The most common electrical energy storage device is the battery. Batteries have been the technology of choice for most applications, because they can store large amounts of energy in a relatively small volume and weight and provide suitable levels of power for many applications. Shelf and cycle life has been a problem with most types of batteries, but people have learned to tolerate this shortcoming due to the lack of an alternative. In recent times, the power requirements in a number of

How do ultracapacitors store energy?

The most common electrical energy storage devices are capacitors and batteries. Capacitors store energy by charge separation. The simplest capacitors store the energy in a thin layer of dielectric material that is supported by metal plates that act as the terminals for the device. The energy stored in a capacitor is given by 1/2 CV2, where C is its capacitance (Farads) and V is the voltage between the terminal plates. The maximum voltage of the capacitor is dependent on the breakdown

Electrochemical capacitors utilizing pseudo-capacitance

For an ideal double-layer capacitor, the charge is transferred into the double-layer and there are no Faradaic reactions between the solid material and the electrolyte. In this case, the capacitance (dQ/dV) is a constant and independent of voltage. For devices that utilize pseudo-capacitance, most of the charge is transferred at the surface or in the bulk near the surface of the solid electrode material. Hence, in this case, the interaction between the solid material and the electrolyte

What is the present and projected status of ultracapacitor technology?

There is currently research and development on ultracapacitors underway in the United States, Japan, and Europe. Much of this work is directed toward electric and hybrid vehicle applications, but some of work is for medical and consumer electronics applications. A summary of ultracapacitor research and development around the world is given in Table 3. It is clear from the table that devices using a wide range of materials and construction approaches have been fabricated. Only a few of the

Key design and cost issues

Research and development of ultracapacitors underway for nearly 10 years has been showing significant progress, but as yet no devices are available that are both technically and economically attractive. For vehicle applications, it is desirable to have devices with high energy density (greater than 5 W h/kg), high power density (that is low resistance), long cycle and shelf life, and reasonably low cost (less than US$2–3/W h). During the past 10 years, the difficulties associated with the

Summary

The physics/chemistry of how ultracapacitors operate has been reviewed for a number of different electrode materials, including carbon, metal oxides, and doped conducting polymers. The special characteristic that differentiates ultracapacitors from other types of capacitors is their high energy density (W h/kg). As shown in Table 8, ultracapacitors are presently available with an energy density of 5–6 W h/kg and projections of improved performance indicate that future devices could have energy