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.