By Sunil Bhardwaj

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It was discovered by Edison in 1899.

Principle: The stored energy is converted into electrical energy by means of chemical reaction.

Construction: It is consist of PVC container, containing electrolyte about 20% KOH sol. The negative electrode (anode) is iron plate and cathode (positive) electrode is Ni plate impregenated with Ni2O3.

Working: When this is connected in the circuit the reaction starts as follows:

When this is connected in the circuit the reaction starts as follows:

At L.H.E : $$Fe \longrightarrow { Fe }^{ +2 } + 2{ e }^{ - }\qquad (ox).$$ $${ Fe }^{ +2 } + 2{ OH }^{ - } \longrightarrow FeO (s) + { H }_{ 2 }O$$ Net Reaction at anode,$$Fe + 2{ OH }^{ - } \longrightarrow FeO (s) + { H }_{ 2 }O + 2{ e }^{ - }\qquad (ox).$$ At R.H.E : $${ Ni }_{ 2 }{ O }_{ 3 }(s) + 3{ H }_{ 2 }O \longrightarrow 2{ Ni }^{ +3 } + 6{ OH }^{ - }$$ $$2{ Ni }^{ +3 } + 2{ e }^{ - } \longrightarrow 2{ Ni }^{ +2 }\qquad (red).$$ $${ Ni }^{ +2 } + 4{ OH }^{ - } \longrightarrow 2NiO (s) + 2{ H }_{ 2 }O$$ The Net Reaction at RHE : $${ Ni }_{ 2 }{ O }_{ 3 }(s) + { H }_{ 2 }O + 2{ e }^{ - } \longrightarrow 2NiO (s) + 2{ OH }^{ - }$$ The Net Cell Reaction is, $$Fe + { Ni }_{ 2 }{ O }_{ 3 }(s) \longrightarrow FeO (s) + 2NiO (s)$$ On applying the law of mass action to above cell reaction, $$K = \frac { { a }_{ FeO (s) } \times { a }_{ NiO (s) }^{ 2 } }{ { a }_{ Fe (s) } \times { a }_{ { Ni }_{ 2 }{ O }_{ 3 }(s) } } = 1$$ All the species are solid and activity of each is one.

On applying the Nernst expression for emf of the cell $${ E }_{ cell } = { E }_{ cell }^{ 0 } - \frac { RT }{ nF } \ln { K }$$ $${ E }_{ cell } = { E }_{ cell }^{ 0 } - \frac { RT }{ 2F } \ln { \left( 1 \right) }$$ $${ E }_{ cell } = { E }_{ cell }^{ 0 } - \frac { RT }{ 2F } \left( 0 \right)$$ $${ E }_{ cell } = { E }_{ cell }^{ 0 }$$

Uses:

(1) They are more economical. Electrodes used in this case are Fe and Ni which are cheaper than Pb.

(2) They are used in Railways.

Representation: $$\circleddash Fe, FeO(s) | \underset { 20% }{ KOH } |NiO(s), { Ni }_{ 2 }{ O }_{ 3 } (s), Ni\oplus$$

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