Basically electric motors, but it burns hydrogen gas instead of carrying a huge flammable battery. The main issues is that hydrogen is very explosive, and not very rich in energy, but it is very clean when everything works, and could be useful in cities if it wasn't for the huge explosion risk.

Either way, by focusing on that part without having to worry about how to build the rest of the truck from scratch, they have a better chance of success. Personally I prefer diesel.