Membrane electrode assembly

Abstract

Provided is a membrane electrode assembly capable of suppressing hydrogen crossover. A membrane electrode assembly for polymer electrolyte membrane water electrolysis which comprises an anode having a catalyst layer, a cathode having a catalyst layer and a polymer electrolyte membrane provided between the anode and the cathode wherein at least one of the catalyst layer in the anode and the catalyst layer in the cathode comprises a fluorinated polymer having ion exchange groups and comprising units having a cyclic ether structure.

Description

TECHNICAL FIELD

[0001] The present invention relates to a membrane electrode assembly.BACKGROUND ART

[0002] Polymer electrolyte membrane water electrolyzers (hereinafter also referred to as PEM water electrolyzers) have been used to store surplus electricity for later use by converting it into a gas.

[0003] For example, Patent Document 1 discloses a PEM water electrolyzer having a catalyst layer containing a fluorinated polymer.PRIOR ART DOCUMENTSPatent Documents

[0004] Patent Document 1: JP-A-2023-041182DISCLOSURE OF INVENTIONTechnical Problem

[0005] PEM water electrolyzers with low hydrogen crossover are demanded from a safety aspect.

[0006] Hydrogen crossover means permeation of hydrogen gas generated in the cathode compartment through the polymer electrolyte membrane to the anode compartment during water electrolysis using a PEM electrolyzer. When the mixing ratio of the hydrogen gas permeated to the anode compartment to the oxygen gas generated in the anode compartment exceeds a certain level, there is a risk of explosion.

[0007] The present inventors assessed a PEM water electrolyzer having the catalyst layer containing a fluorinated polymer disclosed in Patent Document 1 in the cathode or anode and found that it needs improvement for lower hydrogen crossover.

[0008] The present invention aims to provide a membrane electrode assembly which can suppress hydrogen crossover.Solution to Problem

[0009] As a result of their extensive studies, the present inventors have found the following solutions to the above-mentioned problem.[1]

[0010] A membrane electrode assembly for polymer electrolyte membrane water electrolysis which comprises an anode having a catalyst layer, a cathode having a catalyst layer and a polymer electrolyte membrane provided between the anode and the cathode,

[0011] wherein at least one of the catalyst layer in the anode and the catalyst layer in the cathode comprises a fluorinated polymer having ion exchange groups and comprising units having a cyclic ether structure.[2]

[0012] The membrane electrode assembly according to [1], wherein the polymer electrolyte membrane has a thickness of at least 25 μm.[3]

[0013] The membrane electrode assembly according to [1] or [2], wherein the fluorinated polymer has an ion exchange capacity of from 0.90 to 1.60 meq / g dry resin.[4]

[0014] The membrane electrode assembly according to any one of [1] to [3], wherein the content of units having a cyclic ether structure is at least 30 mol % relative to all the units in the fluorinated polymer.[5]

[0015] The membrane electrode assembly according to any one of [1] to [4], wherein the fluorinated polymer has a softening temperature of at least 135° C.[6]

[0016] The membrane electrode assembly according to any one of [1] to [5], wherein the catalyst layer in the anode comprises the fluorinated polymer and a catalyst, and

[0017] the catalyst comprises an iridium-containing catalyst.[7]

[0018] The membrane electrode assembly according to [6], wherein the mass ratio of the amount of the fluorinated polymer to the amount of iridium in the iridium-containing catalyst is from 0.05 to 0.30.[8]

[0019] The membrane electrode assembly according to any one of [1] to [7], wherein the catalyst layer in the cathode comprises the fluorinated polymer and a catalyst, and

[0020] the catalyst comprises a carbon support and a platinum-containing catalyst.[9]

[0021] The membrane electrode assembly according to [8], wherein the mass ratio of the amount of the fluorinated polymer to the amount of the carbon support is from 0.2 to 1.5.

[10]

[0022] The membrane electrode assembly according to [8] or [9], wherein the carbon support has a specific surface area of at least 60 m2 / g.Advantageous Effects of Invention

[0023] According to the present invention, it is possible to provide a membrane electrode assembly which can suppress hydrogen crossover.BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 A schematic cross-sectional view of an embodiment of the membrane electrode assembly of the present invention.DESCRIPTION OF EMBODIMENTS

[0025] The terms used herein have the following meanings.

[0026] A numerical range expressed by using “to” includes the figures before and after “to” as the lower limit and the upper limit. In a series of numerical ranges described in the present specification, the upper limit or lower limit of a numerical range may be replaced by the upper limit or lower limit of another numerical range in the same series. The upper limit or lower limit of any numerical range described in the present specification may be replaced by any of figures indicated in Examples.

[0027] An “ion exchange group” refers to a group containing at least one ion exchangeable for a different ion, such as a sulfonic acid functional group or a carboxylic acid functional group.

[0028] A “sulfonic acid functional group” means a sulfonic acid group in acid form (—SO3H) and in salt form (—SO3M, where M is an alkali metal or a quaternary ammonium cation) collectively.

[0029] A “carboxylic acid functional group” means a carboxylic acid group in acid form (—COOH) and in salt form (—COOM, where M is an alkali metal or a quaternary ammonium cation) collectively.

[0030] A “group convertible to an ion exchange group” means a group which can be converted to an ion exchange group by treatments such as hydrolysis and acidification and is sometimes referred to as a “precursor group”.

[0031] A “group convertible to a sulfonic acid functional group” means a group which can be converted to a sulfonic acid functional group by treatments such as hydrolysis and acidification.

[0032] A “group convertible to a carboxylic acid functional group” means a group which can be converted to a carboxylic acid functional group by treatments such as hydrolysis and acidification.

[0033] A “unit” in a polymer means an atomic group derived from one molecule of a monomer by polymerization. A unit may be an atomic group directly formed by a polymerization reaction, or may be an atomic group having a partially different structure formed by polymerization followed by partial modification. A structural unit derived from a certain monomer is sometimes denoted by adding “unit” after the name of the monomer.

[0034] The content (mol %) of each species of units in a polymer relative to all the units in the polymer is determined by analyzing the polymer by nuclear magnetic resonance spectrometry.

[0035] A unit represented by the formula (u11) is referred to as a unit (u11). Units represented by other formulae are also referred to in the same manner.[Membrane Electrode Assembly]

[0036] The membrane electrode assembly of the present invention is a membrane electrode assembly for polymer electrolyte membrane water electrolysis which comprises an anode having a catalyst layer, a cathode having a catalyst layer and a polymer electrolyte membrane provided between the anode and the cathode,

[0037] wherein at least one of the catalyst layer in the anode and the catalyst layer in the cathode comprises a fluorinated polymer (hereinafter also referred to as a “polymer H”) having ion exchange groups and comprising units having a cyclic ether structure (hereinafter also referred to as “units A”).

[0038] The membrane electrode assembly of the present invention contains a polymer H comprising units A and having ion exchange groups in the catalyst layer of the anode or the cathode, and it is speculated that the membrane electrode assembly can suppress hydrogen and oxygen crossover to the opposite electrode sides owing to the characteristic units and functional groups in the polymer H.

[0039] FIG. 1 is a cross-sectional view of an example of the membrane electrode assembly of the present invention.

[0040] The membrane electrode assembly 10 comprises an anode 13 having a catalyst layer 11 and a gas diffusion layer 12, a cathode 14 having a catalyst layer 11 and a gas diffusion layer 12, and a polymer electrolyte membrane 15 sandwiched between the catalyst layer 11 in the anode 13 and the catalyst layer 11 in the cathode 14.

[0041] At least one of the catalyst layer 11 in the anode 13 and the catalyst layer 11 in the cathode 14 comprises a polymer H.

[0042] Although the membrane electrode assembly 10 shown in FIG. 1 comprises gas 30 diffusion layers 12, gas diffusion layers are optional, and hence the anode 13 and the cathode 14 may comprise no gas diffusion layers.[Anode and Cathode]<Catalyst Layers>

[0043] The membrane electrode assembly comprises an anode having a catalyst layer and a cathode having a catalyst layer.(Catalyst)

[0044] The catalyst layers contain a catalyst.

[0045] The catalyst layer in the anode preferably contains an iridium-containing catalyst (a catalyst containing iridium). As the iridium-containing catalyst, an iridium oxide catalyst, a double oxide catalyst comprising iridium and another metal such as ruthenium, titanium, zirconium, hafnium, niobium, tantalum, molybdenum, tungsten, platinum and gold, an iridium oxide alloy-based catalyst and an iridium oxide-based core-shell catalyst may, for example, be mentioned.

[0046] The catalyst may be held on an insoluble support such as a known metal oxide support.

[0047] The iridium-containing catalyst is preferably in the form of particles.

[0048] When the catalyst layer comprises a polymer H and an iridium-containing catalyst, the mass ratio of polymer H to iridium (the content of polymer H / the content of iridium, hereinafter referred to as “I / Ir”) is preferably at least 0.05, more preferably at least 0.10, and preferably at most 0.40, more preferably at most 0.30. The mass ratio is preferably from 0.05 to 0.40.

[0049] The catalyst layer in the cathode preferably contains a platinum-containing catalyst (a catalyst containing platinum), more preferably contains a carbon support and a platinum-containing catalyst, further preferably a supported catalyst comprising a carbon support and a platinum-containing catalyst held on the carbon support.

[0050] As the platinum-containing catalyst, elemental platinum, a platinum alloy and a core-shell platinum alloy may, for example, be mentioned. As other metal elements in the platinum alloy, non-platinum metals in the platinum group (such as ruthenium, rhodium, palladium, osmium and iridium), gold, silver, chromium, iron, titanium, manganese, cobalt, nickel, molybdenum, tungsten, aluminum, silicon, zinc and tin may, for example, be mentioned.

[0051] As the carbon support, a solid carbon support and a hollow carbon support may be mentioned. The carbon support may contain pores (such as macropores, mesopores and micropores). The carbon support may, for example, be Ketjenblack, acetylene black, graphitized carbon, mesoporous carbon, carbon fiber or carbon nanotube.

[0052] The catalyst layer in the cathode may comprise a catalyst containing no platinum group metals, such as a metal oxide catalyst, a metal nitride catalyst and a carbon alloy catalyst.

[0053] The platinum-containing catalyst is preferably in the form of particles.

[0054] The average particle diameter (number-based median diameter D50) of the platinum-containing catalyst is preferably at least 1.0 nm, more preferably at least 2.0 nm, and preferably at most 10.0 nm, more preferably at most 5.0 nm.

[0055] The average particle diameter of the platinum-containing catalyst is measured by TEM (transmission electron microscopy) or by SAXS (small-angle X-ray scattering).

[0056] The amount of the platinum-containing catalyst in a supported catalyst is at least 20.0 mass %, more preferably at least 30.0 mass %, and preferably at most 70.0 mass %, more preferably at most 60.0 mass %, relative to the whole catalyst.

[0057] In the case of a catalyst layer comprising a polymer H and a platinum-containing catalyst, the mass ratio of the polymer H to platinum (the content of the polymer H / the content of platinum) is preferably at least 1.2, more preferably at least 1.4 and preferably at most 3.0.

[0058] In the case of a catalyst layer comprising a carbon support, the mass ratio of a polymer H to the carbon (the content of the polymer H / the content of the carbon support, hereinafter referred to as “I / C”) is preferably at least 0.4, more preferably at least 0.6 and preferably at most 1.5, more preferably at most 1.2, in view of adhesiveness. The mass ratio is also preferred to be from 0.2 to 1.5 because the mass transfer in the cathode increases as the void in the cathode increases.

[0059] In the case of a catalyst layer comprising a carbon support, the specific surface area of the carbon support is preferably at least 60 m2 / g, more preferably at least 100 m2 / g, further preferably at least 150 m2 / g, and preferably at most 1500 m2 / g, more preferably at most 1,200 m2 / g.

[0060] The thicknesses of the catalyst layer in the anode and the catalyst layer in the cathode are preferably from 0.1 to 25 μm, more preferably from 3 to 20 μm.

[0061] The thicknesses of the anode and the cathode are preferably at least 5 μm, and preferably at most 100 μm, more preferably at most 50 μm, further preferably at most 30 μm, particularly preferably at most 15 μm.

[0062] The thicknesses of the catalyst layers, the anode and the cathode are measured on a magnified cross-sectional image of a membrane electrode assembly cut along the thickness direction under a laser microscope, and the arithmetical mean is calculated from measurements at arbitrary 20 points.(Polymer H)

[0063] At least one of the catalyst layers in the anode and the catalyst layer in the cathode comprises a polymer H. A polymer H may be contained in both the catalyst layer in the anode and the catalyst layer in the cathode.

[0064] A polymer H is a fluorinated polymer comprising units A and having ion exchange groups.

[0065] The cyclic ether structure of units A preferably has a 5-membered ring or a 6-membered ring. The cyclic ether structure having a 5-membered ring or a 6-membered ring preferably contains one or two ethereal oxygen atoms.

[0066] More specifically, the units having a cyclic ether structure are at least one species of units selected from the group consisting of units (u11), units (u12), units (u21), units (u22) and units (u24) shown below.

[0067] R11 is a bivalent perfluoroalkylene group which may have an ethereal oxygen atom, and the number of carbon atoms in the perfluoroalkylene group is preferably from 1 to 10, more preferably from 2 to 4.

[0068] The number of ethereal oxygen atoms, if present, in the perfluoroalkylene group may be 1 or at least two. Such an ethereal oxygen atom may be inserted between carbon atoms in the perfluoroalkylene group or may be bonded to a terminal carbon atom. The perfluoroalkylene group may be linear or branched and is preferably linear.

[0069] Each of R12, R13, R15 and R16 is independently a monovalent perfluoroalkyl group which may have an ethereal oxygen atom or a fluorine atom. The number of carbon atoms in the perfluoroalkyl group is preferably from 1 to 5, more preferably from 1 to 3.

[0070] It is preferred that at least one of R15 and R16 is a fluorine atom, and it is more preferred that both are fluorine atoms, in view of polymerizability.

[0071] R14 is a monovalent perfluoroalkyl group which may have an ethereal oxygen atom, a fluorine atom or a group represented by —R11(SO2X(SO2Rf)a)−M+. The number of ethereal oxygen atoms, if present, in the perfluoroalkyl group may be 1 or at least two. Such an ethereal oxygen atom may be inserted between carbon atoms in the perfluoroalkyl group or may be bonded to a terminal carbon atom. The perfluoroalkyl group may be linear or branched and is preferably linear. The number of carbon atoms in the perfluoroalkyl group is preferably from 1 to 10, more preferably from 2 to 4. When the formula (u11) contains two R11's, they may be identical to or different from each other.

[0072] M+ is H+, a monovalent metal cation (such as a potassium ion or a sodium ion) or ammonium ion which may have at least one hydrocarbon group (such as a methyl group or an ethyl group) instead of a hydrogen atom and is preferably H+ for high electrical conductivity.

[0073] Rf is a linear or branched perfluoroalkyl group which may have an ethereal oxygen atom. The number of carbon atoms in the perfluoroalkyl group is preferably from 1 to 8, particularly preferably from 1 to 6. When the formula (u11) contains two or more Rf's, they may be identical to or different from one another.

[0074] X is an oxygen atom, a nitrogen atom or a carbon atom, and a is 0 when X is an oxygen atom, a is 1 when X is a nitrogen atom, and a is 2 when X is a carbon atom.

[0075] As the —(SO2X(SO2Rf)a)−M+ group, a sulfonic acid group (a —SO3−M+ group), a sulfonimide group (a —SO2N(SO2Rf)−M+ group) or a sulfone methide group (a —SO2C(SO2Rf)2)−M+ group) may be mentioned specifically.

[0076] Units (u11) are preferably units (u11-1).

[0077] M+ in units (u11-1) is the same as M+ in the formula (u11).

[0078] R21 is a C1-6 perfluoroalkylene group or a C2-6 perfluoroalkylene group which has an ethereal oxygen atom between carbon atoms.

[0079] The number of ethereal oxygen atoms, if present, in the perfluoroalkylene group may be 1 or at least two. The perfluoroalkylene group may be linear or branched and is preferably linear.

[0080] R22 is a fluorine atom, a C1-6 perfluoroalkyl group, a C2-6 perfluoroalkyl group which has an ethereal oxygen atom between carbon atoms or a group represented by —R21(SO2X(SO2Rf)a)−M+. The number of ethereal oxygen atoms, if present, in the perfluoroalkyl group may be 1 or at least two. The perfluoroalkyl group may be linear or branched and is preferably linear. When the formula (u12) contains two R21's, they may be identical to or different from each other.

[0081] M+, Rf, X and a are the same as M+, Rf, X and a in the formula (u11), respectively.

[0082] As specific examples of units (u12), units (u12-1) and units (u12-2) may be mentioned. In the formulae, M+ is the same as M+ in the formula (u11).

[0083] Each of R41, R42, R43, R44, R45 and R46 is independently a monovalent perfluoroalkyl group which may have an ethereal oxygen atom or a fluorine atom.

[0084] The number of ethereal oxygen atoms, if present, in the perfluoroalkyl group may be 1 or at least two. Such an ethereal oxygen atom may be inserted between carbon atoms in the perfluoroalkyl group or may be bonded to a terminal carbon atom. The perfluoroalkyl group may be linear or branched and is preferably linear. The number of carbon atoms in the perfluoroalkyl group is preferably from 1 to 5, more preferably from 1 to 3.

[0085] It is preferred that at least one of R45 and R46 is a fluorine atom, and it is more preferred that both are fluorine atoms, in view of polymerizability.

[0086] Units (u21) are preferably units (u21-1).

[0087] s is 0 or 1, preferably 0.

[0088] Each of R51 and R52 is independently a fluorine atom or a C1-5 perfluoroalkyl group or R51 and R52 are bonded together to form a spiro ring (when s is 0).

[0089] Each of R53 and R54 is independently a fluorine atom or a C1-5 perfluoroalkyl group.

[0090] R55 is a fluorine atom, a C1-5 perfluoroalkyl group or a C1-5 perfluoroalkoxy group. R55 is preferably a fluorine atom in view of polymerizability.

[0091] The perfluoroalkyl group and the perfluoroalkoxy group may be linear or branched and are preferably linear.

[0092] Units (u22) are preferably units (u22-1).

[0093] Each of R71 to R76 is independently a monovalent perfluoroalkyl group which may have an ethereal oxygen atom or a fluorine atom.

[0094] The number of ethereal oxygen atoms, if present, in the perfluoroalkyl group may be 1 or at least two. Such an ethereal oxygen atom may be inserted between carbon atoms in the perfluoroalkyl group or may be bonded to a terminal carbon atom. The perfluoroalkyl group may be linear or branched and is preferably linear. The number of carbon atoms in the perfluoroalkyl group is preferably from 1 to 5, more preferably from 1 to 3.

[0095] R71 to R74 are preferably fluorine atoms in view of polymerizability.

[0096] Units A preferably comprise units of at least one species of units selected from the group consisting of units (u21), units (u22) and units (u24) for provision of a catalyst layer with higher oxygen permeability, and are particularly preferably units (u22).

[0097] The content of units A is preferably at least 30 mol %, more preferably at least 50 mol %, further preferably at least 55 mol %, particularly preferably at least 60 mol %, relative to all the units in the polymer H, and preferably at most 87 mol %, more preferably at most 80 mol %, further preferably at most 75 mol %, relative to all the units in the polymer H, for formation of crack-resistant catalyst layers.

[0098] The polymer H may comprise only one species of units A or two or more species of units A. In the latter case, the content of such units means the total content of these species.

[0099] The polymer H preferably comprises units having an ion exchange group and no cyclic ether structure (hereinafter also referred to as “units B”).

[0100] The number of ion exchange groups in a unit B is preferably at least 1, and more preferably at least 2 for easy production of a polymer having a high molecular weight without lowering the content of units A and is more preferably 2 for easy production of the monomer.

[0101] Units B are preferably perfluoromonomer units having an ion exchange group, more preferably perfluoromonomer units having a sulfonic acid functional group.

[0102] The perfluoromonomer units are preferably units (u31), units (A-1), units (A-2) or units (A-3), more preferably units (u31) or units (A-2).

[0103] In the formula (u31), Z is a fluorine atom or a trifluoromethyl group, q is 0 or 1, m is an integer of from 0 to 3, p is 0 or 1, n is an integer of from 1 to 12, provided that m+p>0.

[0104] In the formula (u31), M+ is H+, a monovalent metal cation (such as a potassium ion or a sodium ion) or an ammonium ion which may have at least one hydrocarbon group (such as a methyl group or an ethyl group) instead of a hydrogen atom and is preferably H+.

[0105] When the formula (u31) contains two or more Z's, they may be identical to or different from each other.

[0106] In the formulae (A-1) to (A-3), each of RF1 and RF2 is independently a C1-3 perfluoroalkylene group or a bivalent group derived by replacing —CF2— in a perfluoroalkylene group with an ethereal oxygen atom.

[0107] In the bivalent group, such an ethereal oxygen atom may be bonded to a terminal carbon atom or may be inserted between carbon atoms in the perfluoroalkylene group.

[0108] The number of carbon atoms in the bivalent group is preferably from 1 to 3, more preferably 2 or 3.

[0109] Specific examples of RF1 and RF2 include —CF2—, —CF2CF2—, —CF(CF3)—, —CF2CF2CF2—, —CF(CF2CF3)—, —CF(CF3)CF2—, —CF2CF(CF3)—, —C(CF3)(CF3)—, —CF2OCF2CF2— and —OCF2CF2—.

[0110] Each of RF1 and RF2 is preferred to be independently a C1-2 perfluoroalkylene group, —CF2OCF2CF2— or —OCF2CF2—, for easy production of a polymer H with a higher ion exchange capacity from an inexpensive raw material. A C2 perfluoroalkylene group is preferred to be linear. Specifically speaking, —CF2—, —CF2CF2— or —CF(CF3)— is preferred, —CF2— or —CF2CF2— is more preferred, and —CF2— is further preferred.

[0111] In the formula (A-2), RF3 is a C1-6 perfluoroalkylene group.

[0112] Specific examples of RF3 may be —CF2—, —CF2CF2—, —CF(CF3)—, —CF2CF2CF2—, —CF(CF2CF3)—, —CF(CF3)CF2—, —CF2CF(CF3)— and —C(CF3)(CF3)—.

[0113] RF3 is preferably a C1-3 perfluoroalkylene group, for easy production of a polymer H with a higher ion exchange capacity from an inexpensive raw material. Specifically speaking, —CF2—, —CF2CF2— or —CF2CF(CF3)— is preferred, and —CF2— or —CF2CF(CF3)— is more preferred.

[0114] In the formula (A-2), m is 0 or 1.

[0115] The content of units B is preferably at least 13 mol % relative to all the units in the polymer H for good proton conductivity, and is preferably at most 50 mol % relative to all the units in the polymer H to secure enough water repellency for good water drainage necessary to achieve a high power efficiency.

[0116] The polymer H may comprise only one species of units B or two or more species of units B. In the latter case, the content of such units means the total content of these species.

[0117] The polymer H preferably comprises tetrafluoroethylene (TFE) units (hereinafter also referred to as “units C”).

[0118] Units C improve water repellency.

[0119] The content of units C is preferably at least 5 mol % and at most 35 mol % relative to all the units in the polymer H.

[0120] The total content of units A, units B and units C is preferably at least 90 mol %, more preferably at least 99 mol %, and preferably at most 100 mol %.

[0121] The polymer H may comprise units based on a monomer having two or more polymerizable unsaturated bonds (hereinafter also referred to as “units D”).

[0122] Herein, units based on a monomer having a cyclic ether structure and two or more polymerizable unsaturated bonds are categorized as units D.

[0123] Examples of polymerizable unsaturated bonds are double bonds between carbon atoms (C═C) and triple bonds between carbon atoms (C≡C).

[0124] The number of polymerizable unsaturated bonds in a monomer having at least two polymerizable unsaturated bonds is preferably from 2 to 6, more preferably 2 or 3, particularly preferably 2, in view of polymerizability.

[0125] The monomer having at least two polymerizable unsaturated bonds is preferably a fluorine-containing monomer, more preferably a perfluoromonomer.

[0126] Units D are preferably units represented by the formula (u41) for formation of crack-resistant catalyst layers. A unit represented by the formula (u41) may crosslink two polymer chains or may be present in a single polymer chain.

[0127] In the formula (u41), Q4 is an oxygen atom or a bivalent perfluoroalkylene group which may have an ethereal oxygen atom.

[0128] The number of ethereal oxygen atoms, if present, in the perfluoroalkylene group as Q4 may be 1 or at least two. Such an ethereal oxygen atom may be inserted between carbon atoms in the perfluoroalkylene group or may be bonded to a terminal carbon atom.

[0129] The perfluoroalkylene group may be linear or branched and is preferably linear. The number of carbon atoms in the perfluoroalkylene group is preferably from 1 to 10, more preferably from 2 to 8, particularly preferably 3 or 4.

[0130] As specific examples of units D, units (u41-1), units (u41-2) and units (u41-3) may be mentioned. Units (u41-2) are preferred for formation of crack-resistant catalyst layers.

[0131] In the above units, m1 and m3 are integers of from 2 to 8, particularly preferably 3 or 4.

[0132] In the above units, each of m2 and m4 is independently an integer of from 0 to 5, provided that m2+m4≥1.

[0133] The content of units D is preferably at least 0.001 mol %, more preferably at least 0.005 mol %, further preferably 0.01 mol %, relative to all the units in the polymer H, for formation of crack-resistant catalyst layers, and is preferably at most 10 mol %, more preferably at most 5 mol %, further preferably at most 1 mol %, relative to all the units in the polymer H, for good power generation characteristics.

[0134] The polymer H may comprise only one species of units D or two or more species of units D. In the latter case, the content of such units means the total content of these species.

[0135] The polymer H may comprise units other than those mentioned above (hereinafter referred to as “optional units”).

[0136] Specific examples of such units include units based on such a monomer as perfluoro(3-butenyl vinyl ether), perfluoro(allyl vinyl ether), a perfluoro α-olefin (like hexafluoropropylene) or a perfluoro(alkyl vinyl ether).

[0137] The softening temperature of the polymer H is preferably at least 135° C., more preferably at least 150° C., further preferably at least 160° C., and is preferably at most 300° C., more preferably at most 250° C., further preferably at most 200° C. for formation of crack-resistant catalyst layers.

[0138] The softening temperature of the polymer H is measured as described later in the Examples.

[0139] The ion exchange capacity of the polymer H is preferably at least 0.80 meq / g dry resin, more preferably at least 0.90 meq / g dry resin in view of ion conductivity and is preferably at most 1.60 meq / g dry resin, more preferably at most 1.50 meq / g dry resin, and is further preferably at most 1.20 meq / g dry resin, particularly preferably at most 1.10 meq / g dry resin for a low electrolysis voltage.

[0140] When the catalyst layer in the anode comprises a polymer H, the total content of the polymer H and a catalyst is preferably at least 90 mass %, at least 99 mass % and is preferably at most 100 mass %, relative to the total mass of the catalyst layer in the anode.

[0141] When the catalyst layer in the cathode comprises a polymer H, the total content of the polymer and a catalyst is preferably at least 90 mass %, more preferably at least 99 mass % and is preferably at most 100 mass %, relative to the total mass of the catalyst layer in the cathode.

[0142] The method for producing a polymer H will be described in reference to a polymer H having sulfonic acid groups in acid form.

[0143] A polymer H may be produced, for example, from a precursor polymer (hereinafter also referred to as a “polymer F”) having precursor groups (specifically speaking, groups represented by —SO2F) instead of sulfonic acid groups in acid form (—SO3−H+) in a polymer H, by converting the precursor groups into sulfonic acid groups in acid form.

[0144] The conversion of precursor groups represented by —SO2F to sulfonic acid groups in acid form may be carried out, for example, by hydrolyzing groups representer by —SO2F in a polymer F to sulfonic acid groups in salt form and then converting the sulfonic acid groups from the salt form to the acid form.

[0145] The TQ value of the polymer F is preferably at most 300° C., more preferably at most 290° C. When the TQ value is at the above-mentioned upper limit or below, the resulting polymer H dissolves or disperses well in a liquid medium during preparation of a catalyst ink.

[0146] The TQ value of the polymer F is preferably at least 100° C., more preferably at least 130° C., further preferably at least 160° C. When the TQ value is at the above-mentioned lower limit or above, the resulting polymer H has a sufficiently high molecular weight and can make a strong catalyst layer.

[0147] TQ value correlates with the molecular weight of a polymer and is the temperature at which the polymer has a volume flow rate of 100 mm3 / sec. The volume flow rate is the volume in mm3 / sec of a polymer melt-extruded through a nozzle (inner diameter: 1 mm, length: 1 mm) under a pressure of about 3 MPa at a constant temperature. TQ value is an index of molecular weight of a polymer, and the higher the TQ value of a polymer, the higher the molecular weight of the polymer. The TQ value of the polymer F is determined as described later in the Examples.(Optional Components)

[0148] The catalyst layers may comprise optional components other than those described above.

[0149] As such optional components, hydrophobic particles, hydrophilic particles, a dispersant and carbon fiber may, for example, be mentioned.

[0150] As hydrophobic particles, polytetrafluoroethylene particles may, for example, be mentioned.

[0151] As hydrophilic particles, metal oxide particles may, for example, be mentioned.

[0152] The catalyst layers may be formed by applying a catalyst ink comprising a catalyst, a liquid medium and, if necessary, a polymer H onto a base film to make a wet coating, and drying the wet coating to remove the liquid medium. The base film is preferably a releasable liner (such as an ethylene-tetrafluoroethylene sheet).

[0153] As the catalyst, those mentioned above for the catalyst layers may be mentioned.

[0154] The liquid medium preferably contains at least one liquid selected from the group consisting of water and alcohols, and more preferably contains both water and an alcohol.

[0155] Specific examples of the alcohol include methanol, ethanol, propanols (such as 1-propanol and 2-propanol), 1-butanol, 2-methyl-1-propanol, 2-butanol, 2-methyl-2-propanol, 2,2,2-trifluoroethanol, 2,2,3,3,3-pentafluoro-1-propanol, 2,2,3,3-tetrafluoro-1-propanol, 4,4,5,5,5-pentafluoro-1-pentanol, 1,1,1,3,3,3-hexafluoro-2-propanol, 3,3,3-trifluoro-1-propanol, 3,3,4,4,5,5,6,6,6-nonafluoro-1-hexanol and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-1-octanol.

[0156] The alcohol preferably contains a propanol. The alcohol may be a single species or a combination of two or more species.

[0157] The catalyst layers may be formed by applying a catalyst ink onto a polymer electrolyte membrane, a gas diffusion layer, which will be described later, or a base film to form a wet coating, and drying the wet coating to remove the liquid medium from the wet coating.

[0158] The catalyst layers thus formed may be further subjected to heat treatment such as hot pressing.<Gas Diffusion Layer>

[0159] The anode and the cathode in the membrane electrode assembly may have a gas diffusion layer.

[0160] A gas diffusion layer serves to rapidly diffuse the gas generated in a catalyst layer out of the catalyst layer and also serves as a current collector. As a gas diffusion layer, carbon paper, carbon cloth, carbon felt, a sintered product of titanium oxide fiber, a sintered product of titanium oxide particles or a titanium mesh may, for example, be mentioned.

[0161] It is preferred to use a sintered product of titanium oxide fiber or a sintered product of titanium oxide particles or a titanium mesh on the anode side because a carbon material would oxidize on the anode side where the electrical potential is high. A sintered product of titanium oxide and a titanium mesh may be plated with platinum or iridium, if necessary.

[0162] The gas diffusion layer in the cathode may have a water-repellent finish of polytetrafluoroethylene.[Polymer Electrolyte Membrane]

[0163] The membrane electrode assembly of the present invention has a polymer electrolyte membrane between the anode and the cathode.

[0164] The polymer electrolyte membrane is preferably a membrane comprising a polymer with high proton conductivity, more preferably a membrane comprising a polymer having ion exchange groups.

[0165] As a polymer having ion exchange groups, a polymer comprising units B and units C, and optionally units D may be mentioned.

[0166] The polymer electrolyte membrane may be formed, for example, by applying a liquid composition comprising a polymer and a liquid medium onto a base film or a catalyst layer 11 and drying it (casting).

[0167] It is preferred to treat the polymer electrolyte membrane with heat to stabilize it. The heat treatment temperature is preferably from 130 to 200° C., although it can be adjusted depending on the species of the polymer.

[0168] The polymer electrolyte membrane may be treated with an aqueous hydrogen peroxide solution, if necessary.

[0169] The polymer electrolyte membrane may be reinforced with a reinforcing material.

[0170] The reinforcing material may, for example, be a porous body, fibers, a woven fabric or a non-woven fabric. The reinforcing material may be made of polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylene copolymer, a tetrafluoroethylene-perfluoro (alkyl vinyl ether) copolymer, polyethylene, polypropylene, polyphenylene sulfide or the like.

[0171] The polymer electrolyte membrane may further comprise atoms of at least one member selected from the group consisting of cerium and manganese to further improve durability. Cerium and manganese are preferred because they decompose hydrogen peroxide responsible for deterioration of the polymer electrolyte membrane. Cerium and manganese in the polymer electrolyte membrane are preferably in the form of ions in any state.

[0172] The polymer electrolyte membrane may comprise a humectant such as silica or a heteropoly acid (such as zirconium phosphate, phosphomolybdic acid or phosphotungstic acid) to prevent the membrane from drying.

[0173] The thickness of the polymer electrolyte membrane is preferably at least 5 μm, more preferably at least 15 μm, further preferably at least 25 μm, particularly preferably at least 40 μm, and is preferably at most 300 μm, more preferably at most 150 μm.

[0174] The thickness of the polymer electrolyte membrane is measured on a magnified cross-sectional image of a membrane electrode assembly cut along the thickness direction under a laser microscope, and the arithmetical mean is calculated from measurements at arbitrary 8 points.

[0175] The membrane electrode assembly may be produced, for example, as follows.

[0176] (i) By forming catalyst layers (the catalyst layer in the anode and the catalyst layer in the cathode) on the polymer electrolyte membrane and sandwiching the resulting membrane-catalyst layer assembly between gas diffusion layers; or

[0177] (ii) By forming catalyst layers (the catalyst layer in the anode and the catalyst layer in the cathode) on gas diffusion layers to form electrodes (an anode and a cathode) and sandwiching a polymer electrolyte membrane between the electrodes.

[0178] The membrane electrode assembly is used in a PEM water electrolyzer.

[0179] Such a PEM electrolyzer having the membrane electrode assembly includes a water supply unit for supplying water to the anode compartment and a power supply unit electrically connected to the anode compartment and the cathode compartment.

[0180] In such a PEM electrolyzer, when a direct-current voltage is applied by the power supply unit between the anode and the cathode while water is supplied to the anode compartment by the water supply unit, water decomposes into oxygen and protons in the anode compartment, generating a flow of electrons towards the power supply unit. The protons transfer to the cathode compartment through the membrane electrode assembly, and receive electrons supplied from power supply unit to generate hydrogen gas.

[0181] Such a water electrolyzer may have the same components as those of a known water electrolyzer (for example, an oxygen recovery member to recover the generated oxygen and a hydrogen recovery member to recover the generated hydrogen), in addition to the above-mentioned components.EXAMPLES

[0182] Now, the present invention will be described in further detail with reference to Examples. Example 1 to 2 correspond to examples of the present invention; and Example 3 corresponds to a comparative example.

[0183] Appropriate modifications may be added to the Examples in materials, amounts and ratios of materials, operations and the procedures for operations as long as they do not depart from the gist of the present invention. Therefore, it should be understood that the present invention is by no means restricted to the Examples described below.[Measuring Methods][Ion Exchange Capacity]

[0184] Polymer F-1 and Polymer F-2 were press-molded into membranes under a pressure of 4 MPa (gauge pressure) at 210° C. for Polymer F-1 and at a temperature 10° C. higher than the TQ value for Polymer F-2. The membrane of Polymer F-1 and the membrane of Polymer F-2 were soaked in aqueous alkaline solutions shown in Table 2, respectively, at 80° C. for 16 hours to hydrolyze —SO2F in Polymer F-1 and Polymer F-2 to —SO3K. Each membrane of polymers F was subjected to 5 cycles of 30 minutes of soaking in 3 mol / L aqueous hydrochloric acid at 50° C. and 30 minutes of soaking in ultrapure water at 80° C. to convert —SO3K in each polymer F to —SO3H. The membranes were washed with ultrapure water repeatedly until the ultrapure water washings reached pH 7. Each membrane of polymers F was sandwiched between filter papers and air-dried to obtain a membrane of Polymer H-1 and a membrane of Polymer H-2.

[0185] Each membrane of polymers H was dried under a nitrogen stream in a glove box to a constant mass and weighed. Each membrane of polymers H was soaked in a 0.85 mol / g sodium hydroxide solution (solvent:water / methanol=10 / 90 (mass ratio)) at 60° C. for at least 72 hours to convert sulfonic acid groups from the acid form to the sodium salt form. The ion exchange capacity (meq / g dry resin) of each polymer H was obtained from the amount of the remaining sodium hydroxide in the sodium hydroxide solution measured by back titration with 0.1 mol / L hydrochloric acid.[Ratio of Each Species of Units]

[0186] The ratios of units based on each monomer in polymers F were determined by 19F-NMR. The contents of respective species of units in polymers H are about the same as those in the corresponding polymers F.

[0187] 19F-NMR measurements were made at a frequency of 282.7 MHz, using hexafluorobenzene as the solvent and CFCl3 as the chemical shift standard. For composition analysis, polymers F were dissolved in hexafluorobenzene to a concentration of 10 mass %.[TQ value]

[0188] By means of a flow tester (CFT-500A, manufactured by Shimadzu Corporation) equipped with a nozzle having an inner diameter of 1 mm and a length of 1 mm, a polymer F was extruded at a pressure of 2.94 MPa (gauge pressure) at various temperatures through the nozzle, to obtain the temperature (TQ value) at which the polymer F would be extruded at 100 mm3 / sec. A TQ value exceeding 300° C. was obtained by extrapolating from the extrusion rates at four temperatures of 300° C. or lower, using a logarithmic approximate equation correlating extrusion rate with the reciprocal of absolute temperature. In the case of polymers F having the same composition, the higher the TQ value, the larger the molecular weight.[Softening Temperature]

[0189] The softening temperature of a polymer H was measured as follows.

[0190] A liquid composition S (Liquid Compositions S-1 or S-2) having a polymer H dispersed in a liquid medium was poured into a petri dish and annealed to obtain a polymer electrolyte membrane. The dynamic viscoelasticity of the membrane was measured with a dynamic viscoelastic analyzer (DVA-225, manufactured by IT Keisoku Seigyo K.K.) in the tension mode with a sample width of 5.0 mm and a grip-to-grip length of 15 mm at a frequency of 1 Hz and a heating rate of 2° C. / min. The tan δ (loss tangent) was calculated from the ratio (E″ / E′) of loss modulus E″ to storage modulus E′, and a tan δ-temperature curve was prepared. The temperature at which a peak appeared between −100° C. and 200° C. on the tan-temperature curve was taken as the softening temperature of the polymer H. Softening temperature is defined as the temperature at which transition from a glassy state into a rubbery state occurs. The dimensions and thickness of the membrane used for calculation were measured at a temperature of 23° C. at a relative humidity of 50% RH.[Synthesis of Polymers][Abbreviations]TFE: tetrafluoroethylene

[0192] PFB: CF3CF2CF2C(O)OOC(O)CF2CF2CF3

[0193] HFC-52-13p: CF3(CF2)5H

[0194] HCFC-225cb: CClF2CF2CHClF

[0195] HCFC-141b: CH3CCl2F[Polymer F-1]

[0196] A 2575 mL stainless steel autoclave was deaerated on an ice bath, then loaded with 1116.02 g of Compound 1, 340.92 g of Compound 2, 407.78 mg of 3.2 mass % PFB solution in HFC-52-13p and 172.0 g of HFC-52-13p by suction and deaerated again. After addition of 56.90 g of TFE, the temperature was raised to 24° C. to initiate the reaction. During the reaction, the reaction mixture was stirred with a double helical ribbon blade impeller at a rotational speed of 60 rpm for the initial 2.5 hours, at a rotational speed of 30 rpm between 2.5 hours and 3.5 hours, at a rotational speed of 10 rpm between 3.5 hours and 10 hours. After 10 hours of stirring, the remainer of Compound 2 and TFE was purged from the autoclave, and the reaction product was diluted with HFC-52-13p, and coagulated by adding a mixture of HFC-52-13p and methanol (mass ratio=8:2) and filtered to collect the polymer. The polymer was washed in a mixture of HFC-52-13p and methanol (mass ratio=7:3). The solid matter was separated by filtration and dried at 80° C. and vacuum-dried at 210° C. to obtain Polymer F-1.[Polymer F-2]

[0197] An autoclave (made of Hastelloy and having an inner volume of 230 mL) was loaded with 123.8 g of Compound 3, 35.2 g of HCFC-225cb as a liquid medium and 63.62 mg of 2,2′-azobis(isobutyronitrile) as a polymerization initiator, then cooled with liquid nitrogen and deaerated. Then, the temperature was raised to 70° C., and TFE was introduced to raise the pressure in the system to 1.14 MPaG. The polymerization was conducted while the pressure was maintained constant at 1.14 MPaG by continuous addition of TFE until the addition of TFE reached 12.4 g after 7.9 hours. Then, the polymerization was terminated by cooling the autoclave and purging the gas in the system.

[0198] The resulting polymer solution was diluted with HCFC-225cb and coagulated by adding HCFC-141b. The coagulate was washed with HCFC-225cb and with HCFC-141b and then dried to obtain 25.1 g of a copolymer of TFE and Compound 3, which is designated as Polymer F-2.TABLE 1Species of Polymer FF-1F-2Compound 1 units [mol %]150Compound 2 units (Cyclic600ether units) [mol %]Compound 3 units [mol %]018TFE units [mol %]2582TQ value [°C]290225[Synthesis of Polymer H]

[0199] Powders of Polymers H-1 and H-2 were obtained from the respective polymers F.

[0200] Each polymer F was cooled with dry ice and pulverized. Each pulverized polymer F was soaked in an aqueous alkaline solution shown in Table 2 at 80° C. for 40 hours to hydrolyze —SO2F in each polymer F to —SO3K. Each of the resulting polymers was subjected to 10 cycles of 30 minutes of soaking in 3 mol / L aqueous hydrochloric acid at 80° C. and 30 minutes of soaking in ultrapure water at 80° C. to convert —SO3K in each polymer to —SO3H. The polymers were washed with ultrapure water repeatedly until the ultrapure water washings reached pH 7. Each polymer was dried in a nitrogen stream to obtain powders of polymers H.

[0201] In Table 2, Solution A is a 15 / 30 / 55 (mass ratio) mixture of potassium hydroxide, dimethyl sulfoxide and water, and Solution B is a 15 / 20 / 65 (mass ratio) mixture of potassium hydroxide, methanol and water.

[0202] The softening temperatures and ion exchange capacities in Table 2 were measured as previously described.TABLE 2Polymer FF-1F-2Aqueous alkaline solutionSolution ASolution BPolymer HSpeciesH-1H-2Ion exchange capacity1.091.10[meq / g dry resin]Softening temperature [°C]172105[Preparation of Liquid Composition][Liquid Composition S-1]

[0203] In a 0.2 L glass autoclave, 18.70 g of polymer (powder) (comprising 18.22 g of Polymer H-1 and 0.48 g of water and having a solid content of 97.4 mass %), 22.81 g of ultrapure water and 54.36 g of 1-propanol were stirred at 300 rpm at 115° C. for 13 hours. After dilution with 31.0 g of ultrapure water, the mixture was stirred at 110° C. for 1 hour and allowed to cool. The solution was taken out of the autoclave, diluted with 25.6 g of ultrapure water and 25.6 g of 1-propanol, stirred at 110° C. for 1 hour, then allowed to cool and filtered through a pressure filtration module (filter paper: PF040 manufactured by ADVANTEC Toyo Kaisha, Ltd.) to obtain a 10.2 mass % dispersion of Polymer H-1 in a liquid medium, which is designated as Liquid Composition S-1.[Liquid Composition S-2]

[0204] From Polymer F-2, a dispersion of Polymer H-2 (solid content of 26.0 mass %, in ethanol / water=60 / 40 (mass ratio)), which is designated as Liquid Composition S-2, was obtained in the same manner as in Preparation Example 4 in JP-A-2018-55877.[Preparation of Catalyst Ink][Cathode Catalyst Ink CI-1]

[0205] 3.0 g of a supported catalyst comprising 47.1 mass % platinum supported on carbon powder (“TEC10E50E” manufactured by TANAKA PRECIOUS METAL TECHNOLOGIES Co., Ltd., containing a carbon support having a specific surface area of 800 m2 / g and an average particle diameter of 2.8 nm) was mixed with 23.6 g of water and 13.9 g of 1-propanol. To the mixture, 11.1 g of Liquid Composition S-1 was added so that the I / C would be 0.7, and the solid content would be 8.0 mass %. The resulting mixture was ground by means of a planetary bead mill (Model: LP-4, manufactured by Ito Seisakusho Co., Ltd.) containing zirconia beads with a diameter of 5 mm at a rotation speed of 300 rpm for 90 minutes and then diluted with 10.5 g of water and 6.9 g of 1-propanol to a solid content of 6.0 mass % to obtain Cathode Catalyst Ink CI-1.[Cathode Catalyst Ink CI-2]

[0206] 85.0 g of a supported catalyst comprising 46.2 mass % platinum supported on carbon powder (“TEC10E50E” manufactured by TANAKA PRECIOUS METAL TECHNOLOGIES Co., Ltd., containing a carbon support having a specific surface area of 800 m2 / g and an average particle diameter of 2.8 nm) was mixed with 498.4 g of water and 361.1 g of ethanol. To the mixture, 247.9 g of a liquid mixture preliminarily prepared by mixing 160.2 g of Liquid Composition S-2, 87.4 g of ethanol and 51.1 g of Zeorora H (manufactured by ZEON CORPORATION) was added so that the I / C would be 0.7, and the solid content would be 10.1 mas %, and the resulting mixture was ground by means of a planetary bead mill (Model: LP-4, manufactured by Ito Seisakusho Co., Ltd.) containing zirconia beads with a diameter of 5 mm at a rotation speed of 300 rpm for 90 minutes to obtain Cathode Catalyst Ink CI-2.[Anode Catalyst Ink AI-1]

[0207] Liquid Composition S-1 (9.2 g) was mixed with ethanol (6.4 g) and water (6.5 g), and an iridium oxide catalyst (manufactured by TANAKA PRECIOUS METAL TECHNOLOGIES Co., Ltd.) (5.0 g) containing 74.8 mass % iridium and having a specific surface area of 100 m2 / g was added so that the I / Ir would be 0.25. The resulting mixture was ground by means of a planetary bead mill (rotation speed 300 rpm) for 90 minutes to obtain Anode Catalyst Ink AI-1 having a solid content of 21.9 mass %.[Anode Catalyst Ink AI-2]

[0208] Liquid Composition S-2 (17.0 g) was mixed with ethanol (9.2 g) and Zeorora H (manufactured by ZEON CORPORATION) (5.4 g) in a planetary centrifugal mixer (THINKY MIXER, manufactured by THINKY CORPORATION) at 2200 rpm for 15 minutes. The resulting mixture (13.4 g) was mixed with ethanol (11.7 g) and water (18.9 g) and further mixed with an iridium oxide catalyst (manufactured by TANAKA PRECIOUS METAL TECHNOLOGIES Co., Ltd., 5.0 g) containing 74.8 mass % iridium and having a specific surface area of 100 m2 / g so that the I / Ir would be 0.25. The resulting mixture was ground by means of a planetary bead mill (rotation speed 300 rpm) for 90 minutes to obtain Anode Catalyst Ink AI-2 having a solid content of 22.0 mass %.Example 1

[0209] Cathode Catalyst Ink CI-1 was applied onto an ETFE (ethylene tetrafluoroethylene) sheet with a die coater and dried at 80° C. for 10 minutes and treated with heat at 150° C. for 15 minutes to obtain Cathode Catalyst Layer CC-1 having a thickness of 14 μm with a platinum content of 0.4 mg / cm2.

[0210] Anode Catalyst Ink AI-2 was applied onto another ETFE sheet with an applicator so that the iridium content would be 1.0 mg / cm2, dried at 80° C. for 10 minutes and treated with heat at 150° C. for 15 minutes to obtain Anode Catalyst Layer AC-2 having a thickness of 14 μm.

[0211] As a polymer electrolyte membrane, an ion exchange membrane having a 50-μm thickness made of a polymer having sulfonic acid groups in acid-form (ion exchange capacity: 1.25 meq / g dry resin) was obtained by converting SO2F groups in a copolymer of TFE and Compound 3 to SO3H groups.

[0212] Anode Catalyst Layer AC-2 cut into a 4.0 cm square, the polymer electrolyte membrane cut into a 7 cm square and Cathode Catalyst Layer CC-1 cut into a 4.0 cm square were bonded by hot pressing at a press temperature of 130° C. for 10 minutes under a pressure of 2.6 MPa with the anode catalyst layer in contact with one surface of the polymer electrolyte membrane and the cathode catalyst layer in contact with the other surface of the polymer electrolyte membrane. After cooling to 70° C. and pressure release, the resulting laminate was taken out. The ETFE sheets were peeled off from the anode catalyst layer and the cathode catalyst layer to obtain Membrane Electrode Assembly ME-1 having an electrode area of 16 cm2.Examples 2 to 3

[0213] Membrane Electrode Assemblies ME-2 and ME-3 were obtained in the same manner as in Example 1 by using the anode catalyst layers, polymer electrolyte membranes and cathode catalyst layers shown later in the table by arranging them in this order.[Hydrogen Crossover]

[0214] Each membrane electrode assembly was evaluated in terms of hydrogen crossover by measuring the hydrogen concentration in the anode gas as follows.

[0215] A membrane electrode assembly was sandwiched between platinum-coated sintered products of titanium fiber having a 0.25-mm thickness and a 60 vol % porosity (manufactured by Bekaeart) so that the electrode area would be under a pressure of 1.3 MPa, and the assembly was mounted in a single cell having an electrode area of 16 cm2 with platinum-coated titanium plates having straight channels as separators and evaluated.

[0216] Next, pure water at 80° C. and ordinary pressure with an electrical conductivity of at most 1.0 μS / cm was supplied to both the anode and cathode compartments at a flow rate of 50 mL / min for 4 hours to fully hydrate the polymer electrolyte membrane and the fluorinated polymers in the electrodes. Then, water electrolysis was carried out for 4 hours as a preliminary run at an electric current of 32 A (current density 2 A / cm2) by supplying pure water at 80° C. with an electrical conductivity of at most 1.0 μS / cm to the anode compartment at a flow rate of 50 mL / min with back pressures in the anode compartment and in the cathode compartment kept at ordinary pressure, under current control by a high current potentio / garvanostat HCP-803 (manufactured by BioLogic).

[0217] Then, pure water at 80° C. and ordinary pressure with an electrical conductivity of at most 1.0 μS / cm was supplied to the cell at a flow rate of 50 mL / min with the back pressures in the anode compartment and in the cathode compartment kept at ordinary pressure, while the electric current was kept at 3.2 A (current density 0.2 A / cm2) for 11 hours, at 8 A (current density 0.5 A / cm2) for 7 hours, at 16 A (current density 1 A / cm2) for 4 hours and at 32 A (current density 2 A / cm2) for 4 hours under current control by a high current potentio / garvanostat HCP-803 (manufactured by BioLogic). At the end of each galvanostatic operation, the gas discharged from the anode compartment was dehydrated, and the hydrogen concentration in the gas was measured with a Micro GC (Agilent 490, manufactured by Agilent Technologies, Inc.). Suppression of hydrogen crossover was rated on the following scale based on the final hydrogen concentration in the gas (vol) at 32 A (current density 2 A / cm2).

[0218] “A”: Anode gas contained hydrogen gas in a concentration of at most 0.3 vol %

[0219] “B”: Anode gas contained hydrogen gas in a concentration of over 0.3 vol % and at most 0.7 vol %

[0220] “C”: Anode gas contained hydrogen gas in a concentration of over 0.7 vol %TABLE 3Membrane Electrode AssemblyCathode Catalyst LayerSpecific Surface ofIon ExchangeliquidCarbon SupportSpecies ofCapacity of Polymer HSpeciesSpeciescomposition[m2 / g]Polymer H[meq / g dry resin]I / CEx. 1ME-1CC-1S-1800H-11.090.7Ex. 2ME-2CC-2S-2800H-21.100.7Ex. 3ME-3CC-2S-2800H-21.100.7Membrane Electrode AssemblyAnode Catalyst LayerSpecific Surface ofIridium OxideIon ExchangeEvaluationliquidCatalystSpecies ofCapacity of Polymer HHydrogenSpeciescomposition[m2 / g]Polymer H[meq / g dry resin]I / IrCrossoverEx. 1AC-2S-2100H-21.100.25AEx. 2AC-1S-1100H-11.090.25BEx. 3AC-2S-2100H-21.100.25C

[0221] It is demonstrated that the membrane electrode assemblies of the present invention can suppress hydrogen crossover (Examples 1 and 2).

[0222] It is also demonstrated that the suppression of hydrogen crossover is remarkable when the catalyst layer in the cathode contains a polymer H (Example 1).

[0223] This application is a continuation of PCT Application No. PCT / JP2024 / 042573 filed on Dec. 2, 2024, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2023-205868 filed on Dec. 6, 2023. The contents of those applications are incorporated herein by reference in their entireties.REFERENCE SYMBOLS10 Membrane electrode assembly

[0225] 11 Catalyst layer

[0226] 12 Gas diffusion layer

[0227] 13 Anode

[0228] 14 Cathode

[0229] 15 Polymer electrolyte membrane

Claims

1. A membrane electrode assembly for polymer electrolyte membrane water electrolysis which comprises an anode having a catalyst layer, a cathode having a catalyst layer and a polymer electrolyte membrane provided between the anode and the cathode,wherein at least one of the catalyst layer in the anode and the catalyst layer in the cathode comprises a fluorinated polymer having ion exchange groups and comprising units having a cyclic ether structure.

2. The membrane electrode assembly according to claim 1, wherein the polymer electrolyte membrane has a thickness of at least 25 μm.

3. The membrane electrode assembly according to claim 1, wherein the fluorinated polymer has an ion exchange capacity of from 0.90 to 1.60 meq / g dry resin.

4. The membrane electrode assembly according to claim 1, wherein the content of units having a cyclic ether structure is at least 30 mol % relative to all the units in the fluorinated polymer.

5. The membrane electrode assembly according to claim 1, wherein the fluorinated polymer has a softening temperature of at least 135° C.

6. The membrane electrode assembly according to claim 1, wherein the catalyst layer in the anode comprises the fluorinated polymer and a catalyst, andthe catalyst comprises an iridium-containing catalyst.

7. The membrane electrode assembly according to claim 6, wherein the mass ratio of the amount of the fluorinated polymer to the amount of iridium in the iridium-containing catalyst is from 0.05 to 0.40.

8. The membrane electrode assembly according to claim 1, wherein the catalyst layer in the cathode comprises the fluorinated polymer and a catalyst, andthe catalyst comprises a carbon support and a platinum-containing catalyst.

9. The membrane electrode assembly according to claim 8, wherein the mass ratio of the amount of the fluorinated polymer to the amount of the carbon support is from 0.2 to 1.5.

10. The membrane electrode assembly according to claim 8, wherein the carbon support has a specific surface area of at least 60 m2 / g.

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