# What is the electrochemical reaction of this battery?

I've watched a YouTube video "Soda Can Battery - Power From Trash - DIY Project" by Source Port Services. I understand that the anode can be oxidised, but what is the reduction reaction at the cathode? They are using a salt water solution $$\ce{Na+}$$ $$\ce{Cl-}$$ and a copper cathode.

$$\begin{array}{cc} \hline \text{Half-reaction} & E^\circ/\pu{V} \\ \hline \ce{Cu^2+ + 2 e− <=> Cu(s)} & 0.337 \\ \ce{Na+ + e- <=> Na(s)} & -2.71 \\ \ce{Al^3+ + 3 e− <=> Al(s)} & -1.662 \\ \hline \end{array}$$

Thus we can see that the aluminium is oxidised, but there are no $$\ce{Cu^2+}$$ ions to be reduced also $$\ce{Na+}$$ reduction would lead to a negative $$E_\mathrm{cell}$$ standard and a positive Gibbs energy. Is it an air cathode where $$\ce{O2}$$ is being reduced?

• I took a liberty to convert a list of reduction potentials for elements into a table. Standard potentials determined for half-reactions, not elements. – andselisk Nov 1 '19 at 6:06

These experiments were done in the 17th century by Volta. Atoms and ions did not exist then. These are contact potentials and you do not need a redox system to begin the experiment (metal dipping in its salt). Let us say you start with Al, Al will start to dissolve in a corrosive medium (salt is very corrosive) to form Al ions. You have a valid point, there are no copper ions to reduce? What else do we have in the system? Water, it has some ionized $$\ce{H+}$$, they start to reduce, one should see hydrogen bubbles at the copper electrode. Certainly you can draw small currents to power a LED from such batteries but not for long.