Ca(BH4)2-LiBH4-MgH2: A novel ternary hydrogen storage system with superior long-term cycling performance

Article abstract of DOI:10.1039/c3ta12472j

A ternary hydrogen storage system, of superior cyclic stability and high capacity, was developed from a mixture of Ca(BH₄)₂, LiBH₄ and MgH₂ in molar ratios of 1:2:2. Investigation on both non-isothermal and isothermal hydrogen desorption/absorption properties shows that the hydrogen desorption starts from 320 °C and completes at 370 °C under a heating rate of 2 °C min^-1, releasing ca. 8.1 wt% H₂. The finishing temperature of desorption is much lower and the capacity much higher than any of the two-hydride mixtures in the ternary system. In particular, hydrogenation of the ternary system initiates at an extremely low temperature of ca. 75 °C and the onset dehydrogenation temperature is significantly reduced by 90 °C after the initial dehydrogenation/ hydrogenation cycle, which is ascribed to the formation of an active dual-cation hydride of CaMgH_3.72 for dehydrogenation in the hydrogenation process. There is ca. 7.6 wt% H₂ absorbed at 350 °C and 90 bar H₂ for 18 h for the system post-dehydrogenated at 370 °C for 30 min, demonstrating a reversibility of over 94%. The capacity seems to fade mainly in the initial few cycles and stabilizes after further cycling. The reversibility is as high as 97% and a dehydrogenation capacity of ca. 6.2 wt% H₂ at the 10^th cycle. Favourable kinetics and thermodynamics of hydrogen desorption/absorption are achieved, which are responsible for the low completion temperature and the superior cycling performance. Mechanisms of the improved dehydrogenation/hydrogenation properties including the cyclic behaviour of the system are also proposed in relation to microstructural analyses.

Full text of DOI:10.1039/c3ta12472j

Journal of
Materials Chemistry A
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Ca(BH4)2–LiBH4–MgH2: a novel ternary hydrogen
storage system with superior long-term cycling
Cite this: J. Mater. Chem. A, 2013, 1,
12285
performance†
a
a
a
Mingxia Gao, Jian Gu, Hongge Pan, Yiliu Wang,a Yongfeng Liu,a Chu Lianga
*
and Zhengxiao Guob
A ternary hydrogen storage system, of superior cyclic stability and high capacity, was developed from a
mixture of Ca(BH4)2, LiBH4 and MgH2 in molar ratios of 1 : 2 : 2. Investigation on both non-isothermal
and isothermal hydrogen desorption/absorption properties shows that the hydrogen desorption starts
from 320 ꢀC and completes at 370 ꢀC under a heating rate of 2 ꢀC minꢁ1, releasing ca. 8.1 wt% H2. The
finishing temperature of desorption is much lower and the capacity much higher than any of the two-
hydride mixtures in the ternary system. In particular, hydrogenation of the ternary system initiates at an
extremely low temperature of ca. 75 ꢀC and the onset dehydrogenation temperature is significantly
reduced by 90 ꢀC after the initial dehydrogenation/hydrogenation cycle, which is ascribed to the
formation of an active dual-cation hydride of CaMgH3.72 for dehydrogenation in the hydrogenation
process. There is ca. 7.6 wt% H2 absorbed at 350 ꢀC and 90 bar H2 for 18 h for the system post-
dehydrogenated at 370 ꢀC for 30 min, demonstrating a reversibility of over 94%. The capacity seems to
fade mainly in the initial few cycles and stabilizes after further cycling. The reversibility is as high as 97%
and a dehydrogenation capacity of ca. 6.2 wt% H2 at the 10th cycle. Favourable kinetics and
thermodynamics of hydrogen desorption/absorption are achieved, which are responsible for the low
completion temperature and the superior cycling performance. Mechanisms of the improved
dehydrogenation/hydrogenation properties including the cyclic behaviour of the system are also
proposed in relation to microstructural analyses.
Received 25th June 2013
Accepted 7th August 2013
DOI: 10.1039/c3ta12472j
www.rsc.org/MaterialsA
Introduction
Ever-growing needs in energy, the cumulative depletion of fossil
fuels and the rapid increase of greenhouse gases have prompted
the development of clean and renewable energy sources.
Hydrogen is considered one of the most promising clean energy
carriers. The US Department of Energy's (DOE) targets renewed
in 2009 proposed that >5.5 wt% hydrogen storage capacity with
favourable cycling properties has been deemed necessary for
commercial development of vehicles powered by H2/O2 proton
exchange membrane fuel cells.1 Alkali and alkali-earth metal
borohydrides, which possess high gravimetric and volumetric
hydrogen densities, have attracted extensive attention as
promising hydrogen storage materials.2–4 Among these, lithium
borohydride, LiBH4, has been widely investigated due to its high
gravimetric hydrogen density (18.5 wt%), but its dehydrogena-
tion is hampered by unfavourable thermodynamics (74 kJ
molꢁ1 H2), leading to a desorption temperature over 400 ꢀC. Its
absorption kinetics is also sluggish, even at 150–350 bar of
hydrogen at 600–650 ꢀC.5 The calculated dehydrogenation
reaction enthalpy of calcium borohydride, Ca(BH4)2, is 32 kJ
molꢁ1 H2,6 which is slightly lower than the appropriate value for
hydrogen desorption reaction for fuel cells (ca. 39 kJ molꢁ1 H2).7
aState Key Laboratory of Silicon Materials & Department of Materials Science and
Engineering, Zhejiang University, Hangzhou, 310027, P.R. China. E-mail: hgpan@
zju.edu.cn; Fax: +86-571-87952615; Tel: +86-571-87952615
bDepartment of Chemistry, University College London, London, WC1H 0AJ, UK
† Electronic supplementary information (ESI) available: XRD pattern of the
as-synthesized Ca(BH4)2, XRD patterns and FTIR spectra of the selected
dehydrogenation intermediates and product of the Ca(BH4)2 + 2LiBH4 + 2MgH2
ternary system at different dehydrogenation temperatures; SEM images of the
as-milled Ca(BH4)2
+
2LiBH4
+
2MgH2 mixture, and its products
dehydrogenated at 370 ꢀC for 30 min, and re-hydrogenated at 350 ꢀC for 18 h
aer different cycles; XRD patterns of the dehydrogenation and
re-hydrogenation products of the ternary system dehydrogenated at 370 ꢀC for
30 min, and hydrogenated at 350 ꢀC for 18 h for different cycles, as-synthesized
CaMgH3.72, CaMgH3.72 heated to 260 ꢀC and the 1st round re-hydrogenated
product of the ternary system heated to 260 ꢀC and dwelling for 10 min;
dehydrogenation curves of the individual CaMgH3.72 and Ca(BH4)2 + 2LiBH4
+
2MgH2 system of the 1st and the 2nd round cycles, and the initial
dehydrogenation curves of (1 ꢁ 1.5x)Ca(BH4)2 + 2LiBH4 + (2 ꢁ x)MgH2
+
xCaMgH3.72 (x ¼ 0.1, 0.2) systems; tables of desorption rates of the main
desorption stages of the different systems and the apparent activation energy
(Ea) and the dehydrogenation reaction enthalpy (OH) of the as-milled different
systems. See DOI: 10.1039/c3ta12472j
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Both the gravimetric and volumetric hydrogen densities (ca. Ca(BH4)2 + MgH2 system was signicantly improved compared
11.4 wt% and 130 kg mꢁ3, respectively) of Ca(BH4)2 are also with that of Ca(BH4)2, despite the different synthesis methods
favourable as a hydrogen storage material.8,9
and dehydrogenation processes of the systems tested.23,24 An
Ca(BH4)2 decomposes into CaB6 and CaH2 with 9.6 wt% of initial reversibility of 60% was obtained at 350 ꢀC and 90 bar of
hydrogen evolved upon heating to 500 C.10 The onset decom- H2 for 24 h without association of catalysts for the Ca(BH4)2 +
ꢀ
position temperature is theoretically <100 ꢀC at 1 bar H2,11 MgH2 system post-dehydrogenated at 400 ꢀC, whereas Ca(BH4)2
which seems to satisfy the requirement for practical applica- was not reversible under the same conditions.24 However, the
tions.1 However, its sluggish kinetics, due to a high reaction reversibility of the Ca(BH4)2 + MgH2 system is still too harsh,
barrier, leads to a high operating temperature (350 ꢀC) and poor even without considering the long-term cyclic properties.
reversibility (e.g., an initial reversibility of 60% requires hydro-
genation at 400 C for 48 h under 700 bar of H2 with the addi- sorption capacity, reversibility and reducing the operating
tion of TiCl3 and Pd as catalysts).12
So far, progress has been made in improving hydrogen
ꢀ
temperature of LiBH4 and Ca(BH4)2 systems, via catalysis,
To lower the operating temperature and improve the hydride mixing, and reactive hydride composites, etc. However,
reversibility of Ca(BH4)2, various methods have been adopted, previous investigations of the reversibility focus mostly on the
such as the addition of catalysts,13–15 combination with metal initial cycle. In the present study, we develop a ternary system of
amide hydrides16,17 and other borohydrides.18–20 Our recent work Ca(BH4)2 + 2LiBH4 + 2MgH2 (in molar ratios). By simply mixing
reveals that the main desorption temperature of Ca(BH4)2 is these three hydrides, the obtained system possesses favourable
reduced by more than 50 ꢀC via an in situ formed super-ne TiO2 overall hydrogen storage performance, including a relatively
and a porous structure by the introduction of Ti(OEt)4.14 narrow temperature range and a low completion temperature of
However, the reversibility and reversible capacity still need to be dehydrogenation, a signicantly low hydrogenation tempera-
further improved. The combination of Ca(BH4)2 with LiBH4 was ture, especially
a remarkably reduced dehydrogenation
found to possess a decreased dehydrogenation temperature and temperature aer the initial cycle with superior long-term cyclic
an increased reversible hydrogen storage capacity, controver- stability. A systematic study was performed on the non-
sially due to the existence of eutectic melting18 or the formation isothermal and isothermal hydrogen desorption/absorption
of a dual-cation borohydride LiCa(BH4)3.19 Enhanced kinetics is properties of the ternary system, along with microstructural
also obtained from a mixture of Ca(BH4)2 and Mg(BH4)2, which characterization during desorption/absorption. A mechanism
is superior to either of the individual constituents.20 However, for the sorption process is proposed for further improvement of
the previous reported improvements in hydrogen desorption/ the system.
absorption properties of binary Ca(BH4)2 systems with other
hydrides usually require catalysts,16,18 which reduces the overall
capacity to some extent. Even for some high capacity systems,
Experimental section
the reversibility is still very limited. For example, only 3.2 wt% LiBH4 (95%) and MgH2 (98%) were commercially purchased
H2 was absorbed for a 0.02NbF5-doped 0.4LiBH4 + 0.6Ca(BH4)2 from Sigma-Aldrich and Alfa-Aesar, respectively. Ca(BH4)2
composite under 400 ꢀC and 90 bar of H2 in the rst (96%) was synthesized by the metathesis reaction of CaCl2 and
cycle,18 though its theoretical hydrogen content is as high as ca. NaBH4 in tetrahydrofuran according to our previous work.14
A
8.0 wt%.
mixture of Ca(BH4)2 + 2LiBH4 + 2MgH2 in molar ratios was ball-
The hydrogen storage capacity of MgH2 is 7.6 wt%, lower milled for 24 h on a planetary ball mill (QM-3SP4, Nanjing)
than those of either LiBH4 or Ca(BH4)2. The onset and rotating at 500 rpm. The ball-to-powder weight ratio was 120 : 1.
completion dehydrogenation temperatures of pristine MgH2 are
X-ray diffraction (XRD, Phillips X0 pert Pro) using Cu Ka
high, and its kinetics is relatively low.21 However, MgH2 plays radiation at 40 kV/40 mA with a step width of 0.05ꢀ was carried
effective roles in improving the hydrogen sorption properties of out to determine the phase structure of the as-milled Ca(BH4)2 +
LiBH4 and Ca(BH4)2. Vajo et al.22 proposed rstly a concept of 2LiBH4 + 2MgH2 and its products at different desorption/
destabilizing the 2LiBH4 + MgH2 system so that LiBH4 can be absorption stages and cycling conditions. Fourier transform
reversibly dehydrogenated and re-hydrogenated with a reduced infrared (FTIR) spectra were obtained on a Bruker Tensor 27
reaction enthalpy by the addition of MgH2. It is suggested that unit. A scanning electron microscope (SEM, Hitachi-4800,
the formation of MgB2 in the system due to the introduction of Japan) equipped with an energy-dispersive spectrometer (EDS)
MgH2 stabilizes the dehydrogenated state and destabilizes was applied for morphology and composition characterization
effectively the LiBH4. This approach is also helpful in identi- of the as-milled Ca(BH4)2 + 2LiBH4 + 2MgH2 and its desorption/
fying ways of improving the sorption properties of Ca(BH4)2. For absorption products at different cycles.
the Ca(BH4)2 + MgH2 system, studies from different groups23,24
A homemade temperature-programmed desorption (TPD)
show that the dehydrogenation pathway, in other words, the instrument was employed to measure the temperature-depen-
dehydrogenation product, depends on the dehydrogenation dent hydrogen desorption of the as-milled product. The sample
parameters, such as the presence of hydrogen in the reaction was loaded into a tube reactor housed in aꢀtube furnace, and
atmosphere, or the addition of additives/catalysts. The onset heated from RT (room temperature) to 550 C. To evaluate the
dehydrogenation temperature of the Ca(BH4)2 + MgH2 system is apparent activation energy (Ea), various heating rates (1, 2, 3, 5,
not visibly different from that of Ca(BH4)2 alone. However, the 7.5 ꢀC minꢁ1) were used. DSC (differential scanning calorim-
initial reversibility of the dehydrogenation/hydrogenation of the etry) measurement was performed on a Netzsch STA 200 F3
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instrument to obtain the thermal properties and enthalpy milling process, which is in agreement with the results reported
change during the hydrogen desorption from RT to 500 C. A by other researchers.9,13 Different from the individual Ca(BH4)2,
ꢀ
constant ow of pure argon was supplied as protective gas for the polymorphic Ca(BH4)2 in the ternary Ca(BH4)2 + 2LiBH4 +
TPD and DSC tests during heating. Both non-isothermal and MgH2 system is transformed to single g-Ca(BH4)2 aer ball
isothermal sorption properties were evaluated by volumetric milling. Whereas, for the ball-milled Ca(BH4)2 + MgH2 system,
method on a homemade Sieverts-type apparatus at a tempera- besides the dominant phase of g-Ca(BH4)2, a small amount of
ꢀ
ture range of RT to 320–550 C. In the non-isothermal testing, b-Ca(BH4)2 remains, and the peaks of g-Ca(BH4)2 are broader
ꢁ1
ꢀ
the temperature was elevated at a rate of 2 C min for dehy- than those in the ternary system, indicating improved crystal-
drogenation and a rate of 1 ꢀC minꢁ1 for hydrogenation. Static linity of g-Ca(BH4)2 in the ternary system. Therefore, it is
vacuum and a hydrogen pressure of 90 bar were used for inferred that the introduction of either single MgH2 or the
dehydrogenation and hydrogenation, respectively. For LiBH4 + MgH2 mixture renders a crystal transformation from a/
isothermal testing, samples for dehydrogenatioꢁn1and hydroge- b-Ca(BH4)2 to g-Ca(BH4)2 during the present ball milling.
nation were all heated at a rate of 10 ꢀC min to the given Conversion from a/b- to g-Ca(BH4)2 by ball milling was also
15
temperature and dwelt during the measurement. For hydroge- mentioned in the VF4-doped Ca(BH4)2 and Ca(BH4)2
+
18
nation testing, sꢁa1mples were post-dehydrogenated at a heating xLiBH4 systems. However, differently, major b-Ca(BH4)2 with
rate of 2 ꢀC min to ꢀ370 ꢀC and dwelling for 30 min, and then minor g-Ca(BH4)2 were obtained in a ball-milled Ca(BH4)2
+
hydrogenated at 350 C for 18 h.
MgH2 system reported by Minella et al.,26 which is probably due
For comparison, selected parallel experiments were also to its shorter ball-milling time (3 h) than the present (24 h).
performed on the individual Ca(BH4)2, 2LiBH4 + MgH2, Moreover, the diffraction peaks in Fig. 1 and S1† show that
Ca(BH4)2 + MgH2 and Ca(BH4)2 + 2LiBH4 mixtures, which were except for the polymorphic transformation of Ca(BH4)2,
ball-milled under identical conditions to the ternary Ca(BH4)2 + diffraction peaks of LiBH4 and MgH2 are visible in either the
2LiBH4 + 2MgH2 system. To avoid air and moisture contami- ternary system or the binary systems aer ball milling, indi-
nation, all of the samples were manipulated in a glovebox cating that there is almost no chemical reaction taking place
(MBRAUN). The concentration of O2 and H2O in the glovebox between the different hydrides during ball milling.
was below 1 ppm.
Hydrogen desorption/absorption properties
Results and discussion
Phase transformation during ball milling
Fig. 2 shows the TPD (a) and volumetric desorption (b) results of
the ball-milled Ca(BH4)2 + 2LiBH4 + 2MgH2 system, along with
the corresponding performances of the ball-milled Ca(BH4)2,
Ca(BH4)2 + MgH2 and 2LiBH4 + MgH2 systems. As seen from
Fig. 2a, all systems exhibit a distinguishable multi-step dehy-
drogenation process. The ternary system starts to dehydroge-
nate at ca. 320 ꢀC and completes at ca. 370 ꢀC in a narrow
Fig. 1 shows the XRD pattern of the ball-milled ternary Ca(BH4)2
+ 2LiBH4 + 2MgH2 system. For comparison, those of the ball-
milled Ca(BH4)2, Ca(BH4)2 + MgH2 and 2LiBH4 + MgH2 systems
are also shown. The as-synthesized Ca(BH4)2 is composed of a, b
and g polymorphic hybrid with b-Ca(BH4)2 as the main phase,14
as shown in Fig. S1.† However, the pattern of the ball-milled
Ca(BH4)2 (Fig. 1) is identical to single b-Ca(BH4)2,25 indicating a
phase transformation from a/g- to b-Ca(BH4)2 during the ball-
ꢀ
temperature range of ca. 50 C. Three peaks, centered at 339,
Fig. 2 TPD (a) and volumetric dehydrogenation (b) curves of the ball-milled
Ca(BH4)2 + 2LiBH4 + 2MgH2, Ca(BH4)2, Ca(BH4)2 + MgH2 and 2LiBH4 + MgH2
systems.
Fig. 1 XRD patterns of the ball-milled Ca(BH4)2 + 2LiBH4 + 2MgH2, Ca(BH4)2,
Ca(BH4)2 + MgH2 and 2LiBH4 + MgH2 systems.
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349 and 358 ꢀC, which are partially overlapped, are observed for
the ternary system. The onset dehydrogenation temperature of
the ternary system is almost the same as those of the individual
Ca(BH4)2 and Ca(BH4)2 + MgH2 systems, and is slightly higher
than that of the 2LiBH4 + MgH2 system as seen from Fig. 2, and
is comparable to pristine MgH2,21 but is certainly lower than
LiBH4.5 Though the rst step of the individual Ca(BH4)2 and
Ca(BH4)2 + MgH2 systems, which is also the main dehydroge-
nation step, is also located in the same temperature range of
320–370 ꢀC as that of the ternary system, broad humps are
vaguely seen in their TPD curves at temperatures higher than
ꢀ
ꢀ
370 C in a large temperature range up to at least 450 C. The
second-step dehydrogenation of the two systems completes at a
much higher temperature than the completion dehydrogena-
tion temperature of the ternary system (370 ꢀC). For the 2LiBH4
+ MgH2 system, there are two distinguishable peaks visibly
Fig. 3 DSC curves of the as-milled Ca(BH4)2 + 2LiBH4 + 2MgH2, Ca(BH4)2,
Ca(BH4)2 + MgH2, 2LiBH4 + MgH2 systems and the Ca(BH4)2 + 2LiBH4 + 2MgH2
system after 1st round hydrogenation (a); the Gaussian–Lorentzian fitting line of
the DSC curve for the Ca(BH4)2 + 2LiBH4 + 2MgH2 system (b). Regions of A–D are
the domains of integration in calculating the reaction enthalpy.
ꢀ
observed. The relatively sharp peak at 310–340 C is proposed
from the decomposition of MgH2 and the broad peak ranging
from 350–420 ꢀC is from the dehydrogenation of LiBH4 by
reference to a previous result obtained from a catalyzed LiBH4–
MgH2 system,27 in which LiBH4 and MgH2 interacted with each ascribed to the phase conversion of LiBH4 from orthorhombic
other and lowered the main dehydrogenation temperature of (o) to hexagonal (h) according to a previous report.27 Similar
both of them. It is further seen in Fig. 2a that the decomposition peaks also exist in the 2LiBH4 + MgH2 system. The peak
peak of MgH2 occurring in the 2LiBH4 + MgH2 system is not centered at 200 ꢀC is assigned to the eutectic melting of
observed in the same temperature range in both the Ca(BH4)2 + Ca(BH4)2 and LiBH4.18 The formation of the eutectic melt may
2LiBH4 + 2MgH2 and Ca(BH4)2 + MgH2 systems. This indicates be the reason why the LiBH4 in the ternary system is not
that the existence of either the Ca(BH4)2 + LiBH4 mixture or the effective in lowering the dehydrogenation temperature of MgH2
individual Ca(BH4)2 does not facilitate the decomposition of as it does in the 2LiBH4 + MgH2 system. The sharp peak
MgH2 toward a low temperature as LiBH4 does in the 2LiBH4 + centered at 340 ꢀC and the broad peak around 360 ꢀC corre-
MgH2 system. But anyway, it is clear that the Ca(BH4)2 + 2LiBH4 spond to different dehydrogenation stages. Moreover, careful
+ 2MgH2 ternary system possesses a considerably lower observation shows that the broad peak is composed of two
completion dehydrogenation temperature than the other partially overlapping peaks. The appearance of three endo-
systems. The dehydrogenation of the ternary system is not a thermic reactions for dehydrogenation of the ternary system is
simple accumulation of the desorption of its composed indi- also consistent with the appearance of three peaks in its TPD
vidual hydrides. The composed hydrides interacted with each curve where the two in the high temperature range are mostly
other in the dehydrogenation process and played a synergetic overlapped. The sharp peak occurring at ca. 290 ꢀC in the
role in lowering the dehydrogenation temperature range.
2LiBH4 + MgH2 system, which is ascribed to the melting of h-
The volumetric dehydrogenation curves shown in Fig. 2b LiBH4 according to the previous report,27 does not occur in the
conrms that the main dehydrogenation of the ternary system ternary system, due probably to its eutectic melting with
almost completes at 370 ꢀC with ca. 8.1 wt% H2 released, which Ca(BH4)2 at a low temperature of 200 ꢀC as mentioned before. In
is 83% of the theoretical capacity of the system. Further addition, the sharp peak centered at 340 ꢀC in the ternary
increasing the temperature to 550 ꢀC only results in an addi- system is located at a slightly higher temperature than a similar
tional desorption of 1.6 wt% H2. However, for the individual one in the 2LiBH4 + MgH2 system, which is also consistent with
Ca(BH4)2 and Ca(BH4)2 + MgH2 systems, there is only 5.7 and the TPD result that the 1st-step dehydrogenation of the ternary
6.2 wt% H2, respectively, upon heating to 370 ꢀC. For the 2LiBH4 system is at a slightly higher temperature than that of the
+ MgH2 system, dehydrogenation liberates only 2.7 wt% H2 in 2LiBH4 + MgH2 system. In addition, the rst endothermic peak
the 1st step at 300–330 ꢀC, originated from the decomposition of of either the Ca(BH4)2 + MgH2 system or the individual Ca(BH4)2
MgH2 according to a previous study,27 only ca. 4.0 wt% H2 in is located at higher temperature than the one of the ternary
total up to 370 ꢀC, and 8.1 wt% H2 till ca. 420 ꢀC. Therefore, the system. The existence of the downward humps at ca. 400 and
ternary system not only shows lower completion temperature of 430 ꢀC in the Ca(BH4)2 + MgH2 system and individual Ca(BH4)2,
dehydrogenation, but also much higher dehydrogenation respectively, and the broad peak in the 2LiBH4 + MgH2 system
capacity at a given temperature than the others.
DSC curves of the as-milled Ca(BH4)2 + 2LiBH4 + 2MgH2, dehydrogenation obtained from their TPD results.
Ca(BH4)2, Ca(BH4)2 + MgH2 and 2LiBH4 + MgH2 systems are The isothermal desorption curves of the Ca(BH4)2 + 2LiBH4 +
are also in good agreement with the features of the 2nd-step
presented in Fig. 3. It is seen that all reactions involved in the 2MgH2 and the reference systems performed at 320 ꢀC are
as-milled ternary system are endothermic, indicating that the shown in Fig. 4. It is seen that the hydrogen desorption kinetics
system may be mostly reversible. The peak centered at 120 ꢀC is of the ternary system is much superior to the others. There is ca.
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initiates at ca. 75 ꢀC for the ternary system, and there is already
3.8 wt% H2 absorbed into the post-dehydrogenated product
upon heating to 350 ꢀC. There is almost no hydrogen absorbed
for the individual Ca(BH4)2 under the same heating conditions.
Fig. 5b further shows that the post-dehydrogenated Ca(BH4)2 +
2LiBH4 + 2MgH2 system absorbs 3.3 wt% H2 within 60 min at
350 ꢀC (including the ca. half an hour of heating process),
meanwhile, the absorbed H2 is only 0.8 wt% for the individual
Ca(BH4)2. When the absorption time is prolonged to 1080 min,
there is ca. 7.6 wt% H2 absorbed for the ternary system, which is
equivalent to 94% of the hydrogen evolved during the initial
dehydrogenation (8.1 wt% as seen from Fig. 2b), whereas there
is only 3.4 wt% H2 absorbed for the individual Ca(BH4)2 under
the same conditions (Fig. 5b). In addition, the contents of
hydrogen absorbed aer 18 h of dwelling at 350 ꢀC with
different heating rates are almost the same for both the two
systems, as seen from Fig. 5a and b. The present result shows
that both the initial reversibility and hydrogenation capacity of
the ternary system are evidently higher than those of the
reported catalyzed-Ca(BH4)2 and Ca(BH4)2 combination systems
to date, such as the 0.02NbF5 + Ca(BH4)2 (a hydrogenation
capacity of 3.0 wt% H2 at 400 ꢀC and 90 bar for 24 h with a
reversibility around 40%),13 0.02NbF5 + 0.6Ca(BH4)2 + 0.4LiBH4
(a hydrogenation capacity of 3.2 wt% H2 at 400 ꢀC and 90 bar for
20 h with a reversibility around 40%)18 and Ca(BH4)2 + MgH2
(a hydrogenation capacity of 4.9 wt% H2 at 350 ꢀC and 90 bar H2
for 24 h with a reversibility around 60%)24 systems, despite the
facts that longer hydrogenation time with higher or at least the
same hydrogenation temperature were used in the reported
systems.
Fig. 4 Isothermal desorption curves of the ball-milled Ca(BH4)2 + 2LiBH4
+
2MgH2, Ca(BH4)2, Ca(BH4)2 + MgH2, 2LiBH4 + MgH2 and Ca(BH4)2 + 2LiBH4
systems at 320 ꢀC.
6.9 wt% H2 evolved within 110 min for the ternary system,
whereas there are only 5.4, 5.6, 3.6 and 3.4 wt% H2 evolved for
the Ca(BH4)2, Ca(BH4)2 + MgH2, Ca(BH4)2 + 2LiBH4 and 2LiBH4
+ MgH2 systems, respectively, under the same conditions. The
desorption rates of the main desorption stages of the different
systems obtained from the tangent slopes of the main desorp-
tion parts of the curves are summarized in Table S1 in ESI.† The
much higher dehydrogenation rate of the ternary system also
indicates that mutual interaction exists among the Ca(BH4)2,
LiBH4 and MgH2 phases during the isothermal process which
leads to the evident improvement of dehydrogenation kinetics.
Fig. 5 represents the absorption curves upon heating to
350 ꢀC at a heating rate of 1 ꢀC minꢁ1 and dwelling for 18 h at 90
bar of H2 of the Ca(BH4)2 + 2LiBH4 + 2MgH2 system as well as
the individual Ca(BH4)2 which are all post-dehydrogenated at
370 ꢀC for 30 min (a), and their isothermalꢁ1hydrogenation
The cyclic properties of the Ca(BH4)2 + 2LiBH4 + 2MgH2
system are shown in Fig. 6a. It is found that the reversible
hydrogen capacity decreases from 8.1 to 6.6 wt% in the rst 5
cycles, a capacity fading of less than 0.4 wt% H2 per cycle,
and then tends to be stable in the following cycles (from 6.6 to 6.2
wt% in the further 5 cycles), a capacity fading of less than 0.1 wt%
H2 per cycle. Retention of 77% is attained aer 10 cycles. The
retention aer 10 cycles of the present system is much higher
than the initial reversibility of the 0.02NbF5 + Ca(BH4)2, 0.02NbF5
ꢀ
ꢀ
curves heating to 350 C at a rate of 10 C min and dwelling
for 18 h at 90 bar of H2 (b). As shown in Fig. 5a, hydrogenation
Fig. 6 Cycling properties of the Ca(BH4)2 + 2LiBH4 + 2MgH2 system: (a) the
dehydrogenation capacity at different dehydrogenation/hydrogenation cycles;
Fig. 5 Hydrogenation curves of the Ca(BH4)2 + 2LiBH4 + 2MgH2 system as well as
individual Ca(BH4)2 upon heating to 350 ꢀC at a heating rate of 1 ꢀC minꢁ1 and
dwelling for 18 h (a) and isothermal dwelling at 350 ꢀC for 18 h with a heating
rate of 10 ꢀC minꢁ1 to 350 ꢀC (b).
(b) dehydrogenation curves at the 1st, 2nd and 10th cycles upon heating to 370 ꢀ
C
and dwelling for 30 min; (c) hydrogenation curves of the 1st and 9th cycles upon
heating to 350 ꢀC and dwelling for 18 h at 90 bar H2.
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+ 0.6Ca(BH4)2 + 0.4LiBH4 and Ca(BH4)2 + MgH2 systems as However, FTIR test (Fig. S3†) shows that B–H bonds exist in the
mentioned previously. intermediates, indicating the formation of amorphous B–H
An interesting nding worthy of note is that, aer the rst containing intermediates accompanying the evident liberation of
desorption/absorption cycle, the starting dehydrogenation H2 (Fig. 2b). For the intermediates at 350–370 ꢀC, CaH2 and Mg
temperature of the ternary system in the following cycles up to are still the only crystal phases detected, and a B–H containing
the 10 cycles is remarkably lowered to 230 ꢀC, reduced by ca. 90 phase is also detected by FTIR test (Fig. S3†) in the intermediates
ꢀC compared with the initial one, as shown in Fig. 6b, the at 350 ꢀC, whereas there is almost no B–H containing phase
dehydrogenation curves of the 1st, 2nd and 10th cycles upon detected in intermediates at 370 C. Combining its dehydroge-
ꢀ
heating to 370 ꢀC and then dwelling for 30 min. Moreover, Fig. 6b nation curve (Fig. 2b), this is reasonable, as dehydrogenation has
further shows that the dehydrogenation features of the 2nd and almost completed at 370 C. The B–H containing phase decom-
ꢀ
10th cycles are almost identical in the initial dehydrogenation posed and no boron-containing phase is detected by XRD in the
process at temperature lower than ca. 300 ꢀC. Slightly decreased intermediates of Ca(BH4)2 at 400 ꢀC due to its amorphous
capacity and kinetics occur only at temperature over ca. 300 C. feature.28 For the dehydrogenation product at 550 ꢀC, besides the
ꢀ
This phenomenon indicates that there should be some highly remaining CaH2 and Mg phases, CaB6 and LiH are also detected.
reversible hydrides with comparatively low dehydrogenation Fig. S2b† shows its amplied pattern, in which diffractions of
temperature aer the rst round hydrogenation. The 1st and 9th CaB6 and LiH are clearly seen. Due to the poor crystallinity of
hydrogenation curves in the heating and dwelling process are CaB6, their diffraction peaks are broad. Certainly, there is no B–H
shown in Fig. 6c. It is seen that the hydrogen absorbed at the 1st containing phase detected by FTIR (Fig. S3†) due to the full
and 9th cycles are 3.8 wt% and 2.7 wt%, respectively, during the decomposition of the B–H containing phase. Formation of
heating to 350 ꢀC. The difference in the amount of hydrogen nanocrystalline CaB6 was also reported by Kim et al.24 in the
absorbed between the 1st and 9th cycles is slightly increased with dehydrogenation product of Ca(BH4)2 at 480 ꢀC.
the temperature, indicating that the content of hardly hydroge-
Based on the above analyses, the overall reaction process of
nated phase increases with cycling. With a further isothermal the ternary system is proposed to be:
hydrogenation at 350 ꢀC for 18 h, the total amounts of hydrogen
absorbed are 7.6 and 6.2 wt% for the 1st and 9th cycles, respec-
RT-550 ꢀ
C
CaðBH4Þ þ 2LiBH4 þ 2MgH2
ƒƒƒƒƒƒ!
2
tively, which are also almost the equilibrium values. Combining
the dehydrogenation capacity of the 9th cycle shown in Fig. 6a, it
is found that the reversibility of the 9th cycles is 97%, which is
slightly higher than that of the initial one. It reveals that the
reversibility of the ternary system is signicantly high, especially
aer the initial several cycles. Such superior long-term cycle
performance and high capacity are rarely found in the previous
literature related to the light metal boron hydride systems based
on the best of our knowledge.
1=3CaH2 þ 2=3CaB6 þ 2LiH þ 2Mg þ 26=3H2
The theoretical dehydrogenation capacity based on the equa-
tion is calculated to be 10.1 wt%, which is well in line with the
experimental result (9.8 wt%, Fig. 2b).
Values of the apparent activation energy (Ea) of the ternary
system determined by the Kissinger method,29 with those of the
other systems for comparison, are listed in Table S2.† The Ea
value of ca. 162 kJ molꢁ1 of the 1st-step dehydrogenation of the
ternary system is close to those of the others, being in good
agreement with their almost identical onset temperatures.
However, the Ea value of the 2nd-step dehydrogenation of the
ternary system, ca. 147 kJ molꢁ1, is 20–60 kJ molꢁ1 lower than
those of the other systems. That of the 3rd step of the ternary
system is ca. 143 kJ molꢁ1. The close and low Ea values of the
three reaction stages of the ternary system also explain well the
narrow dehydrogenation temperature region, which also
provides extra evidence that Ca(BH4)2, LiBH4 and MgH2
interact, forming intermediates which have a synergetic effect
on the improvement of its kinetics and high available dehy-
drogenation capacity at relatively low temperature. Moreover,
the Ea value of the 2nd cycle dehydrogenation of the ternary
system is calculated to be ca. 121 kJ molꢁ1, which is lowered by
40 kJ molꢁ1 compared with that of the 1st cycle. The result gives
the reason for the signicantly lowered onset dehydrogenation
temperature of ca. 230 ꢀC of the 2nd cycle compared with that of
Chemical reactions in the dehydrogenation/hydrogenation
process and mechanism of the improved hydrogen storage
properties
XRD analysis of the selected dehydrogenation intermediates of
the Ca(BH4)2 + 2LiBH4 + 2MgH2 ternary system are shown in
ꢀ
Fig. S2a.† The intermediates at 250 C are still composed of g-
Ca(BH4)2, LiBH4 and MgH2. There is no newly formed crystal
ꢀ
phase detected in the intermediates at 250–320 C, though ca.
0.67 wt% H2 is released (Fig. 2b). A small amount of H2 releasing
from Ca(BH4)2 might not change the main crystal structure of
Ca(BH4)2,14 or it is probable that an amorphous phase is formed.
However, when the temperature is increased by only 10 ꢀC to 330
ꢀC, new diffraction peaks originating from CaH2 and Mg appear,
along with those of the remaining Ca(BH4)2, LiBH4 and MgH2.
The relative peak intensity of Ca(BH4)2 is rapidly weakened,
whereas that of the LiBH4 phase remains the same. The
phenomenon implies that Ca(BH4)2 reacts rstly with partial
MgH2, generating CaH2, Mg and H2. Further elevating the
temperature by only another 10 ꢀC to 340 ꢀC, the diffraction
peaks of Ca(BH4)2, LiBH4 and MgH2 all disappear in the inter-
mediates, leaving only those of the CaH2 and Mg phases.
st
ꢀ
320 C for the 1 cycle (Fig. 6b).
The overall reaction enthalpies of the different systems
determined by integrating the endothermic peaks in their DSC
curves marked as A–D in Fig. 3a are also listed in Table S2.† The
dehydrogenation reaction enthalpy for the ternary system is
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Journal of Materials Chemistry A
40.3 kJ molꢁ1 H2, which is 5–12 kJ molꢁ1 H2 lower than those of experimental testing process, which was also reported by
the others. However, it is noteworthy that desorption of the Minella et al.26 in the Ca(BH4)2 + MgH2 system, as magnesium
ternary system is a multi-step process. If separating its over- metal forms oxide exothermally (ꢁ601 kJ molꢁ1).32
lapped DSC curve with a peak tting program based on the
It is worth noting that a newly formed dual-cation hydride of
Gaussian–Lorentzian function and guring out the hydrogen MgCaH3.72 is found to be always the predominant crystal phase
released in each step based on the desorption curve (Fig. 2b), in the hydrogenated products of the ternary system aer different
the separated reaction enthalpy changes of the 1st, 2nd and 3rd- cycles by XRD analysis (Fig. S5b†), demonstrating that CaMgH3.72
steps of the ternary system were calculated to be 50.1, 38.3 and is a highly reversible phase. g-Ca(BH4)2, LiBH4 and MgH2 are
39.6 kJ molꢁ1 H2, respectively. The separated 1st-step reaction evidently detected with relatively high intensity in the hydroge-
enthalpy of the ternary system is almost identical to the overall nated product of the 1st cycle. However, the relative intensities of
value of the individual Ca(BH4)2 (52.2 kJ molꢁ1 H2), related to the diffraction peaks of these three phases are gradually weak-
their similar starting dehydrogenation temperatures. However, ened with cycling as seen from the representative patterns of the
the much lower values of the 2nd/3rd stages contribute to the 3rd and 9th hydrogenated products (Fig. S5b†). CaMgH3.72 formed
decrease in the completion temperature of the ternary system. as the main phase in the hydrogenated product rather than the
SEM images of the as-milled Ca(BH4)2 + 2LiBH4 + 2MgH2 full conversion of only Ca(BH4)2, LiBH4 and MgH2 aer the rst
system and its products aer different cycles of dehydrogena- hydrogenation. To ascertain the action of CaMgH3.72 in the re-
tion/hydrogenation are shown in Fig. S4a–f.† The as-milled dehydrogenation process, CaMgH3.72 was synthesized according
mixture is composed of particles with somewhat smooth to our previous work.33 The synthesized CaMgH3.72 is stable in
surface and size in the range of 1–4 mm (Fig. S4a†). However, the heating process up to 260 ꢀC as known from its XRD pattern
aer 1st dehydrogenation, the particles show smaller size and (Fig. S6†) and its dehydrogenation curve (Fig. S7†). Dehydroge-
irregular shape (Fig. S4b†). Aer 1st round hydrogenation, the nation of CaMgH3.72 only occurs at ca. 320 C (Fig. S7†), being
ꢀ
ꢀ
particles tend to grow and agglomerate (Fig. S4c†), being larger much higher than 260 C for the ternary system aer the initial
than the starting mixture. The 2nd cycle dehydrogenated cycle (Fig. 6b). However, if a small amount of CaMgH3.72 is
product (Fig. S4d†) also shows smaller size than that before introduced into the ternary system to form (1 ꢁ 1.5x)Ca(BH4)2 +
dehydrogenation, but almost in the same order as that of the 1st 2LiBH4 + (2 ꢁ x)MgH2 + xCaMgH3.72 mixture (x ¼ 0.1, 0.2) by ball-
dehydrogenation product. The reduced particle size of the milling, it is found that the onset dehydrogenation temperature
dehydrogenated product should favour the hydrogenation of of the system is lowered by more than 30 ꢀC, as shown in Fig. S8.†
the system, as small particle sizes commonly favour the Particularly, the dehydrogenation curve of the system with x ¼ 0.2
hydrogen storage properties due to easier diffusion.30,31 Though is almost identical to the 2nd cycle dehydrogenation curve of the
the particle size increases aer the initial hydrogenation, the Ca(BH4)2 + 2LiBH4 + 2MgH2 system. Moreover, XRD analysis of
product still possesses lower onset dehydrogenation tempera- the product of the 1st round hydrogenated product of the ternary
ture (Fig. 6b). There must be other factors that compete in the system, which is composed of CaMgH3.72–Ca(BH4)2–LiBH4–
behavior. In addition, there is a belt-like phase found (marked MgH2, heated to 260 ꢀC and dwelling for 10 min shows that the
by arrow 1 in Fig. S4d† and amplied in its inset) in the 2nd relative peak intensities of both Ca(BH4)2 and MgH2 are weak-
dehydrogenation product. In the following cycles, the ened and that of CaMgH3.72 is strengthened, compared with
morphology of the dehydrogenation/hydrogenation products is those of the starting material (Fig. S9†). This result indicates that
not visibly changed, as typically shown in Fig. S4e and f† for the the CaMgH3.72 favours the decomposition of re-generated
9th hydrogenation and 10th dehydrogenation, respectively, Ca(BH4)2 and MgH2, playing an important role in lowering the
except that there are slightly larger amounts of belt-like phase onset dehydrogenation temperature of the present ternary
formed in the dehydrogenated product with cycling, from an system aer the 1st cycle, which is also the reason that though the
overview of the SEM observation. EDS analysis of the belt-like particles grow aer the 1st re-hydrogenation, the onset dehydro-
phase showed that it is both Mg- and O-rich.
genation temperature of the re-hydrogenated product is still
XRD analysis of the dehydrogenation and hydrogenation signicantly lowered.
products at different cycles shows that CaH2 and Mg are the
major phases in the dehydrogenation products, and they are the
Conclusions
only detected crystal phase in the 1st dehydrogenation product.
The patterns of the 1st, 4th and 10th dehydrogenation products A ternary hydrogen storage system, Ca(BH4)2 + 2LiBH4 + 2MgH2,
are representatively shown in Fig. S5a.† In addition, MgO is was synthesized via simply ball milling the three hydrides. The
detected in the dehydrogenation product of several cycles, as ternary system dehydrogenates from 320 to 370 ꢀC, releasing ca.
shown in the 4th cycle. The peaks assigned to MgO are very 8.1 wt% H2, 83% of the theoretical capacity of the system. The
evident in the dehydrogenated product of 10th cycle. Consid- completion temperature is much lower and the capacity
ering the Mg/O-rich belt-like phase in the SEM images of the considerably higher than any of the single or binary systems of
dehydrogenated products mentioned above, it is concluded that the three hydrides studied so far. Enhanced desorption/
the belt-like phase in the dehydrogenation product is MgO. The absorption kinetics and lowered reaction enthalpy are also
content of MgO increases with cycling as known from XRD and obtained for the ternary system. Particularly, the reversibility
SEM/EDS analysis. The formation of MgO is considered to be and the cyclic stability of the ternary system are superior to the
due to the inevitable contact with air and moisture during the single and binary counterparts. There is about 7.6 wt% H2
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absorbed at 350 ꢀC and 90 bar H2 for 18 h for the ternary system 12 E. Ronnebro and E. H. Majzoub, J. Phys. Chem. B, 2007, 111,
initially post-dehydrogenated at 370 ꢀC for 30 min, which is
12045–12047.
equivalent to a reversibility of 94%. The capacity fading for each 13 J. H. Kim, J. H. Shim and Y. W. Cho, J. Power Sources, 2008,
cycle decreases with cycling. A reversibility of 97% and a dehy- 181, 140–143.
drogenation capacity of 6.2 wt% are attained for the 10th cycle. 14 J. Gu, M. X. Gao, H. G. Pan, Y. F. Liu, B. Li, Y. J. Yang,
To the best of our knowledge, this is the best cycle stability for
light metal boron hydride systems reported to date. Ca(BH4)2,
C. Liang, H. L. Fu and Z. X. Guo, Energy Environ. Sci., 2013,
6, 847–858.
LiBH4 and MgH2 in the ternary system are all reversible during 15 C. B. Minella, S. Garroni, C. Pistidda, R. Gosalawit-Utke,
cycling. Moreover, hydrogenation of the ternary system post-
G. Barkhordarian, C. Rongeat, I. Lindemann, O. Guteisch,
T. R. Jensen, Y. Cerenius, J. Christensen, M. D. Baro,
R. Bormann, T. Klassen and M. Dornheim, J. Phys. Chem.
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ꢀ
dehydrogenation at 370 C for 30 min initiates at an extremely
low temperature of ca. 75 ꢀC, and the onset dehydrogenation
temperature of the re-hydrogenated product is as low as 230 ꢀC
aer the 2nd cycle, with a signicant reduction of 90 ꢀC 16 H. L. Chu, Z. T. Xiong, G. T. Wu, J. P. Guo, X. L. Zheng, T. He,
compared with the initial one, which is ascribed to the forma-
tion of an active dual-cation hydride of CaMgH3.72 in the 17 H. L. Chu, G. T. Wu, Y. Zhang, Z. T. Xiong, J. P. Guo, T. He
hydrogenation process. The CaMgH3.72 is highly reversible and P. Chen, J. Phys. Chem. C, 2011, 115, 18035–18041.
during cycling, and favours the decomposition of Ca(BH4)2 and 18 J. Y. Lee, D. Ravnsbaek, Y. S. Lee, Y. Kim, Y. Cerenius,
C. Z. Wu and P. Chen, Chem.–Asian J., 2010, 5, 1594–1599.
MgH2. This work reveals that developing multicomponent
hydrogen storage systems can be an effective approach to obtain
favourable overall hydrogen storage performance.
J. H. Shim, T. R. Jensen, N. H. Hur and Y. W. Cho, J. Phys.
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19 Z. Z. Fang, X. D. Kang, J. H. Luo, P. Wang, H. W. Li and
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20 A. A. Ibikunle and A. J. Goudy, Int. J. Hydrogen Energy, 2012,
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Acknowledgements
The work was supported by the “973” program from the 21 S. A. Jin, J. H. Shim, J. P. Ahn, Y. W. Cho and K. W. Yi, Acta
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Distinguished Youth Scholars of China (no. 51025102), The
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National Natural Science Foundation of China (no. 51371158), 23 G. Barkhordarian, T. R. Jensen, S. Doppiu, U. Bosenberg,
The Key Science and Technology Innovation Team of Zhejiang
Province (2010R50013) and The overseas top disciplinary part-
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