Battery

Battery:

It is a galvanic cell (an electrochemical cell) in which the chemical energy is converted into electrical energy. It consists of one or more than one cell connected in series.

There are mainly two types of batteries.

1. Primary batteries
2. Secondary batteries

Primary batteries:

This type of battery becomes useless after using a period of time. It can not be recharged again. Examples of these types of batteries are Dry cell and Mercury cell.

1. Dry cell: It consists of a graphite rod which acts as a cathode. The rod is surrounded by powdered manganese oxide (MnO₂) and carbon. The outer portion of cell is made up of zinc which acts as anode. The space between the anode and cathode is filled by paste of NH₄Cl₂ and ZnCl₂. The following reactions occur at electrodes.

At anode: Zn(s) → Zn²⁺ + 2e^–

At cathode: MnO₂ + NH₄⁺ → MnO(OH) + NH₃  

The cell has a potential of 1.5 V.

2. Mercury Cell: It consists of a paste of HgO and carbon as a cathode and   mercury-zinc amalgam as anode. A paste of KOH and ZnO acts as an electrolyte. The following reactions occur at electrodes.

At anode: Zn (Hg) + 2OH^– → ZnO(s) + H₂O + 2e^–

At cathode: HgO + H₂O + 2e^– → Hg (l) + 2OH^–

And the overall reaction is

Zn (Hg) + HgO (s) → ZnO(s) + Hg (l)

Secondary batteries:

This type of battery can be recharged again. It can be recharged by passing the current in the opposite direction. Examples of these types of batteries are Lead storage battery and Nickel-Cadmium battery.

1. Lead storage battery:It consist of a grid of lead with lead dioxide as cathode and lead as an anode. The solution of sulphuric acid is used as an electrolyte. The following reactions occur at electrodes.

At Anode: Pb(s) + SO₄^2-(aq) → PbSO₄(S) + 2e^–

At cathode: PbO₂(S) + SO₄^2-(aq) + 4H⁺(aq) + 2e^– →PbSO₄(s) + 2H₂O(l)   

The overall reaction is

Pb(s) + PbO₂(S) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)

2. Nickel-Cadmium battery:It consists of Nickel hydroxide as a positive electrode plate and metallic cadmium as negative electrode plate. A separator is placed between the two electrodes which consist of a layer soaked in moist sodium of potassium hydroxide. Potassium hydroxide acts as an electrolyte.

The overall reaction is

Cd + 2Ni(OH)₃ → CdO + 2Ni(OH)₂ + H₂O