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Effects of amorphous Co–C on the structural and electrochemical characteristics of La0.8Mg0.2Ni0.8Mn0.1Co0.5Al0.1 hydrogen storage alloy
Journal of Alloys and Compounds 467 (2009) L16–L20
Letter
Effects of amorphous Co–C on the structural and electrochemical characteristics of La0.8Mg0.2Ni0.8Mn0.1Co0.5Al0.1 hydrogen storage alloy Yunyun Zhang, Lifang Jiao, Huatang Yuan ∗ , Dawei Song, Yijing Wang, Yanhui Zhang Institute of New Energy Material Chemistry, Engineering Research Center of Energy Storage & Conversion (Ministry of Education) and Key Laboratory of Energy-Material Chemistry (Tianjin), Nankai University, Tianjin 300071, PR China Received 4 September 2007; received in revised form 15 November 2007; accepted 18 December 2007 Available online 5 March 2008
Abstract Amorphous Co–C powder prepared by ball milling was introduced to improve the performance of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 hydrogen storage alloy. The structural and electrochemical properties of the as-prepared La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –x wt.% Co–C composites were investigated systematically. Scanning electron microscopic images show La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 alloy was coated by Co–C particles. X-ray diffraction patterns suggest that the composite almost remained original phase structures of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C in both charge and discharge processes. The maximum discharge capacity of the composites reached 414 mAh g−1 at a current density of 50 mA g−1 at 298 K. The cyclic stability and the discharge capacities of the composite electrodes were noticeably improved in comparison with single La–Mg–Ni-based alloy due to increased corrosion resistance and the catalysis of the Co–C powder. Cyclic voltammogram and potentiodynamic polarization studies on the composite indicate that the electrochemical kinetics was improved and the corrosion resistance was increased. The cycling performance of the composite electrode at high current density is good as well. © 2008 Elsevier B.V. All rights reserved. Keywords: Electrode materials; Mechanical alloying; Electrochemical reactions
1. Introduction Recently, the AB3 -type La–Mg–Ni-based hydrogen storage alloys were extensively studied as candidates of negative electrode material for Ni–MH secondary batteries due to their high discharge capacities and good electrode properties [1–11]. For example, the investigations on La0.7 Mg0.3 (Ni0.85 Co0.15 )3.5 alloy electrode by Pan et al. [3] revealed that the maximum discharge capacity reached 396 mAh g−1 , which was much higher than that of conventional AB5 -type hydrogen storage alloys. Liao et al. [4] investigated the electrochemical properties of Lax Mg3−x Ni9 (x = 1.0–2.0) alloys and found that the La2 MgNi9 alloy exhibited the highest discharge capacity of 403 mAh g−1 . Chen et al. [5] investigated the structural and electrochemical characteristics of the La–Ca–Mg–(NiM)9 (M=Al, Mn) alloys which can provide a maximum discharge capacity of 356 mAh g−1 . However, these AB3 -type La–Mg–Ni-based hydrogen storage alloys
∗
Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (H. Yuan).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.097
suffer from serious degradation of discharge capacity during the charge–discharge cycles. For the practical applications of La–Mg–Ni-based hydrogen storage alloy in secondary batteries, higher discharge capacities and good cycling life are required. Generally, simple elemental substitution and compound doping are regarded as the most effective ways to resist the corrosion and to increase the cycle capacity [12]. Recently, Co and Co-contained alloys have been synthesized and investigated on hydrogen storage. For example, Chung et al. [13] reported the electrochemical hydrogenation of two kinds of crystalline cobalt powder. The maximum discharge capacity reached 415 mAh g−1 (1.54 wt.%) and 428 mAh g−1 (1.58 wt.%), the high discharge capacity was attributed to the hydrogenation and the phase transition of cobalt. Wang et al. [14] reported the electrochemical hydrogen absorption–desorption properties of Co–B, the reversible electrochemical hydrogen storage capacity is about 300 mAh g−1 . CoP [15] and CoSi [16] materials were also successfully synthesized and their electrochemical hydrogen storage properties were revealed. In this work, we prepared amorphous Co–C (which is represented as B) by ball milling and introduced it
Y. Zhang et al. / Journal of Alloys and Compounds 467 (2009) L16–L20
to La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 (which is represented as A) alloy with the expecting of improving the electrochemical properties of La–Mg–Ni-type hydrogen storage alloys. Compared with boron, silicon and phosphorus elements, carbon element possesses particular characteristics of low-cost, light weight and high stability. Experiments show materials prepared by ball milling have high specific surface area, which may offer some catalytic characteristics. 2. Experimental 2.1. Preparation and structural characterization Pure Co and C powders were mixed according to the composition of B on a planetary ball mill (ZKX-2B, Nanjing) in a stainless steel vessel at a speed of 450 rpm under argon atmosphere for 10 h. The weight ratio of milling balls to reagent powders is 20:1. The A alloy was prepared with the method reported by Li et al. [11], and annealed for 7 h at 1073 K. The as-prepared alloy was mechanically crushed and ground in the air into fine powders. Powders of 200 meshes were used for electrochemical tests. The A–x wt.% B composites were prepared by simply mixing A and B in the weight ratio of 9:1, 7:3, 5:5, 3:7, 1:9 respectively. The crystal structure of the alloys were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, CuK␣ radiation; 40 kV; 20 mA), and scanning electron microscopy (SEM, on a Hitachi X-650 scanning electron microscope).
2.2. Electrochemical measurements The charge–discharge measurements were performed on an LAND automatic battery-testing system as reported by our previous work [16]. CHI 660b
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electrochemical workstation was used for the cyclic voltammogram (scan rate: 1 mV s−1 , scan range: between −1.2 and −0.2 V vs. HgO/Hg) and Potentiodynamic polarization (scan rate: 1 mV s−1 , scan range: between −1.2 and −0.2 V vs. HgO/Hg) tests.
3. Results and discussion 3.1. The phase structure and morphology of the composites As shown in Fig. 1, the particle size of A alloy (20–50 m in diameter) is larger than that of B alloy (1–2 m in diameter). The SEM images of A and B composite alloy is shown in Fig. 1(A + B). Since there was no pulverization during mixing, the A and B can be easily distinguished from their particle size. It is obvious that the large A particles were coated with smaller B particles. B particles prepared by ball milling have higher specific surface area which can be absorbed on the A surface. This configuration is helpful for catalytic and anti-corrosion characteristics. Moreover, the addition of smaller B particles reduces the agglomeration of A particles, which can enhance the hydrogen diffusion. Fig. 2 shows the X-ray diffraction patterns of A and B alloys. A alloy mainly consists of the LaNi3 phase of the PuNi3 -type rhombohedral structure (space group R-3m) and the LaNi5 phase of the CaCu5 -type hexagonal structure (space group P6/mmm). B powder ball milling for 10 h mainly shows cobalt and carbon phases. Small graphite peaks were detected, indicating that the
Fig. 1. SEM images of A (A), B (B) and A–50 wt.% B (A + B).
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Y. Zhang et al. / Journal of Alloys and Compounds 467 (2009) L16–L20 Table 1 The maximum discharge capacity by experimentation and by calculated values according to the weight percentage (w) A–x wt.% B of composites
Fig. 2. XRD patterns of the A and B alloys.
regular crystalline structure of the graphite has been destroyed during milling. 3.2. Electrochemical properties Fig. 3 exhibits the discharge capacities of the A, B and A–x wt.% B (x = 90, 70, 50, 30, 10) composite alloys at 298 K. All the alloys were fully activated within five charge–discharge cycles. The maximum discharge capacity of A alloy reached 363 mAh g−1 . The maximum discharge capacity of B powder is 277 mAh g−1 , which is higher than that of the Co(OH)2 /Co faradic reaction 80 mAh g−1 [13]. When x = 50, 30, the maximum discharge capacity reached 407 and 414 mAh g−1 respectively, which is higher than pure A and B alloys. If there is no interaction between A and B in the composite, the calculated values according to the weight percentage (w) of A–x wt.% B composites cannot reach that value (Table 1). Since the experiments prove that the discharge capacity has been improved when incorporate A and B with appropriate ratio, we consider there were interactions between A and B alloys. When x = 10,
Fig. 3. Discharge capacity of the A, B and A–x wt.% B (x = 90, 70, 50, 30, 10) composite alloys at 298 K. Discharge current density: 50 mA g−1 .
Alloy
Maximum discharge capacity (experimentation)
Maximum discharge capacity (calculation)
A B A–50 wt.% B A–30 wt.% B
363 mAh g−1 277 mAh g−1 407 mAh g−1 414 mAh g−1
320 mAh g−1 337 mAh g−1
90 the cyclic stabilities of the composite electrodes are greatly enhanced compared with that of A alloy, although their maximum discharge capacity are lower than that of A alloy, indicating that A–10 wt.% B and A–90 wt.% B had the better cycle lives among this series. Fig. 4 illustrates CV curves of A and A–50 wt.% B alloy electrodes at 298 K. For each sample, the oxidation peak appeared at the potential of around −0.7 V (vs. HgO/Hg). The reduction peak appeared at the potential of around −1.0 V (vs. HgO/Hg). The oxidation peak was attributed to the hydrogen desorption from the interior to the surface of the alloy particles [15,17]. As seen in Fig. 4, the height and area of the oxidation peak of A–50 wt.% B alloy is higher and larger than pure A, which indicated that the discharge kinetics and capacity could be significantly improved. The result is consistent with the charge–discharge measurements. From SEM images, B powders have more powder–particle boundaries and small pores than A particles which can help trapping hydrogen in the sample. So the B powder may play a certain catalytic role for the hydrogen absorption of A, or dissociation of H2 near the A surface. Potentiodynamic polarization experiment was employed to investigate the corrosive behavior of the alloy electrodes. Fig. 5 shows the potentiodynamic polarization curves for A and A–50 wt.% B alloys. The Tafel fitting results were summarized in Table 2. The result reveals that Ecorr (corrosion potential) value of A–50 wt.% B alloy is higher than that of A, and the corrosion rate Icorr (corrosion current) is lower. These results suggest that B improves the anti-corrosive behavior of A alloy, and B coat-
Fig. 4. CV curves of the A and A–50 wt.% B electrode alloys.
Y. Zhang et al. / Journal of Alloys and Compounds 467 (2009) L16–L20
Fig. 5. Potentiodynamic polarization curves of the A and A–50 wt.% B composite alloys. Table 2 Tafel fitting data of the A and A–50 wt.% B alloy electrode Electrode
A
A–50 wt.% B
Ecorr/V Icorr/A
−0.923 3.00E−05
−0.898 2.93E−05
ing on the A powder particles can inhibit the formation of a new oxide layer on the powder surfaces. 3.3. Hydrogenation mechanism The XRD patterns of A and A–50 wt.% B alloys electrode at different charge and discharge states are shown in Fig. 6. The A–50 wt.% B powder basically maintains the A and B original phase structures, the intensity of LaNi3 peaks is stronger than LaNi5 peaks in both charge and discharge processes. After charge (Fig. 6 a, b), no new phase was formed in A alloy and the A–50 wt.% B composite alloy. However, after discharge
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Fig. 7. The 5th cycle discharge potential curves of A, B and A–50 wt.% B electrodes at 298 K.
(Fig. 6d), the A–50 wt.% B showed some characteristic peaks of Co(OH)2 , and these XRD peaks of Co(OH)2 were still presented and remained their intensities almost unchanged after recharge (Fig. 6e), implying that the Co(OH)2 was not reduced at the charging potential interval. This phenomenon suggest that the reversible reaction of Co(OH)2 + 2e− ⇔ Co + 2OH− is a subsidiary reaction in the charge/discharge processes. The high discharge capacities obtained in the present study should be attributed primarily to the dehydrogenation of A–50 wt.% B alloy. The 5th cycle discharge potential curves of A, B and A–50 wt.% B alloys at 298 K are shown in Fig. 7. They have different discharge potentials, −0.87 V for A alloy electrode, −0. 77 V for B alloy electrode and −0.80 for A–50 wt.% B alloy electrode at 50% DOD. The discharge potential of A–50 wt.% B electrode is higher than B alloy electrode and lower than A alloy electrode. As there was only one discharge potential plateau of A–50 wt.% B alloy electrode, we conclude that the discharge process is not generated by A and B alloys separately. Although the A–50 wt.% B alloy was prepared by simply mixing A and B alloys, there exist interactions between A and B alloys. From the above analysis we suppose B powder may play a certain catalytic role in the hydrogen absorption of A. 3.4. High rate capability
Fig. 6. XRD patterns of the A (1st cycle charge and discharge) and A–50 wt.% B (1st cycle charge and discharge, 2nd charge) alloys.
Fig. 8 shows the cycling performance of the A–50 wt.% B electrode at current densities of 50, 250 and 500 mA g−1 . The reversible discharge capacity became lower at higher current densities, and the maximum discharge capacity of A–50 wt.% B electrode at current density of 250 and 500 mA g−1 is 323 and 265 mAh g−1 respectively. But the cycle lives become better at higher current densities. The main reason on this phenomenon possibly is that passivation layer forms on the surface of the alloy when the current rate is high enough and further corrosion is prevented by this layer. This powder can be a potential candidate in higher current density.
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Y. Zhang et al. / Journal of Alloys and Compounds 467 (2009) L16–L20
cess is not generated by La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C alloys separately, there are interactions between La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C alloys. The La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –50 wt.% Co–C electrode at higher current density show good cycling life. Acknowledgments This work was supported by NSFC (50571046, 50631020, 50701025 and 20573058) ,Tianjin Nature Science Fund (07JCYBJC03500), Doctoral Fund of Ministry of Education (20070055064) and 863 programs (2007AA05Z149, 2007AA05Z108, 2006AA05Z110). References Fig. 8. Discharge capacity of the A–50 wt.% B electrode alloys at large current density of 50 mA g−1 , 250 mA g−1 , and 500 mA g−1 (298 K).
4. Conclusion In this work, Co–C powder by ball milling was introduced into La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 hydrogen storage alloy. The maximum discharge capacity and the cycle life were significantly improved. Scanning electron microscopic images show the large La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 particles were coated with smaller Co–C particles. The addition of small Co–C particles reduces the agglomeration of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 particles, which can enhance the hydrogen diffusion. Co–C coating on the La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 powder particles can inhibit the formation of a new oxide layer on the powder surfaces. These results were further proved by the cyclic voltammogram and potentiodynamic polarization studies. X-ray diffraction patterns show that the composite almost kept original phase structures of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C in both charge and discharge processes. The charge–discharge processes of the La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –50 wt.% Co–C electrode are predominated through electrochemical hydrogen storage mechanism. The electrochemical oxidation of cobalt contributes only a negligible part to the reversible electrochemical capacity. As there is only one potential plateau of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –50 wt.% Co–C alloy electrode, we conclude that the discharge pro-
[1] T. Kohno, H. Yoshida, F. Kawashima, T. Inaba, I. Sakai, M. Yamamoto, M. Kanda, J. Alloys Compd. 311 (2000) L5–L7. [2] K. Kadir, T. Sakai, I. Uehare, J. Alloys Compd. 302 (2000) 112–117. [3] H.G. Pan, Y.F. Liu, M.X. Gao, Y.Q. Lei, Q.D. Wang, J. Electrochem. Soc. 150 (5) (2003) A565. [4] B. Liao, Y.Q. Lei, G.L. Lu, L.X. Chen, H.G. Pan, Q.D. Wang, J. Alloys Compd. 356–357 (2003) 746. [5] J. Chen, N. Kuriyama, H.T. Takeshita, H. Tanaka, T. Sakai, M. Haruta, Electrochem. Solid-State Lett. 3 (6) (2000) 249–252. [6] H.G. Pan, Y.F. Liu, M.X. Gao, Y.Q. Lei, Q.D. Wang, J. Electrochem. Soc. 152 (2005) A326. [7] H.G. Pan, Y.F. Liu, M.X. Gao, Y.F. Zhu, Y.Q. Lei, Q.D. Wang, J. Alloys Compd. 351 (2003) 228–234. [8] B.W. Li, H.P. Ren, Y.H. Zhang, X.P. Dong, J.Y. Ren, X.L. Wang, J. Alloys Compd. 425 (2006) 399. [9] Y.F. Liu, H.G. Pan, Y.J. Yue, X.F. Wu, N. Chen, Y.Q. Lei, J. Alloys Compd. 395 (2005) 291. [10] Y.F. Liu, H.G. Pan, M.X. Gao, Y.Q. Lei, Q.D. Wang, J. Alloys Compd. 403 (2005) 296. [11] M. Li, S.M. Han, Y. Li, W. Guan, L.R. Mao, L. Hu, J. Electrochim. Acta 51 (2006) 5926–5931. [12] Y. Zhang, S.K. Zhang, L.X. Chen, Y.Q. Lei, Q.D. Wang, J. Hydrogen Energy 26 (2001) 801–806. [13] S.R. Chung, K.W. Wang, T.P. Perng, J. Electrochem. Soc. 153 (6) (2006) A1128–A1131. [14] Y.D. Wang, X.P. Ai, H.X. Yang, J. Chem. Mater. 16 (2004) 5194–5197. [15] Y.L. Cao, W.C. Zhou, X.Y. Li, X.P. Ai, X.P. Gao, H.X. Yang, J. Electrochim. Acta 51 (2006) 4285–4290. [16] G. He, L.F. Jiao, H.T. Yuan, Y.Y. Zhang, Y.J. Wang, J. Electrochem. Comm. 8 (2006) 1633–1638. [17] Y.F. Liu, H.G. Pan, M.X. Gao, R. Li, Y.Q. Lei, J. Alloys Compd. 376 (2004) 304–313.
Letter
Effects of amorphous Co–C on the structural and electrochemical characteristics of La0.8Mg0.2Ni0.8Mn0.1Co0.5Al0.1 hydrogen storage alloy Yunyun Zhang, Lifang Jiao, Huatang Yuan ∗ , Dawei Song, Yijing Wang, Yanhui Zhang Institute of New Energy Material Chemistry, Engineering Research Center of Energy Storage & Conversion (Ministry of Education) and Key Laboratory of Energy-Material Chemistry (Tianjin), Nankai University, Tianjin 300071, PR China Received 4 September 2007; received in revised form 15 November 2007; accepted 18 December 2007 Available online 5 March 2008
Abstract Amorphous Co–C powder prepared by ball milling was introduced to improve the performance of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 hydrogen storage alloy. The structural and electrochemical properties of the as-prepared La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –x wt.% Co–C composites were investigated systematically. Scanning electron microscopic images show La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 alloy was coated by Co–C particles. X-ray diffraction patterns suggest that the composite almost remained original phase structures of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C in both charge and discharge processes. The maximum discharge capacity of the composites reached 414 mAh g−1 at a current density of 50 mA g−1 at 298 K. The cyclic stability and the discharge capacities of the composite electrodes were noticeably improved in comparison with single La–Mg–Ni-based alloy due to increased corrosion resistance and the catalysis of the Co–C powder. Cyclic voltammogram and potentiodynamic polarization studies on the composite indicate that the electrochemical kinetics was improved and the corrosion resistance was increased. The cycling performance of the composite electrode at high current density is good as well. © 2008 Elsevier B.V. All rights reserved. Keywords: Electrode materials; Mechanical alloying; Electrochemical reactions
1. Introduction Recently, the AB3 -type La–Mg–Ni-based hydrogen storage alloys were extensively studied as candidates of negative electrode material for Ni–MH secondary batteries due to their high discharge capacities and good electrode properties [1–11]. For example, the investigations on La0.7 Mg0.3 (Ni0.85 Co0.15 )3.5 alloy electrode by Pan et al. [3] revealed that the maximum discharge capacity reached 396 mAh g−1 , which was much higher than that of conventional AB5 -type hydrogen storage alloys. Liao et al. [4] investigated the electrochemical properties of Lax Mg3−x Ni9 (x = 1.0–2.0) alloys and found that the La2 MgNi9 alloy exhibited the highest discharge capacity of 403 mAh g−1 . Chen et al. [5] investigated the structural and electrochemical characteristics of the La–Ca–Mg–(NiM)9 (M=Al, Mn) alloys which can provide a maximum discharge capacity of 356 mAh g−1 . However, these AB3 -type La–Mg–Ni-based hydrogen storage alloys
∗
Corresponding author. Tel.: +86 22 23498089; fax: +86 22 23502604. E-mail address: [email protected] (H. Yuan).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.097
suffer from serious degradation of discharge capacity during the charge–discharge cycles. For the practical applications of La–Mg–Ni-based hydrogen storage alloy in secondary batteries, higher discharge capacities and good cycling life are required. Generally, simple elemental substitution and compound doping are regarded as the most effective ways to resist the corrosion and to increase the cycle capacity [12]. Recently, Co and Co-contained alloys have been synthesized and investigated on hydrogen storage. For example, Chung et al. [13] reported the electrochemical hydrogenation of two kinds of crystalline cobalt powder. The maximum discharge capacity reached 415 mAh g−1 (1.54 wt.%) and 428 mAh g−1 (1.58 wt.%), the high discharge capacity was attributed to the hydrogenation and the phase transition of cobalt. Wang et al. [14] reported the electrochemical hydrogen absorption–desorption properties of Co–B, the reversible electrochemical hydrogen storage capacity is about 300 mAh g−1 . CoP [15] and CoSi [16] materials were also successfully synthesized and their electrochemical hydrogen storage properties were revealed. In this work, we prepared amorphous Co–C (which is represented as B) by ball milling and introduced it
Y. Zhang et al. / Journal of Alloys and Compounds 467 (2009) L16–L20
to La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 (which is represented as A) alloy with the expecting of improving the electrochemical properties of La–Mg–Ni-type hydrogen storage alloys. Compared with boron, silicon and phosphorus elements, carbon element possesses particular characteristics of low-cost, light weight and high stability. Experiments show materials prepared by ball milling have high specific surface area, which may offer some catalytic characteristics. 2. Experimental 2.1. Preparation and structural characterization Pure Co and C powders were mixed according to the composition of B on a planetary ball mill (ZKX-2B, Nanjing) in a stainless steel vessel at a speed of 450 rpm under argon atmosphere for 10 h. The weight ratio of milling balls to reagent powders is 20:1. The A alloy was prepared with the method reported by Li et al. [11], and annealed for 7 h at 1073 K. The as-prepared alloy was mechanically crushed and ground in the air into fine powders. Powders of 200 meshes were used for electrochemical tests. The A–x wt.% B composites were prepared by simply mixing A and B in the weight ratio of 9:1, 7:3, 5:5, 3:7, 1:9 respectively. The crystal structure of the alloys were characterized by X-ray diffraction (XRD, Rigaku D/Max-2500, CuK␣ radiation; 40 kV; 20 mA), and scanning electron microscopy (SEM, on a Hitachi X-650 scanning electron microscope).
2.2. Electrochemical measurements The charge–discharge measurements were performed on an LAND automatic battery-testing system as reported by our previous work [16]. CHI 660b
L17
electrochemical workstation was used for the cyclic voltammogram (scan rate: 1 mV s−1 , scan range: between −1.2 and −0.2 V vs. HgO/Hg) and Potentiodynamic polarization (scan rate: 1 mV s−1 , scan range: between −1.2 and −0.2 V vs. HgO/Hg) tests.
3. Results and discussion 3.1. The phase structure and morphology of the composites As shown in Fig. 1, the particle size of A alloy (20–50 m in diameter) is larger than that of B alloy (1–2 m in diameter). The SEM images of A and B composite alloy is shown in Fig. 1(A + B). Since there was no pulverization during mixing, the A and B can be easily distinguished from their particle size. It is obvious that the large A particles were coated with smaller B particles. B particles prepared by ball milling have higher specific surface area which can be absorbed on the A surface. This configuration is helpful for catalytic and anti-corrosion characteristics. Moreover, the addition of smaller B particles reduces the agglomeration of A particles, which can enhance the hydrogen diffusion. Fig. 2 shows the X-ray diffraction patterns of A and B alloys. A alloy mainly consists of the LaNi3 phase of the PuNi3 -type rhombohedral structure (space group R-3m) and the LaNi5 phase of the CaCu5 -type hexagonal structure (space group P6/mmm). B powder ball milling for 10 h mainly shows cobalt and carbon phases. Small graphite peaks were detected, indicating that the
Fig. 1. SEM images of A (A), B (B) and A–50 wt.% B (A + B).
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Y. Zhang et al. / Journal of Alloys and Compounds 467 (2009) L16–L20 Table 1 The maximum discharge capacity by experimentation and by calculated values according to the weight percentage (w) A–x wt.% B of composites
Fig. 2. XRD patterns of the A and B alloys.
regular crystalline structure of the graphite has been destroyed during milling. 3.2. Electrochemical properties Fig. 3 exhibits the discharge capacities of the A, B and A–x wt.% B (x = 90, 70, 50, 30, 10) composite alloys at 298 K. All the alloys were fully activated within five charge–discharge cycles. The maximum discharge capacity of A alloy reached 363 mAh g−1 . The maximum discharge capacity of B powder is 277 mAh g−1 , which is higher than that of the Co(OH)2 /Co faradic reaction 80 mAh g−1 [13]. When x = 50, 30, the maximum discharge capacity reached 407 and 414 mAh g−1 respectively, which is higher than pure A and B alloys. If there is no interaction between A and B in the composite, the calculated values according to the weight percentage (w) of A–x wt.% B composites cannot reach that value (Table 1). Since the experiments prove that the discharge capacity has been improved when incorporate A and B with appropriate ratio, we consider there were interactions between A and B alloys. When x = 10,
Fig. 3. Discharge capacity of the A, B and A–x wt.% B (x = 90, 70, 50, 30, 10) composite alloys at 298 K. Discharge current density: 50 mA g−1 .
Alloy
Maximum discharge capacity (experimentation)
Maximum discharge capacity (calculation)
A B A–50 wt.% B A–30 wt.% B
363 mAh g−1 277 mAh g−1 407 mAh g−1 414 mAh g−1
320 mAh g−1 337 mAh g−1
90 the cyclic stabilities of the composite electrodes are greatly enhanced compared with that of A alloy, although their maximum discharge capacity are lower than that of A alloy, indicating that A–10 wt.% B and A–90 wt.% B had the better cycle lives among this series. Fig. 4 illustrates CV curves of A and A–50 wt.% B alloy electrodes at 298 K. For each sample, the oxidation peak appeared at the potential of around −0.7 V (vs. HgO/Hg). The reduction peak appeared at the potential of around −1.0 V (vs. HgO/Hg). The oxidation peak was attributed to the hydrogen desorption from the interior to the surface of the alloy particles [15,17]. As seen in Fig. 4, the height and area of the oxidation peak of A–50 wt.% B alloy is higher and larger than pure A, which indicated that the discharge kinetics and capacity could be significantly improved. The result is consistent with the charge–discharge measurements. From SEM images, B powders have more powder–particle boundaries and small pores than A particles which can help trapping hydrogen in the sample. So the B powder may play a certain catalytic role for the hydrogen absorption of A, or dissociation of H2 near the A surface. Potentiodynamic polarization experiment was employed to investigate the corrosive behavior of the alloy electrodes. Fig. 5 shows the potentiodynamic polarization curves for A and A–50 wt.% B alloys. The Tafel fitting results were summarized in Table 2. The result reveals that Ecorr (corrosion potential) value of A–50 wt.% B alloy is higher than that of A, and the corrosion rate Icorr (corrosion current) is lower. These results suggest that B improves the anti-corrosive behavior of A alloy, and B coat-
Fig. 4. CV curves of the A and A–50 wt.% B electrode alloys.
Y. Zhang et al. / Journal of Alloys and Compounds 467 (2009) L16–L20
Fig. 5. Potentiodynamic polarization curves of the A and A–50 wt.% B composite alloys. Table 2 Tafel fitting data of the A and A–50 wt.% B alloy electrode Electrode
A
A–50 wt.% B
Ecorr/V Icorr/A
−0.923 3.00E−05
−0.898 2.93E−05
ing on the A powder particles can inhibit the formation of a new oxide layer on the powder surfaces. 3.3. Hydrogenation mechanism The XRD patterns of A and A–50 wt.% B alloys electrode at different charge and discharge states are shown in Fig. 6. The A–50 wt.% B powder basically maintains the A and B original phase structures, the intensity of LaNi3 peaks is stronger than LaNi5 peaks in both charge and discharge processes. After charge (Fig. 6 a, b), no new phase was formed in A alloy and the A–50 wt.% B composite alloy. However, after discharge
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Fig. 7. The 5th cycle discharge potential curves of A, B and A–50 wt.% B electrodes at 298 K.
(Fig. 6d), the A–50 wt.% B showed some characteristic peaks of Co(OH)2 , and these XRD peaks of Co(OH)2 were still presented and remained their intensities almost unchanged after recharge (Fig. 6e), implying that the Co(OH)2 was not reduced at the charging potential interval. This phenomenon suggest that the reversible reaction of Co(OH)2 + 2e− ⇔ Co + 2OH− is a subsidiary reaction in the charge/discharge processes. The high discharge capacities obtained in the present study should be attributed primarily to the dehydrogenation of A–50 wt.% B alloy. The 5th cycle discharge potential curves of A, B and A–50 wt.% B alloys at 298 K are shown in Fig. 7. They have different discharge potentials, −0.87 V for A alloy electrode, −0. 77 V for B alloy electrode and −0.80 for A–50 wt.% B alloy electrode at 50% DOD. The discharge potential of A–50 wt.% B electrode is higher than B alloy electrode and lower than A alloy electrode. As there was only one discharge potential plateau of A–50 wt.% B alloy electrode, we conclude that the discharge process is not generated by A and B alloys separately. Although the A–50 wt.% B alloy was prepared by simply mixing A and B alloys, there exist interactions between A and B alloys. From the above analysis we suppose B powder may play a certain catalytic role in the hydrogen absorption of A. 3.4. High rate capability
Fig. 6. XRD patterns of the A (1st cycle charge and discharge) and A–50 wt.% B (1st cycle charge and discharge, 2nd charge) alloys.
Fig. 8 shows the cycling performance of the A–50 wt.% B electrode at current densities of 50, 250 and 500 mA g−1 . The reversible discharge capacity became lower at higher current densities, and the maximum discharge capacity of A–50 wt.% B electrode at current density of 250 and 500 mA g−1 is 323 and 265 mAh g−1 respectively. But the cycle lives become better at higher current densities. The main reason on this phenomenon possibly is that passivation layer forms on the surface of the alloy when the current rate is high enough and further corrosion is prevented by this layer. This powder can be a potential candidate in higher current density.
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cess is not generated by La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C alloys separately, there are interactions between La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C alloys. The La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –50 wt.% Co–C electrode at higher current density show good cycling life. Acknowledgments This work was supported by NSFC (50571046, 50631020, 50701025 and 20573058) ,Tianjin Nature Science Fund (07JCYBJC03500), Doctoral Fund of Ministry of Education (20070055064) and 863 programs (2007AA05Z149, 2007AA05Z108, 2006AA05Z110). References Fig. 8. Discharge capacity of the A–50 wt.% B electrode alloys at large current density of 50 mA g−1 , 250 mA g−1 , and 500 mA g−1 (298 K).
4. Conclusion In this work, Co–C powder by ball milling was introduced into La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 hydrogen storage alloy. The maximum discharge capacity and the cycle life were significantly improved. Scanning electron microscopic images show the large La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 particles were coated with smaller Co–C particles. The addition of small Co–C particles reduces the agglomeration of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 particles, which can enhance the hydrogen diffusion. Co–C coating on the La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 powder particles can inhibit the formation of a new oxide layer on the powder surfaces. These results were further proved by the cyclic voltammogram and potentiodynamic polarization studies. X-ray diffraction patterns show that the composite almost kept original phase structures of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 and Co–C in both charge and discharge processes. The charge–discharge processes of the La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –50 wt.% Co–C electrode are predominated through electrochemical hydrogen storage mechanism. The electrochemical oxidation of cobalt contributes only a negligible part to the reversible electrochemical capacity. As there is only one potential plateau of La0.8 Mg0.2 Ni0.8 Mn0.1 Co0.5 Al0.1 –50 wt.% Co–C alloy electrode, we conclude that the discharge pro-
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