Electricity: electrical systems and devices – Electrostatic capacitors – Fixed capacitor
Reexamination Certificate
2003-01-23
2004-09-21
Reichard, Dean A. (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
Fixed capacitor
C361S305000, C361S502000, C361S504000, C361S508000, C361S512000, C429S199000, C429S213000, C252S062200
Reexamination Certificate
active
06795293
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrical energy storage devices such as advanced supercapacitors and batteries and, more specifically, to such devices that use polymer modified electrodes.
BACKGROUND OF THE INVENTION
Secondary current sources (storage batteries) make it possible to accumulate, store and give up electric power to an external electric circuit. Among these are conventional batteries, conventional capacitors and electrochemical capacitors (also called Supercapacitors or Ultracapacitors)—[B. E. Conway,
Electrochemical Supercapacitors
// Kluwer Acad. Plen. Publ., NY, 1999, 698 p.].
A conventional electrochemical supercapacitor usually includes a hermetically sealed housing filled with electrolyte, a positive electrode (anode) and negative electrode (cathode) placed inside said housing, a separator that separates anode space from cathode space and special lead terminals connecting the supercapacitor to external electric circuits.
Electrochemical supercapacitors are based on the capacitive (not battery type) or Faradic (battery type) method for storing electric power. In the capacitive type supercapacitors, the capacity of the double electric layer formed at the electrolyte/electrode boundary is used for accumulating energy. Carbon materials having a large specific surface are usually employed as the electrode in such supercapacitors. No chemical or phase changes take place on the electrode surface or in the electrode space during the charge/discharge process in such a device.
In Faraday type supercapacitors, the charge/discharge process is accompanied by redox reactions on the electrode surfaces. In contrast to conventional batteries, these processes take place in a thin layer of electrically active substance on the electrode surface. The surface of electrodes in many known supercapacitors of this type is covered with metal oxides.
Both above mechanisms of energy accumulation exist in known energy storage devices, which are usually classified by the mechanism that makes the major contribution to the energy accumulation and storage process. Electrochemical supercapacitors have very high specific power (as high as 10 kW/kg) and long service life (up to 1 million charge/discharge cycles). These features open a wide range of potential applications for electrochemical supercapacitors [Supercapacitor Market Survey, World Markets, Technologies & Opportunities: 1999-2004 Technical-Economic Analysis for 2000, Tyra T. Buczkowski, ISBN#1-893211-05-32].
Nevertheless, known electrochemical supercapacitors are not free from disadvantages. In particular, they have low specific energy capacity. The value of specific energy capacity for commercially available electrochemical supercapacitors lies within the relatively low range of 1-10 W·h/kg.
The highest value of specific energy capacity was claimed for electrochemical supercapacitors of Faradic type that include carbon electrodes with ruthenium oxide on their surface. It is around 30 W·h/kg [U.S. Pat. No. 6,383,363]. However, high cost of ruthenium would impede the wide application of such devices.
The maximum values of specific energy capacity of known supercapacitors are limited primarily by the nature of materials used for electrode manufacture—i.e. metal oxides. Metal oxides require supplement of conductive additives, which increase the weight of the system and, therefore, reduce the specific energy capacity. These materials also contribute to the high cost of these devices.
Several attempts have been made to obtain fundamentally new materials and technologies for the design and manufacture of electrochemical supercapacitors. These attempts include chemical modification of electrodes—for example, by immobilizing conducting polymers on the inert electrode surface.
Conducting polymers are subdivided into two groups [B. E. Conway, Electrochemical Supercapacitors// Kluwer Acad. Plen. Publ., NY, 1999, 698 p]:
1) The so-called “organic metals” or conducting polymers—these are polymers with a conduction mechanism similar to that of metals;
2) Redox polymers—i.e. compounds in which electron transfer is effected mainly due to redox reactions between adjacent fragments of polymer chain.
Polyacetylene, polypyrrole, polythiophene and polyaniline represent examples of “organic metals”. In partially oxidized form, these polymers offer an even greater degree of conduction, and they can be considered as salts consisting of positively charged “ions” of polymer and counter-ions evenly distributed over its structure (these counter-ions support the overall electrical neutrality of a system).
The polaron theory of conduction is acknowledged to be the main model of charge transfer in conducting polymers [Charge Transfer in Polymeric Systems //Faraday Discussions of the Chemical Society. 1989. V.88]. In solid state physics, a polaron is a cation radical which is partially delocalized over a polymer fragment. The polaron becomes stable, thus polarizing its environment. (#Paragraph 1)
“Organic metals” can be produced by electrochemical oxidation of appropriate monomers on an inert electrode surface. These polymers can be converted from a conducting state (i.e. oxidized state) into a non-conducting state (i.e. reduced state) through variation of the electrode potential. Transition of a polymer from the oxidized state into the neutral reduced state is accompanied by the egress of charge-compensating counter-ions from the polymer into the electrolyte solution, in which the process is conducted. The reverse is also possible.
Both purely organic systems and polymer metal complexes (i.e. metal organic compounds) fall into the category of redox polymers [H. G. Cassidy and K. A. Kun. Oxidation Reduction Polymer //Redox Polymers. Wiley—Interscience, New York, 1965]. Polymers containing metals are better conductors than those without.
As a rule, polymer metal complex compounds are produced via electrochemical polymerization of source monomer complex compounds with octahedral or square-planar configurations, wherein electrochemical polymerization being performed on inert electrodes. As will be shown below, the spatial configuration of monomers plays a crucial role in the formation of polymer structures suitable for use in supercapacitor. Polypyridine complexes of composition poly-[Me(v-bpy)
x
(L)
y
], where:
Me═Co, Fe, Ru, Os;
v-bpy=4-vinyl-4′-methyl-2,2′-bipyridine;
L=v-bpy (4-vinyl-4′-methyl-2,2′-bipyridine), phenanthroline-5,6-dione, 4-methyl phenanthroline, 5-aminophenanthroline, 5-chlorophenanthroline; (x+y=3) represent an example of redox polymers formed using octahedral source complex compounds [Hurrel H. C., Abruna H. D. Redox Conduction in Electropolymerized Films of Transition Metal Complexes of Os, Ru, Fe, and Co //Inorganic Chemistry. 1990. V.29. P.736-741].
Metal ions that may be in different states of charge represent redox centers—i.e. atoms participating in redox reactions in a polymer. Metal complexes having only one possible state of charge (zinc, cadmium) do not produce redox polymers. Conduction of redox polymers requires the presence of a branched system of conjugated Π-bonds that serve as conducting “bridges” between redox centers in a ligand environment of complexes. When a redox polymer is completely oxidized or completely reduced (i.e. all its redox centers are in one state of charge), charge transfer along the polymer chain is impossible and redox polymer conduction is close to zero. When redox centers are in different states of charge, exchange of electrons is possible between them (this proceeds in the same manner as in solution in the course of redox reactions). Therefore, conduction of redox polymers is proportional to the constant of electron self-exchange between redox centers (k
co
) and to concentrations of oxidized [Ox] and reduced [Red] centers in a polymer. In other words, the redox polymer conduction is ~k
co
[Ox] [Red&rs
Kogan Sam
Logvinov Sergey
Shkolnik Nikolay
Timonov Alexander
EnGen Group, Inc.
Ha Nguyen T.
Houston Eliseeva LLP
Reichard Dean A.
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