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Microbial Fuel Cells: Optimization Of The Anode Compartment For Improved Electron Transfer

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14.11.2003

A microbial fuel cell mimics a biological system, in which bacteria do not directly transfer the energy-rich electrons gained out of the feeding to their characteristic electron acceptor. Instead, the electrons are diverted towards an electrode (anode) and subsequently conducted over a resistance or power user, and a cathode (see figure). At the cathode, these electrons are used to reduce oxygen with the formation of water. This way, bacterial energy is directly converted to electrical energy.

Microbial fuel cells have so far known limited success because of the low output observed. The maximum attainable potential over a biofuel cell, based on the potential difference between the redox couple, is 1.15V. However, the real fuel cell potential is mostly lower due to the potential losses observed at both the anode and the cathode, and the internal resistance of the fuel cell. Lowering these losses at the anode can be obtained chemically through enlargement of the specific electrode surface or the use of redox mediators, and biologically by the selection of adapted bacteria.

The internal resistance is mainly caused by the resistance of the electrolytes and of the proton exchange membrane (PEM), and can be lowered by increasing the reactor turbulence and the electrolyte/PEM conductivity.

The Laboratory for Microbial Ecology and Technology (LabMET) and the Laboratory for Non-Ferrous Metallurgy cooperate to obtain chemical and biological anode optimization.

The biological optimisation has been performed through selection of suitable microbial consortia. Electrochemical active bacteria were selected from a bacterial culture originating from anaerobic sludge by repetitive bacterial transfer into new fuel cells. The culture was able to transfer electrons efficiently to the graphite electrodes, and could supply a considerably higher output than previously reported, up to 4,31W/m2 of electrode surface (664 mV, 30.9 mA). A series of tests was performed to elucidate the behaviour of the biofuel cell in relation to several glucose loading rates, clarifying operational parameters. Molecular analysis was performed to determine the nature of the bacteria present in the biofuel cell. The identified bacteria were mainly facultative anaerobic, capable of hydrogen production. Cyclic voltammetry showed an evolution towards an electrochemically more active mixed bacterial culture during the experimental period.

Chemical optimization is the next step in the research. The effect of chemical redox mediators, inserted into the electrode matrix, onto the electron transfer can be of significant importance to further boost up the biofuel cell output. Preliminary tests have indicated the viability of this approach.

The results obtained at LabMET open perspectives towards future applications. The first application will very likely involve the use of microbial fuel cells to generate electricity out of plant juices, obtained on site. This way, a forest can become a bio-power plant. Long term research will focus on low power mobile applications. Hence, the question at the restaurant might one day be: "Waiter, one sugar cube for my coffee, and one for my mobile phone..."

Korneel RABAEY | Source: alphagalileo

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