Question

What are primary cells, secondary cells and fuel cells? Explain one example of each.

Accepted Answer

Primary cells are those in which the redox reaction occurs only once and the cell become dead after some time, and cannot be used again.

Example: Dry cell and mercury cell.

Dry cell (Leclanche cell) consists of a cylindrical zinc container which acts as the anode. A graphite rod placed in the centre acts as the cathode. The space between the anode and the cathode are packed in such a way that zinc container is in touch with the paste of ${NH}_{4} Cl$ and ${ZnCl}_{2}$ and graphite rod is surrounded by powdered ${MnO}_{2}$ and carbon. The graphite rod is fitted with a metal cap and the cylinder is sealed at the top with pitch.

Energy is released during the following cell reactions:

At anode: $Zn (s) \to {Zn}^{2 +} (aq) + 2 e^{-}$

At cathode: $2 {MnO}_{2} (s) + 2 {NH}_{4}^{+} (aq) + 2 e^{-} \to {Mn}_{2} O_{3} (s) + 2 {NH}_{3} (g) + H_{2} O$

The ammonia formed combines immediately with ${Zn}^{2 +}$ ion and ${Cl}^{-}$ ion to form $[Zn {({NH}_{3})}_{2} {Cl}_{2}]$ complex. The voltage of the cell is ranges from $1.25 - 1.50 V$ .

Secondary cells are those in which can be recharged by passing an electric current through them and hence can be reused.

Example: Lead-storage battery, Nickel-cadmium battery.

Nickel-Cadmium storage cell (Nicad cell) is a secondary cell. In this cell, a metal grid containing nickel (IV) oxide is the cathode. Cadmium electrode act as anode. The electrolyte used is the $KOH$ solution.

The electrode reactions occurring during discharge are as follows:

At anode: $Cd (s) + 2 {OH}^{-} (aq) \to Cd {(OH)}_{2} + 2 e^{-}$

At cathode: ${NiO}_{2} (s) + 2 H_{2} O + 2 e^{-} \to Ni {(OH)}_{2} (s) + 2 {OH}^{-} (aq)$

The approximate voltage of the Nickel-Cadmium storage cell is $1.4 V$ .

Fuel cells are the devices which convert the energy produced during the combustion of fuels like hydrogen, methane, methanol, etc., directly into electrical energy.

Example: $H_{2} - O_{2}$ fuel cell

$H_{2} - O_{2}$ fuel cell consists of porous carbon electrodes containing catalysts (finally divided platinum or palladium metal) incorporated in them. Concentrated $KOH$ or $NaOH$ solution is placed between the electrodes to act as electrolyte. $H_{2}$ and $O_{2}$ are bubbled through porous electrodes into the electrolyte solution.

The electrode reactions are takes place as follows:
At anode: $2 H_{2} (g) + 4 {OH}^{-} (aq) \to 4 H_{2} O (l) + 4 e^{-}$

At cathode: $O_{2} (g) + 2 H_{2} O (l) + 4 e^{-} \to 4 {OH}^{-} (aq)$

Overall reaction: $2 H_{2} (g) + O_{2} (g) \to 2 H_{2} O (l)$

In this process, the reactants are fed continuously to the electrode and the products are removed continuously from the electrolyte compartment.

	List-I		List-II
A	$Cd (s) + 2 Ni {(OH)}_{3} (s) \to CdO (s) + 2 Ni {(OH)}_{2} (s) + H_{2} O (l)$	I	Primary battery
B	$Zn (Hg) + HgO (s) \to ZnO (s) + Hg (l)$	II	Discharging of secondary battery
C	$2 {PbSO}_{4} (s) + 2 H_{2} O (I) \to Pb (s) + {PbO}_{2} (s) + 2 H_{2} {SO}_{4} (aq)$	III	Fuel cell
D	$2 H_{2} (g) + O_{2} (g) \to 2 H_{2} O (l)$	IV	Charging of secondary battery

What are primary cells, secondary cells and fuel cells? Explain one example of each.

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