Abstract
The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3–5 Wh/kg with a power density of 300–500 W/kg for high efficiency (90–95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1–2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 Ω cm2.
Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.
Introduction
Electrical energy storage is required in many applications — telecommunication devices, such as cell phones and pagers, stand-by power systems, and electric/hybrid vehicles. The specifications for the various energy storage devices are given in terms of energy stored (W h) and maximum power (W) as well as size and weight, initial cost and life. A storage device to be suitable for a particular application must meet all the requirements. As power requirements for many applications become more demanding, it is often reasonable to consider separating the energy and power requirements by providing for the peak power by using a pulse power device (capacitor) that is charged periodically from a primary energy storage unit (battery). For applications in which significant energy is needed in pulse form, traditional capacitors as used in electronic circuits cannot store enough energy in the volume and weight available. For these applications, the development of high energy density capacitors (ultracapacitors or electrochemical capacitors) has been undertaken by various groups around the world. This paper considers in detail why such capacitors are being developed, how they function, and the present status and projected development of ultracapacitor technology.
Why are ultracapacitors being developed?
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
References
J. DeGaynor, PRI 100 Volts Ultracapacitor Testing (Final Report), Prepared by the AeroVironment, for the California…
Examples of advanced PRI ultracapacitor product development
Development of PRI ultracapacitors for SLI and other automotive applications
