JP2002033249A - Activated carbon for electric double layer capacitors - Google Patents

Abstract

(57) [Summary] [Problem] A high output density per volume, and a small decrease in output density even if charge / discharge cycles are repeated under a large current or a constant voltage is continuously applied for a long time. An activated carbon suitable for an electric double layer capacitor is provided. The activated carbon is obtained by carbonizing coconut.
BET specific surface area of 2,000 m ² / g or more and 2500 m ² / g
Or less, the average pore diameter is 1.95 nm (19.5 °) or more and 2.20 nm (22.0 °) or less, and the pore volume between 50 ° and 300 ° calculated by the Cranston inclay method is 0. electric double layer activated carbon capacitor, wherein 05cm ³ / g or more 0.15 cm ³ / g or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to activated carbon for electric double layer capacitors. More specifically, the present invention relates to activated carbon for electric double layer capacitors, which has a high output density and is excellent in durability, using coconut as a raw material.

[0002]

2. Description of the Related Art In recent years, developments have been made in the automotive field to further improve fuel efficiency and further purify exhaust gas from the viewpoint of global environmental measures. As part of this development effort, technology development for hybrid vehicles and electric vehicles is underway. In connection with these technical developments, the practical use of electric double layer capacitors has attracted attention for applications such as drive system power assist or energy regeneration. Electric double layer capacitors are based on the principle that electric charge is stored in the electric double layer formed at the interface between the polarizable electrode and the electrolyte, and have a higher current than secondary batteries such as lead-acid batteries and nickel-metal hydride secondary batteries. The advantage is that rapid charging and discharging can be performed. Activated carbon is usually used as the material of the polarizable electrode because of its large interface and excellent conductivity. Electric double layer capacitors required for hybrid and electric vehicle applications have not only capacitance (F) but also high output (W) that can be repeatedly charged and discharged with a large current of the order of 100 amperes
Density is required and leaves many technical challenges. Therefore, in order to increase the output density, the thickness of the porous electrode mainly composed of activated carbon, the current collector, and the separator is optimized (JP-A-11-317332), or the composite electrode of activated carbon impregnated with aluminum is separated. Polar electrodes (Tokuheihei 10-5)
No. 09560) has been proposed.

On the other hand, the use of a carbonaceous substance typified by activated carbon as an electrode material of an electric double layer capacitor has
JP-B-60-15138, JP-A-63-1876
No. 14, JP-A-1-321620, JP-A-3
No. -180013, Japanese Patent Publication No. 6-56827,
This is disclosed in Japanese Patent Publication No. 4-44407, Japanese Patent Publication No. 4-70770, and the like. JP-A-1-321620 discloses that powdered activated carbon heat-treated at 1000 ° C. is used as an electrode material for a capacitor, and JP-A-3-180013 discloses that the content of oxygen in activated carbon is 25%. It is stated that the capacitance of the electric double layer capacitor can be improved by setting the content to ~ 35% by weight, but none of the publications describes the type and physical properties of the activated carbon used.

Japanese Patent Publication No. Hei 4-70770 discloses that using activated carbon having an average pore diameter of 1.5 nm (15 °) or more as an electrode material for a capacitor has excellent temperature characteristics, and Japanese Patent Application Laid-Open No. 63-187614. According to the publication, powdered activated carbon exhibiting high capacitance has a specific surface area of 1800 to 3500 m ^2.
/ G, the average pore diameter is 0.5 to 1.5 nm (5 to 15 mm), and the pore volume of 2.0 nm (20 mm) or more in the total pore volume is 20 to 40% of the total pore volume. However, these publications disclose an average pore diameter of 1.9 as in the present invention.
A specific range of 5 nm (19.5 °) or more and 2.20 nm (22.0 °) or less is selected, and the pore diameter is 5.0 nm (50 °).
The pore volume between 〜30.0 nm (300 °) is 0.05 cm ³ /
It does not disclose that the content is not less than g and not more than 0.15 cm ³ / g.

As for fibrous carbon, Japanese Patent Publication No. 60-15 / 1985
No. 138 discloses that fibrous carbon obtained by carbonizing a phenolic fiber and then performing an activation treatment is suitable as an electrode material for a capacitor. A carbon fiber or a carbon powder having a concentration of 1.0 meq / g or less is used as an electrode material for a capacitor, and a material having a specific surface area of 500 m ² / g or more is preferable. Specifically, a phenol resin-based activated carbon fiber is used. It is described. Further, Japanese Patent Publication No. 4-44407 describes that a material obtained by carbonizing a specific phenol resin foam is used as an electrode material for a capacitor. Activated carbon obtained by carbonizing a phenolic resin material described in JP-B-60-15138, JP-B-6-56827, JP-B4-44407 and the like, and then performing an activation treatment, Although the surface area is almost the same as that in the present invention, generally, the phenol resin-based carbonized material serving as an activation raw material is a uniform amorphous carbon, and thus, pores formed in activated carbon obtained by activating the same. Most have a pore diameter of 15 ° or less.
Therefore, as shown in Comparative Example 4 described later, the average pore diameter is usually about 18 ° or less, which is smaller than the average pore diameter in the present invention, and is in a relatively large pore region of 5.0 to 30.0 nm. It has no substantial pore distribution.

Moreover, the electrode materials for electric double layer capacitors described in these publications are intended to increase the capacitance per volume or per weight (F / g, F / cm ³ ). Therefore, in the charge / discharge test in the specific example, the discharge current density per unit area of the electrode is as small as about 0.1 to ² mA / cm ² , and the high current density is, for example, 10 mA / cm 2. ^No effect as an electrode material at a current density of ² or more has been confirmed at all, and they do not disclose any activated carbon suitable for the above-mentioned high-power use. Also,
The activated carbon carbon fiber woven fabric made from a phenolic resin described in Japanese Patent Publication No. 60-15138 is characterized in that its electric resistance is smaller than an electrode obtained by molding or coating powdered activated carbon. In that sense, it is preferable because high output can be expected. However, there is a drawback that the bulk density of the electrode is small, the output per weight is large, but the capacity per volume is small.

Accordingly, the electric double-layer capacitors using the activated carbon described in these publications for the electrode material have a relatively large capacitance, so that the energy density increases, but the output density per unit volume is necessarily large. I can't say. Furthermore, the characteristics required of the electric double layer capacitor are that it has higher durability such as a larger number of charge / discharge cycles that can be used and a smaller capacity drop during a continuous voltage application test, as compared with conventional secondary batteries. . In the electric double layer capacitor described in Japanese Patent Application Laid-Open No. Hei 3-180013, in which activated carbon containing a large amount of oxygen is used as an electrode material, a large capacity drop occurs due to an increase in resistance during charging and discharging, and therefore, the durability is poor. Has a big challenge. Even in the electrode material using the known activated carbon as described above, it is possible to reduce the internal resistance of the electrode by improving the electrode manufacturing method and increase the output density to some extent, but it has a high output density, It can be said that activated carbon for electric double layer capacitors having excellent durability has not been found yet.

[0008]

SUMMARY OF THE INVENTION The present invention has a high output density per unit volume, and has a high output density even when a charge / discharge cycle under a large current is repeated or a constant voltage is continuously applied for a long time. It is an object of the present invention to provide an activated carbon suitable for an electric double layer capacitor with a small decrease in density.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, activated carbon obtained by activating a specific raw material under appropriate conditions is converted into an appropriate fine carbon. It has pore distribution, specific surface area, surface condition, and electrochemical properties, so that the power density per volume is large,
In addition, they have found that they have excellent durability, and have reached the present invention.
That is, the gist of the present invention is an activated carbon obtained by carbonizing coconut, having a BET specific surface area of 2,000 m ² / g or more.
00m ² / g or less, and the average pore diameter is 1.95 nm (1
9.5 mm) or more and 2.20 nm (22.0 mm) or less and a pore diameter of 5.0 nm (5 mm) calculated by the Cranston inclay method.
0Å) to 30.0 nm (300Å) with a pore volume of 0.05
Activated carbon for electric double layer capacitors characterized in that the carbon content is not less than cm ³ / g and not more than 0.15 cm ³ / g.

As a preferred embodiment of the present invention, in the activated carbon for an electric double layer capacitor described above, the oxygen content per 1 g of activated carbon is 1 mg or more and 20 mg or less, and the natural potential of the counter electrode lithium in the non-aqueous electrolyte is 2 or less. .8
5 V or more and 3.03 V or less, and that the activated carbon is obtained by activating steam of coconut charcoal.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The activated carbon for an electric double layer capacitor of the present invention is activated carbon obtained by carbonizing coconut, and has a BET specific surface area of 200.
0 m ² / g or more and 2500 m ² / g or less, and the average pore diameter is 1.95 nm (19.5 °) or more and 2.20 nm (22.0 °).
And the pore diameter calculated by the Cranston inclay method is 5.0 nm (50 °) to 30.0 nm (300 °).
Pore volume of 0.05cm ³ / g or more 0.15cm ³ / g
It is essential to satisfy each of the following physical properties,
Preferably, it further satisfies physical properties in which the oxygen content per 1 g of activated carbon is 1 mg or more and 20 mg or less, and the natural potential at the counter electrode lithium in the nonaqueous electrolyte is 2.85 V or more and 3.03 V or less.

In the electric double layer capacitor using the activated carbon of the present invention satisfying these physical properties as a polarizable electrode material,
The bulk density of the activated carbon is high, and the ionic conductivity of the electrolyte ions and solvent molecules of the electrolyte solution present in the pores of the activated carbon is increased, and a sufficiently high output is exhibited even when charging and discharging with a large current. It is possible. The durability of the electric double layer capacitor is further improved by adjusting the oxygen content of the activated carbon to an appropriate amount and adjusting the natural potential of the activated carbon electrode in the non-aqueous electrolyte to an appropriate range. It is possible to do. Furthermore, the present invention can exhibit such excellent characteristics from activated carbon made from coconut husk, so that productivity and cost efficiency are higher than those made from expensive synthetic resin such as phenol resin. It has excellent characteristics.

In the activated carbon for an electric double layer capacitor of the present invention, if the specific surface area of the activated carbon is too large, the bulk density is reduced and the output per unit volume is reduced. Surface area is 200
0 m ² / g or more and 2500 m ² / g or less, preferably 2000 m ² / g or more and 2400 m ² / g or less, more preferably 2050 m ² / g or more and 225 or more.
0 m ² / g or less. If the specific surface area exceeds this range and is too small, the output per unit weight is undesirably reduced. The activated carbon for an electric double layer capacitor of the present invention has an average pore diameter of 1.95 nm (19.5 °) or more and 2.20 nm (22.20 nm).
0Å) It is mandatory to be less than or equal to. If the average pore diameter is too small, it is not suitable for high-output applications because the electrical resistance, which is thought to be due to the diffusion resistance of electrolytic ions in the pores during charging and discharging under a large current, is not suitable for high-output applications. Is not preferred because the bulk density decreases and the output per unit volume decreases. Preferably it is 2.00 nm (20.0 °) or more and 2.15 nm (21.5 °) or less, more preferably 2.
It is not less than 02 nm (20.2 °) and not more than 2.15 nm (21.5 °).

Further, the activated carbon for an electric double layer capacitor of the present invention has a pore volume between 5.0 nm (50 °) and 30.0 nm (300 °) calculated by the Cranston inlay method of 0.05 cm. ³ / g to 0.15 cm ³ / g is essential. If the pore volume is too large, the bulk density of the electrode is reduced, and the output per volume is undesirably reduced. On the other hand, if it is too small, the internal resistance of the electrode increases, and as a result, the output density decreases. Preferably 0.07-0.
13 cm ³ / g, and more preferably 0.08 to 0.5 cm ³ / g.
12 cm ³ / g.

The activated carbon for an electromagnetic double layer capacitor of the present invention essentially requires that the raw material be palm. Usually, the raw material of activated carbon other than palm is petroleum pitch, petroleum coke, fiber spun tar tar, synthetic polymer, phenol resin, furan resin, polyvinyl chloride resin, polyvinylidene chloride resin, polyimide resin, Various types are available, such as polyamide resin, liquid crystal polymer, plastic waste, and waste tire. For example, when phenol resin is used as a raw material of activated carbon, it is possible to obtain a resin having a specific surface area of 2000 m ² / g or more. However, it is difficult to adjust the average pore diameter and the pore volume between 5.0 nm (50 °) and 30.0 nm (300 °) to the specific ranges in the present invention.

The activated carbon of the present invention can be obtained by carbonizing coconut pods and then activating them. Activation methods are roughly classified into a gas activating method and a chemical activating method. In the gas activation method, chemical activation is chemical activation, whereas physical activation is also called physical activation, and the carbonized raw material is contact-reacted with steam, carbon dioxide, oxygen, and other oxidizing gases at high temperatures. This produces activated carbon. The chemical activation method is a method in which a raw material is uniformly impregnated with an activation chemical, heated in an inert gas atmosphere, and activated carbon is obtained by a dehydration and oxidation reaction of the chemical. The chemicals used include zinc chloride, phosphoric acid, sodium phosphate, calcium chloride, potassium sulfide, potassium hydroxide, sodium hydroxide, potassium carbonate, sodium carbonate, sodium sulfate, potassium sulfate, calcium carbonate and the like. The method for producing activated carbon is not particularly limited, and is not limited to the above method, as long as the generated activated carbon satisfies the above-mentioned characteristics. Among these activation methods, activated carbon obtained by the steam activation method is used for the durability of the electric double layer capacitor. The steam activation method is advantageous because it has excellent characteristics and low production cost. The activated carbon has various shapes such as a crushed shape, a granular shape, a granule, a fiber, a felt, a woven fabric, and a sheet shape, and any of them can be used in the present invention.

The activated carbon obtained by the steam activation method in the present invention is obtained by pulverizing and sizing coconut charcoal, and then carbonizing (dry-distilling) the coconut char in an inert atmosphere at 800 ° C. Not less than 1300 ° C., preferably 850 ° C.
Not less than 1200 ° C and more preferably not less than 900 ° C and 11
It can be obtained by heat treatment at 00 ° C. or lower in an inert gas such as nitrogen, argon, or combustion exhaust gas containing a steam gas atmosphere of 30% by volume or more and 100% by volume or less. The activated carbon before activation, or activated char, and the activated carbon obtained by the activation treatment are washed in an aqueous acid solution such as hydrochloric acid, nitric acid, and sulfuric acid to remove metal impurities, ash, and the like contained in carbon. Those removed are also included in the present invention. Activated carbon after activation treatment,
Heat treatment under an inert atmosphere of nitrogen, argon, helium, xenon, etc. at 500 to 2500 ° C, preferably 700 to 1500 ° C to remove unnecessary surface functional groups or develop carbon crystallinity to conduct electrons. May be increased. In the case of granular activated carbon, the average particle diameter is preferably 30 μm or less, more preferably 7 to 20 μm, from the viewpoint of improving the bulk density of the electrode and reducing the internal resistance.

The activated carbon for an electric double layer capacitor according to the present invention is characterized in that, in an electric double layer capacitor using a non-aqueous electrolytic solution, the natural potential in the electrolytic solution is 2.85 V when Li / Li ⁺ is used as a counter electrode. The voltage is preferably 3.03 V or less, and more preferably 2.90 to 3.00 V. If the natural potential is too large, for example, when 2.5 V or more is applied to an electric double layer capacitor assembled using activated carbon as a positive electrode, the potential after charging of the positive electrode becomes about 4.3 V (vs. Li / Li ⁺ ).
, And reaches the oxidative decomposition potential of the electrolytic solution (4.3 V or more). As a result, a decomposition reaction of the electrolytic solution occurs, and the durability of the electric double layer capacitor is reduced. Note that the natural potential is 2.8.
Those having a voltage lower than 5 V are not usually obtained in the above-mentioned production method. The measurement of the spontaneous potential of the carbonaceous electrode of the positive electrode in the present invention is performed using a usual electrochemical technique. In the measurement of the potential of a non-aqueous electrolyte, a potential reference such as a standard hydrogen electrode in an aqueous solution is not strictly defined, but actually, electrodes such as a silver-silver chloride electrode, a platinum electrode, and a lithium electrode are used. The method is generally widely used, and can be measured by the same method in the present invention.

Since the amount of oxygen contained in the activated carbon affects the durability of the electric double layer capacitor, it is preferable to adjust the oxygen content to an appropriate amount. In the present invention, the oxygen content per 1 g of activated carbon is preferably 1 mg or more and 20 mg or less, more preferably 2 mg or more and 10 mg or less. The oxygen content of the present invention is defined as 10 units of activated carbon in a vacuum or in an inert gas atmosphere such as argon gas or nitrogen gas.
Heat treatment is performed at around 00 ° C., and carbon monoxide (CO) and carbon dioxide (CO ₂ ) contained in the decomposition gas generated at that time are quantified, and the total amount of oxygen contained in these molecules is indicated. . Here, since the thermal decomposition temperature is around 1000 ° C., this oxygen amount corresponds to oxygen-containing functional groups in the activated carbon, that is, carboxyl groups, phenol groups, ketones, and the like. The oxygen content in the
It can be said that this is an index indicating the total amount of oxygen-containing functional groups contained in the activated carbon. If the oxygen content in 1 g of activated carbon exceeds the above range and is too large, the electric resistance due to gas generation presumed to be due to decomposition of oxygen-containing functional groups or reaction with the electrolytic solution in the cell during charging and discharging of the electric double layer capacitor. And the durability of the capacitor deteriorates, which is not preferable. On the other hand, if the amount is less than the above range, the affinity for the electrode binder decreases during the production of the electrode, and as a result, the bulk density of the electrode decreases.

The case where an electric double layer capacitor is formed by using the activated carbon of the present invention will be described below. The polarizable electrode mainly composed of activated carbon is formed by an ordinary method, and is mainly composed of activated carbon and a binder. However, a conductive substance may be further added to impart conductivity to the electrode. Activated carbon can be formed by a conventionally known method. For example, a molded article can be obtained by adding and mixing polytetrafluoroethylene to a mixture of activated carbon and acetylene black, followed by press molding. Also, after adding and mixing activated carbon with coal pitch having a relatively high softening point as a binder,
It is also possible to obtain a molded body by firing in an inert atmosphere to a temperature not lower than the thermal decomposition temperature of the binder. Furthermore, it is also possible to obtain a polarizable electrode by sintering only activated carbon without using a conductive agent and a binder. The electrode may be any of a thin coating film, a sheet-like or plate-like molded body, and a plate-like molded body made of a composite. Prior to mixing with the binder, the activated carbon can be pulverized into desired granules, but the activated carbon is mixed with the binder with a relatively large particle size and pulverized together to obtain a mixed slurry or a dry pulverized product. You can also.

As the conductive agent used for the activated carbon electrode,
At least one conductive agent selected from the group consisting of carbon black such as acetylene black and Ketjen black, natural graphite, thermally expanded graphite, carbon fiber, and metal fiber such as ruthenium oxide, titanium oxide, aluminum and nickel is preferable. Acetylene black and Ketjen black are particularly preferred in that the conductivity is effectively improved with a small amount, and the blending amount with activated carbon is different depending on the bulk density of the activated carbon. The amount of activated carbon is preferably 5 to 50%, particularly preferably about 10 to 30%.

Examples of the binder include polytetrafluoroethylene, polyvinylidene fluoride, carboxycellulose, methylcellulose, a fluoroolefin copolymer crosslinked polymer, polyvinyl alcohol, polyacrylic acid,
It is preferable to use at least one of polyimide, petroleum pitch, coal pitch, and phenol resin. The current collector may be electrochemically and chemically resistant to corrosion, and is not particularly limited.Examples of the positive electrode include stainless steel, aluminum, titanium, and tantalum.
For the negative electrode, stainless steel, nickel, aluminum, copper and the like are preferably used.

The electrolyte is preferably a non-aqueous electrolyte. Non-aqueous
As the solute of the electrolytic solution, R_FourN⁺, R _FourP⁺(However, R is
C_nH_{2n + 1}Alkyl group represented by: n = 1 to 4), trie
Quaternary onion represented by tyl methyl ammonium ion, etc.
Um cation and BF_Four ^-, PF₆ ^-, ClO_Four ^-, SbF₆ ^-Or CF_ThreeS
O_Three ^-Or Katio
A lithium salt whose lithium is a lithium ion is used. Lichiu
The salt used is LiBF_Four, LiClO_Four, LiPF₆,
LiSbF₆, LiAsF₆, LiCF_ThreeSO_Three, LiC
(CF_ThreeSO_Two)_Three, LiB (C₆H_Five)_Four, LiC_FourF₉SO
_Three, LiC₈F₁₇SO_Three, LiN (CF_ThreeSO_Two)_TwoChoose from
One or more substances are preferred. In particular, electrical conductivity,
From the viewpoint of qualitativeness and low cost, R
_FourN⁺(However, R is C_nH_{2n + 1}An alkyl group represented by: n
= 1-4) and triethylmethylammonium ion,
BF as an anion_Four ^-, PF₆ ^-, ClO_Four ^-, And SbF₆ ^-Set
Combined salts are preferred.

The solute concentration in these non-aqueous electrolytes is
To ensure that the characteristics of the double-layer capacitor can be fully derived,
It is preferably from 3 to 2.0 mol / liter, particularly preferably 0.7 mol / liter.
At concentrations between Torr and 1.9 mol / L, high electricity
It is preferable because conductivity is obtained. Especially at low temperature below -20 ℃
When charging and discharging, at a concentration of 2.0 mol / liter or more,
It is not preferable because the electric conductivity of the electrolytic solution decreases. 0.3 mol /
Less than 1 liter has low electrical conductivity both at room temperature and at low temperatures
Not very good. Tetraethylammonium as electrolyte
Umtetrafluoroborate (Et_FourNB _Four) Propylene
A carbonate solution is preferred, and Et_FourNB_FourAs the concentration of
Is preferably 0.5 to 1.0 mol / liter.

Although the solvent of the non-aqueous electrolyte is not particularly limited, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate,
Methyl ethyl carbonate, diethyl carbonate, sulfolane, methyl sulfolane, γ-butyrolactone, γ
-An organic solvent comprising at least one selected from valerolactone, N-methyloxazolidinone, dimethylsulfoxide and trimethylsulfoxide is preferred. One or more types selected from propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, sulfolane, methyl sulfolane, and γ-butyrolactone from the viewpoint of excellent electrochemical and chemical stability and electric conductivity. Is especially preferred. However, a high melting point solvent such as ethylene carbonate alone cannot be used because it becomes a solid at a low temperature and must be a mixed solvent of propylene carbonate and the like with a low melting point solvent. The water content in the non-aqueous electrolyte is preferably 200 ppm or less, more preferably 50 ppm or less so as to obtain a high withstand voltage.

[0026]

EXAMPLES Hereinafter, the present invention will be further described with reference to specific examples, but the present invention is not limited to the following examples unless it exceeds the gist of the present invention.

Examples 1 to 6 and Comparative Examples 1 to 5 Production examples of activated carbon and charcoal char (50 g) were placed in a rotary kiln through which a nitrogen gas containing steam was passed, and steam activation was performed. The activation temperature, the activation time, and the water vapor concentration in the nitrogen gas are as shown in Table 1 below. The obtained activator was washed in hydrochloric acid and then repeatedly washed with demineralized water. After washing, the activators were dried, and after drying, these activators were pulverized to obtain activated carbon powder having an average particle size of 10 to 20 μm (Examples 1 to 10).
6, Comparative Examples 1 and 3). In Comparative Example 2, pulverized bituminous coal was used instead of coconut char, and in Comparative Examples 4 and 5, a phenolic resin (novolak) cured product was carbonized at 700 ° C. in a nitrogen atmosphere. Activated carbon powder was produced under the same conditions as in Example 1 except that was used.

[0028]

[Table 1]

Measurement of Physical Properties of Activated Carbon BET specific surface area and pore diameter of the obtained activated carbon powder were 5.0.
The pore volume between nm (50 °) and 30.0 nm (300 °) and the total pore volume were measured using a Soapmatic 1800 type (manufactured by Fison) under the relative pressures at the liquid nitrogen temperature of the activated carbon powder. Was calculated from the adsorption isotherm of activated carbon obtained by measuring each equilibrium adsorption amount of nitrogen gas. The average pore diameter of the activated carbon powder is based on the assumption that the pore shape of the activated carbon is cylindrical,
The total pore volume obtained from the above nitrogen gas adsorption isotherm and B
It was calculated from the ET specific surface area. The oxygen content in activated carbon is
It was determined as follows. A reaction tube made of quartz glass containing about 1 g of activated carbon powder was heated to about 1000 ° C., and the gas generated at that time was injected into a gas chromatograph analyzer to quantify the CO and CO ₂ components in the generated gas. Quantified C
Calculate the sum of the oxygen content of each of O and CO ₂ and obtain 1 g of activated carbon.
The oxygen content per unit was calculated. Table 2 shows the physical property values of the activated carbon calculated as described above.

[0030]

[Table 2]

Test Example 1 (Evaluation of Capacitor Characteristics of Activated Carbon-1: Initial Performance) Activated carbon powder was used for each activated carbon obtained in the above-mentioned activated carbon production example.
After kneading a mixture consisting of 80% by weight, acetylene black 10% by weight, and polytetrafluoroethylene 10% by weight,
Using a tablet press (manufactured by JASCO Corporation), use a hydraulic press to create a diameter
Pressure molding was performed at a pressure of 50 kgf / cm ² so as to have a thickness of 10 mm and a thickness of 0.5 mm to obtain a disk-shaped molded body, which was used as a positive electrode molded body. In the same manner, one more molded body was produced, and this was used as a negative electrode molded body. The two molded bodies obtained were dried at 300 ° C. for 3 hours in a vacuum of 0.1 torr or less. After the two dried compacts were transferred into a dry box in a nitrogen gas atmosphere, the positive and negative electrode compacts were vacuum impregnated with a propylene carbonate solution of (C ₂ H ₅ ) ₄ NBF _{4 as} an electrolytic solution. Next, the positive electrode molded body and the negative electrode molded body impregnated with the electrolytic solution were opposed to each other via a polyethylene separator, and were then swaged and sealed in a stainless steel coin cell to obtain a coin cell type electric double layer capacitor.

After applying a voltage of 2.5 V to the obtained coin cell type electric double layer capacitor in a 25 ° C. constant temperature bath with a HJ-201 type charge / discharge test apparatus (manufactured by Hokuto Denko Corporation), a current density of 20 mA / The battery was discharged at a constant current of ² cm ² . The initial capacitance (F) was determined from the gradient between 2.5 V and 1.0 V of the obtained discharge curve. In addition, the voltage drop (I
R drop: IR-drop) was read from the discharge curve. IR
If the -drop is large, the energy density that can be obtained as a result is undesirably small. The results are shown in Table-3.

[0033]

[Table 3]

Test Example 2 (Evaluation of Capacitor Characteristics of Activated Carbon-2: Initial Performance) A mixture of 8 parts by weight of each activated carbon, 3 parts by weight of conductive carbon black, and 3 parts by weight of a cellulosic binder obtained in the production example of activated carbon was distilled. After adding water, these were kneaded to obtain a paste for electrode application. The obtained paste was applied to an etched aluminum foil and dried to obtain an electrode body having a activated carbon paste film thickness of 40 μm. Effective electrode area 7.07 cm × 7.07 cm (50 cm ² ) from the above electrode body
Were obtained and used as a positive electrode and a negative electrode, respectively. The positive electrode and the negative electrode were opposed to each other with the activated carbon electrode membrane inside through a cellulose-based separator to obtain an electrochemical device.
The device was sandwiched between glass plates, and the outside of the glass plate was sandwiched between stainless steel plates. The device was fixed with bolts and nuts to obtain an electric double layer capacitor device. The obtained capacitor element was heated and dried in vacuum to remove impurities. Next, a device was impregnated with a propylene carbonate solution of (C ₂ H ₅ ) ₄ NBF ₄ as an electrolytic solution to obtain an electric double layer capacitor.

The obtained electric double layer capacitor was discharged in a thermostat at 25 ° C. after applying 2.5 V using a commercially available charge / discharge tester. From the discharge curve, the capacitance (F / cm
³ ) The internal resistance (Ω) and the output density (W / cm ³ ) when discharging between 2.5 V and 1.5 V for 2 seconds were calculated. However, the capacitor characteristics per unit volume (F /
cm ³ , W / cm ³ ) were calculated by converting the capacitance (F) and output (W) obtained from the obtained discharge curve into the volume of the activated carbon electrode membrane. Table 4 shows the calculated capacitor characteristics.

[0036]

[Table 4]

Test Example 3 (Evaluation of Capacitor Characteristics of Activated Carbon-3: Durability) A durability test of an electric double layer capacitor was performed as follows. For each activated carbon obtained in the above activated carbon production example,
After kneading a mixture consisting of 80% by weight of activated carbon powder, 10% by weight of acetylene black, and 10% by weight of polytetrafluoroethylene, a tablet press (manufactured by JASCO Corporation) was used to make a 10 mm diameter and 0.5 mm thickness using a hydraulic press. 50kgf / cm ²
Pressure molding was carried out at a pressure of 5 ° C. to obtain a disk-shaped molded body, which was used as a positive electrode molded body. In the same manner, one more molded body is produced,
This was used as a molded negative electrode. 0.1 ton of the two molded bodies obtained
It was dried at 300 ° C. for 3 hours in a vacuum of orr or less. Dry 2
After transferring the molded bodies into a dry box in a nitrogen gas atmosphere, the molded bodies of the positive and negative electrodes were made of an electrolytic solution of (C ₂ H ₅ ) ₄
Was vacuum impregnated with a propylene carbonate solution of NBF _4. Next, the positive electrode molded body and the negative electrode molded body impregnated with the electrolytic solution were opposed to each other via a polyethylene separator, and were then swaged and sealed in a stainless steel coin cell to obtain a coin cell type electric double layer capacitor.

After applying a voltage of 2.8 V to the obtained coin cell type electric double layer capacitor in a constant temperature bath at 70 ° C. using a HJ-201 type charge / discharge test apparatus (manufactured by Hokuto Denko Corporation), a current density of 10 mA / The battery was discharged at a constant current of ² cm ² . The initial capacitance (F) was determined from the obtained discharge curve. Next, 2.
After continuously applying 8 V for 500 hours, the battery was discharged, and the capacitance (F) after the durability test was determined. Table 5 shows the change rate (%) of the capacitance after the durability test with respect to the initial stage.

[0039]

[Table 5]

Test Example 4 (Measurement of Natural Potential of Activated Carbon) A mixture consisting of 80% by weight of activated carbon powder, 10% by weight of acetylene black, and 10% by weight of polytetrafluoroethylene was kneaded with each activated carbon obtained in the production example of activated carbon. Thereafter, using a tablet press (manufactured by JASCO Corporation), a pressure was applied at a pressure of 50 kgf / cm ² by a hydraulic press to a diameter of 10 mm and a thickness of 0.5 mm to obtain a disc-shaped molded body. 300 ° C in a vacuum of 0.1torr or less
For 3 hours. Activated carbon electrode after drying and diameter 10mm
The metal lithium foil having a thickness of 0.5 mm punched into the above was opposed to each other via a polyethylene separator (manufactured by Mitsubishi Chemical Corporation), and then the activated carbon electrode and the metal lithium foil were sandwiched from outside with a platinum plate as a current collector. . Further collector, was sandwiched by Teflon plate with activated carbon electrodes, four bolt holes at two 5mm thick from the outermost to the separator is good contact, which 1 mol / liter concentration LiBF ₄ in Was immersed in a beaker containing a propylene carbonate solution. Next, the collector was connected between the activated carbon electrode side and the current collector on the metal lithium foil side via a potentiometer, and the natural potential of the activated carbon electrode was measured. Table 2 shows the natural potential (V vs Li / L ^{i +} ) when the counter electrode of each activated carbon electrode was lithium.

[0041]

By using activated carbon having a suitable pore distribution, specific surface area, surface state, and electrochemical characteristics from the palm of the present invention as a raw material, the output density per volume is large, and In addition, it is possible to provide an electric double layer capacitor in which a decrease in output density is small even when a charge / discharge cycle is repeated under a large current or a constant voltage is continuously applied for a long time.

Continued on the front page (72) Inventor Yoshitaka Takeda 1000 Kamoshita-cho, Aoba-ku, Yokohama-shi, Yokohama-shi Mitsubishi Chemical Corporation Yokohama Research Laboratory (72) Inventor Kazuyuki Toki 1-Toyota-cho, Toyota-shi, Aichi Prefecture Toyota Motor Corporation F term (reference) 4G046 HA01 HB05 HB07 HC09

Claims (3)

[Claims] 1. Activated carbon obtained by carbonizing coconut, having a BET specific surface area of at least 2,000 m ² / g and 2,500 m ^2.
/ G or less, an average pore diameter of 1.95 nm (19.5 °) or more and 2.20 nm (22.0 °) or less, and a pore diameter of 5.0 nm (50 °) calculated by the Cranston inclay method from 30 nm. The pore volume between 0 nm (300 °) is 0.05 cm ³ /
activated carbon for an electric double layer capacitor, wherein the activated carbon is not less than g and not more than 0.15 cm ³ / g. 2. The method according to claim 1, wherein the oxygen content per 1 g of activated carbon is 1 mg or more and 20 mg or less, and the natural potential of the counter electrode lithium in the non-aqueous electrolyte is 2.85 V or more and 3.03 V or less. Item 6. The activated carbon for an electric double layer capacitor according to Item 1. 3. The activated carbon for an electric double layer capacitor according to claim 1, wherein the activated carbon is obtained by activating steam of palm.