Background of the Invention

This invention relates to high bulk density packing of carbon fibers oriented in a parallel array and methods for the production of this material. It also describes the use of this material in a variety of devices, including electric double layer capacitors containing both single and bipolar carbon electrodes, fuel cells, batteries, and other devices.

The following patent summaries are indicative of the prior art.

U.S. Patent No. 5,102,745 describes a family of composites that are characterized as a network of a first fiber and at least a second fiber, where at least the first fibers have multiplicity of bonded junctions at their point of crossing. The largest class has metals as one or both of the fibers, although the second fiber can be of materials such as carbon, ceramics, and high surface area materials. The composites can be simply prepared and manifest enormous

variation in such properties as void volume, pore size, and electrical properties generally.

U.S. Patent No. 5,099,398 describes an electric double layer capacitor having a pair of polarized electrodes in the form of electrode bodies each comprising a porous sintered body, and a pair of current collectors for collecting electric charges stored in the polarized electrodes, the current collectors being in the form of electrically conductive films. The electrically conductive films have surfaces dissolved by a solvent and joined to the electrode bodies with dissolved portions of the electrically conductive films being present in the pores of the electrode bodies. The polarized electrodes and the current collectors are held in stable contact with each other, with a reduced contact resistance there between. The electric double layer capacitor thus has a small internal resistance.

U.S. Patent No. 5,096,663 describes composites of a matrix of metal fibers and carbon fibers interlocked in and interwoven among a network of fused metal fibers that are inherently capable of displaying a broad range of values of a particular physical property. Where the composite is made by sintering a preform of the fiber network dispersed in a matrix of an organic binder, the value of the physical property of the resulting composite is a function of several independent variables which can be controlled during composite fabrication. With

particular regard to the capacitance of a stainless steel-carbon fiber electrode, there is described a method of optimizing capacitance during electrode fabrication.

U.S. Patent No. 5,080,963 describes a new class of composites resulting from a matrix of carbon fibers, including graphite fibers, interwoven in a network of fused metal fibers. The composites can be fabricated to have varying surface area, void volume, and pore size while maintaining high electrical conductivity. Composites are readily prepared from a preform of a dispersion of carbon fibers, metal fibers, and an organic binder such as cellulose, by heating the preform at a temperature sufficient to fuse the metal fibers and to volatilize at least 90 percent of the binder with a loss of less than about 25 percent, and usually under 10 percent, by weight of carbon fiber.

U.S. Patent No. 5,077,634 describes an electric double layer capacitor employing a paste electrode between collector electrodes. The paste electrode is compressed with little pressure by compression plates in only an area corresponding to the paste electrode. With the paste electrode thus compressed, the internal resistance of the capacitor is reduced and the capacitance of the capacitor is stable. The electric double layer capacitor comprises a stack of basic cells of capacitor elements. The basic cells are held closely together by a bolt which extends through central holes in the basic cells

and interconnects the compression plates, so that the basic cells are compressed under a uniform and appropriate pressure.

5 Carbon that is used in capacitors and electrodes of the known art is used in the form of powders and granules or fibers. Carbon powder or granules are usually activated and then randomly packed in a container and often

10 held together with a matrix material. Both types of packing lead to large interparticle spaces and consequent high void volumes, resulting in a reduction of the active site concentration and thus reducing the effectiveness and efficiency of

15 their use.

In fabric-like structures, carbon fibers are typically added to the structure. These fibers can either be activated before being added to the 20 structure or after formation of the hybrid carbon fiber-fabric structure. In either case, the result is a low bulk density of carbon fibers. As with powders and granules, there are large interfiber spaces and a lowered active site concentration. 25 This structure is also accompanied by a high electrical contact resistance due to the parallel orientation of the fibers in relation to electrical collectors or contacts.

30 An alternative is to process activated carbon fibers into a fabric form. However, these fibers are too fragile for processing by traditional textile fabrication techniques. This inherent weakness of the carbon fibers is due to

the high degree of porosity introduced into the fibers by activating them. As a result, stronger fibers, often of metal, or other processing aids are added. These additives then reduce the concentration of activated sites even further, thus resulting in a similar situation as is the case with the above-described powders or granules and woven fabrics.

Therefore, it is an object of the invention to provide a carbon fiber structure having small, but continuous, uninterrupted interparticle spaces and low void volumes. Another object of the invention is to provide a carbon fiber structure without additives that can be assembled after fiber activation. Yet another object of this invention is to assemble a high density, parallel array of carbon fibers that can be manufactured after fiber activation, thereby providing the necessary high concentration of activated carbon sites. Still another object of this invention is to orient the high density, parallel array of carbon fibers of this structure perpendicular to the current collector, thereby optimizing the electronic conductivity between the current collector and the active sites.

Summary of the Invention

The invention described herein is a high bulk density, parallel array of carbon fibers.

This structure has a low interfiber void volume and a resultant high activated site concentration when the fibers are activated. Fibers are con¬ densed to high bulk density after activation, the

resultant structure appearing as a solid cylin¬ drical block upon cutting it in cross-section, i.e., perpendicular to the axis of the fibers, and then taking the form of a wafer. The structure can be shaped to cross-sectional forms other than a cylinder during condensation.

The high bulk density, parallel carbon fiber structure can be used in a number of applications. The structure can be used to store increased amounts of electric charge in a capacitor. It can be used as an electrode in a fuel cell. It can also be used as an electrode in a high power chemical battery, such as a lithium- based battery. More generally, it can be used for storage and separation of gases and liquids by means of adsorption, for transporting thermal energy, and for use in friction and structural applications. The included applications are given as examples only and they are not meant to be limiting in any sense.

By preparing carbon fibers in such a manner, the previously noted problems of high void volume and the low concentration of active sites are much reduced. This increased concentration of active sites in the structure transforms the structure into one with multiple advantages over the prior art that can be employed in a variety of applications in which carbon is used.

Other objects, features, and advantages will be apparent from the following detailed

description of preferred embodiments taken in conjunction with the accompanying drawings in which:

Brief Description of the Drawings

Fig. 1 is a cross-sectional view of the carbon fiber array enclosed in a container prior to compression.

Fig. 2 is a cross-sectional view of the carbon fiber array enclosed in a container after compression.

Fig. 3 is a side view of a diagrammatic drawing of a double layer capacitor that is con¬ structed with the high bulk density, parallel carbon fiber structure before welding the two component wafers together.

Fig. 4 is a side view of a diagrammatic drawing of a double layer capacitor that is con¬ structed with the high bulk density, parallel carbon fiber structure after welding the two component wafers together.

Detailed Description of Preferred

Embodim ents The invention consists of a structure comprising an array of carbon fibers (herein referred to as a carbon fiber array) oriented parallel to one another whose structure has been compressed, thereby increasing its density. Parallel alignment of the array can be accomplished by simply bundling unactivated or

activated carbon fiber tow, with or without the use of a container. Parallel alignment can also be accomplished by actions such as twisting, cabling, braiding, a combination of these tech- niques, and other like methods.

The carbon fibers used to construct the structure of this invention have widths of between 0.1 and 100 micrometers. The carbon fibers can be of any suitable shape. When activated carbon fibers are used, the resulting structure has a surface area of between 0.1 and 4000 square meters per gram. Similarly, this structure also has interfiber void volumes between 5 and 25 percent. The carbon fibers are activated by known methods in the art.

The structure of this invention can be enclosed in a container comprised of various materials. In a preferred embodiment, a thermoplastic tube is used for compressing the array. The tube can be made of plastics, such as chlorinated polyvinylchloride, or other similar materials like polypropylene. Materials other than plastic can also be used to comprise the tube. The tube can be filled with the array by pulling the carbon fibers through the tube or by extruding the tube over the fibers.

The structure of this invention can be produced by heating a thermoplastic container which holds the array of activated fibers oriented parallel to one another to the glass transitional temperature (Tg) of the container.

When softened, a pressure of between 1,000 and 40,000 pounds per square inch is applied in order to deform the container inwardly, thereby compressing the array and increasing its density. The compression force is maintained while the tube is cooled to its rigid temperature.

Fig. 1 shows the bundled carbon fiber tow (10) enclosed in a deformable container (12) before compression. Fig. 2 shows the resultant structure of this invention (14) after the encased carbon fiber tow of Fig. 1 (10) has been compressed by the deformed container (16).

The structure described in this invention can be used in many applications. The following are meant to be illustrative examples of its use and, by their delineation, its use is not intended to be limited only to these applications. Rather, the structure itself is claimed as a part of any device in which it can be used.

Electrochemical Applications. In electrochemical applications, such as capacitors, fuel cells, and batteries, the structure of this invention enables high electrical conductivity and fast transport of electrolytes along the individual carbon fiber axes to be achieved. The low void volume within the fibers also allows a large amount of energy to be stored. This provides high electronic conductivity and power densities.

The collectors and electrodes of the following electrochemical applications that use the structure of this invention can be formed of thin slices, on the order of 0.25 to 10 millimeters, by slicing or wafering the structure perpendicular to the fiber axes. The structure can be sliced or wafered using a laser, a diamond wire saw, an internal diameter (ID) saw, or a like cutting apparatus.

The slice can then be attached to the charge or current collector by bonding the slice and the collector using a variety of methods. For example, metals can be deposited on the fiber tips by means of electroplating, sputtering, plasma spray, and other techniques. Alternatively, thermoset conductive polymers can be cross-linked, thermoplast conductive polymer can be welded, or thermal sprayed to fiber tips. These methods are used to reduce contact resistance between the tips and the collectors.

Capacitors. Double layer capacitors can be produced using the structure described in this invention. An energy density of 5.2 Watt-hours per liter has been observed when the capacitance density of the activated carbon fibers is 34 Farad per gram at a carbon fiber density of 1.1 gram per cubic centimeter at 1 volt, using an electrolyte consisting of 7 moles of potassium hydroxide per liter of water.

The orientation of the individual, activated carbon fibers is in a direction perpendicular to the current collector. This structure has the ability to store large amounts of static electric charge at the interface of the electrolyte and the carbon fibers while retaining both low electrical resistance and high bulk density. Bipolar capaci¬ tors can also be made from this structure by bonding two carbon wafer electrodes on the opposing sides of a single current collector. Both the single and bipolar electrodes show efficient electrical storage properties. Pseudocapacitance is observed with the structure of this invention, and can be further enhanced by modifying the carbon surface species.

Fig. 3 shows a double layer capacitor before bonding together the two wafers of the structure of this invention. The high bulk density, parallel array of carbon fibers that has been compressed (20) is shown diagrammatically. This structure is enclosed in a container (22). The structure of this invention enclosed in this container has been attached to the current collectors (24). A porous, electrically insulating separator (26) has been placed between the two wafers of this structure. Fig. 4 shows the same structure after the two wafers have been welded together with the charge separator (26) separating the wafers.

Such a device enables fast discharge and has no life cycle limits apparently due to the absence of a redox reaction. In applications

requiring high power output, such a device would complement other electrical storage devices by acting as an accumulator for fast charging and discharging at high output levels during high demand.

Example 1 : 4 kilograms of oxidized 10 micron continuous acrylic fibers were carbonized at 700 degrees Centigrade under a blanket of nitrogen and steam for twenty minutes. These carbonized fibers were then drawn into a polyvinylchloride pipe with an inside diameter of 1 centimeter. The pipe with its enclosed carbon fibers were heated to 100 degrees Centigrade and a hydraulic force of 6,000 pounds per square inch was applied uniformly to the pipe and its enclosed carbon fibers to compress the overall diameter by 4 percent. From one end of the resulting structure, a 1 millimeter length of the structure was cut perpendicular to the long axis. The resulting disk contained highly compressed carbon fibers within the cylindrical polyvinylchloride pipe section. The carbon fiber array possessed an inter-fiber porosity of 17 percent. When filled with 7 moles of potassium hydroxide per liter water, the array showed a capacitance at 1 volt that was equivalent to 70 Farads per gram of carbon.

Batteries. The structure of this invention is also suitable for use in high power batteries.

This suitability is due to the high conductivity of electrodes made from this structure and the absence of metals in this structure. These

batteries show reduced series resistance, greater charge/discharge rates, and improved life cycles.

The structure of this invention can be engineered to optimize the intercalation of lithium ions within it. Graphitic, non-activated carbon fibers with a density of 1.7 grams per cubic centimeter can be used is making this carbon fiber-lithium ion anode. The cathode is most typically made from manganese or cobalt oxide.

Fuel Cell. The structure described in this invention can be used as an alternative to structures that are employed in fuel cells. A fuel cell electrode made from the structure of this invention can be engineered with the desired porosity. The high porosity, low void volume structure described in this invention can assist in heat and mass transfer at the electrode interface and within the component high density, carbon fibers. Thermal and electrical conductivity is about a factor of two greater along the fiber axes than across them. The ordered array of fibers and the interfiber voids allow optimization of the transport of the reactant through the electrode.

Gas Storage. Adsorption of various gases is a desirable goal in their intermediate storage and separation. The key to the use of the struc- ture of this invention in the efficient adsorption and desorption of gases is in engineering the microporosity of the carbon fiber to approximately four times the molecular size of the adsorbate gas. This must be done while

maintaining high surface area and reducing the interfiber void volume in order to maintain accessibility of the entire carbon bed for gas adsorption and desorption.

The goal in natural gas storage is to adsorb as great a volume of gas as possible per unit volume of the container. Currently, the best storage capacity using activated carbon is 150 gas volumes to one carbon fiber array volume at 500 pounds per square inch with the bulk density of the activated carbon fibers being 0.6 grams per cubic centimeter. The bulk density of 1 gram per cubic centimeter and greater seen with the structure of this invention will allow a storage capacity of at least 200 natural gas volumes to one carbon fiber array volume. Using this structure for natural gas storage, mainly methane can be stored at a reduced pressure of 500 pounds per square inch.

Other gas adsorption examples are ammonia adsorption and desorption using low- grade heat that can be used for refrigeration. Also, hydrogen can be stored at cryogenic temperatures.

Separation. The high surface area of the carbon fiber array described in this invention makes it suitable for separation of volatile organic compounds. The microporosity of this structure creates capillary condensation that is useful for the absorption and separation of these compounds.

It will now be apparent to those skilled in the art that other embodiments, improvements, details, and uses can be made consistent with the letter and spirit of the foregoing disclosure and within the scope of this patent, which is limited only by the following claims, construed in accordance with the patent law, including the doctrine of equivalents.

What is claimed is: