Hydroxy ion conductive membrane, method for producing hydroxy ion conductive membrane, membrane electrode assembly, hydrogen production method, and hydrogen production system

Abstract

Provided are: a hydroxy ion conductive membrane containing a porous base material and a hydroxy ion conductive polymer disposed at least in pores of the porous base material, wherein the hydroxy ion conductive film has a thickness of 5 μm or more and less than 50 μm, and the polymer contains a constituent component (I) derived from a polyfunctional polymerizable monomer having two or more atoms of at least one kind of atom among oxygen atom, sulfur atom and nitrogen atom in total in a structural moiety other than a polymerizable group by 50 mol% or more of the constituent components of the polymer; a method for producing the hydroxy ion conductive membrane; a membrane electrode assembly, and a hydrogen production method and a hydrogen production system comprising the membrane electrode assembly.

Description

Hydroxy ion conductive membrane, method for producing a hydroxy ion conductive membrane, membrane electrode assembly, hydrogen production method, and hydrogen production system

[0001] The present invention relates to a hydroxy ion conductive membrane, a method for producing a hydroxy ion conductive membrane, a membrane electrode assembly, a hydrogen production method, and a hydrogen production system.

[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 exchange membrane water electrolysis (AEMWE) has also been proposed, which uses an anion exchange 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 precious metal, metal oxide, or the like is optionally supported on a carrier such as carbon, an ionomer that functions as a binder and an ion conductor, and a liquid medium, applying the mixture to a gas-permeable and electronically conductive conductive substrate, and drying the coating to remove the liquid medium. Next, an anion exchange membrane (hydroxy ion conductive membrane) is sandwiched between the formed anode catalyst layer and cathode catalyst layer, and the anion exchange membrane is bonded 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. For example, Patent Document 1 describes the following membrane electrode assembly.

[0005] a membrane electrode assembly for use in a water electrolysis apparatus, the membrane electrode assembly comprising: an electrolyte membrane made of an anion exchange membrane; catalyst layers disposed on both sides of the electrolyte membrane; and a conductive substrate laminated on a surface of the catalyst layer opposite to the surface facing the electrolyte membrane, wherein the catalyst layer comprises a catalyst and an anionic ionomer, at least a portion of the anionic ionomer being in contact with the electrolyte membrane while being embedded in the electrolyte membrane, and the membrane electrode assembly has a mixed region in which the anionic ionomer and the electrolyte membrane are mixed together near a boundary between the catalyst layer and the electrolyte membrane.

[0006] The technology described in Patent Document 1 is said to increase the adhesion between the electrolyte membrane and the catalyst layer, thereby reducing the resistance to hydroxide ion migration at the interface between the electrolyte membrane and the catalyst layer, and as a result, reducing the resistance of the membrane electrode assembly.

[0007] Japanese Patent Application Laid-Open No. 2023-104047

[0008] To achieve the low resistance (high hydroxy ion permeability) of the MEA, it is conceivable to increase the hydroxy ion conductivity of the hydroxy ion conductive membrane as an inherent material property and to minimize the distance between the cathode and the anode. However, shortening the distance between the cathode and the anode requires reducing the thickness of the hydroxy ion conductive membrane, which results in a decrease in the mechanical strength of the hydroxy ion conductive membrane. When the hydroxy ion conductive membrane is combined with a catalyst layer or gas diffusion layer with slight irregularities and pressurized, voids or micro-holes are formed, which can easily cause unexpected gas flow, water flow, and poor contact of the catalyst layer. Therefore, the hydroxy ion conductive membrane must have sufficient mechanical strength even when formed thin. Furthermore, as a basic property of a diaphragm, the hydroxy ion conductive membrane must have low gas permeability even when formed into a thin film as described above.

[0009] Furthermore, solar power generation systems, which are widely used as power generation systems that utilize renewable energy, operate during the day and do not operate at night when the sun sets. In other words, when water electrolysis is performed using a solar power generation system as a power source, there are time periods when power is supplied and time periods when it is not supplied. Even during the time periods when power is supplied, the amount of power supplied fluctuates depending on the weather. This tendency for output to fluctuate is common to other power generation systems that utilize renewable energy. Therefore, when water electrolysis is performed using a power generation system that utilizes renewable energy as a power source, it is necessary to increase or decrease the amount of water supplied depending on the amount of power supplied, or to stop the water supply during times when power is not supplied. Therefore, the MEA is also required to have the ability to maintain low resistance even when the amount of supplied power or water supply fluctuates.

[0010] An object of the present invention is to provide a hydroxy ion conductive membrane that has excellent hydroxy ion conductivity, excellent mechanical strength despite being thin, and sufficiently low air permeability, and an MEA formed by sandwiching this hydroxy ion conductive membrane between a cathode and an anode can sufficiently maintain low resistance even when current or liquid transport is repeatedly turned on and off, and a method for producing the same. Another object of the present invention is to provide a membrane electrode assembly in which the hydroxy ion conductive membrane sandwiched between the cathode and the anode has excellent hydroxy ion conductivity, is thin but has excellent mechanical strength, and has low resistance that can sufficiently maintain low resistance even when current or liquid transport is repeatedly turned on and off, and a hydrogen production method and hydrogen production system that use the membrane electrode assembly.

[0011] The above-mentioned problems of the present invention are solved by the following means. [1] A hydroxy ion conductive membrane comprising a porous substrate and a hydroxy ion conductive polymer disposed at least in the pores of the porous substrate, wherein the hydroxy ion conductive polymer contains a component (I) derived from a polyfunctional polymerizable monomer having a total of two or more atoms of at least one of oxygen, sulfur, and nitrogen atoms in a structural portion other than a polymerizable group, and the proportion of component (I) to all components of the hydroxy ion conductive polymer is 50 mol % or more, and the thickness of the hydroxy ion conductive membrane is 5 μm or more and less than 50 μm. [2] The hydroxy ion conductive membrane according to [1], wherein the elemental composition of the hydroxy ion conductive polymer satisfies the following formula (a): [Formula (a)] [Total of concentrations of oxygen, sulfur, and nitrogen (at %)] / [Concentration of carbon (at %)] ≧ 0.05 [3] The hydroxy ion conductive membrane according to [1] or [2], wherein the water retention capacity of the hydroxy ion conductive polymer is 5 mass % or more. [4] The hydroxy ion conductive membrane according to any one of [1] to [3], wherein the component (I) has any one of the following structural units (i) to (viii): n is an integer of 1 or more. L represents a single bond, an alkylene group, or an arylene group. * represents a linking moiety. [5] The hydroxy ion conductive membrane according to any one of [1] to [4], wherein the constituent component (I) is a nonionic constituent component. [6] The hydroxy ion conductive membrane according to any one of [1] to [5], wherein the hydroxy ion conductive membrane has a tensile strength of 10 MPa or more and a breaking elongation of 100% or more. [7] The hydroxy ion conductive membrane according to any one of [1] to [6], wherein the hydroxy ion conductive membrane has an air permeability of 1000 seconds or more. [8] The hydroxy ion conductive membrane according to any one of [1] to [7], wherein the porous substrate has a porosity of 40 to 60%. [9] The hydroxy ion conductive membrane according to any one of [1] to [8], wherein the porous substrate has a tensile strength of 20 MPa or more and a breaking elongation of 100% or more.

[10] The hydroxy ion conductive membrane according to any one of [1] to [9], wherein a constituent material of the porous substrate comprises a hydrocarbon polymer.

[11] A method for producing a hydroxy ion conductive membrane, comprising impregnating a porous substrate with a polymerizable monomer-containing liquid containing 50 mol % or more of polyfunctional polymerizable monomers, each of which has, in total, two or more atoms of at least one kind selected from oxygen, sulfur, and nitrogen atoms in structural moieties other than polymerizable groups, and then polymerizing the polyfunctional polymerizable monomers to obtain a hydroxy ion conductive membrane having a thickness of 5 μm or more and less than 50 μm.

[12] The method for producing a hydroxy ion conductive membrane according to

[11] , wherein the polymerizable monomer-containing liquid has a solids content of 30 mass % or more.

[13] The method for producing a hydroxy ion conductive membrane according to

[11] or

[12] , wherein the porous substrate has been surface-modified by corona treatment or plasma treatment.

[14] A membrane / electrode assembly having a structure in which a cathode catalyst layer, the hydroxy ion conductive membrane according to any one of [1] to

[10] , and an anode catalyst layer are laminated in this order.

[15] A hydrogen production method using the membrane electrode assembly according to

[14] .

[16] A hydrogen production system including the membrane electrode assembly according to

[14] .

[0012] The hydroxy ion conductive membrane of the present invention has excellent hydroxy ion conductivity, excellent mechanical strength despite its thin film, and sufficiently low air permeability. An MEA formed by sandwiching this hydroxy ion conductive membrane between a cathode and an anode can sufficiently maintain low resistance even when current or liquid transport is repeatedly turned on and off. The method for producing a hydroxy ion conductive membrane of the present invention can provide a hydroxy ion conductive membrane that has excellent hydroxy ion conductivity, excellent mechanical strength despite its thin film, and sufficiently low air permeability. The membrane electrode assembly of the present invention has a hydroxy ion conductive membrane sandwiched between a cathode and an anode that has excellent hydroxy ion conductivity, and is thin but also has excellent mechanical strength. The membrane electrode assembly has low resistance and can sufficiently maintain low resistance even when current or liquid transport is repeatedly turned on and off. The hydrogen production method and hydrogen production system of the present invention use a hydroxy ion conductive membrane or membrane electrode assembly that has the above-mentioned properties, and can produce hydrogen while sufficiently maintaining low resistance even when current or liquid transport is repeatedly turned on and off.

[0013] Fig. 1 is a schematic diagram showing an outline of one embodiment of a method for producing a hydroxy ion conductive membrane, and Fig. 2 is a longitudinal sectional view showing a schematic basic layer structure of one embodiment of a membrane electrode assembly according to the present invention.

[0014] [Hydroxy ion conductive membrane] The hydroxy ion conductive membrane of the present invention can be widely used as a hydroxy ion conductive membrane in, for example, water electrolysis, fuel cells, toluene electrolytic reduction, carbon dioxide reduction, etc. In particular, the hydroxy ion conductive membrane of the present invention is suitable as a diaphragm separating an anode and a cathode in water electrolysis. The hydroxy ion conductive membrane of the present invention has hydroxy ion conductivity, and, despite being a thin film, has excellent mechanical strength and sufficiently low air permeability.

[0015] In the present invention, "having hydroxy ion conductivity" means that the ionic conductivity obtained by the following method is 0.1 mS / cm or more. -Ionic Conductivity- A 10 mm x 50 mm rectangular test piece is clamped between an Electro Chem BT-115 or a jig (attachment with electrodes) described in Electrochemistry, 85, 40, and AC impedance is measured by the four-terminal method. This AC impedance measurement is performed in a 1 M KOH aqueous solution heated to 45°C, at a voltage of 10 mV and a frequency in the range of 1 MHz to 0.1 Hz. The intercept (an extrapolated value is also acceptable) on the real axis of the Nyquist plot is taken as the resistance value R of the test piece, and the ionic conductivity σ is calculated using the following formula. Ion conductivity σ (S / cm) = D (cm) / (W (cm) × T (cm) × resistance value R (Ω)), D: length (cm) of test piece, W: width (cm) of test piece, T: thickness (cm) of test piece. The hydroxy ion conductivity of the hydroxy ion conductive membrane of the present invention is preferably 0.1 mS / cm to 10 S / cm, more preferably 10 mS / cm to 1 S / cm, and even more preferably 10 mS / cm to 500 mS / cm.

[0016] The hydroxy ion conductive membrane of the present invention comprises a porous substrate and a hydroxy ion conductive polymer disposed at least within the pores of the porous substrate. The hydroxy ion conductive polymer may be disposed at least within the pores of the porous substrate, and the hydroxy ion conductive polymer may be disposed not only within the pores of the porous substrate but also in portions other than the pores. For example, the hydroxy ion conductive polymer may cover the surface of the porous substrate.

[0017] <Hydroxy Ion Conductive Polymer> The hydroxy ion conductive polymer has a specific structural component in a specific amount. Specifically, the polymer contains 50 mol% or more of the component (I) derived from a polyfunctional polymerizable monomer (hereinafter simply referred to as "polyfunctional polymerizable monomer IA") having a total of two or more atoms of at least one of oxygen, sulfur, and nitrogen atoms (hereinafter, these three atoms are collectively referred to as "heteroatoms") in a structural portion other than the polymerizable group. That is, the hydroxy ion conductive polymer is a polymer obtained by polymerizing a polymerizable monomer-containing liquid containing 50 mol% or more of the polyfunctional polymerizable monomer IA among all the monomers. This polymerization forms a three-dimensional network structure, producing the hydroxy ion conductive polymer. The hydroxy ion conductive polymer containing the component (I) exhibits excellent hydroxy ion conductivity because the heteroatom-containing structural portion effectively increases the polarity of the polymer, allowing it to stably retain more water.

[0018] The elemental composition of the hydroxy ion conductive polymer preferably satisfies the following formula (a), more preferably the following formula (a-1), even more preferably the following formula (a-2), and even more preferably the following formula (a-3): Increasing the proportion of heteroelements can impart higher hydroxy ion conductivity to the hydroxy ion conductive polymer or a hydroxy ion conductive membrane containing this polymer. [Formula (a)] [total of the concentrations of oxygen, sulfur and nitrogen elements (at%)] / [carbon element concentration (at%)] ≧ 0.05 [Formula (a-1)] 50.00 ≧ [total of the concentrations of oxygen, sulfur and nitrogen elements (at%)] / [carbon element concentration (at%)] ≧ 0.06 [Formula (a-2)] 40.00 ≧ [total of the concentrations of oxygen, sulfur and nitrogen elements (at%)] / [carbon element concentration (at%)] ≧ 0.07 [Formula (a-3)] 38.00 ≧ [total of the concentrations of oxygen, sulfur and nitrogen elements (at%)] / [carbon element concentration (at%)] ≧ 0.08 The element concentration (at%) of each element means the element concentration of each element in the hydroxy ion conductive polymer, which is 100 at% of the total element concentration of all elements. The concentration (at%) of each element contained in the hydroxy ion conductive polymer can be measured using an organic elemental analyzer.

[0019] The polyfunctional polymerizable monomer IA from which the above-mentioned component (I) is derived will now be described.

[0020] In the present invention, the term "polyfunctional polymerizable monomer" refers to a monomer having two or more polymerizable groups in one molecule. In other words, "polyfunctional" refers to having two or more polymerizable groups in one molecule. When the polymerizable group is a chain-polymerizable group, the polyfunctional polymerizable monomer IA is preferably a divalent to hexavalent polyfunctional polymerizable monomer (i.e., a monomer having 2 to 6 chain-polymerizable groups in one molecule), and preferably a divalent to tetravalent polyfunctional polymerizable monomer. When the polymerizable group is a step-polymerizable group, the polyfunctional polymerizable monomer IA is preferably a divalent to hexavalent polyfunctional polymerizable monomer (i.e., a monomer having 2 to 6 step-polymerizable groups in one molecule), and preferably a divalent to pentavalent polyfunctional polymerizable monomer, preferably a divalent to tetravalent polyfunctional polymerizable monomer, and more preferably a divalent to trivalent polyfunctional polymerizable monomer. By using a polyfunctional polymerizable monomer, more crosslinked structures tend to be formed in the resulting polymer compared to when a monofunctional polymerizable monomer is used, resulting in a denser film.

[0021] The polymerizable group of the polyfunctional polymerizable monomer IA is not particularly limited as long as it can cause a polymerization reaction, and may be a chain polymerizable group (e.g., a vinyl group (CH ２ ═CH—), vinyl ether group (CH ２ ═CH—O—), vinyl thioether group (CH ２ =CH-S-), acryloyl group (CH ２ ═CH—C(═O)—), methacryloyl group (CH ２ =C(CH ３)-C(═O)-) and derivatives thereof, and other groups having a carbon-carbon double bond; cyclic ether groups such as an epoxy group (oxiranyl group) or an oxetanyl group); and groups capable of step-growth polymerization include groups that undergo condensation polymerization or polyaddition (hydroxy group, unsubstituted amino group, carboxy group, sulfo group, isocyanate group, acid anhydride group, etc.). The polyfunctional polymerizable monomer IA preferably has two or more chain-polymerizable groups. The polyfunctional polymerizable monomer IA may also have both a chain-polymerizable group and a step-growth polymerizable group. It is also preferable that the polyfunctional polymerizable monomer IA has two or more chain-polymerizable groups and does not have a step-growth polymerizable group. In the present invention, the term "step-growth polymerizable group" refers to a group that reacts with another group to undergo step-growth polymerization under the polymerization reaction conditions of the polyfunctional polymerizable monomer IA to obtain a hydroxy ion-conductive polymer. That is, under these polymerization reaction conditions, if, for example, the above-mentioned "other groups" are not present and step-growth polymerization cannot occur (i.e., if they remain as unreacted groups in the resulting hydroxy ion conductive polymer), even if the polyfunctional polymerizable monomer IA contains groups listed as groups capable of step-growth polymerization, such as the above-mentioned hydroxy group, unsubstituted amino group, carboxy group, sulfo group, isocyanate group, and acid anhydride group, these groups are not "step-growth polymerizable groups." Even if a group is listed as a group capable of step-growth polymerization, if step-growth polymerization cannot occur under the above-mentioned polymerization reaction conditions, the oxygen atom, nitrogen atom, etc. constituting this group will constitute a heteroatom present in a structural part other than the polymerizable group of the polyfunctional polymerizable monomer IA.

[0022] The "polymerizable group" is preferably a group selected from a vinyl group, a vinyl ether group, a vinyl thioether group, an acryloyl group, a methacryloyl group, an epoxy group, and an oxetanyl group, and more preferably a group selected from a vinyl group, an acryloyl group, a methacryloyl group, and an epoxy group. When the polyfunctional polymerizable monomer IA has a styryl group, the "polymerizable group" is a vinyl group. That is, the benzene ring constituting the styryl group constitutes the "structural moiety other than the polymerizable group" defined in the present invention. Similarly, for groups containing a vinyl group other than a vinyl ether group, a vinyl thioether group, an acryloyl group, and a methacryloyl group, the polymerizable group is the vinyl group, and the portion other than the vinyl group constitutes the "structural moiety other than the polymerizable group" defined in the present invention. On the other hand, in the present invention, the chain-polymerizable groups, such as the ether group (-O-) possessed by the vinyl ether group, the thioether group (-S-) possessed by the vinyl thioether group, and the carbonyl group (-C(=O)-) possessed by the acryloyl group and the methacryloyl group, as well as the heteroatoms contained in the sequentially polymerizable groups, such as the hydroxy group, the amino group, the carboxy group, the isocyanate group, the sulfo group, and the acid anhydride, are all considered to be part of the structure of the "polymerizable group" possessed by the polyfunctional polymerizable monomer IA. Therefore, the heteroatoms constituting these polymerizable groups are not atoms constituting the "structural moiety other than the polymerizable group" defined in the present invention.

[0023] As described above, the polyfunctional polymerizable monomer IA has a total of two or more atoms of at least one of oxygen atoms, sulfur atoms, and nitrogen atoms in the structural portion other than the polymerizable group (i.e., the total number of oxygen atoms, sulfur atoms, and nitrogen atoms contained in the structural portion other than the polymerizable group is two or more). The combination of these atoms is not particularly limited, and may be a structure having two or more oxygen atoms in the structural portion other than the polymerizable group and no sulfur atoms and no nitrogen atoms, a structure having two or more sulfur atoms in the structural portion other than the polymerizable group and no oxygen atoms and no nitrogen atoms, or a structure having two or more nitrogen atoms in the structural portion other than the polymerizable group and no oxygen atoms and no sulfur atoms. In addition, two or more different types of heteroatoms may be combined to have two or more (for example, a structure having one sulfur atom and one oxygen atom). In the polyfunctional polymerizable monomer IA, although it depends on the size of the structural moiety other than the polymerizable group, the total number of oxygen atoms, sulfur atoms, and nitrogen atoms contained in the structural moiety other than the polymerizable group is preferably 2 to 160, more preferably 2 to 140, even more preferably 4 to 100, still more preferably 6 to 80, and particularly preferably 6 to 60. In addition, the total number of oxygen atoms, sulfur atoms, and nitrogen atoms contained in the structural moiety other than the polymerizable group may also be 10 to 60.

[0024] The structural portion of the polyfunctional polymerizable monomer IA other than the polymerizable group typically contains atoms other than oxygen, sulfur, and nitrogen atoms. The atoms other than oxygen, sulfur, and nitrogen atoms contained in this structural portion are not particularly limited. The atoms other than oxygen, sulfur, and nitrogen atoms contained in this structural portion are typically carbon and hydrogen atoms. Furthermore, when this structural portion has a cationic or anionic structure, it may contain atoms other than oxygen, sulfur, and nitrogen atoms as counter ions. For example, when the structural portion other than the polymerizable group of the polyfunctional polymerizable monomer IA is a cationic structure having a quaternary ammonium group, it may contain a halogen ion, hydroxy ion, carbonate ion, or the like as counter ions. It is preferable that the polyfunctional polymerizable monomer IA does not contain fluorine atoms in its structure. The absence of fluorine atoms allows for both high anionic conductivity and high gas barrier properties to be achieved.

[0025] The structural portion of the polyfunctional polymerizable monomer IA other than the polymerizable group may be linear, may have a branched structure, or may have a cyclic structure.

[0026] In the structural portion other than the polymerizable group of the polyfunctional polymerizable monomer IA, the number of atoms constituting the shortest link structure connecting two adjacent heteroatoms is preferably 6 or less, more preferably 5 or less, even more preferably 4 or less, and even more preferably 3 or less. For example, in structures (i) and (ii) described below, the atom constituting the shortest link structure between two adjacent oxygen atoms is a carbon atom, and the number of atoms (number of carbon atoms) constituting the shortest link structure is 2. In addition, in structure (iii), the atom constituting the shortest link structure between two adjacent oxygen atoms is a carbon atom, and the number of atoms (number of carbon atoms) constituting the shortest link structure is 1. In addition, in structure (iv), when L is phenylene in which the two linking sites are in a para-position relationship with respect to each other, the atom constituting the shortest link structure between the sulfur atom and oxygen atom adjacent to each other via this phenylene is a carbon atom, and the number of atoms (number of carbon atoms) constituting the shortest link structure is 4. In addition, in structure (iv), when (-S(=O) ２ There are no atoms between the two adjacent heteroatoms in the (-S(=O) ２ In the structure (v), the number of atoms constituting the above-mentioned "shortest link structure" is 0. In structures (v) to (vii), the atoms constituting the shortest link structure between two nitrogen atoms are carbon atoms, and the number of atoms (number of carbon atoms) constituting the shortest link structure is 2. In structure (vii), the atoms constituting the shortest link structure between two nitrogen atoms are carbon atoms, and the number of atoms (number of carbon atoms) constituting the shortest link structure is 6.

[0027] The structural portion of the polyfunctional polymerizable monomer IA other than the polymerizable group has two or more groups selected from oxygen atoms, sulfur atoms, and nitrogen atoms, thereby forming a highly polar structure. Specific examples of the structure having a total of two or more atoms of at least one kind of oxygen atoms, sulfur atoms, and nitrogen atoms that the structural portion has include, for example, a polyalkyleneoxy structure, a carbonate structure (—O—C(═O)—O—), a sulfonyl group (—S(═O) ２a structure having one or more sulfonyl groups (-S(=O) ２ -) and an ether group (-O-) (for example, -O-(CH ２ ) ２ -S (=O) ２ - (CH ２ ) ２ -O-), a structure having two or more imino groups (for example, a polyethyleneimine structure), a structure having two or more quaternary ammonium groups, etc. When the polyfunctional polymerizable monomer IA has two or more groups selected from oxygen atoms, sulfur atoms, and nitrogen atoms in a structural portion other than the polymerizable group, a highly polar structure is formed, and a hydroxy ion conductive polymer obtained by polymerizing this or a hydroxy ion conductive membrane containing this polymer can retain more water and have higher hydroxy ion conductivity.

[0028] The chemical formula weight of the structural portion other than the polymerizable groups of the polyfunctional polymerizable monomer IA is preferably 30 to 4,000, more preferably 50 to 3,000, even more preferably 100 to 2,000, and particularly preferably 100 to 1,000. The value obtained by dividing the "chemical formula weight of the structural portion other than the polymerizable groups" by the "total number of oxygen atoms, sulfur atoms, and nitrogen atoms contained in the structural portion other than the polymerizable groups" ("chemical formula weight of the structural portion other than the polymerizable groups" / "total number of oxygen atoms, sulfur atoms, and nitrogen atoms contained in the structural portion other than the polymerizable groups") of the polyfunctional polymerizable monomer IA is preferably 2 to 100, more preferably 2 to 80, even more preferably 4 to 60, and particularly preferably 5 to 50. The molecular weight of the polyfunctional polymerizable monomer IA is preferably 150 to 5,000, more preferably 180 to 4,500, and even more preferably 200 to 3,000.

[0029] The polyfunctional polymerizable monomer IA may have, in a structural portion other than the polymerizable group, at least one of the following structural portions (i) to (viii).

[0030]

[0031] In the above structural portions (i) to (iii) and (vii), n is an integer of 1 or more, preferably 1 to 50, more preferably 2 to 35, even more preferably 3 to 30, still more preferably 4 to 25, still more preferably 5 to 20, still more preferably 5 to 15, and particularly preferably 5 to 10.

[0032] L represents a single bond, an alkylene group, or an arylene group. When L represents an alkylene group, the alkylene group preferably has 1 to 10 carbon atoms, more preferably 1 to 6 carbon atoms, even more preferably 1 to 3 carbon atoms, and still more preferably 1 or 2 carbon atoms. When L represents an arylene group, the arylene group preferably has 6 to 18 carbon atoms, more preferably 6 to 14 carbon atoms, and still more preferably 6 to 10 carbon atoms. The arylene group is preferably a phenylene group.

[0033] * indicates the linkage site to other structures.

[0034] The structural portion represented by formula (iv) is preferably a structure represented by the following formula (ix), and is also preferably a structural portion represented by formula (x).

[0035]

[0036] The above-mentioned constituent component (I) (a constituent unit derived from the polyfunctional polymerizable monomer IA) of the hydroxy ion conductive polymer is preferably a nonionic constituent component from the viewpoint of durability against continuous high-temperature operation under alkaline conditions. For example, the constituent component (I) preferably has at least one of the above-mentioned structural units (i) to (iv), and more preferably has structural unit (i) or (ii).

[0037] The hydroxy ion conductive polymer can have one or more types of constituent (I). Here, structural unit (vii) has one nitrogen atom in the structure enclosed in parentheses, but when constituent (I) has only structural unit (vii), n in structural unit (vii) is an integer of 2 or more, and when constituent (I) has structural unit (vii) and any of (i) to (vi) and (viii) in combination, n in structural unit (vii) is an integer of 1 or more.

[0038] Preferred specific examples of the polyfunctional polymerizable monomer IA are shown below, where n is an integer of 1 or more. In the preferred specific examples of the polyfunctional polymerizable monomer IA, the hydroxy ion may be a halogen ion (e.g., a chloride ion) or a carbonate ion.

[0039]

[0040]

[0041] The hydroxy ion conductive polymer may be a polymer composed of component (I), or may contain, in addition to component (I), component (II) derived from a monomer other than the polyfunctional polymerizable monomer IA. Component (II) can be appropriately employed as long as it does not impair the effects of the present invention. Examples include, but are not limited to, components derived from monofunctional or polyfunctional polymerizable monomers having one heteroatom in the structural portion other than the polymerizable group, components derived from monofunctional polymerizable monomers having a total of two or more heteroatoms of at least one type in the structural portion other than the polymerizable group, and components derived from monofunctional or polyfunctional polymerizable monomers having no heteroatoms in the structural portion other than the polymerizable group. The polymerizable group that can be included in the polymerizable monomer from which component (II) is derived is the same as the polymerizable group in the polyfunctional polymerizable monomer IA, and the preferred form of the polymerizable group is also the same. The polymerizable group that can be included in the polymerizable monomer from which component (II) is derived is a polymerizable group that can undergo a polymerization reaction with the polymerizable group in the polyfunctional polymerizable monomer IA.

[0042] The molecular weight of the monomer from which component (II) is derived is preferably 80 to 1,000, more preferably 90 to 500, and even more preferably 100 to 300.

[0043] As component (II), a structure having an aryl group is preferred, and the aryl group may further have a substituent. Of these, component (II) is preferably a structure having a benzene ring, and more preferably a structure having one benzene ring. As the substituent, a group having an ammonium group, a group having a primary to tertiary amine group, or the like is preferred.

[0044] The proportion of the component (I) in the total components of the hydroxy ion conductive polymer is 50 mol% or more. The proportion of the component (I) in the total components of the hydroxy ion conductive polymer is preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, may be 90 mol% or more, or may be 95 mol% or more, and all of the components of the hydroxy ion conductive polymer may be the component (I). By increasing the proportion of the component (I), gas permeability can be further reduced.

[0045] The hydroxy ion conductive polymer can retain a large amount of water due to the highly polar structure of component (I). The water retention capacity of the hydroxy ion conductive polymer is preferably 5% by mass or more, more preferably 5 to 30% by mass, more preferably 5 to 27% by mass, even more preferably 10 to 27% by mass, and particularly preferably 18 to 25% by mass. The water retention capacity of the hydroxy ion conductive polymer can be measured by the method described in the examples.

[0046] <Porous substrate> The porous substrate is not particularly limited as long as it has pores that allow water and ions to pass through. The porous substrate preferably has a large number of fine pores that penetrate from the front surface to the back surface. For example, a porous substrate that can be used as a separator for water electrolysis can be used. The constituent material of the porous substrate preferably contains a resin. Examples of constituent materials of the porous substrate include hydrocarbon polymers (preferably polyolefins, more preferably polyethylene or polypropylene), fluoropolymers (polytetrafluoroethylene, etc.), polystyrene, cellulose, polyacrylonitrile, ceramics, etc., and it is preferable to use a hydrocarbon polymer. The porous substrate may be any substrate with a porous structure. For example, it may be a membrane (porous membrane) formed by forming a resin into a film and stretching it to form pores, or it may be a woven fabric or a nonwoven fabric. When the porous substrate is a porous membrane, the porous substrate may be manufactured by a dry method or a wet method, and it is preferable that it is manufactured by a dry method. Here, dry method is the method of forming molten resin into a film, heat treating it, and then stretching it to form pores, and wet method is the method of melting resin and plasticizer (solvent), forming it into a film, stretching it, and then extracting plasticizer to form pores.In the present invention, porous substrate is preferably polyethylene porous membrane, polypropylene porous membrane, polyethylene nonwoven fabric or polytetrafluoroethylene nonwoven fabric, and more preferably polyethylene porous membrane.

[0047] The thickness of the porous substrate is sufficient as long as a hydroxy ion conductive membrane of 5 μm or more and less than 50 μm is obtained. Therefore, it is preferably 5 μm or more and less than 50 μm, more preferably 6 μm or more and 40 μm or less, even more preferably 6 μm or more and 35 μm or less, even more preferably 7 μm or more and 30 μm or less, even more preferably 7 μm or more and 25 μm or less, even more preferably 7 μm or more and 20 μm or less, and particularly preferably 7 μm or more and 15 μm or less. The thickness of the porous substrate is measured after drying the porous substrate.

[0048] The porosity of the porous substrate is preferably 20 to 80%, more preferably 25 to 70%, even more preferably 30 to 60%, even more preferably 35 to 60%, even more preferably 37 to 55%, even more preferably 38 to 50%, and particularly preferably 40 to 50%. The porosity can be measured by the method described in the examples.

[0049] The tensile strength of the porous substrate is preferably 10 MPa or more, more preferably 20 MPa or more, and even more preferably 30 MPa or more. The tensile strength of the porous substrate is usually 400 MPa or less. Therefore, the tensile strength of the porous substrate is preferably 10 to 400 MPa, more preferably 20 to 400 MPa, and even more preferably 30 to 400 MPa. The breaking elongation of the porous substrate is preferably 30% or more, more preferably 50% or more, even more preferably 70% or more, and even more preferably 100% or more. The breaking elongation of the porous substrate is usually 400% or less. Therefore, the breaking elongation of the porous substrate is preferably 30 to 400%, more preferably 50 to 400%, even more preferably 70 to 400%, and even more preferably 100 to 400%. The tensile strength and breaking elongation of the porous substrate can be measured by the method described in the Examples. It is preferable that the porous substrate have both the above tensile strength and breaking elongation. Therefore, it is preferable that the tensile strength is 20 MPa or more and the elongation at break is 100% or more.

[0050] The average pore diameter of the porous substrate is preferably 5 nm or more, more preferably 10 nm or more, and even more preferably 20 nm or more. The average pore diameter of the porous substrate is usually 100 μm or less, preferably 50 μm or less, and even more preferably 1 μm or less. By using a porous substrate within this range, a good hydroxy ion conductive membrane that combines strength and low resistance can be produced. Therefore, the average pore diameter of the porous substrate is preferably 5 nm to 100 μm, more preferably 10 nm to 50 μm, even more preferably 20 nm to 30 μm, even more preferably 20 nm to 27 μm, even more preferably 20 nm to 20 μm, even more preferably 20 nm to 10 μm, even more preferably 20 nm to 1 μm, and particularly preferably 20 nm to 100 nm. The average pore diameter of the porous substrate can be evaluated using mercury intrusion porosimetry. Specifically, it can be measured by the method described in the Examples.

[0051] The porous substrate is preferably surface-modified. Surface modification allows a physical or chemical bond to be formed with the hydroxy ion-conductive polymer during synthesis of the hydroxy ion-conductive polymer, thereby further enhancing the adhesion between the hydroxy ion-conductive polymer and the porous substrate. Examples of surface modification methods include corona treatment and plasma treatment. Corona treatment and plasma treatment can be performed by conventional methods.

[0052] As the porous substrate, for example, the “porous substrate film” and “laminated porous substrate film” described in paragraphs

[0018] to

[0023] of JP-A No. 2018-127506, and the “microporous membrane” described in paragraph

[0022] of JP-A No. 2022-181107 are also suitable.

[0053] The thickness of the hydroxy ion conductive membrane is 5 μm or more but less than 50 μm, preferably 6 to 40 μm, more preferably 6 to 35 μm, even more preferably 7 to 30 μm, even more preferably 7 to 25 μm, even more preferably 7 to 20 μm, and particularly preferably 7 to 15 μm. The thickness of the hydroxy ion conductive membrane is determined by measuring the thickness at 10 random locations and calculating the arithmetic mean of the 10 measurements. The thickness of the porous substrate described above is also determined in a similar manner. The thickness of the hydroxy ion conductive membrane may be the same as or thicker than the thickness of the porous substrate. During synthesis of the hydroxy ion conductive polymer, the thickness of the hydroxy ion conductive membrane may be the same as the thickness of the porous substrate, or may be approximately 110 to 120% or approximately 120 to 140% of the thickness of the porous substrate. When the thickness of the hydroxy ion conductive membrane is greater than that of the porous substrate, the polymer filling the internal pores is introduced at a rate greater than the proportion of the pores (for example, the porous substrate is elongated (expanded) to fill a large amount of polymer, or the hydroxy ion conductive polymer is also provided on the outer surface of the porous substrate). The thickness of the hydroxy ion conductive membrane is measured after drying the hydroxy ion conductive membrane to remove water.

[0054] The tensile strength of the hydroxy ion conductive membrane is preferably 10 MPa or more, more preferably 15 MPa or more, and even more preferably 20 MPa or more. The tensile strength of the hydroxy ion conductive membrane is usually 300 MPa or less. Therefore, the tensile strength of the hydroxy ion conductive membrane is preferably 10 to 300 MPa, more preferably 15 to 300 MPa, and even more preferably 20 to 300 MPa. The breaking elongation of the hydroxy ion conductive membrane is preferably 30% or more, more preferably 50% or more, even more preferably 70% or more, and even more preferably 100% or more. The breaking elongation of the hydroxy ion conductive membrane is usually 400% or less. Therefore, the breaking elongation of the hydroxy ion conductive membrane is preferably 30 to 400%, more preferably 50 to 400%, even more preferably 70 to 400%, and even more preferably 100 to 400%. The tensile strength and breaking elongation of the hydroxy ion conductive membrane can be measured by the methods described in the examples. The hydroxy ion conductive membrane preferably has both the above-mentioned tensile strength and elongation at break, and therefore preferably has a tensile strength of 10 MPa or more and a elongation at break of 100% or more.

[0055] The air permeability of the hydroxy ion conductive membrane is preferably 500 seconds or more, more preferably 800 seconds or more, even more preferably 1000 seconds or more, still more preferably 10,000 seconds or more, and even more preferably 50,000 seconds or more. The air permeability of the hydroxy ion conductive membrane is usually 99,999 seconds (measurement limit) or less. The air permeability of the hydroxy ion conductive membrane is preferably 500 to 99,999 seconds, more preferably 1000 to 99,999 seconds. The air permeability of the hydroxy ion conductive membrane can be measured by the method described in the Examples. Low air permeability means that it takes a long time for gas to permeate. In other words, the lower the air permeability, the more difficult it is for gas to pass through.

[0056] The water retention capacity of the hydroxy ion conductive membrane is preferably 2% by mass or more, more preferably 2 to 25% by mass, more preferably 5 to 20% by mass, even more preferably 7 to 20% by mass, and particularly preferably 9 to 15% by mass. The water retention capacity of the hydroxy ion conductive membrane can be measured by the method described in the Examples.

[0057] The hydroxy ion conductive membrane preferably has a differential voltage of 0.50 V or less, more preferably 0.30 V or less, and even more preferably 0.25 V or less, as measured by the method described in the Examples ("Resistance Evaluation") below. The differential voltage is usually greater than 0.00 V. Therefore, the differential voltage is preferably greater than 0.00 V and 0.50 V or less, more preferably greater than 0.00 V and 0.30 V or less, and even more preferably greater than 0.00 V and 0.25 V or less. The differential voltage can be measured by the method described in the Examples in a state where the hydroxy ion conductive membrane is incorporated into a water electrolysis cell.

[0058] The hydroxy ion conductive membrane preferably has a hydrogen gas permeability (permeability of hydrogen gas generated on the cathode side to the anode side) of less than 1.000 vol% (less than 1 vol%), more preferably less than 0.500 vol%, and even more preferably less than 0.100 vol%. The hydrogen gas permeability is usually preferably 0.001 vol% or more. Therefore, the hydrogen gas permeability is preferably 0.001 vol% or more and less than 1.000 vol%, more preferably 0.001 vol% or more and less than 0.500 vol%, and even more preferably 0.001 vol% or more and less than 0.100 vol%. The hydrogen gas permeability can be measured by the method described in the Examples after incorporating the hydroxy ion conductive membrane into a water electrolysis cell.

[0059] The hydroxy ion conductive membrane preferably exhibits a voltage change rate of 2% or less, more preferably 1% or less, and even more preferably 0.5% or less, when subjected to 500 cycles of current application and interruption after the initial current application, using the method described in the Examples below. The voltage change rate is usually 0.001% or more. Therefore, the voltage change rate is preferably 0.001 to 2%, more preferably 0.001 to 1%, and even more preferably 0.001 to 0.5%.

[0060] [Method of Manufacturing Hydroxy Ion Conductive Membrane] A method of manufacturing the hydroxy ion conductive membrane of the present invention will be described. The hydroxy ion conductive membrane of the present invention can be obtained by impregnating a porous substrate with a polymerizable monomer-containing solution containing 50 mol % or more of a polyfunctional polymerizable monomer IA, followed by polymerization of the polymerizable monomer. FIG. 1 is a schematic diagram showing an outline of the method of manufacturing a hydroxy ion conductive membrane. The left side of FIG. 1 shows a porous substrate 1a. The right side of the arrow in FIG. 1 shows a hydroxy ion conductive membrane 1 obtained by impregnating the porous substrate 1a with the polymerizable monomer-containing solution and polymerizing it. In this hydroxy ion conductive membrane 1, a hydroxy ion conductive polymer 1b is disposed within the pores and on the surface of the porous substrate 1a. As described above, the hydroxy ion conductive polymer 1b may be disposed within the pores of the porous substrate 1a, and may or may not cover the surface of the porous substrate 1a.

[0061] The polymerizable monomers contained in the polymerizable monomer-containing liquid (polyfunctional polymerizable monomer IA and polymerizable monomers other than polyfunctional polymerizable monomer IA that are blended as needed) are as described above. In the polymerizable monomer-containing liquid, the proportion of polyfunctional polymerizable monomer IA in all polymerizable monomers is the same as the "proportion of component (I) in all constituent components of the hydroxy ion conductive polymer" described above. That is, the proportion of polyfunctional polymerizable monomer IA in all polymerizable monomers is 50 mol% or more, preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, may be 90 mol% or more, or may be 95 mol% or more, and all polymerizable monomers may be polyfunctional polymerizable monomer IA.

[0062] The polymerizable monomer-containing liquid may contain a solvent or may not contain a solvent. Examples of solvents include water, isopropyl alcohol, methanol, ethanol, acetonitrile, tetrahydrofuran (THF), etc. A mixture of these may also be used. When a solvent is used, the solid content of the polymerizable monomer-containing liquid is preferably 10% by mass or more, more preferably 30% by mass or more, even more preferably 50% by mass or more, even more preferably 70% by mass or more, and even more preferably 75% by mass or more. It is also preferable that the polymerizable monomer-containing liquid does not contain a solvent.

[0063] The polymerizable monomer-containing liquid may contain a polymerization initiator in addition to the polymerizable monomer. When the polymerizable group of the polymerizable monomer is a chain-polymerizable group, the polymerizable monomer-containing liquid usually contains a polymerization initiator. The polymerization initiator can be appropriately selected depending on the polymerizable group possessed by the polymerizable monomer. The polymerization initiator is preferably selected from photopolymerization initiators, thermal polymerization initiators, photocationic polymerization initiators, and thermal cationic polymerization initiators, and combinations of these can also be used. The polymerization initiator can be one that is commonly used for polymerizing polymerizable monomers having chain-polymerizable groups. The content of the polymerization initiator in the polymerizable monomer-containing liquid is preferably 0.1 to 15 parts by mass, more preferably 0.5 to 10 parts by mass, and more preferably 1 to 8 parts by mass, per 100 parts by mass of the total polymerizable monomers.

[0064] The method of impregnating a porous substrate with a polymerizable monomer-containing liquid is not particularly limited. For example, the porous substrate may be immersed in a polymerizable monomer-containing liquid to allow the polymerizable monomer-containing liquid to penetrate at least into the pores, or the polymerizable monomer-containing liquid may be sprayed or applied to the porous substrate to allow the polymerizable monomer-containing liquid to penetrate at least into the pores. For example, a porous substrate is laid on a resin substrate such as a polyethylene terephthalate (PET) film, and an appropriate amount of polymerizable monomer-containing liquid is placed on top of it, and then applied with an applicator. Then, another PET film is placed on the porous substrate, and pressure is applied from above the PET film to extrude excess polymerizable monomer-containing liquid, thereby obtaining an impregnated membrane in which a desired amount of polymerizable monomer-containing liquid is impregnated at least into the pores.

[0065] The method for polymerizing the polymerizable monomer is not particularly limited and can be appropriately selected depending on the type of polymerizable group possessed by the polymerizable monomer and the type of polymerization initiator. This polymerization reaction forms a hydroxy ion conductive polymer having a three-dimensional network structure. In the polymerization reaction, it is preferable to polymerize the polymerizable monomer by either light or heat. The light irradiation conditions and heating conditions can be appropriately set.

[0066] In this way, the polymerizable monomer impregnated in the porous substrate is polymerized to obtain a hydroxy ion conductive membrane containing a hydroxy ion conductive polymer at least in the pores of the porous substrate. During the polymerization reaction, a portion of the polymerizable monomer may be chemically bonded to functional groups on the surface of the porous substrate.

[0067] [Membrane Electrode Assembly] The membrane electrode assembly of the present invention has a structure in which a cathode catalyst layer, a hydroxy ion conductive membrane of the present invention, and an anode catalyst layer are laminated in this order. In addition to the cathode catalyst layer, the hydroxy ion conductive membrane of the present invention, and the anode catalyst layer, the membrane electrode assembly may also include a pair of gas diffusion layers on the outside of each of the cathode catalyst layer and the anode catalyst layer. FIG. 2 shows a preferred embodiment of the membrane electrode assembly of the present invention. The membrane electrode assembly 4 of FIG. 2 has a cathode catalyst layer 2c formed on one side of a hydroxy ion conductive membrane 1, an anode catalyst layer 2a formed on the other side, and a gas diffusion layer 3 formed on each of these catalyst layers on the side opposite the hydroxy ion conductive membrane 1. This membrane electrode assembly 4 has a laminated structure in which the gas diffusion layer 3, the cathode catalyst layer 2c, the hydroxy ion conductive membrane 1, the anode catalyst layer 2a, and the gas diffusion layer 3 are laminated in this order. The hydroxy ion conductive membrane 1 has a porous substrate and a hydroxy ion conductive polymer disposed in the pores of the substrate, but is shown as an integrated layer in FIG. 2. The anode catalyst layer 2a includes a particulate anode catalyst 21 and an ionomer resin 23 that also functions as a binder, and the anode catalyst 21 is bound by the ionomer resin 23. The cathode catalyst layer 2c includes a particulate cathode catalyst 22 and the ionomer resin 23 that also functions as a binder, and the cathode catalyst 22 is bound by the ionomer resin 23. In Figure 2, a bipolar plate 5 is further formed on the surface 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.

[0068] Each member constituting the membrane electrode assembly will be described below.

[0069] <Cathode Catalyst Layer> The cathode catalyst layer can be a cathode catalyst layer that can be used in a conventional membrane electrode assembly. The cathode catalyst layer is preferably a layer containing a cathode catalyst and an ionomer resin. The cathode catalyst layer may be formed by adhering, baking, and firing a catalyst layer onto a metal mesh or net. Corrosion-resistant stainless steel (SUS) or nickel is preferred for the mesh or net. The cathode catalyst may be any catalyst that electrolyzes water to produce hydrogen (gas). Examples of cathode catalysts that can be used include platinum-supported carbon particles, platinum-coated titanium, palladium-supported carbon particles, cobalt glyoxime, and nickel glyoxime. Non-noble metal catalysts such as iron, iron-rare earth, iron-nickel alloys, and nickel may also be used. The cathode catalyst is preferably in particulate form.

[0070] The ionomer resin is preferably anionic ionomer.As the ionomer resin, sulfonated plastic electrolytes such as perfluoroalkanesulfonic acid, sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfone, sulfonated polysulfide, sulfonated polyphenylene, etc., sulfoalkylated plastic electrolytes such as sulfoalkylated polyether ether ketone, sulfoalkylated polyether sulfone, sulfoalkylated polyether ether sulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide, sulfoalkylated polyphenylene, etc. can be used.

[0071] The cathode catalyst layer can be formed by preparing an ink for forming a catalyst layer containing a cathode catalyst, an ionomer resin, and, if necessary, a solvent, and applying and drying this ink.

[0072] <Anode Catalyst Layer> The anode catalyst layer can be an anode catalyst layer that can be used in a typical membrane electrode assembly. The anode catalyst layer is preferably a layer containing an anode catalyst and an ionomer resin. The anode catalyst layer may be formed by adhering, baking, and firing a catalyst layer onto a metal mesh or net. Corrosion-resistant materials such as stainless steel or nickel are preferred for the mesh or net. Any anode catalyst can be used as long as it electrolyzes water to produce oxygen and water. Examples of anode catalysts that can be used include iridium oxide, iridium oxide-coated titanium, iridium ruthenium cobalt oxide, iridium ruthenium tin oxide, iridium ruthenium iron oxide, iridium ruthenium nickel oxide, iridium tin oxide, iridium zirconium oxide, ruthenium titanium oxide, ruthenium zirconium oxide, ruthenium tantalum oxide, and ruthenium titanium cerium oxide. Alternatively, non-noble metal catalysts such as nickel, nickel-based alloys, iron, nickel-coated iron, nickel-cobalt oxide, cobalt oxide, lanthanum-doped cobalt oxide, lanthanum strontium cobalt oxide, and zinc cobalt oxide may be used. The anode catalyst is preferably in particulate form. The ionomer resin may be the same as that described for the cathode catalyst layer.

[0073] The anode catalyst layer can be formed by preparing an ink for forming a catalyst layer containing an anode catalyst, an ionomer resin, and, if necessary, a solvent, and applying and drying this ink.

[0074] <Gas Diffusion Layer> The gas diffusion layer can be a layer that can transport gas and moisture and has electronic conductivity. The gas diffusion layer can be a gas diffusion layer that can be used in a normal membrane electrode assembly. Examples of materials that can be used for the gas diffusion layer include porous materials such as 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.

[0075] The membrane electrode assembly can be formed by applying a catalyst layer-forming ink to a hydroxy ion conductive membrane, or by bonding a pre-formed catalyst layer to a hydroxy ion 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-attached gas diffusion layer, which may then be bonded to the hydroxy ion conductive membrane.

[0076] [Hydrogen Production Method] The hydrogen production method of the present invention is the same as a conventional hydrogen production method (water electrolysis) except that the membrane electrode assembly of the present invention is used as the membrane electrode assembly. In the hydrogen production method of the present invention, since the hydroxy ion conductive membrane or membrane electrode assembly of the present invention has high hydroxy ion conductivity, water electrolysis can be performed using not only a high-concentration alkaline aqueous solution but also a lower-concentration alkaline aqueous solution or water.

[0077] [Hydrogen Production System] The hydrogen production system of the present invention is a system (apparatus) suitable for carrying out the hydrogen production method of the present invention. The hydrogen production system of the present invention is similar to a conventional hydrogen production system except that it includes the membrane electrode assembly of the present invention as the membrane electrode assembly. The hydrogen production system of the present invention can be, for example, a water electrolysis system in which the membrane electrode assembly of the present invention is further combined with components used in general water electrolysis, such as bipolar plates.

[0078] 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.

[0079] [Preparation of Ink for Forming Catalyst Layer] <Preparation of Ink 1-a1i for Forming Anode Catalyst Layer> In a 5 mL vial, 1.2 g of Nafion dispersion (anionic ionomer, manufactured by Fujifilm Wako Chemical Co., Ltd., 5% Nafion dispersion DE521 (trade name)) and 0.27 g of iridium oxide powder (manufactured by Tanaka Kikinzoku Kogyo K.K., TEC77100 (trade name), iridium content 75% by mass) were mixed and dispersed for 5 minutes using an ultrasonic homogenizer UH-300 (trade name, manufactured by SMT Co., Ltd.) at a power of 30 W, cooled at 5°C for 10 minutes, and then dispersed again for 5 minutes at a power of 30 W. In this way, Ink 1-a1i for forming anode catalyst layer was prepared.

[0080] <Preparation of Ink 1-b1i for Forming Cathode Catalyst Layer> In a 10 mL vial, 2.9 g of Nafion dispersion (anionic ionomer, manufactured by Fujifilm Wako Chemical Co., Ltd., 5% Nafion dispersion DE521 (trade name)) and 0.34 g of platinum carbon powder (manufactured by Tanaka Kikinzoku Kogyo K.K., TEC10E50E (trade name), platinum content 47% by mass) were mixed, and the mixture was dispersed for 5 minutes at a power of 30 W using an ultrasonic homogenizer UH-300 (trade name, manufactured by SMT Co., Ltd.), cooled at 5°C for 10 minutes, and then dispersed again for 5 minutes at a power of 30 W, thereby preparing ink 1-b1i for forming a cathode catalyst layer.

[0081] [Preparation of Gas Diffusion Layer (GDL) with Catalyst Layer] <Preparation of Gas Diffusion Layer 1-a2 with Anode Catalyst Layer 1-a1> Anode catalyst layer-forming ink 1-a1i was applied to a carbon fiber nonwoven fabric having a microporous layer (manufactured by SGL, GDL-39BB (product name)) using an applicator in such a way that the iridium coating amount was 1.0 mg / cm. ２ This was allowed to dry naturally to obtain a gas diffusion layer 1-a2 with an anode catalyst layer 1-a1. <Preparation of gas diffusion layer 1-b2 with cathode catalyst layer 1-b1> The ink 1-b1i for forming a cathode catalyst layer was applied to a carbon fiber nonwoven fabric having a microporous layer (manufactured by SGL, GDL-39BB (product name)) using an applicator so that the platinum coating amount was 1.0 mg / cm ２ This was allowed to dry naturally to obtain a gas diffusion layer 1-b2 with a cathode catalyst layer 1-b1 attached thereto.

[0082] [Preparation of Hydroxy Ion Conductive Membrane] Example 1 Preparation of Polymerizable Monomer-Containing Liquid A 200 mL three-neck flask was charged with 20.0 g of pure water, 6.0 g of isopropyl alcohol, and 72.0 g of Monomer m1 as a polyfunctional polymerizable monomer to prepare a solution. Under yellow light, 1.0 g of 2-hydroxy-2-methylpropiophenone (manufactured by TCI) and 1.0 g of ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (manufactured by TCI) were further added as polymerization initiators to prepare polymerizable monomer-containing liquid 1-m1 with a solids content of 74 mass%. - Preparation of hydroxy ion conductive membrane - Under yellow light, a 7 μm thick, 40% porosity polyethylene porous membrane (manufactured by Toray Industries, Inc., under the trade name Setira (product name)) (without corona treatment) was placed as a porous substrate on a 10 μm thick PET film, and an appropriate amount of polymerizable monomer-containing liquid 1-ml was placed on top of the porous substrate and applied with an applicator. After that, another PET film was placed on top of the coated polyethylene porous membrane, and the excess polymerizable monomer-containing liquid that had not impregnated into the polyethylene porous membrane was pushed out, forming an impregnated membrane in which the polymerizable monomer-containing liquid had impregnated the pores of the porous substrate. This impregnated membrane was then irradiated with 100 mJ / cm at 365 nm. ２ A UV irradiator (EXECURE3000 (trade name), manufactured by HOYA Corporation) adjusted to an integrated light dose of 800 mJ / cm 2 was used. ２ The hydroxy ion conductive polymer was then disposed in the pores of the porous substrate, and the surface of the porous substrate was also covered with the hydroxy ion conductive polymer. Thus, a hydroxy ion conductive membrane 1-c1 having a thickness of 10 μm was produced.

[0083] Example 2 A hydroxy ion conductive membrane 1-c2 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, a polyethylene porous membrane (manufactured by Toray Industries, Inc., under the trade name of Setira (product name)) (not corona-treated) with a thickness of 17 μm and a porosity of 59% was used as the porous substrate, and the thickness of the resulting hydroxy ion conductive membrane was 19 μm.

[0084] Example 3 A hydroxy ion conductive membrane 1-c3 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, a polypropylene porous membrane (CE2500 (product name) manufactured by Celgard Inc.) (without corona treatment) having a thickness of 25 μm and a porosity of 41% was used as the porous substrate, that the polymerization was performed by heating at 80° C. for 5 minutes in addition to UV irradiation, and that the thickness of the resulting hydroxy ion conductive membrane was 27 μm.

[0085] Example 4 A 200 mL three-neck flask was charged with 20.0 g of pure water, 6.0 g of isopropyl alcohol, and 36.0 g of monomer m1 and 15.4 g of monomer Xm1 as polyfunctional polymerizable monomers to prepare a solution. Under yellow light, 1.0 g of 2-hydroxy-2-methylpropiophenone (manufactured by TCI) and 1.0 g of ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (manufactured by TCI) were further added as polymerization initiators to prepare polymerizable monomer-containing solution 1-m4 with a solids content of 67 mass%. A hydroxy ion conductive membrane 1-c4 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, polymerizable monomer-containing solution 1-m4 was used as the polymerizable monomer-containing solution.

[0086] Example 5 A 200 mL three-neck flask was charged with 50.0 g of pure water, 25.0 g of isopropyl alcohol, and 17.4 g of Monomer m2 and 36.0 g of Monomer m6 as polyfunctional polymerizable monomers to prepare a solution. Under yellow light, 1.0 g of 2-hydroxy-2-methylpropiophenone (manufactured by TCI Corporation) as a polymerization initiator, 1.0 g of ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (manufactured by TCI Corporation), and 1.0 g of an azo-based polymerization initiator (V-501 (trade name), manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were further added to prepare a polymerizable monomer-containing solution 1-m5 having a solids content of 43% by mass. A hydroxy ion conductive membrane 1-c5 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, polymerizable monomer-containing liquid 1-m5 was used as the polymerizable monomer-containing liquid, that the polymerization was carried out by heating at 80°C for 10 minutes in addition to UV irradiation, and that the thickness of the resulting hydroxy ion conductive membrane was 9 μm.

[0087] Example 6 A 200 mL three-neck flask was charged with 72.0 g of monomer m3 as a polyfunctional polymerizable monomer, and under yellow light, 5.0 g of diphenyl[4-(phenylthio)phenyl]sulfonium hexafluorophosphate (Tokyo Chemical Industry Co., Ltd.) was further added as an initiator to prepare a polymerizable monomer-containing liquid 1-m6 having a solids content of 100 mass %. A hydroxy ion conductive membrane 1-c6 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, polymerizable monomer-containing liquid 1-m6 was used as the polymerizable monomer-containing liquid and that heating at 100°C for 10 minutes was performed in addition to UV irradiation during polymerization.

[0088] Example 7 A hydroxy ion conductive membrane 1-c7 was obtained in the same manner as in Example 6, except that in the preparation of the polymerizable monomer-containing liquid of Example 6, monomer m4 was used as the polyfunctional polymerizable monomer, 8.6 g of acetonitrile was used as the solvent to obtain a polymerizable monomer-containing liquid 1-m7 with a solid content of 90 mass %, and in the preparation of the hydroxy ion conductive membrane, polymerizable monomer-containing liquid 1-m7 was used as the polymerizable monomer-containing liquid.

[0089] Example 8 A hydroxy ion conductive membrane 1-c8 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, a polyethylene nonwoven fabric (Elfino (product name) manufactured by JNC Corporation, corona treated) with a thickness of 35 μm and a porosity of 65% was used as the porous substrate, and the thickness of the hydroxy ion conductive membrane was 45 μm.

[0090] Example 9 A hydroxy ion conductive membrane 1-c9 was obtained in the same manner as in Example 1, except that in the preparation of the polymerizable monomer-containing liquid in Example 1, 500 g of pure water and 172 g of isopropyl alcohol were used and the solid content was adjusted to 10 mass % to obtain a polymerizable monomer-containing liquid 1-m9, that in the preparation of the hydroxy ion conductive membrane, the polymerizable monomer-containing liquid 1-m9 was used as the polymerizable monomer-containing liquid, and that the thickness of the obtained hydroxy ion conductive membrane was 9 μm.

[0091] Example 10 A hydroxy ion conductive membrane 1-c10 was obtained in the same manner as in Example 7, except that in the preparation of the polymerizable monomer-containing liquid in Example 7, monomer m5 was used as the polyfunctional polymerizable monomer to obtain a polymerizable monomer-containing liquid 1-m10, and in the preparation of the hydroxy ion conductive membrane, polymerizable monomer-containing liquid 1-m10 was used as the polymerizable monomer-containing liquid, and the thickness of the obtained hydroxy ion conductive membrane was 7 μm.

[0092] Example 11 A hydroxy ion conductive membrane 1-c11 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, a polytetrafluoroethylene (PTFE) nonwoven fabric (PTFE nanofiber (product name) manufactured by Valqua Corporation) (corona-treated) having a thickness of 40 μm and a porosity of 65% was used as the porous substrate, and the thickness of the obtained hydroxy ion conductive membrane was 45 μm.

[0093] Example 12 A 200 mL three-neck flask was charged with 72.0 g of monomer m7 as a polyfunctional polymerizable monomer, and under yellow light, 5.0 g of diphenyl[4-(phenylthio)phenyl]sulfonium hexafluorophosphate (manufactured by Tokyo Chemical Industry Co., Ltd.) was further added as an initiator to prepare a polymerizable monomer-containing liquid 1-m12 having a solids content of 100 mass %. A hydroxy ion conductive membrane 1-c12 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, polymerizable monomer-containing liquid 1-m12 was used as the polymerizable monomer-containing liquid, a polyethylene porous membrane (manufactured by Toray Industries, Inc., under the trade name Cetela (product name)) (without corona treatment) having a thickness of 17 μm and a porosity of 59% was used as the porous substrate, heating was performed at 90°C for 10 minutes in addition to UV irradiation during polymerization, and the thickness of the resulting hydroxy ion conductive membrane was 20 μm.

[0094] Example 13 A 200 mL three-neck flask was charged with 16.0 g of pure water, 10.0 g of isopropyl alcohol, and 72.0 g of Monomer m8 as a polyfunctional polymerizable monomer to prepare a solution. Under yellow light, 1.0 g of 2-hydroxy-2-methylpropiophenone (manufactured by TCI) and 1.0 g of ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (manufactured by TCI) were further added as polymerization initiators to prepare a polymerizable monomer-containing solution 1-m13 with a solids content of 74 mass%. A hydroxy ion conductive membrane 1-c13 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, polymerizable monomer-containing liquid 1-m13 was used as the polymerizable monomer-containing liquid, a polyethylene porous membrane (manufactured by Toray Industries, Inc., under the trade name of Ceteera) (not corona-treated) with a thickness of 17 μm and a porosity of 59% was used as the porous substrate, and the thickness of the resulting hydroxy ion conductive membrane was 21 μm.

[0095] Example 14 A hydroxy ion conductive membrane 1-c14 was obtained in the same manner as in Example 13, except that in the preparation of the polymerizable monomer-containing liquid in Example 13, monomer m9 was used as the polyfunctional polymerizable monomer to obtain a polymerizable monomer-containing liquid 1-m14, and in the preparation of the hydroxy ion conductive membrane, polymerizable monomer-containing liquid 1-m14 was used as the polymerizable monomer-containing liquid.

[0096] Example 15 A hydroxy ion conductive membrane 1-c15 was obtained in the same manner as in Example 13, except that in the preparation of the polymerizable monomer-containing liquid in Example 13, monomer m10 was used as the polyfunctional polymerizable monomer to obtain a polymerizable monomer-containing liquid 1-m15, and in the preparation of the hydroxy ion conductive membrane, polymerizable monomer-containing liquid 1-m15 was used as the polymerizable monomer-containing liquid.

[0097] Example 16 A hydroxy ion conductive membrane 1-c16 was obtained in the same manner as in Example 12, except that in the preparation of the polymerizable monomer-containing liquid in Example 12, monomer m11 was used as the polyfunctional polymerizable monomer to obtain a polymerizable monomer-containing liquid 1-m16, and in the preparation of the hydroxy ion conductive membrane, polymerizable monomer-containing liquid 1-m16 was used as the polymerizable monomer-containing liquid.

[0098] Comparative Example 1 A 200 mL three-neck flask was charged with 10 g of pure water, 5 g of isopropyl alcohol, and 35 g of monomer m1 and 22.4 g of monomer Xm1 as polyfunctional polymerizable monomers to prepare a solution. Under yellow light, 1 g of 2-hydroxy-2-methylpropiophenone (manufactured by TCI) and 1 g of ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (manufactured by TCI) were further added as polymerization initiators to prepare polymerizable monomer-containing solution 1-xm1 with a solids content of 80 mass%. A hydroxy ion conductive membrane c-x1 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, polymerizable monomer-containing solution 1-xm1 was used as the polymerizable monomer-containing solution.

[0099] Comparative Example 2 A hydroxy ion conductive membrane c-x2 was obtained in the same manner as in Example 1, except that in the preparation of the polymerizable monomer-containing liquid in Example 1, a mixed solution of water and isopropanol (water:isopropanol=10:3 (mass ratio)) was added to adjust the solids content to 80 mass %, and in the preparation of the hydroxy ion conductive membrane, two stacked polypropylene porous membranes (CE2500 (trade name) manufactured by Celgard Inc.) (without corona treatment) having a thickness of 25 μm and a porosity of 41% were used as the porous substrate, and the thickness of the resulting hydroxy ion conductive membrane was 60 μm. The column for the thickness of the porous substrate in Table 1 shows the total thickness of the two polypropylene porous membranes.

[0100] Comparative Example 3: A 200 mL three-neck flask was charged with 64.8 g of Monomer Xm1 and 7.2 g of Monomer Xm2 as polyfunctional polymerizable monomers to prepare a solution. Under yellow light, 1.0 g of 2-hydroxy-2-methylpropiophenone (manufactured by TCI) and 1.0 g of ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (manufactured by TCI) were further added as polymerization initiators to prepare polymerizable monomer-containing solution 1-xm3 with a solids content of 100 mass%. A hydroxy ion conductive membrane c-x3 was obtained in the same manner as in Example 1, except that in the preparation of the hydroxy ion conductive membrane of Example 1, polymerizable monomer-containing solution 1-xm3 was used as the polymerizable monomer-containing solution.

[0101] Comparative Example 4 A hydroxy ion conductive membrane c-x4 was obtained in the same manner as in Example 1, except that in the preparation of the polymerizable monomer-containing liquid in Example 1, a mixed solution of water and isopropanol (water:isopropanol=10:3 (mass ratio)) was added to adjust the solid content to 80 mass %, and in the preparation of the hydroxy ion conductive membrane, no porous substrate was used, and the polymerizable monomer-containing liquid was sandwiched between two PET films to obtain a 50 μm-thick film made of a hydroxy ion conductive polymer alone.

[0102] The polymerizable monomers used are shown below.

[0103]

[0104]

[0105]

[0106] The hydroxy ion conductive membranes obtained above all had an ion conductivity of 0.1 mS / cm to 10 S / cm.

[0107] [Preparation of Water Electrolysis Cell] <Water Electrolysis Cell 101> The gas diffusion layer 1-a2 with the anode catalyst layer 1-a1 and the gas diffusion layer 1-b2 with the cathode catalyst layer 1-b1 prepared above were each ２ The laminate was punched out to the same shape so that the resulting hydroxy ion conductive membrane 1-c1 was sandwiched between the plates, with each catalyst layer facing inward, and pressed at a surface pressure of 4 MPa. The pressed laminate was sandwiched between two Ni bipolar plates having flow channels and constrained with bolts to maintain a confining pressure of 1 MPa. In this manner, a water electrolysis cell 101 was obtained, which has the same layer structure as the water electrolysis cell 10 shown in FIG. 2 , which has a layer structure of bipolar plate-gas diffusion layer-anode catalyst layer-hydroxy ion conductive membrane-cathode catalyst layer-gas diffusion layer-bipolar plate. This water electrolysis cell 101 has a structure including a membrane electrode assembly (anode catalyst layer-hydroxy ion conductive membrane-cathode catalyst layer, or gas diffusion layer-anode catalyst layer-hydroxy ion conductive membrane-cathode catalyst layer-gas diffusion layer).

[0108] <Water electrolysis cells 102 to 116 and c101 to c104> Water electrolysis cells 102 to 116 and c101 to c104 were obtained in the same manner as in the preparation of the water electrolysis cell 101, except that the hydroxy ion conductive membranes 1-c2 to 1-c16 and c-x1 to c-x4 produced in Examples 2 to 16 and Comparative Examples 1 to 4, respectively, were used.

[0109] The obtained hydroxy ion conductive polymer, porous substrate, hydroxy ion conductive membrane, and water electrolysis cell were evaluated as follows.

[0110] [Measurement of the elemental concentrations of oxygen, sulfur, nitrogen, and carbon contained in the hydroxy ion conductive polymer] To calculate the elemental concentrations of oxygen, sulfur, nitrogen, and carbon contained in the hydroxy ion conductive polymer, elemental analysis was performed using an organic elemental analyzer Vario EL cube (manufactured by Elementar). For elemental analysis, 30 mg of hydroxy ion conductive polymer was used, which was collected by scraping it from the hydroxy ion conductive membrane or dissolving it in dimethyl sulfoxide (DMSO) or tetrahydrofuran (THF). From the obtained elemental concentrations, the ratio of hetero elements was calculated using the following formula: Hetero element ratio = [total of the elemental concentrations of oxygen, sulfur, and nitrogen (at%)] / [carbon element concentration (at%)]

[0111] [Evaluation of Air Permeability] The air permeability of the hydroxy ion conductive membrane was determined as follows. The air permeability (air resistance) of the hydroxy ion conductive membrane was measured using an Oken air permeability meter (EYO-651MR (trade name), manufactured by Asahi Seiko Co., Ltd.) according to the method described in JIS P8117:2009. The hydroxy ion conductive membrane was pretreated by leaving it at 25°C and a relative humidity of 45% for 24 hours. The measurement was carried out at room temperature (25°C) and a relative humidity of 45%, and the average value of three measurements was determined as the air permeability.

[0112] [Evaluation of Water Retention Capacity] The water retention capacity of the hydroxy ion conductive polymer and the hydroxy ion conductive membrane was measured as follows. To measure the water retention capacity of the hydroxy ion conductive polymer, a 50 μm-thick film of the hydroxy ion conductive polymer alone was obtained by sandwiching a polymerizable monomer-containing solution between two PET films instead of using a porous substrate in the preparation of the hydroxy ion conductive membrane. Test pieces measuring 5 cm long x 5 cm wide were cut out from the film of the hydroxy ion conductive polymer alone and the hydroxy ion conductive membrane, and measurements were performed using these test pieces. For the test pieces, the mass of the test piece after drying at 60°C under vacuum for 12 hours was designated as WA, and the mass of the test piece after 8 hours in a state where the dried test piece was completely immersed in 0.2 M potassium hydroxide (KOH) was designated as WB. The water retention capacity was calculated using the following formula: Water retention capacity (%) = (WB - WA) / WB × 100

[0113] [Evaluation of Porosity of Porous Substrate] The volume of the porous substrate (the volume of the entire porous substrate including the volume of the pores) was calculated as X (cm ３ ), the weight of the porous substrate is W (g), the true density of the porous substrate is d (g / cm ３ ) and X and the theoretical volume Y (cm ３ ) (Y=W / d) was calculated using the following formula: Porosity (%)=[1-(Y / X)]×100

[0114] [Evaluation of average pore size of porous substrate] The average pore size of the high-humidity resistant substrate (referred to as "pore size" in Table 1) was measured using a pore size distribution measuring device (Autopore V 9620 (trade name), manufactured by Micromeritics). Each porous substrate was cut into a 2 cm square and used as a sample.

[0115] [Evaluation of Mechanical Properties] As indicators of the mechanical properties of the porous substrate and the hydroxy ion conductive membrane, the tensile strength and elongation at break were measured as follows. The tensile strength and elongation at break were measured at 25°C using an MX-500N electric measuring stand and a ZTA series force gauge (both trade names, manufactured by Imada Co., Ltd.). Each of the porous substrates used above and each of the hydroxy ion conductive membranes obtained above was cut into a 5 mm x 15 mm strip to obtain a test piece. Two parallel benchmark lines were marked at the center of the obtained test piece at 5 mm intervals, and both ends of the test piece were clamped and pulled at a rate of 0.5 mm / min. The maximum tensile stress at this time was taken as the tensile strength. The elongation at break was calculated using the following formula. In this way, the tensile strength and elongation at break were measured. Elongation at break (%) = 100 x (L - L ０ ) / L ０ L ０ : Gauge line distance before test, L: Gauge line distance at break For the porous substrate of Comparative Example 2, two porous substrates used to obtain the hydroxy ion conductive membrane c-x2 were stacked on top of each other, and the above measurement was carried out.

[0116] [Evaluation of Water Electrolysis Cells] A 0.2 M KOH aqueous solution heated to 60°C was supplied to the cathode catalyst layer and the anode catalyst layer of each of the water electrolysis cells obtained above at a flow rate of 10 mL / min, while a current of 0.1 A / cm ２ The water electrolysis cells after initial energization were used to carry out the various evaluations described below.

[0117] <Resistance Evaluation> The above initial current (0.1 A / cm ２ The initial voltage (V0) of each water electrolysis cell immediately after the initial energization (energization for 4 hours at 1.0 A / cm) was measured, and a 0.2 M KOH aqueous solution heated to 60°C was supplied to each water electrolysis cell after the initial energization. ２A current was applied at 100 V for 10 minutes and the voltage (V1) immediately thereafter was measured. From the measured values, V1 - V0 was calculated as the differential voltage. The obtained differential voltage was applied to the following evaluation criteria to evaluate the resistance. The higher the differential voltage, the higher the resistance (lower the hydroxy ion conductivity). - Evaluation criteria for differential voltage - A: More than 0.00 V and 0.25 V or less B: More than 0.25 V and 0.30 V or less C: More than 0.30 V and 0.50 V or less D: More than 0.50 V

[0118] <Evaluation of Hydrogen Gas Permeability> After the initial energization, a 0.2 M KOH aqueous solution heated to 60°C was supplied to each water electrolysis cell, and a current of 0.1 A / cm ２ Electricity was applied for 10 minutes at 10°C, and the gas recovered from the flow path of the bipolar plate on the anode side was analyzed to evaluate the extent to which hydrogen gas generated on the cathode side permeated to the anode side. Specifically, the gas recovered from the flow path of the bipolar plate on the anode side during the above 10-minute period of electric current application was cooled to 10°C, and after removing water vapor, the gas was passed through a gas chromatograph (GC3210G (trade name), manufactured by GL Sciences, column: MS13X (trade name)) to measure the ratio of hydrogen gas contained in the recovered gas. The obtained hydrogen gas ratio was evaluated according to the following evaluation criteria. -Evaluation criteria for hydrogen gas permeability (hydrogen gas ratio)- A: Less than 0.100 vol% B: 0.100 vol% or more and less than 0.500 vol% C: 0.500 vol% or more and less than 1.000 vol% D: 1.000 vol% or more

[0119] <Evaluation of repeated current durability> After the initial current application, each water electrolysis cell was subjected to a current application test at 0.1 A / cm while supplying a 0.2 M KOH aqueous solution heated to 60°C. ２After energizing for 1 hour, the liquid transfer and energization were stopped and the device was left for 3 hours. This constitutes one cycle, and energization and de-energization were repeated 500 times. The voltage after 1 cycle and the voltage after 500 cycles were measured to determine the rate of change in voltage. The obtained rate of change in voltage was applied to the following evaluation criteria to evaluate the durability of repeated energization. Rate of change in voltage = [(voltage after 500 cycles - voltage after 1 cycle) / voltage after 1 cycle] x 100 (%) - Evaluation criteria for durability of repeated energization - A: 0.5% or less B: More than 0.5% and 1% or less C: More than 1% and 2% or less D: More than 2% and 5% or less E: More than 5%, or unable to operate before 500 cycles

[0120]

[0121]

[0122] "-": Indicates that the component was not used. HMP: 2-hydroxy-2-methylpropiophenone (manufactured by TCI) EPTBP: ethyl phenyl(2,4,6-trimethylbenzoyl)phosphinate (manufactured by TCI) DPPSH: diphenyl[4-(phenylthio)phenyl]sulfonium hexafluorophosphate (manufactured by TCI) V-501: 4,4'-azobis(4-cyanovaleric acid) (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Solid content: Indicates the solid content in the polymerizable monomer-containing solution. Polymerization method: When the polymerization of the monomer was carried out by UV exposure, it is indicated as "UV", and when it was carried out by heating, it is indicated as "thermal". Heteroelement ratio: [total of the elemental concentrations of oxygen, sulfur and nitrogen elements (at%)] / [carbon element concentration (at%)] Component of porous substrate: When a polyethylene porous membrane was used, it was "PE-m", when a polypropylene porous membrane was used, it was "PP-m", when a polyethylene nonwoven fabric was used, it was "PE-f", when a polytetrafluoroethylene nonwoven fabric was used, it was "PTFE-f". Also, when a porous substrate was not used, it was described as "none".

[0123]

[0124] The hydroxy ion conductive membrane c-x1 of Comparative Example 1 does not satisfy the requirements of the present invention because it contains a hydroxy ion conductive polymer in which the proportion of component (I) derived from polyfunctional polymerizable monomer IA to all the constituent components of the polymer is 40 mol %. When this hydroxy ion conductive membrane was used as a membrane electrode assembly and further as a water electrolysis cell, it resulted in poor hydrogen gas permeability and repeated current durability. The hydroxy ion conductive membrane c-x2 of Comparative Example 2 had a thickness of 60 μm and did not satisfy the requirements of the present invention. When this hydroxy ion conductive membrane was used as a membrane electrode assembly and further as a water electrolysis cell, the differential voltage exceeded 0.50 V and the resistance was high. This is thought to be due to the long distance between the cathode and anode. Furthermore, in a repeated current durability test, the voltage after 500 cycles was approximately 3% higher than the initial voltage. The hydroxy ion conductive membrane c-x3 of Comparative Example 3, which contained a hydroxy ion conductive polymer without the component (I) derived from the polyfunctional polymerizable monomer IA, was used as a membrane electrode assembly and further as a water electrolysis cell. In a repeated current durability test, the voltage after 500 cycles was nearly 3% higher than the initial voltage. It is believed that the hydroxy ion conductive polymer decomposed between the start and end of current application. The hydroxy ion conductive membrane c-x4 of Comparative Example 4, which was a membrane composed of a hydroxy ion conductive polymer and did not have a porous substrate, had a breaking elongation of 5% and was poor in mechanical strength. When this hydroxy ion conductive membrane was used as a membrane electrode assembly and further as a water electrolysis cell, it short-circuited within 500 cycles in a repeated current durability test, becoming inoperable. In contrast, the hydroxy ion conductive membranes 1-c1 to 1-c16 of the present invention had a thickness of less than 50 μm, yet had a tensile strength of 25 MPa or more, a breaking elongation of 30% or more, and excellent mechanical properties. They also had excellent air permeability, with an air permeability of 800 seconds or more. Furthermore, when these hydroxy ion conductive membranes were used as membrane electrode assemblies and further as water electrolysis cells, the hydrogen gas permeability evaluation showed a gas permeability of less than 1.000 vol %, the resistance evaluation showed a voltage difference of 0.50 V or less, and the voltage change rate in the repeated current application durability evaluation was 2% or less.In particular, when the component (I) derived from the polyfunctional polymerizable monomer defined in the present invention was contained in a larger amount, hydrogen gas permeability could be further suppressed, the resistance was low, and the durability against repeated energization tended to be improved (comparison of Example 1, Example 4, and Comparative Example 1). In Example 1, it is thought that the presence of many highly polar structures and the formation of many crosslinks resulted in the formation of a dense membrane, which enabled the suppression of hydrogen gas permeation.

[0125] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0126] This application claims priority based on Japanese Patent Application No. 2023-161729, filed in Japan on September 25, 2023, the contents of which are incorporated herein by reference as part of the present specification.

[0127] REFERENCE SIGNS LIST 1 Hydroxy ion 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. A hydroxy ion conductive membrane comprising a porous substrate and a hydroxy ion conductive polymer disposed at least in the pores of the porous substrate, wherein the hydroxy ion conductive polymer contains a component (I) derived from a polyfunctional polymerizable monomer having a total of two or more atoms of at least one of oxygen atoms, sulfur atoms and nitrogen atoms in a structural portion other than a polymerizable group, and the proportion of the component (I) in the total components of the hydroxy ion conductive polymer is 50 mol % or more, and the thickness of the hydroxy ion conductive membrane is 5 μm or more and less than 50 μm.

2. The hydroxy ion conductive membrane according to claim 1, wherein the elemental composition in the hydroxy ion conductive polymer satisfies the following formula (a): [Formula (a)] [Total of the concentrations of oxygen, sulfur and nitrogen (at %)] / [Concentration of carbon (at %)]≧0.05 3. The hydroxy ion conductive membrane according to claim 1, wherein the water retention capacity of the hydroxy ion conductive polymer is 5 mass % or more.

4. The hydroxy ion conductive membrane according to claim 1, wherein the component (I) has any one of the following structural units (i) to (viii): n is an integer of 1 or more. L is a single bond, an alkylene group, or an arylene group. * is a linking site.

5. The hydroxy ion conducting membrane according to claim 4, wherein the component (I) is a non-ionic component.

6. The hydroxy ion conductive membrane according to any one of claims 1 to 5, wherein the hydroxy ion conductive membrane has a tensile strength of 10 MPa or more and a breaking elongation of 100% or more.

7. The hydroxy ion conductive membrane according to claim 6, wherein the air permeability of the hydroxy ion conductive membrane is 1000 seconds or more.

8. The hydroxy ion conductive membrane according to claim 7, wherein the porosity of the porous substrate is 40 to 60%.

9. The hydroxy ion conductive membrane according to claim 8, wherein the porous substrate has a tensile strength of 20 MPa or more and a breaking elongation of 100% or more.

10. The hydroxy ion conducting membrane according to claim 9, wherein the material of the porous substrate comprises a hydrocarbon polymer.

11. A method for producing a hydroxyl ion conductive membrane, comprising impregnating a porous substrate with a polymerizable monomer-containing liquid containing 50 mol % or more of polyfunctional polymerizable monomers having a total of two or more atoms of at least one of oxygen atoms, sulfur atoms, and nitrogen atoms in structural parts other than polymerizable groups, and then polymerizing the polyfunctional polymerizable monomers to obtain a hydroxyl ion conductive membrane having a thickness of 5 μm or more and less than 50 μm.

12. The method for producing a hydroxy ion conductive membrane according to claim 11, wherein the solid content of the polymerizable monomer-containing liquid is 30 mass % or more.

13. The method for producing a hydroxy ion conductive membrane according to claim 11 or 12, wherein the porous substrate is surface-modified by corona treatment or plasma treatment.

14. A membrane electrode assembly having a structure in which a cathode catalyst layer, the hydroxy ion conductive membrane according to claim 6, and an anode catalyst layer are laminated in this order.

15. A method for producing hydrogen, comprising using the membrane electrode assembly according to claim 14.

16. A hydrogen production system comprising the membrane electrode assembly according to claim 14.

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