Electrochemical capacitor

Details Electrochemical capacitor Electrochemical capacitor

1999-11-25

2002-07-30

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrolytic systems or devices

Liquid electrolytic capacitor

C361S504000

Reexamination Certificate

active

06426863

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to electrochemical double-layer capacitors, and more particularly to structure and method of manufacture of such capacitors utilizing polymer electrolytes with increased energy and power densities, improved stability, lower leakage, lower manufacturing cost and improved form factor.
Increase in volumetric energy density, high cycle life, greater reliability and low cost are some of the most important requirement for capacitors utilized in various military and commercial applications. Conventional dielectric capacitors such as plastic film capacitors and ceramic capacitors can accumulate and deliver electric charge very rapidly, i.e. they can operate in pulse mode with pulse widths in the nanosecond (ns) scale. However, their charge storage capability is rather poor compared to electrochemical capacitors. A dielectric capacitor with planar metal plates has capacitance in the range of pico to nano farads (pF, nF, resp.) per square centimeter (cm
2
) (B. E. Conway, Journal of the Electrochemical Society, Volume 138, p. 1539, (1991); I. D. Raistrick, Electrochemical Capacitors, LA-UR-90-39 (January 1990); B. E. Conway, “Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications” Kluwer Academic/Plenum Publishers (1999).
Plastic film capacitors can be tailored for very high voltages simply by adjusting the film or dielectric thickness in the capacitor. The energy density of commercial film capacitors based on polyester or polypropylene is less than 1 joule per cubic centimeter (J/cc). Impregnated film capacitors have a very narrow operating temperature range while the metallized version can operate up to 100° C. with the exception of polyphenylene sulfide and Teflon™ that can reach an operating temperature range of 200° C.
Ceramic capacitors have an attractive form factor, high capacitance-voltage (CV) density, very good thermal withstanding, and have been widely used as miniature devices in low stress applications. Unfortunately, in power applications that require large capacitance, high voltage and excellent volumetric efficiencies, ceramic capacitors have not met expectations.
Electrolytic capacitors, as exemplified by the aluminum and tantalum electrolytics, also suffer from a number of performance limitations. The dielectric constants of the aluminum oxide and tantalum oxide dielectrics are about 10 and 28, respectively. Their breakdown voltages are at least an order of magnitude lower than polymers, however, offering little if any net advantage. Their maximum operating voltage is about 400 volts (V). Highest practical energy density achieved has been about 3 J/cc. They suffer from relatively very high leakage, very high dissipation factor (DF), hydrogen and electrolyte outgassing, reforming periodically, high equivalent series resistance (ESR) and form factor. At frequencies above 200 kilohertz (KHz), electrolytic capacitors fail from dielectric instability and poor impedance response.
Electrochemical capacitors are symmetric devices in which the electrolyte is placed between two identical electrode systems. While electrochemical capacitors can store and deliver charge in the time scale of the order of several tens of seconds, their ability to deliver charge at short times is dictated by kinetics of the surface redox (oxidation-reduction) reactions and combined resistivity of the matrix and electrolyte. Electrochemical capacitors fall into two broad categories: (1) double layer capacitors which rely solely on interfacial charge separation across the electrical double layer; and (2) pseudocapacitors which have enhanced charge storage (similar to a battery, but to a lesser extent) derived from faradaic charge transfer in parallel with the double layer. The double layer, created naturally at an electrode/electrolyte interface, has a thickness of about 10 Angstroms (A). For a high area electrode, the capacitance per unit geometric area is amplified by the roughness factor, which could approach 100,000 times. The specific capacitance is further increased in electrode systems having a substantial potential region over which a faradaic reaction (similar to a battery reaction, but to a lesser degree) takes place. Thus electrochemical capacitors, unlike their electrostatic counterparts, can accumulate substantial charge, because of the molecular level charge separation coupled with the high charge density associated with the surface redox processes on high area electrodes.
The projected energy density for electrochemical capacitors is two orders of magnitude lower than that of batteries, but power densities are several orders of magnitude higher. Energy density is much better than for conventional film capacitors but in terms of power, the electrochemical capacitors are more suitable for relatively long discharges (milliseconds (ms) to several seconds) and low to intermediate power applications. Carbon capacitors exhibit high cycle life and good stability, thus making them useful in applications such as lightweight electronic fuses, backup power sources for calculators, and surge-power delivery devices for electric vehicles. Recently, carbon capacitors have been used in small toy cars. Carbon based capacitors utilize very thick electrodes in their construction, resulting in poor particle to particle contact of the agglomerate and high ionic resistance from the electrolyte distributed in the microporous structure. The electrodes are made highly porous allowing for air and sulfuric acid to penetrate deep into the porous structure to achieve the full benefit of the surface area. Although this results in high capacitance and energy density, the ESR increases as a result of the highly porous and thick structure.
Although the pseudocapacitors utilizing valve metal oxide electrodes such as ruthenium or iridium oxide possess very high double layer capacities emanating from the intrinsic high surface areas and redox processes, leading to energy densities as high as 10 to 20 J/cc, they suffer from the same limitations as the carbon capacitors with high ESR. Ruthenium oxide has a high double layer capacity of about 150 microfarads per real square centimeter (&mgr;F/real cm
2
). Since the intrinsic surface area of this material is very high, it is probable that the intrinsic capacitance will also be extremely high. The superior, demonstrated performance of the RuO
2
-based capacitor is a consequence of the high exchange current density of the RuO
2
/Ru
2
O
3
reaction, although this advantage is negated by the porous nature of the RuO
2
matrix used in such devices. Craig, in Canadian Patent No. 1,196,683 (1985), describes a supercapacitor based on ruthenium oxide and mixtures of ruthenium and tantalum oxides and reported capacitances as high as 2.8 F/cm
2
. Increase in the ESR of the capacitor is a consequence of the reduction in the exchange current density. This may be overcome if the capacitor is designed with ultra-thin electrodes and highly conductive thin film electrolytes.
Electrochemical capacitors based on RuO
2
and solid polymer electrolyte have been studied at Giner, Inc (MA). The use of a solid polymer electrolyte leads to a leak-free system that contains no corrosive liquid electrolyte. This concept was based on the use of a hydrated ionomer membrane such as DuPont's Nafion™. The composite structure ensured a continuous proton-conducting ionomer linkage throughout a single cell, thus facilitating proton transport from one electrode to the other. The performance of this capacitor containing only hydrated water dropped off abruptly below the freezing point of water and in addition, the ESR was fairly high at about 0.3 ohm-cm
2
. Subsequent use of sulfuric acid improved the proton conductivity within the particulate by accessing pores down to 100 A diameter.
This study was interesting and demonstrated that high proton conductivity and materials based on very high exchange current densities is effectively required for lowering the ESR. However, the problem with using Nafion™ type memb

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