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J. Fu et al. / Journal of Alloys and Compounds 525 (2012) 73–77
ambiguous. Since Balema et al. attributed the high catalytic activity
of TiCl4 to the in situ formation of a microcrystalline Al3Ti phase
[15], the effect of Al3Ti and other TM trialuminides on LiAlH4 was
investigated [8,17,18].
From the literature reports it can be concluded that doping
LiAlH4 by high-energy ball milling causes
a serious draw-
capacity available for later use is considerably reduced.
Further, it has to be noted that most studies on the dehydrogena-
tion kinetics of TM-doped LiAlH4 were performed either in vacuum
or in argon atmosphere [3,8–12,14,16], which is different from real-
istic operation conditions where a hydride reservoir should provide
hydrogen for a fuel cell at elevated H2 pressures.
In order to overcome this disadvantage of doping LiAlH4 by ball
milling, the purpose of this study was to dope pre-milled LiAlH4
with TMs by low-energy grinding in order to prevent unwanted
dehydrogenation during preparation. Three typical TM chlorides
(ZrCl4, TiCl3 and NiCl2) were chosen as additives. Their effect on
promoting the dehydrogenation properties of LiAlH4 (i.e. reduc-
ing the onset temperature of dehydrogenation and enhancing the
dehydrogenation rate) and their influence on the total amount of
conditions in combination with hydrogen fuel cells. In this regard,
the dehydrogenation of TM-doped LiAlH4 was examined under
isothermal conditions at 80 ◦C, which is the operating temperature
of hydrogen fuel cells [19]. Furthermore, the long-term dehydro-
genation behavior of TM-doped LiAlH4 at room temperature was
monitored up to seven months in order to test its storability.
Fig. 1. XRD patterns of (a) dehydrated NiCl2, (b) as-received LiAlH4, (c) pre-milled
LiAlH4, (d) NiCl2-doped LiAlH4, (e) ZrCl4-doped LiAlH4 and (f) TiCl3-doped LiAlH4.
2.6. Room temperature dehydrogenation
The freshly doped samples have been kept in gas-tight vials (6 ml) inside an Ar
glove box for up to 35 days to test their stability at room temperature (25 ◦C). The
dehydrogenation kinetics was monitored based on the samples’ weight loss (sample
mass: 1 g) over time via an electronic balance (Sartorius, precision: 1 mg).
2. Materials and methods
TM-doped LiAlH4
2.1. General
doped LiAlH4. LiCl peaks are observed in as-received LiAlH4 as
impurity (Fig. 1b). After pre-milling of the LiAlH4 powder for
30 min, no decomposition can be detected (Fig. 1c). The peaks of
ZrCl4 and TiCl3 can be easily found in the XRD patterns of ZrCl4-
doped LiAlH4 (Fig. 1e) and TiCl3-doped LiAlH4 (Fig. 1f), which
under the applied grinding conditions. However, a partial reaction
cannot be eliminated owing to the potential formation of nanopar-
ticles and their relatively small molar fraction. The XRD pattern of
NiCl2-doped LiAlH4 (Fig. 1d) shows only the characteristic peaks of
LiAlH4 but no NiCl2 peaks. This can be explained by the fact that
the dehydrated NiCl2 used for doping is amorphous according to
the XRD pattern of Fig. 1a. In no case the formation of Li3AlH6 was
detected which indicates that TM chlorides do not immediately
All samples were prepared and handled in a glovebox (MBraun) under argon
(5.0 purity) to prevent unwanted oxidation.
2.2. Materials
Powdery lithium aluminum hydride (99% purity) and zirconium (IV) chloride
(98% purity) were purchased from Alfa-Aesar. Titanium (III) chloride (≥98.5% purity)
was purchased from Sigma–Aldrich. Nickel (II) chloride hexahydrate (≥98% purity)
was obtained from VWR and dehydrated under vacuum for 3 h at 250 ◦C. The color
change from blue to yellow indicated the formation of anhydrous Nickel (II) chloride.
2.3. Pre-milling and doping procedure
steel vial and steel balls with a ball-to-powder weight ratio of 20:1 and a rotation
speed of 300 rpm. For low-energy doping, the pre-milled LiAlH4 was ground by hand
with a pestle in a mortar for 10 min with 2 mol% ZrCl4, TiCl3 and NiCl2, respectively.
In addition, 5 mol% TiCl3 was doped into LiAlH4 in the same manner. This 5 mol%
TiCl3-doped LiAlH4 was only used for comparison of the dehydrogenation kinetics
with the 2 mol% TiCl3-doped LiAlH4 sample (Fig. 5).
SEM investigations reveal that the as-received LiAlH4 powder
tion ranging from a few micrometers up to several ten micrometers
(Fig. 2a). For comparison, the particle size of pre-milled LiAlH4
doped with different TM chlorides is in the range of several
micrometers. For example, Fig. 2(b) exhibits the SEM micrograph of
pre-milled LiAlH4 doped with ZrCl4 in BSE mode. Obviously, most of
the powder particles are smaller than 10 m and they are agglom-
erated. A few large LiAlH4 particles with dimensions in the 30 m
range are still found after pre-milling. The ZrCl4 particles (bright
spots in Fig. 2b, exemplarily indicated by arrows) are homoge-
neously distributed in the LiAlH4 powder bed. Similar results in
view of particle size and dopant distribution have been observed
for the other samples. Therefore, effects like catalyst distribution
2.4. Structural analysis
X-ray diffraction (XRD) was performed with a Bruker D8 Advance using Cu-K␣
radiation at a tube voltage of 40 kV and a tube current of 40 mA. The scanning range
of the diffraction angle (2ꢀ) was 10–100◦. All samples were permanently covered
with a capton foil to avoid any unwanted oxidation. The morphology of the powder
has been analyzed in an EVO 50 ZEISS scanning electron microscope (SEM) using
detectors for back-scattered electrons (BSE).
2.5. Dehydrogenation kinetics
Measurements of dehydrogenation kinetics were carried out in a magnetic sus-
pension balance (Rubotherm) with a precision of 10 g under a hydrogen (6.0 purity)
back pressure of 1 bar. The respective sample mass was about 250 mg. A heating rate
of 1 K/min was chosen from 0 ◦C to 220 ◦C.