Microbial fuel cell

A microbial fuel cell ( MBZ ) ( engl. Microbial fuel cell ) can live microorganisms , which, within its energy metabolism process organic substances, use directly for energy production.

The electrons generated during metabolism are transferred by these microorganisms to an electrode and thus enable electricity to be generated .

The microorganisms fulfill the function of a biocatalyst in the MBZ .

Microbial fuel cells are used to generate energy from wastewater and waste, but the current densities that can currently be achieved do not yet permit economically sensible use on a larger scale.

history

Michael Cressé Potter carried out the first investigations into electricity production during the breakdown of organic substances in 1911. The professor of botany at the University of Durham succeeded in transferring electrons from E. coli bacteria . However, the current densities were low and the work received little attention.

Barnett Cohen ( Johns Hopkins Medical School , Baltimore ) developed microbial half-cells in 1931 which, connected in series, generated a voltage of up to 35 volts , but with a current of only two milli amperes .

construction

A microbial fuel cell typically consists of two separate areas, the anode and cathode compartments, which are separated by a proton exchange membrane (PEM).

Microorganisms that oxidize organic substrates such as acetate live in the anode area . So-called exoelectrogenic microorganisms are able to transfer the electrons generated during this process directly to the anode , and thus enable electricity to be generated from organic substances.

Carbon dioxide is produced as an oxidation product .

While the electrons pass through an external circuit, the protons generated migrate through the PEM or a salt bridge directly to the cathode . This is where the reduction of an electron acceptor with electrons and protons from the anode takes place.

Electron acceptors

Depending on the electron acceptor, a distinction is made between an anaerobic and an aerobic cathode reaction . The aerobic cathode reaction, in which atmospheric oxygen acts as an electron acceptor, is widespread.

The advantage of atmospheric oxygen is its almost unlimited supply and the comparatively high redox potential .

The preferred mechanism of oxygen reduction is the synthesis of water according to the following reaction equation:

{\ displaystyle O_ {2} + 4H ^ {+} + 4e ^ {-} \ to 2 {H_ {2}} O.}

In addition, electrons can also be transferred to anaerobic cathode materials such as iron cyanide. However, since the electron acceptor is used up over time, it has to be renewed or regenerated regularly, so that this type of cathode is almost insignificant in use:

{\ displaystyle {Fe (CN) _ {6}} ^ {3 -} + e ^ {-} \ to {Fe (CN) _ {6}} ^ {4-}.}

Electron transfer to the anode

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

Mediators

In previous studies on microbial fuel cells, external chemicals, so-called mediators, were regularly added. These are substances such as neutral red , anthraquinone-2,6-disulfonate (AQDS), thionine, potassium hexacyanidoferrate (III) , methyl viologen , and others that take on the function of electron acceptors. Electrons are released from the microorganisms directly to the mediators, which in turn release electrons to the anode.

Some microorganisms are able to produce mediators themselves. An example of these so-called endogenous mediators is pyocyanin , which is produced by the bacterium Pseudomonas aeruginosa .

Nanowires

Bacteria of the genera Geobacter and Shewanella form conductive appendages, the so-called 'nanowires'. The electrical conductivity of these extensions can be demonstrated with the aid of scanning tunneling microscopy .

Direct contact

A third possible electron transfer mechanism is direct contact between the cell wall and the anode. This mechanism has not yet been studied in detail. However, tests show that Shewanella oneidensis bacteria cultivated under anaerobic conditions show two to five times more adhesion to iron surfaces than in aerobic cultivation. While in the aerobic case electron transfer to atmospheric oxygen is possible, in the former case an electron transfer to the iron electrodes must take place. The increased adhesion allows the assumption that the transfer takes place via direct contact between the cell and the iron electrode.

Use as a biosensor

Since the maximum current in a microbial fuel cell u. a. Depending on the energy content of the medium and the fuel it contains, MBTs can be used to measure the concentration of organic substrates. In this case, the fuel cell serves as a biosensor .

The assessment of the pollution of wastewater is often based on the so-called biochemical oxygen demand (BOD). This indicates the amount of oxygen that is required for the biotic breakdown of organic substances in the water. A microbial fuel cell can be used as a sensor to record BOD values in real time.

It must be ensured, however, that all or a large part of the electrons are released to the anode of the fuel cell and the influence of second electron acceptors is minimized as far as possible. This is achieved by cutting off aerobic and nitrate breathing by adding oxidase inhibitors such as cyanides and azides .

These BOD sensors are commercially available.

Further application scenarios

In addition to BOD sensors that are already in use, microbial fuel cells have a variety of other potential applications. Almost any organic material that can be biodegraded can be used as fuel.

Sediment fuel cell

The sediment fuel cell uses sediment deposits on the seabed and in rivers that contain organic substances and sulphides . By placing the anode of the fuel cell in the sediment and the cathode in the oxygen-containing water above, electrical energy can be generated. This energy can be used, for example, in measuring stations that record the pH value, water temperature, currents, etc.

Hydrogen production

In addition to generating electricity, microbial fuel cells can also be used to produce hydrogen . Under normal operating conditions, the protons generated at the anode react with atmospheric oxygen to form water according to the above reaction equation. By applying an external voltage, however, the energetically less favorable reaction path can be preferred, in which the protons combine directly with electrons to form gaseous hydrogen.

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

Individual evidence

↑ ^a ^b ^c ^d Bruce E. Logan: Microbial Fuel Cells. John Wiley & Sons; Edition: 1st edition (February 8, 2008), ISBN 978-0470239483 .
↑ MC Potter: Electrical effects accompanying the decomposition of organic compounds. In: Proceedings of the Royal Society B, Volume 84, 1911, pp. 260-276
^ B. Cohen: The Bacterial Culture as an Electrical Half-Cell. In: J. Bacteriol. Volume 21, No. 1, 1931, pp. 18–19 ( PDF file; 6.1 MB )
↑ ^a ^b ^c H. Rismani-Yazdi et al .: Cathodic limitations in microbial fuel cells: An overview. In: J. Power Sources , Volume 180, No. 2, 2008, pp. 683-694
↑ M. Zhou et al .: An overview of electrode materials in microbial fuel cells. In: J. Power Sources , Volume 196, No. 10, 2011, pp. 4427-4435
↑ ^a ^b B. E. Logan: Microbial fuel cells: Methodology and Technology. In: Environ. Sci. & Technol. , Volume 40, No. 17, 2006, pp. 5181-5192
^ Y. Luo et al .: Power generation using carbon mesh cathodes with different diffusion layers in microbial fuel cells. In: J. Power Sources , Volume 196, No. 22, 2011, pp. 9317-9321
↑ JM Myers and CR Myers: Genetic complementation of an outer membrane cytochrome omcB mutant of Shewanella putrefaciens MR-1 requires omcB plus downstream DNA. In: Appl. Environ. Microbiol. , Volume 68, No. 6, 2002, pp. 2781-2793.
↑ K. Rabaey et al .: Biofuel cells select for microbial consortia that self-mediate electron transfer. In: Appl. Environ. Microbiol. , Volume 70, No. 9, 2004, pp. 5373-5382.
↑ YA Gorby et al .: Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms. In: PNAS , Volume 103, No. 30, 2006, pp. 11358-11363.
↑ SK Lower et al .: Bacterial recognition of mineral surfaces: nanoscale interactions between Shewanella and α-FeOOH. In: Science , Volume 292, No. 5520, pp. 1360-1363.
↑ BH Kim: Novel BOD (biological oxygen demand) sensor using mediator-less microbial fuel cell. In: Biotechnology Letters , Volume 25, No. 7, 2003, pp. 541-545.
^ IS Chang: Improvement of a microbial fuel cell performance as a BOD sensor using respiratory inhibitors In: Biosensors & Bioelectronics , Volume 20, No. 9, 2005, pp. 1856-1859
^ H. Liu et al .: Electrochemically assisted microbial production of hydrogen from acetate. In: Environ. Sci. Technol. , 2005, pp. 4317-4320.
↑ Z. Du et al .: A state of the art Review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy. In: Biotechnology Advances , Volume 25, 2007, pp. 464-482.

[LoganBuch-1] Bruce E. Logan: Microbial Fuel Cells. John Wiley & Sons; Edition: 1st edition (February 8, 2008), ISBN 978-0470239483 .

[2] MC Potter: Electrical effects accompanying the decomposition of organic compounds. In: Proceedings of the Royal Society B, Volume 84, 1911, pp. 260-276

[3] B. Cohen: The Bacterial Culture as an Electrical Half-Cell. In: J. Bacteriol. Volume 21, No. 1, 1931, pp. 18–19 ( PDF file; 6.1 MB )

[CathodicLimitations-4] H. Rismani-Yazdi et al .: Cathodic limitations in microbial fuel cells: An overview. In: J. Power Sources , Volume 180, No. 2, 2008, pp. 683-694

[5] M. Zhou et al .: An overview of electrode materials in microbial fuel cells. In: J. Power Sources , Volume 196, No. 10, 2011, pp. 4427-4435

[LoganPaper-6] B. E. Logan: Microbial fuel cells: Methodology and Technology. In: Environ. Sci. & Technol. , Volume 40, No. 17, 2006, pp. 5181-5192

[7] Y. Luo et al .: Power generation using carbon mesh cathodes with different diffusion layers in microbial fuel cells. In: J. Power Sources , Volume 196, No. 22, 2011, pp. 9317-9321

[8] JM Myers and CR Myers: Genetic complementation of an outer membrane cytochrome omcB mutant of Shewanella putrefaciens MR-1 requires omcB plus downstream DNA. In: Appl. Environ. Microbiol. , Volume 68, No. 6, 2002, pp. 2781-2793.

[9] K. Rabaey et al .: Biofuel cells select for microbial consortia that self-mediate electron transfer. In: Appl. Environ. Microbiol. , Volume 70, No. 9, 2004, pp. 5373-5382.

[10] YA Gorby et al .: Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms. In: PNAS , Volume 103, No. 30, 2006, pp. 11358-11363.

[11] SK Lower et al .: Bacterial recognition of mineral surfaces: nanoscale interactions between Shewanella and α-FeOOH. In: Science , Volume 292, No. 5520, pp. 1360-1363.

[12] BH Kim: Novel BOD (biological oxygen demand) sensor using mediator-less microbial fuel cell. In: Biotechnology Letters , Volume 25, No. 7, 2003, pp. 541-545.

[13] IS Chang: Improvement of a microbial fuel cell performance as a BOD sensor using respiratory inhibitors In: Biosensors & Bioelectronics , Volume 20, No. 9, 2005, pp. 1856-1859

[14] H. Liu et al .: Electrochemically assisted microbial production of hydrogen from acetate. In: Environ. Sci. Technol. , 2005, pp. 4317-4320.

[15] Z. Du et al .: A state of the art Review on microbial fuel cells: A promising technology for wastewater treatment and bioenergy. In: Biotechnology Advances , Volume 25, 2007, pp. 464-482.