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Effect of Temperature on Structure and Electrode Properties of LaNi3.8Co0.6Mn0.3M0.3 Hydrogen Storage Alloys
Rare Metal Materials and Engineering Volume 44, Issue 7, July 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(7): 1591-1596
ARTICLE
Effect of Temperature on Structure and Electrode Properties of LaNi3.8Co0.6Mn0.3M0.3 Hydrogen Storage Alloys Li Si,
Zhou Huaiying,
Liu Wenping,
Zhang Huaigang,
Deng Jianqiu,
Wang
Zhongmin Guangxi Experiment Center of Information Science, Guilin University of Electronic Technology, Guilin 541004, China
Abstract: A series of experiments have been performed to investigate the temperature effects on the structure and the electrochemical properties of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) hydrogen storage alloys at different temperatures of 238, 273, 303 and 323 K. Samples A, B and C were used to represent LaNi4.1Co0.6Mn0.3 (Ni substituted alloy), LaNi3.8Co0.6Mn0.3Al0.3 (Al substituted alloy) and LaNi3.8Co0.6Mn0.3Cu0.3 (Cu substituted alloy), respectively. The structures and the electrochemical properties of A, B and C hydrogen storage alloys were investigated by XRD and simulated battery test, respectively. The results reveal that all of the alloys are composed of the homogeneous LaNi5 phase with a CaCu5-type hexagonal structure. The low-temperature properties of the Cu substituted alloy are improved, and the high-temperature discharge capacity of the Al substituted alloy is enhanced. Electrochemical impedance spectroscopy (EIS) analysis shows that the improvement of high-temperature discharge capacity of alloy electrode B is attributed to the formation of dense oxide film to protect the active material from corrosion, which is not favorable to charge-transfer reaction at the interface of electrode-electrolyte. The deterioration in the high-rate dischargeability (HRD) of alloy electrodes B and C are attributed to the degradation of electrochemical kinetics, including both charge-transfer reactions on the electrode surface and hydrogen diffusion in the bulk. Furthermore, the good low-temperature performance of alloy electrode C is due to the improvement of charge-transfer reaction. Key words: hydrogen storage; LaNi3.8Co0.6Mn0.3M0.3 alloy; electrode property; high-rate dischargeability (HRD)
AB5-type alloys as negative electrode materials for Ni-MH rechargeable batteries have been commercialized, which is due to their high discharge capacity, good high-rate dischargeability (HRD), long cyclic stability, less memory effect and environmental friendliness[1,2]. To be used in extreme environment, Ni-MH batteries must have better properties over a wide operating temperature range[3]. However, it is well-known that the operating temperature has an striking effect on the electrochemical performance. That is to say, Ni-MH batteries may suffer a substantial deterioration of electrochemical performance at temperatures below ca. 253 K (winter conditions) or above ca. 313 K (summer conditions). Therefore, it is necessary to improve the electrochemical properties of hydrogen storage alloys as negative electrodes in
a wide temperature range[4,5]. In previous literatures, the addition of Mo could improve the low temperature charge efficiency, due to the enhancement of surface charge-transfer reaction[6,7]. Yang et al. systematically investigated the effects of the replacement of Ni by Fe and Co on electrochemical properties at low temperature[8]. Shen et al. reported the high-temperature (333 K) electrochemical performances of La0.8-xCexMg0.2Ni3.5 alloys[9]. So the elemental substitution in hydrogen storage alloys is one of the methods to improve the electrochemical performance in a wide temperature range. The effects of Al and Cu substitution on the electrochemical properties of AB5 type alloy have been studied by some authors[10,11]. However, there are few reports about the effects of Al and Cu substitution on the wide-temperature electrochemical
Received date: July 14, 2014 Foundation item: National Natural Science Foundation of China (51261003); National Key Basic Research Development Program of China (“973” Program) (2010CB631303); Natural Science Foundation of Guangxi (2012 GXNSFGA060002, 2011GXNSFD018004) Corresponding author: Wang Zhongmin, Ph. D., Professor, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, P. R. China, Tel: 0086-773-2291956, E-mail: [email protected], [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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Li Si et al. / Rare Metal Materials and Engineering, 2015, 44(7): 1591-1596
2 2.1
Results and Discussion Crystal structure
(220)
(301)
(112) (211) (202) (300)
(111) (002) (201)
(110) (200)
The LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloys were prepared by arc-furnace melting at 0.4 MPa argon atmosphere, in order to ensure a high homogeneity; the ingots were turned over and re-melted twice. The purities of all elements are above 99.5wt%. A part of alloy samples were mechanically crushed and ground into the powder of 300 mesh size for electrode preparation and X-ray diffraction (XRD) measurements. The D8 ADVANCED (Cu- Kα radiation) X-ray diffraction was used to study the crystal structure of the alloys. The negative electrodes (metal hydride, MH) were fabricated by the following procedures: 0.2 g of alloy powder and 0.2 g of nickel powder were mixed together by adding 0.02 g of ploytetrafluoroethylene (PTFE). The mixture was pasted onto both sides of nickel foam sheet (1 cm×1 cm), and then the nickel foam was cold pressed at 30 MPa for 3 min. Electrochemical properties were tested in a two-electrode open cell, consisting of a metal hydride working electrode, and a commercial Ni(OH)2 counter. The electrodes were immersed in 6 mol/L KOH solution for about 12 h to ensure that the alloys were wetted completely. Charge and discharge tests were carried out on a computer- controlling Kikusui PFX40W-08 battery testing instrument. For every cycle a charge test was conducted with constant current of 60 mA/g for 6 h; after resting 10 min a discharge test was conducted at the same current to cut-off potential of 0.9 V. To compare the high rate dischargeability (HRD) of the alloy electrodes, the discharge capacities at several specific discharge current densities were measured. The electrochemical impedance spectroscopy (EIS) measurements were conducted from 1 mHz to 100 kHz with an amplitude of 5 mV using the Solartron Modulab electrochemical workstation. Before the EIS measurements, the electrode was firstly discharged to a depth of discharge (DOD) of 50% at 0.2 C. The linear polarization curves were measured by scanning electrode potential at a rate of 0.1 mV·s-1 from –5 mV to 5 mV. The hydrogen diffusion coefficient was determined by a potential step method, which was performed under the constant potential of +500 mV (vs. HgO/Hg) electrode for 2500 s at the fully charged state.
(101)
Experiment
(001)
1
The XRD patterns of A, B and C alloy electrodes are shown in Fig.1. All of the alloys have homogenous LaNi5 phase with a hexagonal CaCu5 crystal structure. The crystal parameters of A, B and C alloys are summarized in Table 1. The lattice parameters a, c and cell volume V are increased by the substitution of Ni with Al and Cu, which is due to the metallic radius of Al (0.118 nm)＞Cu (0.117 nm)＞Ni (0.115 nm). For the value of c/a, C (0.79224)＜A (0.79343)＜B (0.79833), which is attributed to the different sites of Cu and Al occupied. The results suggest that Al-substituted alloy may promote hydrogen atoms to get in and out of the crystal and lower the equilibrium hydrogen absorption pressure[12]. 2.2 Maximum discharge capacity and high-rate dischargeability Fig.2 shows the maximum discharge capacity (Cmax) of alloy electrodes A, B and C at different temperatures. It can be observed that the maximum discharge capacity (Cmax) of the alloy electrode A, B and C increases in the order of A＞B＞C at 273 and 303 K, which may be related to the decrease of interstitial sites to accommodate hydrogen atoms through the substitution of Al and Cu for Ni[13]. The maximum discharge capacity (Cmax) of electrode B is 302 mAh/g, which displays the highest high-temperature discharge capacity among these alloy electrodes at 323 K. However, the discharge capacity of alloy electrode C is the highest at 238 K. The discharge capacity of alloy electrodes A, B and C are 298, 291 and 315 mAh/g, respectively, at 238 K. The effects of temperature on Cmax of alloy electrodes are significant, especially at low and high temperature, which is further investigated using electrochemical impedance spectroscopy. The high-rate dischargeability (HRD) is defined as following:
Intensity/a.u.
properties of AB5-type hydrogen storage alloys. In the present paper, in order to improve the electrochemical properties of hydrogen storage alloys in a wide temperature range, we studied the structure and electrochemical properties of LaNi3.8Co0.6Mn0.3M0.3 by replacing some content of Ni by Al and Cu at different testing temperatures of 238, 273, 303 and 323 K. The corrosion behaviors and the electrochemical kinetics of the electrodes were systematically investigated.
Cu Al
Ni
20
30
40
50
60
70
80
2θ/(°) Fig.1
XRD patterns of LaNi3.8Co0.6Mn0.3M0.3 alloys (sample A, M=Ni; sample B, M=Al; sample C, M=Cu)
Table 1
Crystallographic parameters of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloys
Sample
a/nm
c/nm
c/a
V/nm3
A (M=Ni)
0.50284
0.39897
0.7934
0.08736
B (M=Al)
0.50311
0.40191
0.7989
0.08810
C (M=Cu)
0.50367
0.39903
0.7922
0.08767
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400
Cmax/mAh·g
-1
350 300
298 291
315
336 331
317
334
318 315
302 266
250
density of 2100 mA/g are 48.2%, 39.7% and 25.5%, respectively. Moreover, the values are decreased to 45.9%, 36.3% and 18.8%, respectively, at 323 K at the same discharge current density. The high-rate dischargeability of these electrodes is decreased by Al and Cu substitution, while the discharge ability of alloy electrode C is improved at 238 K. The discharge ability of electrode C gradually becomes better than those of the other two electrodes and the discharge ability of alloy electrode B deteriorates fast with temperature decreasing. The high-rate dischargeability is mainly affected by the charge-transfer reaction on electrode surface and hydrogen diffusion ability of the alloy particles. We will discuss the impact factors of Al and Cu substitution on the high-rate dischargeability of alloy electrodes in 238~323 K temperature range in the following section. 2.3 Electrochemical impedance spectroscopy Fig.4a shows the impedance spectra of alloy electrodes A, B and C (DOD of 50%) at 303 K. The small semicircle at high frequency is attributed to the electrode passivation or an oxide layer on the electrode surface and the large depressed semicircle at middle-frequency region stands for charge transfer on surface, while the straight line relates to the diffusion process[14-17]. The proposed model of equivalent circuit[8] is shown in Fig.4b, where Rb is the resistance of electrolyte, Col and Rol are the oxide layer capacitance and resistance, respectively. Cdl is double layer capacitance, Rct is the charge transfer resistance and Zw is attributed to the diffusion of hydrogen. The constant phase elements Qdl and Qol with the associated impedance, ZCPE=1/[YCPE(jω)n] [18,19] (where j is the
Ni Al Cu 264
200 150 100 50 0
273
238 235
303
323
Temperature/K Fig.2
Cmax of LaNi3.8Co0.6Mn0.3M0.3 alloys electrodes (sample A, M=Ni; sample B, M=Al; sample C, M=Cu) at different temperatures
Fig.3
60 40 20 0
0
300 600 900 1200150018002100 -1 Discharge Current Density/mA·g Ni Al Cu
100 80
c
60 40 20 0
0
150 300 450 600 750 900 1050 -1 Discharge Current Density/mA·g
High Rate Dischargeability/%
80
a
-1
Ni Al Cu
100
Discharge Capacity/mAh·g
High Rate Dischargeability/%
High Rate Dischargeability/%
HRDnC=[CnC/(CnC+C0.2C)]×100% (1) where CnC denotes the capacity at the discharge current density of nC, and C0.2 C is the capacity at the discharge current density of 0.2 C. The discharge current density varies from 0.5 C to 7 C with an interval of 0.5 C at 303 and 323 K, but only varies to 3 C at 273 K. However, the discharge ability at 238 K is inferior. The testing range is only from 0.1 C to 0.5 C, and capacity denotes the high-rate dischargeability. Fig.3 shows the high-rate dischargeability of samples A, B and C at 323, 303, 273 and 238 K. It can be shown that all of the electrodes have their best high-rate dischargeability at 303 K among all the testing temperatures. At 303 K, the values of HRD of the three electrodes A, B, C at a discharge current
Ni Al Cu
100 80
b
60 40 20 0
0
300 600 900 1200 1500 1800 2100 -1 Discharge Current Density/mA·g
340 320 0.1 C 300 0.2 C 280 260 240 220 0.5 C 200 180 160 Ni
d
Al Element
Cu
High-rate dischargeability (HRD) of the LaNi3.8Co0.6Mn0.3M0.3 alloy electrodes alloys (sample A, M=Ni; sample B, M=Al; sample C, M=Cu) at different temperatures: (a) 323 K, (b) 303 K, (c) 273 K, and (d) 238 K
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0.8
-Zim/·cm2
0.7
0.075
0.6
0.050
0.5
0.025
0.4
Ni a Al Cu Fitted data
0.100
1.259 mHz 1.259 mHz
1.259 mHz
0.000 0.375 0.400 0.425 0.450 0.475 0.500 0.525
0.3 0.2 0.1 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2 Zreal/·cm Rb Cdl Col b Rol
Fig.4
Zw
Rct
Nyquist plots of EIS spectra of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloy electrodes at 50% SOC after activation at 303 K (a) and the equivalent circuit of EIS using the circuit description code R(QR)(Q(RQ)) (b)
imaginary number, ω is the angular frequency, ZCPE is the impedance of the constant-phase element, and YCPE is a constant), are introduced to replace Cdl and Col because of the dispersion phenomena. Aw is the modulus of Zw, which is defined by the following equation: Zw=Aw(jω)-n (2) where n is the coefficient of the diffusion impedance. As shown in Fig.4a, the fitted lines are in good agreement with the measured data. The fitted results of alloy electrodes A, B and C at 303 K are shown in Table 2. It is indicated that the Rol of sample B increases and that of sample C decreases, which is Table 2 Sample
-1
Current Density/mA ·g
100 80 60 40 20 0 –-20 –-40 –-60 –-80 –-100
Fitting parameters for EIS spectra of LaNi3.8Co0.6Mn0.3M0.3（M=Ni, Al, Cu）alloy electrodes at 303 K
Qol Yol/Ω ·s ·cm-2 0.51 0.54 0.40 -1
A (M=Ni) B (M=Al) C (M=Cu)
ascribed to the formation of oxide film on the alloy surface. The formation of the dense oxide film on the electrode B results in the grain to be hard corrosion during the charge/ discharge processes. It influences the electrochemical activity on the electrode surface, and thus the Rct increases. The previous studies[20,21] have confirmed the increase of charge-transfer resistance due to the formation of Al-containing oxide film. However, for the alloy electrode C, some segregation of Cu particles filled on the alloy surface is profitable to the charge transfer on the interface of electrode-electrolyte[22,23]. Gamboa et al. [24] have attributed the capacity loss at higher temperature to the slow charge transfer at the interface because of the low conductivity of the electrolyte. While Shen et al. [9] have taken the severe corrosion of electrode active material caused by hot alkaline electrolyte into consideration, which inevitably led to some significant electrochemical performance changes of the alloy electrodes. In the present work, the electrode B has the highest Rct value among all three alloy electrodes, meaning the charge transfer at the interface of electrode-electrolyte is degraded. So the good high-temperature discharge capacity of alloy electrode B is due to the dense oxide film on the electrode B surface, which can protect active material from corrosion. The improvement of the charge-transfer reaction on surface of alloy electrode C may be one of the main factors of improved discharge capacity at low temperature. 2.4 Electrochemical kinetics The exchange current density (I0) of the alloy electrodes is used to characterize the activity of electro-catalytic for chargetransfer reaction on the electrode surface. The polarization resistance (Rp) can be obtained from the slope of the linear polarization curve of an electrode (see Fig.5). I0 can be
-n
nol 0.80 0.84 0.83
Col/μF·cm-2
Rol/Ω·cm2
0.33 0.41 0.23
0.31 0.56 0.17
238 K 273 K 303 K 323 K Ni Ni Ni Ni Al Al Al Al Cu Cu Cu Cu
–-5 –-4 –-3 –-2 –-1 0
1 2 Overpotential/mV
3
4
5
Fig.5 Linear polarization curves of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al,
Qdl Ydl/Ω ·s ·cm-2 0.03 0.05 0.03 -1
-n
ndl 0.80 0.68 0.81
Rct 0.06 0.12 0.06
Warburg (Zw) Yol/Ω ·s-n·cm-2 nol 0.51 0.80 0.54 0.84 0.40 0.83 -1
calculated by the following equation: I0 = RT/FRp (3) where R is the gas constant, T is the absolute temperature and F is the Faraday constant. The results are listed in Table 3. As Rp decreases, I0 increases with increasing temperature from 238 K to 303 K, which indicates the acceleration of charge-transfer reaction on the surface. However, the degradation of chargetransfer reaction at 323 K is due to an increasing hydrogen equilibrium pressure at elevated temperature leading to loss in capacity before the reaction fully takes place[25]. The I0 of electrode C is the highest among all three alloy electrodes at the same temperature as a result of the improvement of the conductivity on the alloy electrode surface.
Cu) alloy electrodes at different temperature
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Table 3 Sample
Electrochemical data of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloy electrodes
Temperature/K
Polarization resistance, Rp/×10-3 Ω
Exchange current density, I0/×10-3A·g-1
Hydrogen diffusion coefficient, D/a2 (×10-4 s-1)
238
330.8
61.9
0.23
273
131.3
179.1
0.51
303
38.5
677.3
0.92
323
40.1
694.7
1.51
238
360.2
56.9
0.11
273
139.6
168.4
0.29
303
45.7
571.0
0.81
323
63.0
441.4
1.16
238
180.9
113.3
0.15
273
115.9
202.8
0.22
303
29.5
882.5
0.62
323
34.4
808.0
0.93
A (M=Ni)
B (M=Al)
C (M=Cu)
Activation energy/ kJ·mol-1
13.8
17.8
13.5
Note: a of hydrogen diffusion coefficient is atomic radius
3
Conclusions
1) The discharge capacity for sample B is improved at high temperature, while that for C at low temperature. The high-rate dischargeability of electrodes B and C are all decreased. The charge transfer resistance of sample C is decreased, but it is increased for B. 2) The degradation of high-temperature high-rate dischargeability is due to the deterioration of charge transfer on electrode surface. The lower high-rate dischargeability at low temperature is attributed to lower charge transfer reaction on electrode surface and hydrogen diffusivity in the alloy bulk. The influencing factors of electrochemical kinetics performance by substitution of Al and Cu for Ni are different. 3) The decrease of high-rate dischargeability for alloy electrode B is attributed to the degradation of the charge-transfer on alloy electrode surface, while the inferior high-rate dischargeability of alloy electrode C is due to the decrease of hydrogen diffusion rate of electrodes. The good low-temperature properties of alloy electrode C are attributed to the improvement of the interface of electrode-electrolyte, where the charge transfer is good.
References 1 Zhang W C, Han S M, Hao J S. Electrochimica Acta[J], 2009, 54: 1383 2 Guo S H, Wang G Q, Zhao D L et al. Rare Metal Materials and Engineering[J] 2011, 40(1): 189 (in Chinese) 3 Nelson R F. Journal of Power Sources[J], 2000, 91: 2 4 Li R, Wu J M, Zhou S X et al. Journal of Rare Earths[J], 2006, 24: 341 5 Wang Z M, Tsai P J, Chan S L I et al. International Journal of Hydrogen Energy[J], 2010, 35: 2033 6 Senoh H, Hara Y, Inoue H et al. Electrochimica Acta[J], 2001,
46: 967 7 Young K, Ouchi T, Reichman B et al. Journal of Alloys Compounds[J], 2011, 509: 3995 8 Yang H, Chen Y G, Tao M D et al. Electrochimica Acta[J], 2010, 55: 648 9 Shen X Q, Chen Y G, Tao M D et al. International Journal of Hydrogen Energy[J], 2009, 34: 3395 10 Bliznakov S, Lefterova E, Dimitrov N. Journal of Power Sources[J], 2008, 176: 381 11 Zhang Y H, Ma Z H, Zhao D L et al. Rare Metal Materials and Engineering[J], 2011, 40: 1693 (in Chinese) 12 Takao M, Satoshi S, Naofumi S. Europe Patent, 1200175: 032 [P], 2001 13 Wei X D, Dong H, Liu Y N. Journal of Alloys Compounds[J], 2009, 481: 687 14 Reilly J J, Adzic G D, Johnson J R et al. Journal of Alloys Compounds[J], 2002, 293: 569 15 Khaldi C, Mathlouthi H, Lamloumi J. Journal of Alloys Compounds[J], 2009, 479: 284 16 Yuan X X, Xu N X. Journal of Alloys Compounds[J], 2001, 316: 113 17 Mathlouthi H, Khaldi C, Moussa M B et al. Journal of Alloys Compounds[J], 2004, 375: 297 18 Sánchez M, Gregori J, Alonso C et al. Electrochimica Acta[J], 2007, 52: 7634 19 Jorcin J B, Orazem M E, Pébère N et al. Electrochimica Acta[J], 2006, 51: 1473 20 Wang Z M, Li C Y V, Zhou H Y. Materials Characterization[J], 2008，59 : 468 21 Wang Z M, Li C Y V, Zhou H Y. International Journal of Hydrogen Energy[J], 2009, 34 : 5422 22 Bliznakov S, Lefterova E, Dimitrov N et al. Journal of Power Sources[J], 2008, 176: 381
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23 Shen X Q, Chen Y G, Tao M D et al. Electrochimica Acta[J], 2009, 54: 2581 24 Gamboa S A , Sebastian P J, Geng M et al. International Journal
of Hydrogen Energy[J], 2001, 26: 1315 25 Raju M, Ananth M V, Vijayaraghavan L. Electrochimica Acta[J], 2009, 54: 1368
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ARTICLE
Effect of Temperature on Structure and Electrode Properties of LaNi3.8Co0.6Mn0.3M0.3 Hydrogen Storage Alloys Li Si,
Zhou Huaiying,
Liu Wenping,
Zhang Huaigang,
Deng Jianqiu,
Wang
Zhongmin Guangxi Experiment Center of Information Science, Guilin University of Electronic Technology, Guilin 541004, China
Abstract: A series of experiments have been performed to investigate the temperature effects on the structure and the electrochemical properties of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) hydrogen storage alloys at different temperatures of 238, 273, 303 and 323 K. Samples A, B and C were used to represent LaNi4.1Co0.6Mn0.3 (Ni substituted alloy), LaNi3.8Co0.6Mn0.3Al0.3 (Al substituted alloy) and LaNi3.8Co0.6Mn0.3Cu0.3 (Cu substituted alloy), respectively. The structures and the electrochemical properties of A, B and C hydrogen storage alloys were investigated by XRD and simulated battery test, respectively. The results reveal that all of the alloys are composed of the homogeneous LaNi5 phase with a CaCu5-type hexagonal structure. The low-temperature properties of the Cu substituted alloy are improved, and the high-temperature discharge capacity of the Al substituted alloy is enhanced. Electrochemical impedance spectroscopy (EIS) analysis shows that the improvement of high-temperature discharge capacity of alloy electrode B is attributed to the formation of dense oxide film to protect the active material from corrosion, which is not favorable to charge-transfer reaction at the interface of electrode-electrolyte. The deterioration in the high-rate dischargeability (HRD) of alloy electrodes B and C are attributed to the degradation of electrochemical kinetics, including both charge-transfer reactions on the electrode surface and hydrogen diffusion in the bulk. Furthermore, the good low-temperature performance of alloy electrode C is due to the improvement of charge-transfer reaction. Key words: hydrogen storage; LaNi3.8Co0.6Mn0.3M0.3 alloy; electrode property; high-rate dischargeability (HRD)
AB5-type alloys as negative electrode materials for Ni-MH rechargeable batteries have been commercialized, which is due to their high discharge capacity, good high-rate dischargeability (HRD), long cyclic stability, less memory effect and environmental friendliness[1,2]. To be used in extreme environment, Ni-MH batteries must have better properties over a wide operating temperature range[3]. However, it is well-known that the operating temperature has an striking effect on the electrochemical performance. That is to say, Ni-MH batteries may suffer a substantial deterioration of electrochemical performance at temperatures below ca. 253 K (winter conditions) or above ca. 313 K (summer conditions). Therefore, it is necessary to improve the electrochemical properties of hydrogen storage alloys as negative electrodes in
a wide temperature range[4,5]. In previous literatures, the addition of Mo could improve the low temperature charge efficiency, due to the enhancement of surface charge-transfer reaction[6,7]. Yang et al. systematically investigated the effects of the replacement of Ni by Fe and Co on electrochemical properties at low temperature[8]. Shen et al. reported the high-temperature (333 K) electrochemical performances of La0.8-xCexMg0.2Ni3.5 alloys[9]. So the elemental substitution in hydrogen storage alloys is one of the methods to improve the electrochemical performance in a wide temperature range. The effects of Al and Cu substitution on the electrochemical properties of AB5 type alloy have been studied by some authors[10,11]. However, there are few reports about the effects of Al and Cu substitution on the wide-temperature electrochemical
Received date: July 14, 2014 Foundation item: National Natural Science Foundation of China (51261003); National Key Basic Research Development Program of China (“973” Program) (2010CB631303); Natural Science Foundation of Guangxi (2012 GXNSFGA060002, 2011GXNSFD018004) Corresponding author: Wang Zhongmin, Ph. D., Professor, School of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, P. R. China, Tel: 0086-773-2291956, E-mail: [email protected], [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
1591
Li Si et al. / Rare Metal Materials and Engineering, 2015, 44(7): 1591-1596
2 2.1
Results and Discussion Crystal structure
(220)
(301)
(112) (211) (202) (300)
(111) (002) (201)
(110) (200)
The LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloys were prepared by arc-furnace melting at 0.4 MPa argon atmosphere, in order to ensure a high homogeneity; the ingots were turned over and re-melted twice. The purities of all elements are above 99.5wt%. A part of alloy samples were mechanically crushed and ground into the powder of 300 mesh size for electrode preparation and X-ray diffraction (XRD) measurements. The D8 ADVANCED (Cu- Kα radiation) X-ray diffraction was used to study the crystal structure of the alloys. The negative electrodes (metal hydride, MH) were fabricated by the following procedures: 0.2 g of alloy powder and 0.2 g of nickel powder were mixed together by adding 0.02 g of ploytetrafluoroethylene (PTFE). The mixture was pasted onto both sides of nickel foam sheet (1 cm×1 cm), and then the nickel foam was cold pressed at 30 MPa for 3 min. Electrochemical properties were tested in a two-electrode open cell, consisting of a metal hydride working electrode, and a commercial Ni(OH)2 counter. The electrodes were immersed in 6 mol/L KOH solution for about 12 h to ensure that the alloys were wetted completely. Charge and discharge tests were carried out on a computer- controlling Kikusui PFX40W-08 battery testing instrument. For every cycle a charge test was conducted with constant current of 60 mA/g for 6 h; after resting 10 min a discharge test was conducted at the same current to cut-off potential of 0.9 V. To compare the high rate dischargeability (HRD) of the alloy electrodes, the discharge capacities at several specific discharge current densities were measured. The electrochemical impedance spectroscopy (EIS) measurements were conducted from 1 mHz to 100 kHz with an amplitude of 5 mV using the Solartron Modulab electrochemical workstation. Before the EIS measurements, the electrode was firstly discharged to a depth of discharge (DOD) of 50% at 0.2 C. The linear polarization curves were measured by scanning electrode potential at a rate of 0.1 mV·s-1 from –5 mV to 5 mV. The hydrogen diffusion coefficient was determined by a potential step method, which was performed under the constant potential of +500 mV (vs. HgO/Hg) electrode for 2500 s at the fully charged state.
(101)
Experiment
(001)
1
The XRD patterns of A, B and C alloy electrodes are shown in Fig.1. All of the alloys have homogenous LaNi5 phase with a hexagonal CaCu5 crystal structure. The crystal parameters of A, B and C alloys are summarized in Table 1. The lattice parameters a, c and cell volume V are increased by the substitution of Ni with Al and Cu, which is due to the metallic radius of Al (0.118 nm)＞Cu (0.117 nm)＞Ni (0.115 nm). For the value of c/a, C (0.79224)＜A (0.79343)＜B (0.79833), which is attributed to the different sites of Cu and Al occupied. The results suggest that Al-substituted alloy may promote hydrogen atoms to get in and out of the crystal and lower the equilibrium hydrogen absorption pressure[12]. 2.2 Maximum discharge capacity and high-rate dischargeability Fig.2 shows the maximum discharge capacity (Cmax) of alloy electrodes A, B and C at different temperatures. It can be observed that the maximum discharge capacity (Cmax) of the alloy electrode A, B and C increases in the order of A＞B＞C at 273 and 303 K, which may be related to the decrease of interstitial sites to accommodate hydrogen atoms through the substitution of Al and Cu for Ni[13]. The maximum discharge capacity (Cmax) of electrode B is 302 mAh/g, which displays the highest high-temperature discharge capacity among these alloy electrodes at 323 K. However, the discharge capacity of alloy electrode C is the highest at 238 K. The discharge capacity of alloy electrodes A, B and C are 298, 291 and 315 mAh/g, respectively, at 238 K. The effects of temperature on Cmax of alloy electrodes are significant, especially at low and high temperature, which is further investigated using electrochemical impedance spectroscopy. The high-rate dischargeability (HRD) is defined as following:
Intensity/a.u.
properties of AB5-type hydrogen storage alloys. In the present paper, in order to improve the electrochemical properties of hydrogen storage alloys in a wide temperature range, we studied the structure and electrochemical properties of LaNi3.8Co0.6Mn0.3M0.3 by replacing some content of Ni by Al and Cu at different testing temperatures of 238, 273, 303 and 323 K. The corrosion behaviors and the electrochemical kinetics of the electrodes were systematically investigated.
Cu Al
Ni
20
30
40
50
60
70
80
2θ/(°) Fig.1
XRD patterns of LaNi3.8Co0.6Mn0.3M0.3 alloys (sample A, M=Ni; sample B, M=Al; sample C, M=Cu)
Table 1
Crystallographic parameters of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloys
Sample
a/nm
c/nm
c/a
V/nm3
A (M=Ni)
0.50284
0.39897
0.7934
0.08736
B (M=Al)
0.50311
0.40191
0.7989
0.08810
C (M=Cu)
0.50367
0.39903
0.7922
0.08767
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400
Cmax/mAh·g
-1
350 300
298 291
315
336 331
317
334
318 315
302 266
250
density of 2100 mA/g are 48.2%, 39.7% and 25.5%, respectively. Moreover, the values are decreased to 45.9%, 36.3% and 18.8%, respectively, at 323 K at the same discharge current density. The high-rate dischargeability of these electrodes is decreased by Al and Cu substitution, while the discharge ability of alloy electrode C is improved at 238 K. The discharge ability of electrode C gradually becomes better than those of the other two electrodes and the discharge ability of alloy electrode B deteriorates fast with temperature decreasing. The high-rate dischargeability is mainly affected by the charge-transfer reaction on electrode surface and hydrogen diffusion ability of the alloy particles. We will discuss the impact factors of Al and Cu substitution on the high-rate dischargeability of alloy electrodes in 238~323 K temperature range in the following section. 2.3 Electrochemical impedance spectroscopy Fig.4a shows the impedance spectra of alloy electrodes A, B and C (DOD of 50%) at 303 K. The small semicircle at high frequency is attributed to the electrode passivation or an oxide layer on the electrode surface and the large depressed semicircle at middle-frequency region stands for charge transfer on surface, while the straight line relates to the diffusion process[14-17]. The proposed model of equivalent circuit[8] is shown in Fig.4b, where Rb is the resistance of electrolyte, Col and Rol are the oxide layer capacitance and resistance, respectively. Cdl is double layer capacitance, Rct is the charge transfer resistance and Zw is attributed to the diffusion of hydrogen. The constant phase elements Qdl and Qol with the associated impedance, ZCPE=1/[YCPE(jω)n] [18,19] (where j is the
Ni Al Cu 264
200 150 100 50 0
273
238 235
303
323
Temperature/K Fig.2
Cmax of LaNi3.8Co0.6Mn0.3M0.3 alloys electrodes (sample A, M=Ni; sample B, M=Al; sample C, M=Cu) at different temperatures
Fig.3
60 40 20 0
0
300 600 900 1200150018002100 -1 Discharge Current Density/mA·g Ni Al Cu
100 80
c
60 40 20 0
0
150 300 450 600 750 900 1050 -1 Discharge Current Density/mA·g
High Rate Dischargeability/%
80
a
-1
Ni Al Cu
100
Discharge Capacity/mAh·g
High Rate Dischargeability/%
High Rate Dischargeability/%
HRDnC=[CnC/(CnC+C0.2C)]×100% (1) where CnC denotes the capacity at the discharge current density of nC, and C0.2 C is the capacity at the discharge current density of 0.2 C. The discharge current density varies from 0.5 C to 7 C with an interval of 0.5 C at 303 and 323 K, but only varies to 3 C at 273 K. However, the discharge ability at 238 K is inferior. The testing range is only from 0.1 C to 0.5 C, and capacity denotes the high-rate dischargeability. Fig.3 shows the high-rate dischargeability of samples A, B and C at 323, 303, 273 and 238 K. It can be shown that all of the electrodes have their best high-rate dischargeability at 303 K among all the testing temperatures. At 303 K, the values of HRD of the three electrodes A, B, C at a discharge current
Ni Al Cu
100 80
b
60 40 20 0
0
300 600 900 1200 1500 1800 2100 -1 Discharge Current Density/mA·g
340 320 0.1 C 300 0.2 C 280 260 240 220 0.5 C 200 180 160 Ni
d
Al Element
Cu
High-rate dischargeability (HRD) of the LaNi3.8Co0.6Mn0.3M0.3 alloy electrodes alloys (sample A, M=Ni; sample B, M=Al; sample C, M=Cu) at different temperatures: (a) 323 K, (b) 303 K, (c) 273 K, and (d) 238 K
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0.8
-Zim/·cm2
0.7
0.075
0.6
0.050
0.5
0.025
0.4
Ni a Al Cu Fitted data
0.100
1.259 mHz 1.259 mHz
1.259 mHz
0.000 0.375 0.400 0.425 0.450 0.475 0.500 0.525
0.3 0.2 0.1 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2 Zreal/·cm Rb Cdl Col b Rol
Fig.4
Zw
Rct
Nyquist plots of EIS spectra of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloy electrodes at 50% SOC after activation at 303 K (a) and the equivalent circuit of EIS using the circuit description code R(QR)(Q(RQ)) (b)
imaginary number, ω is the angular frequency, ZCPE is the impedance of the constant-phase element, and YCPE is a constant), are introduced to replace Cdl and Col because of the dispersion phenomena. Aw is the modulus of Zw, which is defined by the following equation: Zw=Aw(jω)-n (2) where n is the coefficient of the diffusion impedance. As shown in Fig.4a, the fitted lines are in good agreement with the measured data. The fitted results of alloy electrodes A, B and C at 303 K are shown in Table 2. It is indicated that the Rol of sample B increases and that of sample C decreases, which is Table 2 Sample
-1
Current Density/mA ·g
100 80 60 40 20 0 –-20 –-40 –-60 –-80 –-100
Fitting parameters for EIS spectra of LaNi3.8Co0.6Mn0.3M0.3（M=Ni, Al, Cu）alloy electrodes at 303 K
Qol Yol/Ω ·s ·cm-2 0.51 0.54 0.40 -1
A (M=Ni) B (M=Al) C (M=Cu)
ascribed to the formation of oxide film on the alloy surface. The formation of the dense oxide film on the electrode B results in the grain to be hard corrosion during the charge/ discharge processes. It influences the electrochemical activity on the electrode surface, and thus the Rct increases. The previous studies[20,21] have confirmed the increase of charge-transfer resistance due to the formation of Al-containing oxide film. However, for the alloy electrode C, some segregation of Cu particles filled on the alloy surface is profitable to the charge transfer on the interface of electrode-electrolyte[22,23]. Gamboa et al. [24] have attributed the capacity loss at higher temperature to the slow charge transfer at the interface because of the low conductivity of the electrolyte. While Shen et al. [9] have taken the severe corrosion of electrode active material caused by hot alkaline electrolyte into consideration, which inevitably led to some significant electrochemical performance changes of the alloy electrodes. In the present work, the electrode B has the highest Rct value among all three alloy electrodes, meaning the charge transfer at the interface of electrode-electrolyte is degraded. So the good high-temperature discharge capacity of alloy electrode B is due to the dense oxide film on the electrode B surface, which can protect active material from corrosion. The improvement of the charge-transfer reaction on surface of alloy electrode C may be one of the main factors of improved discharge capacity at low temperature. 2.4 Electrochemical kinetics The exchange current density (I0) of the alloy electrodes is used to characterize the activity of electro-catalytic for chargetransfer reaction on the electrode surface. The polarization resistance (Rp) can be obtained from the slope of the linear polarization curve of an electrode (see Fig.5). I0 can be
-n
nol 0.80 0.84 0.83
Col/μF·cm-2
Rol/Ω·cm2
0.33 0.41 0.23
0.31 0.56 0.17
238 K 273 K 303 K 323 K Ni Ni Ni Ni Al Al Al Al Cu Cu Cu Cu
–-5 –-4 –-3 –-2 –-1 0
1 2 Overpotential/mV
3
4
5
Fig.5 Linear polarization curves of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al,
Qdl Ydl/Ω ·s ·cm-2 0.03 0.05 0.03 -1
-n
ndl 0.80 0.68 0.81
Rct 0.06 0.12 0.06
Warburg (Zw) Yol/Ω ·s-n·cm-2 nol 0.51 0.80 0.54 0.84 0.40 0.83 -1
calculated by the following equation: I0 = RT/FRp (3) where R is the gas constant, T is the absolute temperature and F is the Faraday constant. The results are listed in Table 3. As Rp decreases, I0 increases with increasing temperature from 238 K to 303 K, which indicates the acceleration of charge-transfer reaction on the surface. However, the degradation of chargetransfer reaction at 323 K is due to an increasing hydrogen equilibrium pressure at elevated temperature leading to loss in capacity before the reaction fully takes place[25]. The I0 of electrode C is the highest among all three alloy electrodes at the same temperature as a result of the improvement of the conductivity on the alloy electrode surface.
Cu) alloy electrodes at different temperature
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Table 3 Sample
Electrochemical data of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) alloy electrodes
Temperature/K
Polarization resistance, Rp/×10-3 Ω
Exchange current density, I0/×10-3A·g-1
Hydrogen diffusion coefficient, D/a2 (×10-4 s-1)
238
330.8
61.9
0.23
273
131.3
179.1
0.51
303
38.5
677.3
0.92
323
40.1
694.7
1.51
238
360.2
56.9
0.11
273
139.6
168.4
0.29
303
45.7
571.0
0.81
323
63.0
441.4
1.16
238
180.9
113.3
0.15
273
115.9
202.8
0.22
303
29.5
882.5
0.62
323
34.4
808.0
0.93
A (M=Ni)
B (M=Al)
C (M=Cu)
Activation energy/ kJ·mol-1
13.8
17.8
13.5
Note: a of hydrogen diffusion coefficient is atomic radius
3
Conclusions
1) The discharge capacity for sample B is improved at high temperature, while that for C at low temperature. The high-rate dischargeability of electrodes B and C are all decreased. The charge transfer resistance of sample C is decreased, but it is increased for B. 2) The degradation of high-temperature high-rate dischargeability is due to the deterioration of charge transfer on electrode surface. The lower high-rate dischargeability at low temperature is attributed to lower charge transfer reaction on electrode surface and hydrogen diffusivity in the alloy bulk. The influencing factors of electrochemical kinetics performance by substitution of Al and Cu for Ni are different. 3) The decrease of high-rate dischargeability for alloy electrode B is attributed to the degradation of the charge-transfer on alloy electrode surface, while the inferior high-rate dischargeability of alloy electrode C is due to the decrease of hydrogen diffusion rate of electrodes. The good low-temperature properties of alloy electrode C are attributed to the improvement of the interface of electrode-electrolyte, where the charge transfer is good.
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