Microbial fuel cells are one of the most well-known devices in a steadily expanding research field called bio-electrochemistry. As diverse and promising as technologies bridging the gap between electrochemistry and bio-economy may be, bringing them to market is fraught with challenges. Two recent collaborative efforts, TexKoMBZ and TextESys, thoroughly investigated how to develop components for these complex systems.
Microbial fuel cells work on the same thermodynamic principles as conventional fuel cells, although their core component is a microbial instead of a (precious) metal catalyst. Biocatalysts oxidize organic matter and transfer electrons released during the process to a solid electrode surface area. From a microbiological perspective, you might say that bacterial species use the fuel cell anode instead of dissolved oxygen or sulfate to “breathe.”
It is this special ability that provides an entire range of opportunities for new fuels. Virtually all substances that, from a microbiological point of view, can be consumed will be. The most popular fuel is wastewater (see fig. 1).
The goal, however, was not to design a powerful biobattery but to combine energy generation or recovery and wastewater treatment to lower demand for conventional yet expensive activated sludge processes. Laboratory experiments had already resulted in microbial fuel cells with high enough power densities to make wastewater treatment more economical. However, it often proved impossible to maintain those densities when scaling up entire systems. The lesson learned was that it would require a more profound understanding of the fuel cell’s individual components to guarantee success. It was the reason for TexKoMBZ’s focus on methods to develop and scale anodes.
What a bioanode looks like
The combination of microbial catalyst and electrode is called a bioanode. Microorganisms cover the electrode material with a conductive biofilm, which is between 100 micrometers and 200 micrometers thin. One well-known species of electrochemically active bacteria is of type Geobacter sulfurreducens. When they grow into mature biofilms, these microbes convert acetic acid into carbon dioxide and electric power at nearly 100 percent efficiency. At this stage, they ingest most of the fuel to produce energy and little of it to grow. In actual wastewater, acetic acid is part of the food chain developing naturally within the microbial community of the biofilm. Every organism that is required for conversion will come directly from the wastewater and accumulate on the electrode surface.