Bacteria that are able to grow on an electrode can learn how to communicate with it - i.e., get better at transferring electrons from the cathode to the anode. Based on this premise and Darwin's theory of natural selection, researchers at Ghent Univ.'s (Ghent, Belgium; http://labmet.ugent.be) Laboratory of Microbial Ecology and Technology have increased the power output of microbial fuel cell systems from about 0.6 W/m^sup 2^ using an anaerobic inoculum to 4.31 W/m^sup 2^ of electrode surface (664 mV, 30.9 mA) with a selected microbial consortium.
Korneel Rabaey and Willy Verstraete of Ghent Univ.'s faculty of bio-engineering first inoculated the fuel cell with anaerobic, methane-generating sludge (nourished by a biodegradable food source) to ensure a broad-spectrum inoculum. "Those bacteria that were able to transfer electrons to the graphite electrode most efficiently gained a selective advantage, because they could get more energy by losing electrons in that way," explains Rabaey.
Over the course of 70-80 d, analysis of the microbial community via a denaturing gradient gel electrophoresis (DGGE), a molecular technique that reveals how many different types of bacteria are present and which ones are dominant, showed that these bacteria did dominate the population, says Rabaey. Consequently, the fuel cells' power output increased, peaking after 92 d of enrichment. Some of the bacteria produced a highly efficient redox mediator that may have facilitated electron transfer, but Rabaey and Verstraete have yet to figure out how to identify the bacteria that are responsible for electron transfer.
Next, the team will decrease the reactor volume (i.e., achieve the same power outputs with smaller "battery" sizes) by adding other redox mediators that further assist electron flow. "Preliminary tests of this strategy have proved promising," says Rabaey. To charge a mobile phone, for instance, the system would need about 1.6 g of glucose at the current efficiency.
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