But look at energy per volume [1]:

Diesel: 35.8 MJ/L

Hydrogen: 9.17 MJ/L at 700 bar (10,000 psi).

It will require a tank of 4x the volume for the same energy which comes with many additional challenges. Plus, a tank that can withstand 700 bar will have to be extremely strong and heavy which hurts your energy/mass ratio.

1. https://en.wikipedia.org/wiki/Energy_density#Energy_densitie...

The problem is that you need a big heavy tank to keep all of the Hydrogen molecules from escaping, especially if you're keeping it under pressure. Hydrogen is much more problematic to work with.