Secondary Cells

Secondary Cells

In the secondary cells, the reactions can be reversed by an external electrical energy source. Therefore, these cells can be recharged by passing electric current and used again and again. These are also celled storage cells. Examples of secondary cells are, lead storage battery and nickel – cadmium storage cell.

In charged

Lead storage cell

Alkali cell

 

 

 

 

 

 

Positive electrode

Perforated lead plates coated with PbO2

Perforated steel plate coated with Ni(OH)4

Negative electrode

Perforated lead plates coated with pure lead

Perforated steel plate coated with Fe

Electrolyte

dil. H2SO4

20% solution of KOH + 1% LiOH

During charging

Chemical reaction

At anode : PbSO4 + 2H+ + 2e® Pb + H2SO4

At cathode : PbSO4 + SO4– – + 2H2O – 2e® PbO2

+ 2H2SO4

Specific gravity of H2SO4 increases and when specific gravity becomes 1.25 the cell is fully charged.

Emf of cell: When cell is fully charged then E = 2.2 volt

Chemical reaction

At anode : Ni (OH)2 + 2OH+ – 2e® Ni(OH)4

At cathode : Fe(OH)2 + 2K+ + 2e® Fe + 2KOH

Emf of cell : When cell is fully charged then E = 1.36 volt

 

During discharging

Chemical reaction

At anode : Pb + SO4– – – 2e® PbSO4

At cathode : PbO2 + 2H+ + 2e + H2SO4® PbSO4 +

2H2O

Specific gravity of H2SO4 decreases and when specific gravity falls below 1.18 the cell requires recharging.

Emf of cell : When emf of cell falls below 1.9 volt the cell requires recharging.

Chemical reaction

At anode : Fe + 2OH­ – 2e® Fe(OH)2

At cathode : Ni(OH)4 + 2K+ + 2e® Ni(OH)2 +

2KOH

Emf of cell : When emf of cell falls below 1.1 V it requires charging.

Efficiency

80%

60%

 

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