What can we do with all the waste produced by private households? One possible solution is to feed it into a fridge-sized tank in the cellar or garage that can convert waste into electricity. Dr. Sven Kerzenmacher and Dr. Johannes Gescher from the University of Freiburg are hoping that one day such a vision will be made possible through the use of bacteria. The two researchers whose work revolves around so-called exoelectrogenic microorganisms which they put into batteries, have recently been awarded a 1.3 million euro grant from the German Ministry of Education and Research. In future, this method might also be used to turn wastewater into a sustainable source of energy. What stage of development have bacterial fuel cells reached? What needs to be optimised? And how does it all work?
Bacterial fuel cells were first talked about in 1912 when microbes that were able to generate electrical power from a carbon source such as sugar were first identified. In the 1960s and 1970s, the idea of bacterial fuel cells came up again as part of aerospace programmes. Exoelectrogenic bacteria are able to turn any type of organic waste into electricity, which makes them suitable for use in the batteries on board a space station. However, the concept only became a major research priority in the last few years when governments discovered the potential that the small organisms had in terms of finding a solution for the global energy problem. "Nowadays, everybody is working on finding alternatives to fossil energy carriers," said Dr. Sven Kerzenmacher from the Institute of Microsystems Technology (IMTEK) at the University of Freiburg. "In order to make this happen, we need to focus on a mixture of renewable energy sources such as wind, sun, biomass as well as electricity-producing bacteria."
In 2008, Kerzenmacher and his colleague Dr. Johannes Gescher from the Institute of Biology at the University of Freiburg carried out experiments to investigate the power generating performance of the bacterial species Shewanella. The microbes produce electricity because they oxidise carbon compounds during the cellular energy generation process of the respiratory chain. This is similar to the energy generation processes used by humans and it leads to electrons that humans normally transfer to oxygen. In bacteria, however, part of the electrons is transported outside of the cell by way of complicated protein machineries in the bacteria's envelope. It is possible to tap this flow of electrons with two electrodes, a wire that guides the electrons from the negative pole to the positive pole and a solution through with the positive charges flow back to the negative pole. Exoelectrogenic bacteria like Shewanella are found in many places in nature. "In principle, immersing two graphite rods into the wastewater of a sewage plant is enough to spark off the process," said Kerzenmacher. "It is not long before different bacterial species start growing on the rods and the organic residues contained in the broth can be used to make electricity."
In 2009, Kerzenmacher and Gescher took part in the "Bioenergy - breaking new ground" idea competition launched as part of the BMBF's BioEnergy 2021 initiative. Their project - "EmBBark - Highly efficient microbial fuel cells based on regenerative carbon sources" won a five-year grant worth 1.3 million euros. Gescher, a biologist, is investigating the molecular processes involved in the electron transport in the bacterial cells. Which proteins are involved? How can the flow of current be maximised? Kerzenmacher is approaching the topic from the technical side. What material needs to be used for the two electrodes of a bacterial fuel cell in order for the bacteria to grow effectively? What alternatives are available to replace expensive platinum or similar noble metals that are used as catalysts in other types of fuel cells? What is the optimal distance between the two electrodes in order to prevent too great a loss of voltage? The two researchers attach great importance to interdisciplinary cooperation. "Each side needs to understand the other in order to make the system as functionally efficient and cost efficient as possible," said Kerzenmacher.
The fuel cells created by Kerzenmacher and Gescher's interdisciplinary team produce between one and two Watts per square metre. The two scientists envisage that they will eventually be able to achieve up to five to ten Watts, enough for the requirements of a normal household. This would pave the way to turning the researchers' vision of a fridge-sized battery in the cellar of a house into reality. This battery would be able to generate electrical power from biological waste. In theory, there is nothing to stop the construction of large-scale plants. "However, so far nobody has been able to show whether this works," said Kerzenmacher. At the moment, scientists can only guess at the problems that might arise when the system is developed on a larger scale: For example, in big tanks the organic compounds might not be able to mix by way of diffusion and the energy required for a pump would of course change the cost-benefit calculations. Nevertheless, Kerzenmacher and Gescher hope to come up with a so-called demonstrator, a prototype battery that will be used to understand and optimise the processes. "Based on this prototype, we will then be able to provide information on the performance of the bacterial fuel cell, its realistic lifespan and where additional research needs to be carried out," said Kerzenmacher.
In order to complete the prototype, Kerzenmacher's team is carrying out a huge number of simultaneous experiments. The scientists have developed software and constructed systems in order to automate tests involving different electrode types. However, the researchers are not only focusing on fuel cells. In another project, funded by the Baden-Württemberg Foundation, Gescher, Kerzenmacher and their five colleagues are investigating the potential of bacterial energy generation from municipal wastewater. In cooperation with a small municipal sewage plant in the Freiburg region, the researchers are testing different inexpensive electrode materials. What kind of surface do electrodes need to have in order for bacteria to grow effectively? Which bacterial species will degrade organic waste and which bacteria will use the smaller molecules to generate electrical current? What steps are needed to create the sought-after biofilm? How can these steps be optimised? Many questions are still to be answered. But one thing is clear: it is a matter of time before bacterial fuel cells become one of the many elements of regenerative energy sources of the future.
Further information:Dr. Sven KerzenmacherDepartment of Application Development Institute of Micosystems Technology (IMTEK)University of Freiburg Georges-Koehler-Allee 10379110 FreiburgTel.: +49 (0)761 / 203-7328Fax: +49 (0)761 / 203-7322