The electrochemical charge-discharge properties of ZrCrNi hydrogen storage alloys

Journal of

AND COMPOUNDS ELSEVIER

Journal of Alloys and Compounds 231 (1995) 573-5??

The electrochemical charge-discharge properties of Zr-Cr-Ni hydrogen storage alloys Lei Yongquan, Yang Xiaoguang, Wu Jing, Wang Qidong Department of Material,: Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People's Republic of China

Abstract

Zr(CrxNi 1_x)2(0.15 ~
1. Introduction N i - M H secondary batteries which adopt hydrogen storage alloys as negatiw,~ electrodes are replacing the conventional N i - C d batteries, because the former batteries exhibit a higher electrochemical capacity, no m e m o r y effect, higher ability to stand overcharge and overdischarge, no heavy metal environmental poilution and compatibility of voltage characteristics with the latter. Until now many electrochemical studies have been carried out on the negative-electrode materials such as the AB 5 type L a - N i , A B or AB2-type T i - N i and AB2-type Zr--Ni systems [1-3]. Shatiel et al. [4] studied the hydrogenation characteristics of Zr-based Laves phase alloys and found that the compound ZrV2 had a high hydrogen storage capacity, i.e. 6 H - Z r V 2. On this basis, Wakao et al. [5] added the active electrochemical catalytic element Ni to ZrV2 to reduce the stability of its hydrides and found that alloys in the specific composition range Zr(V0.33Ni0.67)2+ a, 0 ~< a -'G0.4, which formed the pseudobinary Laves phase of ~the C15 type, exhibited larger 0925-8388/95/$09.50 © 1995 EJsevier Science S.A. All rights reserved SSD1 0925-8388(95)01730-5

electrochemical discharge capacities of 2973 1 1 m A h g -1 at a current density of 1 0 m A c m -2 at 298 K and that the partial molar enthalphy of hydriding increased as the degree of non-stoichiometry increased; they proposed a mechanism for the improvement of the polarization performance. Later a study on the partial substitution of A and B in Z r V - N i system alloys was carried out and a Zr0.sTi0.5(Vo.375Ni0.625)2 alloy hydride electrode, which had a very large discharge capacity of 349 m A h g- 1 at a current density of 1 6 m A g -1 at 2 9 8 K [6-8] was found. The hydrogen capacity of this alloy was satisfactory but its electrode cycle stability was poor. Ti did not have a good effect on its cycle life. It was the severe dissolution of the element V in K O H solution within the scope of practical electrode potential that caused the electrode life to deteriorate. It appears to us that it is worthwhile to carry out the study on the electrochemical performances of Z r - C r - N i system alloy electrodes since ZrCr 2 has a large hydrogenabsorbing capacity, which is only next to that of ZrVz, and Cr is the resistant to K O H solution. The present

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L. Yongquan et al. / Journal of Alloys and Compounds 231 (1995) 573-577

paper is a summary of our investigation and some results of X-ray photoelectron (XPS) analysis are also reported.

2. E x p e r i m e n t a l

details

The alloys designated as Zr(CrxNil_x)2(0.15~x~< 0.65) were prepared by arc melting under an argon atmosphere. The purities of the constituent metals Zr, Cr and Ni were above 99.9%. The alloy in as-cast state was crushed mechanically, and the electrode powder was made to pass through 360 mesh sieve. Powder X-ray diffraction (XRD) data were obtained from - 3 6 0 mesh inactivated alloy powders using a Rigaku C-max-III B diffractometer with Cu K a radiation (A = 0.154 459 8 nm) and a nickel diffracted-beam filter. The hydride electrodes were prepared by cold pressing the mixtures of different powder (-360 mesh) with powdered electrolytic copper (300 mesh) in the weight ratio of 1:2 to form porous pellets of 1 0 m m diameter in copper holders. Electrochemical chargedischarge tests were carried out in an opened standard trielectrode electrolysis cell, in which the counterelectrode was nickel oxyhydroxide, the reference electrode was H g / H g O / 6 M KOH, and the electrolyte used in all experiments was 6 M K O H solution. The discharge capacities of hydride electrodes were determined by the galvanostatic charge-discharge method. The electrodes were fully charged at a current density of 100 m A g-~ and discharged at 50 m A g-~ the cut-off potential was set at -0.6 V. Discharge curves were recorded with a y - t recorder, The alloy electrode samples were analyzed by XPS before and after electrochemical charge-discharge cycles. The measurements were performed using a VG Escalab MkII electron spectrometer. After the electrodes had undergone 30 charge-discharge cycles the K O H solution was first rinsed off the electrodes with distilled water; the electrodes were then dried in air and subsequently placed in an ultrahigh vacuum chamber at about 10 -9 torr. AI K a radiation was used for XPS. The spectra were recorded with a constant pass energy of 50 eV corresponding to an energy resolution of about 2 eV. The sputter depth profiles (Ar ~, 4KeV) were evaluated using a standard rate of 8 nm min- ~

3. Results and discussion Figs. l(a) and l(b) represents the XRD patterns for x =0.35 and x = 0.60 respectively of Zr(Cr/Ni~_~) 2 alloys. The pattern for Zr(Cro.35Nio.65)2 indicates that the major phase of this alloy is C15 Laves phase, and the three extra small peaks around 20 = 37.84, 39.02

^ ~.z

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Fig. 1. XRD for Zr(Cr~N~ ~)2 alloys; (a) x = 0.35; (b) x = 0.6.

and 40.22 ° are identified to be those of orthorhombic ZrTNilo phase, the lattice parameters of which are a-= 1.238 nm, b = 0.9284 nm and 0.9093 nm [9]. Fig. l(b) shows a C14 hexagonal structure without the ZrNilo phase. The fact that no Zr7Ni~o phase exists is reasonable as the concentration of Ni is lower. Transition from the C15 f.c.c, structure to the C14 hexagonal structure occurs in the approximate x range 0.45-0.50. In Table 1 the major phases and lattice parameters of the Zr(CrxNi~_x) 2 alloys are listed. The lattice constants increase linearly as the Cr content increases. Fig. 2 represents the dependence of the electrochemical discharge capacity and hydrogen-desorbing capacity on x. It is found that the discharge capacity reaches a maximum of 305 m A h g-~ at the composition Zr(Cro.35Nio.65)2 at the discharge current density of 50 m A g-1 and 298 K. The result is similar to that of Zr-V-Ni system alloys (297 mA h g-~ for Table 1 Crystal structures and their lattice constants in Zr(Cr~Ni~ ,)2 system alloys Alloy Main phase a (nm) c (nm) Zr(Cro.zoNio 8o)2

C15

Zr(Cro.zsNio.75)2

C15

0.7023

Zr(Cro.33Ni0.67)2

C15

0.7051

Zr(Cro ~5Nio 65)2

C15

0.7054

Zr(Cr014oNioi~o): Zr(Cr045Nio~5)2 Zr(Cro.5oNioso)2 Zr(Cro55Nio45)z Zr(Cr°6°Ni°4°):

C15 C15 C14 C14 C14

0.70082

0.7057 0.7068 0.5009 0.5016 0.5025

0.8329 0.8347 0.8344

575

L. Y o n g q u a n et al. / J o u r n a l o f A l l o y s a n d C o m p o u n d s 231 (1995) 5 7 3 - 5 7 7

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x Fig. 2. Capacity charge with composition for Zr(Cr Ni1 x)2: -m-, discharge at 50 mA g i, 298 K; - - - , desorption at 473 K. Zr(W0.335Ni0.625)2 [5]). There is a sharp decrease in electrochemical discharge capacities in the Cr concentration regions x < 0.133 and x > 0.50. The hydrogen capacities desorbed in gaseous state were measured on the condition that the activated hydride powders released hydrogen at 475; K and 1 atm after being fully charged in hydrogen (purity, 99.9999%) at 4.5 MPa and room temperature. Electrochemical capacities were calculated from the amount of hydrogen gas desorbed. It is evident from the figure that the increase in Cr concentration leads to a significant increase in hydrogen storage capacity. In particular, when the C14 phase begins to form at x = 0.50, there is a high rise in capacity. It seems that the C14 hexagonal structure, which has larger tetrahedral interstitial cavities, is beneficial to hydrogen storage capacity, However, its electrochemical discharge capacity decreases obviously. We t12.¢to ascribe this to the formation of more stable hydrides with higher Cr contents and less catalytically active sites of Ni. The activity of hydride electrodes is an important factor for electrode performances. Fig. 3 shows the dependence of activity on particle size of Zr(Cr0~sNi0.5)2. The alloy particles were crashed mechanically to obtain +140 mesh and -360 mesh respectively. The electrode with + 140 mesh powder in the first cycle exhibited discharge ability comparable with that of -360 mesh powder, but its C i / C m a x increases slowly in the subsequent 15 cycles. The electrode containing fine powder shows no discharge capacity in the first 5 cycles. In comparison with the electrode formed with powder ground mechanically, another electrode was formed with the same alloy powder pulverized by cycling in hydrogen atmosphere to a particle size below 2;60 mesh sieve. It can be seen in the same figure that tlbese electrodes show the best activity. It is interesting to see that the electrodes containing powder of the: same size (-360 mesh)have

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N/cycle humor Fig. 3. Activity property dependence of alloy particle size of Zr(Cr0.sNi0.5)2: - - - , +140 mesh; -~-, -360 mesh;.... 360 mesh (pulverized by cycling in hydrogen gas).

a similar slope of increment d ( C i [ C m a x ) / d l V , The improvement in the electrode with powder pulverized in H 2 gas is tentatively attributed to the formation of many more defects during cycling in the H 2 atmosphere. Fig. 4 shows the cathodic polarization curves and discharge curvesofZr(Cr0.35Ni0.65)2 electrodes treated with H F - H 2 0 2 ) . The alloy powder was obtained by cycling in a H 2 atmosphere. Both electrodes were prepared by first pasting a colloidal mixture of alloy powder and poly(vinyl alcohol)glue of 3wt.% concentration onto a foamed Ni plate and then pressed cold. It can be seen that, at the same cathodic current density, the untreated electrode exhibits a larger overpotential than the treated electrode does. This indicates that the untreated electrode has difficulty in absorbing hydrogen in K O H solution to form hydride because of the generation of hydrogen gas. The same phenomenon for anodic polarization potentials in Fig. 4(b) can be observed. It is noteworthy that the initial discharge capacity of the treated sample reaches l l 0 m A h g -1 while the untreated sample almost shows no capacity at all. The charge-density cycle number for reaching the maximum discharge capacity C~nax is obviously less for the treated sample than for the untreated sample. It can be concluded that the treatment with H F - H 2 0 2 improves the activity, or that the surface state of alloy electrodes affects the polarization performance and the hydrogen storage ability. Fig. 5 depicts the relation of discharge capacity versus cycle number. Both electrodes were charged at 360 mA g-1 for 1 h and discharged at 180 mA g-~ for 1 h, with a depth of discharge of around 60-70% (i.e. Cm,x = 300 mA h g-~). The discharge capacity was measured at a discharge rate of 50 mA g-~ every 50 cycles. The figure shows that after 250 cycles both

L. Yongquan et al. / Journal of Alloys and Compounds 231 (1995) 573-577

576

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Fig. 4. (a) Cathodic polarization curves (1 mV s-l; 298 K). (b) Discharge curves for different numbers of cycles of the ZrCro.7Nit. 3 electrode (50 m A g t; 298 K); . . . . , untreated; - - - , surface treated with H F - H 2 0 2 solution.

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870

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250

300

Fig. 6. X-ray photoelectron surfaces (curves A) virgin and activated electrode surfaces (curves B).

N / c y c l e number

Fig. 5. Capacity changes with numbers of cycle at 5 0 m A g

(303 K): Zr(Cr0.35Ni0.65)z; . . . . . . . Zr(Cr0.sNi05)2. electrodes degrade only slightly. The excellent cycle stability is probably due to the good resistance of Cr and Zr against KOH solution, Fig. 6 represents the Zr3ds/2, Cr3p3/2, Ni3P3/2 X-ray photoelectron spectra of the Zr(Cr0.sNi0.5) 2 electrode before and after electrochemical cyclings, The X-ray photoelectron spectra of the original surface consist of Zr 3d5/2 at 186.5 eV, Cr 3p3/2 at 575.5 eV and Ni3p3/2 at 856.2 eV, and for the activated electrode surface after 30charge-discharge cycles the peak positions of Zr and Ni shift to 183.66 eV and 856.6 eV, and the peak of Cr 3P3/2 cannot be detected at all. As the element Zr has a low electronegativity (an Allred-Rochow value of 1.22), it usually exists in the oxide state (i.e. ZrO2) in air or KOH solution. We consider that the shift in the Zr 3d5/2 peak position is due to the difference in structure of the Zr oxide surface layers of two kinds of electrode, instead of the discrepancy in Zr chemical valences. The O ls peaks

also revealed two chemical states namely that at 02 and O H - in the primative and activated surfaces respectively. After sputtering off 7 nm by Ar + bombardment, the Zr 3d5/2 peaks of both electrodes arrive at the same position of 183.0 eV. It appears that the surface layer within about 5nm plays an important role in the activation process. This layer can be removed by being immersed in HF-H202 according to the following reaction: ZrO 2 + 6HF--->H2ZrF6 + HzO

(1)

In the mean time, the transitional elements Cr and Ni would react with H202 and be dissolved away from the alloy surface. This makes the surface porous. In this case the H 2 0 molecule can penetrate the surface layer without hindrance. The surface constitution of electrodes was analyzed. For the unactivated electrodes, the composition of component elements on the surface was identical with its bulk composition if the O content was not consid-

L. Yongquan et al. / Journal of Alloys and Compounds 231 (1995) 573-577

ered and, for the activated electrodes, a comparatively higher concentration of Zr on the surface was detected owing to the dissolution of Cr and Ni. As the electrodes were prepared by pressing together the spherical alloy particles, metallic Zr, Ni and Cr and their oxides coexist even when a surface layer of 60 nm was sputtered off.

577

of Zr on activated electrode samples; the structures of surface oxides are believed to be important for activation.

Acknowledgment The authors are grateful for financial support provided by the National 863 (8.5) Program.

4. Conclusions (1) The main crystal structure of Zr(CrxNi~_x)2(0.15 ~
References

ing the electrodes in H F - H 2 0 2 solution as well as cycling in gaseous H2. (4) Z r - C r - N i alloy e l e c t r o d e s h a v e long e l e c t r o chemical charge-discharge cycle lives. (5) The XPS analyse,; demonstrates a segregation

[6] H. Sawa et al., Denki Kagaku, 58(9) (1990) 862. [7] H. Sawa et al., Denki Kagaku, 59(11) (1991) 945.

[11 J.J. Willems, Philips J. Res., 39(1) (1984) 1. [2] S.Wakao, H. Sawa et al., J. Less-Common Met., 131 (1987) 311.

[3] K. Sapru et al., US Pat. 4,551,400, 1985. [4] D. Shatiel et al., J. Less-Common Met., 53 (1972) 117. [5] H. Sawa et al., Materials Trans., Jpn. Inst. Met., 31(6) (1990) 487.

[8] H. Sawa et al., Oenki Kagaku, 59(11) (1991) 950.

[9] Yu Jingyun, Ph.D. Thesis, Zhejiang University, 1994, p. 6.