Most electric generation is fossil. Of that, most is coal. Never mind the CO2, coal is extremely dirty (sulfur and heavy metals). After the battery goes flat (and many people are getting charge exhaustion at mileages in the mid to low 30's), Volt fuel economy is unimpressive (the CD is not outstanding and the vehicle is very heavy).
Anybody that's surprised by this low score on a Green Index hasn't been paying attention. Or thinking.