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Nanotubes increase capacitor capacity

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RESEARCHERS at MIT’s Laboratory for Electromagnetic and Electronic Systems (LEES) are using nanotube structures to improve the energy storage capacity of ultracapacitors.

Although ultracapacitors have been around since the 1960s, they are relatively expensive and only recently began being manufactured in sufficient quantities to become cost-competitive. Today, ultracapacitors can be found in a range of electronic devices, from computers to cars.

However, despite their inherent advantages – a 10-year-plus lifetime, indifference to temperature change, high immunity to shock and vibration, and high charging and discharging efficiency – physical constraints on electrode surface area and spacing have limited ultracapacitors to an energy storage capacity around 25 times less than a similarly sized lithium-ion battery.

The LEES ultracapacitor has the capacity to overcome this energy limitation by using vertically aligned, single-wall carbon nanotubes – one thirty-thousandth the diameter of a human hair and 100,000 times as long as they are wide.

It was developed by Joel E Schindall, the Bernard Gordon Professor of Electrical Engineering and Computer Science (EECS) and LEES associate director; John G Kassakian, EECS professor and LEES director; and PhD candidate Riccardo Signorelli.

“Storage capacity in an ultracapacitor is proportional to the surface area of the electrodes. Today’s ultracapacitors use electrodes made of activated carbon, which is extremely porous and therefore has a very large surface area. However, the pores in the carbon are irregular in size and shape, which reduces efficiency,” Schindall explained.

“The vertically aligned nanotubes in the LEES ultracapacitor have a regular shape, and a size that is only several atomic diameters in width. The result is a significantly more effective surface area, which equates to significantly increased storage capacity.

“This configuration has the potential to maintain and even improve the high performance characteristics of ultracapacitors while providing energy storage densities comparable to batteries.

“Nanotube-enhanced ultracapacitors would combine the long life and high power characteristics of a commercial ultracapacitor with the higher energy storage density normally available only from a chemical battery.”

The researchers’ work has been funded by the MIT/Industry Consortium on Advanced Automotive Electrical/Electronic Components and Systems, and a grant from the Ford -MIT Alliance.

If you are involved in or know of a project that uses innovative Australian electronics, then why not enter it into the 2006 EDN Innovation Awards? For criteria and entry forms, visit www.ferret.com.au/FerretAwards/EDNAwards.asp.

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