A series of mixed-metal borohydrides

Full text of DOI:10.1002/anie.200903030

Angewandte
Chemie
DOI: 10.1002/anie.200903030
Hydrogen Storage
A Series of Mixed-Metal Borohydrides**
Dorthe Ravnsbæk, Yaroslav Filinchuk,* Yngve Cerenius, Hans J. Jakobsen,
Flemming Besenbacher, Jørgen Skibsted, and Torben R. Jensen*
The transition towards a sustainable and reliable energy
system capable of meeting the increasing energy demands is
considered one of the greatest challenges in the 21st century.
However, one of the major obstacles is that renewable energy
sources are unevenly distributed both geographically and
over time, and most countries need to integrate several
different sources. Hydrogen is a potential, extremely inter-
esting energy carrier system,[1] but a major challenge in a
future “hydrogen economy” is the development of a safe,
compact, robust, and efficient means of hydrogen storage, in
particular for mobile applications.[2] No single material has yet
been identified that fulfills all the criteria for hydrogen
storage, despite considerable research and technological
efforts.[3]
Borohydride-based materials have recently received great
attention owing to their high gravimetric hydrogen contents,
but the utilization of this class of materials in real, practical
technological applications is often hampered by unfavorable
thermodynamic and kinetic properties.[4,5] Much research has
focused on improving the properties of known interesting
hydrogen storage materials such as LiBH4, which possesses an
extremely high hydrogen content (18.4 wt%) but unfortu-
nately has a high enthalpy of decomposition (ꢀ67 kJmolꢀ1)
and therefore a high decomposition temperature. Thus,
researchers have tried to improve the thermodynamic proper-
ties by design of a reactive hydride composite 2LiBH4/MgH2,
which reduces the enthalpy of decomposition from ꢀ67 to
ꢀ42 kJmolꢀ1 and which is still capable of reversibly storing
11.5 wt% H2.[6,7] Recently, cation substitution in LiBH4 has
also been realized by, for example, the synthesis of LiK-
(BH4)2, which unfortunately possesses the same high thermo-
dynamic stability as LiBH4 and KBH4.[8] The kinetic proper-
ties of alanates ([AlH4]ꢀ)[9] and magnesium-based systems
have been successfully improved by the exploitation of a
variety of catalytic additives, but these materials appear less
efficient than borohydride-based systems owing to their high
chemical reactivity.[10] Numerous other improvements of
known materials have been explored, for example, incorpo-
ration of LiBH4 into nanoporous scaffolds,[11] but there has
been no major breakthrough that allows the synthesis of ideal
hydrogen storage materials with high hydrogen content, low
hydrogen decomposition temperature (i.e. appropriate ther-
modynamic properties), and fast “refueling” of the material
(i.e. good kinetic properties). Therefore, there is a great need
for new types of compounds, such as ternary borohydrides
involving a combination of very different elements, such as
alkali metals and transition metals.[12]
Herein, we report the synthesis and detailed structural,
physical, and chemical characterization of a new series of
borohydride-based materials, LiZn2(BH4)5, NaZn2(BH4)5,
and NaZn(BH4)3. These materials have completely novel
structures, very high hydrogen contents, and low decomposi-
tion temperatures (Table 1). Our findings are useful as
Table 1: Structural data, hydrogen content, and decomposition temper-
atures for the new alkali-metal zinc borohydrides.
[*] Dr. Y. Filinchuk
LiZn2(BH4)5
NaZn2(BH4)5
NaZn(BH4)3
Swiss–Norwegian Beam Lines at ESRF
BP-220, 38043 Grenoble (France)
Fax: (+33)476-88-2694
space group
a [ꢀ]
b [ꢀ]
c [ꢀ]
b [8]
Cmca
P21/c
P21/c
8.6244(3)
17.8970(8)
15.4114(8)
90
9.397(2)
16.635(3)
9.1359(16)
112.658(19)
4
1318.0(5)
227.98
1.15
8.2714(16)
4.5240(7)
18.757(3)
101.689(11)
4
687.3(2)
132.91
1.28
E-mail: yaroslav.filinchuk@esrf.fr
D. Ravnsbæk, Prof. H. J. Jakobsen, Dr. J. Skibsted, Dr. T. R. Jensen
Interdisciplinary Nanoscience Center (iNANO) and
Department of Chemistry, University of Aarhus
8000 Aarhus C (Denmark)
Fax: (+45)8619-6199
E-mail: trj@chem.au.dk
Z
8
V [ꢀ3]
2378.76(19)
211.93
1.18
112.6
9.51
M [gmolꢀ1
]
1 [gcmꢀ3
]
1v(H2) [kgH2 mꢀ3
1m(H2) [wt%]
Tdec [8C]
]
101.6
8.84
95
116.9
9.10
103
Dr. Y. Cerenius
MAX-lab, Lund University, 22100 Lund (Sweden)
127
Prof. F. Besenbacher
Interdisciplinary Nanoscience Center (iNANO) and
Department of Physics and Astronomy
University of Aarhus, 8000 Aarhus C (Denmark)
general guidelines and inspiration for the design and synthesis
of novel materials for hydrogen storage.
Mechanochemical methods, such as ball milling (BM), are
among the most commonly used preparative methods for
synthesis of borohydride hydrogen storage materials. Several
chemical reactions may occur simultaneously during BM,
[**] The authors are grateful to the Swiss–Norwegian Beam Lines for the
provision of in-house beam time. The Danish Research Councils are
acknowledged for funding to the Instrument Centre for Solid-State
NMR Spectroscopy and DanScatt.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200903030.
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6659

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depending on the physical and chemical conditions, and
hydrides. This similarity to MOFs suggests directionality
and some covalent character of the metal–BH4 interaction.
usually a mixture of phases is obtained. We have synthesized
lithium and sodium zinc borohydrides according to the
reaction in Equation (1):
ꢀ
Indeed, the Zn H bonds in LiZn2(BH4)5 are very short; the
refined distances are all below 2 ꢀ, reaching 1.652(15) ꢀ at
ꢀ
the lower limit. Zn B contacts are also very short (2.108(10)–
5 MBH4 þ 2 ZnCl2 ! MZn2ðBH4Þ5 þ 4 MCl ðM ¼ Li or NaÞ
ð1Þ
ꢀ
2.312(9) ꢀ). The average Zn B distance of 2.17 ꢀ in LiZn2-
(BH4)5 is longer than the corresponding 1.97 ꢀ average in
Be(BH4)2 for the trigonally coordinated Be atom,[13] but it is
considerably shorter than in all other metal borohydrides.[5] It
was recently found that the three-dimensional framework in
Mg(BH4)2 contains empty voids large enough to accommo-
date small molecules like H2O.[16] MZn2(BH4)5 structures,
although not porous, reveal another case of strong and
directional metal–BH4 bonding that defines the structural
architecture.
The Zn atoms in NaZn(BH4)3 have a distorted tetrahedral
coordination with CN = 4, while the saddle-like coordination
of the Na atom by BH4 groups is similar to that in MZn2-
(BH4)5. Metal atoms and BH4 groups in NaZn(BH4)3 form a
3D framework. The change in coordination number from
The ideal LiBH4:ZnCl2 ratio of 2.5:1 gives complete
conversion to LiZn2(BH4)5, which is the only new phase
observed for other ratios as well. In the NaBH4/ZnCl2 system
a competitive reaction [Eq. (2)] occurs, and mixtures of
NaZn2(BH4)5 and NaZn(BH4)3 are obtained for different
NaBH4:ZnCl2 ratios. ZnCl2-rich compositions also yield
Na2ZnCl4 as one of the products.
3 NaBH4 þ ZnCl2 ! NaZnðBH4Þ3 þ 2 NaCl
ð2Þ
The new M-Zn-BH4 phases reveal remarkable structural
diversity, as shown by synchrotron radiation powder X-ray
diffraction (SR-PXD). They represent two novel types of
structures, NaZn(BH4)3 and MZn2(BH4)5, which have no
distinct analogues among other known inorganic compounds.
The structure of NaZn2(BH4)5 was identified as a monoclini-
cally distorted derivative of the lithium-containing analogue.
Two independent zinc atoms in MZn2(BH4)5 (M = Li, Na)
have nearly planar trigonal coordination (coordination
number CN = 3) by three BH4
ꢀ
three to four for Zn leads to much longer Zn B separations,
ꢀ
varying from 2.43(6) to 3.16(6) ꢀ. The average Zn B
separation of 2.74 ꢀ in NaZn(BH4)3 is much longer than the
ꢀ
2.42 ꢀ average of the Mg B separation in hexagonal Mg-
(BH4)2, in which the metal atoms are also tetrahedrally
coordinated by the BH4 groups.[16] Interestingly, while one of
groups, similar to the Be atoms
in Be(BH4)2.[13] The Li and Na
atoms in these two related struc-
tures have a saddle-like coordi-
nation (CN = 4), which has not
been observed earlier for alkali-
metal atoms in borohydrides.[5]
All BH4 groups in MZn2(BH4)5
are linearly coordinated by two
metal atoms (the angles in the
two structures cover the range
164.5(16)–179.6(8)8), similar to
the Mg(BH4)2 structures.[14–16]
The BH4 groups are coordinated
through the two opposite tetra-
hedral edges, bridging either two
Zn atoms or one Zn and one M
atom.
It is remarkable that MZn2-
(BH4)5 consists of two identical
doubly interpenetrated three-
dimensional (3D) frameworks,
which implies that there are no
covalent bonds between them
(Figure 1 and Supporting Infor-
mation, Table S1). This type of
structural topology is common
for coordination polymers involv-
Figure 1. Crystal structures of a) LiZn2(BH4)5 and b) NaZn(BH4)3. The doubly interpenetrated three-
dimensional framework is highlighted in blue and light brown. c) Zn2+ and [BH4]ꢀ units are strongly
associated into isolated [Zn2(BH4)5]ꢀ ions of trigonal-planar Zn centers in the Zn-rich MZn2(BH4)5
compounds. d) The more alkali-metal-rich NaZn(BH4)3 contains 1D anionic [{Zn(BH4)3}n]nꢀ chains with
ing organic ligands, also known as
metal–organic frameworks
(MOFs),[17] but here it is observed
for the first time in metal tetrahedrally coordinated Zn atoms. Zn blue, B brown, M dark gray (M=Li, Na), H light gray.
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the BH4 groups in NaZn(BH4)3 exhibits a nearly linear
coordination by two metal atoms, the other two show
trigonal-planar coordination. The latter is quite unusual, as
it has been observed only in b-Ca(BH4)2.[18,19]
intensities from selected phases were integrated and normal-
ized (Figure 3b) to reveal changes in the relative amounts of
the individual phases. In general, diffraction lines of MZn2-
7
The M/Zn/BH4 phases were also characterized using Li,
11B, and 23Na magic-angle spinning (MAS) NMR spectrosco-
py. Generally, the 11B resonances from these borohydride
units exhibit a small, characteristic shift towards lower
frequency (d(11B) ꢁ ꢀ42 to ꢀ46 ppm) compared to the
corresponding resonances from LiBH4 (d(11B) = ꢀ41.2 ppm)
and NaBH4 (d(11B) = ꢀ42.0 ppm), which may reflect the
coordination to the Zn2+ ions. An 11B MQMAS NMR
spectrum[20] of the NaZn(BH4)3 sample resolves two reso-
nances in a 1:2 ratio, which indicates that two of the three
distinct 11B sites are quite similar. The complexity of the
reaction products is apparent from the 23Na MAS NMR
spectrum of the NaZn(BH4)3 sample (Figure 2b), which
shows overlapping resonances for the two 23Na sites in
Na2ZnCl4 and the single site from NaCl in the range ꢀ2 to
10 ppm, a resonance from NaBH4 at d(23Na) = ꢀ8.4 ppm, and
a peak with second-order quadrupolar lineshape for a single
23Na site in NaZn(BH4)3 at lower frequency. This distinct
lineshape, as demonstrated by the simulation (Figure 2c),
shows that NaZn(BH4)3 is present in the sample as a highly
crystalline phase, while the observation of a unique 23Na site is
in agreement with the crystal structure derived from SR-PXD
(Figure 1). The additional line broadening for NaZn(BH4)3 in
the spectrum recorded without 1H decoupling (Figure 2a) and
the low-frequency shift of the resonance demonstrate that the
Na+ ion is coordinated to the [BH4]ꢀ units.
The decomposition of all synthesized new materials was
investigated by in situ SR-PXD (Figure 3a). The diffracted
Figure 3. a) In situ SR-PXD data measured for LiZn2(BH4)5 heated
from room temperature to 2008C, DT/Dt=48Cminꢀ1, and
l=1.06476 ꢀ. The integrated normalized intensities for selected
reflections of LiZn2(BH4)5 and Zn are shown in (b) to visualize the
~
*
*
decomposition reactions. Symbols: LiCl, Zn, & Li2ZnCl4 , ZnCl2.
&
Unmarked peaks in (a) and in (b) refer to LiZn2(BH4)5.
(BH4)5, MCl, and occasionally small amounts of ZnCl2 (and
Na2ZnCl4 for M = Na) are visible in the powder patterns.
Formation of M2ZnCl4 from MCl and ZnCl2 is also observed.
The main phase MZn2(BH4)5 decomposes to metallic zinc and
MBH4 (see Table 1 for decomposition temperatures). Simul-
taneously, the produced MBH4 reacts with M2ZnCl4 to form
Zn and MCl. Diffraction from NaBH4 is observed in a short
temperature interval during the decomposition of NaZn2-
(BH4)5. As indicated, the decomposition of MZn2(BH4)5
occurs through several coupled reactions, which overall can
be described by the reaction in Equation (3):
Figure 2. 23Na MAS NMR spectra (7.05 T) of the sample including
NaZn(BH4)3 obtained a) without and b) with high-power 1H decoupling
(gB2/2pꢁ100 kHz) using a spinning speed of 10.0 kHz. c) Optimized
simulation of the central transition for the unique 23Na site in NaZn-
(BH4)3, corresponding to the isotropic chemical shift
2 MZn2ðBH4Þ5 þ M2ZnCl4 ! 5 Zn þ 4 MCl þ 5 B2H6 þ 5 H2
ð3Þ
for M = Li and Na. The formation of diborane is verified by
thermogravimetric analysis (TGA), which gave calculated
and observed mass losses of 16.5 and 14.6%, respectively.
diso =ꢀ11.2(2) ppm (relative to 1.0m NaCl(aq)), and the quadrupole
coupling parameters CQ =1.16(5) MHz, hQ =0.63(2).
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DSC (heating rate 28Cminꢀ1, RT to 4508C, helium atmosphere,
corundum crucibles).
Furthermore, LiZn2(BH4)5 was found to decompose
slowly at room temperature when stored in argon atmos-
phere. In argon atmosphere LiZn2(BH4)5 turns gray owing to
the formation of metallic zinc after approximately one week
at room temperature or several months at ꢀ358C. The other
new material, NaZn2(BH4)5, was found to slowly decompose
to NaZn(BH4)3 both at room temperature and at ꢀ328C. At
elevated temperatures the thermal decomposition of NaZn2-
(BH4)5 and NaZn(BH4)3 occurs in a similar manner to that
described above for LiZn2(BH4)5, that is, first NaBH4 is
formed, which then reacts with Na2ZnCl4 (at ca. 1108C), and
the final products are Zn and NaCl. In a previous study of a
ball-milled 2NaBH4/ZnCl2 sample, assigned to Zn(BH4)2, the
mass spectroscopic measurements revealed evolution of
diborane at T> 858C.[21] Our studies indicate that the
sample investigated in that work mainly consisted of NaZn2-
(BH4)5 and that Zn(BH4)2 has not yet been isolated.
The samples were initially investigated by powder X-ray diffrac-
tion (CuKa radiation) and subsequently by synchrotron radiation
1
powder X-ray diffraction (SR-PXD). Data were collected at beamline
BM01A at the European Synchrotron Radiation Facility, Grenoble,
France. An MAR345 detector was used, and the selected X-ray
wavelengths were 0.769748 and 0.699846 ꢀ. All 2D SR-PXD data
were integrated into 1D patterns. Structures of LiZn2(BH4)5 and
NaZn(BH4)3 were solved ab initio from PXD data. The structures
were solved by direct-space methods using simulated annealing in the
program FOX[26] and refined by the Rietveld method. The structure
of NaZn2(BH4)5 was identified as a monoclinically distorted deriva-
tive of the Li-containing analogue and refined by the Rietveld
method. The agreement factors are: Rwp (not corrected for back-
ground) = 1.09%, Rp (corrected for background) = 5.30%, RB =
3.56% for LiZn2(BH4)5; Rwp = 3.22%, Rp = 9.39%, RB = 7.51% for
NaZn2(BH4)5; Rwp = 3.23%, Rp = 12.1%, RB = 7.27% for NaZn-
(BH4)3. Cell parameters and space-group symmetry for the three
new phases are listed in Table 1.
The significant structural diversity and low decomposition
temperatures for the novel series of alkali-metal zinc bor-
ohydrides may be attributed to the ability to form more
In situ time-resolved SR-PXD data were collected at I711 at the
synchrotron MAX II, Lund, Sweden in the research laboratory MAX-
Lab (1.09 mm o.d. sapphire tubes, l = 1.0648 ꢀ, RT to 2008C, heating
rate 48Cminꢀ1).
ꢀ
covalent bonds between Zn and BH4 units than M BH4
bonds in alkali-metal borohydrides. The Pauling electro-
negativity of zinc is higher than those of the alkali metals, and
this difference may contribute to the lower stability of Zn-
based borohydrides. An apparent linear correlation between
Pauling electronegativities and decomposition temperatures
for borohydrides has been found,[22] which suggests that
metals with a lower electronegativity should be used in
borohydride materials. In the Zn-rich MZn2(BH4)5, Zn and
BH4 units are strongly associated in isolated [Zn2(BH4)5]ꢀ
anions (Figure 1c) with M+ countercations, similar to the
isolated [Sc(BH4)4]ꢀ units in LiSc(BH4)4.[23,24] This arrange-
ment stabilizes the phase, thereby enabling its synthesis at
room temperature. The more alkali-metal-rich NaZn(BH4)3 is
even more stable, although it contains 1D anionic [{Zn-
(BH4)3}n]nꢀ chains based on longer Zn···B distances. Variation
in the ratio between the alkali metal and the transition metal,
as well as the use of different metals, enables tuning of the
hydrogen storage properties in alkali-metal/transition-met-
al/BH4 systems. Therefore, a variety of novel mixed-cation
transition-metal-based borohydrides may be discovered in the
near future.
Received: June 5, 2009
Published online: August 5, 2009
Keywords: borohydrides · hydrogen storage · metathesis ·
.
solid-state structures · X-ray diffraction
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