Hydrogen doesn't really make much sense to me. It's hard to work with so if you've got a bunch of it laying around why not combine it with some CO2 frozen out

Hydrogen doesn't really make much sense to me. It's hard to work with so if you've got a bunch of it laying around why not combine it with some CO2 frozen out of the air to form methane, something you can pack much more densely in tanks and which won't tend to leak through so many things? You'll produce CO2 when you burn it but no more than what went in to creating it in the first place. Some back of the envelope math says you're losing about 25% of the energy in the Sabatier process to do that but it makes all the transportation and storage of the fuel so much simpler.