A method for making near-net-shape, monolithic carbon electrodes for energy storage devices. The method includes the controlled pyrolysis and activation of a pressed shape of methyl cellulose powder with pyrolysis being carried out in two stages; pre-oxidation, preferably in air at a temperature between 200.degree.-250.degree. C., followed by carbonization under an inert atmosphere. An activation step to adjust the surface area of the carbon shape to a value desirable for the application being considered, including heating the carbon shape in an oxidizing atmosphere to a temperature of at least 300.degree. C., follows carbonization.
STATEMENT OF GOVERNMENT INTEREST
This invention was made with Government support under contract no. DE - AC04- 94AL8500 awarded by the U. S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention.
BACKGROUND OF THE INVENTION
This invention pertains generally to energy storage devices, particularly high specific power and high energy density capacitors, supercapacitors, and specifically to a method of making electrodes for the same.
There is a need for a rechargeable energy source that can provide high power, can be recharged quickly, has a high cycle life and is environmentally benign for a myriad of applications including defense, consumer goods and electric vehicles. Double layer capacitors are rechargeable charge storage devices that fulfill this need.
Double layer capacitors are devices in which two electrodes are electronically insulated from one another and which contain an electrolyte which permits ionic but not electronic conductivity. Applying an electric potential across the electrodes causes charge to build up in the double layer which exists at the electrode/electrolyte interface. This process can continue until a condition of equilibrium has been reached, i.e., the current drops to zero. The capacitance, or amount of charge that a capacitor can store, is directly related to the surface area of the electrodes. Therefore, electrodes made from conductive materials and that possess high surface area (>100 m.sup.2 /g) are desirable. By employing various materials and fabrication means capacitors have been developed which are capable of delivering very high specific power and energy densities.
Because carbon is chemically inert, has a high electronic conductivity, is environmentally benign and is relatively inexpensive, it is a desirable material for fabricating electrodes for supercapacitors. High surface area carbon powders are presently preferred for use in fabricating supercapacitor electrodes. The internal resistance of carbon powder electrodes is dependent upon the extent and quality of particle-to-particle contact. As the quality and extent of these contacts decreases the internal resistance of the electrode increases which in turn reduces the usable stored charge in the capacitor. In some applications the electrodes are often maintained under high compression in an attempt to make them more conductive. Binders are often used to fabricate freestanding electrodes from carbon powders. However, the binders, generally being of higher resistance than the carbon particles they surround, will increase the particle-to-particle resistance thereby degrading the performance of the electrodes.
In addition to methods well known in the art for fabricating high surface area carbon electrodes such as employing a binder, the use of carbon paste electrodes or high pressure, other methods of fabricating these electrodes to improve their conductivity have been developed. U.S. Pat. Nos. 5,150,283 and 4,327,400 disclose electrodes composed of electrically conducting substrates into which or upon which carbon powder in various forms is impressed. A method of fabricating electrodes which have high specific surface area is disclosed in U.S. Patent No. 4,597,028. Here activated carbon fibers are woven into a fabric which is used to fabricate electrodes. Compounds which improve the conductivity of carbon powder electrodes have been also employed as disclosed in U.S. Pat. No. 4,633,372. All these methods suffer from the disadvantage that they require additional fabrication steps which can be expensive and complex.
It has been recognized that one way to overcome the problems associated with carbon powder electrodes is to develop carbon in the form of a continuous, monolithic structure and prepared in such a way as to possess the desirable properties of high surface area and low electrical resistance. As illustrated in U.S. Pat. Nos. 5,260,855; 5,021,462; 5,208,003; 4, 832,881; 4,806,290 and 4,775,655 carbon foams, aerogels and microcellular carbons have been developed which are useful as electrode materials in high energy density capacitor applications, because they possess high surface area, low electrical resistance and adequate mechanical strength. While these materials represent an improvement over conventional carbon powder electrodes for supercapacitors they have several disadvantages in comparison with the present invention. Methods used to prepare carbon foams, aerogels and microcellular carbons require elaborate processing steps to prepare the precursor materials; among other things, the solvents must be completely removed from the precursor prior to the carbonization step. In order not to disrupt the microstructure of the polymer precursor the solvent removal step must be done under carefully controlled conditions using, for example, freeze drying or supercritical extraction. Furthermore, the solvents must either be disposed of or purified prior to reuse. In addition, before the carbonized product produced by these prior art processes can be used additional fabrication steps, such as machining, must be employed. The method disclosed in the present invention overcomes the disadvantages of prior art processes for producing high surface area, continuous structure carbon electrodes.
It is well known in the art that the surface area of carbons, in the form of powder or a monolithic structure, can be increased by a process known as activation. Generally, the process involves exposing carbon to an oxidant which can be a gas or an oxidizing chemical. U.S. Pat. Nos. 3,652,902 and 4,327,400, for example, disclose a process for activating carbon powder by heating in steam or oxygen. While it has been recognized that activation could be used to enhance the surface area of carbon, heretofore little, if any, attempt has been made to control the process. The present invention employs a carefully controlled activation step to produce monolithic carbons having superior properties for use in double layer capacitors.