Hydrogen release reactions of Al-based complex hydrides enhanced by vibrational dynamics and valences of metal cations

Article abstract of DOI:10.1039/c6cc05199e

Hydrogen release from Al-based complex hydrides composed of metal cation(s) and [AlH₄]^- was investigated using inelastic neutron scattering viewed from vibrational dynamics. The hydrogen release followed the softening of translational and [AlH₄]^- librational modes, which was enhanced by vibrational dynamics and the valence(s) of the metal cation(s).

Full text of DOI:10.1039/c6cc05199e

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Inelastic neutron scattering spectra of LiCa(AlH4)3 upon heating and the local atomic
structure around [AlH4]–

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Hydrogen release reactions of Al-based complex hydrides
enhanced by vibrational dynamics and valences of metal cations
T. Sato,a A. J. Ramirez–Cuesta,b L. Daemen,b Y. –Q. Cheng,b K. Tomiyasu,c S. Takagi,a and
S. Orimo*ad
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Hydrogen release from Al-based complex hydrides composed of key issue to understand the functionality of complex hydrides,
metal cation(s) and [AlH4]– was investigated using inelastic the relationship has not been elucidated properly yet.
neutron scattering viewed from vibrational dynamics. Hydrogen
Conventionally, Raman, infrared (IR) and inelastic neutron
release followed the softening of translational and [AlH4]− scattering (INS) spectroscopies are employed for the studies of
librational modes, which was enhanced by vibrational dynamics vibrational dynamics. Although Raman and IR spectroscopies
and the valence of the metal cation(s).
are convenient, they prevent the observation of all vibrational
modes due to the selection rules. In contrast, INS provides
access to all vibrational modes in materials because it provides
information on the dynamics across the Brillouin zone and has
no selection rules. Furthermore, hydrogen has larger
incoherent neutron scattering cross-section (80.26 barns; 1
barn = 1 × 10−24 cm2)7 than most elements (less than 10 barns),
which gives higher INS intensity. Then, hydrogen-contributed
vibrational dynamics can be clearly observed using INS.6,7,9,10
In this study, we therefore performed INS experiments on
alanates with systematically different valences of metal
cation(s); LiAlH4, Ca(AlH4)2 and LiCa(AlH4)3 at 10–400 K with a
broad frequency range and a high resolution to elucidate the
relationship between vibrational dynamics and hydrogen
release reaction kinetics viewed from dependences of valences
of metal cation(s). LiCa(AlH4)3 is formally classified as a tri-
valent cation system, and adopts a mixed local atomic
arrangement around [AlH4]– of LiAlH4 and Ca(AlH4)2 (Fig. 1).11,12
Although vibrational spectroscopic studies on LiAlH45,6,7d,9d and
Complex hydrides, which are composed of metal cation(s),
such as Li+ and Mg2+, and complex anion(s), such as [AlH4]– and
[BH4]–, with covalent bonding between the central atom and
hydrogen, have attracted significant attention because of their
potential use as high gravimetric and volumetric hydrogen
storage materials and ionic conduction materials, among other
useful applications.1–4 For the reasons, numerous studies
attempting to gain a fundamental understanding of such
physical properties and functionalities have been reported.
Particularly, Ti doping on an Al-based complex hydride
NaAlH4 (sodium alanates), which is of interest in high
gravimetric hydrogen storage applications, is acknowledged to
enhance hydrogen release and uptake reaction kinetics of
NaAlH4.1a,1b Owing to clarification of Ti effects, hydrogen
dynamics has been studied.5 According to vibrational
spectroscopic studies, Ti was proposed to be at Na+ site, on
which vibrational modes were softened (shifted to lower
frequencies) compared with a pure NaAlH4.5a,5b,5d–f Recently,
we observed Intrinsic anharmonicity of librational modes of
the complex anion [AlH4]– on LiAlH4 before hydrogen release.
Such phenomenon was also observed in a complex hydride,
LiBH4, with Li+ ionic conduction.7 Although the vibrational
dynamics (librational modes of a complex anion) could be one
13
Ca(AlH4)2 were reported, our results firstly provide effects of
different metal cation(s) on their vibrational dynamics and
hydrogen release reaction kinetics using INS with broad
frequency range and high resolution combined with
theoretical calculations, which were not performed previously.
The INS experiments were performed using the VISION
spectrometer14 (BL-16B) at the Spallation Neutron Source, Oak
Ridge National Laboratory, USA. The detailed methods are
described in the electronic supplementary information (ESI).
We assigned the vibrational modes of LiAlH4, Ca(AlH4)2 and
LiCa(AlH4)3 in the INS spectra (Fig. 1). These spectra can be
mainly divided into three regions: the frequency ranges 0–550,
600–1100 and 1600–2000 cm−1. Using the aCLIMAX program,15
the experimental INS spectra observed at 10 K were compared
with the phonon density of states on LiAlH4, Ca(AlH4)2 and
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Fig. 1 INS spectra of (top) LiAlH4, (middle) Ca(AlH4)2 and
(bottom) LiCa(AlH4)3 at 10 K. Inset figures are the local atomic Fig. 2 INS spectra of (left) LiAlH4, (middle) Ca(AlH4)2 and (right)
structure around [AlH4]– for each alanate. Red, purple, yellow LiCa(AlH4)3 at the frequency region of 0–600 cm–1 with partial
and blue circles indicate Li, Ca, Al and H atoms, respectively. phonon density of states generated from aCLIMAX. The area
The external and internal motions of [AlH4]– are observed at 0– fractions obtained from each partial phonon density of states
550 and 600–2000 cm−1, respectively.
are superimposed on the INS spectra. Black, red, purple,
yellow and blue lines indicates experimentally obtained INS
LiCa(AlH4)3 calculated using the CASTEP program.16 The spectrum and partial phonon density of states for Li, Ca, Al and
calculated INS spectra are in reasonably good agreement with H, respectively. Arrows indicate the weight shifting from
the experimental INS spectra (Figs. S6–S8 in the ESI). translational mode to [AlH4]– librational modes.
Combining the experimental and calculated results, we refer to
the motions at 0–550 cm−1 as the external motion of [AlH4]–, exhibiting of Li+ ionic conduction.
namely, the translational and [AlH4]– librational modes, and
The internal motion of [AlH4]– comprises the Al–H bending
the motion above 600 cm−1 as the internal motion of [AlH4]–, modes (600–1100 cm–1) and the Al–H stretching modes (1600–
namely, the Al–H bending and stretching modes in [AlH4]–. 2000 cm–1). In particular, the Al–H stretching modes are often
It is difficult to experimentally separate the translational and used for the identification of [AlH4]– because they are one of
[AlH4]– librational modes, although there is separation the most characteristic and simplest motions of [AlH4]–. An
between the modes, similar to [AlH4]– in NaAlH4.17 To resolve evaluation of the local atomic structure around [AlH4]– (see Fig.
these modes in the experimentally obtained INS spectra, the 1) shows that H coordinated to two cations (Li+) results in a
INS spectra calculated from the partial phonon density of lower frequency of Al–H stretching modes and longer Al–H
states and the area fractions for the external motions of distances than H coordinated to one cation (Fig. 3 and Table 1).
[AlH4]– are compared with the experimental spectra (Fig. 2). The higher frequency of the Al–H stretching modes reflects a
Increasing H contribution and decreasing Al contribution to the shorter interatomic distance between Al and H, since motion
area fractions with increasing frequency suggest the weight involving shorter interatomic distances requires more
shifting from translational mode to [AlH4]– librational modes energy.10e,21 Therefore, the larger number of cations around H
(marked arrows in Fig. 2). This is consistent with vibrational might attract H atoms farther away from Al, inducing longer
motions calculated using the CASTEP. Therefore, the [AlH4]– Al–H distances. This increased bond length in turn leads to
librational modes in LiAlH4, Ca(AlH4)2 and LiCa(AlH4)3 are lower frequency Al–H stretching modes.
observed around approximately 200, 250 and 100 cm−1
,
We now describe the hydrogen release reactions in terms of
respectively. Although a more highly charged cation(s) allows the temperature dependence of the INS spectra. Upon heating,
easier libration of the complex anions (lower peak
frequency),18 the [AlH4]– librational modes in LiAlH4 are
observed at lower frequencies than those for Ca(AlH4)2. This
originates from the coupling between the translational modes
of light mass of Li+ and librational modes of [AlH4]–. The
coupling can be found that the Li+ contribution in the area
fraction of LiAlH4 or LiCa(AlH4)3 is synchronized with the
increase in H contribution to the [AlH4]– librational modes as
shown in Fig. 2. Such coupling and softening (discussed later) Fig. 3 INS spectra of (left) LiAlH4, (middle) Ca(AlH4)2 and (right)
of translational and complex anion librational modes have LiCa(AlH4)3 in the 1400–2200 cm–1 frequency region. The blue
been previously observed in complex hydrides with ionic and green spectra, which are generated from aCLIMAX,
conduction (e.g. LiBH4).7,19 In LiAlH4, the mobility of Li+ is much indicate the contributions from H atoms with one and two
lower than that of LiBH4.20 This is hypothesized to be due to cation(s) around H, respectively.
the lack of an order–disorder transition of Li+ during the
2 | J. Name., 2012, 00, 1-3
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Table 1 Average Al–H distances (experimental values for LiAlH4
and Ca(AlH4)2,11 and theoretical values for LiCa(AlH4)310) and
peak frequencies of Al–H stretching modes. The peak
frequencies are estimated from the maxima of the lowest
calculated main peaks of the Al–H stretching modes.
Averaged Al–H Peak frequency of Al–H
stretching modes (cm–1)
distance (Å)
LiAlH4
1.61 (one Li)
1.63 (two Li)
1.61
1794
1651
1798
1830
1660
Ca(AlH4)2
LiCa(AlH4)3
1.62 (one Ca)
1.66 (two Li)
peak frequencies below 550 cm–1, corresponding to the
external motion of [AlH4]–, clearly shift to lower frequencies,
termed the softening of the external motion of [AlH4]– in all
alanates (Fig. 4). Although prominent softening is confirmed in
[AlH4]– librational modes as observed in LiAlH4,6 a shift in the
translational modes to lower frequencies are also observed
(see Fig. 4) in this study because of the high resolution of INS
spectroscopy. The softening of the external motion suggests
appearing an intrinsic anharmonicity of the translational and
the librational modes upon heating that would be related to
hydrogen release reaction.6 In contrast, no such shift is
observed above 600 cm–1, corresponding to the internal
motion of [AlH4]–. Note that the peak intensities and widths
are clearly decreased and broadened (Figs. S9–S11 in the ESI).
This is due to the fact that chemical bonding of Al–H in [AlH4]–
is rigidly maintained though thermal motions of H atom are
increased. This is consistent with our previous results on local
atomic structural analysis of LiAlH4.21 In addition to the
softening on the three alanates, spectroscopic studies on Ti
doped NaAlH4 have been shown the softening of the external
motions correlated with the improvement of hydrogen release
reaction kinetics.1a,1b,5 Therefore, the softening of the external
motion of [AlH4]– observed above 150 K, which is equivalent to
weakening of chemical bonding between the metal cation(s)
and [AlH4]–, could be observed as an initial stage phenomenon
to enhancing hydrogen release reaction kinetics of alanates.
Such phenomena would be observed in complex hydrides (e.g.
LiBH4) since softening of [BH4]– librational modes in LiBH4
observed during the exhibiting of Li+ ionic conduction.7
Although softening of vibrational modes on complex transition
metal hydrides (e.g. Mg2NiH4) have not been reported, they
would be a similar with complex hydrides with [AlH4]– and
[BH4]– because of similarities of their vibrational dynamics.5d
At 400 K, the background of the INS spectra decreases in 3 h
owing to hydrogen release (Figs. S12–14 in the ESI). LiCa(AlH4)3
particularly exhibited the fastest hydrogen release reaction
kinetics among the three alanates. Focusing on the peak
frequencies of the [AlH4]– librational modes because of
obvious metal cation(s) and temperature dependences of the
external motion of [AlH4]–, interestingly, LiCa(AlH4)3 with faster
hydrogen release reaction kinetics shows lower [AlH4]–
librational mode peak frequencies than LiAlH4 and Ca(AlH4)2
due to the coupling of Li+ translational modes and the valences
of the metal cation(s). Such relationship has been also
Fig. 4 INS spectra of (top) LiAlH4, (middle) Ca(AlH4)2 and
(bottom) LiCa(AlH4)3 at 10–400 K. The intensity was divided by
the Bose factor, B(E, T) = 1 + 1/[exp{E/kBT} – 1].5c E: energy
transfer in meV (= 8.066 × in cm–1), T: temperature in kelvin
and kB: the Boltzmann constant in J/K. The maxima of the
[AlH4]– librational modes at 10 K are denoted and connected
with curve at each temperature.
reported in water in ionic exchanged smectite clay minerals by
different cations with different valences, in which their
hydration enthalpies were depended on frequencies of water
molecules librational modes with a trend of metal cations.22
Therefore, the hydrogen release reaction kinetics on alanates
was speculated to be enhanced by the vibrational dynamics
and the valences of the metal cation(s) because they affected
for peak shifting to lower frequencies of [AlH4]– librational
modes. Thus, frequency of [AlH4]– librational modes would
imply a relevant to their hydrogen release reaction kinetics.
In summary, we investigated vibrational dynamics on LiAlH4,
Ca(AlH4)2 and LiCa(AlH4)3 with systematically different metal
cation(s) and a [AlH4]– complex anion by inelastic neutron
scattering (INS) spectroscopy to elucidate the relationships
between their vibrational dynamics and hydrogen release
reaction kinetics viewed from dependences of valences of
metal cation(s). The INS spectra were divided into two regions
corresponding to two different types of motion, external (<550
cm–1) and internal motion (>600 cm–1) of [AlH4]–. In the
external motion of [AlH4]–, which are referred to as
translational and [AlH4]– librational modes, the [AlH4]–
librational modes appeared at lower peak frequencies because
of vibrational dynamics and the valence of the metal cation(s).
In addition, the INS experiments upon heating revealed that
the external motion of [AlH4]– was softened equivalent to
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78, 094302; (b) A. M. Racu, J. Schoenes, Z. Lodziana, A.
weakening of the chemical bonding between the metal cations
and [AlH4]– above 150 K before hydrogen release reaction at
400 K, while the internal motion of [AlH4]– is maintained. Such
phenomena were also observed on Ti doped NaAlH4. Those
facts indicated that the softening of the external motion of
[AlH4]– was observed as an initial stage phenomenon for the
hydrogen release. In addition, isothermal INS experiment at
400 K suggested that hydrogen release reaction kinetics
showed to be faster with lower frequency of [AlH4]– librational
modes originated from the vibrational dynamics and the
valences of the metal cation(s). Such phenomena were shown
in water molecule in smectite clay minerals. Therefore,
hydrogen release reaction kinetics could be enhanced by
vibrational dynamics and the valences of the metal cation(s).
We are grateful for technical support from H. Ohmiya and
N. Warifune, and the use of SR16000 supercomputing
resources at the Center for Computational Materials Science of
the Institute for Materials Research, Tohoku University. This
work was supported by JSPS KAKENHI (Grant No. 16K06766,
16H06119 and 25220911), and Collaborative Research Center
on Energy Materials in IMR (E-IMR). This work benefited from
the use of the VISION beamline (IPTS-12290) at ONRL’s
Spallation Neutron Source and the VirtuES (Virtual
Experiments in Spectroscopy) project, (LDRD 7739), which are
supported by the Scientific User Facilities Division, Office of
Basic Energy Sciences, U.S. Department of Energy, under
Contract No. DE-AC0500OR22725.
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