Hydrogen can be released from fossil fuels, and is an energy supply that is very low in carbon emissions. Breaking water down into its component parts with electricity (electrolysis) releases hydrogen that is renewable and gives off no carbon emissions.
It is said that one gallon of hydrogen gas gives off the same amount of energy as one gallon of petrol whilst giving out only water vapour as its emission. Cars powered by hydrogen convert the potential chemical energy of hydrogen to mechanical energy in one of two ways. The least common of which is through an internal combustion engine in a slightly modified way to the one used in normal cars. However the most widely used is through hydrogen fuel-cells.
Fuel-cell powered cars use the energy to run motors that turn the wheels by converting the chemical energy of the hydrogen fuel-cell to electricity. The most common kind of full cell is the polymer electrolyte membrane that has an electrolyte membrane between two electrodes, one positive, a cathode the other, negative, an anode. Between the two electrodes oxygen and hydrogen collide and the oxygen molecules taking with them electrons from the hydrogen molecules. These pass through a circuit to form a cycle. The movement of the electrons causes the current needed to power the car.
Hydrogen can be used as a fuel like petrol put into the take as a pure substance, but this is not safe and could explode with heat or flames present so, hydrogen is added to methanol, ethanol, or natural gas.
Before hydrogen cars can become widespread, the cars need to become more reliable, these are currently only at the prototype stage. But more importantly than that, the equivalent of a petrol station that gives out hydrogen, so that people can fill their cars and know when they next need to fill up, there will be a suitable place to do so and they won’t just be left high and dry in the middle of nowhere. Currently the cars cannot be refuelled and so have a much shorter range than would be acceptable for the everyday user.