Hydrogen is an obvious Fuel for the future. It's ChemicalEnergy can be easily released by Combustion, which can then by harnessed in a number of different ways, most obiously being a TurbineGenerator. The main CombusionProduct from this is water, which is about as unpolluting as you can get. There is also an enormous amount of researching happening into FuelCells, which reverse the process of Electrolysis, producing Electricity.
As a Fuel, though, Hydrogen presents a number of problems. The first one is that it is a very low Density fuel. You need a lot of space to store the Fuel to produce a relatively small amount of Energy. There are also some safety concerns; it is highly explosive in air. On these grounds it gets a bad reputation, mostly because of the Hindenberg—a little unfair really as it wasn't being used as a fuel in this case.
The main problem with Hydrogen as a Fuel, is how we produce it in the first place.