Journal of Solid State Chemistry 190 (2012) 68–72

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Journal of Solid State Chemistry

0022-45

doi:10.1

n Corr

Yanshan

E-m

journal homepage: www.elsevier.com/locate/jssc

Investigations on hydrogen storage properties of Mg2Niþx wt% LaMg2Ni(x¼0, 10, 20, 30) composites

Xin Zhao a,b, Shumin Han a,b,n, Xilin Zhu b, Baozhong Liu a,c, Yanqing Liu b

a State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR Chinab College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR Chinac School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China School of Materials Science and Engineering,

Henan Polytechnic University, Jiaozuo 454000, China

a r t i c l e i n f o

Article history:

Received 21 December 2011

Received in revised form

3 February 2012

Accepted 6 February 2012Available online 10 February 2012

Keywords:

Hydrogen absorbing materials

Mechanical alloying

Microstructure

Thermal analysis

96/$ - see front matter Crown Copyright & 2

016/j.jssc.2012.02.010

esponding author at: College of Environment

University, Qinhuangdao 066004, PR. China

ail address: hanshm@ysu.edu.cn (S. Han).

a b s t r a c t

Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30) composites have been prepared by ball milling Mg2Ni and

LaMg2Ni hydrides. X-ray Diffraction indicates that the composites consist of LaH3 and Mg2NiH4 phases.

Mg2NiH4 phase transforms between with Mg2Ni phase during hydriding/dehydriding cycling, while the

LaH3 phase exists still after dehydriding process. Backscatter Electron results reveal that the LaH3

phase, which is decomposed from hydrided LaMg2Ni, distributes in Mg2Ni alloy homogeneously after

ball milling procedure. Hydriding/Dehydriding measurements indicate significant improvement in

reversible hydrogen storage properties of the composites over Mg2Ni at low temperature. At 473 K, the

hydrogen storage capacity of Mg2Niþ20 wt% LaMg2Ni composite reaches 3.22 wt% and can reversely

desorb hydrogen completely, while the pure Mg2Ni hydride is hardly desorbs hydrogen at this

temperature. The improvement in the hydrogen storage properties is attributed to the existence of

LaH3 phase in the composites.

Crown Copyright & 2012 Published by Elsevier Inc. All rights reserved.

1. Introduction

Mg is considered to be one of the most promising hydrogenstorage materials due to its large hydrogen capacity and low cost.However, the high hydriding/dehydriding temperature and poorkinetic properties have been blocking its application. Since pastdecades, Mg has been alloyed with various metals to form Mg-based alloys to improve its hydrogen storage properties [1],among which Mg2Ni has been considered to be a potential alloythanks to its mild hydriding/dehydriding temperature of around573 K [2, 3] and high hydrogen storage capacity of 3.6 wt% [4].However, the utilization of Mg2Ni is still restricted by low kineticproperties and high hydrogen absorption/desorption tempera-ture. Many preparation methods have been utilized to improvethe hydrogen storage characteristics of Mg2Ni alloy includingmelt spinning, gravity casting, mechanical ball milling etc, inwhich adding catalysts and Ni partial substituted by transitionmetals were also involved [5–7]. These investigations generallyfocused on enhancing the hydriding kinetic properties of Mg2Nialloy, whereas, dehydriding properties at low temperature havebeen scarcely reported. Since Darriet et al. found that La–Mg

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based alloys exhibited better dehydriding properties than Mg-transition metal alloys [8], La–Mg-based alloys have been inves-tigated extensively. LaH3–MgH2 composite characterizes a fasthydrogen desorption proceeding at 673 K, and its hydrogendesorption ratio reaches 5.4 wt% [9]. The structure transformationof LaH3 has been studied using the diamond anvil cell (DAC)technique in the pressure range from ambient to 25 GPa [10], andthe results shown that the LaH3 phase was stable under the abovepressure range. Moreover, Zhu et al. [11] found that introducingNi into La2Mg17 significantly enhanced hydrogen storage kineticproperties of La–Mg–Ni-based alloys significantly. The addition ofLa into Mg2Ni alloy creates a new ternary Mg-based compound,LaMg2Ni [12], which can dramatically improve dehydridingproperties at low temperature. Chio et al. [13] reported the phasetransformation of LaMg2Ni in the hydriding/dehydriding process,indicating that LaMg2Ni phase decomposed into La-hydride phaseand Mg2Ni-hydride phase during hydriding, and the LaMg2Niphase reappeared in the dehydriding process at 983 K. Ouyanget al. [14] found that the LaH2.46 phase existed in the wholehydriding/dehydriding process, which has been helpful toimprove the hydriding kinetics of LaMg2Ni alloys. Accordingto the investigations of LaMg2Ni alloy, the La hydride contributesto improve the hydrogen storage properties of Mg2Ni hydride.However, hydrogen storage capacity of pure LaMg2Ni alloy onlyreaches 1.96 wt%, which is barely enough for application.The degradation on hydrogen storage capacity of LaMg2Ni alloy

rights reserved.

^ ^^^

# Mg2NiH4* Mg2NiH0.3^ LaH (a)

(b)

(c)

(d)

3**#

#

# ##

#

Rel

ativ

e In

tens

ity (a

.u.)

X. Zhao et al. / Journal of Solid State Chemistry 190 (2012) 68–72 69

is due to the high content of La element, but reducing La content inLaMg2Ni alloy will lead to multiphase structure in the meltingprocess [15, 16]. In order to introduce different amount of Lahydride into Mg2Ni hydride and study the effect of La hydride onthe hydrogen storage properties of Mg2Ni, the Mg2Niþx wt%LaMg2Ni (x¼0, 10, 20, 30) composites have been prepared by ball-milling the mixture of LaMg2Ni and Mg2Ni hydrides in this paper.This new method utilizes the hydriding characteristics of LaMg2Ni[17] and Mg2Ni [18] alloys, which are summarized as follows:

LaMg2Ni ��!Hydrogenation

LaH3þMg2NiH4

Mg2Ni ��!Hydrogenation

Mg2NiH4

The microstructure and hydriding/dehydriding properties ofthe composites have been investigated.

10 20 30 40 50 60 70 80

2 Theta (degree)

Fig. 1. XRD patterns of hydrided Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30)

composites (a) x¼0; (b) x¼10; (c) x¼20; (d) x¼30.

10 20 30 40 50 60 70 80

#^^

^^^

^

####

#

#

#

#

#

#

# Mg2Ni^ LaH3

#

Rel

ativ

e In

tens

ity (a

.u.)

2. Experimental

The original Mg2Ni and LaMg2Ni alloys were prepared byinduction melting under the protection of pure argon atmosphereand were mixed according to the designated proportion followedby hydriding treatment under the pressure of 5 MPa at 623 K for 2 h.Then hydrided alloys were mechanically milled to form theMg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30) composites. The ballmilling process was operated in a 65 ml vial on the SPEX 8000 ballmilling machine with ball (10 mm in diameter)-to-powder ratio of10:1 at 1000 rpm for 2 h. All the handing of the powders wasperformed in a glove box under purified argon atmosphere (withconcentrations of both oxygen and water at less than 1 ppm).

The hydrogen absorption/desorption properties of theobtained Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30) compositeswere measured on a pressure-composition-temperature automa-tically controlled equipment (made by Suzuki Shokan in Japan).The hydriding/dehydriding properties were measured at 473 K,523 K and 573 K, respectively. Before measurement, each samplehad been dehydrided for 2 h at 623 K, and then went through onehydriding/dehydriding cycle at 573 K as activation. The micro-structure was determined by X-ray diffraction (XRD) with Cu Ka

radiation and Backscatter Electron (BSE) (HITACHI S3400N).

2 Theta (degree)

Fig. 2. XRD pattern of dehydrided Mg2Niþ30 wt% LaMg2Ni composite at 523 K.

3. Result and discussion

3.1. Microstructure

XRD patterns of the hydrogenated Mg2Niþx wt% LaMg2Ni(x¼0, 10, 20, 30) composites are presented in Fig. 1. It can beseen from the figure that the pure Mg2Ni hydride consisted ofMg2NiH0.30 phase and Mg2NiH4 phase. There were remarkablepeak splitting of the strong reflex around 23 degree, which wasbelonged to the different type of Mg2Ni phase. With the additionof LaMg2Ni hydride the Mg2NiH0.30 phase disappeared, and therewere only LaH3 phase and Mg2NiH4 phase, which indicates thatthe Mg2Niþx wt% LaMg2Ni (x¼10, 20, 30) composites are easierto react with hydrogen than the pure Mg2Ni alloy.

To clarify the phase transformation during the desorptionprocess, the XRD patterns of the dehydrided Mg2Niþ30 wt%LaMg2Ni composite at 523 K is demonstrated in Fig. 2. Thecomposite consisted of LaH3 phase and Mg2Ni phase, whichindicates that after dehydrogenation process, the Mg2NiH4 phasecompletely decomposed into Mg2Ni phase and H2, while the LaH3

phase still existed after dehydriding process.Fig. 3 shows the BSE micrographs of the dehydrided Mg2Niþ

x wt% LaMg2Ni (x¼10, 20, 30) composites. There were smallquantities of bright areas existing in dark areas in the micrographs.

The Energy Dispersive Spectrometers (EDS) results indicated that thebright regions in micrographs were La rich regions, the dark regionswere composed of Mg2Ni phase and a little amount of La. With theincreasing of LaMg2Ni, the content of LaH3 in Mg2Ni phase of thecomposites was 0.25 wt%,10 wt% and 14 wt%, respectively. The EDSanalysis was not in conflict with the XRD results, the latter was theaverage result of the amount of LaH3 in the composites, while theEDS results presented the content of LaH3 in Mg2Ni part. It isproposed that the LaH3 phase distributed in Mg2Ni phase homo-genously by the ball milling process.

3.2. Hydriding/dehydriding properties

The hydriding/dehydriding properties of Mg2Niþx wt%LaMg2Ni (x ¼0, 10, 20, 30) composites were evaluated bymeasuring P–C–T at 523 K and 473 K as revealed in Fig. 4. It canbe seen from the figure that the maximum hydrogen capacity ofthe composites was lower than that of pure Mg2Ni alloy at both523 K and 473 K. This should be due to the fact that LaH3 phase inthe composites did not take part in the hydrogenation cycles.However, with the temperature decreasing from 523 K to 473 K,the max hydrogen storage capacity of the pure Mg2Ni alloy

Fig. 3. BSE micrographs of the dehydrided Mg2Niþx wt% LaMg2Ni composites (a) x¼10; (b) x¼20; (c) x¼30.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01E-3

0.01

0.1

1

10

x = 0x = 10x = 20x = 30

Pres

sure

(MPa

)

Absorption Weight (wt .%)

0 1 2 3 41E-3

0.01

0.1

1

10

x = 0 x = 10 x = 20 x = 30

Pres

sure

(MPa

)

Absorption Weight (wt .%)

Fig. 4. P–C–T curves of Mg2Ni-x wt% LaMg2Ni (x¼0, 10, 20, 30) composites (a) 523 K (b) 473 K.

X. Zhao et al. / Journal of Solid State Chemistry 190 (2012) 68–7270

reduced by 10.82% and that of the composites was only about2.30%. It indicates that the LaH3 phase in the composites benefitsthe stability of the hydrogen storage capacity.

With the introducing of LaMg2Ni, the hydriding plateau of thecomposites was higher than that of pure Mg2Ni alloy at both523 K and 473 K. The reversible hydrogen capacity of the compo-sites (x¼10, 20, 30) at 473 K was 2.477 wt%, 3.223 wt% and2.377 wt%, respectively, while that of the pure Mg2NiH4 was only0.82 wt% at this temperature. This indicates that the compositehydrides gain significant improvement in dehydriding propertycompare with pure Mg2NiH4, which may be ascribed to that thelanthanum hydride provides abundant hydrogen diffusion path-ways along the Mg2NiH4 phase, the pathways provide lacunaswhich is benefit to the release of H atom from Mg2NiH4 [19].

3.3. Hydriding kinetic properties

Hydriding curves of Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30)composites at 473 K and 523 K are shown in Fig. 5. It can be seenthat the maximum hydrogen capacity decreased with increasingamount of LaMg2Ni in the composites, while the hydriding rateincreased obviously and the fit lines were plotted in Fig. 6, which

were fitted by Jander Diffusion Model (JDM) (Eq. (1)) [20].The expression is shown as follows:

gðaÞ ¼ kt¼ ½1�ð1�aÞ1=3�2 ð1Þ

Where a is the ratio of reacted composite to total composite, t isthe reaction time, k in expressions is defined as diffusioncoefficient, which relates with rate of reaction. In Fig. 6, it canbe clearly seen that, k increased significantly with the increasingamount of LaMg2Ni, which means that the LaH3 phase in thecomposites is beneficial to hydriding kinetics properties.

In order to obtain the thermodynamic parameters of the dehy-driding reaction of Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30)composites, the plateau pressure and temperature curves wereplotted according to Van’t Hoff equation (Eq. (2)).

lnKy¼�

DHy

RTþDSy

Rð2Þ

Where Ky is standard equilibrium constant, Ky¼PH2 in dehydriding

process.The Van’t Hoff plot for Mg2Niþ20 wt% LaMg2Ni composite in

dehydriding process is demonstrated in Fig. 7 and the detaileddates are summarized in Table 1. The thermodynamic parameters

0 20 40 60 80 100 120 140 160 180 2001.0

1.5

2.0

2.5

3.0

3.5

4.0

x = 0x = 10x = 20x = 30

At 473 K

Hyd

roge

n C

apac

ity (w

t. %

)

Time (s)0 50 100 150 200 250 300

1.0

1.5

2.0

2.5

3.0

3.5

4.0

x = 0x = 10x = 20x = 30

At 523 K

Hyd

roge

n C

apac

ity (w

t. %

)

Time (s)

Fig. 5. Hydriding curves of Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30) composites at 473 K and 523 K.

0 20 40 60 80 100

0.0

0.1

0.2

0.3

0.4

0.5

0.6

g (α

)

Time (s)

x = 0 R2 = 0.9961x = 10 R2 = 0.9953x = 20 R2 = 0.9970x = 30 R2 = 0.9913

______ Fit line

Fig. 6. g(a) vs. time for Mg2Niþx wt% LaMg2Ni composites (x¼0, 10, 20, 30) at

473 K.

0.0017 0.0018 0.0019 0.0020 0.0021-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

ln(P

H) (

MPa

)

1/T (1/K)

R2 = 0.9805

Fig. 7. The Van’t Hoff plot for the dehydrogenated Mg2Niþ20 wt% LaMg2Ni

composite.

Table 1The Van’t Hoff analysis for the dehydrogenated

Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30) composites.

Component DH (kJ/mol) DS (J/K mol)

x¼0 64.50 123.1

x¼10 58.15 83.78

x¼20 59.33 86.33

x¼30 60.23 87.97

X. Zhao et al. / Journal of Solid State Chemistry 190 (2012) 68–72 71

of the composites were much lower than those of the pure Mg2Nialloy, which indicates that the LaH3 phase contributes to reducingthe enthalpy and entropy for the composites, but with furtherincreasing of the LaH3 content, both of the enthalpy and entropyincrease very slightly. Compared with the Mg2Niþ20 wt%LaMg2Ni composite, the increase of thermodynamic parametersof the Mg2Niþ30 wt% LaMg2Ni composite is marginal, however,

its reversible hydrogen storage properties is lower than expected.This could attribute to the agglomeration of LaH3 in the compo-site, which block the release of H2 in the composite.

4. Conclusions

Mg2Niþx wt% LaMg2Ni (x¼0, 10, 20, 30) composites wereprepared successfully by ball milling LaMg2Ni and Mg2Nihydrides. BES result indicates that the La hydride, which wasdecomposed from hydrided LaMg2Ni, distributed in the compo-sites homogeneously by ball milling process. Compared with thepure Mg2Ni alloy, the composites revealed obvious improvementin hydrogen storage properties. The reversible hydrogen capacityof the composites was beyond 2.23 wt%, in which the x¼20composite reached 3.22 wt% at 473 K, while that of the pureMg2Ni was only 0.82 wt%. These meliorations were ascribed tothe existence of LaH3, which did not absorb/desorb hydrogen inthe hydrogenation cycle of the composites. Thermodynamicanalysis indicated that the enthalpy and entropy of the compo-sites were both lower than those of the pure Mg2Ni alloy.The hydrogen diffusion coefficient of the composites whichcalculated by Jander Diffusion Model was enhanced.

Acknowledgments

This work was financially supported by High-Tech Research andDevelopment (863) Program of China (2007AA05Z117), the NationalNature Science Foundation of China (50971112, 51171165), ChinaPostdoctoral Science Foundation funded project (20100470990) andthe Natural Science Foundation of Hebei Province of China(E2010001170).

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