Simple metal-air cell

Inspired by Sulaiman's cell constructing here:

http://www.sciencemadness.org/talk/viewthread.php?tid=65452

I wanted to experiment with a very low cost metal-air cell that exploits a cathodic protection setup, except that the small currents flowing continuously are put to work in a small load.

Here is my morning's project made from a baked beans tin, some scrap zinc as anode and an iron wool cathode exposed to air, i.e. using oxygen as oxidant.

The electrolyte was prepared by dissolving 50g ZnCl2 in 100ml water.

Then an insulated copper wire was soldered onto the scrap zinc block with a mass of 112g and the contact point sealed thoroughly with melt glue from a glue gun.

This anode block was then placed in a cotton rag bound with a cable tie to prevent it from electrically shorting against the metal can wall.

This pouch was placed at the bottom of the empty tin can and the electrolyte poured in. The empty volume was then filled with builders sand with much tapping. At the top, iron wool was half buried in the moist sand and wetted by the electrolyte by pressing it down.

The cell delivered an open circuit emf of 400mV and a current of 33mA when shorted. The theoretical capacity is a little over 90Ah.

After being left outside in the hot sun while shorted for the day, I returned to a much drier cell that delivered negligible power. I added more water to replenish what had evaporated which slightly improved matters, but only slightly. It looks somewhat dead.

I attribute the low cell potential to the very large activation overpotential for oxygen over iron, namely 750mV (production overpotential from Wikipedia's article on overpotential) which effectively algebraically cancel's out the half potential that zinc can generate:

Zn => Zn2+ + 2e-
E° = +0.76V

That leaves the half cell potential which oxygen can generate at the cathode. using the basic form of the oxygen half reaction, one arrives at the observed cell emf:

O2 + 2H2O + 4e- => 4OH-
E° = +0.4V

By my estimates, had I used magnesium metal as anode, I could bump up the emf to about 1.2V in such a cell.

For your enjoyment, here are some pictures:



EDIT: Swopped cathode and anode words, I keep using it wrongly an also fixed the magnesium cell's expected emf which is slightly higher

[Edited on 2-3-2016 by deltaH]

Quote: Originally posted by aga

Brilliant stuff deltaH ! I do believe i possess the raw material to make a magnesium version, and will give it a go soon. Sorry to be dense, but is the Mg replacing the Fe or the Zn ?

Thanks, but not so brilliant, it doesn't work to well after a while, but it's a start that I'm sure can be refined.

Yes indeed, I just realised that I should have written that magnesium is meant to replace the zinc.

Ideally you would use MgCl2 as the salt for the electrolyte with magnesium, but you can use CaCl2 as well.

Don't forget to leave your cell in the shorted contact position once built, else that iron wool wetted by chloride will rust in hours!

Also make sure the iron wool is moistened by the electrolyte by pressing down on it. You want it slightly moist, but not water-logged.

I think slightly a greater volume of electrolyte would be more ideal than the amount I used.

You can use cotton wool as the filler instead of the sand if you want a light cell. Also a plastic jar is probably a wiser choice than a tin can

[Edited on 1-3-2016 by deltaH]

Dratz, I used anode and cathode the wrong way around AGAIN. As soon as I say cathodic protection, I keep using the order incorrectly, I fixed it in my opening post, hopefully didn't miss one. My apologies!

I have a question about the oxygen activation overpotential. Wikipedia's wiki on it reports it as the oxygen evolution overpotential and quotes a value of +750mV, but forward and reverse activation energies are not the same value, so have I erred in using a value of -750mV to explain my cell's potential of 400mV? I'm thoroughly confused.


[Edited on 2-3-2016 by deltaH]