Energy Storing Devices:

INTRODUCTION
In electrochemical cells, the chemical energy is converted into electrical energy. The cell
potential is related to free energy change (G). In an electrochemical cell, the system does work
by transferring electrical e
of the maximum useful work, that can be obtained from a chemical reaction.
i.e., G = maximum work
But we know that
maximum work = nFE
When a cell operates, work is done on the surroundings (flow of electricity).
G = – G < 0
Decrease in free energy is indicated by (–)ve sign.
One of the main uses of the galvanic cells is the generation of portable electrical energy.
These cells are known as batteries.

Definition
A battery is an arrangement of several electrochemical cells connected in series, that can be
used as a source of direct electric current.
Thus,
A cell : Contains only one anode and cathode.

A Battery : Contains several anodes and cathodes.

Requirements of a battery
A useful battery should fulfill the following requirements.
1. It should be light and compact for easy transport.
2. It should have long life both when it is being used and when it is not used.
3. The voltage of the battery should not vary appreciably during its use.

TYPES OF BATTERY
1. Primary Battery (or) Primary cells
In these cells, the electrode and the electrode reactions cannot be reserved by passing
an external electrical energy. The reactions occur only once and after use they become
dead. Therefore, they are not chargeable.
Example : Dry cell, mercury cell.
2. Secondary Battery (or) Secondary cells
In these cells, the electrode reactions can be reversed by passing an external electrical
energy. Therefore, they can be recharged by passing electric current and used again
and again. These are also called Storage cells (or) Accumulators.
Example : Lead acid storage cell, Nickel - cadmium cell.
3. Flow battery (or) Fuel cell
In these cells, the reactants, products and electrolytes are continuously passing through
the cell. In this chemical energy gets converted into electrical energy.
 Example : Hydrogen - oxygen fuel cell.

IMPORTANT PRIMARY BATTERIES DRY CELL OR LECLANCHE’S CELL

Description
A dry cell consists of a Zinc cylinder, which acts as anode. This zinc cylinder is filled with
an electrolyte consisting of NH4Cl, ZnCl2 and MnO2 in the form of paste using starch and
water. A carbon rod(graphite), acts as cathode, is immersed in the electrolyte in the centre of the
cell. The zinc cylinder has an outer insulation of cardboard case. During use, the zinc cylinder
gets consumed and at the end, it will develop holes which are responsible for leakage.
M et al cap (posit ive)
+
I nsulat ing washer

Zinc cup (negat ive)

M ixt ur e of manganese(I V)
oxide, gr aphit e, ammonium
chlor ide and zinc chlor ide

Car bon r od

M et al cover (negat ive)

-

Leclanche’s cell
Working
When the cell is working, zinc loses electrons and Zn2+ ions gets dissolved in the
electrolyte. The electrons pass through the circuit and are consumed at cathode. This causes
discharge of ions from the electrolyte.

Cell reactions :
At anode : Zn  Zn 2  2e
NH 4 (aq )  MnO 2(s)  2e   MnO(OH  )  NH 3
At cathode :

Zn  NH 4 (aq )  MnO2(s)  Zn 2 MnO(OH)   NH 3

Overall Reaction :

In cathode reaction, Mn is reduced from +4 oxidation state to +3 oxidation state. The

liberation of NH3 gas, which disrupts the current flow, is prevented by a reaction of NH 3(g) with
Zn2+ (from ZnCl2).

ZnCl 2  2NH 3  [ Zn (NH 3 ) 2 ] Cl 2(s)

The voltage of Leclanche’s cell is about 1.5V.

Examples for drycell : Silver cell, Lithium cell.
Disadvantages
1. This dry cell does not have an indefinite life, because NH 4Cl being acidic corrodes the
zinc container, even if it is not used.
2. When current is drawn rapidly from it, products build up on the electrodes, so voltage
drop occurs.
Uses
It is used in transistor radios, calculators, Flash lights, torches
etc.,

MERCURY CELL
It is a small form of dry cell, used in watches, hearing aids.

Uses
It consists of a zinc anode and a mercury (II) oxide cathode. The electrolyte is a paste of
KOH and ZnO.

Cell reactions :
At anode :
Zn  2OH   ZnO(s)  H 2O  2e
At cathode :
HgO(s)  H 2O  2e  Hg (1)  2OH 

Overall Reaction :

Zn  H gO(s)  ZnO(s)  H g (1)

Since the overall reaction does not involve any ion in solution, it has the advantage that its
potential remains almost constant throughout its life.
The voltage of mercury cell is about 1.35 V.

ALKALINE BATTERY
Description
It is an improved form of the drycell, in which the electrlyte NH 4Cl is replaced by KOH.
In alkaline battery, the powdered zinc is mixed with KOH & MnO2 to get a gel. A carbon rod
(grpahite), acts as cathode, is immersed in the electrolyte in the centre of the cell. The outside
cylinderical Zinc body is made of Zinc.

Cell reactions :
At anode :
Zn (s)  2OH  (aq)  Zn (OH) 2(s)  2e
At cathode :
 2MnO2( s )  H 2O( I )  2e  Mn 2O3( s )  2OH  ( aq )

Overall Cell Reaction :

Zn( s)  2MnO2( s)  H 2O( I )  Zn(OH)2( s)  Mn 2O3( s)

Advantages of alkaline battery over dry Battery

The main advantages of alkaline battery overy dry battery are
i) Zinc does not dissolve readily in a basic medium.
ii) The life of alkaline battery is longer than the dry battery, because there is no corrosion
on Zn.
iii) Alkaline battery maintains its voltage, as the current is drawn from it.
Uses
It is used in calculators, watches etc.,

IMPORTANT SECONDARY BATTERIES

LEAD STORAGE CELL OR LEAD ACCUMULATOR OR ACID STORAGE CELL

Storage Cell
A storage cell is the one, which can operate both as a voltaic cell and as an electrolytic cell.
When it acts as a voltaic cell, it supplies electrical energy and becomes “run down”. When it is
recharged, the cell operates as an electrolytic cell.

Description
A lead - acid storage battery consists of a number of (3 to 6) voltaic cells connected in
series to get 6 to 12 V battery. In each cell, the anode is made of lead. The cathode is made of
lead dioxide PbO2 or a grid made of lead, packed with PbO2. A number of lead plates (anodes)
are connected in parallel and a number of PbO2 plates (cathodes) are also connected in parallel.
Various plates are separated from the adjacent ones by insulators like rubber or glass fibre. The
entire combinations is then immersed in dil. H2SO4 (38% by mass) having a
- Anode
density of 1.30 + Cat hode gm/ml.

Pb plat es PbO2 plat es

Aqueous
H 2SO4
 Fig. 5.2 Lead storage cell

The cell may be represented as;

Pb / PbSO4 // H2SO4(aq) / PbO2 / Pb

Working (Discharging)
When the lead - acid storage battery operates, at the anode lead is oxidised to Pb2+ ions
and insoluble PbSO4 is formed. At the cathode PbO2 is reduced to Pb2+ ions and PbSO4 is
formed.

Cell reactions :
At anode : Lead is oxidised to Pb2+ ions, which further combines with forms insoluble PbSO4.
Pb( s )  Pb2 ( aq )  2e 

Pb2 ( aq )  SO24 ( aq )  PbSO4 ( s)

Overall anode Reaction :
Pb( s)  SO24 ( aq )  PbSO4( s)  2e

At cathode :
PbO2 gains electrons ie., Pb undergoes reduction at the cathode from +4 to +2. The Pb 2+
ions then combines with ions forms insoluble PbSO4.

PbO2( s )  4H   2e   Pb2 ( aq )  2H 2O
Pb2 ( aq )  SO24 ( aq )  PbSO4( s)
Overall
PbO2( s )  4H   SO24  2e  PbSO4  2H 2O
Cathode
Reaction

Overall cell reaction during use (discharging)

Pb (s)  SO24 (aq ) 2PbSO4(s)  2e

At anode :

PbO2 (s)  4H   SO24  2e PbSO 4 (s)  2H 2 O

At cathode :
Overall cell reaction :

Pb (s)  PhO2(s)  2H 2SO4(aq) 2PbSO 4 (s) 2H 2O  Energy

 From the above cell reaction it is clear that, PbSO 4 is precipitated at both the electrodes
and H2SO4 is used up. As a result, the concentration of H 2SO4 decreases and hence the density
of H2SO4 falls below 1.2 gm/ml. So the battery needs recharging.

Recharging the Battery

The cell can be charged by passing electric current in the opposite direction. The electrode
reaction gets reversed. As a resulty, Pb is deposited on anode and PbO 2 on the cathode. The
density of H2SO4 also increases.
The net reaction during charging is
2PbSO4(s)  2H 2O  Energy Pb (s)  PbO 2(s)
+ 4 H+ + 2SO42–
Uses
1. Lead storage cell is used to supply current mainly in automobile such as cars, buses,
trucks, etc.,
2. It is also used in gas engine ignition, telephone exchanges, hospitals, power stations
etc.,

NICKEL - CADMIUM CELL

This is also a rechargeable battery.
Description
Nickel - cadmium cell consists of a cadmium anode and a metal grid containing a paste of
NiO2 acting as a cathode. The electrolyte in this cell is KOH.
It is represented as : Cd / Cd(OH)2 // KOH(aq) / NiO2 / Ni
Working (Discharging)
When the Nicad battery operates, at the anode cadmium is oxidised to Cd2+ ions and
insoluble Cd(OH)2 is formed. It produces about 1.4V.
Cell reactions
At anode : Cadmium is oxidised to Cd2+ and further it combines with OH- ions to form
Cd(OH)2.

Cd (s)  2OH  Cd (OH) 2(s)  2e

At cathode : NiO2 gains electrons, i.e., Ni undergoes reduction at the cathode from +4 to +2.
The Ni2+ ions then combine with OH- ions to form Ni(OH)2.

NiO2(s)  2H 2O  2e Ni (OH) 2(s)  2OH 

Cell reactions during use (discharge)

At anode :
 Cd (s)  2OH  Cd (OH) 2(s)  2e
At cathode :

NiO2(s)  2H 2 O  2e Ni (OH) 2(s)  2OH   Ener gy

Overall cell reaction :

Cd (s)  NiO2(s)  2H 2O Cd (OH ) 2(s)  Ni (OH ) 2(s)

+ Energy

From the above cell reactions it is clear that, there is no formation of gaseous products, the
products Cd(OH)2 and Ni(OH)2 adhere well to the surfaces. This can be reconverted by
recharging the cell.

Recharging the Battery

The charging process is similar to lead storage battery. When the current is passed in the
opposite direction, the electrode reaction gets reversed. As a result, Cd gets deposited on anode
and NiO2 on the cathode.
The net reaction during charging is

Cd (OH) 2(s)  Ni (OH) 2(s)  Ener gy Cd (s)  NiO2(s)  2H 2O

Advantages
1. It is smaller and lighter.
2. It has longer life than lead storage cell.
3. Like a dry cell, it can be packed in a sealed container.
Disadvantages
It is more expensive than lead storage battery.
Uses
It is used in calculators, electronic flash units, transistors and cordless appliances.

LITHIUM BATTERY
Lithium battery is a solid state battery because instead of liquid or a paste electrolyte, solid
electrolyte is used.
Construction:
The lithium battery consists of a lithium anode and a TiS 2 cathode. A solid electrolyte,
generally a polymer, is packed in between the electrodes. The electrolyte (polymer) permits the
passage of ions but not that of electrons.
 + -

Anode Cat hode

L i+
Li Solid TiS2
elect r olyt e

-
L i ++e- TiS 2 +e-
-
Li TiS2
Solid state lithium battery
Various reactions
The various electrode reactions are
 
At anode : Li(s)  Li  e
TiS2( s )  e   TiS2
At cathode :

Overall cell reaction :

Li(s)  TiS2( s )  Li   TiS2

Li   TiS2  LiTiS2

This cell is rechargeable and produces a cell voltage of 3V.

Othertypes of Secondary Lithium Batteries

i) Li / MnO2
ii) Li / V2O5
iii) Li / MoO2
iv) Li / Cr3O8

Advantages of Li battery
It is the cell of future, why?
i) Its cell voltage is high, 3V.
ii) Since Li is a light-weight metal, only 7g (1 mole) material is required to produce 1
mole of electrons.
iii) Since Li has the most negative Eo value, it generates a higher voltage than the other
types of cells.
iv) Since all the constituents of the battery are solids there is no risk of leakage from the
battery.
v) This battery can be made in a variety of sizes and shapes.
Lithium - Sulphur Battery :
Lithium - Sulphur battery is a rechargeable battery. Its anode is made of Li. Sulphur is the
electron acceptor, the electron from Li is conducted to S by a graphite cathode. -Alumina
(NaAl11O17) is used as the solid electrolyte.
This solid electrolyte allows the Li+ ions to migrate to equalize the charge, but will not
allow the big poly sulphide product ions.
This battery is operated at high temperatures as Li and S should be in their molten states.

Various reactions
The various electrode reactions are
 
At anode : 2Li  2Li  2e
 2
At cathode : S  2e  S
 2
Overall cell reaction : 2Li  S  2Li  S

The S2- ions, formed, react with elemental sulphur to form the polysulphide ion.
S2  nS  [Sn  1 ]2
The direct reaction between Li and S is prevented by the alumina present in the cell.

Advantages of Li-S battery

1. Li-S battery has light weight unlike the lead acid battery.
2. It possess a high energy density.
3. It is used in electric cars.

FUEL CELLS
Definition
Fuel cell is a voltaic cell, which converts the chemical energy of the fuels directly into
electricity without combustion. It converts the energy of the fuel directly into electricity.In these
cells, the reactants, products and electrolytes pass through the cell.
Fuel + Oxygen Oxidation products + Electricity

Examples : Hydrogen - oxygen fuel cell; Propane - oxygen fuel cell; Methyl alcohol - oxygen
fuel cell.

Hydrogen - Oxygen fuel cell

Hydrogen - oxygen fuel cell is the simplest and most successful fuel cell, in which the fuel
- hydrogen and the oxidiser - oxygen and the liquid electrolyte are continuously passed through
the cell.
Description
It consists of two porous electrodes anode and cathode. These porous electrodes are made
of compressed carbon containing a small amount of catalyst (Pt, Pd, Ag). In between the two
electrodes an electrolytic solution such as 25% KOH or NaOH is filled. The two electrodes are
connected through the voltmeter.

H 2O
Anode - + Cat hode

H2 O2
OH
Elect r olyt e

Por ous car bon

elect r odes

H2 - O2 Fuel cell
Working
Hydrogen (the fuel) is bubbled through the anode compartment, where it is oxidised. The
oxygen (oxidiser) is bubbled through the cathode compartment, where it is reduced.
At cathode
The electrons produced at the anode pass through the external wire to the cathode, where it
is absorbed by oxygen and water to produce hydroxide ions.
O2  2H2O  4e  4OH 

At anode
Hydrogen molecules are oxidised at the anode with the liberation of electrons, which then
combine with hydroxide ions to form water.
2H2  4OH  4H2O  4e

Cell reactions
At anode :
2H2  4OH  4H2O  4e
At cathode :
O2  2H2O  4e  4OH 
Overall cell reaction :
2H 2  O2  2H 2O
The emf of the cell = 0.8 to 1.0V

Fuel Battery
 When a large number of fuel cells are connected in series, it form fuel battery.

Characteristics (or) advantages

1. Continuous source of energy : There is no electrode material to be replaced as in
ordinary battery. The fuel is continuously supplied to produce power.
2. High efficiency : The fuel cell converts the energy of a fuel directly into the
electricity, they are more efficient (70%) than the conventional methods (40%) of
generating electricity.
3. Pollution free working : There are no objectionable by-products and therefore, they
do not cause pollution problems (like noise, vibration, heat transfer, thermal
pollution).

Uses
1. H2 - O2 fuel cells are used as auxilliary energy source in space vehicles, submarines
or other military - vehicles.
2. In case of H2 - O2 fuel cells, the product of water is proved to be a valuable source of
fresh water by the astronauts.

PHOTOGALVANIC CELL OR SOLAR CELL

Definition
Photogalvanic cell is the one, which converts the solar energy (energy obtained from the
sun) directly into electrical energy.
Solar battery
When a large number of photogalvanic cells are inter connected, forms solar battery.

Description

P-Type semiconduct or

e e
n-Type semiconduct or

Solar Cell

Photogalvanic cell consists of a p-type semiconductor (such as Si doped with B) and a n-

type semi conductor (such as Si doped with P). They are in close contact with each other, so that
a limited extent of electrons (from n-type semiconductor) and positive holes (from-type
semiconductor) can cross the junction between the two types of semiconductors.

Working
When the solar rays fall on the outer layer of p-type semiconductor, the electrons in the
valence band get promoted to the conduction band by absorbing the light energy. Since the
conduction electrons, unlike the positive holes, can easily cross the p-n junction into the n-type
semiconductor, a potential difference between two layers is created. This potential difference
causes flow of electrons (ie., an electric current). The potential difference and hence current
increases as more solar energy falls on the surface of the outer layer. When this p-and n-layers
are connected to an external circuit, electrons flow from n-layer to p-layer, there by the current is
generated.

QUESTIONS
1. What is Leclanche’s cell? How does it work?
2. Write notes on (i) Mercury cell (ii) Alkaline battery.
3. Explain discharging and charging process of lead storage cell.
4. Explain description and working of NICAD battery.
5. Write notes on lithium battery.
6. What are fuel cells? Explain the construction and working of hydrogen - oxygen fuel
cell.