Hydrogen Storage in LiBH4-MgH2-Al
Bjarne R. S. Hansena, Dorthe B. Ravnsbæka, Jørgen Skibstedb, Carsten Gundlachc & Torben R. Jensena
a Center
for Materials Crystallography and Department of Chemistry, University of Aarhus, Langelandsgade 140, DK-8000 Aarhus C, Denmark
b Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, and Interdisciplinary Nanoscience Center (iNANO), Aarhus
University, Langelandsgade 140, DK-8000 Aarhus C, Denmark
c MAX-II laboratory, Lund University, Ole Römers väg 1, 223 63, S-22100 Lund, Sweden
Abstract
LiBH4 is an interesting hydrogen storage material, as it has a high gravimetric hydrogen content of 18.5 wt% [1]. However, the utilization is hampered by lack
of reversibility and high decomposition temperatures. One way of improving metalborohydrides, is the utilization of reactive hydride composites, which has
be achieved by adding for instance Al [2] or MgH2 [3]. By adding both MgH2 and Al the LiBH4-MgH2-Al composite further improves the hydrogen release and
uptake properties of LiBH4 [4]. In this study the decomposition reactions of LiBH4-MgH2-Al in molar ratios (4:1:1) and (4:1:5) are investigated using in situ
Synchrotron Radiation Powder X-ray Diffraction (SR-PXD) and thermal analysis (TGA/DSC) coupled with mass spectroscopy (MS), Sieverts Measurements
(PCT), Fourier transform infrared spectroscopy (ATR-FTIR) and solid state 11B Magic Angle Spinning Nuclear Magnetic Resonance (11B MAS NMR)
Mechanochemical Synthesis: Fritsch Pulverisette no. 4 with main disk speed: 400 rpm., Relative ratio: -2.25, Mill time: 5 min, Pause time: 2 min, Ball-toPowder mass ratio: 35:1, Repetitions: 24 (Total mill time: 120 min).
λ = 1.103671 Å
λ = 1.102050 Å
Mg/Mg0.9Al0.1
and Al Bragg
peak overlap
Decomposition reactions (LiBH4-MgH2-Al (4:1:5, S2))
Decomposition reactions (LiBH4-MgH2-Al (4:1:1, S1))
Figures show SR-PXD of LiBH4-MgH2-Al (4:1:1). MgH2 decompose at T = 300 °C, and react
with Al to form Mg17Al12 and Mg0.9Al0.1. MgAlB4 is observed at T = 380 °C. This formation
might be slow, since diffracted intensity from Mg0.9Al0.1 increase as Mg17Al12 decompose. LiAl
is observed at T = 460 °C. From the molar composition LiAl should not form. However, since
the formation of MgAlB4 is relatively slow, it is possible that LiAl forms instead since both Al
and Li are present.
MgH2 decompose at T = 290 °C, but unlike the LiBH4-MgH2-Al (4:1:1) sample Mg17Al12 is not
observed. Mg/Mg0.9Al0.1 is however observed after MgH2 decomposition. The formation of
MgAlB4 occur at a lower temperature compared to (4:1:1), i.e. T = 300 °C, and seems to form
in two different rates. Diffracted intensity from LiAl is observed at T = 410 °C, which is ~50 °C
earlier than in (4:1:1). An unknown compound (1) is observed at T = 490 °C and is possibly a
stable impurity, as it continues unaltered throughout the experiment.
Thermal analysis (comparison)
TGA/DSC-MS analysis complement the decomposition pathway
observed from in situ data well.
No diborane was detected with MS during decomposition of
either sample (not shown). At T = 300 °C the decomposition of
MgH2 is observed in sample LiBH4-MgH2-Al (4:1:1), but not in
the (4:1:5)-sample, although a release of hydrogen is observed
in the MS data for both samples.
The formation of MgAlB4 is observed as a broad endotherm and
the formation of LiAl is seen at T = 410 °C. In LiBH4-MgH2-Al
(4:1:1) and LiBH4-MgH2 (2:1) the formation of LiMg is observed.
This is however not observed in the in situ SR-PXD
measurement (it was only heated to 500 °C)
More Al  lower Tonset
Tonset
Tpeak
Tend
LiBH4-Al (2:1)
390
425
470
LiBH4-Al (2:3)
325
375
450
LiBH4-MgH2 (2:1)
350
420
440 (565)
LiBH4-MgH2-Al (4:1:1)
275
415
440 (550)
LiBH4-MgH2-Al (4:1:5)
275
350/400
485
Comparison with the LiBH4-Al and LiBH4-MgH2 samples
show that the onset temperature for hydrogen release
is lowered up to ~75 °C.
No B2H6 detected
S1: LiBH4-MgH2-Al (4:1:1)
S2: LiBH4-MgH2-Al (4:1:5)
Temperature (°C)
11B
ATR-FTIR
MAS NMR
S2, LiBH4-MgH2-Al (4:1:5)
after 3 PCT cycles
Centerband resonance
from LiBH4 (-41.3 ppm)
Centerband resonance
consistent with Li2B12H12
(-9 ppm)
*
*
*
*
Reversibility
PCT desorptions of the LiBH4-MgH2-Al samples show a decrease
in hydrogen storage capacity. In S2 (4:1:5) three gas release steps
are observed. The release related MgAlB4 is decreasing the most.
Hence, the capacity drop is likely related to a boron-containing
species. This is also the case in S1 (4:1:1), but LiAl is not observed.
LiBH4-MgH2-Al (4:1:1)
λ = 1.54056 Å
LiBH4-MgH2-Al (4:1:5)
λ = 1.54056 Å
Diffractograms were obtained after the first desorption and
first absorption. These diffractograms confirm the reactions
proposed from the PCT desorptions. Unknown 1 is also
observed after absorption. Diffracted intensity from LiBH4
is weak compared to the diffractograms of the ball milled
samples.
LiBH4-MgH2-Al (4:1:1)
After 3 PCT cycles
λ = 1.54056 Å
LiBH4-MgH2-Al (4:1:5)
After 3 PCT cycles
λ = 1.54056 Å
*
*
*
*
*
*
*
*
* = spinning sidebands/satellite transitions
The 11B MAS NMR spectra of S2 after three hydrogen
release and uptake cycles show a centerband
resonance from LiBH4, and a chemical shift consistent
with Li2B12H12 which was not detected by PXD. This is
also the case in S1. Li2B12H12 is a stable closo-borate and
it does not seem to react/rehydrogenate at the physical
conditions used in this study. Li2B12H12 was also
observed by FT-IR in the cycled samples.
Hence, Li2B12H12 is likely responsible for the
hydrogen storage capacity loss in the samples.
More information: brsh@chem.au.dk
Acknowledgements and references
Sincere acknowledgements are directed to iNANO, Danscatt , MAX-Lab
Bor4Store (EU) and Center for Materials Crystallography (CMC)
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G. Barkhordarian, N. Eigen, A. Borgschulte, T. R. Jensen, Y. Cerenius, O. Gutfleisch, T. Klassen, M. Dornheim, Acta Materialia 2007, 55, 3951–3958; [4] Y. Zhang,
Q. Tian, H. Chu, J. Zhang, L. Sun, J. Sun, Z. Wen, J. Phys. Chem. C 2009, 113, 21964–21969