Why Fuel Cars with Hydrogen?

There are three major reasons why one would want to power automobiles with hydrogen instead of traditional gasoline:

1. Reduce greenhouse gas (CO₂) emissions from cars and trucks.
2. Reduce our dependence on foreign oil, and help secure our energy
3. Reduce air pollution

In an age of global warming, it is critical to reduce carbon dioxide emission from mobile sources. Hydrogen can be produced from a number of domestic sources (electrolysis of water, reforming natural gas, nuclear and solar high-temperature processes, coal gasification), thereby diversifying the energy source beyond petroleum to fuel our transportation needs. In addition, it is possible to create highly pure sources of hydrogen, devoid of other impurities that lead to pollutant emissions upon combustion. Hydrogen can also be used in emerging technologies like fuel cells, which are battery-like devices that produce electricity when hydrogen is combined with oxygen. Fuel cells produce only water as tailpipe emission. Since hydrogen burns cleanly, producing only water vapor as a product, hydrogen combustion eliminates CO₂ emission from automotive tailpipes.

Despite these advantages, automobiles are a severe technical challenge for hydrogen storage, due primarily to the limited volume available on cars. The "chemical" energy contained in 1 gallon of gasoline is approximately the same as the combustion energy contained in 1 kg of hydrogen. Therefore, if one wants to store the energy equivalent of 10 gallons of gasoline (typical of a car), then one needs to be able to store 10 kg (or 23 lbs) of hydrogen on a car in a commercially attractive way.

Hydrogen-fuel-cell vehicles are more than twice as efficient as conventional gasoline internal combustion engines, thereby reducing the hydrogen requirement (in this example) from 10 gallons of gasoline equivalent to 5 gallons of gasoline equivalent, ~5 kg of hydrogen.

One can store hydrogen on the automobile in the form of compressed gas. However, 5 kg of hydrogen would occupy 60 gallons at a high pressure of 5000 psi (neglecting the volume of the tank and associated equipment). Research and development is proceeding in the industry for developing high pressure storage tanks operating at 10,000 psi, which would reduce the required volume. Alternatively, one could store hydrogen in the form of liquid H₂. In the liquid state, 5 kg of hydrogen would occupy 19.5 gallons (ignoring the volume for tankeage, etc.), which is more volumetrically attractive. There is currently interest in developing advanced cryogenic tanks that would minimize evaporative losses of the cryogenic hydrogen.

A third option is to store the hydrogen in a material that readily absorbs and releases gaseous hydrogen when heated slightly. One such material is AlH₃ and is an example of a class of materials called Metal Hydrides. The compound AlH₃ can store 5kg of hydrogen in a volume of ~17 gallons and weigh ~110lbs (ignoring hardware). This hydrogen can be released if the material is heated to about 100°C.

Metal Hydrides

Moving to a Metal Hydride can improve the volumetric storage density while maintaining nearly room-temperature operation. However, for true commercial viability, the storage properties of the metal hydrides need to be improved with regard to:

1. Optimal volumetric and weight storage density.
2. Enhanced hydrogen reversibility (the ability to re-absorb the hydrogen back into the material).
3. Reduced desorption temperatures.
4. Enhanced kinetics for hydrogen absorption and desorption (the speed with which hydrogen can be released and re-absorbed).

To help guide research in this area, the FreedomCAR and Fuels Partnership has established targets for on-board storage of hydrogen, which provide quantitative targets for the improvements listed above. These targets can be found in the following DOE document (PDF).

In order to improve the usefulness of metal hydrides for automotive storage of hydrogen, Sandia is performing extensive research into the applied and fundamental properties of these materials. Most of this work is performed within the DOE Metal Hydride Center of Excellence (MHCoE), one of three national centers of excellence devoted to the problem of hydrogen storage in materials. This work is funded by the DOE Office of Energy Efficiency and Renewable Energy (EERE).

Contact:

Dr. Lennie Klebanoff, Sandia National Laboratories
Phone: (925) 294-3471
Email: lekleba@sandia.gov