POLYMER ELECTROLYTE MEMBRANE, MEMBRANE ELECTRODE ASSEMBLY, WATER ELECTROLYZER, AND METHOD FOR PRODUCING POLYMER ELECTROLYTE MEMBRANE

Description

TECHNICAL FIELD

The present invention relates to a polymer electrolyte membrane, a membrane electrode assembly, a water electrolyzer, and a method for producing the polymer electrolyte membrane.

BACKGROUND ART

In the so-called power-to-gas technology for storing and using surplus electric power by converting it to gas, the utilization of polymer electrolyte membrane water electrolyzers (PEM water electrolyzers) has been explored.

For example, Patent Document 1 discloses a water electrolyzer comprising a polymer electrolyte membrane comprising a fluorinated polymer having ion exchange groups and a woven fabric.

PRIOR ART DOCUMENTS

Patent Documents

• • Patent Document 1: WO2019/088299

DISCLOSURE OF INVENTION

Technical Problem

In a water electrolyzer, which is one of applications of polymer electrolyte membranes, the polymer electrolyte membrane as the partition between the anode compartment and the cathode compartment is under high pressure. Therefore, polymer electrolyte membranes need excellent strength.

Further, in a water electrolyzer using a polymer electrolyte membrane, the membrane resistance is preferably as low as possible.

Still further, polymer electrolyte membranes are preferably such that only a polymer having ion exchange groups is observed on their surface. For example, in a case where a polymer electrolyte membrane contains a reinforcing member, preferably no reinforcing member is observed on the surface. Hereinafter a property of a polymer electrolyte membrane such that only a polymer is observed on its surface and no reinforcing member is observed, will be referred to as excellent surface property.

The present inventors have evaluated the polymer electrolyte membrane as disclosed in Patent Document 1 and a water electrolyzer having the polymer electrolyte membrane and as a result found that the above three objects have not been successfully balanced together and that further improvements are required.

Under these circumstances, the object of the present invention is to provide a polymer electrolyte membrane which is excellent in the strength and the surface property, and which can achieve a low membrane resistance when applied to a water electrolyzer.

Another object of the present invention is to provide a membrane electrode assembly, a water electrolyzer and a method for producing the polymer electrolyte membrane.

Solution to Problem

The present inventors have conducted extensive studies on the above objects and as a result found that the objects can be achieved by the following constitutions.

[1] A polymer electrolyte membrane comprising a fluorinated polymer having ion exchange groups and a nonwoven fabric, wherein

• • the nonwoven fabric contains polyphenylene sulfide fibers,
  • in a fiber diameter distribution histogram of fibers constituting the nonwoven fabric, the maximum frequency peak appears in a range of 100 to 900 nm, and
  • the polymer electrolyte membrane has a thickness of 30 to 90 μm.
    [2] The polymer electrolyte membrane according to [1], wherein the ion exchange groups of the fluorinated polymer are sulfonic acid functional groups.
    [3] The polymer electrolyte membrane according to [1] or [2], wherein the fluorinated polymer comprises at least one type selected from the group consisting of units represented by the formula (1-1) described later, units represented by the formula (1-2) described later, units represented by the formula (1-3) described later and units represented by the formula (1-4) described later.
    [4] The polymer electrolyte membrane according to any one of [1] to [3], which contains at least one element selected from the group consisting of platinum, palladium, cerium and manganese.
    [5] The polymer electrolyte membrane according to any one of [1] to [4], wherein the content of the nonwoven fabric is 5 mass % or more and 40 mass % or less to the total mass of the polymer electrolyte membrane.
    [6] The polymer electrolyte membrane according to any one of [1] to [5], which is for water electrolysis.
    [7] A membrane electrode assembly comprising:
  • the polymer electrolyte membrane as defined in any one of [1] to [6];
  • a cathode catalyst layer provided on one surface of the polymer electrolyte membrane; and
  • an anode catalyst layer provided on the other surface of the polymer electrolyte membrane.
    [8] A water electrolyzer comprising:
  • the membrane electrode assembly as defined in [7];
  • 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 to supply water to the anode catalyst layer side.
    [9] A method for producing the polymer electrolyte membrane as defined in any one of [1] to [6], which comprises:
  • step 1 of converting, in a precursor membrane containing a polymer having groups convertible to ion exchange groups and the nonwoven fabric, the groups convertible to ion exchange groups to ion exchange groups, and
  • step 2 of drying the membrane obtained in step 1 while its edges are kept fixed to obtain the polymer electrolyte membrane.
    [10] The polymer electrolyte membrane production method according to [9], which further has, before step 1, step 3 of sandwiching the nonwoven fabric between a plurality of polymer membranes containing a polymer having groups convertible to ion exchange groups, to obtain the precursor membrane.
    [11] The polymer electrolyte membrane production method according to [10], which further has, before step 3, step 4 of kneading a compound containing at least one element selected from the group consisting of platinum, palladium, cerium and manganese and the polymer having groups convertible to ion exchange groups, and extruding the kneaded product to obtain the precursor membrane.

Advantageous Effects of Invention

The present invention provides a polymer electrolyte membrane which is excellent in the strength and the surface property, and which can achieve a low membrane resistance when applied to a water electrolyzer.

The present invention also provides a membrane electrode assembly, a water electrolyzer, and a method for producing the polymer electrolyte membrane.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a sectional view schematically illustrating an embodiment of the membrane electrode assembly of the present invention.

DESCRIPTION OF EMBODIMENTS

The following definitions of terms apply throughout the present specification and claims unless otherwise specified.

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 as described below.

A “sulfonic acid functional group” refers to a sulfonic acid group (—SO₃H) or a sulfonate group. The sulfonate group may be, for example, (—SO₃⁻)Ma⁺, (—SO₃⁻)₂Mb²⁺ or (—SO₃⁻)₃Mc³⁺ (where Ma⁺ is an alkali metal ion or a quaternary ammonium cation; Mb²⁺ is a divalent metal ion; Mc³⁺ is a trivalent metal ion). When there are two ligands, the number of ion exchange groups is counted as 2, and when there are three ligands, the number of ion exchange groups is counted as 3.

A “carboxylic acid functional group” refers to a carboxylic acid group (—COOH) or a carboxylate group. The carboxylate group may be, for example, (—COO⁻)Ma⁺, (—COO⁻)₂Mb²⁺ or (—COO⁻)₃Mc³⁺ (where Ma⁺ is an alkali metal or a quaternary ammonium cation; Mb²⁺ is a divalent metal ion; and Mc³⁺ is a trivalent metal ion). When there are two ligands, the number of ion exchange groups is counted as 2, and when there are three ligands, the number of ion exchange groups is counted as 3.

A “precursor membrane” refers to a membrane containing a polymer having groups convertible to ion exchange groups.

A “group convertible to an ion exchange group” refers to a group that can be converted to an ion exchange group by treatments such as hydrolysis and conversion to acid form.

A “group convertible to a sulfonic acid functional group” refers to a group that can be converted to a sulfonic acid functional group by treatments such as hydrolysis and conversion to acid form.

A “group convertible to a carboxylic acid functional group” refers to a group that can be converted to a carboxylic acid functional group by known treatments such as hydrolysis and conversion to acid form.

A “unit” in a polymer refers to an atomic group derived from a single monomer molecule, which is formed by polymerization reaction of the monomer. A unit may be an atomic group formed directly by polymerization or may be an atomic group formed by polymerization followed by modification to a partially different structure.

A numerical range expressed using “to” means a range including numerical values described before and after “to” as the lower and upper limits. 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 numerical values indicated in Examples.

[Electrolyte Membrane]

The polymer electrolyte membrane (hereinafter also simply referred to as “electrolyte membrane”) of the present invention contains a fluorinated polymer having ion exchange groups (hereinafter also simply referred to as “fluorinated polymer (I)”), and a nonwoven fabric, wherein the nonwoven fabric contains polyphenylene sulfide fibers; in a fiber diameter distribution histogram of fibers constituting the nonwoven fabric, the maximum frequency peak appears in a range of 100 to 900 nm; and the polymer electrolyte membrane has a thickness of 30 to 90 μm.

As described above, the electrolyte membrane is excellent in the strength and the surface property and can achieve a low membrane resistance when applied to a water electrolyzer.

The reasons as to why the above effects are achieved are as follows. The electrolyte membrane is excellent in the surface property by using a nonwoven fabric containing thin fibers. As shown in Comparative Example 1 described later, if the electrolyte membrane contains a woven fabric, intersections of warp yarns and weft yarns are likely to be exposed to the surface, and thus the electrolyte membrane tends to be inferior in the surface property. Further, by the fibers contained in the nonwoven fabric having diameters within a predetermined range, the membrane resistance tends to be low when the electrolyte membrane is applied to a water electrolyzer. Further, by the nonwoven fabric containing polyphenylene sulfide fibers, the electrolyte membrane is excellent in the strength.

The electrolyte membrane may have a single-layer structure or a multilayer structure. Specific embodiments of the electrolyte membrane having a multilayer structure include a laminated structure of electrolyte layers differing in ion exchange capacity.

The thickness of the electrolyte membrane is 30 to 90 μm in view of high mechanical strength and low cell resistance, and is preferably 40 to 90 μm, more preferably 50 to 90 μm. In a case where the electrolyte membrane has a multilayer structure, the thickness of the electrolyte membrane means the total thickness of the respective layers.

The thickness of the electrolyte membrane is measured on a magnified cross-sectional image of the electrolyte membrane taken (at an objective lens magnification of 50) by a laser microscope (model “VK-X1000”, manufactured by KEYENCE CORPORATION) under the conditions of a temperature of 25° C. and a relative humidity of 50% RH.

Now, the respective constituents in the polymer electrolyte membrane will be described in detail.

<Nonwoven Fabric>

The nonwoven fabric has functions to improve the dimensional stability, the strength, the handling efficiency, etc., of the electrolyte membrane.

The nonwoven fabric contains polyphenylene sulfide fibers. The nonwoven fabric may be formed solely of polyphenylene sulfide fibers or may be formed of polyphenylene sulfide fibers and fibers other than the polyphenylene sulfide fibers. It is preferred that the nonwoven fabric is formed solely of polyphenylene sulfide fibers.

In a fiber diameter distribution histogram of the fibers constituting the nonwoven fabric, the maximum frequency peak appears in a range of 100 to 900 nm. Particularly, the maximum frequency peak appears preferably in a range of 350 to 850 nm, more preferably in a range of 600 to 800 nm.

The maximum frequency peak can be obtained by preparing a histogram of the frequency of the number of the fibers constituting the nonwoven fabric and the distribution of the fiber diameters.

First, the fiber diameters and the number of fibers are measured to prepare the histogram. The fiber diameters and the number of fibers are obtained from two-dimensional images covering the entire nonwoven fabric observed by a scanning electron microscope (magnification of 20000). The number of the fibers is measured by counting each continuous fiber within the fields of view of the obtained two-dimensional images as one fiber. To measure the fiber diameters, the length in a direction orthogonal to the fiber length direction in the two-dimensional images is taken as the fiber diameter. This measurement is conducted repeatedly until 100 or more fiber diameters are measured while changing the positions to be observed by the scanning electron microscope.

From the distribution of the numbers for each fiber diameter above-obtained, a histogram of the fiber diameter distribution is prepared. The histogram is prepared by plotting the fiber diameter (nm) on the X axis and the frequency on the Y axis.

The class interval of the histogram is set to 20 nm. For example, in a case where the maximum fiber diameter is 2000 nm and the minimum fiber diameter is 20 nm, by setting the class interval of the histogram to be 20 nm, the number of bins (the number of histogram bars) is (2000-20)/20=99 (bins). The maximum frequency peak in the fiber diameter distribution histogram means the class (interval) with the maximum frequency in the fiber diameter distribution histogram. The fiber diameter corresponding to the maximum frequency peak is meant for the smallest fiber diameter in the class with the maximum frequency. For example, in a case where the class with the maximum frequency covers diameters of 100 nm or more and less than 120 nm, the fiber diameter at the maximum frequency peak is 100 nm.

The fabric weight of the nonwoven fabric is preferably 1 to 50 g/m², more preferably 3 to 30 g/m² in view of high mechanical strength and low cell resistance.

The fabric weight of the nonwoven fabric is calculated by measuring the mass of the nonwoven fabric cut in a 0.15 m square by a precision electronic balance and converting the mass into the mass per 1 m².

The thickness of the nonwoven fabric is preferably 5 to 70 μm, more preferably 5 to 50 μm, further preferably 15 to 40 μm, in view of high mechanical strength and low cell resistance.

The thickness of the nonwoven fabric is measured with respect to the nonwoven fabric placed on a dial gauge stand 7002 (Mitutoyo Corporation) and by using a dial gauge 543-250 (Mitutoyo Corporation) having a flat terminal with a diameter of 5 mm attached to the tip.

The method for producing the nonwoven fabric is not particularly limited, and a method in which a sea/island composite fiber as disclosed in JP-A-2019-197702 is produced and the sea component is dissolved and removed to prepare a nanofiber thereby to produce a wet nonwoven fabric, a method of producing a nonwoven fabric by electrospinning method as disclosed in JP-A-2017-197876, and a method of producing a nonwoven fabric by a known melt blow method as disclosed in JP-A-H2-80651, may be mentioned.

The content of the nonwoven fabric to the total mass of the electrolyte membrane is preferably 3 mass % or more, more preferably 5 mass % or more, and is preferably 50 mass % or less, more preferably 40 mass % or less, still more preferably 30 mass % or less.

<Fluorinated Polymer (I)>

The ion exchange capacity of the fluorinated polymer (I) is preferably 0.90 meq/g dry resin or more, more preferably more than 1.10 meq/g dry resin, still more preferably 1.15 meq/g dry resin or more, particularly preferably 1.20 meq/g dry resin or more, most preferably 1.25 meq/g dry resin or more, with a view to achieving a lower electrolysis voltage when the electrolyte membrane is applied to a water electrolyzer.

The ion exchange capacity of the fluorinated polymer (I) is preferably 2.00 meq/g dry resin or less, more preferably 1.50 meq/g dry resin or less, still more preferably 1.43 meq/g dry resin or less, in consideration of the strength of the membrane electrode assembly in a hydrated state.

The fluorinated polymer (I) may be one type or a laminate or mixture of two or more types.

Although the electrolyte membrane may contain a polymer other than the fluorinated polymer (I), it is preferred that the polymer in the electrolyte membrane substantially consists of the fluorinated polymer (I). Here, the expression “substantially consist of the fluorinated polymer (I)” means that the content of the fluorinated polymer (I) is 95 mass % or more to the total mass of the polymers in the electrolyte membrane. The upper limit of the content of the fluorinated polymer (I) is 100 mass % to the total mass of the polymers in the electrolyte membrane.

As examples of the polymer other than the fluorinated polymer (I), there may be mentioned one or more types of polyazole compounds selected from the group consisting of polymers of heterocyclic compounds each containing one or more nitrogen atoms in the ring and polymers of heterocyclic compounds each containing one or more nitrogen atoms and oxygen and/or sulfur atoms in the ring.

Specific examples of the polyazole compounds include polyimidazole compounds, polybenzimidazole compounds, polybenzobisimidazole compounds, polybenzoxazole compounds, polyoxazole compounds, polythiazole compounds and polybenzothiazole compounds.

In consideration of the oxidation resistance of the electrolyte membrane, a polyphenylene sulfide resin and a polyphenylene ether resin may also be mentioned as examples of the other polymer.

The fluorinated polymer (I) has ion exchange groups. Specific examples of the ion exchange groups include sulfonic acid functional groups and carboxylic acid functional groups. Sulfonic acid functional groups are preferred to achieve a lower electrolysis voltage when the electrolyte membrane is applied to a water electrolyzer.

In the following, embodiments of a fluorinated polymer having sulfonic acid functional groups (hereinafter also referred to as fluorinated polymer(S)) will be discussed mainly.

The fluorinated polymer(S) preferably contains units based on a fluorinated olefin and units based on a fluorine-containing monomer having a sulfonic acid functional group.

The fluorinated olefin may be, for example, a C_2-3 fluoroolefin having one or more fluorine atoms in the molecule. Specific examples of the fluoroolefin include tetrafluoroethylene (hereinafter also referred to as “TFE”), chlorotrifluoroethylene, vinylidene fluoride, vinyl fluoride and hexafluoropropylene. Among others, TFE is preferred in terms of the monomer production cost, the reactivity with other monomers and the ability to give the fluorinated polymer(S) with excellent properties.

The fluorinated olefin may be one type alone or a combination of two or more types.

The units based on the fluorine-containing monomer having a sulfonic acid functional group are preferably units represented by the formula (1).

Here, L is a (n+1)-valent perfluorohydrocarbon group which may contain an etheric oxygen atom.

The etheric oxygen atom may be located at a terminal end of the perfluorohydrocarbon group or may be located between carbon atoms of the perfluorohydrocarbon group.

The number of carbon atoms in the (n+1)-valent perfluorohydrocarbon group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

As L, preferred is a (n+1)-valent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom. More preferred is a divalent perfluoroalkylene group which may contain an etheric atom oxygen atom as corresponding to an embodiment of n=1, or a trivalent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom as corresponding to an embodiment of n=2.

The above-mentioned divalent perfluoroalkylene group may be linear or branched.

M is a hydrogen atom, an alkali metal or a quaternary ammonium cation.

n is 1 or 2.

As the units represented by the formula (1), units represented by the formula (1-1), units represented by the formula (1-2), units represented by the formula (1-3) or units represented by the formula (1-4) are preferred.

Here, R^f1 is 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.

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.

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.

r is 0 or 1.

m is 0 or 1.

M is a hydrogen atom, an alkali metal or a quaternary ammonium cation.

As the units represented by the formula (1-1) and the units represented by the formula (1-2), units represented by the formula (1-5) are more preferred.

Here, x is 0 or 1, y is an integer of 0 to 2, z is an integer of 1 to 4, Y is F or CF₃, and M is as described above.

Specific examples of the units represented by the formula (1-1) include the following units where w is an integer of 1 to 8, x is an integer of 1 to 5, and M is as described above.

Specific examples of the units represented by the formula (1-2) include the following units where w is an integer of 1 to 8, and the definition of M is as described above.

As the units represented by the formula (1-3), preferred are units represented by the formula (1-3-1) where the definition of M is as described above.

R^f4 is a C_1-6 linear perfluoroalkylene group, and R^f5 is a single bond or a C_1-6 linear perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The definitions of r and M in the formula are as described above.

Specific examples of the units represented by the formula (1-3-1) include the following units.

As the units represented by the formula (1-4), preferred are units represented by the formula (1-4-1) where the definitions of R^f1, R^f2 and M are as described above.

Specific examples of the units represented by the formula (1-4-1) include the following units.

The fluorine-containing monomer having a sulfonic acid functional group may be one type alone or a combination of two or more types.

The fluorinated polymer (I) may contain units based on an additional monomer, other than the units based on the fluorinated olefin and the units based on the fluorine-containing monomer having a sulfonic acid functional group.

Specific examples of the additional monomer include CF₂═CFR^f6 (where R^f6 is a C_2-10 perfluoroalkyl group), CF₂═CF—OR^f7 (where R^f7 is a C_1-10 perfluoroalkyl group) and CF₂═CFO(CF₂)_vCF═CF₂ (where v is an integer of 1 to 3).

The content of the units based on the additional monomer is preferably 30 mass % or less to all the units in the fluorinated polymer (I) to ensure a certain level of ion exchange performance.

The content of the fluorinated polymer (I) is preferably 95 to 100 mass % to the total mass of the electrolyte membrane.

<Other Components>

The electrolyte membrane preferably contains at least one element selected from the group consisting of platinum, palladium, cerium and manganese. Such an element may be present in the electrolyte membrane as the metal itself, or as an oxide, an alloy, an ion, a salt or the like.

The platinum may be present in the electrolyte membrane as platinum itself, platinum oxide, a composite metal oxide containing platinum, or a platinum alloy. That is, the electrolyte membrane may contain a platinum-containing material.

Specific examples of the composite metal oxide containing platinum include M_xPt₃O₄ (where M is at least one metal atom selected from the group consisting of Li, Na, Mg, Ca, Zn, Cd, Co, Ni, Mn, Cu, Ag, Bi and Ce; and x is a value greater than 0 and smaller than or equal to 1).

Specific examples of the platinum alloy include alloys each containing platinum and at least one metal selected from the group consisting of transition metals and noble metals other than platinum.

The form of the platinum-containing material can be, for example, particulate form or sheet form. The platinum-containing material, when in particulate form, may be core-shell particles. Examples of the core-shell particles include those having a core made of carbon or containing a metal other than platinum and a shell containing platinum.

The platinum-containing material may be supported on a carrier. Specific examples of the carrier include carbon carriers such as carbon black powders, graphitized carbon, carbon fibers, carbon nanotubes and the like.

In the case where the platinum-containing material is supported on a carrier, the amount of the platinum-containing material supported to the total mass of the platinum-containing material and the carrier is preferably 10 mass % or more, more preferably 20 mass % or more, still more preferably 30 mass % or more, and is preferably 50 mass % or less.

The palladium may be present in the electrolyte membrane as palladium itself, palladium oxide, a composite metal oxide containing palladium, or a palladium alloy. That is, the electrolyte membrane may contain a palladium-containing material.

Cerium and manganese decompose hydrogen peroxide, a hydroxy radical or a hydroperoxyl radical, which causes deterioration of the polymer electrolyte membrane. Cerium and manganese are present preferably as ions in the polymer electrolyte membrane, and so long as they are present as ions, they may be present in any form in the polymer electrolyte membrane.

<Method for Producing Electrolyte Membrane>

For example, the following method (Method 1) may be mentioned as a method for producing the electrolyte membrane.

The fluorinated polymer (I) is first produced by using a polymer (hereinafter also referred to as “fluorinated polymer (I′)”) of a fluorinated monomer having a group convertible to an ion exchange group (hereinafter also referred to as “fluorinated monomer (I′)”) and converting the groups convertible to ion exchange groups in the fluorinated polymer (I′) to ion exchange groups.

Next, the nonwoven fabric is impregnated with a dispersion having the fluorinated polymer (I) dispersed therein, and then dried to obtain an electrolyte membrane containing the fluorinated polymer (I) and the nonwoven fabric.

The following method (Method 2) may also be mentioned as another method for producing the electrolyte membrane.

Using a composition containing the fluorinated polymer (I), membranes P1 containing the fluorinated polymer (I) are obtained.

The membrane P1, the nonwoven fabric and the membrane P1 are stacked in this order and laminated together by laminating rolls or by a vacuum lamination apparatus, to obtain the electrolyte membrane containing the fluorinated polymer (I) and the nonwoven fabric.

The following method (Method 3) may also be mentioned as another method for producing the electrolyte membrane.

Using a composition containing the fluorinated polymer (I′), membranes P2 containing the fluorinated polymer (I′) are obtained.

The membrane P2, the nonwoven fabric and the membrane P2 are stacked in this order and laminated together by laminating rolls or by a vacuum lamination apparatus, to obtain a precursor membrane containing the fluorinated polymer (I′) and the nonwoven fabric.

The groups convertible to ion exchange groups in the fluorinated polymer (I′) in the obtained precursor member are converted to ion exchange groups thereby to obtain the electrolyte membrane containing the fluorinated polymer (I) and the nonwoven fabric.

To produce the electrolyte membrane containing at least one element selected from the group consisting of platinum, palladium, cerium and manganese, for example, in the case of the above Method 1, a method may be mentioned in which at least one element selected from the group consisting of platinum, palladium, cerium and manganese is incorporated in the dispersion having the fluorinated polymer (I) dispersed.

In the case of the above Method 2, a method may be mentioned in which at least one element selected from the group consisting of platinum, palladium, cerium and manganese is incorporated in the composition containing the fluorinated polymer (I).

In the case of the above Method 3, a method may be mentioned in which at least one element selected from the group consisting of platinum, palladium, cerium and manganese is incorporated in the composition containing the fluorinated polymer (I′).

As the fluorinated polymer (I′), preferred is a polymer (hereinafter also referred to as “fluorinated polymer (S′)”) of a fluorinated monomer having a group convertible to a sulfonic acid group (hereinafter also referred to as “fluorinated monomer (S′)”). Particularly preferred is a copolymer of a fluorinated olefin and a fluorine-containing monomer having a group convertible to a sulfonic acid functional group.

The fluorinated polymer (S′) will be now described in detail below.

As a copolymerization method for production of the fluorinated polymer (S′), a known method such as solution polymerization, suspension polymerization or emulsion polymerization may be employed.

The fluorinated olefin can be any of those mentioned above, and is preferably TFE in terms of the monomer production cost, the reactivity with other monomers and the ability to give the fluorinated polymer(S) with excellent properties.

The fluorinated olefin may be one type alone or a combination of two or more types.

The fluorinated monomer (S′) may be a compound having at least one fluorine atom in the molecule, having an ethylenic double bond and having a group convertible to a sulfonic acid functional group.

As the fluorinated monomer (S′), a compound represented by the formula (2) is preferred in terms of the monomer production cost, the reactivity with other monomers and the ability to give the fluorinated polymer(S) with excellent properties.

In the formula (2), the definitions of L and n are as described above.

A is a group convertible to a sulfonic acid functional group. As the group convertible to a sulfonic acid functional group, preferred is a functional group that can be converted to a sulfonic acid functional group by hydrolysis. Specific examples of the group convertible to a sulfonic acid functional group include —SO₂F, —SO₂Cl and —SO₂Br.

As the compound represented by the formula (2), a compound represented by the formula (2-1), a compound represented by the formula (2-2), a compound represented by the formula (2-3) or a compound represented by the formula (2-4) is preferred.

In these formulas, the definitions of R^f1, R^f2, r and A are as described above.

In this formula, the definitions of R^f1, R^f2, R^f3, r, m and A are as described above.

As the compound represented by the formula (2-1) and the compound represented by the formula (2-2), a compound represented by the formula (2-5) is preferred.

The definitions of x, y, z and Y in the formula are as described above.

Specific examples of the compound represented by the formula (2-1) include the following compounds where w is an integer of 1 to 8 and x is an integer of 1 to 5.

Specific examples of the compound represented by the formula (2-2) include the following compounds where w is an integer of 1 to 8.

As the compound represented by the formula (2-3), a compound represented by the formula (2-3-1) is preferred.

In this formula, the definitions of R^f4, R^f5, r and A are as described above.

Specific examples of the compound represented by the formula (2-3-1) include the following compounds.

As the compound represented by the formula (2-4), a compound represented by the formula (2-4-1) is preferred.

In this formula, the definitions of R^f1, R^f2 and A are as described above.

Specific examples of the compound represented by the formula (2-4-1) include the following compound.

The fluorinated monomer (S′) may be one type alone or a combination of two or more types.

For production of the fluorinated polymer (S′), an additional monomer may be used in addition to the fluorinated olefin and the fluorinated monomer (S′). The additional monomer can be any of those mentioned above.

The ion exchange capacity of the fluorinated polymer (I′) can be adjusted by changing the content of groups convertible to ion exchange groups in the fluorinated polymer (I′).

The conversion of groups convertible to ion exchange groups to ion exchange groups may be carried out by, for example, subjecting the precursor membrane to treatment such as hydrolysis or conversion to acid form.

In particular, preferred is contact treatment of the precursor membrane with an aqueous alkaline solution.

The contact treatment of the precursor membrane with the aqueous alkaline solution may be performed by, for example, immersion of the precursor membrane in the aqueous alkaline solution, spraying of the aqueous alkaline solution onto the surface of the precursor membrane, or the like.

The temperature of the aqueous alkaline solution is preferably 30° C. or higher, more preferably 40° C. or higher, and is preferably 100° C. or lower. The contact time of the precursor membrane and the aqueous alkaline solution is preferably 3 minutes or more, more preferably 5 minutes or more, and is preferably 150 minutes or less, more preferably 50 minutes or less.

The aqueous alkaline solution preferably contains an alkali metal hydroxide, a water-soluble organic solvent and water.

Specific examples of the alkali metal hydroxide include sodium hydroxide and potassium hydroxide.

In the present specification, the water-soluble organic solvent refers to an organic solvent easily soluble in water. Specifically, preferred is an organic solvent with a solubility of 0.1 g or more in 1000 ml of water (20° C.), and an organic solvent with a solubility of 0.5 g or more is more preferred. The water-soluble organic solvent preferably contains at least one kind selected from the group consisting of aprotic organic solvents, alcohols and amino alcohols, and more preferably contains an aprotic organic solvent.

The water-soluble organic solvent may be one type alone or a combination of two or more types.

Specific examples of the aprotic organic solvents include dimethyl sulfoxide, N,N-dimethylformamide, N, N-dimethylacetamide, N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone. Preferred is dimethyl sulfoxide.

Specific examples of the alcohols include methanol, ethanol, isopropanol, butanol, methoxyethoxyethanol, butoxyethanol, butylcarbitol, hexyloxyethanol, octanol, 1-methoxy-2-propanol and ethylene glycol.

Specific examples of the amino alcohols include ethanolamine, N-methyl ethanolamine, N-ethyl ethanolamine, 1-amino-2-propanol, 1-amino-3-propanol, 2-aminoethoxyethanol, 2-aminothioethoxyethanol and 2-amino-2-methyl-1-propanol.

The concentration of the alkali metal hydroxide in the aqueous alkaline solution is preferably 1 mass % or higher, more preferably 3 mass %, and is preferably 60 mass % or lower, more preferably 55 mass % or lower.

The content of the water-soluble organic solvent in the aqueous alkaline solution is preferably 1 mass % or more, more preferably 3 mass % or more, and is preferably 60 mass % or less, more preferably 55 mass % or less.

The concentration of water in the aqueous alkaline solution is preferably 39 to 80 mass %.

After the contact treatment of the precursor membrane with the aqueous alkaline solution, any treatment for removing the aqueous alkaline solution may be performed. The aqueous alkaline solution may be removed by, for example, washing, with water, the precursor membrane contact-treated with the aqueous alkaline solution.

After the contact treatment of the precursor membrane with the aqueous alkaline solution, the thus-obtained membrane may be further treated by contact with an aqueous acidic solution to convert the ion exchange groups to acid form.

The contact treatment of the precursor membrane with the aqueous acidic solution may be performed by, for example, immersion of the precursor membrane in the aqueous acidic solution or spraying of the aqueous acidic solution onto the surface of the precursor membrane.

The aqueous acidic solution preferably contains an acid component and water.

Specific examples of the acid component include hydrochloric acid and sulfuric acid.

As a preferred embodiment of the method for producing the polymer electrolyte membrane of the present invention, a method may be mentioned which comprises step 1 of converting the groups convertible to ion exchange groups in the precursor membrane containing the polymer having groups convertible to ion exchange groups and the nonwoven fabric, to ion exchange groups, and step 2 of drying the membrane obtained in step 1 while its edges are kept fixed, to obtain the polymer electrolyte membrane.

As a specific procedure of step 1, the above described method of applying a treatment such as hydrolysis or conversion to acid form to the precursor membrane may be mentioned.

As a specific procedure of step 2, for example, a method may be mentioned in which the membrane is dried at a temperature of the polymer softening point or higher while the edges of the membrane obtained in step 1 are kept fixed by a metal fixing frame.

Further, before step 1, step 3 of sandwiching the nonwoven fabric between a plurality of polymer membranes containing a polymer having groups convertible to ion exchange groups to obtain the precursor membrane may further be conducted.

Further, before step 3, step 4 of kneading a compound containing at least one element selected from the group consisting of platinum, palladium, cerium and manganese and the polymer having groups convertible to ion exchange groups, and extruding the kneaded product to obtain the precursor membrane may further be conducted.

[Membrane Electrode Assembly]

The membrane electrode assembly of the present invention includes the above-mentioned electrolyte membrane, a cathode catalyst layer provided on one surface of the electrolyte membrane and an anode catalyst layer provided on the other surface of the electrolyte membrane.

FIG. 1 is a cross-sectional view illustrating an example of the membrane electrode assembly of the present invention in the case where the electrolyte membrane has a single-layer structure. In the example of FIG. 1, a membrane electrode assembly 20 includes: an anode 22 with a catalyst layer 26 and a gas diffusion layer 28; a cathode 24 with a catalyst layer 26 and a gas diffusion layer 28; and an electrolyte membrane 10 disposed between the anode 22 and the cathode 24 in contact with the respective catalyst layers 26.

Although the electrolyte membrane has a single-layer structure in the example of FIG. 1, the electrolyte membrane may alternatively have a multilayer structure.

<Anode and Cathode>

Each of the anode and the cathode has a catalyst layer. In the example of FIG. 1, each of the anode 22 and the cathode 24 has a catalyst layer 26 and a gas diffusion layer 28.

The catalyst layer may be, for example, a layer containing a catalyst and a polymer having ion exchange groups.

Specific examples of the catalyst include: a supported catalyst having platinum, a platinum alloy or a platinum-based core-shell catalytic material supported on a carbon carrier; an iridium oxide catalyst; a composite oxide catalyst containing iridium and a different metal element; an iridium oxide alloy-based catalyst; and an iridium oxide-based core-shell catalytic material. As the carbon carrier, a carbon black powder may be mentioned.

As the polymer having ion exchange groups, a fluorinated polymer having ion exchange groups may be mentioned. Specific examples of the fluorinated polymer having ion exchange groups may be the same as those of the fluorinated polymer (I) mentioned above.

The mass of the catalyst metal per 1 cm² of the catalyst layer is preferably 0.05 mg/cm² or more, more preferably 0.2 mg/cm² or more, and is preferably 4 mg/cm² or less, more preferably 2 mg/cm² or less, still more preferably 1 mg/cm² or less.

The mass ratio of the catalyst to the polymer having ion exchange groups (mass of catalyst/mass of polymer having ion exchange groups) in the catalyst layer is preferably 2 to 6.

The gas diffusion layer functions to promptly diffuse gas generated from the catalyst layer to the outside of the catalyst layer, and also functions as a current collector. For example, the gas diffusion layer can be in the form of a carbon paper, a carbon cloth, a carbon felt, a sintered product of titanium oxide fiber, a sintered product of titanium oxide particles, or the like. In view of the facts that: the anode side is high in potential; and carbon materials are oxidized during use, it is preferred to use a sintered product of titanium oxide fiber or a sintered product of titanium oxide particles. A sintered product of titanium oxide may be plated with platinum or the like as necessary.

The cathode gas diffusion layer may be made water repellent by applying PTFE or the like.

Although the membrane electrode assembly of FIG. 1 has gas diffusion layers 28, the gas diffusion layers are optional, and the membrane electrode assemblies may have no gas diffusion layers.

With a view to obtaining more excellent effects of the present invention, each of the thicknesses of the anode and the cathode is preferably 5 μm or more, and is preferably 100 μm or less, more preferably 50 μm or less, still more preferably 30 μm or less, particularly preferably 15 μm or less.

Each of the thicknesses of the anode and the cathode is determined, by observing, with a laser microscope, a cross section of the membrane electrode assembly cut in the thickness direction, as an arithmetic mean of thickness measurements at arbitrary 20 points on the cross-sectional image of the membrane electrode assembly.

<Method for Producing Membrane Electrode Assembly>

A method for producing the membrane electrode assembly includes forming the cathode catalyst layer on one surface of the electrolyte membrane and forming the anode catalyst layer on the other surface of the electrolyte membrane.

For example, the membrane electrode assembly may be produced by using a laminate having the anode catalyst layer on a releasable base material (such as an ETFE sheet) and a laminate having the cathode catalyst layer on a releasable base material (such as an ETFE sheet), bonding the catalyst layers to both sides of the electrolyte membrane and peeling off the releasable base materials.

In each of the above-mentioned laminates, the gas diffusion layer may be provided between the catalyst layer and the releasable base material. In this case, the gas diffusion layer is arranged on the opposite side of the catalyst layer from the electrolyte membrane.

The catalyst layers may be formed by applying a catalyst layer forming ink to predetermined areas (such as surfaces of the releasable base materials), followed by drying as necessary. The catalyst layer forming ink is a dispersion of the polymer having ion exchange groups and the catalyst in a dispersion medium.

<Applications>

The membrane electrode assembly of the present invention is suitably applicable to a polymer electrolyte membrane water electrolyzer.

[Water Electrolyzer]

The water electrolyzer of the present invention includes the above-mentioned membrane electrode assembly, a water supply unit for supplying water to the anode catalyst layer side and a power supply unit for electrical connection between the anode catalyst layer side and the cathode catalyst layer side.

In the water electrolyzer of the present invention, when a direct-current voltage is applied by the power supply unit in a state that water is supplied to the anode catalyst layer side by the water supply unit, water is decomposed into oxygen and protons on the anode catalyst layer side. The protons transferring to the cathode catalyst layer side through the electrolyte membrane receive electrons to form hydrogen on the cathode catalyst layer side.

The water electrolyzer of the present invention may have the same configuration as that 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) except that it has the above members.

EXAMPLES

Now, the present invention will be described in further detail with reference to Examples. Ex. 1 to 11 correspond to Examples of the present invention; and Ex. 12 to correspond to Comparative Examples. It should be understood that the present invention is by no means restricted thereto.

[Ion Exchange Capacity of Fluorinated Polymer]

The weight of a fluorinated polymer was measured, after 24 hours of incubation in a glove box purged with dry nitrogen, as the dry mass of the fluorinated polymer. Then, the fluorinated polymer was immersed in a 2 mol/L aqueous sodium chloride solution at 60° C. for 1 hour. After the fluorinated polymer was washed with ultrapure water and recovered, the solution in which the fluorinated polymer had been immersed was titrated with 0.1 mol/L aqueous sodium hydroxide to determine the ion exchange capacity of the fluorinated polymer (meq/g dry resin).

[Method of Measuring Fineness of Spun Fiber]

The weight of the spun fiber is measured by a precision electronic balance and the fineness dtex is obtained from the following formula.

Fineness = ( 10000 × weight ⁢ ( g ) ) / length ⁢ ( m )

[Maximum Frequency Peak in Fiber Diameter Distribution Histogram of Fibers Constituting Nonwoven Fabric (or Woven Fabric)]

Regarding nonwoven fabrics A1 to A6 and A9 described later, fiber diameters of 100 fibers were measured by observation with a scanning electron microscope with a magnification of 20000. A histogram of fiber diameter distribution (class interval 20 nm) was prepared from the distribution of the numbers for the respective fiber diameters. In the prepared fiber diameter distribution histogram, the class with the highest frequency was employed as the maximum frequency peak. The fiber diameter corresponding to the maximum frequency peak is the smallest fiber diameter in the class with the highest frequency.

Further, regarding woven fabric A7 and nonwoven fabric A8 described later, with a magnification of 300, fiber diameters of 100 fibers were measured by observation with a microscope. A histogram of fiber diameter distribution (class interval 1000 nm) was prepared from the distribution of the numbers for the respective fiber diameters. In the prepared fiber diameter distribution histogram, the class with the highest frequency was employed as the maximum frequency peak. The fiber diameter corresponding to the maximum frequency peak is the smallest fiber diameter in the class with the highest frequency.

[Fiber Weight of Nonwoven Fabric (or Woven Fabric)]

The nonwoven fabric (or woven fabric) was cut into a 0.15 m square, and its mass was measured by a precision electronic balance, and the mass was converted to the mass per 1 m². This mass was taken as the fabric weight (g/m²) of the nonwoven fabric (or woven fabric).

[Thickness of Nonwoven Fabric (or Woven Fabric)]

The nonwoven fabric was placed on a dial gauge stand 7002 (Mitutoyo Corporation), and the thickness of the nonwoven fabric (or woven fabric) was measured by a dial gauge 543-250 (Mitutoyo Corporation) having a flat terminal with a diameter of 5 mm attached to the tip.

[Thickness of Electrolyte Membrane]

The thickness of each electrolyte membrane was measured on a magnified cross-sectional image of the electrolyte membrane taken (at an objective lens magnification of 50) by a laser microscope (model “VK-X1000”, manufactured by KEYENCE CORPORATION) at a temperature of 23° C. under a relative humidity of 50% RH.

[Surface Property of Electrolyte Membrane]

The surface property of the electrolyte membrane was evaluated on an image of the electrolyte membrane cut into a 3 cm square, taken by a laser microscope (model “VK-X1000”, manufactured by KEYENCE CORPORATION) at a temperature of 25° C. under a relative humidity of 50% RH, based on the following standards.

• • ◯: no exposure of the nonwoven fabric to the surface of the electrolyte membrane observed
  • x: exposure of the nonwoven fabric to the surface of the electrolyte membrane observed

In Ex. 12, whether exposure of the woven fabric was observed or not was confirmed.

[Strength]

The electrolyte membrane was immersed in pure water and left to stand in an oven at 80° C. for 16 hours. The electrolyte membrane was then left to stand at room temperature for 3 hours or longer, and punched by a No. 7 dumbbell to prepare a hydrated electrolyte membrane sample.

The sample was sandwiched between chucks and subjected to a tensile test using TENSILON RTI-1225 (A&D Company, Limited) in an oven set at 80° C. under a relative humidity of 90%, at a rate of 50 mm/min, and the coefficient of elasticity was calculated from the obtained stress-strain curve and evaluated based on the following standards.

In the tensile test, the nonwoven fabric or the woven fabric was stretched in the MD direction.

• • ◯: coefficient of elasticity being 15 MPa or more
  • x: coefficient of elasticity being less than 15 MPa

[Membrane Resistance]

A membrane electrode assembly was sandwiched between platinum-coated sintered titanium fiber layers (manufactured by Bekaert) having a thickness of 0.25 mm and a porosity of 60%, and then, mounted for evaluation in a single cell having an electrode area of 16 cm² and using platinum-coated titanium plates with straight channels as separators. The membrane electrode assembly was, when sandwiched, tightened so as to apply a pressure of 1.5 MPa to the electrode area.

To fully hydrate the polymer electrolyte membrane and the ionomers in the electrodes, pure water at 60° C. and ordinary pressure with an electrical conductivity of 1.0 uS/cm or less was supplied to both the anode catalyst layer side and the cathode catalyst layer side at a flow rate of 50 mL/min for 8 hours. Then, the supply of water to the cathode catalyst layer side was terminated, and water electrolysis was carried out as preliminary operation for 4 hours by supplying pure water at 60° C. with an electrical conductivity of 1.0 μS/cm or less to the anode catalyst layer side at a flow rate of 50 mL/min, with the back pressures on the anode catalyst layer side and the cathode catalyst layer side kept at ordinary pressure, while maintaining the electric current at 16 A (current density 1 A/cm²) by a high current potentio/garvanostat HCP-803 (manufactured by BioLogic). The membrane resistance was determined by conducting electrochemical impedance measurement with current control while maintaining the electric current at 32 A (current density of 2 A/cm²) as a direct current component. In the Nyquist plot obtained from the electrochemical impedance measurement, the membrane resistance was defined as the value of the real part at the point where the absolute value of the imaginary part of the impedance becomes smallest in the high-frequency region, multiplied by the electrode area, and was evaluated based on the following standards.

• • ◯: higher than 70 mOhm·cm²
  • X: 70 mOhm·cm² or lower

<Production of Fluorinated Polymer (I1)>

CF₂═CF₂ and the compound represented by the following formula (X) (hereinafter also simply referred to as “monomer (X)”) were copolymerized to obtain fluorinated polymer (I1) (ion exchange capacity: 1.25 meq/g dry resin).

<Production of Fluorinated Polymer (I2)>

CF₂=CF₂ and the compound represented by the following formula (Y) (hereinafter also simply referred to as “monomer (Y)”) were copolymerized to obtain fluorinated polymer (I2) (ion exchange capacity: 1.80 meq/g dry resin).

<Production of Fluorinated Polymer (I3)>

CF₂=CF₂ and the monomer (X) were copolymerized to obtain fluorinated polymer (I3) (ion exchange capacity: 1.00 meq/g dry resin).

<Production of Fluorinated Polymer (I4)>

CF₂=CF₂ and the monomer (X) were copolymerized to obtain fluorinated polymer (I4) (ion exchange capacity: 1.10 meq/g dry resin).

<Production of film-attached base material Y1>

To a base material made of polyethylene terephthalate, the fluorinated polymer (I1) was attached by melt-extrusion to obtain film-attached base material Y1 having a film α1 (film thickness: 30 μm) made of the fluorinated polymer (I1) formed on the base material.

<Production of Film-Attached Base Material Y2>

To a base material made of polyethylene terephthalate, the fluorinated polymer (I1) was attached by melt-extrusion to obtain film-attached base material Y2 having a film α2 (film thickness: 15 μm) made of the fluorinated polymer (I1) formed on the base material.

<Production of Film-Attached Base Material Y3>

To a base material made of polyethylene terephthalate, the fluorinated polymer (I1) was attached by melt-extrusion to obtain film-attached base material Y3 having a film α3 (film thickness: 45 μm) made of the fluorinated polymer (I1) formed on the base material.

<Production of Film-Attached Base Material Y4>

To a base material made of polyethylene terephthalate, the fluorinated polymer (I2) was attached by melt-extrusion to obtain film-attached base material Y4 having a film α4 (film thickness: 30 μm) made of the fluorinated polymer (I2) formed on the base material.

<Production of Film-Attached Base Material Y5>

To a base material made of polyethylene terephthalate, the fluorinated polymer (I3) was attached by melt-extrusion to obtain film-attached base material Y5 having a film α5 (film thickness: 30 μm) made of the fluorinated polymer (I3) formed on the base material.

<Production of Film-Attached Base Material Y6>

To a base material made of polyethylene terephthalate, the fluorinated polymer (I4) was attached by melt-extrusion to obtain film-attached base material Y6 having a film α6 (film thickness: 30 μm) made of the fluorinated polymer (I4) formed on the base material.

<Production of Film-Attached Base Material Y7>

The fluorinated polymer (I1) and cerium oxide were kneaded and pelletized by a twin screw extruder so that the cerium oxide concentration would be 0.7 wt %. Using the pellets, to a base material made of polyethylene terephthalate, the cerium oxide and the fluorinated polymer (I1) were attached by melt-extrusion to obtain film-attached base material Y7 having a film α7 (film thickness: 30 μm) containing the fluorinated polymer (I1) and the cerium oxide formed on the base material.

<Production of Film-Attached Base Material Y8>

The fluorinated polymer (I1), cerium oxide and a platinum powder were kneaded and pelletized by a twin screw extruder so that the cerium oxide concentration would be 0.7 wt % and the platinum concentration would be 0.1 wt %. Using the pellets, to a base material made of polyethylene terephthalate, the cerium oxide, the platinum and the fluorinated polymer (I1) were attached by melt-extrusion to obtain film-attached base material Y8 having a film α8 (film thickness: 30 μm) made of the fluorinated polymer (I1), the cerium oxide and the platinum formed on the base material.

<Production of Nonwoven Fabric A1>

As an island component, polyphenylene sulfide synthesized by a known synthesis method is used, and as a sea component, polyethylene terephthalate obtained by copolymerizing sodium 5-sulfoisophthalate and polyethylene glycol is used.

The polyphenylene sulfide as an island component is melted at 340° C. and the polyethylene terephthalate as an island component is melted at 280° C. The molten island component and sea component are merged in a spinneret with 720 islands so that the mass ratio of islands to sea becomes 70:30, and a sea/island composite fiber is discharged at a spinning temperature of 320° C., which is treated in a heating zone below the spinneret with a heating length of 90 mm at an ambient temperature of 450° C. and in a cooling device, and then oiled and wound at a spinning rate of 1000 m/min.

The obtained undrawn yarn is drawn while the spinning discharge rate and the draw ratio are adjusted to obtain drawn yarn with a fineness of 5 dtex.

The obtained drawn yarn is cut to a length of 0.3 mm, and immersed in a 5 mass % aqueous sodium hydroxide solution at 80° C. for 30 minutes to remove the sea component, to obtain PPS fibers with an average fiber diameter of about 700 nm.

A slurry is prepared by mixing 80 wt % of the PPS fibers with 20 wt % of undrawn PPS fibers having a fiber diameter of 6 μm and adding water, which is then subjected to wet paper making and dried, and flattened by heat calendering, to prepare nonwoven fabric A1 with a fabric weight of 20 g/m² and a thickness of 37 μm.

<Production of Nonwoven Fabric A2>

PPS fibers are prepared in the same manner as for the nonwoven fabric A1, and nonwoven fabric A2 with a nonwoven fabric weight of 10 g/m² and a thickness of 19 μm is obtained while the processing rate at the wet paper making is adjusted.

<Production of Nonwoven Fabric A3>

PPS fibers with an average fiber diameter of 100 nm are obtained in the same manner as for the nonwoven fabric A1 except that at the time of spinning, the number of islands is 980, the mass ratio of islands to sea is 10:90 and the fineness of the drawn yarn is 1 dtex.

A slurry is prepared by mixing 80 wt % of the PPS fibers with 20 wt % of undrawn PPS fibers having a fiber diameter of 6 μm and adding water, and nonwoven fabric A3 with a fabric weight of 20 g/m² and a thickness of 37 μm is prepared by wet paper-making.

<Production of Nonwoven Fabric A4>

PPS fibers with an average fiber diameter of 900 nm are obtained in the same manner as for the nonwoven fabric A1 except that at the time of spinning, the number of islands is 400.

A slurry is prepared by mixing 80 wt % of the PPS fibers with 20 wt % of undrawn PPS fibers having a fiber diameter of 6 μm and adding water, and nonwoven fabric A4 with a fabric weight of 20 g/m² and a thickness of 37 μm is prepared by wet paper-making.

<Production of Nonwoven Fabric A5>

PPS fibers with an average fiber diameter of 395 nm are obtained in the same manner as for the nonwoven fabric A1 except that at the time of spinning, the number of islands was 950, the mass ratio of islands to sea is 40:60 and the fineness of the drawn yarn is 3.8 dtex.

A slurry is prepared by mixing 80 wt % of the PPS fibers with 20 wt % of undrawn PPS fibers having a fiber diameter of 6 μm and adding water, and nonwoven fabric A5 with a fabric weight of 20 g/m² and a thickness of 37 μm is prepared by wet paper-making.

<Production of Nonwoven Fabric A6>

PPS fibers are prepared in the same manner as for the nonwoven fabric A1.

The obtained PPS fibers are mixed with water to prepare a slurry, and nonwoven fabric A6 with a fabric weight of 20 g/m² and a thickness of 61 μm is prepared by wet paper-making while adjusting the pressure applied by the heat calendering.

<Production of Woven A7>

Using 18.6-denier yarns of PFA as warp yarns and weft yarns, woven fabric A7 was produced by plain weaving such that the density of the PFA yarns was 100 yarns/inch. The fabric weight of the woven fabric A7 was 16.3 g/m².

<Production of Nonwoven Fabric A8>

Polyphenylene sulfide was melted at 340° C. and spun using a spinning apparatus to obtain undrawn yarn with a fineness of 2.0 dtex. The obtained undrawn yarn was drawn to obtain drawn PPS fibers with an average fiber diameter of 10000 nm.

A slurry was prepared by mixing 60 wt % of the drawn PPS fibers with 40 wt % of undrawn PPS fibers and adding water, and nonwoven fabric A8 with a fabric weight of 10 g/m² and a thickness of 18 μm was prepared by wet paper-making.

<Production of Nonwoven Fabric A9>

Polyethersulfone (PES) polymerized by a known method was dissolved in dimethylacetamide at a solid content concentration of 25 wt %. The obtained solution was supplied to an electrostatic spinning apparatus with an inner diameter of 0.44 mm and a distance from a metal plate of 5.5 cm and spun at a resin discharge rate of 17 mg/min with a voltage applied of 12 kV to prepare fibers, which were collected on the metal plate surface to prepare nonwoven fabric A9 with an average fiber diameter of 350 nm, a fabric weight of 3.2 g/m² and a thickness of 15 μm.

<Preparation of Cathode Catalyst Layer Decal>

Water (59.4 g) and ethanol (39.6 g) were added to a catalyst (manufactured by TANAKA PRECIOUS METAL TECHNOLOGIES Co., Ltd., “TEC10E50E”) (11 g) having 46 mass % of platinum supported on a carbon powder and mixed and pulverized by an ultrasonic homogenizer to obtain a catalyst dispersion.

A polymer (ion exchange capacity: 1.10 meq/g dry resin) produced by copolymerization of TFE and the monomer (X), followed by conversion to acid form through hydrolysis and acid treatment, was dispersed in a solvent of water/ethanol in a ratio of 40/60 (mass %) to obtain a dispersion with a solid content of 26.0% (hereinafter also referred to also as “dispersion Y”). The dispersion Y (20.1 g), ethanol (11 g) and

ZEORORA H (manufactured by Zeon Corporation) (6.3 g) were mixed and kneaded to prepare a mixture (29.2 g), which was added to the catalyst dispersion. Further, water (3.66 g) and ethanol (7.63 g) were added to the dispersion, and mixed by a paint conditioner for 60 minutes to obtain a cathode catalyst ink with a solid content of 10.0 mass %.

The cathode catalyst ink was applied to an ETFE sheet by a die coater, dried at 80° C. and heat-treated at 150° C. for 15 minutes, thereby obtaining a cathode catalyst layer decal with a platinum content of 0.4 mg/cm².

<Preparation of Anode Catalyst Layer Decal>

The dispersion Y (33.0 g) was mixed with ethanol (18.06 g) and ZEORORA H (manufactured by Zeon Corporation) (10.58 g) by a planetary centrifugal mixer (THINKY MIXER, manufactured by THINKY CORPORATION) at 2200 rpm for 5 minutes. The mixed composition (54.06 g) was mixed with ethanol (46.44 g) and water (75.75 g) and further mixed with an iridium oxide catalyst (40.0 g) (manufactured by TANAKA PRECIOUS METAL TECHNOLOGIES Co., Ltd.) containing 74.8 mass % iridium and having a specific surface area of 100 m²/g. The resulting mixed composition was processed in a planetary bead mill (rotation speed: 300 rpm) for 90 minutes to obtain an anode catalyst ink with a solid content of 22 mass %.

The anode catalyst ink was applied to an ETFE sheet by an applicator so that the iridium content would be 1.0 mg/cm², dried at 80° C. for 10 minutes and heat-treated at 150° C. for 15 minutes, thereby obtaining an anode catalyst layer decal.

[Ex. 1]

The film-attached base material Y1, the nonwoven fabric A1 and the film-attached base material Y1 are stacked in this order. At this time, the film-attached base materials Y1 are arranged such that the films α1 of the film-attached base materials Y1 are brought into contact with the nonwoven fabric A1.

These stacked members are thermally press-bonded together by a flat press machine at a temperature of 200° C. under a surface pressure of 5 MPa for 5 minutes, and then, the base materials on both sides are peeled off at a temperature of 50° C. to obtain a precursor membrane.

The precursor membrane is immersed in a solution of dimethyl sulfoxide/potassium hydroxide/water=30/5.5/64.5 (mass ratio) at 80° C. for 16 hours to convert groups convertible to sulfonic acid functional groups in the precursor membrane by hydrolysis to K-type sulfonic acid functional groups, and washed with water. The thus-obtained membrane is immersed in 3M aqueous sulfuric acid solution to convert the terminal groups from K-type to H-type, washed with water and dried to obtain an electrolyte membrane X1 (thickness: 60 μm). Evaluations of the surface property and the mechanical strength are made on the obtained electrolyte membrane. The results are shown in Table 1.

The catalyst layer of the anode catalyst layer decal cut into a 4 cm square is arranged to face one surface of the electrolyte membrane X1 cut into a 7 cm square, and the catalyst layer of the cathode catalyst layer decal cut into a 4 cm square is arranged to face the other surface of the electrolyte membrane X1, and they are hot pressed for 10 minutes at a press temperature of 150° C. under a pressure of 3 MPa. After cooling to 70° C. and releasing the pressure, the resulting laminate is taken out, and the ETFE sheets of the anode catalyst layer decal and the cathode catalyst layer decal are peeled off to obtain a membrane electrode assembly having an electrode area of 16 cm². The evaluation of the membrane resistance is made on the obtained membrane electrode assembly. The evaluation results are shown in Tables 1 and 2.

<Ex. 2 to 15>

An electrolyte membrane and a membrane electrode assembly were obtained in the same manner as in Ex. 1 except that the film-attached base materials and the nonwoven fabric or the woven fabric were changed as identified in Table 1. The evaluations were made on the obtained electrolyte membrane and membrane electrode assembly. The evaluation results are shown in Tables 1 and 2.

10 In Tables 1 and 2, in the “constituent units” row, X represents the monomer (X) and Y represents the monomer (Y).

In Tables 1 and 2, in the “additives” row, Ce represents cerium and Pt represents platinum.

	TABLE 1
	Ex. 1	Ex. 2	Ex. 3	Ex. 4	Ex. 5	Ex. 6	Ex. 7	Ex. 8

Electrolyte	Fluorinated	I1	I1	I1	I1	I1	I1	I1	I1
	polymer
	Film-attached	Y1	Y2	Y1	Y1	Y3	Y3	Y7	Y8
	base material
	Constituent units	TFE/X	TFE/X	TFE/X	TFE/X	TFE/X	TFE/X	TFE/X	TFE/X
	Ion exchange	1.25	1.25	1.25	1.25	1.25	1.25	1.25	1.25
	capacity (meq/g
	dry resin)
Reinforcing	Nonwoven fabric	A1	A2	A3	A4	A5	A6	A2	A2
member	or woven fabric
	Type	Nonwoven	Nonwoven	Nonwoven	Nonwoven	Nonwoven	Nonwoven	Nonwoven	Nonwoven
		fabric	fabric	fabric	fabric	fabric	fabric	fabric	fabric
	Fiber material	PPS	PPS	PPS	PPS	PPS	PPS	PPS	PPS
	Maximum	700	700	100	900	380	700	700	700
	frequency peak
	(nm)
	Fabric weight	20	10	20	20	20	20	10	10
	(gm−2)
	Thickness (μm)	37	19	37	37	37	61	19	19
Electrolyte	Thickness (μm)	60	30	60	60	90	90	60	60
membrane	Additives	—	—	—	—	—	—	Ce	Ce, Pt
Evaluation	Surface property	∘	∘	∘	∘	∘	∘	∘	∘
results	Membrane	∘	∘	∘	∘	∘	∘	∘	∘
	resistance
	Strength	∘	∘	∘	∘	∘	∘	∘	∘

	TABLE 2
	Ex. 9	Ex. 10	Ex. 11	Ex. 12	Ex. 13	Ex. 14	Ex. 15

Electrolyte	Fluorinated polymer	I2	I3	I4	I1	I1	I1	I1
	Film-attached base	Y4	Y5	Y6	Y3	Y1	Y1	Y3
	material
	Constituent units	TFE/Y	TFE/X	TFE/X	TFE/X	TFE/X	TFE/X	TFE/X
	Ion exchange	1.80	1.00	1.10	1.25	1.25	1.25	1.25
	capacity (meq/g dry
	resin)
Reinforcing	Nonwoven fabric or	A1	A1	A1	A7	A8	A9	—
member	woven fabric
	Type	Nonwoven	Nonwoven	Nonwoven	Woven	Nonwoven	Nonwoven	—
		fabric	fabric	fabric	fabric	fabric	fabric
	Fiber material	PPS	PPS	PPS	PFA	PPS	PES	—
	Maximum frequency	700	700	700	30000	10000	340	—
	peak (nm)
	Fabric weight (gm−2)	20	20	20	16.3	10	3	—
	Thickness (μm)	37	37	37	60	18	15	—
Electrolyte	Thickness (μm)	60	60	60	90	60	60	90
membrane	Additives	—	—	—	—	—	—	—
Evaluation	Surface property	∘	∘	∘	x	∘	∘	∘
results	Membrane	∘	∘	∘	—	x	∘	∘
	resistance
	Strength	∘	∘	∘	—	∘	x	x

As shown in the above Tables, it was confirmed that the polymer electrolyte membrane of the present invention has desired effects.

This application is a continuation of PCT Application No. PCT/JP2024/011665, filed on Mar. 25, 2024, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2023-052010 filed on Mar. 28, 2023. The contents of those applications are incorporated herein by reference in their entireties.

REFERENCE SYMBOLS

• • 10: electrolyte membrane
  • 20: membrane electrode assembly
  • 22: anode
  • 24: cathode
  • 26: catalyst layer
  • 28: gas diffusion layer