We all know that yeast is used to produce beer and bread – but electricity? Dean Madden from the National Centre for Biotechnology Education, University of Reading, UK, shows how it works.
For decades, microbes that produce electricity were a biological curiosity. Now, however, researchers foresee a use for them in watches and cameras, as power sources and for bioreactors to generate electricity from organic waste. The microbial fuel cell described here generates an electrical current by diverting electrons from the electron transport chain of yeast. It uses a ‘mediator’ (in this case, methylene blue) to pick up the electrons and transfer them to an external circuit. This process is not very efficient, and this demonstration fuel cell will generate only a very small current. In the classroom, this can provide a stimulating introduction to the study of respiration and permit the study of some of the factors that influence microbial respiration. More recently, mediator-less fuel cells of greater efficiency have been developed, in which the micro-organisms donate electrons directly to the fuel-cell electrodes.
Needed by each student or working group
Important: All of the solutions listed below must be made up in 0.1 M phosphate buffer, pH 7.0, not in water.
Microbial fuel cells of this type typically generate 0.4–0.6 V and 3–50 mA. If the cell is topped up with solutions as necessary, it will continue to generate electricity for several days.
Potassium hexacyanoferrate (III) is poisonous. Eye protection should be worn when handling this material. If the solution comes into contact with the eyes, flood them with water and seek medical attention. If swallowed, give plenty of water to drink and seek medical attention. If spilled on the skin, the solution should be washed off promptly with water. Local regulations should be observed when disposing of used solution.
To make 0.1 M phosphate buffer, pH 7.0, dissolve 4.08 g Na2HPO4 and 3.29 g
NaH2PO4 in 500 ml distilled water.
Solutions of the reagents may be prepared in advance. Note, however, that the glucose solution should be prepared no sooner than 24 hours before the work is to be carried out, as the solution is not sterile and would therefore support the growth of contaminating micro-organisms.
Pre-soak the cation exchange membrane in distilled water for 24 h before use.
Dried yeast can be rehydrated as the fuel cell is assembled, although it is important to add the dried yeast to buffer solution first, then to add glucose solution to the yeast slurry. If you try to rehydrate the yeast directly in glucose solution, osmotic effects will delay the process. (If using fresh yeast, simply make a thick slurry of that with the buffer solution before adding the glucose solution.)
It takes about 30 min from the assembly of the fuel cell to the generation of electricity.
Several fuel cells may be joined together in series to give a greater voltage; the current produced will remain the same, however. Conversely, increasing the size of the cell (or the electrode area) will increase the current generated, but not the voltage.
Different mediators and/or types of yeast, such as wine-makers’ or bakers’ yeast, may be used. Note that for safety reasons, the use of this fuel cell with other micro-organisms is not recommended.
Investigate the effect of temperature on the action of the fuel cell (remember to consider what ‘controls’ are necessary when making comparisons of this type).
Microbial fuel cells suitable for school investigations as described here are available from the National Centre for Biotechnology Education (NCBE) at the University of Reading, UKw1.
For those who prefer to build their own fuel cells, following the instructions in this article, the cation exchange membrane and carbon-fibre tissue electrodes are also available from the NCBE. The cation exchange membrane can also be purchased from VWRw2.
Low-current motors suitable for use with a fuel cell such as the one described here are expensive and difficult to find.
Potassium hexacyanoferrate (III) solution is poisonous. Local regulations should be observed when disposing of used solution.
The potassium hexacyanoferrate (III) solution is light-sensitive and should therefore be stored in a light-proof bottle or in a bottle wrapped in aluminium foil. It should not be kept for more than six months.
You may wish to store the cation exchange membrane in a bottle of distilled water so that it is ready to use. The water should be replaced from time to time if the membrane is stored for an extended period.
Dried yeast, even in a sealed container, has a limited shelf life. The supplier’s ‘best before’ date should therefore be observed.
The microbial fuel cell was developed by Dr Peter Bennetto, formerly of the Department of Chemistry, King’s College, London, UK. It has been adapted for school use by John Schollar and Dean Madden.