Careful pairing of bacteria can create a microbial fuel cell (MFC) that consumes cellulose and produces electricity, according to researchers at Penn State.
No currently known bacteria that allow termites and cows to digest cellulose can power a microbial fuel cell; those bacteria that can produce electrical current cannot eat cellulose. The Penn State researchers, led by John M. Regan, Assistant Professor of Environmental Engineering, put the two together in a syntrophic co-culture that produced a working, celluose-fed MFC. They reported the results of their study in the journal Environmental Science and Technology.
We have gotten microbial fuel cells to work with all kinds of biodegradable substances including glucose, wastewater and other organic wastes. But, cellulose is tricky. There is no known microbe that can degrade cellulose and reduce the anode. We 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.—John Regan
Microbial fuel cells work through the action of bacteria that can pass electrons to an anode. (Earlier post.) 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.
The researchers, who include Regan; Thomas E. Ward, research associate; and Zhiyong Ren, graduate student, looked at Clostridium cellulolyticum, a bacterium that ferments cellulose, and Geobacter sulfurreducens, an electroactive bacterium. Both are anaerobic. 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. It worked, but not as well as the two specifically paired bacteria.—John Regan
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.
The researchers 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. Current fuel cell designs produce about ten times that.—John Regan
Options for improving the power densitsy include finding 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; and to improve the design of the oxygenless fuel cell.
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.
The US Department of Agriculture supported this work.
“Electricity Production from Cellulose in a Microbial Fuel Cell Using a Defined Binary Culture”; Zhiyong Ren, Thomas E. Ward, and John M. Regan; Environ. Sci. Technol., 41 (13), 4781 -4786, 2007. 10.1021/es070577h S0013-936X(07)00577-9
“Electricity-producing bacterial communities in microbial fuel cells”; Bruce E. Logan and John M. Regan; TRENDS in Microbiology Vol.14 No.12 doi:10.1016/j.tim.2006.10.003