Sönke Schmachtel
Aug 28, 2017 · 2 min read

This is indeed an interesting topic to discuss, as it really seems that nobody can really explain this effect in a really convincing way.

And I must say that I have observed myself something similar experimentally, which was very surprising:

It is described on this German webpage:
http://www.chemieunterricht.de/dc2/tip/11_98.htm

You take Zn powder, add it to an alkaline solution and add copper coins. Due to miraculus circumstances, Zn forms on the copper coin, after which the coin can be heated and forms a brass layer.

And it notes also that there is a tendency to form brass. In that sense I have been thinking about how alloying affects the potentials of electrochemical cells. Definitely there is energy which frees form bass formation, and it should make a difference if youd plate Zn on Cu or onto some other material that does not form an alloy with Zn.

Funny though is that also in this case we are talking about a thick layer of Zn and not about a few nm which wouldnt look like Zn.

Now I would consider two cases:

  1. A dense layer of Zn.
    We have Zn powder in alkaline solution on both sides and Zn metal on both “electrodes”. This would mean that there is no energy gain or loss in the deposition of Zn, but the energy comes from the energy that is freed by alloying.
  2. A layer of Zn that leaves small holes and Cu is exposed:
    Cu dissolves when Zn plates and somehow again there is energy freed by alloying Cu with Zn.

Since I was familiar with these kind of effects, I looked up the Cu Li phase diagram. Et voila, similar to Cu — Zn there is a large solubility of Li in Cu:
https://link.springer.com/article/10.1007/s11669–010–9840–3

I can also imagine that Li could be very mobile inside Cu, if it moves interstitially. If you try to formulate it with Kröger-Vink notation (defekt reactions) https://en.wikipedia.org/wiki/Kr%C3%B6ger%E2%80%93Vink_notation

Then you could even think about Li+ moving interstitially into Cu followed by a free electron to keep the charge balance. This kind of thinking is very complex as it shades light on something in between a battery and a capacitor.

Anyway …. what this would basically mean is that there would be an alloy layer with concentration gradients and a potential most probably changing accross the whole layer, such that anyway there would be a small excess charge that would give intergrated over the alloy layer the measured potentials.

What seems logical is that accross both boundaries Li steel and Li Cu there could be forming space charge layers and potential differences, and they could be different for both sides. But then it is very difficult to imagine that the differences could be so large, because junction potenital differences related to thermal gradients are only in the order of 10s of µVs.

I’m preety sure this could be somehow investigated by space charge layer calculations or similar

And another way to find out to look into the dG of the Cu Li alloy formation. Because at equilibrium that exact energy could be visible as potential.

)