Zinc-air batteries, which use zinc metal as the anode and an alkaline paste as the electrolyte, are simple, inexpensive, nontoxic, and long-lasting. But engineers haven't been able to figure out how to recharge them.

Cody Friesen, an associate professor of materials science at Arizona State University, solved the problem by using a porous electrode and a liquid solution of zinc ions and additives as the electrolyte. He cofounded Fluidic Energy in 2007 to commercialize the design, and outside testing of its commercial prototype is planned for late fall. Within two years, Friesen hopes to be selling batteries that can hold twice as much energy as the lithium-ion batteries used in laptops and, increasingly, in electric cars. He calculates that his rechargeable metal-air batteries could ultimately hold 10 times as much energy as lithium-ion devices at a much lower cost. The first market he's aiming for is storage of wind- and solar-generated electricity, but the batteries could also be useful in hybrid electric vehicles and portable electronics. --Neil Savage

Pump it up: In a standard zinc-air battery (bottom left), oxygen from the air reacts with the zinc anode, releasing electrons. Reversing the process creates spiky zinc crystals that ruin the battery. In Friesen's design (above), a pump pushes a solution of zinc ions and proprietary additives through the anode, a scaffold that contains an array of different-sized pores. Both the arrangement of pores and the additives inhibit crystal formation during charge and discharge.
Credit: Arthur Mount