IIRC big weather balloon payloads *are* hydrogen.
I f you want to take a metric ton of prototype Xray or gamma ray telescope to the edge of space you need something pretty substantial and if swept up further (the outer atmosphere can expand and contract by a factor of 10) it can burst and down comes your hardware. This needs a big balloon and on that scale H2 is a much better deal.
I'm not sure if they tank it in or generate on site however. On site GH2 generation is quite simple (with the right materials) and highly controllable. None of the starting materials are a gas either.
Biting the hand that feeds IT
