Site News  |   Monitor Keywords  |   Monitor Archive  |   Organizer  |   Account Info  |

Electrode and current collector for electrochemical capacitor having double electric layer and double electric layer electrochemical capacitor formed therewith

Electrode and current collector for electrochemical capacitor having double electric layer and double electric layer electrochemical capacitor formed therewith description/claims

This application is a continuation of U.S. patent application Ser. No. 11/358,980, filed Feb. 22, 2006, now U.S. Pat. No. 7,443,650, the disclosure of which is incorporated herein by reference as if fully recited herein, and which claims benefit of PCT Application Serial No. PCT/RU05/000350, filed on Jun. 24, 2005.

The present invention relates to an electrode for use in an electrochemical capacitor. More particularly, the electrode of the present invention is ideal for use in an electrochemical capacitor of high energy storage capacity, wherein the capacitor has a double electric layer. Such capacitors are often referred to as “ultracapacitors” or “supercapacitors,” however, they will simply be referred to herein as “capacitors.”

There is an increasing focus on the use of capacitors as a means for storing electrical energy. These capacitors can efficiently store and redistribute a large amount of electrical energy. For purposes of illustration, and not limitation, such capacitors may be used: as a main power supply at a particular location; as a back-up power supply at a particular location; for power quality assurance (i.e., to compensate for short-term power “surges”, “spikes”, and “skips” common to a utility-supplied source of electrical power); to provide load-leveling by storing an amount of electrical energy provided during off-peak hours and thereafter re-distributing said electrical energy during periods of peak demand; and as a primary or secondary power source for a variety of vehicles.

A double electric layer (DEL) capacitor typically comprises a pair of electrodes residing in a spaced apart relationship, between which is an electrolyte. The electrolyte can be either aqueous or non-aqueous in nature, depending on the composition of the electrodes. A separator typically also resides in the space between the electrodes. One or both of the electrodes may store electrical energy through a double layer electrochemical mechanism. In a double electric layer storage process, a layer of electrons forms at the electrode side of the electrode/electrolyte interface. A layer of positive ions also forms on the electrolyte side of the electrode/electrolyte interface. The voltage across the electrode/electrolyte interface increases with charge accumulation, and is eventually released during discharge of the capacitor.

One or both of the electrodes of a DEL capacitor may generally be polarizable electrodes. The polarizable electrode may comprise, for example, an active material and a current collector to which the active material is affixed. The most commonly employed active material is likely one of a plurality of activated carbon materials. Activated carbon materials are inexpensive and have a high specific surface area per unit mass. Electrodes are typically formed from activated carbon materials in the form of an activated carbon powder and a binder, or from woven or non-woven activated carbon fiber materials. However, preparation of DEL electrodes from an activated carbon powder is often preferable due to its lower cost.

As stated above, in a typical capacitor, one or both of the electrodes may be polarizable. However, it has been found that constructing a DEL capacitor with one polarizable electrode and one non-polarizable electrode provides the DEL capacitor with a specific energy capacity that is greater than that of a capacitor with two polarizable electrodes. In such a DEL capacitor, charge storage at the non-polarizable electrode occurs as a result of oxidation and reduction reactions at the interface of the non-polarizable electrode and the electrolyte. Such an electrode is commonly said to exhibit Faradaic pseudocapacitive behavior.

Each of the electrodes of such a DEL capacitor is typically affixed by some means to a current collector. Current collectors are commonly constructed of a material that exhibits electrical conductivity—typically a metal. As at least a portion of the current collector, along with the electrode material, must reside in the electrolyte, it must be ensured that the current collector material will not react adversely thereto. For example, the electrolyte of a DEL capacitor may consist of an aqueous sulfuric acid. In such a case, certain precautions such as, for example, coating or otherwise protecting the portion of the current collector exposed to the electrolyte must generally be undertaken, as the sulfuric acid electrolyte may corrode or erode the current collector material.

While various embodiments of DEL capacitors are currently known, each typically has one or more inherent disadvantages. For example, the activated carbon powders used to form the electrodes of common DEL capacitors are often derived from the processing of coal raw material. Such an activated carbon powder will generally exhibit a high ash percentage (e.g., 15 weight percent or more). Additionally, these activated carbon powders also typically contain an unacceptably high quantity of admixtures of transition metals. This high ash percentage and large quantity of admixtures of transition metals present in the activated carbon powder will, of course, eventually become a part of any electrode formed therefrom. The presence of these impurities in an electrode limits the voltage to which a DEL capacitor employing the electrode can be charged. For example, the presence of admixtures of transition metals can reduce the decomposition voltage of an acid electrolyte and, thereby, decrease the operating voltage of a capacitor.

There are also other disadvantages to known DEL capacitor designs. For example, many of the activated carbon materials employed to form the electrodes of such capacitors require the addition of a large quantity of binder material. The use of more binder material results in a corresponding reduction in the amount of activated carbon material present in the resultant electrode. A reduction in the amount of activated carbon present in the electrode, subsequently diminishes the capacitance and electrical energy storage capabilities of a capacitor to which the electrode is installed. Additionally, steel and similar metals are often used to form the current collectors of a DEL capacitor. Unfortunately, steel and many other metals are not resistant to an acid electrolyte. For example, in the presence of a sulfuric acid electrolyte, a steel current collector will degrade, such as by corrosion. Corrosion of the current collectors can have a negative effect on the cycling capacity and service life of a capacitor. Consequently, to reduce or avoid degradation of such current collectors, known DEL capacitor designs have employed a protective coating that is resistant to the electrolyte used in the capacitor. The protective coating, depending on its composition, can be applied to the current collector by a variety of methods. As one example, a steel current collector may utilize a protective layer of graphite foil. While certain of these coating materials may offer acceptable resistant to the electrolyte in which they reside, there has been a great deal of difficulty in obtaining adequate adhesion between the protective coatings and the subjacent electrode materials. As a result, the electrolyte will often eventually intrude between the protective coating and the current collector. It should be realized that any degradation or erosion of such a metal current collector can adversely effect performance of a DEL capacitor. For example, when a sulfuric acid electrolyte is used, even substantially insignificant quantities of iron present therein can harshly decrease the decomposition voltage of the electrolyte and result in a significant reduction in the operating voltage of the capacitor. Hence, the degradation of the current collector should be avoided.

As can be understood from the foregoing discussion, there are several disadvantages associated with known DEL capacitor designs. The electrode of the present invention utilizes an improved design that substantially reduces or eliminates many of the problems associated with known DEL capacitors. The design of the electrode of the present invention can be used to produce a DEL capacitor having an increased specific energy, better reliability, greater cycling capacity, and an increased service life.

The present invention includes a novel electrode and a DEL capacitor formed therewith. The electrode of the present invention is contemplated to be a polarizable electrode. The composition of the electrode preferably includes an activated carbon material that has an ash percentage of substantially zero, and that is also substantially free of admixtures of transition metals. Such activated carbon materials are typically obtained from a synthetic base material, such as, for example, by the carbonization and subsequent activation of a furane tar or resin. Other activated carbon materials may also be acceptably used, however, as will be described in more detail below. Preferably, a fluorine-containing binder such as polytetrafluoroethylene (PTFE), or a similar polymer substance, is added to the activated carbon material during production of the electrode material. In constructing a DEL capacitor employing the electrode of the present invention, a non-polarizable electrode is also preferably utilized. In one embodiment of such a DEL capacitor, a non-polarizable electrode composed of lead dioxide/lead sulfate compound is preferably employed.

The present invention further contemplates a DEL capacitor employing an electrode of the present invention. It is expected that such a DEL capacitor may utilize an acid-based electrolyte, such as an aqueous sulfuric acid electrolyte. Consequently, the present invention also includes a current collector for use with the above-described electrode. The current collector of the present invention preferably consists essentially of a base material that will provide for the desired operating voltage window. A number of metallic and non-metallic materials may be successfully employed for this purpose. However, based on its ability to meet various physical requirements while simultaneously being low in cost, lead or a lead compound is especially appealing as a current collector base material. The base material is preferably coated with a protective material that is resistant to an acid-based electrolyte. Preferably, the protective coating material is formed from a polymer base and a conductive dope that may thereafter be applied to the current collector base material by a variety of methods. When such a current collector is manufactured using lead or a lead compound as a base material, the current collector is less expensive and more stable in an acid-based electrolyte than a steel current collector. Further, the protective coating contemplated by the present invention is more reliable than known protective coatings, such as foil coatings that are commonly attached to the current collector base material via an adhesive.

Thus, the electrode of the present invention allows a DEL capacitor formed therewith to overcome many of the disadvantages of known DEL capacitors. Additional details of the electrode and DEL capacitor of the present invention will become apparent upon a reading of the following description and reference to the accompanying drawings.

In addition to the features mentioned above, other aspects of the present invention will be readily apparent from the following descriptions of the drawings and exemplary embodiments, wherein like reference numerals across the several views refer to identical or equivalent features, and wherein:

• Provisional Patent
• Utility Patent

• What Is a Patent?
• What Is a Trademark or Servicemark?
• What Is a Copyright?
• Patent Laws