PEM fuel cells, the ones being developed for cars are only about 40% efficient, doesn’t sound all that great until you compare it to the cars we drive now, only about 10% of the energy ends up moving the car.
Modern diesel engines have about 50% thermal efficiency.
Just sayin'.
That sounds great, until you realize that the efficiency number is just for generating the electricity.
You still have to drive a motor and a vehicle drive train with it to make the vehicle move, and there is a lot of efficiency loss there.