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Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3 composite hydrogen storage alloys prepared by two-step re-melting
International Journal of Hydrogen Energy 32 (2007) 4939 – 4942 www.elsevier.com/locate/ijhydene
Activation characteristics and microstructure of Mg2 Ni/Mm0.3Ml0.7Ni3.55Co0.75Mn0.4 Al0.3 composite hydrogen storage alloys prepared by two-step re-melting Liu Xiangdong, Huang Lihong ∗ , Tian Xiao, Feng Hongwei, Chi Bo School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, PR China Received 25 June 2007; received in revised form 25 July 2007; accepted 25 July 2007 Available online 12 September 2007
Abstract In the present work, novel composites Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) for hydrogen storage were prepared by two-step re-melting and their activation characteristic and microstructure were investigated. The influence of Mg2 Ni content on the activation characteristics was analyzed by electrochemical method. With the increasing content of Mg2 Ni, activation characteristics and maximum discharge capacities of composites increase first and then decrease. The composite with 5% Mg2 Ni has the least cycle number for activation and the highest discharge capacity. It is activated after only 6 cycles (Cn = 6) at room temperature and its maximum discharge capacity (Cmax ) reaches 274.4 mAh/g. However, the composite contained 30 wt% Mg2 Ni is difficult to be activated at room temperature. It is also found that it is easier to be activated for the composites at Ic = 60 mA/g and Id = 60 mA/g than that at Ic = 100 mA/g and Id = 60 mA/g, but their discharge capacity decay slightly at the condition of Ic = 60 mA/g and Id = 60 mA/g. The XRD and SEM analysis show that, with the increasing Mg2 Ni content, the microstructure of the composites varies gradually from lamellar (x = 5), acicular (x = 10) to massive (x = 30), and the activity of the composite declines as a result of the grain size of phase Mg2 Ni grows up. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Composite hydrogen storage alloy; Activation characteristic; Mg2 Ni; Microstructure
1. Introduction Rechargeable nickel–metal hydride batteries with hydrogen storage alloy as the negative electrode material have been lucubrated and used extensively for their several advantages [1]: high energy density, good high-rate discharge ability, long cycle life, no memory effect and environmental accept ability. In addition, the rare-earth-based AB5 -type hydrogen storage alloys would be expanded and shrunk excessively during hydrogen absorption/desorption. So they could not meet the application need of nickel–metal hydride batteries with high performance any more for their capacity releasing so fast and short cycle life. Mg–Ni based alloys are advantageous in terms of low weight and high hydrogen storage capacity. However, they can be activated only at higher temperature and the sorption kinetics ∗ Corresponding author. Tel.: +86 015848123209; fax: +86 471 6575712.
E-mail address: [email protected] (H. Lihong).
is poor. So far, their potential ascendancy has not still been exerted in practical application. In order to obtain excellent comprehensive performance, some novel composites are manufactured. Presently, most of composite hydrogen storage materials are obtained by hydride combustion synthesis [2] and mechanical alloying [3–7]. However, there are two disadvantages such as long mechanical alloying time and short cycle stability. In addition, phases cannot be joined closely with each other together in the mechanical alloying process, particle surface is easy to be oxidized and polluted, as well as some impurity is also easy to be introduced into alloy in the processing. In order to improve electrochemical properties of hydrogen storage alloys, novel composites Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) from the two kinds of alloy were prepared by two-step re-melting which can avoid above disadvantages in this present work. The effects of Mg2 Ni content on their structure and activate characteristics of the composites were investigated.
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.07.052
4940
L. Xiangdong et al. / International Journal of Hydrogen Energy 32 (2007) 4939 – 4942
The phase structure and its chemical compositions as well as microstructure of the composites were analyzed by XRD, EDS and SEM.
15 14
The composites Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) were synthesized by two-step remelting. Firstly, the master alloy ingots of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 and Mg2 Ni were prepared by vacuum nonconsumable arc-melting. Every kind of bowl-ingots was remelted three times to ensure the homogeneous composition, and then crushed and mixed in designed proportion. Finally, the mixture was re-molten by electromagnetic levitation melting and the composite was obtained. The purity of the raw materials (Ni, Co, Mn, Al, Mg) used in this work was at least 99.5%. Hydrogen storage powders were prepared by mechanical pulverization to a particle size that less than 200 SI unit, mixed with 10 wt% analytical polyvinyl alcohol (PVA) solution and pasted uniformly on to a 10 mm ×10 mm foamed nickel screen, dried and compressed by a 10 MPa pressure into a pellet, and then the coarse side was cut down and served as a testing anode. The PVA to powder weight ratio was 3:97. Each test cell comprised of a working electrode (MH electrode), a sintered Ni(OH)2 /NiOOH-counter electrode and a reference electrode (Hg/HgO electrode). A non-woven nylon-cloth separator was inserted between two counter electrodes. Electrodes were tightly bound in an organic plate glass holder, charge/discharge cycles were applied in 6 mol L−1 KOH solution at 25 ◦ C and discharged to −0.60 V vs. Hg/HgO at the active condition that Ic =100 mA/g, Id =60 mA/g and Ic =60 mA/g, Id =60 mA/g, respectively. The activation characteristics of the composites were measured by battery testing instrument type DC-5. The crystal structure of the alloy powder was identified by X-ray diffractmeter (Bruker-D 8) with Cu-K radiation. The microstructure was observed by a scanning electron microscope (Holand, type QUANTA400) and chemical compositions were analyzed by an energy dispersive spectroscopy.
Cycle number (N)
13
2. Experimental
12
Ic=100mA/g and Id=60mA/g Ic=60mA/g and Id=60mA/g
11 10 9 8 7 6 5 0
2
4 6 Mg2Ni content (wt.%)
8
10
Fig. 1. Influences of Mg2 Ni content on the activities of investigated alloy electrodes.
difficult to be activated at room temperature because there is no change for its discharge capacity in the activation process. Fig. 2 shows discharge capacities of investigated alloys electrodes vary with cycle numbers. At Ic = 100 mA/g and Id = 60 mA/g, discharge capacities of the composites contained 0.10 wt% Mg2 Ni do not decay notably during cycling. However, it is also found that the composite contained 30 wt% Mg2 Ni is difficult to be activated at room temperature. Obviously, this result shows that the composite contained excess of Mg2 Ni is hardly activated and it has a very low discharge capacity at room temperature, as shown in Fig. 2. Adsorption ability and surface activity of the composite fall down due to Mg or RE oxides forming easily on the alloy surface. On the other hand, at Ic = 60 mA/g and Id = 60 mA/g, the composites contained 0.5 wt% Mg2 Ni have high discharge capacities which do not decline noticeably during cycling. But the composites with 10 and 30 wt% Mg2 Ni both have very low discharge capacities, in spit that the former has good cycle stability during charge–discharge cycling. 3.2. Maximum discharge capacity
3. Results and discussion 3.1. Activation characteristics The influences of Mg2 Ni content on the activities of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) alloy electrodes at room temperature under different activation conditions are shown in Fig. 1. When x is 0, 5 and 10, the composites are activated after 12, 13, and 15 cycles at Ic = 100 mA/g and Id = 60 mA/g, but only after 6, 6, 8 cycles at Ic = 60 mA/g and Id = 60 mA/g, separately. It illustrates that it is easier to be activated for the composites at Ic = 60 mA/g and Id = 60 mA/g than that at Ic = 100 mA/g and Id = 60 mA/g. Cycle number for the composites to be activated increases at room temperature with the increasing content of Mg2 Ni in matrix. It is resulted in Mg2 Ni which is
It is found that the electrode from the composite Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /5 wt% Mg2 Ni has the highest capacity, achieves 274.4 mAh/g at Ic = 100 mA/g and Id = 60 mA/g, as well as 260.6 mAh/g at Ic =60 mA/g and Id =60 mA/g, respectively, as shown in Fig. 3. However, the discharge capacity of the composite Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /30 wt% Mg2 Ni is only 90.72 and 111.7 mAh/g under the above two conditions, individually. Meanwhile, its discharge capacity at Ic =60 mA/g and Id = 60 mA/g decays lower than that at Ic = 100 mA/g and Id = 60 mA/g. 3.3. Phase structure The XRD patterns of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni alloys are shown in Fig. 4. The result shows that
L. Xiangdong et al. / International Journal of Hydrogen Energy 32 (2007) 4939 – 4942
4941
300 280 250
240 Capacity (mAh/g)
Discharge capacity (mAh/g)
260
220 200 x=0 x=5 x=10 x=30
180 160
200
150
140
Ic=100mA/g and Id=60mA/g Ic=60mA/g and Id=60mA/g
120
100
100 0
5
10 Cycle number (N)
15
0
20
5
10 15 20 Mg2Ni content (wt.%)
25
30
Fig. 3. Maximum discharge capacities of investigated alloy electrodes as a function of x Mg2 Ni content.
260 x=0 x=5 x=10 x=30
220 200
LaNi5 Mg2Ni AlLaNi4
180
x=30
160 140
x=10
intensity
Discharge capacity (mAh/g)
240
120 100
x=5
80 0
2
4 6 Cycle number (N)
8
10
Fig. 2. Relationship of discharge capacities of investigated alloys electrodes with cycle numbers; (a) Ic = 100 mA/g and Id = 60 mA/g, (b) Ic = 60 mA/g and Id = 60 mA/g.
the main phase of all the alloys is LaNi5 with a hexagonal type CaCu5 and space group P6/mmm. The second phase Mg2 Ni (Mg2 Ni has a space group P62 22, and its crystal volume is larger than that of LaNi5 ) in alloys exists when x 5 and its diffraction intensity increases with increasing x value. Also there appears some AlLaNi4 phase contained in the composites. The hydride formation heat has relation to the crystal volume of inter-metallic compounds [8]. The larger the crystal volume is, the bigger the tetrahedron clearance is, and the more negative the hydride formation heat is. It is steady enough not to break up the hydride while discharging, so that parts of hydrogen existed in hydrides cannot be deoxidized. It is resulted in that the existence of Mg2 Ni phase reduces the discharge capacity of composite hydrogen storage alloy. From Fig. 4, it is clear that the diffraction intensity of Mg2 Ni in the composite as x = 30 is greater than the others, it shows that it has minimum capacity resulted in excess of Mg2 Ni content. Moreover, Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 alloy without adding Mg2 Ni has a narrower and sharper diffraction peak
x=0 20
25
30
35
40
45 50 2θ / deg
55
60
65
70
Fig. 4. X-ray diffraction patterns of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30).
in contrast with the others. This illustrates that the grain size of the composite becomes smaller when adding 5 wt% Mg2 Ni into Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 alloy. It is obvious that two-step re-melting plays role in reduction in grain size. However, the phase in grains grows up noticeably as Mg2 Ni content excess 5 wt%. 3.4. Microstructure It could be confirmed that, there exist LaNi5 , Mg2 Ni and Al or Ni-rich phase in the matrix from the analytic results of XRD and SEM. Fig. 5 displays the microstructure of the composites. With x increasing, the microstructure of the composites varies gradually from lamellar (x = 5), acicular (x = 10) to massive (x = 30), and the grains of Mg2 Ni phase grow slightly, which results in declining of activity of the composite. It is
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L. Xiangdong et al. / International Journal of Hydrogen Energy 32 (2007) 4939 – 4942
Fig. 5. SEM micrographs of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0.30) composites; (a) x = 0, (b) x = 5, (c) x = 10 and (d) x = 30.
obvious that the electrochemical and thermodynamic performance of the composite vary distinctively as the result of various microstructures as adding different amount of Mg2 Ni. It is attributed to the massive microstructure for the poor activation characteristic of the composite with 30% Mg2 Ni.
Acknowledgment
4. Conclusions
References
With the increasing of x value, activation characteristic and maximum discharge capacity of the hydrogen storage alloys first increase and then decrease. The composite with 5% Mg2 Ni has the least activation number and the highest discharge capacity. It is activated after only 6 cycles (Cn = 6) at room temperature. The maximum discharge capacity (Cmax ) reaches 274.4 mAh/g. However, the composite contained 30 wt% Mg2 Ni is difficult to be activated at room temperature because there is no change for its discharge capacity in the activation process. Furthermore, it is easier to be activated for the composites when Ic = 60 mA/g and Id = 60 mA/g than that at Ic = 100 mA/g and Id = 60 mA/g, but the discharge capacity descends slightly. The microstructure of the composites varies gradually from lamellar (x = 5), acicular (x = 10) to massive (x = 30) as x increasing, and the grains of phase Mg2 Ni grow slightly, corresponding to the decline of activity of the composite hydrogen storage materials.
[1] Yu MS, Kolomiets LL, Solonin SM. et al. Development of hydrogenating power alloys for the electrodes of alkali batteries. Powder Metall Met Ceram 2003;42(9–10):491–6. [2] Li Q, Lin Q, Jiang LJ. et al. Optimization of technics for La1.5 Ni0.5 Mg17 by hydriding combustion synthesis. J Chin Rare Earth Soc 2003;21(6): 652–5 [in Chinese]. [3] Liang G, Huot J, VanNeste A. et al. Hydrogen storage in mechanically milled Mg.LaNi5 and MgH2 .LaNi5 . J Alloys Compds 2000;297:261–5. [4] Wang W, Chen CP, Chen LX. et al. Change in structure and hydrogen storage properties of La2 Mg17 Ni alloys after modification by mechanical grinding in tetrahydrofuran. J Alloys Compds 2002;339:175. [5] Liu YN. Effect of lanthanum additions on electrode properties of Mg2 Ni. J Alloys Compds 1998;267:231–4. [6] Li Q, Jiang LJ. Hydriding and dehydriding characteristics of mechanically alloyed LaMg17 Ni composite material. J Rare Earths 2003;21(3):337–40. [7] Jiang HZ, Kong FQ, Han L. et al. Progress in investigation of RE–Mg–Ni hydrogen storage electrode materials. Chin Rare Earths 2005;26(1):60–6 [in Chinese]. [8] Osumi Y. Properties and applications of metal hydride [Wu Y, Miao Y, Trans.]. Beijing: Chemical Industry Press; 1990. p. 73–9 [in Chinese].
This work was supported by the Talents Development Foundation of Inner Mongolia Autonomous Region, PR China under the Contract No. 200606.
Activation characteristics and microstructure of Mg2 Ni/Mm0.3Ml0.7Ni3.55Co0.75Mn0.4 Al0.3 composite hydrogen storage alloys prepared by two-step re-melting Liu Xiangdong, Huang Lihong ∗ , Tian Xiao, Feng Hongwei, Chi Bo School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, PR China Received 25 June 2007; received in revised form 25 July 2007; accepted 25 July 2007 Available online 12 September 2007
Abstract In the present work, novel composites Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) for hydrogen storage were prepared by two-step re-melting and their activation characteristic and microstructure were investigated. The influence of Mg2 Ni content on the activation characteristics was analyzed by electrochemical method. With the increasing content of Mg2 Ni, activation characteristics and maximum discharge capacities of composites increase first and then decrease. The composite with 5% Mg2 Ni has the least cycle number for activation and the highest discharge capacity. It is activated after only 6 cycles (Cn = 6) at room temperature and its maximum discharge capacity (Cmax ) reaches 274.4 mAh/g. However, the composite contained 30 wt% Mg2 Ni is difficult to be activated at room temperature. It is also found that it is easier to be activated for the composites at Ic = 60 mA/g and Id = 60 mA/g than that at Ic = 100 mA/g and Id = 60 mA/g, but their discharge capacity decay slightly at the condition of Ic = 60 mA/g and Id = 60 mA/g. The XRD and SEM analysis show that, with the increasing Mg2 Ni content, the microstructure of the composites varies gradually from lamellar (x = 5), acicular (x = 10) to massive (x = 30), and the activity of the composite declines as a result of the grain size of phase Mg2 Ni grows up. 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Composite hydrogen storage alloy; Activation characteristic; Mg2 Ni; Microstructure
1. Introduction Rechargeable nickel–metal hydride batteries with hydrogen storage alloy as the negative electrode material have been lucubrated and used extensively for their several advantages [1]: high energy density, good high-rate discharge ability, long cycle life, no memory effect and environmental accept ability. In addition, the rare-earth-based AB5 -type hydrogen storage alloys would be expanded and shrunk excessively during hydrogen absorption/desorption. So they could not meet the application need of nickel–metal hydride batteries with high performance any more for their capacity releasing so fast and short cycle life. Mg–Ni based alloys are advantageous in terms of low weight and high hydrogen storage capacity. However, they can be activated only at higher temperature and the sorption kinetics ∗ Corresponding author. Tel.: +86 015848123209; fax: +86 471 6575712.
E-mail address: [email protected] (H. Lihong).
is poor. So far, their potential ascendancy has not still been exerted in practical application. In order to obtain excellent comprehensive performance, some novel composites are manufactured. Presently, most of composite hydrogen storage materials are obtained by hydride combustion synthesis [2] and mechanical alloying [3–7]. However, there are two disadvantages such as long mechanical alloying time and short cycle stability. In addition, phases cannot be joined closely with each other together in the mechanical alloying process, particle surface is easy to be oxidized and polluted, as well as some impurity is also easy to be introduced into alloy in the processing. In order to improve electrochemical properties of hydrogen storage alloys, novel composites Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) from the two kinds of alloy were prepared by two-step re-melting which can avoid above disadvantages in this present work. The effects of Mg2 Ni content on their structure and activate characteristics of the composites were investigated.
0360-3199/$ - see front matter 䉷 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.07.052
4940
L. Xiangdong et al. / International Journal of Hydrogen Energy 32 (2007) 4939 – 4942
The phase structure and its chemical compositions as well as microstructure of the composites were analyzed by XRD, EDS and SEM.
15 14
The composites Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) were synthesized by two-step remelting. Firstly, the master alloy ingots of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 and Mg2 Ni were prepared by vacuum nonconsumable arc-melting. Every kind of bowl-ingots was remelted three times to ensure the homogeneous composition, and then crushed and mixed in designed proportion. Finally, the mixture was re-molten by electromagnetic levitation melting and the composite was obtained. The purity of the raw materials (Ni, Co, Mn, Al, Mg) used in this work was at least 99.5%. Hydrogen storage powders were prepared by mechanical pulverization to a particle size that less than 200 SI unit, mixed with 10 wt% analytical polyvinyl alcohol (PVA) solution and pasted uniformly on to a 10 mm ×10 mm foamed nickel screen, dried and compressed by a 10 MPa pressure into a pellet, and then the coarse side was cut down and served as a testing anode. The PVA to powder weight ratio was 3:97. Each test cell comprised of a working electrode (MH electrode), a sintered Ni(OH)2 /NiOOH-counter electrode and a reference electrode (Hg/HgO electrode). A non-woven nylon-cloth separator was inserted between two counter electrodes. Electrodes were tightly bound in an organic plate glass holder, charge/discharge cycles were applied in 6 mol L−1 KOH solution at 25 ◦ C and discharged to −0.60 V vs. Hg/HgO at the active condition that Ic =100 mA/g, Id =60 mA/g and Ic =60 mA/g, Id =60 mA/g, respectively. The activation characteristics of the composites were measured by battery testing instrument type DC-5. The crystal structure of the alloy powder was identified by X-ray diffractmeter (Bruker-D 8) with Cu-K radiation. The microstructure was observed by a scanning electron microscope (Holand, type QUANTA400) and chemical compositions were analyzed by an energy dispersive spectroscopy.
Cycle number (N)
13
2. Experimental
12
Ic=100mA/g and Id=60mA/g Ic=60mA/g and Id=60mA/g
11 10 9 8 7 6 5 0
2
4 6 Mg2Ni content (wt.%)
8
10
Fig. 1. Influences of Mg2 Ni content on the activities of investigated alloy electrodes.
difficult to be activated at room temperature because there is no change for its discharge capacity in the activation process. Fig. 2 shows discharge capacities of investigated alloys electrodes vary with cycle numbers. At Ic = 100 mA/g and Id = 60 mA/g, discharge capacities of the composites contained 0.10 wt% Mg2 Ni do not decay notably during cycling. However, it is also found that the composite contained 30 wt% Mg2 Ni is difficult to be activated at room temperature. Obviously, this result shows that the composite contained excess of Mg2 Ni is hardly activated and it has a very low discharge capacity at room temperature, as shown in Fig. 2. Adsorption ability and surface activity of the composite fall down due to Mg or RE oxides forming easily on the alloy surface. On the other hand, at Ic = 60 mA/g and Id = 60 mA/g, the composites contained 0.5 wt% Mg2 Ni have high discharge capacities which do not decline noticeably during cycling. But the composites with 10 and 30 wt% Mg2 Ni both have very low discharge capacities, in spit that the former has good cycle stability during charge–discharge cycling. 3.2. Maximum discharge capacity
3. Results and discussion 3.1. Activation characteristics The influences of Mg2 Ni content on the activities of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30) alloy electrodes at room temperature under different activation conditions are shown in Fig. 1. When x is 0, 5 and 10, the composites are activated after 12, 13, and 15 cycles at Ic = 100 mA/g and Id = 60 mA/g, but only after 6, 6, 8 cycles at Ic = 60 mA/g and Id = 60 mA/g, separately. It illustrates that it is easier to be activated for the composites at Ic = 60 mA/g and Id = 60 mA/g than that at Ic = 100 mA/g and Id = 60 mA/g. Cycle number for the composites to be activated increases at room temperature with the increasing content of Mg2 Ni in matrix. It is resulted in Mg2 Ni which is
It is found that the electrode from the composite Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /5 wt% Mg2 Ni has the highest capacity, achieves 274.4 mAh/g at Ic = 100 mA/g and Id = 60 mA/g, as well as 260.6 mAh/g at Ic =60 mA/g and Id =60 mA/g, respectively, as shown in Fig. 3. However, the discharge capacity of the composite Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /30 wt% Mg2 Ni is only 90.72 and 111.7 mAh/g under the above two conditions, individually. Meanwhile, its discharge capacity at Ic =60 mA/g and Id = 60 mA/g decays lower than that at Ic = 100 mA/g and Id = 60 mA/g. 3.3. Phase structure The XRD patterns of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni alloys are shown in Fig. 4. The result shows that
L. Xiangdong et al. / International Journal of Hydrogen Energy 32 (2007) 4939 – 4942
4941
300 280 250
240 Capacity (mAh/g)
Discharge capacity (mAh/g)
260
220 200 x=0 x=5 x=10 x=30
180 160
200
150
140
Ic=100mA/g and Id=60mA/g Ic=60mA/g and Id=60mA/g
120
100
100 0
5
10 Cycle number (N)
15
0
20
5
10 15 20 Mg2Ni content (wt.%)
25
30
Fig. 3. Maximum discharge capacities of investigated alloy electrodes as a function of x Mg2 Ni content.
260 x=0 x=5 x=10 x=30
220 200
LaNi5 Mg2Ni AlLaNi4
180
x=30
160 140
x=10
intensity
Discharge capacity (mAh/g)
240
120 100
x=5
80 0
2
4 6 Cycle number (N)
8
10
Fig. 2. Relationship of discharge capacities of investigated alloys electrodes with cycle numbers; (a) Ic = 100 mA/g and Id = 60 mA/g, (b) Ic = 60 mA/g and Id = 60 mA/g.
the main phase of all the alloys is LaNi5 with a hexagonal type CaCu5 and space group P6/mmm. The second phase Mg2 Ni (Mg2 Ni has a space group P62 22, and its crystal volume is larger than that of LaNi5 ) in alloys exists when x 5 and its diffraction intensity increases with increasing x value. Also there appears some AlLaNi4 phase contained in the composites. The hydride formation heat has relation to the crystal volume of inter-metallic compounds [8]. The larger the crystal volume is, the bigger the tetrahedron clearance is, and the more negative the hydride formation heat is. It is steady enough not to break up the hydride while discharging, so that parts of hydrogen existed in hydrides cannot be deoxidized. It is resulted in that the existence of Mg2 Ni phase reduces the discharge capacity of composite hydrogen storage alloy. From Fig. 4, it is clear that the diffraction intensity of Mg2 Ni in the composite as x = 30 is greater than the others, it shows that it has minimum capacity resulted in excess of Mg2 Ni content. Moreover, Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 alloy without adding Mg2 Ni has a narrower and sharper diffraction peak
x=0 20
25
30
35
40
45 50 2θ / deg
55
60
65
70
Fig. 4. X-ray diffraction patterns of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0, 5, 10, 30).
in contrast with the others. This illustrates that the grain size of the composite becomes smaller when adding 5 wt% Mg2 Ni into Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 alloy. It is obvious that two-step re-melting plays role in reduction in grain size. However, the phase in grains grows up noticeably as Mg2 Ni content excess 5 wt%. 3.4. Microstructure It could be confirmed that, there exist LaNi5 , Mg2 Ni and Al or Ni-rich phase in the matrix from the analytic results of XRD and SEM. Fig. 5 displays the microstructure of the composites. With x increasing, the microstructure of the composites varies gradually from lamellar (x = 5), acicular (x = 10) to massive (x = 30), and the grains of Mg2 Ni phase grow slightly, which results in declining of activity of the composite. It is
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L. Xiangdong et al. / International Journal of Hydrogen Energy 32 (2007) 4939 – 4942
Fig. 5. SEM micrographs of Mm0.3 Ml0.7 Ni3.55 Co0.75 Mn0.4 Al0.3 /x wt% Mg2 Ni (x = 0.30) composites; (a) x = 0, (b) x = 5, (c) x = 10 and (d) x = 30.
obvious that the electrochemical and thermodynamic performance of the composite vary distinctively as the result of various microstructures as adding different amount of Mg2 Ni. It is attributed to the massive microstructure for the poor activation characteristic of the composite with 30% Mg2 Ni.
Acknowledgment
4. Conclusions
References
With the increasing of x value, activation characteristic and maximum discharge capacity of the hydrogen storage alloys first increase and then decrease. The composite with 5% Mg2 Ni has the least activation number and the highest discharge capacity. It is activated after only 6 cycles (Cn = 6) at room temperature. The maximum discharge capacity (Cmax ) reaches 274.4 mAh/g. However, the composite contained 30 wt% Mg2 Ni is difficult to be activated at room temperature because there is no change for its discharge capacity in the activation process. Furthermore, it is easier to be activated for the composites when Ic = 60 mA/g and Id = 60 mA/g than that at Ic = 100 mA/g and Id = 60 mA/g, but the discharge capacity descends slightly. The microstructure of the composites varies gradually from lamellar (x = 5), acicular (x = 10) to massive (x = 30) as x increasing, and the grains of phase Mg2 Ni grow slightly, corresponding to the decline of activity of the composite hydrogen storage materials.
[1] Yu MS, Kolomiets LL, Solonin SM. et al. Development of hydrogenating power alloys for the electrodes of alkali batteries. Powder Metall Met Ceram 2003;42(9–10):491–6. [2] Li Q, Lin Q, Jiang LJ. et al. Optimization of technics for La1.5 Ni0.5 Mg17 by hydriding combustion synthesis. J Chin Rare Earth Soc 2003;21(6): 652–5 [in Chinese]. [3] Liang G, Huot J, VanNeste A. et al. Hydrogen storage in mechanically milled Mg.LaNi5 and MgH2 .LaNi5 . J Alloys Compds 2000;297:261–5. [4] Wang W, Chen CP, Chen LX. et al. Change in structure and hydrogen storage properties of La2 Mg17 Ni alloys after modification by mechanical grinding in tetrahydrofuran. J Alloys Compds 2002;339:175. [5] Liu YN. Effect of lanthanum additions on electrode properties of Mg2 Ni. J Alloys Compds 1998;267:231–4. [6] Li Q, Jiang LJ. Hydriding and dehydriding characteristics of mechanically alloyed LaMg17 Ni composite material. J Rare Earths 2003;21(3):337–40. [7] Jiang HZ, Kong FQ, Han L. et al. Progress in investigation of RE–Mg–Ni hydrogen storage electrode materials. Chin Rare Earths 2005;26(1):60–6 [in Chinese]. [8] Osumi Y. Properties and applications of metal hydride [Wu Y, Miao Y, Trans.]. Beijing: Chemical Industry Press; 1990. p. 73–9 [in Chinese].
This work was supported by the Talents Development Foundation of Inner Mongolia Autonomous Region, PR China under the Contract No. 200606.