During the charging and discharging of a nickel-iron cell

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Examoogle · Accepted Answer

water is neither formed nor absorbed

Explanation: In a Nickel-Iron (Edison) storage cell, the electrolyte (Potassium Hydroxide, $KOH$) acts only as an ion carrier. The chemical reactions at the electrodes involve the transfer of hydroxyl ions between the nickel and iron plates without consuming or producing water molecules.

Working Principle: During charge and discharge, the reaction at the positive plate is $2Ni(OH)_3 + 2e^- \leftrightarrow 2Ni(OH)_2 + 2OH^-$ and at the negative plate is $Fe + 2OH^- \leftrightarrow Fe(OH)_2 + 2e^-$. Summing these reactions shows that the net change involves the exchange of $OH^-$ ions, resulting in no net change in the water content of the electrolyte.

Key Formulas: $2Ni(OH)_3 + Fe \leftrightarrow 2Ni(OH)_2 + Fe(OH)_2$ — Represents the overall reversible chemical reaction in the cell.

Key Points: The specific gravity of the electrolyte remains practically constant throughout charge and discharge.; The battery is robust and has a long service life compared to lead-acid batteries.; It is primarily used in heavy-duty industrial applications.; Internal resistance is higher than that of lead-acid cells.

Advantages: Extremely durable and rugged construction; Can be left in a discharged state without damage

Disadvantages: Lower efficiency than lead-acid cells; High initial cost

Applications: Railway carriage lighting and ventilation; Mining lamps and industrial trucks

Additional Info: Option A is incorrect because Nickel-Iron cells do not release corrosive fumes in normal operation.. Option C is incorrect as the Nickel Hydroxide is indeed active and changes valence state during the process.. Option D is incorrect because the e.m.f. of a Nickel-Iron cell drops significantly during discharge, unlike some constant-voltage sources.

Answer: B is correct — During the electrochemical process in a Nickel-Iron cell, the net reaction results in neither the formation nor the consumption of water, meaning the electrolyte concentration remains stable.

Exam Tip: Always remember that unlike the Lead-Acid cell where specific gravity changes with charge state, the Nickel-Iron cell's electrolyte (KOH) does not participate in the net mass-balance change of water, making hydrometer readings useless for SoC.

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