Batteries_and_fuel_Cells_for engineering chemistry

Batteries and fuel cells

1. SESSION OUTCOME
“GAIN KNOWLEDGE OF BATTERIES”

2. ASSESSMENT CRITERIA’S
ASSIGNMENT
QUIZ
MID TERM EXAMINATION –II
END TERM EXAMINATION

3. What is a “Battery“?
•PortableSourceofElectricalPower
•EnergyStorage/ConversionDevice
•ConvertsChemicalEnergyIntoElectricalEnergy
•WorksonElectrochemistryPrinciples
•Voltainventedin1800

4. Battery :-
Definition :
A battery is a storage device used for the storage of chemical energy and for
the transformation of chemical energy into electrical energy
Battery consists of group of two or more electric cells connected electrically
in series.

5. Chemical vs. Electrochemical
Chemical
•Chemical energy converted to thermal energy
•Energy producing chemicals: Fuel and Oxidant
•Fuel and oxidant are brought together. Resultant combustion reaction produces heat
•Fuel + Oxidant → Heat
Electrochemical
•Chemical energy converted to electrochemical energy
•Energy Producing chemicals: Anode Material, Cathode material
•Anode and cathode materials are kept separately
•electrons pass through an outer loop
•electrolyte to complete circuit

6. Electrochemical Cell Configuration

7. Thepotentialdifferencedevelopedwhenanelectrodeofan
elementisplacedinasolutioncontainingionsofthatelement.
Thepotentialdifferencebetweenananodeandacathodecanbe
measuredwithavoltagemeasuringdevice.Theabsolutepotential
oftheanodeandcathodecannotbemeasureddirectly.Defininga
standardelectrode,allotherpotentialmeasurementscanbe
madeagainstthisstandardelectrode.Ifthestandardelectrode
potentialissettozero,thepotentialdifferencemeasuredcanbe
consideredastheabsolutepotential.
Electrode potential

8. Battery Terminology
•Cell-Basic Unit: Contains Anode, Cathode, Electrolyte, Separator etc.
•Battery: Contains 2 or more cells in series or parallel
•Discharging: Removing energy from the cell/Battery
•Charging: Returning energy to the cell/battery

9. Basic Electrochemical Cell: Five essential
components of a cell
•The Anode
•The Cathode
•The Ionic Conductor (electrolyte)
•The Metallic Conductor (electrical connection)
•The separator

10. The Anode
•The anode has the lowest potential and is oxidized in the process by a
loss of electrons:
M→ M
z+
+ ze-
Anodic reaction
Oxidation reaction
Electron generation
The Separator
•Electrical insulator membrane, allowing ionic transfer and solvent
wetting.

11. The Cathode
•The cathode has a high potential, leading to a consumption of
electrons.
MO + ze

→ [MO]
z-
Cathodic reaction
Reduction reaction
Electron consumption
The Electrolyte
•A solution conducting ions

12. Electrical Connections
•Theanodeandcathodeinanelectrochemicalcellmustbein
electricalcontactinordertogeneratepowerandenergy.Difference
infreeenergiesbetweentheanodeandthecathodeproduces
electricalpotentialwhichisthedrivingforceforelectrochemical
reaction.
Therelationbetweencellpotential,Gibbsenergyandequilibrium
constantaredirectlyrelatedinthefollowingmulti-partequation:
ΔG
o
=−RTlnK
eq
=−nFE
o
cell

14. Batteries are of 3 types.
1.Primary Batteries (or) Primary Cells
2.Secondary Batteries (or) Secondary Cells
3.Fuel Cells (or) Flow Batteries

15. 1. Primary Batteries (or) Primary Cells
Thosecellsinwhichthechemicalreactionoccursonlyonceandthecellbecomes
deadaftersometimeanditcannotbeusedagain.
ThesebatteriesareusedassourceofDCpower.
Eg. Dry cell (LeclancheCell) and Mercury cell.
Requirements of Primary cell:It should satisfy these requirements
1)It must be convenient to use.
2)Cost of discharge should be low.
3)Stand-by power is desirable.

16. Dry cell (LeclancheCell)
ItconsistsofacylindricalZinccontainerthatactsas
ananode.
Agraphiterodplacedinthecentre(butnottouching
thebase)actsasacathode.
Thespacebetweenanodeandcathodeispacked
withthepasteofNH
4
ClandZnCl
2
andthegraphite
rodissurroundedbypowderedMnO
2
andcarbon.
Thecelliscalleddrycellbecauseoftheabsenceof
anyliquidphase,eventheelectrolyteconsistsof
NH
4
Cl,ZnCl
2
andMnO
2
towhichstarchisaddedto
makeathickpastewhichpreventsleakage.
Thegraphiterodisfittedwithametalcapandthe
cylinderissealedatthetopwithapitch.

17. Working of Dry cell (LeclancheCell)
•The Zn-MnO
2
cell (dry cell) is represented as Zn/Zn
+2
,NH4
+
/MnO
2
/C (EMF = 1.5V)
•Atanode: (Oxidation)
Zn(s) Zn
+2
(aq) + 2e
-
•AtCathode: (Reduction)
2MnO
2
(s) + H
2
O + 2e
-
Mn
2
O
3
(s) + 2OH
–
•The net cell reaction:
Zn(s) + 2MnO
2
(s) + H
2
O Zn
2+
+ Mn
2
O
3
+ 2OH
-
TheresultingOH
-
ionsreactwithNH
4
CltoproduceNH
3
whichisnotliberatedasgasbutimmediately
combineswiththeZn
2+
andtheCl
-
ionstoformacomplexsalt[Zn(NH
3
)
2
Cl
2
](diamminedichlorozinc).
2NH
4
Cl + 2OH
-
2NH
3
+ Cl
-
+ 2H
2
O
Zn
2+
+ 2NH
3
+ 2Cl
-
[Zn(NH
3
)2Cl
2
]

18. Advantages:
1)Thesecellshavevoltagerangingfrom1.25vto1.50v.
2)Primarycellsareusedinthetorches,radios,transistors,hearingaids,
watchesetc.
3)Priceislow.
Disadvantages:
Thesecellsdoesnothavealonglife,becausetheacidicNH
4
Clcorrodesthe
containerevenwhenthecellisnotinuse.

19. 2. Secondary Cells (or) Accumulator batteries
Thesecellscanberechargedbypassinganelectriccurrentthroughthemand
canbeusedagainandagain.
Eg:A.Leadstoragebattery
B.Nickel-Cadmiumbattery
C.Lithium-ioncellbattery
Secondarycellsarewidelyusedincars,trains,motors,electricclocks,power
stations,laboratories,emergencylights,telephoneexchange,digitalcameras,
laptopsetc.
Thesearereversiblecells;theybehaveasgalvaniccellwhiledischargingandas
electrolyticcellwhilecharging.

20. Toimprovetheperformanceofbatteryforcommercialpurpose
a)Theanodesandcathodeswithverysmallseparationtoconservespaceare
used.
b)Currentdischargeshouldbehighatlowtemperature.
c)Itshouldhavelessvariationinvoltageduringdischarge.
d)Itshouldhavehighenergyefficiency.
%energyefficiency=energyreleasedondischargex100
energyrequiredforcharge
e)Itshouldbereliable.
f)Itshouldhavetolerancetoshock,temperatureetc.
g)Itshouldhavenumberofcharginganddischargingcyclesbeforefailureof
battery(Cyclelife)

21. Lead –acid battery:
Ifseveralcellsareconnectedinseries,thearrangement
iscalledabattery.
Theleadstoragebatteryisoneofthemostcommon
batteriesthatisusedintheautomobiles.
A12Vleadstoragebatteryisgenerallyused,which
consistsofsixcellseachproviding2V.
Eachcellconsistsofaleadanodeandagridoflead
packedwithleadoxideasthecathode.
Theseelectrodesarearrangedalternately,separatedbya
thinwoodenpieceandsuspendedindil.H
2
SO
4
(38%),
whichactsasanelectrolyte.HenceitiscalledLead-acid
battery.
•Anode:Pb
•Cathode:PbO
2
•Electrolyte:diluteH
2
SO
4
(20.22%)
•EMF=2V

22. Toincreasethecurrentoutputofeachcell,thecathodeandtheanodeplatesare
joinedtogether,keepingtheminalternatepositions.
Thecellsareconnectedparalleltoeachother(anodetoanodeandcathodeto
cathode).Thecellisrepresentedas
Pb|PbSO
4
(s),H
2
SO
4
(aq.)|PbSO
4
(s),Pb
Intheprocessofdischarging,i.e.whenbatteryproducescurrent,thereactionsat
theelectrodesareasfollows:
At anode:
PbPb
+2
+ 2e

Pb
+2
(s) + SO
4
2
(aq.) PbSO
4
(s)
At cathode:
PbO
2
(s) + SO
4
2
(aq.) + 4H
+
(aq.) + 2e
–
PbSO
4
(s) + 2H
2
O
•Overall reaction is
Pb(s) + PbO
2
(s) + 4H
2
SO
4
(aq.) 2PbSO
4
(s) + 2H
2
O

23. Duringdischargingthebattery,H
2
SO
4
isconsumed,andasaresult,thedensityofH
2
SO
4
falls;whenitfallsbelow1.20
g/cm
3
,thebatteryneedsrecharging.
InDischarging,thecellactsasavoltaiccellwhereoxidationofleadoccurs.
Duringrecharging,thecellisoperatedlikeanelectrolyticcell,i.e.electricalenergyissuppliedtoitfromanexternalsource.
Theelectrodereactionsarethereverseofthosethatoccurduringdischarge.
PbSO
4
(s) + 2e
–
Pb(s) + SO
4
–2
(aq.)
PbSO
4
(s) + 2H
2
O PbO
2
(s) + 2H
2
SO
4
+ 2e
–
--------------------------------------------------------------------------------------------------------------
2PbSO
4
(s) + 2H
2
O Pb(s) + PbO
2
(s) + 2H
2
SO
4
(aq.)
Duringthisprocess,leadisdepositedatthecathode,PbO
2
isformedattheanodeandH
2
SO
4
isregeneratedinthecell.
Advantages:Leadacidbatteriesareusedforsupplyingcurrenttorailways,mines,laboratories,hospitals,automobiles,power
stations,telephoneexchange,gasengineignition,Ups(stand-bysupplies).Otheradvantagesareitsrecharge-ability,
portabilityandItsrelativelyconstantpotential&lowcost.
Disadvantages:UseofConc.H
2
SO
4
isdangerous,Useofleadistoxic.

24. Fully Discharged Fully Charged

25. Nickel–cadmium cell (NiCad cell):
Itisrechargeablesecondarycell.
Itconsistsofcadmiumasthenegativeelectrode(anode)andNiO(OH)actingasapositive
electrode(cathode).
Potassiumhydroxide(KOH)isusedasanelectrolyte.
Thecellreactionduringcharginganddischargingareasfollows.
•Anode:Cd
•Cathode: NiO(OH)
•Electrolyte:KOH
•EMF = 1.4V
At Anode
Cd(S) + 2OH

(aq) Cd(OH)
2
(s) + 2e

At Cathode
2NiO(OH) (s)+ 2H
2
O + 2e

2Ni(OH)
2
+ 2OH

(aq)
Overall reaction
Cd(s) + 2NiO(OH) + 2H
2
O Cd(OH)
2
(s) + 2Ni(OH)
2
(s)

26. Advantages and uses
1.The Nickel-Cadmium cell has small size and high-rate charge/discharge capacity,
which makes it very useful.
2.It has also very low internal resistance and wide temperature range (up to 70°C).
3.It produces a potential about 1.4 volt and has longer life than lead storage cell.
4.These cells are used in electronic calculators, electronic flash units, transistors
etc.
5.Ni-Cd cells are widely used in medical instrumentation and in emergency lighting,
toys etc.

27. 3. Fuel Cell
Definition:AFuelcellisanelectrochemicalcellwhichconvertschemicalenergycontainedin
readilyavailablefueloxidantsystemintoelectricalenergy.
Principle:
Thebasicprincipleofthefuelcellissameasthatofelectrochemicalcell.Theonlydifference
isthatthefuel&oxidantarestoredoutsidethecell.
FuelandOxidantaresuppliedcontinuouslyandseparatelytotheelectrodesatwhichthey
undergoredoxreactions.
Fuelcellsarecapableofsupplyingcurrentaslongasreactantsarereplenished.
Fuel + Oxidant Oxidation Products + electricity
Eg: 1) H
2
-O
2
fuel cell
2) Propane -O
2
fuel cell
3) CH
3
OH-O
2
fuel cell

28. Hydrogen –Oxygen fuel cell
OneofthemostsuccessfulfuelcellisH
2
–O
2
fuelcell.
Thecellconsistsoftwoinertporouselectrodesmadeofgraphiteimpregnatedwithfinely
divided‘Pt’(or)Ni(or)Pd–Agalloyandasolutionof2.5%KOHaselectrolyte.
Anode Cathode
Hydrogen
Water
Oxygen
Electrolyte
Load
Electron flow
O
-
H
H
2
&O
2
gasesarebubbledthroughanode&cathodecompartments,respectively.The
followingreactionstakesplace.
CellReaction:
Atanode: 2H
2
(g)+4OH

4H
2
O+4e

AtCathode:O
2
(g)+2H
2
O+4e

4OH

NetReaction:2H
2
(g)+O
2
(g)2H
2
O,Ecell=1.23V

29. Alargenoofthesecellsconnectedinseriesformafuel-cellbattery.Inthe
productionofelectricitybythismethod,thebyproductsareheat,CO
2
,water,
whichwillnotcausepollutionoftheenvironment.
Applications
:
1.Theseareusedasauxiliaryenergysourceinspacevehicles,submarinesand
othermilitaryvehicles.
2.Theproductwaterproducedisavaluablesourceoffreshwaterforastronauts.
3.Fuelcellispreferredinspacecraftbecauseofitslightness.

30. Advantages:
1)Fuelcellshavehighefficiency.Itisnearly70%whileothersourceshaveefficiency15-
20%(gasolineengine)and30-35%(dieselengine).
2)Theefficiencyofthefuelcelldoesnotdependonthesizeofthepowerplant.
3)Maintenancecostisverylow.
4)Fuelcellsaremoreefficientinproducingthemechanicalpowertodrivethevehicles
andrequirelessenergyconsumption.
Disadvantages:
1)Initialcostoffuelcellishigh.
2)Lifetimeoffuelcellisnotknownaccurately.
3)Thereisaproblemofdurabilityandstorageoflargeamountofhydrogen