Iron-Nickel (Edison) Battery Chemistry FAQ

Iron Nickel Cells

Anode: Iron

Cathode: Nickel oxyhydroxide

Electrolyte: Potassium hydroxide

Applications:

This battery was introduced by Thomas Edison. It is a very robust battery: it can withstand overcharge, overdischarge, and remaining discharged for long periods of time without damage. It is good for high depths of discharge and can have very long life even if so treated. It has low energy density, a high self-discharge rate, and evolves hydrogen during both charge and discharge. It is often used in backup situations where it can be continuously charged and can last for 20 years or longer.

The chemistry involves the movement of oxygen from one electrode to the other: 3Fe + 8NiOOH + 4H2O=8 Ni(OH)2 +Fe3O4.

Half Reaction	Voltage
Fe + 2OH- —> Fe(OH)2 +2e-
3Fe(OH)2 + 2OH- —> Fe3O4 + 4H2O + 2e-

The open circuit voltage of this system is 1.4 V, and the discharge voltage is about 1.2 V. The electrolyte is 30% KOH solution, with some additives.

The ability of this system to survive frequent cycling is due to the low solubility of the reactants in the electrolyte. The formation of metallic iron on charge is slow because of the low solubility of the Fe₃O₄, which is good and bad. It is good because the slow formation of iron crystals preserves the electrode morphology. It is bad because it limits the high rate performance: these cells take a charge slowly, and give it up slowly.