Thursday, May 22, 2008

More-Powerful Fuel Cells

Continued from page 1

By Katherine Bourzac

Hammond synthesizes fuel-cell membranes using a technique called layer-by-layer assembly. She starts with a very thin membrane of the polymer used in conventional fuel cells. She dips it into a water solution of a positively charged polymer, then into one of a negatively charged polymer; the process is repeated until many layers are built up. The result, explains Hammond, is "a polymer backbone that resists the permeation of methanol" while still conducting protons.

The resulting 100-nanometer-thick membrane conducts two orders of magnitude less methanol than conventional, 50-micrometer-thick membranes do. And fuel cells incorporating it have a greater power output.

Hammond says that methanol is a better candidate to power portable fuel cells than hydrogen because it's a liquid and not nearly as flammable. "It's a dense power source that's safe to carry around," she says.

Savinell says that Hammond's work could have applications beyond methanol fuel cells. By picking the right polymers and varying assembly conditions including pH, says Savinell, "you can customize and optimize [the films] for any application." Layer-by-layer films might be used to improve the conductivity of hydrogen fuel-cell membranes and to increase the efficiency of ethanol fuel cells. Ethanol is safer than methanol but has similar drawbacks as a feedstock for fuel cells: ethanol seeps across the polymer membranes.

"The real promise is the power of the technology to make new materials," says Savinell. Hammond is now working on new fuel-cell membranes that contain none of the expensive conventional polymer.