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Elaboration and electrochemical characterization of LaTi2Cr4Ni5 -based metal hydride alloys
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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
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Elaboration and electrochemical characterization of LaTi2Cr4Ni5 -based metal hydride alloys Marwa Ayari, Ouassim Ghodbane*, Mohieddine Abdellaoui Institut National de Recherche et d'Analyse Physico-Chimique, Laboratoire des Materiaux Utiles, P^ole Technologique de Sidi Thabet, 2020 Sidi Thabet, Tunisia
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abstract
Article history:
The present paper focuses on the elaboration of LaTi2Cr4Ni5 -based intermetallic com-
Received 21 March 2015
pounds by mechanical alloying from LaNi5 and TiCr2 precursors and their characterization
Received in revised form
as negative electrode materials in Nickel-metal hydride (Ni-MH) batteries. The structural
26 June 2015
properties of the phases were determined by X-ray diffraction and quantified from the
Accepted 29 June 2015
Rietveld refinement data. The increase of the milling time up to 25 h leads to the highest
Available online 26 July 2015
abundance of the LaTi2Cr4Ni5 phase, estimated at 66 wt%, and a complete elimination of the LaNi5 intermetallic precursor. The electrochemical techniques were applied to char-
Keywords:
acterize the electrochemical behavior of prepared LaTi2Cr4Ni5 -based compounds. The
Ni-metal hydride battery
maximum discharge capacity was 115 mAh g1 and the average capacity retention was
Mechanical alloying
equal to 77% upon 60 charge/discharge cycles, indicating a high electrochemical stability in
Hydrogen absorptionedesorption
the alkaline solution. The values of the hydrogen diffusion coefficient are equal to
properties
4.18 108 and 7.17 108 cm2 s1 after 8 h and 20 h of milling durations, respectively.
Discharge capacity
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Research on Ni-MH batteries knows a significant growth due to their superior safety and endurance as hydrogen storage devices [1]. The distinct alloy families that have been investigated as negative electrodes for these batteries are AB5, AB2, AB and AB0.5 [2,3], where A represents a metallic element with a strong affinity for hydrogen (a transition metal or rare-earth element), and B a metallic element with a weak affinity for hydrogen (a last transition metal). Intermetallic compounds formed with A and B elements are able to reversibly absorb hydrogen. Concerning AB5-type materials, LaNi5 is the most popular compound investigated as a hydrogen storage material [4e7]. The hydrogenation of LaNi5 leads to the ternary
hydride compound LaNi5H6, with a hydrogen equilibrium pressure of 1.7 atm at the room temperature [8,9]. Unfortunately, its poor hydrogenation at low temperatures, its low kinetic and its short cycle life have a significant impact on the battery commercialization. On the other hand, among AB2type materials, TiCr2 alloys attracted a great attention as a potential candidate for the hydrogen storage [10,11]. They react directly and reversibly with hydrogen to form two nonstoichiometric hydride phases having the nominal compositions of TiCr1.9H2.5 and TiCr1.9H3.5 [11]. Unfortunately, their corresponding plateau pressures at 78 C are equal to 0.2 and 30 atm, respectively [11]. Furthermore, AB and AB0.5 -type compounds are the least adapted for electrochemical applications since the hydrogen desorption is hindered by the high stability of their corresponding hydrides.
* Corresponding author. Tel.: þ216 71 537 666; fax: þ216 71 537 688. E-mail address: [email protected] (O. Ghodbane). http://dx.doi.org/10.1016/j.ijhydene.2015.06.169 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
In the last decade, a number of studies demonstrated that AB3-type alloys are promising alternatives for the reversible hydrogen storage [12e14]. They gained a considerable development owing to their high capacity of hydrogen storage and good electrochemical properties compared to AB5-type alloys [15,16]. In this context, the AB3-type structure can be prepared from the intergrowth of AB5-type compounds with AB2 units following the reaction scheme [12]:
AB5 þ 2 AB2 / 3 AB3 (1) This approach aims to overcome the drawbacks of AB2 and AB5 precursors and, in the same way, to take profit from their hydrogenation characteristics in order to obtain new materials with enhanced hydrogen storage properties. In the present work, we investigate the electrochemical hydrogenation of a novel AB3-type LaTi2Cr4Ni5 -based alloys. To the best of our knowledge, LaTi2Cr4Ni5 -based materials are investigated for the first time in the literature. They crystallize in the rhombohedral structure of PuNi3-type with a long-range stacking arrangement similar to either AB5 or AB2 alloys [1]. LaTi2Cr4Ni5 was prepared by mechanical alloying from LaNi5 and TiCr2 intermetallic precursors. The crystal structure was determined and refined from X-ray powder diffraction data. The hydrogen absorption/desorption properties were determined by electrochemical measurements.
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XPERT PRO MPD diffractometer operating with Cu Ka radiation (l ¼ 0.15406 nm). Diffraction patterns were analyzed by the Rietveld refinement method using the FullProf software [17]. The powder morphology of the samples was characterized with a FEI Quanta 200 environmental scanning electron microscope (SEM). For the electrochemical measurements, the alloys ingot was ground mechanically and saved to less than 63 mm in the glove box under argon atmosphere. The working electrodes were prepared according to the “latex” technology [18]: 90% of the alloy powder is mixed with 5% of black carbon, to increase the electrode conductivity, and 5% of polytetrafluoroethylene (PTFE) as a binder. An overall surface of 1 cm2 of this latex was pressed on each side of a nickel grid to prevent the electrode plate from breaking into pieces during the chargeedischarge cycling [19]. This set forms the negative electrode of Ni/MH accumulator. Electrochemical tests were performed with a VMP system and using a conventional threeelectrode cell, in the presence of an aqueous 1 M KOH electrolyte solution, which was deoxygenated by bubbling Ar gas during every measurement. The counter electrode was formed by Ni oxyhydroxide Ni(OH)2, whereas the reference electrode was Hg/HgO immersed in 1 M KOH solution, prepared with deionized water. The discharge capacity of the electrode was determined by a galvanostatic chargingedischarging at C/3 and D/6 regime, respectively. Every cycle was carried out by charging fully to 1.3 V and discharging to 0.6 V vs. Hg/HgO at room temperature.
Results and discussion Experimental section Structural characterizations Starting alloys LaNi5 and TiCr2 were synthesized by ultra high frequency (UHF) induction melting of the pure elements (La 99.9%, Ni 99.9%, Ti 99.99%, Cr 99.99%) into a water cooled copper crucible under argon atmosphere. Every alloy was melted five times and inverted between each melting to ensure homogeneity. The chemical composition of the intermetallic precursors was determined by Electron Probe Micro Analysis (EPMA) in a Cameca SX-100 device. A sample was taken from each alloy and polished for optical and electronic metallographic examination and EPMA. The composite LaTi2Cr4Ni5 was made by mechanical alloying (MA) following this reaction:
LaNi5þ 2 TiCr2 / LaTi2Cr4Ni5 (2) A mixture of the precursors LaNi5 (0.59 g) and TiCr2 (0.42 g), with a molecular ratio of 1:2, was sealed into a stainless steel vial (50 cm3 in volume) with 5 stainless steel balls (12 mm in diameter and 7.0 g in mass) in a glove box filled with purified argon gas. The ball-to-powder weight ratio was equal to 35:1. The MA experiments were performed at room temperature using a Retsh PM400 planetary ball miller. The disc rotation speed and the vial rotation speed were equal to 320 and 640 rpm, respectively. These milling conditions correspond to an injected shock power of 6.175 W g1. The crystallographic characterization of synthesized powders was carried out by XRD using a (qe2q) Panalytical
The microprobe analyses of TiCr2 and LaNi5 are presented in Fig. 1(a,b), and show a constant composition for both samples. The expected compositions of TiCr2 and LaNi5 are x ¼ B/A ¼ 2/ 1 and 5/1, respectively. The EPMA allows to confirm that phase compositions observed for both compounds are TiCr(1.9 ± 0.2) and LaNi(5.5 ± 0.8), in agreement with the expected stoichiometries. On the other hand, XRD measurements were used for the identification of as-prepared precursors. Fig. 2 shows the XRD patterns of the UHF melted TiCr2 and LaNi5 samples together with the mechanically alloyed compounds. For TiCr2 and LaNi5 samples, the XRD measurements showed welldefined peaks corresponding to a long-range crystalline order in these samples. The XRD pattern of TiCr2 is indexed in a hexagonal structure with P63/mmc space group and the calculated unit cell parameters are a ¼ 4.9155(6) Å and c ¼ 7.9810(1) Å. For LaNi5, the pattern is indexed in a hexagonal CaCu5-type structure with P6/mmm space group and the calculated unit cell parameters are a ¼ 5.0120(1) Å and c ¼ 3.9808(9) Å. The XRD patterns of mechanically alloyed powders are shown in Fig. 2 as a function of the alloying duration. Obvious modifications are observed on the pattern after only 2 h of milling time. At this stage, the mechanical alloying process results in broadening of main diffraction peaks of the precursors. Two factors may explain such a behavior: (i) the decrease of the crystallite size and, in a lesser extent, (ii) the increase of the lattice strains [20]. The main diffraction peak of
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parameters are a ¼ 4.9713(7) Å and c ¼ 22.5607(1) Å. The main diffraction peaks of LaNi5 and TiCr2 precursors are still present on the pattern following the 2 milling hours, but their intensities are highly diminishing. It should be noticed that new diffraction pics appear at 2q ¼ 31.50 , 35.7 and 48.46 , corresponding to NiLa2O4, Fe2O3 and Cr, respectively. The formation of the AB3 phase can be explained by assuming some thermodynamic considerations. In our previous works [21,22], we reported that whatever the shock energy consumption percent, the formation of a new phase starting from precursors proceeds above a minimum power input and below a certain maximum power input. The power released by the ball to powders is the product of the frequency with the kinetic energy (Ek). It is given by the following expression [22]: P¼
1 T
ZT dEk ¼ f $Ek
(1)
0
For low shock powers, the increase of the free energy of the precursors, due to the increase of the defect concentration induced by plastic deformation, is not high enough to promote the formation of a new phase. Therefore, the input crystalline phases do not react at such shock powers. Only their grain size decreases with increasing the milling duration. However, for a medium shock power level, a new phase is able to be formed [23,24]. In fact, the free energy of the new phase (DGc(AB3)) is lower than the free energy of the mixture of the precursors, as explained by the following equation: Fig. 1 e Results from the microprobe analysis (EPMA) for TiCr2 (a) and LaNi5 (b). the new LaTi2Cr4Ni5 phase (AB3) appears upon 2 milling hours at 2q (hkl) ¼ 43.18 (116). The as-prepared AB3 compound crystallizes in a rhombohedra ordered LaMg2Ni9-type structure with R-3m space group and the calculated unit cell
Fig. 2 e XRD patterns of TiCr2 and LaNi5 precursors prepared by the UHF technique, and AB3-based samples prepared by mechanical alloying. The alloying durations were varied from 0 to 25 h.
%LaNi5 DGc(LaNi5) þ % TiCr2 DGc(TiCr2) þ DGdefect(LaNi5,TiCr2) > DGc(AB3)
(2)
where: DGc is the free energy of the crystalline phase and DGdefect is the increase in the free energy due to defects introduced by the mechanical alloying. Fig. 2 shows that, after 5 h of milling time, the intensity of the main diffraction peak of TiCr2 (2q ¼ 43 ) becomes lower than the one relative to LaTi2Cr4Ni5 (2q ¼ 44 ). When the alloying durations exceed 8 h, all the diffraction peaks of LaNi5 are completely disappeared. At this stage of milling, new peaks are observed at 2q ¼ 64.34 and 81.32 , both corresponding to Cr crystallized in a cubic structure with Im-3m space group. The XRD patterns were fitted using the Rietveld method. Numerical details of the Rietveld refinement are listed in Table 1. It should be noticed that the diffracting grain size corresponds only to the crystallized grains that diffract in the pattern. Fig. 3 presents the variation of weight content of the precursors and the formed compound. The weight content of LaNi5 sharply decreases from 3 h of milling time and becomes equal to zero after a period of 8 h. However, the TiCr2 precursor content slowly decreases and remains present even for long alloying durations. On the other hand, the weight content of LaTi2Ni5Cr4 progressively increases upon 2 h of milling and reaches its maximum value of 66% at 25 h. The AB3 phase becomes the predominant phase after 5 h. Such a result proves that the mechanical alloying technique is efficient for the formation of the AB3-type compound from TiCr2 and LaNi5 precursors. It was reported [25,26], that during the ball-milling
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Table 1 e Rietveld refinement results as a function of the milling time. Milling time (h)
0 2
3
5
8
12
20
25
Phases
LaNi5 TiCr2 LaNi5 TiCr2 AB3 Cr La2NiO4 Fe2O3 LaNi5 TiCr2 AB3 Cr La2NiO4 Fe2O3 LaNi5 TiCr2 AB3 Cr La2O3 TiCr2 AB3 Cr La2NiO4 Fe2O3 TiCr2 AB3 Cr La2O3 TiCr2 AB3 Cr La2O3 TiCr2 AB3 Cr La2O3
Content (wt%)
58.73 (0.65) 41.26 (0.65) 45.84 (0.43) 9.86 (0.48) 11.14 (0.19) 9.77 (0.32) 4.63 (0.03) 8.77 (0.29) 39.70(1.52) 16.67(1.39) 14.68( 0.79) 23.91(1.01) 2.86(0.26) 2.17( 0.17) 15.63(1.43) 20.27(0.54) 39.62(0.77) 23.20(0.41) 1.28(0.21) 17.88(1.16) 39.50(0.78) 33.53(0.52) 3.95(0.11) 0.54(0.13) 19.41( 0.07) 41.74(0.27) 33.36(0.31) 5.50(0.08) 12.63(1.66) 61.49(1.80) 16.35(1.14) 9.52(0.29) 9.01(0.60) 66.02(0.57) 15.15(0.20) 9.82(0.11)
Unit-cell parameters a (Å)
c (Å)
V (Å )
5.0120(1) 4.9155(6) 5.0120(f) 4.9155(f) 4.9713(6) 4.589(1) 3.869(6) 5.034(9) 5.0120(f) 4.9155(f) 4.9714(6) 4.588(1) 3.870(1) 5.034(9) 5.0120(f) 4.9155(f) 4.759(2) 4.572(1) 3.957(3) 4.9155(f) 4.688(2) 2.899(2) 3.931(1) 5.066(2) 4.9155(f) 4.723(4) 2.889(1) 4.011(3) 4.9155(f) 4.723(8) 2.886(2) 3.927(6) 4.9155(f) 4.718(4) 2.899(1) 3.99(1)
3.9808(9) 7.9810(1) 3.9808(f) 7.9810(f) 22.557(7) 4.589(1) 12.517(7) 13.719(5) 3.9808(f) 7.9810(f) 22.560(6) 4.588(9) 12.513(1) 13.719(5) 3.9808(f) 7.9810(f) 24.919(9) 4.572 (8) 5.823(6) 7.9810(f) 25.867(6) 2.899(2) 12.378(7) 13.536(8) 7.9810(f) 25.155(8) 2.889(1) 5.817(1) 7.9810(f) 25.835(8) 2.886(2) 6.136(9) 7.9810(f) 25.107(7) 2.899(1) 5.76(4)
86.606 167.009 86.606 167.00 482.87 96.576 187.51 301.74 86.606 167.00 482.87 96.576 187.55 301.75 86.606 167.00 488.81 95.621 79.007 167.00 493.51 24.371 384.99 301.15 167.00 486.33 24.118 81.161 167.00 478.51 24.118 81.161 167.00 484.51 24.360 79.517
process, the amount of formed phases depends on the cumulated kinetic energy of milling (Ecum) released from the ball to the powders. The value of Ecum is given by the product of the shock power by the ball milling duration value [25,26]:
RBragg
Rf
c2
Diffracting grain size (nm)
0.163 0.163 0.648 1.120 0.982 1.650 0.982 1.330 1.400 0.735 0.639 1.400 1.610 1.520 1.250 1.350 0.864 1.760 1.150 0.786 1.030 0.281 1.220 0.723 0.754 0.202 0.208 0.324 0.825 0.211 0.217 0.699 0.174 0.167 0.820 0.228
0.158 0.158 0.401 0.606 0.590 0.649 0.590 0.534 1.180 0.504 0.517 0.971 0.781 1.340 0.987 0.953 0.805 1.840 1.080 0.491 0.554 0.227 1.360 0.464 0.376 0.166 0.137 0.241 0.386 0.211 0.125 0.483 0.112 0.122 0.345 0.194
3.251
79 56 8 13 9 7 13 7 5 7 5 8 11 8 5 8 4 24 5 5 4 11 17 54 13 7 13 6 13 6 13 13 8 4 19 6
3
Ecum ¼ Pinj Dt
1.101
1.012
1.103
1.420
1.012
1.210
1.013
(3)
where Ecum is expressed in (Wh g1), Pinj is the injected shock power expressed in (W g1) and Dt is the alloying duration expressed in (h). In the present work, for Pinj equals to 6.175 W g1, when the milling time varies from 2 to 25 h, the cumulated energy varies from 12.35 to 154.37 Wh g1 and the AB3 weight content varies from 10 to 66%. It should be noticed that the Cr content remains constant at about 16% for milling times higher than 8 h.
Life time cycle of the electrodes
Fig. 3 e Variation of the weight content of the alloy components at progressive ball-milling durations.
Fig. 4 shows the effect of the milling duration on the charge/ discharge curves, the hydrogen discharge capacity and the cycling stability of the LaTi2Ni5Cr4 alloys, formed upon various milling durations. For the estimation of the specific discharge capacity of LaTi2Ni5Cr4, values are only calculated from the charge/discharge curves (Fig. 4a) for alloys prepared during at least 8 h of milling times, in order to avoid any hydrogen absorption by LaNi5. In fact, in these conditions, the LaNi5 precursor is completely consumed as affirmed by the XRD data (Figs. 2 and 3). Fig. 4a indicates that, during the charging
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Fig. 4 e (a) Galvanostatic charging/discharging curves of LaTi2Ni5Cr4 -based alloys elaborated by mechanical alloying during 8, 12, 20 and 25 h. (b) Variation of the capacity of as-prepared alloys during the galvanostatic cycling. (c) Variation of the specific capacity relative to LaTi2Ni5Cr4 during the galvanostatic cycling.
regime, the potential value decreases until 1.3 V while the amount of absorbed hydrogen increases. For the discharging regime, the potential value increases up to 0.6 V and the amount of hydrogen trapped in the alloy lattice decreases. Fig. 4b shows that the maximum discharge capacity of each alloy electrode was obtained at the first charge/discharge cycle. During the galvanostatic cycling, the capacities slightly decreased during the initial 10 cycles before being stabilized for the remaining 50 cycles. The alloys prepared upon 25 and 20 h of milling times present similar capacities, and their values are higher than those reached with the alloy prepared during 8 and 12 milling hours. This result demonstrates that the discharge capacity increases when the LaTi2Ni5Cr4 content increases. Hence, the reversible absorption of hydrogen during the charge/discharge cycling only occurs in the host sites of the LaTi2Ni5Cr4 structure. On the other hand, the average capacity retention (C60/Cmax) of the alloy electrodes calculated upon 60 cycles is equal to 77%. Such a good cycling behavior demonstrates that LaTi2Ni5Cr4 is highly stable and may be beneficial to practical applications comparatively to LaNi5 and other AB3-type compounds investigated elsewhere [27,28]. For example, after 60 cycles of charging and discharging the AB3-phase (La0.67Mg0.33)1xTixNi2.75Co0.25, a poor capacity retention of 41% was obtained for x ¼ 0.2 [29]. In the same way, the capacity retention estimated for the AB3-phase La0.67Mg0.18Ca0.15Ni2.75Co0.25 is about 46% after 80 charge/ discharge cycles [30]. The discharge capacity is calculated in Fig. 4c according to the weight content of LaTi2Ni5Cr4, which was evaluated by the Rietveld refinement from the
corresponding XRD pattern (Fig. 3). Independently on the milling duration, LaTi2Ni5Cr4 exhibits nearly the same discharge capacity that remains stable at a value of 80 mAh g1.
Cyclic voltammetry Fig. 5 shows the cyclic voltammogram (CV) curves of LaTi2Ni5Cr4 -based alloys carried out after 60 charge/discharge activation cycles at scan rates varying between 10 and 60 mV s1. These LaTi2Ni5Cr4 -based materials were obtained by mechanical alloying after 8 h (Fig. 5a) and 20 h (Fig. 5b). During the cathodic scan, an onset of the hydrogen evolution reaction is observed at about 0.97 V. Then, the current decreases continuously until 1.1 V. When the potential is reversed to the opposite direction, the current increases and the observed anodic peak is corresponding to the oxidation of the hydrogen atoms absorbed in the composite lattice [31,32]. The anodic peak current increases and the potential slightly shifts to the positive direction with increasing the potential scan rate (w). Fig. 6 demonstrates that the anodic peak potential (Eap), determined at the maximum current, has a linear dependence on log (w). In this case, the anodic peak potential can be given by the following equation [33,34]: dEap 2:3RT ¼ anF dlogðwÞ
(4)
where R is the constant of rare gas, T the temperature of the electrochemical cell, a the charge transfer coefficient, n the
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Fig. 5 e Cyclic voltammograms of the LaTi2Ni5Cr4 -based electrode obtained at potential rates of 10, 20, 30, 40, 50 and 60 mV s¡1. LaTi2Ni5Cr4 was mechanically alloyed during (a) 8 h and (b) 20 h.
spontaneous only for positive current due to the positive Ni(OH)2/NiOOH electrode potential (þ0.49 V), which is higher than the one of MH/M negative electrode potential. Thus, the current of the oxidation anodic peak can be expressed as follows [33,34]: Iap ¼ 0:496a1=2 ðnFÞ3=2 SCo
1=2 wD RT
(5)
where S is the geometric surface of the electrode (cm2), Co the concentration of diffusion species (mol cm3), and D the hydrogen diffusion coefficient (cm2 s1). The concentration Co is calculated by Eq. (6): CO ¼ Fig. 6 e Variation of the anodic peak potential of cyclic voltammograms of the LaTi2Ni5Cr4 -based electrodes as a function of log (y). The LaTi2Ni5Cr4 compounds were mechanically alloyed during 8 h (open squares) and 20 h (open circles).
QM mFVM
(6)
where M is the molar mass, m the effective mass of material, VM the molecular volume of material and Q the charge of anodic oxidation of hydrogen generated during the anodic sweep of cyclic voltammetry. According to Eq. (5), the hydrogen diffusion coefficient in the as-prepared metal
number of exchanged electron, and F the Faraday constant. Based on this formula, the charge transfer coefficient was calculated and presented in Table 2 for LaTi2Ni5Cr4 compounds prepared during 8 and 20 h of milling times. On the other hand, Fig. 7 shows a linear variation of the anodic peak current with the square root of the scan rate (w1/2). Such a behavior reflects a semi-infinite hydrogen diffusion and an irreversible charge transfer. The discharging process is
Table 2 e The charge transfer “a”, the hydrogen diffusion coefficient “D” and the sphere radius of the hydride alloy particle “a” for the LaTi2Ni5Cr4 -based alloys milled during 8 and 20 h. Milling time (h) 8 20
a
D (cm2 s1)
a (mm)
0.34 0.37
4.18 108 7.17 108
2.6 4.8
Fig. 7 e Variation of the anodic peak current of the cyclic voltammograms of the LaTi2Ni5Cr4 -based electrodes as a function of square root of potential scan rate. The LaTi2Ni5Cr4 compounds were mechanically alloyed during 8 h (open squares) and 20 h (open circles).
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hydride electrodes can be deduced from the slope of the curve Iap ¼ f(w1/2) (Fig. 7). The values of a and DH (Table 2) indicate that, at ambient temperature and pressure, the electrochemical reaction is characterized by a suitable kinetic in the LaTi2Ni5Cr4 material in terms of reversibility and diffusivity of hydrogen. In fact, a values ranging from 0.3 to 0.7 can be approximated to 0.5 [33,34], which means that the charge and discharge reactions are reversible and the system presents the same tendency for the charge as for the discharge process [33,34]. On the other hand, the hydrogen diffusion coefficients of as-prepared LaTi2Ni5Cr4 -based alloys, varying between 4.18 108 and 7.17 108 cm2 s1, are much higher than those reported in the literature [31,32,35,36], indicating that the hydrogen diffusion rate is more facilitated in the LaTi2Ni5Cr4 lattice. In fact, the values of DH reported by Liao et al. for LaxMg3xNi9 (x ¼ 1.6e2.0) range from 7 1010 to 9 1010 cm2 s1 [37], while those reported for La2Mg(Ni0.8x Co0.2 Alx)9 (x ¼ 0e0.03) range from 2 1010 to 4 1010 cm2 s1 [38].
Chronoamperometry While the hydride alloy particles are supposed to present a spherical shape [39,40], the diffusion of hydrogen in the bulk material can be expressed, in the spherical coordinate, as follows: vðrCÞ v2 ðrCÞ ¼D vt vr2
(7)
where C is the hydrogen concentration in the alloy, t the time, D the average diffusion coefficient of hydrogen in the bulk and r the distance from the center of the sphere. Eq. (8) gives the solution of the diffusion equation established by Crank as indicated by Weixiang [40], under different boundaries, as: ∞ C C0 2a X ð 1Þn npr D n2 p2 t exp sin ¼1þ 2 pr n¼1 a a Cs C0 n
(8)
where a is the sphere radius, Co and Cs are the initial hydrogen concentrations in the bulk material and the alloy surface, respectively. The resulting diffusion current varies with time as follows: ∞ X 6FD n2 p2 Dt exp i ¼ ± 2 ðC CoÞ 2 da a n¼1
(9)
where d is the density of the material. The ± sign indicates the charge state for the minus sign and the discharge state for the plus sign. For a long time, Eq. (9) can be written accordingly: 6FD p2 D t logðiÞ ¼ log ± 2 ðCo CsÞ da 2:303a2
Fig. 8 e Variation of the diffusion current of LaTi2Ni5Cr4 -based electrode as a function of time after 60 charge/ discharge activation cycles. LaTi2Ni5Cr4 was mechanically alloyed during (a) 8 h and (b) 20 h.
(10)
The D=a2 ratio can be calculated from the slope the plot of log(i) versus time t (Fig. 8). Taking into account that the value of the hydrogen diffusion coefficient “D” was determined by cyclic voltammetry (Table 2), the radius “a” of the particles involved in the electrochemical reaction can be deduced from the value of this slope. Fig. 8 shows the semi-logarithmic curves for the mechanically alloyed LaTi2Ni5Cr4 -based hydrogen storage alloy. After the application of a constant potential of 0.6 V, a drastic decrease of current is obtained indicating a depletion of hydrogen on the composite surface
[36]. Each plot can be divided into two regions: for time t < 3000 s, the current decreases rapidly under a chargetransfer control [41]. For longer time t > 3000 s, the diffusion current decreases linearly with time, reflecting that the diffusion current obeys to Eq. (10). Table 2 indicates that the average radius of the LaTi2Ni5Cr4 -based alloy involved in the electrochemical reaction is about 2.6 and 4.8 mm after 8 and 20 h of milling time, respectively. As mentioned in the Experimental Section: prior to the charge/ discharge process, the alloy ingots were ground mechanically and their diameter saved to less than 63 mm in a glove box for the preparation of the negative electrode. Hence, It is assumed that the decrease of the particle size during long-term cycling and repetitive hydriding is due to the pulverization of powders induced by the expansion the lattice volume [31,40]. The particle size of LaTi2Ni5Cr4 -based alloys involved in the electrochemical reaction is lower than those reported for other AB3 etype alloys elaborated with melting techniques by Mokbli et al. (19e31 mm) [34] and Zheng et al. (15 mm) [39]. Such a result emphasizes the benefit effect of the mechanical alloying process for controlling the particle size comparatively to the melting techniques.
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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
Conclusions New AB3-type rich alloys, consisting in LaTi2Ni5Cr4 -based materials, were successfully elaborated by using the mechanical alloying technique. The synthesis reaction was performed from pure AB5-type (LaNi5) and AB2-type (TiCr2) intermetallic precursors. The increase of the milling duration enhances the weight content of LaTi2Ni5Cr4 in the alloy composition and, in the same way, the overall hydrogen absorption capacity of the system. For an injected shock power of 6.175 W g1, the variation of the milling time from 2 to 25 h results in an increase of the cumulated energy from 12.35 to 154.37 Wh g1, accompanied by an enhancement of the AB3 weight content from 10 to 66%. The refinement of corresponding XRD patterns by the Rietveld method confirms that the LaNi5 precursor was completely consumed after 8 h of milling time. However, 10 wt% of residual TiCr2 precursor was present in the alloy composition after 25 h of milling time. The cycle-life investigation of as-prepared LaTi2Ni5Cr4 -based materials shows a promising electrochemical stability following 60 charge/discharge cycles in alkaline medium. The values of the charge transfer coefficient of the alloys obtained after 8 and 20 milling hours are, respectively, equal to 0.34 and 0.37. On the other hand, the values of the hydrogen diffusion coefficient for the alloys obtained after 8 and 20 milling hours are equal to 4.18 108 and 7.17 108 cm2 s1, respectively. These results indicate a good absorptionedesorption kinetic and a high hydrogen diffusivity through the LaTi2Ni5Cr4 rich alloys. However, the specific discharge capacity of LaTi2Ni5Cr4 was estimated at 80 mAh g1 and further works are needed to improve such hydrogenation capacity.
Acknowledgments The authors would like to thank the Laboratory of Chimie tallurgique des Terres Rares (CMTR), the Institut de Chimie Me riaux Paris-Est (ICMPE) and the Centre National et des Mate pour la Recherche Scientifique (CNRS, Thiais) for the elaboration of LaNi5 and TiCr2 precursors by UHF induction melting, rie Paul-Boncour and Junxian and Pr. Michel Latroche, Dr. Vale Zhang for helpful discussions.
references
[1] Li Yiming, Ren Huiping, Zhang Yanghuan, Liu Zhuocheng, Zhang Hanwei. Hydrogen induced amorphization behaviors of multiphase La0.8Mg0.2Ni3.5 alloy. Int J Hydrogen Energy 2015;40:7093e102. gan A. [2] Cuevas F, Joubert JM, Latroche M, Parcheron-Gue Intermetallic compounds as negative electrodes of Ni/MH batteries. J Appl Phys 2001;72:225e38. [3] Marzouki M, Ghodbane O, Abdellaoui M. Study of hydrogen absorption kinetics of Mg2Ni-based powders produced by high-injected shock power mechanical alloying and subsequent annealing. Mater Renew Sustain 2013;2:9. [4] Seta S, Uchida H. Hydrogen solubility in LaNi5. J Alloy Compd 1995;231:448e53.
10941
[5] Uchida H, Sato M, Moriwaki O. The hydrogen absorption and desorption isotherms in the solid solution regions of the LaNi5-H system. J Alloy Compd 1997;253e254:235e7. [6] Termsuksawad P, Niyomsoan S, Mishra AAB, Olson Z, Gavra Z, Kaydanov VI. Prediction of hydrogen absorption behavior in AB5 hydrogen storage alloys by electronic techniques. Mater Sci Eng 2005;117:45e51. [7] Chumphongphan S, Paskevicius M, Sheppard Drew A, Buckley CE. Cycle life and hydrogen storage properties of mechanical alloyed Ca1xZrxNi5y Cry; (x ¼ 0, 0.05 and y ¼ 0, 0.1). Int J Hydrogen Energy 2012;37:7586e93. [8] Giza K, Iwasieczko W, Pavlyuk VV, Bala H, Drulis H. Thermodynamical properties of LaeNieT (T ¼ Mg, Bi and Sb) hydrogen storage systems. J Power Sources 2008;181:38e40. [9] Han SM, Zhao MS, Zhang Z, Zhang YZ, Jing TF. Effect of AB2 alloy addition on the phase structures and electrochemical characteristics of LaNi5 hydride electrode. J Alloy Compd 2005;392:268e73. [10] Johnson JR, Reilly JJ. Reaction of hydrogen with the lowtemperature form (C15) of titanium-chromium (TiCr2). Inorg Chem 1987;17:3103e8. [11] Johnson JR. Reaction of hydrogen with the high temperature (C14) form of TiCr2. J Less-Common Met 1980;73:345e54. gan A. Structural and [12] Latroche M, Percheron-Gue thermodynamic studies of some hydride forming RM3-type compounds (R ¼ lanthanide, M ¼ transition metal). J Alloy Compd 2003;356e357:461e8. [13] Kadir K, Sakai T. Structural investigation and hydrogen storage capacity of LaMg2Ni9 and (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9 of the AB2C9 type structure. J Alloy Compd 2003;302:112e7. [14] Chen J, Takeshita HT, Tanaka H, Kuriyama N, Sakai T, Uehara I, et al. Hydriding properties of LaNi3 and CaNi3 and their substitutes with PuNi3-type structure. J Alloy Compd 2000;302:304e13. [15] Pan HG, Liu YF, Gao MX, Lei YQ, Wang QD. Electrochemical properties of the La0.7Mg0.3Ni2.65xMn0.1Co0.75(x ¼ 0e0.5) hydrogen storage alloy electrodes. J Electrochem Soc 2005;152:326e32. [16] Lim Kean Long, Liu Yongning, Zhang Qing-An, Chan Sammy Lap Ip. Effects of partial substitutions of cerium and aluminum on the hydrogenation properties of La(0.65x)CexCa1.03Mg1.32Ni(9y) Aly alloy. Int J Hydrogen Energy 2014;39:10537e45. [17] Rodriguez-Carvajal J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys B 1993;192:55e69. gan A. [18] Mathlouthi H, Lamloumi J, Latroche M, Percheron-Gue Study of poly- substituted intermetallic hydrides electrochemical applications. Ann Chim Sci Matter 1997;22:241e4. [19] Chuan-Jian L, Feng-Rong W, Wen-Hao C, Wei L, Wen-Tong Z. The influence of high-rate quenching on the cycle stability and the structure of the AB -type hydrogen storage alloys with different Co5 content. J Alloy Compd 2001;315:218e23. [20] Krill III CE, Haberkorn R, Birringer R. Fabrication and spectroscopic characterization of organic nanocrystals. In: Nalwa HS, editor. Handbook of nanostructured materials and nanotechnology, Chap. 8. San Diego: Academic Press; 2000. p. 387e412. E-Publishing Inc. [21] Abdellaoui M, Gaffet E. A mathematical and experimental dynamical phase diagram for ballmilled Ni10Zr7. J Alloy Compd 1994;209:351e61. [22] Abdellaoui M, Gaffet E. The physics of mechanical alloying in a planetary ball mill: mathematical treatment. Acta Metal Mater 1995;43:1087e98. [23] Brimhall L, Kissinger HE. Amorphous phase formation in irradiated intermetallic compounds. Radiat Eff 1983;77:273e93.
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[24] Schwarz RB, Koch CC. Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and powders of intermetallics. Appl Phys Lett 1986;49:146. [25] Dehouche Z, Djaozandry R, Goyette J, Bose TK. Evaluation techniques of cycling effect on thermodynamic and crystal structure properties of Mg2Ni alloy. J Alloy Compd 1999;288:269e76. [26] Nohara S, Inoue H, Fukumoto Y. Compositional and structural characteristics of MgNi alloy prepared by mechanical alloying for use as negative electrodes in nickelmetal hydride batteries. J Alloy Compd 1997;259:183e5. [27] Liu Y, Pan H, Gao M, Miao H, Lei Y, Wang Q. Function of Al on the cycling behavior of the LaeMgeNieCo-type alloy electrodes. Int J Hydrogen Energy 2008;33:124e33. [28] Zhang YH, Li BW, Ren HP, Cai Y, Dong XP, Wang XL. Cycle stabilities of the La0.7Mg0.3Ni2.55xCo0.45Mx (M ¼ Fe, Mn, Al; x ¼ 0, 0.1) electrode alloys prepared by casting and rapid quenching. J Alloy Compd 2008;458:340e5. [29] Zhenwei D, Yaoming W, Liqun M, Xiao S, LiMin DW. Influences of low-Ti substitution for La and Mg on the electrochemical and kinetic characteristics of AB3-type hydrogen storage alloy electrodes. Sci China Ser E 2010;53:242e7. [30] Zhenwei D, Yaoming W, Liqun M, Lidong W, Xiaodong S, Limin W. Microstructure and electrochemical hydrogen storage characteristics of La0.67Mg0.33xCaxNi2.75Co0.25 (x ¼ 0e0.15) electrode alloys. Int J Hydrogen Energy 2011;36:3050e5. [31] Ben Moussa M, Abdellaoui M, Mathlouthi H, Lamloumi J, gan A. Investigation of the cycle stability and Percheron-Gue diffusivity of hydrogen in the MmNi3.55Mn0.4Al0.3Co0.6Fe0.15 compound. J Alloy Compd 2006;407:256e62. [32] Ben Moussa M, Abdellaoui M, Mathlouthi H, Lamloumi J, gan A. Electrochemical properties of the Percheron-Gue MmNi3.55Mn0.4Al0.3Co0.75xFex (x ¼ 0.55 and 0.75) compounds. J Alloy Compd 2008;458:410e4.
thodes et Applications. [33] Bard AJ, Fulkner LR. Principes, Me Paris: Masson; 1998. p. 249. E- Publishing Inc. [34] Mokbli S, Abdellaoui M, Zarrouk H, Latroche M, Percherongan A. Hydriding and electrochemical properties of Gue amorphous rich MgxNi100x nanomaterial obtained by mechanical alloying starting from Mg2Ni and MgNi2. J Alloy Compd 2008;460:432e9. [35] Ben Moussa M, Abdellaoui M, Mathlouthi H, Lamloumi J, gan A. Investigation on the structure, Percheron-Gue thermodynamic and electrochemical properties of the MmNi3.55Mn0.4Al0.3Fe0.75 compound used as negative electrode in NieMH batteries. J Alloy Compd 2013;575:414e8. [36] Zhao X, Ding Y, Yang M, Ma L. Electrochemical properties of MmNi3.8Co0.75Mn0.4Al0.2 hydrogen storage alloy modified with nanocrystalline nickel. Int J Hydrogen Energy 2008;33:6727e33. [37] Liao B, Lei YQ, Chena LX, Lub GL, Pan HG, Wang QD. Effect of the La/Mg ratio on the structure and electrochemical properties of LaxMg3xNi9 (x ¼ 1.6e2.2) hydrogen storage electrode alloys for nickelemetal hydride batteries. J Power Sources 2004;129:358e67. [38] Liao B, Lei YQ, Chena LX, Lub GL, Pan HG, Wang QD. The structural and electrochemical properties of La2Mg(Ni0.8xCo0.2Alx)9 (x ¼ 0e0.03) hydrogen storage electrode alloys. J Alloy Compd 2006;415:239e43. [39] Zheng G, Popov BN, White RE. Electrochemical determination of the diffusion coefficient of hydrogen through an LaNi4.25Al0.75 electrode in alkaline aqueous solution. J Electrochem Soc 1995;142:2695e8. [40] Weixiang C. Cyclic voltammetry and electrochemical impedance of MmNi3.6Co0.7Mn0.4Al0.3 alloy electrode before and after treatment with a hot alkaline solution containing reducing agent. J Power Sources 2000;90:201e5. [41] Mathlouthi H, Khaldi C, Ben Moussa M, Lamloumi J, gan A. Electrochemical study of monoPercheron-Gue substituted and poly-substituted intermetallic hydrides. J Alloy Compd 2004;375:297e304.
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Elaboration and electrochemical characterization of LaTi2Cr4Ni5 -based metal hydride alloys Marwa Ayari, Ouassim Ghodbane*, Mohieddine Abdellaoui Institut National de Recherche et d'Analyse Physico-Chimique, Laboratoire des Materiaux Utiles, P^ole Technologique de Sidi Thabet, 2020 Sidi Thabet, Tunisia
article info
abstract
Article history:
The present paper focuses on the elaboration of LaTi2Cr4Ni5 -based intermetallic com-
Received 21 March 2015
pounds by mechanical alloying from LaNi5 and TiCr2 precursors and their characterization
Received in revised form
as negative electrode materials in Nickel-metal hydride (Ni-MH) batteries. The structural
26 June 2015
properties of the phases were determined by X-ray diffraction and quantified from the
Accepted 29 June 2015
Rietveld refinement data. The increase of the milling time up to 25 h leads to the highest
Available online 26 July 2015
abundance of the LaTi2Cr4Ni5 phase, estimated at 66 wt%, and a complete elimination of the LaNi5 intermetallic precursor. The electrochemical techniques were applied to char-
Keywords:
acterize the electrochemical behavior of prepared LaTi2Cr4Ni5 -based compounds. The
Ni-metal hydride battery
maximum discharge capacity was 115 mAh g1 and the average capacity retention was
Mechanical alloying
equal to 77% upon 60 charge/discharge cycles, indicating a high electrochemical stability in
Hydrogen absorptionedesorption
the alkaline solution. The values of the hydrogen diffusion coefficient are equal to
properties
4.18 108 and 7.17 108 cm2 s1 after 8 h and 20 h of milling durations, respectively.
Discharge capacity
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Research on Ni-MH batteries knows a significant growth due to their superior safety and endurance as hydrogen storage devices [1]. The distinct alloy families that have been investigated as negative electrodes for these batteries are AB5, AB2, AB and AB0.5 [2,3], where A represents a metallic element with a strong affinity for hydrogen (a transition metal or rare-earth element), and B a metallic element with a weak affinity for hydrogen (a last transition metal). Intermetallic compounds formed with A and B elements are able to reversibly absorb hydrogen. Concerning AB5-type materials, LaNi5 is the most popular compound investigated as a hydrogen storage material [4e7]. The hydrogenation of LaNi5 leads to the ternary
hydride compound LaNi5H6, with a hydrogen equilibrium pressure of 1.7 atm at the room temperature [8,9]. Unfortunately, its poor hydrogenation at low temperatures, its low kinetic and its short cycle life have a significant impact on the battery commercialization. On the other hand, among AB2type materials, TiCr2 alloys attracted a great attention as a potential candidate for the hydrogen storage [10,11]. They react directly and reversibly with hydrogen to form two nonstoichiometric hydride phases having the nominal compositions of TiCr1.9H2.5 and TiCr1.9H3.5 [11]. Unfortunately, their corresponding plateau pressures at 78 C are equal to 0.2 and 30 atm, respectively [11]. Furthermore, AB and AB0.5 -type compounds are the least adapted for electrochemical applications since the hydrogen desorption is hindered by the high stability of their corresponding hydrides.
* Corresponding author. Tel.: þ216 71 537 666; fax: þ216 71 537 688. E-mail address: [email protected] (O. Ghodbane). http://dx.doi.org/10.1016/j.ijhydene.2015.06.169 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
In the last decade, a number of studies demonstrated that AB3-type alloys are promising alternatives for the reversible hydrogen storage [12e14]. They gained a considerable development owing to their high capacity of hydrogen storage and good electrochemical properties compared to AB5-type alloys [15,16]. In this context, the AB3-type structure can be prepared from the intergrowth of AB5-type compounds with AB2 units following the reaction scheme [12]:
AB5 þ 2 AB2 / 3 AB3 (1) This approach aims to overcome the drawbacks of AB2 and AB5 precursors and, in the same way, to take profit from their hydrogenation characteristics in order to obtain new materials with enhanced hydrogen storage properties. In the present work, we investigate the electrochemical hydrogenation of a novel AB3-type LaTi2Cr4Ni5 -based alloys. To the best of our knowledge, LaTi2Cr4Ni5 -based materials are investigated for the first time in the literature. They crystallize in the rhombohedral structure of PuNi3-type with a long-range stacking arrangement similar to either AB5 or AB2 alloys [1]. LaTi2Cr4Ni5 was prepared by mechanical alloying from LaNi5 and TiCr2 intermetallic precursors. The crystal structure was determined and refined from X-ray powder diffraction data. The hydrogen absorption/desorption properties were determined by electrochemical measurements.
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XPERT PRO MPD diffractometer operating with Cu Ka radiation (l ¼ 0.15406 nm). Diffraction patterns were analyzed by the Rietveld refinement method using the FullProf software [17]. The powder morphology of the samples was characterized with a FEI Quanta 200 environmental scanning electron microscope (SEM). For the electrochemical measurements, the alloys ingot was ground mechanically and saved to less than 63 mm in the glove box under argon atmosphere. The working electrodes were prepared according to the “latex” technology [18]: 90% of the alloy powder is mixed with 5% of black carbon, to increase the electrode conductivity, and 5% of polytetrafluoroethylene (PTFE) as a binder. An overall surface of 1 cm2 of this latex was pressed on each side of a nickel grid to prevent the electrode plate from breaking into pieces during the chargeedischarge cycling [19]. This set forms the negative electrode of Ni/MH accumulator. Electrochemical tests were performed with a VMP system and using a conventional threeelectrode cell, in the presence of an aqueous 1 M KOH electrolyte solution, which was deoxygenated by bubbling Ar gas during every measurement. The counter electrode was formed by Ni oxyhydroxide Ni(OH)2, whereas the reference electrode was Hg/HgO immersed in 1 M KOH solution, prepared with deionized water. The discharge capacity of the electrode was determined by a galvanostatic chargingedischarging at C/3 and D/6 regime, respectively. Every cycle was carried out by charging fully to 1.3 V and discharging to 0.6 V vs. Hg/HgO at room temperature.
Results and discussion Experimental section Structural characterizations Starting alloys LaNi5 and TiCr2 were synthesized by ultra high frequency (UHF) induction melting of the pure elements (La 99.9%, Ni 99.9%, Ti 99.99%, Cr 99.99%) into a water cooled copper crucible under argon atmosphere. Every alloy was melted five times and inverted between each melting to ensure homogeneity. The chemical composition of the intermetallic precursors was determined by Electron Probe Micro Analysis (EPMA) in a Cameca SX-100 device. A sample was taken from each alloy and polished for optical and electronic metallographic examination and EPMA. The composite LaTi2Cr4Ni5 was made by mechanical alloying (MA) following this reaction:
LaNi5þ 2 TiCr2 / LaTi2Cr4Ni5 (2) A mixture of the precursors LaNi5 (0.59 g) and TiCr2 (0.42 g), with a molecular ratio of 1:2, was sealed into a stainless steel vial (50 cm3 in volume) with 5 stainless steel balls (12 mm in diameter and 7.0 g in mass) in a glove box filled with purified argon gas. The ball-to-powder weight ratio was equal to 35:1. The MA experiments were performed at room temperature using a Retsh PM400 planetary ball miller. The disc rotation speed and the vial rotation speed were equal to 320 and 640 rpm, respectively. These milling conditions correspond to an injected shock power of 6.175 W g1. The crystallographic characterization of synthesized powders was carried out by XRD using a (qe2q) Panalytical
The microprobe analyses of TiCr2 and LaNi5 are presented in Fig. 1(a,b), and show a constant composition for both samples. The expected compositions of TiCr2 and LaNi5 are x ¼ B/A ¼ 2/ 1 and 5/1, respectively. The EPMA allows to confirm that phase compositions observed for both compounds are TiCr(1.9 ± 0.2) and LaNi(5.5 ± 0.8), in agreement with the expected stoichiometries. On the other hand, XRD measurements were used for the identification of as-prepared precursors. Fig. 2 shows the XRD patterns of the UHF melted TiCr2 and LaNi5 samples together with the mechanically alloyed compounds. For TiCr2 and LaNi5 samples, the XRD measurements showed welldefined peaks corresponding to a long-range crystalline order in these samples. The XRD pattern of TiCr2 is indexed in a hexagonal structure with P63/mmc space group and the calculated unit cell parameters are a ¼ 4.9155(6) Å and c ¼ 7.9810(1) Å. For LaNi5, the pattern is indexed in a hexagonal CaCu5-type structure with P6/mmm space group and the calculated unit cell parameters are a ¼ 5.0120(1) Å and c ¼ 3.9808(9) Å. The XRD patterns of mechanically alloyed powders are shown in Fig. 2 as a function of the alloying duration. Obvious modifications are observed on the pattern after only 2 h of milling time. At this stage, the mechanical alloying process results in broadening of main diffraction peaks of the precursors. Two factors may explain such a behavior: (i) the decrease of the crystallite size and, in a lesser extent, (ii) the increase of the lattice strains [20]. The main diffraction peak of
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parameters are a ¼ 4.9713(7) Å and c ¼ 22.5607(1) Å. The main diffraction peaks of LaNi5 and TiCr2 precursors are still present on the pattern following the 2 milling hours, but their intensities are highly diminishing. It should be noticed that new diffraction pics appear at 2q ¼ 31.50 , 35.7 and 48.46 , corresponding to NiLa2O4, Fe2O3 and Cr, respectively. The formation of the AB3 phase can be explained by assuming some thermodynamic considerations. In our previous works [21,22], we reported that whatever the shock energy consumption percent, the formation of a new phase starting from precursors proceeds above a minimum power input and below a certain maximum power input. The power released by the ball to powders is the product of the frequency with the kinetic energy (Ek). It is given by the following expression [22]: P¼
1 T
ZT dEk ¼ f $Ek
(1)
0
For low shock powers, the increase of the free energy of the precursors, due to the increase of the defect concentration induced by plastic deformation, is not high enough to promote the formation of a new phase. Therefore, the input crystalline phases do not react at such shock powers. Only their grain size decreases with increasing the milling duration. However, for a medium shock power level, a new phase is able to be formed [23,24]. In fact, the free energy of the new phase (DGc(AB3)) is lower than the free energy of the mixture of the precursors, as explained by the following equation: Fig. 1 e Results from the microprobe analysis (EPMA) for TiCr2 (a) and LaNi5 (b). the new LaTi2Cr4Ni5 phase (AB3) appears upon 2 milling hours at 2q (hkl) ¼ 43.18 (116). The as-prepared AB3 compound crystallizes in a rhombohedra ordered LaMg2Ni9-type structure with R-3m space group and the calculated unit cell
Fig. 2 e XRD patterns of TiCr2 and LaNi5 precursors prepared by the UHF technique, and AB3-based samples prepared by mechanical alloying. The alloying durations were varied from 0 to 25 h.
%LaNi5 DGc(LaNi5) þ % TiCr2 DGc(TiCr2) þ DGdefect(LaNi5,TiCr2) > DGc(AB3)
(2)
where: DGc is the free energy of the crystalline phase and DGdefect is the increase in the free energy due to defects introduced by the mechanical alloying. Fig. 2 shows that, after 5 h of milling time, the intensity of the main diffraction peak of TiCr2 (2q ¼ 43 ) becomes lower than the one relative to LaTi2Cr4Ni5 (2q ¼ 44 ). When the alloying durations exceed 8 h, all the diffraction peaks of LaNi5 are completely disappeared. At this stage of milling, new peaks are observed at 2q ¼ 64.34 and 81.32 , both corresponding to Cr crystallized in a cubic structure with Im-3m space group. The XRD patterns were fitted using the Rietveld method. Numerical details of the Rietveld refinement are listed in Table 1. It should be noticed that the diffracting grain size corresponds only to the crystallized grains that diffract in the pattern. Fig. 3 presents the variation of weight content of the precursors and the formed compound. The weight content of LaNi5 sharply decreases from 3 h of milling time and becomes equal to zero after a period of 8 h. However, the TiCr2 precursor content slowly decreases and remains present even for long alloying durations. On the other hand, the weight content of LaTi2Ni5Cr4 progressively increases upon 2 h of milling and reaches its maximum value of 66% at 25 h. The AB3 phase becomes the predominant phase after 5 h. Such a result proves that the mechanical alloying technique is efficient for the formation of the AB3-type compound from TiCr2 and LaNi5 precursors. It was reported [25,26], that during the ball-milling
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Table 1 e Rietveld refinement results as a function of the milling time. Milling time (h)
0 2
3
5
8
12
20
25
Phases
LaNi5 TiCr2 LaNi5 TiCr2 AB3 Cr La2NiO4 Fe2O3 LaNi5 TiCr2 AB3 Cr La2NiO4 Fe2O3 LaNi5 TiCr2 AB3 Cr La2O3 TiCr2 AB3 Cr La2NiO4 Fe2O3 TiCr2 AB3 Cr La2O3 TiCr2 AB3 Cr La2O3 TiCr2 AB3 Cr La2O3
Content (wt%)
58.73 (0.65) 41.26 (0.65) 45.84 (0.43) 9.86 (0.48) 11.14 (0.19) 9.77 (0.32) 4.63 (0.03) 8.77 (0.29) 39.70(1.52) 16.67(1.39) 14.68( 0.79) 23.91(1.01) 2.86(0.26) 2.17( 0.17) 15.63(1.43) 20.27(0.54) 39.62(0.77) 23.20(0.41) 1.28(0.21) 17.88(1.16) 39.50(0.78) 33.53(0.52) 3.95(0.11) 0.54(0.13) 19.41( 0.07) 41.74(0.27) 33.36(0.31) 5.50(0.08) 12.63(1.66) 61.49(1.80) 16.35(1.14) 9.52(0.29) 9.01(0.60) 66.02(0.57) 15.15(0.20) 9.82(0.11)
Unit-cell parameters a (Å)
c (Å)
V (Å )
5.0120(1) 4.9155(6) 5.0120(f) 4.9155(f) 4.9713(6) 4.589(1) 3.869(6) 5.034(9) 5.0120(f) 4.9155(f) 4.9714(6) 4.588(1) 3.870(1) 5.034(9) 5.0120(f) 4.9155(f) 4.759(2) 4.572(1) 3.957(3) 4.9155(f) 4.688(2) 2.899(2) 3.931(1) 5.066(2) 4.9155(f) 4.723(4) 2.889(1) 4.011(3) 4.9155(f) 4.723(8) 2.886(2) 3.927(6) 4.9155(f) 4.718(4) 2.899(1) 3.99(1)
3.9808(9) 7.9810(1) 3.9808(f) 7.9810(f) 22.557(7) 4.589(1) 12.517(7) 13.719(5) 3.9808(f) 7.9810(f) 22.560(6) 4.588(9) 12.513(1) 13.719(5) 3.9808(f) 7.9810(f) 24.919(9) 4.572 (8) 5.823(6) 7.9810(f) 25.867(6) 2.899(2) 12.378(7) 13.536(8) 7.9810(f) 25.155(8) 2.889(1) 5.817(1) 7.9810(f) 25.835(8) 2.886(2) 6.136(9) 7.9810(f) 25.107(7) 2.899(1) 5.76(4)
86.606 167.009 86.606 167.00 482.87 96.576 187.51 301.74 86.606 167.00 482.87 96.576 187.55 301.75 86.606 167.00 488.81 95.621 79.007 167.00 493.51 24.371 384.99 301.15 167.00 486.33 24.118 81.161 167.00 478.51 24.118 81.161 167.00 484.51 24.360 79.517
process, the amount of formed phases depends on the cumulated kinetic energy of milling (Ecum) released from the ball to the powders. The value of Ecum is given by the product of the shock power by the ball milling duration value [25,26]:
RBragg
Rf
c2
Diffracting grain size (nm)
0.163 0.163 0.648 1.120 0.982 1.650 0.982 1.330 1.400 0.735 0.639 1.400 1.610 1.520 1.250 1.350 0.864 1.760 1.150 0.786 1.030 0.281 1.220 0.723 0.754 0.202 0.208 0.324 0.825 0.211 0.217 0.699 0.174 0.167 0.820 0.228
0.158 0.158 0.401 0.606 0.590 0.649 0.590 0.534 1.180 0.504 0.517 0.971 0.781 1.340 0.987 0.953 0.805 1.840 1.080 0.491 0.554 0.227 1.360 0.464 0.376 0.166 0.137 0.241 0.386 0.211 0.125 0.483 0.112 0.122 0.345 0.194
3.251
79 56 8 13 9 7 13 7 5 7 5 8 11 8 5 8 4 24 5 5 4 11 17 54 13 7 13 6 13 6 13 13 8 4 19 6
3
Ecum ¼ Pinj Dt
1.101
1.012
1.103
1.420
1.012
1.210
1.013
(3)
where Ecum is expressed in (Wh g1), Pinj is the injected shock power expressed in (W g1) and Dt is the alloying duration expressed in (h). In the present work, for Pinj equals to 6.175 W g1, when the milling time varies from 2 to 25 h, the cumulated energy varies from 12.35 to 154.37 Wh g1 and the AB3 weight content varies from 10 to 66%. It should be noticed that the Cr content remains constant at about 16% for milling times higher than 8 h.
Life time cycle of the electrodes
Fig. 3 e Variation of the weight content of the alloy components at progressive ball-milling durations.
Fig. 4 shows the effect of the milling duration on the charge/ discharge curves, the hydrogen discharge capacity and the cycling stability of the LaTi2Ni5Cr4 alloys, formed upon various milling durations. For the estimation of the specific discharge capacity of LaTi2Ni5Cr4, values are only calculated from the charge/discharge curves (Fig. 4a) for alloys prepared during at least 8 h of milling times, in order to avoid any hydrogen absorption by LaNi5. In fact, in these conditions, the LaNi5 precursor is completely consumed as affirmed by the XRD data (Figs. 2 and 3). Fig. 4a indicates that, during the charging
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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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
Fig. 4 e (a) Galvanostatic charging/discharging curves of LaTi2Ni5Cr4 -based alloys elaborated by mechanical alloying during 8, 12, 20 and 25 h. (b) Variation of the capacity of as-prepared alloys during the galvanostatic cycling. (c) Variation of the specific capacity relative to LaTi2Ni5Cr4 during the galvanostatic cycling.
regime, the potential value decreases until 1.3 V while the amount of absorbed hydrogen increases. For the discharging regime, the potential value increases up to 0.6 V and the amount of hydrogen trapped in the alloy lattice decreases. Fig. 4b shows that the maximum discharge capacity of each alloy electrode was obtained at the first charge/discharge cycle. During the galvanostatic cycling, the capacities slightly decreased during the initial 10 cycles before being stabilized for the remaining 50 cycles. The alloys prepared upon 25 and 20 h of milling times present similar capacities, and their values are higher than those reached with the alloy prepared during 8 and 12 milling hours. This result demonstrates that the discharge capacity increases when the LaTi2Ni5Cr4 content increases. Hence, the reversible absorption of hydrogen during the charge/discharge cycling only occurs in the host sites of the LaTi2Ni5Cr4 structure. On the other hand, the average capacity retention (C60/Cmax) of the alloy electrodes calculated upon 60 cycles is equal to 77%. Such a good cycling behavior demonstrates that LaTi2Ni5Cr4 is highly stable and may be beneficial to practical applications comparatively to LaNi5 and other AB3-type compounds investigated elsewhere [27,28]. For example, after 60 cycles of charging and discharging the AB3-phase (La0.67Mg0.33)1xTixNi2.75Co0.25, a poor capacity retention of 41% was obtained for x ¼ 0.2 [29]. In the same way, the capacity retention estimated for the AB3-phase La0.67Mg0.18Ca0.15Ni2.75Co0.25 is about 46% after 80 charge/ discharge cycles [30]. The discharge capacity is calculated in Fig. 4c according to the weight content of LaTi2Ni5Cr4, which was evaluated by the Rietveld refinement from the
corresponding XRD pattern (Fig. 3). Independently on the milling duration, LaTi2Ni5Cr4 exhibits nearly the same discharge capacity that remains stable at a value of 80 mAh g1.
Cyclic voltammetry Fig. 5 shows the cyclic voltammogram (CV) curves of LaTi2Ni5Cr4 -based alloys carried out after 60 charge/discharge activation cycles at scan rates varying between 10 and 60 mV s1. These LaTi2Ni5Cr4 -based materials were obtained by mechanical alloying after 8 h (Fig. 5a) and 20 h (Fig. 5b). During the cathodic scan, an onset of the hydrogen evolution reaction is observed at about 0.97 V. Then, the current decreases continuously until 1.1 V. When the potential is reversed to the opposite direction, the current increases and the observed anodic peak is corresponding to the oxidation of the hydrogen atoms absorbed in the composite lattice [31,32]. The anodic peak current increases and the potential slightly shifts to the positive direction with increasing the potential scan rate (w). Fig. 6 demonstrates that the anodic peak potential (Eap), determined at the maximum current, has a linear dependence on log (w). In this case, the anodic peak potential can be given by the following equation [33,34]: dEap 2:3RT ¼ anF dlogðwÞ
(4)
where R is the constant of rare gas, T the temperature of the electrochemical cell, a the charge transfer coefficient, n the
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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
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Fig. 5 e Cyclic voltammograms of the LaTi2Ni5Cr4 -based electrode obtained at potential rates of 10, 20, 30, 40, 50 and 60 mV s¡1. LaTi2Ni5Cr4 was mechanically alloyed during (a) 8 h and (b) 20 h.
spontaneous only for positive current due to the positive Ni(OH)2/NiOOH electrode potential (þ0.49 V), which is higher than the one of MH/M negative electrode potential. Thus, the current of the oxidation anodic peak can be expressed as follows [33,34]: Iap ¼ 0:496a1=2 ðnFÞ3=2 SCo
1=2 wD RT
(5)
where S is the geometric surface of the electrode (cm2), Co the concentration of diffusion species (mol cm3), and D the hydrogen diffusion coefficient (cm2 s1). The concentration Co is calculated by Eq. (6): CO ¼ Fig. 6 e Variation of the anodic peak potential of cyclic voltammograms of the LaTi2Ni5Cr4 -based electrodes as a function of log (y). The LaTi2Ni5Cr4 compounds were mechanically alloyed during 8 h (open squares) and 20 h (open circles).
QM mFVM
(6)
where M is the molar mass, m the effective mass of material, VM the molecular volume of material and Q the charge of anodic oxidation of hydrogen generated during the anodic sweep of cyclic voltammetry. According to Eq. (5), the hydrogen diffusion coefficient in the as-prepared metal
number of exchanged electron, and F the Faraday constant. Based on this formula, the charge transfer coefficient was calculated and presented in Table 2 for LaTi2Ni5Cr4 compounds prepared during 8 and 20 h of milling times. On the other hand, Fig. 7 shows a linear variation of the anodic peak current with the square root of the scan rate (w1/2). Such a behavior reflects a semi-infinite hydrogen diffusion and an irreversible charge transfer. The discharging process is
Table 2 e The charge transfer “a”, the hydrogen diffusion coefficient “D” and the sphere radius of the hydride alloy particle “a” for the LaTi2Ni5Cr4 -based alloys milled during 8 and 20 h. Milling time (h) 8 20
a
D (cm2 s1)
a (mm)
0.34 0.37
4.18 108 7.17 108
2.6 4.8
Fig. 7 e Variation of the anodic peak current of the cyclic voltammograms of the LaTi2Ni5Cr4 -based electrodes as a function of square root of potential scan rate. The LaTi2Ni5Cr4 compounds were mechanically alloyed during 8 h (open squares) and 20 h (open circles).
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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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
hydride electrodes can be deduced from the slope of the curve Iap ¼ f(w1/2) (Fig. 7). The values of a and DH (Table 2) indicate that, at ambient temperature and pressure, the electrochemical reaction is characterized by a suitable kinetic in the LaTi2Ni5Cr4 material in terms of reversibility and diffusivity of hydrogen. In fact, a values ranging from 0.3 to 0.7 can be approximated to 0.5 [33,34], which means that the charge and discharge reactions are reversible and the system presents the same tendency for the charge as for the discharge process [33,34]. On the other hand, the hydrogen diffusion coefficients of as-prepared LaTi2Ni5Cr4 -based alloys, varying between 4.18 108 and 7.17 108 cm2 s1, are much higher than those reported in the literature [31,32,35,36], indicating that the hydrogen diffusion rate is more facilitated in the LaTi2Ni5Cr4 lattice. In fact, the values of DH reported by Liao et al. for LaxMg3xNi9 (x ¼ 1.6e2.0) range from 7 1010 to 9 1010 cm2 s1 [37], while those reported for La2Mg(Ni0.8x Co0.2 Alx)9 (x ¼ 0e0.03) range from 2 1010 to 4 1010 cm2 s1 [38].
Chronoamperometry While the hydride alloy particles are supposed to present a spherical shape [39,40], the diffusion of hydrogen in the bulk material can be expressed, in the spherical coordinate, as follows: vðrCÞ v2 ðrCÞ ¼D vt vr2
(7)
where C is the hydrogen concentration in the alloy, t the time, D the average diffusion coefficient of hydrogen in the bulk and r the distance from the center of the sphere. Eq. (8) gives the solution of the diffusion equation established by Crank as indicated by Weixiang [40], under different boundaries, as: ∞ C C0 2a X ð 1Þn npr D n2 p2 t exp sin ¼1þ 2 pr n¼1 a a Cs C0 n
(8)
where a is the sphere radius, Co and Cs are the initial hydrogen concentrations in the bulk material and the alloy surface, respectively. The resulting diffusion current varies with time as follows: ∞ X 6FD n2 p2 Dt exp i ¼ ± 2 ðC CoÞ 2 da a n¼1
(9)
where d is the density of the material. The ± sign indicates the charge state for the minus sign and the discharge state for the plus sign. For a long time, Eq. (9) can be written accordingly: 6FD p2 D t logðiÞ ¼ log ± 2 ðCo CsÞ da 2:303a2
Fig. 8 e Variation of the diffusion current of LaTi2Ni5Cr4 -based electrode as a function of time after 60 charge/ discharge activation cycles. LaTi2Ni5Cr4 was mechanically alloyed during (a) 8 h and (b) 20 h.
(10)
The D=a2 ratio can be calculated from the slope the plot of log(i) versus time t (Fig. 8). Taking into account that the value of the hydrogen diffusion coefficient “D” was determined by cyclic voltammetry (Table 2), the radius “a” of the particles involved in the electrochemical reaction can be deduced from the value of this slope. Fig. 8 shows the semi-logarithmic curves for the mechanically alloyed LaTi2Ni5Cr4 -based hydrogen storage alloy. After the application of a constant potential of 0.6 V, a drastic decrease of current is obtained indicating a depletion of hydrogen on the composite surface
[36]. Each plot can be divided into two regions: for time t < 3000 s, the current decreases rapidly under a chargetransfer control [41]. For longer time t > 3000 s, the diffusion current decreases linearly with time, reflecting that the diffusion current obeys to Eq. (10). Table 2 indicates that the average radius of the LaTi2Ni5Cr4 -based alloy involved in the electrochemical reaction is about 2.6 and 4.8 mm after 8 and 20 h of milling time, respectively. As mentioned in the Experimental Section: prior to the charge/ discharge process, the alloy ingots were ground mechanically and their diameter saved to less than 63 mm in a glove box for the preparation of the negative electrode. Hence, It is assumed that the decrease of the particle size during long-term cycling and repetitive hydriding is due to the pulverization of powders induced by the expansion the lattice volume [31,40]. The particle size of LaTi2Ni5Cr4 -based alloys involved in the electrochemical reaction is lower than those reported for other AB3 etype alloys elaborated with melting techniques by Mokbli et al. (19e31 mm) [34] and Zheng et al. (15 mm) [39]. Such a result emphasizes the benefit effect of the mechanical alloying process for controlling the particle size comparatively to the melting techniques.
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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
Conclusions New AB3-type rich alloys, consisting in LaTi2Ni5Cr4 -based materials, were successfully elaborated by using the mechanical alloying technique. The synthesis reaction was performed from pure AB5-type (LaNi5) and AB2-type (TiCr2) intermetallic precursors. The increase of the milling duration enhances the weight content of LaTi2Ni5Cr4 in the alloy composition and, in the same way, the overall hydrogen absorption capacity of the system. For an injected shock power of 6.175 W g1, the variation of the milling time from 2 to 25 h results in an increase of the cumulated energy from 12.35 to 154.37 Wh g1, accompanied by an enhancement of the AB3 weight content from 10 to 66%. The refinement of corresponding XRD patterns by the Rietveld method confirms that the LaNi5 precursor was completely consumed after 8 h of milling time. However, 10 wt% of residual TiCr2 precursor was present in the alloy composition after 25 h of milling time. The cycle-life investigation of as-prepared LaTi2Ni5Cr4 -based materials shows a promising electrochemical stability following 60 charge/discharge cycles in alkaline medium. The values of the charge transfer coefficient of the alloys obtained after 8 and 20 milling hours are, respectively, equal to 0.34 and 0.37. On the other hand, the values of the hydrogen diffusion coefficient for the alloys obtained after 8 and 20 milling hours are equal to 4.18 108 and 7.17 108 cm2 s1, respectively. These results indicate a good absorptionedesorption kinetic and a high hydrogen diffusivity through the LaTi2Ni5Cr4 rich alloys. However, the specific discharge capacity of LaTi2Ni5Cr4 was estimated at 80 mAh g1 and further works are needed to improve such hydrogenation capacity.
Acknowledgments The authors would like to thank the Laboratory of Chimie tallurgique des Terres Rares (CMTR), the Institut de Chimie Me riaux Paris-Est (ICMPE) and the Centre National et des Mate pour la Recherche Scientifique (CNRS, Thiais) for the elaboration of LaNi5 and TiCr2 precursors by UHF induction melting, rie Paul-Boncour and Junxian and Pr. Michel Latroche, Dr. Vale Zhang for helpful discussions.
references
[1] Li Yiming, Ren Huiping, Zhang Yanghuan, Liu Zhuocheng, Zhang Hanwei. Hydrogen induced amorphization behaviors of multiphase La0.8Mg0.2Ni3.5 alloy. Int J Hydrogen Energy 2015;40:7093e102. gan A. [2] Cuevas F, Joubert JM, Latroche M, Parcheron-Gue Intermetallic compounds as negative electrodes of Ni/MH batteries. J Appl Phys 2001;72:225e38. [3] Marzouki M, Ghodbane O, Abdellaoui M. Study of hydrogen absorption kinetics of Mg2Ni-based powders produced by high-injected shock power mechanical alloying and subsequent annealing. Mater Renew Sustain 2013;2:9. [4] Seta S, Uchida H. Hydrogen solubility in LaNi5. J Alloy Compd 1995;231:448e53.
10941
[5] Uchida H, Sato M, Moriwaki O. The hydrogen absorption and desorption isotherms in the solid solution regions of the LaNi5-H system. J Alloy Compd 1997;253e254:235e7. [6] Termsuksawad P, Niyomsoan S, Mishra AAB, Olson Z, Gavra Z, Kaydanov VI. Prediction of hydrogen absorption behavior in AB5 hydrogen storage alloys by electronic techniques. Mater Sci Eng 2005;117:45e51. [7] Chumphongphan S, Paskevicius M, Sheppard Drew A, Buckley CE. Cycle life and hydrogen storage properties of mechanical alloyed Ca1xZrxNi5y Cry; (x ¼ 0, 0.05 and y ¼ 0, 0.1). Int J Hydrogen Energy 2012;37:7586e93. [8] Giza K, Iwasieczko W, Pavlyuk VV, Bala H, Drulis H. Thermodynamical properties of LaeNieT (T ¼ Mg, Bi and Sb) hydrogen storage systems. J Power Sources 2008;181:38e40. [9] Han SM, Zhao MS, Zhang Z, Zhang YZ, Jing TF. Effect of AB2 alloy addition on the phase structures and electrochemical characteristics of LaNi5 hydride electrode. J Alloy Compd 2005;392:268e73. [10] Johnson JR, Reilly JJ. Reaction of hydrogen with the lowtemperature form (C15) of titanium-chromium (TiCr2). Inorg Chem 1987;17:3103e8. [11] Johnson JR. Reaction of hydrogen with the high temperature (C14) form of TiCr2. J Less-Common Met 1980;73:345e54. gan A. Structural and [12] Latroche M, Percheron-Gue thermodynamic studies of some hydride forming RM3-type compounds (R ¼ lanthanide, M ¼ transition metal). J Alloy Compd 2003;356e357:461e8. [13] Kadir K, Sakai T. Structural investigation and hydrogen storage capacity of LaMg2Ni9 and (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9 of the AB2C9 type structure. J Alloy Compd 2003;302:112e7. [14] Chen J, Takeshita HT, Tanaka H, Kuriyama N, Sakai T, Uehara I, et al. Hydriding properties of LaNi3 and CaNi3 and their substitutes with PuNi3-type structure. J Alloy Compd 2000;302:304e13. [15] Pan HG, Liu YF, Gao MX, Lei YQ, Wang QD. Electrochemical properties of the La0.7Mg0.3Ni2.65xMn0.1Co0.75(x ¼ 0e0.5) hydrogen storage alloy electrodes. J Electrochem Soc 2005;152:326e32. [16] Lim Kean Long, Liu Yongning, Zhang Qing-An, Chan Sammy Lap Ip. Effects of partial substitutions of cerium and aluminum on the hydrogenation properties of La(0.65x)CexCa1.03Mg1.32Ni(9y) Aly alloy. Int J Hydrogen Energy 2014;39:10537e45. [17] Rodriguez-Carvajal J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys B 1993;192:55e69. gan A. [18] Mathlouthi H, Lamloumi J, Latroche M, Percheron-Gue Study of poly- substituted intermetallic hydrides electrochemical applications. Ann Chim Sci Matter 1997;22:241e4. [19] Chuan-Jian L, Feng-Rong W, Wen-Hao C, Wei L, Wen-Tong Z. The influence of high-rate quenching on the cycle stability and the structure of the AB -type hydrogen storage alloys with different Co5 content. J Alloy Compd 2001;315:218e23. [20] Krill III CE, Haberkorn R, Birringer R. Fabrication and spectroscopic characterization of organic nanocrystals. In: Nalwa HS, editor. Handbook of nanostructured materials and nanotechnology, Chap. 8. San Diego: Academic Press; 2000. p. 387e412. E-Publishing Inc. [21] Abdellaoui M, Gaffet E. A mathematical and experimental dynamical phase diagram for ballmilled Ni10Zr7. J Alloy Compd 1994;209:351e61. [22] Abdellaoui M, Gaffet E. The physics of mechanical alloying in a planetary ball mill: mathematical treatment. Acta Metal Mater 1995;43:1087e98. [23] Brimhall L, Kissinger HE. Amorphous phase formation in irradiated intermetallic compounds. Radiat Eff 1983;77:273e93.
10942
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 4 0 ( 2 0 1 5 ) 1 0 9 3 4 e1 0 9 4 2
[24] Schwarz RB, Koch CC. Formation of amorphous alloys by the mechanical alloying of crystalline powders of pure metals and powders of intermetallics. Appl Phys Lett 1986;49:146. [25] Dehouche Z, Djaozandry R, Goyette J, Bose TK. Evaluation techniques of cycling effect on thermodynamic and crystal structure properties of Mg2Ni alloy. J Alloy Compd 1999;288:269e76. [26] Nohara S, Inoue H, Fukumoto Y. Compositional and structural characteristics of MgNi alloy prepared by mechanical alloying for use as negative electrodes in nickelmetal hydride batteries. J Alloy Compd 1997;259:183e5. [27] Liu Y, Pan H, Gao M, Miao H, Lei Y, Wang Q. Function of Al on the cycling behavior of the LaeMgeNieCo-type alloy electrodes. Int J Hydrogen Energy 2008;33:124e33. [28] Zhang YH, Li BW, Ren HP, Cai Y, Dong XP, Wang XL. Cycle stabilities of the La0.7Mg0.3Ni2.55xCo0.45Mx (M ¼ Fe, Mn, Al; x ¼ 0, 0.1) electrode alloys prepared by casting and rapid quenching. J Alloy Compd 2008;458:340e5. [29] Zhenwei D, Yaoming W, Liqun M, Xiao S, LiMin DW. Influences of low-Ti substitution for La and Mg on the electrochemical and kinetic characteristics of AB3-type hydrogen storage alloy electrodes. Sci China Ser E 2010;53:242e7. [30] Zhenwei D, Yaoming W, Liqun M, Lidong W, Xiaodong S, Limin W. Microstructure and electrochemical hydrogen storage characteristics of La0.67Mg0.33xCaxNi2.75Co0.25 (x ¼ 0e0.15) electrode alloys. Int J Hydrogen Energy 2011;36:3050e5. [31] Ben Moussa M, Abdellaoui M, Mathlouthi H, Lamloumi J, gan A. Investigation of the cycle stability and Percheron-Gue diffusivity of hydrogen in the MmNi3.55Mn0.4Al0.3Co0.6Fe0.15 compound. J Alloy Compd 2006;407:256e62. [32] Ben Moussa M, Abdellaoui M, Mathlouthi H, Lamloumi J, gan A. Electrochemical properties of the Percheron-Gue MmNi3.55Mn0.4Al0.3Co0.75xFex (x ¼ 0.55 and 0.75) compounds. J Alloy Compd 2008;458:410e4.
thodes et Applications. [33] Bard AJ, Fulkner LR. Principes, Me Paris: Masson; 1998. p. 249. E- Publishing Inc. [34] Mokbli S, Abdellaoui M, Zarrouk H, Latroche M, Percherongan A. Hydriding and electrochemical properties of Gue amorphous rich MgxNi100x nanomaterial obtained by mechanical alloying starting from Mg2Ni and MgNi2. J Alloy Compd 2008;460:432e9. [35] Ben Moussa M, Abdellaoui M, Mathlouthi H, Lamloumi J, gan A. Investigation on the structure, Percheron-Gue thermodynamic and electrochemical properties of the MmNi3.55Mn0.4Al0.3Fe0.75 compound used as negative electrode in NieMH batteries. J Alloy Compd 2013;575:414e8. [36] Zhao X, Ding Y, Yang M, Ma L. Electrochemical properties of MmNi3.8Co0.75Mn0.4Al0.2 hydrogen storage alloy modified with nanocrystalline nickel. Int J Hydrogen Energy 2008;33:6727e33. [37] Liao B, Lei YQ, Chena LX, Lub GL, Pan HG, Wang QD. Effect of the La/Mg ratio on the structure and electrochemical properties of LaxMg3xNi9 (x ¼ 1.6e2.2) hydrogen storage electrode alloys for nickelemetal hydride batteries. J Power Sources 2004;129:358e67. [38] Liao B, Lei YQ, Chena LX, Lub GL, Pan HG, Wang QD. The structural and electrochemical properties of La2Mg(Ni0.8xCo0.2Alx)9 (x ¼ 0e0.03) hydrogen storage electrode alloys. J Alloy Compd 2006;415:239e43. [39] Zheng G, Popov BN, White RE. Electrochemical determination of the diffusion coefficient of hydrogen through an LaNi4.25Al0.75 electrode in alkaline aqueous solution. J Electrochem Soc 1995;142:2695e8. [40] Weixiang C. Cyclic voltammetry and electrochemical impedance of MmNi3.6Co0.7Mn0.4Al0.3 alloy electrode before and after treatment with a hot alkaline solution containing reducing agent. J Power Sources 2000;90:201e5. [41] Mathlouthi H, Khaldi C, Ben Moussa M, Lamloumi J, gan A. Electrochemical study of monoPercheron-Gue substituted and poly-substituted intermetallic hydrides. J Alloy Compd 2004;375:297e304.