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  Electrochemical double layer capacitorThe Patent Description & Claims data below is from USPTO Patent Application 20080106850. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] The present application is a divisional application and claims the benefit of U.S. patent application Ser. No. 11/609,291, filed Dec. 11, 2006 and entitled "Pretreated Porous Electrode," which is currently pending and which is a continuation application of U.S. patent application Ser. No. 11/087,409, filed Mar. 23, 2005, entitled "Pretreated Porous Electrode and Method for Manufacturing Same," which issued as U.S. Pat. No. 7,147,674 on Dec. 12, 2006. Each of these applications are incorporated by reference in their entirety as though fully set forth herein. FIELD OF THE INVENTION [0002] The present invention generally relates to processing of porous materials. More specifically, the present invention relates to porous electrodes and to energy storage devices, such as electrochemical double layer capacitors, fabricated using porous electrodes. BACKGROUND [0003] Electrodes are widely used in many devices that store electrical energy, including primary (non-rechargeable) battery cells, secondary (rechargeable) battery cells, fuel cells, and capacitors. Important characteristics of electrical energy storage devices include energy density, power density, maximum charging rate, internal leakage current, equivalent series resistance (ESR), and durability, i.e., the ability to withstand multiple charge-discharge cycles. For a number of reasons, double layer capacitors, also known as supercapacitors and ultracapacitors, are gaining popularity in many energy storage applications. The reasons include availability of double layer capacitors with high power densities (in both charge and discharge modes), and with energy densities approaching those of conventional rechargeable cells. [0004] Double layer capacitors use electrodes immersed in an electrolyte (an electrolytic solution) as their energy storage element. Typically, a porous separator immersed in and impregnated with the electrolyte ensures that the electrodes do not come in contact with each other, preventing electronic current flow directly between the electrodes. At the same time, the porous separator allows ionic currents to flow between the electrodes in both directions. As discussed below, double layers of charges are formed at the interfaces between the solid electrodes and the electrolyte. Double layer capacitors owe their descriptive name to these layers. [0005] When electric potential is applied between a pair of electrodes of a double layer capacitor, ions that exist within the electrolyte are attracted to the surfaces of the oppositely-charged electrodes, and migrate towards the electrodes. A layer of oppositely-charged ions is thus created and maintained near each electrode surface. Electrical energy is stored in the charge separation layers between these ionic layers and the charge layers of the corresponding electrode surfaces. In fact, the charge separation layers behave essentially as electrostatic capacitors. Electrostatic energy can also be stored in the double layer capacitors through orientation and alignment of molecules of the electrolytic solution under influence of the electric field induced by the potential. [0006] In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for these volumetric and weight efficiencies. First, the charge separation layers are very narrow. Their widths are typically on the order of nanometers. Second, the electrodes can be made from a porous material, having very large effective surface area per unit volume. Because capacitance is directly proportional to the electrode area and inversely proportional to the widths of the charge separation layers, the combined effects of the large effective surface area and narrow charge separation layers result in capacitance that is very high in comparison to that of conventional capacitors of similar size and weight. High capacitance of double layer capacitors allows the capacitors to receive, store, and release large amounts of electrical energy. [0007] Electrical energy storage capability of a capacitor is determined using a well-known formula, to wit: E = C * V 2 2 . ( 1 ) In this formula, E represents the stored energy, C stands for the capacitance, and V is the voltage of the charged capacitor. Thus, the maximum energy (E.sub.m) that can be stored in a capacitor is given by the following expression: E m = C * V r 2 2 , ( 2 ) where V.sub.r stands for the rated voltage of the capacitor. It follows that a capacitor's energy storage capability depends on both (1) its capacitance, and (2) its rated voltage. Increasing these two parameters is therefore important to capacitor performance. Indeed, because the total energy storage capacity varies linearly with capacitance and as a second order of the voltage rating, increasing the voltage rating is the more important of the two objectives for improving capacitors. [0008] Voltage ratings of double layer capacitors are generally limited by chemical reactions (reduction, oxidation) and breakdown that take place within the electrolytic solutions in presence of electric field induced between capacitor electrodes. Electrolytic solutions currently used in double layer capacitors are of two kinds. The first kind of electrolytic solutions includes organic solutions, such as propylene carbonate. Long lifetime prior art double layer capacitors made with organic electrolytes can boast voltage ratings approaching 2.5 volts. [0009] Double layer capacitors may also be made with aqueous electrolytic solutions, for example, potassium hydroxide and sulfuric acid solutions. Double layer capacitor cells manufactured using aqueous electrolytes and activated carbon are typically rated at or below 1.2 volts in order to achieve a commercially acceptable number of charge-discharge cycles. Even small increases above 1.2 volts tend to reduce substantially the number of charge-discharge cycles that the capacitors can withstand without significant deterioration in performance. [0010] The 2.5 volt rating is considerably below voltage rating theoretically achievable in organic electrolyte-based double layer capacitors. According to some calculations, double layer capacitors made with an organic electrolyte and activated carbon should perform reliably at voltages ranging to about 3.2-3.5 volts. Achieving this range, however, has been an elusive goal because of early decomposition and breakdown of the electrolyte. The problem results, at least in part, from presence of impurities within the activated carbon and within the electrolyte. Water is one of these impurities. [0011] Trace amounts of water and other impurities can be found in most electrolytes, and they may affect capacitor reliability, durability, and voltage rating. Highly purified electrolytes, however, are commercially available at reasonable cost. [0012] The active material of the electrode--activated carbon or another porous material, for example--almost invariably has some impurities, including water. Water may be present in the raw carbon, and it may be introduced or added during the electrode manufacturing process. In practice, purifying activated carbon has proven to be a much more difficult task than purifying electrolyte. Water molecules can attach to the carbon in several ways, including by means of VanderWaal's forces responsible for physical bonding, and chemical (covalent and hydrogen) bonding forces. [0013] Whatever the nature of the bond between a water molecule and activated carbon, a high energy "excited site" is formed around it. Electrolyte interacts with the water molecules and decomposes more readily near such sites than elsewhere in the capacitor. The trapped water functions deleteriously at the capacitor's working potential, so that the maximum application voltage is affected by the water devolution voltage. This is believed to be a major contributing cause to the lower actual-versus-theoretical breakdown voltage of double layer capacitors. [0014] It would be desirable to increase actual breakdown voltage of double layer capacitors. It would also be desirable to increase reliability and durability of double layer capacitors, as measured by the number of charge-discharge cycles that a capacitor can withstand without significant deterioration in its operating characteristics. Because capacitor breakdown voltage and durability are both compromised by interaction between electrolyte and water molecules trapped in the activated carbon, it would be desirable to reduce the interactions or eliminate the interactions altogether. More generally, it would be desirable to provide a method for preventing impurities attached to porous materials from interacting with surrounding gas or liquid in which the porous material is immersed. SUMMARY [0015] A need thus exists for methods for preventing or reducing exposure of high energy excited sites within porous materials to outside agents. Another need exists for porous materials with reduced exposure of water and other impurities trapped in the materials to outside agents. A further need exists for electrodes made from porous materials having reduced content of water molecules that can interact with surrounding gas or liquid in which the electrodes are immersed. Still another need exists for double layer capacitors and other electrical energy storage devices that employ electrodes made from these materials. [0016] Various embodiments of the present invention are directed to methods, electrodes, electrode assemblies, and energy storage devices that satisfy one or more of these needs. An exemplary embodiment of the invention herein disclosed is a method for processing porous material. According to this method, the porous material is treated with a sealing coating capable of sealing impurities in micropores of the porous material. The porous material is then dried, so that the coating seals water molecules in the micropores. Treatment may involve, for example, immersing the porous material in the sealing coating, and then draining the sealing coating from the porous material before the material is dried. The coating may be such that it does not seal at least a majority of mesapores of the porous material as measured by surface area, while sealing at least a predetermined percentage of water molecules in the micropores of the material. [0017] In other aspects of the invention, the porous material includes activated carbon in particulate form, fibril-forming binder, and conductive carbon. These ingredients may be blended, for example, dry-blended, and subjected to high-shear forces in order to fibrillize the material. The high-shear forces may be applied using non-lubricated techniques. [0018] To make an electrode, the porous material processed as described above may be coated onto one or both sides of a current collector so that film or films of the material are formed on the current collector when the material is dried. To densify the films, the electrode may be calendered. The electrode may then be used in a double layer capacitor, for example, by providing a second electrode, interposing a porous separator between the two electrodes, and immersing the separator and the two electrodes in an electrolyte. [0019] In another aspect, an electrode assembly is made by coating a porous separator with the porous material processed as described above, so that films of the material are formed on the separator. Current collectors may then be attached to the surfaces of the films that are not in contact with the separator. The resulting electrode assembly may be calendered to densify the films. A double layer capacitor is obtained when the assembly is immersed in an electrolyte. [0020] In one embodiment, a method for processing porous material comprises steps of: providing a porous material, at least some of the porous material comprising micropores, at least some of the micropores having impurities disposed therein; treating the porous material with a sealing coating to seal the impurities in micropores of the porous material; and drying the porous material. The treating step may comprise immersing the porous material in the sealing coating, the method further comprising: draining the sealing coating from the porous material before the drying step. The sealing coating may be such that it does not seal at least majority of mesapores of the porous material as measured by surface area. The sealing coating may be capable of sealing water molecules in micropores of the porous material. The step of providing the porous material may comprise providing activated carbon. The step of providing the porous material may comprise: providing a fibril-forming binder; providing conductive carbon; blending the activated carbon, the fibril-forming binder, and the conductive carbon, thereby obtaining blended active electrode material; and applying high-shear forces to the blended active electrode material to fibrillize the blended active electrode material. The step of providing the porous material further may comprise: providing a fibril-forming binder; providing conductive carbon; dry-blending the activated carbon, the fibril-forming binder, and the conductive carbon, thereby obtaining blended active electrode material; and applying a non-lubricated high-shear force technique to the blended active electrode material to dry fibrillize the blended active electrode material. The method may comprise processing porous material, wherein the step of providing activated carbon comprises providing the activated carbon in particulate form; providing a current collector; and coating the current collector with the fibrillized active electrode material before the step of drying, thereby obtaining the electrode. The step of coating the current collector may comprise coating both sides of the current collector with the fibrillized active electrode material so that first and second films of active electrode material are formed on both sides of the current collector. The method may comprise calendering the current collector with the films after the step of drying. The method may comprise: making first and second electrodes by providing a porous separator; disposing the separator between the first and second electrodes so that active electrode material is interposed between the separator and respective current collector of the electrodes; and immersing the electrodes and the separator in an electrolyte. The method may comprise providing processed porous material wherein the step of providing activated carbon comprises providing the activated carbon in particulate form; providing a porous separator; and coating the porous separator with the porous material before the step of drying; whereby the electrode assembly is obtained. The step of coating the porous separator may comprise coating both sides of the porous separator with the active electrode material so that a first film of active electrode material is formed on a first side of the porous separator and a second film of active electrode material is formed on a second side of the porous separator. The method may comprise attaching a first current collector to the first film so that the first film is disposed between the first current collector and the porous separator; and attaching a second current collector to the second film so that the second film is disposed between the second current collector and the porous separator. The method may further comprise calendering the electrode assembly. The method may comprise comprising: making the electrode assembly of claim; and immersing the electrode assembly in an electrolyte. The method for providing film of active electrode material may comprise providing processed porous material and calendering the processed porous material to obtain the film of active electrode material. The method may comprise providing processed porous material and calendering the processed porous material to obtain a first film of active electrode material and a second film of active electrode material; providing a porous separator; providing a first current collector and a second current collector; attaching the first film to the porous separator and to the first current collector so that the first film is disposed between the porous separator and the first current collector; attaching the second film to the porous separator and to the second current collector so that the second film is disposed between the porous separator and the second current collector, and the porous separator is disposed between the first and second films; and immersing the porous separator and the first and second films in an electrolyte. Continue reading... Full patent description for Electrochemical double layer capacitor Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Electrochemical double layer capacitor patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Electrochemical double layer capacitor or other areas of interest. ### Previous Patent Application: Modular energy storage device and method of making the same Next Patent Application: Capacitor Industry Class: Electricity: electrical systems and devices ### FreshPatents.com Support Thank you for viewing the Electrochemical double layer capacitor patent info. 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