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New approaches for rare earth-magnesium based hydrogen storage alloys
Progress in Natural Science: Materials International (xxxx) xxxx–xxxx
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Progress in Natural Science: Materials International journal homepage: www.elsevier.com/locate/pnsmi
Review
New approaches for rare earth-magnesium based hydrogen storage alloys☆ ⁎
Huaiwei Zhanga, Xinyao Zhenga, Xiao Tianb, Yang Liuc, , Xingguo Lia, a b c
⁎
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China College of physics and electronic information, Inner Mongolia Normal University, Huhehaote 010022, China Department of Food Science, Beijing Union University, Beijing 100101 China
A R T I C L E I N F O
A BS T RAC T
Keywords: Hydrogen storage Rare earth Ni/ MH battery Crystal structures Electrochemical performance
As the most possibility applied to the next generation negative electrode materials of Ni/ MH second battery, rare earth (RE)-magnesium (Mg) based alloys have been developed over the last few years. Recent advances about the RE-Mg based intermetallic compounds on the crystal structures, hydrogenation behaviors and electrochemical performances are reviewed in the paper. On the other hand, new results about the preparation and modification methods of the alloys are also covered in details.
1. Introduction At the modern times, the development and utilization of new clean energy has become an extremely important issue along with environment worsening and fossil fuel, such as oil and natural gas, drying up. Hydrogen is a new kindly energy, highly abundant and non toxic renewable. The most commonly used method for hydrogen storage and its capacities were exhibited in Fig. 1 [1]. Since the AB5-type alloys were used in Ni/MH batteries as electrode the higher capacity hydrogen storage alloys are concerned more and more. Mg-containing rare earth-based superlattice MH alloys with higher storage capacity, lower self-discharge, and extended cycle stability have attracted a lot of attentions as the replacements for conventional AB5 alloys [2–4]. Metal or alloys hydride thermodynamics are usually measured by pressure-composition-temperature (PCT) curves, and the reaction enthalpy (ΔH) and entropy (ΔS) could be obtained through fitting the PCT data with van’t Hoff equation. The ideal decomposition enthalpy of hydrogen storage alloys which could be applied in Ni/MH batteries was 39–40 kJ mol−1H2 (H2 pressure of 1–10 bar at 353 K). As we all know, one interest point for the RE-Mg based alloys is its crystal feature as it relates to the storage and release of hydrogen. Some researchers [5,6] pointed out that Mg substitutes La position in the ABx model, where x=(5 m+4)/(m+2), m an integer, and A is La or Mg; B is Ni, which could be described by the stacking along the c axis of [AB5] and [A2B4] units following two possible sequences: R3m rhombohedral symmetry (3R) and the hexagonal P63/mmc space group(2H). This could be used as a gift for designing the RE-Mg based
hydrogen storage alloys. In this review, the recent research progress with respect to crystal structures, hydrogenation behaviors, electrochemical properties, and the preparation and modification methods have been summarized for the rare earth (RE)-Magnesium (Mg) based intermetallics compound. Therefore, there are great need to investigate the competitiveness from the commonness of the alloy to provide guide and suggestion to the development for the hydrogen storage alloys. 2. Characters, gas-solid reactions and electrochemical properties 2.1. AB2 type The AB2 type hydrogen storage alloys with Laves phases caused more and more attentions for those higher capacities. As early as 1980, Oesterreicher et al. [7] firstly prepared the single-phase alloys with the C15-type (MgCu2 type) structure of La1−xMgxNi2 alloys by induction melting under argon atmosphere. They also found that the alloy structures would transformed into C36- type (MgNi2 type) as x is greater than 0.67. Mg-rich La1−xMgxNi2 alloys could form the highly stable hydride at room temperature. So far there are only one type of Mg based AB2 alloys, REMgNi4 (where RE= La, Ce, Pr, Nd, Y, Gd and Sm), with the MgCuAl2 structures [8]. Cheng et al. [9] investigated one LaMgNi4 compound used first principles density functional theory. The La atoms form an fcc sublattice and occupy the (4a: 0, 0, 0) sites, the Mg atoms and Ni4 tetrahedra were in the tetrahedral sites (4c: 0.25, 0.25, 0.25) with an
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding authors. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.pnsc.2016.12.011 Received 9 October 2016; Accepted 30 November 2016 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Zhang, H., Progress in Natural Science: Materials International (2016), http://dx.doi.org/10.1016/j.pnsc.2016.12.011
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process from polycrystalline to amorphous for LaMgNi4 alloy, and proved this phenomenon through TEM analysis as shown in Fig. 3. The electrochemical properties of LaMgM4 (M=Co, Mn, Cu, Al) with AB2-type alloy were studied [15]. With the increasing of the grain size of the alloys the electrochemical kinetics was decreasing usually. The activation capabilities of the LaMgNi4-xCox alloys were also studied [13]. The alloys could be completely activated after 4 cycles. The discharge capacities could increase from about 250 mAh g−1 (x=0) to about 300 mAh g−1 (x=0.5) with the difference of Co content. But there were low cycle stability for all the samples. 2.2. AB3 type For the AB3 type of RE-Mgbased hydrogen storage alloys, the series of REMg2Ni9 with PrNi3 structure (R-3m space group) and REMg2Cu9 with CeNi3 structure (P63/mmc space group) were the most common forms [19–21]. The element substitution type of (Ca0.5Y0.5)(CaMg)Ni9 have been particularly studied because of the higher capacity. The alloy structure could be generalized as AB3 type made by stacking of AB5 and AB2 type along the c axis [22]. Chen et al. [23,24] prepared various kinds of RE-Mg-Ni based intermetallics compounds with PuNi3 structure, such as LaMgCaNi9, LaCaMgNi6Al3 and LaMgCaNi6Mn3, et al. Mg atoms are only located at 6c sites while the Ca and La atoms occupy the 6c and 3a sites. The PCT curves of LaMg2Ni9 alloy and (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9were investigated by Kadir et al. [21] at room temperature, 3.3 MPa H2 pressure. A poor hydrogen absorption/desorption abilities with about 0.33 wt% for the pure LaMg2Ni9 alloy were exhibited in the picture. Otherwise, the (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9-Hx system obviously contains two phases of (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9 and (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9H13, respectively, and the hydrogen storage capacity was 1.87 wt% H/M. The research showed that the substitution of La and Mg with Ca could promote the hydrogen storage capacities [20–22]. Yartys et al. [25] have investigated the performances for the La3xMgxNi9(x=0.5, 0.7, 1.0, 1.5 and 2.0) hydrides. They found that the hydrogen ab/desorption equilibrium pressures of the Mg-rich LaMg2Ni9 were much higher than the Mg-poor La2.3Mg0.7Ni9 alloys. Yartys et al. [26] also studied the relationship between the structure
Fig. 1. Stored hydrogen per mass and per volume [1].
ordered way. There are three well-defined hydride structures: α-cubic, β-orthorhombic distorted and γ-cubic symmetry as shown in Fig. 2. Wang et al. [10] studied the electrochemical performances of the REMgNi4 alloys and t the capacities decreased with the order of PrMgNi4, CeMgNi4, NdMgNi4, LaMgNi4. The typical PCI curves of LaMgNi4 at various temperatures (373, 398 and 423 K) are shown in Fig. 5. There are two plateaus at the 373 K, but one at 423 K and the maximum hydrogen content is 1.45%. It is not common to see this phenomenon in other REMgNi4 compounds [11,12]. Tan et al. [13] prepared the LaMgNi4−xCox (x=0, 0.3, 0.5) compounds by the method of levitation melting. They found that almost the same results would be appeared for the substitution. Yang et al. [14,15] studied the hydrogen storage properties of LaMgNi3.6M0.4 (M=Cu, Mn,Co,Ni and Al). The phase compositions were LaMgNi4 and LaNi5 and LaAlNi for M=Al. The maximum hydrogen capacities for these alloys were all about 1.7 wt%, and the cycle stability increased in the following Mn, Cu, Ni, Co, Al. Otherwise, the microstructure of these hydrogen storage alloys could be altered between polycrystalline and amorphous during a reversible hydrogenation process, especially for AB2 C15-Laves phase alloys [16,17]. Young et al. [18] investigated the phase transformation
Fig. 2. Structures of LaMgNi4 compounds (a), α-LaMgNi4H (b), β-LaMgNi4H4 (c) and γ-LaMgNi4H7 (d).
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Fig. 3. TEM micrograph for LaMgNi4 alloy after 30 PCT cycles.
desorption for RE2MgNi9H13 hydrides show variations ranging from about 28.6 kJ mol−1 H2 for Nd2MgNi9H12, 31 kJ/mol for Pr2MgNi9H12 and 35.9 kJ mol−1 for La2MgNi9H13 [27]. Some other papers [28,29] almost obtained the same conclusions based on the PCT curves and discharge abilities. The discharge capacities promoted with the increasing of La/Mg mole ratio and the maximum value of 397.5 mAh g−1 at x=2.0 [28]. For the alloys of La2MgMn0.3Ni8.7-x (Co0.5Al0.5)x (x=0, 1.0, 2.0 and 3.0) [30], the cyclic stability increased from about 60% (x=0) to about 80% (x=3.0) after 60 cycles. High ratio discharge property are best when x value was 2.0. The electrochemical impedance spectroscopy (EIS) curves revealed that the charge-transfer resistance was increased with increasing Co and Al because of the formation of Al2O3 film around the alloy surface which could prevent the sample further corrosion. Latroche et al. [31] revealed the mechanism of charge/discharge process for La2MgNi9 alloy through in-situ neutron diffraction method. They thought that the alloy reversible capacity is lost because of the electrochemically stable of α-H solid solution formation and the incompleteness transition of the metal hydride into the β hydride phase during the charging process. Volodin et al. [32] calculated the hydrogen diffusion coefficient in the La1.5Nd0.5MgNi9 alloy electrode using low amplitude potentiostatic data treatment. Hydrogen diffusion coefficient (DH) changed with the content of hydrogen atoms in the alloys and had a max value of 2×10–11 cm2 s−1 at 85% of discharge. They pointed out that it should not only decrease the resistance losses in charge/discharge process, but also alter hydride stability and the intrinsic properties.
Fig. 4. La2MgNi9D13.1 crystal structure.
and properties for the RE3-xMgxNi9H10–13 hydrides. The results showed that La2MgNi9 has the highest hydrogen storage capacity among the La-Mg-Ni compounds because D atoms occupy both CaCu5-type and Laves slabs in the crystal structure of La2MgNi9, as shown in Fig. 4. Yartys et al. also studied the enthalpies of hydrogen
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Fig. 5. a-Crystal structures of the β-phase of La2−xMgxNi7; b-Hexagonal and rhombohedral structure of the A2B7 structure. Table 1. Electrochemical properties of various A2B7-type alloys. Alloy
DCD (mA g−1)
MDC (mAh·g−1)
CR
Ref.
La0.75Mg0.25Ni3.5 La0.7Mg0.3Ni3.5 La0.7Mg0.3(Ni0.85Co0.15)3.5 La0.7Mg0.3(Ni0.9Co0.1)3.5 La0.7Mg0.3Ni2.65Mn0.1Co0.75 La0.7Mg0.3Ni2.9(Al0.5Mo0.5)0.6 La0.7Mg0.3Ni3.4(MnAl2)0.1 La0.7Mg0.3Ni2.45Mn0.1Co0.75Al0.2 La0.8−xSmxMg0.2Ni3.35Al0.1Si0.05(x = 0–0.4) La0.6Gd0.2Mg0.2Ni3.0Co0.4Al0.1 La0.6Gd0.2Mg0.2Ni3.0Co0.1Al0.4 La0.95Sm0.66Mg0.40Ni6.25Al0.42Co0.32 La0.8Mg0.2Ni3.3Co0.2Si0.15 La0.8Mg0.2Ni3.15Co0.2Al0.1Si0.05 La0.4Nd0.4Mg0.2Ni3.15Co0.2Al0.1Si0.05 (La1−xDyx)0.8Mg0.2Ni3.4Al0.1 (x=0–0.2) Ml1−xMgxNi2.80Co0.50Mn0.1Al0.1(x=0.08–0.28)
60 50 60 60 60 60 100 60 60 100 100 70 60 100 100 300 300
343.7 352.8 395.6 392.1 403.1 397.6 355.2 370 about 362–395 381.8 339.4 372 about 340 about 370 about 380 367.5–390.2 322–375
S100=55.8% S100=58.4% S60=45.9% S100=93.5% S90=36.9% S70=70.8% S100=40.3% S100=60.7% S10≈(72–89)% S100=91.5% S100=81.1% S100=85% S100=88% S100=65% S100=85% S100≈(81–83)% S100 < 79%
[47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [56] [57] [58] [59] [59] [60] [61]
DCD: Discharge Current Density; MDC: Maximum Discharge Capcity; CR: Capacity Retention.
two such as Nd2Ni7 and La2Ni7 [40,41], the similar results was also obtained by some other researchers [42]. Two other potential structures which could be used in RE-Mg based alloys were Sm2Co7 and Gd2Co7 type. Cao et al. [43] studied the structures and hydrogen absorption performance of Sm2Co7. The alloy transforms to the β-phase Sm2Co7H2.9 firstly, and then was the γ-phase Sm2Co7H6.4. The dehydrogenation enthalpies was 48.5 and 42.0 kJ mol−1, respectively. Zhang et al. [44] studied the (Nd1.5Mg0.5)Ni7 compound. In both 2H- and 3R-type A2B7 structures, Mg atoms occupy Nd sites of AB2 subunits rather than AB5 because of the differences of ionic bond energy. The hydrogen absorption capacity of (Nd1.5Mg0.5)Ni7 is about 1.2 wt% at 298 K, which is slightly less than the 1.4 wt% for (La1.5Mg0.5)Ni7 alloy [45]. Iwatake et al. [46] found a new crystal
2.3. A2B7 type In the past few years, details about the structures of Ce2Ni7H4–4.7 [33,34], La2Ni7H6.5 [35] and La2Ni7Hx(x=6.4, 10.8) [36] have been reported. For the La2Ni7 and Ce2Ni7 deuterides [33,35], D atoms are situated exclusively within the AB2 units, while in La1.63Mg0.37Ni7D8.8 both the AB2 and AB5 structural slabs host D atoms (see Fig. 5a). The RE2M7-type alloys with rhombohedral and hexagonal structures are also studied by some other researchers as shown in Fig. 5b [37,38]. The structural properties and hydrogen storage capacities of Y2Ni7Dx compounds (x=2.1, 4.1, and 8.8) with Gd2Co7 structure were also investigated by Charbonnier et al. [39]. The same as the usual structure, the stacking of one A2B4 subunits and two AB5 along with the c axis were defined. There are three plateaus of Y2Ni7 rather than
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(3R) and Pr5Co19-type phases by sintering the powder mixture of LaNi4.8 and Mg2Ni at 1173 K for 10 days. It showed a high capacity, but the cyclic stability needs to further improve. Liu's program achieved almost identical results [67]. Zhao et al. [68] investigated the La0.84Mg0.16Ni3.80 alloy with the single Pr5Co19-type phase (Fig. 7a). The discharge performances were also studied comparing with the similar type alloys (Fig. 7b) Electrochemical studies showed no signs of higher discharge ability for the composition with single phase. To improve the electrochemical properties, especially the discharge capacity, Ce2Ni7-type or LaNi5 secondary phase should be introduced into the alloys. For some other type RE-Mg based alloys, LaMg2Ni was one of the compounds with stable structure. Earliest research on the structures and hydrogenation behaviors of LaMg2Ni alloys were reported by Karonik et al. [69]. The structure could be considered as a transition metal filled variant of the CaIn2 type [70]. A tetrahedral network was built up by the magnesium atoms. LaMg2Ni shows the transition from metal to non-metal upon hydrogenation process because of a charge transfer of conduction electrons which could stabilize [NiH4]4- complexes, with a closed shell electron configuration [71]. For the La0.7Mg0.3(Ni0.85Co0.15)x (x=2.5–5.0) alloys, the platform range become larger and the pressure value remain unchanged when x increases from 2.5 to 3.5, while the plateau pressure rises and the width decreases gradually when x value further increases to 5.0 [72]. But there is poor cycling durability in alkaline electrolytes for all of these alloys. After 60 cycles, the best capacity retention rate is only 45.9% for the La0.7Mg0.3 (Ni0.85Co0.15)3.5. A new type of Mg-transition metal-based alloys of LaMg8.40Ni2.34xAlx (x=0 and 0.20) which composed of La2Mg17, LaMg2Ni and Mg2Ni phases was also prepared [73]. The reversible hydrogen storage capacity of LaMg8.40Ni2.14Al0.20 alloy is 3.22 wt% at 558 K, which is much higher than the LaMg8.40Ni2.34 alloy. Amorphous Mg70(RE25Ni75)30 (RE=La, Pr, Nd) samples were prepared by melt spinning [74]. The max hydrogen absorption and desorption capacities were 4.21 wt% or Mg70(Nd25Ni75)30 and 2.74 wt% or Mg70(Pr25Ni75)30. Abe et al. [75] studied the structure of Mg30Co2Y9 alloys, the results showed that a highly stable structure with dense-packing (73.2%) atoms was appeared in a large hexagonal lattice (P63/mmc).
structure of the (Nd,Mg)2(Ni,Al)7 alloy. A sub-block layer of the AB3 stoichiometry and a sub-block layer of the A5B19 were alternately stacked on the top of each other in the stacking of the ABC-type rather than one A2B4 unit layer and two AB5 unit layers. As the most possible materials to apply in Ni-MH batteries, the charge/discharge performances of A2B7-type hydrogen storage alloys were studied by many researchers. Optimized A2B7-type alloys with Mg element have been produced for the electrode of Ni/MH batteries, as shown in Table 1. From the table, the discharge capacity could reach more than 400mAh/g, and this is the higher level of the various types about RE-Mg based hydrogen storage alloys. But the limited for its application was the lower capacity retention. 2.4. Other types More and more studies have been carried out about the A5B19-type hydrogen storage alloy because of the higher capacity and more excellent cycle stability all over the world recently. The Pr5Co19-type (3R) structures have three blocks along the c axis; each block is composed of one layer of MgZn2-type cells and three layers of CaCu5type cells. Takeda et al. researched the crystal structures of Sm5Ni19.10. Some super lattice structures (2H, 3R, 4H, 6H, 9R and 12R) were also found by TEM [62]. Akiba et al. [63] investigated the structure of La-Mg-Ni (Co) with Pr5Co19-Type. They thought that Mg could occupy only the [La2Ni4] layer and it makes the lattice stable both hydrogenation and dehydrogenation process. Nakamura et al. [64] researched the structures of La4MgNi19 compound (Fig. 6a), the main phase was La4MgNi19 (5:19R and 5:19H), and the mass fractions were 70%. The La4MgNi19Dx phase structure is shown in Fig. 6b. The D/M value could be obtained from the D sites occupancies was 0.91(5). For the past few years, the studies for the A5B19-type hydrogen storage alloys were focused on the preparation of Pr5Co19-type and Ce5Co19-type phases because of the good electrochemical properties, especially high rate discharge ability. Ozaki et al. [65] studied that the La0.8Mg0.2Ni3.4−xCo0.3 (MnAl)x (0≤x≤0.4) alloys with Ce5Co19-type (3R) and Pr5Co19-type (2H) structures showed the excellent discharge capacity and cyclic stability. Ferey et al. [66] obtained Ce5Co19-type
Fig. 6. Crystal structures of (a) the alloy and (b) the hydride of 5:19R phase, (c) Hydrogen interstitial sites in the AB2, AB5−1, and AB5−2 cells.
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Fig. 7. TEM images of the La0.84Mg0.16Ni3.80 alloy (a) and Discharge capacity (b).
Fig. 8. LaMg12-x alloy structures.
method for preparing the RE-Mg based hydrogen storage alloys. Guo et al. [80] found that La2-xTixMgNi9 consisted of LaNi5, LaMg2Ni9 (3R) and LaNi3, and the cycle stability was great promoted after annealing because of the improvement of the compositional homogeneity. Gao et al. [81] studied the effect of annealed treatment process on microstructure for La-Mg-Ni alloys. They pointed that the both Pr5Co19-type and Ce2Ni7-type phases were all with good stability and the electrochemical cyclic stability was mainly affected by the LaNi5 phase in the alloys. The cooperation of these factors makes the maximum discharge capacity and the retention more than 320mAh/g and 90% after 100 cycles with 3d annealed treatment. Liu et al. [82] investigated the effects of anneal temperatures to the La-Mg-Ni based (La,Mg)2Ni7-phase La0.75Mg0.25Ni3.5 alloys. An obvious structure transformation from 2H-type to 3R-type was appeared when the annealing temperature was 1223 K, and adjusting LaNi5-phase content could improve the hydrogen storage and electrochemical performances. Rapidly quenching technique has much advantage of simple
Another type of compounds composed of rare earth and magnesium were attract us much more attention for the higher hydrogen storage capacities. Denys et al. [76] prepared an intermetallic compound of LaMg12-x. There was a giant unit cell with a volume exceeding 8000 Å3 in orthorhombic structures. La sublattice of LaMg12-x alloy was shown in Fig. 8. Ouyang et al. [77] produced a type of alloy with a formulation of Mg3LaNi0.1, and it showed the excellent performances in hydriding and dehydriding kinetics. The adding of Pd element in the La2(Mg,Pd)17 alloys [78] could promote the equilibrium pressures, but reduced the H2 capacity. 3. New evolution for preparation and modification The hydrogen storage and electrochemical properties were significantly affected by the microstructures, compositions and phase homogeneity of alloys. For the Mg-based compounds, the Mg content is hard to control because of its higher vapor pressure [79]. Induction melting following the annealed treatment process is the most commonly used 6
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operation and high efficiency for keeping the phase during the hydrogen storage alloys preparation process. It is generally used to prepare the amorphous material with high free volume which could reduce the hydride formation enthalpy [83]. Zhang et al. researched the La2Mg(Ni0.85Co0.15)9 alloy with rapidly quenching, the reserved PuNi3type and LaNi5-type phase could enhance the electrochemical performances because of the formation of the amorphous phase [84]. Yartys et al. [76] studied the rapidly solidified process of LaMg12 and LaMg11Ni alloys in different solidification cooling rates, the higher cooling rate, the faster hydrogenation kinetics. They also pointed that rapid solidification process could significantly improve the hydrogen absorption/desorption performances because of the catalytic influence of the Ni-containing Mg2Ni intermetallics with nanostructures. Yan et al. prepared the Mg70 (Ni3La) 30 and Mg70 (Ni3.5La) 30 alloys by melt spinning process, and compare with the as-melting Mg70 (Ni3La) 30 alloys [85]. The results indicated that “three-dimensional interface reaction process” in the hydrogenation process and “geometrical contraction model” in the dehydrogenation occurred. High-energy ball milling has grown to become one of the most frequently used methods for producing the hydrogen storage alloys and metal hydrides, as it was simply, low cost, efficient and practical. Zhu et al. [86] prepared the Ml0.7Mg0.3Ni3.2 alloy with nanocrystalline through high energy ball milling followed by annealing progress. The AB3-typephase tended to transform into an AB5-type with increase milling duration. La0.7Mg0.3Ni3.5-Ti0.17Zr0.08V0.35Cr0.1Ni0.3 composites were produced by ball milling process in the argon atmosphere [87]. Qi et al. studied the modification of La0.7Mg0.25Ti0.05Ni2.975Co0.525 hydrogen storage alloy with polyaniline (PANI) through ball milling process [88]. This process could improve the electrochemical cycle propertied of AB3-type alloys due to the surface nano-film. The surface treatment of nanocrystalline/amorphous LaMg11Ni alloy ball milled with Ni powder was investigated [89]. The addition of Ni could enhance the discharge capacity of the alloy significantly. Adding a suitable amount of LiBr in La2Mg17/Ni composites through ball milling also could promote the alloy electrochemical properties [90]. The sintering process is an effective method to prepare the RE-Mg based alloys. Starting from two precursors LaNi5 and LaMgNi4, Zhang et al. [91] prepared several alloys with various phase structures by powder sintering technique. The mole ratio of LaNi5/LaMgNi4 (x) could significantly affect the phase structures and electrochemical performances, and the LaNi5 phase could react with LaMgNi4 phase generating (La,Mg) Ni3 and (La,Mg)2Ni7 phases. Recent studies have mainly concentrated in exerting external physico-chemical field during the sintering or melting process. Si et al. found that the activation and stable cycle performance were all promoted through the laser sintering process for the La0.7Mg0.3Ni3.5 alloys [92]. Serin et al. synthesized the La0.65Nd0.15Mg0.20Ni3.5 alloy by spark plasma sintering and studied the atomic scale structure [38]. A single phase 2:7 alloy with the 3 R structure could be prepared, as shown as Fig. 9. The La0.1Nd0.075Mg0.04Ni0.65Co0.12 hydrogen storage alloys were prepared through rapidly quenching with exerting a 0.18 T static magnetic field [93]. They pointed that the Lorentz Force induced by the magnetic field leaded to grain refinement and composition homogeneity. Microwave assisted sintering process of hydrogen storage materials was also studied by some researcher [94]. Otherwise, the charging/discharging process is very affected by the solid-liquid interface properties of alloys used in Ni/MH battery electrode materials. Several research groups have reported that alloys surface coating was an effective method to improve their electrochemical performances especially the cycle stability, the coating materials included Ni, Cu, Co, polymer et al. A highly catalytic layer could be formed in coating process. Zhang et al. [95] developed a simple method for improving the hydrogen storage properties through adding nano Ni-Al compounds using as a stable catalyst to the La2Mg17 alloys by magnetron sputtering methods, and the hydrogen absorption and desorption rate are all substantially improved. Li et al. [96] studied
Fig. 9. High magnification HAADF image of the La0.65Nd0.15Mg0.20Ni3.5 sample.
the effects of nano Mo-Ni compound additives by chemical reduction method to the La-Mg-Ni based hydrogen storage alloys. It showed a high electro-catalytic activity, and worked as a catalyst to charge the transfer reaction at the alloy surface. Nanocrystalline or nanofilm coating exhibited remarkable positive effects in terms of the utilization of hydrogen absorption and desorption of the Mg-based alloy electrodes, which has been attributed to the outstanding catalytic activity and electrical conductivity. 4. Conclusion and future prospects Rare earth-magnesium based alloys played more and more important roles in the solid state hydrogen storage field which could be used as Ni/MH electrode materials. These materials have excellent advantage for hydrogen storage and are worth to further investigate in the future. In this review we have focused on the structural characteristics, gas-solid reactions, electrochemical performances, preparation and modification techniques of RE-Mg based hydrogen storage alloys with various types. The formation of rare earth-magnesium-nickel based hydrogen storage alloys were mainly (La,Mg)Ni3 phase with a rhombohedral type structure or a (La,Mg)2Ni7 phase with a hexagonal type structure. These structures were stacked by the AB5 unit (CaCu5-type structure) and the AB2 unit (Laves structure) with a given ratio along the c-axis direction. The special stacking structure about this type of alloys could provide guidance for the further development of hydrogen storage alloys with new structures. The presence of Mg in the hydrogen storage alloys lead to a higher hydrogen storage and electrochemical capacity, but the lower cycle stability because of the alloy surface corrosion. For practical applications, lots of efforts to improve the overall electrochemical performances about the RE-Mg based alloys have been adopted, including composition optimization, heat and ball milling treatment, surface coating treatment, surface nanocrystallization, electrolyte modification, and so on. It is clear that a great deal of progress has been achieved over the last decade in the field of RE-Mg based hydrogen storage alloys. Finally, for RE-Mg based hydrogen storage alloys to be widely used in Ni/MH second batteries, the development of ideal materials with high and stability capacity was still an enormous challenge to all the researchers. Acknowledgments This work is supported by the Ministry of Science and Technology 7
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Review
New approaches for rare earth-magnesium based hydrogen storage alloys☆ ⁎
Huaiwei Zhanga, Xinyao Zhenga, Xiao Tianb, Yang Liuc, , Xingguo Lia, a b c
⁎
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China College of physics and electronic information, Inner Mongolia Normal University, Huhehaote 010022, China Department of Food Science, Beijing Union University, Beijing 100101 China
A R T I C L E I N F O
A BS T RAC T
Keywords: Hydrogen storage Rare earth Ni/ MH battery Crystal structures Electrochemical performance
As the most possibility applied to the next generation negative electrode materials of Ni/ MH second battery, rare earth (RE)-magnesium (Mg) based alloys have been developed over the last few years. Recent advances about the RE-Mg based intermetallic compounds on the crystal structures, hydrogenation behaviors and electrochemical performances are reviewed in the paper. On the other hand, new results about the preparation and modification methods of the alloys are also covered in details.
1. Introduction At the modern times, the development and utilization of new clean energy has become an extremely important issue along with environment worsening and fossil fuel, such as oil and natural gas, drying up. Hydrogen is a new kindly energy, highly abundant and non toxic renewable. The most commonly used method for hydrogen storage and its capacities were exhibited in Fig. 1 [1]. Since the AB5-type alloys were used in Ni/MH batteries as electrode the higher capacity hydrogen storage alloys are concerned more and more. Mg-containing rare earth-based superlattice MH alloys with higher storage capacity, lower self-discharge, and extended cycle stability have attracted a lot of attentions as the replacements for conventional AB5 alloys [2–4]. Metal or alloys hydride thermodynamics are usually measured by pressure-composition-temperature (PCT) curves, and the reaction enthalpy (ΔH) and entropy (ΔS) could be obtained through fitting the PCT data with van’t Hoff equation. The ideal decomposition enthalpy of hydrogen storage alloys which could be applied in Ni/MH batteries was 39–40 kJ mol−1H2 (H2 pressure of 1–10 bar at 353 K). As we all know, one interest point for the RE-Mg based alloys is its crystal feature as it relates to the storage and release of hydrogen. Some researchers [5,6] pointed out that Mg substitutes La position in the ABx model, where x=(5 m+4)/(m+2), m an integer, and A is La or Mg; B is Ni, which could be described by the stacking along the c axis of [AB5] and [A2B4] units following two possible sequences: R3m rhombohedral symmetry (3R) and the hexagonal P63/mmc space group(2H). This could be used as a gift for designing the RE-Mg based
hydrogen storage alloys. In this review, the recent research progress with respect to crystal structures, hydrogenation behaviors, electrochemical properties, and the preparation and modification methods have been summarized for the rare earth (RE)-Magnesium (Mg) based intermetallics compound. Therefore, there are great need to investigate the competitiveness from the commonness of the alloy to provide guide and suggestion to the development for the hydrogen storage alloys. 2. Characters, gas-solid reactions and electrochemical properties 2.1. AB2 type The AB2 type hydrogen storage alloys with Laves phases caused more and more attentions for those higher capacities. As early as 1980, Oesterreicher et al. [7] firstly prepared the single-phase alloys with the C15-type (MgCu2 type) structure of La1−xMgxNi2 alloys by induction melting under argon atmosphere. They also found that the alloy structures would transformed into C36- type (MgNi2 type) as x is greater than 0.67. Mg-rich La1−xMgxNi2 alloys could form the highly stable hydride at room temperature. So far there are only one type of Mg based AB2 alloys, REMgNi4 (where RE= La, Ce, Pr, Nd, Y, Gd and Sm), with the MgCuAl2 structures [8]. Cheng et al. [9] investigated one LaMgNi4 compound used first principles density functional theory. The La atoms form an fcc sublattice and occupy the (4a: 0, 0, 0) sites, the Mg atoms and Ni4 tetrahedra were in the tetrahedral sites (4c: 0.25, 0.25, 0.25) with an
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding authors. E-mail address: [email protected] (X. Li). http://dx.doi.org/10.1016/j.pnsc.2016.12.011 Received 9 October 2016; Accepted 30 November 2016 1002-0071/ © 2017 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Zhang, H., Progress in Natural Science: Materials International (2016), http://dx.doi.org/10.1016/j.pnsc.2016.12.011
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process from polycrystalline to amorphous for LaMgNi4 alloy, and proved this phenomenon through TEM analysis as shown in Fig. 3. The electrochemical properties of LaMgM4 (M=Co, Mn, Cu, Al) with AB2-type alloy were studied [15]. With the increasing of the grain size of the alloys the electrochemical kinetics was decreasing usually. The activation capabilities of the LaMgNi4-xCox alloys were also studied [13]. The alloys could be completely activated after 4 cycles. The discharge capacities could increase from about 250 mAh g−1 (x=0) to about 300 mAh g−1 (x=0.5) with the difference of Co content. But there were low cycle stability for all the samples. 2.2. AB3 type For the AB3 type of RE-Mgbased hydrogen storage alloys, the series of REMg2Ni9 with PrNi3 structure (R-3m space group) and REMg2Cu9 with CeNi3 structure (P63/mmc space group) were the most common forms [19–21]. The element substitution type of (Ca0.5Y0.5)(CaMg)Ni9 have been particularly studied because of the higher capacity. The alloy structure could be generalized as AB3 type made by stacking of AB5 and AB2 type along the c axis [22]. Chen et al. [23,24] prepared various kinds of RE-Mg-Ni based intermetallics compounds with PuNi3 structure, such as LaMgCaNi9, LaCaMgNi6Al3 and LaMgCaNi6Mn3, et al. Mg atoms are only located at 6c sites while the Ca and La atoms occupy the 6c and 3a sites. The PCT curves of LaMg2Ni9 alloy and (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9were investigated by Kadir et al. [21] at room temperature, 3.3 MPa H2 pressure. A poor hydrogen absorption/desorption abilities with about 0.33 wt% for the pure LaMg2Ni9 alloy were exhibited in the picture. Otherwise, the (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9-Hx system obviously contains two phases of (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9 and (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9H13, respectively, and the hydrogen storage capacity was 1.87 wt% H/M. The research showed that the substitution of La and Mg with Ca could promote the hydrogen storage capacities [20–22]. Yartys et al. [25] have investigated the performances for the La3xMgxNi9(x=0.5, 0.7, 1.0, 1.5 and 2.0) hydrides. They found that the hydrogen ab/desorption equilibrium pressures of the Mg-rich LaMg2Ni9 were much higher than the Mg-poor La2.3Mg0.7Ni9 alloys. Yartys et al. [26] also studied the relationship between the structure
Fig. 1. Stored hydrogen per mass and per volume [1].
ordered way. There are three well-defined hydride structures: α-cubic, β-orthorhombic distorted and γ-cubic symmetry as shown in Fig. 2. Wang et al. [10] studied the electrochemical performances of the REMgNi4 alloys and t the capacities decreased with the order of PrMgNi4, CeMgNi4, NdMgNi4, LaMgNi4. The typical PCI curves of LaMgNi4 at various temperatures (373, 398 and 423 K) are shown in Fig. 5. There are two plateaus at the 373 K, but one at 423 K and the maximum hydrogen content is 1.45%. It is not common to see this phenomenon in other REMgNi4 compounds [11,12]. Tan et al. [13] prepared the LaMgNi4−xCox (x=0, 0.3, 0.5) compounds by the method of levitation melting. They found that almost the same results would be appeared for the substitution. Yang et al. [14,15] studied the hydrogen storage properties of LaMgNi3.6M0.4 (M=Cu, Mn,Co,Ni and Al). The phase compositions were LaMgNi4 and LaNi5 and LaAlNi for M=Al. The maximum hydrogen capacities for these alloys were all about 1.7 wt%, and the cycle stability increased in the following Mn, Cu, Ni, Co, Al. Otherwise, the microstructure of these hydrogen storage alloys could be altered between polycrystalline and amorphous during a reversible hydrogenation process, especially for AB2 C15-Laves phase alloys [16,17]. Young et al. [18] investigated the phase transformation
Fig. 2. Structures of LaMgNi4 compounds (a), α-LaMgNi4H (b), β-LaMgNi4H4 (c) and γ-LaMgNi4H7 (d).
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Fig. 3. TEM micrograph for LaMgNi4 alloy after 30 PCT cycles.
desorption for RE2MgNi9H13 hydrides show variations ranging from about 28.6 kJ mol−1 H2 for Nd2MgNi9H12, 31 kJ/mol for Pr2MgNi9H12 and 35.9 kJ mol−1 for La2MgNi9H13 [27]. Some other papers [28,29] almost obtained the same conclusions based on the PCT curves and discharge abilities. The discharge capacities promoted with the increasing of La/Mg mole ratio and the maximum value of 397.5 mAh g−1 at x=2.0 [28]. For the alloys of La2MgMn0.3Ni8.7-x (Co0.5Al0.5)x (x=0, 1.0, 2.0 and 3.0) [30], the cyclic stability increased from about 60% (x=0) to about 80% (x=3.0) after 60 cycles. High ratio discharge property are best when x value was 2.0. The electrochemical impedance spectroscopy (EIS) curves revealed that the charge-transfer resistance was increased with increasing Co and Al because of the formation of Al2O3 film around the alloy surface which could prevent the sample further corrosion. Latroche et al. [31] revealed the mechanism of charge/discharge process for La2MgNi9 alloy through in-situ neutron diffraction method. They thought that the alloy reversible capacity is lost because of the electrochemically stable of α-H solid solution formation and the incompleteness transition of the metal hydride into the β hydride phase during the charging process. Volodin et al. [32] calculated the hydrogen diffusion coefficient in the La1.5Nd0.5MgNi9 alloy electrode using low amplitude potentiostatic data treatment. Hydrogen diffusion coefficient (DH) changed with the content of hydrogen atoms in the alloys and had a max value of 2×10–11 cm2 s−1 at 85% of discharge. They pointed out that it should not only decrease the resistance losses in charge/discharge process, but also alter hydride stability and the intrinsic properties.
Fig. 4. La2MgNi9D13.1 crystal structure.
and properties for the RE3-xMgxNi9H10–13 hydrides. The results showed that La2MgNi9 has the highest hydrogen storage capacity among the La-Mg-Ni compounds because D atoms occupy both CaCu5-type and Laves slabs in the crystal structure of La2MgNi9, as shown in Fig. 4. Yartys et al. also studied the enthalpies of hydrogen
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Fig. 5. a-Crystal structures of the β-phase of La2−xMgxNi7; b-Hexagonal and rhombohedral structure of the A2B7 structure. Table 1. Electrochemical properties of various A2B7-type alloys. Alloy
DCD (mA g−1)
MDC (mAh·g−1)
CR
Ref.
La0.75Mg0.25Ni3.5 La0.7Mg0.3Ni3.5 La0.7Mg0.3(Ni0.85Co0.15)3.5 La0.7Mg0.3(Ni0.9Co0.1)3.5 La0.7Mg0.3Ni2.65Mn0.1Co0.75 La0.7Mg0.3Ni2.9(Al0.5Mo0.5)0.6 La0.7Mg0.3Ni3.4(MnAl2)0.1 La0.7Mg0.3Ni2.45Mn0.1Co0.75Al0.2 La0.8−xSmxMg0.2Ni3.35Al0.1Si0.05(x = 0–0.4) La0.6Gd0.2Mg0.2Ni3.0Co0.4Al0.1 La0.6Gd0.2Mg0.2Ni3.0Co0.1Al0.4 La0.95Sm0.66Mg0.40Ni6.25Al0.42Co0.32 La0.8Mg0.2Ni3.3Co0.2Si0.15 La0.8Mg0.2Ni3.15Co0.2Al0.1Si0.05 La0.4Nd0.4Mg0.2Ni3.15Co0.2Al0.1Si0.05 (La1−xDyx)0.8Mg0.2Ni3.4Al0.1 (x=0–0.2) Ml1−xMgxNi2.80Co0.50Mn0.1Al0.1(x=0.08–0.28)
60 50 60 60 60 60 100 60 60 100 100 70 60 100 100 300 300
343.7 352.8 395.6 392.1 403.1 397.6 355.2 370 about 362–395 381.8 339.4 372 about 340 about 370 about 380 367.5–390.2 322–375
S100=55.8% S100=58.4% S60=45.9% S100=93.5% S90=36.9% S70=70.8% S100=40.3% S100=60.7% S10≈(72–89)% S100=91.5% S100=81.1% S100=85% S100=88% S100=65% S100=85% S100≈(81–83)% S100 < 79%
[47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [56] [57] [58] [59] [59] [60] [61]
DCD: Discharge Current Density; MDC: Maximum Discharge Capcity; CR: Capacity Retention.
two such as Nd2Ni7 and La2Ni7 [40,41], the similar results was also obtained by some other researchers [42]. Two other potential structures which could be used in RE-Mg based alloys were Sm2Co7 and Gd2Co7 type. Cao et al. [43] studied the structures and hydrogen absorption performance of Sm2Co7. The alloy transforms to the β-phase Sm2Co7H2.9 firstly, and then was the γ-phase Sm2Co7H6.4. The dehydrogenation enthalpies was 48.5 and 42.0 kJ mol−1, respectively. Zhang et al. [44] studied the (Nd1.5Mg0.5)Ni7 compound. In both 2H- and 3R-type A2B7 structures, Mg atoms occupy Nd sites of AB2 subunits rather than AB5 because of the differences of ionic bond energy. The hydrogen absorption capacity of (Nd1.5Mg0.5)Ni7 is about 1.2 wt% at 298 K, which is slightly less than the 1.4 wt% for (La1.5Mg0.5)Ni7 alloy [45]. Iwatake et al. [46] found a new crystal
2.3. A2B7 type In the past few years, details about the structures of Ce2Ni7H4–4.7 [33,34], La2Ni7H6.5 [35] and La2Ni7Hx(x=6.4, 10.8) [36] have been reported. For the La2Ni7 and Ce2Ni7 deuterides [33,35], D atoms are situated exclusively within the AB2 units, while in La1.63Mg0.37Ni7D8.8 both the AB2 and AB5 structural slabs host D atoms (see Fig. 5a). The RE2M7-type alloys with rhombohedral and hexagonal structures are also studied by some other researchers as shown in Fig. 5b [37,38]. The structural properties and hydrogen storage capacities of Y2Ni7Dx compounds (x=2.1, 4.1, and 8.8) with Gd2Co7 structure were also investigated by Charbonnier et al. [39]. The same as the usual structure, the stacking of one A2B4 subunits and two AB5 along with the c axis were defined. There are three plateaus of Y2Ni7 rather than
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(3R) and Pr5Co19-type phases by sintering the powder mixture of LaNi4.8 and Mg2Ni at 1173 K for 10 days. It showed a high capacity, but the cyclic stability needs to further improve. Liu's program achieved almost identical results [67]. Zhao et al. [68] investigated the La0.84Mg0.16Ni3.80 alloy with the single Pr5Co19-type phase (Fig. 7a). The discharge performances were also studied comparing with the similar type alloys (Fig. 7b) Electrochemical studies showed no signs of higher discharge ability for the composition with single phase. To improve the electrochemical properties, especially the discharge capacity, Ce2Ni7-type or LaNi5 secondary phase should be introduced into the alloys. For some other type RE-Mg based alloys, LaMg2Ni was one of the compounds with stable structure. Earliest research on the structures and hydrogenation behaviors of LaMg2Ni alloys were reported by Karonik et al. [69]. The structure could be considered as a transition metal filled variant of the CaIn2 type [70]. A tetrahedral network was built up by the magnesium atoms. LaMg2Ni shows the transition from metal to non-metal upon hydrogenation process because of a charge transfer of conduction electrons which could stabilize [NiH4]4- complexes, with a closed shell electron configuration [71]. For the La0.7Mg0.3(Ni0.85Co0.15)x (x=2.5–5.0) alloys, the platform range become larger and the pressure value remain unchanged when x increases from 2.5 to 3.5, while the plateau pressure rises and the width decreases gradually when x value further increases to 5.0 [72]. But there is poor cycling durability in alkaline electrolytes for all of these alloys. After 60 cycles, the best capacity retention rate is only 45.9% for the La0.7Mg0.3 (Ni0.85Co0.15)3.5. A new type of Mg-transition metal-based alloys of LaMg8.40Ni2.34xAlx (x=0 and 0.20) which composed of La2Mg17, LaMg2Ni and Mg2Ni phases was also prepared [73]. The reversible hydrogen storage capacity of LaMg8.40Ni2.14Al0.20 alloy is 3.22 wt% at 558 K, which is much higher than the LaMg8.40Ni2.34 alloy. Amorphous Mg70(RE25Ni75)30 (RE=La, Pr, Nd) samples were prepared by melt spinning [74]. The max hydrogen absorption and desorption capacities were 4.21 wt% or Mg70(Nd25Ni75)30 and 2.74 wt% or Mg70(Pr25Ni75)30. Abe et al. [75] studied the structure of Mg30Co2Y9 alloys, the results showed that a highly stable structure with dense-packing (73.2%) atoms was appeared in a large hexagonal lattice (P63/mmc).
structure of the (Nd,Mg)2(Ni,Al)7 alloy. A sub-block layer of the AB3 stoichiometry and a sub-block layer of the A5B19 were alternately stacked on the top of each other in the stacking of the ABC-type rather than one A2B4 unit layer and two AB5 unit layers. As the most possible materials to apply in Ni-MH batteries, the charge/discharge performances of A2B7-type hydrogen storage alloys were studied by many researchers. Optimized A2B7-type alloys with Mg element have been produced for the electrode of Ni/MH batteries, as shown in Table 1. From the table, the discharge capacity could reach more than 400mAh/g, and this is the higher level of the various types about RE-Mg based hydrogen storage alloys. But the limited for its application was the lower capacity retention. 2.4. Other types More and more studies have been carried out about the A5B19-type hydrogen storage alloy because of the higher capacity and more excellent cycle stability all over the world recently. The Pr5Co19-type (3R) structures have three blocks along the c axis; each block is composed of one layer of MgZn2-type cells and three layers of CaCu5type cells. Takeda et al. researched the crystal structures of Sm5Ni19.10. Some super lattice structures (2H, 3R, 4H, 6H, 9R and 12R) were also found by TEM [62]. Akiba et al. [63] investigated the structure of La-Mg-Ni (Co) with Pr5Co19-Type. They thought that Mg could occupy only the [La2Ni4] layer and it makes the lattice stable both hydrogenation and dehydrogenation process. Nakamura et al. [64] researched the structures of La4MgNi19 compound (Fig. 6a), the main phase was La4MgNi19 (5:19R and 5:19H), and the mass fractions were 70%. The La4MgNi19Dx phase structure is shown in Fig. 6b. The D/M value could be obtained from the D sites occupancies was 0.91(5). For the past few years, the studies for the A5B19-type hydrogen storage alloys were focused on the preparation of Pr5Co19-type and Ce5Co19-type phases because of the good electrochemical properties, especially high rate discharge ability. Ozaki et al. [65] studied that the La0.8Mg0.2Ni3.4−xCo0.3 (MnAl)x (0≤x≤0.4) alloys with Ce5Co19-type (3R) and Pr5Co19-type (2H) structures showed the excellent discharge capacity and cyclic stability. Ferey et al. [66] obtained Ce5Co19-type
Fig. 6. Crystal structures of (a) the alloy and (b) the hydride of 5:19R phase, (c) Hydrogen interstitial sites in the AB2, AB5−1, and AB5−2 cells.
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Fig. 7. TEM images of the La0.84Mg0.16Ni3.80 alloy (a) and Discharge capacity (b).
Fig. 8. LaMg12-x alloy structures.
method for preparing the RE-Mg based hydrogen storage alloys. Guo et al. [80] found that La2-xTixMgNi9 consisted of LaNi5, LaMg2Ni9 (3R) and LaNi3, and the cycle stability was great promoted after annealing because of the improvement of the compositional homogeneity. Gao et al. [81] studied the effect of annealed treatment process on microstructure for La-Mg-Ni alloys. They pointed that the both Pr5Co19-type and Ce2Ni7-type phases were all with good stability and the electrochemical cyclic stability was mainly affected by the LaNi5 phase in the alloys. The cooperation of these factors makes the maximum discharge capacity and the retention more than 320mAh/g and 90% after 100 cycles with 3d annealed treatment. Liu et al. [82] investigated the effects of anneal temperatures to the La-Mg-Ni based (La,Mg)2Ni7-phase La0.75Mg0.25Ni3.5 alloys. An obvious structure transformation from 2H-type to 3R-type was appeared when the annealing temperature was 1223 K, and adjusting LaNi5-phase content could improve the hydrogen storage and electrochemical performances. Rapidly quenching technique has much advantage of simple
Another type of compounds composed of rare earth and magnesium were attract us much more attention for the higher hydrogen storage capacities. Denys et al. [76] prepared an intermetallic compound of LaMg12-x. There was a giant unit cell with a volume exceeding 8000 Å3 in orthorhombic structures. La sublattice of LaMg12-x alloy was shown in Fig. 8. Ouyang et al. [77] produced a type of alloy with a formulation of Mg3LaNi0.1, and it showed the excellent performances in hydriding and dehydriding kinetics. The adding of Pd element in the La2(Mg,Pd)17 alloys [78] could promote the equilibrium pressures, but reduced the H2 capacity. 3. New evolution for preparation and modification The hydrogen storage and electrochemical properties were significantly affected by the microstructures, compositions and phase homogeneity of alloys. For the Mg-based compounds, the Mg content is hard to control because of its higher vapor pressure [79]. Induction melting following the annealed treatment process is the most commonly used 6
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operation and high efficiency for keeping the phase during the hydrogen storage alloys preparation process. It is generally used to prepare the amorphous material with high free volume which could reduce the hydride formation enthalpy [83]. Zhang et al. researched the La2Mg(Ni0.85Co0.15)9 alloy with rapidly quenching, the reserved PuNi3type and LaNi5-type phase could enhance the electrochemical performances because of the formation of the amorphous phase [84]. Yartys et al. [76] studied the rapidly solidified process of LaMg12 and LaMg11Ni alloys in different solidification cooling rates, the higher cooling rate, the faster hydrogenation kinetics. They also pointed that rapid solidification process could significantly improve the hydrogen absorption/desorption performances because of the catalytic influence of the Ni-containing Mg2Ni intermetallics with nanostructures. Yan et al. prepared the Mg70 (Ni3La) 30 and Mg70 (Ni3.5La) 30 alloys by melt spinning process, and compare with the as-melting Mg70 (Ni3La) 30 alloys [85]. The results indicated that “three-dimensional interface reaction process” in the hydrogenation process and “geometrical contraction model” in the dehydrogenation occurred. High-energy ball milling has grown to become one of the most frequently used methods for producing the hydrogen storage alloys and metal hydrides, as it was simply, low cost, efficient and practical. Zhu et al. [86] prepared the Ml0.7Mg0.3Ni3.2 alloy with nanocrystalline through high energy ball milling followed by annealing progress. The AB3-typephase tended to transform into an AB5-type with increase milling duration. La0.7Mg0.3Ni3.5-Ti0.17Zr0.08V0.35Cr0.1Ni0.3 composites were produced by ball milling process in the argon atmosphere [87]. Qi et al. studied the modification of La0.7Mg0.25Ti0.05Ni2.975Co0.525 hydrogen storage alloy with polyaniline (PANI) through ball milling process [88]. This process could improve the electrochemical cycle propertied of AB3-type alloys due to the surface nano-film. The surface treatment of nanocrystalline/amorphous LaMg11Ni alloy ball milled with Ni powder was investigated [89]. The addition of Ni could enhance the discharge capacity of the alloy significantly. Adding a suitable amount of LiBr in La2Mg17/Ni composites through ball milling also could promote the alloy electrochemical properties [90]. The sintering process is an effective method to prepare the RE-Mg based alloys. Starting from two precursors LaNi5 and LaMgNi4, Zhang et al. [91] prepared several alloys with various phase structures by powder sintering technique. The mole ratio of LaNi5/LaMgNi4 (x) could significantly affect the phase structures and electrochemical performances, and the LaNi5 phase could react with LaMgNi4 phase generating (La,Mg) Ni3 and (La,Mg)2Ni7 phases. Recent studies have mainly concentrated in exerting external physico-chemical field during the sintering or melting process. Si et al. found that the activation and stable cycle performance were all promoted through the laser sintering process for the La0.7Mg0.3Ni3.5 alloys [92]. Serin et al. synthesized the La0.65Nd0.15Mg0.20Ni3.5 alloy by spark plasma sintering and studied the atomic scale structure [38]. A single phase 2:7 alloy with the 3 R structure could be prepared, as shown as Fig. 9. The La0.1Nd0.075Mg0.04Ni0.65Co0.12 hydrogen storage alloys were prepared through rapidly quenching with exerting a 0.18 T static magnetic field [93]. They pointed that the Lorentz Force induced by the magnetic field leaded to grain refinement and composition homogeneity. Microwave assisted sintering process of hydrogen storage materials was also studied by some researcher [94]. Otherwise, the charging/discharging process is very affected by the solid-liquid interface properties of alloys used in Ni/MH battery electrode materials. Several research groups have reported that alloys surface coating was an effective method to improve their electrochemical performances especially the cycle stability, the coating materials included Ni, Cu, Co, polymer et al. A highly catalytic layer could be formed in coating process. Zhang et al. [95] developed a simple method for improving the hydrogen storage properties through adding nano Ni-Al compounds using as a stable catalyst to the La2Mg17 alloys by magnetron sputtering methods, and the hydrogen absorption and desorption rate are all substantially improved. Li et al. [96] studied
Fig. 9. High magnification HAADF image of the La0.65Nd0.15Mg0.20Ni3.5 sample.
the effects of nano Mo-Ni compound additives by chemical reduction method to the La-Mg-Ni based hydrogen storage alloys. It showed a high electro-catalytic activity, and worked as a catalyst to charge the transfer reaction at the alloy surface. Nanocrystalline or nanofilm coating exhibited remarkable positive effects in terms of the utilization of hydrogen absorption and desorption of the Mg-based alloy electrodes, which has been attributed to the outstanding catalytic activity and electrical conductivity. 4. Conclusion and future prospects Rare earth-magnesium based alloys played more and more important roles in the solid state hydrogen storage field which could be used as Ni/MH electrode materials. These materials have excellent advantage for hydrogen storage and are worth to further investigate in the future. In this review we have focused on the structural characteristics, gas-solid reactions, electrochemical performances, preparation and modification techniques of RE-Mg based hydrogen storage alloys with various types. The formation of rare earth-magnesium-nickel based hydrogen storage alloys were mainly (La,Mg)Ni3 phase with a rhombohedral type structure or a (La,Mg)2Ni7 phase with a hexagonal type structure. These structures were stacked by the AB5 unit (CaCu5-type structure) and the AB2 unit (Laves structure) with a given ratio along the c-axis direction. The special stacking structure about this type of alloys could provide guidance for the further development of hydrogen storage alloys with new structures. The presence of Mg in the hydrogen storage alloys lead to a higher hydrogen storage and electrochemical capacity, but the lower cycle stability because of the alloy surface corrosion. For practical applications, lots of efforts to improve the overall electrochemical performances about the RE-Mg based alloys have been adopted, including composition optimization, heat and ball milling treatment, surface coating treatment, surface nanocrystallization, electrolyte modification, and so on. It is clear that a great deal of progress has been achieved over the last decade in the field of RE-Mg based hydrogen storage alloys. Finally, for RE-Mg based hydrogen storage alloys to be widely used in Ni/MH second batteries, the development of ideal materials with high and stability capacity was still an enormous challenge to all the researchers. Acknowledgments This work is supported by the Ministry of Science and Technology 7
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