Hydrogen balloons are fun to pop.
10% H2 in N2 is not good for lifting... let alone 4%.
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Oh, and, tri-atomic CO2 is very heavy (44 g/mol). So, waving the ideal gas law around, it really doesn't make sense to use that for lifting purposes at all when you have other, smaller things around.
> The flammability range for hydrogen in air is 4% - 74%, and even though it ignites with difficulty at dilutions higher than 10%, you would still have to dilute it like mad in order to get a provably non-flammable lifting gas
But it could not combust inside the balloon because H doesn't combine with N (at least I don't think it does!). It would have to be released, and then mixed with air to burn. If the balloon popped would it disperse faster than it could burn? I don't know, and simple chemistry would not provide an answer.
One would have to do experiments.
1. Pure nitrogen is lighter than air. Not enough to lift a balloon, but enough to make the balloon lighter than an air filled one. So it would be a good gas to mix with the hydrogen. Nitrogen gas is cheap, too.
2. Helium could be mixed in with the hydrogen, so the hydrogen would not be concentrated enough to ignite. This would reduce the quantity of helium needed.