Comparative Investigations on Hydrogen AbsorptionâDesorption

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Comparative Investigations on Hydrogen Absorption-Desorption Properties of Sm-Mg-Ni Compounds: The Effect of [SmNi]/[SmMgNi] Unit Ratio 5

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Qingan Zhang, Ziliang Chen, Yongtao Li, Fang Fang, Dalin Sun, Liuzhang Ouyang, and Min Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00279 • Publication Date (Web): 12 Feb 2015 Downloaded from http://pubs.acs.org on February 17, 2015

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The Journal of Physical Chemistry

Comparative Investigations on Hydrogen Absorption-Desorption Properties of Sm−Mg−Ni Compounds: The Effect of [SmNi5]/[SmMgNi4] Unit Ratio Qingan Zhang,*,† Ziliang Chen,† Yongtao Li,† Fang Fang,‡ Dalin Sun,‡ Liuzhang Ouyang,§ and Min Zhu*,§ †

School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, P. P.R. China ‡

§

Department of Materials Science, Fudan University, Shanghai 200433, P.R. China

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, P.R. China

ACS Paragon Plus Environment

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ABSTRACT

To understand the effect of [RNi5]/[RMgNi4] unit ratio on hydrogen storage properties of layered R−Mg−Ni compounds, the hydrogen absorption−desorption characteristics of Sm−Mg−Ni compounds have been investigated comparatively in this work. As the [SmNi5]/[SmMgNi4] unit ratio increases, the parameters of [SmNi5] and [SmMgNi4] units in layered Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 compounds decrease gradually; leading to the elevation of equilibrium pressure, enlargement of slope and reduction of hysteresis of their P–C isotherms as well as the decline of enthalpy change for hydrogen absorption-desorption. The hydrogen capacities of the layered Sm−Mg−Ni compounds at 298 K are higher than those of SmMgNi4 and SmNi5 and the cycle stability can be improved by the increase of [SmNi5]/[SmMgNi4] unit ratio.

KEYWORDS: Crystal structure; Hydrogen storage; Sm−Mg−Ni compound


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1. INTRODUCTION R−Mg−Ni (R = rare earth metals) compounds have been widely investigated due to their interesting crystal structures, specific hydrogen absorption-desorption characteristics and good electrochemical properties.1−15 At present the most active research topics are mainly focused on R2MgNi9, R3MgNi14 and R4MgNi19,16−25 which have layered structures where one [RMgNi4] and n [RNi5] (n = 1, 2 and 3, respectively) units stack along the c-axis alternatively according to different patterns.16−23 Mg atoms occupy the R sites of Laves-type [RMgNi4] units rather than those of [RNi5] units because such a site-occupation configuration more significantly strengthens the ionic bond in the systems.17 Generally, R2MgNi9 compounds have a rhombohedral 3R structure (PuNi3-type) while both R3MgNi14 and R4MgNi19 compounds have two variants, viz., a hexagonal 2H structure (Ce2Ni7-type and Pr5Co19-type, respectively) and a rhombohedral 3R-type structure (Gd2Co7-type and Ce5Co19-type, respectively).7, 22 Sometimes the polymorphic 2H and 3R phases coexist in R3MgNi14 and R4MgNi19 alloys because the polymorphic transitions are very sluggish.26 R2MgNi9, R3MgNi14 and R4MgNi19 have been developed from corresponding binary RNi3, R2Ni7 and R5Ni19 compounds, respectively.1−3 However, the occupations of H atoms in their hydrides are quite different. In the Mg-free hydrides such as La2Ni7H6.5, Ce2Ni7H4 and CeNi3H2.8, H atoms reside only in [RNi2] units, not in [RNi5] units; leading to large anisotropic lattice expansions along the c-axis.27, 28 In contrast, for the Mg-containing hydrides like La4MgNi19H21.8, La1.5Mg0.5Ni7H9.1 and La2MgNi9H13, H atoms occupy the interstitial sites of [RMgNi4] and [RNi5] units as well as sites at unit borders, introducing isotropic lattice expansions.19−21 This implies that the [RMgNi4] unit size has a great influence on hydrogen distributions in [RNi5] units. Furthermore, the hydrogen distributions in [RMgNi4] units of the Mg-containing layered hydrides are somewhat different from those in individual RMgNi4 hydrides,18 indicating that the [RNi5] units also affect the hydrogen distributions in [RMgNi4] units. Hence, the [RNi5]/[RMgNi4] unit ratio is one of most important factors in determining hydrogen


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storage properties of layered R−Mg−Ni compounds. Actually, the hydrogen absorption-desorption characteristics and electrochemical performances of layered R−Mg−Ni compounds have been studied extensively over the last decade.14 However, there is a lack of systematic investigation of the [RNi5]/[RMgNi4] unit ratio effect because hydrogen storage properties of layered R−Mg−Ni compounds are related to R species and R/Mg ratio besides [RNi5]/ [RMgNi4] unit ratio. To fully understand the effect of [RNi5]/[RMgNi4] unit ratio on hydrogen storage properties, the hydrogen absorption−desorption characteristics of Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 (with [SmNi5]/[SmMgNi4] unit ratios of 1, 2 and 3, respectively) have been investigated comparatively in this work considering that the Sm/Mg ratio in the [SmMgNi4] unit is fixed as 1/1 and the individual SmMgNi4 compound has a good structure stability without the occurrence of hydrogen-induced amorphization or decomposition during hydrogen absorption-desorption cycles.29 To validate the current study, the hydrogen absorption−desorption properties of individual SmMgNi4 and SmNi5 compounds (with [SmNi5]/[SmMgNi4] unit ratios of 0 and ∞, respectively) have also been investigated as references.

2. EXPERIMENTAL SECTION 2.1 Sample Preparation SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples were prepared by following methods. At first, appropriate amounts of pure metals were induction melted under an argon atmosphere and each sample was re-melted three times to ensure homogeneity. To compensate for the losses of rare earth metal and magnesium during the melting and subsequent sintering process, about 5 wt.% of Sm and 20 wt.% of Mg were excessively added. Then the as-cast ingots were pulverized into 300-mesh powders and pressed into pellets which were subsequently wrapped in a tantalum foil and sintered via a step-heating process at different temperatures (see Table 1, holding 3 h for each step) under argon


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atmosphere (about 0.6 MPa) in order to prevent impurity phases with low melting points from melting during efficient homogenization. The as-sintered alloys were finally polished to remove the oxide layer and mechanically crushed into 300-mesh powders in a glove box under a dry argon atmosphere for structural characterization and performance testing. 2.2 Structural Characterization To identify the phase structures, XRD measurements were carried out on a Rigaku D/Max 2500VL/PC diffractometer with Cu Kα radiation at 50 kV and 200 mA. The XRD profiles were analyzed with the Rietveld refinement program RIETAN−2000.30 In order to evaluate the lattice strains caused by hydrogen absorption and desorption, a pseudo-Voigt function containing a Gaussian part and a Lorentzian part was chosen to fit the XRD peak profiles in the Rietveld refinement.31,32 From the strain parameters in the Rietveld analysis, lattice strain for each phase was obtained. 2.3 Hydrogen Absorption and Desorption To investigate the hydrogen absorption and desorption properties of the Sm–Mg–Ni compounds, the powder samples were loaded into stainless steel containers and then placed in stainless steel autoclaves. The pressure–composition (P–C) isotherms were measured using a Sieverts-type apparatus (Suzuki Shokan Co. Ltd., Japan) at different temperatures ranging from 273 K to 348 K. Prior to formal measurements, powder samples were heated in a vacuum at 373 K for 1 h and then activated by three hydrogenation-dehydrogenation cycles at 298 K. During each cycle, the samples were hydrogenated under a hydrogen pressure of 9 MPa for 2 h and subsequently dehydrogenated against a backpressure of 0.001 MPa for 2 h.

In addition, the activated samples were also subjected

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hydrogenation-dehydrogenation cycles at 298 K to examine their cycle durability. 2.4 Mechanical Property The mechanical properties of sintered samples were measured at 298 K using a G200 nanoindenter with a load resolution of 0.1 nN and a displacement resolution of 0.01 nm. A Berkovitch indenter (triangular pyramid) was selected and a maximum load of 20 mN was adopted at loading rate of 1 mN/s


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during the measurement. On the basis of the Oliver and Pharr analysis,33 the elastic modulus E and nanoindentation hardness H were calculated from the penetration load–displacement curves. Each sample was tested ten times to obtain the average values.

3. RESULTS AND DISCUSSION 3.1 Structural Parameters Figure 1a–e shows the Rietveld refinements of the XRD patterns of SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples, respectively. For the refinements, starting structure models were taken from reported data,7, 12, 15, 34, 35 which are illustrated in Figure 1f. The abundance of each phase in the prepared samples obtained by the Rietveld refinements is listed in Table 2 from which the following two features can be seen : (i) The coexistence of polymorphic 2H and 3R phases occurs in Sm3MgNi14 and Sm4MgNi19 alloys, similar to that in the Nd−Mg−Ni system.17 Given that polymorphic 2H- and 3R-R3MgNi14 phases, as well as 2H- and 3R-R4MgNi19, show almost the same behavior with regard to hydrogen absorption-desorption due to close unit parameters,16 the hydrogen storage properties of 2H- and 3R-Sm3MgNi14 (as well as 2H- and 3R-Sm4MgNi19) can be approximately deemed as equals. (ii) Though pure Sm2MgNi9, Sm4MgNi19 and SmNi5 compounds are not obtained in this work, the impurity phase in each sample is no more than 8 wt% and also reversibly absorbs hydrogen. Hence, the three samples can be approximately regarded as pure ones during evaluation of hydrogen storage properties. Generally, it is reasonable and feasible to use unit parameters, instead of lattice parameters, of different R−Mg−Ni superlattice structures for comparison.16, 19, 22 Figure 2 compares the unit parameters a, c/N and (c/N)/a of SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 compounds; where a and c are lattice parameters except for SmMgNi4 (with a = aC15/√2 and c = aC15/√3), and N is total unit number in one cell except for SmMgNi4 and SmNi5 (with N = 1). It can be seen that the unit parameters of polymorphic 2H and 3R phases are very close, which is in agreement with the previously reported ACS Paragon Plus Environment

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result as mentioned above.16 Note that the unit parameters a and c/N decrease with the increase of [SmNi5]/[SmMgNi4] unit ratio (see Figure 2a and b), and the decrease of (c/N)/a is more sensitive (see Figure 2c). Such a result confirms that obtained by Density Function Theory (DFT) calculations.22 As we know, the a value of [SmMgNi4] unit for the individual SmMgNi4 is larger than that of [SmNi5] unit for the individual SmNi5. In the layered Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 structures, therefore, the mismatch between [SmMgNi4] and [SmNi5] units should be accommodated to a same a-axis length; leading to contraction of [SmMgNi4] and expansion of [SmNi5] relative to those for the individual SmMgNi4 and SmNi5 compounds, respectively (see Figure 3). This constraint of adjacent units may explain the decrease of unit parameters with increasing [SmNi5]/[SmMgNi4] unit ratio. 3.2 Hydrogen Absorption-Desorption Thermodynamics To compare the hydrogen absorption-desorption thermodynamics, the P–C isotherms of activated SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples were measured at different temperatures. From the results shown in Figure 4a–e, the following three features are observable: (i) For Sm3MgNi14 and Sm4MgNi19, only one plateau is visible on each curve at each temperature which suggests that the polymorphic 2H and 3R phases have quite close equilibrium pressures upon hydrogen absorption-desorption although the plateau shows a large sloping characteristic similar to 2H-type (La1.5Mg0.5)Ni7.20 Such a result is in agreement with that of La4MgNi19 as mentioned above.16 Actually, equilibrium pressures of hydrogen absorption and desorption are dependent on unit parameters. Figure 4f presents the correlation of equilibrium pressures in hydriding and dehydriding processes at 298 K with the average unit volume. As expected, the equilibrium pressures are elevated with the decrease of average unit volume due to the increase of [SmNi5]/[SmMgNi4] unit ratio. (ii) SmMgNi4 and SmNi5 have flat plateaus but the layered compounds Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 show sloping ones. Usually the slope can be evaluated by plateau slope factor Sf = ln(P0.75Hmax/P0.25Hmax) where P0.75Hmax and P0.25Hmax are the hydrogen pressures at hydrogen contents with 75% and 25% of maximum hydrogen capacity, respectively.36 Figure 5 compares the plateau slope


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factors of hydriding curves at 298 K, indicating that all the layered compounds have a large Sf value which shows an upward tendency with raising [SmNi5]/[SmMgNi4] unit ratio from 1 to 3. This phenomenon can be explained by the change in inequivalent environments of interstitial sites for hydrogen occupation.37, 38 The previous investigations showed that in layered R−Mg−Ni compounds there exist various interstitial sites with different environments and hydrogen affinities for hydrogen occupation,19−21 and the variation of environments becomes more complicated as unit ratio increases from 1 to 3.18 (iii) The layered compounds Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 present a smaller hysteresis between hydrogen absorption and desorption than SmMgNi4 and SmNi5. If the hysteresis extent is expressed as hysteresis factor Hf = ln(Pa/Pd) (where Pa and Pd are the equilibrium pressures of hydrogen absorption and desorption at hydrogen content with 50% of maximum hydrogen capacity, respectively),36 the difference among these compounds is clearly visible (see Figure 6). Although the cause of hysteresis has not been fully understood and various explanations have been proposed at present,39−45 lattice expansion (leading to an irreversible plastic deformation in the matrix) upon hydriding is generally believed to be of impotance.37, 46 Unfortunately, the lattice expansions of these samples after full hydrogenation are not obtained in this work, however the reported volume expansions of SmMgNi4H4 (15.6%),34 La2MgNi9H13.1 (26.7%),21 La3MgNi14H18.2 (26.3%),20 La4MgNi19H21.8 (25.3%),19 and SmNi5H4 (13.2%),47 can be considered as a reference. If only from the perspective of lattice expansion, evidently, the change in hysteresis as shown in Figure 6 is inconsistent with the reported data. Nevertheless, it should be noted that the layered R−Mg−Ni hydrides retain their own host structures while the orthorhombic SmMgNi4H4 and monoclinic SmNi5H4 do not.19−21, 34, 47 This may be the reason why only Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 comply with the lattice expansion hypothesis that the hysteresis factor decreases with the reduction of volume expansion due to the increase of [SmNi5]/[SmMgNi4] unit ratio from 1 to 3. Figure 7a–e shows the van’t Hoff plots for the SmMgNi4−H2, Sm2MgNi9−H2, Sm3MgNi14−H2,


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Sm4MgNi19−H2 and SmNi5−H2 systems, respectively. From these plots, the enthalpy changes for hydrogen absorption and desorption can be determined (see Figure 7f and Table 3). It can be seen that the hydriding enthalpy (absolute value) decreases with increasing [SmNi5]/[SmMgNi4] unit ratio due to the reduction of unit volume. This result is in accord with Lundin’s theory that the hydrogen absorption-desorption thermodynamics is strongly dependent on structural parameters (interstitial hole size).48 3.3 Hydrogen Capacity and Cycle Stability Figure 8 compares the hydrogen absorption capacities of activated SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples measured at 298 K. It can be seen that SmMgNi4 can absorb hydrogen to form SmMgNi4H3.9 (hydrogen-to-metal ratio, H/M = 0.65) which is consistent with SmMgNi4H4 measured at 273 K.34 For SmNi5, however, only β hydride SmNi5H4.4 (H/M = 0.73) is observed from the P–C isotherm under the present experimental conditions because the γ hydride SmNi5H6 will be formed above 10 MPa at 298 K.47, 49 Relatively the layered compounds have a higher hydrogen capacity (H/M: 1.03 for Sm2MgNi9H12.4, 1.04 for Sm3MgNi14H18.7, and 0.93 for Sm4MgNi19H22.4), which may be related to more interstitial holes suitable for hydrogen occupation caused by the mutual constraints of [SmNi5] and [SmMgNi4] units.18 Interestingly, Sm4MgNi19H22.4 has a slightly smaller hydrogen content than Sm2MgNi9H12.4 and Sm3MgNi14H18.7 due to its lower stability (see Figure 7f), similar to the scenarios as previously reported in the La−Mg−Ni hydrides.19−21 This means that the hydrogen capacity of layered R−Mg−Ni compounds decreases to some extent when the [RNi5]/[RMgNi4] unit ratio increases to 3. The cycle stabilities of activated SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples are presented in Figure 9. After 30 absorption-desorption cycles, the capacity retention rate increases with raising [SmNi5]/[SmMgNi4] unit ratio. Although the complex causes of degradation behavior are not fully understood, capacity degradation is usually linked to the formation of dislocations, occurrence of partial disproportionation or amorphization, and so on during cycles.50−52 To clarify the degradation


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mechanism under present circumstances, the XRD profiles of the dehydrogenated samples after 30 cycles are examined and shown in Figure 10a–e. Obviously, neither disproportionation nor amorphization occurs during the hydrogen absorption-desorption cycles. From the refined XRD profile parameters, the lattice strains caused by the cycles can be extracted (see Figure 10f). It can be seen that the lattice strain gradually decreases with the increase of [SmNi5]/[SmMgNi4] unit ratio. Hence, it seems likely that the formation of dislocations plays a significant role in the capacity degradation because the accumulation of dislocations can lead to the enlargement of lattice strain displayed as broadening of XRD peaks.51, 52 Figure 11 shows the mechanical properties of SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples. It can be seen that elastic modulus E and nanoindentation hardness H increase with raising [SmNi5]/[SmMgNi4] unit ratio, consisting with the theoretical calculation result.22 This implies that the increase of [SmNi5]/[SmMgNi4] unit ratio can enhance dislocation formation energy, which is more favorable for pulverization than introduction of dislocations during hydrogenation.52 Therefore, the cycle stability can be improved by increasing [SmNi5]/[SmMgNi4] unit ratio.

4. CONCLUSIONS A comparative investigation of the structural parameters and hydrogen absorption−desorption properties of Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 (as well as SmMgNi4 and SmNi5 as references) was conducted. The present work shows that the unit parameters of layered Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 compounds decrease with increasing [SmNi5]/[SmMgNi4] unit ratio, leading to the increase of equilibrium pressure, enlargement of slope and reduction of hysteresis for their P–C isotherms. Hence, the hydriding enthalpy varies from −34.4 to −29.5 kJ/mol H2 as the [SmNi5]/[SmMgNi4] unit ratio increases from 1 to 3. The hydrogen capacities of the layered Sm−Mg−Ni compounds at 298 K are larger than those of SmMgNi4 and SmNi5 though the increase of [SmNi5]/[SmMgNi4] unit ratio to 3 leads to a slight decrease of capacity. Furthermore, the increase of [SmNi5]/[SmMgNi4] unit ratio ACS Paragon Plus Environment

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improves cycle stability by enhancing elastic modulus and hardness. These findings provide an important guidance for developing layered R−Mg−Ni compounds as electrode materials for Ni/MH batteries.

AUTHER INFORMATION Corresponding Author *Email: [email protected] (Q.Z.); [email protected] (M.Z.). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 51271002).


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Table 1.

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Sintering Temperatures (K) of Sm− −Mg− −Ni Alloys Sm2MgNi9 Sm3MgNi14 Sm4MgNi19 SmNi5 Alloy SmMgNi4 Step I 723 723 723 723 1223 Step II 1023 1023 1123 1123 Step III 1123 1173 1223 1223

Table 2. Structural Parameters and Phase Abundance for SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19, and SmNi5 Alloys Lattice parameters (Å) Space Abundance Alloy Phase group (wt.%) a c SmMgNi4 Sm2MgNi9

__

SmMgNi4

F43m __

3R-Sm2MgNi9

R3m

3R-Sm3MgNi14 Sm3MgNi14 2H-Sm3MgNi14 3R-Sm3MgNi14 Sm4MgNi19 2H-Sm4MgNi19 3R-Sm4MgNi19 SmNi5

Table 3.

SmNi5 SmNi5 3R-Sm5Ni19

7.0566(1)

100

4.9694(4)

24.253(2)

92

R3m

4.9779(5)

36.207(8)

8

P63/mmc

4.9586(6) 4.9592(6)

24.102(3) 36.131(4)

58 42

4.9501(3) 4.9494(2)

32.020(5) 47.978(4)

33 59

4.9312(4) 4.9309(4) 4.9638(8)

3.9645(4) 3.9646(2) 48.376(2)

8 93 7

__

__

R3m P63/mmc __

R3m P6/mmm P6/mmm __

R3m

Hydriding and Dehydriding Properties for Smn+1MgNi5n+4−H2 Systems System Hydriding Dehydriding Enthalpy Entropy Enthalpy Entropy ACS Paragon Plus Environment

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SmMgNi4−H2 Sm2MgNi9−H2 Sm3MgNi14−H2 Sm4MgNi19−H2 SmNi5−H2

(kJ/mol H2) −38.1 −34.4 −31.4 −29.5 −23.8

(J/K mol H2) −128.5 −120.3 −118.5 −117.6 −114.9

(kJ/mol H2) 37.8 34.2 31.4 29.3 24.6

(J/K mol H2) 128.1 117.5 116.4 115.2 112.9

Figure Captions:

Figure 1. Rietveld refinements of observed XRD patterns for (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5 samples. (f) Structure stacking models for SmMgNi4, 3R-type Sm2MgNi9, 2H- and 3R-type Sm3MgNi14, 2H- and 3R-type Sm4MgNi19, and SmNi5.


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Figure 2. Unit parameters a, c/N and (c/N)/a of SmMgNi4, 3R-type Sm2MgNi9, 2H- and 3R-type Sm3MgNi14, 2H- and 3R-type Sm4MgNi19, and SmNi5.

Figure 3. [SmMgNi4] and [SmNi5] unit volumes as well as the average unit volumes for SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5. The values of 2H- and 3R-Sm3MgNi14 (as well as 2Hand 3R-Sm4MgNi19) are averaged.

Figure 4. P–C isotherms of hydrogen absorption and desorption for (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5 samples. (f) Correlation of the equilibrium pressures in hydriding and dehydriding processes at 298 K with the average unit volume.

Figure 5. Plateau slope factors of hydriding curves at 298 K.

Figure 6. Hysteresis factors on P–C isotherms at 298 K.

Figure 7. van’t Hoff plots for (a) SmMgNi4–H2, (b) Sm2MgNi9–H2, (c) Sm3MgNi14–H2, (d) Sm4MgNi19–H2, and (e) SmNi5–H2 systems. (f) Relationship between the hydriding-dehydriding enthalpy and the average unit volume.


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Figure 8. Hydrogen absorption capacities of activated SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples at 298 K.

Figure 9. Cycle stabilities of activated SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples at 298 K: (a) retention rate and (b) hydrogen amount.

Figure 10. Rietveld refinements of observed XRD patterns for the dehydrogenated (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5 samples after 30 cycles. (f) Lattice strains extracted from the refined XRD profile parameters.

Figure 11. Elastic modulus E and nanoindentation hardness H of (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5.

SYNOPSIS TOC As the [SmNi5]/[SmMgNi4] unit ratio increases, the unit parameters of Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 decrease gradually; leading to the increase of equilibrium pressure, enlargement of slope, and reduction of hysteresis for their P–C isotherms as well as the decline of hydriding enthalpy. Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 have a larger hydrogen capacity than SmMgNi4 and SmNi5 and the cycle stability can be improved by the increase of [SmNi5]/[SmMgNi4] unit ratio.


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Figures


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Figure 1. Rietveld refinements of observed XRD patterns for (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5 samples. (f) Structure stacking models for SmMgNi4, 3R-type Sm2MgNi9, 2H- and 3R-type Sm3MgNi14, 2H- and 3R-type Sm4MgNi19, and SmNi5.


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Figure 2. Unit parameters a, c/N and (c/N)/a of SmMgNi4, 3R-type Sm2MgNi9, 2H- and 3R-type Sm3MgNi14, 2H- and 3R-type Sm4MgNi19, and SmNi5.

Figure 3. [SmMgNi4] and [SmNi5] unit volumes as well as the average unit volumes for SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5. The values of 2H- and 3R-Sm3MgNi14 (as well as 2Hand 3R-Sm4MgNi19) are averaged.


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Figure 4. P–C isotherms of hydrogen absorption and desorption for (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5 samples. (f) Correlation of the equilibrium pressures in hydriding and dehydriding processes at 298 K with the average unit volume.


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Figure 5. Plateau slope factors of hydriding curves at 298 K.

Figure 6. Hysteresis factors on P–C isotherms at 298 K.


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Figure 7. van’t Hoff plots for (a) SmMgNi4–H2, (b) Sm2MgNi9–H2, (c) Sm3MgNi14–H2, (d) Sm4MgNi19–H2, and (e) SmNi5–H2 systems. (f) Relationship between the hydriding-dehydriding enthalpy and the average unit volume.


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Figure 8. Hydrogen absorption capacities of activated SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples at 298 K.

Figure 9. Cycle stabilities of activated SmMgNi4, Sm2MgNi9, Sm3MgNi14, Sm4MgNi19 and SmNi5 samples at 298 K: (a) retention rate and (b) hydrogen amount.


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Figure 10. Rietveld refinements of observed XRD patterns for the dehydrogenated (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5 samples after 30 cycles. (f) Lattice strains extracted from the refined XRD profile parameters.


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Figure 11. Elastic modulus E and nanoindentation hardness H of (a) SmMgNi4, (b) Sm2MgNi9, (c) Sm3MgNi14, (d) Sm4MgNi19, and (e) SmNi5.

SYNOPSIS TOC As the [SmNi5]/[SmMgNi4] unit ratio increases, the unit parameters of Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 decrease gradually; leading to the increase of equilibrium pressure, enlargement of slope, and reduction of hysteresis for their P–C isotherms as well as the decline of hydriding enthalpy. Sm2MgNi9, Sm3MgNi14 and Sm4MgNi19 have a larger hydrogen capacity than SmMgNi4 and SmNi5 and the cycle stability can be improved by the increase of [SmNi5]/[SmMgNi4] unit ratio.


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Comparative Investigations on Hydrogen AbsorptionâDesorption

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