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Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics
Journal of Rare Earths xxx (2018) 1e6
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Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics* Jiasheng Wang a, c, Yuan Li b, Ting Liu b, Dandan Peng b, Shumin Han a, b, * a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China c Department of Environmental Engineering, North China Institute of Science and Technology, PO Box 206, Yanjiao, Beijing 101601, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2017 Received in revised form 6 February 2018 Accepted 6 February 2018 Available online xxx
In this work, a Mg-based composite material with in-situ formed LaH3, Mg2NiH4-LiBH4 þ 20 wt% LaH3, was prepared by ball milling LiBH4 and hydrogenated LaMg2Ni and Mg2Ni powder mixture, followed by heat treatment at 573 K. The onset dehydrogenation temperature of the composite is reduced by 50 K compared with that of Mg2NiH4-LiBH4. The LaH3-doped composite shows faster kinetics, absorbing 1.43 wt% hydrogen within 100 s at 423 K, which is 6.5 times faster than Mg2NiH4-LiBH4. Moreover, the composite releases 1.24 wt% hydrogen within 500 s at 573 K, 0.69 wt% higher than Mg2NiH4-LiBH4. The activation energy of the composite is reduced by 8.2 and 80 kJ/mol compared with that of Mg2NiH4LiBH4 and commercial MgH2, respectively. The improvement in hydrogen storage properties is attributed to the fact that LaH3 promotes the generation of nano-sized spongy Mg structure, which has good catalytic activity during the subsequent hydrogenation/dehydrogenation process. © 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Mg2NiH4 LiBH4 Composite Hydrogen storage properties Lanthanum hydride Rare earths
1. Introduction Hydrogen is considered to be a very promising energy carrier due to its high combustion heat and clean burning. Hydrogen storage, especially a high capacity storage under moderate conditions, is the key for utilization of hydrogen energy. Thus, it is important to develop a material which is able to absorb/desorb hydrogen efficiently. Among different kinds of materials, metal/ intermetallic hydride storage materials are considered as promising candidates.1e4 Magnesium is widely investigated for onboard hydrogen storage, especially for vehicular applications, due to its high hydrogen storage capacity (up to 7.6 wt%), light weight and low cost.5,6 However, the sluggish hydrogen absorbing/desorbing kinetics and high hydride stability prevent practical applications of Mg-based hydrogen storage materials. To improve the kinetics and destabilize Mg hydride, various metals
* Foundation item: Project supported by the National Natural Science Foundation of China (51771164, 51571173), China Postdoctoral Science Foundation (2016M601281), Scientific Research Projects in Colleges and Universities in Hebei Province, China (ZD2014004, QN2016002). * Corresponding author. State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China. E-mail address: [email protected] (S. Han).
have been used to form alloys with Mg.7e10 Among these alloys, Mg2Ni has been considered a potential choice for hydrogen storage applications because of its lower hydriding/dehydriding temperature around 573 K11e13 and high hydrogen storage capacity of 3.6 wt%.14 For further improvement in hydrogen storage ability of Mg2Ni, partial or complete substitution of nickel by other suitable elements has been attempted. Substitution with Co15 and Cu16 has been carried out on Mg2Ni alloy, and it was found that the presence of Co and Cu weakened the interactions between magnesium and nickel, as well as interactions between nickel and hydrogen. Consequently, the alloy was destabilized and the materials showed a remarkable decrease of the onset dehydrogenation temperature. Rare earth elements have been doped into Mg2Ni alloy as hydrogen absorbing elements and found to be effective in improving hydrogen storage properties due to their role as reactive destabilizer.17,18 The addition of La into Mg2Ni alloy created a new ternary Mg-based compound, LaMg2Ni,19e21 which showed significant improvement in its dehydriding properties at low temperature. Previous reports have shown that LiBH4 and Mg2NiH4 undergo the following reaction (1), providing an easy dehydrogenation process.
5Mg2 NiH4 þ 4LiBH4 /2MgNi2:5 B2 þ 4LiH þ 8Mg þ 16H2
(1)
https://doi.org/10.1016/j.jre.2018.02.005 1002-0721/© 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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J. Wang et al. / Journal of Rare Earths xxx (2018) 1e6
The decomposition products were identified as LiH, Mg, MgNi2.5B2, and H2. The dehydrogenation reaction of the composite began at 550 K, which was a lower onset dehydrogenation temperature compared with that of Mg2NiH4. Moreover, the hydrogenation/dehydrogenation rate of the composite was faster than that of Mg2NiH4. This improvement makes the composite a potential candidate as a hydrogen storage material. In our previous work, LaH3 was demonstrated to be effective in catalyzing the dehydrogenation of Mg-based alloys by expanding/ contracting the lattices.22,23 Here, we have prepared a Mg-based composite material, Mg2NiH4-LiBH4 þ 20 wt% LaH3, by a solid state reaction in which LaH3 is formed in-situ in the system. Subsequently, the microstructures and reversible hydrogen storage properties of the Mg2NiH4-LiBH4/LaH3 composite were investigated. 2. Experimental Commercial LiBH4 (95%, Alfa Aesar) powder was used asreceived. The Mg2Ni and LaMg2Ni alloys were prepared by induction melting of the components, metallic La, Mg and Ni (99.9% purity) under protection of highly pure argon atmosphere (99.9% purity). The as-cast Mg2Ni and LaMg2Ni alloys were mechanically crushed into 100 mesh powders. A mixture of Mg2Ni (2.417 g) and LaMg2Ni (1.000 g) alloys was then hydrogenated under 3 MPa hydrogen pressure at 673 K for 8 h to ensure complete transformation into Mg2NiH4 and LaH3 (in-situ reaction). The mixture of LiBH4 (0.27 g)/Mg2NiH4-LaH3 (2.13 g) hydride system was prepared by ball-milling the as-received LiBH4 powder and as-prepared Mg2NiH4 powder in a molar ratio of 4:5, and the weight percentage of LaH3 was 20 wt% of the total material weight. The ball-milling process was performed in a stainless steel vial with a ball-to-powder weight ratio of 20:1 using a QM-ISP2 planetary mill at 500 r/min for 2 h. The composite was then heated to 573 K for 2 h under a dynamic vacuum environment to generate Mg. To avoid air-exposure, all handlings were carried out in an Ar-filled glove box equipped with a purification system, in which the typical O2/H2O levels were below 1 ppm. The curves of hydrogen absorption/desorption quantity of the samples were measured on a pressure-composition-temperature characteristic measurement equipment. The temperature programmed desorption (TPD) properties were determined by heating the sample from ambient temperature to 500 C at a heating rate of 5 C/min under a hydrogen back pressure below 0.0001 MPa. Before the TPD measurement, the composite was placed under vacuum for 10 min to remove hydrogen atoms physically adsorbed onto the composite surface. For the hydrogen absorption/desorption tests, the composite was firstly dehydrogenated at 623 K for 2 h to get completely dehydrogenated species. The phase composition of the samples was characterized by X-ray diffraction (XRD, D/ MAX-2500/PC, Cu Ka radiation). The hydrogen thermodynamic performance was studied by a simultaneous thermal analyzer (DTG-60A) under an Ar flow rate of 100 mL/min from the room temperature to 773 K with the heating rates of 5, 10, 15 and 20 K/ min, respectively. 3. Results and discussion 3.1. Hydrogenation characteristics and morphology of LaMg2Ni and Mg2Ni alloys Fig. 1 presents the XRD patterns of the fully hydrogenated LaMg2Ni-Mg2Ni mixture. In the patterns, only diffraction peaks corresponding to Mg2NiH4 phase and LaH3 phase are observed. It
Fig. 1. XRD patterns of the LaMg2Ni and Mg2Ni alloys hydride.
indicates that the following reactions (2) and (3) occur during the hydrogenation process.
Mg2 Ni þ H2 /Mg2 NiH4
(2)
LaMg2 Ni þ H2 /Mg2 NiH4 þ LaH3
(3)
In the hydrogenated samples, no LaMg2NiH7 phase is detected. It has been suggested by Renaudin et al.24 that reaction of LaMg2Ni in hydrogen atmosphere is generally affected by temperature, and the compound LaMg2Ni will decompose into LaH3 and an amorphous phase when the temperature is higher than 573 K. The phase transformation here is similar to that reported by Ouyang et al.25 Through the above solid-state reaction of LaMg2Ni in hydrogen atmosphere, a composite consisting of Mg2NiH4 and LaH3 can be obtained. However, LaH3 is a stable hydride and it is difficult to release hydrogen from this hydride. Therefore, too much LaH3 hydride will lead to a low reversible capacity for the system. Thus, the ratio of Mg2Ni to LaMg2Ni was adjusted to obtain a composite with an appropriate amount of LaH3, and LiBH4 was added to achieve a high capacity and reversible hydrogen absorbing/desorbing system by accomplishing reaction (1) according to a previous report.26 Fig. 2 presents the decomposition behaviors of Mg2NiH4-LiBH4 composite and Mg2NiH4-LaMg2Ni-LiBH4 composite. The amount of hydrogen evolution as a function of temperature was examined using a PCT apparatus. According to Fig. 2, the onset decomposition temperature of the Mg2NiH4-LaMg2Ni-LiBH4 composite decreases to 500 K, which is 50 K lower than that of the Mg2NiH4-LiBH4 composite. The results indicate that the in-situ formed LaH3 facilitates the hydrogen desorption process of Mg2NiH4-;LiBH4 system at a relatively low temperature (see Fig. 3). To reveal the phase structure of the LaH3-containing composite and the mechanism of effect of LaH3 in the hydrogenation/dehydrogenation process, different hydrogenation state species were collected, including as-milled powders, hydrogenated powders and dehydrogenated powders, and characterized by XRD analysis. XRD results show that the as-milled powders consist of Mg2NiH4 phase and LaH3 phase, with no detection of any new phases, indicating that no chemical reactions occurred upon ball milling. Moreover, the diffraction peaks of LiBH4 are not observed, which can be ascribed to its fine grain and amorphous nature after the ball milling treatment.27 After ball milling, the species was subjected to dehydrogenation at 623 K, and the XRD patterns for the
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
J. Wang et al. / Journal of Rare Earths xxx (2018) 1e6
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This result is similar to that in our previous report,28 and the floccule-shaped species are Mg particles. From the comparison, it is concluded that addition of lanthanum hydride into the composite promotes formation of flocculent Mg. TEM characterization was performed and is shown in Fig. 5. It is seen that the dehydrogenated Mg2NiH4-LiBH4 composite has few flocculent Mg-covered lamellar structures. However, the dehydrogenated Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite consists mainly of flocculent Mg interlinked together to form a spongy-shaped bulk structure. It implies that morphology of the product of reaction (1) is modified by lanthanum hydride. The lanthanum hydride provides proper sites for flocculent Mg to grow around, and then sponge-structured Mg with abundant holes forms in the LaH3containing Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite. 3.2. Hydrogen absorption/desorption kinetic performances
Fig. 2. TPD patterns of the Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites.
dehydrogenated sample indicate appearance of Mg and MgNi2.5B2 phases, which is consistent with the chemical reaction in Eq. (1).21 In the subsequent hydrogenation/dehydrogenation cycles, Mg from the above chemical reaction converts between Mg and MgH2. However, MgNi2.5B2 and LaH3 remain unchanged and promote hydrogenation/dehydrogenation of the composite. As magnesium is the active hydrogen absorbing/desorbing material in the composite, and the reversibility of the transformation between Mg and MgH2 is excellent, the composite then undergoes a stable hydrogenation/dehydrogenation cycle. Morphology of the species with and without lanthanum hydride at different hydrogenation stages was observed using SEM, and the corresponding results are shown in Fig. 4. The composite without lanthanum hydride appears as small particles which are agglomerated together obviously. After hydrogenation/dehydrogenation cycles, the agglomeration is more noticeable, and a lamellar-shape structure is observed. The as-milled Mg2NiH4LiBH4 þ 20 wt% LaH3 composite shows smaller particles, and after dehydrogenation, fine particles with less agglomeration are observed. Comparing Fig. 4(d) with Fig. 4(c), it is found that the main textures of the two composites are the same, however, some floccule-shaped species cover the lamellar structure in Fig. 4(d).
Hydrogenation/dehydrogenation properties of the composites with and without lanthanum hydride were tested. The isothermal hydrogen absorption/desorption kinetics profiles are plotted in Fig. 6, which illustrates the hydrogen absorption curves of Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites at 423 K under 3.0 MPa. Before the hydrogenation/dehydrogenation test, it was ensured that reactions (1), (2) and (3) were performed completely. And the actual hydrogenation/dehydrogenation reactions are between Mg and MgH2. So variation in hydrogen content for the two composites below is calculated by Mg/MgH2. As shown in Fig. 6, Mg2NiH4-LiBH4 composite absorbs 1.43 wt% (weight ratio of the composite) of hydrogen within 750 s. However, the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite absorbs the same content of hydrogen within 100 s, which is 650 s faster compared with the composite without LaH3. Obviously, the hydrogen absorption rate is remarkably improved, indicating that LaH3 plays an effective role in improving hydriding kinetics of the LiBH4-Mg2NiH4 composite. The results of hydrogen desorption kinetics measured at 573 K are presented in Fig. 7. It is seen that the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite displays higher desorption rate. In the first 500 s, only 0.55 wt% of hydrogen is desorbed from the Mg2NiH4-LiBH4 composite. However, 1.24 wt% of hydrogen can be released from the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite in the same time period. The Mg2NiH4-LiBH4 composite releases 0.93 wt% hydrogen within 1600 s, whereas the Mg2NiH4-LiBH4 þ 20 wt% LaH3 releases 0.93 wt % hydrogen within only 200 s. These results indicate that hydrogen desorption has been remarkably promoted with addition of lanthanum hydride. 3.3. Dehydrogenation thermodynamic performances
Fig. 3. XRD patterns of the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite. (1) Ball-milled; (2) Hydrogenated; (3) Dehydrogenated.
Fig. 8 represents the temperature programmed desorption (TPD) test of the Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites. The starting temperature of rapid hydrogen desorption for the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is 530 K, which is 43 K lower than that of the Mg2NiH4-LiBH4 composite. The measurements on Mg2NiH4-LiBH4 and Mg2 NiH4-LiBH4 þ 20 wt% LaH3 composites were performed after completion of reactions (2) and (3), and the actual hydrogen absorbing material is the generated flocculent MgH2. The different particle sizes of MgH2 affect its hydrogen desorption properties, and it has been previously reported that smaller MgH2 particle sizes can noticeably destabilize its hydride. The generated MgH2 in the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite has a significantly smaller particle size, which facilitates the destabilization of its hydride and leads to a lower onset decomposition temperature.29
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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J. Wang et al. / Journal of Rare Earths xxx (2018) 1e6
Fig. 4. SEM images of ball milled and dehydrogenated Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites. (a) Ball-milled Mg2NiH4-LiBH4; (b) Dehydrogenated Mg2NiH4-LiBH4; (c) Ball-milled Mg2NiH4-LiBH4 þ 20 wt% LaH3; (d) Dehydrogenated Mg2NiH4-LiBH4 þ 20 wt% LaH3.
Fig. 5. TEM images of dehydrogenated Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites. (a) Dehydrogenated Mg2NiH4-LiBH4; (b) Dehydrogenated Mg2NiH4LiBH4 þ 20 wt% LaH3.
Obviously, LaH3 plays a key role in catalyzing the dehydrogenation of Mg-based alloys by expanding/contracting the lattices,22,23 as well as in catalyzing the reaction between Mg2NiH4 and LiBH4. DTA tests were performed at different heating rates and the results are shown in Fig. 9. The decomposition temperature of the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is obviously decreased. Compared with the Mg2NiH4-LiBH4 composite, the dehydrogenation peak temperature for Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite at a heating rate of 10 K/min is lowered by 61.67 K. The activation energy (Ea) of the Mg2NiH4-LiBH4 and Mg2NiH4LiBH4 þ 20 wt% LaH3 composites are calculated by using Kissinger's method,29 which is expressed by the below equation.
h . i 2 d ln a Tm d ð1=Tm Þ
¼
Ea R
(4)
Where a is the heating rate, Tm is the maximum desorption temperature, and R is the gas constant. The activation energy values of different composites were evaluated from the DTA data obtained in the present work employing the formula given above. Ea values were calculated based on the plot of ln(a/T2m) versus 1000/Tm (Fig. 10) and are listed in Table 1. The activation energy for the dehydrogenation of Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is calculated to be 80.03 kJ/mol, which is 8.2 kJ/mol lower than that of the Mg2NiH4-LiBH4 composite and ~80 kJ/mol lower than that of
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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Fig. 6. Comparison of the absorption kinetics of Mg2NiH4-LiBH4 and Mg2NiH4LiBH4 þ 20 wt% LaH3 composites at 423 K.
Fig. 9. DTA patterns of Mg2NiH4-LiBH4 (a) and Mg2NiH4-LiBH4 þ 20 wt% LaH3 (b) composites after annealing at various heating rates. Fig. 7. Comparison of the desorption kinetics of Mg2NiH4-LiBH4 and Mg2NiH4LiBH4 þ 20 wt% LaH3 composites at 573 K.
Fig. 10. Kissinger plot of first-order dehydrogenation of different samples at various heating rates.
Fig. 8. TPD patterns of the Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites after annealing.
the commercial MgH2 (~160 kJ/mol),10 suggesting that the LaH3doped composite can lower the energy barrier of Mg2NiH4-LiBH4 composite and pure MgH2 in the dehydrogenation process, resulting in enhanced desorption kinetics.
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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Table 1 Tm and Ea values corresponding to two samples. Samples
Heating rate used in DTA (K/min)
Tm (K)
Ea (kJ/mol)
Mg2NiH4-LiBH4
5 10 15 20 5 10 15 20
566.51 577.61 596.51 604.21 529.42 548.27 561.48 567.22
88.23
Mg2NiH4-LiBH4 þ 20 wt% LaH3
80.03
4. Conclusions In this work, a Mg-based composite hydrogen storage system was prepared by solid reaction between LaMg2Ni, Mg2Ni and LiBH4 in hydrogen atmosphere (denoted as Mg2NiH4-LiBH4 þ 20 wt% LaH3). The reversible hydrogen storage properties and reaction mechanism of Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite were investigated. The SEM results indicate that the in-situ formed LaH3 is beneficial to the homogeneous generation of flocculent Mg on MgNi2.5B2 blocks. The Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite after annealing exhibits a significant decrease in onset decomposition temperature, the temperature being 530 K, which is 43 K lower than that of the Mg2NiH4-LiBH4. The activation energy for dehydrogenation of the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is 80.03 kJ/mol, which is 8.2 kJ/mol lower than that of the Mg2NiH4LiBH4 composite and 80 kJ/mol lower than that of commercial MgH2. Furthermore, the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite absorbs 1.46 wt% hydrogen in just 100 s (at 423 K), 650 s faster than that of the composite without LaH3, while it releases 0.93 wt% hydrogen (at 573 K) in 200 s, which is 1400 s faster than that of the undoped composite. References 1. Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy. 2007;32(9):1121. 2. Li X, Zhang YH, Yang T, Xu JY, Zhao DL. Hydriding/dehydriding properties of NdMgNi alloy with catalyst CeO2. J Rare Earths. 2016;34(4):407. 3. Sahli I, Ghodbane O, Abdellaoui M. Electrochemical hydrogenation of CeZr2Cr4Ni5ebased alloys. Mater Res Bull. 2017;85:10. 4. Ley MB, Jepsen LH, Lee YS, Cho YW, von Colbe JMB, Dornheim M, et al. Complex hydrides for hydrogen storageenew perspectives. Mater Today. 2014;17(3): 122. 5. Wang Y, Wang YJ. Recent advances in additive-enhanced magnesium hydride for hydrogen storage. Prog Nat Sci-Mater. 2017;27(1):41. 6. Varin RA, Jang M, Czujko T, Wronski ZS. The effect of ball milling under hydrogen and argon on the desorption properties of MgH2 covered with a layer of Mg(OH)2. J Alloys Compd. 2010;493(1e2):L29. 7. Yuan ZM, Yang T, Bu WG, Shang HW, Qi Y, Zhang YH. Structure, hydrogen storage kinetics and thermodynamics of Mg-base Sm5Mg41 alloy. Int J Hydrogen Energy. 2016;41(14):5994.
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Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics* Jiasheng Wang a, c, Yuan Li b, Ting Liu b, Dandan Peng b, Shumin Han a, b, * a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China c Department of Environmental Engineering, North China Institute of Science and Technology, PO Box 206, Yanjiao, Beijing 101601, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2017 Received in revised form 6 February 2018 Accepted 6 February 2018 Available online xxx
In this work, a Mg-based composite material with in-situ formed LaH3, Mg2NiH4-LiBH4 þ 20 wt% LaH3, was prepared by ball milling LiBH4 and hydrogenated LaMg2Ni and Mg2Ni powder mixture, followed by heat treatment at 573 K. The onset dehydrogenation temperature of the composite is reduced by 50 K compared with that of Mg2NiH4-LiBH4. The LaH3-doped composite shows faster kinetics, absorbing 1.43 wt% hydrogen within 100 s at 423 K, which is 6.5 times faster than Mg2NiH4-LiBH4. Moreover, the composite releases 1.24 wt% hydrogen within 500 s at 573 K, 0.69 wt% higher than Mg2NiH4-LiBH4. The activation energy of the composite is reduced by 8.2 and 80 kJ/mol compared with that of Mg2NiH4LiBH4 and commercial MgH2, respectively. The improvement in hydrogen storage properties is attributed to the fact that LaH3 promotes the generation of nano-sized spongy Mg structure, which has good catalytic activity during the subsequent hydrogenation/dehydrogenation process. © 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Keywords: Mg2NiH4 LiBH4 Composite Hydrogen storage properties Lanthanum hydride Rare earths
1. Introduction Hydrogen is considered to be a very promising energy carrier due to its high combustion heat and clean burning. Hydrogen storage, especially a high capacity storage under moderate conditions, is the key for utilization of hydrogen energy. Thus, it is important to develop a material which is able to absorb/desorb hydrogen efficiently. Among different kinds of materials, metal/ intermetallic hydride storage materials are considered as promising candidates.1e4 Magnesium is widely investigated for onboard hydrogen storage, especially for vehicular applications, due to its high hydrogen storage capacity (up to 7.6 wt%), light weight and low cost.5,6 However, the sluggish hydrogen absorbing/desorbing kinetics and high hydride stability prevent practical applications of Mg-based hydrogen storage materials. To improve the kinetics and destabilize Mg hydride, various metals
* Foundation item: Project supported by the National Natural Science Foundation of China (51771164, 51571173), China Postdoctoral Science Foundation (2016M601281), Scientific Research Projects in Colleges and Universities in Hebei Province, China (ZD2014004, QN2016002). * Corresponding author. State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China. E-mail address: [email protected] (S. Han).
have been used to form alloys with Mg.7e10 Among these alloys, Mg2Ni has been considered a potential choice for hydrogen storage applications because of its lower hydriding/dehydriding temperature around 573 K11e13 and high hydrogen storage capacity of 3.6 wt%.14 For further improvement in hydrogen storage ability of Mg2Ni, partial or complete substitution of nickel by other suitable elements has been attempted. Substitution with Co15 and Cu16 has been carried out on Mg2Ni alloy, and it was found that the presence of Co and Cu weakened the interactions between magnesium and nickel, as well as interactions between nickel and hydrogen. Consequently, the alloy was destabilized and the materials showed a remarkable decrease of the onset dehydrogenation temperature. Rare earth elements have been doped into Mg2Ni alloy as hydrogen absorbing elements and found to be effective in improving hydrogen storage properties due to their role as reactive destabilizer.17,18 The addition of La into Mg2Ni alloy created a new ternary Mg-based compound, LaMg2Ni,19e21 which showed significant improvement in its dehydriding properties at low temperature. Previous reports have shown that LiBH4 and Mg2NiH4 undergo the following reaction (1), providing an easy dehydrogenation process.
5Mg2 NiH4 þ 4LiBH4 /2MgNi2:5 B2 þ 4LiH þ 8Mg þ 16H2
(1)
https://doi.org/10.1016/j.jre.2018.02.005 1002-0721/© 2018 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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The decomposition products were identified as LiH, Mg, MgNi2.5B2, and H2. The dehydrogenation reaction of the composite began at 550 K, which was a lower onset dehydrogenation temperature compared with that of Mg2NiH4. Moreover, the hydrogenation/dehydrogenation rate of the composite was faster than that of Mg2NiH4. This improvement makes the composite a potential candidate as a hydrogen storage material. In our previous work, LaH3 was demonstrated to be effective in catalyzing the dehydrogenation of Mg-based alloys by expanding/ contracting the lattices.22,23 Here, we have prepared a Mg-based composite material, Mg2NiH4-LiBH4 þ 20 wt% LaH3, by a solid state reaction in which LaH3 is formed in-situ in the system. Subsequently, the microstructures and reversible hydrogen storage properties of the Mg2NiH4-LiBH4/LaH3 composite were investigated. 2. Experimental Commercial LiBH4 (95%, Alfa Aesar) powder was used asreceived. The Mg2Ni and LaMg2Ni alloys were prepared by induction melting of the components, metallic La, Mg and Ni (99.9% purity) under protection of highly pure argon atmosphere (99.9% purity). The as-cast Mg2Ni and LaMg2Ni alloys were mechanically crushed into 100 mesh powders. A mixture of Mg2Ni (2.417 g) and LaMg2Ni (1.000 g) alloys was then hydrogenated under 3 MPa hydrogen pressure at 673 K for 8 h to ensure complete transformation into Mg2NiH4 and LaH3 (in-situ reaction). The mixture of LiBH4 (0.27 g)/Mg2NiH4-LaH3 (2.13 g) hydride system was prepared by ball-milling the as-received LiBH4 powder and as-prepared Mg2NiH4 powder in a molar ratio of 4:5, and the weight percentage of LaH3 was 20 wt% of the total material weight. The ball-milling process was performed in a stainless steel vial with a ball-to-powder weight ratio of 20:1 using a QM-ISP2 planetary mill at 500 r/min for 2 h. The composite was then heated to 573 K for 2 h under a dynamic vacuum environment to generate Mg. To avoid air-exposure, all handlings were carried out in an Ar-filled glove box equipped with a purification system, in which the typical O2/H2O levels were below 1 ppm. The curves of hydrogen absorption/desorption quantity of the samples were measured on a pressure-composition-temperature characteristic measurement equipment. The temperature programmed desorption (TPD) properties were determined by heating the sample from ambient temperature to 500 C at a heating rate of 5 C/min under a hydrogen back pressure below 0.0001 MPa. Before the TPD measurement, the composite was placed under vacuum for 10 min to remove hydrogen atoms physically adsorbed onto the composite surface. For the hydrogen absorption/desorption tests, the composite was firstly dehydrogenated at 623 K for 2 h to get completely dehydrogenated species. The phase composition of the samples was characterized by X-ray diffraction (XRD, D/ MAX-2500/PC, Cu Ka radiation). The hydrogen thermodynamic performance was studied by a simultaneous thermal analyzer (DTG-60A) under an Ar flow rate of 100 mL/min from the room temperature to 773 K with the heating rates of 5, 10, 15 and 20 K/ min, respectively. 3. Results and discussion 3.1. Hydrogenation characteristics and morphology of LaMg2Ni and Mg2Ni alloys Fig. 1 presents the XRD patterns of the fully hydrogenated LaMg2Ni-Mg2Ni mixture. In the patterns, only diffraction peaks corresponding to Mg2NiH4 phase and LaH3 phase are observed. It
Fig. 1. XRD patterns of the LaMg2Ni and Mg2Ni alloys hydride.
indicates that the following reactions (2) and (3) occur during the hydrogenation process.
Mg2 Ni þ H2 /Mg2 NiH4
(2)
LaMg2 Ni þ H2 /Mg2 NiH4 þ LaH3
(3)
In the hydrogenated samples, no LaMg2NiH7 phase is detected. It has been suggested by Renaudin et al.24 that reaction of LaMg2Ni in hydrogen atmosphere is generally affected by temperature, and the compound LaMg2Ni will decompose into LaH3 and an amorphous phase when the temperature is higher than 573 K. The phase transformation here is similar to that reported by Ouyang et al.25 Through the above solid-state reaction of LaMg2Ni in hydrogen atmosphere, a composite consisting of Mg2NiH4 and LaH3 can be obtained. However, LaH3 is a stable hydride and it is difficult to release hydrogen from this hydride. Therefore, too much LaH3 hydride will lead to a low reversible capacity for the system. Thus, the ratio of Mg2Ni to LaMg2Ni was adjusted to obtain a composite with an appropriate amount of LaH3, and LiBH4 was added to achieve a high capacity and reversible hydrogen absorbing/desorbing system by accomplishing reaction (1) according to a previous report.26 Fig. 2 presents the decomposition behaviors of Mg2NiH4-LiBH4 composite and Mg2NiH4-LaMg2Ni-LiBH4 composite. The amount of hydrogen evolution as a function of temperature was examined using a PCT apparatus. According to Fig. 2, the onset decomposition temperature of the Mg2NiH4-LaMg2Ni-LiBH4 composite decreases to 500 K, which is 50 K lower than that of the Mg2NiH4-LiBH4 composite. The results indicate that the in-situ formed LaH3 facilitates the hydrogen desorption process of Mg2NiH4-;LiBH4 system at a relatively low temperature (see Fig. 3). To reveal the phase structure of the LaH3-containing composite and the mechanism of effect of LaH3 in the hydrogenation/dehydrogenation process, different hydrogenation state species were collected, including as-milled powders, hydrogenated powders and dehydrogenated powders, and characterized by XRD analysis. XRD results show that the as-milled powders consist of Mg2NiH4 phase and LaH3 phase, with no detection of any new phases, indicating that no chemical reactions occurred upon ball milling. Moreover, the diffraction peaks of LiBH4 are not observed, which can be ascribed to its fine grain and amorphous nature after the ball milling treatment.27 After ball milling, the species was subjected to dehydrogenation at 623 K, and the XRD patterns for the
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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This result is similar to that in our previous report,28 and the floccule-shaped species are Mg particles. From the comparison, it is concluded that addition of lanthanum hydride into the composite promotes formation of flocculent Mg. TEM characterization was performed and is shown in Fig. 5. It is seen that the dehydrogenated Mg2NiH4-LiBH4 composite has few flocculent Mg-covered lamellar structures. However, the dehydrogenated Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite consists mainly of flocculent Mg interlinked together to form a spongy-shaped bulk structure. It implies that morphology of the product of reaction (1) is modified by lanthanum hydride. The lanthanum hydride provides proper sites for flocculent Mg to grow around, and then sponge-structured Mg with abundant holes forms in the LaH3containing Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite. 3.2. Hydrogen absorption/desorption kinetic performances
Fig. 2. TPD patterns of the Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites.
dehydrogenated sample indicate appearance of Mg and MgNi2.5B2 phases, which is consistent with the chemical reaction in Eq. (1).21 In the subsequent hydrogenation/dehydrogenation cycles, Mg from the above chemical reaction converts between Mg and MgH2. However, MgNi2.5B2 and LaH3 remain unchanged and promote hydrogenation/dehydrogenation of the composite. As magnesium is the active hydrogen absorbing/desorbing material in the composite, and the reversibility of the transformation between Mg and MgH2 is excellent, the composite then undergoes a stable hydrogenation/dehydrogenation cycle. Morphology of the species with and without lanthanum hydride at different hydrogenation stages was observed using SEM, and the corresponding results are shown in Fig. 4. The composite without lanthanum hydride appears as small particles which are agglomerated together obviously. After hydrogenation/dehydrogenation cycles, the agglomeration is more noticeable, and a lamellar-shape structure is observed. The as-milled Mg2NiH4LiBH4 þ 20 wt% LaH3 composite shows smaller particles, and after dehydrogenation, fine particles with less agglomeration are observed. Comparing Fig. 4(d) with Fig. 4(c), it is found that the main textures of the two composites are the same, however, some floccule-shaped species cover the lamellar structure in Fig. 4(d).
Hydrogenation/dehydrogenation properties of the composites with and without lanthanum hydride were tested. The isothermal hydrogen absorption/desorption kinetics profiles are plotted in Fig. 6, which illustrates the hydrogen absorption curves of Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites at 423 K under 3.0 MPa. Before the hydrogenation/dehydrogenation test, it was ensured that reactions (1), (2) and (3) were performed completely. And the actual hydrogenation/dehydrogenation reactions are between Mg and MgH2. So variation in hydrogen content for the two composites below is calculated by Mg/MgH2. As shown in Fig. 6, Mg2NiH4-LiBH4 composite absorbs 1.43 wt% (weight ratio of the composite) of hydrogen within 750 s. However, the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite absorbs the same content of hydrogen within 100 s, which is 650 s faster compared with the composite without LaH3. Obviously, the hydrogen absorption rate is remarkably improved, indicating that LaH3 plays an effective role in improving hydriding kinetics of the LiBH4-Mg2NiH4 composite. The results of hydrogen desorption kinetics measured at 573 K are presented in Fig. 7. It is seen that the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite displays higher desorption rate. In the first 500 s, only 0.55 wt% of hydrogen is desorbed from the Mg2NiH4-LiBH4 composite. However, 1.24 wt% of hydrogen can be released from the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite in the same time period. The Mg2NiH4-LiBH4 composite releases 0.93 wt% hydrogen within 1600 s, whereas the Mg2NiH4-LiBH4 þ 20 wt% LaH3 releases 0.93 wt % hydrogen within only 200 s. These results indicate that hydrogen desorption has been remarkably promoted with addition of lanthanum hydride. 3.3. Dehydrogenation thermodynamic performances
Fig. 3. XRD patterns of the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite. (1) Ball-milled; (2) Hydrogenated; (3) Dehydrogenated.
Fig. 8 represents the temperature programmed desorption (TPD) test of the Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites. The starting temperature of rapid hydrogen desorption for the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is 530 K, which is 43 K lower than that of the Mg2NiH4-LiBH4 composite. The measurements on Mg2NiH4-LiBH4 and Mg2 NiH4-LiBH4 þ 20 wt% LaH3 composites were performed after completion of reactions (2) and (3), and the actual hydrogen absorbing material is the generated flocculent MgH2. The different particle sizes of MgH2 affect its hydrogen desorption properties, and it has been previously reported that smaller MgH2 particle sizes can noticeably destabilize its hydride. The generated MgH2 in the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite has a significantly smaller particle size, which facilitates the destabilization of its hydride and leads to a lower onset decomposition temperature.29
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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Fig. 4. SEM images of ball milled and dehydrogenated Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites. (a) Ball-milled Mg2NiH4-LiBH4; (b) Dehydrogenated Mg2NiH4-LiBH4; (c) Ball-milled Mg2NiH4-LiBH4 þ 20 wt% LaH3; (d) Dehydrogenated Mg2NiH4-LiBH4 þ 20 wt% LaH3.
Fig. 5. TEM images of dehydrogenated Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites. (a) Dehydrogenated Mg2NiH4-LiBH4; (b) Dehydrogenated Mg2NiH4LiBH4 þ 20 wt% LaH3.
Obviously, LaH3 plays a key role in catalyzing the dehydrogenation of Mg-based alloys by expanding/contracting the lattices,22,23 as well as in catalyzing the reaction between Mg2NiH4 and LiBH4. DTA tests were performed at different heating rates and the results are shown in Fig. 9. The decomposition temperature of the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is obviously decreased. Compared with the Mg2NiH4-LiBH4 composite, the dehydrogenation peak temperature for Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite at a heating rate of 10 K/min is lowered by 61.67 K. The activation energy (Ea) of the Mg2NiH4-LiBH4 and Mg2NiH4LiBH4 þ 20 wt% LaH3 composites are calculated by using Kissinger's method,29 which is expressed by the below equation.
h . i 2 d ln a Tm d ð1=Tm Þ
¼
Ea R
(4)
Where a is the heating rate, Tm is the maximum desorption temperature, and R is the gas constant. The activation energy values of different composites were evaluated from the DTA data obtained in the present work employing the formula given above. Ea values were calculated based on the plot of ln(a/T2m) versus 1000/Tm (Fig. 10) and are listed in Table 1. The activation energy for the dehydrogenation of Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is calculated to be 80.03 kJ/mol, which is 8.2 kJ/mol lower than that of the Mg2NiH4-LiBH4 composite and ~80 kJ/mol lower than that of
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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Fig. 6. Comparison of the absorption kinetics of Mg2NiH4-LiBH4 and Mg2NiH4LiBH4 þ 20 wt% LaH3 composites at 423 K.
Fig. 9. DTA patterns of Mg2NiH4-LiBH4 (a) and Mg2NiH4-LiBH4 þ 20 wt% LaH3 (b) composites after annealing at various heating rates. Fig. 7. Comparison of the desorption kinetics of Mg2NiH4-LiBH4 and Mg2NiH4LiBH4 þ 20 wt% LaH3 composites at 573 K.
Fig. 10. Kissinger plot of first-order dehydrogenation of different samples at various heating rates.
Fig. 8. TPD patterns of the Mg2NiH4-LiBH4 and Mg2NiH4-LiBH4 þ 20 wt% LaH3 composites after annealing.
the commercial MgH2 (~160 kJ/mol),10 suggesting that the LaH3doped composite can lower the energy barrier of Mg2NiH4-LiBH4 composite and pure MgH2 in the dehydrogenation process, resulting in enhanced desorption kinetics.
Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005
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Table 1 Tm and Ea values corresponding to two samples. Samples
Heating rate used in DTA (K/min)
Tm (K)
Ea (kJ/mol)
Mg2NiH4-LiBH4
5 10 15 20 5 10 15 20
566.51 577.61 596.51 604.21 529.42 548.27 561.48 567.22
88.23
Mg2NiH4-LiBH4 þ 20 wt% LaH3
80.03
4. Conclusions In this work, a Mg-based composite hydrogen storage system was prepared by solid reaction between LaMg2Ni, Mg2Ni and LiBH4 in hydrogen atmosphere (denoted as Mg2NiH4-LiBH4 þ 20 wt% LaH3). The reversible hydrogen storage properties and reaction mechanism of Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite were investigated. The SEM results indicate that the in-situ formed LaH3 is beneficial to the homogeneous generation of flocculent Mg on MgNi2.5B2 blocks. The Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite after annealing exhibits a significant decrease in onset decomposition temperature, the temperature being 530 K, which is 43 K lower than that of the Mg2NiH4-LiBH4. The activation energy for dehydrogenation of the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite is 80.03 kJ/mol, which is 8.2 kJ/mol lower than that of the Mg2NiH4LiBH4 composite and 80 kJ/mol lower than that of commercial MgH2. Furthermore, the Mg2NiH4-LiBH4 þ 20 wt% LaH3 composite absorbs 1.46 wt% hydrogen in just 100 s (at 423 K), 650 s faster than that of the composite without LaH3, while it releases 0.93 wt% hydrogen (at 573 K) in 200 s, which is 1400 s faster than that of the undoped composite. References 1. Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy. 2007;32(9):1121. 2. Li X, Zhang YH, Yang T, Xu JY, Zhao DL. Hydriding/dehydriding properties of NdMgNi alloy with catalyst CeO2. J Rare Earths. 2016;34(4):407. 3. Sahli I, Ghodbane O, Abdellaoui M. Electrochemical hydrogenation of CeZr2Cr4Ni5ebased alloys. Mater Res Bull. 2017;85:10. 4. Ley MB, Jepsen LH, Lee YS, Cho YW, von Colbe JMB, Dornheim M, et al. Complex hydrides for hydrogen storageenew perspectives. Mater Today. 2014;17(3): 122. 5. Wang Y, Wang YJ. Recent advances in additive-enhanced magnesium hydride for hydrogen storage. Prog Nat Sci-Mater. 2017;27(1):41. 6. Varin RA, Jang M, Czujko T, Wronski ZS. The effect of ball milling under hydrogen and argon on the desorption properties of MgH2 covered with a layer of Mg(OH)2. J Alloys Compd. 2010;493(1e2):L29. 7. Yuan ZM, Yang T, Bu WG, Shang HW, Qi Y, Zhang YH. Structure, hydrogen storage kinetics and thermodynamics of Mg-base Sm5Mg41 alloy. Int J Hydrogen Energy. 2016;41(14):5994.
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Please cite this article in press as: Wang J, et al., Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics, Journal of Rare Earths (2018), https://doi.org/10.1016/j.jre.2018.02.005