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Hydrogen storage properties and phase structures of RMg2Ni (R = La, Ce, Pr, Nd) alloys
Materials Science and Engineering B 177 (2012) 1589–1595
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Hydrogen storage properties and phase structures of RMg2 Ni (R = La, Ce, Pr, Nd) alloys Lichao Pei a,b , Shumin Han a,b,∗ , Jiasheng Wang b , Lin Hu b , Xin Zhao b , Baozhong Liu a,c a b c
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China School of Materials Science and Engineering, Henan Polytechic University, Jiaozuo 454000, PR China
a r t i c l e
i n f o
Article history: Received 20 March 2012 Received in revised form 15 May 2012 Accepted 13 August 2012 Available online 28 August 2012 Keywords: Hydrogen storage alloy Rare earth hydride Phase structure Hydriding kinetic Thermodynamic property
a b s t r a c t RMg2 Ni alloys were prepared by inductive melting where R is rare earth (R = La, Ce, Pr, Nd). X-ray diffraction (XRD) patterns revealed a single-phase composition of RMg2 Ni phase when R was one of the three elements (La, Pr, Nd), and a double-phase composition of CeMg2 Ni and CeMg3 phases when R was Ce. In the hydriding process, RMg2 Ni phases transformed to rare earth hydrides (R-H) and Mg2 NiH4 phase, and for CeMg3 phase, it is decomposed to CeH2.74 and MgH2 phases. The enthalpy change of Mg2 Ni phase in RMg2 Ni alloys during the hydriding/dehydriding process was smaller compared with that of pristine Mg2 Ni alloy, which could be attributed to the existence of R-H. The hydrogen storage properties of RMg2 Ni alloys changed with different R compositions in R-H. At 573 K, the NdMg2 Ni alloy had the highest hydrogen storage capacity and dehydriding plateau, and the descending order of hysteresis was PrMg2 Ni < NdMg2 Ni < CeMg2 Ni < LaMg2 Ni, which suggested that the PrMg2 Ni alloy exhibited a better cycling stability and reversibility than the other three alloys. At 523 K, the uptake time of RMg2 Ni alloys to reach 90% of the maximum hydrogen storage capacity was 75 s, 34 s, 65 s and 52 s, respectively, compared with 110 s of pristine Mg2 Ni alloy. Therefore, we believed the R-H in the alloys not only improved their thermodynamic properties but also accelerated their hydriding kinetics. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Intensive studies on hydrogen as an alternative energy source have been carried out around the world in the past decade, and many hydrogen storage materials have been discovered for storing and transporting hydrogen safely and economically. Due to the high hydrogen storage capacity (7.60 wt.%), rich content in the earth’s crust and the low specific weight (1.74 g cm−3 ), magnesium has been considered as one of the most promising candidates among different hydrogen storage materials. However, the poor hydriding/dehydriding kinetics and the excessively high onset dehydriding temperature have limited its industrial applications [1]. Numerous attempts have been made to overcome these challenges in which of the most practical methods is to alloy Mg with transition metals (Ni, Cu, Ti, etc.) to form alloys such as Mg2 Ni [2,3]. The Mg2 Ni alloy absorbs hydrogen up to Mg2 NiH0.3 in a solid solution and forms Mg2 NiH4 at higher hydrogen concentration
∗ Corresponding author at: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China. Tel.: +86 335 8061569. E-mail address: [email protected] (S. Han). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.003
[4]. Despite the lower hydrogen storage capacity of the Mg2 Ni alloy compared with that of the MgH2 alloy, it has a much faster hydriding/dehydriding kinetics. However, the hydride is still too stable, which requires high temperature of 573 K to release H2 [5]. Further improvement in absorption/desorption conditions can be obtained by alloying Mg2 Ni with rare earth elements, as the rare earth hydride can effectively catalyze hydriding/dehydriding reactions [6,7]. For example, an optimum amount of Nd could provide many amorphous phases and refined microstructures which enhance the absorption kinetics of the Mg–Ni–Nd alloys [8]. Additionally, the rare earth hydride can effectively work as catalyst for the hydrogenation reactions of Mg. Spassov et al. [9] observed an improvement in the hydrogenation kinetics of Mg63 Ni30 Y7 alloy because of the catalytic effect of yttrium. Since the ultrafine crystalline phases (like yttrium hydrides) is helpful to stabilize the nanocrystalline structure by preventing further crystal coarsening during the thermal process, Mg–Cu–Ni–Y [10] alloys, which are prepared by melt spinning, show excellent hydrogen sorption kinetics. Some other methods aim to improve the performances of hydrogen storage alloys are also reported, including ball milling with catalyst [11–13], surface modification [14] and several new sample preparing methods such as hydriding combustion synthesis (HCS) [15], melting-spun [16], and spark plasma sintering [17].
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Recently, attentions have been moved to the LaMg2 Ni alloy. The pristine LaMg2 Ni alloy was produced through inductive melting method [18], and the maximum hydrogen storage capacity could reach to 1.96 wt.% at 564 K. It was discovered that the LaMg2 Ni phase decomposed to Mg2 NiH4 and LaH2.46 phases after the hydriding process. Moreover, the excellent performance in hydrogen storage properties could be attributed to the existence of the LaH2.46 phase, which was useful to improve the hydriding kinetics of LaMg2 Ni alloy. Zhao et al. [19] reported that the reversible hydrogen storage capacity of Mg2 Ni + 20 wt.% LaMg2 Ni complexes reached 3.22 wt.% at 473 K, while the pristine Mg2 Ni hydride could not desorb hydrogen at this temperature. The improvement could be explained by the existence of LaH3 phase in the composite alloys. Therefore, it can be concluded that rare earth hydrides may also be able to ameliorate the hydrogen storage properties of Mg2 Ni phase. In order to study on the catalysis of rare earth hydrides, we explored the phase structures and hydrogen storage properties of RMg2 Ni (R = La, Ce, Pr, Nd) alloys, as well as the effect of R-H in the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys. Fig. 1. XRD patterns of LaMg2 Ni alloy: (a) simulated and (b) as-cast.
2. Experimental methods RMg2 Ni and Mg2 Ni ingots were prepared by inductive melting of rear earth metals, such as La, Ce, Pr, Nd, Mg and Ni (purity more than 99.9%) in a magnesia crucible under argon atmosphere. The raw materials were added to the crucible according to the melting points and the densities of these metals, and the repeated melting at positive pressure protection had to be taken to ensure composition homogeneity during master alloy ingot preparation. A slightly excess of Mg was added to compensate the evaporative Mg loss in the melting procedure [20] and the ingots were annealed at 738 K for 6 h. The compositions of these alloys were measured by inductive coupled plasma (ICP) emission spectrometer. The phase structures of as-cast alloys were tested in a D/max-2500/PC X-ray diffract meter with Cu K␣ radiation and the images were analyzed with Jade-5 software. Scanning electron microscopy (FESEM) images of RMg2 Ni alloys were obtained by HITACHI S-4800 scanning electron microscope with an energy dispersive X-ray spectrometer (EDS). The hydriding/dehydriding process was studied by pressure–composition–temperature (P–C–T) characteristic measurement equipment (made by Suzuki Shokan in Japan) with the measurement parameters set as: delay time 300 s, maximum pressure 3.0 MPa. The sample weight for P–C–T measurement was set as around 2.0 g. Hydriding kinetics of the as-cast alloys were also tested by P–C–T characteristic measurement equipment under the initial hydrogen pressure of 3.0 MPa. The alloy powder was hydrogenated under 3.0 MPa hydrogen pressure for 2 h, followed by the dehydriding process in vacuum for 2 h at 623 K.
cannot be found in the ICDD database. As these four rare elements (La, Ce, Pr, Nd) have the similar atomic structures and physicochemical properties, the XRD patterns of RMg2 Ni (R = Ce, Pr, Nd) phases should be similar to that of LaMg2 Ni phase. The XRD patterns of these alloys are shown in Fig. 2. It can be clearly seen that although the intensities are slightly different from each other, the locations of characteristic peaks of RMg2 Ni (R = Pr, Nd) alloys in XRD patterns are the same as those of LaMg2 Ni alloy, which reveals that the phase compositions of PrMg2 Ni and NdMg2 Ni alloys are PrMg2 Ni phase and NdMg2 Ni phase, respectively. However, the XRD pattern of CeMg2 Ni alloy indicates that it consists of CeMg2 Ni phase and CeMg3 phase, which results from the multi-chemical valences of Ce element. It is difficult for the Ce element to form only one compound with a high chemical valence and thus generate CeMg3 phase. The SEM images of RMg2 Ni alloys are shown in Fig. 3. It is clear to find that RMg2 Ni (R = La, Pr, Nd) alloys contain only single phase structures, while CeMg2 Ni alloy consists of two crystallography phases (the gray region (labeled as A) and the bright region (labeled as B)). The EDS analysis result of CeMg2 Ni alloy is listed in Table 1, which indicates that region A and B are CeMg2 Ni phase and CeMg3 phase, respectively. To further understand the phase
3. Results and discussion 3.1. Phase structure Until now, there is no standard powder diffraction pattern available for LaMg2 Ni alloy in the International Centre for Diffraction Data (ICDD) database. Renaudin et al. [21] previously investigated the cell parameters and atom positions of the single crystal LaMg2 Ni alloy. As shown in Fig. 1(a), it is the simulated XRD pattern of LaMg2 Ni phase that is calculated by using material studio software. The experimental XRD pattern of LaMg2 Ni alloy prepared by inductive melting is presented in Fig. 1(b). Obviously, it can be found that the as-cast LaMg2 Ni alloy only contains LaMg2 Ni phase by the comparison between those two XRD patterns. In the past decades, few studies on RMg2 Ni (R = Ce, Pr, Nd) alloys were reported and thus the powder diffraction patterns of the corresponding single phases
Fig. 2. XRD patterns of RMg2 Ni (R = La, Ce, Pr, Nd) alloys.
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Fig. 3. SEM micrographs of RMg2 Ni (R = La, Ce, Pr, Nd) alloys: (a) LaMg2 Ni, (b) CeMg2 Ni, (c) PrMg2 Ni and (d) NdMg2 Ni.
transitions of those alloys during the hydrogenation process, the XRD patterns of RMg2 Ni (R = La, Ce, Pr, Nd) alloys after hydrogenation at 573 K were collected and shown in Fig. 4. It can be seen that during the hydriding process, RMg2 Ni phases transform to R-H and Mg2 NiH4 phases, and CeMg3 phase decompounds to CeH2.74 and MgH2 phases. 3.2. Thermodynamic properties The hydrogen storage performances of RMg2 Ni alloys are evaluated by measuring the pressure–composition–temperature (P–C–T) at different temperatures. Fig. 5 shows the P–C–T curves of RMg2 Ni alloys measured at 623 K, 603 K, 573 K and 523 K, respectively. This set of data was collected right after the first activation at 623 K. According to Fig. 5, the P–C–T curve of CeMg2 Ni alloy is different from others in type structure. During hydriding/dehydriding processes, CeMg2 Ni alloy shows two plateaus while others only have one. Combining with the previous XRD analysis data, this difference can be explained by the fact that CeMg2 Ni alloy contains CeMg2 Ni phase and CeMg3 phase, while others only contain a single phase. The actual hydrogen absorption phase of RMg2 Ni (R = La, Pr,
Table 1 The EDS analysis of CeMg2 Ni alloy. Region
Elements
Composition (at.%)
A
Ce Mg Ni
24.53 50.85 24.62
B
Ce Mg Ni
24.67 74.21 1.12
Fig. 4. XRD patterns of RMg2 Ni (R = La, Ce, Pr, Nd) alloys after hydrogenation at 573 K.
Nd) alloys is Mg2 Ni phase, and when it changes to CeMg2 Ni alloy, the effective components are Mg and Mg2 Ni phases. As shown in Table 2, the hydrogen storage capacity, hydriding/dehydring plateau (only for Mg2 Ni phase) and hysteresis of the alloys at 573 K are collected in order to obtain the hydrogen storage properties of RMg2 Ni alloys more clearly. It can be seen that NdMg2 Ni alloy has the largest hydrogen storage capacity and the highest dehydriding plateau among those alloys. The descending order of hysteresis is PrMg2 Ni < NdMg2 Ni < CeMg2 Ni < LaMg2 Ni, which suggests that the cycle stability and reversibility of PrMg2 Ni alloy are better than the others.
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Fig. 5. P–C–T curves of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at different temperatures: (a) LaMg2 Ni, (b) CeMg2 Ni, (c) PrMg2 Ni and (d) NdMg2 Ni.
In addition, the P–C–T curves of these alloys are plotted at different temperatures to study the effect of R-H on the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys. The plateau pressure and temperature are plotted according to the Van’t Hoff equation (Eq. (1)). The enthalpy for the hydriding/dehydriding Mg2 Ni phase in RMg2 Ni alloys are calculated and drawn in Fig. 6. ln K =
S H − RT R
(1)
where K is the equilibrium constant (K = 1/PH2 in the hydriding process and K = PH2 in the dehydriding process).
As shown in Fig. 6, the enthalpy change of hydriding/dehydriding Mg2 Ni phase in RMg2 Ni alloys is smaller than that of pristine Mg2 Ni alloy. According to the XRD analysis of these alloys, RMg2 Ni phases transform to R-H and Mg2 NiH4 phases during the hydriding process. The improvement on the hydrogen storage properties of Mg2 NiH4 phase is because of the existence of R-H, which can reduce the enthalpy change of the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys. The main reason for these experimental results is that R-H can increase the reactive surface area and decrease the diffusion length of hydrogen dramatically. Moreover, the enthalpy change for the hydriding/dehydriding process of Mg2 Ni phase in PrMg2 Ni alloy is much smaller than the others, which demonstrates
Table 2 The hydrogen storage properties of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 573 K. Alloys
Hydrogen storage capacity (wt.%)
Hydriding plateau (MPa)
Dehydriding plateau (MPa)
Hysteresis
LaMg2 Ni CeMg2 Ni PrMg2 Ni NdMg2 Ni
1.92 1.78 1.84 2.07
0.2443 0.3617 0.3241 0.3269
0.1205 0.1884 0.2005 0.2021
0.701 0.652 0.480 0.491
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Table 3 The diffusion coefficient (D) of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 423 K. Samples
Diffusion coefficient (m2 /s)
LaMg2 Ni CeMg2 Ni PrMg2 Ni NdMg2 Ni
2.35 × 10−8 4.38 × 10−8 1.27 × 10−8 3.83 × 10−8
The hydrogen storage properties of RMg2 Ni alloy can be further improved by adjusting the ratio of R to Mg2 Ni. Generally, the reaction mechanism can be analyzed by comparing the observed hydriding rate curves with the rate equations derived from different reaction mechanisms. It is found that the hydrogen absorption processes of RMg2 Ni alloys at 423 K are best fitted to Avrami–Erofeev equation (Eq. (2)). a = 1 − exp(−kt m )
Fig. 6. The enthalpy of RMg2 Ni (R = La, Ce, Pr, Nd) and Mg2 Ni alloys.
that Pr-H has better catalytic effect than other R-H (R = La, Ce, Nd). 3.3. Hydriding kinetic properties Typical hydriding kinetic curves of RMg2 Ni alloys and pristine Mg2 Ni alloy at 523 K are shown in Fig. 7. Since the hydrogen storage capacities of these alloys are different from each others, we use normalization method to evaluate the hydriding kinetics of RMg2 Ni alloys. It can be seen that RMg2 Ni alloys show excellent hydrogen absorption kinetic properties. At 523 K, the uptake time of RMg2 Ni (R = La, Ce, Pr, Nd) alloys to reach 90% of the maximum hydrogen storage capacity is 75 s, 34 s, 65 s and 52 s, respectively, while the time of the pristine Mg2 Ni alloy is 110 s. The CeMg2 Ni alloy performs better hydriding kinetic property than other alloys. The improvement of faster hydrogen absorption kinetics in RMg2 Ni alloys owes to the existence of R-H, which can shorten the diffusion distance of H atom and increase the reaction surface area in those alloys. However, the hydrogen absorption capacity is substantially decreased upon the addition of rare earths in Mg2 Ni alloy.
(2)
where ˛ is the reacted fraction vs. time t, k is the rate constant, and m is the order of this reaction. The fitting hydriding kinetic curves of RMg2 Ni alloys are shown in Fig. 8. It is obvious that the fitting curves are in good agreement with the experimental data. The fitting results indicate that the reaction mechanisms of RMg2 Ni alloys are nucleation and growth processes. In order to further study the hydrogen diffusion process of RMg2 Ni alloys in detail, the lines plotted by ln(1 − ˛) and t of RMg2 Ni alloys are shown in Fig. 9. On the basis of Fick’s diffusion law, the diffusion rate of H atoms in the hydrides phase can be expressed as: 6 1 exp 2 i2 ∞
1−a=
i=1
−i2 2 Dt R2
(3)
where ˛ is the reacted fraction vs. time t, D is the hydrogen diffusion coefficient and R is the radius of hydrogen storage alloy particle. The assumption condition of Eq. (3) is that the alloy particles are spherical structures. When the reaction time is sufficiently long, Eq. (3) can be simplified as: 1−a=
6 exp(−KD t) 2
(4)
The expression of KD is shown below: KD =
2 D R2
(5)
The logarithmic form of Eq. (4) can be written as: −ln(1 − a) = KD t − ln
Fig. 7. Hydrogen absorption curves of RMg2 Ni (R = La, Ce, Pr, Nd) alloys and pristine Mg2 Ni alloy at 523 K.
6 2
(6)
As Eq. (6) reveals a linear relationship between ln(1 − ˛) and reaction time t, the hydrogen diffusion coefficients can be determined from the slope of those lines. The diffusion coefficients (D) of RMg2 Ni alloys are calculated and listed in Table 3. The experimental data were collected at 423 K. According to Table 3, it can be found that the CeMg2 Ni alloy has a larger diffusion coefficient than other alloys, which is consistent with the studies on the hydriding kinetic of RMg2 Ni alloys. This phenomenon is attributed to the multiphase structures including MgH2 , Mg2 NiH4 and CeH2.74 phases in CeMg2 Ni alloy after the hydrogenation process. The CeMg2 Ni alloy shows better hydriding kinetic resulting results from the more diffusion paths for H atoms compared with other alloys.
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Fig. 8. Fit lines of hydriding kinetics curves of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 423 K: (a) LaMg2 Ni, (b) CeMg2 Ni, (c) PrMg2 Ni and (d) NdMg2 Ni.
4. Conclusions In this paper, the effects of R-H in the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni (R = La, Ce, Pr, Nd) alloys were investigated and the hydrogen storage properties of these alloys were studied. The main results can be summarized as follows.
Fig. 9. ln(1 − ˛) vs. t plots of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 423 K.
(1) The XRD patterns demonstrate that RMg2 Ni (R = La, Pr, Nd) alloys have a single-phase structure which correspond to RMg2 Ni phase, and CeMg2 Ni alloy has two phase structures containing CeMg2 Ni phase and CeMg3 phase. (2) The R-H plays an important role in improving hydrogen storage properties of Mg2 Ni phase in RMg2 Ni alloys. The enthalpy change in the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys is smaller than that of pristine Mg2 Ni alloy, which suggests that the R-H could reduce the enthalpy of the Mg2 Ni phase. Meanwhile, R-H can accelerate the hydriding rate of Mg2 Ni phase. The uptake time for the hydrogen content to reach 90% of their maximum hydrogen storage capacity is shorter than that of pristine Mg2 Ni alloy. (3) The hydrogen storage properties of RMg2 Ni alloys change with different R-H. The hydrogen storage capacity of NdMg2 Ni alloy
L. Pei et al. / Materials Science and Engineering B 177 (2012) 1589–1595
is largest and the PrMg2 Ni alloy shows best reversibly properties at 573 K. In addition, Pr-H exhibits better catalytic effect than other R-H (R = La, Ce, Nd) in reducing the enthalpy of Mg2 Ni phase. CeMg2 Ni alloy has a faster hydriding rate compared with the other three alloys at 523 K. (4) The hydrogen absorption process of RMg2 Ni alloys can be perfectly fitted to Avrami–Erofeev equation at 423 K. The fitting results indicate that the reaction mechanisms of RMg2 Ni alloys are nucleation and growth processes. Acknowledgements This work was financially supported by High-Tech Research and Development (863) Program of China (2007AA05Z117), the National Nature Science Foundation of China (50971112 and 51001043) and the Natural Science of Foundation of Hebei Province (E201001170). References [1] L. Hu, S.M. Han, J.H. Li, C. Yang, Y. Li, M.Z. Wang, Materials Science and Engineering B 166 (2010) 209–212. [2] S.L. Cheng, Y.Y. Chen, S.W. Lee, Thin Solid Films 517 (2009) 4745–4748. [3] A. Ebrahimi-Purkani, S.F. Kashani-Bozorg, Journal of Alloys and Compounds 456 (2008) 211–215.
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[4] D. Vojtˇech, P. Novák, J. Cˇizkovsky, V. Knotek, F. Prusa, Journal of Physics and Chemistry of Solids 68 (2007) 813–817. [5] S. Orimo, H. Fujii, Applied Physics A 72 (2001) 167–186. [6] L.Z. Ouyang, X.S. Yang, H.W. Dong, Scripta Materialia 61 (2009) 339–342. [7] K. Tanaka, T. Miwa, K. Sasaki, Journal of Alloys and Compounds 478 (2009) 308–316. [8] L.J. Huang, G.Y. Liang, Z.B. Sun, Y.F. Zhou, Journal of Alloys and Compounds 432 (2007) 172–176. [9] T. Spassov, V. Rangelova, N. Neykov, Journal of Alloys and Compounds 334 (2002) 219–223. [10] S. Kalinichenka, L. Rontzsch, T. Riedl, T. Gemming, T. Weißagrber, B. Kieback, International Journal of Hydrogen Energy 36 (2011) 1592–1600. [11] H. Gu, Y.F. Zhu, L.Q. Li, International Journal of Hydrogen Energy 34 (2009) 1405–1410. [12] M. Porcu, A.K. Petford-long, J.M. Sykes, Journal of Alloys and Compounds 453 (2008) 341–346. [13] I.P. Jain, C. Lal, A. Jain, International Journal of Hydrogen Energy 35 (2010) 5133–5144. [14] M.D. Chio, A. Ziggiotti, M. Baricco, Intermetallics 16 (2008) 102–106. [15] X.F. Liu, Y.F. Zhu, L.Q. Li, Journal of Alloys and Compounds 425 (2006) 235–238. [16] H.J. Lin, L.Z. Ouyang, H. Wang, J.W. Liu, M. Zhu, International Journal of Hydrogen Energy 37 (2012) 1145–1150. [17] J. Liu, X.P. Song, P. Pei, G.L. Chen, Journal of Alloys and Compounds 486 (2009) 338–342. [18] L.Z. Ouyang, L. Yao, H.W. Dong, L.Q. Li, M. Zhu, Journal of Alloys and Compounds 485 (2009) 507–509. [19] X. Zhao, S.M. Han, X.L. Zhu, B.Z. Liu, Y.Q. Liu, Journal of Solid State Chemistry 190 (2012) 68–72. [20] S.M. Han, L. Hu, Y. Li, C. Yang, Preparation for rare earth-Mg-transition metal hydrogen storage alloy, China, ZL, CN101597711. [21] G. Renaudin, L. Guenee, L. Yvon, Journal of Alloys and Compounds 350 (2003) 145–150.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Hydrogen storage properties and phase structures of RMg2 Ni (R = La, Ce, Pr, Nd) alloys Lichao Pei a,b , Shumin Han a,b,∗ , Jiasheng Wang b , Lin Hu b , Xin Zhao b , Baozhong Liu a,c a b c
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, PR China School of Materials Science and Engineering, Henan Polytechic University, Jiaozuo 454000, PR China
a r t i c l e
i n f o
Article history: Received 20 March 2012 Received in revised form 15 May 2012 Accepted 13 August 2012 Available online 28 August 2012 Keywords: Hydrogen storage alloy Rare earth hydride Phase structure Hydriding kinetic Thermodynamic property
a b s t r a c t RMg2 Ni alloys were prepared by inductive melting where R is rare earth (R = La, Ce, Pr, Nd). X-ray diffraction (XRD) patterns revealed a single-phase composition of RMg2 Ni phase when R was one of the three elements (La, Pr, Nd), and a double-phase composition of CeMg2 Ni and CeMg3 phases when R was Ce. In the hydriding process, RMg2 Ni phases transformed to rare earth hydrides (R-H) and Mg2 NiH4 phase, and for CeMg3 phase, it is decomposed to CeH2.74 and MgH2 phases. The enthalpy change of Mg2 Ni phase in RMg2 Ni alloys during the hydriding/dehydriding process was smaller compared with that of pristine Mg2 Ni alloy, which could be attributed to the existence of R-H. The hydrogen storage properties of RMg2 Ni alloys changed with different R compositions in R-H. At 573 K, the NdMg2 Ni alloy had the highest hydrogen storage capacity and dehydriding plateau, and the descending order of hysteresis was PrMg2 Ni < NdMg2 Ni < CeMg2 Ni < LaMg2 Ni, which suggested that the PrMg2 Ni alloy exhibited a better cycling stability and reversibility than the other three alloys. At 523 K, the uptake time of RMg2 Ni alloys to reach 90% of the maximum hydrogen storage capacity was 75 s, 34 s, 65 s and 52 s, respectively, compared with 110 s of pristine Mg2 Ni alloy. Therefore, we believed the R-H in the alloys not only improved their thermodynamic properties but also accelerated their hydriding kinetics. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Intensive studies on hydrogen as an alternative energy source have been carried out around the world in the past decade, and many hydrogen storage materials have been discovered for storing and transporting hydrogen safely and economically. Due to the high hydrogen storage capacity (7.60 wt.%), rich content in the earth’s crust and the low specific weight (1.74 g cm−3 ), magnesium has been considered as one of the most promising candidates among different hydrogen storage materials. However, the poor hydriding/dehydriding kinetics and the excessively high onset dehydriding temperature have limited its industrial applications [1]. Numerous attempts have been made to overcome these challenges in which of the most practical methods is to alloy Mg with transition metals (Ni, Cu, Ti, etc.) to form alloys such as Mg2 Ni [2,3]. The Mg2 Ni alloy absorbs hydrogen up to Mg2 NiH0.3 in a solid solution and forms Mg2 NiH4 at higher hydrogen concentration
∗ Corresponding author at: State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, PR China. Tel.: +86 335 8061569. E-mail address: [email protected] (S. Han). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.003
[4]. Despite the lower hydrogen storage capacity of the Mg2 Ni alloy compared with that of the MgH2 alloy, it has a much faster hydriding/dehydriding kinetics. However, the hydride is still too stable, which requires high temperature of 573 K to release H2 [5]. Further improvement in absorption/desorption conditions can be obtained by alloying Mg2 Ni with rare earth elements, as the rare earth hydride can effectively catalyze hydriding/dehydriding reactions [6,7]. For example, an optimum amount of Nd could provide many amorphous phases and refined microstructures which enhance the absorption kinetics of the Mg–Ni–Nd alloys [8]. Additionally, the rare earth hydride can effectively work as catalyst for the hydrogenation reactions of Mg. Spassov et al. [9] observed an improvement in the hydrogenation kinetics of Mg63 Ni30 Y7 alloy because of the catalytic effect of yttrium. Since the ultrafine crystalline phases (like yttrium hydrides) is helpful to stabilize the nanocrystalline structure by preventing further crystal coarsening during the thermal process, Mg–Cu–Ni–Y [10] alloys, which are prepared by melt spinning, show excellent hydrogen sorption kinetics. Some other methods aim to improve the performances of hydrogen storage alloys are also reported, including ball milling with catalyst [11–13], surface modification [14] and several new sample preparing methods such as hydriding combustion synthesis (HCS) [15], melting-spun [16], and spark plasma sintering [17].
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Recently, attentions have been moved to the LaMg2 Ni alloy. The pristine LaMg2 Ni alloy was produced through inductive melting method [18], and the maximum hydrogen storage capacity could reach to 1.96 wt.% at 564 K. It was discovered that the LaMg2 Ni phase decomposed to Mg2 NiH4 and LaH2.46 phases after the hydriding process. Moreover, the excellent performance in hydrogen storage properties could be attributed to the existence of the LaH2.46 phase, which was useful to improve the hydriding kinetics of LaMg2 Ni alloy. Zhao et al. [19] reported that the reversible hydrogen storage capacity of Mg2 Ni + 20 wt.% LaMg2 Ni complexes reached 3.22 wt.% at 473 K, while the pristine Mg2 Ni hydride could not desorb hydrogen at this temperature. The improvement could be explained by the existence of LaH3 phase in the composite alloys. Therefore, it can be concluded that rare earth hydrides may also be able to ameliorate the hydrogen storage properties of Mg2 Ni phase. In order to study on the catalysis of rare earth hydrides, we explored the phase structures and hydrogen storage properties of RMg2 Ni (R = La, Ce, Pr, Nd) alloys, as well as the effect of R-H in the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys. Fig. 1. XRD patterns of LaMg2 Ni alloy: (a) simulated and (b) as-cast.
2. Experimental methods RMg2 Ni and Mg2 Ni ingots were prepared by inductive melting of rear earth metals, such as La, Ce, Pr, Nd, Mg and Ni (purity more than 99.9%) in a magnesia crucible under argon atmosphere. The raw materials were added to the crucible according to the melting points and the densities of these metals, and the repeated melting at positive pressure protection had to be taken to ensure composition homogeneity during master alloy ingot preparation. A slightly excess of Mg was added to compensate the evaporative Mg loss in the melting procedure [20] and the ingots were annealed at 738 K for 6 h. The compositions of these alloys were measured by inductive coupled plasma (ICP) emission spectrometer. The phase structures of as-cast alloys were tested in a D/max-2500/PC X-ray diffract meter with Cu K␣ radiation and the images were analyzed with Jade-5 software. Scanning electron microscopy (FESEM) images of RMg2 Ni alloys were obtained by HITACHI S-4800 scanning electron microscope with an energy dispersive X-ray spectrometer (EDS). The hydriding/dehydriding process was studied by pressure–composition–temperature (P–C–T) characteristic measurement equipment (made by Suzuki Shokan in Japan) with the measurement parameters set as: delay time 300 s, maximum pressure 3.0 MPa. The sample weight for P–C–T measurement was set as around 2.0 g. Hydriding kinetics of the as-cast alloys were also tested by P–C–T characteristic measurement equipment under the initial hydrogen pressure of 3.0 MPa. The alloy powder was hydrogenated under 3.0 MPa hydrogen pressure for 2 h, followed by the dehydriding process in vacuum for 2 h at 623 K.
cannot be found in the ICDD database. As these four rare elements (La, Ce, Pr, Nd) have the similar atomic structures and physicochemical properties, the XRD patterns of RMg2 Ni (R = Ce, Pr, Nd) phases should be similar to that of LaMg2 Ni phase. The XRD patterns of these alloys are shown in Fig. 2. It can be clearly seen that although the intensities are slightly different from each other, the locations of characteristic peaks of RMg2 Ni (R = Pr, Nd) alloys in XRD patterns are the same as those of LaMg2 Ni alloy, which reveals that the phase compositions of PrMg2 Ni and NdMg2 Ni alloys are PrMg2 Ni phase and NdMg2 Ni phase, respectively. However, the XRD pattern of CeMg2 Ni alloy indicates that it consists of CeMg2 Ni phase and CeMg3 phase, which results from the multi-chemical valences of Ce element. It is difficult for the Ce element to form only one compound with a high chemical valence and thus generate CeMg3 phase. The SEM images of RMg2 Ni alloys are shown in Fig. 3. It is clear to find that RMg2 Ni (R = La, Pr, Nd) alloys contain only single phase structures, while CeMg2 Ni alloy consists of two crystallography phases (the gray region (labeled as A) and the bright region (labeled as B)). The EDS analysis result of CeMg2 Ni alloy is listed in Table 1, which indicates that region A and B are CeMg2 Ni phase and CeMg3 phase, respectively. To further understand the phase
3. Results and discussion 3.1. Phase structure Until now, there is no standard powder diffraction pattern available for LaMg2 Ni alloy in the International Centre for Diffraction Data (ICDD) database. Renaudin et al. [21] previously investigated the cell parameters and atom positions of the single crystal LaMg2 Ni alloy. As shown in Fig. 1(a), it is the simulated XRD pattern of LaMg2 Ni phase that is calculated by using material studio software. The experimental XRD pattern of LaMg2 Ni alloy prepared by inductive melting is presented in Fig. 1(b). Obviously, it can be found that the as-cast LaMg2 Ni alloy only contains LaMg2 Ni phase by the comparison between those two XRD patterns. In the past decades, few studies on RMg2 Ni (R = Ce, Pr, Nd) alloys were reported and thus the powder diffraction patterns of the corresponding single phases
Fig. 2. XRD patterns of RMg2 Ni (R = La, Ce, Pr, Nd) alloys.
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Fig. 3. SEM micrographs of RMg2 Ni (R = La, Ce, Pr, Nd) alloys: (a) LaMg2 Ni, (b) CeMg2 Ni, (c) PrMg2 Ni and (d) NdMg2 Ni.
transitions of those alloys during the hydrogenation process, the XRD patterns of RMg2 Ni (R = La, Ce, Pr, Nd) alloys after hydrogenation at 573 K were collected and shown in Fig. 4. It can be seen that during the hydriding process, RMg2 Ni phases transform to R-H and Mg2 NiH4 phases, and CeMg3 phase decompounds to CeH2.74 and MgH2 phases. 3.2. Thermodynamic properties The hydrogen storage performances of RMg2 Ni alloys are evaluated by measuring the pressure–composition–temperature (P–C–T) at different temperatures. Fig. 5 shows the P–C–T curves of RMg2 Ni alloys measured at 623 K, 603 K, 573 K and 523 K, respectively. This set of data was collected right after the first activation at 623 K. According to Fig. 5, the P–C–T curve of CeMg2 Ni alloy is different from others in type structure. During hydriding/dehydriding processes, CeMg2 Ni alloy shows two plateaus while others only have one. Combining with the previous XRD analysis data, this difference can be explained by the fact that CeMg2 Ni alloy contains CeMg2 Ni phase and CeMg3 phase, while others only contain a single phase. The actual hydrogen absorption phase of RMg2 Ni (R = La, Pr,
Table 1 The EDS analysis of CeMg2 Ni alloy. Region
Elements
Composition (at.%)
A
Ce Mg Ni
24.53 50.85 24.62
B
Ce Mg Ni
24.67 74.21 1.12
Fig. 4. XRD patterns of RMg2 Ni (R = La, Ce, Pr, Nd) alloys after hydrogenation at 573 K.
Nd) alloys is Mg2 Ni phase, and when it changes to CeMg2 Ni alloy, the effective components are Mg and Mg2 Ni phases. As shown in Table 2, the hydrogen storage capacity, hydriding/dehydring plateau (only for Mg2 Ni phase) and hysteresis of the alloys at 573 K are collected in order to obtain the hydrogen storage properties of RMg2 Ni alloys more clearly. It can be seen that NdMg2 Ni alloy has the largest hydrogen storage capacity and the highest dehydriding plateau among those alloys. The descending order of hysteresis is PrMg2 Ni < NdMg2 Ni < CeMg2 Ni < LaMg2 Ni, which suggests that the cycle stability and reversibility of PrMg2 Ni alloy are better than the others.
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Fig. 5. P–C–T curves of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at different temperatures: (a) LaMg2 Ni, (b) CeMg2 Ni, (c) PrMg2 Ni and (d) NdMg2 Ni.
In addition, the P–C–T curves of these alloys are plotted at different temperatures to study the effect of R-H on the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys. The plateau pressure and temperature are plotted according to the Van’t Hoff equation (Eq. (1)). The enthalpy for the hydriding/dehydriding Mg2 Ni phase in RMg2 Ni alloys are calculated and drawn in Fig. 6. ln K =
S H − RT R
(1)
where K is the equilibrium constant (K = 1/PH2 in the hydriding process and K = PH2 in the dehydriding process).
As shown in Fig. 6, the enthalpy change of hydriding/dehydriding Mg2 Ni phase in RMg2 Ni alloys is smaller than that of pristine Mg2 Ni alloy. According to the XRD analysis of these alloys, RMg2 Ni phases transform to R-H and Mg2 NiH4 phases during the hydriding process. The improvement on the hydrogen storage properties of Mg2 NiH4 phase is because of the existence of R-H, which can reduce the enthalpy change of the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys. The main reason for these experimental results is that R-H can increase the reactive surface area and decrease the diffusion length of hydrogen dramatically. Moreover, the enthalpy change for the hydriding/dehydriding process of Mg2 Ni phase in PrMg2 Ni alloy is much smaller than the others, which demonstrates
Table 2 The hydrogen storage properties of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 573 K. Alloys
Hydrogen storage capacity (wt.%)
Hydriding plateau (MPa)
Dehydriding plateau (MPa)
Hysteresis
LaMg2 Ni CeMg2 Ni PrMg2 Ni NdMg2 Ni
1.92 1.78 1.84 2.07
0.2443 0.3617 0.3241 0.3269
0.1205 0.1884 0.2005 0.2021
0.701 0.652 0.480 0.491
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Table 3 The diffusion coefficient (D) of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 423 K. Samples
Diffusion coefficient (m2 /s)
LaMg2 Ni CeMg2 Ni PrMg2 Ni NdMg2 Ni
2.35 × 10−8 4.38 × 10−8 1.27 × 10−8 3.83 × 10−8
The hydrogen storage properties of RMg2 Ni alloy can be further improved by adjusting the ratio of R to Mg2 Ni. Generally, the reaction mechanism can be analyzed by comparing the observed hydriding rate curves with the rate equations derived from different reaction mechanisms. It is found that the hydrogen absorption processes of RMg2 Ni alloys at 423 K are best fitted to Avrami–Erofeev equation (Eq. (2)). a = 1 − exp(−kt m )
Fig. 6. The enthalpy of RMg2 Ni (R = La, Ce, Pr, Nd) and Mg2 Ni alloys.
that Pr-H has better catalytic effect than other R-H (R = La, Ce, Nd). 3.3. Hydriding kinetic properties Typical hydriding kinetic curves of RMg2 Ni alloys and pristine Mg2 Ni alloy at 523 K are shown in Fig. 7. Since the hydrogen storage capacities of these alloys are different from each others, we use normalization method to evaluate the hydriding kinetics of RMg2 Ni alloys. It can be seen that RMg2 Ni alloys show excellent hydrogen absorption kinetic properties. At 523 K, the uptake time of RMg2 Ni (R = La, Ce, Pr, Nd) alloys to reach 90% of the maximum hydrogen storage capacity is 75 s, 34 s, 65 s and 52 s, respectively, while the time of the pristine Mg2 Ni alloy is 110 s. The CeMg2 Ni alloy performs better hydriding kinetic property than other alloys. The improvement of faster hydrogen absorption kinetics in RMg2 Ni alloys owes to the existence of R-H, which can shorten the diffusion distance of H atom and increase the reaction surface area in those alloys. However, the hydrogen absorption capacity is substantially decreased upon the addition of rare earths in Mg2 Ni alloy.
(2)
where ˛ is the reacted fraction vs. time t, k is the rate constant, and m is the order of this reaction. The fitting hydriding kinetic curves of RMg2 Ni alloys are shown in Fig. 8. It is obvious that the fitting curves are in good agreement with the experimental data. The fitting results indicate that the reaction mechanisms of RMg2 Ni alloys are nucleation and growth processes. In order to further study the hydrogen diffusion process of RMg2 Ni alloys in detail, the lines plotted by ln(1 − ˛) and t of RMg2 Ni alloys are shown in Fig. 9. On the basis of Fick’s diffusion law, the diffusion rate of H atoms in the hydrides phase can be expressed as: 6 1 exp 2 i2 ∞
1−a=
i=1
−i2 2 Dt R2
(3)
where ˛ is the reacted fraction vs. time t, D is the hydrogen diffusion coefficient and R is the radius of hydrogen storage alloy particle. The assumption condition of Eq. (3) is that the alloy particles are spherical structures. When the reaction time is sufficiently long, Eq. (3) can be simplified as: 1−a=
6 exp(−KD t) 2
(4)
The expression of KD is shown below: KD =
2 D R2
(5)
The logarithmic form of Eq. (4) can be written as: −ln(1 − a) = KD t − ln
Fig. 7. Hydrogen absorption curves of RMg2 Ni (R = La, Ce, Pr, Nd) alloys and pristine Mg2 Ni alloy at 523 K.
6 2
(6)
As Eq. (6) reveals a linear relationship between ln(1 − ˛) and reaction time t, the hydrogen diffusion coefficients can be determined from the slope of those lines. The diffusion coefficients (D) of RMg2 Ni alloys are calculated and listed in Table 3. The experimental data were collected at 423 K. According to Table 3, it can be found that the CeMg2 Ni alloy has a larger diffusion coefficient than other alloys, which is consistent with the studies on the hydriding kinetic of RMg2 Ni alloys. This phenomenon is attributed to the multiphase structures including MgH2 , Mg2 NiH4 and CeH2.74 phases in CeMg2 Ni alloy after the hydrogenation process. The CeMg2 Ni alloy shows better hydriding kinetic resulting results from the more diffusion paths for H atoms compared with other alloys.
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Fig. 8. Fit lines of hydriding kinetics curves of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 423 K: (a) LaMg2 Ni, (b) CeMg2 Ni, (c) PrMg2 Ni and (d) NdMg2 Ni.
4. Conclusions In this paper, the effects of R-H in the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni (R = La, Ce, Pr, Nd) alloys were investigated and the hydrogen storage properties of these alloys were studied. The main results can be summarized as follows.
Fig. 9. ln(1 − ˛) vs. t plots of RMg2 Ni (R = La, Ce, Pr, Nd) alloys at 423 K.
(1) The XRD patterns demonstrate that RMg2 Ni (R = La, Pr, Nd) alloys have a single-phase structure which correspond to RMg2 Ni phase, and CeMg2 Ni alloy has two phase structures containing CeMg2 Ni phase and CeMg3 phase. (2) The R-H plays an important role in improving hydrogen storage properties of Mg2 Ni phase in RMg2 Ni alloys. The enthalpy change in the hydriding/dehydriding process of Mg2 Ni phase in RMg2 Ni alloys is smaller than that of pristine Mg2 Ni alloy, which suggests that the R-H could reduce the enthalpy of the Mg2 Ni phase. Meanwhile, R-H can accelerate the hydriding rate of Mg2 Ni phase. The uptake time for the hydrogen content to reach 90% of their maximum hydrogen storage capacity is shorter than that of pristine Mg2 Ni alloy. (3) The hydrogen storage properties of RMg2 Ni alloys change with different R-H. The hydrogen storage capacity of NdMg2 Ni alloy
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is largest and the PrMg2 Ni alloy shows best reversibly properties at 573 K. In addition, Pr-H exhibits better catalytic effect than other R-H (R = La, Ce, Nd) in reducing the enthalpy of Mg2 Ni phase. CeMg2 Ni alloy has a faster hydriding rate compared with the other three alloys at 523 K. (4) The hydrogen absorption process of RMg2 Ni alloys can be perfectly fitted to Avrami–Erofeev equation at 423 K. The fitting results indicate that the reaction mechanisms of RMg2 Ni alloys are nucleation and growth processes. Acknowledgements This work was financially supported by High-Tech Research and Development (863) Program of China (2007AA05Z117), the National Nature Science Foundation of China (50971112 and 51001043) and the Natural Science of Foundation of Hebei Province (E201001170). References [1] L. Hu, S.M. Han, J.H. Li, C. Yang, Y. Li, M.Z. Wang, Materials Science and Engineering B 166 (2010) 209–212. [2] S.L. Cheng, Y.Y. Chen, S.W. Lee, Thin Solid Films 517 (2009) 4745–4748. [3] A. Ebrahimi-Purkani, S.F. Kashani-Bozorg, Journal of Alloys and Compounds 456 (2008) 211–215.
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