Primary Cells

Primary cells are cells with a fixed amount of each reactant which, once used up, cannot be recharged. The Leclanché Cell (Dry Cell) is the most common primary cell and powers small devices.

The cell is set up such that there is a solid Zn anode which is oxidised and a MnO2/NH4Cl electrolyte paste where the MnO2 is reduced at the carbon cathode.

The Zn anode and C cathode are separated by a porous membrane which allows for the flow of ions between the half-cells and forces the electrons to flow through an external circuit.

Secondary Cells

Secondary cells are cells that have a fixed amount of each reactant, and once used up, can be recharged by applying an external voltage. The Lead-Acid Accumulator cell is a type of secondary cell and is typically used for car batteries.

The cell is set up such that there is a sponge-like Pb grid as the anode and another Pb grid packed with powdered PbO2 as the cathode. The anode and cathode are dipped in an H2SO4 electrolyte, and the half-reactions are separated by a porous membrane.

At the anode, the Pb is oxidised with the SO4-2 ions to produce PbSO4, and at the cathode, the PbO2 is reduced with the SO4-2 ions to also produce PbSO4.

When recharging the cell, an external circuit is applied and the PbSO4 that coats both the anode and cathode is forced to be reduced and oxidised back into their original reactants.

Hydrogen Fuel Cell

Fuel cells are cells that have their reactants continually fed into them and will therefore continually produce electricity as long as their fuel is supplied. The reactants consist of hydrogen and oxygen gas.

When the H2 gas enters into the cell it is oxidised at the anode to form water, and when the O2 gas enters into the cell on the other side, it will be reduced at the cathode to also form water.

The electrolyte used in a hydrogen fuel cell can either be an acidic electrolyte or a basic one. The overall reaction and the voltage produced will always be the same at standard conditions.

Corrosion

Corrosion is a spontaneous redox reaction that has widespread real-world impacts. In this content page, we'll focus on the corrosion of iron (rust).

The corrosion of iron results from the presence of oxygen and water (or water vapour) in the air. The entire process occurs in four stages: