Page 241 - Materials Chemistry, Second Edition

228 3 Metals

2. Covalent (molecules containing covalently bound hydrogen to nonmetals, with
individual molecules held together by intermolecular forces, e.g.,CH 4 , SiH 4 )
3. Metallic/interstitial (hydrogen molecules are contained in vacant interstitial sites
of a transition-metal lattice, e.g., PdH 0.6 )
To be successful for hydrogen storage in energy devices, the following five
parameters must be met [25] :
1. The solid material must be able to adsorb/desorb at least 9 wt.% (system
gravimetric capacity: “specific energy”) and 81 g L 1 (system volumetric capac-
ity: “energy density”) of hydrogen gas;
2. The storage system cost should be <$2/kWh ($67/kg H 2 );
3. The decomposition temperature necessary for generation of hydrogen from the

material should be in the range of 60–90 C;
4. The absorption/desorption of hydrogen from the material should be reversible;
5. The material should be low cost, precluding the use of noble-metal alloys;
6. The storage solid should be nontoxic and inert under environmental conditions.
That is, the solid should not react with water, oxygen, nitrogen, etc.
Table 3.7 lists some important metals and alloys that have been studied for
hydrogen storage applications. To date, no material (neither metals nor nonmetals)
has been discovered that satisfies all of the above five constraints. Although hydrides
exist for most elements of the Periodic Table, only the light elements (e.g., Li, Mg, Al)
are able to meet criterion 1 above. High surface-area carbonaceous materials such
as activated carbons or aerogels have been shown to store significant concentrations
[26]
of H 2 , but are limited by their low storage packing densities (SPDs). More
recently, a metal-organic framework (MOF) has been shown to adsorb and pack
more H 2 in its cavities at 77K than any unpressurized structure to date, likely due to
the presence of unsaturated metal centers. [27]
The most widespread application for hydrogen storage materials continues to be
for the negative electrode (cathode) in rechargeable alkaline nickel–metal hydride
(Ni–MH) batteries – used extensively in portable electronic devices and electric
vehicles. The most common metal hydrides used for battery applications are inter-
metallic species such as LaNi 5 , LaMg 12 , and complex AB 2 alloys (see Table 3.7). [28]
The development of complex alloys was necessary to circumvent the high equilibrium
pressures that early batteries exhibited at room temperature. The composition of metal
hydrides may now be fine-tuned to offer low operating pressures, corrosion resistance,
and reversible H 2 storage.
The decomposition temperature, T dec , for binary metal hydrides (MH x ) is found to
correlate strongly with the standard reduction potential, E (Figure 3.44). In partic-

ular, the easier it is to reduce the metal (i.e., a larger reduction potential), the lower
the temperature that is required to decompose the solid into the metal and hydrogen
gas (Eqs. 25–27):

ð25Þ M nþ þ ne ! M 0

ð26Þ 2n H ! 2n e þ nH 2
________________________________
0

ð27Þ Overall: 2 M nþ þ 2n H ! 2M þ nH 2