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www.scienceinschool.org32 Science in School Issue 14 : Spring 2010

IntrouctionFor decades, microbes that produce

electricity were a biological curiosity.Now, however, researchers foresee ause for them in watches and cameras,as power sources and for bioreactorsto generate electricity from organicwaste. The microbial fuel celldescribed here generates an electricalcurrent by diverting electrons fromthe electron transport chain of yeast.It uses a mediator (in this case,

methylene blue) to pick up the elec-trons and transfer them to an externalcircuit. This process is not very effi-cient, and this demonstration fuel cellwill generate only a very small cur-rent. In the classroom, this can pro-vide a stimulating introduction to thestudy of respiration and permit thestudy of some of the factors that influ-ence microbial respiration. Morerecently, mediator-less fuel cells ofgreater efficiency have been devel-

oped, in which the micro-organismsdonate electrons directly to the fuel-cell electrodes.

Equipment an materialsNeeded by each student or working

group

Equipment

Perspex fuel cell, cut from 4 mmthick Perspex sheet

2 neoprene gaskets Cation exchange membrane, cut to

fit between the chambers of the

fuel cell. The membrane may be re-

used indefinitely, but will melt if itis autoclaved.

2 x 10 ml plastic syringes, for dis-pensing liquids

Petri dish base or lid, on which tostand the fuel cell

2 electrical leads with crocodileclips

05 V voltmeter or multimeterand/or low-current motor

Scissors.

Materials 2 carbon-fibre tissue electrodes, cut

to fit inside the fuel cell

2 pieces of J-Cloth or similar fab-ric, cut to fit inside the fuel cell (thepurpose of this cloth is simply toprevent the electrodes from touch-ing the cation exchange membraneand short-circuiting the cell)

Important: All of the solutions listedbelow must be made up in 0.1 Mphosphate buffer, pH 7.0, not in water.

Dried yeast, made into a thick slur-ry in 0.1 M phosphate buffer (donot add glucose solution withoutfirst rehydrating the yeast inbuffer)

5 ml methylene blue solution (10mM)

5 ml glucose solution (1 M) 10 ml potassium hexacyanoferrate

(III) solution (0.02 M) (also calledpotassium ferricyanide).

Proceure1. Cut out two carbon-fibre electrodes

as shown in on page 34.

ImagecourtesyofJobalou/iStockphoto

Crocodile clips

We all know that yeast is used to produce beerand bread but electricity? dean Maen fromthe National Centre for Biotechnology Education,University of Reading, UK, shows how it works.

The microbial fuel cell:

electricity from yeast

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Teaching activities

www.scienceinschool.org 33Science in School Issue 14 : Spring 2010

bon-fibre electrodes are not touch-ing the cation exchange membrane.

Typical resultsMicrobial fuel cells of this type typi-

cally generate 0.40.6 V and 350 mA.If the cell is topped up with solutionsas necessary, it will continue to gener-ate electricity for several days.

SafetyPotassium hexacyanoferrate (III) is

poisonous. Eye protection should beworn when handling this material. Ifthe solution comes into contact withthe eyes, flood them with water andseek medical attention. If swallowed,

give plenty of water to drink and seekmedical attention. If spilled on theskin, the solution should be washed

off promptly with water. Local regula-tions should be observed when dis-posing of used solution.

Recipes

To make 0.1 M phosphate buffer,pH 7.0, dissolve 4.08 g Na2HPO4 and3.29 g NaH2PO4 in 500 ml distilledwater.

Preparation an timingSolutions of the reagents may be

prepared in advance. Note, however,that the glucose solution should beprepared no sooner than 24 hoursbefore the work is to be carried out,as the solution is not sterile and

would therefore support the growthof contaminating micro-organisms.

Pre-soak the cation exchange mem-

MicrobiologyPhysics

Energy

This article describes a lab-

oratory practical for

demonstrating the electron

transport chain. The practi-

cal is highly relevant for

biology lessons on micro-

bial respiration. It seems

obvious to use this practi-

cal as an extension of fer-

mentation exercises.

The practical can be used

interdisciplinarily at the

interface of biotechnology

and physics, demonstrating

the use of micro-organisms

for energy production. It

could also be related to the

production of bioethanol,

as an example of an alter-

native biotechnologicalway of producing energy.

Niels Bonderup Dohn,

DenmarkREvIEW

2. Cut out two pieces of J-Cloth to fitinside the fuel cell.

3. Assemble the fuel cell as shown onpage 35.

4. Stand the assembled fuel cell on a

Petri dish base or lid, to catch anyliquid that may leak from the cell.

5. Combine equal (5 ml) volumes of theyeast slurry, glucose and methyleneblue solutions. Syringe this mixtureinto one chamber of the fuel cell.

6. Syringe potassium hexacyanofer-rate (III) solution into the otherchamber of the cell.

7. Connect a voltmeter or multimeter(via the crocodile clips) to the elec-trode terminals. A current should

be produced immediately if themeter registers zero, check the con-nections and ensure that the car-

Image courtesy of Dean MaddenThe microbial fuel cell

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www.scienceinschool.org34 Science in School Issue 14 : Spring 2010

brane in distilled water for 24 hbefore use.

Dried yeast can be rehydrated asthe fuel cell is assembled, although itis important to add the dried yeast tobuffer solution first, then to add glu-cose solution to the yeast slurry. Ifyou try to rehydrate the yeast directlyin glucose solution, osmotic effects

will delay the process. (If using freshyeast, simply make a thick slurry ofthat with the buffer solution beforeadding the glucose solution.)

It takes about 30 min from theassembly of the fuel cell to thegeneration of electricity.

Scope for open-eneinvestigations

Several fuel cells may be joinedtogether in series to give a greater

voltage; the current produced willremain the same, however.Conversely, increasing the size of

the cell (or the electrode area) willincrease the current generated, butnot the voltage.

Different mediators and/or types ofyeast, such as wine-makers or bakersyeast, may be used. Note that for safe-ty reasons, the use of this fuel cellwith other micro-organisms is notrecommended.

Investigate the effect of temperatureon the action of the fuel cell (rememberto consider what controls are necessarywhen making comparisons of this type).

SuppliersMicrobial fuel cells suitable for

school investigations as describedhere are available from the NationalCentre for Biotechnology Education(NCBE) at the University of Reading,UKw1.

For those who prefer to build theirown fuel cells, following the instruc-tions in this article, the cation exchange

membrane and carbon-fibre tissueelectrodes are also available from theNCBE. The cation exchange membranecan also be purchased from VWRw2.

Low-current motors suitable for usewith a fuel cell such as the onedescribed here are expensive anddifficult to find.

disposal of waste an recyclingof materials

Potassium hexacyanoferrate (III)solution is poisonous. Local regula-tions should be observed when dis-posing of used solution.

Storage of materialsThe potassium hexacyanoferrate

(III) solution is light-sensitive andshould therefore be stored in alight-proof bottle or in a bottle

wrapped in aluminium foil. Itshould not be kept for more thansix months.

Images courtesy of Dean Madden

The carbon fibre used to make the elec-trodes has a grain. To ensure that thelong tail of the electrode does not tearand that it fits easily through the hole inthe fuel cell, it is necessary to cut and foldthe electrode as shown here

How the microbial fuel cell worksIn one chamber of the cell, yeast cells are fed on glucose solution. Amediator, methylene blue, enters the yeast cells and takes electrons fromthe yeasts electron transport chain. The electrons are then passed to anelectrode (anode). The electrons pass through the external circuit and areaccepted by potassium hexacyanoferrate (III) in the second chamber ofthe cell. Hydrogen ions pass through a cation exchange membrane whichseparates the two chambers.Microbial fuel cells of this type typically generate 0.4-0.6 V and 3-50 mA.This is sufficient to power a very low-current motor. If several such cells arejoined in series, it is possible to light a light-emitting diode (LED)

Electrons

Anode

Cation exchange membrane

Carbon dioxide

Glucose

Cathode

Potassiumhexacyanoferrate

Width of fuelcell chamber

Directionof grain

Scrap

Cut about half-way through the upper piece asshown, then fold it in half, then half again toform a tail on the electrode

Methylene blue(reduced)

Methylene blue(oxidised)

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Teaching activities

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You may wish to store the cationexchange membrane in a bottle of dis-tilled water so that it is ready to use.The water should be replaced fromtime to time if the membrane is storedfor an extended period.

Dried yeast, even in a sealed con-tainer, has a limited shelf life. Thesuppliers best before date should

therefore be observed.

AcknowlegementsThe microbial fuel cell was devel-

oped by Dr Peter Bennetto, formerly

of the Department of Chemistry,

Kings College, London, UK. It has

been adapted for school use by John

Schollar and Dean Madden.

Web references

w1 To learn more about theNational Centre for Biotechnology

Education (NCBE) and to order

their fuel cells, see: www.ncbe.read-

ing.ac.uk

w2 To contact VWR, the supplier of

the cation exchange membrane, see:

www.vwr.com

Resources

Bennetto P (1987) Microbes come topower. New Scientist 114: 3640

Bennetto HP (1990) Electricity genera-tion by micro-organisms. BIO/tech-nology Education 1: 163168. Thisarticle can be downloaded from theNCBE website:www.ncbe.reading.ac.uk or here:http://tinyurl.com/ncf6ql

Lovley DR (2006) Bug juice: harvest-ing electricity with micro-organ-isms. Nature Reviews Microbiology 4:497508. doi:10.1038/nrmicro1442

Sell D (2001) Bioelectrochemical fuel

cells. In: Biotechnology. Volume 10:Special Processes (Second edition).Rehm H-J and Reed G (Eds).

Frankfurt am Main, Germany:Wiley-VCH. ISBN: 9783527620937

For a complete list of all teachingactivities published in Science inSchool, see:www.scienceinschool.org/teaching

Dr Dean Madden is a biologist

working for the National Centre forBiotechnology Education (NCBE)w1 atthe University of Reading, UK. TheNCBE was established in 1984 andhas since gained an international rep-utation for the development of inno-vative educational resources. Its mate-rials have been translated into manylanguages including German,Swedish, French, Dutch and Danish.

How to assemble a microbialfuel cell (the exact dimensionsare unimportant the one shownhere is roughly 55 mm x 55 mm)

The finishedmicrobialfuel cell

Images courtesy of Dean Madden

Hole throughwhich chamberis filled

Neoprenegasket

Terminal ofcarbon fibre

electrode

Chamber glued to endplate with Superglue

Cation exchangemembrane

J-Cloth preventselectrode from

touching membrane