[tt] Nanotube Superbatteries

[Eugen Leitl]

http://www.technologyreview.com/printer_friendly_article.aspx?id=21938&channel=energy&section=

Friday, January 09, 2009

Nanotube Superbatteries

Dense films of carbon nanotubes store large amounts of energy.

By Katherine Bourzac

Researchers at MIT have made pure, dense, thin films of carbon nanotubes that
show promise as electrodes for higher-capacity batteries and supercapacitors.
Dispensing with the additives previously used to hold such films together
improved their electrical properties, including the ability to carry and
store a large amount of charge.

Carbon nanotubes can carry and store more charge than other forms of carbon,
in part because their nanoscale structure gives them a very large surface
area. But conventional methods for making them into films leave significant
gaps between individual nanotubes or require binding materials to hold them
together. Both approaches reduce the films' conductivity--the ability to
convey charge--and capacitance--the ability to store it.

The MIT group, led by chemical-engineering professor Paula Hammond and
mechanical-engineering professor Yang Shao-Horn, made the new nanotube films
using a technique called layer-by-layer assembly. First, the group creates
water solutions of two kinds of nanotubes: one type has positively charged
molecules bound to them, and the other has negatively charged molecules. The
researchers then alternately dip a very thin substrate, such as a silicon
wafer, into the two solutions. Because of the differences in their charge,
the nanotubes are attracted to each other and hold together without the help
of any glues. And nanotubes of similar charge repel each other while in
solution, so they form thin, uniform layers with no clumping.

The resulting films can then be detached from the substrate and baked in a
cloud of hydrogen to burn off the charged molecules, leaving behind a pure
mat of carbon nanotubes. The films are about 70 percent nanotubes; the rest
is empty space, pores that could be used to store lithium or liquid
electrolytes in future battery electrodes. The films "can store a lot of
energy and discharge it rapidly," says Hammond. The capacitance of the MIT
films--that is, their ability to store electrical charge--is one of the
highest ever measured for carbon-nanotube films, says Shao-Horn. This means
that they could serve as electrodes for batteries and supercapacitors that
charge quickly, have a high power output, and have a long life.

The MIT group is not the first to use the layering technique to create
nanotube films. But previously, researchers using the method layered a
positively charged polymer with negatively charged nanotubes, resulting in
films that were only half nanotubes. No polymer can equal the electrical
conductivity of carbon nanotubes, so these films' electrical properties
weren't as impressive as those of Hammond and Shao-Horn. Others have made
films by growing the nanotubes from the substrate up, but the resulting
forest of vertically aligned nanotubes is insufficiently dense.

"I see particular importance of these findings for supercapacitors, because
all-nanotube materials can potentially store a greater amount of charge,"
says Nicholas Kotov, a professor of chemical engineering and materials
science at the University of Michigan.

In addition to their high capacitance, the nanotube films have other
advantages as electrode materials, says Shao-Horn. Conventional
high-energy-density electrodes are made of carbon powder held together with a
binder. But particles of the binder in the surface of the electrode reduce
its active area and make it difficult to modify. With carbon nanotubes, says
Shao-Horn, "you have systematic control of surface chemistry." Adding charged
molecules to the electrodes' surface, for example, could increase their
capacitance and energy density.

"Many researchers are pursuing thin films of carbon nanotubes for diverse
applications in electronics, energy storage, and other areas," says John
Rogers, a professor of materials science and engineering at the University of
Illinois at Champaign-Urbana. The MIT group is primarily focused on
developing the films for electrochemical applications like batteries, but the
layering technique is versatile. By varying the pH of the nanotube solutions
and the number of layers in the films, it's possible to tailor the films'
electrical properties. This is "an attractive feature of this approach," says
Rogers. The technique could be used to make nanotube films for flexible
electronics, for example. Kotov also sees other potential uses of the
nanotube films. When immersed in liquid, the films swell. "This will be
useful, because it changes both the conductivity and capacity of the
material, which opens up a lot of prospects for sensing applications and
smart coatings," says Kotov.

The layer-by-layer method is time consuming, however. Typical electrodes are
10 to 100 micrometers thick; those that the MIT group has made so far are
only about 1 micrometer thick. But Hammond, a pioneer in layer-by-layer
assembly of polymers, has developed a layer-by-layer spraying technique that
should be adaptable to nanotubes. "This reduces the time it takes by an order
of magnitude, which will be necessary for commercial development," says
Shao-Horn.

Copyright Technology Review 2009.