JPH09275042A - Activated charcoal for organic solvent-based electric double layer capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase electric double layer capacity by clarifying the elements governing static density of activated charcoal by baking vinyl chloride-based resin and then activating alkali. SOLUTION: At first, vinyl chloride-based resin is baked and carbonized, and it is desired to give a temperature of more than 2000 deg.C for the baking which is higher than the first stage weight decrease start temperature by heat analysis. This carbonized matter is activated with alkali and activated charcoal for electrode can be created but potassium hydroxide is best suited as an activating agent to be used for alkali activation. Also, activating temperature of 500 to 1000 deg.C and activating time of 1 to 20 hours are desired. The activated charcoal thus obtained is able to improve the weight static density of electrode when used in a capacitor if the mode of pore distribution is located between 10 and 20 angstrom.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to activated carbon used as an electrode of an electric double layer capacitor using an organic electrolytic solution.

[0002]

2. Description of the Related Art An electric double layer capacitor has a farad-class large capacity and excellent charge / discharge cycle characteristics, and is therefore used for applications such as backup power supplies for electronic equipment and in-vehicle batteries. Activated carbon with fine pores is used for the electrodes of electric double layer capacitors, but in order to make electric double layer capacitors smaller, lighter and larger in capacity, activated carbon with a large electrostatic density is used. It has been demanded. Therefore, various characteristics of activated carbon have been studied in order to obtain activated carbon having a large electrostatic density. For example, "the weight electrostatic density per activated carbon weight in the electrode and the specific surface area of the activated carbon are in a substantially linear proportional relationship. , And the electric double layer capacity on the activated carbon electrode is almost constant without being affected by carbon species and pore characteristics (Electrochemistry, 59 , No. 7, p. 607). -613,199
1).

However, since the electric double layer capacity observed at a mercury electrode or the like is about 20 μF / cm ² ,
The electric double layer capacity of activated carbon having a surface area of 0 m ² / cc should be theoretically 75 F / cc, whereas the actual electric double layer capacity is only about 13 F / cc. In addition, the electric double layer capacities may be completely different between activated carbons having the same specific surface area. Therefore, the present inventors have verified that the above hypothesis is based on the specific surface area measured by the BET method by nitrogen gas adsorption, which has a limit of analysis of about 10 Å. Therefore, a special image analysis that can analyze even fine pores of less than 10 angstroms was performed, and the relationship between the electrostatic density and the specific surface area was investigated for activated carbon for electrodes with different specific surface areas. It was concluded that there is no proportional relationship and that there are other factors that influence the capacitance. The results of testing the conventional activated carbon for electrodes in the same manner as in the following examples are shown in FIG.

[0004]

SUMMARY OF THE INVENTION The present invention has been made under such a technical background, and it has been sought to obtain a capacitor having a large electric double layer capacity by investigating the factors that influence the electrostatic density of activated carbon. To aim.

[0005]

The present invention provides an activated carbon for an organic solvent type electric double layer capacitor electrode, which is characterized in that a vinyl chloride resin is fired and then activated with an alkali.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION The starting material for the activated carbon of the present invention is vinyl chloride resin. In order to produce activated carbon from vinyl chloride resin, first, vinyl chloride resin is fired and carbonized.

Thermal analysis of vinyl chloride resins usually shows a two-step weight loss. The initial weight loss is due to the decomposition of the resin, where some carbon skeleton is formed. The second stage weight loss that appears next is chlorine, hydrogen,
This is due to detachment of side branches. For this reason, in the present invention, it is preferable that the firing temperature is not less than the weight reduction start temperature of the first step by thermal analysis and not more than 2,000 ° C. Furthermore, since the pores at the molecular level generated by the weight reduction in the second stage bring about an improvement in the capacitance of the electric double layer capacitor, the firing temperature is not less than the weight reduction start temperature and is 1.00.
More preferably, the temperature is 0 ° C. or lower. If the firing temperature exceeds 2,000 ° C, activation may be difficult to proceed.

As a firing method, a vinyl chloride resin is used.
Any heating rate may be applied up to the weight loss start temperature of the stage, but from the weight loss start temperature to the weight loss end temperature is 20 to 150 ° C / hour, preferably 40 to 100 ° C.
The temperature is preferably raised at a heating rate of / hour. Further, from the weight reduction end temperature to the firing temperature, 100 to 300 ° C.
It is preferable to raise the temperature at a heating rate of / hour, preferably 150 to 250 ° C / hour. The firing time is 0 to 2
4 hours are preferable, and 0.5 to 5 hours are particularly preferable. Here, the firing time is the time after the firing temperature is reached. The firing atmosphere is preferably an inert gas such as nitrogen gas or argon gas.

In the present invention, a charcoal-based material obtained from a vinyl chloride resin is activated with an alkali to form an activated carbon for an electrode. As the activator used for alkali activation, since it is easy to obtain pores having a pore diameter of 20 Å or less, salts or hydroxides of metal ions such as lithium, sodium and potassium are preferable, and potassium hydroxide is particularly preferable. It is suitable.

The alkali activation method can be carried out by an ordinary method. For example, after mixing a carbide and an activator,
A method performed by heating in an inert gas stream, a method of carrying an activator in advance on a vinyl chloride resin that is a material of a carbide and then heating it, a step of carbonizing and activating, a method of heating the carbide such as steam. A method of activating with a gas activation method and then surface-treating with an activator can be mentioned.

The carbide has a particle size of 0.1 to 300 μm, especially 1 to 100 before activation in order to uniformly activate the particles.
It is preferable to pulverize to μm. Particle size 0.1 μm
When it is less than 300 μm, the self-discharge characteristics are adversely affected when it is used as an electrode of a capacitor, while when it exceeds 300 μm, it may be difficult to uniformly activate particles.

When a monovalent base such as potassium hydroxide is used as the activator, the weight ratio of the charcoal to the activator is preferably 1: 1 to 4, more preferably 1: 1.
5 to 3 is more preferable, and 1: 1.8 to 2.2 is most preferable. The ratio of the activator to 1 part by weight of carbide is
If it is less than 1 part by weight, the activation will not proceed sufficiently, while
If it exceeds 4 parts by weight, the electrostatic capacity per volume may decrease.

The activation temperature is preferably 500 to 1,000 ° C., more preferably around 800 ° C. If the activation temperature is less than 500 ° C, the activation will not proceed and the capacitance will be small, while if it exceeds 1,000 ° C, the activation rate will be extremely lowered, which is not preferable.

The activation time is preferably 1 to 20 hours, more preferably 10 to 15 hours. When the activation time is less than 1 hour, the internal resistance when used as an electrode of the capacitor increases, while when it exceeds 20 hours, the electrostatic capacity per unit volume decreases.

The activated carbon thus obtained preferably has a pore size distribution having a mode value in the range of 10 to 20 angstroms. When the pore diameter is such a value, when it is used for a capacitor, an improvement in the weight electrostatic density of the electrode is recognized. This means that most of the pores of the activated carbon are in a state in which electrolytic ions that are solvated in the electrolytic solution are easily adsorbed, for example, have an appropriate size, and the proportion of the pores that contribute to the electric double layer is It is believed that this is due to the fact that it will be significantly larger. A conceptual diagram is shown in FIG.

Particularly, the activated carbon of the present invention preferably has a pore size distribution having a mode value near 13 angstroms. The mode value referred to here is the pore diameter showing the highest frequency among the relative frequencies. Also, 1
In the strict sense, "near 3 angstroms" does not necessarily mean that the pore diameter is 13 angstroms, but includes a range around 13 angstroms, for example, a range of about 13 ± 2 angstroms. Since the mode of the pore diameter is around 13 angstroms, the pores are suitable for the ionic diameter of currently known organic solvent-based electrolytic solution for electric double layer capacitors. That is, by having the smallest pore size that allows ions to be rapidly adsorbed,
The capacitance per volume becomes large. Therefore, if the number of pores of activated carbon having a pore diameter of less than 10 angstroms increases, the amount of adsorbed ions decreases and the capacitance decreases, while if the number of pores exceeding 20 angstroms increases, the volume of the activated carbon decreases. The capacitance per hit decreases.

The pore distribution of activated carbon is an image obtained by binarizing an image of a transmission electron microscope (hereinafter referred to as "TEM") (hereinafter referred to as "binarized image").
It is obtained by a method of performing an image analysis by a power spectrum obtained by the Fourier transform of the above (hereinafter, this method is referred to as a "TEM image analysis method").

According to the TEM image analysis method, it is possible to analyze even minute pores of less than 10 angstrom, which cannot be analyzed by the conventional nitrogen gas adsorption method, and it is possible to specify the shape of the pores. It will be possible.

The analysis of pores can be carried out by TEM as long as it is possible to analyze fine pores of less than 10 Å.
The method is not limited to image analysis. Made in this way,
The activated carbon for an electrode of the present invention is molded using a binder or the like and used as an organic solvent-based electric double layer capacitor electrode.

As the electrolyte used at this time, perchloric acid, hexafluorophosphoric acid, tetrafluoroboric acid, tetraalkylammonium salt of tetrafluoromethanesulfonic acid of trifluoroalkylsulfonic acid, amine salt, or the like is used. Can be mentioned. The electrolyte is propylene carbonate,
It is used in a state of being dissolved in a polar solvent such as butylene carbonate, γ-butyrolactone, acetonitrile, dimethylformamide, 1,2-dimethoxyethane, sulfolane and nitroethane. The concentration of the electrolytic solution is preferably 0.1 to 3 mol / liter, and particularly 0.5 to 1.5.
More preferably mol / l.

As the separator, a sheet of polyolefin such as polyethylene or polypropylene, polyester, PVDF or cellulose is used.

FIG. 17 shows an example of an organic solvent type electric double layer capacitor using the activated carbon for electrodes of the present invention.

[0023]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. The following parts and percentages are by weight unless otherwise specified.

Examples 1 to 5 Preparation of Activated Carbon Vinyl chloride resin (PVC) was fired in a nitrogen atmosphere at a firing temperature profile shown in FIG. 2 to obtain a carbide, and this carbide was crushed. To this charcoal, twice the weight of potassium hydroxide was added and mixed, and the charcoal was alkali-activated under the nitrogen gas convection under the activation conditions shown in Table 1 to obtain activated carbon for electrodes. After the furnace was cooled, the activated carbon was thoroughly washed with water to remove the alkali content and then dried. A thermogravimetric measurement chart of this vinyl chloride resin is shown in FIG.

With respect to the obtained activated carbon for electrodes, the pore distribution was measured by the TEM image analysis method and the specific surface area was measured by the nitrogen gas adsorption method. The pore distribution is shown in Figs.
The specific surface area is shown in FIG. 9 and Table 2. The pore distribution and the specific surface area were measured by the following methods.

(A) Measurement of pore distribution With a transmission electron microscope [CM120, manufactured by Phillips, Inc.] with an accelerating voltage of 120 kV, an image of activated carbon for an electrode having a magnification of 200,000 times was obtained and further magnified 30 times. Then, the original image was printed on photographic paper. Next, the original image is scanned with a high-resolution image scanner [AGFA, Stud
io Scan II Si] to obtain a binary image. The area of each pore expressed in the obtained binarized image, the vertical direction of the pore, and the image data in the horizontal direction were obtained, and these data were analyzed to determine the vertical direction of each pore. After specifying the size and shape in the direction and the lateral direction, the pores were classified by shape, and the pore diameters of all pores classified by the shape were statistically aggregated to express the pore distribution.

(B) Measurement of Specific Surface Area The specific surface area was measured by a BET single point method using a specific surface area measuring device [Flow Sorb II 2300 manufactured by Micromeritics].

Charge / Discharge Cycle Test On 81 parts of the obtained activated carbon for electrodes, 9 parts of Denka Black [granular manufactured by Denki Kagaku Kogyo KK] as a conductive material and a fluororesin [Mitsui DuPont Fluorochemicals Co. ), Teflon 6J] was added and mixed, and this was pressed to produce an electrode having a diameter of 20 mm. This electrode
A test cell was assembled using a PVDF sheet [JMWP manufactured by Nippon Millipore Corporation] as a separator and a propylene carbonate solution of tetraethylammonium tetrafluoride (1 mol / 1 liter) as an electrolyte. This battery cell was repeatedly charged and discharged at a charge end potential of 4 V, a discharge end potential of 0 V, and a charge / discharge current of 5 mA. The electrode density and capacitance of this electrode were measured. The results are shown in FIGS. 10 to 11 and Table 2.

Comparative Examples 1 to 4 Activated carbons for electrodes were prepared in the same manner as in Example 1 except that the materials shown in Table 1 other than vinyl chloride resin were used as starting materials for the activated carbons, and battery cells were prepared. Assembled The pore distribution of activated carbon for electrodes by TEM image analysis method is shown in FIGS.
5 and Table 2 show the specific surface areas by the nitrogen gas adsorption method. In addition, the electrode density and the capacitance after the charge / discharge cycle were measured. The results are shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

According to FIGS. 4 to 8 and Table 1, in all of the activated carbons for electrodes of Examples 1 to 5, the cumulative relative frequency of pore diameters of 10 to 20 angstroms is more than half of the whole, and around 13 angstroms. The resulting pore size distribution has a mode of. Therefore, it has been found that according to the present invention, activated carbon having a desired pore distribution can be easily obtained. Also,
According to Table 2, it was found that the electrodes using the activated carbon for electrodes of Examples 1 to 5 were excellent in electrostatic density, particularly in volume electrostatic density.

Further, according to FIG. 9, the specific surface area by the measuring method using nitrogen gas showed a correlation with the activation time in Examples 1 to 3 in which the activation time was up to 10 hours. Example 4 in which the time exceeds 10 hours
No correlation was observed in No. 5. On the other hand, according to FIG. 10, the density of the activated carbon for electrodes decreased as the activation time became longer. Therefore, as is clear from FIG. 11, it was found that the electrostatic capacity per unit volume exhibited a maximum value in Examples 3 to 4 in which the activation time was 10 to 15 hours. From this, it can be said that in the activated carbon for electrodes of Examples 3 to 4, the pores for adsorbing ions are opened without lowering the density.

On the other hand, a sufficient electrostatic density was not obtained except for the activated carbon for electrodes of Comparative Example 3 which showed a desired pore distribution. In addition, the activated carbon for an electrode of Comparative Example 3 is the same as that of Examples 1 to 5.
It was found that the weight electrostatic density was significantly larger than that of Examples 1 to 5 and was not so different from Examples 1 to 5, but the volume electrostatic density was significantly inferior to any of Examples 1 to 5.

Examples 6 to 11 Preparation of activated carbon A vinyl chloride resin (PVC) was used in a nitrogen stream under pressure of 60.
It was kept at 0 ° C. for 30 minutes to obtain a carbide. After crushing this carbide, 1.6 times weight of potassium hydroxide was added and mixed, put in an Inconel 601 crucible, and alkali activated under the condition of keeping for 4 hours at each temperature under nitrogen flow, and after washing , Activated carbon.

Charge / Discharge Cycle Test To 85 parts of the obtained activated carbon for electrodes, 10 parts of Denka Black [manufactured by Denki Kagaku Kogyo Co., Ltd.] as a conductive material, and a fluororesin [Mitsui DuPont Fluorochemicals Co. ), Teflon 6J] was added and mixed, and this was pressed to produce an electrode having a diameter of 20 mm. Methyl ethylpyrrolidinium tetrafluoroborate (MEPY / B)
A test cell was assembled using a propylene carbonate solution of F ₄ (1 mol / 1 liter) as an electrolytic solution.
In this cell, charge end potential; 3.5 V, discharge end potential;
Charging / discharging was performed at 0 V and a charging / discharging current of 5 mA, and the electrostatic capacity of the activated carbon in the electrode was measured. The results are shown in Fig. 16. As is clear from the above and FIG. 16, by optimizing the addition amount of potassium hydroxide and the activation time at the time of alkali activation,
It can be seen that the capacitance is greatly improved.

[0037]

EFFECTS OF THE INVENTION According to the present invention, it is possible to reliably obtain activated carbon for an electrode having excellent electrostatic density. Further, by performing activation under a constant activation condition, activated carbon for an electrode having a particularly excellent volume capacitance can be obtained, so that a capacitor having a high energy density can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the concept of how electrolytic ions are adsorbed by electrodes in an electrolyte.

FIG. 2 is a firing temperature profile of the vinyl chloride resin of the example.

FIG. 3 is a chart of thermogravimetric measurement of vinyl chloride resin used in Examples.

FIG. 4 is a chart showing the pore distribution of the activated carbon for electrodes of Example 1.

FIG. 5 is a chart showing the pore distribution of the activated carbon for electrodes of Example 2.

FIG. 6 is a chart showing the pore distribution of the activated carbon for electrodes of Example 3.

FIG. 7 is a chart showing the pore distribution of the activated carbon for electrodes of Example 4.

FIG. 8 is a chart showing the pore distribution of the activated carbon for electrodes of Example 5.

FIG. 9 is a chart showing the relationship between the specific surface area of activated carbon for electrodes of Examples 1 to 5 measured by a nitrogen gas adsorption method and the activation time.

FIG. 10 is a chart showing the relationship between volume electrode density and activation time of electrodes using the activated carbon for electrodes of Examples 1 to 5.

FIG. 11 is a chart showing the relationship between the weight electrode density and the activation time of the electrodes using the activated carbon for electrodes of Examples 1 to 5.

12 is a chart showing the pore distribution of activated carbon for electrodes of Comparative Example 1. FIG.

13 is a chart showing the pore distribution of activated carbon for electrodes of Comparative Example 2. FIG.

FIG. 14 is a chart showing the pore distribution of activated carbon for electrodes of Comparative Example 3.

FIG. 15 is a chart showing the pore distribution of activated carbon for electrodes of Comparative Example 4.

FIG. 16 is a chart showing the relationship between the activation temperature of electrodes using the activated carbon for electrodes of Examples 6 to 11 and the electrostatic capacity per volume of activated carbon.

FIG. 17 is a partial cross-sectional view structural diagram showing an example of an organic solvent-based electric double layer capacitor using the activated carbon for electrodes of the present invention.

FIG. 18 is a chart showing the relationship between electrostatic capacity and specific surface area of conventional activated carbon for electrodes.

Claims (4)

[Claims] 1. An activated carbon for an organic solvent-based electric double layer capacitor electrode, which is obtained by firing a vinyl chloride resin and then activating it with an alkali. 2. The activated carbon for an organic solvent-based electric double layer capacitor electrode according to claim 1, wherein the mode of the pore distribution of the activated carbon by the TEM image analysis method is in the range of 10 to 20 Å. 3. The mode of pore distribution of activated carbon measured by TEM image analysis is 13 ± 2 angstroms.
Activated carbon for an organic solvent-based electric double layer capacitor electrode as described. 4. A vinyl chloride resin is first analyzed by thermal analysis.
After calcination at the weight loss temperature of the second stage to 2,000 ° C., alkali activation is carried out using potassium hydroxide at an activation temperature of 50.
The activated carbon for an organic solvent type electric double layer capacitor electrode according to any one of claims 1 to 3, which is performed at 0 to 1,000 ° C for 1 to 20 hours.