MXPA97002485A - Capacitors of double layer, of high functioning that includes composite electrodes of aluminum ycar - Google Patents

Abstract

A high performance double layer capacitor is described, having an electric double layer formed at the interface between the activated carbon and an electrolyte. The high-performance double-layer capacitor includes a pair of carbon-impregnated aluminum-impregnated electrodes having a uniformly distributed and continuous aluminum-impregnated path inside a preform of activated carbon fiber saturated with a high-performance electrolytic solution. The high-performance double-layer capacitor is capable of supplying at least 5 Wh / kg of useful energy at power ratings of at least 600 W /

Description

DOUBLE LAYER CAPACITORS. HIGH FUNCTIONING INCLUDING ALUMINUM AND CARBON COMPOSITE ELECTRODES DESCRIPTION OF THE INVENTION The present invention relates generally to a double-layer, electric capacitor and more particularly to a high-performance double capacitor, comprising electrodes composed of aluminum and carbon and a high performance electrolytic solution and also includes a method to manufacture them. Dual-layer capacitors are energy storage devices that are capable of storing more energy by weight than traditional capacitors and can typically supply power at a higher power rating than many rechargeable beetteries. The double-layer capacitors consist of two porous electrodes that are isolated from the electrical contact by a porous separator. Both of the separator and the electrodes are impregnated with an electrolytic solution. This allows the ionic current to flow between the electrodes while preventing electrical current from short-circuiting the cell. On the back of each of these active electrodes there is a plate that collects the current. One purpose of the plate that collects the current is to reduce the ohmic losses in the double layer capacitor. If these current collection plates are not porous, they can also be used as part of the capacitor seal. When the electrical power is applied through the electrodes in a double layer capacitor, the ionic current flows due to the attraction of anions for the positive electrode and cations for the negative electrode. Upon reaching the electrode surface, the charge is absorbed in the liquid-solid interface region. This is done by absorption of the same cargo species or by realignment of the bipoles of the solvent molecule. The absorbed load is maintained in the region by the opposite charges on the solid electrode. The use of carbon electrodes in electrochemical capacitors represents a significant advantage in this technology, because coal has a low atomic weight and carbon electrodes can be manufactured with very high surface areas. The manufacture of double-layer capacitors with carbon electrodes has been known in the art for some time, as evidenced by U.S. Patent Nos. 2,800,616 (Becker), and 3,648,126 (Boos et al.). A major problem in many carbon electrode capacitors is that operation is often limited, due to the high internal resistance of most carbon electrodes. This high internal resistance is mainly due to the high contact resistance of the carbon-carbon contacts. This high resistance translates into large ohmic losses in the capacitor during the discharge phase. Decreasing this internal resistance in the double layer capacitors is mainly achieved through the reduction of the electronic resistance in the electrode. It is also difficult to simultaneously achieve a combination of large surface area and sufficient control of the porosity of the carbon electrode. The porosity of the carbon electrode is translated to the degree of accessibility of the electrolyte to the surface of the carbon atoms. To increase the operating voltage of many double-layer capacitors, individual cells are often stacked in series. The current routes between the cells must be reduced to a minimum to reduce the ohmic losses. The optimal design is to have adjacent cells, separated only by a single current collection plate. This plate must not be porous, in such a way that no electrolytic solution is shared between the cells. The separation avoids losses due to the current path between the cells. This type of design is called bipolar. In a bipolar double layer capacitorone side acts as a positive electrode and the other side acts as a negative electrode for an adjacent cell. U.S. Patent No. 3,536,963, issued to D. L. Boos is an example of this bipolar double layer capacitor. Another dual-layer capacitor design that has recently become popular is a spirally wound cell. U.S. Patent No. 5,150,283 issued to Yoshida et al., Describes an example of the spirally wound cell. In the Yoshida et al. Patent, a double-layer, electric capacitor has a pair of polarizable electrodes composed of electrically conductive substrates. The substrates are coated in layers of a mixture of activated carbon with an agglutination agent based on water-soluble material. The electrodes face each other interposed by a separator impregnated with an electrolyte. This capacitor has advantageous characteristics of both of the conventional capacitors, which use aqueous electrolytes and capacitors which typically use organic solvent electrolytes. One advantage of the spirally wound cell of the double layer capacitor is that the large surface area electrodes can be wound up in a small housing. The large electrodes greatly reduce the internal resistance of the capacitor and the housing greatly simplifies the capacitor seal or the seal required for the double layer capacitor. In a bipolar design, each cell must be sealed around the perimeter of the electrode. However, in a winding design, only the exterior may require sealing. This design is not as efficient as a bipolar design, when the cells are stacked in series, because the resistance of the wire cables will add to the ohmic losses. The present invention, however, is more related to electrochemical or double layer capacitors that have aluminum / carbon composite electrodes. The metal / carbon compounds and more particularly, the aluminum / carbon composite electrodes, tend to minimize the internal resistance of the electrode. The teachings in the related art that are of particular importance involve fabrication methods of the aluminum / carbon composite electrodes, methods of making and adhering the current collector to the composite electrode and the suitable electrolytes that can be used with double layer capacitors. high performance. Several manufacturing techniques to reduce the internal resistance of carbon composite electrodes have been described in recent years. For example, Yoshida et al., In the patent (U.S. Patent No. 5,150,283) discloses a method of manufacturing an aluminum / carbon composite electrode by depositing carbon dust and other agents that improve electrical conductivity in a substrate of aluminum. Another related approach is described in U.S. Patent No. 4,597,028 (Yoshida et al.), Which teaches that incorporation of metals such as aluminum into carbon fiber electrodes can be accomplished by means of woven metal fibers within preforms of carbon fibers. U.S. Patent No. 4,562,511 (Nishino et al.) Describes a different approach, where the carbon fiber is submerged in an aqueous solution, so that a layer of conductive metal oxide and preferably a transition metal oxide, is formed in the pores of carbon fibers. Nishino et al. they also describe the formation of metal oxides, such as tin oxide or indium oxide by vapor deposition. Still another related method is described in U.S. Patent Nos. 5,102,745, 5,304,330 and 5,080,963 (Tatarchuk et al.). These descriptions demonstrate that the metal fibers can be intertwined with the carbon preform and sintered to create a structurally stable conductive matrix, which can be used as a composite electrode. The patents of Tatarchuk et al. they also teach a process that reduces the electrical resistance in the composite electrode by reducing the number of carbon-carbon contacts, which current must flow through them to reach the metal conductor. This approach works well, if the nickel or stainless steel fibers are used as the metal. However, this approach has not been successful when the aluminum fibers are used due to the formation of aluminum carbide during the sintering or heating of the composite electrode. The use of aluminum in the manufacturing process of double layer capacitors is important because aluminum is the optimal metal in terms of cost, availability and operation. For example, with an aluminum / carbon composite electrode in a double layer capacitor with a non-aqueous electrolyte, it is quite possible to achieve an operating voltage of 3.0 volts. However, with nickel or stainless steel instead of aluminum, the operating voltage must be reduced to less than 2.0 volts. Related designs of double layer capacitors are also discussed in U.S. Patent No. 4,438,481 issued to Phillips et al .; U.S. Patent No. 4,597,028 issued to Yoshida, et al .; U.S. Patent No. 4,709,303 issued to Fuji ara, et al .; U.S. Patent No. 4,725,927, issued to Morimoto; and U.S. Patent No. 5,136,472, issued to Tsuchiya, et al.

Another area of great interest in the manufacture of double-layer capacitors is related to the method of manufacturing the current collector plate and attaching the current collector plate to the electrode. This is important because the interface between the electrode and the current collector plate is another source of internal resistance of the double layer capacitor. The patent Nishino et al. (Patent of the States No. 4,562,511) suggests plasma spraying of molten metals such as aluminum on one side of the polarizable electrode, thereby forming an appropriate layer which acts as the current collector. This patent also considers alternative techniques for joining and / or forming the current collector that includes arc sprinkling, vacuum deposition, cathodic sublimation, non-electrolytic electrodeposition and the use of conductive paints. The patents of Tatarchuk et al. (Patents of the United States Nos. ,102,745, 5,304,330 and 5,080,963) shows the connection of a current collector of a thin sheet of metal to the electrode by sintering the junction of the thin metal sheet to the electrode element. U.S. Patent No. 5,142,451 (Kurabayashi et al.) Discloses a method for attaching the current collector to the electrode by a heat curing process, such that the material from the current collectors enters the pores of the electrode. the electrode elements. U.S. Patent No. 5,099,398 (Kurabayashi et al.) Discloses a method of attaching the current collector to the electrode by chemically bonding a thin film collector, such that some of the material of the current collectors enters the the pores of the electrode elements. This patent further discloses some other conventional methods of joining the current collector to the electrode include the use of electrically conductive adhesives and bonding under pressure and heat. Still another technique related to the method of making and adhering the current collector plates can be found in US Pat. Nos. 5,065,286.; 5,072,335; 5,072,336; 5,072,337; and 5,121,301 all issued to Kurabayashi et al. Accordingly, there is a continuing need to improve double layer capacitors with carbon / aluminum composite electrodes. These improved double layer capacitors need to supply large amounts of useful power at very high power output and power density indices. These improved double-layer capacitors must also have a relatively low internal resistance and still be able to produce a relatively high operating voltage.In addition, it is also apparent that improvements are needed in the techniques and manufacturing methods of double-layer capacitors with aluminum / carbon composite electrodes in an effort to decrease the internal resistance of the double-layer capacitor and to maximize the operation. Since the energy density of the capacitor increases with the square of the operating voltage, the higher operating voltages translate into greater operation of the ceipacitores, due to the significantly higher energy densities and power output indices. The present invention is a high performance double layer capacitor, having an electric double layer formed at the interface between the activated carbon and an electrolyte. The high-performance double-layer capeicitor includes a pair of carbon-impregnated aluminum-impregnated electrodes having a uniformly distributed and continuous path of aluminum impregnated within an activated carbon fiber preform and saturated with a high-performance electrolytic solution. The high-performance double-layer capacitor is capable of supplying at least 5 Wh / kg of useful energy at power ratings of at least 600 W / kg. The present invention also identifies methods for manufacturing the high performance double layer capacitor.

The present invention further includes an improved method for manufacturing high performance double layer capacitors by impregnating cast aluminum in carbon fiber preforms. The present invention also identifies several improvements in the fabrication of aluminum / carbon composite electrodes and techniques for attaching the current collector plate to the electrode. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed description, reference will be made to the accompanying drawings, in which: FIGURE 1 is a sectional view of a high-performance, double-layer, single-cell capacitor in accordance with the present invention; FIGURE 2 is a sectional representation of an aluminum / bipolar carbon composite electrode according to the present invention; and FIGURE 3 is a sectional representation of a series stack of high performance bipolar capacitors of the double layer capacitor type. The high performance double layer capacitor described herein is preferably a large bipolar capacitor of the double layer capacitor type which can supply a large amount of useful energy at very high power output and power density indices. Specifically, the high performance double layer capacitor is capable of supplying at least 5 Wh / kg of useful energy at power ratings of at least 600 W / kg. In addition, the preferred high-performance double-layer capacitor demonstrates a relatively low internal resistance, producing a charge / discharge efficiency ratio of at least 90% and also demonstrates relatively high operating voltages of approximately 3.0 volts for a capacitor of a single cell. When configured as a serial stack of bipolar-type double-layer capacitors, the high-performance capacitor stack operates, for example, as much as 350 volts and will store approximately 1.8 MJ of energy. The high performance double layer capacitor preferably includes advanced aluminum / carbon composite electrodes with a high performance electrolytic solution. Of particular importance is the advantageous method of manufacturing the aluminum / carbon composite electrodes and the method of adhering the current collector that are employed with the present high performance double layer capacitors. With reference to FIGURE 1, a high-performance, single-cell double-layer capacitor 10 is illustrated including a cell holder 11, a pair of aluminum / carbon composite electrodes 12 and 14, an electronic separator 18, an electrolyte 20, a pair of current collector plates 22 and 24, electrical cables 28 and 29, which extend from the current collector plates 22 and 24. The pair of aluminum / carbon composite electrodes 12 and 14 are preferably formed from a preformed carbon cloth, porous or carbon paper sheet which is impregnated with molten aluminum. The porosity of the aluminum / carbon composite electrodes 12 and 14 can be intimately controlled during the impregnation process, to subsequently allow a sufficient quantity of the electrolyte 20 to be introduced into the double layer capacitor 10 and penetrate the pores of the fibers of coal. The pair of current collector plates 22 and 24 are attached to the back of each aluminum / carbon composite electrode 12 and 14. Preferably, the current collector plates 22 and 24 are thin layers of a thin sheet of aluminum. In this configuration of the single-cell capacitor, the current collection plates 22 and 24 are preferably non-porous. In such a way that they can also be used as part of the external capacitor seal. An electronic separator 18 is placed between the opposing aluminum / carbon composite electrodes 12 and 14. The electronic separator 18 is preferably made of a highly porous material, which acts as an electrical insulator between the aluminum / carbon 12 composite electrodes and 14. The purpose of the electronic separator 18 is to ensure that the opposing electrodes 12 and 14 are never in contact with each other. The contact between the electrodes results in a short circuit and rapid exhaustion of the charges stored in the electrodes. The porous nature of the electronic separator 18 allows for the movement of the ions in the electrolyte 20. The preferred electronic separator 18 is a porous polypropylene disk approximately 25.4 μm thick. The polypropylene separator is initially soaked in the electrolyte 20 before inserting it between the aluminum / carbon composite electrodes 12 and 14. The fastener of the cell 11 can be any known packaging means commonly used with double layer capacitors. To maximize the energy density of double-layer capacitors, it is an advantage to minimize the weight of the packaging medium. Packed double layer capacitors are typically expected to weigh no more than 25% of the double layer capacitor without packing. The electrical cables 28 and 29 extend from the current collector plates 22 and 24 through the fastener of the cell 11 and are adapted for connection to an electrical circuit (not shown). As seen in FIGURE 2 and FIGURE 3, a bipolar carbon / aluminum composite electrode 30 and a corresponding series stack of high-performance bipolar double layer capacitors 40 are illustrated. The aluminum / bipolar carbon composite electrode 30 comprises a polarized aluminum / carbon composite body, separated with a non-porous current collector plate 36. Attached to a surface 37 of the current collector plate 36 is an electrode 32 charged for a first cell. Attached to the opposite surface 38 of the current collector plate 36, there is an oppositely charged electrode 34 for a second cell. In other words, if the first electrode 32 is a negative electrode for a first capacitor cell "A", the second electrode 34 is then a positive electrode for an adjacent cell "B". As seen more clearly in FIGURE 3, a high-performance bipolar double-layer capacitor 40 stack includes a plurality of cells (A, B, C, and D), which are preferably connected in series. Each cell includes a pair of porous electrodes 42 and 44 composed of carbon impregnated with aluminum facing each other with an ionically conductive separator 46 placed therebetween. A plurality of non-porous current collectors 98 are placed between each cell, with each current collector 48 having two adjacent polarized electrodes 42 and 44 of different cells attached thereto, as described herein. In addition, a sufficient amount of an electrolyte 50 is introduced into each cell, such that the electrolyte 50 saturates the composite electrodes 42 and 44 and the separator 46 within each cell. The external current collection plates 49 are placed at each end of this stack. The individual aluminum / carbon composite electrodes 42 and 44 are preferably formed in a manner similar to the process described above. Each electrode is manufactured from a carbon fabric preform or carbon paper preform, which is impregnated with molten aluminum. As in the above, the porosity of the aluminum / carbon composite electrodes 42 and 44 should be controlled closely during the impregnation process to subsequently allow a sufficient amount of the electrolyte 50 to be introduced into the capacitor cell and penetrate the pores of the fibers of coal. The composite carbon electrodes impregnated with aluminum 42 and 44 are sufficiently porous and preferably have a uniformly continuous and evenly distributed aluminum path within the activated carbon fibers, such that the equivalent series resistance of each composite electrode, when used in a three volt cell is approximately 1.5 omega cm2 and the capacitance of each composite electrode 42 and 44 is approximately 30 F / cm3 or greater.

The internal current collector plates 48 of each bipolar electrode are preferably non-porous layers of an aluminum foil designed to separate the electrolyte 50 from adjacent cells. The external current collector plates 49 are also non-porous so that they can be used as part of the external capacitor seal, if necessary. An electronic separator 46 is placed between the opposing aluminum / carbon composite electrodes 42 and 44 within a particular capacitor cell. The electronic separator 46 is preferably a porous polypropylene disk similar to the electronic separators used in the configuration of a single cell. Many of the advantages of the present dual-layer capacitor result from the preferred methods of manufacturing the aluminum / carbon composite electrodes, the preferred method of adhering the current collector and the use of high-performance electrolytes. Each of these aspects of the invention are discussed in more detail in the paragraphs that follow. ALUMINUM / CARBON COMPOSITE ELECTRODE As identified above, the aluminum / carbon composite electrode is preferably made of a porous carbon fiber cloth preform, or a carbon fiber paper preform, which is impregnated with cast aluminum. The preform can be made of any suitable activated carbon fiber material, such as carbon fiber felt or other activated carbon fiber substrates having sufficient porosity to receive the impregnated molten aluminum and the electrolytic solution. The impregnated aluminum is uniformly and continuously distributed throughout the preform, in such a way as to provide a low resistance current path within the electrode. The aluminum / carbon composite electrode also remains sufficiently porous, so that an electrolytic solution, preferably a non-aqueous electrolytic solution, infiltrates the pores of the activated carbon fibers. The manufacturing process of the aluminum / carbon composite electrodes of the double layer capacitor starts with the manufacture of a carbon fiber electrode preform. The carbon fiber electrode preform is typically manufactured from a paper or fabric preform using carbon fibers of high surface area. The surface area of these carbon fibers can be in the range of approximately 500 to 3000 m2 / g. The carbon fiber paper preform is constructed with the standard paper mixing equipment using carbon fibers of approximately 8-10 μm in diameter, which are cut to a length of about 2 to 7 mm. Cellulose fibers of comparable size can also be added to the preform to act as a binding agent and to control the porosity of the resulting preform. The carbon fiber fabric preform is preferably a commercially available fabric, which uses woven carbon fibers that also have a surface area of about 500 to 3000 m2 / g and a diameter of about 8-10 μm. The carbon fiber cloth preform is typically more expensive than the carbon fiber paper preform, but the carbon fiber cloth preform has more structural stability than the carbon fiber paper preform. The surface area and other dimensions of the carbon fibers, however, can easily be tailored to meet the requirements of the application in which they are used. The impregnation of the carbon fiber preforms with molten aluminum is preferably carried out using a plasma spray technique, or alternatively using a liquid infiltration technique or immersion technique. In the plasma spray technique, the molten aluminum is preferably sprayed on both sides of the carbon fiber preform. The molten metal for plasma spray has previously been used in a double layer capacitor construction but has typically been used only as a means to form the current collector. The plasma spray technique is optimized to penetrate into the carbon fiber cloth preform and form a uniformly distributed porous aluminum matrix. This optimization is done by adjusting the electric current to the spray unit, the temperature and pressure of the molten aluminum, the distance of the plasma spray unit from the carbon fiber preform and the sweep of the plasma spray unit. In the technique of liquid infiltration, the molten aluminum is impregnated in the structure by bathing or immersing the carbon fiber preform in a molten aluminum bath. The molten aluminum, however, does not moisten the surface of the carbon fiber easily, so it does not get into the pores of the carbon fiber. Due to the low wetting properties of the carbon, special techniques are necessary to properly impregnate the molten aluminum within the interstices of the carbon fibers. These techniques that increase the impregnation of the electrode, imply variations of the impregnation techniques that are used to create graphite and aluminum compounds in the aerospace industry. These impregnation techniques are only adapted and modified for the present method of manufacturing the aluminum / carbon composite electrodes. For example, it is necessary to precisely control the impregnation process, in such a way that the electrode of the composite material remains adequately porous so that the electrolytic solution carries ionic current. One such technique of improving impregnation uses ultrasonic vibration to improve the wettability of carbon fibers by molten aluminum. As the carbon fiber preforms are being immersed in the molten bath to be impregnated with the molten aluminum, the ultrasonic vibrations are directed to the impregnation site. When these vibrations oscillate, the pressure in the liquid causes localized cavitations. At some particular frequencies, the molten aluminum is pumped into the interstices of the carbon fibers. By varying the frequency of ultrasonic vibrations, the level of impregnation can be controlled, thereby ensuring the resulting porous product. Another technique for improving the impregnation of molten aluminum within the preform during the fabrication of the aluminum / carbon composite electrode, involves other means for forming cycles of the external pressure on the molten aluminum that is being impregnated. The increase and decrease of the pressure creates a pumping action that will help the molten aluminum enter the spaces between the carbon fibers. The temperature of the molten aluminum is often increased to further assist the aluminum in the filling spaces between the carbon fibers. The wetting agents are also used as an additional medium that aids in the impregnation of the molten aluminum in the carbon preform during the fabrication of the aluminum / carbon composite electrode. The wettability of the carbon fiber is increased by initially immersing the carbon fiber in a filtered molten metal composed of wetting agents such as tin-titanium or a molten sodium-copper-tin-titanium alloy. When the carbon preform is removed from the metal filtrate, it is immersed in a bath of molten aluminum. The molten aluminum is leached from the wetting agent of the carbon fibers, which allows the aluminum to fill the interstices of the carbon fibers. Other suitable wetting agents such as tantalum, titanium-carbon, titanium-nitrogen, titanium-carbon-nitrogen, or silicon-carbon can also be introduced into the carbon fiber preforms to assist with the impregnation of the molten aluminum. Alternative techniques for improving the wettability of carbon fibers are also contemplated for use in the manufacturing process of aluminum / carbon composite electrodes for double layer capacitors. Such alternative means of improving wettability include, for example, coating carbon fibers with a thin layer of metals such as silver., cobalt, copper or nickel. However, it is important to note that any of the foreign agents or other contaminants used in the manufacture of the composite electrodes for the double layer capacitors, must be either substantially eliminated before using the capacitor or the presence of such agents should not severely limit the physical characteristics or performance of the double-layer capacitor. The ultrasonic vibrations, as described above or the external pressure cycles may also be used with or without a wetting agent as a means of improving the impregnation process of the molten aluminum. In addition, variations of the plasma spray process, as described above, can also be used with or without a wetting agent as a means of improving the impregnation process of the molten aluminum. The control of the impregnation process allows the control of the porosity of the electrode. The porosity of the aluminum / carbon composite electrode is closely controlled during the impregnation process to subsequently allow the electrolyte solution to enter the pores of the carbon fibers unimpeded and thus form a sufficiently large interface between the electrolyte and the fibers of the carbon fiber. carbon. By introducing aluminum into the electrode as described above, it alters the electrolytic path for the ion current in the electrode / electrolyte interface region. This altered electrolytic path, however, does not add significantly to the internal resistance of the double layer capacitor because most of the internal resistance remains in the small pores of the carbon fibers. The porosity of the aluminum / carbon composite electrode can best be expressed as a ratio by weight of aluminum to activated carbon. It is important, however, that the aluminum be uniformly and continuously distributed throughout the preform, so as to provide a low resistance current path within the composite electrode. The preferred weight ratio of aluminum to carbon is in the range of approximately 1.3 to 0.5, preferably less than 1.0. As identified in the above, the control of the impregnation process and the resulting control of the porosity of the electrode can be carried out in several ways including the use of wetting agents, cycles of the external pressure of the molten aluminum, and / or introduction of the vibrations. ultrasonic during liquid infiltration. By adjusting the frequency and magnitude of the ultrasonic vibrations and pressure variations during the infiltration of the liquid, the impregnation of the molten aluminum can be varied. In addition, the control of external parameters in plasma spray processes will affect the resulting porosity of the electrode. For example, by adjusting such external parameters as the electric current directed to the plasma spray unit, the sweep rate of the plasma spray unit, the separation distance between the plasma spray unit and the carbon fiber preform; and the temperature and pressure of the supply of the molten aluminum, optimum porosity can be achieved. Alternatively or together, by varying the amount of cellulose used in the carbon fiber paper preforms, the porosity of the aluminum / carbon composite electrodes can be controlled. Specifically, porosity control is performed by carbonization or sintering of cellulose fibers, thereby causing their removal after the molten aluminum is impregnated into the carbon fiber preform. EXAMPLE 1 The following description is representative of the preparation and manufacture of the aluminum / carbon composite electrodes and the high performance double layer capacitor. This example together with the previous detailed description represents a better mode currently contemplated to carry out the invention. This description is not to be taken in a limiting sense, if it is not done solely for the purpose of describing some general principles of the invention. The scope of the invention should be determined with reference to the claims. The carbon fiber preformis are made using activated carbon fibers of approximately 5 mm in length and approximately 8 μm in diameter. The activated carbon fibers have a surface area of approximately 2500 m2 / g. The cellulose fibers of approximately 5 mm in length and approximately 8 μm in diameter were also incorporated into the carbon fiber preforms. The cellulose fibers were added as an agglutination agent and to control the porosity of the electrode. The percentage of aggregated cellulose fibers consisted of between about 9.0 to 50% by weight of the preform and more preferably about 15% by weight. Alternatively, the carbon fiber preforms can be obtained from a commercial source. These carbon fiber preforms are typically activated carbon fabrics, in which the individual carbon fibers are wound into bundles called a tow. The preferred fabric employs a tow consisting of carbon fibers that were about 8 μm in diameter and had a surface area of about 2500 m2 / g. The tow was woven to create a fabric that was approximately 432 μm thick. The carbon fiber preforms were impregnated with molten aluminum using a plasma spray technique. The spraying process was optimized to penetrate uniformly into the carbon fiber preforms by adjusting the flow to the spray unit, the spray pressure, the separation distance of the preform spray, the distance of the vertical stage and the speed of sweep of the spray. The optimum conditions for this example were determined to be 65 amperes of current for the spray unit, a spray pressure of 3.52 kg / cm2 with a separation distance of approximately 50.8 cm. The sweep rate of the plasma spray unit was approximately 161.5 cm per second and the distance from the vertical stage was approximately 2.54 cm. Each aluminum / carbon composite electrode contains approximately 0.2 grams of carbon fibers to approximately 0.24 grams of aluminum.

After the spraying process is completed, the composite electrode discs are drilled from the impregnated carbon preform. Each composite electrode has a diameter of approximately 5.1 cm (2 inches) and a thickness of approximately 432 μm. This results in a surface area of approximately 20.3 cm2. The cellulose fibers are removed from the composite electrode by sintering the electrode at approximately 200 ° C-300 ° C in a reducing atmosphere. A sheet of 50.8 μm thick aluminum foil is bonded to each aluminum / carbon composite electrode at a temperature between approximately 360 ° C-600 ° C and an external pressure of 0.84 kg / cm2, in the presence of an atmosphere inert or slightly reducing. The composite aluminum / carbon electrodes finished with the current collector plate was disk-shaped device with a surface area of approximately 20.3 cm2 and a thickness of approximately 0.048 cm. The assembly of the single cell capacitor also includes a porous polypropylene separator approximately 25.4 μm thick, which was placed between the aluminum / carbon composite electrodes to act as the electronic separator. An electrolytic solution of tetrafluoroborate tetraethylammonium 1.4 M in acetonitrile is then impregnated into the carbon / aluminum composite electrodes and the separator using a vacuum infiltration technique. Then the capacitor is sealed externally. Table 1 identifies several examples of the activated carbon / aluminum composite electrode and its performance characteristics. It is important to note that ID No. 071994A uses a different carbon cloth than the fabric preforms described above and similarly has a greater thickness than the other samples. The resistance and capacitance measurements were made for comparison purposes only.

Table 1 Performance Characteristics of the Aluminum / Carbon Composite Electrode ADHESION OF THE CURRENT COLLECTOR TO THE COMPOUND ELECTRODE After the aluminum is impregnated into the carbon fiber preform, a thin sheet of aluminum is secured to the back side of the electrode. In such a process, the diffusion of the preform of carbon impregnated with aluminum is attached to the thin sheet of aluminum so it creates a weak bond between the composite electrode and the current collector. The thin aluminum sheet functions as a current collector or conductive electrode of the capacitor. Specifically, the bond diffusion is performed by first modifying or removing the oxide layer on the thin layer of aluminum and then heating the electrode and the aluminum foil structure under pressure in an inert atmosphere. This joining process involves the combination of high temperature and moderate pressure in an inert atmosphere to bring the surfaces of the composite electrode and the current collector together. These stages are performed so that the aluminum atoms fill the spaces in the interface to adhere the current collector to the composite electrode. In a bipolar capacitor stack, the thin sheet of bonded metal must be non-porous to separate the electrolyte solutions between the cells. The thin metal sheet should be thick enough to ensure that there are no holes or other defects. A thin sheet of metal with thickness between about 12.7 μm and 76.2 μm is preferred for bipolar electrodes. Aluminum is not a material that is very suitable for the diffusion of the union. The difficulty occurs from the strong oxide layer that is normally present on an aluminum surface. This oxide layer tends to retard the transfer of aluminum between the surfaces to be joined. Most bond diffusion techniques involve an aluminum that requires high external pressure and a bonding temperature that is just below the melting point of aluminum. The pieces or structures that are to be joined typically must be maintained in this state in an inert atmosphere for a prolonged period of time. The bonding conditions are not acceptable for the electrodes composed of activated carbon / aluminum, because the high external pressure sprays the activated carbon fibers in the electrode. In addition, a high temperature for a prolonged period of time results in the formation of aluminum carbides. The formation of aluminum carbide significantly reduces the effectiveness of the electrode. The present process joins the aluminum / carbon composite electrodes to a thin sheet of aluminum with low external pressure, at a lower temperature and for a relatively shorter amount of time. The present method also significantly reduces the amount of aluminum carbide formed during the bonding process and allows bonding to form without physically damaging the activated carbon fibers in the electrode. The present preferred bonding process allows the attachment of the aluminum foil to the aluminum / carbon composite electrode at a temperature in the range of about 300 ° C to 600 ° C and more preferably to about 360 ° C + 50 ° C. The connection is achieved with an external pressure of approximately 0.84 kg / cm2. This improved bonding technique, performed under advantageous conditions, is achieved by physically removing or modifying the oxide layer on the aluminum foil before joining the foil to the electrode of an inert atmosphere. The oxide layer is removed using a technique of cathodic sublimation of argon ions. Alternatively, the oxide layer can be modified by etching of the aluminum foil in a solution of sodium dichromate in sulfuric acid, for example [Na2 (Cr202) in H2SO4]. In any technique, the aluminum oxide layer is significantly reduced. Before the aluminum foil is bonded to the aluminum / carbon composite electrode, any of the cellulose fibers present in the aluminum / carbon composite electrode can be removed by carbonizing or sintering the cellulose fibers by heating them in an inert atmosphere or alternatively reducing them in chemical form. By varying the amount of cellulose used in the preforms, the resulting porosity of the aluminum / carbon composite electrodes can be controlled. For the bipolar carbon / aluminum composite electrode like each pair of carbon fiber preforms impregnated with aluminum, they are removed from the impregnation process, aligned together and bonded to a single layer of aluminum foil. The preferred process simultaneously involves spreading the pair of aligned pair of aluminum / carbon composite electrodes to an aluminum foil stream collector under low pressure, a relatively low temperature and in an inert atmosphere. As in the above, the process avoids the formation of aluminum carbide and other contaminants and avoids physical damage to carbon fibers. The quality of the joint is increased by first chemically attacking both surfaces of the aluminum foil or by removing any of the aluminum oxide layers that may be present. EXAMPLE 2 The following description is representative of the diffusion binding process and its preparation. This example, together with the previous detailed description, represents a better mode currently contemplated for carrying out the invention. This description, however, is not to be taken in a limiting sense, but is made solely for the purpose of describing some general principles of the invention. The scope of the invention should be determined with reference to the claims. A bath for chemical attack is prepared to chemically attack the aluminum foil by combining approximately 60 g of Na2 (CR202) 2H § with approximately 173 ml of concentrated H2SO4, approximately 1.9 grams of aluminum powder, and enough water to form a liter of solution. The chemical attack bath is heated to approximately 60 ° C. In addition, a water bath to rinse the thin aluminum sheet chemically attacked, it is prepared and heated to approximately 60 ° C. The thin aluminum sheet is immersed in the chemical etch bath for approximately 15 minutes. The thin sheet is then removed from the etchant bath and immersed in the water bath to rinse off the thin sheet of metal. The thin sheet of metal is allowed to dry in an oven for approximately 30 minutes. The electrodes and the thin sheet of metal are chemically attacked, then assembled for diffusion bonding. A carbon cloth preform is used as a release sheet and the Hastoloy X plates are used to apply moderate pressure of between about 0.21 kg / cm2 to 28.1 kg / cm2 and preferably about 0.84 kg / cm2 to the electrode assembly / thin sheet of metal. The electrode / thin sheet metal assembly is placed in a stainless steel reactor for real diffusion bonding. The present diffusion bonding process involves heating the electrode assembly and thin sheet metal inside the stainless steel reactor, while under moderate pressure in an inert atmosphere. After the aluminum is impregnated into the carbon fiber preform, thereby forming the composite electrodes, the electrodes are dried for about 30 minutes. The electrodes are then purged with hydrogen (550 ml / min) and argon (1000 ml / min) at approximately 100 ° C. Next, the cellulose fibers were carbonized at 300 ° C for about 30 minutes, while the electrodes were still purged with hydrogen (550 ml / min) and argon (1000 ml / min). Then the hydrogen is turned off and the argon velocity is increased to about 1500 ml / min. The binding temperature is then achieved by increasing the temperature of the reactor, as required from 300 ° C to the final binding or sintering temperature, which is between about 300 ° C-600 ° C (see Table 2). This binding temperature was maintained for a prescribed binding time of about 1 to 5 hours as indicated in Table 2. Then the rejector is turned off and the electrodes are allowed to cool for approximately 90 minutes. After the time of 90 minutes, the reactor is allowed to cool with water and the electrodes are removed. Table 2 identifies the capacitance, series resistance and other performance characteristics of various electrode / thin sheet metal assemblies that were spliced in accordance with the bonding process described above. The resistance and capacitance measurements were made for comparison purposes only.

Table 2 Electrode Bonding Features / Thin Metal Sheet HIGH-PERFORMANCE ELECTROLYTES The performance of double-layer capacitors is very much dependent on the choice of the electrolytic solutions used. Traditional aqueous electrolytes typically have lower strength than non-aqueous electrolyte solutions. On the other hand, non-aqueous electrolytic solutions often have higher ionic conductivity and thus increase the operating voltage of double-layer capacitors. In particular, these non-aqueous electrolytic solutions have allowed the voltage of double-layer, single-cell capacitors to be increased to approximately three volts. The present invention considers the use of some advanced electrolyte solutions. These advanced electrolyte solutions often fall into three types or classes of solutions. First, there are solutions of ammonia which uses gaseous ammonia as the solvent for the electrolyte. Preferred ammonium solutions result when certain salts are combined with gaseous ammonia to form highly conductive liquids at room temperature. Due to their high conductivity, voltage stability and temperature range, these solutions are good candidates for electrolytes in high performance double layer capacitors. Examples of some ammonium solutions which are suitable for use as electrolytes in high performance double layer capacitors include: [NH4N03] 1.3 [NH J; [Nal] 3.3 [NH ^; and [LiCl04] 4 [NH ^. The second class of electrolytes is made up of solutions based on sulfur dioxide. Sulfur dioxide, which is a gas at room temperature, is used as the solvent for the electrolyte. Sulfur dioxide dissolves some salts to form liquid electrolytes at room temperature. These solutions based on sulfur dioxide, usually have higher conductivity than the corresponding ammonium solutions, but also tend to be more corrosive. These electrolytes are obtained when the sulfur dioxide is used to dissolve tetrachloroaluminates of lithium, cadmium, sodium or strontium. These electrolyte solutions can be characterized generally as follows: M [A1C14] x SO 2 where x is between 2.5 and 6.0; and where M is selected from Li, Ca, Na, or Mr. The third class of enhanced electrolyte, which are suitable for use in high-performance double-layer capacitors, are electrolytes from molten salts. Molten salt electrolytes are formed from ionic salts that have been liquefied at elevated temperatures. High temperatures are usually in the range of 450 ° C and higher. These high temperature molten salt electrolytes have the highest ionic conductivity and interruption voltage of any of the electrolyte solutions. The main disadvantage for these solutions is that they require high operating temperatures and in many are highly corrosive liquids.

An example of a high temperature molten salt electrolyte that is suitable for use in high performance double layer capacitors is a mixture of potassium chloride and / or lithium chloride. A high-performance double-layer capacitor that uses such electrolytes must have an operating voltage of above 4 volts and an ionic conductivity of approximately 1.6 S / cm to approximately 450 ° C. In addition, there are some alkali metal tetrachloroaluminates and tetrabroaluminates, which are molten salt electrolytes having operating temperatures between about 100 ° C-400 ° C with conductivities in the range of 0.15 to 0.45 S / cm. There are also several molten chloroaluminate salts, which are liquid at room temperature, which are considered to be useful as electrolytes in the high performance double layer capacitor described herein. The electrolyte solutions evaluated for use with a high-performance, double-layer, 3-volt capacitor are identified in Table 3. The carbon fiber preforms impregnated with aluminum and the separator are preferably saturated with the electrolyte solutions described by means of a process of vacuum infiltration.

Table 3 Non-Aqueous Electrolytes From the foregoing, it should be appreciated that the present invention in this manner provides a double layer capacitor with aluminum / carbon composite electrolytes and high performance electrolytes and a method for manufacturing the same. Furthermore, it will be apparent that various changes can be made in the form, construction and arrangement of their parts without departing from the spirit and scope of the invention or sacrificing all of their material advantages, the forms described in the foregoing are only exemplary embodiments thereof. For this purpose, it is not intended that the scope of the invention be limited to the specific embodiments and processes described. On the contrary, it is intended that the scope of this invention be determined by the appended claims and their equivalents.

Claims (42)

1. CLAIMS 1. A double-layer capacitor, characterized in that it comprises: a pair of porous aluminum impregnated carbon composite electrodes consisting of a preform of activated carbon fibers, of high surface area impregnated with molten aluminum; a pair of current collectors, each current collector placed on a surface that does not face the respective composite electrode; an ionically conductive separator placed on the front surface of the composite electrodes; and a non-aqueous electrolytic solution, which saturates the composite electrodes and the separator.
2. 2. The double-layer capacitor according to claim 1, characterized in that each of the electrodes composed of carbon impregnated with aluminum, further comprises a preform of activated carbon fibers of high surface area having a uniformly distributed and continuous path of impregnating aluminum and having an internal resistivity of less than 1.5 omega cm.
3. The double-layer capacitor according to claim 1 and 2, characterized in that the double-layer capacitor is capable of supplying at least 5 Wh / kg of useful energy at power ratings of at least 600 W / kg.
4. 4. The double layer capacitor according to claim 1, 2 and 3, characterized in that the composite electrode has a capacitance of at least 30 F / cm3.
5. 5. The double-layer capacitor according to claims 1 to 4, characterized in that each of the composite electrodes of carbon, impregnated with aluminum further comprises a porous electrode having a uniformly distributed and continuous path of aluminum impregnated within a preform of carbon cloth.
6. 6. The double-layer capacitor according to claims 1 to 4, characterized in that each of the electrodes composed of carbon impregnated with aluminum, further comprises a porous electrode having a uniformly distributed and continuous aluminum impregnated path inside a preform of carbon paper.
7. The double-layer capacitor according to claims 1 to 4, characterized in that the electrodes composed of carbon impregnated with aluminum further comprise activated carbon fibers having a surface area in a range of about 500-3000 m / g.
8. 8. The double-layer capacitor according to claims 1 to 4, characterized in that the electrodes composed of carbon impregnated with aluminum, further comprise activated carbon fibers having a surface area greater than 2000 m2 / g.
9. 9. The double layer capacitor according to claims 1 to 4, characterized in that the electrodes composed of carbon impregnated with aluminum, furthermore comprise a weight ratio of aluminum to carbon in a range of about 1.3 to 0.5.
10. 10. The double layer capacitor according to claims 1 to 4, characterized in that the electrodes composed of carbon impregnated with aluminum, furthermore comprise a weight ratio of aluminum to carbon of less than 1.0.
11. 11. The double-layer capacitor according to claim 1, further characterized in that it comprises a housing means for sealingly packing the double-layer capacitor.
12. 12. The double-layer capacitor according to claims 1 to 4, characterized in that the separator is a porous polypropylene separator, saturated with non-aqueous electrolytic solution.
13. The double-layer capacitor according to claims 1 to 4, characterized in that each of the current collectors comprises a thin layer of a thin sheet of metal bonded to the surface that does not face the respective electrodes.
14. The double layer capacitor according to claim 13, characterized in that the current collector comprises a thin layer of a thin sheet of aluminum bonded to the surfaces that do not face the respective electrodes.
15. 15. The double layer capacitor according to claim 1, characterized in that the electrolyte solution is a molten salt solution.
16. 16. The double-layer capacitor according to claim 15, characterized in that the electrolytic solution of molten salt is a mixture of potassium chloride and lithium chloride.
17. 17. The double layer capacitor according to claim 15, wherein the molten salt electrolytic solution is characterized by a formula: [A1C13] X where M is an alkali metal; and wherein X is selected from a group consisting of chlorine or bromine.
18. 18. The double-layer capacitor according to claim 1, characterized in that the electrolytic solution is an ammonium solution.
19. 19. The double layer capacitor according to claim 18, characterized in that the ammonium electrolyte solution is selected from a group consisting of [NH4N03] 1.3 [NH 3; [Nal] 3.3 [NH] $ and [LiCl04] 4 [NH J.
20. 20. The double layer capacitor according to claim 1, wherein the electrolyte solution is a solution of sulfur dioxide characterized by a formula: M [AlCl4] nx SO2 where x is between 2.5 and 6.0; and where M is selected from a group consisting of lithium, cadmium, sodium or strontium.
21. 21. A method for manufacturing a double layer capacitor characterized in that it comprises the steps of: impregnating a preform of activated carbon fibers, of high surface area with a uniformly distributed and continuous path of aluminum to form a composite electrode of carbon impregnated with aluminum , porous; attaching a current collector plate on a surface of the electrode composed of carbon impregnated with aluminum; Align a couple of electrodes composed of carbon impregnated with aluminum, so that the electrodes composed of carbon impregnated with aluminum have unbonded surfaces that face each other; placing an ionically conductive separator between the faces facing the electrodes composed of carbon impregnated with aluminum; and saturate the electrodes composed of carbon impregnated with aluminum and a separator with a non-aqueous electrolytic solution.
22. 22. The method for manufacturing a double-layer capacitor according to claim 21, in the step of attaching the current collector plate to the carbon-impregnated electrode impregnated with aluminum, further characterized by comprising the steps of: removing any of the layers of aluminum oxide from a current collector of a thin sheet of aluminum; and pressing the composite electrode against the aluminum foil at a bonding temperature well below a melting point of aluminum in an inert atmosphere, such that the aluminum atoms will fill the gaps at an interface of the composite electrode and the Thin aluminum sheet to adhere the thin aluminum sheet to the composite electrode, while avoiding the formation of aluminum carbide at the interface.
23. 23. The method for manufacturing a double layer capacitor according to claims 21 and 22, further characterized in that it comprises the step of sealing packing the double layer capacitor within a housing.
24. 24. The method for manufacturing a double-layer capacitor according to claims 21 and 22, further characterized in that it comprises the step of controlling the porosity of the electrodes composed of carbon impregnated with aluminum during the impregnation step.
25. 25. The method for manufacturing a double-layer capacitor according to claim 24, characterized in that the step of impregnating a preform of activated carbon fibers of high surface area with a uniform and continuous aluminum distributed path further comprises plasma spraying cast aluminum in the preform of activated carbon fiber.
26. 26. The method for manufacturing a double layer capacitor according to claim 25, characterized in that the step of controlling the porosity of the electrodes composed of carbon impregnated with aluminum further includes adjusting a separation distance between a plasma spray unit and the carbon fiber preform.
27. 27. The method for manufacturing a double-layer capacitor according to claim 25, characterized in that the step of controlling the porosity of the electrodes composed of carbon impregnated with aluminum, further includes controlling a scanning speed of a plasma spray unit in relation to the carbon fiber preform.
28. 28. The method for manufacturing a double-layer capacitor according to claim 25, characterized in that the step of controlling the porosity of the composite electrode of carbon impregnated with aluminum, it also includes controlling the temperature and supply pressure of the molten aluminum to the activated carbon fiber preform.
29. 29. The method for manufacturing a double layer capacitor according to claim 24, characterized in that the step of impregnating a high surface area preform of activated carbon fibers with a uniform and continuous aluminum distributed path, further comprises submerging the preform of activated carbon fiber inside a molten aluminum bath.
30. 30. The method for manufacturing a double layer capacitor according to claim 24, characterized in that the step of controlling the porosity of the electrodes composed of carbon impregnated with aluminum, further includes pressure cycles of the molten aluminum, while impregnating the preform of activated carbon fiber.
31. The method for manufacturing a double-layer capacitor according to claim 24, characterized in that the step of controlling the porosity of the electrodes composed of carbon impregnated with aluminum also includes applying ultrasonic vibrations to the preform of the activated carbon fiber. during the impregnation stage.
32. 32. The method for manufacturing a double layer capacitor according to claims 21 and 22, characterized in that it comprises the step of immersing the activated carbon fiber preform of high surface area in a molten metal infiltrate, composed of a wetting agent before impregnation of the activated carbon fiber preform with aluminum.
33. The method for manufacturing a double layer capacitor according to claim 32, characterized in that the wetting agent is selected from the group consisting of tin-titanium, copper-tin-titanium, tantalum, titanium-carbon, titanium-nitrogen, titanium-nitrogen-carbon, silicon-carbon and their mixtures.
34. 34. The method for manufacturing a double-layer capacitor according to claims 21 and 22, characterized in that the saturation step of the electrodes composed of carbon impregnated with aluminum and the separator with the electrolytic solution, further comprises saturating the electrodes composed of carbon impregnated with aluminum and the separator with the solution >; n electrolytic using a vacuum infiltration process'.
35. 35. The method for manufacturing a double layer capacitor according to claims 21 and 22, characterized in that the junction temperature is between approximately 300 ° C and 600 ° C.
36. 36. The method for manufacturing a double-layer capacitor according to claim 35, characterized in that the junction temperature is approximately 360 ° C.
37. 37. The method for manufacturing a double layer capacitor according to claim 22, further characterized in that it comprises the step of chemically attacking the surfaces of the aluminum foil before bonding.
38. 38. The method for manufacturing a double-layer capacitor according to claim 37, characterized in that the etching step of the aluminum foil further comprises immersing the foil in a solution of sodium dichromate in sulfuric acid.
39. 39. The method for manufacturing a double-layer capacitor according to claim 22, further characterized in that the step of removing any of the aluminum oxide layers from the aluminum foil with a cathodic sublimation technique of argon ion.
40. 40. The method for manufacturing a double-layer capacitor according to claim 22, further characterized in that it includes the step of pressing the composite electrode against the aluminum foil having a thickness between about 12.7 μm and 76.2 μm.
41. 41. The method for manufacturing a double layer capacitor according to claim 22, characterized in that the step of pressing the composite electrode against the aluminum foil is performed at a joint pressure of between approximately 0.21 kg / cm2 and 28.1 kg / cm2.
42. 42. The method for manufacturing a double-layer capacitor according to claim 41, characterized in that the step of pressing the composite electrode against the aluminum foil is performed at a joint pressure of approximately 0.84 kg / cm2.