JP7819476B2 - Electrolyte membrane, catalyst layer-coated electrolyte membrane, membrane electrode assembly, and polymer electrolyte fuel cell - Google Patents

Description

The present invention relates to an electrolyte membrane, a catalyst-coated electrolyte membrane in which catalyst layers are sandwiched between both sides of the electrolyte membrane, a membrane electrode assembly including the electrolyte membrane, and a polymer electrolyte fuel cell including the electrolyte membrane.

It is known that solid polymer electrolyte membranes used in fuel cells, redox flow batteries, and the like are composited with a porous substrate to enhance durability. For example, electrolyte membranes have been proposed in which a polymer electrolyte layer is provided on one or both sides of a composite layer formed by impregnating (filling) a porous substrate with a polymer electrolyte (Patent Documents 1 to 4).

Republished Patent No. WO2014/034415 Japanese Patent Application Laid-Open No. 2007-329015 Japanese Patent Application Laid-Open No. 2006-202532 Japanese Patent Application Laid-Open No. 2005-44611

However, when the above-mentioned conventional electrolyte membranes are applied to, for example, polymer electrolyte fuel cells, they have insufficient mechanical properties in a hydrated state, resulting in a lack of physical durability.

In light of this background, the object of the present invention is to provide an electrolyte membrane that has good mechanical properties in both a dry and hydrated state.

In order to solve the above problems, the electrolyte membrane of the present invention has the following configuration:
The electrolyte membrane includes a porous substrate and a polymer electrolyte, and the porous substrate has a streaky structure.

Also preferred is a catalyst-coated electrolyte membrane having catalyst layers laminated on both sides.

Furthermore, a membrane electrode assembly containing the above-mentioned electrolyte membrane can be produced, and a polymer electrolyte fuel cell containing the above-mentioned electrolyte membrane can be produced.

The present invention can provide an electrolyte membrane that has good mechanical properties in both dry and wet states. By applying the electrolyte membrane of the present invention to a fuel cell, the dry-wet cycle durability of the fuel cell can be improved.

1 is a digital microscope image of the electrolyte membrane of the present invention, viewed from the surface side coated with the polymer electrolyte.

The present invention will be described in detail below. Hereinafter, in this specification, the term "to" indicates a range that includes both the numerical values at both ends.

[Electrolyte membrane]
The electrolyte membrane of the present invention comprises a porous substrate and a polymer electrolyte, and the porous substrate has a streaky structure (hereinafter, this streaky structure may be referred to as "the present streaky structure").

Figure 1 shows an image of the surface of an electrolyte membrane according to one embodiment of the present invention, on the side where the polymer electrolyte is present, observed with a digital microscope. In the image, multiple streak structures (1a) to (1f) present in the porous substrate can be observed through the electrolyte membrane. In Figure 1, the main streak structures are labeled with symbols.

If the thickness of the polymer electrolyte layer present on the outside of the porous substrate is small, unevenness reflecting this streak-like structure may appear on the surface of the polymer electrolyte. It is preferable that at least one of these streak-like structures has a length of 10 μm or more. Having this streak-like structure of 10 μm or more in length can suppress swelling of the electrolyte membrane when it absorbs water, thereby improving wet-dry cycle durability.

In the present invention, the streaky structure refers to a structural feature present in the porous substrate of the electrolyte membrane. When the electrolyte membrane of the present invention is observed under a microscope, the streaky structure is characterized by linear structures having a width of 0.2 μm or more and an average width of 0.35 μm or more.

The electrolyte membrane of the present invention preferably has 100 to 2000 streak structures with a width of 0.5 μm or more per ^mm2 of surface, more preferably 200 to 2000, and particularly preferably 200 to 1000. When the number of streak structures is within the above-mentioned preferred range, an electrolyte membrane can be obtained that has high proton conductivity and excellent wet-dry cycle durability by suppressing swelling and softening when hydrated.

The average length of this streak structure is more preferably 10 to 200 μm, even more preferably 10 to 100 μm, and particularly preferably 15 to 70 μm. The average width (diameter) of this streak structure is preferably 0.4 to 7.0 μm, more preferably 0.4 to 5.0 μm, even more preferably 0.4 to 3.0 μm, and particularly preferably 0.5 to 3.0 μm. By using a porous substrate with such a streak structure, it is possible to obtain an electrolyte membrane that has high proton conductivity and excellent wet-dry cycle durability by suppressing swelling and softening when hydrated.

From the perspective of suppressing softening of the electrolyte membrane in a hydrated state, the value obtained by dividing the average length of this streak-like structure by the average width is preferably 5 times or more, more preferably 10 times or more, and particularly preferably 20 times or more. Furthermore, this ratio is preferably 100 times or less, more preferably 80 times or less, and particularly preferably 50 times or less. By using a porous substrate with this streak-like structure, it is possible to obtain an electrolyte membrane that has high proton conductivity and excellent wet-dry cycle durability by suppressing swelling and softening when hydrated.

Preferably, multiple streak structures are arranged approximately parallel to any one direction parallel to the surface of the porous substrate. When the electrolyte membrane is in the form of a roll or long sheet, it is more preferable for multiple streak structures to be arranged approximately parallel to the longitudinal direction. This arrangement makes it easier to suppress swelling and softening when hydrated when formed into a polymer electrolyte membrane, and also gives the porous substrate high strength in the longitudinal direction, making it easier to suppress deformation of the porous substrate due to conveying tension during the production of the porous substrate or electrolyte membrane.

Here, the longitudinal direction (sometimes referred to as "MD") is the direction (conveyance direction) in which the porous substrate, coater, or support substrate is moved in the electrolyte membrane manufacturing process described below. On the other hand, the direction perpendicular to the longitudinal direction on the membrane surface is the width direction (sometimes referred to as "TD").

It is preferable that 1 to 20 of these streaky structures are observed per 100 μm of length in one axial direction (e.g., width direction), more preferably 2 to 20, and particularly preferably 2 to 10. If there is less than 1 structure, it may be difficult to suppress swelling and softening when hydrated. If there are more than 20 structures, proton conductivity may decrease.

The electrolyte membrane of the present invention comprises a porous substrate and a polymer electrolyte, which are laminated together. Furthermore, a portion of the polymer electrolyte penetrates the porous substrate (the portion (layer) that penetrates the porous substrate is sometimes referred to as a "composite layer"), while the other portion (the portion (layer) that does not penetrate the porous substrate is sometimes referred to as a "single layer") generally exists outside the porous substrate. The single layer may be provided on only one side of the porous substrate or on both sides, but is preferably provided on both sides. The presence of a single layer facilitates the formation of a catalyst layer on the surface of the polymer electrolyte membrane and reduces the interfacial resistance between the catalyst layer or electrode layer and the polymer electrolyte membrane, thereby facilitating improved fuel cell performance and durability.

When monolayers are provided on both sides of a porous substrate, it is important that the streaky structure be observed when observing at least one of the monolayers using a microscope, but it is preferable that the streaky structure be observed when observing the monolayer on either side. If the streaky structure is observed when observing from either side, the composite layer is located near the center in the thickness direction of the polymer electrolyte membrane. Furthermore, by making the thickness of the composite layer larger relative to the thickness of the polymer electrolyte membrane, swelling and softening when hydrated are more easily suppressed.

In light of the above, one preferred embodiment of the electrolyte membrane of the present invention is one in which the porous substrate and polymer electrolyte are made of different materials (polymer compounds). As described above, it is important in the present invention that the porous substrate has a relatively large streak-like structure in the portion that corresponds to the composite layer.

The thickness of the electrolyte membrane of the present invention is not particularly limited, but is preferably 1 μm to 1000 μm, more preferably 2 μm to 500 μm, and particularly preferably 3 μm to 200 μm.

The electrolyte membrane of the present invention preferably has a haze value of 20% to 45% when light is transmitted through it in the thickness direction, more preferably 25% to 45%, even more preferably 30% to 45%, and particularly preferably 35% to 45%. An electrolyte membrane with a haze value within the above range has an appropriate range for the density and shape of the streak-like structure of the present invention, has high proton conductivity, and exhibits excellent wet-dry cycle durability by suppressing swelling and softening when hydrated.

[Composite layer]
The composite layer is a layer formed by impregnating a porous substrate with a polymer electrolyte, and is a composite layer of the polymer electrolyte and the porous substrate, and can be obtained, for example, by filling (impregnating) a porous substrate with the polymer electrolyte.

The polymer electrolyte filling rate in the composite layer is preferably 50% or more, and more preferably 60% or more. When the polymer electrolyte filling rate is 50% or more, a continuous proton conduction path is formed throughout the electrolyte membrane, thereby improving power generation performance. The polymer electrolyte filling rate in the composite layer is a value that indicates the proportion of the polymer electrolyte to the total volume of the composite layer, and can be calculated from the ion exchange capacity (IEC). Specifically, this can be done using the method described in the Examples.

The electrolyte membrane of the present invention has a composite layer, which can reduce the dimensional change rate in the in-plane direction. By reducing the dimensional change rate in the in-plane direction, when used as an electrolyte membrane for a fuel cell, stress caused by swelling and shrinkage that occurs at the edge portions of the electrolyte membrane during dry-wet cycling can be reduced, making it easier to improve durability. The dimensional change rate λ _xy of the electrolyte membrane in the in-plane direction is preferably 10% or less, more preferably 8% or less, and even more preferably 5% or less.

Furthermore, it is preferable that the anisotropy of the dimensional change rate in the plane direction of the electrolyte membrane is small in the longitudinal and width directions. Specifically, it is preferable that the ratio of the dimensional change rate in the longitudinal direction λ _MD to the dimensional change rate in the width _direction λ TD (λ _MD /λ _TD ) satisfies 0.5≦(λ _MD /λ _TD )≦2.0.

When the ratio of the dimensional change rate in the MD λ _MD to the dimensional change rate in the width direction λ _TD (λ _MD /λ _TD ) satisfies 0.5≦(λ _MD /λ _TD )≦2.0, the cell design of the fuel cell is not restricted, and even at edges perpendicular to the direction of the larger dimensional change in the MD or TD direction, stress concentration due to swelling and shrinkage is small. Therefore, the edges are less likely to become the starting point for electrolyte membrane rupture, and dry-wet cycle durability can be improved.

Here, the dimensional change rate is an index that represents the change in the dimensions of the electrolyte membrane in a dry state and in a wet state, and specific measurements can be performed using the method described in the examples.

The thickness of the composite layer in the electrolyte membrane is not particularly limited, but is preferably 0.5 μm to 50 μm, and more preferably 1 μm to 40 μm. When the composite layer is 0.5 μm to 50 μm thick, a composite polymer electrolyte membrane with high proton conductivity, which can suppress electrical short circuits and fuel permeation and has excellent physical durability can be obtained.

[Polymer electrolyte]
The polymer electrolyte used in the present invention is a polymer having proton conductivity due to the presence of ionic groups, and fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes are known, and any of these can be used.

Here, a fluoropolymer electrolyte refers to one in which most or all of the hydrogen atoms in the hydrocarbon groups in the molecule have been replaced with fluorine atoms. Representative examples of fluoropolymer electrolytes with ionic groups include commercially available products such as "Nafion" (registered trademark) (manufactured by Chemours), "Aquivion" (registered trademark) (manufactured by Solvay), "Flemion" (registered trademark) (manufactured by AGC), and "Aciplex" (registered trademark) (manufactured by Asahi Kasei Corporation).

These fluorine-based polymer electrolytes are generally very expensive and tend to have large gas crossovers. Therefore, from the viewpoints of mechanical strength, dry-wet cycle durability, chemical stability, and the like, it is preferable that the polymer electrolyte used in the present invention is a hydrocarbon-based polymer electrolyte. That is, in the present invention, it is preferable that the polymer electrolyte is a hydrocarbon-based polymer electrolyte having ionic groups. In particular, in the composite layer of the present invention, an embodiment in which the polymer electrolyte is a hydrocarbon-based polymer electrolyte is particularly preferable, as it has excellent mechanical properties in both the dry state and the hydrated state.

Aromatic hydrocarbon-based polymer electrolytes having aromatic rings in the main chain are preferred. Here, aromatic rings may include not only hydrocarbon-based aromatic rings consisting only of carbon atoms and hydrogen atoms, such as benzene rings and naphthalene skeletons, but also heterocycles such as pyridine rings, imidazole rings, and thiol rings. Furthermore, the polymer may also contain some aliphatic units in addition to aromatic ring units.

Specific examples of aromatic hydrocarbon-based polymer electrolytes include polymers having a structure selected from polysulfone (PSU), polyethersulfone (PES), polyphenylene oxide (PPO), polyarylene ether-based polymers, polyphenylene sulfide (PPS), polyphenylene sulfide sulfone, polyparaphenylene (PPP), polyarylene-based polymers, polyarylene ketone, polyether ketone (PEK), polyarylene phosphine oxide, polyether phosphine oxide, polybenzoxazole (PBO), polybenzthiazole (PBT), polybenzimidazole (PBI), polyamide (PA), polyimide (PI), polyetherimide (PEI), and polyimide sulfone in the main chain together with an aromatic ring. The terms polysulfone, polyethersulfone, polyetherketone, etc., used here are general terms for structures having sulfone bonds, ether bonds, or ketone bonds in the molecular chain, and include polyetherketoneketone, polyetheretherketone, polyetheretherketoneketone, polyetherketoneetherketoneketone, polyetherketonesulfone, etc. The hydrocarbon skeleton may have multiple of these structures. Of these, polymers having a polyetherketone skeleton, i.e., polyetherketone-based polymers, are most preferred as aromatic hydrocarbon-based polymers.

Polymer electrolytes that form a bicontinuous or lamellar phase-separated structure are preferred. Such phase-separated structures can be realized, for example, in molded articles composed of a blend of two or more incompatible polymers, such as a hydrophilic polymer having an ionic group and a hydrophobic polymer not having an ionic group, or in block copolymers composed of two or more incompatible segments, such as a segment (S _E1 ) containing an ionic group and a segment (S _E2 ) not containing an ionic group. In bicontinuous or lamellar phase-separated structures, both the hydrophilic and hydrophobic domains form continuous phases, forming continuous proton-conducting channels and facilitating the production of electrolyte membranes with excellent proton conductivity. At the same time, the formation of a continuous hydrophobic backbone facilitates the production of electrolyte membranes with excellent mechanical properties, not only in the dry state but also in the hydrated state. Here, "domain" refers to a mass formed by the aggregation of similar substances or segments within a single membrane.

As the polymer electrolyte, a block copolymer having one or more segments (S _E1 ) containing an ionic group and one or more segments (S _E2 ) not containing an ionic group is particularly preferred. Here, a segment refers to a partial structure in a copolymer polymer chain consisting of repeating units that exhibit specific properties and has a molecular weight of 2,000 or more. The use of a block copolymer makes it possible to develop a bicontinuous or lamellar phase-separated structure with finer domains than a polymer blend, thereby achieving better power generation performance and wet-dry cycle durability.

Hereinafter, a segment (S _E1 ) or polymer containing an ionic group may be referred to as an “ionic block,” and a segment (S _E2 ) or polymer not containing an ionic group may be referred to as a “non-ionic block.” However, the expression “not containing an ionic group” in this specification does not exclude an embodiment in which the segment or polymer contains a small amount of ionic groups to the extent that the formation of a phase-separated structure is not inhibited.

In such block copolymers, the molar composition ratio (S _E1 /S _E2 ) of the ionic block to the nonionic block is preferably 0.20 or more, more preferably 0.33 or more, and even more preferably 0.50 or more. Furthermore, the molar composition ratio (S _E1 /S _E2 ) is preferably 5.00 or less, more preferably 3.00 or less, and even more preferably 2.50 or less. When the molar composition ratio (S _E1 /S _E2 ) is 0.20 or more or 5.00 or less, a composite polymer electrolyte membrane can be obtained that exhibits high proton conductivity under low humidification conditions and excellent hot water resistance and dry-wet cycle durability. Here, the molar composition ratio (S _E1 /S _E2 ) refers to the ratio of the number of moles of repeating units present in the ionic block to the number of moles of repeating units present in the nonionic block. The "number of moles of repeating units" refers to the value obtained by dividing the number average molecular weight of the ionic block and the nonionic block by the molecular weight of the corresponding structural unit.

The ionic group possessed by the polymer electrolyte may be any ionic group having proton exchange capacity. Examples of such functional groups that are preferably used include sulfonic acid groups, sulfonimide groups, sulfate groups, phosphonic acid groups, phosphate groups, and carboxylic acid groups. Two or more types of ionic groups can be contained in the polymer. From the viewpoint of high proton conductivity, it is more preferable for the polymer to have at least one group selected from sulfonic acid groups, sulfonimide groups, and sulfate groups, and from the viewpoint of raw material costs, it is most preferable for the polymer to have a sulfonic acid group.

The ion exchange capacity (IEC) of the polymer electrolyte is preferably 0.1 to 5.0 meq/g in order to balance proton conductivity and water resistance. The IEC is more preferably 1.4 meq/g or higher, and even more preferably 1.7 meq/g or higher. The IEC is more preferably 3.5 meq/g or lower, and even more preferably 3.0 meq/g or lower. When the IEC is within the above preferred range, excellent proton conductivity and water resistance can both be achieved.

Here, IEC refers to the molar amount of ionic groups introduced per unit dry weight of the polymer electrolyte; a larger value indicates a larger amount of ionic groups introduced. In the present invention, IEC is defined as a value determined by neutralization titration. IEC calculation by neutralization titration can be performed using the method described in the Examples.

In the present invention, it is particularly preferable to use an aromatic hydrocarbon-based block copolymer as the polymer electrolyte, and a polyether ketone-based block copolymer is even more preferable. In particular, polyether ketone-based block copolymers containing a segment including a structural unit (S1) containing an ionic group and a segment including a structural unit (S2) not containing an ionic group, as shown below, are particularly preferably used.

In general formula (S1), Ar ¹ to Ar ⁴ represent any divalent arylene group, Ar ¹ and/or Ar ² contain an ionic group, and Ar ³ and Ar ⁴ may or may not contain an ionic group. Ar ¹ to Ar ⁴ may optionally contain a group other than an ionic group, and two or more types of arylene groups may be used independently. * represents a bonding site with another structural unit represented by general formula (S1) or another structural unit.

In general formula (S2), Ar ⁵ to Ar ⁸ represent any divalent arylene group and do not contain an ionic group. Ar ⁵ to Ar ⁸ may optionally have a group other than an ionic group, and two or more types of arylene groups may be used independently. * represents a bonding site with another structural unit represented by general formula (S2) or another structural unit.

Here, preferred divalent arylene groups for Ar ¹ to Ar ⁸ include hydrocarbon arylene groups such as phenylene, naphthylene, biphenylene, and fluorenediyl groups, and heteroarylene groups such as pyridinediyl, quinoxalinediyl, and thiophenediyl, but are not limited to these. Here, the phenylene group may be of three types: o-phenylene, m-phenylene, and p-phenylene, and unless otherwise specified, these will be used as a general term in this specification. The same applies to other divalent arylene groups, such as naphthylene and biphenylene groups. Ar ¹ to Ar ⁸ are preferably phenylene groups containing a phenylene group and an ionic group, and most preferably p-phenylene groups containing a p-phenylene group and an ionic group. Furthermore, Ar ⁵ to Ar ⁸ may be substituted with groups other than ionic groups, but it is more preferable that they are unsubstituted in terms of proton conductivity, chemical stability, and dry-wet cycle durability.

In addition, in order to obtain sufficient dimensional stability, mechanical strength, dry/wet cycle durability, fuel barrier properties, and solvent resistance, it is preferable that the polymer electrolyte be an aromatic hydrocarbon-based polymer electrolyte that has crystallinity. Here, "crystalline" means that the polymer electrolyte has the property of being crystallizable when heated, or that the polymer electrolyte has already crystallized.

The presence or absence of crystallinity is confirmed by differential scanning calorimetry (DSC) or wide-angle X-ray diffraction. In the present invention, it is preferable that the heat of crystallization measured by DSC after film formation is 0.1 J/g or more, or the crystallinity measured by wide-angle X-ray diffraction is 0.5% or more. In other words, if no crystallization peak is observed in DSC, this may mean that the polymer electrolyte has already crystallized or that it is amorphous. If the polymer electrolyte has already crystallized, the crystallinity measured by wide-angle X-ray diffraction will be 0.5% or more.

Crystalline aromatic hydrocarbon-based polymer electrolytes may have poor processability for forming electrolyte membranes. In such cases, protecting groups may be introduced into the aromatic hydrocarbon-based polymer electrolyte to temporarily suppress crystallinity. Specifically, by forming a membrane with protecting groups introduced and then deprotecting the membrane, a crystalline aromatic hydrocarbon-based polymer electrolyte can be used as the polymer electrolyte in the present invention.

[Porous base material]
The porous substrate used in the present invention has a streaky structure in the porous substrate when formed into an electrolyte membrane. As described above, this streaky structure (the present streaky structure) is a structural feature present in the porous substrate in the electrolyte membrane, and is characterized in that, when the electrolyte membrane is observed under a microscope, linear structures each having a width of 0.2 μm or more have an average width of 0.35 μm or more.

The porous substrate preferably has a fibril structure with a small average width in addition to the streak-like structure, and more preferably has a structure in which the streak-like structures are connected by the fibrils.

A porous substrate containing this streak structure with a width at least twice the average fibril width is preferred, a porous substrate containing this streak structure with a width at least three times the average fibril width is more preferred, and a porous substrate containing this streak structure with a width at least five times the average fibril width is even more preferred. Also preferred are porous substrates containing this streak structure with a width at most 20 times the average fibril width, and a porous substrate containing this streak structure with a width at most 10 times the average fibril width is more preferred. This streak structure can be observed by microscopic observation of the electrolyte membrane surface using a digital microscope or other such device, or by taking a tomographic photograph and processing the image to obtain an image viewed from the electrolyte membrane surface. Its length and width can be determined using the method described in the Examples section.

There are no particular restrictions on the material that can be used to form the porous substrate, as long as it has this streaky structure. Suitable materials for forming the porous substrate include, for example, hydrocarbon-based porous substrates whose main component is a hydrocarbon-based polymer compound, and fluorine-based porous substrates whose main component is a fluorine-based polymer compound.

Examples of hydrocarbon polymer compounds include polyethylene (PE), polypropylene (PP), polystyrene (PS), polyacrylate, polymethacrylate, polyvinyl chloride (PVC), polyvinylidene chloride (PVdC), polyester, polycarbonate (PC), polysulfone (PSU), polyethersulfone (PES), polyphenylene oxide (PPO), polyarylene ether polymers, polyphenylene sulfide (PPS), polyphenylene sulfide sulfone, polyparaphenylene (PPP), polyarylene polymers, polyarylene ketone, polyether ketone (PEK), polyarylene phosphine oxide, polyether phosphine oxide, polybenzoxazole (PBO), polybenzthiazole (PBT), polybenzimidazole (PBI), polyamide (PA), polyimide (PI), polyetherimide (PEI), and polyimide sulfone (PIS).

Examples of fluorine-based polymer compounds include polytetrafluoroethylene (PTFE), polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVdF), polychlorotrifluoroethylene (PCTFE), perfluoroalkoxy fluororesin (PFA), and ethylene-chlorotrifluoroethylene copolymer (ECTFE).

From the standpoint of water resistance, chemical resistance, and mechanical properties, PE, PP, PPS, PEK, PBI, PTFE, polyhexafluoropropylene, FEP, and PFA are preferred.

Fluorine-based porous substrates generally consist primarily of highly hydrophobic fluorine-based polymer compounds. Therefore, by combining them with a polymer electrolyte, they impart water resistance to the electrolyte membrane, improve mechanical properties in a hydrated state, and effectively suppress dimensional change. Furthermore, fluorine-containing polymer compounds generally have low solubility in chemicals and are stable against chemical reactions, making them preferable for imparting chemical resistance and chemical durability to the electrolyte membrane. From the perspective of chemical resistance and chemical durability, PTFE, polyhexafluoropropylene, FEP, and PFA are more preferred, with PTFE being particularly preferred due to its high mechanical strength achieved through molecular orientation.

From the viewpoint of water resistance, when the total mass of the porous substrate is taken as 100 mass%, the total mass of fluorine atoms contained in the porous substrate is preferably 50 mass% or more, more preferably 60 mass% or more, and particularly preferably 70 mass% or more. The fluorine atom content in the porous substrate is a value measured by ion chromatography of a solution in which gas generated by combustion of the porous substrate has been absorbed, and specifically, can be measured by the method described in the examples below.

The oxygen atom content of the porous substrate, as measured by X-ray photoelectron spectroscopy (XPS), is preferably 10% by mass or less, more preferably 8% or less, and even more preferably 5% or less. When the oxygen atom content is 10% or less, the porous substrate has good water resistance, thereby suppressing dimensional changes due to water absorption and swelling of the electrolyte membrane. The oxygen atom content of the porous substrate can be measured specifically by the method described in the examples below.

These fluorine-based porous substrates, particularly those with a fluorine atom content of 50% by mass or more and those with an oxygen atom content of 10% by mass or less, have excellent water resistance as described above and can suppress dimensional change when the electrolyte membrane absorbs water. Therefore, by including this streaky structure in the porous substrate, water absorption and the accompanying dimensional change in the electrolyte membrane can be significantly reduced, further improving the wet-dry cycle durability of the electrolyte membrane.

When analyzing the porous substrate present in an electrolyte membrane, it is possible to extract only the porous substrate by immersing the electrolyte membrane in a solvent that dissolves only the polymer electrolyte.

Porous substrates can be embodied in various ways, including (a) stretched microporous membranes, which are made by stretching a pore-forming film in the plane direction to form micropores; (b) wet-coagulated microporous membranes, which are made by preparing a solution containing the raw materials for the porous substrate, forming a membrane, and then immersing the membrane in a poor solvent for the raw materials while still containing the solvent, and coagulating the membrane; (c) nonwoven fabrics made from solution-spun fibers spun from a raw material solution; and (d) nonwoven fabrics made from melt-spun fibers spun from the raw materials for the porous substrate.

Methods of solution spinning include dry spinning, in which high pressure is applied from a spinneret to extrude the raw material solution into fibers and then dried with hot air; wet spinning, in which the raw material solution extruded into fibers is immersed in a poor solvent for the raw material and solidified; and electrospinning, in which the raw material solution is extruded into a space where high voltage is applied and then pulled into fibers using static electricity.

An example of a melt spinning method is melt-blown spinning, in which molten porous substrate raw material is extruded in the form of fibers from a spinneret.

Among the above porous substrate forms, stretched microporous membrane (a) is preferred because it is easy to form a streaky structure.

The electrolyte membrane of the present invention is characterized in that the porous substrate has this striated structure. In order to ensure that this striated structure is present when the polymer electrolyte membrane is formed, the porous substrate used in the present invention should have structural characteristics corresponding to this striated structure before being impregnated with the polymer electrolyte.

This streaky structure can be confirmed by observing the surface of the porous substrate using a scanning electron microscope (SEM).

A porous substrate having this streaky structure can be produced, for example, by stretching a film with pore-forming properties. As a stretching method, relatively strong stretching in a uniaxial direction facilitates the formation of this streaky structure in a direction perpendicular to the stretching direction. The stretching ratio is preferably 5 to 30 times, more preferably 10 to 25 times, and particularly preferably 12 to 20 times.

Methods for producing porous substrates using pore-forming films include: (i) a dry method in which the resin material of the porous substrate and a crystal nucleating agent or fine particles are mixed using a kneader or the like, then formed into a film using an extruder or the like, and stretched to open the interface between the resin material and the crystal nucleating agent/fine particle surface, thereby forming, expanding, and connecting pores; and (ii) a wet method in which the resin material of the porous substrate and a plasticizer are mixed using a kneader or the like, then formed into a film using an extruder or the like, the plasticizer is removed to form countless fine pores, and the pores are expanded and connected by stretching. However, the wet method is preferred because it is easier to produce a film with uniform pore size and film thickness.

When using the wet method, the plasticizer is not particularly limited as long as it can be uniformly mixed with the resin material of the porous substrate without causing phase separation and can be removed by means such as evaporation or extraction. For example, hydrocarbon compounds such as liquid paraffin, naphtha, toluene, xylene, and kerosene, as well as alcohols, ketones, esters, and fluorine-based solvents can be used.

When using the wet method, after film formation, the film may be rolled using a flat press or roll press to adjust the film thickness appropriately before removing the plasticizer. It is also preferable to stretch the film while it contains plasticizer, as this improves the strength and elastic modulus of the film.

Methods for removing plasticizers in wet processes include, but are not limited to, extraction using a solvent that dissolves only the plasticizer or volatilizing only the plasticizer by heating. Extraction methods include, but are not limited to, immersing the film containing the plasticizer in the solvent or pouring the solvent over it. Drying methods include, but are not limited to, hot air drying, vacuum drying, and heating by infrared radiation. Among these, removal by drying is preferred from the standpoint of safety and cost. Hot air drying and heating by infrared radiation are more preferred because they facilitate the production of roll film by continuous conveyance. Hot air drying is particularly preferred because it is less likely to deform the film containing the plasticizer.

After producing a film with pore-forming properties in this manner, the porous substrate of the present invention can be obtained by stretching the film using the method described above.

The stretching method may be uniaxial or biaxial, but biaxial stretching is preferred because it reduces the MD and TD anisotropy of the porous substrate and the polymer electrolyte membrane composited with the porous substrate.

When biaxially stretching, for example, relatively strong stretching in one direction (e.g., TD) makes it easier to form the streaky structure in the perpendicular direction (e.g., MD). Specifically, the TD stretch ratio is preferably 5 to 30 times, more preferably 10 to 25 times, and particularly preferably 12 to 20 times. The MD stretch ratio is preferably 1.2 to 10 times, more preferably 1.5 to 5 times, and particularly preferably 1.7 to 4 times.

The porous substrate preferably has thin, fibrous fibrils that connect the streak structures in the in-plane direction to form a ladder-like structure. It is more preferable that the fibrils include those that are aligned approximately perpendicular to the streak structures. By having this structure, an electrolyte membrane can be obtained that has excellent mechanical properties, particularly in a hydrated state, not only in the in-plane direction parallel to the streak structures but also in the direction perpendicular to the streak structures.

There are no particular restrictions on the thickness of the porous substrate used in the present invention, and this should be determined depending on the application of the electrolyte membrane. However, a thickness of 0.5 μm to 50 μm is practically used, and a thickness of 1 μm to 40 μm is preferred.

The porosity of the porous substrate before it is composited with the polymer electrolyte is not particularly limited, but from the viewpoint of achieving both proton conductivity and mechanical strength in the resulting electrolyte membrane, it is preferably 50 to 98%, and more preferably 70 to 98%. The porosity Y1 (volume %) of the porous substrate is defined as the value calculated using the following formula:

Y1=(1-Db/Da)×100
(Here, Da is the specific gravity of the raw material constituting the porous substrate, and Db is the specific gravity of the entire porous substrate.)
[Preferred embodiment of composite layer]
In the composite layer, which is a layer formed by impregnating the porous substrate with a polymer electrolyte and which is a composite of the porous substrate and the polymer electrolyte, it is important that the porous substrate has this streaky structure, as this can improve the dry-wet cycle durability by suppressing swelling of the electrolyte membrane when it contains water.From this perspective, one preferred embodiment of the composite layer is, for example, an embodiment in which the porous substrate and the polymer electrolyte are made of different materials (polymer compounds).

The porous substrate and polymer electrolyte can be selected from different combinations of the aforementioned polymer compounds. Examples of such different combinations include a combination of a fluorine-based porous substrate with a hydrocarbon-based electrolyte, and a combination of a hydrocarbon-based porous substrate with a fluorine-based polymer electrolyte.

Even if the porous substrate has this striated structure, it may be destroyed when the polymer electrolyte is impregnated. Therefore, among the above combinations, a combination of a fluorine-based porous substrate and a hydrocarbon-based electrolyte is preferred, as this makes it less likely that the striated structure will be destroyed in the composite layer when the polymer electrolyte is impregnated, and it also improves the water resistance and mechanical properties of the electrolyte membrane.

Preferred fluorine-based porous substrates include those containing polytetrafluoroethylene (PTFE), polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and perfluoroalkoxy fluororesin (PFA), with PTFE-containing porous substrates being particularly preferred. Furthermore, uniaxially or biaxially stretched fluorine-based porous substrates are preferred, with biaxially stretched fluorine-based porous substrates being even more preferred.

As the hydrocarbon-based electrolyte, an aromatic hydrocarbon-based electrolyte (polymer) having an aromatic ring in the main chain is preferred, and in particular, a block copolymer having one or more segments (S _E1 ) containing an ionic group and one or more segments (S _E2 ) not containing an ionic group is preferred.

It is also preferable that the combination of the above-mentioned fluorine-based porous substrate and hydrocarbon-based electrolyte further contains a nonionic fluorine-based surfactant.

Fluorine-based porous substrates and hydrocarbon-based electrolytes do not have a high affinity, so while this is effective in preventing the streaky structure of the porous substrate from shrinking or becoming smaller during the composite layer formation process (impregnation process), it is not a desirable combination for composite formation. Using a nonionic fluorine-based surfactant for composite formation makes it easier to fill (impregnate) the fluorine-based porous substrate with the hydrocarbon-based electrolyte without gaps and with high efficiency.

It is also preferable to use a polymer electrolyte membrane having two or more composite layers. This embodiment can be obtained by stacking multiple porous substrates. In this case, the multiple composite layers may be adjacent to each other, or a single layer, as described below, may be present between the composite layers.

[Nonionic Fluorosurfactant]
The electrolyte membrane of the present invention preferably contains a nonionic fluorine-based surfactant. The use of a nonionic fluorine-based surfactant improves the affinity between the polymer electrolyte and the porous substrate, making it easier to produce the electrolyte membrane. This not only strengthens the interfacial bonding between the polymer electrolyte and the porous substrate in the electrolyte membrane, but also suppresses interfacial peeling between the two due to volume changes in the electrolyte membrane during wet-dry cycles, thereby improving the wet-dry cycle durability of the fuel cell. In particular, when a hydrocarbon-based polymer electrolyte and a fluorine-based porous substrate are used, this is more preferable because it can significantly improve the affinity between the two.

When nonionic fluorochemical surfactants are used, they have lower solubility in water and acid compared to cationic and anionic surfactants, making them less likely to dissolve in the water and acid generated during the operation of electrochemical cells such as fuel cells. This prevents leaching from the electrolyte membrane and allows the affinity between the polymer electrolyte and the porous substrate to be maintained for a long period of time.

Preferably, the nonionic fluorine-based surfactant is a compound having a fluorine-containing group consisting of a fluorinated alkyl group, a fluorinated alkenyl group, or a fluorinated aryl group, in which hydrogen atoms in an alkyl group, alkenyl group, or aryl group are substituted with fluorine atoms, and a nonionic solvent-philic group (hydrophilic group or lipophilic group).

The fluorine-containing group is preferably a perfluoroalkyl group, perfluoroalkenyl group, or perfluoroaryl group in which all hydrogen atoms in the alkyl group, alkenyl group, or aryl group have been substituted with fluorine atoms.

As the fluorine-containing group, a fluorinated alkenyl group or a fluorinated aryl group is more preferred due to their excellent surfactant effect, and a fluorinated alkenyl group is even more preferred due to their flexible structure and high surfactant activity.

The fluorine-containing group preferably has two or more carbon atoms, more preferably four or more, and particularly preferably six or more. It also preferably has 20 or fewer carbon atoms, more preferably 15 or fewer, and particularly preferably 10 or fewer. When the carbon number is within the above preferred range, the fluorine-containing group has adequate volatility and water solubility, remains in the electrolyte membrane, and exhibits excellent wet-dry cycle durability, while also being resistant to phase separation from the polymer electrolyte, resulting in excellent wet-dry cycle durability.

Specific examples of fluorinated alkyl groups include, but are not limited to, ethyl fluoride, propyl fluoride, butyl fluoride, pentyl fluoride, hexyl fluoride, heptyl fluoride, octyl fluoride, nonyl fluoride, and decyl fluoride.

Furthermore, examples of fluorinated alkenyl groups include, but are not limited to, ethenyl fluoride, propenyl fluoride, butenyl fluoride, pentenyl fluoride, hexenyl fluoride, heptenyl fluoride, octenyl fluoride, nonenyl fluoride, and decenyl fluoride.

Among these, hexyl fluoride, heptyl fluoride, octyl fluoride, nonyl fluoride, decyl fluoride, hexenyl fluoride, heptenyl fluoride, octenyl fluoride, nonenyl fluoride, and decenyl fluoride are more preferred due to their low volatility and water solubility, making them more likely to remain in the electrolyte membrane. Here, the ethyl fluoride group can be classified into five types of functional groups depending on the number of fluorine atoms contained in one functional group: monofluoroethyl, difluoroethyl, trifluoroethyl, tetrafluoroethyl, and pentafluoroethyl. In this specification, the term ethyl fluoride group is used as a general term for all of these. The same applies to other functional groups such as propyl fluoride and butyl fluoride. Furthermore, in the case of the difluoroethyl group, which is a functional group containing two fluorine atoms, there are three structural isomers: the 1,1-difluoroethyl group, the 1,2-difluoroethyl group, and the 2,2-difluoroethyl group. In this specification, the term "difluoroethyl group" is used as a general term for these. The same applies to other groups such as the "trifluoroethyl group" and "tetrafluoroethyl group."

The structure of the fluorine-containing group can be linear, branched, cyclic, etc., but a branched structure is preferred because it weakens the interaction between fluorine compounds and makes it easier to reduce surface tension. In the present invention, surfactants having a fluorine-containing group with a structure represented by the following formula (F1) are particularly preferred.

In formula (F1), * indicates the bonding point to other atomic groups.

As a nonionic fluorine-based surfactant, a compound having 10% by mass or more of fluorine atoms per molecule is preferably used. Compounds having 20% by mass or more of fluorine atoms are more preferred, and compounds having 40% by mass or more of fluorine atoms are even more preferred. When the fluorine atom content per molecule is 10% by mass or more, the affinity between the polymer electrolyte and the porous substrate is good, and an electrolyte membrane with excellent power generation performance can be obtained. In addition, due to its low solubility in water and acid, it is less likely to dissolve in water or acid generated during the operation of an electrochemical cell such as a fuel cell, preventing elution from the electrolyte membrane and maintaining the affinity between the polymer electrolyte and the porous substrate for a long period of time.

The philic group can be a hydrophilic group or a lipophilic group, but is preferably a hydrophilic group.

The hydrophilic group is not particularly limited as long as it is a nonionic group containing a hydrophilic element selected from the group consisting of oxygen, nitrogen, phosphorus, sulfur, and boron. However, groups containing polyether groups, carboxylic acid ester groups, sulfonate ester groups, phosphite ester groups, or phosphate ester groups are preferred, and groups containing polyether groups are more preferred because they form hydrogen bonds with the ionic group, thereby providing excellent affinity with polymer electrolytes and chemical stability. Among these, it is preferred that the hydrophilic group be a group having a polyalkyl ether structure shown in general formula (C1) below or a polyacrylate structure shown in general formula (C2) below. Polyalkyl ethers shown in general formula (C1) below are more preferred because of their particularly excellent affinity with polymer electrolytes.

In general formula (C1), q and r are natural numbers satisfying the equation r = 2q, and s is an integer between 1 and 1,000 representing the number of repeating alkyl ether structures. In general formula (C2), R is at least one group selected from an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 1 to 20 carbon atoms, and an aryl group having 6 to 20 carbon atoms, and t is an integer between 1 and 1,000 representing the number of repeating acrylate structures. When s or t is 2 or greater in (C1) or (C2), the multiple alkyl ether structures or acrylate structures may be the same or different.

Lipophilic groups include alkyl groups, alkenyl groups, alkynyl groups, and phenyl groups that do not contain fluorine atoms.

In the present invention, the nonionic fluorine-based surfactant is preferably a compound with a vapor pressure of less than 2 kPa at 150°C, more preferably a compound with a vapor pressure of 1 kPa or less at 150°C, and most preferably a compound with no boiling point, i.e., a compound that begins thermal decomposition without boiling. In particular, in the present invention, it is preferable that the surfactant is a compound with a 5% weight loss temperature of 150°C or higher in thermogravimetric differential thermal analysis. Such surfactants do not volatilize or decompose during membrane formation, and can therefore be left in the electrolyte membrane.

In the present invention, the nonionic fluorosurfactant is preferably a compound with a weight-average molecular weight of 1,000 or more, more preferably 1,500 or more, and even more preferably 2,000 or more. Furthermore, the nonionic fluorosurfactant is preferably a compound with a weight-average molecular weight of 10,000 or less, more preferably 8,000 or less, and even more preferably 5,000 or less. When the weight-average molecular weight of the surfactant is within the above preferred range, it has moderate volatility and is less soluble in solvents such as water. This maintains affinity between the polymer electrolyte and the porous substrate and the catalyst layer described below, making the interface between the two less susceptible to peeling and rupture, resulting in excellent durability. Meanwhile, the surfactant is less likely to aggregate alone within the electrolyte membrane to form an island-like phase-separated structure, making the interface between the surfactant and the polymer electrolyte or porous substrate less susceptible to rupture, resulting in excellent durability.

In nonionic fluorine-based surfactants, the molecular weight of the fluorine-containing group is preferably 200 or more, more preferably 400 or more, and even more preferably 1,000 or more. Furthermore, the molecular weight of the fluorine-containing group is preferably 5,000 or less, more preferably 3,000 or less, and even more preferably 2,000 or less. When the molecular weight of the fluorine-containing group is within the above preferred range, the molecular chain of the fluorine-containing group has excellent flexibility and freedom, maintaining affinity with the porous substrate and catalyst layer, and resisting peeling and rupture at the interface with the polymer electrolyte, resulting in excellent durability. Meanwhile, the surfactant is less likely to aggregate alone in the electrolyte membrane and form an island-like phase-separated structure, resulting in resisting rupture at the interface between the surfactant and the polymer electrolyte or porous substrate, resulting in excellent durability.

In the case of surfactants having a hydrophilic group as the solvent group, the molecular weight of the hydrophilic group is preferably 100 or more, more preferably 200 or more, and even more preferably 500 or more. Furthermore, the molecular weight of the hydrophilic group is preferably 4,000 or less, more preferably 2,500 or less, and even more preferably 1,500 or less. When the molecular weight of the hydrophilic group is within the above preferred range, the molecular chain of the hydrophilic group has excellent flexibility and freedom, maintaining affinity with the polymer electrolyte and resisting breakage at the interface with the porous substrate, resulting in excellent durability. Meanwhile, the appropriate water solubility makes it difficult to dissolve in water or acid generated during operation of an electrochemical cell such as a fuel cell, preventing elution from the electrolyte membrane. This maintains affinity between the polymer electrolyte and the porous substrate, resisting breakage at the interface between the polymer electrolyte and the porous substrate during dimensional changes, resulting in excellent durability.

The nonionic fluorine-based surfactant is preferably a compound that is insoluble in water, 10% sulfuric acid, or 10% aqueous sodium hydroxide solution, more preferably a compound that is insoluble in water, and most preferably a compound that is insoluble in none of these. Here, "insoluble" means that the saturated solubility at 25°C is less than 0.1% by mass. If the surfactant is insoluble in the solvent and/or solution, it becomes less soluble in water or acid generated during operation of an electrochemical cell such as a fuel cell, preventing leaching from the electrolyte membrane. This maintains the affinity between the polymer electrolyte and the porous substrate or catalyst layer, thereby preventing a decrease in durability due to peeling or rupture at the interface between the polymer electrolyte and the porous substrate or catalyst layer during dimensional changes.

Nonionic fluorine-based surfactants that can be used in the present invention include, for example, "Megafac" (registered trademark) F-251, F-253, F-281, F-430, F-477, F-551, F-552, F-553, F-554, F-555, F-556, F-557, F-558, F-559, F-560, F-561, F-562, F-563, F-564, F-565, F-566, F-567, F-568, F-569, F-570, F-571, F-572, F-573, F-574, F-575, F-576, F-577, F-578, F-579, F-580, F-581, F-582, F-583, F-584, F-585, F-586, F-587, F-588, F-589 ...9, F- F-563, F-565, F-568, F-570, F-572, F-574, F-575, F-576, R-40, R-40-LM, R-41, R-94, RS-56, RS-72-K, RS-75, RS-76-E, RS-76-NS, DS-21, F444, TF-2066, and Surflon (registered trademark) S-141 manufactured by AGC Inc. S-145, S-241, S-242, S-243, S-386, S-420, S-611, S-651, and "Ftergent" (registered trademark) 251, 208M, 212M, 215M, 250, 209F, 222F, 245F, 208G, 218GL, 240G, 212P, 220P, 228P, FTX-218, and DF manufactured by Neos Corporation. X-18, 710FL, 710FM, 710FS, 730FL, 730FM, 610FM, 683, 601AD, 601ADH2, 602A, 650AC, 681, EF-PP31N04, EF-PP31N09, EF-PP31N15, EF-PP31N22 manufactured by Mitsubishi Materials Electronic Chemicals Co., Ltd., FC-4430, FC-4432, OMNOVA manufactured by 3M Examples include PF-151N, PF-636, PF-6320, PF-656, PF-6520, PF-652-NF, and PF-3320 manufactured by SOLUTIONS, TG-9131 and "Zeffle" (registered trademark) GH-701 manufactured by Daikin Industries, Ltd., and "Fluorolink" (registered trademark) A10-P manufactured by Solvay Japan Co., Ltd.

The content of the nonionic fluorosurfactant in the electrolyte membrane is preferably 0.005 or more, more preferably 0.01 or more, expressed as a mass ratio to the total amount of polymer electrolyte contained in the electrolyte membrane. Furthermore, this mass ratio is preferably 0.20 or less, more preferably 0.10 or less. When the mass ratio is within the preferred range, the affinity between the polymer electrolyte and the porous substrate and catalyst layer is maintained, and the interfaces between the polymer electrolyte and the porous substrate and catalyst layer are less likely to peel or break during dimensional changes, resulting in excellent durability. Meanwhile, the amount of nonionic fluorosurfactant is appropriate, resulting in excellent proton conductivity of the electrolyte membrane. Note that the content of nonionic fluorosurfactant here refers to the amount of nonionic fluorosurfactant remaining in the completed electrolyte membrane, excluding the amount of nonionic fluorosurfactant that falls off during the manufacturing process.

One method for analyzing nonionic fluorosurfactants contained in electrolyte membranes is to dissolve the surfactant in a specified solvent along with the polymer electrolyte membrane. The polymer electrolyte membrane solution and the dried product from which the solvent has been removed can be subjected to infrared spectroscopy (IR), 1H nuclear magnetic resonance (NMR), 19F-NMR, MALDI-MS, and pyrolysis GC/MS analysis to analyze the chemical structure of various surfactants and calculate the surfactant content. It is also suitable to subject the solution and dried product to common purification processes such as solvent extraction and reprecipitation to extract only the nonionic fluorosurfactant before analyzing.

The solvent used in the analysis of the nonionic surfactant can be selected appropriately depending on the type of polymer electrolyte that constitutes the electrolyte membrane. For example, aprotic polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, sulfolane, 1,3-dimethyl-2-imidazolidinone, and hexamethylphosphontriamide; carbonate-based solvents such as ethylene carbonate and propylene carbonate; alkylene glycol monoalkyl ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether; alcohol-based solvents such as methanol, ethanol, propanol, and butanol; acetone; Suitable solvents include ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, etc.; ester solvents such as gamma-butyrolactone, ethyl acetate, butyl acetate, ethyl lactate, etc.; hydrocarbon solvents such as hexane and cyclohexane; aromatic hydrocarbon solvents such as benzene, toluene, and xylene; halogenated hydrocarbon solvents such as chloroform, dichloromethane, 1,2-dichloroethane, perchloroethylene, chlorobenzene, and dichlorobenzene; ether solvents such as diethyl ether, tetrahydrofuran, and 1,4-dioxane; nitrile solvents such as acetonitrile; nitrated hydrocarbon solvents such as nitromethane and nitroethane; and water. Mixed solvents containing two or more of these solvents may also be used. Here, "propanol" has a structure in which one hydrogen atom in the propane molecule is replaced with a hydroxyl group. Two structural isomers, 1-propanol and 2-propanol, exist; however, "propanol" is used herein as a general term for both. The same applies to other solvents such as "butanol" and "xylene."

If the electrolyte membrane is insoluble, different analytical methods can be used. The chemical structure and content of the surfactant can be analyzed by performing IR analysis, solid-state NMR analysis, MALDI-MS analysis, and pyrolysis GC/MS analysis on the electrolyte membrane itself, which contains a nonionic fluorosurfactant. It is also suitable to immerse the electrolyte membrane in a solvent to dissolve and extract only the nonionic fluorosurfactant before analyzing it.

The nonionic fluorosurfactant used in the present invention preferably does not contain metals as impurities. Here, metals include, but are not limited to, simple metal elements, metal ions, nonionic metal compounds, and metal oxides. When metals are not included, interactions between the hydrophilic and lipophilic groups of the nonionic fluorosurfactant and the metals are less likely to occur. This allows the surfactant to retain its functionality and maintain a high degree of freedom even within the polymer electrolyte membrane, thereby suppressing a decrease in durability due to peeling or rupture at the interface between the polymer electrolyte and the porous substrate or catalyst layer during dimensional changes.

[Single layer]
The single layer, which is a portion (layer) where the polymer electrolyte does not penetrate the porous substrate, is present adjacent to the aforementioned composite layer. The single layer is preferably formed on both sides of the porous substrate. The presence of the single layer can improve the bonding strength between the electrolyte membrane and the electrodes and suppress peeling at the interface.

Examples of polymer electrolytes used in the single layer include those similar to those used in the composite layer described above. The polymer electrolytes used in the single layer and the composite layer may be the same or different; that is, a polymer electrolyte layer may be formed using two or more types of polymer electrolytes, but it is preferable that the same polymer electrolyte be used in the single layer and the composite layer.

Furthermore, when polymer electrolyte layers are provided on both sides of the composite layer, the polymer electrolyte in one of the polymer electrolyte layers can be the same as that in the composite layer, while a different polymer electrolyte can be used in the other polymer electrolyte layer. In this case, the ion exchange capacity (IEC) of the polymer electrolyte used in the other polymer electrolyte layer can be larger or smaller than the ion exchange capacity (IEC) of the polymer electrolyte in the composite layer, but a smaller IEC is preferred.

The thickness of each polymer electrolyte layer is preferably 0.1 to 20.0 μm, more preferably 0.3 to 10.0 μm, and particularly preferably 0.5 to 5.0 μm.

[Method of manufacturing electrolyte membrane]
The electrolyte membrane of the present invention can generally be produced by impregnating a porous substrate with a polymer electrolyte from one or both sides. The polymer electrolyte solution to be impregnated preferably has a suitable fluidity and can be used as a solution or dispersion. In this case, the monolayer can be formed by impregnating the porous substrate with the polymer electrolyte so as to leave a portion of the porous substrate unimpregnated, thereby forming a composite layer and an adjacent monolayer in a single operation, or by impregnating the porous substrate with the polymer electrolyte to form a composite layer, and then applying a polymer electrolyte to the side on which the composite layer is formed, thereby forming a composite layer and an adjacent monolayer.

The method for applying the polymer electrolyte will be described in more detail below.

The composite layer can be obtained, for example, by impregnating a porous substrate with a polymer electrolyte solution and then drying it to remove the solvent contained in the polymer electrolyte solution.

The impregnation method includes the following methods.
(1) A method of immersing a porous substrate in a bath of polymer electrolyte solution and removing excess polymer electrolyte solution from the pulled-out porous substrate. (2) A method of casting the polymer electrolyte solution onto a porous substrate. (3) A method of attaching a porous substrate to a support substrate onto which the polymer electrolyte solution has been cast and impregnating the substrate. When applying the polymer electrolyte solution to the porous substrate, the polymer electrolyte solution may be cast onto one side of the porous substrate and then cast onto the other side, or cast onto both sides simultaneously.

Methods that can be used to cast and apply the polymer electrolyte solution include knife coating, direct roll coating, Mayer bar coating, gravure coating, reverse coating, air knife coating, spray coating, brush coating, dip coating, die coating, vacuum die coating, curtain coating, flow coating, spin coating, screen printing, and inkjet coating. Here, the device used to cast and apply the polymer electrolyte solution is called a coater.

After the polymer electrolyte solution is applied to the support substrate, a drying process is performed to form a composite layer. In the drying process, the coating film of the polymer electrolyte solution impregnated into the porous substrate is heated to evaporate the solvent. The heating means is not particularly limited as long as it can evaporate the solvent, but for example, heating devices such as ovens and heaters, or devices that control the temperature near the composite layer using infrared rays or hot air can be used. Heat can also be conducted to the coating film via the substrate. The heating temperature range is preferably close to the boiling point of the solvent and below the glass transition temperature of the electrolyte. It is also possible to remove the solvent without heating by simply reducing the pressure or introducing an airflow.

The drying time and temperature during the drying process can be determined experimentally as appropriate, but it is preferable to dry the film to the extent that it remains a self-supporting film even when peeled off from the support substrate. Known drying methods can be selected, such as heating the support substrate, using hot air, or using an infrared heater. Taking into account the decomposition of the polymer electrolyte and surfactant, the drying temperature is preferably 200°C or less, and more preferably 150°C or less.

The polymer electrolyte in the solution may be one in which the ionic groups form salts with alkali metal or alkaline earth metal cations. In this case, it is also preferable to include a step of exchanging the alkali metal or alkaline earth metal cations for protons after forming a film on a substrate and drying it. This step is more preferably a step of contacting the formed film with an acidic aqueous solution. Furthermore, this contact is even more preferably a step of immersing the formed film in the acidic aqueous solution. In this step, the protons in the acidic aqueous solution are replaced with cations ionically bonded to the ionic groups, and residual water-soluble impurities, residual monomers, solvents, residual salts, etc. are simultaneously removed. The acidic aqueous solution is not particularly limited, but sulfuric acid, hydrochloric acid, nitric acid, acetic acid, trifluoromethanesulfonic acid, methanesulfonic acid, phosphoric acid, citric acid, etc. are preferably used. The temperature and concentration of the acidic aqueous solution should also be determined appropriately. From the perspective of productivity, however, it is preferable to use a sulfuric acid aqueous solution with a concentration of 3% by mass or more and 30% by mass or less at a temperature of 0°C to 80°C.

In the composite layer manufacturing process (impregnation process), the concentration of the polymer electrolyte solution is preferably 3 to 40% by mass, and more preferably 5 to 25% by mass. A concentration within this range can reduce the number of coating cycles required to sufficiently fill the voids in the porous substrate with polymer electrolyte to around 1 or 2 times, and also makes it easier to obtain a composite layer with excellent surface smoothness.

The viscosity of the polymer electrolyte solution is preferably 100 to 50,000 mPa·s, and more preferably 300 to 10,000 mPa·s. If the viscosity is less than 100 mPa·s, the thickness of the composite layer may become non-uniform. If the viscosity is within the above preferred range, the polymer electrolyte can easily fill the voids in the porous substrate, it is easy to achieve a uniform thickness for the composite layer, and the surface smoothness of the composite layer is excellent.

The solvent used in the polymer electrolyte solution can be selected appropriately depending on the type of polymer. Examples of solvents include aprotic polar solvents such as N,N-dimethylacetamide, N,N-dimethylformamide, N-methyl-2-pyrrolidone, dimethyl sulfoxide, sulfolane, 1,3-dimethyl-2-imidazolidinone, and hexamethylphosphontriamide; carbonate solvents such as ethylene carbonate and propylene carbonate; alkylene glycol monoalkyl ethers such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, and propylene glycol monoethyl ether; and alcohol solvents such as methanol, ethanol, propanol, and butanol. Suitable solvents include ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone; ester solvents such as gamma-butyrolactone, ethyl acetate, butyl acetate, and ethyl lactate; hydrocarbon solvents such as hexane and cyclohexane; aromatic hydrocarbon solvents such as benzene, toluene, and xylene; halogenated hydrocarbon solvents such as chloroform, dichloromethane, 1,2-dichloroethane, perchloroethylene, chlorobenzene, and dichlorobenzene; ether solvents such as diethyl ether, tetrahydrofuran, and 1,4-dioxane; nitrile solvents such as acetonitrile; nitrated hydrocarbon solvents such as nitromethane and nitroethane; and water. Mixed solvents of two or more of these solvents may also be used.

In the dipping and casting methods, the thickness of the single layer can be adjusted by the amount of polymer electrolyte solution removed or applied.

The single layer can also be produced by separately preparing a polymer electrolyte membrane and then laminating it onto the surface on which the composite layer is formed.

[Catalyst layer]
The catalyst layer is a layer containing catalyst particles formed on both sides of the electrolyte membrane, and is generally a layer containing catalyst particles and a polymer electrolyte made of a fluorine-based polymer electrolyte. As the fluorine-based polymer electrolyte, the above-mentioned ones can be used.

In the catalyst-layered electrolyte membrane of the present invention, it is preferable that the catalyst layer contain a perfluoropolymer having ionic groups, from the viewpoints of power generation performance and chemical durability. By using a perfluoropolymer having ionic groups, the acidity of the ionic groups contained in the catalyst layer is increased, further improving proton conductivity, and the large number of chemically stable C-F bonds further improves the chemical durability of the catalyst layer.

Catalyst-supported carbon particles, in which a catalytic substance is supported on carbon particles, are commonly used as catalyst particles. Examples of catalytic substances include not only platinum, but also platinum group elements such as palladium, ruthenium, iridium, rhodium, and osmium, as well as metals such as iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, as well as alloys, oxides, and double oxides of these metals. There are no particular limitations on the type of carbon particles, as long as they are fine particles, electrically conductive, and do not corrode or deteriorate when reacted with the catalyst. Examples of usable carbon particles include carbon black, graphite, activated carbon, carbon fiber, carbon nanotubes, and fullerene particles.

The mass ratio of the polymer electrolyte content to the catalyst particle content in the catalyst layer is preferably in the range of 0.2 to 2.5, and particularly preferably 0.5 to 2.0. When the mass ratio of the polymer electrolyte content to the catalyst particle content is in the above preferred range, the catalyst layer is less likely to crack, while gas diffusion properties are not impaired, resulting in excellent power generation performance.

The thickness of the catalyst layer is preferably 0.03 μm to 2,000 μm. To facilitate good power generation performance and durability, a catalyst layer thickness of 0.5 μm or more is more preferable, and a thickness of 1 to 30 μm is particularly preferable.

[Method of manufacturing a catalyst-layered electrolyte membrane]
The catalyst coated electrolyte membrane (CCM) of the present invention comprises an electrolyte membrane of the present invention and a catalyst layer formed on both sides thereof. The method for forming the catalyst layer is not particularly limited, but a method of applying a catalyst layer ink and drying it, or a method of transferring a catalyst layer using a catalyst layer decal in which the catalyst layer has been formed in advance on a decal base material and then drying it is preferred, as these methods are simple and can reduce process costs.

When applying catalyst layer ink, there are no particular limitations on the application method as long as it can be applied in the desired shape, and the methods described above for the application process of the polymer electrolyte solution can be used.

The solvent contained in the catalyst layer ink is not particularly limited as long as it disperses the polymer electrolyte and catalyst-supported carbon particles, but a solvent that is easily removed by evaporation when heated is preferred. For example, a solvent with a boiling point of 140°C or less is preferred. Specific examples of solvents that can be used for the catalyst layer ink include water; alcohols such as methanol, ethanol, propanol, butanol, and pentanol; ketones such as acetone, methyl ethyl ketone, pentanone, hexanone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, and diisobutyl ketone; ethers such as tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, and dibutyl ether; esters such as methyl acetate, ethyl acetate, n-propyl acetate, isopropyl acetate, butyl acetate, methyl lactate, ethyl lactate, and butyl lactate; and mixtures of two or more of the following: dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol.

In the transfer method using a catalyst layer decal, a catalyst layer decal is first prepared by applying catalyst layer ink to a substrate and, if necessary, drying the decal. The electrolyte membrane is then sandwiched between the catalyst layer decal on the cathode electrode side and the catalyst layer decal on the anode electrode side, and hot-pressed so that the catalyst layer surfaces of both decals are in contact with the electrolyte membrane, resulting in a catalyst-layered electrolyte membrane. The hot-pressing temperature and pressure can be selected appropriately based on the thickness and moisture content of the electrolyte membrane, the catalyst layer, and the decal substrate. From the perspectives of industrial productivity and suppressing thermal decomposition of the electrolyte membrane material, hot-pressing is preferably performed at a temperature between 0°C and 250°C, and more preferably above the glass transition temperature of the polymer electrolyte contained in the catalyst layer but below 200°C. The pressure applied during hot-pressing should be as low as possible to protect the electrolyte membrane and electrodes. In the case of flat-plate pressing, a pressure of 10 MPa or less is preferred.

The decal substrate used when applying the catalyst layer ink can be a resin film or substrate similar to the substrate used when forming the electrolyte membrane, or a fluororesin such as PTFE, polyhexafluoropropylene, ETFE, ethylene-hexafluoropropylene copolymer, FEP, PFA, or PVdF. It is preferable to use a fluororesin film because of its heat resistance, solvent resistance, chemical stability, and releasability.

The catalyst layer can be dried using the same method as described above for drying the polymer electrolyte solution.

[Application]
The electrolyte membrane of the present invention can be used in a variety of applications. For example, it can be used in medical applications such as artificial skin, filtration applications, ion exchange resin applications such as chlorine-resistant reverse osmosis membranes, various structural materials, electrochemical applications, humidifying membranes, anti-fogging membranes, antistatic membranes, oxygen-removing membranes, solar cell membranes, and gas barrier membranes. Among these, it is particularly suitable for various electrochemical applications. Examples of electrochemical applications include solid polymer fuel cells, redox flow batteries, water electrolysis devices, chloroalkali electrolysis devices, electrochemical hydrogen pumps, and water electrolysis hydrogen generation devices.

In solid polymer fuel cells, electrochemical hydrogen pumps, and water electrolysis hydrogen generators, electrolyte membranes are used with catalyst layers, electrode substrates, and separators laminated in that order on both sides. In particular, an electrolyte membrane with catalyst layers and gas diffusion substrates laminated in that order on both sides (i.e., a membrane electrode assembly with a layer structure of gas diffusion substrate/catalyst layer/electrolyte membrane/catalyst layer/gas diffusion substrate) is called a membrane electrode assembly (MEA). The electrolyte membrane of the present invention is suitable for use as the electrolyte membrane that constitutes the CCM and MEA.

The membrane electrode assembly of the present invention and the polymer electrolyte fuel cell of the present invention use the electrolyte membrane of the present invention.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these. The various measurement conditions are as follows:

(1) Molecular Weight of Polymer The number average molecular weight and weight average molecular weight of the polymer solution were measured by GPC. An HLC-8022 GPC manufactured by Tosoh Corporation was used as an integrated device combining an ultraviolet detector and a differential refractometer. Two TSK gel Super HM-H GPC columns (inner diameter 6.0 mm, length 15 cm) manufactured by Tosoh Corporation were used. Measurements were performed in N-methyl-2-pyrrolidone solvent (N-methyl-2-pyrrolidone solvent containing 10 mmol/L of lithium bromide) at a flow rate of 0.2 mL/min, and the number average molecular weight and weight average molecular weight were calculated in terms of standard polystyrene.

(2) Ion exchange capacity (IEC)
Measurement was carried out by neutralization titration. The measurement was carried out three times and the average value was taken.

After proton exchange and thorough washing with pure water, the electrolyte membrane was wiped to remove moisture from its surface, and then vacuum dried at 100°C for at least 12 hours, after which the dry weight was determined.

50 mL of a 5% by mass aqueous solution of sodium sulfate was added to the electrolyte membrane and left to stand for 12 hours to allow for ion exchange.

The resulting sulfuric acid was titrated using 0.01 mol/L aqueous sodium hydroxide solution. 0.1 w/v% commercially available phenolphthalein solution for titration was added as an indicator, and the point at which the color turned pale reddish-purple was defined as the endpoint.

The ion exchange capacity (IEC) was calculated using the following formula:

IEC (meq/g) = [concentration of aqueous sodium hydroxide solution (mmol/mL) × amount dropped (mL)] / dry weight of sample (g)
(3) Cross-sectional SEM Measurement of Electrolyte Membrane Cross-sectional SEM measurement was performed under the following conditions: From the obtained image, the central white area was regarded as the composite layer, and the adjacent black areas were regarded as the polymer electrolyte layers, and their thicknesses were measured.

Apparatus: Field emission scanning electron microscope (FE-SEM) S-4800 (Hitachi High-Technologies Corporation)
Acceleration voltage: 2.0 kV
Pretreatment: A cross-sectional sample prepared by the BIB method was coated with Pt and then measured.

BIB method: A cross-sectional sample preparation device using an argon ion beam.

A shielding plate is placed directly above the sample, and a broad argon ion beam is irradiated from above to etch the sample, creating an observation and analysis surface (cross section).

(4) Filling rate of polymer electrolyte in composite layer (filling rate of composite layer)
According to the above (3), the cross section of the electrolyte membrane was observed using an SEM, and the thickness T1 of the composite layer consisting of the polymer electrolyte and the porous substrate, and if there was a polymer electrolyte layer on the outside of the composite layer, their thicknesses T2 and T3 were calculated (if a single layer exists on only one side, T2 is its thickness and T3 is 0). The specific gravity of the polymer electrolyte forming the composite layer was D1, the specific gravity of the polymer electrolyte forming the polymer electrolyte layer was D2 and D3 (D2 is the specific gravity of the single layer that gives the thickness T2, and D3 is the specific gravity of the single layer that gives the thickness T3), and the specific gravity of the electrolyte membrane was D. The IEC of the polymers forming each layer was I1, I2, and I3 (I1 is the IEC of the composite layer, I2 is the IEC of the single layer that gives the thickness T2, and I3 is the IEC of the single layer that gives the thickness T3), and the IEC of the electrolyte membrane was I. The filling rate Y2 (volume%) of the polymer electrolyte in the composite layer was calculated using the following formula.

Y2=[(T1+T2+T3)×D×I−(T2×D2×I2+T3×D3×I3)]/(T1×D1×I1)×100.

(5) Observation of phase separation structure by transmission electron microscope (TEM) tomography A sample piece of the electrolyte membrane was immersed in a 2% by mass aqueous solution of lead acetate as a staining agent and left to stand at 25°C for 48 hours for staining treatment. The stained sample was removed, embedded in epoxy resin, and fixed by irradiating with visible light for 30 seconds. A 100 nm thin section was cut using an ultramicrotome at room temperature and observed under the following conditions.

Equipment: Field emission electron microscope (HRTEM) JEOL Ltd. JEM2100F
Image acquisition: Digital Micrograph
System: Marker method Accelerating voltage: 200 kV
Magnification: 30,000x Tilt angle: +61° to -62°
Reconstruction resolution: 0.71 nm/pixel
The marker method was used for the 3D reconstruction process. Gold colloid particles attached to a collodion film were used as alignment markers for the 3D reconstruction. A total of 124 TEM images were acquired from a series of tilted images, in which the sample was tilted in 1° increments from +61° to -62° relative to the markers, and the 3D phase-separated structure was observed.

(6) Measurement of dimensional change rate (λ _xy ) by hot water test An electrolyte membrane was cut into a square of approximately 5 cm x approximately 5 cm, and left to stand for 24 hours in a temperature- and humidity-controlled atmosphere at a temperature of 23°C ± 5°C and a humidity of 50 ± 5% RH. The MD length and TD length (MD1, TD1, respectively) were measured with a vernier caliper. The electrolyte membrane was immersed in hot water at 80°C for 8 hours, and the MD length and TD length (MD2, TD2, respectively) were measured again with a vernier caliper. The dimensional change rates of the MD and TD in the in-plane direction (λ _MD and λ _TD ) and the dimensional change rate in the in-plane direction (λ _xy ) (%) were calculated using the following formula.

λ _MD = (MD2-MD1)/MD1×100
λ _TD = (TD2-TD1)/TD1×100
λ _xy = (λ _MD + λ _TD )/2.

(7) Proton Conductivity The electrolyte membrane was immersed in pure water at 25°C for 24 hours, then held in a thermo-hygrostat at 80°C and 25% RH for 30 minutes, and the proton conductivity was measured by a constant-potential AC impedance method. A Solartron electrochemical measurement system (Solartron 1287 Electrochemical Interface and Solartron 1255B Frequency Response Analyzer) was used as the measurement device, and constant-potential impedance measurements were performed using the two-terminal method to determine the proton conductivity. The AC amplitude was 50 mV. A membrane measuring 10 mm in width and 50 mm in length was used as the sample. The measurement jig was made of phenolic resin, and the measurement area was left open. Platinum plates (100 μm thick, two sheets) were used as electrodes. The electrodes were arranged at an inter-electrode distance of 10 mm on the front and back sides of the sample membrane, parallel to each other and perpendicular to the longitudinal direction of the sample membrane.

(8) Preparation of Membrane Electrode Assembly (MEA) Using Electrolyte Membrane A catalyst ink prepared by mixing platinum catalyst-supported carbon particles TEC10E50E (platinum support rate: 50 mass%) manufactured by Tanaka Kikinzoku Kogyo K.K. and "Nafion" (registered trademark) manufactured by Chemours Corporation in a mass ratio of 2:1 was applied to a commercially available polytetrafluoroethylene film so that the platinum amount was 0.3 mg/ ^cm2 , to prepare a catalyst decal.

Two of the above catalyst decals were cut into 5 cm squares and stacked facing each other with the electrolyte membrane sandwiched between them. They were then hot-pressed at 150°C and 5 MPa for 3 minutes to obtain a catalyst-coated electrolyte membrane. A commercially available gas diffusion electrode 24BCH manufactured by SGL was cut into a 5 cm square and sandwiched between the resulting catalyst-coated electrolyte membrane to produce an MEA, with one side serving as the cathode and the other as the anode.

(9) Wet-dry cycle durability The MEA prepared in (8) above was set in a JARI standard cell "Ex-1" (electrode area 25 cm ² ) manufactured by Eiwa Co., Ltd., and a cycle was repeated in which nitrogen at 160% RH was supplied to both electrodes for 2 minutes at a cell temperature of 80°C, and then nitrogen at 0% RH (dew point -20°C or lower) was supplied to both electrodes for 2 minutes. The amount of hydrogen permeation was measured every 1,000 cycles, and the point at which the hydrogen permeation current exceeded 10 times the initial current was taken as the wet-dry cycle durability.

To measure hydrogen permeation, hydrogen was supplied as fuel gas to one electrode and nitrogen to the other, and the test was conducted under humidification conditions: hydrogen gas 90% RH, nitrogen gas 90% RH. The open circuit voltage was maintained at 0.2 V or less, and the voltage was swept from 0.2 to 0.7 V at 1 mV/sec, and the current value at 0.7 V was taken as the hydrogen permeation current.

(10) Measurement of fluorine atom content contained in porous substrate According to the following conditions, a porous substrate sample was weighed and burned in a combustion tube of an analyzer, and the generated gas was absorbed into a solution, and then a portion of the absorbed solution was analyzed by ion chromatography.
<Combustion and absorption conditions>
System: AQF-2100H, GA-210 (Mitsubishi Chemical Corporation)
Electric furnace temperature: Inlet 900℃, Outlet 1000℃
Gas: Ar/O ₂ 200 mL/min, O ₂ 400 mL/min
Absorption solution: H ₂ O ₂ 0.1%, internal standard Br 8 μg/mL
Amount of absorbed liquid: 20 mL
<Ion chromatography/anion analysis conditions>
System: ICS1600 (DIONEX)
Mobile phase: 2.7 mmol/L Na ₂ CO ₃ /0.3 mmol/L NaHCO ₃
Flow rate: 1.50mL/min
Detector: electrical conductivity detector Injection volume: 20 μL.

(11) Chemical Structure Analysis of Surfactants The chemical structures of various surfactants were analyzed by infrared spectroscopy (IR), ^1H nuclear magnetic resonance (NMR), ^19F -NMR, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and pyrolysis gas chromatography mass spectrometry (GC/MS), and the contents of fluorine atoms and hydrophilic elements (total of oxygen, nitrogen, phosphorus, sulfur, and boron) were calculated.

(12) Measurement of Weight-Average Molecular Weight of Surfactant The weight-average molecular weight of the surfactant was measured by gel permeation chromatography (GPC) analysis under the following conditions.

Apparatus: Gel permeation chromatography GPC (instrument No. GPC-27)
Detector: UV-visible absorption spectroscopic detector UV (Shimadzu Corporation SPD-20AV)
Column: TSKgel Super HZM-N (2 columns), Super HZ4000, 2500, 1000 (1 column each) Solvent: Tetrahydrofuran (THF)
Flow rate: 0.45mL/min
Column temperature: 40°C
Injection volume: 0.02mL
Standard samples: Tosoh Corporation and Agilent monodisperse polyethylene glycol (PEG)
Data processing: GPC data processing system manufactured by Toray Research Center, Inc. (13) Amount of surfactant contained in electrolyte membrane According to the following conditions, the electrolyte membrane was weighed and burned in the combustion tube of the analyzer, the generated gas was absorbed in a solution, and a portion of the absorbed solution was analyzed by ion chromatography. The contribution of the surfactant was calculated by excluding the contribution of the surfactant-free polymer electrolyte measured in advance from this analysis value and the contribution of the porous substrate measured in advance by (10). The amount of surfactant contained in the composite electrolyte membrane was calculated from the amount of fluorine atoms contained in the surfactant, and the ratio of surfactant to polymer electrolyte contained in the composite membrane (surfactant/polymer electrolyte) was determined.

<Combustion and absorption conditions>
System: AQF-2100H, GA-210 (Mitsubishi Chemical Corporation)
Electric furnace temperature: Inlet 900℃, Outlet 1000℃
Gas: Ar/O ₂ 200 mL/min, O ₂ 400 mL/min
Absorption solution: H ₂ O ₂ 0.1%, internal standard Br 8 μg/mL
Amount of absorbed liquid: 20 mL
<Ion chromatography/anion analysis conditions>
System: ICS1600 (DIONEX)
Mobile phase: 2.7 mmol/L Na ₂ CO ₃ /0.3 mmol/L NaHCO ₃
Flow rate: 1.50mL/min
Detector: electrical conductivity detector Injection volume: 20 μL.

(14) Measurement of oxygen content of porous substrate by XPS A porous substrate previously cut into a size of 5 mm square was rinsed with ultrapure water, dried at room temperature and 67 Pa for 10 hours, cooled with liquid nitrogen for 30 minutes, and treated twice in a freeze-pulverizer for 5 minutes to prepare a sample. The composition of the prepared sample was measured, and the oxygen atom content was calculated. The measurement device and conditions were as follows.

Measuring device: Quantera SXM
Excitation X-ray: monochromatic Al Kα ₁ , Kα ₂ ray (1,486.6 eV)
X-ray diameter: 200 μm
Photoelectron escape angle: 45°.

(15) SEM Observation of Porous Substrate The porous substrate to be used as a specimen was cut into a size of 2 mm x 2 mm, and after platinum ion sputtering, SEM observation was performed to observe the morphology of the porous substrate in the plane direction. The orientation of the bundles and fibrils contained in the porous substrate was determined based on the structure observed during SEM observation in an arbitrary 15 μm x 15 μm range. The average length and average diameter of the bundles and the average diameter of the fibrils were calculated by measuring the length and diameter of the bundles and the diameter of the fibrils at 100 points in an arbitrary 15 μm x 15 μm range and calculating the average value. Ion sputtering and SEM observation were performed using the following equipment and conditions.
<Ion sputtering>
Sputtering device: Ion sputter E-1045 (manufactured by Hitachi High-Technologies Corporation)
Metal type: Pt
Ion emission current: 15mA
Sputtering time: 60 seconds <SEM observation>
Measuring device: Scanning electron microscope S-5500 (Hitachi High-Technologies Corporation)
Acceleration voltage: 5 kV
Observation magnification: 2,000 to 20,000 times.

(16) Digital Microscope Observation of Electrolyte Membrane The specimen electrolyte membrane was cut to a size of 10 cm x 10 cm, and digital microscope observation was performed from the surface on which the polymer electrolyte layer was provided. The light reflection image was taken to observe the streak-like structures derived from the porous substrate. In an image in which a light-colored portion derived from the polymer electrolyte and a dark-colored portion derived from the porous substrate were observed, the boundary where the color changed was used as the boundary between the polymer electrolyte and the porous substrate, and the width of the portion derived from the porous substrate was measured. The number of streak-like structures having a width of 0.5 μm or more per 1 mm ² was calculated by counting the number of streak-like structures having a width of 0.5 μm or more in an arbitrary 120 μm x 150 μm range and converting it to the number per mm ^2. For streak-like structures whose width varies depending on the position, the width was measured at any five positions, and the arithmetic average was calculated to determine the width of each streak-like structure. The orientation of the streak-like structure was determined based on the structure observed under a digital microscope in an arbitrary 120 μm x 150 μm range. The average length and average width (diameter) of the streaky structures were calculated by measuring 100 points each in an arbitrary 120 μm × 150 μm area and calculating the arithmetic mean. Digital microscope observation was performed using the following equipment and conditions.
Measuring device: KH-1300 (manufactured by Hirox Co., Ltd.)
Lens used: MXG-2500REZ High-Range (Hirox Co., Ltd.)
Observation magnification: 2000x (17) Measurement of Haze Value and Diffuse Transmittance of Electrolyte Membrane A sample electrolyte membrane was cut into a size of 5 cm x 5 cm, and a haze meter HGM-20P (manufactured by Suga Test Instruments Co., Ltd.) was used to measure the total luminous transmittance (Tt) (%) and diffuse transmittance (Td) (%) by irradiating the surface on which the polymer electrolyte layer was formed with light from a C light source and using an integrating sphere on the detector side. The haze value Hz was calculated using the following formula. The total luminous transmittance is known as the ratio of the amount of all light captured by the integrating sphere to the amount of incident light, and the diffuse transmittance is known as the ratio of the amount of diffused light (transmitted light other than transmitted light parallel to the incident light) captured by the integrating sphere to the amount of incident light.
Hz=Td/Tt×100(%)
[Synthesis Example 1]
(Synthesis of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane (K-DHBP) represented by the following chemical formula (G1))
A 500 mL flask equipped with a stirrer, thermometer, and distillation tube was charged with 49.5 g of 4,4'-dihydroxybenzophenone, 134 g of ethylene glycol, 96.9 g of trimethyl orthoformate, and 0.50 g of p-toluenesulfonic acid monohydrate, and dissolved. The mixture was then stirred and kept at 78-82°C for 2 hours. The internal temperature was gradually raised to 120°C, and the mixture was heated until the distillation of methyl formate, methanol, and trimethyl orthoformate completely stopped. After cooling the reaction mixture to room temperature, the reaction mixture was diluted with ethyl acetate, and the organic layer was washed with 100 mL of a 5% aqueous potassium carbonate solution and separated. The solvent was then distilled off. 80 mL of dichloromethane was added to the residue to precipitate crystals, which were then filtered and dried to obtain 52.0 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane represented by the following chemical formula (G1). GC analysis of the crystals revealed that they were 99.9% 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane and 0.1% 4,4'-dihydroxybenzophenone.

[Synthesis Example 2]
(Synthesis of disodium-3,3'-disulfonate-4,4'-difluorobenzophenone represented by the following chemical formula (G2))
109.1 g of 4,4'-difluorobenzophenone (Sigma-Aldrich Japan (same) reagent) was reacted in 150 mL of fuming sulfuric acid (50% SO ₃ ) (Fujifilm Wako Pure Chemical Industries, Ltd. reagent) at 100°C for 10 hours. Thereafter, the mixture was poured little by little into a large amount of water, neutralized with NaOH, and then 200 g of table salt (NaCl) was added to precipitate a synthesized product. The resulting precipitate was filtered and recrystallized from an aqueous ethanol solution to obtain disodium-3,3'-disulfonate-4,4'-difluorobenzophenone represented by the following chemical formula (G2). The purity was 99.3%.

[Synthesis Example 3]
(Synthesis of oligomer a1 not containing an ionic group represented by the following general formula (G3))
A 1,000 mL three-neck flask equipped with a stirrer, nitrogen inlet tube, and Dean-Stark trap was charged with 16.59 g of potassium carbonate (Sigma-Aldrich Japan (same manufacturer) reagent, 120 mmol), 25.8 g (100 mmol) of K-DHBP obtained in Synthesis Example 1, and 20.3 g of 4,4'-difluorobenzophenone (Sigma-Aldrich Japan (same manufacturer) reagent, 93 mmol). After nitrogen substitution, 300 mL of N-methylpyrrolidone (NMP) and 100 mL of toluene were added, and the mixture was dehydrated at 160°C. The temperature was then raised to remove the toluene, and polymerization was carried out at 180°C for 1 hour. Purification by reprecipitation into a large amount of methanol yielded an oligomer (terminal: hydroxyl group) containing no ionic groups. The number average molecular weight was 10,000.

1.1 g of potassium carbonate (Sigma-Aldrich Japan (same) reagent, 8 mmol) and 20.0 g (2 mmol) of the above oligomer (terminal: hydroxyl group) without ionic groups were placed in a 500 mL three-neck flask equipped with a stirrer, nitrogen inlet tube, and Dean-Stark trap. After replacing the atmosphere with nitrogen, 100 mL of NMP and 30 mL of toluene were added. After dehydration at 100°C, the mixture was heated to remove the toluene, and 4.0 g of decafluorobiphenyl (Sigma-Aldrich Japan (same) reagent, 12 mmol) was added and reacted at 105°C for 1 hour. Purification was carried out by reprecipitation with a large amount of isopropyl alcohol to obtain oligomer a1 (terminal: fluoro group) without ionic groups, represented by the following general formula (G3). The number average molecular weight was 11,000.

[Synthesis Example 4]
(Synthesis of oligomer a2 containing an ionic group represented by the following general formula (G4))
A 1,000 mL three-neck flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap was charged with 27.6 g of potassium carbonate (Sigma-Aldrich Japan (same manufacturer) reagent, 200 mmol), 12.9 g (50 mmol) of K-DHBP and 9.3 g of 4,4'-biphenol (Sigma-Aldrich Japan (same manufacturer) reagent, 50 mmol) obtained in Synthesis Example 1, 39.3 g (93 mmol) of disodium-3,3'-disulfonate-4,4'-difluorobenzophenone obtained in Synthesis Example 2, and 17.9 g of 18-crown-6 (Fujifilm Wako Pure Chemical Industries, Ltd. reagent, 82 mmol), and after nitrogen substitution, 300 mL of NMP and 100 mL of toluene were added. After dehydration at 170°C, the mixture was heated to remove toluene, and polymerization was carried out at 180°C for 1 hour. The resulting product was purified by reprecipitation with a large amount of isopropyl alcohol to obtain an ionic group-containing oligomer a2 (terminal: hydroxyl group) represented by the following general formula (G4). The number average molecular weight was 16,000.

(In general formula (G4), M represents H, Na, or K.)
[Synthesis Example 5]
(Synthesis of neopentyl 3-(2,5-dichlorobenzoyl)benzenesulfonate represented by the following chemical formula (G5))
245 g (2.1 mol) of chlorosulfonic acid was placed in a 3 L three-neck flask equipped with a stirrer and a condenser, followed by the addition of 105 g (420 mmol) of 2,5-dichlorobenzophenone, and the reaction was carried out for 8 hours in an oil bath at 100°C. After the prescribed time, the reaction solution was slowly poured into 1,000 g of crushed ice and extracted with ethyl acetate. The organic layer was washed with brine and dried over magnesium sulfate, after which the ethyl acetate was distilled off to obtain pale yellow crude crystals of 3-(2,5-dichlorobenzoyl)benzenesulfonic acid chloride. The crude crystals were used directly in the next step without purification.

41.1 g (462 mmol) of 2,2-dimethyl-1-propanol (neopentyl alcohol) was added to 300 mL of pyridine and cooled to approximately 10°C. The crude crystals obtained above were gradually added over approximately 30 minutes. After the entire amount was added, the mixture was stirred for an additional 30 minutes to allow the reaction to proceed. After the reaction, the reaction solution was poured into 1,000 mL of aqueous hydrochloric acid, and the precipitated solid was collected. The resulting solid was dissolved in ethyl acetate, washed with aqueous sodium bicarbonate and brine, dried over magnesium sulfate, and the ethyl acetate was distilled off to obtain crude crystals. These were recrystallized from methanol to obtain white crystals of neopentyl 3-(2,5-dichlorobenzoyl)benzenesulfonate, represented by the following chemical formula (G5).

[Synthesis Example 6]
(Synthesis of an oligomer not containing an ionic group represented by the following general formula (G6))
Into a 1 L three-necked flask equipped with a stirrer, thermometer, condenser, Dean-Stark tube, and a three-way stopcock for nitrogen introduction, 49.4 g (0.29 mol) of 2,6-dichlorobenzonitrile, 88.4 g (0.26 mol) of 2,2-bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluoropropane, and 47.3 g (0.34 mol) of potassium carbonate were weighed. After nitrogen substitution, 346 mL of sulfolane and 173 mL of toluene were added and stirred. The flask was placed in an oil bath and heated to reflux at 150 °C. The water produced by the reaction was azeotroped with toluene and removed from the system using a Dean-Stark tube. After approximately 3 hours, water production was barely noticeable. The reaction temperature was gradually increased to remove most of the toluene, and the reaction was continued at 200 °C for 3 hours. Next, 12.3 g (0.072 mol) of 2,6-dichlorobenzonitrile was added, and the mixture was further reacted for 5 hours.

After allowing the resulting reaction liquid to cool, it was diluted with 100 mL of toluene. The by-product inorganic compound precipitate was removed by filtration, and the filtrate was poured into 2 L of methanol. The precipitated product was filtered, collected, dried, and then dissolved in 250 mL of tetrahydrofuran. This was reprecipitated in 2 L of methanol, yielding 107 g of the target oligomer represented by the following general formula (G6). The number average molecular weight was 11,000.

[Synthesis Example 7]
(Synthesis of polyethersulfone (PES)-based block copolymer precursor b4′ consisting of a segment represented by the following chemical formula (G8) and a segment represented by the following general formula (G9))
1.62 g of anhydrous nickel chloride and 15 mL of dimethyl sulfoxide were mixed and adjusted to 70° C. 2.15 g of 2,2′-bipyridyl was added thereto, and the mixture was stirred at the same temperature for 10 minutes to prepare a nickel-containing solution.

1.49 g of 2,5-dichlorobenzenesulfonic acid (2,2-dimethylpropyl) and 0.50 g of Sumikaexcel PES5200P (Sumitomo Chemical Co., Ltd., Mn = 40,000, Mw = 94,000), represented by the following general formula (G7), were dissolved in 5 mL of dimethyl sulfoxide to obtain a solution, to which 1.23 g of zinc powder was added and the temperature was adjusted to 70°C. The nickel-containing solution was poured into the solution, and a polymerization reaction was carried out at 70°C for 4 hours. The reaction mixture was added to 60 mL of methanol, followed by the addition of 60 mL of 6 mol/L hydrochloric acid and stirring for 1 hour. The precipitated solid was separated by filtration and dried to obtain 1.62 g of a grayish-white block copolymer precursor b4' containing segments represented by the following general formula (G8) and the following general formula (G9), in a yield of 99%. The weight-average molecular weight was 231,000.

[Polymer electrolyte solution A] A polymer electrolyte solution consisting of a block copolymer b1 obtained by copolymerizing an oligomer represented by the general formula (G4) as a segment containing an ionic group and an oligomer represented by the general formula (G3) as a segment not containing an ionic group. A 500 mL three-necked flask equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap was charged with 0.56 g of potassium carbonate (Sigma-Aldrich Japan (same) reagent, 4 mmol), 16 g (1 mmol) of ionic group-containing oligomer a2 (terminal: hydroxyl group) obtained in Synthesis Example 4, and after nitrogen substitution, 100 mL of N-methylpyrrolidone (NMP) and 30 mL of cyclohexane were added, and the mixture was dehydrated at 100 ° C., heated to remove cyclohexane, and 11 g (1 mmol) of ionic group-free oligomer a1 (terminal: fluoro group) obtained in Synthesis Example 3 was added, and the reaction was carried out at 105 ° C. for 24 hours. Purification by reprecipitation into a large amount of isopropyl alcohol gave block copolymer b1. The weight average molecular weight was 343,000. The ion exchange capacity (IEC) of this block copolymer b1 was 2.1.

The resulting 5% by mass N-methylpyrrolidone (NMP) solution containing block copolymer b1 was directly centrifuged at 25°C for 30 minutes at a centrifugal force of 20,000 G using an inverter compact high-speed refrigerated centrifuge (model 6930) manufactured by Kubota Manufacturing Co., Ltd., equipped with an angle rotor RA-800. The precipitated solids (cake) and the supernatant liquid (coating liquid) were clearly separated, and the supernatant liquid was recovered. The mixture was then distilled under reduced pressure at 80°C while stirring, and pressure filtered using a 1 μm polypropylene filter to obtain polymer electrolyte solution A (polymer electrolyte concentration: 13% by mass). The viscosity of polymer electrolyte solution A was 1,300 mPa·s.

[Polymer electrolyte solution B] A polymer electrolyte solution comprising block copolymer b2 obtained by copolymerizing an oligomer represented by general formula (G4) as a segment containing an ionic group and an oligomer represented by general formula (G3) as a segment not containing an ionic group. Block copolymer b2 was synthesized in the same manner as block copolymer b1, except that 20 g (1.25 mmol) of oligomer a2 having an ionic group obtained in Synthesis Example 4 was used. The weight-average molecular weight of this block copolymer b2 was 372,000, and the ion exchange capacity (IEC) was 2.4. The viscosity of polymer electrolyte solution B, obtained in the same manner as polymer electrolyte solution A except that block copolymer b2 was used instead of block copolymer b1, was 1,600 mPa s.

[Polymer electrolyte solution C] A polymer electrolyte solution comprising a block copolymer b3 obtained by copolymerizing an oligomer represented by the general formula (G4) as a segment containing an ionic group and an oligomer represented by the general formula (G3) as a segment not containing an ionic group. Block copolymer b3 was synthesized in the same manner as polymer electrolyte solution A, except that 25.6 g (1.6 mmol) of oligomer a2 having an ionic group obtained in Synthesis Example 4 was used. The weight-average molecular weight of this block copolymer b3 was 389,000, and the ion exchange capacity (IEC) was 2.7. The viscosity of polymer electrolyte solution C, obtained in the same manner as polymer electrolyte solution A except that block copolymer b3 was used instead of block copolymer b1, was 1,800 mPa s.

[Polyelectrolyte Solution D] Polyelectrolyte Solution D Containing Random Copolymer
A 5 L reaction vessel equipped with a stirrer, a nitrogen inlet tube, and a Dean-Stark trap was charged with 129 g of 2,2-bis(4-hydroxyphenyl)-1,3-dioxolane synthesized in Synthesis Example 1, 93 g of 4,4'-biphenol (Sigma-Aldrich Japan (same) reagent), and 422 g (1.0 mol) of disodium-3,3'-disulfonate-4,4'-difluorobenzophenone synthesized in Synthesis Example 2. After nitrogen substitution, 3,000 g of N-methyl-2-pyrrolidone (NMP), 450 g of toluene, and 232 g of 18-crown-6 (Fujifilm Wako Pure Chemical Industries, Ltd. reagent) were added. After confirming that all the monomers had dissolved, 304 g of potassium carbonate (Sigma-Aldrich Japan (same) reagent) was added. The mixture was refluxed at 160°C for dehydration, then heated to remove toluene, and subjected to desalting polycondensation at 200°C for 1 hour. The resulting random copolymer a had a weight average molecular weight of 317,000 and an ion exchange capacity (IEC) of 2.1.

Next, NMP was added to dilute the polymerization stock solution to a viscosity of 500 mPa·s. The polymerization stock solution was directly centrifuged at 25°C for 30 minutes at a centrifugal force of 20,000 G using an angle rotor RA-800 mounted on a Kubota Manufacturing Co., Ltd. inverter compact high-speed refrigerated centrifuge (model number 6930). The precipitated solids (cake) and the supernatant liquid (coating liquid) were clearly separated, and the supernatant liquid was recovered. Next, the mixture was distilled under reduced pressure at 80°C while stirring, and NMP was removed until the polymer concentration reached 14% by mass. The mixture was then pressure-filtered through a 5 μm polyethylene filter to obtain polymer electrolyte solution D. The viscosity of this polymer electrolyte solution D was 1,000 mPa·s.

[Polymer Electrolyte Solution E] Polymer Electrolyte Solution Comprising a Polyarylene Block Copolymer Represented by the Following General Formula (G10) 540 mL of dried N,N-dimethylacetamide (DMAc) was added under nitrogen to a mixture of 135.0 g (0.336 mol) of neopentyl 3-(2,5-dichlorobenzoyl)benzenesulfonate, 40.7 g (5.6 mmol) of the oligomer not containing an ionic group and represented by general formula (G6) synthesized in Synthesis Example 6, 6.71 g (16.8 mmol) of 2,5-dichloro-4′-(1-imidazolyl)benzophenone, 6.71 g (10.3 mmol) of bis(triphenylphosphine)nickel dichloride, 35.9 g (0.137 mol) of triphenylphosphine, 1.54 g (10.3 mmol) of sodium iodide, and 53.7 g (0.821 mol) of zinc.

The reaction system was heated with stirring (ultimately reaching 79°C) and allowed to react for 3 hours. An increase in viscosity was observed during the reaction. The polymerization reaction solution was diluted with 730 mL of DMAc, stirred for 30 minutes, and filtered using Celite as a filter aid.

The filtrate was concentrated using an evaporator, and 43.8 g (0.505 mol) of lithium bromide was added to the filtrate. The mixture was allowed to react at an internal temperature of 110°C for 7 hours under a nitrogen atmosphere. After the reaction, the mixture was cooled to room temperature and poured into 4 L of acetone to coagulate. The coagulated material was collected by filtration, air-dried, pulverized in a mixer, and washed with 1,500 mL of 1N hydrochloric acid while stirring. After filtration, the product was washed with ion-exchanged water until the pH of the washings reached 5 or higher and then dried overnight at 80°C to obtain 23.0 g of the desired polyarylene-based block copolymer a. After deprotection, the weight-average molecular weight of this polyarylene-based block copolymer was 192,000 and the ion exchange capacity (IEC) was 2.0. The resulting polyarylene-based block copolymer a was dissolved in an organic solvent (N-methyl-2-pyrrolidone/methanol = 30/70 (mass%)) to a concentration of 0.1 g/g to obtain polymer electrolyte solution E. The viscosity of polymer electrolyte solution E was 1,200 mPa·s.

[Polymer electrolyte solution F] Polymer electrolyte solution F made of polyethersulfone-based block copolymer
0.23 g of the block copolymer precursor b4' obtained in Synthesis Example 7 was weighed and added to a mixed solution of 0.16 g of lithium bromide monohydrate and 8 mL of NMP, and the mixture was allowed to react at 120°C for 24 hours. The reaction mixture was poured into 80 mL of 6 mol/L hydrochloric acid and stirred for 1 hour. The precipitated solid was separated by filtration. The separated solid was dried to obtain a gray-white block copolymer b4 composed of a segment represented by the general formula (G9) and a segment represented by the following chemical formula (G11). The weight-average molecular weight of the obtained polyethersulfone-based block copolymer was 191,000, and the ion exchange capacity (IEC) was 2.0. The obtained polyethersulfone-based block copolymer b4 was dissolved in an organic solvent of N-methyl-2-pyrrolidone/methanol = 30/70 (mass %) to a concentration of 0.1 g/g, to obtain polymer electrolyte solution F. The viscosity of polymer electrolyte solution F was 1,300 mPa s.

[Polytetrafluoroethylene (ePTFE) porous substrate P1]
"POREFLON" (registered trademark) WP-010-80 (manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) was stretched 15 times in the transverse direction (corresponding to TD) and 2 times in the longitudinal direction (corresponding to MD), and then heat-treated at 365 ° C. to produce an ePTFE porous substrate P1 with a film thickness of 8 μm and a porosity of 85%. As a result of SEM observation, a streak structure with an average diameter of 0.85 μm and an average length of 25 μm was formed, which was approximately parallel to the longitudinal direction (MD), and had a structure with fibrils with an average diameter of 0.15 μm approximately parallel to the transverse direction (TD). The number of streak structures per mm ² of substrate area was 550.

[Polytetrafluoroethylene (ePTFE) porous substrate P2]
"POREFLON" (registered trademark) HP-045-30 (manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) was simultaneously biaxially stretched 3 times in the longitudinal and transverse directions to produce an ePTFE porous substrate P2 with a film thickness of 8 μm and a porosity of 89%. SEM observation revealed that the structure was one in which fibrils with an average diameter of 0.3 μm formed an irregular spider web-like structure, and no streaky structure was present.

[Hydrophilized ePTFE porous base material P2']
In a glove box with a dew point of -80°C, immerse ePTFE porous substrate P2 in the solution that consists of 30g of metal sodium-naphthalene complex/tetrahydrofuran (THF) 1% solution and 70g of THF, and after 3 seconds, pull it out, and immediately use THF to thoroughly wash, prepare the hydrophilic ePTFE porous substrate P2' with a film thickness of 8 μm and porosity of 88%.

[Polytetrafluoroethylene (ePTFE) porous substrate P3]
"POREFLON" (registered trademark) WP-010-80 (manufactured by Sumitomo Electric Fine Polymer Co., Ltd.) was stretched 20 times in the transverse direction (corresponding to TD) and 1.5 times in the longitudinal direction (corresponding to MD), and then heat-treated at 365 ° C. to produce an ePTFE porous substrate P3 with a film thickness of 8 μm and a porosity of 84%. As a result of SEM observation, a streak structure with an average diameter of 0.81 μm and an average length of 24 μm was formed, which was approximately parallel to the longitudinal direction (MD), and had a structure with fibrils with an average diameter of 0.14 μm approximately parallel to the transverse direction (TD). The number of streak structures per mm ² of substrate area was 540.

<Production of electrolyte membrane>
[Example 1]
In 100 g of polymer electrolyte solution A, 0.26 g of polyoxyethylene ether surfactant "Ftergent" (registered trademark) 208G (manufactured by Neos Co., Ltd.) was dissolved as a nonionic fluorine-based surfactant, and a mass ratio of polymer electrolyte to surfactant (hereinafter referred to as "surfactant / electrolyte") of 0.02 was prepared as an electrolyte-surfactant mixed solution. Using a knife coater, this electrolyte-surfactant mixed solution was cast onto a glass substrate, and an ePTFE porous substrate P1 was attached to the coated film. After holding at room temperature for 1 hour, the ePTFE porous substrate P1 was sufficiently impregnated with the electrolyte-surfactant mixed solution A, and then dried at 100 ° C. for 4 hours. After drying, the electrolyte-surfactant mixed solution was cast onto the surface opposite the glass substrate side of the ePTFE substrate, and then held at room temperature for 1 hour, and then dried at 100 ° C. for 4 hours. The resulting composite, in which the polymer electrolyte was impregnated into the ePTFE porous substrate, was peeled off from the glass substrate, and then immersed in a 10% by mass aqueous sulfuric acid solution at 80°C for 24 hours to cause proton substitution and deprotection reactions, and then immersed in a large excess of pure water for 24 hours to thoroughly wash, thereby obtaining an electrolyte membrane (thickness 9 μm). This electrolyte membrane has a single layer (thickness per layer: 3 μm) on both sides of the composite layer.

As a supplementary explanation, the impregnation and drying of the electrolyte-surfactant mixed solution caused the porous substrate to shrink in the thickness direction, resulting in a total electrolyte membrane thickness of 9 μm (the same applies to other examples and comparative examples).

[Example 2]
An electrolyte membrane (thickness: 9 μm) was obtained in the same manner as in Example 1, except that an electrolyte-surfactant mixed solution with a surfactant/electrolyte ratio of 0.10 was used. This electrolyte membrane had single layers (each layer having a thickness of 3 μm) on both sides of the composite layer.

[Example 3]
An electrolyte membrane (thickness: 9 μm) was obtained in the same manner as in Example 1, except that an electrolyte-surfactant mixed solution with a surfactant/electrolyte ratio of 0.01 was used. This electrolyte membrane had single layers (each layer having a thickness of 3 μm) on both sides of the composite layer.

[Example 4]
An electrolyte membrane (thickness: 9 μm) was obtained in the same manner as in Example 1, except that polymer electrolyte solution B was used instead of polymer electrolyte solution A. This electrolyte membrane had single layers (each layer having a thickness of 3 μm) on both sides of the composite layer.

[Example 5]
An electrolyte membrane (thickness: 9 μm) was obtained in the same manner as in Example 1, except that polymer electrolyte solution C was used instead of polymer electrolyte solution A. This electrolyte membrane had single layers (each layer having a thickness of 3 μm) on both sides of the composite layer.

[Example 6]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Example 1, except that polymer electrolyte solution D was used instead of polymer electrolyte solution A. This electrolyte membrane had single layers (each layer having a thickness of 3.5 μm) on both sides of the composite layer.

[Example 7]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Example 1, except that polymer electrolyte solution E was used instead of polymer electrolyte solution A. This electrolyte membrane had a single layer (each layer having a thickness of 3.5 μm) on both sides of the composite layer.

[Example 8]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Example 1, except that polymer electrolyte solution F was used instead of polymer electrolyte solution A. This electrolyte membrane had single layers (each layer having a thickness of 3.5 μm) on both sides of the composite layer.

[Example 9]
An electrolyte membrane (thickness 10 μm) was obtained in the same manner as in Example 1, except that the nonionic fluorine-based surfactant was changed to a polyoxyethylene ether-based surfactant "Ftergent" (registered trademark) FTX-218 (manufactured by Neos Corporation). This electrolyte membrane had single layers (each 3.5 μm thick) on both sides of the composite layer.

[Example 10]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Example 1, except that the nonionic fluorine-based surfactant was changed to a low-metal-content polyoxyethylene ether surfactant "Ftergent" (registered trademark) DFX-18 (manufactured by Neos Corporation). This electrolyte membrane had single layers (each 3.5 μm thick) on both sides of the composite layer.

[Example 11]
An electrolyte membrane (thickness 10 μm) was obtained in the same manner as in Example 1, except that the nonionic fluorine-based surfactant was changed to a fluorine-containing hydrophilic group/lipophilic group-containing oligomeric surfactant "Ftergent" (registered trademark) 710FS (manufactured by Neos Corporation). This electrolyte membrane had single layers (each 3.5 μm thick) on both sides of the composite layer.

[Example 12]
Except for using ePTFE porous substrate P3 instead of ePTFE porous substrate P1, an electrolyte membrane (thickness: 9 μm) was obtained in the same manner as in Example 1. This electrolyte membrane had single layers (each layer having a thickness of 3 μm) on both sides of the composite layer.

[Comparative Example 1]
Using a knife coater, 100 g of polymer electrolyte solution A was cast onto a glass substrate and then dried at 100° C. for 4 hours to obtain a film-like polymer. The film was then immersed in a 10% by mass aqueous sulfuric acid solution at 80° C. for 24 hours to undergo proton substitution and deprotection reactions, and then thoroughly washed by immersion in a large excess of pure water for 24 hours to obtain an electrolyte membrane (thickness: 10 μm).

[Comparative Example 2]
An attempt was made to prepare an electrolyte membrane in the same manner as in Example 1, except that the nonionic fluorine-based surfactant was not used. However, the polymer electrolyte solution A did not penetrate into the porous substrate, and an electrolyte membrane could not be obtained.

[Comparative Example 3]
Except for using ePTFE porous substrate P2 instead of ePTFE porous substrate P1, an electrolyte membrane (thickness: 8 μm) was obtained in the same manner as in Example 1. This electrolyte membrane had single layers (each layer having a thickness of 2.5 μm) on both sides of the composite layer.

[Comparative Example 4]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Comparative Example 1, except that polymer electrolyte solution B was used instead of polymer electrolyte solution A.

[Comparative Example 5]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Comparative Example 1, except that polymer electrolyte solution C was used instead of polymer electrolyte solution A.

[Comparative Example 6]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Comparative Example 1, except that polymer electrolyte solution D was used instead of polymer electrolyte solution A.

[Comparative Example 7]
An electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Comparative Example 1, except that polymer electrolyte solution E was used instead of polymer electrolyte solution A.

[Comparative Example 8]
A composite electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Comparative Example 1, except that polymer electrolyte solution F was used instead of polymer electrolyte solution A.

[Comparative Example 9]
Except for using a hydrophilized ePTFE porous substrate P2' instead of the ePTFE porous substrate P1, an electrolyte membrane (thickness: 10 μm) was obtained in the same manner as in Comparative Example 2. This electrolyte membrane had single layers (each layer having a thickness of 3.5 μm) on both sides of the composite layer.

[Comparative Example 10]
An attempt was made to prepare an electrolyte membrane in the same manner as in Example 1, except that the nonionic fluorine-based surfactant was changed to the non-fluorine-based surfactant "Triton" (registered trademark) X-100. However, the polymer electrolyte solution A did not permeate into the porous substrate, and an electrolyte membrane could not be obtained.

[evaluation]
The electrolyte membranes produced in Examples 1 to 12, Comparative Example 1, and Comparative Examples 3 to 9 were evaluated for ion exchange capacity (IEC), polymer electrolyte packing ratio in the composite layer, dimensional change rate λ _xy , proton conductivity, average length of streaky structures, average diameter of streaky structures, ratio of average length to average diameter, number of streaky structures per ^mm2 of membrane area, haze value, diffuse transmittance, and wet-dry cycle durability. The ionic group-containing polymer membrane constituting the electrolyte membrane was also evaluated for the presence or absence of a phase-separated structure, and the porous substrate was evaluated for fluorine atom content, oxygen atom content, and porosity. The results of these evaluations are shown in Tables 1 and 2. Regarding wet-dry cycle durability, if the hydrogen permeation current did not exceed 10 times the initial current even after 30,000 cycles, the evaluation was terminated at 30,000 cycles.

Note 1) Polymer electrolyte A: block copolymer b1, polymer electrolyte B: block copolymer b2, polymer electrolyte C: block copolymer b3, polymer electrolyte D: random copolymer a, polymer electrolyte E: polyarylene-based block copolymer a, polymer electrolyte F: polyethersulfone-based block copolymer b4
Note 2) Porous substrate P1: Polytetrafluoroethylene (ePTFE) with a film thickness of 8 μm and a porosity of 85%; Porous substrate P2: Polytetrafluoroethylene (ePTFE) with a film thickness of 8 μm and a porosity of 89%; Porous substrate P2': Hydrophilized ePTFE with a film thickness of 8 μm and a porosity of 88%; Porous substrate P3: Polytetrafluoroethylene (ePTFE) with a film thickness of 8 μm and a porosity of 84%.

1a-1f: Striated bundles observed on the surface of a polyelectrolyte layer using a digital microscope

Claims (13)

An electrolyte membrane comprising a porous substrate and a polymer electrolyte, wherein the porous substrate has a streak structure , the streak structure having an average length of 15 μm to 70 μm and an average width of 0.5 μm to 3.0 μm, and wherein the streak structures having a width of 0.5 μm or more are present in an amount of 200 to 1000 per ^mm2 of the surface of the porous substrate. 2. The electrolyte membrane according to claim 1 , wherein the value obtained by dividing the average length of the streak structures by the average width is 5 to 100 . 3. The electrolyte membrane according to claim 1, wherein a plurality of said streak structures are arranged substantially parallel to one another in any one direction parallel to the surface of the porous substrate. 4. The electrolyte membrane according to claim 1 , wherein the porous substrate contains a fluororesin. 5. The electrolyte membrane according to claim 4 , wherein the total mass of fluorine atoms contained in the porous substrate is 50 mass% or more when the total mass of the porous substrate is taken as 100 mass%. 6. The electrolyte membrane according to claim 1 , wherein the polymer electrolyte is a hydrocarbon polymer having an ionic group. 7. The electrolyte membrane according to claim 6 , wherein the hydrocarbon-based polymer is an aromatic hydrocarbon-based polymer having an aromatic ring in the main chain. The electrolyte membrane according to any one of claims 1 to 7 , further comprising a nonionic fluorine-based surfactant. 9. The electrolyte membrane according to claim 8 , wherein the nonionic fluorine-based surfactant is a compound having a fluorine-containing group, which includes a fluorinated alkyl group, a fluorinated alkenyl group, or a fluorinated aryl group, and a nonionic solvent-philic group. 10. The electrolyte membrane according to claim 1 , wherein the haze value when light is transmitted through the membrane in the thickness direction is 20% or more and 45% or less. A catalyst-layered electrolyte membrane, comprising the electrolyte membrane according to any one of claims 1 to 10 , and catalyst layers laminated on both sides of the electrolyte membrane. A membrane electrode assembly comprising the electrolyte membrane according to any one of claims 1 to 10 . A polymer electrolyte fuel cell comprising the electrolyte membrane according to any one of claims 1 to 10 .