Batteries

A battery is one galvanic cell or several in series. A useful one is light, compact and keeps a nearly steady voltage while in use.

Primary cells react once and are then spent. The dry (Leclanché) cell has a zinc container as anode and a graphite rod in MnO₂ and carbon as cathode, with a moist paste of NH₄Cl and ZnCl₂ between them.

Dry cell: anode Zn → Zn²⁺ + 2e⁻; cathode MnO₂ + NH₄⁺ + e⁻ → MnO(OH) + NH₃, reducing Mn from +4 to +3. The NH₃ binds Zn²⁺ as [Zn(NH₃)₄]²⁺. Potential about 1.5 V.

The mercury cell, for low-current devices such as hearing aids and watches, has a Zn-Hg amalgam anode, a HgO + carbon paste cathode and a KOH-ZnO paste electrolyte: Zn(Hg) + 2OH⁻ → ZnO + H₂O + 2e⁻ and HgO + H₂O + 2e⁻ → Hg + 2OH⁻.

Its overall reaction Zn(Hg) + HgO → ZnO + Hg involves no ion whose concentration changes, so its 1.35 V stays constant over its life.

Secondary cells are recharged by driving current the opposite way, over many cycles. The lead storage battery (vehicles, inverters) has a lead anode, a lead grid packed with PbO₂ as cathode and 38% sulphuric acid.

Lead storage, discharge: anode Pb + SO₄²⁻ → PbSO₄ + 2e⁻; cathode PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O; overall Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O. Charging turns the PbSO₄ back into Pb and PbO₂.

The nickel-cadmium cell lasts longer than the lead cell but costs more to make: Cd + 2Ni(OH)₃ → CdO + 2Ni(OH)₂ + H₂O on discharge.