Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 92

6.2.1 Storage Capacities

Both the gravimetric and volumetric storage capacities are still the most
important parameters to compare when measuring the storage performance
of different materials. The ideal storage capacity of a metal hydride is deter-
mined by the stoichiometry of the particular hydride. For a metal hydride
MH x , the ideal gravimetric storage capacity

′

ρ M is determined as

xM

′ =

H

ρ M × 100 wt%,

(6.1)

xM + M

H M

where M H and M M are the atomic mass of hydrogen and metal (or alloy) M.
The volumetric storage capacity

′

ρ V is defined as

m
V

xM
M /

′ =

H H

ρ = ,

(6.2)

V

ρ

M M M

where m H is the mass of hydrogen stored in the metal with a volume V M , and
ρ M is the mass density of metal M. Here, the definition does not consider the
lattice expansion during hydrogenation of a metal. A more rigorous definition
that can be applied for any chemical storage materials is,

m
V

xM

′ =

H H

ρ = . (6.3)

V

M / ρ

MH MH MH

x x x

In practice, the hydrogen storage capacity may vary since there may be
impurities or defects in the materials, and hydrogen may adsorb into the
material through physical interactions. Furthermore, the thermodynamics
and kinetics of hydrogenation and dehydrogenation processes will determine
the real hydrogen storage capacity under specific conditions.

6.2.2 Thermodynamics and Reversible Storage Capacity

The hydrogenation and dehydrogenation are the mutual reverse processes for
a metal hydride and only occur at a certain temperature (preferably at room
temperature) and pressure range, that is, depending on the thermodynamic
nature of the metal and metal hydride. When exposed to hydrogen, the metal
or metal alloy (M) will form metal hydride through the following reaction,

x H

M +

2

MH + Q ,

(6.4)

x

2