The high-temperature performance of low-cost LaNiFe based hydrogen storage alloys with Si substituting

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

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The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting Wanhai Zhou a, Qiannan Wang a, Ding Zhu b,**, Chaoling Wu a, Liwu Huang a, Zhewen Ma a, Zhengyao Tang a, Yungui Chen a,b,* a

Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610065, PR China b Institute of New Energy and Low-Carbon Technology, Sichuan University, Chengdu 610065, PR China

article info

abstract

Article history:

Various properties of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix (x ¼ 0, 0.05, 0.075 and 0.1)

Received 2 April 2016

hydrogen storage alloys at 20e80  C have been investigated systematically. With the in-

Received in revised form

crease of Si content, the hydrogen storage capacity increases and the equilibrium pressure

21 June 2016

decreases, thus the improved thermodynamic stability of the metal hydrides makes the Si-

Accepted 23 June 2016

containing alloys more suitable for working at higher temperatures. Appropriate addition

Available online xxx

of Si (x ¼ 0e0.075) improves the anti-corrosion capability of the alloy, as a result, the temperature insensitivity, high-temperature dischargeability and high-temperature

Keywords:

recoverability increase accordingly, particularly the cycling stability (S150) increases from

AB5-type hydrogen storage alloy

43.7% to 71.8%. In addition, the charge-transfer process is suppressed gradually by Si

High-temperature electrochemical

substituting (x ¼ 0e0.075) at 20  C, but gets promoted due to the improved surface catalytic

performance

ability when that at 60  C, consequently the high-rate dischargeability and high-power

Silicon substitution

delivery deteriorate at room temperature, but increase gradually at higher temperature.

Metal hydride nickel battery

Therefore, an optimum alloy electrode is got when x ¼ 0.075, the deterioration of x ¼ 0.1

Peak power density

alloy on high-temperature performance is attributed to the appearance of the second A2B7 phase. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Up to now, nickel/metal hydride (Ni/MH) batteries have been the most widely used energy storage systems for hybrid electrical vehicles (HEV). With more than ten million HEV built to-date, some improvements in cost reductions [1,2], long

cycle life [3,4], high specific energy [5,6], low self-discharge [7,8] and wide temperature performance [9e13] were reported to resist serious competitions from lithium (Li)-ion batteries and other advanced secondary batteries. Generally, large numbers of improvements for Ni/MH batteries are conventionally tested at or near room temperature [1e8]. However, the U.S. Advanced Battery Consortium (USABC) has

* Corresponding author. Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610065, PR China. Tel./fax: þ86 028 85466916. ** Corresponding author. Tel./fax: þ86 028 85466916. E-mail addresses: [email protected] (D. Zhu), [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.ijhydene.2016.06.206 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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required the batteries served in EVs, HEVs and PHEVs to be worked satisfactorily at the temperature up to 60  C [14]. In addition, satisfy requirements of energy storage for other power tool applications, military devices and scorching areas provide the Ni/MH battery with much potential market competitiveness. In the last two decades, much progress in improving the charge efficiency or discharge capacity of the positive electrode of Ni/MH battery working at elevated temperature using additive and alpha-gamma phase were reported [10,15e18]. However, the improvements of hydrogen storage electrode alloy in negative electrode should also be the key technology in spite of the over-fill capacity of the negative electrode, since problems such as decline of charge efficiency, resistance to overcharging, discharge capacity and cycling stability of the hydrogen storage electrode alloy would appear at elevated temperature [19e21]. Recently, various feasible economical approaches including adding corrosion inhibitors into the electrolyte [22,23], modifying of active material surface [24], multi-alloying of the hydrogen storing alloy [11,19,25e30] and optimization of preparing technology [20,29] were employed to suppress capacity attenuation of the negative electrode at elevated temperature. Generally, multi-alloying is deemed to the most economical method for practical application. By now, the effects of substituting Ni by Fe [19], Co [25], Mn [26], Al [11,12,27] and V [28] etc. on its comprehensive hightemperature properties have been researched systematically. Iron, which is a relatively inexpensive 3d-transition metal, is deemed to the potential element to replace Co in AB5-type hydrogen storage alloys [2,19]. Chao et al. [19] reported that excellent cycling stability, charge retention and other hightemperature performance were obtained for high-Fe alloy due to its relatively good anti-corrosion ability. In addition, it reported that heat treatment was in favor of improving the metal hydride stability, high-temperature capacity and highrate dischargeability of LaeNieFe based hydrogen storage alloys, but exacerbated the degradation of cycling life at 60  C [20]. According to our previous study [11,12], Al content should be appropriately high to form protective layers due to the conspicuous dissolution at elevated temperature. However, although Fe and Al had a synergetic action among the unit cell volume, cycling stability and high rate discharge property, the higher Fe content usually means the lower Al content [30]. Low-cost Si is considered to have the similar effect with Al on suppression of the pulverization and improving the cycling stability [31e34]. Iwakura et al. [33] reported that the Fe and Si contained in Co-free alloys more or less suppressed the dissolution of Al and the lattice expansion with hydrogen absorption, leading to the long cycle life at 20  C, and this effect was strengthened by the simultaneous substitution with Si and Fe. Here, the Si-substituted LaeNieFe based hydrogen alloy alloys were designed to improve the high-temperature cycling stability of the electrodes. What is more, there is still no relevant report about the effect of Si on high-temperature performance of hydrogen storage alloys, a revisiting of the subject of low-cost with Si substituting is necessary. In this study, in order to research the effect of Si on hightemperature performance and further improve the hightemperature performance of the low-cost LaeNieFe based

hydrogen storage alloys, La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17 Fe0.5
xSix (x ¼ 0, 0.05, 0.075 and 0.1) alloys were prepared. And their electrochemical properties at 20  C (room temperature) and 60  C (higher temperature) as well as its mechanism have been studied systematically.

Experimental La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix (x ¼ 0, 0.05, 0.075 and 0.1) hydrogen storage alloys were selected as the active material. The purity of the rare earth metals La and Ce (CHN. KEBAIRUI) was over 99.9 wt%, and that of all the other raw materials (Ni, Co, Mn, Al, Fe and Si, CHN. JIFENG) in this experiment was over 99.5 wt%. The alloys were prepared by arc-melting under argon atmosphere and re-melted for four times to keep compositional homogeneity. Thenceforth, crush part of the as-cast ingot into powders mechanically and screened into different sizes. Alloy powders with a dimension of 200-mesh (<75 mm) were used for electrochemical tests, and those with a size of 400-mesh (<38 mm) were used for X-ray diffractometer (XRD) analysis (DX-2000 equipment with Cu Ka radiation at 35 kV and 25 mA). The microstructure of the alloys was examined using a scanning electron microscope (SEM, JSM-7500F). The micro-area compositions of the alloys were analyzed by an energy disperse X-ray analyzer (EDS-51XMX0019). Thermodynamic properties of the as-cast alloys were measured by an automated Sievert's apparatus (PCTPro-2000 from Hy-Energy LLC). In the PCT analysis, each sample was first activated by several hydrogen absorption/desorption cycles (absorption under 40 bar and desorption under primary vacuum). Then PCT curves were measured at hydrogen pressure of 0.01e40 bar in the temperature range of 40e80  C. Details of both electrode and cell preparations methods had been reported previously [19]. The discharge capacities of the electrodes were measured by galvanostatic method. Each electrode was fully activated at a 0.2 C (60 mA g
1) charge/ discharge current density. And the discharge cut-off potential of the electrode is
0.6 V (vs. Hg/HgO), except for
0.55 V at the discharge rate above 5 C (1500 mA g
1). To measure the peak power density of the electrodes, the electrodes were fully charged at 0.2 C firstly, and then discharged at 0.2 C to 80% state of charge (SOC), subsequently discharged at various current densities for 60 s after 1 h standing. The test at higher temperature was conducted after laying the testing system at constant temperature for 3 h using a digital thermostat water bath pot. Electrochemical impedance spectra (EIS) measurements of the electrodes were measured using a sine perturbation signal of 5 mV in the frequency range of 10 KHz e 5 mHz. Exchange current density (I0) was measured by linear polarization (LP) curves, scanning electrode potential at the rate of 0.1 mV s
1 from
5 to 5 mV. Prior to all of the electrochemical kinetics analysis (performed by Parstat-2273 electrochemical potentiostat), the electrodes were discharged to 50% SOC and then rest for 2 h. Hydrogen diffusion coefficient (D) was estimated by the constant potential step (CPS) method, and the test

Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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electrodes were discharged at a constant potential of E ¼
0.6 V (vs. Hg/HgO) for 3600 s after 100% SOC.

Results and discussions Microstructure Fig. 1(a) shows XRD patterns of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix (x ¼ 0, 0.05, 0.075 and 0.1) hydrogen storage alloys. It can be confirmed from the plots that the specimens (x ¼ 0e0.075) present a single hexagonal LaNi5 phase with a hexagonal CaCu5 crystal structure. However, some La2Ni7 phase starts to appear when x ¼ 0.1. Here, Rietveld refinement of XRD patterns for La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.4Si0.1 was conducted to measure the abundance and lattice parameters of the phase, as shown in Fig. 1(b). The abundance of the La2Ni7 phase in La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.4Si0.1 alloy is about 5.6%. The calculated lattice parameters of alloys are listed in Table 1. It can be seen that the unit-cell lengths of a-axis and c-axis have a slight increase by the addition of Si element, and the c/a aspect ratio of each alloy is almost similar. This may be related to the metallic bond length and preference of the site occupied, which Si atoms occupies only 3g sites in AB5-type alloys whereas Fe atoms occupies 3g sites and some 2c sites [33], although the atomic radius of Si (1.46  A) is smaller than that of Fe (1.72  A). The corresponding increment in unit cell volume implies that the Si-containing alloys may have stronger metalehydrogen bond strength [11,19]. In addition, as shown in

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Table 1, an increase of Si content leads to an decrease of fullwidth at half-maximum (FWHM), due to a increase of crystallite size (see in Table 1) as well as a drop of lattice strain and defects [35]. And the crystallographic difference of x ¼ 0.1 alloy is due to the change of the micro-composition by the appearance of the second phase. SEM backscattered electron images (BEI) of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5-xSix (x ¼ 0.075, 0.1) are presented in Fig. 1(c and d). The compositions of the four studied alloys in several areas identified numerically in the micrographs were studied by EDS and the results are listed in Table 2. When x  0.075, the BEI analysis results for the alloys is consistent with that of XRD, namely no second phase can be observed. It can be seen that all elements are successfully doped into the matrix AB5 phase with a composition close to the design value. But when x ¼ 0.1, the second phase (A2B7 phase identified by EDS with white image in Fig. 1(d)) start to appear. Zhang et al. [36] and Dong et al. [37] have reported that the existence of Si would induce the appearing of the second phase (A2B7). It can be seen from Table 2 that all of the Si and Fe elements are doped into the matrix, and no Si presents in the second phase when x ¼ 0.1. In addition, the slight composition deviation of the main AB5 phase is observed due to the appearance of a small amount of second A2B7 phase.

Hydrogen desorption performance Generally, the metal hydride used in Ni/MH battery worked at higher temperature need a relatively higher thermodynamic stability to prevent the spontaneous overflow of hydrogen from

Fig. 1 e XRD patterns of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5¡xSix (a), rietveld profile refinement of XRD patterns of x ¼ 0.1 alloy (Rf ¼ 9.19%) (b) and BEI analysis of (c) x ¼ 0.075 alloy and (d) x ¼ 0.1 alloy. Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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Table 1 e Lattice parameters of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5¡xSix hydrogen storage alloys. Samples

x x x x

¼ ¼ ¼ ¼

0 0.05 0.075 0.1

Phase

LaNi5 LaNi5 LaNi5 LaNi5 La2Ni7

Lattice constants a/ A

c/ A

5.030 (5) 5.030 (6) 5.031 (7) 5.035 (6) 5.086 (3)

4.045 (9) 4.046 (8) 4.050 (8) 4.048 (8) 24.709 (8)

Cell volume/ A3

FWHM (111)

Crystallite size/ A

88.67 88.69 88.82 88.91 553.6

0.257 0.252 0.244 0.238 e

347 356 370 381 e

c/a 0.804 0.804 0.805 0.804 e

(3) (4) (1) (0)

Table 2 e EDS analysis results for La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5¡xSix hydrogen storage alloys. Samples x x x x

¼ ¼ ¼ ¼

0 0.05 0.075 0.1

Phase

La

Ce

Ni

Co

Mn

Al

Fe

Si

Matrix Matrix Matrix Matrix Second phase

0.781 0.783 0.781 0.772 1.839

0.219 0.217 0.219 0.228 0.161

3.726 3.732 3.731 3.722 5.504

0.297 0.302 0.298 0.289 0.795

0.308 0.305 0.297 0.301 0.402

0.172 0.164 0.165 0.168 0.299

0.497 0.448 0.432 0.417 e

e 0.049 0.077 0.104 e

the negative electrode. Fig. 2 shows hydrogen desorption pressure-composition-temperature (PCT) isotherms obtained with La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix (x ¼ 0e0.1) alloys at 40, 60 and 80  C. The information obtained from the PCT study is summarized in Table 3. With increasing Si content, it can be seen from Table 3 that the hydrogen desorption capacity (ude) increases at any particular temperature, corresponding to the relatively wider hydrogen desorption plateau in Fig. 2. The increase is due to the larger unit cell resulting from Si-addition, increasing the number of sites available for hydrogen storage. In addition, the hydrogen desorption plateau pressure (Pde)

truly increases with the rising of temperature, since the stability of the metal hydride drops accordingly at higher temperatures. Furthermore, at each temperature, the Pde value decreases with the increase of Si content, especially at 80  C, drops down from 1.70 bar (x ¼ 0) to 1.02 bar (x ¼ 0.1). The van't Hoff plot conducted to obtain thermodynamic parameters of the hydrogen storage alloy, fitted by the van't Hoff equation:

ln

Peq DH DS
¼ RT R Pq

(1)

Fig. 2 e Thermodynamic performance of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5¡xSix alloys:(a), (b), (c) desorption PCT curves at 40, 60, 80  C respectively; (d) Van't Hoff plot. Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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Table 3 e Thermodynamic parameters of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.50¡xSix hydrogen storage alloys. Pdea/bar

Samples

Desorption capacity/wt%

40  C

60  C

80  C

40  C

60  C

80  C

0.51 0.39 0.28 0.21

0.93 0.78 0.57 0.46

1.70 1.32 1.16 1.02

1.090 1.134 1.144 1.155

1.063 1.100 1.127 1.143

1.016 1.036 1.052 1.068

x x x x

¼ ¼ ¼ ¼

a

The hydrogen desorption plateau pressure at 0.6 wt% H-storage.

0 0.05 0.075 0.1

where R is the ideal gas constant, T is the absolute temperature, DH represents the changes in enthalpy and DS is the change in entropy, is present in Fig. 2(d). The values of DH and DS were evaluated from the slope and the intercept of van't Hoff plot, respectively. It can be seen from Table 3 that
DH value increases from 27.55 kJ mol
1 (x ¼ 0) to 36.19 kJ mol
1 (x ¼ 0.1). The drop of Pde shown in Table 3 corresponds to the increase of the
DH value. It means the stability of the hydrides increases as Si content increases, i.e., MeH bond strengthens. Generally, the hydrogen desorption pressure is directly related to the unit cell volume of hydrogen storage alloys. The alloy which has a large unit cell volume will possess a relatively lower dissociation plateau pressure for hydrogen desorption, since its strong MeH bond. Furthermore,
DS measured from these alloys increases from 82.34 J mol
1 K
1 (x ¼ 0) to 102.50 J mol
1 K
1 (x ¼ 0.1), it implies a more ordered b hydride phase produces by Si substituting [38]. The Gibbs free energy (DG) listed in Table 3 was calculated according to DG ¼ DHeTDS. It can be found out that all of the DG value at 60  C is positive, but turns into a negative when the temperature at 80  C. This means the hydrogen will overflow from metal hydride spontaneously after the temperature higher 80  C. In addition, the DG value increases with the increase of Si content, it indicates that the Si-containing alloy is more suitable for working at higher temperatures.

Electrochemical performance Charge and discharge behavior As shown in Table 4, the activation times at 20  C are 4, 4, 5, and 4 for the specimens x ¼ 0, 0.05, 0.075, and 0.1, respectively. Obviously, it is more difficult to fully activate the x ¼ 0.075 alloy than the others. After substituted for Fe by Si, the maximum discharge capacities of 305.1, 307.7, 294.4 and 315.8 mAh g
1 are obtained for the x ¼ 0, 0.05, 0.075 and 0.1 alloys at 20  C with a discharge current rate of 0.2 C, respectively. And the discharge capacity of 1 C decreases from 268.4 mAh g
1 (x ¼ 0) to 248.6 mAh g
1 (x ¼ 0.075) and then increases to 285.3 mAh g
1 (x ¼ 0.1). Although, the higher discharge capacities of 0.2 C for x ¼ 0.05 and 0.1 alloys are consistent with the improvement of ude as found from the PCT analysis (Table 3), the discharge capacities for the x ¼ 0.075 alloy is contrary, particularly at 1 C. This can be attributed to the addition of Si adjusts the charge/discharge plateau potential (Emid,c/Emid,d). As shown in Fig. 3(a and b), the charge/discharge polarization (Emid,ceEmid,d) increases with the increase of Si from x ¼ 0 to x ¼ 0.075, due to the formation of oxide film by Si [33,34], particularly when x ¼ 0.075. In


DH kJ mol
1

27.55 31.44 32.67 36.19


DS J mol
1 K
1

82.34 92.48 93.70 102.50

DG/kJ mol
1 40  C

60  C

80  C

1.77 2.48 3.33 4.09

0.12 0.63 1.45 2.04


1.53
1.22
0.42
0.01

addition, when x ¼ 0.1, the existence of A2B7 phase has a positive effect on polarization drop [20,39]. Compared with 20  C, the activation property of the alloys improves at 60  C. This can be attributed to the greater electrocatalytic activity of hydride-forming electrodes and the extreme dissolution at higher temperature. The x ¼ 0.075 alloy needs three cycles to reach its maximum discharge capacity, which is still higher than the others. The discharge capacity values of the electrodes at 60  C with 0.2 C and 1 C are also listed in Table 4. Obviously, the discharge capacity is depressed at 60  C both with 0.2 C and 1 C except when x ¼ 0.075, the discharge capacity increases from 248.6 mAh g
1 (20  C) to 253.1 mAh g
1 (60  C) at 1 C. On the one hand, as the increase of temperature, hydrogen desorption capacity (ude) decreases gradually and equilibrium plateau pressure (Peq) increases (Table 3), which may result in the hydrogen releasing from MH during discharge and further decreasing of hydrogen participating electrochemical discharge reaction. On the other hand, it related to the decline of charge acceptance ability. The Emid,c and EHER is deemed to the potential of hydride-formation reaction and hydrogen evolution reaction (HER) respectively, and the difference between EHER and Emid,c (EHER
Emid,c) can be expected to characterize the charge acceptance ability [11,27]. As shown in Fig. 3(c), the charge potentials decline and the discharge potentials ascend at 60  C, indicating that the charge/discharge polarization gets small at 60  C. Thereby, EHER drops with the temperature increases, making the value of EHER
Emid,c is much smaller, and consequently results in lower charge acceptance ability (Compared Fig. 3(bed)). Furthermore, the deterioration in the conductivity of the electrolyte affecting the charge transfer mechanism at the interface, and the increase in IR losses at higher temperatures cannot be neglected [40]. In addition, as shown in Table 4, the discharge capacity decreases from 267.2 mAh g
1 (x ¼ 0) to 256.7 mAh g
1 (x ¼ 0.05), and then increases to 259.5 mAh g
1 of x ¼ 0.075 and 279.5 mAh g
1 of x ¼ 0.1. The decrease of the capacity is due to the rise of charge/discharge polarization by Si substituting. However, the addition of Si makes the Peq drop and the ude rise, increasing the stability of the metal hydride. In addition, Si increases the value of EHER
Emid,c, which means the Sicontaining alloys have better charge acceptance ability. Furthermore, severe dissolution of A2B7 phase at higher temperature by galvanic corrosion has a good effect on the declining of charge/discharge polarization [11]. The capacity declining rate at elevated temperature can be evaluated by high-temperature dischargeability (HTD). The calculation method of HTD60 has been reported by Chao et al.

Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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Table 4 e Electrochemical properties of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5-xSix hydrogen storage alloys. Cmax,0.2Cb/mAh g
1

Naa

Samples

20  C 60  C

Cmax,1C/mAh g
1

20  C

60  C

20  C

60  C

305.1 307.7 294.4 315.8

267.2 256.7 259.1 279.5

268.4 267.2 248.6 285.3

252.9 264.6 253.1 275.9

HTD60c/% HTRd/%

S150e/%

20  C 60  C 20  C 60  C

x x x x

¼ ¼ ¼ ¼

a

The cycle numbers needed to activate the electrode. The maximum discharge capacity at a discharge current density of 60 mA g
1, where 1C ¼ 300 mA g
1. The high-temperature dischargeability at 60  C. The high-temperature recoverability obtained from the equation of previous research [18]. The cycling capacity retention rate at 150th cycle. The high rate dischargeability at a discharge current density of 900 mA g
1. The peak power density.

b c d e f g

0 0.05 0.075 0.1

4 4 5 4

2 2 3 2

HRD900f/%

87.58 83.42 88.01 88.51

88.42 90.91 93.86 91.98

89.6 91.5 94.6 84.8

43.7 57.7 71.8 35.7

39.54 37.58 34.79 40.29

69.64 73.70 79.61 71.92

Ppeakg/W kg
1 20  C

60  C

2404.2 2243.5 2033.3 2627.1

3183.6 3917.3 4261.1 3431.3

Fig. 3 e Charge/discharge profile at 20  C (a) and 60  C (c), middle charging/discharging potential and HER potential at 20  C (b) and 60  C (d), discharge capacities at different temperatures (e), and high-temperature recoverability at 20  C (f). Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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[19]. As shown in Table 4, the HTD60 value of the x ¼ 0, 0.05, 0.075 and 0.1 alloys are 87.58%, 83.42%, 88.01% and 88.51%, respectively. This means the proper addition of Si is beneficial to improving the temperature insensitivity of the electrodes. Fig. 3(e) illustrates the discharge capacity of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix alloy electrodes at 20e80  C. It can be seen that the electrode of x ¼ 0.05 shows the fastest capacity falling rate, but further adding Si is in favor of reducing the capacity falling rate, particularly when the temperature above 60  C. After the temperature above 60  C, the capacity drops rapidly, especially for those alloys with high hydrogen desorption pressure and low corrosion resistance, owing to the deteriorated stability of the counterpart metal hydride and severe corrosion and dissolution. Fig. 3(f) presents the high-temperature recoverability (HTR) of the alloy electrodes. According to our previous work [11,19], HTR after high-temperature cycle has a direct influence on the service life of the battery, since the electrode alloys have suffered irreversible damage from high-temperature cycles. Similarly, it can be seen in this research, the discharge capacity at 20  C of the electrode has some decline (87e90%) after 10 cycles at 60  C at first, and then recovers to a relatively stable value (<94%) in the subsequent cycles at 20  C. Furthermore, as shown in Table 4, the addition of Si contribute significantly to the improvement of HTR, the HTR value can increase from 88.42% of x ¼ 0 alloy to 93.86% of x ¼ 0.075 alloy. These imply that the proper substitution of Si is in favor of improving the anti-corrosion property of alloys at high temperature, and improved the HTR property consequently. However, the appearance of the second phase is detrimental to HTR.

High-temperature cycling stability Fig. 4 illustrates the cycle life curves of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix alloy electrodes at 20  C and 60  C, and the cycling stability (S150) calculated from the equation of previous research [19] is listed in Table 4. It is seen that, the increase of Si content is in favor of enhancing cycling stability both at 20  C and 60  C, except for when x ¼ 0.1. After 150 cycles at 20  C, the S150 increases from 89.6% for x ¼ 0 alloy to 94.6% for x ¼ 0.075 alloy, then decreases to 84.8% for x ¼ 0.1 alloy. More apparently, the S150 increases from 43.7% for x ¼ 0 alloy to 71.8% for x ¼ 0.075 alloy, then decreases to 35.7% for x ¼ 0.1 alloy at the elevated temperature (60  C). It is reported that Si-containing alloy shown great corrosion resistance, since Si forms fine oxide film on alloys surface easily [31e34].

7

In addition, the relatively lower discharge capacity of x ¼ 0.075 alloy (253.1 mAh g
1) is in favor of reducing the lattice expansion during charge/discharge cycles. Meanwhile, the existence of A2B7 phase would increase the corrosion of the alloy by galvanic corrosion, particularly at higher temperatures, and consequently, the worst cycling stability is obtained when x ¼ 0.1. It is worth noticing that the capacity of the x ¼ 0.1 electrode alloy drops rapidly at the former 18 cycles, but keeps a relatively slower attenuation rate later. The same thing has happened in our previous research, the rapid drop is believed to the heavy severe dissolution of A2B7 and the slower rate is deemed to the protection of oxide film [12]. As is well known, the different capacity degradation rates of the investigated alloys are directly related to the alloy compositions and working conditions, which affect the oxidation and corrosion of the surface layer exposed to the electrolyte. Generally, corrosion potential (Ecorr) presented corrosion tendency and corrosion current density (Icorr) denoted corrosion rate are the effective parameters to evaluate the corrosion behavior of the electrode alloy. Chao et al. [19] has reported the effects of Fe on the changes of Ecorr and Icorr in LaeNieFe based hydrogen storage alloys, and indicated that the substitution of Fe is in favor of reducing the corrosion tendency and the corrosion rate of alloy electrodes at 60  C. Here, the technique of potentiodynamic polarization was conducted, and the software CorrView was employed to analyze the Tafel curves. As shown in Fig. 5(aec), it is no doubt that the Ecorr moves to the negative position and Icorr increases with the temperature increases, due to the severe corrosion at elevated temperature. In addition, by increasing the Si content, the Ecorr shifts towards to positive direction firstly, then shifts towards to negative direction when x ¼ 0.1, particularly at higher temperature (60  C). This indicates that the corrosion tendency of alloys with an appropriate amount of Si is less obvious than that of Si-free alloy. Furthermore, it can be noticed from Fig. 5(c) that the Icorr decreases firstly, and then rise up when x ¼ 0.1, i.e. the corrosion rate of Si-containing (x ¼ 0.05 and 0.075) alloy is smaller than that of Si-free alloy. As a consequence, better high-temperature recoverability and cycling stability are obtained for the x ¼ 0.075 alloy electrode. The degradation of corrosion resistance of x ¼ 0.1 alloy is attributed to the appearance of A2B7, which increases the corrosion tendency and the corrosion rate of the alloy electrode.

Fig. 4 e Cycle life curves of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5¡xSix alloy electrodes at: (a) 20  C and (b) 60  C. Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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presented in Table 4 and Fig. 6(a and b). As shown in Fig. 6(a), the HRD of the alloy electrodes at room temperature (20  C) decays with the increase of Si content from x ¼ 0 to x ¼ 0.075 firstly, but the highest HRD value is obtained when x ¼ 0.1, corresponding to the changes of Emid,ceEmid,d values in Fig. 3(a). At elevated temperature (60  C), the HRD value of each electrode improves distinctly, particularly for those with high charge/discharge polarization. More interestingly, the HRD property of electrode is different to that at room temperature. Instead, the HRD improves with the increase of Si content from x ¼ 0 to x ¼ 0.075 firstly, then slightly decreases when x ¼ 0.1. The HRD900 value increases from 69.64% (x ¼ 0) to 79.61% (x ¼ 0.075) and then drops to 71.92% (x ¼ 0.1). Meanwhile, the elevated HRD at higher temperature also means the excellent power delivery, which will be discussed below. For the practice application of Ni/MH power battery, the peak power seems to be more important than the high-rate dischargeability. It is because the peak power directly relates to the power demand of the equipment [14]. Fig. 6(ced) show the V ~ i correlation and output power of the alloy electrodes at 20  C and 60  C. Here, the specific power (P) is calculated from the following expression: P ¼ Un;60  In

Fig. 5 e Evolution of the Tafel curves as x value: (a) at 20  C; (b) at 60  C; (c) corrosion potential and corrosion current density.

(3)

where In denotes the discharge current density of n mA g
1, and Un,60 is the average output voltage at the corresponding current density for 60 s. The peak power (Ppeak) values of the electrodes are listed in Table 4. It can be noticed from Fig. 6(c) that the output voltage of each electrode almost decreases linearly with the increase of output current density, thus a peak power is observed at high output current density. Corresponding to HRD at 20  C, when current density increases from 60 to 3900 mAh g
1, the output voltage decreases with the Si content increasing from x ¼ 0 to x ¼ 0.075 firstly, but the highest output voltage is obtained when x ¼ 0.1. As a result, the Ppeak decreases from 2404.2 W kg
1 (x ¼ 0) to 2033.3 W kg
1 (x ¼ 0.075), then increases to 2627.1 W kg
1 (x ¼ 0.1) at 20  C. In addition, at 60  C, the electrodes get higher output voltage than that at 20  C, and consequently higher peak current (Ipeak) are observed in Fig. 6(d), Ipeak increases from 12 C (3600 mA g
1) at 20  C to 15 C (4500 mA g
1) at 60  C. As a result, the Ppeak increases from 2627.1 W kg
1 at 20  C to 3431.3 W kg
1 at 60  C. This mainly associates with the decline of the electrode polarization and the improvement of electrochemical kinetics at 60  C. Furthermore, it is notable that the partial Si substituting is in favor of improving the power output performance of the electrodes at 60  C. The Ppeak increases from 3183.6 W kg
1 (x ¼ 0) to 4261.1 W kg
1 (x ¼ 0.075), then decreases to 3431.3 W kg
1 (x ¼ 0.1) at 60  C, which agrees well with the HRD results.

High-rate performance The high-rate dischargeability (HRD), which represents the discharge capability of the batteries at high discharge current densities, can be calculated by the following formula: Cn  100 HRDn ð%Þ ¼ Cn þ C60

(2)

where Cn is the discharge capacity at the discharge current density of n mA g
1, C60 is the residue discharge capacity at a discharge current density of 60 mA g
1, and results are

Electrochemical kinetics Generally, temperature and the surface condition of the alloy directly affect the electrochemical kinetics. During electrochemical charge/discharge process, the electrochemical kinetics of the electrodes is mainly determined by the chargetransfer process which happens at the interface between the electrolyte and alloy powder and the diffusion process of the

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

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Fig. 6 e High-rate dischargeability at 20  C (a) and at 60  C (b) and output power of the alloy electrodes at 20  C (c) and at 60  C (d).

hydrogen absorbed into the bulk of alloy particles. The former can be characterized by charge-transfer resistance (Rct) or the exchange current density (I0), while the latter can be determined by hydrogen diffusion coefficient (D). Here, the Rct of the hydrogen storage alloy electrode can be estimated from EIS. According to previous work [12,41], the high frequency intercept at the horizontal axis is designated as bulk resistance (Rb) of the cell, including the resistance of the electrolyte, separator and electrode, as well as the contact resistance; the small semicircle in the high-frequency region is attributed to the resistance (Rol) and capacitance (Qol), which is related to the behavior of the surface oxide layers of particles; and the large semicircle in the middle frequency region is related to the charge-transfer resistance (Rct) and double-layer capacitance (Qdl) on the alloy particles. And W represents the Warburg diffusion impedance of the straight line in the low frequency region. The fitting of the impedance model to the experimental data is performed using software ZSimpWin 3.21 (Fig. 7(a and b)), irrespective of the Warburg resistance, and the results are listed in Table 5. It can be seen from Fig. 7 the Rct and Rol values decrease with the increase of temperature. The decrease of Rol is attributed to the improvement of electric conductivity of oxide layer at higher temperature. When x ¼ 0.075, the Rct decreases from 1.666 U at 20  C to 0.442 U at 60  C, which means a faster charge transfer rate at higher temperature, leading to a great activation and highrate performance at 60  C. As shown in Table 5, the Rol increases with the increase of Si content at 20  C and 60  C, indicating the acceleration of the

protective passive film formation by partially Si substituting as conjecture before. In addition, the Rct is enlarged gradually with the increase of Si content (x ¼ 0e0.075) at 20  C, but become small when that at 60  C, coinciding the result of HRD and Ppeak. Moreover, the value of Rct decreases obviously for the Si-containing electrode alloys after the temperature over 40  C (Fig. 7(c)), indicating the Si-containing electrode alloys exhibit a great electrochemical kinetics at elevated temperature, and consequently, excellent high-temperature dischargeability, high-temperature HRD and high-temperature Ppeak are obtained. This may be attributed to the severe dissolution of oxide formed by Si and Al, due to their large solubility in 6 M hot alkaline [12,29,33], improving the activity of electrode. However, excessive Si content (x ¼ 0.1) will result in more accumulation of oxide, and the galvanic corrosion caused by the appearance of the second phase cannot be ignored. Exchange current density (I0) which is another method to study the charge-transfer process is presented in Table 5 and Fig. 7(d). It can be seen that I0 increases with the increase of temperature, and decreases with Si content (x ¼ 0e0.075) increases at 20  C, but increases with Si content (x ¼ 0e0.075) increasing at 60  C, in accordance with the change tendency of Rct, HRD and Ppeak at different temperatures. Therefore, high temperature is in favor of improving the electrochemical kinetics of the electrode alloys, and improving the activation and high-rate performance accordingly. The hydrogen diffusion coefficient D in the bulk electrodes is estimated by CPS technique, and the result is listed in Table

Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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Fig. 7 e Nyquist plots and exchange current density of the alloy electrodes after activated: (a) EIS at 20  C; (b) EIS at 60  C; (c) Rct and (d) exchange current density. 5. According to our previous study [11,19,20], the chargetransfer reaction on alloy surface is the primary control step of the electrode reaction process, which is also contributing to the La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix. It can be seen that the order of hydrogen diffusion coefficient is not consistent with that of HRD and Ppeak, whereas the change of Rct and I0 are well matched.

Conclusions In summary, the following conclusions can be drawn: (1) The La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5
xSix (x ¼ 0, 0.05, 0.075 and 0.1) hydrogen storage alloys consist of single hexagonal CaCu5-type LaNi5 phase until the appearance of A2B7 phase when x ¼ 0.1. The substitution of Fe by Si increases the hydrogen desorption capacity, decreases the hydrogen desorption plateau

pressure and improves the stability of the metal hydrides, which mean the Si-containing alloy is more suitable for working at higher temperature. (2) When the Si content increases from x ¼ 0 to x ¼ 0.075, the high-rate dischargeability (HRD) and high-power delivery (Ppeak) deteriorate at room temperature, but increase gradually at higher temperature, The HRD900 value increases from 69.64% to 79.61% and Ppeak increases from 3183.6 W kg
1 to 4261.1 W kg
1 effectively at 60  C. Electrochemical kinetics analysis reveals that the chargetransfer process is suppressed gradually by Si substituting (x ¼ 0e0.075) at 20  C, but gets promoted due to the improved surface catalytic ability when that at 60  C. (3) The appropriate addition of Si (x ¼ 0e0.075) improves the anti-corrosion capability of the alloy, as a result, the temperature insensitivity, high-temperature dischargeability and high-temperature recoverability increase accordingly, particularly the cycling stability (S150) increases from 43.7% to 71.8% at 60  C.

Table 5 e Kinetics parameters of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5-xSix hydrogen storage alloys. Samples

x x x x

¼ ¼ ¼ ¼

0 0.05 0.075 0.1

Rol (mU)

I0 (mA g
1)

Rct (U)

D/10
12 cm2 s
1

20  C

60  C

20  C

60  C

20  C

60  C

20  C

60  C

153.4 163.1 172.4 184.3

108.2 121.4 141.3 152.4

1.314 1.549 1.666 1.146

0.573 0.507 0.442 0.524

78.95 65.80 46.31 81.04

144.45 164.71 194.76 155.02

2.07 1.32 1.99 1.86

1.82 1.79 1.85 1.76

Please cite this article in press as: Zhou W, et al., The high-temperature performance of low-cost LaeNieFe based hydrogen storage alloys with Si substituting, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.06.206

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(4) Considering overall electrochemical properties of the alloy electrodes at various temperatures, an optimum alloy electrode was got when x ¼ 0.075, the deterioration of x ¼ 0.1 alloy on high-temperature performance is attributed to the appearance of the second A2B7 phase.

Acknowledgment This work is financially supported by the National High Technology Research and Development Program of China (863Program, No. 2011AA03A408) and the Ni/MH Batteries and Materials R & D Coordination Project (15H1110).

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