`color{green}text(Definition) :` 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.

`color{green}text(Need of Fuel Cells) :`

`=>` 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.

`=>` Then, this is used to run a turbine to produce electricity.

`=>` Since 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.

`color{red}text(Example) :` One of the most successful fuel cells uses the reaction of hydrogen with oxygen to form water (Fig. 3.12).

`=>` 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 :

`color{green}("Cathode")` : `color{red}(O_2 (g) +2H_2O (l) +4 e^(-) → 4 OH^(-) (aq))`

`color{green}("Anode")` : `color{red}(2H_2 (g) +4OH^(-) (aq) → 4H_2O (l) +4 e^(-))`.

`color{green}("Overall reaction being")` : `color{red}(2H_2(g) +O_2 (g) → 2H_2O(l))`.

`=>` The cell runs continuously as long as the reactants are supplied.

`=>` Fuel cells produce electricity with an efficiency of about `70%` compared to thermal plants whose efficiency is about `40%`.

`=>` There has been tremendous progress in the development of new electrode materials, better catalysts and electrolytes for increasing the efficiency of fuel cells.

`=>` These have been used in automobiles on an experimental basis.

`=>` Fuel cells are pollution free and in view of their future importance, a variety of fuel cells have been fabricated and tried.

Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of the metal.

`color{red}("Example")` : The rusting of iron, tarnishing of silver, development of green coating on copper and bronze.

`=>` It causes enormous damage to buildings, bridges, ships and to all objects made of metals especially that of iron. We lose crores of rupees every year on account of corrosion.

`=>` In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides.

`=>` Corrosion of iron (commonly known as rusting) occurs in presence of water and air.

`=>` The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon.

`=>` At a particular spot (Fig. 3.13) of an object made of iron, oxidation takes place and that spot behaves as anode and we can write the reaction :

`color{green}("Anode")` : `color{red}(2Fe (s) → 2Fe^(2+) +4 e^(-) \ \ \ \ \ \ \ E_(Fe^(2+)//Fe)^(⊖) = -0.44V)`.

`=>` Electrons released at anodic spot move through the metal and go to another spot on the metal and reduce oxygen in presence of `H^+` (which is believed to be available from `H_2CO_3` formed due to dissolution of carbon dioxide from air into water. Hydrogen ion in water may also be available due to dissolution of other acidic oxides from the atmosphere). This spot behaves as cathode with the reaction

`color{green}("Cathode")` : `color{red}(O_2(g) +4H^(+)(aq) + 4e^(-) → 2H_2O (l) \ \ \ \ E_(H^+ | O_2|H_2O)^(⊖) = 1.23V)`.

`=>` `color{green}("The overall reaction is")` : `color{red}(2Fe(s) +O_2 (g) +4H^(+) (aq) → 2Fe^(2+) (aq) +2H_2O (l) \ \ \ \ E_text(cell)^(⊖) = 1.67 V)`

`=>` The ferrous ions are further oxidised by atmospheric oxygen to ferric ions which come out as rust in the form of hydrated ferric oxide `(Fe_2O_3. x H_2O)` and with further production of hydrogen ions.

`color(green}text(Prevention of Corrosion) :`

`=>` Prevention of corrosion is of prime importance. It not only saves money but also helps in preventing accidents such as a bridge collapse or failure of a key component due to corrosion.

`=>` One of the simplest methods of preventing corrosion is to prevent the surface of the metallic object to come in contact with atmosphere. This is done by covering the surface with paint or by some chemicals (e.g. bisphenol). `color{green}("BARRIER PROTECTION")`

`=>` Another simple method is to cover the surface by other metals (`Sn`, `Zn`, etc.) that are inert or react to save the object.

`=>` An electrochemical method is to provide a sacrificial electrode of another metal (like `Mg`, `Zn`, etc.) which corrodes itself but saves the object. `color{green}("SACRIFICIAL PROTECTION")`