Work at MIT's Laboratory for Electromagnetic and Electronic Systems (LEES)
holds out the promise of the first technologically significant and
economically viable alternative to conventional batteries in more than 200
years.
Battery advances, particularly big breakthroughs, are widely seen as
complementary to renewable energy technologies, which could benefit from
improvements in electricity storage. A major battery breakthrough could
also have major implications in the realm of plug-in hybrid electric
vehicles whose batteries could be charged partly by renewable energy such
as solar or wind.
Joel E. Schindall, the Bernard Gordon Professor of Electrical Engineering
and Computer Science (EECS) and associate director of the Laboratory for
Electromagnetic and Electronic Systems; John G. Kassakian, EECS professor
and director of LEES; and Ph.D. candidate Riccardo Signorelli are using
nanotube structures to improve on an energy storage device called an
ultracapacitor.
Capacitors store energy as an electrical field, making them more efficient
than standard batteries, which get their energy from chemical reactions.
Ultracapacitors are capacitor-based storage cells that provide quick,
massive bursts of instant energy. They are sometimes used in fuel-cell
vehicles to provide an extra burst for accelerating into traffic and
climbing hills.
However, ultracapacitors need to be much larger than batteries to hold the
same charge.
The LEES invention would increase the storage capacity of existing
commercial ultracapacitors by storing electrical fields at the atomic
level.
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 you can find
ultracapacitors 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. How does it work? 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. 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.
The new nanotube-enhanced ultracapacitors could be made in any of the
sizes currently available and be produced using conventional technology.
"This configuration has the potential to maintain and even improve the
high performance characteristics of ultracapacitors while providing energy
storage densities comparable to batteries," Schindall said. "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."
This work was presented at the 15th International Seminar on Double Layer
Capacitors and Hybrid Energy Storage Devices in Deerfield Beach, Fla., in
December 2005. The work has been funded in part by the MIT/Industry
Consortium on Advanced Automotive Electrical/Electronic Components and
Systems and in part by a grant from the Ford-MIT Alliance.
Article courtesy MIT News Service