Proton conductor for electrolyte membrane of solid polymer fuel cell, electrolyte membrane for solid polymer fuel cell, membrane electrode assembly, solid polymer fuel cell, manufacturing methods therefor, and proton conductor for electrolyte membrane of solid polymer water electrolysis device

Abstract

The present invention relates to: a proton conductor for an electrolyte membrane of a solid polymer fuel cell, the proton conductor making it possible to obtain an electrolyte membrane for a solid polymer fuel cell, the electrolyte membrane having excellent proton conductivity at high temperatures and being unlikely to decrease in proton conductivity at high temperatures; an electrolyte membrane for a solid polymer fuel cell, the electrolyte membrane containing said proton conductor; a membrane electrode assembly having said electrolyte membrane for a solid polymer fuel cell; a solid polymer fuel cell having said membrane electrode assembly; manufacturing methods for a proton conductor for an electrolyte membrane of a solid polymer fuel cell, an electrolyte membrane for a solid polymer fuel cell, a membrane electrode assembly, and a solid polymer fuel cell; and a proton conductor for an electrolyte membrane of a solid polymer water electrolysis device.　This proton conductor for an electrolyte membrane of a solid polymer fuel cell contains the following component (A) and component (B).　Component (A): fine fibrous cellulose having a fiber width of 1,000 nm or less. Component (B): a polymer compound having one phosphorus oxoacid group.

Description

Proton conductors for electrolyte membranes in polymer electrolyte fuel cells, electrolyte membranes for polymer electrolyte fuel cells, membrane electrode assemblies, polymer electrolyte fuel cells, and methods for manufacturing the same, as well as proton conductors for electrolyte membranes in polymer electrolyte water electrolysis devices.

[0001] The present invention relates to a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell, an electrolyte membrane for a polymer electrolyte fuel cell, a membrane electrode assembly, a polymer electrolyte fuel cell, and methods for manufacturing the same, as well as a proton conductor for the electrolyte membrane of a polymer electrolyte water electrolysis apparatus.

[0002] Fuel cells have a low environmental impact because they can generate energy using hydrogen. Among fuel cells, polymer electrolyte fuel cells (PEFCs) are attracting attention as an energy source for stationary power plants and automobiles because they can operate in a wide temperature range and have high power generation efficiency.

[0003] For example, Patent Document 1 describes a solid polymer electrolyte membrane containing a proton-conducting polymer that is stable at temperatures above 100°C, and a hydrogen or direct methanol fuel cell equipped with the electrolyte membrane, with the aim of providing a solid polymer electrolyte that maintains stable and appropriate ion conductivity at temperatures up to at least 200°C.

[0004] Japanese Patent Application Publication No. 11-503262

[0005] From the perspective of suppressing carbon monoxide poisoning when using fuel cells, it is desirable to increase the operating temperature of the fuel cell. Furthermore, if a fuel cell has excellent ionic conductivity at high temperatures, the need for strict temperature control will be eliminated, and the entire system incorporating the fuel cell can be simplified and miniaturized.

[0006] The solid polymer electrolyte membrane in the fuel cell (solid polymer fuel cell) described in Patent Document 1, which contains a proton-conducting polymer that is stable at temperatures exceeding 100°C, does not exhibit sufficient proton conductivity at high temperatures. Further improvement in proton conductivity at high temperatures is desired to improve the power generation efficiency of the battery. Furthermore, from the viewpoint of extending the battery life, the electrolyte membrane of the solid polymer fuel cell is required to have properties that prevent a decrease in proton conductivity at high temperatures. In addition, further improvement in proton conductivity at high temperatures and properties that prevent a decrease in proton conductivity at high temperatures are also required for proton conductors used in the electrolyte membranes of solid polymer water electrolyzers. The present invention aims to provide a proton conductor for electrolyte membranes of polymer electrolyte fuel cells that exhibits excellent proton conductivity at high temperatures and whose proton conductivity does not decrease easily at high temperatures; an electrolyte membrane for polymer electrolyte fuel cells containing the proton conductor; a membrane electrode assembly having the electrolyte membrane for polymer electrolyte fuel cells; a polymer electrolyte fuel cell having the membrane electrode assembly; a method for manufacturing the proton conductor for electrolyte membranes of polymer electrolyte fuel cells, an electrolyte membrane for polymer electrolyte fuel cells, a membrane electrode assembly, and a polymer electrolyte fuel cell; and a proton conductor for electrolyte membranes of polymer electrolyte water electrolyzers.

[0007] The inventors of the present invention have discovered that by using fine fibrous cellulose with a fiber width of 1,000 nm or less and a polymer compound having a phosphorus oxoacid group as an electrolyte membrane, an electrolyte membrane with excellent proton conductivity at high temperatures can be obtained, and that the proton conductivity does not decrease easily at high temperatures, thus completing the present invention.

[0008] In other words, the present invention relates to the following <1> to <24>. <1> A proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell, comprising the following components (A) and (B): Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less Component (B): Polymer compound having 1 phosphorus oxoacid group <2> The proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to <1>, wherein the mass ratio of component (B) to component (A) (component (B) / component (A)) is 20 / 80 or more and 80 / 20 or less. <3> The proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to <1> or <2>, wherein the degree of polymerization of component (A) is 350 or more. <4> The proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to any one of <1> to <3>, wherein component (A) has 2 phosphorus oxoacid groups. <5> A polymer electrolyte membrane for a polymer electrolyte fuel cell, comprising a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell as described in any one of <1> to <4> and the following component (C): Component (C): Membrane component <6> The polymer electrolyte membrane for a polymer electrolyte fuel cell as described in <5>, wherein the ratio of the total mass of component (A) and component (B) to the mass of component (C) ((component (A) + component (B)) / component (C)) is 5 / 95 or more and 55 / 45 or less. <7> A membrane electrode assembly comprising a positive electrode catalyst layer, a polymer electrolyte membrane for a polymer electrolyte fuel cell as described in <5> or <6>, and a negative electrode catalyst layer joined in this order. <8> A polymer electrolyte fuel cell having the membrane electrode assembly as described in <7>. <9> A method for producing a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell, comprising the following preparation step I and polymerization step in this order. Preparation step I: A step to obtain a mixed solution I containing the following components (A), (b), and a solvent. Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less. Component (b): Monomer having 1 phosphorus oxoacid group. Polymerization step: A step to obtain a proton conductor for electrolyte membranes of a solid polymer fuel cell, containing component (A) and the following component (B). Component (B): Polymer compound having 1 phosphorus oxoacid group. <10> A method for producing an electrolyte membrane for a solid polymer fuel cell, comprising the following preparation step II and film formation step in this order.Preparation step II: A step to obtain a mixture II containing the following component (C), a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell obtained by the method for manufacturing a polymer electrolyte membrane of a polymer electrolyte fuel cell described in <9>, and a solvent. Component (C): Membrane component. Film formation step: A step to obtain an electrolyte membrane for a polymer electrolyte fuel cell by removing the solvent from the mixture II. <11> A method for manufacturing a membrane electrode assembly, comprising the step of joining a positive electrode catalyst layer to one side of an electrolyte membrane for a polymer electrolyte fuel cell obtained by the method for manufacturing an electrolyte membrane of a polymer electrolyte fuel cell described in <10>, and joining a negative electrode catalyst layer to the other side. <12> A method for manufacturing a polymer electrolyte fuel cell, comprising the step of incorporating a membrane electrode assembly obtained by the method for manufacturing a membrane electrode assembly described in <11>. <13> A proton conductor for the electrolyte membrane of a polymer electrolyte water electrolyzer, comprising the following components (A) and (B). Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less Component (B): Polymer compound having 1 phosphorus oxoacid group <14> A proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus according to <13>, wherein the mass ratio of component (B) to component (A) (component (B) / component (A)) is 20 / 80 or more and 80 / 20 or less. <15> A proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus according to <13> or <14>, wherein the degree of polymerization of component (A) is 350 or more. <16> A proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus according to any one of <13> to <15>, wherein component (A) has 2 phosphorus oxoacid groups. <17> An electrolyte membrane for a solid polymer water electrolysis apparatus, comprising a proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus as described in any one of <13> to <16>, and the following component (C). Component (C): Membrane component <18> An electrolyte membrane for a solid polymer water electrolysis apparatus as described in <17>, wherein the ratio of the total mass of component (A) and component (B) to the mass of component (C) ((component (A) + component (B)) / component (C)) is 5 / 95 or more and 55 / 45 or less. <19> A membrane electrode assembly comprising a positive electrode catalyst layer, an electrolyte membrane for a solid polymer water electrolysis apparatus as described in <17> or <18>, and a negative electrode catalyst layer joined in this order.<20> A solid polymer water electrolysis apparatus having the membrane electrode assembly described in <19>. <21> A method for producing a proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus, comprising the following preparation step I and polymerization step in this order: Preparation step I: A step to obtain a mixed solution I containing the following components (A), (b), and a solvent. Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less. Component (b): A monomer having 1 phosphorus oxoacid group. Polymerization step: A step to obtain a proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus, comprising polymerizing component (b) in the mixed solution I to obtain a proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus containing component (A) and the following component (B). Component (B): A polymer compound having 1 phosphorus oxoacid group. <22> A method for producing an electrolyte membrane for a solid polymer water electrolysis apparatus, comprising the following preparation step II and film formation step in this order. Preparation step II: A step to obtain a mixed solution II containing the following component (C), a proton conductor for the electrolyte membrane of a solid polymer water electrolysis device obtained by the method for producing a proton conductor for the electrolyte membrane of a solid polymer water electrolysis device described in <21>, and a solvent. Component (C): Membrane component. Film formation step: A step to obtain an electrolyte membrane for a solid polymer water electrolysis device by removing the solvent from the mixed solution II. <23> A method for producing a membrane electrode assembly, comprising the step of joining a positive electrode catalyst layer to one side of the electrolyte membrane for a solid polymer water electrolysis device obtained by the method for producing an electrolyte membrane for a solid polymer water electrolysis device described in <22>, and joining a negative electrode catalyst layer to the other side. <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 described in <23>.

[0009] According to the present invention, it is possible to provide a proton conductor for electrolyte membranes of polymer electrolyte fuel cells that has excellent proton conductivity at high temperatures and whose proton conductivity does not decrease easily at high temperatures, an electrolyte membrane for polymer electrolyte fuel cells containing the proton conductor, a membrane electrode assembly having the electrolyte membrane for polymer electrolyte fuel cells, and a polymer electrolyte fuel cell having the membrane electrode assembly, as well as methods for manufacturing the proton conductor for electrolyte membranes of polymer electrolyte fuel cells, the electrolyte membrane for polymer electrolyte fuel cells, the membrane electrode assembly, and polymer electrolyte fuel cells, and a proton conductor for electrolyte membranes of polymer electrolyte water electrolyzers.

[0010] Figure 1 is a graph showing the relationship between the amount of NaOH added to a fine fibrous cellulose dispersion containing two phosphorus oxoacid groups and the pH.

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

[0012] [Proton Conductor for Electrolyte Membrane of Polymer Electrolyte Fuel Cell] The proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to this embodiment (hereinafter also simply referred to as "proton conductor") is a composition containing the following components (A) and (B). Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less Component (B): Polymer compound having 1 phosphorus oxoacid group The components contained in or that can be contained in the proton conductor of this embodiment may be used individually or in combination of two or more. The proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell and the electrolyte membrane for a polymer electrolyte fuel cell according to this embodiment, and the proton conductor for the electrolyte membrane of a polymer electrolyte water electrolysis device according to this embodiment, which will be described later, have the same configuration except for their intended use. Hereinafter, "proton conductor" means an electrolyte membrane for a polymer electrolyte fuel cell or an electrolyte membrane for a polymer electrolyte water electrolysis device. Also, "electrolyte membrane" means an electrolyte membrane for a polymer electrolyte fuel cell or an electrolyte membrane for a polymer electrolyte water electrolysis device. Using the proton conductor of this embodiment, it is possible to obtain an electrolyte membrane for polymer electrolyte fuel cells that exhibits excellent proton conductivity at high temperatures (especially in operating environments above 100°C where it is difficult to increase humidity without sealing, even with humidification), and in which proton conductivity does not easily decrease at high temperatures.

[0013] The reason why an electrolyte membrane for polymer electrolyte fuel cells with excellent proton conductivity at high temperatures and less susceptibility to degradation at high temperatures can be obtained by using the membrane component and the proton conductor of this embodiment is not entirely clear, but it is thought to be as follows. An electrolyte membrane for polymer electrolyte fuel cells containing component (A), component (B), and membrane component can be obtained by using the membrane component and the proton conductor of this embodiment. Component (B) itself has excellent proton conductivity, which enables excellent proton conductivity at high temperatures and is thought to be less susceptible to degradation. Furthermore, when an acidic molecule is used, the interaction between the phosphorus oxoacid group 1 of component (B) and the acidic molecule suppresses the elution of acidic molecules from the electrolyte membrane for polymer electrolyte fuel cells even in high-temperature environments, thus enabling even better proton conductivity and further reducing the likelihood of degradation. The proton conductor of this embodiment will be described in detail below.

[0014] [Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less] The upper limit of the fiber width of the fine fibrous cellulose is 1,000 nm or less. The fiber width of the fine fibrous cellulose is preferably 2 nm or more and 1,000 nm or less, more preferably 100 nm or less, even more preferably 50 nm or less, even more preferably 40 nm or less, even more preferably 30 nm or less, even more preferably 20 nm or less, and even more preferably 10 nm or less. By setting the fiber width of the fine fibrous cellulose to 2 nm or more, the dissolution of cellulose molecules in water can be suppressed, and the strength, rigidity, and dimensional stability of the electrolyte membrane for polymer electrolyte fuel cells can be improved. The fiber width of the fine fibrous cellulose can be measured, for example, by electron microscope observation.

[0015] The average fiber width of the fine fibrous cellulose is, for example, 1,000 nm or less. Preferably, the average fiber width of the fine fibrous cellulose is 2 nm or more and 1,000 nm or less, more preferably 100 nm or less, even more preferably 50 nm or less, even more preferably 40 nm or less, even more preferably 30 nm or less, even more preferably 20 nm or less, and even more preferably 10 nm or less. By setting the average fiber width of the fine fibrous cellulose to 2 nm or more, the dissolution of cellulose molecules in water can be suppressed, and the strength, rigidity, and dimensional stability of the electrolyte membrane for polymer electrolyte fuel cells can be improved. Furthermore, by setting the average fiber width of the fine fibrous cellulose to below the above upper limit, the surface of the electrolyte membrane for polymer electrolyte fuel cells can be made smooth. A smooth surface for the electrolyte membrane for polymer electrolyte fuel cells facilitates the movement of protons between the positive electrode catalyst layer, the electrolyte membrane for polymer electrolyte fuel cell, and the negative electrode catalyst layer, which can improve the proton conductivity at high temperatures. The fine fibrous cellulose is, for example, monofilamentous cellulose.

[0016] The average fiber width of fine fibrous cellulose can be measured, for example, using an electron microscope as follows: First, an aqueous suspension of fine fibrous cellulose with a concentration of 0.05% to 0.1% by mass is prepared, and this suspension is cast onto a hydrophilic carbon film-coated grid to prepare a sample for transmission electron microscopy (TEM) observation. If the sample contains wide fibers, a scanning electron microscope (SEM) image of the surface cast on glass may be observed. Next, observation of the electron microscope image is performed at a magnification 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 should be adjusted to satisfy the following conditions: (1) A straight line X is drawn at any point in the observation image, and 20 or more fibers intersect with this line X. (2) A straight line Y is drawn perpendicular to the line X in the same image, and 20 or more fibers intersect with this line Y.

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

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

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

[0020] The axial ratio (fiber length / fiber width) of the microfibrous cellulose is not particularly limited. For example, it is preferably 20 or more and 10,000 or less, more preferably 50 or more and 1,000 or less. By setting the axial ratio to be not less than the above lower limit value, it is easy to form an electrolyte membrane for a solid polymer fuel cell. Also, sufficient thickening property is easily obtained when preparing a solvent dispersion. By setting the axial ratio to be not more than the above upper limit value, for example, when handling microfibrous cellulose as an aqueous dispersion, it is preferable in terms of easy handling such as dilution.

[0021] From the viewpoints of mechanical strength and heat resistance for making the electrolyte membrane for a solid polymer fuel cell into a self-supporting membrane, the degree of polymerization of the microfibrous cellulose is preferably 350 or more, more preferably 400 or more, still more preferably 450 or more, and from the viewpoint of ease of production, it is preferably 700 or less, more preferably 650 or less, still more preferably 600 or less. The microfibrous cellulose can be depolymerized by a depolymerization treatment such as an ozone treatment step, an enzyme treatment step, a hypochlorous acid treatment step, a subcritical water treatment step, a radiation irradiation treatment step, etc.

[0022] The degree of polymerization of the microfibrous cellulose is a value calculated from the viscosity measured according to Tappi T230. Specifically, after measuring the viscosity (designated as η1) of the fibrous cellulose to be measured dissolved in an aqueous solution of copper ethylenediamine and the blank viscosity (designated as η0) measured with only the aqueous solution of copper ethylenediamine, the specific viscosity (ηsp) and the intrinsic viscosity ([η]) are measured according to the following formulas. ηsp = (η1 / η0) - 1 [η] = ηsp / (c(1 + 0.28×ηsp)) Here, c in the formula indicates the concentration (g / mL) of the fibrous cellulose at the time of viscosity measurement. Further, the degree of polymerization (DP) is calculated from the following formula. DP = 1.75×[η] Since this degree of polymerization is the average degree of polymerization measured by the viscosity method, it is sometimes referred to as the "viscosity average degree of polymerization".

[0023] In terms of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell at high temperatures, the fine fibrous cellulose in the present embodiment preferably has a phosphonooxy group 2. In this specification, the "phosphonooxy group 2" shall include substituents derived from the phosphonooxy group. Also, the "counter ion" of the phosphonooxy group 2 shall include dissociable protons. As the counter ion, for example, β of the substituent represented by the following formula (1) described later ｂ＋ (proton (H ＋ ) or a monovalent or higher cation composed of an organic or inorganic substance) can be mentioned. Among them, from the perspective of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell at high temperatures, it is preferable that the counter ion contains at least one selected from the group consisting of H ＋ and ions of an alkali metal (such as sodium, potassium, or lithium, etc.), and it is more preferable to contain H ＋ and / or sodium ions (Na ＋ ). From the perspective of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell at high temperatures, it is even more preferable that the counter ion contains H ＋ . Also, from the perspective of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell at high temperatures, the content (mass) of cations other than H ＋ contained in the counter ion 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.

[0024] The phosphonooxy group 2 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 substituents represented by the following formula (1) introduced in plurality may be the same or different from each other. The substituent represented by the following formula (1) is preferably bonded directly to the carbon atoms at the 2-position, 3-position, and / or 6-position of cellulose through "-O-".

[0025]

[0026] In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (provided that a = b × m). At least one of the n α and α' is O － and the rest are R or OR. Note that all of each α and α' may be O － The n α may all be the same or different from each other. β ｂ＋ is H ＋ or a monovalent or higher-valent cation composed of an organic or inorganic substance.

[0027] Each R is a hydrogen atom, a saturated - straight-chain hydrocarbon group, a saturated - branched-chain hydrocarbon group, a saturated - cyclic hydrocarbon group, an unsaturated - straight-chain hydrocarbon group, an unsaturated - branched-chain hydrocarbon group, an unsaturated - cyclic hydrocarbon group, an aromatic group, or a derivative group thereof. In formula (1), n is preferably 1.

[0028] Examples of the saturated - straight-chain hydrocarbon group include a methyl group, an ethyl group, an n - propyl group, or an n - butyl group, etc., but are not particularly limited. Examples of the saturated - branched-chain hydrocarbon group include an i - propyl group, or a t - butyl group, etc., but are not particularly limited. Examples of the saturated - cyclic hydrocarbon group include a cyclopentyl group, or a cyclohexyl group, etc., but are not particularly limited. Examples of the unsaturated - straight-chain hydrocarbon group include a vinyl group, or an allyl group, etc., but are not particularly limited. Examples of the unsaturated - branched-chain hydrocarbon group include an i - propenyl group, or a 3 - butenyl group, etc., but are not particularly limited. Examples of the unsaturated - cyclic hydrocarbon group include a cyclopentenyl group, a cyclohexenyl group, etc., but are not particularly limited. Examples of the aromatic group include a phenyl group, or a naphthyl group, etc., but are not particularly limited.

[0029] Furthermore, the derivative group in R is a functional group obtained by adding or substituting at least one functional group selected from carboxyl groups, carboxylate groups (-COO-), hydroxyl groups, amino groups, and ammonium groups to the main chain or side chain of the various hydrocarbon groups mentioned above, but is not particularly limited. Also, the number of carbon atoms constituting the main chain of R is not particularly limited, but is preferably 20 or less, 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 phosphorus oxoacid group 2 can be set within an appropriate range, which facilitates penetration into the fiber raw material and can also increase the yield of fine fibrous cellulose. Note that when there are multiple Rs in formula (1) or when multiple substituents represented by formula (1) are introduced into the fine fibrous cellulose, the multiple Rs may be the same or different.

[0030] β ｂ＋ H ＋ Alternatively, it is a monovalent or more cation composed of organic or inorganic substances. Examples of monovalent or more cations composed of organic substances include organic onium ions. Examples of organic onium ions include organic ammonium ions and organic phosphonium ions. Examples of organic ammonium ions include aliphatic ammonium ions and aromatic ammonium ions, and examples of organic phosphonium ions include aliphatic phosphonium ions and aromatic phosphonium ions. Examples of monovalent or more cations composed of inorganic substances include alkali metal ions such as sodium, potassium, or lithium, divalent metal ions such as calcium or magnesium, hydrogen ions, ammonium ions, etc. Note that β is included in formula (1). ｂ＋ If multiple β atoms exist, or if multiple substituents represented by the above formula (1) are introduced into the fine fibrous cellulose, then multiple β atoms exist. ｂ＋ These may be the same or different.

[0031] More specifically, as phosphorus oxoacid group 2, phosphate group (-OPO ３ H ２ ), phosphate group salt, phospholite group (phosphonic acid group) (-OPO２ H ２ Examples include phosphate groups (phosphonic acid groups) and salts of phosphonotic groups. Furthermore, phosphorus oxoacid group 2 can be a group formed by the condensation of a phosphate group (e.g., pyrophosphate group), a group formed by the condensation of a phosphonic acid (e.g., polyphosphonic acid group), a phosphate ester group (e.g., monomethyl phosphate group, polyoxyethylene alkyl phosphate group), or an alkylphosphonic acid group (e.g., methylphosphonic acid group).

[0032] The amount of phosphorus oxoacid group 2 introduced into the fine fibrous cellulose is preferably 0.50 mmol / g or more and 2.50 mmol / g or less per 1 g (mass) of fine fibrous cellulose, more preferably 0.80 mmol / g or more, even more preferably 1.00 mmol / g or more, and even more preferably 1.20 mmol / g or more, and preferably 2.30 mmol / g or less, and more preferably 2.10 mmol / g or less. From the viewpoint of improving the proton conductivity of the electrolyte membrane for polymer electrolyte fuel cells at high temperatures, it is preferable that the amount of phosphorus oxoacid group 2 introduced is above the lower limit, and from the viewpoint of improving the strength, rigidity, and dimensional stability of the electrolyte membrane for polymer electrolyte fuel cells, it is preferable that the amount of phosphorus oxoacid group 2 introduced is below the upper limit. Here, the denominator in the unit mmol / g is the counterion of phosphorus oxoacid group 2 being a hydrogen ion (H ＋ This shows the mass of the fine fibrous cellulose when ).

[0033] The amount of phosphorus oxoacid groups 2 introduced into fine fibrous cellulose (the amount of phosphorus oxoacid groups 2 present in the fine fibrous cellulose) can be measured, for example, by neutralization titration. In neutralization titration, the amount introduced is determined by adding an alkali such as an aqueous sodium hydroxide solution to a slurry containing the obtained fine fibrous cellulose and observing the change in pH. Figure 1 is a graph showing the relationship between the amount of NaOH added to fine fibrous cellulose containing phosphorus oxoacid groups 2 and pH.

[0034] Figure 1 is a graph showing the relationship between the amount of NaOH added to a slurry containing fine fibrous cellulose having phosphorus oxoacid group 2 and pH. The amount of phosphorus oxoacid group 2 introduced into the fine fibrous cellulose is measured, for example, as follows. First, the slurry containing fine fibrous cellulose is treated with a strongly acidic ion exchange resin. If necessary, a defibration treatment similar to the defibration treatment step described later may be performed on the sample before treatment with the strongly acidic ion exchange resin. Next, the change in pH is observed while adding an aqueous sodium hydroxide solution to obtain a titration curve as shown in the upper part of Figure 1. In the titration curve shown in the upper part of Figure 1, the measured pH is plotted against the amount of alkali added, and in the titration curve shown in the lower part of Figure 1, the increment (derivative value) (1 / mol) of pH with respect to the amount of alkali added is plotted. In this neutralization titration, two points are observed in the curve plotting the measured pH against the amount of alkali added where the increment (derivative value of pH with respect to the amount of alkali added) is maximum. Of these, the first maximum increment obtained after starting to add alkali is called the first endpoint, and the next maximum increment obtained 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. 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 solid content (g) of the slurry being titrated is the amount of phosphorus oxoacid group 2 introduced (mol / g). Note that when simply referred to as the amount of phosphorus oxoacid group 2 introduced (or amount of phosphorus oxoacid group 2), it refers to the amount of first dissociated acid. In Figure 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, if phosphorus oxoacid group 2 is a phosphate group and this phosphate group undergoes condensation, the amount of weakly acidic group in phosphorus oxoacid group 2 (also referred to as the amount of second dissociated acid in this specification) appears to decrease, and the amount of alkali required in the second region becomes less than the amount of alkali required in the first region. On the other hand, the amount of strongly acidic group in phosphorus oxoacid group 2 (also referred to as the amount of first dissociated acid in this specification) is equal to the amount of phosphorus atoms, regardless of whether condensation occurs or not. Also, if phosphorus oxoacid group 2 is a phosphorous acid group, there is no weakly acidic group in phosphorus oxoacid group 2, so the amount of alkali required in the second region becomes less, or in some cases, the amount of alkali required in the second region becomes zero. In this case, there is only one point on the titration curve where the pH increment is maximum. The above-mentioned amount of phosphorus oxoacid group 2 introduced (mol / g) represents the amount of phosphorus oxoacid group 2 present in the acid-type microfibrous cellulose, since the denominator represents the mass of acid-type microfibrous cellulose (hereinafter referred to as phosphorus oxoacid group 2 amount (acid type)). On the other hand, if the counterion of phosphorus oxoacid group 2 is substituted with an arbitrary cation C such that it has an equivalent charge, the amount of phosphorus oxoacid group 2 present in the microfibrous cellulose with cation C as the counterion can be determined by converting the denominator to the mass of microfibrous cellulose when cation C is the counterion (hereinafter referred to as phosphorus oxoacid group 2 amount (C type)). That is, it is calculated using the following formula. Amount of two phosphorus oxoacid groups (C type) = Amount of two phosphorus oxoacid groups (acid type) / {1 + (W - 1) × A / 1000} A [mol / g]: Total amount of anions derived from two phosphorus oxoacid groups in the fine fibrous cellulose (the sum of the strongly acidic and weakly acidic amounts of the two phosphorus oxoacid groups) W: Formula weight per unit of cation C (for example, Na is 23, Al is 9).

[0035] In the titration method for measuring phosphorus oxoacid groups 2, if the amount of sodium hydroxide aqueous solution added is too large or the titration interval is too short, accurate values ​​may not be obtained, resulting in a lower-than-actual value for phosphorus oxoacid groups 2. Appropriate titration amounts and intervals include, for example, titrating with 10 to 50 μL of 0.1 N sodium hydroxide aqueous solution over 5 to 30 seconds. Furthermore, to eliminate the influence of carbon dioxide dissolved in the fine fibrous cellulose-containing slurry, it is desirable to blow an inert gas such as nitrogen gas into the slurry from 15 minutes before the start of the titration until the end of the titration while measuring. The above method for measuring the amount of phosphorus oxoacid groups 2 is applicable to fine fibrous cellulose with a fiber width of 50 nm or less. When measuring the amount of phosphorus oxoacid groups 2 in pulp fibers with a fiber width exceeding 50 nm, the pulp fibers should be pulverized before measurement.

[0036] In this embodiment, the fine fibrous cellulose may be obtained by removing some of the phosphorus oxoacid groups 2 from the fine fibrous cellulose that has been defibrated by introducing the phosphorus oxoacid groups 2 as described above.

[0037] [Method for producing fine fibrous cellulose] (Fiber raw materials) The fiber raw materials are fiber raw materials containing cellulose, and are not particularly limited, but examples include wood pulp, non-wood pulp, and deinked pulp. Wood pulp is not particularly limited, but examples include chemical pulps such as bleached hardwood kraft pulp (LBKP), bleached softwood kraft pulp (NBKP), sulfite pulp (SP), dissolved 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 chemical groundwood pulp (CGP); and mechanical pulps such as crushed wood pulp (GP) and thermomechanical pulp (TMP, BCTMP). Non-wood pulp is not particularly limited, but examples include cotton pulps such as cotton linters and cotton lint, and non-wood pulps such as hemp, straw, bamboo, and bagasse. The deinked pulp is not particularly limited, but examples include deinked pulp made from recycled paper. Among these, bleached hardwood kraft pulp (LBKP) and bleached softwood kraft pulp (NBKP) are preferred from the viewpoint of availability and the ionic conductivity of the electrolyte membrane for polymer electrolyte fuel cells at high temperatures, with bleached softwood kraft pulp (NBKP) being more preferred. The raw material pulp in this embodiment may be one of the above types used alone, or two or more types may be used in mixture. The NBKP content in the raw material pulp is preferably 80% by mass or more, more preferably 85% by mass or more, even more preferably 90% by mass or more, and even more preferably 95% by mass or more, and 100% by mass or less, from the viewpoint of the ionic conductivity of the electrolyte membrane for polymer electrolyte fuel cells at high temperatures.

[0038] To obtain the fine fibrous cellulose into which the phosphorus oxoacid group 2 has been introduced as described above, it is preferable to have the following steps in this order: a phosphorus oxoacid group 2 introduction step in which the phosphorus oxoacid group 2 has been introduced into the fibrous raw material containing the cellulose described above, a washing step, an alkali treatment step (neutralization step), a defibration treatment step, and a pH adjustment step (ion exchange step). An acid treatment step may be included instead of the washing step, or in addition to the washing step. Furthermore, the low polymerization degree treatment described above may be performed before or after the defibration treatment step.

[0039] (Phosphorus Oxoacid Group 2 Introduction Step) The phosphorus oxoacid group 2 introduction step is a step in which at least one compound (hereinafter also referred to as "compound A") selected from compounds that can introduce phosphorus oxoacid group 2 by reacting with the hydroxyl group present in the cellulose-containing fiber raw material is reacted with the cellulose-containing fiber raw material. This step yields phosphorus oxoacid group 2 introduced fibers. In the phosphorus oxoacid group 2 introduction step according to this embodiment, the reaction between the cellulose-containing fiber raw material and compound A may be carried out in the presence of at least one compound (hereinafter also referred to as "compound B") selected from urea and its derivatives. Alternatively, the reaction between the cellulose-containing fiber raw material and compound A may be carried out in the absence of compound B. An example of a method for reacting compound A with the fiber raw material in the presence of compound B is a method of mixing compound A and compound B with a fiber raw material in a dry state, a wet state, or a slurry state. Of these, it is preferable to use a fiber raw material in a dry state or a wet state because the uniformity of the reaction is high, and it is particularly preferable to use a fiber raw material in a dry state. The form of the fiber raw material is not particularly limited, but it is preferably, for example, cotton-like or thin sheet-like. Compounds A and B can be added to the fiber raw material in powder form, as a solution dissolved in a solvent, or after being heated above their melting point and melted. Of these, it is preferable to add them as a solution dissolved in a solvent, especially as an aqueous solution, because this provides high reaction uniformity. Compounds A and B may be added to the fiber raw material simultaneously, separately, or as a mixture. The method of adding compounds A and B is not particularly limited, but if compounds A and B are in solution form, the fiber raw material may be immersed in the solution and then removed, or the solution may be added dropwise to the fiber raw material. Alternatively, the required amount 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 separately, and then the excess compound A and compound B may be removed by pressing or filtration.

[0040] Compound A used in this embodiment may be any compound having a phosphorus atom and capable of forming an ester bond with cellulose, and is not particularly limited to phosphoric acid or its salts, phosphorous acid or its salts, dehydrated condensed phosphoric acid or its salts, or phosphoric anhydride (phosphorus pentoxide). As phosphoric acid, compounds of various purities can be used, for example, 100% by mass phosphoric acid (orthophosphoric acid) or 85% by mass phosphoric acid can be used. As phosphorous acid, for example, 99% by mass phosphorous acid (phosphonic acid) can be used. Dehydrated condensed phosphoric acid is obtained by condensing two or more molecules of phosphoric acid through a dehydration reaction, and can be pyrophosphoric acid, polyphosphoric acid, etc. As phosphates, phosphites, and dehydrated condensed phosphates, examples include lithium salts, sodium salts, potassium salts, and ammonium salts of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid, and these can be neutralized to various degrees. Of these, from the viewpoint of improving the proton conductivity of electrolyte membranes for polymer electrolyte fuel cells at high temperatures, phosphoric acid or its salts, or phosphorous acid or its salts are preferred. From the viewpoint of high efficiency in introducing phosphate groups, easier improvement in the defibration process described later, 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, but for example, when the amount of compound A added is converted to the amount of phosphorus atoms, the amount of phosphorus atoms added per 100 parts by mass (oven-dry mass) of fiber raw material 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 keeping 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 keeping the amount of phosphorus atoms added to the fiber raw material below the above upper limit, a balance can be struck between the effect of improving yield and cost.

[0041] 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, it is preferable to use compound B 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 to 100 parts by mass (oven-dry mass) of fiber raw material is not particularly limited, but for example, it is preferably 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.

[0042] In the reaction of cellulose-containing fiber raw materials with compound A, in addition to compound B, for example, amides or amines may be included in the reaction system. 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 act as a particularly good reaction catalyst.

[0043] In the phosphorus oxoacid group 2 introduction step, it is preferable to add or mix compound A or the like to the fiber raw material and then subject the fiber raw material to heat treatment. The heat treatment temperature is preferably selected to efficiently introduce phosphorus oxoacid group 2 while suppressing thermal decomposition and hydrolysis reactions of the fibers. For example, the heat treatment temperature is preferably 50°C to 300°C, more preferably 100°C to 250°C, and even more preferably 130°C to 200°C. Furthermore, various heat transfer devices can be used for the heat treatment, such as agitation dryers, rotary dryers, disc dryers, roll-type heaters, plate-type heaters, fluidized bed dryers, band-type dryers, filtration dryers, vibrating fluidized bed dryers, airflow dryers, vacuum dryers, infrared heaters, far-infrared heaters, microwave heaters, and high-frequency dryers.

[0044] In the heat treatment according to this embodiment, for example, a method can be employed in which compound A is added to a thin sheet-like fiber raw material by impregnation or other methods, and then heated, or a method can be employed in which the fiber raw material and compound A are kneaded or stirred while heating. This makes it possible to suppress uneven concentration of compound A in the fiber raw material and to introduce phosphate groups more uniformly to the surface of the cellulose fibers contained in the fiber raw material. This is thought to be because, as water molecules move to the surface of the fiber raw material during drying, dissolved compound A is attracted to the water molecules by surface tension and similarly moves to the surface of the fiber raw material (i.e., uneven concentration of compound A is produced), and this can be suppressed. Furthermore, it is preferable that the heating device used for the heat treatment is a device that can constantly discharge the moisture held in the slurry and the moisture generated by the dehydration condensation (phosphate esterification) reaction between compound A and hydroxyl groups contained in cellulose etc. in the fiber raw material to the outside of the device system. Examples of such heating devices include a forced-air oven. By constantly draining moisture from the apparatus system, the hydrolysis reaction of phosphate ester bonds, which is the reverse reaction of phosphate esterification, can be suppressed, as can the acid hydrolysis of sugar chains in the fibers. This makes it possible to obtain fine fibrous cellulose with a high axial ratio. The heating time is, for example, preferably 1 second to 300 minutes, more preferably 1 second to 1,000 seconds, and even more preferably 10 seconds to 800 seconds, after substantially all moisture has been removed from the fiber raw material. In this embodiment, by setting the heating temperature and heating time within an appropriate range, the amount of phosphorus oxoacid group 2 introduced can be kept within a preferred range.

[0045] The phosphorus oxoacid group 2 introduction step only needs to be performed at least once, but it can also be repeated two or more times. By performing the phosphorus oxoacid group 2 introduction step two or more times, a large number of phosphorus oxoacid groups 2 can be introduced into the fiber raw material. In this embodiment, one example of a preferred embodiment is the case in which the phosphorus oxoacid group 2 introduction step is performed twice.

[0046] When introducing phosphorus oxoacid group 2 into fiber raw materials, the cellulose contained in the fiber raw material may be treated with an alkaline solution to alkalize the cellulose. This treatment causes some of the hydroxyl groups of the cellulose to ionically dissociate, 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. For their versatility, it is preferable to use, for example, sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, or tetrabutylammonium hydroxide. Alkalicelluloseization may be performed simultaneously with the introduction of phosphorus oxoacid group 2, as a preliminary step, or at both timings.

[0047] The solution temperature at which alkali cellulose formation is initiated is preferably 0°C to 50°C, more preferably 5°C to 40°C, and even more preferably 10°C to 30°C.

[0048] The concentration of the alkaline solution is preferably 0.01 mol / L or more and 4 mol / L or less as a molar concentration, 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 processing temperature is below 10°C, it is preferably 1 mol / L or more and 2 mol / L or less.

[0049] The alkali cellulose treatment time is preferably 1 minute to 6 hours, more preferably 10 minutes or more, even more preferably 30 minutes or more, and more preferably 5 hours or less, and even more preferably 4 hours or less.

[0050] By adjusting the type of alkaline solution, treatment temperature, concentration, and immersion time as described above, the penetration of the alkaline solution into the crystalline region of cellulose can be suppressed, making it easier to maintain the type I crystalline structure of cellulose and increasing the yield of fine fibrous cellulose.

[0051] If the introduction of phosphorus oxoacid group 2 and alkali cellulose formation are not carried out simultaneously, it is preferable to separate the alkali cellulose obtained from the alkali treatment into solid and liquid by general deliquidation methods such as centrifugation or filtration to remove moisture. This improves the reaction efficiency in the subsequent phosphorus oxoacid group 2 introduction step. The cellulose fiber concentration after solid and liquid separation is preferably 5% by mass or more and 50% by mass or less, more preferably 10% by mass or more and 40% by mass or less, and even more preferably 15% by mass or more and 35% by mass or less.

[0052] (Washing Process) In the method for producing fine fibrous cellulose according to this embodiment, a washing process can be performed on the phosphorus oxoacid group 2-introduced fibers as needed. The washing process is carried out, for example, by washing the phosphorus oxoacid group 2-introduced fibers with water or an organic solvent. The washing process may also be performed after each of the processes described later, and the number of washes performed in each washing process is not particularly limited.

[0053] (Acid Treatment Process) When producing fine fibrous cellulose, an acid treatment may be performed on the fiber raw material between the step of introducing phosphorus oxoacid group 2 and the defibration treatment process described later. For example, the phosphorus oxoacid group 2 introduction step, the acid treatment step, the alkali treatment step, and the defibration treatment step may be performed in this order. The method of acid treatment is not particularly limited, but for example, one method is to immerse the fiber raw material in an acidic solution containing acid. The concentration of the acidic solution used is not particularly limited, but for example, it is preferably 10% by mass or less, more preferably 5% by mass or less. The pH of the acidic solution used is not particularly limited, but for example, it is preferably 0 to 4, more preferably 1 to 3. As the acid contained in the acidic solution, for example, inorganic acids, sulfonic acids, carboxylic acids, etc., can be used. 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, boric acid, etc. 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 in the acid treatment is not particularly limited, but for example, it is preferably 5°C to 100°C, and more preferably 20°C to 90°C. The immersion time in the acid solution in the acid treatment is not particularly limited, but for example, it is preferably 5 minutes to 120 minutes, and more preferably 10 minutes to 60 minutes. The amount of acid solution used in the acid treatment is not particularly limited, but for example, it is preferably 100 parts by mass to 100,000 parts by mass, and more preferably 1,000 parts by mass to 10,000 parts by mass, per 100 parts by mass (dry mass) of fiber raw material.

[0054] (Alkali Treatment Process) When producing fine fibrous cellulose, an alkali treatment may be performed on the fiber raw material between the phosphorus oxoacid group 2 introduction process and the defibration treatment process described later. The alkali treatment method is not particularly limited, but for example, one method is to immerse the phosphorus oxoacid group 2-introduced fiber in an alkali solution. The alkali compound contained in the alkali solution is not particularly limited and may be an inorganic alkali compound or an organic alkali compound. In this embodiment, for its high versatility, it is preferable to use sodium hydroxide or potassium hydroxide as the alkali compound. The solvent contained in the alkali solution may be either water or an organic solvent. Among these, the solvent contained in the alkali solution is preferably water or a polar solvent including a polar organic solvent such as alcohol, and more preferably an aqueous solvent containing at least water. As the alkali solution, for its high versatility, for example, an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide is preferred. The temperature of the alkali solution in the alkali treatment process is not particularly limited, but for example, it is preferably 5°C to 80°C, more preferably 10°C to 60°C. The immersion time of the phosphorus oxoacid group 2-introduced fiber in the alkaline solution during the alkaline treatment process is not particularly limited, but is preferably 5 minutes to 30 minutes, more preferably 10 minutes to 20 minutes. The amount of alkaline solution used in the alkaline treatment is not particularly limited, but is preferably 100 parts by mass to 100,000 parts by mass, more preferably 1,000 parts by mass to 10,000 parts by mass, per 100 parts by mass (dry mass) of phosphorus oxoacid group 2-introduced fiber. The alkaline treatment may be a neutralization treatment or ion exchange treatment of the phosphorus oxoacid group 2. The temperature of the alkaline solution is preferably room temperature.

[0055] To reduce the amount of alkaline solution used in the alkaline treatment process, the phosphorus oxoacid group 2-introduced fibers may be washed with water or an organic solvent after the phosphorus oxoacid group 2 introduction process and before the alkaline treatment process. After the alkaline treatment process and before the defibration process, it is preferable to wash the alkali-treated phosphorus oxoacid group 2-introduced fibers with water or an organic solvent to improve handling.

[0056] (Fibration Process) By defibrating the phosphorus oxoacid group 2-introduced fibers in the defibration process, fine fibrous cellulose is obtained. In the defibration process, for example, a defibration processing device can be used. The defibration processing device is not particularly limited, but for example, a high-speed defibrator, grinder (stone mill type grinder), high-pressure homogenizer or ultra-high-pressure homogenizer, high-pressure impact grinder, ball mill, bead mill, disc type refiner, conical refiner, twin-screw kneader, vibrating mill, homomixer under high-speed rotation, ultrasonic disperser, or beater can be used. Among the above defibration processing devices, it is preferable to use a high-speed defibrator, high-pressure homogenizer, or ultra-high-pressure homogenizer, which have less influence from the grinding media and less risk of contamination.

[0057] In the defibration process, for example, it is preferable to dilute the phosphorus oxoacid group 2-introduced fiber with a dispersion medium to form a slurry. As the dispersion medium, one or more selected from water and organic solvents such as polar organic solvents can be used. The polar organic solvent is not particularly limited, but for example, alcohols, polyhydric alcohols, ketones, ethers, esters, and aprotonic polar solvents are preferred. 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).

[0058] The solid content concentration of the fine fibrous cellulose during the defibration process can be set as appropriate. Furthermore, the slurry obtained by dispersing the phosphorus oxoacid group 2-modified fibers in a dispersion medium may contain solid components other than the phosphorus oxoacid group 2-modified fibers, such as hydrogen-bonding urea.

[0059] (pH adjustment step) After the defibration treatment, a step may be provided to adjust the pH of the slurry containing fine fibrous cellulose. For example, an ionic substituent may be introduced into the cellulose fibers, and the counterion of this ionic substituent may be Na ＋In this case, the slurry containing the fine fibrous cellulose after defibration will be weakly alkaline. If heated in this state, monosaccharides, which are one of the causes of discoloration, may be generated due to the decomposition of cellulose, so it is preferable to adjust the pH of the slurry to 8 or below. Similarly, monosaccharides may also be generated under acidic conditions, so it is preferable to adjust the pH of the slurry to 3 or above.

[0060] The means of adjusting the pH are not particularly limited, but for example, an acidic or alkaline component may be added to a slurry containing fine fibrous cellulose. The acidic component may be either an inorganic acid or an organic acid. Examples of inorganic acids include sulfuric acid, hydrochloric acid, nitric acid, and phosphoric acid. Examples of organic acids include formic acid, acetic acid, citric acid, malic acid, lactic acid, adipic acid, sebacic acid, stearic acid, maleic acid, succinic acid, tartaric acid, fumaric acid, and gluconic acid. The alkaline component may be an inorganic alkali compound or an organic alkali compound. Examples of inorganic alkali compounds include lithium hydroxide, sodium hydroxide, potassium hydroxide, lithium carbonate, lithium bicarbonate, potassium carbonate, potassium bicarbonate, sodium carbonate, and sodium bicarbonate. Examples of organic alkali compounds include ammonia, hydrazine, methylamine, ethylamine, diethylamine, triethylamine, propylamine, dipropylamine, butylamine, diaminoethane, diaminopropane, diaminobutane, diaminopentane, diaminohexane, cyclohexylamine, aniline, tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide, benzyltrimethylammonium hydroxide, pyridine, and N,N-dimethyl-4-aminopyridine.

[0061] Furthermore, in the pH adjustment process, ion exchange treatment may be performed to adjust the pH. For ion exchange treatment, a strongly acidic cation exchange resin or a weakly acidic ion exchange resin can be used. By treating with an appropriate amount of cation exchange resin for a sufficient amount of time, a slurry containing fine fibrous cellulose at the desired pH can be obtained. In addition, the pH adjustment process may be combined with the addition of acidic or alkaline components and ion exchange treatment.

[0062] [Component (B): Polymer compound having phosphorus oxoacid group 1] Specific examples of phosphorus oxoacid group 1 include phosphonic acid group, phosphonic acid monoester group, phosphite group, phosphate group, phosphate monoester group, and phosphate diester group. Among these, the phosphonic acid group is preferred from the viewpoint of improving the ionic conductivity of electrolyte membranes for polymer electrolyte fuel cells at high temperatures.

[0063] The polymer compound having phosphorus oxoacid group 1 may be a sequential polymer or a chain polymer, but a chain polymer is preferred from the viewpoint of improving the ionic conductivity of the electrolyte membrane for polymer electrolyte fuel cells at high temperatures. The polymer may be a homopolymer or a copolymer. Furthermore, the polymerization form of the chain polymer may be block or random. Specific examples of component (B) include poly(vinylphosphonic acid), copolymers containing constituent units derived from vinylphosphonic acid, and the following polymers having phosphorus oxoacid group 1. Polyethersulfone resin, polyetheretherketone resin, linear phenol-formaldehyde resin, cross-linked phenol-formaldehyde resin, linear polystyrene resin, cross-linked polystyrene resin, linear poly(trifluorostyrene) resin, cross-linked (trifluorostyrene) resin, poly(2,3-diphenyl-1,4-phenylene oxide) resin, poly(allyl etherketone) resin, poly(arylene ethersulfone) resin, poly(phenylquinoxaline) resin, poly(benzylsilane) resin, polystyrene-grafted ethylenetetrafluoroethylene resin, polystyrene-grafted polyvinylidene fluoride resin, polystyrene-grafted tetrafluoroethylene resin. Among these, poly(vinylphosphonic acid) is preferred from the viewpoint of improving the ionic conductivity of electrolyte membranes for polymer electrolyte fuel cells at high temperatures.

[0064] In the proton conductor, the mass ratio of component (B) to component (A) (component (B) / component (A)) is preferably 20 / 80 or more and 80 / 20 or less, more preferably 30 / 70 or more, even more preferably 40 / 60 or more, and even more preferably 70 / 30 or less, and even more preferably 60 / 40 or less, from the viewpoint of improving the ionic conductivity of the electrolyte membrane for polymer electrolyte fuel cells at high temperatures.

[0065] The total content of components (A) and (B) in the proton conductor is preferably 85% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and even more preferably 98% by mass or more, and 100% by mass or less, from the viewpoint of improving the ionic conductivity of the electrolyte membrane for the polymer electrolyte fuel cell at high temperatures.

[0066] <Other Component 1> The proton conductor of this embodiment may contain components other than components (A) and (B) (other component 1) within a range that does not impair the effects of the present invention. The content of other component 1 in the proton conductor is preferably 15% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, even more preferably 2% by mass or less, and 0% by mass or more.

[0067] [Electrolyte membrane for polymer electrolyte fuel cell] The electrolyte membrane for polymer electrolyte fuel cell of this embodiment contains the proton conductor and component (C): membrane component of this embodiment. The electrolyte membrane for polymer electrolyte fuel cell is also simply referred to as "electrolyte membrane".

[0068] [Component (C): Membrane Component] The electrolyte membrane of this embodiment contains component (C): a membrane component, from the viewpoint of improving ionic conductivity at high temperatures. Component (C) is preferably a polymer from the viewpoint of forming an electrolyte membrane, and may be a step-polymer or a chain-polymer, with a step-polymer being preferred from the viewpoint of improving ionic conductivity at high temperatures. The polymer may be a homopolymer or a copolymer. The polymerization form of the chain-polymer may be block or random. As component (C), a polymer having a repeating unit of a 5 or 6-membered heterocycle in which at least one of the ring constituent atoms is a nitrogen atom (hereinafter also referred to as a "heterocyclic polymer") is preferred. The number of nitrogen atoms in the ring constituent atoms is preferably 1 or 2. The ring constituent atoms of the heterocyclic polymer may have heteroatoms other than nitrogen atoms, and specific examples of heteroatoms include oxygen atoms and sulfur atoms. When the ring constituent atoms have heteroatoms other than nitrogen atoms, the number of heteroatoms other than nitrogen atoms is preferably 1. Specific examples of "a 5- or 6-membered heterocycle in which at least one of the ring constituent atoms is a nitrogen atom" include imidazole rings, thiazole rings, oxazole rings, oxadiazole rings (1,2,3-oxadiazole ring, 1,2,4-oxadiazole ring, 1,2,5-oxadiazole ring, 1,3,4-oxadiazole ring), pyrazine rings, thiadiazole rings (1,2,3-thiadiazole ring, 1,2,4-thiadiazole ring, 1,2,5-thiadiazole ring, 1,3,4-thiadiazole ring), pyridine rings, and pyrimidine rings. Among these, imidazole rings are preferred from the viewpoint of improving ionic conductivity at high temperatures. That is, a polymer having imidazole rings as repeating units (hereinafter also referred to as "polyimidazole polymer") is preferred as component (C).

[0069] The polyimidazole polymer is preferably a compound having a repeating unit represented by any of the following formulas (I) to (IV), and more preferably a compound having a return unit represented by the following formula (III).

[0070]

[0071] In the formula, R１ ~R ８ Each of these independently represents either a hydrogen atom or a sulfonic acid group, and it is preferable that it represents a hydrogen atom. １ and L ２ Each of these independently represents a phenylene group, a pyridinediyl group, and a pyrimidinediyl group, with a preference for representing a phenylene group and a preference for representing an m-phenylene group.

[0072] The content of repeating units represented by any of formulas (I) to (IV) in all constituent units of the polyimidazole polymer is preferably 85% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and 100% by mass or less.

[0073] Specific examples of heterocyclic polymers include polybenzimidazole, polybenzothiazole, polybenzoxazole, polyoxadiazole, polyquinoxaline, polythiadiazole, poly(pyridine), poly(pyrimidine), and poly(tetrazapyrene). Among these, polybenzimidazole is preferred from the viewpoint of improving ionic conductivity at high temperatures.

[0074] In the electrolyte membrane for polymer electrolyte fuel cells, the ratio of the total mass of components (A) and (B) to the mass of component (C) ((component (A) + component (B)) / component (C)) is preferably 5 / 95 or more and 55 / 45 or less from the viewpoint of improving ionic conductivity at high temperatures.

[0075] [Component (D): Acidic Molecules] The electrolyte membrane of this embodiment preferably contains component (D): acidic molecules, from the viewpoint of improving ionic conductivity at high temperatures. The embodiment of the electrolyte membrane containing component (D) is also called an acid-doped electrolyte membrane. The acidic molecules can be either organic acids or inorganic acids.

[0076] Examples of organic acids include organic sulfonic acids and organic phosphonic acids. Specific examples of organic sulfonic acids include alkyl sulfonic acids such as methanesulfonic acid, ethanesulfonic acid, hexanesulfonic acid, octylsulfonic acid, dodecylsulfonic acid, cetylsulfonic acid, sulfosuccinic acid, sulfoglutaric acid, sulfoadipic acid, sulfopimeric acid, sulfoseveric acid, sulfoazelaic acid, and sulfosebacic acid; perfluoroalkyl sulfonic acids such as trifluoromethanesulfonic acid, pentafluoroethanesulfonic acid, and heptafluoropropylsulfonic acid; and aromatic sulfonic acids such as benzenesulfonic acid, 1,3-benzenesulfonic acid, toluenesulfonic acid, octylbenzenesulfonic acid, 2-methyl-5-isopropylbenzenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, naphthalenesulfonic acid, chlorobenzenesulfonic acid, phenolsulfonic acid, trichlorobenzenesulfonic acid, nitrotoluenesulfonic acid, nitrobenzenesulfonic acid, and sulfobenzoic acid. Specific examples of organic phosphonic acids include aromatic phosphonic acids such as phenylphosphonic acid and 1,3-dicarboxyphenylphosphonic acid; and aliphatic phosphonic acids such as 1-hydroxyethane-1,1-diphosphonic acid and vinylphosphonic acid.

[0077] Specific examples of inorganic acids include phosphoric acid, polyphosphate, sulfuric acid, nitric acid, hydrofluoric acid, hydrochloric acid, and hydrobromic acid.

[0078] Component (D) is preferably an inorganic acid, more preferably an inorganic strong acid, further preferably phosphoric acid, polyphosphoric acid, sulfuric acid, nitric acid, hydrochloric acid, and hydrobromic acid, even more preferably phosphoric acid and sulfuric acid, and even more preferably phosphoric acid.

[0079] The amount of component (D) contained in the acid-doped electrolyte membrane, calculated from the following formula, is preferably 150% by mass or more and 450% by mass or less, more preferably 175% by mass or more, even more preferably 200% by mass or more, and even more preferably 400% by mass or less, and even more preferably 370% by mass or less. Amount of component (D) (mass%) = 100 × (W ＰＡ -W ｄｒｙ ) / W ｄｒｙ WＰＡ : Component (D) Mass measurement of the doped electrolyte membrane at 60°C W ｄｒｙ : Mass measurement of the electrolyte membrane at 60°C before adding component (D)

[0080] <Other Components 2> The electrolyte membrane of this embodiment may contain components other than the components contained in the proton conductor of this embodiment, component (C) and component (D) (other components 2), to the extent that the effects of the present invention are not impaired. The content of other components 2 in the electrolyte membrane is preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less, and 0% by mass or more.

[0081] <Film Thickness> The film thickness of the electrolyte membrane in this embodiment can be appropriately selected according to the size of the membrane electrode assembly incorporating the electrolyte membrane for the polymer electrolyte fuel cell and the polymer electrolyte fuel cell. For example, the film thickness of the electrolyte membrane for the polymer electrolyte fuel cell in this embodiment can be 1 μm or more and 300 μm or less, or 5 μm or more and 75 μm or less.

[0082] The electrolyte membrane of this embodiment can conduct protons via phosphorus oxoacid groups even without containing sufficient water. Therefore, even when used in a polymer electrolyte fuel cell operating in an environment of 100°C or higher, the battery can be operated without increasing the humidity of the operating environment. A polymer electrolyte fuel cell equipped with the electrolyte membrane of this embodiment can be used in environments of 100°C or higher, as well as in environments of 120°C or higher, and even in environments of 150°C or higher. The upper limit is, for example, 160°C or lower.

[0083] The proton conductivity of the electrolyte membrane in this embodiment at 150°C is preferably 4.5 × 10⁻⁶. －２ S / cm or more, more preferably 4.8 × 10 －２ S / cm or more, more preferably 5.0 × 10 －２ S / cm or more, more preferably 5.2 × 10 －２ The density is S / cm or greater. The upper limit is not particularly limited from a manufacturing standpoint, but for example, 2.0 × 10 －１ It is less than or equal to S / cm. The proton conductivity of the electrolyte membrane in this embodiment is calculated by the method described in the example.

[0084] The retention rate of the proton conductivity of the electrolyte membrane of this embodiment when maintained at 150°C for 24 hours is preferably 45% or more, more preferably 50% or more, and even more preferably 55% or more. There is no particular upper limit, but from a manufacturing standpoint, for example, it is 99% or less. The retention rate of the proton conductivity of the electrolyte membrane of this embodiment is calculated by the method described in the examples.

[0085] [Proton conductor for electrolyte membrane of polymer electrolyte water electrolysis apparatus] As described above, the proton conductor for the electrolyte membrane of the polymer electrolyte fuel cell in this embodiment and the proton conductor for the electrolyte membrane of the polymer electrolyte water electrolysis apparatus in this embodiment have the same configuration except for their intended use.

[0086] [Electrolyte membrane for solid polymer water electrolysis device] As described above, the electrolyte membrane for solid polymer fuel cell of this embodiment and the electrolyte membrane for solid polymer water electrolysis device of this embodiment have the same configuration except for their intended use.

[0087] [Membrane Electrode Assembly (Equipped with Electrolyte Membrane for Polymer Electrolyte Fuel Cell)] The membrane electrode assembly (MEA) of this embodiment is formed by joining a positive electrode catalyst layer, the electrolyte membrane for polymer electrolyte fuel cell of this embodiment, and a negative electrode catalyst layer in this order. The membrane electrode assembly of this embodiment can have the same configuration as known membrane electrode assemblies that can be used in polymer electrolyte fuel cells, except that it has the electrolyte membrane for polymer electrolyte fuel cell of this embodiment. For example, see Japanese Patent Application Publication No. 2022-190524 for details on membrane electrode assemblies. The membrane electrode assembly of this embodiment may have a gas diffusion layer on the outside of the positive electrode catalyst layer (the side of the positive electrode catalyst layer opposite to the side of the positive electrode catalyst layer that has the electrolyte membrane for polymer electrolyte fuel cell). The membrane electrode assembly of this embodiment may also have a gas diffusion layer on the outside of the negative electrode catalyst layer (the side of the negative electrode catalyst layer opposite to the side of the negative electrode catalyst layer that has the electrolyte membrane for polymer electrolyte fuel cell). The thicknesses of the positive electrode catalyst layer, the negative electrode catalyst layer, and the gas diffusion layer can be appropriately determined according to the size of the polymer electrolyte fuel cell.

[0088] [Membrane Electrode Assembly (Equipped with Electrolyte Membrane for Polymer Electrolyte Water Electrolyzer)] Another membrane electrode assembly (MEA) of this embodiment is formed by joining a positive electrode catalyst layer, the electrolyte membrane for polymer Electrolyte Water Electrolyzer of this embodiment, and a negative electrode catalyst layer in this order. The membrane electrode assembly of this embodiment can have the same configuration as known membrane electrode assemblies that can be used in polymer Electrolyte Water Electrolyzers, except that it has the electrolyte membrane for polymer Electrolyte Water Electrolyzer of this embodiment. For membrane electrode assemblies, see, for example, Electrochemistry, 85(1), 28-33(2017), Japanese Patent Application Publication No. 2023-41182, and International Publication No. 2014 / 157389. The membrane electrode assembly of this embodiment may have a gas diffusion layer on the outside of the positive electrode catalyst layer (the side of the positive electrode catalyst layer opposite to the side having the electrolyte membrane for polymer Electrolyte Water Electrolyzer). Furthermore, the membrane electrode assembly of this embodiment may have a gas diffusion layer on the outside of the negative electrode catalyst layer (the side of the negative electrode catalyst layer opposite to the side having the electrolyte membrane for the polymer electrolyte water electrolysis apparatus). The thicknesses of the positive electrode catalyst layer, the negative electrode catalyst layer, and the gas diffusion layer can be appropriately determined according to the size of the polymer electrolyte water electrolysis apparatus.

[0089] [Solid Polymer Fuel Cell] The solid polymer fuel cell of this embodiment can have the same configuration as known solid polymer fuel cells, except that it has the membrane electrode assembly of this embodiment (equipped with an electrolyte membrane for solid polymer fuel cells).

[0090] [Solid Polymer Water Electrolyzer] The solid polymer water electrolyzer of this embodiment (solid polymer electrolyte water electrolyzer, hydrogen production apparatus) can have the same configuration as known solid polymer water electrolyzers, except that it has other membrane electrode assemblies (equipped with electrolyte membranes for solid polymer water electrolyzers) of this embodiment. For known solid polymer water electrolyzers, see, for example, Electrochemistry, 85(1), 28-33(2017), Japanese Patent Application Publication No. 2023-41182, and International Publication No. 2014 / 157389.

[0091] [Method for manufacturing proton conductors for electrolyte membranes of polymer electrolyte fuel cells] The method for manufacturing proton conductors for electrolyte membranes of polymer electrolyte fuel cells according to this embodiment comprises the following preparation step I and polymerization step in this order. Preparation step I: A step to obtain a mixed solution I containing the following components (A), (b), and a solvent. Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less. Component (b): Monomer having 1 phosphorus oxoacid group. Polymerization step: A step to obtain a proton conductor for electrolyte membranes of polymer electrolyte fuel cells containing component (A) and the following component (B), by polymerizing component (b) in the mixed solution I. Component (B): Polymer compound having 1 phosphorus oxoacid group. Component (A) and component (B) used in the method for manufacturing proton conductors according to this embodiment are the same as component (A) and component (B) contained in the proton conductor of this embodiment, respectively.

[0092] [Preparation Step I] In Preparation Step I, component (A), component (b), and solvent can be mixed by a conventional method to obtain mixed solution I. It is preferable to use an organic solvent, and specific examples of organic solvents include dimethylformamide, ethyl acetate, and butyl acetate.

[0093] [Polymerization Process] In the polymerization process, component (b) is polymerized in the mixed liquid I to obtain a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell containing component (A) and component (B). Preferably, at least a portion of each fine fibrous cellulose constituting component (A) is coated with component (B). In the polymerization process, component (b) can be polymerized by conventional methods using a polymerization initiator and a chain transfer agent, depending on the type of component (B) and the desired weight-average molecular weight. The following explanation will take vinylphosphonic acid as component (b) as an example to describe how to obtain a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell containing component (A) and component (B): poly(vinylphosphonic acid).

[0094] Component (b) is polymerized by adding a chain transfer agent and a polymerization initiator to mixture I and stirring. Examples of chain transfer agents include O-ethyl-S-(1-ethoxycarbonyl)-ethyl dithiocarbonate, methyl (methoxycarbonothiothio)acetate, methyl (ethoxycarbonothiothio)acetate, methyl (isopropyloxycarbonothiothio)acetate, O-ethyl-S-(1-phenylethyl) dithiocarbonate, methyl (ethoxycarbonothiothio) pivalate, and 1-(ethoxycarbonothiothio)acetate. Either a thermal polymerization initiator or a photopolymerization initiator may be used as the polymerization initiator. Specific examples of polymerization initiators include 2,2'-azobis(isobutyronitrile), 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), benzoyl peroxide, tert-butyl peroxide, 2,2'-azobis(2-methylpropionic acid)dimethyl, 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone, and lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate. The stirring time is, for example, 12 hours or more and 36 hours or less. Stirring may be carried out while heating the reaction system, and the heating temperature is, for example, 45°C or more and 90°C or less.

[0095] [Method for manufacturing proton conductors for electrolyte membranes in solid polymer water electrolysis devices] The method for manufacturing proton conductors for electrolyte membranes in solid polymer water electrolysis devices according to this embodiment is the same as the method for manufacturing proton conductors for electrolyte membranes in solid polymer fuel cells.

[0096] [Method for Manufacturing an Electrolyte Membrane for a Solid Polymer Fuel Cell] The method for manufacturing an electrolyte membrane for a solid polymer fuel cell according to this embodiment comprises the following preparation step II and film formation step in this order. Preparation step II: A step to obtain a mixed solution II containing the following component (C), a proton conductor for a solid polymer fuel cell obtained by the method for manufacturing a proton conductor according to this embodiment, and a solvent. Component (C): Membrane component Film formation step: A step to remove the solvent from the mixed solution II to obtain an electrolyte membrane for a solid polymer fuel cell.

[0097] [Preparation Step II] In Preparation Step II, a mixture II can be obtained by mixing component (C), the proton conductor obtained by the method for producing a proton conductor of this embodiment, and the solvent by a conventional method.

[0098] [Film Formation Process] In the film formation process, the mixed solution II is coated onto a substrate such as a glass plate and dried to obtain the electrolyte film of this embodiment. The drying temperature can be selected according to the boiling point of the solvent. For example, the drying temperature is 45°C to 90°C.

[0099] [Acid Doping Step] The electrolyte membrane manufacturing method of this embodiment preferably includes an acid doping step in which acidic molecules are added to the electrolyte membrane, from the viewpoint of improving the putron conductivity at high temperatures. In the electrolyte membrane manufacturing method of this embodiment, an acid-doped electrolyte membrane is obtained by incorporating acidic molecules into the electrolyte membrane obtained in the film formation step through the acid doping step. The addition of acidic molecules to the electrolyte membrane can be done, for example, by immersing the electrolyte membrane in an aqueous solution of acidic molecules. From the viewpoint of efficiently incorporating acidic molecules into the electrolyte membrane, the concentration of the aqueous solution of acidic molecules is, for example, 50% by mass or more and 95% by mass or less. The aqueous solution of acidic molecules may be heated, and the temperature of the aqueous solution of acidic molecules is, for example, 45°C or more and 90°C or less.

[0100] [Method for manufacturing electrolyte membranes for solid polymer water electrolysis devices] The method for manufacturing electrolyte membranes for solid polymer water electrolysis devices in this embodiment is the same as the method for manufacturing electrolyte membranes for solid polymer fuel cells.

[0101] [Method for manufacturing a membrane electrode assembly (equipped with an electrolyte membrane for polymer electrolyte fuel cell)] The method for manufacturing the membrane electrode assembly of this embodiment includes the steps of bonding a positive electrode catalyst layer to one side of the electrolyte membrane for polymer electrolyte fuel cell obtained by the method for manufacturing the electrolyte membrane of this embodiment, and bonding a negative electrode catalyst layer to the other side of the electrolyte membrane for polymer electrolyte fuel cell. The "bonding" can be carried out by a conventional method. For a method for manufacturing a membrane electrode assembly, see, for example, Japanese Patent Application Publication No. 2022-190524.

[0102] [Method for manufacturing a membrane electrode assembly (equipped with an electrolyte membrane for a polymer electrolyte water electrolysis device)] Another method for manufacturing a membrane electrode assembly according to this embodiment includes the steps of bonding a positive electrode catalyst layer to one side of the electrolyte membrane for a polymer electrolyte water electrolysis device obtained by the electrolyte membrane manufacturing method of this embodiment, and bonding a negative electrode catalyst layer to the other side of the electrolyte membrane for a polymer electrolyte water electrolysis device. The "bonding" can be carried out by conventional methods. For more information on the method for manufacturing a membrane electrode assembly, see, for example, Electrochemistry, 85(1), 28-33(2017), Japanese Patent Application Publication No. 2023-41182, and International Publication No. 2014 / 157389.

[0103] [Method for Manufacturing a Polymer Electrode Fuel Cell] The method for manufacturing a polymer electrolyte fuel cell according to this embodiment includes the step of incorporating the membrane electrode assembly obtained by the method for manufacturing a membrane electrode assembly according to this embodiment into a polymer electrolyte fuel cell. For information on the method for manufacturing a polymer electrolyte fuel cell, see, for example, Japanese Patent Application Publication No. 2022-190524.

[0104] [Method for Manufacturing a Solid Polymer Water Electrolyzer] The method for manufacturing the solid polymer water electrolyzer of this embodiment includes the step of incorporating the membrane electrode assembly obtained by the method for manufacturing the membrane electrode assembly of this embodiment into a solid polymer water electrolyzer. The solid polymer water electrolyzer of this embodiment can be manufactured, for example, by referring to Electrochemistry, 85(1), 28-33(2017) and Japanese Patent Application Publication No. 2023-41182.

[0105] The features of the present invention will be further described below with reference to examples and comparative examples. The materials, amounts used, proportions, processing content, and processing procedures shown in the following examples can be modified 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. In addition, the operations in the examples and comparative examples were carried out under conditions of room temperature (20 to 25°C) and normal humidity (40 to 50% RH (relative humidity)) unless otherwise specified.

[0106] Example 1 [Manufacturing of electrolyte membrane for polymer electrolyte fuel cell] [Preparation process I] <Component (A): Manufacturing of fine fibrous cellulose> (Phosphorus oxoacid group 2 introduction process) As raw material pulp, bleached softwood kraft pulp (solid content 93% by mass, basis weight 245 g / m²) manufactured by Oji Paper Co., Ltd. ２ A sheet-like material with a Canadian standard filtration efficiency (CSF) of 700 mL (measured according to JIS P 8121-2:2012) was used. This raw material pulp was subjected to phosphorus oxooxidation treatment as follows: First, 100 parts by mass (oven-dry mass) of the raw material pulp was mixed with an aqueous solution containing 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water to obtain chemically impregnated pulp. Next, the obtained chemically impregnated pulp was heated in a hot air dryer at 165°C for 250 seconds to obtain pulp (phosphorylated pulp) in which phosphorus oxoacid groups 2 were introduced into the cellulose in the pulp.

[0107] (Washing Process) Next, the obtained phosphorylated pulp was subjected to a washing process. The washing process was carried out by repeatedly stirring the pulp dispersion obtained by pouring 10 L of deionized water over 100 g (absolute dry mass) of phosphorylated pulp, ensuring uniform dispersion of the pulp, and then filtering and dewatering it. The washing process was terminated when the electrical conductivity of the filtrate was 100 μS / cm or less.

[0108] (Alkali treatment process (neutralization process)) Next, the washed phosphorylated pulp was subjected to neutralization treatment as follows. First, the washed phosphorylated pulp was diluted with 10 L of deionized water, and then a 1 N sodium hydroxide aqueous solution was gradually added while stirring to obtain a phosphorylated pulp dispersion with a pH of 12 to 13. Next, the phosphorylated pulp dispersion was dehydrated and washed with deionized water to obtain phosphorylated pulp that had undergone neutralization treatment. The obtained phosphorylated pulp was subjected to infrared absorption spectrum measurement using FT-IR. As a result, 1,230 cm⁻¹ was obtained. －１Absorption based on the P=O of phosphorus oxoacid group 2 was observed in the vicinity, confirming that phosphorus oxoacid group 2 was added to the pulp. Furthermore, when the obtained phosphorylated pulp was tested and analyzed using an X-ray diffractometer, typical peaks were observed at two locations: around 2θ = 14° to 17° and around 2θ = 22° to 23°, confirming the presence of type I cellulose crystals.

[0109] (Fibration Treatment Process) Ion-exchanged water was added to the obtained phosphorylated pulp to prepare a dispersion with a solid content of 2% by mass. This dispersion was treated five times at a pressure of 200 MPa in a wet atomizing device (Sugino Machine Co., Ltd., Starburst) to obtain a fine fibrous cellulose dispersion (1) containing fine fibrous cellulose. The counterion of phosphorus oxoacid group 2 contained in the fine fibrous cellulose dispersion (1) is Na ＋ X-ray diffraction confirmed that the obtained fibrous cellulose maintained a type I cellulose crystal structure. Furthermore, the fiber width of the fibrous cellulose was measured using a transmission electron microscope and was found to be 3–5 nm. The average fiber width of the fibrous cellulose was 3.5 nm. The amount of phosphate groups (amount of first dissociated acid) measured by the measurement method described in [Measurement of phosphorus oxoacid group amount 2] below was 1.45 mmol / g. The total amount of dissociated acid was 2.45 mmol / g.

[0110] (Ion exchange process) 25 g of a fine fibrous cellulose dispersion (1) with a solid content concentration of 2% by mass was mixed with ion-exchanged water to make 100 g of a fine fibrous cellulose dispersion (2) with a solid content concentration of 0.5% by mass. 5 g by mass of strongly acidic ion exchange resin (manufactured by Mitsubishi Chemical Corporation, conditioned) was added to the obtained fine fibrous cellulose dispersion (3), and the mixture was stirred for 2 hours. The ion exchange resin and the dispersion were then separated by centrifugation to obtain a fine fibrous cellulose dispersion (4). The degree of polymerization of the fine fibrous cellulose was 485.

[0111] <Preparation of Mixture I> To the fine fibrous cellulose dispersion (4), 20 times the volume (by volume) of ethanol was added and the mixture was stirred for 3 hours. This liquid was filtered using a suction filtration apparatus equipped with filter paper, and the residue on the filter paper was dried on a hot plate set to 30°C to obtain dried fine fibrous cellulose. Next, 0.15 g of the dried fine fibrous cellulose and 0.15 g of vinylphosphonic acid (purity of 95% by mass or more) were added to 4 g of dimethylformamide and stirred to obtain Mixture I.

[0112] [Polymerization Process] To the above mixture I, 3.09 mg of dithiocarbonate O-ethyl-S-(1-ethoxycarbonyl)-ethyl was added as a RAFT reagent (chain transfer agent), and 0.912 mg of 2,2'-azobis(isobutyronitrile) was added as a polymerization initiator. The vinylphosphonic acid was polymerized by heating this mixture to 75°C and stirring at 800 rpm for 24 hours. Next, 10 times the amount of acetone was added to the stirred mixture to precipitate the solids, and the supernatant was removed. This process was repeated three times. Furthermore, the supernatant was removed by centrifugation under the conditions of (5,000 rpm, 10 minutes), and the solids were recovered as precipitate. The recovered solids were vacuum dried at 40°C to obtain a proton conductor for electrolyte membranes of polymer electrolyte fuel cells, containing component (A): fine fibrous cellulose and component (B): a polymer compound having 1 phosphorus oxoacid group (poly(vinylphosphonic acid)). In the proton conductor for electrolyte membranes, the ratio of the mass of component (B) to the mass of component (A) (fine fibrous cellulose) (component (B) / component (A)) is 50 / 50.

[0113] <Component (C): Synthesis of the membrane component> 90 g of polyphosphate was prepared as the synthesis solvent, and 2 g of 3,3'-diaminobenzidine and 1.6 g of isophthalic acid were added as raw materials for the synthesis of the membrane component. This mixture was heated to 210°C under a nitrogen flow and stirred for 36 hours to obtain a membrane component solution. Ten times the volume (based on volume) of deionized water was added to this membrane component solution to precipitate it, and the membrane component was recovered as a solid. To neutralize the remaining polyphosphate, 500 times the volume (based on volume) of sodium bicarbonate solution (saturated solution) was added to this solid and allowed to stand for 16 hours. Next, the sodium bicarbonate solution was removed, and 500 times the volume (based on volume) of deionized water was added and allowed to stand for 16 hours. Finally, the deionized water was removed and the mixture was dried to obtain component (C): membrane component (poly(2,2'-(m-phenylene)-5,5'-benzimidazole), a polymer having the repeating structure shown below).

[0114]

[0115] [Preparation Step II] The above component (C): membrane component was added to dimethylacetamide to a concentration of 4% by mass, and the mixture was treated with a rotational-orbital stirring device (Sinky Co., Ltd., AR-100) at a rotational speed of 2,000 rpm for 6 hours to obtain a membrane component solution. A proton conductor for electrolyte membranes was added to this membrane component solution, and the mixture was treated with a rotational-orbital stirring device (Sinky Co., Ltd., AR-100) at a rotational speed of 2,000 rpm for 30 minutes to obtain a composite material for film formation (mixture II). In the composite material for film formation, the proton conductor for electrolyte membranes was added so that the ratio of the total mass of component (A): fine fibrous cellulose and component (B): poly(vinylphosphonic acid) to the mass of component (C): membrane component ((component (A) + component (B)) / component (C)) was 50 / 50.

[0116] [Film Formation Process] The obtained composite material for film formation was coated onto a glass plate using a film applicator with a clearance of 2 mm. Then, it was cast-dried at 80°C to obtain an electrolyte film. The thickness of the obtained electrolyte film was 25 μm.

[0117] [Acid Doping Process] The electrolyte membrane was immersed in an aqueous phosphoric acid solution (85% by mass) heated to 80°C and left to stand for 24 hours. The electrolyte membrane was then removed from the aqueous phosphoric acid solution and dried at 60°C for 24 hours to obtain a phosphoric acid-doped electrolyte membrane.

[0118] Example 2 In the preparation step II of Example 1, a proton conductor for the electrolyte membrane was added to the composite material for film formation so that the mass ratio "(component (A) + component (B)) / component (C)" was 25 / 75. The other procedures were the same as in Example 1 to obtain an electrolyte membrane with a thickness of 20 μm and a phosphate-doped electrolyte membrane.

[0119] Example 3 In the preparation step II of Example 1, a proton conductor for the electrolyte membrane was added to the composite material for film formation so that the mass ratio "(component (A) + component (B)) / component (C)" was 10 / 90. The other procedures were the same as in Example 1 to obtain an electrolyte membrane with a thickness of 15 μm and a phosphate-doped electrolyte membrane.

[0120] Comparative Example 1 In the preparation step II of Example 1, no proton conductor for the electrolyte membrane was added. The other procedures were the same as in Example 1, and an electrolyte membrane with a thickness of 20 μm, consisting of component (C): membrane component alone, and a phosphate-doped electrolyte membrane were obtained.

[0121] [Measurement Method] [Measurement of the amount of phosphorus oxoacid groups in microfibrous cellulose] The amount of phosphorus oxoacid groups in microfibrous cellulose was measured by diluting the target microfibrous cellulose dispersion with ion-exchanged water to a microfibrous cellulose content of 0.2% by mass, preparing a microfibrous cellulose-containing dispersion, treating it with an ion-exchange resin, and then performing an alkali titration. The treatment with the ion-exchange resin was performed by adding 1 / 10 the volume of strongly acidic ion-exchange resin (Organo Corporation, Amberjet 1024, conditioned) to the microfibrous cellulose dispersion, shaking for 1 hour, and then pouring it onto a mesh with a mesh opening of 90 μm to separate the ion-exchange resin from the dispersion. The alkali titration was performed by adding 10 μL of 0.1 N sodium hydroxide aqueous solution to the microfibrous cellulose dispersion after treatment with the ion-exchange resin at 5-second intervals, and measuring the change in the pH value of the dispersion.

[0122] In this neutralization titration, two points are observed in the curve plotting the measured pH against the amount of sodium hydroxide added, where the increment (the derivative of pH with respect to the amount of sodium hydroxide added) is maximum. Of these, the first increment maximum obtained after starting to add sodium hydroxide is called the first endpoint, and the next increment maximum obtained is called the second endpoint (Figure 1). The amount of sodium hydroxide required from the start of the titration to the first endpoint is equal to the amount of the first dissociated acid in the dispersion used for titration. Also, the amount of sodium hydroxide required from the start of the titration to the second endpoint is equal to the total amount of dissociated acid in the dispersion used for titration. Note that the amount of 2 phosphorus oxoacid groups (mol / g) was defined as the amount of sodium hydroxide (mol) required from the start of the titration to the first endpoint divided by the solid content (g) in the dispersion being titrated.

[0123] [Measurement of the degree of polymerization of fine fibrous cellulose] The degree of polymerization of fine fibrous cellulose was calculated from the viscosity measured according to Tappi T230. Specifically, the viscosity (η1) measured by dissolving the fine fibrous cellulose to be measured in a 1 mol / L copper ethylenediamine aqueous solution, and the blank viscosity (η0) measured using only the above copper ethylenediamine aqueous solution were measured. Then, the specific viscosity (ηsp) and intrinsic viscosity ([η]) were calculated according to the following formulas: ηsp = (η1 / η0) - 1 [η] = ηsp / (c(1 + 0.28 × ηsp)) Here, c in the formula represents the concentration of fine fibrous cellulose at the time of viscosity measurement. Furthermore, the degree of polymerization (DP) was calculated from the following formula: DP = 1.75 × [η]

[0124] [Evaluation Method] [Amount of Phosphate Adsorption of Electrolyte Membrane] The mass of the obtained electrolyte membrane and the phosphate-doped electrolyte membrane was measured, and the amount of phosphate adsorption of the electrolyte membrane was calculated according to the following formula: Amount of phosphate adsorption (mass%) = 100 × (W ＰＡ -W ｄｒｙ ) / W ｄｒｙ W ＰＡ : Mass measurement of phosphate-doped electrolyte membrane at 60°C W ｄｒｙ : Mass measurement of electrolyte membrane at 60°C

[0125] [Proton Conductivity of Phosphate-Doped Electrolyte Membrane] The proton conductivity of the obtained phosphate-doped electrolyte membrane was measured in an unhumidified state at a temperature of 100-160°C using an impedance analyzer (IM3570, HIOKI E.E. CORPORATION, frequency: 4.6-4.6 × 10⁻¹⁰) within an environmental control device (tabletop constant temperature and humidity chamber, SH-222, ESPEC Corporation). ６ The AC impedance was measured using the Hz, four-terminal method. Proton conductivity was measured after standing at 100°C for 1 hour, then raising the temperature to a predetermined temperature, and standing for 20 minutes after reaching the predetermined temperature. From the Cole-Cole plot, the inflection point was considered to be the bulk resistance of each electrolyte membrane. The proton conductivity σ (S / cm) was calculated using the following formula: σ = d / (Rs × S) d (cm): distance between gold wires Rs (Ω): bulk impedance (Ω) S (cm)２ ): Proton conduction area calculated by film thickness × width

[0126] [Maintenance of proton conductivity of phosphate-doped electrolyte membrane] A phosphate-doped electrolyte membrane was subjected to an impedance analyzer (IM3570, HIOKI E.E. CORPORATION, frequency: 4.6 to 4.6 × 10⁻¹) inside an environmental control device (tabletop constant temperature and humidity chamber, SH-222, ESPEC Corporation) set to 150°C. ６ The device was set to Hz (four-terminal method), and the proton conductivity was measured and calculated using the method described above immediately after setting, and after 1 hour, 2 hours, 6 hours, 12 hours, and 24 hours. The ratio (%) of the proton conductivity after the predetermined time to the proton conductivity immediately after setting was defined as the proton conductivity maintenance rate.

[0127]

[0128]

[0129] (Table Notes) Actual: Example Comparison: Comparative Example

[0130] Tables 1 and 2 show that the electrolyte membrane for polymer electrolyte fuel cells (phosphate-doped electrolyte membrane) of the present invention exhibits excellent proton conductivity at high temperatures and is resistant to degradation at high temperatures (Examples 1-3). In contrast, the phosphate-doped electrolyte membrane produced using only component (C): poly(vinylphosphonic acid) showed a similar amount of phosphate adsorption as Example 1, but exhibited low proton conductivity at high temperatures and a significantly reduced retention rate of proton conductivity at high temperatures (Comparative Example 1). Furthermore, Tables 1 and 2 show that the electrolyte membrane for polymer electrolyte water electrolysis of this embodiment, which includes a proton conductor for the electrolyte membrane of a polymer electrolyte water electrolysis device having the same configuration as the proton conductor for the electrolyte membrane of the polymer electrolyte fuel cell of the present invention, also exhibits excellent proton conductivity at high temperatures and is resistant to degradation at high temperatures. Furthermore, it is understood that a membrane electrode assembly using the proton conductor for the electrolyte membrane of the polymer electrolyte water electrolysis apparatus of this embodiment, and a polymer electrolyte water electrolysis apparatus are provided, as well as a method for manufacturing a proton conductor for the electrolyte membrane of a polymer electrolyte water electrolysis apparatus that achieves the above effects, a method for manufacturing an electrolyte membrane for polymer electrolyte water electrolysis, a method for manufacturing a membrane electrode assembly, and a method for manufacturing a polymer electrolyte water electrolysis apparatus are provided.

Claims

1. A proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell, containing the following components (A) and (B): Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less; Component (B): Polymer compound having 1 phosphorus oxoacid group.

2. The proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to claim 1, wherein the mass ratio of component (B) to component (A) (component (B) / component (A)) is 20 / 80 or more and 80 / 20 or less.

3. A proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to claim 1 or 2, wherein the degree of polymerization of component (A) is 350 or higher.

4. A proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to claim 1 or 2, wherein component (A) has two phosphorus oxoacid groups.

5. A polymer electrolyte membrane for a polymer electrolyte fuel cell, comprising a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to claim 1 or 2, and the following component (C). Component (C): Membrane component 6. The electrolyte membrane for a polymer electrolyte fuel cell according to claim 5, wherein the ratio of the total mass of component (A) and component (B) to the mass of component (C) ((component (A) + component (B)) / component (C)) is 5 / 95 or more and 55 / 45 or less.

7. A membrane electrode assembly comprising a positive electrode catalyst layer, an electrolyte membrane for a polymer electrolyte fuel cell as described in claim 5, and a negative electrode catalyst layer, joined in this order.

8. A polymer electrolyte fuel cell having the membrane electrode assembly described in claim 7.

9. A method for producing a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell, comprising the following preparation step I and polymerization step in this order: Preparation step I: A step to obtain a mixed solution I containing the following components (A), (b), and a solvent. Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less. Component (b): Monomer having 1 phosphorus oxoacid group. Polymerization step: A step to obtain a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell containing component (A) and the following component (B). Component (B): Polymer compound having 1 phosphorus oxoacid group.

10. A method for manufacturing an electrolyte membrane for a polymer electrolyte fuel cell, comprising the following preparation step II and film formation step in this order: Preparation step II: A step to obtain a mixed solution II containing the following component (C), a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell obtained by the method for manufacturing a proton conductor for the electrolyte membrane of a polymer electrolyte fuel cell according to claim 9, and a solvent. Component (C): Membrane component Film formation step: A step to obtain an electrolyte membrane for a polymer electrolyte fuel cell by removing the solvent from the mixed solution II.

11. A method for manufacturing a membrane electrode assembly, comprising the steps of bonding a positive electrode catalyst layer to one side of an electrolyte membrane for a polymer electrolyte fuel cell obtained by the method for manufacturing an electrolyte membrane for a polymer electrolyte fuel cell described in claim 10, and bonding a negative electrode catalyst layer to the other side.

12. A method for manufacturing a polymer electrolyte fuel cell, comprising the step of incorporating a membrane electrode assembly obtained by the method for manufacturing a membrane electrode assembly described in claim 11.

13. A proton conductor for the electrolyte membrane of a solid polymer water electrolysis apparatus, containing the following components (A) and (B): Component (A): Fine fibrous cellulose with a fiber width of 1,000 nm or less; Component (B): Polymer compound having 1 phosphorus oxoacid group.

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