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This Defense Intelligence Reference Document from the Defense Intelligence Agency, dated 1 November 2010, was one of a series of advanced technology reports produced in FY 2010 under the Advanced Aerospace Weapons System Applications (AAWSA) Program. It reviews ultracapacitors as energy and power storage devices. It covers how they work, their materials, their commercial and military uses, and new developments such as carbon nanotubes, thin films and magnetic capacitors. It concludes that the U.S. government must understand and adopt advanced ultracapacitors.
UNCLASSIFIED/ /F81il 8FFI1il.t.k Wfili &,.kY showing the electrochemical double layer responsible for the characteristic performance. 5 The basic principles of operation have not changed with improved technologies, only the materials and cell design. The two electrode plates are traditionally the same material, which is usually a carbonaceous material, such as activated carbon. Each layer is capable of storing a low voltage, and multiple layers serve to increase the potential. Individual cells are placed in series to create a higher voltage, in much the same manner as a battery. Recent advances in ultracapadtors have moved toward employing dissimilar electrodes, which create a higher potential; since the electrodes are now different, these are sometimes referred to as "pseudo" or hybrid capacitors. The pseudocapacitor uses a battery-like electrode to replace one of the carbon electrodes, yielding a high-energy-storage electrode and a highly capacitive electrode within the same system. The capacitance of an ultracapacitor can be determined by the Helmholtz equation (equation 1), which describes the relationship between the electrolyte and the electrode: C - EA/d ( 1) Here, E is the dielectric constant of the electrolyte, A is the available surface area, and d represents the distance between the center of the double layer and the electrode surface. Increasing the double-layer capacitance in an ultracap is generally accomplished by either manipulating the electrode (carbon) surface area or the electrolyte. Energy density (equation 2) is the product of the capacitance and the square of the voltage: E = ½ cv 2 (2) Strategies for increasing the stored energy target improvements to both the electrodes and the electrolyte. Changing from an aqueous electrolyte to an organic electrolyte with a higher dielectric constant will increase the voltage from approximately 1 volt to more than 2.5 volts. Increasing the surface area of the electrode is another approach to increasing storage capability. There is a tradeoff between porosity and surface area that must be considered when constructing an electrode with an extremely high surface area. An activated carbon approaching a measured surface area of 3,000 m 2/g may have less than half that as accessible or useable area. 6 Increasing the pore size sacrifices surface area but provides more accessible material. By providing more interfacial area between the electrode and the electrolyte, a better electrochemical double layer can be produced, yielding a better ultracapacitor. 2 UNCLASSIFIED/ ,'1'81l 81'1'1@111it 1!1!11!! 8HLV
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Report, from the dia collection. The PDF is mirrored here; the original link is above. 34 pages are in the text index: search them above, or from the library's search.