When a current of electricity is passed through a cell containing an electrolytic solution, chemical changes are produced and electrical energy is converted into chemical energy. Such an electrochemical cell can be used as a source of electrical energy for different electrical purposes. These cells are calledcommercial cells.
CLASSIFICATION OF COMMERCIAL CELL:
They are of two types:-
1. Primary Cell/Commercial Voltaic Cell.
2. Secondary Cell/Commercial Electrolytic Cell.
Here we are going to discuss secondary cell or commercial electrolytic cell.
Commercial Electrolytic Cell: The cell in which electrical energy is stored as chemical energy by passing electric current from the external source and then the chemical energy is converted into electrical energy is called the commercial electrolytic cell.
Examples:Lead accumulator, Silver-zinc accumulator, Nickel-cadmium storage battery etc.
Example: Lead accumulator, silver zinc accumulator are common examples of such accumulator.
Figure: Lead Accumulator Cell
CONSTRUCTION: A lead accumulator or battery consisting of a cell each having positive and negative plates. A cover provided of each of the cells and cell connector interconnect the cell electrically and mechanically. The cell connectors are disposed below the upper edge of cell covers and provide a rigid interconnected between cells. Separators between the positive and negative plates stop short circuit. Separators also obstruct the flow of ions between the plates and the increase in internal ions between the plates and the increase in internal resistance of the cell.
OPERATION: In the charged state, each cell contains element lead (Pb) and lead (IV) oxide (PbO2) an electrolyte of approximately 33.5% V/V (4.2 molars) sulfuric acid (H2SO4). In the discharged state both electrodes turn into lead (II) sulfate (PbSO4) and the electrolyte losses its H2SO4 and become primarily water. The electrode reactions are,
At left electrode (Anode):
Pb + HSO4- = pbSO4 + H+ + 2e-
At the right electrode (Cathode):
PbO2 + 3H+ + 2e- = PbSO4 + 2H2O
In order to recharge the accumulator, the reaction occurring in the discharging state is reversed (Charge). During this state Pb is deposited at the cathode, PbO2 is in anode and H2SO4 is regenerated in the cell. It is a reversible cell having a voltage of 2.3 V.
USE: They are widely used as a battery in an automobile.
1) It is rechargeable.
2) It supplies large energy.
3) Six cells are connected.
1) It is large in size.
2) It is very big, so difficult to carry.
Nickel-Cadmium (NiCad) Battery
Figure: Nickel-Cadmium storage battery.
The nickel-cadmium or NiCad battery is used in small electrical appliances and devices like drills, portable vacuum cleaners and PM/FM digital tuners. It is water primarily based cell with a Cd anode and an extremely oxidized nickel cathode that’s sometimes represented as the nickel(III) oxo-hydroxide, NiO(OH).
The electrode reactions throughout the discharge of a nickel-cadmium accumulator battery are as follows:
A variation in the NiCad battery is the nickel metal hydride battery (NiMH) used in hybrid automobiles, wireless communication devices and mobile computing. The overall chemical equation for this kind of battery is as follows:
NiO(OH)(s) + MH = Ni(OH)2(s) + M(s)
1) They are rechargeable.
2) The design of these battery maximizes the surface area of the electrodes and minimizes the distance between them which gives the battery both a high discharge current and a high capacity.