Fluorine-containing polymer, electrolyte film, membrane electrode assembly, water electrolysis device, production method for hydrogen, and production method for fluorine-containing polymer

Abstract

The present invention addresses the problem of providing a fluorine-containing polymer that makes it possible to produce a polymer film that: includes a fluorine-containing polymer that has a group that can be converted to an ion exchange group; and has few pinholes when a laminate in which the polymer film is sandwiched between transfer substrates has been hot-pressed and then the transfer substrates have been removed from the laminate. The present invention also addresses the problem of providing an electrolyte film, a membrane electrode assembly, and a water electrolysis device that are obtained using the fluorine-containing polymer.　This fluorine-containing polymer includes a unit based on tetrafluoroethylene and has a group that can be converted to an ion exchange group. No endothermic peak is observed in the range of 300°C–350°C when the fluorine-containing polymer is assessed by differential scanning calorimetry.

Description

Fluorine-containing polymer, electrolyte membrane, membrane electrode assembly, water electrolysis device, method for producing hydrogen, and method for producing fluorine-containing polymer

[0001] The present disclosure relates to a fluoropolymer, an electrolyte membrane, a membrane electrode assembly, a water electrolysis device, a method for producing hydrogen, and a method for producing a fluoropolymer.

[0002] Fluorine-containing polymers (hereinafter also referred to as precursor polymers) having groups that can be converted into ion-exchange groups are widely used, for example, in the production of electrolyte membranes used in water electrolysis devices and the like. Patent Document 1 discloses the following method as a method for producing such an electrolyte membrane. First, a PET film, a film made of a precursor polymer, a woven fabric, a film made of the precursor polymer, and a PET film are laminated in this order to obtain a laminate. Next, the obtained laminate is hot-pressed, and the PET films on both sides of the laminate are peeled off to obtain a precursor membrane. Next, the groups that can be converted into ion-exchange groups in the obtained precursor membrane are subjected to hydrolysis treatment and acid-form treatment to obtain an electrolyte membrane containing a fluoropolymer having ion-exchange groups.

[0003] International Publication No. 2020 / 162511

[0004] In recent years, further improvements in the performance of electrolyte membranes have been required. In order to improve the performance of electrolyte membranes, the present inventors focused on suppressing pinholes that occur in electrolyte membranes. A pinhole is a defect formed by a hole penetrating through the membrane. When a pinhole occurs, for example, in electrolysis or battery applications, the amount of material moving between the anode side and the cathode side or the cathode side and the anode side, which are separated by the electrolyte membrane, increases, and the pinhole may cause damage or a short circuit. When the present inventors manufactured an electrolyte membrane based on the method described in the above-mentioned Patent Document 1, they found that there was room for improvement in terms of suppressing pinhole formation.

[0005] The present disclosure has been made in view of the above problems, and an object of one embodiment of the present invention is to provide a fluoropolymer and a method for producing a fluoropolymer that can produce a polymer membrane with few pinholes when a laminate in which a polymer membrane containing a fluoropolymer having groups convertible to ion-exchange groups is sandwiched between transfer substrates is subjected to a heat press treatment and then the transfer substrate is peeled from the laminate. Another object of one embodiment of the present invention is to provide an electrolyte membrane, a membrane electrode assembly, and a water electrolysis device obtained using the fluoropolymer. Another object of one embodiment of the present invention is to provide a method for producing hydrogen using the water electrolysis device.

[0006] The present disclosure includes the following aspects. [1] A fluoropolymer containing units based on tetrafluoroethylene and having groups that can be converted into ion-exchange groups, wherein no endothermic peak is observed in the range of 300 to 350°C as measured by differential scanning calorimetry. [2] The fluoropolymer according to [1], wherein the fluoropolymer contains units based on a compound represented by formula (1): Formula (1) CF ２ =CF-L-(A) ｎ In formula (1), L is an (n+1)-valent perfluorohydrocarbon group having 3 or more carbon atoms which may contain an etheric oxygen atom, A is a group which can be converted into a sulfonic acid functional group, and n is 1 or 2. [3] A fluorine-containing polymer according to [1] or [2], which contains a unit based on a compound represented by formula (1-3) described later or a unit based on a compound represented by formula (1-4) described later. In formulas (1-3) and (1-4), R ｆ１ is a perfluoroalkylene group which may contain an oxygen atom between the carbon atoms, and R ｆ２ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms, and R ｆ３is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms, r is 0 or 1, m is 0 or 1, and A is a group which can be converted into a sulfonic acid type functional group. [4] An electrolyte membrane comprising a fluoropolymer according to any one of [1] to [3], in which a group convertible to an ion exchange group in the fluoropolymer has been converted into an ion exchange group. [5] The electrolyte membrane according to [4], further comprising a reinforcing material. [6] A membrane electrode assembly comprising the electrolyte membrane according to [4] or [5], a cathode catalyst layer disposed on one side of the electrolyte membrane, and an anode catalyst layer disposed on the other side of the electrolyte membrane. [7] A water electrolysis device comprising the membrane electrode assembly according to [6], a power supply unit connected to the cathode catalyst layer side and the anode catalyst layer side of the membrane electrode assembly, and a water supply unit which supplies water to the anode catalyst layer side. [8] A method for producing hydrogen by electrolyzing water using the water electrolysis device according to [7]. [9] A method for producing a fluoropolymer, comprising polymerizing tetrafluoroethylene and a fluoromonomer having a group that can be converted into an ion-exchange group in a reactor to produce a fluoropolymer, wherein the upper surface outside the reactor is covered with a heat insulating material during the polymerization.

[10] A method for producing a fluoropolymer according to [9], wherein aggregates are removed from the upper surface inside the reactor after the polymerization.

[0007] According to one embodiment of the present invention, there are provided a fluoropolymer and a method for producing a fluoropolymer, which can produce a polymer membrane with few pinholes when a laminate in which a polymer membrane containing a fluoropolymer having groups convertible to ion-exchange groups is sandwiched between transfer substrates is subjected to a heat press treatment and then the transfer substrate is peeled off from the laminate. Another embodiment of the present invention also provides an electrolyte membrane, a membrane electrode assembly, and a water electrolysis device obtained using the above-mentioned fluoropolymer. Another embodiment of the present invention also provides a method for producing hydrogen using the above-mentioned water electrolysis device.

[0008] 1 is a cross-sectional view schematically illustrating an example of a membrane electrode assembly according to the present disclosure.

[0009] The definitions of the following terms apply throughout the present specification and claims unless otherwise specified. An "ion exchange group" is a group that can exchange at least a portion of the ions contained in this group with other ions, and examples thereof include the sulfonic acid functional group and carboxylic acid functional group shown below. A "sulfonic acid functional group" is a sulfonic acid group (-SO ３ Here, the form of the sulfonate group is, for example, (—SO ３ － ) Ma ＋ , (-SO ３ － ) ２ Mb ２＋ , and (-SO ３ － ) ３ Mc ３＋ (However, Ma ＋ is an alkali metal ion or a quaternary ammonium cation, and Mb ２＋ is a divalent metal ion, Mc ３＋ is a trivalent metal ion.) When there are two ligands, the number of ion exchange groups is counted as two, and when there are three ligands, the number of ion exchange groups is counted as three. "Carboxylic acid type functional group" means a carboxylic acid group (-COOH) or a carboxylic acid salt group. Here, the form of the carboxylic acid salt group can be, for example, (-COO － ) Ma ＋ , (-COO － ) ２ Mb ２＋ , and (-COO － ) ３ Mc ３＋ (However, Ma ＋ is an alkali metal ion or a quaternary ammonium cation, and Mb ２＋ is a divalent metal ion, Mc ３＋is a trivalent metal ion.) Note that when there are two ligands, the number of ion exchange groups is counted as two, and when there are three ligands, the number of ion exchange groups is counted as three. A "precursor membrane" is a membrane containing a polymer having a group that can be converted into an ion exchange group. A "group that can be converted into an ion exchange group" means a group that can be converted into an ion exchange group by known treatments such as hydrolysis treatment and acidification treatment. A "group that can be converted into a sulfonic acid functional group" means a group that can be converted into a sulfonic acid functional group by known treatments such as hydrolysis treatment and acidification treatment. A "group that can be converted into a carboxylic acid functional group" means a group that can be converted into a carboxylic acid functional group by known treatments such as hydrolysis treatment and acidification treatment.

[0010] A "unit" in a polymer refers to an atomic group derived from one molecule of a monomer formed by polymerization of the monomer. The unit may be an atomic group formed directly by the polymerization reaction, or may be an atomic group in which part of the atomic group is converted into a different structure by treating the polymer obtained by the polymerization reaction. In the following, units derived from individual monomers may be referred to by the name of the monomer followed by "unit" in some cases.

[0011] A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples.

[0012] [Fluorine-Containing Polymer (I')] The fluoropolymer of the present disclosure (hereinafter also referred to as "fluoropolymer (I')") is a fluoropolymer containing units based on tetrafluoroethylene (hereinafter also referred to as "TFE") and having groups (hereinafter also referred to as "precursor groups") that can be converted to ion-exchange groups, and no endothermic peak is observed in the range of 300 to 350°C as measured by differential scanning calorimetry (hereinafter also referred to as "DSC"). If the fluoropolymer (I') is used, a polymer membrane (e.g., precursor membrane, electrolyte membrane) with few pinholes can be produced. The details of the reason for this have not been made clear, but it is presumed to be due to the following reason. When a fluoropolymer having precursor groups is produced using TFE, fine particles of polytetrafluoroethylene (hereinafter also referred to as "PTFE"), which is a homopolymer of TFE, or fine particles of a high-melting-point crystalline component mainly composed of TFE units, may be produced as by-products. For this reason, the above-mentioned fine particles may be mixed into a fluoropolymer having precursor groups produced using TFE. When a film is formed using a fluoropolymer containing such fine particles, and the film is sandwiched between transfer substrates to form a laminate, the resulting laminate is hot-pressed, and then the transfer substrate is peeled off to obtain a precursor film, the fine particles present in the film may be peeled off along with the transfer substrate. In this case, pinholes caused by the fine particles will occur in the resulting precursor film. As a result, it is believed that the pinholes that existed in the precursor film will also be present in the electrolyte membrane formed using the precursor film. Here, the endothermic peak in the range of 300 to 350 ° C measured by DSC is a peak due to PTFE, but the fluoropolymer (I') does not observe an endothermic peak in the range of 300 to 350 ° C measured by DSC. Therefore, even when a film of the fluoropolymer (I') is sandwiched between transfer substrates and hot-pressed, and then the transfer substrate is peeled off, it is presumed that pinholes caused by the fine particles will be less likely to occur in the resulting polymer membrane (e.g., precursor membrane, electrolyte membrane).Furthermore, when the cross section of the fluoropolymer (I') formed into an electrolyte membrane is observed with a scanning electron microscope, it is preferable that no fine particles greater than 100 nmΦ are observed, more preferably only fine particles of 50 to 100 nmΦ are observed, and particularly preferably no fine particles are observed. Even when no endothermic peak is observed by DSC, a very small amount of by-products may be produced to such an extent that no endothermic peak is observed by DSC. When no fine particles greater than 100 nmΦ are observed in the cross section of the electrolyte membrane, it is thought that the probability of pinhole formation can be further reduced. The method for observing the fine particles is as described in the Examples below.

[0013] The fluoropolymer (I') is not particularly limited as long as it is a polymer containing TFE units and having a precursor group, but a copolymer of TFE units and units based on a fluoromonomer having a precursor group (for example, a group that can be converted into a sulfonic acid type functional group and a group that can be converted into a carboxylic acid type functional group) is preferred, and a copolymer of TFE units and units based on a fluoromonomer having a group that can be converted into a sulfonic acid type functional group (hereinafter also referred to as "fluoropolymer (S')") is more preferred from the viewpoint that the electrolysis voltage when the electrolyte membrane is applied to a water electrolysis device can be further reduced.

[0014] The fluorine-containing monomer (S') may be a compound having one or more fluorine atoms in the molecule, an ethylenic double bond, and a group that can be converted into a sulfonic acid functional group. As the fluorine-containing monomer (S'), a compound represented by formula (1) is preferred in terms of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer (S). Formula (1) CF ２ =CF-L-(A) ｎ

[0015] L is an (n+1)-valent perfluorohydrocarbon group which may contain an etheric oxygen atom. The etheric oxygen atom may be located at the terminal of the perfluorohydrocarbon group or between carbon atoms. The number of carbon atoms in the (n+1)-valent perfluorohydrocarbon group is preferably 1 or more, more preferably 2 or more, even more preferably 3 or more, and is preferably 20 or less, more preferably 10 or less. L is preferably an (n+1)-valent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom, and more preferably a divalent perfluoroalkylene group which may contain an etheric oxygen atom in the embodiment where n = 1, or a trivalent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom in the embodiment where n = 2. The divalent perfluoroalkylene group may be either linear or branched.

[0016] A is a group that can be converted into a sulfonic acid functional group. The group that can be converted into a sulfonic acid functional group is preferably a functional group that can be converted into a sulfonic acid functional group by hydrolysis. Specific examples of groups that can be converted into a sulfonic acid functional group include -SO ２ F, -SO ２ Cl, —SO ２ Br is an example.

[0017] n is 1 or 2. When n is 2, two As may be the same or different.

[0018] The compound represented by formula (1) is preferably a compound represented by formula (1-1), a compound represented by formula (1-2), a compound represented by formula (1-3), or a compound represented by formula (1-4). ２ =CF-O-R ｆ１ -A Formula (1-2) CF ２ =CF-R ｆ１ -A

[0019]

[0020]

[0021] R ｆ１is a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and even more preferably 3 or more, and is preferably 20 or less, more preferably 10 or less.

[0022] R ｆ２ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0023] R ｆ３ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0024] r is 0 or 1. m is 0 or 1.

[0025] The definition of A in the formula is as described above.

[0026] As the compound represented by formula (1-1) and the compound represented by formula (1-2), a compound represented by formula (1-5) is preferred. ２ =CF-(CF ２ ) ｘ -(OCF ２ CFY) ｙ -O-(CF ２ ) ｚ -SO ３ F x is 0 or 1, y is an integer from 0 to 2, z is an integer from 1 to 4, and Y is F or CF ３ is.

[0027] Specific examples of the compound represented by formula (1-1) include the following compounds. In the formula, w is an integer of 1 to 8, and x is an integer of 1 to 5. CF ２ =CF-O-(CF ２ ) ｗ -SO ２ FCF ２ =CF-O-CF ２CF (CF ３ )-O-(CF ２ ) ｗ -SO ２ FCF ２ =CF-[O-CF ２ CF (CF ３ )] ｘ -SO ２ F

[0028] Specific examples of the compound represented by formula (1-2) include the following compounds: In the formula, w is an integer of 1 to 8. CF ２ =CF-(CF ２ ) ｗ -SO ２ FCF ２ =CF-CF ２ -O-(CF ２ ) ｗ -SO ２ F

[0029] The compound represented by formula (1-3) is preferably a compound represented by formula (1-3-1).

[0030]

[0031] R ｆ４ is a linear perfluoroalkylene group having 1 to 6 carbon atoms, and R ｆ５ represents a single bond or a linear perfluoroalkylene group having 1 to 6 carbon atoms which may contain an oxygen atom between the carbon atoms. The definitions of r and A are as described above.

[0032] Specific examples of the compound represented by formula (1-3-1) include the following.

[0033]

[0034] The compound represented by formula (1-4) is preferably a compound represented by formula (1-4-1).

[0035]

[0036] R in the formula ｆ１ , R ｆ２ and A are defined as above.

[0037] Specific examples of the compound represented by formula (1-4-1) include the following.

[0038]

[0039] The fluorine-containing monomer (S') may be used alone or in combination of two or more kinds.

[0040] In the production of the fluoropolymer (S'), other monomers may be used in addition to TFE and the fluoromonomer (S'). Specific examples of the other monomers include CF ２ = CFR ｆ６ (However, R ｆ６ is a perfluoroalkyl group having 2 to 10 carbon atoms, CF ２ =CF-OR ｆ７ (However, R ｆ７ is a perfluoroalkyl group having 1 to 10 carbon atoms, CF ２ = CFO (CF ２ ) ｖ CF = CF ２ (wherein v is an integer of 1 to 3.) The content of units based on other monomers is preferably at most 30 mass% based on all units in the fluoropolymer (I') (preferably the fluoropolymer (S')) from the viewpoint of maintaining ion exchange performance.

[0041] The ion exchange capacity of the fluoropolymer (I') can be adjusted by changing the content of groups convertible to ion exchange groups in the fluoropolymer (I'). The content of TFE units relative to all units contained in the fluoropolymer (S') is preferably 5 to 35 mol %. The content of units based on the fluoromonomer (S') relative to all units contained in the fluoropolymer (S') is preferably 65 to 95 mol %.

[0042] When the fluoropolymer (I') is measured by DSC, no endothermic peak is observed in the range of 300 to 350°C. Here, the presence or absence of an endothermic peak in the range of 300 to 350°C by DSC is judged based on a DSC curve. A DSC curve is a curve showing changes in the amount of heat absorbed and released by a sample (fluoropolymer) with temperature on the horizontal axis and heat flow on the vertical axis, and is measured using a differential scanning calorimeter by a method referring to JIS K 7123-1987. Detailed measurement conditions are as described in the Examples section.

[0043] <Method for producing fluoropolymer (I')> The method for producing fluoropolymer (I') will be explained using the method for producing fluoropolymer (S') as an example.

[0044] An example of the method for producing the fluoropolymer (S') is a method in which TFE and the fluoromonomer (S') are copolymerized in a reactor in the presence of a polymerization initiator. Specific examples of the copolymerization method include bulk polymerization, solution polymerization, suspension polymerization, and emulsion polymerization.

[0045] In the case of the solution polymerization method, specific examples of the polymerization solvent include solvents (fluorine-containing solvents) such as chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, and hydrofluoroethers.

[0046] Specific examples of the polymerization initiator include diacyl peroxides (disuccinic acid peroxide, benzoyl peroxide, perfluorobenzoyl peroxide, lauroyl peroxide, bis(pentafluoropropionyl) peroxide, etc.), azo compounds (2,2'-azobis(2-amidinopropane) hydrochlorides, 4,4'-azobis(4-cyanovaleric acid), dimethyl 2,2'-azobisisobutyrate, azobisisobutyronitrile, etc.), peroxyesters, peroxydicarbonates (diisopropyl peroxydicarbonate, bis(2-ethylhexyl)peroxydicarbonate, etc.), hydroperoxides (diisopropylbenzene hydroperoxide, t-butyl hydroperoxide, etc.), dialkyl peroxides (di-t-butyl peroxide, perfluoro-di-t-butyl peroxide), etc.

[0047] The amount of the polymerization initiator added is preferably 0.0001 part by mass or more, and preferably 3 parts by mass or less, more preferably 2 parts by mass or less, per 100 parts by mass of the monomer components. By setting the amount of the polymerization initiator to 3 parts by mass or less, the molecular weight of the fluoropolymer can be increased.

[0048] In the process for producing the fluoropolymer (S'), in addition to the polymerization initiator, a molecular weight modifier or the like used in ordinary solution polymerization may be added.

[0049] The monomers (TFE, fluoromonomer (S')) and the polymerization initiator may be added continuously or successively to the reactor. The amounts of TFE and fluoromonomer (S') added may be appropriately determined so that the contents of the respective monomer units in the fluoropolymer (S') fall within the above-mentioned ranges.

[0050] When a solution polymerization method is employed, the polymerization temperature is preferably from the boiling point at normal pressure of the polymerization solvent (particularly the above-mentioned fluorine-containing solvent) having the lowest boiling point at normal pressure to 70°C or less. If the polymerization temperature is 70°C or less, the molecular weight of the obtained fluorine-containing polymer will be relatively high. The polymerization temperature is more preferably 65°C or less, and even more preferably 60°C or less.

[0051] The polymerization pressure (gauge pressure) is preferably 0.1 to 5.0 MPaG, more preferably 0.5 to 3.0 MPaG. When the polymerization pressure (gauge pressure) is within the above range, the rate of the polymerization reaction can be maintained at a practically satisfactory rate, and a high-molecular-weight fluoropolymer can be obtained.

[0052] In the process for producing the fluoropolymer (I') (fluoropolymer (S')), TFE is used, so that by-products such as PTFE and aggregates of high-melting-point crystalline components mainly composed of TFE units may be produced. In particular, such by-products are likely to be produced in the upper part of the reactor, and the by-products produced in the upper part of the reactor may settle and be mixed with the fluoropolymer. Since the above-mentioned problems are likely to occur when by-products are mixed into the fluoropolymer, the process for producing the fluoropolymer (I') preferably includes a treatment to prevent the by-products from being mixed into the fluoropolymer (I'). By including a treatment to prevent the by-products from being mixed in, a fluoropolymer in which no endothermic peak is observed in the range of 300 to 350°C as measured by DSC can be easily obtained.

[0053] Specific examples of the treatment for suppressing the incorporation of the by-products include a method of installing a heater on the upper surface outside the reactor to adjust the temperature to prevent the formation of by-products such as PTFE, a method of insulating the upper surface outside the reactor to adjust the temperature to prevent the formation of by-products such as PTFE, and a method of peeling off and removing by-products such as PTFE deposited on the upper surface inside the reactor using a rod or comb. A method of insulating the upper surface outside the reactor includes a method of covering it with a heat insulating material. In this case, it is preferable to cover the surface outside the reactor by preferably 30% or more, more preferably 50% or more of the area ratio of the outer surface of the reactor from the top of the reactor. When adjusting the temperature to prevent the formation of PTFE, it is preferable to adjust the temperature outside the reactor and inside the heat insulating material so that it is preferably 20°C or higher, more preferably 30°C or higher.

[0054] [Electrolyte Membrane] The electrolyte membrane of the present disclosure contains a fluoropolymer (hereinafter also referred to as "fluoropolymer (I)") in which the precursor groups in the above-mentioned fluoropolymer (I') have been converted to ion-exchange groups.

[0055] Specific examples of the ion-exchange group possessed by the fluoropolymer (I) include a sulfonic acid type functional group and a carboxylic acid type functional group, with the sulfonic acid type functional group being preferred from the viewpoint of further reducing the electrolysis voltage when the electrolyte membrane is applied to a water electrolysis device. Below, embodiments of the fluoropolymer having a sulfonic acid type functional group (hereinafter also referred to as "fluoropolymer (S)") will be mainly described in detail.

[0056] The fluoropolymer (S) preferably contains TFE units and units having a sulfonic acid type functional group and a fluorine atom.

[0057] As the unit having a sulfonic acid type functional group and a fluorine atom, a unit represented by formula (2) is preferred. ２ -CF(-L-(SO ３ M) ｎ ) )]— In formula (2), L and n are defined as above, and M is a hydrogen atom, an alkali metal, or a quaternary ammonium cation.

[0058] The unit represented by formula (2) is preferably a unit represented by formula (2-1), a unit represented by formula (2-2), a unit represented by formula (2-3), or a unit represented by formula (2-4). ２ -CF(-O-R ｆ１ -SO ３ M)] - Formula (2-2) - [CF ２ -CF(-R ｆ１ -SO ３ M) ]-

[0059]

[0060] R in the formula ｆ１ , R ｆ２ , r and M are as defined above.

[0061]

[0062] R in the formula ｆ１ , R ｆ２ , R ｆ３ , r, m and M are as defined above.

[0063] As the unit represented by formula (2-1) and the unit represented by formula (2-2), a unit represented by formula (2-5) is more preferred. ２ -CF(-(CF ２ ) ｘ -(OCF ２ CFY) ｙ -O-(CF ２ ) ｚ -SO ３ M)]—wherein x, y, z, Y and M are as defined above.

[0064] Specific examples of the unit represented by formula (2-1) include the following units. In the formula, w, x, and M are defined as above. -[CF ２ -CF(-O-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF(-O-CF ２ CF (CF ３ )-O-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF(-(O-CF ２ CF (CF ３ )) ｘ -SO ３ M) ]-

[0065] Specific examples of the unit represented by formula (2-2) include the following units. The definitions of w and M in the formula are as described above. -[CF ２ -CF(-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF (-CF ２ -O-(CF ２ ) ｗ -SO ３ M) ]-

[0066] As the unit represented by formula (2-3), a unit represented by formula (2-3-1) is preferred.

[0067]

[0068] R in the formula ｆ４ , R ｆ５ , r and M are as defined above.

[0069] Specific examples of the unit represented by formula (2-3-1) include the following.

[0070]

[0071] As the unit represented by formula (2-4), a unit represented by formula (2-4-1) is preferred.

[0072]

[0073] R in the formula ｆ１ , R ｆ２ and M are defined as above.

[0074] Specific examples of the unit represented by formula (2-4-1) include the following.

[0075]

[0076] The unit having a sulfonic acid type functional group and a fluorine atom may be used alone or in combination of two or more.

[0077] The fluoropolymer (I) may comprise units based on other monomers other than TFE units and units having sulfonic acid type functional groups and fluorine atoms.Specific examples of other monomers are as described above.From the viewpoint of maintaining ion exchange performance, the content of units based on other monomers is preferably 30 mass% or less relative to the total units in the fluoropolymer (I).

[0078] The content of the fluoropolymer (I) is preferably from 95 to 100% by mass based on the total mass of the electrolyte membrane.

[0079] The ion exchange capacity of the fluoropolymer (I) is preferably 0.90 milliequivalents / gram dry resin or more, more preferably greater than 1.10 milliequivalents / gram dry resin, even more preferably 1.15 milliequivalents / gram dry resin or more, particularly preferably 1.20 milliequivalents / gram dry resin or more, and most preferably 1.25 milliequivalents / gram dry resin or more, from the viewpoint of being able to further reduce the electrolysis voltage when applied to a water electrolysis device. The ion exchange capacity of the fluoropolymer (I) is preferably 2.00 milliequivalents / gram dry resin or less, more preferably 1.50 milliequivalents / gram dry resin or less, even more preferably 1.43 milliequivalents / gram dry resin or less, from the viewpoint of the strength of the membrane electrode assembly when water is contained. Only one type of fluoropolymer (I) may be used, or two or more types may be used in a laminated or mixed form.

[0080] The electrolyte membrane may be reinforced with a reinforcing material. Examples of the reinforcing material include porous bodies, fibers, woven fabrics, and nonwoven fabrics. Examples of the reinforcing material include polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymers, polyethylene, polypropylene, and polyphenylene sulfide.

[0081] The thickness of the electrolyte membrane is preferably 30 μm or more, more preferably 50 μm or more, even more preferably 70 μm or more, particularly preferably 90 μm or more, and preferably 300 μm or less, more preferably 200 μm or less, even more preferably 150 μm or less, and particularly preferably 120 μm or less. When the electrolyte membrane has a multilayer structure, the thickness of the electrolyte membrane refers to the total thickness of each layer. The thickness of the electrolyte membrane is measured using a magnified image (e.g., 100x) of the cross section of the electrolyte membrane taken with an optical microscope (product name "BX-51", manufactured by Olympus Corporation). When the electrolyte membrane has an uneven surface, the thickness of 10 concave portions and the thickness of 10 convex portions are measured, and the arithmetic mean value of the thicknesses at the 20 points is taken as the thickness of the electrolyte membrane. However, when the convex portions contain threads that constitute the woven fabric, the thickness of the convex portions is the value obtained by subtracting the thickness of the threads present in the convex portions.

[0082] <Method for producing electrolyte membrane> An example of a method for producing an electrolyte membrane is the following method. First, a membrane P1 containing the fluoropolymer (I') is obtained using the fluoropolymer (I'). Next, a laminate obtained by laminating a transfer substrate, the membrane P1, a reinforcing material, the membrane P1, and the transfer substrate in this order is hot-pressed, and the transfer substrates arranged on both sides of the laminate are peeled off to obtain a precursor membrane in which the membrane P1, the reinforcing material, and the membrane P1 are laminated in this order. Next, the precursor groups in the precursor membrane are converted into an ion-exchange membrane to obtain an electrolyte membrane containing the fluoropolymer (I) and the reinforcing material. Examples of methods for forming the membrane P1 from the fluoropolymer (I') include a method of forming a membrane containing the fluoropolymer (I') by melt extrusion or hot press molding, a method of forming a plurality of membranes containing the fluoropolymer (I') by melt extrusion or hot press molding and then laminating them, and a method of adhering the fluoropolymer (I') to a substrate by melt extrusion or hot press molding. The fluoropolymer (I') is preferably in the form of pellets or powder, and its shape is preferably uniform.

[0083] Specific examples of the transfer substrate include polyethylene terephthalate film (PET film), polyethylene film, polypropylene film, and polystyrene film.

[0084] In the above-mentioned method for producing an electrolyte membrane, a known heat press device such as a flat press or a roll press can be used for the heat press. When the heat press is performed using a flat press, the surface pressure is preferably 2 MPa or more, more preferably 3 MPa or more, from the viewpoint of adhesion between the films, and preferably 5 MPa or less, more preferably 4 MPa or less, from the viewpoint of releasability of the transfer substrate. When the heat press is performed using a roll press, the linear pressure is preferably 20 kg / cm or more, more preferably 30 kg / cm or more, from the viewpoint of adhesion between the films, and preferably 50 kg / cm or less, more preferably 40 kg / cm or less, from the viewpoint of releasability of the transfer substrate. The heating temperature in the heat press is preferably 120°C or more, more preferably 130°C or more, from the viewpoint of adhesion between the films, and preferably 200°C or less, more preferably 180°C or less, from the viewpoint of releasability of the transfer substrate. The pressing time in the heat press is preferably 1 minute or more, more preferably 2 minutes or more, and is preferably 5 minutes or less, more preferably 4 minutes or less.

[0085] The transfer substrate can be peeled off by any known method, and is not particularly limited. The surface temperature of the transfer substrate when peeling it off is preferably 0° C. or higher, more preferably 20° C. or higher, from the viewpoint of suppressing cracking of the electrolyte membrane, and is preferably 50° C. or lower, more preferably 40° C. or lower, from the viewpoint of releasability of the transfer substrate.

[0086] Specific examples of methods for converting precursor groups in the precursor membrane into ion-exchange groups include methods of subjecting the precursor membrane to hydrolysis treatment, acidification treatment, etc. Among these, a method of contacting the precursor membrane with an alkaline aqueous solution is preferred.

[0087] Specific examples of the method for contacting the precursor film with the alkaline aqueous solution include immersing the precursor film in the alkaline aqueous solution and spraying the alkaline aqueous solution onto the surface of the precursor film. The temperature of the alkaline aqueous solution is preferably 30° C. or higher, more preferably 40° C. or higher, and preferably 100° C. or lower. The contact time between the precursor film and the alkaline aqueous solution is preferably 3 minutes or longer, more preferably 5 minutes or longer, and preferably 150 minutes or shorter, more preferably 50 minutes or shorter.

[0088] The alkaline aqueous solution preferably contains an alkali metal hydroxide, a water-soluble organic solvent, and water. Examples of alkali metal hydroxides include sodium hydroxide and potassium hydroxide. In this specification, the water-soluble organic solvent refers to an organic solvent that is easily soluble in water. Specifically, an organic solvent having a solubility of 0.1 g or more in 1000 ml of water (20°C) is preferred, and an organic solvent having a solubility of 0.5 g or more is particularly preferred. The water-soluble organic solvent preferably contains at least one selected from the group consisting of aprotic organic solvents, alcohols, and aminoalcohols, and particularly preferably contains an aprotic organic solvent. One water-soluble organic solvent may be used alone, or two or more may be used in combination.

[0089] Specific examples of aprotic organic solvents include dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and N-ethyl-2-pyrrolidone, with dimethyl sulfoxide being preferred. Specific examples of alcohols include methanol, ethanol, isopropanol, butanol, methoxyethoxyethanol, butoxyethanol, butylcarbitol, hexyloxyethanol, octanol, 1-methoxy-2-propanol, and ethylene glycol. Specific examples of aminoalcohols include ethanolamine, N-methylethanolamine, N-ethylethanolamine, 1-amino-2-propanol, 1-amino-3-propanol, 2-aminoethoxyethanol, 2-aminothioethoxyethanol, and 2-amino-2-methyl-1-propanol.

[0090] The concentration of the alkali metal hydroxide in the alkaline aqueous solution is preferably 1% by mass or more, more preferably 3% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less. The content of the water-soluble organic solvent in the alkaline aqueous solution is preferably 1% by mass or more, more preferably 3% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less. The concentration of water in the alkaline aqueous solution is preferably 39 to 80% by mass.

[0091] After the precursor film is brought into contact with the alkaline aqueous solution, a treatment for removing the alkaline aqueous solution may be carried out. As a method for removing the alkaline aqueous solution, for example, a method for washing the precursor film that has been brought into contact with the alkaline aqueous solution with water may be mentioned.

[0092] After contacting the precursor membrane with the alkaline aqueous solution, the resulting membrane may be contacted with an acidic aqueous solution to convert the ion exchange groups to an acid form. Specific examples of methods for contacting the precursor membrane with the acidic aqueous solution include immersing the precursor membrane in the acidic aqueous solution and spraying the acidic aqueous solution onto the surface of the precursor membrane. The acidic aqueous solution preferably contains an acid component and water. Specific examples of the acid component include hydrochloric acid and sulfuric acid.

[0093] Although the above-mentioned method for producing an electrolyte membrane has been described using an example in which the electrolyte membrane contains a reinforcing material, the present invention is not limited thereto, and an electrolyte membrane may be produced without using a reinforcing material. Also, the present invention is not limited thereto, and only one membrane P1 may be used, or three or more membranes P1 may be used. Furthermore, when two or more membranes P1 are used, the fluoropolymers (I') contained in the membranes P1 may be the same as or different from each other.

[0094] [Membrane Electrode Assembly] The membrane electrode assembly of the present disclosure includes the electrolyte membrane, a cathode catalyst layer disposed on one side of the electrolyte membrane, and an anode catalyst layer disposed on the other side of the electrolyte membrane. The membrane electrode assembly of the present disclosure is suitable for use in a solid polymer water electrolysis device.

[0095] 1 is a cross-sectional view schematically illustrating an example of a membrane electrode assembly according to the present disclosure. In the example of Fig. 1, the membrane electrode assembly 20 includes an anode 22 having a catalyst layer 26 and a gas diffusion layer 28, a cathode 24 having the catalyst layer 26 and the gas diffusion layer 28, and an electrolyte membrane 10 disposed between the anode 22 and the cathode 24 in contact with the catalyst layer 26.

[0096] The details of the electrolyte membrane 10 are as described above.

[0097] The anode 22 and cathode 24 each have a catalyst layer 26 and a gas diffusion layer 28 .

[0098] A specific example of the catalyst layer 26 is a layer containing a catalyst and a polymer having an ion exchange group. Specific examples of the catalyst include a supported catalyst in which a carbon support supports a catalyst containing platinum, a platinum alloy, or a platinum catalyst having a core-shell structure, an iridium oxide catalyst, a composite oxide catalyst containing iridium and another metal element, an alloy containing iridium oxide, and a catalyst containing iridium oxide having a core-shell structure. An example of the carbon support is carbon black powder. The polymer having an ion exchange group is not particularly limited, and for example, a known fluorine-containing polymer having an ion exchange group can be used.

[0099] The gas diffusion layer 28 functions to rapidly diffuse gas generated from the catalyst layer out of the catalyst layer and also functions as a current collector. Specific examples of gas diffusion layers include carbon paper, carbon cloth, carbon felt, sintered titanium oxide fibers, and sintered titanium oxide particles. The anode side has a high potential, and using carbon materials would result in oxidation. Therefore, it is preferable to use sintered titanium oxide fibers or sintered titanium oxide particles. The sintered titanium oxide may be plated with platinum or other metals as needed. The gas diffusion layer 28 of the cathode 24 may be treated with PTFE or other metals to provide water repellency. While the membrane electrode assembly of FIG. 1 includes a gas diffusion layer 28, the gas diffusion layer is an optional component and may not be included in the membrane electrode assembly.

[0100] <Method for Manufacturing Membrane Electrode Assembly> The method for manufacturing a membrane electrode assembly involves forming a cathode catalyst layer on one side of an electrolyte membrane and an anode catalyst layer on the other side of the electrolyte membrane. One example of a method for manufacturing a membrane electrode assembly involves using a laminate having an anode catalyst layer and a releasable substrate (e.g., an ETFE sheet) and another laminate having a cathode catalyst layer and a releasable substrate (e.g., an ETFE sheet), bonding catalyst layers to both sides of the electrolyte membrane, and then peeling off the releasable substrate. The laminate may have a gas diffusion layer between the catalyst layer and the releasable substrate. In this case, the gas diffusion layer can be formed on the side of the catalyst layer opposite the electrolyte membrane. The method for manufacturing a catalyst layer involves applying a catalyst layer-forming coating liquid to a predetermined position (e.g., the surface of the releasable substrate) and drying it as necessary. The catalyst layer-forming coating liquid is a liquid in which a polymer having ion exchange groups and a catalyst are dispersed in a dispersion medium.

[0101] [Water Electrolysis Apparatus] The water electrolysis apparatus of the present disclosure includes the membrane electrode assembly described above, a water supply unit that supplies water to the anode catalyst layer side, and a power supply unit that is electrically connected to the anode catalyst layer side and the cathode catalyst layer side. In the water electrolysis apparatus of the present disclosure, when a DC voltage is applied by the power supply unit while water is supplied to the anode catalyst layer side by the water supply unit, water is decomposed on the anode catalyst layer side to generate oxygen and protons. On the cathode catalyst layer side, protons that have migrated to the cathode catalyst layer side through the electrolyte membrane gain electrons to generate hydrogen. The water electrolysis apparatus of the present disclosure may have the same configuration as known water electrolysis apparatuses (e.g., an oxygen recovery member that recovers the generated oxygen, a hydrogen recovery member that recovers the generated hydrogen), except for the components described above.

[0102] [Method for Producing Hydrogen] The method for producing hydrogen according to the present disclosure produces hydrogen by electrolyzing water (electrolyte) using the water electrolysis device described above. The method for producing hydrogen according to the present disclosure can produce hydrogen efficiently because it uses the water electrolysis device described above.

[0103] The present invention will be described in detail below with reference to examples. Examples 1 to 3 are working examples, and Examples 4 and 5 are comparative examples. However, the present invention is not limited to these examples.

[0104] [Differential Scanning Calorimetry (DSC)] The fluoropolymer was subjected to temperature increase or decrease in a cycle of 0°C → 350°C → 0°C → 350°C at a rate of 10°C / min using a differential scanning calorimeter (trade name "DSC2500", manufactured by TA Instruments) in accordance with a method based on JIS K 7123-1987, and a DSC curve was measured to confirm the presence or absence of an endothermic peak between 300°C and 350°C.

[0105] [Ion exchange capacity of fluoropolymer] The fluoropolymer was placed in a glove box filled with dry nitrogen for 24 hours, and the dry mass of the fluoropolymer was measured. Thereafter, the fluoropolymer was immersed in a 2 mol / L aqueous sodium chloride solution at 60°C for 1 hour. The fluoropolymer was washed with ultrapure water and then removed. The solution in which the fluoropolymer had been immersed was titrated with a 0.1 mol / L aqueous sodium hydroxide solution to determine the ion exchange capacity (milli-equivalent / gram dry resin) of the fluoropolymer.

[0106] [Pinholes] Using a pinhole inspection device (product name "TRS-70", manufactured by Sanko Electronics Laboratory Co., Ltd.), the number of pinholes (holes) in the electrolyte membrane was counted and evaluated according to the following criteria: A: No pinholes. B: 1 to 2 pinholes. C: 3 or more pinholes.

[0107] [Presence or absence of fine particles] A cross section of the electrolyte membrane was observed at a 10,000x field of view using a scanning electron microscope (SU8230 manufactured by Hitachi High-Tech Corporation, acceleration voltage: 1 kV, emission current: 10 μA, probe current: Normal, detector conditions: LA100(U)) to obtain an observation image, and the size of the fine particles in the observation image was evaluated according to the following criteria. It is considered that the fewer the number of fine particles observed, the less fine particles of PTFE or high-melting-point crystalline components mainly composed of TFE units are mixed into the fluoropolymer, and the more likely it is that pinholes will be suppressed in the electrolyte membrane. A: No fine particles were observed. B: Fine particles of 50 to 100 nmΦ were observed. C: Fine particles of more than 100 nmΦ were observed.

[0108] (Abbreviation) TFE: tetrafluoroethylene Compound 1: CF２ =CF-O-CF ２ CF (CF ３ )-O-CF ２ CF ２ -SO ２ F tBPO: (CH ３ ) ３ COOC (CH ３ ) ３ HFC-52-13p:CF ３ (CF ２ ) ５ H HFE-347pc-f:CF ３ CH ２ OCF ２ CF ２ H HFC-245fa: CHF ２ CH ２ CF ３ HCFC-225cb: CClF ２ CF ２ CHClF

[0109] Compound 2: Monomer represented by the following formula m32-1

[0110]

[0111] [Example 1] 1,371 g of Compound 1 was placed in an autoclave (internal volume 2 L, stainless steel reactor) equipped with a stirrer, cooled with liquid nitrogen, and degassed. The autoclave was heated in an oil bath until the internal temperature reached 65°C, and nitrogen was sealed into the autoclave to set the pressure to 0.2 MPa (gauge pressure). 178 g of TFE was introduced into the autoclave, and the pressure became 1.27 MPa (gauge pressure). 60 g of a mixture of tBPO, a polymerization initiator, and HFC-52-13p, with a concentration of 0.24 mass%, was injected into the autoclave through the injection line. TFE was continuously added while maintaining the pressure at 1.27 MPa (gauge pressure), and polymerization was carried out. When the amount of TFE added reached 60 g after 3 hours, the autoclave was cooled to stop the polymerization, and the gas in the system was purged. Here, 50% or more of the surface of the upper part of the autoclave was covered with a glass wool heat insulating material, and the temperature at the upper part inside the autoclave was maintained at 30°C or higher during polymerization. Furthermore, when the polymerization was stopped and the gas in the system was purged, the aggregates formed at the upper part of the autoclave were removed with a metal scraper. The reaction liquid obtained from removing the aggregates was diluted with HFC-52-13p, and then HFE-347pc-f was added to aggregate the polymer and filter it. Thereafter, an operation of stirring the polymer in HFC-52-13p and re-agglomerating it with HFE-347pc-f was repeated twice. The polymer was vacuum dried at 80°C for 16 hours, and a fluoropolymer (I'-1), which is a copolymer of units based on TFE and units based on Compound 1, was recovered. This operation constitutes one batch, and fluoropolymer (I'-1) was produced in 10 consecutive batches. The obtained fluoropolymer (I'-1) was measured by DSC, and no endothermic peak was observed in the range of 300 to 350° C. Furthermore, the ion exchange capacity of the fluoropolymer (I'-1) was measured, and it was found to be 1.25 meq / g dry resin.

[0112] The aggregates formed in the upper part of the autoclave were analyzed using a micro-Raman spectrometer under the following conditions, and were confirmed to be polytetrafluoroethylene (PTFE): Device: LabRAM HR Evolution (manufactured by Horiba, Ltd.) Excitation wavelength: 1064 nm Power: Approximately 250 mW Objective lens: 50x, NA = 0.9 Confocal pinhole: 150 μm Grating: 150 gr / mm Measurement time: 2 sec x 20 times

[0113] Next, the obtained fluoropolymer (I'-1) was pulverized, and then the fluoropolymer (I'-1) was adhered to a transfer substrate made of a linear low-density polyethylene (LLDPE) film (melting point: 110 to 120°C) by a melt extrusion method, to obtain a film-attached substrate Y1 in which a film α1 (film thickness: 45 μm) was formed on the transfer substrate.

[0114] In addition, 18.6 denier PFA yarns were used as the warp and weft yarns, and plain weaving was performed so that the density of the PFA yarns was 100 threads / inch to obtain a woven fabric A1. The weight of the woven fabric A1 was 16.3 g / m ２ The warp and weft yarns were made of monofilaments, and the woven fabric A1 was not subjected to heat pressing alone.

[0115] The film-attached substrate Y1 / woven fabric A1 / film-attached substrate Y1 were stacked in this order. The film-attached substrate Y1 was positioned so that the film α1 in the film-attached substrate Y1 was in contact with the woven fabric A1. The stacked components were heated and pressed for 10 minutes using a flat press at a temperature of 160°C and a surface pressure of 30 MPa. The transfer substrates on both sides were then peeled off at a temperature of 50°C to obtain a precursor membrane. The precursor membrane was immersed in a solution of dimethyl sulfoxide / potassium hydroxide / water = 30 / 5.5 / 64.5 (mass ratio) at 95°C for 30 minutes, and the groups in the precursor membrane that could be converted to sulfonic acid functional groups were hydrolyzed to K-type sulfonic acid functional groups, and then washed with water. The resulting membrane was then immersed in 1M sulfuric acid, the terminal groups were converted from K-type to H-type, and then dried to obtain an electrolyte membrane 1 (thickness 90 μm) of Example 1. The resulting electrolyte membrane 1 was subjected to the above-mentioned evaluation. The results are shown in Table 1.

[0116] [Example 2] 5.77 g of compound 1, 74.62 g of compound 2, 8.44 g of HFC-245fa as a solvent, and 24.0 mg of a 50% by mass solution of t-butyl peroxypivalate dissolved in HCFC-225cb were charged into a 125 mL stainless steel autoclave, and the mixture was thoroughly degassed under cooling with liquid nitrogen. After heating to 55°C, TFE was introduced and the pressure was adjusted to 0.88 MPaG. TFE was continuously supplied while maintaining the temperature and pressure constant. 2.5 hours after the start of polymerization, the autoclave was cooled to terminate the polymerization reaction. Here, 50% or more of the upper surface of the autoclave was covered with a glass wool insulating material, and the temperature at the upper part of the autoclave was maintained at 30°C or higher during polymerization. The resulting reaction solution was diluted with HFC-52-13p, and then HFE-347pc-f was added to coagulate the polymer, which was then filtered. Thereafter, the polymer was stirred in HFC-52-13p and re-coagulated with HFE-347pc-f. This procedure was repeated twice. The polymer was vacuum dried at 180°C for 16 hours to recover a fluoropolymer (I'-2), which is a copolymer of units based on TFE, units based on Compound 1, and units based on Compound 2. This procedure constitutes one batch, and 10 batches of fluoropolymer (I'-2) were produced continuously. When the fluoropolymer (I'-2) was measured by DSC, no endothermic peak was observed in the range of 300 to 350°C. Furthermore, when the ion exchange capacity was measured using the fluoropolymer (I'-2), it was found to be 1.79 meq / g dry resin.

[0117] An electrolyte membrane 2 (thickness: 90 μm) of Example 2 was obtained in the same manner as in Example 1, except that fluoropolymer I′-2 was used instead of fluoropolymer I′-1. The obtained electrolyte membrane 2 was subjected to the above-mentioned evaluations. The results are shown in Table 1.

[0118] Example 3 Ten batches of fluoropolymer (I'-3) were produced continuously in the same manner as in Example 1, except that the aggregates formed in the upper part of the autoclave were not removed when the polymerization was stopped and the gas in the system was purged in Example 1. When the fluoropolymer (I'-3) was measured by DSC, no endothermic peak was observed in the range of 300 to 350°C. Furthermore, when the ion exchange capacity was measured using the fluoropolymer (I'-3), it was found to be 1.25 meq / g dry resin.

[0119] An electrolyte membrane 3 (thickness 90 μm) of Example 3 was obtained in the same manner as in Example 1, except that fluoropolymer (I′-3) was used instead of fluoropolymer (I′-1). The obtained electrolyte membrane 3 was subjected to the above-mentioned evaluations. The results are shown in Table 1.

[0120] Example 4 Ten batches of fluoropolymer (I'-4) were produced continuously in the same manner as in Example 1, except that the top of the autoclave was not insulated with a heat insulating material and that the aggregates formed in the top of the autoclave were not removed when the polymerization was stopped and the gas in the system was purged. When the fluoropolymer (I'-4) was measured by DSC, an endothermic peak was observed in the range of 300 to 350°C. Furthermore, when the ion exchange capacity of the fluoropolymer (I'-4) was measured, it was found to be 1.25 meq / g dry resin.

[0121] An electrolyte membrane 4 (thickness 90 μm) of Example 4 was obtained in the same manner as in Example 1, except that fluoropolymer (I′-4) was used instead of fluoropolymer (I′-1). The obtained electrolyte membrane 4 was subjected to the above-mentioned evaluations. The results are shown in Table 1.

[0122] Example 5 Ten batches of fluoropolymer (I'-5) were produced continuously in the same manner as in Example 2, except that the top of the autoclave was not insulated with a heat insulating material. When the fluoropolymer (I'-5) was measured by DSC, an endothermic peak was observed in the range of 300 to 350°C. Furthermore, when the ion exchange capacity of the fluoropolymer (I'-5) was measured, it was found to be 1.79 meq / g dry resin.

[0123] An electrolyte membrane 5 (thickness: 90 μm) of Example 5 was obtained in the same manner as in Example 1, except that fluoropolymer I′-5 was used instead of fluoropolymer I′-1. The obtained electrolyte membrane 5 was subjected to the above-mentioned evaluations. The results are shown in Table 1.

[0124]

[0125] As shown in Table 1, it was confirmed that an electrolyte membrane in which the generation of pinholes was suppressed could be obtained when a fluoropolymer containing TFE units and having precursor groups was used, which did not show an endothermic peak in the range of 300 to 350°C measured by DSC (Examples 1 to 3). In Examples 1 to 3, it is presumed that the amount of PTFE aggregates generated was reduced by insulating the upper part of the autoclave with a heat insulating material. Furthermore, in comparison with Example 1 and Example 3, when both insulating the upper part of the autoclave with a heat insulating material and removing the aggregates were performed, the generation of pinholes was further suppressed. Note that in Example 2, no aggregates visible to the naked eye were generated.

[0126] On the other hand, when a fluoropolymer containing TFE units and precursor groups was used, which showed an endothermic peak in the range of 300 to 350°C as measured by DSC, the generation of pinholes in the electrolyte membrane could not be sufficiently suppressed (Examples 4 to 5). In Examples 4 and 5, it is presumed that the amount of PTFE aggregates generated increased because the upper part of the autoclave was not insulated with an insulating material. In Example 4, aggregates were visually confirmed in all of the batches. In Example 5, aggregates were visually confirmed in the production of the final batch (10th batch).

[0127] The entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2023-189963 filed on November 7, 2023 are hereby incorporated by reference as the disclosure of the present invention.

[0128] 10 Electrolyte membrane 20 Membrane electrode assembly 22 Anode 24 Cathode 26 Catalyst layer 28 Gas diffusion layer

Claims

1. A fluoropolymer which contains units based on tetrafluoroethylene and has groups which can be converted into ion exchange groups, and which does not show an endothermic peak in the range of 300 to 350°C as measured by differential scanning calorimetry.

2. The fluoropolymer according to claim 1, which contains a unit based on a compound represented by formula (1). Formula (1) CF ２ = CF-L-(A) ｎ In formula (1), L is an (n+1) valent perfluorohydrocarbon group having 3 or more carbon atoms which may contain an etheric oxygen atom, A is a group which can be converted into a sulfonic acid type functional group, and n is 1 or 2.

3. The fluorine-containing polymer according to claim 1, which contains a unit based on a compound represented by formula (1-3) or a unit based on a compound represented by formula (1-4). In formula (1-3) and formula (1-4), R ｆ１ is a perfluoroalkylene group which may contain an oxygen atom between the carbon atoms, R ｆ２ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between the carbon atoms, R ｆ３ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms, r is 0 or 1, m is 0 or 1, and A is a group which can be converted into a sulfonic acid type functional group.

4. An electrolyte membrane comprising the fluoropolymer according to any one of claims 1 to 3, in which groups convertible to ion-exchange groups in the fluoropolymer have been converted into ion-exchange groups.

5. The electrolyte membrane of claim 4, further comprising a reinforcing material.

6. A membrane electrode assembly comprising the electrolyte membrane according to claim 4, a cathode catalyst layer disposed on one side of the electrolyte membrane, and an anode catalyst layer disposed on the other side of the electrolyte membrane.

7. A water electrolysis device comprising: the membrane electrode assembly according to claim 6; a power supply unit connected to the cathode catalyst layer side and the anode catalyst layer side of said membrane electrode assembly; and a water supply unit for supplying water to said anode catalyst layer side.

8. A method for producing hydrogen, comprising electrolyzing water using the water electrolysis device according to claim 7 to produce hydrogen.

9. A method for producing a fluoropolymer, comprising polymerizing tetrafluoroethylene and a fluorine-containing monomer having a group that can be converted into an ion exchange group in a reactor to produce a fluoropolymer, wherein, during the polymerization, the upper surface outside the reactor is covered with a heat insulating material.

10. The process for producing a fluoropolymer according to claim 9, wherein after the polymerization, agglomerates are removed from the upper surface inside the reactor.