Batteries 02

7/28/2019 Batteries 02

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BATTERIES:

A device, comprises an electrochemical cell or a series of

electrochemical cells which convert chemical energy of a

spontaneous redox reaction into direct electric current at constant

voltage. Batteries are in continuous demand for power sources as

they are compact, dimensionally adaptable, able to operate over a

wide temperature range and highly dependable. There are

following three types of batteries.

A. Primary batteries

B. Secondary batteriesC. Reserve batteries


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A. Primary Batteries

A primary battery can be defined as a device in which the cell reaction is

non-reversible and it cannot be recharged. So, the amount of energy it can

deliver is limited to that obtainable from the reactants that were placed in it

at the time of manufacture.

The most common examples of these types of batteries are

leclanche cell, Alkaline cell,

button cells,

lithium battery.

So that electrical energy can be obtained as long as the active materials arestill present. Once these have been consumed, the cell can not be profitably

or readily rejuvenated and must be discarded. Or in other words, they

cannot be recharged and re-used.


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B. Secondary batteries/Storage cells

/Accumulators

A secondary battery once used can be recharged by passing current

through it so that it can be used over and over again. Its electrode reactions

are reversible so can proceed in either direction. During charging electrical

work is done on the cell to provide the free energy needed to force the

reaction in the non-spontaneous direction so that redox reaction gets

reversed during recharging. Thus electrical energy is stored in the form ofchemical energy and utilized for supplying the current when needed.

Whereas, a primary cell acts only as a galvanic or voltaic cell, a secondary

cell can act both as galvanic cell and electrolytic cell. During discharging it

acts as a galvanic cell, converting chemical energy into electrical energy

and during charging it acts as an electrolytic cell converting electrical energyinto chemical energy. e.g

Lead-acid battery,

Nickle-Cadmium battery.


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C. RESERVE BATTERIES:

In this type of batteries, a key component is separated from the rest of thebattery prior to activation.

These batteries designed in such a way that they are capable to store in aninactive state by isolating/inactivating some vital cell components (usuallyelectrolyte) and made ready prior to use by adding/activating thatcomponent which capable them for long-term storage

e.g thermal batteries are activated by melting solidified electrolyte whichthen become conductive,

water activated batteries, the electrolyte solute is already contained in thecell while only water is added for activation.

Batteries which use highly active component material are designed in this

form to withstand deterioration in storage and to eliminate self dischargeprior to use. They are usually used to deliver high power for relatively shortperiods of time needed as in missiles, torpedoes and other weaponsystems.


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A. Primary Batteries

1. Dry Cell or Leclanche cell


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Cell Reactions

A. Primary Reactions (Electrode reactions)At Anode

Zn Zn2+ + 2e- [Oxi Half Reaction]

At Cathode2MnO2 + H2O + 2e- Mn2O3 + 2OH- [Red Half Reaction]

Net primary cell reactionZn + 2MnO2 + H2O Zn2++ Mn2O3 + 2OH-

Cell Working Mechanism

Zn | ZnCl2, NH4Cl || MnO2 | C

2e- 2e_

Zn2+ 2OH_


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As discharging going on

In Anolyte Cations become in excess

In Catholyte Anions become in excess

This Cation and Anion disbalance retards further performance of

anodic and cathodic cells so excess cations and anions are removed

by secondary reactions.

B. Secondary reactions (side reactions)

2NH4Cl + 2OH 2Cl + 2NH3 + 2H2O 2NH4OH

Zn2+ + 2NH3 +2Cl [Zn(NH3)2Cl2]


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NH4Cl is used to convert OH in undissociated form as NH4OH which is a weak base.If NaCl is used than OH would not convert in undissociated form as NaOH is strongbase.

As dry cell has low water concentration so most of ammonia do not dissolved in waterbut this excess ammonia react with ZnCl2 to form a Zinc Complex.

NH4Cl in paste ofZnCl2, can attack on Zn metal cylinder to liberate H2 gas as follow

Zn + 2NH4Cl [Zn(NH3)2Cl2] + H2

During discharging, in anodic region electrode is consumed while in cathodic region

electrolyte is consumed i.e reason they taken in larger quantities as compared to theircounterpart for increasing duration of cylinder.

Anode take part in redox reaction so undergoes chemical changes while cathode donot undergo chemical changes i.e reason Anode should be of high surface area andmore in quantity as compared to cathode.

Characteristic Properties:

Its very cost effective to give voltage of 1.5 volts but have limited shelf life due to selfdischarge.


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2. ALKALINE BATTERY:

It is modern version of dry cell.

Its called so b/c instead of NH4Cl, an alkaline electrolytic solution

(usually KOH & NaOH) is used which permits cell

to deliver higher current, avoids corrosive effects of acidic ammonium ion

increase shelf life 5-8 times more than dry cell

and supply constant voltage of throughout its lifetime as conc of

OH- does not change in electrolyte during discharging.


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Cell Reactions

At Anode

Zn + 2OH- ZnO + H2O + 2e- [Oxi Half Reaction]At Cathode

2MnO2+ H2O + 2e- Mn2O3+ 2OH- [Red Half reaction]

Net cell reaction

Zn + 2MnO2 ZnO + Mn2O3


ZnO

2e_

2e_

2OH_

+ Mn2O3

Zn | KOH || MnO2 | C


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ii- Mercuric oxide Zinc alkaline cell /

Mercury button or coin cell

It has high electric storage density

Cell output is 1.34 Volts

Anodic Area Cathodic area

Anode = Powdered Zinc incontact with steel cell cap

Cathode = Mercury in contactwith steel cell casing

Electrolyte = Paste of KOH oraqueous solution of NaOH

Electrolyte = HgO

Cell Configuration

Zn|KOH || HgO| Hg

Anodic cell Cathodic cell


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Cell Reactions

At Anode: Zn + 2OH-

ZnO + H2O + 2e- [Oxi Half Reaction]At Cathode: HgO + H2O + 2e- Hg + 2OH- [Red Half Reaction]

Net cell reaction: Zn + HgO ZnO + Hg


ZnO

2e_

2e_

2OH_

+ Hg

Zn | KOH || HgO | Hg


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iii. Silver Oxide Zinc alkaline cell /

Silver button / coin cell

Anodic Area Cathodic Area

Anode = powdered Zinc in

contact with steel cell cap

Cathode = silver in contact

with steel cell casing

Electrolyte = Paste of KOH oraqueous solution of NaOH

Electrolyte = Ag2O

Cell Configuration

Zn| KOH || Ag2O| Ag



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Cell Reactions

At Anode: Zn + 2OH- ZnO + H2O + 2e- [Oxi Half reaction]At Cathode: Ag2O + H2O + 2e- 2Ag + 2OH- [Red Half reaction]

Net cell reaction: Zn + Ag2O ZnO + 2Ag


This cell gives an emf of 1.6 volts with KOH as an electrolyte.

ZnO

2e_

2e_

2OH_

+ 2Ag

Zn | KOH || Ag2O | Ag


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KOH offers less resistance to cell current flow so make it efficientunder higher current drains. Therefore KOH types are better for theshort bursts of higher current drain,e.g LCD watches with backlights.

On other hand NaOH offers comparatively more resistance to cellcurrent flow so make it suitable for low current drain which last twoto three years e.g analog digital watches or digital watches withoutbacklight.


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3. Lithium Batteries:

The term lithium battery refers to a large family of batteries with a

common feature of having lithium negative electrode (anode). Thedifferent lithium battery systems differ in the choice of electrolytemedium and of positive electrode system. Lithium is an ideal anodematerial owning to its light weight, high oxidation potential and goodconductivity.

The electrolyte used in lithium batteries can not be aqueoussolutions, because of the high reactivity of lithium with water.Following non aqueous solutions are used

LiX + Aprotic polar organic solvent (propylene carbonate, THF,ethers & acetonitrile)

LiX + Aprotic inorganic solvent ( thionyl chloride, sulfuryl chloride)

An ion conducting Li salt electrolyte (LiI or Li + Al2O3)

LiX + Ion conducting organic polymers (Polyethylene Oxide).

Molten mixture of Lithium salt (LiCl + KCl)


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Li cells have longer shell life for over 10 up to 20 years b/c it have

ability to react with its surrounding electrolyte layer to form a stable

passivation layer (e.g. lithium chloride, lithium dithoinite, lithium

hydroxide & lithium carbonate etc. which depend upon type of

electrolyte used) which retard further dissolution of electrode. Inspite of passivation film lithium electrode may be activated quickly &

easily by applying electrical load. Which break down passivation film

quickly within fractions of seconds. Protic solvents are not used in

lithium cells b/c they are not able to produce a sufficiently stable and

really passivating layer with lithium electrode.


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i- Lithium Manganese dioxide

Battery

Anodic Area Cathodic Area

Anode = Lithium metal Cathode = Carbon

Electrolyte = KOH paste Electrolyte = MnO2

Cell Configuration

Li|KOH || MnO2| C



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Cell Reactions

At Anode Li + OH- LiOH + e- [Oxi Half Reaction]

At Cathode MnO2 + 2H2O + e- Mn(OH)3 +OH- [Red Half reaction]

Net cell reaction Li + MnO2 + 2H2O LiOH + Mn(OH)3


LiOH

e_

e_

OH_

+ Mn(OH)3

Li | KOH || MnO2 | C


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Advantages of Li- manganese battery

i- High energy density is stored as 1F is release by dissolution of 7gof the metal.

ii- Long shelf life.

iii- Low self-discharge rate (less than half of Ni based batteries).

iv- Needs less maintenance.v- Can provide very high current and voltage (upto 4 v) depend uponcathode material.

vi- No memory and scheduled cycling is required to prolong thebatterys life.

vii- Cause little harm when disposed.viii- Retain constant voltage and resistance throughout discharging

process.


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Limitations of Li- manganese battery

i- Show aging effect even if not in use but this can

reduce by storing charged battery in a cool space.

ii- Air transportation is restricted due to fire hazards

iii- Its manufacturing cost is 40% higher than nickel-

cadmium battery.


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B- Secondary batteries / Storage cells / Accumulators

1- Pb-Acid battery:


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Anode = Lead frame grids consist of spongy lead

Cathode = Lead frame grids consists of PbO2

Separator = Insulating sheet of micro porous polyethylene divider

Electrolyte = About 4 M H2SO4 solution saturated with PbSO4

Cell Configuration :

Pb | PbSO4

, H2

SO4

|| PbSO4

, H2

SO4

| PbO2

| Pb



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CELL REACTION:

1. Discharging

At Anode i. Pb Pb+2 + 2e-ii. Pb+2 + SO4-2 PbSO4

Net anode reaction Pb + SO4-2 PbSO4 + 2e- Eoanode= - 0.36V

At Cathode i. PbO2 + 4H+ + 2e- Pb+2 + 2H2O

ii. Pb+2 + SO4-2 PbSO4Net cathode PbO2 + 2H+ + H2SO4 + 2e- PbSO4 + 2H2Oreaction Eocathode = +1.69V

Net cell Pb + PbO2 + 2H2SO4 PbSO4 + PbSO4+ 2H2O +Eelectrical

Discharging reaction

Eocell= Eocathode - Eoanode = +1.69 -(- 0.36) = 2.05 V


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As each electrode pair develops a potential of 2 V, a number of cell

electrode pairs are connected in series to obtain higher potential. So

voltage of Pb-Acid battery is always in multiple of 2 volts. Usually six

electrode plates pairs are used to make a battery of 12 volts output.

Cell discharging mechanism

Pb | H2SO4, PbSO4 || PbSO4, H2SO4 | PbO2 | Pb

2e_

2e_

Oxidation Reduction


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As discharging process consumed H2SO4 and produced water while

H2SO4 is about twice dense than water so as the cell is discharge,mass density (specific gravity) of electrolyte solution also decreased

therefore voltage of battery drop and it becomes less efficient b/c as

H2SO4 concentration decreased, rate of discharging would also

decreased. Therefore state of charge or efficiency of a battery can

be estimated by measuring the mass density of electrolyte solutionthrough hydrometer. e.g

When Conc of H2SO4 is 39.2 %, 21.4%, 7.4% than electrode

potential of single pair would be 2.14 V, 2.0 V, 0.9 V respectively.


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Recharging:At Cathode (previous anode):

PbSO4 + 2e- Pb + SO4-2

At Anode (previous cathode):PbSO4 + 2H2O PbO2 + SO4-2+ 4H+ + 2e-

Net cell recharge reaction:

2PbSO4 + 2H2O Pb + PbO2 + 2H2SO4

Cell recharging mechanism

Pb | H2SO4, Pb2+

SO4 || Pb2+

SO4, H2SO4 | Pb4+

O2 | Pb

2e_2e

_|_ +|

Pb2+ Pb

2+

Reduction Oxidation

Discharging

RechargingPb + PbO2 + 2H2SO4 2PbSO4 + 2H2O + EelectricalOverall cell reaction


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Limitation:

Heavy weight battery due to weight of Pb.

In cold weather efficiency decreases b/c as electrolyte solution becomemore viscous, it inhibit flow of ions b/w plates and reduce current delivery.

It have tendency to self discharge.

Fast charging of battery cause electrolysis of water molecule hence release

H2 gas which can tear off PbO2 from cathode. This will settle down at bottomof battery and can cause short circuit of electrode

Some of PbSO4 ppt do not deposited at electrode hence do not convertedinto PbO2 during charging which decrease battery capacity.

Overcharging may damage electrodes which may lead to explosion.

These batteries pose a significant danger if leakage occurs, as theelectrolyte used in lead-acid batteries is very corrosive sulfuric acid.

Full discharge should be avoided as during discharging both electrodes aresulfated and if cell is completely discharged then both lead electrode plateswould entirely sulfated and become identical so terminal voltage collapseand reaction become irreversible.


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Effect of solubility of electrode discharge

product in secondary batteries

It is an important factor which influences stability of electrode structure,hence determined number of discharge /charge cycle that a battery can

undergoes. If electrode discharge product is highly soluble then discharging

cause loss of electrode initial structure by dissolution of electrode to a larger

extent which leads to problems during recharging b/c redeposition of

electrode material is fast at bottom where concentration/density of solutionis high. Hence deformation/distortion of electrode structure occur which

reduce efficiency & cycle life of battery. Moreover during recharging

precipitated electrode material can disintegrate to form dendrites which may

penetrate into separator and reached the opposite electrode, thus gradually

establishing a short circuit e.g. Zn & Li metals discharge electrode product

are highly soluble thus not fit to use as an efficient electrode for secondary

batteries.


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But if discharging electrode product is extremely insoluble (e.g.Cd(OH)2 .PbSO4 etc, Pb+2 ions have solubility in order of 10-6

mole/dm3 in presence of H2SO4 or SO4-2 ions) then it leads to

formation of insulating covering layer on electrode surface. Thus

discharge reactions come to halt as this passivating layer completely

covered electrode surface. So only a thin layer of active material

surface can reacts. This passivation problem is encountered byincreasing surface area through physical changes in electrode

structure i.e. a spongy electrode structure is used which has a large

surface area. This will lead to precipitations of reaction product

within the pores of active material surface i.e. close to place of their

origin hence structure of electrode remain nearly stable.


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2. SECONDRY LITHIUM BATTERIES:

Initially lithium batteries were not considered safe for use insecondary modes as they had some safety problems. During

discharge lithium anode not only change it oxidation state but also

migrates quantitatively to the cathode so that structure of anode

completely decomposed. During recharging Lithium metal built upagain through uncontrolled granular deposition at anode thus

yielding large microscopic surfaces of high lithium reactivity.

Moreover if Li deposited as metals within pores of separator then it

cause serious cell short circuit. Overcharging of Li batteries also

leads to explosion.


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i. Li ion battery:

Li ion technology offered advantages of light weight and high energy

density in a rechargeable and safe mode.

Chemistry ED (Wh/L) ES (Wh/kg)

NiCd 120-150 40-50

NiMH 250-300 70-80

Li ion 280-320 110-130


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The lithium-ion technique operates with host lattices for both anodeand cathode, between which lithium ions are exchanged duringdischarge and charge. The anode of carbon in the graphitic or cokeform, contains the lithium in the charged state and delivers it to thecathode made from transitional metal oxide MeO2 during dischargewhere it is able to build in a maximum amount of lithiumcorresponding to the formula LiMeO2. The lithium ions migrateduring cycling forth and back between the two host lattices of Cx

and MeO2. The following reaction scheme shows this in a simplifiedmanner:

Anode: LiC6 C6 + Li+ + e-

Cathode: Me+4O2 + Li+ + e- LiMe + 3O2

Cell LiC6 + MeO2 C6 + LiMeO2 E0 = 4V

Where MeO2 = CoO2, NiO2 & Mn2O4

For recharge the arrows have to be reversed. This back and forth of the lithiumions is named rockingchair orswing Principle.


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Anodic material:


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Anodic material:

Carbon is a candidate for anodic host lattice due to its ability to build up stable, highly

Porous particles of graphitic-layered structure which consist of parallel staggered layers

of carbon hexagons units arranged in two dimensional network like structure as honey

comb. Each carbon layer consists of sigma bond skeleton mounted by bond structure.

Two adjacent carbon planes are held together by weak Vander waal forces i.e. no

chemical bond cross linking is present. Hence normal distance b/w adjacent carbon layer

of 3.35 Angstrom may be widened easily by incorporation of foreign atoms/ions.

This behavior of graphite enables it to use as intercalating electrodes for intercalation of

lithium. Moreover incorporation of lithium cations b/w graphitic carbon layers meansaccumulation of positive charges which than compensated by intake of electrons into

conducting band of -bonding system. of lithium ions for incorporation is limited to

formula LiC6 to LiC2 As lithium cations are of very smaller size(A0) so a slight increase

in distance b/w carbon layers is required for incorporation of lithium ions. This thing

justified by x-rays analysis of pure graphite and lithium containing graphite. So only a

little activation energy is needed for ion-out migration i.e in-excorporaton of Li due to this

a small over voltage is needed is for recharge when Li ions are inserted again in graphite.

Therefore redox potential for lithium incorporation is near to potential of electrochemical

solution for deposition of pure metallic lithium. Consequently lithium ion cells deliver high

CCV values similar to Li primary cells, which are based on a pure lithium metal anode.


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Cathodic Material:In Li ion batteries that substances should be used as cathode which,

can intercalate and discharge lithium ions at a highly positive potential

compared to the intercalations into carbon cathode.

Offers low kinetic hindrance for insertion and release of Li ions. Can

hold & release electrons to compensate for Li-cation in-excorporation.


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These requirements fulfilled by transitional metal oxides MeO2 like CoO2, NiO2

& Mn2O4 etc. Crystals of these metal oxides have layered structure with cubic

closet packages. During discharge of the cell, Li+ ions are inserted b/w these

layers whereas geometry of their crystal is changed only slightly by Li in-and

excorporations. Thus offering low kinetic hindrance. Transitional metal atoms of

these compounds can easily change their oxidation state due to easy

transactions of valence electrons in their 3dn valence bands, which is

necessary to compensate for simultaneous intake or release of Li ions.Although transitional metal halides and sulfides have above mentioned

Properties but they are poor electronic conductor than oxides, so not used as a

cathode material. Whereas Mn2O4 fulfilled all cathodic material requirements

but practically it is not in use b/c in the case of Mn2O4 cathode over

discharging make battery irreversible a/c to following discharging step

mechanism.


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(i) Discharging

Mn2O4 + Li+ + e LiMn2O4

This Li ion incorporation takes place at a cell voltage of about 4V and it

is reversible

(ii) Over dischargingLiMn2O4 + Li+ + e Li2Mn2O4

This Li incorporation takes place at a cell voltage of about 3V but it is

irreversible.


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Electrolyte for Li-ion batteries:

Electrolyte consists of electrolytic salt and solvents which should have following

quality criteria. The quality criteria for electrolyte solvent are

high dielectric constant for good solvation of salt.

Low viscosity for increasing ionic mobility or conductance.

Low freezing point for good low temperature performance.

Inert to electrodes thus avoid exfoliation.

(Exfoliation is the destruction of anode graphite structure by incorporation of solvent

molecules thus destroying cohesion of carbon layers during discharge-charge cycles.

Cyclic esters like propylene carbonate (PC) and Ethylene carbonate (EC) are used as

electrolyte solvent due to high dielectric constant but they are highly viscous b/c of their

polarity. To compensate for it, low viscous straight alkyl esters like dimethyl carbonate(DMC) & diethyl carbonate (DEC) are added. Today lithium-ion batteries with electrolyte

mixture of PC + DEC and EC + DMC are found in the market. Lithium salts with

voluminous anions are used as electrolyte salts, to kinetically retard anions mobility e.g Li

ClO4, LiBF4, LiAsF6, LiPF6 and LiAlCl4


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Protection against overcharging:

As overcharging may deposit metallic Lithium at anodes which is

dangerous b/c of its high reactivity. As a protective measure a slight

charging at voltage over to 4.8 V. Thus creates overpressure inhermetically sealed cell by its gaseous reaction products. A suitably

designed membrane bulges consequently and interrupts the contact

b/w cathode and batterys positive pole.


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Protection against overheating:

One substantial problem of Li technology is lower Melting point of Li

metal at 180oC. In contrast to solid state, molten Li metal stronglyreact with cathode and electrolyte material, releasing huge amountof heat which leads to blast of cell case. Cell lid contains positiveelectrode which has outer contact through thermo switch foil(polyswith ). Polyswith is made of polymer foil which isconductive at normal temperature but becomes non-conductive at a

defined threshold temperature. Thus gives protection againstoverheating. These cells also comprises thermally soft or weakseparators which close their pores at a defined high temperatureand stop any further discharge reactions which hinders thermalrunaway if the cell is shortened.

In new secondary Li-ion systems, safety is controlled by electronic

charge and discharge control devices to avoid any damage by faultyoperation. This achieve by limitation of charge and dischargevoltages, thermo control devices, switches and membranes to avoidoverpressure due to overheating.


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ii. Lithium ion polymer battery.

This class of battery is similar to Li-ion battery except having dry solid gelled

Polymer electrolyte, which replaces conventional porous separator soaked withelectrolyte. Solid electrolyte does not conduct electricity but allows ionic

conductance lower than fluid electrolyte b/c of low ionic mobility. Hence these

cells tolerate very low load b/c of higher cell resistance. On other hand solid

electrolyte reduced side reactions (like self discharge) to negligible. Therefore

this battery system shows high reliability during shelf lives and operationaltimes for many years. This cell fit best only for low rate long time applications

with A current for a number of years e.g cardiac pacemaker battery. These

batteries are smaller in size and light weight as gelled electrolytes enable

Simplified packaging by eliminating metal shell in battery structure hence make

it fit for portable use. It has less safety hazards as chances for electrolyteleakage are rear.


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Limitations:

They have low energy density storage

Manufacturing cost is higher

Cost to energy ratio is also higher than Lithium-ion


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Example: Lithium Iodide solid state cell

Anode: LithiumCathode: Iodine absorbed in dry solid polymer gel of Poly-2-Vinyl Pyridine.

P2VP acts as conducting agent and absorber for Iodine so thatvapour pressure of Iodine reduced.

Electrolyte: Li ICell configuration: Li | Li I || I2 (in polymer)

Cell reactions:At anode 2Li 2Li+ + 2e- At Cathode

At cathode P2VP. nI 2 + 2e- P2VP. (n-1)I 2 + 2I - At Anode

Net reactions 2Li + P2VP. nI 2 P2VP. (n-1)I 2 + 2Li I

discharging

discharging

discharging

recharging

recharging

recharging


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Cell working Mechanism:

2Li | 2LiI || I2

2e_ 2e_

2Li+ 2I-

During discharging

2Li | 2LiI || I2

2e_

2e_

2Li+ 2I-

|_ +|

During recharging

CathodeAnode Anode


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3. Ni-Cd battery

Cell configuration:Cd | Cd(OH)2, KOH || NiO2, Ni(OH)2 | Ni

2NiO(OH)

Cathodic cell electrolyte is not an exactly defined chemical compound

but a mixture of Ni2+, Ni3+ and Ni4+. Thus 2.NiO(OH) in the charged

state more precisely can be written as u.NiO2v.NiOOHw.H2O where

u,v and w are factors that describe the share of three components.


Anode = grid covered with Cd Cathode =Ni frame grids

Electrolyte = Cd(OH)2, KOH Electrolyte = Mixture of NiO2 &Ni(OH)2


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Ni ions remain in their solid crystalline state structure in spite theiroxidation /reduction during charging & discharging cyclerespectively. To preserve electrical neutrality, a correspondingnumber of H+ ions (proton) must migrate into crystal lattice duringdischarge. When nickel electrode is charged (oxidized) these

protons must leave the crystal lattice otherwise local space chargeswould immediately bring the reaction to a standstill. Small H+ ionshave high mobility which facilitates such migrations but requires alarge surface area of the electrode active material in order to keeppenetration distance low i.e. spongy electrode is used.

An important feature of Ni-Cd battery is that no KOH is consumedduring charging or discharging except small amount of K+ ions thatare incorporated into nickel hydroxide during overcharging.


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Cell reactions:

At anode: Cd + 2OH- Cd (OH)2 + 2e- At Cathode

At Cathode: 2NiO(OH) + 2H2O + 2e- 2Ni(OH)2 + 2OH- At Anode

Net Cell reaction

2Ni+3O(OH) +2H2O + Cd 2Ni+2(OH)2 + Cd+2(OH)2

discharging

recharging

recharging

recharging

discharging

discharging


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Cell working mechanism:

Discharging

Recharging

Cd | Cd(OH)2,KOH || 2NiO(OH) | Ni

2e_

2e_

Cd++ 2OH

-+ 2Ni(OH)2

Anode Cathode

Cd | Cd(OH)2, KOH || 2NiO(OH) | Ni

2e_

2e_|_ +|

Cd++

2OH- 2OH

-+ 2Ni(OH)2

AnodeCathode


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This has cell nominal potential 1.2V so12V NiCd batteries are made up of10 cells connected in series. This is compact way of storing electricalenergy reversibly thus have lower weight for a given quantity of storedenergy. It used in small consumer devices. They have good chargingefficiency(as charging is endothermic reaction so cause cooling of NiCdbatteries), small variation in terminal voltage during discharge (It is considerdrawback b/c it is difficult to detect when the battery charge is low), lowinternal resistance and non critical charging conditions.

They can deliver higher surge current and undergoes hundred of charge &discharge cycles but their use is being discouraged b/c Cd is anenvironmental toxin more over it show sever memory effect and mAh ratingis not high enough to keep device running for a long period.

The alkaline electrolyte (KOH) is not consumed in cell reactions andtherefore it specific gravity is not a guide to its state of charge as in lead

acid batteries.

4. Nickel Metal hydride cells (Ni - MH Cells)


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Cell Configuration: MH | KOH || NiOH | Ni

Ni MH cell is similar to Ni-Cd Cell except it uses hydrogen,

adsorbed in a metal alloy for active negative material in

place of Cd in Ni-Cd cell.


Anode = Ni wire gauze grid framecoated With highly purpose &spongy metal alloys of AB2 & AB5class like LaNi5, ZrNi2 etc. Which

hold H2 gas as their hydrides

Cathode = A highly poroussintered or felt Ni substrate

Electrolyte = conc. KOH solutionabsorbed in non woven syntheticmaterial separator.

Electrolyte = Nickel oxyhydroixideimpregnate into cathode


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Cell Configuration: MH | KOH || NiOH | Ni

Ni MH cell is similar to Ni-Cd Cell except it uses hydrogen, adsorbedin a metal alloy for active negative material in place of Cd in Ni-Cd cell.

Anodic material:

In this cell H2 is not stored as a gas but hold by Anode at low internal

pressure so do not required a cell container that can with stand high

pressure.

There are two types of transitional metals which formed metallic

hydrides or Interstitial hydrides. Strong-hydride forming metals e.g

lanthanum, Titanium, Zirconium & Vanadium etc.A + H2 AH

(most of Hydrogen exist in hydride form)


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Weak-hydride forming metals Ni, Pd, Pt, Al etc.

B + H2 B.H2

(most of Hydrogen exist in diatomic molecule form)


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If any one material used as anode it make cell irreversible so inter

metallic compounds are formed by making alloys of both metals in AB2

& AB5 system while A is strong hydride forming metals and B is weak

hydride forming metals. e.g LaNi5.ZrNi2, TiNi2, LaNi4.1Al0.3 etc

A + B. H2 AH + B

This intermetallic anode absorbed H2 gas as hydride during charging

while desorbed H2 gas during discharging. So both metals A& B also

acts as catalytic electrode surface for absorption & desorption of H2gas respectively.

absorption

desorption


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Cell Reactions

Discharging

At anode i) MxH XM + H2

ii) H2 + OH- H2O + e-

At Cathode NiO(OH) + H2O + e- Ni (OH)2 + OH-

Net cell discharge MxH + Ni3+O(OH) xM0 + Ni2+(OH)2reactions is

desorption

absorption


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Discharging mechanism:

Hydrogen desorption is an endothermic process so cooling effect is

observed when cell is discharge. Nominal cell discharge voltage is 1.3V


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Recharging:

At Anode Ni(OH)2

+ OH - NiO(OH) + H2

O + e-At Cathode i) H2O + e- H2 +OH-

ii) xM + H2 MxH

Net cell recharging

Reaction is Ni+2(OH)2 + xM0 Mx+1 H + Ni+3O(OH)

Discharging mechanism:

5 Zinc Air cell


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5. Zinc-Air cell

Zinc-Air battery has very compact size catalytic cathode which used

oxygen directly from atmosphere as an active material of cathode toproduce electrochemical energy by delivering electrons for reduction

of oxygen (O2 + 4e- 2O2-). A very thin cathode of the cell (about

0.5 mm) permits the use of very large zinc anode. This reduce

volume requirement for cathodic half cell so more space is available

for anodic part while cell capacity now depend open anodic part. AsZn is used as anode which is light weight metal and offers a very

negative reduction potential so weight & volume of battery are

reduced while cell capacity & energy density are increased.

Anode = granulated powder of ZnElectrolyte = Aqueous alkaline electrolyte (30%KOH)


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Cathode =Gas diffusion electrode (GDE).

It consist of

catalyst layer contains carbon blended with oxides of manganesewhich mixed with wet proofing agent coated on Ni plated steelmesh support to form conducting medium

an outer layer of gas permeable Teflon.

A number of holes which provides a path for oxygen to enter the

cell and diffuse to cathode catalyst site.Both electrodes are separated by an electrolyte absorbent separator.

Cell Configuration Zn| KOH || O2 | Cathode Catalyst


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Cell reactions:At anode i) Zn Zn2+ + 2e-

ii) Zn2+ + 2OH ZnO + H2ONet anode Reaction Zn + 2OH ZnO + H2O + 2e-

At cathode O2 + H2O + 2e- 2OH-

Net cell reactions Zn + O2 ZnO

During cell reaction, alkaline electrolyte and cathode remain invariant.

H2OZnO

Zn2+

Zn | KOH || O2 | Cathode Catalyst

2e_

2e_

2OH_


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Zn-air cell has OCV of 1.4V. Although it has high energy density,

longer shelf life if sealed, no environmental hazards and lower

cost but has limited power out put and short operational life. Zn-air cells are rechargeable but two problems arise in respect to

recharging.

High solubility of Zn causes shape distortion in each cycle

Intensive oxygen evolution that occurs during charge which may

harm the positive electrode.

Due to this, Zn electrode can stand only a very limited number of

cycles (


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1. Phosphoric acid fuel cell (FAFC)

Electrolyte: Concentrated phosphoric acid absorbed into a solidmatrix of silicon carbide.

Operating temperature: 150 200oC

Intermediate high operating temperature b/c at low temperature

phosphoric acid has poor ionic conductivity and carbon monoxide

poisoning of platinum electro-catalyst becomes sever at anode.

Anode Reaction: H2 2H+ + 2e-

Cathode Reaction: O2 + 2H+ + 2e- H2O

Net cell reaction : H2 + O2 H2O

Advantages:


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Advantages:

Its most mature & commercially available fuel cell technology.

PAFCs generate electricity at more than 40% efficiency while 85 % of

produced steam can economically used for co-generation. Excellent thermal, chemical and electro chemical stability of electrolyte.

Relatively low volatility of phosphoric acid above 150 0C compared to otheracid, so It has very low water vaporization rate so steady state waterremoval by reactant gases will always equal to water production rate somanaging water level is not a problem.

Due to intermediate high temperature CO is catalytically oxidized by waterCO + H2O CO2 + H2

so it can use impure hydrogen fuel containing CO up to 1.5% so broadensthe fuels choice range.

Disadvantages It generate low power and current

Larger size and weight

In this cathode performance is sluggish.


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1- ALKALINE FUEL CELL

Electrolyte : 30 45 % w/v aq. KOHOperating temperature : 100 250oC

In alkaline fuel cell Ionic conductance provided by hydroxyl (OH-) ions.

Rather than acid electrolyte, alkaline electrolyte cause more rapid

reduction of oxygen at cathode and feasibly allow use of wide range of

non noble metals electro-catalyst at cathode such as nickel, silver,

metal oxides, spinals and noble metals.

Anode reaction: H2 2H+ + 2e-

2H+ + 2OH- 2H2O

H2 + 2OH- 2H2O + 2e-Cathode reaction: O2 + H2O + 2e- 2OH-

Net cell reaction: H2 + O2 H2O

Ad t


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Advantages

It gives highest cell voltage of all fuel cells due to enhancement ofcathode performance

It tends to be low cost as choice for electrode material widen to nonnoble metals.

It used in space crafts & submarines for generating electricity &drinking water.

Limitations:

Fuel & oxidant must be free from CO2 as it from K2CO3 ppt withalkaline electrolyte which severely limits the cells performance by

blocking electrolyte pathways and electrode pores.

CO2 + 2KOH K2CO3 + H2O

Therefore carbonaceous fuels and atmospheric air cannot used in itas final redox products would contain CO2 so it only restricted topure H2 & O2 gases. In alkaline electrolyte ionic conductor is OH- ion& water is produced at anode, while in acid electrolyte ionicconductor is H+ ion & water is produced at cathode.

Electrode Material:


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Electrode Material:

Expensive pt based catalyst seem to be the only catalyst capable ofaccelerating late of both electrode half reactions platinum is unique

b/c it is sufficiently reactive in bonding H and O intermediate asrequired to facilitate the electorate process and also capable ofeffectively releasing the intermediate to form the final product. Thisrequires optimized bonding to H & O atoms, not too weak and nottoo strong, which is unique feature of Pt catalyst e.g. when hydrogengas diffused at anode through tortuous prose & carbonaceouspathways until a platinum particle encountered which catalyticallydissociate H2 molecule into two hydrogen atoms (H), bonded them totwo neighboring pt atoms them each H atom release an electron toform a hydrogen ion (H+) current flow in circuit as there H+ ions areconducted to cathode through electrolyte.

H2 + 2pt 2ptH

2ptH 2pt + 2H

2ptH 2pt + 2H+ + 2e-


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Realizingthat the pt (best catalytic material for electrode) is very

expensive, lowering pt catalyst levels in electrode is an on-going

effort to reduce cost of fuel cell. This accomplish by constructing

catalyst layers with the highest possible surface area by

impregnating porous carbon with very small size Pt particles, about

2 nanometers in diameter. So enormously large surface area of Pt is

accessible to gas molecules while small amount of Pt used. Thisallow electrode reactions to proceed at many Pt surface sites

simultaneously, which generate significant electron flow i.e current

in a fuel cell. As both Pt & C are good conductor so current is

conducted to external circuit.


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Water management:

Effective operation of fuel cell depends upon ionic mobility/conductance b/w both electrodes through electrolyte which in turn

depends upon hydration level of Anode-Electrolyte-cathode

assembly b/c solvation of ions by H2O molecules increased ionic

mobility. Although water is produced during cell operation but due to

high operating temperature it is carried out with some additionalwater from electrolyte in form of steam which adversely loose ionic

conductivity in electrolyte and cell current drop. This thing rectify by

prehumidifying both fuel & air, entering the fuel cell. On other hand if

fuel / air pass slowly through their respective electrodes than it

cannot carryout all H2O produced at electrode hence electrode

floods. Cell performance is hurt b/c no enough fuel/air is able to

penetrate the excess liquid water to reach electrode catalyst sites.

3 Proton Exchange/Polymer Electrolyte


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3. Proton Exchange/Polymer Electrolyte

Membrane Fuel Cell (PEMFC)

Electrolyte used is a solid organic polymer Poly-Perflouro Sulfonic Acid. Itconsists of fluorocarbon polymer backbone (similar to Teflon) to whichsulfonic acid groups are attached. These acid groups are fixed to thepolymer and can not leak out but protons on these groups are free tomigrate though membrane. Therefore although this membrane is an

electronic insulator due to organic nature but an excellent conductor ofhydrogen ions. So it acts as electrolyte which provides efficient hydrogenions communication b/w anode & cathode thus increased power densityrange from 50 to 250 kw.

Being a solid electrolyte based on teflon it is stable and can not change,

move about or vaporize from the system and also serves as a goodseparator for the both reactant gases, even can withstand elevatedpressure, thus cell can vary their out put quickly to meet shifts in powerdemand.


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Proper water management in PEM is crucial for efficient fuel cell

operation as dehydration of membrane reduces proton conductivity

and excess of water can lead to flooding of electrodes. This thingencounter by using prehumidified reactant gases and adjusting low

operating temperature at 80oC so that water does not vaporize into

reaction air steam faster than it is produced. Low operating

temperatures can not eliminate lethal effect of CO therefore fuel

should be free from CO. Thus choice of fuel limited to pure hydrogenand hydrogen rich gases obtained by decomposition of hydrides.

PEM freezes at about 0oC and undergoes a freeze-drying

phenomena which make PEMFC non operational at lower

temperature. The only liquid in PEMFC is water, which minimize

corrosion thus increases cell life. Due to solid electrolyte, cell issimple to fabricate but electrolyte is very expensive which increased

cell cost.


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4. Molten carbonate fuel cell (MCFC)

Electrolyte is the mixture of alkali metal carbonates (32% Li2CO3 + 28%

K2CO3/Na2CO3 absorbed in molten state in a porous inorganic matrix of 40%LiAlO2).

MCFC operational temperature is 600700oC which is above than M.P ofthese carbonates, hence all alkali metal carbonates would be melted andcause ionic conductance by carbonate anions. High operational

temperature increases efficiency & gives flexibility to feasibly employedinexpensive non noble metal catalysts and to use more type of fuels asbreaking of carbon bonds in larger hydrocarbon fuels occur much faster astemperature is increased.

Porous Ni or Ni-Cr alloy serve as anode while cathode is porous NiO. Highoperating temperature convert any CO in fuel into hydrogen at anode via

water gas shift reaction.CO + H2O CO2 + H2

Therefore MCFC have been operated on H CO CH propane


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Therefore MCFC have been operated on H2, CO, CH4, propane,landfill gas, marine Diesel and simulated coal gasification productsbut have low tolerance to sulfur & chloride compounds. On other

hand high operating temperature enhance corrosion and imposesevere constraints on cell components hence shortened cell life.

Anode reactions: 2H2 + 2CO3-2 2CO2 + 2H2O + 4e-

If CO is present CO + CO3-2 2CO2 + 2e-

or CO + H2O CO2 + H2

Cathode reaction O2 + 2CO2 + 4e- 2CO3-22H2 + O2 2H2O

It can gives voltage as high as 0.9 V while CO2 recycled from anodeexhaust to cathode inlet.

They gives high fuel to electricity efficiencies, about 60% normallyand 85% with cogeneration b/c waste heat is available at a relativelyhigh temperature(~500oC), enabling its use in bottoming or industrialheating cycles.

5 Solid oxide fuel cell (SOFC)


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5. Solid oxide fuel cell (SOFC)

It employed a solid, nonporous metal oxide electrolyte which usually consistof hard ceramic material of Zirconium oxide (Zirconia/ZrO2) doped with smallamount of yttrium oxide (Y2O3/ ytrria). The cell operates at about 1000oCwhere ionic conduction takes place by migration of O2- anion across theelectrolyte through vacant lattice sites. High operating temperature avoidsexpensive catalyst. Anode is made up of Co-ZrO2 or Ni-ZrO2 cermets whilecathode consists of praseodymium oxide or indium oxide or Sr doped with

LaMnO3

. It has high tolerance to impurities so it can be operated by using hydrogenor carbonaceous reformed fuels consist of CO.

Anode reactions: H2 + O2- H2OIf CO is present CO + O2- CO2 + 2e-Cathode reaction: O2 + 2e- O2-

Net cell reaction: H2 + O2 H2O

It does not require CO2 recycle to the cathode. Power generatingefficiencies could reach 60-85 % with cogeneration but cell output relativelyless efficient up to 100 KW.

5 P t i C i F l C ll (PCFC)


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5. Protonic Ceramic Fuel Cell (PCFC)

Most PEM (proton exchange membrane) fuel cells use a polymermembrane, such as Nafion which requires humidity and a low operating

temperature to operate efficiently. As the temperature rises, the polymer

becomes unstable and the membrane dehydrates, leading to loss of

performance. However, operation of PEM fuel cells at higher temperatures

would be more desirable, as the reaction rates at the catalysts areincreased while potential poisoning of the catalysts is reduced.

Ceramics electrolyte material that exhibit high protonic conductively at

elevated temperatures have been proposed as good candidates for use in

PEM fuel cells because of their thermal, chemical, and mechanical stability,

and lower material costs. Although protonic conductivity of mineralmembranes(ceramics) is low in comparison with the perfluorosulfonate-

based membranes but the chemical reactions that create the electricity are

more efficient at higher temperatures.


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Proton conducting ceramic membranes consist of metal-oxane e.g

ferroxane which consist of iron nanoparticles stabilized by carboxylic acid

with lepidocrocite. Another ceramic membrane consists of ferroxane-derived

with PVA (polyvinyl alcohol) calcined at 500C (PVA Fe500) showed the

best performance.

PCFCs share the thermal and kinetic advantages of high temperature

operation at 700oC with molten carbonate and solid oxide fuel cells, while

exhibiting all of the intrinsic benefits of proton conduction in polymerelectrolyte and phosphoric acid fuel cells moreover PCFCs have a solid

electrolyte so the membrane can not dry out as with PEMFC or liquid cant

leak out as with PAFCs. PCFCs can operate at more high temperature for

hydrocarbon fossil fuels to electrochemically oxidize them directly at anode.

This eliminates the intermediate step of producing hydrogen through costlyreforming process.


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B. Zn Air Fuel Cell (ZAFC)

It has similar operations as Zn Air cell but additionally has a closed-loop system consist of Zinc fuel tank and a Zinc regenerator that

automatically & silently, replace & regenerate Zinc fuel. As fuel is

used up, the system is connected to the grid source and process is

reversed to once again give pure Zinc fuel pellets. This reversing

process takes only about 5 minute to complete, so the battery

recharging time hang up is not an issue.

C DIRECT METHANOL FUEL CELL (DMCF)


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C. DIRECT METHANOL FUEL CELL (DMCF)

1. alkaline type:Anode: Pt or Pd catalyst doped on porous nickel sheet.

Cathode: Silver impregnated in porous nickel sheet.

Electrolyte: KOH aqueous solution

Fuel: Humidified methanol

Oxidant: Air

Operating temperature: ~100oC

In DMFC, large amount of expensive Pt catalyst is used at anode so

that it can itself draw hydrogen from liquid methanol thus eliminatingthe need for a fuel reforming sub system and a bulky & heavyhydrogen storage system


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Electrode reactions:

At anode: CH3OH + 6OH- CO2 + 5H2O + 6e-

At Cathode: 3/2O2

+ 2H2

O + 6e- 6OH-

Fuel cell reaction: CH3OH + 3/2O2 CO2 + 2H2O

Alkaline electrolyte has a disadvantage that CO2 release at anode,absorbed & converted into K2CO3 by alkaline electrolyte.

2KOH + CO2 K2CO3 + H2O

which decrease ionic conductivity of electrolyte & increaseconcentration polarization at the electrode surface thus decrease cellefficiency to 40%. The major problem is fuel crossing over from anode

to cathode through electrolyte without producing electricity whichwastes fuel and decreases the performance of the air electrode i.ecathode. Therefore acid polymer electrolyte membrane is better option.


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2. Acid type:

Electrolyte: Proton (H+) conductivity solid polymer membrane.

Electrode reactions

At anode: CH3OH + H2O CO2 + 6H+ + 6e-

At cathode: 6H+ + 6e- + 3/2O2 3H2O

Fuel cell reaction: CH3OH + 3/2O2 CO2 + 2H2O

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