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U.S. Department of Energy Energy Efficiency and Renewable Energy

Basics

Storing enough hydrogen on-board a vehicle to achieve a driving range of greater than 300 miles is a significant challenge. On a weight basis, hydrogen has nearly three times the energy content of gasoline (120 MJ/kg for hydrogen versus 44 MJ/kg for gasoline). However, on a volume basis the situation is reversed (8 MJ/liter for liquid hydrogen versus 32 MJ/liter for gasoline). On-board hydrogen storage in the range of 5–13 kg H2 is required to encompass the full platform of light-duty vehicles.

How is Hydrogen Stored?

Hydrogen can be stored in a variety of ways, but for hydrogen to be a competitive fuel for vehicles, the hydrogen vehicle must be able to travel a comparable distance to conventional hydrocarbon-fueled vehicles.

Photo of hydrogen conformable hydrogen storage tank.

Compressed Gas and Cryogenic Liquid Storage

Hydrogen can be physically stored as either a gas or a liquid. Storage as a gas typically requires high-pressure tanks (5000–10,000 psi tank pressure). Storage of hydrogen as a liquid requires cryogenic temperatures because the boiling point of hydrogen at one atmosphere pressure is -252.8°C.

Materials-Based Hydrogen Storage

Hydrogen can also be stored on the surfaces of solids (by adsorption) or within solids (by absorption). In adsorption, hydrogen is attached to the surface of a material either as hydrogen molecules or as hydrogen atoms. In absorption, hydrogen is dissociated into H-atoms, and then the hydrogen atoms are incorporated into the solid lattice framework.

The graphic contains text that reads: Hydrogen can be stored in different forms, in tanks. The text is followed by an illustration showing a box labeled compressed gas containing drawings of approximately 20 hydrogen molecules. A second illustration shows a box labeled "cryogenic liquid" and shows approximately 100 representations of tightly packed hydrogen molecules. The text under the illustration reads: And in materials…Hydrogen can be stored on the surfaces of solids by adsorption or within solids by absorption. In adsorption (a) hydrogen attaches to the surface of a material either as hydrogen molecules (H2) or hydrogen atoms (H). In absorption (b), hydrogen molecules dissociate into hydrogen atoms that are incorporated into the solid lattice framework—this method may make it possible to store larger quantities of hydrogen in smaller volumes at low pressure and temperatures close to room temperature. Finally, hydrogen can be strongly bound within molecular structures, as chemical compounds containing hydrogen atoms (c). An illustration below the text shows an arrow pointing from left to right labeled "increasing density." Above the arrow are four illustrations labeled from left to right: (a) surface adsorption, (b) intermetallic hydride, (b) complex hydride, and (c) chemical hydride. The illustrations show cartoon drawings of hydrogen atoms and hydrogen molecules.

Hydrogen storage in solids may make it possible to store larger quantities of hydrogen in smaller volumes at low pressure and at temperatures close to room temperature. It is also possible to achieve volumetric storage densities greater than liquid hydrogen because the hydrogen molecule is dissociated into atomic hydrogen within the metal hydride lattice structure.

Finally, hydrogen can be stored through the reaction of hydrogen-containing materials with water (or other compounds such as alcohols). In this case, the hydrogen is effectively stored in both the material and in the water. The term "chemical hydrogen storage" or chemical hydrides is used to describe this form of hydrogen storage. It is also possible to store hydrogen in the chemical structures of liquids and solids.

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