I have to agree with Jake-the-peg. Hydrogen is the only real contender in the long run. Produced as he says, by algae, or electrolysis of water from green sources (wind farms etc).
The exhaust gas is water which can be split again. The oxygen from the electrolysis could be used for medical purposes etc.
I think governments should be throwing more resources into hydrogen cell research and using the taxes they keep claiming are to reduce global warming, to do so.