Something I haven’t seen addressed here. For hydrogen to be a practical fuel, at least for transport purposes, it has to be highly compressed, or possibly liquefied. (I’m a little vague on the details.)
Unless I’m even more vague than I think, compressing gases this much requires a LOT of energy. Which might pretty dramatically impact the “efficiency” of the system as a whole.
In fact, I’ve seen plans for cars designed to simply run on the energy stored in highly compressed air.
New hydrogen storage technologies that do not require compression of the gas are being developed. There are materials that ‘soak up’ hydrogen like a sponge does water and hold it inside a molecular matrix until it is released by heat or some other trigger mechanism.
Storing pure hydrogen, gas or liquid, presents problems in the storage and transportation arena. Metals exposed to pure hydrogen for prolonged periods develop ‘hydrogen embrittlement’ as the hydrogen atoms are small enough to migrate right through the solid metal and escape, like termites in a 2x4.....................