# Magnesium air-fuel cell or battery?

I'm a bit confused. I've got this "toy" ... with the help of this site I was abled to find out that the reaction stops with a lack of air. Link to the old question.

The battery/fuel cell or however you call it consists out of following components:

• magnesium plate
• isolator which consists out of something like paper, the surface feels more like cotton
• the black plate is called "black cathode" in the manual, my hypothesis is that it might be rubbered graphite or a polymer, because it has a very low electrical resistance.
• the electrolyte is a $\ce{NaCl}$ dispension- I think it just need to lower $\ce{H2O}$'s electrical resistance
• plastic stuff is only the case of the "cell"

In the manual they call it a fuel cell. My chemistry teacher says that in his hypothesis it is impossible to be a fuel cell and has to be a battery. -Because of the great answer to my last question I doubt it or at least I'm too confused to agree.

## My research results:

►Battery:

• news about a battery for cars

►Fuel cell:

• a kind of datasheet of a "battery" which is in development

►My hypothesis for beeing called fuel cell

→ because fuel cells require according to their definition a constant source of fuel, it might be called fuel cell because of the fact that the "thing" requires air to react.

►My hypothesis for beeing called battery

→ oxygen as fuel?- some people might see it different and call it battery because of that (then there's no constant fuel source →battery) .

►My idea about the chemical equation is:

$\ce{Mg + H2O → Mg(OH)2 + H2}$ (but I doubt it, because I can't imagine that it is responsible to sell a toy which emitts $\ce{H2}$ - or what else happens with it?-maybe the black cathode has to do something with it?)

If you've got a better idea about the chemical equation- let me know.

## I would like to know if there's a mistake in my thoughts- and if it's actually a battery or a fuel cell.(the main problem).

And if you've got a better idea about the black cathode component's material- let me know please.

please don't mark it as duplicate to my older question-there's a difference because I added information about the theories and reasearched a lot about it and found the reasons/arguments I've posted here out- the last question was doing it by its components.

What you have is classified as a Mg-air battery. You can read more about it here : https://en.wikipedia.org/wiki/Category:Metal-air_batteries

These are a class of primary(i.e. not rechargeable) batteries, where the anodic reaction is a metal being oxidised and the cathodic reaction is oxygen reduction.

The reactions are:

$\ce{Mg → Mg^{2+} + 2 e^{-}}$ on the anode

$\ce{O2 + 4 e^{-} + 4 H+ → 2 H2O }$ on the cathode, if the reduction goes to completion. Most of the time, it is only reduced to the peroxide or the superoxide which is considered incomplete reduction.

For it to be classified as a fuel cell, the fuel for the anodic reaction would need be brought from the outside(source: anectodal, personal thought).

• from where do you get the H+ ? – leAthlon Jun 19 '15 at 7:13
• From water dissociation. The potential and the kinetics of the reaction is pH dependent. As I mentioned, in most practical applications, the reaction only proceeds to superoxide which doesn't involve H+ – Burak Ulgut Jun 19 '15 at 7:57
• so from the OH- or H3O+ ? – leAthlon Jun 19 '15 at 7:57
• H3O+ if you will. – Burak Ulgut Jun 19 '15 at 8:45

It is a fuel cell because the magnesium anode is consumed by the oxidation reaction and is then replaced by a new anode. This example being a toy is a very simplified version where the full scale fuelcell has a continuous roll of magnesium foil, once the surface is oxidized the roll slowly advances until it is entirely expended and then replaced to start again. If the container were not excess able for cartridge replacement it would be disposable and would be considered a primary cell.

I would say more : $$\ce{O2 + 2H2O + 4e- → 4OH-}$$ $$\ce{2Mg + 3OH- → 2 Mg2+ + 4e-}$$ (2.93V global potential so 6,46 kWh/kg theoritical energy density)

It is a potential fuel cell by adding magnesium pellets continuously with oxygen from the air.

Great research anyway and good scientific spirit, I'm now looking for information about KIST car but can't find. Maybe it has a link with the fact that the autonomy is twice compared with today's electric cars?