Hydrogen storage and hydrogen generation properties of CaMg2-based alloys

Article abstract of DOI:10.1016/j.jallcom.2016.08.307

The hydrogen storage and hydrogen generation of CaMg₂and CaMg_1.9Ni_0.1alloys were investigated by X-ray diffraction (XRD) and pressure-composition-isotherm (PCI) measurements. The results confirmed that the CaMg₂alloy cannot absorb hydrogen at room temperature. While the addition of Ni to the CaMg₂-based alloys resulted in room-temperature hydrogen absorption without an activation process, and a maximum hydrogen-absorption capacity of 5.65?wt%. The hydrolysis performance of hydrogenated CaMg₂and hydrogenated CaMg_1.9Ni_0.1(abbreviated as H-CaMg₂and H-CaMg_1.9Ni_0.1hereafter) was also evaluated. The hydrolysis reaction of H-CaMg₂occurred rapidly in water and resulted in a hydrogen yield of 800?mL/g. Furthermore, the hydrolysis properties of H-CaMg_1.9Ni_0.1were significantly enhanced with the addition of Ni, as evidenced by a hydrogen yield of 1053?mL/g in 12?min, this yield is 94.7% of the theoretical hydrogen yield of H-CaMg_1.9Ni_0.1.

Full text of DOI:10.1016/j.jallcom.2016.08.307

Accepted Manuscript
Hydrogen storage and hydrogen generation properties of CaMg -based alloys
2
Miaolian Ma, Ruoming Duan, Liuzhang Ouyang, Xiaoke Zhu, Zhiling Chen,
Chenghong Peng, Min Zhu
PII:
S0925-8388(16)32709-8
10.1016/j.jallcom.2016.08.307
JALCOM 38809
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 5 July 2016
Revised Date: 28 August 2016
Accepted Date: 29 August 2016
Please cite this article as: M. Ma, R. Duan, L. Ouyang, X. Zhu, Z. Chen, C. Peng, M. Zhu, Hydrogen
storage and hydrogen generation properties of CaMg -based alloys, Journal of Alloys and Compounds
2
(2016), doi: 10.1016/j.jallcom.2016.08.307.
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ACCEPTED MANUSCRIPT
Hydrogen storage and hydrogen generation properties of
CaMg2-based alloys
Miaolian Maa, b, Ruoming Duana, b, Liuzhang Ouyanga, b, c*, Xiaoke Zhua,
Zhiling Chena, Chenghong Penga, Min Zhua, b
a School of Materials Science and Engineering and Key Laboratory of Advanced Energy Storage Materials of
Guangdong Province, South China University of Technology, Guangzhou, 510641, PR China
b China-Australia Joint Laboratory for Energy & Environmental Materials, South China University of Technology,
Guangzhou, 510641, PR China
c Key Laboratory of Fuel Cell Technology of Guangdong Province, South China University of Technology,
Guangzhou 510641, PR China
Abstract:
The hydrogen storage and hydrogen generation of CaMg2 and CaMg1.9Ni0.1 alloys were
investigated by X-ray diffraction (XRD) and pressure-composition-isotherm (PCI) measurements.
The results confirmed that the CaMg2 alloy cannot absorb hydrogen at room temperature. While
the addition of Ni to the CaMg2-based alloys resulted in room-temperature hydrogen absorption
without an activation process, and a maximum hydrogen-absorption capacity of 5.65 wt.%. The
hydrolysis performance of hydrogenated CaMg2 and hydrogenated CaMg1.9Ni0.1 (abbreviated as
H-CaMg2 and H-CaMg1.9Ni0.1 hereafter) was also evaluated. The hydrolysis reaction of H-CaMg2
occurred rapidly in water and resulted in a hydrogen yield of 800 mL/g. Furthermore, the
hydrolysis properties of H-CaMg1.9Ni0.1 were significantly enhanced with the addition of Ni, as
evidenced by a hydrogen yield of 1053 mL/g in 12 min, this yield is 94.7% of the theoretical
hydrogen yield of H-CaMg1.9Ni0.1.
Keywords: Hydrogen storage; CaMg2-based alloys; Hydrogen absorption; Kinetics; Hydrolysis.
* Corresponding author: Liuzhang Ouyang, E-mail: meouyang@scut.edu.cn, Tel.: 86-20-87114253, Fax:
86-20-87112762.

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1. Introduction
Hydrogen is a promising renewable energy for replacing increasingly depleted fossil fuels
due to its high energy density and clean emissions [1, 2]. As a hydrogen storage materials,
magnesium-based materials are well suited for use as energy carrier media owing to their high
theoretical hydrogen-storage capacities (7.6 wt.% for MgH2), natural abundance, and low cost [3].
However, the practical applications of these materials are blocked by the kinetic and
thermodynamic properties. In the last few decades, several methods, such as ball-milling [4],
addition of catalysts [5], surface modification [6] or element substitution [7], and a combination
thereof, have been used to develop Mg-based alloys. Development of a new type of Mg-based
alloys with relatively high hydrogen capacity at moderate temperatures and hydrogen pressure is
therefore essential.
Recent studies have reported that magnesium alloys can react with hydrogen at room
temperature. For example, Mg-rare-earth-based alloys (such as Mg3Pr, Mg3La, and Mg3Mm with
reversible hydrogen capacities of 2.2 wt.%, 2.89 wt.%, and 2.91 wt.%) absorb hydrogen at room
temperature [8-10]; Mg-based pseudo-binary alloys (Mg0.67Ca0.33)Ni2 can reversibly absorb and
desorb 1.4 mass% hydrogen at room-temperature [11]. However, rare-earth metals are extremely
expensive and their hydrogen capacities are inadequate for commercial application. Therefore, the
development of alloys with satisfactory hydrogen capacity under moderate reaction conditions is
crucial.
Chiotti et al. [12] found that CaMg2 reacts with hydrogen and then transforms to CaH2 and
Mg at 450 °C. The CaMg2 alloy is expected to absorb more than 6.3 wt.% of hydrogen, at an H/M
ratio of two. It is well known that the transition metal Ni has catalytic effect of hydrogen on the Ni

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surface during the hydriding process of alloys. Researchers have reported some excellent
hydrogen storage properties of CaMg2-based alloys added with Ni. For example, Lupu et al. [13]
reported the CaMg1.8Ni0.5 alloy can absorb 5.7 wt.% of hydrogen at 338 °C with fast hydriding
process. Terashita et al. [14] found that the (Ca0.8La0.2)Mg2.2Ni0.1 alloy can absorb 5.1 wt.%
hydrogen at room temperature. To obtain a higher hydrogen capacity of Ca-Mg-Ni alloys and
reduce costs, we designed CaMg2-based alloys by adding a small amount of Ni.
In this work, the CaMg2-based alloys were prepared by induction melting and the
hydrogen-storage behavior of the resulting CaMg1.9Ni0.1 alloy was described by evaluating the
phase-structure transformation during hydrogenation and dehydrogenation, de/hydriding kinetics,
apparent activation energy, and thermodynamic parameters (enthalpy and entropy) of the
de/hydriding reaction. The hydrolysis properties of CaMg2, CaMg1.9Ni0.1, and their hydrides,
which constitute promising candidates for either hydrogen-storage or hydrolysis-induced
hydrogen-generating materials, are also considered in this study.
2. Experiment
2.1. Sample preparation
Raw materials are high purity Mg (≥99.9%), Ni (≥99.9%), and Ca (≥95%). The
CaMg2 alloy was prepared via the sealed tube method: placing an argon-filled pure iron
pipe in a resistance furnace maintained at 900 °C for 20 h. The CaMg1.9Ni0.1 alloy was
prepared via induction melting in an alumina crucible under the protection of pure argon
atmosphere. This melting process was repeated at least three times. Approximately 6–8 wt.%
of extra Mg and Ca was calculated for the loss of evaporation. The resulting Mg and Ni

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ingots were pulverized by saws and the oxide layer was polished with fine-grit sandpaper.
The as-melted ingots were broken into small particles and filtered through a 300-mesh
sieve. This filtering and subsequent storage of the samples were performed in an
argon-filled glove box equipped with a recirculation system, in order to prevent sample
oxidation and/or hydroxide formation. The CaMg2 and CaMg1.9Ni0.1 alloys were
hydrogenated by using an Advanced Materials Corporation (AMC) gas reaction controller.
2.2. Hydrolysis experiment
Hydrogen-generation measurements were performed using in-house-developed
equipment [15]. During these measurements, 0.1–0.2 g of CaMg2, CaMg1.9Ni0.1, and/or
their hydrides reacted with pure water in a 50-mL Pyrex glass reactor that has three
openings: one for water addition, one for inserting the thermometer, and one for hydrogen
exhausting. Hydrolysis began with contact between the sample and the water. The
generated hydrogen was passed through a Tygon tube and then through a Monteggia
washing bottle filled with room-temperature water, in order to condense the water vapor.
The amount of hydrogen generated was determined by extracting water into a beaker,
which was then placed on an electronic scale. The weight changes, over time, were
recorded and the quantity of hydrogen was thereby determined. The hydrogen-generation
rate and yield can be calculated from the reaction time and hydrogen volume that was
measured and analyzed by the computer. For the sake of reproducibility, each experiment
was repeated at least twice.
2.3. Sample characterization

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The chemical composition of the alloys was determined by energy dispersive x-ray
spectroscopy (EDX) accessory attached to a Philips XL-30 FEG scanning electron
microscope (SEM). Moreover, the phase structure of alloys was determined via X-ray
diffraction (XRD; Philips X’Pert MPD) using Cu-Kα radiation (λ=0.154056 nm). Scans
were performed over a 2
θ range of 20–80°, at an accelerating voltage, current, and
scanning rate of 40 kV, 15 mA, and 0.02°/s, respectively. The samples were wrapped with
liquid paraffin to prevent oxidation during XRD measurements. In addition, the hydrogen
absorption–desorption properties of the CaMg2 and CaMg1.9Ni0.1 alloys were evaluated at
different temperatures. The measurement temperature of each sample was controlled, to an
accuracy of ±2 K, via induction heating in the reaction cell. The
△H and △S were
determined from the van’t Hoff plot. For these calculations, the plateau pressure was taken
as the hydrogen pressure at the midpoint of the desorption pressure plateau of the measured
PCI curves.
3. Results and discussion
3.1. Phase transition of CaMg2 and CaMg1.9Ni0.1 during hydrogenation and dehydrogenation
The Ca-Mg-Ni alloy, with an EDX-determined chemical composition of 62.3 at%
Mg, 33.9 at% Ca, and 3.8 at% Ni, is referred to as CaMg1.9Ni0.1 alloy. The XRD patterns of
the as-melted, hydrogenated, and dehydrogenated CaMg2 and CaMg1.9Ni0.1 alloys are
shown in Fig. 1 and Fig. 2, respectively. The pattern shown in Fig. 1(a) indicates that
induction melting of pure Ca and Mg metals yields a CaMg2 compound consisting of C14
Laves phases and described by the space group, P63/mmc. Based on the ICDD file, the

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peaks indexed to the CaMg2 compound are attributed to a close-packed hexagonal structure
(hcp) phase with lattice constants of a=b=0.6225 nm and c=1.028 nm. Fig. 1(b) shows the
XRD pattern of the hydrogenated CaMg2 alloy (abbreviated as H-CaMg2) at 350
℃. The
peaks in this pattern indicate that the CaMg2 alloy reacts with hydrogen during the
hydriding process, thereby forming MgH2 and Ca4Mg3H14 phases, and a small amount of
Mg. The Mg peaks in the pattern of the dehydrogenated alloy (Fig. 1(c)), are sharper than
their counterparts in the pattern of H-CaMg2 (Fig. 1(b)). In addition, peaks corresponding
to MgH2 and Ca4Mg3H14 are absent, but new peaks corresponding to CaH2, are observed.
The CaH2 phase in H-CaMg2 arises from the dehydrogenation reaction of Ca4Mg3H14,
which proceeds as follows:
CaꢀMgꢁHꢂꢀ → CaHꢃ + Mg + Hꢃ ↑
(1)
Owing to the splitting catalytic effect of hydrogen on the Ni surface, the addition of
Ni may lead to accelerated hydrogenation of the CaMg2 alloy. CaMg1.9Ni0.1 alloy can
absorb hydrogen at room temperature and a hydrogen pressure of 3.8 MPa. Therefore, the
addition of Ni may accelerate dehydrogenation of the CaMg2-base alloy, even under mild
conditions. Fig. 2(a) shows the XRD pattern obtained from the CaMg1.9Ni0.1 alloy prepared
via induction melting. The diffraction peaks can all be indexed as CaMg2 and MgNi2. The
strongest diffraction peaks of CaMg2 are left-shifted by 0.26°, indicative of increases in the
lattice constant and volume. In other words, the Ni catalyst and lattice expansion facilitate
hydrogenation of the CaMg1.9Ni0.1 alloy, even at room temperature. Peaks corresponding to
the CaMg2 and MgNi2 phases occur in the XRD patterns (Fig. 2(b)) of the hydrogenated
CaMg1.9Ni0.1 alloy. In addition, several sharp peaks, which appear inconsistent with the

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ICDD file, are attributed to Ca5Mg9H28, based on the XRD, PCI and EDX measurements.
The Ca5Mg9H28 phase, whose hydrogen absorption/desorption properties differ from those
of the H-CaMg2 system, is a new Ca-Mg hydride that merits further study. Furthermore,
except for the occurrence of peaks corresponding to the MgNi2 phase, the pattern of the
dehydrogenated CaMg1.9Ni0.1 alloy (Fig. 2(c)) is identical to that of the dehydrogenated
H-CaMg2 alloy (Fig. 1(c)). The phase transformation of the CaMg1.9Ni0.1 alloy during
hydrogenation and dehydrogenation will be discussed further in the following section.
Fig. 1 XRD patterns of CaMg2 alloys at different stages: (a) as-melted, (b) hydrogenation
and (c) dehydrogenation
Fig. 2 XRD patterns of CaMg1.9Ni0.1 alloys at different stages: (a) as-melted, (b)
hydrogenation and (c) dehydrogenation
3.2. Dehydriding thermodynamic properties of CaMg2 and CaMg1.9Ni0.1 alloys

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The thermodynamic parameters associated with the dehydriding reaction of
H-CaMg1.9Ni0.1 were determined from pressure-composition isotherm (PCI) curves. Fig.
3(a) shows the PCI curves obtained at 573, 593, 613, and 633 K during hydrogen
desorption of this alloy. The maximum hydrogen desorption capacity (2.57 wt.%) can be
obtained at 633 K. Moreover, two plateaus occur in each PCI desorption curve, indicating
that the dehydrogenation process of H-CaMg1.9Ni0.1 can be divided into two steps. The
high flat plateau occurs at high pressure and is composed of a few data points, indicating
that the desorption process occurs more easily in the initial stage than in subsequent stages.
The dehydrogenation mechanism of H-CaMg1.9Ni0.1 was further investigated via quasi-in
situ XRD analysis.
The dehydrogenation reaction was stopped when the first point (denoted as I in Fig.
3(a)) appeared on plateau 1 of the PCI curve. As shown in Fig. 4(a), peaks corresponding
to MgCaH3.72, MgH2, and Mg phases appear in the pattern during the first step of
dehydrogenation. Therefore, the initial plateau region corresponds to the decomposition of
Ca5Mg9H28, which yields H2, Mg, MgH2, and MgCaH3.72. Plateau 2 forms during the
dehydrogenation reaction; the XRD pattern of the resulting product (denoted as point II in
Fig. 3(a)) is shown in Fig. 4(b). The relative strength of the peaks corresponding to the Mg
and CaH2 phases increased, whereas the MgH2 phase nearly disappeared. Therefore, the
second plateau region corresponds to the dehydrogenation of MgH2 and MgCaH3.72, which
subsequently undergo complete transformation to Mg and CaH2 phases. The dehydriding
pressure of CaMg1.9Ni0.1 is higher than that of pure Mg at the same temperature, and is
indicative of unstable dehydriding thermodynamics.

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Fig. 3 (a) PCI curves for the hydrogen desorption and (b) Van’t Hoff plots for the two plateaus
of H-CaMg1.9Ni0.1 composite.
Fig. 4 XRD patterns of the dehydrogenation products: (a) plateau
1 and (b) plateau 2.
The van’t Hoff plots (Fig. 3(b)) of the H-CaMg1.9Ni0.1 sample were obtained from the
aforementioned PCI curves. Furthermore, the enthalpy and entropy of the dehydriding
process were determined from the Van’t Hoff equation (Eq. (2)), which is given as follows:
∆ꢈꢇ
ꢉꢊ
∆ꢋꢇ
ꢉ
ꢄꢅꢆꢇ = −
+
(2)
where Kθ is the standard equilibrium constant, Kθ= PH2 during dehydriding, and K = 1/PH2
during hydriding. The midpoint of the plateau was used as the equilibrium pressure for the
calculation. Enthalpy and entropy values of 94.80 kJ/mol H2 and 160.2 J/(K·mol) H2,
respectively, were obtained for the dehydrogenation plateau of Ca5Mg9H28. Corresponding

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values of 97.34 kJ/mol H2 and 168.2 J/(K·mol) H2 were obtained for the dehydrogenation
reaction of MgCaH3.72 and MgH2.
3.3. Hydriding properties of CaMg2 and CaMg1.9Ni0.1 alloys
Fig. 5(a) shows the hydriding-kinetic curves for the CaMg2 alloy at 573 K and
CaMg1.9Ni0.1 alloy at different temperatures. A capacity of 5.10 wt.% was realized for the
hydrogenation of CaMg1.9Ni0.1 alloy at room temperature. The maximum hydrogen
capacity of 5.65 wt.% in this alloy were attained at 353 K. However, the hydriding kinetics
of the CaMg2 alloy were sluggish, as shown in Fig. 5, even after 16 h, the hydrogenation
process was incomplete.
The hydriding-kinetic curves were analyzed by using the Johnson–Mehl–Avrami–
Kolmogorov (JMAK) model [16, 17]:
η
ꢌ ꢎ
α
ꢍ = ꢏ − ꢐꢑꢒ[−ꢌꢓꢍꢎ ]
(3)
where
α
(
t
), k, and
η
are the reaction fraction at time
t, rate constant, and Avrami exponent,
respectively. The JMAK equation is widely used to describe the time-dependent kinetic
behavior of isothermal solid-state reactions [18, 19]. In most cases, Eq. (2) is rewritten as:
[
ꢌ
ꢎ]
ꢌꢍꢎ =
ꢄꢅ − ꢄꢅ ꢏ −
α
η
ꢄꢅꢍ +
η
ꢄꢅꢔ
(4)
A hydrogen absorption fraction ranging from 0.2–0.5 was used to fit the kinetic
curves. By plotting ln[-ln(1- ))] versus ln , the Avrami exponent and the rate constant
can be obtained from the slope and intercept, respectively, of the fitted line as shown in
α
(
t
t
η
ꢔ
Fig. 5(b). The
η values obtained (i.e., 0.92–1.25) were close to unity, indicating that the
hydriding reaction of the sample follows a diffusion-controlled mechanism [20]. In

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addition, the apparent activation energy (Ea) of hydrogenation was determined by
substituting the values of
ꢔ
into the Arrhenius equation (Eq. (5)):
ꢗꢘꢙ
ꢔ = ꢕ ꢐꢑꢒ ꢖ ꢚꢛ
ꢜ
(5)
where
absolute temperature, respectively. The hydrogen absorption of CaMg1.9Ni0.1 exhibits
significant linearity when ln is plotted as a function of 1/ . Moreover, the hydrogenation
A, R, and T are the pre-exponential factor, gas constant (R=8.314 J/(mol·K)), and
k
T
Ea (41.74 kJ/mol) is significantly lower than those of pure MgH2 ( 100 kJ/mol) [21, 22]
and Mg2Ni ( 50 kJ/mol) [23-25].
Fig. 5 (a) Isothermal hydriding kinetic curves obtained at different temperatures, (b) JMAK
plots, and (c) Arrhenius plot for determining the activation energy of the CaMg1.9Ni0.1
alloy.
3.4. Hydrolysis properties of CaMg2, CaMg1.9Ni0.1 alloys, and their hydrides

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Owing to their room-temperature hydrogen-absorption capacity, CaMg1.9Ni0.1 alloys
can store hydrogen released from the factory. CaMg2 and CaMg1.9Ni0.1 alloys have
theoretical hydrogen-storage capacities of 6.4 wt.% and 5.9 wt.%, respectively. However,
hydrolysis-induced hydrogen generation is twice as high as the storage capacity, and values
of up to 12.8 wt.% (1434 ml/g H2) and 11.8 wt.% (1327 mL/g H2) have been realized in
this work. We investigated the hydrolysis properties of CaMg2-based alloys and hydrides
in order to determine the effect of hydrogen addition on the CaMg2 system. The hydrolysis
behavior of the as-milled CaMg2 and CaMg1.9Ni0.1 alloys, at 298 K, is described by the
hydrogen evolution curves shown in Fig. 6(a). A comparison of the hydrolysis kinetics of
these alloys reveals that, in the first 30 min, the hydrogen-generation rate of CaMg2 was
higher than that of CaMg1.9Ni0.1 and slower thereafter. The hydrolysis of CaMg2 yielded (at
a constant rate) 131 mL/g, 179.3 mL/g, and 351.7 mL/g of hydrogen in 20, 30, and 72 min,
respectively. During the hydrolysis of CaMg1.9Ni0.1, 100 mL/g of hydrogen was generated
in 20 min. However, the hydrolysis rate increased thereafter and hence 171.4 mL/g, 442.9
mL/g, and 571 mL/g of hydrogen were generated in 30, 72, and 120 min, respectively. The
increased reactivity of CaMg1.9Ni0.1, after the first 30 min, stems from the addition of Ni,
which has a low hydrogen overpotential and (as a cathode material) induces severe
galvanic corrosion of alloys [26].
Fig. 5(b) shows the hydrogen generation curves that describe the hydrolysis behavior
of the H-CaMg2 and H-CaMg1.9Ni0.1 hydrides. The hydrolysis rate of H-CaMg1.9Ni0.1 is
higher than that of H-CaMg2. In fact, 872 mL/g, 968 mL/g, and 1053 mL/g (9.4 wt.%) of
hydrogen were generated in 3 min, 5 min, and 12 min, respectively, in a sustained and

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rapid hydrogen-release process. The hydrolysis of H-CaMg2 was initially very rapid,
lasting for only 50 s and producing 800 mL/g (7.14 wt.%) of hydrogen. The max
hydrolysis rate achieved (2640 mL/(min·g)) is, to the best of our knowledge, the fastest
hydrolysis rate ever reported for the Ca-Mg-H system. The addition of Ni significantly
enhanced the hydrolysis properties of the CaMg2 alloy. Therefore, in 12 min,
H-CaMg1.9Ni0.1 produces 1053 mL/g of hydrogen, which is 94.7% of its theoretical
hydrogen yield.
Fig. 6 Hydrogen evolution curves associated with hydrolysis of (a) as-prepared CaMg2 and
CaMg1.9Ni0.1 alloys, (b) H-CaMg2 and H-CaMg1.9Ni0.1 hydrides at 298 K.
A comparison of the hydrolysis properties and phase content of H-CaMg2 and
H-CaMg1.9Ni0.1 reveals that the hydrolysis properties of the Ca5Mg9H28 phase are far
superior to those of the Ca4Mg3H14 phase. Complete hydrogenation of CaMg2 yields 65%
of Ca4Mg3H14. Furthermore, the theoretical hydrogen generation yield of hydrogenated
CaMg2 (come from 65% Ca4Mg3H14 is 825 mL/g hydrogen) is similar to that of H-CaMg2
(800 mL/g). The hydrolysis curves revealed that the hydrolysis of H-CaMg2 is incomplete.
However, the addition of Ni resulted in significantly enhanced hydrolysis properties, as
evidenced by the hydrogen yield (1053 mL/g in 12 min) of H-CaMg1.9Ni0.1. The rapid and
constant hydrolysis rate of this alloy can exceed 365 mL·min-1·g-1 in the first 2 min.

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Table 1 shows list of the technologies for the preparation of Mg-Ca based materials
and the hydrogen yields in previous works. Compared to the methods used in those studies,
the method employed in this work involves milder reaction conditions, a simpler reaction
process, and higher hydrogen yields.
Table 1 Hydrolysis-generated hydrogen yields of CaMg2-based materials
Reactants
Conditions
Max hydrogen yield (298 K)
Spex 8000 shaker mill/10 h/1
76% of hydrolyzed fraction in
90 min
MgH2 and 20.3 mol% Ca [27]
atm Ar
magnetic stirrer
Spex 8000 shaker mill/10 h/1
MgH2 and 20.3 mol% CaH2
[27]
76% of hydrolyzed fraction in
30 min
atm Ar
magnetic stirrer
50% of hydrolyzed fraction in
30 min, 57% in 45min and 62%
in 80min
Spex 8000 shaker mill/15 h
Spex 8000 shaker mill/15 h
MgH2-25 wt% Ca [28]
65% of hydrolyzed fraction in
20 min, 70% in 30 min and 76%
in 80 min
MgH2-25 wt% CaH2 [28]
201 mL/g, 400 mL/g and
732.8 mL/g in 10 min,
respectively.
320.6 mL/g, 549.6 mL/g and
755.7 mL/g in 60 min,
respectively.
Resistance furnace
melting/Spex 8000 shaker
mill/2 h/hydrogenated at 673
K and 4-5 MPa H2.
x wt.% Mg-Ca (x=10, 20 and
30 ) alloy hydride [29]
Resistance furnace
melting/broken/300-mesh
sieved
179.3 mL/g in 30 min, and
351.7 mL/g H2 in 72 min
CaMg2 alloy (in this work)
442.9 mL/g in 72 min, and
571 mL/g (80.9%)H2in 120
min
CaMg1.9Ni0.1 alloy (in this
work)
Induction melting/broken/300
mesh sieved
Resistance furnace
melting/broken/300 mesh
sieved/hydrogenated at 573 K
and 3.8 MPa H2.
CaMg2 hydride (in this work)
800 mL/g in 1 min
Induction melting/broken/300 968 mL/g in 5 min, and1053
CaMg1.9Ni0.1 hydride (in this
work)
mesh sieved/hydrogenated
at 298 K and 3.8 MPa H2.
mL/g (94.7%) hydrogen in
12 min
3.5. The hydrolysis mechanisms of H-CaMg2 and H-CaMg1.9Ni0.1

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The hydrogen evolution mechanism for the hydrolysis of H-CaMg2 and
H-CaMg1.9Ni0.1 alloys at 298 K was also analyzed by using the Avrami–Erofeev equation
[30]. Fig. 7(a) and (b) show the hydrogen evolution curves by hydrolysis of H-CaMg2 and
H-CaMg1.9Ni0.1 alloys at 298 K, respectively. The hydrogen production is expressed as
conversion yield (%), which is defined as the volume of produced hydrogen over the
theoretical volume. As shown in Fig. 7, the experimental curves describing the hydrolysis
of H-CaMg2 and H-CaMg1.9Ni0.1 are well-fitted to the Avrami–Erofeev equation (Eq. (6)),
which is deduced from the nucleation and growth process:
ꢠ
ꢌ ꢎ
ꢝ ꢞ = 1 − expꢌ−ꢟ ∗ ꢞ ꢎ
(6)
ꢌ ꢎ
where ꢡ ꢢ is the reaction rate, i.e., the ratio of reacted material to total material, B and m
are constants, and t is the reaction time. The values of B and m, which were obtained via
fitting, estimated error S (i.e., the standard error), and r, the correlation coefficient, are
shown in Fig. 7. The low values of S and r confirm the close correspondence between the
fitted and experimental data. The fitting confirms that the hydrolysis reaction of H-CaMg2
and H-CaMg1.9Ni0.1 is governed by the nucleation growth mechanism. However, owing to
the difference in their m values, the rate-controlling steps of the nucleation and growth
reaction are different for different alloys. For example, m value of 0.62 is the
one-dimensional diffusion process, while 1.07 indicates the three-dimensional interface
reaction process [31]. The m value of H-CaMg2 (0.54) is close to 0.62, indicating that the
reaction is basically a one-dimensional diffusion process. While, an m value of 0.9 is close
to 1.07 (see Fig. 6(b)), indicating that the hydrolysis reaction of H-CaMg1.9Ni0.1 is a
three-dimensional interface reaction process. This may result from the catalytic effect of

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fine Ni particles, which uniformly distribute in the matrix of the hydrides, and promote the
hydrolysis reaction around the interface region.
Fig. 7 Kinetic curves associated with hydrogen evolution of the (a) H-CaMg2 and (b)
H-CaMg1.9Ni0.1 composites at 298 K.
4. Conclusions
Hydrogen storage and hydrogen generation properties of CaMg2-based alloys were
investigated. CaMg2 alloy did not absorb hydrogen at 298 K, but, at 573 K, absorbed 4.48 wt.% of
hydrogen and transformed to Ca4Mg3H14 and MgH2 under 3.8 MPa of hydrogen. The addition of
Ni enabled room-temperature absorption of hydrogen. In fact, the CaMg1.9Ni0.1 alloy had a
room-temperature hydrogen-absorption capacity of 5.65 wt.% with an apparent activation energy
of 41.74 kJ/mol. Furthermore, in the case of Ca5Mg9H28, values of 94.80 kJ/mol H2 and 160.2
J/(K·mol) H2 were obtained for the enthalpy and entropy, respectively, of dehydrogenation;
corresponding values of 97.34 kJ/mol H2 and 168.2 J/(K·mol) H2 were obtained for MgCaH3.72
.
Hydrolysis of the H-CaMg1.9Ni0.1 alloy occurs rapidly at room temperature, even without the
addition of a catalyst, leading to a hydrolysis yield of 94.7%, in 12 min, and 1053 mL/g of
hydrogen. The reaction mechanism of H-CaMg1.9Ni0.1 is governed by a three-dimensional
interface reaction process. CaMg1.9Ni0.1 alloy can be industrially synthesized via

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conventional induction melting of abundant and cheap Mg, Ca, and a small ratio of Ni.
This alloy can absorb hydrogen at room-temperature and mild hydrogen pressure. These
attributes suggest that the CaMg1.9Ni0.1 alloy, a new low-cost hydrogen storage and highly
efficient hydrolysis material, has significant potential for use in portable fuel cells or other
commercial hydrogen-production systems.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China
(Nos. 51431001, 51271078 and U120124), by International Science & Technology
Cooperation Program of China (2015DFA51750) and by the Project Supported by
Guangdong Natural Science Foundation (2014A030311004, 2014GKXM011). Author
Ouyang also thanks Guangdong Province Universities and Colleges Pearl River Scholar
Funded Scheme (2014).
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Highlights
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The hydrogen storage properties and transition mechanism of CaMg2-based
alloys are clarified.
The hydrolysis performance of hydrogenated CaMg2 hydrides is discussed for the
first time.
CaMg1.9Ni0.1 alloys absorb 5.65 wt.% hydrogen at 25 with Ea being 41.74
kJ/mol.
An undefined hydride in Mg-Ca system has been determined to be Ca5Mg9H28.