Electrochemical Capacitors

Executive Summary

Electrochemical capacitors (ECs) – sometimes referred to as “electric double-layer” capacitors – also appear under trade names like “Supercapacitor” or “Ultracapacitor.” The phrase “double-layer” refers to their physically storing electrical1. An adjective meaning “pertaining to electricity”. Electrical Engineer. 2. Related to, pertaining to or associated with electricity but not having its properties or characteristics. chargeThe process of injecting energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... to be stored into the storage system. at a surface-electrolyte interface of high-surface-area carbon electrodes. There are two types of ECs, symmetric and asymmetric, with different properties suitable for different applications.

Markets and applications for electrochemical capacitors are growing rapidly and applications related to electricity grid will be part of that growth.

Discussion

When the two electrodes of an EC are connected in an external current path, current flows until complete charge balance is achieved. The capacitor can then be returned to its charged state by applying voltage. Because the charge is stored physically, with no chemical or phase changes taking place, the process is fast and highly reversible and the discharge-charge cycleOne sequence of storage charging and discharging. Also known as charge-discharge cycle. can be repeated over and over again, virtually without limit. Because of the high surface area and the small thickness of the double layer, these devices can have very high specific and volumetric capacitances. This enables them to combine a previously unattainable capacitance density with an essentially unlimited 

charge-discharge cycle lifeThe number of charge-discharge cycles after which storage becomes inoperable or unusable for a given application. In practice, storage may be inoperable or unusable when it can still deliver a portion of its initial rated energy and/or powerThe rate at which energy is generated, converted, transmitted, distributed or delivered. ratingThe operational limits of a transmissionAn interconnected group of lines and associated equipment for the movement or transfer of electric energy between points of supply and points at which it is transformed for delivery to customers or is delivered to other electric systems. system element under a set of specified conditions... The operational voltage of one cell is limited only by the breakdown potential of the electrolyteFor electrochemical batteries; A chemical compound which, when fused or dissolved in certain solvents, usually water, will conduct an electric current. All electrolytes in the fused state or in solution give rise to ions which conduct the electric... and is usually less than 3 V. Thus, cells are connected in series for higher voltage operation, exactly like battery1. Two or more electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. cells connected together electrically. In common usage, the term “battery” is also applied to a single cell, such as a household battery. 2. A system comprised of identical electrochemical cells. cells.

There are two types of ECs: those with 1) symmetric designs, where both positive and negative electrodes are made of the same high-surface-area carbon and 2) asymmetric designs with different materials for the two electrodes, one high-surface-area carbon and the other a higher capacityThe rate at which equipment can either generate, convert or transfer energy. battery-like electrodeAn electrical conductor through which an electric current enters or leaves a conducting medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. For electrolytic solutions, many solids, and molten masses, an electrode is an.... Symmetric ECs have specific energyThe amount of energy that a storage system can deliver per unit mass of the system. values up to ~6 Wh/kg and higher power performance than asymmetric capacitors where designs having specific energy values approach 20 Wh/kg. There are other differences in the characteristics and performance of these two types leading to use in different applications.

Because of their high power, long cycle life, good reliabilityThe degree of performance of the elements of the bulk electric system that results in electricity being delivered to customers within accepted standards and in the amount desired. May be measured by the frequency, duration and magnitude of adverse..., and other characteristics, the market and applications for ECs have been steadily increasing. There are dozens of manufacturers and more are entering the market because of market growth. Applications range from portable electronics and medical devices to heavy hybrid and other transportation uses. ECs are better suited than batteries for applications requiring high cycle life and charge or dischargeThe process of extracting stored energyThe energy available in the storage system to perform physical work through the conversion of its chemical or mechanical energy, stated in kWh or MWh. from the storage system. times of 1 second or less. The largest barrier to market growth has been the lack of understanding of the technology and the applications for which it is best suited. Aqueous electrolyte asymmetric EC technology offers opportunities to achieve exceptionally low-cost bulk energy storage.

There are difference requirements for energy storage in different electricity grid-related applications from voltage support and loadAn end-use device or an end-use customer receiving electric power and using electric energy from the electrical system (grid). Note: The term load is sometimes treated as a synonym for demand, which is the measure of power that a load receives or... following to integration of wind generationThe manner in which electricity is generated. The electricity that flows through California and time-shifting. Symmetric ECs have response times on the order of 1 second and are well-suited for short duration high-power applications related to both grid regulation and frequency regulation. Asymmetric ECs are better suited for grid energy storage applications that have long duration, for instance, charge-at-night/use-during-the-day storage (i.e., bulk energy storage). Some asymmetric EC products have been optimized for ~5 hour charge with ~5 hour discharge. Advantages of ECs in these applications include long cycle life, good efficiency, low life-cycle costs, and adequate energy densityThe amount of energy that a storage system can store per unit volume occupied by the system..