|MadSci Network: Chemistry|
This is out of my area but I will hazard a guess. I don't think that grouping or alignment of ions is the explanation. Unless the coins were cleaned with steel wool or a scouring pad, and well rinsed, it is likely that the coins that the questioner started with had a layer of oxide (or skin oil scum from handling) on them. As the electrochemical reaction proceeded PERHAPS the oxide/scum was diminished (I hesitate to say etched away, maybe consumed in some reaction is a better way to express this). This would improve the surface contact between the metal of the coins and the electrolyte (potato starch). This improved surface contact might lead to an increased voltage from the potato battery. The questioner can test this by STARTING with coins that have been cleaned, water rinsed, maybe then wiped with rubbing alcohol, and then rinsed well again with water, and then placed in the potato with out delay. If these cleaned coins display a relatively high and CONSTANT voltage then my explanation is more likely to be correct. Since this is an educated guess the Mad Scientist Administrator might want to poll someone on the network who lists electrochemistry or batteries as their area of expertise.
Try the links in the MadSci Library for more information on Chemistry.