The TiV3Ni0.56 hydride electrode: its electrochemical and cycle life characterization

Journal of

AILLOY5 AND COMPOUNDS ELSEVIER

Journal of Alloys and Compounds 231 (1995) 616-620

The TiV3Nio.56 hydride electrode: its electrochemical and cycle life characterization Makoto Tsukahara a, Kunio Takahashi a, Takahiro Mishima a, Hiroshi Miyamura b, Tetsuo Sakai b, Nobuhiro Kuriyama b, Itsuki Uehara b alMRA Material R&D Co., Ltd., 5-50 Hachiken-cho, Kariya-shi, Aichi 448, Japan bOsaka National Research Institute, Midorigaoka, Ikeda-shi, Osaka 563, Japan

Abstract Charge/discharge cycle tests for the TiV3Nios6 electrode were conducted in order to investigate the processes of activation and deterioration. Cracking formation was observed to occur across the alloy grain during the activation process. After a few more cycles following the activation, smaller cracks were observed between the matrix and a secondary phase and also within the secondary phase. In the deterioration process titanium and vanadium were found to dissolve selectively from both phases. When the electrode lost its discharge ability, titanium and vanadium in the secondary phase had almost disappeared, leaving the nickel layer, while the matrix phase still retained the original composition and hydrogen storage ability. Keywords: Hydrogen electrode; Vanadium based alloy; Cycle life; Discharge capacity; Microstructure

L Introduction Vanadium hydrides have been attracting interests because of the high mobility of hydrogen at room temperatures [1]. Pressure-composition isotherms (PCT) for the reaction between w n 1 and V H 2 have a pressure plateau at moderate pressure and temperature [2]. The hydriding properties of vanadium were controlled and improved by alloying with other metals [3-5]. Such applications as hydrogen compressors, metal hydride heat pumps and isotope separation were suggested [4]. We have investigated the electrode properties [6] and microstructures [7] of the vanadium based allo~,s containing titanium and nickel. The alloy of TiV3Ni0.56 consisted of the fl-phase matrix and the 8-secondary phase, where the main fl-phase was a (flTi, V) based solid solution with a body-centered cubic (bcc) structure and the 6-secondary phase was a TiNi based compound with a bcc structure [8]. This alloy showed a maximum discharge capacity of 420 Ah kg -1. The secondary phase was precipitated along the grain boundary of the matrix and formed a three-dimensional network. The formation of the secondary phase network was important in order to enhance the elec0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSD1 0925-8388(95)01738-0

trode kinetics because it could work as a catalyst and/or a current collector [9]. In the present paper the activation and deterioration processes of the alloy electrode are reported with regard to changes in the microstructure and the alloy composition.

2. Experimental details The alloy sample of TiV3Ni0.56 was prepared by arc melting of titanium, vanadium and nickel on a watercooled copper hearth in argon gas. The as-cast sample was once hydrided under hydrogen pressure lower than 3.3 MPa by heating up to 673 K before being mechanically crushed into fine powder (less than 100 mesh). The alloy electrodes were prepared by mixing 20 mass% copper coated alloy with 10 mass% FEP binder (Dalkin Co. tetrafluoroethylene-hexafluoropropylene copolymer) and then hot-pressing the mixture on a nickel mesh at 573 K. The charge/discharge cycle test of each electrode was conducted in a half cell at 293 K using an HglHgO reference electrode, a 6 M KOH solution and an Ni(OH)2 counter electrode.

617

M. Tsukahara et al. I Journal o f Alloys and Compounds 231 (1995) 616-620

The electrode was charged at 100 A kg -~ and discharged at 50 A kg -~ to the cut off voltage of - 0.7 V vs. HglHgO. Each electrode sample; was withdrawn from the cell after 5, 10, 20, 30, 50 and 77 cycles, rinsed with pure water and dried under vacuum at 333 K. Cross-sections of the electrodes were examined by scanning electron microscopy (SEM) with electron probe X-ray microanalysis (EPMA) in order to determine the alloy composition of each phase. Crystal structures were examined by X-ray powder diffraction (XRD) analysis for the samples which were degassed under vacuum at 673 K. The electrolyte solution in the test cells was analyzed after cycle tests by inductively coupled plasma-atomic emission spectroscopy (ICP-AES)in order to determine the contents of dissolved elements,

3. Results and discussion Discharge capacity (Ca) vs. charge/discharge cycle curves up to 5, 10, 20, 3(}, 50 and 77 cycles are plotted in Fig. 1. The repeatability of the seven curves seems good enough to examine the capacity deterioration of the alloy. The electrode was activated so easily in the K O H solution that a maximum discharge capacity was obtained after the first: few cycles. The discharge

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capacity, however, rapidly decreased and vanished after 77 cycles. The electrode was fully activated by a couple of cycles and showed the maximum Cj value. A crosssection of the electrode sample after the first five cycles is shown in Fig. 2. The matrix in the alloy was a vanadium rich phase with the bcc structure, while a secondary phase was a TiNi based phase with the bcc structure [9]. The network-forming secondary phase was identified as a titanium and nickel rich phase in Fig. 2(b) and (d). During the activation process cracking across the powder grain occurred, making a fresh surface on the alloy. Fig. 3 shows the cross-section of the electrode alloy after ten cycles. Smaller cracking was observed between the matrix and the secondary phase and also within the secondary phase. The cracking could be ascribed to the difference in expansion ratios between the two phases. Fig. 4 shows the cross-section of the electrode after

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618

M. Tsukahara et al. / Journal of Alloys and Compounds 231 (1995) 616-620

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77 cycles. The secondary phase almost disappeared, as shown in Fig. 4(a). The characteristic X-ray images of titanium, vanadium and nickel show that titanium and vanadium have almost dissolved, leaving the nickel layer. However, the grains of the matrix still retained the original alloy composition though the matrix grains

showed rounded edges and diminished grain size. Fig. 5 shows the composition changes of the matrix and the secondary phase with the cycle number by EPMA analysis. These compositions were obtained by measuring several points in each phase. The composition in the matrix did not change with the cycle number, while the contents of titanium and vanadium in the secondary phase rapidly decreased with the cycle number, Concentration changes of titanium, vanadium, nickel and copper in the K O H solution with cycle number are shown in Table 1. The concentrations of titanium and vanadium increased with the cycle number. Nickel was detected in the solution after 50 cycles in very low concentration. This result is in good agreement with that by the above EPMA analysis on the cross-sections. Copper was not detected through the cycle test. The coated copper was not dissolved into the K O H solution. It is clear that titanium and vanadium dissolve into the K O H solution with the charge/discharge cycling, leaving a nickel layer on the grain boundaries, The X R D patterns in Fig. 6 show that the intensity of peaks due to nickel, titanium oxide and the vana-

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50 0m Fig. 4. Scanning electron micrograph image with back scattered electron (a) and characteristic X-ray micrograph images of Ti Ka (b), V Ka (c) and Ni Ka (d) for the TiV3Ni056 electrode crosssection after the 77th cycle.

dium oxide increased with the cycle number. The crystal structure and its lattice constants for the /3phase matrix of TiVaNi0.56 were unchanged though the peak width became broad with increasing cycle number. The peaks due to the secondary phase were not clear because of the broadness of the peaks due to the matrix. The deposition of titanium oxide and vanadium oxide suggests that both elements of titanium and vanadium are oxidized and partially dissolved into the electrolyte solution, as shown in Table 1. During the cycle test the matrix kept its composition and crystal structure. As described in the previous paper [9], it was confirmed that even after 77 cycles the alloy reacted with hydrogen gas with almost the same equilibrium pressure as that before the cycle tests. The rapid capacity decay of the alloy during cycles could be ascribed to the surface deterioration because the matrix is not significantly corroded. A lot of small cracks were observed between the matrix and the secondary phase and within the sec-

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Journal of Alloys and Compounds 231 (1995) 616-620

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Summary

80 The activation and deterioration processes of the TiV3Nio.56 electrode could be described as follows:

(b)

(1) The first few cycles cause cracks across the grain. Fig. 5. Composition changes of the matrix (a)and the secondary phase (b) obtained by EPMA analysis with cycle number, Table 1 Concentration changes of metal elements in KOH solution with the charge/discharge cycle number Cycle number, n C(Ti) C(V) C(Ni) C(Cu)

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ondary phase. When the discharge capacity was lost, the secondary phase almost disappeared. Titanium and vanadium in the secondary phase were selectively oxidized and dissolved into the KOH solution, leaving the nickel layer. It is concluded that the secondary phase has an important role in the electrochemical reaction as a catalyst and/or a current collector, while the matrix w o r k s as a m a i n h y d r o g e n storage phase.

The cracking between the matrix and the secondary p h a s e a n d within the s e c o n d a r y p h a s e w o u l d acceler-

A fresh surface is f o r m e d and fully activates the electrode. T i t a n i u m and v a n a d i u m begin to dissolve

into the KOH solution. (2) After a few more cycles following activation, cracking occurs b e t w e e n the matrix and the s e c o n d a r y

phase and also within the secondary phase. This type Of cracking w o u l d be caused by the different expan-

sion ratios of the matrix and the secondary phase due to hydrogen absorption. (3) Titanium and vanadium are dissolved into the KOH solution from the surface of the matrix, reducing the grain size. Titanium and vanadium of the secondary phase are dissolved into the solution, leaving the nickel layer. T h e dissolved titanium and v a n a d i u m

are partly deposited as oxides on the electrode. (4) After the secondary phase has disappeared, the discharge ability is lost although the matrix still has hydrogen storage ability.

References [1] T. Schober and H. Wenzl, in G. Alefeld and J.V61kl (eds.), Hydrogen in Metals H, Topics in Applied Physics, Vol. 29, Springer-Verlag, Berlin, Heidenberg, 1978, p. 11. [2] J.J. Reilly and R.H. Wiswall, Inorg. Chem,, 9 (1970) 1678.

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M. Tsukahara et al. / Journal o f Alloys and Compounds 231 (1995) 616-620

[3] A.J. Maeland, G.G. Libowitz, J.F. Lynch and G. Rak, J. LessCommon Met., 104 (1984) 133. [4] G.G. Libowitz and A.J. Mealand, Mater. Sci Forum, 31 (1988) 177. [5] A. Kagawa, E. Ono, T. Kusakabe and Y. Sakamoto, J. LessCommon Met, 172 (1991) 64. [6] M. Tsukahara, K. Takahashi, T. Mishima, H. Miyamura, N. Kuriyama, T. Sakai and I. Uehara, Abstr. ll3th Meet. Japan Inst. Metal, 1993, p. 480; M. Tsukahara, K. Takahashi, T. Mishima, A. Isomura, I. Uehara, K. Oguro, T. Sakai and N. Kuriyama, Japanese Patent H6-228699.

[7] M. Tsukahara, K. Takahashi, T. Mishima, H. Miyamura, N. Kuriyama, T. Sakai and I.Uehara, Abstr. i14th Meet. Japan Inst. Metal, 1993, p. 95. [8] V.N. Eremenko, L.A. Tret'chenko, S.B. Prima and E.L. Semenova, Soy. Powder Metall. Met. Ceram., 23 (1984) 613. [9] M. Tsukahara, K. Takahashi, T. Mishima, T. Sakai, H. Miyamura, N. Kuriyama and I.Uehara, J. Alloys Comp., in preparation.