Phase structures and electrochemical behaviors of V2.1TiNi0.5Hf0.05Crx (x=0–0.152) hydrogen storage alloys

Journal of Alloys and Compounds 358 (2003) 223–227

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Phase structures and electrochemical behaviors of V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) hydrogen storage alloys a a, a a b a Rui Guo , Li-Xin Chen *, Yong-Quan Lei , Shi-Qun Li , Yue-Wu Zeng , Qi-Dong Wang a

Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China b Central Laboratory, Zhejiang University, Hangzhou 310028, PR China Received 25 November 2002; accepted 10 December 2002

Abstract The phase structures and electrochemical behaviors of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50, 0.074, 0.113 and 0.152) alloys have been investigated. It is found that the addition of Cr gives rise to a decrease in the amount of the V-based solid solution main phase and an increase in the amount of C14-type secondary phase without any change in the pattern of the three-dimensional network structure. On adding Cr, the unit cell of the main phase contracts, while that of the secondary phase significantly expands. Electrochemical measurements show that the maximum discharge capacity of the Cr-added alloys (x50.074–0.152) is only 368–395 mAh / g, much less than that of the V2.1 TiNi 0.5 Hf 0.05 alloy (444 mAh / g), but their high-rate dischargeability and cycle stability are markedly improved. For the alloy with the Cr content of x50.152, the best high-rate dischargeability and best cycle stability are obtained.  2003 Elsevier B.V. All rights reserved. Keywords: Transition metal alloys; Hydrogen absorbing materials; Electrode materials; Metal hydrides; Electrochemical reactions

1. Introduction Owing to their large hydrogen storage capacity, the vanadium-based solid solution alloys have had potential application in the Ni–MH battery since Tsukahara et al. discovered and reported the new dual phase hydride electrode alloy V3 TiNi 0.56 [1]. However, the poor cycle stability of these alloys in KOH electrolyte keeps them from being industrialized. Recently, Tsukahara et al. [2,3] and Zhang et al. [4–6] investigated the influence of various elements added to V3 TiNi 0.56 and V3 TiNi 0.56 Hf 0.24 alloys to improve their electrochemical properties, especially cycling stability. Their research has shown that multicomponent alloying is an effective method for improving the overall properties of this kind of electrode alloys. V2.1 TiNi x and V2.1 TiNi 0.5 Hf x alloys have been studied and reported in our previous works [7,8]. V2.1 TiNi 0.5 Hf 0.05 was found to have a high discharge capacity of 444 mAh / g and a poor capacity retention of 27.66% after 30 charging–discharging cycles. In order to improve its *Corresponding author. Tel.: 186-571-8795-1152; fax: 186-5718795-1152. E-mail address: [email protected] (L.-X. Chen). 0925-8388 / 03 / $ – see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00073-2

electrochemical properties further, we chose Cr as the added alloying element to inhibit the dissolution of vanadium into and the corrosion of vanadium by the KOH electrolyte [9]. The phase structures and electrochemical behaviors of the Cr-added V2.1 TiNi 0.5 Hf 0.05 alloys are reported in this paper.

2. Experimental details The V2.1 TiNi 0.5 Hf 0.05 Cr x (x50, 0.074, 0.113, 0.152) alloy samples were prepared by vacuum induction melting under argon atmosphere and each batch was remelted three times to ensure high homogeneity. For metallographic studies, samples were etched with a solution containing 10% HF, 40% HNO 3 and 50% H 2 O (by volume). The metallographic microstructures were examined by scanning electron microscopy (SEM) and the chemical compositions determined with an energy dispersive X-ray spectrometer (EDS). Samples were pulverized by first hydriding at a high temperature (673 K) under high-pressure hydrogen (#2.5 MPa) and then by mechanical crushing into a powder of 300 mesh after being dehydrided. The crystal structures

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circuit potential) with a Solartron SII287 Potentionstat. The quantities of metallic elements in the KOH electrolyte dissolved from each negative electrode after 30 charging– discharging cycles were measured by emission spectrochemical analysis.

3. Results and discussion

3.1. Phase structures

Fig. 1. X-ray diffraction patterns of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloys: (a) x50, (b) x50.074, (c) x50.113, (d) x50.152.

and lattice parameters were determined by X-ray powder diffraction (XRD) using Cu Ka radiation. The pressure– composition isotherms (P–C–T curves) were plotted by converting the data obtained with the electrochemical method. Each test electrode was prepared by mixing 0.1 g alloy powders for investigation with copper powder in the weight ratio of 1:2 and then the mixture was cold-pressed into a pellet of 10-mm diameter. The electrochemical properties of each pellet (used as the negative electrode) were measured in a tri-electrode open cell at 298 K with a sintered Ni(OH) 2 / NiOOH positive counter electrode and a Hg / HgO reference electrode. The negative electrodes were charged at 100 mA / g for 6.5 h and discharged at 25–500 mA / g to the cutoff potential of 20.70 V versus Hg / HgO. After each charging and before discharging, the circuit was kept open for 10 min. The exchange current densities I0 of alloys were calculated from the slopes of micro-polarization curves, which were determined by scanning the electrode potential at 0.1 mV/ s from 25 to 5 mV (vs. open

Fig. 1 shows XRD patterns of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50, 0.074, 0.113, 0.152) alloys. All alloys consist of two phases: a main phase of V-based solid solution with b.c.c. structure and a secondary phase of C14-type Laves phase. It can be seen from the increase in the relative intensities of the C14 diffraction peaks that the amount of the secondary phase increases with the increase in Cr content. The calculated lattice parameters of each phase are shown in Table 1. It can be seen that the lattice parameter of the main phase decreases from 0.30626 to 0.30560 nm, while those of the secondary phase significantly increase with Cr content x increasing from 0 to 0.152. The SEM micrographs of these alloys are shown in Fig. 2. It can be seen that the secondary phase precipitates along the grain boundaries of the main phase. With the increase in Cr content, the grain size of the main phase decreases. Based on the compositions determined by EDS analysis, the molar fractions of the main and secondary phases are calculated [10] and shown in Table 1. It can be seen that the increase in Cr content gives rise to a decrease in the amount of the main phase and an increase in the amount of the secondary phase, which is in good agreement with the result from XRD analysis. In addition, because the added Cr mainly goes into the V-based solid solution, causing the total content of the elements with larger atom radius such as Hf (0.158 nm) and Ti (0.146 nm) to decrease, the unit cell of the main phase contracts with increasing Cr content, which is also in agreement with XRD analysis.

Table 1 The characteristics of the phase structures in the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloys Alloy

V2.1 TiNi 0.5 Hf 0.05 V2.1 TiNi 0.5 Hf 0.05 Cr 0.074 V2.1 TiNi 0.5 Hf 0.05 Cr 0.113 V2.1 TiNi 0.5 Hf 0.05 Cr 0.152

Phase

Main phase Secondary phase Main phase Secondary phase Main phase Secondary phase Main phase Secondary phase

Composition (at.%) V

Ti

Ni

Hf

Cr

75.23 15.98 77.02 23.48 76.67 24.80 76.61 27.17

19.44 44.51 17.15 40.49 15.87 40.00 15.29 39.37

5.28 36.01 4.46 31.98 3.81 30.80 3.21 28.35

0.04 3.50 0 3.24 0 2.80 0 2.36

– – 1.37 0.81 3.65 1.60 4.89 2.76

Molar fraction

Lattice parameters (nm)

0.69 0.31 0.62 0.38 0.60 0.40 0.57 0.43

a50.30626 a50.50147, c50.80933 a50.30625 a50.50249, c50.82206 a50.30604 a50.50201, c50.82124 a50.30560 a50.50200, c50.82060

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Fig. 2. Scanning electron micrographs of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50– 0.152) alloys.

3.2. Electrochemical properties Fig. 3 shows the maximum discharge capacity of the alloys with various Cr content at the discharge current density of 25 mA / g. It can be seen that V2.1 TiNi 0.5 Hf 0.05

Fig. 3. The maximum discharge capacity of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloy electrodes.

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alloy has the highest discharge capacity of 444 mAh / g, much higher than that of the Cr-added alloys (x50.074– 0.113) of 390–395 mAh / g. When increasing the Cr content x further to 0.152, the maximum discharge capacity decreases to 368 mAh / g. The electrochemical P–C–T curves (298 K) of the above alloys (Fig. 4) indicate that the plateau pressure of the P–C–T curves rises with the addition of Cr, while the width of plateau pressure reduces. We believe that the decrease in the discharge capacity due to the Cr addition can be ascribed to two factors: the decrease in the molar fractions and the lattice parameters of the main phase. The former is mainly responsible for the first noticeable decrease (from 444 to 390–395 mAh / g), while the latter is more responsible for the second decrease (from 390–395 to 368 mAh / g), because compared with V2.1 TiNi 0.5 Hf 0.05 , the equilibrium hydrogen pressure of V2.1 TiNi 0.5 Hf 0.05 Cr 0.074 or V2.1 TiNi 0.5 Hf 0.05 Cr 0.113 is almost the same, while that of V2.1 TiNi 0.5 Hf 0.05 Cr 0.152 is distinctly higher, as a smaller cell volume generally leads to a higher equilibrium pressure. The relationships between discharge capacity and discharge current density for the V2.1 TiNi 0.5 Hf 0.05 Cr x alloys are shown in Fig. 5. At higher current densities (300–500 mA / g), the discharge capacities of the Cr-added alloys are much higher than that of V2.1 TiNi 0.5 Hf 0.05 alloy. From the high-rate dischargeability (HRD400 5 C400 /C25 3 100%) of these alloys at the discharge current density of 400 mA / g (Table 2), we can see that the alloy with x50.152 has the highest high-rate dischargeability (HRD 400 of 73.40%) among the alloys studied. These results are also supported by the results obtained from the micro-polarization curves of the alloys (Fig. 6) and the magnitude of the exchange current density I0 with various Cr content (Table 2) obtained in this study. Fig. 7 shows the cycling capacity degradation curves of

Fig. 4. Pressure–composition isotherms of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x5 0–0.152) alloys at 298 K.

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Fig. 5. Relationships between discharge capacity and discharge current for the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloy electrodes. Table 2 The high-rate dischargeability (HRD 5 C400 /C25 3 100%) and exchange current densities I0 (mA / g) of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloys x

0

0.074

0.113

0.152

HRD (%) I0 (mA / g)

26.50 96.14

63.14 135.2

58.68 120.5

73.40 138.7

the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloy electrodes at the discharge current of 50 mA / g in 100 charge–discharge cycles. All the alloys studied were activated within three charge–discharge cycles. The discharge capacity of V2.1 TiNi 0.5 Hf 0.05 reduces rapidly and its capacity retention after 30 charging–discharging cycles is only 27.66%. With the increase in Cr content, the degradation of the Cr-added alloy slows down. As shown in Table 3, the capacity

Fig. 6. Micro-polarization curves of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50– 0.152) alloy electrodes.

Fig. 7. Discharge capacity versus cycle number for the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloy electrodes charged at 100 mA / g and discharged at 50 mA / g.

retention of V2.1 TiNi 0.5 Hf 0.05 Cr 0.152 is 78.76% after 30 charge–discharge cycles and 42.64% after 100 charge– discharge cycles, much better than that of V2.1 TiNi 0.5 Hf 0.05 . This indicates that the increase in Cr content is helpful in improving the cycling stability of the V2.1 TiNi 0.5 Hf 0.05 Cr x alloy electrodes. Although the dissolution of vanadium in the alloy into KOH electrolyte is fast, Cr restricts the dissolution of vanadium and noticeably improves the corrosion resistance of this series of alloys in KOH electrolyte solution [9].

4. Conclusions All alloys examined consist of a main phase of V-based solid solution with b.c.c. structure and a secondary phase of C14-type Laves phase. The C14 Laves phase precipitates along the grain boundaries of the main phase in the form of a three-dimensional network and increases with increasing Cr content. After adding Cr, the unit cell of the main phase contracts and that of the secondary phase significantly expands. As for the electrochemical properties, the Cr-added alloys (x50.074–0.152) only have a maximum discharge capacity of 368–395 mAh / g, significantly less than that of the V2.1 TiNi 0.5 Hf 0.05 alloy which is 444 mAh / g. The decrease in the molar fractions and the lattice parameters of the main phase are thought to be the main reasons. On the other hand, introducing Cr is beneficial for the highrate dischargeability and the cycle stability of the V2.1 TiNi 0.5 Hf 0.05 Cr x alloys. Among the alloys studied, the V2.1 TiNi 0.5 Hf 0.05 Cr 0.152 alloy shows a HRD400 of 73.40% and a capacity retention rate of 78.76% after 30 charging– discharging cycles.

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Table 3 The capacity retention of the V2.1 TiNi 0.5 Hf 0.05 Cr x (x50–0.152) alloys after 30 and 100 charge–discharge cycles x Capacity retention (%)

After 30 cycles After 100 cycles

Acknowledgements The authors wish to express their gratitude and appreciation for the support from The National Natural Science Foundation of China (No. 50271064) and The National High Technology Research and Development Program of China.

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