Introduction to Microbial Fuel Cells

Microbial fuel cells are devices that use bacteria to turn the energy stored in chemical bonds into electrical current that we can use without the need for combustion. Essentially, we are harnessing the power of metabolism for electricity. While the energy produced is not yet practical for a scale larger than simple demonstrations, a thorough understanding is helpful to truly appreciate how bioenergy works and how it could one day be integrated into our energy generation systems. Microbial fuel cells get right to the heart of many of the principles we run into in the discussion of bioenergy. They are an excellent way to illustrate electron transfer principles and to discuss subjects such as reduction and oxidation.

In brief, microbial fuel cells work by creating situations in which bacteria can feed off a substrate or food (such as an agricultural or industrial waste or human sewage waste). The bacteria are kept in an environment lacking oxygen which would normally slow down their growth. This is because oxygen normally acts as an electron acceptor. What happens in the metabolism of the substrate from the waste stream is that electrons in sugars, fats, proteins, or other bioavailable molecules are taken by the bacteria and transferred through a bacterial metabolic pathway in such a way that the electrons can be used by the cells for energy. Oxygen acts as the driving force in this process as it has a high affinity for the electrons and it is the ultimate acceptor of the electrons. So without oxygen present, none of this electron transfer (known as aerobic respiration) can occur. The cells have ways of using the substrate molecules in the waste stream in the absence of oxygen, but the amount of energy they receive is greatly reduced and growth under these conditions is limited.

An electrode provided by a microbial fuel cell solves this lack of oxygen problem for the bacteria. Bacteria transfer their electrons to the electrode that is linked by a wire to a second electrode in an oxygen-containing environment. The electrons ultimately get transferred to oxygen, which is what the bacteria need for optimal growth, but not before we utilize those electrons in the form of an electric current to power an electronic device. Of course we could always burn the substrate molecules and use the energy to heat water in a steam powered generator to create electricity, but this would not be as efficient and the energy return is much less.

At this point, we should introduce a few terms. All batteries or fuel cells have two electrodes: one where electrons enter the system (the anode) and one where electrons exit (the cathode). Bacteria transfer electrons to the anode in the oxygen-free compartment and the electrons are then transferred to oxygen in the cathode compartment.

A simple example:

We start with a simple culture of E. coli bacteria suspended in water.

When we add a dye such as methylene blue, the tube turns blue.

After a few minutes, the tube turns white again except for a thin layer of blue at the top of the liquid inside the tube where the liquid meets the air. What has happened is that the bacteria have transferred the electrons they have taken from stored energy reserves to the methylene blue molecules. When the methylene blue molecules receive these electrons, they become reduced, and the blue color is not longer visible. At the very top of the tube where the liquid meets the air, the methylene blue molecules have transferred those electrons to oxygen and have regained their blue color.

If we were to shake the tube, oxygen would be distributed to all the methylene blue molecules and the entire tube would turn blue again. This example illustrates how electrons can be passed from a cell to an external mediator such as methylene blue. In our next example we will see how those electrons are passed to an electrode and create a current that we can measure.

Further Reading:

One of the current pioneers in microbial fuel cell research is Dr. Derek Lovely at the University of Massachusetts, Amherst. His programs website can be found here.

The previous demonstrations were adapted from an excellent article by H. P. Bennetto which can be found here.