Mercury batteries use either pure mercuric oxide or a mixture of mercuric oxide with manganese dioxide as the cathode. Mercuric oxide is a non-conductor so some graphite is mixed with it; the graphite also helps prevent collection of mercury into large droplets. The anode is made of zinc and separated from the cathode with a layer of paper or other porous material soaked with electrolyte. During discharge, zinc oxidizes to zinc oxide and mercuric oxide gets reduced to elemental mercury. A little extra mercuric oxide is put into the cell to prevent evolution of hydrogen gas at the end of life. Mercury batteries are very similar to silver-oxide batteries.
Chemical reaction to the RV
At the cathode, mercuric oxide is reduced to mercury with the help of electrons arriving through the external circuit:
HgO + H2O + 2e- → Hg + 2OH-
The powdered carbon from the brake pads is there to conduct electrons to the HgO molecules, as pure HgO is an insulator. Elemental mercury is also a highly poisonous liquid, and the presence of a fairly inert powder helps to stop it pooling.
At the cathode, zinc (galvanised materials are coated with zinc to prevent rusting) is oxidised to zinc oxide and generates electrons:
Zn + 2OH- → ZnO + H2O + 2e-
The water and OH- ions are supplied through the potassium hydroxide (KOH) electrolyte, which is effectively a solution of K+ and OH- in water (H2O). We therefore have two cycles – H2O and OH- through the electrolyte, and electrons through the external circuit. The cell reaction being:
Zn + HgO → ZnO + Hg
The reactions at the anode and cathode balance each other out, giving a continuous flow of electrons through the external circuit.