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ACURRICULUMforHIGHSCHOOLSCIENCEEDUCATION:

MICROBIALFUELCELLS:ALivingBatteryBrettBarronHighSchoolScienceTeacher,HazelwoodCentralHighSchool,St.Louis,MOEmail:bbarron@hazelwoodschools.orgMiriamRosenbaum,PhDResearchAssociate–MicrobialFuelCellResearchDepartmentofBiologicalandEnvironmentalEngineering,CornellUniversityEmail:mr625@cornell.eduPhyllisBalcerzak,PhDScienceEducationSpecialistScienceOutreach,WashingtonUniversityinSt.Louis,Email:pbalcerz@biology2.wustl.eduLargusT.Angenent,PhDAssociateProfessorBiologicalandEnvironmentalEngineering,CornellUniversity214Riley‐RobbHall,Ithaca,NY14853Email:la249@cornell.eduPhone:607‐255‐2480Weblink:http://angenent.bee.cornell.edu/

ThisworkwasmadepossiblethroughaNSFCAREERgranttoLarsAngenent(#0939882).

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MICROBIALFUELCELLS:ALivingBattery

MOTIVATION Microbialelectrochemistryprovidesauniquemethodforpromotinganinterdisciplinaryapproachtoteachingscience. Mostsciencecurriculatoday,adaptingtothegrowingdemandto align course content to state standards, veer away from the integration of biology,chemistry, and physics. Many times, this does a great disservice to our students. The lifeprocessesstudiedinthesecoursesdefyourattemptstosimplifyandcategorizethem–theyareinfinitely complex, and often these complex problems require the integration of biology,chemistryandphysicstofullyexplainthem.Inatypicalhighschoolsciencecourse,thestudentacquiresfragmentedideas(factoids)aboutthesubjectofstudy,andwillvaryintheamountofinformation they have acquired. Frequently, the average student has great difficulty inconceptualizing the patterns and relationships between the various science disciplines. Forexample, students can master the basics of oxidation‐reduction from their introductorychemistry class, yetwhen calledupon tobe familiarwith themetabolic pathwaysof cellularrespirationinabiochemistryunit,failtorecognizehowredoxchemistrygovernsthebiologicalprocesses.

Ourgoalisfirsttodemonstratehowanintegratedcomplexsystem(e.g.,amicrobialfuelcell)canbeusedtosuccessfullyteachscience(biology/chemistry/physics),andthenprogresstohow science can be used to solve complex problems (e.g., energy‐neutral wastewatertreatment). Essentially, we hope to provide opportunities for students to transform factualknowledge intousable information forproblemsolving. Often, this typeof investigationnotonlystimulatesthecuriosityofstudentsinbiotechnologyanditsrelationtothebasicsciences,butalsoencouragesamoregeneralflexibilityofscientificthinkingintheirinterpretations.

Demonstrationsofmicrobialelectricitydevicesoftencapturetheattentionofhighschoolaudiences,andtheintentionofthiscurriculumistoprovidethetools forinstruction and some relatively simple experiments that can be performed in ahigh school laboratory. Some over­simplification of the basic biochemical andmicrobiologicalprocessesisperhapsinevitable,andwherenecessaryreferencesto standard texts should be tailored to the needs of your students. Whereapplicable,attentionwillbedrawntotherationaleforeachactivitybydescribinghow the students can link their understanding of each process to theunderstandingofafunctionalMFC.

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INTRODUCTION Asaresultofglobalpopulationgrowthandcumulativeindustrializationprocesses,twotremendouschallengesinourworldtodayarethegrowingdemandforenergy,andecologicallyandenvironmentallysoundmethodsforwastedisposal.While,arguably,wepossessadequatemethods forwaste treatmentanddisposal, theyarequiteenergy‐intensiveprocesses. Somecurrentresearch,especiallyintomicrobialfuelcells,asksthequestionifwastewatertreatmentcan be transformed into energy producing processes. As fossil fuels are exhausted, couldsmaller, tailoredsolutionsdesignedfor individualenergydemandscometoreplacethehuge,centralized power plant? In our evolving energy conscious environment, could wastes evercometoplayadistinguishedroleasaresourcefortheprovisionofelectricalpower?Thesearequestions researchers in the fieldofmicrobial fuel cellshope toanswer.Wastes, contrary toconventional wisdom, are not useless – as the namemay imply – rather, they are valuableenergysources.Municipalwastewaterpossessesanorganicloadof~200mg/LCOD,andwhilethis isnotenoughrawenergyforwastewatertoeverbeamajorproducingsource,therearesome that argue the treatment could become an energy‐neutral process rather than energyconsuming. Special industrial wastewaters, such as from the food industry (dairy, brewery,sugar industry), being much richer in organic content, may potentially hold an even morepromisingenergyproductionfuture.

Anovelapproachtofacilitatedirectenergyproductionfromwastewateristhemicrobialfuel cell. The first studies into the application of direct transformation of chemical intoelectrical energy by the exploitation ofmicrobial processes arose during the 1970s. Mainlybecauseoftheabundanceoffossiloilandgas,itreceivedonlytemporaryattention.Itwasnotuntilthe1990s,wheninterestinsustainableandrenewableenergysourcesexpanded,thattheideawasrevivedandtheresearchintensified.Althoughmuchworkremainstobedoneattheresearch level tounwrapthebiochemistryof theelectricalactivityofmicroorganisms, recentstudiesofmicrobialfuelcellshavegreatlyadvancedourunderstandingofmicrobialelectricitygeneration.Additionally,studyingthesemicrobialfuelcellscanleadtoadeeperunderstandingof electron transfer processes in general and the application of this knowledge to otheralternativefuelpossibilities.ForinformationontheworldwidestatusofMFCresearch,consultthewebpageoftheMFCresearchcommunity:www.microbialfuelcell.org.

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Chapter1:WhatisaBattery?

BATTERYBASICS Havethestudentsgivetheirdefinitionofabattery.(NOTE:Thepubliccallsita“battery,”buttheindustryreferstoitasa“cell.”)A battery is an electrochemical device that contains two or more power cells connectedelectricallysothatchemicalenergycanbeconvertedintoelectricity. Simplystated,abatterypowers products that require electricity to work. They are useful because they allow us totransportelectricityanduseproductsinlocationswheretherearenoelectricaloutlets,suchasbeaches,sportingevents,picnics,etc.Inabattery,eachcell thatstorestheelectricalenergy inachemicalstatehastwoelectrodesthatreactwiththechemicalandeachothertoreleaseenergy.Thebattery’stwometalendsarecalled terminals. Usually one terminal is flat (negative end) and the other is button‐shaped(positiveend).InatypicalCarbon‐Zincbattery,thepositiveelectrodeisacarbonrodandthenegativeelectrode isthezinccase.Thecase is importantalsobecause itkeepsthechemicalsfromleakingout. Lookfortheterminalsonabattery.Howaretheymarked?

Theendsaremarkedwitha+and‐.

MFCswillalsohavepositiveandnegativeterminals.OurspecificlaboratorymodelMFC,instead of having just two terminals at opposite ends, will have three terminals (twonegativeandonepositive)allcomingoutofthetopoftheMFC.Theterminalswillbetheconductive carbon electrodes (rods) that emerge from the two anolytic (negative)chambers and the catholytic (positive) chamber. The two negative electrodes will belinkedbyalligatorclipsandcopperwire,essentiallycreatingonenegativeterminal fromthetwo.

HowdoesunderstandingbatterieshelpstudentsunderstandMFCs?Since our microbial fuel cell effectively acts as a battery, it is beneficial to begin bydemonstratinghowitfunctionsasabattery.WhenstudentsconceptualizethattheMFCisactingasabattery,theycaninternalizethatalloftheprocessesthatoccurwithinatypicaldrycellbatteryarealsooccurringwithintheMFC. Thetopicsbeginwitha reviewof thedesignandoperationofabasicbatteryandthenapplythoseconceptstotheMFC.Partsofthistopicmaybeunnecessaryifthestudentshavepriorknowledgeofbatteriesandcircuitsgainedatthemiddleschoolorintroductoryphysicalsciencelevel.

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Eachbatteryconsistsoffourmainparts:a)PositiveElectrode(Cathode)‐theelectronacceptor.b)NegativeElectrode(Anode)‐theelectrondonor.c)Electrolyte‐apaste‐likesubstanceorsolutionthatcontainschargedparticlesthatcanmoveorconductanelectriccurrent.

d)Separator‐materialthatprovidesseparationofthe(+)fromthe(–)chargesandinsulation.

The electrochemical reaction that fuels the battery occurs at the interface of positive andnegativeelectrodesandtheelectrolytes.Electricitycreatedbyabatteryconsistsofastreamoftinyinvisibleparticlescalledelectronsflowingfromonemetalendofthebatterytotheothermetalend, just likea liquid.Thepath it follows iscalledacircuit.Whenelectricity flows inacircuit,itiscalledcurrent.Electricityonlyflowswhenitcangofromoneterminaltoanother.The electrons must have a pathway or circuit to follow from the positive to the negativeelectrode.

When thecircuit is complete,electricity flows fromanareaof low (morenegative)electricalpotentialtooneofhigher(lessnegativeorevenpositive)potential.Thedifferenceinelectricalpotentialmakestheelectricitymove.Electricitycanflowthroughsomethings,butnotthroughothers.Materialsthatallowelectricitytoflowthroughitarecalledconductors.Metalsusuallymakegoodconductors.Ifelectricitycannotflowthroughthematerial,itiscalledaninsulator. Whatiselectricity?

Electricity or electric current is a movement or flow of electrically chargedparticles,calledelectrons.

Canyounamesomematerialsthatmakegoodconductorsandinsulators?

Good conductors include aluminum, carbon, copper, salt water, and zinc.Insulatorsincludedrysalt,glass,plastic,andwood.

Theelectricalsymbolforabatteryis:

The longer line indicates positive and the shorter line indicate negative. All batteries have astrengthindicatedbyanumberfollowedbytheletter“V”.The“V”standsforvolts.Thevoltisan electrical unit thatmeasures the potential difference between two points in an electricalcircuit.Thepotentialdifferencebetweentwopointsistheamountofworkthatwouldneedtobedoneonaunitofelectricchargetomoveitfromonepointtotheotheragainstanelectricfield.

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Thevoltagenumberforabatterygivesarelativemeasureofhowhardtheelectronsarebeingpulledthroughthecircuitfromanareaoflowelectricalpotential(moreelectronrich)tooneofhigher potential. For instance, a 1.5 V battery contains a single cell whereas a 9 V batterycontainssixcells,withthecurrentmovingthroughallsixcells. Whatdothenumbersonabatterymean?Howdotheyrelatetothestrengthofthebattery?Canyousuggesthowvoltagecouldbeincreased?

The numbers indicate voltage. Batteries with higher numbers have a higherstrength.Youcanmakeavoltagestrongerbylinkingseveralbatteries.

PRIMARYANDSECONDARYCELLSThemostcommonhouseholdbatteryisadrycellbattery.Adrycellbatteryischaracterizedbyapasty,lowmoistureelectrolyte.Onekindofdrycellisaprimarycellbattery.Thesebatteriesautomaticallyconvertchemicalenergy intoelectricalenergy.ThiskindofbatteryCANNOTberechargedbecauseithasadefinitefuelstock.Itisdesignedtobeusedonce.After thechemicals in theelectrolyte solution (that transmit theelectric currents)havebeenused up, the energy is no longer available and the battery is said to be exhausted, used, or“dead,”andisthendiscarded.Anexampleofsuchaprimarycellbatterywouldbethealkalinebattery,usedinmostremotecontrols.Theanodeofthisbatteryisamixtureofpowderedzincand a concentrated solution of potassium hydroxide. The cathode is a mixture of solidmanganese (IV) oxide and graphite. The anode and cathode chambers are separated by aporousdivider.Themostcommonsizesofalkalinebatteriesarethe“roundcells”,suchasAAA,AA,C,andD(Figure1.1).

Figure1.1.TypicalalkalinebatteriesdepictingD,C,AA,andAAAsizes.

Activity1.1(Demonstration)–LightItUpShowthebrightnessofabulbresultingfromoneandtwobatteriesaddedinaseriescircuit.Then,useamultimeterorvoltmetertoshowhowthevoltageisadditive.

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AsecondarycellbatteryCANberechargedandusedrepeatedly.Thedischargedenergycanberestoredbyforcingelectronstoflowintheoppositedirectionbyutilizinganexternalelectricalenergysource.Anexampleofasecondarybatterythatcanbeusedforanumberofyearsisthelead‐acidbatteryorleadstoragecellfoundinautomobiles(Figure1.2).Themainfunctionofacar battery is to store electrical energy for starting the car and operating electrical deviceswhenthecarisnotrunning.Bothelectrodesintheleadstoragecellcontainaleadgrid.Theanodesareimpregnatedwithaspongyleadmetal,whileinthecathode,thegridispackedwithred‐brown,lead(IV)oxide.Adilutesulfuricacidsolutionservesastheelectrolyte.Thebatteryisconstantlybeingdischargedeverytimetheengineisstarted.Duringthischemicalreaction,sulfuricacidisturnedintowaterandbothelectrodesareconvertedtoleadsulfate.Whilethecar isbeingdriven,electricalenergygeneratedbythealternatorreversesthedecomposition,andthebatteryiscontinuouslyrecharged.

Figure1.2.Atypicalautomotivelead­acidbattery

CathodeReaction(reduction):

PbSO4(s)+5H20(l)↔PbO2(s)+3H3O+(aq)+HSO4

–(aq)+2e–

AnodeReaction(oxidation):

PbSO4(s)+H3O+(aq)+2e–↔Pb(s)+HSO4

–(aq)+H2O(l)

Oneofthemostrecentlydevelopedrechargeablebatteriesisthelithiumionbatterycommonlyusedincellphonesandlaptopcomputers(Figure1.3).TheanodeconsistsofLi+ionsthathavebeen inserted reversibly into layers of graphite. The cathode contains lithium cobalt oxide(LiCoO2).Whenthecelldischarges,Li+ionsspontaneouslymigratefromthegraphiteanodetothecathode,andelectrons flowthroughtheexternalcircuit. Whenthebattery is recharged,cobaltionsareoxidizedandLi+ionsmovebackintothegraphite.

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Figure1.3.Atypicalcellphonelithiumionbattery

Havestudentsresearchvarioustypesofsecondarycellbatteriesandreportonsuchtopicsashowitworks,thelifespanofthebatteries,usesofthebatteries,andcost.

ELECTRICALCIRCUITS

Anelectric circuit is an interconnectingpath,external to thebattery,whichallowscharge toflowbetweenthepositiveandnegativeterminalsofthebattery.Asimplecircuitmayconsistofasinglestrandofmetalwirelinkingtheterminals.However,amorerealisticcircuitpossessesmultiple branch points so that charge can take many different paths between the twoterminals. Although there canbemanydifferent paths through the external circuit that thecharge could take, the electrical energy that the charge acquires in making this journey isalwaysthesame.Since,whenanalyzingelectricalcircuits,weareprimarilyinterestedinenergy(i.e., in the transformation of the chemical energy of the battery into heat energy in someelectricheatingelements,ormechanicalenergy in someelectricmotors,etc.), it follows thatthepropertyofabattery,whichprimarilyconcernsus, is itsvoltage. Recall that thevoltagenumberV for a battery gives a relativemeasure of howhard the electrons are being pulledthroughthecircuit.

The rate at which charge flows out of the positive terminal is termed the electric currentflowingoutofthebattery.Likewise,therateatwhichchargeflowsintothenegativeterminalistermedthecurrentflowingintothebattery.Ofcourse,thesetwocurrentsmustbethesameotherwise charge would build up in either the battery or the circuit. Electric current,representedbytheletter“I”,ismeasuredinunitsofamperesoramps,abbreviated“A”.

MFCscannotbe“recharged”byreversingtheflowofelectrons,butsincetheyareatypeof fuel cell, theywill run indefinitely as long as a fuel feed is constantly supplied. Thisensures the bacteria are constantly metabolizing glucose to produce electricity. Thepotassium ferricyanide solution can be recirculated in the cathode chamber for anextendedperiodoftimeaswell,butshouldbereplacedregularly.

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Though,inactuality,thecurrentiscarriedbynegativecharges(i.e.,byelectrons)flowingintheopposite direction, conventionally, the direction of the current is taken to be the directionpositive chargeswould have tomove to account for the flow of charge. The current at allpoints in the external circuit must remain constant over time.We call this type of circuit adirectcurrentorDCcircuitbecausethecurrentalwaysflowsinthesamedirection.Thereisasecondtypeofcircuit,whichiscalledanalternatingcurrentorACcircuit,inwhichthecurrentperiodicallyswitchesdirection.

A simple circuit can be described as being somewhat analogous to a small ski resort. Thecharges flowing around the external circuit are like people skiing down the skislope. Thecharges flowdown a gradient of electric potential just as the people ski down a gradient ofgravitational potential. Note that the good skiers who ski directly down the slope acquireexactlythesamegravitationalenergyasthepoorskierswhoskifromsidetoside.Inbothcases,the total acquired energy depends only on the difference in height between the top andbottom of the slope. Likewise, charges flowing around an external circuit acquire the sameelectricalenergynomatterwhatroutetheytakebecausetheacquiredenergyonlydependsonthepotentialdifferencebetweenthetwoterminalsofthebattery.Oncethepeopleinourskiresortreachthebottomoftheslope,theymustbeliftedtothetopinaskiliftbeforetheycanskidown it again.Thus, the skilift inour resortplaysananalogous role to thebattery in ourcircuit.Ofcourse,theskiliftmustexpendnon‐gravitationalenergytoliftskierstothetopoftheslope, in just the same manner as the battery must expend non‐electrical energy to movechargesupapotentialgradient.Iftheskiliftrunsoutofenergy,thenthecirculationofskiersinthe resort rapidly stops. Likewise, if thebattery runsoutof energy (i.e., if thebattery ``runsdown''),thenthecurrentintheexternalcircuitstopsflowing.

OHM’SLAWConsider,again,asimplecircuit inwhichasteadycurrentI flowsthroughasingleconductingwire connecting the positive and negative terminals of a battery of voltage V. What is therelationshipbetweenthecurrent I flowing in thewireand thepotentialdifferenceVappliedacross the two ends of the wire by the battery? If we were to investigate this relationshipexperimentally,wewouldquicklyconcludethattheelectricalcurrentI isdirectlyproportionaltothepotentialdifferenceV.Inotherwords,

V=IRwheretheconstantofproportionality“R”istermedthe(electrical)resistanceofthewire.Theabove formula is calledOhm's law after its originator, the early nineteenth centuryGermanphysicistGeorgSimonOhm.Theunitofelectricalresistanceistheohm(Ω).

Goodconductorsofelectricity(i.e.,copper,silver,aluminium,andmostothermetals)possessnon‐zero electrical resistances. However, these resistances are generally so small that if wewere to connect the terminals of a battery together using a wire fashioned out of a goodconductor,thenthecurrent,whichwouldflowinthewire,accordingtoOhm'slaw,wouldbeso

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largethatitwoulddamageboththewireandthebattery.Weusuallycallsuchacircuitashortcircuit. To prevent excessively large currents from flowing, conventional electric circuitscontaincomponentscalledresistors,whoseelectricalresistance ismanyordersofmagnitudegreaterthanthatoftheconductingwiresinthecircuit.WhenweapplyOhm'slaw,V=IR,toacircuit,weusuallyonlycountthenetresistanceRofalltheresistorsinthecircuit,andneglecttheresistancesoftheinterconnectingwires.Thismeansthatallofthemajordropsinelectricpotential,aswetravelaroundthecircuit fromoneterminalof thebattery to theother, takeplaceinsidetheresistors. Thedropinpotential intheconductingwiresthemselves isusuallynegligible. Thus, forall intentsandpurposes,goodconductors,andwiresmadeoutofgoodconductors,actasiftheyhavezeroresistance.

SampleOhm’sLawProblems Anine‐voltbatterysuppliespowertoacordlesscurlingironwitharesistanceof

18ohms.Howmuchcurrentisflowingthroughthecurlingiron?0.5ampsA110‐voltwalloutletsuppliespowertoastrobelightwitharesistanceof2200ohms.Howmuchcurrentisflowingthroughthestrobelight?0.05amps

CIRCUITDIAGRAMS

Electric circuits, whether simple or complex, can be described with mere words – sayingsomething like, "A light bulb is connected to a D‐cell" is a sufficient amount of words todescribeasimplecircuit.But,anothermeansofdescribingacircuitistosimplydrawit. Suchdrawings,calledcircuitdiagrams,provideaquickermentalpictureoftheactualcircuit.

Describing Circuits withWords

"A circuit contains a light bulbanda1.5VoltD‐cell."

DescribingCircuitswithDrawings

MFCscommonlyachieveaworkingvoltageof0.3–0.7V.Thevoltage(orcellpotential)isafunctionoftheexternalresistanceorloadonthecircuitandthecurrent.Ohm’slawcanalso be used to calculate the cell potential for theMFC. The current produced from asingleMFCisusuallysosmallthatwhenconstructedinalaboratory,thecurrentfromtheMFCisnotmeasured,butiscalculatedfromthemeasuredvoltagedropacrosstheresistoras I=V/R. Thehighestvoltageproduced in theMFC is called theopencircuitvoltage,OCV,which ismeasuredwith the circuit disconnected (infinite resistance, zero current).As the resistances are decreased, the voltage decreases. The power at any time iscalculatedasP=IV,whichwillbedescribedinmoredetaillater(seeChapter1–PowerinElectricalCircuits).

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A final means of describing an electric circuit is by use of conventional circuit symbols toprovidea schematicdiagramof thecircuitand its components.Somecircuit symbolsused inschematicdiagramsareshownbelow.

SingleCellBatteriesConnectingWireResistorSwitch(open)Switch(closed)

Asinglecellorotherpowersource,aspreviouslynoted, isrepresentedbya longandashortparallelline.Acollectionofcells,orbatteries,isrepresentedbyacollectionoflongandshortparallellines.Inbothcases,thelonglineisrepresentativeofthepositiveterminaloftheenergysourceandtheshortlinerepresentsthenegativeterminal.Astraightlineisusedtorepresentaconnectingwirebetweenanytwocomponentsofthecircuit.Anelectricaldevice,whichoffersresistancetotheflowofcharge(aresistor),isrepresentedbyazigzagline.Anopenswitchisgenerally represented by providing a break in a straight line by lifting a portion of the lineupwardatadiagonal.Asanillustrationoftheuseofelectricalsymbolsinschematicdiagrams,considerthefollowingtwoexamples.

Example1:Descriptionusingwords: ThreeD‐cell batteries areused topower a circuit containing threelightbulbs.DrawingofCircuitSchematicDiagramofCircuit

Using theverbaldescription,onecanacquireamentalpictureof thecircuitbeingdescribed.Thisverbaldescriptioncanthenberepresentedbyadrawingofthreecellsandthreelightbulbsconnectedbywires.Finally,thecircuitsymbolspresentedabovecanbeusedtorepresentthesamecircuit.Notethatthreesetsoflongandshortparallellineshavebeenusedtorepresentthe threeD‐cells. Alsonote thateach lightbulb is representedby itsown individual resistorsymbol. Straight lines have been used to connect the two terminals of the battery to theresistorsandtheresistorstoeachother.

Theabovecircuitspresumedthatthethreelightbulbswereconnectedinsuchawaythatthecharge flowing through the circuit would pass through each one of the three light bulbs inconsecutivefashion.Thepathofapositivechargeleavingthepositiveterminalofthebatteryand traversing the external circuit would involve a passage through each one of the threeconnectedlightbulbsbeforereturningtothenegativeterminalofthebattery.Butisthistheonlywaythatthreelightbulbscanbeconnected?Dotheyhavetobeconnectedinconsecutive

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fashionasshownabove? Absolutelynot! Infact,example2belowcontainsthesameverbaldescriptionwiththedrawingandtheschematicdiagramsbeingdrawndifferently.

Example2:Descriptionusingwords: ThreeD‐cell batteries areused topower a circuit containing threelightbulbs.DrawingofCircuitSchematicDiagramofCircuit

Again, using the verbal description, one can acquire a mental picture of the circuit beingdescribed,however this time, the connectionsof lightbulbsaredone inamanner such thatthere is a point on the circuit where the wires branch off from each other. The branchinglocationisreferredtoasanode. Eachlightbulbisplacedinitsownseparatebranch. Thesebranchwireseventuallyconnecttoeachothertoformasecondnode.Asinglewireisusedtoconnectthissecondnodetothenegativeterminalofthebattery.

Thesetwoexamplesillustratethetwocommontypesofconnectionsmadeinelectriccircuits.When two or more resistors are present in a circuit, they can be connected in series or inparallel.

Activity1.2–CreatingElectricalCircuitsStudents will sketch and construct several circuits and draw corresponding circuitdiagramstoinvestigateparallelandseriescircuits.

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Activity1.2–CreatingElectricalCircuits

ProblemA

TypeofCircuit:_____________________________

Sketch and construct an electric circuit inwhichoneD‐typebattery can light twobulbsasbrightlyasonebulb.

ProblemB

TypeofCircuit:_____________________________

Sketch and construct an electric circuit inwhichonebulbis litbrighterwithtwoD‐typebatteriesthanwithoneD‐typebattery.

ProblemC

TypeofCircuit:_____________________________

Sketch and construct an electric circuit inwhichonebulbislightednobrighterwithtwoD‐typebatteriesthanwithoneD‐typebattery.

ProblemD

TypeofCircuit:_____________________________

Sketch and construct an electric circuit inwhichoneD‐typebatterylightstwobulbslessbrightlythanonebulb.

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Activity1.2–CreatingElectricalCircuits(withanswers)

ProblemA

PARALLEL

Sketch and construct an electric circuit inwhichoneD‐typebattery can light twobulbsequallyasbrightasonebulb.

Rationale:Twobulbswiredinparallelareequallybright and have the same brightness as thestandard. Thevoltageisevenlydistributedacrosstheentirecircuit.

ProblemB

SERIES

Sketch and construct an electric circuit inwhichonebulbislightedbrighterwithtwoD‐typebatteriesthanwithoneD‐typebattery.

Rationale: Two batteries wired in series havetwice the voltage as a single cell. The voltage isadditive.

ProblemC

PARALLEL

Sketch and construct an electric circuit inwhichonebulbislightednobrighterwithtwoD‐typebatteriesthanwithoneD‐typebattery.

Rationale: Two batteries wired in parallel havethesamevoltageasasinglecell,but lasttwiceaslong. Thevoltage isevenlydistributedacross theentirecircuit.

ProblemD

SERIES

Sketch and construct an electric circuit inwhichoneD‐typebatterylightstwobulbslessbrightlythanonebulb.

Rationale: Twobulbswired inseriesareequallybright, but dimmer than the standard. Thevoltageisadditive.

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Activity1.2–CreatingElectricalCircuits(InstructorNotes)

• Have the studentsobserveboth current andvoltagewithamultimeter as they construct

thecircuits.

• AskthemtomakeobservationsaboutthedifferencesbetweenparallelandseriesconnectionsastheyrelatetoOhm’sLaw(takingintoaccounttheresistanceofthebulbs).

­­QuestionsforAnalysis­­ Whatarethebasiccomponentsofanelectricalcircuit?

Twoormoreinterconnectedcomponentsconsistingofasourceofcurrentorvoltageandaresistor.

Whatisthedifferencebetweenseriesandparallelcircuits?

Aseriescircuitisasinglepathforelectriccurrentthroughallofitscomponents.Severalbatteries connected in series canproducegreatervoltage thana singlebatterycan.Aparallel circuit isadifferentpath forcurrent througheachof itscomponents. A parallel circuit provides the same voltage across all itscomponents.

YoumayhavenoticedthatanentirestringofoldChristmaslightsgoesoutifonebulbburnsoutorisremoved.Explain.

Christmaslightsareconnectedinseries,sowhenonegoesoutorisremoved,anopencircuitiscreated.Anopencircuitisanincompletecircuitduetoabreakinthecontinuousconnectionofconductorsfromoneendofanelectricalsourcetotheother.

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SERIESANDPARALLELCIRCUITS

Aseriescircuitisacircuitinwhichresistorsarearrangedinachainsothatthecurrenthasonlyone path to take. The current is the same through each resistor. The total resistance of thecircuitisfoundbysimplyaddinguptheresistance(orvoltage)valuesoftheindividualresistors:

equivalentresistanceofresistorsinseries:R=R1+R2+R3+...

R1R2R3V

I

Aseriescircuitisshowninthediagramabove.Thecurrentflowsthrougheachresistorinturn.Ifthevaluesofthethreeresistorsare:

R1=8Ω,R2=8Ω,andR3=4Ω,thenthetotalresistanceis8+8+4=20Ω.

Witha10Vbattery,byV=IR,thetotalcurrentinthecircuitis:

I=V/R=10/20=0.5A.Thecurrentthrougheachresistorwouldbe0.5A.

Aparallel circuit is a circuit in which the resistors are arrangedwith their heads connectedtogether, and their tails connected together. The current in a parallel circuit breaksup,withsomeflowingalongeachparallelbranch,andre‐combiningwhenthebranchesmeetagain.Thevoltageacrosseachresistorinparallelisthesame.

Thetotalresistanceofasetofresistorsinparallelisfoundbyaddingupthereciprocalsoftheresistancevalues,andthentakingthereciprocalofthetotal:

equivalentresistanceofresistorsinparallel:1/R=1/R1+1/R2+1/R3+...

I1R1

II2R2I

I3R3

V

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Aparallelcircuitisshowninthediagramabove.Inthiscase,thecurrentsuppliedbythebatterysplitsup,andtheamountgoingthrougheachresistordependsontheresistance.Ifthevaluesofthethreeresistorsare:

R1=8Ω,R2=8Ω,andR3=4Ω,thenthetotalresistanceisfoundby

1/R=1/8+1/8+1/4=1/2.ThisgivesR=2Ω.

Witha10Vbattery,byV=IR,thetotalcurrentinthecircuitis:I=V/R=10/2=5A.

TheindividualcurrentscanalsobefoundusingI=V/R.Thevoltageacrosseachresistoris10V,andtherefore:

I1=10/8=1.25AI2=10/8=1.25AI3=10/4=2.5A

Notethatthecurrentsaddtogetherto5A,thetotalcurrent.

Inamanneranalogoustoresistors,batteries(orsourcesofvoltage)canalsobeplacedwithincircuits in seriesand inparallel. Ineach instance,batteriesadded in series increasevoltage,whilebatteriesaddedinparallelincreasecurrent.

EMFANDINTERNALRESISTANCEReal batteries are constructed from materials, which possess non‐zero resistivities, so realbatteriesarenotjustpurevoltagesources.Theyalsopossessinternalresistances.Incidentally,apurevoltagesourceisusuallyreferredtoasanemf(whichstandsforelectromotiveforce).Of

course,emfisalsomeasuredinunitsofvolts.Abatterycanbemodeledasanemfεconnectedinserieswitharesistorr,whichrepresentsitsinternalresistance.SupposethatsuchabatteryisusedtodriveacurrentIthroughanexternalloadresistorRasshowninthecircuitdiagram

below. Note that in circuitdiagrams, anemfε is representedas twoclosely spacedparallellinesofunequallength.Aresistorisrepresentedasazig‐zagline.

ThereasonourexperimentalMFChastwoanodechambersisanalogoustotheconnectionof twobatteries in series –more voltage is generated. To light the LED, approximately0.45 V is necessary, and a single anode chamber often is not capable of supplying thatmuchvoltage.Thereasontheanodechambersaresituatedoneithersideofthecathodechamber is to provide increased surface area at the interface between the anode andcathodeforthemigrationofpositiveionsthroughtheion‐exchangemembrane.

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Battery B ε r A R I

Considerthebatteryabove.ThevoltageVofthebatteryisdefinedasthedifferenceinelectricpotentialbetweenitspositiveandnegativeterminals:i.e.,thepointsAandB,respectively.AswemovefromBtoA,theelectricpotentialincreasesby+ εvoltsaswecrosstheemf,butthendecreasesby1/rvoltsaswecross the internal resistor. Thus, thevoltageVof thebattery isrelatedtoitsemfεandinternalresistancervia:

V = ε - IR

Now, we usually think of the emf of a battery as being essentially constant (since it onlydependsonthechemicalreactiongoingoninsidethebattery,whichconvertschemicalenergyinto electrical energy). Therefore, we must conclude that the voltage of a battery actuallydecreasesasthecurrentdrawnfromitincreases.Infact,thevoltageonlyequalstheemfwhenthecurrentisnegligiblysmall.Ifweshort‐circuitabattery,byconnectingitspositiveandnegativeterminalstogetherusingaconductingwireofnegligibleresistance,thecurrentdrawnfromthebatteryislimitedonlybyits internal resistance. In fact, in this case, the current is equal to the maximum possible

current. A real battery is usually characterized in termsof its emfε (i.e., its voltage at zerocurrent)andthemaximumcurrentthatitcansupply.Forinstance,astandarddrycell(i.e.,thesort of battery used to power remote controls) is usually rated at 1.5V and (around) 0.1A.Thus,nothingreallycatastrophic isgoingtohappenifweshort‐circuitadrycell. Wewillrunthebatterydowninacomparativelyshortperiodoftime,butnodangerouslylargecurrentisgoingtoflow.Ontheotherhand,acarbatteryisusuallyratedat12Vandsomethinglike200A(this isthesortofcurrentneededtooperatethestartermotor). It isclearthatacarbatterymusthaveamuchlower internalresistancethanadrycell. It followsthat ifwewerefoolishenoughtoshort‐circuitacarbattery,theresultwouldbefairlycatastrophic(imagineallofthe

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energyneededtoturnover theengineofacargoing intoa thinwireconnectingthebatteryterminalstogether). Problem:What is the terminal (final) voltage supplied by a cell of emf 2.0 V with aninternalresistanceof1.0Ωwhenitisconnectedtoa9.0Ωresistor? BeginbyusingOhm’slawtofindthecurrentsuppliedbythebattery. I=V/R=2.0/9.0=0.22A Then,usetheinternalresistanceformulatosolvetherestoftheproblem.

V = ε - IR V=2.0–(0.22)(1)=1.8VPOWERINELECTRICALCIRCUITS

Inadditiontovoltageandcurrent,anothermeasureofelectronactivityinacircuitispower.Inelectric circuits, power is a function of both voltage and current. Not surprisingly, thisrelationshipbearsastrikingresemblancetoOhm’slaw:

P=IV

Inthisequation,powerP isexactlyequaltothecurrentImultipliedbythevoltageV. Whenusingthisformula,theunitofmeasurementforpoweristhewatt,abbreviatedwiththeletter"W."

It must be understood that neither voltage nor current by them selves constitute power.Rather, power is the combination of both voltage and current in a circuit. Remember thatvoltage is the specificwork (orpotentialenergy)perunit charge,whilecurrent is the rateat

It isnotunusual tonotesignificantdifferences involtageoutputbetweendifferingMFCassemblies based on specific system architecture. The main reason is because of theinternalresistanceofthereactorcomparedtothemaximumpossiblepotentialduetothechemicalreactionsattheanodeandcathode,orthecellpotential,Eemf (seeChapter1–StandardReductionPotentials).Todeterminethetotalvoltagecapableofbeingproducedbythecell,wemustviewtheMFCashavingcurrentthroughtworesistorslinkedinseries,withonebeingtheexternalloadandtheothertheinternalresistance.

BeingthattheoperationalconditionswithintheMFCaredefinedbytherequirementsofthemicrobesforoptimalgrowthandmetabolismandarenotnecessarilycompatiblewiththemostfavorableelectrochemicalconditions,akeychallengeforthoselaboratoriesandscientists working to improveMFC voltage (and resulting power) outputs is to identifypossible sources of internal resistance within the specific MFC design and significantlyreducetheseresistances.

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whichelectric chargesmove throughaconductor. Voltage isanalogous to theworkdone inlifting aweight against the pull of gravity. Current is analogous to the speed atwhich thatweight is lifted. Thus, the rule is thepower inacircuit is theproductof thevoltageand thecurrent.According to the formula, a circuitwithhigh voltage and low currentmaybedissipating thesameamountofpowerasacircuitwithlowvoltageandhighcurrent. Neithertheamountofvoltagealone,northeamountofcurrentalone, indicatestheamountofpower inanelectriccircuit.

In an open circuit, a circuit contains no external resistors, voltage is present between theterminalsofthebattery,andthereiszerocurrent.Asaresult,thereiszeropowerdissipated,nomatterhowgreatthatvoltagemaybe.SinceP=IVandI=0andanythingmultipliedbyzero is zero, thepowerdissipated in anyopen circuitmustbe zero. Likewise, ifwewere tohaveashortcircuitconstructedofa loopofconductingwire (essentiallyzeroresistance),wecouldhaveaconditionofcurrentintheloopwithzerovoltageand,likewise,nopowerwouldbedissipated.

Thepowerformuladoesnotjustapplytobatteries.Itcouldalsoapplytoaresistorexternaltothebattery.Sucharesistorisreferredtoasaloadresistor.Itcouldbeeitheranelectriclight,anelectricheatingelement,or,maybe,anelectricmotor.Thebasicpurposeofthecircuitistotransfer energy from thebattery to the load,where it actually does somethinguseful for us(e.g.,lightingalightbulborliftingaweight).Thepower transferbetweena voltage source andanexternal load (or load resistor) ismostefficientwhentheresistanceoftheloadmatchestheinternalresistanceofthevoltagesource.If the load resistance is too low, then most of the power output of the voltage source isdissipatedasheatinsidethesourceitself.Electricalenergyisconvertedintoheat(i.e.,randommotion of the atoms that make up the voltage source) as the electrically accelerated freeelectrons inside the source collide with the atoms and, thereby, transfer all of their kineticenergy to the atoms. If the load resistance is too high, then the current,which flows in thecircuit,istoolowtotransferenergytotheloadatanappreciablerate.Theoptimalcaseexistswhentheloadresistanceisequaltotheinternalresistanceofthevoltagesource,however,onlyhalfofthepowerofthevoltagesourceistransferredtotheload.Theotherhalfisdissipatedasheatinsidethesource.

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TomakeMFCs useful as amethod to generate power, it is necessary to first optimize thesystemforpowerproduction.PoweriscalculatedfromthevoltageandthecurrentasP=IE, where “E” stands for the cell potential (in some instances, “V” is not used for voltagebecausethesymbolandtheunitsV=Voltscanleadtoconfusion).

ThepowergeneratedbyanMFC is calculated fromthemeasuredvoltage,EMFC, across theloadand the currentusingP= IEMFC. The currentproducedby a laboratory‐scaleMFC iscommonlycalculated,aspreviousstated,bymeasuringthepotentialacrosstheload(i.e.,theexternalresistor,Rext),andusingI=EMFC/Rext.Thus,poweroutputcanbefoundusing

BasedontherelationshipI=EMFC/Rext,wecanalternativelyexpresspoweroutputintermsofthecalculatedcurrentas:

P=I2Rext

Activity1.3.A(Assessment)–TheShoeboxRoomActivity1.3.B(Assessment)–CanYouLightYourWay?Students will demonstrate an understanding of the requirements of making an electricalcircuit;will be able to build a working series circuit and parallel circuit;will be able todescribe how both types of circuits work;will be able to describe the advantages anddisadvantages of parallel and series circuits;will understand that electrical energy can beconvertedtoheat, light,soundandmotion;andwillwireashoeboxhousewithseriesandparallelcircuitsto operatelightsandotherresistorsthatarecontrolledbyswitches.

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Activity1.3.A–TheShoeboxRoom

Hello,mynameisStuart.Iliveinaverysmallhouse,andyouwon’tfindmecomplainingaboutthat. I’msuremylittlehouseismuchmorecomfortablethanyouraveragecaveormudhut.Houses,as I’msureyou’venoticed, seemtobegetting smallerand smallerand smalleruntileventually,Iimagine,wewillallbelivinginshoeboxes.Notjustme.

Yes,althoughtheentirehousehasnotyetshrunkintosomethingyoucouldeasilywalkaroundwith,mybedroomisridiculouslytiny.Itisbyfarthesmallestinthehouse.

Iamlookingtodosomeredecoratingandremodelinginmybedroomandwouldliketoincludeafewnewamenitiesandupgradestoitscurrentdesign:

• Analarmthatsoundswhenthedooropens• Aceilingfanthatcanrunonmorethanonespeedwithalight• Areadinglampwithmorethanonelevelofbrightness

Beingthatmostinteriordecoratingbusinessesandelectricalengineeringfirmsdon’tdealwithrooms as small as shoeboxes, Stuart has sought you out to design and construct his newbedroom.

Youshouldbeginbysketchingouta floorplan for the room, including the locationofall thefurnitureandfixtures,thendrawingoutcircuitdiagramsforthenecessaryelectricalwork,and,finally,completingtheelectricalworkwithintheroom.

InadditiontocreatingafullyfunctioningshoeboxroomforStuart,hehasalsoaskedthatyouprepare a report detailing how the electrical energy is being transferred and transformedthroughout his room. The report should also include a discussion ofwiring choices, resistorarrangements,andenergyflow.

ThefollowingmaterialswillbeprovidedforyoutouseinthedesignofStuart’sshoeboxroom:

Fortheteachertodevelop:NeedtodevelopmaterialslistNeedtodevelopscoringrubric

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Activity1.3.B–CanYouLightYourWay?(afterProf.G.Watson,UniversityofDelaware,www.pysics.udel.edu)

"What'swrongwiththecarbidelantern?"

BeforeChriscouldutterareplytoPat'squestion,darknesssuddenlydescended.

"Great!I'vebeenbeggingmyfolksforanewlampsincethelasttimewewentcaving.Toolatenow!Let'sfireuponeofourcandles."

It took a bit of rummaging through the pack to find one in the shroud of darkness. Finallyready, Pat struck a match but it immediately flickered out. This happened again with thesecondmatch,andagainwiththethird.ThereasonsuddenlyoccurredtoChris.

"You've gotta be kidding me ‐‐ I think the air is bad down here! The guidebook didn't sayanythingaboutthat,didit?"

"Not that I remember. You know, my breath does seem a little short right now that youmentionit.Thinkwemadeawrongturn?"

"Don'tknow,butweneedtogetoutofhererightnow.ShineyourflashlightoverherewhileItakeanotherlookatthemap."

AminutepassedwhilePatrifledthroughthepackagain.

"Badnews!Imusthavelefttheflashlightlyingonthegroundwhereweateourlunch."

"You'dbetterbejoking!Wehavewalkedforfourhourssinceweate.Wecan'tmakeitbacktherewithoutalight."

"I'mserious.Iremembertakingitouttogetthesandwiches.GuessIforgottoputitback."

"Nevermind,Ikeepasmallflashlightinmyjacket.Hey,whereisit?Darnit,mylittlebrotherisalwaysstealingmystuff!"

"Calmdown!Gettingupsetwon’thelpanything!"

"OK.OK.You’reright.Whatdowehaveforalight?"

"Well, Idohaveasparebulb formy lantern inmypocket.But Ididn'tbringasparebattery;they'resoheavy."

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"Do you have anything else with batteries in it? I've got two extra AAA batteries for theflashlightthatlittlethieftook."

"I’vegotnothingelsewithbatteries.Idon’tcarryalittlebitofeverythinglikeyoudo."

"GoodthingIdo!Butthebulbisfora6Vbattery.DoyouthinkitwillworkwithtwoAAAcells?"

"Even if it does, do you think itwill burn long enough for us to get out? I'm starting to getworried..."

"Maybewecanuseoneatatimetomakethemlastlonger.Itwon'tbeasbrightandwe’llhavetowalk slower, butmaybe one at a timewould be better. Hey, you’ve been learning aboutelectricalcircuitinschool.Whatshouldwedo?"

TheExerciseYourlabexercisecomesintwoparts.

1. YourgroupwillhaveavailabletwoAAAcellsandthebulbdescribedabove. Inthelabyouwillhaveavailabletwomultimeters,lengthsofwire,andachoiceofresistors.Makethemeasurementsnecessarytoanswerthequestionsintheprecedingstory.Youmaywishtoconsultadditionalonlineresourcesforsupportingdata.

2. Designaflashlightusingmaterialsthatyouwishyouhadinyourpockets.Becreative‐assumethatyouarenotcarryingalengthofwire!Also,aswitchingmechanismwouldbeadesirablefeature.Inthenextclassperiod,yourgroupwillbegivenfiveminutestoassembleaworkingflashlight.Andwemayeventurnoutthelights!!!

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ELECTROCHEMICALCELLSAnelectrochemicalcellgeneratesanelectromotiveforceoremf(voltage)andelectriccurrentfrom chemical reactions. The current is caused by the chemical reactions releasing andaccepting electrons at the different ends of a conductor. Electrochemistry is the branch ofchemistryinvolvedwiththestudyofelectricityproducedbychemicalreactions.There are two different kinds of electrochemical cells. Electrolytic cells are those in whichelectrical energy causes nonspontaneous chemical reactions to occur, and voltaic cells arethose in which spontaneous chemical reactions produce electricity and pass it through anexternal circuit. In a voltaic cell, electrical current canbe conducted throughmetalwiresoralongmetalsurfacesandthroughpureliquidelectrolytesorsolutionsthatcontainelectrolytes.When a metallic conductor is employed, current is conducted through the metal withoutcausinganychemicalchange.Electrolyticconductionoccursviathemotionofionsthroughthesolutionorpureliquid.Thechemicalreactionsinbatteriesoccurwhentwodissimilarmaterials,suchasaluminumandcopper (calledelectrodes) react togetherwhen inserted into a chemical conduction solutioncalled an electrolyte. The electrolyte solution begins to slowly dissolve the aluminumelectrodes,yieldingpositivealuminumionsandelectrons.Theseelectronsmustremainboundto an electron conductingmaterial and cannotmove into the electrolyte. The positive ionsaccumulateintheelectrolyteandflow(viaconcentrationgradient)towardsthecathode.Theelectrons travel around, from the anode, through the conducting wires, and complete thecircuit at the cathode. At the cathode, the positive aluminum ions in combinationwith theelectronscausealuminumtobedepositedonthecopperelectrode.

Activity1.4–AHomemade“Lemony”BatteryStudentswillconstructafunctionalbatteryfromseveralcommonhouseholditems.

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Activity1.4–AHomemade“Lemony”BatteryMaterials:lemons,coins(suchascopperpennies),papertowels,aluminumfoil(orcoins,suchasdimes),bowls, scissors, lemon juicer, wire strippers, plastic tape, paper tube (toilet paper or papertoweltubewillwork),plastic‐coveredelectricalwireProcedure:1.Wrapfoiloveroneendofthepapertubeandthensecureitbytapingitdown.2.Withthewirecutters,strip1‐2’’oftheplasticfromthewire.Tapeoneendtothefoil.Then,setupthepapertubewiththefoildownandtheopeningontop.3.Squeezethejuicefromthelemonsintoadish.Soakthepapertowelsinthejuice.Then,startfillingthepapertubewithasmallrolledpieceoftoweling,thenacoin,followedbyapieceoffoil.Continuetolayerthesethreematerials,fillingthetubeandendingwithacoin.Tapeasecondstrippedwiretothecoin.4.Moistenafingertiponeachhandandtouchtheendsofthetwowires.Studentswillexperienceasmallshockortinglebutitwillbeveryharmless.StudentsmayalsotryattachinganLEDtoeachendofthebattery.­­QuestionsforAnalysis­­ IfyouarenotgeneratingenoughelectricitytolighttheLED,whatcouldyoudotogeneratemorevoltage?

Connectseveralhomemadebatteriestogetherinseries.

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Thehomemadebatteryisanexampleofawetcell.Thelemonjuiceactsastheelectrolytethatconductstheelectricitycreatedbythecoinsandthefoil.Wetcellsarecharacterizedbyliquidelectrolytes.

Activity1.4–TheHandBattery

Activity1.5–TheHandBatteryMaterials:multimeter* or voltmeter (for instructions on operations and settings consultwww.extech.com/instrument/products/400_450/manuals/EX410_UM.pdf or your instructionmanual), copperplateandaluminumplate (eachabout the sizeofyourhand), twoelectricalleadwireswith alligator clips, a piece ofwood or other nonmetallic surface, plates of othermetals,suchastinorzinc(optional)*Themultimetermustbecapableofmeasuring lowrangecurrent. Industrialgaugescannotalwaysreadinmilliampranges.Multimeterssensitiveenoughforthisactivitycanbepurchasedatelectronicsstores,suchasRadioShack.Procedure:

1. Mount bothmetal plates on a piece ofwood or simply clamp them to a nonmetallicsurface.(Ifyouprefer,youdon'tevenhavetomountthetwoplates.Youcanattachthewires as described below and then simply hold one plate in each hand. This has thebenefitofallowingyoutosubstituteothermetalseasily.)

Activity1.5–TheHandBatteryStudentswillobservethatvoltageoutput(i.e.,batteries)canresultfrommanydifferentmaterialsaslongastheprinciplesofelectronflowareobserved.

Whilelemonjuiceactedasthesoleelectrolyteinthishomemadebattery,ourMFCwillbeusingtwodifferentelectrolytes,oneintheanode(negative)chamberandoneinthecathode(positive)chamber.Theanodewillcontainamixtureofsoilbacteriainaglucosenutrient growthmediawith an added redoxmediator (methylene blue). The cathodewillcontainasolutionofasolublechemicalelectronacceptor(oxidizer)calledpotassiumferricyanide. TheMFCmustutilize twodifferentelectrolytesbecauseof thenatureofthe reactions occurring in each chamber. In the anode chamber, there is no oxygenpresent (anaerobic conditions), while there typically is oxygen present in the cathodechamber. Some MFC designs can also have anaerobic cathode chambers, e.g. withchemicalelectronacceptors,suchasferricyanide.

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2. Usingtheclipleads,connectoneplatetooneofthemeter'sterminalsandconnecttheotherplatetotheotherterminal.Atthispoint,itdoesn'tmatterwhichplateattachestowhichterminal.

3. Placeonehandoneachplate.Youshouldnoticeareadingonthemeter. If themeterdoesn't show an electrical current, simply reverse your connections, attaching thecopper plate to the terminal that the aluminumwas connected to and vice versa. Ifthere isstillnocurrent,checktheconnectionsandthewiring. If thatdoesn'tproducecurrent,trycleaningtheplateswithapencileraserorsteelwooltoremoveoxidation.

4. Experiment with different metals to find out what combination produces the mostcurrent(amps).Trypressingharderontheplates.Getyourhandswetandtryagain.

5. Haveonepersonputahandonthecopperplateandanotherpersonputahandonthealuminumplate,andthenhavethemjointheirfreehands.

6. Also,experimentwithmeasuringtheresistance(ohms)ofwetvs.dryhands.7. Vigorously exercise (30 push ups or 30 jumping jacks), and once again measure the

resistance.

­­QuestionsforAnalysis­­

Isthereadifferencebetweendryandwethandsinthevoltageorcurrent?Whyorwhynot?

Wethands(lessresistance)shouldyieldahighercurrentandlowervoltageduetoOhm’slaw.

Whendidyouobtainthehighestandthelowestresistance?

Wet(lowresistance),Dry(higherresistance) Istheresistanceofyourskinafunctionofmoistureinyourskin?

Yes Suggesthowthehandbatterycouldbeutilizedasasimpletypeofliedetector.

Onecouldassumethatapersonwhoislyingmaybenervous,causingtheirhandstoperspire.

Discussion:

When you touch the two metal plates, the thin film of sweat on your hands acts as theelectrolyte, reacting with the copper plate and with the aluminum plate. In one of thesereactions, yourhand takesnegatively chargedelectrons away from the copperplate, leavingpositivechargesbehind.Intheotherreaction,yourhandgiveselectronstothealuminumplate,causingittobecomenegativelycharged.

Thisdifferenceinchargebetweenthetwoplatescreatesaflowofelectricalcharge,orelectricalcurrent.Sinceelectronscanmovefreelythroughmetals,theexcesselectronsonthealuminumplate flow through themeter on their way to the copper plate. (Inmetals, positive charges

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cannotmove.) In yourbody,bothpositiveandnegative ionsmove.Negativeelectronsmovethroughyourbodyfromthehandtouchingthecoppertothehandtouchingaluminum.Atthesametime,positiveionsmoveintheoppositedirection.Aslongasthereactionscontinue,thechargeswillcontinuetoflowandthemeterwillshowasmallcurrent.

Yourbody resists the flowofcurrent.Mostof this resistance is inyour skin.Bywettingyourskin,youcandecreaseyourresistanceandincreasethecurrentthroughthemeter.Sincetwopeopleholdinghandshavemoreresistancethanoneperson,theflowofcurrentwillbeless.

If you disconnect one of the wires to the multimeter, the aluminum becomes negativelycharged.Electronspileuponthealuminumsidebecausetheycannotcrossthegapinthewire.Thecopperbecomespositivelychargedasyourhandremoveselectronsfromthemetal.Thesepilesofchargecreateavoltage,whichismeasuredwhenthemultimeterissettoreadvolts.

Most batteries use two different metals and an electrolyte solution to create a potentialdifferenceand,thus,avoltage.Whentheterminalsofthebatteryareconnectedwithawire,thisvoltageproducesacurrent.

Youcanuseotherpairsofdifferentmetals inacircuit toproduceacurrent.Thesuccessyouhaveusingvariousmetalswilldependonametal'selectricpotential,thatis,itsabilitytogainorlose charges. Try various metals to see which produces the highest current reading. Anelectromotiveseriestable(foundinmostchemistrytextbooks)showstheelectricpotentialsofmetals and allows you to predictwhichmetalswillworkwell inmaking a handbattery (seetable,page33).

Youcansometimesgetasmallcurrentevenbetweentwoplatesmadeofthesamemetal.Eachplate has a slightly different coating of oxides, salts, and oils on its surface. These coatingscreate slight differences in the surfacesof themetals, and thesedifferences canproduce anelectricalcurrent.

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OXIDATION–REDUCTIONREACTIONS

Batteriesrelyonchemicalreactionstogeneratevoltageandcurrent.Thecurrentiscausedbythe reactions releasing and accepting electrons at the different ends of a conductor. Thereactionisalwaysanoxidation‐reduction(orredox)reactioninvolvingatransferofelectrons.Redox reactions can be broken down into two half‐reactions: (1)oxidation at the anode aselectronsaretransferredfromanelectrondonortotheelectrode,and(2)reductionatcathodeas an electron acceptor gains electrons from the electrode. Thus, in a spontaneous redoxreaction, electrons flow from theoxidized reactant (electrondonor) to the reduced reactant(electronacceptor). Theelectrodesaresimplytheordinarymetallicsurfacesuponwhichtheoxidationorreductionhalf‐reactionsoccur.

Redoxprocessesoccurinsingledisplacementorsubstitutionreactions.Theredoxcomponentofthistypeofreactionisthechangeofoxidationstate(charge)oncertainatoms,nottheactualexchangeofatomsinthecompounds.

Inelectrochemicalcells,spontaneousoxidation‐reductionreactionsgenerateelectricalenergy.Forexample,inthereactionbetweenironandcopper(II)sulfatesolution:

Fe + CuSO4 → FeSO4 + Cu

Theionicequationforthisreactionis:

Fe + Cu2+ → Fe2+ + Cu

Astwohalf‐equations,itisseenthattheironisoxidized(electrondonor):

Fe → Fe2+ + 2e–

Andthecopperisreduced(electronacceptor):

Cu2+ + 2e– → Cu

Ifwedipan ironstrip intoacopper(II)sulfatesolution,the ironwillbeeatenaway,aspongylayerofmetalliccopperwillplateoutonthe ironstrip,andthedeepbluecolorofcopper(II)sulfate will gradually fade. [Iron(II) sulfate, which is formed, is colorless.] In contrast, if weimmerseacopperstripinaniron(II)sulfatesolution,noreactionwilloccurbecausethereversereactionishighlynonspontaneous.

Inthisarrangement,thespontaneoustransferofelectronsfromirontocopperisnotusefulforgeneratingelectricalenergybecausetheenergyreleasedisdissipatedasheat.Itisanalogoustoburningaspoonfulofsugarwithamatchinsteadofeatingitandconvertingthefreeenergyofoxidationintousefulmusclework(seeChapter2–MetabolismasaRedoxProcess).Ifsomemeans could be found to separate the removal of electrons from iron (oxidation) from the

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donation of electrons to copper ions (reduction), then the electrons may be made to dosomethingusefulalongtheway.

Whenthetwohalf‐reactionsareseparated,this flowofelectrons, insteadofoccurringatthesurfaceofthemetal,occursthroughanexternalcircuitandelectriccurrentisgenerated.Thisiscalledavoltaiccell,andisexactlyhowabatteryworks.Thetwohalvesoftheredoxreactionarereferredtoashalf‐cells.

A battery, like the ones found in a flashlight or calculator, contain oxidizing and reducingsubstances. As the electrons are transferred from anode to cathode, they are “tapped” toprovidethevoltagenecessarytopowertheelectricaldevice.

VOLTAICCELLS

Voltaic cells consist of two half‐cells connected by a salt bridge. Each half cell is typically ametalelectrodeina1.0molar(1mol/LorM)aqueoussolutionofthemetal’ssalt.Thehalfcellsareconnectedbyametalwireandavoltmetercanbeplaced intothecircuittomeasurethepotentialdifferencebetweenthetwoelectrodes.Thesaltbridgeisoftenemployedtoprovideelectrical contact between two half‐cells with very different electrolytes – to prevent thesolutionsfrommixing–andismadeofsomemediumthroughwhichionscanslowlypassandmaintainchargebalance in the twohalf‐cell solutions. A simple saltbridgeconsistsof filterpapersoakedwitharelativelyinertelectrolyte,usuallypotassiumchlorideorsodiumchloride.Another type of salt bridge consists of U‐shaped glass tubes‐ filled potassium chloride orsodiumchloride.Agarisoftenusedforgelificationwithinthetube.

The two electrodes, connecting wire, and salt bridge form a closed circuit, with electronsmoving from negative electrode to positive electrode through the wire, and positive andnegativeionsmovingthroughthesaltbridge.Thus,electronswillflowspontaneouslyfromtheanode, where oxidation is occurring, to the cathode, where the reduction is occurring.Electricity in the voltaic cell is generated due to electric potential difference between twoelectrodes. The potential difference between the electrodes provides the driving force thatpushestheelectronsthroughtheexternalcircuit.Thispotential,orEMF(electromotiveforce),of a cell is a measure of the tendency of the electrons to flow, and is dependant on thedifferenceinoxidizingorreducingstrengthofthetwohalf‐cells.

A good analogy for the flow of electrons is the flow of water. Water flowsspontaneouslydownhill. Damsandwaterwheelsareexamplesofwaysthattheenergyof flowingwater is tapped togeneratepower. Cellpotential is roughlyanalogous to a height difference between two reservoirs of water that areconnected. Water will have a tendency (potential) to flow between thereservoirs,andthatflowcanbetappedtodowork.

To maintain electrical neutrality and complete the circuit in the iron‐copper cell describedabove, two Cl– ions from the salt bridgemigrate into the anode solution for every Fe2+ ion

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formed. Simultaneously, twoK+ ionsmigrate into thecathodesolution to replaceeveryCu2+ionreduced.

Voltaiccellscanberepresentedinshorthandform:

Fe|Fe2+(1.0M)||Cu2+(1.0M)|Cu

Theanode(Fe)isshownontheleftsideandthecathode(Cu)ontherightside.Theelectrodesurfacesandthespecies(andtheirconcentrations) incontactwiththeelectrodesurfacesareseparatedbysingleverticallines.Doubleverticallinesrepresentthesaltbridge.

STANDARDREDUCTIONPOTENTIALS

ThereiscompetitionbetweenFe2+andCu2+intheiron‐coppervoltaiccellforelectrons.ItturnsoutthatCu2+iseasiertoreducethanFe2+andwinsthebattlefortheelectrons.Thisisknownbecauseeachhalf‐cellhasacharacteristicvoltageorpotential.

Itwouldbedesirabletoseparatetheindividualcontributionseachhalf‐cellreactionmakestothe total cell potential. This would allow us to determine the relative tendencies of theparticular oxidation or reduction half‐reactions to occur. However, it is not possible todetermine experimentally the potential of a single electrode since every oxidation must beaccompanied by a reduction; that is, the electrons must have somewhere to go. As aconsequence,itisnecessarytoestablishsomearbitrarystandard.

Byinternationalagreement,thereferenceelectrodewasselectedtobethestandardhydrogenelectrode(SHE).Thisstandardhydrogenelectrodeconsistsofapieceofmetalelectrolyticallycoatedwithagrainy,black surfaceof inertplatinummetal immersed ina1.0MH+ solution.Hydrogengas,H2, is thenbubbledover theelectrodeatoneatmosphere (atm)pressure. Byconvention,theSHEisassignedapotentialof0.00V,andthenallotherhalf‐cellpotentialsaremeasuredrelativetothatassignedvalue.

IntheMFC,electronsaregeneratedattheanodeasglucoseisoxidizedthroughaprocesscalledglycolysis(seeChapter2–Step1–Glycolysis). Theelectronsarethenshuttledtothe anodewith the help of a chemical redoxmediator calledmethylene blue. Copperwires, connected to the electrodes by alligator clips, conduct the electrons, potentiallythrougharesistor(LED),tothecathode.Inthecathodechamber,electronsaredischargedfrom the cathode to potassium ferricyanide, or, more specifically, the ferricyanide ion,Fe(CN)6

–3, is reduced to the ferrous cyanide ion, Fe(CN)6–4. In our MFC, the circuit is

closed, not by a salt bridge, but rather by semi‐porous, ion‐exchange membranes thatseparatetheanodeandcathodechambers.Ifthepotentialdifferencebetweentheanodeandcathodechambersislargeenough(over0.45Vrequired),theMFCcanpowertheLED.

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H2(g)→2H+(aq)+2e– Eo=0.00V (SHEasanode) 2H+(aq)+2e–→H2(g) Eo=0.00V (SHEascathode)

TheCopper–SHECell

To determine the half‐cell potential for the reduction of copper, a voltaic cell is constructedusing a copper electrode and the standard hydrogen electrode. As the half‐cell reactionsensue,thecopperelectrodedecreasesinmassandtheconcentrationofCu2+ionsdecreasesinthe solution around the copper electrode. Simultaneously, gaseous hydrogen decreases inmass and theH+ ion concentration increases in the solution of the SHE. The total electricalpotential for this cell has been experimentally determined to be +0.34 V at the start of thereaction.

(anode) H2→2H++2e– Eo=0.00V(cathode) Cu2+→2e–+Cu Eo=?

(cellreaction)H2+Cu2+→2H++Cu Eo=0.34V

Thus,thehalf‐cellpotentialforthereductionofcopperis

Cu2++2e–→CuEo=+0.34V

TheIron–CopperCell

As described earlier, in this cell, copper deposits on one electrode as the iron electrodedecreasesinmass.Thetotalelectricalpotentialofthiscellhasbeencalculatedtobe+0.78V.

Fe→Fe2++2e– Eo=?Cu2++2e–→Cu Eo=+0.34VFe+Cu2+→Fe2++Cu Eo=+0.78V

Thus,thehalf‐cellpotentialfortheoxidationofironis

Fe→Fe2++2e–Eo=+0.44V

Thereactionforthehalf‐cellisalwayswrittenasareduction(gainofelectrons)

Zn2++2e–→ZnEo=–0.76V

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By measuring the potentials of other standard electrodes versus the SHE or some otherstandardhalf‐cellwhoseelectrodepotentialisknown,aseriesofstandardelectrodepotentialscanbeestablished.Whentheelectrodesinvolvemetalsornonmetalsthatareincontactwiththeir ions, the resulting series is called the electromotive series or activity series of theelements(seethetablebelow).Thosespeciesatthetopoftheseriesandontherightsideofthe reduction half‐reaction are the most active and possess the greatest tendency to beoxidizedorloseelectrons(thebestreducingagents).Thespeciesatthebottomoftheseriesonthe left side of the equation undergo reduction easily to gain electrons (the best oxidizingagents). Any specieson the left sideofagivenhalf‐reactionwill react spontaneouslywithasubstance that is on the right side in half‐reaction above it, and the voltaic cell will have apositivepotential.

StandardElectrodePotentialsat25oC

TheElectromotiveSeriesElectrodeReaction Eo

Li+(aq)+e–→Li(s) –3.05VK+(aq)+e–→K(s) –2.93VNa+(aq)+e–→Na(s) –2.71VMg2+(aq)+2e–→Mg(s) –2.38VAl3+(aq)+3e–→Al(s) –1.66VZn2+(aq)+2e–→Zn(s) –0.76VCr3+(aq)+3e–→Cr(s) –0.74VFe2+(aq)+2e–→Fe(s) –0.44VCd2+(aq)+2e–→Cd(s) –0.40VNi2+(aq)+2e–→Ni(s) –0.25VSn2+(aq)+2e–→Sn(s) –0.14VPb2+(aq)+2e–→Pb(s) –0.13V2H+(aq)+2e–→H2(s) 0.00VCu2+(aq)+2e–→Cu(s) +0.34VI2(s)+2e

–→2I–(aq) +0.54VFe3+(aq)+e–→ Fe2+(aq) +0.77VHg2+(aq)+2e

–→Hg(l) +0.79VAg+(aq)+e–→Ag(s) +0.80VBr2(l)+2e

–→2Br–(aq) +1.07VCl2(g)+2e

–→2Cl–(aq) +1.36VAu3+(aq)+3e–→Au(s) +1.50VF2(g)+2e

–→2F–(aq) +2.87V

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Problem:Whatisthespontaneouselectrochemicalreactionthatoccurswhenastandardcopperhalf‐celliscombinedwithastandardsilverhalf‐cellandwhatisthevalueofEoforthisvoltaiccell?

FromthetableweknowthatCu2++2e–→CuEo=+0.337VAg++e–→AgEo=+0.800V

Combiningbothhalf‐cells,wehavethefollowingequationtogivetheanswerCu→Cu2++2e–Eo=–0.337V2Ag++2e–→2AgEo=+0.800VCu+2Ag+→Cu2++2AgEo=+0.463V

Notethatdoublingthesilverhalf‐cellreactionbalancestheelectrons,butdoesnotaffectthevalueofEoforthehalf‐reaction.

NON­STANDARDSTATEREDUCTIONPOTENTIALSTheNernstequationwasderivedtoobtainelectrodepotentialsforconcentrationsandpartialpressuresotherthanthestandardstatevalues.

Ecell=Eo‐2.303(RT/nF)logQ

Ecell=theelectrodepotentialundernonstandardconditionsEo=thestandardelectrodepotentialR=thegasconstant,8.314J/moleKT=theabsolutetemperature,inKn=thenumberofmolesofelectronstransferredpermoleofcellreactionF=theFaradayconstant,96485J/Vmolee–Q=thereactionquotient(concentrationofproductsdividedbyconcentrationofreagents)

PuttingthevaluesgivenaboveintotheNernstequationat298K(25oC)simplifiestheequationtoEcell=E

o–(0.0592/n)logQ.

Problem:CalculatethereductionpotentialfortheFe3+/Fe2+electrodeiftheconcentrationofFe2+isfivetimeslargerthanthatofFe3+.

Fromatableofstandardreductionpotentials,weknowthatFe3++e–→Fe2+Eo=+0.771V

UsingtheNernstequation,wecansolvethealgorithmtofindtheanswerEcell=E

o–(0.0592/n)logQEcell=+0.771V–(0.0592/1)log5=+0.771V–0.0592(0.699)Ecell=+0.771V‐0.041V=+0.730V

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Problem: A voltaic cell is prepared by combining the Fe3+/Fe2+ couplewith theMnO4–

/Mn2+couple. Inonecompartmentof thecell, [Fe3+]=1.00M and [Fe2+]=0.100M. In theothercompartment,[MnO4

–]=1.00x10–2M,[Mn2+]=1.00x10–4,and[H+]=1.00x10–3M.ThecellreactionisMnO4

–+8H++5Fe2+→Mn2++4H2O+5Fe3+,withEo=+0.74V.Whatisthe

cellpotential,E,forthecell?

Ecell=Eo–(0.0592/n)log[Mn2+][Fe3+]5/[MnO4

–][H+]8[Fe2+]5Ecell=+0.74–(0.0592/5)log(1.00x10

–4)(1.00)5/(1.00x10–2)(1.00x10–3)8(1.00x10–1)5Ecell=+0.74–(0.0592/5)log(1.00x10

27)=+0.74V–0.32VEcell=+0.42V

Problem:Thenickel‐leadvoltaiccellundergoestheelectrochemicalreactionNi+Pb2+→Ni2++PbwithEo=+0.124V. If theconcentrationofPb2+ in thecell is1.00Mand thecellpotential,E,is0.183V,whatistheconcentrationofNi2+inthecell?

Ecell=Eo–(0.0592/n)log[Ni2+]/[Pb2+]

0.183=0.124–(0.0592/2)log[Ni2+]/(1.00)0.059=–0.0296log[Ni2+]/(1.00)0.059/(–0.0296)=–2.0=log[Ni2+]/(1.00)[Ni2+]/(1.00)=1.0x10–2[Ni2+]=1.0x10–2M

ThevoltagegeneratedbyanMFC(andtheresultingpoweroutput)isfarmorecomplicatedtopredict or calculate than thatof a standard voltaic cell or even a chemical fuel cell. In anMFC, the bacteria colonizing the anode chambermust grow andmanufacture enzymes orstructures capable of transferring electrons outside the cell (see Chapter 3 –MediatorlessMFCs). In amixed culture, different bacteria can grow at different rates, setting differentpotentials.Additionally,themaximumvoltagesthatcanbegeneratedbytheMFCarebasedon the specific electrochemical relationships between the electrondonors (substrates) andacceptors(oxidizers).As noted previously, the open circuit voltage, OCV, measured for an MFC represents themaximumvoltagethatcanbeobtainedwiththesystem.TheOCVissubjecttothelimitationsimposed by the specific bacterial community and the inherent electrochemical restrictionsdefinedby the electron donor and acceptors. For anMFC, aswith any power source, theobjective is tomaximize power output and, therefore, obtain the highest current possibleunder the conditionsofmaximumvoltage. TheOCV isonlyachievedundera conditionofinfiniteresistance.Whentheresistanceisreduced,thevoltagedropsproportionately.Thus,tomaximize the power production, the key is to determine the smallest possible drop involtageasthecurrentisincreasedoveraspecificrangeofcurrentinterest.To achieve this, a polarization curve is generated to characterize current as a function ofvoltage (Figure 1.4). By changing the circuit external resistance (load), a new voltage isobtained, and consequently, a new current at that resistance. Therefore, to obtain apolarization curve, a series of different resistance on the circuit is used and the voltage

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measured at each resistance. The current is calculated using I = E /Rext and the voltageversusthecurrentplottedtogeneratethecurve.ThepolarizationcurveindicateshowwelltheMFCmaintainsitsvoltageasafunctionofthecurrentproduction.Itshouldbenotedthatapolarizationcurveisveryeasilygeneratedusingsimpleresistorsthatcan be purchased at a retail electronics supply store, such as Radio Shack, and thenconnectingtheresistorstothecircuitoftheMFCassembly.

ApowerdensitycurveisthencalculatedfromthemeasuredvoltageasP=E2MFC/Rextor,alternatively, asP= I2 Rext. MFC researchers typically use the top of the power curve toreportthe“maximumpower”capableofbeinggeneratedbyanMFC.

Figure1.4. (A)Thecellpotential(voltage)wasplottedversuscurrentdensity to give the polarization curve, and thenmultipliedwith eachother(B)toobtainthepowerdensitycurve(P=E*I).Maximumpowerisindicatedas1.0mWcm–2.

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Activity1.6.A–TheVirtualVoltaicCell

Materials:copiesofstudentworksheet(downloadablefromsite),onePCperpairofstudentsProcedure:

1. Ifyouhavethecapability,thetutorialcanbeshowntotheentireclassandtimegivenforclassdiscussion.

2. Directstudentstothefollowingwebsite:www.blackgold.ab.ca/ict/Division4/Science/Div.%204/Voltaic%20Cells/Voltaic.htm

3. Uponcompletionofthesimulation,studentsshouldcompletethestudentworksheetincludedinthewebsite.

Activity1.6.B–VoltaicCellsLab

IntroductionAn electrochemical reaction is a chemical reaction that involves reduction and oxidation (aredox reaction). The energy released in a spontaneous redox reaction can be used to doelectricalwork. This is accomplished using a voltaic (or galvanic) cell, a device inwhich thetransfer of electrons occurs through an external pathway rather than directly betweenreactants.Aspontaneousredoxreactiontakesplacewhenastripofzincisputintoasolutioncontainingcoppersulfate.ThereactionisZn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s).Carryingoutthereactioninthisway,however,willnotpermittheobtainingofusefulelectricalenergy.Instead,azincstripisplacedintoasolutionofzincsulfateinonecontainer.Acopperstripisplacedintoa solutionofcoppersulfate inanothercontainer. Anelectrical conductingwire is connected

Activity1.6–DemonstrationofaVoltaicCell:Option1.6.A(simulation)–Afterviewingavoltaiccellanimation,studentswilluseaflashapplettosimulatevariousmetal/electrolytecombinationstocreateavoltaiccellwiththegreatestcellpotential.Option1.6.B(wetlab)–Studentswillconstructandevaluatethepotentialofseveralvoltaic cells and compare their values to the accepted values found in a chemistrytextbook.

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between the zinc and copper strips. A salt bridge containing an electrolyte capable ofmaintainingchargebalanceineachsolutionisplacedbetweenthetwosolutions.Avoltmetercanbeplacedinthecircuittomeasurethepotentialgeneratedinthereaction.ZincisoxidizedtoZn2+inonecompartment.Thezincstripiscalledtheanode.Cu2+isreducedtocopperintheothercompartment. Thecopperstrip iscalledthecathode. TheelectronslostbythezinctoformZn2+travelfromtheanodethroughtheexternalcircuittothecathodeandarepickedupbyCu2+ionsastheyformcopperatomsontheelectrode.Adiagramofthisvoltaiccellisshowninthefigurebelow: Theelectrochemicalreactioncanberepresentedastwohalf‐reactions: Zn(s)→Zn2+(aq)+2e‐ anode Cu2+(aq)+2e‐→Cu(s) cathode

Zn(s)+Cu2+(aq)→Zn2+(aq)+Cu(s)overallreactionIfallthecomponentsofthecellareintheirstandardstates(with[Zn2+]=1.0Mand[Cu2+]=1.0M), thecell iscalledastandardcell. Thevoltagegeneratedbythestandardcell iscalledthestandardcellpotential(E°).Itisconvenienttohaveashorthandmechanismfordesignatingvoltaiccells.Thestandardzinc‐coppercellcanbewrittenas:

Zn(s)|Zn2+(1.0M)||Cu2+(1.0M)|Cu(s)In this designation, the anode, or oxidation half‐cell, is represented on the left side and thecathode,orreductionhalf‐cell,isrepresentedontherightside.Theelectrodesinthetwohalf‐reactionsareelectricallyconnectedusingasaltbridgerepresentedbytwovertical lines. The

40

cellelectrodesarerepresentedatthetwoendsinthisnotation.Asingleverticallineshowsthephaseboundarybetweenthesolidelectrodeandtheelectrolytesolutionineachhalf‐cell.Pre‐LabProblem:WhatisthecellpotentialinaZn/Cucellthathasa[Zn2+]of0.10MintheZnhalf‐cellanda[Cu2+]of0.010MintheCuhalf‐cell?Materials:

Filterpaperdisks DropperbottlesVoltmeter Stripsofthefollowingmetals:copper,lead,silver,andzinc1.0 Msolutionsofthefollowingsalts:CuSO4,Pb(NO3)2,AgNO3,ZnSO4,NaNO30.010Msolutionsofthefollowingsalts:CuSO4,Pb(NO3)2,AgNO3

I. StandardCells

Six standard voltaic cells will be set up and the potential generated by each will bemeasured.However,insteadofsettingupthehalf‐cellsinbeakers,theywillbesetuponapieceoffilterpaper.Thehalf‐cellsareestablishedbyplacingthreedropsof1.0Msolutionscontainingtherequiredmetal ionsatfourspotsaroundthefilterpaper,andthensettingapieceoftheappropriatemetalincontactwiththesolution.Twodropsof1.0MNaNO3areplacedonthecenterofthefilterpapertoserveasasaltbridge.Procedure:Obtainadiskoffilterpaper.Placethreedropsof1.0MCuSO4solutionatthetopofthefilter paper (at 12 o’clock). Put a strip of copper in contactwith the CuSO4 solution.Placethreedropsof1.0MPb(NO3)2solutionontherightsideofthefilterpaper(at3o’clock).PutapieceofleadincontactwiththePb(NO3)2solution.Transferthreedropsof1.0MAgNO3solutiontothebottomofthefilterpaper(at6o’clock).PutapieceofsilverincontactwiththeAgNO3solution.Transferthreedropsof1.0MZnSO4solutiontotheleftsideofthefilterpaper(at9o’clock).PutazincstripincontactwiththeZnSO4solution.Puttwodropsof1.0MNaNO3inthecenterofthefilterpapertoserveasthesaltbridge.Thearrangementwillbeasrepresentedinthefollowingfigure:

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Turnthevoltmeterontothe2voltsDCrange.PlacetheleadsofthevoltmetersuccessivelyontheCuandPbstrips,CuandAgstrips,CuandZnstrips,PbandAgstrips,PbandZnstrips,and,finally,AgandZnstripstomeasurethepotentialsoftheCu/Pb,Cu/Ag,Cu/Zn,Pb/Ag,Pb/Zn,andAg/Znstandardvoltaiccells,respectively.Ifyougetanegativepotential,switchtheleadsonthemetalssothatyougetapositivepotential.Themetalthatyouconnecttotheblackleadofthevoltmetertogetapositivereadingistheanodeofthecell.Recordthepotentialsforeachofthecellsonthedatasheet,andcomparethemtotheliteraturevalues.

II. NonstandardCells

Ifoneormoreofthecomponentsofavoltaiccellisnotinitsstandardstate,thecellisnotastandardcell.ThepotentialthatisexpectedtobegeneratedinsuchacellcanbecalculatedfromtheNernstequation,E=E°‐0.0592logQ.

n

Inthisequation,E°isthestandardcellpotential,Eisthecellpotential,nisthenumberofelectronstransferredinthereaction,andQisthereactionquotient.Procedure:Obtainafilterpaperdiskandputthreedropsof1.0MAgNO3atthetop(at12o’clock).PutapieceofsilverincontactwiththeAgNO3solution.Transferthreedropsof0.010MCuSO4 to the right sideof the filterpaper (at3o’clock). Seta stripof copperon theCuSO4solution. Place threedropsof0.010MPb(NO3)2 solutionat thebottomof thefilterpaper(at6o’clock).PutapieceofleadincontactwiththePb(NO3)2solution.Puttwodropsof1.0MNaNO3solutioninthecenterofthefilterpaper.PresstheleadsofthevoltmeterontothemetalstripsandmeasurethepotentialsoftheAg/CuandAg/Pbnonstandard voltaic cells. Record the values of the potentials on the data sheet.CalculatevaluesforthepotentialsexpectedusingtheNernstequation.

III. ConcentrationCell

Aconcentrationcellisavoltaiccellinwhichtheanodeandcathodecompartmentsarethesameexceptthattheconcentrationsofspeciespresentaredifferent.Forexample,a silver concentration cell contains a silver electrode in contact with a silver nitratesolution of a certain concentration in one compartment, and a silver electrode incontact with a silver nitrate solution of a different concentration in the othercompartment.Thehalf‐cellpotentialgeneratedineachofthehalf‐cellsisgivenbytheNernstequation,E=E°Ag‐0.0592log[Ag

+]. n

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Thepotentialoftheconcentrationcellisthedifferencebetweenthetwohalf‐cellpotentials,

Ecell=E°Ag–(0.0592/1)log[Ag+]concentrated–(E°Ag–(0.0592/1)log[Ag

+]diluted),or

Ecell=0.0592log[Ag+]concentrated/[Ag

+]dilutedProcedure:Useadiskof filterpaper,andputthreedropsof1.0MAgNO3on it. Placeapieceofsilverincontactwiththe1.0MAgNO3solution.Transferthreedropsof0.010MAgNO3to a different spot on the filter paper. Put a piece of silver on the 0.010M AgNO3solution. Addtwodropsof1.0MNaNO3betweentheothertwosolutions. Presstheleadsof the voltmeter against the two silver strips, andmeasure thepotential of thecell.Recordthevalueofthepotentialonthedatasheet.CalculatetheexpectedvaluesofthepotentialfromtheNernstequation.

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Activity1.6.B–StudentDataSheet

I. StandardCells

VoltaicCellExperimentalPotential(Volts)

LiteratureValue(Volts)

Pb(s)|Pb2+(1.0M)||Cu2+(1.0M)|Cu(s)

Cu(s)|Cu2+(1.0M)||Ag+(1.0M)|Ag(s)

Zn(s)|Zn2+(1.0M)||Cu2+(1.0M)|Cu(s)

Pb(s)|Pb2+(1.0M)||Ag+(1.0M)|Ag(s)

Zn(s)|Zn2+(1.0M)||Pb2+(1.0M)|Pb(s)

Zn(s)|Zn2+(1.0M)||Ag+(1.0M)|Ag(s)

II. NonstandardCells

VoltaicCellExperimentalPotential(Volts)

LiteratureValue(Volts)

Cu(s)|Cu2+(0.010M)||Ag+(1.0M)|Ag(s)

Pb(s)|Pb2+(0.010M)||Ag+(1.0M)Ag(s)

III.ConcentrationCell

VoltaicCellExperimentalPotential(Volts)

LiteratureValue(Volts)

Ag(s)|Ag+(1.0M)||Ag+(0.010M)|Ag(s)

Post­LabQuestions:1. Whywouldyouexpectnopotentialtobegeneratedbetweentwohalf‐cells inthis

experimentuntil theNaNO3solutionmigratestoapointwhere itoverlapsbothofthehalf‐cellsolutions?

2. Discusssomepossiblesourcesoferrorthatcouldaccountforthedifferencesintheexperimentalvaluesofthecellpotentialsyouobtainedandtheliteraturevalues.

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Chapter 2: How do organisms obtainenergy?orWhydon’tIrunonbatteries?

METABOLISMASAREDOXPROCESS

Burningafuelisanoxidativeprocessandtheformationofafuelisareductiveprocess.Theseprocessesareperpetuallylinked.Thereisalwaysadonorandanacceptor.Asonesubstancebecomes oxidized, the other substance is reduced and vice versa. The common featurebetween the two processes is the transfer of electrons. (In biology classes, the transfer ofelectrons,as itoccurs in livingcells, isalmostalways referred toaselectron transport.) Thesubstance that is being reduced accepts electrons donated by the substance that is beingoxidized.Althoughtheactofgaininganelectron,whichiscalledareduction,maybeconfusingat first, the terminology starts to make more sense when we remember that electrons arenegatively charged. Reduction implies becoming less and, obviously, accepting somethingnegativecanbeinterpretedasadecrease.

Respiration, whether carried out by humans or plants, is essentially the same process of“burning” or oxidizing organic compounds. The carbon containing compounds burned inrespirationaremostlikelytobesugars. Thecombustionismorecontrolled(byenzymesthatkeepthereactionsgoingatnondestructivetemperatures),butenergyisproducedandcarbondioxideisliberatedjustasitwouldbeifsugarwereburnedonafire.

Somebiologicaloxidationsaresimilartotraditionalcombustionreactions inthatthey involvethe direct addition of oxygen, but more often in living organisms, oxidation occurs by theremovalofhydrogens.Becausehydrogeniscomposedofaproton(i.e.,ahydrogenion,H+)andanelectron,e–,thetransferofhydrogensalwaysinvolvesthetransferofelectrons.

H→H++e–

(atom)(proton)(electron)Inmetabolism,thereareoftenlongsequences,orchains,ofhydrogen(orelectron)acceptors.Thisisthereasonwhyburningsugarinrespiration,althoughiteventuallyinvolvestheadditionofoxygen,doesnotliberateenergyquiteasdramaticallyasitdoesifsugarisburnedonafire.Instead,energy isreleasedinatrickleratherthanaflood. Inaddition,muchoftheenergy isnot released at all, immediately, but is conserved, as chemical energy, in “high‐energy”substances,suchasadenosinetriphosphateorATP.

ThecharacteristicsofATPmake itexceptionallyusefulas thebasicenergysourceofall cells.The three phosphate groups of ATP are the key to its ability to store and release energy.Wheneverthechemicalbondbetweenthesecondandthirdphosphategroupisbroken,energyisreleased,fuelingavarietyofcellularactivities.Theresultingadenosinediphosphate(ADP)is

45

identicaltoATPexceptithastwophosphategroupsinsteadofthree. Whenthepathwaysofcellularmetabolismcreatethosesmallamountsofenergy,alittleatatime,itcanbestoredbyaddingaphosphategrouptoADPmolecules,producingATP.

Activity2.1–FruitSnackTerminator

MATERIALS

Onesmallpackageoffruitsnacks(anyvarietywillwork),ringstandandclamp,onemediumsizedpyrextesttube,around2.5gofsolidpotassiumchlorate(KClO3),safetyshieldandgoggles,Bunsenburner&striker,tongsorlongforceps

PROCEDURE

Fillthetesttubetoadepthofaboutoneinchwithpotassiumchlorate.Clampthetesttubeinplace at an approximately 45° angle, directed away from any person. Set up the ring stand‐clamp‐testtubeassemblybehindasafetyshieldinfrontoftheclass.Connecttheburnersothatthetesttubecanbeheatedeasily.

Lighttheburnerandheatthetesttubeatthebottomuntilthesolidmelts(m.p.isaround350°C). Standbehind the safety shieldandcarefullydropone fruit snack into the test tubeusingtongsorforceps.Aviolentflame‐shootingreactionensuesandlastsforaboutoneminute.

DISCUSSION

The fruit snack ismostly sugar,which is easily oxidized by something likemolten potassiumchlorate. Ideally, a balanced equation would show sucrose (C12H22O11) being converted tocarbondioxideandwaterwhiletheKClO3becomesKCl.Theactualreactiondoesnotseemtogo to totalcompletionsince there isusuallya littlegunky residue leftbehind fromgelatinoragaradditives.

Activity2.1(Demonstration)–FruitSnackTerminatorStudentswillobservetheactionofastrongoxidizer,KClO3,onsugarsandgainaveryvisualappreciationfortheamountofenergythatcanbeliberatedfromsugars.

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HAZARDS

MoltenKClO3cancauseverysevereburns.Exercisegoodsafetytechniquewhilepresentingthisdemonstration.Thereisalsoalotofsmokeproducedduringtheoxidation,sothisexperimentshouldonlybedoneinaroomwithgoodventilation(becarefulinroomswithsmokedetectors).

THEROLEOFGLUCOSEINMETABOLISM

We,andallother living creatures, requirea continuous sourceof chemicalenergy to remainalive. This is the reason for eating:We take in highly orderedmolecules that have high freeenergy,andejectdisorderedmoleculeswithlowfreeenergy.

All nutrition is based on onemolecule, glucose (C6H12O6). Evenmore remarkable, all life onEarth uses the samemetabolicmachinery to extract free energy from glucose ‐ not just thesameoverallreactions,butthesamesteps,thesameintermediates,andthesamecontrollingenzymes.Theonlydifferenceisthatnoteveryorganismusestheentirescheme.

THESTAGESOFMETABOLISM

Metabolismisalwaysinitiatedbythepathwaycalledglycolysis.Glycolysisreleasesonlyasmallamountofenergy. Ifoxygenispresent,glycolysis leadstotwootherpathwaysthatreleaseagreatdealmoreenergy. Ifoxygen isnotpresent,however,glycolysis is followedbydifferentpathways.

METABOLISMWITHOXYGEN=CELLULARRESPIRATION

Inthepresenceofoxygen,glycolysisisfollowedbytheKreb’scycleandtheelectrontransportchain. When linked together, thesepathwaysmakeup a process called cellular respiration.

Itcannotbeemphasizedtoostronglythatthischapterisnotintendedtobeanexerciseinmemorization. The focus is the pathways of energy flow that living organisms (andspecifically microorganisms) use to stay alive. It is far less important that studentsrememberhow towrite the conversionofonemolecule intoanother, than for them tolook at the two molecules, understand what happened between one and other, andrecognizehowenergywasliberated.Ourpurposeistoindicatehowaseriesofchemicalreactions can be said to have a strategy. The specific strategy utilized specifies thedirection inwhichelectronswill travel. In theMFC,wecanexploit thesestrategiesanddirecttheflowofelectrons(energy)forourpurposes.

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Cellularrespirationistheoxidationofglucose(C6H12O6)toCO2andthereductionofoxygentowater.Thesummaryequationforcellrespirationis:

C6H12O6+6O2→6CO2+6H2O+energy

Donotbemisledbythesimplicityofthisequation.Ifcellularrespirationtookplaceinjustonestep,alloftheenergyfromglucosewouldbereleasedatonce,andmostofitwouldbeintheformoflightandheat(justlikeinthefruitsnackdemo).Thekeyforalivingcell istocontrolthat energy. It cannot simply start a fire, butmust release theexplosive chemical energy insugarsalittlebitatatime.ThecellthenneedstotrapthoselittlebitsofenergybyusingthemtoproduceATP,the"molecularcurrency"ofintracellularenergytransfer.

­­QuestionsforReview­­ Whatiscellularrespiration?

Cellular respiration is the breakdown of food molecules, such as sugars, intosimplercompoundswithareleaseofenergy.

Howoftendoescellularrespirationoccurincells?

Cellularrespirationoccurscontinuouslyinlivingcells.Ifcellularrespirationstops,acelldies.

Createapictureorposterthatshows(a)thewordequationforthecellularrespiration,(b)thechemicalformula,and(c)anon‐linguisticrepresentationoftheprocess.

C6H12O6+6O2→6CO2+6H2O+energyglucose+oxygen→carbondioxide+water+energy

STEP1–GLYCOLYSIS

Thefirststepintheoverallschemeofenergyextractioninmostorganismsisglycolysis.Inthefirst step, one glucosemolecule is degraded (essentially broken in half) to twomolecules ofpyruvicacid(CH3‐CO‐COOH),a3‐carboncompound,withtheproductionofrelativelylittleATP.

Althoughglycolysis is anenergy releasingprocess, the cell needs to input a small amountofenergytogettheprocessstarted.Atthebeginningofthepathway,twomoleculesofATPareusedup. In away, theprocess of glycolysis is analogous to a savings account at a bank. Apersonhastodepositmoneyintotheaccounttoearninterest,justasacellmustputtwoATPinto the “account” to earn the interest of additional molecules of ATP. When glycolysis iscomplete,fourATPmoleculeshavebeenproducedfromeachmoleculeofglucose.ThisgivesthecellanetgainoftwoATPmolecules.

Inmorecriticalreactionsoftheglycolysispathway,fourhigh‐energyelectronsareremovedandthenpassed toanelectron carrier calledNAD+, ornicotinamideadeninedinucleotide. Each

48

moleculeofNAD+ canacquire twoelectrons; that is,be reducedby twoelectrons.However,only one proton accompanies the reduction. The other proton produced as two hydrogenatomsareremovedfromthemoleculebeingoxidizedisliberatedintothesurroundingmedium.ForNAD+,thereactionisthus:

NAD++2H→NADH+H+

Consequently,theoverallreactionthatoccursduringtheprocessofglycolysisisasfollows:

C6H12O6+2NAD+→2C3H4O3+2NADH+2H

++2ATP

­­QuestionsforReview­­ What is the source of the energy used by the cell to split the glucose molecule intopyruvate?

TwoATPmoleculesprovidetheenergy.

Howmanypyruvatemoleculesareproducedfromeachglucosemolecule?Two

WhatisaddedwhenNAD+becomesNADH?

Electrons

Whataretheendproductsofglycolysis?Pyruvateandenergy

PGAL=3‐carbonintermediate;(Glyceraldehyde3‐phosphate)

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STEPS2&3–KREB’SCYCLEANDTHEELECTRONTRANSPORTCHAIN

The second step in themachinery of cellular respiration ismuchmore efficient in extractingenergy. Pyruvic acid (orpyruvate) enters theKreb’s cycle, orcitric acid cycle (because citricacidistheproductofthefirstreaction),whereitisbrokendowntoCO2,withhydrogenatomsbeing used to reduceNAD+ toNADH. Some additional ATP also ismade along theway. TheNADHfromthecitricacidcycleflowsintothethirdprocess,theelectrontransportchain.HereNADHisreoxidizedtoNAD+andisrecycled.Thehydrogenatomsaretranslocatedoverthecellmembrane from "inside" to "outside", establishing a concentration gradient across themembrane, which temporarily stores the energy released in the chemical reactions. Thispotentialenergy(thefreeenergythatisliberated)isstoredintheformofATP.Thehydrogenatoms ultimately are added to O2 to make water. The overall process ‐ the combustion ofglucosewithoxygen‐carriedoutintheseseriesofsmallstepssothatthemaximumamountofenergyfromthereactioncanbesaved–yields38moleculesofATPpermoleculeofglucose.

AEROBICVS.ANAEROBICMETABOLISM

Atthispointitbecomesnecessarytoclearupafewmisconceptionsandterminologyerrorsthat frequently occur. It is not uncommon for the terms anaerobic respiration andfermentationtobeusedinterchangeably,butthisisincorrect.Fermentationisthemetabolicprocess that occurs in microorganisms (and occasionally in human muscle tissue) in theabsenceofoxygen.Duringfermentation,thepyruvategeneratedbyglycolysisgoesthrougha series of enzymatic transformations to form reducedmetabolites, such as lactic acid orethanol. During anaerobic respiration, glycolysis is followed by the Kreb’s cycle and theelectron transport chain, just as in normal aerobic respiration, however, the final electronacceptorisamoleculeotherthanoxygen.

Anaerobicrespirationisakeyelementtotheunderstandingandthefunctioningofpracticalapplicationmicrobial fuel cells (see Chapter 2 –Anaerobic Respiration). Fermentation is acriticalcomponentofoursimpleexperimentalMFC.

Under nonstrenuous conditions, when the oxygen supply is ample, glucose is degraded topyruvateduringglycolysis,andpyruvateisbrokendowntoCO2intheKreb’scycleandelectronstransferredtooxygenintherespirationchain,withayieldof38moleculesofATPpermoleculeofglucose.However,duringvigorousexercisewhereO2demandisveryhighinthemuscles,aninteresting reaction called fermentation is responsible for allowing our muscles to keepworkinghardevenwhenoxygenstartstorunlow.

As the cell supplies large amounts of ATP (energy) from glycolysis to muscle cells duringexercise,itrunsintoaproblem.Injustafewseconds,allofthecell’savailableNAD+moleculesarefilledupwithelectrons(becomingNADH).TheNADHcannotbere‐oxidizedbacktoNAD+fast enough since the oxygen,which functions as the final electron acceptor in the electrontransport chain, is limited. Without NAD+, the cell cannot keep glycolysis going, and ATPproductionstops.Duringfermentationinhumanmuscle,insteadofenteringtheKreb’scycle,

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thepyruvategeneratedbyglycolysis is reducedto lactate. Thisactionconvertsall theNADHbackintotheelectroncarrierNAD+,allowingglycolysistocontinue:

Thisisaredoxreactionthatconsistsoftwohalfreactions:

(1) pyruvate+2H++2e–→lactate pyruvategainse–(isreduced)

(2) NADH+H+→NAD++2H++2e– NADHlosese–(isoxidized)

TheNAD+thatisgeneratedisthenavailabletobeusedinglycolysisagaintogeneratemoreATPforthemuscles.The lactate(lacticacid)generatedbythisreactioncausesacidificationoftheblood(loweredpH),whichisbelievedtoberesponsibleforthe"burn"thatyoufeelinmusclesthat youworked toohard. Because this fermentationprocessdoesnot requireoxygen, it issaidtobeanaerobic.

ADETAILEDLOOKATFERMENTATION

Itispossible(orevenroutine)forotherorganismstoalsoproduceenergywithoutusingoxygento“burn”glucose.In1861,LouisPasteurdiscoveredthatyeastcellsarecapableofconvertingglucosetocarbondioxideandethylalcohol(ethanol).Thisreactionyieldsonlyabout1/20theamount of energy released when glucose is metabolized all the way to carbon dioxide andwater through the use of oxygen (aerobic respiration). This is not a significant problem,however,asyeastdonothaveashighanenergyrequirementasmorecomplexorganisms.Thefoodindustrymakesuseofthis(mostdelightful)reactiontoproduceavarietyofconsumable(andquitetasty)products.

Ethanol fermentation (performedbyyeastandsome typesofbacteria)breaksthe pyruvate down into ethanol and carbon dioxide. It is important in bread‐making,brewing,andwine‐making.Usuallyonlyoneoftheproductsisdesired;inbreadthealcoholisbakedout,andinalcoholproductionthecarbondioxideisreleasedintotheatmosphere.

pyruvic acid 2CH3—C—COOH + 2NADH + 2H+

O

2CH3—CH—COOH + 2NAD+

OH

lactic acid

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Lacticacidfermentationbreaksdownthepyruvateintolacticacid.Aspreviouslydiscussed,itoccursinthemusclesofanimalswhentheyneedenergyfasterthanthebloodcansupplyoxygen.However,italsooccursinsomebacteriaandsomefungi. It is this type of bacteria that convert lactose into lactic acid in yogurt,givingititssourtaste.

Ethanoland lacticacidaretypicalexamplesof fermentationproducts. However,moreexoticcompoundscanbeproducedby fermentation, suchasmolecularhydrogen,butyricacid, andacetone. For livingcells, theproductsproducedby fermentationareactuallywasteproductscreatedduringthereductionofpyruvatetoregenerateNAD+intheabsenceofoxygen.In our experimental MFC, we will use the “waste” products of anaerobic bacterialfermentation as a source of electrons. Molecular hydrogen will be oxidized to generateelectronsandprotons(H+)inthefollowingreaction:

H2→2H++2e–

The electrons are subsequently transported to the anode and dropped into the electricalcircuitbytheactionofasolubleelectrontransportmediatorcalledmethyleneblue.

Humans can use anaerobic respiration and fermentation in much the same way yeast do,includingusing the samepathways. However, unlike yeast that canexcrete alcohol into thesurrounding medium, our cells (if we possessed the same enzymes) would have to excretealcohol intoourbloodstream. Thenwewouldhavetoeliminateitormetabolize itquicklytoavoidbeingcontinuouslydrunk.Instead,ourbodies,lackingthenecessaryenzymestoproducealcohol,followanalternativepathway,whichyieldsasubstancecalledlacticacid.Whilelacticacidislesstoxicthanalcohol,itnonethelesscannotbetoleratedattoohighaconcentration.

Asmuscle activity increases,we needmore energy, somore glucose ismetabolized. Aswebegintouseoxygen(foraerobicrespiration)ataratethatexceedswhatourlungscanprovide,locally inthemusclecellsaswitchtothefermentationpathwayoccursandlacticacidquicklybuildsup.WerapidlybecomefatiguedandourmusclesstarttoacheandburnasthebloodpHlowersinresponsetotheincreasingconcentrationoflacticacid.Iftheproductionoflacticacidwouldcontinue,wewouldsoonbeunabletomoveatall. Aswelieinaheaponthegroundpantingforair,ourheartandlungsworkespeciallyhardtodeliverasmuchoxygenaspossibleto our starvedmuscle cells. At this point, the lactic acid can bemetabolized further by theoxygenthroughtheremainderofthecellularrespirationpathway.Thismeanswegetthefullbenefitofalltheenergyreleasedwhenglucosegoestolacticacidandthentocarbondioxideandwater.

To summarize, glycolysis carried out in both aerobic and anaerobic conditions producespyruvate. The difference is that without oxygen, the pyruvate is turned into lactic acid or

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ethanol,and,whenoxygenispresent,thepyruvatecontinueson(throughtheKreb’scycleandelectrontransport)toeventuallygivecarbondioxideandwater.

­­QuestionsforReview­­ Describe the role of NAD+ (nicotinamide adenine dinucleotide) in the cell. Write achemicalequationforthereductionofNAD+.

ThemainfunctionofNAD+ istoactasanelectroncarrierintheredoxreactionsthat occur in the cell duringmetabolism. It is found in two forms:NAD+ is anoxidizing agent – [it accepts electrons from other molecules and becomesreduced,thisreactionformsNADH,whichcanthenbeusedasareducingagenttodonateelectrons.]

NAD++2H→NADH+H+

Summarizewhathappensduring fermentation, and thenexplain the importanceof theprocess(especiallyintermsofNAD+).

Duringstrenuousexercise(inhumans)orinorganismthatexistunderanaerobicconditions, theoxygendebtmakes it impossible for thepyruvategeneratedbyglycolysis to be fully metabolized to carbon dioxide, water, and ATP.Subsequently,allNAD+moleculesinthecellbecomesaturatedwithelectronsandareconvertedtoNADH.WithouttheaerobiccellularrespirationprocessesoftheKreb’scycleandelectrontransporttorecycleNADHtoNAD+,glycolysisandtheproductionofATPwouldcease.Instead,underanaerobicconditions,organismsmake a switch and follow‐up glycolysis with fermentation. In fermentation,which does not require oxygen, the pyruvic acid is reduced to lactic acid (inhumans)orethanol (inyeastsandsomebacteria),andtheelectronsgeneratedare used to oxidize NADH back to NAD+. Thus, NAD+ is again available forglycolysistocontinue.

Activity2.2–LacticAcidSit­OutChallenge

Findalongwall.Havethestudentslineupagainstthewallandplacetheirbacksagainstthewall.Theythenslidedownthewallintoasittingposition,withtheirthighsparalleltothefloorandtheirshinsparalleltothewall.Theobjectiveofthegameistoseewhocanmaintain that position for the longest time. Soon after being in the position, thestudentsfeeltheburnofthelacticacidaccumulation.

Analyze the results of who had the longest wall time, and analyze what the burningfeelingisandwhyaburningfeelingbecameaproblem.Thewinnersgetacandybar,andthentherecanbemorediscussionaboutwhatwillhappenuponeatingthecandybar.

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Activity2.3.A–FermentationbyYeastDuring the fermentation process by yeast cells, ethanol is produced and carbon dioxide isreleased.Thechemicalformulaforfermentationinyeastis:

Chemicalequation

C6H12O6→2CH3CH2OH+2CO2+2ATP

Wordequation

Sugar(glucose)→alcohol(ethanol)+carbondioxide+energy(ATP)Many of the biochemical reactions in the fermentation of glucose require magnesium ions.Magnesium ions activate several enzymes involved in fermentation. To test the effect ofmagnesium ions on the breakdown of glucose, extramagnesium ionsmay be added to thereaction.Magnesiumionsmayberemovedfromthereactionsolutionbyaddingacompoundcontainingfluoride.Fluoridecausesmagnesiumionstoprecipitatefromsolutions.Studentsshouldformulateahypothesisforthelabbeforeconductingtheexperiment.[Ifextramagnesiumionsareaddedtothereactionsolution,thenfermentationshouldoccuratafaster

rate; if removed, fermentationwill slowdown.] Reviewallsafetyprecautionsassociatedwiththislab.Becarefulwhenusingsodiumfluoride.Itcanbetoxicifingestedorinhaled,andmay irritate the skin. Ifany spilloccursonskinorclothes,itshouldbewashedoffimmediatelywithlarge amounts ofwater. Develop a plan to dispose ofwastematerialsand informstudentsof theplanbeforeexecutingtheexperiment.Organize the class into groups of four students. Workwith the class to design the experimental procedures.Inform the groups that they will have available thefollowingmaterials:five50‐mLbeakers,greasepencilsorsharpies, five fermentation tubes (Figure 2.1 or similargascollectionapparatus),six10‐mLgraduatedcylinders,yeastsuspension*,distilledwater,10%glucosesolution,

Activity2.3.A–FermentationbyYeastStudents will measure CO2 production by fermenting yeast. Then demonstrate howmagnesium ionsandcompounds containing fluorideenhance, inhibit, or slowenzymeactivity.

Figure2.1.Fermentationtube(http://www.enasco.com/product/SA08681M)

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0.01Mmagnesiumsulfatesolution,0.06Msodiumfluoridesolution,0.20Msodiumfluoridesolution,andametricruler.* To prepare the yeast suspension,mix the entire contents of one fresh package of baker’syeastin90mLofdiH2Oand10mLglucosesolution.Then,incubatethesuspensionat37oCfor12hourstodeveloptheyeastcolonies.Guidethestudentssothateachbeakercontainsthefollowingmaterials:

Beaker#1‐10mLyeastsuspension,20mLdistilledwaterBeaker#2‐10mLyeastsuspension,10mLdistilledwater,10mLglucoseBeaker#3‐10mLyeastsuspension,10mLglucose,10mLof0.10MmagnesiumsulfateBeaker#4‐10mLyeastsuspension,10mLglucose,10mLof0.06MsodiumfluorideBeaker#5‐10mLyeastsuspension,10mLglucose,10mLof0.20Msodiumfluoride

Havestudentspourthecontentsofeachbeakerintoafermentationtubenumberedthesameasthebeaker.Afterthegroupshavesetuptheexperiment,allowittorunovernight.Thenextday(Day2),writethefollowingclassdatachartonthechalkboard:

CO2ProductionYeast(mL)Tube1 Tube2 Tube3 Tube4 Tube5

Group#1 Group#2 Group#3 Group#4 Group#5 Group#6

Group#7,etc. Avg.mLof

CO2produced

Ranking 5 2 1 3 4SpecialNote:Theresultsforrankingarebasedontheexpectedoutcomeifthetubescontainthematerialsaslistedaboveineachnumberedbeakerandtransferredtothetubewiththecorrespondingnumber.OnDay2,havestudentsmeasuretheamountofcarbondioxidecollectedusingametricrulerorreadthevolumeofcarbondioxide(inmL)producedifthefermentationtubeisgraduated.Havestudentsusearatingscaleforeachofthetubes.Use#1forthetubethatproducedthegreatestaverageamountofcarbondioxideand#2 for the tubewith the2nd largestaverageamountofcarbondioxide.Continuethisrankingmethoduntilthetubewithnocarbondioxideproducedisrated#5.Theexpectedoutcomebasedonthecontentsofthefermentationtubesas listed above is: Tube three should be #1 with the greatest amount of carbon dioxideproduced,Tube twoshouldbe#2with the2ndgreatest carbondioxideproduced,Tube four

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should be #3, Tube five should be #4, and Tube one should be #5 with no carbon dioxideproduced.Assigneachgroupanumberandhaveamemberofeachgroupwritetheirresultsintheclassdatachart.Havestudentscomparetheirresultswiththoseofothergroupsusingtheclassdatachart.Lookforcommonresponses.Iftheclassdatadonotagree,askstudentstoaccountforanydifferences.Cometoanagreementontherankingforeachtube.AnotherapplicationtothislabwouldbetouseTES‐TAPE(availableatmostpharmaciesfor$3‐$5)tomeasuretherelativeamountofglucoseinthetubesbeforeandafterincubation.TES‐TAPEisaglucosetestingstripusedbydiabeticstomonitortheamountofbloodsugarinurine,but it will also work to visually confirm that the amount of glucose in the tubes has beenreducedovernightastheyeastmetabolizeitintoCO2.ANAEROBICRESPIRATION

Certain bacteria, which thrive in chemically‐rich but nearly oxygen‐free environments, areadaptedtousesubstancesotherthanoxygenasterminalelectronacceptorsduringrespiration.In anaerobic respiration, as the electrons from the electron donor (e.g., sugars and otherorganic compounds) are transported down the electron transport chain to the terminalelectronacceptor,protonsaretranslocatedoverthecellmembranefrom"inside"to"outside",establishing a concentration gradient across the membrane, which temporarily stores theenergyreleasedinthechemicalreactions.ThispotentialenergyisthenconvertedintoATPbythe same enzyme used during aerobic respiration. If given a choice, oxygen is always thepreferred final electronacceptor, however,otherpossibleelectronacceptors (electron sinks)for anaerobic respiration include NO3

–, SO42–, Fe+3, and MnO2. Anaerobic respiration is a

commonoccurrenceinnaturebutisrestrictedtoprokaryoticorganisms(e.g.,bacteria).

The property of specific bacterial genera (e.g., Geobacter and Shewanella) that makes the microbial fuel cell without artificial mediators, such as methylene blue, possible is their ability to perform anaerobic respiration and use an external electron receptor, as opposed to fermentation, which utilizes the internally generated electron acceptor, pyruvate. When no dissolved electron acceptors are available, these bacteria are able to transport electrons to extracellular undissolved electron acceptors, such as some metals and electrodes (i.e., anodes), and thus create an external

Activity2.3.B(Extention)–OtherVariablesinFermentationHavestudentsdesignandcarryoutaninquiryexperimenttotestahypothesisabouttheeffects of sweeteners (instead of, and/or in addition to, glucose) on carbon dioxideproductionbyyeast.Studentsshouldproposeahypothesisabouthowdifferenttypesandamountsofsweetenerswillaffecttheamountofcarbondioxideproductionbyyeast.Haveavailablevarioussweeteners,suchastablesugar,cornsyrup,molasses,honey,fruitjuiceconcentrate, saccharine, aspartame, etc. Remind students to set up a control. Havestudentsanalyzethedataanddrawconclusions.Discusstheoutcomeandresults.

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voltage potential between the solution and the undissolved electron acceptors (i.e., the anode). Another critical adaptation of these bacteria is that in the absence of soluble electron acceptors, they automatically colonize and grow to cover the electrode that is acting as the electron acceptor (see Chapter 3 – Bacterial Biofilms).

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Chapter3:WhatisaMicrobialFuelCell?

PRINCIPLESOFFUELCELLS

Afuelcellisanelectrochemicaldevicecapableofthedirectconversionofchemicalenergyintoelectricalenergy.Itproduceselectricityfromanexternalfuel(ontheanodeside)fromwhichelectronsarewithdrawn,andthentransferredtoanoxidant(onthecathodeside).Thecircuitisclosedbyanionorprotonexchangeconnectionbetweenthefuelandoxidantchamber.Fuelcellsaredifferentfrombatteriesinthattheyconsumereactant,whichmustbeconstantlyfedintothedevice,whereas,batteriesstoreelectricalenergychemically inaclosedsystem.Fuelcellscanoperatevirtuallycontinuouslyas longas thenecessary flowsaremaintained. Manycombinationsoffuelandoxidantarepossibleinafuelcell.Ahydrogenfuelcelluseshydrogenasfuelandoxygenasoxidant(Figure3.1).

Figure3.1.Abasicchemical(orhydrogen)fuelcell

Inessence,afuelcellworksbycatalyticallyseparatingthecomponentelectronsandprotonsofthereactantfuel,andthenbyforcingtheelectronstotravelthroughacircuit,convertingthemtoelectricalpower.Theprotonsmovethroughaseparator(ionconductivemembrane)tothecathodetomaintainelectroneutrality.Thecatalyst is typicallycomprisedofaplatinumgroupmetal,oralloy, thathasbeencoatedontoa carbonorgraphiteelectrode. Another catalyticprocesstakestheelectronsbackin,combiningthemwiththeprotonsandtheoxidanttoformwasteproducts(typicallysimplecompoundslikewaterandcarbondioxide).

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The effectiveness of the electron transfer at the electrode surface, and subsequent fueloxidationattheanodeandthereductionoftheoxidantatthecathodesurface,aredeterminedbytheeffectivenessofthecatalyst.Thedifferencesinthestandardelectrodepotentialsfortheanodicoxidationofthefuelandforthecathodicreductionoftheelectronacceptordeterminethecontentofstoredchemicalenergyinthefuelcell. Thelargerthedifferencebetweentheindividualelectrodepotentials, themoreelectricalenergycanbegenerated (seeChapter1–StandardReductionPotentials).

Todeliverthedesiredamountofenergy,thefuelcellscanbecombinedinseriesandparallelcircuits,whereseriesyieldshighervoltage,andparallelallowsastrongercurrenttobedrawn(seeChapter1–ElectricalCircuits).Suchadesigniscalledafuelcellstack.

Whatisthechiefdifferencebetweenabatteryandafuelcell?

Batterieshaveadefinite fuel stock,whichwilleventuallybecomedepletedandthe battery will be “dead”. Fuel cells can run indefinitely because fuel isconstantlyfedintothedevice.

PRINCIPLESOFBIOFUELCELLSIn a biofuel cell, electrons aremade accessible from a non‐electroactive fuel by the use ofbiocatalysts.Biofuelcellscanbecharacterizedaseitherenzymaticfuelcellsormicrobialfuelcells(MFCs),dependingonthekindofbiocatalystused–enzymesorbacteria,respectively.EnzymaticFuelCellsEnzymatic fuel cells usually employ immobilized enzymes (commonly redox enzymes) ascatalysts to accelerate highly specific reactions. Purely enzymatic fuel cells have the greatadvantage of being very small in scale. Due to the small size of the enzymes and the highspecificityof theanodeandcathode reactions, theyhavehigh turnover ratesand, thus,highpower densities (and a membrane separation of the electrodes is often not necessary).Therefore, they give the possibility to construct low energy power supply units for smallelectricaldevicesaswellaspossiblein‐vivoapplications(e.g.,medicalimplantations).

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MicrobialFuelCellsMicrobial fuel cells (MFCs) are as diverse chemically as the bacteria that power them. In anMFC,theoxidationreactionsoccurinsidethebacteria,andelectronsmustthenbetransferredtotheextracellularanode.Thoughtheyoperateonthesameprinciples,thedesigngoalsoftheMFCaredifferentfromthoseofachemicalfuelcell.Ratherthananon‐renewablesource,suchashydrogengas,MFCsusebiomassasthesubstrate(metabolicoxidation‐reductionreactions)andmicroorganismsasthecatalyst,exploitingwholelivingcellsinanaimtogainenergy. Describetheoxidationandreductionreactionsthatoccurinbacteria.

Bacteria oxidize glucose during the process of glycolysis, producing two, three‐carbonmoleculesofpyruvate.Underaerobicconditions,thepyruvatecontinuesthrough the pathways of the Kreb’s cycle and electron transport to eventuallyyieldH2Oasmolecularoxygen(O2) is reduced. Underanaerobicconditions (noO2),thepyruvateundergoesfermentationtoyieldeitherethanolorlacticacidoranaerobicrespiration(seeChapter2–AerobicRespirationvs.Fermentation).

BIOLOGICALPRINCIPLESOFMFCsIn terms of the chemistry, every biological degradation of organic matter is an oxidationprocess.However,whenwekeepthedegradationanaerobic,wegetthechancetoexploitthisprocess for electron recovery (power production). Anaerobic conversion of sugars is realizedeither by bacterial fermentation leading to the formation of small, reduced energy‐richmetabolic products, such as ethanol, acetate, or hydrogen or by anaerobic respiration usinganotherterminalelectronacceptorinsteadofoxygentotakeuptheelectronscomingfromthesugar. Different MFC techniques allow us to utilize both of these anaerobic metabolicprocesses.

Activity3.1–CanaFuelCellRunonCoke?

Havethestudentsreadthearticlesfoundat:http://blog.wired.com/gadgets/2007/03/like_programmer.html

http://www.slu.edu/x14605.xmlhttp://www.slu.edu/readstory/more/4479

http://www.slu.edu/readstory/newsinfo/2474

Have students discuss how an enzymatic fuel cell functions. After reading the articles,discusstheadvantagesanddisadvantagesofenzymaticfuelcelltechnology.Doyouthinkthistechnologyhasanypracticalapplicationsinthenearfuture?Whyorwhynot?

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WhatmustbedonetoensureananaerobicenvironmentintheanodechamberoftheMFC?

Wemustnotallowoxygenintothechamber.(Note:Studentsshouldbeinformedthatitwillbenearlyimpossibletokeepthechambercompletelyanaerobicduetotheopenings(ports)intheMFCbodies.) Thebacteriawillbegrowninasealedcontainer,sonooxygencangetin.WhenthebacterialsolutionisaddedtotheMFC,itmustbedonequicklytolimitoxygenexposure.

AtypicalMFCconsistsoftwoseparatechambers ,whichcanbe inoculatedwith liquidmedia(Figure3.2).Thesechambers,ananaerobicanodechamberandanaerobiccathodechamber,aregenerallyseparatedbyanion‐exchangemembrane. AMFCsuchasthiscanbeclassifiedintotwotypes. Onetypegenerateselectricityfromtheadditionofartificialelectronshuttles(mediators)toaccomplishelectrontransferfrombacterialcytoplasmtotheanode.TheothertypedoesnotrequiretheseadditionsofexogenouschemicalsandcanbelooselydefinedasamediatorlessMFC. MediatorlessMFCscanbeconsideredtohavemorecommercialpotentialthenMFCs that requiremediators because the typicalmediators are expensive. Onemajorchallenge is that, if oxygenwill beused as the final electron acceptor, the cathode chamberneedstobefilledwithasolutionandaeratedtoprovideampleoxygentothecathode.

Figure3.2.Abasicmicrobialfuelcell(reprintedwithpermissionofCellPress)

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ELECTRICITYGENERATIONINTHEMFCInnormalmicrobialmetabolism (seeChapter2), a carbohydrate (glucose) isoxidized initiallywithout the participation of oxygenwhen its electrons are released by enzymatic reactions.Theelectronsarestoredasintermediates,whichbecomereduced,and,inthisstate,theyareused to fuel the reactions, which provide the living cell with energy for maintenance andgrowth. Theultimate“electronsink”(orendpointoftheredoxreaction) ismolecularoxygen(dioxygen,O2).Toanelectrochemist,asimplifiedrepresentationoftheanodehalf‐cellreactioninvolvedintheoxidationofglucosebyawholebacterialcellwouldbeasfollows:

C6H12O6+6H2O→6CO2+24H++24e–

The largeharvestof electrons is storedas reduced intermediates,but theeventual terminustherespiratorychainisoxygen,asdemonstratedinthecathodehalf‐cellreaction:

6O2+24H++24e–→12H2O

pHEFFECTSINTHEMFC

The term pH refers to the concentration of hydrogen ions (H+) in a solution. An acidicenvironmentisenrichedinhydrogenions,whereas,abasicenvironmentisrelativelydepletedofhydrogen ions. ThepHofbiological systems is an important factor thatdetermineswhichmicroorganismsareabletosurviveandoperateinaparticularenvironment,suchastheanodechamberofamicrobialfuelcell.MostmicroorganismspreferpHvaluesthatapproximatethatofdistilledwater,aneutralsolution.

Thehydrogen ionconcentrationcanbedeterminedempiricallyandexpressedasthepH.ThepHscalerangesfrom0to14,with1beingthemostacidicand14beingthemostbasic.ThepHscale isa logarithmicscale.That is,eachdivision isdifferent fromtheadjacentdivisionsbyafactorof10.Forexample,asolutionthathasapHof5is10timesasacidicasasolutionwithapHof6.

Therangeofthe14‐pointpHscaleisenormous.DistilledwaterhasapHof7(neutral).ApHof0correspondsto10millionmorehydrogenionsperunitvolumeandisthepHofbatteryacid.ApHof14correspondsto110‐millionthasmanyhydrogenionsperunitvolume,comparedtodistilledwater,andisthepHofliquiddraincleaner.

Compounds that contribute hydrogen ions to a solution are called acids. For example,hydrochloric acid (HCl) is a strong acid. This means that the compounds dissociate easily insolutiontoproducetheionsthatcomprisethecompound(H+andCl–).Thehydrogenionisalso

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aproton.Themoreprotonsthereareinasolution,thegreatertheacidityofthesolution,andthelowerthepH.

Mathematically,pHiscalculatedasthenegativelogarithmofthehydrogenionconcentration.Forexample,thehydrogenionconcentrationofdistilledwateris10–7and,hence,purewaterhasapHof7.

pH=–log[H+]=–log10–7=7

In theMFC,maintaining the pH ofmicrobial anode chamber and the anode feed solution isimportanttoensurethatgrowthofthetargetmicrobesoccurs.Also,pHneedstobemonitoredastheanodeandcathodereactionsareoccurring.IfthepHvariestoowidely,thegrowthandmetabolismofthemicroorganismcanbehalted.Thisinhibitionisduetoanumbersofreasons,such as the change in shape of proteins due to the presence ofmore hydrogen ions. If thealteredproteinceasestoperformavitalfunction,thenormalfunctioningandeventhesurvivalofthemicroorganismcanbethreatened.

Ion exchangemembranes pose an important challenge in pHmaintenance in theMFC. Ionexchangemembranesseparatethebiologicalanodefromthecathodereactions,while,atthesame time, facilitating the transport of ions through the membrane to maintain electro‐neutralityinthesystemandproperbacterialrespiration.Iontransferbetweentheanodeandcathode isnecessarybecause ofthemovementofnegativelychargedelectronsfromthe anodeto the cathode. To achieve a counterbalance, either negative charge equivalents(anions/hydroxide ions) travel fromthecathode to theanode,orpositivechargeequivalents (cations/protons)movefromtheanodetothecathode, dependingontheselectionoftheion‐exchangemembranematerial.Since thecathode reactionsofMFCsconsumeprotons inequalamountsaselectrons, ideallyonly protons are transported through the ion exchange membrane. In this way, electro‐neutralityisobservedwithoutpHchangestakingplaceatthecathode.However,becauseMFCsoperate near neutral pH in the anode and cathode chambers, the concentration of cationsotherthanprotons(e.g.,Na+,K+,NH4

+)aretypically105timeshigherthanH+ ionsinsolution.ThiscompetitivetransportofcationsotherthanprotonssignificantlyeffectsMFCperformance.When the substrate is degraded (metabolized), protons are produced at the anode andconsumed at the cathode. However, if due to competition and concentrations gradients,protonscannotmigrateatasufficientratefromtheanodetothecathode,thepHwilldecreaseattheanodeandincreaseatthecathodewhilechargebalanceismaintainedbythemigrationofothercations.ThepHdecreaseattheanodeaffectsbacterialrespirationand,thus,currentgeneration. Utilizingwell‐bufferedsolutions intheMFCcanoffsetthesepHchanges,buttheaddition of chemical buffer solutions is not sustainable and it is not clear to what extentlocalizedpHchangesintheMFCmayaffectpowergeneration.

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MEDIATORLESSMFCs(1)TheuseofbacterialbiofilmsBacterialbiofilmMFCsarebasedonthedirectphysicalandelectronicinteractionbetweenthemicroorganisms and the anode surface (Figure 3.3). A biofilm is a community ofmicroorganismsadheringtoasurface.InthecaseofMFCtechnology,thesurfaceistheanode(i.e.,electron‐acceptingelectrode‐seeChapter2‐AnaerobicRespiration).TherearecurrentlythreemaintheoriestowhichmostMFCresearcherssubscribe:(1)Thebacteriaadheringtotheanode have electrochemically active redox enzymes on their outermembranes that containironmolecules. The ironmoleculesorient themselves insuchawayas to traverse theoutermembraneofthebacterium,allowingthedirecttransferofelectronstoexternalmaterialsand,therefore, do not require any chemical assistance to accomplish electron transfer to theelectrode; (2) Some evidence suggests that electrons may be transferred directly to theelectrodeviaconductive,hair‐likeproteinappendages(orpilli)foundonthesurfaceofcertainbacterialgenera;and(3)Certainbacterialspeciescansynthesizemicromolaramountsoftheirownredoxmediatorsthatcanthenbeusedbyawholerangeofbacteriainamixedculture.

Themethodologynotwithstanding,whenthesebacteriaoxidizetheorganicmatterpresent inthesubstrate,theelectronsareshuttledtotheanode.Oxygen,thehydrogenprotons,andtheelectronsthatareconnectedbyacircuitfromtheanodetothecathode,arethencatalyticallycombinedwith(routinely)aplatinumcatalysttoformwater(Figure3.3).(Platinumisacritical

At this point, it may be necessary to explain how certain bacteria can transfer electronsdirectly to an electrode and why, in some instances, mediators are necessary. Sinceelectrodesaresolidentitiesthatcannotpenetratethebacterialcells,amajorrequirementisthat electrons are to be transferred from the inside of themicrobial cellmembrane to itsoutside – either via the physical transfer of reduced compounds, or via electron hoppingacrossthemembraneusingmembrane‐boundredoxenzymes.Ananalogyofeatingacookiecan be used to explore how the processworks. Whenwe eat a cookie, the sugars in thecookie are acting as electron donors as they are oxidized. The electron acceptor, then, isoxygen.Studentsshouldunderstandthatthesourceofoxygenistheairaroundthemastheyrespire. As theybreathe inoxygen, it isavailable tobeusedasanelectronacceptor. It ismoredifficult, though, tousethesameanalogy forbacteria thataretransmittingelectronsdirectly to an electrode. The electron donor is again sugar (glucose), but how can anelectrodebe“breathedin”?Howcantheelectronsbeinggeneratedbythereactionsinsidethe cell be transported across cell membranes to the electrode? These questions will beansweredtoanextentinthefollowingsections,butitshouldbenotedthatthisisabuddingarea of MFC research and not all of the mechanisms of direct electron transfer to theelectrodeareknownorfullyunderstood.

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catalystatthecathodeandnoalternativemetalhasbeenproventocatalyzethecombinationofoxygen,thehydrogenproton,andtheelectroninamoreefficientmanner.)InMFC research,microbial biofilms are quite common, however the start‐up phase of suchbiofilm‐basedMFCanodesnormallytakesdaysorweeksand,therefore,cannotberealizedinshort‐terminvestigations(butmaybesuitableforalong‐termschoolproject).

Figure3.3.BacteriafixedinabiofilmatanMFCanode

(2)Directoxidationofsecondaryfuelsattheanode

Asecond,highlypromisingMFCapproachisbasedonthepreparationandinvestigationofelectrocatalyticanodes,capableofanefficientanddirectoxidationofbacterialendproductsunderthediverseandcomplexmicrobialgrowthconditions (seeChapter2–AerobicRespirationvs.Fermentation).Again,platinumcanbeusedasacatalystfortheutilization of molecular hydrogen by fermentative microorganisms at the anode.However,atthisstage,thelowstabilityandhighcostsofplatinumpreventlarge‐scaleapplications.

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Figure3.4.Transformationofaprimarysubstrateintoasecondaryfuelfordirectoxidationattheanode(Ptorothercatalystrequired)

Platinum‐coatedcarbonelectrodescouldbeusedasboththeanodeandcathodeinourMFC(Figure3.4).Attheanode,platinumwouldfunctionasanexcellentelectronacceptorduringthe oxidation of molecular hydrogen, H2, present as a byproduct of the initial anaerobicprocessing of glucose. In the cathode, platinum is capable of donating electrons duringoxygen reduction toproducewater. Thereare several reasonswehavechosennot touseplatinum electrodes in our experimental MFC. (1) Platinum electrodes are very easilypoisonedby theproductsofmicrobialmetabolism if theyarenot coatedwithaprotectivepolymerprior touse in theMFC. Sulfides, carbonmonoxide,and, toa lesserextent,evencarbondioxidepresentintheanodechambercancausethemoleculestobindtightlytothesurfaceoftheplatinumelectrode,renderingituselessforelectrontransport. Theplatinumand polymer coating is quite expensive and the preparation of the electrodes is a verydetailed process requiring specific equipment and computer software that does not lenditself to simple application in a high school setting. (2) The reduction of oxygen at aplatinum‐coatedcathode,thoughnotunworkable,doesfurthercomplicatethesetupoftheMFC.Anaquariumpump(orsimilarapparatus)mustbeusedtoprovideaconstantsourceofoxygenintothecathodechamber.Thiscanbeaccomplishedbyattachingathinneedletothetubingofthepumpandtheninsertingtheneedleintothetopportofthecathodechamber.

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THEROLEOFMEDIATORSDirect electron transfer frommost bacteria to an electrode is hampered by overpotentials,whichcanbedescribedastransferresistances.Toreducetheseresistances,thesurfaceareaofthe electrodes needs to be increased, and/or redox mediators need to be added to thesolution.Aredoxmediatorisacompoundthatcanbereversiblyoxidizedorreduced.Bacteriacan use redox mediators to deposit their electrons onto an electron acceptor they cannotdirectlyreduce(Figure3.5).In the absence of oxygen, electronsmay be diverted from the respiratory chain by a redoxmediator,which enters the outer cellmembrane, becomes reduced, and leaves again in thereduced state. The reducedmediator then shuttles the “stolen”electrons to theanode. Tocompletethecircuit,asecond(oxidizing)electrode(thecathode)isrequired,againfunctioningastheelectronsink.Theoxidizingmaterialcanagainbeoxygengas,butismoreconvenientforschoolpurposestouseasimplesolubleoxidizingagent,suchaspotassiumferricyanide.In early MFC research, chemical redox‐mediators (often dyes, such as methylene blue orneutral red) were used to shuttle metabolic energy (in the form of electrons) from thecytoplasmofbacteriatoananode.Today,theseexpensiveandtoxicdyesnolongerplayaroleforthedevelopmentofpracticalMFCs,mainlybecauseincontinuoussystems,thesemediatorswould have to be added and recycled permanently. However, thesemediator‐assistedMFCsarestillaverygoodacademicdevicetostudyelectrontransferprocessesofmicroorganisms.

Figure3.5.OurexperimentalMFCdesignusingamethyleneblueredoxmediatorintheanodeandferricyanideinthecathode

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Activity3.2–MethyleneBlueDyeReductionTest

Students will conduct a simple experiment to show that anaerobically active bacteriacollectedfromthesoilhavethecapabilityofreducingmethyleneblue.

Activity3.2–MethyleneBlueDyeReductionTestINTRODUCTION

Though it has lost popularity with the advent of new technologies, themethylene blue dyereductiontest remainsa fairlyaccurate indicatorof thebacterialcontentofmilk. Thetest isbasedonthefactthatthecolorimpartedtomilkbytheadditionofadye,suchasmethylenebluewilldisappearmoreorlessquicklydependingonthenumberofbacterialcellspresent.Theremoval of the oxygen frommilk and the formation of reducing substances during bacterialmetabolismcausesthecolortodisappear.Theagentsresponsiblefortheoxygenconsumptionare the bacteria. Though certain species of bacteria have considerably more influence thanothers,itisgenerallyassumedthatthegreaterthenumberofbacteriainmilk,thequickertheoxygenwillbeconsumed,andinturn,thesoonerthebluecolorwilldisappear.Thus,thetimeofreductionistakenasameasureofthenumberoforganismsinmilk.

Thedairyindustryclassifies(grades)themilkbasedonbacterialcontent:

Class1.Excellent,notdecolorizedin8hours.

Class2.Good,decolorizedinlessthan8hours,butnotlessthan6hours.

Class3.Fair,decolorizedinlessthan6hours,butnotlessthan2hours.

Class4.Poor,decolorizedinlessthan2hours.

In this activity, we will use the methylene blue reduction test to qualitatively assess ouranaerobic bacterial cultures’ (which will later be used in our experimental MFC) ability toreducemethyleneblue. Thiswillvisuallyconfirmthatmethyleneblue,when introduced intothe anode chamber of ourMFC,will be capable ofmediating the transfer of electrons frominsideofthebacterialcellmembranetotheanode.

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MATERIALS

Fertile soil (free of chemicals), anaerobic bacterial growthmedium, 10mMmethylene bluesolution,solutionbottleswithcaps,glassPetridish,oven,15mLconical tubes,30oC(optimalgrowthtemperatureforClostridium)waterbathapparatusorincubator,pipettes,pipettorsandtips

PROCEDURE

Preparationstobedoneatleastonedayinadvanceofexperiment:

• Preparationofrequiredsolutionso Anaerobicbacterialgrowthmedium

For1LofdiH2Oadd:- 2.0g NH4HCO3- 3.6g KH2PO4- 0.1g MgSO4x7H2O- 0.01g NaCl- 0.01g Na2MoO4x2H2O- 0.01g CaCl2x2H2O- 0.015gMnSO4x7H2O- 0.00278gFeCl2- 2.0g yeastextract- 5.0g glucose

AdjustpHto6.5±0.5andautoclave(alternatively,forschoollab,usefreshlyboiled,cooleddownwaterforpreparation–tomaintainsterilityuntiluse,donotletwaterstandopen.).Note:Soilculturemustbegrownovernight(orlonger)tobeactiveduringexperiments!

o MethyleneBlue(syntheticelectronmediator)

- Preparea10mMsolution(0.032g/10mL)

• Preparationofanaerobicsoilculture1. Put some fertile soil inaglassPetridishandheatuncovered inanovenat

approximately120°Cforonehour.2. Fillatightlysealablebottlewithbacterialgrowthmedia(about150mL)and

addasmallamount(about1tsp)ofheatpre‐treatedsoil.3. Grow the bacterial culture overnight at 30°C (or longer at room

temperature).Thesolutionshouldturncloudyandstarttofoam.Caution:Ifgrowinglongerthanoneday,build‐upofgaspressurecouldcausebottletorupture!

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Note: We are working with an environmental sample of microorganisms, some ofwhichmayalsobepathogenic.Thus,usegloveswhenhandlingthemicrobialsolutionandwashyourhandscarefullywhenyouarefinished.Cleanupspillswithbleachanddecontaminatesolutionwithbleachattheendofyourexperiment.

Dayofexperiment:

1. Measureincreasingamountsofmethyleneblueintoaseriesof15mLconicaltubes:Tube1–nomethyleneblue,MB(control)Tube2–0.25mLMBTube3–0.50mLMBTube4–1.0mLMBTube5–2.0mLMBTube6–1.0mLMB+H2O(negativecontrol)

2. Add12–13mL(filltubecompletely)ofbacterialsuspensiontoeachtube1–5and12

mLwatertotube6;tightlyscrewoncap.Gentlyinverttubeoncetoensuremixingofbacteriawiththemethyleneblue.Donotshaketubes!Shakingintroducesoxygenintotheanaerobiccultures.

3. Immediately place tubes in 30oC incubator or water bath. Record this time as thebeginningoftheincubationperiod.Covertubestokeepoutlight.

4. Check samples for decolorization after 15 minutes of incubation. Make subsequentreadingsat30minuteintervalsthereafter.Decolorizationisdeterminedbycomparisonofthethemethylenebluecontainingtubestothecontrol(noMB).

5. After each reading, remove decolorized tubes and then slowly make one completeinversionofremainingtubes.Again,noshaking.

6. Recordreduction(decolorization)timeinminutesforeachsample.Forexample,ifthesamplewasstillblueafter15minutesbutwasdecolorizedatthe45‐minutereading,thereductiontimeshouldberecordedastheaveragetimebetweenthetwoobservations,or30minutes.

Factors affecting the test. Many factors affect the methylene blue reduction test, andthereforethestepsofoperationshouldbeuniform.

Sincetheoxygencontentmustbeusedupbeforethecolordisappears,anymanipulationthatincreasestheoxygenaffectsthetest.Thetubesshouldbecompletelyfilledwiththebacterialsolution (even to thepointofoverflowing). Donot shake the tubes, andmixonlybygentleinversion.

Light hastens reduction, and therefore the tests should be kept covered. The initialconcentrationofthedye(10mM)shouldbeuniformasanincreasedconcentrationlengthens

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the time of reduction. Increasing the incubation temperature augments the activity of thebacteria,andthereforeshortensthereductiontime.

Overtimethebacteriawillsettletothebottomofthetube.Thisfactorcausesvariationsinthereduction time, since the bacteria are not evenly distributed. The accuracy of the test isincreased, reduction time shortened, and decolorization more uniform if the samples areperiodicallyinvertedduringincubation.

How does the methylene blue reduction test show that our soil culture bacteria are“active”?

Ifthebacteriaarecapableofdecolorizingthemethyleneblue,itmeanstheyareactively undergoing anaerobic metabolism. The bacteria are oxidizing theglucoseinthenutrientbrothandgeneratingelectrons.Whenthebacterialcellstakeupthemethyleneblue intotheiroutercellmembrane, thoseelectronsareusedforreducingthedyetothecolorlessformofthemethylenebluemolecule.

How can themethylene blue reduction test show thatwe have an anaerobic bacterialculturecapableofgeneratingelectricityintheMFC?

The reduction of the methylene blue we are witnessing is exactly the samereactionthebacteriawillbecarryingoutintheanodecompartmentofourMFC.Only in theMFC, the electrons generated by bacterialmetabolism do not staywith the methylene blue molecules. Molecules of methylene blue are back‐oxidized at the anode and the electrons are subsequently dropped into theelectricalcircuitoftheMFC.

ADDITIONALDEMONSTRATIONAt the conclusion of the dye reduction test, pool all of the decolorized tubes into alargersolutionbottle(largeenoughthatthebottleislessthanhalffull).Byswirlingthecontents of the bottle, oxygen trapped in container will become dissolved in thebacterial solution and will back‐oxidize the methylene blue, returning it to the blue(oxidized form). This isagreatvisualdemonstrationtousewiththestudentssotheycanunderstandwhythemethyleneblueretains itscolor intheMFC. IntheMFC;themethylene blue is only an electron carrier and does not remain in the reduced(colorless) form. As the methylene blue is reduced by electrons generated byfermentation, theelectronsare subsequentlydropped into (donated to) theelectricalcircuitattheanode.

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ELECTRONFLOWINTHEMFCRemember thatduringglycolysis,glucose isbrokendown intocarbondioxide inanoxidationreaction(seeChapter2–Step1–Glycolysis).Oxidationreactionsarealwayscharacterizedbyalossofelectrons.

C6H12O6+6H2O→6CO2+24H++24e–

The electrons released have to be deposited in some sort of electron sink and expelled aswaste. If thecellcouldnotgetridof itselectrons, itwouldbuildupahugenegativecharge.Frequently,duetoitsavailability,oxygenfunctionsastheelectronsink.Amoleculeofoxygentakesupelectronsandcombineswithprotons(H+)tomakewater.

6O2+24H++24e–→12H2O

The electron sink, however, is not limited to oxygen. During anaerobic respiration, the finalelectronacceptor couldbeanymolecule capableof acceptingelectronsprovided theproperenzymatic pathway is available. So, how do we know in which direction (and to whatmolecules)theelectronswillflowinourMFC?Theanswerisrelatedtothereductionpotentialof the various electron carriers involved in themetabolic pathways of the bacteria and themediatorswehaveintroducedintoourMFC.Thetablebelowgivestheapparentbiologicalstandardreductionpotentials(E°’atpH7,insteadofE°atstandardconditions,seeChapterI–StandardReductionpotentials)ofsomeimportantbiological half‐reactions. Electrons flow spontaneously from the more readily oxidizedsubstance (theonewith themorenegative reductionpotential) to themore readily reducedsubstance (the one with the more positive reduction potential). Therefore, more negativepotentials are assigned to reactions that have a greater tendency to donate electrons (i.e.,reactionsthattendtooxidizemosteasily).

StandardReductionPotentialsofInterestinMFCsHalfCellReactions StandardReductionPotential(E°’)

2H++2e–→H2 –0.42VNAD++2H++2e–→NADH+H+ –0.32VAcetylaldehyde+2H++2e–→Ethanol –0.20VPyruvate+2H++2e–→Lactate –0.19VMB(ox)+2H

++2e–→MB(red) +0.01VFe(CN)6

–3+e–→Fe(CN)6–4 +0.44V

NO3–+2e–+2H+→NO2

–+H2O +0.42VMnO2+4e

–+4H+→Mn2++2H2O +0.60VFe3++e–→Fe2+ +0.76V½O2+2H

++2e–→H2O +0.82V

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It is important to note thedirectionof all these reactions is alwayswritten in the formof areductionorgainofelectrons.That'snotimportantwhenitcomestodeterminingthedirectionofelectronflow.Forexample,notethatthereductionofprotonstohydrogenisatthetopofthe list (Eo=–0.43V).Electronsreleasedby theoxidationofmolecularhydrogenwill flowtoanyhalfreactionthathasahigher(lessnegative)standardreductionpotential.Inthiscase,theelectronsendupinNADH(Eo=–0.32V). Noticethereductionofoxygeniswaydownatthebottomofthelist.That'swhyit'saneffectiveelectronsinkforgettingridofelectrons.Othernaturalelectronsinksincludenitrate(NO3

–),iron(Fe3+),andmanganese(MnO2)(seeChapter2–AnaerobicRespiration).In our experimental MFC, fermentative bacteria will convert the glucose‐rich media intomainlybutyrate,acetate,andmolecularhydrogen.Whenthisbacterialsolution,initsredoxstate, is filled into the anode chamber together with the methylene blue synthetic redoxmediator, themediator itselfwillbereducedbythebacterialcells,aswellasbymolecularhydrogen. If the electric circuit to the cathode is closed (thatmeans both electrodes areconnected by a wire), the reduced mediator is reversibly oxidized at the anode and itselectronsaredroppedintotheelectriccircuit.Bymakingmethyleneblue,whichhasahigherstandardreductionpotential,availabletothebacteria,wecandiverttheelectronsawayfromthenormalpathwaysofethanolorlactatefermentation. Theelectronsthenflowthroughtheclosedcircuitfromanodetocathodedue,again,tothedifferencesinelectricalpotential.Thepotassiumferricyanideintroducedintothecathodechambercanthenactastheelectronsinkastheferricyanideionisreducedtotheferrocyanideion.Again,oxygen(ataplatinumcoatedelectrode)couldbeusedasthefinalelectronacceptorinourexperimentalMFC,butan apparatus for the continuous delivery of oxygen into the cathode chamber (aquariumpump)isnecessary. WhateffectwouldusingoxygenasthefinalelectronacceptoratthecathodehaveontheelectricitygeneratingabilitiesoftheMFC?

Usingoxygenwould(theoretically)result inmorepowerproductionintheMFC.Since the oxygen half‐reaction has a more positive reduction potential, thepotentialdifferencebetweentheoxidationandreductionhalfreactionswouldbegreater.Alargerpotentialdifferencemeanslargervoltageacrossthecircuit(seeChapter1–ReductionPotentials).

Activity3.3–TheMFCinAction

StudentswillconstructtheexperimentalMFC,loadtheanodeandcathodechamberswiththeappropriateoxidantandreductant,andharvesttheelectricitygeneratedtopoweranLED.Note:StudentsfrequentlyhavedifficultlyassemblingtheMFC.Sometimeshouldbegivenpriortothedayofexperimentationforpractice. Agoodideamaybetoreviewthestep‐by‐step procedure one day prior to the actual date of experimentation, and have thestudentsassemblea“dry”MFCusingtheFigures3.6,3.7,3.8,and3.9providedasguides.

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Activity3.3–TheMicrobialFuelCellinAction

MATERIALSTheexperimentwillbeconductedinamodifiedmodelMFCcellcommerciallyavailableattheNCBE,UniversityofReading,UK(http://www.ncbe.reading.ac.uk/).ThemodelMFCsetscontaintheacrylicMFCbodies,screwstoassemblethem,rubbergaskets,andionexchangemembraneandcarbonclothmaterialforthepreparationofelectrodes.Note: TosetuptheMFCasdescribedinthefollowingprotocol,twocompleteMFCkitsareneeded.AssemblingtheMFCwithasingleanodeandcathodechamberwilldemonstratecellpotential,butitisnotsignificantenoughtolighttheLED.IfthedesiredoutcomeistoallowstudentstheopportunitytopowertheLED,twoanodechambersmustbeassembled(Figure3.6andFigure3.7).

Figure3.6.The3­chamberedMFCinexplosionview

Hole through which chamber is filled

Neoprene gasket

Neoprene gasket

Terminal of carbon fibre electrode

Chamber glued to end plate with Superglue

Hole through which chamber is filled

Ion exchange membrane

Ion exchange membrane

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Preparationsdoneinadvanceofsession:

• Presoaktheionexchangemembraneindistilledwaterfor24hourspriortouse.

• Allelectrodepreparationandmodificationo Carbonclothcuttorightsizeforcellcompartmentso Pieces of carbon cloth glued to graphite rods* for external connection of

electrodesusingconductivecarboncement**o For the preparation of electrodes in the high school (no graphite rods or

conductivecementneeded),thecarbonclothcouldbecutinsuchawaythatathin strip of cloth can be led through the connection hole in the acrylic body,providinganexternalconnection(Figure3.7).

Figure3.7.TheassembledMFC

*Graphiterodsarehighcarboncontentpencilrods(verysoft)purchasedfromtheartsupplysectionofthecampusbookstore.**Conductivecarboncementisavailableat:http://www.emsdiasum.com/microscopy/products/chemicals/adhesive.aspx#12664

• Preparationofrequiredsolutionso Anolyte–bacterialgrowthmedium

For1LofdiH2Oadd:- 2.0g NH4HCO3- 3.6g KH2PO4- 0.1g MgSO4x7H2O- 0.01g NaCl

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- 0.01g Na2MoO4x2H2O- 0.01g CaCl2x2H2O- 0.015gMnSO4x7H2O- 0.00278gFeCl2- 2.0g yeastextract- 5.0g glucose

Adjust pH to 6.5 ± 0.5 and autoclave (Alternatively, for school lab, use freshlyboiled, cooled‐downwater for preparation – tomaintain sterility until use, donotletwaterstandopen.).

Note:Soilculturemustbegrownovernight(24–36hours)tobeactiveduring experiments! Bacteria tend to slow metabolism (and theirresultingelectroactivity)after48hoursasglucoseisdepleted.

o MethyleneBlue(syntheticelectronmediator)

- Preparea10mMsolution(0.032g/10mL)o Catholyte–ferricyanide:

- Preparea100mMsolutionofpotassiumferricyanide(8.23g/250mL)CAUTION:Potassiumferricyanideishazardousandshouldnotcomeincontactwith theeyes. Eyeprotection isnecessarywhenhandling thismaterial.Ifthesolutiondoescomeincontactwiththeeyes,floodthemwith water and seek medical attention. Do not let Potassiumferricyanidereactwithstrongacids(poisonformation).Followdisposalguidelinesattheendofthisprotocol.

Additionalmaterialsneeded:

• Two–30mLsyringesfordeliveryofsolutionsintoanodeandcathodechambers(Figure3.9)

• Glasswareforpreparingandstoringsolutions• TubeconnectorsandmatchingtubestoconnectsyringestoMFCcells• Fertilesoil(withoutchemicalfertilizers)asasourceofanaerobicbacteria• GlassPetridishfortheheatpre‐treatmentofsoil• Handheldmultimetersfortheobservationofcellpotentialandcurrent• Largetraytopreventspillsofsolutions• “Real”loads,e.g.LEDlightorsmallfans• AlligatorclipsandcopperwireformakingconnectionsbetweenchambersandtoLED• Glovesandsafetygoggles

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PROCEDUREAtleastonedaypriortoinvestigation:

1. Putsomefertilesoil inaglassPetridishorclaycrucibleandheatuncovered inanovenatapproximately120°Cforonehour.

2. Fillatightlysealablebottlewithbacterialgrowthmedia(about150mL)andaddasmallamount(about1tsp)ofheatpre‐treatedsoil.

3. Growthebacterialcultureovernightat30°C(orlongeratroomtemperature).Thesolutionshouldturncloudyandstarttofoam.Caution:Ifgrowinglongerthanoneday,build‐upofgaspressurecouldcausebottletorupture!

Dayofinvestigation:

1. AssembletheMFCcellsaccordingtotheexplosionFigure3.8–wewillcreatethreecompartments(anode/cathode/anode). Separatethechamberswith ionexchangemembrane and use gaskets on both sides of the membrane to prevent leaking.Severalfuelcellsmaybejoinedtogethertogivegreatervoltage.

Figure3.8.The3­chamberedMFC(topview)

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2. Attachthetubingviatubeconnectorstotheinfluentport(side,bottom)oftheMFCchambers.

3. Usingalligatorclipsandcopperwire,connectthetwoanodesinparallel(sothatthetwochambersbecomeelectricallyone).

4. Connect the handheld multimeter in parallel to the MFC to investigate thedevelopmentof cellpotentialover time. Attachone lead to thecathodeandoneleadtoeitheranode.

5. Add 0.3mL (300 uL) ofmethylene blue to each anode chamber through the topeffluentport.

6. Fillthetwo30mLsyringeswithcatholyteandanolytesolution(asfastaspossibletolimit aeration) and connect them to the respective tubings at the influent ports(Figure3.9).

Note:Toeliminatespacingissues,theinfluentportsforbothanodechambersshouldbeoneandthesameside,andtheinfluentportforthecathodechambershouldbeon the opposite side. Additionally, both anode chambers should be connected bytubingandaT‐shapedconnectorsotheycanbefilledsimultaneouslyfromthesamesyringe.

7. Fill the cathode cell first (limits the exposure to oxygen in the anode chambers)followed by both anode cells (from the same syringe) taking care not to overfillthem.

8. Observe the cell potential on the multimeter and if it is significant (>450 mV),directlyconnecttheMFCtotheLEDinshortcircuit.

9. The LED will flash for a short time until the potential drops below the requiredvoltagenecessarytopowertheLED.

10. DisconnecttheLED,waitforrecoveryofthecellpotential,andtryitagain.

Disposalandclean‐up:

• TostoretheMFCset‐upsandkeeptheelectrodesreusableandactive,itisnecessary

tocleananddisinfect thewholecellandespecially theelectrodes intensivelywithethanolaftereachexperiment.

• Afteraddingasmallamountofbleachandashortincubation,spentbacterialbrothsolutionscanbedraineddownthesink.

• Cathodicferricyanidesolutionishazardousandmayproducetoxicfumesincontactwithstrongacids! Itmaybeusedseveral timesandgets lightertocolorless if theelectronacceptorisspent.Localregulationsshouldbeobservedwhendisposingoftheusedsolution.

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Figure3.9.Theassembled3­chamberedmicrobialfuelcellDISCUSSIONTheheatpre‐treatmentof thesoil killedall livingcellsand just thesporesofbacteria strainssurvived.FermentativeClostridiumspeciesrepresentalargegroupofthesebacteriainsoil.Ifthesoilisinoculatedandincubatedanaerobicallyinarichbacterialmedium,thesporesofonlytheanaerobicbacteriagerminateandformamixedbacterialculture.Astheygrowovernight,the fermentativeClostridium convert theglucose in thenutrientmediamainly intobutyrate,acetate, andmolecular hydrogen. When this bacterial solution is introduced into the anodechamber,themethylenebluemediatorisreducedbythebacterialcellsaswellasbymolecularhydrogen. When the electrical circuit is closed, the methylene blue is back‐oxidized at theanodebydroppingitselectronsintotheelectricalcircuit.Theelectronsflowthroughthecircuitto the cathode chamber where the ferricyanide acts as the final electron acceptor. If thepotentialdifferencebetweenthereducedanodesideandtheferricyanideonthecathodesideislargeenough,wecanpowertheLED.

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AnalternativeprotocolthatalsomayassistwithstudentsunderstandingofthefunctionalMFCwould be to add methylene blue to the growing bacterial solution prior to the day ofinvestigation,asopposedtoadding itdirectly intotheanodechamberoftheMFCduringtheactualinvestigation.Asthebacteriagrowandmetabolize,theywillreducethemethylenebluetoitscolorlessform.Whenthebacterialsolution(plusmethyleneblue)isthenintroducedintotheMFC,themethylenebluewouldbecomeblueagainwhenitisbackoxidizedattheanode.Thestudentswouldthenhaveavisualrepresentation(acolorchange)oftheelectronsbeingmediatedtotheanodebythemethyleneblue.FURTHERINVESTIGATIONSIftimeallows,abacterialbiofilmcouldbedevelopedwithintheanodechambertocomparethefunctionofthemethylenebluemediatedMFCtoamediatorlessMFC.

• Preparetheanodicfeedsolutiono Anolyte–bacterialfeedsolutionforbiofilmMFC

For1LofdiH2Oadd:- 0.8g sucrose- 3g KH2PO4- 0.1g yeastextract- 0.033gNH4Cl- 0.06g K2SO4- 0.033gFeCl2x4H2O- 0.011giron(III)citrate- 0.5gNaCl- 0.1gKCl- 0.1g CaCl2- 0.1g MgCl2x6H2O

AdjustpHto6.5±0.5andautoclave(alternatively,forschoollab,usefreshlyboiled,cooled‐downwaterforpreparation–tomaintainsterilityuntiluse,donotletwaterstandopen.).

• Assemble the MFC kit as in the previous experiment, but in addition, seal the holearound the external anode connections (influent and effluent ports) to prevent theleakingofliquidandoxygenationoftheanodesolution.(Tip:Aquariumsiliconesealantworkswell.)Also,tubingshouldbeconnected(andsealed)tothetop(effluent)porttoallowtheanodesolutiontobe flushedoutdaily. Tomaintainanaerobicconditions inthe anode chamber, be sure the tubing in the top (effluent) port remains closed (byclamp)exceptduringthetimeofsolutionreplacement.

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• Mixverydifferentsamplesofanaerobicsoils,sludges,andwaterfromstagnantpuddlesto get an anaerobic bacterial inoculum. Inject some of the inoculum into the anodechamberoftheMFC(throughinfluentport).

• Asinthepreviousexperiment,fillthetwosyringeswithanolyteandcatholytesolution,

connect them to their respective tubings, and fill both cells with solution (throughinfluentport).

• Addfreshanodesolutiononceperday. By initiallyconnecting (andsealing) tubingto

thetop(effluent)portoftheanodechambers,(thespentsolutioncaneasilybeforcedoutasfreshsolutionisaddeddaily).

• Connectthemultimeter inparallel totheMFCasbeforeandperiodicallymeasurecell

potential.Ifsignificantcellpotentialdevelops,connecttheMFCtoanexternalresistor(e.g., 100 Ohms, available at Radio Shack or similar electronics store). Wait until astablepotentialisformedatthisresistance(couldtakedaysorweeks).Ifthepotentialisstableovertime,theresistorcanbedisconnectedfor24hoursto“collect”potentialbetweenthetwopoles.Ifthepotentialgoesover0.45V,theLEDdevicewillflash.ItisalsopossibletoperformapolarizationtestwithaseriesofdifferentexternalresistorsasitwasdescribedinChapter1,page37.

The anaerobic bacteria from the sludge mixture will grow in the anode compartment byutilizing the offered sucrose (sugar) in the feed solution. If the anodic chamber is keptair/oxygenfree,someofthebacteriawillbeginanaerobicrespirationbychoosingtheanodeastheterminalelectronacceptor(seeChapter2–AnaerobicRespiration).Abiofilmwillthenbeslowlyestablishedattheanodethatdeliverselectronsintotheelectricalcircuit.MFCprotocol forschoolexperimentswasadaptedfromBioscienceExplainedbyD.MaddenandJ.Schollar(http://www.bioscience­explained.org/ENvol1_1/pdf/FulcelEN.pdf)

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Chapter4:SoNowWhat?–MFCChallengesandApplicationsTECHNICALCHALLENGESSystemarchitecture.TheideaofusingbacteriainMFCstocaptureelectricityhasbeenaroundforsometime,butinearlysystems,powerproductionwasverylowandrequiredtheadditionof extracellularmediators to shuttle electrons from inside to outside of the bacterial cell. Innewersystems(e.g.,mediatorlessMFCsthatemploybiofilms);mediatorsarenotneeded,andpower production fromMFCs has increased dramatically in just the past few years, in partbecauseofdesignsthatlowerinternalresistance(seeChapter3–MediatorlessMFCs).

Resistancecanbedescribedasanythingthatopposes the flowofelectriccurrent. Oftenthesystem components of theMFC themselves are regarded as the principle (internal) resistors(e.g., transfer resistance between electronmediator and anode or electrolyte and cathode).Forexample,justa0.1Ohmresistanceoccurringatacurrentof2ampscanyielda0.2VdropintheelectricalpotentialoftheMFC(seeChapter1–ElectricalCircuits).Thetheoreticalpower(orEMF)generatedbyMFCsisafunctionoftheindividualelectrodepotentialswhenthecircuitis open, but the actual power generation of theMFC is often less thanwhat is theoreticallypossible.

Theanodepotential issetbytherespiratoryenzymesofthebacteriaanddoesnotappeartovarysubstantially indifferentsystemsorwithdifferentsubstrates (fuels).Ontheotherhand,the cathodepotential varies dependingon the catholyte andoxidant. Experimentally, lowerthanexpectedvoltagesareusuallyobtainedwhenoxygenisusedasthefinalelectronacceptor.Withferricyanideasthefinalelectronacceptorinsteadofoxygen,lessinternalresistanceandhigher cell voltages have been documented, however, power generationwith ferricyanide isnotsustainable–theferricyanidemustbeexternallyregeneratedovertime.

Withoxygenastheelectronacceptoratthecathode,themaintechnicalchallengeinimprovingpowergeneration is tocreatea systemarchitecture thatminimizes the internal resistanceoftheMFCbut,atthesametime,allowsforcontinuousflowthroughthesystem.Onepossibleremedy could be the use of an air cathode. Exposing the cathode directly to air (an aircathode), insteadoftodissolvedair(oxygen)inwater,hasresultedinsubstantial increasesinsystemperformance (Figure4.1). Clearly, though,maximizingpowergeneration inMFCswillrequire innovative flow patterns and increased electrode – electrolyte interactions thatminimize internal resistance.Additionally, findingmethods to increase thecathodepotential,withoxygenastheelectronacceptor,couldhaveasubstantialimpactonpowergeneration.

Materials. The cost of materials used to construct MFCs will also be a key factor for thesuccessfulapplicationofthetechnologyonalargerscale.Verylargesurfaceareasareneeded

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for supporting bacterial biofilms, and the structuremust be able to bear the weight of thewaterandbiofilm.Electrodematerialsrangefromcarbonclothandcarbonpaper,tographiterods,plates,andgranules.Cathodesaremadefromthesamematerials,buttheyalsocontainpreciousmetals,suchasplatinum,whenoxygenisusedastheelectronacceptor.

Somematerialsarenotexpectedtobesuitable forscale‐upbecauseoftheir inherent lackofdurability or structural strength (e.g., carbon paper), or cost (e.g., platinum catalysts, ionexchange membranes). Future designers will need to consider conductive coatings onstructurallystrongsupportingmaterials.Systemscale‐upwillalsorequirethatthedesignandapplicationofthesematerialsbeadaptabletomass‐manufacturingapproaches.

Microbiology.Ourunderstandingofelectrochemicallyactivemicrobesisstillinitsinfancy,butclearlyawholenewfieldofmicrobialecologyisemergingthatisbasedonanodophilicbacteria(bacteria that will adhere to an anode) and possible interspecies electron transfer. Thesebacteria may be referred to as electricigens based on their ability to exocellularly releaseelectrons. The initial understandingof electron transfer bybacteria to electrodes came fromstudies of metal‐reducing bacteria, such as Geobacter and Shewanella species, which canproduceelectricityinMFCs.Biochemicalandgeneticcharacterizationsindicatedthatforsomebacterial species, outer‐membrane cytochromes (membrane‐bound proteins containing ironthatcarryoutelectrontransport)canbeinvolvedinextracellularelectrontransfer.Also,somebacteriaproduceandusesolubleelectronshuttles thateliminate theneed fordirectcontactbetweenthebacterialcellandtheelectronacceptor(Figure4.1).

The recent discovery of nanowires introduces a whole new dimension to the study ofextracellular electron transfer. These conductive, pilli‐like structures identified, thus far, incertainGeobacterspeciesappeartobedirectlyinvolvedinextracellularelectrontransfer(seeChapter 3 –MediatorlessMFCs). Thesenanowire structures allow thedirect reductionof adistantelectronacceptor.Thisremovestheneedforsolublemediatorsthatwouldbelostinacontinuous‐flowMFCandmayallowfordirectinterspecieselectrontransfers.

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Figure4.1.Summaryofmethodsforelectrontransfertotheanode(withpermissionfromtheNaturePublishingGroup,License#2342510598123)

Glucose serves as an example fuel. a | An indirect microbial fuel cell. A fermentativemicroorganism convertsglucose to an end product, hydrogen, which can react with the anode to produce electrons and protons. Thisprocess only partially recovers the electrons available in the organic fuel as electricity, and results in theaccumulationoforganicproducts in theanodechamber. b |Amediator‐drivenmicrobial fuel cell.Anelectron‐shuttlingmediatoracceptselectronsfromreducedcellconstituentsandtransferstheelectronstotheanode.Thereoxidizedmediatorcanthenundergorepeatedcyclesofreductionandoxidation.Inmostinstances,thecellsthathave been used in such fuel cells only incompletely oxidize their organic fuels as shown. c | The oxidation ofglucosetocarbondioxidewithdirectelectrontransfertotheelectrodesurface.Glucoseistakenintothecellandoxidizedtocarbondioxidebytypicalmetabolicpathways.Electronsderivedfromglucoseoxidationaretransferredacross the innermembrane and outer membrane through electron transport proteins, such as iron‐containingcytochromes. In thisexample, the system is illustratedwithanair cathode rather thana cathodesubmerged inwater.d|Atwo‐chamberedmicrobialfuelcell.Thissystemisnotoptimizedformaximumpowerproductionbutisconvenientformicrobiologicalstudies.

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Activity4.1–Electricigens:MicrobialEnergizers

Studentsshouldreadthearticlefoundat:

http://www.asm.org/microbe/index.asp?bid=43711

After reading, have students answer the questions that follow. This reading assignmentwill also provide some background information on current MFC applications andtechnologythatwillassistwiththeposterproject. Whatareelectricigens?

Microorganismswiththeabilitytooxidizeorganiccompoundstocarbondioxidewhiletransferringelectronstoelectrodeswithhighefficency.

Howare theactionsofelectricigens similar to the functioningofa traditionalhydrogenfuelcell?

Botharecapableofconvertingafuelintoelectricitywithoutlosingsubstantialamountsofenergyasheat.Ahydrogenfuelcelloxidizeshydrogenandreducesoxygentowaterwhileproducingelectricity.Electricigensutilizetheirowncellularmetabolicpathwaystooxidizesugarsandreduceoxygen(oranotherfinalelectronacceptor).

What are the differences between a hydrogen fuel cell and MFCs that employelectricigens?

Hydrogenfuelcellsrequireaverypuresourceofahighlyexplosivegasthatisdifficulttostore and distribute, and hydrogen is derived mainly from fossil fuel rather thanrenewable sources. In contrast, the energy sources formicrobial fuel cells that utilizeelectricigens are renewable organics, which are stable and very cheap. In terms ofpowerproduction,MFCswillneverbeabletocompete(intermsofelectricityproduction)withthetraditionalfuelcellbecausethebiomassfueloftheMFCwillalwaysbeinlowerconcentrations thanwhat is fed into a hydrogen fuel cell. Thus, the power capable ofbeinggeneratedbyMFCswillalwaysbeordersofmagnitudelowerthanhydrogenfuelcells.

Activity4.2–MFCApplicationsPoster

IndividualstudentsorstudentgroupswillresearchthevariousapplicationsofMFCtechnologycurrentlyinuseandthenpresenttheirfindingsinapostersession.

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Activity4.2–MFCApplicationsPoster

MATERIALS

Internet/collegelibraryaccess,posterboard,markers,coloredpencils

PROCEDURE

IndividuallyorincooperativegroupsthestudentsshouldresearchtheapplicationsofMFC(orBES)technologythatarecurrentlyinuseorthathavebeenproposed.Havestudentsperformsome initial research into possible applications, or alternatively, present the information onapplicationsthatfollowthisactivityduringclasslecture.Then,eachstudent(orgroup)shouldselect their topic. After a pre‐determined period of time, the students should present theirfindingstotheclass.

Havethestudentsfocustheirresearchtoansweringthesequestions:

• WhatisthespecificapplicationoftheMFC,orwhatdoestheMFCpowerorproduce?• Where(location)isthistechnologyinuse?• Howlonghasitbeenusedandhowsuccessfulistheventure?• BasedonourstudyoftheMFC,describehowtheMFCisbeingusedinthistechnology?

Forexample:o Isitmediatorlessorisamediatorused?o WhatreactionsareoccurringattheanodeandcathodeoftheMFC?

Asuggestedrubricforgradingtheposterisprovidedattheendofthischapter.

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WASTEWATERANDOTHERAPPLICATIONS

Whenanewtechnologyisintroducedintothemarketplace,thegreatestlikelihoodforsuccessoccurswhen themost immediately profitable application is targeted first. As the technologydevelops, becomes better understood, and improves, more difficult applications can beundertaken.ThemostimmediateandusefulapplicationsofMFCsappeartobeforwastewatertreatmentandother,similarnicheapplications.Renewableenergyproductionisalonger‐termprospect that will require substantial technical and manufacturing advances. But researchadvances during the last three years also showed promising other applications, besidesmicrobiallyinducedpowergeneration,asinthecaseofanMFC.Tonameallnewtechnologiesthatarebasedonthesamefunctionalprinciples,butcanbeusedfordifferentapplications,thetermBioelectrochemicalSystem(BES)isgettingpromoted.

Wastewater treatment. Worldwide, more than 2 billion people do not have adequatesanitation, in largepartbecauseofa lackofstart‐upcapitalaswellasoperatingcosts. IntheU.S.,approximately$25billionisspentannuallyforwaterandwastewatertreatment.Overthenext 20 years,water andwastewater infrastructure demandswill require over $2 trillion forbuilding, maintaining, and operating these systems. In the U.S., close to 4% of the totalelectricity produced is used for the operation of the whole water and wastewaterinfrastructure.AtreatmentsystembasedonanMFCprovidesagreatopportunitytodevelopthetechnologybecausethesubstrateis“free”andwastewatermustbetreated.

Currentmethods forwastewater treatment utilize a filtration/straining procedure to removelarge objects from primarywastewater and then employ an aerobic treatment of secondarywastewater to degrade the biological content of the sewage. Large amounts of oxygen arepumped into secondarywastewater, at an immense economical and electrical cost, creatingwhat the industry refers to as activated sludge. Essentially, bacteria undergo aerobicrespirationandmetabolizethewastecomponentsofthesewage,usingitasafoodsource.

MFCscouldbeusedinatreatmentsystemasareplacementfortheexistingenergy‐demandingaerobictreatment;however,wedonotyetknowhowtoeconomicallyscaleupanMFCorwhatthecostswouldbetoreplaceaconventionalsystemwithanMFC‐baseddesign.Scale‐upandmaterials issues are the greatest challenges in the application of MFCs for wastewatertreatment,butpotentially,energyrecoveryatwastewatertreatmentplantscouldleadnotonlyto a sustainable system based on energy requirements (an energy neutral wastewatertreatmentfacility),butpossiblytotheproductionofanetexcessofenergy.

Environmentalsensors.Dataonthenaturalenvironmentcanbehelpfulinunderstandingandmodeling ecosystem responses, but sensors distributed in the natural environment requirepowerforoperation.MFCscanpossiblybeusedtopowersuchdevices,particularlyinriveranddeep‐water environments where it is difficult to routinely access the system to replacebatteries.Sedimentfuelcellsarebeingdevelopedtomonitorenvironmentalsystems,suchascreeks,rivers,andoceans. Powerforthesedevicescanbeprovidedbyorganicmatter inthesediments. Power densities are low in sediment fuel cells because of both the low organic

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matter concentrations and their high intrinsic internal resistance; however, the low powerdensitycanbeoffsetbyenergystoragesystemsthatreleasedata inburststocentralsensors(Figure4.2).

Figure4.2.Asedimentmicrobialfuelcellforuseasanenvironmentalsensor(withpermissionfromtheNaturePublishingGroup,License#2342510598123)

a|Aschematicofasedimentmicrobialfuelcell.OrganismsinthefamilyGeobactercanoxidizeacetateandotherfermentationproducts,andtransfertheelectronstographiteelectrodesinthesediment.Theseelectronsflowtothecathodeintheoverlyingaerobicwaterwheretheyreactwithoxygen.b|Anactualsedimentfuelcellbeforedeployment.

Bioremediation. AnMFCcanbemodifiedininterestingandusefulways,andthiscanleadtonew types of fuel cell‐based technologies.With suchmodifications, however, these systemsmaynolongerbetruefuelcellsbecausetheydonotproduceelectricity.Onesuchapplicationisthemodificationofthebasictwo‐electrodesystemforbioremediation.TheMFCisnotusedtoproduce electricity; instead, power can be put into the system to drive desired reactions toremoveordegradedangerousortoxicchemicalstosaferorusefulforms.

Renewableelectricityproduction frombiomass. Becauseofuncertaintyabout thematerialsneededandtheircosts,combinedwithcomparativelylowcostsforoil,theapplicationofMFCsforrenewableenergyproductionfromcrops,suchascorn,isnotlikelyintheimmediatefuture.In the near term, MFCs will have to compete with more mature renewable‐energytechnologies, such as wind and solar power. The operating costs needed for electricityproductionwithMFCswillprobablybetoogreatifthesubstratefortheMFCisgrownasacropinamannersimilartothatforethanolproductionfromcorn.

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Microbialelectrolysiscells(MEC).Inadditiontogeneratingelectricalenergy,MFCscouldalsobeused inaBEStogenerate industrialchemicals fromwasteproductswithexternal inputofsmallamountsofenergy.MECsareveryversatilesystemsinwhichatleastoneoftheanodicorcathodic oxidation‐reduction reactions are microbially catalyzed. They are capable ofgenerating products , such as hydrogen, methane, and ethanol. Likewise, they have thepotential to catalytically degrade less desired reactants and contaminants into moreeconomicallyprofitableorecologicallysoundproducts.BESshavealsobeenusedsuccessfully toremovesulfurcomponents,promotedenitrification,andreduceperchloratesandchlorinatedorganiccompounds.

EDUCATION

Someof themost immediateandusefulapplicationsofMFCsare in theclassroom:Studentsfindelectricitygenerationbybacteriabothfascinatingandfun!MFCshavebeenfoundtobeaneffectiveeducationaltooltocapturestudentinterest.Thesesmallandportablesystemsserveasawonderfulplatformformotivatingstudentstostudyandunderstandcomplexconceptsofcell respiration, microbial ecology, electrochemistry, and materials science. Additionally,processes that can couple sustainable energy production with waste treatment have innateappealtoenvironmentallymindedstudents.TheMFCisamodelsystemforscienceinstructionthat dissolves disciplinary boundaries and shows how technology can help solve significantsocialandenvironmentalissues.Directevidencefortheappealofthistechnologycanbeseenthroughrecentscience‐fairprojectsonMFCsbystudentsinmiddleandhighschoolsaroundtheworld.Atuniversity level, theMFChasbeenused inundergraduateandgraduate laboratorycourses in environmental microbiology to teach students methods of microbial‐communityanalysis.

1

RubricforMFCApplicationsPoster

Criteria 1 2 3 4

Organization

• Cluttered,nodefinitivesections,allovertheplace

• Noheadings,butsectioned• Hardtofollow,requiresassistance

• Obviousrefinementrequired

• Headingspresentbutunclear

• Mustrereadforclarity• Someevidenceofrefinement

• Definedsections• Clearheadings• Flowsnicelytoassistthereaderwithouthelp

• Finishedproduct

Creativity

• Bland,novariability• Nouseofcolorordiagrams• Boringtolookat,doesnotcatchyourattention

• Interest,motivation,effortandtimeobviouslyabsent

• Verylittleuseofcolororpicturesbutenoughtoengageandholdattention

• Someuseofcolor,diagrams,etc.

• Willengagebutwillnotstimulate

• Interesting,engaging,visuallystimulating

• Aestheticallyappealinguseofcolor,diagramsandtext

• Interest,motivation,effortandtimeobviouslypresent

ContentandLiteracy

• Noanalysisoftopic• Noexplanations• NospecificconnectionstoMFCunit

• Nouseofresources

• Poorexplanations• InaccurateconnectionstoMFCunit

• Misinterpretsthetechnology

• Oneresourceforsure

• Adequateexplanations• MFCunitconnectionspresentbutcouldbedevelopedfurther

• Morethanoneresourcepresent

• Conceptfullyandproperlyexplained

• Insightpresent• MFCunitspecificconnectionsmade

• Contentisaccurate,comprehensiveandwellsupported

• Excellentuseofresources

LevelandDifficultyofUnderstanding

(DepthofThought)

• Difficulty(depthofthought)notsuitableforgradelevel/notrelatedtoclassdiscussions(tooeasy)

• Superficial/irrelevant

• Explanationdescribesminimallevelofvalidity

• Needsseriousrefinement

• Difficultycouldbeincreasedordeveloped

• Somelevelofunderstandingshown

• Difficultyappropriateforgradelevel

• Understandingpresentandapparent