富山大学水素同位体科学研究センター研究報告 23: 43-52, 2003.

Addition of Some Transition Elements and their Effects
on the Electrochemical Properties of MgNi
Tiejun Ma, Yuji Hatano, Takayuki Abe and Kuniaki Watanabe
Hydrogen Isotope Research Center
Toyama University
Gofuku 3190, Toyama 930-8555, Japan(Received March 15th, 2004; Accepted March 15th, 2005 )Abstract
In this study, some transition elements, Ag, Co, Pd, Re, Ru and Ti, at 6 and 10mol%
compositions, were selected to ball mill with Mg2Ni and Ni, and the effects of transition
elements on the electrochemical properties of amorphous MgNi were investigated. Three
distinct effects on the electrochemical properties of MgNi were found: Ti and Pd
dissolved into the bulk of MgNi during ball milling and the degradation of discharge
capacity was decreased by addition of Ti and Pd. Ag and Co also dissolved in the MgNi
powder, but there was no appreciable influence on the electrochemical behavior of MgNi.
Re and Ru neither dissolved into MgNi nor improved the properties of MgNi.

1. Introduction
Amorphous MgNi is attractive as an anode material of rechargeable Ni/MH batteries
because of its high hydrogen storage capacity, low material cost, rich mineral resource,
light weight and so on. However, practical application of MgNi is impeded by the rapid
degradation of discharge capacity with charge/discharge cycle in alkaline solution, which
was regarded as a result of the formation of Mg(OH)2 during the charge/discharge1-4 ). In
our previous research, it was found that the formation of Mg(OH)2 is not necessarily due
to electrochemical reaction, but it is simply formed by chemical reactions with alkaline
electrolyte2, 3) .
Surface modification has been proved to be an effective method to improve theelectrochemical properties of MgNi. Ball-milling of MgNi with graphite for a short
time5-7) and vacuum deposition of carbon on MgNi 8) increased the discharge capacity
and the cycle life of MgNi. We also found that ball-milling aromatic compounds such as
benzene (C6H6) and perylene (C20H12) with MgNi improved the discharge capacity
and/or the cycle life of MgNi 9). Other studies also reported that composition

43馬鉄軍・波多野雄治・阿部孝之・渡辺国昭

modification (e.g., by alloying Ti 6, 10-12), V 6, 11, 12), Zr, Al11), and Y13) with MgNi) resulted
in a certain extent of improvement on the electrochemical properties of MgNi. These
results suggest that the electrochemical properties, especially the cycle life of MgNi,
could be improved if the formation of Mg(OH)2 is suppressed by modification.
Alloying is one of the methods to improve physicochemical properties of hydrogen
absorbing materials. As for amorphous MgNi, Orimo and Fujii14) had systemicallystudied hydriding properties of amorphous MgNi by Ni-site substitution with
3d-transition metals and Mg-site substitution by Zr, where the hydriding properties were
examined on the basis of thermal analysis. However, no study on their electrochemical
properties were carried out. In this study, some transition metal elements were selected asaddition for MgNi modification, and the effects of these elements on the electrochemical
properties of MgNi were investigated.

2. Experimental
In this study, Ag, Co, Pd, Re, Ru and Ti were used to modify MgNi. The purity and sizeof the metal powders are listed in the Table 1. Two compositions of addition wereselected, 6mol% and 10mol%. The transition elements, Ag, Co, Pd, Ti, Re or Ru, weremixed with Ni and Mg2Ni (the molar ratio of Ni and Mg2Ni is 1:1) in prescribedweight fractions with the total weight of 2gram. Each mixture powder was ball milled for
80 hr under the same conditions as described previously 9,15). The discharge capacity ofeach kind of modified MgNi powders was also measured under the same conditions as
before 9,15).
Table.1 Basic characteristic of the powders in this study.
Ag
Purity

99.9%

Size

44µm

Co

5µm

Pd

1µm

Re

Ru

Ti

Ni

99.99%

99.9%

99.9%

99.8%

149µm

3-7µm

4-5µm

Mg2Ni

74µm

In order to examine the effects on prevention of Mg(OH)2 formation, some testing
powder was immersed into 6M KOH for 168 hr, and XRD patterns of these powders
before and after the immersion were measured by Kα radiation of Cu by PhillipsPW1825 Automated Powder Diffractometer operated at 40 kV and 30 mA.

3. Results

44Addition of Some Transition Elements and their Effects on the Electrochemical Properties of MgNi

Discharge Capacity ( mAH/g)

The effects of 6mol% addition of transition elements on the discharge capacity are
shown in Fig.1. For bare MgNi, the discharge capacity at the first cycle was 360mAh/g,
and it decreased quickly with the increasing number of cycle to become less than
100mAh/g after about 10th cycles. Comparing with the bare MgNi, Pd and Ti modified
MgNi showed almost the same
450
discharge capacity at the first cycle.
Bare MgNi
6mol%
400
Pd
Ag
Re
Co
Their
discharge
capacities
350
Ti
Ru
decreased with cycle number but
300
the rates were slower than that of250
bare MgNi. For other elements, the200discharge capacity at the first cycle150
did not increase, and decreased at100
almost the similar rate to that of the
50
0bare MgNi.
0
2
4
6
8
10
12
Cycle Number
Fig.2 shows the XRD patterns of
the bare MgNi and modified MgNi
by 6mol% addition. The bare MgNi Fig．1 Discharge capacity of modified MgNi byshowed the broad peak aroundtransition elements at 6 mol%.
Bare MgNi

Intensity (arb. unit)

Ni

20

30

40

50

60

70

Ag

Co

Pd

Re

Ru

Ti

80 20

30

40

50

60

70

80

２θ
Fig. 2

XRD patterns of modified MgNi by 6 mol% transition elements.

41.5° assigned to amorphous structure and sharp peaks at 44.5° and 51.8° due toremaining crystalline Ni. After ball milling with Ag, Co, Ti and Pd, there was noobservable peaks of these elements in the patterns of XRD, which indicates that theseelements dissolved into the bulk of amorphous MgNi powder during ball milling.

45

馬鉄軍・波多野雄治・阿部孝之・渡辺国昭

However, for Re and Ru modification, the peaks of Re and Ru were very strong evenafter 80 hr ball milling and there appeared a broad peak at about 42° due to the formationof amorphous MgNi. These results suggest that amorphous MgNi was formed by ball
milling Mg2Ni and Ni, but at least a most part of Re and Ru did not dissolve into the bulkof MgNi; they were still in their crystalline states after ball milling.
In our previous studies 2-4, 8, 9, 15), it has been concluded that, 1) the degradation of thedischarge capacity of MgNi in alkaline solution was due to formation of Mg(OH)2, and2) the cycle life of MgNi could be increased if the formation of Mg(OH)2 is suppressed.
From this point, it is very important to understand whether these transition metals canprevent the formation of Mg(OH)2 .
After the modified MgNi being immersed into 6M KOH for 168 hr, XRD patterns oftesting powders were characterized as shown in Fig.3. For the bare MgNi, it is very clearthat Mg(OH)2 peaks appeared after immersion, and the board peak of amorphous
structure became weaker than that for before immersion using the peak strength of Ni at
47° as a reference. This suggests that some of MgNi lost its amorphous structure because
Bare MgNi

Mg(OH)2

Intensity (arb. Unit)

Ni

20

30

40

50

60

Ag

Co

Pd

Re

Ru

Ti

70

80 20

30

40

50

60

70

80

２θ
Fig. 3

XRD patterns of modified MgNi by 6 mol% transition elements after

immersion in 6M KOH for 168 hr.

of the transformation of Mg into Mg(OH)2, resulting in disability of containing hydrogenatoms anymore.
For Ag and Co addition, the strength of the peaks of Mg(OH)2 were almost same as thatin bare MgNi , indicating that the modification did not prevent the formation ofMg(OH)2. The loss of amorphous structure could also be observed. Together with the46Addition of Some Transition Elements and their Effects on the Electrochemical Properties of MgNi

Discharge Capacity (mAH/g)

results in Fig. 1, it is clear that 6mol% Ag and Co did not affect the corrosion receptivityof MgNi in alkaline solution.
With regard to Re and Ru addition, Mg(OH)2 also was formed after immersion. In thesecases, it was difficult to compare the strength of peaks of Mg(OH)2 with that of bare
MgNi, since the peaks of crystalline structure of Re and Ru were very strong. It shouldbe mentioned, however, that the formation of Mg(OH)2 was still in considerable extent.
As for Ti and Pd, however, it was found that the formation of Mg(OH)2 was not muchappreciable as the bare one. These observations correspond to the slower decay rate ofdischarge capacity after Pd and Ti addition.
Fig.4 shows the effects of 10mol% addition on the discharge capacity, where one cansee very similar features to those of 6mol% addition. For Pd and Ti, the discharge
capacity of modified-MgNi was a little higher than that of bare one; the decay rate ofdischarge capacity with cycles was lower than that of bare MgNi, and even lower thanthat of 6mol% addition. These results indicate that 10mol% addition is more effective
than 6mol% addition for Pd and Ti addition.
For Co addition, the discharge capacity for the first cycle was almost same as bare one,but the decrease of discharge capacity with cycles became slightly slower in comparison
with bare MgNi. It was not so
450
Bare MgNi
significantly improved, however, inPd
Ag
10mol%
400
Re
Co
comparison with the case of Pd and
Ti
Ru
350
Ti addition. Ag addition also
300
decreased the decay rates a little.
250
Meanwhile, it seems that the effects
200
of Ag and Co on the properties of150
MgNi had almost no clear
100
dependence on the amount of Co
50
and Ag addition.
0
0
2
4
6
8
10
12
As to Re and Ru addition, theCycle Number
discharge capacity for the firstcycle was lower than that of bare Fig．4 Discharge capacity of modified MgNi by
MgNi, and the ability of hydrogen transition elements at 10 mol%.
absorption also decreased with thecycles at the similar rates as bare MgNi. These observations mean that Re and Ru
addition had almost no effect on the electrochemical properties of MgNi.
The XRD patterns of MgNi modified by 10mol% transition metals were also recorded
before and after the immersion in 6M KOH for 168 hr. The results are showed in Fig. 5.

47

馬鉄軍・波多野雄治・阿部孝之・渡辺国昭

In the case of Ag, there was no change in amorphous structure after 10mol% Ag addition,
which is very similar to that of 6mol% Ag addition. There were also no significant peaksof Mg(OH)2 after 168 hr immersion in 6M KOH, but the discharge capacity stilldecreased with the cycle number. As for Co, there was no observable difference between
10mol% Co-added MgNi and bare MgNi from XRD patterns. It means that Co candissolve into the bulk of MgNi. However, stronger peaks of Mg(OH)2 than that of MgNi
could be seen after the immersion which might be responsible for the reduction in the
discharge capacity with cycles.
For Ti and Pd (not listed in the figure), there was no difference between Ti (or Pd)
added MgNi and bare MgNi from XRD, indicating that Ti (or Pd) dissolved into the bulkof MgNi powder during ball milling. In addition, there appeared no observable Mg(OH)2
peaks after 168 hr immersion, suggesting that the 10mol% addition of Ti and Pd is very
effective to prevent the formation of Mg(OH)2, as 6mol% Ti and 6mol% Pd.
Regarding Re and Ru, 10mol% addition gave very similar results as 6mol% addition.
Namely, strong metallic peaks of Re or Ru could be found after ball milling with Mg2Niand Ni, and Mg(OH)2 was also formed during the immersion in KOH solution. These
results indicate that Re or Ru addition in MgNi has no effects on the electrochemical
properties of MgNi except for the lower discharge capacity at the first cycle.

4. Discussion
As mentioned above, the effects of compositional modification of the amorphous
MgNi on the electrochemical properties could be classified into three distinct kinds.
These effects are discussed in the present section.
1) Modification by Pd and Ti:
The discharge capacity of modified MgNi did not change much at the first cycle, but
the degradation of discharge capacity was significantly suppressed by Ti or Pd addition.
In this respect, Pd was more effective than Ti. The effectiveness of addition was greaterfor 10mol% than 6mol% addition for both Ti and Pd. These electrochemical properties
corresponded well with the properties that the addition of Ti and Pd into MgNi could
prevent the formation of Mg(OH)2.

48

Addition of Some Transition Elements and their Effects on the Electrochemical Properties of MgNi

Mg(OH)2

Bare MgNi

Bare MgNi

Intensity (arb. unit)

Ni

20

40

Ag

Ag

Ti

Ti

60

80 20

Co

40

60

80

Co

Mg(OH)2

Intensity (arb. unit)

Ni

20

40

Re

Re

Ru

Ru

60

80 20

40

60

80

２θ
Fig. 5

XRD patterns of modified MgNi by 10 mol% transition elements before (left)

and after (right) immersion in 6M KOH for 168 hr.

Han et al. 10) also observed that substitution of Ti for Mg in amorphous MgNi could
increase the lifetime of MgNi in alkaline solution because the formation of titaniumoxide layer on the surface of powder prevents the further oxidation of Mg. In the present
study, Ti replaced both Mg and Ni in MgNi bulk. It was also found that Ti could preventthe oxidation of Mg. We also found that substitution of Ni by Ti (10mol%) resulted in
not only the increase in lifetime of MgNi but also the significant increase in the
discharge capacity (540mAh/g at first cycle, almost 1.5 times of that of bare MgNi forfirst cycle). These results indicate that the properties of MgNi after Ti modification areclosely related with the composition and substitution site of Ti atoms in MgNi. Moreexperimental work, however, is required in this aspect.
49

馬鉄軍・波多野雄治・阿部孝之・渡辺国昭

For the Mg-Ni alloy system, effects of Pd on Mg2Ni were studied by Zaluski et al.16)
and Kohno et al. 17), and they believed that the improved hydrogen absorption ability by
Pd addition is attributable to the catalyst effect of Pd. However, as for amorphous MgNi,
our research15) proved that the improvement by Pd-modification was due to the
suppression of Mg(OH)2 formation in alkaline media. The XRD results after long time
immersion in this paper confirmed our explanation.
2) Modification by Ag and Co:
These two elements, Ag and Co, could dissolve into the bulk of MgNi during ball millingwith Mg2Ni and Ni at both 6 and 10mol% concentrations. There was, however, nosignificant effect on the electrochemical properties of MgNi; only some slower
degradation in the discharge capacity was observed. According to Orimo and Fujii14), Co
addition gave neither the change in the structure of MgNi nor the hydriding properties.
These observations were very similar to the results of the present study.
3) Modification by Re and Ru:
For Re and Ru, ball milling Re and Ru with Mg2Ni and Ni did not make Re and Ru
dissolve into the MgNi powder. Most parts of Re and Ru were still in metallic state evenafter 80 hr ball milling for both 6 and 10mol%. No significant effect on theelectrochemical properties of MgNi was observed, except for the lower discharge
capacity at the first cycle in comparison with the bare MgNi.
Table. 2

Lattice type and radius of some transition elements.
Mg

Ni

Ag

Co

Pd

Ti

Re

Ru

Structure

A3

A1

A1

A3

A1

A3

A3

A3

Radius(Á)

1.60

1.25

1.45

1.25

1.38

1.45

1.37

1.33

The lattice structure and radius of transition elements used in this study are listed in
the Table 2. It can be seen that for all of the transition elements studied the radiuses are
between those of Mg and Ni. But there appears any clear relation between the radius ofadded-atom and/or the lattice structure and the properties of modified powders. In ourprevious research15), it was found that after surface modification of MgNi by Pd
prepared by ball milling, 6 and 10mol% Pd dissolved into the lattice of MgNi uniformly
after 2 hr ball milling, and this transformation was very important for the improvementof the cycle life of MgNi in alkaline solution. However, as for Re, it’s atomic radius is
1.37A, which is almost the same as that of Pd. But Re did not dissolve into MgNi andwas still in crystal state even after ball milling with Mg2Ni and Ni for 80 hr. Addition of50

Addition of Some Transition Elements and their Effects on the Electrochemical Properties of MgNi

Re had no effects on the electrochemical properties of MgNi. The roles of Ru are very
similar with that of Re. Meanwhile, it was found 14) that Co and Ti can dissolve into theamorphous structure of MgNi. These results suggest that the relationship between thelattice structure and/or radius of the transition elements with their effects on the MgNielectrode properties is still open to question.

5. Summery
In this study, some transition elements at 6 and 10mol% compositions were selected toball mill with Mg2Ni and Ni to modify amorphous MgNi, and three distinct effects on theelectrochemical properties of MgNi were observed: 1) the degradation of discharge
capacity was greatly suppressed by Ti and Pd addition after Ti and Pd dissolved into thebulk of MgNi, 2) Ag and Co also dissolved in the MgNi powder, but there was no
considerable influence on the electrochemical properties of MgNi, 3) Re and Ru neither
dissolved in the MgNi nor improved the properties of MgNi.

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.

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