When hydrogen is burned, it doesn’t emit harmful substances. Basically, it reacts to oxygen without burning and the energy it releases can be used to generate electricity used to drive an electric motor. Also, it doesn’t generate carbon dioxide when burnt, not unlike other power sources.
Compared to diesel or gas, hydrogen is much more fuel efficient as it can produce more energy per pound of fuel. This means that if a car is fueled by hydrogen, it can go farther than a vehicle loaded with the same amount of fuel but using a more traditional source of energy.
the process of making hydrogen requires energy - often from fossil fuel sources. So hydrogen's green credentials are under question.
And then there is the question of safety. Hydrogen is a notoriously explosive gas.