NEW YORK: A material consisting of sheets of carbon just one atom thick, could be very useful for storing renewable energy, says a new study.

The material, 'graphene', is a super-thin version of graphite, giving it a relatively enormous surface area: just one gram is enough to cover 2,630 square metres, around the size of a football field.

Though graphene was first discovered in the 1970s, it has only recently been applied to energy storage.

Charge and discharge

Researchers at the University of Texas, in Austin, U.S., have now tested the the material in ultracapacitors, which, like batteries, are storage devices for electrical energy.

While batteries use chemical reactions to store the electricity, ultracapacitors typically use activated carbon, a porous form of carbon that can absorb electrically charged particles.

Existing ultracapacitors only store about five per cent as much energy as an equivalent-sized lithium battery. But they deliver energy faster and can typically be charged and discharged 600,000 times as compared to a few thousand times for a battery.

Ultracapacitors are made by sandwiching an electrically conductive solution, called an electrolyte, between two plates made of activated carbon. The carbon material absorbs and stores electrically charged ions present in the electrolyte.

They are therefore ideal for systems that require energy storage with high power or a long lifespan, such as: electric and hybrid cars, buses and trains – and for controlling the flow of power to or from an electricity grid.

Ultracapacitors are particularly useful for storing energy from renewable sources such as solar panels and wind turbines, which do not produce a steady stream of power.

More area = more storage

The energy storage in an ultracapacitor depends on the surface area of the carbon material in contact with the electrolyte: more area equals more storage. Therefore, as graphene's surface area can be significantly greater than that of activated carbon, it has a much higher electricity storage potential than found in existing ultracapacitors, say the researchers.

"The unique properties of this material have the potential to significantly increase the amount of energy stored in ultracapacitors from their current levels," said Rod Ruoff, a physical chemist at the University of Texas, the lead researcher on the project.

Ruoff's team constructed and tested graphene-based ultracapacitor cells using a variety of electrolytes. The new material rivalled the performance of current commercial carbons in the amount of energy it could store.

Mathematical modelling further suggested that graphene may even potentially double the energy capacity of carbon. Ruoff said the next step is to continue fundamental research to see if these estimates are correct.

The results were published in a recent issue of the American Chemical Society journal Nano Letters.

"Major boon"

While graphene ultracapacitors may be used in any system requiring energy storage, they may be especially useful in renewable energy devices, said Ruoff. Electricity could be stored when the wind is blowing or the Sun shining, and saved for night-time or a still day when no power is being generated.

Graeme Bethune a spokesperson for EnergyQuest, an energy advisory firm based in Adelaide and Melbourne, said that "any cost-effective way of storing electricity would be a major boon."

He noted that, in Australia, wind only contributes energy around nine per cent of the time that it is required, so "cost-effective storage would be a major advance".

Joel Schindall, an electrical engineer at the Massachusetts Institute of Technology in Boston, U.S., and expert in ultracapacitor technology, called the work "credible", and added that graphene "does seem to exhibit some very promising properties."