Energy producing cells

The energy producing cells are of two types - primary cells and secondary cells.

1) Primary cells
In a primary cell the reaction occurs only once and it becomes dead after a period of time and hence cannot be used again. Dry cell, mercury cell etc are examples of primary cells.
Dry cell The anode is zinc vessel and the cathode rod. It is surrounded by powdered Mno2. The space between the electrode is filled with a paste of NH4Cl and ZnCl2, the electrolyte. The potential is 1.5 V
Zn ------> Zn2+ + 2 electron (anode)
NH4+ + MnO2 + electron ------> MnO(OH) + NH3 (cathode)
It is of our common experience that dry cells do not have a long life. It is because the acidic ammonium chloride corrodes the zinc container even if the acidic ammonium chloride corrodes the zinc container even if the cell is not in use. In the leak proof cells, the zinc vessel is protected by an outer steel covering. In these cells recharging is not possible.

2) Secondary cells
These are cells which can be recharged and can be used again and again. Eg: Lead storage cell.
Lead storage cell This is a secondary cell in which lead acts as the anode and a grid of lead packed with lead dioxide acts as the cathode. A solution of sulphuric acid (38% by mass and density 1.3 g/ml) is used as the electrolyte.
Anode reaction : Pb + So4 2- -------> PbSO4 + 2 electron
Cathode reaction : PbO2 + 4H+ + SO4 2- + 2 electron ------> PbSO4 + 2H2O
Overall reaction : Pb + PbO2 + 2H2SO4 + 2 electron ------> PbSO4 + 2H2O
When the battery is recharged, the reverse of the reaction takes place .
A fully charged cell has a voltage of 2.2 V

Fuel cell
Fuel cells are galvanic cells in which chemical energy from combustion of fuels are converted to electrical energy. Eg: Hydrogen-oxygen fuel cell
Hydrogen-Oxygen fuel cell The cell consists of three compartments separated by graphite electrodes. H2 is passed through one compartment and O2 is passed through another compartment. Water containing small amount of NaOH is taken in the central compartment.
Anode reaction : [ H2 + 2OH- -----> 2H2O + 2electron] *2
Cathode reaction : O2 + 2H2O + 4electron -----> 4OH-
Net reaction : 2H2 + O2 -----> 2H2O
Fuel cells are very efficient and free from pollution. They produce continuous supply of energy. Fuel cells are used in space ships and in military equipments.

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