Chapter 12
Hydrogen in Light-Metal Cage Assemblies:
Towards a Nanofoam Storage
Fedor Y. Naumkin and David J. Wales
Abstract Isomeric alternatives to usual metal-hydrides as hydrogen-storage materials are considered. Presented are results of ab initio calculations for Be n (n ≤ 18)
clusters with up to two endohedral H 2 molecules which undergo in-cage dissociation. The systems structures and stabilities are discussed, including energy barriers
for hydrogen exit from the cage. The origin of the observed metastability, allowing
for a lower-temperature release of H 2 , is explored. Preservation of the cage integrity
and hydrogen confinement is investigated when such core-shell units are merged
into larger assemblies structurally resembling fragments of hydrogen-filled metal
nanofoams, possible isomeric forms of metal-hydride solid. Different “nanofoam”
isomers are composed of pairs or single H atoms suspended electrostatically inside
the metal cage units (“nanobubbles”). Interesting features include simultaneous exit
of two H atoms, etc. Structural extrapolations suggest potential hydrogen storage
capacity up to ∼10 weight-%.
12.1 Introduction
Reliable storage of hydrogen with an easy release on demand is a bottleneck problem of hydrogen-based energy solutions. Solid metal hydrides (e.g. MgH 2 ) offer
high capacity but are so far problematic due to strong metal-H bonds needing high
temperature (>300 °C for MgH 2 ) for releasing H 2 [1].
Corresponding clusters face similar problem, while being smaller and less rigid,
both factors reducing the hydrogen-desorption temperature—see, e.g., recent advances for MgH 2 -related species [2, 3]. Another class of such systems is represented
by mixed/doped metal clusters M n A k (M = Be, B, Al; A = Li, Na, Mg, B, P, etc.),
both physi- and chemisorbing hydrogen [4–7]. Here the hydrogen binding energies
can be close to estimated ideal ∼0.5 eV [1], larger or smaller up to negative values
(corresponding to metastable systems, e.g. for CAl 12 or SiAl 12 substrates [6, 7]),
usually with ad-/desorption barrier of ∼1 eV.
F.Y. Naumkin (B)
Faculty of Science, UOIT, Oshawa, ON L1H 7K4, Canada
e-mail: fedor.naumkin@uoit.ca
M. Hotokka et al. (eds.), Advances in Quantum Methods and Applications in
Chemistry, Physics, and Biology, Progress in Theoretical Chemistry and Physics 27,
DOI 10.1007/978-3-319-01529-3_12,
© Springer International Publishing Switzerland 2013
211
Hydrogen in Light-Metal Cage Assemblies:
Towards a Nanofoam Storage
Fedor Y. Naumkin and David J. Wales
Abstract Isomeric alternatives to usual metal-hydrides as hydrogen-storage materials are considered. Presented are results of ab initio calculations for Be n (n ≤ 18)
clusters with up to two endohedral H 2 molecules which undergo in-cage dissociation. The systems structures and stabilities are discussed, including energy barriers
for hydrogen exit from the cage. The origin of the observed metastability, allowing
for a lower-temperature release of H 2 , is explored. Preservation of the cage integrity
and hydrogen confinement is investigated when such core-shell units are merged
into larger assemblies structurally resembling fragments of hydrogen-filled metal
nanofoams, possible isomeric forms of metal-hydride solid. Different “nanofoam”
isomers are composed of pairs or single H atoms suspended electrostatically inside
the metal cage units (“nanobubbles”). Interesting features include simultaneous exit
of two H atoms, etc. Structural extrapolations suggest potential hydrogen storage
capacity up to ∼10 weight-%.
12.1 Introduction
Reliable storage of hydrogen with an easy release on demand is a bottleneck problem of hydrogen-based energy solutions. Solid metal hydrides (e.g. MgH 2 ) offer
high capacity but are so far problematic due to strong metal-H bonds needing high
temperature (>300 °C for MgH 2 ) for releasing H 2 [1].
Corresponding clusters face similar problem, while being smaller and less rigid,
both factors reducing the hydrogen-desorption temperature—see, e.g., recent advances for MgH 2 -related species [2, 3]. Another class of such systems is represented
by mixed/doped metal clusters M n A k (M = Be, B, Al; A = Li, Na, Mg, B, P, etc.),
both physi- and chemisorbing hydrogen [4–7]. Here the hydrogen binding energies
can be close to estimated ideal ∼0.5 eV [1], larger or smaller up to negative values
(corresponding to metastable systems, e.g. for CAl 12 or SiAl 12 substrates [6, 7]),
usually with ad-/desorption barrier of ∼1 eV.
F.Y. Naumkin (B)
Faculty of Science, UOIT, Oshawa, ON L1H 7K4, Canada
e-mail: fedor.naumkin@uoit.ca
M. Hotokka et al. (eds.), Advances in Quantum Methods and Applications in
Chemistry, Physics, and Biology, Progress in Theoretical Chemistry and Physics 27,
DOI 10.1007/978-3-319-01529-3_12,
© Springer International Publishing Switzerland 2013
211