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Structures and electrochemical hydrogen storage behaviours of La0.75−xPrxMg0.25Ni3.2Co0.2Al0.1 (x = 0–0.4) alloys prepared by melt spinning
international journal of hydrogen energy 34 (2009) 6335–6342
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Structures and electrochemical hydrogen storage behaviours of La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys prepared by melt spinning Yang-huan Zhanga,b,*, Hui-ping Renb, Bao-wei Lib, Shi-hai Guoa, Qing-chun Wangb, Xin-lin Wanga a
Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, PR China School of Material, Inner Mongolia University of Science and Technology, Baotou 014010, PR China
b
article info
abstract
Article history:
In order to improve the electrochemical performance of the La–Mg–Ni system A2B7-type
Received 24 February 2009
electrode alloys, La in the alloy was partially substituted by Pr and melt spinning tech-
Received in revised form
nology was used for preparing La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0, 0.1, 0.2, 0.3, 0.4) elec-
27 May 2009
trode alloys. The microstructures and electrochemical performance of the as-cast and spun
Accepted 2 June 2009
alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the
Available online 30 June 2009
as-cast and spun alloys have a multiphase structure which consists of two main phases (La, Mg)Ni3 and LaNi5 as well as a residual phase LaNi2. The substitution of Pr for La leads to
Keywords:
an obvious increase of the (La, Mg)Ni3 phase and a decrease of the LaNi5 phase in the alloys.
A2B7-type electrode alloy
The results of the electrochemical measurement indicate that the discharge capacity of the
Substitution of Pr for La
alloys first increases and then decreases with variation of the Pr content. The cycle stability
Melt spinning
of the alloy monotonically rises with increasing Pr content. When the Pr content rises from
Microstructure
0 to 0.4, the discharge capacity increases from 389.4 (x ¼ 0) to 392.4 (x ¼ 0.1) and then drops
Electrochemical performance
to 383.7 mAh/g (x ¼ 0.4) for the as-cast alloy. Discharge capacity increases from 393.5 (x ¼ 0) to 397.9 (x ¼ 0.1), and then declines to 382.5 mAh/g for the as-spun (5 m/s) alloys. The capacity remaining after 100 cycles increases from 65.32 to 79.36% for the as-cast alloy, and from 73.97 to 93.08% for the as-spun (20 m/s) alloy. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, the European community and major developed countries world-wide banned the use of cadmium in batteries, effectively preventing continuing use of Ni–Cd power. This provides a golden opportunity for the development of the NiMH battery. However, as negative electrode material of the NiMH battery, none of the currently commercialized electrode
alloys including AB5 and AB2-types can meet the demand of the power battery owing to the limitation of their properties. The discharge capacity of the AB5-type electrode alloy is comparatively low, and the activation capability of the AB2-type Laves phase electrode alloys is very poor. Therefore, it has been one of the serious challenges faced by researchers in this area to find new electrode alloys with a higher capacity and longer cycle life. Recently, some of the new series of
* Corresponding author. Department of Functional Material Research, Central Iron and Steel Research Institute, 76 Xueyuan Nan Road, Haidian District, Beijing 100081, PR China. Tel.: þ86 010 62187570; fax: þ86 010 62182296. E-mail address: [email protected] (Y.-h. Zhang). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.06.007
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RE–Mg–Ni-based (where RE is a rare earth or Y, Ca) AB3 and A2B7-type alloys were considered to be the most promising candidates owing to their higher discharge capacities (360–410 mAh/g) and low production costs. Kadir et al. [1–4] revealed that this kind of alloy holds a PuNi3-type rhombohedral structure. Kohno et al. [5] found that the La5Mg2Ni23type La0.7Mg0.3Ni2.8Co0.5 electrode alloy has a capacity of 410 mAh/g, and good cycle stability during 30 charge– discharge cycles. Pan et al. [6] investigated the structures and electrochemical characteristics of the La0.7Mg0.3(Ni0.85Co0.15)x (x ¼ 3.15 w 3.80) alloy system and obtained a maximum discharge capacity of 398.4 mAh/g, but the cycle stability of the alloy needs to be improved further. Liao et al. researched the influence of element additive and substitution on the structure and electrochemical behaviours of the alloys, and the results showed that the addition and substitution of elements Al, Cu, Fe, Mn, Co and Zr significantly improved the electrochemical performances of the alloys [7,8]. It is well known that element substitution is one of the effective methods for improving the overall properties of hydrogen storage alloys. In addition, the preparation technology is also extremely important for improving the performances of the alloys. Therefore, it is expected that the combination of an optimized amount of Pr substitution for La with a proper melt spinning technique may lead to an alloy with a high discharge capacity and good cycling stability. In this paper, the La in the alloy was partially substituted by Pr in order to improve the electrochemical properties of the A2B7-type electrode alloys. The A2B7-type La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0–0.4) alloys were prepared by melt spinning and their structure and electrochemical characteristics were examined in detail.
2.
Experimental
The alloy ingots were prepared by using a vacuum induction furnace in a helium atmosphere under a pressure of 0.04 MPa. 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 because it is too difficult to measure a real spinning rate, i.e. cooling rate of the sample during spinning. The spinning rates used in the experiment were 5, 10, 15 and 20 m/s, respectively. The nominal compositions of the experimental alloys were La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0, 0.1, 0.2, 0.3, 0.4). For convenience, the alloys were denoted with Pr content as Pr0, Pr1, Pr2, Pr3 and Pr4, respectively. The cast ingot and spun ribbons were mechanically crushed and ground into powder of 300 mesh size for X-ray diffraction (XRD). The phase structures and compositions of the alloys were determined by an XRD diffractometer of D/ max/2400. Diffraction, with experimental parameters of 160 mA, 40 kV and 10 /min respectively, was performed with CuKa1 radiation filtered by graphite. The morphologies of the as-cast alloys were examined by SEM. The thin film samples of the as-spun alloys were prepared by ion etching for observing the grain morphology with TEM, and for determining the crystalline state of the samples with selected area electron diffraction (ED).
Round electrode pellets of 15 mm diameter were prepared by cold pressing a mixture of alloy powder and carbonyl nickel powder in the weight ratio of 1:4 with a pressure of 35 MPa. After drying for 4 h, the electrode pellets were immersed in a 6 M KOH solution for 24 h in order to wet fully the electrodes before the electrochemical measurement. A tri-electrode open cell, consisting of a metal hydride electrode, a NiOOH/Ni(OH)2 counter electrode and a Hg/HgO reference electrode, was used for testing the electrochemical performance of the experimental electrodes. The electrolyte was a 6 M KOH solution. The voltage between the negative electrode and the reference electrode was defined as the discharge voltage. In every cycle, the alloy electrode was firstly charged with a constant current density. After resting for 15 min, it was discharged at the same current density to a cutoff voltage of 0.500 V. The environment temperature of the measurement was kept at 30 C. After the alloy electrodes were completely activated, the electrochemical impedance spectroscopy (EIS) of the alloys was measured by an advanced electrochemical system (PARSTAT 2273) at 50% depth of discharge (DOD) and in the frequency range of 10 kHz–5 mHz with an amplitude of 5 mV under the open-circuit condition.
3.
Results and discussion
3.1.
Structural characteristics
XRD patterns of the cast and spun (5 m/s) alloys are shown in Fig. 1, revealing that the as-cast and spun alloys hold a multiphase structure, composed of two major phases (La, Mg)Ni3 and LaNi5 as well as a residual phase LaNi2. The substitution of Pr for La exerts an unconscious influence on the phase composition of the alloys. Listed in Table 1 are the lattice parameters of the (La, Mg)Ni3 and the LaNi5 major phases in the as-cast and spun (5 m/s) alloys, which were calculated from the XRD data by the software of Jade 6.0. The results show that the substitution of Pr for La obviously reduces the lattice constants and cell volumes of the (La, Mg)Ni3 and the LaNi5 major phases in the alloys. This is due to the fact that the atom radius of Pr is smaller than that of La. It is derived by Table 1 that the substitution of Pr for La causes an increase of the (La, Mg)Ni3 phase and a decrease of the LaNi5 phase. In order to distinctly show the influence of Pr content on the abundances of the two major phases in the alloys, the Pr content dependence of the abundances of the LaNi5 and the (La, Mg)Ni3 major phases in the as-cast and spun (5 m/s) alloys is plotted in Fig. 2. It can clearly be seen in Fig. 2 that melt spinning leads to an increase of the LaNi5 phase and a decrease of the (La, Mg)Ni3 phase. SEM images of the as-cast alloys are shown in Fig. 3. The result obtained by SEM with energy dispersive spectrometry (EDS) indicates that all the experimental alloys are of multiphase structure, containing both the (La, Mg)Ni3 and the LaNi5 phases, which is in agreement with the results by XRD. Because the amount of the LaNi2 phase is small and it attaches itself to the (La, Mg)Ni3 or the LaNi5 phase in the process of growing, it is difficult to observe the morphology of the LaNi2 phase. It can be seen in Fig. 3 that the substitution of Pr for La leads to obvious refined grains of the as-cast alloys.
international journal of hydrogen energy 34 (2009) 6335–6342
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capacities at the first charging–discharging cycle. The substitution of Pr for La does not effect the activation capability of the alloys. The Pr content dependence of the maximum discharge capacities of the alloys is shown in Fig. 6 with a charge– discharge current density of 100 mA/g. It can clearly be seen in Fig. 6 that with the incremental change of the Pr content the discharge capacities of the alloys first increase and then decrease. When the Pr content rises from 0 to 0.4, the discharge capacity increases from 389.4 (x ¼ 0) to 392.4 (x ¼ 0.1), and then drops to 383.7 mAh/g (x ¼ 0.4) for the as-cast alloy. And it mounts up from 393.5 (x ¼ 0) to 397.9 (x ¼ 0.1), and then declines to 382.5 mAh/g for the as-spun (5 m/s) alloys. The discharge capacity of the alloy depends on multiple factors, involving its crystal structure, phase composition and structure, grain size, composition uniformity and surface state, etc. A decrease of a cell volume caused by the substitution of Pr for La is harmful for enhancing the discharge capacity of the alloy. Therefore, it is understandable that the substitution of Pr for La leads to a decrease of the discharge capacities of the as-cast and spun alloys. In fact, the discharge capacities of the as-cast and spun alloys have the maximum values with the variation of Pr content, which is related to the change of the phase abundances and structure of the alloys caused by Pr substitution. The amount of the (La, Mg)Ni3 phase in the alloy increases with increasing Pr content (Fig. 1), which is advantageous to the discharge capacity of the alloy due to the fact that the discharge capacity of the (La, Mg)Ni3 phase is more than that of the LaNi5 phase [9,10]. It is the above mentioned contrary effects that result in an optimum Pr content for the discharge capacities of the alloys.
3.2.2. High rate discharge (HRD) ability and electrochemical impedance spectroscopy (EIS)
Fig. 1 – XRD patterns of the as-cast and spun La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys: (a) ascast, (b) as-spun (5 m/s).
The morphologies and the crystalline states of the as-spun alloys were examined by TEM as shown in Fig. 4. The figure indicates that the as-spun (5 m/s) Pr0 and Pr4 alloys exhibit a nano-crystalline and microcrystalline structure. The electron diffraction patterns of the alloys show the same result.
3.2.
Electrochemical performances
3.2.1.
Activation capability and discharge capacity
The activation capability was characterized by the number of charging–discharging cycles required for attaining the greatest discharge capacity through a charging–discharging cycle at a constant current density of 100 mA/g. The fewer the number of charging–discharging cycles, the better the activation performance. The evolution of the discharge capacities of the as-cast and spun alloys with the cycle number is plotted in Fig. 5. It indicates that all the alloys display excellent activation performances, attaining their maximum discharge
The high rate discharge (HRD) ability of the alloy electrode, which is mainly determined by the kinetic property, is calculated according to the following formula: HRD ¼ C600;max =C100;max 100%; where C600,max and C100,max are the maximum discharge capacities of the electrode charged–discharged at current densities of 600 and 100 mA/g, respectively. The HRDs of the alloys as a function of the Pr content are shown in Fig. 7. It can be seen that the HRDs of the alloys obtain the maximum value with variation of the Pr content. When the Pr content increases from 0 to 0.4, the HRD of the as-cast alloy increases from 87.16 to 90.21% and then decreases to 84.69%, and it rises from 88.33 to 92.02% and then falls to 83.35% for the as-spun (5 m/s) alloy. The HRD of the alloy is a dynamical property of hydrogen absorption/desorption of the alloy electrode, which is influenced mainly by electrochemical reaction kinetics on the alloy powder surface and the diffusion rate of hydrogen in the bulk of the alloy [11]. The grain refinement produced by Pr substitution enhances the diffusion capability of hydrogen in the alloy [12], but the decrease of cell volume caused by Pr substitution lowers the diffusion capability of hydrogen [13]. It seems to be self-evident that the above mentioned contrary effects consequentially lead to an optimum Pr content for the HRDs of the alloys. Fig. 8 shows the electrochemical impedance spectra of the as-cast alloy electrodes at 50% DOD at 303 K. As shown in
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international journal of hydrogen energy 34 (2009) 6335–6342
Table 1 – Lattice constants and abundances of the LaNi5 and (La, Mg)Ni3 major phases. Conditions
As-cast
Alloys
Pr0 Pr1 Pr2 Pr3 Pr4
As-spun (5 m/s)
Pr0 Pr1 Pr2 Pr3 Pr4
Major phases
(La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5
Lattice constants (nm) a
c
0.5204 0.5197 0.5157 0.5109 0.5108 0.5085 0.5093 0.5027 0.5012 0.4982 0.5201 0.5194 0.5139 0.5103 0.5106 0.5078 0.5084 0.5025 0.5009 0.4979
2.4401 0.4178 2.4312 0.4124 2.4284 0.4098 2.4209 0.4049 2.4185 0.4013 2.4403 0.4178 2.4313 0.4126 2.4284 0.4099 2.4211 0.4051 2.4187 0.4014
Fig. 8, each EIS spectrum contains two semicircles. Kuriyama et al. [14] explained that the smaller semicircle in the high frequency region is attributed to the contact resistance between the alloy powder and the conductive material, while the larger semicircle in the low frequency region is attributed to the charge-transfer resistance on the alloy surface. So the larger the radius of the semicircle in the low frequency region, the larger the charge-transfer resistance of the alloy electrode. In Fig. 8, the radius of the large semicircle in the low frequency first decreases when the Pr content x increases from 0.0 to 0.1 and then increases when x reaches 0.2. This indicates that the charge-transfer resistance of the alloy electrode first decreases and then increases. As has been mentioned above, the grain of the as-cast alloy significantly refines with increasing Pr content, and the grain boundary leads to an increase of new reaction surface areas of the alloy electrode.
Fig. 2 – Evolution of the abundances of the major phases LaNi5 and (La, Mg)Ni3 with Pr content.
Cell volume (nm3)
Phase abundance (wt.%)
0.5723 0.0977 0.5599 0.0932 0.5487 0.0918 0.5438 0.0886 0.5261 0.0863 0.5717 0.0976 0.5559 0.0930 0.5482 0.0915 0.5419 0.0885 0.5255 0.0861
74.35 24.07 75.65 22.94 77.04 21.73 78.59 20.13 80.02 18.90 73.57 25.02 74.95 23.89 76.61 22.14 78.02 20.93 79.65 19.11
And thus the surface charge-transfer resistance is reduced. However, the increase of the (La, Mg)Ni3 phase caused by Pr substitution is not beneficial for decreasing the surface charge-transfer resistance because the Ni element has good electrocatalytic activity in the alkaline electrolyte and is favorable for increasing the electrochemical hydrogen reaction rate [15]. Therefore, there is an optimum amount of Pr substitution for decreasing the surface charge-transfer resistance. This is in good agreement with the HRDs of the alloys.
3.2.3.
Cycle stability
The capacity remaining (Sn) is introduced for accurately evaluating the cycle stability of the alloy. It is defined as Sn ¼ Cn =Cmax 100%; where Cmax is the maximum discharge capacity, and Cn is the discharge capacity of the nth cycle at a current density of 600 mA/g. The capacity remaining (S100) of the as-cast and spun alloys as a function of Pr content is illustrated in Fig. 9, indicating that the substitution of Pr for La leads to a significant increase of the capacity remaining (S100). When the Pr content rises from 0 to 0.4, the capacity remaining (S100) increases from 65.32 to 79.36% for the as-cast alloy, and from 73.97 to 93.08% for the as-spun (20 m/s) alloy. Similar results were obtained by Li et al. and Pan et al. [16,17]. It can also be derived from Fig. 9 that for a fixed Pr content the capacity remaining (S100) of the alloys rises with the increase of the spinning rate, meaning that melt spinning clearly improves the cycle stability of the alloys. When the spinning rate increases from 0 (as-cast was defined as a spinning rate of 0 m/s) to 20 m/s, the capacity remaining (S100) increases from 65.32 to 73.97% for the Pr0 alloy, and from 79.36 to 93.08% for the Pr4 alloy. In order to clearly see the process of capacity degradation of the alloy electrode, the evolution of the capacity remaining of the as-cast and spun (20 m/s) alloys with the cycle number is shown in Fig. 10. A rough tendency can be seen in Fig. 10 in which the substitution of Pr for La
international journal of hydrogen energy 34 (2009) 6335–6342
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Fig. 3 – SEM images of the as-cast La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys: (a) Pr0, (b) Pr1, (c) Pr2, (d) Pr4.
causes an obvious decrease of the decay rates of the discharge capacities of the as-cast and spun alloys, suggesting that the substitution of Pr for La enhances the cycle stability of the alloy. The cycle stability of the electrode alloy is a decisive factor of the life of the Ni-MH battery. The main cause leading to
battery efficacy loss is due to the negative electrode rather than the positive electrode. Electrode failure is characterized by the decay of the discharge capacity and the drop of the discharge voltage. It was confirmed by the literature [18,19] that the fundamental reasons for the capacity decay of the electrode alloy are pulverization and oxidation of the alloy
Fig. 4 – The morphologies and ED of the as-spun (5 m/s) La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys taken by TEM: (a, b) morphologies of Pr0 and Pr4 alloys, (c, d) ED of Pr0 and Pr4 alloys.
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international journal of hydrogen energy 34 (2009) 6335–6342
Fig. 7 – Evolution of the high rate discharge capabilities (HRDs) of the La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with Pr content.
Fig. 5 – The activation capability of the as-cast and spun (5 m/s) La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys at 303 K: (a) as-cast, (b) as-spun.
Fig. 6 – Evolution of the discharge capacities of the La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with Pr content.
during the charging–discharging cycle. The lattice stress and the expansion of the cell volume, which are inevitable when hydrogen atoms enter into the interstitial sites of the lattice, are the real driving force that leads to the pulverization of the alloy. The decrease of cell volume caused by the substitution of Pr for La increases the ratios of expansion/contraction of the alloys in the process of hydrogen absorption/desorption, which means impairing the anti-pulverization capability of the alloy. However, the substitution of Pr for La significantly improves the cycle stability of the alloy electrode (Fig. 9), for which the refined grain caused by Pr substitution is responsible. The positive impact of melt spinning on the cycle stability of the alloy is primarily ascribed to the significant refinement of the grains caused by melt spinning. The antipulverization capability of the alloy basically depends on its grain size. Therefore, it is understandable that the cycle stability of the alloy increases with increasing spinning rate.
Fig. 8 – Electrochemical impedance spectra of the as-cast La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloy electrodes measured at 50% DOD and 303 K.
international journal of hydrogen energy 34 (2009) 6335–6342
4.
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Conclusions
The structures and electrochemical performances of the ascast and spun La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0, 0.1, 0.2, 0.3, 0.4) electrode alloys were investigated, and the conclusions obtained are summarized as follows:
Fig. 9 – Evolution of the capacity retaining rate (S100) of the as-cast and spun La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with Pr content.
1. The substitution of Pr for La does not change the phase compositions of the alloys, but it leads to an increases of the (La, Mg)Ni3 phase and a decrease of the LaNi5 phase in the alloys, and obviously reduces the lattice constants and cell volumes of the alloys. 2. For a fixed spinning rate, the discharge capacities of the alloys first increase and then decrease with increasing Pr content. The substitution of Pr for La does affect the activation performances of the alloys, but it significantly enhances the cycle stability of the as-cast and spun alloys. 3. Melt spinning significantly improves the cycle stability of the alloys, which is attributed to grain refinement of the alloys produced by melt spinning.
Acknowledgements This work was supported by Hi-Tech Research and Development Programs of China (2006AA05Z132 and 2007AA03Z227), National Natural Science Foundation of China (50871050 and 50701011), Natural Science Foundation of Inner Mongolia, China (200711020703) and High Education Science Research Project of Inner Mongolia, China (NJzy08071).
references
Fig. 10 – Evolution of the capacity retaining rates of the La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with the cycle number at 303 K: (a) as-cast, (b) as-spun (20 m/s).
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Structures and electrochemical hydrogen storage behaviours of La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys prepared by melt spinning Yang-huan Zhanga,b,*, Hui-ping Renb, Bao-wei Lib, Shi-hai Guoa, Qing-chun Wangb, Xin-lin Wanga a
Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, PR China School of Material, Inner Mongolia University of Science and Technology, Baotou 014010, PR China
b
article info
abstract
Article history:
In order to improve the electrochemical performance of the La–Mg–Ni system A2B7-type
Received 24 February 2009
electrode alloys, La in the alloy was partially substituted by Pr and melt spinning tech-
Received in revised form
nology was used for preparing La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0, 0.1, 0.2, 0.3, 0.4) elec-
27 May 2009
trode alloys. The microstructures and electrochemical performance of the as-cast and spun
Accepted 2 June 2009
alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the
Available online 30 June 2009
as-cast and spun alloys have a multiphase structure which consists of two main phases (La, Mg)Ni3 and LaNi5 as well as a residual phase LaNi2. The substitution of Pr for La leads to
Keywords:
an obvious increase of the (La, Mg)Ni3 phase and a decrease of the LaNi5 phase in the alloys.
A2B7-type electrode alloy
The results of the electrochemical measurement indicate that the discharge capacity of the
Substitution of Pr for La
alloys first increases and then decreases with variation of the Pr content. The cycle stability
Melt spinning
of the alloy monotonically rises with increasing Pr content. When the Pr content rises from
Microstructure
0 to 0.4, the discharge capacity increases from 389.4 (x ¼ 0) to 392.4 (x ¼ 0.1) and then drops
Electrochemical performance
to 383.7 mAh/g (x ¼ 0.4) for the as-cast alloy. Discharge capacity increases from 393.5 (x ¼ 0) to 397.9 (x ¼ 0.1), and then declines to 382.5 mAh/g for the as-spun (5 m/s) alloys. The capacity remaining after 100 cycles increases from 65.32 to 79.36% for the as-cast alloy, and from 73.97 to 93.08% for the as-spun (20 m/s) alloy. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, the European community and major developed countries world-wide banned the use of cadmium in batteries, effectively preventing continuing use of Ni–Cd power. This provides a golden opportunity for the development of the NiMH battery. However, as negative electrode material of the NiMH battery, none of the currently commercialized electrode
alloys including AB5 and AB2-types can meet the demand of the power battery owing to the limitation of their properties. The discharge capacity of the AB5-type electrode alloy is comparatively low, and the activation capability of the AB2-type Laves phase electrode alloys is very poor. Therefore, it has been one of the serious challenges faced by researchers in this area to find new electrode alloys with a higher capacity and longer cycle life. Recently, some of the new series of
* Corresponding author. Department of Functional Material Research, Central Iron and Steel Research Institute, 76 Xueyuan Nan Road, Haidian District, Beijing 100081, PR China. Tel.: þ86 010 62187570; fax: þ86 010 62182296. E-mail address: [email protected] (Y.-h. Zhang). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.06.007
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RE–Mg–Ni-based (where RE is a rare earth or Y, Ca) AB3 and A2B7-type alloys were considered to be the most promising candidates owing to their higher discharge capacities (360–410 mAh/g) and low production costs. Kadir et al. [1–4] revealed that this kind of alloy holds a PuNi3-type rhombohedral structure. Kohno et al. [5] found that the La5Mg2Ni23type La0.7Mg0.3Ni2.8Co0.5 electrode alloy has a capacity of 410 mAh/g, and good cycle stability during 30 charge– discharge cycles. Pan et al. [6] investigated the structures and electrochemical characteristics of the La0.7Mg0.3(Ni0.85Co0.15)x (x ¼ 3.15 w 3.80) alloy system and obtained a maximum discharge capacity of 398.4 mAh/g, but the cycle stability of the alloy needs to be improved further. Liao et al. researched the influence of element additive and substitution on the structure and electrochemical behaviours of the alloys, and the results showed that the addition and substitution of elements Al, Cu, Fe, Mn, Co and Zr significantly improved the electrochemical performances of the alloys [7,8]. It is well known that element substitution is one of the effective methods for improving the overall properties of hydrogen storage alloys. In addition, the preparation technology is also extremely important for improving the performances of the alloys. Therefore, it is expected that the combination of an optimized amount of Pr substitution for La with a proper melt spinning technique may lead to an alloy with a high discharge capacity and good cycling stability. In this paper, the La in the alloy was partially substituted by Pr in order to improve the electrochemical properties of the A2B7-type electrode alloys. The A2B7-type La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0–0.4) alloys were prepared by melt spinning and their structure and electrochemical characteristics were examined in detail.
2.
Experimental
The alloy ingots were prepared by using a vacuum induction furnace in a helium atmosphere under a pressure of 0.04 MPa. 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 because it is too difficult to measure a real spinning rate, i.e. cooling rate of the sample during spinning. The spinning rates used in the experiment were 5, 10, 15 and 20 m/s, respectively. The nominal compositions of the experimental alloys were La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0, 0.1, 0.2, 0.3, 0.4). For convenience, the alloys were denoted with Pr content as Pr0, Pr1, Pr2, Pr3 and Pr4, respectively. The cast ingot and spun ribbons were mechanically crushed and ground into powder of 300 mesh size for X-ray diffraction (XRD). The phase structures and compositions of the alloys were determined by an XRD diffractometer of D/ max/2400. Diffraction, with experimental parameters of 160 mA, 40 kV and 10 /min respectively, was performed with CuKa1 radiation filtered by graphite. The morphologies of the as-cast alloys were examined by SEM. The thin film samples of the as-spun alloys were prepared by ion etching for observing the grain morphology with TEM, and for determining the crystalline state of the samples with selected area electron diffraction (ED).
Round electrode pellets of 15 mm diameter were prepared by cold pressing a mixture of alloy powder and carbonyl nickel powder in the weight ratio of 1:4 with a pressure of 35 MPa. After drying for 4 h, the electrode pellets were immersed in a 6 M KOH solution for 24 h in order to wet fully the electrodes before the electrochemical measurement. A tri-electrode open cell, consisting of a metal hydride electrode, a NiOOH/Ni(OH)2 counter electrode and a Hg/HgO reference electrode, was used for testing the electrochemical performance of the experimental electrodes. The electrolyte was a 6 M KOH solution. The voltage between the negative electrode and the reference electrode was defined as the discharge voltage. In every cycle, the alloy electrode was firstly charged with a constant current density. After resting for 15 min, it was discharged at the same current density to a cutoff voltage of 0.500 V. The environment temperature of the measurement was kept at 30 C. After the alloy electrodes were completely activated, the electrochemical impedance spectroscopy (EIS) of the alloys was measured by an advanced electrochemical system (PARSTAT 2273) at 50% depth of discharge (DOD) and in the frequency range of 10 kHz–5 mHz with an amplitude of 5 mV under the open-circuit condition.
3.
Results and discussion
3.1.
Structural characteristics
XRD patterns of the cast and spun (5 m/s) alloys are shown in Fig. 1, revealing that the as-cast and spun alloys hold a multiphase structure, composed of two major phases (La, Mg)Ni3 and LaNi5 as well as a residual phase LaNi2. The substitution of Pr for La exerts an unconscious influence on the phase composition of the alloys. Listed in Table 1 are the lattice parameters of the (La, Mg)Ni3 and the LaNi5 major phases in the as-cast and spun (5 m/s) alloys, which were calculated from the XRD data by the software of Jade 6.0. The results show that the substitution of Pr for La obviously reduces the lattice constants and cell volumes of the (La, Mg)Ni3 and the LaNi5 major phases in the alloys. This is due to the fact that the atom radius of Pr is smaller than that of La. It is derived by Table 1 that the substitution of Pr for La causes an increase of the (La, Mg)Ni3 phase and a decrease of the LaNi5 phase. In order to distinctly show the influence of Pr content on the abundances of the two major phases in the alloys, the Pr content dependence of the abundances of the LaNi5 and the (La, Mg)Ni3 major phases in the as-cast and spun (5 m/s) alloys is plotted in Fig. 2. It can clearly be seen in Fig. 2 that melt spinning leads to an increase of the LaNi5 phase and a decrease of the (La, Mg)Ni3 phase. SEM images of the as-cast alloys are shown in Fig. 3. The result obtained by SEM with energy dispersive spectrometry (EDS) indicates that all the experimental alloys are of multiphase structure, containing both the (La, Mg)Ni3 and the LaNi5 phases, which is in agreement with the results by XRD. Because the amount of the LaNi2 phase is small and it attaches itself to the (La, Mg)Ni3 or the LaNi5 phase in the process of growing, it is difficult to observe the morphology of the LaNi2 phase. It can be seen in Fig. 3 that the substitution of Pr for La leads to obvious refined grains of the as-cast alloys.
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capacities at the first charging–discharging cycle. The substitution of Pr for La does not effect the activation capability of the alloys. The Pr content dependence of the maximum discharge capacities of the alloys is shown in Fig. 6 with a charge– discharge current density of 100 mA/g. It can clearly be seen in Fig. 6 that with the incremental change of the Pr content the discharge capacities of the alloys first increase and then decrease. When the Pr content rises from 0 to 0.4, the discharge capacity increases from 389.4 (x ¼ 0) to 392.4 (x ¼ 0.1), and then drops to 383.7 mAh/g (x ¼ 0.4) for the as-cast alloy. And it mounts up from 393.5 (x ¼ 0) to 397.9 (x ¼ 0.1), and then declines to 382.5 mAh/g for the as-spun (5 m/s) alloys. The discharge capacity of the alloy depends on multiple factors, involving its crystal structure, phase composition and structure, grain size, composition uniformity and surface state, etc. A decrease of a cell volume caused by the substitution of Pr for La is harmful for enhancing the discharge capacity of the alloy. Therefore, it is understandable that the substitution of Pr for La leads to a decrease of the discharge capacities of the as-cast and spun alloys. In fact, the discharge capacities of the as-cast and spun alloys have the maximum values with the variation of Pr content, which is related to the change of the phase abundances and structure of the alloys caused by Pr substitution. The amount of the (La, Mg)Ni3 phase in the alloy increases with increasing Pr content (Fig. 1), which is advantageous to the discharge capacity of the alloy due to the fact that the discharge capacity of the (La, Mg)Ni3 phase is more than that of the LaNi5 phase [9,10]. It is the above mentioned contrary effects that result in an optimum Pr content for the discharge capacities of the alloys.
3.2.2. High rate discharge (HRD) ability and electrochemical impedance spectroscopy (EIS)
Fig. 1 – XRD patterns of the as-cast and spun La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys: (a) ascast, (b) as-spun (5 m/s).
The morphologies and the crystalline states of the as-spun alloys were examined by TEM as shown in Fig. 4. The figure indicates that the as-spun (5 m/s) Pr0 and Pr4 alloys exhibit a nano-crystalline and microcrystalline structure. The electron diffraction patterns of the alloys show the same result.
3.2.
Electrochemical performances
3.2.1.
Activation capability and discharge capacity
The activation capability was characterized by the number of charging–discharging cycles required for attaining the greatest discharge capacity through a charging–discharging cycle at a constant current density of 100 mA/g. The fewer the number of charging–discharging cycles, the better the activation performance. The evolution of the discharge capacities of the as-cast and spun alloys with the cycle number is plotted in Fig. 5. It indicates that all the alloys display excellent activation performances, attaining their maximum discharge
The high rate discharge (HRD) ability of the alloy electrode, which is mainly determined by the kinetic property, is calculated according to the following formula: HRD ¼ C600;max =C100;max 100%; where C600,max and C100,max are the maximum discharge capacities of the electrode charged–discharged at current densities of 600 and 100 mA/g, respectively. The HRDs of the alloys as a function of the Pr content are shown in Fig. 7. It can be seen that the HRDs of the alloys obtain the maximum value with variation of the Pr content. When the Pr content increases from 0 to 0.4, the HRD of the as-cast alloy increases from 87.16 to 90.21% and then decreases to 84.69%, and it rises from 88.33 to 92.02% and then falls to 83.35% for the as-spun (5 m/s) alloy. The HRD of the alloy is a dynamical property of hydrogen absorption/desorption of the alloy electrode, which is influenced mainly by electrochemical reaction kinetics on the alloy powder surface and the diffusion rate of hydrogen in the bulk of the alloy [11]. The grain refinement produced by Pr substitution enhances the diffusion capability of hydrogen in the alloy [12], but the decrease of cell volume caused by Pr substitution lowers the diffusion capability of hydrogen [13]. It seems to be self-evident that the above mentioned contrary effects consequentially lead to an optimum Pr content for the HRDs of the alloys. Fig. 8 shows the electrochemical impedance spectra of the as-cast alloy electrodes at 50% DOD at 303 K. As shown in
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Table 1 – Lattice constants and abundances of the LaNi5 and (La, Mg)Ni3 major phases. Conditions
As-cast
Alloys
Pr0 Pr1 Pr2 Pr3 Pr4
As-spun (5 m/s)
Pr0 Pr1 Pr2 Pr3 Pr4
Major phases
(La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5
Lattice constants (nm) a
c
0.5204 0.5197 0.5157 0.5109 0.5108 0.5085 0.5093 0.5027 0.5012 0.4982 0.5201 0.5194 0.5139 0.5103 0.5106 0.5078 0.5084 0.5025 0.5009 0.4979
2.4401 0.4178 2.4312 0.4124 2.4284 0.4098 2.4209 0.4049 2.4185 0.4013 2.4403 0.4178 2.4313 0.4126 2.4284 0.4099 2.4211 0.4051 2.4187 0.4014
Fig. 8, each EIS spectrum contains two semicircles. Kuriyama et al. [14] explained that the smaller semicircle in the high frequency region is attributed to the contact resistance between the alloy powder and the conductive material, while the larger semicircle in the low frequency region is attributed to the charge-transfer resistance on the alloy surface. So the larger the radius of the semicircle in the low frequency region, the larger the charge-transfer resistance of the alloy electrode. In Fig. 8, the radius of the large semicircle in the low frequency first decreases when the Pr content x increases from 0.0 to 0.1 and then increases when x reaches 0.2. This indicates that the charge-transfer resistance of the alloy electrode first decreases and then increases. As has been mentioned above, the grain of the as-cast alloy significantly refines with increasing Pr content, and the grain boundary leads to an increase of new reaction surface areas of the alloy electrode.
Fig. 2 – Evolution of the abundances of the major phases LaNi5 and (La, Mg)Ni3 with Pr content.
Cell volume (nm3)
Phase abundance (wt.%)
0.5723 0.0977 0.5599 0.0932 0.5487 0.0918 0.5438 0.0886 0.5261 0.0863 0.5717 0.0976 0.5559 0.0930 0.5482 0.0915 0.5419 0.0885 0.5255 0.0861
74.35 24.07 75.65 22.94 77.04 21.73 78.59 20.13 80.02 18.90 73.57 25.02 74.95 23.89 76.61 22.14 78.02 20.93 79.65 19.11
And thus the surface charge-transfer resistance is reduced. However, the increase of the (La, Mg)Ni3 phase caused by Pr substitution is not beneficial for decreasing the surface charge-transfer resistance because the Ni element has good electrocatalytic activity in the alkaline electrolyte and is favorable for increasing the electrochemical hydrogen reaction rate [15]. Therefore, there is an optimum amount of Pr substitution for decreasing the surface charge-transfer resistance. This is in good agreement with the HRDs of the alloys.
3.2.3.
Cycle stability
The capacity remaining (Sn) is introduced for accurately evaluating the cycle stability of the alloy. It is defined as Sn ¼ Cn =Cmax 100%; where Cmax is the maximum discharge capacity, and Cn is the discharge capacity of the nth cycle at a current density of 600 mA/g. The capacity remaining (S100) of the as-cast and spun alloys as a function of Pr content is illustrated in Fig. 9, indicating that the substitution of Pr for La leads to a significant increase of the capacity remaining (S100). When the Pr content rises from 0 to 0.4, the capacity remaining (S100) increases from 65.32 to 79.36% for the as-cast alloy, and from 73.97 to 93.08% for the as-spun (20 m/s) alloy. Similar results were obtained by Li et al. and Pan et al. [16,17]. It can also be derived from Fig. 9 that for a fixed Pr content the capacity remaining (S100) of the alloys rises with the increase of the spinning rate, meaning that melt spinning clearly improves the cycle stability of the alloys. When the spinning rate increases from 0 (as-cast was defined as a spinning rate of 0 m/s) to 20 m/s, the capacity remaining (S100) increases from 65.32 to 73.97% for the Pr0 alloy, and from 79.36 to 93.08% for the Pr4 alloy. In order to clearly see the process of capacity degradation of the alloy electrode, the evolution of the capacity remaining of the as-cast and spun (20 m/s) alloys with the cycle number is shown in Fig. 10. A rough tendency can be seen in Fig. 10 in which the substitution of Pr for La
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Fig. 3 – SEM images of the as-cast La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys: (a) Pr0, (b) Pr1, (c) Pr2, (d) Pr4.
causes an obvious decrease of the decay rates of the discharge capacities of the as-cast and spun alloys, suggesting that the substitution of Pr for La enhances the cycle stability of the alloy. The cycle stability of the electrode alloy is a decisive factor of the life of the Ni-MH battery. The main cause leading to
battery efficacy loss is due to the negative electrode rather than the positive electrode. Electrode failure is characterized by the decay of the discharge capacity and the drop of the discharge voltage. It was confirmed by the literature [18,19] that the fundamental reasons for the capacity decay of the electrode alloy are pulverization and oxidation of the alloy
Fig. 4 – The morphologies and ED of the as-spun (5 m/s) La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys taken by TEM: (a, b) morphologies of Pr0 and Pr4 alloys, (c, d) ED of Pr0 and Pr4 alloys.
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Fig. 7 – Evolution of the high rate discharge capabilities (HRDs) of the La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with Pr content.
Fig. 5 – The activation capability of the as-cast and spun (5 m/s) La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys at 303 K: (a) as-cast, (b) as-spun.
Fig. 6 – Evolution of the discharge capacities of the La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with Pr content.
during the charging–discharging cycle. The lattice stress and the expansion of the cell volume, which are inevitable when hydrogen atoms enter into the interstitial sites of the lattice, are the real driving force that leads to the pulverization of the alloy. The decrease of cell volume caused by the substitution of Pr for La increases the ratios of expansion/contraction of the alloys in the process of hydrogen absorption/desorption, which means impairing the anti-pulverization capability of the alloy. However, the substitution of Pr for La significantly improves the cycle stability of the alloy electrode (Fig. 9), for which the refined grain caused by Pr substitution is responsible. The positive impact of melt spinning on the cycle stability of the alloy is primarily ascribed to the significant refinement of the grains caused by melt spinning. The antipulverization capability of the alloy basically depends on its grain size. Therefore, it is understandable that the cycle stability of the alloy increases with increasing spinning rate.
Fig. 8 – Electrochemical impedance spectra of the as-cast La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloy electrodes measured at 50% DOD and 303 K.
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4.
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Conclusions
The structures and electrochemical performances of the ascast and spun La0.75xPrxMg0.25Ni3.2Co0.2Al0.1 (x ¼ 0, 0.1, 0.2, 0.3, 0.4) electrode alloys were investigated, and the conclusions obtained are summarized as follows:
Fig. 9 – Evolution of the capacity retaining rate (S100) of the as-cast and spun La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with Pr content.
1. The substitution of Pr for La does not change the phase compositions of the alloys, but it leads to an increases of the (La, Mg)Ni3 phase and a decrease of the LaNi5 phase in the alloys, and obviously reduces the lattice constants and cell volumes of the alloys. 2. For a fixed spinning rate, the discharge capacities of the alloys first increase and then decrease with increasing Pr content. The substitution of Pr for La does affect the activation performances of the alloys, but it significantly enhances the cycle stability of the as-cast and spun alloys. 3. Melt spinning significantly improves the cycle stability of the alloys, which is attributed to grain refinement of the alloys produced by melt spinning.
Acknowledgements This work was supported by Hi-Tech Research and Development Programs of China (2006AA05Z132 and 2007AA03Z227), National Natural Science Foundation of China (50871050 and 50701011), Natural Science Foundation of Inner Mongolia, China (200711020703) and High Education Science Research Project of Inner Mongolia, China (NJzy08071).
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Fig. 10 – Evolution of the capacity retaining rates of the La0.75LxPrxMg0.25Ni3.2Co0.2Al0.1 (x [ 0–0.4) alloys with the cycle number at 303 K: (a) as-cast, (b) as-spun (20 m/s).
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