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Microbial fuel cells are devices capable of converting organic fuels such as acetate into electrical energy. Their operation principle is similar to that of a fuel cell but they use living organisms to catalyse the conversion of the fuel into electricity at the anode. The cathode can be a conventional oxygen diffusion electrode. During the operation of the fuel cell, the fuel is converted into CO ₂ and O ₂ is reduced to water, in an overall reaction equal to the direct combustion of the fuel but without direct contact of the reagents. So, electrical energy is spontaneously produced. 

Although the power output of these devices is still rather low, they are especially well suited to be used in the degradation of waste waters jointly with the cogeneration of electrical energy.

The main aspect that needs improvement is the connectivity between bacteria and the electrode surface. Different strategies are planned to improve this problem during the course of this project.

The aim of the proposed research is not to replace large power generation facilities such as coal-fired power stations, but to provide the possibility of a self-sustained process where the chemical energy stored in the pollutant would be invested on its own remediation treatment. This is possible employing a new approach that makes use of recently available nanotechnology and microbiological concepts.

Ideas that led to this project

Electrical interfacing to bacterial cell has been a matter of study during decades due to the high potential that microorganisms have in biotechnological applications. During the last 5 years a fascinating finding has induced a paradigmatic change in the field of microbial electrochemistry: the discovery of electro-active microorganisms with the outstanding ability to oxidise organic compounds whilst exchanging electrons with external solid surfaces. This has opened a wide spectrum of possibilities to exploit bacterial metabolic capabilities: The control and development of electrochemically-assisted bioremediation processes, the development of new biosensors and the possibility of bacterial electricity production are now feasible and relevant examples of application of electro-active bacteria/electrode interfaces have been described.

What microorganisms can be used?

Most of the relevant electro-active bacteria are Fe(III)-reducing species that have evolved exquisite mechanisms to drive electrons to extracellular solid metal oxides. Although they conserve energy for growth using oxides as the final electron acceptor, it has been shown that they can also exchange electrons with a polarized electrode. Electrodes usually employed in electrochemical experimentation and applications such as gold or carbon are not natural electron acceptors for these bacteria. This implies an intrinsic energy loss due to the imperfect catalysis of oxidation/reduction reactions at the bacteria/electrode interface that must be improved for practical applications.
Among Fe(III)-reducing bacteria, Geobacter sulfurreducens combine biodegradation and electrogenic capabilities making this organism the most appropriate for this project.
 

Why apply our approach to wastewaters?

Wastewaters containing high amount of organic matter is a potential energy storage that actually represent almost 10-fold the energy invested in treating the wastewaters themselves. Wastewaters are classically processed under anaerobic conditions to transform organic matter into acetate and short-chain organic acids. Acetate is further converted into methane through an additional treatment that requires a shift in the operation conditions of the bioreactor. The present proposal suggests replacing such a methanogenic stage by an advanced microbial fuel cell system able to convert acetate into electricity at high rate by means of a highly inter-connected nanoengineered electrogenic biofilms. This method avoids the formation of methane as a dangerous bioremediation product, providing an efficient path for the direct coupling of the degradation process with the energy production.