Hydrogen has an awkward physical property.
It holds more energy per kilogram than any other fuel and almost none per litre, which means a vehicle running on it has to carry gas compressed to hundreds of times atmospheric pressure.
The tank stops being a container and becomes an engineering problem.
That is why cylinder design decides whether hydrogen mobility works.
A steel vessel strong enough to hold 700 bar is too heavy to be worth carrying, and the weight eats the range the fuel was supposed to provide.
