Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1999-03-16
2001-10-09
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C361S502000, C423S447100, C423S447800, C429S231800
Reexamination Certificate
active
06299850
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the preparation of a type of electrolytic double layer capacitor that can store very high levels of electrical energy in the smallest possible volume and can deliver very high power. More particularly, the invention relates to the processing of the carbon electrodes used in double layer capacitors.
DESCRIPTION OF THE PRIOR ART
Double layer capacitors, also known as “supercapacitors” or “ultracapacitors”, have been in commercial use in the electronics and computer industries and are now under development as hybrid motive power sources for electric vehicles (Proceedings of the “Third International Seminar on Double Layer Capacitors and Similar Energy Storage Devices”, Dec. 6-8, 1993, Deerfield Beach, Fla.).
Conventionally, many double layer capacitors employ activated carbon as electrode material, because of its large internal surface area. This same property makes activated carbon effective in the adsorption and removal of impurities from drinking and waste water, as well as from many other liquids and gases used in industrial processes. When destined to adsorption purposes, carbon is usually prepared by heating a carbonaceous material such as coal, coconut shell, wood, cellulose, or certain man-made polymeric materials, to drive off non-carbon components, e.g., water, oxygen, hydrogen, and nitrogen. Further treatment of the product with air, steam, or carbon dioxide at 70 to 950° C. expands and extends the pore network, but also burns a considerable portion of carbon, which causes the carbon to assume very low density and to become very fragile. These processes are documented worldwide in the scientific and patent literature and have been reviewed in many articles and books (See, for example, “Carbon Adsorption Handbook”, P. N. Cheremisinoff and F. Ellerbusch, Ann Arbor Science Publishers, Inc., 1978).
For carbon used in double layer capacitor, the objective has been to maximize the porosity in order to obtain very high capacitance and energy density, while preserving structural stability. Besides the traditional activation process used to achieve the desired properties, other methods have been tried for making better double layer capacitors through the use of carbon foams. Thus, J. L. Kashmitter et al. claim that a capacitance of one to ten farads per cubic centimeter of monolithic carbon foam is possible (U.S. Pat. No. 5,260,855, issued on Nov. 9, 1993).
In conventionally activated carbons, most of the internal surface exists in micropores, which are effective in adsorbing many chemical species but are too small for electrical double layer formation. Longer, more intensive conventional activation can generate more pore area but does not significantly change the pore size distribution. As a result, excessive activation will not improve but rather will cause the quality of the carbon electrodes to deteriorate, due particularly to a very substantial weakening of the carbon structure.
It is an object of the present invention, therefore, to develop a process for increasing the available surface of carbon electrodes for double layer capacitors. It is also an object to make possible the manufacture of double capacitors having very high capacitance and very high energy storage capabilities for use in electronic devices and in hybrid motive power sources for electric vehicles. Another object is to provide high electrical energy storage in the smallest possible volume. A further object is to provide a process that preserves the mechanical strength of carbon electrodes. A further object is to make possible the assembly of very large double layer capacitors for use in electric vehicles. A still further object is to create a process that is easy to use and can achieve uniform product quality. Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned in the practice of the invention.
SUMMARY OF THE INVENTION
In order to accomplish the objects of the invention, there has been developed a new process which employs the same operating principles as conventional activations, namely oxidation and gasification of carbonized material.
According to the new process, carbon films obtained by carbonization of a polymeric material having a high degree of molecular directionality, e.g., polyimide films, are oxidized in one to five molar aqueous nitric acid at 25° C. for 5 to 15 minutes and are then pyrolyzed at a temperature of at least 350° C. for a period of at least 15 minutes. These treatments are repeated in that sequence 7 to 10 times or until the desired capacitance is obtained. A capacitance of about 300 farads per cubic centimeter can be achieved and densities within the range of 0.6 to 1.2 g/cm
3
can be obtained.
REFERENCES:
patent: 5891922 (1999-04-01), Oyama et al.
patent: 5993969 (1999-11-01), Tan
patent: 6060424 (2000-05-01), Alford
Doughty Daniel H.
Eisenmann Erhard T.
Caress Virginia B.
Durkis James C.
Hendrickson Stuart L.
McMillan Armand
The United States of America as represented by the Department of
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