Batteries

Electrochemical cell vs Electrolytic cell

Electrochemical cell	Electrolytic cell
It is a device which converts chemical energy into electrical energy.	It is a device which converts electrical energy into chemical energy.
The redox reaction is spontaneous and is responsible for the production of electrical energy.	The redox reaction is non-spontaneous and electrical energy is supplied to make the reaction to occur.
The two half cells are set up in different containers and are connected through salt bridge or porous partition.	Both the electrodes are placed in the solution or molten electrolyte in the same container.
In electrochemical cell, anode is negative and cathode is positive electrode.	In electrolytic cell, anode is positive and cathode is negative electrode.
The electrons move from anode to cathode in external circuit.	The electrons are supplied by the external battery and enter through cathode and come out through anode.

Batteries

A battery is an arrangement of electrochemical cells used as an energy source. The basis of an electrochemical cell is an oxidation-reduction reaction.

A useful battery should also fulfil the following requirements

1) It should be light and compact so that it can be easily transported.

2) It should have reasonably 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 commercial cells

There are mainly two types of commercial cells:

1) Primary batteries or cells

2) Secondary batteries or cells

Primary batteries or cells

In these cells, the electrode reactions cannot be reversed by an external electric energy source. In these cells, reactions occur only once and after use they become dead. Therefore, they are not chargeable.

Some common examples are dry cell, mercury cell.

Secondary batteries or cells

In the secondary cells, the reactions can be reversed by an external electric energy source. Therefore, these cells can be recharged by passing electric current and used again and again. These are also called storage cells.

Examples of secondary cells are lead storage battery and nickel-cadmium storage cell.

1) Primary Cells

1) Dry cells

 

These are used in torches, toys, flash lights, calculators, tape recorders and many other devices.

Construction of Dry Cell

a) It consists of a zinc cylinder. This is filled with a moist paste of NH₄Cl and little of ZnCl₂.

b) The zinc cylinder acts as a anode. The cathode is a graphite (carbon) rod.

c) The carbon rod is surrounded by a black paste of manganese dioxide (MnO₂) and carbon powder.

d) During use, the zinc case gets consumed and in the end, it will develop holes which are responsible for leakages.  The leak proof cells or dry cells have an iron or steel sheet covering the zinc.

Working of Dry Cell

Zinc loses electrons and Zn²⁺ ions dissolve in the electrolyte. The electrons pass around the external circuit and are taken up at cathode. This causes discharge of NH₄⁺ ions from the electrolyte.

The reactions taking place at the electrodes are:

Anode

Zn —–> Zn²⁺ + 2e‾

Cathode 

NH₄⁺(aq) + MnO₂(s) +2e¯ ——-> MnO(OH) + NH₃

—————————————————————-

Zn + 2NH₄⁺ (aq) + 2MnO₂(s) ——->Zn²⁺ + 2MnO(OH) +2NH₃

In the cathode reaction, manganese is reduced from +4 Oxidation state to +3 oxidation state. Ammonia is not liberated as a gas but it Combines with some of the Zn²⁺ ions produced from the anode to form Complexion having the formula  [Zn(NH₃)₂]²⁺

It gives voltage of approximately 1.2 to 1.5 V. This dry cell does not have an indefinite life because NH₄Cl being acidic corrodes the zinc Container even when not in use.

2) Mercury cell

A new type of cell which has found use in small electrical circuits such as hearing aids, Watches and cameras is the mercury cell.

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

 

The reactions occurring in the cell are as follows :

Anode:

Zn(Hg) + 2OH¯ → ZnO(s) + H₂O+ 2e¯

Cathode:

HgO(s) + H₂O+ 2e¯ → Hg(l) + 2OH¯

———————————————-
Zn + HgO(s) ——-> ZnO(s) + Hg(l)

(Amalgam) 

It has the advantage that its potential remains almost constant throughout its life. The voltage of mercury cell is approximately 1.35 V.

2) Secondary Cells

There are some cells which can be recharged. These are called accumulators or storage cells. These can be recharged and used again and again as a source of electric current.

Lead Storage Cell

Each battery consists of a number of voltaic cells connected in series. Three to six such cells are generally combined to get 6 to 12 volt battery.

a) The anode is a grid of lead packed with finely divided spongy lead and the cathode is a grid of lead packed with PbO₂.

b) The electrolyte is aqueous solution of sulphuric acid.

c) 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.

The following reactions take place in the lead storage cell:

 

At anode

The lead loses two electrons and is oxidised to Pb²⁺ ions.

Pb(s) ——-> Pb²⁺(aq) +2 e¯

Pb²⁺(aq) + SO₄^2-(aq) ———–> PbSO₄(s)

The overall anode reaction may be written as :

Pb(s) + SO₄^2-(aq) ———–> PbSO₄(s) + 2e¯

At cathode

The PbO₂ is reduced as:

PbO₂ (s) + 4 H⁺ +2 e‾ ————> Pb²⁺(aq) + 2 H₂O

PbO₂ (s) + SO₄^2-(aq) ——->PbSO₄(s)

The overall cathode reaction is :

PbO₂ (s) + 4 H⁺ +2 e‾ + SO₄^2-(aq) ———-> PbSO₄(s) + 2 H₂O

Pb(s) + SO₄^2-(aq) ———–> PbSO₄(s) + 2e¯

PbO₂ (s) + 4 H⁺ +2 e‾ + SO₄^2-(aq) ———-> PbSO₄(s) + 2 H₂O

———————————————————————————————
Pb(s)  + PbO₂ (s) + 2 H₂SO₄ (aq) ——–> 2 PbSO₄(s) + 2 H₂O
———————————————————————————————

During the working of the cell PbSO₄ is formed at each electrode and sulphuric acid is used up. As a result, the concentration of H₂SO4 decreases and the density of the solution also decreases.

Recharging the Battery

The cell can be charged by passing electric current of a suitable voltage in the opposite direction. The electrode reaction gets reversed. As a result, the flow of electrons gets reversed and lead is deposited on anode and PbO₂ on the cathode. The density of sulphuric acid also increases.

The reaction may be Written as

2PbSO₄ (s)+2H₂O ——>Pb(s) + PbO₂ (s) +2H₂SO₄

When it is used to start the engine of the automobile, it acts as a voltaic cell and produces electric energy. During recharging, it acts as an electrolytic cell.

Nickel Cadmium Storage Cell

This is also a rechargeable cell. It has longer life than the lead storage cell but more expensive than lead storage battery. However, it has some advantages because it is smaller and lighter. It can be used in portable and cordless appliances.

1) It consists of a cadmium anode and a metal grid containing NiO₂ acting as a cathode.

2) The electrolyte in this cell is KOH.

The reactions taking place during discharge and charge are:

Anode:

Cd(s)+2OH¯ ⇔  CdO(s) +H₂O(l)+ 2e¯

Cathode:

2Ni(OH₃) (s) + 2e¯ ⇔ 2Ni(OH)₂(s) +2OH¯ (aq)

——————————————————–
Cd(s)+ 2Ni(OH₃) (s) ⇔ CdO(s) + 2Ni(OH)₂(s) + H₂O(l)
———————————————————-

The reaction products generally remain sticking to the electrodes and can be reconverted by recharging the cell. The charging process is similar to lead storage battery.

It produces a potential of about 1.4 V. 

 

Fuel Cells

These are voltaic cells in which the reactants are continuously supplied to the electrodes. These are designed to convert the energy from the combustion of fuels such as H, CO, CH₄, etc. directly into electrical energy.

1) In this cell, hydrogen and oxygen are bubbled through a porous carbon electrode into concentrated aqueous sodium hydroxide or potassium hydroxide.

2) Hydrogen (the fuel) is fed into the anode compartment where it is oxidised.

3) The oxygen is fed into cathode compartment where it is reduced.

The reactions are:

Anode:

2[H₂(g)+ 2OH¯(aq) ———>2H₂O(l)+2e¯]

Cathode

O₂(g)+ 2H₂O(l) + 4e¯ ————>4OH¯ (aq)

————————————————————
2H₂(g) +0₂(g)————-> 2H₂O (l)
————————————————————

Each electrode is made of porous compressed carbon containing a small amount of catalyst (Pt, Ag or CoO).These fuel cells are more efficient than conventionally used methods of generating electricity on a large scale by burning hydrogen, carbon fuels because these fuel cells convert the energy of the fuel directly into electricity.

Advantages of fuel cells over ordinary batteries:

1) High efficiency

The fuel cells convert the energy of a fuel directly into electricity and therefore, they are more efficient than the conventional methods of generating electricity on a large scale by burning hydrogen, carbon fuels.The conventional methods of production of electrical energy involve combustion of a fuel to liberate heat which is then used to produce electricity. The efficiency of these methods is only about 40%.

2) Continuous source of energy

There is no electrode material to be replaced as in ordinary battery. The fuel can be fed continuously to produce power.

(3) Pollution free working

There are no objectionable by-products and, therefore, they do not cause pollution problems.