JP2003224038A - Electrode material for electric double layer capacitor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode material of high capacitance electric double layer capacitor. <P>SOLUTION: The electrode material comprises a carbon material obtained by carbonizing and activating cellulose fibers adsorbing copper ions. Preferably, the cellulose fibers are cotton fibers and adsorption quantity of copper ions is not smaller than 0.3 mass% for the mass of cellulose fibers. Preferably, thin holes formed on the surface of the carbon material satisfy the following conditions (i)-(iv) between the radius (Rp) and the volume (Vp). (i) In the range where the Rp is smaller than 0.5 nm, (dVp/dRp) has a maximum value. (ii) In the range of 5≤Rp≤1.0 nm, (dVp/dRp) has a maximum value. (iii) A first peak has a value larger than that of a second peak. (iv) The second peak is not smaller than 580 mm<SP>3</SP>/nm.g. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrode material for an electric double layer capacitor made of a carbon material. In particular, the present invention relates to an electrode material for an electric double layer capacitor, which has specificity in the pore radius distribution of a carbon material and as a result can realize a high capacitance.

[0002]

2. Description of the Related Art An electric double layer capacity is composed of an electrode material constituting a positive electrode and a negative electrode, a separator inserted between the positive electrode and the negative electrode, and an electrolytic solution which freely moves in the electrode material and the separator. There is. Then, by applying a voltage between the positive electrode and the negative electrode, the anions in the electrolytic solution move to the positive electrode and the cations move to the negative electrode, and the opposite charges of each ion are stored in the positive electrode and the negative electrode, thereby forming an electric double layer capacitor. It works.

Conventionally, activated carbon has been used as an electrode material for electric double layer capacitors. The reason why activated carbon is used is that activated carbon has innumerable pores on its surface and its specific surface area is extremely large. That is,
Since electric charges are stored on the surface of activated carbon, the larger the surface area, the larger the amount of storage, and the electric double layer capacitor having a high electrostatic capacitance can be obtained.

However, even when activated carbon having a large specific surface area is used, the electrostatic capacity corresponding to the specific surface area may not be obtained. The reason for this is that there are various sizes (pore diameters) of pores on the surface of activated carbon,
This is because the cations or anions in the electrolytic solution may not easily enter the small-sized pores, and the surface inside the pores is not used for storing electric charges. In order to solve such a problem, it is considered that the size of the pores formed in the activated carbon should be increased as a whole, but then, the specific surface area of the activated carbon will be reduced and the high capacitance This means that the electric double layer capacitor can not be obtained.

For this reason, it has been proposed to obtain an electric double layer capacitor having a high electrostatic capacity by preventing the total specific surface area from decreasing, and relatively increasing the pore size in a slightly larger specific range. (International publication number WO00 / 11
688 publication). That is, the number of pores having a pore diameter of 1 nm (0.5 nm when converted to a pore radius) is reduced, and the pore volume in the range of pore diameters of 2.0 to 11 nm is set to 2.0.
An electrode material made of activated carbon having a pore volume in the range of up to 50 nm of 15% or more has been proposed. Such activated carbon is obtained by adsorbing a transition metal such as copper, iron, cobalt, or nickel onto a carbon material precursor such as coconut shell, coke, charcoal, resin, or bamboo, and then carbonizing and activating the precursor. Is what you get.

[0006]

Under these technical circumstances, the present inventors have found that the international publication number WO00 / 11
Cellulose fibers, which are not used in the technology described in Japanese Patent No. 688 (hereinafter, referred to as “the technology”), adsorb specific transition metal ions, that is, copper ions, to carbonize and activate the cellulose fibers. I made him
What had a pore size distribution different from that obtained by the above technique was obtained. That is, a carbon material having the largest number of pores having a pore radius of less than 0.5 nm was obtained. It was found that this carbon material can be used as an electrode material having a high capacitance, contrary to the theory described in the above-mentioned technology. The present invention is based on such knowledge.

[0007]

Means for Solving the Problems That is, the present invention provides an electric double layer comprising a carbon material obtained by adsorbing copper ions on cellulose fibers, heating the carbon fibers to carbonize them, and then activating them. The present invention relates to a capacitor electrode material. In addition, in this carbon material, the pore radius (Rp) and the pore volume (Vp) generally have the following (i) to
It satisfies each condition of (iv). (I) In the range where Rp is less than 0.5 nm, (dVp / dRp) has a maximum value (hereinafter, this maximum value is referred to as "first peak value"), and (ii) Rp is 0. 5 ≦ Rp <1.
In the range of 0 nm, (dVp / dRp) has a maximum value (hereinafter, this maximum value is referred to as "second peak value").
Say. ), (Iii) the first peak value has a larger value than the second peak value, and (iv) the second peak value is
It satisfies all the conditions of 580 mm ³ / nm · g or more.

First, in the present invention, copper ions are adsorbed on the cellulose fibers. As the cellulose fibers, natural cellulose fibers such as cotton fibers and hemp fibers, regenerated cellulose fibers such as rayon fibers and polynosic fibers, or solvent-spun cellulose fibers such as Lyocell can be used. In particular, it is preferable to use natural cellulose fiber,
Most preferably, cotton fibers are used. When coconut shell, coal, wood, resin or bamboo used in the above-mentioned technique is used instead of the cellulose fiber, the number of pores having a pore radius of less than 0.5 nm is the most, and the second peak of the present invention is the same. It becomes difficult to obtain the existing carbon material.

[0009] In order to adsorb copper ions to the cellulose fibers, the cellulose fibers may be dipped in an aqueous copper solution. As the copper aqueous solution, an aqueous solution containing copper ions such as a copper sulfate aqueous solution or a copper chloride aqueous solution is used. The concentration of the copper solution is
The amount of copper ions to be adsorbed on the cellulose fibers can be appropriately determined according to the amount of adsorption, but generally the copper ion concentration is 2 to 20 g /
It is preferably about 1 and most preferably about 3 to 17 g / l. The amount of the aqueous copper solution applied to the cellulose fibers is arbitrary, but it is not preferable that the amount is an excessive amount that cannot hold the aqueous copper solution. Generally, after immersing in the aqueous copper solution and squeezing the cellulose fibers, It is preferable to apply about 60% by mass of the copper aqueous solution to the mass. In the case of immersing in a copper aqueous solution, the cellulose fibers may be in a lump form or in a sheet form such as a knitted woven fabric or a non-woven fabric.

A predetermined amount of aqueous copper solution is applied to the cellulose fibers and then dried. As a result, water in the copper aqueous solution evaporates, and copper ions are adsorbed on the cellulose fibers. For drying, the cellulose fiber to which the aqueous copper solution has been applied may be introduced into a dryer such as a warm air dryer or an infrared dryer. The amount of copper ions adsorbed on the cellulose fibers is arbitrary, but is generally preferably 0.3% by mass or more. When the copper ion content is less than 0.3% by mass, the number of pores having a pore radius of 0.5 nm or more and less than 1.0 nm is small on the surface of the obtained carbon material, and an electrode material having a high capacitance is obtained. It becomes difficult to obtain. Further, when other metal ions such as iron ions, cobalt ions, and nickel ions are adsorbed in place of the copper ions, the number of pores having a pore radius of less than 0.5 nm becomes small, and high staticity is achieved. It is not preferable because it is difficult to obtain an electrode material having a capacitance.

After copper ions are adsorbed on the cellulose fibers, the cellulose fibers are carbonized. To carbonize cellulosic fibers, generally 250 to 1 under an inert atmosphere.
It is performed by heating to about 100 ° C. Here, the term "inert atmosphere" means an atmosphere in which the inflow of air is blocked, and it is sufficient if the cellulose fibers are not burned and disappear. By the heat treatment, hydrogen atoms and oxygen atoms are removed from the cellulose molecule, and a carbonized cellulose fiber composed almost entirely of carbon atoms is obtained.

After carbonizing the cellulose fibers, activation is carried out. The activation is generally 600 to 600 while supplying steam.
It is performed by heating to a temperature near 1100 ° C. By this activation, water vapor captures the carbon atoms of the carbonized cellulose fiber, and the carbon atoms are removed. Therefore, the removed portions become pores, and activated carbonized cellulose fibers are obtained. This activated carbonized cellulose fiber is used as it is or is crushed to obtain a carbon material, which is used as an electrode material having a predetermined shape.

In the present invention, since copper ions are adsorbed on the cellulose fibers to carry out carbonization and activation, the pores formed on the surface of the obtained carbon material generally have the following (i) )-(Iv) are satisfied. First, (i) (dVp / dRp) has a maximum value in the range where Rp is less than 0.5 nm. Here, Rp is a pore radius, and its unit is nm. Vp is a pore volume per unit weight, and its unit is mm ³ / g. The maximum value referred to here is the first value in the present invention.
It is called a peak value. In addition, in the range where Rp is less than 0.5 nm, when there are two or more maximum values, the one having the largest value is set as the first peak value. This (i) means that the carbon material of the present invention has many pores having a radius of less than 0.5 nm. If the condition (i) is not satisfied, the specific surface area of the carbon material will be small, and an electrode material having a high electrostatic capacity will not be obtained.

Next, (ii) when Rp is 0.5 nm or more, 1.
This means that (dVp / dRp) has a maximum value in the range of less than 0 nm. The maximum value here is referred to as a second peak value in the present invention. Further, in the range where Rp is 0.5 nm or more and less than 1.0 nm, when there are two or more maximum values, the one having the largest value is set as the second peak value. The meanings of Rp and Vp are as described above. This (ii) means that the carbon material in the present invention has many pores having a radius of 0.5 nm or more and less than 1.0 nm. This (i
If the condition of i) is not satisfied, anions or cations having a relatively large size in the electrolytic solution are less likely to enter the pores, and an electrode material having a high capacitance tends to be unobtainable.

The relationship between the first peak value and the second peak value is
This means that the first peak value is larger (iii).
That is, in the carbon material of the present invention, the number of pores having a small radius is relatively large. When the second peak value is larger, the specific surface area becomes relatively small, and it tends to be difficult to obtain an electrode material having a high capacitance.

The first peak value is about 1000 mm ³ / nm
・ In some cases, more than g, 3000 mm ³ / nm ・ g
In some cases, the above may occur. On the other hand, the second peak value is 580 mm
³ / nm · g or more (iv), especially 590 to 800 m
It is preferably about m ³ / nm · g. When the second peak value is less than 580 mm ³ / nm · g, the number of pores having a relatively large radius decreases, and anions or cations having a relatively large size in the electrolytic solution enter the pores. It becomes difficult to obtain an electrode material having a high electrostatic capacity.

The carbon material having pores satisfying the above-mentioned conditions (i) to (iv) is formed into a predetermined shape and used as an electrode material for a positive electrode and a negative electrode. When an electric double layer capacitor is produced using this electrode material, a capacitor having a high electrostatic capacity can be obtained.

[0018]

EXAMPLES The present invention will be described below with reference to examples.
The invention is not limited to the examples. The present invention is
The carbon material obtained by carbonizing and activating the cellulose fiber having copper ions adsorbed is interpreted as being based on the finding that it exhibits excellent capacitance as an electrode material of an electric double layer capacitor. Should be.

Comparative Example 100 g of scoured cotton cloth (cloth woven using cotton fibers)
Was stored in a kettle and heated to 750 ° C. over 1 hour in an inert atmosphere in which the inflow of air was blocked to carbonize the cotton cloth. Next, add 300 liters / h of steam to the pot.
The activation treatment was performed for 90 minutes while maintaining the temperature at 750 ° C. while introducing at a rate of. After the activation treatment, cooling to room temperature while blocking the inflow of air into the kettle, activated carbonized cotton 7.6
g was obtained. This activated carbonized cotton is the carbon material referred to in the present invention.

Example 1 Copper sulfate pentahydrate was dissolved in water to prepare an aqueous copper sulfate solution having a copper sulfate pentahydrate concentration of 13 g / l. The copper ion concentration in this copper sulfate aqueous solution is 3.402 g / l. Then, after immersing 100 g of the cotton cloth used in the comparative example in this copper sulfate aqueous solution, it was squeezed with a squeezing ratio of 60%, and 60 mass% of the copper sulfate aqueous solution was applied to the weight of the cotton cloth. Then, it dried with the dryer and made copper cloth adsorb copper ion. The amount of copper ions adsorbed was 0.2267% by mass based on the mass of the cotton cloth as measured by an atomic absorption method. After that, carbonization and activation were carried out in the same manner as in Comparative Example to obtain 12.4 g of activated carbonized carbon (carbon material).

Example 2 The copper sulfate pentahydrate concentration of the aqueous copper sulfate solution used was 26 g /
Carbon material 1 was produced in the same manner as in Example 1 except that the carbon material 1 was used.
2.4 g was obtained. The amount of copper ions adsorbed was 0.4039% by mass based on the mass of the cotton cloth.

Example 3 The copper sulfate pentahydrate concentration of the copper sulfate aqueous solution used was 39 g /
Carbon material 1 was produced in the same manner as in Example 1 except that the carbon material 1 was used.
4.1 g was obtained. The amount of copper ions adsorbed was 0.5909% by mass based on the mass of the cotton cloth.

Example 4 The copper sulfate pentahydrate concentration of the copper sulfate aqueous solution used was 52 g /
Carbon material 1 was produced in the same manner as in Example 1 except that the carbon material 1 was used.
5.5 g was obtained. The amount of copper ions adsorbed was 0.7423% by mass based on the mass of the cotton cloth.

Example 5 The copper sulfate pentahydrate concentration of the aqueous copper sulfate solution used was 65 g /
Carbon material 1 was produced in the same manner as in Example 1 except that the carbon material 1 was used.
6.1 g was obtained. The amount of copper ions adsorbed was 0.9294% by mass based on the mass of the cotton cloth.

[Measurement of Rp and (dVp / dRp)] A sample having a predetermined mass was prepared from the carbon materials obtained in Comparative Examples and Examples 1 to 5 under the trade name "BELSOAP 2" manufactured by Bell Japan.
8 SP ”was used to obtain a nitrogen gas adsorption isotherm. This isotherm is the DH (Dollimore-Heal) method +
The relationship between the pore radius Rp and (dVp / dRp) was obtained by calculation from the MP (Micro-Pore) method. Figure 1
~ Shown in FIG. Referring to FIGS. 1 to 6, all have a first peak value in the range where Rp is less than 0.5 nm, and a second peak value in the range where Rp is 0.5 nm or more and less than 1.0 nm. You can see that it has a value. Further, specific values of the first peak value and the second peak value can be calculated based on FIGS. The specific surface area of each sample can also be measured by "BELSOAP 28 SP". The results are shown in Table 1 below.

[Table 1] ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 1st peak value 2nd peak value Specific surface area ━ ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Comparative example 3600 or more 560 1110 Example 1 3200 or more 400 1028 Example 2 2700 or more 760 1097 Example 3 3000 or more 720 1077 Example 4 1070 or more 800 1103 Example 5 2000 or more 590 1088 ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ ━━━━ The unit of the first and second peak values is mm ³
/ Nm · g, and the value is an approximate value.
Further, the unit of the specific surface area is m ² / g, and “BELS
OAP 28 SP ".

[Measurement of Capacitance (F / g)] Using the carbon materials obtained in Comparative Examples and Examples 1 to 5, the capacitance was measured by the following method. First, Comparative Examples and Examples 1-5
Each of the carbon materials obtained in 1. was crushed to a particle size of about 75 μm in a mortar and used as a crushed sample. Then, for each crushed sample of a predetermined amount,
An appropriate amount of 30 mass% sulfuric acid aqueous solution was added and mixed until a slurry was formed. Thereafter, the vacuum depressurized state was maintained for 30 minutes, and an aqueous sulfuric acid solution was impregnated into each crushed sample to obtain an electrode material. This electrode material was incorporated into the simple electric double layer capacitor shown in FIG. 7 (3 and 3'in FIG. 7 are electrode materials, respectively). Then, a charge / discharge test was conducted in an air constant temperature bath kept at 30 ° C., and a charge / discharge measuring device (BTS2004:
A charge / discharge curve was obtained by using Nagano Co., Ltd.). The charging curve is 0 at the constant voltage (0.5mA)
It was obtained by increasing the voltage from V and reaching 0.9 V, while maintaining the voltage as it is for 30 minutes. On the other hand, the discharge curve is obtained by discharging at constant current (0.1 mA) until the voltage becomes 0 V after charging. This charging / discharging was repeated 100 cycles continuously, and the electrostatic capacity for each cycle was changed from 0.54 V to 0.45 V of the discharge curve.
It calculated from the inclination of the curve in V, and each capacitance in 100 cycles was calculated | required. These are divided by the mass of the crushed sample used to obtain the capacitance per unit mass of 10
The values at the 0th cycle are shown in Table 2 as a representative value.

Here, the simple electric double layer capacitor shown in FIG. 7 used for measuring the electrostatic capacitance has the following constitution. That is, as the positive electrode current collector 1 and the negative electrode current collector 1 ', a disc-shaped platinum current collector having a thickness of 1 mm and an outer diameter of 18 mm is used. Each current collector 1,
1 ', thickness 0.5mm, inner diameter 3mm, outer diameter 18mm
The hollow disk-shaped silicon sheets 2 and 2'are pressure-bonded. The electrode material 3, 3 ′ is filled in each hollow portion of each silicon sheet 2, 2 ′. A disc-shaped polypropylene separator 5 having a thickness of 25 μm and an outer diameter of 18 mm is inserted between the opposing electrode materials 3 and 3 ′ and the silicon sheets 2 and 2 ′. Terminal plates 4 and 4'are laminated on the positive electrode current collector 1 and the negative electrode current collector 1 ', and a pressure of about 385 kPa is applied between the terminal plates 4 and 4'and fixed as a whole. . Such a simple electric double layer capacitor uses an aqueous solution of sulfuric acid added to a ground sample as an electrolytic solution.

[0029]   [Table 2]               ━━━━━━━━━━━━━━━━━━━                               Capacitance (F / g)               ━━━━━━━━━━━━━━━━━━━                   Comparative Example 43                   Example 1 82                   Example 2 122                   Example 3 122                   Example 4 150                   Example 5 140               ━━━━━━━━━━━━━━━━━━━

As is clear from the above results, Example 1
It can be seen that the carbon materials obtained in Examples Nos. 5 to 5 exhibit higher capacitance when used as an electrode material of an electric double layer capacitor than the carbon materials obtained in Comparative Examples. In particular, it can be seen that the carbon materials obtained in Examples 2 to 5, in which the amount of copper ions attached to the cellulose fiber is large, show higher capacitance.

[0031]

INDUSTRIAL APPLICABILITY In the present invention, when a carbon material obtained by carbonizing and activating a cellulose fiber having copper ions adsorbed is used as an electrode material for an electric double layer capacitor, its action is not clear, but As demonstrated by the comparative examples and the examples, the electrode material exhibiting a high capacitance is obtained.

[Brief description of drawings]

FIG. 1 shows Rp and (dVp / of a carbon material obtained in a comparative example.
3 is a graph showing the relationship with dRp).

FIG. 2 shows Rp and (dVp of the carbon material obtained in Example 1.
5 is a graph showing the relationship with / dRp).

FIG. 3 shows Rp and (dVp of the carbon material obtained in Example 2.
5 is a graph showing the relationship with / dRp).

FIG. 4 shows Rp and (dVp of the carbon material obtained in Example 3;
5 is a graph showing the relationship with / dRp).

FIG. 5 shows Rp and (dVp of the carbon material obtained in Example 4.
5 is a graph showing the relationship with / dRp).

FIG. 6 shows Rp and (dVp of the carbon material obtained in Example 5.
5 is a graph showing the relationship with / dRp).

FIG. 7 is a schematic side view of a simple electric double layer capacitor used for measuring the capacitance of an electrode material.

[Explanation of symbols]

1 Positive electrode current collector 1'Negative electrode current collector 2,2 'Silicon sheet 3,3 'electrode material 4,4 'terminal board 5 separator

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Tomitaka Toyama             No. 1 Nitta, Nishibiwajima-machi, Nishi-Kasugai-gun, Aichi Prefecture             No. 2 Tokai Senkou Co., Ltd. (72) Inventor Yusaku Sakata             2-3-3 Gakunan-cho, Okayama City, Okayama Prefecture (72) Inventor Akinori Muto             1-2, Tsushima-Nakamachi, Okayama City, Okayama Prefecture

Claims (7)

[Claims] 1. An electrode material for an electric double layer capacitor comprising a carbon material obtained by adsorbing copper ions on cellulose fibers, heating the carbon fibers to carbonize them, and then activating the carbon fibers. 2. The cellulose fiber is a cotton fiber.
The electrode material for an electric double layer capacitor described. 3. The electrode material for an electric double layer capacitor according to claim 1, wherein 0.3 mass% or more of copper ion is adsorbed with respect to the mass of cellulose fiber. 4. An electrode material for an electric double layer capacitor comprising a carbon material having pores on its surface, wherein the relationship between the pore radius (Rp) and the pore volume (Vp) of the carbon material is as follows: An electrode material for an electric double layer capacitor, which satisfies the conditions (i) to (iv). (I) When Rp is less than 0.5 nm, (dVp
/ DRp) has a maximum value (hereinafter, this maximum value is referred to as "first peak value"). (Ii) In the range of Rp of 0.5≤Rp <1.0 nm, (dVp / dRp) has a maximum value (hereinafter, this maximum value is referred to as "second peak value"). (Iii) The first peak value has a larger value than the second peak value. (Iv) The second peak value should be 580 mm ³ / nm · g or more. 5. The electrode material for an electric double layer capacitor according to claim 4, wherein the carbon material is obtained by carbonizing and activating a cellulose fiber having copper ions adsorbed thereon. 6. The cellulosic fiber is a cotton fiber.
The electrode material for an electric double layer capacitor described. 7. The electrode material for an electric double layer capacitor according to claim 5, wherein 0.3 mass% or more of copper ions are adsorbed with respect to the mass of cellulose fiber.