Ultimate low temperature -273C or Kelvin as often in science is approached by liquid Hydrogen -258C. Metal tanks can get brittle at this low temperature.
This and valency (atomic bonding strength) are physical roadblocks so far.
Here is a plane, Toyota has an engine - no fuel at anything like competitive cost. It is most unlikely in our lifetimes.
Especially as there is a world wide glut of easier to handle LNG.
