Oh, it can be done. There are no scientific laws that say you can’t run a car on water. In fact, I have personally made fire from water on a number of occasions. A Japanese company is the latest to claim they are running a car on water. See the video here:
However, what you can’t do is run a car on water without overall energy inputs greater than you get from splitting the water. In simple terms, let’s say you split water to create 10 BTUs of hydrogen. You can then use that to burn in the car, or to operate a fuel cell. When you burn the hydrogen, it reacts with oxygen to again form water. But if you want to take the water and turn it back into hydrogen, it will always take more than the 10 BTUs that you released in the first place.
So let’s say it takes 12 BTUs of input to produce 10 BTUs of hydrogen from water. What’s wrong with that? Well, why wouldn’t you just use those 12 BTUs directly, instead of going through the step of cracking the water? This would be sort of like using gasoline in your car to produce steam to drive a steam engine that actually runs the car. But it’s a lot more efficient to cut out the middleman and use the gasoline directly.
There are two possible scenarios in which this sort of scheme might make sense. One is if the conversion allowed you to operate a more efficient motor; say an electric motor instead of an internal combustion engine. The second is when it is more convenient to keep the fuel in a solid form, as was the case for my carbide lamp example. Since the fuel is only produced when water drips on the solid, there isn’t a large inventory of flammable gas or liquid that can catch fire or explode.
However, it is important to keep in mind that there is a catch. There is a way to mask the energy input, and that is what the Japanese company is doing. I had to do a bit of research, but I finally found this:
Within the story is the key to what’s going on:
Though the company did not reveal any more detail the company president said that they had “succeeded in adopting a well-known process to produce hydrogen from water to the MEA”, similar to the mechanism that produces hydrogen by a reaction of metal hydride and water.
That clued me in as to how they were pulling this off. Metal hydrides will react with water to produce hydrogen. For instance, sodium hydride (NaH) reacts spontaneously with water as follows:
NaH + H2O → H2 (gas) + NaOH ΔH = −83.6 kJ/mol, ΔG = −109.0 kJ/mol
So, if you had NaH in your car, and you dripped water on it, you would produce hydrogen from the water. What’s the catch? Metal hydrides that react with water don’t occur naturally, because they would have already reacted. This is the same reason hydrogen doesn’t occur naturally on earth. So, it takes energy inputs to make the metal hydrides. And there is the hidden energy source in the water car. The car isn’t really running on water. It is running on a combination of water and a very reactive compound that must be replenished.
Here’s what the laws of thermodynamics tell you. Back to the 10 BTUs of energy we liberated for the water car; it would have taken well more than 10 BTUs to produce the metal hydride required for that reaction. (Note that they may not be using metal hydrides; there are other compounds that react with water to liberate hydrogen. Again, none occur naturally on earth in the reactive form, and all require significant energy inputs to produce).
So, the moral is: Sometimes it appears that the lunch is free, but the bill eventually comes anyway – when you have to replenish the catalyst. (Note: As others have correctly pointed out, the proper term here would be reagent instead of catalyst since it is almost certainly undergoing a transformation from one compound to another. I merely used the term Genepax used to describe the system.)