Zinc–Manganese Dioxide Alkaline Cells

The most prevalent primary batteries are the traditional alkaline and dry cell batteries that are based on reactions involving manganese dioxide at the cathode and zinc metal at the anode. The half-cell reactions are as follows:

Cathodic reaction:
2MnO₂ + 2H₂O + 2e⁻ ↔ 2MnO(OH) + 2OH⁻    E^o = 0.132 V

Anodic reaction:
Zn(NH₃)₂²⁺ + 2H₂O + 2e⁻ ↔ Zn + 2OH⁻ + 2NH₄⁺    E^o = −1.17 V

Overall reaction:
Zn + 2MnO₂ + 2NH₄⁺ ↔ 2MnOOH + Zn(NH₃)₂²⁺    E_cell^o = 1.30 V

Figure: Cross-sectional schematic of a Zinc–Manganese Dioxide Alkaline Cell. The zinc anode (Zn) is on one side, the MnO₂ cathode mixed with carbon is on the other, and the central paste contains NH₄Cl and ZnCl₂ electrolyte with separator.

The term dry cell is misleading. Dry cell electrodes are placed in a wet paste. The paste consists of particles such as flour and starch as well as aqueous electrolyte. The electrolyte consists of around 28% ammonium chloride, 16% zinc chloride, and the balance is water. The particles immobilize the electrolyte, retaining water and preventing leakage. Consequently, the battery acts as a dry cell from a handling point of view.

The cathode consists of fine particles of carbon (acetylene black) and manganese dioxide. The anode consists of zinc in the form of foil or fine particles. The normal overall cell voltage is usually around 1.6 V, though this varies depending on the electrolyte composition.