A quick though, maybe the rubber is too porous and nitrogen can get through? THe molecule is bonded really close togheter (triple bond) and is also lighter than the CO2 bond (which is 14 grams per mol heavier) and is also quite a lot bigger (3 atoms, bonded with double bonds).
Just a quickie though. Pure hydrogen is after all VERY hard to store, since it can go through most of the materials, but it is really REALLY small after all.