In the race to find an alternative to gasoline for powering our cars, converting corn, wood chips, and other biomass to ethanol is all the rage these days. But new work from researchers in the United States suggests that turning biomass into hydrogen gas may be a simpler and potentially cheaper alternative in the end.
Researchers have long known that microbes in devices called microbial fuel cells can break down organics to generate electricity directly. If the electrons that carry this charge could be combined with protons that the microbes also produce in the process, they could be used to generate hydrogen gas (H2), which could be used in fuel cells or specially designed internal combustion engines. But microbial fuel cells don't produce electrons with a high enough voltage to allow them to pair up with protons. Two years ago, researchers led by Bruce Logan, an environmental engineer from Pennsylvania State University in State College, showed that it was possible to get around this by giving the electrons generated by microbes a little external energy boost.
The original process didn't produce enough hydrogen to be useful. So for their current work, Logan and graduate student Shaoan Cheng placed bacteria collected from soil or wastewater in a liquid-filled vessel separated into two chambers by a thin membrane. The microbes sit on a positively charged electrode in one chamber and break down acetic acid (the sour-smelling compound in vinegar), glucose, and other organics, producing electrons and protons. The protons diffuse toward a negatively charged electrode in the second chamber. The electrons, meanwhile, travel through a wire from the positive to the negative electrode, getting their energy boost from an external power supply before they meet back up with the protons and pair up to form hydrogen.
To crank out more hydrogen, Logan and Cheng improved the membrane to allow protons to move to the cathode about twice as easily as before. They also improved their positively charged electrode, giving it a higher surface area and treating it so that bacteria could more easily adhere to it. The upshot is the apparatus produces about 1000 times the amount of H2 as their previous system, the researchers report online this week in the Proceedings of the National Academy of Sciences. It also comes out ahead in efficiency: The gas contains nearly three times as much energy as added from the external power supply to get the electrons and protons to combine.
"It's a very nice paper," says Bruce Rittmann, an environmental engineer at Arizona State University in Tempe. "You spend a little energy to get [the H2], but you get much more in return," he says. That's likely to make it a far more efficient fuel overall than turning biomass to ethanol, Logan says. Still, Logan acknowledges that H2 has a long way to go before it's used to power all of our cars, because that would require an entirely new infrastructure be built for shipping and storing hydrogen gas. In the meantime, he says, the hungry microbes could be used in wastewater treatment plants, producing H2 that could then be sold for industrial use.