- Public funding: At the low end, public investment of $10 billion is needed to get just 2 million hydrogen cars on the road by 2025. Other studies show it will take $55 billion.
- Filling stations: Classic chicken-and-egg problem here. Who will build these before there's a market to make them profitable? Shell currently has six hydrogen stations globally. BP shut theirs down in 2007 to focus on biofuels.
- CO2 emissions: Yes, the only emission from a hydrogen-powered car is water. But when extracting hydrogen from natural sources, CO2 would be created.
- Low energy density: One expert called hydrogen "just about the worst possible vehicle fuel." Alternatives like methanol are more energy rich and easier to store, transport.
(Image of the Cadillac Provoq, a GM hydrogen car, by alan_D via Flickr)