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 Whrkg Ž . with a power density of 300-500 Wrkg for high efficiency 90-95% chargerdischarges. Projections of future developments using carbon indicate that energy densities of 10 Whrkg or higher are likely with power densities of 1-2 kWrkg. 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 V cm 2 . 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 chargerdischarge efficiency. q

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People today are very familiar with different battery technologies, from small zinc-air button cells to AAA alkali cells to spiral wound lithium ion laptop batteries to fifty-pound lead acid batteries found in automobiles. This situation has come about because people rely heavily on battery power. Common battery applications include, for instance, power for cell phones, Sony Walkman, PDAs, laptop computers, Ipod players, digital cameras, and on and on. In sharp contrast, people today are much less familiar with capacitor technologies, although capacitors also store energy.

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This article is concerned with the design and characteristics of electrochemical double layer capacitors (EDLCs), and in particular the role of nanostructured carbons and how they function.