New La–Fe–B ternary system hydrogen storage alloys

New La–Fe–B ternary system hydrogen storage alloys

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New LaeFeeB ternary system hydrogen storage alloys H.Z. Yan a,b,*, F.Q. Kong a,b, W. Xiong a,b, B.Q. Li a,b, J. Li a,b, L. Wang a,b a b

Baotou Research Institute of Rare Earths, No.36, Huanghe Street, Rare Earth Development Zone, Baotou 014030, PR China National Engineering Research Centre of Rare Earth Metallurgy and Functional Materials, Baotou 014030, PR China

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abstract

Article history:

The new La8Fe28B24-, La15Fe77B8- and La17Fe76B7-type alloys have multiphase structures

Received 20 January 2010

including LaNi5, La3Ni13B2 and (Fe, Ni) phases. The amount of La3Ni13B2 phase increased

Received in revised form

and that of (Fe, Ni) phase decreased with an increasing La/(Fe þ B) atomic ratio. The

25 February 2010

measurement of PeCeI curves revealed that the maximum hydrogen capacity exceeded

Accepted 26 February 2010

1.12 wt% at 313 K in the pressure range of 103 MPae2.0 MPa. The alloys exhibited good

Available online 3 April 2010

absorption/desorption kinetics at room temperature, and electrochemical experiments showed that all of the alloy electrodes exhibited good activation characteristics, high-rate

Keywords: Hydrogen storage materials

dischargeability (HRD) and low-temperature (233 K) dischargeability (LTD). ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

LaeFeeB system alloys Electrochemical properties High-rate dischargeability Low-temperature dischargeability Ni-MH battery

1.

Introduction

Nickel-metal hydride (Ni-MH) secondary cells have been widely adopted in various portable electronic devices, electric hand tools and hybrid electric vehicles [1e4]. Several types of metal hydride electrode materials have been intensively investigated, including rare-earth-based AB5type alloys [1,4], Ti- or Zr-based AB2-type alloys [5,6], Mgbased alloys [7,8] and V-based solid solution alloys [9]. Among the hydrogen storage alloys, AB2-type alloys suffer from slow activation and a relatively low rate capacity, while Mg- and V-based solid solution alloys are less stable and have a shorter cycling life in alkaline solution. To date, almost all commercial Ni-MH batteries employ LaNi5-type alloys as the negative electrode material because of their relatively long life and good electrode kinetics and activation [3,4].

The Ni-MH battery is presently appointed as the most promising system for electric and hybrid electric vehicle (HEV) propulsion in the short- and mid-term. Its high-rate dischargeability (HRD) and its low-temperature dischargeability (LTD) are the crucial performance requirements in HEV applications. Therefore, investigations of new types of hydrogen storage alloys with better HRD and LTD properties are extremely important in both the science and engineering of Ni-MH batteries. As shown in our previous Chinese patents [10,11], the new LaeFeeB system alloys showed good HRD and LTD as electrode materials compared with the LaNi5-type alloys. The LaeFeeB system has many chemical compositions, including La2Fe14B, La2FeB3, La2Fe23B3, La5Fe2B6, La5Fe18B18, La8Fe28B24(La2Fe7B6), La8Fe27B24, La8Fe86B6, La15Fe77B8, La17Fe76B7 and La19Fe68B68. In the present paper, the structure and hydrogen storage properties of the La8Fe27B24, La15Fe77B8 and La17Fe76B7

* Corresponding author at: Baotou Research Institute of Rare Earths, No.36, Huanghe Street, Rare Earth Development Zone, Baotou 014030, PR China. Tel.: þ86 472 5179370; fax: þ86 472 5179330. E-mail address: [email protected] (H.Z. Yan). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.02.127

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hydrogen storage alloys were investigated. Their electrochemical properties, including cycling stability, rate dischargeability and performance at low temperatures, were evaluated.

2.

Experimental

The chemical compositions of the investigated LaeFeeB system alloys are La8Fe2Ni42B2Mn4Al (La8Fe27B24), La15Fe2Ni72Mn7B2Al2 (La15Fe77B8) and La17Fe3Ni73B2Mn3Al2 (La17Fe76B7). Corresponding with the La content, these alloys are named L8, L15 and L17, respectively. These alloys were prepared in a 0.05-MPa argon atmosphere using a vacuum induction-quenching furnace with a rotating copper wheel. In this work, the quenching rate, which was expressed by the linear velocity of the copper wheel, was 4.33 m s1. For a comparison with the three LaeFeeB alloys, one LaNi5-type hydrogen storage alloy (composition MlNi3.9Co0.25Mn0.45 Al0.20Cu0.20 and named as L1, Ml to represent the lanthanumrich mischmetal) was prepared at the same time. The purities of the component metals and master alloys (such as BFe) were at least 99 wt%. The alloys were annealed under vacuum of 102 Pa at 1223 K for 3 h and then at 873 K for 3 h. The prepared alloy flakes were mechanically pulverised into powder particles, ranging from 50 mm to 76 mm in size, for the electrochemical measurements. The structure of the alloys was characterised by x-ray diffraction (XRD) and scanning electron microscopy (SEM) linked with an energy-dispersive X-ray spectrometer (EDS), as described in a previous paper [12]. The phase composition of alloys was also analysed using an electro-probe microanalyser (EPMA, Jeol JXA-8100) to confirm the location of the B element in the alloy. The hydrogen absorptionedesorption and the electrochemical properties of the alloys were investigated, as reported in a previous paper [12]. Each electrode was charged at 70 mA g1 for 6 h followed by a 10-min break and then discharged at 70 mA g1 to the cut-off potential of 1.0 V versus the counter electrode. The high-rate dischargeability (HRD) was measured in the range of discharge current densities, 350 mAe10,500 mA g1. To investigate the low-temperature dischargeability (LTD) of the alloy electrodes at 233 K, the discharge capacities at discharge current densities of 70 mA g1 and 350 mA g1 were measured. The Tafel polarisation curves of the electrodes were measured as described in the literature [13].

3.

Results and discussion

3.1.

Alloy structure characteristics

The XRD patterns of the three LaeFeeB alloys are shown in Fig. 1. All of the alloys have multiphase structures, composed of LaNi5 phase with a hexagonal structure (SG: P6/mmm), La3Ni13B2 phase with a hexagonal structure (SG: P6/mmm) and (Fe, Ni) phase with a cubic structure (SG: Fm-3m). The amount of La3Ni13B2 phase increases, while that of the (Fe, Ni) phase decreases, in the alloys with an increasing La/(Fe þ B) atomic

Fig. 1 e XRD patterns of the three LaeFeeB hydrogen storage alloys.

ratio (in turn, L8, L15 and L17). According to the composing characteristics of the hydrogen storage alloy [14], the LaNi5 phase in the alloys should be responsible for the major part of hydrogen absorption and desorption, while the (Fe, Ni) phase should control the catalytic activities for hydrogen absorption and desorption. The electrochemical test revealed that the hydrogen storage capacity of the La3Ni13B2 alloy is 157 mAh g1, so the La3Ni13B2 phase should also work as a hydrogen reservoir. Fig. 2 shows the SEM images of the three LaeFeeB alloys. All alloys exhibit multiphase images. The EDS and EPMA analyses show that these phases are LaNi5 (grey region), La3Ni13B2 (white region) and (Fe, Ni) (black region), respectively. The L8 alloy consisted mainly of LaNi5 phase with LaNi4.5Fe0.2Mn0.2Al0.1 composition and (Fe, Ni) phase with Fe0.1Ni0.6Mn0.2B0.2 composition, whereas the L17 alloy mainly consisted of LaNi5 phase with LaNi4.2Fe0.2Mn0.3Al0.2 composition and La3Ni13B2 phase with La3Ni11.3Fe0.6Mn0.4Al0.2B1.2 composition. With little difference between the two alloys above, the L15 alloy consisted of three phases (LaNi5 phase with LaNi4.5Fe0.1Mn0.3Al0.1 composition, La3Ni13B2 phase with La3Ni13.1Fe0.2Mn0.6Al0.3B1.2 composition and (Fe, Ni) phase with Fe0.1Ni0.6Mn0.3 composition), which was consistent with the results characterised by XRD. The EPMA analysis results showed that the B element could not be detected in the LaNi5 phase. So, while it was doped into the LaNi5-type alloy, the B element did not enter into the LaNi5 composition. It is possible that the B element formed a second phase with some element in the LaNi5 composition. As a result, the capacity evidently decreased and the kinetics obviously improved for the LaNi5type alloy doped with the B element [15].

3.2.

PeC isotherms

Fig. 3 shows the PeC isotherms of L8, L15 and L17 at 313 K compared to an L1 alloy with low cost (low-Co) and good kinetics, which was one of the commercial LaNi5-type alloys. The pressure of desorption equilibrium in the three LaeFeeB alloys ranged between 0.01 and 0.05 MPa, which fit

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Fig. 3 e The absorption/desorption PeC isotherms of the three LaeFeeB hydrogen storage alloys at 313 K.

Fig. 4 e The absorption/desorption kinetics curves of the three LaeFeeB hydrogen storage alloys at 313 K.

Fig. 2 e SEMs of the three LaeFeeB alloys.

for the negative material in the Ni-MH battery. When the La/(Fe þ B) atomic ratio increased from L8 to L17, the hydrogen storage capacity increased from 1.12 wt% to 1.28 wt%. The hydrogen storage capacity of the alloys was smaller than that (1.34 wt%) of the L1 alloy. This difference could relate to the multiphase structure of the three LaeFeeB alloys with different compositions.

To determine the kinetic characteristics of the three LaeFeeB alloys, Fig. 4 shows the absorption/desorption weight percent of L8, L15, L17 and L1 alloys at different times according to the correlative data obtained in the PeCeI measurements at 313 K. From Fig. 4, it is apparent that the kinetic absorption/desorption property of the three LaeFeeB alloys, especially the kinetic desorption property, was superior to that of the LaNi5-type alloy. The absorption/desorption of the L8 and L15 alloys was completed in 40 min. The desorption velocity of the L8, L15 and L17 alloys was about 4, 3 and 1.6 times larger than that of the LaNi5-type alloy, respectively. The good kinetic characteristics of the LaeFeeB alloys were probably attributed to the catalysis (Fe, Ni) phase and abundant phase interfaces in the alloys. The kinetic order of the L8, L15 and L17 alloys decreased with increasing La/(Fe þ B) atomic ratio, which could be related to the decrease of the (Fe, Ni) phase in turn.

3.3.

Electrochemical characteristics

The L1 alloy was selected from among the LaNi5-type alloys as a reference alloy for comparison because the L1 alloy excels

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Fig. 5 e The discharge potential curves of the three LaeFeeB alloy electrodes at discharge density of 70 mA gL1 (298 K).

over the conventional LaNi5-type alloy (MlNi3.50Co0.75 Mn0.45Al0.30) in both high-rate dischargeability (HRD) and lowtemperature dischargeability (LTD). Fig. 5 shows the discharge curves of the three LaeFeeB alloy electrodes and where they correspond to the maximum discharge capacity. The discharge curves of the three LaeFeeB alloy electrodes had a wide plateau region, and no other potential step attributable to the side reactions was observed. This result suggests that only the hydrogen adsorptionedesorption reactions took place in this potential range. The mid-discharge potential (about 1.31 V) was slightly larger than that (about 1.30 V) of the LaNi5-type alloy. The discharge potential equalled the potential difference between the anode and the cathode in the electrochemical test system with two electrodes, and all Ni(OH)2-/NiOOH-positive electrodes with an excess capacity have the same potential. Therefore, the change of discharge potential was caused by different negative electrodes. This result indicates that the potential of the three LaeFeeB alloy electrodes is more negative than that of the LaNi5-type alloy, which helps to improve the power property of the LaeFeeB alloy electrodes. Fig. 6 shows the discharge capacity retention versus the number of charge/discharge cycles for the three LaeFeeB alloy electrodes. The discharge capacity retention means the ratio of the discharge capacity and the maximum capacity

Fig. 6 e Discharge capacity retention vs. cycle number for the three LaeFeeB alloy electrodes.

during the cycle test, which can represent the cyclic stability of the alloy electrode. The electrochemical data about activation, maximum capacity and cyclic stability are listed in Table 1. All of the alloys could be easily activated to reach the maximum capacity within two cycles. The capacity of the LaeFeeB alloy electrodes was close to or larger than the 300 mAh g1 and a little less than that of the LaNi5-type alloy. The cyclic stability of the alloy electrode was almost the same as that of the LaNi5-type alloy. The high-rate dischargeability (HRD) was defined and calculated as reported in a previous paper [12]. In this paper, the discharge current density Id was 350, 1750, 3500, 7000 and 10,500 mA g1, respectively. Fig. 7 shows the HRDs of the LaeFeeB alloy electrodes. Note that the HRD of the three LaeFeeB alloy electrodes was larger than that of the LaNi5type alloy, which implies that the LaeFeeB alloy electrodes have good electrochemical kinetic properties owing to their multiphase structure. The HRD was influenced mainly by charge transfer on the surface of the alloy electrodes and hydrogen diffusion in the alloy particles. To determine the kinetics of hydrogen absorptionedesorption, the Tafel polarisation curves were plotted. Fig. 8(a) shows the Tafel polarisation curves of the three LaeFeeB alloy electrodes. Each Tafel polarisation curve contained an anode polarisation region, which corresponded to the hydrogen desorption process of the electrode, and

Table 1 e The electrochemical properties of the three LaeFeeB alloy electrodes. Samples

Alloy system

L8 L15 L17 L1

La8Fe27B24 La15Fe77B8 La 17Fe76B7 LaNi5

Alloy

Na

Cmax (mAh g1)b

S50 (%)c

La8Fe2Ni42B2Mn4Al La15Fe2Ni72Mn7B2Al2 La17Fe3Ni73B2Mn3Al2 MlNi3.9Co0.25Mn0.45Al0.20Cu0.20

2 2 2 2

297 309 321 343

75 77 77 78

a N represents the number of cycles needed to activate the electrodes. b Cmax represents the maximum discharge capacity of alloy electrodes. c S50 represents the capacity retention ratio after 50 cycles.

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Fig. 7 e High-rate dischargeability of the three LaeFeeB alloy electrodes at 298 K.

Fig. 9 e Low-temperature (233 K) discharge properties of the LaeFeeB alloy electrodes.

a cathode polarisation region, which corresponded to the hydrogen absorption process into the electrode [13]. During the anodic polarisation process, the anodic current increased first with increasing overpotential and reached a maximum, which was defined as the limiting current

density IL [16]. The IL is influenced by charge transfer, hydrogen diffusion and passivation of the active composition [17] and is mainly related to the diffusion rate of the hydrogen in the hydrogen storage alloy [18], where the larger the IL value, the faster the diffusion of hydrogen atoms in the particles. The IL values obtained from the Tafel polarisation curves are shown in Fig. 8(b). Note that the limiting current density IL of the LaeFeeB alloy electrodes was larger than that of the LaNi5-type alloy electrode, indicating a drastic increase in electrochemical kinetics of the LaeFeeB electrode alloys. The discharge capacities of the three LaeFeeB alloy electrodes at a discharge current densities of 70 and 350 mA g1 at 233 K are shown in Fig. 9. The discharge capacity of the LaeFeeB alloy electrodes was 1.5 times larger than that of LaNi5-type alloy at 70 mA g1. Although the discharge capacity of the LaeFeeB alloy electrodes at 350 mA g1 decreased significantly, it was still larger than that of the LaNi5-type alloy. Zhang et al. [19] thought that the desorption of hydrogen in a metal hydride was an endothermal reaction and that the hydrogen diffusion in the alloy probably became the ratedetermining factor for low-temperature dischargeability (LTD).

4.

Fig. 8 e Tafel polarisation curves for the three LaeFeeB alloy electrodes with a scan rate of 5 mV sL1 measured at the 50% DOD and 298 K: (a) Tafel polarisation curves; (b) limiting current density IL.

Conclusions

La8Fe27B24-, La15Fe77B8- and La17Fe76B7-type alloys were investigated in this work. They exhibited multiphase structures composed of the phases of LaNi5, La3Ni13B2 and (Fe, Ni). The amount of La3Ni13B2 phase increased and that of (Fe, Ni) phase decreased with increasing La/(Fe þ B) atomic ratio. These alloys have good hydrogen absorption/desorption plateau characteristics. The hydrogen storage capacity increased from 1.12 wt% to 1.28 wt%, with the La/(Fe þ B) atomic ratio increasing from La8Fe27B24-type to La17Fe76B7type. The desorption velocities of the La8Fe27B24-, La15Fe77B8and La17Fe76B7-type alloys were about 4, 3 and 1.6 times larger than that of LaNi5-type alloy, respectively. The HRD of the LaeFeeB alloy electrodes was observably larger than that of the LaNi5-type alloy, and the LaeFeeB alloy electrodes

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exhibited a larger value of the limiting current density than the LaNi5 alloy electrode, which implies that the LaeFeeB alloy electrodes have good electrochemical kinetic properties. Moreover, the discharge capacity of the LaeFeeB alloy electrodes at a discharge current density of 70 mA g1 at 233 K was 1.5 times larger than that of LaNi5-type alloy. The good HRD and LTD values of the LaeFeeB alloy electrodes were attributed to the catalysis (Fe, Ni) phase accelerating the electrochemical hydrogen reactions on the surface of the alloys and the abundant phase interfaces favouring hydrogen diffusion in the alloy particles. The LaeFeeB alloy with a multiphase structure demonstrated outstanding potential as new typenegative material of Ni-MH secondary cells.

Acknowledgements This work was supported by the Key Projects in International Science and Technology Cooperation from the Ministry of Science and Technology of the PRC (2006DFB52550). The authors are indebted to Dr. Wei-Kang Hu from Stockholm University for his work on the project.

references

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