Comparative study on the reversibility of pure metal borohydrides

Article abstract of DOI:10.1016/j.jallcom.2013.03.264

Improvement of the reversibility of metal borohydrides is a key issue for hydrogen storage applications. In this study, we carefully investigated the first rehydrogenation of pure Mg(BH₄)₂ and Ca(BH ₄)₂ under a hydrogen pressure of 40.0 MPa. Mg(BH ₄)₂ is produced even at a relatively low temperature of 473 K, and its amount increases with the temperature up to 673 K, leading to the increased rehydrogenation content and a maximum (7.6 mass%, equivalent to 51% of Mg(BH₄)₂) at 673 K. Under the same condition, more than 90% of rehydrogenation is confirmed through the formation of α-Ca(BH ₄)₂ as clearly observed by XRD and NMR measurements. Comparison of the rehydrogenation properties of Mg(BH₄)₂ and Ca(BH₄)₂ suggests that control of dehydrogenation products would be an important approach to improve the reversibility of metal borohydrides.

Full text of DOI:10.1016/j.jallcom.2013.03.264

Journal of Alloys and Compounds 580 (2013) S292–S295
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Comparative study on the reversibility of pure metal borohydrides
Hai-Wen Li a,b, , Etsuo Akiba a,b,c, Shin-ichi Orimo d,e
⇑
a
International Research Center for Hydrogen Energy, Kyushu University, Fukuoka 819-0395, Japan
International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan
Department of Mechanical Engineering, Kyushu University, Fukuoka 819-0395, Japan
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
b
c
d
e
WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 6 April 2013
Improvement of the reversibility of metal borohydrides is a key issue for hydrogen storage applications.
In this study, we carefully investigated the first rehydrogenation of pure Mg(BH
hydrogen pressure of 40.0 MPa. Mg(BH is produced even at a relatively low temperature of 473 K, and
its amount increases with the temperature up to 673 K, leading to the increased rehydrogenation content
and a maximum (7.6 mass%, equivalent to 51% of Mg(BH ) at 673 K. Under the same condition, more
than 90% of rehydrogenation is confirmed through the formation of -Ca(BH as clearly observed by
4 2
XRD and NMR measurements. Comparison of the rehydrogenation properties of Mg(BH and Ca(BH )
4 2 4 2
) and Ca(BH ) under a
4 2
)
Keywords:
Hydrogen storage
Borohydride
Reversibility
Hydrogen
4 2
)
a
4 2
)
4
)
2
suggests that control of dehydrogenation products would be an important approach to improve the
reversibility of metal borohydrides.
Hydride
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
to improve the rehydrogenation of Mg(BH
was produced by hydrogenation of the dehydrogenated product
MgB at 673 K under a hydrogen pressure of 95.0 MPa for 108 h
[16]. The hydrogenation of MgB can be significantly promoted
by introduction of the nanostructures with defects [17,18].
Approximately 60% of Ca(BH have been produced by rehydroge-
nation at 623–713 K under a hydrogenation pressure of 9.0–
70.0 MPa with various additions, such as NbF [19], TiF [20] or
TiCl + Pd [21]. It is noteworthy to mention that most reports focus
on the rehydrogenation of doped Mg(BH and Ca(BH without
4 2 4 2
) [15]. Also, Mg(BH )
Metal borohydrides M(BH
4
)
n
have been regarded as potential
2
hydrogen storage materials owing to their high hydrogen densities.
Most of M(BH suffer from their relatively high thermodynamics
2
4 n
)
and sluggish kinetics. In the last decade, great efforts have been
devoted to solve these problems, and lots of progresses have been
achieved such as using electronegativity as an indicator for esti-
mating the thermodynamics [1]; making a composite system with
other element or their compounds (also called reactive hydride
composites) [2,3], and nanoconfinement technology [4], as sum-
marized in recent review papers [5–8].
4 2
)
5
3
3
4
)
2
4 2
)
comparison with those of pure samples. Therefore, it is of great
importance to systematically investigate the rehydrogenation of
Reversibility (reversible dehydrogenation/rehydrogenation) of
4 2 4 2
pure Mg(BH ) and Ca(BH ) as a reference point for comparison
M(BH
cation. Generally dehydrogenation/rehydrogenation reactions pro-
ceed through decomposition and recombination of M(BH itself
together with the formation of intermediate compounds such as
12 regarded as one of the main reasons causing the degrada-
tion of reversibility [9–11]. Rehydrogenation of LiBH has been
4
)
n
is an important issue for onboard hydrogen storage appli-
prior to depict the improvement effect of additives.
In this study, we carefully investigate the rehydrogenation of
4 2 4 2
pure Mg(BH ) and Ca(BH ) under a hydrogen pressure of
40.0 MPa by using thermogravimetric (TG) analysis. Also, the
phase changes after dehydrogenation and rehydrogenation are
analyzed by X-ray diffraction (XRD) and nuclear magnetic reso-
nance (NMR). Moreover, the rehydrogenation properties are com-
pared in order to provide insightful information for substantial
improvement of reversibility of metal borohydrides.
4
)
n
MB12
H
4
reported to proceed at a temperature higher than 873 K under a
hydrogen pressure of 15.0 or 35.0 MPa [12,13]. The first rehydroge-
nation experiment of Mg(BH
rehydrogenated phase at 543 K under a hydrogen pressure of
0.0 MPa [14]. The addition of 5 mol% TiF and ScCl was reported
4 2
) found that MgB12H12 was the main
4
3
3
2. Experimental
⇑
Corresponding author at: International Research Center for Hydrogen Energy,
Kyushu University, Fukuoka 819-0395, Japan. Tel.: +81 92 802 3226; fax: +81 92
8
Mg(BH
(Aldrich; purity, 95%) and Na(BH4) (Aldrich; purity, 99.99%), as described in the pre-
vious report [22]. The single high-temperature phase b-Mg(BH was used as the
starting material. Ca(BH was synthesized by heating the commercial reagent
4 2 2
) was synthesized by means of the metathesis reaction of MgCl
02 3235.
E-mail address: li.haiwen.305@m.kyushu-u.ac.jp (H.-W. Li).
4 2
)
4 2
)
0
925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.03.264

H.-W. Li et al. / Journal of Alloys and Compounds 580 (2013) S292–S295
S293
Ca(BH
sists of b-Ca(BH
under vacuum at 623 K and kept for 6 h, and the obtained product was used as the
starting material of rehydrogenation. The dehydrogenated Ca(BH samples were
prepared under 1.0 MPa Ar at 643 and 743 K, respectively, and kept for 3 h. The
rehydrogenation was carried out under a hydrogen pressure of 40.0 MPa in a
specially designed pressure-resistant reaction tube.
4
)
2
Á2THF (Sigma–Aldrich) under vacuum at 503 K for 16 h. This sample con-
of rehydrogenation content. At 773 K, no evidence of Mg(BH
be found and the main phase becomes MgB12 12 and MgB
firmed at À15 and 100 ppm (not shown), respectively [15]. The
4 2
change in the chemical composition of MgH , MgB12 12, Mg(BH )
4
)
2
can
)
4 2
phase mainly. The dehydrogenation of Mg(BH was performed
4 2
)
H
2
as con-
4 2
)
2
H
and MgB dominates not only the rehydrogenation content but also
2
the sample colors, as given in Fig. 1.
The rehydrogenation content examined by TG increases and
reaches the maximum (7.6 mass%, equivalent to 51% of the total
The crystal structures were examined via XRD (PANalytical X’PERT with Cu K
a
radiation) at room temperature. The hydrogen content was analyzed by using ther-
mogravimetry (TG, Rigaku TG-8210) under a He flow of 150 ml/min at a heating
rate of 5 K/min. The chemical bonding states of boron atoms were further investi-
gated via 11B MAS NMR measurement at room temperature (JEOL Ltd., JNM-
ECA600 spectrometer operated at a magnetic field of 14.1 T; resonance frequency,
4 2
hydrogen content in Mg(BH ) ) when increasing the temperature
from 473 to 673 K, while reduces when the temperature higher
than 673 K. The increased hydrogen content attributes to the
improved kinetics because higher temperature would thermally
activate the rehydrogenation reaction. On the other hand, if the
temperature is higher than the decomposition temperature under
1
92.57 MHz; spinning rate of 4.0 mm diameter sample rotor, 16 kHz). Spectra were
obtained by using a single pulse sequence without a high power 1H decoupling dur-
ing signal acquisition. The pulse width of 1 s was used, which was set equivalent
to a /9 of the solution /2 pulse to minimize the nutation effect on the quantitative
peak area analysis of the spectrum. For each spectrum, 6000 scans were accumu-
lated with repetition time of 2 s. Chemical shift was referenced to BF O in ether
l
p
p
a hydrogen pressure of 40.0 MPa, Mg(BH
4 2
) becomes thermody-
3
ÁEt
2
namically unstable and results in the reduced hydrogen content.
solution as 0 ppm. All the samples were always handled in a glove box filled with
purified Ar/He gas (water and oxygen concentration, <1 ppm) in order to avoid
(
hydro-)oxidation.
3
.2. Rehydrogenation property of Ca(BH
4 2
)
3
. Results and discussion
XRD profiles and NMR spectra of Ca(BH ) and its dehydroge-
4 2
nated products at 643 and 743 K are shown in Figs. 2 and 3, respec-
tively. Both the diffraction peaks and the chemical shift at
3
.1. Rehydrogenation property of Mg(BH )
4 2
À32 ppm consistently indicate that b-Ca(BH
of Ca(BH
are identified in the XRD profiles of Ca(BH
643 K, while only CaH
4
)
2
is the main phase
and CaB [24]
dehydrogenated at
The dehydrogenated product of Mg(BH
not shown) and B MAS NMR (Fig. 1). Only diffraction peaks of
4
)
2
are analyzed by XRD
4
)
2
Á2THF after desolvation [23]. CaH
2
2 x
H
1
1
(
4
)
2
Mg are observed in the XRD profile. MgB12
H
12 reported to be amor-
2
is confirmed as dehydrogenated product
phous are confirmed in 11B MAS NMR spectra, as evidenced by the
peak at approximately À15 ppm. Thus the dehydrogenated prod-
uct is a mixture consisting of crystalline Mg and amorphous MgB12-
at 743 K. The presence of CaO may originate from the impurities
in the lines and reaction tube. A series of signals at ꢀ15, ꢀ2,
ꢀÀ14 and ꢀÀ30 ppm are observed for dehydrogenated samples
1
1
H
12, which are used as the starting material for rehydrogenation.
After rehydrogenation at 473–773 K, diffraction peaks of MgH
are cleared observed in all the XRD profiles. Also, MgB is confirmed
when rehydrogenated at 773 K, suggesting the interaction between
Mg and MgB12 12. Some selected rehydrogenated samples are
examined by B MAS NMR and the spectra are shown in the inset
of Fig. 1. At 473 K, the formation of small amount Mg(BH is evi-
at both 643 K and 743 K. In reference to the B MAS NMR spec-
trum of pure CaB
to CaB , which is also clearly detected by Raman analysis (not
shown here). The peak at À30 ppm appears in between
- (À29 ppm) and b-Ca(BH (À32 ppm), suggesting the quite
similar chemical environment with Ca(BH . Also, the intensity
decreases significantly when the dehydrogenation temperature
6
[25], the broad peak at 15 ppm should belong
2
2
6
H
a
4 2
)
1
1
4 2
)
4 2
)
denced by a very weak peak at À40 ppm. With increasing the rehy-
drogenation temperature up to 543 K, the intensity ratio of
4 2
Mg(BH ) to MgB12H12 increases largely, suggesting the increment
4
0 MPa H , 12 h
2
8
6
4
2
MgB12H12
(
i)
ii)
iii)
iv)
(
(
(
(v)
(
vi)
2
0
0
-20 -40 -60 -80
Chemical Shift (ppm)
0
4
50
500
550
600
650
700
750
800
Rehydrogenation Temperature (K)
Fig. 1. Rehydrogenation content (black close circle) as a function of rehydrogena-
tion temperature under a hydrogen pressure of 40.0 MPa for 12 h. Photos of
rehydrogenated product powders are shown to compare the color changes. Inset
presents the 11B MAS NMR spectra of selected samples: (i) dehydrogenated product,
rehydrogenated products at (ii) 473 K, (iii) 543 K, (iv) 623 K, (v) 673 K and (vi)
4 2 4 2-
Fig. 2. Powder XRD profiles of (a) Ca(BH ) prepared by desolvation of Ca(BH )
Á2THF, dehydrogenated products at (b) 643 K and (c) 743 K, as well as their
rehydrogenation products (d and e), respectively, under a hydrogen pressure of
40.0 MPa at 673 K. , b, triangle, circle and square symbols indicate
a
4 2
a-Ca(BH ) , b-
7
73 K.
Ca(BH , CaH , CaB H and CaO, respectively.
4
)
2
2
2 x

S294
H.-W. Li et al. / Journal of Alloys and Compounds 580 (2013) S292–S295
(
4 2
7.7 mass%) of the starting Ca(BH ) prepared in this study, demon-
strating the superior rehydrogenation property than the previous
reports [11,19–21,23]. The loss of rehydrogenation content may
originate from the impurities included in the lines and reaction
12
tube, or from the very small amount formation of stable CaB12H .
3
.3. Comparison of reversibility of Mg(BH
)
4 2
and Ca(BH
4 2
)
Rehydrogenation of both Mg(BH and Ca(BH )
4
)
2
4 2
are thermody-
namically favorable, whereas need to overcome the kinetic obsta-
cle originating from the migration of constituent element and the
rearrangement of chemical bonding. Under the same rehydrogena-
tion condition (40 MPa H
ibility in the first rehydrogenation than that of Mg(BH
than 90% formation of Ca(BH and only 51% formation of
Mg(BH . This may originate from the different dehydrogenation
products, especially for the main boron-related species: MgB12
in the case of Mg(BH (Fig. 1i), while CaB and CaB in Ca(BH
Fig. 3b). This is the case similar to the previous report that MgB
2
, 673 K), Ca(BH
4
)
2
shows superior revers-
4 2
)
, i.e. more
4 2
)
4 2
)
H
12
4
)
2
6
2
H
x
4
)
3
H
2
8
(
and MgB
2
shows better reactivity with hydrogen to produce
than that of MgB12 12 [16,26]. Effective control of
dehydrogenation pathway to avoid the formation of too stable
intermediate compounds like MB12 12, therefore, is an important
approach to develop metal borohydrides with good reversibility.
Mg(BH
4
)
2
H
Fig. 3. 11B MAS NMR spectra of (a) Ca(BH
) prepared by desolvation of Ca(BH )
4 2 4 2-
H
Á2THF, dehydrogenated products at (b) 643 K and (c) 743 K, scale adjusted by 2
times, as well as their rehydrogenation products (d and e), respectively.
4
. Conclusions
increased from 643 to 743 K. These results suggest that the peak at
À30 ppm could be residual Ca(BH
4 2
) , as observed in the previous
The dehydrogenated product consisting of crystalline Mg and
reports [23,25]. In addition, the peak at À14 ppm can be assigned
amorphous MgB12
Mg(BH after hydrogenation under a hydrogen pressure of
0.0 MPa at 473–673 K. With increasing the rehydrogenation tem-
perature, the composition ratio of Mg(BH to MgB12 12 in-
creased, leading to the increased hydrogen content and its
maximum reached 7.6 mass% (equivalent to 51% of Mg(BH ) at
73 K. More than 90% of rehydrogenation were confirmed through
the formation of -Ca(BH by hydrogenation of the dehydroge-
nated products including CaH , CaB and/or CaB at 673 K. The
different boron-related species between the dehydrogenated
Mg(BH and Ca(BH would afford different rehydrogenation
2
H12 transformed into MgH , MgB12H12 and
11
to CaB12
H12 in reference to the
B MAS NMR spectrum
4 2
)
(
À15.6 ppm) of pure CaB12
H
12 [25]. A very broad signal centered
4
at around ꢀ0 ppm suggests the presence of element boron for
4
)
2
H
the dehydrogenated at 743 K [25].
The rehydrogenated products are identified mainly as
a-
4 2
)
Ca(BH from both XRD profiles and NMR spectra, regardless of
4 2
)
6
dehydrogenation temperature, as shown in Figs. 2 and 3. Surpris-
ingly, no obvious traces of dehydrogenated products including
a
4 2
)
2
6
2 x
H
CaH
NMR profiles. This indicates the good rehydrogenation property
of Ca(BH at present experimental condition, which is well sup-
2 6 2 x
, CaB , CaB12H12 and CaB H are observed in the XRD and
4
)
2
4 2
)
4 2
)
performance, suggesting that control of dehydrogenation products
is an effective approach to develop metal borohydrides with good
reversibility.
ported by the rehydrogenation content shown in Fig. 4. The rehy-
drogenation contents are 7.4 and 7.0 mass%, respectively, for
samples dehydrogenated at 643 and 743 K. These values are
equivalent to 96% and 91% of the dehydrogenation amount
Acknowledgements
The authors would like to thank Drs. S. Towata, K. Miwa, M.
Aoki, M. Matsumoto, and T. Noritake for their valuable discussions.
This study was partially supported by NEDO, ‘Development for
Hydrogen Production, Transportation and Storage System’ Project,
JSPS KAKENHI (23760658) and The International Institute for
Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the
Japanese Ministry of Education, Culture, Sports, Science and
Technology.
References
[
1] Y. Nakamori, K. Miwa, A. Ninomiya, H.W. Li, N. Ohba, S.I. Towata, A. Zuttel, S.I.
Orimo, Phy. Rev. B 74 (2006) 045126.
[
[
2] J.J. Vajo, S.L. Skeith, F. Mertens, J. Phy. Chem. B 109 (2005) 3719–3722.
3] G. Barkhordarian, T.R. Jensen, S. Doppiu, U. Bo1senberg, A. Borgschulte, R.
Gremaud, Y. Cerenius, M. Dornheim, T. Klassen, R. Bormann, J. Phys. Chem. C
112 (2008) 2743–2749.
[
[
4] P.E. de Jongh, P. Adelhelm, ChemSusChem 3 (2010) 1332–1348.
5] S.I. Orimo, Y. Nakamori, J.R. Eliseo, A. Zuttel, C.M. Jensen, Chem. Rev. 107
(
2007) 4111–4132.
Fig. 4. Hydrogen content examined by TG of Ca(BH
4
)
2
prepared by desolvation of
[6] H.W. Li, Y.G. Yan, S. Orimo, A. Zuttel, C.M. Jensen, Energies 4 (2011) 185–214.
[7] E. Ronnebro, Curr. Opin. Solid St. M 15 (2011) 44–51.
4 2
Ca(BH ) Á2THF, rehydrogenated products after dehydrogenation at 643 and 743 K.

H.-W. Li et al. / Journal of Alloys and Compounds 580 (2013) S292–S295
S295
[
[
8] L.H. Rude, T.K. Nielsen, D.B. Ravnsbaek, U. Bosenberg, M.B. Ley, B. Richter, L.M.
Arnbjerg, M. Dornheim, Y. Filinchuk, F. Besenbacher, T.R. Jensen, Phys. Status
Solidi A 208 (2011) 1754–1773.
9] H.W. Li, K. Miwa, N. Ohba, T. Fujita, T. Sato, Y. Yan, S. Towata, M.W. Chen, S.
Orimo, Nanotechnology 20 (2009) 204013.
[18] C. Pistidda, S. Garroni, F. Dolci, E.G. Bardaji, A. Khandelwal, P. Nolis, M.
Dornheim, R. Gosalawit, T. Jensen, Y. Cerenius, S. Surinach, M.D. Baro, W.
Lohstroh, M. Fichtner, J. Alloys Comp. 508 (2010) 212–215.
[19] J.H. Kim, J.H. Shim, Y.W. Cho, J. Power Sources 181 (2008) 140–143.
[20] C. Rongeat, V. D’Anna, H. Hagemann, A. Borgschulte, A. Zuttel, L. Schultz, O.
Gutfleisch, J. Alloys Comp. 493 (2010) 281–287.
[21] E. Ronnebro, E.H. Majzoub, J. Phy. Chem. B 111 (2007) 12045–12047.
[22] H.W. Li, K. Kikuchi, Y. Nakamori, K. Miwa, S. Towata, S. Orimo, Scr. Mater. 57
(2007) 679–682.
[23] Y. Kim, S.J. Hwang, J.H. Shim, Y.S. Lee, H.N. Han, Y.W. Cho, J. Phy. Chem. C 116
(2012) 4330–4334.
[24] M.D. Riktor, M.H. Sorby, K. Chlopek, M. Fichtner, B.C. Hauback, J. Mater. Chem.
19 (2009) 2754–2759.
[
[
10] V. Ozolins, E.H. Majzoub, C. Wolverton, J. Am. Chem. Soc. 131 (2009) 230–237.
11] L.L. Wang, D.D. Graham, I.M. Robertson, D.D. Johnson, J. Phy. Chem. C 113
(
2009) 20088–20096.
[
[
[
[
12] S. Orimo, Y. Nakamori, G. Kitahara, K. Miwa, N. Ohba, S. Towata, A. Zuttel, J.
Alloys Comp. 404 (2005) 427–430.
13] P. Mauron, F. Buchter, O. Friedrichs, A. Remhof, M. Bielmann, C.N. Zwicky, A.
Zuttel, J. Phy. Chem. B 112 (2008) 906–910.
14] H.W. Li, K. Kikuchi, Y. Nakamori, N. Ohba, K. Miwa, S. Towata, S. Orimo, Acta
Mater. 56 (2008) 1342–1347.
15] R.J. Newhouse, V. Stavila, S.J. Hwang, L.E. Klebanoff, J.Z. Zhang, J. Phy. Chem. C
[25] C.B. Minella, S. Garroni, D. Olid, F. Teixidor, C. Pistidda, I. Lindemann, O.
Gutfleisch, M.D. Baro, R. Bormann, T. Klassen, M. Dornheim, J. Phy. Chem. C 115
(2011) 18010–18014.
1
14 (2010) 5224–5232.
[
[
16] G. Severa, E. Ronnebro, C.M. Jensen, Chem. Commun. 46 (2010) 421–423.
17] H.W. Li, T. Matsunaga, Y.G. Yan, H. Maekawa, M. Ishikiriyama, S. Orimo, J.
Alloys Comp. 505 (2010) 654–656.
[26] M. Chong, A. Karkamkar, T. Autrey, S. Orimo, S. Jalisatgi, C.M. Jensen, Chem.
Commun. 47 (2011) 1330–1332.