By Davide Castelvecchi
From Science News, March 31st, 2007; Vol.171 #13 (p. 197)
Using enzymes commonly found in living cells, a new type of fuel cell produces small amounts of electricity from sugar. If the technology becomes viable for mass production, a few drops of your favorite soft drink will be all you need to recharge your cell phone.
In fuel cells, chemical reactions generate electrical currents. The process usually relies on precious metals, such as platinum, acting as catalysts. In living cells, enzymes perform a similar job, breaking down sugars to extract electrons and produce energy.
When researchers previously used enzymes in fuel cells, they had trouble keeping them humming, says Shelley D. Minteer of St. Louis University. Whereas biological cells continually produce fresh enzymes, there’s no mechanism in fuel cells to replace enzymes as they quickly degrade.
Minteer and Tamara Klotzbach, also of St. Louis University, have now developed polymers that wrap around an enzyme and preserve it in a microscopic pocket. “We tailor these pockets to provide the ideal microenvironment” for the enzyme, Minteer says. The polymers keep the enzyme active for months instead of days.
In the new fuel cell, tiny polymer bags of enzyme are embedded in a membrane that coats one of the electrodes. When glucose from a sugary liquid penetrates a pocket, the enzyme oxidizes it, releasing electrons and protons. The electrons cross the membrane and enter a wire through which they travel to the other electrode, where they react with oxygen in the atmosphere to produce water. This flow of electrons through the wire constitutes an electrical current that can generate power.
“The elimination of noble metals is saving cost, but [using enzymes] also widens the range of fuels that can be used,” says Paul Kenis, a chemical engineer at the University of Illinois at Urbana-Champaign.
Enzymatic fuel cells developed by other research groups typically run on more conventional fuels, such as ethanol. Direct use of sugars as fuel would be more energy efficient than fermenting corn, sugarcane, or other crops to turn their sugars into ethanol, Minteer says.
The current version of Minteer’s fuel cell oxidizes glucose only partially, so it yields only small amounts of power. “Still,” Kenis says, “just getting it to work is a major accomplishment.”
Minteer’s team is now working to embed a set of different enzymes in its fuel cells to extract more of the sugar’s energy.
Another potential advantage of biological fuel cells—compared to ordinary fuel cells or batteries—is that they might become a mass-produced power source that’s completely biodegradable, Minteer says.
It could take as little as 3 years to bring the technology into consumer products, Minteer predicts. The U.S. Department of Defense, which is funding the research, is also interested in using sugar as a densely packed energy source on the battlefield.
Klotzbach presented the current work this week at the American Chemical Society meeting in Chicago.