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

Abstract

The purpose of the present invention is to provide: Provided are: an electrolyte membrane for a solid polymer fuel cell, which has excellent proton conductivity; a membrane-electrode assembly comprising the electrolyte membrane for a solid polymer fuel cell; a solid polymer fuel cell comprising the membrane-electrode assembly; and an electrolyte membrane for a solid polymer fuel cell. Various methods for producing a membrane electrode assembly and a solid polymer fuel cell; and an electrolyte membrane for solid polymer water electrolysis. This electrolyte membrane for a solid polymer fuel cell contains a woody or herbaceous microfibrous cellulose, the average fiber width of the woody or herbaceous microfibrous cellulose being 50 nm or less, and the microfibrous cellulose having a phosphorus oxyacid group.

Description

TECHNICAL FIELD

[0001] The present application relates to an electrolyte membrane for a solid polymer fuel cell, a membrane electrode assembly, a solid polymer fuel cell, and a manufacturing method thereof, and an electrolyte membrane for solid polymer water electrolysis. BACKGROUND

[0002] A fuel cell is capable of generating energy using hydrogen, and therefore has a small environmental burden. In a fuel cell, in particular, a solid polymer fuel cell (PEFC) is capable of operating at low temperatures and has high power generation efficiency, and therefore is attracting attention as an energy supply source for mobile phones and / or automobiles, and the like. In addition, research and development of solid polymer water electrolysis as a manufacturing method for supplying hydrogen to a solid polymer fuel cell is also being conducted.

[0003] In the past, perfluorosulfonic acid polymers have been widely used as a constituent material of an electrolyte membrane for a solid polymer fuel cell. In recent years, however, research and development aimed at improving cell performance by using cellulose, which is naturally abundant and has a small environmental burden compared to perfluorosulfonic acid polymers, as an electrolyte membrane forming raw material is being conducted. For example, Patent Literature 1 describes a fuel cell characterized by containing a nanocellulose membrane as an electrolyte membrane, in a form in which OH of a hydroxymethyl group (CH2OH) constituting the nanocellulose membrane is replaced with a sulfonic acid group (SO3H) that is directly bonded to the nanocellulose membrane. In addition, Patent Literature 2 describes an electrolyte membrane composition, a solid polymer fuel cell and a water electrolysis device provided with a solid polymer electrolyte membrane obtained from the electrolyte membrane composition, with the aim of providing a composition capable of easily obtaining a polymer electrolyte membrane that is excellent in durability and capable of suppressing temporal reduction in power generation performance and water electrolysis performance.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent No. 6721952

[0007] Patent Literature 2: International Publication No. 2014 / 157389 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION

[0009] The proton conductivity of the nanocellulose film serving as the electrolyte film of the fuel cell (solid polymer fuel cell) described in Patent Literature 1 cannot be said to be sufficient, and it is desirable to improve the proton conductivity of the electrolyte film from the viewpoint of improving the power generation efficiency and / or the long life of the cell. Also, for the water electrolysis device described in Patent Literature 2, it is also desirable to improve the proton conductivity of the solid polymer electrolyte film from the viewpoint of improving the efficiency of hydrogen production.

[0010] An object of the present application is to provide a solid polymer fuel cell electrolyte film having excellent proton conductivity, a membrane electrode assembly having the solid polymer fuel cell electrolyte film, a solid polymer fuel cell having the membrane electrode assembly, various production methods of a solid polymer fuel cell electrolyte film, a membrane electrode assembly, and a solid polymer fuel cell, and a solid polymer water electrolysis electrolyte film.

[0011] Approach for solving the problem

[0012] The present inventors have attempted to apply, to a solid polymer fuel cell electrolyte film and a solid polymer water electrolysis electrolyte film, among fibrous celluloses derived from various fiber raw materials, especially fine fibrous celluloses derived from woody or herbaceous plants, which are easily uniformly dispersed in water after introduction of a phosphorus-containing oxyacid group. The present inventors have found that by using a fine fibrous cellulose derived from woody or herbaceous plants, having a specific average fiber width, and having a phosphorus-containing oxyacid group for an electrolyte film, an electrolyte film having excellent proton conductivity can be obtained, and the present application has been completed as a result.

[0013] That is, the present application relates to the following <1> to <24>.

[0014] <1> A solid polymer fuel cell electrolyte film containing a fine fibrous cellulose derived from woody or herbaceous plants,

[0015] The fine fibrous cellulose derived from woody or herbaceous plants has an average fiber width of 50 nm or less, and the fine fibrous cellulose derived from woody or herbaceous plants has a phosphorus-containing oxyacid group.

[0016] <2> The solid polymer fuel cell electrolyte film according to <1>, wherein the counter ion of the phosphorus-containing oxyacid group contains H + and / or Na + .

[0017] <3> The solid polymer fuel cell electrolyte film according to <1> or <2>, wherein the counter ion of the phosphorus-containing oxyacid group contains H + .

[0018] <4> The electrolyte membrane for a solid polymer fuel cell according to any one of <1> to <3>, wherein the amount of introduction of phosphorus-containing oxyacid groups in the microfibrous cellulose derived from a woody or herbaceous plant is 0.50 mmol / g or more.

[0019] <5> The electrolyte membrane for a solid polymer fuel cell according to any one of <1> to <4>, wherein the content of the microfibrous cellulose derived from a woody or herbaceous plant in the solid component of the electrolyte membrane for a solid polymer fuel cell is 80 mass% or more.

[0020] <6> The electrolyte membrane for a solid polymer fuel cell according to any one of <1> to <4>, wherein the content of the microfibrous cellulose derived from a woody or herbaceous plant in the solid component of the electrolyte membrane for a solid polymer fuel cell is 30 mass% or more.

[0021] <7> A membrane electrode assembly which is formed by sequentially joining a positive electrode catalyst layer, the electrolyte membrane for a solid polymer fuel cell according to any one of <1> to <6>, and a negative electrode catalyst layer.

[0022] <8> A solid polymer fuel cell which has the membrane electrode assembly according to <7>.

[0023] <9> A method for producing an electrolyte membrane for a solid polymer fuel cell, which sequentially has the following preparation step and film formation step,

[0024] the microfibrous cellulose derived from a woody or herbaceous plant in the preparation step has an average fiber width of 50 nm or less and has phosphorus-containing oxyacid groups.

[0025] Preparation step: a step of preparing a dispersion liquid of microfibrous cellulose derived from a woody or herbaceous plant

[0026] Film formation step: a step of forming a film of the dispersion liquid of microfibrous cellulose derived from a woody or herbaceous plant

[0027] <10> The method for producing an electrolyte membrane for a solid polymer fuel cell according to <9>, wherein the method has the following ion exchange step between the preparation step and the film formation step,

[0028] Ion exchange step: a step of treating the dispersion liquid of microfibrous cellulose derived from a woody or herbaceous plant with a cation exchange resin.

[0029] <11> A method for manufacturing a membrane electrode assembly, comprising the steps of: joining a positive electrode catalyst layer to one face of a solid polymer fuel cell electrolyte membrane obtained by the method for manufacturing a solid polymer fuel cell electrolyte membrane according to <9> or <10>, and joining a negative electrode catalyst layer to the other face of the solid polymer fuel cell electrolyte membrane.

[0030] <12> A method for manufacturing a solid polymer fuel cell, comprising the step of incorporating a membrane electrode assembly obtained by the method for manufacturing a membrane electrode assembly according to <11>.

[0031] <13> A solid polymer electrolyte membrane for water electrolysis, comprising a fine fibrous cellulose derived from a woody or herbaceous plant,

[0032] The fine fibrous cellulose derived from a woody or herbaceous plant has an average fiber width of 50 nm or less, and the fine fibrous cellulose derived from a woody or herbaceous plant has a phosphorus-containing oxyacid group.

[0033] <14> The solid polymer electrolyte membrane for water electrolysis according to <13>, wherein the counter ion of the phosphorus-containing oxyacid group comprises H + and / or Na + .

[0034] <15> The solid polymer electrolyte membrane for water electrolysis according to <13> or <14>, wherein the counter ion of the phosphorus-containing oxyacid group comprises H + .

[0035] <16> The solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <15>, wherein the amount of the phosphorus-containing oxyacid group introduced into the fine fibrous cellulose derived from a woody or herbaceous plant is 0.50 mmol / g or more.

[0036] <17> The solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <16>, wherein the content of the fine fibrous cellulose derived from a woody or herbaceous plant in the solid component of the solid polymer electrolyte membrane for water electrolysis is 80 mass% or more.

[0037] <18> The solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <16>, wherein the content of the fine fibrous cellulose derived from a woody or herbaceous plant in the solid component of the solid polymer electrolyte membrane for water electrolysis is 30 mass% or more.

[0038] <19> A membrane electrode assembly which is formed by sequentially joining a positive electrode catalyst layer, the solid polymer electrolyte membrane for water electrolysis according to any one of <13> to <18>, and a negative electrode catalyst layer.

[0039] <20> A solid polymer water electrolysis device having the membrane electrode assembly according to <19>.

[0040] <21> A method for producing a solid polymer electrolyte membrane for water electrolysis, which sequentially has the following preparation step and film formation step,

[0041] The average fiber width of the fine fiber-like cellulose derived from a woody or herbaceous system in the preparation step is 50 nm or less, and the fine fiber-like cellulose derived from a woody or herbaceous system has a phosphorus-containing oxyacid group.

[0042] Preparation step: a step of preparing a fine fiber-like cellulose derived from a woody or herbaceous system dispersion liquid

[0043] Film formation step: a step of forming a fine fiber-like cellulose derived from a woody or herbaceous system dispersion liquid into a film.

[0044] <22> The method for producing a solid polymer electrolyte membrane for water electrolysis according to <21>, wherein the preparation step and the film formation step have the following ion exchange step therebetween,

[0045] Ion exchange step: a step of treating a fine fiber-like cellulose derived from a woody or herbaceous system dispersion liquid with a cation exchange resin.

[0046] <23> A method for producing a membrane electrode assembly, which has a step of joining a positive electrode catalyst layer to one face of a solid polymer electrolyte membrane for water electrolysis obtained by the method for producing a solid polymer electrolyte membrane for water electrolysis according to <21> or <22>, and joining a negative electrode catalyst layer to the other face of the solid polymer electrolyte membrane for water electrolysis.

[0047] <24> A method for producing a solid polymer water electrolysis device, which has a step of incorporating a membrane electrode assembly obtained by the method for producing a membrane electrode assembly according to <23>.

[0048] Effects of the Invention

[0049] According to the present application, it is possible to provide a solid polymer electrolyte membrane for fuel cells, a membrane electrode assembly having the same, a solid polymer fuel cell having the same, various production methods for a solid polymer electrolyte membrane for fuel cells, a membrane electrode assembly, and a solid polymer fuel cell, and a solid polymer electrolyte membrane for water electrolysis. Attached Figure Description

[0050] Figure 1 This is a graph showing the relationship between the amount of NaOH added and pH relative to a fine fibrous cellulose dispersion containing phosphorus oxyacid groups. Detailed Implementation

[0051] The preferred embodiments of the present invention will now be described. It should be noted that, in this specification, "X~Y" indicating a range means "above X and below Y". Furthermore, in this specification, the upper and lower limits of the numerical range can be arbitrarily combined.

[0052] Electrolyte membranes for solid polymer fuel cells

[0053] The electrolyte membrane for solid polymer fuel cells in this embodiment contains microfibrillated cellulose derived from woody or herbaceous plants. The average fiber width of the microfibrillated cellulose derived from woody or herbaceous plants is less than 50 nm, and the microfibrillated cellulose has phosphorus oxyacid groups.

[0054] The electrolyte membrane for solid polymer fuel cells in this embodiment may contain or may contain one type of component alone, or two or more types in combination.

[0055] The electrolyte membrane for a solid polymer fuel cell in this embodiment has the same structure as the electrolyte membrane for water electrolysis in this embodiment described later, except for its application. Hereinafter, "electrolyte membrane" refers to either the electrolyte membrane for a solid polymer fuel cell or the electrolyte membrane for water electrolysis. In addition, "fine fibrous cellulose derived from woody or herbaceous plants" is sometimes simply referred to as "fine fibrous cellulose".

[0056] The reason for the excellent proton conductivity of the solid polymer electrolyte membrane for fuel cells in this embodiment is not yet certain, but it can be considered that the proton conductivity within the membrane is improved for the following reasons: the fine fibrous cellulose derived from woody or herbaceous plants is relatively hydrophilic even after the introduction of phosphorus oxyacid groups; the fine fibrous cellulose derived from woody or herbaceous plants with phosphorus oxyacid groups is easily and uniformly dispersed in water, and can form a smooth solid polymer electrolyte membrane for fuel cells; and most of the phosphorus oxyacid groups do not undergo dehydration condensation within the fine fibrous cellulose molecules, and exist in the solid polymer electrolyte membrane for fuel cells as divalent anionic groups.

[0057] The electrolyte membrane for a solid polymer fuel cell according to this embodiment will be described in detail below.

[0058] <Fine fibrous cellulose derived from woody or herbaceous plants>

[0059] The fiber width of the microfibrous cellulose is preferably 2 nm or more and 50 nm or less, and more preferably 40 nm or less, further preferably 30 nm or less, further preferably 20 nm or less, particularly preferably 10 nm or less. By setting the fiber width of the microfibrous cellulose to 2 nm or more, it is possible to suppress the dissolution in water in the form of cellulose molecules, and it is easier to exhibit effects such as an increase in strength and / or rigidity, dimensional stability, and the like, which are brought about by the microfibrous cellulose.

[0060] Note that the fiber width of the microfibrous cellulose can be measured by electron microscope observation or the like, for example.

[0061] The average fiber width of the microfibrous cellulose is 50 nm or less. The average fiber width of the microfibrous cellulose is preferably 2 nm or more and 50 nm or less, and more preferably 40 nm or less, further preferably 30 nm or less, further more preferably 20 nm or less, particularly preferably 10 nm or less. By setting the average fiber width of the microfibrous cellulose to 2 nm or more, it is possible to suppress the dissolution in water in the form of cellulose molecules, and it is easier to exhibit effects such as an increase in strength and / or rigidity, dimensional stability, and the like, which are brought about by the microfibrous cellulose. In addition, by setting the average fiber width of the microfibrous cellulose to 50 nm or less, it is possible to make the surface of the solid polymer electrolyte membrane for a fuel cell of the present embodiment smooth. By making the surface of the solid polymer electrolyte membrane for a fuel cell smooth, protons easily move between the positive electrode catalyst layer-solid polymer electrolyte membrane for a fuel cell-negative electrode catalyst layer, and it is possible to improve proton conductivity.

[0062] Note that the microfibrous cellulose is, for example, a single-fiber cellulose.

[0063] The average fiber width of the microfibrous cellulose is measured using an electron microscope as follows, for example. First, an aqueous suspension of the microfibrous cellulose having a concentration of 0.05 mass% or more and 0.1 mass% or less is prepared, and the suspension is cast on a hydrophilic-treated carbon film cover grid to prepare a sample for transmission electron microscope (TEM) observation. In the case of containing fibers having a wide width, it is also possible to observe a scanning electron microscope (SEM) image of the surface cast on glass. Next, observation based on an electron microscope image is performed at any of 1,000 times, 5,000 times, 10,000 times, or 50,000 times, according to the width of the fibers that become the observation target. Among them, the sample, the observation conditions, and / or the magnification are adjusted in such a manner as to satisfy the following conditions.

[0064] (1) A straight line X is drawn at an arbitrary position in the observation image, and 20 or more fibers cross the straight line X.

[0065] (2) draw a straight line Y that intersects the straight line X perpendicularly within the same image, and 20 or more fibers intersect the straight line Y.

[0066] For the observed images that satisfy the above conditions, the width of the fibers that intersect the straight line X and the straight line Y is visually read. This operation is performed for 3 or more sets of mutually non-overlapping surface portions. Then, for each image, the width of the fibers that intersect the straight line X and the straight line Y is read. Thus, the width of 120 or more fibers is read. Then, the average of the read fiber widths is taken as the average fiber width of the microfibrous cellulose.

[0067] The fiber length of the microfibrous cellulose is preferably, for example, 0.1 μm or more and 1,000 μm or less, more preferably 0.1 μm or more and 800 μm or less, and further preferably 0.1 μm or more and 600 μm or less. By setting the fiber length within the above range, the destruction of the crystalline region of the microfibrous cellulose can be suppressed. In addition, the viscosity of a slurry (dispersion) of the microfibrous cellulose can be set within an appropriate range. Note that the fiber length of the microfibrous cellulose can be determined, for example, by image analysis based on TEM, SEM, or atomic force microscopy (AFM).

[0068] The microfibrous cellulose preferably has a crystal structure of type I. Here, that the microfibrous cellulose has a crystal structure of type I can be identified from a diffraction pattern obtained from a wide-angle X-ray diffraction photograph using CuKa (λ = 1.5418 A) monochromatized with graphite. Specifically, since typical peaks are present at both around 2θ = 14° or more and 17° or less and around 2θ = 22° or more and 23° or less, identification is possible.

[0069] The proportion of the crystal structure of type I in the microfibrous cellulose is preferably, for example, 30% or more, more preferably 40% or more, and further preferably 50% or more. The crystallinity degree is determined from an X-ray diffraction pattern, and is determined according to the pattern and by a conventional method (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).

[0070] The aspect ratio (fiber length / fiber width) of the microfibrous cellulose is not particularly limited, and is preferably, for example, 20 or more and 10,000 or less, more preferably 50 or more and 1,000 or less. By setting the aspect ratio to be the lower limit value or more, it is easy to form an electrolyte membrane for a solid polymer fuel cell. In addition, it is easy to obtain sufficient thickening properties when preparing a solvent dispersion. By setting the aspect ratio to be the upper limit value or less, it is preferable from the viewpoint of easily performing dilution and the like when the microfibrous cellulose is handled in the form of an aqueous dispersion.

[0071] The microfibrous cellulose in the present embodiment has a phosphorus oxoacid group. Note that, in the present specification, the "phosphorus oxoacid group" includes a substituent derived from a phosphorus oxoacid group. In addition, the "counter ion" of the phosphorus oxoacid group includes a dissociative proton.

[0072] As the counter ion, for example, the β b+ (proton (H + ) or a monovalent or more cation formed of an organic substance or an inorganic substance) can be given. Among them, from the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the counter ion preferably contains at least one selected from the group consisting of H + and an ion of an alkali metal (sodium, potassium, or lithium, etc.), more preferably contains H + and / or a sodium ion (Na + ), from the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, reducing the temperature dependence of the proton conductivity, and reducing the activation energy of the proton conduction, the counter ion further preferably contains H + .

[0073] In addition, from the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, reducing the temperature dependence of the proton conductivity, and reducing the activation energy of the proton conduction, the content (mass) of the cation other than H + contained in the counter ion is preferably 100,000 ppm or less, more preferably 1,000 ppm or less, further preferably 100 ppm or less, and is 0 ppm or more, preferably 0 ppm.

[0074] The phosphorus oxoacid group is, for example, a substituent represented by the following formula (1). A plurality of substituents represented by the following formula (1) can be introduced into each microfibrous cellulose. At this time, the plurality of substituents represented by the following formula (1) introduced can be the same or different. The substituent represented by the following formula (1) is preferably directly bonded to the carbon atom at position 2, 3, and / or 6 of cellulose with "-O-".

[0075]

[0076] In formula (1), a, b, and n are natural numbers, and m is an arbitrary number (where a = b x m). At least one of n α and α' is O - The rest are R or OR. Note that each α and α' can also be O - The n α can all be the same or can each be different. β b+ is a cation of valence 1 or more formed from an organic or inorganic substance.

[0077] 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 thereof. In formula (1), n is preferably 1.

[0078] As the saturated-straight-chain hydrocarbon group, a methyl group, an ethyl group, a n-propyl group, or a n-butyl group, etc. can be cited, without particular limitation. As the saturated-branched-chain hydrocarbon group, an iso-propyl group or a t-butyl group, etc. can be cited, without particular limitation. As the saturated-cyclic hydrocarbon group, a cyclopentyl group or a cyclohexyl group, etc. can be cited, without particular limitation. As the unsaturated-straight-chain hydrocarbon group, a vinyl group or an allyl group, etc. can be cited, without particular limitation. As the unsaturated-branched-chain hydrocarbon group, an iso-propenyl group or a 3-butenyl group, etc. can be cited, without particular limitation. As the unsaturated-cyclic hydrocarbon group, a cyclopentenyl group, a cyclohexenyl group, etc. can be cited, without particular limitation. As the aromatic group, a phenyl group or a naphthyl group, etc. can be cited, without particular limitation.

[0079] Further, as the derivative in R, a functional group in a state in which at least one functional group selected from a carboxyl group, a carboxylate group (-COO-), a hydroxyl group, an amino group, and an ammonium group, etc. is added to or replaced in the main chain or side chain of the above-described various hydrocarbon groups can be cited, without particular limitation. Further, the number of carbon atoms constituting the main chain of R is not particularly limited, and is preferably 20 or less, more preferably 10 or less. By setting the number of carbon atoms constituting the main chain of R to the above-described range, the molecular weight of the phosphorus oxoacid group can be set to an appropriate range, making penetration into the fiber raw material easy, and the yield of the microfibrous cellulose can also be improved. Note that in the case where multiple R's exist in formula (1) and / or multiple substituents represented by the above-described formula (1) are introduced into the microfibrous cellulose, the multiple R's that exist can each be the same or different.

[0080] β b+ is H +or a cation of one valence or more formed from an organic substance or an inorganic substance. As the cation of one valence or more formed from an organic substance, an organic onium ion can be exemplified. As the organic onium ion, for example, an organic ammonium ion and / or an organic phosphonium ion can be exemplified. As the organic ammonium ion, for example, an aliphatic ammonium ion and / or an aromatic ammonium ion can be exemplified, and as the organic phosphonium ion, for example, an aliphatic phosphonium ion and / or an aromatic phosphonium ion can be exemplified. As the cation of one valence or more formed from an inorganic substance, an ion of an alkali metal such as sodium, potassium or lithium and / or an ion of a divalent metal such as calcium or magnesium, a hydrogen ion, an ammonium ion and the like can be exemplified. Note that in the case where a plurality of β b+ may be different. From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, β b+ may be the same or different. From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, β b+ is preferably composed of at least one selected from the group consisting of H + and an ion of an alkali metal, more preferably H + and / or a sodium ion (Na + ). From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, reducing the temperature dependence of the proton conductivity, and reducing the activation energy of the proton conduction, β b+ is further preferably composed of H + .

[0081] As the phosphorus-containing oxyacid group, more specifically, a phosphoric acid group (-OPO3H2), a salt of the phosphoric acid group, a phosphorous acid group (phosphonic acid group) (-OPO2H2), a salt of the phosphorous acid group (phosphonic acid group) can be exemplified. In addition, the phosphorus-containing oxyacid group can include a group condensed with a phosphoric acid group (e.g., a pyrophosphoric acid group), a group condensed with a phosphonic acid (e.g., a polyphosphonic acid group), a phosphoric acid ester group (e.g., a phosphoric acid monomethyl ester group, a polyoxyethylene alkyl phosphoric acid group), an alkyl phosphonic acid group (e.g., a methyl phosphonic acid group).

[0082] The amount of the phosphorus-containing oxyacid group introduced with respect to the microfibrous cellulose is, for example, preferably 0.50 mmol / g or more and 2.50 mmol / g or less, more preferably 0.80 mmol / g or more, further preferably 1.00 mmol / g or more, particularly preferably 1.20 mmol / g or more, and more preferably 2.30 mmol / g or less, further preferably 2.10 mmol / g or less, per 1 g (mass) of the microfibrous cellulose.

[0083] From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the amount of introduction of the phosphorus oxoacid group is preferably 0.1 mmol / g or more, and from the viewpoint of improving the strength and / or rigidity and the dimensional stability of the electrolyte membrane for a solid polymer fuel cell, the amount of introduction of the phosphorus oxoacid group is preferably 10 mmol / g or less.

[0084] Here, the denominator in the unit of mmol / g indicates the mass of the fine fibrous cellulose when the counter ion of the phosphorus oxoacid group is a hydrogen ion (H + ).

[0085] The amount of introduction of the phosphorus oxoacid group with respect to the fine fibrous cellulose can be determined, for example, using a neutralization titration method. In the determination using the neutralization titration method, the change in pH is found while adding an alkali such as an aqueous sodium hydroxide solution to the obtained slurry containing the fine fibrous cellulose, and thereby the amount of introduction is determined.

[0086] Figure 1 is a graph showing the relationship between the amount of addition of NaOH and the pH with respect to the fine fibrous cellulose having the phosphorus oxoacid group.

[0087] Figure 1 is a graph showing the relationship between the amount of addition of NaOH and the pH with respect to the slurry containing the fine fibrous cellulose having the phosphorus oxoacid group. The amount of introduction of the phosphorus oxoacid group with respect to the fine fibrous cellulose can be determined, for example, as follows.

[0088] First, the slurry containing the fine fibrous cellulose is treated with a strongly acidic ion exchange resin. Note that, as needed, the same fibrillation treatment as the fibrillation treatment process described later can be performed on the determination object before the treatment with the strongly acidic ion exchange resin.

[0089] Next, while adding an aqueous sodium hydroxide solution, the change in pH is observed, and a titration curve shown in the upper side of Figure 1 is obtained. In the titration curve shown in the upper side of Figure 1 , the measured pH is plotted against the amount of addition of the alkali, and the amount of introduction of the phosphorus oxoacid group with respect to the fine fibrous cellulose is determined from the point at which the pH becomes 7. Figure 1In the titration curve shown in the lower side of FIG. 1, the increment (derivative value) of pH with respect to the amount of base added (1 / mmol) is plotted. In the curve obtained by plotting the measured pH against the amount of base added in the neutralization titration, two points at which the increment (derivative value of pH with respect to the amount of base added) reaches a maximum are confirmed. The point at which the increment first reaches a maximum upon addition of base is referred to as the first end point, and the point at which the increment next reaches a maximum is referred to as the second end point. The amount of base required from the start of the titration to the first end point is equal to the first dissociation acid amount of the fine fibrous cellulose contained in the pulp used for the titration, the amount of base required from the first end point to the second end point is equal to the second dissociation acid amount of the fine fibrous cellulose contained in the pulp used for the titration, and the amount of base required from the start of the titration to the second end point is equal to the total dissociation acid amount of the fine fibrous cellulose contained in the pulp used for the titration. Then, the value obtained by dividing the amount of base required from the start of the titration to the first end point by the solid content (g) in the pulp subjected to the titration is the phosphorus-containing oxyacid group introduction amount (mmol / g). Note that, in the case of a simple term "phosphorus-containing oxyacid group introduction amount" (or "phosphorus-containing oxyacid group amount"), the first dissociation acid amount is meant.

[0090] Note that, in the case of a simple term "phosphorus-containing oxyacid group introduction amount" (or "phosphorus-containing oxyacid group amount"), the first dissociation acid amount is meant. Figure 1 In the case where the phosphorus-containing oxyacid group is a phosphoric acid group, in the case where the phosphoric acid group undergoes condensation, the amount of weakly acidic groups in the phosphorus-containing oxyacid group (also referred to as the second dissociation acid amount in the present specification) apparently decreases, and the amount of base required in the second region decreases as compared with the amount of base required in the first region. On the other hand, the amount of strongly acidic groups in the phosphorus-containing oxyacid group (also referred to as the first dissociation acid amount in the present specification) coincides with the amount of phosphorus atoms regardless of condensation. In the case where the phosphorus-containing oxyacid group is a phosphorous acid group, there is no weakly acidic group in the phosphorus-containing oxyacid group, and thus, the amount of base required in the second region can be less or zero. In this case, in the titration curve, the point at which the increment of pH reaches a maximum is one.

[0091] Note that, since the denominator indicates the mass of the fine fibrous cellulose in the acid form, the above-described phosphorus-containing oxyacid group introduction amount (mmol / g) indicates the amount of phosphorus-containing oxyacid groups possessed by the fine fibrous cellulose in the acid form (hereinafter referred to as the phosphorus-containing oxyacid group amount (acid form)). On the other hand, in the case where the counter ion of the phosphorus-containing oxyacid group is replaced with an arbitrary cation C in a manner so as to become a charge equivalent, by converting the denominator to the mass of the fine fibrous cellulose in which the cation C is the counter ion, the amount of phosphorus-containing oxyacid groups possessed by the fine fibrous cellulose in which the cation C is the counter ion (hereinafter referred to as the phosphorus-containing oxyacid group amount (C form)) can be calculated.

[0092] That is, the amount of phosphorus-containing oxyacid group is calculated by the following calculation formula.

[0093] Amount of phosphorus-containing oxyacid group (C type) = Amount of phosphorus-containing oxyacid group (acid type) / {1 + (W - 1) x A / 1000}

[0094] A [mmol / g]: Total anion amount of phosphorus-containing oxyacid group possessed by the microfibrillar cellulose (value obtained by adding the amount of strong acidic group of the phosphorus-containing oxyacid group and the amount of weak acidic group)

[0095] W: Formula weight per 1 valence of cation C (for example, Na is 23, and Al is 9)

[0096] Note that in the measurement of the phosphorus-containing oxyacid group by the titration method, in a case where the amount of the addition of 1 drop of the aqueous sodium hydroxide solution is too much and / or the titration interval is too short, sometimes the phosphorus-containing oxyacid group and the like become lower than originally, and an accurate value cannot be obtained. As the appropriate amount of addition and the titration interval, for example, 10 to 50 μL of the 0.1 N aqueous sodium hydroxide solution is preferably titrated at a time within 5 to 30 seconds, and the like. In addition, in order to eliminate the influence of carbon dioxide dissolved in the pulp containing the microfibrillar cellulose, for example, it is preferable to perform the measurement while blowing a non-active gas such as nitrogen into the pulp from 15 minutes before the start of the titration to the end of the titration, and the like.

[0097] The measurement of the amount of the phosphorus-containing oxyacid group performed by the above-described method is applied to the microfibrillar cellulose having a fiber width of 50 nm or less, and in a case where the amount of the phosphorus-containing oxyacid group of the pulp fiber having a fiber width exceeding 50 nm is measured, the pulp fiber is microfibrillated and then the measurement is performed.

[0098] In the present embodiment, the microfibrillar cellulose can be a substance obtained by removing a part of the phosphorus-containing oxyacid group from the microfibrillar cellulose into which the above-described phosphorus-containing oxyacid group is introduced and defibrillated.

[0099] [Method for producing microfibrillar cellulose]

[0100] [Woody or herbaceous fiber raw material containing cellulose]

[0101] The microfibrillar cellulose derived from the woody or herbaceous is produced from a woody or herbaceous fiber raw material containing cellulose (hereinafter, the woody or herbaceous fiber raw material is simply referred to as "fiber raw material"). From the viewpoint of easily introducing the phosphorus-containing oxyacid group, increasing the amount of introduction of the phosphorus-containing oxyacid group, and improving the proton conductivity of the electrolyte membrane for the solid polymer fuel cell, the microfibrillar cellulose derived from the woody or herbaceous is preferable.

[0102] As the woody fiber raw material containing cellulose, woody pulp can be given. As the woody pulp, for example, chemical pulp such as broad-leaved tree bleached kraft pulp (LBKP), needle-leaved tree bleached kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), alkaline pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP), semi-chemical pulp such as semi-chemical pulp (SCP) and chemical groundwood pulp (CGP), mechanical pulp such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP), and the like can be given.

[0103] As the herbaceous fiber raw material containing cellulose, for example, cotton pulp such as linter and cotton lint, hemp, wheat straw, bamboo, and sugar cane residue can be given.

[0104] Among the above-mentioned pulps, from the viewpoint of easiness of obtaining, woody pulp is preferable. In addition, among the woody pulps, from the viewpoint that the cellulose ratio is large, the yield of the fine fibrous cellulose at the time of defibrating treatment is high, and the fine fibrous cellulose having a small decomposition of cellulose in the pulp and a large aspect ratio can be obtained, for example, chemical pulp is more preferable, kraft pulp is further preferable, and LBKP and NBKP are particularly preferable.

[0105] The pulp of the present embodiment can use one of the above-mentioned 1 kind alone, or two or more kinds in combination.

[0106] In order to obtain the fine fibrous cellulose into which the phosphorus-containing oxyacid group is introduced as described above, it is preferable to sequentially have the following processes: a phosphorus-containing oxyacid group introduction process of introducing a phosphorus-containing oxyacid group into the above-mentioned fiber raw material containing cellulose, a washing process, an alkali treatment process (neutralization process), and a defibrating treatment process, and it is also possible to have an acid treatment process instead of the washing process or in addition to the washing process.

[0107] (Phosphorus-containing oxyacid group introduction process)

[0108] The phosphorus-containing oxyacid group introduction process is a process in which at least one compound selected from compounds capable of introducing a phosphorus-containing oxyacid group (hereinafter also referred to as "compound A") is allowed to act on the fiber raw material containing cellulose by reacting with the hydroxyl group possessed by the fiber raw material containing cellulose. By this process, a phosphorus-containing oxyacid group-introduced fiber can be obtained.

[0109] In the phosphorus-containing oxyacid group introduction process of the present embodiment, the reaction of the fiber raw material containing cellulose with the compound A can be performed in the presence of at least one selected from urea and derivatives thereof (hereinafter also referred to as "compound B"). On the other hand, the reaction of the fiber raw material containing cellulose with the compound A can also be performed in the absence of the compound B.

[0110] As an example of the method of allowing compound A to act on the fiber raw material in the presence of compound B, a method of mixing compound A and compound B with a fiber raw material in a dry state or a wet state or in a pulp state can be exemplified. Among them, from the viewpoint of high reaction uniformity, it is preferable to use a fiber raw material in a dry state or a wet state, and particularly preferably a fiber raw material in a dry state. The form of the fiber raw material is not particularly limited, and for example, a cotton-like and / or flake-like form is preferable. A method in which compound A and compound B are added to a fiber raw material in a powder form or in a solution form in which they are dissolved in a solvent or in a state in which they are melted by heating to a temperature above the melting point can be exemplified. Among them, from the viewpoint of high reaction uniformity, it is preferable to add them in a solution form in which they are dissolved in a solvent, and particularly preferably in a water solution state. In addition, compound A and compound B can be added simultaneously with respect to the fiber raw material, can be added separately, or can be added in a mixture form. As the method of adding compound A and compound B, there is no particular limitation, and in the case where compound A and compound B are in a solution form, the fiber raw material can be soaked in the solution to allow it to absorb the solution and then taken out, or the solution can be added dropwise to the fiber raw material. In addition, the required amount of compound A and compound B can be added to the fiber raw material, or an excess amount of compound A and compound B can be added separately to the fiber raw material, and then the excess amount of compound A and compound B can be removed by pressing and / or filtration.

[0111] As compound A used in the present embodiment, a compound having a phosphorus atom and capable of forming an ester bond with cellulose can be used, and phosphoric acid or a salt thereof, phosphorous acid or a salt thereof, dehydrated condensed phosphoric acid or a salt thereof, phosphoric anhydride (phosphorus pentoxide), and the like can be exemplified, and there is no particular limitation. As the phosphoric acid, various purity substances can be used, and for example, 100% phosphoric acid (orthophosphoric acid), 85% phosphoric acid can be used. As the phosphorous acid, for example, 99% phosphorous acid (phosphonic acid) can be exemplified. The dehydrated condensed phosphoric acid is a substance in which phosphoric acid is condensed by a dehydration reaction with 2 or more molecules, and for example, pyrophosphoric acid, polyphosphoric acid, and the like can be exemplified. As the phosphoric acid salt, phosphorous acid salt, dehydrated condensed phosphoric acid salt, lithium salt, sodium salt, potassium salt, ammonium salt, and the like of phosphoric acid, phosphorous acid, or dehydrated condensed phosphoric acid can be exemplified, and they can be provided in various degrees of neutralization.

[0112] Among them, from the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, phosphoric acid or a salt thereof, phosphorous acid or a salt thereof are preferable, from the viewpoints of high introduction efficiency of the phosphoric group, easy improvement of the fibrillation efficiency in the fibrillation process described later, low cost, and easy application in industry, phosphorous acid, phosphoric acid, sodium salt of phosphoric acid, potassium salt of phosphoric acid, or ammonium salt of phosphoric acid are more preferable, and phosphorous acid, phosphoric acid, sodium dihydrogen phosphate, or ammonium dihydrogen phosphate are further preferable.

[0113] The amount of addition of the compound A with respect to the fiber raw material is not particularly limited, and for example, in the case where the amount of addition of the compound A is converted into the amount of phosphorus atoms, the amount of addition of the phosphorus atoms with respect to 100 parts by mass (absolute dry mass) of the 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 further preferably 2 parts by mass or more and 30 parts by mass or less. By setting the amount of addition of the phosphorus atoms with respect to the fiber raw material to the above range, the yield of the microfibrous cellulose can be further improved. On the other hand, by setting the amount of addition of the phosphorus atoms with respect to the fiber raw material to the above upper limit value or less, the balance between the effect of improving the yield and the cost can be achieved.

[0114] The compound B used in the present embodiment is at least one selected from urea and derivatives thereof, as described above. As the compound B, for example, urea, biuret, 1-phenylurea, 1-benzylurea, 1-methylurea, and 1-ethylurea, and the like can be exemplified.

[0115] From the viewpoint of improving the uniformity of the reaction, the compound B is preferably used in the form of an aqueous solution. In addition, from the viewpoint of further improving the uniformity of the reaction, an aqueous solution in which both the compound A and the compound B are dissolved is preferably used.

[0116] The amount of addition of the compound B with respect to 100 parts by mass (absolute dry mass) of the fiber raw material is not particularly limited, and 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 further preferably 100 parts by mass or more and 350 parts by mass or less.

[0117] In the reaction of the fiber raw material containing cellulose and the compound A, in addition to the compound B, an amide or an amine, for example, can be contained in the reaction system. As the amide, for example, formamide, dimethylformamide, acetamide, dimethylacetamide, and the like can be exemplified. As the amine, for example, methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, hexamethylenediamine, and the like can be exemplified. Among them, it is known that, in particular, triethylamine functions as a good reaction catalyst.

[0118] In the phosphorus-containing oxyacid group-introducing step, it is preferable to perform heat treatment on the fiber raw material after adding or mixing Compound A or the like to the fiber raw material. As the heat treatment temperature, a temperature at which the phosphorus-containing oxyacid group can be introduced while suppressing thermal decomposition and / or hydrolysis reaction of the fiber is preferably selected. The heat treatment temperature is, for example, preferably 50°C or higher and 300°C or lower, more preferably 100°C or higher and 250°C or lower, and further preferably 130°C or higher and 200°C or lower. In the heat treatment, various types of heating devices can be used, and, for example, stirring dryers, rotary dryers, disc dryers, roller-type heating devices, plate-type heating devices, fluidized bed dryers, belt-type dryers, filtration dryers, vibration fluidized bed dryers, air flow dryers, reduced pressure dryers, infrared heating devices, far infrared heating devices, microwave heating devices, and high frequency dryers can be used.

[0119] In the heat treatment of the present embodiment, for example, after adding Compound A to the fibrous raw material in the form of a sheet by impregnation or the like, a method of heating and / or a method of heating while kneading or stirring the fibrous raw material and Compound A using a kneader or the like can be employed. By this, the concentration unevenness of Compound A in the fibrous raw material can be suppressed, and the phosphoric acid group can be introduced more uniformly to the surface of the cellulose fiber contained in the fibrous raw material. It is considered that this is because the following phenomenon can be suppressed: when water molecules move to the surface of the fibrous raw material as drying proceeds, the dissolved Compound A is drawn by the water molecules due to surface tension, and similarly moves to the surface of the fibrous raw material (i.e., concentration unevenness of Compound A occurs).

[0120] In addition, the heating device used in the heat treatment is, for example, preferably a device that can discharge water held by the slurry and water produced by the dehydration condensation (phosphoric esterification) reaction of Compound A and the hydroxyl group or the like contained in the cellulose or the like in the fibrous raw material to the outside of the device system at all times. As such a heating device, for example, an oven of the air blowing type or the like can be exemplified. By discharging water in the device system at all times, the hydrolysis reaction of the phosphoric ester bond, which is the reverse reaction of the phosphoric esterification, can be suppressed, and the acid hydrolysis of the sugar chain in the fiber can also be suppressed. Thus, a fine fibrous cellulose having a high aspect ratio can be obtained.

[0121] The time of the heat treatment is, for example, preferably 1 second or more and 300 minutes or less after substantially removing water from the fibrous raw material, more preferably 1 second or more and 1,000 seconds or less, and further preferably 10 seconds or more and 800 seconds or less. In the present embodiment, by setting the heat temperature and the heat time to an appropriate range, the amount of the phosphorus-containing oxyacid group to be introduced can be set to a preferable range.

[0122] The phosphorus oxoacid group introduction step can be performed at least once, and can be repeated two or more times. By performing the phosphorus oxoacid group introduction step two or more times, a large amount of phosphorus oxoacid groups can be introduced into the fiber raw material. In the present embodiment, as an example of a preferred mode, the case where the phosphorus oxoacid group introduction step is performed twice can be cited.

[0123] In the introduction of the phosphorus oxoacid group into the fiber raw material, an alkali solution can be allowed to act on cellulose contained in the fiber raw material, and the cellulose can be subjected to alkali cellulose treatment. By this treatment, a part of the hydroxyl groups of the cellulose ionically dissociate, and the nucleophilicity (reactivity) can be increased. The alkali compound contained in the alkali solution is not particularly limited, and can be an inorganic alkali compound or an organic alkali compound. From the viewpoint of high versatility, it is preferable to use, for example, sodium hydroxide, potassium hydroxide, tetraethylammonium hydroxide, or tetrabutylammonium hydroxide. The alkali cellulose treatment can be performed simultaneously with the introduction of the phosphorus oxoacid group, can be performed as a preceding step thereof, or can be performed at both times.

[0124] The solution temperature at the start of the alkali cellulose treatment is preferably 0°C or higher and 50°C or lower, more preferably 5°C or higher and 40°C or lower, and further preferably 10°C or higher and 30°C or lower.

[0125] The concentration of the alkali solution is preferably 0.01 mol / L or higher and 4 mol / L or lower, more preferably 0.1 mol / L or higher and 3 mol / L or lower, and further preferably 1 mol / L or higher and 2.5 mol / L or lower, in terms of molarity. In particular, in the case where the treatment temperature is lower than 10°C, it is preferable to be 1 mol / L or higher and 2 mol / L or lower.

[0126] The treatment time of the alkali cellulose treatment is preferably 1 minute or longer and 6 hours or shorter, more preferably 10 minutes or longer, further preferably 30 minutes or longer, and more preferably 5 hours or shorter, and further preferably 4 hours or shorter.

[0127] By adjusting the type of the alkali solution, the treatment temperature, the concentration, and the immersion time as described above, the penetration of the alkali solution into the crystalline region of the cellulose can be suppressed, the crystal structure of cellulose I can be easily maintained, and the yield of the microfibrous cellulose can be increased.

[0128] In the case where the introduction of the phosphorus oxoacid group and the alkali cellulose treatment are not performed at the same time, the alkali cellulose obtained in the alkali treatment is preferably subjected to solid-liquid separation by a general liquid removal method such as centrifugation and / or filtration, and the moisture is removed in advance. Thereby, the reaction efficiency in the phosphorus oxoacid group introduction step performed next is increased. The concentration of the cellulose fiber after the solid-liquid separation is preferably 5% or higher and 50% or lower, more preferably 10% or higher and 40% or lower, and further preferably 15% or higher and 35% or lower.

[0129] (Washing step)

[0130] In the production method of the microfibrous cellulose of the present embodiment, a washing step can be performed on the phosphorus oxoacid group-introduced fiber as needed. The washing step is performed by washing the phosphorus oxoacid group-introduced fiber with, for example, water and / or an organic solvent. In addition, the washing step can be performed after each of the steps described later, and the number of times of washing performed in each washing step is not particularly limited.

[0131] (Alkali treatment step)

[0132] In the case of producing the microfibrous cellulose, an alkali treatment can be performed on the fiber raw material between the phosphorus oxoacid group-introduction step and the defibrillation treatment step described later. The method of the alkali treatment is not particularly limited, and, for example, a method in which the phosphorus oxoacid group-introduced fiber is immersed in an alkali solution can be exemplified.

[0133] The alkali compound contained in the alkali solution is not particularly limited, and can be an inorganic alkali compound or an organic alkali compound. In the present embodiment, from the aspect of high versatility, for example, sodium hydroxide or potassium hydroxide is preferably used as the alkali compound. In addition, the solvent contained in the alkali solution can be any of water or an organic solvent. Among them, the solvent contained in the alkali solution is preferably water or a polar solvent containing a polar organic solvent exemplified by an alcohol or the like, and more preferably an aqueous solvent containing at least water. As the alkali solution, from the aspect of high versatility, for example, an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution is preferable.

[0134] The temperature of the alkali solution in the alkali treatment step is not particularly limited, and, for example, 5°C or higher and 80°C or lower, and more preferably 10°C or higher and 60°C or lower are preferable. The immersion time of the phosphorus oxoacid group-introduced fiber in the alkali solution in the alkali treatment step is not particularly limited, and, for example, 5 minutes or longer and 30 minutes or shorter, and more preferably 10 minutes or longer and 20 minutes or shorter are preferable. The amount of use of the alkali solution in the alkali treatment is not particularly limited, and, for example, 100 parts by mass or more and 100,000 parts by mass or less, and more preferably 1,000 parts by mass or more and 10,000 parts by mass or less, relative to 100 parts by mass (absolute dry mass) of the phosphorus oxoacid group-introduced fiber are preferable.

[0135] The alkali treatment can be a neutralization treatment / ion exchange treatment of the phosphorus oxoacid group. The temperature of the alkali solution is preferably room temperature.

[0136] In order to reduce the amount of use of the alkali solution in the alkali treatment step, the phosphorus oxoacid group-introduced fiber can be washed with water and / or an organic solvent after the phosphorus oxoacid group-introduction step and before the alkali treatment step. After the alkali treatment step and before the defibrillation treatment step, from the viewpoint of improving the treatment properties, it is preferable to wash the phosphorus oxoacid group-introduced fiber subjected to the alkali treatment with water and / or an organic solvent.

[0137] (acid treatment step)

[0138] In the case of producing the microfibrous cellulose, the fiber raw material can be subjected to acid treatment between the step of introducing the phosphorus-containing oxyacid group and the defibrillation treatment step described later. For example, the phosphorus-containing oxyacid group introduction step, the acid treatment step, the alkali treatment step, and the defibrillation treatment step can be performed in this order.

[0139] As the method of acid treatment, there is no particular limitation, and for example, a method of immersing the fiber raw material in an acid liquid containing an acid can be exemplified. The concentration of the acid liquid used is not particularly limited, and for example, it is preferably 10% by mass or less, and more preferably 5% by mass or less. In addition, the pH of the acid liquid used is not particularly limited, and for example, it is preferably 0 or higher and 4 or lower, and more preferably 1 or higher and 3 or lower. As the acid contained in the acid liquid, for example, inorganic acids, sulfonic acids, carboxylic acids, and the like can be used. As the inorganic acid, for example, sulfuric acid, nitric acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, hypochlorous acid, chlorous acid, chloric acid, perchloric acid, phosphoric acid, boric acid, and the like can be exemplified. As the sulfonic acid, for example, methanesulfonic acid, ethanesulfonic acid, benzenesulfonic acid, p-toluenesulfonic acid, trifluoromethanesulfonic acid, and the like can be exemplified. As the carboxylic acid, for example, formic acid, acetic acid, citric acid, gluconic acid, lactic acid, oxalic acid, tartaric acid, and the like can be exemplified. Among them, the use of hydrochloric acid or sulfuric acid is particularly preferred.

[0140] The temperature of the acid solution in the acid treatment is not particularly limited, and for example, it is preferably 5°C or higher and 100°C or lower, and more preferably 20°C or higher and 90°C or lower. The immersion time in the acid solution in the acid treatment is not particularly limited, and for example, it is preferably 5 minutes or longer and 120 minutes or shorter, and more preferably 10 minutes or longer and 60 minutes or shorter. The amount of use of the acid solution in the acid treatment is not particularly limited, and for example, it is preferably 100 parts by mass or more and 100,000 parts by mass or less, and more preferably 1,000 parts by mass or more and 10,000 parts by mass or less, with respect to 100 parts by mass (absolute dry mass) of the fiber raw material.

[0141] (defibrillation treatment step)

[0142] By subjecting the phosphorus-containing oxyacid group-introduced fiber to defibrillation treatment in the defibrillation treatment step, the microfibrous cellulose can be obtained.

[0143] In the defibrillation treatment step, a defibrillation treatment device, for example, can be used. The defibrillation treatment device is not particularly limited, and a high-speed defibrillator, a grinder (stone mill type pulverizer), a high-pressure homogenizer and / or an ultrahigh-pressure homogenizer, a high-pressure impact type pulverizer, a ball mill, a bead mill, a disk refiner, a conical refiner, a twin-screw kneader, a vibration mill, a homogenizer mixer under high-speed rotation, an ultrasonic disperser, or a beater, for example, can be used. Among the above defibrillation treatment devices, a high-speed defibrillator, a high-pressure homogenizer, and an ultrahigh-pressure homogenizer, which have less influence of a pulverizing medium and are less likely to be contaminated, are preferably used.

[0144] In the defibrillation treatment step, the phosphorus-containing oxyacid group is preferably introduced into the fibers by diluting the fibers with a dispersion medium to make a slurry, for example. As the dispersion medium, one or two or more selected from water and polar organic solvents, and the like, can be used. As the polar organic solvent, there is no particular limitation, and alcohols, polyhydric alcohols, ketones, ethers, esters, aprotic polar solvents, and the like, are preferable, for example. As the alcohols, methanol, ethanol, isopropanol, n-butanol, isobutanol, and the like, can be exemplified. As the polyhydric alcohols, ethylene glycol, propylene glycol, glycerol, and the like, can be exemplified. As the ketones, acetone, methyl ethyl ketone (MEK), and the like, can be exemplified. As the ethers, diethyl ether, tetrahydrofuran, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether, and the like, can be exemplified. As the esters, ethyl acetate, butyl acetate, and the like, can be exemplified. As the aprotic polar solvents, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), and the like, can be exemplified.

[0145] The solid content concentration of the microfine fibrous cellulose at the time of defibrillation treatment can be appropriately set.

[0146] In addition, in the slurry obtained by dispersing the fibers into which the phosphorus-containing oxyacid group is introduced in a dispersion medium, a solid content other than the fibers to which the phosphorus-containing oxyacid group is introduced, such as urea having hydrogen bonding, can also be contained.

[0147] In one embodiment of the electrolyte membrane for a solid polymer fuel cell, from the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the content of the microfine fibrous cellulose in the solid content is preferably 80% by mass or more and 100% by mass or less, more preferably 85% by mass or more, further preferably 90% by mass or more, and can be 95% by mass or less.

[0148] In other embodiments of the electrolyte membrane for a solid polymer fuel cell, the content of the fine fibrous cellulose in the solid component is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more, further preferably 30% by mass or more, and more preferably 70% by mass or less, further preferably 60% by mass or less.

[0149] In the case where the electrolyte membrane for a solid polymer fuel cell of the present embodiment contains the cellulose nanocrystal covered with poly(vinyl phosphonic acid) described later, the content of the fine fibrous cellulose in the solid component of the electrolyte membrane for a solid polymer fuel cell is preferably within the aforementioned range.

[0150] In the aforementioned case, from the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell and the mechanical strength of the electrolyte membrane for a solid polymer fuel cell, the content of the fine fibrous cellulose in the solid component of the electrolyte membrane for a solid polymer fuel cell is preferably the above lower limit value or more, and from the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, preferably the above upper limit value or less.

[0151] <Polymers having sulfonic acid groups>

[0152] From the viewpoint of the mechanical strength and thinning of the electrolyte membrane for a solid polymer fuel cell, the electrolyte membrane for a solid polymer fuel cell of the present embodiment can contain a filler material. As the filler material, for example, polymers having sulfonic acid groups, sulfonated silica nanoparticles, carbon nanotubes, MOF (Metal Organic Frameworks), and preferably polymers having sulfonic acid groups can be exemplified. As the polymers having sulfonic acid groups, for example, perfluorosulfonic acid polymers, sulfonated polyether ether ketone, sulfonated polybenzimidazole, sulfonated polyether ether ketone, polystyrene sulfonic acid, and sulfonated polytrifluorostyrene can be exemplified.

[0153] The content of the filler material in the electrolyte membrane for a solid polymer fuel cell can be, for example, less than 10% by mass, can be 5% by mass or less, and is 0% by mass or more.

[0154] <Cellulose nanocrystal covered with poly(vinyl phosphonic acid)>

[0155] From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the electrolyte membrane for a solid polymer fuel cell of the present embodiment preferably contains a cellulose nanocrystal covered with poly(vinyl phosphonic acid). In the cellulose nanocrystal covered with poly(vinyl phosphonic acid), part or all of the surface of the cellulose nanocrystal is covered with poly(vinyl phosphonic acid).

[0156] The cellulose nanocrystal can be obtained by acid treatment of a cellulose fiber. The cellulose fiber is a fiber raw material containing cellulose, and there is no particular limitation on the cellulose fiber, and from the viewpoint of easy availability and low cost, pulp is preferably used. As the pulp, for example, wood pulp, non-wood pulp, and deinked pulp can be cited. As the wood pulp, there is no particular limitation, and for example, chemical pulp such as broadleaf kraft pulp (LBKP), coniferous kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen-bleached kraft pulp (OKP), semi-chemical pulp such as semi-chemical pulp (SCP) and chemical groundwood pulp (CGP), mechanical pulp such as groundwood pulp (GP) and thermomechanical pulp (TMP, BCTMP), and the like can be cited. As the non-wood pulp, there is no particular limitation, and for example, cotton-based pulp such as cotton linter and cotton lint, non-wood-based pulp such as hemp, straw, bamboo, and sugarcane bagasse can be cited. As the deinked pulp, there is no particular limitation, and for example, deinked pulp using old paper as a raw material can be cited. The pulp of the present embodiment can use one of the above alone, or two or more in combination.

[0157] From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the cellulose nanocrystal can have an ionic functional group, and as a specific example of the ionic functional group, a phosphorus oxoacid group can be cited.

[0158] The cellulose nanocrystal can be manufactured, for example, with reference to Japanese Patent Application Publication No. 2022-132151.

[0159] The mass ratio of poly (vinyl phosphonic acid) to cellulose nanocrystal (poly (vinyl phosphonic acid) / cellulose nanocrystal) in the poly (vinyl phosphonic acid)-coated cellulose nanocrystal is preferably 1 / 99 or more and 40 / 60 or less, more preferably 5 / 95 or more, further preferably 15 / 85 or more, and more preferably 30 / 70 or less, and further preferably 25 / 75 or less.

[0160] From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the above mass ratio (poly (vinyl phosphonic acid) / cellulose nanocrystal) is preferably the above lower limit value or more, and from the viewpoint of the mechanical strength of the electrolyte membrane for a solid polymer fuel cell, the above upper limit value or less is preferable.

[0161] In the case where the electrolyte membrane for a solid polymer fuel cell contains the poly(vinyl phosphonic acid)-coated cellulose nanocrystal, the content of the poly(vinyl phosphonic acid)-coated cellulose nanocrystal in the solid component of the electrolyte membrane for a solid polymer fuel cell is preferably 10% by mass or more and 80% by mass or less, more preferably 20% by mass or more, further preferably 30% by mass or more, and more preferably 75% by mass or less, further preferably 70% by mass or less.

[0162] From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the content of the poly(vinyl phosphonic acid)-coated cellulose nanocrystal is preferably the above lower limit value or more, and from the viewpoint of the mechanical strength of the electrolyte membrane for a solid polymer fuel cell, the content is preferably the above upper limit value or less.

[0163] In the case where the electrolyte membrane for a solid polymer fuel cell contains the poly(vinyl phosphonic acid)-coated cellulose nanocrystal, the mass ratio of the poly(vinyl phosphonic acid)-coated cellulose nanocrystal to the microfibrous cellulose (poly(vinyl phosphonic acid)-coated cellulose nanocrystal / microfibrous cellulose) in the electrolyte membrane for a solid polymer fuel cell is preferably 20 / 80 or more and 80 / 20 or less, more preferably 30 / 70 or more, further preferably 40 / 60 or more, and more preferably 70 / 30 or less, further preferably 60 / 40 or less.

[0164] From the viewpoint of improving the proton conductivity of the electrolyte membrane for a solid polymer fuel cell, the mass ratio (poly(vinyl phosphonic acid)-coated cellulose nanocrystal / microfibrous cellulose) is preferably the above lower limit value or more, and from the viewpoint of the mechanical strength of the electrolyte membrane for a solid polymer fuel cell, the mass ratio is preferably the above upper limit value or less.

[0165] The poly(vinyl phosphonic acid)-coated cellulose nanocrystal can be produced by a conventional method. Specifically, for example, reference can be made to ACS Applied Materials & Interfaces 2022, 14(6), 8353-8360, Nanoscale Advances 2022, 4(22), 4714-4723, Energy Fuels 2022, 36, 13924-13929.

[0166] <water>

[0167] In addition, from the viewpoint of improving proton conductivity, the electrolyte membrane for a solid polymer fuel cell of the present embodiment preferably contains water in an amount of 3% by mass or more and 20% by mass or less, more preferably 5% by mass or more, further preferably 7% by mass or more, and more preferably 16% by mass or less, further preferably 12% by mass or less.

[0168] <Other components>

[0169] The electrolyte membrane for a solid polymer fuel cell of the present embodiment can contain components other than the fine fibrous cellulose, the polymer having a sulfonic acid group, the poly(vinyl phosphonic acid)-coated cellulose nanocrystal, and water, within a range that does not impair the effects of the present application. As such components, for example, hydrophilic polymers, hydrophilic low-molecular-weight substances, paper strength enhancers, thermoplastic resins, surfactants, organic ions, coupling agents, inorganic layered compounds, inorganic compounds, leveling agents, preservatives, antifoaming agents, organic particles, lubricants, antistatic agents, ultraviolet ray protection agents, dyes, pigments, stabilizers, magnetic powders, orientation promoters, plasticizers, dispersants, anti-coloring agents, polymerization inhibitors, pH adjustors, and crosslinking agents, etc. can be exemplified. The content of the above components in the electrolyte can be, for example, 5% by mass or less, 3% by mass or less, and 0% by mass or more.

[0170] <Membrane thickness>

[0171] The membrane thickness of the electrolyte membrane for a solid polymer fuel cell of the present embodiment can be appropriately determined depending on the size of the membrane electrode assembly incorporated in the electrolyte membrane for a solid polymer fuel cell and / or the solid polymer fuel cell. The membrane thickness of the electrolyte membrane for a solid polymer fuel cell of the present embodiment can be, for example, 10 μm or more and 100 μm or less, or 25 μm or more and 75 μm or less.

[0172] <Water contact angle>

[0173] The water contact angle of the electrolyte membrane for a solid polymer fuel cell of the present embodiment is preferably low, and specifically, 80° or less, more preferably 60° or less, and further preferably 40° or less, and the lower limit value thereof is not particularly limited, and is preferably 30° or more.

[0174] The water contact angle is a value obtained by dropping 4 μL of distilled water on the surface of the electrolyte membrane for a solid polymer fuel cell using a dynamic water contact angle tester (Fibro Co., Ltd., 1100 DAT) in accordance with JIS R 3257: 1999, and measuring the value after 30 seconds from the dropping. The water contact angle is measured on both surfaces of the electrolyte membrane for a solid polymer fuel cell, and in the case where the water contact angles on the both surfaces are different from each other, the average value is used as the water contact angle.

[0175] [Solid polymer type water electrolysis electrolyte membrane]

[0176] As described above, the solid polymer type fuel cell electrolyte membrane of the present embodiment is the same in constitution except for the use as the solid polymer type water electrolysis electrolyte membrane of the present embodiment.

[0177] [Membrane electrode assembly (provided with solid polymer type fuel cell electrolyte membrane)]

[0178] The membrane electrode assembly (MEA) of the present embodiment is made by sequentially joining a positive electrode catalyst layer, the solid polymer type fuel cell electrolyte membrane of the present embodiment, and a negative electrode catalyst layer. The membrane electrode assembly of the present embodiment can have the same constitution as the publicly known membrane electrode assembly that can be used for a solid polymer type fuel cell, except for having the solid polymer type fuel cell electrolyte membrane of the present embodiment. As for the membrane electrode assembly, for example, refer to Japanese Patent Application Publication No. 2022-190524.

[0179] The membrane electrode assembly of the present embodiment can have a gas diffusion layer on the outer side of the positive electrode catalyst layer (the side of the positive electrode catalyst layer opposite to the side having the solid polymer type fuel cell electrolyte membrane). In addition, the membrane electrode assembly of the present embodiment can have a gas diffusion layer on the outer side of the negative electrode catalyst layer (the side of the negative electrode catalyst layer opposite to the side having the solid polymer type fuel cell electrolyte membrane). The thickness of the positive electrode catalyst layer, the negative electrode catalyst layer, and the gas diffusion layer can be appropriately determined depending on the size of the solid polymer type fuel cell.

[0180] [Membrane electrode assembly (provided with solid polymer type water electrolysis electrolyte membrane)]

[0181] The other membrane electrode assembly (MEA) of the present embodiment is made by sequentially joining a positive electrode catalyst layer, the solid polymer type water electrolysis electrolyte membrane of the present embodiment, and a negative electrode catalyst layer. The membrane electrode assembly of the present embodiment can have the same constitution as the publicly known membrane electrode assembly that can be used for a solid polymer type water electrolysis device, except for having the solid polymer type water electrolysis electrolyte membrane of the present embodiment. As for the membrane electrode assembly, for example, refer to Electrochemistry, 85(1), 28-33 (2017), Japanese Patent Application Publication No. 2023-41182, International Publication No. 2014 / 157389.

[0182] The membrane-electrode assembly of the present embodiment can have a gas diffusion layer on the outer side of the positive electrode catalyst layer (the side of the positive electrode catalyst layer opposite the side having the solid polymer-type water electrolysis electrolyte membrane). In addition, the membrane-electrode assembly of the present embodiment can have a gas diffusion layer on the outer side of the negative electrode catalyst layer (the side of the negative electrode catalyst layer opposite the side having the solid polymer-type water electrolysis electrolyte membrane). The thickness of the positive electrode catalyst layer, the negative electrode catalyst layer, and the gas diffusion layer can be appropriately determined in accordance with the size of the solid polymer-type water electrolysis device.

[0183] [Solid polymer-type fuel cell]

[0184] The solid polymer-type fuel cell of the present embodiment can have the same configuration as a publicly known solid polymer-type fuel cell, in addition to the membrane-electrode assembly of the present embodiment.

[0185] [Solid polymer-type water electrolysis device]

[0186] The solid polymer-type water electrolysis device (solid polymer electrolyte water electrolysis device, hydrogen manufacturing device) of the present embodiment can have the same configuration as a publicly known solid polymer-type water electrolysis device, in addition to the other membrane-electrode assembly of the present embodiment. With respect to the publicly known solid polymer-type water electrolysis device, for example, reference can be made to Electrochemistry, 85 (1), 28-33 (2017), Japanese Patent Application Publication No. 2023-41182, International Publication No. 2014 / 157389.

[0187] [Method for manufacturing electrolyte membrane]

[0188] The method for manufacturing an electrolyte membrane of the present embodiment successively has a preparation step in which the average fiber width of the fine fiber-shaped cellulose derived from a woody or herbaceous plant is 50 nm or less, and a film formation step, and the fine fiber-shaped cellulose derived from a woody or herbaceous plant has a phosphorus-containing oxyacid group.

[0189] Preparation step: Step of preparing fine fiber-shaped cellulose derived from a woody or herbaceous plant dispersion liquid

[0190] Film formation step: Step of forming fine fiber-shaped cellulose derived from a woody or herbaceous plant dispersion liquid into a film

[0191] The preparation step can be performed, for example, in the same manner as the above-described “method for manufacturing fine fiber-shaped cellulose”. In the case where the electrolyte membrane of the present embodiment contains a component other than the fine fiber-shaped cellulose, for example, a fine fiber-shaped cellulose dispersion liquid containing the desired component can be obtained by mixing the component other than the fine fiber-shaped cellulose in the dispersion liquid containing the fine fiber-shaped cellulose obtained by going through the method for manufacturing fine fiber-shaped cellulose.

[0192] The film forming step is a method known in the art.

[0193] From the viewpoint of improving the proton conductivity of the obtained electrolyte membrane, reducing the temperature dependence of the proton conductivity, and reducing the activation energy of the proton conduction, the electrolyte membrane production method of the present embodiment preferably has the following ion exchange step between the preparation step and the film forming step.

[0194] Ion exchange step: a step of treating the fine fibrillar cellulose dispersion derived from a woody or herbaceous plant with a cation exchange resin

[0195] The cation exchange resin can be any of a strong acid cation exchange resin and a weak acid cation exchange resin, and is preferably a strong acid cation exchange resin.

[0196] [Method for producing membrane-electrode assembly (provided with electrolyte membrane for solid polymer fuel cell)]

[0197] The method for producing a membrane-electrode assembly of the present embodiment has a step of joining a positive electrode catalyst layer to one face of the electrolyte membrane for a solid polymer fuel cell obtained by the method for producing an electrolyte membrane of the present embodiment, and a negative electrode catalyst layer to the other face of the electrolyte membrane for a solid polymer fuel cell. The "joining" can be performed by a conventional method. For the method for producing a membrane-electrode assembly, for example, refer to Japanese Patent Application Publication No. 2022-190524.

[0198] [Method for producing membrane-electrode assembly (provided with electrolyte membrane for solid polymer water electrolysis)]

[0199] The other method for producing a membrane-electrode assembly of the present embodiment has a step of joining a positive electrode catalyst layer to one face of the electrolyte membrane for solid polymer water electrolysis obtained by the method for producing an electrolyte membrane of the present embodiment, and a negative electrode catalyst layer to the other face of the electrolyte membrane for solid polymer water electrolysis. The "joining" can be performed by a conventional method. For the method for producing a membrane-electrode assembly, for example, refer to Electrochemistry, 85(1), 28-33 (2017), Japanese Patent Application Publication No. 2023-41182, and International Publication No. 2014 / 157389.

[0200] [Method for producing solid polymer fuel cell]

[0201] The manufacturing method of the solid polymer fuel cell of the present embodiment has a step of incorporating the membrane-electrode assembly obtained by the manufacturing method of the membrane-electrode assembly of the present embodiment into the solid polymer fuel cell. For the manufacturing method of the solid polymer fuel cell, for example, refer to Japanese Patent Application Publication No. 2022-190524.

[0202] [Manufacturing method of solid polymer water electrolysis device]

[0203] The manufacturing method of the solid polymer water electrolysis device of the present embodiment has a step of incorporating the membrane-electrode assembly obtained by the manufacturing method of the membrane-electrode assembly of the present embodiment into the solid polymer water electrolysis device. The solid polymer water electrolysis device of the present embodiment can be manufactured, for example, with reference to Electrochemistry, 85(1), 28-33 (2017), Japanese Patent Application Publication No. 2023-41182.

[0204] Example

[0205] The features of the present application are more specifically described below with reference to examples and comparative examples. The materials, amounts, ratios, contents of treatment, steps of treatment, and the like shown in the following examples can be appropriately changed as long as the gist of the present application is not deviated. Therefore, the scope of the present application should not be interpreted limitatively by the specific examples shown below. In addition, unless otherwise specified, the operations of the examples and comparative examples are performed under the conditions of room temperature (20 to 25°C) and normal humidity (40 to 50% RH (relative humidity)).

[0206] Example 1

[0207] [Preparation step]

[0208] (Phosphorus oxoacid group introduction step)

[0209] As the raw pulp, a coniferous bleached kraft pulp (solid content: 93 mass%, basis weight: 245 g / m 2 of sheet shape, having a Canadian Standard Freeness (CSF) of 700 ml measured according to JIS P 8121-2:2012) manufactured by Prince Paper Co., Ltd. was used. The raw pulp was subjected to phosphorus oxoacid treatment as follows. First, to 100 parts by mass (absolute dry mass) of the above raw pulp, a mixed aqueous solution of 45 parts by mass of ammonium dihydrogen phosphate, 120 parts by mass of urea, and 150 parts by mass of water was added, to obtain a chemical solution-impregnated pulp. Next, the obtained chemical solution-impregnated pulp was heated with a hot air drier at 165°C for 250 seconds, to obtain a pulp in which phosphorus oxoacid groups were introduced into cellulose in the pulp (phosphorized pulp).

[0210] (Washing step)

[0211] Next, the obtained phosphonated pulp was subjected to a washing treatment. The washing treatment was performed by repeatedly performing stirring of a pulp dispersion liquid obtained by injecting 100 g (absolute dry mass) of the phosphonated pulp into 10 L of ion exchange water, in a manner that the pulp was uniformly dispersed, and then performing filtration and dewatering. The time at which the conductivity of the filtrate reached 100 μS / cm or less was taken as the end point of the washing.

[0212] (neutralization treatment)

[0213] Next, the washed phosphonated pulp was subjected to a neutralization treatment as follows. First, the washed phosphonated pulp was diluted with 10 L of ion exchange water, and then 1 N sodium hydroxide aqueous solution was added little by little while stirring, thereby obtaining a phosphonated pulp dispersion liquid having a pH of 12 or more and 13 or less. Next, the phosphonated pulp dispersion liquid was subjected to dewatering and washing, thereby obtaining a phosphonated pulp subjected to a neutralization treatment.

[0214] The obtained phosphonated pulp was subjected to measurement of infrared absorption spectrum using FT-IR. As a result, absorption based on P=O of phosphorus oxoacid group was observed at 1230 cm -1 nearby, and it was confirmed that the pulp was provided with a phosphorus oxoacid group. In addition, the obtained phosphonated pulp was subjected to analysis using an X-ray diffractometer, and as a result, typical peaks were confirmed at both positions of 2θ = 14° or more and 17° or less, and 2θ = 22° or more and 23° or less, and it was confirmed that it had a cellulose I crystal.

[0215] (fibrillation treatment step)

[0216] Ion exchange water was added to the obtained phosphonated pulp, and a dispersion liquid having a solid content concentration of 2 mass% was prepared. This dispersion liquid was treated 5 times at a pressure of 200 MPa using a wet-type microparticulation device (Star Burst manufactured by SUGINO MACHINE LIMITED), and a microfine fibrous cellulose dispersion liquid (1) containing microfine fibrous cellulose was obtained. The counter ions of the phosphorus oxoacid group contained in the microfine fibrous cellulose dispersion liquid (1) were Na + .

[0217] It was confirmed by X-ray diffraction that the obtained microfine fibrous cellulose maintained a cellulose I crystal structure. In addition, the fiber width of the microfine fibrous cellulose was measured using a transmission electron microscope, and the result was 3 to 5 nm. In addition, the average fiber width of the microfine fibrous cellulose was 3.5 nm. Note that the amount of phosphoric acid group (1st dissociation acid amount) measured using the measurement method described in [Measurement of amount of phosphorus oxoacid group] described later was 1.45 mmol / g. Note that the total dissociation acid amount was 2.45 mmol / g.

[0218] [ion exchange step]

[0219] To 25 g of the microfibrous cellulose dispersion (1) having a solid content of 2 mass%, ion exchange water was added to prepare 100 g of a microfibrous cellulose dispersion (A) having a solid content of 0.5 mass%. To the obtained microfibrous cellulose dispersion (A), 5 g of a strongly acidic ion exchange resin (manufactured by Mitsubishi Chemical Corporation, adjusted) was added, and stirring treatment was performed for 2 hours. Then, the ion exchange resin and the dispersion were separated by centrifugal separation to obtain a microfibrous cellulose dispersion (B).

[0220] [membrane formation step]

[0221] The obtained microfibrous cellulose dispersion (B) was poured into a petri dish, and cast drying was performed at 20°C and 30% RH to obtain an electrolyte membrane. The thickness of the obtained electrolyte membrane was 48 μm. In addition, the counter ion of the phosphorus oxyacid group possessed by the microfibrous cellulose contained in the electrolyte membrane was H + .

[0222] Example 2

[0223] A microfibrous cellulose dispersion (A) 100 g was prepared in the same manner as in Example 1, without performing the treatment using a strongly acidic ion exchange resin. The microfibrous cellulose dispersion (A) 100 g was poured into a petri dish, and cast drying was performed at 20°C and 30% RH to obtain an electrolyte membrane. The thickness of the obtained electrolyte membrane was 50 μm. In addition, the counter ion of the phosphorus oxyacid group possessed by the microfibrous cellulose contained in the electrolyte membrane was Na + .

[0224] Example 3

[0225] In the [preparation step] of Example 1, the (phosphorus oxyacid group introduction step) and the (washing step) were further added one time each, in this order, to the (washed) phosphorized pulp. The other steps were the same as in Example 1, and an electrolyte membrane having a thickness of 50 μm was obtained. The counter ion of the phosphorus oxyacid group possessed by the microfibrous cellulose contained in the electrolyte membrane was H + .

[0226] Note that infrared absorption spectrum measurement was performed on the neutralized phosphorized pulp using FT-IR. As a result, a peak at 1230 cm -1The absorption based on P=O of the phosphorus-containing oxyacid group was observed near 1230 cm"1, confirming that the phosphorus-containing oxyacid group was added to the pulp. In addition, the obtained phosphated pulp was subjected to analysis using an X-ray diffractometer, and as a result, typical peaks were confirmed at both positions near 2Θ = 14° or more and 17° or less and 2Θ = 22° or more and 23° or less, confirming that it had a cellulose I crystal.

[0227] In addition, the fiber width of the microfibrous cellulose obtained in the defibrillation treatment step was measured using a transmission electron microscope, and the result was 3 to 5 nm. In addition, the average fiber width of the microfibrous cellulose was 3.5 nm. Note that the amount of phosphoric acid group (1st dissociation acid amount) measured using the measurement method described in the aforementioned [Measurement of the amount of phosphorus-containing oxyacid group] was 1.99 mmol / g. Note that the total dissociation acid amount was 3.29 mmol / g.

[0228] Example 4

[0229] In the [preparation step] (phosphorus-containing oxyacid group introduction step) of Example 1, the conditions when the pulp was heated to impregnate with the chemical solution were changed to heating for 200 seconds with a hot air dryer at 145°C. The other steps were the same as in Example 1, and an electrolyte membrane having a thickness of 50 μm was obtained. The counter ion of the phosphorus-containing oxyacid group possessed by the microfibrous cellulose contained in the electrolyte membrane was H + .

[0230] Note that the infrared absorption spectrum of the phosphated pulp after the neutralization treatment was measured using FT-IR. As a result, the absorption based on P=O of the phosphorus-containing oxyacid group was observed near 1230 cm"1, confirming that the phosphorus-containing oxyacid group was added to the pulp. In addition, the obtained phosphated pulp was subjected to analysis using an X-ray diffractometer, and as a result, typical peaks were confirmed at both positions near 2Θ = 14° or more and 17° or less and 2Θ = 22° or more and 23° or less, confirming that it had a cellulose I crystal. -1

[0231] In addition, the fiber width of the microfibrous cellulose obtained in the defibrillation treatment step was measured using a transmission electron microscope, and the result was 3 to 5 nm. In addition, the average fiber width of the microfibrous cellulose was 3.9 nm. Note that the amount of phosphoric acid group (1st dissociation acid amount) measured using the measurement method described in the aforementioned [Measurement of the amount of phosphorus-containing oxyacid group] was 0.87 mmol / g. Note that the total dissociation acid amount was 1.51 mmol / g.

[0232] Example 5

[0233] [Preparation of a poly(vinylphosphonic acid)-coated cellulose nanocrystal aqueous dispersion] ​

[0234] O-ethyl-S-(1-ethoxycarbonyl)ethyl dithiocarbonate was synthesized as a RAFT agent (chain transfer agent) according to ACS Applied Materials & Interfaces 2022, 14(6), 8353-8360. Next, cellulose nanocrystals covered with poly(vinyl phosphonic acid) were produced according to Nanoscale Advances 2022, 4(22), 4714-4723 and Energy Fuels 2022, 36, 13924-13929. In the cellulose nanocrystals covered with poly(vinyl phosphonic acid), the mass ratio of cellulose nanocrystals to poly(vinyl phosphonic acid) (cellulose nanocrystals: poly(vinyl phosphonic acid)) was 4:1. The resulting cellulose nanocrystals covered with poly(vinyl phosphonic acid) were dispersed in ion exchange water to obtain a cellulose nanocrystals covered with poly(vinyl phosphonic acid) aqueous dispersion.

[0235] [Preparation of cellulose aqueous dispersion]

[0236] The cellulose nanocrystals covered with poly(vinyl phosphonic acid) aqueous dispersion and the fine fibrous cellulose dispersion (B) obtained in Example 1 were mixed in such a manner that the solid content amount of each was 1:1 by mass, to obtain a mixed solution.

[0237] [Membrane formation step]

[0238] The above mixed solution was injected into a petri dish, and after that, cast drying was performed at 20°C, 30% RH to produce a sheet. The resulting sheet was subjected to press treatment at 10 MPa for 5 minutes to obtain an electrolyte membrane. The thickness of the resulting electrolyte membrane was 50 μm. In addition, the counter ion of the phosphorus-containing oxyacid group possessed by the fine fibrous cellulose contained in the electrolyte membrane was H + .

[0239] Example 6

[0240] In the [preparation step] (phosphorus-containing oxyacid group introduction step) of Example 1, phosphorous acid (phosphonic acid) 33 parts by mass was used instead of ammonium dihydrogen phosphate. The other steps were the same as in Example 1, and an electrolyte membrane having a thickness of 50 μm was obtained. The counter ion of the phosphorus-containing oxyacid group possessed by the fine fibrous cellulose contained in the electrolyte membrane was H + .

[0241] Note that infrared absorption spectroscopy was measured for the phosphorous acid-converted pulp after neutralization treatment using FT-IR. As a result, a peak at 1210 cm -1The absorption of P=O based on the tautomer of phosphite group as phosphonic group was observed in the vicinity, confirming that the pulp was additionally provided with phosphite group (phosphonic group). In addition, it was confirmed by X-ray diffraction that the obtained microfine fibrous cellulose maintained the cellulose I crystal. In addition, the fiber width of the microfine fibrous cellulose obtained in the defibrillation treatment step was measured using a transmission electron microscope, and the result was 3 to 5 nm. In addition, the average fiber width of the microfine fibrous cellulose was 3.8 nm. Note that the amount of phosphite group (1st dissociation acid amount) measured by the measurement method described in [Measurement of the amount of phosphorus oxoacid group] described later was 1.51 mmol / g, and the total dissociation acid amount was 1.54 mmol / g.

[0242] Comparative Example 1

[0243] To 10 g of a microfine fibrous cellulose dispersion liquid (SUGINO MACHINE LIMITED, WFo-10005, average fiber width: 10 to 50 nm) having a solid content concentration of 5.0% by mass, ion exchange water was added to prepare 100 g of a fibrous cellulose dispersion liquid (C) having a solid content concentration of 0.5% by mass. The obtained microfine fibrous cellulose dispersion liquid (C) was poured into a petri dish, and cast drying was performed at 20°C and 30% RH to obtain an electrolyte membrane. The thickness of the obtained electrolyte membrane was 60 μm. In addition, the microfine fibrous cellulose contained in the electrolyte membrane did not have any ionic functional groups.

[0244] [Measurement method]

[0245] [Measurement of the amount of phosphorus oxoacid group of microfine fibrous cellulose]

[0246] The amount of phosphorus oxoacid group of microfine fibrous cellulose was measured as follows: a dispersion liquid containing microfine fibrous cellulose prepared by diluting a microfine fibrous cellulose dispersion liquid serving as the object with ion exchange water in such a manner that the content of microfine fibrous cellulose became 0.2% by mass was treated using an ion exchange resin, and titration was performed using a base, whereby the measurement was performed.

[0247] The treatment using the aforementioned ion exchange resin was performed as follows: 1 / 10 by volume of a strongly acidic ion exchange resin (ORGANO CORPORATION, Amberjet 1024, adjusted) was added to the aforementioned microfine fibrous cellulose dispersion liquid, and after 1 hour of shaking treatment, the ion exchange resin was separated from the dispersion liquid by pouring onto a mesh having a pore size of 90 μm.

[0248] In addition, the titration using the aforementioned base was performed as follows: while 10 μL of a 0.1 N aqueous sodium hydroxide solution was added every 5 seconds to the microfine fibrous cellulose dispersion liquid after the treatment with the ion exchange resin, the change in the pH exhibited by the dispersion liquid was measured.

[0249] In the neutralization titration, in a plot of the measured pH against the amount of added sodium hydroxide, two points at which the increment (the differential value of the pH with respect to the amount of added sodium hydroxide) reached a maximum were observed. The point at which the increment first reached a maximum upon the addition of sodium hydroxide was referred to as the 1st end point, and the point at which the increment next reached a maximum was referred to as the 2nd end point. Figure 1 The amount of sodium hydroxide required from the start of the titration to the 1st end point was equal to the amount of the 1st dissociation acid in the dispersion liquid used in the titration. In addition, the amount of sodium hydroxide required from the start of the titration to the 2nd end point was equal to the amount of the total dissociation acid in the dispersion liquid used in the titration. Note that the value obtained by dividing the amount of sodium hydroxide (mmol) required from the start of the titration to the 1st end point by the solid content (g) in the dispersion liquid subjected to the titration was used as the phosphorus-containing oxyacid group amount (mmol / g).

[0250] [Method of Evaluation]

[0251] [Proton conductivity of electrolyte membrane]

[0252] The proton conductivity of the obtained electrolyte membrane at a relative humidity of 95% and a temperature of 20 to 80°C was measured by an alternating current (AC) impedance method using an impedance analyzer (IM3570, SOKUHAN Co., Ltd., frequency: 4.6 to 4.6 x 10 6 Hz, 4-terminal method) in an environmental control device (a table-top constant temperature and humidity device, SH-222, ESPEC CORPORATION). The measurement of the proton conductivity was performed after the electrolyte membrane was sufficiently left to be saturated with moisture absorption and the proton conductivity was stabilized. According to a Cole-Cole plot, the inflection point was regarded as the bulk resistance of each electrolyte membrane. The proton conductivity σ (S / cm) was calculated using the following formula. The greater the proton conductivity, the more the battery performance such as power generation efficiency can be improved.

[0253] σ = d / (Rs x S)

[0254] d (cm): distance between metal wires

[0255] Rs (Ω): bulk impedance = Rs (Ω)

[0256] S (cm 2 ): proton conduction area calculated from the film thickness x width

[0257] [Temperature dependence of proton conductivity of electrolyte membrane]

[0258] The activation energy of proton conduction (Ea) was calculated by dividing the proton conductivity at 80°C by the proton conductivity at 20°C.

[0259] [Activation energy (Ea) of proton conduction of electrolyte membrane]

[0260] The activation energy of proton conduction Ea (eV) was calculated using the Arrhenius equation represented below. The smaller the value of the activation energy of proton conduction, the smaller the change in the proton conductivity in the temperature change, and the battery performance such as the electric energy conversion efficiency and / or the energy density can be improved by the reduction in the change rate of the proton conductivity and / or the voltage / current density.

[0261] σ x T = σ0exp(-Ea / kT)

[0262] σ (S / cm): Proton conductivity

[0263] T (K): Absolute temperature

[0264] σ0: Pre-exponential factor

[0265] k: Boltzmann constant (8.6171 x 10 -5 eV / K)

[0266] [Table 1]

[0267]

[0268] The electrolyte membrane containing the fine fibrous cellulose derived from the ligneous or herbaceous system (average fiber width of 50 nm or less, the fine fibrous cellulose having a phosphorus-containing oxyacid group) defined in the present application is excellent in proton conductivity (Examples 1 to 5). Furthermore, the electrolyte membrane containing the fine fibrous cellulose having a phosphoric acid group of H + The temperature dependence of the proton conductivity and the activation energy of proton conduction of the electrolyte membrane are also excellent (Examples 1, 3 to 5).

[0269] Note that, although not described in Table 1, the proton conductivity, the temperature dependence of the proton conductivity, and the activation energy of proton conduction of Example 6 are equal to those of Example 4.

[0270] On the other hand, the electrolyte membrane containing the fine fibrous cellulose derived from the ligneous or herbaceous system having an average fiber width of 50 nm or less but not having a phosphorus-containing oxyacid group is low in proton conductivity, and the temperature dependence of the proton conductivity is also large (Comparative Example 1).

Claims

1. A solid polymer electrolyte membrane for fuel cells, comprising finely fibrous cellulose derived from woody or herbaceous plants. The average fiber width of microfibrillary cellulose derived from woody or herbaceous plants is less than 50 nm, and microfibrillary cellulose derived from woody or herbaceous plants has phosphorus oxyacid groups.

2. The electrolyte membrane for a solid polymer fuel cell according to claim 1, wherein, The counterions of phosphorus oxyacid groups include H + and / or Na + .

3. The electrolyte membrane for a solid polymer fuel cell according to claim 1 or 2, wherein, The counterions of phosphorus oxyacid groups include H + .

4. The electrolyte membrane for a solid polymer fuel cell according to any one of claims 1 to 3, wherein, The amount of phosphorus oxyacid groups introduced from the fine fibrous cellulose derived from woody or herbaceous plants is above 0.50 mmol / g.

5. The electrolyte membrane for a solid polymer fuel cell according to any one of claims 1 to 4, wherein, In the solid composition of electrolyte membranes for solid polymer fuel cells, the content of fine fibrous cellulose derived from woody or herbaceous plants is above 80% by mass.

6. The electrolyte membrane for a solid polymer fuel cell according to any one of claims 1 to 4, wherein, In the solid composition of electrolyte membranes for solid polymer fuel cells, the content of fine fibrous cellulose derived from woody or herbaceous plants is more than 30% by mass.

7. A membrane electrode assembly, which is formed by sequentially joining a positive electrode catalyst layer, a solid polymer electrolyte membrane for fuel cells according to any one of claims 1 to 6, and a negative electrode catalyst layer.

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

9. A method for manufacturing an electrolyte membrane for a solid polymer fuel cell, comprising the following preparation steps and film formation steps in sequence: The microfiber cellulose derived from woody or herbaceous plants used in the following preparation processes has an average fiber width of less than 50 nm and contains phosphorus oxyacid groups. Preparation process: The process of preparing a fine fibrous cellulose dispersion derived from woody or herbaceous plants; Film-forming process: The process of forming a film from a fine fibrous cellulose dispersion derived from woody or herbaceous plants.

10. The method for manufacturing an electrolyte membrane for a solid polymer fuel cell according to claim 9, wherein, The following ion exchange process occurs between the preparation process and the film formation process. Ion exchange process: The process of treating a fine fibrous cellulose dispersion derived from woody or herbaceous plants with a cation exchange resin.

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

12. A method for manufacturing a solid polymer fuel cell, comprising: a step of assembling a membrane electrode assembly obtained by the method for manufacturing a membrane electrode assembly according to claim 11.

13. A solid polymeric electrolyte membrane for water electrolysis, comprising finely fibrous cellulose derived from woody or herbaceous plants. The average fiber width of microfibrillary cellulose derived from woody or herbaceous plants is less than 50 nm, and microfibrillary cellulose derived from woody or herbaceous plants has phosphorus oxyacid groups.

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