Researchers have created an elegant and affordable new way to build a material with powerful capacities for energy storage.
For the past several years, scientists at George Washington University's Institute for Nanotechnology have been learning how to synthesize graphene, a sheetlike, orderly network of carbon atoms so thin that its third dimension is almost undetectable. Until recently, they had focused on synthesizing "nanotubes," tiny curls of graphene that are strong conductors of electricity. But in 2010, says Michael Keidar, the institute's director and an author of Tuesday's report, "We tried to grow nanotubes and we accidentally grew graphene" sheets at the same time.
Since the nanotubes and sheets have complementary properties that are valuable for energy storage, the researchers spent the next four years perfecting their accident to create the two materials simultaneously. What they made is a messy but powerful jumble of the two, which can be painted directly onto paper.
Graphene sheets and nanotubes come from the same raw material: solid graphite that is heated to become a gas and then a plasma. Cradled within a magnetic field, the plasma bundle releases jets of material that precipitate as either sheets or tubes, depending on the field.
By applying a non-uniform magnetic field, the scientists were able to make the plasma release jets containing both fully-formed nanotubes and still-fluid plasma that forms graphene on contact with a surface. This mixture, either painted or printed onto sheets of paper, becomes an ultracapacitor: two plates which hold large amounts of energy between them in an electric field.
"Generally speaking, [nanotubes] and graphene both have unique and excellent electronic, thermal, and mechanical properties," states the report, published April 22 in the Journal of Applied Physics. Of the two materials, graphene sheets have a higher surface area, which translates to higher energy storage capacity, and lower resistance, which means that electricity skates more freely across it. But graphene flakes don't necessarily form a cohesive surface alone.
"The nanotubes interconnect this entire network," says Dr. Keidar. Thanks to their stickiness and the graphene's many excellent qualities, he says, "the power per unit volume would be larger" than a capacitor made out of either material alone.
It would be cheaper, too, since the team has figured out how to produce a ready-made blend of the two materials, in one single step – no mixing or purifying needed.
Ultracapacitors are great at delivering quick bursts of power, and they are far more durable than chemical batteries. But until now, their high cost and low storage capacity have prevented them from replacing batteries.
The work of Keidar and his colleagues stands to make them both smaller and cheaper. "That's what makes it very exciting," he says.