Download Chemistry-Unit-2-Battery-Technology-Cells-and-Battery

BATTERY TECHNOLOGY
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Commercial Cells.
Galvanic cells used as source of electric energy
for various consumer, industrial and military
applications.
Classification:
Primary Cells. Eg.: Dry cell
Secondary Cells. Eg.: Lead acid cell.
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Objectives:
Describe the major features of commercial cells.
Know the two major types of batteries.
Distinguish between primary & secondary
battery types.
Know the various applications of Dry cell, Nicad
cell, Lead acid cell, H2-O2 fuel cell and CH3OH – O2
fuel cell.
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Basic Requirements Of Primary Cell.
 Compactness and lightweight.
 Fabricated from easily available raw materials.
 Economically priced.
 High energy density and constant voltage.
 Benign environmental properties
 Longer shelf life and discharge period.
 Leak proof containers and variety of design options.
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Basic Requirements Of Secondary Cell.
Long shelf-life and cycle life.
High power to weight ratio
Short time for recharging
Tolerance to service condition.
High voltage & high energy density.
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Primary Cells.
Produce electricity from chemicals that are sealed
into it.
Cannot be recharged as the cell reaction cannot be
reversed efficiently by recharging. The cell must be
discarded after discharging.
e.g. Zinc - manganese dioxide cell (Dry cell)
Mercuric oxide – Zinc cell.
Silver oxide – zinc cell.
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Secondary Cells
Generation of electric energy, that can be restored
to its original charged condition after its discharge
by passing current flowing in the opposite
direction.
These cells have a large number of cycles of
discharging and charging.
They are known as rechargeable cells, storage cells,
or accumulators.
e.g. Lead storage cell.
Nickel- cadmium cell.
Lithium- ion batteries.
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Differences
Primary Batteries
 Cell reaction is irreversible





Secondary Batteries
Cell reaction is reversible.
Must be discarded after use. May be recharged
Have relatively short shelf life Have long shelf life.
Function only as galvanic
Functions both galvanic
cells .
Cell & as electrolytic cell.
They cannot be used as
They can be used as energy
storage devices
storage devices (e.g. solar/
thermal energy
converted to electrical energy)
They cannot be recharged
They can be recharged.
e.g.
Dry cell.
Li-MnO2battery.Lead acid,
Ni-Cd battery.
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DRY CELL(LECLANCHE CELL)
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• Anode: Zinc metal container.
• Cathode: MnO2 + Carbon (powdered graphite)
• Electrolyte: Aqueous paste of NH4Cl and
ZnCl2
• Cell Scheme:
Zn(s)/ ZnCl2(aq),NH4Cl(aq),MnO2(s)/C
• O.C.V. = 1.5 V
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Working.
Primary Electrode Reactions:
Anode: Zn(s)→Zn2+ (aq)+ 2eCathode:
2MnO2(s)+H2O(l) + 2e- → Mn2O3(s) + 2OH-(aq)
Net Reaction: Zn(s)+2MnO2(s)+ H2O(l) →
Zn2+(aq)+Mn2O3(s)+2OH-(aq)
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Secondary Reactions:
NH4+(aq)+OH-(aq) → NH3(g)+H2O(l)
Zn2+(aq)+2NH3(s)+2Cl - → [Zn(NH3)2 Cl2]
Zn + 2MnO2 + 2NH4Cl →[Zn(NH3)2Cl2]+ H2O+
Mn2O3
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Applications:

In small portable appliances where small
amount of current is needed.

In consumer electronic devices- quartz wall
clocks, walkman etc.
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Advantages.
Dry cell is cheap.
Normally works without leaking (leak proof
cells).
Has a high energy density.
It is not toxic
It contains no liquid electrolytes.
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Disadvantages.
Voltage drops due to build up of reaction products
around the electrodes when current is drawn
rapidly from it .
It has limited shelf life because the zinc is corroded
by the faintly acid,ammonium chloride.
The shelf life of dry cell is 6-8 months.
They cannot be used once they get discharged.
Its emf decreases during use as the material is
consumed.
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Lead-acid battery:
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LEAD STORAGE BATTERY.
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• Anode: Spongy lead on lead grid.
• Cathode: Porous PbO2.
• Electrolyte: H2SO4(aq)( 20 %)
(density 1.21-1.30g/ml)
• Cell Scheme:
Pb/PbSO4;H2SO4(aq);PbSO4;PbO2/Pb
O.C.V. = 2V (Pair of plates)
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Reactions during discharging.
• Anode: Pb (s) → Pb2+ (aq) + 2ePb2+(aq) + SO42-(aq) → PbSO4(s)
Pb(s)+ SO42-(aq) → PbSO4(aq) + 2e• Cathode:PbO2(s)+ 4H+(aq)+2e- →Pb2+(aq)+ 2H2O(l)
Pb2+(aq)+SO42-(aq)→PbSO4(s)
PbO2(s)+4H+(aq)+SO42-(aq)+2e- → PbSO4(s)+
2H2O(l)
• Overall: Pb (s)+PbO2 (s)+4H+(aq)+ 2SO42-(aq) →
2PbSO4 (s)+2H2O(l)
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Charging the Lead-acid battery:
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Charging reactions
• Cathode:
PbSO4(s)+2H2O(l)→PbO2(s)+ SO42-(aq)+4H+(aq) +2e• Anode :
PbSO4(s) + 2e- → Pb(s)+ SO42- (aq)
• Net:2PbSO4 (s)+ 2H2O(aq) → Pb(s)+ PbO2(s)
+2H2SO4
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Limitations.
• Self discharge: They are subject to self discharge with H2
evolution at negative plates and O2 evolution at positive
plates.
Pb +H2SO4
PbSO4 + H2
PbO2 + H2SO4
PbSO4 +H2O +1/2 O2
SO42- +2 H+ (From dissociation of water)
H2O
H+ +OH-
H2SO4
• Loss of Water: Due to evaporation, self discharge and
electrolysis of water while charging. Hence water content
must be regularly checked and distilled water must be
added.
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• Sulfation: If left in uncharged state, for a prolonged period, or
operated at too high temperatures or at too high acid
concentrations, transformation of porous PbSO4 into dense
and coarse grained form by re crystallization.
* This results in passivation of negative plates inhibiting
their charge acceptance.
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• Corrosion of Grid: Can occur due to overcharging
when grid metal gets exposed to the electrolyte. This
weakens the grid and increases the internal
resistance of the battery.
• Effectiveness of battery is reduced at low
temperature due to increase in the viscosity of
electrolyte.
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• Recent years have seen the introduction of
“maintenance – free batteries” without a gas –
release vent. Here the gassing is controlled by
careful choice of the composition of the lead alloys
used i.e. by using a Pb-Ca (0.1 % ) as the anode which
inhibits the electrolysis of water
•
Alternatively, some modern batteries contain a
catalyst (e.g. a mixture of 98% ceria (cerium oxide) &
2% platinum, heated to 1000o C) that combines the
hydrogen and oxygen produced during discharge
back into water. Thus the battery retains its potency
and requires no maintenance. Such batteries are
sealed as there is no need to add water and this
sealing prevents leakage of cell materials.
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Applications.
*Automative: For starting, lighting and
ignition of IC engine driven vehicles.
*Consumer Applications: Emergency
lighting, security alarm system.
*Heavy duty Application: Trains, lift trucks,
mining machines etc.
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Advantages:
A lead storage battery is highly efficient. The voltage
efficiency of the cell is defined as follows.
Voltage efficiency = average voltage during discharge
average voltage during charge
The voltage efficiency of the lead – acid cell is about
80 %.
The near reversibility is a consequence of the faster
rate of the chemical reactions in the cell i.e. anode
oxidizes easily and cathode reduces easily leading to
an overall reaction with a high negative free energy
change.
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 A lead – acid battery provides a good service for
several years. Its larger versions can last 20 to 30
years, if carefully attended (i.e. longer design life)
 It can be recharged. The number of recharges
possible range from 300 to 1500, depending on the
battery’s design and conditions. The sealed leadacid batteries can withstand upto 2000 –
rechargings. Generally the most costly, largest,
heaviest cells are the longest–lived.
 The battery’s own internal self – discharging is low.
 The length of time that is generally required for recharging process is less i.e. recharge time is 2-8
hours depending on the status of battery.
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Low environmental impact of constituent
materials is an added advantage
It has sensitivity to rough handling and good
safety characteristics.
Ease of servicing as indicated by several local
battery service points.
It is a low- cost battery with facilities for
manufacture throughout the world using
cheap materials.
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NICKEL- CADMIUM CELL
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Anode: Porous cadmium powder
compressed to cylindrical pellets.
Cathode: Ni(OH)3 or NiO(OH) mixed with 20%
graphite powder
Electrolyte: 20-28% Aq. KOH jelled with a
jelling agent.
Cell Scheme:
Cd/Cd(OH)2,KOH,Ni(OH)2, Ni(OH)3/Ni
O.C.V. = 1.25V
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Reactions during discharging.
Anode:
Cd(s)+2OH-(aq)→Cd(OH)2(s)+ 2eCathode:
2Ni(OH)3(s)+2e- → 2Ni(OH)2(s)+2OH-(aq)
• Net Reaction:
Cd(s)+2Ni(OH)3(s)→ 2Ni(OH)2(s)+ Cd(OH)2(s)
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Charging reactions:
Anode:
Cd(OH)2(s)+2e-→ Cd(s) +2OH-(aq)
Cathode:
2Ni(OH)2(s) +2OH-(aq)→2Ni(OH)3(s)+2eNet:
2Ni(OH)2(s)+Cd(OH)2(s)→2Ni(OH)3(s)+ Cd(s)
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Discharging reaction:
Anode: Cd(s)+2OH-(aq) → Cd(OH)2(s) + 2e-
Cathode: 2NiO (OH) (s) + 2 H2O + 2 e- →
2Ni (OH)2(s) + 2OH-(aq)
Net Reaction: Cd(s) + 2NiO (OH) (s) +
2H2O → 2 Ni(OH)2 (s) + Cd(OH)2(s)
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Charging reactions:
-ve pole: Cd(OH)2 (s) + 2e-→ Cd(s) + 2OH-(aq)
+ve pole: 2 Ni(OH)2(s) + 2OH-(aq) → 2 NiO(OH)
(s) + 2H2O+2eOverall reaction: 2 Ni(OH)2 (s) + Cd(OH)2(s) →
2 NiO(OH) (s) + Cd(s) +2H2O(l)
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Applications.
In flash lights, photoflash units and portable
electronic equipments.
In emergency lighting systems, alarm systems.
In air crafts and space satellite power systems.
For starting large diesel engines and gas
turbines etc.,
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Advantages.
Can be recharged many times.
They maintain nearly constant voltage level throught their
discharge. There is no change in the electrolyte composition
during the operation.
It can be left unused for long periods of time at any state of
charge without any appreciable damage (i.e. long shelf life).
It can be encased as a sealed unit like the dry cell because
gassing will not occur during nominal discharging or
recharging.
They exhibit good performance ability at low temperatures.
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They can be used to produce large instantaneous
currents as high as 1000-8000 A for one second.
It is a compact rechargeable cell available in three
basic configurations – button, cylindrical and
rectangular.
They have low internal resistance.
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Disadvantages.
 It poses an environmental pollution hazard due to
higher toxicity of metallic cadmium than lead.
 Cadmium is a heavy metal and its use increases the
weight of batteries, particularly in larger versions.
 Cost of cadmium metal and hence the cost of
construction of NiCad batteries is high.
 The KOH electrolyte used is a corrosive hazardous
chemical.
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Fuel Cells.
A fuel cell is a galvanic cell in which chemical
energy of a fuel – oxidant system is converted
directly into electrical energy in a continuous
electrochemical process.
• Cell Schematic Representation:
Fuel;electrode/electrolyte/electrode/oxidant.
e.g. H2-O2; CH3OH-O2
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• The reactants (i.e. fuel + oxidant) are constantly supplied from
outside and the products are removed at the same rate as
they are formed.
• Anode:
Fuel+ oxygen → Oxidation products+ ne• Cathode:
Oxidant + ne- → Reduction products.
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Requirements Of Fuel Cell.
• Electrodes: Must be stable, porous and
good conductor.
• Catalyst: Porous electrode must be
impregnated with catalyst like Pt,
Pd, Ag or Ni, to enhance
otherwise slow electrochemical
reactions.
• Optimum Temperature: Optimum.
• Electrolyte: Fairly concentrated.
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Hydrogen – Oxygen Fuel Cell
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• Anode: Porous graphite electrodes impregnated with finely
divided Pt/Pd.
• Cathode: Porous graphite electrodes impregnated with finely
divided Pt/Pd.
• Electrolyte: 35-50% KOH held in asbestos matrix.
• Operating Temperature: 90oC.
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• Anode :
2H2(g) +40H- (aq)→ 4H2O(l)+4e• Cathode:
O2(g)+2H2O(l)+4e- →4OH(aq)
• Net Reaction:
2H2(g)+O2(g)→2H2O(l).
*Water should be removed from the cell.
*O2should be free from impurities.
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Applications.
• Used as energy source in space shuttles e.g.
Apollo spacecraft.
• Used in small- scale applications in
submarines and other military vehicles.
• Suitable in places where, environmental
pollution and noise are objectionable.
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CH3OH-O2 Fuel Cell
• Both electrodes: Made of porous nickel
plates impregnated with finely- divided
Platinum.
• Fuel: Methyl alcohol.
• Oxidant: Pure oxygen / air.
• Electrolyte: Conc.Phosphoric acid/Aq.KOH
• Operating Temperature: 150-200oC.
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• The emf of the cell is 1.20 V at 25oC.
• MeOH is one of the most electro active organic fuels in the
low temperature range as
*It has a low carbon content
*It posseses a readily oxidizable OH group
*It is miscible in all proportions in
aqueous
electrolytes.
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•
At anode:
CH3OH + 6OH- →CO2 + 5H2O + 6e• At cathode:
3/2 O2 +3H2O + 6e- →6OHNet Reaction:
CH3OH +3/2O2 →CO2 + 2H2O.
It is used in millitary applications and in large scale power
production. It has been used to power television relay stations.
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Advantages Of Fuel Cells.
• High efficiency of the energy conversion
process.
• Silent operation.
• No moving parts and so elimination of wear
and tear.
• Absence of harmful waste products.
• No need of charging.
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Limitations Of Fuel Cells.
• Cost of power is high as a result of the cost of
electrodes.
• Fuels in the form of gases and O2 need to be
stored in tanks under high pressure.
• Power output is moderate.
• They are sensitive to fuel contaminants such
as CO,H2S, NH3 & halides, depending on the
type of fuel cell.
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Differences.
• Fuel Cell
Galvanic Cell
*Do not store chemical
Stores chemical energy
energy
*Reactants are fed from
The reactants form an
outside continuously. integral part of it.
*Need expensive noble
These conditions are
metal catalysts.
not required
*No need of charging
Get-discharged when stored
– up energy is exhausted.
*Never become dead
Limited life span in use
*Useful for long-term
Useful as portable power services
electricity generation.
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