JPH0213925B2 - - Google Patents

Abstract

PURPOSE:To make a polarized electrode made of active carbon with which an electric double layer capacitor or secondary battery having excellent volume capacity efficiency can be produced by treating the active carbon by nitric acid. CONSTITUTION:An active carbon fiber cloth 13 is exposed to a nitric acid vapor. Specifically, after concentrate nitric acid 11 is put in a round bottom flask 10, the concentrate nitric acid 11 is heated using a mantle heater 12. The thus produced nitric acid gas touches the active carbon fiber cloth 13 located in the upper area of the flask 10. A reflux cooling tube 14 is installed over the flask 10. The cloth 13 is treated by the nitric acid vapor for 3hr by using this device. Following that, the treated cloth 13 is washed with water for 1hr before being dried at 110 deg.C for 1hr. After that, a plasma flame coating layer of aluminum is formed on one surface of the dried cloth 13, thereby mak ing a polarized electrode which is then used to assemble a capacitor.

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a method for producing polarizable electrodes made of activated carbon used in electric double layer capacitors, double batteries, and the like. Industrial Application Fields Electric double layer capacitors basically consist of an insulating separator and a separator placed opposite each other.
It consists of a pair of polarizable electrodes impregnated with an electrolyte.
Polarizable electrodes using activated carbon have been developed, and typical configurations include cylindrical electrodes and flat electrodes. The former is made by winding a pair of polarizable electrodes with activated carbon and an organic binder such as polytetrafluoroethylene or polyvinylpyrrolidone applied to the surface of an aluminum net as a current collecting electrode with a separator in between. The entire device is housed in a cylindrical metal case and sealed with a rubber cap, and a pair of electrode leads are led out from the rubber cap. As the electrolytic solution, a propylene carbonate solution of tetraethylammonium perchlorate or the like is used. The second type uses activated carbon fibers as polarizable electrodes, and has the structure shown in Figure 1.
A separator 3 is arranged between polarizable electrodes 1 and 2 made of activated carbon fiber cloth, and the separator 3 is sealed with metal cases 4 and 5 and an insulating ring 6. 7 and 8 are collector electrodes made of an aluminum sprayed layer provided on the back surfaces of the polarizable electrodes 1 and 2. The electrolyte is the same as the first type. As a secondary battery using an activated carbon electrode and a reactive metal electrode, for example, a polarizable electrode 1 shown in FIG.
A device has been devised that replaces the lithium electrode with a lithium electrode. A propylene carbonate solution in which lithium perchlorate is dissolved is used as the electrolyte. These polarizable electrodes made of activated carbon can be used for charging,
The physical relationship between the activated carbon surface and electrolyte ions during discharge,
Chemical reactions govern capacitors, battery capacity values, charging/discharging characteristics, etc. The reaction between activated carbon and electrolyte ions will be described below using an electric double layer capacitor as an example. In an electric double layer capacitor, when an electric field is applied to the activated carbon electrode, an electric double layer is formed at the interface between the activated carbon surface and the electrolyte, and the storage capacity of this part becomes the capacitor capacity C, and its size is calculated by the following formula: expressed. C=ε·S/4πδ Here, ε is the dielectric constant of the electrolytic solution, S is the surface area of activated carbon, and δ is the thickness of the electric double layer. As can be seen from this equation, the capacity C increases as the activated carbon surface area S increases. However, S is the surface area of activated carbon on which an electric double layer is formed, and the surface electric double layer capacity of Hg with a smooth surface is
In the second type of capacitor described above, using a 14 mm diameter activated carbon cloth made of activated carbon fibers with a specific surface area of ~2500 m ² /g, judging from 40 μF/cm ² ,
A capacity of 10 to 20 F per cell should be obtained, but in reality, the capacity is only 20 to 30% of this value at most. That is, 2 to 3 on the surface of activated carbon
This means that only a relatively effective electric double layer is formed. The following two points are considered to be the cause of this. (1) The pores of activated carbon often do not have a sufficient diameter (20 Å or more) to form an electric double layer (thickness 2 to 5 Å) on the inner wall surface, resulting in poor surface utilization. is low. (2) There is a part where the activated carbon surface has poor affinity with the electrolyte ions, and an electric double layer is not formed in this part. Among the above causes, the first cause is attributed to the pore size distribution and pore volume distribution of the activated carbon, and the second cause is attributed to the types of functional groups present on the activated carbon surface. Up to now, several attempts have been made to improve the utilization efficiency of the activated carbon surface from these two viewpoints, and a typical example is high-temperature treatment of activated carbon. This treatment causes the pores to undergo a so-called sintering phenomenon, which reduces the specific surface area of the activated carbon, but increases the proportion of pores with large diameters. The capacity can be increased by about 10 to 20% compared to the previous version. There has also been an attempt to effectively utilize the pores by using an electrolyte with a small molecular diameter, such as 1,2-dimethoxyethane. Furthermore, even in a system that combines activated carbon and an alloy containing lithium, an electric double layer is formed and erased on the surface of the activated carbon during charging and discharging, and the function of activated carbon and the causes that prevent it are similar to those of the capacitor described above. They are almost the same, with pore size and surface functional groups governing their properties. OBJECTS OF THE INVENTION It is an object of the present invention to provide an activated carbon polarized electrode that provides an electric double layer capacitor and a secondary battery with much better volumetric capacity efficiency than conventional ones. Structure of the Invention The present invention is characterized in that activated carbon is treated with nitric acid. According to the present invention, many NO ₂ groups exist on the surface of activated carbon that has been oxidized with nitric acid, and the gases adsorbed after the activation treatment are removed by oxidation with nitric acid, so that the original clean surface of activated carbon is maintained. can get.
Therefore, the surface of the activated carbon is effectively brought into contact with the electrolytic solution, and the ratio of the surface on which an electric double layer is formed at the interface increases due to charging. Furthermore, the nitric acid treatment slightly oxidizes and decomposes the inner walls of small-diameter pores, so although the total surface area decreases, the proportion of large-diameter pores increases and the effective use of pores is promoted. Therefore, capacitors and batteries using activated carbon obtained by the treatment of the present invention as polarizable electrodes have greatly improved volume capacity efficiency. Description of Examples Example 1 5 pieces of activated carbon fiber cloth with a basis weight of 150 g/m ² were added to concentrated nitric acid.
Soak for an hour. Next, after washing for 1 hour under running water,
Dry in an atmosphere at ℃ for at least 1 hour. One side of the treated cloth is coated with a thickness of 100% by plasma spraying.
Form a 300μm aluminum layer and
Punch out a 10mm circle. A capacitor having the configuration shown in FIG. 1 is assembled using the above polarizable electrodes. Two types of electrolytes were used: a solution of tetraethylammonium perchlorate dissolved in propylene carbonate, and a 20% aqueous solution of caustic potash. Example 2 Instead of the concentrated nitric acid used in Example 1, a solution obtained by diluting concentrated nitric acid twice is used. Example 3 A 1% aqueous nitric acid solution is used in place of the concentrated nitric acid in Example 1. Example 4 Using the apparatus shown in FIG. 2, the same activated carbon fiber cloth as in Example 1 is exposed to nitric acid vapor. That is, concentrated nitric acid 11 is placed in a round-bottomed flask 10, and the concentrated nitric acid is heated by a mantle heater 12. The evaporated nitric acid gas contacts the activated carbon fiber cloth 13 placed on top of the round bottom flask. A cooling pipe 14 for reflux cooling is provided above the flask. Nitric acid gas treatment is carried out using this device for 3 hours, followed by washing with water for 1 hour and drying at 110°C for 1 hour. As in Example 1, a plasma sprayed layer of aluminum is applied to one side, and a capacitor is assembled using an electrode having a diameter of 10 mm. Example 5 An activated carbon fiber cloth weighing 150 g/m ² is soaked in a concentrated nitric acid solution for 5 hours. Next, wash with water for 1 hour and dry for 1 hour. Subsequently, it is treated in an atmosphere of 250° C. and 1 mmHg for 3 hours. A capacitor is assembled with the same configuration as in Example 1. Example 6 A chopped activated carbon fiber having a diameter of 10 μm and a length of 3 mm is immersed in concentrated nitric acid for 3 hours. Subsequently, it was washed under running water for 3 hours and dried at 110°C for 3 hours. The activated carbon fibers and bulbs thus produced are mixed in the same weight ratio and paper is made. The basis weight of the paper is 100g/ ^m2 . An aluminum sprayed layer was formed on one side of the resulting paper, and a circle with a diameter of 10 mm was punched out to obtain a capacitor having the same structure as in Example 1. Example 7 From a cylinder into a container containing powdered activated carbon
Supply NO ₂ gas for 1 hour. Polytetrafluoroethylene powder was added at a weight ratio of 1 to the obtained activated carbon powder.
% and pressurize this mixture to form a diameter of 10 mm and a thickness of 1.
Make a molded body of mm. Using this shaped electrode, the first
Create a capacitor with the configuration shown in the figure. As the electrolytic solution, a solution prepared by dissolving tetraethylammonium perchlorate in propylene carbonate or a 20% aqueous solution of caustic potash was used. Example 8 An aluminum plasma sprayed layer 21 was formed on one side of an activated carbon cloth 20 that had been treated in the same manner as in Example 1,
This is cut into a size of 2 cm x 5 cm, and as shown in FIG. 3, an etched aluminum foil 22 with a thickness of 50 μm is stacked so as to be in contact with the plasma sprayed layer 21. These two electrodes and the separator are wound spirally to assemble a cylindrical capacitor.
In addition, 23 is an aluminum lead terminal,
It is caulked and bonded to the aluminum foil 22. Example 9 Activated carbon powder treated in the same manner as in Example 1 was mixed with an aqueous sulfuric acid solution, and this was pressure-molded into a disk shape with a diameter of 10 mm and a thickness of 1 mm. Using this electrode, a capacitor having the configuration shown in FIG. 1 is assembled. Comparative Example 1 A capacitor similar to that of Example 1 was assembled using the activated carbon fiber cloth of Example 1 which was not treated with nitric acid. Comparative Examples 2 to 4 In Examples 6, 8, and 9, capacitors were assembled using activated carbon that was not treated with nitric acid. These are referred to as Comparative Examples 2, 3, and 4, respectively. The following table shows a comparison of the capacitance and impedance values at 25°C and -25°C for the above capacitors.

[Table] Example 10 An activated carbon fiber cloth with a basis weight of 150 g/m ² is immersed in a concentrated nitric acid solution for 5 hours. Next, after washing under running water for 1 hour,
Dry for at least 1 hour in an atmosphere of 110℃. A secondary battery was constructed by using a 50 mm x 50 mm treated cloth as a negative electrode and a sintered nickel electrode of the same size as a positive electrode by immersing it in a 30% caustic potash aqueous solution. Curve A in FIG. 4 is the discharge curve of the negative electrode of this secondary battery, and curve B shows the discharge curve of the negative electrode constructed using activated carbon fibers not subjected to nitric acid treatment. Effects of the Invention As described above, according to the present invention, the pore diameter in activated carbon has an optimal distribution for forming an electric double layer, and the activated carbon surface has excellent affinity with the electrolyte. When such activated carbon is used alone or mixed with a suitable binder as an electrode to construct an electric double layer capacitor or a secondary battery, a larger capacity than conventional capacitors and batteries can be obtained.

[Brief explanation of drawings]

FIG. 1 is a vertical cross-sectional view of an electric double layer capacitor according to an example, FIG. 2 is a schematic vertical cross-sectional view showing the configuration of a processing device according to an example, FIG. 3 is a perspective view of a polarizable electrode according to an example, and FIG. FIG. 2 is a diagram showing the discharge characteristics of a negative electrode of a battery.

Claims (1)

[Claims] 1. A method for producing a polarizable electrode, which comprises treating activated carbon with nitric acid. 2. The method for producing a polarizable electrode according to claim 1, wherein the activated carbon is fibrous, granular, or powdery. 3. The method for manufacturing a polarizable electrode according to claim 1, wherein the nitric acid treatment is performed by immersing activated carbon in an aqueous solution of nitric acid. 4. The method for manufacturing a polarizable electrode according to claim 1, wherein the nitric acid treatment is performed by exposing activated carbon to nitric acid vapor. 5. A method for producing a polarizable electrode, which comprises treating activated carbon with nitric acid, followed by treatment in a high-temperature, reduced-pressure atmosphere. 6. A method for producing a polarizable electrode characterized by treating activated carbon with NO ₂ gas.