J. Plasma Fusion Res. SERIES, Vol. 10 (2013)

Hydrogen Storage Properties of Nanocrystalline Mg2Ni
Based Alloys Prepared by Ball-Milling
Yifu XIONG, Jingwen BA, Wuwen QINGˈWenyong JING
China Academy of Engineering Physics, Mianyang, 621900, China(Received: 15 May 2012 / Accepted: 26 November 2012)

Nanocrystalline hydrides are a new class of material in which outstanding hydrogen sorption may be obtainedby proper engineering of the microstructure and surface. The nanocrystalline Mg2Ni alloy is a promising hydrogenstorage material. In the present work, nanocrystalline Mg2Ni alloy powders with grain size of about 50 nm wereprepared by high-energy ball-milling, and its phase, crystal structure and hydrogen storage properties wereinvestigated by X-ray diffraction analysis, transmission electron microscopy and pressure-composition isotherms,respectively. The hydrogen storage characteristics of Mg2Ni are also presented. Nanocrystalline Mg2Ni can readily
absorb hydrogen at temperature lower than 523 K. The reversible hydrogen capacity is up to 3.5 wt.%.
Keywords: Mg2Ni, nanocrystalline, mechanical alloying, ball-milling, hydrogen storage

elemental Mg and Ni powders under hydrogen
atmosphere. Mg2Ni alloy powder has been followed by
X-ray diffraction (XRD) and transmission electron
microscopy (TEM), and the values of enthalpy andentropy changes for hydrogen absorption and desorption
were evaluated from the pressure-composition isotherms
on van’t Hoff equation.

1. Introduction

Many metallic materials are known to form hydrides
reversibly. Intermetallic Mg2Ni with its high hydrogencapacity (up to 3.6 wt.%) is the prime candidates amonghydrogen storage systems [1–5]. However, Mg2Ni cannot
absorb hydrogen under normal conditions (i.e. room
temperature and atmospheric pressure), and the ability ofhydrogenation appears in temperature range from
523–623 K with hydrogen pressure of 1.5–5 MPa.
Moreover, even if Mg2Ni is heated more than 523 K, it
also needs to be activated before hydrogenation. The
activation process usually involves a heating process at ahigh temperature (598 K) under a high hydrogen pressure(2 MPa). The activation process has to be carried out
several times to obtain reproducible hydrogen
absorption/desorption characteristics. Improvement of thehydrogenation conditions of Mg2Ni is therefore essential
to produce a suitable material for practical hydrogenstorage [6–8].
The mechanical alloying (MA) may give various
homogeneous composite particles. Since 1970, the MA
process via the ball-milling (BM) technique has beenused to prepare several dispersion-strengthened alloy
powders. The application of the MA process has been
expanded recently to produce several hydrogen storage
metals using a method called the reactive ball-milling.
Metal hydrides are usually prepared by passing a flowof reactive hydrogen over the metallic material underhigh pressure and at temperatures usually above room
temperature. In our study, the single-phase Mg2NiH4
alloy powders with grain size about 50nm has beensynthesized by milling an equiatomic mixture of

2. Experimental

In order to produce Mg2Ni, a mixture of magnesium(99.95%, 150 mesh) and nickel (99.95%, 250 mesh),purified hydrogen and argon gas (H2O and O2, less than
20 ppm) has been used. The mixed powders were charged
and sealed in a cylindrical stainless steel shell together
with stainless steel rods in a laboratory high-energy mill
SPEX 8000 (from SPEX Industries). The rod-to-powderweight ratio was controlled to be about 20 to 1.The inletof the shell was connected to a rotary pump andevacuated for about 4 Pa. After evacuation, a flow ofhydrogen gas was passed into the rod mill through abellows pipe. Once the rod mill was filled withhydrogen at 0.25 MPa, the inlet of the vial was closed
and the reactive rod-milling (RRM) process wascarried out at ambient temperature by mounting therod mill on a rotator at the rate of 2.0 s-1. The RRMwas stopped at selected intervals and a small amountof the rod-milled powder was taken out under argon
atmosphere in a glove box.

3. Results and discussion

Mg2Ni is difficult to prepare by conventional
metallurgy because of the great difference in melting

author’s e-mail: xiongyifu@yeah.net

94©2013 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Y. Xiong et al., Hydrogen Storage Properties of Nanocrystalline Mg2Ni Based Alloys Prepared by Ball-Milling

temperature and vapor pressures of Mg and Ni.
Mechanical alloying by ball-milling requires no melt
process of the elements and yields the compound througha solid-state reaction. Although this ball-milling is quite acommon technique, published results on ball-milled Mg
and Ni powders have not yet been shown to produce
single-phase [2,5].
Under the ball-milling conditions (high-energy milland high ball-to-powder mass ratio) in the present work,
Mg2Ni alloy powder was formed directly. The
ball-milling parameters were as follows: The ratio of rod
and metallic powder was 20 to 1, rotational speed ofball-milling per minute was 0.2 s-1 and processing
control reagent is stearic acid.
Fig. 1 shows the X-ray diffraction patterns of Mg2Ni
after mechanically ball-milling for 2 h and 50 h underhydrogen atmosphere. Fig. 1 (a) shows an intermediatestage of mechanical alloying reaction (after 2 h ofball-milling), and this diffraction pattern includes thepeaks of the Mg2Ni compound, Mg and Ni. After millingfor 50 h, the diffraction peaks of Mg2NiH4 become
broader and none of the peaks corresponding to MgH2and NiH2 are seen. It is worth noting that the broadness ofthese peaks can be attributed to the formation ofMg2NiH4 solid solution powder, as shown in Fig. 1 (b).
We should emphasize that this solid solution phase has
not changed to any other phase, even after milling for as
long as 50 hours.
The size of nanocrystalline Mg2Ni alloy was calculatedby Scherrer formula as follows:
D = KO/E1/2cosT, (1)where D is the size of nanocrystalline, K is Scherrer
constant (0.89), O is the wavelength of CuKD X-rays(0.15405 nm), E1/2 is the width of half-height for
diffraction peak (rad). Those of (101) and (105) ofcommon Mg2Ni alloy are 0.3062° and 0.2879°,
respectively. In our work, those of (101) and (105) of
nanocrystalline Mg2Ni alloy were 0.3204° and 0.2950°,
respectively. The size of nanocrystalline Mg2Ni alloy wasevaluated from XRD patterns to be about 50 nm.
Detailed TEM analyses were performed in order tounderstand the change in the fine structure ofmechanically alloyed Mg2NiH4 particles during
above-mentioned stages of milling (Fig. 2). The powder
have large grains of about 50 nm diameter and theselected-area diffraction patterns (SADPs) taken at thecenter of this micrograph shows sharp rings coexisting
1with spot patterns of nanocrystalline Mg2Ni alloy powderin Fig. 2 (c).
Figs. 3 and 4 show pressure-composition isotherms ofnanocrystalline Mg2Ni prepared with 50 h of ball-milling.
The absorption/desorption temperature was in a range
from 473 K to 623 K, and the nanocrystalline Mg2Ni-H2
system has obvious and wide plateau region. Reversiblehydrogen capacity amounts to 3.5 wt.% within the 95Fig. 1 XRD patterns of Mg-Ni alloys prepared byball-milling for 2 h and 50 h.

111Fig.2 TEM images of Mg-Ni alloys prepared byball-milling under different conditions(a-2h, 120 K˗b-50h, 150 K˗c-electron
diffraction pattern n (50h)).
applied hydrogenation time, and the capacity agreed withthe literature [5]. The amount of absorbed hydrogen isusually 3.0–3.45 wt.% in the reversible hydrogenation
cycles as reported in [5, 6]. The hydrogenation of thehigh-temperature phase of Mg2NiH4 is apparently mucheasier than for the low-temperature phase. The
low-temperature phase of Mg2NiH4 is very difficult toobtain by the direct hydrogenation under moderatepressure conditions. The plateau pressures difference
between absorption and desorption at the same
temperature was observed (Fig. 5). The phenomenon wasreferred to hysteresis (its represents by hysteresis factor
Hf), Hf increases with temperature. The hysteresis
originates duo to the presence of transformation strains
during both hydride formation and decomposition.

Y. Xiong et al., Hydrogen Storage Properties of Nanocrystalline Mg2Ni Based Alloys Prepared by Ball-Milling

The van’t Hoff plots of the nanocrystalline Mg2Nialloy (log(PH2) versus 1/T) were shown in Fig. 6. The
best–fitting equations for 'H 4 and 'S 4 are given byhydrogen absorption process:log(P / 0.1 MPa) = í3843/T + 7.012, (2)and by hydrogen desorption process:log(P / 0.1 MPa) = í3992/T + 7.173. (3)The values of enthalpy and entropy changes for hydrogenabsorption are í73.58 kJ·mol-1 and í134.26 J·mol-1·K-1,
and those for hydrogen desorption are 76.44 kJ·mol-1 and137.34 J·mol-1·K-1, respectively. The results agreed wellwith the literature [8].

Fig. 3 Pressure-composition isotherms of hydrogen
absorption for nanocrystalline Mg2Ni alloy.

Fig. 6 Temperature dependence of plateau fornanocrystalline Mg2Ni alloy.

Fig. 4 Pressure-composition isotherms of hydrogen
desorption for nanocrystalline Mg2Ni alloy.

Hydrogen absorption properties for ball-milled
nanocrystalline Mg2Ni alloy were better than those forconventional Mg2Ni alloy. The as-produced powder
readily absorbs hydrogen with no activation, and the final
hydrogen content was found to be about 3.5 wt.% after
the first hydrogenation cycle at 573 K (Fig. 7). In
successive cycles, the absorption kinetics of Mg2Ni
prepared was reproducible and about four times fasterthan that of conventional Mg2Ni. The results imply thatthe nanocrystalline Mg2Ni alloys would be a promisinghydrogen storage material.

hydrogen content(x)Fig. 5 The relationship between plateau pressure andtemperature for nanocrystalline Mg2Ni alloy.
According to van’t Hoff equation, the relationshipbetween  temperature and hydrogen equilibriumpressure can be expressed as 'H 4' S 4 , (1)log( P H 2 )RTR
where 'H 4 is the standard enthalpy changes (J·mol-1)
of hydriding reaction, ' S 4 is the standard entropychanges (J·mol-1·K-1) of the reaction, R is the gas
constant, T is the given measurement temperature, andPH2 is the equilibrium hydrogen pressure.

4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5

AB

0200 400 600 800 1000t(min)Fig.7 Absorption rate of hydrogen at 573K
A-nanocrystalline Mg2Ni alloy (50h)and B- conventional Mg2Ni alloy96

Y. Xiong et al., Hydrogen Storage Properties of Nanocrystalline Mg2Ni Based Alloys Prepared by Ball-Milling

4. Conclusions

The Mg2Ni alloys were prepared by ball-milling,which was composed of nanoparticles with grain size of
about 50 nm. The structures and the hydriding behaviorof Mg2Ni prepared were studied. The absorption anddesorption thermodynamic properties of Mg2Ni alloys
were improved by a ball-milling method. The reversiblehydrogen capacity was found to be to 3.5 wt.%. Thevalues of the enthalpy and the entropy for hydrogenabsorption are í73.58 kJ·mol-1 and í134.26 J·mol-1·K-1,
and those for hydrogen desorption are 76.44 kJ·mol-1 and137.34 J·mol-1·K-1, respectively. The nanocrystalline
Mg2Ni alloy will be a promising hydrogen storage
material for hydrogen energy and fuel cell systems.
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[6] S. Orimo et al., J. Alloys Compd., 253, 94 (1997).
[7] H. Imamura et al., J. Alloys Compd., 386, 211 (2005).
[8] P. Selvam et. al, Int. J. Hydrogen Energy, 11,169 (1986).

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