In the process of biological electricity generation, the two electrodes- anode (negative terminal) and cathode (positive terminal) are usually connected by conductive materials. Micro-organisms produce the electrons, after which transferred to cathode using electron mediators and nano-wires (Bose et al. 2018). Bioremediation is the process of using micro-organisms to neutralize or remove contamination from waste. However, nowadays it is also an important technique used to produce electricity from organic and inorganic matter catalyzed (Cookson 2011). Microbial Fuel Cells or Bio-Electrochemical Systems (BES) combine biological catalytic redox activity with abiotic electrochemical reactions and physics. However, in 1911, Potter deduced the idea of electricity generation using microbes. Furthermore, studies by Santoro et al. (2017) has led to the improvement of concept and practical developments. Thus, by using organic substrates, such as domestic wastewater, animal wastewater, oil wastewater, and wasted sludge microbial fuel cells, integrated with anaerobic and aerobic treatments using bacteria as catalysts. Therefore they can lead to effective bioelectricity generation (Bose et al. 2018).
Microbial fuel cells (MFC)
Microbial fuel cells are the bioelectrochemical devices that use microorganisms as catalysts in the conversion of chemical energy into electrical current. However, they are possible from organic compounds and equally responsible for simultaneous wastewater treatment. Different types of microbial fuel cells (Kumar et al. 2017);
- Double-chamber (DCMFC)– It consists of an anode and a cathode separated by PEM. A defined anode substrate and cathode solution are used to generate energy.
- Single-chamber (SCMFC) – It consists of one chamber that has both anode and cathode separated by PEM.
- Upflow MFC – It is a cylinder-shaped MFC with cathode chamber at the top and the anode at the bottom.
- Stacked MFC – It is a combination of some MFC that are either coupled in series or in parallel to enhance the power output.
List of organisms for harvesting electricity
|Geobacter SPP||Two-chamber with carbon paper anode||Jung & Regan (2007)|
|Betaproteo bacterium||Two-chamber||Zhou et al. (2011)|
|GeobacterSPP (Firmicutes)||Two-chamber with carbon paper anode||Jung & Regan (2007)|
|G. sulfurreducens||Two-chamber with carbon paper anode||Jung & Regan (2007)|
|Escherichia coli||Single chamber with platinum and polyanilineco-modified anode||Franks & Nevin (2010)|
|Escherichia coli||Single chamber with composite electrode (graphite/PTFE) anode||Pisciotta & Dolceamore Jr (2016)|
|Electrochemically active bacteria||Two chamber (H-type MFC) with teflon treated carbon fiber paper anode||Pisciotta & Dolceamore Jr (2016)|
|Mixed culture||2-chamber air-cathode MFC with graphite plate anode||Rahimnejad et al. (2011)|
|Deltaproteo bacterium||Two-chamber with graphite anode||Bond et al. (2002)|
|Desulfurmonas SPP||Two-chamber with non-corroding graphite anode||Tender et al. (2002)|
|Escherichia coli||Single chamber with graphite anode||Park & Zeikus (2003)|
|Escherichia coli||DCMFC with carbon paper with PPY-CNTs anode||Pisciotta & Dolceamore Jr (2016)|
Physiology of electrochemically active bacteria
A basic microbial fuel cell is, however, a double-chambered device consisting of anode and cathode separated by an ion exchange membrane (PEM or a salt bridge), connected to an external circuit (Bose et al. 2018). Furthermore, a biofilm of anaerobic bacteria acts as a catalyst and coated on the anode electrode immersed in an organic matter (fuel). Thus, it acts as an electron donor. Similarly, a biofilm may be optionally used in the cathode compartment but of a different bacterial species (typically aerobic) than anode (Rahimnejad et al. 2015).
However, the oxidation reaction takes place in the anode compartment and the reduction reaction in the cathode compartment.
- Fuel is oxidized by the bacteria, via degradative metabolism responsible for the release of energy. Thereafter generating free electrons, protons and other cations in the solutions.
- Furthermore, the transfer of electrons to the anode substrate.
- Electrons thus passed to cathode compartment via an external circuit.
- Lastly in the cathode compartment, migrated protons and electrons combine with oxygen forming water.
Due to the depletion of already existing energy sources marks the urgent need for alternative sources to overcome the energy crisis. However, microbial fuel cells are of different types and have different applications. The limitations comprise of high-temperature requirement and insufficient power generation to run a sensor or transmitter. Furthermore, biofuels and fuel cells are the future faces of new sources of energy without any pollution, unlike fossil fuels. Active biocatalysts, one of the newest energy forms of microbial fuel cells thus, helps to generate electricity.
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