We don't even need the "petrol" as they say. Hydrogen alone can be used to power future vehicles.
The reason you don't see hydrogen cars and fueling stations all over is that to separate the hydrogen from water you have to put in energy. So, why bother to use that energy to split water molecules when we can use the energy dirctly as fuel or electricity?
Since the process described requires splitting water molecules, it is no more efficient than the harvesting of hydrogen from water alone. In fact, it requires even more steps.
So, it's not a solution.