JPH0385711A - Acrylonitrile system active carbon fiber for polarizing electrode material - Google Patents

Abstract

PURPOSE:To obtain a polarizing electrode which is suitable for an electric double-layer capacitor having a large capacitance and little self-discharge by a method wherein specific acrylonitrile system active carbon fiber is employed as polarizing electrode material. CONSTITUTION:The iron content of acrylonitrile system active carbon fiber, icluding a form of compound, must be less than 300ppm. If the iron content exceeds 300ppm, the tendency of self-discharge is increased and electric charge stored in a capacitor is rapidly reduced. Further, the total zinc content must be 500-4000ppm. If the zinc content is less than 500ppm, the electric resistance of the electrode material is increased and, if the zinc content exceeds 4000ppm, the increase of self-discharge can not neglected. Basically, the acrylonitrile system active carbon fiber is obtained by a known method wherein acrylonitrile system fiber is subjected to an oxidizing treatment in an oxidizing atmosphere and then subjected to an activating treatment in active gas.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention is an electric double layer connector that uses an electric double layer formed between a polarizable electrode and an electrolyte interface. When used as a polar electrode material, an electric double layer capacitor with a large electrostatic capacity and low self-discharge can be obtained by adjusting the contents of nitrogen, iron, and zinc.

(Conventional technology and new knowledge) Although electric double layer capacitors are capacitors, they have a large capacity similar to that of batteries, and demand has been increasing in recent years for applications such as bank-up power supplies for microcomputers.

The capacitance of an electric double layer capacitor basically increases in proportion to the surface area of the polarizable electrode, so in order to obtain a large capacitance, the polarizable electrode must have a high specific surface area and be It is common to use activated carbon with a

Conventionally, as activated carbon for polarizable electrode materials, carbon mu, especially activated carbon mH (ACF), has been used in the form of flutes, crosses,
It has been widely used in paper form or milled form.

When fibrous 8C is used as an electrode material, as disclosed in Japanese Patent Publication No. 60-15138,
Processability, utilization efficiency, and capacity per unit volume can be improved.

In addition, even when milled ACF is used, as disclosed in JP-A-62-179711,
Since ACF with a larger specific surface area can be used, an increase in capacity can be expected.

As described above, ACF is suitable as an electrode material, but in order to obtain electric double layer capacitors with higher capacitance and less self-discharge, many proposals have been made regarding its usage and electrode fabrication methods. Ta.

However, there has been no example focusing on the composition of ACF, which is the electrode material itself.

For example, since it is an electrical material, if it contains a conductive metal, it is naturally expected that the content will have a large effect on the electrical properties, but when referring to the metal content of 80F, Although there is Japanese Unexamined Patent Publication No. 59-172230, the purpose of proposing the coexistence of a certain metal salt during activation is to obtain an ACF with improved pore size distribution, and does not specify the composition of the ACF itself. It wasn't something to do.

In view of this, the present inventors investigated the increase in capacitance and the decrease in self-discharge of electric double layer capacitors from the viewpoint of the material of polarizable electrodes, and found that a specific acrylonitrile-based AC
We have discovered that F is extremely effective in improving these properties, and have arrived at the present invention based on this new finding.

(Object of the Invention) The present invention relates to an electrode material used for a polarizable electrode of an electric double layer capacitor that utilizes an electric double layer formed by a polarizable electrode and an electrolytic solution interface, particularly a material having a large capacitance. Another object of the present invention is to provide an electrode material for a polarizable electrode that causes less self-discharge.

[Structure of the invention]

The present invention consists of the following configuration.

An acrylonitrile activated carbon fiber for a polarizable electrode material of an electric double layer capacitor, characterized by having a nitrogen content of 4.5% by weight or less, an iron content of 3 ooppm or less, and a zinc content of 500 to 4000 ppm.

In the present invention, the above-mentioned specific acrylonitrile-based AC
It is necessary to be F.

Other carbonaceous materials cannot be used alone at least because they have different specific carbon-containing molecular structures and crystal structures that exhibit preferable electrical properties.

Acrylonitrile-based ACF has a nitrogen valve derived from the raw material acrylonitrile, and any content can be obtained within the range of 0 to 30% by weight, but for the purpose of the present invention, nitrogen content of 4% is preferable. It is necessary that the amount is .5% by weight or less.

As the nitrogen content becomes larger than this value, the capacitance when used as a polarizable electrode of an electric double layer capacitor rapidly decreases.

Acrylonitrile-based ACF can contain various metal compounds, but iron and zinc or their compounds are particularly noteworthy as they have the greatest influence on the electrical performance of electric double layer capacitors.

Among these, in the present invention, AC "with a total iron content of 300 ppIIILJ, including those in the form of compounds,
It needs to be below.

If the iron content exceeds this value, self-discharge increases5 and the amount of electricity held by the capacitor rapidly decreases over time, making it difficult to maintain the capacity within a range that is advantageous for practical use. .

Moreover, regarding the zinc content, it is necessary that the total amount of zinc contained in the ACF is 500 to 4000 ppm.

When the zinc content is less than 500 ppm, the electrical resistance of the electrode material increases, and the electrical resistance is sensitive to the zinc content and other impurities and easily fluctuates. When the zinc content exceeds 4000 ppm, the increase in self-discharge cannot be ignored.

When the zinc content is 500 to 4000 ppm, the electrode material stably has a low resistance that is acceptable for practical use, and the deterioration in discharge characteristics as seen with iron is relatively small.

Acrylonitrile AC used in the present invention is basically obtained by a known method of oxidizing acrylonitrile fibers in an oxidizing atmosphere and then activating them in an active gas.

-〇The raw material acrylonitrile fiber here is made of a polymer or copolymer containing at least 80% by weight of acrylonitrile in order to satisfy the nitrogen content of the present invention, and has an ash content of 08001~0° after removing excess metals. 100% by weight
These acrylonitrile fibers can be obtained by strengthening the purification of acrylonitrile and monomers and by strengthening the cleaning of the fibers after spinning.

The oxidation treatment of the acrylonitrile-based m fibers is carried out by heating the fibers at 200 to 400°C in an oxidizing atmosphere. Air, oxygen, hydrogen chloride, sulfur dioxide gas, or a mixture of these gases are used as the medium for the oxidizing atmosphere, but air or a mixture of air and nitrogen gas is used for the main binding because of economical efficiency and process stability. It is optimal from this point of view.

The method for activating the obtained oxidized fibers can be either a batch method or a continuous method, but a continuous method in which the oxidized fiber is continuously supplied into the activation furnace is preferable.

As the activating gas, steam, carbon dioxide, etc. are used, but it is preferable to use a mixed gas of carbon dioxide and/or nitrogen, which is mainly steam.

Activation temperature is 800-1100℃, especially 900-1000℃
°C is preferred.

The activation time varies depending on the activation humidity, but is preferably 1 to 120 minutes.

In the present invention, it is preferable to adjust the nitrogen content of the resulting acrylonitrile system to a required value by adjusting the activation temperature and activation time.

At that time, the specific surface area of ACF is 500 to 1200 m'/
It is preferable that it is in the range of g.

There are various methods to achieve the iron and zinc content specified in the present invention.

For example, there is a concern that the iron content may be excessive due to contamination from manufacturing equipment, so in each process of polymerization, spinning, flame resistance, and activation, restrictions are placed on the use of iron in equipment, and the use of corrosion-resistant materials. , painting, and sealing the process are effective measures.

In order to adjust the zinc content to the predetermined content of the present invention, Togane,
It is necessary to add zinc or a compound thereof to ACF or a precursor thereof, or a stock solution thereof at each step of spinning, flame resistance, and activation.

Examples of zinc compounds that can be added include salts such as zinc chloride, zinc hydroxide, zinc sulfate, and zinc nitrate, as well as zinc oxide, but it is recommended to add zinc compounds that are soluble in the stock solution at the precursor stock solution stage. This method is desirable from the viewpoint of obtaining ACF with a uniform composition.

If the iron or zinc content of the obtained ACF exceeds the value specified in the present invention, it is washed with hydrochloric acid in order to reduce these contents to the specified value.

Furthermore, it is also possible to add a carbonaceous material such as powdered charcoal to the above-mentioned ACF or its finely pulverized product, and to use the mixture well for the electrode material.

At that time, in order to ensure uniform mixing of both, the average particle size was 50
It is desirable that the particles be pulverized to a size of 15 μm or less, preferably 15 μm or less, and then mixed.

The carbonaceous feed such as powdered charcoal mentioned here includes charcoal, coconut shell charcoal, coal or its burned products, burned products of coal and petroleum pitch, or rayon fibers, phenolic fibers, pitch fibers, acrylonitrile fibers, etc. Carbon I obtained by firing
! Although it is possible to use miscellaneous or activated carbon IM fibers,
Its composition is less than 300 ppm iron content and 50 ppm zinc content.
0 to 4000 ppm is required, and the ash content is preferably 5% by weight or less.

In that case, the overall composition of the mixture of ACF and powdered coal is the same as in the case of ACF alone, with a nitrogen content of 4.5% by weight or less, an iron content of 300ppm or less, and a zinc content of 500 to 4%.
It is necessary that the content satisfies 000 ppm.

[Effects of the Invention] By using the ACF of the present invention, it is possible to obtain a polarizable electrode suitable for an electric double layer capacitor having a large capacity and little self-discharge. That is, increasing the capacitance by 0- is useful for downsizing the electric double layer capacitor, and reducing self-discharge is useful for improving reliability, etc., and this greatly facilitates the industrial application of the electric double layer capacitor.

[Examples and comparative examples]

Evaluation of the electrode material for an electric double layer capacitor in the present invention was performed by the following method.

That is, felt made of ACF was cut out to a size of 20 m x 2 cm, these two sheets were used as polarizable electrodes, a Teflon membrane filter (pore diameter 3 μm) was sandwiched between these two sheets, and both ends of the sheet were cut out. This was held down with a stainless wire mesh having a stainless steel wire as a lead wire, and placed in 30 m2 of a 1 mo Q/2 lithium perchlorate-propylene carbonate solution to prepare an electric double layer capacitor.

Examples of the present invention will be shown below. Note that these are examples, and the present invention is not limited to these examples.

Example 1 Zinc chloride was added to a copolymer composition consisting of 91.51% acrylonitrile, 8.0% by weight of methyl methacrylate, and 0.5% by weight of acrylic acid, spun, and spun using 2% hydrochloric acid. After washing, an acrylic fiber tow (single yarn Ili degree i, 5d) having an ash content o, oos weight %, and 540,000 denier was obtained. This tow was heated in air at 240°C for 2 hours, and then at 270°C for 0.5 hours at a free shrinkage rate of 70 to 80.
When the fibers were oxidized under a tension such that the amount of oxygen bonded was 18.5% by weight, an oxidized fiber with an oxygen bond amount of 18.5% by weight was obtained.

Furthermore, this oxidized fiber was activated at a temperature of 920°C and a furnace pressure of 0.0.
05 kg/cm' with activation gas (20/CO
'/N2=5/1/1).

At that time, five types of ACF)-U(A-E) with different nitrogen contents were obtained by changing the activation time.

During manufacturing, equipment with anti-corrosion measures was used after thorough purification to prevent contamination with impurities.

The obtained ACF has a fiber diameter of 5 to 14 μm and a density of 1.9 to 14 μm.
It was in the range of 2.1 g/cm'. After making felt from this ACF tow, it was made into an electric double layer capacitor according to the method described above, and its capacitance (F/+1>) was measured.

The capacitance is as shown in Table 1, and it is advantageous that the lower the nitrogen content, the larger the capacitance can be obtained even with a capacitor of the same structure.

Table 1 Example 2 Among the ACF felts obtained in Example 1, the sample (A) was immersed in a 5% by weight iron chloride aqueous solution (
F), immersed in 5% by weight zinc chloride aqueous solution [G],
A large amount of 10% by weight salt 3-hydrogen hydride aqueous solution was boiled to obtain the product.

The results of measuring the self-discharge rate of each sample are shown in Table 2 along with their iron and zinc contents.

Only the F felt that satisfies the iron and zinc content of the present invention has a low self-discharge (rate), retains its capacity even after charging for a long time, and is excellent in practicality.

Table 2 Here, the self-discharge rate is a value expressed by the following formula.

Example 3 Of the 〇F tow obtained in Example 1, (A) was used as a sample, and this was pulverized to 14-mm in a ball mill to obtain ACF powder with an average particle size of 3 μm.

Separately, coconut husk charcoal powder was prepared which had an ash content of 1.3% silver, an iron content of 210 ppm, a zinc content of 70 ppm, and an average particle size of 2.5 μm.

Add coconut husk charcoal powder to ACF powder at a ratio of 2:1,
An artificial latex made of polytetrafluoroethylene was added to the resulting mixture, stirred, dried, and then pressure-molded in a press, which was then used as a polarizable electrode.
An electric double layer capacitor was prepared by the method described above.

In the obtained electric double layer capacitor, the capacitance per unit mass of the carbonaceous mixture contained in the electrode was determined by the third method, along with the mixing ratio of ACF powder and coconut husk charcoal in the carbonaceous mixture, and its nitrogen content. Shown in the table.

From the results in Table 3, even if coconut husk charcoal is mixed with ACF powder, it can be used as an electrode material as long as it satisfies the above-mentioned specific composition.
It is clear that there is no inferiority in capacitance.

Table 1 As is clear from these tables, by using 80F of the present invention, it is possible to obtain a polarizable electrode suitable for electric double layer capacitors having a large capacity and little self-discharge.

Claims (1)

[Claims] Nitrogen content 4.5% by weight or less, iron content 300ppm
An acrylonitrile-based activated carbon fiber for a polarizable electrode material of an electric double layer capacitor, which has the following properties and a zinc content of 500 to 4000 ppm.