BioEd Online

New materials for advanced electronics are usually expensive, high-tech substances. But a team of researchers in France has shown that energy-storage components called supercapacitors can be made from a remarkably cheap and humble material: baked seaweed.

Francois Béguin of the CNRS Research Centre on Divided Matter in Orlans, France, and his co-workers say that seaweed, when burned to a charcoal-like form, is just the right stuff for making the electrodes in state-of-the-art supercapacitors. It performs as well as the carbon-based substances currently used in commercial devices, the researchers say.

"People working on carbons are always looking for improved properties," says Mildred Dresselhaus, a specialist in carbon materials at the Massachusetts Institute of Technology. She points out that coconut shells are already used as a source of porous carbon for water filtration and other applications. "Low-tech routes are commonly used when they do the job," Dresselhaus says.

The seaweed-derived polymer that Béguin has hit upon, called alginate, is non-toxic, and is widely used as a thickener in foods and cosmetics: 20,000 tonnes of it are extracted from seaweed for this purpose every year. This makes the material very cheap.

Emergency power

Supercapacitors provide an alternative to batteries for power storage in portable electronics. They consist of a pair of plates, or electrodes, loaded with electrical charge that can be subsequently tapped, producing a current.

Capacitors can provide more power higher voltages or currents than batteries, but tend to store less total electrical energy.

They are predicted to find applications as emergency power sources for computers, or as supplementary sources in electric vehicles where, for example, they might store energy captured during braking.

Charged up

The amount of energy stored in a capacitor depends on how much charge can be built up on the electrodes without the material they are made from breaking down.

Many supercapacitors currently available have electrodes made of a porous form of graphite-like substance, called activated carbon, which is cheap and can store charge. But the porosity is a drawback, because storing a lot of charge in a low-density substance requires a large volume of material, and that's bad news for making miniaturized power sources.

Bguin and his colleagues reasoned that what is needed is a form of carbon that is relatively dense, electrically conductive, and capable of holding a lot of charge. The researchers thought that cellulose, the basis of plant matter, might be suitable. This carbohydrate contains plenty of charge-holding oxygen atoms, but most of the oxygen is lost when cellulose is heated.

They then struck upon alginate, abundant in brown seaweeds, which is chemically similar to cellulose but holds on to its oxygen when heated.

Fried circuits

The French team cooked alginate in an air-free enclosure, turning it into a black powder. They then combined this with a polymer binder to make a hard material, which they shaped into electrodes for supercapacitors.

The amount of electrical charge and energy that these devices can hold is comparable to that of capacitors made from commercial activated carbons. But the seaweed capacitors can be charged to voltages twice as high without breaking down, and the material is twice as dense. They hold up well over time, too: their charge-storage capacity declines by only 15% after 10,000 cycles of charging and discharging.

When can we expect to see burnt seaweed helping to drive your laptop? Bguin thinks that commercialization of the carbon material could happen very quickly. "We are in touch with a company interested in this development," he says.

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