Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2000-10-19
2003-07-15
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C502S427000
Reexamination Certificate
active
06592838
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric double layer capacitor having a high capacitance density per electrode volume, excellent low temperature properties, and a high-current electricity discharging capability.
2. Related Art of the Invention
An electric double layer capacitor is a large-capacity capacitor using an activated carbon powder or an activated carbon fiber as a polarized electrode and also using an electric double layer created at the activated carbon—electrolyte solution interface. It is widely used for, for example, backup of microcomputers, memories and timers as a miniature, large-capacity capacitor.
The conventional electric double layer capacitors as described above are roughly divided into two types: those which use aqueous solution-based electrolyte solutions, such as aqueous solution of sulfuric acid, and those which use organic solvent-based electrolyte solutions, such as propylene carbonate, obtained by adding electrolytes to organic solvents.
The electric double layer capacitors is basically characterized by, for example, electrostatic capacitance, withstand voltage and internal resistance, and of them, electrostatic capacitance is the most important characteristic.
The principle of the electric double layer capacitors is using an electric double layer created at the electrode—electrolyte solution interface. The electrostatic capacitance of the ordinary plate capacitor is proportional to the area of the plates and is inversely as the distance between the plates; but on the other hand, for the electric double layer capacitors, the area of the electrode corresponds to the above area of the plates and the distance between the ions in the electrolyte solution and the electrons or holes in the electrode corresponds to the above distance between the plates.
Accordingly, it is obvious that the surface area of the electrode should be increased in order to increase the electrostatic capacitance. At present, the activated carbon having a specific surface area of as large as 1000 m
2
/g or more is used as the chief material of the electrode.
To produce activated carbon for use in the electric double layer capacitors, the process has been taken which is known as steam activation. The steam activation is a process in which, first, a carbon raw material (coconut shell, organic resin, petroleum pitch, etc.) is heated to 900 to 1000° C. in an inert gas, commonly in nitrogen gas to be carbonized, then steam is introduced and collided against the carbon raw material, so as to obtain a carbon material having an extremely high specific surface area (that is, activated carbon). For the activated carbon obtained by this steam activation process, however, there are limits to its capacitance density.
On the other hand, an alkali activation process has been devised in recent years which enable the attainment of capacitors with higher capacitance compared with in cases where the steam activation process is used.
Typical examples of the patents which refer to the alkali activation process and aim at attaining a larger-capacity capacitor include: for example, a process in which petroleum cork is added with a 2 to 4 times amount of sodium hydroxide then the mixture is subjected to temporary baking at 300 to 600° C. and activated at 700 to 1200° C., so that the specific surface area of the activated carbon, of which limits have been 1500 m
2
/g when adopting the steam activation process, is greatly improved to 2000 to 3500 m
2
/g and the yield is also improved (Japanese Patent No. 2548546); a process in which sodium hydroxide is added to coconut shells and the mixture is subjected to heat treatment at 400 to 500° C. (Japanese Patent Application Laid-Open No. 3-812203); a process in which petroleum pitch, as the carbon raw material, is subjected to melt spinning and activated with an aqueous solution of alkali metal hydroxide (Japanese Patent Application Laid-Open No. 5-258996); a process in which phenolic laminated sheets as the paper base are pulverized and carbonated at 500 to 900° C., then activated with alkali metal hydroxide at 400 to 900° C. (Japanese Patent Application Laid-Open No. 8-51045); a process in which a carbon material is activated with alkali metal hydroxide and subjected to heat treatment in an inert gas (Japanese Patent Application Laid-Open No. 9-213590); a process in which alkali metal compound is added to a mesophase pitch carbon fiber and the mixture is subjected to heat treatment at 550 to 900° C. (Japanese Patent Application Laid-Open No. 10-121336); a process in which petroleum cork or petroleum pitch is carbonated and, after controlling the volatile component produced and the atom ratio of hydrogen to carbon, subjected to alkali activation (Japanese Patent Application Laid-Open No. 10-199767); a process in which saccharide is subjected to dehydration and partial decomposition and activated at 700 to 1000° C. (Japanese Patent Application Laid-Open No. 10-335188); a process in which metal impalpable powder is previously added to saccharide and the mixture is activated (Japanese Patent Application Laid-Open No. 10-335189); halogenated resin is activated with a molten alkali salt (Japanese Patent Application Laid-Open No. 11-121299); and a process in which mesophase pitch of which surface is coated with a graphite laminated structure layer or is partially oxidized is activated with alkali metal hydroxide at 550 to 850° C. (Japanese Patent Application Laid-Open No. 11-135380). There is presented no detailed description on the pore distribution of the activated carbon after activation, as herein described later, in the documents above.
In the steam activation process which has a longer history than the alkali activation process, inventions have been made from the viewpoint of the pore distribution of the activated carbon. Typical examples of the patents include, for example, the activated carbon of which volume of the pores 20 Å or larger in inside diameter accounts for 40% or more (Japanese Patent Application Laid-Open No. 61-102023); carbon-based material of which specific surface area is 1800 to 3500 m
2
/g, average pore diameter is 5 to 15 Å, and volume ratio of the pores 20 Å or larger in inside diameter to the total pores is 20 to 40% (Japanese Patent Application Laid-Open No. 63-187614); a process in which pores having a slit larger than the diameter of the solvated ion (Japanese Patent Application Laid-Open No.7-220985); the activated carbon of which specific surface area of the pores 20 Å or larger accounts for 10% or less (Japanese Patent Application Laid-Open No. 9-74053); the activated carbon having peaks at the inside diameters 10 to 20 Å and 20 to 100 Å (Japanese Patent Application Laid-Open No. 9-328308); the activated carbon which has a peak at the inside diameter of 8 Å or smaller and the pore diameter of 15 Å or small of which pore volume accounts for 65% or more (Japanese Patent Application Laid-Open No. 10-279303); and the activated carbon of which pore diameter is equal to or larger than the maximum ion diameter of electrolyte solution and smaller than the 5.0 fold diameter of the same (Japanese Patent Application Laid-Open No. 11-11921).
The inventions of controlling the pore distribution in the above steam activation process have been made mainly from the viewpoint of the energy density.
Meanwhile, devices capable of storing electrical energy just like the electric double layer capacitor include, for example, a secondary battery. Generally the electric double layer capacitor has a lower energy density than the secondary battery, and it has been used as a substitute of the secondary battery solely in the field where a long life is required, like an electrolytic capacitor. In recent years, however, there have been demands for the electric double layer capacitor to have excellent high-power output and low-temperature resistance in, for example, vehicle applications.
In such a situation, the present inventors concentrated their energy o
Hirose Eri
Nomoto Susumu
Yoshioka Kaneharu
Bos Steven
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
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