Redox (II): Storage cells: the nickel cadmium cell.

Edexcel A level Chemistry (2017)

Topic 14: Redox (II): Storage cells: the nickel cadmium cell.

14/15. To be able to understand the application of electrode potentials to storage cells.

The nickel cadmium rechargeable cell

The nickel-cadmium cell is an alkaline storage cell and also a rechargeable cell.  It has longer shelf life than the zinc–carbon cell.

The cell consists of a cadmium anode and a cathode composed of a paste of nickel(IV)oxide (NiO₂).  

The electrolyte used is 20-25% potassium hydroxide (KOH) solution. The cell notation is as follows:

Cd │ [OH^–, Cd(OH)₂ ] ││ [NiO₂, Ni(OH)₂ ] │Ni

At the cadmium electrode (cathode (—)), cadmium is oxidized and the cadmium (II) ions (Cd²⁺) are precipitated as solid cadmium(II)hydroxide (Cd(OH)_2.

Cd(s) + 2OH^–(aq) ⟶ Cd(OH)₂(s) + 2e^–

At the nickel electrode (anode (+)), the nickel(IV) oxide (NiO₂) is reduced to nickel(II) hydroxide (Ni(OH)₂) which is then precipitated. 

NiO₂(s) + 2H₂O(l) + 2e^–  ⟶  2Ni(OH)₂(s) + 2OH^–(aq)

The overall cell reaction is as follows:

NiO₂(s) + Cd(s) + 2H₂O(l) ⟶  Cd(OH)₂(s) + 2Ni(OH)₂(s)

Notice that in the oxidation of cadmium 2 moles of hydroxide ions form but in the reduction of nickel(IV) the same two moles of hydroxide ions are consumed. 

The overall result is that the alkaline electrolyte is not consumed in this reaction.

The reaction can be readily reversed, because the reaction products, nickel(II) hydroxide (Ni(OH)₂) and cadmium(II)hydroxide (Cd(OH)₂) adhere to the electrode surfaces.

Ni-Cd cell is a portable, rechargeable cell and its cell voltage is fairly constant at 1.4 V. It can be left for long periods of time without any appreciable deterioration, since no gases are produced during discharging (or charging).

The reactions employed to charge the cell are as follows:

Cathode: Cd(OH)₂ + 2e^– → Cd + 2OH^–

Anode: Ni(OH)₂ + 2OH^–→ NiO₂ + 2H₂O + 2e^–

The main advantage of a rechargeable Ni–Cd cell is it can charge in as short a time as 15 min.

Ni–Cd cells are used in electronic calculators, phones, electronic flash units, cordless electronic shavers, transistors and other battery powered small tools. They are also used widely in space applications in combination with solar cells.

The Solar and Heliospheric Observatory (SOHO), which is positioned close to the sun, allowing hundreds of astronomers to study and analyze photographs of solar eruptions, has been powered with Ni-Cd batteries. 

Other well-known satellites such as Envisat, ERS, ISO and XMM (infra-red telescope Rx) also used Ni-Cd type batteries.