A photo-chemical fuel to solve the energy and pollution problems once and for all, and at the same time.
Imagine a liquid that releases oxygen in the presence of sun light, this liquid has also the property of remaining liquid at very high temperatures. You burn this fuel in an external combustion engine, then you collect the exhaust gas and condense it into a reservoir where it is then exposed to the sun light in order to release the oxygen added during the combustion process, once free of oxygen it returns to the fuel tank and the process repeats itself indefinitely. There is no pollution since nothing escapes to the atmosphere, and there is no need to refuel either since the exhaust gases are turned into fuel again by the sun light, so both problems (energy and pollution) are solved simultaneously. The recommendation for an external combustion engine is due to the fact that internal combustion engines may add oil vapor to the exhaust gas contaminating this way the fuel. The fuel must remain liquid at high temperatures in order to condensate it quickly once burned, but also to keep it from vaporizing when exposed to the sun light.
I understand this kind of process may not be suitable for a vehicle, but I think it can suit quite well many industrial processes.
I am not a chemical engineer and don't know if such a thing is possible, but I suspect it is, and if it is, it would be one of the most dramatic breakthroughs of human history.