22:00 12 November 2007
NewScientist.com news service
Mason Inman
A new microbe-powered device can extract up to 99% of the available hydrogen from biological compounds that have stumped previous attempts to ferment fuel from plant waste. The secret is to give the bugs a helping hand with a kick of electric charge.
Hydrogen is an attractive environmentally friendly fuel because burning it creates only water as a waste product. But finding an efficient, clean way to produce hydrogen in the first place is difficult.
Fermenting organic material using microbes is one possibility, but generally produces poor yields. Microbes reach a chemical dead end once sugar from material has been broken down into acetic acid, carbon dioxide and hydrogen. That releases at best only a third of the hydrogen in a molecule of the sugar glucose, for example.
"There is no known biological route to ferment glucose to [get] any more hydrogen than that," says Bruce Logan of Pennsylvania State University in University Park, US, who with colleague Shaoan Cheng has demonstrated a way around that the problem.
Reversed process
Their microbial electrolysis cells (MECs) enable microbes to break down organic materials completely, to just water, carbon dioxide, and hydrogen. The latest design can release as much as 99% of the hydrogen in acetic acid, a common dead end to fermentation.
MECs are modified versions of microbial fuel cells, which are used to harvest electrons produced by metabolising microbes as they feed to generate electricity. The electrochemical reactions are balanced when the used electrons are combined oxygen and hydrogen ions also released by the microbes to form water.
Logan's MECs are like microbial fuel cells in reverse. Instead of charge being drawn out, it is pumped in, and the hydrogen ions combine with electrons alone to form hydrogen gas. Applying roughly 0.5 volts provides enough energy to drive thermodynamically unlikely chemical reactions that break down the dead-end products that limited previous attempts to ferment hydrogen.
MECs can also break down other by-products of fermentation that put an end to the process, such as lactic, valeric, and butyric acids, says Logan. In tests it was fed cellulose and glucose and broke them down completely without problems.
'Great news'
The researchers were able to generate up to 1.23 cubic meters of hydrogen per day for every cubic meter of hydrogen fuel cell. This rate of hydrogen production is about 275 times faster than their earlier MEC.
In tests, the system produced hydrogen that, if fed into a hydrogen fuel cell that was 50% efficient, could generate between 1.2 and 3.4 times as much electricity as was fed into the system. By comparison, hydrogen extracted from water can only pay back about 25 to 30% of the energy used to extract it.
"It is surprising that such high hydrogen yields can so readily be obtained," says Patrick Hallenbeck of the University of Montreal in Canada. "The net energy yield appears much higher than what people are getting in other biofuel production processes ? bioethanol, for example," he adds.
But the process is still much too slow to be practical, Hallenbeck adds. Logan and colleagues are currently working on improving the speed. The performance of the MECs exceeded the expectations of Lars Angenent, of Washington University in St. Louis, Missouri, US, who is also interested in using microbes to make fuel. "This is great news," he says
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