Hydrogen Storage
Another problem with the hydrogen idea is that the tanks are a b**ger to get right, particularly if you're thinking of trying to use them over say the life of an airliner, and make something light enough to build into an aircraft structure. You run up against all sorts of problems with hydrogen being adsorbed into the metal matrix an making it a lot more brittle over time. Plus anyone remember the DC-X SSTO demonstrator? I may stand to be corrected, but IIRC one of the major problems that killed that beastie was trying to build a composite fuel tank that would hold hydrogen, even at cryogenic temperatures.