Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1999-01-14
2001-07-10
Hendrickson, Stuart L. (Department: 2831)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S448000
Reexamination Certificate
active
06258337
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a carbonaceous material for electric double layer capacitors, more particularly a carbonaceous material suitable as a capacitor electrode material having a large capacity, a low electric resistivity and a large bulk density, and a process for production thereof.
In electronic apparatus equipped with IC memories, such as micro-computers and personal computers, back-up power supplies are extensively used for obviating mal-function of or loss of recorded data from such electronic apparatus due to momentary power interruption or voltage drop. As such back-up power supplies, Ni—Cd batteries and lithium batteries have been used hitherto, but in recent years, an electric double layer capacitor is becoming frequently used because of its excellent instantaneous charge/discharge characteristics and little liability of performance lowering during repetitive use. The above-mentioned type of electronic apparatus is required to reduce its weight and size and exhibit better reliability particularly in recent years, and an electric double layer capacitor mounted therein is expected to have a larger energy density so as to be adapted to such reduction in weight and size. Further, an electric double layer capacitor is expected to be used as a power source for electric vehicles because of its instantaneous charge/discharge characteristics, and also for that use, is required to have a further increased energy density.
An electric double layer capacitor is a device formed by immersing a pair of solid electrodes in an electrolytic solution containing electrolyte ions and operated on a principle that when a DC voltage is applied between the electrodes, anions in the solution are attracted to a positively polarized electrode and cations in the solution are attracted to a negatively polarized electrode respectively electrostatically to form a spatial charge layer called an electric double layer at a boundary of each electrode with the electrolytic solution so that an electric energy of the charges accumulated in the electric double layers is utilized. Accordingly, in order to provide an electric double layer capacitor with an increased capacity, it is indispensable to develop a material capable of forming a totally larger area of electric double layers. A polarizable electrode proposed and used heretofore is a sheet principally comprising a carbonaceous material in a state of activated carbon. This is because activated carbon has a large specific surface area and chemical stability which are properties suitable for capacitor electrode material. For capacitors, there have been proposed, e.g., powdery activated carbon having a specific surface area of 1800-3500 m
2
/g, an average pore diameter of 5-15 Å, and a ratio of volume of pores having inner diameters of at least 20 Å to a total volume of pores of 20-40% (e.g., Japanese Laid-Open Patent Appln. (JP-A) 63-187614), and fibrous activated carbon obtained by carbonizing and activating phenolic resin fiber (e.g., Japanese Patent Publication (examined) (JP-B) 60-15138). However, an electric double layer capacitor obtained by using activated carbon powder having a large specific surface area exhibits a large capacitance per unit weight of the activated carbon but does not necessarily exhibit a large capacitance per unit volume of the activated carbon because it provides an electrode of a small bulk density. Such powdery activated carbon may be formed into a sheet-form electrode by using an appropriate binder, but in this case, a quick charge/discharge characteristic of the electric double layer capacitor is liable to be lost. On the other hand, a woven cloth of fibrous activated carbon prepared from phenolic resin fiber provides an advantageous feature of lower resistivity because it uses no binder than a sheet electrode formed from powdery activated carbon together with a binder. In this case, however, the resultant electrode has a small bulk density so that it has a large capacitance per weight but has a small capacitance per volume.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a carbonaceous material suitable as an electrode material for electric double layer capacitors having a large capacitance per volume, a low electrical resistivity and a large bulk density.
Another object of the present invention is to provide a process for producing such a carbonaceous material.
As a result of our study on electrode materials for high-performance electric double layer capacitors for achieving the above-mentioned objects, it has been found that a carbonaceous material obtained by subjecting a certain carbon precursor to an appropriate activation treatment has appropriate level of specific surface area and layer structure and, because of these features, provides an electrode material having a large capacitance, a large bulk density and a low resistivity, thus arriving at the present invention.
According to the present invention, there is provided a carbonaceous material for electric double layer capacitors, having a specific surface area of 800-2000 m
2
/g as measured by the nitrogen adsorption BET method, and an average layer-plane spacing of at most 0.36 nm as measured by the X-ray diffraction method.
According to another aspect of the present invention, there is provided a process for producing the above-mentioned carbonaceous material for electric double layer capacitors, comprising: activating a carbon precursor of pitch origin in an oxidizing gas atmosphere at a temperature of 700-1500° C. Preferably, the carbon precursor of pitch origin is produced through steps of: mixing pitch with an additive comprising an aromatic compound having two to three aromatic rings and a boiling point of at least 200° C. to form a shaped pitch product; extracting the additive from the shaped pitch product with a solvent showing a lower dissolving power to the pitch and a higher dissolving power to the additive, thereby to leave a porous pitch product; and oxidizing the porous pitch product to form a thermally infusible porous carbon precursor to be subjected to the activation.
Compared with conventional activated carbon obtained by using a phenolic resin as the starting material, for example, the carbonaceous material according to the present invention has a relatively small specific surface area (a relatively low degree of activation) and a relatively small layer spacing (d
002
) (a relatively high degree of graphitization), and correspondingly has a relatively low resistivity, thus effectively functioning as an electric double layer capacitor electrode material having a relatively high bulk density. This is presumably because the carbon precursor before the activation is provided with a diversity of microtexture due to a diversity of components contained in the starting pitch and a nongraphitizable texture portion is preferentially dissipated by oxidation in the carbonization and activation step to result in a graphite-rich microtexture. In contrast thereto, in the case of carbonizing and activating a phenolic resin, it is assumed that a relatively uniform carbon precursor is produced, and on further progress of the activation, the effect of preferential dissipation of non-graphitizable portion to leave a graphite-rich microtexture cannot be substantially expected.
Incidentally, while the mechanism has not been fully clarified, in the infusibilized porous carbon precursor of pitch origin produced in the above-mentioned preferable process, it is assumed that the diversity of components in the starting pitch is enhanced and a layer of non-graphitizable carbon precursor is preferentially present at the pore surfaces, so that the layer of nongraphitizable carbon precursor is preferentially dissipated in the carbonization and activation stage, to result in a graphite-rich micro-texture even at a relatively low level of activation treatment.
The thus-formed carbonaceous material according to the present invention retains numerous fine pores and therefor can p
Aita Tomoyuki
Iwaida Manabu
Komazawa Eisuke
Nagai Aisaku
Noguchi Minoru
Hendrickson Stuart L.
Kureha Kagaku Kogyo Kabushiki Kaisha
Wenderoth , Lind & Ponack, L.L.P.
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