Your car already runs on hydrogen. It's just that it's attached to carbon atoms so that it can be stored in a more handy form (petrol!). Everyone looks at the heat of combustion of hydrogen, thinks "Wow! Three times that of petrol!" and stops there. Unfortunately, it being thte lightest gas in the universe, you can't fit terribly much of it in your car. The most feasible method we have is to liquify it (at ruinous energy cost) and even then, it still has less than half the energy in a litre than the equivalent litre of petrol. Your hydrogen car will have less range than conventional ones, alas. Then there's the fact that it's a cryogenic liquid, at minus 253 C; this is not something that easy to handle. Given the number of muppets infesting our roads, just how many do you want to be dealing with liquid hydrogen! What could possibly go wrong?
