Why hydrogen will not save the planet
Hydrogen is a gas used used as a fuel. Hydrogen gas used in fuel cells is ideally obtained from water by using an electric current to break the water molecules apart into hydrogen and oxygen, and the breaking apart of water uses more energy than is gained when hydrogen is burned. This makes hydrogen more comparable to a battery than to oil. You put more energy in than you get out. The electricity used to break the water has to come from somewhere. If it comes from coal, then your hydrogen-powered car is effectively running on coal. The energy must come from renewable sources if a hydrogen powered vehicle is to be considered sustainable.
Unfortunately, one of the main commercial sources for hydrogen today is natural gas. Natural gas is a fossil fuel and therefore nonrenewable. Given that energy must be used in order to separate out the hydrogen, it might make more sense to have a natural gas powered car in the first place.
In addition to this problem, hydrogen is a gas at room temperature. In order to have relatively small fuel tank such as can fit in a vehicle, it has to be either compressed or liquefied. In order to liquefy hydrogen, it must be refrigerated down to -159.14C, although compressing it can raise the temperature at which it is liquid somewhat. As you can imagine, doing this in a car does present technical challenges. It is easier to do in something the size of a bus. That said, it can and has been done. What happens is the technical challenges involved make the vehicle expensive. There is, however, a safety issue.
Hydrogen is not only a gas, it is a flammable and explosive one. Think of the Hindenburg. Do you really want to be driving something that is powered by an explosive gas that has been tightly compressed? What happens when you get rear-ended, or have a head-on collision? The tanks have to be very, very strong, and they had better not have any flaw or weakness. While such tanks can and are being built, there is a high potential for any failure to produce nasty results.
It might be easier, safer and simpler to have plug-in electric vehicles powered by wind, solar, hydro or geothermal energy. Or for shorter trips, that wonderful invention, the bicycle.