Biofilm batteries

A review of the article;
Erable, B. & Bergel, A. (2009) First air-tolerant stainless steel microbial anode obtained from a natural marine biofilm Bioresource Technology 100 3302-3307

Marine sediments have high concentrations of organic compounds, but are often low in oxygen. As a result, fermentations products such as acetate are often in high concentrations. Oxidation of these highly reduced fermentation products yields considerable energy.

In recent years, researchers have attempted to harness this energy and create microbial fuel cells (MFCs). Early attempts were had limited success in transferring the electrons produced from the oxidation of acetate directly to the anode. However the development of these early fuel cells was hindered by practical considerations. These fuel cells were also highly sensitive to oxygen. If oxygen is present in the fuel cells, it can be reduced by contact with the anode, decreasing the energy yield. These early MFCs also used graphite electrodes which were fragile and not tractable for large-scale or long-term requirements. As such, stainless steel electrodes were tested, but were deemed ineffective due to interference by surface oxide layers.

This study by Erable & Bergel presents a considerable step forward in the development of this promising energy source. The primary difference lies in the way the authors prepared the anodes. Previously attempts at creating MFC’s have added pure cultures in solution to form biofilms on the anode. This study differed in that natural biofilms were removed intact and used as the inoculum. The biofilm was collected from a floating bridge that was in contact with the sediment for 6 hours a day, so the biofilm frequently experienced both the sediment’s anaerobic condtions and exposure to the air.

Using this inoculum, the authors were able to create biofilms that were relatively impermeable to oxygen. As oxygen was unable to directly contact the surface of the anode there was no energy decrease due to oxygen reduction. As oxide layers were unable to form, the anodes were made of stainless steel and hence were much more robust. These improvements yielded a considerable increase in the energy produced by the cells. In addition, the shift to steel has implications for design of future MFC’s. The high tensile strength of steel means that anodes with higher surface area can be built, increasing the charge possible from the same volume.

This paper is concise and presents an important step in the development of this energy source. However, throughout the paper it was not explicitly clear why the MFC created by the authors performed so much more efficiently. It is possible that the causal mechanism is not yet elucidated but this was unclear. Negative controls were used extensively to determine that the improvements in performance observed were due to the changes made by the researchers. This strengthened the experimental design leading to robust conclusions and a bright future for this area of research.

Posted by Kristian McConville, March 2011