A battery is a galvanic cell. It's no more than an anode and a cathode... separated by an electrolyte, right?
|― Walter White to Jesse Pinkman|
A mercury battery (also called mercuric oxide battery, or mercury cell) is a non-rechargeable electrochemical battery, a primary cell. In Breaking Bad, Walter White builds a mercury battery to jump start the RV when he and Jesse Pinkman were stranded on the desert for four days ("").
- "On one side, you have mercuric oxide... and graphite from your brake pads. This is the cathode. This is the positive terminal. This is where the supply of current flows out from, you see? Then... here, I'll show you. On the opposite side is our anode. This. It's zinc. It's what we find in our coins and anything galvanized."
- ―Walter White to Jesse Pinkman
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 anode, 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.