I think Ammonia, it’s very dense in the solid state, which is quite easy to achieve at Trav TL’s, and while it’s toxic and corrosive; we’ve been safely handling it since TL6 or so...
Frozen (-80C) it has a density of 817 kg/m~3, so 14.1 m^3 == 11520 kg Ammonia.
11520 kg NH3 == 676 kMol NH3 (17.03 kg/kmol)
1 mol NH2 = 1.5 mol H2
676 * 1.5 = 1014 kMols H2. == 2028 kg H2.
If
you're going to be refining it anyway, ship water. If my calculations are correct (*), 1 dTon of water is equivalent to 1.556 tonnes of hydrogen. And, of course, you'll need to ship some water anyway, even if the station is fully automatic.
David Shaw