Matthew Davey, of Paraparaumu Beach School, asks :-
How can hydrogen be separated from oxygen in a car run by water?
Jim Pearce, a chemist with interests in electrolysis at Victoria University of Wellington, responded.
Let me begin by stating that it is impossible to run a car by water alone. Every so often you see someone on TV or in a magazine claim that they can do so. Such claims are never quite truthful because water is not a fuel. It is important to distinguish between the actual energy source and something that contributes to producing motion from the energy source.
At first sight, a steam engine appears to run on water but the energy source is whatever is used to generate the steam. A family friend once improved the performance of his car by fitting a second carburettor that added a small amount of water to the petrol/air mixture. The water merely contributed to a more efficient use of the energy released by burning the petrol. Until quite recently, some passenger aircraft used water injected into the jet engine to boost power during takeoff. Again the water was not the fuel but the conversion of water to steam increased the thrust and hence the performance.
In terms of your question, the car is run on hydrogen, not water but the hydrogen can be produced from water by a process called electrolysis in which an electric current is used to split water into the gases hydrogen and oxygen. Separating hydrogen from oxygen is not difficult because the two gases are generated in different parts of the electrolysis cell. Although sensible to do so, in principle, it is not essential to separate the two gases. A mixture of hydrogen and oxygen is dangerously explosive but, with suitable precautions, can be stored safely. In 1978, a Sydney inventor demonstrated a welding torch that used a hydrogen/oxygen mixture generated by electrolysis as distinct from the two gases being mixed in the torch itself. Fortunately, his car fitted with tanks of this mixture, was banned from the road by local authorities who feared for public safety in the event of an accident.
While separating hydrogen from oxygen is not difficult, what would be more difficult is storing enough hydrogen to allow a car to travel a reasonable distance between fill-ups. The hydrogen would have to be compressed into a tank similar to those used for CNG but you would need approximately three times as much hydrogen as CNG to travel the same distance. That would mean more tanks or higher pressures in stronger/heavier tanks. There are other methods for storing hydrogen but they are less practical at the moment. In theory, hydrogen atoms can be stored chemically in special alloys at a density similar to liquid hydrogen but with out the need for extreme low temperatures. Such a method may become practicable in the future.
From a clean air viewpoint, hydrogen is an attractive fuel for the internal combustion engine because it produces no particulate matter and no carbon monoxide but is not entirely non-polluting because when burnt in air it is likely to produce oxides of nitrogen just as with hydrocarbon fuels. This difficulty is overcome if the hydrogen is used in a fuel cell to generate electricity for an electric car. Fuel cells have the added advantage over the internal combustion engine in that a much larger proportion of the energy content of the fuel can be turned into useful work. Despite intense research activity, I suspect that cars powered by fuel cells are some years off yet.
As a final comment, there is a danger associated with compressed hydrogen of which the majority of people are unaware. Most gases cool as they expand. This is the principle used in refrigeration and heat pumps. Except at very low temperatures, hydrogen does the opposite, it gets hotter. The hydrogen injection system on a hydrogen powered engine would need cooling but, more importantly, any leak in the fuel system would not only release an explosive gas, it would also get hotter, increasing the risk of burns or ignition.