Mechanochemical synthesis of erbium borohydride: Polymorphism, thermal decomposition and hydrogen storage

Article abstract of DOI:10.1016/j.jallcom.2013.07.012

A new erbium borohydride Er(BH₄)₃ was synthesized from 3LiBH4 and ErCl3 through mechanochemical processing. This rare-earth metal borohydride presents a primitive cubic structure with a = 10.74(1) A, which is isostructural with the one previously reported for R(BH₄)₃ (R = Y, Dy and Gd). During heating Er(BH₄)₃ exhibits a reversible structural transformation at about 220 °C, analogous to that observed for Y(BH₄)₃. Combination of thermal hydrogen desorption, DSC, XRPD and FTIR measurements allows to determine that thermal decomposition of Er(BH₄)₃ starts at 230 °C and leads to 3.2 wt% of hydrogen release obtaining ErH₂, an unknown intermediate compound and ErB₄. It was observed that Er(BH ₄)₃ is partially reversible under 6.0 MPa of H₂ at 400 °C and absorbs about 20% of the total hydrogen capacity obtained experimentally.

Full text of DOI:10.1016/j.jallcom.2013.07.012

Journal of Alloys and Compounds 581 (2013) 192–195
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Letter
Mechanochemical synthesis of erbium borohydride: Polymorphism,
thermal decomposition and hydrogen storage
⇑
F.C. Gennari
Consejo Nacional de Investigaciones Científicas y Técnicas, CONICET, Argentina
Centro Atómico Bariloche (CNEA) and Instituto Balseiro, U.N. Cuyo R8402AGP, S. C. de Bariloche, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
A new erbium borohydride Er(BH4)3 was synthesized from 3LiBH4 and ErCl3 through mechanochemical
processing. This rare-earth metal borohydride presents a primitive cubic structure with a = 10.74(1) Å,
which is isostructural with the one previously reported for R(BH4)3 (R = Y, Dy and Gd). During heating
Er(BH4)3 exhibits a reversible structural transformation at about 220 °C, analogous to that observed for
Y(BH4)3. Combination of thermal hydrogen desorption, DSC, XRPD and FTIR measurements allows to
determine that thermal decomposition of Er(BH4)3 starts at 230 °C and leads to 3.2 wt% of hydrogen
release obtaining ErH2, an unknown intermediate compound and ErB4. It was observed that Er(BH4)3 is
partially reversible under 6.0 MPa of H2 at 400 °C and absorbs about 20% of the total hydrogen capacity
obtained experimentally.
Received 29 March 2013
Received in revised form 20 June 2013
Accepted 2 July 2013
Available online 13 July 2013
Keywords:
Rare-earth metal
Borohydrides
Mechanical milling
Phase transition
Hydrogen storage
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
In the search of new metal borohydrides with lower thermody-
namic stabilities than those with ionic character, the rare-earth
Metal borohydrides M(BH4)n (where n indicates the valence of
metal M) are being actively investigated as potential hydrogen
storage materials [1,2]. Alkali and alkali-earth metal borohydrides
such as LiBH4, Mg(BH4)2 and Ca(BH4)2 are of great interest due to
their high gravimetric hydrogen storage capacities [3–5]. Unfortu-
nately, decomposition temperatures are usually high and rehydrid-
ing only occurs under severe conditions due to the strong ionic
interactions between the metal cation and the complex [BH4]ꢀ an-
ion. In the last years, acorrelation between thermodynamical sta-
bilities of metal borohydrides M(BH4)n (for M = Li, Na, K, Cu, Mg,
Zn, Sc, Zr, Hf and Ti) and cation electronegativity was experimen-
tally and theoretically investigated [6]. The results revealed that
the charge transfer from Mn+ cations to [BH4]ꢀ anions is a key fea-
ture for the stability of M(BH4)n. The hydrogen desorption temper-
ature of M(BH4)n decreases with increasing electronegativity of M.
Therefore, the electronegativity is an indicator to estimate the sta-
bility of M(BH4)n. This suggests that development and investiga-
tion of new metalborohydrides may be a fruitful approach to
obtain more favorable thermodynamic properties and the hydro-
gen release temperature.
metals result interesting due to their electronegativities compris-
ing values between 1.12 and 1.25. In the past, some of these com-
plex hydrides were prepared by wet-chemistry routes, for example
forming Ln(BH4)3ꢁ3THF complex (Ln = La, Nd, Gd, Er and Y) [7,8].
However, the elimination of the THF solvated molecules occurs
at about the same temperature that hydrogen is evolved, leading
to the irreversible decomposition of the borohydrides. In this con-
text, the application of a solvent-free synthesis route such as the
dry mechanochemical technique constitutes a powerful alterna-
tive[9]. This approach has recently been applied for the synthesis
of rare-earth metal borohydrides R(BH4)3 (R = Y, Dy, Gd) by ball
milling of anhydrous RCl3 and LiBH4 [10]. Their crystal structure
ꢀ
was shown to have a primitive cubic structure (space group Pa3),
with the [BH4]ꢀ complex anions located on the edges of a distorted
cube composed of R3+. Frommen et al. synthesized a high-temper-
ature (HT) structure of Y(BH4)3 (space group Fm3c) by heating of
a
-Y(BH4)3 (low temperature, LT) structure to 203 °C under
10 MPa of hydrogen for 60 h and then cooling to ambient condi-
tions [11]. In addition, HT–Y(BH4)3 was also observed at room tem-
perature after milling of the LiBH4–YCl3 mixture [12,13] as well as
after heating of LT–Y(BH4)3 to 194–216 °C [14], simultaneously
with the LT–Y(BH4)3. Several studies showed that Y(BH4)3 desorbs
hydrogen at temperatures ꢂ200–220 °C, evidencing a lower ther-
modynamic stability respect to both alkali and alkali-earth metal-
borohydrides [11–16]. Similar thermodynamic stability was
displayed by Gd(BH4)3, with decomposition starting at ꢂ210 °C
⇑
Address: Instituto Balseiro (U. N. Cuyo), Centro Atómico Bariloche (CNEA) and
Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), R8402AGP, S.
C. de Bariloche, Argentina. Tel.: +54 2944 445118; fax: +54 2944 445190.
E-mail address: gennari@cab.cnea.gov.ar
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.07.012

F.C. Gennari / Journal of Alloys and Compounds 581 (2013) 192–195
193
[17]. Recently, the formation of new cubic structure of Ce(BH4)3
Er(BH4 3
)
(A)
was reported, which starts to release hydrogen at about 200 °C
[18]. Following this work, the formation/decomposition of
La(BH4)3 with the same structure of Ce(BH4)3, was investigated
[19]. In a further study was clarified that the mechanochemical
reaction between LiBH4 and CeCl3 leads to the formation of
LiCe(BH4)3Clcompound, the first mixed-metal and anion-substi-
tuted rare earth borohydride. LiCe(BH4)3Cl crystallizes in the cubic
x LiCl
X
X
X
X
ꢀ
ꢀ
space group I43m [20]. No additional information was reported in
the literature regarding other rare-earth metal borohydrides.
In the present paper, the new solvent-free Er(BH4)3 borohydride
is synthesized by mechanochemical processing. Its crystal struc-
ture, thermal decomposition reaction and hydrogen storage prop-
erties are reported. Combination of XRD, thermal hydrogen
desorption, DSC and FTIR was employed to understand the decom-
position pathway and to identify the occurrence of a structural
transition in Er(BH4)3.
10
20
30
40
50
60
2θ (deg)
(B)
2. Experimental
The starting materials LiBH4 (purity P90%), NaBH4 (purity P99.9%) and anhy-
drous ErCl3 (purity: 99.99%) were purchased from Sigma-Aldrich. The 3LiBH4–ErCl3
and 3NaBH4–ErCl3 mixtures were mecanochemically milled at 200 rpm in argon
atmosphere during 5 h using a Frisch P6 planetary mill. To avoid temperature rising
during the experiment, milling times of 10 min were alternated with 10 min of rest.
All materials were handled in an argon-filled glove box (MBraum Unilab) with
moisture and oxygen levels below 1 ppm.
Dehydriding experiments were conducted under non-isothermal conditions
(heating rates of 5 °C/min or 10 °C/min) from 20 to 400 °C using a modified Sie-
verts-type device coupled with a mass flow controller. The hydrogen pressure dur-
ing desorption was fixed at a constant value of 0.02 MPa or 0.5 MPa. Hydrogen
absorption was performed at 400 °C and 6.0 MPa of hydrogen pressure. The thermal
behavior of the samples was analyzed by DSC using a heating ramp of 5 °C/minꢀ1
and argon flow rate of 122 ml/min. About 5–7 mg of sample was loaded into alumi-
num capsules hermetically closed in the glove box. Structural changes were studied
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
Fig. 1. (A) X-ray powder diffraction pattern of as-synthesized LT–Er(BH4)3 by
mechanochemical processing of the 3LiBH4–ErCl3 mixture. (B) The FTIR spectra of
the LT–Er(BH4)3.
in the same cubic structure than b-Y(BH4)3, i.e. the high-
temperature polymorph of Y(BH4)3. Therefore, in a similar way
to previous investigations on Y(BH4)3 [12,13], the formation of
high-temperature HT–Er(BH4)3 structure was possible via mechano-
chemical processing.
by X-ray Powder Diffraction (XRPD, Philips PW 1710/01 Instruments), using CuK
a
radiation and graphite monochromator. Cell parameter of the as-milled product
was calculated with CELREF software [21]. The samples were sealed in a special
holder inside the glovebox to completely prevent the reaction with air during XRPD
measurements. IR spectra were obtained using FTIR Perkin Elmer Spectrum 400
spectrometer in the range of 800–4000 cmꢀ1. The gas phase released during milling
and non-isothermal heating of the 3LiBH4–ErCl3 mixture was collected in a de-
gassed quartz optical cell with NaCl windows and gas phase spectra at room tem-
perature were taken. For solid-state IR spectroscopy measurements, the selected
samples were grounded with dry KBr under purified argon atmosphere, pressed
to pellets and put in a specially designed cell. Handling was done inside the glove
box to avoid contact with air.
When NaBH4 is used as starting material instead of LiBH4, the
mechanochemical synthesis was unsuccessful to produce Er(BH4)3:
3 NaBH4 þ ErCl3—X ! ErðBH4Þ3 þ 3 NaCl
ð2Þ
The XRPD pattern of the post-milled sample (Supplementary mate-
rial, Fig. S2) evidences the strong amorphization of the sample,
while the most intense diffraction peaks of NaBH4 and YCl3 are
hardly identified. The reaction yield was unnoticeable for this sam-
ple after 5 h of milling, while no evidence of the NaCl formation was
detected.
3. Results and discussion
Synthesis of Er(BH4)3 was also confirmed by infrared spectros-
copy. The vibrational spectra of LT–Er(BH4)3 is shown in Fig. 1B.
The BAH stretching modes are split into three groups at about
2556, 2482 and 2304 cmꢀ1 (Fig. 1B). In addition, strong BAH bend-
ing bands at 1127 and 1211 cmꢀ1 are identified as well as an incip-
ient band around 1352 cmꢀ1. The position of these bands yields
excellent resemblance with the previously reported for LT–
Y(BH4)3 produced by ball milling [16].
Thermal decomposition of the LT–Er(BH4)3 was investigated by
combination of non-isothermal hydrogen desorption and DSC
measurements. Fig. 2 shows the amount of hydrogen desorbed un-
der 0.02 MPa of hydrogen pressure. The hydrogen desorption
started at 230 °C; this temperature was similar than the initial
dehydriding temperature for analogous rare-earth metal borohy-
dride [11–20]. The total amount of hydrogen released after heating
up to 400 °C was 3.2 wt%, taking into account the total mass of the
Er(BH4)3–LiCl mixture. This amount corresponds to 5.1 wt% when
the estimation is made respect to LT–Er(BH4)3. Diborane (B2H6)
and other gas impurities were not detected by gas FTIR during
thermal treatment up to 400 °C, thus only hydrogen release take
place during heating process. This is one of the conditions for
reversible hydrogen storage in borohydrides. Moreover, multiple
peaks observed in DSC curve evidence the complexity of the
Fig. 1A shows the XRPD profiles of the 3LiBH4–ErCl3 mixture
after 5 h of milling. In this Figure, a new set of unidentified Bragg
peaks and those corresponding to LiCl are observed. The new set
of diffraction lines were assigned to a single phase compound
Er(BH4)3. The structure belongs to the primitive cubic cell with a
lattice constant of a = 10.74(1) Å, which is isostructural to the pre-
viously reported by Sato et al. [10] for other trivalent rare-earth
metal borohydrides R(BH4)3, with R = Y, Dy and Gd. Cell parameter
of Er(BH4)3 exhibits a decrease in comparison with Gd(BH4)3 and
Dy(BH4)3, in correlation with a reduction of ionic radii. No traces
of the starting materials are detected. These results clearly indicate
that after ball milling of 3LiBH4–ErCl3 mixture for 5 h, LiBH4 had
completely reacted with ErCl3 yielding a mixture composed by
LT–Er(BH4)3 and LiCl. The metathesis reaction activated by mecha-
nochemical processing can be described as:
3 LiBH4 þ ErCl3 ! ErðBH4Þ3 þ 3 LiCl
ð1Þ
Minor traces of B2H6 were detected by FTIR analysis in the
gaseous atmosphere of the milling chamber. Additional milling for
7 h induces the apparition of some new diffraction peaks
(see Supplementary material, Fig. S1). These peaks were indexed

194
F.C. Gennari / Journal of Alloys and Compounds 581 (2013) 192–195
thermal decomposition process of LT–Er(BH4)3, similar to those
occurring for other complex borohydrides [11–20].
o
&
?
$
X LiCl
ErH2
$
ErB4
Unknown
HT-Er(BH4)3
X
Extra XRPD measurements were done to obtain information on
the dehydriding steps. The LT–Er(BH4)3 was heated to different
temperatures in correlation with DSC curve and subsequently cool-
ing down to room temperature. The XRPD profiles of the samples at
each temperature are shown in Fig. 3. In all XRPD patterns the
presence of LiCl is clearly identified. As compared to the diffraction
peaks of the as-milled LT–Er(BH4)3 (Fig. 3a), some additional dif-
fraction peaks appears in the sample heated at 220 °C (Fig. 3b).
Taking into account the absence of hydrogen desorption up to
230 °C, the presence of an endothermic peak before this tempera-
ture (Fig. 2) and the previous structural transformation observed
during milling, these peaks can be indexed based on the same cu-
bic cell as reported for b-Y(BH4)3 [11]. Then, the endothermic peak
between 210 and 230 °C could be at least partially associated with
the LT to the HT structural transition of Er(BH4)3. After non-
isothermal heating up to 250 °C, a 0.6 wt% of hydrogen release
occurs and a second endothermic peak is identified. In correlation,
the diffraction peaks of Er(BH4)3 disappeared while those of
the ErH2 together with some unidentified peaks are detected
(Fig. 3c). These unidentified peaks could imply the formation of
an intermediate phase. It seems likely that Er–B–H containing spe-
cies are formed during the decomposition of Er(BH4)3, by analogy
to those found in other borohydrides [19,20]. When the tempera-
ture is increased up to 400 °C, the complete decomposition of
Er(BH4)3 occurs. The diffraction peaks observed in Fig. 3d are weak
and the background suggests the presence of an amorphous phase.
From Fig. 3e, this background could partially be associated with the
formation of ErB4, which is visibly detected as a crystalline phase at
500 °C. In opposition, in these last two XRD patterns (Fig. 3d and e)
the peaks corresponding to ErH2 are not clearly observed, while the
most intense diffraction peak of ErH2 (i.e. 2h = 30.19°) is superim-
posed with that of LiCl. In fact, the progress of the Er(BH4)3 decom-
position between 250 °C and 400 °C could involve the complete
disappearance of the Er–B–H intermediate compound as well as
the partial consumption of the ErH2. Then, as temperature in-
creases the amount of ErH2 decreases, probably influencing the
crystallinity of ErH2. Taking into account that from XRPD the for-
mation of ErH2 and ErB4 is identified (Fig. 2d) and total hydrogen
released at 400 °C is 3.2 wt% (Fig. 1A), the following reaction could
be proposed as overall dehydrogenation path for Er(BH4)3:
X
$
X
$
$
$
$
$
X
$
(e)
(d)
&
$
500 oC
400 oC
250 oC
?
o
?
o
o
?
(c)
o
&
&
&
&
&
&
&
(b)
(a)
220 oC
&
&
as-milled
10
20
30
40
50
60
2θ (deg)
Fig. 3. X-ray powder diffraction patterns of: (a) as-synthesized LT–Er(BH4)3 and
heated up to, (b) 220 °C, (c) 250 °C, (d) 400 °C and (e) 500 °C.
Additional DSC measurements were performed to corroborate
the nature of the first endothermic event observed at about
220 °C (Fig. 2). The as-milled LT–Er(BH4)3 was submitted to two
consecutive thermal cycling (Fig. 4): first, a heating up to 225 °C
and cooling down to 80 °C; second, a heating up to 400 °C. For
comparison, the reference DSC curve for the as-synthesized LT–
Er(BH4)3 is included (Fig. 4a). It is clearly observed from Fig. 4b that
the first endothermic peak at 220 °C is reversible during cooling. In
fact, the second cycle clearly displays practically the same thermal
decomposition curve than that of the as-milled sample (Fig. 4a).
This experimental behavior, in combination with XRPD data
(Fig. 3b), demonstrate that the first endothermic peak involves
the structural transition from the low to high temperature modifi-
cation of Er(BH4)3. However, due to the shape of the first peak
where two events seems to be superimposed (see Fig. 4a), the
simultaneous occurrence of the Er(BH4)3 melting cannot be ruled
out. On the other hand, Fig. 4c shows the thermal behavior of the
sample heated at 250 °C and cooled down at room temperature
(XRPD shown in Fig. 3c). The thermal behavior corresponds to
the third endothermic peak, verifying that this sample constitutes
an intermediate phase containing hydrogen. Therefore, XRPD and
ErðBH4Þ3ðsÞ ! 0:25 ErH2ðsÞ þ 0:75 ErB4ðsÞ þ 5:75 H2ðgÞ
ð3Þ
Theoretical amount of hydrogen released from Er(BH4)3 according
to reaction (3) is about 3.4 wt%, when the presence of LiCl is consid-
ered (5.4 wt% respect to Er(BH4)3). The actually measured value of
3.2 wt% is in good agreement with that calculated for reaction (3).
(c)
cooling 1
(b)
heating 2
100
200
300
400
230 oC
heating 1
250oC
0
-1
-2
-3
-4
(a)
ENDO
Heating ramp: 5 oC/min
100
50
100
150
200
250
300
C)
350
400
200
300
400
Temperature (o
Temperature (oC)
Fig. 4. DSC curves of: (a) as-synthesized LT–Er(BH4)3; (b) as-synthesized LT–
Er(BH4)3 heated to 225 °C, cooled down to 80 °C and heated again to 400 °C; (c) LT–
Er(BH4)3 pre-heated at 250 °C.
Fig. 2. Non-isothermal hydrogen desorption and DSC curve of as-synthesized LT–
Er(BH4)3.

F.C. Gennari / Journal of Alloys and Compounds 581 (2013) 192–195
195
DSC studies (Figs. 3 and 4) evidence the multi-step nature of the
Acknowledgments
dehydriding process of Er(BH4)3.
Hydrogen storage reversibility of Er(BH4)3–3LiCl was explored
using a Sieverts-type apparatus. The sample was first heated up
to 400 °C at a heating rate of 10 °C/min under 0.5 MPa of hydrogen
pressure, and 3.4 wt% of hydrogen was released (about 5.4 wt% of
hydrogen respect Er(BH4)3). After rehydriding at 400 °C and
6 MPa of hydrogen pressure during 6 h, a second dehydriding run
was performed up to 400 °C under 0.5 MPa. The hydrogen des-
orbed after the second run was 0.7 wt% (see Supplementary mate-
rial, Fig. S3), which corresponds to ꢂ20% of the initial hydrogen
release obtained after the first dehydriding run and demonstrates
that the system is only partially reversible. Similar behavior was
previously observed for analogous trivalent rare-earth metal boro-
hydrides [13,15,17]. To transform the 3LiBH4–ErCl3 mixture in a
more attractive material for hydrogen storage and by analogy with
LiBH4–RCl3 systems (R = Ce, Gd, La, Y) [13,22,23], the stoichiometry
of the initial mixture could be changed to influence the nature of
the decomposition products and to improve the reversibility.
This study has been partially supported by CONICET (National
Council of Scientific and Technological Research), CNEA (National
Commission of Atomic Energy), ANPCyT (PICT No. 1049) and Insti-
tuto Balseiro (University of Cuyo).
Appendix A. Supplementary material
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jallcom.2013.
07.012.
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spectroscopy and volumetric measurements. The material was
found to exist in a low-temperature distorted modification that
exhibits a phase transition to an ordered high-temperature phase,
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reabsorbs about 20% of its original hydrogen content at 400 °C
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