Electrical Engineering ⇒ Topic : Nickel-lron Cell or Edison Cell
Nickel-lron Cell or Edison Cell
Nickel-iron cell was developed by American scientist Thomas A. Edison in 1909. It has lesser weight and longer life than that of a lead-acid cell. As a result, these cells are very suitable for portable work. The e.m.f. of this cell is about 1.36 V.
Construction. When nickel-iron cell is in the charged condition, the active material on positive plates is Ni(OH)4 and that on the negative plates is iron (Fe). The positive and negative plates are held in a nickel-plated steal container; the plates being insulated from each other by hard rubber strips.The container contains 21 percent solution of KOH (electrolyte) to which is added a small amount of lithium hydrate (Li0H) for increasing the capacity of the cell.
(1) The positive plates are in the form of perforated nickel-plated steel tubes filled with Ni(OH)4 and flakes of metallic nickel; the addition of flakes of nickel reduces the internal resistance of the cell.
(2) The negative plates are also in the form of perforated nickel-plated steel tubes filled with powdered iron oxide and a little mercuric oxide. The purpose of mercuric oxide is to decrease the internal resistance of the cell.
Chemical changes. The molecules of electrolyte (KOH) dissociate into K+ and OH- ions.
KOH → K+ + OH -
(i) During discharging, the K+ ions move towards the positive plate (anode) and reduce Ni(OH)4 to Ni(OH)2. The OH - ions travel towards the negative plate (cathode) and oxidise iron. The chemical changes during discharging can be represented by the following equations :
Positive plate : Ni(OH)4 + 2K > Ni(OH)2 + 2KOH
Negative plate : Fe + 20H > Fe(OH)2
(ii) During recharging, the K+ ions move towards negative plate (cathode) and OH - ions go to positive plate (anode) causing the following chemical changes
Positive plate : Ni(OH)2 + 20H → Ni(OH)4
Negative plate : Fe(OH)2 + 2K → Fe + 2KOH
The chemical reactions during discharging and recharging can be summed up in a single reversible equation as under
It may be observed from the above equation that no water is formed in the reaction. Consequently, the specific gravity of the electrolyte (KOH) remains unchanged during charging or discharging. For this reason, a nickel-iron cell is not damaged if left in a fully discharged condition for a considerable period of time.
Since the electrolyte (KOH) does not undergo any change in specific gravity during charge or discharge, the state of charge of this cell cannot be determined by the specific gravity of the electrolyte. Instead, a voltmeter is employed to ascertain whether the cell is charged up to its rated voltage.
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