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Development of the high performance magnesium basedhydrogen storage alloy

Mustafa Anik*, Fatma Karanfil, Nilufer Kucukdeveci

Department of Metallurgical and Materials Engineering, Eskisehir Osmangazi University, 26480 Eskisehir, Turkey

a r t i c l e i n f o

Article history:

Received 23 July 2011

Received in revised form

14 September 2011

Accepted 16 September 2011

Available online 15 October 2011

Keywords:

MgNi

Mechanical alloying

Electrochemical hydrogen storage

Impedance spectroscopy

Hydrogen diffusion coefficient

* Corresponding author. Tel.: þ90 222 239 37E-mail address: manik@ogu.edu.tr (M. An

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.09.057

a b s t r a c t

Series of MgNi type alloys with Ti, Al, Zr, Pd and Co additive elements were synthesized by

mechanical alloying and their electrochemical hydrogen storage characteristics were

investigated. Systematical alloy designing indicated that Mg0.80Ti0.15Al0.05Zr0.05Ni0.95 alloy

has the best electrode performance. The atomic fractions in this alloy were believed to be

optimum to get the reasonable amount of hydrogen storage with the improved cyclic

stability. Titanium was estimated to enter into Mg(OH)2 layer during the discharging

process and make this barrier layer more penetrable by elemental hydrogen. Al and/or Al-

oxides were predicted to dissolve selectively throughout the barrier hydroxide layer and

thus reduce the stability of this layer. The main contribution of Zr was estimated to arise

from its large atomic size that Zr atoms can create extra sites for the elemental hydrogen in

the alloy structure. As the alloy charge transfer resistances decreased, the alloy retention

rates increased. Improvement in the alloy capacity retaining rate was also closely related

with the hydrogen diffusion coefficients in the alloy.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction capacity of amorphous MgNi alloy was observed to be higher

The potential of metal hydrides to be used as hydrogen

storage media especially in the automotive industry makes

Mg very promising material due to its high theoretical

hydrogen storage capacity (7.6 wt%) and low cost [1]. The slow

absorption kinetics and an elevated hydride decomposition

temperature, however, hinder the utilization of Mg effectively

[2]. Alloying of Mg with transition metals appears to be the

onlyway to overcome the poor kinetics ofMg at the expense of

a reduced hydrogen storage capacity. Especially Ni is known

with its beneficial effects to improve kinetics and reduce the

hydride formation enthalpy of Mg.

Mg2Ni and MgNi alloys synthesized with mechanical

alloying have attracted great attention of many researchers

involved in the hydrogen storage [3e6]. The discharge

50; fax: þ90 222 239 3613.ik).2011, Hydrogen Energy P

than that of nanocrystalline Mg2Ni alloy [5,6]. The increase

in Ni content was believed to accelerate the amorphization

process in the alloy and create relatively catalytic alloy

surface for the hydrogen reactions [5,6]. Despite the limited

improvements in the surface kinetics, the cyclic stability of

MgeNi alloys was still far below the desired level [6]. Addition

of the third, fourth or even fifth alloying element to MgeNi

alloys has become common method to improve the cyclic

stability of the alloy [7e22].

Titanium is the most beneficial alloying element for Mg-

based hydrogen storage alloys since it improves the cyclic

stability of the alloy without causing any considerable

reduction in the alloy discharge capacity [7e9,12,17e19,21,22].

Aluminum also improves the alloy cyclic stability but it brings

about significant reduction in the initial discharge capacity

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Fig. 1 e Overlaid XRD patterns of Mg0.80Ti0.15Al0.10Ni0.95,

Mg0.80Ti0.20Al0.05Zr0.05Ni0.90 and Mg0.80Ti0.15Al0.05Zr0.05Ni0.95 alloys.

0

100

200

300

400

500

0 5 10 15 20 25

Disch

arg

e C

ap

acity / m

A h

g

-1

Cycle Number / n

Mg0.85Ti0.10Zr0.05Ni

Mg0.85Ti0.10Pd0.05Ni

Mg0.85Ti0.10Al0.05Ni

Mg0.85Ti0.10Co0.05Ni

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

Cn

/C

maxx

10

0

Cycle Number / n

Mg0.85Ti0.10Zr0.05Ni

Mg0.85Ti0.10Pd0.05Ni

Mg0.85Ti0.10Al0.05Ni

Mg0.85Ti0.10Co0.05Ni

a

b

Fig. 2 e (a) Discharge capacity and (b) cyclic stability as

a function of cycle number for Mg0.85Ti0.10(M)0.05Ni (M [ Al,

Co, Pd and Zr) quaternary alloys.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8300

[15e17,19,21]. Zirconium is known with its limited positive

contribution to the alloy cyclic stability and the initial

discharge capacity [15e19,21]. The beneficial effect of palla-

dium, which is a very well known catalyst for hydrogen, as the

alloying element is not as high as expected [20]. In fact if Pd is

added more than a critical value, it degrades the electrode

performance of the Mg-based alloys [20]. Although they are

not very common additive elements, Co and Fe are reported to

have considerable contribution to the hydrogen storage

performance [8,10,21]. Boron is another disappointing additive

element for the Mg-based alloys since it induces poor cyclic

reversibility [19].

In this work series of MgNi type alloys with Ti, Al, Zr, Pd

and Co additive elements were synthesized by mechanical

alloying to investigate the electrochemical hydrogen storage

characteristics. Alloy compositions were designed to get the

maximum electrode performance by combining the positive

contributions of the additive elements. Atomic ratios of both

Mg and Ni were reduced in the alloy designing to synthesize

four- or five-component alloys.

0

100

200

300

400

500

0 5 10 15 20 25

Disch

arg

e C

ap

acity / m

A h

g

-1

Cycle Number / n

Mg0.85Ti0.15Zr0.05Ni0.95

Mg0.85Ti0.15Pd0.05Ni0.95

Mg0.85Ti0.15Al0.05Ni0.95

Mg0.85Ti0.15Co0.05Ni0.95

0

0

10

20

30

40

50

60

70

80

90

100

5 10 15 20 25

Cn /C

max*100

Cycle Number / n

Mg0.85Ti0.15Zr0.05Ni0.95

Mg0.85Ti0.15Pd0.05Ni0.95

Mg0.85Ti0.15Al0.05Ni0.95

Mg0.85Ti0.15Co0.05Ni0.95

a

b

Fig. 3 e (a) Discharge capacity and (b) cyclic stability as

a function of cycle number for Mg0.85Ti0.15(M)0.05Ni0.95(M [ Al, Co, Pd and Zr) quaternary alloys.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8 301

2. Materials and methods

Elemental Mg, Ni, Ti, Al, Zr, Pd and Co powders (�325 mesh

powders with at least 99.9% purity were obtained from Alfa

Aesar) were mixed in various compositions and charged into

the stainless steel vials under the high purity Ar atmosphere

(in the glove box). The diameter of the stainless steel balls was

5mmand the ball to powder weight ratio was selected as 20:1.

The mechanical alloying was performed with a planetary ball

mill (Fritsch, Pulverisette P-7) and the milling speed was

500 rpm. The ball milling duration was selected as 25 h. The

mechanical alloying was carried out by milling for 30 min in

the forward direction then cooling for 15min and thenmilling

for 30 min in the reverse direction. By this procedure the

increase in the temperature of the vial was prevented and the

0

100

200

300

400

500

0 5 10 15 20 25

Dis

ch

arg

e C

ap

ac

ity

/ m

A h

g

-1

Cycle Number / n

Mg0.80Ti0.15Al0.10Ni0.95

Mg0.80Ti0.15Al0.05Zr0.05Ni0.95

Mg0.80Ti0.15Al0.05Pd0.05Ni0.95

Mg0.80Ti0.15Al0.05Co0.05Ni0.95

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

Cn /C

maxx

10

0

Cycle Number / n

Mg0.80Ti0.15Al0.10Ni0.95

Mg0.80Ti0.15Al0.05Zr0.05Ni0.95

Mg0.80Ti0.15Al0.05Pd0.05Ni0.95

Mg0.80Ti0.15Al0.05Co0.05Ni0.95

a

b

Fig. 4 e (a) Discharge capacity and (b) cyclic stability as

a function of cycle number for Mg0.80Ti0.15Al0.05(M)0.05Ni0.95(M [ Al, Co, Pd and Zr) five-component alloys.

better homogeneity was obtained. The vials, which have the

special lids, were vacuumed and the high purity Ar gas was re-

charged into them at every 10 h during the ball milling to

prevent the possible oxidation of the alloy powders.

The phase structure of the alloy powders was examined by

the X-ray diffractometer (Bruker axs D8) using Cu Ka

radiation.

Working electrodes were prepared by mixing 0.2 g alloy

powder with 0.6 g nickel powder and then cold pressing into

pellets of 10 mm in diameter, under a pressure of 10 ton cm�2.

NiOOH/Ni(OH)2 counter electrode and a Hg/HgO reference

electrodewere used to set up a three-electrode cell in 6 M KOH

solution. Tests were performed with PARSTAT Model 2273

potentiostat/galvanostat unit. The charge current density was

100 mA g�1 and the charging was carried out down to the

severe gassing potential. The discharge current density was

0

100

200

300

400

500

0 5 10 15 20 25

Disch

arg

e C

ap

acity / m

A h

g

-1

Cycle Number / n

Mg0.85Ti0.15Al0.10Ni0.90

Mg0.85Ti0.15Al0.05Zr0.05Ni0.90

Mg0.80Ti0.20Al0.05Zr0.05Ni0.90

Mg0.85Ti0.20Al0.05Zr0.05Ni0.85

Mg0.85Ti0.25Al0.05Zr0.05Ni0.80

Mg0.80Ti0.30Al0.05Zr0.05Ni0.80

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

Cn /C

maxx100

Cycle Number / n

Mg0.85Ti0.15Al0.10Ni0.90

Mg0.85Ti0.15Al0.05Zr0.05Ni0.90

Mg0.80Ti0.20Al0.05Zr0.05Ni0.90

Mg0.85Ti0.20Al0.05Zr0.05Ni0.85

Mg0.85Ti0.25Al0.05Zr0.05Ni0.80

Mg0.80Ti0.30Al0.05Zr0.05Ni0.80

a

b

Fig. 5 e (a) Discharge capacity and (b) cyclic stability as

a function of cycle number for various MgeTieAleZreNi

five-component alloys.

Fig. 6 e Comparison of the 1st and 20th cycle discharge capacities of the alloys. Alloys are sorted from the highest to the

lowest (from left to right) according to (a) the discharge capacity at 20th cycle and (b) the alloy capacity retaining rate.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8302

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8 303

25 mA g�1 and the discharge cut-off potential was �0.6 VHg/

HgO. The excitation voltage was 10 mV (peak to peak) and the

applied frequency varied from 100 kHz to 0.01 Hz in the elec-

trochemical impedance measurements.

3. Experimental results and discussion

3.1. The structural characteristics of alloys

The previously reported ball milling time dependence of the

alloy discharge capacities indicated that the Mg-based alloys

shouldbemilledfor25hfor theoptimumelectrodeperformance

[19]. Therefore all the investigated alloys in this study were

synthesized by 25 hmilling. The XRD patterns of the developed

structures of Mg0.80Ti0.15Al0.10Ni0.95, Mg0.80Ti0.20Al0.05Zr0.05Ni0.90and Mg0.80Ti0.15Al0.05Zr0.05Ni0.95 alloys are compared in Fig. 1.

Only these three alloys were selected to present the XRD

patterns since they perfectly reflect the XRD patterns of all

the synthesized alloys in this study. All the alloys have a broad

peak around 41.5� which can be assigned to amorphous/nano-

crystalline structure in Fig. 1. The presence of small Ni peaks

at Mg0.80Ti0.15Al0.10Ni0.95 and Mg0.80Ti0.15Al0.05Zr0.05Ni0.95 alloy

patterns shows that the additive elements (Ti, Al, Zr) slightly

0

100

200

300

400

500

600

Dis

char

ge C

apac

ity

/ mA

h g

-1

Fig. 7 e Comparison of the 1st and 20th cycle discharge capacitie

(from left to right) by considering the discharge capacity at 20th c

decrease the solubility ofNi in themain phase [19]. If Ni content

is reduced slightly, however, then these small Ni peaks tend

to disappear, as in Mg0.80Ti0.20Al0.05Zr0.05Ni0.90 alloy pattern

in Fig. 1.

Powder morphologies and the average powder size of the

synthesized alloys were not presented in this study since they

are exactly same with those of the alloys reported in our very

recent publications [19e22].

3.2. The cyclic stabilities of the alloys

The variations in the discharge capacities of Mg0.85Ti0.10(M)0.05Ni (M ¼ Al, Co, Pd and Zr) quaternary alloys

depending on the charge/discharge cycle numbers are indi-

cated in Fig. 2a. The cyclic stabilities of the alloys are also

evaluated by observing the capacity retaining rates as in

Fig. 2b. Aluminum-including alloy exhibits the best perfor-

mance in Fig. 2a and b. Despite its high initial discharge

capacity (Fig. 2a), Co-including alloy has relatively low

capacity retaining rate (Fig. 2b). The electrode performances of

Zr- and Pd-including alloys stay between that of Al-including

and Co-including alloys.

If the atomic fraction of Mg is kept same and that of Ti is

increased by reducing the same amount of atomic fraction of

s of the alloys. Alloys are sorted from the best to the worst

ycle and alloy capacity retaining rate are equally important.

a

b

Fig. 8 e (a) Experimental and simulated impedance

diagrams of the selected alloys at 100% depth of discharges

(DOD) and (b) the equivalent circuit used in the simulation

of the experimental impedance data of the alloys.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8304

Ni (Mg0.85Ti0.15(M)0.05Ni0.95 (M ¼ Al, Co, Pd and Zr)) as in Fig. 3a

and b, both the initial discharge capacities (Fig. 3a) and

capacity retaining rates (Fig. 3b) look improved for all the

alloys. When the relative effects of additive elements (Al, Co,

Pd, Zr) on the alloy electrode performance are compared, the

results appear very similar to those in Fig. 2a and b.

The alloy electrode performances in Figs. 2 and 3 indicate

that Al additive element must definitely present in the

Mg-based alloys. By taking into account this observation

Mg0.80Ti0.15Al0.05(M)0.05Ni0.95 (M ¼ Al, Co, Pd and Zr) five-

component alloys were synthesized and their electrode

performances are compared in Fig. 4a and b. Aluminum and

Pd-including alloys need few cycles to reach their maximum

discharge capacities. All the alloys, except Co-including alloy,

have more than 90% capacity retaining rate at 20th charge/

discharge cycle in Fig. 4b. The initial discharge capacities,

however, looks reduced slightly in Fig. 4a. Upon comparison, it

is clearly observable in Fig. 4a that Zr-including five-compo-

nent alloy has very distinctive performance.

Overall result of Figs. 2e4 is that five-component

alloy should include Ti, Al and Zr additive elements in

addition to main Mg and Ni elements. The relative atomic

fractions of these elements can be optimized to get the best

electrode performance. Various MgeTieAleZreNi alloys

were synthesized and their performances are compared as

in Fig. 5a and b. If the atomic fractions of Al and Zr are kept

constant at 0.05, then Ti atomic fraction shouldn’t exceed

0.20 in order to get the best combination of the initial

discharge capacity and the alloy capacity retaining rate.

Another important point is that the atomic fractions of Mg

and Ni should be comparable.

Comparison of all the synthesized alloys in the same figure

may be more helpful especially in determining the optimum

atomic fractions of the best performance alloy. For this purpose

the 1st and 20th cycle discharge capacities of all the alloys are

compared in Fig. 6a and b. Alloys are sorted from the highest to

the lowest (from left to right) according to the discharge

capacity at 20th cycle and the alloy capacity retaining rate in

Fig. 6a and b, respectively. By considering the discharge

capacity at 20th cycle and the alloy capacity retaining rate are

equally important, the alloys are re-sorted from the best to the

worst in Fig. 7. Mg0.80Ti0.15Al0.05Zr0.05Ni0.95 alloy appears to have

the best electrode performance with at least 90% capacity

retaining rate at 20th cycle and the relatively high initial

discharge capacity (420 mA h g�1) in Fig. 7. The performance of

this alloy is very promising in developing the commercial

negative electrodes for the NieMH secondary batteries.

In five-componentMgeTieAleZreNi alloys, Mg is themain

hydride forming element. Magnesium atomic fraction

shouldn’t be less than 0.80 in order to get reasonable amount

of hydrogen storage. The poor cyclic stability of the alloys

arises also from the presence of Mg. Formation of Mg(OH)2layer in the strongly basic media makes in and out hydrogen

diffusion very difficult [15e21]. Thickening of Mg(OH)2 layer

and the formation of new Mg(OH)2 layers on the freshly

opened surfaces (due to the volume expansion) upon charge/

discharge cycles are globally accepted as the main reasons in

the degradation of the Mg-based alloy electrodes [1e22].

Therefore increase in the Mg atomic ratio (for example above

0.80) causes alloys to have poor cyclic stability.

Nickel, as the transition metal, reduces the stability of the

hydride to facilitate the cyclic reversibility and provides the

electro-active sites for hydrogen reactions. Therefore its

presence is very critical and according to the results exhibited

in Fig. 7 its atomic fraction shouldn’t be less than 0.90 in order

to get desired level of the cyclic stability.

Titanium, which is another hydride forming element,

probably enters into Mg(OH)2 layer during the discharging

process and makes this barrier layer more penetrable by

elemental hydrogen [7,19]. The stability of Ti and/or Ti-

oxides is lower than that of Mg(OH)2 layer in basic media

[17]. Therefore disseminated Ti and/or Ti-oxides may

decrease the stability of the hydroxide layer and make it

more permeable for the elemental hydrogen. According to

Fig. 7 the optimum atomic fraction of Ti looks as 0.15 or

maximum 0.20. Increasing the atomic fraction of Ti above

0.20 requires a decrease in the content of other useful

elements. In fact if selective dissolution occurs, then the

overall stability of the electrode may decrease with the

increase in Ti content.

The dissolution rate of Al and/or Al-oxides are extremely

higher in basic media [17]. Therefore Al and/or Al-oxides may

dissolve selectively throughout the barrier hydroxide layer

and reduce the stability of this layer [15,17,19]. Of course,

selective dissolution of Al and/or Al-oxides is expected to be

muchmore pronounced than that of Ti and/or Ti-oxides. Thus

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8 305

exposition of the underlying electro-catalytic Ni sites upon

selective dissolution is probably more effective in the pres-

ence of Al additive element [15,17,19,23]. Due to its extreme

dissolution rate the atomic fraction of Al additive element

shouldn’t exceed 0.05 in five-component alloy. But according

to Fig. 7 if Zr additive element is excluded (four-component

alloy), then the increase in the Al atomic fraction up to 0.10

can be considered.

The positive contribution of Zr to the electrode perfor-

mance of the five-component alloy is limited. The selective

dissolution of Zr and/or Zr-oxides may also be expected in

the basic media [17]. But the selective dissolution cannot

be the only contribution of Zr since this contribution

Fig. 9 e (a) The calculated charge transfer resistances for all the in

charge transfer resistances and the alloy capacity retaining rate

cannot be observed in the presence of Al. Probably the

extra contribution of Zr arises from its large atomic size

[15e19]. With larger atomic size Zr atoms can create extra

sites for the elemental hydrogen in the alloy structure and

thus the hydrogen storage capacity of the alloy increases

[15e19]. The atomic fraction of 0.05 looks enough for Zr in

Fig. 7.

Finally, the presence of Co and Pd additive elements

appears to be not necessary to develop a high performance

Mg-based hydrogen storage alloy. It was also observed in our

previous studies that Fe, B and C additive elements cannot

make any critical contribution to the electrode performance of

the Mg-based alloys [21].

vestigated alloys and (b) the relationship between the alloy

s.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8306

3.3. Electrochemical impedance spectroscopy (EIS)analysis of the alloys

The AC impedance spectra measured at 100% depth of

discharge (DOD) are given for Mg0.85Ti0.15Zr0.05Ni0.95, Mg0.85-

Ti0.10Co0.05Ni, Mg0.80Ti0.15Al0.05Pd0.05Ni0.95 and Mg0.80Ti0.15Al0.05-

Zr0.05Ni0.95 alloys in Fig. 8a. Only the spectra of these four alloys

were selected since all the other alloys have similar impedance

spectra. The impedance data in Fig. 8a (data points) can be

modeled by the equivalent circuit in Fig. 8b [24] and then the

value of the each component in the equivalent circuit can be

calculated by the curve fitting method. The charge transfer

Fig. 10 e (a) Potentiostatic discharge plots of the selected alloys

from discharge plots, for all the investigated alloys.

resistances calculated according to the equivalent circuit in

Fig. 8b are presented for all the alloys investigated in this study

in Fig. 9a. As reported previously [19] the big portion of the alloy

degradation takes place during the final stage of the discharge.

Therefore the comparison of the charge transfer resistances

obtainedat100%DOD isveryuseful inevaluationof the effect of

the additive elements on the alloy performance.

The calculated charge transfer resistances are sorted from

the lowest to the highest in Fig. 9a. The alloy ranking in Fig. 9a

is very similar to that in Fig. 6b which presents the ranking in

alloy retaining rate from the highest to the lowest. Obviously

as the alloy charge transfer resistances decrease, i.e., as the

and (b) the dependence of D/a2 ratios, which are obtained

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 2 9 9e3 0 8 307

alloy surfaces become electro-catalytic, the alloy retaining

rates increase. This inverse relationship is clearly exhibited in

Fig. 9b. Therefore in getting high performance electrode the

alloy design should be made such that the surface charge

transfer resistance should be as low as possible. This criterion,

in this study, is fulfilled by getting the unstable hydroxide

barrier layer through selective dissolutions and allowing the

underlying electro-catalytic Ni sites to expose [19].

3.4. Determination of the hydrogen diffusion coefficients

Zheng at al. proposed a potentiostatic polarization discharge

technique to estimate an average diffusion coefficient of

hydrogen over a defined concentration range in the metal

hydride electrodes [25]. They reported that during the poten-

tiostatic polarization of fully charged electrode the current

becomes totally diffusion controlled after certain discharging

period [25]. In 100% diffusion controlled regime the constant

slope of log iet (i: current density in A g�1; t is time in second)

plot yields D/a2 ratio where D is the average diffusion coeffi-

cient of hydrogen in cm2 s�1 and a is the average particle

radius in cm [25].

Mg0.85Ti0.15Zr0.05Ni0.95, Mg0.85Ti0.10Co0.05Ni, Mg0.80Ti0.15Al0.05Pd0.05Ni0.95 and Mg0.80Ti0.15Al0.05Zr0.05Ni0.95 alloys are selected

as an example and their log iet plots are presented in Fig. 10a.

D/a2 ratios, which are obtained from the slope of log iet plots

as in Fig. 10a, are provided for all the investigated alloys in

Fig. 10b. If the average particle radius (a) is accepted same for

all the alloys, then the data in Fig. 10b can be assumed to

reflect the average hydrogen diffusion coefficient values.

The calculated D/a2 ratios are sorted from the highest to the

lowest in Fig. 10b. The alloy ranking in Fig. 10b is again very

similar to that in Fig. 6b. Clearly improved alloy capacity

retaining rate is closely related with hydrogen diffusion coef-

ficient, as presented one more time in Fig. 11, that as the

average hydrogen diffusion coefficient increases the alloy

capacity retaining rate also increases. It has to be pointed out,

however, that there are large data scattering in Figs. 9b and 11.

Obviously the improved performances of the designed alloys in

this study cannot only be explained by the charge transfer

Fig. 11 e Relationship between the D/a2 ratios and the alloy

capacity retaining rates.

resistances and average hydrogen diffusion coefficients. The

detailed alloy surface characterizations, like XPS characteriza-

tions, are also needed formuchmore satisfactory explanations.

This kind of study, of course, should be subject of another work

which is going to be conducted on the same alloys.

4. Conclusions

Series of MgNi type alloys with Ti, Al, Zr, Pd and Co additive

elements were synthesized by mechanical alloying and their

electrochemical hydrogen storage characteristics were

investigated. The following conclusions may be deduced:

� Systematical designing showed that Mg0.80Ti0.15Al0.05Zr0.05Ni0.95 alloy has the best electrode performance among

the synthesized alloys.

� Increase in the atomic fraction of Mgwas accompaniedwith

higher hydrogen storage but lower cyclic stability. Therefore

the competition between the hydrogen storage and the

cyclic stability indicated that Mg atomic fraction should be

at around 0.80 in five-component alloys.

� The atomic fraction of nickel, which facilitated the cyclic

reversibility in alloys, was observed to be not less than 0.90

in five-component alloys.

� Titanium was estimated to enter into Mg(OH)2 layer during

the discharging process and make this barrier layer more

penetrable by elemental hydrogen. The optimum atomic

fraction of Ti was predicted to be 0.15 or 0.20 in five-

component alloys.

� Al and/or Al-oxides were believed to dissolve selectively

throughout the barrier hydroxide layer and thus reduce the

stability of this layer. Due to its extreme dissolution rate the

atomic fraction of Al additive element was selected as 0.05

in five-component alloys.

� The main contribution of Zr was estimated to arise from its

large atomic size that Zr atoms can create extra sites for the

elemental hydrogen in the alloy structure. The atomic

fraction of Zr additive element was selected as 0.05 in five-

component alloys.

� The presence of Co and Pd additive elements were believed

to not necessary to develop a high performance Mg-based

hydrogen storage alloy.

� As the alloy charge transfer resistances decreased, the alloy

retention rates increased.

� Improvement in the alloy capacity retaining ratewas closely

related with hydrogen diffusion coefficients in the alloy.

r e f e r e n c e s

[1] Selvam P, Viswanathan B, Swamy CS, Srinivasan V.Magnesium and magnesium alloy hydrides. Int J HydrogenEnergy 1986;11:169e92.

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