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 electrical charge 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.
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 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 life. The operational voltage of one cell is limited only by the breakdown potential of the electrolyte and is usually less than 3 V. Thus, cells are connected in series for higher voltage operation, exactly like battery 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 capacity battery-like electrode. Symmetric ECs have specific energy 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 reliability, 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 discharge 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 load following to integration of wind generation 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 density.