There's some problems with that concept based on how fuel cells currently work. The hydrogen feeding into the fuel cells needs to be very dry and pure, necessi

There's some problems with that concept based on how fuel cells currently work. The hydrogen feeding into the fuel cells needs to be very dry and pure, necessitating ultrapure water. Furthermore, the hydrogen would need to still be stored on the vehicle. Storage on the vehicle is actually not that big of a problem. Hydrogen is explosive, but people forget that gasoline is also extremely explosive... and yet we drive around sloshing tanks with very few issues (unlike the movies where the slightest fender-bender results in a fireball). The fuel cells are the hard part. The cheap ones are fickle, with narrow active temperatures and intolerance for contaminants. The most efficient ones run at very high temperatures, are heavy, and involve nasty chemicals... very useful for sealed stationary applications, not so for cars.