side note: I don't think there's any issue with the tiny amount of left over hydrogen, and I don't think the Elektron should/has to be a perfect conversion back (some loss in the conversion would be acceptable as a way to give the part a certain lifespan before it needs topping up. Hydrogen is a pain to keep stored, so some loss would be expected..) but not at quite the ratio exhibited, especially when it ends up producing even more O2.
Fuel cells operate at a ratio of 2:1 hydrogen & oxygen -> electricity + water.
The Elektron which reverses the process, however converts with a 1:2 ratio, effectively transmogrifying hydrogen into oxygen.
From testing:
This looks to have been a typo, as the config files have opposite ordering of hydrogen/oxygen between fuel cells and the Elektron.
https://github.com/linuxgurugamer/universal-storage-2/blob/master/GameData/UniversalStorage2/Parts/Processors/Elektron.cfg#L75-L88
https://github.com/linuxgurugamer/universal-storage-2/blob/master/GameData/UniversalStorage2/Parts/Electrical/FuelCellSmall.cfg#L63-L74
side note: I don't think there's any issue with the tiny amount of left over hydrogen, and I don't think the Elektron should/has to be a perfect conversion back (some loss in the conversion would be acceptable as a way to give the part a certain lifespan before it needs topping up. Hydrogen is a pain to keep stored, so some loss would be expected..) but not at quite the ratio exhibited, especially when it ends up producing even more O2.