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In this chapter, we will discuss the classification, construction & working of few

important batteries and fuel cells. We will also learn the advantages & disadvantages

of batteries.

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Cell: single arrangement of 2 electrodes & an electrolyte to provide electricity

due to redox reaction.

Battery: Combination of 2 or more cells arranged in series or parallel to produce

more voltage.

Charging : process in which battery is restored to its original charged condition

by reversal of current flow.

Discharging: Process by which a battery delivers electrical energy to an external

load.

Separator : physical barrier between anodes & cathodes in a battery to prevent

internal short circuiting. Separators must be ionically conducting but

electronically insulating & inert in the battery environment.

Fuel cell: Fuel & the oxidizer are supplied to the cell continuously while it is

operating to generate electricity.

Voltage: Voltage can be defined as the amount of pressure of electrons that pass from a negative electrode to a positive electrode. Mathematically,

V = IR Where V = Voltage; I = Current; R= Resistance Current: Current is a measure of the rate at which the battery is discharging. Capacity: The capacity is the charge or the amount of electricity that may be

obtained

from the battery and is given in ampere hours (Ah). Capacity =

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Where I = Current t= time Electricity storage density: Electrical storage density is the amount of

electricity per unit weight which the storer can hold, its the capacity per unit

weight of the battery.

Power (Watts): Power is the power per unit weight of the battery. Power (Watts) = EI, Where E = Voltage, I = Current

Cycle life: Primary batteries are designed for single discharge, but a secondary battery is rechargeable. The cycle life is the number of complete

charge/discharge cycles a battery can perform before its capacity fails below

80% of its initial capacity.

Shelf life: Time an inactive battery can be stored before it becomes unusable i.e,the length of time a battery can remain without losing its energy capacity

Design life: Elapsed time before a battery becomes unusable whether it is in active use or inactive.

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Classification of commercial cells:

Primary Cells: They are galvanic cells which produce electricity from chemicals that

are sealed into it when it is made. This type of cells cannot be recharged as the cell

reaction cannot be reversed efficiently by recharging. Once the cell reaction has

reached equilibrium the cell must be discarded. No more electricity is generated &

we say the battery is dead. These are also known as throw-away batteries or

irreversible batteries. e.g. Zinc - manganese dioxide cell, Dry cell, Lithium copper

sulfide cell

Secondary cells: A secondary cell is rechargeable by passing current through it. In

the charging process, an external source of electricity reverses the spontaneous cell

reaction & restores a non- equilibrium mixture of reactants. After charging, the cell

can be used for supplying current when required, as the reaction sinks toward

equilibrium again. Thus as secondary cell can be used through a large number of

cycles of discharging & charging. Such cells are also known as rechargeable cells,

storage cells, or accumulators. e.g.Lead-acid batteries, Nickel-cadmium cell,

Lithium- ion battery

Similarities:

Rechargeable batteries are available in exactly the same sizes as disposable

ones & replace them in all devices-AA/AAA/C/D/9v

CE is stored in the form of electro-active materials (high energy anodic &

cathodic materials)

Produce EE at the expense of stored-up CE

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Both types are commercially exploited (available easily in the market)

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*Importance of Batteries: * Wall clock * Wrist Watch * Motor Vehicle * UPS- Backbone * Remote Control * Laptop Computer * Cameras, Battery operated gas lighter/bell * Cell Phones, Children toys * Cardiac Pacemaker, Key Chain Laser Car Battery - Large Current - short time Pacemaker battery Steady current long time UPS battery Consistent & longer back-up Laptop Computer Flexible sheetEnergy intensive

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Construction:

Primary cell. The standard dry cell consists of a cylindrical zinc container that serves

as the anode & a graphite rod at the center serves as an inexpensive inertcathode.

The electrolyte is a moist paste of ammonium chloride, zinc chloride, manganese

dioxide & carbon particles. Carbon powder is mixed to improve conductivity. MnO2

acts as the depolarizing agent. A gel coated paper membrane / stretched fabric

electrically separate the two electrodes. Finally, the top of the cell is covered with

plastic cover to prevent drying & a metal cap placed at the center to provide electrical

contact. The cell has a nominal potential of l.5 V. Ammonium chloride is used as a

paste rather than as a dry solid. Solid ionic compounds do not conduct electricity - the

ions are firmly held by strong electrostatic forces & cannot move. When NH4Cl is

dissolved in water it loosens its structure & their ions are free to more & can carry an

electric current.

Working: Primary Reactions:

Anode:

Zn(s) Zn2+ (aq) +2e_ Cathode:

2MnO2(s) + H2O (l) + 2e- Mn2O3(s) + 2OH-(aq)

Net Reaction:

Zn(s) + 2MnO2(s) + H2O (l) Zn2+(aq) +Mn2O3 (s)+ 2OH

-(aq)

Secondary Reactions:

NH4+(aq) + OH(aq) NH3(g) + H2O (l)

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

Cell Reaction: Zn + 2MnO2 + NH4Cl [Zn(NH3)2Cl2] + H2O + Mn2O3

Explanation:

At the anode, zinc atoms give up two electrons & become positively charged zinc ions

(Zn2+) as the cell discharges. At the cathode, electrons reduce manganese from +4

oxidation state to +3 state & hydroxyl ions react with ammonium chloride to produce

ammonia. The ammonia released at the cathode diffuses to the anode where it reacts

with zinc ions to form the stable complex, [ZnNH3)2 Cl2]. It cannot be recharged

because the primary product Zn2+ is irretrievably converted into a stable complex.

This cell produces about 1.5 V due to the primary electrode reactions but with use its

potential falls to about 0.8 V as the reaction products accumulate inside.

The overall discharge reaction is

Zn + 2MnO2 + 2NH4Cl Zn(NH3)2 Cl2+ H2O + Mn2O3

Applications: Used in small portable appliances such as flashlights, transistors,

calculators, & toys where small amount of current is needed. It has large use in

consumer electronic devices like quartz wall clocks & walkman.

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Advantages:

Ordinary dry cell is cheap to make & has low price. e.g. Pentorch cell (about 10

rupees each).

It has a high energy density.

It is not toxic

It contains no liquid electrolytes.

Disadvantages

Its voltage drop when current is drawn rapidly from it due to the building up of

reaction products around the electrodes. So,itis not suitable for use in a

photoflash unit of a camera.

It has limited shelf life because the zinc is corroded by the faintly acid,

ammonium chloride. Consequently, the zinc container disintegrates and the

paste comes out through the outer covering. The shelf life of dry cell is 6-

8months.

They cannot be used once they get discharged. Attempts to recharge them results

in explosion and fire hazards.

Its emf decreases during use as the material is consumed and falls to 0.8 V as the

reaction products accumulate inside it.

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