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Electrochemical hydrogen storage of ball-milled Mg-rich Mg–Nd alloy with Ni powders
Journal of Alloys and Compounds 433 (2007) 269–273
Electrochemical hydrogen storage of ball-milled Mg-rich Mg–Nd alloy with Ni powders Z.W. Lu, S. Sun, G.R. Li, T.Y. Yan, J.Q. Qu, X.P. Gao ∗ Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai University, Tianjin 300071, China Received 31 May 2006; received in revised form 13 June 2006; accepted 13 June 2006 Available online 1 August 2006
Abstract The Mg-rich Mg–Nd alloy was synthesized by salt-cover-melting. It is found that the Nd5 Mg41 phase with tetragonal structure exists as the main phase in the as-cast Mg-rich Mg–Nd alloy. The microstructure and electrochemical hydrogen storage of the composites prepared by ball-milling the Mg-rich Mg–Nd alloy with Ni powders are investigated. The result reveals that the addition of a larger amount of metallic Ni during the ball-milling process is beneficial to the formation of highly dispersed Ni nanocrystalline on the amorphous Mg-rich Mg–Nd alloy matrix of the composites. The composite, prepared by ball-milling the Mg-rich Mg–Nd alloy with 200 wt.% Ni, is demonstrated to have the discharge capacity of about 953 mAh/g in the first cycle. The appearance of stronger hydrogen adsorption peaks in cyclic voltammograms is very sensitive to the surface Ni-rich dispersion of the composites. The rate-determining step for the electrochemical reaction is transformed from a mixed process of the hydrogen diffusion and surface charge-transfer to a single process of the surface charge-transfer as the Ni amount in the Mg-rich Mg–Nd alloy–Ni composites is increased. © 2006 Elsevier B.V. All rights reserved. Keywords: Rare earth alloys; Hydrogen absorbing materials; Electrochemical reactions
1. Introduction Nickel–metal hydride (Ni–MH) batteries using metal hydrides as negative electrode materials have been commercialized due to their high reversible energy storage capacity, excellent long-term cycling stability and good electrochemical reaction kinetics. Numerous investigations on a new hydrogen storage alloy with a high discharge capacity have been performed to further increase the energy density of Ni/MH batteries. Mgbased alloys have attracted much interest because of their high hydrogen storage capacity and low cost [1–3]. Mechanical alloying (MA) is an effective way to synthesize Mg-based alloys or Ti-based alloys with an amorphous or nanocrystalline structure for electrochemical applications [4–8]. Recently, the composites prepared by ball-milling rare earths–Mg alloys with Ni or Co powders have been demonstrated to be potential materials for the electrochemical hydrogen storage in alkaline solutions [9–12]. For example, it was found that the initial discharge capacity of ball-milled La2 Mg17 and LaMg12 alloy with Ni pow-
der can reach to over 1000 mAh/g (La2 Mg17 or LaMg12 alloys as active materials) [9,10]. The ball-milled Ce2 Mg17 –Ni and La2 Mg17 –Co composites were reported to have a high electrochemical capacity for hydrogen storage [11,12]. Furthermore, the similar result was also obtained in the PrMg12 –Ni composites [13]. The Mg–Nd system consists of the intermetallic compounds Mg12 Nd, Mg41 Nd5 , Mg3 Nd, Mg2 Nd and MgNd [14,15]. Mg-rich Mg–Nd or Mg–Ni–Nd alloys were reported to absorb hydrogen up to 4.7 mass% above 423 K and completely desorb it above 453 K [16,17]. However, the electrochemical hydrogen storage of the Mg-rich Mg–Nd or Mg–Ni–Nd alloys has not been reported so far. In this work, the Mg-rich Mg–Nd alloy was synthesized by salt-cover-melting. Electrochemical properties of the composites, prepared through ball-milling the Mg-rich Mg–Nd alloy with Ni powders, were investigated in order to explore new materials of rare earth–Mg alloys for electrochemical hydrogen storage. 2. Experimental
∗
Corresponding author. Tel.: +86 22 23500876; fax: +86 22 23500876. E-mail address: [email protected] (X.P. Gao).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.051
The Mg-rich Mg–Nd (with the composition of Nd5 Mg41 ) alloy was synthesized through a salt-cover-melting process by melting the stoichiometric mixture
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of metallic Nd and Mg, and then the ingot was pulverized to 200 meshes. The negative electrode materials were prepared by further mechanical alloying of the Mg-rich Mg–Nd alloy with the different weight percentage of carbonyl nickel powders (255 INCO, 200, 150 and 100 wt.%). All ball-milling processes were performed with a weight ratio of ball to powders of 20:1 at 290 rpm for 11–16 h under Ar atmosphere in a cyclohexane solution in a planetary-type ball-mill. The microstructure of the alloy powders were analyzed by X-ray diffraction (XRD, Rigaku D/max-2500) and transmission electron microscopy (TEM, FEI Tecnai 20). The negative electrodes were constructed by mixing the as-prepared composites with carbonyl nickel powder in a weight ratio of 1:3. The mixture powder was pressed under 30 MPa into a small pellet of 10 mm diameter and 1.5 mm thickness. The electrochemical measurements were conducted in a three-compartment cell using a Land Battery Test instrument. A sintered nickel electrode with a large capacity and an Hg/HgO electrode in 6 M KOH solution served as counter and reference electrodes, respectively. The electrodes were charged at a current density of 1000 mA/g for 80 min, and then discharged at current density of 50 mA/g to −0.6 V (versus Hg/HgO) after resting for 30 min at room temperature. When the discharge capacity was calculated, only the Mg-rich Mg–Nd alloy was considered as the active material. The cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) experiments were taken after the electrodes were fully charged using an IM6e Electrochemical workstation. The parameters of EIS plots were fitted by the Zview2 software. The working electrodes for CV experiments were constructed from an apparent surface of 1.0 cm2 porous nickel without addition of carbonyl nickel powders.
3. Results and discussion XRD patterns of the as-cast Mg-rich Mg–Nd alloy and the composites prepared by ball-milling the Mg-rich Mg–Nd alloy with Ni powders are shown in Fig. 1. It is shown from the XRD pattern that the as-cast Mg-rich Mg–Nd alloy has a multiphase structure as reported previously [18]. The Nd5 Mg41 phase exists as the main phase with the Ce5 Mg41 -type tetragonal structure (I4/m, JCPDS Card No 65-1632). However, some weak diffraction peaks are also detected in XRD patterns except for the Nd5 Mg41 phase. Two peaks at 2θ angles of 20.6◦ and 40.2◦ are indexed as the cubic phase of NdMg3 alloy (JCPDS Card No 26-426), which is in accordance with the diffraction peaks of REMg3 alloy (RE = La, Ce, Pr, Tb and Gd). The small peak at 2θ
Fig. 1. XRD patterns of the as-cast Nd5 Mg41 alloy and the composites prepared by ball-milling Nd5 Mg41 alloy with Ni powders (() NdMg3 ; () unconfirmed phase).
angle of 28.3◦ cannot be identified with any known phases. Similar to the RE–Mg alloys system [9–13] investigated previously, the XRD diffraction peaks of the composites after ball-milling are broadened and merged, indicating the formation of amorphous or nanocrystalline structure of the composites. In the meantime, diffraction peaks of the metallic Ni are obviously observed in the ball-milled Mg5 Nd41 –100 wt.% Ni composite. The addition of metallic Ni powders with a larger amount is the important factor to the amorphism or nanocrystallization of the Mg-rich Mg–Nd alloy–Ni composites. TEM images of the ball-milled composites are indicated in Fig. 2. From the TEM bright field (BF) images, it is found that Ni nanocrystallines with 20–40 nm (dark area) are dispersed in an amorphous Mg–Nd alloy matrix of the ball-milled Mg5 Nd41 –100 wt.% Ni composite according to the mass contrast (Fig. 2a). At the same ball-milling condition, fine Ni nanocrystallites with smaller size of 5–10 nm in the amorphous alloy matrix of the ball-milled Mg5 Nd41 –200 wt.% Ni composite can still be identified from the TEM BF image (Fig. 2c). This result reveals that the addition of a large amount of metallic Ni during the ball-milling process is beneficial to the formation of highly dispersed Ni nanocrystallites with smaller size in the amorphous Mg-rich Mg–Nd alloy matrix of the composites. The first charge–discharge cycle curves of the composites are presented in Fig. 3. After the ball-milling, the discharge capacity of the Mg5 Nd41 –200 wt.% Ni, Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –100 wt.% Ni composites reach 953 mAh/g, 645 mAh/g and 167 mAh/g (Nd5 Mg41 as active material), respectively. It is clear that the Mg5 Nd41 –200 wt.% Ni composite with highly dispersed nanocrystalline Ni has the highest electrochemical capacity. Moreover, the larger irreversible capacity may exist as compared to the first charge and discharge curves of the ball-milled Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –100 wt.% Ni composites, respectively. It is well known that the metallic Ni served as an unstable hydride element may lead to the decrease of metal hydride stability in the hydrogen storage alloy system. Obviously, the large addition of Ni can decrease the metal hydride stability of the composites, which leads to the excellent reversible electrochemical hydrogen storage of the ball-milled Mg5 Nd41 –200 wt.% Ni composite. To further investigate the electrochemical hydrogen storage of Mg-rich Mg–Nd alloy–Ni composites, the EIS and CV tests were performed. In the CV curves as shown in Fig. 4, the anodic oxidation peaks of hydrogen can be found in the potential range of about −0.60 to −0.55 V (versus Hg/HgO). The cathodic adsorption peak of hydrogen is observed around −0.95 V (versus Hg/HgO), similar to that of nanocrystalline Mg1.9 Ti0.05 Zr0.05 Ni alloy [19]. The oxidation peak area of ball-milled Mg5 Nd41 –200 wt.% Ni composite is larger, contributed to the higher electrochemical hydrogen storage capacity. However, ball-milled Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –100 wt.% Ni composites were demonstrated to have much stronger adsorption peaks of hydrogen in the cathodic process. Obviously, the appearance of stronger hydrogen adsorption peaks is not related to the electrochemical hydrogen storage capacity of the composites, but is more sensitive to the surface
Z.W. Lu et al. / Journal of Alloys and Compounds 433 (2007) 269–273
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Fig. 3. The first charge and discharge curves of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders (() x = 100, () x = 150 and () x = 200).
Ni-rich dispersion of the composites as detected by XRD and TEM. In the electrochemical hydrogen storage reaction, the Ni on the alloy surface serves as the site of hydrogen adsorption in the alkaline solution, which was confirmed in the Ni-rich AB2 alloy after F-treatment [20]. Electrochemical impedance spectra of the composites at fully charged state are shown in Fig. 5. Electrochemical impedance spectra of the composite electrodes are consisted of two semicircles and a straight line. Calculated electrochemical parameters from impedance spectra of the composites are shown in Table 1, according to the proposed equivalent circuit reported previously [13,21,22]. It is shown that the charge-transfer resistance increases slightly and the Warburg impedance decreases dramatically with increasing the Ni amount in the Mg-rich Mg–Nd alloy–Ni composites. This suggests that the rate-determining step is transformed from a mixed process of hydrogen diffu-
Fig. 2. TEM image of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders: (a) x = 100, (b) x = 150 and (c) x = 200. Fig. 4. Cyclic voltammograms of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders at a scan rate of 10 mV/s ((a) x = 200, (b) x = 150 and (c) x = 100).
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Fig. 5. Electrochemical impedance spectra of the composites prepared by ballmilling Nd5 Mg41 alloy with x wt.% Ni powders after fully charged (() x = 100, () x = 150 and () x = 200).
Fig. 6. Cycle performance of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders (() x = 150; () x = 200).
4. Conclusions sion and surface charge-transfer to a single process of surface charge-transfer, as Ni amount in the Mg-rich Mg–Nd alloy–Ni composites is increased. As discussed above in XRD patterns and TEM observation, the increase of the Ni amount results in the formation of Ni nanocrystallites embedded into the amorphous Mg-rich Mg–Nd alloy matrix of the composites. Therefore, the hydrogen diffusion ability in the composites is enhanced due to the large amount of grain boundary and crystalline defects distributed through the amorphous alloy matrix, where the electron density is the lowest [9,23]. In the meantime, the surface chargetransfer resistance, related to electrocatalytic activity sites on the alloy surface, decreases slightly because of the decrease of the dispersed metallic Ni in the amorphous alloy matrix. The cyclic behavior of the ball-milled Mg-rich Mg–Nd alloy–Ni composites in 6 M KOH solutions is shown in Fig. 6. After 20 cycles, the capacity retention of ball-milled Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –200 wt.% Ni composites falls to 40% and 24%, respectively. The result indicates that the addition of the large amount of metallic Ni can improve the cycle behavior to some extent. However, the cycle behavior of the ball-milled Mg-rich Mg–Nd alloy–Ni composites is still not satisfactory as reported previously for the composites of ball-milled rare earth–Mg alloy with Ni powders. Therefore, the cycle stability of the composites of ball-milled rare earth–Mg alloy with Ni powders may be improved further by the modification of non-metallic materials or hydrogen storage alloys, which have a good corrosion resistance in alkaline solution. Table 1 Electrochemical parameters calculated from electrochemical impedance spectra of the composites Sample
Rct ()
Zw ()
Nd5 Mg41 –100 wt.% Ni Nd5 Mg41 –150 wt.% Ni Nd5 Mg41 –200 wt.% Ni
4.837 4.871 5.419
116.2 20.7 6.64
Rct is defined as the charge-transfer resistance and Zw is defined as the Warburg impedance.
The as-cast Mg-rich Mg–Nd alloy has a multiphase structure. After ball-milling Mg-rich Mg–Nd alloy with Ni powders, amorphous or nanocrystalline structure of the composites can be found. The addition of metallic Ni powders with a larger amount is found to be important factor for the amorphisation or nanocrystallization of the Mg-rich Mg–Nd alloy–Ni composites. These changes of the microstructure may have great influence on the electrochemical hydrogen storage of the composites. The addition of large amount of metallic Ni can decrease the metal hydride stability of the composites, which leads to the excellent reversible electrochemical hydrogen storage of the ball-milled Mg5 Nd41 –200 wt.% Ni composite. The existence of the dispersed Ni-rich surface of the composites is beneficial to the stronger hydrogen adsorption and low charge-transfer resistance. The formation of Ni nanocrystallites embedded into the amorphous Mg-rich Mg–Nd alloy matrix contributes to the enhanced hydrogen diffusion ability in the composites. Acknowledgements This work is supported by the 973 Program (2002CB211800), NCET (040219) and NSFC (50371038) of China. References [1] L. Schlapbach, A. Z¨uttel, Nature 414 (2001) 353. [2] R.A.H. Niessen, P.H.L. Notten, Electrochem. Solid-State Lett. 8 (2005) A534. [3] H.G. Pan, Y.F. Liu, M.X. Gao, Y.F. Zhu, Y.Q. Lei, Int. J. Hydrogen Energy 28 (2003) 1219. [4] Y.Q. Lei, Y.M. Wu, Q.M. Yang, J. Wu, Q.D. Wang, Z. Phys. Chem. (Munich) 183 (1994) 379. [5] N. Cui, P. He, J.L. Luo, Electrochim. Acta 44 (1999) 3549. [6] L. Sun, G.X. Wang, H.K. Liu, D.H. Bradhurst, S.X. Dou, Electrochem. Solid-State Lett. 3 (2000) 121. [7] M. Jurczyk, E. Jankowska, M. Nowak, J. Jakubowicz, J. Alloys Compd. 336 (2002) 265. [8] X.Z. Xiao, X.H. Wang, L.H. Gao, L. Wang, C.P. Chen, J. Alloys Compd. 413 (2006) 312.
Z.W. Lu et al. / Journal of Alloys and Compounds 433 (2007) 269–273 [9] X.P. Gao, Y. Wang, Z.W. Lu, W.K. Hu, F. Wu, D.Y. Song, P.W. Shen, Chem. Mater. 16 (2004) 2515. [10] X.P. Gao, Z.W. Lu, Y. Wang, F. Wu, D.Y. Song, P.W. Shen, Electrochem. Solid-State Lett. 7 (2004) A102. [11] L. Wang, X.H. Wang, C.P. Chen, X.H. Xiao, J. Rare Earths 23 (2005) 382. [12] L. Wang, X.H. Wang, L.X. Chen, C.P. Chen, X.Z. Xiao, L.H. Gao, Q.D. Wang, J. Alloys Compd. 414 (2006) 248. [13] Y. Wang, Z.W. Lu, Y.L. Wang, T.Y. Yan, J.Q. Qu, X.P. Gao, P.W. Shen, J. Alloys Compd. 421 (2006) 236. [14] S. Gorsse, C.R. Hutchinson, B. Chevalier, J.F. Nie, J. Alloys Compd. 392 (2005) 253. [15] C.P. Guo, Z.M. Du, Z. Metallkd. 97 (2006) 130.
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Electrochemical hydrogen storage of ball-milled Mg-rich Mg–Nd alloy with Ni powders Z.W. Lu, S. Sun, G.R. Li, T.Y. Yan, J.Q. Qu, X.P. Gao ∗ Institute of New Energy Material Chemistry, Department of Materials Chemistry, Nankai University, Tianjin 300071, China Received 31 May 2006; received in revised form 13 June 2006; accepted 13 June 2006 Available online 1 August 2006
Abstract The Mg-rich Mg–Nd alloy was synthesized by salt-cover-melting. It is found that the Nd5 Mg41 phase with tetragonal structure exists as the main phase in the as-cast Mg-rich Mg–Nd alloy. The microstructure and electrochemical hydrogen storage of the composites prepared by ball-milling the Mg-rich Mg–Nd alloy with Ni powders are investigated. The result reveals that the addition of a larger amount of metallic Ni during the ball-milling process is beneficial to the formation of highly dispersed Ni nanocrystalline on the amorphous Mg-rich Mg–Nd alloy matrix of the composites. The composite, prepared by ball-milling the Mg-rich Mg–Nd alloy with 200 wt.% Ni, is demonstrated to have the discharge capacity of about 953 mAh/g in the first cycle. The appearance of stronger hydrogen adsorption peaks in cyclic voltammograms is very sensitive to the surface Ni-rich dispersion of the composites. The rate-determining step for the electrochemical reaction is transformed from a mixed process of the hydrogen diffusion and surface charge-transfer to a single process of the surface charge-transfer as the Ni amount in the Mg-rich Mg–Nd alloy–Ni composites is increased. © 2006 Elsevier B.V. All rights reserved. Keywords: Rare earth alloys; Hydrogen absorbing materials; Electrochemical reactions
1. Introduction Nickel–metal hydride (Ni–MH) batteries using metal hydrides as negative electrode materials have been commercialized due to their high reversible energy storage capacity, excellent long-term cycling stability and good electrochemical reaction kinetics. Numerous investigations on a new hydrogen storage alloy with a high discharge capacity have been performed to further increase the energy density of Ni/MH batteries. Mgbased alloys have attracted much interest because of their high hydrogen storage capacity and low cost [1–3]. Mechanical alloying (MA) is an effective way to synthesize Mg-based alloys or Ti-based alloys with an amorphous or nanocrystalline structure for electrochemical applications [4–8]. Recently, the composites prepared by ball-milling rare earths–Mg alloys with Ni or Co powders have been demonstrated to be potential materials for the electrochemical hydrogen storage in alkaline solutions [9–12]. For example, it was found that the initial discharge capacity of ball-milled La2 Mg17 and LaMg12 alloy with Ni pow-
der can reach to over 1000 mAh/g (La2 Mg17 or LaMg12 alloys as active materials) [9,10]. The ball-milled Ce2 Mg17 –Ni and La2 Mg17 –Co composites were reported to have a high electrochemical capacity for hydrogen storage [11,12]. Furthermore, the similar result was also obtained in the PrMg12 –Ni composites [13]. The Mg–Nd system consists of the intermetallic compounds Mg12 Nd, Mg41 Nd5 , Mg3 Nd, Mg2 Nd and MgNd [14,15]. Mg-rich Mg–Nd or Mg–Ni–Nd alloys were reported to absorb hydrogen up to 4.7 mass% above 423 K and completely desorb it above 453 K [16,17]. However, the electrochemical hydrogen storage of the Mg-rich Mg–Nd or Mg–Ni–Nd alloys has not been reported so far. In this work, the Mg-rich Mg–Nd alloy was synthesized by salt-cover-melting. Electrochemical properties of the composites, prepared through ball-milling the Mg-rich Mg–Nd alloy with Ni powders, were investigated in order to explore new materials of rare earth–Mg alloys for electrochemical hydrogen storage. 2. Experimental
∗
Corresponding author. Tel.: +86 22 23500876; fax: +86 22 23500876. E-mail address: [email protected] (X.P. Gao).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.051
The Mg-rich Mg–Nd (with the composition of Nd5 Mg41 ) alloy was synthesized through a salt-cover-melting process by melting the stoichiometric mixture
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Z.W. Lu et al. / Journal of Alloys and Compounds 433 (2007) 269–273
of metallic Nd and Mg, and then the ingot was pulverized to 200 meshes. The negative electrode materials were prepared by further mechanical alloying of the Mg-rich Mg–Nd alloy with the different weight percentage of carbonyl nickel powders (255 INCO, 200, 150 and 100 wt.%). All ball-milling processes were performed with a weight ratio of ball to powders of 20:1 at 290 rpm for 11–16 h under Ar atmosphere in a cyclohexane solution in a planetary-type ball-mill. The microstructure of the alloy powders were analyzed by X-ray diffraction (XRD, Rigaku D/max-2500) and transmission electron microscopy (TEM, FEI Tecnai 20). The negative electrodes were constructed by mixing the as-prepared composites with carbonyl nickel powder in a weight ratio of 1:3. The mixture powder was pressed under 30 MPa into a small pellet of 10 mm diameter and 1.5 mm thickness. The electrochemical measurements were conducted in a three-compartment cell using a Land Battery Test instrument. A sintered nickel electrode with a large capacity and an Hg/HgO electrode in 6 M KOH solution served as counter and reference electrodes, respectively. The electrodes were charged at a current density of 1000 mA/g for 80 min, and then discharged at current density of 50 mA/g to −0.6 V (versus Hg/HgO) after resting for 30 min at room temperature. When the discharge capacity was calculated, only the Mg-rich Mg–Nd alloy was considered as the active material. The cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) experiments were taken after the electrodes were fully charged using an IM6e Electrochemical workstation. The parameters of EIS plots were fitted by the Zview2 software. The working electrodes for CV experiments were constructed from an apparent surface of 1.0 cm2 porous nickel without addition of carbonyl nickel powders.
3. Results and discussion XRD patterns of the as-cast Mg-rich Mg–Nd alloy and the composites prepared by ball-milling the Mg-rich Mg–Nd alloy with Ni powders are shown in Fig. 1. It is shown from the XRD pattern that the as-cast Mg-rich Mg–Nd alloy has a multiphase structure as reported previously [18]. The Nd5 Mg41 phase exists as the main phase with the Ce5 Mg41 -type tetragonal structure (I4/m, JCPDS Card No 65-1632). However, some weak diffraction peaks are also detected in XRD patterns except for the Nd5 Mg41 phase. Two peaks at 2θ angles of 20.6◦ and 40.2◦ are indexed as the cubic phase of NdMg3 alloy (JCPDS Card No 26-426), which is in accordance with the diffraction peaks of REMg3 alloy (RE = La, Ce, Pr, Tb and Gd). The small peak at 2θ
Fig. 1. XRD patterns of the as-cast Nd5 Mg41 alloy and the composites prepared by ball-milling Nd5 Mg41 alloy with Ni powders (() NdMg3 ; () unconfirmed phase).
angle of 28.3◦ cannot be identified with any known phases. Similar to the RE–Mg alloys system [9–13] investigated previously, the XRD diffraction peaks of the composites after ball-milling are broadened and merged, indicating the formation of amorphous or nanocrystalline structure of the composites. In the meantime, diffraction peaks of the metallic Ni are obviously observed in the ball-milled Mg5 Nd41 –100 wt.% Ni composite. The addition of metallic Ni powders with a larger amount is the important factor to the amorphism or nanocrystallization of the Mg-rich Mg–Nd alloy–Ni composites. TEM images of the ball-milled composites are indicated in Fig. 2. From the TEM bright field (BF) images, it is found that Ni nanocrystallines with 20–40 nm (dark area) are dispersed in an amorphous Mg–Nd alloy matrix of the ball-milled Mg5 Nd41 –100 wt.% Ni composite according to the mass contrast (Fig. 2a). At the same ball-milling condition, fine Ni nanocrystallites with smaller size of 5–10 nm in the amorphous alloy matrix of the ball-milled Mg5 Nd41 –200 wt.% Ni composite can still be identified from the TEM BF image (Fig. 2c). This result reveals that the addition of a large amount of metallic Ni during the ball-milling process is beneficial to the formation of highly dispersed Ni nanocrystallites with smaller size in the amorphous Mg-rich Mg–Nd alloy matrix of the composites. The first charge–discharge cycle curves of the composites are presented in Fig. 3. After the ball-milling, the discharge capacity of the Mg5 Nd41 –200 wt.% Ni, Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –100 wt.% Ni composites reach 953 mAh/g, 645 mAh/g and 167 mAh/g (Nd5 Mg41 as active material), respectively. It is clear that the Mg5 Nd41 –200 wt.% Ni composite with highly dispersed nanocrystalline Ni has the highest electrochemical capacity. Moreover, the larger irreversible capacity may exist as compared to the first charge and discharge curves of the ball-milled Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –100 wt.% Ni composites, respectively. It is well known that the metallic Ni served as an unstable hydride element may lead to the decrease of metal hydride stability in the hydrogen storage alloy system. Obviously, the large addition of Ni can decrease the metal hydride stability of the composites, which leads to the excellent reversible electrochemical hydrogen storage of the ball-milled Mg5 Nd41 –200 wt.% Ni composite. To further investigate the electrochemical hydrogen storage of Mg-rich Mg–Nd alloy–Ni composites, the EIS and CV tests were performed. In the CV curves as shown in Fig. 4, the anodic oxidation peaks of hydrogen can be found in the potential range of about −0.60 to −0.55 V (versus Hg/HgO). The cathodic adsorption peak of hydrogen is observed around −0.95 V (versus Hg/HgO), similar to that of nanocrystalline Mg1.9 Ti0.05 Zr0.05 Ni alloy [19]. The oxidation peak area of ball-milled Mg5 Nd41 –200 wt.% Ni composite is larger, contributed to the higher electrochemical hydrogen storage capacity. However, ball-milled Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –100 wt.% Ni composites were demonstrated to have much stronger adsorption peaks of hydrogen in the cathodic process. Obviously, the appearance of stronger hydrogen adsorption peaks is not related to the electrochemical hydrogen storage capacity of the composites, but is more sensitive to the surface
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Fig. 3. The first charge and discharge curves of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders (() x = 100, () x = 150 and () x = 200).
Ni-rich dispersion of the composites as detected by XRD and TEM. In the electrochemical hydrogen storage reaction, the Ni on the alloy surface serves as the site of hydrogen adsorption in the alkaline solution, which was confirmed in the Ni-rich AB2 alloy after F-treatment [20]. Electrochemical impedance spectra of the composites at fully charged state are shown in Fig. 5. Electrochemical impedance spectra of the composite electrodes are consisted of two semicircles and a straight line. Calculated electrochemical parameters from impedance spectra of the composites are shown in Table 1, according to the proposed equivalent circuit reported previously [13,21,22]. It is shown that the charge-transfer resistance increases slightly and the Warburg impedance decreases dramatically with increasing the Ni amount in the Mg-rich Mg–Nd alloy–Ni composites. This suggests that the rate-determining step is transformed from a mixed process of hydrogen diffu-
Fig. 2. TEM image of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders: (a) x = 100, (b) x = 150 and (c) x = 200. Fig. 4. Cyclic voltammograms of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders at a scan rate of 10 mV/s ((a) x = 200, (b) x = 150 and (c) x = 100).
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Fig. 5. Electrochemical impedance spectra of the composites prepared by ballmilling Nd5 Mg41 alloy with x wt.% Ni powders after fully charged (() x = 100, () x = 150 and () x = 200).
Fig. 6. Cycle performance of the composites prepared by ball-milling Nd5 Mg41 alloy with x wt.% Ni powders (() x = 150; () x = 200).
4. Conclusions sion and surface charge-transfer to a single process of surface charge-transfer, as Ni amount in the Mg-rich Mg–Nd alloy–Ni composites is increased. As discussed above in XRD patterns and TEM observation, the increase of the Ni amount results in the formation of Ni nanocrystallites embedded into the amorphous Mg-rich Mg–Nd alloy matrix of the composites. Therefore, the hydrogen diffusion ability in the composites is enhanced due to the large amount of grain boundary and crystalline defects distributed through the amorphous alloy matrix, where the electron density is the lowest [9,23]. In the meantime, the surface chargetransfer resistance, related to electrocatalytic activity sites on the alloy surface, decreases slightly because of the decrease of the dispersed metallic Ni in the amorphous alloy matrix. The cyclic behavior of the ball-milled Mg-rich Mg–Nd alloy–Ni composites in 6 M KOH solutions is shown in Fig. 6. After 20 cycles, the capacity retention of ball-milled Mg5 Nd41 –150 wt.% Ni and Mg5 Nd41 –200 wt.% Ni composites falls to 40% and 24%, respectively. The result indicates that the addition of the large amount of metallic Ni can improve the cycle behavior to some extent. However, the cycle behavior of the ball-milled Mg-rich Mg–Nd alloy–Ni composites is still not satisfactory as reported previously for the composites of ball-milled rare earth–Mg alloy with Ni powders. Therefore, the cycle stability of the composites of ball-milled rare earth–Mg alloy with Ni powders may be improved further by the modification of non-metallic materials or hydrogen storage alloys, which have a good corrosion resistance in alkaline solution. Table 1 Electrochemical parameters calculated from electrochemical impedance spectra of the composites Sample
Rct ()
Zw ()
Nd5 Mg41 –100 wt.% Ni Nd5 Mg41 –150 wt.% Ni Nd5 Mg41 –200 wt.% Ni
4.837 4.871 5.419
116.2 20.7 6.64
Rct is defined as the charge-transfer resistance and Zw is defined as the Warburg impedance.
The as-cast Mg-rich Mg–Nd alloy has a multiphase structure. After ball-milling Mg-rich Mg–Nd alloy with Ni powders, amorphous or nanocrystalline structure of the composites can be found. The addition of metallic Ni powders with a larger amount is found to be important factor for the amorphisation or nanocrystallization of the Mg-rich Mg–Nd alloy–Ni composites. These changes of the microstructure may have great influence on the electrochemical hydrogen storage of the composites. The addition of large amount of metallic Ni can decrease the metal hydride stability of the composites, which leads to the excellent reversible electrochemical hydrogen storage of the ball-milled Mg5 Nd41 –200 wt.% Ni composite. The existence of the dispersed Ni-rich surface of the composites is beneficial to the stronger hydrogen adsorption and low charge-transfer resistance. The formation of Ni nanocrystallites embedded into the amorphous Mg-rich Mg–Nd alloy matrix contributes to the enhanced hydrogen diffusion ability in the composites. Acknowledgements This work is supported by the 973 Program (2002CB211800), NCET (040219) and NSFC (50371038) of China. References [1] L. Schlapbach, A. Z¨uttel, Nature 414 (2001) 353. [2] R.A.H. Niessen, P.H.L. Notten, Electrochem. Solid-State Lett. 8 (2005) A534. [3] H.G. Pan, Y.F. Liu, M.X. Gao, Y.F. Zhu, Y.Q. Lei, Int. J. Hydrogen Energy 28 (2003) 1219. [4] Y.Q. Lei, Y.M. Wu, Q.M. Yang, J. Wu, Q.D. Wang, Z. Phys. Chem. (Munich) 183 (1994) 379. [5] N. Cui, P. He, J.L. Luo, Electrochim. Acta 44 (1999) 3549. [6] L. Sun, G.X. Wang, H.K. Liu, D.H. Bradhurst, S.X. Dou, Electrochem. Solid-State Lett. 3 (2000) 121. [7] M. Jurczyk, E. Jankowska, M. Nowak, J. Jakubowicz, J. Alloys Compd. 336 (2002) 265. [8] X.Z. Xiao, X.H. Wang, L.H. Gao, L. Wang, C.P. Chen, J. Alloys Compd. 413 (2006) 312.
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