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Tuesday, September 16, 2008

Nanoflowers Improve Ultracapcitors

Nanoflower power: A transmission electron microscope image shows a flowerlike manganese oxide nanoparticle deposited at the junction of crossed carbon nanotubes. Used as an electrode material, this nanotube-manganese-oxide composite could improve the energy-storage ability of ultracapacitors, which show promise as powerful, long-lasting replacements for batteries.
Credit: American Chemical Society

Imagine a cell-phone battery that recharges in a few seconds and that you would never have to replace. That's the promise of energy-storage devices known as ultracapacitors, but at present, they can store only about 5 percent as much energy as lithium-ion batteries. An advance by researchers at the Research Institute of Chemical Defense, in China, could boost ultracapacitors' ability to store energy.

A capacitor consists of two electrodes with opposite charges, often separated by an insulator that keeps electrons from jumping directly between them. The researchers have developed an electrode that can store twice as much charge as the activated-carbon electrodes used in current ultracapacitors. The new electrode contains flower-shaped manganese oxide nanoparticles deposited on vertically grown carbon nanotubes.

The electrode design promises to deliver more power, says Hao Zhang, lead author of the Nano Letters paper describing the new work. The electrode's longevity also compares with that of activated-carbon electrodes, Zhang says: discharging and recharging the electrodes 20,000 times reduced the capacitor's energy-storage capacity by only 3 percent.

In a typical ultracapacitor, two aluminum electrodes are suspended in an electrolyte. A voltage applied to the electrodes separates the positive and negative ions in the electrolyte, which get attracted to the oppositely charged electrodes. How much energy the ultracapacitor can store largely depends on the electrodes' surface area: the more area, the more space to store charge. Coating the electrodes with activated carbon increases their surface area, since a teaspoonful of the porous, spongelike material has about the surface area of a football field. Ultracapacitors can store millions of times more energy than the tiny capacitors used in electronic circuits.

But their performance still pales beside that of batteries, which store energy using chemical reactions. "If I gave you a cell phone with an ultracapacitor battery, you'd never replace the battery, and you could recharge it in a few seconds, but it would only last half an hour," says Joel Schindall, an electrical-engineering professor at MIT.

So far, ultracapacitors have been limited to niche applications that require high power and quick, repetitive recharging. For example, the devices provide quick bursts of power to buses, trucks, and light-rail trains over short stretches, and braking replenishes them. If they could store more energy, however, they could be a powerful, long-lasting replacement for batteries in hybrid-electric vehicles and portable electronics.

Researchers have long sought to boost energy storage in ultracapacitors by improving electrode design. Schindall and his colleagues are trying to make electrodes coated with carbon nanotubes, which have a greater surface area than activated carbon and are excellent conductors. Other research groups are using better charge-storing materials, such as manganese oxide and conducting polymers.

The new electrode combines the advantages of these two methods. First, the researchers grow an array of carbon nanotubes on a foil made from the metal tantalum, which is commonly used in capacitors. Then they grow 100-nanometer-wide flower-shaped nanoparticles directly on the array. The nanotubes grow more or less vertically, but they're not very stiff and tend to fall across each other. The nanoflowers grow mostly at the junctions of multiple nanotubes and have a large surface area (236 square meters per gram) compared with typical particles of manganese oxide.

"Each manganese oxide nanoflower is connected directly with the tantalum foil by two or more electron superhighways, the carbon nanotubes," says Gaoping Cao, Zhang's coleader on the project. "This superior conducting network allows for efficient charge transport." When current flows through the tantalum foil, charges quickly get transferred to and stored in the manganese oxide: the electrode stores twice as much charge as the same volume of activated carbon. The nanotubes' high conductivity could also give them a greater power output than current ultracapacitors have, the researchers say.

"The way of growing manganese oxide on carbon nanotube arrays is new and has produced beautiful structures," says Yury Gogotsi, a materials-science and engineering professor at Drexel University. Gogotsi says that combining the high conductivity of the carbon nanotubes with the charge-storage capacity of manganese oxide is an attractive approach. But, he adds, "it is not practical for large volume, such as automotive applications, because the use of carbon nanotube arrays and tantalum foil makes them expensive."

Indeed, says Schindall, cost could be the main barrier to ultracapacitors with nanostructured electrodes. "They've found a way to grow these structures," he says, "but now they've got to be able to grow them densely enough and economically enough to be practical."