hydriding characteristics of a novel Mg18In1Ni3 alloy

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 4 0 3 3 e1 4 0 3 8

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Reversible de-/hydriding characteristics of a novel Mg18In1Ni3 alloy Y.S. Lu a, M. Zhu b, H. Wang a,*, Z.M. Li a, L.Z. Ouyang c, J.W. Liu d a

School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, China Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou, 510641, China c Key Laboratory of Fuel Cell Technology of Guangdong Province, South China University of Technology, Guangzhou, 510641, China d China-Aus Joint Laboratory for Energy & Environmental Materials, South China University of Technology, Guangzhou, 510641, China b

article info

abstract

Article history:

The present work demonstrates the reversible hydrogen storage properties of the ternary

Received 16 May 2014

alloy Mg18In1Ni3, which is prepared by ball-milling Mg(In) solid solution with Ni powder.

Received in revised form

The two-step dehydriding mechanism of hydrogenated Mg18In1Ni3 is revealed, namely the

2 July 2014

decomposition of MgH2 is involved with different intermetallic compounds or Ni, which

Accepted 3 July 2014

leads to the formation of Mg2Ni(In) solid solution or unknown ternary MgeIneNi alloy

Available online 28 July 2014

phase. As a result, the alloy Mg18In1Ni3 shows improved thermodynamics in comparison with pure Mg. The Ni addition also results in the kinetic improvement, and the minimum

Keywords:

desorption temperature is reduced down to 503 K, which is a great decrease comparing

Hydrogen storage

with that for MgeIn binary alloy. The composition and microstructure of MgeIneNi ternary

MgH2

alloy could be further optimized for better hydrogen storage properties.

MgeIneNi ternary alloy

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Dehydrogenation mechanism

Introduction Mg shows great potential for solid-state hydrogen storage because of its advantages in the high hydrogen capacity, good reversibility, and plentiful resource [1e3]. The magnesium hydride MgH2 releases 7.6 wt.% H during decomposition, which however requires over-high dehydriding temperature up to 350
C. This drawback hampers its practical hydrogen storage applications, particularly on-board hydrogen storage for fuel cell vehicles [4e6].

In essence, the high thermodynamic stability of MgH2 is responsible for the unfavourable dehydrogenation temperature. Therefore, numerous attempts have been made to improve the hydrogen storage properties of Mg through alloying Mg with transition metals or rare earth metal [7e16]. Normally, these Mg-based alloys were hydrogenated into the nanocomposite of MgH2 and elemental hydrides, which showed excellent hydrogen absorption kinetics at lowered temperature [17e20]. For example, the Mg91.9Ni4.3Y3.8 alloy with long-period-ordered-stacking structure decomposed into a mixture of nanosized MgH2, Mg2NiH4 and YH3 in

* Corresponding author. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.07.016 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

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hydrogenation, in which the in-situ formed Mg2NiH4 and YH3 played great catalytic role on the dehydrogenation of MgH2 [21]. However, most of Mg-based alloys could not be recovered in the dehydrogenation and thus the dehydriding thermodynamics of MgH2 remains unchanged. Recently, some reversible structural changes in dehydriding were reported in the MgeAg [22,23], MgeCd [24], and MgeIn Refs. [25,26] binary alloy systems. It is found that the alloying elements Ag, Cd, and In are non-hydride-forming elements, which would combine with Mg to form intermetallic compounds, other than stable element hydrides. The Mg95In5 solid solution alloy, as an example [25], was hydrogenated into MgH2 and MgIn compound. In the dehydrogenation, the MgH2 reacted with the MgIn and leads to the recovery of Mg(In) solid solution. As a result, the thermodynamics of MgH2 is destabilized. On the basis of Mg(In) binary solid solution, our group successfully realized reversible hydrogen storage in the MgeIneAl ternary solid solution [27]. In the Mg90In5Al5 alloy, it was found the existing In extended the dehydriding reversibility of Al, because the Al preferred to get dissolved in the MgIn compound upon hydrogenation, rather than forming free Al in the hydriding of Mg(Al) solid solution [28]. Although the addition of Al further improved the thermodynamics of Mg(In) binary solid solution to a certain extent, the dehydriding temperature of Mg90In5Al5 alloy was still higher than 573 K, which is due to the fact that Al might slow the diffusion rate of In in dehydriding. Nickel is a widely used alloying component to improve both kinetics and thermodynamics of hydrogen storage alloys, such as Mg2Ni, LaNi5 [29]. Further, the hydride of Ni is highly unstable, which may be beneficial for the structural reversibility of Mg-base alloys in dehydriding. In the present work, we investigate the microstructural and hydrogen storage properties of a ternary MgeIneNi alloy, the reversible structural evolution in de-/hydriding is demonstrated.

Experimental The ternary alloy, with a designed composition of Mg18In1Ni3, was firstly prepared by sintering the mixture of Mg and In powders at 773 K for 6 h in a tube furnace under the protection of Ar, and then the sintered MgeIn alloy was ball-milled with Ni powder on a QM-3SP2 planetary mill at 250 rpm for 20 h with ball-to-powder weight ratio 20:1. The composition of the milled alloy was measured by energy-dispersive spectrometer, which is well consistent with the designed composition. Phase structure of alloy at different states was investigated by X-ray diffraction (XRD) with Cu-Ka radiation at 40 kV and 40 mA (Philips X’Pert MPD X-ray diffractometer). The microstructure was observed by scanning electron microscope (SEM, Zeiss EVO 18). The hydrogen storage properties including the pressure-composition isotherm (PCI) and isothermal hydrogenation and dehydrogenation kinetics were measured using automatic Sievert-type apparatus. Sample of ca. 0.3 g was fully activated through three hydrogenation/ dehydrogenation cycles at 593 K prior to measurements. For the PCI desorption test, the hydrogen pressure was varied from 40 atm to 0.05 atm, while for the kinetic test, the initial

pressure of sample cell was about 40 atm for hydrogenation, and vacuum for the dehydrogenation. All sample handlings were performed in the glove box filled with Ar atmosphere with the water concentration less than 3 ppm.

Results and discussions The XRD patterns of Mg18In1Ni3 alloy at different states are shown in Fig. 1. It is seen in Fig. 1(a) that the milled alloy consists of Mg(In) solid solution and Ni. There is no any new phase formed during milling, the dissolving of Ni in the Mg(In) solid solution is not observed. As seen from Fig. 1(b), MgH2 and L10-structured MgIn compound (also denoted as b00 phase) are identified in the hydrogenated products, which are same with those for the Mg(In) binary alloy [25,26]. In addition, an unknown phase (hereafter denoted as X1) is present. Since the relative intensity of Ni peaks is decreased in comparison with milled sample, this unknown phase is therefore deduced to be a MgeIneNi ternary alloy, which has not been reported in the literature. After a hydrogenationedehydrogenation cycle, the phase composition of Mg18In1Ni3 alloy is not recovered to the original Mg(In) solid solution and Ni according to Fig. 1(c). In addition to the presence of Mg instead of Mg(In) solid solution, the solid solution Mg2Ni(In) is identified because the Mg2Ni peaks slightly shifted toward the lower values of the diffraction angel, this shift is verified by adding Si powder as interior substance for 2q calibration. Ouyang et al. [30] also reported the formation of Mg2Ni(In) phase in the milled mixture of Mg2Ni and In. It is noted that the diffraction intensity of Ni is further decreased, whilst X1 phase is absent and the other unknown phase (hereafter denoted as X2) is present. Therefore, the X2 phase should belong to the other MgeIneNi ternary alloy. Reversible microstructure evolution of Mg18In1Ni3 alloy is investigated by SEM observation. Fig. 2(a) shows the backscattering SEM micrograph of Mg18In1Ni3 alloy at fully hydrogenated state. There appear two characteristic contrasts in the SEM micrograph, in which the grey matrix is the MgH2,

Fig. 1 e XRD patterns of Mg18In1Ni3 alloy at different states: (a) ball-milled, (b) hydrogenated, and (c) dehydrogenated.

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Fig. 2 e SEM back scattered electron images of (a) hydrogenated and (b) dehydrogenated Mg18In1Ni3.

while bright spots embedding in the MgH2 particles could not precisely assigned to Ni, b00 or X1 phase. At dehydrogenated state, the SEM image (Fig. 2(b)) shows more homogeneous microstructure, indicating finer distribution of the elements In and Ni after dehydrogenation. This result also indicates longrange diffusion of In and Ni in the dehydrogenation of Mg18In1Ni3 alloy. It has been proved that the abovementioned phase transformation of Mg18In1Ni3 alloy could be reproduced during subsequent multiple hydridingedehydriding cycles. Obviously, the decomposition process of MgH2 in the Mg18In1Ni3 alloy becomes more complicated due to the addition of Ni, which is involved with the MgeIneNi ternary alloy and the metal Ni. Therefore, the dehydriding thermodynamics and kinetics of MgH2 in the Mg18In1Ni3 alloy is changed. Fig. 3(a) shows the desorption PCI curves of Mg18In1Ni3 alloy at different temperatures. Two evident plateaus are present, suggesting two-step dehydrogenation process of Mg18In1Ni3 alloy. The overall reversible hydrogen storage capacity is ca. 3.8 wt.%, and ca. 2.8 wt.% H capacity is contributed by the low-pressure plateau. It is noted that the minimum hydrogen desorption temperature of Mg18In1Ni3 alloy is reduced down to 503 K, much lower than the value 586 K for the Mg95In5 binary solid solution or 588 K for pure Mg [27]. The fitted van't Hoff plots are shown in Fig. 3(b), by which the desorption enthalpy DH and desorption entropy DS for the low-pressure and high-pressure plateau are calculated and shown in Table 1. The results imply the altered dehydriding thermodynamics of Mg18In1Ni3 alloy, which will be discussed in a later part of this paper. To further investigate the dehydriding mechanism of Mg18In1Ni3 alloy, quasi-in-situ XRD analysis was carried out on the alloy at different dehydriding stages of PCI test, namely the PCI desorption test was manually terminated, and the sample holder was taken out from the furnace but keeping the hydrogen pressure of sample cell. The sample was quickly cooled and taken out for XRD analysis at room temperature. The XRD results are shown in Fig. 4, in which the profiles (a, b, c, d, e) respectively corresponds to the dehydriding points (a, b, c, d, e) as marked in the PCI curve at 503 K (Fig. 3(a)). It is of first importance that the diffractions intensity of Ni becomes weakening with the dehydrogenation process, indicating the participation of Ni in the dehydriding reaction of MgH2. When the dehydriding of Mg18In1Ni3 alloy proceeds to

the point b in Fig. 3(a), i.e. the middle-point of high-pressure plateau region, at which the Mg2Ni(In) solid solution and small amount of Mg are present according to Fig. 4b. Meanwhile the b00 phase is absent. At the point c, i.e. the endpoint of high-pressure plateau, Fig. 4c shows that the Mg2Ni(In) peaks becomes mildly intensifying, while the MgH2 peaks becomes further weakening. At this stage, X1 phase almost remains unchanged, and X2 phase is absent. Therefore, for the highpressure plateau, it is deduced that a part of MgH2 reacts

Fig. 3 e Pressure-composition isotherms (a) and van't Hoff plots (b) for the hydrogen desorption of Mg18In1Ni3 alloy.

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Table 1 e Comparison of desorption enthalpy DH and entropy DS of different reactions. DH kJ/(mol H2)

DS J/(K mol H2)

Equilibrium pressure at 573 K (atm)

Ref.

71.7 79.7 74.7 64.5 68.1

135.8 144.7 130.0 122.3 125.5

3.62 1.97 0.96 3.22 2.23

Present Present [31] [8] [25]

Eq. (1) Eq. (2) MgH2 / Mg þ H2 Mg2NiH4 / Mg2Ni þ H2 MgH2 þ MgIn / Mg95In5 þ H2

with the b00 phase and Ni, which leads to the formation of Mg2Ni(In) and Mg, and minor hydrogen release, the dehydriding reaction is as following: MgH2 þ b0 0 þ Ni / Mg þ Mg2Ni(In) þ H2

(1)

When the dehydriding process reaches the middle-point of low-pressure plateau region, the peaks of X1 completely disappear, whilst the X2 phase is present according to Fig. 4d. Meanwhile, the MgH2 and Ni peaks become further weakening, which is accompanied with the intensified Mg peaks. At the point e, the full dehydrogenation is proved by the disappearance of MgH2. Obviously, the decomposition of most MgH2 occurs at the low-pressure plateau, explaining its relatively larger plateau length. It is also noted and the diffraction peaks of X2 phase are slightly shifted toward lower angles comparing with those in Fig. 4d, which is calibrated by the MgH2 or added Si. Accordingly, the dehydriding reaction corresponding to the low-pressure plateau can be expressed as following: MgH2 þ X1 þ Ni / Mg þ X2 þ H2

(2)

The changed decomposition reaction pathway of MgH2 in the Mg18In1Ni3 alloy explains the altered thermodynamic properties. Table 1 compares the enthalpy change DH and entropy change DS for the two dehydriding reactions of

Mg18In1Ni3 alloy with those of Mg, Mg2Ni, and Mg95In5 alloys. Evidently, the reaction Eq. (2) shows a great increase in DH and DS if comparing with pure MgH2. Despite this, the equilibrium pressure Peq at 573 K for Eq. (2), which is calculated by the van't Hoff equation, is higher than that of pure MgH2. The reason is that the DS experiences more increase than the DH. Comparatively, the reaction Eq. (1) shows greater equilibrium pressure, which is also higher than those for Mg2Ni and Mg95In5 alloys. The elevated equilibrium pressure definitely indicates that the dehydriding thermodynamics of MgH2 is destabilized for this MgeIneNi ternary alloy. Fig. 5 displays the isothermal hydrogenation and dehydrogenation kinetic curves of Mg18In1Ni3 alloy at different temperatures. As illustrated in Fig. 5(a), the Mg18In1Ni3 alloy shows excellent hydrogen absorption kinetic property. At 553 K, the hydrogen absorption content reaches the maximum in less than 5 min, while at 453 K the alloy can absorb ca. 2.0 wt.% H within 60 min. With respect to the dehydrogenation (Fig. 5(b)), a maximum 3.8 wt.% hydrogen release content is obtained within 20 min at 553 K. Even at a lower temperature of 493 K, ca. 1.0 wt.% H is desorbed within 10 min. It is stated that the hydrogenation and dehydrogenation rate of Mg18In1Ni3 alloy is much faster than pure Mg [32] and Mg95In5 solid solution alloy [27]. The dehydriding kinetic curves of Mg18In1Ni3 alloy at 533 K, 553 K and 583 K were analysed using JohnsoneMehleAvramieKolmogorov (JMAK) model [33,34]:

Fig. 4 e XRD patterns of Mg18In1Ni3 at different PCI desorption stages at 503 K.

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Fig. 6 e Arrhenius plot for the dehydrogenation of Mg18In1Ni3 alloy.

Fig. 5 e Isothermal hydrogenation curves (a) and isothermal dehydrogenation curves (b) of Mg18In1Ni3 alloy at different temperatures.

h

a ¼ 1  eðktÞ

Conclusions (3)

where a is the reaction fraction, k the rate constant, t the reaction time, and h the reaction order. The reaction fraction a chosen for fitting ranges 0.3e0.7, because of the linear relationship between the desorption fraction and desorption time at this range. The fitting results show that all h values at different temperatures are close to 1, suggesting that the dehydrogenation of Mg18In1Ni3 alloy follows one dimensional growth model. The obtained k values are further used for the calculation of the apparent activation energy Ea by Arrhenius equation: Ea= RT

k ¼ Ae

storage alloy with excellent hydriding kinetics, played great catalytic role on the dehydrogenation of Mg90In10 alloy, in which the MgH2 even could be fully discharged in a temperature range 120e150
C. The additive TiMn2 is believed to promote the diffusion, dissociation and recombination of hydrogen. In the present work, the Ni component not only participates the decomposition reaction of MgH2, but the excess Ni in the Mg18In1Ni3 alloy also has similar catalytic role as the TiMn2 on the dehydrogenation of MgH2. Further, it is worth stating that the long-range diffusion of metal atoms In and Ni in the multi-step dehydriding reactions of Mg18In1Ni3 alloy appears to not be the kinetic barrier, however, the mutual effect of In and Ni diffusion in this MgeIneNi ternary alloy remains to be further clarified.

The alloy Mg18In1Ni3 was prepared by ball-milling the Mg(In) solid solution with Ni powder. This alloy showed reversible hydrogen storage capacity of ca. 3.8 wt.%, and improved de-/ hydriding thermodynamic and kinetic properties, which resulted in the minimum hydrogen desorption temperature of 503 K. Two new MgeIneNi ternary alloy phases were reversibly formed in the two-step hydriding and dehydriding process. Our work shows the potential of MgeIneNi ternary alloy for reversible hydrogen storage. The structural determination and the composition optimization of MgeIneNi alloy for better hydrogen storage properties are under investigation.

(4)

where A the pre-exponential factor, R the gas constant, and T the Kelvin temperature. The Arrhenius plot are shown in Fig. 6, and the Ea for dehydrogenation is determined to be 107 kJ/mol, much lower than the value ~160 kJ/mol for pure MgH2 [35,36], or 145 kJ/mol for the Mg95In5 solid solution alloy [27]. Undoubtedly, the kinetic improvement should be attributed to the Ni addition. With respect to the kinetic improvement of MgeIn alloy, Fang et al. [26] reported that the addition of TiMn2, a hydrogen

Acknowledgements This work was financially supported by the Ministry of Science and Technology of the People's Republic of China under grant no. 2010CB631302, Natural Science Foundation of China under grant no. U1201241, 51071068, 51271078, U1201241, KLGHEI (KLB11003), and the Fundamental Research Funds for the Central Universities of China.

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