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Catalytic effect of in situ formed nano-Mg2Ni and Mg2Cu on the hydrogen storage properties of Mg-Y hydride composites
Journal of Alloys and Compounds 782 (2019) 242e250
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Catalytic effect of in situ formed nano-Mg2Ni and Mg2Cu on the hydrogen storage properties of Mg-Y hydride composites Cheng Xu a, c, Huai-Jun Lin b, f, *, Yunlei Wang a, c, Peng Zhang b, Yuying Meng b, Yao Zhang c, d, f, Yana Liu a, c, Jiguang Zhang a, c, Liquan Li a, c, Qian Shi e, Wei Li b, Yunfeng Zhu a, c, ** a
College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, China Institute of Advanced Wear & Corrosion Resistance and Functional Materials, Jinan University, Guangzhou, 510632, China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 210009, China d School of Materials Science and Engineering, Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing, 211189, China e Guangdong Institute of New Materials, Guangzhou, 510651, China f Guangdong Provincial Key Laboratory of Advance Energy Storage Materials, South China University of Technology, Guangzhou, 510640, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2018 Received in revised form 14 December 2018 Accepted 17 December 2018 Available online 18 December 2018
Two hydrides nanocomposites, consisting of MgH2-Mg2NiH4-YH2 and MgH2-MgCu2-YH2 phases, were prepared by hydrogenation upon the melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 amorphous alloys, respectively. The catalytic effect of nano-Mg2Ni/Mg2NiH4 and Mg2Cu/MgCu2 particles on hydrogen storage properties of Mg-Y hydride composites have been studied. It is shown that the activated meltspun Mg80Y5Ni15 alloy exhibits higher hydrogen absorption capacity and faster hydriding/dehydriding kinetics than those of the activated melt-spun Mg80Y5Cu15 alloy. The activated melt-spun Mg80Y5Ni15 alloy absorbs about 4.2 wt% hydrogen in 10 min at 200 C, and almost fully desorbs hydrogen within 5 min at 280 C. In contrast, the activated melt-spun Mg80Y5Cu15 alloy absorbs about 4 wt % of hydrogen in 2 h at 280 C, and desorbs 3.7 wt% of hydrogen in 80 min at 300 C. The apparent activation energy of dehydrogenation for the activated melt-spun Mg80Y5Ni15 alloy was calculated to be 83.9 kJ/mol, which is significantly lower than that of 132.3 kJ/mol for the activated melt-spun Mg80Y5Cu15 alloy. The excellent hydrogen storage properties of the activated melt-spun Mg80Y5Ni15 alloy can be attributed to the multiple cracks and refined particle size generated during hydrogenation, and the uniform distributed catalytic Mg2Ni/Mg2NiH4 nanoparticles in the Mg/MgH2 matrix. In contrast, a large amount of Mg2Cu/ MgCu2 phase significantly aggregations, and the separation between Mg2Cu/MgCu2 and Mg/MgH2 matrix in the activated melt-spun Mg80Y5Cu15 alloy, which greatly decrease the catalytic effect of Mg2Cu for the activated melt-spun Mg80Y5Cu15 alloy. © 2018 Elsevier B.V. All rights reserved.
Keywords: Mg-based alloys Hydrogen storage Catalytic effect Kinetics Melt spinning
1. Introduction Hydrogen is widely considered as a promising energy barrier for the sustainable development of our society, and hydrogen storage is a critical issue on the way of realizing the hydrogen economy [1]. Mg is one of the promising hydrogen storage materials due to its high hydrogen capacity (7.6 wt%), abundance in magnesium
* Corresponding author. Institute of Advanced Wear & Corrosion Resistance and Functional Materials, Jinan University, Guangzhou, 510632, China. ** Corresponding author. College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, China. E-mail addresses: [email protected] (H.-J. Lin), [email protected] (Y. Zhu). https://doi.org/10.1016/j.jallcom.2018.12.223 0925-8388/© 2018 Elsevier B.V. All rights reserved.
minerals and harmlessness to the environment [2e4]. Nevertheless, the practical utilization of Mg-based alloys is inhibited by the slow reaction rate for the hydrogen absorption and desorption, and high hydrogen desorption temperature [4,5]. Until now, vast efforts have been made to overcome the drawbacks, such as alloying [6e9], nanostructuring [10e12], fabricating composites [13e16] and doping catalysts [17e20]. Great achievements have been obtained but further investigation is still needed. Previous studies proved that the hydrogen desorption rate of MgH2 can be significantly enhanced by introducing catalytically active elements such as transition metals (TM), rare earth elements (RE) and metal oxides, etc [16,21e24]. Doping with rare earth elements (RE) is reported to have a beneficial effect on the
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dehydrogenation of MgH2 owing to the catalytic effect of RE hydrides or RE oxides. Li et al. [25] combined both first-principles calculations and experimental observation to investigate the catalytic effect of YH2 on Mg/MgH2, which can be ascribed to the high interfacial energy of YH2/Mg, the low diffusion energy barrier for H at the YH2/Mg interface, and the high affinity between YH2 and H during the absorption process, and can act as the hydrogen pump during desorption process. Lin et al. [26] induced symbiotic CeH2.73/ CeO2 catalyst in the MgH2 matrix, which exhibits high catalytic activity for the MgH2 decomposition. Hydrogen can be easily released in the interface region of symbiotic CeH2.73/CeO2 nanoparticles on account of the efficient hydrogen pump. It was also shown that transition metals could effectively destabilize the MgH2 phase. The introduction of Ni and Cu shows catalytic effect on the hydrogen sorption properties of Mg. Ni forms Mg2Ni with Mg, which dissociates hydrogen molecule easily, leading to the high absorption kinetics. During the desorption process, Mg2NiH4 could promote hydrogen release of MgH2 due to the volume contraction after desorption, resulting in a contraction strain on the adjacent MgH2, which triggers dehydrogenation of MgH2 [27,28]. Unlike Ni, Cu forms two intermetallic phases with Mg: Mg2Cu and MgCu2. Mg2Cu reacts with hydrogen via a disproportionation reaction: 2Mg2Cu þ 3H2 % 3MgH2 þ MgCu2
(1)
MgCu2 is unable to react with hydrogen, leading to a theoretical hydrogen storage capacity of only 2.6 wt% [29]. The catalytic effect of Mg2Cu has been reported in the existing studies [30]. Jiang et al. [31] investigated a Y-Mg-Cu-H nanocomposite obtained by the hydrogen-induced decomposition of YMg4Cu. The nanocomposites exhibited excellent hydrogen absorption kinetics owing to the in situ formed well-dispersed catalysts of Mg2Cu and YH2. However, the different catalytic effects of Mg2Ni and Mg2Cu, and the corresponding products of Mg2NiH4 and MgCu2 after hydrogen absorption remain unclear and need to be further identified. In this work, two amorphous alloys with chemical compositions of Mg80Y5Ni15 and Mg80Y5Cu15 were fabricated by melt spinning. The melt-spun alloys were then treated by a hydrogenation activation treatment to induce MgH2-Mg2NiH4-YH2 and MgH2-MgCu2YH2 nanocomposites. The difference between catalytic effects of Mg2Ni/Mg2NiH4 and Mg2Cu/MgCu2 on the hydrogen storage properties of Mg/MgH2 were comparatively studied.
initial pressure of 3 MPa H2 for the absorption and 0.005 MPa H2 for the desorption at 200 C, 250 C, 280 C and 300 C, respectively, and the nonisothermal dehydriding properties were performed by differential scanning calorimetry (DSC, TA Q2000) and heated to 500 C under 50 ml/min Ar flow with a heating rate of 5 C/min, 8 C/min, 10 C/min and 13 C/min. For the isothermal hydrogen absorption and desorption tests, about 0.35 g of the alloys were loaded into a stainless steel sample chamber in a glove-box filled with Ar atmosphere. Morphologies of the alloys after hydrogen absorption and desorption process were characterized by field emission scanning electron microscopy (FESEM, Ultra55) equipped with a backscattered electron detector operated at 30 kV. TEM was performed on high resolution transmission electron microscope (HRTEM, JEM-2010 UHR) operated at 200 kV. The samples were dispersed onto a copper grid and examined at highly vacuum. 3. Results and discussion 3.1. Phase structure and crystallization characteristic Fig. 1 shows the XRD patterns of melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys. Melt-spun Mg80Y5Cu15 alloy shows a board hump, indicating its fully amorphous structure, while XRD pattern of the melt-spun Mg80Y5Ni15 alloy consists of a main amorphous peak and small amount of Ni2Y3, Ni5Y crystalline phases. Crystallization processes of the melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys are conducted by DSC measurement as shown in Fig. 2, which displays two or three exothermic peaks. Evidently, Mg80Y5Ni15 exhibits slightly higher thermal stability than Mg80Y5Cu15. The structure characterization of Mg80Y5Ni15 using XRD indicated that the first exothermic peak at 192.04 C should be assigned to the crystallization of Mg2Ni phase as shown in Fig. 3. The residual amorphous phase is still stable after the second exothermic peak at 285.82 C and the crystallization step belongs to the formation of Mg and Ni2Y3 phases. The final crystallization products are Mg, Ni3Y, Ni2Y3 and Mg2Ni phases. The crystallization of Mg80Y5Cu15 consists of two steps, and the first crystallization reaction at 193.48 C, leading to the formation of Mg, Mg2Cu, Cu2Y together with several unidentified phases. After the second crystallization peaking at 295.91 C, the amorphous Mg80Y5Cu15 alloy transforms into alloys consisting of Mg, Mg2Cu, Mg24Y5 and Cu2Y
2. Experimental YNi3 and YCu3 intermediate alloys were firstly fabricated by arcmelting. Mg80Y5Ni15 and Mg80Y5Cu15 ingots were then prepared by conventional induction-melting of a mixture of YNi3 and YCu3 intermediate alloys and pure Mg metal in a quartz crucible under argon atmosphere. Afterwards, the ingots were re-melted and then injected through a nozzle (F ¼ 1 mm) onto the surface of a rotating copper wheel at a linear velocity of 27.2 m/s. The obtained ribbons was ~3 mm in width and ~50 mm in thickness. The ribbons were crushed into powders via ball milling with a speed of 220 rpm for 1 h, and then screened through 100 mesh sieves. The ball-topowder weight ratio was 10:1. The phase structure analysis was characterized by an ARL X’TRA diffractometer with Cu-Ka radiation. All XRD patterns were obtained with 0.02 step and scan steep of 10 /min. Crystallization behaviors were investigated using Differential scanning calorimetry (DSC, TA Q2000). The melt-spun alloys were heated from room temperature to 500 C with a heating rate of 10 C/min under 1 atm Ar atmosphere. The hydrogen storage properties of alloys were measured on a Sieverts type pressurecompositiontemperature (PCT) volumetric apparatus (GRC, Advanced Materials Co.) under
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Fig. 1. XRD patterns of melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys.
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Fig. 2. DSC curves of melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys under Ar atmosphere at a heating rate of 10 C/min.
phases. 3.2. Hydrogenation activation properties Fig. 4 shows the hydrogenation activation cycles of the meltspun Mg80Y5Ni15 and Mg80Y5Cu15 alloys. The hydrogen absorption and desorption processes were carried out at 350 C, under 3 MPa H2 for 5 h and under 0.005 MPa H2 for 0.5 h, respectively. In the first hydrogenation process, both Mg80Y5Ni15 and Mg80Y5Cu15 alloys show poor hydrogen absorption kinetics, and significantly improved hydrogenation kinetics occurs at the second cycle. In the case of Mg80Y5Ni15, the activation process lasts five cycles to reach a maximum absorption capacity of about 4.7 wt%. The activation of Mg80Y5Cu15 continues for three cycles and shows a reversible capacity of about 4.1 wt%. Mg80Y5Cu15 requires fewer activation cycles than Mg80Y5Ni15, which might be attributed to the higher thermal stability of the Mg80Y5Ni15 alloy compared with that of the Mg80Y5Cu15 alloy (Fig. 2). It is found that the maximum absorption capacities of the Mg80Y5Ni15 and Mg80Y5Cu15 alloys are slightly lower than the corresponding theoretical capacities. The reason can be attributed to the high stability of the yttrium hydride under the experimental conditions. 3.3. Hydrogenation and dehydrogenation properties After the activation cycles, the hydrogen absorption and desorption properties were tested at different temperatures from 200 C to 300 C. Fig. 5 shows the hydrogen absorption kinetics of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys at different temperatures under 3 MPa H2. No evident difference in hydrogen absorption kinetics of the Mg80Y5Ni15 alloy at the experimental temperatures was observed and it takes about 10 min to achieve the absorption capacity of about 4.2 wt% and finally absorbs about 4.5 wt% hydrogen in 2 h, while the Mg80Y5Cu15 alloy shows much poorer absorption kinetics. The hydrogen content increases rapidly in the first 5 min at 300 C and then gradually increases to about 4 wt% within 2 h. The hydrogen absorption capacity reaches only about 3 wt% at 200 C within 2 h.
Fig. 3. XRD patterns of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 after crystallization at different temperatures.
Fig. 6 shows the hydrogen desorption kinetics of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys under 0.005 MPa H2 at 300, 280, 250 and 200 C, respectively. The hydrogen desorption kinetics of the Mg80Y5Ni15 alloy are quite fast at 300 and 280 C, reaching almost fully dehydrogenated within 5 min, while the dehydrogenation rate slows down at 250 C. Only about 1 wt% hydrogen is released when the temperature drops to 200 C. The Mg80Y5Cu15 alloy shows much poorer hydrogen desorption properties, even at 300 C it takes about 80 min to reach a saturated desorption capacity of about 3.7 wt%. The dehydrogenation capacity decreases to 2.3 wt% within 2 h at 280 C and no significant amount of hydrogen is desorbed at 200 C. After hydrogen absorption, the initial amorphous/ nanocrystalline-amorphous structure of melt-spun Mg80Y5Cu15 and Mg80Y5Ni15 alloys transforms into crystalline hydrides as shown in Fig. 7. Mg80Y5Ni15 transforms into hydrides composites consisting of MgH2, Mg2NiH4, YH2 and YH3 after the hydrogenation for 2 h, while Mg80Y5Cu15 transforms into MgH2, MgCu2 and YH2
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Fig. 4. The activation cycles of melt-spun Mg80Y5Ni15 and (b) Mg80Y5Cu15 (at 350 C; 3/ 0.005 MPa H2 for 5/0.5 h).
and YH3. Weak peaks of YH3 phase indicate the small amount of YH2/YH3 transformation and could be attributed to the short hydrogenation time, resulting in the unsaturated absorption, which is consistent with the results reported by Yang et al. [9]. MgCu2 is transformed from Mg2Cu and unable to react with hydrogen to form hydride, leading to the lower absorption capacity of Mg80Y5Cu15. The poorer hydrogen absorption kinetics of Mg80Y5Cu15 can be attributed to the different catalytic effects of Mg2Cu and Mg2Ni. Both Mg2Cu and Mg2Ni are considered to accelerate the dissociation of hydrogen molecular and promote the hydrogenation of Mg phase, playing a significant catalytic role in the formation of MgH2 phase [32,33]. Cho et al. [11] reported the excellent hydrogen storage properties of 3D Ni doped Mg crystals encapsulated by rGO layers. The Ni-doped rGO-Mg can absorb 6.5 wt% hydrogen at 200 C, of which about 90% was completed within 2.5 min. The fantastic hydrogen absorption property can be owing to the synergistic effect of nanosizing, rGO encapsulation and Ni doping. Mg-Ni nanoalloy (including Mg2Ni) crystallites formed in the hydrogen absorption/desorption tests play a crucial role in the catalytic phenomenon. Zhang et al. [34] investigated the substitution of Cu for Ni on the hydrogen sorption behavior of Mg20Ni10-xCux and found that Mg2Cu can act as an active site for hydriding, as well as it increases the lattice parameter of the alloy, leading to faster H-diffusion. Even though the catalytic effect of Mg2Ni and Mg2Cu on the hydrogenation of Mg phase as reported in the literatures, it seems that Mg2Ni shows higher catalytic activity than Mg2Cu during the hydrogen absorption process. The dehydrogenation products after the dehydrogenation for 2 h using XRD identified are present in Fig. 7. The dehydrogenated Mg80Y5Ni15 exhibits three phases: Mg, Mg2Ni and YH2 while Mg80Y5Cu15 consists of Mg, Mg2Cu and YH2. Both Mg2NiH4 and MgCu2 are reported to be beneficial for the dehydrogenation of MgH2. As reported by Xie et al. [35], the activation energy of desorption for Mg-5.0 wt% Ni-2.3 wt% V nanocomposite was calculated to be 85.1 kJ/mol. The enhanced hydrogen storage properties of the Mg-Ni-V nanocomposite are attributed to the synergistic catalytic effects of Mg2Ni/Mg2NiH4 and V/VH2 formed after activization process. Milanese et al. [36] investigated the MgNi-Cu mixtures to analyze the influence of Ni and Cu on the hydrogen storage properties. The activation energy decreases of 1.7 times for Mg: Ni: Cu ¼ 80: 10: 10 (wt.%) and of 2.4 times for Mg: Ni:
Fig. 5. Hydrogen absorption kinetics of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 after activation at different temperatures under 3 MPa H2.
Cu ¼ 40: 30: 30 (wt.%). The addition of Cu and Ni is beneficial in decreasing the energetic barrier for the MgH2 desorption and the dissociation of MgCu2 improves the dehydrogenation kinetics. However, there have been literatures reporting that the addition of Cu inhibits the de-/hydrogenation properties. Kalinichenka et al. [37,38] comparatively studied the hydrogen storage properties of the melt-spun Mg-Y-Ni and Mg-Y-Ni-Cu alloys, and found that the Mg-Y-Ni alloys exhibit higher hydrogen storage capacity and faster dehydrogenation rate than the Mg-Cu-Ni-Y alloys, which is surmised to be due to the in direct contact of MgCu2 with Mg2NiH4 forming co-domains embedded in MgH2 matrix, leading to the small specific interface area between matrix and catalytic phases. Here, the poor hydrogen storage properties of Mg80Y5Cu15 may also be due to the difference of the catalytic phase distribution in MgH2 matrix compared with Mg80Y5Ni15, which will be discussed in detail in later section. DSC analysis was performed to further investigate the dehydrogenation property of the Mg80Y5Ni15 and Mg80Y5Cu15 alloys as shown in Fig. 8a and b, and Fig. 8c and d are the dehydrogenation activation energy calculated using Kissinger's equation.
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Fig. 6. Hydrogen desorption kinetics of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 after activation at different temperatures under 0.005 MPa H2.
"
!#, 1 ¼ EA =R d ln 2 d T TP P 4
(2)
where Tp is the endothermic peak temperature, 4 is the heating rate, R is the gas constant and EA is the activation energy. The hydrogenated Mg80Y5Ni15 released hydrogen at the peak temperature of 280.50 C in the case of 5 C/min and showed about 70 C lower than the hydrogenated Mg80Y5Cu15 at each heating rate. The EA for the Mg80Y5Ni15 alloy was calculated to be 83.9 kJ/mol from the slop of the Kissinger's plot; this value is significantly lower than 132.3 kJ/ mol for the Mg80Y5Cu15 alloy, suggesting that the catalytic effect of Mg2NiH4 phase on the MgH2 decomposition shows lower kinetic barrier than that of MgCu2 phase. The prominent catalytic effect of Mg2NiH4 on the MgH2 desorption has been reported elsewhere. Chen et al. [18] studied the catalytic effect Ni/C nanoparticles on the hydrogen storage properties of MgH2. The in situ formed Mg2Ni and Mg2NiH4 are regarded as the catalytically active species and the uniform distributions of Mg2Ni and Mg2NiH4 are responsible for a favorable and lasting catalytic efficiency. Therefore, the huge
Fig. 7. XRD patterns of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 alloys after hydrogenation and dehydrogenation.
difference of hydrogen sorption properties between the melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys may be related to the microstructure difference, which need to be further discussed. 3.4. Microstructures of the hydrogenated and dehydrogenated alloys In order to characterize the microstructures of activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys after hydrogenation, BSE-FESEM was employed to investigate the catalytic phase distribution in the matrix. Fig. 9a exhibits the multiple cracks and plenty of small particles, indicating the severe pulverization of the hydrogenated Mg80Y5Ni15 due to the repeated volume expansion and contraction of Mg/MgH2 and Mg2Ni/Mg2NiH4 during the hydrogen absorption and desorption tests. No significant pulverization was observed in the hydrogenated Mg80Y5Cu15 alloy as shown in Fig. 9c and it remains coarse particles after hydrogenation. After the hydrogen absorption/desorption tests, multiple cracks and refined particle size are regarded to be beneficial for the hydrogen sorption kinetics [39]. Besides, the uniform distribution of Mg2NiH4 pieces was
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Fig. 8. DSC curves of hydrogenated (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 under Ar atmosphere at different heating rates (5, 8, 10, 13 C/min); Kissinger's plots of hydrogenated (c) Mg80Y5Ni15 and (d) Mg80Y5Cu15 derived from the DSC profiles.
Fig. 9. BSE-FESEM images of hydrogenated (a, b) Mg80Y5Ni15 and (c, d) Mg80Y5Cu15 after activation.
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observed to be dispersed in the MgH2 matrix, while the MgCu2 is aggregated into dendritic structure, leading to the small specific interface area between MgCu2 and MgH2. The sufficient interface between catalytic and matrix shows great importance on the hydrogen storage properties and the huge particle size is definitely harmful for the hydrogen sorption properties [40]. Further structural observation of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys after hydrogen absorption/desorption was conducted by TEM analysis as shown in Figs. 10 and 11. Fig. 10a and b shows small pieces of Mg2NiH4 and YH2 embedded in the MgH2 matrix, while the MgCu2 phase is aggregated together to form coarse grains (Fig. 10c and d), which is consistent with the FESEM results as shown in Fig. 9c and d. The insufficient contact of MgCu2 and MgH2 may be account for the poor hydrogen storage properties of the Mg80Y5Cu15 alloy. Wu et al. [41] comparatively studied two kinds of LaNiO3 with different morphologies (porous and powder) on the hydrogen storage properties of MgH2, and found the porous LaNiO3 showed much better catalytic effect on hydrogen storage properties of MgH2 than that of powder LaNiO3, which can be attributed to the uniform distribution of powder LaNiO3 with high BET surface in the matrix. Fig. 11 shows the TEM images of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys after hydrogen desorption. Fig. 11a shows that Mg2Ni and YH2 is relatively well-distributed in the Mg matrix, but a certain amount of Mg2Ni aggregations can be seen in Fig. 11b. However, plenty of Mg2Cu aggregations can be observed in Fig. 11d and in some particles as shown in Fig. 11e, almost all the Mg2Cu phase is aggregated on the boundary of the particles and no evident Mg2Cu is found to distributed inside the Mg matrix. Such heterogeneous distribution of Mg2Cu phase and the separation between Mg2Cu and Mg would decrease the catalytic effect on hydrogen storage properties, leading to the lower absorption kinetics. The above results clearly indicate that the huge microstructure difference between the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys could significantly affect the hydrogenation/dehydrogenation kinetics. It is deduced that the superior hydrogen storage properties of the activated Mg80Y5Ni15 alloy might be owing to the following reasons.
Firstly, Mg2Ni/Mg2NiH4 and Mg2Cu/MgCu2 exhibit intrinsic catalytic effect differences on the hydrogen storage properties of Mg-Y hydride composites. Mg2Ni is reported to promote the dissociation of the hydrogen molecule, resulting in a faster nucleation of magnesium hydride [42]. During the dehydriding process, Mg2NiH4 and MgH2 exhibit a synergetic effect of hydrogen desorption. Mg2NiH4 releases hydrogen prior to MgH2 to form Mg2Ni, leading to a severe volume contraction and lattice strain, which provide the driving force to trigger the dehydrogenation of MgH2 [27]. In the case of the catalytic effect of Mg2Cu/MgCu2 as previous studies proved, the enhanced hydrogenation kinetics can be due to the easily dissociation of hydrogen molecule, improved hydrogen atoms diffusion in bulk Mg2Cu or along the Mg/Mg2Cu interface, and the addition of stable nucleation sites along the Mg/ Mg2Cu interface [43]. Regarding the catalytic effect in desorption process, on the one hand, MgCu2 can react with MgH2 according to reaction (1), promoting the cleavage of the Mg-H bond. On the other hand, the existence of MgCu2 provides an oxide-reduced surface, enhancing hydrogen recombination. However, slow hydrogen diffusion from MgH2 through MgCu2 to the external surface may be account for the poor dehydriding kinetics [44]. Secondly, the activated Mg80Y5Ni15 is subject to the severe pulverization owing the repeated volume expansion and contraction of Mg2Ni/Mg2NiH4, while Mg2Cu/MgCu2 transformation shows no apparent volume change. Multiple cracks and refined particle size are induced, providing more active sites and reducing the hydrogen diffusion distance. The coarse dendritic MgCu2, no significant pulverization and huge particle size result in the slow sorption properties. Finally, Mg2Ni/Mg2NiH4 remains small grain size welldistributed in matrix, providing sufficient contract between Mg2Ni/Mg2NiH4 and Mg/MgH2, which provides the prominent catalytic effect. In contrast, Mg2Cu/MgCu2 is more likely to aggregated together on the boundaries of the matrix. Therefore, the extremely small specific interface area between Mg2Cu/MgCu2 and the primary matrix could decrease the catalytic effect of Mg2Cu/ MgCu2.
Fig. 10. TEM images of the activated (a, b) Mg80Y5Ni15 and (c, d) Mg80Y5Cu15 after hydrogen absorption.
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Fig. 11. TEM images of the activated (aec) Mg80Y5Ni15 and (def) Mg80Y5Cu15 after hydrogen desorption.
4. Conclusion
Acknowledgements
In summary, melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys were fabricated to obtain an amorphous or nanocrystalline-amorphous precursor. Afterwards, two hydrides nanocomposites consisting of MgH2-Mg2NiH4-YH2 and MgH2-MgCu2-YH2 phases were prepared by hydrogenation. The melt-spun Mg80Y5Ni15 alloy exhibits higher thermal stability than Mg80Y5Cu15, leading to more activation cycles for the Mg80Y5Ni15 alloy than the Mg80Y5Cu15 alloy. The Mg80Y5Ni15 alloy shows superior hydrogen absorption and desorption kinetics, which absorbs about 4.2 wt% hydrogen in 10 min and finally absorbs about 4.5 wt% hydrogen in 2 h, and can desorb almost all the hydrogen within 5 min at 300 and 280 C. In contrast, the Mg80Y5Cu15 alloy only absorbs 3 wt % of hydrogen in 20 min, and needs about 80 min to reach a saturated desorption capacity of about 3.7 wt% at 300 C. Furthermore, the activation energy for the Mg80Y5Ni15 alloy is calculated to be 83.9 kJ/mol, which is much lower than that of 132.3 kJ/mol for the Mg80Y5Cu15 alloy. It is suggested that the superior hydrogen storage properties of Mg80Y5Ni15 should be attributed to the multiple cracks and refined particle size emerged during hydrogen sorption, and the uniform distributed Mg2Ni/Mg2NiH4 in Mg/MgH2 matrix, compared with the vast Mg2Cu/MgCu2 aggregations and the separation between Mg2Cu/MgCu2 and Mg/MgH2 matrix, which decreases the catalytic effect of Mg2Cu/MgCu2 on hydrogen storage properties of the Mg80Y5Cu15 alloy.
This work was supported by the National Natural Science Foundation of China (Nos. 51601090, 51471087, 51571112), Science and Technology Program of Guangzhou (No. 201607010091), the Open Fund of the Guangdong Provincial Key Laboratory of Advance Energy Storage Materials, National Key R&D Program of China (2017YFB0305100) and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. References [1] L. Schlapbach, A. Züttel, Hydrogen-storage materials for mobile applications, Nature 414 (2001) 353. [2] T. Liu, C. Wang, Y. Wu, Mg-based nanocomposites with improved hydrogen storage performances, Int. J. Hydrogen Energy 39 (2014) 14262e14274. [3] R. Mohtadi, S.-i. Orimo, The renaissance of hydrides as energy materials, Nat Rev. Mater. 2 (2016) 16091. [4] H. Shao, L. He, H. Lin, H.-W. Li, Progress and trends in magnesium-based materials for energy-storage research: a review, Energy Technol. 6 (2018) 445e458. [5] X. Yu, Z. Tang, D. Sun, L. Ouyang, M. Zhu, Recent advances and remaining challenges of nanostructured materials for hydrogen storage applications, Prog. Mater. Sci. 88 (2017) 1e48. [6] L.Z. Ouyang, X.S. Yang, H.W. Dong, M. Zhu, Structure and hydrogen storage properties of Mg3Pr and Mg3PrNi0.1 alloys, Scr. Mater. 61 (2009) 339e342. [7] H. Shao, K. Asano, H. Enoki, E. Akiba, Fabrication, hydrogen storage properties and mechanistic study of nanostructured Mg50Co50 body-centered cubic alloy, Scr. Mater. 60 (2009) 818e821. [8] S. Luo, H. Han, H. Huang, J. Zhang, Y. Liu, Y. Zhu, Y. Zhang, B. Xu, L. Li, Effect of Al* generated in situ in hydriding on the dehydriding properties of Mg-Al
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Catalytic effect of in situ formed nano-Mg2Ni and Mg2Cu on the hydrogen storage properties of Mg-Y hydride composites Cheng Xu a, c, Huai-Jun Lin b, f, *, Yunlei Wang a, c, Peng Zhang b, Yuying Meng b, Yao Zhang c, d, f, Yana Liu a, c, Jiguang Zhang a, c, Liquan Li a, c, Qian Shi e, Wei Li b, Yunfeng Zhu a, c, ** a
College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, China Institute of Advanced Wear & Corrosion Resistance and Functional Materials, Jinan University, Guangzhou, 510632, China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing, 210009, China d School of Materials Science and Engineering, Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing, 211189, China e Guangdong Institute of New Materials, Guangzhou, 510651, China f Guangdong Provincial Key Laboratory of Advance Energy Storage Materials, South China University of Technology, Guangzhou, 510640, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 September 2018 Received in revised form 14 December 2018 Accepted 17 December 2018 Available online 18 December 2018
Two hydrides nanocomposites, consisting of MgH2-Mg2NiH4-YH2 and MgH2-MgCu2-YH2 phases, were prepared by hydrogenation upon the melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 amorphous alloys, respectively. The catalytic effect of nano-Mg2Ni/Mg2NiH4 and Mg2Cu/MgCu2 particles on hydrogen storage properties of Mg-Y hydride composites have been studied. It is shown that the activated meltspun Mg80Y5Ni15 alloy exhibits higher hydrogen absorption capacity and faster hydriding/dehydriding kinetics than those of the activated melt-spun Mg80Y5Cu15 alloy. The activated melt-spun Mg80Y5Ni15 alloy absorbs about 4.2 wt% hydrogen in 10 min at 200 C, and almost fully desorbs hydrogen within 5 min at 280 C. In contrast, the activated melt-spun Mg80Y5Cu15 alloy absorbs about 4 wt % of hydrogen in 2 h at 280 C, and desorbs 3.7 wt% of hydrogen in 80 min at 300 C. The apparent activation energy of dehydrogenation for the activated melt-spun Mg80Y5Ni15 alloy was calculated to be 83.9 kJ/mol, which is significantly lower than that of 132.3 kJ/mol for the activated melt-spun Mg80Y5Cu15 alloy. The excellent hydrogen storage properties of the activated melt-spun Mg80Y5Ni15 alloy can be attributed to the multiple cracks and refined particle size generated during hydrogenation, and the uniform distributed catalytic Mg2Ni/Mg2NiH4 nanoparticles in the Mg/MgH2 matrix. In contrast, a large amount of Mg2Cu/ MgCu2 phase significantly aggregations, and the separation between Mg2Cu/MgCu2 and Mg/MgH2 matrix in the activated melt-spun Mg80Y5Cu15 alloy, which greatly decrease the catalytic effect of Mg2Cu for the activated melt-spun Mg80Y5Cu15 alloy. © 2018 Elsevier B.V. All rights reserved.
Keywords: Mg-based alloys Hydrogen storage Catalytic effect Kinetics Melt spinning
1. Introduction Hydrogen is widely considered as a promising energy barrier for the sustainable development of our society, and hydrogen storage is a critical issue on the way of realizing the hydrogen economy [1]. Mg is one of the promising hydrogen storage materials due to its high hydrogen capacity (7.6 wt%), abundance in magnesium
* Corresponding author. Institute of Advanced Wear & Corrosion Resistance and Functional Materials, Jinan University, Guangzhou, 510632, China. ** Corresponding author. College of Materials Science and Engineering, Nanjing Tech University, Nanjing, 210009, China. E-mail addresses: [email protected] (H.-J. Lin), [email protected] (Y. Zhu). https://doi.org/10.1016/j.jallcom.2018.12.223 0925-8388/© 2018 Elsevier B.V. All rights reserved.
minerals and harmlessness to the environment [2e4]. Nevertheless, the practical utilization of Mg-based alloys is inhibited by the slow reaction rate for the hydrogen absorption and desorption, and high hydrogen desorption temperature [4,5]. Until now, vast efforts have been made to overcome the drawbacks, such as alloying [6e9], nanostructuring [10e12], fabricating composites [13e16] and doping catalysts [17e20]. Great achievements have been obtained but further investigation is still needed. Previous studies proved that the hydrogen desorption rate of MgH2 can be significantly enhanced by introducing catalytically active elements such as transition metals (TM), rare earth elements (RE) and metal oxides, etc [16,21e24]. Doping with rare earth elements (RE) is reported to have a beneficial effect on the
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dehydrogenation of MgH2 owing to the catalytic effect of RE hydrides or RE oxides. Li et al. [25] combined both first-principles calculations and experimental observation to investigate the catalytic effect of YH2 on Mg/MgH2, which can be ascribed to the high interfacial energy of YH2/Mg, the low diffusion energy barrier for H at the YH2/Mg interface, and the high affinity between YH2 and H during the absorption process, and can act as the hydrogen pump during desorption process. Lin et al. [26] induced symbiotic CeH2.73/ CeO2 catalyst in the MgH2 matrix, which exhibits high catalytic activity for the MgH2 decomposition. Hydrogen can be easily released in the interface region of symbiotic CeH2.73/CeO2 nanoparticles on account of the efficient hydrogen pump. It was also shown that transition metals could effectively destabilize the MgH2 phase. The introduction of Ni and Cu shows catalytic effect on the hydrogen sorption properties of Mg. Ni forms Mg2Ni with Mg, which dissociates hydrogen molecule easily, leading to the high absorption kinetics. During the desorption process, Mg2NiH4 could promote hydrogen release of MgH2 due to the volume contraction after desorption, resulting in a contraction strain on the adjacent MgH2, which triggers dehydrogenation of MgH2 [27,28]. Unlike Ni, Cu forms two intermetallic phases with Mg: Mg2Cu and MgCu2. Mg2Cu reacts with hydrogen via a disproportionation reaction: 2Mg2Cu þ 3H2 % 3MgH2 þ MgCu2
(1)
MgCu2 is unable to react with hydrogen, leading to a theoretical hydrogen storage capacity of only 2.6 wt% [29]. The catalytic effect of Mg2Cu has been reported in the existing studies [30]. Jiang et al. [31] investigated a Y-Mg-Cu-H nanocomposite obtained by the hydrogen-induced decomposition of YMg4Cu. The nanocomposites exhibited excellent hydrogen absorption kinetics owing to the in situ formed well-dispersed catalysts of Mg2Cu and YH2. However, the different catalytic effects of Mg2Ni and Mg2Cu, and the corresponding products of Mg2NiH4 and MgCu2 after hydrogen absorption remain unclear and need to be further identified. In this work, two amorphous alloys with chemical compositions of Mg80Y5Ni15 and Mg80Y5Cu15 were fabricated by melt spinning. The melt-spun alloys were then treated by a hydrogenation activation treatment to induce MgH2-Mg2NiH4-YH2 and MgH2-MgCu2YH2 nanocomposites. The difference between catalytic effects of Mg2Ni/Mg2NiH4 and Mg2Cu/MgCu2 on the hydrogen storage properties of Mg/MgH2 were comparatively studied.
initial pressure of 3 MPa H2 for the absorption and 0.005 MPa H2 for the desorption at 200 C, 250 C, 280 C and 300 C, respectively, and the nonisothermal dehydriding properties were performed by differential scanning calorimetry (DSC, TA Q2000) and heated to 500 C under 50 ml/min Ar flow with a heating rate of 5 C/min, 8 C/min, 10 C/min and 13 C/min. For the isothermal hydrogen absorption and desorption tests, about 0.35 g of the alloys were loaded into a stainless steel sample chamber in a glove-box filled with Ar atmosphere. Morphologies of the alloys after hydrogen absorption and desorption process were characterized by field emission scanning electron microscopy (FESEM, Ultra55) equipped with a backscattered electron detector operated at 30 kV. TEM was performed on high resolution transmission electron microscope (HRTEM, JEM-2010 UHR) operated at 200 kV. The samples were dispersed onto a copper grid and examined at highly vacuum. 3. Results and discussion 3.1. Phase structure and crystallization characteristic Fig. 1 shows the XRD patterns of melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys. Melt-spun Mg80Y5Cu15 alloy shows a board hump, indicating its fully amorphous structure, while XRD pattern of the melt-spun Mg80Y5Ni15 alloy consists of a main amorphous peak and small amount of Ni2Y3, Ni5Y crystalline phases. Crystallization processes of the melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys are conducted by DSC measurement as shown in Fig. 2, which displays two or three exothermic peaks. Evidently, Mg80Y5Ni15 exhibits slightly higher thermal stability than Mg80Y5Cu15. The structure characterization of Mg80Y5Ni15 using XRD indicated that the first exothermic peak at 192.04 C should be assigned to the crystallization of Mg2Ni phase as shown in Fig. 3. The residual amorphous phase is still stable after the second exothermic peak at 285.82 C and the crystallization step belongs to the formation of Mg and Ni2Y3 phases. The final crystallization products are Mg, Ni3Y, Ni2Y3 and Mg2Ni phases. The crystallization of Mg80Y5Cu15 consists of two steps, and the first crystallization reaction at 193.48 C, leading to the formation of Mg, Mg2Cu, Cu2Y together with several unidentified phases. After the second crystallization peaking at 295.91 C, the amorphous Mg80Y5Cu15 alloy transforms into alloys consisting of Mg, Mg2Cu, Mg24Y5 and Cu2Y
2. Experimental YNi3 and YCu3 intermediate alloys were firstly fabricated by arcmelting. Mg80Y5Ni15 and Mg80Y5Cu15 ingots were then prepared by conventional induction-melting of a mixture of YNi3 and YCu3 intermediate alloys and pure Mg metal in a quartz crucible under argon atmosphere. Afterwards, the ingots were re-melted and then injected through a nozzle (F ¼ 1 mm) onto the surface of a rotating copper wheel at a linear velocity of 27.2 m/s. The obtained ribbons was ~3 mm in width and ~50 mm in thickness. The ribbons were crushed into powders via ball milling with a speed of 220 rpm for 1 h, and then screened through 100 mesh sieves. The ball-topowder weight ratio was 10:1. The phase structure analysis was characterized by an ARL X’TRA diffractometer with Cu-Ka radiation. All XRD patterns were obtained with 0.02 step and scan steep of 10 /min. Crystallization behaviors were investigated using Differential scanning calorimetry (DSC, TA Q2000). The melt-spun alloys were heated from room temperature to 500 C with a heating rate of 10 C/min under 1 atm Ar atmosphere. The hydrogen storage properties of alloys were measured on a Sieverts type pressurecompositiontemperature (PCT) volumetric apparatus (GRC, Advanced Materials Co.) under
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Fig. 1. XRD patterns of melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys.
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Fig. 2. DSC curves of melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys under Ar atmosphere at a heating rate of 10 C/min.
phases. 3.2. Hydrogenation activation properties Fig. 4 shows the hydrogenation activation cycles of the meltspun Mg80Y5Ni15 and Mg80Y5Cu15 alloys. The hydrogen absorption and desorption processes were carried out at 350 C, under 3 MPa H2 for 5 h and under 0.005 MPa H2 for 0.5 h, respectively. In the first hydrogenation process, both Mg80Y5Ni15 and Mg80Y5Cu15 alloys show poor hydrogen absorption kinetics, and significantly improved hydrogenation kinetics occurs at the second cycle. In the case of Mg80Y5Ni15, the activation process lasts five cycles to reach a maximum absorption capacity of about 4.7 wt%. The activation of Mg80Y5Cu15 continues for three cycles and shows a reversible capacity of about 4.1 wt%. Mg80Y5Cu15 requires fewer activation cycles than Mg80Y5Ni15, which might be attributed to the higher thermal stability of the Mg80Y5Ni15 alloy compared with that of the Mg80Y5Cu15 alloy (Fig. 2). It is found that the maximum absorption capacities of the Mg80Y5Ni15 and Mg80Y5Cu15 alloys are slightly lower than the corresponding theoretical capacities. The reason can be attributed to the high stability of the yttrium hydride under the experimental conditions. 3.3. Hydrogenation and dehydrogenation properties After the activation cycles, the hydrogen absorption and desorption properties were tested at different temperatures from 200 C to 300 C. Fig. 5 shows the hydrogen absorption kinetics of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys at different temperatures under 3 MPa H2. No evident difference in hydrogen absorption kinetics of the Mg80Y5Ni15 alloy at the experimental temperatures was observed and it takes about 10 min to achieve the absorption capacity of about 4.2 wt% and finally absorbs about 4.5 wt% hydrogen in 2 h, while the Mg80Y5Cu15 alloy shows much poorer absorption kinetics. The hydrogen content increases rapidly in the first 5 min at 300 C and then gradually increases to about 4 wt% within 2 h. The hydrogen absorption capacity reaches only about 3 wt% at 200 C within 2 h.
Fig. 3. XRD patterns of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 after crystallization at different temperatures.
Fig. 6 shows the hydrogen desorption kinetics of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys under 0.005 MPa H2 at 300, 280, 250 and 200 C, respectively. The hydrogen desorption kinetics of the Mg80Y5Ni15 alloy are quite fast at 300 and 280 C, reaching almost fully dehydrogenated within 5 min, while the dehydrogenation rate slows down at 250 C. Only about 1 wt% hydrogen is released when the temperature drops to 200 C. The Mg80Y5Cu15 alloy shows much poorer hydrogen desorption properties, even at 300 C it takes about 80 min to reach a saturated desorption capacity of about 3.7 wt%. The dehydrogenation capacity decreases to 2.3 wt% within 2 h at 280 C and no significant amount of hydrogen is desorbed at 200 C. After hydrogen absorption, the initial amorphous/ nanocrystalline-amorphous structure of melt-spun Mg80Y5Cu15 and Mg80Y5Ni15 alloys transforms into crystalline hydrides as shown in Fig. 7. Mg80Y5Ni15 transforms into hydrides composites consisting of MgH2, Mg2NiH4, YH2 and YH3 after the hydrogenation for 2 h, while Mg80Y5Cu15 transforms into MgH2, MgCu2 and YH2
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Fig. 4. The activation cycles of melt-spun Mg80Y5Ni15 and (b) Mg80Y5Cu15 (at 350 C; 3/ 0.005 MPa H2 for 5/0.5 h).
and YH3. Weak peaks of YH3 phase indicate the small amount of YH2/YH3 transformation and could be attributed to the short hydrogenation time, resulting in the unsaturated absorption, which is consistent with the results reported by Yang et al. [9]. MgCu2 is transformed from Mg2Cu and unable to react with hydrogen to form hydride, leading to the lower absorption capacity of Mg80Y5Cu15. The poorer hydrogen absorption kinetics of Mg80Y5Cu15 can be attributed to the different catalytic effects of Mg2Cu and Mg2Ni. Both Mg2Cu and Mg2Ni are considered to accelerate the dissociation of hydrogen molecular and promote the hydrogenation of Mg phase, playing a significant catalytic role in the formation of MgH2 phase [32,33]. Cho et al. [11] reported the excellent hydrogen storage properties of 3D Ni doped Mg crystals encapsulated by rGO layers. The Ni-doped rGO-Mg can absorb 6.5 wt% hydrogen at 200 C, of which about 90% was completed within 2.5 min. The fantastic hydrogen absorption property can be owing to the synergistic effect of nanosizing, rGO encapsulation and Ni doping. Mg-Ni nanoalloy (including Mg2Ni) crystallites formed in the hydrogen absorption/desorption tests play a crucial role in the catalytic phenomenon. Zhang et al. [34] investigated the substitution of Cu for Ni on the hydrogen sorption behavior of Mg20Ni10-xCux and found that Mg2Cu can act as an active site for hydriding, as well as it increases the lattice parameter of the alloy, leading to faster H-diffusion. Even though the catalytic effect of Mg2Ni and Mg2Cu on the hydrogenation of Mg phase as reported in the literatures, it seems that Mg2Ni shows higher catalytic activity than Mg2Cu during the hydrogen absorption process. The dehydrogenation products after the dehydrogenation for 2 h using XRD identified are present in Fig. 7. The dehydrogenated Mg80Y5Ni15 exhibits three phases: Mg, Mg2Ni and YH2 while Mg80Y5Cu15 consists of Mg, Mg2Cu and YH2. Both Mg2NiH4 and MgCu2 are reported to be beneficial for the dehydrogenation of MgH2. As reported by Xie et al. [35], the activation energy of desorption for Mg-5.0 wt% Ni-2.3 wt% V nanocomposite was calculated to be 85.1 kJ/mol. The enhanced hydrogen storage properties of the Mg-Ni-V nanocomposite are attributed to the synergistic catalytic effects of Mg2Ni/Mg2NiH4 and V/VH2 formed after activization process. Milanese et al. [36] investigated the MgNi-Cu mixtures to analyze the influence of Ni and Cu on the hydrogen storage properties. The activation energy decreases of 1.7 times for Mg: Ni: Cu ¼ 80: 10: 10 (wt.%) and of 2.4 times for Mg: Ni:
Fig. 5. Hydrogen absorption kinetics of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 after activation at different temperatures under 3 MPa H2.
Cu ¼ 40: 30: 30 (wt.%). The addition of Cu and Ni is beneficial in decreasing the energetic barrier for the MgH2 desorption and the dissociation of MgCu2 improves the dehydrogenation kinetics. However, there have been literatures reporting that the addition of Cu inhibits the de-/hydrogenation properties. Kalinichenka et al. [37,38] comparatively studied the hydrogen storage properties of the melt-spun Mg-Y-Ni and Mg-Y-Ni-Cu alloys, and found that the Mg-Y-Ni alloys exhibit higher hydrogen storage capacity and faster dehydrogenation rate than the Mg-Cu-Ni-Y alloys, which is surmised to be due to the in direct contact of MgCu2 with Mg2NiH4 forming co-domains embedded in MgH2 matrix, leading to the small specific interface area between matrix and catalytic phases. Here, the poor hydrogen storage properties of Mg80Y5Cu15 may also be due to the difference of the catalytic phase distribution in MgH2 matrix compared with Mg80Y5Ni15, which will be discussed in detail in later section. DSC analysis was performed to further investigate the dehydrogenation property of the Mg80Y5Ni15 and Mg80Y5Cu15 alloys as shown in Fig. 8a and b, and Fig. 8c and d are the dehydrogenation activation energy calculated using Kissinger's equation.
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Fig. 6. Hydrogen desorption kinetics of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 after activation at different temperatures under 0.005 MPa H2.
"
!#, 1 ¼ EA =R d ln 2 d T TP P 4
(2)
where Tp is the endothermic peak temperature, 4 is the heating rate, R is the gas constant and EA is the activation energy. The hydrogenated Mg80Y5Ni15 released hydrogen at the peak temperature of 280.50 C in the case of 5 C/min and showed about 70 C lower than the hydrogenated Mg80Y5Cu15 at each heating rate. The EA for the Mg80Y5Ni15 alloy was calculated to be 83.9 kJ/mol from the slop of the Kissinger's plot; this value is significantly lower than 132.3 kJ/ mol for the Mg80Y5Cu15 alloy, suggesting that the catalytic effect of Mg2NiH4 phase on the MgH2 decomposition shows lower kinetic barrier than that of MgCu2 phase. The prominent catalytic effect of Mg2NiH4 on the MgH2 desorption has been reported elsewhere. Chen et al. [18] studied the catalytic effect Ni/C nanoparticles on the hydrogen storage properties of MgH2. The in situ formed Mg2Ni and Mg2NiH4 are regarded as the catalytically active species and the uniform distributions of Mg2Ni and Mg2NiH4 are responsible for a favorable and lasting catalytic efficiency. Therefore, the huge
Fig. 7. XRD patterns of melt-spun (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 alloys after hydrogenation and dehydrogenation.
difference of hydrogen sorption properties between the melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys may be related to the microstructure difference, which need to be further discussed. 3.4. Microstructures of the hydrogenated and dehydrogenated alloys In order to characterize the microstructures of activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys after hydrogenation, BSE-FESEM was employed to investigate the catalytic phase distribution in the matrix. Fig. 9a exhibits the multiple cracks and plenty of small particles, indicating the severe pulverization of the hydrogenated Mg80Y5Ni15 due to the repeated volume expansion and contraction of Mg/MgH2 and Mg2Ni/Mg2NiH4 during the hydrogen absorption and desorption tests. No significant pulverization was observed in the hydrogenated Mg80Y5Cu15 alloy as shown in Fig. 9c and it remains coarse particles after hydrogenation. After the hydrogen absorption/desorption tests, multiple cracks and refined particle size are regarded to be beneficial for the hydrogen sorption kinetics [39]. Besides, the uniform distribution of Mg2NiH4 pieces was
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Fig. 8. DSC curves of hydrogenated (a) Mg80Y5Ni15 and (b) Mg80Y5Cu15 under Ar atmosphere at different heating rates (5, 8, 10, 13 C/min); Kissinger's plots of hydrogenated (c) Mg80Y5Ni15 and (d) Mg80Y5Cu15 derived from the DSC profiles.
Fig. 9. BSE-FESEM images of hydrogenated (a, b) Mg80Y5Ni15 and (c, d) Mg80Y5Cu15 after activation.
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observed to be dispersed in the MgH2 matrix, while the MgCu2 is aggregated into dendritic structure, leading to the small specific interface area between MgCu2 and MgH2. The sufficient interface between catalytic and matrix shows great importance on the hydrogen storage properties and the huge particle size is definitely harmful for the hydrogen sorption properties [40]. Further structural observation of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys after hydrogen absorption/desorption was conducted by TEM analysis as shown in Figs. 10 and 11. Fig. 10a and b shows small pieces of Mg2NiH4 and YH2 embedded in the MgH2 matrix, while the MgCu2 phase is aggregated together to form coarse grains (Fig. 10c and d), which is consistent with the FESEM results as shown in Fig. 9c and d. The insufficient contact of MgCu2 and MgH2 may be account for the poor hydrogen storage properties of the Mg80Y5Cu15 alloy. Wu et al. [41] comparatively studied two kinds of LaNiO3 with different morphologies (porous and powder) on the hydrogen storage properties of MgH2, and found the porous LaNiO3 showed much better catalytic effect on hydrogen storage properties of MgH2 than that of powder LaNiO3, which can be attributed to the uniform distribution of powder LaNiO3 with high BET surface in the matrix. Fig. 11 shows the TEM images of the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys after hydrogen desorption. Fig. 11a shows that Mg2Ni and YH2 is relatively well-distributed in the Mg matrix, but a certain amount of Mg2Ni aggregations can be seen in Fig. 11b. However, plenty of Mg2Cu aggregations can be observed in Fig. 11d and in some particles as shown in Fig. 11e, almost all the Mg2Cu phase is aggregated on the boundary of the particles and no evident Mg2Cu is found to distributed inside the Mg matrix. Such heterogeneous distribution of Mg2Cu phase and the separation between Mg2Cu and Mg would decrease the catalytic effect on hydrogen storage properties, leading to the lower absorption kinetics. The above results clearly indicate that the huge microstructure difference between the activated Mg80Y5Ni15 and Mg80Y5Cu15 alloys could significantly affect the hydrogenation/dehydrogenation kinetics. It is deduced that the superior hydrogen storage properties of the activated Mg80Y5Ni15 alloy might be owing to the following reasons.
Firstly, Mg2Ni/Mg2NiH4 and Mg2Cu/MgCu2 exhibit intrinsic catalytic effect differences on the hydrogen storage properties of Mg-Y hydride composites. Mg2Ni is reported to promote the dissociation of the hydrogen molecule, resulting in a faster nucleation of magnesium hydride [42]. During the dehydriding process, Mg2NiH4 and MgH2 exhibit a synergetic effect of hydrogen desorption. Mg2NiH4 releases hydrogen prior to MgH2 to form Mg2Ni, leading to a severe volume contraction and lattice strain, which provide the driving force to trigger the dehydrogenation of MgH2 [27]. In the case of the catalytic effect of Mg2Cu/MgCu2 as previous studies proved, the enhanced hydrogenation kinetics can be due to the easily dissociation of hydrogen molecule, improved hydrogen atoms diffusion in bulk Mg2Cu or along the Mg/Mg2Cu interface, and the addition of stable nucleation sites along the Mg/ Mg2Cu interface [43]. Regarding the catalytic effect in desorption process, on the one hand, MgCu2 can react with MgH2 according to reaction (1), promoting the cleavage of the Mg-H bond. On the other hand, the existence of MgCu2 provides an oxide-reduced surface, enhancing hydrogen recombination. However, slow hydrogen diffusion from MgH2 through MgCu2 to the external surface may be account for the poor dehydriding kinetics [44]. Secondly, the activated Mg80Y5Ni15 is subject to the severe pulverization owing the repeated volume expansion and contraction of Mg2Ni/Mg2NiH4, while Mg2Cu/MgCu2 transformation shows no apparent volume change. Multiple cracks and refined particle size are induced, providing more active sites and reducing the hydrogen diffusion distance. The coarse dendritic MgCu2, no significant pulverization and huge particle size result in the slow sorption properties. Finally, Mg2Ni/Mg2NiH4 remains small grain size welldistributed in matrix, providing sufficient contract between Mg2Ni/Mg2NiH4 and Mg/MgH2, which provides the prominent catalytic effect. In contrast, Mg2Cu/MgCu2 is more likely to aggregated together on the boundaries of the matrix. Therefore, the extremely small specific interface area between Mg2Cu/MgCu2 and the primary matrix could decrease the catalytic effect of Mg2Cu/ MgCu2.
Fig. 10. TEM images of the activated (a, b) Mg80Y5Ni15 and (c, d) Mg80Y5Cu15 after hydrogen absorption.
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Fig. 11. TEM images of the activated (aec) Mg80Y5Ni15 and (def) Mg80Y5Cu15 after hydrogen desorption.
4. Conclusion
Acknowledgements
In summary, melt-spun Mg80Y5Ni15 and Mg80Y5Cu15 alloys were fabricated to obtain an amorphous or nanocrystalline-amorphous precursor. Afterwards, two hydrides nanocomposites consisting of MgH2-Mg2NiH4-YH2 and MgH2-MgCu2-YH2 phases were prepared by hydrogenation. The melt-spun Mg80Y5Ni15 alloy exhibits higher thermal stability than Mg80Y5Cu15, leading to more activation cycles for the Mg80Y5Ni15 alloy than the Mg80Y5Cu15 alloy. The Mg80Y5Ni15 alloy shows superior hydrogen absorption and desorption kinetics, which absorbs about 4.2 wt% hydrogen in 10 min and finally absorbs about 4.5 wt% hydrogen in 2 h, and can desorb almost all the hydrogen within 5 min at 300 and 280 C. In contrast, the Mg80Y5Cu15 alloy only absorbs 3 wt % of hydrogen in 20 min, and needs about 80 min to reach a saturated desorption capacity of about 3.7 wt% at 300 C. Furthermore, the activation energy for the Mg80Y5Ni15 alloy is calculated to be 83.9 kJ/mol, which is much lower than that of 132.3 kJ/mol for the Mg80Y5Cu15 alloy. It is suggested that the superior hydrogen storage properties of Mg80Y5Ni15 should be attributed to the multiple cracks and refined particle size emerged during hydrogen sorption, and the uniform distributed Mg2Ni/Mg2NiH4 in Mg/MgH2 matrix, compared with the vast Mg2Cu/MgCu2 aggregations and the separation between Mg2Cu/MgCu2 and Mg/MgH2 matrix, which decreases the catalytic effect of Mg2Cu/MgCu2 on hydrogen storage properties of the Mg80Y5Cu15 alloy.
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