Because of the hydrogen fuel cell, the battery pack is smaller and lighter, letting the car drive for only 20 miles on electric only as opposed to 40 miles in the first version. The fuel cell system itself is also lightweight and is considered GM's 5th generation device.
There are a few important considerations about this vehicle. Due to the lack of hydrogen refueling infrastructure, pure hydrogen vehicles are impractical for among other reasons, inablity for drivers to refuel. The Hydrogen Volt, will allow drivers to do more driving without hydrogen, and refueling can be much less freuqent. Another issue is that we are aware that Big Oil and Big Goverment are pushing the hydrogen economy. This vehicle will allow GM access to government funding dollars to help develop the car. Remember, the pure electric car isnt a favorite of the goverment and big oil. Third, the fact that the vehicle could be built with different non-electric fuel sources, demonstrates the flexiblilty and robustness of the E-flex platform and allows a "roadmap" of future vehicle to roll out on the platform, with the ICE Volt coming out first.