Although molecular hydrogen has very high energy density on a mass basis, partly because of its low molecular weight, as a gas at ambient conditions it has very low energy density by volume. If it is to be used as fuel stored on board the vehicle, pure hydrogen gas must be pressurized or liquefied to provide sufficient driving range. Increasing gas pressure improves the energy density by volume, making for smaller, but not lighter container tanks (see pressure vessel). Achieving higher pressures necessitates greater use of external energy to power the compression. Alternatively, higher volumetric energy density liquid hydrogen or slush hydrogen may be used. However, liquid hydrogen is cryogenic and boils at 20.268 K (–252.882 °C or –423.188 °F). Cryogenic storage cuts weight but requires large liquification energies. The liquefaction process, involving pressurizing and cooling steps, is energy intensive. The liquefied hydrogen has lower energy density by volume than gasoline by approximately a factor of four, because of the low density of liquid hydrogen — there is actually more hydrogen in a liter of gasoline (116 grams) than there is in a liter of pure liquid hydrogen (71 grams). Liquid hydrogen storage tanks must also be well insulated to minimize boil off. Ice may form around the tank and help corrode it further if the liquid hydrogen tank insulation fails.
The mass of the tanks needed for compressed hydrogen reduces the fuel economy of the vehicle. Because it is a small, energetic molecule, hydrogen tends to diffuse through any liner material intended to contain it, leading to the embrittlement, or weakening, of its container.
Distinct from storing molecular hydrogen, hydrogen can be stored as a chemical hydride or in some other hydrogen-containing compound. Hydrogen gas is reacted with some other materials to produce the hydrogen storage material, which can be transported relatively easily. At the point of use the hydrogen storage material can be made to decompose, yielding hydrogen gas. As well as the mass and volume density problems associated with molecular hydrogen storage, current barriers to practical storage schemes stem from the high pressure and temperature conditions needed for hydride formation and hydrogen release. For many potential systems hydriding and dehydriding kinetics and heat management are also issues that need to be overcome.
A third approach is to absorb molecular hydrogen into a solid storage material. Unlike in the hydrides mentioned above, the hydrogen does not dissociate/recombine upon charging/discharging the storage system, and hence does not suffer from the kinetic limitations of many hydride storage systems. Hydrogen densities similar to liquefied hydrogen can be achieved with appropriate absorption media. Some suggested absorbers include MOFs, nanostructured carbons (including CNTs) and clathrate hydrate.
The most common method of on board hydrogen storage in today's demonstration vehicles is as a compressed gas at pressures of roughly 700 bar (70 MPa).
Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen are stored in underground caverns by ICI for many years without any difficulties.[38] The storage of large quantities of hydrogen underground can function as grid energy storage which is essential for the hydrogen economy.
Additional to the codes and standards (RCS) for hydrogen technology products, there are codes and standards for hydrogen safety, for the safe handling of hydrogen and the storage of hydrogen for example the Standard for the installation of stationary fuel cell power systems from the National Fire Protection Association.
Hydrogen has the widest explosive/ignition mix range with air of all the gases except acetylene. That means that whatever the mix proportion between air and hydrogen, a hydrogen leak will lead to an explosion, not a mere flame. This makes the use of hydrogen particularly dangerous in closed areas (tunnels, parkings)..[47] Some differences with common fuels include the fact that pure hydrogen-oxygen flames burn in the ultraviolet color range and are nearly invisible to the naked eye, thus it requires a flame detector to detect if a hydrogen leak is burning. Hydrogen is odorless and leaks cannot be detected by smell.
One of the measures on the roadmap is to implement higher safety standards like early leak detection with hydrogen sensors.[48] The Canadian Hydrogen Safety Program concluded that hydrogen fueling is as safe as, or safer than, CNG fueling.[49] The European Commission emphasizes the critical importance of education in lifting technical safety barriers to development of the hydrogen economy and has funded World's First Higher Educational Programme in Hydrogen Safety Engineering. It is expected that the general public will be able to use hydrogen technologies in everyday life with at least the same level of safety and comfort as with today's fossil fuels.