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Effect of LaH3–TiH2 composite additive on the hydrogen storage properties of Mg2Ni alloys
Journal of Alloys and Compounds 581 (2013) 270–274
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of LaH3–TiH2 composite additive on the hydrogen storage properties of Mg2Ni alloys Xin Zhao a,b, Shumin Han a,b,⇑, Xilin Zhu a, Baozhong Liu b,c, Xiaocui Chen a, Yanqing Liu a, Ruibing Wang d a
College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China c School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, PR China d Nordion Inc., 447 March Road, Ottawa, Ontario, Canada K2K 1X8 b
a r t i c l e
i n f o
Article history: Received 13 March 2013 Received in revised form 21 April 2013 Accepted 2 May 2013 Available online 16 May 2013 Keywords: Hydrogen storage Mechanical alloying Microstructure Kinetic properties
a b s t r a c t A novel LaH3–TiH2 composite additive was prepared by ball milling LaH3 and TiH2 at a 1:1 weight ratio. Kinetic measurements on the hydrogenation reaction indicated that the LaH3–TiH2 composite additive could significantly improve the hydrogen absorption properties of Mg2Ni. In comparison with that of a pure Mg2Ni alloy and a composite material combining Mg2Ni + 10 wt.% LaH3, the hydrogen storage capacity of the Mg2Ni + 10 wt.% (LaH3–TiH2) composite was 0.251 wt.% and 0.559 wt.% higher at 423 K within 100 s, respectively. Moreover, the composite additive revealed an excellent effect on ameliorating the thermodynamic properties of the Mg2Ni hydride. The decomposition onset temperature of the Mg2Ni + 10 wt.% (LaH3–TiH2) composite hydride was 17 and 13 K lower than that of the pure Mg2NiH4 and Mg2Ni + 10 wt.% TiH2 composite, respectively. X-ray diffraction (XRD) results indicated that the LaH3 in the LaH3–TiH2 composite additive were converted to LaH2 by the catalytic effect of TiH2 during the hydrogenation cycles and that the process enhanced the kinetic properties of hydrogen storage for Mg2Ni alloy. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Mg–Ni-based alloys are considered as one of the most promising materials for hydrogen storage applications due to their high storage density, light weight and reversible hydrogenation properties [1–3]. It is known that the applications of Mg–Ni-based alloys are restricted by their high reversible hydrogenation temperature and low kinetic properties. There have been many studies that attempted to overcome the obstacles that hinder development of Mg–Ni-based alloys for practical applications [4–6]. As a representative of these alloys, Mg2Ni-type alloys have been investigated extensively in recent decades to clarify its hydrogen storage mechanism and to improve its hydrogen uptake characteristics. Zhang et al. [7] found that the substitution of Co for Ni significantly improved the hydrogen storage kinetics of the Mg2Ni alloys, due to the formation of the MgCo2 and Mg phases. The effect of Cu alloyed with Mg2Ni hydride was investigated by the first principle planewave pseudopotential method [8], which revealed that the presence of Cu weakened the interactions between magnesium and nickel, nickel and hydrogen that reduced the decomposition temperature of the hydride. An investigation on a ball milled Mg2 ⇑ Corresponding author at: College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China. Tel./fax: +86 335 8074648. E-mail address: [email protected] (S. Han). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.053
Ni + 10 wt.% Cu composite also proved that the presence of Cu increased the likelihood of comparatively unstable hydrides being formed [9]. Recently, titanium and its alloys have been considered as promising additives to decrease the decomposition temperatures of Mg-based alloy hydrides [10–13]. When it was mixed with Mg–Ni based alloys, Ti readily formed Ti–Ni phases with the Ni element such as TiNi, TiNi2 and TiNi3 in the Mg–Ni alloys, and these new phases enhanced the hydrogen absorption kinetic properties of the alloys [14–16]. However, the use of Ti additives did not yield a significant reduction of the decomposition temperature of Mg–Ni hydrides. Meanwhile, it has been demonstrated that rare earth elements can significantly improve the hydrogen desorption properties of Mg–Ni hydrides. Delogu et al. [17] reported that the presence of La hydride in LaMg2Ni alloy hydride dramatically improved the hydrogen desorption properties of the Mg2NiH4 phase at low temperature. We have previously investigated the effect of varying the La hydride content in the Mg2Ni phase on its catalytic properties In that investigation, the optimal La hydride content was determined and the obtained composite underwent complete dehydrogenation completely at 473 K [18]. According to the investigations mentioned above, Ti and its compounds improve the hydrogenation kinetic properties of Mg2 Ni alloys, and La hydrides enhance the thermodynamic properties of alloys. To further advance the hydrogen storage properties of the Mg2Ni alloys, we have synthesized a LaH3–TiH2 composite
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Powders of LaH3 and TiH2 were mixed together at a 1:1 weight ratio and subsequently ball milled for 2 h to produce the LaH3–TiH2 composite additive. The ball milling process was performed in a 65 ml vial using a SPEX 8000 ball milling machine with ball (10 mm in diameter)-to-powder ratio of 10:1 at 1000 rpm. The original Mg2Ni alloy was prepared by induction melting under the protection of pure argon atmosphere using the Mg and Ni ingots (99.9% pure). The Mg2Ni alloys were then hydrogenated under a 5 MPa hydrogen atmosphere at 673 K for 3 h to ensure the completeness of hydrogenation process. The powder of TiH2 (99% purity, mean grain size of 50 lm) was purchased from Alfa Aesar and the LaH3 powder was prepared by hydrogenating La metal under 10 MPa hydrogen atmosphere at 773 K. The Mg2Ni hydride and its additives were ball milled at a 9:1 weight ratio to produce Mg2Ni + 10 wt.% M (M = LaH3–TiH2, LaH3, TiH2) composites. The duration of the ball milling treatment was 2 h, while the ball-to-powder ratio was 10:1 at 1000 rpm. All handing of the powders were performed in a glove box under purified argon atmosphere (with concentrations of both oxygen and water at less than 1 ppm). The hydrogen absorption/desorption properties were measured on a pressurecomposition-isotherm (PCI) automatically controlled device (manufactured by Suzuki Shokan in Japan). The Temperature Programmed Desorption (TPD) properties were measured with the PCI device, which was equipped with a homemade programmed heater, and the heating rate was 2°/min. Before measurements were performed, each sample had been dehydrogenated for 2 h at 623 K, and then went through one hydrogen absorb/desorb cycle at 573 K to activate the sample. The microstructure was determined via X-ray diffraction (XRD) with Cu Ka radiation and Scanning Electron Microscope (SEM) (HITACHI S3400 N). The hydrogenated samples were processed in a 3 MPa hydrogen atmosphere at 573 K. The dehydrogenated samples were decomposed in a 0.01 MPa hydrogen atmosphere at 573 K. All the hydrogenated/dehydrogenated samples were collected after hydrogenation characteristic tests.
3. Results and discussion
Hydrogen storage capacity (wt.%)
2. Experimental
0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40
0
500
1000
1500
2000
2500
3000
3500
4000
Time (s) Fig. 2. Hydrogen desorption curves of LaH3–TiH2 composite additive at 573 K.
Mg2 NiH4 TiH 2
LaH3
TiNiH
LaH3-TiH 2
Relative Intensity (a.u.)
additive. The effects of this additive on the hydrogenation thermodynamics and kinetic properties of the Mg2Ni alloys are investigated in this paper.
TiH2
LaH3
3.1. Microstructure
^ LaH3 & TiH2 # LaH2 # &
Intensity (a.u.)
#
#
# &
10
Dehydrided & & & #
20
30
40
*
* * *
&
^
^
^&
Hydrided & &
70
80
* Mg2 Ni # TiH2 ^ LaH3 & TiNi @LaH2
* *
^ ^
*
^
^
*
# @ #
@ @ #
LaH3 *
LaH3-TiH2
# &
# &
60
Fig. 3. XRD patterns of the hydrided Mg2Ni + 10 wt.% M (M = LaH3–TiH2, LaH3, TiH2) composites.
^
^
50
2 Theta (degree)
Relative Intensity (a.u.)
Fig. 1 shows the XRD patterns of the hydrogenated and dehydrogenated LaH3–TiH2 composite additives. It can be seen that the hydrogenated composite additive consisted of a LaH3 phase and a TiH2 phase. After the hydrogen desorption process was performed at 573 K, the LaH3 phase became transformed to the LaH2 phase, while the TiH2 phase remained stable. Fig. 2 shows the hydrogen desorption curves of the LaH3–TiH2 composite additive at 573 K. The composite additive decomposed 0.338 wt.% of hydrogen in 3600 s, which is consistent with the theoretical calculation (0.352 wt.%).
10
20
30
40
# &
50
60
TiH2
70
&
80
2 Theta (degree) Fig. 4. XRD patterns of the dehydrided Mg2Ni + 10 wt.% M (M = LaH3–TiH2, LaH3, TiH2) composites.
10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. XRD patterns of the hydrided and dehydrided LaH3–TiH2 composite additives.
Fig. 3 shows the X-ray diffraction patterns of Mg2Ni + 10% M (M = LaH3, TiH2, LaH3–TiH2) hydrides. A TiNiH phase appeared in the Mg2Ni + 10 wt.% TiH2 composite hydride. The appearance of the TiNiH phase indicated that the TiH2 additive formed an alloy
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a
b C
A
B A C B
Fig. 5. SEM micrographs of the dehydrided Mg2Ni + 10% TiH2 (a) and Mg2Ni + 10 wt.% (LaH3–TiH2) (b) composites.
Samples
Element content (wt.%)
Mg2Ni + 10 wt.% TiH2 A B C
La
Ti
Mg
Ni
6.68 17.11 27.16
7.02 19.63 25.32
33.25 26.58 18.92
53.04 36.68 28.60
9.38 12.25 7.67
31.48 33.71 36.67
59.14 54.04 55.66
Mg2Ni + 10 wt.% (LaH3–TiH2) A B C
2.0 1.8
Hydrogen storage capacity (wt.%)
Table 1 The EDS results of the dehydrided Mg2Ni + 10% TiH2 (a) and Mg2Ni + 10 wt.% (LaH3– TiH2) (b) composites.
1.6 1.4 1.2 1.0 0.8 0.6
Mg2 Ni Mg2 Ni+10 wt.% TiH2 Mg2 Ni+10 wt.% (LaH3-TiH2) Mg2 Ni+10 wt.% LaH3
0.4 0.2 0.0 0
1000
2000
3000
4000
5000
6000
7000
Time (s)
3.5
Fig. 7. Hydrogen absorption curves of Mg2Ni alloys and Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composites at 373 K.
3.0 2.5
0.1 0.0
2.0
Mg2 Ni Mg2 Ni+10 wt.% TiH 2 Mg2 Ni+10 wt.% (LaH3-TiH2) Mg2 Ni+10 wt.% LaH3
1.5 1.0 0.5 350
400
450
500
550
600
Temperature (K) Fig. 6. Hydrogen absorption capacity of Mg2Ni alloys and Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composites at different temperature.
with Ni in the Mg2Ni hydride during the ball milling process [15]. In contrast, this new phase was absent when the Mg2Ni hydride was ball milled with the LaH3–TiH2 composite additive. The unit cell volume of LaH3 in the composite additive is 0.1773 nm3, while that of the Mg2Ni + 10 wt.% LaH3 composite hydride is 0.1756 nm3. This indicates that the LaH3 is influenced by the TiH2 in the composite additive. That effect leads to the increased volume of the unit cell for the LaH3 phase. To characterize the phase transformation during the hydrogen absorbption/desorbption cycles, the XRD patterns of the dehydrogenated composites are shown in Fig. 4. After the hydrogen desorption process, the Mg2NiH4 phase transformed to Mg2Ni phase completely, the TiNiH phase in the Mg2Ni + 10 wt.% TiH2 composite
Hydrogen storage capacity (wt.%)
Hydrogen storage capacity (wt.%)
4.0
Mg2 Ni Mg2 Ni+10 wt.% TiH 2 Mg2 Ni+10 wt.% (LaH3-TiH2) Mg2 Ni+10 wt.% LaH3
-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2
0
1000
2000
3000
4000
5000
Time (s) Fig. 8. Hydrogen desorption curves of Mg2Ni hydride and Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composites at 523 K.
decomposed to yield a TiNi phase. The LaH3 phase in the Mg2 Ni + 10 wt.% LaH3 composite did not exhibit any transformation. However, the LaH3 phase in the Mg2Ni + 10 wt.% (LaH3–TiH2) composite hydride exhibited a significant change, with the LaH3 phase becoming transformed to a LaH2 phase. According to the phase structure, which is shown in Fig. 3, the LaH2 had apparently reverted to LaH3 during the hydrogenation process. This indicated
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0.016 Mg2 Ni Mg2 Ni+10 wt.% TiH2 Mg2 Ni+10 wt.% (LaH3-TiH2 ) Mg2 Ni+10 wt.% LaH3
0.014
[1-(1-a )1/3]2
0.012 0.010
temperature was decreased from 573 to 373 K, the hydrogen storage capacity of the samples decreased. The hydrogen storage capacity of the Mg2Ni alloys was more responsive to changes in the temperature than were the composites. The composites Mg2 Ni + 10 wt.% TiH2 and Mg2Ni + 10 wt.% (LaH3–TiH2) absorbed 1.77 wt.% and 1.17 wt.%, respectively, of hydrogen at 373 K. This capacity was better than that provided by the Mg2Ni + 10 wt.% LaH3 composite and the Mg2Ni alloys. To explain this improvement, the hydrogen absorption curves at 373 K shown in Fig. 7 are taken as an example. During the first 10 s, the hydrogen storage capacities of the composites increased more rapidly than those of the Mg2Ni alloys. This was because H2 molecules were easily adsorbed onto the particle surfaces, and the additives increased the active surfaces of the Mg2Ni particles. Because of the diffusion effect, the H atoms subsequently entered the crystals of the Mg2Ni phase to form Mg2NiH4, which was observed as the lentamente increasing process of the hydrogen storage capacity. The hydrogen absorption rates of the composites including TiH2 and LaH3–TiH2 additives preceded that of pure Mg2Ni during the H diffusion process. This behavior indicated that the phase transformation effect of the additives enhanced the diffusion of H in the Mg2Ni phase and increased the hydrogen storage capacity of the composites at lower temperatures. Fig. 8 reveals the hydrogen desorption curves of the hydrogenated samples at 523 K. It can be seen that the hydrogen desorption capacity of the Mg2Ni + 10 wt.% TiH2 hydride and the Mg2Ni + 10 wt.% (LaH3–TiH2) hydride reached 1.07 wt.% and 0.96 wt.%, respectively, in 2 h at 523 K. Meanwhile that of pure Mg2NiH4 was 0.49 wt.% and that of the Mg2Ni + 10 wt.% LaH3 hydride was 0.56 wt.%. However, the foremost hydrogen desorption kinetic properties of the composites did not exhibit difference clearly because of the approximately hydrogen desorption capacity in the first 100 s. To clarify the hydrogen diffusion properties of the hydrogenated samples during the first 100 s, Fig. 9 shows the fitted hydrogen desorption kinetic curves of the hydrogenated samples. The three-dimensional diffusion model was chosen as the mechanism function of the kinetics [19]. The term a represents the ratio of reacted composite to total composite and k is the diffusion coefficient. This relationship reveals that the k term for the Mg2 Ni + 10 wt.% (LaH3–TiH2) composite hydride reached 12.22 104, which was twice of that of the pure Mg2Ni hydride.
-5
k=6.83x10 -5 k=8.79x10 k=12.22x10-5 k=6.22x10-5
_____Fit line
0.008 0.006 0.004 0.002 0.000
0
10
20
30
40
50
60
70
80
90
100 110
Time (s) Fig. 9. Fitted curves of the hydrogen desorption kinetic properties of the Mg2Ni hydride and the Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composite hydrides at 523 K by three-dimensional diffusion model.
that the presence of LaH3 in the composite additive was influenced by the TiH2 phase and participated in the hydrogenation cycles during the phase transformation between LaH3 and LaH2. Fig. 5 reveals the SEM micrographs of the dehydrogenated Mg2Ni + 10 wt.% TiH2 and Mg2Ni + 10 wt.% (LaH3–TiH2) composites. It can be seen that the particle conglomeration of the Mg2Ni + 10 wt.% TiH2 composite was distinct, as shown in Fig. 5a. The Energy Dispersive Spectrometers (EDSs) results indicated that TiH2 was distributed homogeneously throughout the Mg2Ni alloy. The Mg2 Ni + 10 wt.% (LaH3–TiH2) composite exhibited a incompact state with a smaller particle size than that of the Mg2Ni + 10 wt.% TiH2 composite, as shown in Fig. 5b. The weight proportion of La and Ti in the white points observed from the EDS analysis was approximately 1:1, which is consistent with the proportion of the composite additive. The EDS results of the areas singed in the Fig. 5 are listed in the Table. 1. 3.2. Hydrogen absorption/desorption properties The hydrogen absorption properties of the composites at various temperatures are shown in Fig. 6. It can be clearly seen that as the 0.5 Hydrogen storage capacity (wt.%)
0.10
Hydrogen storage capacity (wt.%)
0.0 -0.5 -1.0
0.05 0.00 -0.05 -0.10 -0.15 -0.20 420
-1.5
430
440
450
460
470
480
490
500
Temperature (K)
-2.0 -2.5 -3.0 -3.5 400
Mg2 Ni Mg2 Ni+10 wt.% TiH2 Mg2 Ni+10 wt.% (TiH2 -LaH 3) Mg2 Ni+10 wt.% LaH3
450
500
550
600
650
700
750
Temperature (K) Fig. 10. TPD curves of the Mg2Ni hydride and the Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composite hydrides.
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Meanwhile, the Mg2Ni + 10 wt.% TiH2 composite hydride exhibited a k value of 8.79 104. This trend indicated that the LaH3–TiH2 composite additive was more effective than the TiH2 additive on the catalysis of hydrogen desorption property for the Mg2Ni hydride. To investigate the relationship between the hydrogen desorption performance and the temperature, Fig. 10 reveals the TPD curves of the hydrogenated samples. This relationship shows that the decomposition onset temperature of the hydrogenated Mg2Ni + 10 wt.% (LaH3–TiH2) composite was 464 K, which was 13 K lower than that of the Mg2Ni + 10 wt.% TiH2 composites and 17 K lower than that of the pure Mg2Ni, respectively. In addition, this temperature was also 7 K lower than that of the Mg2Ni + 10 wt.% LaH3. This trend indicates that the La hydride can effectively decrease the decomposition temperature of Mg2Ni hydride, which has been investigated previously [17,18]. In the Mg2Ni + 10 wt.% (LaH3–TiH2) composite, the presence of TiH2 changes the catalytic mechanism of the LaH3 phase by promoting a phase transformation between LaH3 and LaH2, which could be described by the following equation: TiH2
4LaH3 þ Mg2 Ni ! 4LaH2 þ Mg2 NiH4
ð1Þ
4. Conclusions A LaH3–TiH2 composite additive for the Mg2Ni alloys was prepared by ball milling the La hydride and the Ti hydride. The LaH3–TiH2 composite additive provided a more effective catalysis on the hydrogenation thermodynamics properties of the Mg2Ni alloys than was provided by the TiH2 additives. This improved performance by the LaH3–TiH2 additive could be attributed to the presence of the La hydride. The LaH3–TiH2 additive was also more effective than the LaH3 additive for improving the hydrogenation kinetic properties because of the phase transformation of the La hydride. The XRD analyses indicate that the phase transformation between the LaH3 phase and the LaH2 phase was catalyzed by the TiH2 phase present in the LaH3–TiH2 composite additive. The obtained Mg2Ni + 10 wt.% (LaH3–TiH2) composite could absorb 3.174 wt.% hydrogen at 423 K and 1.17 wt.% hydrogen at 373 K,
and the decomposition onset temperature of its hydride reached 464 K. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (No. 50971112), the Natural Science Foundation of Hebei Province (No. E2010001170) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Hebei Province (20100501). References [1] J.W. Kim, J.P. Ahn, D.H. Kim, H.S. Chung, J.H. Shim, Y.W. Cho, K.H. Oh, Scr. Mater. 62 (2010) 701–704. [2] X.B. Yu, Y.H. Guo, Z.X. Yang, Z.P. Guo, H.K. Liu, S.X. Dou, Scr. Mater. 61 (2009) 469–472. [3] C. Milanese, A. Girella, G. Bruni, P. Cofrancesco, V. Berbenni, P. Matteazzi, A. Marini, Intermetallics 18 (2010) 203–211. [4] M. Chourashiya, D.C. Yang, C.N. Park, C.J. Park, Int. J. Hydrogen Energy 37 (2012) 4238–4245. [5] L.H. Gao, C.P. Chen, L.X. Chen, Q.D. Wang, C.Y. Wang, Y. An, J. Alloys Comp. 424 (2006) 338–341. [6] P. Palade, S. Sartori, A. Maddalena, G. Principi, S.L. Russo, M. Lazarescu, G. Schinteie, V. Kuncser, G. Filoti, J. Alloys Comp. 415 (2006) 170–176. [7] Y.H. Zhang, C.H. Song, H.P. Ren, Z.G. Li, F. Hu, D.L. Zhao, Trans. Nonferr Met. Soc. China 21 (2011) 2002–2009. [8] J. Zhang, D.W. Zhou, P. Peng, J.S. Liu, Rare Metal Mater. Eng. 37 (2008) 1336– 1340. [9] D. Vyas, P. Jain, J. Khan, V. Kulshrestha, A. Jain, I.P. Jain, Int. J. Hydrogen Energy 37 (2012) 3755–3760. [10] K. Asano, E. Akiba, J. Alloys Comp. 481 (2009) L8–L11. [11] O.G. Ershova, V.D. Dobrovolsky, Y.M. Solonin, O.Y. Khyzhun, A.Y. Koval, J. Alloys Comp. 464 (2008) 212–218. [12] S.W.H. Eijt, H. Leegwater, H. Schut, A. Anastasopol, W. Egger, L. Ravelli, C. Hugenschmidt, B. Dam, J. Alloys Comp. 509 (2011) S567–S571. [13] N. Novakovic, G.J. Novakovic, L. Matovic, M. Manasijevic, I. Radisavljevic, B.P. Mamula, N. Ivanoic, Int. J. Hydrogen Energy 35 (2010) 598–608. [14] R. Olmez, G. Cakmak, T. Ozturk, Int. J. Hydrogen Energy 35 (2010) (1965) 11957–11965. [15] G. Liang, J. Huot, S. Boily, A.V. Neste, R. Schulz, J. Alloys Comp. 282 (1999) 286– 290. [16] C.K. Lin, C.K. Wang, P.Y. Lee, H.C. Lin, K.M. Lin, Mat. Sci. Eng. A-Struct. 449–451 (2007) 1102–1106. [17] F. Delogu, G. Mulas, Int. J. Hydrogen Energy 34 (2009) 3026–3031. [18] X. Zhao, S.M. Han, X.L. Zhu, B.Z. Liu, Y.Q. Liu, J. Solid State Chem. 190 (2012) 68–72. [19] Q. Li, K.C. Chou, Q. Lin, L.J. Jiang, F. Zhan, Int. J. Hydrogen Energy 29 (2004) 843–849.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of LaH3–TiH2 composite additive on the hydrogen storage properties of Mg2Ni alloys Xin Zhao a,b, Shumin Han a,b,⇑, Xilin Zhu a, Baozhong Liu b,c, Xiaocui Chen a, Yanqing Liu a, Ruibing Wang d a
College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China c School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, PR China d Nordion Inc., 447 March Road, Ottawa, Ontario, Canada K2K 1X8 b
a r t i c l e
i n f o
Article history: Received 13 March 2013 Received in revised form 21 April 2013 Accepted 2 May 2013 Available online 16 May 2013 Keywords: Hydrogen storage Mechanical alloying Microstructure Kinetic properties
a b s t r a c t A novel LaH3–TiH2 composite additive was prepared by ball milling LaH3 and TiH2 at a 1:1 weight ratio. Kinetic measurements on the hydrogenation reaction indicated that the LaH3–TiH2 composite additive could significantly improve the hydrogen absorption properties of Mg2Ni. In comparison with that of a pure Mg2Ni alloy and a composite material combining Mg2Ni + 10 wt.% LaH3, the hydrogen storage capacity of the Mg2Ni + 10 wt.% (LaH3–TiH2) composite was 0.251 wt.% and 0.559 wt.% higher at 423 K within 100 s, respectively. Moreover, the composite additive revealed an excellent effect on ameliorating the thermodynamic properties of the Mg2Ni hydride. The decomposition onset temperature of the Mg2Ni + 10 wt.% (LaH3–TiH2) composite hydride was 17 and 13 K lower than that of the pure Mg2NiH4 and Mg2Ni + 10 wt.% TiH2 composite, respectively. X-ray diffraction (XRD) results indicated that the LaH3 in the LaH3–TiH2 composite additive were converted to LaH2 by the catalytic effect of TiH2 during the hydrogenation cycles and that the process enhanced the kinetic properties of hydrogen storage for Mg2Ni alloy. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Mg–Ni-based alloys are considered as one of the most promising materials for hydrogen storage applications due to their high storage density, light weight and reversible hydrogenation properties [1–3]. It is known that the applications of Mg–Ni-based alloys are restricted by their high reversible hydrogenation temperature and low kinetic properties. There have been many studies that attempted to overcome the obstacles that hinder development of Mg–Ni-based alloys for practical applications [4–6]. As a representative of these alloys, Mg2Ni-type alloys have been investigated extensively in recent decades to clarify its hydrogen storage mechanism and to improve its hydrogen uptake characteristics. Zhang et al. [7] found that the substitution of Co for Ni significantly improved the hydrogen storage kinetics of the Mg2Ni alloys, due to the formation of the MgCo2 and Mg phases. The effect of Cu alloyed with Mg2Ni hydride was investigated by the first principle planewave pseudopotential method [8], which revealed that the presence of Cu weakened the interactions between magnesium and nickel, nickel and hydrogen that reduced the decomposition temperature of the hydride. An investigation on a ball milled Mg2 ⇑ Corresponding author at: College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China. Tel./fax: +86 335 8074648. E-mail address: [email protected] (S. Han). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.053
Ni + 10 wt.% Cu composite also proved that the presence of Cu increased the likelihood of comparatively unstable hydrides being formed [9]. Recently, titanium and its alloys have been considered as promising additives to decrease the decomposition temperatures of Mg-based alloy hydrides [10–13]. When it was mixed with Mg–Ni based alloys, Ti readily formed Ti–Ni phases with the Ni element such as TiNi, TiNi2 and TiNi3 in the Mg–Ni alloys, and these new phases enhanced the hydrogen absorption kinetic properties of the alloys [14–16]. However, the use of Ti additives did not yield a significant reduction of the decomposition temperature of Mg–Ni hydrides. Meanwhile, it has been demonstrated that rare earth elements can significantly improve the hydrogen desorption properties of Mg–Ni hydrides. Delogu et al. [17] reported that the presence of La hydride in LaMg2Ni alloy hydride dramatically improved the hydrogen desorption properties of the Mg2NiH4 phase at low temperature. We have previously investigated the effect of varying the La hydride content in the Mg2Ni phase on its catalytic properties In that investigation, the optimal La hydride content was determined and the obtained composite underwent complete dehydrogenation completely at 473 K [18]. According to the investigations mentioned above, Ti and its compounds improve the hydrogenation kinetic properties of Mg2 Ni alloys, and La hydrides enhance the thermodynamic properties of alloys. To further advance the hydrogen storage properties of the Mg2Ni alloys, we have synthesized a LaH3–TiH2 composite
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Powders of LaH3 and TiH2 were mixed together at a 1:1 weight ratio and subsequently ball milled for 2 h to produce the LaH3–TiH2 composite additive. The ball milling process was performed in a 65 ml vial using a SPEX 8000 ball milling machine with ball (10 mm in diameter)-to-powder ratio of 10:1 at 1000 rpm. The original Mg2Ni alloy was prepared by induction melting under the protection of pure argon atmosphere using the Mg and Ni ingots (99.9% pure). The Mg2Ni alloys were then hydrogenated under a 5 MPa hydrogen atmosphere at 673 K for 3 h to ensure the completeness of hydrogenation process. The powder of TiH2 (99% purity, mean grain size of 50 lm) was purchased from Alfa Aesar and the LaH3 powder was prepared by hydrogenating La metal under 10 MPa hydrogen atmosphere at 773 K. The Mg2Ni hydride and its additives were ball milled at a 9:1 weight ratio to produce Mg2Ni + 10 wt.% M (M = LaH3–TiH2, LaH3, TiH2) composites. The duration of the ball milling treatment was 2 h, while the ball-to-powder ratio was 10:1 at 1000 rpm. All handing of the powders were performed in a glove box under purified argon atmosphere (with concentrations of both oxygen and water at less than 1 ppm). The hydrogen absorption/desorption properties were measured on a pressurecomposition-isotherm (PCI) automatically controlled device (manufactured by Suzuki Shokan in Japan). The Temperature Programmed Desorption (TPD) properties were measured with the PCI device, which was equipped with a homemade programmed heater, and the heating rate was 2°/min. Before measurements were performed, each sample had been dehydrogenated for 2 h at 623 K, and then went through one hydrogen absorb/desorb cycle at 573 K to activate the sample. The microstructure was determined via X-ray diffraction (XRD) with Cu Ka radiation and Scanning Electron Microscope (SEM) (HITACHI S3400 N). The hydrogenated samples were processed in a 3 MPa hydrogen atmosphere at 573 K. The dehydrogenated samples were decomposed in a 0.01 MPa hydrogen atmosphere at 573 K. All the hydrogenated/dehydrogenated samples were collected after hydrogenation characteristic tests.
3. Results and discussion
Hydrogen storage capacity (wt.%)
2. Experimental
0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40
0
500
1000
1500
2000
2500
3000
3500
4000
Time (s) Fig. 2. Hydrogen desorption curves of LaH3–TiH2 composite additive at 573 K.
Mg2 NiH4 TiH 2
LaH3
TiNiH
LaH3-TiH 2
Relative Intensity (a.u.)
additive. The effects of this additive on the hydrogenation thermodynamics and kinetic properties of the Mg2Ni alloys are investigated in this paper.
TiH2
LaH3
3.1. Microstructure
^ LaH3 & TiH2 # LaH2 # &
Intensity (a.u.)
#
#
# &
10
Dehydrided & & & #
20
30
40
*
* * *
&
^
^
^&
Hydrided & &
70
80
* Mg2 Ni # TiH2 ^ LaH3 & TiNi @LaH2
* *
^ ^
*
^
^
*
# @ #
@ @ #
LaH3 *
LaH3-TiH2
# &
# &
60
Fig. 3. XRD patterns of the hydrided Mg2Ni + 10 wt.% M (M = LaH3–TiH2, LaH3, TiH2) composites.
^
^
50
2 Theta (degree)
Relative Intensity (a.u.)
Fig. 1 shows the XRD patterns of the hydrogenated and dehydrogenated LaH3–TiH2 composite additives. It can be seen that the hydrogenated composite additive consisted of a LaH3 phase and a TiH2 phase. After the hydrogen desorption process was performed at 573 K, the LaH3 phase became transformed to the LaH2 phase, while the TiH2 phase remained stable. Fig. 2 shows the hydrogen desorption curves of the LaH3–TiH2 composite additive at 573 K. The composite additive decomposed 0.338 wt.% of hydrogen in 3600 s, which is consistent with the theoretical calculation (0.352 wt.%).
10
20
30
40
# &
50
60
TiH2
70
&
80
2 Theta (degree) Fig. 4. XRD patterns of the dehydrided Mg2Ni + 10 wt.% M (M = LaH3–TiH2, LaH3, TiH2) composites.
10
20
30
40
50
60
70
80
2 Theta (degree) Fig. 1. XRD patterns of the hydrided and dehydrided LaH3–TiH2 composite additives.
Fig. 3 shows the X-ray diffraction patterns of Mg2Ni + 10% M (M = LaH3, TiH2, LaH3–TiH2) hydrides. A TiNiH phase appeared in the Mg2Ni + 10 wt.% TiH2 composite hydride. The appearance of the TiNiH phase indicated that the TiH2 additive formed an alloy
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a
b C
A
B A C B
Fig. 5. SEM micrographs of the dehydrided Mg2Ni + 10% TiH2 (a) and Mg2Ni + 10 wt.% (LaH3–TiH2) (b) composites.
Samples
Element content (wt.%)
Mg2Ni + 10 wt.% TiH2 A B C
La
Ti
Mg
Ni
6.68 17.11 27.16
7.02 19.63 25.32
33.25 26.58 18.92
53.04 36.68 28.60
9.38 12.25 7.67
31.48 33.71 36.67
59.14 54.04 55.66
Mg2Ni + 10 wt.% (LaH3–TiH2) A B C
2.0 1.8
Hydrogen storage capacity (wt.%)
Table 1 The EDS results of the dehydrided Mg2Ni + 10% TiH2 (a) and Mg2Ni + 10 wt.% (LaH3– TiH2) (b) composites.
1.6 1.4 1.2 1.0 0.8 0.6
Mg2 Ni Mg2 Ni+10 wt.% TiH2 Mg2 Ni+10 wt.% (LaH3-TiH2) Mg2 Ni+10 wt.% LaH3
0.4 0.2 0.0 0
1000
2000
3000
4000
5000
6000
7000
Time (s)
3.5
Fig. 7. Hydrogen absorption curves of Mg2Ni alloys and Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composites at 373 K.
3.0 2.5
0.1 0.0
2.0
Mg2 Ni Mg2 Ni+10 wt.% TiH 2 Mg2 Ni+10 wt.% (LaH3-TiH2) Mg2 Ni+10 wt.% LaH3
1.5 1.0 0.5 350
400
450
500
550
600
Temperature (K) Fig. 6. Hydrogen absorption capacity of Mg2Ni alloys and Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composites at different temperature.
with Ni in the Mg2Ni hydride during the ball milling process [15]. In contrast, this new phase was absent when the Mg2Ni hydride was ball milled with the LaH3–TiH2 composite additive. The unit cell volume of LaH3 in the composite additive is 0.1773 nm3, while that of the Mg2Ni + 10 wt.% LaH3 composite hydride is 0.1756 nm3. This indicates that the LaH3 is influenced by the TiH2 in the composite additive. That effect leads to the increased volume of the unit cell for the LaH3 phase. To characterize the phase transformation during the hydrogen absorbption/desorbption cycles, the XRD patterns of the dehydrogenated composites are shown in Fig. 4. After the hydrogen desorption process, the Mg2NiH4 phase transformed to Mg2Ni phase completely, the TiNiH phase in the Mg2Ni + 10 wt.% TiH2 composite
Hydrogen storage capacity (wt.%)
Hydrogen storage capacity (wt.%)
4.0
Mg2 Ni Mg2 Ni+10 wt.% TiH 2 Mg2 Ni+10 wt.% (LaH3-TiH2) Mg2 Ni+10 wt.% LaH3
-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2
0
1000
2000
3000
4000
5000
Time (s) Fig. 8. Hydrogen desorption curves of Mg2Ni hydride and Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composites at 523 K.
decomposed to yield a TiNi phase. The LaH3 phase in the Mg2 Ni + 10 wt.% LaH3 composite did not exhibit any transformation. However, the LaH3 phase in the Mg2Ni + 10 wt.% (LaH3–TiH2) composite hydride exhibited a significant change, with the LaH3 phase becoming transformed to a LaH2 phase. According to the phase structure, which is shown in Fig. 3, the LaH2 had apparently reverted to LaH3 during the hydrogenation process. This indicated
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0.016 Mg2 Ni Mg2 Ni+10 wt.% TiH2 Mg2 Ni+10 wt.% (LaH3-TiH2 ) Mg2 Ni+10 wt.% LaH3
0.014
[1-(1-a )1/3]2
0.012 0.010
temperature was decreased from 573 to 373 K, the hydrogen storage capacity of the samples decreased. The hydrogen storage capacity of the Mg2Ni alloys was more responsive to changes in the temperature than were the composites. The composites Mg2 Ni + 10 wt.% TiH2 and Mg2Ni + 10 wt.% (LaH3–TiH2) absorbed 1.77 wt.% and 1.17 wt.%, respectively, of hydrogen at 373 K. This capacity was better than that provided by the Mg2Ni + 10 wt.% LaH3 composite and the Mg2Ni alloys. To explain this improvement, the hydrogen absorption curves at 373 K shown in Fig. 7 are taken as an example. During the first 10 s, the hydrogen storage capacities of the composites increased more rapidly than those of the Mg2Ni alloys. This was because H2 molecules were easily adsorbed onto the particle surfaces, and the additives increased the active surfaces of the Mg2Ni particles. Because of the diffusion effect, the H atoms subsequently entered the crystals of the Mg2Ni phase to form Mg2NiH4, which was observed as the lentamente increasing process of the hydrogen storage capacity. The hydrogen absorption rates of the composites including TiH2 and LaH3–TiH2 additives preceded that of pure Mg2Ni during the H diffusion process. This behavior indicated that the phase transformation effect of the additives enhanced the diffusion of H in the Mg2Ni phase and increased the hydrogen storage capacity of the composites at lower temperatures. Fig. 8 reveals the hydrogen desorption curves of the hydrogenated samples at 523 K. It can be seen that the hydrogen desorption capacity of the Mg2Ni + 10 wt.% TiH2 hydride and the Mg2Ni + 10 wt.% (LaH3–TiH2) hydride reached 1.07 wt.% and 0.96 wt.%, respectively, in 2 h at 523 K. Meanwhile that of pure Mg2NiH4 was 0.49 wt.% and that of the Mg2Ni + 10 wt.% LaH3 hydride was 0.56 wt.%. However, the foremost hydrogen desorption kinetic properties of the composites did not exhibit difference clearly because of the approximately hydrogen desorption capacity in the first 100 s. To clarify the hydrogen diffusion properties of the hydrogenated samples during the first 100 s, Fig. 9 shows the fitted hydrogen desorption kinetic curves of the hydrogenated samples. The three-dimensional diffusion model was chosen as the mechanism function of the kinetics [19]. The term a represents the ratio of reacted composite to total composite and k is the diffusion coefficient. This relationship reveals that the k term for the Mg2 Ni + 10 wt.% (LaH3–TiH2) composite hydride reached 12.22 104, which was twice of that of the pure Mg2Ni hydride.
-5
k=6.83x10 -5 k=8.79x10 k=12.22x10-5 k=6.22x10-5
_____Fit line
0.008 0.006 0.004 0.002 0.000
0
10
20
30
40
50
60
70
80
90
100 110
Time (s) Fig. 9. Fitted curves of the hydrogen desorption kinetic properties of the Mg2Ni hydride and the Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composite hydrides at 523 K by three-dimensional diffusion model.
that the presence of LaH3 in the composite additive was influenced by the TiH2 phase and participated in the hydrogenation cycles during the phase transformation between LaH3 and LaH2. Fig. 5 reveals the SEM micrographs of the dehydrogenated Mg2Ni + 10 wt.% TiH2 and Mg2Ni + 10 wt.% (LaH3–TiH2) composites. It can be seen that the particle conglomeration of the Mg2Ni + 10 wt.% TiH2 composite was distinct, as shown in Fig. 5a. The Energy Dispersive Spectrometers (EDSs) results indicated that TiH2 was distributed homogeneously throughout the Mg2Ni alloy. The Mg2 Ni + 10 wt.% (LaH3–TiH2) composite exhibited a incompact state with a smaller particle size than that of the Mg2Ni + 10 wt.% TiH2 composite, as shown in Fig. 5b. The weight proportion of La and Ti in the white points observed from the EDS analysis was approximately 1:1, which is consistent with the proportion of the composite additive. The EDS results of the areas singed in the Fig. 5 are listed in the Table. 1. 3.2. Hydrogen absorption/desorption properties The hydrogen absorption properties of the composites at various temperatures are shown in Fig. 6. It can be clearly seen that as the 0.5 Hydrogen storage capacity (wt.%)
0.10
Hydrogen storage capacity (wt.%)
0.0 -0.5 -1.0
0.05 0.00 -0.05 -0.10 -0.15 -0.20 420
-1.5
430
440
450
460
470
480
490
500
Temperature (K)
-2.0 -2.5 -3.0 -3.5 400
Mg2 Ni Mg2 Ni+10 wt.% TiH2 Mg2 Ni+10 wt.% (TiH2 -LaH 3) Mg2 Ni+10 wt.% LaH3
450
500
550
600
650
700
750
Temperature (K) Fig. 10. TPD curves of the Mg2Ni hydride and the Mg2Ni + 10 wt.% M (M = LaH3–TiH2, TiH2, LaH3) composite hydrides.
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Meanwhile, the Mg2Ni + 10 wt.% TiH2 composite hydride exhibited a k value of 8.79 104. This trend indicated that the LaH3–TiH2 composite additive was more effective than the TiH2 additive on the catalysis of hydrogen desorption property for the Mg2Ni hydride. To investigate the relationship between the hydrogen desorption performance and the temperature, Fig. 10 reveals the TPD curves of the hydrogenated samples. This relationship shows that the decomposition onset temperature of the hydrogenated Mg2Ni + 10 wt.% (LaH3–TiH2) composite was 464 K, which was 13 K lower than that of the Mg2Ni + 10 wt.% TiH2 composites and 17 K lower than that of the pure Mg2Ni, respectively. In addition, this temperature was also 7 K lower than that of the Mg2Ni + 10 wt.% LaH3. This trend indicates that the La hydride can effectively decrease the decomposition temperature of Mg2Ni hydride, which has been investigated previously [17,18]. In the Mg2Ni + 10 wt.% (LaH3–TiH2) composite, the presence of TiH2 changes the catalytic mechanism of the LaH3 phase by promoting a phase transformation between LaH3 and LaH2, which could be described by the following equation: TiH2
4LaH3 þ Mg2 Ni ! 4LaH2 þ Mg2 NiH4
ð1Þ
4. Conclusions A LaH3–TiH2 composite additive for the Mg2Ni alloys was prepared by ball milling the La hydride and the Ti hydride. The LaH3–TiH2 composite additive provided a more effective catalysis on the hydrogenation thermodynamics properties of the Mg2Ni alloys than was provided by the TiH2 additives. This improved performance by the LaH3–TiH2 additive could be attributed to the presence of the La hydride. The LaH3–TiH2 additive was also more effective than the LaH3 additive for improving the hydrogenation kinetic properties because of the phase transformation of the La hydride. The XRD analyses indicate that the phase transformation between the LaH3 phase and the LaH2 phase was catalyzed by the TiH2 phase present in the LaH3–TiH2 composite additive. The obtained Mg2Ni + 10 wt.% (LaH3–TiH2) composite could absorb 3.174 wt.% hydrogen at 423 K and 1.17 wt.% hydrogen at 373 K,
and the decomposition onset temperature of its hydride reached 464 K. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (No. 50971112), the Natural Science Foundation of Hebei Province (No. E2010001170) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Hebei Province (20100501). References [1] J.W. Kim, J.P. Ahn, D.H. Kim, H.S. Chung, J.H. Shim, Y.W. Cho, K.H. Oh, Scr. Mater. 62 (2010) 701–704. [2] X.B. Yu, Y.H. Guo, Z.X. Yang, Z.P. Guo, H.K. Liu, S.X. Dou, Scr. Mater. 61 (2009) 469–472. [3] C. Milanese, A. Girella, G. Bruni, P. Cofrancesco, V. Berbenni, P. Matteazzi, A. Marini, Intermetallics 18 (2010) 203–211. [4] M. Chourashiya, D.C. Yang, C.N. Park, C.J. Park, Int. J. Hydrogen Energy 37 (2012) 4238–4245. [5] L.H. Gao, C.P. Chen, L.X. Chen, Q.D. Wang, C.Y. Wang, Y. An, J. Alloys Comp. 424 (2006) 338–341. [6] P. Palade, S. Sartori, A. Maddalena, G. Principi, S.L. Russo, M. Lazarescu, G. Schinteie, V. Kuncser, G. Filoti, J. Alloys Comp. 415 (2006) 170–176. [7] Y.H. Zhang, C.H. Song, H.P. Ren, Z.G. Li, F. Hu, D.L. Zhao, Trans. Nonferr Met. Soc. China 21 (2011) 2002–2009. [8] J. Zhang, D.W. Zhou, P. Peng, J.S. Liu, Rare Metal Mater. Eng. 37 (2008) 1336– 1340. [9] D. Vyas, P. Jain, J. Khan, V. Kulshrestha, A. Jain, I.P. Jain, Int. J. Hydrogen Energy 37 (2012) 3755–3760. [10] K. Asano, E. Akiba, J. Alloys Comp. 481 (2009) L8–L11. [11] O.G. Ershova, V.D. Dobrovolsky, Y.M. Solonin, O.Y. Khyzhun, A.Y. Koval, J. Alloys Comp. 464 (2008) 212–218. [12] S.W.H. Eijt, H. Leegwater, H. Schut, A. Anastasopol, W. Egger, L. Ravelli, C. Hugenschmidt, B. Dam, J. Alloys Comp. 509 (2011) S567–S571. [13] N. Novakovic, G.J. Novakovic, L. Matovic, M. Manasijevic, I. Radisavljevic, B.P. Mamula, N. Ivanoic, Int. J. Hydrogen Energy 35 (2010) 598–608. [14] R. Olmez, G. Cakmak, T. Ozturk, Int. J. Hydrogen Energy 35 (2010) (1965) 11957–11965. [15] G. Liang, J. Huot, S. Boily, A.V. Neste, R. Schulz, J. Alloys Comp. 282 (1999) 286– 290. [16] C.K. Lin, C.K. Wang, P.Y. Lee, H.C. Lin, K.M. Lin, Mat. Sci. Eng. A-Struct. 449–451 (2007) 1102–1106. [17] F. Delogu, G. Mulas, Int. J. Hydrogen Energy 34 (2009) 3026–3031. [18] X. Zhao, S.M. Han, X.L. Zhu, B.Z. Liu, Y.Q. Liu, J. Solid State Chem. 190 (2012) 68–72. [19] Q. Li, K.C. Chou, Q. Lin, L.J. Jiang, F. Zhan, Int. J. Hydrogen Energy 29 (2004) 843–849.