Cells: The cells are also popularly known as batteries. Therefore, a battery is an arrangement of electrochemical cells used as an energy source. The basis of an electrochemical cell is an oxidation- reduction reaction. There are mainly two types of commercial cells namely-
Primary 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 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. e.g. lead storage battery and nickel- cadmium storage cell.
Dry cells: The most familiar commercial cells are dry cells. These are used in torches, toys, flash lights, calculators, tape recorders and many other devices. These are based on Leclanche cell invented by G. Leclanche in 1868. A dry cell is consists of a zinc cylinder. This is filled with a paste of NHCl4 and little of ZnC12 in the form of a paste in water. The zinc cylinder acts as an anode. The cathode is a graphite (carbon) rod. The carbon rod is surrounded by a black paste of manganese dioxide (MnO2) and carbon powder. The zinc case has an outer insulation of cardboard case. 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. When the cell is working, 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(s) ———–> Zn2+(aq) + 2e
Cathode: NH4+(aq) + MnO2(s) + 2e ———–> MnO(OH) + NH3
Overall: Zn + NH4+ (aq) + MnO2(s) + 2e ————> Zn2+ + MnO(OH) + NH3
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 complex ion having the formula [Zn(NH3)2]2+. This dry cell does not have an indefinite life because NH4C1 being acidic corrodes the zinc container even when not in use.
Lead Storage Cell: This is the most commonly used battery in automobiles. 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. In each cell, the anode is a grid of lead packed with finely divided spongy lead and the cathode is a grid of lead packed with PbO2. The electrolyte is aqueous solution of sulphuric acid (38% by mass) having a density 1.30 g/ml sulphuric acid. When the lead plates are kept for sometimes, a deposit of lead sulphate is formed on them.
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) + 2e
Pb2+(aq) + SO42- (aq) ———-> PbSO4 (s)
Pb (s) + SO42- (aq) ———–> PbSO4 (s) + 2e —————- (1)
At cathode, the PbO2 is reduced as
Pb O2 (s) +4H+ + 2e ———-> Pb2+(aq) +2H2O
Pb2+ (aq) + SO42+ (aq) ———–>PbSO4 (s)
PbO2 (s) + SO42+ (aq) +4H+ + 2e ———-> PbSO4 (s) + 2H2O—– (2)
Thus, the complete electrode reactions are
Pb (s) + PbO2 (s) + 2H2SO4 (aq) ———>2PbSO4 (s) + 2H2O
It is clear from the above reactions that 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 decrease. When density of H2SO4 falls below 1.2 g/ml, then battery needs recharging.
Fuel Cells: The common example is hydrogen-oxygen fuel cell. In fuel cell, hydrogen and oxygen are bubbled through a porous carbon electrode into concentrated aqueous sodium hydroxide or potassium hydroxide. Hydrogen (the fuel) is fed into the anode compartment where it is oxidized. The oxygen is fed into cathode compartment where it is reduced. The diffusion rates of the gases into the cell are carefully regulated to get maximum efficiency. The net reaction of hydrogen and oxygen to form water are given-
2H2 (g) + O2 (g) ———>2H2O (l)
The advantages of fuel cells over ordinary batteries are- High efficiency, continuous source of energy and pollution free working.