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Hydrogen makers

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11:16am, November 26, 2007
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By putting bacteria to work, a new bioreactor produces hydrogen hundreds of times as fast as previous prototypes.

In a microbial fuel cell, bacteria break down organic matter, releasing electrons and protons into a solution. The protons migrate through a membrane, while the electrons enter a cathode and pass through a circuit that delivers them to the protons on the other side. There, protons—ionized hydrogen—and electrons react with oxygen to produce water, at the same time generating a voltage that keeps the electrons flowing, so the device produces a small amount of electric power (SN: 2/4/06, p. 72).

In the absence of oxygen, and with the help of a metal catalyst, the protons and electrons will instead combine into hydrogen gas. However, such hydrogen-producing bioreactors require an external voltage to pull the electrons from one side to the other, and so far have been very inefficient: A 1-liter bioreactor would normally produce 4 milliliters of hydrogen per day, says Bruce Logan of Pennsylvania State University in University Park.

By switching to a different membrane and using phosphates to ferry protons through it, Logan and his colleague Shaoan Cheng have now created a prototype bioreactor that can produce hydrogen 300 times as fast as before, with bacteria that can feed on a variety of foods, including glucose and cellulose. It produces almost three times as much energy—in the form of hydrogen gas—as it uses electrically, the researchers write in the Nov. 20 Proceedings of the National Academy of Sciences.

"This is an important step," says Derek Lovley of the University of Massachusetts in Amherst, who's working on genetically engineered microbes that can produce hydrogen more efficiently.

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