Abstract:
Methods and apparatus for an electrochemical double-layer capacitor for hostile environments. An electrochemical double-layer capacitor includes two electrodes wetted with an electrolyte, each electrode being attached to or in contact with or coated onto a current collector and separated from each other by a separator porous to the electrolyte, the electrodes, electrolyte and current collector containing less than 1,000 parts per million (ppm) of impurities, while exhibiting a leakage current less than 1 amp per liter of volume over a range of operating temperatures and at a voltage up to a rated voltage.

Description:
BACKGROUND OF THE INVENTION 
     The invention generally relates to electrochemical double-layer capacitors, and more specifically to an electrochemical double-layer capacitor for high temperature applications. 
     An electric double-layer capacitor, also known as a “supercapacitor,” “supercondenser,” “pseudocapacitor,” “electrochemical double layer capacitor (EDLC  10 ),” or “ultracapacitor,” is an electrochemical capacitor that has an unusually high energy density when compared to common capacitors, typically on the order of thousands of times greater than a high capacity electrolytic capacitor. 
     Industries such as oil-drilling, aerospace, aviation, military and automotive have some applications that require electrical components to work continuously at temperatures typically exceeding 80° C. Capacitors are one of the key components in any electronic device and system; their main functions include power supply voltage smoothing, supporting the energy source, and filtering. 
     Reliability and safety is a key requirement in typical aerospace and defense applications—where engines, turbo fans, and control and sensing electronics are placed near outer shells of rockets and space shuttles. Automotive applications, such as small gearboxes or embedded alternators/starters, also require reliability and long life at elevated temperatures. 
     In the oil and gas industries, high temperature capacitors are needed for the Dc/dc converters used in drilling heads that experience rising ambient temperatures the deeper they drill. For example, oil drilling applications typically require capacitors for use in control circuits at 3.3 V (digital) and 5/15 V (analog). The oil industry also has specific service interval requirements. 
     Automotive applications typically require 12 and 24 V for power lines that, allowing for de-rating, meaning voltages from 25 V and up must be provided. In combination with high temperature, this is a real challenge. 
     SUMMARY OF THE INVENTION 
     The present invention discloses an electrical double layer capacitor for high temperature applications. 
     In general, in one aspect, the invention features an electrochemical double-layer capacitor including two electrodes wetted with an electrolyte, each electrode being attached to or in contact with or coated onto a current collector and separated from each other by a separator porous to the electrolyte, the electrodes, electrolyte and current collector containing less than 1,000 parts per million (ppm) of impurities, while exhibiting a leakage current less than 1 amp per liter of volume over a range of operating temperatures and at a voltage up to a rated voltage. 
     Other features and advantages of the invention are apparent from the following description, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more fully understood by reference to the detailed description, in conjunction with the following figures, wherein: 
         FIG. 1  is a block diagram. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     This invention discloses an electrochemical double layer capacitor, also referred to as an “ultracapacitor” or “supracapacitor,” designed for high temperature applications, such as those applications performing at temperatures exceeding 80° C. Example applications that can utilize the disclosed ultracapacitor herein are oil-drilling, aerospace, aviation, military, automotive and so forth. 
     As shown in  FIG. 1 , an exemplary electrochemical double-layer capacitor (EDLC)  10  in accordance with this invention includes two electrodes  12 ,  14 , each with a double layer of charge at an electrolyte interface, supported on conductive metal plates  16 ,  18 . The electrodes  12 ,  14  are the two electrically conducting parts of the EDLC  10 . 
     The exemplary EDLC  10  includes charge collectors  20 ,  22 , respectively, an electrolyte  24  and a separator  26 . In general, the electrolyte  24  is a chemical compound (salt) that disassociates into electrically charged ions. A solvent that that dissolves the ions might be present. The resulting electrolytic solution is an ionic liquid that conducts electricity by ionic transport. 
     The exemplary EDLC  10  is either a wound or prismatic and then packaged into a cylindrical or prismatic enclosing body. The enclosing body is hermetically sealed. In one example, the package is hermetically sealed by laser, ultrasonic, or welding. 
     In general, the separator  26  is a thin structural material (usually a sheet) used to separate the electrodes  12 ,  14 , of a divided electrochemical cell into two or more compartments. The electrolyte  24  occupies pores in the electrodes  12 ,  14  and separator  26 . 
     In the exemplary EDLC  10 , the electrodes  12 ,  14  can be provided by and include activated carbon, carbon fibers, rayon, graphene, aerogel, carbon cloth, and/or carbon nanotubes. Activated carbon electrodes can be manufactured, for example, by producing a carbon base material by carrying out a first activation treatment to a carbon material obtained by carbonization of a carbon compound, producing a formed body by adding a binder to the carbon base material, carbonizing the formed body, and finally producing an active carbon electrode by carrying out a second activation treatment to the carbonized formed body. Others methods may also be used. 
     Carbon fiber electrodes can be produced, for example, by using paper or cloth preform with high surface area carbon fibers. Other methods may be used. 
     In one specific example, multiwall carbon nanotubes (MWNT) on a variety of substrates using chemical vapor deposition (CVD) are fabricated for use in the electrodes  12 ,  14  of EDLC  10 . In a preferred embodiment, low-pressure chemical vapor deposition (LPCVD) is used. The fabrication process uses a gas mixture of acetylene, argon, and hydrogen, and an iron catalyst deposited on the substrate using electron beam deposition and or sputtering deposition. 
     The electrolyte  24  includes a pairing of a cation  30  and an anion  32  and may include a solvent. Various combinations of each are used. In the exemplary EDLC  10 , the cation  30  can include 1-(3-Cyanopropyl)-3-methylimidazolium, 1,2-Dimethyl-3-propylimidazolium, 1,3-Bis(3-cyanopropyl)imidazoliu, 1,3-Diethoxyimidazolium, 1-Butyl-1-methylpiperidinium, 1-Butyl-2,3-dimethylimidazolium, 1-Butyl-3-methylimidazolium, 1-Butyl-4-methylpyridinium, 1-Butylpyridinium, 1-Decyl-3-methylimidazolium, 1-Ethyl-3-methylimidazolium, 3-Methyl-1-propylpyridinium, and combinations thereof. 
     In the exemplary EDLC  10 , the anion  32  can include bis(trifluoromethanesulfonate)imide, tris(trifluoromethanesulfonate)methide, dicyanamide, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate, bis(pentafluoroethanesulfonate)imide, thiocyanate, trifluoro(trifluoromethyl)borate, and combinations thereof. 
     The solvent can be Acetonitrile, Amides, Benzonitrile, Butyrolactone, Cyclic Ether, Dibutyl carbonate, Diethyl carbonate, Diethylether, Dimethoxyethane, Dimethyl carbonate, Dimethylformamide, Dimethylsulfone, Dioxane, Dioxolane, Ethyl formate, Ethylene carbonate, Ethylmethyl carbonate, Lactone, Linear Ether, Methyl formate, Methyl propionate, Methyltetrahydrofuran, Nitrile, Nitrobenzene, Nitromethane, N-methylpyrrolidone, Propylene carbonate, Sulfolane, Sulfone, Tetrahydrofuran, Tetramethylene sulfone, Thiophene, Ethylene glycol, Diethylene glycol, Triethylene glycol, Polyethylene glycols, Carbonic acid ester, γ-Butyrolactone, Nitrile, Tricyanohexane, or any combination thereof. 
     The separator  26  can be fabricated from non-woven glass. The separator  26  can also be fabricated from fiberglass, flouro-polymers, Telfon®, PTFE, and ceramics. For example, using non-woven glass, the separator  26  can include main fibers and binder fibers each having a fiber diameter smaller than that of each of the main fibers and allowing the main fibers to be bonded together. 
     Once the EDLC  10  is fabricated, it may be used in high temperature applications with little or no leakage current. The EDLC  10  described herein can operate efficiently at temperatures from −20 C to 300 C with leakage currents normalized over the volume of the device less than 1 amp per liter (A/L) of volume of the device within the entire operating voltage and temperature range. One key to this performance at −20 C to 300 C with leakage currents less than 1 A/L is the assembly process itself, which produces a finished EDLC  10  having a moisture concentration in the electrolyte of less than 500 parts per million (ppm) over the weight and volume of the electrolyte and an amount of impurities less than 1000 ppm. 
     More specifically, the activated carbon, carbon fibers, rayon, carbon cloth, and/or nanotubes making up the two electrolytes  12 ,  14 , is dried at elevated temperature in a vacuum environment. The separator  26  is dried at elevated temperature in a vacuum environment. The electrolyte  24  is dried at elevated temperature in a vacuum environment. Once the electrodes  12 ,  14 , the separator  26 , and electrolyte  24  are dried under vacuum, they are packaged without a final seal or cap in an atmosphere with less than 50 parts per million (ppm) of water. The uncapped EDLC  10  is dried under vacuum over a temperature range of 100 C to 300 C. Once this final drying is complete, the EDLC  10  is sealed in an atmosphere with less than 50 ppm of moisture. 
     In addition, impurities, such as halide ions (chloride, bromide, fluoride, iodide), free amines (ammonia), sulfate, metal cations (Ag, Al, Ba, Br, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, Pb, Sr, Ti, Zn), and so forth, are kept to below 1000 ppm. Further, impurities from solvents and precursors used in the synthesis process are kept below 1000 ppm and can include, for example, bromoethane, chloroethane, 1-bromobutane, 1-chlorobutane, 1-methylimidazole, ethyl acetate, methylene chloride and so forth. 
     Impurities can be measured using a variety of techniques, such as, for example, Atomic Absorption Spectometry (AAS), Inductively Coupled Plasma-Mass Spectometry (ICPMS), or simplified solubilizing and electrochemical electrochemical sensing trace heavy metal oxide particulates based on a task specific ionic liquid. 
     AAS is a spectro-analytical procedure for the qualitative and quantitative determination of chemical elements employing the absorption of optical radiation (light) by free atoms in the gaseous state. The technique is used for determining the concentration of a particular element (the analyte) in a sample to be analyzed. AAS can be used to determine over 70 different elements in solution or directly in solid samples. 
     ICPMS is a type of mass spectrometry that is highly sensitive and capable of the determination of a range of metals and several non-metals at concentrations below one part in 10 12  (part per trillion). It is based on coupling together an inductively coupled plasma as a method of producing ions (ionization) with a mass spectrometer as a method of separating and detecting the ions. ICP-MS is also capable of monitoring isotopic speciation for the ions of choice. 
     By reducing the moisture content in the EDLC  10  to less than 500 part per million (ppm) over the weight and volume of the electrolyte and the impurities to less than 1,000 ppm, the EDLC  10  can efficiently operate over a temperature range of −20 C to 300 C with a leakage current less than 1 Amp per Liter within the temperature and voltage range. 
     It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.