Biomedical Engineering Reference
In-Depth Information
the large binding energy involved would make the adsorption/
desorption cycles hardly achievable. However, in storage experiments
a significant amount of hydrogen molecules could experience
bonding phenomena as soon as the thermal energy is large enough;
therefore hydrogen chemisorption must be considered explicitly.
8.5.1.1
Graphene
Chemisorption of atomic hydrogen induces on graphene layers
the appearance of magnetic properties often accompanied by local
lattice distortions [13]. Moreover, in this case atomic hydrogen
diffuses easily because of the flatness of the potential energy
landscape felt and, as a consequence, H atoms may easily recombine
and form molecular hydrogen that is immediately desorbed from the
graphene [72].
According to the Stoner picture, hydrogen chemisorption
induces a magnetic ordering that is driven by the exchange energy
between the
orbitals of the carbon sites where H atoms are
adsorbed; thus either ferromagnetism or antiferromagnetism arises
depending on the lattice sites where localized states are introduced
by chemisorption [73].
Full hydrogen coverage of both sides of an isolated graphene layer
forms a stable structure where each carbon atom has a tetrahedral
coordination due to sp
p
z
3
atomic orbital hybridization. The situation
is different at intermediate coverage and depends strongly on the
overall magnetization; thus, at least in principle, then substate
magnetization could be used to tune and control the adsorption
dynamics [18].
8.5.1.2
Fullerenes
Novel organometallic molecules based on fullerenes have been deeply
studied for hydrogen storage. To this aim, Kim and co-workers [52]
have investigated light elements, either in interstitial sites (Li and F)
or in substitutional sites (N, B and Be), as doping species of C
and
36
C
total energy calculations, with both LDA
and GGA. Hydrogen adsorption occurs with large binding energies
only for fullerenes doped with B and Be at substitutional sites (0.40
and 0.65 eV for B and Be, respectively) due to the strong interaction
between the B (Be)
by means of
ab initio
60
orbital and the hydrogenσmolecular orbital. The
charge transfer analysis, performed along the direction orthogonal
p
z
