Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 98

where k is treated as the apparent reaction constant and n depends on the
geometry and dimensionality of the reaction. The reaction rate k is thermally
activated and follows the Arrhenius relation,

E
k T

a

k = k

0 exp

− , (6.12)

B

where E a is the reaction activation energy and k B is the Boltzmann constant.
The activation energy for adsorption and desorption may be different for the
same metal hydride, depending on the structure, morphology, and catalyst
used. To lower the activation energy, different catalysts are usually added to
the metal hydride systems.

6.2.4 Cycling Stability

For practical applications, the ability of the metal hydride to retain its revers-
ible storage capacity during repeated hydrogenation and dehydrogenation
cycles is an important parameter to consider. Different applications may
require different cycling times. For example, the 2015 DOE storage target
specifies 1500 cycles for a good hydrogen storage material [7]. There are
intrinsic and extrinsic degradations governing the cycling stability of a metal
hydride. The intrinsic degradation is caused by the physical and chemical
changes of the metal hydrides during the cycling process. The degradation
includes hydride decomposition into more thermally stable products, forma-
tion of defects and dislocations, loss of reversible storage capacity, and loss
of material components during the cycling process. In particular, the decrepi-
tation, a self-pulverization of metal hydrides into smaller powder due to
volume change during cycling, not only will change the heat transfer and
gas flow inside the storage tank, but also will induce tank rupture and affect
the purity of the released hydrogen. These intrinsic degradation processes
are irreversible. The extrinsic degradation is due to impurities in the H 2 gas
during the hydrogenation process. Gas impurities, such as O 2 , CO, CO 2 , SO 2 ,
H 2 O, H 2 S, NH 3 , hydrocarbons, formaldehyde, and formic acid, could react
with and poison or corrode the metal or metal hydride surfaces, reducing
hydrogen adsorption/desorption. If the hydrides are poisoned, the material
could be regenerated by using high purity H 2 cleaning and high temperature
annealing.

6.2.5 Activation

Usually, the surfaces of metals are covered by a layer of oxide, or ad-
sorbed with a layer of gas or water vapor, which makes them hard to be