富山大学水素同位体科学研究センター研究報告 22: 35-46, 2002.

Effects of Pd Addition on the Electrochemical Properties of MgNi
Tiejun Ma

Yuji Hatano

Takayuki Abe

Kuniaki Watanabe

Hydrogen Isotope Research Center, Toyama University
Gofuku 3190, Toyama 930-8555, Japan(Received January 14, 2003; Accepted January 23, 2004)

Abstract
Two kinds of Pd-modified MgNi were prepared: sample I, ball-milling Pd (1, 6 and10
mol%) with amorphous MgNi for 10 min, 2 hr and 10 hr respectively, and sample II,ball-milling 10mol% Pd with Mg2Ni and Ni for 80 hr. Effects of Pd addition onelectrochemical properties of MgNi were investigated. For sample I, effects of Pd
depended on both Pd ratio and the time of ball-milling. During ball-milling, Pd became
amorphous and distributed uniformly on the surface of MgNi powder. At 6mol% and
10mol% Pd, the degradation of discharge capacity of MgNi was significantly decreased.
As for sample II, cycle life of MgNi was remarkably improved by Pd modification. The
roles of Pd on behavior of MgNi were analyzed and the improvements of MgNi
properties arising from both modifications by Pd were regarded as prevention of theformation of Mg(OH)2.

Introduction
Amorphous MgNi is attractive to be used as anode material of rechargeable Ni/MH
batteries and has been extensively studied1-6). However, the discharge capacity of MgNi
decreases rapidly with the cycles1-7) because of the formation of Mg(OH)2 during
charge/discharge cycles7). The present authors have found that the formation ofMg(OH)2 is not necessarily due to charge/discharge cycles, but it is simply formed bychemical reactions with electrolyte solution8) . More experimental results 9,10) suggested
that the properties of MgNi would be improved by surface modification and/or bulk
modification if the formation of Mg(OH)2 could be inhibited.
Surface modification is an effective method to improve the properties of MgNi.

35

Tiejun Ma : Yuji Hatano : Takayuki Abe : Kuniaki Watanabe

Ball-milling graphite with MgNi for short time11-13) and vacuum evaporating carbon on
MgNi14) increased the discharge capacity and cycle life of MgNi. The present authors
also found that ball-milling some aromatic compounds such as benzene (C6H6) andperylene (C20H12) increased discharge capacity and/or cycle life of MgNi15). Meanwhile
composition modification, such as alloying Ti 6, 12, 16, 17), V 12, 17, 18), Zr, Al 16), and Y19)
with MgNi, could result in a certain extent of improvement on electrochemical
properties of MgNi.
Pd is one of catalysts studied in metal-hydrogen reaction. For Mg-Ni alloy system,
Zaluski et al.20) found that small amount of Pd (less than 1mass%) addition not only
improved the hydrogen absorption kinetics of nanocrystalline Mg2Ni at 200℃, but also
caused hydrogen absorption of nanaocrystalline Mg2Ni with relatively good kinetics
even at room temperature without activation. Kohno et al.21) reported that nano-size Pdparticles ball-milled with Mg2Ni at 1:1 molar ratio improved the hydrogen storage
properties of Mg2Ni in alkaline solution. The improvements on Mg2Ni by Pd were
attributed to catalyst effect 20, 21) on hydrogen absorption reaction, but there was no
detailed analysis. In this study, two kinds of Pd-modified MgNi were prepared, effectsof Pd on electrochemical properties of MgNi were investigated and the roles of Pd on
behavior of MgNi were analyzed.

Experimental
Mg2Ni (below 200 mesh) and Ni (about 3 - 7 µm) powders was mixed at molar ratio of
1:1. Two grams of the mixture powder were put into a chromium steel vessel of 12 cm3
together with eighteen balls of chromium steel (5.5 mm diameter) in argon atmosphereby using a glove box. A planetary ball mill apparatus (Fritsch P5) was used to grind
mechanically the mixture powder at a rotation speed of 350 rpm. After 80 hr ofball-milling, the MgNi powder was taken out from the vessel and kept in argon.
In this study, two kinds of Pd-modified MgNi were prepared. For sample I, after
amorphous MgNi was prepared, pure Pd powder (about 1 µm) at 1, 6 and 10 mol% wasmixed with MgNi and ball milled again for 10 min, 2 hr and 10 hr under sameball-milling conditions as MgNi preparation. Prepared powders of sample I will be
indicated as MgNi-x%Pd or MgNi-x%Pd-yhr (x=1, 6, 10 and y= 10 min, 2 and 10 hr)
hereafter. As for Sample II, Pd was mixed with Mg2Ni and Ni in10:45:45 molar ratio,

36Effects of Pd Addition on the Electrochemical Properties of MgNi

then the mixed powder was ball milled for 80 hr under above mentioned conditions andthe achieved powder will be called (MgNi)90Pd10 in this paper.
Each of Pd-modified MgNi powder, as well as bare MgNi, was mixed with electrolytic
copper powder (2 - 3 µm) in a mass ratio of 1:3 respectively, and 1 g of such mixture
was compressed at 590 MPa for 10 min into a disc with 10 mm in diameter in argon.
The disc was sandwiched in two nickel meshes and fixed by spot-welding. The
charge-discharge characteristics of each sample were measured in 6M KOH solution at
25 ℃ controlled by a water bath. Two-electrode system was used, where thesandwiched sample disc acted as the negative electrode and sintered Ni(OH)2/NiOOH
the counter electrode. For each charge-discharge cycle, the sample was charged at 100
mA·g-1 for 10 hr, rested for 5 min and then discharged at 20 mA·g-1 to a cut off voltageof 1.0 V.
XRD patterns of testing powders were measured by Kα radiation of Cu on PhillipsPW1825 Automated Powder Diffractmeter System at 40 kV and 30 mA. Morphology of
powders was observed by Scanning Electron Microscope, Hitachi, S-3500H with
Oxford EDS system.

Results
1. Sample I
Fig.1 shows the XRD patterns of
Ni

Pd

10hr

Intensity (arb. unit)

sample I containing 6mol% Pd,
MgNi-6%Pd. For bare MgNi, thebroad peak around 41.5° was
assigned to amorphous structure andsharp peaks at 44.5° and 51.8° were
due to Ni remaining in a crystalline
state. After 10 min ball-milling withPd, strong Pd peaks could beobserved, indicating that Pd mainly
existed as metallic state after short

2hr
10min
Bare MgNi
20

time ball-milling. As ball-milling
time increasing, the intensity of Pd

Fig.1

40

２θ

60

XRD patterns of sample I containing

6mol% Pd, MgNi-6%Pd, after ball-milling.

37

80

Tiejun Ma : Yuji Hatano : Takayuki Abe : Kuniaki Watanabe

peaks became weak, and after 10 hr only the strongest peak of Pd at 40.1° could be
identified, while there was no noticeable change on amorphous peak of MgNi. The
XRD patterns of MgNi-1%Pd and MgNi-10%Pd showed similar results as MgNi-6%Pd,
even the peaks of Pd for MgNi-1%Pd were much weaker than those of MgNi-6%Pd
because of lower Pd concentration.

MgNi-6%Pd

Mg

Ni

Pd

10min

2hr

10hr

Fig. 2

SEM morphology and concentration distribution of Mg, Ni and Pd for MgNi-6%Pd

after different time of ball-milling.

Fig.2 shows the morphology and concentration distribution of Mg, Ni and Pd for
MgNi-6%Pd observed by SEM with EDS. There was no observable difference between
the morphologies of powders before (not shown in the figure) and after Pd modification,and distributions of Mg and Ni were very uniform for all cases. The distribution of Pd,
however, was very different with the time of ball-milling. After 10 min ball-milling with
MgNi, Pd was mainly concentrated at some bright points which should be original Pd
powders while only small amount of Pd distributed on the surface of MgNi powders.
After 2 hr, more Pd distributed rather uniformly on the surface of powders, but there
were still some Pd-enriched points. After longer time ball-milling, 10 hr, distribution of
Pd became very uniform on the surface, and the concentration of Pd measured by EDS

38

Effects of Pd Addition on the Electrochemical Properties of MgNi

was some higher than that after 2 hr ball-milling.

Discharge Capacity (mAhg-1)

Effects of Pd on discharge
capacity of MgNi is shown(a)
Bare MgNi
400
as Fig.3. It was found that
MgNi-1% Pd-10min
MgNi-1% Pd-2hr
the effects of Pd were
MgNi-1% Pd-10hr
300
depended on both Pd ratio
and ball-milling time. For
200
MgNi-1%Pd, as in Fig. 3(a),small amount of Pd had no100
significant
effectsonelectrochemical properties
0of MgNi: the discharge(b)Bare MgNi400
MgNi-6% Pd-10mincapacities
foreach
MgNi-6% Pd-2hrball-milling time for the
MgNi-6%
Pd-10hr300
first cycle were almost
same as that of bare MgNi,
200
and all decreased with thecycle number at similar rate100
as bare MgNi. However,
the results for 6mol% Pd
0
0
2
4
6
8
10
12
modification were very
Cycle Number (n)different. In Fig. 3(b), after
10 min ball-milling, thedischarge capacity wasFig. 3 Effects of Pd on discharge capacity of MgNi forincreased at the first cycle
sample I, (a) 1%Pd and (b) 6%Pd.
by Pd, but still decreased
with the cycles at similarrate as bare MgNi. After 2 hr ball-milling, even discharge capacity for the fist cycle wasalmost same as the bare one, the degradation of discharge capacity was significantly
decreased: at the 10th cycles, the discharge capacity for Pd-modified MgNi was about
220mAhg-1 while about 80mAhg-1 for bare MgNi. The results of 10 hr ball-milling werevery similar with those of 2 hr. In addition, for MgNi-10% Pd, the results were very
similar with those of MgNi-6%Pd.

39

Tiejun Ma : Yuji Hatano : Takayuki Abe : Kuniaki Watanabe-1capacity, about 250mAhg , became
about 60-65% of the first cycle
even after 30 cycles .

Intensity (arb. unit)(a)(MgNi)90Pd10

Bare MgNi

20

40

60

２θ

-1

Discharge Capacity (mAhg )2. Sample II
The XRD pattern of (MgNi)90Pd10,
ball-milling 10mol% Pd with
Mg2Ni and Ni for 80 hr, wasshown in Fig. 4(a). There is nodifference comparing with that ofbare MgNi, suggesting that
10mol% Pd addition did notchange the amorphous structure of
MgNi. For (MgNi)90Pd10, Pd
additioncausedremarkable
improvement on the properties of
MgNi as shown in Fig.4(b).
Discharge capacity at the firstcycle was increased slightly, more
importantly, the discharge capacity
decreased very slowly with cycles
comparing with bare MgNi. For
MgNi, after about 10 cycles, thedischarge capacity became about
25% of the first cycle, however, for(MgNi)90Pd10,
thedischarge

80

(b)

400

(MgNi)90Pd10

300
200

Bare MgNi
100
0
0

10

20

30

Cycle Number (n)Fig. 4

Effects of Pd of sample II, ball-milling 10mol%Pd

with Mg2Ni and Ni for 80 hr, on the properties of MgNi: (a)XRD pattern and (b) discharge capacity.

Discussion
It was found that 1mass% Pd addition improved the hydrogen absorption kinetics ofnanocrystalline Mg2Ni even at room temperature20), and high concentration Pd(50mol%) increased the discharge capacity and cycle life of Mg2Ni in alkaline
solution21). In these studies Pd was regarded as catalyst for hydrogen absorption.
However, in this study, 1mol% Pd showed negligible effects on properties of amorphous
MgNi in 6M KOH and effects of 6 and 10mol% Pd were strongly depended onball-milling time and the method of Pd modification. These results suggest that the roles

40Effects of Pd Addition on the Electrochemical Properties of MgNiof Pd on the properties of MgNi are different with that on Mg2Ni. In addition, it was
found that during ball-milling with MgNi, Pd gradually lost its crystal structure, became
into atom group (nanostructure) or atom and dissolved into amorphous MgNi and/or
attached on the surface of MgNi. This transformation seems very crucial for theproperties of MgNi.

The ability of Pd for hydrogen
absorption/desorption
wasmeasured. Pd powder withsame weight as in MgNi-6%Pd,
was ball milled with graphite(total weight of graphite and Pd
was 2 gram) for 10 hr undersame
conditions
asPdball-milling with MgNi, andthe discharge capacity of
Pd-graphite was measured. The
results are shown in Fig. 5. A
weak broad peak around 45°
was observed from XRD

(a)

40

Pd ball milled with graphite for 10hr

20

0
0

2

4

6

8

10

12

Cycle Number (n)20graphitePd(b)Intensity (abs. Unit)

works were carried out toexamine which might be thedominant mechanism of the
improvements.

Dishcharge Capacity(mAhg-1)

In our previous studies8-10, 14, 15), it has been concluded that, 1) the discharge capacity of
MgNi at the first cycle is determined by the amount of hydrogen absorbed during charge
process, and 2) cycle life of MgNi could be increased if the formation of Mg(OH)2 were
inhabited. In the case of this study, Pd might affect the properties of MgNi in threepossible aspects: 1) Pd itself can absorb and desorb hydrogen during charge/discharge
cycle, 2) Pd atoms, distributing on the surface of MgNi powder, acted as catalyst to
improve the electron transmission or hydrogen absorption, and 3) Pd atoms in MgNi
structure or on the surface could prevent the formation of Mg(OH)2. Some experimental

40

60

80

２θ

Fig. 5

Discharge capacity and XRD results of Pd ballmilled with graphite.

patterns in Fig. 5(b), suggesting
there might be small amount of Pd and/or graphite transformed into amorphous41

Tiejun Ma : Yuji Hatano : Takayuki Abe : Kuniaki Watanabe

structure, while strong sharp peaks of Pd indicated most of Pd was in crystalline state
after ball-milling. This result was very similar with those of MgNi-6%Pd-10min and
MgNi-10%Pd-10min. Discharge capacity for Pd ball-milling with graphite was about
14-17 mAhg-1 for all cycles under current testing conditions as shown in Fig. 5(a)indicating that the ability of hydrogen absorption of crystalline Pd in 6M KOH is much
lower than that of MgNi. Meanwhile it was found nanocrsystalline Pd had lower
hydrogen absorption ability than grained (crystalline) Pd at 298K 22). These results
suggest that discharge capacity of Pd is very lower comparing with amorphous MgNi
and the improvement on discharge capacity of Pd-modified MgNi as shown in Fig. 3(b)and Fig 5(b) is not due to the hydrogen absorption ability of Pd.
In order to understand whether Pd on the surface of MgNi causes significant increase inhydrogen absorption ability, a Pd layer was prepared on the surface of MgNi powder byBarrel Sputtering Technique23). Two grams of MgNi were put on the inner wall of theIntensity (abo. Unit)(a)

Pd Sputtering

Bare MgNi-1

Discharge Capacity (mAhg )

20

40

400

60

２θ

80

(b)

Pd Sputtering
Bare MgNi

300
200
100
0
0

2

4

6

8

10

12

Cycle Number (n)Fig. 6

Discharge capacity and XRD patterns of

MgNi after Pd sputtering for 15 min.

42

barrel installed in the sputteringchamber and the chamber was
evacuated below 1×10-4 Pa at room
temperature. Then the powder wascaused to tumble synchronically withthe rotation of the barrel. The surfaceof MgNi powder was first cleaned byargon plasma for 5 min, and thendeposited for 15 min by Pd by usinga Pd-target at the center of thechamber. The sputtering was carriedout by argon plasma generated by RFdischarge of 100 W in an argon flowof 100 sccm at 8×10-1 Pa. During thecleaning and sputtering, the rotating
speed of the chamber was kept at 100
rpm. After the sputtering, thethickness of this Pd layer was
estimated to be in the order ofmagnitude of 10-8 m.

Effects of Pd Addition on the Electrochemical Properties of MgNi

Another possible explanation for Pd
improving properties of MgNi is thatball-milling Pd with MgNi could
prevent the formation of Mg(OH)2.
Some testing powders had been
immersed in 6M KOH solution for
120 hr, which is about the time for
4-5 charge/discharge cycles, and
XRD patterns were recorded after
immersion. The results are shown in
Fig. 7. For 6mol% Pd modified
MgNi of sample I, the peaks of
Mg(OH)2 was found on the patterns

Intensity (arb. unit)

Effects of deposited Pd on the properties of MgNi are shown in Fig. 6. After sputtering,no difference was found comparing that of bare MgNi from XRD results. The discharge
capacity of MgNi was increased by deposited Pd from 360 to 405mAhg-1 at the firstcycle, but the degradation of discharge capability had not been suppressed. In ourprevious research, improved properties of MgNi by carbon evaporation14) was regarded
as that surface-applied carbon worked as active positions on the surface of MgNi
powder to improve the electron translation and hydrogen absorption. By Barrel
Sputtering Technique, the thin Pd layer on the MgNi surface only increased discharge
capacity at the first cycle but did not inhabit the decay of discharge capacity with cycles,
which was very similar with that of MgNi-6%Pd-10min in Fig. 3(b) and carbon
modification14), indicating that Pd
atoms on the surface of MgNi acts as
Mg(OH)2
MgNi-6%Pd
Pd
active position to improve hydrogen
10hr
Ni
absorption.
However,the2hr
improvement by Pd in thismechanism is not significant enough10min
to improve the cycle life as theBare MgNi
results observed in this study, such as
MgNi-6%Pd-2hr, MgNi-6%Pd-10hr
20
40
60
80
and (MgNi)90Pd10.
Mg(OH)2
Ni(MgNi)90Pd10

Bare MgNi

20

40

60

80

２θ
Fig. 7

XRD results of some tested powders

after immersion in 6M KOH solution for 120
hr: (a) MgNi-6%Pd of sample I and (b) sampleII.

of bare MgNi and MgNi-6mol%Pd-10min, which showed decreased discharge capacity

43Tiejun Ma : Yuji Hatano : Takayuki Abe : Kuniaki Watanabewith cycles; while for 2 hr and 10 hr ball-milling, which showed very lower degradation
speed than bare MgNi, there was no observable Mg(OH)2 peaks. Combining the results
shown in Fig.1, Fig.2, and Fig.3, it can be concluded that uniform distribution of Pd andsome extends concentration (at least 6mol% in this study) is very effective to preventthe formation of Mg(OH)2 in alkaline solution. For (MgNi)90Pd10, sample II, after
immersion in 6M KOH for 120 hr, there was no Mg(OH)2 peaks on XRD pattern,indicating that Pd addition was very effective to prevent the formation of Mg(OH)2 on
MgNi, and this should be the reason for the remarkable increase in cycle life of MgNi.
Mg(OH)2, formed in alkaline solution, retarded electron-transfer reactions and resulted
in the reduction of hydrogen absorption by MgNi 8). Ball-milling compounds 15) andgraphite 11-13) with MgNi protected Mg atoms on the surface of MgNi powder to form
Mg(OH)2 and improved the cycle life of MgNi. Improved properties of MgNi by Ti 6, 16) ,Y 19) and V18) addition were also regarded as suppressing the corrosion of Mg in alkaline
media. In this study, even it was found that Pd can absorb some hydrogen and can
improve the hydrogen absorption ability of MgNi, the most important effect of Pd on
improvement of cycle life of MgNi might be due to suppressing the Mg(OH)2
formation.
In this study, at same Pd concentration, 10mol%, the effects of sample II is better thansample I. This might relate to distribution of Pd atom(s) on surface and/or in lattice of
MgNi powder. Even the morphology and XRD patterns of both kinds of Pd-modified
MgNi showed very similar results, different modification procedures are very possible
to result in some detailed differences on the distribution and/or position of Pd atoms in
MgNi powder. Although Pd modification is very effective to prevent the formation ofMg(OH)2, the detailed distribution of Pd in MgNi, structure of MgNi after modificationand how Pd prevents the Mg(OH)2 are still under investigation.

Conclusion
In this study, two kinds of Pd-modified MgNi were prepared and the effects of Pd
addition on electrochemical properties of MgNi were investigated. It was found that, forsample I, after ball-milling Pd (1, 6 and10mol%) with amorphous MgNi for 10 min, 2
hr and 10 hr, effects of Pd depended on both Pd ratio and ball-milling time. Duringball-milling, Pd became amorphous and distributed uniformly on the surface of MgNi

44

Effects of Pd Addition on the Electrochemical Properties of MgNi

powder. At 6 and 10mol% Pd, the degradation of discharge capacity of MgNi wassignificantly decreased. As for sample II, ball-milling 10mol% Pd with Mg2Ni and Ni
for 80 hr, cycle life of MgNi was remarkably improved by Pd modification. The
improvements, arising from both modifications by Pd were regarded as prevention of
the formation of Mg(OH)2.

Acknowledgements
This work has been supported in part by a Grant-in-Aid for Scientific Research on
Priority Areas A of "New Protium Function" of the Ministry of Education, Science,Sports and Culture of Japan. Thanks are offered to Japan Metals and Chemicals Co. forthe gift of Mg2Ni powder and Matsushita Electric Industrial Co. for positive electrodematerial. The authors would like to express their sincere thanks to Prof. Ikeno and Prof.
Matsuda of Toyama University for the help of SEM analyses.

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