There are a number of algae which off-gas hydrogen in certain circumstances, so water + sunlight = hydrogen with no energy input. The difficulty for scientists is to increase the efficiency of the process to commercially viable levels.
The blanket ban on the sale of vehicles with internal combustion engines may actually stifle a really promising area of research, because there are other algae which in controlled circumstances produce bio-diesel with no energy input or fossil fuel content. Again, water + sunlight = diesel. Commercialisation of the process, which isn't easy, could produce a limitless fuel with no CO2 emmissions used directly in a current vehicle without modification.