- Email: [email protected]
Microstructures and electrochemical characteristics of LaNi3.70Co0.2−xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x hydrogen storage alloys
Journal of Alloys and Compounds 641 (2015) 148–154
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructures and electrochemical characteristics of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x hydrogen storage alloys Junling Sun a, Yanping Fan a, Baozhong Liu a,⇑, Liqiang Ji b, Yongguang Wang b, Mingjie Ma a,⇑ a b
School of Materials Science & Engineering, Henan Polytechnic University, Jiaozuo 454000, China Inner Mongolia Rare Earth Ovonic Metal Hydride Co. Ltd., Baotou 014030, China
a r t i c l e
i n f o
Article history: Received 27 February 2015 Received in revised form 6 April 2015 Accepted 7 April 2015 Available online 11 April 2015 Keywords: Hydrogen storage alloy Electrochemical properties Kinetics Microstructure MoFe alloy
a b s t r a c t Electrochemical hydrogen storage properties of LaNi3.70Co0.2Mn0.30Al0.15Cu0.65 alloy are improved by substituting Co with Mo0.46Fe0.54, rather than pure Mo and Fe. Microstructures and electrochemical properties of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) hydrogen storage alloys are investigated. X-ray diffraction and backscattered electron results indicate that the pristine alloy is LaNi5 phase with a hexagonal CaCu5-type structure, while the alloys containing Mo0.46Fe0.54 consist of LaNi5 matrix phase and Mo secondary phase. The relative abundance of Mo phase increases with the increase in x value. The lattice parameters a, c, c/a and cell volume V of LaNi5 phase increase with increasing x value. As x increases from 0 to 0.20, maximum discharge capacity of the alloy electrodes monotonically decreases from 335.4 (x = 0) to 324.2 mA h/g (x = 0.20). The high-rate dischargeability of the alloy electrodes at the discharge current density of 1200 mA/g first increases from 59.8% (x = 0) to 69.6% (x = 0.15), and then decreases to 64.0% (x = 0.20). The cycling capacity retention rate at the 100th cycle decreases from 80.4% (x = 0) to 61.9% (x = 0.20), which should be ascribed to the deterioration of the corrosion resistance of alloy electrode with increasing x value. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is expected to be a promising new energy source to replace the conventional fossil for solving the shortage of fossil energy resources and the global warming in the near future. Hydrogen storage is one of many important processes in order for hydrogen to become a viable solution to the energy crisis and the environment problem [1]. Among different ways to store hydrogen, absorption in solid to form hydride is very attractive since it allows safe storage at pressure and temperature close to ambient conditions [2,3]. Nickel/metal hydride (Ni/MH) secondary batteries have been adopted in various portable electronic devices because of their good electrochemical performance and environment compatibility [4,5]. Almost all of the commercial Ni/MH batteries employ AB5-type alloys as negative electrode materials due to their good overall electrode properties [6]. However, high cost of AB5-type alloys slows down the rhythms of wide applications. Substantial efforts have been dedicated to reduce their costs by eliminating Nd and Pr [7] or substituting Co with low-cost elements, such as, Mn, Fe, Cu, and Si [8,9]. Co element contributes to the high-rate dischargeability due to excellent catalytic and ⇑ Corresponding authors. Tel./fax: +86 335 3989859. E-mail addresses: [email protected] (B. Liu), [email protected] (M. Ma). http://dx.doi.org/10.1016/j.jallcom.2015.04.058 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
conductive performance [10], as well as to cycling stability due to the anti-pulverization [11,12]. However, it is very difficult to keep excellent electrochemical properties of the alloys after the substitution of Co by single foreign elements. In comparison, the two elements, such as, MoAl [13,14], and WAl [15], were used together to substitute Co, and the less loss of high-rate dischargeability and cycling stability of the alloys can be obtained. Unfortunately, the substitution with pure element is detriment to the decrease in the alloy cost. In our previous investigations [16,17], the commercial B0.57Fe0.43 and V0.81Fe0.19 alloys, rather than pure B, V or Fe, was used to substitute the Co, which obviously decreases the cost of the alloys. Thus, it is feasible that the commercial alloys, rather than pure metallic elements, are used to substitute Co for improving the performance-price ration of the alloys. Yeh et al. [18] stressed that the substitution of Mo for Co increased both the hydrogen absorption capacity and equilibrium plateau pressure of rare-earth based AB5-type hydrogen storage alloy at 313 K. Ye and Zhang [19] have reported that the addition of Mo improves the high-rate dischargeability due to the increase in the exchange-current density without the degradation of the discharge capacity of alloy at low current density. Obviously, the addition of Mo element is effective to enhance discharge capacity, activation property and high-rate dischargeability [20]. Moreover, it is reported that Fe introduction in AB5 alloy can remarkably
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
as electrolyte. Each electrode was charged for 7 h at 60 mA/g and discharged to 0.6 V versus Hg/HgO at 60 mA/g at 298 K. After every charging/discharging, the rest time was 10 min. In evaluating the high-rate dischargeability, discharge capacity of the alloy electrodes at different discharge current density was measured. The high-rate dischargeability (HRD, %) was defined as
improve cycling stability due to the improvement in the anti-pulverization of the alloy electrode [21,22]. More importantly, the commercial Mo0.46Fe0.54 alloy is cheaper than pure Mo, and the melt point of Mo0.46Fe0.54 alloy is lower compared with pure Mo, which contributes to the homogeneity of the alloys. Herein, on the basis of the merits of Mo0.46Fe0.54 and the belief that the Fe and Mo additions may result in some noticeable modifications of electrochemical hydrogen storage performance, microstructures and electrochemical hydrogen storage characteristics of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) alloys have been investigated.
HRD ¼ C d =C max 100%
LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) alloys were synthesized by induction melting of the metal elements (La, Ni, Co, Mn, Al, Cu: 99.9% in purity and commercial Mo–Fe alloy contained 46 at.% Mo and the other were Fe and trace impurities) in argon atmosphere. The ingots were annealed by using vacuum heat-treating furnace. Before heating, the sample cell was vacuumized until pressure is less than 10 Pa and then the argon was charged into the sample cell until the pressure was 0.8 MPa. The samples were heated from room temperature to 1223 K at the rate of 10 K/min and then kept in 1223 K for 2 h followed by natural cooling in the furnace. Sample powders of 300 mesh size were used for X-ray diffraction (XRD) test, which was carried out on a D/Max-2500/PCX with Cu Ka radiation in the range from 20° to 80° with 0.02°/min. Backscattered electron (BSE) images were obtained by using HITACHI-4800 scanning electron microscope with an energy dispersive Xray spectrometer (EDS). The hydrogen storage properties of the alloys were studied by using pressure-composition-temperature (PCT) (made by Suzuki Shokan in Japan) at 314 K. Before the PCT measurement, the alloys were activated by 5 hydriding/dehydriding cycles. The delay time was 180 s and maximum pressure was 2 MPa. The alloy powders of measuring electrodes were obtained by grinding the inner part of alloy ingots in the Ar atmosphere. All measuring electrodes for test were prepared by cold pressing the mixture of 0.15 g alloy powders of 200–400 mesh and 0.75 g nickel carbonyl powders into a pellet of 10 mm in diameter under 15 MPa. Electrochemical measurements were performed at 298 K in a standard tri-electrode system, consisting of a working electrode (metal hydride), a counter electrode (Ni(OH)2/NiOOH), and a reference electrode (Hg/HgO) with 6 mol/L KOH solution
3. Results and discussions 3.1. Crystal structure Rietveld refinement is an effective method to analyze the XRD data for obtaining the lattice parameters and the phase abundance of the alloys [23–25]. Fig. 1(a) presents XRD patterns of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys. Fig. 1(b) shows the XRD pattern and Rietveld analysis pattern of LaNi3.70Co0.05Mn0.30Al0.15Cu0.65(Mo0.46Fe0.54)0.15 alloy. It can be seen that the pristine alloy is LaNi5 phase with CaCu5 structure, and the alloys containing Mo0.46Fe0.54 are composed of LaNi5 matrix phase and a spot of Mo phase. The lattice parameter, cell volume and phase abundance are listed in Table 1. The abundance of Mo phase increases with increasing x value. It is noted that the a, c, c/a and V of the alloys increase monotonously with increasing x value, which should be ascribed to larger atomic radius of Mo (2.01 Å) and Fe (1.72 Å) than that of Co (1.67 Å).
$ x = 0.20 $ $
$
$
$ LaNi5 $
ð1Þ
where Cd was the discharge capacity at the discharge current density Id (Id = 60, 300, 600, 900 and 1200 mA/g, respectively), and Cmax was the maximum discharge capacity at the discharge current density of 60 mA/g. The linear polarization curves, potential-step discharging curves and Tafel curves were obtained on a PARSTAT 2273 Advanced Potentiostat/Galvanostat station. The linear polarization curves conducted by scanning the electrode potential at the rate of 0.1 mV/s from 5 to 5 mV (versus open circuit potential) at 50% depth of discharge (DOD) at 298 K. As for the potential step measurement, the electrodes in fully charged state were discharged at +500 mV potential step for 3600 s. Tafel curves were tested by scanning the electrode potential at the rate of 5 mV/s from 250 to 250 mV (versus open circuit potential) at 100% (DOD).
2. Experimental procedure
(a)
Mo
$
$$$
$
$$
Intensity (a.u.)
$ x = 0.15 $ $
$
$ $ $ $
$$$
$
$$
x = 0.10 $ $
$
$
$
$
$$$
$
$$
$
$$$
$
$$
$
$$$
$
$$
$ x = 0.05 $ $
$
$
$
$ x=0 $
20
$
30
$
$
40
$
149
50
60
70
80
2θ (Degree)
(b)
Fig. 1. XRD patterns of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys (a) XRD patterns for x = 0–0.20 and (b) Rietveld analysis pattern for x = 0.15.
150
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
Table 1 Lattice parameters and phase LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys. Samples
Phases
x=0 x = 0.05
LaNi5 LaNi5 Mo LaNi5 Mo LaNi5 Mo LaNi5 Mo
x = 0.10 x = 0.15 x = 0.20
Lattice parameter (Å)
abundance
Cell volume (Å3)
Abundance (%) 100 98.1 1.9 92.6 3.7 90.3 9.4 89.1 11.9
a = 5.0591 a = 5.0601
c = 4.0304 c = 4.0317
89.19 89.43
a = 5.0607
c = 4.0327
89.71
a = 5.0613
c = 4.0332
89.89
a = 5.0620
c = 4.0344
90.04
of
Table 2 Electrochemical properties of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
a b
Fig. 2a shows BSE image of LaNi3.70Mn0.30Al0.15Cu0.65(Mo0.46 Fe0.54)0.2 alloy, where the LaNi3.55Mn0.25Al0.15Cu0.75(Mo0.46 Fe0.54)0.2 alloy consists of shallow black area (C), white gray region (B) and white round area (A). Fig. 2b–d shows EDS result of A, B and C area in LaNi3.70Mn0.30Al0.15Cu0.65(Mo0.46Fe0.54)0.2 alloy. According to EDS result, it is reasonable to believe that the A area is Mo phase, the B area is La-rich segregation and C area is LaNi5 matrix phase. This is agreement with the results proposed by Young et al. [26].
3.2. Activation capability and maximum discharge capability The number of cycles (Na) needed to activate the electrodes and maximum discharge capacity (Cmax) of LaNi3.70Co0.2xMn0.30Al0.15 Cu0.65(Mo0.46Fe0.54)x alloy electrodes are given in Table 2. It is noted
(a)
x
Cmax (mA h/g)
Nab
HRD1200a (%)
S100 (%)
0 0.05 0.10 0.15 0.20
335.4 332.0 330.1 327.4 324.2
5 4 3 2 2
59.8 62.4 66.8 69.6 64.0
80.4 77.5 73.0 67.4 61.9
The high-rate dischargeability at the discharge current density of 1200 mA/g. The number of cycles needed to activate the electrode.
that the Na of the alloy electrodes decreases from 5 (x = 0) to 2 (x = 0.20) with increasing x value, which indicates that the substitution of Co by Mo0.46Fe0.54 significantly improves the activation performance. The phase interface or grain boundary contributes to the activation of alloy electrode because the interface or boundary is a buffer area of the releasing of the lattice stress and strain energy formed in the process of hydrogen absorbed [27]. In our results, the XRD and BSE results show that the relative abundance of Mo phase increases with the increment of x value. Increasing Mo phase causes the increase in volume fraction of phase boundary correspondingly, which decreases the lattice distortion and strain energy formed in the process of hydrogen absorption, and then contributes to the activation property. Furthermore, it is believed that the electrocatalytic activity of the alloy surface is beneficial to the rapid activation [28]. Yeh et al. [19] pointed out that doping Mo can increase the surface electrocatalytic activity of alloy
(b)
Element MoL
Wt% 100.0
At% 100.0
(d)
Element AlK LaL MnK FeK NiK CuK
Wt% 00.83 37.26 04.60 00.52 46.08 10.31
At% 02.28 19.97 06.23 00.69 58.43 12.08
A
B C
(c)
Element AlK LaL MnK FeK NiK CuK
Wt% 00.64 43.13 03.29 00.40 42.88 08.72
At% 01.85 24.28 04.68 00.56 57.12 10.74
Fig. 2. BSE and EDS results of LaNi3.70Mn0.30Al0.15Cu0.65 (Mo0.46Fe0.54)0.2 alloy (a) BSE, (b) EDS of A area, (c) EDS of B area and (d) EDS of C area.
151
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
100
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0 .20
HRD (%)
90
80
70
60 0
200
400
600
800
1000
1200
Discharge current density (mA/g) Fig. 4. HRD of La LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
40
Current density (mA/g)
surface. With increasing x value, the increase in Mo content gives rise to the electrochemical activity of alloy surface, and therefore improves the activation property. On the other hand, metalic Co in the alloy surface acts as an electrocatalyst for the rapid activation [29]. Co content on the surface of the alloy electrode decreases with increasing x value, which is detrimental to the activation properties. Thus, the advantageous factors are prominent for the improvement in activation performance of LaNi3.70Co0.2-xMn0.30 Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. The maximum discharge capacity (Cmax) of the alloy electrodes decreases monotonically from 335.4 (x = 0) to 324.2 mA h/g (x = 0.20) with increasing x value. The decrease in the Cmax of the alloy electrodes may be ascribed to the following factor. Firstly, PCT curves of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys are showed in Fig. 3, it can see that the hydrogen storage capacity decreases with increasing x value, and the plateau pressure gradually decreases with increasing x value, which indicates that the stability of the hydrides increases with increasing Mo0.46Fe0.54 content. Those are detrimental to the maximum discharge capacity of alloy electrodes. Secondly, Co contributes to the charge-transfer reaction at the electrode/electrolyte interface due to excellent electrocatalytic activity and makes the hydrogen diffusion through the surface more easily due to good electrical conductivity [30]. As x increases, the decrement of Co content degrades the charge-transfer reaction on the alloy surface and leads to a decrease of the discharge capacity. Thirdly, the increase in Fe content makes the surface oxide film become thicker, which degrades the chargetransfer reaction on the alloy surface. The oxide film also decreases the activity sites on the alloy surface, and then makes hydrogen diffuse from the inner to the surface more difficultly.
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
20
0
-20
3.3. High-rate dischargeability and electrochemical kinetics Fig. 4 shows the relationship between the high-rate dischargeability (HRD) and the discharge current density of LaNi3.70Co0.2x Mn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. The HRD of the alloy electrodes first increases with increasing x from 0 to 0.15, and then decreases when x increases to 0.20. The HRD at the discharge current density of 1200 mA/g (HRD1200) is listed in Table 2. It can be seen that HRD1200 first increases from 59.8% (x = 0) to 69.6% (x = 0.15), and then decreases to 64.0% (x = 0.20). Fig. 5 shows the linear polarization curves of LaNi3.55Co0.2x Mn0.25Al0.15Cu0.75(Mo0.46Fe0.54)x alloy electrodes at 50% DOD and 298 K. The polarization resistances (Rp) is calculated through estimating the slopes of linear polarization curves, and listed in Table 3. The Rp value of the alloy electrodes first decreases from
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
p (MPa)
1
-40
-4
2
4
Fig. 5. Linear polarization of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
Table 3 Electrochemical kinetic characteristics LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
of
x
Rp (mX g)
I0 (mA/g)
D (1011 cm2/s)
0 0.05 0.10 0.15 0.20
166.7 156.5 133.0 118.3 142.1
154.0 164.1 193.1 217.1 180.7
5.91 7.01 9.26 12.08 8.91
0.01
I0 ¼
1E-3 0.2
0
Overpotential (mV)
166.7 mX g (x = 0) to 118.3 mX g (x = 0.15), and then increases to 142.1 mX g (x = 0.20) with increasing x value. Furthermore, the exchange current density (I0) can also describe the charge-transfer process. The I0 value can be calculated according to the following formula [31].
0.1
0.0
-2
0.4
0.6
0.8
1.0
1.2
1.4
Hydrogen capacity (wt.%) Fig. 3. PCT of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
RT FRp
ð2Þ
where R, T, F, Rp are the gas constant, absolute temperature, Faraday constant and the polarization resistance, respectively. The I0 values are calculated by Eq. (2) and listed in Table 3. It is clear that the I0 first increases from 154.0 mA/g (x = 0) to 217.1 mA/g (x = 0.15), and then decreases to 180.7 mA/g (x = 0.20). Iwakura et al. [32] pointed
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
out that the addition of Mo leads to the improvement of the electrocatalytic activity on the alloy surface. Thus, the secondary phase Mo increases with increasing Mo0.46Fe0.54 content, which is beneficial to the electrocatalytic activity of the surface of alloy electrodes and improves the charge-transfer reaction. Unfortunately, the increase in Fe and the decrease of Co with increasing x value will cause the increase of surface oxide film and then degrade the charge-transfer reaction on the alloy surface. Fig. 6 shows the semi-logarithmic plots of the anodic current versus the time response of LaNi3.70Co0.2xMn0.30Al0.15 Cu0.65(Mo0.46Fe0.54)x alloy electrodes. Zheng et al. [33] reported that in a large anodic potential-step test, after a long discharge time, the diffusion current varies with time according to the following equation:
6FD p2 D lg i ¼ lg ðC C Þ t 0 s 2 2:303 a2 da D¼
ð3Þ
2:303a2 d lg i dt p2
ð4Þ
where i is anodic current density (A/g), D is the hydrogen diffusion coefficient (cm2/s), d is the density of the alloy (g/cm3), a is the radius of the alloy particle, C0 is the initial hydrogen concentration in the bulk of the alloy (mol/cm3), Cs is the surface hydrogen concentration of the alloy (mol/cm3) and t is the discharge time (s). Assuming that the alloy has a similar particle distribution with an average particle radius of 13 lm according to previous study [16], D is calculated and summarized in Table 3. The D of LaNi3.70 Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes first increases from 5.91 1011 (x = 0) to 12.08 1011 cm2/s (x = 0.15), and then decreases to 8.91 1011 cm2/s (x = 0.20). As mentioned above, phase boundaries provide extra tunnels for the diffusion of hydrogen atoms. The increase in the phase boundary due to Mo phase is beneficial to hydrogen diffusion. On the contrary, Khaldi et al. [34] have reported that the oxidation of Fe on the alloy surface restricted the hydrogen diffuse from the surface to the bulk. As mentioned above, the increase of Fe content causes the increment of the surface oxide film, which degrades the hydrogen diffusion. Therefore, it is certain that the diffusion coefficient has a maximum value with increasing x value.
is expressed as Sn (%) = Cn/Cmax 100 (where Cn is the discharge capacity at the nth cycle). The cycling capacity retention rate of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes as a function of cycle number is shown in Fig. 7. Cycling stability decreases with increasing x from 0 to 0.20. The S100 of the alloy electrodes are listed in Table 2. It can be seen that S100 monotonically decreases from 80.4% (x = 0) to 61.9% (x = 0.20). Generally, the capacity decay of the alloy electrode is ascribed to the pulverization and corrosion of the alloy during the charging/discharging cycles [35], as well as to the decrease of the electrochemical kinetics [36]. It is pointed out that the increase in c/a ratio facilitates hydrogen atoms from going in and out of the crystal and therefore decreases lattice stress during charging/discharging cycles [37]. The c/a ratio of LaNi5 phase increases with increasing x value, which contributes to reducing lattice stress, and then strengthens anti-pulverization property of the alloy electrodes. Furthermore, as mentioned above, the formation of the secondary phase Mo increases the number of phase boundary as buffer areas for the release of distortion and stress of crystal lattice, restricted the pulverization and then improves cycling stability. On the other hand, Fe easily oxidized due to the lower surface energy and forms coarse oxide film. The increase in Fe content not only causes the deterioration of the corrosion resistance and then increases the loss of the alloy, but also degrades the electrochemical kinetics on the alloy
100
90
S (%)
152
80
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
70
60 20
3.4. Cycling stability
60
80
100
Cycling number (n)
Cycling stability is an extremely important factor for the service life of hydrogen storage alloys. The cycling capacity retention rate
0.5
Fig. 7. Cycling stability of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
0.0
0.1
Current (A)
-0.5
lg (i, A/g)
40
-1.0 -1.5
0.01
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
1E-3
-2.0 1E-4 -2.5 0
500
1000
1500
2000
2500
3000
3500
-1.2
-1.1
Fig. 6. Semilogarithmic curves of anodic current versus time of response of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
-1.0
-0.9
-0.8
-0.7
Potential (V, vs. SCE)
Time (s) Fig. 8. Tafel electrodes.
curves
of
LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x
alloy
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154 Table 4 Corrosion potential and current of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. Sample
x=0
x = 0.05
x = 0.10
x = 0.15
x = 0.20
Potential (V) Current (mA)
1.012 29.33
1.034 35.51
1.046 36.99
1.058 38.11
1.067 38.61
surface, which is also detrimental to the cycling stability. In addition, Fig. 8 presents the Tafel curves of LaNi3.70Co0.2xMn0.30 Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. Corrosion potential and current are listed in Table 4. Clearly the corrosion potential of alloy electrodes becomes more negative and the corrosion current increases with increasing x value, indicating the corrosion resistance of alloy electrodes decreases with increasing x value. Thus, it is reasonable to believe that the decrease in corrosion resistance of alloy electrode is one of the critical reasons for the degradation of cycling stability in present work.
4. Conclusions In this paper, microstructures and electrochemical characteristics of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) hydrogen storage alloys are systematically investigated. The following conclusions can be drawn: 1. Analysis of X-ray diffraction profiles and backscattered electron results show that the pristine alloy is LaNi5 phase, while the alloys containing Mo0.46Fe0.54 consist of LaNi5 matrix phase and secondary Mo phase. The relative abundance of Mo phase increases with the increase in x value. The lattice parameters a, c, c/a and V of LaNi5 phase increase with increasing x value. 2. The activation property of the alloy electrodes is improved by substituting Co with Mo0.46Fe0.54. The maximum discharge capacity of alloy electrodes monotonically decreases from 335.4 (x = 0) to 324.2 (x = 0.20) mA h/g. 3. HRD1200 first increases from 59.8% (x = 0) to 69.6% (x = 0.15), and then decreases to 64.0% (x = 0.20), which is determined by the exchange current density and hydrogen diffusion coefficient. 4. The cycling capacity retention rate at the 100th cycle decreases from 80.4% (x = 0) to 61.9% (x = 0.20), which should be ascribed to the deterioration of corrosion resistance of alloy electrode in the charging–discharging cycle.
Acknowledgements This research is financially supported by the National Natural Science Foundation of China (51471065, U1304522), Program for New Century Excellent Talents in University — China (NCET-110943), Plan for Scientific Innovation Talent of Henan Province — China (144100510009) and Foundation for University Key Teacher in the University of Henan Province — China (2011GGJS052).
References [1] L.Z. Ouyang, C.H. Peng, M. Zhu, A new type of Mg based metal hydride with promising hydrogen storage properties, Int. J. Hydrogen Energy 32 (2007) 3929–3935. [2] X.B. Zhang, D.Z. Sun, W.Y. Yin, Y.J. Chai, M.S. Zhao, Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni3x(Al0.5Mo0.5)x (x = 0–0.4) hydrogen storage alloys, Electrochim. Acta 50 (2005) 3407–3413. [3] Z.J. Cao, L.Z. Ouyang, H. Wang, J.W. Liu, D.L. Sun, Q.A. Zhang, M. Zhu, Structural characteristics and hydrogen storage properties of Sm2Co7, J. Alloys Comp. 608 (2014) 14–18.
153
[4] L.Z. Ouyang, Y.J. Xu, H.W. Dong, L.X. Sun, M. Zhu, Production of hydrogen via hydrolysis of hydrides in Mg–La system, Int. J. Hydrogen Energy 34 (2009) 9671–9676. [5] Y. Fukumoto, M. Miyamoto, M. Matsuoka, C. Iwakura, Effect of the stoichiometric ratio on electrochemical properties of hydrogen storage alloys for nickel-metal hydride batteries, Electrochim. Acta 40 (1995) 845–848. [6] L.Z. Ouyang, Z.J. Cao, L.L. Li, H. Wang, J.W. Liu, D. Min, Y.W. Chen, F.M. Xiao, R.H. Tang, M. Zhu, Enhanced high-rate discharge properties of La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 with added graphene synthesized by plasma milling, Int. J. Hydrogen Energy 39 (2014) 12765–12772. [7] X.D. Wei, P. Zhang, H. Dong, Y.N. Liu, J.W. Zhu, G. Yu, Electrochemical performances of a Co-free La(NiMnAlFe)5 hydrogen storage alloy modified by surface coating with Cu, J. Alloys Comp. 458 (2008) 583–587. [8] C. Iwakura, K. Ohkawa, H. Senoh, H. Inoue, Electrochemical and crystallographic characterization of Co-free hydrogen storage alloys for use in nickel-metal hydride batteries, Electrochim. Acta 46 (2001) 4383–4388. [9] B. Zhang, W.Y. Wu, B. Xue, G.F. Tu, Investigations on the kinetics properties of the electrochemical reactions of MlNi3.55Co0.75xMn0.4Al0.3(Cu0.75P0.25)x (x = 0.0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys, J. Alloys Comp. 538 (2012) 189– 192. [10] X. Tian, G.H. Yun, H.Y. Wang, T. Shang, Z.Q. Yao, W. Wei, X.X. Liang, Preparation and electrochemical properties of La–Mg–Ni-based La0.75Mg0.25Ni3.3Co0.5 multiphase hydrogen storage alloy as negative material of Ni/MH battery, Int. J. Hydrogen Energy 39 (2014) 8474–8481. [11] L. Wang, H.Z. Yan, W. Xiong, B.Q. Li, J. Li, F.Q. Kong, The influence of boron content on the structural and electrochemical properties of the La15Fe77B8type hydrogen storage alloy, J. Power Sources 259 (2014) 213–218. [12] Q.R. Yao, H.Y. Zhou, Z.M. Wang, S.K. Pan, G.H. Rao, Electrochemical properties of the LaNi4.5Co0.25Al0.25 hydrogen storage alloy in wide temperature range, J. Alloys Comp. 606 (2014) 81–85. [13] X.B. Zhang, D.Z. Sun, W.Y. Yin, Y.J. Chai, M.S. Zhao, Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni4.5x(Al0.5Mo0.5)x (x = 0–0.8) hydrogen storage alloys, Eur. J. Inorg. Chem. 11 (2005) 2235–2238. [14] X.B. Zhang, D.Z. Sun, W.Y. Yin, Y.J. Chai, M.S. Zhao, Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni3.5x(Al0.5Mo0.5)x (x = 0–0.8) hydrogen storage alloys, J. Power Sources 154 (2006) 290–297. [15] P. Zhang, Y.N. Liu, J.W. Zhu, X. Wei, G. Yu, Effect of Al and W substitution for Ni on the microstructure and electrochemical properties of La1.3CaMg0.7 Ni9x(Al0.5W0.5)x hydrogen storage alloys, Int. J. Hydrogen Energy 32 (2007) 2488–2493. [16] Y.P. Fan, B.Z. Liu, B.Q. Zhang, L.Q. Ji, Y.G. Wang, Z. Zhang, Microstructures and electrochemical properties of LaNi3.55Co0.2xMn0.35Al0.15Cu0.75(Fe0.43B0.57)x (x = 0–0.20) hydrogen storage alloys, Mater. Chem. Phys. 138 (2013) 803–809. [17] B.Z. Liu, X.Y. Peng, Y.P. Fan, L.Q. Ji, B.Q. Zhang, Z. Zhang, Microstructures and electrochemical properties of LaNi3.55Co0.2xMn0.35Al0.15Cu0.75(V0.81Fe0.19)x hydrogen storage alloys, Int. J. Eletrochem. Sci. 7 (2012) 11966–11977. [18] M.T. Yeh, V.M. Beibutian, S.E. Hsu, Effect of Mo additive on hydrogen absorption of rare-earth based hydrogen storage alloy, J. Alloys Comp. 293– 295 (1999) 721–723. [19] H. Ye, H. Zhang, Development of hydrogen-storage alloys for high-power nickel-metal hydride batteries, Adv. Eng. Mater. 3 (2001) 481–485. [20] S.E. Hsu, V.M. Beibutian, M.T. Yeh, Preparation of hydrogen storage alloys for applications of hydrogen storage and transportation, J. Alloys Comp. 330–332 (2002) 882–885. [21] X.D. Wei, S.S. Liu, H. Dong, P. Zhang, Y.N. Liu, J.W. Zhu, G. Yu, Microstructures and electrochemical properties of Co-free AB5-type hydrogen storage alloys through substitution of Ni by Fe, Electrochim. Acta 52 (2007) 2423–2428. [22] S.X. Yang, Y. Li, Z.P. Liu, S.Q. Yang, S.M. Han, Effect of partial substitution of Cu by Fe on the phase structure and electrochemical property of low-Co AB5 type hydrogen storage alloys, Acta Phys.-Chim. Sin. 26 (2010) 2144–2150. [23] L.Z. Ouyang, J.M. Huang, H. Wang, Y.J. Wen, Q.A. Zhang, D.L. Sun, M. Zhu, Excellent hydrolysis performances of Mg3RE hydrides, Int. J. Hydrogen Energy 38 (2013) 2973–2978. [24] L.Z. Ouyang, Z.J. Cao, H. Wang, J.W. Liu, D.L. Sun, Q.A. Zhang, M. Zhu, Dualtuning effect of In on the thermodynamic and kinetic properties of Mg2Ni dehydrogenation, Int. J. Hydrogen Energy 38 (2013) 8881–8887. [25] L.Z. Ouyang, X.S. Yang, M. Zhu, J.W. Liu, H.W. Dong, D.L. Sun, J. Zou, X.D. Yao, Enhanced hydrogen storage kinetics and stability by synergistic effects of in situ formed CeH2.73 and Ni in CeH2.73–MgH2–Ni nanocomposites, J. Phys. Chem. C 118 (2014) 7808–7820. [26] K. Young, T. Ouchi, B. Reichman, J. Koch, M.A. Fetcenko, Effects of Mo additive on the structure and electrochemical properties of low-temperature AB5 metal hydride alloys, J. Alloys Comp. 509 (2011) 3995–4001. [27] C.Y. Seo, S.J. Choi, J. Choi, C.N. Park, J.Y. Lee, Effect of V and Zr on the electrochemical properties of La-based AB5-type metal hydride electrodes, J. Alloys Comp. 35 (2003) 255–263. [28] R. Li, J.M. Wu, H. Su, S.X. Zhou, Microstructure and electrochemical performance of vanadium-containing AB5-type low-Co intermetallic hydrides, J. Alloys Comp. 421 (2006) 258–267. [29] L.Z. Ouyang, F.X. Qin, M. Zhu, Hydrogen storage behavior of Mg3La and Mg3LaNi0.1, Scr. Mater. 55 (2006) 1075–1078. [30] S.R. Ovshinsky, M.A. Fetcenko, J. Rossm, A nickel metal hydride battery for electric vehicles, Science 260 (1993) 176–181. [31] P.H.L. Notten, P. Hokkeling, Double-phase hydride forming compounds. A new class of highly electrocatalytic materials, J. Electrochem. Soc. 138 (1991) 1877– 1885.
154
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
[32] C. Iwakura, H. Senoh, K. Morimot, Y. Hara, H. Inoue, Influence of temperature on discharge process of mischmetal-based hydrogen storage alloy electrodes, Electrochemistry 70 (2002) 2–7. [33] G. Zheng, B.N. Popov, R.E. White, Electrochemical determination of the diffusion coefficient of hydrogen through a LaNi4.25Al0.75 electrode in alkaline aqueous solution, J. Electrochem. Soc. 142 (1995) 2695–2698. [34] C. Khaldi, H. Mathlouthi, J. Lamloumi, A. Percheron-Guegan, Effect of partial substitution of Co with Fe on the properties of LaNi3.55Mn0.4Al0.3Co0.75-xFex (x=0, 0.15, 0.55) alloys electrodes, J. Alloys Comp. 360 (2003) 266–271.
[35] D. Chartouni, F. Meli, A. Züttel, K. Gross, L. Schlapbach, The influence of cobalt on the electrochemical cycling stability of LaNi5-based hydride forming alloys, J. Alloys Comp. 241 (1996) 160–166. [36] B.Z. Liu, G.X. Fan, Y.C. Wang, G.F. Mi, Y.M. Wu, L.M. Wang, Crystallographic and electrochemical characteristics of melt-spun Ti–Zr–Ni–Y alloys, Int. J. Hydrogen Energy 33 (2008) 5801–5805. [37] M. Takao, S. Satoshi, S. Naofumi, Hydrogen absorbing alloy and nickel-metal hydride rechargeable battery, European Patent 1,075,032, B1, 2007.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Microstructures and electrochemical characteristics of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x hydrogen storage alloys Junling Sun a, Yanping Fan a, Baozhong Liu a,⇑, Liqiang Ji b, Yongguang Wang b, Mingjie Ma a,⇑ a b
School of Materials Science & Engineering, Henan Polytechnic University, Jiaozuo 454000, China Inner Mongolia Rare Earth Ovonic Metal Hydride Co. Ltd., Baotou 014030, China
a r t i c l e
i n f o
Article history: Received 27 February 2015 Received in revised form 6 April 2015 Accepted 7 April 2015 Available online 11 April 2015 Keywords: Hydrogen storage alloy Electrochemical properties Kinetics Microstructure MoFe alloy
a b s t r a c t Electrochemical hydrogen storage properties of LaNi3.70Co0.2Mn0.30Al0.15Cu0.65 alloy are improved by substituting Co with Mo0.46Fe0.54, rather than pure Mo and Fe. Microstructures and electrochemical properties of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) hydrogen storage alloys are investigated. X-ray diffraction and backscattered electron results indicate that the pristine alloy is LaNi5 phase with a hexagonal CaCu5-type structure, while the alloys containing Mo0.46Fe0.54 consist of LaNi5 matrix phase and Mo secondary phase. The relative abundance of Mo phase increases with the increase in x value. The lattice parameters a, c, c/a and cell volume V of LaNi5 phase increase with increasing x value. As x increases from 0 to 0.20, maximum discharge capacity of the alloy electrodes monotonically decreases from 335.4 (x = 0) to 324.2 mA h/g (x = 0.20). The high-rate dischargeability of the alloy electrodes at the discharge current density of 1200 mA/g first increases from 59.8% (x = 0) to 69.6% (x = 0.15), and then decreases to 64.0% (x = 0.20). The cycling capacity retention rate at the 100th cycle decreases from 80.4% (x = 0) to 61.9% (x = 0.20), which should be ascribed to the deterioration of the corrosion resistance of alloy electrode with increasing x value. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is expected to be a promising new energy source to replace the conventional fossil for solving the shortage of fossil energy resources and the global warming in the near future. Hydrogen storage is one of many important processes in order for hydrogen to become a viable solution to the energy crisis and the environment problem [1]. Among different ways to store hydrogen, absorption in solid to form hydride is very attractive since it allows safe storage at pressure and temperature close to ambient conditions [2,3]. Nickel/metal hydride (Ni/MH) secondary batteries have been adopted in various portable electronic devices because of their good electrochemical performance and environment compatibility [4,5]. Almost all of the commercial Ni/MH batteries employ AB5-type alloys as negative electrode materials due to their good overall electrode properties [6]. However, high cost of AB5-type alloys slows down the rhythms of wide applications. Substantial efforts have been dedicated to reduce their costs by eliminating Nd and Pr [7] or substituting Co with low-cost elements, such as, Mn, Fe, Cu, and Si [8,9]. Co element contributes to the high-rate dischargeability due to excellent catalytic and ⇑ Corresponding authors. Tel./fax: +86 335 3989859. E-mail addresses: [email protected] (B. Liu), [email protected] (M. Ma). http://dx.doi.org/10.1016/j.jallcom.2015.04.058 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
conductive performance [10], as well as to cycling stability due to the anti-pulverization [11,12]. However, it is very difficult to keep excellent electrochemical properties of the alloys after the substitution of Co by single foreign elements. In comparison, the two elements, such as, MoAl [13,14], and WAl [15], were used together to substitute Co, and the less loss of high-rate dischargeability and cycling stability of the alloys can be obtained. Unfortunately, the substitution with pure element is detriment to the decrease in the alloy cost. In our previous investigations [16,17], the commercial B0.57Fe0.43 and V0.81Fe0.19 alloys, rather than pure B, V or Fe, was used to substitute the Co, which obviously decreases the cost of the alloys. Thus, it is feasible that the commercial alloys, rather than pure metallic elements, are used to substitute Co for improving the performance-price ration of the alloys. Yeh et al. [18] stressed that the substitution of Mo for Co increased both the hydrogen absorption capacity and equilibrium plateau pressure of rare-earth based AB5-type hydrogen storage alloy at 313 K. Ye and Zhang [19] have reported that the addition of Mo improves the high-rate dischargeability due to the increase in the exchange-current density without the degradation of the discharge capacity of alloy at low current density. Obviously, the addition of Mo element is effective to enhance discharge capacity, activation property and high-rate dischargeability [20]. Moreover, it is reported that Fe introduction in AB5 alloy can remarkably
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
as electrolyte. Each electrode was charged for 7 h at 60 mA/g and discharged to 0.6 V versus Hg/HgO at 60 mA/g at 298 K. After every charging/discharging, the rest time was 10 min. In evaluating the high-rate dischargeability, discharge capacity of the alloy electrodes at different discharge current density was measured. The high-rate dischargeability (HRD, %) was defined as
improve cycling stability due to the improvement in the anti-pulverization of the alloy electrode [21,22]. More importantly, the commercial Mo0.46Fe0.54 alloy is cheaper than pure Mo, and the melt point of Mo0.46Fe0.54 alloy is lower compared with pure Mo, which contributes to the homogeneity of the alloys. Herein, on the basis of the merits of Mo0.46Fe0.54 and the belief that the Fe and Mo additions may result in some noticeable modifications of electrochemical hydrogen storage performance, microstructures and electrochemical hydrogen storage characteristics of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) alloys have been investigated.
HRD ¼ C d =C max 100%
LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) alloys were synthesized by induction melting of the metal elements (La, Ni, Co, Mn, Al, Cu: 99.9% in purity and commercial Mo–Fe alloy contained 46 at.% Mo and the other were Fe and trace impurities) in argon atmosphere. The ingots were annealed by using vacuum heat-treating furnace. Before heating, the sample cell was vacuumized until pressure is less than 10 Pa and then the argon was charged into the sample cell until the pressure was 0.8 MPa. The samples were heated from room temperature to 1223 K at the rate of 10 K/min and then kept in 1223 K for 2 h followed by natural cooling in the furnace. Sample powders of 300 mesh size were used for X-ray diffraction (XRD) test, which was carried out on a D/Max-2500/PCX with Cu Ka radiation in the range from 20° to 80° with 0.02°/min. Backscattered electron (BSE) images were obtained by using HITACHI-4800 scanning electron microscope with an energy dispersive Xray spectrometer (EDS). The hydrogen storage properties of the alloys were studied by using pressure-composition-temperature (PCT) (made by Suzuki Shokan in Japan) at 314 K. Before the PCT measurement, the alloys were activated by 5 hydriding/dehydriding cycles. The delay time was 180 s and maximum pressure was 2 MPa. The alloy powders of measuring electrodes were obtained by grinding the inner part of alloy ingots in the Ar atmosphere. All measuring electrodes for test were prepared by cold pressing the mixture of 0.15 g alloy powders of 200–400 mesh and 0.75 g nickel carbonyl powders into a pellet of 10 mm in diameter under 15 MPa. Electrochemical measurements were performed at 298 K in a standard tri-electrode system, consisting of a working electrode (metal hydride), a counter electrode (Ni(OH)2/NiOOH), and a reference electrode (Hg/HgO) with 6 mol/L KOH solution
3. Results and discussions 3.1. Crystal structure Rietveld refinement is an effective method to analyze the XRD data for obtaining the lattice parameters and the phase abundance of the alloys [23–25]. Fig. 1(a) presents XRD patterns of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys. Fig. 1(b) shows the XRD pattern and Rietveld analysis pattern of LaNi3.70Co0.05Mn0.30Al0.15Cu0.65(Mo0.46Fe0.54)0.15 alloy. It can be seen that the pristine alloy is LaNi5 phase with CaCu5 structure, and the alloys containing Mo0.46Fe0.54 are composed of LaNi5 matrix phase and a spot of Mo phase. The lattice parameter, cell volume and phase abundance are listed in Table 1. The abundance of Mo phase increases with increasing x value. It is noted that the a, c, c/a and V of the alloys increase monotonously with increasing x value, which should be ascribed to larger atomic radius of Mo (2.01 Å) and Fe (1.72 Å) than that of Co (1.67 Å).
$ x = 0.20 $ $
$
$
$ LaNi5 $
ð1Þ
where Cd was the discharge capacity at the discharge current density Id (Id = 60, 300, 600, 900 and 1200 mA/g, respectively), and Cmax was the maximum discharge capacity at the discharge current density of 60 mA/g. The linear polarization curves, potential-step discharging curves and Tafel curves were obtained on a PARSTAT 2273 Advanced Potentiostat/Galvanostat station. The linear polarization curves conducted by scanning the electrode potential at the rate of 0.1 mV/s from 5 to 5 mV (versus open circuit potential) at 50% depth of discharge (DOD) at 298 K. As for the potential step measurement, the electrodes in fully charged state were discharged at +500 mV potential step for 3600 s. Tafel curves were tested by scanning the electrode potential at the rate of 5 mV/s from 250 to 250 mV (versus open circuit potential) at 100% (DOD).
2. Experimental procedure
(a)
Mo
$
$$$
$
$$
Intensity (a.u.)
$ x = 0.15 $ $
$
$ $ $ $
$$$
$
$$
x = 0.10 $ $
$
$
$
$
$$$
$
$$
$
$$$
$
$$
$
$$$
$
$$
$ x = 0.05 $ $
$
$
$
$ x=0 $
20
$
30
$
$
40
$
149
50
60
70
80
2θ (Degree)
(b)
Fig. 1. XRD patterns of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys (a) XRD patterns for x = 0–0.20 and (b) Rietveld analysis pattern for x = 0.15.
150
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
Table 1 Lattice parameters and phase LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys. Samples
Phases
x=0 x = 0.05
LaNi5 LaNi5 Mo LaNi5 Mo LaNi5 Mo LaNi5 Mo
x = 0.10 x = 0.15 x = 0.20
Lattice parameter (Å)
abundance
Cell volume (Å3)
Abundance (%) 100 98.1 1.9 92.6 3.7 90.3 9.4 89.1 11.9
a = 5.0591 a = 5.0601
c = 4.0304 c = 4.0317
89.19 89.43
a = 5.0607
c = 4.0327
89.71
a = 5.0613
c = 4.0332
89.89
a = 5.0620
c = 4.0344
90.04
of
Table 2 Electrochemical properties of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
a b
Fig. 2a shows BSE image of LaNi3.70Mn0.30Al0.15Cu0.65(Mo0.46 Fe0.54)0.2 alloy, where the LaNi3.55Mn0.25Al0.15Cu0.75(Mo0.46 Fe0.54)0.2 alloy consists of shallow black area (C), white gray region (B) and white round area (A). Fig. 2b–d shows EDS result of A, B and C area in LaNi3.70Mn0.30Al0.15Cu0.65(Mo0.46Fe0.54)0.2 alloy. According to EDS result, it is reasonable to believe that the A area is Mo phase, the B area is La-rich segregation and C area is LaNi5 matrix phase. This is agreement with the results proposed by Young et al. [26].
3.2. Activation capability and maximum discharge capability The number of cycles (Na) needed to activate the electrodes and maximum discharge capacity (Cmax) of LaNi3.70Co0.2xMn0.30Al0.15 Cu0.65(Mo0.46Fe0.54)x alloy electrodes are given in Table 2. It is noted
(a)
x
Cmax (mA h/g)
Nab
HRD1200a (%)
S100 (%)
0 0.05 0.10 0.15 0.20
335.4 332.0 330.1 327.4 324.2
5 4 3 2 2
59.8 62.4 66.8 69.6 64.0
80.4 77.5 73.0 67.4 61.9
The high-rate dischargeability at the discharge current density of 1200 mA/g. The number of cycles needed to activate the electrode.
that the Na of the alloy electrodes decreases from 5 (x = 0) to 2 (x = 0.20) with increasing x value, which indicates that the substitution of Co by Mo0.46Fe0.54 significantly improves the activation performance. The phase interface or grain boundary contributes to the activation of alloy electrode because the interface or boundary is a buffer area of the releasing of the lattice stress and strain energy formed in the process of hydrogen absorbed [27]. In our results, the XRD and BSE results show that the relative abundance of Mo phase increases with the increment of x value. Increasing Mo phase causes the increase in volume fraction of phase boundary correspondingly, which decreases the lattice distortion and strain energy formed in the process of hydrogen absorption, and then contributes to the activation property. Furthermore, it is believed that the electrocatalytic activity of the alloy surface is beneficial to the rapid activation [28]. Yeh et al. [19] pointed out that doping Mo can increase the surface electrocatalytic activity of alloy
(b)
Element MoL
Wt% 100.0
At% 100.0
(d)
Element AlK LaL MnK FeK NiK CuK
Wt% 00.83 37.26 04.60 00.52 46.08 10.31
At% 02.28 19.97 06.23 00.69 58.43 12.08
A
B C
(c)
Element AlK LaL MnK FeK NiK CuK
Wt% 00.64 43.13 03.29 00.40 42.88 08.72
At% 01.85 24.28 04.68 00.56 57.12 10.74
Fig. 2. BSE and EDS results of LaNi3.70Mn0.30Al0.15Cu0.65 (Mo0.46Fe0.54)0.2 alloy (a) BSE, (b) EDS of A area, (c) EDS of B area and (d) EDS of C area.
151
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
100
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0 .20
HRD (%)
90
80
70
60 0
200
400
600
800
1000
1200
Discharge current density (mA/g) Fig. 4. HRD of La LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
40
Current density (mA/g)
surface. With increasing x value, the increase in Mo content gives rise to the electrochemical activity of alloy surface, and therefore improves the activation property. On the other hand, metalic Co in the alloy surface acts as an electrocatalyst for the rapid activation [29]. Co content on the surface of the alloy electrode decreases with increasing x value, which is detrimental to the activation properties. Thus, the advantageous factors are prominent for the improvement in activation performance of LaNi3.70Co0.2-xMn0.30 Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. The maximum discharge capacity (Cmax) of the alloy electrodes decreases monotonically from 335.4 (x = 0) to 324.2 mA h/g (x = 0.20) with increasing x value. The decrease in the Cmax of the alloy electrodes may be ascribed to the following factor. Firstly, PCT curves of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloys are showed in Fig. 3, it can see that the hydrogen storage capacity decreases with increasing x value, and the plateau pressure gradually decreases with increasing x value, which indicates that the stability of the hydrides increases with increasing Mo0.46Fe0.54 content. Those are detrimental to the maximum discharge capacity of alloy electrodes. Secondly, Co contributes to the charge-transfer reaction at the electrode/electrolyte interface due to excellent electrocatalytic activity and makes the hydrogen diffusion through the surface more easily due to good electrical conductivity [30]. As x increases, the decrement of Co content degrades the charge-transfer reaction on the alloy surface and leads to a decrease of the discharge capacity. Thirdly, the increase in Fe content makes the surface oxide film become thicker, which degrades the chargetransfer reaction on the alloy surface. The oxide film also decreases the activity sites on the alloy surface, and then makes hydrogen diffuse from the inner to the surface more difficultly.
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
20
0
-20
3.3. High-rate dischargeability and electrochemical kinetics Fig. 4 shows the relationship between the high-rate dischargeability (HRD) and the discharge current density of LaNi3.70Co0.2x Mn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. The HRD of the alloy electrodes first increases with increasing x from 0 to 0.15, and then decreases when x increases to 0.20. The HRD at the discharge current density of 1200 mA/g (HRD1200) is listed in Table 2. It can be seen that HRD1200 first increases from 59.8% (x = 0) to 69.6% (x = 0.15), and then decreases to 64.0% (x = 0.20). Fig. 5 shows the linear polarization curves of LaNi3.55Co0.2x Mn0.25Al0.15Cu0.75(Mo0.46Fe0.54)x alloy electrodes at 50% DOD and 298 K. The polarization resistances (Rp) is calculated through estimating the slopes of linear polarization curves, and listed in Table 3. The Rp value of the alloy electrodes first decreases from
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
p (MPa)
1
-40
-4
2
4
Fig. 5. Linear polarization of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
Table 3 Electrochemical kinetic characteristics LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
of
x
Rp (mX g)
I0 (mA/g)
D (1011 cm2/s)
0 0.05 0.10 0.15 0.20
166.7 156.5 133.0 118.3 142.1
154.0 164.1 193.1 217.1 180.7
5.91 7.01 9.26 12.08 8.91
0.01
I0 ¼
1E-3 0.2
0
Overpotential (mV)
166.7 mX g (x = 0) to 118.3 mX g (x = 0.15), and then increases to 142.1 mX g (x = 0.20) with increasing x value. Furthermore, the exchange current density (I0) can also describe the charge-transfer process. The I0 value can be calculated according to the following formula [31].
0.1
0.0
-2
0.4
0.6
0.8
1.0
1.2
1.4
Hydrogen capacity (wt.%) Fig. 3. PCT of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
RT FRp
ð2Þ
where R, T, F, Rp are the gas constant, absolute temperature, Faraday constant and the polarization resistance, respectively. The I0 values are calculated by Eq. (2) and listed in Table 3. It is clear that the I0 first increases from 154.0 mA/g (x = 0) to 217.1 mA/g (x = 0.15), and then decreases to 180.7 mA/g (x = 0.20). Iwakura et al. [32] pointed
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
out that the addition of Mo leads to the improvement of the electrocatalytic activity on the alloy surface. Thus, the secondary phase Mo increases with increasing Mo0.46Fe0.54 content, which is beneficial to the electrocatalytic activity of the surface of alloy electrodes and improves the charge-transfer reaction. Unfortunately, the increase in Fe and the decrease of Co with increasing x value will cause the increase of surface oxide film and then degrade the charge-transfer reaction on the alloy surface. Fig. 6 shows the semi-logarithmic plots of the anodic current versus the time response of LaNi3.70Co0.2xMn0.30Al0.15 Cu0.65(Mo0.46Fe0.54)x alloy electrodes. Zheng et al. [33] reported that in a large anodic potential-step test, after a long discharge time, the diffusion current varies with time according to the following equation:
6FD p2 D lg i ¼ lg ðC C Þ t 0 s 2 2:303 a2 da D¼
ð3Þ
2:303a2 d lg i dt p2
ð4Þ
where i is anodic current density (A/g), D is the hydrogen diffusion coefficient (cm2/s), d is the density of the alloy (g/cm3), a is the radius of the alloy particle, C0 is the initial hydrogen concentration in the bulk of the alloy (mol/cm3), Cs is the surface hydrogen concentration of the alloy (mol/cm3) and t is the discharge time (s). Assuming that the alloy has a similar particle distribution with an average particle radius of 13 lm according to previous study [16], D is calculated and summarized in Table 3. The D of LaNi3.70 Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes first increases from 5.91 1011 (x = 0) to 12.08 1011 cm2/s (x = 0.15), and then decreases to 8.91 1011 cm2/s (x = 0.20). As mentioned above, phase boundaries provide extra tunnels for the diffusion of hydrogen atoms. The increase in the phase boundary due to Mo phase is beneficial to hydrogen diffusion. On the contrary, Khaldi et al. [34] have reported that the oxidation of Fe on the alloy surface restricted the hydrogen diffuse from the surface to the bulk. As mentioned above, the increase of Fe content causes the increment of the surface oxide film, which degrades the hydrogen diffusion. Therefore, it is certain that the diffusion coefficient has a maximum value with increasing x value.
is expressed as Sn (%) = Cn/Cmax 100 (where Cn is the discharge capacity at the nth cycle). The cycling capacity retention rate of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes as a function of cycle number is shown in Fig. 7. Cycling stability decreases with increasing x from 0 to 0.20. The S100 of the alloy electrodes are listed in Table 2. It can be seen that S100 monotonically decreases from 80.4% (x = 0) to 61.9% (x = 0.20). Generally, the capacity decay of the alloy electrode is ascribed to the pulverization and corrosion of the alloy during the charging/discharging cycles [35], as well as to the decrease of the electrochemical kinetics [36]. It is pointed out that the increase in c/a ratio facilitates hydrogen atoms from going in and out of the crystal and therefore decreases lattice stress during charging/discharging cycles [37]. The c/a ratio of LaNi5 phase increases with increasing x value, which contributes to reducing lattice stress, and then strengthens anti-pulverization property of the alloy electrodes. Furthermore, as mentioned above, the formation of the secondary phase Mo increases the number of phase boundary as buffer areas for the release of distortion and stress of crystal lattice, restricted the pulverization and then improves cycling stability. On the other hand, Fe easily oxidized due to the lower surface energy and forms coarse oxide film. The increase in Fe content not only causes the deterioration of the corrosion resistance and then increases the loss of the alloy, but also degrades the electrochemical kinetics on the alloy
100
90
S (%)
152
80
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
70
60 20
3.4. Cycling stability
60
80
100
Cycling number (n)
Cycling stability is an extremely important factor for the service life of hydrogen storage alloys. The cycling capacity retention rate
0.5
Fig. 7. Cycling stability of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
0.0
0.1
Current (A)
-0.5
lg (i, A/g)
40
-1.0 -1.5
0.01
x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20
1E-3
-2.0 1E-4 -2.5 0
500
1000
1500
2000
2500
3000
3500
-1.2
-1.1
Fig. 6. Semilogarithmic curves of anodic current versus time of response of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes.
-1.0
-0.9
-0.8
-0.7
Potential (V, vs. SCE)
Time (s) Fig. 8. Tafel electrodes.
curves
of
LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x
alloy
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154 Table 4 Corrosion potential and current of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. Sample
x=0
x = 0.05
x = 0.10
x = 0.15
x = 0.20
Potential (V) Current (mA)
1.012 29.33
1.034 35.51
1.046 36.99
1.058 38.11
1.067 38.61
surface, which is also detrimental to the cycling stability. In addition, Fig. 8 presents the Tafel curves of LaNi3.70Co0.2xMn0.30 Al0.15Cu0.65(Mo0.46Fe0.54)x alloy electrodes. Corrosion potential and current are listed in Table 4. Clearly the corrosion potential of alloy electrodes becomes more negative and the corrosion current increases with increasing x value, indicating the corrosion resistance of alloy electrodes decreases with increasing x value. Thus, it is reasonable to believe that the decrease in corrosion resistance of alloy electrode is one of the critical reasons for the degradation of cycling stability in present work.
4. Conclusions In this paper, microstructures and electrochemical characteristics of LaNi3.70Co0.2xMn0.30Al0.15Cu0.65(Mo0.46Fe0.54)x (x = 0–0.20) hydrogen storage alloys are systematically investigated. The following conclusions can be drawn: 1. Analysis of X-ray diffraction profiles and backscattered electron results show that the pristine alloy is LaNi5 phase, while the alloys containing Mo0.46Fe0.54 consist of LaNi5 matrix phase and secondary Mo phase. The relative abundance of Mo phase increases with the increase in x value. The lattice parameters a, c, c/a and V of LaNi5 phase increase with increasing x value. 2. The activation property of the alloy electrodes is improved by substituting Co with Mo0.46Fe0.54. The maximum discharge capacity of alloy electrodes monotonically decreases from 335.4 (x = 0) to 324.2 (x = 0.20) mA h/g. 3. HRD1200 first increases from 59.8% (x = 0) to 69.6% (x = 0.15), and then decreases to 64.0% (x = 0.20), which is determined by the exchange current density and hydrogen diffusion coefficient. 4. The cycling capacity retention rate at the 100th cycle decreases from 80.4% (x = 0) to 61.9% (x = 0.20), which should be ascribed to the deterioration of corrosion resistance of alloy electrode in the charging–discharging cycle.
Acknowledgements This research is financially supported by the National Natural Science Foundation of China (51471065, U1304522), Program for New Century Excellent Talents in University — China (NCET-110943), Plan for Scientific Innovation Talent of Henan Province — China (144100510009) and Foundation for University Key Teacher in the University of Henan Province — China (2011GGJS052).
References [1] L.Z. Ouyang, C.H. Peng, M. Zhu, A new type of Mg based metal hydride with promising hydrogen storage properties, Int. J. Hydrogen Energy 32 (2007) 3929–3935. [2] X.B. Zhang, D.Z. Sun, W.Y. Yin, Y.J. Chai, M.S. Zhao, Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni3x(Al0.5Mo0.5)x (x = 0–0.4) hydrogen storage alloys, Electrochim. Acta 50 (2005) 3407–3413. [3] Z.J. Cao, L.Z. Ouyang, H. Wang, J.W. Liu, D.L. Sun, Q.A. Zhang, M. Zhu, Structural characteristics and hydrogen storage properties of Sm2Co7, J. Alloys Comp. 608 (2014) 14–18.
153
[4] L.Z. Ouyang, Y.J. Xu, H.W. Dong, L.X. Sun, M. Zhu, Production of hydrogen via hydrolysis of hydrides in Mg–La system, Int. J. Hydrogen Energy 34 (2009) 9671–9676. [5] Y. Fukumoto, M. Miyamoto, M. Matsuoka, C. Iwakura, Effect of the stoichiometric ratio on electrochemical properties of hydrogen storage alloys for nickel-metal hydride batteries, Electrochim. Acta 40 (1995) 845–848. [6] L.Z. Ouyang, Z.J. Cao, L.L. Li, H. Wang, J.W. Liu, D. Min, Y.W. Chen, F.M. Xiao, R.H. Tang, M. Zhu, Enhanced high-rate discharge properties of La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 with added graphene synthesized by plasma milling, Int. J. Hydrogen Energy 39 (2014) 12765–12772. [7] X.D. Wei, P. Zhang, H. Dong, Y.N. Liu, J.W. Zhu, G. Yu, Electrochemical performances of a Co-free La(NiMnAlFe)5 hydrogen storage alloy modified by surface coating with Cu, J. Alloys Comp. 458 (2008) 583–587. [8] C. Iwakura, K. Ohkawa, H. Senoh, H. Inoue, Electrochemical and crystallographic characterization of Co-free hydrogen storage alloys for use in nickel-metal hydride batteries, Electrochim. Acta 46 (2001) 4383–4388. [9] B. Zhang, W.Y. Wu, B. Xue, G.F. Tu, Investigations on the kinetics properties of the electrochemical reactions of MlNi3.55Co0.75xMn0.4Al0.3(Cu0.75P0.25)x (x = 0.0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys, J. Alloys Comp. 538 (2012) 189– 192. [10] X. Tian, G.H. Yun, H.Y. Wang, T. Shang, Z.Q. Yao, W. Wei, X.X. Liang, Preparation and electrochemical properties of La–Mg–Ni-based La0.75Mg0.25Ni3.3Co0.5 multiphase hydrogen storage alloy as negative material of Ni/MH battery, Int. J. Hydrogen Energy 39 (2014) 8474–8481. [11] L. Wang, H.Z. Yan, W. Xiong, B.Q. Li, J. Li, F.Q. Kong, The influence of boron content on the structural and electrochemical properties of the La15Fe77B8type hydrogen storage alloy, J. Power Sources 259 (2014) 213–218. [12] Q.R. Yao, H.Y. Zhou, Z.M. Wang, S.K. Pan, G.H. Rao, Electrochemical properties of the LaNi4.5Co0.25Al0.25 hydrogen storage alloy in wide temperature range, J. Alloys Comp. 606 (2014) 81–85. [13] X.B. Zhang, D.Z. Sun, W.Y. Yin, Y.J. Chai, M.S. Zhao, Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni4.5x(Al0.5Mo0.5)x (x = 0–0.8) hydrogen storage alloys, Eur. J. Inorg. Chem. 11 (2005) 2235–2238. [14] X.B. Zhang, D.Z. Sun, W.Y. Yin, Y.J. Chai, M.S. Zhao, Crystallographic and electrochemical characteristics of La0.7Mg0.3Ni3.5x(Al0.5Mo0.5)x (x = 0–0.8) hydrogen storage alloys, J. Power Sources 154 (2006) 290–297. [15] P. Zhang, Y.N. Liu, J.W. Zhu, X. Wei, G. Yu, Effect of Al and W substitution for Ni on the microstructure and electrochemical properties of La1.3CaMg0.7 Ni9x(Al0.5W0.5)x hydrogen storage alloys, Int. J. Hydrogen Energy 32 (2007) 2488–2493. [16] Y.P. Fan, B.Z. Liu, B.Q. Zhang, L.Q. Ji, Y.G. Wang, Z. Zhang, Microstructures and electrochemical properties of LaNi3.55Co0.2xMn0.35Al0.15Cu0.75(Fe0.43B0.57)x (x = 0–0.20) hydrogen storage alloys, Mater. Chem. Phys. 138 (2013) 803–809. [17] B.Z. Liu, X.Y. Peng, Y.P. Fan, L.Q. Ji, B.Q. Zhang, Z. Zhang, Microstructures and electrochemical properties of LaNi3.55Co0.2xMn0.35Al0.15Cu0.75(V0.81Fe0.19)x hydrogen storage alloys, Int. J. Eletrochem. Sci. 7 (2012) 11966–11977. [18] M.T. Yeh, V.M. Beibutian, S.E. Hsu, Effect of Mo additive on hydrogen absorption of rare-earth based hydrogen storage alloy, J. Alloys Comp. 293– 295 (1999) 721–723. [19] H. Ye, H. Zhang, Development of hydrogen-storage alloys for high-power nickel-metal hydride batteries, Adv. Eng. Mater. 3 (2001) 481–485. [20] S.E. Hsu, V.M. Beibutian, M.T. Yeh, Preparation of hydrogen storage alloys for applications of hydrogen storage and transportation, J. Alloys Comp. 330–332 (2002) 882–885. [21] X.D. Wei, S.S. Liu, H. Dong, P. Zhang, Y.N. Liu, J.W. Zhu, G. Yu, Microstructures and electrochemical properties of Co-free AB5-type hydrogen storage alloys through substitution of Ni by Fe, Electrochim. Acta 52 (2007) 2423–2428. [22] S.X. Yang, Y. Li, Z.P. Liu, S.Q. Yang, S.M. Han, Effect of partial substitution of Cu by Fe on the phase structure and electrochemical property of low-Co AB5 type hydrogen storage alloys, Acta Phys.-Chim. Sin. 26 (2010) 2144–2150. [23] L.Z. Ouyang, J.M. Huang, H. Wang, Y.J. Wen, Q.A. Zhang, D.L. Sun, M. Zhu, Excellent hydrolysis performances of Mg3RE hydrides, Int. J. Hydrogen Energy 38 (2013) 2973–2978. [24] L.Z. Ouyang, Z.J. Cao, H. Wang, J.W. Liu, D.L. Sun, Q.A. Zhang, M. Zhu, Dualtuning effect of In on the thermodynamic and kinetic properties of Mg2Ni dehydrogenation, Int. J. Hydrogen Energy 38 (2013) 8881–8887. [25] L.Z. Ouyang, X.S. Yang, M. Zhu, J.W. Liu, H.W. Dong, D.L. Sun, J. Zou, X.D. Yao, Enhanced hydrogen storage kinetics and stability by synergistic effects of in situ formed CeH2.73 and Ni in CeH2.73–MgH2–Ni nanocomposites, J. Phys. Chem. C 118 (2014) 7808–7820. [26] K. Young, T. Ouchi, B. Reichman, J. Koch, M.A. Fetcenko, Effects of Mo additive on the structure and electrochemical properties of low-temperature AB5 metal hydride alloys, J. Alloys Comp. 509 (2011) 3995–4001. [27] C.Y. Seo, S.J. Choi, J. Choi, C.N. Park, J.Y. Lee, Effect of V and Zr on the electrochemical properties of La-based AB5-type metal hydride electrodes, J. Alloys Comp. 35 (2003) 255–263. [28] R. Li, J.M. Wu, H. Su, S.X. Zhou, Microstructure and electrochemical performance of vanadium-containing AB5-type low-Co intermetallic hydrides, J. Alloys Comp. 421 (2006) 258–267. [29] L.Z. Ouyang, F.X. Qin, M. Zhu, Hydrogen storage behavior of Mg3La and Mg3LaNi0.1, Scr. Mater. 55 (2006) 1075–1078. [30] S.R. Ovshinsky, M.A. Fetcenko, J. Rossm, A nickel metal hydride battery for electric vehicles, Science 260 (1993) 176–181. [31] P.H.L. Notten, P. Hokkeling, Double-phase hydride forming compounds. A new class of highly electrocatalytic materials, J. Electrochem. Soc. 138 (1991) 1877– 1885.
154
J. Sun et al. / Journal of Alloys and Compounds 641 (2015) 148–154
[32] C. Iwakura, H. Senoh, K. Morimot, Y. Hara, H. Inoue, Influence of temperature on discharge process of mischmetal-based hydrogen storage alloy electrodes, Electrochemistry 70 (2002) 2–7. [33] G. Zheng, B.N. Popov, R.E. White, Electrochemical determination of the diffusion coefficient of hydrogen through a LaNi4.25Al0.75 electrode in alkaline aqueous solution, J. Electrochem. Soc. 142 (1995) 2695–2698. [34] C. Khaldi, H. Mathlouthi, J. Lamloumi, A. Percheron-Guegan, Effect of partial substitution of Co with Fe on the properties of LaNi3.55Mn0.4Al0.3Co0.75-xFex (x=0, 0.15, 0.55) alloys electrodes, J. Alloys Comp. 360 (2003) 266–271.
[35] D. Chartouni, F. Meli, A. Züttel, K. Gross, L. Schlapbach, The influence of cobalt on the electrochemical cycling stability of LaNi5-based hydride forming alloys, J. Alloys Comp. 241 (1996) 160–166. [36] B.Z. Liu, G.X. Fan, Y.C. Wang, G.F. Mi, Y.M. Wu, L.M. Wang, Crystallographic and electrochemical characteristics of melt-spun Ti–Zr–Ni–Y alloys, Int. J. Hydrogen Energy 33 (2008) 5801–5805. [37] M. Takao, S. Satoshi, S. Naofumi, Hydrogen absorbing alloy and nickel-metal hydride rechargeable battery, European Patent 1,075,032, B1, 2007.