Electrolyte membrane containing fine cellulose fibers

Abstract

An electrolyte membrane containing fine cellulose fibers, polyvinyl alcohol, and a crosslinking agent.

Description

Electrolyte membrane containing fine cellulose fibers

[0001] This invention relates to an electrolyte membrane containing fine cellulose fibers.

[0002] Well-known electrochemical devices include secondary batteries such as lithium-ion batteries (LiB), electric double-layer capacitors (EDLCs), fuel cells, and water electrolyzers. Of these, lithium-ion batteries (LiB) and electric double-layer capacitors utilize organic solvents as electrolytes due to their voltage resistance. However, organic solvents are flammable and volatile, raising concerns about safety and environmental impact. To reduce environmental impact, development of electrochemical devices using aqueous electrolytes is also progressing.

[0003] Patent Document 1 discloses an electric double-layer capacitor that uses a gel-like solid electrolyte consisting of a polymer compound that is soluble in water and exhibits a gel-like state in response to acidic or basic substances, and an aqueous electrolyte, and states that it has the advantage of not being susceptible to degradation failure due to electrolyte leakage.

[0004] Japanese Patent Publication No. 2003-257794

[0005] The gel-like solid electrolyte described in Patent Document 1 was not a self-supporting membrane, making it difficult to handle. Furthermore, it required acidic substances such as sulfuric acid or basic substances such as potassium hydroxide, resulting in a high environmental impact.

[0006] Furthermore, in electrochemical devices such as water electrolyzers, ions move between the anode chamber and the cathode chamber during the electrolysis of water. Therefore, in order to perform electrolysis efficiently, it is desirable that the membrane separating the anode chamber and the cathode chamber not only be water-resistant but also have high ionic conductivity.

[0007] Therefore, the present invention aims to provide an electrolyte membrane that has a low environmental impact, excellent handling, water resistance and ionic conductivity, and, when used in electrochemical devices, possesses durability that can withstand continuous use in practical applications, while also enabling efficient ion conduction.

[0008] As a result of diligent research to achieve this objective, the inventors of the present invention found that using fine cellulose fibers, which are biomass-derived materials, is extremely effective, and thus completed the present invention.

[0009] The present invention provides the following: (1) An electrolyte membrane comprising fine cellulose fibers, polyvinyl alcohol, and a crosslinking agent. (2) The electrolyte membrane according to (1), wherein the crosslinking agent is an organotitanium compound or an organozirconium compound. (3) The electrolyte membrane according to (1) or (2), wherein the amount of fine cellulose fibers is 50 to 98 parts by mass per 100 parts by mass of the electrolyte membrane. (4) The electrolyte membrane according to (1) or (2), wherein the amount of polyvinyl alcohol is 2 to 49 parts by mass per 100 parts by mass of the electrolyte membrane. (5) The electrolyte membrane according to (1) or (2), wherein the amount of the crosslinking agent is 0.1 to 100 parts by mass per 100 parts by mass of the polyvinyl alcohol. (6) The electrolyte membrane according to (1) or (2), wherein the ionic conductivity is 0.01 to 100 mS / cm. (7) The electrolyte membrane according to (1) or (2), wherein the metal cation content is 0.1 to 10.0 mmol / g.

[0010] According to the present invention, it is possible to provide an electrolyte membrane that has a low environmental impact, excellent handling properties, water resistance and ionic conductivity, and, when used in electrochemical devices, possesses durability that can withstand continuous use in practical applications, while also enabling efficient ion conduction.

[0011] The present invention will now be described in detail. In this invention, "~" includes the endpoints. That is, "X~Y" includes the values ​​X and Y at both ends.

[0012] (Electrolyte membrane) The electrolyte membrane of the present invention comprises fine cellulose fibers, polyvinyl alcohol, and a crosslinking agent.

[0013] (Fine Cellulose Fibers) Examples of fine cellulose fibers used in the electrolyte membrane of the present invention include cellulose nanofibers and microfibrillated cellulose obtained by micronizing cellulose raw materials. Methods for producing fine cellulose fibers include micronizing cellulose raw materials with mechanical shear force, performing enzymatic or chemical treatment on cellulose raw materials followed by mechanical micronization, and using microorganisms to produce fine cellulose fibers, such as bacterial cellulose. In the present invention, from the viewpoint of the resulting electrolyte membrane having excellent ionic conductivity, it is preferable to use ion-modified fine cellulose fibers obtained by chemically treating cellulose raw materials to introduce ionic functional groups into the molecular chains of cellulose, and then performing defibrillation on ion-modified cellulose.

[0014] (Cellulose Raw Materials) The type of cellulose raw material used to obtain the fine cellulose fibers used in the present invention is not particularly limited. For example, bleached or unbleached mechanical pulp (e.g., thermomechanical pulp (TMP), wood pulp) or chemical pulp (e.g., sulfite pulp, kraft pulp) made from coniferous trees, broad-leaved trees, cotton, straw, bamboo, hemp, jute, kenaf, etc., as well as dissolved pulp, regenerated cellulose, microcrystalline cellulose with the amorphous region removed, etc., and any of these can be used as cellulose raw materials.

[0015] (Ionic Modified Cellulose) Ionic modified cellulose can be prepared by introducing ionic functional groups into a cellulose raw material. The ionic functional group may be anionic or cationic. There are no particular limitations on the method of introducing these ionic functional groups. For example, the method described below can be used.

[0016] (Anionically Modified Cellulose) Cellulose into which anionic groups have been introduced as ionic functional groups is called "anionically modified cellulose." The method of introducing anionic groups is not particularly limited, but examples include directly oxidizing the hydroxyl group of the pyranose ring of cellulose to a carboxyl group, as described later, or introducing anionic groups by etherification or esterification reactions at the hydroxyl portion of the pyranose ring.

[0017] (Carboxylated Cellulose) As an example of anionic modified cellulose, carboxylated cellulose having a carboxyl group can be cited. The carboxyl group (-COOH) is -COOM (metal salt type) (wherein M is a metal ion) or -COOH when the metal ion is ionized in the electrolyte membrane. － It may take the form shown. Carboxylated cellulose can be obtained using known methods for oxidizing the hydroxyl group of the pyranose ring of cellulose to a carboxyl group. Examples of methods for oxidizing cellulose include oxidizing cellulose in water using an oxidizing agent in the presence of an N-oxyl compound such as 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) and a bromide and / or iodide, or oxidizing cellulose by contacting the cellulose raw material with a gas containing ozone as an oxidizing agent.

[0018] The amount of carboxyl groups in carboxylated cellulose is preferably 0.4 to 3.0 mmol / g, more preferably 0.6 to 2.0 mmol / g, even more preferably 1.0 to 2.0 mmol / g, and even more preferably 1.1 to 2.0 mmol / g, relative to the oven-dry mass of carboxylated cellulose. The amount of carboxyl groups in carboxylated cellulose can be adjusted by controlling reaction conditions such as the amount of oxidizing agent added and the reaction time. The amount of carboxyl groups can be measured by the following method: Prepare 60 mL of a 0.5% by mass slurry (aqueous dispersion) of carboxylated cellulose, add a 0.1 M aqueous hydrochloric acid solution to make the pH 2.5, then add a 0.05 N aqueous sodium hydroxide solution dropwise until the pH becomes 11 and measure the electrical conductivity. From the amount of sodium hydroxide consumed in the neutralization stage of the weak acid, where the change in electrical conductivity is gradual (a), the amount of carboxyl groups [mol / g carboxylated cellulose] = a [mL] × 0.05 / mass of carboxylated cellulose [g].

[0019] (Carboxyalkylated cellulose) As an example of anionic modified cellulose, carboxyalkylated cellulose having a carboxyalkyl group can be cited. In this specification, carboxyalkyl group (-RCOOH) (wherein R is an alkylene group such as a methylene group or an ethylene group) is -RCOOM (metal salt type) (wherein M is a metal ion) or -RCOO obtained by ionization of the metal ion in the electrolyte membrane. － It may take the form shown. As carboxyalkylated cellulose, carboxymethylated cellulose in which R is a methylene group is most preferred (hereinafter, "carboxymethyl" will be referred to as "CM"). Carboxyalkylated cellulose can be obtained using a known method in which a cellulose raw material is treated with a mercerizing agent and then treated with a carboxyalkylating agent to introduce a carboxyalkyl group.

[0020] The degree of carboxyalkyl substitution per anhydrous glucose unit of carboxyalkylated cellulose (hereinafter also simply referred to as "degree of substitution") is not particularly limited, but from the viewpoint of obtaining carboxyalkylated cellulose that can easily maintain its fibrous form even in water, it is preferably less than 0.40.

[0021] Carboxyalkylated cellulose, which has a low degree of substitution and easily maintains its fibrous form even in water, can be converted into fibrillated cellulose or cellulose nanofibers by reducing the fiber diameter through defibration, as described later. Fibrillated cellulose and cellulose nanofibers, which have a small fiber diameter, are preferable from the viewpoint of ionic conductivity because they are easily dispersed uniformly in an electrolyte membrane. The degree of substitution in carboxyalkylated cellulose, which has a low degree of substitution and easily maintains its fibrous form even in water, is preferably 0.10 or more and less than 0.40, more preferably 0.10 or more and 0.35 or less, and even more preferably 0.15 or more and 0.30 or less.

[0022] Note that anhydrous glucose units refer to individual anhydrous glucose (glucose residues) that make up cellulose, and the degree of substitution indicates the proportion of hydroxyl groups (-OH) in the glucose residues that make up cellulose that are substituted with carboxyalkyl groups (-ORCOOH or -ORCOOM) (the number of carboxyalkyl groups per glucose residue). The degree of substitution can be adjusted by controlling reaction conditions such as the amount of mercerizing agent and reaction time. For example, the degree of CM substitution can be measured by the following method: Accurately weigh about 2.0 g of CM-modified cellulose (dry) and place it in a 300 mL stoppered Erlenmeyer flask. Add 100 mL of a solution made by adding 100 mL of special grade concentrated nitric acid to 900 mL of methanol, and shake for 3 hours to convert the salt-type CM-modified cellulose to hydrogen-type CM-modified cellulose. Accurately weigh 1.5 g to 2.0 g of hydrogen-type CM-modified cellulose (dry) and place it in a 300 mL stoppered Erlenmeyer flask. Wet the hydrogenated CM-modified cellulose with 15 mL of 80% by mass methanol, add 100 mL of 0.1 N NaOH, and shake at room temperature for 3 hours. Use phenolphthalein as an indicator and 0.1 N H ２ SO ４ The excess NaOH is back-titrated. The degree of CM substitution (DS) is calculated by the following formula: A = [(100 × F' - (0.1 N H) ２ SO ４) (mL) × F) × 0.1] / (Dry mass of hydrogen-type CM-modified cellulose (g)) DS = 0.162 × A / (1 - 0.058 × A) A: Amount of 1N NaOH required to neutralize 1g of hydrogen-type CM-modified cellulose (mL) F: 0.1N H ２ SO ４ Factor F': Factor of 0.1N NaOH The degree of substitution of carboxyalkyl groups other than the CM group can also be measured using the same method as described above.

[0023] (Phosphate-esterified cellulose) Phosphate-esterified cellulose can be cited as an example of anionically modified cellulose. Phosphate-esterified cellulose can be obtained by introducing phosphate groups derived from phosphate compounds into cellulose by mixing a powder or aqueous solution of a phosphate compound with the cellulose raw material described above, or by adding an aqueous solution of a phosphate compound to a slurry of cellulose raw materials. Examples of phosphate compounds include phosphoric acid, polyphosphate, phosphorous acid, hypophosphorous acid, phosphonic acid, polyphosphonic acid, or esters or salts thereof. Specifically, examples include, but are not limited to, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium phosphite, potassium phosphite, sodium hypophosphite, potassium hypophosphite, sodium pyrophosphate, sodium metaphosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, potassium metaphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, ammonium metaphosphate, etc. Phosphate groups derived from phosphate compounds can be introduced into cellulose by using one or more of these in combination. In this specification, phosphate groups derived from phosphate compounds include phosphate groups, phosphite groups, hypophosphite groups, pyrophosphate groups, metaphosphate groups, polyphosphate groups, phosphonic acid groups, and polyphosphonic acid groups. Phosphate-esterified cellulose includes cellulose in which one or more of these phosphate groups are introduced into the molecular chain. When reacting cellulose raw materials with phosphate compounds, it is desirable to use the phosphate compound as an aqueous solution in order to ensure uniform reaction and high efficiency in introducing the above groups, and in this case, the pH of the aqueous solution is preferably pH 3 to 7. Nitrogen-containing compounds such as urea may also be added.

[0024] The degree of phosphate group substitution per glucose unit in phosphated cellulose (hereinafter simply referred to as "phosphate group substitution degree") is preferably 0.001 or more and less than 0.40. The degree of phosphate group substitution per glucose unit can be measured by the following method: Prepare a slurry of phosphated cellulose with a solid content of 0.2% by mass. Add 1 / 10 by volume of strongly acidic ion exchange resin (Amberjet 1024; Organo, conditioned) to the slurry, shake for 1 hour, and then pour onto a mesh with a mesh opening of 90 μm to separate the resin and slurry and obtain hydrogen-type phosphated cellulose. Next, add 50 μL of 0.1 N sodium hydroxide aqueous solution to the slurry after treatment with the ion exchange resin once every 30 seconds, and measure the change in the electrical conductivity value of the slurry. The amount of alkali (moles) required in the region where the electrical conductivity rapidly decreases from the measurement results is divided by the amount of solids (g) in the slurry being titrated to calculate the amount of phosphate groups (moles / g) per gram of hydrogen-type phosphated cellulose. Furthermore, the degree of phosphate group substitution (DS) per glucose unit of phosphated cellulose is calculated using the following formula: DS = 0.162 × A / (1 - 0.079 × A) where A is the amount of phosphate groups per gram of hydrogen-type phosphated cellulose (moles / g).

[0025] (Sulfate-esterified cellulose) As an example of anionically modified cellulose, sulfate-esterified cellulose can be mentioned. Sulfate-esterified cellulose can be obtained by reacting the above-mentioned cellulose raw material with a sulfate-based compound, thereby introducing sulfate-based groups derived from the sulfate-based compound into the cellulose. Examples of sulfate-based compounds include sulfuric acid, sulfamic acid, chlorosulfonic acid, sulfur trioxide, or esters or salts thereof. Among these, sulfamic acid is preferred because it has low cellulose solubility and low acidity.

[0026] For example, when sulfamic acid is used as the sulfate compound, the amount of sulfamic acid used can be appropriately adjusted considering the amount of anionic group introduced into the cellulose chain. For example, it can be used in an amount of preferably 0.01 to 50 mol per mol of glucose units in the cellulose molecule, and more preferably in an amount of 0.1 to 3.0 mol.

[0027] The amount of sulfate groups per glucose unit in sulfated cellulose (hereinafter simply referred to as "amount of sulfate groups") is preferably 0.1 to 3.0 mmol / g. The amount of sulfate groups per glucose unit can be measured by the following method: An aqueous dispersion of sulfated cellulose is solvent-substituted with ethanol and then t-butanol, and then freeze-dried. 15 mL of ethanol and 5 mL of water are added to 200 mg of the obtained sample and stirred for 30 minutes. Then, 10 mL of 0.5 N aqueous sodium hydroxide solution is added and stirred at 70°C for 30 minutes, and then stirred at 30°C for 24 hours. Next, phenolphthalein is added as an indicator and titrated with hydrochloric acid, and the amount of sulfate groups is calculated using the following formula: Amount of sulfate groups [mol / g sample] = (5 - (0.1 × volume of hydrochloric acid titration [mL] × 2)) / 0.2.

[0028] (Cationized Cellulose) Cellulose into which a cationic group has been introduced as an ionic functional group is called "cationized cellulose." The method of introducing the cationic group is not particularly limited. For example, it can be obtained by a known method in which the aforementioned carboxylated cellulose is reacted with a cationizing agent such as glycidyltrimethylammonium chloride, 3-chloro-2-hydroxypropyltrialkylammonium halide or its halohydrin type, and an alkali metal hydroxide catalyst (such as sodium hydroxide or potassium hydroxide) in the presence of water or an alcohol having 1 to 4 carbon atoms.

[0029] The degree of cation substitution per glucose unit in cation-modified cellulose is preferably 0.02 to 0.50. The degree of cation substitution can be adjusted by the amount of cationizing agent added and the composition ratio of water or C1-C4 alcohol. The degree of cation substitution per glucose unit can be measured by the following method: After drying the cation-modified cellulose, the nitrogen content is measured using a total nitrogen analyzer (TN-10, manufactured by Mitsubishi Chemical Corporation), and the degree of cation substitution (average number of moles of substituents per mole of anhydrous glucose unit) is calculated using the following formula: Degree of cation substitution = (162 × N) / (1 - 151.6 × N) N: Nitrogen content.

[0030] (Fibrillation) The ion-modified cellulose obtained as described above is preferably subjected to a defibrillation process, which reduces the diameter of the cellulose fibers, to finely pulverize it into ion-modified fine cellulose fibers before being used in an electrolyte membrane. Known defibrillation methods include, but are not limited to, wet or dry processing using high-pressure homogenizers, microfluidizers, refiners, grinders, shear-type agitators, colloid mills, beaters, kneaders, dispersers, freeze-drying and grinding, ultrasonic defibrillation, etc.

[0031] The degree of defibration is not particularly limited. For example, ion-modified cellulose before defibration (pulp that has undergone ion modification) is often, though not limited to, a fiber diameter of 10 to 50 μm and a fiber length of 0.5 to 5.0 mm. However, by defibrating it, or by partially fluffing the fiber surface (fibrillation), it can be made into fibrillated cellulose with an average fiber diameter of 1 to 10 μm. The average fiber diameter of fibrillated cellulose is more preferably about 5 to 10 μm. The average fiber length of fibrillated cellulose is not particularly limited, but is about 0.2 to 3.0 mm, more preferably about 0.3 to 2.5 mm, and even more preferably about 0.5 to 2.0 mm. The average fiber diameter and average fiber length of fibrillated cellulose can be determined, for example, by an image analysis type fiber analyzer such as the L&W Fiber Tester Plus manufactured by ABB Corporation or the Fractionator manufactured by Valmet Corporation. Specifically, the following method can be used for measurement: A fibrillated cellulose aqueous dispersion diluted to a solid content concentration of 0.25% by mass is subjected to a fractionator, and the length-weighted fiber width and length-weighted average fiber length are determined, respectively (n=2).

[0032] In the specification, ionically modified cellulose that has been fibrillated is sometimes referred to as ionically modified fibrillated cellulose. Similarly, for example, anionally modified cellulose that has been fibrillated is sometimes referred to as anionally modified fibrillated cellulose.

[0033] Ion-modified cellulose may optionally be defibrated until its diameter is smaller than that of fibrillated cellulose, i.e., until the average fiber diameter is less than 500 nm, to obtain ion-modified cellulose nanofibers (hereinafter, cellulose nanofibers are referred to as "CNF"). When obtaining ion-modified CNF, it is preferable to use a wet high-pressure homogenizer. The average fiber diameter of ion-modified CNF is more preferably about 3 to 500 nm, even more preferably about 3 nm to 150 nm, and even more preferably about 3 nm to 20 nm. The aspect ratio of ion-modified CNF is preferably 30 or more, more preferably 50 or more, and even more preferably 100 or more. There is no upper limit to the aspect ratio, but it is generally around 500 or less. The average fiber diameter and average fiber length of CNF can be measured by analyzing 200 randomly selected fibers using an atomic force microscope (AFM) when the diameter is less than 20 nm, or a field emission scanning electron microscope (FE-SEM) when the diameter is 20 nm or more, and calculating the average. Furthermore, the aspect ratio can be calculated using the following formula: Aspect ratio = Average fiber length / Average fiber diameter.

[0034] In this specification, ion-modified cellulose obtained by defibrating it to a CNF with an average fiber diameter of less than 500 nm is referred to as ion-modified CNF. Similarly, for example, CNF obtained by defibrating anion-modified cellulose is sometimes called anion-modified CNF, and CNF obtained by defibrating carboxylated cellulose is sometimes called carboxylated CNF.

[0035] (Polyvinyl alcohol) From the viewpoint of durability, the polyvinyl alcohol used in the electrolyte membrane of the present invention preferably has an average degree of polymerization of 500 to 3000. Partially saponified polyvinyl alcohol, intermediately saponified polyvinyl alcohol, and fully saponified polyvinyl alcohol can also be used. The polyvinyl alcohol is, for example, a vinyl alcohol-vinyl acetate copolymer, a vinyl alcohol-vinyl butyral copolymer, or an ethylene-vinyl alcohol copolymer, and preferably a vinyl alcohol-vinyl acetate copolymer.

[0036] The copolymerization ratio of polyvinyl alcohol is expressed as the degree of saponification. From the viewpoint of crosslinking stability and chemical stability, the degree of saponification of polyvinyl alcohol is preferably 80-99 mol%, and more preferably 90-99 mol%. If the degree of saponification of polyvinyl alcohol is less than 80 mol%, the physical strength of the electrolyte membrane will be reduced. The degree of saponification of polyvinyl alcohol can be determined by the amount of alkali consumed for hydrolysis of copolymer units such as vinyl acetate, or by compositional analysis using NMR.

[0037] In the electrolyte membrane of the present invention, the amount of polyvinyl alcohol per 100 parts by mass of fine cellulose fibers is preferably 1 to 49 parts by mass, and more preferably 2 to 35 parts by mass, from the viewpoint of ionic conductivity. If the amount of polyvinyl alcohol is too high, the ionic conductivity may decrease excessively, and if it is too low, crosslinking may not proceed, which may worsen the durability and water resistance of the membrane.

[0038] (Crosslinking agent) In the present invention, the crosslinking agent plays the role of crosslinking polyvinyl alcohols together. Examples of crosslinking agents that can be used in the electrolyte membrane of the present invention include organotitanium compounds such as titanium lactate and titanium lactate ammonium salt, and organozirconium compounds such as zirconyl chloride compounds and zirconium lactate ammonium salt. From the viewpoint of ease of crosslinking reaction, it is preferable to use organotitanium compounds, and among them, it is preferable to use titanium lactate from the viewpoint of availability and ease of handling.

[0039] The amount of crosslinking agent per 100 parts by mass of the electrolyte membrane of the present invention is preferably 1 to 20 parts by mass from the viewpoint of membrane durability. Furthermore, the amount of crosslinking agent per 100 parts by mass of polyvinyl alcohol is preferably 0.1 to 100 parts by mass, and more preferably 5 to 60 parts by mass, from the viewpoint of reactivity.

[0040] (Electrolyte Membrane) The electrolyte membrane of the present invention is a membrane containing fine cellulose fibers, polyvinyl alcohol, and a crosslinking agent, and is preferably a self-supporting membrane. As used herein, a self-supporting membrane means a membrane that can maintain its shape as a membrane even without the presence of other supports. The manufacturing method of the electrolyte membrane of the present invention is not particularly limited. For example, the electrolyte membrane can be obtained by mixing an aqueous dispersion of fine cellulose fibers, an aqueous solution of polyvinyl alcohol, and other components used as necessary, adding and mixing a crosslinking agent thereto, applying the resulting composition onto a support to form a coating film, heating the obtained coating film at 25 to 105 °C for 12 hours to 15 days to remove a solvent such as water and crosslink, and then peeling the cured membrane from the support. The obtained self-supporting membrane may be additionally crosslinked by heating at 50 to 200 °C for 0.1 to 24 hours.

[0041] (Other Components) The electrolyte membrane of the present invention may contain other components alone or in combination of two or more, as long as they do not impair the effects of the present invention. Examples of other components include cellulose and cellulose derivatives other than fine cellulose fibers, chitin, chitosan, polyethylene glycol, polyvinylidene fluoride, polyethylene terephthalate, polymethyl methacrylate resin, polyethylene, polypropylene, polyethylene oxide, polyvinylidene fluoride-hexafluoropropylene, polyparabenic acid resin, polyether resin, polyester resin, polyether derivative, and polymers added to improve mechanical strength such as polyimine resin, polyamide resin, and polycarbonate resin, as well as ionic liquids, flexible ionic crystals, electrolytes other than ionic modified cellulose (for example, organic acids and their salts, inorganic acids and their salts, and polymers such as perfluorosulfonic acid polymers). Examples of electrolytes include inorganic electrolytes such as glass ceramics, electrolyte salts (e.g., lithium salts such as lithium hexafluorophosphate, quaternary ammonium salts), ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, vinylene carbonate, sulfolane, 1,3-propanesultone, methanesulfonic acid (R)-1-methyl-2-propynyl, cyclohexylbenzene, tert-amylbenzene, adiponitrile, phthalate esters, adipic acid esters, trimellitic acid esters, polyesters, phosphate esters, citrate esters, epoxidized vegetable oils, sebacate acid esters, azelaic acid esters, maleic acid esters, and benzoic acid esters. It is preferable that other components make up 50% by mass or less of the total electrolyte membrane.

[0042] The film thickness of the electrolyte membrane of the present invention is preferably 0.1 to 100 μm, and more preferably 0.1 to 30 μm, from the viewpoint of energy storage capacity and electrical resistance. The film thickness of the electrolyte membrane can be adjusted by the amount and concentration of the coating solution applied.

[0043] With respect to 100 parts by mass of the electrolyte membrane of the present invention, the amount of microcrystalline cellulose fibers is preferably 50 to 98 parts by mass, more preferably 60 to 95 parts by mass, from the viewpoints of water resistance and ionic conductivity. If the amount of microcrystalline cellulose fibers is too large, there is a risk of reduced water resistance, and if it is too small, there is a risk of reduced ionic conductivity.

[0044] With respect to 100 parts by mass of the electrolyte membrane of the present invention, the amount of polyvinyl alcohol is preferably 2 to 49 parts by mass, more preferably 5 to 40 parts by mass, from the viewpoint of ionic conductivity. If the amount of polyvinyl alcohol is too large, there is a risk of excessive reduction in ionic conductivity, and if it is too small, there is a risk that crosslinking will not proceed and the durability and water resistance of the membrane will deteriorate.

[0045] (Water swelling ratio) From the viewpoint of water resistance, the water swelling ratio of the electrolyte membrane of the present invention is preferably 1500% or less, more preferably 1200% or less, and even more preferably 1000% or less. The lower limit is 300% or more. If the water swelling ratio is too higher than the above upper limit value, there is a risk of inferior water resistance. The water swelling ratio can be adjusted by the addition amount of the crosslinking agent. In the present invention, the water swelling ratio can be calculated by the following formula. Water swelling ratio (%) = mass of the membrane in the wet state (mg) / mass of the membrane in the dry state (mg) × 100

[0046] (Ionic conductivity) From the viewpoint of ionic conductivity, the ionic conductivity in the thickness direction of the electrolyte membrane measured by the method described in the examples described later of the electrolyte membrane of the present invention is preferably 0.01 to 100 mS / cm, more preferably 0.1 to 100 mS / cm, and even more preferably 0.5 to 100 mS / cm. If the ionic conductivity is too lower than the above lower limit value, there is a risk of deterioration in the performance of the resulting electrochemical device. The ionic conductivity can be adjusted by the amount of microcrystalline cellulose fibers, water swelling, and addition of other components.

[0047] (Amount of Ionic Functional Groups) From the viewpoint of ionic conductivity, the electrolyte membrane of the present invention preferably has an amount of ionic functional groups of 0.1 mmol / g or more per gram of electrolyte membrane, more preferably 0.5 mmol / g or more, and even more preferably 0.9 mmol / g or more. If the amount of ionic functional groups in the electrolyte membrane is too small, there is a risk of a decrease in ionic conductivity. The amount of ionic functional groups can be adjusted by the amount of fine cellulose fibers. In the present invention, when carboxylated CNF is used as the fine cellulose fibers, the amount of ionic functional groups per gram of electrolyte membrane can be calculated by the following formula: Amount of ionic functional groups (mol / g) = Amount of CNF (mg) / Amount of electrolyte membrane (mg) × Amount of carboxyl groups (mol / g) Here, the amount of carboxyl groups in the above formula represents the amount of carboxyl groups (mol / g) in 1 g of carboxylated CNF used.

[0048] (Amount of Metal Cations) From the viewpoint of ionic conductivity, the electrolyte membrane of the present invention preferably contains 0.1 to 10.0 mmol / g of metal cations, including sodium ions, per gram of electrolyte membrane, and more preferably 1.0 to 6.0 mmol / g. Too much metal cation may reduce the water resistance of the electrolyte membrane, while too little may reduce ionic conductivity. The amount of metal cations can be adjusted by the amount of fine cellulose fibers and the addition of other components. The amount of sodium ions will be referred to as the amount of Na below. The counterions of the fine cellulose fibers do not all need to be metal cations; some may be hydrogen ions. The amount of hydrogen ions can be adjusted, for example, by the amount of chemicals used for neutralization when manufacturing the fine cellulose fibers.

[0049] Examples of metal cations include monovalent cations such as sodium ions, potassium ions, and lithium ions, and polyvalent cations such as zinc ions, magnesium ions, calcium ions, copper ions, and aluminum ions. From the viewpoint of ionic conductivity, sodium ions, potassium ions, and lithium ions are preferred.

[0050] The amount of metal cations in the electrolyte membrane can be measured, for example, by ICP emission spectroscopy. If the electrolyte membrane contains only metal cations derived from ion-modified fine cellulose fibers, the amount of metal cations (mol / g) contained in 1 g of electrolyte membrane can be calculated using the following formula (1). If the amount of metal cations is increased by the addition of other components, the amount of metal cations (mol / g) contained in 1 g of electrolyte membrane can be calculated using the following formula (2). (1) Amount of metal cations (mol / g) = Amount of ionic functional groups {Amount of CNF (mg) / Amount of electrolyte membrane (mg) × Amount of carboxyl groups (mol / g)} / Metal cation valence (2) Amount of metal cations (mol / g) = Amount of ionic functional groups [Amount of CNF (mg) / {Amount of electrolyte membrane (mg) + Amount of other components added (mg)} × Amount of carboxyl groups (mol / g)] / Metal cation valence + Amount of metal cations of other components (mol / g) Here, the amount of carboxyl groups in the above formulas (1) and (2) represents the amount of carboxyl groups (mol / g) in 1 g of carboxylated CNF used.

[0051] The electrolyte membrane of the present invention is obtained by crosslinking a fine cellulose fiber-based electrolyte membrane with polyvinyl alcohol and a crosslinking agent, and has a structure in which fine cellulose fibers, preferably ion-modified fine cellulose fibers, are constrained by a network of polyvinyl alcohol. Therefore, compared to membranes made solely from fine cellulose fibers such as CNF, the water resistance is significantly improved. As a result, when used with an aqueous electrolyte or in other aqueous environments, problems such as strength reduction and leaching are greatly reduced.

[0052] Furthermore, the electrolyte membrane of the present invention maintains the high ionic conductivity of fine cellulose fibers, preferably ion-modified CNF, while being improved to enhance water resistance. This enables efficient electrical conduction in electrochemical devices such as fuel cells, capacitors, and secondary batteries.

[0053] Furthermore, because the electrolyte membrane of the present invention uses biomass-derived materials such as CNF as fine cellulose fibers, the environmental burden is reduced throughout the process from the manufacture to the disposal of the electrolyte membrane. By selecting components with minimal environmental impact for use as crosslinking agents, the overall environmental burden is further reduced.

[0054] (Applications) The electrolyte membrane of the present invention can be suitably used as an electrolyte membrane for electrochemical devices used in an aquatic environment. Examples of such electrochemical devices include electric double-layer capacitors (EDLCs), secondary batteries, fuel cells, and water electrolyzers.

[0055] When the electrolyte membrane of the present invention is used in an EDLC, for example, the electrolyte membrane of the present invention and a separator as needed are placed between the positive electrode and the negative electrode, and after adding water or an organic solvent to the electrolyte as needed to homogenize it, the EDLC can be formed by sealing these in an outer case. Although separators are generally placed to prevent direct contact between the positive electrode and the negative electrode, in the present invention the electrolyte membrane can perform the same role as a separator, so the EDLC may not have a separator. When the electrolyte membrane is used as a separator, its thickness can be approximately 0.1 μm to 100 mm. If the film thickness is less than 0.1 μm, the two electrodes tend to short-circuit easily, and if the film thickness exceeds 100 mm, the volume of the electrochemical device tends to increase and the resistance between the two electrodes tends to increase.

[0056] When the electrolyte membrane of the present invention is used in a secondary battery, as in the case of forming an EDLC, there are no particular limitations on the components other than the electrolyte membrane, and ordinary components can be used. That is, a secondary battery can be formed by placing a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and the electrolyte membrane between them, and sealing them in an outer casing. The electrolyte membrane can be homogenized by adding water or an organic solvent as needed, and the homogenized membrane can be placed between the positive and negative electrodes. The positive electrode can be manufactured, for example, by coating a paste made by mixing a positive electrode active material, a binder, a solvent, a conductive additive, etc., onto a positive electrode current collector. The negative electrode can be manufactured similarly, for example, by coating a paste made by mixing a negative electrode active material, a binder, a solvent, a conductive additive, etc., onto a negative electrode current collector. The separator may or may not be provided, as in the case of an EDLC.

[0057] The electrolyte membrane of the present invention can be suitably used not only in the electrochemical devices mentioned above, but also in primary batteries and fuel cells, and can further be applied to energy conversion devices such as water electrolyzers. Furthermore, the improved water resistance makes it possible to use the membrane in specific environments that were previously difficult, thus expanding its range of applications to new areas.

[0058] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to these examples. Unless otherwise specified, the measurement / calculation methods for each numerical value in each example are as described in the specification. In the following description, "parts" refers to parts by mass unless otherwise specified, and "%" refers to mass percent unless otherwise specified.

[0059] (Example 1) (Preparation of fine cellulose fibers) 500 g (absolutely dry) bleached, unbeaten kraft pulp (whiteness 85%) derived from coniferous trees was added to 50 L of an aqueous solution in which TEMPO (Sigma Aldrich) (0.025 mmol / g relative to the cellulose raw material) and sodium bromide (1.0 mmol / g relative to the cellulose raw material) were dissolved, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous sodium hypochlorite solution was added to the reaction system at a concentration of 5.2 mmol / g relative to the cellulose raw material to start the oxidation reaction. During the reaction, the pH in the system decreased, but the pH was adjusted to 10 by sequentially adding an aqueous 3M sodium hydroxide solution. The reaction was terminated when the sodium hypochlorite was consumed and the pH in the system no longer changed. The mixture after the reaction was filtered through a glass filter to separate the pulp, and the pulp was thoroughly washed with water to obtain oxidized pulp. The pulp yield at this time was 90%, and the time required for the oxidation reaction was 100 minutes. The oxidized pulp (carboxylated cellulose) obtained in the above process was adjusted to 0.5% (w / v) with water and subjected to five defibration treatments in a high-pressure homogenizer (20°C, 150 MPa) to obtain an aqueous dispersion of carboxylated CNF. The amount of carboxyl groups in the obtained carboxylated CNF was 1.30 mmol / g.

[0060] (Manufacture of Electrolyte Membrane) A 0.5% by mass aqueous dispersion of carboxylated CNF obtained as described above and polyvinyl alcohol (Kuraray Co., Ltd., product name: Kuraray Poval PVA117) (diluted with water to a solid content of 5.7% by mass) were mixed in the proportions shown in Table 1 on a solid content basis and thoroughly mixed. After degassing under reduced pressure, titanium lactate (Matsumoto Fine Chemical Co., Ltd., product name: Orgatics TC-310) (approximately 40% by mass solid content, water / IPA solution) was added as a crosslinking agent and thoroughly mixed again. 30 g of the resulting mixture was placed in a polystyrene petri dish with an inner diameter of 90 mm and spread out, then dried at 50°C for two nights. The resulting dried film was removed from the polystyrene petri dish, placed on a stainless steel tray, and dried at 105°C for 1 hour. By further crosslinking with polyvinyl alcohol, a dried electrolyte membrane was obtained and subjected to the evaluation described below.

[0061] Furthermore, it was confirmed that the obtained electrolyte membrane has a structure in which polyvinyl alcohol is crosslinked with titanium lactate as a crosslinking agent. Therefore, the obtained electrolyte membrane has a structure in which carboxylated CNF exists within a network of crosslinked polyvinyl alcohol.

[0062] (Evaluation) (Amount of ionic functional groups) Based on the amount of ionic functional groups in the fine cellulose fibers used, the amount of ionic functional groups per gram of the prepared electrolyte membrane was calculated as the amount of carboxyl groups. A higher value indicates a higher ionic conductivity. In this invention, the amount of ionic functional groups can be calculated using the following formula: Amount of ionic functional groups (mol / g) = {Amount of CNF (mg) / Amount of electrolyte membrane (mg)} × Amount of carboxyl groups (mol / g) Here, the amount of carboxyl groups in the above formula represents the amount of carboxyl groups (mol / g) in 1 g of carboxylated CNF used.

[0063] (Water Swelling Rate) A dry electrolyte membrane was immersed in deionized water for 3 hours. The membrane was removed from the water, and excess water on the wet membrane was absorbed and removed by sandwiching it between paper wipers to obtain a wet membrane. The obtained wet membrane was weighed. Next, this membrane was dried at 105°C for 1 hour, and the dry membrane was weighed. The water swelling rate was calculated using the following formula, and the results are shown in Table 1. A lower water swelling rate indicates higher and better water resistance. Water swelling rate (%) = Mass of wet membrane (mg) / Mass of dry membrane (mg) × 100

[0064] (Ionic Conductivity) A sample with a diameter of 16 mm was punched out from the dry electrolyte membrane and immersed in ion-exchanged water at room temperature for 10 minutes. The membrane was taken out of the water, and the excess moisture adhering to the wet membrane was sandwiched with a paper wiper and absorbed and removed to obtain a wet membrane. The obtained membrane was sandwiched between gold-plated electrodes with a diameter of 10 mm, and an alternating current impedance spectrum measurement was performed at room temperature using an electrochemical measurement device (Keysight 4294A) with an alternating current of 100 mHz to 100 Hz. The resistance value of the electrolyte membrane was obtained from the obtained Nyquist plot, and the ionic conductivity (mS / cm) of the electrolyte membrane was calculated using the following formula. The results are shown in Table 1. σ = {L / (R × A)} × 1000 σ: Conductivity (mS / cm) L: Thickness of the electrolyte membrane (cm) R: Resistance of the electrolyte membrane (Ω) A: Electrode area (cm ２ )

[0065] (Fragility) The dry electrolyte membrane was immersed in ion-exchanged water at room temperature for 10 minutes. The membrane was taken out of the water, and the excess moisture adhering to the wet membrane was sandwiched with a paper wiper and absorbed and removed to obtain a wet membrane. The obtained membrane was pinched with tweezers and evaluated according to the following criteria based on the angle at which it could be bent without cracking. The results are shown in Table 1. A: No cracks occurred even when bent at 180°. B: No cracks occurred even when bent at 90°, but cracks occurred when bent at 180°. C: Cracks occurred when bent at 90°.

[0066] (Examples 2 to 5) In the production of the electrolyte membrane, an electrolyte membrane was obtained in the same manner as in Example 1, except that the amount of the crosslinking agent was changed to the ratio shown in Table 1. Further, the obtained electrolyte membrane was evaluated in the same manner as in Example 1.

[0067] (Comparative Example 1) In the production of the electrolyte membrane, an electrolyte membrane was obtained in the same manner as in Example 1, except that neither polyvinyl alcohol nor the crosslinking agent was used. Further, the obtained electrolyte membrane was evaluated in the same manner as in Example 1. In the evaluation of the water swelling ratio, since the membrane collapsed when pinched with tweezers to take the membrane out of the water, it was difficult to lift the swollen membrane from the ion-exchanged water, and the water swelling ratio could not be measured.

[0068] (Comparative Example 2) An electrolyte membrane was obtained in the same manner as in Example 1, except that a crosslinking agent was not used in the production of the electrolyte membrane. The obtained electrolyte membrane was evaluated in the same manner as in Example 1. However, in the evaluation of the water swelling rate, the membrane disintegrated when it was picked up with tweezers to remove it from the water, making it difficult to remove the swollen membrane from the deionized water, and thus the water swelling rate could not be measured.

[0069] (Comparative Example 3) An electrolyte membrane was obtained in the same manner as in Example 1, except that the amounts of carboxylated CNF and polyvinyl alcohol were changed to the proportions shown in Table 1 in the manufacturing of the electrolyte membrane, and no crosslinking agent was used. The obtained electrolyte membrane was evaluated in the same manner as in Example 1. However, in the evaluation of the water swelling rate, the membrane disintegrated when it was picked up with tweezers to remove it from the water, making it difficult to remove the swollen membrane from the deionized water, and therefore the water swelling rate could not be measured.

[0070]

[0071] From Table 1, it can be seen that the electrolyte membranes of the present invention, which contain fine cellulose fibers, polyvinyl alcohol, and a crosslinking agent obtained in Examples 1 to 5, exhibit ionic conductivity equivalent to that of Comparative Examples 1 to 3. This indicates that the electrolyte membranes maintain sufficient ionic conductivity while exhibiting excellent water resistance and membrane strength.

Claims

1. An electrolyte membrane containing fine cellulose fibers, polyvinyl alcohol, and a crosslinking agent.

2. The electrolyte membrane according to claim 1, wherein the crosslinking agent is an organotitanium compound or an organozirconium compound.

3. The electrolyte membrane according to claim 1 or 2, wherein the amount of fine cellulose fibers is 50 to 98 parts by mass per 100 parts by mass of the electrolyte membrane.

4. The electrolyte membrane according to claim 1 or 2, wherein the amount of polyvinyl alcohol is 2 to 49 parts by mass per 100 parts by mass of the electrolyte membrane.

5. The electrolyte membrane according to claim 1 or 2, wherein the amount of the crosslinking agent is 0.1 to 100 parts by mass per 100 parts by mass of the polyvinyl alcohol.

6. The electrolyte membrane according to claim 1 or 2, wherein the ionic conductivity is 0.01 to 100 mS / cm.

7. The electrolyte membrane according to claim 1 or 2, wherein the amount of metal cations is 0.1 to 10.0 mmol / g.

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