Generally, after using Electro chemical cells once, they cannot be used again, that is, the chemical reactions that take place in the cell cannot be reversed by passing an electric current. But some such cells are also used now, which can be used again after being used once, such cells are called storage cells.
Lead storage cell or battery is most suitable among the accumulator cells in general use. It is used for driving car, vehicle etc and for many domestic works.
Therefore, the cells that are re-charged are called storage cells. These types of cells have two lead electrodes. One of which is covered with lead oxide on the electrode. Dilute sulfuric acid is used as an electrolyte.
When the cell is used, lead gets deposited on the lead oxide electrode and electric current is generated and when electric current is passed through the cell, lead and sulfuric acid are produced again as a result of the opposite reaction. This reaction is reversible, which takes place as follows.
Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O
This is the reason that storage cells can be charged again and again, thus they can be used again and again.
Structure of Lead Storage Cell: — Lead storage cell is a vessel made of hard rubber, in which there are positive charge plates made of lead paroxide (PbO₂). All positive charge plates are connected to a terminal, which is called positive charge terminal. Negative charge plates made of spangi lead are attached, which is connected to the negative charge terminal of the storage cell.
In the storage cell, positive charge plates and negative charge plates are arranged in an alternate order, that is, a positive charge plate is followed by a negative charge plate and a negative charge plate is followed by a positive charge plate.
Separating plates of porous wood, glass or rubber are kept between each positive charge plate and negative charge plate. Dilute sulfuric acid (H₂SO₄) acts as the storage cell’s electrolyte. Lead acid cell is represented as follows –
O⁻ [Pb | PbSO₄(s)]
O⁻ [H₂SO₄(40%) | PbO₂(s)]
Working of Lead Storage Cell — The mechanism of the lead storage cell is based on an instantaneous oxidation reduction reaction, in which electrons are transferred from the “lead anode” to the ‘lead dioxide’ cathode by an external circuit.
The following reactions take place when a storage cell is used –
H₂SO₄ → 2H⁺ + SO₄ ⁻
PbO₂(s) + 2H⁺ + 2e⁻ + H₂SO₄ → PbSO₄(s) + 2H₂O
Pb + SO₄⁻ → PbSO₄ + 2e⁻
Therefore, during use, both the electrodes of the cell are covered with a layer of lead sulfate and the relative density of the electrolyte decreases to 1: 1 and the voltage of the battery decreases. In this case the battery is declared disabled.
Recharging of Battery :- At the time of recharging the lead storage cell, by taking some resistance in the middle, connect the anode of the immersed cell to the main wire. That is, the electrode which acts as a cathode, acts as an anode when charged. During this the following reaction takes place –
PbSO₄ (s) + 2H⁺ + 2e⁻ → Pb (s) + H₂SO₄
PbSO₄ (s) + SO₄⁻⁻ + 2H₂O → PbO₂ (s) + 2H₂SO₄ + 2e⁻
Complete Reaction :
2PbSO₄ (s) + 2H₂O (l) → Pb (s) + PbO₂ + 4H⁺ (aq) + 2SO₄⁻ (aq)
Once charged, the positive plate becomes lead paroxide (PbO₂) and the negative plate becomes spongy lead (Pb), meaning the chemical reactions in this cell are variable. After charging the storage cell, the cell is used again as a source of current.
The voltage of the storage cell depends on the number of minus and plus plates present in it.
A cell provides a potential of 2 volts. Generally some cells of 6 or 12 volt are stored.
Some other storage cells are nickel-iron and nickel-cadmium storage cells, which are used in aircrafts.