Catalytic Effect of Nb Nanoparticles for Improving the Hydrogen

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Catalytic Effect of Nb Nanoparticles for Improving the Hydrogen Storage Properties of Mg-based Nanocomposite Tong Liu, Xiujuan Ma, Chunguang Chen, Li Xu, and Xingguo Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03442 • Publication Date (Web): 08 Jun 2015 Downloaded from http://pubs.acs.org on June 12, 2015

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The Journal of Physical Chemistry

Catalytic Effect of Nb Nanoparticles for Improving the Hydrogen Storage Properties of Mg-based Nanocomposite Tong Liua,*, Xiujuan Maa, Chunguang Chena, Li Xub, Xingguo Lic a

Key Laboratory of Aerospace Materials and Performance (Ministry of Education),

School of Materials Science and Engineering, Beihang University, Beijing 100191, China b

State Grid Smart Grid Research Institute，Future science and technology city

Changping, Beijing, 102211, China c

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

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ABSTRACT With the intention of improving the hydrogenation/dehydrogenation kinetics of Mg, Mg-7.5 wt % Nb nanocomposite has been prepared by hydrogen plasma-metal reaction (HPMR) approach. The spherical Nb nanoparticles (NPs) of 12 nm are uniformly decorated on the surface of Mg NPs. The Mg-Nb nanocomposite can quickly uptake 4.0 wt % H2 in 10 min and reach a saturation value of 5.7 wt % H2 in 60 min at 473 K. Furthermore, it can also release 4.0 wt % H2 in 60 min at 573 K. The reversible hydrogen storage capacity is as high as 7.0 wt % at 673 K. Nb NPs transform into NbH during hydrogenation and recover after dehydrogenation process. They restrain the growth of Mg and work as catalysts to accelerate the hydrogen transportation in the Mg-based nanocomposite by decreasing the activation energies of hydrogenation/dehydrogenation to 70.9 and 86.4 kJmol-1, respectively. The catalytic mechanism of Nb NPs is explained in terms of spillover, d-electrons and electronegativity effects. The nanosizing effects of both Mg and Nb and the catalytic effect of Nb NPs give rise to the improved hydrogen storage properties of the Mg-Nb nanocomposite at moderate temperatures. KEYWORDS: Magnesium; Niobium; Nanocomposite; Catalytic effect; Hydrogen storage

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INTRODUCTION Hydrogen is an ideal energy carrier to replace the non-renewable fossil fuels because it is abundant, lightweight, environment-friendly and high energy content of 142 MJ/kg.1-3 Efficient, safe and cost-effective hydrogen storage methods not only enable hydrogen energy to achieve overwhelmingly practical applications, but also protect the environment due to the zero emission of pollutants. The conventional hydrogen storage approaches，such as the compressed hydrogen gas and cryogenic hydrogen liquid, however, fail to meet the 2015 targets of the US Department of Energy (DOE) for on-board applications.3 Compared with these methods, solid-state hydrogen storage materials have the potential advantages of favorable storage capacity and high safety.4,5 Among various hydrogen storage materials, MgH2 is considered as a promising and attractive candidate for hydrogen storage because of its reversibility, low cost and the relatively high theoretical gravimetric capacity of 7.6 wt % H2.6-9 Nevertheless, two major obstacles prevent it from being widely employed in practical applications. Firstly, MgH2 is too thermodynamically stable. The decomposition enthalpy is as high as 75 kJmol-1, which requires an operating temperature above 573 K.10,11 Secondly, poor kinetics of hydrogen absorption and desorption makes it inefficient for on-board hydrogen storage.12 To overcome these barriers, tremendous efforts have been devoted to decreasing the operating temperature and enhancing the sorption kinetics in the past decades. The hydrogenation/dehydrogenation kinetics of Mg can be highly improved by adding transition metals as catalysts, alloying with metal elements and reducing the particle size to nanoscale.4,5,9 Huot et al. found that the desorption activation energy of the milled MgH2 was 36 kJmol-1 lower than that of the unmilled MgH2. 9 It was also demonstrated that 3d-transition elements (Ti, V, Mn, Fe, Ni) exhibited catalytic effects 3



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on the reaction kinetics of Mg and reduced the activation energy of hydrogenation/dehydrogenation.8,13,14 Therefore, more attention has been paid to enhance the reaction kinetics of Mg through mixing Mg /MgH2 with different catalysts by means of high energy ball milling (HEBM). Among these catalysts, many studies have demonstrated that doping a small percentage of Nb can intensively enhance the hydrogen sorption kinetics of Mg without apparently deteriorating its hydrogen storage capacity.15,16 On the basis of the first principle calculations, Song et al. confirmed that the alloying element Nb could destabilize magnesium hydride, because it makes the bond energies between magnesium and hydrogen weak.17 Huot et al. demonstrated that the ball milled MgH2 + 5 at % Nb composite could absorb 5.2 wt % H2 at 573 K within 5 min.12 Yavari et al. also confirmed an obvious increase of hydrogen desorption kinetics when Nb was added to MgH2.18 Castro and coworkers reported that the desorption temperature of the ball milled Mg + 5 at % Nb nanocomposite was 543 K, which was reduced by 40 K in comparison with the pure Mg sample.19 Thus, Nb is considered as an effective catalyst for not only significantly reinforcing hydrogen sorption kinetics, but also reserving the superior hydrogen sorption capacity of the Mg-based composite. With respect to the catalytic mechanism of Nb, Huot et al. claimed that the formation of a metastable NbHx (x≈0.6) phase during dehydrogenation acted as a gate way to accelerate the hydrogen released from MgH2.12 Li et al. argued that the substitution of Nb at the Mg site followed by the clustering of H around Nb atoms can act as a pathway for hydrogen diffusion by using a density functional theory calculation.20 A theoretical study also reported that Nb interstitial atoms in MgH2 provide vacancies, modify the surface geometry and weaken the Mg-H bonds.21 Although some “speculative pathways” generated from intermediate phase or defects have been proposed to be responsible for the enhanced 4


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kinetics in these literatures, the specific catalytic effects of Nb on the dissociation and association of hydrogen and the nucleation of Mg/MgH2 for improving the sorption rate of the Mg-Nb composites have not been elucidated in details. On the other hand, it should be noted that during HEBM, the particle size is still in the range of micrometers even though the grain size can be decreased to nanoscale.22-23 Recent studies suggested that when the particle size of Mg was reduced to nanoscale, both the hydrogen absorption and desorption kinetics can be significantly improved due to the large specific surface area and the short hydrogen diffusion distance.24-26 Aguey-Zinsou et al. reported that Mg NPs with a diameter of 5 nm prepared by electrochemical approach exhibited good hydrogen storage properties near room temperature.27 However, these Mg NPs were prone to be oxidized due to their high reactivity. Jeon and co-workers successfully embedded Mg NPs of 5-15 nm in a polymer matrix to protect them from oxidation.28 The nanocomposite presented a rapid hydrogen sorption rate, which quickly absorbed 4.7 wt % H2 (without including the weight of the polymer) in 10 min at 473K. Nevertheless, since the polymer content in the composite is as high as 40 wt %, the hydrogen storage capacity of the overall nanocomposite can only reach 3.2 wt % H2. More importantly, the polymer did not show catalytic effects on the hydrogenation/dehydrogenation of Mg. Hydrogen plasma-metal reaction (HPMR) is a novel vapor deposition process and suitable for producing metallic NPs with high purity and low cost. Recently, Mg-based nanocomposites with enhanced hydrogen storage properties, such as Mg-Al, Mg-La-Ni and Mg-Ti, have been successfully fabricated by HPMR approach.29-31 Considering the good catalytic effect of Nb on the hydrogen sorption of the milled Mg-Nb composite, in this work, we intend to fabricate the Mg-Nb nanocomposite by HPMR, investigate its hydrogen storage properties at moderate temperatures, and 5



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clarify the catalytic mechanism of Nb NPs for improving the hydrogenation and dehydrogenation of Mg in details. EXPERIMENTAL SECTION Preparation of Mg-Nb nanocomposite: The Mg-Nb nanocomposite was produced by HPMR method in a water-cooled chamber connected with a collecting room. The direct current arc discharge of Mg (purity>99.9%) ingot of 20 g and Nb (purity>99.9%) ingot of 20 g was carried out in a mixture of hydrogen and argon in a ratio of 1:1 with a total pressure of 0.1 MPa. During the process, the discharge current was kept at 260 A. The nanocomposite was collected in Ar and used for the measurement of hydrogen storage properties. Before the characterizations of the crystal structure and particle size, it was passivated with a mixture of argon and air to prevent the particles from burning. Characterization: The hydrogen absorption and desorption properties of the Mg-Nb nanocomposite were evaluated using a Sieverts-type apparatus. Prior to the measurement, the sample of 100 mg was heated to 673 K to experience one hydrogen absorption/desorption cycle at the hydrogen pressure of 4 MPa and vacuum, respectively. Then, the sample was heated up to 373, 473, 523, 573, 623 and 673 K to measure the hydrogen absorption/desorption kinetic curves, respectively. The initial hydrogen pressures for hydrogenation/dehydrogenation were selected as 4 MPa and 100 Pa at each temperature. By using the conventional pressure-volume-temperature technique, the pressure-composition-temperature (PCT) curves were plotted at different temperatures (623, 648 and 673 K) to investigate the thermodynamic property of the nanocomposite. Once the vibration of hydrogen pressure at the certain temperature was less than 20 Pa/s, the hydrogen absorption or desorption measurement for the PCT curves was considered as reaching an equilibrium. 6


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To characterize the phase changes of the Mg-Nb nanocomposite during the hydrogenation/dehydrogenation, X-ray diffraction (XRD) measurements were carried out using a Rigaku X-ray diffractometer with monochromatic Cu Kα radiation at 40 kV and 40 mA. The morphology, size distribution and shape of the NPs were investigated by JEOL-JSM-2100 transmission electron microscopy (TEM) at an accelerating voltage of 200 kV. X-ray energy dispersive spectroscopy (EDS) was used to determine the concentrations of Mg and Nb in the nanocomposite. RESULTS AND DISCUSSION Particle features of the Mg-Nb nanocomposite: Figure 1(a) displays the TEM image of the Mg-Nb nanocomposite prepared by HPMR. Two disparate types of particles can be apparently observed, the big particles in clear hexagonal shape and the small black particles in nearly spherical shape decorating on the surface of big particles. The high-resolution TEM (HR-TEM) image in Figure 1(b) displays the selected zone of one small and black nanoparticle on the surface of one big nanoparticle in Figure 1(a). The lattice fringes are clearly seen in the big particle and the measured lattice spacing is 1.895 Å, which corresponds to the (102) plane of α-Mg (hexagonal close packing, hcp) with a standard spacing of 1.900 Å. It is also observed that the interplanar spacing of the small and spherical particle is measured to be 2.339 Å, which is consistent with the (110) plane of Nb (body centered cubic, bcc) with a standard spacing of 2.336 Å. It can also be confirmed from the lattice fringes that both Mg and Nb NPs are of single crystal. Figure 1(c) presents the size distribution of Mg particles, which range from 40 to 180 nm with an average size of 120 nm. The small Nb NPs possess an average size of 12 nm as shown in Figure 1(d). The pure Mg NPs prepared by HPMR had an average size of 310 nm.32 It can be deduced that the decoration of Nb NPs on the surface of Mg remarkably reduces the 7



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aggregation and growing of Mg particles during the HPMR process. The EDS result in Figure 1(e) displays that the as-prepared Mg-Nb nanocomposite is made of 92.5 wt % Mg and 7.5 wt % Nb. The much lower Nb content in the as-prepared nanocomposite than that in the master ingots is attributed to the higher evaporation rate of Mg than that of Nb during the HPMR process. To understand the impact of Nb NPs, XRD technique is employed to characterize the phase changes of the Mg-Nb nanocomposite after the hydrogenation and dehydrogenation process. Figure 2(a) shows the XRD pattern of the as-prepared nanocomposite. It is composed predominantly of α-Mg phase (JCPDS: 35-0821, Space group: P63/mmc). The weak diffraction peak around 38° belongs to the (110) plane of Nb phase (JCPDS: 35-0789, Space group: I m3m ). No diffraction peaks of MgH2 are found in the present work, which is in good agreement with the TEM observation. Given that Mg and Nb are immiscible and not capable of forming solid solution or intermetallic compound,33 Mg and Nb firstly evaporate individually under 0.05MPa hydrogen pressure at several thousand degrees during the HPMR process. In the cooling stage, Mg and Nb nucleate separately below their melting points. To the best of our knowledge, whether the hydrides of Mg or Nb can be formed is determined by their hydrogen sorption kinetics and thermodynamics. During the HPMR process, the formation of magnesium hydride is strongly suppressed by the low hydrogen pressure (0.05 MPa) and the high cooling rate. Meanwhile, the hydrogenation of Nb is also prevented under the same condition. In general, the metallic NPs are more pyrophoric than the corresponding microscale particles when they come into contact with air. As a result, the diffraction peak of MgO at 42.9° is often discernible in the XRD pattern of the pure Mg NPs.34 MgO layer on the surface of Mg NPs prevents hydrogen atoms from penetrating inside Mg. In this regard, in 8


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order to obtain high hydrogen storage properties, it is better to restrain the formation of MgO layer. Very importantly, it is noticed that the diffraction peaks of MgO are not detectable in the as-prepared Mg-Nb nanocomposite, indicating that the decoration of Nb NPs on the surface of Mg effectively suppresses the oxidation of Mg. After the hydrogenation at 673 K, it is observed that Mg NPs in the Mg-Nb nanocomposite completely transform into MgH2 (Figure 2(b)). For the ball milled Mg-Nb composite, however, the hydrogenation did not complete at 673 K and a small amount Mg remained.19,35 This implies that the addition of Nb NPs through the novel HPMR approach is more effective to catalyze the hydrogen sorption process of Mg. The diffraction peak at 36.6° in Figure 2(b) belongs to NbH, indicating that Nb NPs are able to react with H2 during the hydrogen absorption process. In the case of Mg-Nb composite prepared by ball milling in the hydrogen atmosphere, two kinds of hydrides can be produced, namely NbH and NbH2, whose type depends on the reaction parameters such as hydrogen pressure, rotation speed and reaction time. A high hydrogen pressure of 3 MPa, a high rotation speed of 600 rpm and a long reaction time of 25 h gave rise to the formation of NbH2 other than NbH in the as-milled Mg-Nb composite,19 whereas only NbH was produced in the MgH2-Nb sample milled at 400 rpm and a short reaction time of 2 h under 1 MPa H2 atmosphere.36 In this regard, it is proposed that high hydrogen pressure, high rotation speed and long reaction time are of benefit for the full hydrogenation of Nb. In this work, the formation of NbH rather than NbH2 after hydrogenation can be interpreted by the short reaction time of only 1 h. It can be observed from Figure 2(c) that after the dehydrogenation, MgH2 completely transforms into Mg. Furthermore, the weak diffraction peak of Nb at around 38° can be detected again and the diffraction peak of NbH disappears, indicating that NbH can be effectively dehydrogenated. It seems that 9



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the recovery of Nb is of great importance for sustaining the good hydrogen storage reversibility of the Mg-Nb nanocomposite. It should be noted that the peak of MgO at 42.9° is discernable in both Figure 2(b) and Figure 2(c), which can be attributed to the fact that the NPs are taken out of the chamber without sufficient passivation after the hydrogenation and dehydrogenation. Figure 3(a) shows the particle morphology of the Mg-Nb nanocomposite after hydrogen absorption at 673 K. The small black NPs are dispersed on the surface of the big NPs. Figure 3(b) displays the HR-TEM image of one small particle on the surface of one big particle. The measured lattice spacing of the big particle is 2.519 Å, corresponding to the (101) crystal plane of MgH2, 2.510 Å. The lattice spacing of the small particle is evaluated to be 2.440 Å, which belongs to the (111) plane of NbH with a standard spacing of 2.446 Å. These phases determined from the HR-TEM analyses are in good agreement with the XRD results. It should be noted that the Mg NPs change from the single crystal of the as-prepared sample into the polycrystalline structure after the hydrogenation. It can be seen from the inset histogram in Figure 3(a) that the MgH2 particle size varies from 50 to 200 nm with an average size of 125 nm, which is comparable with the as-prepared Mg. After the dehydrogenation, the Mg-Nb nanocomposite keeps the polyhedral shape (Figure 4(a)). For the big particle in the HR-TEM image of Figure 4(b), the measured lattice fringe spacing is 2.450 Å, corresponding to the (101) plane of α-Mg (hcp) with a standard spacing of 2.452 Å, and no lattice fringe of MgH2 can be detected. The small particles are also homogeneously distributed on the surfaces of Mg. The lattice spacing of the small particle is evaluated to be 2.330 Å, which belongs to the (110) plane of Nb, 2.336 Å. These results further prove that MgH2 and NbH dehydrogenate completely into hcp-Mg and bcc-Nb phases, respectively, which agree well with the 10


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XRD results. It is observed from the inset histogram of Figure 4(a) that the Mg particle size varies from 50 to 200 nm with an average size of 125 nm, which is almost as big as the as-prepared Mg particles. As we know, the NPs tend to sinter and agglomeration at high temperature. However, the size of the Mg-Nb nanocomposite does not change apparently after the hydrogenation/dehydrogenation at 673 K in this work. This is due to the fact that the existence of Nb NPs (Figure 4(b)) on the surface of Mg effectively restrains the growth of Mg NPs. According to the discussion above, the reaction equations of the hydrogen absorption and desorption processes of the Mg-Nb nanocomposite can be summarized as follows.

Mg + Nb + H2 → MgH2 + NbH

(1)

Mg H2 + NbH→ Mg + Nb+ H2

(2)

Hydrogen storage properties: To further understand the catalytic effect of Nb NPs on the hydrogen properties of the Mg-Nb nanocomposite, the hydrogen absorption and desorption capacities and kinetics are tested by a carefully calibrated Sieverts type apparatus. The hydrogenation curves at various temperatures with an initial hydrogen pressure of 4MPa are presented in Figure 5(a). It is observed that the Mg-Nb nanocomposite can absorb 2.0 wt % H2 in 5 min at 373 K, and reach a value of 3.4 wt % H2 in 60 min, much better than the Mg-Ti-V composite which only can absorb less than 1 wt % H2 at the same temperature.32 It is also observed that the Mg-Nb nanocomposite can quickly absorb 4.4 wt% H2 in 15 min and reach a value of 5.7 wt % H2 in 60 min at 473 K, superior to the Mg NPs of 310 nm synthesized by the HPMR method that only absorbed 2.0 wt % H2 at 473 K in 60 min.32 In addition, Norberg also found that Mg NPs of 38 nm can absorb nearly 2.5 wt % H2 in 25 min at 493 K.37 Jeon et al. reported that the Mg NPs of 5 nm embedded in a gas-selective 11



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polymer matrix had a storage capacity of 4 wt % H2 in 80 min at 473 K.28 It can be concluded that the hydrogen sorption capacities of pure Mg NPs and Mg-polymer nanocomposite were still low. Thus, the enhanced hydrogenation properties of the Mg-Nb nanocomposite not only attribute to the nanosizing effect of Mg but also relate to the pronounced catalytic effect of Nb NPs. The Mg-Nb nanocomposite can also quickly absorb 5.8 wt% H2 in 5 min at 573 K, higher than the milled MgH2 + 5 at % Nb composite, which absorbed 5.2 wt % H2 within 5 min at 573 K.12 This indicates that in comparison with the ball milled MgH2 + 5 at % Nb composite, the acquisition of Mg and Nb NPs by using HPMR approach further accelerates the hydrogen absorption kinetics of Mg. The storage capacity of the Mg-Nb nanocomposite reaches 6.7 wt % H2 at 623 K and 7.1 wt % H2 at 673 K in 60 min, respectively. The theoretical gravimetric capacity of the Mg-7.5 wt % Nb nanocomposite is 7.1 wt % H2, indicating that the composite accomplishes full hydrogenation at 673K. Due to the partial oxidation, the pure Mg NPs fabricated by HPMR only absorbed 6.4 wt % H2 at 673 K, far below the theoretical storage capacity of 7.6 wt % H2.32 In this regard, the existence of a small amount of Nb NPs offers a high hydrogen storage capacity of Mg by protecting the Mg NPs from oxidation. The dehydrogenation curves in Figure 5(b) show that the hydrogen desorption rate and capacity of the Mg-Nb nanocomposite also increase remarkably with the increasing temperature from 523 to 673 K. The Mg-Nb nanocomposite can desorb 4.0 wt % H2 at 573 K, 6.3 wt % H2 at 623 K and 7.0 wt % H2 at 673 K within 60 min, respectively. Since the pure Mg NPs synthesized by HPMR can only release 0.8 wt % H2 in 30 min at 573K,32 it can be deduced that the NbH NPs on the surface of MgH2 NPs effectively improve the hydrogen desorption rate and capacity of MgH2. Guo et al. reported that the Mg-MgNb2O3.67 composite desorbed 6.0 wt % H2 at 623 K and 6.1 wt % H2 at 683 K respectively.38 12


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Given that MgNb2O3.67 can not absorb/desorb hydrogen, the existence of MgNb2O3.67 reduced the hydrogen storage capacities of the Mg-based composite. On the other hand, the Mg-Nb nanocomposite maintains high desorption capacity with the contribution of the good reversibility of NbH NPs during dehydrogenation. Hydrogenation/dehydrogenation process can take place through either isotropic diffusion and random nucleation, or preferential nucleation along certain favorable crystal axes.28 The characterization of the mechanism and dimensionality of MgH2 phase formation in the Mg-Nb nanocomposite can be obtained by means of theoretical kinetic model developed by Avrami.39 Assuming that the interface velocity of MgH2 formation is constant, the hydrogen sorption kinetics can be fitted with Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, which is expressed as the following linear equation: 28

ln[ − ln(1 − α )] = η ln k + η ln t

(3)

Where α is the hydrogenated fraction of Mg at time t, k is the phase transformation rate constant, t is the reaction time, and η is the dimensionality of MgH2 growth, namely, Avrami exponent of reaction order. For the experimental hydrogen absorption data at 473, 523, 573, 623 and 673 K fitted with JMAK model, it is observed in Figure 6(a) that the curve for each temperature provides a straight line with a slope η and an intercept ηln(k) by plotting ln[-ln(1-α)] vs. ln(t) curve relationship. The determined reaction order η values for different temperatures are approximately 0.5, indicating that the hydrogenation of the Mg-Nb nanocomposite is mainly controlled by the diffusion rate and belongs to one-dimensional growth.40 The hydrogen desorption data at 573, 623 and 673 K are fitted with JMAK model and shown in Figure 6(b). It is observed that the liner relationship is fitted very well. The reaction order η values of dehydrogenation for 13



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different temperatures are approximately 1, similar with the result of Mg-5 at %Nb nanocomposite, whose rate limiting step lies in H atomic migration through interconnected transformed domains of Mg.15 Given that the rate-limiting process of hydrogenation and dehydrogenation rests with the diffusion and migration of hydrogen through the hydride, it is suggested that the nanosized MgH2 can significantly reduce the

diffusion

distance

of

H

atoms,

and

consequently

accelerate

the

hydrogenation/dehydrogenation rate. After the calculation of the rate constant k values on the basis of the linear equation of hydrogenation and dehydrogenation, the apparent activation energies for the absorption and desorption process can be evaluated from the Arrhenius equation:  k = A • exp  E a  RT  

(4)

where A is a temperature-independent coefficient，Ea represents the activation energy, R is the gas constant (8.314 J mol-1K-1), and T is the absolute temperature. The absorption plot of ln(k) vs. 1000/T is shown in Figure 6(c), and the calculated Ea value for the hydrogen absorption of the Mg-Nb nanocomposite is 70.9 kJmol-1, much smaller than that of the 25 nm Mg particles with a value of 122 kJmol-1.37 Similarly, the desorption plot of ln(k) vs. 1000/T from the experimental data is shown in Figure 6(d). The hydrogen desorption activation energy of the Mg-Nb nanocomposite is 86.4 kJmol-1, smaller than that of the Mg-V NPs, 119.4 kJmol-1.34 It is also quite lower than the dehydrogenation activation energies of the 25 nm Mg particles of 126 kJmol-1 and the Mg(In)-MgF2 composite of 127.7 kJmol-1.37,41 It can be deduced that both the nanosizing effects and the super catalytic effect of Nb NPs contribute to the reduced Ea values for the hydrogenation and dehydrogenation of MgH2. Ouyang et al.42 demonstrated that the desorption activation energy of the in situ formed 59.7 wt % CeH2.73-MgH2-0.3 wt % Ni composite was 63 kJmol-1, lower than 14


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that of the Mg-Nb nanocomposite in this work. This is probably due to the synergetic catalysis of CeH2.73 and Ni during the dehydrogenation of the CeH2.73-MgH2-Ni composite. Nevertheless, the maximum hydrogen absorption content of the CeH2.73-MgH2-Ni composite is 4.03 wt % at 584K much smaller than that of the Mg-Nb nanocomposite (5.8 wt % H2) at 573 K due to the fact that CeH2.73 is unable to reversibly storage hydrogen. The hydrogenation process of Mg contains the following four steps: (1) the hydrogen physisorption at Mg surface; (2) the dissociation of hydrogen molecule; (3) the surface penetration of the hydrogen atoms into Mg and the formation of MgH2; (4) the diffusion of hydrogen through the hydride layer. It is known that the rate-limiting step is either the dissociation of hydrogen molecule at the surface or the diffusion of hydrogen through the hydride, or a combination of both.43 Most of the light metals do not have d-electrons, and hence their hydrogen association/dissociation rates at the surface are low. Mg without catalytic additives does not have sufficient ability to dissociate hydrogen molecules. In this work, the Nb NPs with d-electron shell have the ability of chemisorbing hydrogen and transferring hydrogen to the surface of Mg and effectively enhance the hydrogen sorption kinetics. The schematic mechanism for the catalytic effect of Nb NPs during hydrogenation/dehydrogenation of the Mg-Nb nanocomposite is illustrated in Figure 7. During the hydrogenation process, H2 molecules are initially dissociated into atoms on the surfaces of the Nb NPs due to the fact that these NPs are prone to significantly decrease the activation energy of H2 dissociation. Subsequently, H atoms undergo dissociative chemisorption on the Nb NPs. Considering the sufficient H2 pressure in the reaction environment, after the surfaces of the Nb NPs are fully saturated with dissociative H atoms, the Nb NPs spill over H atoms to the surface of Mg.44 Once the surfaces of the Mg NPs were saturated 15



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with H atoms, MgH2 layer is gradually generated on the interface between Mg and Nb, where the defects usually act as the active nucleation sites for MgH2. At this moment, the particle core is still of Mg phase. The diffusion of hydrogen through the MgH2 layer to the Mg core is rather difficult. Compared with microscale particles, the particles at nanoscale greatly decrease hydrogen diffusion distance and also provide high interfacial energy at the metal/hydride interface due to the large surface area. In this regard, the reduced hydrogenation activation energy of the Mg-Nb nanocomposite is attributed to both the nanostructure of Mg and the catalytic effect of Nb NPs by providing active “catalytic sites” and H “diffusion channels”. The affinity of the metallic element with hydrogen increases when its electronegativity decreases. The electronegativity of Nb is 1.6, which is much higher than that of Mg, 1.3.16 Therefore, Nb-H bond is weaker than Mg-H bond. During the dehydrogenation process, NbH starts to release hydrogen prior to the dehydrogenation of MgH2. It is proposed that MgH2 begins to dehydrogenate after the complete dehydrogenation of NbH. The released hydrogen from MgH2 diffuses into the Nb NPs through the interface of MgH2/Nb to form NbH. Then, NbH releases hydrogen again and transforms into Nb. In this regard, Nb NPs serve as the intermediate transfer carriers during the dehydrogenation process to accelerate the hydrogen desorption process of MgH2. Moreover, the interfaces at the Nb/MgH2 boundaries with high interfacial energy can act as fast hydrogen diffusion paths to accelerate the dehydrogenation process of MgH2. With the help of Nb/NbH hydrogenation/dehydrogenation cycling, MgH2 NPs completely change into Mg NPs finally. The pressure-composition-temperature experiments are carried out to evaluate the thermodynamic properties of the Mg-Nb nanocomposite. The P-C-T curves of the hydrogen absorption/desorption at 623, 648 and 673 K are plotted in Figure 8(a). 16


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Since there is only a small amount of Nb in the Mg-Nb nanocomposite, only one flat plateau is detected for each absorption/desorption process due to the transformation between Mg and MgH2 at each temperature. The equilibrium pressure corresponding to the phase transition between the solid solution dissolved with hydrogen and the metal hydride can be drawn as a function of temperature. The hydrogen equilibrium pressures of the absorption plateaus are 1.87 MPa at 673 K, 1.03 MPa at 648 K and 0.65 MPa at 623K. The hydrogen equilibrium pressures of the desorption plateaus are 1.58 MPa at 673 K, 0.96 MPa at 648 K and 0.57 MPa at 623 K. The Van’t Hoff plot, the logarithm of the equilibrium pressures ln(P) against the inverse temperature 1000/T, is obtained from these data, see Figure 8(b). The Van’t Hoff equation for absorption derived from the fitting line of the experimental data is ln(P) =-8.785/T +15.950. The formation enthalpy (∆Hab) for the Mg-Nb nanocomposites is determined as -73.0 kJmol-1, comparable to the reported enthalpy value for MgH2 (-75 kJmol-1).45 The Van’t Hoff equation for desorption is ln(P) =-8.543/T+15.453. The decomposition enthalpy (∆Hde) for the Mg-Nb nanocomposite is evaluated to be 71.0 kJmol-1. Thus, it is suggested that the addition of Nb NPs does not apparently change the thermodynamics of hydrogenation and dehydrogenation of Mg. The high hydrogen storage capacity and the enhanced hydrogen sorption kinetics are due to the catalytic effect of Nb NPs and the nanosizing effects of both Mg and Nb. CONCLUSIONS The Mg-7.5 wt % Nb nanocomposite has been successfully prepared by HPMR method. The Nb NPs of 12 nm are homogeneously decorated on the surfaces of the Mg NPs and effectively prevent the Mg NPs from growing during both the synthesis stage and the hydrogenation/dehydrogenation process. The Mg-Nb nanocomposite shows fast hydrogen absorption/desorption kinetics and high hydrogen storage 17



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capacity. It can quickly absorb 3.4 wt% H2 at 373 K and 5.7 wt% H2 at 473 K in 60 min. The apparent activation energies for hydrogen absorption and desorption are calculated to be 70.9 and 86.4 kJmol-1, respectively. The enhanced hydrogen storage capacities and the reduced apparent activation energies are due to the prominent catalytic effect of Nb NPs and the nanosizing effects of both Mg and Nb. AUTHOR INFORMATION Corresponding Author *

Tel. & fax: +86 10 8231 6192; [email protected]

ACKNOWLEDGMENTS The authors acknowledge the support of this work by the 973 project of MOST of China (No. 2013CB035503), the Aeronautical Science Foundation of China (No. 2014ZF51069), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. REFERENCES (1) Tozzini, V.; Pellegrini, V. Prospects for Hydrogen Storage in Graphene. Phys. Chem. Chem. Phys. 2013, 15, 80-89. (2) Sevilla, M.; Mokaya, R. Energy Storage Applications of Activated Carbons: Supercapacitors and Hydrogen Storage. Energy Environ. Sci. 2014, 7, 1250-1280. (3) Dalebrook, A. F.; Gan, W. J.; Grasemann, M.; Moret, S.; Laurenczy, G. Hydrogen Storage: Beyond Conventional Methods. Chem. Commun. 2013, 49, 8735-8751. (4) Bérubé, V.; Radtke, G.; Dresselhaus, M.; Chen, G. Size Effects on the Hydrogen Storage Properties of Nanostructured Metal Hydrides: A Review. Int. J. Energy Res. 2007, 31, 637-663. (5) Sakintuna, B.; Lamari-Darkrim, F.; Hirscher, M. Metal Hydride Materials for Solid Hydrogen Storage: A Review. Int. J. Hydrogen Energy. 2007, 32, 1121-1140. 18


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FIGURE CAPTIONS Figure 1. (a) TEM bright-field image of the as-prepared Mg-Nb nanocomposite; (b) HR-TEM image of the selected zone; (c) the size distribution of the big Mg particles; (d) the size distribution of the small black Nb particles; (e) EDS result of the Mg-Nb nanocomposite. Figure 2. XRD patterns of the Mg-Nb nanocomposite: (a) as-prepared; (b) after the hydrogenation under 4 MPa hydrogen pressure at 673 K; (c) after the dehydrogenation under 100 Pa and at 673 K. Figure 3. The Mg-Nb nanocomposite after the hydrogenation under 4 MPa hydrogen pressure at 673 K: (a) TEM bright-field image and the inset histogram shows the MgH2 particle size distribution; (b) HR-TEM image of one small particle on the surface of one big particle. Figure 4. The Mg-Nb nanocomposite after the dehydrogenation under 100 Pa at 673 K: (a) TEM bright-field image and the inset histogram of the Mg particle size distribution; (b) HR-TEM image of one small particle on the surface of one big particle. Figure 5. (a) Hydrogen absorption curves under 4 MPa hydrogen pressure; (b) desorption curves under 100 Pa of the Mg-Nb nanocomposite. Figure 6. Plots of ln[-ln(1-α)] vs. ln(t) for the hydrogenation (a) and the dehydrogenation (b) of the Mg-Nb nanocomposite; plots of ln k vs. 1000/T for the hydrogenation (c) and dehydrogenation (d) of the Mg-Nb nanocomposite. Figure 7. The schematic diagram of the catalytic mechanism of Nb NPs during the hydrogenation/dehydrogenation of the Mg-Nb nanocomposite. Figure 8. P-C isotherm curves at 623, 648 and 673 K (a) and Van't Hoff plots (b) for the Mg-Nb nanocomposite. 24


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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 6

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Figure 8

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Table of Contents (TOC) Image

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Catalytic Effect of Nb Nanoparticles for Improving the Hydrogen

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