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I won this "toy" at a science fair... Now I have to make a short presentation about it in school. On the package description they describe its energy source as "fuel cell"... It consists out of a metal plate(magnesium), a black metal plate with a black coating and a paper-cotton-like isolator which is between those plates. Through putting salt water on the isolator, there's a voltage difference of 1.76V between the black and the magnesium plate. After a couple of seconds on load the voltage stays constant at 1.31V. My theory is that it's only a simple battery, because there's no continious source of fuel which should be required for naming it fuel cell (according to wikipedia).

Is my thought right?

My idea about the black coat is, that it is a rubberd graphite or "coal" cathode, rubberd because coal or graphite would fall apart quickly. It is described as air-cathode in the package description. My idea is that the black coal or graphite coat is used to provide a larger amount of air to the reaction...

The magnesium plate gets damaged through this reaction, the black thing not.

Which material is the black coat?

and what is the full reaction?

a picture of the components

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Your analysis seems correct, i.e. it is only a fuel cell if the magnesium is being oxidized by the air. You can prove if that is the case, and do so as part of your classroom demonstration, by excluding any air, and therefore oxygen, from the fuel cell.

  1. Use distilled water and fairly pure $\ce{NaCl}$ (rather than table salt) to make the salt solution.

  2. Boil the salt water for a minute or so to remove air bubbles (some oxygen from air dissolves into water).

  3. Put the cell with electrolyte in a plastic bag sealed around the wires and a narrow tube such as a plastic coffee stirrer or soda straw that you'll use later to admit air, carefully squeezing out air bubbles. Seal both bag and tube with plasticine modeling clay or soft wax.

  4. Observe the open circuit voltage of the cell.

  5. Then run it for a while with a load (e.g. a 75 mA 1.5 V lamp) or a 100 ohm resistor, which will draw 1.3 mA), depending on the current output of the cell. You would expect some voltage drop over time, either due to cell polarization or due to any residual oxygen being used up.

  6. To prove that this is a true fuel cell oxidizing the magnesium using air, bubble some air into the plastic bag through the (unplugged) tube. *If the voltage goes back up, then drops down until yet more air is added, you've shown that oxygen is being used and it's a true fuel cell.

Please accept my compliments for your thoughtful question; do post your findings here!

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  • $\begingroup$ I accept your compliments thanking you very much, I'll make your experiment and will add it to this question... after that you'll get your laurels $\endgroup$ – leAthlon Jun 15 '15 at 5:05
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    $\begingroup$ Hey, I found out that it stops working without oxygen. The voltage goes back up after fresh air can contact the cell. Could you took a look at this link and tell me if this might be mine? en.wikipedia.org/wiki/Magnesium-air_fuel_cell I think it is, because it would fit perfectly on mine. (Magnesium plate, "unknown" cathode.... If I got it right, this seems to be a fuel cell, right? $\endgroup$ – leAthlon Jun 17 '15 at 21:47
  • $\begingroup$ Sure looks like you proved the point! Thanks for going to the trouble of testing it and updating your post. Hope you get extra credit for the thoughtful analysis, as well as the classroom presentation. $\endgroup$ – DrMoishe Pippik Jun 17 '15 at 23:32

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