JP2006100798A - Electrical double layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrical double layer capacitor capable of raising a withstand voltage and an energy density and enlarging a capacitance. <P>SOLUTION: The electrical double layer capacitor according to this invention has a unit cell 10 filled up with electrolyte 18 between a pair of capacitor electrodes 16 furnishing a collector 12 with a polarizable electrode 14. It is characterized in that silk raw materials are calcined to the polarizable electrode 14 and are carbonized, and there is included a silk calcination body in which a large number of pores are formed on a front surface thereof by carrying out an activation treatment. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric double layer capacitor.

In the electric double layer capacitor, an activated carbon material is used as a polarizable electrode, and an organic electrolyte material or an aqueous electrolyte material is used as an electrolyte. The shape includes a coin type, a wound type, a laminated square type, and a laminated cylindrical type.
These electric double layer capacitors are used as backup power sources for portable devices as small power storage devices. In recent years, large capacitors have been developed for hybrid vehicles, fuel cell vehicles, and countermeasures against momentary power failure from the viewpoint of environmental issues. Has been. An electric double layer capacitor has a drawback that it has a higher power density but a lower energy density than a secondary battery.

This energy density (W) is expressed by W = 1/2 × C × V ² and is proportional to the square of the voltage (C is a capacity). Therefore, in order to increase the energy density, it is effective to increase the withstand voltage (V).
However, the withstand voltage of many electric double layer capacitors currently used is 3.0V or less. This is because the withstand voltage depends on the electrolysis of the electrolyte. For this reason, attempts have been made to improve the withstand voltage by improving the electrolytic solution (Japanese Patent Laid-Open No. 2002-110472). However, in theory, an electrolytic solution that should have a high withstand voltage causes electrolysis at a voltage lower than the theoretical value. This is considered to be because the reaction proceeds on the surface of activated carbon used as a current collector of the electric double layer capacitor.

Therefore, on the other hand, improvement of activated carbon is being promoted. In general, it is said that the withstand voltage can be increased by removing the influence of a functional group containing oxygen present on the surface of activated carbon (JP 2002-362912). However, this method requires a cumbersome process of removing the functional group containing oxygen.
JP2002-110472 JP 2002-362912 A

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an electric double layer capacitor that can increase the withstand voltage, increase the energy density, and increase the capacity.

An electric double layer capacitor according to the present invention is an electric double layer capacitor having a unit cell in which an electrolyte is filled between a pair of capacitor electrodes in which a polarizable electrode is attached to a current collector. It is characterized in that it includes a calcined silk body that has been fired and carbonized and that has been subjected to an activation treatment and has a large number of pores formed on its surface.
Further, the voltage-current characteristic in the cyclic voltammetry method has a characteristic that the current value becomes constant up to about 3.8V.
Further, the electrolytic solution is TEABF4 / PC.

The silk fired body is a silk fired body obtained by firing a silk material in a temperature range of 400 ° C. to 3000 ° C.
Further, the fired silk fired body is characterized in that a nitrogen component of 15 wt% or less remains in the combustion / melting elemental analysis.
The silk fired body is formed into a sheet shape by firing and activating a silk material having a woven, knitted or nonwoven shape.
The silk fired body is in a powder form.
Moreover, the polarizable electrode is characterized in that carbon fired powder or powdered silk fired body is mixed in the silk fired body and is hardened into a sheet with a binder.
The pore size distribution of the silk fired body is characterized in that there are peak distributions in micropores having a diameter of 2 nm or less and mesopores having a diameter of 3 to 6 nm.

  The electric double layer capacitor using the fired silk body as a polarizable electrode according to the present invention can increase the withstand voltage, increase the energy density, and increase the capacity as compared with the conventional product.

Hereinafter, an electric double layer capacitor according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is an explanatory diagram showing an example of a unit cell 10 of an electric double layer capacitor.
A current collector 12 is made of a thin conductive rubber. Reference numeral 14 denotes a polarizable electrode attached to the current collector 12. The current collector 12 and the polarizable electrode 14 constitute a capacitor electrode 16. A pair of capacitor electrodes 16 are arranged with their polarizable electrodes 14 facing each other, and an electrolyte solution 18 is filled between the pair of capacitor electrodes 16 to constitute the unit cell 10.
As the electrolytic solution 18, an organic electrolytic solution material or an aqueous electrolytic solution material is used.
In addition, 20 is a separator and 22 is a charger.
The unit cell 10 is sealed by a gasket made of an insulating resin, and a plurality of unit cells 10 are connected in series to constitute an electric double layer capacitor.

In the present invention, a fired silk body in which a silk material is fired and carbonized and a large number of pores are formed on the surface is used for the polarizable electrode 14.
Here, the silk material is a general term for woven fabrics, knitted fabrics, powders, cotton, yarns, and the like made of rabbits or wild silkworms. These are fired alone or in combination.
The firing temperature of the silk material is set at a high temperature of 400 ° C to 3000 ° C.
The firing atmosphere is performed in an inert gas atmosphere such as nitrogen gas or argon gas or in a vacuum to prevent the silk material from burning and ashing.
In addition, although a calcination temperature is not specifically limited, In order to activate a silk calcination body, the temperature range in 400 to 1400 degreeC is preferable.
Further, as described later, in order to leave a large amount of nitrogen component in the fired body, firing is preferably performed at a temperature of 1200 ° C. or less, particularly preferably 400 ° C. to 1000 ° C.

As for the firing conditions, rapid firing is avoided and firing is performed in a plurality of stages.
For example, in an inert gas atmosphere, the temperature is raised at a moderate heating rate of 100 ° C./hour, preferably 50 ° C./hour or less until the first firing temperature (for example, 400 ° C.). Hold for several hours and perform primary firing. Next, after cooling to room temperature, the temperature is gradually raised to a secondary firing temperature (for example, 700 ° C.) at a moderate temperature increase rate of 100 ° C. or less, preferably 50 ° C. or less per hour. The secondary firing is performed for several hours. Then it is cooled. Similarly, the third baking (for example, 2000 ° C. of final baking) is performed to obtain a silk fired body. The firing conditions are not limited to the above, and can be appropriately changed depending on the type of silk material, the desired function of the silk fired body, and the like.

  As described above, by performing baking in multiple stages, and by heating at a moderate temperature increase rate and baking, a protein in which dozens of amino acids are complicated with an amorphous structure and a crystalline structure. Abrupt decomposition of the higher order structure is avoided, and a black glossy flexible (flexible) silk fired body is obtained.

  By firing the silk material in this manner, a large amount of nitrogen component remains, and the presence of these functional groups facilitates the accumulation of electrons, thereby improving the capacitance of the capacitor. The nitrogen component remains more when the silk material is baked at a low temperature (about 400 to 1200 ° C.). However, on the other hand, when baked at a low temperature, the conductivity decreases. The decrease in conductivity can be adjusted by adding a conductive material such as carbon nanofiber or carbon black. Of course, when the silk material is baked at a high temperature, the conductivity is good, and it is not necessary to add these conductive materials.

Table 1 shows the results of combustion / melting type elemental analysis of a silk fired body obtained by firing rabbit cotton at 500 ° C. A large amount of nitrogen component remains at 13.7 wt%. If firing at a low temperature of about 400 ° C., a large amount of nitrogen component of 15.0 wt% or more remains.
Table 1

表３

Table 2 shows the results of combustion / melting type elemental analysis of a product obtained by activating silk fired body fired from rabbit cotton at 500 ° C. with steam at 750 ° C. Table 3 shows the results of combustion / melting type elemental analysis of a product obtained by activating silk fired body obtained by firing rabbit cotton at 500 ° C. with steam at 850 ° C.
Table 2

Table 3

In any case, the nitrogen component decreases by the activation treatment, but does not disappear. It is preferable that the nitrogen component remains about 15 wt% from the viewpoint of increasing the capacity of the capacitor, but it has been found that even if the remaining 1 wt% remains, it contributes to the increase in capacity.
Thus, in order for the nitrogen component to remain even after the activation treatment, it is preferable to set the firing temperature of the silk material to a temperature range of 400 ° C. to 1200 ° C. as described above.

Table 4 shows the results of combustion / melting type elemental analysis of a silk fired body obtained by firing rabbit cotton at 2000 ° C. The residual amount of nitrogen component became zero.
Table 4

In order to use the silk fired body for the polarizable electrode 14, it is necessary to increase the surface area so that a large amount of charge is charged. Therefore, the silk fired body is activated to form a large number of pores on the surface to increase the surface area.
The activation process of the silk fired body can be performed by exposing the silk fired body to high-temperature water vapor.
Alternatively, chemical activation such as KOH can be performed.
Alternatively, the silk fired body may be activated by microwave treatment. This microwave treatment is performed by irradiating the silk fired body with microwaves (frequency: 2.45 GHz) for several minutes. When irradiating microwaves, it is advisable to sandwich the carbon material with an unglazed plate or the like in order to prevent the carbon material from burning and ashing.

The pore diameter, depth, and density of the pores can be controlled to some extent by the firing conditions and activation treatment conditions of the silk fired body.
By subjecting the silk fired body to steam activation treatment at a temperature range of 700 to 1200 ° C., a large number of pores having a diameter of 10 nm or less can be formed, and a silk fired body having a surface area of 200 to 2000 m ² / g is obtained. Can charge a large amount of charge.
In order to obtain a silk fired body having a large surface area exceeding 2000 m ² / g, an activation treatment using a chemical such as KOH is preferably performed.
As described above, the activation treatment makes it possible to form a large number of pores on the surface of the silk fired body. The pores have micropores having a pore size distribution of 2 nm or less, and 3 It is preferable that the activation treatment is performed so that the peak distribution comes to the site of the mesopores of ˜6 nm.

By adjusting the peak value of the pore diameter to be in the above range, the charge can be reduced in the pores regardless of whether the electrolyte is an aqueous solution or an organic solution. Therefore, it is possible to charge a large amount of charge.
FIG. 2 and FIG. 3 show pore diameter distributions of pores when a fired silk body fired at 700 ° C. is activated with water vapor at 500 ° C. or higher for several hours. It can be seen that there is a distribution peak value around 3.8 nm.
2 and 3 show the pore size distribution, the abscissa indicates the pore radius, and the ordinate indicates the relative amount of the distribution.

The silk fired body, which has been activated as described above and has a large number of pores on the surface, is adjusted to the polarizable electrode 14 as follows.
As the silk fired body used for the polarizable electrode 14, it is preferable to use a silk fired body obtained by firing, activating, and forming a silk material in a woven, knitted or non-woven shape. is there. Since the silk fiber is extremely thin, for example, a silk fired body obtained by firing a woven silk material can be made as thin as about 100 μm to 40 μm. If this sheet-like silk fired body is subjected to compression processing in order to increase the bulk density, the thickness can be further reduced. This silk fired body can be formed into a sheet-like thin polarizable electrode 14 by hardening it with a binder such as PTFE (polytetrafluoroethylene) and further adding CMC (carboxymethylcellulose) or the like as necessary.

In order to obtain a large-capacity capacitor, it is important to have a high specific surface area and a large bulk density. Therefore, a silk fired body powder or activated carbon powder processed into a powder form is kneaded with a binder in a silk fired body. The density may be increased.
Alternatively, the fired silk powder processed into powder and carbon particles such as carbon nanofiber, activated carbon powder or carbon black may be kneaded with a binder to form the polarizable electrode 14 processed into a sheet.

The silk cotton was fired at 400 ° C. in a nitrogen atmosphere, pulverized with a ball mill, and then steam activated at 700 ° C. The fired silk powder (silk activated carbon) was hardened with a PTFE binder to form an electrode material, and this electrode material was also used to form a simple electric double layer capacitor shown in FIG. The electrolyte used was 1 mol / l TEABF ₄ / PC.
As a comparative example, an electric double layer capacitor was formed under the same conditions as in the example except that phenol resin activated carbon was used as an electrode material.

The gas generation state by cyclic voltammetry of the example product (silk activated carbon) and the comparative example product (phenol resin activated carbon) was compared. FIG. 4 is a cyclic voltammogram thereof. The scanning speed was 1 mV / s.
As is apparent from FIG. 4, when the potential is increased, the current value gradually increases in the comparative example product, whereas the current value is substantially flat up to about 3.8 V in the example product. This is considered to be because the reaction of the comparative example product occurred at an early stage on the surface of the electrode material, whereas the electrode material surface of the example product was very stable. The gas generation state was 3.0 V or more, and gas was generated in the comparative example product, whereas almost no gas was generated in the example product. For this reason, the example product can be used stably even at a voltage of 3.0 V or more. That is, the withstand voltage could be increased to around 3.8 V, and the energy density could be greatly improved.

FIG. 5 shows a graph comparing the volume capacities of the comparative product and the example product. The charging current density was 5 mA / cm ² and the discharging current density was 1 mA / cm ² .
As is apparent from FIG. 5, the capacity of the example product is increased compared to the comparative example product.

It is explanatory drawing which shows an example of the unit cell of an electric double layer capacitor. It is a graph which shows the hole diameter distribution of a pore. It is a graph which shows the hole diameter distribution of a pore. It is a cyclic voltammogram of a comparative example product and an example product. It is the graph which compared the volume capacity of the comparative example goods and the Example goods.

Explanation of symbols

10 Unit Cell 12 Current Collector 14 Polarized Electrode 16 Capacitor Electrode 18 Electrolyte 20 Separator 22 Charger

Claims (9)

In an electric double layer capacitor having a unit cell filled with an electrolyte between a pair of capacitor electrodes in which a polarizable electrode is attached to a current collector,
An electric double layer capacitor comprising the polarizable electrode including a silk fired body in which a silk material is fired and carbonized and activated to have a large number of pores formed on the surface.   2. The electric double layer capacitor according to claim 1, wherein the current value is constant up to about 3.8 V in the voltage-current characteristics in the cyclic voltammetry method.   3. The electric double layer capacitor according to claim 1, wherein the electrolytic solution is TEABF4 / PC.   The electric double layer capacitor according to claim 1, wherein the silk fired body is a silk fired body obtained by firing a silk material in a temperature range of 400 ° C. to 3000 ° C. 5.   The electric double layer capacitor according to any one of claims 1 to 4, wherein the fired silk fired body has a nitrogen component of 15 wt% or less remaining in combustion / melting elemental analysis.   6. The electric double layer capacitor according to claim 1, wherein the silk fired body is formed into a sheet shape by firing and activating a silk material having a woven, knitted or nonwoven fabric shape.   The electric double layer capacitor according to claim 1, wherein the silk fired body is in a powder form.   8. The polarizable electrode according to any one of claims 1 to 7, wherein carbon fired powder or powdered silk fired body is mixed in the fired silk fired body, and the polarizable electrode is hardened into a sheet with a binder. 2. The electric double layer capacitor according to item 1. The electric double layer capacitor according to any one of claims 1 to 8, wherein the pore diameter distribution of the silk fired body has a peak distribution in micropores having a diameter of 2 nm or less and mesopores having a diameter of 3 to 6 nm.

Images