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Hydrogen Storage Behaviour of Nanocrystalline and Amorphous Mg20Ni10−xCox (x=0∼4) Alloys by Melt-Spinning
Rare Metal Materials and Engineering Volume 41, Issue 11, November 2012 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2012, 41(11): 1881-1886.
ARTICLE
Hydrogen Storage Behaviour of Nanocrystalline and Amorphous Mg20Ni10-xCox (x=0~4) Alloys by Melt-Spinning Zhang Yanghuan1,2, Xu Yongyan1,2, 1
Guo Shihai2,
Qi Yan2,
Yang Tai1,2,
Chen Licui1,2,
Zhao Dongliang2
Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, 2
Baotou 014010, China; Central Iron and Steel Research Institute, Beijing 100081, China
Abstract: In order to improve the hydriding and dehydriding behaviour of the Mg2Ni-type alloys, Ni in the alloy was partially substituted by Co, and melt-spinning technology was used to prepare the Mg20Ni10-xCox (x=0, 1, 2, 3, 4) hydrogen storage alloys. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydriding and dehydriding kinetics as well as the electrochemical performances of the alloys were measured. The results show that no amorphous phase is detected in the as-spun Co-free alloy, but the as-spun alloys substituted by Co display the presence of an amorphous phase. The substitution of Co for Ni and the melt spinning significantly improve the hydrogen absorption and desorption performances of the as-cast and spun alloys. Meanwhile, the substitution of Co for Ni enhances the discharge capacity and cycle stability of the as-spun alloys dramatically. Key words: Mg2Ni-type alloy; melt-spinning; structure; hydrogen storage characteristics
Mg and Mg-based metallic hydrides are considered to be more promising candidates for hydrogen storage because of their high hydrogen capacity and low price. Unfortunately, the practical applications of these hydrides are deeply frustrated by their sluggish absorption/desorption kinetics and high thermodynamic stability. Therefore, finding ways to improve the hydriding kinetics of Mg-based alloys has been one of the main challenges faced by researchers in this area. Various attempts, particularly mechanical alloying (MA)[1], surface modification[2], and alloying with other elements[3], have been undertaken to improve the activation and hydriding properties. Lei et al. obtained the improved discharge capacity of around 500 mAh/g for Mg2Ni alloys prepared by mechanical alloying (MA) at a current density of 20 mA/g[4]. Iwakura et al.[5] have also improved the discharge capacity of Mg-based alloy with graphite surface modification by mechanical grinding (MG). After the surface modification with Ni powder by ball milling, Kohno et al.[6] obtained a larger discharge capac-
ity of 750 mAh/g at a current density of 20 mA/g for modified Mg2Ni alloys. However, MA Mg-based alloys showed extremely poor electrochemical cycle stability[7]. Alternatively, the meltspinning technique is an effective method to obtain an amorphous and/or nanocrystalline phase and is very suitable for mass-production of amorphous alloys. Huang et al.[8] found that amorphous and nanocrystalline Mg-based (Mg60Ni25)90Nd10 alloy prepared by melt-spinning obtained the highest discharge capacity of 580 mAh/g and the maximum hydrogen capacity of 4.2 wt%. In this paper, the element Ni in Mg20Ni10 alloy was partially substituted by Co in order to improve hydrogen storage properties of the Mg2Ni-type alloys. The ternary Mg2Ni-type nanocrystalline and amorphous Mg20Ni10-xCox (x=0~4) alloys were prepared by melt spinning and their hydrogen storage characteristics were examined in detail.
1
Experiment
Received date: November 25, 2011 Foundation item: National Natural Science Foundations of China (51161015 and 50961009); Natural Science Foundation of Inner Mongolia, China (2011ZD10 and 2010ZD05) Corresponding author: Zhang Yanghuan, Ph. D., Professor, Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, P. R. China, Tel: 0086-10-62183115, E-mail: [email protected] Copyright © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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2
mation of the secondary phase MgCo2, the amount of which increases with the rising of Co content. It is evident that no amorphous phase is detected in the as-spun Co0 alloy, but the as-spun Co4 alloy exhibits the presence of an amorphous phase. The lattice parameters and cell volumes as well as the full width at half maximum (FWHM) values of the main diffraction peaks of the as-cast and spun (30 m/s) alloys which were calculated by Jade 6.0 software are listed in Table 1. It is found that the FWHM values of the main diffraction peaks increase with the rising of Co content considerably. The substitution of Co for Ni renders a visible enlargement of the lattice parameters and cell volumes, which is attributed to the fact that the radius of Co atom is larger than Ni atom. Furthermore, it can be seen from Table 1 that the melt spinning produces broadened diffuse peaks, indicating the refined grain and stored stress in the grains. a
Mg2Ni MgCo2
Co4
Intensity/a.u.
Co3 Co2 Co1 Co0
20
40
60
80
Mg2Ni MgCo2
100 b Co4 Co3
Intensity/a.u.
The compositions of the experimental alloys were Mg20Ni10-xCox (x=0, 1, 2, 3, 4). For convenience, the alloys were denoted with Co content as Co0, Co1, Co2, Co3 and Co4, respectively. The alloy ingots were prepared by using a vacuum induction furnace in a helium atmosphere at a pressure of 0.04 MPa. A part of the as-cast alloys was re-melted and spun by melt-spinning with a rotating copper roller. The spinning rate was approximately expressed by the linear velocity of the copper roller. The spinning rates used in the experiment were 15, 20, 25 and 30 m/s, respectively. The phase structures and compositions of the as-cast and spun alloys were determined by XRD (D/max/2400). The morphologies of the as-cast alloys were examined by SEM (Philips QUANTA 400). The thin film samples of the as-spun alloys prepared by ion etching were observed by HRTEM (JEM-2100F) and the crystalline state of the samples were characterized by electron diffraction (ED). Thermal stability and crystallization of the as-spun alloys were studied by means of DSC instrument (STA449C), and the heating temperature and rate are 600 °C and 10 °C/min, respectively. The hydriding and dehydriding kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The hydrogen absorption was conducted at 1.5 MPa and 200 °C, and the hydrogen desorption was carried out at a pressure of 1×10-4 MPa and 200 °C, respectively. The pulverized amorphous alloys were mixed with carbonyl nickel powder in a mass ratio of 1:4. The mixture was cold pressed at a pressure of 35 MPa into round electrode pellets with a diameter of 10 mm and total mass of about 1 g. A tri-electrode open cell was used for testing the electrochemical characteristics of the experimental alloy electrodes at 30 °C. In every cycle, the alloy electrode was first charged at a current density of 20 mA/g, after resting for 15 min, and it was discharged at the same current density to cut-off voltage of –0.500 V.
Co2 Co1 Co0
Results and Discussion
20
2.1 Microstructure characteristics The XRD patterns of the as-cast and spun alloys are depicted in Fig.1. The results indicate that the substitution of Co for Ni, instead of changing the major phase of Mg2Ni, leads to the for-
40
60
80
100
2θ/(°) Fig.1 XRD patterns of the alloys: (a) as-cast and (b) as-spun (30 m/s)
Table 1 Lattice parameters, cell volume and the FWHM values of the diffraction peaks of the alloys FWHM value Alloy
2θ=20.02°
Lattice parameters and cell volume 2θ=45.14°
a/nm
V/nm3
c/nm
As-cast
As-spun
As-cast
As-spun
As-cast
As-spun
As-cast
As-spun
As-cast
As-spun
Co0
0.122
0.133
0.169
0.182
0.520 97
0.521 06
1.324 4
1.328 7
0.311 28
0.313 41
Co1
0.179
0.206
0.188
0.225
0.521 26
0.521 86
1.325 6
1.332 3
0.311 92
0.314 22
Co2
0.188
0.246
0.239
0.290
0.521 67
0.528 53
1.330 4
1.333 6
0.313 55
0.322 61
Co3
0.192
0.536
0.243
0.329
0.521 83
0.528 68
1.330 9
1.341 2
0.313 84
0.324 64
Co4
0.213
—
0.350
—
0.522 04
—
1.331 2
—
0.314 17
—
* As-spun: spinning rate is 30 m/s
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a
b
20 μm Mg
Co
Intensity/a.u.
Mg2Ni
MgCo2
Co
Ni Mg Ni
Co
0
2
4
6
Energy/keV
Ni
Co
8
0
2
4
6
Ni
8
Energy/keV
Fig.2 SEM images of the as-cast Co0 and Co3 alloys together with typical EDS spectra of Fig.2b: (a) Co0 alloy and (b) Co3 alloy
10
a
b
200 nm
10 nm
Fig.3 HRTEM images and ED patterns of the as-spun (30 m/s) alloys: (a) Co0 alloy and (b) Co3 alloy
30 m/s
Heat Flow Exo
The SEM images of the as-cast Co0 and Co3 alloys are illustrated in Fig.2, showing that the substitution of Co for Ni brings on an evident refinement of the grains of the as-cast alloy, which probably is the reason why the substitution of Co for Ni leads to the main diffraction peaks of the alloys clearly broadened (Fig.1). The morphologies of the alloys change from bulky dendrite structure to massive and rabdoid structure. The result obtained by energy dispersive spectrometry (EDS) indicates that the major phase of the Co0 and Co3 alloys is Mg2Ni phase, but Co substitution yields the secondary phase MgCo2, which is in agreement with the result of the XRD observation. The micrographs of the as-spun (30 m/s) Co0 and Co3 alloys observed by HRTEM are depicted in Fig.3, showing that the as-spun Co0 alloy displays a nanocrystalline structure with grain size of about 10 nm, and its electron diffraction (ED) pattern presents sharp multi-haloes, corresponding to a crystal structure. The morphology of the as-spun Co3 alloy exhibits a feature of the above-mentioned equal sized nanocrystalline structure embedded in the amorphous matrix, and its electron diffraction pattern consists of broad and dull halo, confirming the presence of an amorphous structure. This result agrees very well with the XRD observation shown in Fig.1. 2.2 Thermal stability and crystallization In order to examine the thermal stability and the crystallization of the as-spun amorphous and nanocrystalline/amorphous alloys, DSC analysis was conducted as shown in Fig.4. It reveals that during heating the alloys crystallize completely, and the crystallization process of Co3 alloy consists of two steps. The first crystallization reaction at about 232 °C is connected with a sharp exothermic DSC peak, followed by a smaller and
232.5 °C
25 m/s
232.1 °C
100
200
300
400
Temperature/°C Fig.4 DSC profiles of Co3 alloy spun at 25 and 30 m/s
wider peak (418 °C) corresponding to a second crystallization reaction. It was proved that the first sharper peak corresponds to the crystallization (ordering) of the amorphous into nanocrystalline Mg2Ni[9]. 2.3 Hydriding and dehydriding characteristics The hydrogen absorption kinetic curves of the as-cast and spun Co0 and Co3 alloys are plotted in Fig.5, indicating that all the as-spun alloys possess very fast hydrogen absorption rate and nearly reach their saturation capacities in 5 min. The melt spinning improves the hydrogen absorption properties of the alloys significantly. When the spinning rate grows from 0 to 30 m/s, the hydrogen absorption capacity of the Co0 alloy in 10 min rises from 1.39 wt% to 3.12 wt%, and from 2.71 wt% to 3.23 wt% for the Co3 alloy, indicating that the hydriding kinetics and storage capacity of all the as-spun nanocrystalline and nanocrystalline/amorphous Mg2Ni-type alloys are superior to those of conventional polycrystalline materials with similar composition. Spassov et al.[10] have also confirmed that the melt spinning could improve the hydrogen absorption performance of Mg-based alloy significantly, and obtain the maximum hydrogen capacity of 4.0 wt% for the as-quenched Mg75Ni20Mm5 (Mm=Ce, La-rich mischmetal) alloy. It is found from comparing Fig.5a with 5b that the substitution of Co for Ni improves the hydrogen absorption property of the as-cast Mg2Ni-type alloy dramatically, for which the increased cell volume and the refined grain induced by Co substitution are responsible.
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3.0 2.5 2.0 1.5 1.0 0.5
Hydrogen Absorption/wt%
0.0 0
20
40
60
80
100
120 b
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
20
40
60
80
100
120
Time/min Fig.5 Hydriding kinetic curves of the as-cast and spun Co0 (a) and
Hydrogen Desorption/wt%
Co3 (b) alloys
0.8 0.6 0.4 0.2 0.0
Hydrogen Desorption/wt%
a
1.0
0
30
60
90
120
2.4
b
1.6 1.2 0.8 0.4 0
40
80
120
160
Time/min
Fig.6 Dehydriding kinetic curves of the as-cast and spun Co0 (a) and Co3 (b) alloys
a
140 120 100 80 60 40 20 0 0 350
150
2.0
0.0
Discharge Capacity/mAh·g-1
a
3.5
the hydrogen desorption of the alloys is substantially meliorated by melt spinning. When the spinning rate grows from 0 to 30 m/s, the hydrogen desorption capacity of the Co0 alloy in 20 min increases from 0.19 wt% to 0.89 wt%, and from 1.24 wt% to 1.87 wt% for Co3 alloy, respectively. Fig.6a indicates that as-cast and spun Co0 alloy possesses a low hydrogen desorption capacity and a poor dehydriding kinetics, which is ascribed to the high stability of the crystal Mg-based hydride because melt spinning can not change the crystal state of the Co0 alloy. Correspondingly, the as-spun Co3 alloy exhibits high hydrogen desorption capacity and fast dehydriding rate. The improved dehydriding characteristics can be attributed to two reasons. On one hand, Co substitution evidently intensifies the glass forming ability of Mg2Ni-type alloy because amorphous Mg2Ni shows an excellent hydrogen desorption capability. On the other hand, such substitution decreases the stability of the hydride and makes the desorption reaction easier[1,11]. 2.4 Electrochemical hydrogen storage performances The cycle number dependence of the discharge capacity of the alloys is illustrated in Fig.7. Evidently, all the alloys have an excellent activation capability and attain their maximum discharge capacities at the first charging-discharging cycle. The melt spinning markedly enhances the discharge capacity of the alloys. When the spinning rate increases from 0 to 30 m/s, the discharge capacity grows from 30.26 to 135.51 mAh/g for the Co0 alloy, and from 115.4 to 284.2 mAh/g for
Discharge Capacity/mAh·g-1
Hydrogen Absorption/wt%
The dehydriding kinetic curves of the Co0 and Co3 alloys are presented in Fig.6, from which it is found that the capability of
4
8
12
16
20 b
300 250 200 150 100 50 0
4
8
12
16
20
Cycle Number, n
Fig.7 Evolution of the discharge capacity of the alloys with the cycle number: (a) Co0 alloy and (b) Co3 alloy
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Co3 alloy. The nanocrystalline and the amorphous microstructures formed by melt spinning are extremely helpful for enhancing hydrogen diffusivity and solubility, and thus it seems to be self-evident that the discharge capacity of the alloys grows with the rising of the spinning rate. It is noteworthy that, for the same spinning rate, the discharge capacity of the Co3 alloy is much larger than that of the Co0 alloy, which is ascribed to the glass forming ability of the Mg2Ni-type alloy enhanced by substituting Ni with Co. The capacity retaining rate (Sn) as the symbolization of the cycle stability of an alloy, is defined as Sn=Cn/Cmax×100%, where Cmax is the maximum discharge capacity and Cn is the discharge capacity of the nth charge-discharge cycle, respectively. According to the above-mentioned definition, it can be seen that the larger the capacity retaining rate (Sn) is, the better the cycle stability of the alloy will be. The Sn values of the Co0 and Co3 alloys as functions of the cycle number are illustrated in Fig.8. It is evident that the melt-spinning engenders a negligible influence on the cycle stability of the Co0 alloy, whereas it strengthens the cycle stability of Co3 alloy dramatically, for which the glass forming ability enhanced by Co substitution is mainly responsible. When the spinning rate increases from 0 to 30 m/s, the S20 value rises from 39.02% to 78.51% for the Co3 alloy, but it declines from 36.71% to 27.06% for the Co0 alloy, which is mainly attributed to the grains refined by the melt spinning. It is well known that the essential reason of leading to the loss efficacy of the Mg-based alloy electrodes is the severe corrosion of Mg in the alkaline KOH solution[12-14].
a
100
Sn/%
80
3 Conclusions 1) In the Mg20Ni10-xCox (x=0, 1, 2, 3, 4) alloys, the substitution of Co for Ni significantly enhances the glass forming ability of Mg2Ni-type alloy and leads to the visible refinement of the grains of the as-cast alloy. The substitution of Co for Ni results in the formation of the secondary phase MgCo2 without altering the major phase Mg2Ni in the alloy. 2) The substitution of Co for Ni improves the hydriding and dehydriding characteristics of the alloy dramatically. The increased hydrogen absorption capacity is attributed to the enlarged cell volume and the refined grain produced by Co substitution. The decreased stability of the hydride produced by Co substitution is responsible for the improved hydrogen desorption property. 3) Melt spinning improves the hydriding and dehydriding properties of the alloys significantly. Hydriding and dehydriding capacities and rates of the alloy markedly rise with increasing of the spinning rate, which is mainly attributed to the nanocrystalline and amorphous structure generated by the melt spinning.
References
60
1 Woo J H, Lee K S. J Electrochem Soc[J], 1999, 146(3): 819 2 Liu F J, Suda S. J Alloy Compd[J], 1995, 231: 742
40 20
The grain refinement weakens the anti-corrosion capability of the alloy consequentially due to intercrystalline corrosion being inevitable. Therefore, it is understandable why melt-spinning impairs the cycle stability of the Co0 alloy. The melt-spinning strongly heightens the cycle stability of the Co3 alloy, which is mainly attributed to the formation of an amorphous phase because the amorphous phase improves not only anti-pulverization ability but also anti-corrosion and anti-oxidation abilities of the alloy electrode in a corrosive electrolyte[15].
3 Liu Y N, Zhang X J. J Alloy Compd[J], 1998, 276: 231 4 Lei Y Q, Wu Y M, Yang Q M et al. Z Phys Chem Bd[J], 1994, 2
6
10
14
18
183: 379 b
5 Iwakura C, Nohara S, Inoue H et al. Chem Commun[J], 1996, 15: 1831
90 Sn/%
6 Kohno T, Kanda M. J Electrochem Soc[J], 1997, 144: 2384 7 Nohara S, Fujita N, Zhang S G et al. J Alloy Compd[J], 1998,
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267: 76 8 Huang L J, Liang G Y, Sun Z B et al. J Power Sources[J], 2006,
50 30
160: 684 9 Spassov T, Solsona P, Suriñach S et al. J Alloy Compd[J], 2003, 2
6
10
14
349: 242
18
Cycle Number, n
10 Spassov T, Köster U. J Alloy Compd[J], 1999, 287: 243 11 Takahashi Y, Yukawa H, Morinaga M. J Alloy Compd[J], 1996,
Fig.8 Evolution of the capacity retaining rate of the alloys with cycle number: (a) Co0 alloy and (b) Co3 alloy
242: 98 12
Goo N H, Woo J H, Lee K S. J Alloy Compd[J], 1999, 288: 286
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13 Wang L B, Tang Y H, Wang Y J et al. J Alloy Compd[J], 2002, 336: 297
15 Zhang Y H, Li B W, Ren H P et al. Int J Hydrogen Energy[J], 2007, 32: 4627
14 Jiang J J, Gasik M. J Power Sources[J], 2000, 89(1): 117
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ARTICLE
Hydrogen Storage Behaviour of Nanocrystalline and Amorphous Mg20Ni10-xCox (x=0~4) Alloys by Melt-Spinning Zhang Yanghuan1,2, Xu Yongyan1,2, 1
Guo Shihai2,
Qi Yan2,
Yang Tai1,2,
Chen Licui1,2,
Zhao Dongliang2
Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, 2
Baotou 014010, China; Central Iron and Steel Research Institute, Beijing 100081, China
Abstract: In order to improve the hydriding and dehydriding behaviour of the Mg2Ni-type alloys, Ni in the alloy was partially substituted by Co, and melt-spinning technology was used to prepare the Mg20Ni10-xCox (x=0, 1, 2, 3, 4) hydrogen storage alloys. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydriding and dehydriding kinetics as well as the electrochemical performances of the alloys were measured. The results show that no amorphous phase is detected in the as-spun Co-free alloy, but the as-spun alloys substituted by Co display the presence of an amorphous phase. The substitution of Co for Ni and the melt spinning significantly improve the hydrogen absorption and desorption performances of the as-cast and spun alloys. Meanwhile, the substitution of Co for Ni enhances the discharge capacity and cycle stability of the as-spun alloys dramatically. Key words: Mg2Ni-type alloy; melt-spinning; structure; hydrogen storage characteristics
Mg and Mg-based metallic hydrides are considered to be more promising candidates for hydrogen storage because of their high hydrogen capacity and low price. Unfortunately, the practical applications of these hydrides are deeply frustrated by their sluggish absorption/desorption kinetics and high thermodynamic stability. Therefore, finding ways to improve the hydriding kinetics of Mg-based alloys has been one of the main challenges faced by researchers in this area. Various attempts, particularly mechanical alloying (MA)[1], surface modification[2], and alloying with other elements[3], have been undertaken to improve the activation and hydriding properties. Lei et al. obtained the improved discharge capacity of around 500 mAh/g for Mg2Ni alloys prepared by mechanical alloying (MA) at a current density of 20 mA/g[4]. Iwakura et al.[5] have also improved the discharge capacity of Mg-based alloy with graphite surface modification by mechanical grinding (MG). After the surface modification with Ni powder by ball milling, Kohno et al.[6] obtained a larger discharge capac-
ity of 750 mAh/g at a current density of 20 mA/g for modified Mg2Ni alloys. However, MA Mg-based alloys showed extremely poor electrochemical cycle stability[7]. Alternatively, the meltspinning technique is an effective method to obtain an amorphous and/or nanocrystalline phase and is very suitable for mass-production of amorphous alloys. Huang et al.[8] found that amorphous and nanocrystalline Mg-based (Mg60Ni25)90Nd10 alloy prepared by melt-spinning obtained the highest discharge capacity of 580 mAh/g and the maximum hydrogen capacity of 4.2 wt%. In this paper, the element Ni in Mg20Ni10 alloy was partially substituted by Co in order to improve hydrogen storage properties of the Mg2Ni-type alloys. The ternary Mg2Ni-type nanocrystalline and amorphous Mg20Ni10-xCox (x=0~4) alloys were prepared by melt spinning and their hydrogen storage characteristics were examined in detail.
1
Experiment
Received date: November 25, 2011 Foundation item: National Natural Science Foundations of China (51161015 and 50961009); Natural Science Foundation of Inner Mongolia, China (2011ZD10 and 2010ZD05) Corresponding author: Zhang Yanghuan, Ph. D., Professor, Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, P. R. China, Tel: 0086-10-62183115, E-mail: [email protected] Copyright © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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2
mation of the secondary phase MgCo2, the amount of which increases with the rising of Co content. It is evident that no amorphous phase is detected in the as-spun Co0 alloy, but the as-spun Co4 alloy exhibits the presence of an amorphous phase. The lattice parameters and cell volumes as well as the full width at half maximum (FWHM) values of the main diffraction peaks of the as-cast and spun (30 m/s) alloys which were calculated by Jade 6.0 software are listed in Table 1. It is found that the FWHM values of the main diffraction peaks increase with the rising of Co content considerably. The substitution of Co for Ni renders a visible enlargement of the lattice parameters and cell volumes, which is attributed to the fact that the radius of Co atom is larger than Ni atom. Furthermore, it can be seen from Table 1 that the melt spinning produces broadened diffuse peaks, indicating the refined grain and stored stress in the grains. a
Mg2Ni MgCo2
Co4
Intensity/a.u.
Co3 Co2 Co1 Co0
20
40
60
80
Mg2Ni MgCo2
100 b Co4 Co3
Intensity/a.u.
The compositions of the experimental alloys were Mg20Ni10-xCox (x=0, 1, 2, 3, 4). For convenience, the alloys were denoted with Co content as Co0, Co1, Co2, Co3 and Co4, respectively. The alloy ingots were prepared by using a vacuum induction furnace in a helium atmosphere at a pressure of 0.04 MPa. A part of the as-cast alloys was re-melted and spun by melt-spinning with a rotating copper roller. The spinning rate was approximately expressed by the linear velocity of the copper roller. The spinning rates used in the experiment were 15, 20, 25 and 30 m/s, respectively. The phase structures and compositions of the as-cast and spun alloys were determined by XRD (D/max/2400). The morphologies of the as-cast alloys were examined by SEM (Philips QUANTA 400). The thin film samples of the as-spun alloys prepared by ion etching were observed by HRTEM (JEM-2100F) and the crystalline state of the samples were characterized by electron diffraction (ED). Thermal stability and crystallization of the as-spun alloys were studied by means of DSC instrument (STA449C), and the heating temperature and rate are 600 °C and 10 °C/min, respectively. The hydriding and dehydriding kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The hydrogen absorption was conducted at 1.5 MPa and 200 °C, and the hydrogen desorption was carried out at a pressure of 1×10-4 MPa and 200 °C, respectively. The pulverized amorphous alloys were mixed with carbonyl nickel powder in a mass ratio of 1:4. The mixture was cold pressed at a pressure of 35 MPa into round electrode pellets with a diameter of 10 mm and total mass of about 1 g. A tri-electrode open cell was used for testing the electrochemical characteristics of the experimental alloy electrodes at 30 °C. In every cycle, the alloy electrode was first charged at a current density of 20 mA/g, after resting for 15 min, and it was discharged at the same current density to cut-off voltage of –0.500 V.
Co2 Co1 Co0
Results and Discussion
20
2.1 Microstructure characteristics The XRD patterns of the as-cast and spun alloys are depicted in Fig.1. The results indicate that the substitution of Co for Ni, instead of changing the major phase of Mg2Ni, leads to the for-
40
60
80
100
2θ/(°) Fig.1 XRD patterns of the alloys: (a) as-cast and (b) as-spun (30 m/s)
Table 1 Lattice parameters, cell volume and the FWHM values of the diffraction peaks of the alloys FWHM value Alloy
2θ=20.02°
Lattice parameters and cell volume 2θ=45.14°
a/nm
V/nm3
c/nm
As-cast
As-spun
As-cast
As-spun
As-cast
As-spun
As-cast
As-spun
As-cast
As-spun
Co0
0.122
0.133
0.169
0.182
0.520 97
0.521 06
1.324 4
1.328 7
0.311 28
0.313 41
Co1
0.179
0.206
0.188
0.225
0.521 26
0.521 86
1.325 6
1.332 3
0.311 92
0.314 22
Co2
0.188
0.246
0.239
0.290
0.521 67
0.528 53
1.330 4
1.333 6
0.313 55
0.322 61
Co3
0.192
0.536
0.243
0.329
0.521 83
0.528 68
1.330 9
1.341 2
0.313 84
0.324 64
Co4
0.213
—
0.350
—
0.522 04
—
1.331 2
—
0.314 17
—
* As-spun: spinning rate is 30 m/s
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a
b
20 μm Mg
Co
Intensity/a.u.
Mg2Ni
MgCo2
Co
Ni Mg Ni
Co
0
2
4
6
Energy/keV
Ni
Co
8
0
2
4
6
Ni
8
Energy/keV
Fig.2 SEM images of the as-cast Co0 and Co3 alloys together with typical EDS spectra of Fig.2b: (a) Co0 alloy and (b) Co3 alloy
10
a
b
200 nm
10 nm
Fig.3 HRTEM images and ED patterns of the as-spun (30 m/s) alloys: (a) Co0 alloy and (b) Co3 alloy
30 m/s
Heat Flow Exo
The SEM images of the as-cast Co0 and Co3 alloys are illustrated in Fig.2, showing that the substitution of Co for Ni brings on an evident refinement of the grains of the as-cast alloy, which probably is the reason why the substitution of Co for Ni leads to the main diffraction peaks of the alloys clearly broadened (Fig.1). The morphologies of the alloys change from bulky dendrite structure to massive and rabdoid structure. The result obtained by energy dispersive spectrometry (EDS) indicates that the major phase of the Co0 and Co3 alloys is Mg2Ni phase, but Co substitution yields the secondary phase MgCo2, which is in agreement with the result of the XRD observation. The micrographs of the as-spun (30 m/s) Co0 and Co3 alloys observed by HRTEM are depicted in Fig.3, showing that the as-spun Co0 alloy displays a nanocrystalline structure with grain size of about 10 nm, and its electron diffraction (ED) pattern presents sharp multi-haloes, corresponding to a crystal structure. The morphology of the as-spun Co3 alloy exhibits a feature of the above-mentioned equal sized nanocrystalline structure embedded in the amorphous matrix, and its electron diffraction pattern consists of broad and dull halo, confirming the presence of an amorphous structure. This result agrees very well with the XRD observation shown in Fig.1. 2.2 Thermal stability and crystallization In order to examine the thermal stability and the crystallization of the as-spun amorphous and nanocrystalline/amorphous alloys, DSC analysis was conducted as shown in Fig.4. It reveals that during heating the alloys crystallize completely, and the crystallization process of Co3 alloy consists of two steps. The first crystallization reaction at about 232 °C is connected with a sharp exothermic DSC peak, followed by a smaller and
232.5 °C
25 m/s
232.1 °C
100
200
300
400
Temperature/°C Fig.4 DSC profiles of Co3 alloy spun at 25 and 30 m/s
wider peak (418 °C) corresponding to a second crystallization reaction. It was proved that the first sharper peak corresponds to the crystallization (ordering) of the amorphous into nanocrystalline Mg2Ni[9]. 2.3 Hydriding and dehydriding characteristics The hydrogen absorption kinetic curves of the as-cast and spun Co0 and Co3 alloys are plotted in Fig.5, indicating that all the as-spun alloys possess very fast hydrogen absorption rate and nearly reach their saturation capacities in 5 min. The melt spinning improves the hydrogen absorption properties of the alloys significantly. When the spinning rate grows from 0 to 30 m/s, the hydrogen absorption capacity of the Co0 alloy in 10 min rises from 1.39 wt% to 3.12 wt%, and from 2.71 wt% to 3.23 wt% for the Co3 alloy, indicating that the hydriding kinetics and storage capacity of all the as-spun nanocrystalline and nanocrystalline/amorphous Mg2Ni-type alloys are superior to those of conventional polycrystalline materials with similar composition. Spassov et al.[10] have also confirmed that the melt spinning could improve the hydrogen absorption performance of Mg-based alloy significantly, and obtain the maximum hydrogen capacity of 4.0 wt% for the as-quenched Mg75Ni20Mm5 (Mm=Ce, La-rich mischmetal) alloy. It is found from comparing Fig.5a with 5b that the substitution of Co for Ni improves the hydrogen absorption property of the as-cast Mg2Ni-type alloy dramatically, for which the increased cell volume and the refined grain induced by Co substitution are responsible.
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3.0 2.5 2.0 1.5 1.0 0.5
Hydrogen Absorption/wt%
0.0 0
20
40
60
80
100
120 b
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
20
40
60
80
100
120
Time/min Fig.5 Hydriding kinetic curves of the as-cast and spun Co0 (a) and
Hydrogen Desorption/wt%
Co3 (b) alloys
0.8 0.6 0.4 0.2 0.0
Hydrogen Desorption/wt%
a
1.0
0
30
60
90
120
2.4
b
1.6 1.2 0.8 0.4 0
40
80
120
160
Time/min
Fig.6 Dehydriding kinetic curves of the as-cast and spun Co0 (a) and Co3 (b) alloys
a
140 120 100 80 60 40 20 0 0 350
150
2.0
0.0
Discharge Capacity/mAh·g-1
a
3.5
the hydrogen desorption of the alloys is substantially meliorated by melt spinning. When the spinning rate grows from 0 to 30 m/s, the hydrogen desorption capacity of the Co0 alloy in 20 min increases from 0.19 wt% to 0.89 wt%, and from 1.24 wt% to 1.87 wt% for Co3 alloy, respectively. Fig.6a indicates that as-cast and spun Co0 alloy possesses a low hydrogen desorption capacity and a poor dehydriding kinetics, which is ascribed to the high stability of the crystal Mg-based hydride because melt spinning can not change the crystal state of the Co0 alloy. Correspondingly, the as-spun Co3 alloy exhibits high hydrogen desorption capacity and fast dehydriding rate. The improved dehydriding characteristics can be attributed to two reasons. On one hand, Co substitution evidently intensifies the glass forming ability of Mg2Ni-type alloy because amorphous Mg2Ni shows an excellent hydrogen desorption capability. On the other hand, such substitution decreases the stability of the hydride and makes the desorption reaction easier[1,11]. 2.4 Electrochemical hydrogen storage performances The cycle number dependence of the discharge capacity of the alloys is illustrated in Fig.7. Evidently, all the alloys have an excellent activation capability and attain their maximum discharge capacities at the first charging-discharging cycle. The melt spinning markedly enhances the discharge capacity of the alloys. When the spinning rate increases from 0 to 30 m/s, the discharge capacity grows from 30.26 to 135.51 mAh/g for the Co0 alloy, and from 115.4 to 284.2 mAh/g for
Discharge Capacity/mAh·g-1
Hydrogen Absorption/wt%
The dehydriding kinetic curves of the Co0 and Co3 alloys are presented in Fig.6, from which it is found that the capability of
4
8
12
16
20 b
300 250 200 150 100 50 0
4
8
12
16
20
Cycle Number, n
Fig.7 Evolution of the discharge capacity of the alloys with the cycle number: (a) Co0 alloy and (b) Co3 alloy
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Co3 alloy. The nanocrystalline and the amorphous microstructures formed by melt spinning are extremely helpful for enhancing hydrogen diffusivity and solubility, and thus it seems to be self-evident that the discharge capacity of the alloys grows with the rising of the spinning rate. It is noteworthy that, for the same spinning rate, the discharge capacity of the Co3 alloy is much larger than that of the Co0 alloy, which is ascribed to the glass forming ability of the Mg2Ni-type alloy enhanced by substituting Ni with Co. The capacity retaining rate (Sn) as the symbolization of the cycle stability of an alloy, is defined as Sn=Cn/Cmax×100%, where Cmax is the maximum discharge capacity and Cn is the discharge capacity of the nth charge-discharge cycle, respectively. According to the above-mentioned definition, it can be seen that the larger the capacity retaining rate (Sn) is, the better the cycle stability of the alloy will be. The Sn values of the Co0 and Co3 alloys as functions of the cycle number are illustrated in Fig.8. It is evident that the melt-spinning engenders a negligible influence on the cycle stability of the Co0 alloy, whereas it strengthens the cycle stability of Co3 alloy dramatically, for which the glass forming ability enhanced by Co substitution is mainly responsible. When the spinning rate increases from 0 to 30 m/s, the S20 value rises from 39.02% to 78.51% for the Co3 alloy, but it declines from 36.71% to 27.06% for the Co0 alloy, which is mainly attributed to the grains refined by the melt spinning. It is well known that the essential reason of leading to the loss efficacy of the Mg-based alloy electrodes is the severe corrosion of Mg in the alkaline KOH solution[12-14].
a
100
Sn/%
80
3 Conclusions 1) In the Mg20Ni10-xCox (x=0, 1, 2, 3, 4) alloys, the substitution of Co for Ni significantly enhances the glass forming ability of Mg2Ni-type alloy and leads to the visible refinement of the grains of the as-cast alloy. The substitution of Co for Ni results in the formation of the secondary phase MgCo2 without altering the major phase Mg2Ni in the alloy. 2) The substitution of Co for Ni improves the hydriding and dehydriding characteristics of the alloy dramatically. The increased hydrogen absorption capacity is attributed to the enlarged cell volume and the refined grain produced by Co substitution. The decreased stability of the hydride produced by Co substitution is responsible for the improved hydrogen desorption property. 3) Melt spinning improves the hydriding and dehydriding properties of the alloys significantly. Hydriding and dehydriding capacities and rates of the alloy markedly rise with increasing of the spinning rate, which is mainly attributed to the nanocrystalline and amorphous structure generated by the melt spinning.
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