Electricity: electrical systems and devices – Electrolytic systems or devices – Double layer electrolytic capacitor
Reexamination Certificate
2000-02-28
2002-01-15
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Double layer electrolytic capacitor
C361S503000, C428S469000
Reexamination Certificate
active
06339528
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a metal oxide electrode for a supercapacitor and a method for manufacturing the same, and more particularly to a metal oxide electrode for a supercapacitor including manganese oxide as an active material of the electrode, and a method for manufacturing the same.
2. Description of the Related Art
The recent advancement of scientific civilization accelerates the use of various high technology electronic devices. These devices are essential for modern life, but such devices produce many environmental problems, such as increasing waste and pollution. Considering those problems, a great deal of effort has been expended to develop an alternative energy storing device having high capacity and long durability without pollution. Also, the need for memory devices which can conveniently control various electronic devices has rapidly increased.
Because of the problem that most electronic devices are subject to memory loss, thus causing errors, when an undesired stoppage or even a variation of power occurs, the need for memory back up power continually increases. To meet such need, much research has been undertaken. One of the best solutions of recent research is development of an electrochemical capacitor, which is called a supercapacitor. It has a greatly enhanced storage capacitance which is more than hundreds to thousands of times larger than that of conventional capacitors. Also, the supercapacitor has high energy density and excellent power density which is hundreds of times more than the power density of a battery, thereby providing much stable and powerful energy to electronic devices.
The electrochemical capacitor is generally divided into three categories, such as an Electric Double Layer Capacitor (ELDC), a metal oxide pseudocapacitor and a conducting polymer capacitor, depending on the energy storage mechanism and active materials used in each system. In the metal oxide pseudocapacitor, the active material is generally conductive metal oxide which has high surface area and electrochemical reactivity with working ions in an electrolyte. Electrochemical reduction and oxidation reactions as well as physical charge separation between an electrode and electrolyte interface, are the energy storage mechanisms. On the other hand, the ELDC uses activated carbon with a large surface area as an active material, and physical charge separation between an electrode and an electrolyte interface is the main energy storage mechanism.
It is generally appreciated that the metal oxide pseudocapacitor can obtain higher capacitance than the EDLC because, as mentioned above, the metal oxide pseudocapacitor can get its capacitance using an electrochemical redox reaction as well as the physical charge separation at the electrolyte interface, whereas the EDLC system only obtains its capacitance from the physical charge separation at the electrolyte interface.
The supercapacitor generally consists of porous active material electrodes, a separator, an electrolyte, a current collector, a case and terminals. The current collector can be composed of high electrical conductivity material, such as metal or a conducting film. The case and the terminals should be composed of light materials to reduce the weight of the capacitor. The separator and the electrolyte relate to the ionic conductivity of the capacitor. The current collector and the terminals are concerned with electrical conductivity of the capacitor. The electrical and the ionic conductivities are the main factors in determining the output characteristics of the capacitor.
The manganese oxide with layered structure can be a candidate for an electrode of the metal oxide pseudocapacitor, which has been studied as an electrode of rechargeable batteries. The manganese oxide, including layered structure having potassium ions therein, is obtained by thermally decomposing potassium permanganate or chemical reactions.
The reaction of the supercapacitor is a surface reaction, while the reaction of the manganese oxide is an intercalation reaction. Thus, such manganese oxide may not be applied to the electrode of the supercapacitor because the supercapacitor has a rapidly charging/discharging, wide permissible temperature range and high electrical conductivity. But, depending on a condition of material synthesis, such as cooling rate, surface condition as a mean valence of manganese ion of material can be changed to show good capacitor performance. Moreover, intercalation reaction also may contribute to the total capacitance of the material in special capacitor operation, such as slow charge and discharge conditions.
Ruthenium oxide (RuO
2
) has recently been utilized as an electrode of a capacitor. The capacitor using ruthenium oxide as an electrode has a high capacitance of about 700 F/g, which is much higher than that of conventional capacitors. However, ruthenium oxide is too expensive to apply to the capacitor electrodes, i.e., the manufacturing cost of a ruthenium oxide electrode is hundreds of times higher than that of a conventional electrode. Furthermore, the high capacitance of ruthenium oxide can be obtained only when an acid solution, such as that of sulfuric acid (H
2
SO
4
), is used therewith, which causes a serious environmental hazard.
The present inventors reported that amorphous manganese oxide has good properties as an electrode for a supercapacitor in a neutral electrolyte, such as potassium chloride (refer to
Journal of Solid State Chemistry,
vol. 144, pages 220 to 223, 1999, entitled “SUPERCAPACITOR BEHAVIOR WITH KCl ELECTROLYTE”). However, when amorphous manganese oxide is directly used as the electrode of a supercapacitor, the Equivalent Serial Resistance (ESR) of the supercapacitor may be greatly increased when operating at a high frequency, and the energy loss of the supercapacitor may be seriously increased at a low frequency because amorphous manganese oxide has low conductivity at room temperature.
Although the electrode for the supercapacitor is manufactured by physically mixing conductive carbon having good electrical conductivity with amorphous manganese oxide, the increasing capacitance of the supercapacitor and the reducing volume of the supercapacitor may be limited since little manganese oxide can be included in conductive carbon by specific volume. Also, the physical mixing process has some disadvantages; the contact area between the manganese oxide and the conductive carbon is reduced, and the degree of dispersion of the manganese is limited.
SUMMARY OF THE INVENTION
Considering the above-described problems and disadvantages, it is an object of the present invention to provide a metal oxide electrode for a supercapacitor having high capacitance.
It is another object of the present invention to provide a method for manufacturing a metal oxide electrode for a supercapacitor having high capacitance.
It is still another object of the present invention to provide a metal oxide electrode for a supercapacitor having a low ESR and an enhanced high frequency characteristic in a neutral electrolyte.
It is still another object of the present invention to provide a method for manufacturing a metal oxide electrode for a supercapacitor having a low ESR and an enhanced high frequency characteristic in a neutral electrolyte.
To achieve these objects, the present invention provides a metal oxide electrode comprising manganese oxide powder, conductive material and binder.
Preferably, the binder is composed of or comprises polytetrafluoroethylene.
According to one embodiment of the present invention, the conductive material is conductive carbon and the manganese oxide powder is coated on the conductive carbon. As for the present invention, many kinds of highly conductive materials can replace the conductive carbon because the roles of the conductive carbon of the present invention are making an electrical conduction path and sites of the amorphous manganese oxide coating. Therefore, basically all conductive materials, such as metal oxide, metal nitride, m
Kim Heui-Soo
Kim Sun-Wook
Lee Hee-Young
Seong Woo-Kyeong
Dinkins Anthony
Jacobson & Holman PLLC
Ness Capacitor Co., Ltd.
Thomas Eric W.
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