12 Hydrogen in Light-Metal Cage Assemblies: Towards a Nanofoam Storage
219
(as for H@Be 6 ), apparently due to a lower strain in the beryllium cage. The charge
on each of the H atoms is −1.4e, same as for (2H)@Be 9 .
Similar to (H 2 )@Be n , the relative stabilities of the above (kH)@Be n systems
to separation into the hydrogen and beryllium components translate into their total
dissociation energies D total
e
(corresponding to Be n dissociated into atoms). Inserting
H into Be 6 and H + H 2 into Be 11 (D 3h ) stabilizes these clusters, significantly (by
0.3 eV per Be atom) for the former and slightly for the latter. While insertion of H 2
into Be 9 , Be 10 and Be 11 destabilizes them by about 0.1 eV per Be atom. Overall,
the D total
e
values increase with the system size, as for (H 2 )@Be n , from 1.6 to 2 eV
per Be atom (Table 12.3).
12.4 Conclusions
Beryllium cluster cages form metastable (by a few eV) core-shell systems when
endohedrally doped by molecular hydrogen dissociating due to a strong electron
donation from, and confined in, the cage. When small H 2 @Be n units are merged
together, they can generally preserve shapes and integrity in the larger assemblies. In some cases, this can result in a further stabilization of the system, as for
(H 2 ) 2 @Be 17 . Another feature is a possible higher stability of the assemblies composed of higher-energy isomers of the units, as for (H 2 ) 2 @Be 18 . Energy barriers to
extraction of H 2 can be low (∼0.1 eV for n = 8, 9, 14) to appreciable (∼0.6 eV for
n = 10), suggesting low-temperature conditions for stabilization. Extraction of both
H atoms at once is more likely.
The metastability may offer two benefits: (1) easier release of hydrogen, (2) direct storage of extra energy (∼0.5–2 eV per H 2 molecule here). Hydrogen storage capacity can be increased in cluster assemblies/materials, e.g. nanofoams. This
is confirmed for face-sharing H 2 @Be 10 units, with extrapolated upper bound of
∼8 weight-%. Feasibility of such materials is supported experimentally by a recent
progress reported for Mg [18].
Such an endohedral doping may also offer options for modification of mechanical and electronic characteristics (shape, dipole moment) of clusters. This suggests
potential applications in nanomaterials and molecular electronics.
Be 6 is able to accommodate H atom inside and is significantly stable (by ∼2 eV)
to its release. This stability can be reduced by design via merging such units into assemblies which are metastable to release of molecular H 2 , with a desorption barrier
of ∼0.5 eV (matching the suggested ideal binding energy [1]). Hydrogen storage
capacity of such systems extrapolated to a nanofoam material, composed of facesharing H@Be 6 units filling space, can reach ∼10 weight-%. This may exceed the
value for the counterparts with encapsulated H 2 (e.g. inside Be 10 units) due to higher
symmetry and better packing.
Such cluster-assembled materials could be more straightforward to develop than
macro-assemblies of clusters preserving multiple surface sites for external binding
of H 2 molecules. Firm conclusions, however, would benefit from relevant experiments as well as modelling of molecular dynamics related to hydrogen entering and
219
(as for H@Be 6 ), apparently due to a lower strain in the beryllium cage. The charge
on each of the H atoms is −1.4e, same as for (2H)@Be 9 .
Similar to (H 2 )@Be n , the relative stabilities of the above (kH)@Be n systems
to separation into the hydrogen and beryllium components translate into their total
dissociation energies D total
e
(corresponding to Be n dissociated into atoms). Inserting
H into Be 6 and H + H 2 into Be 11 (D 3h ) stabilizes these clusters, significantly (by
0.3 eV per Be atom) for the former and slightly for the latter. While insertion of H 2
into Be 9 , Be 10 and Be 11 destabilizes them by about 0.1 eV per Be atom. Overall,
the D total
e
values increase with the system size, as for (H 2 )@Be n , from 1.6 to 2 eV
per Be atom (Table 12.3).
12.4 Conclusions
Beryllium cluster cages form metastable (by a few eV) core-shell systems when
endohedrally doped by molecular hydrogen dissociating due to a strong electron
donation from, and confined in, the cage. When small H 2 @Be n units are merged
together, they can generally preserve shapes and integrity in the larger assemblies. In some cases, this can result in a further stabilization of the system, as for
(H 2 ) 2 @Be 17 . Another feature is a possible higher stability of the assemblies composed of higher-energy isomers of the units, as for (H 2 ) 2 @Be 18 . Energy barriers to
extraction of H 2 can be low (∼0.1 eV for n = 8, 9, 14) to appreciable (∼0.6 eV for
n = 10), suggesting low-temperature conditions for stabilization. Extraction of both
H atoms at once is more likely.
The metastability may offer two benefits: (1) easier release of hydrogen, (2) direct storage of extra energy (∼0.5–2 eV per H 2 molecule here). Hydrogen storage capacity can be increased in cluster assemblies/materials, e.g. nanofoams. This
is confirmed for face-sharing H 2 @Be 10 units, with extrapolated upper bound of
∼8 weight-%. Feasibility of such materials is supported experimentally by a recent
progress reported for Mg [18].
Such an endohedral doping may also offer options for modification of mechanical and electronic characteristics (shape, dipole moment) of clusters. This suggests
potential applications in nanomaterials and molecular electronics.
Be 6 is able to accommodate H atom inside and is significantly stable (by ∼2 eV)
to its release. This stability can be reduced by design via merging such units into assemblies which are metastable to release of molecular H 2 , with a desorption barrier
of ∼0.5 eV (matching the suggested ideal binding energy [1]). Hydrogen storage
capacity of such systems extrapolated to a nanofoam material, composed of facesharing H@Be 6 units filling space, can reach ∼10 weight-%. This may exceed the
value for the counterparts with encapsulated H 2 (e.g. inside Be 10 units) due to higher
symmetry and better packing.
Such cluster-assembled materials could be more straightforward to develop than
macro-assemblies of clusters preserving multiple surface sites for external binding
of H 2 molecules. Firm conclusions, however, would benefit from relevant experiments as well as modelling of molecular dynamics related to hydrogen entering and