You might find this paper interesting, looking at different metal combinations (as 'air electrodes') abnd the effect of relative humidity on voltage generation. While I am not suggesting that you have a water vapour effect at all I think the choice of metal pairs here may be of interest.
https://www.nature.com/articles/s41598-020-64409-2
As far as ionisation goes, the only mechanism for this to produce a voltage of the type you see is for positive and negative ions to be differentially attracted to one electrode surface or the other. This would be a function of the Bronsted -Lewis Acid/base nature of the metal surface and associated oxides.
https://www.researchgate.net/p…s_to_in_situ_measurements.
As for the source of the ionisation itself. I'm still wondering.