Joy of Hydrogen (Re: Remember "Bit-Slice" Chips ?)

And once you create it, and pump it into the car's pressure tank
(you'll need a pressure vessel unless the car has a cryo-cooler on
board, and the energy expended by the cryo-cooler would dwarf the
energy needed to propel the car), you are right back to the
'embrittlement' problem again.
>
And consider the explosive force stored in a 350-700 bar (your
AI's number) pressure vessel that becomes brittle enough to go "bang". 
That's one hell of a bang, even without the hydrogen itself explosively
combusting as part of the pressure release.
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Plus, the walls of the pressure vessel quite effectively become a
'fragmentation grenade' in the process of going bang.