Behind the buzz and beyond the hype:
Our Nanowerk-exclusive feature articles
Posted: Aug 09, 2007
From waste to power in one step
(Nanowerk Spotlight) A revolutionary new environmental biotechnology – the Microbial Fuel Cell – turns the treatment of organic wastes into a source of electricity. Fuel cell technology, despite its recent popularity as a possible solution for a fossil-fuel free future, is actually quite old. The principle of the fuel cell was discovered by German scientist Christian Friedrich Schónbein in 1838 and published in 1839. Based on this work, the first fuel cell was developed by Welsh scientist Sir William Robert Grove in 1843. The operating principle of a fuel cell is fairly straightforward. It is an electrochemical energy conversion device that converts the chemical energy from fuel (on the anode side) and oxidant (on the cathode side) directly into electricity. Today, there are many competing types of fuel cells, depending on what kind of fuel and oxidant they use. Many combinations of fuel and oxidant are possible. For instance, hydrogen cell uses hydrogen as fuel and oxygen as oxidant. Other fuels include hydrocarbons and alcohols. An interesting - but not commercially viable yet - variant of the fuel cell is the microbial fuel cell (MFC) where bacteria oxidize compounds such as glucose, acetate or wastewater. Researchers in Spain have fabricated multi-walled carbon nanotube (MWCNT) scaffolds with a micro-channel structure in which bacteria can grow. This scaffold structure could be used as electrodes in microbial fuel cells.
There are a number of research institutes that develop MFCs. One of them, the Center for Environmental Biotechnology at Arizona State University explains the three reasons why the MFC shapes up to become a revolutionary technology:
"First, it makes the treatment of organic pollutants a direct producer of electricity, not a consumer. Second, it expands fuel-cell technology to use renewable organic materials as a fuel; conventional fuel cells use hydrogen gas, which is today produced from fossil fuels. Furthermore, the MFC can use organic fuels that are wet, the usual form for wastes and fuel crops. Third, the MFC, by operating at ambient temperature, can double to triple the electricity-capture efficiency over combustion, while eliminating all the air pollution that comes from combustion."
If you are looking for a good source of links and materials on MFC, check out the Microbial Fuel Cells website.
The link from MFC to nanotechnology comes in the shape of carbon nanotubes (CNTs). One field of research in the CNT area is devoted to fabricating biosensors. Some of the unique properties of CNTs, such as chemical stability, good mechanical properties and high surface area, makes them ideal for the design of sensors. For instance, the surface of CNTs could be coated with antibodies in order to detect cancer cells.
"Given that carbon nanotubes are also suitable supports for cell growth, one could also think about the use of CNT based electrodes in microbial fuel cells," says Dr. Francisco del Monte. " MFCs function on different carbohydrates but also on complex substrates present in wastewaters. As yet there is limited information available about the energy metabolism and nature of the bacteria using the anode as electron acceptor; few electron transfer mechanisms have been established unequivocally. Nonetheless, the efficient electron transfer between the microorganism and the anode (e.g., microorganisms forming a biofilm on graphite fibers) seems to play a major role in the performance of the fuel cell. To further enlarge the electrode surface exposed to the bacterial growth medium (hence, the energy conversion) the design and preparation of three-dimensional architectures through which bacteria can grow and proliferate is indeed of great help to further improve the performance of this sort of device."
(Left) SEM micrographs of cross-sectioned MWCNT scaffolds (bar is 500 µm). Inset shows a detail of the chamber-like structure (bar is 20 µm). (Right) SEM micrograph of a longitudinal section of MWCNT scaffold (bar is 200 µm). The non-structured zones at the upper-left and lower-right corners correspond to the external surfaces of the monolith. (Images: The Royal Society of Chemistry)
"We think that MWCNT scaffolds could offer a self-supported structure with large surface area through which hydrogen producing bacteria (e.g., E. coli) can eventually grow and proliferate" says del Monte. "Our first concern was to study the biocompatibility of the MWCNT scaffolds. MWCNTs have been reported to be
biocompatible for different eukaryotic cells, but no data exist for bacteria."
Del Monte's team found that MWCNT scaffolds have exhibited excellent biocompatibility for immobilization of E. coli.
The Spanish researchers tried to grow bacteria on the scaffolds by two different means: by direct soaking in a bacterial culture medium and by the immobilization of nutrient-containing beads prior to scaffold preparation.
The former approach provided a higher bacteria population, but only in a few layers at the surface of the scaffold, while the latter colonized the whole of the nanostructure.
"Given that full colonization is highly desirable, we are currently focused on the improvement of bacterial viability during the scaffold formation process" says del Monte. "We believe that the efficient proliferation of hydrogen producing bacteria throughout an electron conducting scaffold like this can form the basis for the potential application of these MWCNT scaffolds as electrodes in MFCs."