iron-containing oxide particles (a) is not particularly limited, and is not less than 1.0 m ２ / g or more is preferable, and 10.0m ２ / g or more, and 30.0m ２ / g or more, and more preferably 50.0m ２ / g or more, and more preferably 80.0m ２ / g or more is more preferable. ２ Therefore, the specific surface area of ​​the iron-containing oxide particles (a) is 1.0 to 500.0 m / g. ２ / g is preferred, and 10.0 to 400.0 m ２ / g is more preferable, and 30.0 to 300.0 m ２ / g is more preferable, and 50.0 to 250.0 m ２ / g is more preferable, and 80.0 to 150.0 m ２ The specific surface area of ​​the iron-containing oxide particles (a) can be measured by the method described in the Examples.

[0015] The iron-containing oxide particles (a) may be oxide particles containing iron oxide (iron (II) oxide or iron (III) oxide) as a main component and exhibiting the above volume resistivity, and are preferably particles of ceramics (ferrite) containing iron oxide as a main component. The term "iron-containing oxide" includes iron hydroxide (iron (II) hydroxide (Fe(OH) ２The iron-containing oxide particles (a) are preferably particles of an iron-containing composite oxide represented by a composition formula containing iron and a metal element other than iron. Examples of metal elements other than iron that the iron-containing oxide particles (a) may contain include barium, calcium, strontium, nickel, titanium, sodium, magnesium, cobalt, copper, manganese, zinc, and yttrium. Among these, it is preferable that the iron-containing oxide particles (a) contain at least one of barium, calcium, strontium, nickel, titanium, sodium, magnesium, cobalt, copper, manganese, and zinc, more preferably at least one of barium, strontium, nickel, and calcium, and even more preferably at least one of barium and strontium. The crystal structure of the iron-containing oxide particles (a) is not particularly limited. The crystal structure of the iron-containing oxide particles (a) is a hexagonal close-packed structure (M Ａ Fe １２ O １９ ), spinel structure (M Ａ Fe ２ O ４ ), perovskite structure (M Ａ FeO ３ ), and garnet structure (M Ａ Fe ５ O １２ In each of the above crystal structures, M Ａ indicates the "metal elements other than iron that may be contained in the iron-containing oxide particles (a)" that can constitute the above-mentioned respective crystal structures. The crystal structure of the iron-containing oxide particles (a) preferably has a hexagonal close-packed structure, a spinel structure, or a perovskite structure. Specific examples of the iron-containing oxide particles (a) include the following iron-containing composite oxide particles: Barium ferrite: BaFe １２ O １９ , BaFeO ３ -d Strontium ferrite: SrFe １２ O １９ Nickel ferrite: NiFe ２ O ４ Calcium ferrite: CaFe ２O ４ Yttrium ferrite: Y ３ Fe ５ O １２

[0016] The primary particle size of the iron-containing oxide particles (a) is not particularly limited. The primary particle size of the iron-containing oxide particles (a) is preferably 10.000 μm or less, more preferably 5.000 μm or less, more preferably 3.000 μm or less, more preferably 2.500 μm or less, even more preferably 2.000 μm or less, still more preferably 1.000 μm or less, even more preferably 0.500 μm or less, still more preferably 0.200 μm or less, even more preferably 0.100 μm or less, and still more preferably 0.050 μm or less. The lower limit is not particularly limited, and 0.001 μm is practical. Therefore, the primary particle size of the iron-containing oxide particles (a) is preferably 0.001 to 10.000 μm, more preferably 0.003 to 5.000 μm, even more preferably 0.005 to 3.000 μm, still more preferably 0.007 to 2.500 μm, still more preferably 0.008 to 2.200 μm, still more preferably 0.009 to 2.000 μm, still more preferably 0.010 to 1.000 μm, still more preferably 0.012 to 0.500 μm, still more preferably 0.015 to 0.200 μm, still more preferably 0.020 to 0.100 μm, and still more preferably 0.020 to 0.050 μm. When the iron-containing oxide particles (a) have a hexagonal close-packed structure, the primary particle size of the iron-containing oxide particles (a) is preferably 0.005 to 1.500 μm, more preferably 0.005 to 1.000 μm, even more preferably 0.005 to 0.500 μm, still more preferably 0.005 to 0.100 μm, still more preferably 0.010 to 0.070 μm, still more preferably 0.010 to 0.050 μm, and still more preferably 0.020 to 0.050 μm. When the iron-containing oxide particles (a) have a spinel structure or a perovskite structure, the primary particle size of the iron-containing oxide particles (a) is preferably 0.010 to 3.000 μm, more preferably 0.050 to 1.000 μm, and still more preferably 0.050 to 0.500 μm. The primary particle size of the iron-containing oxide particles (a) can be measured by the method described in the Examples.

[0017] <Conductive particles with a volume resistivity of 5.0 Ω cm or less> The conductive particles with a volume resistivity of 5.0 Ω cm or less (hereinafter also referred to simply as "conductive particles (b)") are not particularly limited as long as they are particles of a conductive material with a volume resistivity of 5.0 Ω cm or less. The volume resistivity of the conductive particles (b) is 5.0 Ω cm or less, preferably 3.0 Ω cm or less, more preferably 1.0 Ω cm or less, and more preferably 5.0×10 －１ Ω cm (0.5 Ω cm) or less is more preferable, and 3.0 × 10 －１ Ω cm (0.3 Ω cm) or less is more preferable, and 2.0 × 10 －１ Ω cm (0.2 Ω cm) or less is more preferable, and 1.0 × 10 －１ The lower limit of the volume resistivity of the conductive particles (b) is not particularly limited, and is preferably 1.0×10 －２ Therefore, the volume resistivity of the conductive particles (b) is 5.0×10 －２ (0.05) to 5.0 Ω cm is preferable, and 5.0 × 10 －２ (0.05) to 3.0 Ω cm is more preferable, and 5.0 × 10 －２ (0.05) to 1.0 Ω cm is more preferable, and 5.0 × 10 －２ (0.05) to 5.0 x 10 －１ (0.5) Ω cm is more preferable, and 5.0 × 10 －２ (0.05) to 3.0 x 10 －１ Ω cm (0.3) Ω cm is more preferable, and 5.0 × 10 －２ (0.05) to 2.0 x 10 －１ Ω cm (0.2) Ω cm is more preferable, and 5.0 × 10 －２ (0.05) to 1.0 x 10 －１Ω·cm (0.1) Ω·cm is more preferable. The conductive particles (b) may be used singly or in combination of two or more types. When two or more types of conductive particles (b) are used, a mixture of two or more types will exhibit the above volume resistivity. The volume resistivity of the conductive particles (b) can be controlled by adjusting the material, particle size, specific surface area, etc. of the conductive particles (b). The volume resistivity of the conductive particles (b) can be measured by the method described in the examples, similar to the volume resistivity of the iron-containing oxide particles (a).

[0018] The conductive particles (b) may be particles of any conductive material as long as they exhibit the above volume resistivity. The conductive particles (b) preferably contain at least one of carbon particles, metal particles, and metal oxide particles (preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more of the conductive particles (b) being at least one of carbon particles, metal particles, and metal oxide particles), and more preferably at least one of carbon particles, metal particles, and metal oxide particles. In particular, the conductive particles (b) preferably contain carbon particles (preferably 50% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, and even more preferably 90% by mass or more of the conductive particles (b) being carbon particles), and it is also preferred that the conductive particles (b) are carbon particles.

[0019] Examples of the carbon particles include graphites such as natural graphite and artificial graphite; carbon blacks such as acetylene black, ketjen black, and furnace black; amorphous carbons such as needle coke; carbon fibers such as vapor-grown carbon fibers and carbon nanotubes; and carbonaceous materials such as graphene and fullerenes. One or more of these can be used. In the present invention, carbon fibers are also included in the term "carbon particles." In terms of structure, the carbon particles are preferably carbon particles having a hollow structure. From this perspective, carbon nanotubes and / or particles of carbonaceous materials having a hollow shell structure (mesopores) are preferred. Examples of carbonaceous materials having a hollow shell structure include ketjen black. Preferred examples of ketjen black include the ECP series manufactured by Lion Corporation and the CNOVEL series manufactured by Toyo Tanso Co., Ltd.

[0020] Examples of the metal particles include gold particles, silver particles, manganese particles, rhodium particles, ruthenium particles, titanium particles, stainless steel (such as SUS316) particles, tin particles, iron particles, palladium particles, platinum particles, and gold-coated silver particles, and one or more of these can be used. Among these, at least one of silver particles, manganese particles, stainless steel particles, gold particles, tin particles, iron particles, and gold-coated silver particles is preferred in terms of large production volumes. Furthermore, from the viewpoint of reducing costs, it is also preferred that the metal particles have a reduced content of precious metals (gold, silver, platinum, palladium, rhodium, ruthenium, osmium, and iridium) or are free of precious metals.

[0021] Examples of the metal oxide particles include antimony-doped tin oxide particles, titanium oxide particles coated with antimony-doped tin oxide, titanium oxide particles coated with antimony-doped indium oxide, gallium-doped zinc oxide, and aluminum-doped zinc oxide.

[0022] In the present invention, from the viewpoint of enhancing catalytic activity, the conductive particles (b) are preferably carbon particles, more preferably carbon nanotubes and / or carbon particles having a hollow shell structure, and even more preferably Ketjen black.

[0023] The primary particle size of the conductive particles (b) is not particularly limited. The primary particle size of the conductive particles (b) is preferably 10 to 1,000 nm, more preferably 10 to 700 nm, even more preferably 10 to 500 nm, even more preferably 10 to 300 nm, even more preferably 10 to 200 nm, even more preferably 10 to 100 nm, even more preferably 20 to 80 nm, and even more preferably 30 to 50 nm. The primary particle size of the conductive particles (b) can be measured in the same manner as the primary particle size of the iron-containing oxide particles (a).

[0024] From the viewpoint of enhancing catalytic activity and durability, the content of iron-containing oxide particles (a) in the water electrolysis catalyst of the present invention (V Ａ The content of the conductive particles (b) relative to the Ｂ ) ratio (V Ｂ / V Ａ ) is preferably 0.01 to 30.00 in volume ratio. The volume ratio is preferably 0.05 to 25.00, more preferably 0.10 to 23.00, even more preferably 0.30 to 20.00, still more preferably 0.50 to 15.00, even more preferably 0.80 to 10.00, and still more preferably 1.00 to 7.00. The volumes of the iron-containing oxide particles (a) and the conductive particles (b) can be measured by the method described in the Examples.

[0025] The water electrolysis catalyst of the present invention may contain a polymer in addition to the iron-containing oxide particles (a) and the conductive particles (b). The polymer is thought to function as a dispersant in the catalyst layer-forming ink and as a binder in the catalyst layer. The polymer may be a conventional polymer used in the anode catalyst layer or cathode catalyst layer of a membrane electrode assembly. An ionomer resin, for example, is preferred. An anionic ionomer is preferred as the ionomer resin. Examples of the ionomer resin include sulfonated plastic electrolytes such as perfluoroalkanesulfonic acid, sulfonated polyetherketone, sulfonated polyethersulfone, sulfonated polyetherethersulfone, sulfonated polysulfone, sulfonated polysulfide, and sulfonated polyphenylene; and sulfoalkylated plastic electrolytes such as sulfoalkylated polyetheretherketone, sulfoalkylated polyethersulfone, sulfoalkylated polyetherethersulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide, and sulfoalkylated polyphenylene. Furthermore, fluoropolymers other than ionomer resins (for example, polytetrafluoroethylene (PTFE)), silicone polymers, etc. can also be used as the polymer.

[0026] When the water electrolysis catalyst of the present invention contains a polymer, the content of the polymer is preferably 1 to 50 mass %, more preferably 5 to 40 mass %, and even more preferably 10 to 35 mass %, of the total solid content.

[0027] The volume resistivity (powder volume resistivity) of the water electrolysis catalyst of the present invention is 5.0 × 10 ４ Ω cm or less is preferable, and 1.0 × 10 ４ Ω cm or less is more preferable, and 1.0 × 10 ３ More preferably, it is 1.0×10 ２ Ω cm or less (100 Ω cm or less) is more preferable, and 1.0 × 10 １ The lower limit of the volume resistivity of the water electrolysis catalyst is not particularly limited, and is preferably 5.0 × 10 －２A practical volume resistivity of the water electrolysis catalyst is 5.0×10 －２ Ω・cm (0.05Ω・cm) ~ 5.0×10 ４ Ω cm is preferred, and 2.0×10 －１ Ω・cm (0.2Ω・cm) ~ 1.0×10 ４ Ω cm is more preferable, and 4.0×10 －１ Ω・cm (0.4Ω・cm) ~ 1.0×10 ３ Ω cm is more preferable, and 5.0×10 －１ Ω・cm (0.5Ω・cm) ~ 1.0×10 ２ Ω cm is more preferable, and 8.0×10 －１ Ω・cm (0.8Ω・cm) ~ 1.0×10 １ Ω cm is more preferable, and 9.0×10 －１ The volume resistivity of the water electrolysis catalyst is preferably Ω cm (0.9 Ω cm) to 5.0 Ω cm. The volume resistivity of the water electrolysis catalyst can be controlled by adjusting the amounts, particle sizes, specific surface areas, etc. of the iron-containing oxide particles (a) and the conductive particles (b). The volume resistivity of the water electrolysis catalyst can be measured in the same manner as the volume resistivity of the iron-containing oxide particles (a).

[0028] The water electrolysis catalyst of the present invention can be suitably used to form a catalyst layer of a membrane electrode assembly. The catalyst layer can be formed by preparing a catalyst layer-forming ink in which iron-containing oxide particles (a), conductive particles (b), and, if necessary, other components such as a binder are dispersed in a dispersion medium or the like, and then applying and drying the ink. Therefore, the catalyst layer of the membrane electrode assembly has a volume resistivity of 1.0 × 10 ３ The catalyst layer comprises a mixture of iron-containing oxide particles having a volume resistivity of 5.0 Ω cm or more and conductive particles having a volume resistivity of 5.0 Ω cm or less. The description of each component in the water electrolysis catalyst of the present invention is cited as an explanation of preferred embodiments of the components constituting the catalyst layer, their quantitative ratios, etc. Preferred embodiments of the membrane electrode assembly will be described below with respect to configurations other than those already described, such as the components and quantitative ratios of the catalyst layer.

[0029] [Membrane Electrode Assembly] The membrane electrode assembly of the present invention has a catalyst layer (also referred to as "catalyst layer X") containing the water electrolysis catalyst of the present invention, in contact with at least one surface of the anion conductive membrane. The membrane electrode assembly of the present invention can be widely used as a membrane electrode assembly for alkaline water electrolysis.

[0030] The membrane / electrode assembly of the present invention preferably includes an anion conductive membrane, a cathode catalyst layer in direct contact with one surface of the anion conductive membrane, and an anode catalyst layer in direct contact with the other surface of the anion conductive membrane. In this case, the membrane / electrode assembly of the present invention includes a catalyst layer X as at least one of the catalyst layers of the cathode catalyst layer and the anode catalyst layer. The catalyst layer X may be a cathode catalyst layer, an anode catalyst layer, or both the cathode catalyst layer and the anode catalyst layer. It is preferable that the catalyst layer X constitutes at least the anode catalyst layer, and it is more preferable that the anode catalyst layer be the catalyst layer X and the cathode catalyst layer be a catalyst layer other than the catalyst layer X. In the present invention, the simple term "catalyst layer" means the anode catalyst layer and / or the cathode catalyst layer.

[0031] In the membrane / electrode assembly of the present invention, when the catalyst layer X is an anode catalyst layer and the cathode catalyst layer is a catalyst layer other than catalyst layer X, or when the catalyst layer X is a cathode catalyst layer and the anode catalyst layer is a catalyst layer other than catalyst layer X, the cathode catalyst layer or anode catalyst layer other than catalyst layer X can be a cathode catalyst layer or anode catalyst layer employed in a typical membrane / electrode assembly.

[0032] In the present invention, the catalyst layer X may have a single layer structure or a multi-layer structure of two or more layers.

[0033] The catalyst layer X can be formed by preparing a catalyst layer-forming ink containing the water electrolysis catalyst of the present invention and, if necessary, a solvent, etc., and applying and drying the ink. When the catalyst layer X has a multi-layer structure, the catalyst layer X can be formed by preparing multiple types of catalyst layer-forming inks with different compositions, applying each catalyst layer-forming ink, drying, and laminating them.

[0034] FIG. 1 shows a preferred embodiment of the membrane electrode assembly of the present invention. In the membrane electrode assembly 4 of FIG. 1, a cathode catalyst layer 2c is formed on one surface of an anion conductive membrane 1, an anode catalyst layer 2a is formed on the other surface, and a gas diffusion layer 3 is formed on each of these catalyst layers on the surface opposite the anion conductive membrane 1. This membrane electrode assembly 4 has a layered structure in which the gas diffusion layer 3 - the cathode catalyst layer 2c - the anion conductive membrane 1 - the anode catalyst layer 2a - and the gas diffusion layer 3 are layered in this order. The anode catalyst layer 2a contains a particulate anode catalyst 21 and a polymer 23 that also functions as a binder, and the anode catalyst 21 is bound by the polymer 23. The cathode catalyst layer 2c contains a particulate cathode catalyst 22 and a polymer 23 that also functions as a binder, and the cathode catalyst 22 is bound by the polymer 23. In Fig. 1 , a bipolar plate 5 is further formed on the surface of each of the two gas diffusion layers 3 of the membrane electrode assembly 4 opposite to the catalyst layer, thereby forming a water electrolysis cell 10. In this membrane electrode assembly, for example, the anode catalyst layer 2a can be the catalyst layer X. In this case, the anode catalyst 21 is a mixture of iron-containing oxide particles (a) and conductive particles (b), but the drawing does not distinguish between the two particles.

[0035] The membrane electrode assembly of the present invention can be suitably used for producing hydrogen by being incorporated into a module such as the water electrolysis cell 10. The membrane electrode assembly of the present invention can be used in the form of a water electrolysis module, for example, in which the membrane electrode assembly of the present invention is further combined with components commonly used in water electrolysis, such as bipolar plates.

[0036] The membrane electrode assembly of the present invention will now be described in more detail.

[0037] <Anion Conducting Membrane> The anion conducting membrane (hydroxy ion conducting membrane) may be any membrane having anion conductivity, and may be a typical anion conducting membrane used in water electrolysis. Examples of anion conducting membranes that may be used include solid polymer membranes and porous membranes. The polymers that constitute the anion conducting membrane may be any of the polymers described above as polymers that may be contained in the water electrolysis catalyst.

[0038] <Gas Diffusion Layer> The membrane / electrode assembly of the present invention may have a gas diffusion layer on the surface of the catalyst layer opposite to the anion conductive membrane. The gas diffusion layer may be a layer capable of transporting gas and moisture and having electronic conductivity. The gas diffusion layer may be a gas diffusion layer used in a typical membrane / electrode assembly. Examples of materials that can be used for the gas diffusion layer include carbon fiber nonwoven fabric, carbon paper, carbon plate, stainless steel (SUS) fiber nonwoven fabric, stainless steel sintered compact, titanium (Ti) fiber nonwoven fabric, titanium sintered compact, nickel (Ni) fiber nonwoven fabric, nickel sintered compact, and materials in which each of these substrates is coated with platinum or gold.

[0039] The membrane electrode assembly can be formed by applying a catalyst layer-forming ink to an anion conductive membrane, or by laminating a pre-formed catalyst layer and an anion conductive membrane. When a gas diffusion layer is provided on the membrane electrode assembly, the catalyst layer-forming ink may be applied to one side of the gas diffusion layer to form a catalyst layer-gas diffusion layer laminate, which may then be bonded to the anion conductive membrane.

[0040] [Water Electrolysis Module] A water electrolysis module (water electrolysis cell) is a basic structural unit having a water electrolysis function, comprising a membrane electrode assembly and peripheral equipment for obtaining hydrogen gas and oxygen gas by electrolysis of water using the membrane electrode assembly. The water electrolysis module of the present invention can have the same configuration as a conventional water electrolysis module, except for the inclusion of the membrane electrode assembly of the present invention. The water electrolysis module preferably comprises a membrane electrode assembly including an anode catalyst layer, a cathode catalyst layer, an anion conductive membrane (hydroxy ion conductive membrane) sandwiched between these catalyst layers, and gas diffusion layers disposed on the outer sides of these catalyst layers, an electrolyte flow path for supplying and / or recovering an electrolyte to the membrane electrode assembly (preferably the gas diffusion layer of the membrane electrode assembly), and a gas-liquid separation mechanism connected to the electrolyte flow path.

[0041] The electrolyte flow path and the gas-liquid separation mechanism can be the same as those typically used in water electrolysis modules. The electrolyte flow path can be formed by providing bipolar plates having flow paths on the outer sides of the two gas diffusion layers of the membrane electrode assembly and sandwiching the membrane electrode assembly between them. The gas-liquid separation mechanism is a mechanism for removing water from hydrogen gas containing a small amount of water obtained from the cathode side. The gas-liquid separation mechanism is a tank or the like.

[0042] [Water Electrolysis Apparatus] The water electrolysis apparatus of the present invention can be a conventional water electrolysis apparatus except for incorporating the membrane electrode assembly of the present invention or the water electrolysis module of the present invention. The water electrolysis apparatus of the present invention may incorporate one or more water electrolysis modules of the present invention. In addition to the water electrolysis module, the water electrolysis apparatus may also include a moisture removal mechanism (a dehumidification device that further removes water vapor from hydrogen gas after water removal by the gas-liquid separation mechanism, or an adsorption tower that removes water vapor contained in the primary dehumidified gas obtained by the dehumidification device using an adsorbent to obtain a secondary dehumidified gas), a dealkalization mechanism, a water supply mechanism, a rectifier, a compressor, etc. The basic configuration of the water electrolysis module and the water electrolysis apparatus can be described, for example, in a report published by the International Renewable Energy Agency (IRENA): IRENA (2020), Green Hydrogen Cost Reduction: Scaling up Electrolysers to Meet the 1.5°C Climate Goal, International Renewable Energy Agency, Abu Dhabi.

[0043] The present invention will be described in more detail below based on examples, but the present invention should not be construed as being limited thereto.

[0044] [Example 1] (A) Preparation of catalyst dispersion (water electrolysis catalyst dispersion) Barium ferrite 1 (BaFe １２ O １９40.0 mg of ketjen black (manufactured by Ion Specialty Chemicals, ECP600JD (trade name)), 28.0 mg of ketjen black (manufactured by Ion Specialty Chemicals, ECP600JD (trade name)), 0.5 ml of 5% Nafion dispersion solution (manufactured by Fujifilm Wako Pure Chemical Industries, DE520 (trade name), 25 mg of Nafion), 8.0 ml of ethanol, and 2.0 ml of pure water were placed in a sample bottle and mixed. This mixture was dispersed for 3 minutes using an ultrasonic homogenizer (SMT Corporation, UH-300 (trade name)) at output 2. In this way, a dispersion of a water electrolysis catalyst (catalyst layer forming ink) was obtained, which is a mixture of iron-containing oxide particles and conductive particles, as in Example 1. In this catalyst dispersion, the ratio of the content of ketjen black to the content of barium ferrite 1 was 2.00 by volume. Barium ferrite 1 was prepared as follows, with reference to Examples 1-7 of WO 2015 / 046496. In the following description, the numbers in the above publication are used as they are. (Method for preparing barium ferrite 1) (1) Preparation of precursor particle solution: Sodium oleate (C １７ H ３３ COONa) and barium hydroxide (Ba(OH) ２ ・8H ２ O), iron(III) nitrate (Fe(NO ３ ) ３ ・9H ２A hydroxide sol for use as a precursor particle solution was prepared by dissolving Ba and Fe in the solution (sol). The concentration of the prepared aqueous solution (sol) (total concentration of Ba and Fe) was 0.01 M, and the Ba / Fe molar ratio was 0.5. The concentration of sodium oleate was 0.1 M. KOH was added in an amount sufficient to adjust the pH of the reaction solution to 4.0. (2) Synthesis of hexagonal ferrite particles by a continuous hydrothermal synthesis process: The aqueous solution (sol) prepared in (1) above was introduced into the liquid tank 2 of the manufacturing apparatus shown in Figure 1 of the above publication. SUS316BA tubing was used for the piping of the manufacturing apparatus. Purified water introduced into the liquid tank 1 was pumped by the high-pressure pump 5a while being heated by the heater 4, thereby circulating high-temperature, high-pressure water through the piping 100. During this process, the temperature and pressure were controlled so that the temperature of the high-temperature, high-pressure water after passing through the heating means 4c was 450°C and the pressure was 30 MPa. Meanwhile, an aqueous solution (sol) was pumped to pipe 101 at 25°C using high-pressure pump 5b and mixed with the high-temperature, high-pressure water in mixing section M1. Subsequently, barium ferrite nanoparticles were synthesized by heating and pressurizing at 400°C and 30 MPa in reactor 6. After the barium ferrite nanoparticle synthesis, the solution containing the barium ferrite nanoparticles was cooled with cold water in cooling section 7 and collected. The collected particles were washed with ethanol and then centrifuged to separate the barium ferrite nanoparticles. XRD confirmed that the obtained barium ferrite 1 had a hexagonal close-packed structure. (B) Preparation of Evaluation Electrode: 10 μL of the prepared catalyst dispersion was dropped onto a 3 mm diameter gold electrode of an electrode chip (manufactured by BioLogic, 094-PTFE-Au / 3 (trade name)) while uniformly mixing, ensuring that the dispersion did not spill over the gold electrode. The gold electrode was held horizontally and left to dry at room temperature (25°C) to produce a catalyst-coated gold electrode. The produced catalyst-coated gold electrode was then wrapped in a laminate sheet and vacuum-sealed, after which it was cold isostatically pressed at 300 MPa using a cold isostatic press (manufactured by Nikkiso Co., Ltd., CL10-55-40 (product name)). The laminate sheet was then removed. In this way, an electrode for evaluation was produced.

[0045] [Example 2] In the preparation of the catalyst dispersion liquid (A) of Example 1, strontium ferrite 1 (SrFe １２ O １９ A catalyst of Example 2 was prepared in the same manner as in Example 1, except that SrCO was used, and an electrode for evaluation was fabricated. Strontium ferrite 1 was prepared as follows, with reference to the method described in the examples of JP-A No. 2023-73303 (method 1 for preparing hexagonal strontium ferrite powder). (Method for preparing strontium ferrite 1) ３ 1707g, H ３ BO ３ 687g, Fe ２ O ３ 1120 g of Al(OH) ３ 45g, BaCO ３ 24 g, CaCO ３ 13 g, and Nd ２ O ３235 g of the above was weighed and mixed in a mixer to obtain a raw material mixture. The resulting raw material mixture was melted in a platinum crucible at a melting temperature of 1,390 ° C. The melt was stirred while heating the tapping port at the bottom of the platinum crucible, and the melt was poured into a rod-like shape at approximately 6 g / sec. The tapped liquid was rolled and quenched with a water-cooled twin roller to produce an amorphous body. 280 g of the produced amorphous body was placed in an electric furnace, heated to 635 ° C (crystallization temperature) at a heating rate of 3.5 ° C / min, and held at the same temperature for 5 hours to precipitate (crystallize) hexagonal strontium ferrite particles. Next, the obtained crystallized material containing hexagonal strontium ferrite particles was coarsely crushed in a mortar, and 1,000 g of zirconia beads with a particle size of 1 mm and 800 mL of a 1% acetic acid aqueous solution were added to a glass bottle and dispersed for 3 hours using a paint shaker. The resulting dispersion was then separated from the beads and placed in a stainless steel beaker. The dispersion was left to stand at a liquid temperature of 100°C for 3 hours to dissolve the glass component, and then precipitated in a centrifuge, washed by repeated decantation, and dried for 6 hours in a heating furnace at a furnace temperature of 110°C to obtain hexagonal strontium ferrite powder. The primary particle size of the obtained hexagonal strontium ferrite powder was 20 nm. It was confirmed by XRD that the obtained strontium ferrite 1 had a hexagonal close-packed structure.

[0046] [Example 3] In the preparation of the catalyst dispersion liquid (A) of Example 1, nickel ferrite 1 (NiFe ２ O ４ A catalyst of Example 3 was prepared in the same manner as in Example 1, except that nickel ferrite 1 (manufactured by Kojundo Chemical Co., Ltd., under the trade name NIF09PB) was used, and an electrode for evaluation was fabricated. The nickel ferrite 1 has a spinel structure.

[0047] [Example 4] In the preparation of the catalyst dispersion liquid (A) of Example 1, barium ferrite 1 was replaced with barium ferrite 2 (BaFeO ３ A catalyst of Example 4 was prepared and an electrode for evaluation was fabricated in the same manner as in Example 1, except that BaFeO was used. The method for preparing barium ferrite 2 is as follows. (Method for preparing barium ferrite 2)３ Chem. Commun., 2018, 54, 6772. Then, 30 zirconia balls with a diameter of 3 mm were placed in a 45 mL zirconia pot under an argon atmosphere, and the prepared BaFeO ３ 1.5 g of ...

[0048] [Example 5] In the preparation of the catalyst dispersion liquid (A) of Example 1, calcium ferrite 1 (CaFe ２ O ４ A catalyst of Example 5 was prepared in the same manner as in Example 1, except that Calcium Ferrite 1 was used, and an electrode for evaluation was fabricated. The method for preparing calcium ferrite 1 is as follows. (Preparation of calcium ferrite 1) 400 mL of pure water, 4.9 g of iron (II) acetate (Tokyo Chemical Industry Co., Ltd.), 2.5 g of calcium acetate monohydrate (Fujifilm Wako Pure Chemical Industries, Ltd.), and 8.4 g of DL-malic acid (Fujifilm Wako Pure Chemical Industries, Ltd.) were weighed and placed in a 1 L three-neck flask and stirred. The mixture was heated to 80°C, dissolved, and then stirred for 2 hours. The solvent was distilled off, and the residue was recovered and dried by heating at 200°C. The resulting residue was calcined in air at 750°C for 6 hours to prepare calcium ferrite. Under an argon atmosphere, 30 zirconia balls with a diameter of 3 mm were placed in a 45 mL zirconia pot, and 1.5 g of the prepared calcium ferrite was weighed out and placed in a ball mill (P-7 (trade name) manufactured by Fritsche). The zirconia balls were removed, and the residue was dried to obtain calcium ferrite 1 with a primary particle size of 0.07 μm. The obtained calcium ferrite 1 was observed by XRD, and it was confirmed that calcium ferrite with a spinel structure had been prepared.

[0049] [Example 6] In the preparation of the catalyst dispersion liquid (A) of Example 1, barium ferrite 1 was replaced with barium ferrite 3 (BaFe１２ O １９ A catalyst of Example 6 was prepared in the same manner as in Example 1, except that a barium ferrite 1 (manufactured by Kojundo Chemical Co., Ltd., under the trade name BAF14PB) was used, and an electrode for evaluation was fabricated. As shown in Table 1, barium ferrite 1 and barium ferrite 3 have the same structural formula but differ in volume resistivity, specific surface area, and particle size. Barium ferrite 3 has a hexagonal close-packed structure.

[0050] [Example 7] A catalyst of Example 7 was prepared in the same manner as in Example 1, except that in the preparation of (A) the catalyst dispersion liquid of Example 1, carbon black (manufactured by MTI, Super P (trade name)) was used instead of Ketjen black, and an electrode for evaluation was fabricated.

[0051] [Example 8] A catalyst of Example 8 was prepared in the same manner as in Example 1, except that in the preparation of (A) catalyst dispersion liquid of Example 1, carbon nanotubes (F9106S (product name) manufactured by KJ Specialty Paper Co., Ltd.) were used instead of Ketjen black, and an electrode for evaluation was fabricated.

[0052] Example 9 A catalyst of Example 9 was prepared in the same manner as in Example 1, except that in the preparation of (A) catalyst dispersion liquid of Example 1, gold particles (manufactured by Nilaco Corporation, AU-174015 (product name)) were used instead of Ketjen Black, and an electrode for evaluation was fabricated.

[0053] [Example 10] A catalyst of Example 10 was prepared in the same manner as in Example 1, except that in the preparation of (A) catalyst dispersion liquid of Example 1, tin oxide (SN-100P (trade name) manufactured by Ishihara Sangyo Kaisha, Ltd.) was used instead of Ketjen Black, and an electrode for evaluation was fabricated.

[0054] [Example 11] A catalyst of Example 11 was prepared in the same manner as in Example 1, except that in the preparation of the catalyst dispersion liquid (A) of Example 1, the content of barium ferrite 1 and the content of ketjen black were changed so that the volume ratio of the content of ketjen black to the content of barium ferrite 1 was 0.05, and an electrode for evaluation was fabricated.

[0055] [Example 12] A catalyst of Example 12 was prepared in the same manner as in Example 1, except that in the preparation of the catalyst dispersion liquid (A) of Example 1, the content of barium ferrite 1 and the content of ketjen black were changed so that the volume ratio of the content of ketjen black to the content of barium ferrite 1 was 0.10, and an electrode for evaluation was fabricated.

[0056] [Example 13] A catalyst of Example 13 was prepared in the same manner as in Example 1, except that in the preparation of the catalyst dispersion liquid (A) of Example 1, the content of barium ferrite 1 and the content of ketjen black were changed so that the volume ratio of the content of ketjen black to the content of barium ferrite 1 was 0.30, and an electrode for evaluation was fabricated.

[0057] [Example 14] A catalyst of Example 14 was prepared in the same manner as in Example 1, except that in the preparation of the catalyst dispersion liquid (A) of Example 1, the content of barium ferrite 1 and the content of ketjen black were changed so that the volume ratio of the content of ketjen black to the content of barium ferrite 1 was 1.00, and an evaluation electrode was fabricated.

[0058] [Example 15] A catalyst of Example 15 was prepared in the same manner as in Example 1, except that in the preparation of the catalyst dispersion liquid (A) of Example 1, the content of barium ferrite 1 and the content of ketjen black were changed so that the volume ratio of the content of ketjen black to the content of barium ferrite 1 was 5.00, and an electrode for evaluation was fabricated.

[0059] [Example 16] A catalyst of Example 16 was prepared in the same manner as in Example 1, except that in the preparation of the catalyst dispersion liquid (A) of Example 1, the content of barium ferrite 1 and the content of ketjen black were changed so that the volume ratio of the content of ketjen black to the content of barium ferrite 1 was 20.00, and an evaluation electrode was fabricated.

[0060] [Example 17] A catalyst of Example 17 was prepared in the same manner as in Example 1, except that in the preparation of the catalyst dispersion liquid (A) of Example 1, the content of barium ferrite 1 and the content of ketjen black were changed so that the volume ratio of the content of ketjen black to the content of barium ferrite 1 was 25.00, and an electrode for evaluation was fabricated.

[0061] Example 18 A catalyst of Example 18 was prepared and an electrode for evaluation was fabricated in the same manner as in Example 1, except that in the preparation of (A) catalyst dispersion liquid of Example 1, a material in which titanium oxide was coated with antimony-doped tin oxide (FT-1000 (product name) manufactured by Ishihara Sangyo Kaisha, Ltd.) (shown in the table as "Titanium oxide / Tin oxide 1") was used instead of Ketjen black.

[0062] [Example 19] In the preparation of the catalyst dispersion liquid (A) of Example 1, yttrium ferrite 1 (Y ３ Fe ５ O １２ A catalyst of Example 19 was prepared in the same manner as in Example 1, except that yttrium ferrite 1 (manufactured by Kojundo Chemical Co., Ltd., under the trade name YYF06PB) was used, and an electrode for evaluation was fabricated. The yttrium ferrite 1 has a garnet structure.

[0063] [Example 20] In Example 1, barium ferrite 1 was replaced with barium ferrite 4 (BaFe １２ O １９A catalyst of Example 20 was prepared in the same manner as in Example 1, except that barium ferrite 1 was used, and an evaluation electrode was fabricated. As shown in Table 1, barium ferrite 1 and barium ferrite 4 have the same structural formula, but differ in volume resistivity, specific surface area, and particle size. Barium ferrite 4 was prepared as follows. (Preparation of barium ferrite 4) Thirty 3 mm diameter zirconia balls were placed in a 45 mL zirconia pot, and 1.0 g of barium ferrite (BAF14PB (trade name) manufactured by Kojundo Chemical Co., Ltd.) and 5.5 g of pure water were weighed out and treated using a ball mill (P-7 (trade name) manufactured by Fritsche). The zirconia balls were removed, and the residue was dried to obtain barium ferrite 4 with a primary particle size of 0.25 μm. The obtained barium ferrite 4 was observed by XRD, and it was confirmed that barium ferrite with a hexagonal close-packed structure had been prepared.

[0064] Comparative Example 1 A catalyst of Comparative Example 1 was prepared in the same manner as in Example 1, except that Ketjen Black was not used in the preparation of (A) the catalyst dispersion liquid of Example 1, and an electrode for evaluation was fabricated.

[0065] [Comparative Example 2] In the preparation of the catalyst dispersion liquid (A) of Example 1, nickel ferrite 2 (the structural formula in the table is "NiO / 9Fe") prepared as follows was used instead of barium ferrite 1. ２ O ３") was used. A catalyst of Comparative Example 2 was prepared in the same manner as in Example 1, and an electrode for evaluation was fabricated. Nickel ferrite 2 was prepared by the following method. (Preparation of nickel ferrite 2) 1.0 g of nickel (II) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) and 9.0 g of iron (III) oxide (manufactured by FUJIFILM Wako Pure Chemical Industries, Ltd.) were placed in a 45 mL zirconia pot, and 15 zirconia balls having a diameter of 10 mm were placed therein. Using a ball mill (manufactured by Fritsche GmbH, P-7), the mixture was stirred at 200 rpm for 15 minutes, cooled, and further stirred at 200 rpm for 15 minutes, after which the zirconia balls were removed to obtain a sample. This sample was pressurized at 10 MPa and molded into a pellet with a diameter of 10 mm, which was then pre-fired at 500°C in a nitrogen atmosphere for 5 hours, and the resulting molded body was pulverized, remolded, and fired at 1000°C in a nitrogen atmosphere, and the resulting molded body was pulverized to prepare nickel ferrite 2. The obtained nickel ferrite 2 was observed by XRD, and it was confirmed that nickel ferrite containing a spinel structure had been prepared.

[0066] [Comparative Example 3] In the preparation of the catalyst dispersion liquid (A) of Example 1, magnesium ferrite 1 (the structural formula in the table is "MgO / 9Fe ２ O ３ A catalyst of Comparative Example 3 was prepared in the same manner as in Example 1, except that NiO (described as "NiO") was used, and an electrode for evaluation was fabricated. The method for preparing magnesium ferrite 1 was as follows. (Preparation of magnesium ferrite 1) Magnesium ferrite 1 was prepared in the same manner as in the preparation of nickel ferrite 2 of Comparative Example 2, except that NiO was changed to MgO in the preparation of nickel ferrite 2 of Comparative Example 2. The obtained magnesium ferrite 1 was observed by XRD, and it was confirmed that a magnesium ferrite containing a spinel structure had been prepared.

[0067] Comparative Example 4 A catalyst of Comparative Example 4 was prepared in the same manner as in Example 1, except that in the preparation of (A) catalyst dispersion liquid in Example 1, a material in which titanium oxide was coated with antimony-doped tin oxide (ET-300W (product name) manufactured by Ishihara Sangyo Kaisha, Ltd.) (shown in the table as "Titanium oxide / Tin oxide 2") was used instead of Ketjen black. An electrode for evaluation was fabricated from the catalyst of Comparative Example 4.

[0068] [Measurement and Evaluation] (1) Measurement Method of Volume Resistivity The volume resistivity (powder volume resistivity) of each iron-containing oxide particle, conductive particle, and water electrolysis catalyst was evaluated using an all-solid-state cell (SB1800 (trade name), manufactured by EC Frontier Corporation). Specifically, 200 mg of powder (sample) to be measured was measured and placed inside the cylindrical tube of the all-solid-state cell. Then, using a press, the sample was pressurized at 185 MPa for 5 minutes. Stainless steel rods with a diameter of 10 mm were inserted from the upper and lower openings of the tube, and the ends of the rods were placed so as to contact the surface of the pressed sample. The bolts were tightened to a confining pressure of 5 MPa, and the sample was constrained to obtain a measurement cell. The thickness T (cm) of the sample in the constrained state was calculated by subtracting the cell thickness before the sample was placed from the thickness of the measurement cell. This measurement cell was connected to an electrochemical measurement system (VMP-300 (trade name) manufactured by BioLogic), and a voltage of 10 mV was applied at room temperature (25°C), and the current value x (A) after 1 hour was measured. Using the obtained sample thickness T and current value x, the powder volume resistivity R (Ω cm) was calculated according to the following formula: R = 0.01 / x × 0.79 / T The volume resistivity of each water electrolysis catalyst was measured using the catalyst dispersion liquid (catalyst layer-forming ink) obtained above that was dried and used as the sample.

[0069] (2) Measurement of Specific Surface Area The specific surface area (BET specific surface area) of each iron-containing oxide particle was measured using a high-precision gas adsorption measurement device (Belsorp mini (trade name), manufactured by Japan Bel Corporation).

[0070] (3) Measurement of Primary Particle Size The primary particle size of each iron-containing oxide particle and each conductive particle was measured as follows. Sample powder was spread on a mesh with an elastic carbon support film manufactured by Stem Co., Ltd., and observed under a transmission electron microscope. The particle sizes (circle-equivalent diameters) of a total of 100 particles in five fields of view were measured, and the arithmetic mean value was calculated as the primary particle size.

[0071] (4) Measurement of the Volume of Iron-Containing Oxide Particles and Conductive Particles The volumes of each iron-containing oxide particle and conductive particle were measured as follows. The volume of iron-containing oxide particles (a) was calculated by measuring the mass and dividing by the true density. The volume of conductive particles (b) was calculated in the same manner. The masses of iron-containing oxide particles (a) and conductive particles (b) were measured at room temperature (25°C). The true densities of iron-containing oxide particles (a) and conductive particles (b) were determined in accordance with the "gas displacement method" described in JIS Z8807:2012.

[0072] (5) Catalytic Activity Evaluation The catalytic activity of each of the obtained water electrolysis catalysts was evaluated by the rotating disk electrode method (RDE method) using each of the evaluation electrodes. Each of the evaluation electrodes prepared above was set in a rotating electrode device (Bluev (trade name) manufactured by Bio Logic), and using a reversible hydrogen electrode (RHE) as a reference electrode, the device was immersed in a 1 M KOH aqueous solution under argon bubbling and driven under the following conditions for evaluation. Details are described below. (i) Initial Adjustment First, as an initial adjustment, cyclic voltammetry was performed under the following conditions. Rotation speed: 0 rpm Starting voltage: 0.8 V Sweep rate: 100 mV / sec Turn-back voltage 1: -0.7 V Turn-back voltage 2: 1.2 V Number of cycles: 100 End voltage: 0.8 V (ii) LSV measurement Following the above initial adjustment, linear sweep voltammetry (LSV) was performed under the following conditions to determine the current value Dcg (mA) at the end voltage. Under the following conditions, each catalyst-equipped electrode acts as an anode, and water electrolysis occurs. Rotation speed: 3600 rpm Starting voltage: 1.2 V Sweep rate: 10 mV / sec End voltage: 1.8 V (i) Initial adjustment and (ii) LSV measurement were performed in the same manner as above, except that a gold electrode without a catalyst layer was used instead of the evaluation electrode (catalyst-attached electrode), and the current value Dg (mA) at the end voltage when only the gold electrode was used was measured. The difference (Dcg-Dg) obtained by subtracting the current value Dg from the obtained current value Dcg was used as the current value Dc at the end voltage when the catalyst alone was used. ０ The obtained current value Dc０ The results were evaluated according to the following evaluation criteria. The higher the current value, the higher the catalytic activity. Evaluation criteria: A: 20 mA or more B: 10 mA or more, less than 20 mA C: 5 mA or more, less than 10 mA D: Less than 5 mA E: Unable to evaluate (current value could not be detected)

[0073] (6) Catalyst durability test After evaluating the catalytic activity under the conditions of (5) above, the catalyst was subsequently driven under the following conditions (iii) to accelerate degradation, and then LSV measurement was carried out to obtain a current value. The ratio to the current value obtained by the LSV measurement of (ii) above was calculated, and durability was evaluated using this current value ratio as an index. Details are explained below. (iii) Degradation acceleration conditions For degradation acceleration, cyclic voltammetry was carried out under the following conditions: Rotation speed: 1800 rpm Start voltage: 1.2 V Sweep rate: 10 mV / sec Turn-around voltage 1: 1.8 V Turn-around voltage 2: 1.2 V Number of cycles: 200 End voltage: 1.2 V After the degradation acceleration of (iii) above, LSV evaluation was carried out under the same conditions as in (ii) above, and the current value Dc at the end voltage of the catalyst alone was calculated. ｌ (mA) was calculated. (5) Current value Dc during catalytic activity evaluation ０ Current value Dc ｌ Current value ratio (Dc ｌ / Dc ０ × 100 (%)) was calculated and evaluated according to the following evaluation criteria. -Evaluation criteria- A: 90% or more B: 70% or more, less than 90% C: 50% or more, less than 70% D: 20% or more, less than 50% E: Less than 20%

[0074]

[0075] <Notes for the table> "Volume resistivity" in the "Composition" column indicates the volume resistivity of the water electrolysis catalyst (composition).

[0076] Volume resistivity is 1.0 x 10 ３ The catalyst of Comparative Example 1, which contains iron-containing oxide particles with a volume resistivity of 5.0 Ω cm or more but does not contain conductive particles with a volume resistivity of 5.0 Ω cm or less, had a current value Dc ０Therefore, the catalyst of Comparative Example 1 was not subjected to a durability test. ３ The catalysts of Comparative Examples 2 and 3, which contained iron-containing oxide particles with a volume resistivity of less than 5.0 Ω cm, had a current value ratio of less than 20%, and were poor in durability, even though they contained conductive particles with a volume resistivity of 5.0 Ω cm or less. ０ The volume resistivity was less than 1.0×10 ３ The catalyst of Comparative Example 4, which contains iron-containing oxide particles having a powder volume resistivity of 10.0 Ω cm or more, had a current value Dc ０ The current value ratio was less than 20%, and the durability was poor. ３ The water electrolysis catalysts of Examples 1 to 20, which contain iron-containing oxide particles with a volume resistivity of 5.0 Ω cm or more and conductive particles with a volume resistivity of 5.0 Ω cm or less, exhibited a current value Dc ０ The current value ratio was 5 mA or more, indicating excellent catalytic activity. Furthermore, the current value ratio was 50% or more, indicating excellent durability. It can be seen that the water electrolysis catalyst of the present invention has excellent catalytic activity and excellent durability of catalytic activity, despite containing inexpensive iron element. When a membrane electrode assembly is formed using the water electrolysis catalyst of the present invention and incorporated into a water electrolysis module and ultimately into a water electrolysis device, water electrolysis can be performed at low cost.

[0077] This application claims priority based on Japanese Patent Application No. 2024-118634, filed on July 24, 2024, the contents of which are incorporated herein by reference as part of the present specification.

[0078] REFERENCE SIGNS LIST 1 anion conductive membrane 2a anode catalyst layer 21 anode catalyst 23 ionomer resin 2c cathode catalyst layer 22 cathode catalyst 23 ionomer resin 3 gas diffusion layer 4 membrane electrode assembly 5 bipolar plate 10 water electrolysis cell

Claims

1. Volume resistivity is 1.0 x 10 ３ A catalyst for water electrolysis comprising a mixture of iron-containing oxide particles having a volume resistivity of Ω·cm or more and conductive particles having a volume resistivity of 5.0 Ω·cm or less.

2. The water electrolysis catalyst according to claim 1, wherein the conductive particles include at least one of carbon particles, metal particles, and metal oxide particles.

3. The water electrolysis catalyst according to claim 2, wherein the conductive particles include carbon particles.

4. The water electrolysis catalyst according to claim 3, wherein the carbon particles are at least one of carbon nanotubes and carbon particles having a hollow shell structure.

5. The water electrolysis catalyst according to claim 4, wherein the ratio of the content of the conductive particles to the content of the iron-containing oxide particles in the water electrolysis catalyst is 0.01 to 30.00 by volume.

6. The specific surface area of ​​the iron-containing oxide particles is 1.0 m ２ The water electrolysis catalyst according to claim 5 , wherein the Mo content is 1 / g or more.

7. The water electrolysis catalyst according to claim 6, wherein the iron-containing oxide particles have a primary particle size of 10,000 μm or less.

8. The water electrolysis catalyst according to claim 7, wherein the iron-containing oxide particles contain at least one of barium, calcium, strontium, nickel, titanium, sodium, magnesium, cobalt, copper, manganese, and zinc.

9. The water electrolysis catalyst according to claim 8, wherein the iron-containing oxide particles have a hexagonal close-packed structure, a spinel structure, or a perovskite structure.

10. The water electrolysis catalyst according to claim 8, wherein the iron-containing oxide particles contain at least one of barium ferrite, calcium ferrite, and strontium ferrite.

11. The specific surface area of ​​the iron-containing oxide particles is 80.0 m ２ The water electrolysis catalyst according to claim 10, wherein the Mo content is 1 / g or more.

12. A membrane electrode assembly having a catalyst layer containing the water electrolysis catalyst according to any one of claims 1 to 11, in contact with at least one surface of an anion conductive membrane.

13. The membrane electrode assembly according to claim 12, further comprising a gas diffusion layer on the side of the catalyst layer opposite to the anion conductive membrane.

14. A water electrolysis module comprising: the membrane electrode assembly according to claim 12; an electrolyte flow path for supplying and / or recovering an electrolyte to the membrane electrode assembly via the gas diffusion layer of the membrane electrode assembly; and a gas-liquid separation mechanism connected to the electrolyte flow path.

15. A water electrolysis device comprising the membrane electrode assembly according to claim 12.

16. A water electrolysis device comprising the water electrolysis module according to claim 14.

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