In Situ Embedding of Mg2NiH4 and YH3 Nanoparticles into Bimetallic

Article pubs.acs.org/JPCC

In Situ Embedding of Mg2NiH4 and YH3 Nanoparticles into Bimetallic Hydride NaMgH3 to Inhibit Phase Segregation for Enhanced Hydrogen Storage Yongtao Li,† Luxing Zhang,† Qingan Zhang,*,† Fang Fang,‡ Dalin Sun,‡ Kongzhai Li,§ Hua Wang,§ Liuzhang Ouyang,∥ and Min Zhu*,∥ †

School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China Department of Materials Science, Fudan University, Shanghai 200433, China § State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming 650093, China ∥ School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China ‡

S Supporting Information *

ABSTRACT: For the first time, component segregation during cycling was demonstrated to be responsible for the deterioration in the kinetics of bimetallic hydride NaMgH3. To solve this problem, we propose a method involving in situ embedding of Mg2NiH4 and YH3 into NaMgH3, which is accomplished with a solid-phase reaction using amorphous Mg12YNi alloy and NaH as starting materials under a H2 atmosphere. Using this novel method, 12 wt % of the Mg2NiH4 phase and 11 wt % of the YH3 phase were homogeneously embedded in the NaMgH3 matrix. In comparison to those of pure NaMgH3, this composite material exhibits no change in thermodynamics but shows greatly enhanced kinetics for hydrogen absorption and desorption cycling. In addition, reasonable mechanisms for the enhanced kinetics have been proposed including the prevention of macroscopic segregation of metallic Na, grain refinement, and a synergistic catalytic effect. All of these mechanisms rely on the intimately interdispersed Mg2NiH4 and YH3 nanoparticles embedded in the NaMgH3 matrix.

1. INTRODUCTION Magnesium (Mg)-based bimetallic hydrides, which are formed via hydrogenation of Mg alloyed with alkali metals,1−3 transition metals,4−7 and rare earth metals,8,9 are considered to be promising materials for solid-state hydrogen storage due to two aspects as follows: (i) the ternary Mg-based hydrides have a high hydrogen gravimetric capacity (>4 wt %),6−9 and (ii) a stable hydride is most likely formed through reversible reactions under a relatively low hydrogen pressure. These properties are superior to those of the H-containing alanate complexes, such as NaAlH4, LiAlH4, and Mg(AlH4)2, due to limited reversibility under rigorous pressure conditions (>10 MPa).10−16 As a prototype of Mg-based bimetallic hydrides, NaMgH3, which has a perovskite-type structure, can absorb and desorb hydrogen through the following two steps: NaMgH3 ↔ NaH + Mg + H 2 (1) NaH ↔ Na + 1/2H 2

hydrogen sorption thermodynamics of NaMgH3 by combining in situ synchrotron X-ray diffraction and high-pressure differential calorimetry with theoretical calculations, and determined that the temperature of the first decomposition reaction is actually higher than the predicted value. This temperature discrepancy strongly suggests the slow kinetics of the desorption reaction 1, which conflicts with the expected fast kinetics due to the high H hopping mobility related to its perovskite structure.18,19 More recently, Sheppard et al.20 noted that, as a solar heat storage medium, NaMgH3 has several thermodynamic advantages, such as a large enthalpy value, wide plateau, and negligible hysteresis. However, NaMgH3 exhibits continuous kinetic degradation upon multicycling. The underlying mechanism is not well understood, but it has been suggested to be related to the liquid Na formation in reaction 2,17,20 restricts the utilization of reaction 2, and limits the efficient capacity of the whole system. Therefore, an approach is needed that promotes the kinetics for reaction 1 and prevents liquid Na migration for reaction 2 to improve the hydrogen storage efficiency of NaMgH3. To the best of our knowledge,

(2)

Corresponding to the two reactions, two plateaus at approximately 0.15 and 0.04 MPa were observed in the P−C isotherms at 400 °C by Orimo et al.,2 which indicates that the hydrogen released from NaMgH3 only occurs above 371 °C at 0.1 MPa. This high thermal stability renders it impractical for low-temperature usage. Baricco et al.17 further studied the © 2014 American Chemical Society

Received: August 19, 2014 Revised: September 25, 2014 Published: September 26, 2014 23635

dx.doi.org/10.1021/jp508395s | J. Phys. Chem. C 2014, 118, 23635−23644

The Journal of Physical Chemistry C

Article

Aldrich) as starting materials. The preparation procedure for NaMgH3 involves mechanical milling of the NaH and MgH2 mixture in a 1:1 molar ratio at ambient temperature under an argon atmosphere for 10 h, followed by isothermal treatment at 350 °C under a hydrogen pressure of 5 MPa for 24 h.32 The asprepared NaMgH3 was characterized using powder X-ray diffraction (XRD), which indicated good purity (see Figure 1).

no studies have reported on the limited kinetics in the sorption of NaMgH3, and the kinetic enhancements have not yet been addressed. Therefore, studies of the kinetic improvements in NaMgH3 are needed from a fundamental perspective as well as for potential application. Previous studies on other chemical hydrides have shown that the in situ formation of active metal hydride nanoparticles has been demonstrated to be a promising approach for improving hydrogen sorption kinetics.21−36 For example, Zhou et al.30 and Liu et al.31 respectively reported that the reversible storage properties of a complex hydride (LiBH4) under relatively milder conditions (below 400 °C and 4 MPa) were substantially improved by in situ formation of MgH2 and LaH3 (or Al). Sun et al.32 noted that an enhanced cycling stability in LiNH2 was achieved by a high capacity retention of approximately 75% after four cycles, which was due to the in situ formation of NaH and MgH2 that served as a “carrier” for effectively conveying N-containing species. Our recent studies further indicated that the hydrogen absorption and desorption kinetics of MgH2 can be significantly enhanced by in situ introduction of Mg2NiH4 and YH3 nanoparticles.33−36 Similar to MgH2, NaMgH3 has a near ideal configuration where the six H atoms around the Mg form a MgH6 octahedron, and the Mg−H bonds are primarily covalent bonds in the MgH6 octahedron.1,37,38 This common structural feature leads us to believe that the poor sorption kinetics of NaMgH3 can be promoted by the in situ embedded Mg2NiH4 and YH3 nanoparticles. In addition, these highly dispersed hydrides may act not only as active species for facilitating hydrogen dissociation and diffusion in the Mg matrix but also as preferential sites in the liquid Na phase for stimulating heterogeneous nucleation and/or as the second-phase particles exerting the “pinning effect” on the solid Na migration. Therefore, studies of the hydrogen absorption and desorption of NaMgH3 with in situ embedded Mg2NiH4 and YH3 nanoparticles have been performed. First, the NaMgH3 with in situ embedded Mg2NiH4 and YH3 nanoparticles was designed and prepared in two consecutive steps as follows: (i) an amorphous Mg-rich Mg−Y−Ni precursor was selected and synthesized by combining induction melting with remelt spinning, which allows Y and Ni to be homogeneously dissolved in the Mg matrix. This step is crucial for obtaining a high dispersion of in situ embedded Mg2NiH4 and YH3; (ii) then, the obtained amorphous Mg−Y−Ni precursor was reacted with NaH via solid-state reaction under a H2 atmosphere to form Mg2NiH4 and YH3 nanoparticles in situ, and these nanoparticles were embedded into the NaMgH3 matrix. To validate the current study, the pure NaMgH3 was prepared by hydrogenation of MgH2 and NaH and used as a reference. Next, a systematic study of the difference in the sorption kinetics of NaMgH3 originating from two hydrogenation samples of MgH2 + NaH (i.e., pure NaMgH3) and Mg12YNi + NaH (i.e., NaMgH3 with in situ embedded Mg2NiH4 and YH3) was performed. Finally, the structural features and element distributions of their resulting products were examined to understand the underlying mechanism responsible for the differences in hydrogen absorption and desorption cycling.

Figure 1. XRD patterns for melt-spun Mg12YNi (a), in situ embedded NaMgH3 (b), and pure NaMgH3 (c).

2.2. Preparation of NaMgH3 with in Situ Embedded Mg2NiH4 and YH3. The formation of NaMgH3 with in situ embedded Mg2NiH4 and YH3 (hereafter denoted as in situ embedded NaMgH3) is accomplished in two consecutive steps. First, the Mg12YNi alloy ingots were prepared by induction melting appropriate amounts of a mixture of pure Mg (99.9% purity) and an intermediate YNi alloy under an argon atmosphere (approximately 0.1 MPa). The as-cast alloy was remelted and quenched by melt spinning with a copper roller at a constant rotating surface velocity of 40 m/s. The melt-spun Mg12YNi alloy consists of long, continuous ribbons that are approximately 3 mm wide and 30−50 μm thick. An extra ∼18 wt % Mg was added to compensate for the loss of Mg during induction melting and melt-spinning. Then, as for the pure NaMgH3, the in situ embedded NaMgH3 was also prepared with a solid-phase reaction method according to reaction 3 using Mg12YNi and NaH (95% purity) as starting materials. The preparation procedure involves mechanical milling of the Mg12YNi and NaH mixture in a 1:1 molar ratio at ambient temperature under an argon atmosphere for 10 h followed by isothermal treatment at 350 °C under a hydrogen pressure of 5 MPa for 24 h. The quality of this in situ embedded NaMgH3 was characterized using XRD, which indicated good purity (see Figure 1). Mg12YNi + 10NaH + 27/2H 2 ↔ 10NaMgH3 + Mg 2NiH4 + YH3

(3)

2.3. Characterizations. To investigate the phase components and structure of the as-prepared samples, X-ray diffraction (XRD) was carried out on a Rigaku D/max 2400 with Cu Kα radiation at 50 kV and 30 mA with further analysis using a Rietveld refinement program (RIETAN-2000).39 The XRD sample was only handled in an Ar-filled glovebox, and the surface of the samples was covered with Scotch tape to prevent any possible reaction with water and oxygen during measure-

2. EXPERIMENTAL SECTION 2.1. Preparation of Pure NaMgH3. The bimetallic hydride NaMgH3 was prepared with a solid-phase reaction using NaH (95% purity, Sigma-Aldrich) and MgH2 (98% purity, Sigma23636



Article

Figure 2. SEM image (a) and corresponding EDX maps (b) of the Na, Mg, Ni, and Y elements for the selected area, as well as TEM micrographs (c), EDX spectra (d), SAED pattern (e), and magnified HRTEM image (f) for the in situ embedded NaMgH3 sample.

Hydrogen desorption at 300, 330, 350, and 400 °C was successively performed against a back pressure of 10 Pa. 2.5. Multiple Cycling Tests. The changes in the kinetics of multiple hydrogen absorption/desorption cycling were studied using Sieverts-type measurements. Approximately 0.5 g of sample was sealed in the autoclave and rapidly heated to 350 or 400 °C, which was maintained at this temperature for hydrogen release. After completion, the sample was pressurized with 5 MPa of H2 for 10 h for hydrogen absorption. The same hydrogen absorption/desorption procedure was repeated several times to examine the cycling performance.

ments. To examine the microstructure and elemental distribution in the in situ embedded NaMgH3 sample before and after multiple cycling, high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) observations were carried out on JEOL JEM-2100 F and Shimadzu SUPERSCAN SSX-550 instruments equipped with an energy dispersive X-ray spectrometer (EDX), respectively. The HRTEM samples were prepared by dispersion in a dried THF solvent followed by spreading on a holey carbon film supported on a copper grid. The thermal desorption behaviors of the as-prepared samples were studied using mass spectroscopy (MS) on a Netzsch STA 409 PC with a ramping rate of 10 °C/min under a flowing Ar atmosphere. All of the samples were moved into the equipment with a device that maintained an Ar overpressure during the transfer process. 2.4. Hydrogen Desorption Kinetic Measurements. The desorption kinetics were measured using an automated Sieverts-type apparatus which allowed for the accurate determination of the amount of evolved hydrogen. Typically, an approximately 0.5 g sample was loaded into a stainless-steel autoclave that was evacuated. Rapid heating of the sample to the desired temperatures was accomplished by immersing the autoclave in a silicon oil bath preheated to a given temperature.

3. RESULTS AND DISCUSSION 3.1. Structural and Morphological Features of NaMgH3 with in Situ Embedded Mg2NiH4 and YH3. The XRD patterns of the melt-spun Mg12YNi, in situ embedded NaMgH3 (i.e., the hydrogenated sample of Mg12YNi and NaH), and pure NaMgH3 are compared in Figure 1. For the melt-spun Mg12YNi (Figure 1a), the hump in the pattern reflects its amorphous state. This disordered structure facilitates the homogeneous dissolution of the Y and Ni elements in the Mg matrix. For the in situ embedded NaMgH3 (Figure 1b), sharp and intense peaks are observed and can be indexed to the NaMgH3 phase, which is consistent with pure NaMgH3 (Figure 23637



Article

Figure 3. Rietveld refinement of the observed XRD patterns for (a) pure NaMgH3 and (b) in situ embedded NaMgH3 before and after hydrogen desorption/absorption at 350 °C. The gray lines at the bottom show the disagreement between the refined and observed patterns.

observed, implying that the pure NaMgH3 sample does not completely decompose within 1 h at 350 °C. In contrast, for the in situ embedded NaMgH3, the peaks corresponding to the NaMgH3 and NaH phases were not detected, and only the peaks corresponding to Na and Mg were observed, which strongly suggests that the slow desorption of NaMgH3 is greatly improved by in situ embedding of the Mg2NiH4 and YH3 nanoparticles. In addition, YH3 and Mg2NiH4 decomposed into YH2 and Mg2Ni upon release of H2. After dehydrogenation, these two dehydrogenated samples were rehydrogenated at 350 °C and 4 MPa of H2 for 10 h. For the pure NaMgH3 sample, the residual Mg (10%) and NaH (5%) phases were observed, and the regenerated NaMgH3 phase was only in 78% abundance, which is much less than the initial abundance of 100%, suggesting that the absorption kinetics were also slow. In contrast, for the in situ embedded NaMgH3 sample, the dehydrogenated products consisting of Na and Mg phases can be nearly completely converted back to NaMgH3 (the capacity retention is close to 96%), and this result is supported by the refined results shown in Table S1 in the Supporting Information. The remaining 2% MgH2 is most likely due to the volatilization of liquid Na (melting at ∼97 °C) during heating at 350 °C. The difference in the phase changes strongly suggests that both the hydrogen desorption and absorption of NaMgH3 are promoted by in situ embedding of Mg2NiH4 and YH3 nanoparticles. To investigate the promotion of hydrogen absorption and desorption, the thermodynamic characteristics of in situ embedded NaMgH3 sample are compared to those of pure NaMgH3. 3.3. Unaltered Thermodynamics for in Situ Embedded NaMgH3 Compared to Pure NaMgH3. The P−C isotherms for the pure NaMgH3 and in situ embedded NaMgH3 systems at 400 °C are shown in Figure 4. For pure NaMgH3, two plateaus were observed where the lower one at ∼0.04 MPa is due to Na + H2 ↔ NaH and the higher one at ∼0.16 MPa corresponds to NaH + Mg + H2 ↔ NaMgH3, which is consistent with the reported values of approximately 0.04 and 0.15 MPa, respectively.2 The in situ embedded NaMgH3 sample exhibited negligible changes including for the pressure and slope of these two plateaus except for an additional plateau at ∼2.28 MPa for absorption and ∼1.38 MPa for desorption, which is due to the reversible reaction Mg2Ni + H2 ↔ Mg2NiH4 (from the in situ embedded Mg2NiH4). These features clearly indicate that the in situ embedding of Mg2NiH4

1c). However, the peaks from the Mg2NiH4 and YH3 phases are relatively broader and weaker, to some extent, which suggests that these in situ formed hydrides are dispersed in fine crystallites in the NaMgH3 matrix. Figure 2 shows the surface morphology, microstructure, and corresponding element distributions for the in situ embedded NaMgH3 sample. The SEM image of in situ embedded NaMgH3 shows coarse aggregates consisting of submicrometer fine particles (see Figure 2a). The corresponding Na and Mg maps are consistent with the morphology of the aggregates, as marked by the dashed square, and the relatively scattered Ni and Y morphologies are in good agreement with the dense Na and Mg maps (see Figure 2b). These features confirm that the in situ formed Mg2NiH4 and YH3 are well-dispersed inside the NaMgH3 matrix. In addition, the representative TEM images along with the corresponding EDX analysis and SAED pattern are shown in Figure 2c−e, which indicates a nearly homogeneous state without clear phase boundaries between the second phases and the matrix even under magnification on the 50 nm scale. The EDX spectra indicate that the composites are composed of Na, Mg, Y, and Ni elements, and the C, O, and Cu elements originate from the testing copper grid supported holey carbon film. The clear rings that are composed of discrete spots in the SAED pattern further demonstrate the coexistence of the NaMgH3, Mg2NiH4, and YH3 phases, which is consistent with the obtained XRD results (Figure 1b). In addition, the HRTEM image (Figure 2f) shows that the YH3 and Mg2NiH4 phases are very fine-grained nanocrystals that are approximately 7 and 10 nm in size, respectively. These results confirm that the in situ formed Mg2NiH4 and YH3 are well embedded into the NaMgH3 matrix. 3.2. Comparison of Phase Evolution between in Situ Embedded NaMgH3 and Pure NaMgH3 upon Hydrogen Absorption/Desorption. Figure 3 shows the differences in the phase components for the in situ embedded NaMgH3 and pure NaMgH3 samples before and after hydrogen desorption/ absorption, and their phase abundance, which was calculated using Rietveld refinements, is summarized in Table S1 in the Supporting Information. In comparison to pure NaMgH3, the extra peaks for the Mg2NiH4 (12%) and YH3 (11%) phases were observed in the in situ embedded NaMgH3. After dehydrogenation at 350 °C, the major peaks for the pure NaMgH3 sample were indexed to the NaH, Na, and Mg phases. However, some peaks from the NaMgH3 phase (9%) were still 23638



Article

liquid Na at higher temperature. (ii) For pure NaMgH3, the kinetics of absorption/desorption continuously decay with the cycling number. In contrast, the in situ embedded NaMgH3 sample exhibited better kinetics with negligible reduction. Such extraordinary results strongly suggest that the kinetic enhancement by in situ embedded Mg2NiH4 and YH3 can be maintained even upon multiple cycling. In comparison to hydrogen absorption, the desorption process exhibited much faster kinetics. Therefore, dehydrogenation was selected to quantitatively elucidate the kinetic difference between pure NaMgH3 and in situ embedded NaMgH3. The isothermal dehydrogenation profiles for the in situ embedded NaMgH3 at 300, 330, 350, and 400 °C and pure NaMgH3 at 330, 350, and 400 °C are compared in Figure 7a and b. The in situ embedded NaMgH3 sample released ∼4.1 wt % H2 in 5 min at 400 °C. In contrast, the pure NaMgH3 sample only released ∼3.0 wt % H2 under the same conditions. At 350 °C, the in situ embedded NaMgH3 sample desorbed ∼4.0 wt % H2 in 30 min but the pure NaMgH3 sample only desorbed ∼2.1 wt % H2 in the same period. More importantly, the in situ embedded NaMgH3 sample exhibited good performance at even lower temperature. This sample released ∼4.0 and 1.2 wt % H2 in 60 min at 330 and 300 °C, respectively. However, the pure NaMgH3 sample did not release any hydrogen at 300 °C (not shown here). These results clearly demonstrate that the dehydrogenation of NaMgH3 is kinetically promoted by in situ embedding of Mg2NiH4 and YH3. The improved dehydrogenation kinetics by in situ embedding of Mg2NiH4 and YH3 can be quantitatively evaluated by determining the activation energy (Ea) for hydrogen release. The activation energy (E a ) for hydrogen release was determined using the JMAK (Johnson−Mehl−Avrami−Kolmogorov) model and Arrhenius analysis.40 On the basis of the JMAK model, the hydrogen release kinetics can be expressed by the following equation:

Figure 4. P−C isotherms for (a) pure NaMgH3 and (b) in situ embedded NaMgH3 systems at 400 °C.

and YH3 does not alter the thermodynamics of NaMgH3; i.e., the enhancements in the absorption/desorption, which are shown in Figure 3, may be caused by kinetics and not by thermodynamics. 3.4. Kinetic Advantage of the in Situ Embedded NaMgH3 over Pure NaMgH3. The cycling curves for the in situ embedded NaMgH3 and pure NaMgH3 at 350 °C for 1000 min are compared in Figure 5. Two notable features were

ln[− ln(1 − α)] = η ln k + η ln t

(4)

where k is the rate constant, α is the reaction fraction transforming from 0 to 1 corresponding to the beginning and completion of the reaction, η is the Avrami exponent of the reaction order, and t is the time. For the experimental sample data, the plot of ln[−ln(1 − α)] as a function of ln(t) was linear for each curve at the various temperatures, as shown in Figure 7c and d. After calculating the rate constant k, the activation energy (Ea) for the hydrogen desorption process can be evaluated from the Arrhenius equation:

Figure 5. Reversible H2 absorption and desorption curves of (a) pure NaMgH3 and (b) in situ embedded NaMgH3 samples at 350 °C within 1000 min. The deep green shadows correspond to the desorption process.

k = k 0e−Ea / RT

observed as follows: (i) For both samples, the absorption/ desorption kinetics of the first cycle is relatively slower than that of the second one. In addition, the absorption/desorption kinetics from the second to the third cycle remains nearly constant, indicating that the first cycle acts as an activation step. (ii) Within 1000 min, two and a half cycles were performed for pure NaMgH3. In contrast, three dehydrogenation/rehydrogenation cycles were completed for in situ embedded NaMgH3 without capacity degradation. This result clearly demonstrates that the kinetics is significantly enhanced by in situ embedding of Mg2NiH4 and YH3 nanoparticles. When the temperature was increased to 400 °C, some additional cycling features were observed, as shown in Figure 6. (i) The capacity degradation for the two samples was substantial compared to that at 350 °C, which may correspond to the loss of active products due to the rapid volatilization of

(5)

where k0 is a temperature-independent coefficient, R is the gas constant (8.314 J/mol/K), and T is the absolute temperature. The linear plots of ln(k) as a function of 1000/T for the hydrogen desorption of in situ embedded NaMgH3 and pure NaMgH3 are shown in Figure 7e. From the slopes (−Ea/R) of the straight lines, the Ea was determined to be 219.3 ± 7.8 kJ/ mol for the in situ embedded NaMgH3, which is 59 kJ/mol less than that for the pure NaMgH3 (i.e., 278.6 ± 4.9 kJ/mol). 3.5. Distinct Cycling-Induced Morphology and Element Distribution between in Situ Embedded NaMgH3 and Pure NaMgH3. Figure 8a−d shows the morphologies with different magnifications and element distributions for selected areas of pure NaMgH3 after the fifth cycle, from which one can see that (i) a typical remelting characteristic was observed where the lamellar particles with contractive edges 23639



Article

Figure 6. Time dependence of desorption and absorption curves of the (a, b) pure NaMgH3 and (c, d) in situ embedded NaMgH3 samples at 400 °C.

Figure 7. Top: Isothermal dehydrogenation curves for (a) pure NaMgH3 and (b) in situ embedded NaMgH3 at different temperatures. Bottom: Plots of ln(−ln(1 − α)) vs ln(t) for the dehydrogenation of (c) pure NaMgH3 and (d) in situ embedded NaMgH3 and (e) the plots of ln k vs 1000/ T for both samples.

stack with each other. It is important to note that each disc has a hump core (marked by a dashed square (1) in Figure 8a,b),

strongly suggesting that the resulting products (i.e., Na and Mg) may agglomerate into heterogeneous domains. (ii) For the 23640



Article

Figure 8. Left: SEM images at different magnifications for pure NaMgH3 (a, b) and in situ embedded NaMgH3 (e, f) after the fifth cycle. Right: Enlarged images (c, d, g) as well as corresponding EDX spectra and elemental mapping for the selected areas, which are marked with dashed squares.

comparison to pure NaMgH3, two distinct characteristics have been demonstrated for the desorbed products of the in situ embedded NaMgH3 samples from section 3.5 as follows: (i) a much smaller particle size of the resulting products (Na and Mg) and (ii) a well-maintained distribution after five cycles with no detectable phase segregation. To further investigate the state of the absorbed products, the microstructure of the in situ embedded NaMgH3 after the fifth hydrogen absorption was examined using HRTEM. The results in Figure 9a indicate that the fine black particles that were homogeneously dispersed in the gray matrix were maintained even after multiple cycling. The matrix was determined to consist of the NaMgH3 phase (see Figure 9b), and the Mg2NiH4 and YH3 with a size of ∼10 nm remain in the NaMgH3 phase (see Figure 9c). In addition, the NaMgH3 nanoparticle is locally surrounded by YH3 nanoparticles (see Figure 9d), which provides evidence for the inhibition of the grain growth and/or pinning phase segregation of NaMgH3 upon hydrogen absorption/desorption. Therefore, the particle aggregation of the resulting products from the decomposition of NaMgH3 was effectively prevented by the in situ embedded Mg2NiH4 and YH3, which explains the faster kinetics observed for the in situ embedded NaMgH3 sample. (b). Synergistic Catalytic Effects of in Situ Embedded Mg2NiH4 and YH3. To gain insight into the role of in situ formed Mg2NiH4 and YH3 in property improvements, the thermal decomposition behavior of the in situ embedded NaMgH3 sample was compared to the behaviors of individual Mg2NiH4 and/or YH3 doped with NaMgH3. Pure NaMgH3 starts to release hydrogen at approximately 360 °C and exhibits a broad peak corresponding to hydrogen at approximately 406 °C (see Figure S1a, Supporting Information). Mg2NiH4-doped, YH3-doped, and Mg2NiH4/YH3-codoped NaMgH3 exhibit similar traces and a corresponding onset and peak shift to lower temperatures (i.e., approximately 350 and 390 °C, respectively) (see Figure S1b−d, Supporting Information). In contrast, both the onset and peak temperatures of the in situ embedded NaMgH3 composite were significantly reduced to approximately 330 and 372 °C, respectively, which are lower

selected area (1) from pure NaMgH3, only one peak for the Na element was detected in the EDX pattern (see Figure 8c), and the Mg peak with a small shoulder for the Na element was observed for the selected area (2) (see Figure 8d). These distinct elemental distributions strongly indicate that phase segregation occurs between the resulting Na and Mg. In particular, Na grows to approximately 3 μm. This phase segregation explains why the kinetics and cycling properties deteriorate for pure NaMgH3, as shown in Figure 6. In contrast, two features were obtained for in situ embedded NaMgH3 samples after the fifth cycle, as shown in Figure 8e−g. (i) The resulting products (i.e., Na and Mg) from the in situ embedded NaMgH3 sample exhibit a much smaller particle size (see Figure 8e). At increased magnification, these significant differences become more obvious where pure NaMgH3 exhibits larger and continuous aggregation (see Figure 8b) but the submicrometer and even nanometer particles can be easily observed in in situ embedded NaMgH3 (see Figure 8f). (ii) From EDX analysis (see Figure 8g), both peaks for the Na and Mg elements are strong, but changes in the elemental distribution were not observed. Therefore, the phase segregation phenomena do not occur in the in situ embedded sample after five cycles. This significant difference strongly suggests the positive effect of in situ embedded Mg2NiH4 and YH3 inhibiting phase segregation. 3.6. Roles of in Situ Embedded Mg2NiH4 and YH3 Nanoparticles. On the basis of the results mentioned above, it is reasonable to deduce that, in the in situ embedded NaMgH3 system, the in situ embedded Mg2NiH4 and YH3 may serve as heterogeneous nucleation sites in the liquid Na phase for grain refinement and/or the second-phase particles for pinning the metallic Na phase segregation as well as chemical catalysts that favor hydrogen dissociation and diffusion in the NaMgH3 matrix. Therefore, the improved performance may result from the synergistic effect of both grain refinement and chemical catalysis. This effect was experimentally validated and is discussed below. (a). Prevention of Particle Agglomeration and Phase Segregation by in Situ Embedded Mg2NiH4 and YH3. In 23641



Article

Figure 9. TEM images of the in situ embedded NaMgH3 samples after the fifth cycle: (a) low magnification and HRTEM images for the (b) NaMgH3 matrix, (c) Mg2NiH4 and YH3 embedded in NaMgH3 matrix, and (d) the surrounding NaMgH3 nanoparticle.

°C) and Mg phases are created from the dehydrogenated NaMgH3. (ii) The in situ embedded Mg2NiH4 and YH3 not only provide more active sites and lower energy barriers for the heterogeneous nucleation of metallic phases (especially for the liquid Na phase) but also act as second-phase particles for restricting grain growth and pinning the Na phase segregation. Therefore, the phase segregation is effectively inhibited, and the desorption kinetics are substantially improved in contrast to the results for pure NaMgH3, which exhibits Na macroscopic segregation and sluggish kinetics. Vice versa, the enhanced absorption kinetics that was observed may be attributed to similar reasons. In this work, the kinetics of NaMgH3 was indeed improved by the method of in situ embedding of hydrides, but it still has a high thermodynamic stability that needs high temperature to release hydrogen. To make it close to practical materials, we further discuss two possible methods for thermodynamic destabilization as follows: (i) introducing indium (In) into Mg to form Mg(In) solid solution,29 which may decrease the enthalpy change of eq 1 (i.e., the decomposition of MgH2), and

than the values of the pristine sample by 30 and 34 °C, respectively (see Figure S1e, Supporting Information). These significant differences strongly suggest that the enhanced dehydrogenation is caused by the synergistic effects of in situ embedded Mg2NiH4 and YH3. The advantage of in situ embedded sample over the coaddition of Mg2NiH4 and YH3 is most likely due to the in situ formed Mg2NiH4 and YH3 being highly active and well dispersed in the NaMgH3 matrix. On the basis of these results, a possible mechanism for the enhanced kinetic behaviors of in situ embedded NaMgH3 is proposed and schematically shown in Figure 10. For the in situ embedded NaMgH3 system, the enhancements in the hydrogen desorption process can be understood as follows: (i) The in situ embedded Mg2NiH4 and YH3 act as catalysts to reduce the high activation energy barrier for the diffusion of hydrogen atoms through the hydride to the interfaces and the recombination of hydrogen atoms to hydrogen molecules, even though the thermodynamic enthalpy (ΔEendo) does not change according to the unaltered plateaus shown in Figure 4. Accompanying the release of H2, the liquid Na (melting point of approximately 97 23642



■

Article

AUTHOR INFORMATION

Corresponding Authors

*Phone and Fax: +86-555-2311 570. E-mail: qazhang@ahut. edu.cn. *Phone and Fax: +86-555-2311 570. E-mail: memzhu@scut. edu.cn. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51271002, 51301002), the Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, and the Key Laboratory of Clean Energy Materials of Guangdong Higher Education Institute under Grant No. KLB11003.

■

(1) Wu, H.; Zhou, W.; Udovic, T. J.; Rush, J. J.; Yildirim, T. Crystal Chemistry of Perovskite-Type Hydride NaMgH3: Implications for Hydrogen Storage. Chem. Mater. 2008, 20, 2335−2342. (2) Ikeda, K.; Kogure, Y.; Nakamori, Y.; Orimo, S. Reversible Hydriding and Dehydriding Reactions of Perovskite-type Hydride NaMgH3. Scr. Mater. 2005, 53, 319−322. (3) Wu, H.; Zhou, W.; Udovic, T.; Rush, J.; Yildirim, T. Crystal Chemistry and Dehydrogenation/ Rehydrogenation Properties of Perovskite Hydrides RbMgH3 and RbCaH3. J. Phys. Chem. C 2009, 113, 15091−15098. (4) Zhang, J.; Cuevas, F.; Zaïdi, W.; Bonnet, J.; Aymard, L.; Bobet, J.; Latroche, M. Highlighting of a Single Reaction Path during Reactive Ball Milling of Mg and TM by Quantitative H2 Sorption Analysis To Form Ternary Complex Hydrides (TM = Fe, Co, Ni). J. Phys. Chem. C 2011, 115, 4971−4979. (5) Zhang, X.; Yang, R.; Qu, J.; Zhao, W.; Xie, L.; Tian, W.; Li, X. The Synthesis and Hydrogen Storage Properties of Pure Nanostructured Mg2FeH6. Nanotechnology 2010, 21, 095706. (6) Danaie, M.; Asselli, A.; Huot, J.; Botton, G. Formation of the Ternary Complex Hydride Mg2FeH6 from Magnesium Hydride (βMgH2) and Iron: An Electron Microscopy and Energy-Loss Spectroscopy Study. J. Phys. Chem. C 2012, 116, 25701−25714. (7) Manivasagam, T.; Magusin, P.; Srinivasan, S.; Krishnan, G.; Kooi, B.; Notten, P. Electrochemical Deuteration of Metastable MgTi Alloys: An Effective Way to Inhibit Phase Segregation. Adv. Energy Mater. 2014, 4, 1300590. (8) Ouyang, L.; Huang, J.; Fang, C.; Zhang, Q.; Sun, D.; Zhu, M. The Controllable Hydrolysis Rate of LaMg12 Hydride. Int. J. Hydrogen Energy 2012, 37, 12358−12364. (9) Ouyang, L.; Yang, X.; Dong, H.; Zhu, M. Structure and Hydrogen Storage Properties of Mg3Pr and Mg3PrNi0.1 Alloys. Scr. Mater. 2009, 61, 339−342. (10) Bogdanović, B.; Schwickardi, M. Ti-doped Alkali Metal Aluminium Hydrides as Potential Novel Reversible Hydrogen Storage Materials. J. Alloys Compd. 1997, 253−254, 1−9. (11) Orimo, S.; Nakamori, Y.; Jennifer, R. E.; Züttel, A.; Jensen, C. M. Complex Hydrides for Hydrogen Storage. Chem. Rev. 2007, 107, 4111−4132. (12) Zheng, S.; Fang, F.; Zhou, G.; Chen, G.; Ouyang, L.; Zhu, M.; Sun, D. Hydrogen Storage Properties of Space-Confined NaAlH4 Nanoparticles in Ordered Mesoporous Silica. Chem. Mater. 2008, 20, 3954−3958. (13) Li, Y.; Zhou, G.; Fang, F.; Yua, X.; Zhang, Q.; Ouyang, L.; Zhu, M.; Sun, D. De-/re-hydrogenation Features of NaAlH4 Confined Exclusively in Nanopores. Acta Mater. 2011, 59, 1829−1838. (14) Liu, X.; Langmi, H.; Beattie, S.; Azenwi, F.; McGrady, G.; Jensen, C. M. Ti-Doped LiAlH4 for Hydrogen Storage: Synthesis, Catalyst Loading and Cycling Performance. J. Am. Chem. Soc. 2011, 133 (39), 15593−15597.

Figure 10. Schematic illustration of the kinetic enhanced mechanism for the hydrogen absorption/desorption of the in situ embedded NaMgH3 system.

(ii) physical limitation at a nanoscale that destabilizes the hydrides by increasing the surface energy and H atom migration.12,13 In this regard, further investigation on the thermodynamics of NaMgH3 with embedded Mg2NiH4 and YH3 nanoparticles is underway in our laboratories.

4. CONCLUSIONS In the current study, NaMgH3 with homogeneously embedded Mg2NiH4 and YH3 nanoparticles was obtained via a solid-phase reaction of an amorphous Mg12YNi alloy with NaH under a H2 atmosphere. The results of systematic studies of hydrogen absorption and desorption cycling indicate that significant improvements in the kinetics and cycling stability were observed in the in situ embedded NaMgH3 sample compared to pure NaMaH3. These results are due to the synergistic effect from the prevention of phase segregation and particle aggregation as well as chemical catalysis by the in situ embedded Mg2NiH4 and YH3 nanoparticles. This method of in situ embedding of hydrides may provide an alternative approach for improving hydrogen storage properties, especially the kinetics and reversibility of NaMgH3 and other Mg-based bimetallic hydride multiphase systems.

■

REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Phase components and their relative abundance for initial NaMgH3, in situ embedded NaMgH3, and their dehydrogenated and rehydrogenated states determined by XRD and Rietveld analysis. MS curves for the pure NaMgH3, Mg2NiH4doped, YH3-doped, Mg2NiH4/YH3-codoped, and in situ embedded NgMgH3 samples. This material is available free of charge via the Internet at http://pubs.acs.org. 23643



Article

(35) Zhang, Q. A.; Zhang, L. X.; Wang, Q. Q. Crystallization Behavior and Hydrogen Storage Kinetics of Amorphous Mg11Y2Ni2 Alloy. J. Alloys Compd. 2013, 551, 376−381. (36) Zhang, Q. A.; Jiang, C. J.; Liu, D. D. Comparative Investigations on the Hydrogenation Characteristics and Hydrogen Storage Kinetics of Melt-spun Mg10NiR (R=La, Nd and Sm) Alloys. Int. J. Hydrogen Energy 2012, 37, 10709−10714. (37) Wagemans, R. W. P.; van Lenthe, J. H.; de Jong, P. E.; van Dillen, A. J.; de Jong, K. P. Hydrogen Storage in Magnesium Clusters: Quantum Chemical Study. J. Am. Chem. Soc. 2005, 127, 16675−16680. (38) Yvon, K.; Bertheville, B. Magnesium Based Ternary Metal Hydrides Containing Alkali and Alkaline-Earth Elements. J. Alloys Compd. 2006, 425, 101−108. (39) Izumi, F.; Ikeda, T. A Rietveld-Analysis Program RIETAN-98 and Its Application to Zeolites. Mater. Sci. Forum 2000, 321−323, 198−205. (40) Liu, T.; Cao, Y.; Qin, C.; Chou, W.; Li, X. Synthesis and Hydrogen Storage Properties of Mg-10.6La-3.5Ni Nanoparticles. J. Power Sources 2014, 246, 277−282.

(15) Fichtner, M.; Engel, J.; Fuhr, Q.; Glöss, A.; Rubner, O.; Ahlrichs, R. The Structure of Magnesium Alanate. Inorg. Chem. 2003, 42 (22), 7060−7066. (16) Yu, H.; Du, A.; Song, Y.; Searles, D. Graphyne and Graphdiyne: Versatile Catalysts for Dehydrogenation of Light Metal Complex Hydrides. J. Phys. Chem. C 2013, 117, 21643−21650. (17) Pottmaier, D.; Pinatel, E. R.; Vitillo, J. G.; Garroni, S.; Orlova, M.; Baró, M. D.; Vaughan, G. B. M.; Fichtner, M.; Lohstroh, W.; Baricco, M. Structure and Thermodynamic Properties of the NaMgH3 Perovskite: A Comprehensive Study. Chem. Mater. 2011, 23, 2317− 2326. (18) Shane, D.; Corey, R.; Bowman, R.; Zidan, R.; Stowe, A.; Hwang, S.; Kim, C.; Conradi, M. NMR Studies of the Hydrogen Storage Compound NaMgH3. J. Phys. Chem. C 2009, 113, 18414−18419. (19) Hao, S.; Sholl, D. Role of Schottky Defects in Hydrogen and Metal Diffusion in NaH, MgH2, and NaMgH3. J. Phys. Chem. Lett. 2010, 1 (19), 2968−2973. (20) Sheppard, D.; Paskevicius, M.; Buckley, C. Thermodynamics of Hydrogen Desorption from NaMgH3 and Its Application as a Solar Heat Storage Medium. Chem. Mater. 2011, 23, 4298−4300. (21) Aizawa, T.; Kuji, T.; Nakano, H. Synthesis of Mg2Ni Alloy by Bulk Mechanical Alloying. J. Alloys Compd. 1999, 291, 248−253. (22) Yermakov, A. Ye.; Mushnikov, N. V.; Unimin, M. A.; Gaviko, V. S.; Tankeev, A. P.; Skipov, A. V.; Soloninin, A. V.; Buzlukov, A. L. Hydrogen Reaction Kinetics of Mg-based Alloys Synthesized by Mechanical Milling. J. Alloys Compd. 2006, 425, 367−372. (23) Orimo, S.; Fujii, H. Materials Science of Mg-Ni-based New Hydrides. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 167−168. (24) Liang, G.; Huot, J.; Boily, S.; Van Neste, A.; Schulz, R. Hydrogen Storage Properties of the Mechanically Milled MgH2-V Nanocomposite. J. Alloys Compd. 1999, 292, 247−252. (25) Janot, R.; Darok, X.; Rougier, A.; Aymard, L.; Nazri, G. A.; Tarascon, J. M. Hydrogen Sorption Properties for Surface Treated MgH2 and Mg2Ni Alloy. J. Alloys Compd. 2005, 404−406, 293−296. (26) Suda, S.; Sun, Y. M.; Liu, B. H.; Zhou, Y.; Morimitsu, S.; Arai, K.; Tsukamoto, N.; Uchida, M.; Candra, Y.; Li, Z. P. Catalytic Generation of Hydrogen by Applying Fluorinated-metal Hydrides as Catalysts. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 209−212. (27) Zheng, S.; Fang, F.; Zhang, J.; Sun, L.; He, B.; Wei, S.; Chen, G.; Sun, D. Study of the Correlation between the Stability of Mg-Based Hydride and the Ti-Containing Agent. J. Phys. Chem. C 2007, 111, 14021−14025. (28) Fang, F.; Li, Y.; Song, Y.; Sun, D.; Zhang, Q.; Ouyang, L.; Zhu, M. Superior Destabilization Effects of MnF2 over MnCl2 in the Decomposition of LiBH4. J. Phys. Chem. C 2011, 115, 13528−13533. (29) Zhou, C.; Fang, Z. Z.; Lu, J.; Zhang, X. Thermodynamic and Kinetic Destabilization of Magnesium Hydride Using Mg-In Solid Solution Alloys. J. Am. Chem. Soc. 2013, 135, 10982−10985. (30) Zhou, Y.; Liu, Y.; Wu, W.; Zhang, Y.; Gao, M.; Pan, H. Improved Hydrogen Storage Properties of LiBH4 Destabilized by in Situ Formation of MgH2 and LaH3. J. Phys. Chem. C 2012, 116, 1588− 1595. (31) Liu, D.; Liu, Q.; Si, T.; Zhang, Q.; Fang, F.; Sun, D.; Ouyang, L.; Zhu, M. Superior Hydrogen Storage Properties of LiBH4 Catalyzed by Mg(AlH4)2. Chem. Commun. 2011, 47, 5741−5743. (32) Li, Y.; Fang, F.; Song, Y.; Li, Y.; Sun, D.; Zheng, S.; Bendersky, L.; Zhang, Q.; Ouyang, L.; Zhu, M. Hydrogen Storage of a Novel Combined System of LiNH2-NaMgH3: Synergistic Effects of in Situ Formed Alkali and Alkaline-Earth Metal Hydrides. Dalton Trans. 2013, 42, 1810−1819. (33) Zhang, Q. A.; Liu, D.; Wang, Q.; Fang, F.; Sun, D.; Ouyang, L.; Zhu, M. Superior Hydrogen Storage Kinetics of Mg12YNi Alloy with a Long-period Stacking Ordered Phase. Scr. Mater. 2011, 65, 233−236. (34) Ouyang, L.; Yang, X.; Zhu, M.; Liu, J.; Dong, H.; Sun, D.; Zou, J.; Yao, X. Enhanced Hydrogen Storage Kinetics and Stability by Synergistic Effects of in Situ Formed CeH2.73 and Ni in CeH2.73-MgH2Ni Nanocomposites. J. Phys. Chem. C 2014, 118, 7808−7820. 23644


In Situ Embedding of Mg2NiH4 and YH3 Nanoparticles into Bimetallic

Recommend Documents