Direct formation of Na3 Al H6 by mechanical milling NaHAl with Ti F3

Article abstract of DOI:10.1063/1.2001756

Na3 Al H6 can be directly formed by mechanical milling NaHAl with Ti F3 under hydrogen atmosphere. The hydrogenation fraction of NaH increases with increasing the milling time, and reaches up to 0.61 after 20 h milling. Thus-formed Na3 Al H6 exhibits unexpected polymorphic transformation and decomposition behaviors. This, together with the unusual hydrogen storage performance of the mechanically prepared materials, provides us a suggestive perspective to probe the favorable modification of the thermodynamics of Na3 Al H6 and nature of active Ti-species in Ti-doped NaAl H4.

Full text of DOI:10.1063/1.2001756

Direct formation of Na 3 Al H 6 by mechanical milling Na H ∕ Al with Ti F 3
P. Wang, X. D. Kang, and H. M. Cheng
Citation: Applied Physics Letters 87, 071911 (2005); doi: 10.1063/1.2001756
View online: http://dx.doi.org/10.1063/1.2001756
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APPLIED PHYSICS LETTERS 87, 071911 ͑2005͒
Direct formation of Na3AlH6 by mechanical milling NaH/Al with TiF3
P. Wang,a͒ X. D. Kang, and H. M. Cheng
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, Shenyang 110016, China
͑Received 24 January 2005; accepted 9 June 2005; published online 12 August 2005͒
Na3AlH6 can be directly formed by mechanical milling NaH/Al with TiF3 under hydrogen
atmosphere. The hydrogenation fraction of NaH increases with increasing the milling time, and
reaches up to 0.61 after 20 h milling. Thus-formed Na3AlH6 exhibits unexpected polymorphic
transformation and decomposition behaviors. This, together with the unusual hydrogen storage
performance of the mechanically prepared materials, provides us a suggestive perspective to probe
the favorable modification of the thermodynamics of Na3AlH6 and nature of active Ti-species in
Ti-doped NaAlH4. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2001756͔
A key technical challenge in advancing “hydrogen
economy” is to develop a viable on-board hydrogen storage
medium. Among various potential approaches, material-
based reversible hydrogen storage has long been appreciated
for its significant advantages on safety, energy efficiency and
cost issues.1 However, decades of extensive research efforts
on metal/alloy hydrides and nanostructured carbon and re-
lated materials have led to no material system which can
provide hydrogen source for on-board fuel cell application
within combined constrains of energy density, operation con-
ditions, and efficiency.1–3 Recently, Bogdanović and
Schwickardi had made a substantial breakthrough on sodium
aluminum hydrides that brings the promise of clean hydro-
gen energy a big step forward.4 It was found that, upon dop-
ing with catalytic amount of selected transition metal com-
pounds, NaAlH4 might undergo reversible hydrogen storage
at moderate temperatures. This finding immediately stimu-
lated worldwide study on developing doped NaAlH4 and
other light-metal hydrides as viable hydrogen storage media
for on-board applications.5–18
pressure hydrogen ͑several hundred bar͒, organic solvent and
complicated purification processes.20–22 Additionally, con-
trolled thermal decomposition of NaAlH4 was also proposed
as an alternative method.22 Recently, Huot et al. reported a
simple solid-state approach, in which ␤-Na3AlH6 was syn-
thesized by mechanical milling the mixture of NaAlH4
+2NaH.23
Quite recently we found that ␤-Na3AlH6 could be di-
rectly formed during mechanical milling of NaH/Al with
TiF3 under hydrogen atmosphere. More significantly, thus-
formed Na3AlH6 presents unexpected polymorphic transfor-
mation and decomposition behaviors, in comparison with the
literature results.23–25 Furthermore, the mechanically pre-
pared materials starting from NaH+Al+4 mol % TiF3 ex-
hibit unusual hydrogen storage performance, which possibly
relates to the in situ hydrogenation of the materials.
The starting materials of NaH ͑95%, 200 mesh͒, Al pow-
der ͑99.95+%, ϳ200 mesh͒, and TiF3 powder were all pur-
chased from Sigma-Aldrich Co. and were used as-received.
The mixture of NaH+Al+4 mol % TiF3 was mechanically
milled by using stainless steel pot and balls in a Fritsch 6
Planetary mill at 400 rpm. The milling was performed under
hydrogen atmosphere, with an initial pressure of about
0.85 MPa. The ball-to-powder ratio was about 40:1. All the
sample operation was performed in an Ar-filled glove-box.
The as-prepared materials were characterized by x-ray
diffraction ͑XRD, Rigaku D/MAX-2500, Cu K␣ radiation͒
and synchronous thermal analyses ͓thermogravimetry ͑TG͒
and differential scanning calorimetry ͑DSC͒, Netzch 449C
Jupier͔. It should be noted that special caution had been
taken to minimize the H2O/O2 contamination during the
measurements. In the thermal analysis measurement, high-
purity Ar ͑purityϾ99.999%͒ was used as purge gas. The
heating rate was 10 K/min. Hydrogen absorption/desorption
behavior was examined by using a carefully calibrated
Sievert’s-type apparatus, the details of which can be found in
Refs. 17 and 18. A typical cyclic experiment entailed desorp-
tion at 150 °C and absorption at 120 °C with an initial pres-
sure conditions of Ͻ1 Torr and ϳ10 MPa, respectively. The
weight of dopant precursor was taken into account in the
determination of H-capacity.
The hydrogen storage in NaAlH4 is accomplished via a
reversible two-step process involving the reactions seen in
Eq. ͑1͒,
NaAlH4 ꢀ 1/3Na3AlH6 + 2/3Al + H2
ꢀNaH + Al + 3/2H2.
͑1͒
Although both steps of de-/hydrogenation processes can be
kinetically enhanced upon doping with Ti species, the prac-
tical availability of NaAlH4 for on-board application is still
largely hampered by the thermodynamic property of
Na3AlH6. According to the literature, the plateau pressure of
Na3AlH6/NaH+Al+H2 equilibrium is less than 1 atm at
temperature below 110 °C.7,10 Therefore, the study on the
intermediate Na3AlH6 stage is critical in developing Ti-
doped NaAlH4 as practical on-board hydrogen storage me-
dia. In this regard, however, the investigations hitherto per-
formed, in particular focusing on the decomposition behavior
and modification of thermodynamics, are rather limited.4,19
Partially, such situation should be attributed to the difficulty
involved in the preparation of Na3AlH6. The traditional
methods of preparing Na3AlH6 involve the utilization of high
Upon doing with TiF3, NaH/Al can be directly hydro-
genated to ␤-Na3AlH6 during mechanical milling under H2
atmosphere. Figure 1 presents the XRD profiles of the mate-
rials milled for a period ranging from 1 to 20 h. It was ob-
a͒
Author to whom correspondence should be addressed; electronic mail:
pingwang@imr.ac.cn
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0003-6951/2005/87͑7͒/071911/3/$22.50 87, 071911-1 © 2005 American Institute of Physics
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071911-2
Wang, Kang, and Cheng
Appl. Phys. Lett. 87, 071911 ͑2005͒
FIG. 1. XRD patterns of NaH+Al+4 mol % TiF3 mechanically milled un-
der H2 atmosphere for different periods.
served that the intensive milling of the first 5 h only led to a
markedly decreased diffraction intensity of the starting ma-
terials, especially that of NaH. When the milling time was
increased to over 10 h, the diffraction peaks ͑220͒ and ͑400͒
of metastable ␤-Na3AlH6 appeared. The as-formed
␤-Na3AlH6 was nanocrystalline, the average grain size of
which was estimated to be about 16 nm according to the
Scherrer equation. In a comparative investigation, we failed
in our attempt to synthesize Na3AlH6 from the mixture of
NaH/Al under identical milling conditions. Therefore, active
Ti-species may be produced in the milling process and play a
critical role in the in situ hydrogenation.
A quantitative characterization of the in situ hydrogena-
tion during mechanical milling was further achieved by TG
measurement of thus-prepared samples. As shown in Fig. 2,
for the samples milled for longer than 2 h, a significant
weight loss was observed to start around 470 K and termi-
nate around 570 K. Clearly, the weight loss should come
from the hydrogen evolution from the formed Na3AlH6, as
was confirmed by the XRD examination ͑the result not
shown here͒ of the samples after being heated to 573 K. The
amount of formed ␤-Na3AlH6 increased with increasing the
milling time. As shown in the inserted figure in Fig. 2, the
hydrogen amount desorbed from the material milled for 20 h
reaches up to 1.11 wt. %, which corresponds to a hydroge-
nation fraction of NaH of 0.61. These values agree well with
the measurement results by using volumetric methods.
The synchronous thermal analyses of thus-formed
␤-Na3AlH6 revealed that its decomposition behavior and
FIG. 3. Synchronous TG/DSC profiles of the material NaH+Al+4 mol %
TiF3 milled for 10 h ͑top͒ and 20 h ͑bottom͒. The heating rate was
10 K/min.
phase transformation during the heating process were much
more complicated than our current understanding.23–25 And
excellent agreement was achieved among the measurements
performed at different times. Figure 3 presents the TG/DSC
profiles of the sample milled for 10 h ͑top͒ and 20 h ͑bot-
tom͒. The broad exothermic peak centered around 410 K,
according to the literature,23–25 should originate from
␤-Na3AlH6→␣-Na3AlH6 and was observed to shift to
higher temperature upon increasing the milling time from
10 to 20 h. While the counter polymorphic transformation
cannot be distinguished from DSC profiles during the fol-
lowing heating process. Actually, an endothermic peak cen-
tered at 525 K that was previously assigned to the polymor-
phic transformation from ␣- to ␤-Na3AlH6 was observed for
both samples.23–25 However, the rapid weight loss detected
by synchronous TG measurement provided clear evidence
that this heat effect should, at least mainly, correspond to the
decomposition of thus-formed Na3AlH6, rather than the
polymorphic transformation. This is further supported by the
XRD examination result that the diffraction peaks of
␤-Na3AlH6 disappeared after the sample being heated up to
573 K. One possible mechanism explaining the phenomenon
is that thus-formed Na3AlH6 is in a highly disordered nano-
structure characterized by a fine grain size and wide energy
distribution. As a result, the small and wide peak arising
from the polymorphic transformation may be masked by the
large heat effect of thermal decomposition. Additionally, for
both samples, another exothermic peak was observed around
490 K, but with the nature still unclear.
It was further found that decomposition behavior of the
Na3AlH6 formed after 20 h milling was substantially differ-
ent from that of 10 h sample. In great contrast to the single
endothermic peak in the case of the 10 h sample, double
endothermic effects were observed to be involved in the ther-
mal decomposition of the Na3AlH6 formed after 20 h mill-
ing. Furthermore, as determined by combination of the calo-
FIG. 2. TG profiles of NaH+Al+4 mol % TiF3 mechanically milled under
hydrogen atmosphere for different periods. The heating rate was 10 K/min.
The inserted figure gives the desorbed H-amount and the corresponding
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hydrogenation fraction of NaH determined by TG measurement.
rimetric value measured in DSC with the weight loss
129.100.58.76 On: Mon, 01 Dec 2014 19:07:11

071911-3
Wang, Kang, and Cheng
Appl. Phys. Lett. 87, 071911 ͑2005͒
In conclusion, for the first time, direct formation of
␤-Na3AlH6 from NaH/Al was achieved by mechanical mill-
ing with TiF3 under H2 atmosphere. The as-formed Na3AlH6
exhibits a series of unexpected phase transformation and de-
composition behaviors. In particular, it was found that vary-
ing the milling time led to a modified thermodynamics of the
dehydriding reaction of Na3AlH6. Additionally, possibly re-
lated to the in situ hydrogenation, the mechanically prepared
Ti-doped materials were observed to undergo a serious deg-
radation on the hydrogen storage performance upon increas-
ing the milling time. A comprehensive understanding of
these phenomena may lead to the elucidation of the nature of
active Ti-species and the means of improving the hydrogen
storage property of Ti-doped NaAlH4.
FIG. 4. 150 °C dehydriding profiles ͑second cycle͒ of NaH+Al+4 mol %
TiF3 mechanically milled under H2 atmosphere for different periods.
The authors are grateful to Ms. S.H. Yang and Mr. Y.
Chen for their assistance in XRD and thermal analysis mea-
surements. The financial support for this research from the
Hundred Talents Project of Chinese Academy of Sciences is
gratefully acknowledged.
detected by TG, the enthalpy change for the solid-state de-
hydriding of Na3AlH6 to NaH/Al was found to decrease
from 35 for the 10 h sample to 28.5 kJ/mol for the 20 h
sample. And both of them are considerably lower than the
literature values.7,10,25 These findings clearly suggest that the
variation of the preparation condition may lead to a change
of the microstructure of Na3AlH6, possibly including local
atomic arrangement, coordination environment and structural
defects. And as a result, the phase transformation, decompo-
sition behavior, and even thermodynamics of Na3AlH6 may
be modified. A better understanding of the involved mecha-
nism, including the possible role of Tix+ and F− played in
such property change, is clearly of importance for achieving
favorable modification of thermodynamics of Na3AlH6.
Of particular interest, we found that the materials pre-
pared by mechanical milling NaH/Al with TiF3 under H2
atmosphere underwent a serious degradation on the hydrogen
storage performance upon increasing the milling time. More-
over, the marked difference on hydrogen storage perfor-
mance arising from the variation of milling time was found
to persist in the following de-/hydrogenation cycles. Figure 4
gives the typical dehydriding profiles ͑second cycle͒ at
150 °C of the materials milled for different periods. A care-
ful examination of the dehydriding profiles found that almost
all the capacity degradation came from the first decomposi-
tion step ͑NaAlH4→Na3AlH6/Al͒, while the second step
͑Na3AlH6→NaH/Al͒ was left almost intact. These results
clearly indicate that the active Ti-species that are involved in
the reversible dehydrogenation of Na3AlH6 may not neces-
sarily contribute to the kinetic enhancement of
NaAlH4/Na3AlH6+Al reactions. The possible reasons for
the aggravated “ “selective” deactivation” of Ti-species upon
increasing the milling time include the modification of
structural/chemical environment of the doped hydride, the
distribution state and even the nature of the active Ti-species.
Here, a key clue is available from the direct formation of
␤-Na3AlH6 because the in situ hydrogenation during the
milling process may produce a critical influence on the dis-
tribution of the active Ti-species. Further investigation in this
respect is currently under way.
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