Microbial fuel cell

A microbial fuel cell ( MFN ) (English microbial fuel cell ) can use living microorganisms that process as part of their energy metabolism organic substances directly for energy.

The resulting electrons are transferred in the metabolism of these microorganisms on an electrode, thus enabling the generation of electricity.

The microorganisms meet here in the MBZ function as a biocatalyst.

Applications of microbial fuel cells are the energy generation from effluents and solid wastes, but the current densities currently achievable allow no economically meaningful use on a larger scale.

• 3.1 mediators
• 3.2 nanowires
• 3.3 Direct Contact

• 5.1 sediment fuel cell
• 5.2 Hydrogen Production

History

First investigations of the electricity production in the degradation of organic matter were carried out in 1911 by M. Potter. M. Potter, a professor of botany at the University of Durham, succeeded electron transfer from E. coli bacteria. However, the current densities were low, and the work took little notice.

Barnet Cohen ( The Johns Hopkins Medical School, Baltimore, MD ), developed in 1931 half microbial cells connected in a voltage up to 35 volts generated, but with a current flow of two milliamps in series.

Construction

Typically, a microbial fuel cell is composed of two separate regions, the anode and the cathode, separated by a proton exchange membrane (PEM).

In the anode region live microorganisms that oxidize organic substrates such as acetate. Exoelektrogene known microorganisms are able to transfer the electrons generated during this process directly to the anode, thus enabling the generation of electricity from the organic substances.

As an oxidation product of carbon dioxide is formed.

As the electrons pass through an external circuit, hiking the protons produced by the PEM or a salt bridge directly to the cathode. To this the reduction of an electron acceptor with electrons and protons from the anode takes place.

Electron

Depending on the electron, a distinction is an anaerobic and an aerobic cathode reaction. Widespread is the aerobic cathode reaction, acts as an electron acceptor in the atmospheric oxygen.

Advantage of atmospheric oxygen is its virtually unlimited storage and the comparatively high redox potential.

Preferably, the mechanism of oxygen reduction is the synthesis of water according to the following reaction equation:

In addition, electrons can also be delivered to anaerobic cathode materials such as iron cyanide. Since the electron is here but it consumes over time, it must be regularly replaced or regenerated, so that this type of cathode in the application is almost insignificant:

Electron transfer to the anode

The process of electron transfer from microorganisms to external acceptors is the subject of current research and not known precisely. The following mechanisms are previously known.

Mediators

In previous studies on microbial fuel cells were regularly external chemicals, called mediators, added. These are substances such as neutral red, anthraquinone -2 ,6- disulfonate ( AQDS ), thionine, Kaliumhexacyanidoferrat (III ), methyl viologen, and others, which take over the function of the electron acceptor. Electrons are thus given by the microorganisms directly to the mediators, which in turn donate electrons to the anode.

Some microorganisms are able to produce their own mediators. An example of this so-called endogenous mediators pyocyanin that is produced by the bacterium Pseudomonas aeruginosa.

Nanowires

Bacteria of the genera Geobacter and Shewanella form conductive appendage of the so-called ' nanowires '. The electrical conductivity of these processes can be detected by means of scanning tunneling microscopy.

Direct Contact

A third electron transfer mechanism, a direct contact between the cell wall and anode can be considered. This mechanism has not yet been studied in detail. Experiments show that under anaerobic conditions Shewanella oneidensis cultured bacteria to a two - five times the adhesion point of iron surfaces, as in an aerobic cultivation. While in the case of aerobic the electron donation of atmospheric oxygen is possible to transfer an electron to the electrode must be made of iron in the former case. The increased adhesion leads to the assumption that the transfer by direct contact between the cell and iron electrode.

Application as a biosensor

Since the maximum current in a microbial fuel cell, among others the energy content of the medium, and the fuel contained therein depends MBZs can serve to measure the concentration of organic substrates. The fuel cell is used in this case as a biosensor.

The assessment of pollution of wastewater is frequently done with the so-called biochemical oxygen demand (BOD). This indicates the amount of oxygen that is required present in the water of organic substances to biotic degradation. A microbial fuel cell may be used as a sensor for receiving BOD values ​​in real time.

However, it must be ensured that all or a majority of the electrons is delivered to the anode of the fuel cell and the effects of secondary electron is largely minimized. This is achieved by aerobic respiration, and nitrate respiration be prevented by adding oxidase inhibitors such as cyanide and azide.

This BOD sensors are commercially available.

Further application scenarios

In addition to BOD sensors, which are already used, microbial fuel cells have a variety of other potential applications. As fuel virtually any organic material comes into question, which may be biodegradable.

Sediment fuel cell

The sediment fuel cell uses sediment deposits on ocean floors and in rivers that contain organic matter and sulfides. In that the anode of the fuel cell in the sediment and the cathode is mounted in overlying oxygen-containing water, electric energy can be obtained. This energy can be used for example in measuring stations, the pH, water temperature, currents, etc. to record.

Hydrogen production

Microbial fuel cells can be used in addition to the generation of electricity and the production of hydrogen. Under normal operating conditions, a reaction of the resulting protons at the anode with oxygen from the air to water according to the above equation takes place. By applying an external voltage but the energetically favorable reaction pathway may be preferred in which the protons combine with electrons directly to gaseous hydrogen.

The theoretically necessary for this external potential is 110 mV, which is far below the potential, which is necessary for direct electrolysis of water at neutral pH (1210 mV).