Stock material or miscellaneous articles – Self-sustaining carbon mass or layer with impregnant or...
Reexamination Certificate
1998-08-17
2001-09-18
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Self-sustaining carbon mass or layer with impregnant or...
C428S403000, C361S502000, C361S301100, C264S029600
Reexamination Certificate
active
06291069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to activated carbon for use in electric double layer capacitors, and a method of manufacturing such activated carbon.
2. Description of the Related Art
When the surface of a solid comes into contact with an electrolytic solution, positive or negative ions are selectively adsorbed to the surface of the solid, positively or negatively charging the surface of the solid. At the same time, an increased number of oppositely charged paired ions are developed in the electrolytic solution. As a result, positive and negative charges are arranged across the interface between the surface of the solid and the electrolytic solution, thereby making up an electric double layer. It is known in the art that the electric double layer can be used as a capacitor with the solid serving as an electrode. Such a capacitor is called an electric double layer capacitor.
In the electric double layer capacitor, the electrostatic capacitance produced when an electric field is applied to the solid electrode is proportional to the surface area of the solid electrode. Generally, electric double layer capacitors generally use a substance having a large specific surface area, such as activated carbon, as electrodes, and the electrodes is impregnated with an electrolytic solution. The electrodes are separated by a separator, and current collectors are attached to the respective electrodes that are disposed one on each side of the separator.
Electric double layer capacitors find use as backup power supplies for electric devices because they have a large capacitance in farads and exhibit excellent charging and discharging cycle characteristics. Efforts are being made to use electric double layer capacitors as a substitute for vehicle-mounted batteries.
As shown in
FIG. 1
of the accompanying drawings, an electric double layer comprises a pair of polarized electrodes
1
made of activated carbon which are positioned to face each other across a separator
2
. The polarized electrodes
1
are impregnated with an electrolytic solution containing tetraalkylammonium salt or the like dissolved in an organic solvent. The polarized electrodes
1
with the separator
2
sandwiched therebetween are housed in a casing
3
of aluminum. The casing
3
is closed by a lid
5
of aluminum. The casing
3
and the lid
5
serve as current collectors held in contact with the polarized electrodes
1
, respectively. Specifically, each of the polarized electrodes
1
is made of a shaped mixture of particles of activated carbon and a binder of tetrafluoroethylene or the like. Alternatively, each of the polarized electrodes
1
may be made of fibers of activated carbon.
In the illustrated electric double layer capacitor, charges in the polarized electrodes
1
close to the current collectors can easily be collected by the current collectors. However, charges in the polarized electrodes
1
remote from the current collectors are reduced due to the internal resistance of the polarized electrodes
1
before they are collected by the current collectors. Therefore, the electric double layer capacitor has a relatively small effective electrostatic capacitance.
Attempts have been made to improve the internal structure of the particles of activated carbon for reducing the internal resistance of the polarized electrodes
1
. For example, some polarized electrodes are made of particles of activated carbon which are mixed with a conductive filler such as of furnace black. According to Japanese laid-open patent publication No. 61-187322, fibers of activated carbon for polarized electrodes have an inner region made of graphite and an outer layer of activated carbon. The inventors of the present invention have proposed to form polarized electrodes of a mixture of particles of activated carbon and a conductive filler by carbonizing an easily graphitizable resin and activating the carbonized resin with an alkali for thereby producing particles of activated carbon, each having a laminated structure of graphite extending inwardly from the surface of the particle (see Japanese patent application No. 8-132050).
However, the improved internal structures of the particles of activated carbon have not been sufficiently effective in reducing the internal resistance of polarized electrodes.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an activated carbon which will be able to reduce the internal resistance of polarized electrodes of an electric double layer capacitor when the polarized electrodes are made of the activated carbon.
Another object of the present invention is to provide a polarized electrode which is made of such an activated carbon.
Still another object of the present invention is to provide an electric double layer capacitor having polarized electrodes which are made of such an activated carbon.
Yet sill another object of the present invention is to provide a method of manufacturing such an activated carbon.
The inventors have reviewed an internal structure of polarized electrodes of an electric double layer capacitor which comprise particles of activated carbon, a conductive filler, and a binder. As a result of the review, the inventors have found that the conductive filler is disposed around the particles of activated carbon and acts to electrically connect the particles of activated carbon. It has also been found that most of the transfer of electric charges between the particles of activated carbon is effected through the conductive filler.
For reducing the internal resistance of the polarized electrodes, therefore, it is considered more effective to reduce the resistance between the particles of activated carbon than to reduce the resistance of the particles of activated carbon themselves by improving the internal structure of the particles of activated carbon.
The conductive filler is in the form of substantially spherical particles having a diameter of about 200 Å which is about {fraction (1/500)} of the diameter of the particles of activated carbon. Therefore, the conductive filler is merely held in point-to-point contact with the particles of activated carbon, and has only a small area of contact with the particles of activated carbon. The conductive filler thus fails to provide a sufficient efficiency for electric contact with the particles of activated carbon. The inventors have found it possible to increase the efficiency for electric contact between the conductive filler and the particles of activated carbon and reduce the resistance between the particles of activated carbon by improving the conductivity of the surface of the particles of activated carbon.
According to Japanese patent application No. 8-132050 filed earlier by the inventors, particles of activated carbon have a laminated structure of graphite extending inwardly from the surface of the particles. Since the graphite which is electrically conductive is exposed on the surface of the particles of activated carbon, the conductive filler held in contact with the graphite reduces the resistance between the particles of activated carbon. However, the laminated structure of graphite extending inwardly from the surface of the particles leaves only 10 to 20% of graphite exposed on the surface of the particles. Therefore, much remains to be improved on the disclosed particles of activated carbon.
To achieve the above objects, activated carbon for use in an electric double layer capacitor according to the present invention comprises particles produced by crushing activated carbon or fibers of activated carbon, the particles having a surface covered at least partly with a laminated structure layer of graphite formed along the surface. The laminated structure layer of graphite is a structure of stacked layers each composed of a two-dimensional linkage of six-membered rings of carbon, the layers being joined to each other by the van der Waals force.
The surface of the particles of the activated carbon is covered with the laminated structure layer of graphit
Noguchi Minoru
Oki Naohiko
Oyama Shigeki
Sato Kenji
Arent Fox Kintner & Plotkin & Kahn, PLLC
Boss Wendy
Honda Giken Kogyo Kabushiki Kaisha
Jones Deborah
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