Electrolyte membrane for solid polymer fuel cell, membrane electrode assembly, solid polymer fuel cell, method for producing same, and electrolyte membrane for solid polymer water electrolysis

Abstract

A purpose of the present invention is to provide: an electrolyte membrane for a solid polymer fuel cell, said electrolyte membrane having excellent proton conductivity; a membrane electrode assembly that includes the electrolyte membrane for the solid polymer fuel cell; a solid polymer fuel cell that includes the membrane electrode assembly; methods for manufacturing the electrolyte membrane for a solid polymer fuel cell, the membrane electrode assembly, and the solid polymer fuel cell; and an electrolyte membrane for solid polymer water electrolysis.　The electrolyte membrane for a solid polymer fuel cell according to the present invention contains fine fibrous cellulose that is derived from wood or plants, wherein the average fiber width of the fine fibrous cellulose that is derived from wood or plants is 50 nm or less, and the fine fibrous cellulose has a phosphorous oxo acid group.

Description

Electrolyte membrane for polymer electrolyte fuel cell, membrane electrode assembly, polymer electrolyte fuel cell, and manufacturing method thereof, and polymer electrolyte membrane for water electrolysis

[0001] The present invention relates to an electrolyte membrane for a polymer electrolyte fuel cell, a membrane electrode assembly, a polymer electrolyte fuel cell, methods for producing these, and a polymer electrolyte membrane for water electrolysis.

[0002] Fuel cells have a low environmental impact because they can generate energy using hydrogen. Among fuel cells, polymer electrolyte fuel cells (PEFCs) can operate at low temperatures and have high power generation efficiency, so they are attracting attention as an energy source for mobile phones, automobiles, and other devices. Research and development is also underway on polymer electrolyte water electrolysis, a method of producing hydrogen to be supplied to polymer electrolyte fuel cells.

[0003] Conventionally, perfluorosulfonic acid polymers have been widely used as a constituent material of the electrolyte membrane of solid polymer fuel cells. However, in recent years, research and development has been underway to improve cell performance by using cellulose, which has a smaller environmental impact than perfluorosulfonic acid polymers and is naturally abundant, as a raw material for forming the electrolyte membrane. For example, Patent Document 1 discloses a fuel cell including a nanocellulose membrane as an electrolyte membrane, in which hydroxymethyl groups (CH ２ OH) is a sulfonic acid group (SO ３ In addition, Patent Document 2 describes a composition for an electrolyte membrane, a solid polymer electrolyte fuel cell, and a water electrolysis device, each of which has a solid polymer electrolyte membrane obtained from the electrolyte membrane composition, with the aim of providing a composition that can easily obtain a polymer electrolyte membrane that is excellent in durability and can suppress deterioration over time in power generation performance and water electrolysis performance.

[0004] Patent No. 6721952 International Publication No. 2014 / 157389

[0005] The nanocellulose membrane used as the electrolyte membrane in the fuel cell (solid polymer fuel cell) described in Patent Document 1 does not have sufficient proton conductivity, and it is desirable to improve the proton conductivity of the electrolyte membrane from the viewpoint of improving the power generation efficiency and extending the life of the cell. Furthermore, it is desirable to improve the proton conductivity of the solid polymer electrolyte membrane in the water electrolysis device described in Patent Document 2 from the viewpoint of improving the hydrogen production efficiency. The present invention aims to provide an electrolyte membrane for solid polymer fuel cells with excellent proton conductivity, a membrane electrode assembly including the solid polymer fuel cell electrolyte membrane, a solid polymer fuel cell including the membrane electrode assembly, a solid polymer fuel cell electrolyte membrane, a membrane electrode assembly, and a solid polymer fuel cell, as well as a solid polymer water electrolysis electrolyte membrane.

[0006] The present inventors attempted to apply fine fibrous cellulose derived from wood or plants, among various fibrous cellulose derived from fiber raw materials, to electrolyte membranes for polymer electrolyte fuel cells and polymer electrolyte membranes for water electrolysis, taking advantage of the fact that fine fibrous cellulose derived from wood or plants is easily dispersed homogeneously in water after phosphorus oxo acid groups are introduced. The present inventors discovered that electrolyte membranes with excellent proton conductivity can be obtained by using fine fibrous cellulose derived from wood or plants, having a specific average fiber width, and containing phosphorus oxo acid groups, and thus completed the present invention.

[0007] That is, the present invention relates to the following items <1> to <24>. <1> An electrolyte membrane for a polymer electrolyte fuel cell containing wood- or plant-derived fine fibrous cellulose, wherein the wood- or plant-derived fine fibrous cellulose has an average fiber width of 50 nm or less and the wood- or plant-derived fine fibrous cellulose has a phosphorus oxo acid group. <2> A counter ion of the phosphorus oxo acid group is H ＋ and / or Na ＋ <3> The electrolyte membrane for a polymer electrolyte fuel cell according to <1>, wherein the counter ion of the phosphorus oxo acid group is H ＋<4> The electrolyte membrane for a polymer electrolyte fuel cell according to any one of <1> to <3>, wherein the amount of phosphorus oxoacid groups introduced in the wood- or plant-derived fine fibrous cellulose is 0.50 mmol / g or more. <5> The electrolyte membrane for a polymer electrolyte fuel cell according to any one of <1> to <4>, wherein the content of the wood- or plant-derived fine fibrous cellulose in the solid content of the electrolyte membrane for a polymer electrolyte fuel cell is 80 mass% or more. <6> The electrolyte membrane for a polymer electrolyte fuel cell according to any one of <1> to <4>, wherein the content of the wood- or plant-derived fine fibrous cellulose in the solid content of the electrolyte membrane for a polymer electrolyte fuel cell is 30 mass% or more. <7> A membrane / electrode assembly comprising a positive electrode catalyst layer, the electrolyte membrane for a polymer electrolyte fuel cell according to any one of <1> to <6>, and a negative electrode catalyst layer bonded together in this order. <8> A polymer electrolyte fuel cell having the membrane electrode assembly according to <7>. <9> A method for producing an electrolyte membrane for a polymer electrolyte fuel cell, the method comprising the following preparation step and membrane formation step in this order: The wood- or plant-derived fine fibrous cellulose in the preparation step has an average fiber width of 50 nm or less, and the wood- or plant-derived fine fibrous cellulose has a phosphorus oxo acid group. Preparation step: preparing a dispersion of wood- or plant-derived fine fibrous cellulose Membrane formation step: forming a membrane from the wood- or plant-derived fine fibrous cellulose dispersion <10> A method for producing an electrolyte membrane for a polymer electrolyte fuel cell according to <9>, the method comprising the following ion exchange step between the preparation step and the membrane formation step. <11> A method for producing a membrane electrode assembly, comprising the steps of bonding a positive electrode catalyst layer to one side of a solid polymer electrolyte fuel cell electrolyte membrane obtained by the method for producing a solid polymer electrolyte fuel cell electrolyte membrane according to <9> or <10>, and bonding a negative electrode catalyst layer to the other side of the solid polymer electrolyte fuel cell electrolyte membrane. <12> A method for producing a solid polymer fuel cell, comprising the step of incorporating the membrane electrode assembly obtained by the method for producing a membrane electrode assembly according to <11>.

[0008] <13> A solid polymer electrolyte membrane for water electrolysis containing wood- or plant-derived fine fibrous cellulose, wherein the wood- or plant-derived fine fibrous cellulose has an average fiber width of 50 nm or less and the wood- or plant-derived fine fibrous cellulose has a phosphorus oxo acid group. <14> A solid polymer electrolyte membrane for water electrolysis containing wood- or plant-derived fine fibrous cellulose, wherein the counter ion of the phosphorus oxo acid group is H ＋ and / or Na ＋ <15> The solid polymer electrolyte membrane for water electrolysis according to <13>, wherein the counter ion of the phosphorus oxoacid group is H ＋<16> The solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <15>, wherein the amount of phosphorus oxoacid groups introduced in the wood- or plant-derived fine fibrous cellulose is 0.50 mmol / g or more. <17> The solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <16>, wherein the content of the wood- or plant-derived fine fibrous cellulose in the solid content of the solid polymer electrolyte membrane for water electrolysis is 80 mass% or more. <18> The solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <16>, wherein the content of the wood- or plant-derived fine fibrous cellulose in the solid content of the solid polymer electrolyte membrane for water electrolysis is 30 mass% or more. <19> A membrane / electrode assembly comprising a positive electrode catalyst layer, the solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <18>, and an anode catalyst layer joined together in this order. <20> A solid polymer water electrolysis device comprising the membrane / electrode assembly according to <19>. <21> A method for producing a solid polymer electrolyte membrane for water electrolysis, comprising the following preparation step and membrane-forming step, in this order: the wood- or plant-derived fine fibrous cellulose in the preparation step has an average fiber width of 50 nm or less, and the wood- or plant-derived fine fibrous cellulose has a phosphorus oxo acid group. Preparation step: preparing a wood- or plant-derived fine fibrous cellulose dispersion; Membrane-forming step: forming a membrane from the wood- or plant-derived fine fibrous cellulose dispersion. <22> A method for producing the solid polymer electrolyte membrane for water electrolysis according to <21>, comprising the following ion exchange step between the preparation step and the membrane-forming step. Ion exchange step: a step of treating a dispersion of fine fibrous cellulose derived from wood or plant materials with a cation exchange resin <23> A method for producing a membrane electrode assembly, comprising the steps of bonding a positive electrode catalyst layer to one surface of a solid polymer electrolyte membrane for water electrolysis obtained by the method for producing a solid polymer electrolyte membrane for water electrolysis according to <21> or <22>, and bonding a negative electrode catalyst layer to the other surface of the solid polymer electrolyte membrane for water electrolysis. <24> A method for producing a solid polymer water electrolysis device, comprising the step of incorporating the membrane electrode assembly obtained by the method for producing a membrane electrode assembly according to <23>.

[0009] According to the present invention, it is possible to provide an electrolyte membrane for a polymer electrolyte fuel cell having excellent proton conductivity, a membrane electrode assembly having the electrolyte membrane for a polymer electrolyte fuel cell, a polymer electrolyte fuel cell having the membrane electrode assembly, methods for producing the electrolyte membrane for a polymer electrolyte fuel cell, the membrane electrode assembly, and the polymer electrolyte fuel cell, and a polymer electrolyte membrane for water electrolysis.

[0010] FIG. 1 is a graph showing the relationship between the amount of NaOH dropped onto a dispersion of fine fibrous cellulose having phosphorus oxo acid groups and pH.

[0011] Preferred embodiments of the present invention will be described below. In this specification, the range "X to Y" means "X or more and Y or less." In addition, in this specification, the upper and lower limits of a numerical range can be combined in any combination.

[0012] [Electrolyte Membrane for Solid Polymer Fuel Cells] The electrolyte membrane for solid polymer fuel cells of this embodiment contains wood- or plant-derived fine fibrous cellulose, the wood- or plant-derived fine fibrous cellulose having an average fiber diameter width of 50 nm or less, and the fine fibrous cellulose having phosphorus oxoacid groups. The components contained or potentially contained in the electrolyte membrane for solid polymer fuel cells of this embodiment may be used alone or in combination of two or more. The electrolyte membrane for solid polymer fuel cells of this embodiment and the electrolyte membrane for solid polymer water electrolysis of this embodiment described below have the same configuration except for their intended use. Hereinafter, "electrolyte membrane" refers to the electrolyte membrane for solid polymer fuel cells or solid polymer water electrolysis. Furthermore, "wood- or plant-derived fine fibrous cellulose" may also be simply referred to as "fine fibrous cellulose."

[0013] Although the reason why the electrolyte membrane for a polymer electrolyte fuel cell of this embodiment has excellent proton conductivity is not clear, it is believed that the proton conductivity within the membrane is enhanced by the following factors: fine fibrous cellulose derived from wood or plants remains relatively hydrophilic even after the introduction of phosphorus oxo acid groups; fine fibrous cellulose derived from wood or plants having phosphorus oxo acid groups can be easily dispersed homogeneously in water to form a smooth electrolyte membrane for a polymer electrolyte fuel cell; and many of the phosphorus oxo acid groups are not dehydrated and condensed within the fine fibrous cellulose molecules but are present in the electrolyte membrane for a polymer electrolyte fuel cell in the form of divalent anionic groups. The electrolyte membrane for a polymer electrolyte fuel cell of this embodiment will be described in detail below.

[0014] <Wood- or plant-derived fine fibrous cellulose> The fiber width of the fine fibrous cellulose is preferably 2 nm or more and 50 nm or less, more preferably 40 nm or less, even more preferably 30 nm or less, even more preferably 20 nm or less, and particularly preferably 10 nm or less. By making the fiber width of the fine fibrous cellulose 2 nm or more, it is possible to suppress the dissolution of cellulose molecules in water, and to more easily achieve the effects of the fine fibrous cellulose, such as improved strength, rigidity, and dimensional stability. The fiber width of the fine fibrous cellulose can be measured, for example, by observation with an electron microscope.

[0015] The average fiber width of the fine fibrous cellulose is 50 nm or less. The average fiber width of the fine fibrous cellulose is preferably 2 nm or more and 50 nm or less, more preferably 40 nm or less, even more preferably 30 nm or less, even more preferably 20 nm or less, and particularly preferably 10 nm or less. By making the average fiber width of the fine fibrous cellulose 2 nm or more, dissolution of the cellulose molecules in water is suppressed, and the effects of the fine fibrous cellulose, such as improved strength, rigidity, and dimensional stability, can be more easily achieved. Furthermore, by making the average fiber width of the fine fibrous cellulose 50 nm or less, the surface of the solid polymer fuel cell electrolyte membrane of this embodiment can be made smooth. A smooth surface of the solid polymer fuel cell electrolyte membrane facilitates proton transfer between the positive electrode catalyst layer, the solid polymer fuel cell electrolyte membrane, and the negative electrode catalyst layer, thereby improving proton conductivity. The fine fibrous cellulose is, for example, monofilamentous cellulose.

[0016] The average fiber width of fine fibrous cellulose is measured, for example, using an electron microscope as follows. First, an aqueous suspension of fine fibrous cellulose with a concentration of 0.05% by mass to 0.1% by mass is prepared, and this suspension is cast on a hydrophilically treated carbon film-coated grid to prepare a sample for transmission electron microscope (TEM) observation. When wide fibers are included, a scanning electron microscope (SEM) image of the surface cast on glass may be observed. Next, electron microscope images are observed at magnifications of 1,000x, 5,000x, 10,000x, or 50,000x, depending on the width of the fibers to be observed. However, the sample, observation conditions, and magnification are adjusted to satisfy the following conditions: (1) A line X is drawn at any location within the observed image, and 20 or more fibers intersect with line X. (2) A line Y is drawn within the same image, perpendicular to line X, and 20 or more fibers intersect line Y.

[0017] For observation images that satisfy the above conditions, the widths of fibers intersecting with lines X and Y are visually read. In this way, three or more sets of observation images of at least the surface portions that do not overlap each other are obtained. Next, for each image, the widths of fibers intersecting with lines X and Y are read. In this way, the widths of at least 20 fibers x 2 x 3 = 120 fibers are read. The average of the read fiber widths is then taken as the average fiber width of the fine fibrous cellulose.

[0018] The fiber length of the fine fibrous cellulose is, for example, preferably 0.1 μm or more and 1,000 μm or less, more preferably 0.1 μm or more and 800 μm or less, and even more preferably 0.1 μm or more and 600 μm or less. By setting the fiber length within the above range, destruction of the crystalline regions of the fine fibrous cellulose can be suppressed. It also becomes possible to set the viscosity of the slurry (dispersion liquid) of the fine fibrous cellulose within an appropriate range. The fiber length of the fine fibrous cellulose can be determined by image analysis using, for example, TEM, SEM, or atomic force microscopy (AFM).

[0019] The fine fibrous cellulose preferably has a type I crystal structure. The fact that the fine fibrous cellulose has a type I crystal structure can be identified by a diffraction profile obtained from a wide-angle X-ray diffraction photograph using CuKα (λ=1.5418 Å) monochromated with graphite. Specifically, it can be identified by the presence of typical peaks at two positions: near 2θ=14° to 17° and near 2θ=22° to 23°. The proportion of type I crystal structure in the fine fibrous cellulose is, for example, preferably 30% or more, more preferably 40% or more, and even more preferably 50% or more. The degree of crystallinity can be determined by measuring the X-ray diffraction profile and using a conventional method from the pattern (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

[0020] The axial ratio (fiber length / fiber width) of the fine fibrous cellulose is not particularly limited, but is preferably 20 or more and 10,000 or less, and more preferably 50 or more and 1,000 or less. By setting the axial ratio at or above the lower limit, it is easy to form an electrolyte membrane for a polymer electrolyte fuel cell. Furthermore, sufficient viscosity is easily obtained when a solvent dispersion is produced. Setting the axial ratio at or below the upper limit is preferable in that, for example, when the fine fibrous cellulose is used as an aqueous dispersion, handling such as dilution is easier.

[0021] The fine fibrous cellulose in this embodiment has a phosphorus oxoacid group. In this specification, the "phosphorus oxoacid group" includes a substituent derived from the phosphorus oxoacid group. Furthermore, the "counter ion" of the phosphorus oxoacid group includes a dissociative proton. Examples of the counter ion include the β of the substituent represented by formula (1) below. ｂ＋ (Proton (H ＋ ) or monovalent or higher cations consisting of organic or inorganic substances). Among them, from the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, it is preferable that the counter ion is H ＋ and alkali metal (sodium, potassium, lithium, etc.) ions, ＋ and / or sodium ions (Na ＋ From the viewpoints of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, reducing the temperature dependency of the proton conductivity, and reducing the activation energy of the proton conduction, it is more preferable that the counter ion contains H ＋ Furthermore, from the viewpoints of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, reducing the temperature dependency of the proton conductivity, and reducing the activation energy of the proton conduction, it is more preferable that the counter ions contain H. ＋ The content (mass) of cations other than the above is preferably 100,000 ppm or less, more preferably 1,000 ppm or less, even more preferably 100 ppm or less, and is 0 ppm or more, preferably 0 ppm.

[0022] The phosphorus oxoacid group is, for example, a substituent represented by the following formula (1). A plurality of types of substituents represented by the following formula (1) may be introduced into each fine fibrous cellulose. In this case, the plurality of introduced substituents represented by the following formula (1) may be the same or different. The substituent represented by the following formula (1) is preferably bonded directly to the carbon atom at the 2-, 3-, and / or 6-position of the cellulose via "-O-".

[0023]

[0024] In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (where a=b×m). At least one of n α and α′ is O. － and the rest are R or OR. Note that all of α and α' are O － The n α's may all be the same or may be different. ｂ＋ is a cation of one or more valences consisting of organic or inorganic substances.

[0025] R is a hydrogen atom, a saturated linear hydrocarbon group, a saturated branched hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated linear hydrocarbon group, an unsaturated branched hydrocarbon group, an unsaturated cyclic hydrocarbon group, an aromatic group, or a group derived therefrom. In formula (1), n ​​is preferably 1.

[0026] Examples of saturated linear hydrocarbon groups include, but are not limited to, methyl, ethyl, n-propyl, or n-butyl groups. Examples of saturated branched hydrocarbon groups include, but are not limited to, i-propyl or t-butyl groups. Examples of saturated cyclic hydrocarbon groups include, but are not limited to, cyclopentyl or cyclohexyl groups. Examples of unsaturated linear hydrocarbon groups include, but are not limited to, vinyl or allyl groups. Examples of unsaturated branched hydrocarbon groups include, but are not limited to, i-propenyl or 3-butenyl groups. Examples of unsaturated cyclic hydrocarbon groups include, but are not limited to, cyclopentenyl or cyclohexenyl groups. Examples of aromatic groups include, but are not limited to, phenyl or naphthyl groups.

[0027] Furthermore, examples of the derivative group in R include, but are not limited to, functional groups in which at least one functional group selected from the group consisting of a carboxy group, a carboxylate group (—COO—), a hydroxy group, an amino group, and an ammonium group is added to or substituted on the main chain or side chain of the above-mentioned hydrocarbon groups. The number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, and more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R within the above range, the molecular weight of the phosphorus oxoacid group can be set within an appropriate range, facilitating penetration into the fiber raw material and increasing the yield of fine fibrous cellulose. When multiple R groups are present in formula (1) or when multiple types of substituents represented by formula (1) are introduced into the fine fibrous cellulose, the multiple R groups may be the same or different.

[0028] β ｂ＋ Is, H ＋Or, it is a monovalent or higher cation made of an organic or inorganic substance. Examples of the monovalent or higher cation made of an organic substance include organic onium ions. Examples of the organic onium ions include organic ammonium ions and organic phosphonium ions. Examples of the organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of the organic phosphonium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of the monovalent or higher cation made of an inorganic substance include ions of alkali metals such as sodium, potassium, or lithium, ions of divalent metals such as calcium or magnesium, hydrogen ions, ammonium ions, etc. It should be noted that in formula (1), β ｂ＋ When a plurality of β ｂ＋ may be the same or different. From the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, β ｂ＋ H ＋ and alkali metal ions, and ＋ and / or sodium ions (Na ＋ ), and from the viewpoints of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, reducing the temperature dependency of the proton conductivity, and reducing the activation energy of the proton conduction, it is more preferable that the electrolyte membrane for a polymer electrolyte fuel cell contains β ｂ＋ H ＋ It is more preferred that the composition comprises:

[0029] More specifically, the phosphorus oxoacid group is a phosphate group (-OPO ３ H ２ ), salts of phosphate groups, phosphite groups (phosphonic acid groups) (-OPO ２ H ２Examples of the phosphorus oxoacid group include a group in which a phosphate group is condensed (e.g., a pyrophosphate group), a group in which a phosphonic acid group is condensed (e.g., a polyphosphonic acid group), a phosphate ester group (e.g., a monomethyl phosphate group, a polyoxyethylene alkyl phosphate group), and an alkyl phosphonic acid group (e.g., a methyl phosphonic acid group).

[0030] The amount of phosphorus oxoacid groups introduced into the fine fibrous cellulose is, for example, preferably 0.50 mmol / g or more and 2.50 mmol / g or less, more preferably 0.80 mmol / g or more, even more preferably 1.00 mmol / g or more, particularly preferably 1.20 mmol / g or more, and more preferably 2.30 mmol / g or less, and even more preferably 2.10 mmol / g or less, per gram (mass) of fine fibrous cellulose. From the viewpoint of improving the proton conductivity of the electrolyte membrane for solid polymer fuel cells, the amount of phosphorus oxoacid groups introduced is preferably equal to or greater than the above-mentioned lower limit, and from the viewpoint of improving the strength, rigidity, and dimensional stability of the electrolyte membrane for solid polymer fuel cells, the amount of phosphorus oxoacid groups introduced is preferably equal to or less than the above-mentioned upper limit. Here, the denominator in the unit mmol / g is calculated based on the ratio of the counter ions of the phosphorus oxoacid groups to hydrogen ions (H ＋ ) indicates the mass of the fine fibrous cellulose when

[0031] The amount of phosphorus oxo acid groups introduced into the fine fibrous cellulose can be measured, for example, by neutralization titration. In measurement by neutralization titration, the amount introduced is measured by determining the change in pH while adding an alkali such as an aqueous sodium hydroxide solution to a slurry containing the obtained fine fibrous cellulose. Figure 1 is a graph showing the relationship between the amount of NaOH added dropwise to the fine fibrous cellulose having phosphorus oxo acid groups and the pH.

[0032] FIG. 1 is a graph showing the relationship between the amount of NaOH added dropwise to a slurry containing fine fibrous cellulose having phosphorus oxo acid groups and pH. The amount of phosphorus oxo acid groups introduced into fine fibrous cellulose is measured, for example, as follows. First, a slurry containing fine fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, the measurement target may be subjected to a defibration treatment similar to the defibration treatment process described below before treatment with the strongly acidic ion exchange resin. Next, the change in pH is observed while adding aqueous sodium hydroxide solution, and a titration curve such as that shown in the upper part of FIG. 1 is obtained. The titration curve shown in the upper part of FIG. 1 plots the measured pH against the amount of alkali added, while the titration curve shown in the lower part of FIG. 1 plots the pH increment (differential value) (1 / mmol) against the amount of alkali added. In this neutralization titration, two points at which the increment (differential value of pH with respect to the amount of alkali added) is maximized are confirmed on the curve plotting the measured pH against the amount of alkali added. Of these, the first maximum point of the increment obtained after starting the addition of alkali is called the first endpoint, and the next maximum point of the increment is called the second endpoint. The amount of alkali required from the start of titration to the first endpoint is equal to the amount of first dissociated acid of the fine fibrous cellulose contained in the slurry used for titration, the amount of alkali required from the first endpoint to the second endpoint is equal to the amount of second dissociated acid of the fine fibrous cellulose contained in the slurry used for titration, and the amount of alkali required from the start of titration to the second endpoint is equal to the total amount of dissociated acid of the fine fibrous cellulose contained in the slurry used for titration. The value obtained by dividing the amount of alkali required from the start of titration to the first endpoint by the solids content (g) in the slurry to be titrated is the amount of phosphorus oxo acid group introduced (mmol / g). Note that when simply referring to the amount of phosphorus oxo acid group introduced (or amount of phosphorus oxo acid group), it refers to the amount of first dissociated acid. In FIG. 1, the region from the start of titration to the first endpoint is called the first region, and the region from the first endpoint to the second endpoint is called the second region.For example, when the phosphorus oxoacid group is a phosphate group and this phosphate group undergoes condensation, the amount of weakly acidic groups in the phosphorus oxoacid group (also referred to herein as the second dissociated acid amount) appears to decrease, and the amount of alkali required in the second region becomes smaller than the amount of alkali required in the first region. On the other hand, the amount of strongly acidic groups in the phosphorus oxoacid group (also referred to herein as the first dissociated acid amount) corresponds to the amount of phosphorus atoms regardless of whether condensation occurs. Furthermore, when the phosphorus oxoacid group is a phosphite group, the phosphorus oxoacid group no longer contains weakly acidic groups, so the amount of alkali required in the second region becomes smaller or may even become zero. In this case, the titration curve has only one point at which the pH increment is maximized. The denominator of the introduced amount of phosphorus oxoacid groups (mmol / g) indicates the mass of the acid-form fine fibrous cellulose, and therefore indicates the amount of phosphorus oxoacid groups in the acid-form fine fibrous cellulose (hereinafter referred to as the amount of phosphorus oxoacid groups (acid form)). On the other hand, when the counter ions of the phosphorus oxoacid groups are substituted with any cation C so as to be charge equivalent, the amount of phosphorus oxoacid groups possessed by the fine fibrous cellulose when the cation C is the counter ion (hereinafter referred to as the amount of phosphorus oxoacid groups (type C)) can be determined by converting the denominator to the mass of the fine fibrous cellulose when the cation C is the counter ion. That is, it is calculated using the following formula: Amount of phosphorus oxoacid groups (type C) = Amount of phosphorus oxoacid groups (acid type) / {1 + (W - 1) × A / 1000}, where A [mmol / g] is the total amount of anions derived from phosphorus oxoacid groups possessed by the fine fibrous cellulose (the sum of the amount of strongly acidic groups and the amount of weakly acidic groups in the phosphorus oxoacid groups), and W is the formula weight per monovalent of the cation C (for example, 23 for Na and 9 for Al).

[0033] In measuring phosphorus oxo acid groups by titration, adding too many drops of sodium hydroxide aqueous solution or titrating too quickly can result in lower phosphorus oxo acid groups than expected, making it difficult to obtain accurate values. An appropriate amount of drop and titration interval is, for example, titrating 10 to 50 μL of 0.1 N sodium hydroxide aqueous solution over 5 to 30 seconds. To eliminate the influence of carbon dioxide dissolved in the fine fibrous cellulose-containing slurry, it is also desirable to perform the measurement while blowing an inert gas such as nitrogen gas into the slurry from 15 minutes before the start of titration until the end of titration. The measurement of the amount of phosphorus oxo acid groups by the above-mentioned method is applicable to fine fibrous cellulose having a fiber width of 50 nm or less. When measuring the amount of phosphorus oxo acid groups in pulp fibers having a fiber width of more than 50 nm, the pulp fibers are first refined before measurement.

[0034] In this embodiment, the fine fibrous cellulose may be obtained by removing some of the phosphorus oxo acid groups from the fine fibrous cellulose that has been defibrated by introducing phosphorus oxo acid groups as described above.

[0035] [Method for producing fine fibrous cellulose] (Wood-based or plant-based fiber raw material containing cellulose) Wood-based or plant-based fine fibrous cellulose is produced from a wood-based or plant-based fiber raw material containing cellulose (hereinafter, the wood-based or plant-based fiber raw material will also be simply referred to as the "fiber raw material"). Wood-based or plant-based fine fibrous cellulose is easy to introduce phosphorus oxo acid groups into, and is preferred from the viewpoint of increasing the amount of phosphorus oxo acid groups introduced and improving the proton conductivity of electrolyte membranes for polymer electrolyte fuel cells. An example of a wood-based fiber raw material containing cellulose is wood pulp. Examples of wood pulp include chemical pulps such as bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP); semi-chemical pulps such as semi-chemical pulp (SCP) and chemi-groundwood pulp (CGP); and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Cellulose-containing plant fiber raw materials include cotton pulps such as cotton linters and cotton lint, hemp, straw, bamboo, and bagasse. Among the above pulps, wood pulps are preferred from the viewpoint of availability. Among wood pulps, chemical pulp is more preferred, kraft pulp is even more preferred, and LBKP and NBKP are particularly preferred, from the viewpoints of having a high cellulose ratio and a high yield of fine fibrous cellulose during defibration treatment, and of obtaining long-fiber fine fibrous cellulose with a large axial ratio due to little decomposition of cellulose in the pulp. The pulp of this embodiment may be one of the above types alone or a mixture of two or more types.

[0036] In order to obtain the above-mentioned fine fibrous cellulose into which phosphorus oxo acid groups have been introduced, it is preferable to have a phosphorus oxo acid group introduction step for introducing phosphorus oxo acid groups into the above-mentioned cellulose-containing fiber raw material, a washing step, an alkali treatment step (neutralization step), and a defibration treatment step in this order, and an acid treatment step may be included instead of or in addition to the washing step.

[0037] (Phosphorus Oxo Acid Group Introduction Step) The phosphorus oxo acid group introduction step is a step of reacting a cellulose-containing fiber raw material with at least one compound (hereinafter also referred to as "compound A") selected from compounds capable of introducing phosphorus oxo acid groups by reacting with hydroxyl groups possessed by the cellulose-containing fiber raw material. This step results in a fiber into which phosphorus oxo acid groups have been introduced. In the phosphorus oxo acid group introduction step according to this embodiment, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the presence of at least one selected from urea and its derivatives (hereinafter also referred to as "compound B"). Alternatively, the reaction of the cellulose-containing fiber raw material with compound A may be carried out in the absence of compound B. An example of a method for reacting compound A with a fiber raw material in the coexistence of compound B is a method in which compound A and compound B are mixed with a dry, wet, or slurried fiber raw material. Of these, using a dry or wet fiber raw material is preferred because it results in high reaction uniformity, and using a dry fiber raw material is particularly preferred. The form of the fiber raw material is not particularly limited, but is preferably, for example, in the form of a cotton or thin sheet. Compound A and compound B can be added to the fiber raw material in the form of a powder, a solution dissolved in a solvent, or a melted state obtained by heating to or above their melting point. Of these, adding them in the form of a solution dissolved in a solvent, particularly an aqueous solution, is preferred because it results in high reaction uniformity. Compound A and compound B may be added to the fiber raw material simultaneously, separately, or as a mixture. The method for adding compound A and compound B is not particularly limited, but when compound A and compound B are in the form of a solution, the fiber raw material may be immersed in the solution to absorb the liquid and then removed, or the solution may be added dropwise to the fiber raw material. Alternatively, the required amounts of compound A and compound B may be added to the fiber raw material, or excess amounts of compound A and compound B may be added to the fiber raw material, and then the excess compound A and compound B may be removed by squeezing or filtration.

[0038] The compound A used in this embodiment may be any compound that has a phosphorus atom and can form an ester bond with cellulose, including, but not limited to, phosphoric acid or a salt thereof, phosphorous acid or a salt thereof, dehydrated condensed phosphoric acid or a salt thereof, and phosphoric anhydride (diphosphorus pentoxide). Phosphoric acid can be used with various purities, such as 100% phosphoric acid (orthophosphoric acid) or 85% phosphoric acid. Phosphorous acid can be, for example, 99% phosphorous acid (phosphonic acid). Dehydrated condensed phosphoric acid is a compound in which two or more molecules of phosphoric acid are condensed by a dehydration reaction, and examples thereof include pyrophosphoric acid and polyphosphoric acid. Phosphates, phosphites, and dehydrated condensed phosphates include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, which can be neutralized to various degrees. Among these, phosphoric acid or a salt thereof, and phosphorous acid or a salt thereof are preferred from the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell. From the viewpoints of high efficiency of introduction of phosphate groups, easier improvement of defibration efficiency in the defibration step described below, low cost, and ease of industrial application, phosphorous acid, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, or ammonium salt of phosphoric acid are more preferred, and phosphorous acid, phosphoric acid, sodium dihydrogen phosphate, or ammonium dihydrogen phosphate are even more preferred. The amount of compound A added to the fiber raw material is not particularly limited. For example, when the amount of compound A added is converted into the amount of phosphorus atoms, the amount of phosphorus atoms added per 100 parts by mass of the fiber raw material (bone dry mass) is preferably 0.5 parts by mass or more and 100 parts by mass or less, more preferably 1 part by mass or more and 50 parts by mass or less, and even more preferably 2 parts by mass or more and 30 parts by mass or less. By setting the amount of phosphorus atoms added to the fiber raw material within the above range, the yield of fine fibrous cellulose can be further improved. On the other hand, by setting the amount of phosphorus atoms added to the fiber raw material to the above upper limit or less, a balance between the yield improvement effect and cost can be achieved.

[0039] As described above, compound B used in this embodiment is at least one selected from urea and its derivatives. Examples of compound B include urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea. From the viewpoint of improving the uniformity of the reaction, compound B is preferably used as an aqueous solution. Furthermore, from the viewpoint of further improving the uniformity of the reaction, it is preferable to use an aqueous solution in which both compound A and compound B are dissolved. The amount of compound B added per 100 parts by mass (bone dry mass) of the fiber raw material is not particularly limited, but is preferably, for example, 1 part by mass or more and 500 parts by mass or less, more preferably 10 parts by mass or more and 400 parts by mass or less, and even more preferably 100 parts by mass or more and 350 parts by mass or less.

[0040] In the reaction of a fiber raw material containing cellulose with compound A, the reaction system may contain, in addition to compound B, for example, amides or amines. Examples of amides include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among these, triethylamine is known to function as a particularly good reaction catalyst.

[0041] In the phosphorus oxo acid group introduction step, it is preferable to add or mix compound A or the like to or with the fiber raw material, and then heat-treat the fiber raw material. The heat treatment temperature is preferably selected so that the phosphorus oxo acid group can be efficiently introduced while suppressing thermal decomposition and hydrolysis of the fiber. The heat treatment temperature is preferably, for example, 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and even more preferably 130°C or higher and 200°C or lower. Various types of equipment having heat transfer media can be used for the heat treatment, including, for example, a stirring dryer, a rotary dryer, a disk dryer, a roll-type heater, a plate-type heater, a fluidized-bed dryer, a band-type dryer, a filtration dryer, a vibration fluidized-bed dryer, a flash dryer, a reduced-pressure dryer, an infrared heater, a far-infrared heater, a microwave heater, and a high-frequency dryer.

[0042] In the heat treatment according to this embodiment, for example, compound A may be added to a thin sheet-like fiber raw material by impregnation or other methods, followed by heating, or heating while kneading or stirring the fiber raw material and compound A in a kneader or the like. This suppresses unevenness in the concentration of compound A in the fiber raw material, enabling more uniform introduction of phosphate groups onto the surface of the cellulose fibers contained in the fiber raw material. This is thought to be due to the fact that, when water molecules move to the surface of the fiber raw material as it dries, dissolved compound A is prevented from being attracted to the water molecules by surface tension and similarly moving to the surface of the fiber raw material (i.e., causing unevenness in the concentration of compound A). Furthermore, the heating device used for the heat treatment is preferably one that can constantly discharge, for example, moisture retained in the slurry and moisture generated in the dehydration condensation (phosphorylation) reaction between compound A and hydroxyl groups contained in cellulose or the like in the fiber raw material. Examples of such a heating device include an oven with a blower system. Constantly discharging moisture from the device system not only suppresses the hydrolysis of phosphate ester bonds, which is the reverse reaction of phosphate esterification, but also suppresses acid hydrolysis of sugar chains in the fiber. This makes it possible to obtain fine fibrous cellulose with a high axial ratio. The heat treatment time is, for example, preferably from 1 second to 300 minutes after moisture has been substantially removed from the fiber raw material, more preferably from 1 second to 1,000 seconds, and even more preferably from 10 seconds to 800 seconds. In this embodiment, by setting the heating temperature and heating time within appropriate ranges, the amount of phosphorus oxo acid groups introduced can be kept within a preferred range.

[0043] The phosphorus oxo acid group introduction step may be carried out at least once, but may also be carried out twice or more. By carrying out the phosphorus oxo acid group introduction step twice or more, a large number of phosphorus oxo acid groups can be introduced into the fiber raw material. In this embodiment, a preferred example is when the phosphorus oxo acid group introduction step is carried out twice.

[0044] When introducing phosphorus oxo acid groups into a fiber raw material, an alkaline solution may be applied to the cellulose contained in the fiber raw material to convert the cellulose into alkali cellulose. This treatment causes ionic dissociation of some of the hydroxyl groups in the cellulose, thereby increasing its nucleophilicity (reactivity). The alkaline compound contained in the alkaline solution is not particularly limited and may be an inorganic alkaline compound or an organic alkaline compound. Due to their high versatility, it is preferable to use, for example, sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, or tetrabutylammonium hydroxide. The conversion into alkali cellulose may be carried out simultaneously with the introduction of phosphorus oxo acid groups, before the introduction, or at both the same time.

[0045] The solution temperature at the start of alkali cellulose formation is preferably 0°C or higher and 50°C or lower, more preferably 5°C or higher and 40°C or lower, and even more preferably 10°C or higher and 30°C or lower.

[0046] The alkaline solution concentration is preferably 0.01 mol / L or more and 4 mol / L or less, more preferably 0.1 mol / L or more and 3 mol / L or less, and even more preferably 1 mol / L or more and 2.5 mol / L or less. In particular, when the treatment temperature is less than 10° C., the concentration is preferably 1 mol / L or more and 2 mol / L or less.

[0047] The treatment time for alkali cellulose formation is preferably 1 minute or more and 6 hours or less, more preferably 10 minutes or more, even more preferably 30 minutes or more, and more preferably 5 hours or less, even more preferably 4 hours or less.

[0048] By adjusting the type of alkaline solution, treatment temperature, concentration, and immersion time as described above, it is possible to suppress penetration of the alkaline solution into the crystalline regions of cellulose, making it easier to maintain the cellulose type I crystal structure and increasing the yield of fine fibrous cellulose.

[0049] When the introduction of phosphorus oxo acid groups and the conversion to alkali cellulose are not carried out simultaneously, the alkali cellulose obtained by the alkali treatment is preferably subjected to solid-liquid separation and water removal by a common deliquoring method such as centrifugation or filtration. This improves the reaction efficiency in the subsequent phosphorus oxo acid group introduction step. The cellulose fiber concentration after solid-liquid separation is preferably 5% to 50%, more preferably 10% to 40%, and even more preferably 15% to 35%.

[0050] (Washing Step) In the method for producing fine fibrous cellulose in this embodiment, a washing step can be carried out on the phosphorus oxo acid group-introduced fibers as needed. The washing step is carried out by washing the phosphorus oxo acid group-introduced fibers with water or an organic solvent, for example. The washing step may be carried out after each step described below, and the number of washings carried out in each washing step is not particularly limited.

[0051] (Alkali Treatment Step) When producing fine fibrous cellulose, an alkali treatment may be performed on the fiber raw material between the phosphorus oxo acid group introduction step and the defibration step described below. The alkali treatment method is not particularly limited, but an example is a method of immersing the phosphorus oxo acid group-introduced fibers in an alkaline solution. The alkaline compound contained in the alkaline solution is not particularly limited, and may be an inorganic alkaline compound or an organic alkaline compound. In this embodiment, sodium hydroxide or potassium hydroxide is preferably used as the alkaline compound because of its high versatility. The solvent contained in the alkaline solution may be either water or an organic solvent. Among these, the solvent contained in the alkaline solution is preferably water or a polar solvent including a polar organic solvent such as an alcohol, and more preferably an aqueous solvent containing at least water. As the alkaline solution, for example, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution is preferred because of its high versatility. The temperature of the alkaline solution in the alkali treatment step is not particularly limited, but is preferably, for example, 5°C to 80°C, and more preferably 10°C to 60°C. The immersion time of the phosphorus oxo acid group-introduced fiber in the alkaline solution in the alkaline treatment step is not particularly limited, but is preferably, for example, 5 to 30 minutes, and more preferably, 10 to 20 minutes. The amount of alkaline solution used in the alkaline treatment is not particularly limited, but is, for example, preferably, 100 to 100,000 parts by mass, and more preferably, 1,000 to 10,000 parts by mass, per 100 parts by mass (bone dry mass) of the phosphorus oxo acid group-introduced fiber. The alkaline treatment may be a neutralization treatment or ion exchange treatment of the phosphorus oxo acid group. The temperature of the alkaline solution is preferably room temperature.

[0052] In order to reduce the amount of alkaline solution used in the alkaline treatment step, the phosphorus oxo acid group-introduced fiber may be washed with water or an organic solvent after the phosphorus oxo acid group-introducing step and before the alkaline treatment step. From the viewpoint of improving handleability, it is preferable to wash the alkali-treated phosphorus oxo acid group-introduced fiber with water or an organic solvent after the alkaline treatment step and before the defibration treatment step.

[0053] (Acid Treatment Step) When producing fine fibrous cellulose, an acid treatment may be performed on the fiber raw material between the step of introducing phosphorus oxo acid groups and the defibration step described below. For example, the phosphorus oxo acid group introduction step, acid treatment step, alkali treatment step, and defibration step may be performed in this order. The acid treatment method is not particularly limited, but an example is a method of immersing the fiber raw material in an acid-containing acid solution. The concentration of the acid solution used is not particularly limited, but is preferably 10% by mass or less, and more preferably 5% by mass or less. The pH of the acid solution used is also not particularly limited, but is preferably 0 to 4, and more preferably 1 to 3. Examples of acids that can be used in the acid solution include inorganic acids, sulfonic acids, and carboxylic acids. Examples of inorganic acids include sulfuric acid, nitric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, and boric acid. Examples of sulfonic acids include methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, and trifluoromethanesulfonic acid. Examples of carboxylic acids include formic acid, acetic acid, citric acid, gluconic acid, lactic acid, oxalic acid, and tartaric acid. Among these, hydrochloric acid or sulfuric acid is particularly preferred. The temperature of the acid solution used in the acid treatment is not particularly limited, but is preferably 5°C or higher and 100°C or lower, and more preferably 20°C or higher and 90°C or lower. The immersion time in the acid solution used in the acid treatment is not particularly limited, but is preferably 5 minutes or higher and 120 minutes or lower, and more preferably 10 minutes or higher and 60 minutes or lower. The amount of acid solution used in the acid treatment is not particularly limited, but is preferably 100 parts by mass or higher and 100,000 parts by mass or lower, and more preferably 1,000 parts by mass or higher and 10,000 parts by mass or lower, per 100 parts by mass of the fiber raw material (bone dry mass).

[0054] (Defibrillation Treatment Step) Fine fibrous cellulose can be obtained by defibrillating the phosphorus oxoacid group-introduced fibers in the defibrillation treatment step. In the defibrillation treatment step, for example, a defibrillation treatment device can be used. The defibrillation treatment device is not particularly limited, but examples that can be used include a high-speed defibrillator, a grinder (stone mill-type grinder), a high-pressure homogenizer, an ultra-high-pressure homogenizer, a high-pressure collision grinder, a ball mill, a bead mill, a disk-type refiner, a conical refiner, a twin-screw kneader, a vibration mill, a homomixer under high-speed rotation, an ultrasonic disperser, or a beater. Among the above defibrillation treatment devices, it is preferable to use a high-speed defibrillator, a high-pressure homogenizer, or an ultra-high-pressure homogenizer, which are less affected by the grinding media and have less risk of contamination.

[0055] In the defibration process, it is preferable to dilute the phosphorus oxo acid group-introduced fibers with a dispersion medium to form a slurry. The dispersion medium can be one or more selected from water and organic solvents such as polar organic solvents. The polar organic solvent is not particularly limited, but examples thereof include alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotic polar solvents. Examples of alcohols include methanol, ethanol, isopropanol, n-butanol, and isobutyl alcohol. Examples of polyhydric alcohols include ethylene glycol, propylene glycol, and glycerin. Examples of ketones include acetone and methyl ethyl ketone (MEK). Examples of ethers include diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol monomethyl ether. Examples of esters include ethyl acetate and butyl acetate. Examples of aprotic polar solvents include dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidinone (NMP).

[0056] The solids concentration of the fine fibrous cellulose during the defibration treatment can be appropriately set. The slurry obtained by dispersing the phosphorus oxo acid group-introduced fibers in a dispersion medium may contain solids other than the phosphorus oxo acid group-introduced fibers, such as urea, which has hydrogen bonding properties.

[0057] In one embodiment of the electrolyte membrane for a polymer electrolyte fuel cell, the content of the fine fibrous cellulose in the solid content is preferably 80% by mass or more and 100% by mass or less, more preferably 85% by mass or more, even more preferably 90% by mass or more, and may be 95% by mass or less, from the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell.

[0058] In another embodiment of the electrolyte membrane for polymer electrolyte fuel cells, the content of the fine fibrous cellulose in the solid content is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more, even more preferably 30% by mass or more, and more preferably 70% by mass or less, and even more preferably 60% by mass or less. When the electrolyte membrane for polymer electrolyte fuel cells of this embodiment contains poly(vinyl phosphonic acid)-coated cellulose nanocrystals described below, the content of the fine fibrous cellulose in the solid content of the electrolyte membrane for polymer electrolyte fuel cells is preferably within the above-mentioned range. In the above-mentioned case, the content of the fine fibrous cellulose in the solid content of the electrolyte membrane for polymer electrolyte fuel cells is preferably equal to or greater than the above-mentioned lower limit from the viewpoint of improving the proton conductivity of the electrolyte membrane for polymer electrolyte fuel cells and the mechanical strength of the electrolyte membrane for polymer electrolyte fuel cells, and is preferably equal to or less than the above-mentioned upper limit from the viewpoint of improving the proton conductivity of the electrolyte membrane for polymer electrolyte fuel cells.

[0059] <Polymer Having Sulfonic Acid Groups> The solid polymer fuel cell electrolyte membrane of this embodiment may contain a filler from the viewpoint of the mechanical strength and thinning of the solid polymer fuel cell electrolyte membrane. Examples of fillers include polymers having sulfonic acid groups, sulfonated silica nanoparticles, carbon nanotubes, and MOFs (metal organic frameworks), with polymers having sulfonic acid groups being preferred. Examples of polymers having sulfonic acid groups include perfluorosulfonic acid polymers, sulfonated polyether ether ketones, sulfonated polybenzimidazoles, sulfonated polyether ether ketones, polystyrene sulfonic acid, and sulfonated polytrifluorostyrenes. The content of the filler in the solid polymer fuel cell electrolyte membrane can be, for example, less than 10% by mass, or 5% by mass or less, or 0% by mass or more.

[0060] <Poly(vinylphosphonic acid)-coated cellulose nanocrystals> From the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, the electrolyte membrane for a polymer electrolyte fuel cell of this embodiment preferably contains poly(vinylphosphonic acid)-coated cellulose nanocrystals. The poly(vinylphosphonic acid)-coated cellulose nanocrystals have a surface that is partially or entirely coated with poly(vinylphosphonic acid).

[0061] Cellulose nanocrystals can be obtained by treating cellulose fibers with acid. Cellulose fibers are a fiber material containing cellulose. While not particularly limited, pulp is preferably used as the cellulose fiber due to its availability and low cost. Examples of pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of wood pulp include, but are not limited to, chemical pulps such as hardwood kraft pulp (LBKP), softwood kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP); semi-chemical pulps such as semi-chemical pulp (SCP) and chemi-groundwood pulp (CGP); and mechanical pulps such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Non-wood pulps include, but are not limited to, cotton-based pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, straw, bamboo, and bagasse. Deinked pulps include, but are not limited to, deinked pulp made from recycled paper. The pulp of this embodiment may be used alone or in combination with two or more of the above. From the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, the cellulose nanocrystals may have an ionic functional group, and a specific example of the ionic functional group is a phosphorus oxoacid group. Cellulose nanocrystals can be produced, for example, by referring to JP 2022-132151 A.

[0062] In the poly(vinylphosphonic acid)-coated cellulose nanocrystals, the mass ratio of poly(vinylphosphonic acid) to cellulose nanocrystals (poly(vinylphosphonic acid) / cellulose nanocrystals) is preferably 1 / 99 or more and 40 / 60 or less, more preferably 5 / 95 or more, even more preferably 15 / 85 or more, and more preferably 30 / 70 or less, and even more preferably 25 / 75 or less. From the viewpoint of improving the proton conductivity of the electrolyte membrane for solid polymer fuel cells, this mass ratio (poly(vinylphosphonic acid) / cellulose nanocrystals) is preferably equal to or greater than the above-mentioned lower limit, and from the viewpoint of the mechanical strength of the electrolyte membrane for solid polymer fuel cells, it is preferably equal to or less than the above-mentioned upper limit.

[0063] When the electrolyte membrane for a polymer electrolyte fuel cell contains poly(vinylphosphonic acid)-coated cellulose nanocrystals, the content of the poly(vinylphosphonic acid)-coated cellulose nanocrystals in the solid content of the electrolyte membrane for a polymer electrolyte fuel cell is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more, even more preferably 30% by mass or more, and more preferably 75% by mass or less, and even more preferably 70% by mass or less. From the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, the content of the poly(vinylphosphonic acid)-coated cellulose nanocrystals is preferably equal to or more than the above-mentioned lower limit, and from the viewpoint of the mechanical strength of the electrolyte membrane for a polymer electrolyte fuel cell, it is preferably equal to or less than the above-mentioned upper limit.

[0064] When the electrolyte membrane for a polymer electrolyte fuel cell contains poly(vinylphosphonic acid)-coated cellulose nanocrystals, the mass ratio of the poly(vinylphosphonic acid)-coated cellulose nanocrystals to the fine fibrous cellulose in the electrolyte membrane for a polymer electrolyte fuel cell (poly(vinylphosphonic acid)-coated cellulose nanocrystals / fine fibrous cellulose) is preferably 20 / 80 or more and 80 / 20 or less, more preferably 30 / 70 or more, even more preferably 40 / 60 or more, more preferably 70 / 30 or less, and even more preferably 60 / 40 or less. From the viewpoint of improving the proton conductivity of the electrolyte membrane for a polymer electrolyte fuel cell, the mass ratio (poly(vinylphosphonic acid)-coated cellulose nanocrystals / fine fibrous cellulose) is preferably at least the lower limit mentioned above, and from the viewpoint of the mechanical strength of the electrolyte membrane for a polymer electrolyte fuel cell, it is preferably at most the upper limit mentioned above.

[0065] Poly(vinylphosphonic acid)-coated cellulose nanocrystals can be produced by conventional methods, specifically by referring to, for example, ACS Applied Materials & Interfaces 2022, 14 (6), 8353-8360, Nanoscale Advances 2022, 4 (22), 4714-4723, and Energy Fuels 2022, 36, 13924-13929.

[0066] <Water> Furthermore, from the viewpoint of improving proton conductivity, the electrolyte membrane for a polymer electrolyte fuel cell of this embodiment preferably contains a certain amount of water or more, and the water content in the electrolyte membrane for a polymer electrolyte fuel cell is preferably 3 mass % or more and 20 mass % or less, more preferably 5 mass % or more, even more preferably 7 mass % or more, and more preferably 16 mass % or less, even more preferably 12 mass % or less.

[0067] <Other Components> The electrolyte membrane for a polymer electrolyte fuel cell according to this embodiment may contain components other than the fine fibrous cellulose, the polymer having sulfonic acid groups, the poly(vinylphosphonic acid)-coated cellulose nanocrystals, and water, provided that the effects of the present invention are not impaired. Examples of such components include hydrophilic polymers, hydrophilic low-molecular-weight compounds, paper strength agents, thermoplastic resins, surfactants, organic ions, coupling agents, inorganic layered compounds, inorganic compounds, leveling agents, preservatives, antifoaming agents, organic particles, lubricants, antistatic agents, UV protection agents, dyes, pigments, stabilizers, magnetic powders, alignment promoters, plasticizers, dispersants, color inhibitors, polymerization inhibitors, pH adjusters, and crosslinking agents. The content of the above components in the electrolyte may be, for example, 5% by mass or less, 3% by mass or less, or 0% by mass or more.

[0068] <Thickness> The thickness of the electrolyte membrane for a polymer electrolyte fuel cell of this embodiment can be appropriately determined depending on the size of the membrane electrode assembly into which the electrolyte membrane for a polymer electrolyte fuel cell is incorporated and the size of the polymer electrolyte fuel cell. The thickness of the electrolyte membrane for a polymer electrolyte fuel cell of this embodiment can be, for example, 10 μm or more and 100 μm or less, or may be 25 μm or more and 75 μm or less.

[0069] <Water Contact Angle> The water contact angle of the electrolyte membrane for a polymer electrolyte fuel cell according to this embodiment is preferably low, specifically, preferably 80° or less, more preferably 60° or less, and even more preferably 40° or less. The lower limit is not particularly limited, but is preferably 30° or more. The water contact angle is measured in accordance with JIS R 3257:1999 using a dynamic water contact angle tester (Fibro, 1100DAT) by dropping 4 μL of distilled water onto the surface of the electrolyte membrane for a polymer electrolyte fuel cell and measuring the angle 30 seconds after the drop. The measurement is performed on both sides of the electrolyte membrane for a polymer electrolyte fuel cell. If the water contact angles on both sides are different, the average is used as the water contact angle.

[0070] [Solid polymer electrolyte membrane for water electrolysis] As described above, the electrolyte membrane for a solid polymer fuel cell of this embodiment and the electrolyte membrane for solid polymer water electrolysis of this embodiment have the same configuration except for their intended use.

[0071] [Membrane Electrode Assembly (Comprising a Polymer Electrolyte Fuel Cell Electrolyte Membrane)] The membrane electrode assembly (MEA) of this embodiment is formed by joining a cathode catalyst layer, a polymer electrolyte fuel cell membrane of this embodiment, and an anode catalyst layer in this order. The membrane electrode assembly of this embodiment may have a configuration similar to that of a known membrane electrode assembly that can be used in a polymer electrolyte fuel cell, except for the presence of the polymer electrolyte fuel cell membrane of this embodiment. For details of the membrane electrode assembly, see, for example, Japanese Patent Application Laid-Open No. 2022-190524. The membrane electrode assembly of this embodiment may have a gas diffusion layer on the outer side of the cathode catalyst layer (the side of the cathode catalyst layer opposite to the side bearing the polymer electrolyte fuel cell membrane). The membrane electrode assembly of this embodiment may also have a gas diffusion layer on the outer side of the anode catalyst layer (the side of the anode catalyst layer opposite to the side bearing the polymer electrolyte fuel cell membrane). The thicknesses of the cathode catalyst layer, anode catalyst layer, and gas diffusion layer can be determined appropriately according to the size of the polymer electrolyte fuel cell.

[0072] [Membrane Electrode Assembly (Equipped with Solid Polymer Water Electrolyte Membrane)] Another membrane electrode assembly (MEA) according to this embodiment is formed by joining a cathode catalyst layer, the solid polymer water electrolysis electrolyte membrane of this embodiment, and an anode catalyst layer in this order. The membrane electrode assembly according to this embodiment may have the same configuration as a known membrane electrode assembly that can be used in a solid polymer water electrolysis device, except for the inclusion of the solid polymer water electrolysis electrolyte membrane of this embodiment. For details of the membrane electrode assembly, see, for example, Electrochemistry, 85(1), 28-33(2017), JP 2023-41182 A, and WO 2014 / 157389. The membrane electrode assembly according to this embodiment may have a gas diffusion layer on the outer side of the cathode catalyst layer (the side of the cathode catalyst layer opposite to the side with the solid polymer water electrolysis electrolyte membrane). The membrane electrode assembly of the present embodiment may also have a gas diffusion layer on the outer side of the anode catalyst layer (the surface of the anode catalyst layer opposite to the surface having the solid polymer water electrolysis electrolyte membrane). The thicknesses of the cathode catalyst layer, the anode catalyst layer, and the gas diffusion layer can be appropriately determined depending on the size of the solid polymer water electrolysis device.

[0073] [Polymer Electrolyte Fuel Cell] The polymer electrolyte fuel cell of this embodiment can have the same configuration as known polymer electrolyte fuel cells, except that it has the membrane electrode assembly of this embodiment.

[0074] [Solid polymer water electrolysis device] The solid polymer water electrolysis device (solid polymer electrolyte water electrolysis device, hydrogen production device) of this embodiment can have the same configuration as a known solid polymer water electrolysis device, except for having another membrane electrode assembly of this embodiment. For known solid polymer water electrolysis devices, see, for example, Electrochemistry, 85(1), 28-33(2017), JP 2023-41182 A, and WO 2014 / 157389.

[0075] [Method for manufacturing electrolyte membrane] The method for manufacturing an electrolyte membrane of this embodiment includes the following preparation step and membrane formation step in this order, in which the wood- or plant-derived fine fibrous cellulose in the preparation step has an average fiber width of 50 nm or less, and the wood- or plant-derived fine fibrous cellulose has phosphorus oxo acid groups. Preparation step: A step of preparing a wood- or plant-derived fine fibrous cellulose dispersion. Membrane formation step: A step of forming a membrane from the wood- or plant-derived fine fibrous cellulose dispersion.

[0076] The preparation step can be carried out, for example, in the same manner as in the above-mentioned "method for producing fine fibrous cellulose." When components other than fine fibrous cellulose are to be contained in the electrolyte membrane of this embodiment, for example, a fine fibrous cellulose dispersion containing the desired components can be obtained by mixing the components other than fine fibrous cellulose with a dispersion containing fine fibrous cellulose obtained through the method for producing fine fibrous cellulose. The membrane formation step is a conventionally known method.

[0077] From the viewpoints of improving the proton conductivity of the resulting electrolyte membrane, reducing the temperature dependence of proton conductivity, and reducing the activation energy of proton conduction, the method for producing the electrolyte membrane of this embodiment preferably includes the following ion exchange step between the preparation step and the membrane formation step: Ion exchange step: A step of treating a dispersion of fine fibrous cellulose derived from wood or plant materials with a cation exchange resin The cation exchange resin may be either a strongly acidic ion exchange resin or a weakly acidic ion exchange resin, with a strongly acidic ion exchange resin being preferred.

[0078] [Method for manufacturing a membrane electrode assembly (including an electrolyte membrane for a polymer electrolyte fuel cell)] The method for manufacturing a membrane electrode assembly of this embodiment includes a step of bonding a cathode catalyst layer to one surface of the electrolyte membrane for a polymer electrolyte fuel cell obtained by the method for manufacturing an electrolyte membrane of this embodiment, and bonding a cathode catalyst layer to the other surface of the electrolyte membrane for a polymer electrolyte fuel cell. The "bonding" can be performed by a conventional method. For the method for manufacturing a membrane electrode assembly, see, for example, JP 2022-190524 A.

[0079] [Method for manufacturing a membrane electrode assembly (including a solid polymer electrolyte membrane for water electrolysis)] Another method for manufacturing a membrane electrode assembly according to this embodiment includes a step of bonding a positive electrode catalyst layer to one surface of the solid polymer electrolyte membrane for water electrolysis obtained by the method for manufacturing an electrolyte membrane according to this embodiment, and bonding a negative electrode catalyst layer to the other surface of the solid polymer electrolyte membrane for water electrolysis. The "bonding" can be performed by a conventional method. For the method for manufacturing a membrane electrode assembly, see, for example, Electrochemistry, 85(1), 28-33(2017), JP 2023-41182 A, and WO 2014 / 157389.

[0080] [Method for manufacturing a polymer electrolyte fuel cell] The method for manufacturing a polymer electrolyte fuel cell of this embodiment includes a step of incorporating the membrane electrode assembly obtained by the method for manufacturing a membrane electrode assembly of this embodiment into a polymer electrolyte fuel cell. For details on the method for manufacturing a polymer electrolyte fuel cell, see, for example, JP 2022-190524 A.

[0081] [Method for manufacturing a solid polymer water electrolysis device] The method for manufacturing a solid polymer water electrolysis device of the present embodiment includes a step of incorporating the membrane electrode assembly obtained by the method for manufacturing a membrane electrode assembly of the present embodiment into a solid polymer water electrolysis device. The solid polymer water electrolysis device of the present embodiment can be manufactured by referring to, for example, Electrochemistry, 85(1), 28-33(2017) and JP2023-41182A.

[0082] The features of the present invention will be explained in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, treatment details, treatment procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below. Furthermore, unless otherwise specified, the operations in the examples and comparative examples were carried out at room temperature (20 to 25°C) and normal humidity (40 to 50% RH (relative humidity)).

[0083] Example 1 [Preparation Step] (Phosphorus Oxo Acid Group Introduction Step) As a raw material pulp, softwood bleached kraft pulp (solid content 93% by mass, basis weight 245 g / m) manufactured by Oji Paper Co., Ltd. was used. ２A sheet-like pulp (disintegrated, with a Canadian Standard Freeness (CSF) of 700 ml as measured in accordance with JIS P 8121-2:2012) was used. This raw pulp was subjected to a phosphorus oxo-oxidation treatment as follows. First, a mixed aqueous solution of 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water was added to 100 parts by mass (bone dry mass) of the raw pulp to obtain a chemical-impregnated pulp. Next, the obtained chemical-impregnated pulp was heated in a hot air dryer at 165°C for 250 seconds to obtain a pulp in which phosphorus oxo acid groups had been introduced into the cellulose in the pulp (phosphated pulp).

[0084] (Washing step) The resulting phosphorylated pulp was then washed. The washing step was carried out by repeatedly adding 10 L of ion-exchanged water to 100 g (bone dry mass) of phosphorylated pulp to obtain a pulp dispersion, stirring the resulting mixture to uniformly disperse the pulp, and then filtering and dehydrating the mixture. The washing was completed when the electrical conductivity of the filtrate reached 100 μS / cm or less.

[0085] (Neutralization Treatment) Next, the washed phosphorylated pulp was neutralized as follows. First, the washed phosphorylated pulp was diluted with 10 L of ion-exchanged water, and then a 1N aqueous solution of sodium hydroxide was added little by little while stirring to obtain a phosphorylated pulp dispersion having a pH of 12 to 13. Next, the phosphorylated pulp dispersion was dehydrated and washed to obtain neutralized phosphorylated pulp. The infrared absorption spectrum of the obtained phosphorylated pulp was measured using FT-IR. As a result, －１ The phosphorylated pulp was analyzed using an X-ray diffractometer, and typical peaks were observed at two positions, around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of cellulose type I crystals.

[0086] (Defibrillation Treatment Step) Ion-exchanged water was added to the obtained phosphorylated pulp to prepare a dispersion having a solid content of 2% by mass. This dispersion was treated five times at a pressure of 200 MPa using a wet pulverization device (Starburst, manufactured by Sugino Machine Co., Ltd.) to obtain a fine fibrous cellulose dispersion (1) containing fine fibrous cellulose. The counter ions of the phosphorus oxoacid groups contained in the fine fibrous cellulose dispersion (1) were Na ＋ X-ray diffraction confirmed that the obtained fine fibrous cellulose maintained a cellulose type I crystal structure. Furthermore, the fiber width of the fine fibrous cellulose was measured using a transmission electron microscope and found to be 3 to 5 nm. The average fiber width of the fine fibrous cellulose was 3.5 nm. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of amount of phosphorus oxo acid group] was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0087] [Ion exchange step] Ion exchange water was added to 25 g of a fine fibrous cellulose dispersion (1) having a solid content concentration of 2% by mass to obtain 100 g of a fine fibrous cellulose dispersion (A) having a solid content concentration of 0.5% by mass. 5 g of a strongly acidic ion exchange resin (manufactured by Mitsubishi Chemical Corporation, conditioned) was added to the obtained fine fibrous cellulose dispersion (A) and stirred for 2 hours. The ion exchange resin and the dispersion were then separated by centrifugation to obtain a fine fibrous cellulose dispersion (B).

[0088] [Membrane formation process] The obtained fine fibrous cellulose dispersion (B) was poured into a Petri dish and then cast-dried at 20°C and 30% RH to obtain an electrolyte membrane. The thickness of the obtained electrolyte membrane was 48 μm. In addition, the counter ion of the phosphorus oxo acid group of the fine fibrous cellulose contained in the electrolyte membrane was H ＋ It was.

[0089] Example 2 100 g of a fine fibrous cellulose dispersion (A) was prepared in the same manner as in Example 1, but without treatment with a strongly acidic ion exchange resin. 100 g of the fine fibrous cellulose dispersion (A) was poured into a Petri dish and cast-dried at 20°C and 30% RH to obtain an electrolyte membrane. The thickness of the obtained electrolyte membrane was 50 μm. The counter ions of the phosphorus oxo acid groups of the fine fibrous cellulose contained in the electrolyte membrane were Na. ＋ It was.

[0090] Example 3 In the preparation step of Example 1, the phosphorylated pulp after washing was further subjected to the phosphorus oxoacid group introduction step and the washing step, in that order, once each. The remaining procedures were the same as in Example 1, and an electrolyte membrane with a thickness of 50 μm was obtained. The counter ions of the phosphorus oxoacid groups of the fine fibrous cellulose contained in the electrolyte membrane were H ＋ The infrared absorption spectrum of the neutralized phosphorylated pulp was measured using FT-IR. －１ Absorption due to the P=O of phosphorus oxoacid groups was observed near the peak, confirming that phosphorus oxoacid groups had been added to the pulp. Furthermore, when the obtained phosphorylated pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. Furthermore, the fiber width of the fine fibrous cellulose obtained in the (defibration treatment step) was measured using a transmission electron microscope and found to be 3 to 5 nm. The average fiber width of the fine fibrous cellulose was 3.5 nm. The amount of phosphate groups (amount of first dissociated acid) measured using the method described in [Measurement of phosphorus oxoacid group amount] above was 1.99 mmol / g. The total amount of dissociated acid was 3.29 mmol / g.

[0091] Example 4 In the preparation step (phosphorus oxo acid group introduction step) of Example 1, the conditions for heating the chemical-impregnated pulp were changed to heating it for 200 seconds in a hot air dryer at 145°C. The other procedures were the same as in Example 1, and an electrolyte membrane with a thickness of 50 µm was obtained. The counter ions of the phosphorus oxo acid groups of the fine fibrous cellulose contained in the electrolyte membrane were H＋ The infrared absorption spectrum of the neutralized phosphorylated pulp was measured using FT-IR. －１ Absorption due to the P=O of phosphorus oxoacid groups was observed near the peak, confirming that phosphorus oxoacid groups had been added to the pulp. Furthermore, when the obtained phosphorylated pulp was analyzed using an X-ray diffractometer, typical peaks were observed at two positions, near 2θ = 14° to 17° and near 2θ = 22° to 23°, confirming the presence of cellulose type I crystals. Furthermore, the fiber width of the fine fibrous cellulose obtained in the (defibration treatment step) was measured using a transmission electron microscope and found to be 3 to 5 nm. The average fiber width of the fine fibrous cellulose was 3.9 nm. The amount of phosphate groups (amount of first dissociated acid) measured using the method described in [Measurement of phosphorus oxoacid group amount] above was 0.87 mmol / g. The total amount of dissociated acid was 1.51 mmol / g.

[0092] Example 5 [Preparation of Poly(vinylphosphonic acid)-Coated Cellulose Nanocrystal Aqueous Dispersion] O-Ethyl-S-(1-ethoxycarbonyl)-ethyl dithiocarbonate was synthesized as a RAFT reagent (chain transfer agent) according to ACS Applied Materials & Interfaces 2022, 14 (6), 8353-8360. Cellulose nanocrystals coated with poly(vinylphosphonic acid) were then prepared according to Nanoscale Advances 2022, 4 (22), 4714-4723 and Energy Fuels 2022, 36, 13924-13929. In the poly(vinylphosphonic acid)-coated cellulose nanocrystals, the mass ratio of cellulose nanocrystals to poly(vinylphosphonic acid) (cellulose nanocrystals:poly(vinylphosphonic acid)) was 4:1. The obtained poly(vinylphosphonic acid)-coated cellulose nanocrystals were dispersed in ion-exchanged water to obtain a poly(vinylphosphonic acid)-coated cellulose nanocrystal aqueous dispersion. [Preparation of cellulose aqueous dispersion] The poly(vinylphosphonic acid)-coated cellulose nanocrystal aqueous dispersion and the fine fibrous cellulose dispersion (B) obtained in Example 1 were mixed so that the solid content of both was 1:1 by mass to obtain a mixed solution. [Membrane formation process] The above mixed solution was poured into a Petri dish and then cast-dried at 20°C and 30% RH to produce a sheet. The obtained sheet was pressed at a pressure of 10 MPa for 5 minutes to obtain an electrolyte membrane. The thickness of the obtained electrolyte membrane was 50 μm. In addition, the counter ion of the phosphorus oxo acid group possessed by the fine fibrous cellulose contained in the electrolyte membrane was H ＋ It was.

[0093] Example 6 In the preparation step (phosphoric acid group introduction step) of Example 1, 33 parts by mass of phosphorous acid (phosphonic acid) was used instead of ammonium dihydrogen phosphate. The other procedures were the same as in Example 1, and an electrolyte membrane with a thickness of 50 μm was obtained. The counter ions of the phosphoric acid groups of the fine fibrous cellulose contained in the electrolyte membrane were H ＋ The infrared absorption spectrum of the phosphorous-containing pulp after neutralization was measured using FT-IR. －１Absorption due to P=O of a phosphonic acid group, which is a tautomer of a phosphorous acid group, was observed near the nucleus, confirming that a phosphorous acid group (phosphonic acid group) had been added to the pulp. Furthermore, X-ray diffraction confirmed that the obtained fine fibrous cellulose maintained cellulose type I crystals. Furthermore, the fiber width of the fine fibrous cellulose obtained in the (defibration treatment step) was measured using a transmission electron microscope and found to be 3 to 5 nm. Furthermore, the average fiber width of the fine fibrous cellulose was 3.8 nm. The amount of phosphorous acid groups (amount of first dissociated acid) measured by the measurement method described below in [Measurement of phosphorus oxo acid group amount] was 1.51 mmol / g, and the total amount of dissociated acid was 1.54 mmol / g.

[0094] Comparative Example 1 Ion-exchanged water was added to 10 g of a fine fibrous cellulose dispersion (WFo-10005, average fiber width 10 to 50 nm, manufactured by Sugino Machine Corporation) having a solids concentration of 5.0% by mass to prepare 100 g of a fibrous cellulose dispersion (C) having a solids concentration of 0.5% by mass. The obtained fine fibrous cellulose dispersion (C) was poured into a Petri dish and then cast-dried at 20°C and 30% RH to obtain an electrolyte membrane. The thickness of the obtained electrolyte membrane was 60 μm. Furthermore, the fine fibrous cellulose contained in the electrolyte membrane did not have any ionic functional groups.

[0095] <Measurement Method> [Measurement of Phosphorus Oxo Acid Group Amount in Fine Fibrous Cellulose] The amount of phosphorus oxo acid groups in fine fibrous cellulose was measured by treating a fine fibrous cellulose-containing dispersion prepared by diluting the target fine fibrous cellulose dispersion with ion exchange water to a fine fibrous cellulose content of 0.2% by mass with ion exchange resin, followed by titration with alkali. The ion exchange resin treatment was performed by adding 1 / 10 by volume of a strongly acidic ion exchange resin (Amberjet 1024, conditioned, manufactured by Organo Corporation) to the fine fibrous cellulose dispersion, shaking for 1 hour, and then pouring the mixture onto a 90 μm mesh to separate the ion exchange resin from the dispersion. The alkali titration was performed by adding 10 μL of 0.1 N aqueous sodium hydroxide solution to the fine fibrous cellulose dispersion after ion exchange resin treatment at 5-second intervals, while measuring the change in pH of the dispersion.

[0096] In this neutralization titration, two maximum points of increment (the differential value of pH with respect to the amount of sodium hydroxide added) are observed on a curve plotting measured pH against the amount of sodium hydroxide added. Of these, the first maximum point of increment obtained after starting to add sodium hydroxide is called the first endpoint, and the next maximum point of increment obtained is called the second endpoint (Figure 1). The amount of sodium hydroxide required from the start of titration to the first endpoint is equal to the amount of first dissociated acid in the dispersion used for titration. Furthermore, the amount of sodium hydroxide required from the start of titration to the second endpoint is equal to the total amount of dissociated acid in the dispersion used for titration. The amount of sodium hydroxide required from the start of titration to the first endpoint (mmol) divided by the solids content (g) in the dispersion to be titrated was defined as the amount of phosphorus oxoacid groups (mmol / g).

[0097] <Evaluation Method> [Proton Conductivity of Electrolyte Membrane] The proton conductivity of the obtained electrolyte membrane at a relative humidity of 95% and a temperature of 20 to 80°C was measured using an impedance analyzer (IM3570, Hioki E.E. Corporation, frequency: 4.6 to 4.6 × 10) in an environmental control device (desktop thermo-hygrostat, SH-222, Espec Corporation). ６Hz, four-terminal method) by an alternating current (AC) impedance method. Proton conductivity measurements were performed after the electrolyte membrane was left to stand for a sufficient time until the water uptake by the membrane was saturated and the proton conductivity stabilized. From the Cole-Cole plot, the inflection point was regarded as the bulk resistance of each electrolyte membrane. Proton conductivity σ (S / cm) was calculated using the following formula. The higher the proton conductivity, the more the cell performance such as power generation efficiency can be improved. σ = d / (Rs × S) d (cm): distance between gold wires Rs (Ω): bulk impedance Rs (Ω) S (cm ２ ): Proton conduction area calculated by film thickness x width

[0098] [Temperature Dependence of Proton Conductivity of Electrolyte Membrane] The proton conductivity was calculated by dividing the proton conductivity at 80°C by the proton conductivity at 20°C.

[0099] [Activation Energy (Ea) of Proton Conduction in Electrolyte Membrane] The activation energy Ea (eV) of proton conduction was calculated using the Arrhenius formula shown below. The smaller the activation energy of proton conduction, the smaller the change in proton conductivity with temperature change, and the lower the rate of change in proton conductivity and voltage / current density, thereby improving battery performance such as electrical energy conversion efficiency and energy density. σ × T = σ ０ exp(-Ea / kT) σ(S / cm): Proton conductivity T(K): Absolute temperature σ ０ : pre-exponential factor k: Boltzmann constant (8.6171 x 10 －５ eV / K)

[0100]

[0101] The electrolyte membrane containing the wood- or plant-derived fine fibrous cellulose (having an average fiber width of 50 nm or less and having phosphorus oxo acid groups) specified in the present invention has excellent proton conductivity (Examples 1 to 5). Furthermore, the counter ions of the phosphate groups are H ＋The electrolyte membrane containing the fine fibrous cellulose also exhibits excellent temperature dependence of proton conductivity and activation energy of proton conduction (Examples 1, 3 to 5). Although not shown in Table 1, the proton conductivity, temperature dependence of proton conductivity, and activation energy of proton conduction of Example 6 were equivalent to those of Example 4. In contrast, an electrolyte membrane containing wood- or plant-derived fine fibrous cellulose with an average fiber width of 50 nm or less but no phosphorus oxoacid groups exhibits low proton conductivity and a large temperature dependence of proton conductivity (Comparative Example 1).

Claims

1. An electrolyte membrane for a polymer electrolyte fuel cell containing fine fibrous cellulose derived from wood or plants, The wood- or plant-derived fine fibrous cellulose has an average fiber width of 50 nm or less, and the wood- or plant-derived fine fibrous cellulose has phosphorus oxo acid groups. Electrolyte membrane for polymer electrolyte fuel cells.

2. The counter ion of the phosphorus oxoacid group is H ＋ and / or Na ＋ The electrolyte membrane for a polymer electrolyte fuel cell according to claim 1, comprising:

3. The counter ion of the phosphorus oxoacid group is H ＋ The electrolyte membrane for a polymer electrolyte fuel cell according to claim 1 or 2, comprising:

4. 3. The electrolyte membrane for a polymer electrolyte fuel cell according to claim 1, wherein the amount of phosphorus oxo acid groups introduced into the wood- or plant-derived fine fibrous cellulose is 0.50 mmol / g or more.

5. 3. The electrolyte membrane for a polymer electrolyte fuel cell according to claim 1, wherein the content of the wood- or plant-derived fine fibrous cellulose in the solid content of the electrolyte membrane for a polymer electrolyte fuel cell is 80 mass % or more.

6. 3. The electrolyte membrane for a polymer electrolyte fuel cell according to claim 1, wherein the content of the wood- or plant-derived fine fibrous cellulose in the solid content of the electrolyte membrane for a polymer electrolyte fuel cell is 30 mass % or more.

7. 3. A membrane electrode assembly comprising a positive electrode catalyst layer, the electrolyte membrane for a polymer electrolyte fuel cell according to claim 1, and a negative electrode catalyst layer joined in this order.

8. A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 7.

9. A method for producing an electrolyte membrane for a polymer electrolyte fuel cell, comprising the following preparation step and membrane formation step in this order: A method for producing an electrolyte membrane for a polymer electrolyte fuel cell, wherein the wood- or plant-derived fine fibrous cellulose in the preparation step described below has an average fiber width of 50 nm or less, and the wood- or plant-derived fine fibrous cellulose has phosphorus oxo acid groups. Preparation step: A step of preparing a dispersion of fine fibrous cellulose derived from wood or plant materials. Film-forming process: A process of forming a film from a dispersion of fine fibrous cellulose derived from wood or plant materials

10. The method for producing an electrolyte membrane for a polymer electrolyte fuel cell according to claim 9 , further comprising the following ion exchange step between the preparation step and the membrane formation step: Ion exchange process: A process of treating a dispersion of fine fibrous cellulose derived from wood or plants with a cation exchange resin.

11. 11. A method for producing a membrane electrode assembly, comprising the steps of bonding a positive electrode catalyst layer to one surface of the electrolyte membrane for a solid polymer fuel cell obtained by the method for producing an electrolyte membrane for a solid polymer fuel cell according to claim 9 or 10, and bonding a negative electrode catalyst layer to the other surface of the electrolyte membrane for a solid polymer fuel cell.

12. A method for producing a polymer electrolyte fuel cell, comprising a step of incorporating a membrane electrode assembly obtained by the method for producing a membrane electrode assembly according to claim 11.

13. A solid polymer electrolyte membrane for water electrolysis containing fine fibrous cellulose derived from wood or plant materials, The wood- or plant-derived fine fibrous cellulose has an average fiber width of 50 nm or less, and the wood- or plant-derived fine fibrous cellulose has phosphorus oxo acid groups. Electrolyte membrane for solid polymer water electrolysis.