Question

Scientists at the University of Massachusetts at Amherst have used a bacterial organism, known as Geobacter species, in a fuel cell to generate electricity. Describe, in detail, the principle and process of electrical energy generation from bacterial fuel cell. Also include in the discussion major advantages and limitations of bacterial fuel cells.

Answer 1

What is the principle behind a bacterial fuel cell?

Microbial fuel cells work by allowing bacteria to do what they do best, oxidize and reduce organic molecules. Bacterial respiration is basically one big redox reaction in which electrons are being moved around. Whenever you have moving electrons, the potential exists for harnessing an electromotive force to perform useful work. A MFC consists of an anode and a cathode separated by a cation specific membrane. Microbes at the anode oxidize the organic fuel generating protons which pass through the membrane to the cathode, and electrons which pass through the anode to an external circuit to generate a current. The trick of course is collecting the electrons released by bacteria as they respire. This leads to two types of MFCs: mediator and mediatorless.

Mediator MFC's

Prior to 1999, most MFCs required a mediator chemical to transfer electrons from the bacterial cells to the electrode. Mediators like neutral red, humic acid, thionine, methyl blue, and methyl viologen were expensive and often toxic, making the technology difficult to commercialize.

Mediatorless MFC's

Research performed by B. H. Kim et al in 1999 led to the development of a new type of MFC's mediatorless MFCs. The Fe (III) reducer Shewanella putrefaciens, unlike most MFC bacteria at the time, were electrochemically active. This bacteria had the ability to respire directly into the electrode under certain conditions by using the anode as an electron acceptor as part of its normal metabolic process. Bacteria that can transfer electrons extracellularly, are called exoelectrogens.

Exoelectrogens - The Living Microbial Catalyst

The most promising MFC's for commercialization in today's energy industry are mediatorless MFC's which use a special type of microorganism termed exoelectrogens. Exoelectrogens are electrochemically active bacteria. While aerobic bacteria use oxygen as their final electron acceptor and anaerobic bacteria use other soluble compounds as their final electron acceptor, exoelectrogens are a special class of bacteria that can use a strong oxidizing agent or solid conductor as a final electron acceptor.

How do bacterial fuel cells generate energy?
In order for any fuel cell to work you need to have a means of completing a circuit. In the case of the MFC you have a cathode and an anode separated by a cation selective membrane and linked together with an external wire. When an organic "fuel" enters the anode chamber, the bacteria set to work oxidizing and reducing the organic matter to generate the life sustaining ATP that fuels their cellular machinery. Protons, electrons, and carbon dioxide are produced as byproducts, with the anode serving as the electron acceptor in the bacteria's electron transport chain.

The newly generated electrons pass from the anode to the cathode using the wire as a conductive bridge. At the same time protons pass freely into the cathode chamber through the proton exchange membrane separating the two chambers. Finally an oxidizing agent or oxygen present at the cathode recombines with hydrogen and the electrons from the cathode to produce pure water, completing the circuit. Replace that wire with a light bulb or some other device that requires electricity and you have effectively harnessed the power of microbes to solve your energy needs.

Advantages:

1) Generation of energy out of Biowaste/ Organic Matter

2) Direct conversion of substrate energy to electricity

3) Omission of gas treatment : Anaerobic processes usually contain toxic gases

Disadvantages:

1) Low power density: The maximum power denstiy reported in literautre is 3600 mW/m^2.

2) High initial cost

3) Scale-up i.e. upscaling problems

4) Ohmic Losses : Due to electrode spacing or due to internal cell resistance, there are losses in efficiency.

5) COncentration losses: Mass transfer limitations are present.