Its easier than that, fuel cells are problematic ina s much as you need to refine the hydrogen first which still uses large amounts of energy.
The daft thing is that most car can run on refined veg based products i.e. alcohol and bio-diesel (not the chip shop variety the refined version which is almost 100% diesel compatible!) and we currently pay farmers to not grow anything on upto 1/3 of their land.....so where is the sense!
On top of this, it is now becoming possible to burn various wood pulps, which can be grown, in power stations......
By growing a crop that is then turned into a useable fuel you will continuously be taking CO2 from the atmosphere as some of the waste is ploughed back into the land which then locks some CO2 away again........
Saabs new 9-5 bio fuel is a really great car, and MORE powerful than the petrol version. Its got to be the way forward.