[SunilNagpal]

Microbial Fuel Cells or in technical words, Bio-electrochemical cells, are the bio-technical analogs of conventional Electrochemical cells. Where as an electrochemical cell converts the energy derived from chemical reaction(s) into electricity, an MFC utilizes the energy derived from biochemical reactions taking place inside the microbes, for converting into electricity.

Let's understand the concept very briefly:
1. Electrochemical cell has two half cells-Anode half and cathode half.
2. Both cells are connected to each other through internal perforated bridge (for ions to flow between the cells) and external wires connecting the electrodes of two cells. This completes the circuit.
3. Chemicals are used as electrolytes. Reaction of electrolyte in anode half creates an excess of electrons which are transferred to the cathode half, where reaction creates deficiency of electron.
4. Electrons thus travel and lead to electricity generation. Following image could help in understanding the concept:

A Microbial Fuel Cell has minor differences. It's anode half has microbial culture (anaerobic condition preferred) instead of electrolyte. Microbes are allowed to grow there. Cathode half is filled with a salt solution (often NaCl) in oxygen excess conditions. Electrodes are of carbon in both cells, usually. When cells grow, their metabolic electrons (viz. from Electron Transport Chain (ETC)) are transferred to Anode half, which get transferred to cathode half to induce H2O production by reaction between H+ and O2. The flow of metabolic electrons through wires thus produces electricity. Please refer image below:

The electricity generated is very low, but competent enough to replace the now common electrocemical batteries.

Developments:

a. Electron Transfer From Bacteria to Anodic Electrode

In the earliest form of MFCs, mediators were used to mediate the transfer of metabolic electrons from bacteria to the electrode. The mediators were REDOX mediators, so that they penetrated the cells in oxidized form and get themselves reduced by the metabolic electrons. The reduced mediator also being permeable through cell membrane, like oxidised mediator, would reach the anode to get oxidized, transferring electrons in the process. The process would keep repeating, generating electricity. Examples of such mediators are: methylene blue (MB), thionine (Th), meldola's blue (MelB), anthraquinone-2,6-disulfonate (AQDS), potassium ferrocyanade etc

Discovery of certain species of bacteria which could directly transfer the electrons to the anode without the need of mediators really revolutionized the idea of making MFCs. Such MFCs were called Mediator Less, and have become a trend among the high school students for science fairs, owing to the ease of developing it.

Examples of such species are: Shewanella putrefaciens, Geobacter metallireducens, Aeromonas hydrophila etc

b. Carbon Cloth & Carbon Nanotubes for Electrodes
Considering the fact that amount of electricity produced is not only limited by the density of bacterial culture ans efficiency of transfer, but also by the surface area available for electron transfer, several scientific groups have tried to use Carbon cloth as an alternative to normal carbon electrodes (Carbon cloth is basically as sheet of carbon). Recently in 2011, a scientific group from the esteemed University of Hawaii, University of Southern California, University of New Mexico, and Saint Louis University (all in United States) published a research on the "Fabrication of macroporous chitosan scaffolds doped with carbon nanotubes and their characterization in microbial fuel cell operation!
Whereas, the microporous scaffold provided very high surface area for interaction with bacterial cells, doping with Carbon Nano-tubes provided extra-ordinary conductivity to the scaffold!

c. Microbial-Enzyme Hybrid MFCs
In a recent research publication of April 2013, a scientific group from The University of New Mexico reported the use of laccase-modified air-breathing cathode, which catalyzed the reduction process for water production at the cathodic half. Media had lactate as carbon source. Shewanella oneidensis was used as the microbe that didn't need any mediator.

The coupling of waste water treatment process with electricity generation using the concept of MFC has been a big hit with many scientific groups across the globe. It is based upon the conversion of waste water treatment vessel into anode and connecting a cathode to it, making it a full-fledged Waste-Water-Treating & Electricity Production hybrid system.

There are numerous videos on sites like youtube citing the MFC models of various students and scientists across the globe. Each has it's own design and it's own efficiency. MFCs are indeed one of the simplest and yet amazing systems, and the best part is that even "you" can make it by hardly spending $2 on it!

The literature will keep updating on every development in the MFC domain, and I hope the next update comes from your side soon!

Here's a link to a channel on youtube, where a student has explained the details on making a robust MFC at Home!:

YouTube Channel: Microbial Fuel Cells

Enjoy!

Thanks