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Bacterial fuel: It’s a gas
As oil prices continue to rise, hydrogen-powered vehicles are drawing increasing attention from the automaking industry. Today, most hydrogen for such uses is produced from nonrenewable fossil fuels, making it not as environmentally friendly as proponents suggest. Enter hydrogen-producing microbes, which, one study suggests, may become an efficient and renewable source of hydrogen for fuel.
To power vehicles, hydrogen gas is burned in a combustion engine or as part of a hydrogen fuel cell, which uses the reaction between hydrogen and oxygen gases to produce electricity to power the car’s motor. The difficulty, however, is where to get the hydrogen. Making hydrogen from fossil fuels, the most common method, produces greenhouse gases just like those from gasoline-powered cars. Hydrogen can also come from renewable sources, such as by breaking water molecules into hydrogen and oxygen, but that method is not energy-efficient.
There is another possible source, says Bruce Logan, a civil engineer from Pennsylvania State University in University Park. Scientists have been working with bacteria such as E. coli to create microbial fuel cells to generate electricity. When the microbes munch on sugars such as glucose, they produce electrons and protons that are channeled through the cell to form an electrical circuit — much like a battery. They also produce hydrogen as a waste product.
So, Logan reasoned, if the goal was actually hydrogen gas, it should be possible to reverse the same microbial setup — producing hydrogen rather than electricity. That hydrogen could potentially become a more efficient source of the gas to power vehicles, he says. “It’s been estimated that we need to double our electricity output to get enough electricity to make hydrogen for all our cars,” Logan says. “These cells could potentially need only one-tenth of that electricity.”
By adding a small voltage to the electricity already produced by the bacteria in a fuel cell — and by eliminating the oxygen-adding step — the end product will be hydrogen gas.
But how much of the gas is produced, relative to how much applied voltage is needed, would be the trick to determining the setup’s efficiency, Logan says.
Logan, with colleague Shaoan Cheng, also at Penn State, tested the efficiency of such a hydrogen-producing fuel cell. More than 0.2 volts were needed to prompt the bacteria to begin producing hydrogen, but the efficiency of the process, and the amount of hydrogen produced, were still relatively low, Logan says. By tinkering with the design of the cell to increase the amount of power produced by the bacteria, however, Logan and Cheng were able to increase the efficiency of hydrogen production to 288 percent, they reported Nov. 13 in Proceedings of the National Academy of Sciences. Hydrogen produced by water electrolysis, comparatively, typically has an efficiency of 50 to 70 percent.
Adapting microbial fuel cells to produce hydrogen power is an innovative idea, says Lars Angenent of Washington University in St. Louis, Mo. “The efficiency was very high, and this is great news,” he says. It may be years before such a hydrogen-producing fuel cell could be a primary fuel source, however, he says, because the initial study only generated a small volume of hydrogen, and thus scaling up to mass production levels will be difficult. Hydrogen-producing fuel cells powered by bacteria may play a role in hydrogen production, he says, but hydrogen will have to come from many sources and different technologies.
