Hydrogen Car Fuel
For a number of years hydrogen fuel has looked like it was going to become the major fuel source for cars. This may still happen but there are now a lot of people doubting the long term viability of hydrogen. Basically there are two ways that you can power a car with hydrogen, you can burn it directly or you can use it in fuel cell. A fuel cell simply combines hydrogen and oxygen to produce water which creates electricity as a byproduct. The major advantage of hydrogen is that theoretically it doesn't produce any greenhouse gas emissions. In practice this isn't quite true. Unfortunately a lot of problems have come up that have slowed the growth of hydrogen as a power source.
The main problem with hydrogen as a power source is that it isn't very efficient. Hydrogen actually contains very little energy for its volume. That means that in order to power a car you are going to need to carry a lot of hydrogen. There is also the fact that right now it is debatable if the power that can be generated from hydrogen is even enough to offset the power required to produce they hydrogen. Clearly a lot more work is going to have to be done in this regard.
The next major issue that hydrogen will need to overcome is the infrastructure problem. In order to use it as fuel source in cars you need to be able to buy it at many locations like we currently do with gas. That requires that hydrogen filling stations be built. This creates a catch 22. Nobody is going to spend the money to build these filling stations until there are enough hydrogen cars on the road to justify the cost. But there won't be many hydrogen cars on the road until the filling stations get built. A related problem is that hydrogen is highly flammable and there are safety issues with the filling procedure.
One more problem that will need to be overcome in order to use hydrogen as a fuel source is actually getting the hydrogen. Right now the vast majority of the hydrogen that we get comes as a byproduct of fossil fuel production. This is a problem since the point of hydrogen is to relieve us of our dependence on fossil fuels. There is also the issue that it takes away from the environmental advantages of hydrogen if we need to produce fossil fuels to get it. We can produce hydrogen by separating it out of water but that is currently only done on a small scale and requires quite a bit of electricity.
There is growing doubt that hydrogen will ever become a major fuel source. There are a lot of hurdles to be overcome and other options are starting to look more promising. That being said none of the problems are so great that they can't be overcome so hydrogen may still play an important role in the future.