While liquid hydrogen as a fuel might have three times the energy content of hydrocarbon fuels per unit of weight, its density is one tenth of the hydrocarbons (0.07 vs. 0.7g/ml) - i.e. the energy content per unit of volume is 30% that of hydrocarbon fuels. Add to it bulky cryogenic storage tanks - and the spatial requirements of liquid hydrogen as a fuel become problematic, unless the amounts used are very large, like in space shuttle.
If an aircraft were not fueled until immediately before deployment, could not the vaporisation of fuel as it is removed from the tank be used to keep the rest of it cool?
Density and cryo storage are the reasons why fuel cells are more practical. Extracting a hydrogen atoms from alcohol or gasoline is way more practical than compressing Hydrogen into the weight and volume necessary to propel a vehicle.
I haven't looked in a while but I thought peroxide plus hydrazine had a nice energy density as well as no carbon. Only problem is it's toxicity to humans (not really a concern for greenies since it's combustion artifacts are very green).