[Tuxedo]

Hydrogen generally will not leak out of compressed gas cylinders. It is however very flammable and it takes very little for it to go boom. It just is not practical... even via electrolysis, which results in a net energy loss. Energy is required to split very strong H-O bonds, which steals efficiency from the engine. Gasoline is much more efficient, and fuel standards help improve combustion efficiency to very high levels. Besides, oil is not just used for fuel. It is the source of many chemicals, some of which are used to produce polymers which find their way into everyday use. As of yet, they are difficult to synthesize from more "enviro-friendly" alternatives.

[DuncanWaring]

Energy is required to split very strong H-O bonds...

Well, yes, but you get the same energy back when the H-O bonds re-coalesce into water.

As an example, look at the fire that comes out of the back of a Space Shuttle's main engines at launch.