Yes, they have invented a car that runs on water, unfortunately it runs on bottled water so it costs more than gasoline. Just kidding. Actually the energy it takes to dissociate hydrogen and oxygen is exactly equal to the energy released when they are burned. Think about it: you start with water, you end with water, energy is neither created or destroyed so the equation has to balance. Hydrogen as a fuel will always just be an energy storage device, the energy to break the water molecule coming from some other source such as the power grid. That is unless a large stash of hydrogen is discovered that can be mined like oil. Then it will be a very cheap source of energy indeed. P.S. that will never happen, at least on this planet.