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Effect of ball-milling with Ni and Raney Ni on surface structural characteristics of TiV2.1Ni0.3 alloy
Journal of Alloys and Compounds 325 (2001) 299–303
L
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Effect of ball-milling with Ni and Raney Ni on surface structural characteristics of TiV2.1 Ni 0.3 alloy Hiroshi Inoue a , Rie Miyauchi a , Toshiyuki Tanaka a , Weon-Kyung Choi a ,1 , Ryuji Shin-ya b , b a, Jun-ichiro Murayama , Chiaki Iwakura * a
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1 -1 Gakuen-cho, Sakai, Osaka 599 -8531, Japan b Sumitomo Metal Technology Inc., Fusocho 1 -8, Amagasaki, Hyogo 660 -0891, Japan Received 30 March 2001; accepted 13 April 2001
Abstract Surface structural characteristics of TiV2.1 Ni 0.3 –Ni and TiV2.1 Ni 0.3 –Raney Ni composites, which were prepared by ball-milling a TiV2.1 Ni 0.3 alloy with Ni or Raney Ni under an Ar atmosphere, were investigated by X-ray diffractometry, Auger electron spectrometric analysis (AES) and so on. The ball-milling with Ni or Raney Ni did not influence the bulk crystal structure of the TiV2.1 Ni 0.3 alloy. The mutual diffusion layer, which was presumed to be responsible for retarding the progress of the corrosion, was found to be formed in the interface region between Ni and alloy components during the ball-milling with Ni or Raney Ni. The AES depth profile of the constituent elements for the TiV2.1 Ni 0.3 –Ni composite, however, was different from the TiV2.1 Ni 0.3 –Raney Ni composite due to the difference in the coverage and the homogeneity of the surface modification, which also influenced the time course of the rest potential. The ball-milling with Ni and Raney Ni greatly decreased the onset temperature of weight loss assigned to dehydriding from dihydride to monohydride and from monohydride to alloy. 2001 Elsevier Science B.V. All rights reserved. Keywords: Nickel–Metal hydride battery; Hydrogen storage materials; Composite materials; Ball-milling; Gas–solid reaction
1. Introduction V-Based alloys with a body-centered cubic (b.c.c.) structure as a main phase are promising as negative electrode materials for nickel–metal hydride batteries and hydrogen reservoirs for fuel cells because of high hydrogen storage capacity per volume. Several papers have reported on the charge–discharge characteristics of the V-based alloys [1–4]. Recently, we have reported that the TiV2.1 Ni 0.3 alloy with a b.c.c. structure as a main phase showed high hydrogen storage capacity and high discharge capacity, whereas cycle durability was poor probably due to the dissolution of the alloy components in a 6 M KOH solution [5,6]. In order to suppress such deterioration of the TiV2.1 Ni 0.3 alloy, we carried out a surface modification of the alloy by
ball-milling with amorphous MgNi [7,8]. The spectroscopic data for the modified TiV2.1 Ni 0.3 alloy showed that ball-milling caused a mutual diffusion between Mg and some of Ni components in the MgNi alloy and Ti and V components in the TiV2.1 Ni 0.3 alloy at the interface of these alloys, and the resulting mutual diffusion layer was important for suppressing the deterioration of the alloy. The surface modification of the TiV2.1 Ni 0.3 alloy by ballmilling with Ni or Raney Ni, which was an effective catalyst for hydriding and dehydriding, was also found to be effective for improving the charge–discharge cycle durability [9]. In this study, we investigate the change in the surface structural characteristics upon ball-milling of the TiV2.1 Ni 0.3 alloy with Ni or Raney Ni, and we relate these results to the improved cycle durability.
2. Experimental *Corresponding author. Tel.: 181-722-54-9283; fax: 181-722-549283. E-mail address: [email protected] (C. Iwakura). 1 Present address: Industrial Chemistry Major, College of Engineering, Dankook University, 29 Anseo-dong, Cheonan, Choongnam 330-714, South Korea.
The TiV2.1 Ni 0.3 powder was prepared according to the previous paper [6]. The resulting powder was sieved to particle sizes of 25–106 mm for surface modification. The Ni powder of 99.99% (Aldrich) for the surface
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01389-5
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modification of the TiV2.1 Ni 0.3 alloy was used as received. On the other hand, the Raney Ni powder was made from Raney Ni alloy powder (Ni content, ca. 50%, Wako Pure Chemical) according to Ref. [10]. The mixture of the TiV2.1 Ni 0.3 alloy powder and 30 wt.% Ni powder or Raney Ni powder was put into a stainless steel pot and ball-milled at a rotating speed of 180 rev. min 21 for 3 h to prepare the TiV2.1 Ni 0.3 –M (M5Ni or Raney Ni) composite. XRD measurements of the TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites were carried out using an X-ray ˚ 40 kV, 20 mA). diffractometer (Cu Ka /l51.541 A, The surface morphology and concentration profiles of the constituent metals for the TiV2.1 Ni 0.3 alloy and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites were analyzed by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The SEM images of the TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites were observed with a field-emission scanning electron microscope (Hitachi S-4500). The AES analyses were carried out with a PHI Model 680 instrument. In a depth profiling, a 3 kV Ar 1 ion gun was used for sputtering. The depth of the sputtering was calculated using the sputtering rate for SiO 2 under the same conditions. The differential thermal analysis (DTA) and thermogravimetry (TG) were carried out at a heating rate of 108C min 21 from room temperature to 4508C under an argon atmosphere. Each sample was hydrided at 308C and 3 MPa for 12 h before DTA and TG. The rest potential was measured versus a Hg / HgO reference electrode using a pellet-type working electrode prepared in the same manner as the previous paper [6]. The electrolyte solution was 6 M KOH aqueous solution and the counter electrode was a NiOOH / Ni(OH) 2 electrode. The measurement was carried out at 308C.
Fig. 1. X-Ray diffraction patterns for TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites.
ball-milling and Ni modified the alloy surface. On the other hand, the TiV2.1 Ni 0.3 –Raney Ni composite particles tended to have smaller size than the TiV2.1 Ni 0.3 alloy
3. Results and discussion Fig. 1 shows the XRD patterns for TiV2.1 Ni 0.3 –M (M5 Ni and Raney Ni) composites. For each composite, the XRD peaks assigned to the body-centered cubic (b.c.c.) main phase of the TiV2.1 Ni 0.3 alloy and Ni or Raney Ni were observed. The XRD peaks for the b.c.c. main phase was retained even after the ball-milling with Ni or Raney Ni, suggesting that the ball-milling with Ni or Raney Ni did not influence the bulk crystal structure of the TiV2.1 Ni 0.3 alloy. The sharp XRD peaks assigned to Ni were observed for the TiV2.1 Ni 0.3 –Ni composite, while for the TiV2.1 Ni 0.3 –Raney Ni composite, the peaks assigned to Ni were broad and small. Fig. 2 shows the SEM images of TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites. For the TiV2.1 Ni 0.3 –Ni composite, the particle size tended to become large compared to that for the TiV2.1 Ni 0.3 alloy, suggesting that the TiV2.1 Ni 0.3 alloy particles did not pulverize during the
Fig. 2. SEM images for TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites.
H. Inoue et al. / Journal of Alloys and Compounds 325 (2001) 299 – 303
particles and the surface of the resulting composite was uneven, suggesting that the Raney Ni modified the surface of the TiV2.1 Ni 0.3 alloy particles pulverized during the ball-milling. The specific surface area of the TiV2.1 Ni 0.3 alloy, TiV2.1 Ni 0.3 –Ni composite and TiV2.1 Ni 0.3 –Raney Ni composite was evaluated by the BET method. The specific surface area was 0.30 m 2 g 21 for the TiV2.1 Ni 0.3 alloy, 0.22 m 2 g 21 for the TiV2.1 Ni 0.3 –Ni composite and 1.22 m 2 g 21 for the TiV2.1 Ni 0.3 –Raney Ni composite. The difference in the specific surface area between the TiV2.1 Ni 0.3 – Ni and TiV2.1 Ni 0.3 –Raney Ni composites seems to be explainable from the SEM images. For investigating the surface structure of the TiV2.1 Ni 0.3 –Ni and TiV2.1 Ni 0.3 –Raney Ni composites, AES depth profiles of the constituent elements for both composites were measured. The results are summarized in Fig. 3. In both cases, a small amount of Fe was observed due to the contamination from the stainless steel pot during the ball-milling. In the case of the TiV2.1 Ni 0.3 –Ni composite, an oxide
Fig. 3. AES depth profiles of constituent elements for TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites.
301
layer of several nanometers covered the surface. Underneath this thin layer much metallic Ni was present, whereas the content of the other alloy components was low, suggesting that the metallic Ni covered the alloy surface widely and homogeneously. The content of Ni decreased with increasing depth of the sputtering, whereas that of Ti and V increased. This suggests that a mutual diffusion layer is formed in the interface region between the TiV2.1 Ni 0.3 alloy and Ni during the ball-milling, as has been observed for the ball-milling of the TiV2.1 Ni 0.3 alloy with MgNi [7,8]. The mutual diffusion layer of about 100 nm is presumed to be responsible for retarding the progress of the corrosion. In the case of the TiV2.1 Ni 0.3 –Raney Ni composite, a surface oxide layer of several nanometers appeared in analogy with the TiV2.1 Ni 0.3 –Ni composite, but underneath this thin layer the content of O was maintained at ca. 10 at% irrespective of the depth of sputtering. The increase in Ti and V contents ended at about 70 nm, which indicates the thickness of the mutual diffusion layer, whereas the Ni content was high even at the depth of the mutual diffusion layer, as was distinct from the TiV2.1 Ni 0.3 –Ni composite. This suggests that the TiV2.1 Ni 0.3 alloy surface is not homogeneously and perfectly covered with Raney Ni, as expected from the uneven surface morphology of the TiV2.1 Ni 0.3 –Raney Ni composite. The imperfect surface modification of the alloy with Raney Ni seems to give a negative effect for the suppression of the corrosion. Fig. 4 shows the time course of the rest potential for the TiV2.1 Ni 0.3 alloy electrode and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composite electrodes immersed in a 6 M KOH solution. In the case of the TiV2.1 Ni 0.3 alloy, the rest potential shifted in the negative direction with time due to hydrogen absorption caused by the local-cell mechanism [11]. The change in the rest potential was greatly retarded by ball-milling with Ni or Raney Ni. This clearly indicates
Fig. 4. Time courses of rest potential for TiV2.1 Ni 0.3 alloy electrode and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composite electrodes immersed in a 6 M KOH solution.
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H. Inoue et al. / Journal of Alloys and Compounds 325 (2001) 299 – 303
the usefulness of the surface modification by ball-milling with Ni and Raney Ni for improving corrosion resistance. In addition, the ball-milling with Ni retarded the negative shift of the rest potential more effectively than that with Raney Ni. The difference seems to be ascribable to that of the coverage and the homogeneity of the surface modification, as can be expected from Fig. 3, which also seems to explain the difference in the charge–discharge cycle durability. Fig. 5 shows DTA and TG curves for the TiV2.1 Ni 0.3 alloy and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites hydrided at 3 MPa for 12 h. In the DTA curve of the hydrided TiV2.1 Ni 0.3 alloy, two endothermic peaks were observed at about 270 and 3908C and in the TG curve the corresponding two weight losses due to the dehydriding were observed. This strongly suggests a two-step dehydriding reaction from the dihydride with f.c.c. structure to alloy with b.c.c. structure via the monohydride with b.c.t. structure. As Fig. 5(b) shows, the onset temperature of dehydriding from dihydride to monohydride markedly decreased in the case of the TiV2.1 Ni 0.3 –Ni and TiV2.1 Ni 0.3 –Raney Ni composites. In particular, the ball-milling with Raney Ni showed a lower onset temperature of dehydriding than that with Ni. Moreover, the second weight loss due to the dehydriding from monohydride to alloy for both composites also began at a lower temperature than that for the
alloy, as can be seen from Fig. 5(b). These results strongly suggest that the ball-milling with Ni and Raney Ni lowers the energy barrier for both dehydriding steps due to the surface modification or the formation of the mutual diffusion layer.
4. Conclusions The results obtained in this work are summarized as follows. 1. Ball-milling with Ni or Raney Ni does not influence the bulk crystal structure of the TiV2.1 Ni 0.3 alloy. 2. A mutual diffusion layer is formed in the interface region between Ni and the alloy components during the ball-milling with Ni or Raney Ni, and is presumed to be responsible for retarding the progress of the corrosion. 3. The depth profile of the constituent elements for the TiV2.1 Ni 0.3 –Ni composite is different from that for the TiV2.1 Ni 0.3 –Raney Ni composite due to the difference in the coverage and the homogeneity of the surface modification. 4. The change in the rest potential was greatly retarded by ball-milling with Ni and Raney Ni, clearly indicating the usefulness of such surface modification. 5. Ball-milling with Ni and Raney Ni greatly decreased the onset temperature of weight loss assigned to dehydriding from dihydride to monohydride and from monohydride to alloy.
Acknowledgements This work has been partially supported by a Grant-inAid for Scientific Research on Priority Areas (A) of ‘New Protium Function’ (No. 10148105) and Scientific Research on Priority Areas (B) of ‘Ionics Devices’ (No. 11229205) from the Ministry of Education, Science, Sports and Culture of Japan.
References
Fig. 5. (a) DTA and (b) TG curves for the TiV2.1 Ni 0.3 alloy and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites hydrided at 308C and 3 MPa for 12 h.
[1] A.J. Mealand, G.G. Libowitz, J.F. Lynch, G. Rak, J. Less-Common Met. 104 (1984) 133. [2] H. Muller, K. Weymann, J. Less-Common Met. 119 (1986) 115. [3] M. Tsukahara, K. Takahashi, T. Mishima, H. Miyamura, T. Sakai, N. Kuriyama, I. Uehara, J. Alloys Comp. 231 (1995) 616. [4] H.-H. Lee, K.-Y. Lee, J.-Y. Lee, J. Alloys Comp. 239 (1996) 63. [5] H. Inoue, W.-K. Choi, C. Iwakura, in: Proceedings of the 3rd Korea–Japan Joint Seminar on Advanced Batteries, 1999, p. 51. [6] C. Iwakura, W.-K. Choi, R. Miyauchi, H. Inoue, J. Electrochem. Soc. 147 (2000) 2503. [7] W.-K. Choi, T. Tanaka, R. Miyauchi, T. Morikawa, H. Inoue, C. Iwakura, J. Alloys Comp. 299 (2000) 141.
H. Inoue et al. / Journal of Alloys and Compounds 325 (2001) 299 – 303 [8] W.-K. Choi, T. Tanaka, T. Morikawa, H. Inoue, C. Iwakura, J. Alloys Comp. 302 (2000) 82. [9] H. Inoue, R. Miyauchi, R. Shin-ya, W.-K. Choi, C. Iwakura, J. Alloys Comp., in press.
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[10] T. Chiba, M. Okimoto, H. Nagai, Y. Takata, Bull. Chem. Soc. Jpn. 56 (1983) 719. [11] C. Iwakura, W.-K. Choi, S.G. Zhang, H. Inoue, Electrochim. Acta 44 (1999) 1677.
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Effect of ball-milling with Ni and Raney Ni on surface structural characteristics of TiV2.1 Ni 0.3 alloy Hiroshi Inoue a , Rie Miyauchi a , Toshiyuki Tanaka a , Weon-Kyung Choi a ,1 , Ryuji Shin-ya b , b a, Jun-ichiro Murayama , Chiaki Iwakura * a
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1 -1 Gakuen-cho, Sakai, Osaka 599 -8531, Japan b Sumitomo Metal Technology Inc., Fusocho 1 -8, Amagasaki, Hyogo 660 -0891, Japan Received 30 March 2001; accepted 13 April 2001
Abstract Surface structural characteristics of TiV2.1 Ni 0.3 –Ni and TiV2.1 Ni 0.3 –Raney Ni composites, which were prepared by ball-milling a TiV2.1 Ni 0.3 alloy with Ni or Raney Ni under an Ar atmosphere, were investigated by X-ray diffractometry, Auger electron spectrometric analysis (AES) and so on. The ball-milling with Ni or Raney Ni did not influence the bulk crystal structure of the TiV2.1 Ni 0.3 alloy. The mutual diffusion layer, which was presumed to be responsible for retarding the progress of the corrosion, was found to be formed in the interface region between Ni and alloy components during the ball-milling with Ni or Raney Ni. The AES depth profile of the constituent elements for the TiV2.1 Ni 0.3 –Ni composite, however, was different from the TiV2.1 Ni 0.3 –Raney Ni composite due to the difference in the coverage and the homogeneity of the surface modification, which also influenced the time course of the rest potential. The ball-milling with Ni and Raney Ni greatly decreased the onset temperature of weight loss assigned to dehydriding from dihydride to monohydride and from monohydride to alloy. 2001 Elsevier Science B.V. All rights reserved. Keywords: Nickel–Metal hydride battery; Hydrogen storage materials; Composite materials; Ball-milling; Gas–solid reaction
1. Introduction V-Based alloys with a body-centered cubic (b.c.c.) structure as a main phase are promising as negative electrode materials for nickel–metal hydride batteries and hydrogen reservoirs for fuel cells because of high hydrogen storage capacity per volume. Several papers have reported on the charge–discharge characteristics of the V-based alloys [1–4]. Recently, we have reported that the TiV2.1 Ni 0.3 alloy with a b.c.c. structure as a main phase showed high hydrogen storage capacity and high discharge capacity, whereas cycle durability was poor probably due to the dissolution of the alloy components in a 6 M KOH solution [5,6]. In order to suppress such deterioration of the TiV2.1 Ni 0.3 alloy, we carried out a surface modification of the alloy by
ball-milling with amorphous MgNi [7,8]. The spectroscopic data for the modified TiV2.1 Ni 0.3 alloy showed that ball-milling caused a mutual diffusion between Mg and some of Ni components in the MgNi alloy and Ti and V components in the TiV2.1 Ni 0.3 alloy at the interface of these alloys, and the resulting mutual diffusion layer was important for suppressing the deterioration of the alloy. The surface modification of the TiV2.1 Ni 0.3 alloy by ballmilling with Ni or Raney Ni, which was an effective catalyst for hydriding and dehydriding, was also found to be effective for improving the charge–discharge cycle durability [9]. In this study, we investigate the change in the surface structural characteristics upon ball-milling of the TiV2.1 Ni 0.3 alloy with Ni or Raney Ni, and we relate these results to the improved cycle durability.
2. Experimental *Corresponding author. Tel.: 181-722-54-9283; fax: 181-722-549283. E-mail address: [email protected] (C. Iwakura). 1 Present address: Industrial Chemistry Major, College of Engineering, Dankook University, 29 Anseo-dong, Cheonan, Choongnam 330-714, South Korea.
The TiV2.1 Ni 0.3 powder was prepared according to the previous paper [6]. The resulting powder was sieved to particle sizes of 25–106 mm for surface modification. The Ni powder of 99.99% (Aldrich) for the surface
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01389-5
300
H. Inoue et al. / Journal of Alloys and Compounds 325 (2001) 299 – 303
modification of the TiV2.1 Ni 0.3 alloy was used as received. On the other hand, the Raney Ni powder was made from Raney Ni alloy powder (Ni content, ca. 50%, Wako Pure Chemical) according to Ref. [10]. The mixture of the TiV2.1 Ni 0.3 alloy powder and 30 wt.% Ni powder or Raney Ni powder was put into a stainless steel pot and ball-milled at a rotating speed of 180 rev. min 21 for 3 h to prepare the TiV2.1 Ni 0.3 –M (M5Ni or Raney Ni) composite. XRD measurements of the TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites were carried out using an X-ray ˚ 40 kV, 20 mA). diffractometer (Cu Ka /l51.541 A, The surface morphology and concentration profiles of the constituent metals for the TiV2.1 Ni 0.3 alloy and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites were analyzed by scanning electron microscopy (SEM) and Auger electron spectroscopy (AES). The SEM images of the TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites were observed with a field-emission scanning electron microscope (Hitachi S-4500). The AES analyses were carried out with a PHI Model 680 instrument. In a depth profiling, a 3 kV Ar 1 ion gun was used for sputtering. The depth of the sputtering was calculated using the sputtering rate for SiO 2 under the same conditions. The differential thermal analysis (DTA) and thermogravimetry (TG) were carried out at a heating rate of 108C min 21 from room temperature to 4508C under an argon atmosphere. Each sample was hydrided at 308C and 3 MPa for 12 h before DTA and TG. The rest potential was measured versus a Hg / HgO reference electrode using a pellet-type working electrode prepared in the same manner as the previous paper [6]. The electrolyte solution was 6 M KOH aqueous solution and the counter electrode was a NiOOH / Ni(OH) 2 electrode. The measurement was carried out at 308C.
Fig. 1. X-Ray diffraction patterns for TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites.
ball-milling and Ni modified the alloy surface. On the other hand, the TiV2.1 Ni 0.3 –Raney Ni composite particles tended to have smaller size than the TiV2.1 Ni 0.3 alloy
3. Results and discussion Fig. 1 shows the XRD patterns for TiV2.1 Ni 0.3 –M (M5 Ni and Raney Ni) composites. For each composite, the XRD peaks assigned to the body-centered cubic (b.c.c.) main phase of the TiV2.1 Ni 0.3 alloy and Ni or Raney Ni were observed. The XRD peaks for the b.c.c. main phase was retained even after the ball-milling with Ni or Raney Ni, suggesting that the ball-milling with Ni or Raney Ni did not influence the bulk crystal structure of the TiV2.1 Ni 0.3 alloy. The sharp XRD peaks assigned to Ni were observed for the TiV2.1 Ni 0.3 –Ni composite, while for the TiV2.1 Ni 0.3 –Raney Ni composite, the peaks assigned to Ni were broad and small. Fig. 2 shows the SEM images of TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites. For the TiV2.1 Ni 0.3 –Ni composite, the particle size tended to become large compared to that for the TiV2.1 Ni 0.3 alloy, suggesting that the TiV2.1 Ni 0.3 alloy particles did not pulverize during the
Fig. 2. SEM images for TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites.
H. Inoue et al. / Journal of Alloys and Compounds 325 (2001) 299 – 303
particles and the surface of the resulting composite was uneven, suggesting that the Raney Ni modified the surface of the TiV2.1 Ni 0.3 alloy particles pulverized during the ball-milling. The specific surface area of the TiV2.1 Ni 0.3 alloy, TiV2.1 Ni 0.3 –Ni composite and TiV2.1 Ni 0.3 –Raney Ni composite was evaluated by the BET method. The specific surface area was 0.30 m 2 g 21 for the TiV2.1 Ni 0.3 alloy, 0.22 m 2 g 21 for the TiV2.1 Ni 0.3 –Ni composite and 1.22 m 2 g 21 for the TiV2.1 Ni 0.3 –Raney Ni composite. The difference in the specific surface area between the TiV2.1 Ni 0.3 – Ni and TiV2.1 Ni 0.3 –Raney Ni composites seems to be explainable from the SEM images. For investigating the surface structure of the TiV2.1 Ni 0.3 –Ni and TiV2.1 Ni 0.3 –Raney Ni composites, AES depth profiles of the constituent elements for both composites were measured. The results are summarized in Fig. 3. In both cases, a small amount of Fe was observed due to the contamination from the stainless steel pot during the ball-milling. In the case of the TiV2.1 Ni 0.3 –Ni composite, an oxide
Fig. 3. AES depth profiles of constituent elements for TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites.
301
layer of several nanometers covered the surface. Underneath this thin layer much metallic Ni was present, whereas the content of the other alloy components was low, suggesting that the metallic Ni covered the alloy surface widely and homogeneously. The content of Ni decreased with increasing depth of the sputtering, whereas that of Ti and V increased. This suggests that a mutual diffusion layer is formed in the interface region between the TiV2.1 Ni 0.3 alloy and Ni during the ball-milling, as has been observed for the ball-milling of the TiV2.1 Ni 0.3 alloy with MgNi [7,8]. The mutual diffusion layer of about 100 nm is presumed to be responsible for retarding the progress of the corrosion. In the case of the TiV2.1 Ni 0.3 –Raney Ni composite, a surface oxide layer of several nanometers appeared in analogy with the TiV2.1 Ni 0.3 –Ni composite, but underneath this thin layer the content of O was maintained at ca. 10 at% irrespective of the depth of sputtering. The increase in Ti and V contents ended at about 70 nm, which indicates the thickness of the mutual diffusion layer, whereas the Ni content was high even at the depth of the mutual diffusion layer, as was distinct from the TiV2.1 Ni 0.3 –Ni composite. This suggests that the TiV2.1 Ni 0.3 alloy surface is not homogeneously and perfectly covered with Raney Ni, as expected from the uneven surface morphology of the TiV2.1 Ni 0.3 –Raney Ni composite. The imperfect surface modification of the alloy with Raney Ni seems to give a negative effect for the suppression of the corrosion. Fig. 4 shows the time course of the rest potential for the TiV2.1 Ni 0.3 alloy electrode and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composite electrodes immersed in a 6 M KOH solution. In the case of the TiV2.1 Ni 0.3 alloy, the rest potential shifted in the negative direction with time due to hydrogen absorption caused by the local-cell mechanism [11]. The change in the rest potential was greatly retarded by ball-milling with Ni or Raney Ni. This clearly indicates
Fig. 4. Time courses of rest potential for TiV2.1 Ni 0.3 alloy electrode and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composite electrodes immersed in a 6 M KOH solution.
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H. Inoue et al. / Journal of Alloys and Compounds 325 (2001) 299 – 303
the usefulness of the surface modification by ball-milling with Ni and Raney Ni for improving corrosion resistance. In addition, the ball-milling with Ni retarded the negative shift of the rest potential more effectively than that with Raney Ni. The difference seems to be ascribable to that of the coverage and the homogeneity of the surface modification, as can be expected from Fig. 3, which also seems to explain the difference in the charge–discharge cycle durability. Fig. 5 shows DTA and TG curves for the TiV2.1 Ni 0.3 alloy and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites hydrided at 3 MPa for 12 h. In the DTA curve of the hydrided TiV2.1 Ni 0.3 alloy, two endothermic peaks were observed at about 270 and 3908C and in the TG curve the corresponding two weight losses due to the dehydriding were observed. This strongly suggests a two-step dehydriding reaction from the dihydride with f.c.c. structure to alloy with b.c.c. structure via the monohydride with b.c.t. structure. As Fig. 5(b) shows, the onset temperature of dehydriding from dihydride to monohydride markedly decreased in the case of the TiV2.1 Ni 0.3 –Ni and TiV2.1 Ni 0.3 –Raney Ni composites. In particular, the ball-milling with Raney Ni showed a lower onset temperature of dehydriding than that with Ni. Moreover, the second weight loss due to the dehydriding from monohydride to alloy for both composites also began at a lower temperature than that for the
alloy, as can be seen from Fig. 5(b). These results strongly suggest that the ball-milling with Ni and Raney Ni lowers the energy barrier for both dehydriding steps due to the surface modification or the formation of the mutual diffusion layer.
4. Conclusions The results obtained in this work are summarized as follows. 1. Ball-milling with Ni or Raney Ni does not influence the bulk crystal structure of the TiV2.1 Ni 0.3 alloy. 2. A mutual diffusion layer is formed in the interface region between Ni and the alloy components during the ball-milling with Ni or Raney Ni, and is presumed to be responsible for retarding the progress of the corrosion. 3. The depth profile of the constituent elements for the TiV2.1 Ni 0.3 –Ni composite is different from that for the TiV2.1 Ni 0.3 –Raney Ni composite due to the difference in the coverage and the homogeneity of the surface modification. 4. The change in the rest potential was greatly retarded by ball-milling with Ni and Raney Ni, clearly indicating the usefulness of such surface modification. 5. Ball-milling with Ni and Raney Ni greatly decreased the onset temperature of weight loss assigned to dehydriding from dihydride to monohydride and from monohydride to alloy.
Acknowledgements This work has been partially supported by a Grant-inAid for Scientific Research on Priority Areas (A) of ‘New Protium Function’ (No. 10148105) and Scientific Research on Priority Areas (B) of ‘Ionics Devices’ (No. 11229205) from the Ministry of Education, Science, Sports and Culture of Japan.
References
Fig. 5. (a) DTA and (b) TG curves for the TiV2.1 Ni 0.3 alloy and TiV2.1 Ni 0.3 –M (M5Ni and Raney Ni) composites hydrided at 308C and 3 MPa for 12 h.
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