Investigation of modification of hydrogenation and structure properties of multi-substituted LaNi5 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 3 7 ( 2 0 1 2 ) 1 3 2 3 4 e1 3 2 4 2

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Investigation of modification of hydrogenation and structure properties of multi-substituted LaNi5 alloys Chubin Wan a, Huizhong Yan b,c, Xin Ju a,*, Yuting Wang a, Miaofeng Huang a, Fanqing Kong b,c, Wei Xiong b,c a

Department of Physics, University of Science and Technology Beijing, No. 30 Xueyuan Road, Haidian District, Beijing, China Baotou Research Institute of Rare Earth, Baotou, China c National Engineering Research Centre of Rare Earth Metallurgy and Function Materials, Baotou, China b

article info

abstract

Article history:

Multi-substituted LaNi5 alloys prepared by annealing, activating, and cycling were inves-

Received 19 December 2011

tigated systematically using synchrotron radiation X-ray diffraction (XRD), extended X-ray

Received in revised form

absorption fine structure spectroscopy (EXAFS), and X-ray photoelectron spectroscopy to

24 February 2012

clarify the evolution of the structural properties of the alloys. XRD and Rietveld analysis

Accepted 1 March 2012

results show that annealing and activation can cause changes in the isotropy of the lattice

Available online 21 April 2012

constants of the LaMnxNiy phase. EXAFS data on the La-L3-edge show variations in atomic distances between LaeMn and LaeNi during annealing. Chemical shifts in the La 3d and Ni

Keywords:

2p core levels induced by activation can be determined with very high levels of accuracy

Hydrogen storage alloy

using energy differences DEB (Ni 2p1/2eLa 2d5/2). The decrease in the cycling capacity may

XRD

also be attributed to the oxidation of La and Ni in the alloys during cycling, which can resist

EXAFS

the reaction of hydrogen absorption.

XPS

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The intermetallic compound LaNi5 is a typical hydrogen storage alloy with high capacity, easy activation, and fast reaction in the hydridingedehydriding process at room temperature and pressure [1e3]. The equilibrium hydrogen pressure of LaNi5 hydride at moderate temperatures is slightly higher than atmospheric pressure, and the hydridingedehydriding cycles of the compound pulverize itself intensively [4]. Substitution in this binary compound has been widely studied to improve the hydrogenation properties of hydrogen storage compounds [5]. Several reports

have explored the in situ neutron powder diffraction of LaNi5 and LaNi5
xMx to investigate their hydrogen occupation, crystal structures, and defects, among others [6,7]. Palumbo et al. investigated the local structures of LaNi4.5Al0.5 and LaNi4.5Al0.5-H [8] and revealed that Al substitution results in an increase in interatomic NieNi distance. Substituting Ni with Fe in LaNi5-type alloys improves their hydrogenation behaviors because the higher electron attractive power and larger atomic size of Fe are more effective for storage capacity [9]. Li et al. studied the hydrogen storage properties of LaNi3.8Al1.0M0.2 (Ni, Cu, Fe, Al, Cr, and Mn) alloys and demonstrated the linear

* Corresponding author. Tel./fax: þ86 10 62333921. E-mail address: [email protected] (X. Ju). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.026

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Experimental

The alloy was prepared from a mixture of pure powder containing La, Ni, Mn, Fe, Al, and B (purity > 99 wt.%) at a stoichiometric ratio of 1:5.4:0.5:0.18:0.19:0.23 using a vacuum induction furnace under an argon atmosphere of 0.05 MPa. The melting process was repeated several times for homogenization. In each cycle, the button ingot was inverted in the furnace. Afterward, the partial as-cast alloy was vacuum annealed at 1223 K for 4 h and 873 K for 3 h. Activation was performed by exposing the alloys to 3.0 MPa H2 at 293 K. After each hydrogen absorptionedesorption cycle, the alloys were evacuated at 453 K for 1 h to ensure that they were fully dehydrogenized, cooled to room temperature, and then brought into hydrogen at 3.0 MPa and 293 K for absorption. Annealed LaeFeeB alloys were cycled for 5, 30, 70, and 100 times. As-cast, annealed, activated, and cycled alloys were selected for subsequent experiments. Synchrotron XRD patterns were measured in the range of 2q ¼ 10 e80 with a step size of D(2q) ¼ 0.02 at Station BL14B1 of the SSRF (Shanghai, China). A synchrotron radiation ˚ was obtained from a double channelwavelength of 1.2438 A cut Si (1 1 1) monochromator. Rietveld refinements were carried out using Toolbar FullProf Suite software (Version 1.00) [13]. Pseudo-Voigt (PV) profile functions were used with backgrounds modeled by interpolating between manually chosen points [14]. EXAFS spectra of the alloys were obtained at the La L-3 edge (5483 eV) in transmission mode. Measurements were carried out using the 1W1B-XAFS station at BSRF (Beijing, China). Standard normalization and background subtraction procedures were performed in transmission mode with WinXas software [15]. Theoretical EXAFS results were simulated using FEFF-7 software [16]. XPS measurements were carried out under an ultimate pressure of 1  10
6 torr with a PHI 1600 ESCA system using Mg Ka radiation. High-resolution scans of elemental lines were recorded at analyzer pass energies of 187.85 eV and 29.35 eV. The binding energy (BE) of the samples was calibrated by setting the measured BE of C1s to 284.6 eV.

Results and discussion

3.1.

Effects of annealing and activation

3.1.1.

Hydrogen behavior

Fig. 1 shows the absorption curves of as-cast and annealed alloys at 298 K after the first 1st and 10th activation. Under identical conditions, both alloys could be activated completely within 40,000 s. Absorption of the as-cast alloy is delayed upon activation and the activation rate of the annealed alloy is higher. Both alloys are fully activated after the 10th hydrogen absorptionedesorption cycle, as shown in Fig. 1(b). As-cast and annealed alloys can absorb about 1.15 wt.% and 1.20 wt.% hydrogen, respectively, within 100 s. Diffusion of hydrogen is believed to influence the hysteresis phenomenon observed. During the first activation, hydrogen atoms must overcome the hydrogen concentration gradient between the outer and inner environments of unit cells. Thus, the first activation has a longer diffusion path. Compared with the ascast alloy, the annealed alloy has a better-ordered and nonerratic structure, such that hydrogen atoms encounter a shorter diffusion path in annealed alloys and enter the unit cell easily. During the activation, the alloys are completely pulverized, which causes numerous lattice defects, such as

a

1.4

as-cast annealed

1.2

Hydrogen content (wt.%)

2.

3.

1.0

0.8

0.6

0.4

0.2

0.0 0

10000

20000

30000

40000

Hydriding time (s)

b 1.4

annealed as-cast

1.2

Hydrogen content (wt.%)

relationship between plateau pressure and cell volume [10]. Partial substitution of Ni with Mn shortens the activation cycle, improves cycling performance, decreases the maximum discharge ability, and increases the overpotential of the resulting alloy electrode [11]. Our previous work [12] revealed that multi-substituted LaNi5 alloys (LaeFeeB alloys) with a multiphase structure demonstrate outstanding potential as a new type of negative material for use in NieMH secondary cells. In the present work, we investigate multi-substituted (Mn, Al, Fe, and B) LaNi5 alloys, including as-cast, annealed, activated, and cycled LaeFeeB alloys. The crystal, electronic, and local structures of these alloys are investigated using synchrotron radiation X-ray diffraction (XRD), extended Xray absorption fine structure spectroscopy (EXAFS), and Xray photoelectron spectroscopy (XPS). The relationship between alloy structures and hydrogen storage properties is also analyzed.

1.0

0.8

0.6

0.4

0.2

0.0 0

50

100

150

200

Hydriding time (s)

Fig. 1 e The (a) 1st and (b) 10th hydrogen activated curves of the as-cast and annealed alloys at 298 K.

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dislocations. The diffusion path of hydrogen thus shortens and the alloys can absorb more hydrogen rapidly. As shown in Fig. 1(b), the kinetic absorption property of the LaeFeeB alloys used in this work is greater than that of LaNi5-type alloys [12], and absorption velocities are approximately several times faster in the former than in the latter; these findings agree with our previous work [12]. Fig. 2 shows the PeC isotherms of as-cast and annealed alloys at 313 K. The pressure of desorption equilibrium in these two LaeFeeB alloys ranges between 0.05 MPa and 0.1 MPa, which is suitable for negative materials in NieMH batteries.

3.1.2.

Structure evolution

The evolution of crystal structures in the LaeNieMn phase was investigated. Fig. 3 shows the XRD patterns of as-cast, annealed, and their activated alloys at different states. All alloys exhibit similar diffraction patterns, indicating that the phase constitutions of the alloys remain almost unchanged during dehydrogenation in different states. The starting structural models were taken from Pearson’s Handbook [17]. We used the La0.99Mn0.32Ni4.71 phase (Space group: P 6/m m m; ˚ , c ¼ 4.057 A ˚ ) to approximately lattice constant: a ¼ 5.076 A refine the LaeNieMn phase in the four alloys. Fig. 4 illustrates the Rietveld analysis pattern of the activated annealed alloy. The lattice parameters of the LaeNieMn phase calculated using the Rietveld method are shown in Table 1. The estimated standard deviations for unit-cell dimensions are shown in parentheses. LaeNieMn phases in the four samples show significant differences in unit-cell dimensions. The unit cell of the LaeNieMn phase remarkably increases after annealing and decreases after activation. Previous studies have also observed that annealing treatment can increase the unit-cell dimensions of alloys [18e20]. Annealing can cause metal atoms to diffuse to other phase site positions and induce changes in the lattice constants and unit cell volumes. Annealed alloys have larger particle sizes than as-cast alloys. In our study, the lattice constants of the a and c axes decrease during activation, which disagrees with previous studies [21,22]. The value of a/c varies at around 1.251 during annealing and activation, indicating that the lattice variation here is isotropic.

as-cast annealed

Pressure (MPa)

1

0.1

0.01

1E-3 0.0

0.2

0.4

0.6

0.8

1.0

1.2

H weight (wt.%)

Fig. 2 e The absorption/desorption PeC isotherms of the as-cast and annealed LaeFeeB alloys at 313 K.

Fig. 3 e XRD patterns of the as-cast, annealed, and their activated alloys.

XPS allows us to access the local environment of atoms and their chemical shifts. In this paper, we used XPS to follow the annealing and activation processes occurring in LaeNieMneFeeAleB alloys. Fig. 5 shows the core level spectra of as-cast alloys after activation. Surface elements of the alloys were detected, except for some trace elements, such as Al, Fe, and B, based on their peak positions. The main contaminants included carbon and oxygen. Atomic concentrations of the different elements in the samples are given in Table 2. The (Mn þ Ni)/La atomic ratio slightly decreases on the surface of the alloys after annealing and then significantly decreases after activation, suggesting that the atomic concentration of La increases by a small degree on the alloy surface during annealing. Annealing is an effective way of obtaining more homogeneous alloy compositions. Thus, the (Mn þ Ni)/La atomic ratio in the annealed alloy may be considered to be close to the true ratio. A distinct increase in the atomic concentration of La after activation is observed. (Mn þ Ni)/La atomic ratios on the surface of the alloys after activation are 0.8 and 0.4, which agrees with findings in previous studies [23e25]. In our study, less Ni and Mn are found on the surface of alloys than La, although Ni is a major constituent of the bulk alloy. The heats of formation of La2O3 [26], NiO, and MnO [27] were
1792 kJ mol
1,
244 kJ mol
1, and
385 kJ mol
1, respectively. Oxidation tends to occur on the surface and may act as the driving force for the surface migration of La. Hydrogenated activation-induced chemical shifts in the La 3d and Ni 2p core levels (Fig. 6) can be determined with very high levels of accuracy using the energy difference DEB (Ni 2p1/ 2eLa 3d5/2) ¼ 35.8 eV and 36.8 eV, which are reduced by 1 eV and 0.8 eV, respectively, compared with the as-cast and annealed alloys. Oxidation of La on the surface of alloys during activation can be attributed to induce chemical shifts. The BE of La 3d5/2 in La2O3 is approximately 834.80 eV [26], whereas that of La is 835.90 eV [28]. Fourier transforms (FTs) of EXAFS oscillations extracted from the La-L3-edge X-ray absorption spectra were measured

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Fig. 4 e Observed (circles) and calculated intensities from Rietveld refinements of the activated annealed alloy. Positions of Bragg reflections are shown with bars for LaMnxNiy, Cu, and Sn (from top).

5.0752 5.0812 5.0749 5.0754

(2) (2) (3) (3)

˚) c (A 4.0576 4.0592 4.0566 4.0581

(3) (2) (4) (4)

3.2.1.

Hydrogen behavior

La 3d 5/2

La 3d 3/2

Ni 2p 1/2 Ni 2p 3/2

4

30 25 20

˚ 3) Volume (A

10

1.2508 1.2518 1.2510 1.2507

90.5120 90.7622 90.5174 90.5307

5

C 1s

15

a/c

Estimated standard deviations for the unit-cell dimensions are shown in parentheses.

Ni 2p 1/2 Ni 2p 3/2

35

O 1s

40

C KLL

45

La 3d 3/2 La 3d 5/2 Mn 2s O KLL

50

Mn 2p

Fig. 8 shows the cyclic hydrogen capacities of the LaeFeeB alloys subjected to cyclic hydrogen absorption at 298 K. The saturated capacities of the 5th hydrogen absorption at 298 K can reach 1.096 wt.% during the first 400 s. However, the value is slightly reduced to 1.053 wt.% in the 30th cycle, which is approximately 96% of the initial value. In the 70th and 100th cycles, the hydrogen absorption capacities decay continuously, reaching 1.0 wt.% and 0.869 wt.%, respectively. After 100

La 4d -Ni 3s Ni 3p O 2s

As-cast Annealed Activated (as-cast) Activated (annealed)

˚) a (A

Effects of cycles

La 3p

Alloy

3.2.

Ni LMM

Table 1 e Lattice parameters and cell volumes of LaeNieMn phases.

agrees with the increase in volume of the cell unit after annealing in the previous XRD analysis. DW factors of the first shell increase after annealing, whereas those of the second shell decrease, indicating that the atomic disorder increases in the first shell and decreases in the second shell. As previously discussed [29], high-temperature annealing may help in the partial recovery of atomic order. In our work, annealing could enhance the atomic order in the first shell coordination around La atoms and reduce the atomic order in the second shell.

Counts / s (* 10 )

in as-cast and annealed alloys (Fig. 7), revealing real space partial atomic distributions relative to La atoms. FTs were ˚
1 and kmax ¼ 10.2 A ˚
1 using performed between kmin ¼ 5.0 A a Gaussian window. The precise distribution function is usually difficult to determine because of the limited k-range; however, the experimental data range is sufficient for identifying the major differences between the relative atomic distributions [29]. Damping of the signal is distinctly observed ˚ in the FT at an approximate radial distance (R) of 2.7 A amplitude for all atomic pairs (not shown). Similar peak shapes reveal that similar local structures exist in the two alloys. The unit cell of LaNi5 (P6/mmm) contains 1 La atom, 3 Ni atoms at the 3g site, and 2 Ni atoms at the 2c site. In our structural configuration, 6 Ni and Mn atoms are located at ˚ from the La atom and 12 Ni a distance of approximately 2.89 A ˚. and Mn atoms are found at a distance of approximately 3.20 A The main peak in the FT is mainly attributed to the single scattering from the two sets of Ni and Mn atoms described above. We used a conventional procedure to analyze EXAFS signals with the two sets of Ni atoms and two sets of Mn atoms as the first two shells. Except for R and the corresponding DebyeeWaller (DW) factor, s2, all other parameters remained fixed in a least squares fit ðS20 ¼ 0:9Þ. As shown in Fig. 7, the overall agreement with the experimental data is relatively good. Near neighbor distances and DW factors from the analysis described above are presented in Table 3. Although the second LaeNi and LaeMn distances show a negligible change after annealing, the first LaeNi and LaeMn distances increase during annealing. This result

0 1000

800

600

400

200

0

Binding Energy (eV)

Fig. 5 e XPS survey spectra of the activated alloy (as-cast alloy after activation). The inset shows a magnification of the spectra between 824 eV and 876 eV.

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a

Table 2 e Atomic concentrations obtained from the survey spectra of the dehydrogenated alloys. Sample

Atomic concentration (%)

As-cast Annealed Activated (as-cast) Activated (annealed)

La

Ni

Mn

(Mn þ Ni)/La

18.2 21.5 57.1 69.6

74.3 70.9 17.9 16.8

7.5 7.6 25.0 13.6

4.5 3.7 0.8 0.4

Without regard to C, O and trace elements.

hydrogen absorptionedesorption cycles, the saturated capacities are degraded to about 80%. The absorption properties of LaeFeeB alloys, particularly their kinetic properties and cycle lives, are apparently superior to that of LaNi5-type alloys; these improved properties may be attributed to the micro-addition of Al and B elements in the former [30e32].

3.2.2.

b

Crystal and electronic structure

The crystal structures of the LaeFeeB alloys were measured after their complete desorption in different cycles. During XRD, whole patterns of the LaeFeeB alloys and their phase

a

40

La 3d 3/2 Ni 2p 3/2

as-cast activated

La 3d 5/2

4

Counts / s (* 10 )

30

20

Fig. 7 e FT EXAFS signals (squares) of the as-cast and annealed LaeFeeB alloys and the best-fit curve obtained from the two sub-shell models (circles).

Ni 2p 1/2 10

36.8 eV 0

35.8 eV 830

840

850

860

870

880

Binding Energy (eV)

b

50

annealed annealed and activated

La 3d 3/2 Ni 2p 3/2

40

4

Counts / s (* 10 )

La 3d 5/2

constitutions remain almost unchanged after completion of different cycles (data not shown). Fig. 9 illustrates the Rietveld analysis of the LaeFeeB alloys after 70 complete cycles. Rietveld refinement results agree well with the experimental data (Rp ¼ 9.1, Rwp ¼ 11.1, and GOF ¼ 2.1). The lattice parameters of the LaMnxNiy phase obtained from the Rietveld analysis are presented in Fig. 10. The a-value of the unit cell shows

30

Table 3 e La L-3 edge EXAFS fitting results of LaeFeeB alloys.

20

Ni 2p 1/2 10

37.6 eV

0

Alloy

Atomic bond

Bond length ˚) (A

DW and ˚ 2) change s2 (10
2A

As-cast

LaeMn LaeNi LaeMn LaeNi LaeMn LaeNi LaeMn LaeNi

2.71 2.85 3.02 3.14 2.76 2.90 3.04 3.14

0.59 1.11 0.68 0.92 0.82 1.25 0.60 0.74

36.8 eV -10 830

840

850

860

870

880

890

Annealed

Binding Energy (eV)

Fig. 6 e XPS La 3d and Ni 2p core level spectra of the LaeFeeB alloys.

13239

1.2

5.072

1.0

5.071

4.054

c-value

a-value

4.052

5.070

0.8

5 30 70 100

0.6

4.050

R (Å)

Hydrogen contents (wt. %)

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 2 3 4 e1 3 2 4 2

5.069

5.068

0.4

4.048

5.067

0.2

4.046 5.066

0.0

0

0

100

200

300

400

Time / s

Fig. 8 e Cyclic hydrogen absorption capacities of the LaeFeeB alloys at 298 K and 3.0 MPa.

a distinct decrease with each increasing cycle, whereas the cvalue increases. The a/c value continues to decrease from 1.2535 to 1.2501 over 100 hydrogen absorptionedesorption cycles, indicating that the lattices of the LaeNieMn phase are distorted during cycling. Absorption and desorption processes induce phase transformations and volume changes [33e36]. Absorption induces the lattice and volume expansion and hydrides release hydrogen gradually, causing the structures to contract by several degrees and successfully regenerating the LaeFeeB alloy. During hydrogen absorption and desorption, the unit cell can generate lattice imperfections and distortion. The volume of the unit cell does not decrease with the cycling process and degradation of absorption capacity, in fact only approximately 0.1% maximum variation in volume was observed. This result differs from the results of the degradation of hydrogen capacity observed in TiVCrMn alloys [37]. XPS analysis was performed on the alloys after completion of the different cycles to clarify the evolution of the surface structure of LaeFeeB. The spectra for Ni (2p3/2) could be deconvoluted into two Gaussian peaks. The peak ranging from

20

40

60

80

100

Cycle numbers (times)

Fig. 10 e Lattice constants (a and c values) of the LaMnxNiy phase obtained from the Rietveld method as a function of the number of cycles. Dashed lines provide a guide for the eyes.

850.8 eV to 851.9 eV can be attributed to metallic Ni, whereas the broad peak ranging from 854.7 eV to 855.1 eV can be attributed to Ni2þ, as shown in Fig. 11 [38]. The profile for La (3d3/ is also separated into three Gaussian peaks at 2) 835.6 eVe836.1 eV, 838.7 eVe839.0 eV, and 832.9 eVe833.7 eV for metallic La, LaH3, and La3þ, respectively (Fig. 11) [38]. The surface atomic ratio was calculated according to the peak area assuming that the atomic sensitivity factors of Ni (2p3/2) and La (3d3/2) were 26.50 and 2.85, respectively. Although peaks attributed to nickel and lanthanum oxide are not present in the XRD patterns of the LaeFeeB alloys during cycle, XPS results show the presence of Ni2þ, La3þ, and LaH3 species on the surface of the cycled alloys. As listed in Table 4, the atomic ratio of Ni2þ/Ni0 increases as cycling proceeds. Ni is likely oxidized in air when removed from the reactor. A new Ni species other than nickel oxide may form on the surface of the alloys during cycling, which agrees with the previous literature [39]. We discuss here the variation in atomic concentration of La species since the absorption capacity seems to depend on

Fig. 9 e Observed (circles) and calculated intensities from Rietveld refinements of the LaeFeeB alloys after 70 cycles measured at 25  C at BSRF. Positions of Bragg reflections are shown with bars for LaMnxNiy, Cu, and Sn (from top). The difference between the observed and calculated intensities is shown with the bottom line.

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a

b

Ni 2p3/2 Ni Ni

Ni 2p3/2

2+

0

La 3d5/2

La

Intensity

Intensity

La 3d5/2 0

LaH La

La

3+

La

825

3

0

835

845

855

825

865

3+

835

845

855

865

B.E.(eV)

B.E.(eV)

c

d

Ni 2p3/2

Ni 2p3/2

La 3d5/2 Intensity

Intensity

La 3d5/2

825

835

845

855

825

865

835

845

855

865

B.E.(eV)

B.E.(eV)

Fig. 11 e La 3d and Ni 2p XPS patterns of LaeFeeB alloys after (a) 5, (b) 30, (c) 70, and (d) 100 hydrogen absorptionedesorption cycles.

Table 4 e XPS analyses for La 3d and Ni 2p in LaeFeeB alloy. Cycling times

Binding energy/eV (surface composition/mol%) 0

Ni 5 30 70 100

850.8 851.9 850.9 851.7

(27.7) (23.9) (20.4) (21.1)

2þ

0

Ni 854.9 854.9 854.7 855.1

(21.8) (25.3) (27.3) (29.7)

3þ

La 835.9 835.9 835.6 836.1

their surface concentration. The 5th desorbed sample reveals that metallic La species are dominant on the surface of LaeFeeB alloys. LaH3 and La3þ are scarcely found in the alloys after five cycles (Fig. 11 and Table 4). As cycling continues, the concentration of metallic La decreases, with the minimum observed after the 70th cycle. The maximum content of La3þ species is achieved after the 70th cycle, and the concentration of LaH3 continues to increase until the 100th cycle. The concentration of metallic La increases in the 100th cycle after it consistently decreases in earlier cycles, whereas that of La3þ species continues to increase. After the maximum oxidation of La, hydrogen causes restoration of the original state of La3þ, suggesting that oxides of La and Ni may influence hydrogen absorption. The formation of LaH3 may be attributed to incomplete

La

(97.8) (47.1) (25.3) (28.4)

833.0 (2.2) 833.7 (41.6) 832.9 (43.1) 833.5 (23.4)

Atomic ratio La (LaH3)

Ni2þ/Ni0

839.0 (11.3) 838.7 (31.6) 838.8 (48.2)

0.79 1.06 1.34 1.41

desorption, which can degrade the absorption capacity of LaeFeeB alloys.

4.

Conclusion

In this research, we investigated the effects of annealing, activating, and cycling on the structures of LaeFeeB alloys. Lattice variations in the LaeNieMn phase are isotropic during annealing and activation, and the lattices of the LaeNieMn phase are distorted during the long cycling procedure. During high-temperature annealing, an increase in the first shell DW factors of LaeNi and LaeMn is observed, corresponding to increases in LaeNieMn distances in the unit cell. Oxides of the alloying element and increases in LaH3 content probably

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 3 2 3 4 e1 3 2 4 2

inhibit the hydriding progressing.

and

dehydriding

reaction

from [16]

Acknowledgments This project was financially supported by the China PostDoctoral Science Foundation (No. 2011M500223) and the National Natural Science Foundation of China (No. 11087011). The authors thank the staff at the BL14B1 station in SSRF for their assistance with XRD measurements. The project team at 1W1B XAFS Station, BSRF, is also gratefully acknowledged for their skillful assistance.

[17] [18]

[19]

[20]

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