Graphitic nanofibers in electrochemical capacitors

Details Graphitic nanofibers in electrochemical capacitors Graphitic nanofibers in electrochemical capacitors

2002-04-24

2003-12-16

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S305000, C361S031000, C361S313000, C361S301500

Reexamination Certificate

active

06665169

ABSTRACT:

FIELD OF THE INVENTION
This application generally relates to electrochemical storage devices, more particularly to the application of graphitic nanofibers as electrodes in electrochemical capacitors.
BACKGROUND OF THE INVENTION
Several publications are referenced in this application. These references describe the state of the art to which this invention pertains, and are incorporated herein by reference.
Electrochemical capacitors (ECs) are gaining acceptance in the electronics industry as system designers become familiar with their attributes and benefits. Compared with conventional capacitors, ECs have extremely high capacitance values, limited frequency response, high equivalent series resistance (ESR) which is directly related to electrode thickness and inversely proportional to the cross sectional area of the electrode, voltage-dependent capacitance, and voltage-dependent self-discharge rate. ECs were originally developed to provide large bursts of driving energy for orbital lasers. In complementary metal oxide semiconductor (CMOS) memory backup applications, for instance, a one-Farad EC having a volume of only one-half cubic inch can replace nickel-cadmium or lithium batteries and provide backup power for months. And in electric vehicle applications, large ECs can “load-level” the power on the battery system and thereby increase battery life and extend vehicle range.
Capacitors store energy in the electric field between two oppositely charged parallel plates, which are separated by an insulator. The amount of energy a capacitor can store increases as the area of conducting plates increases, the distance between the plates decreases, and the dielectric constant (the ability to store charge between the plates) of the insulating material increases.
ECs are distinguishable from traditional electrolytic capacitors which store energy by charge separation across a thin insulating oxide film that is often formed by a controlled electrolytic oxidation process at an appropriate metal.
The high volumetric capacitance density of an EC (10 to 100 times greater than conventional capacitors) derives from using porous electrodes to create a large effective “plate area” and from storing energy in the diffuse double layer. This double layer, created naturally at a solid-electrolyte interface when voltage is imposed, has a thickness of only about 1 nm, thus forming an extremely small effective “plate separation”. In some ECs, stored energy is substantially augmented by so-called “pseudocapacitance” effects, occurring again at the solid-electrolyte interface. Double layer capacitances are commonly of the order of 16-40 &mgr;F cm
−2
while pseudocapacitances associated with EC systems are commonly 10-100 &mgr;F cm
−2
.
The double layer capacitor is based on a high surface area electrode material, such as activated carbon, immersed in an electrolyte. A polarized double layer is formed at each electrode providing double-layer capacitance. The carbon provides a high surface area, A, and the effective d is reduced to an atomic scale, thus providing a high capacitance.
Although the energy storage capability of the double layer was recognized more than 100 years ago, it took the development of low-current-draw volatile computer memories to create a market for ECs.
Conventional electrochemical energy storage is achieved in a galvanic cell or a battery of such cells. The energy corresponds to the charge associated with chemical redox changes that can occur in the battery on discharge, multiplied by the voltage difference between the electrodes of the cell. The discharge process involves a net chemical reaction in the cell associated with passage of a certain number of electron or faradays per mole of reactants.
If an electrochemical reaction, such as a redox process, should occur at or near the electrode, the capacitance may be further increased. This increased capacitance is sometimes termed “pseudocapacitance” and the resulting device, while properly an electrochemical capacitor, is informally called a pseudocapacitor, supercapacitor or ultracapacitor. An electrochemical capacitor will have a different cyclic voltammogram than a pure double-layer capacitor, the pseudocapacitance revealing a Faradaic signature.
Redox systems, especially of RuO
2
.xH
2
O, for electrochemical capacitors have been demonstrated (Zheng, Z. P. and Jow, T. R., “A new charge storage mechanism for Electrochemical Capacitors”, J. Electrochem. Soc., 142, L6 (1995)), but high cost and limited cycle life are continuing impediments to commercial use of such materials. The greater the Faradaic component of the capacitance, the more the discharge curves and life approach those of a battery rather than those of a capacitor. On the other hand, the specific goals of obtaining high power output suitable for EV applications cannot be met by a pure double layer capacitor using known or proposed electrode materials (Eisenmann, E. T., “Design Rules and Reality Check for Carbon-Based Ultracapacitors”, SAND95-0671•UC-400 April 1995).
ECs do not approach the energy density of batteries. For a given applied voltage, capacitatively storage energy associated with a given charge is half that storable in a corresponding battery system for passage of the same charge. This difference is due to the fact that in an ideal battery reaction, involving two-phase systems, charge can be accumulated at constant potential while, for a capacitor, charge must be passed into the capacitor where voltage and charge is being continuously built up. This is why energy storage by a capacitor is half that for the same charge and voltage in battery energy storage under otherwise identical and ideal conditions.
Nevertheless, ECs are extremely attractive power sources. Compared with batteries, they require no maintenance, offer much higher cycle-life, require a very simple charging circuit, experience no “memory effect”, and are generally much safer. Physical rather than chemical energy storage is the key reason for their safe operation and extraordinarily high cycle-life. Perhaps most importantly, capacitors offer higher power density than batteries.
However, readily available EC products are limited in size and power performance, due primarily to their targeted memory backup use. They have capacitance values of up to a few Farads, an equivalent series resistance (ESR) of one to fifty ohms, and a working voltage of 3 to 11 V.
Until recently, ECs suitable for high-power applications have been unavailable. But interest in automotive starting, lighting and ignition (SLI) applications, as well as in electric vehicle (EV) load-leveling, has stimulated product development activities for such high-power devices. The goal is to develop products that can be efficiently charged and then discharged in the time specified for these high-rate applications.
Severe demands are placed on the energy storage system used in an EV. The system must store sufficient energy to provide an acceptable driving range. It must have adequate power to provide acceptable driving performance, notably acceleration rate. In addition, the system must be durable to give years of reliable operation. And finally, the system must be affordable. These four requirements are often in conflict for candidate energy storage technologies. This situation creates significant challenges to developers of EV energy storage systems.
A capacitor offers significant advantages to the EV energy storage system. But to be useful, it must store about 400 Wh of energy, be able to deliver about 40 kW of power for about 10 seconds, provide high cycle-life (>100,000 cycles), and meet specified volume, weight and cost constraints. This capacitor does not exist presently.
Electrochemical capacitors, sometimes called ultracapacitors, or supercapacitors, are of interest in hybrid electric vehicles where they can supplement a battery used in electric cars to provide bursts of power needed for rapid acceleration, the biggest technical hurdle to making battery-powered cars commercially viable. A battery would still be u

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