Why Bio-energy?
As world population continues to grow, so does the demand for energy. Our most prevalent sources of energy include petroleum, natural gas, coal, and nuclear materials. However, long-term use of these conventional fuels has led to public concerns over resource depletion, pollution, national security, and possible climate change implications. We are seeing a comparatively small but increasing use of renewable forms of energy such as hydroelectric, geothermal, wind, solar, and biomass. Bio-energy from renewable agricultural products and waste materials is an attractive possible solution to the energy problem, and a variety of technologies are being developed. Current methods of deriving energy from biomass often rely upon combustion of bio-fuels produced from anaerobic digestion, biological hydrogen production, ethanol fermentation, or vegetable oil transesterification. These methods produce combustible gases such as methane and hydrogen or combustible liquids such as ethanol and biodiesel. These products are then burned to drive engines, turbines, or other power generating devices. The inefficiency of combustion has led to the search for a new method of obtaining clean, renewable, electrical energy directly from biomass.
What is a microbial fuel cell (MFC)?
Microbial fuel cells (MFCs) are non-combustive devices providing electrical energy using microorganisms as biocatalysts and organic materials as substrate – or food for the microorganisms. As these living microbes metabolize the substrate, chemical energy is converted into useable electrical energy. MFCs can use waste materials such as rumen fluid (see “microorganisms section”) as a source for the bio-catalysts and agricultural byproducts such as manure, corn stover, or used straw bedding as a substrate. In this way, MFCs are inexpensive to construct because they depend on materials produced in abundance and conventionally regarded as waste.
Progress made over the past two decades has considerably increased the power output and conversion efficiencies of MFCs. As a result, MFCs are broadly applicable for wastewater treatment, powering marine electronic devices in remote locations, and as biological sensors.
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| Figure 1 (above): Diagram of working MFC. Reference: (Henslee et al. 2004) |
How do MFCs work?
The main components of the microbial fuel cell are electrodes: the negative anode and the positive cathode (see Figure, on right). A two-chambered MFC has the anode and cathode compartments separated by a proton exchange membrane (PEM), a micro filter that allows only very small positively charged molecules to fit through. The anode is the electrode on which microorganisms colonize, and it is submerged in the organic substrate. The anode is also the site of oxidation, where electrons are removed from the organic material by the microbes and passed onto the electrode surface. The anodic and cathodic electrodes are connected by a wire, and electrons from the oxidation reactions in the anode compartment are passed through the wire to the cathode. The cathode can be surrounded by a variety of oxidizing agents including air, water, or potassium ferricyanide. Positively charged hydrogen ions generated at the anode are passed through the PEM to the cathode, and combine with electrons and oxygen on the cathode surface to form water, thus completing the electrical circuit.
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| Figure 2 (Above): Rumen fluid extraction from a fistulated cow |
Microorganisms
At The Ohio State University, researchers use rumen microorganisms as biocatalysts in MFCs. Rumen microorganisms can be obtained from the first chamber of the digestive tract of a ruminant animal. These microorganisms are successful biocatalysts because they efficiently break down cellulose in ruminant animals. Rumen fluid is often collected from a fistulated cow – a cow with a resealable porthole leading directly to its stomach (see Figure 2, on right). A manure slurry, a mixture of manure with water, from a ruminant animal also contains some rumen microorganisms and can be used as a less potent alternative to microbes extracted directly from the animal’s stomach. However, manure slurries usually produce less electricity than rumen fluid because there are fewer electrochemically active microbes in manure and it is a poor environment for the continued growth and reproduction of those microbes. Examples of ruminant animals besides cattle include goats, sheep, antelope, llamas, giraffe, bison, buffalo, and deer. Contact your local zoo, farm, or petting zoo for manure to build your very own MFC!
References
- Henslee, B.E., et al. 2004. Biological fuel cell: Modeling, design, and testing. Final report for ASAE’s G.B. Gunlogson Student Environmental Design Competition. Ohio State University, Columbus, Ohio
- Logan, Bruce E., et al. 2006. Microbial Fuel Cells: Methodology and Technology. Environmental Science & Technology 40, no. 17:5181-5192.
- Rabaey, K., et al. 2004. Biofuel Cells Select for Microbial Consortia That Self-Mediate Electron Transfer. Applied and Environmental Microbiology 70, no. 9:5373.
- Rabaey, K. and W. Verstraete. 2005. Microbial fuel cells: novel biotechnology for energy generation. Trends in Biotechnology 23, no. 6:291.
- Rismani-Yazdi, H., et al. 2007. Electricity generation from cellulose by rumen microorganisms in microbial fuel cells. Biotechnology and Bioengineering 97, no. 6:1398-407.