Effect of Nd on Subunits Structure and Electrochemical Properties of ...

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Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015) 0013-4651/2015/162(10)/A2218/9/$33.00 © The Electrochemical Society

Effect of Nd on Subunits Structure and Electrochemical Properties of Super-Stacking PuNi3 -Type La–Mg–Ni-Based Alloys Lu Zhang,a,b Shengbiao Cao,c Yuan Li,a,b Yumeng Zhao,a,b Wenkai Du,a,b Yanqiao Ding,a,b and Shumin Hana,b,z a College

of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, People’s Republic of China Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, People’s Republic of China c Inner Mongalia Rare Earth Ovonic High-Power MH/Ni Battery Co. Ltd, Baotou 014010, People’s Republic of China b State

The super-stacking PuNi3 -type La0.67-x Ndx Mg0.33 Ni3.0 (x = 0, 0.12) alloys have been prepared using induction melting followed by annealing treatment method. The alloys are composed of PuNi3 -type main phase and CaCu5 -, MgCu4 Sn-, Ce5 Co19 - and Gd2 Co7 type minor phases in as-cast alloy. As annealing temperature increases from 850 to 950◦ C, the minor phases gradually transform to the PuNi3 -type phase via peritectic reactions, forming PuNi3 -type single-phase La0.67 Mg0.33 Ni3.0 alloy. Partial substitution of Nd for La has enhanced the phase stability of Gd2 Co7 -type phase, resulting in the existence of Gd2 Co7 -type phase as a secondary phase with the PuNi3 -type main phase in the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy after annealing treatment. It is found that Nd mainly replace La in [AB5 ] slabs in the PuNi3 -type structure, which increases the volume ratio of [A2 B4 ] slabs in the cell and decreases the volume change of [A2 B4 ] slabs during hydrogenation/dehydrogenation process, thus contributing to the improvement in cycling stability at the 100th cycle of the alloy, from 76.3% to 80.3%. Moreover, the cell volume contraction due to Nd substitution heightens the discharge plateau pressure and potential of the alloy electrode, and helps to the enhancement in high rate dischargeability (HRD) from 55.7% to 68.1% at a 1200 mA g−1 discharge current density. © 2015 The Electrochemical Society. [DOI: 10.1149/2.0981510jes] All rights reserved. Manuscript submitted June 1, 2015; revised manuscript received August 3, 2015. Published August 18, 2015.

In recent years, although lithium ion battery has been rapidly developed in the secondary battery market, the nickel/metal hydride (Ni/MH) battery still occupies a larger market and has a broad potential application because it can offer significant advantages of high capacity, environmental friendly and good safety features.1–3 As negative electrode materials for the Ni/MH battery, La–Mg–Ni-based hydrogen storage with super-stacking structures have been on focus due to their higher discharge capacities than those of the commercially available AB5 -type alloys.4–6 La–Mg–Ni-based alloys have super-stacking structures with [AB5 ] and [A2 B4 ] subunits stacking along c-axis in certain ratios forming various ABx phase, where x = (5n + 4)/(n + 2) and n is an integer (n = 1,2,3. . . ), with A = La, Mg and B = Ni, as shown in Fig. 1. Each ABx phase structure has two kinds of configurations: the hexagonal (2H) and rhombohedral (3R) structures, crystallized in space groups of P63 /mmc (no. 194) and R–3m (no. 166), respectively.7 For n = 1, the structure stands for AB3 -type phase. AB3 -type La–Mg–Ni-based alloys show higher discharge capacities than those of the other superstacking ABx -type alloys, and have also applied as the Ni/MH battery negative electrodes.8 However, the cycling stability and high rate dischargeability of these new Mg-containing alloys are still unsatisfied and need to be further improved. Partial substitution of La by Nd is reported an effective way to improve the electrochemical properties of La–Mg–Ni-based alloys.9–11 Li et al. found that the discharge capacity of the low-Co La0.80−x Ndx Mg0.20 Ni3.20 Co0.20 Al0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloy increases from 290 mAh g−1 (x = 0.20) to 374 mAh g−1 (x = 0.30), then decreases to 338 mAh g−1 (x = 0.60).9 They also found after partial substitution of La by Nd, the capacity retention rate at the 100th cycle of the La0.60 Nd0.20 Mg0.20 (NiCoMnAl)3.5 alloy increased by 13.4% and the high rate dischargeability increased from 22.1% to 49.6%.10 Zhang et al. reported Nd can improve the resistance to oxidation and increase the surface exchange current and bulk diffusion of hydrogen of the (La,Ce,Pr,Nd)2 MgNi9 alloy, thus enhancing its cycle life and high rate dischargeability, respectively.12 Our group recently found that with partial substitution of Nd for La, the CaCu5 type phase pre-existing in the La0.75 Mg0.25 Ni3.3 alloy disappeared and the La0.6 Nd0.15 Mg0.25 Ni3.3 alloy only composed of Ce2 Ni7 -type and Gd2 Co7 -type phases, resulting in the improvement in cycling stability and high rate dischargeability of the alloy.13 Although some studies z

E-mail: [email protected].

have found that Nd substitution for La can significantly improve the electrochemical properties of the alloys, it is essential to make a deep analysis of the impact of Nd substitution for La on the microstructure of the alloys. In this paper, we prepared a PuNi3 -type single-phase La0.67 Mg0.33 Ni3.0 alloy by annealing the induction melting sample. The formation and electrochemical properties of the PuNi3 -type singlephase alloy are studied. More importantly, based on the PuNi3 -type single-phase La0.67 Mg0.33 Ni3.0 alloy, the impact of partial substitution Nd for La on phase subunits structure and electrochemical properties of the alloy are illustrated in detail. Experimental La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys were prepared by induction melting constituent metals, including La, Mg, Nd and Ni, with a purity of 99.5%. The blocky as-cast alloys (∼10 g) were first wrapped in a nickel foil, placed in the tube furnace SK-G05123K and then heated up to 850, 900 and 950◦ C for 12 h (heating rate 4◦ C min−1 below 600◦ C and 1◦ C min−1 above 600◦ C) under 0.04 MPa argon atmosphere, respectively. A slight excess of Mg was used to compensate for the evaporation of Mg during the annealing procedure. The chemical analysis of the samples was performed using inductively coupled plasma (ICP) analyzers. Alloys were crushed mechanically into particles (–400 mesh) for X-ray diffraction (XRD) measurements with a Rigaku D/Max-2500/PC X-ray diffractometer (Cu Kα radiation). The profiles of the alloys were recorded over a range of 10◦ to 80◦ in 2θ by a step of 1◦ min−1 . Then the collected data was analyzed by Rietveld method using RIETICA software.14 Samples were mounted and polished on epoxy blocks, rinsed and dried before entering the SUPRA 55 scanning electron microscopy (SEM) chamber and morphology of the samples was studied by SEM coupled with energy dispersive spectrometer (EDS). For electrochemical measurements, a three-electrode system was used, which consisted of MH electrode as the working electrode, Ni(OH)2 /NiOOH electrode as the counter electrode and Hg/HgO electrode as the reference electrode. To prepare the working electrode, alloys and carbonyl nickel powders were mixed in a ratio of 1:5 (w:w). The mixtures were cold pressed into a pellet of 10 mm in diameter under 15 MPa and then each pellets was welded to a nickel stick. The measurements were performed on an automatic DC-5 battery testing instrument.

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Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015) Results and Discussion

La LaMg Ni

C14 hexa

The ICP results of the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys in atomic percentage are listed in Table I. Samples before and after annealing treatment show approximate composition to the design value, except for a small amount of over-compensation of Mg when annealing at higher temperatures (900 and 950◦ C). The final B/A ratios after annealing are all very close to the target value of 3.0.

C15 cubic CaCu5 hexa

n=1 2H

n=1 3R

n=2 2H

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n=2 3R

n=3 2H

n=3 3R

Figure 1. Schematic structure of the supper-stacking La–Mg–Ni-based alloys.

For the electrochemical pressure–composition test, the alloy electrode was discharged at a current density of 60 mA g−1 for a period of time followed by an interval of 0.5 h. The voltages between every interval were recorded as the equilibrium potential. The equilibrium potential (Eeq ) was converted into hydrogen equilibrium pressure (Peq ) according to the following Nernst equation with the experimental temperature of 298 K:   E eq (versus Hg/HgO) = −0.925 − 0.03 log Peq .

[1]

For the constant-current charge-discharge test, the electrodes were fully charged with an overcharge ratio of approximately 30% at a current density of 60 mA g−1 , and then discharged at 60 mA g−1 with a cutoff potential of −0.6 V. The charge/discharge process was performed for 100 cycles, and the discharge capacity of the alloy electrodes in the first 9 cycles and every 20 cycles afterwards were recorded to study the activation and cycling stability of alloy electrodes, respectively. For high rate dischargeability (HRD) measurement, the batteries were fully charged at a current density of 60 mA g−1 and then discharged at various current densities of 300, 600, 900 and 1200 mA g−1 . All the tests were measured at room temperature. The kinetic properties were performed on ZF-9 potentiostat after activation. The linear polarization curves were performed by scanning the alloy electrodes from −5 mV to +5 mV at a scanning rate of 0.1 mV s−1 . For the potential step experiments, the fully charge electrodes were tested at +500 mV (versus open circuit potential) for 3600 s.

Crystal structure.— Fig. 2 shows the XRD profiles of the as-cast and annealed samples of the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys. Jade 6.0 analyses suggest that the two kinds of the as-cast alloys are all consist of CaCu5 -, Ce5 Co19 -, Gd2 Co7 -, PuNi3 - and MgCu4 Sn-type phases. After annealing, all patterns of the two alloys are dominated by the PuNi3 -type phase. When annealing at the temperature of 850◦ C, the non-superstacking CaCu5 - and MgCu4 Sn-type phases in the two kinds of alloys all disappear, and the alloys contain super-stacking Ce5 Co19 -, Gd2 Co7 and PuNi3 -type phases. Further increasing the annealing temperature to 900◦ C, the Ce5 Co19 -type phase vanished in the double alloys. As the annealing temperature rises to 950◦ C, minor Gd2 Co7 -type phase in La0.67 Mg0.33 Ni3.0 alloy transforms to PuNi3 -type phase and the alloy is with a PuNi3 -type single phase. However, for La0.55 Nd0.12 Mg0.33 Ni3.0 alloy, Gd2 Co7 -type phase still exists steady at the temperature of 950◦ C. Figs. 3a and 3b show the low angle region XRD profiles in the 2θ range between 3◦ and 15◦ for the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys annealed at 950◦ C, respectively. According to PuNi3 -type (PDF41-1129) and Gd2 Co7 -type (PDF271107) structures, the single peak in Fig. 3a at 2θ = 10.86◦ is the (003) lattice plane corresponding to the d = 0.8 nm of the PuNi3 type structure, and the other two peaks in Fig. 3b at 2θ = 7.3◦ and 14.4◦ are (003) and (006) lattice planes belonging to the Gd2 Co7 -type structure with d = 1.2 and 0.6 nm. Those results indicate the singleand double-phase modes for the two alloys, respectively. Figs. 3c and 3d show the Rietveld refinements of the XRD profiles of the above alloys as the typical examples, respectively. Table II summarizes the Rietveld refinement results of all the alloys. For La0.67 Mg0.33 Ni3.0 alloy, the phase abundance of PuNi3 -type main phase continuely increases with increasing anealing temperature. At the temperature of 850◦ C, with the disappearance of CaCu5 - (15 wt%) and MgCu4 Sntype (5 wt%) phases of the as-cast alloy, PuNi3 -type phase content increases from 37 to 62 wt%. Its sustainable growing content from phase transformation of Ce5 Co19 - (8 wt%) and Gd2 Co7 -type (15 wt%) proceeds at 900 and 950◦ C, resulting in the formation of PuNi3 -type single phase. It is evident that annealing treatment is an effective way to improve homogeneity in distribution of alloy’s components. Further characterization is performed by TEM analysis. Figs. 4a and 4b present the magnified image of the alloy particle and selected-area electron diffraction (SAED) pattern, respectively. The result confirms the La0.67 Mg0.33 Ni3.0 crystalline particle (annealed at 950◦ C) is a single crystal pattern on the relevant lattice spacing with PuNi3 -type (R-3m) structure. When using 3 at.% Nd replaces La forming the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy, Gd2 Co7 - and PuNi3 -type phases

Table I. ICP results of the as-cast and annealed La0.67-x Ndx Mg0.33 Ni3.0 (x = 0, 0.12) alloys. Sample La0.67 Mg0.33 Ni3.0

La0.55 Nd0.12 Mg0.33 Ni3.0

As-cast 850◦ C-12h 900◦ C-12h 950◦ C-12h As-cast 850◦ C-12h 900◦ C-12h 950◦ C-12h

La (at.%)

Nd (at.%)

Mg (at.%)

Ni (at.%)

B/A

16.5 16.5 16.7 16.8 13.7 13.9 14.1 13.8

– – – – 3.1 3.1 3.0 3.2

8.4 8.2 7.9 7.9 8.0 7.9 7.7 7.8

75.0 75.3 75.2 75.3 75.2 75.1 75.2 75.2

3.00 3.05 3.03 3.05 3.03 3.02 3.03 3.03

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Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015)

Figure 2. XRD profiles of the as-cast and annealed alloys of the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 in the 2θ range of 10–80◦ (a), zoom of (a) for the La0.67 Mg0.33 Ni3.0 alloy (b) and for the La0.55 Nd0.12 Mg0.33 Ni3.0 (c) in the 2θ range of 20–48◦ .

content in the as-cast alloy increase by 5 and 3 wt% in comparison with those of the La0.67 Mg0.33 Ni3.0 alloy, and the abundances of minor phases of CaCu5 - and MgCu4 Sn-type phases both reduce. At annealing temperature of 850◦ C, the phase transformation of

CaCu5 - and MgCu4 Sn-type minor phases in La0.55 Nd0.12 Mg0.33 Ni3.0 alloy is similar with the alloy of La0.67 Mg0.33 Ni3.0 . However, when increase the annealing temperature to 900◦ C, PuNi3 -type phase content increases by 7 wt% but Gd2 Co7 -type changes slightly. This

Figure 3. Low angle region of XRD profiles in 2θ range between 3–15◦ for the La0.67 Mg0.33 Ni3.0 (a) and La0.55 Nd0.12 Mg0.33 Ni3.0 (b) alloys annealed at 950◦ C. Rietveld refinement of XRD patterns for the La0.67 Mg0.33 Ni3.0 (c) and La0.55 Nd0.12 Mg0.33 Ni3.0 (d) alloys annealed at 950◦ C. Downloaded on 2015-09-15 to IP 183.157.160.33 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015)

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Table II. Lattice constants and phase abundances of the as-cast and annealed La0.67-x Ndx Mg0.33 Ni3.0 (x = 0, 0.12) alloys. Lattice constants Samples La0.67 Mg0.33 Ni3.0

As-cast

850◦ C-12h 900◦ C-12h 950◦ C-12h As-cast

La0.55 Nd0.12 Mg0.33 Ni3.0

850◦ C-12h 900◦ C-12h 950◦ C-12h

Phases

a (nm)

c (nm)

Cell volume V (nm3 )

Phase abundance (wt%)

PuNi3 Gd2 Co7 Ce5 Co19 CaCu5 MgCu4 Sn PuNi3 Gd2 Co7 Ce5 Co19 PuNi3 Gd2 Co7 PuNi3 PuNi3 Gd2 Co7 Ce5 Co19 CaCu5 MgCu4 Sn PuNi3 Gd2 Co7 Ce5 Co19 PuNi3 Gd2 Co7 PuNi3 Gd2 Co7

0.5044 0.5034 0.5030 0.5036 0.7165 0.5034 0.5031 0.5030 0.5034 0.5029 0.5033 0.5030 0.5021 0.5030 0.4982 0.7106 0.5025 0.5034 0.5022 0.5021 0.5024 0.5023 0.5021

2.4451 3.6291 4.8371 0.3988 2.4312 3.6274 4.8350 2.4300 3.6321 2.4295 2.4465 3.6301 4.8299 0.4001 2.4313 3.6288 4.8323 2.4302 3.6264 2.4286 3.6274

0.5387 0.7964 1.0598 0.0876 0.5335 0.7951 1.0594 0.5333 0.7955 0.5329 0.5360 0.7925 1.0583 0.0860 0.5317 0.7963 1.0555 0.5314 0.7927 0.5306 0.7919

37 32 9 15 5 62 30 8 85 15 100 40 37 8 11 4 65 29 6 72 28 76 24

phase structure change keeps on as the annealing temperature further increases to 950◦ C, which PuNi3 -type phase further increases by 5 wt% and the secondary Gd2 Co7 -type phase remains with 24 wt%. From the different phase change results with increasing annealing

(a)

(b) –

[010]

[021]

–

[011] [001] Figure 4. TEM images for magnified morphology image (a) and the selectedarea electron diffraction (SAED) pattern taken along the zone axes of [001] (b) of the La0.67 Mg0.33 Ni3.0 alloy annealed at 950◦ C:.

temperature for the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys, we can conclude that the addition of Nd has increased the phase stability of the Gd2 Co7 -type phase. The reason may be that the bond energy of Nd–Ni/Nd–Nd is higher than La–Ni/La–La ascribing ´ than La (1.88 Å). ´ Therefore, to the smaller radius of Nd (1.82 Å) formation of the Nd–Ni/Nd–Nd contributes to the existence of the secondary super-stacking A2 B7 Gd2 Co7 -type phase after annealing temperature, which has a higher structural stability and lower energy than AB3 PuNi3 -type phase.7 As shown in Table II, the lattice parameter values of the La0.67 Mg0.33 Ni3.0 alloys of the constituent phases vary within narrow ranges and identify well agree with the literature data.15 The cell parameters of the super-staking structure shrink as Nd is added to the alloy, which is also due to the smaller radius of Nd than La. Table III lists the atom coordinates and occupation number of PuNi3 type structure for La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys. According to the refinement results of the atom coordinates and cell parameters, the detailed expansions of lattice parameters, the unit-cell volume, the length along c-axes of the [AB5 ] and [A2 B4 ] slabs and the volume of the [AB5 ] and [A2 B4 ] slabs of the PuNi3 -type structures were calculated and shown in Table IV. Replacement of Nd on La results in the whole cell contracting isotropically by 1.04%. For [AB5 ] and [A2 B4 ] slabs in the whole cell, the [AB5 ] slabs undergo isotropic shrinkage but that for [A2 B4 ] slabs is anisotropic. The length along c-axes of [AB5 ] slabs decreases by 1.21%, while the length along c-axes of [A2 B4 ] slabs increases by 0.45%, which leads to the volume contraction of the [AB5 ] slabs almost ten higher than the [A2 B4 ] slabs. Based on the slab changes, we can conclude that Nd atoms mainly enter into the [AB5 ] slabs. Morphology.— The morphologies of the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys after annealing were studied by SEM, and the backscattering electron images (BEI) for the as-cast alloy and alloys annealed at 850 and 950◦ C are presented in Fig. 5. The compositions in several spots (identified numerically in the micrographs) were studied by EDS. For the as-cast La0.67 Mg0.33 Ni3.0 alloy, the darkest contrast of spots 1 and 3 indicates the areas are AB5 -type phase and the brightest area for spot 6 with the unsmooth surface is the AB2 -type phase. The phase composition of middle contrast spots of 2, 4 and 5 are close to the AB3 -, A2 B7 - and A5 B19 -type phases, respectively.

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Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015)

Table III. Atomic coordinates and occupation number for the PuNi3 -type (space group R-3m) La0.67-x Ndx Mg0.33 Ni3.0 (x = 0, 0.12) alloys.

La0.67 Mg0.33 Ni3.0

La0.55 Nd0.12 Mg0.33 Ni3.0

Atom

Site

x

y

z

Occ.

La1 La2 Mg Ni1 Ni2 Ni3 (La,Nd)1 (La,Nd)2 Mg Ni1 Ni2 Ni3

3a 6c 6c 6c 3b 18h 3a 6c 6c 6c 3b 18h

0 0 0 0 0 0.5018 0 0 0 0 0 0.5059

0 0 0 0 0 0.4982 0 0 0 0 0 0.4941

0 0.14332 0.14332 0.33084 0.5 0.08221 0 0.14845 0.14845 0.32828 0.5 0.08151

1 0.506 0.494 1 1 1 1 0.502 0.498 1 1 1

Table IV. Comparison of crystallographic parameters of the PuNi3 -type phase in La0.67-x Ndx Mg0.33 Ni3.0 (x = 0, 0.12) alloys. Alloys Parameters

La0.67 Mg0.33 Ni3.0

La0.55 Nd0.12 Mg0.33 Ni3.0

a (Å) c (Å) V (Å3 ) VAB5 (Å3 ) VA2B4 (Å3 ) a/a (%) c/c (%) V/V (%) cAB5 /cAB5 (%) cA2B4 /cA2B4 (%) VAB5 /VAB5 (%) VA2B4 /VA2B4 (%)

5.0403 24.373 536.23 88.166 90.575 – – – – – – –

5.0230 24.286 530.66 86.508 90.378 −0.34 −0.36 −1.04 –1.21 +0.47 −1.88 −0.22

For the La0.67 Mg0.33 Ni3.0 alloy annealed at 850◦ C, matrix with darker contrast of spots 2 and 5 (B/A ratio of 4.0) and brighter matrix of spots 3 and 4 (B/A ratio of 3.10) correspond to A2 B7 - and AB3 -type phases, respectively (Fig. 5b). And the embraced area of spot 1 is identified with a B/A ratio of 3.95, suggesting the A5 B19 -type phase. When

(a)

(d)

annealing at 950◦ C, the A5 B19 - and A2 B7 -type areas all disappear in Fig. 5c and the morphology of the alloy appears uniform with the B/A ratio remains ca. 3.05 for spot 1–5, which indicates the single-phase characteristic. Some cracks on the alloy’ surface can be also seen and those are the grain boundaries. For La0.55 Nd0.12 Mg0.33 Ni3.0 alloys, five different compositions for the as-cast sample can be seen (Fig. 5d). The spots of 4 and 6 are the non-supers-tacking phases of AB2 - and AB5 -type, respectively. Super-stacking phases of A5 B19 -, A2 B7 - and AB3 -type are observed as spots 2, 4 and 3, respectively. Fig. 5e shows three phases of A5 B19 - (spot 4), A2 B7 - (spots 2 and 3) and AB3 -type (spots 1 and 5) phases when the alloy is annealed at 850◦ C. At the annealing temperature of 950◦ C, double areas of spots 1 and 3 with the stoichiometry of 2.98 and 3.04 (Fig. 5f) suggest the AB3 -type phases and the stoichiometry of spot 2 with 3.56 indicates the A2 B7 type phase. SEM results are in accordance with the XRD analysis. The discrepancy between EDS and ICP results may be relates to the assumptions in the EDS k-factor calculation. P–C isotherms.— A high hydrogen storage capacity and moderate hydride stability are necessary for hydrogen storage alloys to be used as electrode for nickel metal hydride batteries, which properties can be evaluated from P–C isotherms from the maximum hydrogen storage capacity and the plateau pressure, respectively. The electrochemical discharge P–C isotherms for the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloy electrodes are shown in Figs. 6a and 6b,

(c)

(e)

(f)

Figure 5. SEM backscattering images for the as-cast La0.67 Mg0.33 Ni3.0 alloy (a), annealed La0.67 Mg0.33 Ni3.0 alloy at 850◦ C (b) and 950◦ C (c), and for the as-cast La0.55 Nd0.12 Mg0.33 Ni3.0 alloy (d), annealed La0.55 Nd0.12 Mg0.33 Ni3.0 alloy at 850◦ C (e) and 950◦ C (f). Downloaded on 2015-09-15 to IP 183.157.160.33 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015)

La0.55 Nd0.12 Mg0.33 Ni3.0 alloys annealed at 950◦ C, it can be seen the plateau pressure is elevated to 0.170 MPa from 0.0578 MPa after Nd addition. It indicates the addition of Nd weakens the hydride stability of the alloy. Evidence shows that the discharge plateau pressure of one alloy electrode is closely related to the cell volume of the alloy, that the decrease of the cell volume can lift platform pressure of the alloy electrode.16 Therefore, the reduction of the cell volume after Nd addition should be ascribed to the decreasing plateau pressure. Besides, the maximum hydrogen storage of the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy slightly decreases to 1.21 H/M in comparison with the La0.67 Mg0.33 Ni3.0 alloy when annealed at 950◦ C. However, the plateau region of the La0.55 Nd0.12 Mg0.33 Ni3.0 is wider and flatter than that of the La0.67 Mg0.33 Ni3.0 alloy, indicating Nd can ameliorate the voltage stability of the alloy electrode, which make alloy more suitable for electrode application.17

(a) 10

0.1

0.74 H/M

0.104 Mpa 0.0578 Mpa

0.01 0.77 H/M

2

PH (Mpa)

1

1E-3

As-cast 850 °C-12h 900 °C-12h 950 °C-12h

1E-4 1E-5 1E-6 0.0

0.2

0.4

0.6

0.8

1.0

1.2

H/M

Activation and discharge capacity.— Fig. 7 shows the relationship between discharge capacity and cycle number of the as-cast and annealed La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloy electrodes at 298 K. The electrochemical properties including the activation property (Na ), maximum discharge capacity (Cmax ) and capacity retention rate at 100 cycles (S100 ) of each alloy electrode are summarized in Table V. The alloys all possess excellent activation capability, which can be completely activated at 2–3 cycles. After activation, the maximum discharge capacity of the as-cast La0.67 Mg0.33 Ni3.0 alloy is about 368 mAh g−1 at 0.2 C rate of charge–discharge measurement. The significant improvement on Cmax of the annealed alloys can be observed, which increases to 401 mAh g−1 (Fig. 7a). This is attributed to the changes in phase composition after annealing. The lower hydrogen storage capacity of CaCu5 -type phase and the hard ability to absorb/desorb hydrogen at room temperature of MgCu4 Sn-type phase reduces the discharge capacity of the as-cast alloy. While, after annealing treatment, the disappearance of those non-super-stacking phases (850◦ C) and the increasing abundance of PuNi3 -type phase make the annealed alloys possessing increasing Cmax , especially for the PuNi3 -type single-phase La0.67 Mg0.33 Ni3.0 alloy, whose Cmax attains the maximum. In addition, not only the appropriate phase changes can improve the Cmax after annealing treatment, but the enhancive homogeneity in distribution of the alloying components, reduction in crystal structure defects and refinement in the crystalline grain after annealing can also contribute to the Cmax enhancement.18 The Cmax of the as-cast La0.55 Nd0.12 Mg0.33 Ni3.0 alloy is 359 mAh g−1 , decreasing about 10 mAh g−1 compared to that of the La0.67 Mg0.33 Ni3.0 alloy. The discharge capacity of the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy also increases with the rising annealing temperature, up to 395 mAh g−1 (Fig. 7b), which also can be allowed for the phase transformation. After annealing the alloy at the temperatures of 900 and 950◦ C, the alloys are all composed of PuNi3 - and Gd2 Co7 -type phases. The double structures are with high hydrogen storage capacities. Therefore, the annealed alloys at higher temperatures show higher discharge capacity. However, the Cmax s of the La0.55 Nd0.12 Mg0.33 Ni3.0 alloys are still lower than those of the La0.67 Mg0.33 Ni3.0 alloys due to the reduction of cell volume after addition of Nd, which reduces sites for hydrogen atoms.

(b) 10 1

0.17 Mpa

0.85 H/M

2

PH (Mpa)

0.1 0.01 1E-3

As-cast 850 °C-12h 900 °C-12h 950 °C-12h

1E-4 1E-5 1E-6

0.0

0.2

0.4

0.6

0.8

1.0

A2223

1.2

H/M Figure 6. P–C isotherms of the as-cast and annealed La0.67 Mg0.33 Ni3.0 (a) and La0.55 Nd0.12 Mg0.33 Ni3.0 (b) alloys.

respectively. The discharge plateau pressures from the P–C curves are listed in Table V. As shown in Fig. 6a, the P–C isotherms of the annealed La0.67 Mg0.33 Ni3.0 alloys present just one plateau, while the as-cast one has an additional plateau at ∼0.289 MPa, owing to the existence of LaNi5 phase that has a higher plateau pressure.8 For the annealed alloys, the plateau pressure reduces from 0.124 Mpa to 0.0578 Mpa with the increasing content of PuNi3 -type phase from 62 wt% (850◦ C) to 100 wt% (950◦ C). This suggests that the plateau pressure of PuNi3 -type phase is lower than the other super-stacking phases and its hydride stability is also superior.15 Moreover, the PuNi3 -type single-phase structure possesses good hydrogen storage performance, which the length of the plateau region is 0.77 H/M and its maximum hydrogen storage capacity reaches 1.23 H/M. When Nd was added to the system, the double plateau of the as-cast alloy of La0.55 Nd0.12 Mg0.33 Ni3.0 also converts to single one after annealing. Comparing the isotherms of the La0.67 Mg0.33 Ni3.0 and

Table V. Summary of electrochemical properties of the as-cast and annealed La0.67-x Ndx Mg0.33 Ni3.0 (x = 0, 0.12) alloy electrodes.

Samples La0.67 Mg0.33 Ni3.0

La0.55 Nd0.12 Mg0.33 Ni3.0

As-cast 850◦ C-12h 900◦ C-12h 950◦ C-12h As-cast 850◦ C-12h 900◦ C-12h 950◦ C-12h

N

Cmax. (mAh g−1 )

S100 (%)

Vmid (–V)

Peq (Mpa)

HRD (%)

D (10−10 cm2 s−1 )

Io (mA g−1 )

2 2 2 2 3 2 3 2

368 394 399 401 359 378 393 395

62.8 74.8 75.7 76.3 75.8 79.1 80.3 80.4

0.8854 0.8876 0.8843 0.8842 0.8985 0.8960 0.8961 0.8962

0.173 0.124 0.104 0.058 0.340 0.194 0.191 0.170

54.4 60.1 56.6 55.7 61.5 70.5 68.5 68.1

1.03 1.48 1.44 1.29 1.06 1.67 1.53 1.50

290.24 290.75 343.30 362.03 274.41 302.13 307.61 382.92

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A2224

Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015)

(a) 1.00

(a)

Discharge voltage (-V), vs Hg/HgO

-1

Discharge capacity (mAh g )

400

350 300

250

200

As-cast 850 °C-12h 900 °C-12h 950 °C-12h 0 1 2 3 4 5 6 7 8 9 20

40

60

80

0.80 0.75 0.70

300

As-cast 850 °C-12h 900 °C-12h 950 °C-12h

0.60 0.55

40

0

50

100

150

200

250

300

350

400

350

400

-1

0.95 0.90 0.85 0.80 0.75 0.70

60

80

100

Cycle number Figure 7. Discharge capacity vs. cycle number of the as-cast and annealed La0.67 Mg0.33 Ni3.0 (a) and La0.55 Nd0.12 Mg0.33 Ni3.0 (b) alloy electrodes.

Cycling stability.— As also shown in Fig. 7, annealing treatment can significantly improve the cycle life of both La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys. For La0.67 Mg0.33 Ni3.0 alloy, the cycling stability of the as-cast alloy is poor which the S100 is 62.8%. This is because the alloy contains uneven components MgCu4 Sn-type (La,Mg)Ni2 phase, who is susceptible to corrosion. The enhanced cycling stability after annealing treatment can be attributed to the reduction of the pulverization and oxidation of alloy particles during cycling.19 The gradual homogenized alloy composition via annealing, in which process the disappearance of non-super-stacking MgCu4 Snand CaCu5 -type phases decreases the inconsistencies during lattice expansion/contraction, thereby reducing the pulverization of the alloy particles20 and increasing the cycle life of the alloy to 74.8%. With the increase of annealing temperature from 850 to 950◦ C, S100 slightly enhances from 74.8% to 76.9%, which is contributing to the successive phase transforms of Ce5 Co19 - and Gd2 Co7 -type phase to PuNi3 -type. The existence of mounts of phase boundaries in multiphase alloy makes the alloy more easily to be oxidized when in contact with alkaline. However, single-phase alloy exhibits superior cycling stability in advantages of accordant lattice expansion/contraction and none phase boundary which suppress the alloy pulverization then retrain the oxidation of the active elements of the alloy. When Nd is added to the system, the cycling stabilities of the alloys are improved, which the S100 increased by 13% for the as-cast La0.55 Nd0.12 Mg0.33 Ni3.0 alloy in comparison with the La0.67 Mg0.33 Ni3.0 , and S100 for the annealed La0.55 Nd0.12 Mg0.33 Ni3.0 alloys increases with annealing temperature in the range of 79.1– 80.4%. When annealed the alloys at 900◦ C, both of the alloys contain the similar double phase structures of the PuNi3 -type main phase with Gd2 Co7 -type secondary phase, but the cycling stabil-

As-cast 850 °C-12h 900 °C-12h 950 °C-12h

0.65 0.60 0.55

0 1 2 3 4 5 6 7 8 9 20

As-cast 850 °C-12h 900 °C-12h 950 °C-12h

0.65

Discharge capacty (mAh g )

Discharge voltage (-V), vs Hg/HgO

-1

Discharge capacity (mAh g )

350

200

0.85

(b) 1.00

400

250

0.90

100

Cycle number

(b)

0.95

0

50

100

150

200

250

300 -1

Discharge capacty (mAh g ) Figure 8. The discharge curves of the as-cast and annealed La0.67 Mg0.33 Ni3.0 (a) and La0.55 Nd0.12 Mg0.33 Ni3.0 (b) alloy electrodes.

ity of the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy is 4.6% higher than that of the La0.67 Mg0.33 Ni3.0 alloy. Although no single-phase structure forms when annealing at 950◦ C for the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy, the double-phase PuNi3 - and Gd2 Co7 -type still has superior cycling stability, 4.1% higher than the single-phase La0.67 Mg0.33 Ni3.0 alloy. The effect of the Gd2 Co7 -type secondary phase to the PuNi3 -type main phase to cycling stability can be neglected in terms of the small discrepancy value in either the La0.67 Mg0.33 Ni3.0 alloy or La0.55 Nd0.12 Mg0.33 Ni3.0 alloy. Therefore, it can be deduce that the main factor impacting the cycling stability after substitution of Nd for La is the volume changes. Due to the significant contraction of [AB5 ] slabs after Nd replace La at [AB5 ] slabs, the volume ratio of the [A2 B4 ] slabs in whole cell increases from 50.67 to 51.09%. Previous studies have shown that the capacity degradation of the PuNi3 -type structure alloy correlates with the structure changes after hydrogenation/dehydrogenation, especially for the [A2 B4 ] slabs changes.6,21,22 Denys et al. have shown that in PuNi3 -type structure, 60.3% deuterium atoms enters into the [A2 B4 ] slabs, resulting in the cell contraction of 0.9% higher than that of the [AB5 ] slabs.23 Therefore, increase in original volume ratio of [A2 B4 ] slabs in the structure thanks to the Nd addtion may reduce the volume changes during hydrogenation/dehydrogenation and then depress the hydrogen induced amorphizaiton, thus improving the increase the structure stability and increasing the cycle life of the alloy. Discharge curves.— Fig. 8 shows the discharge curves (the second cycle) of the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloy electrodes. Each curve of the alloys can be divided into two parts: the plateau region and the sloping region, of which the plateau region is controlled by charge the transfer process and the slope region is caused by the rapid consumption of hydrogen alloy surface leading to a potential reduction. Besides, each curve has a wide discharge

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Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015)

(a)

As-cast 850 °C-12h 950 °C-12h 950 °C-12h

100 90

HRD (%)

80 65

HRD (%)

70 60

60

55

50

50

850°C-12h

As-cast

900°C-12h

950°C-12h

Sample

0

200

400

600

800

1000

1200

-1

Discharge current density (mA g )

(b)

As-cast 850 °C-12h 900 °C-12h 950 °C-12h

100

80 72

HRD (%)

HRD (%)

90

70

68 64 60 56

60

850°C-12h 900°C-12h 950°C-12h

As-cast

Sample

0

200

400

600

800

1000

1200

-1

Discharge current density (mA g ) Figure 9. High rate dischargeability (HRD) of the as-cast and annealed La0.67 Mg0.33 Ni3.0 (a) and La0.55 Nd0.12 Mg0.33 Ni3.0 (b) alloy electrodes.

A2225

900◦ C, the HRD significantly reduces to 56.6%. The HRD further weakens with the increasing annealing temperature, lowering down to 55.7%, which in mainly due to the reduction of phase boundaries as decrease phase numbers, thereby decreasing the diffusion channels for hydrogen atoms in the alloy bulk. When Nd is added to the system, the HRD of the alloy electrodes were significantly improved. HRD1200 of the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy electrodes ranges between 61.1% and 70.5%, higher than the values of La0.67 Mg0.33 Ni3.0 alloy, indicating that Nd element benefits for HRD. Similarly, the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy annealed at 850◦ C possesses better HRD, which is 70.5%, while, the alloys with double phase of PuNi3 - and Gd2 Co7 -type show close HRD about 68%. The cell volume contraction due to Nd substitution is the origin, which heightens the discharge plateau pressure and potential of the alloy electrode, helping to the enhancement in high rate dischargeability. It can be conclude that the elemental constituent and phase composition of the alloy are two important factors influencing the high rate dischargeability of an alloy electrode. Conventionally, both surface exchange current (Io ) and the bulk diffusion coefficient (D) were used to study the source of HRD changes.22 The both parameters represent the charge-transfer resistance at the electrode surface and the diffusion rate of hydrogen atoms in the bulk alloy, respectively. From Table V it can be seen that the changes of D values of the both alloys are in coincidence with HRD, which D values firstly increase then decrease with annealing temperature and reaches the maximum value when annealed at 850◦ C. Both Io values of the two alloys rise with the rising annealing temperature. These explained that Ce5 Co19 -type phase can boost the hydrogen diffusion in PuNi3 - and Gd2 Co7 -type phases, acting as a catalytic role, and the bulk diffusion coefficient contributes positively to the HRD performance. Besides, with the reduction of phase number, further decrease of D value illustrates the multiphase structure is favor in hydrogen diffusion due to the plentiful phase boundaries. Comparing D values of the La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 alloys reveal the following fact: the hydrogen diffusion resistance of the former alloy is larger than the later one. Conclusions

potential plateau based on the oxidation of desorbed hydrogen from the hydride after annealing, indicating a faster charge transfer process after annealing, and the results agrees well with the following dynamic results. For the La0.67 Mg0.33 Ni3.0 alloy, the mid-discharge potential of the alloy shifts toward a more positive potential as increase of annealing temperature, which the middle-discharge potential decreases to − 0.8842 V for the PuNi3 -type single-phase alloy from − 0.8876 V for alloy annealed at 950◦ C. Moreover, the discharge plateau of La0.55 Nd0.12 Mg0.33 Ni3.0 alloys all lower than those of La0.67 Mg0.33 Ni3.0 alloys, indicating that the charge efficiency of the former alloy electrodes has been ameliorated than the later one, which coincidence with the subsequent analysis. Electrochemical kinetic characteristics.— Fig. 9 shows the high rate dischargeability (HRD) curves of the alloy La0.67 Mg0.33 Ni3.0 and La0.55 Nd0.12 Mg0.33 Ni3.0 electrodes which reflect the discharge capability of the alloy electrodes at high current density, and HRD can be defined as: HRDn = Cn / (Cn +C60 ) ×100%, where HRDn represents the HRD at a discharge current density of n mA g−1 , Cn represents the discharge capacity at a discharge current density of n mA g−1 and C60 is the residue discharge capacity at a discharge current density of 60 mA g−1 . The HRD1200 is listed in Table III. As can be seen from Fig. 9a, the HRD1200 is significantly improved when annealed at 850◦ C, which increases from 54.4% to 60.1%, but the value then decreases with further increasing annealing temperature. This is due to the phase composition changes, that the disappearance of minor phase of MgCu4 Sn-type and existence of Ce5 Co19 -type who has a higher synergetic effect to other super-stacking phases (Gd2 Co7 -type and PuNi3 -type) with higher hydrogen storage capacity contributes the increasing HRD. While with disappearance of Ce5 Co19 -type phase at

PuNi3 -type single-phase La0.67 Mg0.33 Ni3.0 alloy has been obtained by annealing the induction melting the as-cast sample at 950◦ C for 12 h. The as-cast La0.67 Mg0.33 Ni3.0 alloy consists of CaCu5 -, Ce5 Co19 -, Gd2 Co7 -, PuNi3 - and MgCu4 Sn-type phases. The non-super-stacking phases of CaCu5 - and MgCu4 Sn-type are eliminated at lower annealing temperature of 850◦ C, and the Ce5 Co19 - and Gd2 Co7 -type phases successively transform to the PuNi3 -type phase at 900 and 950◦ C, respectively, forming PuNi3 -type single-phase La0.67 Mg0.33 Ni3.0 alloy. Partial substitution of Nd for La has enhanced the phase stability of Gd2 Co7 -type phase, which Gd2 Co7 -type phase appears as a secondary phase with the PuNi3 -type main phase in the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy after annealing treatment. The partial replacement of Nd for La atoms in the PuNi3 -type structure mainly proceeds in [AB5 ] slabs, resulting in the volume contraction of the isotropic [AB5 ] slabs ten higher than of the anisotropic [A2 B4 ] slabs, and of the isotropic cell by 1.04%. The increase in the volume ratio of [A2 B4 ] slabs in the cell after Nd substitution decrease the volume change of the [A2 B4 ] slabs during hydrogenation/dehydrogenation process, contributing to the cycling stability at the 100th cycle improving from 76.3% for the La0.67 Mg0.33 Ni3.0 alloy to 80.3% for the La0.55 Nd0.12 Mg0.33 Ni3.0 alloy. Moreover, the cell volume contraction due to partial replacement of Nd for La increases the discharge plateau pressure and potential of the alloy electrode, which contribute to the improvement of high rate dischargeability (HRD), increasing from 55.7% to 68.1% at a 1200 mA g−1 discharge density. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NOs. 51171165, 21303157 and

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A2226

Journal of The Electrochemical Society, 162 (10) A2218-A2226 (2015)

U1304522), the Natural Science Foundation of Hebei Province (NOs. B2012203027 and B2014203114), and the Research Fund for the Doctoral Program of Higher Education of China (20131333120008). References 1. K. Young, T. Ouchi, and B. Huang, J. Power Sources, 248, 147 (2014). 2. H. G. Pan, Y. F. Liu, M. X. Gao, Y. Q. Lei, and Q. D. Wang, J. Electrochem. Soc., 152, A326 (2005). 3. J. J. Liu, Y. Li, S. M. Han, S. Q. Yang, W. Z. Shen, X. C. Chen, and Y. M. Zhao, J. Electrochem. Soc., 160, A1139 (2013). 4. B. Liao, Y. Q. Lei, L. X. Chen, G. L. Lu, H. G. Pan, and Q. D. Wang, J. Power Sources, 129, 358 (2004). 5. R. V. Denys and V. A. Yartys, J. Alloys Compd., 509S, S540 (2011). 6. L. Zhang, S. M. Han, D. Han, Y. Li, X. Zhao, and J. J. Liu, J. Power Sources, 268, 575 (2014). 7. J. C. Crivello, J. Zhang, and M. Latroche, J. Phys. Chem. C, 115, 25470 (2011). 8. J. J. Liu, S. M. Han, Y. Li, S. Q. Yang, W. Z. Shen, L. Zhang, and Y. Zhou, J. Alloys Compd., 552, 119 (2013). 9. Y. Li, S. M. Han, J. H. Li, X. L. Zhu, and L. Hu, J. Alloys Compd., 458, 357 (2008). 10. Y. Li, D. Han, S. M. Han, X. L. Zhu, L. Hu, Z. Zhang, and Y. W. Liu, Int. J. Hydrogen Energy, 34, 1399 (2009).

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