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 electrodes deliver five times as much power as activated-carbon electrodes, 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.