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Microbes as an Energy Source

A new generation of microbial fuel cells demonstrates amazingly high current density


Fuel cells could be the energy source of the future for many applications, including in vehicles. The direct transformation of chemical to electrical energy without a detour involving combustion is what makes this technology so environmentally friendly. The breakthrough into broad usage hasn't happened yet, but some clever minds are already contemplating novel concepts for the generation after next. These future systems could even work using self-regenerating raw materials -- microbial fuel cells. Researchers from Greifswald University have now reached an important milestone on the way toward such bio-fuel cells.

While "normal" fuel cells use conventional energy sources like methanol, bio-fuel cells make use of the metabolic activity of microorganisms to produce energy. This ingenious concept has thus far failed in practice because the resulting current has been much too low. This stems from the decidedly difficult task of "wiring" the microbes to the circuit. Uwe Schröder, Juliane Nießen, and Fritz Scholz have now developed a new process for delivering the electrons "tapped" from microbial metabolism to the electrodes. While the microorganisms happily metabolize away in their tank of glucose, the fermentation solution is continuously pumped through a separate anode chamber. The anode compartment is separated from the cathode chamber by a semipermeable membrane. Nothing unusual so far.

The secret of this bio-fuel cell lies in its novel layered anode. A platinum electrode or platinum-coated graphite electrode is coated with a layer of the electrically conducting polymer polyaniline. This layer of plastic has it all; it is biocompatible and electrocatalytically active. It takes electrons from the bacterial metabolism and transfers them to the anode, thus playing an active role in the flow of current. But that is still not all. During operation of the cell, the products of the bacterial metabolism, as well as side-products of the electrocatalytic oxidation process are formed. These accumulate on uncoated anodes and quickly deactivate them. The plastic coating noticeably slows this process, though it cannot completely prevent it. This problem is solved by a further trick; the deposits are chemically converted and dissolved away from the anode surface by means of regular voltage pulses.

"Our novel bio-fuel cell continually produces up to 1.5 mA/cm2," reports Schröder. "We have thus successfully stepped from the micro- into the milliampere range."

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