稀土元素对Ti0.26Zr0.07V0.24Mn0.1Ni0.33 贮氢合金微观结构和电化学性能的影响

2013-08-20李书存赵敏寿焦体峰

无机化学学报 2013年1期

李书存赵敏寿刘研焦体峰

（1 秦皇岛燕山大学环境与化学工程学院，秦皇岛 066004）

（2 秦皇岛燕山大学亚稳材料制备技术与科学实验室，秦皇岛 066004）

(3 中国科学院长春应用化学研究所稀土化学与物理重点实验室，长春 130022）

Recently, nickel-metal hydride (Ni-MH)secondary batteries are competing with the Li-ion batteries as power sources for HEV, electric tools etc.Many performances of hydrogen storage alloys used as the negative electrode materials for the Ni-MH batteries have been widely and extensively studied[1-6],including their discharge capacity, high-rate discharge ability (HRD), cycle stability, fade mechanism and environmental compatibility. Several types of hydrogen storage alloys have been investigated, for example,rare earth-based AB5-type alloys[7], Ti, Zr-based or AB2-type alloys[8-9], Mg-based alloys[10]and V-based solid solutions[11]. Among the hydrogen storage alloys mentioned above, the discharge capacity of the AB5-type alloy electrodes is comparatively low, the activation of the AB2-type laves phase alloy electrodes is very difficult and the cycle stability of the Mgbased alloy electrodes is extremely poor. V-based solid solutions have larger discharge capacity, V can form two types of hydride, monohydride (VH1) and dihydride (VH2), the equilibrium pressure of hydrogenation between V and VH1is too low at room temperature for utilization. But the equilibrium pressure of hydrogenation between VH1and VH2is appropriate for various applications and the theoretical capacity of protium per mass of alloy during this reaction is higher than that of LaNi5transforming with LaNi5H6[12-13]. So vanadium-based solid solution hydrides have attracted significant attention for various applications such as hydrogen storage,hydrogen compressor and heat pump[14]. However, Vbased solid solution phase alone has very low electrochemical discharge capacity in the KOH electrolyte due to the lack of electrochemical catalytic activity in decomposing water into hydrogen atoms and OH-ions. Yet it could be activated to absorb and desorb a large amount of hydrogen with the presence of a secondary phase, such as Ti-Ni phase or C14 Laves phase, which is considered to act both as a micro-current collector and as an electrochemical catalyst. After Laves phase undergoing easy activation treatment, it may be possible that the activation of BCC phase will be easier if the alloys contain such Laves phase. The approach to improve the hydrogen absorbing properties by forming Laves phase in Vbased BCC alloys can make it become more promising materials for electrodes of Ni-MH batteries.

Following the idea mentioned above, in this work, we select Ti0.26Zr0.07V0.24Mn0.1Ni0.33hydrogen storage alloys as norm alloy for investigation.Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01)alloys are obtained. The effect of rare earth elements substitution on microstructures and electrochemical properties of Ti0.26Zr0.07V0.24Mn0.1Ni0.33alloy is studied.

1 Experimental

All samples were prepared by arc melting the constituent metals or master alloy on a water-cooled cuprum hearth under an argon atmosphere. The purity of all the metal, i.e., Ti, V, Mn, Zr ,Ni and rare earth elements Ce, Nd and Gd was higher than 99.9 mass%,respectively. In order to obtain homogeneous alloys,the ingots of alloys were turned over and remelted at least three times. Then the as-cast alloys were divided into two parts. One part was crushed mechanically into particles with the average size of 74 μm (200 mesh) for electrochemical measurement. Another part was crushed mechanically into particles with the average size of 37 μm (400 mesh) for X-ray diffraction analysis.

The crystal structures of the hydrogen storage alloys were determined by X-ray diffraction (XRD)analysis on a Rigaku D/Max 2500PC X-ray diffractometer with Cu Kα radiation after Kα2 stripping (λ=0.154 06 nm ) at 40 kV and 200 mA(Bragg-Brentano geometry, 2θ range 20°～90°, step size 0.02°, backscattered rear graphite monochromator).The XRD patterns were analyzed using RIETAN97 software. Scanning electron microscopy (SEM) was used to study the microstructures of the alloy electrodes.

The alloy electrodes were prepared by mixing the alloy powders with carbonyl nickel powders in a weight ratio of 1∶5 and then cold-pressing the mixture to form pellets of 10 mm in diameter and thickness of 1mm under a pressure of 14 MPa. Prior to electrochemical testing, all alloy electrodes were activated by immersion in 6 mol·L-1KOH aqueous solution for 24 h. The positive electrode was a sintered Ni(OH)2/NiOOH with excessive capacity.

The charge/discharge tests were carried out with DC-5 battery testing instrument under a computer control. Each alloy electrode was charged with a current density of 60 mA·g-1and discharged with a current density of 60 mA·g-1to a cut-off voltage of 0.8V at different temperatures (303, 313, 333, and 343 K, respectively).

To evaluate the HRD (in the range of 60～600 mA·g-1), the charging current density was kept a constant of 60 mA·g-1and the obtained discharge capacity was denoted as Cn(with n=30, 60, 180, 360,and 600 mA·g-1, respectively). HRD is generally defined as Cn×100/(Cn+C30).

2 Results and discussion

2.1 Structure characteristic

The X-ray diffraction patterns of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloys are shown in Fig.1. It is found that these alloys are all composed of V-based solid solution phase with BCC structure and C14 Laves phase with hexagonal structure. Compared to the different X-ray diffraction patterns, we can find that the diffraction peaks of BCC phase and C14 Laves phase shift toward higher angle with rare earth elements substitution. This phenomenon shows that the lattice parameters of alloys are different with different rare earth elements substitution.

Fig.1 XRD patterns of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloy

Table 1 Lattice parameter and cell volume of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloy

The lattice parameters and cell volumes of alloys are shown in Table 1. It can be seen that with rare earth elements substitution, the cell volumes of the BCC phases are increased and the cell volumes of C14 phase with rare earth elements substitution are larger than norm alloy. It is obvious that with rare earth elements substitution, the cell volumes of BCC phase and C14 phase are increased at the same time.

Fig.2. gives the scanning electron micrographs of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01)alloys. It is obvious that all the alloy samples are mainly composed of two distinct crystallographic phases： one is C14 Laves phase, and the other is Vbased solid solution phase as confirmed by EDS analysis, which is in agreement with the XRD results.It can be easily seen that, with rare earth elements substitution, the BCC phase increases and finally forms a three-dimensional phase structure, and a little third phase appears.

Fig.2 FESEM images of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloy

It also can be seen from Fig.2 that there are some white particles with irregular edges distributed near the grain boundaries of BCC phase, which is proved to be Ce-rich particles by energy dispersive Xray spectroscopy (EDS), as indicated by Qiao et al.[15]

2.2 Electrochemical property

The curves of the discharge capacity vs. the cycle number at 303 K for Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloy electrodes are shown in Fig.3. Compared with the alloy electrode without rare earth elements substitution which requires 2 ～3 cycles to be activated, the ones with rare earth elements substitution can reach their maximum capacity in the first cycle, which means that activation of the alloy electrodes is easy and rare earth elements substitution is benefit for the activation property of the alloy electrode. These phenomena may be attributed to the electro-catalytic activity improved by adding the rare earth elements[16]. The rare earth elements have certain effect on the maximum discharge capacity of the alloy. With Ce substitution, the maximum discharge capacity of the alloy increases distinctly,but the cyclic stability decreases; Nd and Gd substitution has little effect on the maximum discharge capacity of the alloy, whereas the cyclic stability increases a little; and with Nd and Gd substitution,maximum discharge capacity decreases distinctly.

Fig.3 Discharge capacity as a function of cycle number for Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd;x=0.01) alloy electrode

Alloys used as negative electrode material in Ni-MH battery should be capable of working at wide temperature range. In order to investigate the temperature-related properties of the hydrogen storage electrodes, the alloys electrodes are fully charged at room temperature and discharged at various temperatures. Discharge capacity as a function of temperature of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce,Nd, Gd; x=0.01) alloy electrodes is shown in Fig.4.

Fig.4 Effect of temperature on the discharge capacity of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd;x=0.01) alloy electrode

The discharge capacity of the alloy electrodes is sensitive to temperature. With increasing temperature,the discharge capacity of the alloy electrodes with the rare earth element substitution increases first and then decreases. The discharge capacity of the alloy electrodes is up to maximum at 333 K except the norm alloy electrode.

It is very important to restrain the decrease of the discharge capacity even at the high charge/discharge current density for practical application of metal hydride electrode. The effect of the discharge current density (60～600 mA·g-1) on the discharge capacity of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01)alloy electrodes is shown in Fig.5. It can be observed that the HRD of the alloy electrodes decreases with increasing discharge current density. The different rare earth elements substitution has the different effect on the high rate dischargebility of the alloy.

Nd substitution can improve the HRD of the alloy. Especially, when the discharge current density is 600 mA·g-1, the HRD of the alloy with Nd substitution is higher than that of the norm alloy. Gd substitution is not beneficial to the high rate discharge ability of the alloy when the discharge current density is small. However, with increasing the discharge current density, the HRD of the alloy can be improved. Ce substitution can not improve the HRD of the alloy.

Fig.5 HRD of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd,Gd; x=0.01) alloy electrode at 303 K

Fig.6 Impedance response for Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce,Nd, Gd; x=0.01) alloy electrode

Fig.6 shows the EIS for Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloy electrodes at 50%depth of discharge (DOD). It can be found that all the electrochemical impedance spectroscopy (EIS) curves consist of two semicircles followed with a straight line.According to the analysis model proposed by Kuriyama et al.[17], the smaller semicircle in the highfrequency region represents the resistance and capacitance between the alloy particles and the current collector, the larger semicircle region represents the charge transfer resistance for electrochemical reaction at the surface, while the straight line is attributed to the Warburg impedance.On the basis of the circuit the charge-transfer resistances Rctare obtained by means of fitting program Z-VIEW. The exchange current density I0is calculated using the following formula when an overpotential is very small and trends to zero, and the results are listed in Table 2.

I0= RT/(FRct)

where R is the gas constant, T is the absolute temperature and F is the Faraday constant.

It is obvious that the radius of the larger semicircle decreases with Nd substitution, which indicate that the charge-transfer resistance for the alloy electrode decreases, Accordingly, the I0increases from 169.5 mA·g-1(x=0.0) to 251.0 mA·g-1(x=0.10), which explains why the HRD of the alloy electrode decreases with Nd substitution.

With different rare earth element substitutions,the performances of the hydrogen storage alloys vary,some improve and some decrease. Because the reasons are complex, it is still difficult to draw a definite law at present, it needs further in-depth and systematic research.

Table 2 Electrochemical kinetic parameter of Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloy electrode

3 Conclusions

In order to im prove the electrochemical properties of Ti-V-based hydrogen storage alloy, the nonstoichiometric composition with different rare earth elements substitution to Ti-Zr-V-Ni-Mn system alloys have been studied. Norm alloy and Ti0.26Zr0.07V0.24-xMn0.10Ni0.33REx(RE=Ce, Nd, Gd; x=0.01) alloys are consisted of V-based solid solution with bcc structure and C14 Laves phase with hexagonal structure. The lattice parameter and the cell volume of BCC phase monotonically increase with rare earth element Ce substitution, which is mainly attributed to the fact that the atomic radius of Ce is larger than that of other elements.

Rare-earth elements substitution can improve the activation characteristics of Ti0.26Zr0.07V0.24Mn0.1Ni0.33alloy electrode and the alloys can reach their maximum discharge capacity in the first cycle with rare earth element Ce substitution. The discharge capacity of the alloys is quite sensitive to temperature.The discharge capacity is up to maximum at 333 K with Rare-earth elements Nd and Gd substitution. And the excessively high temperature makes the capacity of the alloy electrodes degraded. Nd substitution improved the high-rate dischargeability of the alloy electrode and the effect on the high-rate dischargeability of Ti0.26Zr0.07V0.24Mn0.1Ni0.33alloy electrode is the most obvious with the discharge current density of 600 mA·g-1. The EIS results show that the exchange current density I0increases to 251.0 mA·g-1with Nd substitution. Gd and Ce substitution is not beneficial for the HRD of the alloy electrode.

Acknowledgements： This work was financially supported by the Hebei natural science foundation (Grant No.B2010001132) and National Natural Science Foundation of China (Grant No. 20903078).

[1] Song D, Gao X, Zhang Y, et al. J. Alloys Compd., 1994,206：43-46

[2] Willems J J G., Buschow K H J. J. Less-Commen Met.,1987,129：13-30

[3] Sakai T, Miyamura H, Kuriyama N, et al. J. Electrochem.Soc., 1990,137：795-799

[4] Notten P H L, Hokkeling P. J. Electrochem. Soc., 1991,138：1877-1883

[5] Pan H, Chen Y, Wang C, et al. Electrochim. Acta, 1999,44：2263-2269

[6] Wang C, Lei Y, Wang Q. Electrochim Acta, 1998,43：3193-3207

[7] Pan H, Ma J, Wang C, et al. Electrochim Acta, 1999,44：3977-3987

[8] Yu J S, Lee S M, Cho K, et al. J. Electrochem Soc., 2000,147：2013-2019

[9] Kim D, Lee S, Jang K, et a. J. Alloys Compd., 1998,268：241-247

[10]Iwakura C, Inoue H, Zhang S, et al. J Electrochem Soc.,1999,146：1659-1665

[11]Tsukahara M, Kamiya T, Takahashi K, et al. J Electrochem.Soc., 2000,147：2941-2947

[12]Iba. H. Ph. D. Dissertation, Tohoku Univ., Japan, 1997.