Electrochemical cells are batteries, and one of the most common types is the alkaline battery (alkaline because potassium hydroxide is the electrolyte).
At the anode:
Unlike electrochemical cells mentioned in an earlier article, the anode is not a piece of zinc but zinc particles suspended in a KOH electrolyte (Zn–KOH paste). Zinc, being relatively reactive, is readily oxidised to Zn2+ as follows:
Zn (s) + 2OH– (aq) → ZnO (s) + 2H2O (l) + 2e–
The free electrons are collected at the anode collector and flow out of the battery via the anode cap when the battery is connected to an external circuit to power an electronic device (see diagram below).
At the cathode:
Returning electrons enter the battery via the cathode cap and flow along the sides to the cathode collector and then to the cathode where they participate in the following reduction reaction:
2MnO2 (s) + H2O (l) + 2e–→ Mn2O3 (s) + 2OH– (aq)
Like the anode, the cathode is not a piece of manganese (IV) oxide but a paste of manganese (IV) oxide and carbon (carbon is added to improve conductivity). The porous cellulose separator prevents the electrode materials from mixing but allows hydroxide ions produced in the cathode to migrate across it to the anode to maintain electrical neutrality.
As the battery is used to power an electronic device, zinc is gradually consumed (forming zinc oxide) and manganese (IV) oxide is reduced to manganese (III) oxide. This process continues until the active materials are depleted, resulting in an increase in internal resistance and a drop in voltage, at which point the device stops functioning. Unlike rechargeable batteries such as lithium-ion batteries, the solid zinc oxide and manganese (III) oxide formed during discharge cannot easily be converted back into their original forms by applying an external current. Therefore, alkaline batteries are classified as primary (non-rechargeable) cells.
