JPS6331930B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a novel wet-type electric double layer capacitor, and more particularly to a small-sized, large-capacity capacitor that uses activated carbon fibers with a specific structure as polarizable electrodes and has low temperature dependence of internal resistance. Conventionally, powdered activated carbon electrodes have been used in wet-type electric double capacitors, but there are several problems described below, and in order to improve these problems, carbon fibers or activated carbon fiber aggregates have been used as polarizable electrodes. There is an attempt to use it (Japanese Patent Application No. 7768, 1983). However, the characteristics discussed in such prior art are mainly limited to improving the processability of electrodes, improving the utilization rate of electrode layers, improving product variations, and improving capacitance per unit volume. This is merely a description of the properties that naturally arise from the properties of activated carbon fibers. On the other hand, in an electric double layer capacitor, the capacity,
In addition to the value of internal resistance, particular importance is placed on the magnitude of leakage current and its temperature dependence, the temperature dependence of internal resistance, the temperature dependence of capacitance, and the like. The temperature dependence of capacitance and internal resistance is particularly focused on the low temperature side, with temperatures below 20°C.
It is evaluated by comparing the value at -25°C with the value at -25°C, and the standard is often that the capacitance is within 25% and the internal resistance is within twice that. As is clear from this value, the temperature dependence of the internal resistance is particularly remarkable. Since the internal resistance of a capacitor is generally large, there is a strong demand for a capacitor that has less temperature dependence as well as a decrease in its absolute value. In view of these circumstances, we have arrived at the present invention as a result of intensive research. The object of the present invention is to provide a capacitor with a small temperature dependence of internal resistance and a small internal resistance, which is used as a polarizable electrode and has a pore diameter of 30 to 300 Å and a pore volume of 0.36.
It is made using activated carbon fiber with a carbon fiber density of cc/g or more. Incidentally, the main reason why the internal resistance increases at low temperatures is thought to be due to a decrease in ion mobility in the electrolytic solution. Therefore, one way is to use an electrolytic solution whose viscosity is less dependent on temperature, but it also depends greatly on the characteristics of the porous carbon material used for other polarizable electrodes, and it is necessary to select it appropriately. It turns out that something is extremely important. In an electric double layer capacitor, both negative and positive ions in the electrolyte move to the surface of the pores of the microporous carbon material, forming an electric double layer. I'm coming. The speed of ion movement within a pore is highly dependent on the pore diameter. So-called micropores with a diameter of 30 Å or less are very effective in increasing the surface area (increasing the capacitance in capacitors), but the pore diameter is too small from the point of view of the movement of ions within the pore.
So-called transitional pores with a size of 30 Å or more and 1000 Å or less are particularly preferable from the viewpoint of movement. However, as the pore size increases, the surface area decreases, so the upper limit of the pore size is set at 300 Å.
The pore volume of ordinary activated carbon fibers with a pore diameter of 30 to 300 Å is usually 0.35 cc/g or less, and most
It is in the range of 0.1 to 0.2 cc/g. It must be said that even if such activated carbon fibers are used, the temperature dependence of internal resistance is still large. It has been found that a capacitor with extremely low temperature dependence can be obtained only by using a material with a pore diameter of 30 to 300 Å and a pore volume of 0.36 cc/g or more. Activated carbon fibers having such a specific porous structure are produced, for example, by the following method. That is, carbon fibers having a surface area of 30 to 1200 m ² /g, a pore diameter of 30 to 300 Å, and a pore volume of 0.1 cc/g or less are coated with at least one compound selected from compounds of Group A of the Periodic Table and transition metals. It is produced by performing an activation treatment after being supported, that is, a reactivation treatment. As the above-mentioned activation aid, compounds of Periodic Table Group A elements such as magnesium, calcium, and barium, or transition metal elements such as iron, cobalt, nickel, and manganese are used. Water-soluble salts such as magnesium chloride, magnesium acetate, calcium chloride, ferric chloride, cobalt chloride, nickel acetate, and manganese chloride are most easily used. The method for supporting the activation aid includes, but is not limited to, a method in which the starting carbon fiber is immersed in an aqueous solution of the above compound, followed by dehydration, and then dried, or a method in which the aqueous solution is sprayed and then dried. The amount of the activation aid to be impregnated is preferably 0.01 to 40% by weight in terms of metal element. Further, for the reactivation treatment, a method of heating to 650 to 1050°C in an oxidizing gas containing steam, carbon gas, etc. or in a combustion gas can be applied. The reason why pores with a pore diameter of 30 to 300 Å increase when an activation aid is used is as follows.
This is thought to be because the reaction rate between the carbon around the auxiliary agent attached to the walls of the micropores and the activating gas increases significantly, and the micropores expand and coalesce. By selecting the starting carbonaceous fiber in this way, loading it with a specific compound, and performing a reactivation treatment, it became possible for the first time to make it have a pore volume of 30 to 300 Å and 0.36 cc/g or more. Ta. The method of reducing the temperature dependence of internal resistance has been described above, but in order to reduce the level of internal resistance itself, it is necessary to reduce the electrical resistance of the activated carbon fiber itself. The electrical resistivity of activated carbon fibers generally belongs to the semiconductor region and is quite large. Furthermore, since the fibers are porous, they tend to become larger, and at the same time, they vary greatly due to differences in temperature and activation temperature during production. The electrical resistivity of activated carbon single fibers is 10 ^-2 Ω・cm or more, often around 10 ^-1 Ω・cm, which is higher than that of graphite fibers (10 ^-3 Ω・cm), and the temperature during manufacturing is low. Normally, it is within a range of large variations. This makes it difficult to obtain a capacitor with high reliability and low internal resistance. The electrical resistivity of a single fiber is 10 ^-2
By setting the resistance to Ω·cm or less, a capacitor with little variation and small internal resistance can be obtained. Activated carbon fibers having an electrical resistivity of 10 ^-2 Ω・cm or less can be obtained by a method of subjecting activated carbon fibers having a specific porous structure as described above to a temperature of 950°C or higher. . preferably 1000
Processing in an inert gas at temperatures above ℃ is recommended. It should be noted here that the diameter of pores of 30 Å or less is likely to change by heat treatment in the temperature range, but pores of 30 Å or more maintain their diameter. Therefore, by using activated carbon fibers with pore diameters and low electrical resistivity as in the present invention, it is possible for the first time to create a capacitor with low internal resistance and low temperature dependence of internal resistance. Summer. Any known aggregate form of activated carbon fibers used in the polarizable electrode can be used, including felt, woven fabric, knitted fabric, mixed paper, and the like. Next, when activated carbon fiber aggregates are used as electrodes of non-aqueous electrolyte capacitors, and when the current collector of the electrodes is made of aluminum or stainless steel plates, metal spraying is applied to the surface of the fiber aggregates in order to reduce contact resistance. Alternatively, it is desirable to perform metal vapor deposition. In the case of a water electrolyte capacitor that uses strong acid as a solute, a carbon resin plate is used, but in this case, the contact resistance is small and thermal spraying is unnecessary. This polarizable electrode is left alone with a non-polarizable electrode or a polarizable electrode of the same type through a thin porous separator, and is impregnated with an electrolytic solution in which a solute is dissolved to form a capacitor. If necessary, these structural units can be stacked in parallel or in series. The separator uses a porous sheet that is resistant to electrolyte.
Nonwoven fabrics and porous sheets such as polyethylene, polypropylene, and Teflon are good. When the electrolyte is nonaqueous, it is necessary to pay close attention to the water content. You need to dehydrate it. Also, highly pure solutes are dried under reduced pressure below the decomposition temperature to lower their moisture content, and stored in a phosphorus pentoxide desiccator. If the solution has been prepared, add a dehydrating agent and set aside. The electrolytic solution is impregnated by vacuum or heating so that the electrode and separator are sufficiently wetted. Impregnation can be done smoothly by exposing the material to electrolytic vapor in advance to adsorb the electrolyte and then impregnating it. Each characteristic value described in the text is measured by the following method.
This is the calculated value. Pore Diameter and Pore Volume Determine the nitrogen gas adsorption isotherm at liquid nitrogen temperature for a sample dried at a temperature of 120°C for 2 hours under reduced pressure. Then, the Cranston-Inkley calculation method ("Adsorption" by Tominaga Kei, Kyoritsu Zensho) was applied to the adsorption isotherm to calculate the pore size distribution (curve representing pore diameter vs. pore volume).
From this, the pore volume in the pore diameter range of 30 to 300 Å was determined. However, the relationship between the multimolecular adsorption layer thickness and relative pressure is t (Å) = 3.54 [5/1n (Ps/p)] ^1/3 .
The pore volume in the range of 30 to 300 Å in diameter is hereinafter abbreviated as TPV. Electrical specific resistance in the longitudinal direction of single fibers Arrange an appropriate number of sampled single fibers, fix both ends with conductive adhesive, apply electricity, and calculate the resistance R (Ω) of the fibers from the voltage and current values between the adhesives. seek. Also, measure the length L (cm) between the conductive adhesives. If the single fiber is bent, determine the actual fiber length using a microscope, etc. Next, remove the fibers and use a microscope to determine the total cross-sectional area S in the direction perpendicular to the fiber direction.
(cm ² ), and calculate the electrical resistivity ρ (Ω·cm) in the fiber direction using the following formula. ρ=R・S/L However, the measurement shall be performed in a dry atmosphere at room temperature and relative humidity of 5% or less after drying as in the previous section. Comparative Example A twill fabric was produced using a spun yarn made of regenerated cellulose fiber with a single fiber of 2.0 d. This fabric is impregnated with 10% diammonium phosphate based on the weight of the fibers by dipping the fabric in an aqueous solution of diammonium phosphate, squeezing it and drying it.
The activated carbon fiber fabrics A and B were then heated for 90 minutes from 270°C to 850°C, and then activated in nitrogen gas containing 40 vol% of water vapor for different times. Obtained. The pore volume of A and B in the diameter range of 30 to 300 Å is 0.08 cc/g0.15 c.c./g, and the specific resistance of the single fibers is 1.5×10 ^-3 Ω・cm, respectively.
It was 2.2×10 ^-3 Ω・cm. Aluminum rod spraying was applied to one side of these fabrics. The amount of aluminum adhered is 90
g/ ^m2 . Two circular pieces with a diameter of 20 mm are punched out of this aluminum-adhered fabric, each piece is used as a positive and negative polarizable electrode, and a circular polypropylene non-woven fabric with a thickness of 0.12 mm and a diameter of 25 mm is sandwiched between them, and the pieces are placed in an aluminum case with a diameter of 25 mm and a thickness of 5 mm. A sealing gasket and a lid were attached, and a propylene carbonate solution containing 1M LiClO ₄ dissolved therein was injected and crimped to obtain a capacitor. At this time, the aluminum sprayed surface was placed on the case and lid side. The internal resistance of this capacitor at 1KHz at multiple temperatures is as shown in Table 1.

[Table] Table 1 shows that conventional capacitors have a large temperature dependence. Example 1 and Comparative Example 12% of magnesium acetate was added to Fabric A obtained in Comparative Example 1.
After the weight% impregnation, water vapor activation was performed at 850℃ for different times, and the TPV was 0.25cc/g and 0.34cc/g, respectively.
Activated carbon fiber fabrics C, D, and E with a weight of 0.46 cc/g and 0.46 cc/g were obtained, and one side of these fabrics was sprayed with aluminum rod at a rate of 70 to 90 g/m ² to form a polarizable electrode. A capacitor was assembled in the same manner as the comparative example, and the internal resistance was measured. The results are shown in Table 2.

[Table] From Table 2, it can be seen that the product (E) of the present invention has a significantly small temperature dependence of internal resistance. Example 2 Fabric E obtained in Example 1 was heated in an inert gas at a temperature of
When heat treated at 1200℃, the electrical resistivity of the single fiber decreased to 8×10 ^-3 Ω・cm (TPV was 0.46cc/cm).
There was no difference at all from g). When we measured the internal resistance of this in the same way as the comparative example, it was 0.60Ω at 20℃.
It was found that the capacitor has a low internal resistance of 0.68Ω at -25℃ and has little temperature dependence.

Claims (1)

[Claims] 1. In a wet electric double layer capacitor using an electric double layer formed at the interface between a polarizable electrode and an electrolyte, the polarizable electrode has a pore volume of 0.36 cc/g or more with a pore diameter of 30 to 300 Å. A wet electric double layer capacitor characterized by being made of activated carbon fiber. 2 Activated carbon fiber has an electrical specific resistance of 1.0×10 ^-2 Ω・cm
The electric double layer capacitor according to claim 1, which is as follows.