Formation of Multiple-Phase Catalysts for the ... - ACS Publications

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Formation of Multiple-Phase Catalysts for the Hydrogen Storage of Mg Nanoparticles by Adding Flowerlike NiS Xiubo Xie,† Xiujuan Ma,† Peng Liu,† Jiaxiang Shang,† Xingguo Li,‡ and Tong Liu*,† †

Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, No.37 Xueyuan Road, Beijing, 100191, China ‡ Beijing National Laboratory for Molecular Sciences (BNLMS), The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: In order to enhance the hydrogen storage properties of Mg, flowerlike NiS particles have been successfully prepared by solvothermal reaction method, and are subsequently ball milled with Mg nanoparticles (NPs) to fabricate Mg-5 wt % NiS nanocomposite. The nanocomposite displays Mg/NiS core/shell structure. The NiS shell decomposes into Ni, MgS and Mg2Ni multiple-phases, decorating on the surface of the Mg NPs after the first hydrogen absorption and desorption cycle at 673 K. The MgMgS-Mg2Ni-Ni nanocomposite shows enhanced hydrogenation and dehydrogenation rates: it can quickly uptake 3.5 wt % H2 within 10 min at 423 K and release 3.1 wt % H2 within 10 min at 573 K. The apparent hydrogen absorption and desorption activation energies are decreased to 45.45 and 64.71 kJ mol−1. The enhanced sorption kinetics of the nanocomposite is attributed to the synergistic catalytic effects of the in situ formed MgS, Ni and Mg2Ni multiple-phase catalysts during the hydrogenation/dehydrogenation process, the porthole effects for the volume expansion and microstrain of the phase transformation of Mg2Ni and Mg2NiH4 and the reduced hydrogen diffusion distance caused by nanosized Mg. This novel method of in situ producing multiple-phase catalysts gives a new horizon for designing high performance hydrogen storage material. KEYWORDS: magnesium, hydrogen storage, multiple-phase catalysts, catalytic effects, nanocomposite

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INTRODUCTION Magnesium (Mg) exhibits great potential application in the hydrogen storage field because of its lightweight, high theoretical weight storage capacity of 7.6 wt % H2 and volumetric capacity of 109 g H2 L−1. Moreover, Mg is nontoxic, low cost, reversible during the hydrogen storage cycles and abundant in the world.1−3 The practical application for the Mgbased hydrides, however, is strongly hindered by the following two obstacles. The decomposition enthalpy is as high as 75 kJ mol−14 due to the thermodynamically stable of the Mg−H bond, which demands an operating temperature above 573 K;5 Meanwhile, the sluggish hydrogenation and dehydrogenation kinetics make it inefficient for on-board hydrogen storage material.6,7 Over the past decades, tremendous attempts have been undertaken to accelerate the sorption kinetic properties of the hydrogen/magnesium reaction and lower down the operating temperature through reducing the particle size to nanoscale,2,6,8,9 alloying with mental elements,4,10−12 and the most common way, introducing various catalysts.13−17 To date, catalysts additives especially the 3d-transition metals (Nb, V, Ti, Ni, etc.)3,4,14−17 and their oxides18,19 have been steadily studied for their catalytic effects on the hydrogen dissociation and diffusion.20 Mechanical milling is often used to disperse various © 2017 American Chemical Society

catalysts effectively into the Mg/MgH2 system. Liang et al. reported that the sorption kinetic properties of Mg were greatly improved by ball milling Mg and Ni powders.13 Holtz confirmed that the onset temperature of hydrogenation of Mg-5 at% Ni composite can be decreased from 548 to 448 K with an increased hydrogen absorption capacity increased by 50%.21 The ball milled Mg-4 at% Ni nanofibers started to release hydrogen at 416 K, and it can desorb 4.5 wt % H2 within 60 min at 523 K.22 Furthermore, Ouyang and coworkers23 reported that the thermodynamic and kinetic properties of the Mg(In)-MgF2 composite produced by a new approach of dielectric barrier discharge plasma assisted milling can be dual-tuned with enhanced hydrogen storage properties. The activation energy of the composite for dehydrogenation was decreased to 127.7 kJ/mol, lower than the pure MgH2 of about 160 kJ/mol, and it can desorb 4.0 wt % H2 within 15 min at 623 K due to the valid catalytic activities of the MgF2 catalyst. The enthalpy for dehydrogenation of the Mg(In)-MgF2 composite was also decreased to 69.2 kJ/mol H2, slightly smaller than the pure MgH2 of 79.1 kJ/mol H2. Received: October 18, 2016 Accepted: January 25, 2017 Published: January 25, 2017 5937

DOI: 10.1021/acsami.6b13222 ACS Appl. Mater. Interfaces 2017, 9, 5937−5946

Research Article

ACS Applied Materials & Interfaces

Figure 1. XRD pattern (a), SEM (b), and high magnification SEM (c) images of the NiS flowerlike particles.

method of in situ producing multiple-phase catalysts gives a new horizon for designing high performance hydrogen storage material.

Moreover, the dehydrogenation enthalpy of the Mg85In5Al5Ti524 composite prepared by the same method was also decreased to 65.2 kJ/mol H2. It is reported that many catalysts can be in situ formed in the Mg matrix and these uniformly dispersed catalysts show better catalytic effects than the catalysts added by ball milling.11,25,26 Itam Sulaiman and co-workers found that NaMgF3, NaF and Fe were in situ formed during the heating process of MgH2− Na3FeF6 composite, and decreased the dehydrogenation temperature of the composite.25 The in situ formed Ni and CeH 2 catalysts were produced by hydrogenating the Mg80Ce18Ni2 alloy at room temperature, and the MgH2−NiCeH2 composite showed low onset dehydrogenation temperature, fast desorption kinetics and good cycle stability.26 Very recently, many transition metal sulfides, such as MoS2 and Fe3S4, have been added into the Mg/MgH2 system to enhance its hydrogen storage properties. Jia et al.27 reported that the in situ formed MgS catalyst in the Mg-MoS2 system can enhance the hydrogen absorption and desorption kinetics and decrease the temperature of thermodynamics dissociation for Mg/MgH2 system. Zhang and co-workers28 found that new phases of MgS and Fe can be in situ formed in the reactions between MgH2 and Fe3S4 during the ball milling process. These new phases showed strong catalytic activities on the hydrogen absorption and desorption rates of Mg/MgH2. Given that Ni is one of the most effective catalysts for the Mg/MgH2 system,3,15,21,22 the addition of NiS may give rise to in situ formed multiple-phase catalysts including Ni. Hydrogen plasma-metal reaction is a novel method for producing different metallic NPs by arc melting the corresponding metal bulks under H2 and Ar atmosphere. In our previous works, we have systematically investigated the properties of various Mg-based hydrogen storage materials, such as Mg−V, Mg−Ti, Mg−Nb, etc.29−32 prepared by HPMR approach. However, these in situ formed catalysts are single, leading to the limit catalytic effects on the hydrogen storage performances of Mg/MgH2 system. In this work, flowerlike NiS particles have been successfully prepared by solvothermal reaction method, and are subsequently ball milled with Mg NPs to fabricate Mg-5 wt % NiS nanocomposite. The nanocomposite displays Mg/NiS core/shell structure. The NiS shell in situ decomposes into MgS, Ni and Mg2Ni multiple-phases decorating on the surface of the Mg NPs after first hydrogen absorption and desorption cycle at 673 K. The nanocomposite shows enhanced hydrogen storage properties. The catalytic mechanism of these catalysts for improving the hydrogenation and dehydrogenation of Mg is clarified in this work. This novel

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EXPERIMENTAL SECTION

Preparation of Flowerlike NiS Particles. The flowerlike NiS particles were produced by a solvothermal reaction method. NiCl2· 6H2O (1 mmol) was dissolved in 40 mL of distilled water at room temperature. Then, 1 mmol of Na3C6H5O7·3H2O and 3 mmol of thiourea (CH4N2S) were continually added to the above-mentioned solution to form a light blue solution under ultrasonic treatment. The pH value was tuned to 12 by adding ammonia (28 wt %) dropwise. Then, the solution was electromagnetically stirred for 30 min until formation of the dark blue solution without any precipitation. This homogeneous solution was shifted into a 50 mL sealed Teflon-lined autoclave. The reaction of the solution for the formation of NiS was performed at 180 °C for 24 h. After the cooling down of the autoclave to room temperature naturally, the black powder was isolated by centrifugation and washed with distilled water and absolute alcohol three times. The obtained particles were dried by vacuum pumping at 50 °C overnight to remove the residual alcohol. Preparation of Mg/NiS Nanocomposite. The Mg NPs were fabricated by the HPMR method, which was described previously.29 The as made NiS powder was mechanically milled with the Mg NPs (weight ratio, 5:95) under the conditions of a ball to powder ratio of 20:1, and the rotation speed and ball milling time were 220 rpm and 2 h in a H2 atmosphere (99.999%). It is known that Mg powder is difficult to break down for its good plasticity and toughness.33 To avoid agglomeration of the Mg NPs, we added a small amount of brittle MgH2 NPs at a weight ratio of 9:1 (Mg/MgH2). The preparation and collection of the ball milled materials were performed in a glovebox filled with purified argon (99.999%) and a drying agent to prevent the milled material from undergoing hydroxide and/or oxidation formation. Characterization. The measurement of the hydrogen storage properties of the Mg-MgS-Mg2Ni-Ni nanocomposite were performed on a Sieverts-type apparatus with reactor chamber volume of 60 mL. Before the measurement, the as-prepared nanocomposite of 100 mg experienced one hydrogenation and dehydrogenation activation cycle under a hydrogen pressures of 4 MPa and 100 Pa at 673 K. After the Sieverts-type apparatus was pumped to 10−3 Pa at 673 K for more than 1 h to eliminate residual hydrogen in the material, the Mg-MgSMg2Ni-Ni nanocomposite was heated up to the tested temperatures to measure the corresponding hydrogenation and dehydrogenation kinetic curves. The initial hydrogenation and dehydrogenation pressures for each temperature were 4 MPa and 100 Pa, respectively. The thermodynamic property of the nanocomposite was investigated by conventional pressure−volume−temperature equipment. For the pressure−composition (P−C) isotherm curves at a certain temperature, we suggest that the measurement reaches equilibrium if the vibration of hydrogen pressure is below 20 Pa/s. The absorption and 5938


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Figure 2. (a) SEM, (b) high magnification SEM images, (c) EDS, (d) TEM images, (e) HRTEM image of selected zone A, and (f) HRTEM image of selected zone B of the ball milled Mg-5 wt % NiS nanocomposite. desorption cycle performance was determined as follows: before each measurement, the Sieverts-type system was pumped to 10−3 Pa and stayed for more than 1 h at 673 K to remove the absorbed hydrogen in the nanocomposite. The hydrogen absorption process was performed for 0.5 h under the initial hydrogen pressure of 4 MPa, and the hydrogen desorption process was performed for another 0.5 h under 100 Pa H2. The phase changes of the Mg-NiS nanocomposite samples during the hydrogenation/dehydrogenation process were measured by an Xray diffraction (XRD) measurement with monochromatic Cu Kα radiation (Rigaku X-ray diffractometer). The lattice constants and the phase contents of the as prepared nanocomposite were calculated on the basis of the XRD data of the ball milled Mg-NiS powder. Field emission scanning electronic microscopes (SEM, JEM7500, 15 kV) and transmission electron microscopy (TEM, JEOL-JSM-2100, 200 kV) were applied to analyze the morphology and microstructures of the samples. The energy dispersive spectroscopy (EDS) results were obtained to study the elemental distributions.

dependent on the anion of the Ni source. The weak acid CH3COO− would decrease the restrictions of the selective bindings of citrate anions by partial hydrolization and thus the NiS growth into flowerlike rather than nanorod-shaped particles. Moreover, some cracks can be found on the NiS particles, see the circled areas in Figure 1b and c, making them easily broken during the ball milling process. Figure 2a and b display the different magnification SEM images of the Mg-5 wt % NiS nanocomposite after ball milling. The nanocomposite particles are mainly in spherical-like shape and in the range of 200−900 nm with an average value of 400 nm, see Figure 2a. This morphology of the particles is not like the hexagonal shape Mg NPs fabricated by the HPMR method (Figure S2). Small particles can be clearly observed on the surface of the large Mg NPs, see Figure 2b. Since that NiS is a brittle ceramic phase, and there exist cracks on the loose flowerlike NiS particles (see Figure 1c), we suppose that the NiS additive is broken into very small particles and covers on the Mg NPs surface after ball milling in terms of the disappearance of the large scale NiS particles in Figure 2a, suggesting the feasibility of this method to produce core/shell structure materials. Chen and co-workers also fabricated the nanonickel coated Mg2Ni-based hydride by mixing the Mg2Ni and Ni powders through ball milling.38 The EDS results in Figure 2c show that the NiS concentration of the ball milled core/shell Mg/NiS nanocomposite is 5.1 wt %, which coincides with the designed composition. TEM was employed to observe the microstructures further, and the results were shown in Figure 2d−f. The large particles of about 500 nm are covered by small particles, see Figure 2d, similar to the SEM observations (see Figure 2b). The measured lattice fringes of 1.911 Å in the big particle (selected zone A, Figure 2d) correspond to the (102) plane of Mg (hexagonal close packing, hcp) with a standard spacing of 1.900 Å, see Figure 2e. The interplanar spacings with different directions of the particles at the edge of the big particle (selected zone B, Figure 2d) are measured to be 1.205 Å, which is consistent with the (440) plane of β-NiS, 1.202 Å; the interplanar angle of 60° is the same as the standard value, see Figure 2f. To confirm the core−shell structure, we slanted one composite between +25° and −25° in a horizontal direction during the TEM observation, and the results are shown in Figure S3. After the as-prepared

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RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of the as-synthesized NiS particles. The diffraction peaks of the particles agree well with the standard pattern of β-NiS (space group: R3m, a = 9.6200 Å and c = 3.1600 Å). It is reported that α-NiS and β-NiS phases generally coexist under a reaction temperature of 180 °C.34 However, no peaks of any impurity phase of α-NiS and other Ni-S compounds such as Ni2S3, Ni3S4, and Ni9S8 could be observed in Figure 1a, indicating the high purity of the β-NiS product. To observe the surface morphology of the NiS particles, SEM and high magnification SEM are employed, and the images are shown in Figure 1b and c. Generally, according to the Ostwald ripening process,35 the NiS clusters first come into being in the dark blue solution containing Ni(CH3COO)2· 4H2O as a Ni source. Then they assemble together to form small NPs by van der Waals forces and/or intermolecular hydrogen bonds. The assembled NPs can function as crystal nuclei, and the smaller NPs in the solution will dissolve and recrystallize on the larger NPs to minimize the surface free energy.36 At the same time, the selective binding of citrate anions restricts the growth of some crystal planes, leading to the formation of nanorod-shaped NiS particles.37 However, the particles in this work are about 2 μm (Figure 1b) and surprisingly display a flowerlike shape, see Figures 1c and S1. We suggest that the growth of the NiS cluster is strongly 5939


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HPMR and ball milling processes and the packing of the nanocomposite to the Sieverts-type test apparatus are under Ar and/or H2 atmospheres. This can protect the sample from oxidization, and there exists no MgO to hinder the hydrogenation/dehydrogenation. Figure 3b and c display the XRD patterns of the as-prepared nanocomposite during the absorption and desorption processes. After being hydrogenated at 673 K, the Mg NPs in the composite almost completely transformed into MgH2, see Figure 3b. The NiS phase surprisingly disappeared, and the diffraction peaks centering at 23.6 and 39.0° can be brought out by the new phase, Mg2NiH4 (JCPDS No. 37-1414, space group Fm3̅m). It is believed that the addition of a MgS catalyst can enhance the hydrogen storage properties of Mg.27 The formed new phase MgS after the hydrogenation process (Figure 3b) in the nanocomposite may show good catalytic effects on the Mg NPs. Interestingly, a detectable amount of Ni phase can be observed after hydrogenation of the composite. Generally, the Ni element should be reacted with Mg to form a Mg2Ni phase at 633 K in the Mg−Ni system according to the Mg−Ni binary phase diagram.41 We suggest that during the hydrogenation process, the Ni atom partially diffuses into the Mg NPs to form a Mg2NiH4 phase. The other Ni atoms spread outside the Mg core to structure Ni particles due to the prevention of the reaction between Ni and Mg by the new formed MgS phase. After the hydrogenation/dehydrogenation process, the MgH2 totally transforms into Mg, and the Mg2NiH4 changes into Mg2Ni. However, the MgS and Ni phases still exist and remain unchanged during the hydrogenation/dehydrogenation cycle (Figure 3c). Given that the NiS disappeared after the hydrogenation/dehydrogenation process, we refer to the sample as the Mg-MgS-Mg2Ni-Ni nanocomposite hereafter. Moreover, the peak intensity of MgO at 42.9° becomes strong in both Figure 3b and c, because the activated NPs are taken out of the measurement chamber without being passivated sufficiently. To observe the element distribution of the sample after five hydrogen absorption and desorption cycles at 673 K, the SEM and EDS analyses were further carried out, and the results were shown in Figure 4. After absorption and desorption processes, the particle shape changes to irregular and some small particles come into being on the surface of the big Mg NPs. The particle size is similar to that of the as prepared NPs, see Figure 2a. The Mg element map in Figure 4b shows that the big particles belong to Mg, and most of the S and Ni elements distribute uniformly on the surface of Mg NPs, see Figure 4c and d. The Ni and S peaks in the EDS sum spectrum, see Figure S5, confirm the authenticity of Ni and S maps in Figure 4d. By summarizing the analysis above, the hydrogenation reaction of the Mg-MgS-Mg2Ni-Ni nanocomposite can be described in eq 1:

nanocomposite was tilted, the small particles still existed on the surface of the big Mg nanoparticle; see the results in Figure S3a−c. The selected area electron diffraction (SAED) results in Figure S3d of the particle confirm that the nanocomposite contains Mg, NiS, and MgO phases. These results confirm the core/shell morphology of the Mg-5 wt % NiS nanocomposite after ball milling. Figure 3a shows the XRD pattern of the ball milled nanocomposite. The α-Mg (space group P63/mmc), β-NiS,

Figure 3. XRD patterns of the ball milled Mg-5 wt % NiS nanocomposite: as prepared (a), after absorption under 4 MPa at 673 K (b), and desorption under 100 Pa at 673 K (c).

and MgH2 phases dominate the XRD profile without other impurities, indicating that almost no phase changes of NiS happened during the ball milling process. To determine the phase contents and the lattice parameters of the as prepared sample, Rietveld refinement is employed by using GSAS software on the basis of the XRD data, see Figure S4, and the results are summarized in Table 1. The calculated lattice Table 1. Structure Parameters and Phase Contents of AsPrepared Mg-MgH2−NiS Sample lattice parameters (Å) phase

space group

a

c

phase content (wt %)

Mg MgH2 NiS

P63/mmc P42/mnm R3m

3.2088 4.5156 9.6287

5.2096 3.0186 3.1783

77.851 14.980 7.169

parameters of Mg are a = 3.2088 Å and c = 5.2096 Å, comparable to the reported Mg, a = 3.2093 Å and c = 5.2099 Å,23,24 suggesting that no atom dissolved in the Mg lattice. The phase content of MgH2 is 14.980 wt %, larger than the designed concentration of 9.5 wt %, indicating very little Mg transformed into MgH2 during the ball milling process. Varin and coworkers reported that the Mg quickly transformed into MgH2 after a ball milling time of at least 5 h.39 We suggest that the short ball milling time of 2 h in this work leads to the small amount of transformation of Mg into MgH2. Furthermore, the broad diffraction peak at 42.9° can be ascribed to MgO, consisting of the SAED results in Figure S3d. House et al. also found that an ultrafine-grained MgO layer can be generated during the ball milling process.40 The MgO in Figure 3a is attributed to the exposure of the nanocomposite to air during the measurement. It should be noted that the preparation of the Mg NPs and Mg-MgS-Mg2Ni-Ni nanocomposite during the

5Mg + 2NiS + 3H 2 → MgH 2 + Mg 2NiH4 + 2MgS + Ni

(1)

The dehydrogenation reaction of the Mg-MgS-Mg2Ni-Ni nanocomposite can be expressed in eq 2: MgH 2 + Mg 2NiH4 → Mg + Mg 2Ni + 3H 2

(2)

The schematic diagram of the fabrication of the Mg-based nanocomposite and the phase transformation in the first hydrogenation/dehydrogenation cycle at 673 K were shown in Figure 5. The catalyst phases Ni, Mg2Ni, and MgS stay 5940


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Figure 4. SEM image (a) and EDS element maps of Mg (b), S (c), and Ni (d) of the Mg-MgS-Mg2Ni-Ni nanocomposite after 5 hydrogenation/ dehydrogenation cycles at 673 K.

Figure 5. Schematic diagram of the fabrication of the Mg-MgS-Mg2Ni-Ni nanocomposite and its first activization process at 673 K.

unchanged after the first activization process. The multiple phases are expected to exhibit better catalytic function than each individual phase. Figure 6a,b display the hydrogenation and dehydrogenation kinetic curves tested by a carefully calibrated Sieverts-type apparatus at various temperatures. In this work, the multiplephase catalysts of Ni, Mg2Ni, and MgS are in situ formed during the first hydrogenation/dehydrogenation process at 673 K rather than in the ball-milling process. Thus, it is very hard to produce the suitable control specimens for comparison. Therefore, in this work, we compared the Mg-MgS-Ni-Mg2Ni sample with the Mg-based materials containing MgS, Ni, or Mg2Ni phases reported in other works. It is observed in Figure 6a that at an absorption temperature of 373 K, the Mg-MgSMg2Ni-Ni nanocomposite can uptake 1.1 wt % H2 in 5 min and 3.5 wt % H2 within 60 min, better than that of the Mg85Ni15 nanocomposite, which can only absorb less than 0.8 wt % H2 within 5 min at the same tempreture.42 It is also much better than those of the Mg-based samples catalyzed by Nb, V, Ti−V, and Mg17Al12.28,31,32,43 In addition, the Mg-MgS-Mg2Ni-Ni nanocomposite can quickly absorb 3.5 wt % H2 within 10 min at 423 K. Han et al. found that the ball milled Mg-20 wt % Fe3S4 composite absorbed less than 3.0 wt % H2 within 20 min at 423 K.28 Zou et al. found that the Mg-5 wt % Ni composite fabricated by electroless plating only absorbed no more than 1.0 wt % H2 within 30 min at the same temperature.44 It can be

concluded that the in situ formed MgS, Mg2Ni, and Ni additives further improve the hydrogenation kinetics of Mg/ MgH2. Furthermore, compared with the pure Mg NPs (38 nm) and MgH2-20 wt % MgS composites,28,44 the Mg-MgS-Mg2NiNi nanocomposite shows better absorption kinetics at 473 K, which can uptake 4.0 wt % H2 within 5 min. The storage capacities of the Mg-MgS-Mg2Ni-Ni nanocomposite are 4.5, 5.3, and 6.0 wt % in 5 min at the absorption temperatures of 573, 623, and 673 K, respectively, showing faster absorption kinetics than that of the Mg NPs synthesized by the HPMR method.29 Figure 6b shows that with the increase of the temperature from 523 to 673 K, the dehydrogenation rates and capacities of the Mg-MgS-Mg2Ni-Ni nanocomposite increase remarkably. For instance, at 673 K, the composite can release 5.4 wt % H2 in 5 min, superior to the Mg NPs that only desorb less than 3.5 wt % H2 under the same conditions.31 It is worth it to note that the nanocomposite can desorb 5.0 wt % H2 within 10 min at 623 K. However, the MgH2 with Fe3S4, Fe, and MgS additives can only desorb 4.0, 3.5, and 2.5 wt % H2, respectively.28 The MgH2 catalyzed by the in situ formed MgF2 can release 4.0 wt % H2 within 15 min at 623 K.23 We suggest that the MgS, Mg2Ni, and Ni phases produced during the activation process can synergistically enhance the dehydrogenation kinetics and capacity of MgH 2. At a desorption temperature of 573 K, the composite can also release 3.1 wt % H2 in 10 min and 3.7 wt % H2 within 60 min, 5941


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Figure 6. Hydrogen absorption curves under 4 MPa hydrogen pressure (a), desorption curves under 100 Pa (b), plots of ln k vs 1000/T for the hydrogenation (c) and dehydrogenation (d) of the Mg-MgS-Mg2Ni-Ni nanocomposite.

at different temperatures is controlled by either diffusion-rate limited transformation of two-dimensional growth or interfacecontrolled transformation with one-dimensional growth. After the calculation of the rate constant k values on the basis of the slope η and intercept η ln t of hydrogenation and dehydrogenation, the apparent activation energies for the hydrogenation/ dehydrogenation processes can be given from the following Arrhenius equation:

superior to the Mg-14 wt % Ni-6 wt % NbF5 composite and the Mg−V, Mg−Ti, and Mg−Ti−V systems.29−31,45 It is known that the differences in synthesis technique and parameters can have a large impact on the resulting kinetics. Despite this, the performance of the Mg-MgS-Mg2Ni-Ni nanocomposite is sufficiently greater compared to other related Mg-based materials. We suggest that the improvement can be attributed to this new material system rather than the differences in synthesis. We determined the activation energy for hydrogen absorption and desorption by using the Johnson−Mehl− Avrami−Kolmogorov (JMAK) model and the Arrhenius theory. The following linear equation can describe the hydrogen absorption kinetics:30 ln[− ln(1 − α)] = η ln k + η ln t

k = A exp(Ea /RT )

(4)

in which A represents the temperature-independent coefficient, Ea means the activation energy, R is a constant of 8.314472 J mol−1 K−1, and T is the absolute temperature. The ln k versus 1000/T plot for the absorption process is shown in Figure 6c, and the Ea value of the nanocomposite is calculated to be 45.45 kJ mol−1, much smaller than that of the Mg-5 wt % Ni nanocomposite of 88.9 kJ mol−1 and the Mg particles (25 nm) with a value of 122 kJ mol−1.8,44 Similarly, the dehydrogenation activation energy of the Mg-MgS-Mg2Ni-Ni nanocomposite calculated from the desorption plot of ln k versus 1000/T (Figure 6d) is 64.71 kJ mol−1, much smaller than those of the Mg90Ni10 nanocomposite of 88.0 kJ mol−1,47 the MgH2-MoS2 composite of 87.19 kJ mol−1,27 as well as other Mg-based composites.16,23,26,44,48 This clearly demonstrates that the MgS, Mg2Ni, and Ni multiple-phase catalysts efficiently decrease the activation energies and thus improve the hydrogen absorption and desorption rates of Mg/MgH2. To determine the cycle stability of the Mg-MgS-Mg2Ni-Ni nanocomposite, the cyclic hydrogen absorption and desorption performance of the nanocomposite was measured at 673 K, and the results are shown in Figure 7. After five cycles, the sample does not show an apparent decrease of absorption and desorption capacity, implying the good cyclic stability of the

(3)

in which α means the fraction of the hydrogenated Mg at time t, k is a constant of reaction rate, and η stands for the reaction order. On the basis of the experimental hydrogen absorption data of 423, 473, 523, 573, 623, and 673 K fitted with the JMAK model (as shown in Figure S6a), the ln[−ln(1 − α)] versus ln t fitting lines of each temperature display a straight line with a slope η and an intercept η ln k. The reaction order η is affected by the rate-limiting process, growth dimensionality, and nucleation behavior of the hydrides.46 Different η values represent different controlling mechanisms for the rates. The reaction orders for different temperatures are about 0.5 (Figure S6a), implying that the hydrogen absorption process of the MgMgS-Mg2Ni-Ni nanocomposite is mainly determined by diffusion rate and belongs to one-dimensional growth. Similarly, the ln[−ln(1 − α)] versus ln t curves of the dehydrogenation data of 523, 573, and 623 K are plotted in Figure S6b. The reaction order η of about 1 suggests that the dehydrogenation 5942


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unknown, given that the good performance of this material system, it can be deduced that either MgS is sufficiently permeable to hydrogen or the MgS shell is sufficiently porous/ noncontinuous as to allow hydrogen ingress/egress to the Mg/ MgH2. With the constant diffusion of H atoms into the Mg core, the phase transformation of Mg2Ni and Mg2NiH4 near the Mg core accelerates the diffusion rate of the H atoms due to the expansion-stain-absorption mechanism.50 Once the MgH2 layer is generated, the diffusion of hydrogen into the Mg core is hindered as a result of the rather slow diffusion velocity of H atoms in the metal hydrides. Fortunately, the particles in this work on the nanoscale decrease the hydrogen diffusion distance and provide high interfacial energy at the metal/hydride interface due to the large surface area.26,31,49 The enthalpy of the formation for Mg2NiH4 (about 40 kJ mol−1 H2)51 is nearly half that of the MgH2, 75 kJ mol−1 H2.52 Therefore, Mg2NiH4 releases hydrogen before MgH2. The Mg2Ni phase can be regarded as a “hydrogen pump” that continuously transfers the H atoms from the decomposition of MgH2 into H2 molecules. In addition to the fast hydrogen diffusion paths caused by the large numbers of interface boundaries between the nanosized Mg core and the MgS layer, the microstrain of the Mg2Ni and Mg2NiH4 can also accelerate the dehydrogenation process of MgH2. Jia et al. reported that the presence of MgS can effectively enhance the hydrogen release kinetics of MgH2.27 We suggest that the MgS layer in this work also improved the recombination and desorption rate of H2. The thermodynamic performance of the Mg-MgS-Mg2Ni-Ni nanocomposite is evaluated by using the P−C isotherm experiment data. The P−C isotherm curves of the hydrogenation and dehydrogenation processes at different temperatures are plotted in Figure 9a. Generally, different flat plateaus represent different phase transitions between the solid solution dissolved with hydrogen and the metal hydride. Since there is only a small amount of Mg2Ni in the nanocomposite, only one flat plateau can be detected for each absorption/desorption process. The hydrogen equilibrium pressures of the absorption

Figure 7. Cyclic hydrogenation/dehydrogenation performance of the Mg-MgS-Mg2Ni-Ni nanocomposite at 673 K.

Mg-MgS-Mg2Ni-Ni nanocomposite. Figure 8 depicts the suggested schematic drawing of the catalytic effects of the multiple-phases Ni, Mg2Ni, and MgS in the Mg/MgH2 system. On the basis of the gas−solid reaction of the hydrogenation/ dehydrogenation process, the main rate-limiting factor is the dissociation velocity of the H2 molecule at the surface and/or the diffusion of hydrogen through the hydride layer.32 Generally, Mg without any catalysts cannot sufficiently dissociate H2 molecules due to its absence of the d-electrons orbital, which can act as an electron transfer medium during the hydrogenation/dehydrogenation process.33 We propose that the hydrogenation/dehydrogenation mechanism of the MgMgS-Mg2Ni-Ni nanocomposite can be explained as follows: (1) the H2 molecules diffuse to the surface of the nanocomposite and are initially dissociated into atoms by the Ni NPs and the MgS layer. The H atoms are captured by the Mg2Ni NPs to form Mg2NH4. This is different from the Mg−Ni thin film in that the Mg phase absorbs hydrogen prior to Mg2Ni.49 Although the diffusivity of hydrogen through MgS is currently

Figure 8. Schematic diagram of the catalytic mechanism of Ni, Mg2Ni, and MgS catalysts during the hydrogenation/dehydrogenation processes of the nanocomposite. 5943


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Figure 9. P−C isotherm curves at 623, 648, and 673 K (a) and Van’t Hoff plots (b) for the Mg-MgS-Mg2Ni-Ni nanocomposite.

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plateaus at 673, 648, and 623 K are 1.85, 1.15, 0.66 MPa, respectively. The hydrogen equilibrium pressures of the desorption plateaus at 673, 648, and 623 K are 1.59, 0.90, and 0.54 MPa, respectively. Then, the ln P versus 1/T plots (Van’t Hoff plots) for both hydrogenation and dehydrogenation are shown in Figure 9b. The equation obtained from fitting the line of the hydrogenation is ln P = 18.1143 − 8.6713/T with a goodness of linear fit of 0.998. The determined enthalpy for the hydrogenation process of the nanocomposite is −72.09 kJ mol−1, similar to that of the MgH2 (−75 kJ mol−1) in other works.52 The equation obtained from fitting the line of the dehydrogenation in Figure 9b is ln P = 18.5192 − 9.0617/T with a goodness of linear fit of 0.995. And the determined enthalpy for the dehydrogenation process of the nanocomposite is 75.34 kJ mol−1. This suggests that the hydrogenation/dehydrogenation thermodynamics of the Mg-MgSMg2Ni-Ni nanocomposite cannot be modified apparently by adding MgS, Mg2Ni, and Ni catalysts. Thus, the enhanced hydrogen sorption kinetics and capacity are mainly caused by the catalytic effects of the multiple-phase catalysts MgS, Mg2Ni, and Ni.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13222. SEM image of the NiS particle, TEM image of the Mg nanoparticles, tilted TEM, SAED images and Rietveld refinement XRD pattern of the Mg-5 wt % NiS nanocomposite, EDS sum spectrum, and plots of ln[−ln(1 − α)] vs lnt for the hydrogenation and the dehydrogenation of the Mg-MgS-Ni-Mg2Ni nanocomposite (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tong Liu: 0000-0002-1079-3169 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of this work by the Joint Fund of the National Natural Science Foundation of China and Baosteel Group Corporation (No. U1560106), the Aeronautical Science Foundation of China (No. 2014ZF51069), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry), and the support of Mrs. Cui Ni from University of Science and Technology Beijing during the fabrication of NiS.

CONCLUSIONS The Mg-5 wt % NiS nanocomposite was successfully produced by ball milling the Mg NPs prepared by the HPMR approach and flowerlike NiS particles prepared by the solvothermal reaction method. The nanocomposite displayed a Mg/NiS core/shell structure. The NiS shell decomposed into Ni, MgS, and Mg2Ni multiple-phases during the activization process at 673 K. The Mg-MgS-Mg2Ni-Ni nanocomposite showed enhanced hydrogen absorption and desorption properties. It can quickly absorb 3.5 wt % H2 within 10 min and reaches a value of 4.0 wt % H2 within 60 min at 423 K. It can desorb 3.1 wt % H2 within 10 min at 573 K. The multiple-phase catalysts accelerate the hydrogen transportation in the Mg-based nanocomposite by decreasing the activation energies of hydrogenation/dehydrogenation to 45.45 and 64.71 kJ mol−1. The enhanced sorption kinetics of the nanocomposite are attributed to the synergistic catalytic effects of the in situ formed multiple-phases MgS, Ni, and Mg2Ni during the hydrogenation/dehydrogenation process, the microstrain of the phase transformation of Mg2Ni and Mg2NiH4, and the reduced hydrogen diffusion distance caused by nanosized Mg. This novel method of in situ producing multiple-phase catalysts gives a new horizon for designing high performance hydrogen storage material.

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Formation of Multiple-Phase Catalysts for the ... - ACS Publications

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