Environmental Engineering Reference
In-Depth Information
The dehydrogenation of AB was found to occur in three steps that are generally represented by the transformation of AB to poly-
aminoborane (PAB, (H
2
nBH
2
)
x
) and H
2
, PAB to polyiminoborane (PIB, (HnBH)
x
) and H
2
, and PIB to Bn and H
2
. As previously
mentioned, ammonia borane is under significant investigation as a possible hydrogen storage material. While chemical additives
have been shown to lower the temperature for hydrogen release from ammonia borane, this may result in additional complications
in the regeneration cycle. Mechanically alloyed h-Bn (nano-Bn) was shown [65] to facilitate the release of hydrogen from
ammonia borane at lower temperature, with minimal induction time and less exothermicity, and inert nano-Bn may be easily
removed during any regeneration of the spent ammonia borane. The samples were prepared by mechanically alloying ammonia
borane with nano-Bn (i.e., physical mixtures). The
11
B magic angle spinning (MAs) solid state nuclear magnetic resonance
(nMr) spectrum of the decomposition products showed that diammoniate of diborane is present in the mechanically alloyed
mixture, which drastically shortens the induction period for hydrogen release from ammonia borane. Analysis also showed that
all the borazine produced in the reaction comes from ammonia borane and that increasing the nano-Bn surface area results in
increased amounts of borazine. However, under high temperature, for example, approximately 150°C, isothermal conditions, the
amount of borazine released significantly decreases. The electron microscopy of the initial and final nano-Bn additive provided
evidence for loss of crystallinity but not significant chemical changes. The higher concentration of borazine observed for low-
temperature dehydrogenation of AB/nano-Bn mixtures versus neat ammonia borane was attributed to a surface interaction that
favors the formation of precursors, which ultimately result in borazine. This pathway can be avoided through isothermal heating
at temperatures lower than 150°C. several effects of the mixtures of AB:nano-Bn were shown to be beneficial in comparison
with neat ammonia borane: decrease of the dehydrogenation temperature, decrease in nH
3
formation, as well as decrease of the
exothermicity of hydrogen release with increasing nano-Bn concentration. Hydrogen is produced via the following reactions:
→
(
)
+
HNBH
HNBH
H
90 120
−
°
C
,
3
3
2
2
2
x
(
)
→
(
)
+
HNBH
HNBH
x
H
2
120 160
−
°
C
,
2
2
x
x
(
)
→+
HNBHNHwell above
x
500
°
C
.
2
x
The state-of-the-art for hydrogen storage is compressed H
2
at 700 bar and the development of a liquid-phase hydrogen
storage material has the potential to take advantage of the existing liquid-based distribution infrastructure. Luo et al. [66]
described a liquid-phase hydrogen storage material, Bn-methylcyclopentane, that is a liquid under ambient conditions (i.e., at
20°C and 1 atm), air- and moisture-stable, and recyclable; releases H
2
controllably and cleanly at temperatures below or at the
proton exchange membrane fuel cell waste-heat temperature of 80°C; utilizes catalysts that are cheap and abundant for H
2
desorption; features reasonable gravimetric and volumetric storage capacity; and does not undergo a phase change upon H
2
desorption. Among many promising hydrogen-rich boron-containing materials, ammonia borane (nH
3
BH
3
, AB) received much
attention because of its satisfactory air stability, relatively low molecular mass, and remarkably high energy storage densities
(gravimetric and volumetric hydrogen capacities are 19.6 wt.% and 140 g/l, respectively). However, the direct use of pristine
ammonia borane as a hydrogen energy carrier in onboard/fuel-cell applications is prevented beacuse of its very slow dehydro-
genation kinetics below 100°C and the concurrent release of detrimental volatile by-products such as ammonia, borazine, and
diborane. yıldırım [67] discussed different approaches to understand and control the properties of ammonia borane and other
boron-based materials to be practical in terms of reduced dehydrogenation temperatures, accelerated H
2
release kinetics, and/
or minimized borazine release: (i) developing of a new family of metal borohydride ammonia borane complexes (mixed metal
amidoboranes na
2
Mg(nH
2
BH
3
)
4
and Li
2
(BH
4
)
2
nH
3
BH
3
, and Ca(BH
4
)
2
(nH
3
BH
3
)
2
are some examples), from which >11 wt.%
hydrogen can be released; (ii) exploring the effect of nano-confinement and catalytic activity of various metal-organic frame-
works on the hydrogen release of ammonia borane; and (iii) designing and synthesizing a new nanoporous material, the so-called
graphene oxide framework, as a potential storage medium for hydrogen gas.
27.5
otHer Boron-ricH nanoMaterials
Hydrogen adsorption on the pristine boron sheets and nanotubes was investigated [68] by dFT calculations. Both molecular
physisorption and dissociative atomic chemisorption were considered. Molecular hydrogen physisorption energies were found
to be approximately 30-60 meV/molecule, actually lower than in graphene and in carbon nanotubes and far from the energies
of 300-400 meV/molecule necessary for efficient hydrogen storage at room temperature and moderate pressures for onboard
