Reversible hydriding and dehydriding properties of CaSi: Potential of metal silicides for hydrogen storage

Article abstract of DOI:10.1063/1.1773930

The reversible hydriding and dehydriding properties of CaSi were investigated. First-principles calculations were performed to investigate the stability of CaSi hydride by the ultrasoft pseudo-potential method based on the density functional theory. A CaSi sample was examined by pressure-composition isotherm measurement, hydrogen analysis using the inert gas fusion method, and x-ray diffraction analysis. It was found that CaSi reversibly absorbs and desorbs hydrogen in a temperature range of 473-573 K. The reversible hydriding and dehydriding properties of CaSi indicate the potential of metal silicides for hydrogen storage.

Full text of DOI:10.1063/1.1773930

Reversible hydriding and dehydriding properties of CaSi: Potential of metal silicides for
hydrogen storage
M. Aoki, N. Ohba, T. Noritake, and S. Towata
Citation: Applied Physics Letters 85, 387 (2004); doi: 10.1063/1.1773930
View online: http://dx.doi.org/10.1063/1.1773930
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 3
19 JULY 2004
Reversible hydriding and dehydriding properties of CaSi: Potential
of metal silicides for hydrogen storage
M. Aoki,a) N. Ohba, T. Noritake, and S. Towata
Toyota Central R&D Labs, Inc., Nagakute, Aichi 480-1192, Japan
(Received 9 March 2004; accepted 21 May 2004)
We found that CaSi reversibly absorbs and desorbs hydrogen. First-principles calculations
theoretically indicated that CaSi hydride is thermodynamically stable. The hydriding and
dehydriding properties of CaSi were experimentally determined using pressure-composition ͑p-c͒
isotherms and x-ray diffraction analysis. The p-c isotherms clearly demonstrated plateau pressures
in a temperature range of 473–573 K. The maximum hydrogen content was 1.9 wt % under a
hydrogen pressure of 9 MPa at 473 K. The reversible hydriding and dehydriding properties of CaSi
suggest the potential of metal silicides for hydrogen storage. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1773930]
Hydrogen is considered to be one of the new clean en-
ergy sources capable of replacing fossil fuels. The use of
hydrogen-based energy in practical applications such as fuel
cell vehicles, however, requires the development of safe and
efficient hydrogen storage technology. Although metal hy-
drides hold promise for hydrogen storage, those developed
so far do not possess sufficient storage capacity on a unit
weight basis for practical applications.1–3 Accordingly, the
development of hydrides of light metals possessing large hy-
drogen storage capacities is an urgent necessity.
It is well known that ZrNi and LaNi with the CrB-type
structure (space group Cmcm) absorb a large number of hy-
drogen atoms and form ZrNiH3 and LaNiH4,
respectively.4–16 Other alloys with the same structure are also
expected to have high hydrogen storage capacities. There-
fore, we turned our attention to CaSi,17,18 which has the CrB-
type structure, and studied its hydriding and dehydriding
properties both theoretically and experimentally. We ex-
pected the hydrogen storage capacity of CaSi to be much
larger than those of ZrNi and LaNi on a unit weight basis,
because Ca and Si are much lighter elements than Zr, La, and
Ni. In addition, Ca and Si have the advantage of being low-
cost elements.
First, we performed first-principles calculations to inves-
tigate the stability of CaSi hydride by the ultrasoft pseudo-
potential method19 based on the density functional theory.20
The theoretical calculations suggested that the enthalpy of
formation of CaSi hydride is negative. Encouraged by this
result, we conducted an experiment to investigate the hydrid-
ing and dehydriding properties of CaSi. The results are pre-
sented in this letter. The fact that CaSi reversibly absorbs and
desorbs hydrogen is a new find, and neither theoretical nor
experimental studies on this phenomenon have been re-
ported.
The sample was examined by pressure-composition
͑p-c͒ isotherm measurement, hydrogen analysis using the in-
ert gas fusion method (Horiba EMGA-621), and x-ray dif-
fraction (XRD) analysis (Rigaku RINT-TTR). The conven-
tional volumetric method with a Sieverts apparatus (Suzuki
Shokan Co., Ltd.) was used to obtain p-c isotherms when
CaSi was dehydrided at 473, 523, and 573 K. Before each
measurement, the sample was hydrogenated at 473 K under
a hydrogen (purity 99.999 99%) pressure of 9 MPa. The
XRD measurements were carried out with Cu K␣ radiation at
room temperature, and the diffraction patterns were analyzed
by the Rietveld method using the computer program
RIETAN.21
Figure 1 shows the p-c isotherms of CaSi during dehy-
driding at 473, 523, and 573 K. The hydrogen contents at
473 K under 9 MPa and 1ϫ10−3 MPa of hydrogen pressure
were 1.9 wt % ͑CaSiH1.3͒ and 0.4 wt % ͑CaSiH0.26͒, respec-
tively. These values were in agreement with those obtained
by the hydrogen analysis. The hydrogen analysis also re-
vealed that, after the sample was evacuated at 473 K at a
pressure below 1ϫ10−1 Pa, its hydrogen content was
0.2 wt % ͑CaSiH0.13͒. Each of the p-c isotherms in the tem-
perature range selected for this work clearly showed a pla-
teau pressure. The maximum hydrogen content per formula
unit of CaSi in the plateau region (CaSiH) is comparable to
the hydrogen content of ZrNiH.4–7 The enthalpy and entropy
A CaSi sample was prepared by melting a mixture of Ca
(purity 99.5%) and Si (purity 99.999% plus) in a high-purity
graphite crucible placed in a high-frequency induction fur-
nace under an argon pressure of 0.2 MPa. The CaSi melt
weighed ϳ400 g. The alloy was then heat-treated in an argon
atmosphere at 1223 K for 30 h and finally quenched in wa-
ter.
FIG. 1. p-c isotherms of CaSi in dehydriding processes at 473, 523, and
573 K.
a)Electronic mail: e1172@mosk.tytlabs.co.jp
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388
Appl. Phys. Lett., Vol. 85, No. 3, 19 July 2004
Aoki et al.
p-c isotherm measurement was carried out to 1ϫ10−3 MPa
of hydrogen pressure to dehydrogenate the sample, the dif-
fraction peaks of the CaSiH1.3 phase disappeared, and those
of the CaSi phase appeared with 131 and 200 reflections for
the orthorhombic lattice splitting as indicated in Fig. 3. The
lattice parameters estimated by the Rietveld analysis were
a=4.5336͑7͒ Å, b=10.839͑2͒ Å, and c=3.8913͑6͒ Å. These
changes in the lattice parameters are attributed to the hydro-
gen dissolved in the CaSi phase.
We will systematically investigate the detailed structure
of CaSi hydride by synchrotron radiation x-ray diffraction
and neutron diffraction measurements to understand the hy-
driding and dehydriding properties of CaSi, through an ex-
periment and theoretical discussion.
In conclusion, we found that CaSi reversibly absorbs and
desorbs hydrogen in a temperature range of 473–573 K.
CaSi has the same CrB-type structure as ZrNi and LaNi,
however, it shows different hydriding and dehydriding prop-
erties. Therefore, it is important to investigate the detailed
structure of CaSi for improving its performance as a practical
hydrogen storage material. CaSi is a new hydrogen storage
alloy, and its reversible hydriding and dehydriding properties
suggest great potential of metal silicides for hydrogen stor-
age. We are also investigating the hydriding and dehydriding
properties of other metal silicides.
FIG. 2. van’t Hoff plot of CaSi in the dehydriding process.
of hydride formation calculated using a van’t Hoff plot
(shown in Fig. 2) were −62 kJ/mol H2 and
−116 J/mol H2 K, respectively. This value of the enthalpy,
which is less negative than those of ZrNiH, ZrNiH3, and
LaNiH4,5–9 shows that CaSiH1.3 is unstable as compared with
these hydrides.
Figure 3 shows the XRD profiles of the sample as pre-
pared, after hydrogenation and after dehydrogenation at
473 K in all cases. The sample as prepared was composed of
the CaSi phase and a small quantity of CaO phase. The lat-
tice parameters of the CaSi phase determined by a Rietveld
analysis of the XRD profile were a=4.5589͑1͒ Å, b
=10.7250͑2͒ Å, and c=3.8930͑1͒ Å, which are in agreement
with the values reported in the previous work.17,18 After the
sample was hydrogenated at 9 MPa, the diffraction peaks of
the CaSiH1.3 phase appeared, while those of the CaSi phase
disappeared. The CaSiH1.3 phase was indexed on an ortho-
rhombic lattice with the parameters a=11.217͑1͒ Å, b
=14.615͑2͒ Å, and c=3.8165͑3͒ Å. The diffraction intensi-
ties suggested the space group to be Pbcn (No. 60). Rietveld
refinements based on a metal atom sublattice in the space
group Pbcn converged at Rwp=5.7% and RI=11.4%, where
Rwp and RI are the weighted profile reliability factor and the
reliability factor based on integrated intensities, respectively.
The structure of the CaSiH1.3 phase (space group Pbcn) is
different from those of ZrNiH12 and ZrNiH3.12,15 After the
The authors would like to thank K. Miwa for his valu-
able comments and suggestions regarding this study and Y.
Kondo for conducting the valuable hydrogen analysis.
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FIG. 3. X-ray diffraction profiles of CaSi (a) as-prepared, (b) after hydro-
genation under 9 MPa of hydrogen pressure, and (c) after dehydrogenation
as the p-c isotherm measurement was carried out to 1ϫ10−3 MPa of hydro-
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