West Chester University
The DOE Hydrogen Program looks at all aspects of using hydrogen as a fuel. In principle, molecular hydrogen (composed of two hyrogen atoms H2) seems to be an ideal fuel. When it is burned in oxygen (a molecule of two oxygen atoms O2) it forms water (H2O). No carbon dioxide (CO2) is formed.
The problem is that H2 only exists in very small quantities. Hydrogen does exist in very large quantities in nature. However, it tends to be bound up in chemical compounds such as water or in hydrocarbons such as oil, natural gas and coal. Coal gasification is one possible route to H2 production. The problem is that this generates a huge quantity of CO2, just as much as burning the coal in a conventional power plant.
An alternative is to liberate H2 from water. There are two was to do this. The first is electrolysis: by passing electricity of the proper voltage through water, the water is dissociated to form H2 and O2 molecules. The question, of course, is how do we produce the electricity in the first place without generating as much CO2 as we are trying to replace with the H2. If the electricity can be generated by solar panels or wind turbines, then the H2 is produced without significant production of greenhouse gases.
An alternative is to produce the H2 thermochemically from water, perhaps in the presence of a catalyst. The idea here is to put heat up water to the point at which is breaks down to produce H2 and O2. The thermal energy can be provided by concentrated sunlight (solar thermal H2 generation) or by the heat given off by a nuclear reaction (that is, a nuclear power plant used just to generate heat not electricity).Fuel Cells
What is the best way to get the energy back out of H2? We could just burn it, as gasoline and diesel are burned in a conventional internal combustion engine. This is a simple solution that does not require much alteration of current vehicle designs. However, combustion has a limited efficiency and it will always also produce byproduct pollutions such as nitrogen oxides (a component of smog)
A better way to extract energy from H2 is to put it in a fuel cell. A fuel cell is an electrochemical device. It produces H2O from H2 and O2 not by burning it; rather, it uses electron transfer to facilitate the reaction. This is a much more efficient method to extract the energy.
If we had copious inexpensive quantities of H2, the H2 + O2 fuel cell would be our best option. There are many techical hurdles that need to be overcome for incorporation into automobiles, but it would clearly be a good choice. But we do not yet have copious quantities of inexpensive H2. Therefore, scientists are also looking at alternative sources of hydrogen. In effect, other molecules that also contain hydrogen atoms can be used as a carrier of the hydrogen. Some candidates include methanol, ethanol, formic acid, and acetic acid.
NREL has lots more information on hydrogen and fuel cells.