primary cell

A primary cell is a type of cell whose cell reaction cannot be reversed (i.e. cannot be recharged). Dead primary cells are discarded.

Leclanche Cell (Dry Cell)

• Leclanche cells hold charge for a long time—good for emergency backup
• Anode uses zinc to prevent rapid corrosion (esp. in alkaline environemnt).
• Cathode can be either manganese oxide or mercury cathode.
• $\xi \approx 1.5\mathrm{V}$

Acidic cells:

$$\begin{array}{rlrl}\text{Anode:}& & \mathrm{Zn}& \to \mathrm{Zn}{}^{2+}+2\mathrm{e}{}^{-}\\ \text{Cathode:}& & 2\mathrm{NH}{}_4{}^{+}+2\mathrm{MnO}{}_2+2\mathrm{e}{}^{-}& \to \mathrm{Mn}{}_2\mathrm{O}{}_3+2\mathrm{NH}{}_3+\mathrm{H}{}_2\mathrm{O}\end{array}$$

Alkaline cells:

$$\begin{array}{rlrl}\text{Anode:}& & \mathrm{Zn}+2\mathrm{OH}{}^{-}& \to \mathrm{ZnO}+\mathrm{H}{}_2\mathrm{O}+2\mathrm{e}{}^{-}\\ \text{Cathode:}& & 2\mathrm{MnO}{}_2+2\mathrm{e}{}^{-}& \to \mathrm{Mn}{}_2\mathrm{O}{}_3+2\mathrm{OH}{}^{-}\end{array}$$

Button Battery (Silver-Zinc Cell)

• High storage capacity so can be made small
• Suitable for use in small devices (watches, hearing aids, etc)
• $\xi =1.8\mathrm{V}$
• Alkaline cell (hydroxide ions)

$$\begin{array}{rlrl}\text{Anode:}& & \mathrm{Zn}+2\mathrm{OH}{}^{-}& \to \mathrm{ZnO}+\mathrm{H}{}_2\mathrm{O}+2\mathrm{e}{}^{-}\\ \text{Cathode:}& & \mathrm{Ag}{}_2\mathrm{O}(\mathrm{s})+\mathrm{H}{}_2\mathrm{O}(\mathrm{l})+2\mathrm{e}{}^{-}& \to 2\mathrm{Ag}(\mathrm{s})+2\mathrm{OH}{}^{-}(\mathrm{aq})\end{array}$$