Cellulose-based electricity storage material and capacitor

Abstract

To provide a power storage material and a pseudo solid water electric double layer capacitor which are made by using a fiber material of a cellulose molecule obtained from wood / plant fiber (pulp) and capable of storing power in DC and AC.SOLUTION: The present invention provides an electricity storage material including cellulose nanofibers having a mean fiber size of 35nm or less and electrodes, the electricity storage material having an electric double layer composed of quasi-solid water between surfaces of cellulose nanofiber molecules and the electrodes, and a capacitor including the electricity storage material. The cellulose nanofibers are preferably crystallized / amorphous fibers, and are preferably amorphous fibers having atomic vacancies. The cellulose nanofibers preferably have irregularities, and the diameters thereof are preferably 1nm to 35nm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cellulosic capacitor, and more particularly to a cellulose-based capacitor having a quasi-solid water electric double layer. [Background technology]

[0002] Capacitors are electronic components that store and discharge electric charge (electrical energy) through capacitance. They are essential components for mobile electronic devices such as personal computers and mobile phones, providing power supply stability, backup circuits, coupling elements, and noise filters. In recent years, high-performance IT products such as mobile phones and ultra-compact storage devices, as well as batteries for electric vehicles, have rapidly evolved, resulting in increased demand for capacitors that are even smaller, have larger capacities, and offer advanced functions such as memory. In particular, there is a demand for products that are compatible with a smart grid (next-generation power transmission network) society, which is in line with green innovation (low-carbonization) aimed at preventing global warming. The capacitor market, which includes applications in automobiles, IT devices, and energy-saving inverters, has been expanding steadily at an average annual rate of approximately 3.7%, reaching a value of 1 trillion yen.

[0003] It is desirable that such a capacitor does not use flammable elements such as lithium or environmental pollutants.

[0004] Capacitors are broadly divided into those for high-voltage power circuits (heavy electrical) and those for electronic and electrical equipment circuits (low-voltage). Of these, ceramic capacitors are mainly used for low-voltage electronic and electrical equipment circuits, and secondary batteries are also widely used for storing electricity in mobile phones and other devices. In contrast, capacitors for heavy electrical equipment have not yet been put to practical use due to their insufficient voltage resistance and storage capacity.

[0005] Capacitors using conventional electrical lumped constant circuits are widely used as main components of electronic and electrical devices, ranging from 1 pF to several tens of mF. The storage capacitance C (F) is given by: C=Q / V=ε×(A / d) (where Q is the charge, V is the voltage, ε is the dielectric constant, A is the electrode area, and d is the distance between the electrodes) Therefore, the larger the electrode area and the smaller the inter-electrode distance, the higher the charge capacity. However, from the perspective of making electronic and electrical devices lighter, thinner, shorter, and smaller, and of the required storage capacity, it is difficult to increase the electrode area A and reduce the inter-electrode distance d to achieve an extremely large capacity, or to decrease the electrode area A and increase the inter-electrode distance d to achieve an extremely small capacity. In addition, the capacitance of current dielectric-specification capacitors using electrical lumped parameter circuits has already saturated.

[0006] A method using an electric distributed constant circuit is a storage method that surpasses conventional capacitance. For example, an electric double layer capacitor, in which an electrolytic solution is wetted in activated carbon, has recently been put to practical use. However, a solid-state electric double layer capacitor has not yet been used.

[0007] Regarding solid-state electricity storage materials, the present inventors have discovered that electric charges can be stored in Si-(Al,Ti,V) alloys from which Al, Ti, and V have been removed by surface extraction, TiO-coated Ti-Ni-Si-based and AlO-coated Al-Y-based amorphous alloys regardless of whether they are DC or AC (see, for example, Non-Patent Documents 1 to 7 and Patent Documents 1 to 4).

[0008] Furthermore, the present inventors have discovered that when compound particles are 40 nm or less, preferably 10 nm or less, a "quantum size effect" occurs due to electron shielding that occurs on the surface of nanosized solids. Taking advantage of this phenomenon, the present inventors have developed energy storage materials that incorporate nanosized irregularities on the surface of amorphous titania, amorphous alumina, amorphous fluoropolymers, and cellulose nanofibers (see, for example, Non-Patent Documents 4, 7 to 13, and Patent Documents 4 to 6). In these energy storage materials, due to the quantum nanosize effect, the smaller the nanosized diameter of the protrusions, the greater the van der Waals electrostatic force acting as the negative sixth power of the diameter of the protrusions, increasing the electron adsorption capacity of the protrusions (see, for example, Non-Patent Document 7). The work function, which is a measure of the magnitude of electron adsorption capacity, is 5.5 eV for amorphous titania (see, for example, Non-Patent Document 7) and 10.3 eV to 13.35 eV for amorphous fluoropolymers (see, for example, Non-Patent Document 4, Patent Documents 4 and 5).

[0009] All of the above capacitors are artificial compounds of inorganic or organic compounds. Currently, the production of artificial compounds that increase carbon dioxide gas and the production of microplastics that cause marine pollution are being avoided around the world from the perspective of protecting plants and animals and preserving the global environment. From this perspective, developing a biomass capacitor that uses wood and plant fiber (cellulose) obtained from plants, which has a low environmental impact in terms of production and disposal, is lightweight, and has high elasticity, is a timely direction for preserving the global environment.

[0010] Meanwhile, Patent Document 5 has developed a cellulose nanofiber capacitor using a distributed constant circuit that utilizes the same nano-sized surface irregularities as described above. [Prior art documents] [Non-patent literature]

[0011] [Non-Patent Document 1] M. Fukuhara, T. Araki, K. Nagayama and H. Sakuraba, “Electric storage in de-alloyed Si-Al alloy ribbons”, EuroPhys. Lett., 2012, 99, 47001 [Non-Patent Document 2] M. Fukuhara, “Electric Charging / Discharging Characteristics of Capacitor, Using De-alloyed Si-20Al Alloy Ribbons”, Elect. Electr. Eng., 2013, 3(2), p.72-76 [Non-Patent Document 3] M. Fukuhara and H. Yoshida, “AC charging / discharging of de-alloyed Si-Al-V alloy ribbons”, J. Alloys and Comp., 2014, 586, S130-S133 [Non-Patent Document 4] M. Fukuhara, H. Yoshida, M. Sato, K. Sugawara, T. Takeuchi, I. Seki, and T. Sueyoshi, “Superior electric storage in de-alloyed and anodic oxidized Ti-Ni-Si glassy alloy ribbons”, Phys. Stat. Sol. RRL, 2013, 7(7), p.477-480 [Non-Patent Document 5] M. Fukuhara and K. Sugawara, “Electric charging / discharging characteristics of super capacitor, using de-alloying and anodic oxidized Ti-Ni-Si amorphous alloy ribbons”, Nanoscale. Res. Lett., 2014, 9, p.253

Non-licensed Document 6

Non-licensed Document 7

Non-licensed Document 8

Non-licensed literature 9

Non-licensed literature 10

Non-licensed Document 11

non-patent document12

non-patent document13

non-patent document14

patent document

[0012] [Patent Document 1] Patent No. 6498945 [Patent Document 2] Patent No. 6628241 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-134934 [Patent Document 4] Japanese Patent Application Publication No. 2017-41578 [Patent Document 5] Patent No. 7057577 Summary of the Invention [Problem to be solved by the invention]

[0013] None of the prior art documents focuses on the chemical bonding between the electrode material and cellulose molecules in the region of the cellulose layer in contact with the electrode in the cellulose laminate. The present invention aims to provide an energy storage material that can be used to construct capacitors with high energy density, made from environmentally friendly, recyclable wood and plant fiber (pulp). [Means for solving the problem]

[0014] During the research and development of cellulose capacitors, we discovered that oxygen atoms of hydrophilic hydroxyl groups on the equatorial side of cellulose molecules and hydrogen atoms of water molecules attached to the electrode surface are strongly hydrogen-bonded over long distances along the interface between the electrode and cellulose nanofibers. This approximately 0.28 nm monolayer attached to the electrode surface forms a pseudo-solid electric double layer, which creates a large-scale charge storage layer. Specifically, cellulose molecules convert liquid water into pseudo-solid water through the adsorption and compression effect of the enormous number of hydroxyl groups on their sides. This simultaneously forms an electric double layer, acting as a cradle (or container) for storing the pseudo-solid water. Here, pseudo-solid water refers to water that retains a structure and properties similar to ice even above its freezing point. For example, water is known to undergo a phase change at high temperatures in microscopic spaces (Non-Patent Document 14), but its interaction with cellulose is completely unknown. One of the greatest features of this invention is that, unlike conventional capacitors, water is not simply used as an electrolyte solution or a diffusion medium for ions, but rather the true nature of water is trapped in a molecular cradle called CNF, thereby bringing out the inherent potential properties of water.

[0015] That is, the present invention provides the following [1] to [9]. [1] An energy storage material having cellulose nanofibers with an average fiber diameter of 35 nm or less and electrodes, and having an electric double layer composed of quasi-solid water between the cellulose nanofiber molecular surface and the electrodes. [2] The electricity storage material according to [1], wherein the cellulose nanofibers are crystallized amorphous fibers. [3] The electricity storage material according to [1] or [2], wherein the cellulose nanofibers are amorphous fibers having atomic vacancies. [4] The electricity storage material according to any one of [1] to [3], wherein the cellulose nanofibers have irregularities with a diameter of 1 nm to 35 nm on the molecular surface. [5] The electricity storage material according to any one of [1] to [4], wherein the cellulose nanofibers are chemically modified cellulose nanofibers. [6] The electricity storage material according to any one of [1] to [5], wherein the cellulose nanofibers are TEMPO-oxidized cellulose nanofibers. [7] The electricity storage material according to any one of [1] to [6], which is in the form of a sheet. [8] A quasi-solid water electric double layer capacitor, which is a laminate having at least one electricity storage material according to any one of [1] to [6] and a pair of external electrodes located on both sides of the electricity storage material. [9] The quasi-solid water electric double layer capacitor according to [8], wherein the laminate contains two or more electricity storage materials. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide an electricity storage material capable of storing electricity in both DC and AC modes, and a capacitor using the same, such as a highly energy-efficient pseudo-solid water electric double layer capacitor. Because the electricity storage material of the present invention is made from recyclable wood and plant fiber (pulp), it is an environmentally friendly material that can contribute to the realization of sustainable resource circulation. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram showing a model of a quasi-solid water electric double layer representing the bond between the CNF molecules and the electrodes of the electricity storage material of the present invention. [Figure 2] Figure 2 is a graph showing the amount of electricity stored in the dry and wet states of TEMPO-oxidized CNF (chemically defibrated CNF in the figure) in the example, and mechanically defibrated coniferous wood and bamboo as comparative samples. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, an electricity storage material and a double layer capacitor according to an embodiment of the present invention will be described with reference to the drawings and examples.

[0019] [1. Energy storage materials] The electricity storage material has cellulose nanofibers and electrodes. The electricity storage material may have an electrode laminated on one side of the cellulose nanofiber, or a pair of electrodes laminated on both sides of the cellulose nanofiber.

[0020] [1.1 Cellulose nanofiber] Cellulose nanofibers are typically aggregates whose main component is cellulose fibers such as cellulose nanofibers. Cellulose nanofibers are essentially composed of nano-level cellulose fibers, for example, at a concentration of 90% by mass or more, 95% by mass or more, 97% by mass or more, or 99% by mass or more, and preferably 100% by mass. Cellulose nanofibers may also contain wood-derived components such as lignin and hemicellulose polysaccharides.

[0021] Cellulose nanofibers (hereinafter sometimes referred to as "CNF") are cellulose fibers that have been processed to be finer, and are aggregates of cellulose fibers with an average fiber diameter on the nano-order. When an electricity storage material contains CNF, the electrocapillary force due to the minus-first power quantum size effect can be enhanced, thereby increasing the electricity storage capacity.

[0022] In this specification, "CNF" refers to fine fibers with a fiber diameter of approximately 1 to 500 nm, which are obtained by pulp or other cellulose raw materials being refined to the nanometer level. The average fiber diameter and average fiber length of cellulose nanofibers can be obtained by averaging the fiber diameters and fiber lengths obtained from the observation of each fiber using an atomic force microscope (AFM) or a transmission electron microscope (TEM). The values ​​in the examples below are also values ​​measured using this method. Cellulose nanofibers can be obtained by applying mechanical force to cellulose raw materials such as pulp to refine them (defibration), or by defibrating chemically modified cellulose fibers obtained by chemically modifying cellulose raw materials, such as anion-modified cellulose fibers such as carboxylated cellulose fibers (hereinafter also referred to as "oxidized cellulose fibers"), carboxymethylated cellulose fibers, phosphate-esterified cellulose fibers, and phosphite-esterified cellulose fibers.

[0023] The average fiber diameter of CNF is usually 35 nm or less, preferably 30 nm or less, more preferably 25 nm or less, and even more preferably 20 nm or less. The lower limit is not particularly limited, but is, for example, 1 nm or more. The average fiber length (length-weighted average fiber length) is usually 50 to 2,000 nm, preferably 100 to 1,000 nm. The aspect ratio of CNF is usually 10 or more, preferably 50 or more. The upper limit is not particularly limited, but is usually 1,000 or less. The average fiber length and average fiber diameter of fine fibers can be determined by observing each fiber using an atomic force microscope (AFM) or a transmission electron microscope (TEM), and the average aspect ratio can be calculated using the formula: average aspect ratio = average fiber length / average fiber diameter. These values ​​can be adjusted by chemical modification treatment or defibration treatment.

[0024] The average aspect ratio of CNF is usually 50 or more, but is preferably 10 or less, and more preferably 5 or less. The average aspect ratio can be calculated using the following formula. Aspect ratio = average fiber length / average fiber diameter

[0025] [Cellulose raw materials] The type of cellulose raw material used as the raw material for CNF is not particularly limited, and cellulose derived from, for example, plants (e.g., wood, bamboo, hemp, jute, kenaf, agricultural waste, cloth, pulp (e.g., unbleached softwood kraft pulp (NUKP), bleached softwood kraft pulp (NBKP), unbleached hardwood kraft pulp (LUKP), bleached hardwood kraft pulp (LBKP), unbleached softwood sulfite pulp (NUSP), bleached softwood sulfite pulp (NBSP), thermomechanical pulp (TMP), recycled pulp, waste paper, etc.), animals (e.g., ascidians), algae, microorganisms (e.g., acetic acid bacteria (Acetobacter)), microbial products, etc., can be used. Cellulose fibers derived from plants or microorganisms are preferred, and cellulose fibers derived from plants are more preferred.

[0026] The average fiber diameter of the cellulose raw material is not particularly limited. In the case of softwood kraft pulp, which is a common pulp, the average fiber diameter is about 30 to 60 μm, and in the case of hardwood kraft pulp, the average fiber diameter is about 10 to 30 μm. In the case of other pulps, those that have undergone general refining have an average fiber diameter of about 50 μm. For example, when chips or the like several centimeters in size are refined, it is preferable to adjust the average fiber diameter to about 50 μm by mechanical processing using a disintegrator such as a refiner or beater.

[0027] [Denaturation treatment] Cellulose nanofibers may be unmodified or chemically modified. Examples of chemically modified cellulose nanofibers (chemically modified CNF) include anion-modified CNF and cation-modified CNF, with anion-modified CNF being preferred. Anion-modified CNF is a fine fiber obtained by defibrating anion-modified cellulose fibers, in which anionic groups have been introduced into the cellulose molecular chain, to nanoscale fiber diameters.

[0028] Examples of anion-modified cellulose fibers include carboxylated (oxidized) cellulose fibers, carboxymethylated cellulose fibers, phosphated cellulose fibers, and phosphite esterified cellulose fibers. By defibrating these, oxidized cellulose nanofibers, carboxymethylated cellulose nanofibers, phosphated cellulose nanofibers, and phosphite esterified cellulose nanofibers can be obtained, respectively. Among these, carboxylated (oxidized) cellulose nanofibers and carboxymethylated cellulose nanofibers are preferred.

[0029] Anionic groups are introduced into the cellulose raw material to produce anion-modified cellulose fibers. The method for introducing the anionic groups is not particularly limited, but examples include methods for introducing anionic groups into the pyranose rings of cellulose by oxidation or substitution reactions. Specific examples include reactions in which the hydroxyl groups of the pyranose rings are oxidized to convert them into carboxy groups, and reactions in which carboxymethyl groups, phosphate ester groups, or phosphorous ester groups are introduced into the pyranose rings by substitution reactions.

[0030] [Carboxymethylation] As mentioned above, carboxymethylation can be cited as an example of anion-modified cellulose fibers. Carboxymethylated cellulose fibers, which are an example of anion-modified cellulose fibers, may be obtained by carboxymethylating the above-mentioned cellulose raw material using a known method, or may be commercially available products. In either case, the degree of carboxymethyl substitution per anhydroglucose unit of the cellulose is 0.01 to 0.50, preferably 0.01 to 0.44, more preferably 0.02 to 0.40, and even more preferably 0.10 to 0.30. A carboxymethyl substitution degree of 0.50 or less can suppress solubility in media such as water, allowing the cellulose to maintain its fibrous shape. The degree of carboxymethyl substitution of carboxymethylated cellulose fibers is the same as the degree of carboxymethyl substitution of carboxymethylated cellulose nanofibers.

[0031] The degree of carboxymethyl substitution of carboxymethylated cellulose fiber can be measured by the following method. Specifically, approximately 2.0 g of bone-dry carboxymethylated cellulose fiber is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. 100 mL of nitric acid-methanol (a solution of 1000 mL of methanol and 100 mL of special-grade concentrated nitric acid) is added and the mixture is shaken for 3 hours to convert the salt form of carboxymethylated cellulose fiber (hereinafter also referred to as "carboxymethylated cellulose fiber") into acid-form carboxymethylated cellulose fiber. 1.5 to 2.0 g of acid-form carboxymethylated cellulose fiber (bone-dry) is weighed out and placed in a 300 mL Erlenmeyer flask with a stopper. The acid-form carboxymethylated cellulose fiber is moistened with 15 mL of 80% by weight methanol, 100 mL of 0.1 N NaOH is added, and the mixture is shaken at room temperature for 3 hours. Using phenolphthalein as an indicator, excess NaOH is back-titrated with 0.1 N H2SO4. The degree of carboxymethyl substitution (DS) is calculated using the following formula: A = [(100 × F' - (0.1N H2SO4) (mL) × F) × 0.1] / (bone-dry mass of acid-form carboxymethylated cellulose fiber (g)) DS=0.162×A / (1-0.058×A) (Here, A is the amount (mL) of 1N NaOH required to neutralize 1 g of acid-type carboxymethylated cellulose fiber, F is the factor of 0.1N H2SO4, and F' is the factor of 0.1N NaOH.)

[0032] The following method can be given as an example of a method for producing carboxymethylated cellulose fibers. Specifically, first, 3 to 20 times by weight of water and / or a lower alcohol (e.g., water, methanol, ethanol, N-propyl alcohol, isopropyl alcohol, N-butyl alcohol, isobutyl alcohol, tertiary butyl alcohol) is added as a solvent to the cellulose raw material, either alone or as a mixture of two or more of them. When a lower alcohol is mixed with the solvent, the mixing ratio of the lower alcohol is preferably 60 to 95% by mass. Then, 0.5 to 20 times by mole of an alkali metal hydroxide (e.g., sodium hydroxide, potassium hydroxide) is added as a mercerizing agent per anhydroglucose residue of the cellulose raw material. The cellulose raw material, solvent, and mercerizing agent are mixed, and mercerization is carried out at a reaction temperature of 0 to 70°C (preferably 10 to 60°C) for a reaction time of 15 minutes to 8 hours (preferably 30 minutes to 7 hours). Thereafter, a carboxymethylating agent (e.g., monochloroacetic acid or a salt thereof) is added in an amount of 0.05 to 10.0 times the amount of glucose residue in terms of moles, and an etherification reaction is carried out at a reaction temperature of 30 to 90°C (preferably 40 to 80°C) for a reaction time of 30 minutes to 10 hours (preferably 1 to 4 hours).

[0033] "Carboxymethylated cellulose fiber," a type of anion-modified cellulose used in the preparation of anion-modified CNF, maintains at least a portion of its fibrous shape when dispersed in water. Therefore, it is distinct from carboxymethyl cellulose, a type of water-soluble polymer. When observing an aqueous dispersion of "carboxymethylated cellulose fiber" with an electron microscope, fibrous material can be observed. On the other hand, when observing an aqueous dispersion of carboxymethyl cellulose, a type of water-soluble polymer, no fibrous material can be observed. Furthermore, when "carboxymethylated cellulose fiber" is measured by X-ray diffraction, peaks of cellulose type I crystals can be observed, whereas cellulose type I crystals are not observed in the water-soluble polymer carboxymethyl cellulose.

[0034] [Carboxylation (oxidation)] An example of anionic modification is carboxylation (also called oxidation). Carboxylation refers to a reaction in which the hydroxyl groups on the pyranose rings of cellulose are oxidized to carboxyl groups (-COOH (acid type) or -COOM (metal salt type) (M is a metal ion)). In this specification, anion-modified cellulose fibers obtained by carboxylation are also called carboxylated cellulose fibers or oxidized cellulose fibers. Carboxylated cellulose fibers can be obtained by carboxylating (oxidizing) the above-mentioned cellulose raw materials using known methods.

[0035] The amount of carboxy groups in carboxylated cellulose fibers is not particularly limited, but is preferably 0.6 to 3.0 mmol / g, and more preferably 1.0 to 2.0 mmol / g, based on the bone dry mass of the carboxylated cellulose fibers. The amount of carboxy groups can be adjusted by controlling the type and amount of oxidizing agent, the temperature and time of the oxidation reaction, etc. The amount of carboxy groups in carboxylated cellulose fibers is the same as the amount of carboxy groups in carboxylated cellulose nanofibers.

[0036] The amount of carboxyl groups in carboxylated cellulose fibers can be measured by the following method. First, 60 ml of a 0.5% by mass slurry of carboxylated cellulose fibers (medium: water) is prepared, and 0.1 M aqueous hydrochloric acid is added to adjust the pH to 2.5. A 0.05 N aqueous sodium hydroxide solution is added dropwise, and the electrical conductivity is measured until the pH reaches 11. The amount of carboxyl groups in carboxylated cellulose fibers is calculated using the following formula, based on the amount of sodium hydroxide (a) consumed during the neutralization stage of the weak acid, where the change in electrical conductivity is gradual. Amount of carboxyl groups [mmol / g carboxylated cellulose fiber] = a [ml] x 0.05 / mass of carboxylated cellulose fiber [g]

[0037] One example of a carboxylation (oxidation) method is a method in which a cellulose raw material is oxidized in water using an oxidizing agent in the presence of an N-oxyl compound and a compound selected from the group consisting of bromides, iodides, and mixtures thereof. This oxidation reaction selectively oxidizes the primary hydroxyl group at C6 of the glucopyranose ring on the surface of the cellulose, leaving aldehyde groups and carboxyl groups (-COOH) or carboxylate groups (-COO) on the surface. - The concentration of the cellulose raw material in water during the reaction is not particularly limited, but is preferably 5% by mass or less.

[0038] An N-oxyl compound refers to a compound capable of generating a nitroxy radical. Any compound can be used as the N-oxyl compound as long as it promotes the target oxidation reaction. Examples include 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and its derivatives (e.g., 4-hydroxyTEMPO). The amount of the N-oxyl compound used is not particularly limited, as long as it is a catalytic amount capable of oxidizing the cellulose raw material. For example, the amount is preferably 0.01 to 10 mmol, more preferably 0.01 to 1 mmol, and even more preferably 0.05 to 0.5 mmol, per 1 g of bone-dry cellulose raw material. The amount of the N-oxyl compound used is preferably about 0.1 to 4 mmol / L of the entire reaction solution.

[0039] Bromides are compounds containing bromine, examples of which include alkali metal bromides that can dissociate and ionize in water. Iodides are compounds containing iodine, examples of which include alkali metal iodides. The amount of bromide or iodide used can be selected within a range that can promote the oxidation reaction. The total amount of bromide and iodide is, for example, preferably 0.1 to 100 mmol, more preferably 0.1 to 10 mmol, and even more preferably 0.5 to 5 mmol, per 1 g of bone-dry cellulose raw material.

[0040] Known oxidizing agents can be used, such as halogens, hypohalous acids, halous acids, perhalogen acids or their salts, halogen oxides, and peroxides. Among these, sodium hypochlorite is preferred because it is inexpensive and environmentally friendly. The appropriate amount of oxidizing agent used is, for example, preferably 0.5 to 500 mmol, more preferably 0.5 to 50 mmol, even more preferably 1 to 25 mmol, and even more preferably 3 to 10 mmol, per 1 g of bone-dry cellulose raw material. Furthermore, for example, 1 to 40 mol is preferred per 1 mol of the N-oxyl compound.

[0041] The oxidation of cellulose raw materials proceeds efficiently even under relatively mild conditions. Therefore, the reaction temperature may be 4 to 40°C, or may be room temperature, about 15 to 30°C. As the reaction proceeds, carboxyl groups are generated on the cellulose chains, resulting in a decrease in the pH of the reaction solution. To efficiently proceed with the oxidation reaction, it is preferable to add an alkaline solution such as an aqueous sodium hydroxide solution to maintain the pH of the reaction solution at about 8 to 12, preferably about 10 to 11. Water is preferred as the medium for the reaction solution because it is easy to handle and does not easily cause side reactions. The reaction time for the oxidation reaction can be appropriately set depending on the degree of oxidation progression, and is usually 0.5 to 6 hours, for example, about 0.5 to 4 hours.

[0042] The oxidation reaction may be carried out in two stages. For example, the oxidized cellulose fiber obtained by filtration after the completion of the first stage reaction can be oxidized again under the same or different reaction conditions, allowing the oxidation to proceed efficiently without being inhibited by the salt by-produced in the first stage reaction.

[0043] Another example of a carboxylation (oxidation) method is a method in which cellulose raw materials are oxidized by contacting an ozone-containing gas with the raw material. This oxidation reaction oxidizes at least the hydroxyl groups at positions 2 and 6 of the glucopyranose ring to carboxyl groups, and decomposes the cellulose chain. The ozone concentration in the ozone-containing gas is 50 to 250 g / m 3is preferable, and 50 to 220 g / m 3 The amount of ozone added to the cellulose raw material is preferably 0.1 to 30 parts by mass, and more preferably 5 to 30 parts by mass, based on 100 parts by mass of the solids content of the cellulose raw material. The ozone treatment temperature is preferably 0 to 50°C, and more preferably 20 to 50°C. The ozone treatment time is not particularly limited, but is about 1 to 360 minutes, and preferably about 30 to 360 minutes. When the ozone treatment conditions are within these ranges, excessive oxidation and decomposition of cellulose can be prevented, and a good yield of oxidized cellulose fiber can be achieved.

[0044] After the ozone treatment, a further oxidation treatment may be carried out using an oxidizing agent. The oxidizing agent used in the further oxidation treatment is not particularly limited, but examples include chlorine compounds such as chlorine dioxide and sodium chlorite, oxygen, hydrogen peroxide, persulfuric acid, and peracetic acid. For example, the further oxidation treatment can be carried out by dissolving these oxidizing agents in water or a polar organic solvent such as alcohol to prepare an oxidizing agent solution, and then immersing the cellulose raw material in the solution.

[0045] [Esterification] An example of anionic modification is esterification. An example of esterification is the introduction of phosphate groups or phosphite groups into a cellulose raw material. In this specification, anionically modified cellulose fibers obtained by the introduction of phosphate groups are referred to as "phosphate-esterified cellulose fibers," and anionically modified cellulose fibers obtained by the introduction of phosphite groups are referred to as "phosphite-esterified cellulose fibers," and both are collectively referred to as "esterified cellulose fibers."

[0046] A method for producing phosphated cellulose fibers includes mixing a powder or aqueous solution of a compound having a phosphate group with a cellulose raw material or a slurry thereof. Examples of the compound having a phosphate group include phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, 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, and ammonium metaphosphate. These compounds may be used alone or in combination.

[0047] The proportion of the phosphate group-containing compound added to the cellulose raw material is preferably 0.1 to 500 parts by mass, more preferably 1 to 400 parts by mass, and even more preferably 2 to 200 parts by mass, calculated as phosphorus element per 100 parts by mass of the solid content of the cellulose raw material. The reaction temperature is preferably 0 to 95°C, more preferably 30 to 90°C. The reaction time is not particularly limited, but is approximately 1 to 600 minutes, more preferably 30 to 480 minutes. The resulting suspension of phosphated cellulose fibers is preferably dehydrated and then heat-treated at 100 to 170°C, from the viewpoint of suppressing cellulose hydrolysis. The degree of phosphate group substitution per glucose unit of the phosphated cellulose fibers is preferably 0.001 or more and less than 0.40.

[0048] One example of a method for producing phosphite-esterified cellulose fibers is to add an alkali metal ion-containing substance and an additive (A) consisting of at least one of phosphorous acids and metal phosphites (preferably sodium hydrogen phosphite) to a cellulose raw material or a slurry thereof, followed by heating to introduce phosphate ester groups containing inorganic cations into the cellulose fibers. It is more preferable to add an additive (B) consisting of at least one of urea and a urea derivative, followed by heating to introduce phosphate ester groups and carbamate groups containing inorganic cations into the cellulose fibers. Examples of alkali metal ion-containing substances that can be used include hydroxides, metal sulfates, metal nitrates, metal chlorides, metal phosphates, metal phosphites, and metal carbonates. However, metal phosphites that also serve as additive (A) are preferred, with sodium hydrogen phosphite being more preferred.

[0049] The additive (A) comprises at least one of phosphorous acids and metal phosphites. Examples of the additive (A) that can be used include phosphorous acid compounds such as phosphorous acid, sodium hydrogen phosphite, ammonium hydrogen phosphite, potassium hydrogen phosphite, sodium dihydrogen phosphite, sodium phosphite, lithium phosphite, potassium phosphite, magnesium phosphite, calcium phosphite, triethyl phosphite, triphenyl phosphite, and pyrophosphorous acid. These phosphorous acids and metal phosphites can be used alone or in combination. However, sodium hydrogen phosphite, which also contains alkali metal ions, is preferred. The amount of additive (A) added is preferably 1 to 10,000 g, more preferably 100 to 5,000 g, and even more preferably 300 to 1,500 g per kg of the cellulose raw material.

[0050] The additive (B) is composed of at least one of urea and a urea derivative. Examples of additive (B) that can be used include urea, thiourea, biuret, phenylurea, benzylurea, dimethylurea, diethylurea, and tetramethylurea. These ureas or urea derivatives can be used alone or in combination. However, urea is preferred. The amount of additive (B) added is preferably 0.01 to 100 mol, more preferably 0.2 to 20 mol, and even more preferably 0.5 to 10 mol, per mol of additive (A).

[0051] The reaction temperature is preferably 100 to 200°C, more preferably 100 to 180°C. The reaction time is not particularly limited, but is about 10 to 180 minutes, more preferably 30 to 120 minutes. The cellulose fibers into which phosphorous ester groups or the like have been introduced are preferably washed prior to defibration. The degree of substitution of phosphorous groups per glucose unit of the phosphite-esterified cellulose fibers is preferably 0.01 or more and less than 0.23.

[0052] [Defibrillation] Defibration can be carried out by applying a strong shear force to the cellulose fiber dispersion using, for example, a high-speed rotation type, colloid mill type, high-pressure type, roll mill type, or ultrasonic type device. For efficient defibration, it is preferable to use a wet high-pressure or ultra-high-pressure homogenizer that can apply a pressure of 50 MPa or more and a strong shear force. The pressure is more preferably 100 MPa or more, and even more preferably 140 MPa or more. The number of treatments (passes) in the defibration device may be one or two or more times, with two or more being preferred.

[0053] In the dispersion treatment, cellulose fibers are usually dispersed in a solvent. The solvent is not particularly limited as long as it can disperse cellulose fibers, but examples include water, organic solvents (e.g., hydrophilic organic solvents such as methanol), and mixtures thereof, with water being preferred. In the case of anion-modified cellulose fibers, the fibers are hydrophilic, so the solvent is preferably water.

[0054] The solids concentration of the cellulose fiber in the dispersion is usually 0.1% by weight or more, preferably 0.2% by weight or more, and more preferably 0.3% by weight or more. This ensures an appropriate liquid amount relative to the amount of cellulose fiber, which is efficient. The upper limit is usually 10% by weight or less, preferably 6% by weight or less. This allows fluidity to be maintained.

[0055] Furthermore, prior to defibration and dispersion using a high-pressure homogenizer, the cellulose fibers can be pretreated as needed. Pretreatment can be carried out using a high-speed shear mixer or other device for mixing, stirring, emulsifying, and dispersing.

[0056] The CNF obtained after defibration may be in the form of an aqueous dispersion immediately after defibration, or may undergo post-treatment as needed. Examples of post-treatment include reduced pressure treatment (e.g., using an aspirator), degassing treatment (e.g., using a magnetic stirrer or a stirring degassing device), drying (e.g., freeze drying, spray drying, tray drying, drum drying, belt drying, a method in which the CNF is thinly spread on a substrate such as a glass plate or plastic plate and then dried, fluidized bed drying, microwave drying, and a heated fan-type vacuum drying method), redispersion in water (the dispersion device is not limited), and pulverization (e.g., pulverization using equipment such as a cutter mill, hammer mill, pin mill, or jet mill). Stirring, degassing, and drying treatments are preferred, and drying treatment is more preferred. In the case of a method in which the CNF is thinly spread on a substrate and then dried, for example, a mold may be placed on the substrate as needed, and the dispersion is poured into the mold and dried. During drying, the mixture may be allowed to stand at a temperature of 35 to 50°C for 30 to 60 hours, for example.

[0057] The cellulose nanofibers are preferably crystalline amorphous fibers, and preferably have a large proportion of amorphous material with rotational freedom. The cellulose nanofibers are preferably granular or short-axis fibers arranged in a sheet. The fibers may also be amorphous fibers with atomic vacancies. Furthermore, the fibers preferably have irregularities on the molecular surface. The irregularity size is preferably 1 nm in diameter, more preferably 5 nm or more, and even more preferably 10 nm or more. The upper limit is preferably 35 nm or less, more preferably 30 nm or less, and even more preferably 25 nm or less. Therefore, the diameter is preferably 1 to 35 nm, more preferably 5 to 30 nm, and even more preferably 10 to 25 nm.

[0058] (Thickness) The thickness of the cellulose fiber layer is typically 100 μm or less, 50 μm or less, 40 μm or less, 30 μm or less, or 20 μm or less, preferably 10 μm or less or 5 μm or less, and more preferably 3 μm or less, 2 μm or less, or 1 μm or less. This allows for a lightweight device even in the case of a multi-layer laminate. Furthermore, since static electricity is attached and detached on the surface, forming the layer into a thin film allows for increased power density and energy density.

[0059] [1.2 Electrodes] Examples of electrode materials for the electrodes include metal electrode materials and polymer electrode materials. Examples of metal electrode materials include carbon (C), aluminum (Al), copper (Cu), gold (Au), silver (Ag), molybdenum (Mo), chromium (Cr), iron (Fe), zinc (Zn), titanium (Ti), nickel (Ni), lead (Pb), platinum (Pt), tungsten (W), bismuth (Bi), cobalt (Co), silicon wafers (SiO2), Ag alloys, Al alloys, Mo alloys, and Fe alloys (e.g., stainless steel), as well as metal oxides such as ZnO. Examples of polymer electrode materials include conductive resins such as polyacetylene, polythiophene, polyethylenedioxythiophene, polyacene, polyaniline, polypyrrole, polyphenylenevinylene, polyamine, polyferrocene, and polyphthalocyanine. Among metal electrode materials, Al and Cr are preferred in terms of good electrical conductivity, low specific gravity, economy, good corrosion resistance, and ease of bonding to cellulose materials. Among polymer electrode materials, polythiophene and polyethylenedioxythiophene are preferred in terms of good electrical conductivity, low specific gravity, and thin film properties. Each electrode may be made of the same electrode material or different electrode materials. When any electrode material is used, a flexible laminate can be obtained, and when a polymer electrode material is used, a more flexible and lightweight laminate can be obtained.

[0060] The thickness of the electrode is usually 0.01 μm to 500 μm, preferably 1 μm to 100 μm.

[0061] [1.3 Electric double layer] The energy storage material has an electric double layer between the surface of the cellulose nanofiber molecules and the electrode, which is composed of quasi-solid water.

[0062] The electric double layer will be explained using Figure 1 as an example. Figure 1 is a diagram that shows a schematic example of an electric double layer. As shown in the figure, the hydroxyl groups of countless CNF molecules combine with the electrode and adsorbed water at the electrode interface to form pseudo-solid water. The pseudo-solid water forms an electric double layer between the electrode, exerting a quantum size effect due to electrocapillary action and forming a parallel-connected electric distributed parameter circuit, enabling large-capacity storage and discharge.

[0063] [1.4 Other Layers] The electricity storage material is usually a laminate in which a CNF layer and an electrode layer are stacked in contact with each other, but may also contain other layers, such as a substrate.

[0064] The substrate is usually provided in contact with the electrode layer (the outermost electrode layer when two or more electrode layers are used). Examples of materials for the substrate include organic materials such as plastics, and inorganic materials such as silicon and glass.

[0065] [1.5 Shape and size of energy storage materials] The shape and size of the electricity storage material is preferably a thin film of 100 μm or less, more preferably 10 μm or less, in order to achieve a weight reduction effect. In this case, static electricity is attached and detached on the surface, so by making it into a thin film, the power density and energy density can be increased.

[0066] [1.6 Manufacturing method of energy storage materials] Electricity storage materials can be produced by laminating cellulose nanofibers and electrodes. Examples of lamination methods include humidifying cellulose nanofibers and then adhering an electrode material to one or both sides of the nanofibers. The humidification treatment can be carried out so that the moisture content of the cellulose fiber layer after humidification is preferably 2% or more, more preferably 3% or more, and even more preferably 4% or more (the upper limit is usually 10% or less). The lamination method is not particularly limited, and examples include a method of applying a dispersion containing cellulose fibers to an electrode substrate (e.g., electrophoresis, bar coating, spin coating, etc., electrophoresis), and a method of preparing a cellulose fiber sheet in advance and adhering it to the electrode material. The latter method is preferred because it allows for easy adjustment of the moisture content.

[0067] [2. Capacitor] The capacitor of the present invention comprises the above-described energy storage material. This allows for a pseudo-solid water electric double layer and high energy density. The capacitor comprises an energy storage material and a pair of external electrodes. The energy storage material is preferably in the form of a thin film sheet, with a pair of metal electrodes provided on both sides of the energy storage material to sandwich the energy storage material. This makes the capacitor equivalent to a distributed constant capacitor with multiple microcapacitors perpendicular to each metal electrode, corresponding to the number of cellulose molecules. In other words, the micromolecular surface itself exhibits high electrical resistance, which can be expressed as a parallel equivalent circuit of C and R. At least one energy storage material is sufficient, and a combination of two or more is preferred. In the case of a combination of two or more materials, for example, the electrode layer of one energy storage material and the CNF layer of another energy storage material are stacked so that they are in contact with each other. This further improves energy storage efficiency.

[0068] A pair of external electrodes are respectively disposed on the outside of the electricity storage material (or the laminate thereof if there are two or more). Examples of electrode materials constituting the external electrodes are the same as the examples of electrode materials for the electrode layers described above. An insulating layer may be provided between the external electrodes and the electricity storage material. Examples of insulating materials constituting the insulating layer include organic insulating materials (e.g., resins such as epoxy resins and polyimide resins, and rubber) and inorganic insulating materials (e.g., SiO2). Of these, organic insulating materials are preferred because they are easy to manufacture.

[0069] The method for manufacturing the capacitor of the present invention is not particularly limited, but a method using a microelectromechanical system (MEMS) is preferred. Specifically, the capacitor can be manufactured by forming external electrodes on the electricity storage material by a sputtering method or a casting method. In this case, it is preferable that each conductive electrode is closely attached to the surface of the electricity storage material along the shape of the surface. Furthermore, it is preferable that the quasi-solid water capacitor of the present invention can operate at temperatures between -269°C and 300°C.

[0070] When the capacitor according to the present invention includes a laminate in which a plurality of the above-described storage materials are stacked, the laminate can be manufactured by, for example, various MEMS methods. Furthermore, when a plurality of storage materials are used, they may be stacked in parallel. This allows a solid-state quantum direct capacitor in which each parallel equivalent circuit is electrically coupled in a distributed constant manner. The MEMS method can also be used to manufacture a capacitor in which the materials are stacked in parallel.

[0071] The quasi-solid water electric double layer capacitor of the present invention can be used, for example, as an AC capacitor in microelectronic circuits or as a capacitor on the backside of solar panels. It can also be used in various backup power supply modules, coupling elements, noise filters, high-sensitivity acceleration sensors, high-power transformer cutoff prevention devices, and electronic and electrical circuit boards for emergency power supply systems for automobiles or ships. It is also expected to be used as a direct DC storage material for solar energy in outer space in the future.

[0072] [3. Effect] The electricity storage material of the present invention contains cellulose nanofibers, which are cellulose fibers with nano-sized diameters, and can exhibit high energy density electricity storage properties by forming a quasi-solid electric double layer at the interface between the cellulose molecular surface and the electrode. The cellulose nanofibers have the molecular formula (CH 10 O5) n It is a polysaccharide cellulose molecule represented by the formula (I), and contains a large number of hydroxyl groups (-OH). The hydroxyl groups can easily bond with water at the electrode interface through hydrogen bonding to form an electric double layer of pseudo-solid water. This pseudo-solid electric double layer exhibits a physical electrocapillary phenomenon due to the quantum size effect, which allows the creation of a capacitor with high energy density and charge storage capacity. It is believed that the pseudo-solid water bonds to the molecular surface to form an electric double layer. The electric double layer can form a distributed constant circuit, which can be integrated in parallel to create a large-capacity capacitor. Therefore, the present invention overcomes the major weakness of conventional capacitors, their low energy density characteristics.

[0073] The amount of stored electricity is accumulated in proportion to the area, as shown in equation (1). The total amount of stored electricity, C, is the product of n nano-sized capacitors, c. Cellulose, which is polymerized β-glucose, has molecules that easily form sheets due to hydrogen bonding, making it ideal for integrating nanocapacitors, which are the basis of distributed constant circuits.

[0074]

number

[0075] The present invention will be described below with reference to examples. Note that the following examples are provided merely for the purpose of explaining the present invention and for reference of specific embodiments thereof, and are not intended to limit or restrict the scope of the present invention.

[0076] Example 1 <Sample 1> 500 g (bone dry) of bleached, unbeaten kraft pulp (85% brightness) derived from softwood was added to 500 ml of an aqueous solution containing 780 mg of TEMPO (Sigma-Aldrich) and 75.5 g of sodium bromide, and the mixture was stirred until the pulp was uniformly dispersed. An aqueous solution of sodium hypochlorite was added to the reaction system to a concentration of 6.0 mmol / g to initiate the oxidation reaction. The pH of the system decreased during the reaction, but was gradually adjusted to pH 10 by the addition of 3 M aqueous sodium hydroxide. The reaction was terminated when the sodium hypochlorite was consumed and the pH no longer changed. The reaction mixture was filtered through a glass filter to separate the pulp, which was then thoroughly washed with water to obtain oxidized pulp (hereinafter referred to as "TEMPO-oxidized pulp"). The pulp yield was 90%, and the oxidation reaction took 90 minutes.

[0077] The TEMPO-oxidized pulp obtained in the above process was adjusted to 3.0% (w / v) with water and subjected to defibration treatment five times using an ultra-high-pressure homogenizer (20°C, 150 MPa) to obtain a TEMPO-oxidized fine cellulose fiber dispersion (hereinafter referred to as "TEMPO-oxidized CNF"). The obtained TEMPO-oxidized CNF had an average fiber diameter of 4 nm and an aspect ratio of 150. The carboxyl group content of the obtained TEMPO-oxidized CNF was 1.42 mmol / g.

[0078] Ion-exchanged water was added to the obtained TEMPO-oxidized CNF 3% (w / v) dispersion, and the mixture was stirred with a homogenizer at 3000 rpm for 10 minutes, followed by dilution to a concentration of 0.5% (w / v). The dispersion was depressurized with an aspirator, and the dispersion was degassed with a magnetic stirrer at 200 rpm for 60 minutes, and then further degassed with a Mazerustar KK-300SS (Kurabo Industries, Ltd.) at 2000 rpm for 2 minutes. The hydrophilic polyethylene terephthalate film was then placed in a silicone rubber mold (100 cm). 2 ) was placed on the tray, 140 g of the dispersion was poured into it, and the mixture was dried at 40°C for 48 hours to obtain a TEMPO-oxidized CNF sheet.

[0079] Sample 1 was cut into a 1.2cm x 1.5cm piece and subjected to humidification treatment at room temperature and 90% humidity for 5 hours to achieve a moisture content of 5%. 20μm thick aluminum was mechanically attached to the upper and lower electrodes. Figure 2 shows the charge capacity. This result was obtained from the discharge curve obtained by charging at 2mA-10V for 50 seconds and then discharging at a constant 1μA. In Figure 2, "dry" refers to the sample before humidification treatment, and "wet" refers to the sample after the above humidification treatment.

[0080] Comparative Examples 1 and 2 Using CNF made from mechanically defibrated coniferous wood (average fiber diameter 37 nm) and bamboo (average fiber diameter 55 nm), the charge storage capacity of the samples was measured before and after humidification, as in Example 1 (Figure 2).

[0081] As shown in Figure 2, the charge storage capacity of the wet sample was approximately 20-400 times that of the dry sample. On the other hand, the charge storage capacity of wet Sample 1 was approximately 4.3 times that of wet mechanically defibrated coniferous wood and approximately 2.4 times that of wet mechanically defibrated bamboo. These results demonstrate that the charge storage material of the present invention is useful as a material for capacitors that can store electricity at high energy density. [Industrial Applicability]

[0082] The electricity storage material and capacitor of the present invention can be used, for example, as an AC capacitor in a microelectronic circuit or a capacitor on the back surface of a solar panel. They can also be used, for example, in various backup power supply modules for lightning arresters, welding, and over-discharge prevention, as well as in electronic and electrical circuit boards such as coupling elements, noise filters, high-sensitivity acceleration sensors, high-power transformer cutoff prevention devices, and emergency power supply devices for automobiles or ships.

Claims

1. An electricity storage material having cellulose nanofibers with an average fiber diameter of 35 nm or less and electrodes, and having an electric double layer composed of pseudo-solid water between the cellulose nanofiber molecular surface and the electrodes.

2. The electricity storage material according to claim 1, wherein the cellulose nanofibers are crystallized amorphous fibers.

3. The electricity storage material according to claim 1 or 2, wherein the cellulose nanofibers are amorphous fibers having atomic vacancies.

4. The electricity storage material according to claim 1 or 2, wherein the cellulose nanofibers have irregularities on the molecular surface with a diameter of 1 nm to 35 nm.

5. The electricity storage material according to claim 1 or 2, wherein the cellulose nanofibers are chemically modified cellulose nanofibers.

6. The electricity storage material according to claim 1 or 2, wherein the cellulose nanofibers are TEMPO-oxidized cellulose nanofibers.

7. The electricity storage material according to claim 1 or 2, which is in the form of a sheet.

8. A quasi-solid water electric double layer capacitor, which is a laminate having at least one electricity storage material according to claim 1 or 2 and a pair of external electrodes positioned on both sides thereof.

9. The quasi-solid water electric double layer capacitor according to claim 8 , wherein the stack comprises two or more storage materials.

Images