When hydrogen combusts it is turned into h20. H20 can be broken down into hydrogen and oxygen. NASA has been using solar panels power hydrogen generators in space for this purpose. Say you had a vehicle that carried a water tank, a hydrogen generator a solar panel a battery and a hydrogen powered engine. You could use the solar panel to run the H. Gen. Then you could have the o2 flow into the car to breath easier. Here is the question though. Could you run the exhaust through an air cooler and back into the water tank and reuse the hydrogen over and over again. It seems that this would work.