This invention pertains generally to energy storage devices, particularly high specific power and high energy density electrochemical capacitors known as supercapacitors, and specifically to a method of making active materials or 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.
A single-cell double layer capacitor consists of two electrodes which store charge (these are called the xe2x80x9cactivexe2x80x9d materials), separated by a permeable membrane which permits ionic but not electronic conductivity. Each electrode is also in contact with a current collector which provides an electrical path to the external environment. The electrodes and the membrane are infused with an electrolyte, and the entire assembly is contained in inert packaging. Multiple cells may be connected in series or in parallel in the final packaged unit.
Applying an electric potential across the electrodes causes charge to build up in the double layer which exists at the electrode/electrolyte interface of each electrode. 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 that possess high surface area ( greater than 100 m2/g) are desirable. Devices incorporating such electrodes are often referred to as xe2x80x9cdouble-layer capacitorsxe2x80x9d or xe2x80x9csupercapacitorsxe2x80x9d. By employing various materials and fabrication means, supercapacitors 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.
Supercapacitor electrode materials may store charge as a consequence of their high surface area. They may also store charge through a phenomenon known as pseudocapacitance, well-known to those skilled in the art. Pseudocapacitance is a reversible electrochemical reaction which often occurs at or near the surface of an electrode, and whose electrochemical charge/discharge properties exhibit capacitive features, distinguishing them from the processes of conventional batteries. As an example of this phenomenon, it is well known to those skilled in the art that quinone groups bonded to a supercapacitor carbon can electrochemically transform into alcohol groups in a reversible process in an acidic aqueous electrolyte, and it is known that this reaction (pseudocapacitance) provides charge storage. Quinone-containing carbon materials useful in supercapacitors thus simultaneously exhibit two types of charge storagexe2x80x94one associated with its surface area (double-layer charging), and another associated with the quinone electrochemistry (pseudocapacitance).
It is an object of this invention to provide a method for improving the energy storage of carbon materials useful as electrodes in energy storage devices. This is accomplished through a unique and advantageous surface treatment for carbons to increase their charge storage through pseudocapacitance. It consists of sulfonation and preferably sulfonation preceded by hydrogenation. While not desiring to be bound, it is believed this treatment places sulfonic acid groups on the edges of graphite planes. This enhances the carbon""s charge storage through pseudocapacitance; the pseudocapacitive reaction is the reversible electrochemical conversion of a sulfonic acid group to a sulfinic acid group which occurs upon charge and discharge. This sulfonic/sulfinic charge storage mechanism has particular advantages in that a) it occurs at a voltage in the middle of the useful range for aqueous systems, so that sulfonated carbon anodes and cathodes charge to a roughly equal degree when they are worked against one another, and b) sulfonation of any carbon or any other organic material is known to enhance proton transport, facilitating fast electrochemical reactions (and thus fast discharge in a charge storage device).
Sulfonation through direct reaction with concentrated sulfuric acid or fuming sulfuric acid produces desirable improvements in the performance of carbon supercapacitor materials; however, even greater improvements are observed if the carbon is first hydrogenated and then sulfonated. This two-step approach to sulfonation (hydrogenation followed by sulfonation) is an important part of the new art disclosed herein. Hydrogenation is believed to convert oxygen-containing functional groups on the carbon edge-planes to hydrogens, facilitating the subsequent substitution of sulfonic acid groups at these atomic sites.
In the broadest sense of the invention, the sulfonation procedure is applicable to any carbon material, regardless of whether the carbon exists as a porous monolith, powder, fiber, cloth, film, or in some other form. In the present invention, the sulfonation is particularly directed to a structural class of carbon known as fibrils.
Carbon Fibrils
There are many types of carbon structures that are observed on a microscopic scale, such as the crystalline platelets of graphite or the spherical shapes of fullerenes (xe2x80x9cbuckyballsxe2x80x9d). Another structural type is a carbon fibril, which is a thin vermicular (wormlike) carbon deposit having a diameter of about 500 nanometers or less. Carbon fibrils can exist as filaments (solid-core) or tubes (hollow core), and they are frequently referred to as xe2x80x9cnanotubesxe2x80x9d or xe2x80x9cnanofibers.xe2x80x9d Their internal structures vary: some consist of single- or multiple-wall graphite tubes oriented roughly parallel to the fibril axis, others have a graphitic tube core surrounded by a layer of amorphous carbon, and still others are layered solid filaments with varying layer orientations.
Typically, carbon fibrils are prepared through the catalytic decomposition of carbon-containing gases at heated metal surfaces. This is typically done in an inert atmosphere, using gases which only contain carbon and hydrogen (although some preparations introduce gases containing other elements). These preparations have been reported at a wide range of temperatures, from 400xc2x0 C. to 1500xc2x0 C.
The preparation and properties of carbon fibrils are described in a number of patents. Tennet U.S. Pat. No. 4,663,230, issued May 5, 1987, describes carbon fibrils which are free of an amorphous carbon coating and consist of multiple graphitic outer layers substantially parallel to the fibril axis. Tubular fibrils with graphitic layers and diameters between 3.5 and 75 nanometers are described in Tennet et al. U.S. Pat. No. 5,165,909, issued Nov. 24, 1992, Tennet et al. U.S. Pat. No. 5,171,560, issued Dec. 15, 1992, and Mandeville et al. U.S. Pat. No. 5,500,200, issued Mar. 19, 1996. For the purpose of this invention, U.S. Pat. Nos. 4,663,230; 5,165,909; 5,171,560; and 5,500,200 are incorporated herein by reference. Fibrils having different macromorphologies, such as the so-called xe2x80x9cfishbonexe2x80x9d morphology are described in European Patent Application No. 198,558 to J. W. Geus (published Oct. 22, 1986). Fibrils with the fishbone morphology may be characterized as having their c-axes (the axes which are perpendicular to the tangents of the curved layers of graphite) at some angle less than perpendicular to the cylindrical axes of the fibrils.
The surface chemistry of carbon fibrils will depend on their method of preparation. If the fibril is made from gases containing not only carbon, but also oxygen, sulfur, or nitrogen, the latter three elements can be incorporated into surface groups (groups bonded to the edge planes of carbon). Another surface chemistry would result from the following procedure: An existing fibril might be subjected to a high temperature treatment in an inert atmosphere ( greater than 1000xc2x0 C.) to graphitize any amorphous carbon remaining in the fibril. The high graphitization temperature will cleave off surface groups and leave the carbon edge atoms (carbons at the edges of the graphene planes) with a vacant valence (i.e., without a terminating group). When the sample is cooled and removed from the graphitization furnace, these edge carbons will scavenge oxygen from the air to fill the missing valence, creating a carbon fibril with an oxygen-containing surface.
Still another surface chemistry will result from the following preparation: If the carbon fibril is synthesized from a hydrocarbon feedstock containing only carbon and hydrogen using relatively low temperatures (below approx. 800xc2x0 C.), the resulting fibril is expected to contain surface hydrogen. In this case there are no elements except for hydrogen which are available to terminate the edge planes of the carbon fibrils. Furthermore, once the hydrogen becomes attached to these edge sites it should remain in place, because the temperature within the reactor is insufficient to homolytically cleave the carbon-hydrogen bond which has been formed.
Carbon Fibrils as Electrode Materials
Some carbon fibrils have proven to be useful as supercapacitor electrode materials. Some fibrils have large amounts of exposed and electrochemically active surface area, and this accessible, pore-free morphology gives them a desirable characteristic not observed in conventional supercapacitor carbons, namely significant amounts of double-layer capacitance (energy storage) at frequencies above 10 Hz. The frequency associated with a capacitance is a relative measure of the charge/discharge speed, so that higher frequency capacitance indicates higher power output for an energy storage material.
The capacitance of carbon fibrils materials can be greatly enhanced by the sulfonation procedure disclosed herein. Capacitance enhancements of up to 330% (i.e., energy storage improvements of 330%) have been observed in our laboratory when fibrils have been treated with the procedures in this disclosure. Furthermore, capacitance improvements are observed even at very high frequencies (we have made measurements at up to 1000 Hz). It is thus clear that sulfonation not only improves the fibril""s energy storage properties, it also improves their power properties. The degree of improvement for any particular carbon sample will depend on its structure.
The preferred sulfonation procedure for carbon fibrils consists of a hydrogenation followed by a sulfonation. As is the case with other carbons, the hydrogenation step converts oxygen-containing surface groups to hydrogen, creating a surface site which can be more easily sulfonated. However, in those cases where the carbon fibril surface is already hydrogenated as a consequence of its method of preparation, it may not be necessary to carry out the hydrogenation step.
The objects of the present invention together with additional objects, novel features and advantages thereof over existing prior art forms, which will become apparent to those skilled in the art from detailed disclosure of the present invention as set forth herein below, are accomplished by the improvements herein described and claimed.