As already stated you can't run engines that split hydrogen from water and then burn it again.
It's an eternal motion machine and breaks the "conservation of energy"
However here's an adaption that runs your car on hydrogen:
http://www.switch2hydrogen.com/h2.htm
The problem isn't the engine it's fueling - hydrogen is dangerous because it's so explosive and storing it as a gas (even compressed is very inefficient. (short range).
Liquifying it would make the refueling process dangerous from cryogenic burns and increase the lisk of explosion.
So far experiments with hydrides have also been disappointing.
It's one of the big challenges of our time.