|A brush anode and a tube cathode next to an assembled MFC containing one anode and two cathodes. Click to enlarge. Source: Yi Zuo, PSU|
Researchers at Penn State have developed a new graphite brush anode, consisting of graphite fibers wound around a conductive, but noncorrosive metal core, for use in a microbial fuel cell (MFC).
Use of the new anode more than doubles the power output of fuel cells using earlier generations of electrodes. A new membrane-tube air cathode, adapted from existing wastewater treatment equipment, completes the circuit.
When anaerobic bacteria are placed in the oxygen-free anode chamber of an MFC, they attach to the electrode. Because they do not have oxygen, they must transfer the electrons that they obtain from consumption (oxidation) of their food somewhere else than to oxygen, and so they transfer them to the electrode. The two electrodes of the fuel cell are at different potentials, creating a bio-battery (if the system is not refilled) or a fuel cell (if refilled).
In the process, the bacteria consume organic matter in the wastewater and clean the water. The Penn State approach uses the bacteria that naturally occur in wastewater, requiring no special bacterial strains or unusual environmental demands.
Previously, Logan and his team showed that small, rectangular fuel cells that used a carbon fiber paper as anode and a carbon fiber paper with platinum catalyst as cathode could produce electricity and clean water from wastewater. However, commercial scale-up for carbon fiber paper cells was not practical.
Using brush anodes, which have 300 to 1,500 times more surface area than the previously used carbon paper anode, the fuel cells created more than twice the power produced by the fuel cells two years ago. A fuel cell using a small brush about 1 inch in diameter and 1 inch long produced the equivalent of 2.4 watts for every 260 gallons of water using the carbon paper cathode.
Other carbon anodes were problematic because the pores or spaces became clogged with the biofilm and lost efficiency, but because the brush contains very fine fibers with plenty of circulation room around them, dead bacteria do not clog the brush.
The carbon fiber brushes are electrically conducting, very inexpensive to produce and supply large surface area for the bacterial biofilm attachment. These anodes can be made by any existing brush manufacturer in any size or shape desired. The anode is no longer a limiting factor in power production for these cells.—Bruce E. Logan, the Kappe Professor of Environmental Engineering
For the cathode, the researchers looked for a new type that could produce much more surface area. The cathode must have one side exposed to the oxygen to work. The researchers looked at membrane tubes currently used in wastewater treatment applications. Commercially available in a variety of sizes ranging up to 6 to 8 foot tall, these membrane tubes are not electrically conductive.
The team painted the membrane tubes with conducting graphite paint and added a cobalt-based catalyst. The painted tubes did work to produce power, but not as much as the carbon paper doped with platinum.
The researchers tested two cathode configurations, one with the catalyst on the outside of the tubes and one with the catalyst on the inside of the tube. In the best test case, the researchers used a carbon fiber brush anode and two tubular cathodes of about .6 inches in diameter doped with a cobalt catalyst on the inside to produce 18 watts per 260 gallons of water and achieve a charge efficiency of more than 70%.
The newly configured anodes and cathodes also allow for a variety of configurations of the fuel cell.
We showed a proof of concept with these tubes, but now we have to improve the efficiency and reduce costs. With these new anodes and cathodes the design of a wastewater treatment reactor could be as simple as a large tank with the brushes and tubular cathodes inserted into the same tank—Bruce Logan
The results are reported in a pair of papers in Environmental Science and Technology.
The National Science Foundation and the US Department of Agriculture supported this work.
Logan’s team has also explored the use of microbial fuel cells to convert corn stover directly into electricity following the pre-treatment of the biomass to release the sugars. (Earlier post.)
“Graphite Fiber Brush Anodes for Increased Power Production in Air-Cathode Microbial Fuel Cells”; Bruce Logan, Shaoan Cheng, Valerie Watson, and Garett Estadt; Environ. Sci. Technol., ASAP Article 10.1021/es062644y S0013-936X(06)02644-7
“Tubular Membrane Cathodes for Scalable Power Generation in Microbial Fuel Cells”; Yi Zuo, Shaoan Cheng, Doug Call, and Bruce E. Logan; Environ. Sci. Technol., ASAP Article 10.1021/es0627601 S0013-936X(06)02760-X