Scientists develop microbial fuel cell that converts cellulose into electricity by pairing bacteria

Scientists from the Penn State University have found a way to develop a microbial fuel cell (MFC) that produces electricity from cellulose. No currently known bacteria that allow termites and cows to digest cellulose can power a microbial fuel cell, and those bacteria that can produce electrical current cannot eat cellulose. But by carefully pairing two different bacteria the researchers succeeded in creating a fuel cell that consumes cellulose - the biosphere's most abundant organic compound - and converts it into renewable electricity.

They report the results of their study in a open access article in a recent issue of the journal Environmental Science and Technology.

John M. Regan, assistant professor of environmental engineering, said they had got microbial fuel cells to work with all kinds of biodegradable substances including glucose, wastewater and other organic wastes. But converting cellulose is trickier. There is no known microbe that can degrade cellulose and reduce the anode.

The researchers overcame this by putting together a microbe that can degrade and ferment cellulose and an anode-reducing bacterium that can live off the fermentation products.

Microbial fuel cells work through the action of bacteria that can pass electrons to an anode. The electrons flow from the anode through a wire to the cathode, producing an electric current. In the process, the bacteria consume organic matter in the water or sediment. More technically, the Penn State team describes MFCs as follows:

Using electrochemically active microorganisms as biocatalysts, microbial fuel cells are bioelectrochemical reactors that convert organic material directly into electricity. Unlike chemical or enzyme-based fuel cells, which are tailored to oxidize specific electron donors, MFCs have tremendous electron donor versatility. This includes simple substrates such as glucose, acetate, and lactate; complex substrates such as municipal and industrial wastewaters ; and even steam-exploded corn stover hydrolysate. MFCs can also be configured to produce hydrogen instead of electricity using an anaerobic cathode and a small applied voltage to reduce protons in the cathode chamber.

The interesting aspect of the new research is that the MFC works on cellulose, the material that holds so much potential for the production of renewable energy, but that is difficult to work with. Plants produce cellulose to use as their cell walls and to provide rigidity to their structure. Along with lignin and hemicellulose, they make up huge amounts of the biomass produced by plants. Some animals, ruminants and termites for example, can break down cellulose with the aid of bacteria that live in their digestive tract. Humans and most vertebrates derive little nutrition from cellulose.

The researchers, who include Regan, Thomas E. Ward, research associate and Zhiyong Ren, graduate student, looked at Clostridium cellulolyticum, a bacterium that ferments cellulose via its cellulase enzymes, andGeobacter sulfurreducens, an electroactive bacterium:
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Both are anaerobic, living in places where no free oxygen exists. This fermenter produces acetate, ethanol and hydrogen. The electroactive bacteria consumed some of the acetate and ethanol.

"We thought that maybe we did not need a binary setup, maybe uncharacterized bacterial consortia would work" says Regan. "It worked, but not as well as the two specifically paired bacteria."

One problem with anaerobic bacteria - and the reason the researchers looked into an uncharacterized mixture of bacteria - is that currently the most efficient microbial fuel cells use an air cathode. Unfortunately, it is impossible to have an air cathode without some oxygen leaking into the reaction chamber, killing strictly anaerobic bacteria and reducing output. "We tried an aerobic cathode with the binary culture and it will not work," says Regan.

The researchers then settled on a two-chamber fuel cell that produced a maximum of 150 milliwatts per square meter. "We achieved a low power density because of the two chamber system," says Regan. "Current fuel cell designs produce about ten times that."

Currently the researchers are using pure, processed cellulose without any hemicellulose or lignin. They are just beginning to look at other cellulose products so the fuel cells can operate on less manufactured feedstock.

As a proof of concept, the researchers are happy with their results, but they would like to see the power density increase. One approach would be to find a community of bacteria that could tolerate small amounts of oxygen because some of the bacteria use up the oxygen before it reached the anaerobic bacteria. Another approach would be to improve the design of the oxygenless fuel cell.

Image: Structure of the cellulase enzyme Cel9G with which the bacterium Clostridium cellulolyticum breaks down cellulose. Credit: Institut de Biologie et de Chimie des Protéines.

References
Zhiyong Ren, Thomas E. Ward, and John M. Regan, "Electricity Production from Cellulose in a Microbial Fuel Cell Using a Defined Binary Culture", Environ. Sci. Technol., 41 (13), 4781 -4786, 2007. DOI: 10.1021/es070577h S0013-936X(07)00577-9, Web Release Date: June 6, 2007.

Eurekalert: Two bacteria better than one in cellulose-fed fuel cell - July 27, 2007.