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Investigations on AB3-, A2B7- and A5B19-type LaYNi system hydrogen storage 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 x x x ( 2 0 1 6 ) 1 e8
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Investigations on AB3-, A2B7- and A5B19-type Lae YeNi system hydrogen storage alloys Huizhong Yan a,b,c,*, Wei Xiong a,b, Li Wang a,b, Baoquan Li a,b, Jin Li a,b, Xin Zhao a,b a
Baotou Research Institute of Rare Earths, Baotou 014030, PR China National Engineering Research Center of Rare Earth Metallurgy and Functional Materials, Baotou 014030, PR China c Tianjin Baogang Research Institute of Rare Earths Co., Ltd, Tianjin 300300, PR China b
article info
abstract
Article history:
The structure and properties of new LaeYeNi system alloys with high hydrogen-storing
Received 23 July 2016
capacity were investigated using X-ray diffraction (XRD), scanning electron microscope
Received in revised form
(SEM), solid-H2 reactions (P-C-I curves) and electrochemical measurements. The LaY2-
4 September 2016
Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-
Accepted 7 September 2016
type) hydrogen storage alloys were prepared with the induction-melting rapid-quenching
Available online xxx
method and annealed at 1148 K for 16 h. The LaeYeNieMneAl alloys were also compared with commercial AB5-type hydrogen storage alloy with high capacity. Similarly to Lae
Keywords:
MgeNi system hydrogen storage alloy, LaeYeNi system alloys are multiphase structures
LaeYeNi hydrogen storage alloy
and Y element in the LaeYeNi alloys avoid or delay the hydrogen-induced amorphous
Hydride
(HIA) of the alloys in the hydrogenation/dehydrogenization process. The hydrogen
Hydrogenation/dehydrogenation
storage capacities of the A2B7- and A5B19-type alloys at 313 K are 1.48 wt.% and 1.45 wt.%,
Electrochemical properties
respectively, which are larger than that of the AB5-type alloy (1.38 wt.%). The maximum
Ni-MH battery
discharge capacities of the A2B7- and A5B19-type alloy electrodes at 298 K are 385.7 mAh g1 and 362.1 mAh g1, respectively, which are larger than that of the AB5-type alloy (356.1 mAh g1). The maximum discharge capacity of the A2B7-type alloy exceeds the theoretical capacity (372 mAh g1) of the AB5-type alloy. The A2B7- and A5B19-type alloy electrodes have better cycling ability than the AB5-type alloy. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen storage is one of the key technologies for hydrogen energy appliance which provides an ideal solution for the dwindling global energy crisis and ever-increasing environmental pollution [1]. Rare earth hydrogen storage alloys are the most mature hydrogen storage products, which are mainly used for the negative materials of nickel-metal
hydride (Ni-MH) batteries [2,3] and gas-phase hydrogen storage devices [4,5]. AB5-type LaNi5-based alloys are currently the main commercial hydrogen storage materials. However, the theoretically electrochemical capacity (372 mAh g1) or maximal hydrogen storage amount (160 cm3 g1 or 1.43 wt.%) after optimization of this material [6] often hardly satisfies the required hydrogen storage capacity because of its restraining intrinsic structure (CaCu5-type). RE-Mg-Ni (RE ¼ Rare Metals) system AB3-3.8-type metal hydride alloys are of special
* Corresponding author. Baotou Research Institute of Rare Earths, Baotou 014030, PR China. E-mail address: [email protected] (H. Yan). http://dx.doi.org/10.1016/j.ijhydene.2016.09.049 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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concern because of the higher discharge capacity (~400 mAh g1), and a series of significant developments in the study of this material has been made in the past few years [7e12]. RE-Mg-Ni hydrogen storage alloys have been widely used as negative electrode active materials for Ni-MH batteries as substitutes for conventional AB5-type alloys [10e12]. The active Mg element is one of the main compositions in RE-MgNi system alloys. The chemical composition of RE-Mg-Ni system alloys is difficult to control in the high-temperature melting manufacturing process because of the high vapor pressure of Mg. Ultra-fine magnesium powder, which is formed by Mg volatilization, becomes a safety hazard. Thus, new manufacturing techniques are investigated, such as smelting protection with helium gas [7,12,13], various sintering technologies [14,15], high-powered ball-grinding technology [16] and so on. However, the application of these techniques has either a high cost or a complicated process. So, the research and development of Mg-free hydrogen storage alloys with high hydrogen-storing capacity is of great significance. Combining LaNi5 and YNi2 binary compounds with hydrogen storage properties, Baddour-Hadjean et al. [17] studied the LaeYeNi ternary alloy, which is equivalent to an overall substitution of the Mg element in the LaeMgeNi system alloy by a rare earth Y element. By studying AB3-type La1xCexY2Ni9(0 x 1) alloys, it is found that the LaY2Ni9 alloy is a PuNi3-type structure and forms LaY2Ni9H12 after hydrogenation, which excels the hydrogen capacity of LaMg2Ni9 alloy under identical conditions. However, the maximum discharge capacity of LaY2Ni9 alloy electrodes is only 265 mAh g1. Belgacem et al. [18] also tested the electrochemical properties of LaY2Ni9 alloy electrode. The maximum discharge capacity of the alloy electrodes is 258 mAh g1 within five cycles, and 54% of the capacity is maintained after 100 cycles, which is far from meeting the needs of the application. In our previous work [19e21], the reversible hydrogen storage performance of AB3-, A2B7- and A5B19-type LaeYeNi system alloys is noticeably improved by adjusting the alloy composition using the element substitution method. Importantly, LaeYeNi system alloys can be directly prepared using the high-temperature melting method, which solves the preparation problems for Mg-based hydrogen storage alloys. The structure and properties of the representative AB3-, A2B7- and A5B19-type LaeYeNieMneAl hydrogen storage alloys were systematically investigated in this paper and compared with commercial AB5-type hydrogen storage alloy with high capacity.
were at least 99 wt.%. The prepared alloy flakes were annealed in vacuum of 102 Pa at 1148 K for 16 h. The annealed alloy flakes and commercial AB5-type (LaCe)Ni3.8Co0.7Mn0.4Al0.2 hydrogen storage alloy were mechanically pulverized into powder particles of 38e74 mm in size for the electrochemical measurements. The phases of the alloy powders were characterized by Xray diffraction (XRD) using a Philips-PW 1700 X powder diffractometer with Cu Ka radiation at 40 kV, 200 mA in the range from 0 to 80 with 0.02 min1, and the diffraction patterns were analyzed with a Rietveld refinement (using the software MAUD). The morphologies of the alloys were examined by HITACHI S-3400N scanning electron microscope (SEM) linked with an energy dispersive X-ray spectrometer (EDS). Pressure-composition isotherms for the H2 absorption/ desorption reactions were determined over the pressure range of 103 MPa to 2.0 MPa in a Sieverts testing device. The alloy flakes were mechanically broken into small particles of 74 mm1.2 mm in size before testing. Alloy particles with a mass of approximately 5 g were placed in the reaction chamber, evacuated for 60 min at 343 K and then allowed to react with hydrogen gas (99.999% purity) under a pressure of 2 MPa. The chamber was then slowly cooled to room temperature and held at that temperature for 30 min. De-hydriding was performed by heating the chamber to 343 K and evacuating it for 60 min until the hydrogen pressure was below 103 MPa. Five hydriding/de-hydriding cycles were performed to ensure that the alloys were fully activated. Next, the P-C isotherms were measured at 298 K, 313 K, 333 K and 343 K. MH electrodes were prepared by mixing 0.1 g alloy powder with 0.4 g carbonyl nickel powder and then cold-pressed into pellets with 15 mm in diameter under a pressure of 16 MPa. This pellet was then placed between two Ni gauze layers, and the edges were tightly spot-welded to maintain good electrochemical contact between the pellet and the Ni gauze. A Ni lead wire was then attached to the Ni gauze by spot-welding to prepare the hydrogen storage alloy electrode (MH electrode). Electrochemical measurements were performed at 298 K in a half-cell consisting of a prepared MH electrode and a sintered Ni(OH)2/NiOOH counter electrode with an excess capacity immersed in 6 mol L1 KOH electrolyte. The discharge capacity and cycle stability were measured by galvanostatic method as follows: each electrode was charged at 70 mA g1 for 6 h, which was followed by a 5-min break, and then was subsequently discharged at 70 mA g1 to the cut-off potential of 1.0 V versus the counter electrode. All tests were measured at room temperature (298 K).
Results and discussion Experimental
Phase structure
The chemical compositions of the investigated LaeYeNie MneAl alloys are LaY2Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-type). These alloys were prepared in a 0.05 MPa argon atmosphere using a vacuum induction-quenching furnace with a rotating copper wheel. In this work, the linear velocity of the copper wheel was 4.33 m s1. The purities of the component metals
Fig. 1 shows the refined analysis of XRD patterns for the LaY2Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-type) alloys, and the results are listed in Table 1. The alloys have a multiphase microstructure. The AB3-type alloy consists of LaY2Ni9-type phase, Ce2Ni7type phase and a notably small quantity of LaNi5-type phase. The A2B7-type alloy consists of Ce2Ni7-type phase and Gd2Co7-
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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Fig. 1 e Refined analyses of powder XRD data of the LaeYe NieMneAl alloys.
type phase. The A5B19-type alloy contains Ce5Co19-type phase, Gd2Co7-type phase, Ce2Ni7-type phase and LaNi5-type phase. The LaeYeNi and LaeMgeNi system alloys have similar constituent phases and phase structures [10,17,22e24]. PuNi3-type, Ce2Ni7- or Gd2Co7-type and Ce5Co19-type phases are most common phases in LaeYeNi and LaeMgeNi
system alloys. These multi-component alloys consist of alternating structural AB5 and A2B4 slabs stacked along the c-direction. The AB3-, A2B7- and A5B19-type phases exhibit similar super-stacking structures with [AB5] and [A2B4] (Laves) subunits in a ratio of 1:1, 2:1 and 3:1, respectively [8,12]. Therefore, it can be asserted that the Y element and Mg element have similar effects in the two series of hydrogen storage alloys. Both Mg in the LaeMgeNi alloy and Y in the LaeYeNi alloy increase the effective hydrogen storage capacity and structural stability of the corresponding alloys in the hydrogenation/dehydrogenization process and avoid or delay the hydrogen-induced amorphous (HIA) of the alloys [13,24]. Another study has shown that the HIA of the alloy can be suppressed to a certain extent because the Ni element in LaNi3-type alloy is partly substituted by a small amount of Mn and Si elements [25]. According to the research results, the HIA of LaeYeNieMneAl alloys in this work can be effectively suppressed because of the Y element at A-side and Mn element at B-side, which are confirmed by subsequent results of the solid-H2 reaction and electrochemical measurement in this paper. Some changes in microstructure of the alloys occur during the annealing process. For example, the LaY2Ni9-type phase in the AB3-type alloy and alloy composition have identical A/B ratio (1/3), but the A/B ratios of Ce2Ni7-type phase and LaNi5type phase (2/7 and 1/5, respectively) are less than 1/3. The Ce2Ni7-type phase is formed through the peritectic reaction between LaNi5-type and LaY2Ni9-type phases, and some of the LaY2Ni9-type phase transform into Ce2Ni7-type phase because of the partial loss of A-side elements [23]. SEM micrographs of the LaeYeNieMneAl alloys are shown in Fig. 2. EDS results from selected areas in each micrograph reveals the phase composition of the alloy. The phases in the AB3-type alloy are LaY2Ni9- (light grey region), Ce2Ni7-type (dark grey region). The composition of the areas in A2B7-type alloys is identical to that of the alloys, suggesting that the composition of Ce2Ni7- and Gd2Co7-type phases is basically the same. The phases in the A5B19-type alloy are mainly Ce5Co19- (dark grey region), Gd2Co7- and Ce2Ni7-type (light grey region). The LaNi5-type phase (black region, see the inset in Fig. 2) is detected on one side of the alloy flakes near the copper wheel, which may indicate that the annealing of this alloy is not enough.
Table 1 e Characteristics of the phases in the LaeYeNieMneAl alloys. Alloys
AB3
A2B7
A5B19
a
Phase
LaY2Ni9(3R) Ce2Ni7(2H) LaNi5 Ce2Ni7(2H) Gd2Co7(3R) Ce5Co19(3R) Gd2Co7(3R) Ce2Ni7(2H) LaNi5
Parameters of fita Rp ¼ 12.73 Rwp ¼ 16.93 S ¼ 2.54 Rp ¼ 11.73 Rwp ¼ 15.93 S ¼ 2.59 Rp ¼ 11.58 Rwp ¼ 15.66 S ¼ 2.47
Lattice constant
Cell volume ( A3)
Phase abundance (wt.%)
a( A)
c( A)
5.0624 5.0343 4.9752 5.0237 5.0115
24.5314 24.5078 3.9779 24.3722 36.5499
544.44 537.90 85.27 532.67 794.95
51.92(±0.21) 46.89(±3.66) 1.19(±0.13) 81.36(±0.17) 18.64(±0.86)
5.0024 5.0041 4.9497 4.9764
48.4994 36.4173 25.0391 3.9956
1051.02 789.73 531.24 85.69
66.55(±0.11) 14.51(±1.35) 11.79(±1.65) 7.16(±0.31)
Rp ¼ pattern factor, Rwp ¼ weighted pattern factor, S ¼ goodness of fit.
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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Fig. 2 e SEMs of the LaeYeNieMneAl alloys.
Hydrogenation/dehydrogenation reaction characteristics The P-C isotherms of the LaeYeNieMneAl and AB5-type alloys are shown in Fig. 3. The plateau features of hydrogenation/dehydrogenation are decided by the content of La and Ni elements in the LaeNi-based hydrogen storage alloy. It is wellknown that a higher Ni content at the B side corresponds to a higher plateau pressure for the identical composition at the A side owing to the atomic radius of Ni (1.62 A) being somewhat smaller than that of substitute elements, which is in agreement with the decrease of lattice parameters. A higher La content at the A side corresponds to a lower plateau pressure for the identical composition at the B side owing to the atomic radius of La (1.87 A) being somewhat larger than that of substitute elements, which is in agreement with the increase of lattice parameters [26]. The plateau pressure decreases or increases when Ni or La is partly substituted by other elements, respectively. In this paper, the Ni content in three LaeYeNie MneAl alloys with identical compositions (LaY2) at the A side increases from 57.7% and 61.8% to 63.9% with the decrease in A/B ratio. Thus, the plateau pressure of the three alloys increases in order of AB3-type, A2B7-type and A5B19-type at identical temperatures. In Fig. 3, the plateau characteristics of AB3-type alloys are not obvious. The LaY2Ni9- and Ce2Ni7-type phases in the AB3-type alloy have similar equilibrium pressures because of their similar stacking structures [23,27] and cell volumes. The hydrogen storage amount of the AB3-type alloy is only 0.85 wt.%, and about half of them can be released because
AB3
1
A 2 B7 A5B19
PH2 ( MPa)
AB5 0.1
0.01
1E-3 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
[H] ( wt.%) Fig. 3 e Absorption/desorption Pec isotherms of the LaeYe NieMneAl alloys at 313 K.
AB3-type hydride decomposition pressure is very low, about 103 MPa orders of magnitude. Two obvious plateaus are observed for the A2B7-type alloy. The Gd2Co7-type phase with a larger cell unit volume, which can provide more available interstices for H in the lattice, exhibits a lower hydrogen absorption/desorption plateau pressure (approximately 0.01 MPa for the hydride decomposition pressure) [13]. The hydride decomposition pressure at the higher plateau for the Ce2Ni7-type phase with a smaller cell unit volume is approximately 0.04 MPa. The Ce2Ni7-type phase has a wider plateau than the Gd2Co7-type phase because of the significantly greater abundance of the Ce2Ni7-type phase. The hydrogen storage amount (1.48 wt.%; the calculated electrochemical capacity is 397 mAh g1) of the A2B7type alloy is maximal in the three LaeYeNieMneAl alloys, which is distinctly larger than that of AB5-type alloy (1.38 wt.%; the calculated electrochemical capacity is 370 mAh g1). The hydrogen absorption/desorption plateau of the A5B19-type alloy with multiphase structure is not flat, which reflects the hydrogen absorption/desorption characteristics of the phases with different cell volume. The lower plateaus (approximately 0.06 MPa for the hydride decomposition pressure) represent the hydrogen absorption/ desorption characteristics of the Ce5Co19-type phase with large unit cell volume. With the increasing of the plateau pressure (up to approximately 0.10 MPa for the hydride decomposition pressure), the combination of the Gd2Co7-, Ce2Ni7-, and LaNi5-type phase plateaus is reflected in the order of decreasing cell volume [23]. The A5B19-type alloy has a slightly lower hydrogen storage amount (1.45 wt.%) than the A2B7-type alloy but a greater value than the AB5type alloy. Research results on singer-phase LaeMgeNi alloys show that the order of the plateau pressure is La2MgNi9 (AB3type) < La3MgNi14 (A2B7-type) < La4MgNi19 (A5B19-type) alloys [8], which is consistent with the above experimental results. However, the hydrogen storage amount of the alloys decreases with the increase in [AB5]/[A2B4] ratios [8], i.e., the hydrogen storage amount decreases in the order of AB3-type, A2B7-type and A5B19-type alloys. By contrast, the AB3-type alloy has the smallest hydrogen storage amount among all three LaeYeNieMneAl alloys in this work. The main reason has three aspects. First, only approximately half of the AB3type alloy is AB3-type phase (LaY2Ni9). Second, the higher lattice strain of AB3-type phase in the hydrogen absorption process leads to HIA [8,24,25]. Third, the notably low hydrogen desorption plateau pressure of AB3-type alloy indicates the formation of a high-stability hydride phase [24]. The A5B19-
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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and A2B7-type alloys have similar hydrogen storage amounts because the A5B19-type alloy contains approximately 25% of A2B7-type phase with higher hydrogen storage amount and a moderate hydride decomposition pressure. To further understand the thermodynamic properties of the three LaeYeNieMneAl alloys, Fig. 4 shows a Linear fitting Van't Hoff plot of the logarithmic mid-plateau pressure versus the reciprocal of the absolute temperature. The pressure was obtained from the P-C-I curves of the AB3-, A2B7-, A5B19- and AB5-type alloys at different temperatures (298 K, 313 K, 333 K and 343 K). The hydride formation enthalpies DH and entropies DS of the alloys were calculated from the following van't Hoff equation: ln PH2 ¼ DH=RT DS=R and the Gibbs free energy DG was calculated from the following equation: DG ¼ DH TDS Other details refer to a previous paper [28]. The calculated thermodynamic data are listed in Table 2. From the results in Table 2, the DH and DS for the three LaeYeNieMneAl alloys all meet the requirements for a hydrogen storage alloy [29]. As observed in Table 2, the absolute DG value of the AB3-type alloy is obviously greater other alloys, which indicates that the hydrides of the AB3-type alloy exhibit higher thermodynamic stability.
Electrochemical reaction characteristics Fig. 5 shows the discharge curves of three LaeYeNieMneAl alloy electrodes, and the LaNi5-type alloy electrode is a reference alloy for comparison; the discharge curves correspond to the maximum discharge capacity. The mid-discharge potential and maximum discharge capacity of the LaNi5-type alloy are approximately 1.3 V and 356.1 mAh g1, respectively. The A5B19-type alloy has a slightly higher mid-discharge potential and a larger maximum discharge capacity (362.1 mAh g1) than the LaNi5-type alloy. The A2B7-type alloy has a slightly lower mid-discharge potential and a larger maximum discharge capacity (385.7 mAh g1), than the LaNi5-type alloy.
Fig. 4 e Logarithm of the equilibrium pressure (PH2 ) versus the reciprocal of temperature (1/T).
5
This maximum discharge capacity exceeds the theoretical capacity (372 mAh g1) of the LaNi5-type alloy [6], which is largest among these alloys. The AB3-type alloy has the lowest mid-discharge potential (approximately 1.26 V) and smallest maximum discharge capacity (321.4 mAh g1) among these alloys. As expected, the evolution of electrode discharge capacity essentially follows the same trend as the hydrogen storage amount, which is demonstrated in the P-C-I curves. The electrochemical data about the activation, maximum capacity and cyclic stability are summarized in Table 3. Fig. 6(a) shows the activation curves of the three LaeYeNie MneAl alloy electrodes and the LaNi5-type alloy electrode. The A2B7- and A5B19-type alloys, which are identical to the LaNi5-type alloy, have excellent activation capability and can approach or attain maximum capacity within 3 cycles. The AB3-type alloy has relatively poor activation property because of its poor kinetics which is discussed further below. In the charge/discharge process, for the LaY2Ni9-type phase of AB3type alloy, the hydrogen absorption by its La(Y)Ni2 subunits makes the local lattice disorderly and causes the formation of the amorphous phase. Simultaneously, the La(Y)Ni5Hx crystallites, which are formed by the La(Y)Ni5 subunit, start to precipitate [24]. The hydrogen absorption/desorption reaction of the alloy is further catalyzed by the precipitated La(Y)Ni5Hx crystallites [30,31], which makes the AB3-type alloy gradually achieve the maximum capacity through more charge/ discharge cycles but decreases the electrochemical capacity of the AB3-type alloy. Fig. 6(b) shows the discharge capacity retention, which is the ratio of discharge capacity and maximum capacity during the cycle test, versus the number of charge/discharge cycles for the three LaeYeNieMneAl alloy electrodes and the LaNi5type alloy electrode. The discharge capacity retention can represent the cyclic stability of the alloy electrode. Fig. 6(b) and Table 3 show that the discharge capacity retentions of the A2B7-, A5B19-, AB3- and AB5-type alloys are 76.6%, 75.8%, 47.6% and 74.0% in 300 cycles, respectively. The A2B7- and A5B19-type alloys have better cycle stabilities than the AB5-type alloy. The AB3-type has the worst cycle stability among these alloys. The principal factors that affect the cyclic properties of the AB3-, A2B7- and A5B19-type alloys are pulverization and amorphization caused by the strains which are responsible for lattice mismatch in the hydrogen absorption/desorption or charge/discharge process. The pulverization can cause the accelerated corrosion of alloy particles. The amorphization destroys the super-stacking structures of the phases in the alloys. The Y element in LaeYeNi system alloys can restrain the HIA [17,24], and the mismatch between Laves and LaNi5 layers in the ternary phases is reduced [15], so the multielement alloys have better cyclic stability than their corresponding LaeNi binary ones. Based on the research results for the capacity degradation mechanism of single-phase LaeMgeNi system hydrogen storage alloy [8], higher [AB5]/[A2B4] ratios of the A5B19- (3:1) and A2B7-type phase (2:1) can reduce the structural mismatch and decrease the lattice strains of the alloys. Consequently, the amorphization and pulverization of the alloys are restrained in the hydrogenation/dehydrogenization process. As a result, the cycling stability of the alloys is improved. In contrast, the A5B19-type phase has higher [AB5]/[A2B4] ratio
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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Table 2 e Thermodynamic parameters of the LaeYeNieMneAl alloys. Alloys
Hydride decomposition pressure at 313 K (MPa) 3
AB3 A 2B 7
~10 0.01 0.04 0.06 0.10 0.025
A5B19 AB5
DH/kJ mol1 H2
DS/J K1 mol1 H2
DG/kJ mol1 H2 (298 K)
35.36(±1.54) 32.24(±1.39) 31.13(±0.45) 30.90(±0.45) 29.63(±0.35) 32.66(±0.57)
67.03(±4.79) 68.94(±4.40) 75.07(±1.34) 73.15(±1.34) 76.79(±1.15) 75.64(±1.72)
15.39(±2.97) 11.70(±2.71) 8.76(±0.85) 9.10(±0.85) 6.75(±0.70) 10.12(±1.08)
–1
Discharge capacity (mAh g )
400
(a)
350 300 250
AB3
200
A2 B 7
150
A5B19
100
AB5
50 0
2
4
6
8
10
Cycle number (n)
Fig. 5 e Discharge potential curves of the LaeYeNieMneAl alloy electrodes.
than the A2B7-type phase. Accordingly, the A5B19-type alloy should have a better cyclic stability than the A2B7-type alloy [32]. Zhang et al. [15] reported that the samples with a larger amount of A5B19-type phase showed better resistance to corrosion in the charge/discharge process. In fact, the A2B7type alloy has a slightly better cycling stability than the A5B19type alloy in the present paper. This result is attributed to the more phases and phase boundaries in the A5B19-type alloy. The microcracks formed along the phase boundary may accelerate the pulverization process during the charging/discharging cycles [9]. The AB3-type phase in the AB3-type alloy has lowest [AB5]/ [A2B4] ratio (1:1) in the phases with the super-stacking structures, so the poor stability of the AB3-type alloy is attributed to the amorphization of Laves subunits [13,24,27,32] in addition to severe pulverization of the alloy because of great lattice strains during the hydrogenation/dehydrogenization process
Table 3 e Electrochemical properties of the LaeYeNie MneAl alloy electrodes. Alloys AB3 A2B 7 A5B19 AB5 a b c
Composition LaY2Ni8.2Mn0.5Al0.3 LaY2Ni9.7Mn0.5Al0.3 LaY2Ni10.6Mn0.5Al0.3 (LaCe)Ni3.8Co0.7Mn0.4Al0.2
Na Cmaxb (mAh g1) S300c (%) 7 3 3 3
321.4 385.7 362.1 356.1
47.6 76.6 75.8 74.0
N is the required number of cycles to activate the electrodes. Cmax is the maximum discharge capacity of the alloy electrodes. S300 is the capacity retention ratio after 300 cycles.
Discharge capacity retention (%)
100
AB3
(b)
95
A2 B 7
90
A5B19
85
AB5
80 75 70 65 60 55 50 45
50
100
150
200
250
300
Cycle number (n)
Fig. 6 e Activation curves (a) and cyclic stability curves (b) of the LaeYeNieMneAl alloy electrodes.
[14,31]. Furthermore, there is a greater percentage of active elements such as La, Y, Mn and Al in the AB3-type alloy than in A2B7- and A5B19-type alloys. These elements are more easily corroded and enter the electrolyte, which causes a faster capacity degradation [9,13,33]. The high-rate dischargeability (HRD) reflects the electrochemical kinetics of the alloy electrodes. The HRD is defined and calculated according to the following formula: HRD ¼ Cd =ðCd þ C70 Þ 100% where Cd is the discharge capacity at the discharge current density and at the cut off potential of 1.0 V versus Ni(OH)2/
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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NiOOH counter electrode, C70 is the residual discharge capacity at the discharge current density I ¼ 70 mA g1. In the present work, the discharge current density Id is 1750, 1050 and 350 mA g1. Fig. 7 shows the HRDs of the LaeYeNieMne Al and LaNi5-type alloy electrodes. The HRDs of the three Lae YeNieMneAl alloy electrodes increase with the order of AB3type, A2B7-type and A5B19-type. Note that the three LaeYeNie MneAl alloy electrodes have smaller HRDs than the LaNi5type alloy, which is a new subject for further research. However, the A5B19 phase itself is believed to have superior HRD [14]. Compared with some LaeMgeNi alloys [2,22,23,30], the high-rate dischargeability of A5B19-type LaeYeNieMneAl alloy is not inferior. The principal factors that affect the HRD of the hydrogen storage alloy electrodes are charge transfer on the alloy surface and hydrogen diffusion in the alloy bulk [9,10]. Generally, the factors that affect the charge transfer rate have three aspects. First, the Ni element in the alloy is a catalyst for hydrogen absorption and desorption reactions [9,31]. Second, the AB5-type phase in multiphase alloys acts as an catalyst to accelerate the electrochemical reaction [16,27,30,31]. Third, corrosion products on the alloy surface in an alkaline electrolyte can decrease the charge transfer speed [9,33]. The factors that affect the hydrogen diffusion are mainly related to the diffusion channels such as the grain boundary and phase boundary in the alloy [9,22]. Fig. 7 shows that the HRDs of the three LaeYeNieMneAl alloy electrodes increase with the increase in Ni element content in the alloys. From the constituent phases in the alloys, there is more AB5-type phase (approximately 7%), which acts as the electrocatalyst in the A5B19-type alloy. In addition, the A5B19-type alloy includes more phases, which increases the phase boundaries, provides more tunnels for hydrogen diffusion [16]. In any case, the A5B19-type has the best HRD among the three LaeYeNieMneAl alloy electrodes. Liu et al. [14] also reported that the A5B19-type phase has faster hydrogen desorption rate and generates a catalytic effect on the discharge process of the coexisting A2B7-type phase. The AB3-type has the worst HRD among the three LaeYe NieMneAl alloy electrodes. In addition to the two disadvantages of lower Ni content and fewer AB5-type phase in the
7
alloy, the AB3-type alloy is more easily corroded because it has more active elements, and the corrosion products with high resistance on the alloy surface decrease the charge transfer speed. From the analysis above it can be seen that the charge transfer rate on the alloy surface is the control factor of electrochemical kinetic for the LaeYeNi alloys. The three LaeYe NieMneAl alloy electrodes have smaller HRDs than the LaNi5type alloy because the former has lower Ni content than the latter although the former includes more phases than the latter.
Conclusions The present study investigates AB3-, A2B7- and A5B19-type Lae YeNi system hydrogen storage alloys and aims to develop a new type of Mg-free hydrogen storage alloys which has the advantages of high hydrogen-storing capacity and easy preparation. The main results can be summarized as follows. The AB3-type (LaY2Ni8.2Mn0.5Al0.3), A2B7-type (LaY2Ni9.7Mn0.5Al0.3) and A5B19-type (LaY2Ni10.6Mn0.5Al0.3) LaeYeNi system hydrogen storage alloys have a relatively stable multiphase microstructure because of the partial substitution of La by Y and Ni by Mn. The AB3-type alloy mainly consists of LaY2Ni9and Ce2Ni7-type phases. The A2B7-type alloy is composed of Ce2Ni7- and Gd2Co7-type phases. The A5B19-type alloy contains Ce5Co19-, Gd2Co7-, Ce2Ni7- and LaNi5-type phases. At identical temperatures, the plateau pressure of the three alloys has the following order: A5B19-type > A2B7-type > AB3type. The hydride formation enthalpies DH and entropies DS of the three alloys satisfy the requirements of Ni-MH batteries for hydride electrodes. The A2B7-type alloy has the maximum hydrogen storage amount (1.48 wt.%) among the three alloys, which is distinctly greater than that of the AB5-type alloy (1.38 wt.%). The A2B7- and A5B19-type alloys have excellent activation capability. The A2B7-type alloy has the largest maximum discharge capacity (385.7 mAh g1) among these alloys, which exceeds the theoretical capacity of the AB5-type alloy. The A2B7-, A5B19- and AB5-type alloys all show good cyclic stability and 76.6%, 75.8% and 74.0% hydrogen capacity remains within 300 cycles, respectively. It is a pity that the three alloy electrodes have smaller HRD than the LaNi5-type alloy.
Acknowledgement This work was supported by the Key Projects in International Science and Technology Cooperation from Ministry of Science and Technology of the PRC (2010DFB63510, 2013DFR50940) and National Nature Science Foundation of China (51061001).
references
Fig. 7 e High-rate dischargeability of the LaeYeNieMneAl alloy electrodes at 298 K.
[1] Cao ZJ, Ouyang LZ, Wang H, Liu JW, Sun LX, Felderhoff M, et al. Development of Zr-Fe-V alloys for hybrid hydrogen storage system. Int J Hydrogen Energy 2016;41:11242e53.
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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[2] Liu JJ, Han SM, Li Y, Zhao X, Yang SQ, Zhao YM. Cooperative effects of Sm and Mg on electrochemical performance of LaMg-Ni-based alloys with A2B7- and A5B19-type super-stacking structure. Int J Hydrogen Energy 2015;40:1116e27. [3] Yan HZ, Kong FQ, Xiong W, Li BQ, Li J, Wang L. New La-Fe-B ternary system hydrogen storage alloys. Int J Hydrogen Energy 2010;35:5687e92. [4] Zhu M, Peng CH, Ouyang LZ, Tong YQ. The effect of nanocrystalline formation on the hydrogen storage properties of AB3-base Ml-Mg-Ni multi-phase alloys. J Alloys Compd 2006;426:316e21. [5] Anbarasu S, Muthukumar P, Mishra SC. Thermal modeling of LmNi4.91Sn0.15 based solid state hydrogen storage device with embedded cooling tubes. Int J Hydrogen Energy 2014;39(28):15549e62. [6] Bliznakov S, Lefterova E, Dimitrov N, Petrov K, Popov A. A study of the Al content impact on the properties of MmNi4.4xCo0.6Alx alloys as precursors for negative electrodes in NiMH batteries. J Power Sources 2008;176:381e6. [7] Zhai TT, Yang T, Yuan ZM, Zhang YH. An investigation on electrochemical and gaseous hydrogen storage performances of as-cast La1-xPrxMgNi3.6Co0.4 (x ¼ 0-0.4) alloys. Int J Hydrogen Energy 2014;39:14282e7. [8] Liu JJ, Li Y, Han D, Yang SQ, Chen XC, Zhang L, et al. Electrochemical performance and capacity degradation mechanism of single-phase La-Mg-Ni-based hydrogen storage alloys. J Power Sources 2015;300:77e86. [9] Li RF, Wan J, Wang F, Ding CP, Yu RH. Effect of nonstoichiometry on microstructure and electrochemical performance of La0.8GdxMg0.2Ni3.15Co0.25Al0.1(x ¼ 0-0.4) hydrogen storage alloys. J Power Sources 2016;301:229e36. [10] Ouyang LZ, Cao ZJ, Li LL, Wang H, Liu JW, Min D, et al. Enhanced high-rate discharge properties of La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 with added graphene synthesized by plasma milling. Int J Hydrogen Energy 2014;39:12765e72. [11] Cao ZJ, Ouyang LZ, Li LL, Lu YS, Wang H, Liu JW, et al. Enhanced discharge capacity and cycling properties in highsamarium, praseodymium/neodymium-free, and low-cobalt A2B7 electrode materials for nickel-metal hydride battery. Int J Hydrogen Energy 2015;40:451e5. [12] Young K, Ouchi T, Shen H, Bendersky LA. Hydrogen induced amorphization of LaMgNi4 phase in metal hydride alloys. Int J Hydrogen Energy 2015;40:8941e7. [13] Ma ZW, Zhu D, Wu CL, Zhong CL, Wang QN, Zhou WH, et al. Effects of Mg on the structures and cycling properties of the LaNi3.8 hydrogen storage alloy for negative electrode in Ni/ MH battery. J Alloys Compd 2015;620:149e55. [14] Liu JJ, Han SM, Han D, Li Y, Yang SQ, Zhang L, et al. Enhanced cycling stability and high rate dischargeability of (La,Mg)2Ni7type hydrogen storage alloys with (La,Mg)5Ni19 minor phase. J Power Sources 2015;287:237e46. [15] Zhang JX, Villeroy B, Knosp B, Bernard P, Latroche M. Structural and chemical analyses of the new ternary La5MgNi24 phase synthesized by Spark Plasma Sintering and used as negative electrode material for Ni-MH batteries. Int J Hydrogen Energy 2012;37:5225e33. [16] Tian X, Yun GH, Wang HY, Shang T, Yao ZQ, Wei W, et al. Preparation and electrochemical properties of La-Mg-Nibased La0.75Mg0.25Ni3.3Co0.5 multiphase hydrogen storage alloy as negative material of Ni/MH battery. Int J Hydrogen Energy 2014;39:8474e81.
[17] Baddour-Hadjean R, Meyer L, Pereira-Ramos JP, Latroche M, gan A. An electrochemical study of new La1Percheron-Gue xCexY2Ni9 (0 x 1) hydrogen storage alloys. Electrochim Acta 2001;46:2385e93. [18] Belgacem YB, Khaldi C, Boussami S, Lamloumi J, Mathlouthi H. Electrochemical properties of LaY2Ni9 hydrogen storage alloy, used as an anode in nickel-metal hydride batteries. J Solid State Electrochem 2014;18:2019e26. [19] Yan HZ, Xiong W, Wang L, Li BQ, Li J. A kind of yttriumnickel-rare earth system hydrogen storage alloy. Chinese patent 2014; CN201410429187.7. [20] Yan HZ, Li BQ, Xiong W, Wang L, Li J. A kind of yttriumnickel-rare earth system hydrogen storage alloy. Chinese patent 2014; CN201410429202.8. [21] Yan HZ, Li BQ, Xiong W, Wang L, Li J. A kind of yttriumnickel-rare earth system hydrogen storage alloy and secondary battery containing the hydrogen storage alloy. Chinese patent 2014; CN201410427281.9. [22] Zhang JL, Han SM, Li Y, Liu JJ, Yang SQ, Zhang L, et al. Effects of PuNi3- and Ce2Ni7-type phase abundance on electrochemical characteristics of La-Mg-Ni-based alloys. J Alloys Compd 2013;581:693e8. [23] Liu JJ, Han SM, Li Y, Yang SQ, Shen WZ, Zhang L, et al. An investigation on phase transformation and electrochemical properties of as-cast and annealed La0.75Mg0.25Nix (x ¼ 3.0, 3.3, 3.5, 3.8) alloys. J Alloys Compd 2013;552:119e26. [24] Zhang J, Fang F, Zheng SY, Zhu J, Chen GR, Sun DL, et al. Hydrogen-induced phase transitions in RNi3 and RY2Ni9(R ¼ La, Ce) compounds. J Power Sources 2007;172:446e50. [25] Wang W, Chen YG, Wu CL. Hydrogen-induced amorphization in LaNi2.5M0.5 (M ¼ Ni, Fe, Mn, Si) alloys. Rare Met Mater Eng 2011;40:2080e2. [26] Van Mal HH, Buschow KHJ, Miedema AR. Hydrogen absorption in LaNi5 and related compounds: experimental observations and their explanation. J Less Common Met 1974;35:65e76. [27] Gao J, Yan XL, Zhao ZY, Chai YJ, Hou DL. Effect of annealed treatment on microstructure and cyclic stability for La-Mg-Ni hydrogen storage alloys. J Power Sources 2012;209:257e61. [28] Xiong W, Li BQ, Wang L, Li J, Kong FQ, Yan HZ. Investigation of the thermodynamic and kinetic properties of La-Fe-B system hydrogen-storage alloys. Int J Hydrogen Energy 2014;39:3805e9. [29] Hu ZL. Hydrogen storage material. 1st ed. Beijing: Chemical Industrial Press; 2002. [30] Li Y, Han SM, Li JH, Hu L. Study on phase structure and electrochemical properties of Ml1-xMgxNi2.80Co0.50Mn0.10Al0.10 (x ¼ 0.08, 0.12, 0.20, 0.24, 0.28) hydrogen storage alloys. Electrochim Acta 2007;52:5945e9. [31] Liu YF, Pan HG, Gao MX, Lei YQ, Wang QD. XRD study on the electrochemical hydriding/dehydriding behavior of the LaMg-Ni-Co-type hydrogen storage alloys. J Alloys Compd 2005;403:296e304. [32] Liu ZY, Yan XL, Wang N, Chai YJ, Hou DL. Cyclic stability and high rate discharge performance of (La,Mg)5Ni19 multiphase alloy. Int J Hydrogen Energy 2011;36:4370e4. [33] Monnier J, Chen H, Joiret S, Bourgon J, Latroche M. Identification of a new pseudo-binary hydroxide during calendar corrosion of (La, Mg)2Ni7-type hydrogen storage alloys for Nickel-Metal Hydride batteries. J Power Sources 2014;266:162e9.
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Investigations on AB3-, A2B7- and A5B19-type Lae YeNi system hydrogen storage alloys Huizhong Yan a,b,c,*, Wei Xiong a,b, Li Wang a,b, Baoquan Li a,b, Jin Li a,b, Xin Zhao a,b a
Baotou Research Institute of Rare Earths, Baotou 014030, PR China National Engineering Research Center of Rare Earth Metallurgy and Functional Materials, Baotou 014030, PR China c Tianjin Baogang Research Institute of Rare Earths Co., Ltd, Tianjin 300300, PR China b
article info
abstract
Article history:
The structure and properties of new LaeYeNi system alloys with high hydrogen-storing
Received 23 July 2016
capacity were investigated using X-ray diffraction (XRD), scanning electron microscope
Received in revised form
(SEM), solid-H2 reactions (P-C-I curves) and electrochemical measurements. The LaY2-
4 September 2016
Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-
Accepted 7 September 2016
type) hydrogen storage alloys were prepared with the induction-melting rapid-quenching
Available online xxx
method and annealed at 1148 K for 16 h. The LaeYeNieMneAl alloys were also compared with commercial AB5-type hydrogen storage alloy with high capacity. Similarly to Lae
Keywords:
MgeNi system hydrogen storage alloy, LaeYeNi system alloys are multiphase structures
LaeYeNi hydrogen storage alloy
and Y element in the LaeYeNi alloys avoid or delay the hydrogen-induced amorphous
Hydride
(HIA) of the alloys in the hydrogenation/dehydrogenization process. The hydrogen
Hydrogenation/dehydrogenation
storage capacities of the A2B7- and A5B19-type alloys at 313 K are 1.48 wt.% and 1.45 wt.%,
Electrochemical properties
respectively, which are larger than that of the AB5-type alloy (1.38 wt.%). The maximum
Ni-MH battery
discharge capacities of the A2B7- and A5B19-type alloy electrodes at 298 K are 385.7 mAh g1 and 362.1 mAh g1, respectively, which are larger than that of the AB5-type alloy (356.1 mAh g1). The maximum discharge capacity of the A2B7-type alloy exceeds the theoretical capacity (372 mAh g1) of the AB5-type alloy. The A2B7- and A5B19-type alloy electrodes have better cycling ability than the AB5-type alloy. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen storage is one of the key technologies for hydrogen energy appliance which provides an ideal solution for the dwindling global energy crisis and ever-increasing environmental pollution [1]. Rare earth hydrogen storage alloys are the most mature hydrogen storage products, which are mainly used for the negative materials of nickel-metal
hydride (Ni-MH) batteries [2,3] and gas-phase hydrogen storage devices [4,5]. AB5-type LaNi5-based alloys are currently the main commercial hydrogen storage materials. However, the theoretically electrochemical capacity (372 mAh g1) or maximal hydrogen storage amount (160 cm3 g1 or 1.43 wt.%) after optimization of this material [6] often hardly satisfies the required hydrogen storage capacity because of its restraining intrinsic structure (CaCu5-type). RE-Mg-Ni (RE ¼ Rare Metals) system AB3-3.8-type metal hydride alloys are of special
* Corresponding author. Baotou Research Institute of Rare Earths, Baotou 014030, PR China. E-mail address: [email protected] (H. Yan). http://dx.doi.org/10.1016/j.ijhydene.2016.09.049 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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concern because of the higher discharge capacity (~400 mAh g1), and a series of significant developments in the study of this material has been made in the past few years [7e12]. RE-Mg-Ni hydrogen storage alloys have been widely used as negative electrode active materials for Ni-MH batteries as substitutes for conventional AB5-type alloys [10e12]. The active Mg element is one of the main compositions in RE-MgNi system alloys. The chemical composition of RE-Mg-Ni system alloys is difficult to control in the high-temperature melting manufacturing process because of the high vapor pressure of Mg. Ultra-fine magnesium powder, which is formed by Mg volatilization, becomes a safety hazard. Thus, new manufacturing techniques are investigated, such as smelting protection with helium gas [7,12,13], various sintering technologies [14,15], high-powered ball-grinding technology [16] and so on. However, the application of these techniques has either a high cost or a complicated process. So, the research and development of Mg-free hydrogen storage alloys with high hydrogen-storing capacity is of great significance. Combining LaNi5 and YNi2 binary compounds with hydrogen storage properties, Baddour-Hadjean et al. [17] studied the LaeYeNi ternary alloy, which is equivalent to an overall substitution of the Mg element in the LaeMgeNi system alloy by a rare earth Y element. By studying AB3-type La1xCexY2Ni9(0 x 1) alloys, it is found that the LaY2Ni9 alloy is a PuNi3-type structure and forms LaY2Ni9H12 after hydrogenation, which excels the hydrogen capacity of LaMg2Ni9 alloy under identical conditions. However, the maximum discharge capacity of LaY2Ni9 alloy electrodes is only 265 mAh g1. Belgacem et al. [18] also tested the electrochemical properties of LaY2Ni9 alloy electrode. The maximum discharge capacity of the alloy electrodes is 258 mAh g1 within five cycles, and 54% of the capacity is maintained after 100 cycles, which is far from meeting the needs of the application. In our previous work [19e21], the reversible hydrogen storage performance of AB3-, A2B7- and A5B19-type LaeYeNi system alloys is noticeably improved by adjusting the alloy composition using the element substitution method. Importantly, LaeYeNi system alloys can be directly prepared using the high-temperature melting method, which solves the preparation problems for Mg-based hydrogen storage alloys. The structure and properties of the representative AB3-, A2B7- and A5B19-type LaeYeNieMneAl hydrogen storage alloys were systematically investigated in this paper and compared with commercial AB5-type hydrogen storage alloy with high capacity.
were at least 99 wt.%. The prepared alloy flakes were annealed in vacuum of 102 Pa at 1148 K for 16 h. The annealed alloy flakes and commercial AB5-type (LaCe)Ni3.8Co0.7Mn0.4Al0.2 hydrogen storage alloy were mechanically pulverized into powder particles of 38e74 mm in size for the electrochemical measurements. The phases of the alloy powders were characterized by Xray diffraction (XRD) using a Philips-PW 1700 X powder diffractometer with Cu Ka radiation at 40 kV, 200 mA in the range from 0 to 80 with 0.02 min1, and the diffraction patterns were analyzed with a Rietveld refinement (using the software MAUD). The morphologies of the alloys were examined by HITACHI S-3400N scanning electron microscope (SEM) linked with an energy dispersive X-ray spectrometer (EDS). Pressure-composition isotherms for the H2 absorption/ desorption reactions were determined over the pressure range of 103 MPa to 2.0 MPa in a Sieverts testing device. The alloy flakes were mechanically broken into small particles of 74 mm1.2 mm in size before testing. Alloy particles with a mass of approximately 5 g were placed in the reaction chamber, evacuated for 60 min at 343 K and then allowed to react with hydrogen gas (99.999% purity) under a pressure of 2 MPa. The chamber was then slowly cooled to room temperature and held at that temperature for 30 min. De-hydriding was performed by heating the chamber to 343 K and evacuating it for 60 min until the hydrogen pressure was below 103 MPa. Five hydriding/de-hydriding cycles were performed to ensure that the alloys were fully activated. Next, the P-C isotherms were measured at 298 K, 313 K, 333 K and 343 K. MH electrodes were prepared by mixing 0.1 g alloy powder with 0.4 g carbonyl nickel powder and then cold-pressed into pellets with 15 mm in diameter under a pressure of 16 MPa. This pellet was then placed between two Ni gauze layers, and the edges were tightly spot-welded to maintain good electrochemical contact between the pellet and the Ni gauze. A Ni lead wire was then attached to the Ni gauze by spot-welding to prepare the hydrogen storage alloy electrode (MH electrode). Electrochemical measurements were performed at 298 K in a half-cell consisting of a prepared MH electrode and a sintered Ni(OH)2/NiOOH counter electrode with an excess capacity immersed in 6 mol L1 KOH electrolyte. The discharge capacity and cycle stability were measured by galvanostatic method as follows: each electrode was charged at 70 mA g1 for 6 h, which was followed by a 5-min break, and then was subsequently discharged at 70 mA g1 to the cut-off potential of 1.0 V versus the counter electrode. All tests were measured at room temperature (298 K).
Results and discussion Experimental
Phase structure
The chemical compositions of the investigated LaeYeNie MneAl alloys are LaY2Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-type). These alloys were prepared in a 0.05 MPa argon atmosphere using a vacuum induction-quenching furnace with a rotating copper wheel. In this work, the linear velocity of the copper wheel was 4.33 m s1. The purities of the component metals
Fig. 1 shows the refined analysis of XRD patterns for the LaY2Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-type) alloys, and the results are listed in Table 1. The alloys have a multiphase microstructure. The AB3-type alloy consists of LaY2Ni9-type phase, Ce2Ni7type phase and a notably small quantity of LaNi5-type phase. The A2B7-type alloy consists of Ce2Ni7-type phase and Gd2Co7-
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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Fig. 1 e Refined analyses of powder XRD data of the LaeYe NieMneAl alloys.
type phase. The A5B19-type alloy contains Ce5Co19-type phase, Gd2Co7-type phase, Ce2Ni7-type phase and LaNi5-type phase. The LaeYeNi and LaeMgeNi system alloys have similar constituent phases and phase structures [10,17,22e24]. PuNi3-type, Ce2Ni7- or Gd2Co7-type and Ce5Co19-type phases are most common phases in LaeYeNi and LaeMgeNi
system alloys. These multi-component alloys consist of alternating structural AB5 and A2B4 slabs stacked along the c-direction. The AB3-, A2B7- and A5B19-type phases exhibit similar super-stacking structures with [AB5] and [A2B4] (Laves) subunits in a ratio of 1:1, 2:1 and 3:1, respectively [8,12]. Therefore, it can be asserted that the Y element and Mg element have similar effects in the two series of hydrogen storage alloys. Both Mg in the LaeMgeNi alloy and Y in the LaeYeNi alloy increase the effective hydrogen storage capacity and structural stability of the corresponding alloys in the hydrogenation/dehydrogenization process and avoid or delay the hydrogen-induced amorphous (HIA) of the alloys [13,24]. Another study has shown that the HIA of the alloy can be suppressed to a certain extent because the Ni element in LaNi3-type alloy is partly substituted by a small amount of Mn and Si elements [25]. According to the research results, the HIA of LaeYeNieMneAl alloys in this work can be effectively suppressed because of the Y element at A-side and Mn element at B-side, which are confirmed by subsequent results of the solid-H2 reaction and electrochemical measurement in this paper. Some changes in microstructure of the alloys occur during the annealing process. For example, the LaY2Ni9-type phase in the AB3-type alloy and alloy composition have identical A/B ratio (1/3), but the A/B ratios of Ce2Ni7-type phase and LaNi5type phase (2/7 and 1/5, respectively) are less than 1/3. The Ce2Ni7-type phase is formed through the peritectic reaction between LaNi5-type and LaY2Ni9-type phases, and some of the LaY2Ni9-type phase transform into Ce2Ni7-type phase because of the partial loss of A-side elements [23]. SEM micrographs of the LaeYeNieMneAl alloys are shown in Fig. 2. EDS results from selected areas in each micrograph reveals the phase composition of the alloy. The phases in the AB3-type alloy are LaY2Ni9- (light grey region), Ce2Ni7-type (dark grey region). The composition of the areas in A2B7-type alloys is identical to that of the alloys, suggesting that the composition of Ce2Ni7- and Gd2Co7-type phases is basically the same. The phases in the A5B19-type alloy are mainly Ce5Co19- (dark grey region), Gd2Co7- and Ce2Ni7-type (light grey region). The LaNi5-type phase (black region, see the inset in Fig. 2) is detected on one side of the alloy flakes near the copper wheel, which may indicate that the annealing of this alloy is not enough.
Table 1 e Characteristics of the phases in the LaeYeNieMneAl alloys. Alloys
AB3
A2B7
A5B19
a
Phase
LaY2Ni9(3R) Ce2Ni7(2H) LaNi5 Ce2Ni7(2H) Gd2Co7(3R) Ce5Co19(3R) Gd2Co7(3R) Ce2Ni7(2H) LaNi5
Parameters of fita Rp ¼ 12.73 Rwp ¼ 16.93 S ¼ 2.54 Rp ¼ 11.73 Rwp ¼ 15.93 S ¼ 2.59 Rp ¼ 11.58 Rwp ¼ 15.66 S ¼ 2.47
Lattice constant
Cell volume ( A3)
Phase abundance (wt.%)
a( A)
c( A)
5.0624 5.0343 4.9752 5.0237 5.0115
24.5314 24.5078 3.9779 24.3722 36.5499
544.44 537.90 85.27 532.67 794.95
51.92(±0.21) 46.89(±3.66) 1.19(±0.13) 81.36(±0.17) 18.64(±0.86)
5.0024 5.0041 4.9497 4.9764
48.4994 36.4173 25.0391 3.9956
1051.02 789.73 531.24 85.69
66.55(±0.11) 14.51(±1.35) 11.79(±1.65) 7.16(±0.31)
Rp ¼ pattern factor, Rwp ¼ weighted pattern factor, S ¼ goodness of fit.
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Fig. 2 e SEMs of the LaeYeNieMneAl alloys.
Hydrogenation/dehydrogenation reaction characteristics The P-C isotherms of the LaeYeNieMneAl and AB5-type alloys are shown in Fig. 3. The plateau features of hydrogenation/dehydrogenation are decided by the content of La and Ni elements in the LaeNi-based hydrogen storage alloy. It is wellknown that a higher Ni content at the B side corresponds to a higher plateau pressure for the identical composition at the A side owing to the atomic radius of Ni (1.62 A) being somewhat smaller than that of substitute elements, which is in agreement with the decrease of lattice parameters. A higher La content at the A side corresponds to a lower plateau pressure for the identical composition at the B side owing to the atomic radius of La (1.87 A) being somewhat larger than that of substitute elements, which is in agreement with the increase of lattice parameters [26]. The plateau pressure decreases or increases when Ni or La is partly substituted by other elements, respectively. In this paper, the Ni content in three LaeYeNie MneAl alloys with identical compositions (LaY2) at the A side increases from 57.7% and 61.8% to 63.9% with the decrease in A/B ratio. Thus, the plateau pressure of the three alloys increases in order of AB3-type, A2B7-type and A5B19-type at identical temperatures. In Fig. 3, the plateau characteristics of AB3-type alloys are not obvious. The LaY2Ni9- and Ce2Ni7-type phases in the AB3-type alloy have similar equilibrium pressures because of their similar stacking structures [23,27] and cell volumes. The hydrogen storage amount of the AB3-type alloy is only 0.85 wt.%, and about half of them can be released because
AB3
1
A 2 B7 A5B19
PH2 ( MPa)
AB5 0.1
0.01
1E-3 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
[H] ( wt.%) Fig. 3 e Absorption/desorption Pec isotherms of the LaeYe NieMneAl alloys at 313 K.
AB3-type hydride decomposition pressure is very low, about 103 MPa orders of magnitude. Two obvious plateaus are observed for the A2B7-type alloy. The Gd2Co7-type phase with a larger cell unit volume, which can provide more available interstices for H in the lattice, exhibits a lower hydrogen absorption/desorption plateau pressure (approximately 0.01 MPa for the hydride decomposition pressure) [13]. The hydride decomposition pressure at the higher plateau for the Ce2Ni7-type phase with a smaller cell unit volume is approximately 0.04 MPa. The Ce2Ni7-type phase has a wider plateau than the Gd2Co7-type phase because of the significantly greater abundance of the Ce2Ni7-type phase. The hydrogen storage amount (1.48 wt.%; the calculated electrochemical capacity is 397 mAh g1) of the A2B7type alloy is maximal in the three LaeYeNieMneAl alloys, which is distinctly larger than that of AB5-type alloy (1.38 wt.%; the calculated electrochemical capacity is 370 mAh g1). The hydrogen absorption/desorption plateau of the A5B19-type alloy with multiphase structure is not flat, which reflects the hydrogen absorption/desorption characteristics of the phases with different cell volume. The lower plateaus (approximately 0.06 MPa for the hydride decomposition pressure) represent the hydrogen absorption/ desorption characteristics of the Ce5Co19-type phase with large unit cell volume. With the increasing of the plateau pressure (up to approximately 0.10 MPa for the hydride decomposition pressure), the combination of the Gd2Co7-, Ce2Ni7-, and LaNi5-type phase plateaus is reflected in the order of decreasing cell volume [23]. The A5B19-type alloy has a slightly lower hydrogen storage amount (1.45 wt.%) than the A2B7-type alloy but a greater value than the AB5type alloy. Research results on singer-phase LaeMgeNi alloys show that the order of the plateau pressure is La2MgNi9 (AB3type) < La3MgNi14 (A2B7-type) < La4MgNi19 (A5B19-type) alloys [8], which is consistent with the above experimental results. However, the hydrogen storage amount of the alloys decreases with the increase in [AB5]/[A2B4] ratios [8], i.e., the hydrogen storage amount decreases in the order of AB3-type, A2B7-type and A5B19-type alloys. By contrast, the AB3-type alloy has the smallest hydrogen storage amount among all three LaeYeNieMneAl alloys in this work. The main reason has three aspects. First, only approximately half of the AB3type alloy is AB3-type phase (LaY2Ni9). Second, the higher lattice strain of AB3-type phase in the hydrogen absorption process leads to HIA [8,24,25]. Third, the notably low hydrogen desorption plateau pressure of AB3-type alloy indicates the formation of a high-stability hydride phase [24]. The A5B19-
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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and A2B7-type alloys have similar hydrogen storage amounts because the A5B19-type alloy contains approximately 25% of A2B7-type phase with higher hydrogen storage amount and a moderate hydride decomposition pressure. To further understand the thermodynamic properties of the three LaeYeNieMneAl alloys, Fig. 4 shows a Linear fitting Van't Hoff plot of the logarithmic mid-plateau pressure versus the reciprocal of the absolute temperature. The pressure was obtained from the P-C-I curves of the AB3-, A2B7-, A5B19- and AB5-type alloys at different temperatures (298 K, 313 K, 333 K and 343 K). The hydride formation enthalpies DH and entropies DS of the alloys were calculated from the following van't Hoff equation: ln PH2 ¼ DH=RT DS=R and the Gibbs free energy DG was calculated from the following equation: DG ¼ DH TDS Other details refer to a previous paper [28]. The calculated thermodynamic data are listed in Table 2. From the results in Table 2, the DH and DS for the three LaeYeNieMneAl alloys all meet the requirements for a hydrogen storage alloy [29]. As observed in Table 2, the absolute DG value of the AB3-type alloy is obviously greater other alloys, which indicates that the hydrides of the AB3-type alloy exhibit higher thermodynamic stability.
Electrochemical reaction characteristics Fig. 5 shows the discharge curves of three LaeYeNieMneAl alloy electrodes, and the LaNi5-type alloy electrode is a reference alloy for comparison; the discharge curves correspond to the maximum discharge capacity. The mid-discharge potential and maximum discharge capacity of the LaNi5-type alloy are approximately 1.3 V and 356.1 mAh g1, respectively. The A5B19-type alloy has a slightly higher mid-discharge potential and a larger maximum discharge capacity (362.1 mAh g1) than the LaNi5-type alloy. The A2B7-type alloy has a slightly lower mid-discharge potential and a larger maximum discharge capacity (385.7 mAh g1), than the LaNi5-type alloy.
Fig. 4 e Logarithm of the equilibrium pressure (PH2 ) versus the reciprocal of temperature (1/T).
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This maximum discharge capacity exceeds the theoretical capacity (372 mAh g1) of the LaNi5-type alloy [6], which is largest among these alloys. The AB3-type alloy has the lowest mid-discharge potential (approximately 1.26 V) and smallest maximum discharge capacity (321.4 mAh g1) among these alloys. As expected, the evolution of electrode discharge capacity essentially follows the same trend as the hydrogen storage amount, which is demonstrated in the P-C-I curves. The electrochemical data about the activation, maximum capacity and cyclic stability are summarized in Table 3. Fig. 6(a) shows the activation curves of the three LaeYeNie MneAl alloy electrodes and the LaNi5-type alloy electrode. The A2B7- and A5B19-type alloys, which are identical to the LaNi5-type alloy, have excellent activation capability and can approach or attain maximum capacity within 3 cycles. The AB3-type alloy has relatively poor activation property because of its poor kinetics which is discussed further below. In the charge/discharge process, for the LaY2Ni9-type phase of AB3type alloy, the hydrogen absorption by its La(Y)Ni2 subunits makes the local lattice disorderly and causes the formation of the amorphous phase. Simultaneously, the La(Y)Ni5Hx crystallites, which are formed by the La(Y)Ni5 subunit, start to precipitate [24]. The hydrogen absorption/desorption reaction of the alloy is further catalyzed by the precipitated La(Y)Ni5Hx crystallites [30,31], which makes the AB3-type alloy gradually achieve the maximum capacity through more charge/ discharge cycles but decreases the electrochemical capacity of the AB3-type alloy. Fig. 6(b) shows the discharge capacity retention, which is the ratio of discharge capacity and maximum capacity during the cycle test, versus the number of charge/discharge cycles for the three LaeYeNieMneAl alloy electrodes and the LaNi5type alloy electrode. The discharge capacity retention can represent the cyclic stability of the alloy electrode. Fig. 6(b) and Table 3 show that the discharge capacity retentions of the A2B7-, A5B19-, AB3- and AB5-type alloys are 76.6%, 75.8%, 47.6% and 74.0% in 300 cycles, respectively. The A2B7- and A5B19-type alloys have better cycle stabilities than the AB5-type alloy. The AB3-type has the worst cycle stability among these alloys. The principal factors that affect the cyclic properties of the AB3-, A2B7- and A5B19-type alloys are pulverization and amorphization caused by the strains which are responsible for lattice mismatch in the hydrogen absorption/desorption or charge/discharge process. The pulverization can cause the accelerated corrosion of alloy particles. The amorphization destroys the super-stacking structures of the phases in the alloys. The Y element in LaeYeNi system alloys can restrain the HIA [17,24], and the mismatch between Laves and LaNi5 layers in the ternary phases is reduced [15], so the multielement alloys have better cyclic stability than their corresponding LaeNi binary ones. Based on the research results for the capacity degradation mechanism of single-phase LaeMgeNi system hydrogen storage alloy [8], higher [AB5]/[A2B4] ratios of the A5B19- (3:1) and A2B7-type phase (2:1) can reduce the structural mismatch and decrease the lattice strains of the alloys. Consequently, the amorphization and pulverization of the alloys are restrained in the hydrogenation/dehydrogenization process. As a result, the cycling stability of the alloys is improved. In contrast, the A5B19-type phase has higher [AB5]/[A2B4] ratio
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Table 2 e Thermodynamic parameters of the LaeYeNieMneAl alloys. Alloys
Hydride decomposition pressure at 313 K (MPa) 3
AB3 A 2B 7
~10 0.01 0.04 0.06 0.10 0.025
A5B19 AB5
DH/kJ mol1 H2
DS/J K1 mol1 H2
DG/kJ mol1 H2 (298 K)
35.36(±1.54) 32.24(±1.39) 31.13(±0.45) 30.90(±0.45) 29.63(±0.35) 32.66(±0.57)
67.03(±4.79) 68.94(±4.40) 75.07(±1.34) 73.15(±1.34) 76.79(±1.15) 75.64(±1.72)
15.39(±2.97) 11.70(±2.71) 8.76(±0.85) 9.10(±0.85) 6.75(±0.70) 10.12(±1.08)
–1
Discharge capacity (mAh g )
400
(a)
350 300 250
AB3
200
A2 B 7
150
A5B19
100
AB5
50 0
2
4
6
8
10
Cycle number (n)
Fig. 5 e Discharge potential curves of the LaeYeNieMneAl alloy electrodes.
than the A2B7-type phase. Accordingly, the A5B19-type alloy should have a better cyclic stability than the A2B7-type alloy [32]. Zhang et al. [15] reported that the samples with a larger amount of A5B19-type phase showed better resistance to corrosion in the charge/discharge process. In fact, the A2B7type alloy has a slightly better cycling stability than the A5B19type alloy in the present paper. This result is attributed to the more phases and phase boundaries in the A5B19-type alloy. The microcracks formed along the phase boundary may accelerate the pulverization process during the charging/discharging cycles [9]. The AB3-type phase in the AB3-type alloy has lowest [AB5]/ [A2B4] ratio (1:1) in the phases with the super-stacking structures, so the poor stability of the AB3-type alloy is attributed to the amorphization of Laves subunits [13,24,27,32] in addition to severe pulverization of the alloy because of great lattice strains during the hydrogenation/dehydrogenization process
Table 3 e Electrochemical properties of the LaeYeNie MneAl alloy electrodes. Alloys AB3 A2B 7 A5B19 AB5 a b c
Composition LaY2Ni8.2Mn0.5Al0.3 LaY2Ni9.7Mn0.5Al0.3 LaY2Ni10.6Mn0.5Al0.3 (LaCe)Ni3.8Co0.7Mn0.4Al0.2
Na Cmaxb (mAh g1) S300c (%) 7 3 3 3
321.4 385.7 362.1 356.1
47.6 76.6 75.8 74.0
N is the required number of cycles to activate the electrodes. Cmax is the maximum discharge capacity of the alloy electrodes. S300 is the capacity retention ratio after 300 cycles.
Discharge capacity retention (%)
100
AB3
(b)
95
A2 B 7
90
A5B19
85
AB5
80 75 70 65 60 55 50 45
50
100
150
200
250
300
Cycle number (n)
Fig. 6 e Activation curves (a) and cyclic stability curves (b) of the LaeYeNieMneAl alloy electrodes.
[14,31]. Furthermore, there is a greater percentage of active elements such as La, Y, Mn and Al in the AB3-type alloy than in A2B7- and A5B19-type alloys. These elements are more easily corroded and enter the electrolyte, which causes a faster capacity degradation [9,13,33]. The high-rate dischargeability (HRD) reflects the electrochemical kinetics of the alloy electrodes. The HRD is defined and calculated according to the following formula: HRD ¼ Cd =ðCd þ C70 Þ 100% where Cd is the discharge capacity at the discharge current density and at the cut off potential of 1.0 V versus Ni(OH)2/
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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NiOOH counter electrode, C70 is the residual discharge capacity at the discharge current density I ¼ 70 mA g1. In the present work, the discharge current density Id is 1750, 1050 and 350 mA g1. Fig. 7 shows the HRDs of the LaeYeNieMne Al and LaNi5-type alloy electrodes. The HRDs of the three Lae YeNieMneAl alloy electrodes increase with the order of AB3type, A2B7-type and A5B19-type. Note that the three LaeYeNie MneAl alloy electrodes have smaller HRDs than the LaNi5type alloy, which is a new subject for further research. However, the A5B19 phase itself is believed to have superior HRD [14]. Compared with some LaeMgeNi alloys [2,22,23,30], the high-rate dischargeability of A5B19-type LaeYeNieMneAl alloy is not inferior. The principal factors that affect the HRD of the hydrogen storage alloy electrodes are charge transfer on the alloy surface and hydrogen diffusion in the alloy bulk [9,10]. Generally, the factors that affect the charge transfer rate have three aspects. First, the Ni element in the alloy is a catalyst for hydrogen absorption and desorption reactions [9,31]. Second, the AB5-type phase in multiphase alloys acts as an catalyst to accelerate the electrochemical reaction [16,27,30,31]. Third, corrosion products on the alloy surface in an alkaline electrolyte can decrease the charge transfer speed [9,33]. The factors that affect the hydrogen diffusion are mainly related to the diffusion channels such as the grain boundary and phase boundary in the alloy [9,22]. Fig. 7 shows that the HRDs of the three LaeYeNieMneAl alloy electrodes increase with the increase in Ni element content in the alloys. From the constituent phases in the alloys, there is more AB5-type phase (approximately 7%), which acts as the electrocatalyst in the A5B19-type alloy. In addition, the A5B19-type alloy includes more phases, which increases the phase boundaries, provides more tunnels for hydrogen diffusion [16]. In any case, the A5B19-type has the best HRD among the three LaeYeNieMneAl alloy electrodes. Liu et al. [14] also reported that the A5B19-type phase has faster hydrogen desorption rate and generates a catalytic effect on the discharge process of the coexisting A2B7-type phase. The AB3-type has the worst HRD among the three LaeYe NieMneAl alloy electrodes. In addition to the two disadvantages of lower Ni content and fewer AB5-type phase in the
7
alloy, the AB3-type alloy is more easily corroded because it has more active elements, and the corrosion products with high resistance on the alloy surface decrease the charge transfer speed. From the analysis above it can be seen that the charge transfer rate on the alloy surface is the control factor of electrochemical kinetic for the LaeYeNi alloys. The three LaeYe NieMneAl alloy electrodes have smaller HRDs than the LaNi5type alloy because the former has lower Ni content than the latter although the former includes more phases than the latter.
Conclusions The present study investigates AB3-, A2B7- and A5B19-type Lae YeNi system hydrogen storage alloys and aims to develop a new type of Mg-free hydrogen storage alloys which has the advantages of high hydrogen-storing capacity and easy preparation. The main results can be summarized as follows. The AB3-type (LaY2Ni8.2Mn0.5Al0.3), A2B7-type (LaY2Ni9.7Mn0.5Al0.3) and A5B19-type (LaY2Ni10.6Mn0.5Al0.3) LaeYeNi system hydrogen storage alloys have a relatively stable multiphase microstructure because of the partial substitution of La by Y and Ni by Mn. The AB3-type alloy mainly consists of LaY2Ni9and Ce2Ni7-type phases. The A2B7-type alloy is composed of Ce2Ni7- and Gd2Co7-type phases. The A5B19-type alloy contains Ce5Co19-, Gd2Co7-, Ce2Ni7- and LaNi5-type phases. At identical temperatures, the plateau pressure of the three alloys has the following order: A5B19-type > A2B7-type > AB3type. The hydride formation enthalpies DH and entropies DS of the three alloys satisfy the requirements of Ni-MH batteries for hydride electrodes. The A2B7-type alloy has the maximum hydrogen storage amount (1.48 wt.%) among the three alloys, which is distinctly greater than that of the AB5-type alloy (1.38 wt.%). The A2B7- and A5B19-type alloys have excellent activation capability. The A2B7-type alloy has the largest maximum discharge capacity (385.7 mAh g1) among these alloys, which exceeds the theoretical capacity of the AB5-type alloy. The A2B7-, A5B19- and AB5-type alloys all show good cyclic stability and 76.6%, 75.8% and 74.0% hydrogen capacity remains within 300 cycles, respectively. It is a pity that the three alloy electrodes have smaller HRD than the LaNi5-type alloy.
Acknowledgement This work was supported by the Key Projects in International Science and Technology Cooperation from Ministry of Science and Technology of the PRC (2010DFB63510, 2013DFR50940) and National Nature Science Foundation of China (51061001).
references
Fig. 7 e High-rate dischargeability of the LaeYeNieMneAl alloy electrodes at 298 K.
[1] Cao ZJ, Ouyang LZ, Wang H, Liu JW, Sun LX, Felderhoff M, et al. Development of Zr-Fe-V alloys for hybrid hydrogen storage system. Int J Hydrogen Energy 2016;41:11242e53.
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049
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[2] Liu JJ, Han SM, Li Y, Zhao X, Yang SQ, Zhao YM. Cooperative effects of Sm and Mg on electrochemical performance of LaMg-Ni-based alloys with A2B7- and A5B19-type super-stacking structure. Int J Hydrogen Energy 2015;40:1116e27. [3] Yan HZ, Kong FQ, Xiong W, Li BQ, Li J, Wang L. New La-Fe-B ternary system hydrogen storage alloys. Int J Hydrogen Energy 2010;35:5687e92. [4] Zhu M, Peng CH, Ouyang LZ, Tong YQ. The effect of nanocrystalline formation on the hydrogen storage properties of AB3-base Ml-Mg-Ni multi-phase alloys. J Alloys Compd 2006;426:316e21. [5] Anbarasu S, Muthukumar P, Mishra SC. Thermal modeling of LmNi4.91Sn0.15 based solid state hydrogen storage device with embedded cooling tubes. Int J Hydrogen Energy 2014;39(28):15549e62. [6] Bliznakov S, Lefterova E, Dimitrov N, Petrov K, Popov A. A study of the Al content impact on the properties of MmNi4.4xCo0.6Alx alloys as precursors for negative electrodes in NiMH batteries. J Power Sources 2008;176:381e6. [7] Zhai TT, Yang T, Yuan ZM, Zhang YH. An investigation on electrochemical and gaseous hydrogen storage performances of as-cast La1-xPrxMgNi3.6Co0.4 (x ¼ 0-0.4) alloys. Int J Hydrogen Energy 2014;39:14282e7. [8] Liu JJ, Li Y, Han D, Yang SQ, Chen XC, Zhang L, et al. Electrochemical performance and capacity degradation mechanism of single-phase La-Mg-Ni-based hydrogen storage alloys. J Power Sources 2015;300:77e86. [9] Li RF, Wan J, Wang F, Ding CP, Yu RH. Effect of nonstoichiometry on microstructure and electrochemical performance of La0.8GdxMg0.2Ni3.15Co0.25Al0.1(x ¼ 0-0.4) hydrogen storage alloys. J Power Sources 2016;301:229e36. [10] Ouyang LZ, Cao ZJ, Li LL, Wang H, Liu JW, Min D, et al. Enhanced high-rate discharge properties of La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 with added graphene synthesized by plasma milling. Int J Hydrogen Energy 2014;39:12765e72. [11] Cao ZJ, Ouyang LZ, Li LL, Lu YS, Wang H, Liu JW, et al. Enhanced discharge capacity and cycling properties in highsamarium, praseodymium/neodymium-free, and low-cobalt A2B7 electrode materials for nickel-metal hydride battery. Int J Hydrogen Energy 2015;40:451e5. [12] Young K, Ouchi T, Shen H, Bendersky LA. Hydrogen induced amorphization of LaMgNi4 phase in metal hydride alloys. Int J Hydrogen Energy 2015;40:8941e7. [13] Ma ZW, Zhu D, Wu CL, Zhong CL, Wang QN, Zhou WH, et al. Effects of Mg on the structures and cycling properties of the LaNi3.8 hydrogen storage alloy for negative electrode in Ni/ MH battery. J Alloys Compd 2015;620:149e55. [14] Liu JJ, Han SM, Han D, Li Y, Yang SQ, Zhang L, et al. Enhanced cycling stability and high rate dischargeability of (La,Mg)2Ni7type hydrogen storage alloys with (La,Mg)5Ni19 minor phase. J Power Sources 2015;287:237e46. [15] Zhang JX, Villeroy B, Knosp B, Bernard P, Latroche M. Structural and chemical analyses of the new ternary La5MgNi24 phase synthesized by Spark Plasma Sintering and used as negative electrode material for Ni-MH batteries. Int J Hydrogen Energy 2012;37:5225e33. [16] Tian X, Yun GH, Wang HY, Shang T, Yao ZQ, Wei W, et al. Preparation and electrochemical properties of La-Mg-Nibased La0.75Mg0.25Ni3.3Co0.5 multiphase hydrogen storage alloy as negative material of Ni/MH battery. Int J Hydrogen Energy 2014;39:8474e81.
[17] Baddour-Hadjean R, Meyer L, Pereira-Ramos JP, Latroche M, gan A. An electrochemical study of new La1Percheron-Gue xCexY2Ni9 (0 x 1) hydrogen storage alloys. Electrochim Acta 2001;46:2385e93. [18] Belgacem YB, Khaldi C, Boussami S, Lamloumi J, Mathlouthi H. Electrochemical properties of LaY2Ni9 hydrogen storage alloy, used as an anode in nickel-metal hydride batteries. J Solid State Electrochem 2014;18:2019e26. [19] Yan HZ, Xiong W, Wang L, Li BQ, Li J. A kind of yttriumnickel-rare earth system hydrogen storage alloy. Chinese patent 2014; CN201410429187.7. [20] Yan HZ, Li BQ, Xiong W, Wang L, Li J. A kind of yttriumnickel-rare earth system hydrogen storage alloy. Chinese patent 2014; CN201410429202.8. [21] Yan HZ, Li BQ, Xiong W, Wang L, Li J. A kind of yttriumnickel-rare earth system hydrogen storage alloy and secondary battery containing the hydrogen storage alloy. Chinese patent 2014; CN201410427281.9. [22] Zhang JL, Han SM, Li Y, Liu JJ, Yang SQ, Zhang L, et al. Effects of PuNi3- and Ce2Ni7-type phase abundance on electrochemical characteristics of La-Mg-Ni-based alloys. J Alloys Compd 2013;581:693e8. [23] Liu JJ, Han SM, Li Y, Yang SQ, Shen WZ, Zhang L, et al. An investigation on phase transformation and electrochemical properties of as-cast and annealed La0.75Mg0.25Nix (x ¼ 3.0, 3.3, 3.5, 3.8) alloys. J Alloys Compd 2013;552:119e26. [24] Zhang J, Fang F, Zheng SY, Zhu J, Chen GR, Sun DL, et al. Hydrogen-induced phase transitions in RNi3 and RY2Ni9(R ¼ La, Ce) compounds. J Power Sources 2007;172:446e50. [25] Wang W, Chen YG, Wu CL. Hydrogen-induced amorphization in LaNi2.5M0.5 (M ¼ Ni, Fe, Mn, Si) alloys. Rare Met Mater Eng 2011;40:2080e2. [26] Van Mal HH, Buschow KHJ, Miedema AR. Hydrogen absorption in LaNi5 and related compounds: experimental observations and their explanation. J Less Common Met 1974;35:65e76. [27] Gao J, Yan XL, Zhao ZY, Chai YJ, Hou DL. Effect of annealed treatment on microstructure and cyclic stability for La-Mg-Ni hydrogen storage alloys. J Power Sources 2012;209:257e61. [28] Xiong W, Li BQ, Wang L, Li J, Kong FQ, Yan HZ. Investigation of the thermodynamic and kinetic properties of La-Fe-B system hydrogen-storage alloys. Int J Hydrogen Energy 2014;39:3805e9. [29] Hu ZL. Hydrogen storage material. 1st ed. Beijing: Chemical Industrial Press; 2002. [30] Li Y, Han SM, Li JH, Hu L. Study on phase structure and electrochemical properties of Ml1-xMgxNi2.80Co0.50Mn0.10Al0.10 (x ¼ 0.08, 0.12, 0.20, 0.24, 0.28) hydrogen storage alloys. Electrochim Acta 2007;52:5945e9. [31] Liu YF, Pan HG, Gao MX, Lei YQ, Wang QD. XRD study on the electrochemical hydriding/dehydriding behavior of the LaMg-Ni-Co-type hydrogen storage alloys. J Alloys Compd 2005;403:296e304. [32] Liu ZY, Yan XL, Wang N, Chai YJ, Hou DL. Cyclic stability and high rate discharge performance of (La,Mg)5Ni19 multiphase alloy. Int J Hydrogen Energy 2011;36:4370e4. [33] Monnier J, Chen H, Joiret S, Bourgon J, Latroche M. Identification of a new pseudo-binary hydroxide during calendar corrosion of (La, Mg)2Ni7-type hydrogen storage alloys for Nickel-Metal Hydride batteries. J Power Sources 2014;266:162e9.
Please cite this article in press as: Yan H, et al., Investigations on AB3-, A2B7- and A5B19-type LaeYeNi system hydrogen storage alloys, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.09.049