Hydrogen is very combustible: it will react with oxygen and give off a lot of energy. But water is the result: the hydrogen is already bonded to oxygen. It's as if somebody already drained the battery. If you want to use hydrogen to power our cars, or "recharge the battery," you would need to separate the H₂O into H₂ and O₂. That requires a lot of energy. In fact, because of something called entropy, it requires more energy to split water into H₂ and O₂ than you get back when you burn them.

The reaction of hydrogen with oxygen to create water releases energy, just as the reaction of anything that is combustible else with oxygen releases energy. If the reaction has already taken place and the carbon already is in the form of carbon dioxide or the hydrogen already is in the form of water, no additional energy can be gotten out of it because it is already combined with the oxygen.

Now, if you got your energy from another source (e.g. solar or nuclear power), then you could break up water with electricity. What this does is it puts the energy back into the hydrogen so that it can be released again when it is burned. Many of our historic space missions were powered by fuel cells that burned hydrogen that had been produced this way. It is not unlikely that in the future automobiles will run on hydrogen fuel cells in which the hydrogen is produced by hydrolyzing water. As far as environmental practices are concerned, however, we need to switch to a cleaner source of energy for electricity first - it's not much help to run everything on electricity if the power plant is still burning coal.

It takes a Lot of energy to get the hydrogen out of the water, because water is a very stable chemical. So basically there are power plants somewhere making all the energy to break the water into hydrogen and oxygen.