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Great toy! Great Science! Dodgy ecology?

My daughter bought a great self assembly kit for a model motorbike, marketed as the ‘Salt Water Motorcycle’ (by The Source) and it turned out to be a great buy. Here it is fully assembled (length about 200 mm):

It does straight runs, zigzags or runs in circles, as desired by the owner. And it is powered by two magnesium/air oxygen fuel cells connected in series. Here they are, left hand side is opened so you can see the magnesium anode:

The cells are powered by the following redox reactions:

Anodic oxidation:

Mg(s) → Mg2+(aq) + 2 e

Cathodic reduction:

½ O2(g) + H2O(l) + 2 e → 2 OH-(aq)

Because Mg(OH)2 is poorly soluble the overall cell reaction is:

Mg(s) + ½ O2(g) + H2O(l) → Mg(OH)2(s)

The Gibbs Free Energy change at STP for this reaction is ΔGcell = – 596.4 kJ/mol and using Nernst we can deduce that the theoretical cell potential E = – ΔG / (n F) = + 3.09 V.

The cells are made up simply of a carbon containing, flexible cathode laid at the bottom of the cell housing, a piece of magnesium metal on top acting as an anode and a piece of non-woven fabric in between the cathode and anode. During operation the non-woven spacer is soaked in 20 % NaCl solution, acting as the electrolyte. It’s really simple yet ingenuous at the same time. After assembling the fuel cells and loading them with saline solution, I measured a cell potential of about 1.6 V (one fuel cell), using a simple multimeter. Not bad for such a simple assembly!

And once the fuel cells were inserted into the motorbike that thing went like the clappers! That’s also due to the very lightweight construction and the tiniest (about 10 mm x 5mm) DC motor I’ve seen in a while. Much fun was had assembling this kit and riding it.

So where’s the rub? The kit makers make some misleading claims with regards to the ‘ecology’ of their toy. The illusion is created, mainly by creative marketing that the toy runs on saline solution and nothing more. In reality it runs on magnesium metal and air oxygen (the saline really only provides conductivity and water) and while two latter are free, the former certainly isn’t. Even in the most of ideal worlds, to fully recover the magnesium the cell reaction needs to be completely reversed and a minimum of 596.4 kJ/mol of magnesium spent to restore its chemical energy. It’s by no means certain that this energy would be supplied from ‘green’ sources, so the toy’s ‘ecology’ is really only very local and electrical motorbikes don’t run on saline solutions only…

Here’s a report on the serious use of rechargeable magnesium air batteries in real cars (Korea)

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