Splitting water with electricity isn't great efficiency either. High power PWM is
efficient though, >95% isn't implausible.
Thinking about it PWM won't work either, as you get side-reactions if the voltage is too
high (even for a short time).
So you need a buck-converter topology, an inductor capable of 150A at efficient PWM
frequencies is going to have to be wound yourself using litz wire I reckon.
Since the potential to split water is about 1.2V, think many 1000's of amps for electrolysis
on a scale to help power a car - in practice stacks of cells are put in series so the current
is lower and the voltage higher. Perhaps you just need 8 or so cells in series from 12V?