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Hydrogen storage properties of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys
Journal of Alloys and Compounds 617 (2014) 29–33
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Hydrogen storage properties of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys Tai Yang a,b, Tingting Zhai a, Zeming Yuan a, Wengang Bu a, Sheng Xu b, Yanghuan Zhang a,b,⇑ a b
Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, China Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou 014010, China
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 26 July 2014 Accepted 28 July 2014 Available online 7 August 2014 Keywords: LaMgNi4 alloy Hydrogen storage Cycle stability p–c Isotherms Hydrogen-induced amorphisation
a b s t r a c t LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys were prepared through induction melting process. The phase compositions and crystal structures were characterised via X-ray diffraction (XRD). The hydrogen storage properties, including activation performance, hydrogen absorption capacity, cycle stability, alloy particle pulverisation and plateau pressure, were systemically investigated. Results show that Ni, Co, Mn and Cu substitution alloys exhibit multiphase structures comprising the main phase LaMgNi4 and the secondary phase LaNi5. However, the secondary phase of the Al substitution alloy changes into LaAlNi4. The lattice parameters and cell volumes of the LaMgNi4 phase follow the order Ni < Co < Al < Cu < Mn. Activation is simplified through partial substitution of Ni with Al, Cu and Co. The hydrogen absorption capacities of all of the alloys are approximately 1.7 wt.% at the first activation process; however, they rapidly decrease with increasing cycle number. In addition, the stabilities of hydriding and dehydriding cycles decrease in the order Al > Co > Ni > Cu > Mn. Hydriding processes result in numerous cracks and amorphisation of the LaMgNi4 phase in the alloys. The p–c isotherms were determined by a Sieverts-type apparatus. Two plateaus were observed for the Ni, Co and Al substitution alloys, whereas only one plateau was found for Mn and Cu. This result was caused by the amorphisation of the LaMgNi4 phase during the hydriding cycles. Reversible absorption and desorption of hydrogen are difficult to achieve. Substitutions of Ni with Co, Mn, Cu and Al significantly influence the reduction of hysteresis between hydriding and dehydriding. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is a promising energy carrier, known for its high energy density and clean combustion process. In the 21st century, hydrogen storage has become the cornerstone technology for energy utilisation as components of a combined hydrogen economy [1]. Storage of molecular hydrogen has the disadvantage of requiring high pressures and/or low temperatures. By contrast, if the H–H bond is broken using a proper catalyst, the hydrogen atoms are readily absorbed into the metals at standard pressures and temperatures [2]. Thus far, hydrogen uptake materials have progressed considerably [3–7]. However, these methods are uneconomical for practical automotive applications because of their high storage pressure and unfavourable temperature conditions. Intermetallics have been widely investigated for their ability to repeatedly store hydrogen in solid state. Studies on RE–Mg–Ni ⇑ Corresponding author at: Department of Functional Material Research, Central Iron and Steel Research Institute, No. 76 Xueyuannan Road, Haidian District, 100081 Beijing, China. Tel.: +86 10 62183115; fax: +86 10 62187102. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jallcom.2014.07.206 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
(RE = La, Ce, Pr, Nd) hydrogen storage alloys have led to a new series of ternary alloys [8–10]. A series of REMgNi4 (RE = Ca, La, Ce, Pr, Nd, Y) compounds has drawn considerable attention among the compositions studied [11–14]. YMgNi4 synthesis and determination of its hydriding properties by Aono et al. [15] revealed maximum hydrogen content of approximately 1.05 wt.% (H/M 0.6) under hydrogen pressure of 4.0 MPa at 313 K. The enthalpy of hydride formation was 35.8 kJ/mol H2. Kadir et al. [16] and Wang et al. [17] studied AMgNi4 (where A = Ca, La, Ce, Pr, Nd, Y) alloys, and found that these alloys have a cubic SnMgCu4 (AuBe5 type) structure. Among them, the LaMgNi4 alloy had a great potential for applications in future hydrogen energy systems, such as rechargeable batteries and fuel cells. Based on these results, the La–Mg–Ni series AB2-type alloys can also be applied in hydrogen storage. As reported in the literature, element substitution is an important method to improve hydrogen storage properties. Co addition does not only serve as catalyst, it also inhibits the degradation caused by hydriding/dehydriding, thereby significantly improving the hydrogen shortage properties of the alloy [18,19]. Mn partial substitution for Ni decreases the plateau pressure without
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reducing its hydrogen storage capacity [20]. Meanwhile, Al partial substitution for Ni improves the cycling performance and decreases the plateau pressure of the hydride [21,22]. Moreover, partial replacement of Ni by Cu, Fe and Cr decreases the hydrogen plateau pressure to a certain extent [23–25]. Therefore, we hypothesise that the substitution of Ni with Co, Mn, Cu and Al may improve the hydrogen storage characteristics of the LaMgNi4 alloy more noticeably. In the current study, the hydrogenation behaviour of LaMg Ni3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys, including the activation performance, hydrogen absorption capacity, cycle stability, pulverisation and plateau pressure, was studied. The effects of different B-site partial substitutions were also analysed.
phase LaMgNi4 corresponding to the SnMgCu4 (AuBe5)-type structure with F43m (2 1 6) space group and some secondary phase LaNi5 for Ni, Co, Mn and Cu substitution alloys. By contrast, the secondary phase of the Al substitution alloy changes into LaAlNi4. The diffraction peaks of the LaMgNi4 phase are highly consistent with the reports of Kadir et al. [16] and Wang et al. [17]. Table 1 lists the lattice parameters and unit cell volumes of the LaMgNi4 phase. This table shows that the partial substitutions of Ni with Co, Mn, Cu and Al result in the significant increase of unit cell volumes in the following order: Ni < Co < Al < Cu < Mn. The cell volume of the Mn replaced alloy is larger than that of the alloys replaced with other elements. This phenomenon has also been discovered by Li et al. [27] in their investigation of LaNi3.8Al1.0M0.2 alloys and Liu et al. [28] in their investigation of LaNi4.7M0.3 alloys.
2. Experimental The chemical composition of the alloy was LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al). For convenience, the unsubstituted LaMgNi4 alloy was denoted as Ni-substituted LaMgNi3.6M0.4 (M = Ni) alloy. The as-cast alloys were prepared using a vacuum induction furnace in a helium atmosphere at 0.04 MPa pressure to prevent Mg from volatilising. The raw materials were high-purity metals (purity over 99.5%). Afterwards, the prepared alloy ingots were crushed and mechanically ground to fine powders (passed through a 200-mesh sifter). The structural and phase identification of alloys were carried out at room temperature using powder XRD (D/max/2400). The diffraction, with scanning rate of 2 (°)/min and 2h values between 20° and 80°, was performed with Cu Ka radiation filtered by graphite. A scanning electron micrograph (SEM) (QUANTA 400) was used for morphological characterisation of the alloy powders. A Sieverts-type apparatus (Beijing General Research Institute for Nonferrous Metals) was used to measure the hydrogen absorption properties of the alloys. Approximately 1 g powder sample was taken for each test. The hydrogen absorption was conducted at 3 MPa hydrogen pressure (this pressure is the initial pressure of the hydriding process). Our earlier research results showed that the tested alloys could quickly absorb hydrogen even at room temperature. However, the pressure of hydrogen desorption was very low and could hardly desorb hydrogen. Thus, the hydrogen absorption/desorption cycles were performed at 373 K. The p–c isotherm measurements were carried out at 323, 348 and 373 K to investigate the absorption and desorption equilibrium pressure.
3.2. Activation performances Fig. 2 shows the first activation and second hydrogen absorption curves of the LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K. Fig. 2(a) shows that all the investigated alloys have incubation periods. Once they begin absorbing hydrogen, they saturate rapidly and reach their maximum hydrogen capacity. The incubation time increases in the following order: Al < Cu < Co < Ni < Mn. This order denotes that activation becomes easier with partial substitutions of Ni with Al, Cu and Co, and becomes more difficult with Mn substitution. The best activation performance of the Al substituted alloy may be associated with the LaAlNi4 secondary phase. The hydrogen absorption kinetics of the activated alloy is very different from that of the first activation. Fig. 2(b) shows that under the same experimental conditions, the hydrogen absorption rates of the activated alloys are very fast, reaching saturation within 3 min. In addition, Fig. 2(b) shows that the substitutions of M (M = Ni, Co, Mn, Cu, Al) for Ni have a large effect on its hydrogen absorption capacity. This phenomenon will be discussed in the next section.
3. Results and discussion 3.1. Crystal structure characteristics Fig. 1 shows the XRD patterns of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys. As seen in the figure, the diffraction patterns are similar and exhibit sharp peaks. These results indicate long-range crystallographic orders and excellent crystallinities of the alloys [26]. These alloys have multiphase structures containing the major
3.3. Hydrogen absorption capacity and cycling performances To investigate the hydriding and dehydriding cycle stability of the alloy, the variations of saturated hydrogen capacity with cycle number are presented in Fig. 3. As seen in the figure, the hydrogen absorption capacities of all the alloys are approximately 1.7 wt.% at the first activation process. However, it rapidly decreases with increasing cycle number. Approximately half of the maximum capacity of the Mn and Cu substitution alloys remains after one cycle. To more directly demonstrate the effects of M (M = Ni, Co, Mn, Cu, Al) substitution on the cycle stability of the alloy, we introduce the capacity retaining rate (Rn) to evaluate cycle stability. This value can be calculated according to the following formula:
Rn ¼
Cn 100% C max
ð1Þ
where Cmax is the maximum hydrogen absorption capacity and Cn is the hydrogen absorption capacity of the nth hydriding–dehydriding cycle. If the alloy still has a high Rn value after the hydriding– Table 1 Lattice parameters and cell volumes of LaMgNi4 phase in the LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys.
Fig. 1. XRD patterns of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys.
Alloys
a (Å)
V (Å3)
M = Ni M = Co M = Mn M = Cu M = Al
7.176 7.190 7.204 7.196 7.191
369.5 371.2 373.9 372.6 371.9
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Fig. 2. Hydrogen absorption properties of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K: (a) first activation, (b) second hydrogen absorption.
with Co and Al, and can be decreased by substitution with Cu and Mn. The hydrogen absorption capacity and capacity retaining rate are listed in Table 2. SEM images of the alloy particles at different hydriding cycles are examined to investigate the capacity fading mechanism of the alloy. Results are shown in Fig. 4. As seen in the figure, the first activation process leads to numerous cracks in the alloy particles. However, the crush degree is not significantly increased in the following hydriding cycles. The activation capability of the alloy is closely related to the internal energy change of the hydride system before and after hydrogen absorption. As such, larger internal energy change results in poorer activation performance of the alloy [29]. The alloy absorbs significant quantities of hydrogen and the formation of metal hydride leads to lattice expansion, causing cracks and strain energy in the alloy particles. Moreover, the hydrogen atoms have to penetrate the oxide layer surface of the alloy particles before forming metal hydrides. This occurrence explains why the incubation period appeared in the activation curves. However, in the activated alloys, their greatly reduced additive internal energy and large number of new micro-cracks caused a quick saturation of hydriding, as shown in Fig. 2. The XRD patterns of alloy particles after hydrogen absorption/ desorption cycles are illustrated in Fig. 5. Fig. 1 shows that the as-cast (not activated) alloys have sharp and narrow diffraction peaks, corresponding to a perfect crystal structure. However, as shown in Fig. 5, the first hydriding process leads to significantly broadened and short diffraction peaks of the LaMgNi4 phase, which exhibit nanocrystalline and amorphous structures. Fig. 5(b) also shows that such tendency becomes more obvious with the increase of cycle number, which implies that the hydrogen-induced amorphisation process appears during the hydriding process. Oesterreicher et al. also discovered this phenomenon [14,30]. The hydrogen absorption/desorption cycles seem to have minimal influence on structures of the secondary phase (LaNi5 and LaAlNi4). Fig. 5 shows that Mn and Cu substitutions significantly promote the amorphisation of the LaMgNi4 phase. However, Co substitution reduces the amorphisation process of the alloy. This result explains why the Co-substituted alloy has better cycle stability than those of Mn- and Cu-substituted alloys. The diffraction peaks of the LaMgNi4 phase in the Al substitution alloy completely disappear and only those of the LaAlNi4 phase remain after 20 cycles. Notably, LaAlNi4 can also absorb/desorb hydrogen reversibly [27]. Therefore, we can assume that the Al substitution alloy has the best cycle stability. 3.4. p–c isotherms The p–c isotherms (pressure–composition-isotherms) of the alloys are shown in Fig. 6. Three hydriding cycles were carried out before testing. Under identical conditions, the partial replacement of Ni with Co, Mn, Cu and Al remarkably changes the shape of the curves. Two plateaus are clearly observed in the Ni, Co and Al substitution alloys, which correspond to the hydriding process of LaMgNi4 (lower plateau pressure) and LaNi5 or LaAlNi4 phases (higher plateau pressure), respectively. Only one plateau pressure is observed in the Mn and Cu substitution alloys because of amor-
Fig. 3. Evolution of the hydrogen absorption capacity of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K.
dehydriding cycles, we can deduce that it has superior cycle stability. Fig. 3 presents the R5 and R20 values of the alloys. This figure shows that the hydriding and dehydriding cycle stabilities decrease in the following order: Al > Co > Ni > Cu > Mn. Evidently, the cycle stability can be promoted by partial substitution of Ni
Table 2 Hydrogen absorption capacity (Cn) and capacity retaining rate (Rn) of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K. Alloys
C1 (wt.%)
C5 (wt.%)
C20 (wt.%)
R5 (%)
R20 (%)
M = Ni M = Co M = Mn M = Cu M = Al
1.716 1.733 1.800 1.711 1.648
0.847 0.922 0.543 0.652 0.851
0.638 0.682 0.481 0.546 0.783
49.4 53.2 30.2 38.1 51.6
37.2 39.4 26.7 31.9 47.5
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Fig. 4. SEM images of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloy particles before and after hydrogen absorption/desorption cycles at 373 K: (a) before hydriding, (b) after one cycle, (c) after 20 cycles.
Fig. 5. XRD patterns of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys after hydrogen absorption/desorption cycles at 373 K: (a) after one cycle, (b) after 20 cycles.
phisation of the LaMgNi4 phase after three hydriding cycles. Additionally, reversible absorption and desorption of hydrogen are difficult to achieve. Fig. 6(b) shows that the plateau pressure of the alloy was remarkably increased by raising the temperature. All plateaus of the alloys have big slopes caused by the amorphisation of the LaMgNi4 phase in the hydriding/dehydriding process. Thus, calculating the thermodynamic parameters of the hydriding reactions using plateau pressures at different temperatures is difficult.
Fig. 6. Hydrogen absorption/desorption p–c isotherms of the alloys: (a) LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K, (b) LaMgNi4 alloy at 323, 348 and 373 K.
Reversibility is an important performance for the practical application of hydrogen storage alloys. Hysteresis between hydrogen absorption and desorption is usually employed to evaluate the reversibility of the hydriding process. Fig. 6 shows that the substitutions of Ni with Co, Mn, Cu and Al have significant effects on decreasing hysteresis between hydriding and dehydriding. This result should be caused by the increase of the crystal cell volume. The increased crystal cell volume decreases the pressure of hydrogen absorption and elevates the pressure of hydrogen desorption, resulting in the reduction of the hysteresis [31,32]. Fig. 6(a) also
T. Yang et al. / Journal of Alloys and Compounds 617 (2014) 29–33
shows that hysteresis of the Al substitution alloy is significantly reduced because of the enhanced reversibility of its LaAlNi4 phase [27]. Table 1 shows that the Mn substitution alloy has the biggest lattice parameter and cell volume among the substitution alloys. The Mn substitution alloy also has the lowest plateau pressure and poorest hydrogen absorption stability. This result is consistent with the correlation between plateau pressure and unit cell volume previously reported by Lundin et al. [33,34]. Their work also indicated that the hydrogen plateau pressure decreased as the unit cell volume increased. Moreover, Fig. 6 shows that all the p–c isotherms are not closed. This result means that the desorbed hydrogen capacity in one absorption/desorption cycle is less than that of the absorbed one. Results also imply that some of the LaMgNi4 phases transform into amorphous hydrides after every hydriding/ dehydriding cycle. These hydrides cannot be resolved, subsequently decreasing the hydrogen absorption capacity. 4. Conclusions The as-cast LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys have multiphase structures consisting of the main phase LaMgNi4 and the secondary phase LaNi5 for the Ni, Co Mn and Cu substitution alloys. However, the secondary phase of the Al substitution alloy changes into LaAlNi4. The lattice parameters and cell volumes of the LaMgNi4 phase follow the order Ni < Co < Al < Cu < Mn. All the alloys could be activated in 20 min at 373 K, and the substitutions of Ni with Al, Cu and Co improve the activation properties. The hydrogen absorption capacity of the alloys is approximately 1.7 wt.% at the first activation processes. However, it rapidly decreases with increasing cycle number. The first hydrogenation process significantly promotes the kinetics of the alloys, reaching its saturation within 3 min. The hydriding and dehydriding cycle stability descends in the following order: Al > Co > Ni > Cu > Mn. Hydriding process leads to numerous micro-cracks in the alloy particles and the amorphisation of the LaMgNi4 phase in the alloys. Two plateaus are observed in the substitution alloys of Ni, Co and Al, whereas only one plateau is found for those of Mn and Cu. This phenomenon is caused by the amorphisation of the LaMgNi4 phase during the hydriding cycles. The increase of lattice parameters and cell volumes caused by element substitution has a substantial effect on decreasing hysteresis between hydrogenation and dehydrogenation. Acknowledgement The authors gratefully acknowledge the financial support provided by the National Natural Science Foundations of China (51161015 and 51371094).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Hydrogen storage properties of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys Tai Yang a,b, Tingting Zhai a, Zeming Yuan a, Wengang Bu a, Sheng Xu b, Yanghuan Zhang a,b,⇑ a b
Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, China Key Laboratory of Integrated Exploitation of Baiyun Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Baotou 014010, China
a r t i c l e
i n f o
Article history: Received 12 May 2014 Received in revised form 26 July 2014 Accepted 28 July 2014 Available online 7 August 2014 Keywords: LaMgNi4 alloy Hydrogen storage Cycle stability p–c Isotherms Hydrogen-induced amorphisation
a b s t r a c t LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys were prepared through induction melting process. The phase compositions and crystal structures were characterised via X-ray diffraction (XRD). The hydrogen storage properties, including activation performance, hydrogen absorption capacity, cycle stability, alloy particle pulverisation and plateau pressure, were systemically investigated. Results show that Ni, Co, Mn and Cu substitution alloys exhibit multiphase structures comprising the main phase LaMgNi4 and the secondary phase LaNi5. However, the secondary phase of the Al substitution alloy changes into LaAlNi4. The lattice parameters and cell volumes of the LaMgNi4 phase follow the order Ni < Co < Al < Cu < Mn. Activation is simplified through partial substitution of Ni with Al, Cu and Co. The hydrogen absorption capacities of all of the alloys are approximately 1.7 wt.% at the first activation process; however, they rapidly decrease with increasing cycle number. In addition, the stabilities of hydriding and dehydriding cycles decrease in the order Al > Co > Ni > Cu > Mn. Hydriding processes result in numerous cracks and amorphisation of the LaMgNi4 phase in the alloys. The p–c isotherms were determined by a Sieverts-type apparatus. Two plateaus were observed for the Ni, Co and Al substitution alloys, whereas only one plateau was found for Mn and Cu. This result was caused by the amorphisation of the LaMgNi4 phase during the hydriding cycles. Reversible absorption and desorption of hydrogen are difficult to achieve. Substitutions of Ni with Co, Mn, Cu and Al significantly influence the reduction of hysteresis between hydriding and dehydriding. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen is a promising energy carrier, known for its high energy density and clean combustion process. In the 21st century, hydrogen storage has become the cornerstone technology for energy utilisation as components of a combined hydrogen economy [1]. Storage of molecular hydrogen has the disadvantage of requiring high pressures and/or low temperatures. By contrast, if the H–H bond is broken using a proper catalyst, the hydrogen atoms are readily absorbed into the metals at standard pressures and temperatures [2]. Thus far, hydrogen uptake materials have progressed considerably [3–7]. However, these methods are uneconomical for practical automotive applications because of their high storage pressure and unfavourable temperature conditions. Intermetallics have been widely investigated for their ability to repeatedly store hydrogen in solid state. Studies on RE–Mg–Ni ⇑ Corresponding author at: Department of Functional Material Research, Central Iron and Steel Research Institute, No. 76 Xueyuannan Road, Haidian District, 100081 Beijing, China. Tel.: +86 10 62183115; fax: +86 10 62187102. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.jallcom.2014.07.206 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
(RE = La, Ce, Pr, Nd) hydrogen storage alloys have led to a new series of ternary alloys [8–10]. A series of REMgNi4 (RE = Ca, La, Ce, Pr, Nd, Y) compounds has drawn considerable attention among the compositions studied [11–14]. YMgNi4 synthesis and determination of its hydriding properties by Aono et al. [15] revealed maximum hydrogen content of approximately 1.05 wt.% (H/M 0.6) under hydrogen pressure of 4.0 MPa at 313 K. The enthalpy of hydride formation was 35.8 kJ/mol H2. Kadir et al. [16] and Wang et al. [17] studied AMgNi4 (where A = Ca, La, Ce, Pr, Nd, Y) alloys, and found that these alloys have a cubic SnMgCu4 (AuBe5 type) structure. Among them, the LaMgNi4 alloy had a great potential for applications in future hydrogen energy systems, such as rechargeable batteries and fuel cells. Based on these results, the La–Mg–Ni series AB2-type alloys can also be applied in hydrogen storage. As reported in the literature, element substitution is an important method to improve hydrogen storage properties. Co addition does not only serve as catalyst, it also inhibits the degradation caused by hydriding/dehydriding, thereby significantly improving the hydrogen shortage properties of the alloy [18,19]. Mn partial substitution for Ni decreases the plateau pressure without
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T. Yang et al. / Journal of Alloys and Compounds 617 (2014) 29–33
reducing its hydrogen storage capacity [20]. Meanwhile, Al partial substitution for Ni improves the cycling performance and decreases the plateau pressure of the hydride [21,22]. Moreover, partial replacement of Ni by Cu, Fe and Cr decreases the hydrogen plateau pressure to a certain extent [23–25]. Therefore, we hypothesise that the substitution of Ni with Co, Mn, Cu and Al may improve the hydrogen storage characteristics of the LaMgNi4 alloy more noticeably. In the current study, the hydrogenation behaviour of LaMg Ni3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys, including the activation performance, hydrogen absorption capacity, cycle stability, pulverisation and plateau pressure, was studied. The effects of different B-site partial substitutions were also analysed.
phase LaMgNi4 corresponding to the SnMgCu4 (AuBe5)-type structure with F43m (2 1 6) space group and some secondary phase LaNi5 for Ni, Co, Mn and Cu substitution alloys. By contrast, the secondary phase of the Al substitution alloy changes into LaAlNi4. The diffraction peaks of the LaMgNi4 phase are highly consistent with the reports of Kadir et al. [16] and Wang et al. [17]. Table 1 lists the lattice parameters and unit cell volumes of the LaMgNi4 phase. This table shows that the partial substitutions of Ni with Co, Mn, Cu and Al result in the significant increase of unit cell volumes in the following order: Ni < Co < Al < Cu < Mn. The cell volume of the Mn replaced alloy is larger than that of the alloys replaced with other elements. This phenomenon has also been discovered by Li et al. [27] in their investigation of LaNi3.8Al1.0M0.2 alloys and Liu et al. [28] in their investigation of LaNi4.7M0.3 alloys.
2. Experimental The chemical composition of the alloy was LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al). For convenience, the unsubstituted LaMgNi4 alloy was denoted as Ni-substituted LaMgNi3.6M0.4 (M = Ni) alloy. The as-cast alloys were prepared using a vacuum induction furnace in a helium atmosphere at 0.04 MPa pressure to prevent Mg from volatilising. The raw materials were high-purity metals (purity over 99.5%). Afterwards, the prepared alloy ingots were crushed and mechanically ground to fine powders (passed through a 200-mesh sifter). The structural and phase identification of alloys were carried out at room temperature using powder XRD (D/max/2400). The diffraction, with scanning rate of 2 (°)/min and 2h values between 20° and 80°, was performed with Cu Ka radiation filtered by graphite. A scanning electron micrograph (SEM) (QUANTA 400) was used for morphological characterisation of the alloy powders. A Sieverts-type apparatus (Beijing General Research Institute for Nonferrous Metals) was used to measure the hydrogen absorption properties of the alloys. Approximately 1 g powder sample was taken for each test. The hydrogen absorption was conducted at 3 MPa hydrogen pressure (this pressure is the initial pressure of the hydriding process). Our earlier research results showed that the tested alloys could quickly absorb hydrogen even at room temperature. However, the pressure of hydrogen desorption was very low and could hardly desorb hydrogen. Thus, the hydrogen absorption/desorption cycles were performed at 373 K. The p–c isotherm measurements were carried out at 323, 348 and 373 K to investigate the absorption and desorption equilibrium pressure.
3.2. Activation performances Fig. 2 shows the first activation and second hydrogen absorption curves of the LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K. Fig. 2(a) shows that all the investigated alloys have incubation periods. Once they begin absorbing hydrogen, they saturate rapidly and reach their maximum hydrogen capacity. The incubation time increases in the following order: Al < Cu < Co < Ni < Mn. This order denotes that activation becomes easier with partial substitutions of Ni with Al, Cu and Co, and becomes more difficult with Mn substitution. The best activation performance of the Al substituted alloy may be associated with the LaAlNi4 secondary phase. The hydrogen absorption kinetics of the activated alloy is very different from that of the first activation. Fig. 2(b) shows that under the same experimental conditions, the hydrogen absorption rates of the activated alloys are very fast, reaching saturation within 3 min. In addition, Fig. 2(b) shows that the substitutions of M (M = Ni, Co, Mn, Cu, Al) for Ni have a large effect on its hydrogen absorption capacity. This phenomenon will be discussed in the next section.
3. Results and discussion 3.1. Crystal structure characteristics Fig. 1 shows the XRD patterns of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys. As seen in the figure, the diffraction patterns are similar and exhibit sharp peaks. These results indicate long-range crystallographic orders and excellent crystallinities of the alloys [26]. These alloys have multiphase structures containing the major
3.3. Hydrogen absorption capacity and cycling performances To investigate the hydriding and dehydriding cycle stability of the alloy, the variations of saturated hydrogen capacity with cycle number are presented in Fig. 3. As seen in the figure, the hydrogen absorption capacities of all the alloys are approximately 1.7 wt.% at the first activation process. However, it rapidly decreases with increasing cycle number. Approximately half of the maximum capacity of the Mn and Cu substitution alloys remains after one cycle. To more directly demonstrate the effects of M (M = Ni, Co, Mn, Cu, Al) substitution on the cycle stability of the alloy, we introduce the capacity retaining rate (Rn) to evaluate cycle stability. This value can be calculated according to the following formula:
Rn ¼
Cn 100% C max
ð1Þ
where Cmax is the maximum hydrogen absorption capacity and Cn is the hydrogen absorption capacity of the nth hydriding–dehydriding cycle. If the alloy still has a high Rn value after the hydriding– Table 1 Lattice parameters and cell volumes of LaMgNi4 phase in the LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys.
Fig. 1. XRD patterns of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys.
Alloys
a (Å)
V (Å3)
M = Ni M = Co M = Mn M = Cu M = Al
7.176 7.190 7.204 7.196 7.191
369.5 371.2 373.9 372.6 371.9
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T. Yang et al. / Journal of Alloys and Compounds 617 (2014) 29–33
Fig. 2. Hydrogen absorption properties of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K: (a) first activation, (b) second hydrogen absorption.
with Co and Al, and can be decreased by substitution with Cu and Mn. The hydrogen absorption capacity and capacity retaining rate are listed in Table 2. SEM images of the alloy particles at different hydriding cycles are examined to investigate the capacity fading mechanism of the alloy. Results are shown in Fig. 4. As seen in the figure, the first activation process leads to numerous cracks in the alloy particles. However, the crush degree is not significantly increased in the following hydriding cycles. The activation capability of the alloy is closely related to the internal energy change of the hydride system before and after hydrogen absorption. As such, larger internal energy change results in poorer activation performance of the alloy [29]. The alloy absorbs significant quantities of hydrogen and the formation of metal hydride leads to lattice expansion, causing cracks and strain energy in the alloy particles. Moreover, the hydrogen atoms have to penetrate the oxide layer surface of the alloy particles before forming metal hydrides. This occurrence explains why the incubation period appeared in the activation curves. However, in the activated alloys, their greatly reduced additive internal energy and large number of new micro-cracks caused a quick saturation of hydriding, as shown in Fig. 2. The XRD patterns of alloy particles after hydrogen absorption/ desorption cycles are illustrated in Fig. 5. Fig. 1 shows that the as-cast (not activated) alloys have sharp and narrow diffraction peaks, corresponding to a perfect crystal structure. However, as shown in Fig. 5, the first hydriding process leads to significantly broadened and short diffraction peaks of the LaMgNi4 phase, which exhibit nanocrystalline and amorphous structures. Fig. 5(b) also shows that such tendency becomes more obvious with the increase of cycle number, which implies that the hydrogen-induced amorphisation process appears during the hydriding process. Oesterreicher et al. also discovered this phenomenon [14,30]. The hydrogen absorption/desorption cycles seem to have minimal influence on structures of the secondary phase (LaNi5 and LaAlNi4). Fig. 5 shows that Mn and Cu substitutions significantly promote the amorphisation of the LaMgNi4 phase. However, Co substitution reduces the amorphisation process of the alloy. This result explains why the Co-substituted alloy has better cycle stability than those of Mn- and Cu-substituted alloys. The diffraction peaks of the LaMgNi4 phase in the Al substitution alloy completely disappear and only those of the LaAlNi4 phase remain after 20 cycles. Notably, LaAlNi4 can also absorb/desorb hydrogen reversibly [27]. Therefore, we can assume that the Al substitution alloy has the best cycle stability. 3.4. p–c isotherms The p–c isotherms (pressure–composition-isotherms) of the alloys are shown in Fig. 6. Three hydriding cycles were carried out before testing. Under identical conditions, the partial replacement of Ni with Co, Mn, Cu and Al remarkably changes the shape of the curves. Two plateaus are clearly observed in the Ni, Co and Al substitution alloys, which correspond to the hydriding process of LaMgNi4 (lower plateau pressure) and LaNi5 or LaAlNi4 phases (higher plateau pressure), respectively. Only one plateau pressure is observed in the Mn and Cu substitution alloys because of amor-
Fig. 3. Evolution of the hydrogen absorption capacity of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K.
dehydriding cycles, we can deduce that it has superior cycle stability. Fig. 3 presents the R5 and R20 values of the alloys. This figure shows that the hydriding and dehydriding cycle stabilities decrease in the following order: Al > Co > Ni > Cu > Mn. Evidently, the cycle stability can be promoted by partial substitution of Ni
Table 2 Hydrogen absorption capacity (Cn) and capacity retaining rate (Rn) of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K. Alloys
C1 (wt.%)
C5 (wt.%)
C20 (wt.%)
R5 (%)
R20 (%)
M = Ni M = Co M = Mn M = Cu M = Al
1.716 1.733 1.800 1.711 1.648
0.847 0.922 0.543 0.652 0.851
0.638 0.682 0.481 0.546 0.783
49.4 53.2 30.2 38.1 51.6
37.2 39.4 26.7 31.9 47.5
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Fig. 4. SEM images of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloy particles before and after hydrogen absorption/desorption cycles at 373 K: (a) before hydriding, (b) after one cycle, (c) after 20 cycles.
Fig. 5. XRD patterns of LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys after hydrogen absorption/desorption cycles at 373 K: (a) after one cycle, (b) after 20 cycles.
phisation of the LaMgNi4 phase after three hydriding cycles. Additionally, reversible absorption and desorption of hydrogen are difficult to achieve. Fig. 6(b) shows that the plateau pressure of the alloy was remarkably increased by raising the temperature. All plateaus of the alloys have big slopes caused by the amorphisation of the LaMgNi4 phase in the hydriding/dehydriding process. Thus, calculating the thermodynamic parameters of the hydriding reactions using plateau pressures at different temperatures is difficult.
Fig. 6. Hydrogen absorption/desorption p–c isotherms of the alloys: (a) LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys at 373 K, (b) LaMgNi4 alloy at 323, 348 and 373 K.
Reversibility is an important performance for the practical application of hydrogen storage alloys. Hysteresis between hydrogen absorption and desorption is usually employed to evaluate the reversibility of the hydriding process. Fig. 6 shows that the substitutions of Ni with Co, Mn, Cu and Al have significant effects on decreasing hysteresis between hydriding and dehydriding. This result should be caused by the increase of the crystal cell volume. The increased crystal cell volume decreases the pressure of hydrogen absorption and elevates the pressure of hydrogen desorption, resulting in the reduction of the hysteresis [31,32]. Fig. 6(a) also
T. Yang et al. / Journal of Alloys and Compounds 617 (2014) 29–33
shows that hysteresis of the Al substitution alloy is significantly reduced because of the enhanced reversibility of its LaAlNi4 phase [27]. Table 1 shows that the Mn substitution alloy has the biggest lattice parameter and cell volume among the substitution alloys. The Mn substitution alloy also has the lowest plateau pressure and poorest hydrogen absorption stability. This result is consistent with the correlation between plateau pressure and unit cell volume previously reported by Lundin et al. [33,34]. Their work also indicated that the hydrogen plateau pressure decreased as the unit cell volume increased. Moreover, Fig. 6 shows that all the p–c isotherms are not closed. This result means that the desorbed hydrogen capacity in one absorption/desorption cycle is less than that of the absorbed one. Results also imply that some of the LaMgNi4 phases transform into amorphous hydrides after every hydriding/ dehydriding cycle. These hydrides cannot be resolved, subsequently decreasing the hydrogen absorption capacity. 4. Conclusions The as-cast LaMgNi3.6M0.4 (M = Ni, Co, Mn, Cu, Al) alloys have multiphase structures consisting of the main phase LaMgNi4 and the secondary phase LaNi5 for the Ni, Co Mn and Cu substitution alloys. However, the secondary phase of the Al substitution alloy changes into LaAlNi4. The lattice parameters and cell volumes of the LaMgNi4 phase follow the order Ni < Co < Al < Cu < Mn. All the alloys could be activated in 20 min at 373 K, and the substitutions of Ni with Al, Cu and Co improve the activation properties. The hydrogen absorption capacity of the alloys is approximately 1.7 wt.% at the first activation processes. However, it rapidly decreases with increasing cycle number. The first hydrogenation process significantly promotes the kinetics of the alloys, reaching its saturation within 3 min. The hydriding and dehydriding cycle stability descends in the following order: Al > Co > Ni > Cu > Mn. Hydriding process leads to numerous micro-cracks in the alloy particles and the amorphisation of the LaMgNi4 phase in the alloys. Two plateaus are observed in the substitution alloys of Ni, Co and Al, whereas only one plateau is found for those of Mn and Cu. This phenomenon is caused by the amorphisation of the LaMgNi4 phase during the hydriding cycles. The increase of lattice parameters and cell volumes caused by element substitution has a substantial effect on decreasing hysteresis between hydrogenation and dehydrogenation. Acknowledgement The authors gratefully acknowledge the financial support provided by the National Natural Science Foundations of China (51161015 and 51371094).
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