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Crystallographic and electrochemical characteristics of Ti45Zr35Ni17Cu3 quasicrystalline alloy ball-milled with nickel powder
Electrochimica Acta 51 (2006) 3586–3591
Crystallographic and electrochemical characteristics of Ti45Zr35Ni17Cu3 quasicrystalline alloy ball-milled with nickel powder Baozhong Liu a,b , Jianli Wang a,b , Yaoming Wu a , Limin Wang a,∗ a
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, CAS, 5625 Renmin Street, Changchun 130022, China b Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China Received 28 September 2005; received in revised form 12 October 2005; accepted 12 October 2005 Available online 15 November 2005
Abstract Crystallographic and electrochemical characteristics of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi (x = 0, 5, 10, 15 and 20 mass%) composite powders have been investigated. The powders are composed of amorphous, I- and Ni-phases when x increases from 5 to 20. With increasing x, the amount of Ni-phase increases but the quasi-lattice constant decreases. The maximum discharge capacity first increases as x increases from 0 to 15 and then decreases when x increases further from 15 to 20. The high-rate dischargeability and cycling stability increase monotonically with increasing x. The improvement of the electrochemical characteristics is ascribed to the metallic nickel particles highly dispersed in the alloys, which improves the electrochemical kinetic properties and prevents the oxidation of the alloy electrodes, as well as to the mixed structure of amorphous and icosahedral quasicrystalline phases, which enhances the hydrogen diffusivity in the bulk of the alloy electrodes and efficiently inhibits the pulverization of the alloy particles. © 2005 Elsevier Ltd. All rights reserved. Keywords: Ti–Zr–Ni–Cu alloy; Icosahedral quasicrystal; Ball-milling; Electrochemical characteristics; Metal hydride electrode
1. Introduction Icosahedral quasicrystalline phase (I-phase) showing the rotational symmetry of icosahedral point group is likely dominated by local tetrahedral [1]. The dominance of polytetrahedral order makes I-phase potentially used for hydrogen storage application as energy storage media or battery electrode. Ti-based I-phase has attracted much attention because of high hydrogen capacity, low cost and thermodynamical stability [2]. However, it was difficult for practical application because of low equilibrium plateau pressure [3] and the instability of I-phase due to high temperature of hydrogen desorption [1,4]. Electrochemical methods may enable the hydride of Ti–Zr–Ni–Cu I-phase alloy to dissociate at moderate condition and reversibly charge/discharge without undergoing any structural phase transitions due to very low atmospheric pressure can be obtained [5]. Thus, it is very promising that Ti-base I-phase alloy is applied as negative electrode materials of nickel-metal hydride (MH) battery. ∗
Corresponding author. Tel.: +86 431 5262447; fax: +86 431 5698041. E-mail address: [email protected] (L. Wang).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.10.014
In the condition of thermodynamic equilibrium, the hydrogen storage capacity of TiZrNi I-phase alloy can reach two hydrogen atoms per metal atom, which was higher than that of LaNi5 and TiFe compounds [3]. Takasaki et al. [4] have prepared the quasicrystalline Ti45 Zr38 Ni17 powder by mechanical alloying and subsequent annealing, and the maximum absorbing capacity with an atom ratio of hydrogen to metal ([H]/[M]) is 1.5. Majzoub et al. [6] have reported the hydrogen to metal atom ratio of 1.9 was obtained when electrochemical method was used to hydrogenate Ti45 Zr38 Ni17 I-phase alloy. In our previous work, Ti45 Zr35 Ni17 Cu3 single I-phase powder has been synthesized and investigated by electrochemical charging/discharging method, and the results showed I-phase was steady upon charging/discharging cycles. Unfortunately, the maximum discharge capacity of Ti45 Zr35 Ni17 Cu3 I-phase alloy electrode was so far less than the theoretical capacity. It is necessary to further improve its electrochemical properties for the practical application. It is well known that metallic nickel is a good electrocatalyst and it is important to improve electrochemical characteristics of MH electrodes [7]. Furthermore, ball-milling technique is very effective to modify the structure and surface properties for
B. Liu et al. / Electrochimica Acta 51 (2006) 3586–3591
improving physical and chemical properties. Recently, Jurczyk et al. [8] and Lee et al. [9] have investigated the electrochemical properties of Ti/Zr-based AB2 alloys ball-milled with additive nickel powder, and the results showed the multi-phase structure and refined grain efficiently enhanced the electrochemical discharge capacity and cycling stability. On the basis of our previous studies and the belief that the electrochemical characteristics of the Ti45 Zr35 Ni17 Cu3 Iphase alloy electrodes can be enhanced by ball-milling with nickel powder, crystallographic and electrochemical characteristics of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi (x = 0, 5, 10, 15 and 20 mass%) composite alloys have been investigated. 2. Experimental procedures Elemental powders of Ti (100 mesh, 99.9%), Zr (100 mesh, 99.9%), Ni (150 mesh, 99.9%) and Cu (250 mesh, 99.99%) were used as starting materials in this study. A powder mixture with a desired composition of Ti45 Zr35 Ni17 Cu3 was poured into the milling container. Mechanical alloying (MA) was carried out in a vibratory ball-miller under the vibration frequency of 25 Hz and amplitude of 2.5 mm for 280 h in an argon atmosphere. The stainless steel vial and stainless balls were utilized, and the ball to powder mass ratio was 25:1. The powders after MA were sealed under dynamic vacuum (<10−1 Pa) in a fused silica tube, and then annealed at 855 K for 30 min. The Ti45 Zr35 Ni17 Cu3 + xNi (x = 5, 10, 15 and 20 mass%) mixture powders were mechanically milled under high-purity argon in Spex800 ball-miller. The ball to powder mass ratio was 20:1, and the ball-milling time was 180 min. The Ti45 Zr35 Ni17 Cu3 I-phase alloy powder before ball-milling was represented as x = 0. Electrochemical charging/discharging tests were performed on an automatic galvanostatic system (DC-5) at 313 K in 6 M KOH electrolyte using a standard open three-electrode cell. The working electrodes were fabricated by compressing a mixture of 0.15 g alloy powder (200–400 mesh) and 0.75 g nickel carbonyl powder into a pellet of 10 mm diameter under a pressure of 15 MPa. The counter electrode was a Ni(OH)2 /NiOOH electrode and the reference electrode was a Hg/HgO electrode. Each electrode was charged for 7 h at 60 mA/g and discharged to −0.6 V versus Hg/HgO at 30 mA/g. After every charging, the circuit was opened for 5 min. In evaluating the high-rate dischargeability, discharge capacity of the alloy electrode at different discharge current density were measured. The high-rate dischargeability HRD (%) defined as Cn /(Cn + C30 ), was determined from the ratio of the discharge capacity Cn (n = 60, 90, 120, 180 and 240, separately) to the total discharge capacity defined as the sum of Cn and C30 , which was additional capacity measured subsequently at 30 mA/g after Cn was measured. The phase of the alloys was determined by X-ray diffraction (XRD) on a Rigaku D/max 2500PC X-ray diffractometer with Cu K␣ radiation. The electrochemical impedance spectroscopy (EIS) analysis was carried out on a Solartron 1287 Potentiostat/Galvanostat and a Solartron 1255 frequency response analyzer with Z-POLT software. The electrodes were all tested at 10% depth of discharge (DOD) at the fifth cycle, and the frequency range was from 0.1 Hz to 1 MHz. The potentiostatic
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discharge technique was used to evaluate the diffusion coefficient of hydrogen in the bulk of the alloy. After being fully charged followed by 30 min open-circuit lay-aside, the test electrodes were discharged with +500 mV potential-step for 3000 s on the EG&G PARC Model 273 Potentiostat/Galvanostat. 3. Results and discussion 3.1. Phase structure Fig. 1 shows XRD patterns of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi powders. It can be seen that the powders are composed of amorphous, I- and Ni-phases when x increases from 5 to 20. The intensity of diffraction peaks corresponding to the Ni-phase increases with increasing x, which implies the amount of the Ni-phase in the powders increases. The diffraction peaks corresponding to the I-phase broaden, which is due to the particle refinement and the formation of amorphous phase, as well as shift to larger degree with increasing x, which indicates the cell volume of the I-phase decreases. According to the report [10], quasi-lattice constant (αQ ) of the I-phase is calculated and ˚ (x = 0) to 5.17 A ˚ listed in Table 1. The αQ decreases from 5.26 A (x = 20), which is attributed to that ball-milling causes the nickel diffuse into the I-phase. Increasing Ni content in the I-phase will lead to the decrease in the cell volume since the atomic
Fig. 1. XRD patterns of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi powders.
Table 1 Quasi-lattice constant and electrochemical characteristics of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes x
˚ αQ (A)
Cmax (mAh/g)
HRD240 a (%)
Na b
S30 (%)
0 5 10 15 20
5.26 5.24 5.23 5.21 5.17
145 176 192 203 199
54.2 57.3 61.7 62.4 65.3
2 1 1 1 1
41.4 40.9 41.7 45.3 52.8
a b
The high-rate dischargeability at the discharge current density of 240 mA/g. The number of cycles needed to activate the electrode.
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˚ is smaller than that of Ti (1.47 A) ˚ and radius of Ni (1.26 A) ˚ Zr (1.62 A). 3.2. Maximum discharge capacity The number of cycles needed to activate the electrodes and the maximum discharge capacity of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes are given in Table 1. It can be seen that all the alloy electrodes can be activated at two charging/discharging cycles. In addition, it is evident that ball-milling with nickel powder is effective to improve the dischargeability. The maximum discharge capacity of the alloy electrodes first increases from 145 mAh/g (x = 0) to 203 mAh/g (x = 15), and then decreases to 199 mAh/g (x = 20). The previous investigation showed the dischargeability of Ti45 Zr35 Ni17 Cu3 I-phase alloy electrode is inhibited by the high stability of alloy hydride and poor kinetic properties. Gao et al. [11] have reported that the hydride stability decreases with the increase of instable hydride elements in alloy electrode. Ni is instable hydride element in the Ti–Zr–Ni–Cu alloy because the bond strength of NiH is weaker than that of TiH and ZrH. The increase of the Ni content will decrease the stability of the alloy hydrides, and thereby is beneficial to the dischargeability. Furthermore, the metallic nickel particles highly dispersed in the alloys accelerates the charge-transfer reactions, and refined grains and the mixed structure of I- and amorphous-phases are beneficial to the hydrogen diffusion in the bulk of the alloys. The enhancement of kinetic properties is helpful to the increase of the discharge capacity. On the other hand, Li et al. [12] pointed out that hydrogen storage capacity has a linear relationship with the cell volume, as well as the larger the cell volume is, the higher the hydrogen storage capacity is. The cell volume of the I-phase decreases with increasing x and the I-phase is the main phase for hydrogen storage in the composite alloys. The decrease in the cell volume of the I-phase is detrimental to the hydrogen storage capacity of the alloy electrodes. Obviously, the decrease of the hydride stability and the improvement in kinetic properties resulted from the increase in Ni content are favorable to the dischargeability, but the decrease in cell volume of the I-phase is unfavorable. Therefore, it is reasonable to assume that, with additive Ni lower than a certain amount, the favorable effect is dominant and will cause an increase in the maximum discharge capacity. However, when additive Ni exceeds the critical amount, the unfavorable effect will become dominant and give rise to a decrease in the maximum discharge capacity. In our present work, the critical amount of Ni addition is x = 15.
Fig. 2. High-rate dischargeability of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
It is well known that the HRD of the metal hydride electrodes are influenced mainly by the charge-transfer kinetics at the electrode/electrolyte interface and hydrogen diffusion rate in the bulk of alloy [13]. The exchange current density I0 of alloy electrode is commonly used to characterize the catalytic activity for charge-transfer reaction at the electrode/electrolyte interface, and hydrogen diffusion coefficient D is used to characterize hydrogen diffusion rate in the bulk of alloy. In order to obtain I0 and D, electrochemical impedance and potential-step experiment are performed on these alloy electrodes. Fig. 3 shows EIS of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes at 10% DOD and the equivalent circuit. The equivalent circuit is proposed by Kuriyama et al. [14]. R1 is the electrolyte resistance. R2 and C1 characterize the contact resis-
3.3. High-rate dischargeability and kinetic properties It is of importance for hydride electrodes in batteries to exhibit good discharge property even at the high discharge current density. Fig. 2 shows the relationship between the high-rate dischargeability and the discharge current density of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It can be noticed that the HRD increases with increasing x. The HRD at the discharge current density of 240 mA/g is listed in Table 1. The HRD increases from 54.2% (x = 0) to 65.3% (x = 20).
Fig. 3. EIS of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes at 10% DOD and the equivalent circuit.
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Table 2 Electrochemical kinetic parameters of Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes x
0 5 10 15 20
Charge-transfer resistance Rct (m g) 144 115 96.3 91.7 84.4
Exchange current density I0 (mA/g)
Hydrogen diffusion coefficient D (×10−11 cm2 /s)
193.4 241.6 289.0 303.5 329.8
8.19 9.22 10.4 10.6 11.2
tance and the 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 present the charge-transfer resistance and the double-layer capacitance, respectively. W is the Warburg resistance. On the basis of the circuit the charge-transfer resistances Rct are obtained by means of fitting program Z-VIEW. Table 2 gives Rct and I0 calculated by the following equation [14]: RT 1 I0 = (1) F Rct wherein R is the gas constant, T the absolute temperature and F is the Faraday constant. It is clear that the Rct decreases and I0 increases with increasing x. The calculated results show the Rct decreases from 144 m g (x = 0) to 84.4 m g (x = 20) and accordingly the I0 increases from 193.4 mA/g (x = 0) to 329.8 mA/g (x = 20). It is well known that metallic nickel is a good electrocatalyst and it is important to improve charge-transfer reaction of MH electrodes. The metallic nickel particles highly dispersed in the alloys contribute to the decrease in the charge-transfer resistance and the increase in the exchange current density. The content of metallic nickel in the composite alloys increases with increasing x. Therefore, the exchange current density increases. The diffusion coefficient of hydrogen in the bulk of the alloy electrodes is determined with the potential-step method. Fig. 4 shows semi-logarithmic plots of the anodic current versus the time response of the Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It can be observed that after the application of overpotential, the current–time response can be divided into two time domains. According to the model of Zheng et al. [15], the diffusion coefficient of the hydrogen atoms in the bulk can be estimated through the slope of the linear region of the corresponding plots by following formula: 2 6FD(C0 − Cs ) π D Log i = log − t (2) dα2 2.303 α2 wherein D is the hydrogen diffusion coefficient (cm2 /s); α, the radius of the spherical particle (cm); i, the diffusion current density (A/g); C0 , the initial hydrogen concentration in the bulk of alloy (mol/cm3 ); Cs , the hydrogen concentration on the surface of alloy particles (mol/cm3 ); d, the density of the hydrogen storage alloy (g/cm3 ); and t is the discharge time. Assuming that the alloys have a similar particle distribution with an aver-
Fig. 4. Semi-logarithmic plots of the anodic current vs. the time response of ball-milled. Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
age particle radius of 13 m, the hydrogen diffusion coefficient D in the bulk of alloys is estimated by Eq. (2) and also listed in Table 2. The D increases from 8.19 × 10−11 cm2 /s (x = 0) to 11.2 × 10−11 cm2 /s (x = 20). The improvement of the hydrogen diffusivity can be attributed to the following factors: First, refined grains shorten diffusion length, and abundant defects and grain boundaries can provide good channels for the hydrogen diffusion. Second, the mixed structure of amorphous- and I-phases makes hydrogen diffusion more easily. Spassov et al. [16] pointed out the long-range diffusion through an already formed hydride is often the slowest stage of hydrogen diffusion. The amorphous-phase around the I-phase leads to an easier access of hydrogen to the I-phase, avoiding the long-range diffusion through an already formed hydride. Third, the content of nickel at the surface increases with increasing x, which causes the hydrogen diffusion through the alloy surface more easily due to the good electrocatalytic activity and electrical conductivity of nickel. Iwakura et al. [17,18] have reported that if the electrochemical kinetics at the electrode/electrolyte interface is rate-determine, a linear dependence of HRD on the exchange current density should be observed. In contrast, if the diffusion of hydrogen in the bulk alloy is rate-determine, a linear dependence of HRD on the hydrogen diffusion coefficient should be observed. Fig. 5 shows HRD as a function of exchange current density and hydrogen diffusion coefficient for ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It is evident that the HRD increases with the increase of I0 and D, and shows a linear relationship with I0 and D, respectively. This implies that both the charge-transfer at the electrode/electrolyte interface and the hydrogen diffusion in the bulk of the alloys are responsible for the improvement of the HRD at a discharge current density of 240 mA/g. 3.4. Cycling stability Fig. 6 shows discharge capacities as a function of cycle number for the ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It
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alloy electrodes. Zhang et al. [19] have reported that the antipulverization performance depends mainly on the strength and toughness of alloy. The refined grain and the amorphous-phase in the alloys increase the strength and toughness, and accordingly improve the anti-pulverization of the alloys. In addition, the amorphous-phase in the alloys can enhance the anti-corrosion ability of the alloys. 4. Conclusions
Fig. 5. HRD at 240 mA/g as a function of exchange current density and hydrogen diffusion coefficient for ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
is evident that ball-milling with nickel powder enhances cycling stability of Ti45 Zr35 Ni17 Cu3 alloy electrode. The discharge capacity of ball-milled Ti45 Zr35 Ni17 Cu3 + 20Ni alloy electrode after 30 cycles is 105 mAh/g, which is 1.78 times higher than that of alloy electrode before ball-milling. The cycling capacity retention rate, expressed as S30 (%) = C30 /Cmax × 100 (where Cmax is the maximum discharge capacity, C30 is the discharge capacity at the 30th cycle), is listed in Table 1. S30 increases noticeable from 40.9% (x = 0) to 52.8% (x = 20). As reported previously [6], the degradation of surface oxidation and the pulverization of alloy particle lead to the discharge capacity loss of Ti45 Zr35 Ni17 Cu3 I-phase alloy electrode. Ball-milling efficiently increases the content of the nickel in the alloy electrodes, which not only inhibits the formation of oxide film, but also provides the necessary electrical conductivity and catalytic activity in the surface oxide film. Furthermore, refined grain and the mixed structure of I- and amorphous-phases resulted from the ball-milling improve the anti-pulverization property of the
Fig. 6. Discharge capacities as a function of cycle number for ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
Crystallographic and electrochemical characteristics of ballmilled Ti45 Zr35 Ni17 Cu3 + xNi (x = 0, 5, 10, 15 and 20 mass%) composite alloys have been investigated systematically. The XRD reveals that the powders are composed of amorphous-, I- and Ni-phases when x increases from 5 to 20. The abundance of Ni-phase increases with increasing x and the αQ ˚ (x = 0) to 5.17 A ˚ (x = 20). The electrodecreases from 5.26 A chemical measurements show the maximum discharge capacity of the alloy electrodes first increases from 145 mAh/g (x = 0) to 203 mAh/g (x = 15), and then decreases to 199 mAh/g (x = 20). The HRD at the discharge current density of 240 mA/g increases from 54.2% (x = 0) to 65.3% (x = 20). The discharge capacity of Ti45 Zr35 Ni17 Cu3 + 20Ni alloy electrode after 30 cycles is 1.78 times higher than that of alloy electrode before ballmilling and S30 increases noticeably from 40.9% (x = 0) to 52.8% (x = 20). Acknowledgments This work is supported by National Natural Science Foundations of China (50571094) and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. The authors would like to thank Qiuming Peng, Jie Yang and Ning Liu for benefic discussion. References [1] A.M. Viano, R.M. Stroud, P.C. Gibbons, A. McDowell, M.S. Conradi, K.F. Kelton, Phys. Rev. B: Rapid Commun. 51 (1995) 12026. [2] K.F. Kelton, W.J. Kim, R.M. Stroud, Appl. Phys. Lett. 70 (1997) 3230. [3] K.F. Kelton, Mater. Sci. Eng. A 375 (2004) 31. [4] A. Takasaki, K.F. Kelton, J. Alloys Compd. 347 (2002) 295. [5] S.I. Yamaura, H.Y. Kim, H. Kimura, A. Inoue, Y. Arata, J. Alloys Compd. 329 (2002) 230. [6] E.H. Majzoub, J.Y. Kim, R.G. Hennig, K.F. Kelton, P.C. Gibbons, W.B. Yelon, Mater. Sci. Eng. A 294–296 (2000) 108. [7] Y.H. Xu, C.P. Chen, X.L. Wang, Q.D. Wang, Solid State Ionics 146 (2002) 157. [8] M. Jurczyk, W. Rajewski, W. Majchzycki, G. Wojcik, J. Alloys Compd. 274 (1998) 299. [9] S.M. Lee, J.S. Yu, P.S. Lee, J.Y. Lee, J. Alloys Compd. 330–332 (2002) 835. [10] C.L. Henley, V. Elser, Philos. Mag. B 53 (1986) 59. [11] X.P. Gao, Y. Wang, Z.W. Lu, F. Wu, D.Y. Song, P.W. Shen, Chem. Mater. 16 (2004) 2515. [12] Q.A. Li, Y.G. Chen, M.J. Tu, S. Ge, M.G. Han, N. Li, D.X. Tang, Chin. J. Power Sources 24 (2000) 246. [13] Y.F. Liu, H.G. Pan, M.X. Gao, Y.F. Zhu, Y.Q. Lei, J. Alloys Compd. 365 (2004) 246.
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[17] C. Iwakura, M. Miyamoto, H. Inoue, M. Matsuoka, Y. Fukumoto, J. Alloys Compd. 259 (1997) 132. [18] C. Iwakura, T. Oura, H. Inoue, M. Matsuoka, Electrochim. Acta 41 (1995) 117. [19] Y.H. Zhang, G.Q. Wang, X.P. Dong, S.H. Guo, X.L. Wang, J. Power Sources 137 (2004) 309.
Crystallographic and electrochemical characteristics of Ti45Zr35Ni17Cu3 quasicrystalline alloy ball-milled with nickel powder Baozhong Liu a,b , Jianli Wang a,b , Yaoming Wu a , Limin Wang a,∗ a
Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, CAS, 5625 Renmin Street, Changchun 130022, China b Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China Received 28 September 2005; received in revised form 12 October 2005; accepted 12 October 2005 Available online 15 November 2005
Abstract Crystallographic and electrochemical characteristics of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi (x = 0, 5, 10, 15 and 20 mass%) composite powders have been investigated. The powders are composed of amorphous, I- and Ni-phases when x increases from 5 to 20. With increasing x, the amount of Ni-phase increases but the quasi-lattice constant decreases. The maximum discharge capacity first increases as x increases from 0 to 15 and then decreases when x increases further from 15 to 20. The high-rate dischargeability and cycling stability increase monotonically with increasing x. The improvement of the electrochemical characteristics is ascribed to the metallic nickel particles highly dispersed in the alloys, which improves the electrochemical kinetic properties and prevents the oxidation of the alloy electrodes, as well as to the mixed structure of amorphous and icosahedral quasicrystalline phases, which enhances the hydrogen diffusivity in the bulk of the alloy electrodes and efficiently inhibits the pulverization of the alloy particles. © 2005 Elsevier Ltd. All rights reserved. Keywords: Ti–Zr–Ni–Cu alloy; Icosahedral quasicrystal; Ball-milling; Electrochemical characteristics; Metal hydride electrode
1. Introduction Icosahedral quasicrystalline phase (I-phase) showing the rotational symmetry of icosahedral point group is likely dominated by local tetrahedral [1]. The dominance of polytetrahedral order makes I-phase potentially used for hydrogen storage application as energy storage media or battery electrode. Ti-based I-phase has attracted much attention because of high hydrogen capacity, low cost and thermodynamical stability [2]. However, it was difficult for practical application because of low equilibrium plateau pressure [3] and the instability of I-phase due to high temperature of hydrogen desorption [1,4]. Electrochemical methods may enable the hydride of Ti–Zr–Ni–Cu I-phase alloy to dissociate at moderate condition and reversibly charge/discharge without undergoing any structural phase transitions due to very low atmospheric pressure can be obtained [5]. Thus, it is very promising that Ti-base I-phase alloy is applied as negative electrode materials of nickel-metal hydride (MH) battery. ∗
Corresponding author. Tel.: +86 431 5262447; fax: +86 431 5698041. E-mail address: [email protected] (L. Wang).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.10.014
In the condition of thermodynamic equilibrium, the hydrogen storage capacity of TiZrNi I-phase alloy can reach two hydrogen atoms per metal atom, which was higher than that of LaNi5 and TiFe compounds [3]. Takasaki et al. [4] have prepared the quasicrystalline Ti45 Zr38 Ni17 powder by mechanical alloying and subsequent annealing, and the maximum absorbing capacity with an atom ratio of hydrogen to metal ([H]/[M]) is 1.5. Majzoub et al. [6] have reported the hydrogen to metal atom ratio of 1.9 was obtained when electrochemical method was used to hydrogenate Ti45 Zr38 Ni17 I-phase alloy. In our previous work, Ti45 Zr35 Ni17 Cu3 single I-phase powder has been synthesized and investigated by electrochemical charging/discharging method, and the results showed I-phase was steady upon charging/discharging cycles. Unfortunately, the maximum discharge capacity of Ti45 Zr35 Ni17 Cu3 I-phase alloy electrode was so far less than the theoretical capacity. It is necessary to further improve its electrochemical properties for the practical application. It is well known that metallic nickel is a good electrocatalyst and it is important to improve electrochemical characteristics of MH electrodes [7]. Furthermore, ball-milling technique is very effective to modify the structure and surface properties for
B. Liu et al. / Electrochimica Acta 51 (2006) 3586–3591
improving physical and chemical properties. Recently, Jurczyk et al. [8] and Lee et al. [9] have investigated the electrochemical properties of Ti/Zr-based AB2 alloys ball-milled with additive nickel powder, and the results showed the multi-phase structure and refined grain efficiently enhanced the electrochemical discharge capacity and cycling stability. On the basis of our previous studies and the belief that the electrochemical characteristics of the Ti45 Zr35 Ni17 Cu3 Iphase alloy electrodes can be enhanced by ball-milling with nickel powder, crystallographic and electrochemical characteristics of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi (x = 0, 5, 10, 15 and 20 mass%) composite alloys have been investigated. 2. Experimental procedures Elemental powders of Ti (100 mesh, 99.9%), Zr (100 mesh, 99.9%), Ni (150 mesh, 99.9%) and Cu (250 mesh, 99.99%) were used as starting materials in this study. A powder mixture with a desired composition of Ti45 Zr35 Ni17 Cu3 was poured into the milling container. Mechanical alloying (MA) was carried out in a vibratory ball-miller under the vibration frequency of 25 Hz and amplitude of 2.5 mm for 280 h in an argon atmosphere. The stainless steel vial and stainless balls were utilized, and the ball to powder mass ratio was 25:1. The powders after MA were sealed under dynamic vacuum (<10−1 Pa) in a fused silica tube, and then annealed at 855 K for 30 min. The Ti45 Zr35 Ni17 Cu3 + xNi (x = 5, 10, 15 and 20 mass%) mixture powders were mechanically milled under high-purity argon in Spex800 ball-miller. The ball to powder mass ratio was 20:1, and the ball-milling time was 180 min. The Ti45 Zr35 Ni17 Cu3 I-phase alloy powder before ball-milling was represented as x = 0. Electrochemical charging/discharging tests were performed on an automatic galvanostatic system (DC-5) at 313 K in 6 M KOH electrolyte using a standard open three-electrode cell. The working electrodes were fabricated by compressing a mixture of 0.15 g alloy powder (200–400 mesh) and 0.75 g nickel carbonyl powder into a pellet of 10 mm diameter under a pressure of 15 MPa. The counter electrode was a Ni(OH)2 /NiOOH electrode and the reference electrode was a Hg/HgO electrode. Each electrode was charged for 7 h at 60 mA/g and discharged to −0.6 V versus Hg/HgO at 30 mA/g. After every charging, the circuit was opened for 5 min. In evaluating the high-rate dischargeability, discharge capacity of the alloy electrode at different discharge current density were measured. The high-rate dischargeability HRD (%) defined as Cn /(Cn + C30 ), was determined from the ratio of the discharge capacity Cn (n = 60, 90, 120, 180 and 240, separately) to the total discharge capacity defined as the sum of Cn and C30 , which was additional capacity measured subsequently at 30 mA/g after Cn was measured. The phase of the alloys was determined by X-ray diffraction (XRD) on a Rigaku D/max 2500PC X-ray diffractometer with Cu K␣ radiation. The electrochemical impedance spectroscopy (EIS) analysis was carried out on a Solartron 1287 Potentiostat/Galvanostat and a Solartron 1255 frequency response analyzer with Z-POLT software. The electrodes were all tested at 10% depth of discharge (DOD) at the fifth cycle, and the frequency range was from 0.1 Hz to 1 MHz. The potentiostatic
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discharge technique was used to evaluate the diffusion coefficient of hydrogen in the bulk of the alloy. After being fully charged followed by 30 min open-circuit lay-aside, the test electrodes were discharged with +500 mV potential-step for 3000 s on the EG&G PARC Model 273 Potentiostat/Galvanostat. 3. Results and discussion 3.1. Phase structure Fig. 1 shows XRD patterns of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi powders. It can be seen that the powders are composed of amorphous, I- and Ni-phases when x increases from 5 to 20. The intensity of diffraction peaks corresponding to the Ni-phase increases with increasing x, which implies the amount of the Ni-phase in the powders increases. The diffraction peaks corresponding to the I-phase broaden, which is due to the particle refinement and the formation of amorphous phase, as well as shift to larger degree with increasing x, which indicates the cell volume of the I-phase decreases. According to the report [10], quasi-lattice constant (αQ ) of the I-phase is calculated and ˚ (x = 0) to 5.17 A ˚ listed in Table 1. The αQ decreases from 5.26 A (x = 20), which is attributed to that ball-milling causes the nickel diffuse into the I-phase. Increasing Ni content in the I-phase will lead to the decrease in the cell volume since the atomic
Fig. 1. XRD patterns of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi powders.
Table 1 Quasi-lattice constant and electrochemical characteristics of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes x
˚ αQ (A)
Cmax (mAh/g)
HRD240 a (%)
Na b
S30 (%)
0 5 10 15 20
5.26 5.24 5.23 5.21 5.17
145 176 192 203 199
54.2 57.3 61.7 62.4 65.3
2 1 1 1 1
41.4 40.9 41.7 45.3 52.8
a b
The high-rate dischargeability at the discharge current density of 240 mA/g. The number of cycles needed to activate the electrode.
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˚ is smaller than that of Ti (1.47 A) ˚ and radius of Ni (1.26 A) ˚ Zr (1.62 A). 3.2. Maximum discharge capacity The number of cycles needed to activate the electrodes and the maximum discharge capacity of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes are given in Table 1. It can be seen that all the alloy electrodes can be activated at two charging/discharging cycles. In addition, it is evident that ball-milling with nickel powder is effective to improve the dischargeability. The maximum discharge capacity of the alloy electrodes first increases from 145 mAh/g (x = 0) to 203 mAh/g (x = 15), and then decreases to 199 mAh/g (x = 20). The previous investigation showed the dischargeability of Ti45 Zr35 Ni17 Cu3 I-phase alloy electrode is inhibited by the high stability of alloy hydride and poor kinetic properties. Gao et al. [11] have reported that the hydride stability decreases with the increase of instable hydride elements in alloy electrode. Ni is instable hydride element in the Ti–Zr–Ni–Cu alloy because the bond strength of NiH is weaker than that of TiH and ZrH. The increase of the Ni content will decrease the stability of the alloy hydrides, and thereby is beneficial to the dischargeability. Furthermore, the metallic nickel particles highly dispersed in the alloys accelerates the charge-transfer reactions, and refined grains and the mixed structure of I- and amorphous-phases are beneficial to the hydrogen diffusion in the bulk of the alloys. The enhancement of kinetic properties is helpful to the increase of the discharge capacity. On the other hand, Li et al. [12] pointed out that hydrogen storage capacity has a linear relationship with the cell volume, as well as the larger the cell volume is, the higher the hydrogen storage capacity is. The cell volume of the I-phase decreases with increasing x and the I-phase is the main phase for hydrogen storage in the composite alloys. The decrease in the cell volume of the I-phase is detrimental to the hydrogen storage capacity of the alloy electrodes. Obviously, the decrease of the hydride stability and the improvement in kinetic properties resulted from the increase in Ni content are favorable to the dischargeability, but the decrease in cell volume of the I-phase is unfavorable. Therefore, it is reasonable to assume that, with additive Ni lower than a certain amount, the favorable effect is dominant and will cause an increase in the maximum discharge capacity. However, when additive Ni exceeds the critical amount, the unfavorable effect will become dominant and give rise to a decrease in the maximum discharge capacity. In our present work, the critical amount of Ni addition is x = 15.
Fig. 2. High-rate dischargeability of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
It is well known that the HRD of the metal hydride electrodes are influenced mainly by the charge-transfer kinetics at the electrode/electrolyte interface and hydrogen diffusion rate in the bulk of alloy [13]. The exchange current density I0 of alloy electrode is commonly used to characterize the catalytic activity for charge-transfer reaction at the electrode/electrolyte interface, and hydrogen diffusion coefficient D is used to characterize hydrogen diffusion rate in the bulk of alloy. In order to obtain I0 and D, electrochemical impedance and potential-step experiment are performed on these alloy electrodes. Fig. 3 shows EIS of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes at 10% DOD and the equivalent circuit. The equivalent circuit is proposed by Kuriyama et al. [14]. R1 is the electrolyte resistance. R2 and C1 characterize the contact resis-
3.3. High-rate dischargeability and kinetic properties It is of importance for hydride electrodes in batteries to exhibit good discharge property even at the high discharge current density. Fig. 2 shows the relationship between the high-rate dischargeability and the discharge current density of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It can be noticed that the HRD increases with increasing x. The HRD at the discharge current density of 240 mA/g is listed in Table 1. The HRD increases from 54.2% (x = 0) to 65.3% (x = 20).
Fig. 3. EIS of ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes at 10% DOD and the equivalent circuit.
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Table 2 Electrochemical kinetic parameters of Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes x
0 5 10 15 20
Charge-transfer resistance Rct (m g) 144 115 96.3 91.7 84.4
Exchange current density I0 (mA/g)
Hydrogen diffusion coefficient D (×10−11 cm2 /s)
193.4 241.6 289.0 303.5 329.8
8.19 9.22 10.4 10.6 11.2
tance and the 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 present the charge-transfer resistance and the double-layer capacitance, respectively. W is the Warburg resistance. On the basis of the circuit the charge-transfer resistances Rct are obtained by means of fitting program Z-VIEW. Table 2 gives Rct and I0 calculated by the following equation [14]: RT 1 I0 = (1) F Rct wherein R is the gas constant, T the absolute temperature and F is the Faraday constant. It is clear that the Rct decreases and I0 increases with increasing x. The calculated results show the Rct decreases from 144 m g (x = 0) to 84.4 m g (x = 20) and accordingly the I0 increases from 193.4 mA/g (x = 0) to 329.8 mA/g (x = 20). It is well known that metallic nickel is a good electrocatalyst and it is important to improve charge-transfer reaction of MH electrodes. The metallic nickel particles highly dispersed in the alloys contribute to the decrease in the charge-transfer resistance and the increase in the exchange current density. The content of metallic nickel in the composite alloys increases with increasing x. Therefore, the exchange current density increases. The diffusion coefficient of hydrogen in the bulk of the alloy electrodes is determined with the potential-step method. Fig. 4 shows semi-logarithmic plots of the anodic current versus the time response of the Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It can be observed that after the application of overpotential, the current–time response can be divided into two time domains. According to the model of Zheng et al. [15], the diffusion coefficient of the hydrogen atoms in the bulk can be estimated through the slope of the linear region of the corresponding plots by following formula: 2 6FD(C0 − Cs ) π D Log i = log − t (2) dα2 2.303 α2 wherein D is the hydrogen diffusion coefficient (cm2 /s); α, the radius of the spherical particle (cm); i, the diffusion current density (A/g); C0 , the initial hydrogen concentration in the bulk of alloy (mol/cm3 ); Cs , the hydrogen concentration on the surface of alloy particles (mol/cm3 ); d, the density of the hydrogen storage alloy (g/cm3 ); and t is the discharge time. Assuming that the alloys have a similar particle distribution with an aver-
Fig. 4. Semi-logarithmic plots of the anodic current vs. the time response of ball-milled. Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
age particle radius of 13 m, the hydrogen diffusion coefficient D in the bulk of alloys is estimated by Eq. (2) and also listed in Table 2. The D increases from 8.19 × 10−11 cm2 /s (x = 0) to 11.2 × 10−11 cm2 /s (x = 20). The improvement of the hydrogen diffusivity can be attributed to the following factors: First, refined grains shorten diffusion length, and abundant defects and grain boundaries can provide good channels for the hydrogen diffusion. Second, the mixed structure of amorphous- and I-phases makes hydrogen diffusion more easily. Spassov et al. [16] pointed out the long-range diffusion through an already formed hydride is often the slowest stage of hydrogen diffusion. The amorphous-phase around the I-phase leads to an easier access of hydrogen to the I-phase, avoiding the long-range diffusion through an already formed hydride. Third, the content of nickel at the surface increases with increasing x, which causes the hydrogen diffusion through the alloy surface more easily due to the good electrocatalytic activity and electrical conductivity of nickel. Iwakura et al. [17,18] have reported that if the electrochemical kinetics at the electrode/electrolyte interface is rate-determine, a linear dependence of HRD on the exchange current density should be observed. In contrast, if the diffusion of hydrogen in the bulk alloy is rate-determine, a linear dependence of HRD on the hydrogen diffusion coefficient should be observed. Fig. 5 shows HRD as a function of exchange current density and hydrogen diffusion coefficient for ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It is evident that the HRD increases with the increase of I0 and D, and shows a linear relationship with I0 and D, respectively. This implies that both the charge-transfer at the electrode/electrolyte interface and the hydrogen diffusion in the bulk of the alloys are responsible for the improvement of the HRD at a discharge current density of 240 mA/g. 3.4. Cycling stability Fig. 6 shows discharge capacities as a function of cycle number for the ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes. It
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alloy electrodes. Zhang et al. [19] have reported that the antipulverization performance depends mainly on the strength and toughness of alloy. The refined grain and the amorphous-phase in the alloys increase the strength and toughness, and accordingly improve the anti-pulverization of the alloys. In addition, the amorphous-phase in the alloys can enhance the anti-corrosion ability of the alloys. 4. Conclusions
Fig. 5. HRD at 240 mA/g as a function of exchange current density and hydrogen diffusion coefficient for ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
is evident that ball-milling with nickel powder enhances cycling stability of Ti45 Zr35 Ni17 Cu3 alloy electrode. The discharge capacity of ball-milled Ti45 Zr35 Ni17 Cu3 + 20Ni alloy electrode after 30 cycles is 105 mAh/g, which is 1.78 times higher than that of alloy electrode before ball-milling. The cycling capacity retention rate, expressed as S30 (%) = C30 /Cmax × 100 (where Cmax is the maximum discharge capacity, C30 is the discharge capacity at the 30th cycle), is listed in Table 1. S30 increases noticeable from 40.9% (x = 0) to 52.8% (x = 20). As reported previously [6], the degradation of surface oxidation and the pulverization of alloy particle lead to the discharge capacity loss of Ti45 Zr35 Ni17 Cu3 I-phase alloy electrode. Ball-milling efficiently increases the content of the nickel in the alloy electrodes, which not only inhibits the formation of oxide film, but also provides the necessary electrical conductivity and catalytic activity in the surface oxide film. Furthermore, refined grain and the mixed structure of I- and amorphous-phases resulted from the ball-milling improve the anti-pulverization property of the
Fig. 6. Discharge capacities as a function of cycle number for ball-milled Ti45 Zr35 Ni17 Cu3 + xNi alloy electrodes.
Crystallographic and electrochemical characteristics of ballmilled Ti45 Zr35 Ni17 Cu3 + xNi (x = 0, 5, 10, 15 and 20 mass%) composite alloys have been investigated systematically. The XRD reveals that the powders are composed of amorphous-, I- and Ni-phases when x increases from 5 to 20. The abundance of Ni-phase increases with increasing x and the αQ ˚ (x = 0) to 5.17 A ˚ (x = 20). The electrodecreases from 5.26 A chemical measurements show the maximum discharge capacity of the alloy electrodes first increases from 145 mAh/g (x = 0) to 203 mAh/g (x = 15), and then decreases to 199 mAh/g (x = 20). The HRD at the discharge current density of 240 mA/g increases from 54.2% (x = 0) to 65.3% (x = 20). The discharge capacity of Ti45 Zr35 Ni17 Cu3 + 20Ni alloy electrode after 30 cycles is 1.78 times higher than that of alloy electrode before ballmilling and S30 increases noticeably from 40.9% (x = 0) to 52.8% (x = 20). Acknowledgments This work is supported by National Natural Science Foundations of China (50571094) and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. The authors would like to thank Qiuming Peng, Jie Yang and Ning Liu for benefic discussion. References [1] A.M. Viano, R.M. Stroud, P.C. Gibbons, A. McDowell, M.S. Conradi, K.F. Kelton, Phys. Rev. B: Rapid Commun. 51 (1995) 12026. [2] K.F. Kelton, W.J. Kim, R.M. Stroud, Appl. Phys. Lett. 70 (1997) 3230. [3] K.F. Kelton, Mater. Sci. Eng. A 375 (2004) 31. [4] A. Takasaki, K.F. Kelton, J. Alloys Compd. 347 (2002) 295. [5] S.I. Yamaura, H.Y. Kim, H. Kimura, A. Inoue, Y. Arata, J. Alloys Compd. 329 (2002) 230. [6] E.H. Majzoub, J.Y. Kim, R.G. Hennig, K.F. Kelton, P.C. Gibbons, W.B. Yelon, Mater. Sci. Eng. A 294–296 (2000) 108. [7] Y.H. Xu, C.P. Chen, X.L. Wang, Q.D. Wang, Solid State Ionics 146 (2002) 157. [8] M. Jurczyk, W. Rajewski, W. Majchzycki, G. Wojcik, J. Alloys Compd. 274 (1998) 299. [9] S.M. Lee, J.S. Yu, P.S. Lee, J.Y. Lee, J. Alloys Compd. 330–332 (2002) 835. [10] C.L. Henley, V. Elser, Philos. Mag. B 53 (1986) 59. [11] X.P. Gao, Y. Wang, Z.W. Lu, F. Wu, D.Y. Song, P.W. Shen, Chem. Mater. 16 (2004) 2515. [12] Q.A. Li, Y.G. Chen, M.J. Tu, S. Ge, M.G. Han, N. Li, D.X. Tang, Chin. J. Power Sources 24 (2000) 246. [13] Y.F. Liu, H.G. Pan, M.X. Gao, Y.F. Zhu, Y.Q. Lei, J. Alloys Compd. 365 (2004) 246.
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