2.6 Batteries · Part 2

. Commonly used mercury cell. The reducing agent is zinc and the oxidising agent is mercury (II) oxide. . . Secondary Batteries Positive plate Separator Negative plate Another important secondary cell is the nickel-cadmium cell (Fig. . ) which has longer life than the lead storage cell but more expensive to manufacture. We shall not go into details of working of the cell and the electrode reactions during charging and discharging. The overall reaction during discharge is: Cd (s) + 2Ni(OH) (s) ® CdO (s) + 2Ni(OH) (s) + H O (l ) Production of electricity by thermal plants is not a very efficient method and is a major source of pollution. In such plants, the chemical energy (heat of combustion) of fossil fuels (coal, gas or oil) is first used for converting water into high pressure steam. This is then used to run a turbine to produce electricity. We know that a galvanic cell directly converts chemical energy into electricity and is highly efficient. It is now possible to make such cells in which reactants are fed continuously to the electrodes and products are removed continuously from the electrolyte compartment. Galvanic cells that are designed to convert the energy of combustion of fuels like hydrogen, methane, methanol, etc. directly into electrical energy are called fuel cells . One of the most successful fuel cells uses the reaction of hydrogen with oxygen to form water (Fig. . ). The cell was used for providing electrical power in the Apollo space programme. The water vapours produced during the reaction were condensed and added to the drinking water supply for the astronauts. In the cell, hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide solution. Catalysts like finely divided platinum or palladium metal are incorporated into the electrodes for increasing the rate of electrode reactions. The electrode reactions are given below: Cathode: O (g) + 2H O(l) + 4e – ¾® 4OH – (aq) Anode: 2H (g) + 4OH – (aq) ¾® 4H O(l) + 4e – Overall reaction being: 2H