Effect of sintering conditions on the formation of single-phase NdMgNi4 compound and its hydrogen storage properties

Article abstract of DOI:10.1016/j.jallcom.2006.03.094

Effects of different sintering conditions on the formation of single-phase NdMgNi₄ compound and its electrochemical properties have been investigated. XRD analysis shows that an ideal single-phase compound of NdMgNi₄ can be synthesized by sintering the pressed tablets of mixture of Mg, Ni and NdNi powders under 973 K for 5 h; single-phase compounds of REMgNi₄ (RE = La, Ce, Pr) can be synthesized in this way as well. The electrochemical properties were measured by simulated battery tests. The maximum discharge capacity of NdMgNi₄ compound was about 200 mAh/g, and just 78 mAh/g for CeMgNi₄ compound. NdMgNi₄ compound could store 3.5 H/M of hydrogen under 2.5 MPa at 298 K, whilst it is difficult to absorb hydrogen at a higher temperature (473 K).

Full text of DOI:10.1016/j.jallcom.2006.03.094

Journal of Alloys and Compounds 429 (2007) 260–263
Effect of sintering conditions on the formation of single-phase
NdMgNi4 compound and its hydrogen storage properties
Z.M. Wanga,∗, Huaiying Zhoua, R.P. Zoub, Qingrong Yaoa
a Department of Information Material Science and Engineering, Guilin University of Electronic Technology, Guilin, Guangxi 541004, PR China
b School of Material Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia
Received 18 February 2006; received in revised form 28 March 2006; accepted 29 March 2006
Available online 10 July 2006
Abstract
Effects of different sintering conditions on the formation of single-phase NdMgNi4 compound and its electrochemical properties have been
investigated. XRD analysis shows that an ideal single-phase compound of NdMgNi4 can be synthesized by sintering the pressed tablets of mixture
of Mg, Ni and NdNi powders under 973 K for 5 h; single-phase compounds of REMgNi4 (RE = La, Ce, Pr) can be synthesized in this way as well.
The electrochemical properties were measured by simulated battery tests. The maximum discharge capacity of NdMgNi4 compound was about
200 mAh/g, and just 78 mAh/g for CeMgNi4 compound. NdMgNi4 compound could store 3.5 H/M of hydrogen under 2.5 MPa at 298 K, whilst it
is difficult to absorb hydrogen at a higher temperature (473 K).
© 2006 Published by Elsevier B.V.
Keywords: REMgNi4; Ball milling; Sintering; Discharge capacity; Hydrogenation
1. Introduction
system, indicating this compound can also be of interest for
hydrogen storage.
Recently, much work was focused on Mg–Ni based hydro-
gen storage alloys. The theoretical discharge capacity of Mg2Ni
electrode is about 1000 mAh/g, which is about three times that
of LaNi5-based alloys. Therefore, they have a large potential
for application in rechargeable batteries and fuel cells [1,2].
The major limitation to the application of Mg-Ni based alloys
is their poor hydrogen absorption/desorption kinetics and poor
corrosion resistance in alkaline aqueous solution. For a bet-
ter hydrogen storage capacity, other compound in Mg–Ni-RE
systems are regarded as the most promising candidates. For
example, La1−xMgxNi2 compound is superior to LaNi5 type
or Laves-phase compounds in hydrogen absorbing capacity [3].
REMgNi4 (RE = Ca, La, Ce, Pr, Nd and Y) is also a very promis-
ingternarycompoundwithacubicSnMgCu4 structure[4]. Aono
et al. [5] reported synthesis of YMgNi4 and its hydriding prop-
erties, and observed 1.05 wt.% of reversible hydrogen capacity
to YMgNi4 compound with an enthalpy of hydride formation
of −35.8 kJ/mol H2, which is comparable to that of LaNi5-H
Recently we have carried out studies on the synthesis and
electrode properties of amorphous REMgNi4 (RE = La, Ce, Pr,
Nd) [6–8]. This paper is an extension to previous papers in which
heavy emphasis is placed on the synthesis, characterization,
hydrogenation behavior and electrode properties of NdMgNi4
as well as other REMgNi4 compounds.
2. Experimental details
REMgNi4 compoundsweresynthesizedbythefollowingprocedures. Firstly,
RENi (RE = La, Ce, Pr, Nd) alloy were synthesized by melting Ni and RE in a
vacuum melting furnace, turned over and remelted 3 times to assure homogene-
ity; then crushed and grounded mechanically to powders (about 200 mesch).
Secondly, the mixed powders of Mg, Ni and RENi were milled for 7 h, and then
pressed into tablets. The milling process was performed under the protection
of pure argon in a QM-1SP planetary ball mill. The weight ratio of steel ball
to powder was 10:1, and the rotational speed was 150 rpm. Finally, the pressed
tablets were sealed in quartz tube with 0.8 MPa Ar gas and heat-treated at 973 K
for 5 h, then cooled to room temperature for use.
X-ray diffractometer (D8 Advance) using Cu K␣ radiation was used to
determine the phase structure and phase component of the compounds. The
electrode properties of the synthesis compounds were measured by simulated
battery tests with the REMgNi4 compounds as negative electrodes. The nega-
tive electrodes were fabricated by the following procedures: Firstly, the alloy
powder (0.6 g) were mixed with fine Copper powder (1.2 g) and a small amount
∗
Corresponding author. Tel.: +86 773 5601 434; fax: +86 773 560 5683.
E-mail address: zmwang@gliet.edu.cn (Z.M. Wang).
0925-8388/$ – see front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.jallcom.2006.03.094

Z.M. Wang et al. / Journal of Alloys and Compounds 429 (2007) 260–263
of polytetrafluoroethylene (PTFE). Secondly, the mixture was pasted onto both
261
sides of a Nickel foam sheet (2.5 cm × 2.5 cm). Finally, the Nickel sheets pasted
with alloy powder were cold pressed to a pellet at a compacting pressure of
155–175 MPa. Commercial Nickel hydroxide Ni(OH)2 were used as the counter
electrode (positive). The negative electrode was separated from the counter
electrode (Ni(OH)2) by a porous frit in 6 M KOH electrolyte. This type of
test battery construction was designed for a negative capacity-limited system,
i.e. the capacity of the negative electrode was much less than that of positive
electrode. Electrochemical properties of test battery were measured at 298 K
usingacomputer-controlledbatterytestinginstrument(PCBT-138-8D-A). Inthe
charge–discharge measurement, negative electrodes were charged at 150 mA/g
for 3.5 h and discharged at 50 mA/g to the cut-off potential of 0.9 V. The P-
C isotherms were measured using a gas reaction controller. About 1 g powder
sample was taken for each test. The measurement was carried out at a constant
temperature of 298 and 473 K, respectively to investigate the absorption and
desorption equilibrium pressure.
Fig. 2. XRD patterns of NdMgNi4 sample sintered under different sintering
conditions: line a, sintered at 873 K for 5 h; line b, sintered at 973 K for 5 h and
line c, sintered at 1173 K for 5 h.
3. Results and discussion
3.1. Preparation of single-phase NdMgNi4 compound and
REMgNi4 (RE = La, Ce, Pr) compounds
ture ranging from 873 to 973 K. To compare with the reference
data [4], the observed and calculated 2θ, d-values and intensities
of NdMgNi4 phase formed under different sintering conditions
are listed in Table 1, respectively.
A series of REMgNi4 (where RE = La, Ce, Pr) compounds
have also been synthesized in the same way. The XRD spectra of
REMgNi4 compounds are shown in Fig. 3. It reveals that these
compounds are all single-phase with the same cubic structure as
SnMgCu4 [4].
The XRD patterns of NdMgNi4 synthesized by the above-
mentioned method are shown in Fig. 1(b). Fig. 1(a) presents the
diffraction peaks of the mixed powders of Mg, Ni and RENi
milled for 7 h. It can be clearly seen that no solid reaction took
place during the ball milling process. The NdMgNi4 phase was
produced in the subsequent sintering procedure—heat-treated
at 973 K for 5 h. So the formation of NdMgNi4 phase may be
described by the following reactions:
3.2. Effect of sintering condition on electrochemical
properties of NdMgNi4 compounds
Mg + 3Ni + NdNi → NdMgNi4.
Furthermore, effects of different sintering temperature on the
formation of NdMgNi4 phase have been studied. The XRD pat-
terns of NdMgNi4 sample prepared under different sintering
conditions are given in Fig. 2. It can be seen that the single-
phase compound of NdMgNi4 phase were formed after sintering
at 873 K for 5 h, or sintering at 973 K for 5 h, and the Nd2Ni7
phase emerged when the sintering temperature was increased to
1173 K. It is suggested that an ideal single-phase compound of
NdMgNi4 phase can be synthesized by sintering at the tempera-
As above mentioned, formation of NdMgNi4 phase was
significantly influenced by the sintering temperature. Effects
of sintering temperature on the electrochemical properties of
NdMgNi4 compound were investigated. Fig. 4 shows the depen-
dence of the discharge capacity of NdMgNi4 compound syn-
thesized at different sintering temperatures on the number of
charge/discharge cycles. It can be seen that good electrochemi-
cal property was observed in NdMgNi4 compound synthesized
Fig. 1. XRD patterns of NdMgNi4 sample: line a, the mixed powders of Mg,
Ni and NdNi alloy milled for 7 h under the protection of pure argon; line b, the
pressed tablets sintered at 973 K for 5 h.
Fig. 3. XRD patterns of REMgNi4 (RE = La, Ce, Pr, Nd) synthesized by sin-
tering at 973 K for 5 h, respectively: line a, NdMgNi4; line b, LaMgNi4; line c,
CeMgNi4 and line d, PrMgNi4.

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Z.M. Wang et al. / Journal of Alloys and Compounds 429 (2007) 260–263
Table 1
Observed and calculated 2␪, d-values and intensities of NdMgNi4 phase with Si as the internal standard at 293 K
H k l
2θcal
dobs
Iobs
0#
a#
b#
c#
0#
a#
b#
c#
0#
a#
b#
c#
1 1 1
2 0 0
2 2 0
3 1 1
2 2 2
4 2 2
5 1 1
4 4 0
21.589
24.980
35.618
42.033
43.997
63.976
68.371
75.424
21.679
25.081
35.749
42.185
44.154
64.190
68.646
75.705
21.646
25.049
35.730
42.171
44.131
64.208
68.612
75.671
21.652
25.054
35.899
42.201
44.155
64.469
68.650
75.832
4.1134
3.5623
2.5196
2.1488
2.0569
1.4545
1.3709
1.2593
4.096
3.548
2.510
2.141
2.050
1.450
1.366
1.222
4.102
3.552
2.511
2.141
2.050
1.449
1.367
1.256
4.101
3.551
2.500
2.140
2.050
1.444
1.366
1.254
14
18
44
100
34
16
33
22
19.6
24.3
47.8
100
29.2
12.8
20.1
16.3
17.3
22.4
46.6
100
25.6
11.5
22.5
12.4
14.6
14.4
75.4
100
33.4
22.3
18.2
20.9
Note: 0# are the data of NdMgNi4 phase reported in reference, [4] and a#, b#, c# are observed and calculated data of the sample under different sintering conditions
(please refer to Fig. 2).
by being sintered at 873 K for 5 h. The increase of sintering tem-
perature leads to the decrease of discharge capacity. It is likely
that an ideal single-phase compound of NdMgNi4 with good
electrochemical properties can be synthesized by lower sinter-
ing temperature (873 K).
solution than other compounds. Thus, increase in cycle numbers
may result in serious decrease in capacity as active materials
were lost in oxidation.
3.3. The pressure-composition-isotherm curves of
NdMgNi4 compound
In comparison, Fig. 5 shows the dependence of discharge
capacity of REMgNi4 (where RE = La, Ce, Pr, Nd) com-
pounds synthesized by sintering at 973 K for 5 h on the
charge/discharge cycle number. It can be clearly seen that
the maximum values of discharge capacity of these com-
pounds were obtained after 2–5 charge–discharge cycles, and
then slightly decreased with increasing number of cycles. This
phenomenon has also been observed in LaNi5 -based hydro-
gen storage alloys [9]. As to REMgNi4 compounds, a slight
increase in discharge capacity was observed in this order:
NdMgNi4 > PrMgNi4 > LaMgNi4 > CeMgNi4. The maximum
discharge capacity was about 200 mAh/g for NdMgNi4 com-
pound, and 78 mAh/g for CeMgNi4 compound. It is suggested
that CeMgNi4 compound is much easier to oxidize in alkaline
Usually, the gaseous hydrogen storage capacity of an alloy is
known as the indicator of the upper electrochemical energy den-
sity limit in an electrochemical cell. The P-C-I curves (pressure-
composition-isotherm) of NdMgNi4 compound at 298 K and
473 K are shown in Fig. 6, respectively. It clearly shows that
a plateau pressure was around 0.38 MPa in the absorbing pro-
cess at 298 K, and the value of H/M could reach up to 3.0 in
this plateau region, and 3.5 H/M obtained under 2.5 MPa. As
to the reversibility of the hydrogen absorption/desorption reac-
tion, single-phase compound of NdMgNi4 released about 80%
(((3.2 − 0.7)/3.5) × 100%) of stored hydrogen under 0.01 MPa.
When increases the experimental temperature to 473 K (Fig. 7),
no clear plateau pressure could be observed under 4.0 MPa of
hydrogen pressure, and the maximum value of H/M was only
Fig. 4. The variation of discharge capacity with cycle number in the
charge/discharge cycle process of NdMgNi4 compound synthesized under dif-
ferent sintering conditions: line a, sintered at 873 K for 5 h; line b, sintered at
973 K for 5 h and line c, sintered at 1173 K for 5 h.
Fig. 5. The variation of discharge capacities of REMgNi4 compounds with the
cycle number in the charge/discharge cycle process: line a, CeMgNi4; line b,
LaMgNi4; line c, PrMgNi4 and line d, NdMgNi4.

Z.M. Wang et al. / Journal of Alloys and Compounds 429 (2007) 260–263
263
4. Conclusions
Single-phase compound NdMgNi4 has been synthesized by
sintering the pressed tablets of the mixture of Mg, Ni and
NdNi powders under 973 K for 5 h, and other single-phase
compounds of REMgNi4 (RE = La, Ce, Pr) were made in the
same way as well. NdMgNi4 compound was able to store
3.5 H/M of hydrogen under 2.5 MPa at 298 K and release
about 80% (((3.2 − 0.7)/3.5) × 100%) of the stored hydro-
gen under 0.01 MPa, but difficult to absorb hydrogen at a
higher temperature (473 K). As to REMgNi4 compounds in
general, a slight increase in discharge capacity was observed
in this order: NdMgNi4 > PrMgNi4 > LaMgNi4 > CeMgNi4.
The maximum discharge capacity was about 200 mAh/g
for NdMgNi4 compound, and 78 mAh/g for CeMgNi4
compound.
Fig. 6. Pressure-composition isotherms for NdMgNi4 compound (H/M is the
hydrogen-to-metal ratio) tested at 298 K.
Acknowledgements
This work was supported by the National Natural Sci-
ence Foundation of China (Project No.50641001) and Guangxi
provincial Natural Science Foundation (Project No. 0448071,
0443004-43). The authors would like to thank Mr J. Cheng for
his technical assistance in XRD analysis and Ms. Peng from
South China University of Technology for the measurement of
P-C-I curves.
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about 1.1 under 3.75 MPa. As illustrated by Figs. 6 and 7,
it is likely that synthesized NdMgNi4 compound is easier to
absorb/desorb hydrogen at low temperature (298 K), but diffi-
cult to absorb/desorb at higher temperature (473 K).