NICKEL–IRON ALKALINE CELL

It is also known as Edison cell as it was developed by an American scientist Thomson A Edison in 1909.

4.13.1  Construction

It contains two plates, that is, a positive plate (cathode) and a negative plate (anode). The active material of cathode is Ni(OH)₄ and of anode is iron (Fe) when fully charged. The two plates are immersed in the electrolyte, a solution of potassium hydroxide (KOH). The specific gravity of the electrolyte is 1.2. In this case, the container is made of nickel-plated iron to which negative plates are connected. This cell is quit compact as small quantity of electrolyte is used.

4.13.2  Working

When the cell is fully charged, its positive plate is of Ni(OH)₄ and its negative plate is of iron (Fe). The electrolyte is potassium hydroxide (KOH) of specific gravity 1.2. When dissolved in water, the KOH is dissociated into potassium (K⁺) and hydroxyl (OH⁻) ions.

4.13.3  Discharging

When a load (resistor) is connected across the terminals of the cell, the hydroxyl ions go to anode and potassium ions go to cathode. The following chemical action takes places during discharging:

At anode:     Fe + 2OH → Fe(OH)₂

At cathode   Ni(OH)₄ + 2K → 2KOH + Ni(OH)₂

Therefore, cathode is converted from Ni(OH)₄ to Ni(OH)₂ and anode is converted from iron (Fe) to iron hydroxide Fe (OH)₂. However, the strength of the electrolyte remains the same.

4.13.4  Recharging

When the cell is put on charging, the hydroxyl (OH⁻) ions move towards cathode and potassium (K⁺) ions move towards anode. The following chemical actions take place during recharging:

At anode:      Ni(OH)₂ + 2OH → Ni(OH)₄

At cathode:   Fe(OH)₂ + 2K → Fe + 2KOH

Therefore, both the electrodes regain their original chemical composition without changing the strength of the electrolyte.

4.13.5  Electrical Characteristics

1. The emf of a fully charged cell is 1.4 V which decreases to 1.3 V rapidly. However, the average emf of the cell is 1.2 V which decreases to 1.0 V when fully discharged.
2. The internal resistance of this cell is quite high nearly 5 times to that of a lead–acid cell.
3. The A-H efficiency of this cell is nearly 80%, whereas the W-H efficiency is 60%.

4.13.6  Advantages

It has the following advantages in comparison to that of a lead–acid cell:

1. Longer life—about 5 year
2. Its electrolyte (KOH) is not harmful if spilled away.
3. The specific gravity of its electrolyte does not change when discharged; therefore, it can be left in a fully discharged condition for a considerable period of time without damage.
4. Lower weight—nearly half to that of lead–acid cell.
5. It can be discharged and recharged at higher rate for longer period without damage.
6. It can withstand higher temperature.
7. It is more rugged and can withstand more mechanical and electrical stresses.

4.13.7  Disadvantages

1. Higher cost—nearly double.
2. As the emf developed per cell is less (only 1.2 V), more number of cells are required for a particular voltage.
3. Higher internal resistance—nearly 5 times. Therefore, it cannot provide large current and is unsuitable for automobile starting.
4. Lower efficiency.