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Microstructure and electrochemical properties of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy
JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010, p. 100
Microstructure and electrochemical properties of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy PAN Chongchao (潘崇超), CAI Wupeng (蔡吴鹏), Javed Iqbal, WANG Zhen (王 震), GENG Chaoqing (耿朝青), YU Ronghai (于荣海) (Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China) Received 11 May 2009; revised 3 June 2009
Abstract: The effects of rapid solidification on the microstructure and electrochemical properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloy were systematically investigated. The microstructure of alloys was characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and transmission electron microscopy (TEM). It was found that the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons became thinner and the average grain size of the ribbons became smaller with increasing wheel speed. A fraction of amorphous phase was observed for the ribbons melt-spun at high wheel speed (≥20 m/s). Microstructural characterization revealed that two phases: (Nd,Mg)2(Ni,Co)7 main phase (Ce2Ni7 type structure) and NdNi5 second phase (CaCu5 type structure), existed in the samples in cast state and melt-spun. The cycle stability of the melt-spun alloys was significantly enhanced as compared with cast alloy, and the sample prepared at wheel speed of 20 m/s exhibited good comprehensive electrochemical properties. Keywords: hydrogen storage alloy; rapid solidification; microstructure; electrochemical property; rare earths
Hydrogen is thought to be one of the most important energy carriers for the future automotive applications with high energy efficiency and is environmental friendly[1–3]. Compared with AB5 alloys, the La-Mg-Ni-Co type alloys exhibit high hydrogen storage capacity, but they are with short cycle life and low capacity at high discharge rates[4]. A number of techniques has been employed to improve the overall performance of the alloys, such as element substitution, heat treatment and different stoichiometric ratio. However, cycle life and discharge capacity are controversial and could not be enhanced at the same time[5–7]. Tanaka et al.[8] reported that the cycle stability of Mg-Ni-Nd alloys fabricated by melt spinning has been improved. The refinement of the grain size and homogenization of composition were regarded as two main reasons for the improvement of the cycle stability of alloy electrode[9–11]. In this work, we optimized the alloys to a composition of Nd0.8Mg0.2(Ni0.8Co0.2)3.8, which exhibited good overall properties in normal state. Subsequently, the alloys were melt-spun at varying wheel speed to investigate the effect of rapid solidification on the microstructure and electrochemical properties of this hydrogen storage alloy. A considerable enhancement of electrochemical properties was observed, which is closely correlated to the microstructural modification during melt-spinning process.
1 Experimental Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys were prepared by induction
melting under high purity argon atmosphere of 0.03 MPa, and the alloy composition is analytical results. All raw materials are with high purity (99.9 wt.% or above). Then, the ingots were re-melted and melt-spun onto Cu wheel at 10, 20, 30 and 40 m/s, respectively. The samples were crushed, ground into powder, and then sieved through 150 mesh and 300 mesh, respectively, for the measurement of electrochemical properties and structural characterizations. Alloy electrode was prepared by cold pressing the mixture of alloy powder with Cu powder in the mass ratio of 1:3 to form a pellet under the pressure of 250 MPa. Electrochemical measurements were performed in a standard open tri-electrode electrolysis cell which consists of working electrode, sintered Ni(OH)2/NiOOH counter electrode, and a Hg/HgO reference electrode immersed in a solution of 6 mol/L KOH electrolyte. The discharge capacities of alloys were determined by the galvanostatic method. Each electrode was charged with current of 60 mA/g for 7 h, rested for 5 min, and discharged with current of 60 mA/g to the cut-off potential of –0.6 V versus the Hg/HgO reference electrode. To investigate the high rate discharge property, the discharge capacities at different discharge current densities (150, 300, 600, 900 mA/g) were measured. The electrochemical impedance spectroscopy (EIS) were recorded using IM6 electrochemical workstation in open-circuit condition after 8 charge-discharge cycles for all alloy electrodes at 50% depth of discharge (DOD). The scanning frequency was varied from 100 kHz to 10 MHz at AC amplitude of 5 mV, the Z-View electrochemical impedance software was
Foundation item: Project supported by the National Natural Science Foundation of China (50471011, 50525101) Corresponding author: YU Ronghai (E-mail: [email protected]; Tel.: +86-10-62771160) DOI: 10.1016/S1002-0721(09)60060-0
PAN Chongchao et al., Microstructure and electrochemical properties of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 …
adopted to analyze the experimental data. X-ray powder diffraction patterns were measured on Rigaku D/max2500 X-ray diffractometer with Cu Kα radiation at a scanning speed of 6(°)/min in range of 20° to 80°, and the lattice parameters was obtained through Rietveld refinement with Maud software. The operating voltage and current of this XRD machine were 40 kV and 200 mA, respectively. Microstructures of these samples were characterized by JEM-2011 TEM and JSM-6460 SEM. The crystalline state of the samples was examined by selected area electron diffraction (SAD). The SEM samples were corroded in the solution of 15 g FeCl3, 50 ml HCl and 100 ml H2O.
2 Results and discussion
101
ribbons decreases from 60.0 μm to 28.0 μm with increasing wheel speed from 10 m/s to 40 m/s. The average grain sizes for melt-spun alloys with the wheel speeds of 10, 20, 30 and 40 m/s are about 8.9, 4.4, 3.6 and 1 μm, respectively. The rapid solidification leads to the fine grains and homogenous microstructure in the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons. Fig. 3 illustrates the TEM image and SAED pattern of melt-spun alloy fabricated at a wheel speed of 20 m/s. The diffraction spots and diffraction rings coexist, indicating a fraction of amorphous phases present in this melt-spun alloy. Our further TEM observation revealed that the amount of amorphous phases increases with increasing wheel speed. The wheel speed is an important factor which affects the phase composition in melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys.
2.1 Microstructure
2.2 Activation property and discharge capacity
The XRD patterns of the cast and melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys are shown in Fig. 1. It indicates that all the samples are mainly composed of two phases, main phase (NdMg)2(NiCo)7 (Ce2Ni7 type structure) with P63/mmc space group, and second phase NdNi5 (CaCu5 type structure) with the P6/mmm space group. As illustrated in Fig. 1, the typical diffraction peaks of two phases are broadened for the melt-spun samples compared with cast samples. The broadening of diffraction peaks might be partially attributed to the internal stress which could be easily induced by melt-spinning, i.e. fast solidification, especially at high wheel speed. The lattice parameters were calculated by Rietveld refinement with Maud software and the results are listed in Table 1. The values of lattice constants a and c are 0.5009 and 2.4376 nm, respectively, for (NdMg)2(NiCo)7 phase of cast samples. Those two lattice constants decrease for melt-spun samples with increasing wheel speed, and decrease more dramatically at wheel speed higher than 30 m/s. A similar tendency in the change of the lattice constants a and c with wheel speed was also found for NdNi5 phase as shown in Table 1. Fig. 2 shows the SEM images of the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons along the cross section, and the thickness has been measured. The thickness of melt spun
The activation，the maximum discharge capacity and rate properties of the alloy electrodes at 298 K are shown in Fig. 4. The cast alloy electrode is usually activated within three cycles, and all melt-spun alloy electrodes are activated within six cycles. The maximum discharge capacities of cast alloy and melt-spun alloys fabricated at the wheel speed of 10, 20,
Fig. 1 XRD patterns of cast and melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys
Table 1 Lattice parameters of two main phases in the cast and melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys Samples Phase
Lattice parameters a/nm
Cast 10 m/s 20 m/s 30 m/s 40 m/s
c/nm
c/a ratio Cell volume/nm3
(Nd, Mg)2(Ni, Co)7
0.5009 2.4376 4.867
0.529644
NdNi5
0.4989 0.3996 0.804
0.086172
(Nd, Mg)2(Ni, Co)7
0.5011 2.4347 4.858
0.529514
NdNi5
0.4982 0.3989 0.801
0.085725
(Nd, Mg)2(Ni, Co)7
0.5012 2.4342 4.857
0.529517
NdNi5
0.4984 0.3988 0.800
0.085798
(Nd, Mg)2(Ni, Co)7
0.5010 2.4326 4.855
0.528853
NdNi5
0.4983 0.3989 0.801
0.085777
(Nd, Mg)2(Ni, Co)7
0.5009 2.4302 4.851
0.528129
NdNi5
0.4977 0.3988 0.801
0.085581
Fig. 2 SEM images of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons along the cross section (a) 10 m/s; (b) 20 m/s; (c) 30 m/s; (d) 40 m/s
102
30 and 40 m/s are about 342.0, 311.0, 305.0, 285.0 and 275.0 mAh/g, respectively. The discharge capacity of the samples cast and melt-spun decreases with increasing discharge electric current, however, the drops for melt-spun samples are more dramatically compared with cast samples. Rapid solidification induced changes in microstructure: refinement of the grain size, appearance of amorphous phase, decrease of the lattice parameters (Table 1), and those changes are more prominent for the samples melt-spun at high wheel speed. All the microstructural modifications shown above reduced the hydrogen diffusion rate in the alloys, leading to the decrease of the discharge capacity and the rate properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes. Discharge voltage characteristic is an important property of the alloy electrode. The longer and more horizontal of discharge voltage plateau, the better discharge voltage characteristics. The discharge voltage curves of Nd0.8Mg0.2 (Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes at 298 K are shown in Fig. 5. The discharge potentials decrease with increasing wheel speeds, and the discharge capacity does the same. Therefore, the melt-spinning or rapid solidification negatively affects the discharge voltage properties for Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes.
JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010
of the alloys. The finer grain size may result in lower internal stress induced as the hydrogen atoms enter the interstitials sites of lattice. Therefore, the absorption of hydrogen atoms for melt-spun alloys leads to less lattice volume expansion as compared with cast alloys. However, the higher rapid solidification rates, the higher internal stress of lattice, so the maximum discharge capacity decreases with the increase of melt-spun speed. Another advantage of melt-spun alloys is that they have good corrosion resistance due to composition homogeneity and existence of amorphous phase. The electrochemical impedance spectroscopy (EIS) of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes at
2.3 Cycling stability
Fig. 4 Activation, maximum discharge capacity and rate property of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes
Fig. 6 demonstrates the variation of discharge capacity with cycle number measured at 298 K. The slopes of cycle stability curves of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys are smaller than that of the cast alloy. The melt-spun alloy sample with the wheel speed of 20 m/s exhibits good comprehensive properties, cycle stability and discharge capacity. According to the microstructural analysis, the rapid solidification reduces the grain size and lattice parameters, and homogenizes the distribution of the elements. It was reported that the capacity decay of alloy electrodes is attributed to the pulverization and oxidation mechanism[12,13]. The lattice internal stress and cell volume expansion are inevitable for alloy electrodes. The hydrogen proton absorption and adsorption in the lattice and interstitials lead to the pulverization
Fig. 5 Discharge voltage curves of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes
Fig. 3 TEM image and SAED pattern of alloy melt-spun at a wheel speed of 20 m/s
Fig. 6 Cycle property of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes
PAN Chongchao et al., Microstructure and electrochemical properties of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 …
103
References:
Fig. 7 Electrochemical impedance spectra (EIS) of Nd0.8Mg0.2(Ni0.8 Co0.2)3.8 hydrogen storage alloy electrodes measured at 50% DOD and 298 K (inset is equivalent circuit)
50% DOD and 298 K is shown in Fig. 7. All EIS of alloy electrodes consist of four parts: a smaller semicircle at high frequency region, a transition semicircle at mid-low frequency, a bigger semicircle at low frequency region, and a straight line at lower frequency region. According to the equivalent circuit proposed by Kuriyama et al.[14–16], the high frequency semicircle is ascribed to the contact resistance (R1) and the capacitance (C1) between the alloy pellet and the current collector, the mid-low frequency semicircle is ascribed to contact resistance (R2) and capacitance (C2) between alloy particles of pellet, the low frequency semicircle is ascribed to charge-transfer reaction resistance (R3) and capacitance (C3), a straight line at lower frequency is ascribed to the Warburg impedance (W1), Rs is assigned to the electrolyte resistance between the electrodes. Compared with EIS spectra of other alloy electrodes, the melt-spun alloy electrode at a wheel speed of 20 m/s exhibits low contact resistance and reaction resistance.
3 Conclusions The Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys were composed of multi- phase structure: the (Nd Mg)2(Ni Co)7 phase (Ce2Ni7 type structure) with hexagonal symmetry and P63/mmc space group, the NdNi5 phase (CaCu5 type structure) with hexagonal symmetry and the P6/mmm space group, and other phases. The average grain size of the samples decreased with the wheel speed, i.e. solidification rate. For melt-spun samples, distortion and amorphous phases appeared, and the lattice parameters of two main phases for melt-spun alloys decreased with increasing wheel speeds, which negatively affected the maximum discharge capacity and rate properties due to the shrinkage of the lattice volume and the increase of the resistance of hydrogen diffusion inside the lattice. The reaction resistance of alloy electrodes firstly decreased with increasing wheel speeds, and then increased at the wheel speed higher than 20 m/s. Experiments showed that alloy melt-spun at a wheel speed of 20 m/s exhibited good overall electrochemical properties.
[1] Bérubé V, Radtke G, Dresselhaus Mand Chen G. Size effects on the hydrogen storage properties of nanostructured metal hydrides: A review. International Journal of Energy Research, 2007, 31: 637. [2] Penner S S. Steps toward the hydrogen economy. Energy, 2006, 31: 33. [3] Schlapbach L, Zuttel A. Hydrogen-storage materials for mobile applications. Nature, 2001, 414: 353. [4] Liu Y F, Pan H G, Yue Y J, Wu X F, Chen N, Lei Y Q. Cycling durability and degradation behavior of La-Mg-Ni-Cotype metal hydride electrodes. Journal of Alloys and Compounds, 2005, 395: 291. [5] Zhang Y H, Li B W, Ren H P, Cai Y, Dong X P, Wang X L. Effects of substituting Ni with Cu on the microstructures and electrochemical characteristics of the as-cast and quenched La0.7Mg0.3Ni2.55−xCo0.45Cux (x=0.0.4) electrode alloys. International Journal of Hydrogen Energy, 2007, 32: 3420. [6] Pan H G, Liu Y F, Gao M X, Zhu Y F. A study on the effect of annealing treatment on the electrochemical properties of La0.67Mg0.33Ni2.5Co0.5 alloy electrodes. International Journal of Hydrogen Energy, 2003, 28: 113. [7] Pan H G, Liu Y F, Gao M X, Zhu Y F, Lei Y Q. The structural and electrochemical properties of La0.7Mg0.3(Ni0.85Co0.15)x (x=3.0–5.0) hydrogen storage alloys. International Journal of Hydrogen Energy, 2003, 28: 1219. [8] Tanaka K, Kanda Y, Furuhashi M, Saito K, Kuroda K, Saka H. Improvement of hydrogen storage properties of melt-spun Mg-Ni-RE alloys by nanocrystallization. Journal of Alloys and Compounds, 1999, 293-295: 521. [9] Zhang Y H, Li B W, Ren H P, Cai Y, Dong X P, Wang X L. Investigation on structures and electrochemical performances of the as-cast and quenched La0.7Mg0.3Co0.45Ni2.55−xFex (x=0.0.4) electrode alloys. International Journal of Hydrogen Energy, 2007, 32: 4627. [10] Wei F S, Lei Y Q, Chen L X, Lu G L, Wang Q D. Influence of rapid quenching on the microstructure and electrochemical. International Journal of Hydrogen Energy, 2007, 32: 2935. [11] Kuriiwa T, Ohno Y, Nakamura H, Takamura H, Tanaka T, Okadam M. Crystal structure and hydrogen absorption properties of La(Ni, M)x (M=Al, Co, Mn, Si. x= 3–7) meltspun ribbons. Materials Transactions-JIM, 1996, 37(40): 839 [12] Chartouni D, Meli F, Zuttel A, Gross K, Schlapbach L. The influence of cobalt on the electrochemical cycling stability of LaNi5-based hydride forming alloys. Journal of Alloys and Compounds, 1996, 241: 160. [13] Sakai T, Oguro K, Miyamura H, Kurijama N, Kato A, Ishikawa H, Iwakura C. Some factors affecting the cycle lives of LaNi5-based alloy electrodes of hydrogen batteries. Journal of Less-Common Metals, 1990, 161: 193. [14] Kuriyama N, Sakai T, Miyamura H, Uehara I, Ishikawa H, Iwasaki T. Electrochemical impedance spectra and deterioration mechanism of metal hydride electrodes. Journal of Electrochemical Society, 1992, 139: L72. [15] Kuriyama N, Tsukahara M, Takahashi K, Yoshinaga H, Takeshita H T, Sakai T. Deterioration behavior of a multi-phase vanadium-based solid solution alloy electrode. Journal of Alloys and Compounds, 2003, 356-357: 738. [16] Chu H L, Qiu S J, Sun L X, Zhang Y, Xu F, Zhu M, Hu W Y. Electrochemical hydrogen storage properties of La0.7Mg0.3 Ni3.5-Ti0.17Zr0.08V0.35Cr0.1Ni0.3 composites. International Journal of Hydrogen Energy, 2008, 33: 755.
Microstructure and electrochemical properties of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy PAN Chongchao (潘崇超), CAI Wupeng (蔡吴鹏), Javed Iqbal, WANG Zhen (王 震), GENG Chaoqing (耿朝青), YU Ronghai (于荣海) (Key Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China) Received 11 May 2009; revised 3 June 2009
Abstract: The effects of rapid solidification on the microstructure and electrochemical properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloy were systematically investigated. The microstructure of alloys was characterized by scanning electron microscopy (SEM), X-ray diffractometer (XRD) and transmission electron microscopy (TEM). It was found that the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons became thinner and the average grain size of the ribbons became smaller with increasing wheel speed. A fraction of amorphous phase was observed for the ribbons melt-spun at high wheel speed (≥20 m/s). Microstructural characterization revealed that two phases: (Nd,Mg)2(Ni,Co)7 main phase (Ce2Ni7 type structure) and NdNi5 second phase (CaCu5 type structure), existed in the samples in cast state and melt-spun. The cycle stability of the melt-spun alloys was significantly enhanced as compared with cast alloy, and the sample prepared at wheel speed of 20 m/s exhibited good comprehensive electrochemical properties. Keywords: hydrogen storage alloy; rapid solidification; microstructure; electrochemical property; rare earths
Hydrogen is thought to be one of the most important energy carriers for the future automotive applications with high energy efficiency and is environmental friendly[1–3]. Compared with AB5 alloys, the La-Mg-Ni-Co type alloys exhibit high hydrogen storage capacity, but they are with short cycle life and low capacity at high discharge rates[4]. A number of techniques has been employed to improve the overall performance of the alloys, such as element substitution, heat treatment and different stoichiometric ratio. However, cycle life and discharge capacity are controversial and could not be enhanced at the same time[5–7]. Tanaka et al.[8] reported that the cycle stability of Mg-Ni-Nd alloys fabricated by melt spinning has been improved. The refinement of the grain size and homogenization of composition were regarded as two main reasons for the improvement of the cycle stability of alloy electrode[9–11]. In this work, we optimized the alloys to a composition of Nd0.8Mg0.2(Ni0.8Co0.2)3.8, which exhibited good overall properties in normal state. Subsequently, the alloys were melt-spun at varying wheel speed to investigate the effect of rapid solidification on the microstructure and electrochemical properties of this hydrogen storage alloy. A considerable enhancement of electrochemical properties was observed, which is closely correlated to the microstructural modification during melt-spinning process.
1 Experimental Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys were prepared by induction
melting under high purity argon atmosphere of 0.03 MPa, and the alloy composition is analytical results. All raw materials are with high purity (99.9 wt.% or above). Then, the ingots were re-melted and melt-spun onto Cu wheel at 10, 20, 30 and 40 m/s, respectively. The samples were crushed, ground into powder, and then sieved through 150 mesh and 300 mesh, respectively, for the measurement of electrochemical properties and structural characterizations. Alloy electrode was prepared by cold pressing the mixture of alloy powder with Cu powder in the mass ratio of 1:3 to form a pellet under the pressure of 250 MPa. Electrochemical measurements were performed in a standard open tri-electrode electrolysis cell which consists of working electrode, sintered Ni(OH)2/NiOOH counter electrode, and a Hg/HgO reference electrode immersed in a solution of 6 mol/L KOH electrolyte. The discharge capacities of alloys were determined by the galvanostatic method. Each electrode was charged with current of 60 mA/g for 7 h, rested for 5 min, and discharged with current of 60 mA/g to the cut-off potential of –0.6 V versus the Hg/HgO reference electrode. To investigate the high rate discharge property, the discharge capacities at different discharge current densities (150, 300, 600, 900 mA/g) were measured. The electrochemical impedance spectroscopy (EIS) were recorded using IM6 electrochemical workstation in open-circuit condition after 8 charge-discharge cycles for all alloy electrodes at 50% depth of discharge (DOD). The scanning frequency was varied from 100 kHz to 10 MHz at AC amplitude of 5 mV, the Z-View electrochemical impedance software was
Foundation item: Project supported by the National Natural Science Foundation of China (50471011, 50525101) Corresponding author: YU Ronghai (E-mail: [email protected]; Tel.: +86-10-62771160) DOI: 10.1016/S1002-0721(09)60060-0
PAN Chongchao et al., Microstructure and electrochemical properties of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 …
adopted to analyze the experimental data. X-ray powder diffraction patterns were measured on Rigaku D/max2500 X-ray diffractometer with Cu Kα radiation at a scanning speed of 6(°)/min in range of 20° to 80°, and the lattice parameters was obtained through Rietveld refinement with Maud software. The operating voltage and current of this XRD machine were 40 kV and 200 mA, respectively. Microstructures of these samples were characterized by JEM-2011 TEM and JSM-6460 SEM. The crystalline state of the samples was examined by selected area electron diffraction (SAD). The SEM samples were corroded in the solution of 15 g FeCl3, 50 ml HCl and 100 ml H2O.
2 Results and discussion
101
ribbons decreases from 60.0 μm to 28.0 μm with increasing wheel speed from 10 m/s to 40 m/s. The average grain sizes for melt-spun alloys with the wheel speeds of 10, 20, 30 and 40 m/s are about 8.9, 4.4, 3.6 and 1 μm, respectively. The rapid solidification leads to the fine grains and homogenous microstructure in the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons. Fig. 3 illustrates the TEM image and SAED pattern of melt-spun alloy fabricated at a wheel speed of 20 m/s. The diffraction spots and diffraction rings coexist, indicating a fraction of amorphous phases present in this melt-spun alloy. Our further TEM observation revealed that the amount of amorphous phases increases with increasing wheel speed. The wheel speed is an important factor which affects the phase composition in melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys.
2.1 Microstructure
2.2 Activation property and discharge capacity
The XRD patterns of the cast and melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys are shown in Fig. 1. It indicates that all the samples are mainly composed of two phases, main phase (NdMg)2(NiCo)7 (Ce2Ni7 type structure) with P63/mmc space group, and second phase NdNi5 (CaCu5 type structure) with the P6/mmm space group. As illustrated in Fig. 1, the typical diffraction peaks of two phases are broadened for the melt-spun samples compared with cast samples. The broadening of diffraction peaks might be partially attributed to the internal stress which could be easily induced by melt-spinning, i.e. fast solidification, especially at high wheel speed. The lattice parameters were calculated by Rietveld refinement with Maud software and the results are listed in Table 1. The values of lattice constants a and c are 0.5009 and 2.4376 nm, respectively, for (NdMg)2(NiCo)7 phase of cast samples. Those two lattice constants decrease for melt-spun samples with increasing wheel speed, and decrease more dramatically at wheel speed higher than 30 m/s. A similar tendency in the change of the lattice constants a and c with wheel speed was also found for NdNi5 phase as shown in Table 1. Fig. 2 shows the SEM images of the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons along the cross section, and the thickness has been measured. The thickness of melt spun
The activation，the maximum discharge capacity and rate properties of the alloy electrodes at 298 K are shown in Fig. 4. The cast alloy electrode is usually activated within three cycles, and all melt-spun alloy electrodes are activated within six cycles. The maximum discharge capacities of cast alloy and melt-spun alloys fabricated at the wheel speed of 10, 20,
Fig. 1 XRD patterns of cast and melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys
Table 1 Lattice parameters of two main phases in the cast and melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys Samples Phase
Lattice parameters a/nm
Cast 10 m/s 20 m/s 30 m/s 40 m/s
c/nm
c/a ratio Cell volume/nm3
(Nd, Mg)2(Ni, Co)7
0.5009 2.4376 4.867
0.529644
NdNi5
0.4989 0.3996 0.804
0.086172
(Nd, Mg)2(Ni, Co)7
0.5011 2.4347 4.858
0.529514
NdNi5
0.4982 0.3989 0.801
0.085725
(Nd, Mg)2(Ni, Co)7
0.5012 2.4342 4.857
0.529517
NdNi5
0.4984 0.3988 0.800
0.085798
(Nd, Mg)2(Ni, Co)7
0.5010 2.4326 4.855
0.528853
NdNi5
0.4983 0.3989 0.801
0.085777
(Nd, Mg)2(Ni, Co)7
0.5009 2.4302 4.851
0.528129
NdNi5
0.4977 0.3988 0.801
0.085581
Fig. 2 SEM images of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons along the cross section (a) 10 m/s; (b) 20 m/s; (c) 30 m/s; (d) 40 m/s
102
30 and 40 m/s are about 342.0, 311.0, 305.0, 285.0 and 275.0 mAh/g, respectively. The discharge capacity of the samples cast and melt-spun decreases with increasing discharge electric current, however, the drops for melt-spun samples are more dramatically compared with cast samples. Rapid solidification induced changes in microstructure: refinement of the grain size, appearance of amorphous phase, decrease of the lattice parameters (Table 1), and those changes are more prominent for the samples melt-spun at high wheel speed. All the microstructural modifications shown above reduced the hydrogen diffusion rate in the alloys, leading to the decrease of the discharge capacity and the rate properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes. Discharge voltage characteristic is an important property of the alloy electrode. The longer and more horizontal of discharge voltage plateau, the better discharge voltage characteristics. The discharge voltage curves of Nd0.8Mg0.2 (Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes at 298 K are shown in Fig. 5. The discharge potentials decrease with increasing wheel speeds, and the discharge capacity does the same. Therefore, the melt-spinning or rapid solidification negatively affects the discharge voltage properties for Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes.
JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010
of the alloys. The finer grain size may result in lower internal stress induced as the hydrogen atoms enter the interstitials sites of lattice. Therefore, the absorption of hydrogen atoms for melt-spun alloys leads to less lattice volume expansion as compared with cast alloys. However, the higher rapid solidification rates, the higher internal stress of lattice, so the maximum discharge capacity decreases with the increase of melt-spun speed. Another advantage of melt-spun alloys is that they have good corrosion resistance due to composition homogeneity and existence of amorphous phase. The electrochemical impedance spectroscopy (EIS) of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes at
2.3 Cycling stability
Fig. 4 Activation, maximum discharge capacity and rate property of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes
Fig. 6 demonstrates the variation of discharge capacity with cycle number measured at 298 K. The slopes of cycle stability curves of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys are smaller than that of the cast alloy. The melt-spun alloy sample with the wheel speed of 20 m/s exhibits good comprehensive properties, cycle stability and discharge capacity. According to the microstructural analysis, the rapid solidification reduces the grain size and lattice parameters, and homogenizes the distribution of the elements. It was reported that the capacity decay of alloy electrodes is attributed to the pulverization and oxidation mechanism[12,13]. The lattice internal stress and cell volume expansion are inevitable for alloy electrodes. The hydrogen proton absorption and adsorption in the lattice and interstitials lead to the pulverization
Fig. 5 Discharge voltage curves of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes
Fig. 3 TEM image and SAED pattern of alloy melt-spun at a wheel speed of 20 m/s
Fig. 6 Cycle property of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy electrodes
PAN Chongchao et al., Microstructure and electrochemical properties of melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 …
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Fig. 7 Electrochemical impedance spectra (EIS) of Nd0.8Mg0.2(Ni0.8 Co0.2)3.8 hydrogen storage alloy electrodes measured at 50% DOD and 298 K (inset is equivalent circuit)
50% DOD and 298 K is shown in Fig. 7. All EIS of alloy electrodes consist of four parts: a smaller semicircle at high frequency region, a transition semicircle at mid-low frequency, a bigger semicircle at low frequency region, and a straight line at lower frequency region. According to the equivalent circuit proposed by Kuriyama et al.[14–16], the high frequency semicircle is ascribed to the contact resistance (R1) and the capacitance (C1) between the alloy pellet and the current collector, the mid-low frequency semicircle is ascribed to contact resistance (R2) and capacitance (C2) between alloy particles of pellet, the low frequency semicircle is ascribed to charge-transfer reaction resistance (R3) and capacitance (C3), a straight line at lower frequency is ascribed to the Warburg impedance (W1), Rs is assigned to the electrolyte resistance between the electrodes. Compared with EIS spectra of other alloy electrodes, the melt-spun alloy electrode at a wheel speed of 20 m/s exhibits low contact resistance and reaction resistance.
3 Conclusions The Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloys were composed of multi- phase structure: the (Nd Mg)2(Ni Co)7 phase (Ce2Ni7 type structure) with hexagonal symmetry and P63/mmc space group, the NdNi5 phase (CaCu5 type structure) with hexagonal symmetry and the P6/mmm space group, and other phases. The average grain size of the samples decreased with the wheel speed, i.e. solidification rate. For melt-spun samples, distortion and amorphous phases appeared, and the lattice parameters of two main phases for melt-spun alloys decreased with increasing wheel speeds, which negatively affected the maximum discharge capacity and rate properties due to the shrinkage of the lattice volume and the increase of the resistance of hydrogen diffusion inside the lattice. The reaction resistance of alloy electrodes firstly decreased with increasing wheel speeds, and then increased at the wheel speed higher than 20 m/s. Experiments showed that alloy melt-spun at a wheel speed of 20 m/s exhibited good overall electrochemical properties.
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