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In-Depth Information
of possible structures permits the study of the effect of the TM-TM distance on the storage capacity. When the TMs are too close to
one another, TM-TM bonding reduces the capacity. even when separated by distances larger than the normal TM-TM bond length,
delocalization of TM valence electrons can still lower the hydrogen capacity. An optimal TM-TM distance for the structural motifs
studied was approximately 6Å. The study also permitted the evaluation of new TM boride nanostructures. It was predicted that a
low-energy single-walled scandium triboride scB
3
nanostructures can bind approximately 6.1 wt.% hydrogen with the energy of
22-26 kJ/mol. A new family of porous boron-substituted carbon BC
x
materials with controlled structure was investigated [87]. The
chemistry involves a B-precursor polymer containing templates in the form of inorganic additives. Amorphous carbon-like BC
x
materials containing up to 12% B were prepared, which show an extended fused hexagonal ring structure with B-puckered curvature.
This maintains its electron deficiency out of planar B moiety, due to limited
p
-electron delocalization, and exhibits superactivated
properties to enhance H
2
binding energy (20-10 kJ/mol) and adsorption capacity. After removing the inorganic additives by water-
washing, the resulting porous BC
x
shows a surface area of 500-800 m
2
/g. A porous BC
6
material exhibits a reversible hydrogen
physisorption capacity of 0.5 and 3.5 wt.% H
2
per 500 m
2
/g surface area of the material at 293 and 77 K, respectively, under moderate
hydrogen pressure (<100 bars). Both values are more than three times higher than H
2
absorption capacities in the corresponding
carbonaceous materials. The physisorption results were further warranted by absorption isotherms, indicating a binding energy of
hydrogen molecules between 10 and 20 kJ/mol, significantly higher than the 4 kJ/mol reported on most graphitic C surfaces.
The investigation of the interaction of a hydrogen atom with a B
12
P
12
nanocluster based on dFT calculations indicated [88]
that this process is energetically more favorable than that with B
12
n
12
cluster.
Meng et al. [89] investigated the potential for hydrogen storage of a new class of nanomaterials, metal-diboride nanotubes.
These materials have the advantages of a high density of binding sites on the tubular surfaces without the adverse effects of metal
clustering. Using the TiB
2
(8, 0) and (5, 5) nanotubes as prototype examples, it was shown through first-principles calculations
that each Ti atom can host two intact H
2
units, leading to a retrievable hydrogen storage capacity of 5.5 wt.%. Most strikingly,
the binding energies fall in the desirable range of 0.2-0.6 eV/H
2
molecule, endowing these structures with the potential for
room-temperature, near-ambient pressure applications.
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