The importance of electrochemical cells or galvanic cells lies in their ability to provide us with a portable source of electrical energy. We have already studied that indirect redox reaction is, primarily, the basis of all the electrochemical cells Quite often, we use the term battery to represent the arrangement of two or more galvanic cells connected in series.
However, in practice the redox reaction used should give the arrangement which fulfills the following requirements:
• It should be light and compact;
• Its voltage should not vary appreciably during its use;
• It should provide power for a longer period; and
• It should be rechargeable.
The galvanic cells can be broadly classified into two categories, namely; primary cells and secondary cells.
This type of cells become dead over a period of time and the chemical reaction stops. They cannot be recharged or used again. Some common examples are dry cell, mercury cell, etc.
(a) Dry Cell. It is a compact form of Leclanche cell known after its inventor, a French chemist, G Leclanche. In this cell, anode consists of zinc container while cathode is a graphite rod surrounded by powdered :MnO2 and carbon. The space between the electrodes is filled with the paste of NH4Cl and ZnCl2. The arrangement is shown in Fig. 33.9.
The reactions taking place at the electrodes are given in their simplified form as follows:
MnO2 + NH4+ e- à MnO(OH) + NH3
Zn à Zn2+ + 2e-
The zinc ions (Zn2+) so produced combine with ammonia liberated in cathodic reaction to form diammine zinc (II) cation.
Zn2+ + 2NH3 à [Zn(NH3) 2]2+
Dry cells do not have long life as NH4Cl which is acidic, corrodes the zinc container even if the cell is not in use. The cell potential of dry cells lies in the range J. 25 V to 1. 5 V.
(b) Mercury Cell. It is miniature cell which finds a frequent use these days to supply energy for watches, video cameras, hearing aids and other compact devices. In mercury cell the anode is zinc-mercury amalgam and the cathode is a paste of mercury( II) oxide and carbon. Electrolyte is a moist paste of KOH-ZnO. The arrangement in its simple form is shown in Fig. 33.10.
Fig. 33.10. Commonly used mercury cell. The reducing agent is zinc and the oxidising agent s mercury (‘I) oxide
The operating voltage for mercury cell is = 1.35 V and the cell reactions are as follows:
This type of cells can be recharged by passing direct current through them and can be used again and again. Some examples are lead-storage battery, nickel-cadmium storage cell, etc. Let us study the working of lead storage cell.
(a) Lead-storage Battery. It is the most frequently used battery in automobiles. It consists of six voltaic cells connected in series. In each cell anode is made of spongy lead and cathode ‘is a grid of lead packed with lead dioxide (PbO2). The electrolyte is the aqueous solution of H2SO 4 which is 38% by mass. The reactions taking place in this type of cell can be represented as:
During the working of the cell, the concentration of H2SO4 decreases as sulphate ions are consumed to form PbS04. The PbSO4 precipitates and partially gets coated on both the electrodes and water formed dilutes the sulphuric acid. With the decrease in the concentration of H2SO4 the density of the solution also decreases. The condition of the battery can be easily checked by measuring the density of the solution .
To enhance the output of the cell, the anode and cathode plates are arranged in alternating manner and they are separated by sheets of insulating material. The anode and cathode plates are separately connected to each other so as to increase the electrode area in contact with electrolyte solution. This increases current delivering capacity of the cell. The groups of electrodes constitute one cell are shown in Fig. 33.11. The cells are further connected in series so as to increase the voltage of the battery. In 6 volts battery there are 3 cells and in 12 volts battery there are 6 cells.
Recharging the battery. The battery can be recharged by connecting it to an external source of direct current with voltage greater than 12 V. It forces the electrons to flow in opposite directions resulting in the deposition of Pb on the anode and Pb02 on the cathode.
During recharging operation, the cell behaves as electrolytic cell. The recharging reactions are:
Such an operation becomes possible because PbSO4 formed during discharge operation is solid and sticks to the electrodes. Therefore, it is in a position to lose or gain electrons during electrolysis.
The discharging and recharging of lead storage battery has been shown in Figs. 33.12 (a) and 33.12 (b) respectively.
In an automobile, the battery is discharged when the engine is started. While running, the engine powers the alternator which produces electrical energy sufficient enough to recharge the battery. Thus, battery is constantly recharged as long as the automobile is being driven.
Fig. 33.12. Discharging and recharging of lead storage cell
(b) Nickel-cadmium Storage Cell. It is another rechargeable cell. It consists of cadmium anode and the cathode is made of a metal grid containing nickel (IV) oxide.
These are immersed in KOH solution. The reactions occurring are:
The cell is also called nicad cell and has voltage =1.4 V. As is evident, there are no gaseous products, the products formed adhere to the electrodes and can be reconverted by recharging process. This cell is becoming more popular these days and finds use in electronic watches and calculators.