17 September 2003—Before too long, maybe, you won’t bother looking for a power outlet to recharge your dead cellphone battery and instead you’ll go get your sugar bowl. That admittedly far-fetched scenario now seems a little more plausible thanks to scientists who have developed a fuel cell powered by a bacterium that converts sugar into electricity with an 80-plus percent efficiency. Besides providing energy for handheld devices, their microbial fuel cell might also be used to produce electric power on a larger scale from sugars found in waste material—even in sewage in remote communities.
In the October issue of Nature Biotechnology , microbiology professor Derek Lovley and postdoctoral researcher Swades Chaudhuri, both at the University of Massachusetts at Amherst, report that the microbe Rhodoferax ferrireducens can metabolize glucose and other types of sugar into carbon dioxide, producing electrons in the process. The microbe was isolated from marine sediments collected from Oyster Bay, Va.
When the researchers immersed a graphite electrode into a solution containing glucose and the bacteria, the microbes attached to the electrode and began to feed on the sugar. The electrons produced were then collected by the electrode and flowed to an external circuit. Once the glucose was fully consumed, the researchers replaced the solution, and electricity production rapidly resumed.
Similar microbial fuel cells developed previously by other researchers were much less efficient in yielding the energy available in the sugar molecules. ”We are harvesting a much higher percentage of the electrons available,” Lovley told IEEE Spectrum. ”Typically, the efficiency is of 10 percent or less. Ours is over 80 percent.”
Moreover, previous bacteria-powered fuel cells required so-called mediators, chemicals that facilitate the transport of electrons from the medium containing the bacteria to the electrode. ”Microbial fuel cells have been around for a while but really haven’t amounted to anything substantial because they have been dependent on mediators,” Leonard Tender, a research scientist at the Naval Research Laboratory’s Center for Bio/Molecular Science and Engineering (Washington, D.C.), told Spectrum . These mediators, he says, are expensive and toxic, and have to be refueled frequently in the cell.
The device built by the University of Massachusetts’ researchers doesn’t require mediators because the bacteria attach to the electrode’s surface and transfer electrons directly to it. The key advance of the project, Tender says, is finding a microbe that can be highly efficient in harvesting the electrons available and, in addition, doesn’t require mediators. ”That’s outstanding—that’s unprecedented,” he says.
Exploiting ”iron breathers”
The R. ferrireducens is a newly discovered microbe isolated in Lovley’s microbiology lab as part of a previous Department of Energy study on microorganisms that feed on organic matter and transfer electrons to iron oxides. Lovley realized that if these microorganisms—known as iron breathers—could transfer electrons to iron compounds, they should be able to transfer electrons to other metal-like materials, such as graphite. In the laboratory, after flowing through an external circuit, the electrons reach another electrode immersed in a chamber containing oxygenated water. The electrons then combine with oxygen molecules and hydrogen ions to produce more water molecules.
The fuel cell prototype produced on average 0.5 V, enough to power a tiny lamp. But the rate at which the bacteria generate the electrons is still too slow to power other real-life, commercial applications. ”There is a significant amount of engineering [that needs to be done] before you can make a little battery that you dump sugar on and have it run your cellphone,” Lovley says. But, at least in principle, the method would allow a cup of sugar to power a 60-W light bulb for 17 hours, he says.
According to the researchers, the microbial fuel cells could ultimately provide energy to monitor devices located in remote areas, such as the bottom of the ocean, where the bacteria would feed on sugar-containing sediments. ”It’s like a local power source,” Lovley says. ”You’re not setting up a large [power] plant.” In addition, poor communities could use the device to generate electricity from domestic waste and sewage.
Yet another application might be using these fuel cells to power medical implants that require electricity to work, such as a pacemaker or a microfabricated sensor or drug delivery device. The bacteria would be encapsulated in the implants and feed on glucose in the blood.
The researchers are now working to solve a series of engineering problems, such as increasing the intensity of the electric current produced. They know that by having more bacteria in contact with the electrode, they can collect more electrons. Thus, they’re trying to increase the contact surface of the electrode, making it more porous. Ultimately, they expect that the sugar-electricity conversion will be fast enough to power devices like cellphones. In the meanwhile, though, keep the recharger handy.