Since 2000, the study of microbial electro-catalysis in fuel cells has been a rapidly emerging domain of science. The number of publications devoted to MFCs has been increasing exponentially from less than 5 papers per year before 2001 (before the discovery of the concept of direct microbial electro-catalysis) to 80 in 2006 and more than 110 in 2007. 29 patents were taken out between 2000 and 2005, 40% from US, 23% from Korea, and only 20% from Europe.
Microbial electro-catalysis in fuel cells presents many remarkable advantages. Electrochemically active (EA) microorganisms develop naturally on the surface of common electrode materials (graphite, stainless steels, DSA , etc.). Cheap materials can thus be used to construct MFC electrodes, the microorganisms replacing the expensive catalysts required in traditional fuel cells. EA microorganisms can drive electrons to the anode by oxidising the organic matter available in a wide variety of inexpensive “dirty” sources : sediments, starch, domestic or industrial effluents, wastes from food industries or agriculture, etc. It can be said that biofouling of the electrode constitutes the catalytic component here. There is consequently no fouling effect, as fouling itself forms the catalytic layer. MFCs can provide significant current densities, which are stable for some weeks, oxidising inexpensive substrates like acetate, sugars or the organic matter contained in sediments or wastes.
MFCs have consequently been presented as ideal equipment for producing electrical energy from biomass and organic residues in remote locations such as off-shore installations, ocean floors, or developing countries, anywhere far from energy distribution networks. Exciting application niches have been evoked, like powering electronic devices (sensors, phones…) and numerous other applications remain to be devised on the basis of the ubiquitous and innocuous properties of the fuels. Because they consume organic matter, MFCs have also been shown to be effective in enhancing waste water treatments.
Principle of a MFC
Microbial fuel cells (MFCs) convert directly to electrical energy the energy produced by the oxidation of different costless organic compounds contained in sediments, soils, domestic or industrial wastewaters. The catalysis of the oxidation process is achieved by electro-active microorganisms that develop on the anode surface.
Similarly to classical fuel cells, MFCs are basically composed of an anode, on which the fuel is oxidized, and a cathode that wastes the electrons generally by reducing oxygen. MFCs have recently had <outstanding impact, because of the discovery of a completely new
concept : certain microorganisms revealed able to act as of electro-catalyst, oxidizing the organic matter and transferring the electrons produced directly to the electrode.
MFCs have been presented as ideal equipments for producing electrical energy from biomass. Nevertheless, the power density provided still remains weak, around a few W/m2.
(1) A first industrial-scale pilot of MFC has been tested.
Some technological lock does not allow us yet to reach high powers. During the first test, the MFC produced continuous current of 50mA with a voltage of 200mV so a power of 10mW.
Several improvements have already been envisaged to improve the performance of this MFC, this will be our work for the coming years. But there is no doubt that this prototype will provide us with a valuable technological tool in the race to alternative energies.}
(2) Our team has developed a laboratory scale prototype capable of supplying up to 2W/m2 of projected electrode surface with a maximal current density of 10A/m2. This device is constituted by 10 independent MFCs and electricity is generated from the digestion of acetate by marine micro-organisms.}
Alain BERGEL, Professor CNRS