First-principles study of alkaline-earth alanates
O. M. Løvvik∗ and P. N. Molin∗
∗
University of Oslo, Center for Materials Science and Nanotechnology, P. O. Box 1126 Blindern,
N-0318 Oslo, Norway
Abstract. Ground-state properties of magnesium alanate Mg(AlH4 )2 and calcium alanate
Ca(AlH4 )2 have been calculated using the generalized-gradient approximation of density-functional
theory. Most of the differences between the two compounds are easily explained by the different
size of the alkaline-earth ions: bond lengths, coordination, and peak electron density. In addition,
Ca alanate exhibits a relatively strong spherical electron attractor around Ca, the analogue is not
seen in Mg alanate.
Keywords: Metal hydrides, alanates, hydrogen storage, ab inito calculations, density functional
theory, modelling
PACS: 61.50.Ah, 61.66.Fn, 71.20.Ps
INTRODUCTION
After the discovery of reversible hydrogenation of sodium alanate NaAlH4 at moderate
conditions when catalytic amounts of Ti salts were added,[1] there has been a large
focus on alkali alanates as potential hydrogen storage materials.[2, 3, 4, 5, 6, 7, 8] It
has become clear that NaAlH4 is the best hydrogen storage material among the alkali
alanates,[9] and the search has been extended to alkaline-earth alanates.[10, 11, 12,
13, 14] Magnesium alanate and calcium alanate are the most promising candidates in
this group of materials, with 7.0 and 5.9 wt% potentially accessible hydrogen content,
respectively. They release the hydrogen content through two or three dehydrogenation
steps:
3
Ae(AlH4 )2 → AeAl2 H5 + H2
2
3
AeAl2 H5 → AeH2 + 2Al + H2
2
AeH2 → Ae + H2 ,
(1)
(2)
(3)
where Ae is Mg or Ca. This scheme has been proposed for Ca(AlH4 )2 based on thermogravimetric analyses, but the crystal structures of both the Ca-Al containing phases
were unknown.[10]? The intermediate phase MgAl2H5 has not been seen in any of the
experimental studies, hence it is most probable that Mg(AlH4)2 decomposes directly to
MgH2.
The rehydrogenation of either of these alanate phases from the alkaline-earth dihydride has not been reported, so it is unclear whether it is possible to hydrogenate either
of them from the gas phase at moderate conditions. The difficulties experienced so far
may just as well be due to kinetic constrictions as due to the thermodynamics.
85
The present article compares the calculated properties of magnesium alanate and
calcium alanate achieved from density-functional band-structure calculations at 0 K.
This is partially based on previously reported results,[13, 14] but the focus is here
on comparing the two materials, in addition to a thorough presentation of the spatial
behavior of the electronic structure.
METHODOLOGY
Density functional theory (DFT) within the generalized gradient approximation (GGA)
as implemented in the Vienna ab initio simulation package (VASP)[15, 16] was used
in the calculations. The PW91 potential was used for the gradient corrections,[17]
while the electron density was represented using the projector augmented wave (PAW)
method.[18, 19] The number of k-points was sufficient to achieve convergence in the
total energy of 1 meV with respect to the k-point density. The electronic density was
defined to be self-consistent when the difference in calculated total energy between
two consecutive cycles was less than 0.01 meV. The cutoff energy for the plane wave
expansion was 700 and 780 eV for Mg(AlH4)2 and Ca(AlH4 )2 , respectively. The method
being used for the structure determination has been thoroughly described elsewhere.[14]
RESULTS
The crystal structure of Mg(AlH4)2 has been determined previously by combined Xray and neutron powder diffraction measurements,[12] and the calculated structure was
shown to be in very good agreement with the experimental one.[13] The crystal structure
of Ca(AlH4 )2 has been predicted by density functional calculations,[14] and the structure
is consistent with a recently published X-ray diffraction pattern of Ca(AlH4 )2 .[20, 21]
Some details from the predicted crystal structure of Mg(AlH4)2 and Ca(AlH4 )2 are
shown in Table 1.
Similarly to what has been found for the alkali alanates,[9] the AlH4 tetrahedra are
almost identical in the two alkaline-earth alanates, with the same Al-H bond lengths, coordination numbers and degree of distortion from perfect tetrahedra. Also in correspondence with the alkali alanates, the M-H bond length and coordination number increase
when the cation size increases, which may most readily be explained by geometry; the
larger cation takes more place, leading to longer bonds and higher coordination.
The volumetric density of both alanates is higher than the international goal of
70 kg/m3,[22] at least if the tank is not included. The accessible hydrogen content is
also above the international target for mobile applications of 5 wt.% hydrogen[22] for
both the alanates, while the US Department of Energy target of 6 wt% hydrogen including the tank[23] is only possible to reach using Mg alanate.
It has, however, been difficult to hydrogenate Mg alanate from the gas phase,[24]
which may be due to the magnesium alanate being thermodynamically too unstable.
We have calculated the formation enthalpy and reaction enthalpy of the alkaline-earth
alanates, defined as follows:
∆HForm(Ae(AlH4 )2 ) = E(Ae(AlH4 )2 ) − E(Ae) − 2E(Al) − 4E(H2),
86
(4)
TABLE 1. Predicted properties of Mg and Ca alanate. The theoretical total and accessible hydrogen content are achieved by assuming complete composing the alanate to the elements and to
the alkaline-earth dihydride, respectively. The volumetric hydrogen
density is based on the accessible hydrogen content and the calculated volume. The formation and reaction enthalpies are based on
the dehydrogenation down to the elements (defined in Eq. 4) and
down to the dihydride (defined in Eq. 5.)
Space group
Distance (pm)
Coordination
Angles
H wt. %
H vol. density
Formation enthalpy
Reaction enthalpy
M-H min
M-H max
Al-H min
Al-H max
M-H
Al-H
H-Al-H min
H-Al-H max
Total
Accessible
(kg/m3)
(kJ/mol H2 )
(kJ/mol H2 )
Mg(AlH4 )2
Ca(AlH4)2
P3̄m/1
188.6
188.6
160.1
161.9
6
4
105.7
113.0
9.34
7.01
72.3
-21.1
-6.2
Pbca
219.6
227.9
160.8
163.3
8
4
106.8
113.2
7.90
5.92
70.4
-59.4
-20.7
∆HReact (Ae(AlH4 )2 ) = E(Ae(AlH4 )2 ) − E(AeH2 ) − 2E(Al) − 3E(H2 ).
(5)
E(Ae(AlH4 )2 ) is the total free energy of Ae(AlH4 )2 as calculated by VASP, etc., and Ae
is Mg or Ca. If the thermodynamics are responsible for the hydrogenation problems of
magnesium alanate, it is interesting to see in Table 1 that the decomposition of Ca alanate
is significantly more endothermic than that of Mg alanate. The formation enthalpy of Ca
alanate is similar to that of NaAlH4 ,[9] which gives some hope that it may be possible to
hydrogenate reversibly Ca alanate at reasonable conditions. Since this has not yet been
reported, there may be large kinetic barriers to hydrogenation, which could be overcome
by catalysis or surface treatment.
The calculated electron density and electron localization function (ELF) are shown for
Mg(AlH4)2 in Fig. 1 and for Ca(AlH4 )2 in Fig. 2. The plots are taken through planes that
contain or are close to all three atomic species, in order to easily compare the alanates.
Some features are very similar between Mg and Ca alanate: There is a high degree of
charge concentration around the alkaline-earth metals and H, while the electron density
is low around the Al core. A quite large region around the hydrogen cores has strongly
localized electrons, while the electrons near the Al core are delocalized. The absolute
values of the charge density and the ELF are also quite similar, except the highest charge
density near the Mg core, which is considerably higher than that near the Ca core—this is
because of the smaller radius of the Mg atoms. There are also some distinct differences.
While there are relatively large non-connected pockets of space between the atoms with
low electron density in Ca(AlH4 )2 , the similar areas in Mg(AlH4 )2 are connected to form
two-dimensional sheets, reflecting the layered structure of Mg(AlH4 )2 . Furthermore,
87
E le c t r o n d e ns it y
E le c t r o n lo c a liz a t io n
H
H
H
H
Al
H
Al
H
Al
H
Al
H
H
H
H
0.04
H
0.26
0.72
3.4
0.2
7.0
0.4
0.6
0.8
FIGURE 1. The electron density (left panel) and electron localization function (ELF) (right panel) of
Mg(AlH4)2 . The plots are taken through the (101) surface projected on the (110) plane, with Mg atoms at
the corners. The charge density is measured in e/Å3 , and the contour spacing is logarithmic. The ELF is
normalized between 0 and 1, the highest value designating complete localization.
H
E le c t r o n d e ns it y
Al
H
H
H
E le c t r o n lo c a liz a t io n
Al
H
H
Al
H
Al
H
Al
H
Ca
H
H
H
H
H
H
Ca
Al
H
H
H
H
H
Al
Al
Ca
H
H
Ca
H
H
Al
Al
0.05
0.5
1 .7
3.3
H
H
0.2
5 .3
0.4
0.6
0.8
FIGURE 2. The electron density (left panel) and electron localization function (ELF) (right panel) of
Ca(AlH4 )2 . The plots are taken through a (110) plane that contains all three kinds of atoms. The charge
density is measured in e/Å3 , and the contour spacing is logarithmic. The ELF is normalized between 0
and 1, the highest value designating complete localization.
there is a relatively strong electron attractor in a spherical shell around Ca in Ca(AlH4 )2 ,
while the corresponding area around Mg only contains very weakly localized electrons
in Mg(AlH4 )2 . The latter difference may be important for the stability of the compounds,
and could be an important reason why calcium alanate is relatively more stable than
magnesium alanate.
88
CONCLUSIONS
The calculated structural and electronic properties of Mg(AlH4)2 and Ca(AlH4 )2 are
compared, revealing many similarities, but also some important differences. The AlH4
unit is virtually unchanged when the local environment changes, both structurally and
electronically. Most of the differences are thus found around the alkaline-earth metal.
Due to the larger size of the Ca atom, it has longer bonds to the neighbor hydrogen
atoms, higher coordination, and lower peak electron density than the Mg atom. Another
important difference is the existence of a spherical electron attractor around Ca, which
does not exist around Mg. This is probably a main reason why the calcium alanate is
relatively more stable than the magnesium alanate (compared to the respective alkalineearth dihydrides). Since it is quite probable that magnesium alanate is too unstable to
be of interest as a reversible hydrogen storage material, this gives hope that either pure
calcium alanate or a hypothetical mixed magnesium-calcium alanate phase may have
beneficial thermodynamics for reversible hydrogen storage.
ACKNOWLEDGMENTS
Economic support from the Norwegian Research Council via the NANOMAT program
is acknowledged.
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