Metal working – Barrier layer or semiconductor device making – Barrier layer device making
Reexamination Certificate
2002-05-03
2004-03-16
Nguyen, Ha Tran (Department: 2812)
Metal working
Barrier layer or semiconductor device making
Barrier layer device making
C361S502000, C361S503000
Reexamination Certificate
active
06706079
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to electrical engineering and to capacitor engineering in particular, and can be used for manufacturing of high capacitance capacitors utilizing the energy of the electric double layer (EDL). EDL capacitors have found their use as backup power sources in systems requiring uninterrupted electric power supply, such as computers, communication devices, digital programmable lathes, continuous production cycles; for electric starting of internal combustion engines, powering the engines of wheelchairs, golf carts; etc.
DESCRIPTION OF THE PRIOR ART
Several electric power storage devices exist in the form of electric double layer (EDL) capacitors, for example as described in U.S. Pat. Nos. 4,313,084 and 4,562,511 (1985). Such capacitors consist of two porous polarizable electrodes with a porous separator made of a dielectric material between them and current collectors. A liquid electrolyte, which can be either non-aqueous or aqueous, including an aqueous sulfuric acid solution, is retained in the pores of the electrodes and the separator and in the free volume inside the capacitor case. The electric charge is accumulated in the pores on the interface between the electrode material and the electrolyte. Various porous carbon materials are normally used for manufacturing of polarizable electrodes. To increase the capacitance of the electric double layer capacitor, these carbon materials are subjected to prior activation for the purpose of increasing their specific surface area up to 300-3000 m
2
/g.
EDL capacitors have much higher capacitance than the conventional electrostatic and electrolytic capacitors—up to hundreds of farads per gram of active electrode material. However, a disadvantage of these capacitors is their rather low specific energy, not exceeding 3 Wh/l. This maximum value of specific energy for double-layer capacitors is set with non-aqueous electrolytes, where the maximum voltage values are in the range of 3 to 3.5 V. However, such capacitors permit very low discharge and charge currents due to the very low conductivity of non-aqueous electrolytes. Still lower specific energies, 0.5 to 2 Wh/l, have been achieved by double-layer capacitors using aqueous electrolytes with maximum voltage value of approximately 0.9 V. When such double-layer capacitors remain under charge for a prolonged period of time (which is often quite long) at voltages higher than 0.9 V, noticeable oxidation of the positive carbon electrode takes place.
Several methods of preparation of polarizable carbon electrodes for EDL capacitors from activated carbon powder have been described. According to one of them [set forth in European Patent No. 414420, a high-voltage pulse is applied to a layer of activated carbon resulting in heating-up the layer to a temperature of 700-1000° C. and its sintering. Thus, a compact electrode is formed.
An electric double layer capacitor having only one polarizable electrode made of a carbon material, has been described elsewhere (see e.g., Patent of Japan, Accepted Application No. 2-11008, published 1990). The other electrode is a non-polarizable (i.e., storage) one, made of lithium or lithium alloy, the electrolyte being non-aqueous. Such a capacitor has higher specific energy compared to the conventional double-layer capacitor with two polarizable electrodes. However, a drawback of this prototype in Patent of Japan, Accepted Application No. 2-11008, is the very low practical charge and discharge current (0.1 to 1 mA/cm
2
) and, therefore, very low power density as a result of using a non-aqueous electrolyte. Another essential disadvantage of the device in question is its very low cycleability—about 100-200 cycles.
An electric double layer capacitor having only one polarizable electrode made of a fibrous carbonaceous material, has been described elsewhere (see e.g. WO Patent No. 97/07518, Feb. 27, 1997). The other electrode, made of nickel oxide, is slightly polarizable. An aqueous solution of alkaline metal carbonate or hydroxide is used as electrolyte. Such a capacitor excels considerably the double-layer capacitors with two polarizable electrodes in both specific energy (up to 6 Wh/l) and maximum voltage (1.4 V). However, this capacitor has a number of drawbacks, for example it contains emergency valves that make it not entirely hermetical.
A hybrid EDL capacitor, which consists of a polarizable electrode made of porous carbonaceous material, second slightly polarizable electrode employing lead dioxide as an active component, and aqueous solution of sulfuric acid, has been described elsewhere in Patent Application PCT/RU No. 97/00353. It offers considerable advantage over the capacitor in WO Patent No. 97/07518 in its higher maximum charge voltage (approximately 2 V). Thus, specific energy of 10 Wh/kg or 20 Wh/l was achieved.
All the carbonaceous electrodes of the abovementioned capacitors have relatively low specific capacitances. Especially low values of specific capacitance (30-100 F/g) are common for non-aqueous electrolytes due to the larger dimensions of the organic molecules. In A. B. McEwen, H. L. Ngo, J. L. Goldman, T. Blakleay, W. F. Averill, //Proc. 7
th
Int. Seminar on DLC and Similar Energy Storage Devices, Dec. 8-10, 1997, Deerfield Beach, Fla., USA., very high for non-aqueous electrolytes specific capacitances (123-129 F/g) were obtained. In this study, electrolytes based on cyclic aromatic imidazole salts were used. The main disadvantage of the capacitors employing non-aqueous electrolytes is their low specific conductivity, which predetermines their low specific power.
Aqueous solutions provide considerably higher values of capacitance. A list of specific capacitances for 34 different types of activated carbons and cloths measured in 30% solution of electrolyte is given elsewhere H. Shi, Electrochimica Acta, 41 (1996) 1633. These carbons with specific surface area ranging from 552 to 2571 m
2
/g exhibited capacitances from 94 to 413 F/g.
From technical point of view closest to the present invention is the procedure proposed in Yu. Volfkovich, V. M. Mazin, N. A. Urisson, Russian Electrochemistry, 34 (1998) 825, which deals with DEL capacitors employing polarizable activated carbon electrodes and aqueous solutions of H
2
SO
4
and KOH. According to this study, said electrodes were subjected to electrochemical pretreatment consisting of keeping them at certain potential until the current densities dropped to 0.2-0.8 mA/cm
2
. The values of the potential applied ranged from the minimum carbon potential (−0.15 to 0 V vs. reference hydrogen electrode in the same solution, RHE) to the maximum carbon potential (+0.9 to +1.2 V vs. RHE). According to Yu. Volfkovich, V. M. Mazin, N. A. Urisson, Russian Electrochemistry, 34 (1998) 825, widening of the working range was ruled out because of the conjectured generation of hydrogen and oxygen. Same opinion was expressed by other authors (see e.g., N. A. Urisson, G. V. Steinberg, M. R. Tarasevich, N. M. Zagudayeva, Soviet Electrochemistry, 19 (1983) 275). The electrochemical pretreatment or formation of the polarizable activated carbon electrodes described in Yu. Volfkovich, V. M. Mazin, N. A. Urisson, Russian Electrochemistry, 34 (1998) 825 leads to a substantial increase in the specific capacitance of these electrodes, and therefore, to improvement of the resulting properties of the EDL capacitor as a whole. A disadvantage of the pretreatment method proposed in Yu. Volfkovich, V. M. Mazin, N. A. Urisson, Russian Electrochemistry, 34 (1998) 825 and N. A. Urisson, G. V. Steinberg, M. R. Tarasevich, N. M. Zagudayeva, Soviet Electrochemistry, 19 (1983) 275 is the relatively narrow potential range applied, whereas it is known that the larger the potential, the higher the capacitance in F/g.
SUMMARY OF THE INVENTION
Contrary to other studies in this field, we ascertained that hydrogen evolution on carbonaceous materials occurs only at very negative potentials. Depending on the carbonaceous material, the hydrogen evolution reaches noticeable rat
Richagov Alexey Yurievich
Shmatko Pavel Andreevich
Urisson Nina Azrielevna
Volfkovich Yuri Mironovich
C and T Company, Inc.
Kilpatrick & Stockton LLP
Nguyen Ha Tran
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