Batteries Powered to Taste

SEATTLE--It's not only our collection of newfangled electronic gadgets that keeps growing--the variety of batteries that power these contraptions keeps expanding as well. But a new generation of smart batteries could put a stop to that. Prototypes have the potential to power a multitude of devices with different energy needs, as well as recharging in a fraction of the time needed by conventional rechargeable batteries.

Marc Madou, a microengineering expert at the University of California, Irvine, and his colleagues are targeting their efforts at microbatteries, small power cells used to juice devices such as pacemakers, hearing aids, smart cards, and remote sensors. Like all batteries, microbatteries work by shuttling electron-toting compounds from a negatively charged electrode to one that is positively charged, where the electrons are siphoned off. Unlike large batteries, microbattery electrodes are typically made out of thin carbon films stacked on top of one another. One benefit of the film-based electrodes is that their large surface area relative to their volume allows electron carriers such as lithium to ferry charges out quickly, providing a quick burst of power. Unfortunately, their low overall volume means they can't store much of a charge.

To boost storage capacity, microengineers have sought to give their electrodes some extra heft without cutting down on surface area. One strategy has been to try to transform the electrodes from flat sheets into a forest-like array of carbon pillars, which are then wired together in long alternating rows to serve as negative and positive electrodes. For the scheme to pay off, individual pillars must be as tall and skinny as possible, which raises their mass while still keeping their surface area large. But although micromechanical engineers have invented slick ways to make such three-dimensional structures in silicon, they've had less luck with carbon, a standard electrode component.

Madou and postdoc Chunlei Wang broke this logjam by starting with a standard computer chip patterning technology known as photolithography. They shined ultraviolet (UV) light through a stencil pocked with an array of holes. Where the light passed through, it hit an underlying organic film called a photoresist and caused the material to polymerize. Because this photoresist was largely transparent, UV photons penetrated deep within the film, forming tall polymer pillars. The researchers then chemically washed away the unpolymerized material to expose the pillars. A series of heat treatments then burned away much of the polymer, leaving only carbon behind. Finally, they wired up their pillars into alternating rows of anodes and cathodes, submerged their forest with a lithium-containing electrolyte, and showed that it provided 78% more power than a microbattery made with thin-film carbon electrodes, they reported here 13 February at the annual AAAS meeting.

The pillar-based batteries may do more than store more charge. By controlling which individual electrodes are active, it's possible to deliver power at a wide range of voltages and currents--a micro equivalent of a battery that can serve as a 9-volt or a AAA. The electrode's large surface-to-volume ratio may also allow them to be recharged quickly, Madou adds, although his team has yet to measure this effect.

Kevin Drost, a mechanical engineer at Oregon State University, Corvallis, says the new work is impressive. In addition to making batteries, he says, the 3D patterning technique is likely to be useful for making a wide variety of novel carbon-based microstructures.

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