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Electrochemical performance of TiVNi-Quasicrystal and AB3-Type hydrogen storage alloy composite materials
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
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Electrochemical performance of TiVNi-Quasicrystal and AB3-Type hydrogen storage alloy composite materials Wanqiang Liu a,b, Xinlu Wang a,d, Wen Hu a,d, Yoshiteru Kawabe c, Masaharu Watada c, Limin Wang a,* a
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China c GS Yuasa Corporation, Nishinosho, Kisshoin, Minami-ku, Kyoto 601-8520, Japan d Graduate University of the Chinese Academy of Sciences, Beijing 100049, China b
article info
abstract
Article history:
The Ti1.4V0.6Ni ribbon alloy and AB3-type (La0.65Nd0.12Mg0.23Ni2.9Al0.1) alloy ingot are
Received 2 August 2010
prepared by melt-spinning technique and induction levitation melting technique, respec-
Received in revised form
tively. The Ti1.4V0.6Ni þ 20 wt.% AB3 mixture powders are synthesized by ball-milling the
27 September 2010
above prepared alloy ingots, and their structures and the electrochemical hydrogen storage
Accepted 3 October 2010
properties are investigated. It is found that the icosahedral quasicrystal, Ti2Ni, BCC
Available online 9 November 2010
structural solid solution and AB3-type phases are all presented in the composite material. The maximum electrochemical discharge capacity of the composite electrode is
Keywords:
294.7 mAh/g at the discharge current density of 30 mA/g and 303 K. In addition, the elec-
Ti1.4V0.6Ni alloy
trode made of Ti1.4V0.6Ni and AB3 composite holds better high-rate discharge ability than
Icosahedral quasicrystal
that of Ti1.4V0.6Ni.
AB3-type alloy
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage Electrochemical performance
1.
Introduction
Since icosahedral quasicrystalline phase (I-phase) is observed in 1984, lots of attention was paid to investigate their unusual and interesting chemical and physical properties, such as high hardness [1], low surface friction [2], special transport property [3], as well as high hydrogen storage ability [4] which is stem from the relatively plenty of tetrahedral and octahedral sites available for hydrogen accommodation. I-phase has a new type of translational long-range order but a non-crystallographic rotational symmetry [5]. In general, hydrogen atoms enter a normal lattice, and they prefer to occupy tetrahedral or octahedral sites in hydrogen storage alloys. The local structures
of Ti-based I-phase alloys are believed to be related to the Bergman two-shell atomic cluster, which contains 20 tetrahedral interstitials within its inner shell, 120 between its inner and outer shells [6]. Ti-based I-phase alloy is considered as one of the most promising hydrogen storage materials [7]. Recently, much attention has been focused on the hydrogen storage property of TieZreNi I-phase, because the Ti-based quasicrystals are typically less well ordered and metastable except the TieZreNi one. The low hydrogen equilibrium vapour pressure is the main obstacle for the hydrogenation of TieZreNi I-phase. Kelton and co-workers made a significant breakthrough on it, revealing at modest pressures (6.9e13.8 bar) the presence of a relatively flat plateau during the hydrogenating process for
* Corresponding author. Tel./fax: þ86 431 85262447. E-mail address: [email protected] (L. Wang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.017
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
2.
Fig. 1 e XRD patterns of AB3-type (a), Ti1.4V0.6Ni (b) and Ti1.4V0.6Ni D AB3 (c) alloys.
Ti41.5Zr41.5Ni17 I-phase with its hydrogen concentrations up to H/M ¼ 3 [8,9]. A hydrogen concentration of H/M ¼ 1.9 in the Ti45Zr38Ni17 I-phase was achieved by electrolytic loading [10]. In addition, in order to optimize the substitutional Cu element amount for Ni in Ti45Zr38Ni17 I-phase, Liu et al. investigate the Ti45Zr35Ni17Cu3 I-phase alloy and focused their investigation on the kinetic and electrochemical properties [11]. To date, although the electrochemical performance of hydrogen storage alloys with crystalline phase have been widely investigated, such as AB5 type with CaCu5 type structure [12], AB type with the CsCl type structure [13], and AB2 type with cubic C15 or hexagonal C14 structure [14], there is few report about electrochemical performance of composite material made of quasicrystal and crystallized conventional hydrogen storage alloy. In this paper, we will report the electrochemical performance of composite material of Ti1.4V0.6Ni and AB3-type alloy hydrogen storage alloy.
617
Experimental procedures
The Ti1.4V0.6Ni alloy ribbon of about 2.5 mm in width and 35 mm in thickness was prepared by a single roller meltspinning technique under an argon atmosphere. The circumferential velocity of the copper wheel was 34 m/s. The AB3-type alloy with the composition of La0.65Nd0.12Mg0.23Ni2.9Al0.1 was prepared by induction melting technique under an argon atmosphere. The Ti1.4V0.6Ni and AB3-type alloy samples were mechanically crushed and ground to powder of 200e400 mesh, and mechanical alloying (MA) of 80 wt.% Ti1.4V0.6Ni and 20 wt.% AB3-type alloy powders mixture was carried out in a high-energy ball-milling for 30 min. The phase of the as-obtained ribbons was determined by X-ray diffractometry (XRD), and the microstructure was examined by transmission electron microscopy (TEM). Negative electrodes were constructed through mixing as prepared alloy powders with carbonyl nickel powders in a weight ratio of 1:5. The powder mixture was pressed under 15 MPa pressure into a small pellet of 10 mm in diameter and 1.5 mm consolidated thick. For electrochemical measurement, a half-cell was constructed using a Ni(OH)2/NiOOH electrode as counter electrode, a Hg/HgO electrode as reference electrode and a working electrode in 6 M KOH electrolyte. The electrochemical tests were performed on an automatic galvanostatic chargeedischarge apparatus (DC-5) at 303 K. The electrode was charged at 60 mA/g for 6 h and discharged at 30 mA/g to cut-off voltage of 0.6 V (vs. Hg/HgO). After every charging/discharging, the rest time was 5 min. The electrochemical impedance spectroscopy (EIS) analysis was taken at the 50% depth of discharge (DOD) using a Solartron 1287 Potentiostat/Galvanostat and a Solartron 1255B frequency response analyzer with Z-PLOT software for WINDOWS. In this work potentiostatic discharge technique was selected to estimate the hydrogen diffusion coefficient through a given electrode. After being fully charged followed by 30 min opencircuit lay-aside, the test electrodes were discharged with
Fig. 2 e TEM results of the melt-spun Ti1.4V0.6Ni ribbon. (a) Bright-field image, (b) and (c) correspond to the I-phase and BCC phase.
618
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
Fig. 3 e Discharge capacities as a function cycle number for Ti1.4V0.6Ni (a) and Ti1.4V0.6Ni D AB3 (b) alloys.
þ500 mV potential-step for 3600 s on the EG&G PARC Model 273 Potentiostatic/Galvanostat, using the M352 corrosion software.
3.
Results and discussion
3.1.
Phase structure
Fig. 4 e Electrochemical impedance spectra of Ti1.4V0.6Ni and Ti1.4V0.6Ni D AB3 alloy electrodes at 50% DOD, and the equivalent circuit.
Ni3, LaNi5 and Ti2Ni-type FCC phases, the XRD results (Fig. 1) clearly indicate their existence in the composite sample. This phenomenon is possible because the other crystalline phases usually formed from the liquid under different conditions thus precipitated in different regions of the sample.
Fig. 1 shows the XRD patterns of the Ti1.4V0.6Ni, AB3-type and their composite alloys. The AB3-type is given in Fig. 1(a), it can be seen that the alloy mainly consists of the (La, Mg)Ni3 phase with the PuNi3-type rhombohedral structure (space group R3m) and the LaNi5 phase with the CaCu5 type hexagonal structure (space group P6/mmm). The diffraction peaks in Fig. 1(b) could be indexed to the I-phase phase, Ti2Ni-type face centered cubic (FCC) phase and body centered cubic (BCC) solid solution phase. After ball-milling of the mixture of Ti1.4V0.6Ni and AB3-type alloys (Fig. 1(c)), the diffraction peaks of (La, Mg) Ni3 and LaNi5 phases are becoming obviously broaden, which can be attributed to the decrease of the particle size due to the high-energy balling. Fig. 2(a) is the bright-field image displaying the typical growth morphology and the appearance of the I-phase nodules (indicated as I-phase). As for the point group symmetry of the Iphase and crystalline BCC phase, the expected 5-fold and regular electron diffraction patterns [001] are observed as shown in Fig. 2(b) and (c), respectively. Although there is no electron diffraction patterns corresponding to of the (La, Mg)
3.2.
Discharge capacity and stability
Fig. 3 shows the discharge capacities as a function of cycle number of the Ti1.4V0.6Ni and Ti1.4V0.6Ni þ AB3 alloy negative electrodes. The discharge capacity is greatly improved by adding AB3-type alloy. The discharge capacity reaches its maximum of 272.7 mAh/g in the second cycle when AB3-type alloy not be added, after adding the higher hydrogen storage property (340 mAh/g) of the AB3-type alloy, the first cycle reaches a maximum of 294.7 mAh/g. This indicates that there is a synergistic effect between the AB3-type alloy and Ti1.4V0.6Ni alloy in the composite hydrogen storage alloy electrodes. The capacity decay is relatively serious after the initial first or two activation cycles. In this work, after a preliminary test of 30 consecutive cycles of charging and discharging (100% DOD at 30 mA/g), except the corrosion of active constituent Ti and V during cycling in the electrolyte, a degradation of the local icosahedral order by hydrogen incorporation, pulverization of alloy
Table 1 e High-rate discharge abilities of Ti1.4V0.6Ni and Ti1.4V0.6Ni D AB3 alloy electrodes. Current density (mA/g) a (mAh/g) b (mAh/g)
30
60
90
120
180
240
HRD (C240/C30)
276.3 294.7
248.3 272.4
245.9 266.3
244.6 262.1
230.9 240.9
205.1 229.6
74.2% 77.9%
619
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
Fig. 5 e Semi-logarithmic plots of anodic current vs. time responses of Ti1.4V0.6Ni and Ti1.4V0.6Ni D AB3 alloy electrodes.
particles, increasing of passive oxidation layer on the particle surface, and local heating may also account for the discharge capacity degradation. After adding AB3-type alloy, cyclic stability of alloy electrode is little improved. Therefore, the cycle performance of the Ti1.4V0.6Ni alloy needed to be further improved with multicomponent composition and surface modification to fulfil requirement as electrode in the practical application of Ni/MH batteries.
3.3.
electrochemical reaction at the surface, wherein R1 is the electrolyte resistance, R2 and C1 characterized the contact resistance and contact capacitance between the current collector and the alloy pellet, respectively. The contact resistance and the contact capacitance between alloy powders in the electrode pellet are described by R3 and C2, respectively. Rct and C3 presented the charge-transfer resistance and the double-layer capacitance, respectively. Wo is the Warburg resistance. On the basis of the circuit the charge-transfer resistances Rct was obtained by means of fitting program ZVIEW. The calculated results show that the Rct of alloy electrodes increased from 0.148 (b) to 0.218U (a). It is well known that the charge-transfer resistance and the exchange current density are determined by the electro-catalytic activity. Ti1.4V0.6Ni alloy electrode after adding AB3-type alloy led to the decrease in the Rct. However, the results of electrochemical impedance measurement differed from those of HRD, suggesting that the surface charge-transfer might not be the rate-determining step. The diffusion coefficient of hydrogen in the bulk electrodes is determined by the potential-step method. Fig. 5 shows semi-logarithmic plots of the anodic current vs. clearly from the spectra, the current-time response could be divided into three time domains after the application of overpotential. According to the model of Zheng et al. [17], the hydrogen diffusion coefficient in the bulk electrode, which is used to characterize the diffusion rate of hydrogen, can be estimated through the slope of the linear region of the corresponding plots by following formula:
High-rate discharge ability and kinetic properties log i ¼ log
The high-rate discharge abilities (HRD) of Ti1.4V0.6Ni (a) and Ti1.4V0.6Ni þ AB3 (b) negative electrodes are summarized in Table 1. It is found that the electrode made of Ti1.4V0.6Ni and AB3 composite holds better high-rate discharge ability than that of Ti1.4V0.6Ni. It is well known that the HRD of the metal-hydride (MH) electrodes is influenced mainly by the charge-transfer reaction occurring at the electrode/electrolyte interface and by the diffusion rate of hydrogen in the electrodes [15]. To evaluate electrochemical kinetics of the Ti1.4V0.6Ni þ AB3 and Ti1.4V0.6Ni alloy electrodes, electrochemical impedance and potential-step spectra is measured. The electrochemical impedance spectra of all the electrode samples at 50% DOD are illustrated in Fig. 4. It can be found that all the EIS curves consisted of two semicircles followed with a straight line. According to the analysis model proposed by Kuriyama et al. [16], relatively larger semicircle in the mediumfrequency region represents the charge-transfer resistance for
6FDðC0 CS Þ p2 Dt 2 da 2:303a2
(1)
wherein D is the hydrogen diffusion coefficient (cm2/s); a the radius of the spherical particle (cm); i the diffusion current density (A/g); C0 the initial hydrogen concentration in the bulk electrode (mol/cm3); Cs the hydrogen concentration on the surface of alloy particles (mol/cm3); d the density of the hydrogen storage materials (g/cm3); and t is the discharge time. Assuming that the average particle radius was 15 mm, the hydrogen diffusion coefficient D in the bulk electrodes was estimated by Eq. (1) and also listed in Table 2. The D value is in the order of sample Ti1.4V0.6Ni þ AB3 > Ti1.4V0.6Ni, which is consistent with that of the high-rate of discharge ability. This suggested that diffusion processes are dominant in controlling the electrochemical reaction. As mentioned above, the AB3-type alloy facilitated atomic hydrogen transportation, more content would be beneficial to the improvement of electrochemical kinetics.
Table 2 e Electrochemical behaviours of Ti1.4V0.6Ni (a) and Ti1.4V0.6Ni D AB3 (b) alloy electrodes. Electrode a b
Cycling stability Sn (Cn/Cmax) (30)
Charge-transfer resistance, Rct(U)
Hydrogen diffusion coefficient D (1011 cm2/s)
79.4 72.2
0.218 0.196
6.47 7.66
620
4.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
Conclusions
In this work, composite material made of quasicrystal Ti1.4V0.6Ni alloy and conventional AB3 alloy are prepared. It is found that the prepared sample is composed of the icosahedral quasicrystal, Ti2Ni, BCC structural solid solution and AB3-type phases. As for the electrochemical performance, its maximum electrochemical discharge capacity can reach 294.7 mAh/g discharged at 30 mA/g and 303 K. Moreover, the electrode made of Ti1.4V0.6Ni and AB3 composite holds better high-rate discharge ability than that of Ti1.4V0.6Ni. It is considered that this high capacity is due to the AB3 alloy additives, which play a synergistic effect in electrochemical reaction kinetics.
Acknowledgements This work is financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation (20921002) and GS Yuasa Corporation Japan.
references
[1] Yi S, Kim K, Fleury E, Kim W, Kim D. Fabrication of a bulk icosahedral material through mechanical alloying of the powder mixture Ti41.5Zr41.5Ni17. Mater Lett 2002;52:75e9. [2] Zhang L, Zhang H, Zhou Q, Dong C. Friction measurement on AleCueFe quasicrystalline and B2-typematerials. Wear 1999; 225:784e8. [3] Klein T, Berger C, Mayou D, Cyrot-Lackmann F. Proximity of a metal-insulator transition in icosahedral phases of high structural quality. Phys Rev Lett 1991;66:2907e10. [4] Stroud RM, Viano AM, Gibbons PC, Kelton KF. Stable Ti-based quasicrystal offers prospect for improved hydrogen storage. Appl Phys Lett 1996;69:2998e3000.
[5] Levine D, Steinhardt PJ. A new class of ordered structure. Phys Rev Lett 1984;53:2477e80. [6] Takasaki A, Han CH, Furuya Y, Kelton KF. Synthesis of amorphous and quasicrystal phases by mechanical alloying of Ti45Zr38Ni17 powder mixtures, and their hydrogenation. Philos Mag Lett 2002;82:353e61. [7] Kelton KF, Kim WJ, Stroud RM. A stable Ti-base quasicrystals. Appl Phys Lett 1997;70:3230e2. [8] Kelton KF. Ti/Zr/Hf-based quasicrystals. Mat Sci Eng A-Struct 2004;375:31e7. [9] Kelton KF, Hartzell JJ, Hennig RG, Huett VT, Takasaki A. Hydrogen storage in Ti-Zr and Ti-Hf-based quasicrystals. Philos Mag 2006;86:957e64. [10] Majzoub EH, Kim JY, Hennig RG, Kelton KF, Gibbons PC, Yelon WB. Cluster structure and hydrogen in TieZreNi quasicrystals and approximants. Mat Sci Eng A-Struct 2000; 294e296:108e11. [11] Liu BZ, Wu YM, Wang LM. Kinetic and electrochemical properties of icosahedral quasicrystalline Ti45Zr35Ni17Cu3 powder. Int J Hydrogen Energy 2006;31:1394e400. [12] Ivey DG, Northwood DO. Storing energy in metal hydrides: a review of the physical metallurgy. J Mater Sci 1983;18: 321e47. [13] Lee SM, Perng TP. Effects of boron and carbon on the hydrogenation properties of TiFe and Ti1.1Fe. Int J Hydrogen Energy 2000;25:831e6. [14] Prakash M, Ramaprabhu S. Hydrogen absorption studies in ZrMnFe0.7Co0.3. Int J Hydrogen Energy 2000;25:871e8. [15] Sakai T, Miyamura H, Kuriyama N, Kato A, Oguro K, Ishikawa H. The influence of small amounts of added elements on various anode performance characteristics for LaNi2.5Co2.5based alloys. J Less-Common Met 1990;159:127e9. [16] Kuriyama N, Sakai T, Miyamura H, Uehara I, Ishikawa H. Electrochemical impedance and deterioration behavior of metal hydride electrodes. J Alloys Compd 1993;202: 183e97. [17] Zheng G, Popov BN, White RE. Electrochemical determination of the diffusion coefficient of hydrogen through an LaNi4.25Al0.75 electrode in alkaline aqueous Solution. J Electrochem Soc 1995;142:2695e9.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Electrochemical performance of TiVNi-Quasicrystal and AB3-Type hydrogen storage alloy composite materials Wanqiang Liu a,b, Xinlu Wang a,d, Wen Hu a,d, Yoshiteru Kawabe c, Masaharu Watada c, Limin Wang a,* a
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, China c GS Yuasa Corporation, Nishinosho, Kisshoin, Minami-ku, Kyoto 601-8520, Japan d Graduate University of the Chinese Academy of Sciences, Beijing 100049, China b
article info
abstract
Article history:
The Ti1.4V0.6Ni ribbon alloy and AB3-type (La0.65Nd0.12Mg0.23Ni2.9Al0.1) alloy ingot are
Received 2 August 2010
prepared by melt-spinning technique and induction levitation melting technique, respec-
Received in revised form
tively. The Ti1.4V0.6Ni þ 20 wt.% AB3 mixture powders are synthesized by ball-milling the
27 September 2010
above prepared alloy ingots, and their structures and the electrochemical hydrogen storage
Accepted 3 October 2010
properties are investigated. It is found that the icosahedral quasicrystal, Ti2Ni, BCC
Available online 9 November 2010
structural solid solution and AB3-type phases are all presented in the composite material. The maximum electrochemical discharge capacity of the composite electrode is
Keywords:
294.7 mAh/g at the discharge current density of 30 mA/g and 303 K. In addition, the elec-
Ti1.4V0.6Ni alloy
trode made of Ti1.4V0.6Ni and AB3 composite holds better high-rate discharge ability than
Icosahedral quasicrystal
that of Ti1.4V0.6Ni.
AB3-type alloy
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage Electrochemical performance
1.
Introduction
Since icosahedral quasicrystalline phase (I-phase) is observed in 1984, lots of attention was paid to investigate their unusual and interesting chemical and physical properties, such as high hardness [1], low surface friction [2], special transport property [3], as well as high hydrogen storage ability [4] which is stem from the relatively plenty of tetrahedral and octahedral sites available for hydrogen accommodation. I-phase has a new type of translational long-range order but a non-crystallographic rotational symmetry [5]. In general, hydrogen atoms enter a normal lattice, and they prefer to occupy tetrahedral or octahedral sites in hydrogen storage alloys. The local structures
of Ti-based I-phase alloys are believed to be related to the Bergman two-shell atomic cluster, which contains 20 tetrahedral interstitials within its inner shell, 120 between its inner and outer shells [6]. Ti-based I-phase alloy is considered as one of the most promising hydrogen storage materials [7]. Recently, much attention has been focused on the hydrogen storage property of TieZreNi I-phase, because the Ti-based quasicrystals are typically less well ordered and metastable except the TieZreNi one. The low hydrogen equilibrium vapour pressure is the main obstacle for the hydrogenation of TieZreNi I-phase. Kelton and co-workers made a significant breakthrough on it, revealing at modest pressures (6.9e13.8 bar) the presence of a relatively flat plateau during the hydrogenating process for
* Corresponding author. Tel./fax: þ86 431 85262447. E-mail address: [email protected] (L. Wang). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.10.017
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
2.
Fig. 1 e XRD patterns of AB3-type (a), Ti1.4V0.6Ni (b) and Ti1.4V0.6Ni D AB3 (c) alloys.
Ti41.5Zr41.5Ni17 I-phase with its hydrogen concentrations up to H/M ¼ 3 [8,9]. A hydrogen concentration of H/M ¼ 1.9 in the Ti45Zr38Ni17 I-phase was achieved by electrolytic loading [10]. In addition, in order to optimize the substitutional Cu element amount for Ni in Ti45Zr38Ni17 I-phase, Liu et al. investigate the Ti45Zr35Ni17Cu3 I-phase alloy and focused their investigation on the kinetic and electrochemical properties [11]. To date, although the electrochemical performance of hydrogen storage alloys with crystalline phase have been widely investigated, such as AB5 type with CaCu5 type structure [12], AB type with the CsCl type structure [13], and AB2 type with cubic C15 or hexagonal C14 structure [14], there is few report about electrochemical performance of composite material made of quasicrystal and crystallized conventional hydrogen storage alloy. In this paper, we will report the electrochemical performance of composite material of Ti1.4V0.6Ni and AB3-type alloy hydrogen storage alloy.
617
Experimental procedures
The Ti1.4V0.6Ni alloy ribbon of about 2.5 mm in width and 35 mm in thickness was prepared by a single roller meltspinning technique under an argon atmosphere. The circumferential velocity of the copper wheel was 34 m/s. The AB3-type alloy with the composition of La0.65Nd0.12Mg0.23Ni2.9Al0.1 was prepared by induction melting technique under an argon atmosphere. The Ti1.4V0.6Ni and AB3-type alloy samples were mechanically crushed and ground to powder of 200e400 mesh, and mechanical alloying (MA) of 80 wt.% Ti1.4V0.6Ni and 20 wt.% AB3-type alloy powders mixture was carried out in a high-energy ball-milling for 30 min. The phase of the as-obtained ribbons was determined by X-ray diffractometry (XRD), and the microstructure was examined by transmission electron microscopy (TEM). Negative electrodes were constructed through mixing as prepared alloy powders with carbonyl nickel powders in a weight ratio of 1:5. The powder mixture was pressed under 15 MPa pressure into a small pellet of 10 mm in diameter and 1.5 mm consolidated thick. For electrochemical measurement, a half-cell was constructed using a Ni(OH)2/NiOOH electrode as counter electrode, a Hg/HgO electrode as reference electrode and a working electrode in 6 M KOH electrolyte. The electrochemical tests were performed on an automatic galvanostatic chargeedischarge apparatus (DC-5) at 303 K. The electrode was charged at 60 mA/g for 6 h and discharged at 30 mA/g to cut-off voltage of 0.6 V (vs. Hg/HgO). After every charging/discharging, the rest time was 5 min. The electrochemical impedance spectroscopy (EIS) analysis was taken at the 50% depth of discharge (DOD) using a Solartron 1287 Potentiostat/Galvanostat and a Solartron 1255B frequency response analyzer with Z-PLOT software for WINDOWS. In this work potentiostatic discharge technique was selected to estimate the hydrogen diffusion coefficient through a given electrode. After being fully charged followed by 30 min opencircuit lay-aside, the test electrodes were discharged with
Fig. 2 e TEM results of the melt-spun Ti1.4V0.6Ni ribbon. (a) Bright-field image, (b) and (c) correspond to the I-phase and BCC phase.
618
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
Fig. 3 e Discharge capacities as a function cycle number for Ti1.4V0.6Ni (a) and Ti1.4V0.6Ni D AB3 (b) alloys.
þ500 mV potential-step for 3600 s on the EG&G PARC Model 273 Potentiostatic/Galvanostat, using the M352 corrosion software.
3.
Results and discussion
3.1.
Phase structure
Fig. 4 e Electrochemical impedance spectra of Ti1.4V0.6Ni and Ti1.4V0.6Ni D AB3 alloy electrodes at 50% DOD, and the equivalent circuit.
Ni3, LaNi5 and Ti2Ni-type FCC phases, the XRD results (Fig. 1) clearly indicate their existence in the composite sample. This phenomenon is possible because the other crystalline phases usually formed from the liquid under different conditions thus precipitated in different regions of the sample.
Fig. 1 shows the XRD patterns of the Ti1.4V0.6Ni, AB3-type and their composite alloys. The AB3-type is given in Fig. 1(a), it can be seen that the alloy mainly consists of the (La, Mg)Ni3 phase with the PuNi3-type rhombohedral structure (space group R3m) and the LaNi5 phase with the CaCu5 type hexagonal structure (space group P6/mmm). The diffraction peaks in Fig. 1(b) could be indexed to the I-phase phase, Ti2Ni-type face centered cubic (FCC) phase and body centered cubic (BCC) solid solution phase. After ball-milling of the mixture of Ti1.4V0.6Ni and AB3-type alloys (Fig. 1(c)), the diffraction peaks of (La, Mg) Ni3 and LaNi5 phases are becoming obviously broaden, which can be attributed to the decrease of the particle size due to the high-energy balling. Fig. 2(a) is the bright-field image displaying the typical growth morphology and the appearance of the I-phase nodules (indicated as I-phase). As for the point group symmetry of the Iphase and crystalline BCC phase, the expected 5-fold and regular electron diffraction patterns [001] are observed as shown in Fig. 2(b) and (c), respectively. Although there is no electron diffraction patterns corresponding to of the (La, Mg)
3.2.
Discharge capacity and stability
Fig. 3 shows the discharge capacities as a function of cycle number of the Ti1.4V0.6Ni and Ti1.4V0.6Ni þ AB3 alloy negative electrodes. The discharge capacity is greatly improved by adding AB3-type alloy. The discharge capacity reaches its maximum of 272.7 mAh/g in the second cycle when AB3-type alloy not be added, after adding the higher hydrogen storage property (340 mAh/g) of the AB3-type alloy, the first cycle reaches a maximum of 294.7 mAh/g. This indicates that there is a synergistic effect between the AB3-type alloy and Ti1.4V0.6Ni alloy in the composite hydrogen storage alloy electrodes. The capacity decay is relatively serious after the initial first or two activation cycles. In this work, after a preliminary test of 30 consecutive cycles of charging and discharging (100% DOD at 30 mA/g), except the corrosion of active constituent Ti and V during cycling in the electrolyte, a degradation of the local icosahedral order by hydrogen incorporation, pulverization of alloy
Table 1 e High-rate discharge abilities of Ti1.4V0.6Ni and Ti1.4V0.6Ni D AB3 alloy electrodes. Current density (mA/g) a (mAh/g) b (mAh/g)
30
60
90
120
180
240
HRD (C240/C30)
276.3 294.7
248.3 272.4
245.9 266.3
244.6 262.1
230.9 240.9
205.1 229.6
74.2% 77.9%
619
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
Fig. 5 e Semi-logarithmic plots of anodic current vs. time responses of Ti1.4V0.6Ni and Ti1.4V0.6Ni D AB3 alloy electrodes.
particles, increasing of passive oxidation layer on the particle surface, and local heating may also account for the discharge capacity degradation. After adding AB3-type alloy, cyclic stability of alloy electrode is little improved. Therefore, the cycle performance of the Ti1.4V0.6Ni alloy needed to be further improved with multicomponent composition and surface modification to fulfil requirement as electrode in the practical application of Ni/MH batteries.
3.3.
electrochemical reaction at the surface, wherein R1 is the electrolyte resistance, R2 and C1 characterized the contact resistance and contact capacitance between the current collector and the alloy pellet, respectively. The contact resistance and the contact capacitance between alloy powders in the electrode pellet are described by R3 and C2, respectively. Rct and C3 presented the charge-transfer resistance and the double-layer capacitance, respectively. Wo is the Warburg resistance. On the basis of the circuit the charge-transfer resistances Rct was obtained by means of fitting program ZVIEW. The calculated results show that the Rct of alloy electrodes increased from 0.148 (b) to 0.218U (a). It is well known that the charge-transfer resistance and the exchange current density are determined by the electro-catalytic activity. Ti1.4V0.6Ni alloy electrode after adding AB3-type alloy led to the decrease in the Rct. However, the results of electrochemical impedance measurement differed from those of HRD, suggesting that the surface charge-transfer might not be the rate-determining step. The diffusion coefficient of hydrogen in the bulk electrodes is determined by the potential-step method. Fig. 5 shows semi-logarithmic plots of the anodic current vs. clearly from the spectra, the current-time response could be divided into three time domains after the application of overpotential. According to the model of Zheng et al. [17], the hydrogen diffusion coefficient in the bulk electrode, which is used to characterize the diffusion rate of hydrogen, can be estimated through the slope of the linear region of the corresponding plots by following formula:
High-rate discharge ability and kinetic properties log i ¼ log
The high-rate discharge abilities (HRD) of Ti1.4V0.6Ni (a) and Ti1.4V0.6Ni þ AB3 (b) negative electrodes are summarized in Table 1. It is found that the electrode made of Ti1.4V0.6Ni and AB3 composite holds better high-rate discharge ability than that of Ti1.4V0.6Ni. It is well known that the HRD of the metal-hydride (MH) electrodes is influenced mainly by the charge-transfer reaction occurring at the electrode/electrolyte interface and by the diffusion rate of hydrogen in the electrodes [15]. To evaluate electrochemical kinetics of the Ti1.4V0.6Ni þ AB3 and Ti1.4V0.6Ni alloy electrodes, electrochemical impedance and potential-step spectra is measured. The electrochemical impedance spectra of all the electrode samples at 50% DOD are illustrated in Fig. 4. It can be found that all the EIS curves consisted of two semicircles followed with a straight line. According to the analysis model proposed by Kuriyama et al. [16], relatively larger semicircle in the mediumfrequency region represents the charge-transfer resistance for
6FDðC0 CS Þ p2 Dt 2 da 2:303a2
(1)
wherein D is the hydrogen diffusion coefficient (cm2/s); a the radius of the spherical particle (cm); i the diffusion current density (A/g); C0 the initial hydrogen concentration in the bulk electrode (mol/cm3); Cs the hydrogen concentration on the surface of alloy particles (mol/cm3); d the density of the hydrogen storage materials (g/cm3); and t is the discharge time. Assuming that the average particle radius was 15 mm, the hydrogen diffusion coefficient D in the bulk electrodes was estimated by Eq. (1) and also listed in Table 2. The D value is in the order of sample Ti1.4V0.6Ni þ AB3 > Ti1.4V0.6Ni, which is consistent with that of the high-rate of discharge ability. This suggested that diffusion processes are dominant in controlling the electrochemical reaction. As mentioned above, the AB3-type alloy facilitated atomic hydrogen transportation, more content would be beneficial to the improvement of electrochemical kinetics.
Table 2 e Electrochemical behaviours of Ti1.4V0.6Ni (a) and Ti1.4V0.6Ni D AB3 (b) alloy electrodes. Electrode a b
Cycling stability Sn (Cn/Cmax) (30)
Charge-transfer resistance, Rct(U)
Hydrogen diffusion coefficient D (1011 cm2/s)
79.4 72.2
0.218 0.196
6.47 7.66
620
4.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 6 1 6 e6 2 0
Conclusions
In this work, composite material made of quasicrystal Ti1.4V0.6Ni alloy and conventional AB3 alloy are prepared. It is found that the prepared sample is composed of the icosahedral quasicrystal, Ti2Ni, BCC structural solid solution and AB3-type phases. As for the electrochemical performance, its maximum electrochemical discharge capacity can reach 294.7 mAh/g discharged at 30 mA/g and 303 K. Moreover, the electrode made of Ti1.4V0.6Ni and AB3 composite holds better high-rate discharge ability than that of Ti1.4V0.6Ni. It is considered that this high capacity is due to the AB3 alloy additives, which play a synergistic effect in electrochemical reaction kinetics.
Acknowledgements This work is financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation (20921002) and GS Yuasa Corporation Japan.
references
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