Aluminium, Carbon, Aluminium Sulphate battery. What's the reaction?

Question

I made galvanic cells from the following items (while trying to do something else), and they give between 0.6 V and 1 V per cell.

Aluminium foil – negative terminal

Carbon (these work: charcoal sticks, graphite rods, carbon felt) – positive terminal.

Electrolyte – aluminium sulphate $\ce{Al2(SO4)3}$ in water.

With 9 cells – 3 series * 3 parallel – it lights a red LED. Left on overnight it gets very dim, but given some recovery time, it comes back bright again. So the current isn't high, but I wasn't expecting any.

I'm not observing any gas being evolved, or any change in the foil (yet). The battery has been running about a week.

I know the Al sulphate reacts with water to make sulphuric acid + Al hydroxide. Is that what's happening here? What happens next in the reaction? What's the final result?

Thanks for your help. It's a long time since school chemistry lessons.

Answer 1

Very interesting: no gas, no change in the electrode, yet a significant electrical output. This could be the beginning of perpetual power (similar to perpetual motion!).

But no, probably the output power is barely significant, and something is happening somewhere, possibly in the solution. Aluminum sulfate doesn't usually produce a clear solution, so possibly you have missed an increased turbidity.

Possibly oxygen is reacting with the metal under the acidic conditions to produce Al(OH)3 which is removed by the acid solution (without reducing the acidity, because Al(OH)3 is not much of an alkali). The equation could be 4 Al + 3 O2 + 6 H2O --> 4 Al(OH)3. If you restrict oxygen access, the voltage and current should decrease. If you increase it, say with an aquarium pump, you might get a higher output.