The most common storage systems are high pressure gas
cylinders with a maximum pressure of 20 MPa. New light
weight composite cylinders have been developed which
support pressure up to 80 MPa and therefore, the hydrogen
reaches a volumetric density of 36 kg m3, approximately
half as much as in its liquid state. The safety of pressurized
cylinders is an issue of concern especially in highly
populated regions. Liquid hydrogen is stored in cryogenic
tanks at 21.2 K and ambient pressure. Due to the low critical temperature of hydrogen (33 K) liquid hydrogen
can only be stored in open systems. The volumetric density
of liquid hydrogen is 70.8 kg m3, and large volumes
where the thermal losses are small can reach hydrogen
to system mass ratio close to one. The highest volumetric
densities of hydrogen are found in metal hydrides. Fig. 1
shows the volumetric versus gravimetric hydrogen density
of some selected materials. Many metals and alloys are
capable of reversibly absorbing large amounts of hydrogen.
Charging can be done using molecular hydrogen gas
or hydrogen atoms from an electrolyte. Molecular hydrogen
is dissociated at the surface prior to absorption, two H
atoms recombine to H2 in the desorption process. The host
metal dissolves some hydrogen as a Sieverts type solid
solution (a-phase). As the concentration cH of H in the
metal is increased, the H–H interaction becomes locally
important and nucleation and growth of the hydride phase
(b) start.