Instead, the hydrogen is fed into a device in which it is combined with oxygen in a chemical reaction that makes water while also releasing a stream of electricity. That electricity is stored in a battery and used to run the FCX's electric motor.
In this way, the car itself produces no pollution. How much pollution its use ultimately creates depends on how the hydrogen is harvested. While hydrogen is the most abundant element in the universe, it doesn't generally exist by itself. It must be separated out of other substances, a process that takes some power, usually electricity.
Regardless of how the electricity is produced, though, electricity generation is a more efficient way to make power from a fuel than exploding it to push pistons, a process in which most of the power is wasted as heat. So, even if the electricity to separate the hydrogen were to come from a coal-burning power plant, the FCX would still be cleaner than a gasoline-powered car.